Symposium EN11 : Nanomaterials for the Water and Energy Nexus

Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors for terrestrial and space-based photovoltaics (PVs). Due to their high emission efficiencies and readily tunable emission and absorption spectra, colloidal quantum dots (QDs) have emerged as promising LSC fluorophores, superseding dye molecules previously dominating the LSC field. An important advantage of the QDs over dye-based systems is a possibility to greatly reduce losses to re-absorption by displacing the emission band from the onset of strong optical absorption, the approach commonly referred to as "Stokes-shift engineering." Spectral tunability of the QDs also facilitates the realization of stacked multi-layered LSCs wherein the enhanced performance is obtained through spectral splitting of incident sunlight, as in multi-junction PVs. This presentation will discuss several approaches to Stokes-shift engineering with II-VI [1,2] and I-III-VI₂ [3,4] QDs and also describe the first example of a large-area (>200 cm²) tandem LSC based on two types of nearly reabsorption-free QDs spectrally-tuned for optimal solar-spectrum splitting [5]. This prototype device exhibits a record-high optical quantum efficiency of more than 6% for sunlight illumination and solar-to-electrical power conversion efficiency of >3%. Due to their strong performance achievable with low-cost, solution-processible materials, QD-based LSC tandems can provide a viable pathway for further reducing the cost of solar electricity by complementing the existing PV technology with inexpensive, high-efficiency sunlight collectors deployable either as strongly absorbing LSC-PV modules or semi-transparent building-integrated solar windows.

1. Meinardi, F., Colombo, A., Velizhanin, K. A., Simonutti, R., Lorenzon, M., Beverina, L., Viswanatha, R., Klimov, V. I. & Brovelli, S. Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA matrix. Nature Phot. 8, 392-399 (2014).
2. Li, H., Wu, K., Song, H.-J. & Klimov, V. I. Doctor-blade deposition of quantum dots onto standard window glass for low-loss large-area luminescent solar concentrators. Nature Energy 1, 16157 (2016).
3. Meinardi, F., McDaniel, H., Carulli, F., Colombo, A., Velizhanin, K. A., Makarov, N. S., Simonutti, R., Klimov, V. I. & Brovelli, S. Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots. Nature Nanotech. 10, 878-885 (2015).
4. Klimov, V. I., Baker T. A., Lim J., Velizhanin, K. A., McDaniel H. Quality Factor of Luminescent Solar Concentrators and Practical Concentration Limits Attainable with Semiconductor Quantum Dots, ACS Phot. 3, 1138 -1148 (2016)
5. Wu, K., Li, H.,. & Klimov, V. I. Tandem luminescent solar concentrators based on engineered quantum dots, submitted (2017).

To achieve effective solar power generation with photovoltaic arrays and concentrated solar power technologies alike, specialized material coatings are essential for the system-wide control of propagating radiation – specifically for solar-thermal power conversion, a class of high-temperature, air-stable material coatings, characterized by high UV-VIS absorptance and low NIR emittance are favorable for energy efficient operation, functioning to mitigate losses via re-radiated waste heat. The synthesis of intrinsic coatings, as opposed to the use of multilayer heterogeneous configurations, generally offer the benefit of easier fabrication at the cost of slightly reduced absorptive performance. We report on coatings fabricated through hydrothermal and co-precipitation nanomaterial syntheses, in which uni-metallic and bi-metallic oxides (CuO, Co3O4, Cu0.15Co2.84O4, and Cu1.5Mn1.5O4) are synthesized as nanomaterials before being ported to an absorber coating; these reaction methods consistently generate phase-stable products that are viable for bulk manufacturability. To maximize the spectral absorptance capability of the coatings, surface texturing modifications are introduced by embedding sacrificial polymer beads in the coating precursors prior to high-temperature curing and annealing reaction stages. In optimizing the reaction conditions and ensuring best absorptance capability, nanoparticle features are analyzed during various synthesis stages via optical and field-emission scanning electron microscopy data. In scanning for morphological detail in both the spatial and frequency domain, multiresolution analysis is used to approximate particle density, sizing, and distribution. This data is then correlated to the materials’ scattering events using a first-order Harvey-Shack model and differential raytracing methods, which approximate scatter contributions to the spectral irradiance profile of the material, relating surface structuring to the coating’s solar absorptance. Additional performance enhancements are implemented for reducing waste byproducts and minimizing water consumption, to ensure the low-cost of process reactants and ease-of-synthesis for high-purity materials. In this way, coupled simulation and experimental approaches for characterizing preceding nanomaterial syntheses result in absorber coatings tenable for long-term usage in solar power generation systems.

Downconversion, also known as quantum cutting [1], has attracted much attention due to the potential application in photovoltaic cells. The downconversion materials placed on the front surface of solar cell are able to harvest the far above-bandgap light and split the high energy photons into two photons that can still be absorbed by the solar cells. This is an avenue to realize current (and thus efficiency) doubling for the high energy (blue, UV) part of the solar spectrum.
Even though downconversion has been demonstrated for a variety of materials, downconversion materials are still far from practical application. Downconversion materials rely on lanthanide ions. Their low absorption cross sections result in low excitation efficiency and only a small fraction of the solar spectrum can be harvested [2]. Therefore, enhancing the efficiency by means of broadband absorption by so-called sensitizers is crucial. An important class of sensitizers, organic dyes, has been extensively investigated in upconversion materials because of the 3-4 order of magnitude higher absorption cross section [3-4], but so far not for downconversion. In this work, we demonstrate dye sensitized downconversion with a proof-of-concept experiment. Luminescence spectra and decay lifetime measurements shows the occurrence of Förster energy transfer from dye molecules absorbed on the surface on NaYF₄:Pr,Yb nanocrystals. Energy transfer from the dye to Pr³⁺ is followed by energy transfer to two neighboring Yb³⁺ ions and emission of two infrared photons. The NaYF₄:Pr³⁺Yb³⁺ nanocrystals have ~30 times stronger infrared emission intensity after dye sensitization. The decrease in lifetime of the dye emission after absorption at the nanocrystal surface serves as evidence for energy transfer and can be used to quantitatively determine the efficiency of Förster energy transfer. The present study demonstrates the feasibility of dye sensitized downconversion and the strategy can be used to enhance the downconversion efficiency for a wide range of lanthanide doped materials.

References:
[1] Science 1999, 283, 663-666.
[2] Chem. Soc. Rev. 2013, 42, 173-201.
[3] Nat. Photonics 2012, 6, 560-564.
[4] Chem. Soc. Rev. 2017, 46, 4150-4167.

The production of integrated electronic circuits provides examples of the most advanced fabrication and assembly approaches that are generally characterized by large-scale integration of high-performance compact semiconductor elements that rely on rigid and essentially planar form factors. New methods of fabricating micro-scale semiconductor devices provide a set of enabling means to lift these constraints by engendering approaches to device configurations that would be impossible to realize with bulk, wafer-scale materials while retaining capacities for high (or altogether new forms of) electronic and/or optoelectronic performance. An exemplary case of interest in our work includes large-area integrated electro-optical systems for photovoltaic energy conversion that can provide a potentially transformational approach to supplant current technologies with high performance, low cost alternatives. In this talk I will highlight progress made in the collaborative research efforts of the LMI EFRC that illustrates important opportunities for exploiting advances in optical and electronic materials in synergy with physical means of patterning, fabrication, and assembly to advance capabilities for photovoltaic energy conversion and highlight emerging applications for unconventional form factor high efficiency energy conversion and storage technologies. Of particular interest are the materials, and new understandings of science, that will allow an efficient utilization of the full solar resource.

Single-walled carbon nanotubes (SWNT), graphene, and fullerene (C₆₀ and PCBM) would be very efficiently used in lead halide Perovskite solar cells. A film of SWNTs or graphene can be flexible and stretchable transparent-conductive layer. At the same time, this film can be carrier-selective layers, i.e., electron-blocking-layers or hole-blocking-layers, by using adequate polymer-doping. Based on our experiences of using nanotube films for CNT-Si solar cells and organic polymer solar cells [1,2], we have extended the application of SWNT films for organic-inorganic Perovskite solar cells. We have demonstrated the replacement of ITO in inverted-type perovskite solar cells, SWNTs/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Al [3]. The flexible application on polyethylene terephthalate (PET) is also demonstrated [3]. Replacement of electron-blocking-layer and metal electrode in normal-type perovskite solar cells is demonstrated as well. They show high power conversion efficiency (PCE), cost-efficiency, and higher stability. Those devices can have comparable PCE as the conventional design with organic electron-blocking layer and top metal electrode. The normal-type perovskite solar cell, composed of ITO/C₆₀/CH₃NH₃PbI₃/SWNTs, can achieve a PCE of 17 % with spiro-MeOTAD as dopant to SWNTs [4]. This structure with a perovskite layer sandwiched by C₆₀ and SWNTs can lead to the solar cells without hysteresis and with much improved air-stability [4]. The effective passivation of the degradation of perovskite material by moisture can be achieved with C₆₀ and SWNTs [4]. More recent configuration is using a film of SWNTs for both anode and cathode electrode [5]. With adequate polymer-doping, we can fabricate Perovskite solar cells without ITO and metal electrode. Finally, SWNT film and graphene are compared as flexible transparent electrode of inverted Perovskite solar cells [6].
This work was supported by JSPS KAKENHI Grant Numbers JP25107002 and JP15H05760.

References:
[1] I. Jeon, K. Cui, T. Chiba, A. Anisimov, A. Nasibulin, E. Kauppinen, S. Maruyama, Y. Matsuo, J. Am. Chem. Soc., 137, 7982 (2015).
[2] I. Jeon, C. Delacou, A. Kaskela, E. I. Kauppinen, S. Maruyama, Y. Matsuo, Sci. Rep. 6, 31348 (2016).
[3] I. Jeon, T. Chiba, C. Delacou, Y. Guo, A. Kaskela, O. Reynaud, E. I. Kauppinen, S. Maruyama, Y. Matsuo, Nano Lett. 15, 6665 (2015).
[4] N. Ahn, I. Jeon, J. Yoon, E. I. Kauppinen, Y. Matsuo, S. Maruyama, M. Choi, submitted.
[5] I. Jeon, S. Seo, Y. Sato, C. Delacou, A. Anisimov, K. Suenaga, E. I. Kauppinen, S. Maruyama, Y. Matsuo, J. Phys. Chem. C, (2017) [DOI: 10.1021/acs.jpcc.7b10334].
[6] I. Jeon, J. Yoon, N. Ahn, M. Atwa, C. Delacou, A. Anisimov, E. Kauppinen, M. Choi, S. Maruyama, Y. Matsuo, J. Phys. Chem. Lett., 8 (2017), 5395.

The oxygen evolution reaction (OER) at the anodic electrode occurs in electrochemical energy conversion and storage technologies, including rechargeable metal-air batteries, regenerative fuel cells, and water splitting. However, the state-of-the-art are precious metal-based catalyst which suffer from higher cost and makes them unsuitable for large-scale industrial application. Thus, developing low-cost, durable, earth-abundant, and high-performance non-precious metal electrocatalyst for oxygen evolution reaction (OER) is essential to improve the overall efficiency of water splitting. Electrochemically Hydrogen production has gained great interest over the past decades as a cleaner, higher-purity and sustainable production technology for carbon-neutral alternative fuel sources. Hydrogen is crucial for the future hydrogen energy technologies.¹ However, poor OER thermodynamic up-hill reaction limits the efficiency of H₂ production from water electrolysis and photoelectrolysis routes to large-scale energy storage. It is crucial to develop efficient and low-cost material to boost the sluggish kinetics step of four-electron OER process. Despite of a well-documented approach for synthesizing metal borides, its application as an OER electrocatalyst has not gained enough attention. Owing to the presence of boron which diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction of the metal active centers, interest into the application of metal borides as efficient catalyst have emerged for this application and it is highly required to tailor its electrochemical properties.^{2, 3} However, the synergic effect had not been reported for multimetal borides material which was find to be the most OER among the reported metal borides materials.⁴ Herein, we investigate the OER electrocatalytic properties of an amorphous trimetallic borides nanostructure synthetized by a simple, one-step approach, yet exhibits robust electrochemical performance and outstanding stability in harsh alkaline condition. It exhibits an overpotential (η) of 274 mV to deliver a geometric current density (j_geo) of 10 mA cm^-2, a small Tafel slope of 38 mV dec^-1. The impressive electrocatalytic performance originates from the unique amorphous multimetal-metalloid complex nanostructure. From application point of view, this work holds great promise as this process is simple and allows for large scale production of cheap yet efficient material.

Keywords: Solar-driven Catalysis, Water Splitting, Electrocatalyst; Earth-abundant, Metal borides.

References
1. J. A. Turner, Science, 2004, 305, 972-974.
2. J. Masa, P. Weide, D. Peeters, I. Sinev, W. Xia, Z. Sun, C. Somsen, M. Muhler and W. Schuhmann, Adv. energy Mater., 2016, 6, 1502313-n/a.
3. H. Li, P. Wen, Q. Li, C. Dun, J. Xing, C. Lu, S. Adhikari, L. Jiang, D. L. Carroll and S. M. Geyer, Adv. Energy Mater., 2017, 7, 1700513-n/a.
4. J. M. V. Nsanzimana, Y. Peng, Y. Y. Xu, L. Thia, C. Wang, B. Y. Xia and X. Wang, Adv. Energy Mater., 2017, 7, 1701475.

Semiconductor photocatalysis is a promising route for the use of solar energy for the cost-effective treatment of water for the decomposition of contaminants and other applications, including H₂ production, organic chemical synthesis and the conversion of CO₂ to fuel. Bismuth oxides are promising for these applications due to the wide availability of bismuth from other industrial processes and their relatively benign impact on the environment. Bismuth oxide halides (BiOX, where X = F, Cl, Br and I) may offer improvements in any of these aspects: band gap narrowing, change to an indirect band gap and reduced electron-hole recombination rates.
Here, we discuss the investigation of a facile, scalable route for the production of bismuth oxychloride nanoparticles for solar photocatalytic water treatment from a simple solution treatment of Bi₂O₃. Depending on chlorine to bismuth stoichiometry, and the time of reaction, different nanoscale morphologies can be obtained. Under low Cl:Bi stoichiometry, nanosheets of ~5 nm thickness and up to 1 mm in diameter are formed; while, under high Cl:Bi stoichiometry, nanoplates of ~20 nm thickness and ~200 nm in diameter are formed that become thicker with reaction time. The control of particle morphology through reaction parameters and its effect on optical properties and photocatalytic efficiency are discussed. The phase transformation mechanism, and its application to the synthesis of more complex bismuth based nanostructures are also considered.

Thermal energy, its transport and conversion, are critical aspects of the quest to develop the new generation of efficient energy technologies. Yet, traditionally, thermal energy is often not treated with needed care toward maximization of its potential with respect to all: its capturing, utilization, storage and eventually, the level at which it is considered as waste. Here I discuss cases of innovative technologies targeting to alleviate such deficiencies, all involving water as the working or targeted liquid.

To begin with, the Information Technology (IT) industry must play a key role in the global effort to reduce carbon-dioxide emissions with Low-Power High-Performance Computin, as it consumes 2% of the world energy with a strongupward trend. With this as a driver, I will demonstrate, the employment of an emerging, water-cooled processor architecture, exhibiting a higher efficiency in terms of MFlops/W, for the solution of challenging scientific problems and to harness the heat that is inevitably generated as close as possible to the source with a very high temperature level. This allows, for example, the heat to be re-used for space heating or process heat. The goal is to demonstrate a high performance, low power consumption datacenter/computing operation approaching zero net emission. The same approach amf developed devices can be taken to use the captured heat from the water cooling of high concentration photovoltaics for energy deficient applications running in parallel, such as desalination. Note here that geographically rich regions in solar radiation, often suffer simultaneously from fresh water scarcity.

Next, the current global electricity production stands at 23000 TWh with about 3000 TWh generated within Europe and is increasing steadily. It is likely to grow nearly 69% from present levels by 2040. However, more than 80% of the global electricity generation, and 48% of European power generation is based on the steam cycle that uses fossil fuels and nuclear fission^1-3. This results in power generation being the largest contributor to greenhouse gas emissions (GHGs). Enhancing the thermal efficiency of a broad range of water condenser devices requires means of achieving sustainable dropwise condensation on metallic surfaces through surface micro- nanoengineering. To this end, I will discuss a rationally driven, hierarchical micro- nanotexturing process of copper surfaces, guided by fundamental principles of wettability and coalescence, which achieves controlled droplet departure under vapor flow conditions, significantly enhancing phase change thermal transport, by nearly 700% increase in heat transfer coefficients compared to filmwise condensation.

1. OECD Factbook 2015-2016 Economic, Environmental and Social Statistics; Paris, 2016.
2. European Commission. Electricity production, consumption and market overview; 2017.
3. International Energy Outlook 2016; 2016.

The development of a smart surface with reversible wettability between water-attracting (hydrophilic) and water-repelling (hydrophobic) state in response to external stimuli is presented. Stimuli-responsive surfaces are of great interest due to their versatility of applications, including microfluidics, water harvesting, wastewater treatment and oil-water separation. Here, we present an electrochemical approach for fast and reversible wettability control on core-shell Cu-CuO_x dendritic structure by manipulating the oxidation state of the CuO_x shell phase. The wetting switching from superhydrophobic (contact angle > 150°) to superhydrophilic (contact angle < 10°) regime could be accomplished within a few seconds to a few minutes by applying a low voltage (< 1.5 V). The modulation of the magnitude and duration of the applied voltage controls precisely the rate and extent of the transition. The initial superhydrophobicity is fully regained by air drying at room temperature for 1 hour or mild heat drying at 100°C for 30 min. Microstructural analysis based on the scanning transmission electron microscopic high-angle annular dark-field imaging (STEM-HAADF), energy-dispersive X-ray spectroscopy (EDS) mapping and X-ray photoelectron spectroscopy (XPS) revealed the presence of CuO_x surface film shielding the Cu core. Electrochemical analysis showed that the in-situ wetting transition is based on the Faradaic phase transformation of the surface bound CuO_x groups. During electroreduction of the outermost oxide layer, the surface transitioned from low adhesive rolling state (lotus effect), to high adhesive pinning state (petal effect), and eventually to superwetting state with superior water-absorbing ability (fish scale wetting). Gravity-driven separation of oil-water mixtures demonstrated the functionality of the in-situ wettability switching of the Cu-CuO_x core-shell nanostructures. We showed that the as-deposited Cu mesh exhibiting superhydrophobicity and superoleophilicity is effective for heavy oil-water separation. On the other hand, application of a small reduction voltage (< 1.5 V) remediated light-oil contaminated water. The voltage application drives the electrochemical reduction of the CuO_x shell phase, converting the Cu mesh into superhydrophilic/underwater superoleophobic state. Depending on the specific treatment needs, the mesh can be switched between oil-removal and water-removal mode for water purification contaminated with organic solvents of different densities. The separation efficiencies for a series of oil-water mixtures are above 98% for 30 separation cycles, illustrating good recyclability of the mesh for long-term operation at industrial scale. The findings open a new avenue for exploration of various metal oxide materials for redox reaction-mediated wetting and adhesion tuning in areas such as selective droplet transportation, heat transfer manipulation, and controllable drug delivery.

Biomimetic oil-infused surfaces promise great potential in water and energy applications, such as condensation processes. Surface micro/nano-structures facilitate the oil infusion and therefore enhance the water condensate mobility. In this work, we fabricate micro-grooves on surfaces and coat them with nanoparticles to strengthen the oil infusion. We demonstrate directional transport and spontaneous removal of condensing droplets can be achieved with diverging micro-grooved channels. Condensate droplets on the channel walls would submerge into the microgrooves owing to the capillary pressure gradient in the infusing oil. The submerged and confined droplets deform and exhibit a difference between the back and front curvatures, which makes the droplets move along the channel diverging direction. These nano-porous groove surfaces possess the ability to uniformly spread the infusing oil and minimize its drainage. The surface oil-infusion performance is experimentally evaluated by studying the impact of surface structure dimensions and intrinsic wettability on oil drainage. Our findings on spontaneous condensate removal offer great opportunities in enhancing condensation heat transfer.

Microbubbles produced by plasmonic heating or chemical reactions play an important role in emerging and efficient plasmonic-enhanced processes for catalytic conversion, solar energy harvesting, biomedical imaging, and cancer therapy. In this work, the growth dynamics of nucleating bubbles from plamonic heating are studied to determine the exact origin of the occurrence and growth of these bubbles. The microbubbles were measured in air-equilibrated water (AEW) and degassed water (DGW) with fast imaging. Our experimental data reveals that the growth dynamics can be divided into two regimes: an initial bubble nucleation phase and subsequently a bubble growth phase. The explosive growth in regime I is identical for AEW and DGW due to the vaporization of water. However, the slower growth in regime II is distinctly different, which is attributed to the uptake of dissolved gas expelled from the water around the hot nanoparticle. We also experimentally and theoretically examine the growth and detachment dynamics of oxygen bubbles from hydrogen peroxide decomposition catalyzed by gold. We measured the bubble radius R(t) as a function of time by confocal microscopy. The results show that the dynamical evolution of bubbles is influenced by comprehensive effects combining chemical catalysis and physical mass transfer. The size of the bubbles at the moment of detachment is determined by the balance between buoyancy and surface tension and by the detailed geometry at the bubble’s contact line.

Bimetallic alloy nanostructures (NS) have attracted much interest compared to their monometallic counterparts for a variety of catalytic applications. Despite much progress, rapid synthesis of them has still been a big challenge. Herein, we reported a generalized strategy of rapidly synthesizing bimetallic alloy NS in a planar on-chip reactor consisting of polymer-derived porous graphene (G) via a transient laser induced process. In the process, G acts as a reactor that holds metal alloy precursors. When laser scans, G absorbs the light power and then converts it to transient localized heat to direct the nucleation and crystallization of metal alloy NS. Here, Fe-Ni alloy is chosen as a model material to demonstrate this process. In the final hybrid material of Fe-Ni/G, G serves as a conductive support, which benefits the electron transfer between the Fe-Ni alloy NS and the electrodes. The optimized Fe-Ni/G shows a superior oxygen evolution reaction (OER) activity with an overpotential of 281±4 mV at a current density of 10 mA/cm² and a low Tafel slope of 51 mV/dec. This work would offer a universal methodology for synthesizing various metal alloy NS with controlled structures and properties, laying foundations for their widespread applications in energy storage and conversion.

Solar driven photoelectrochemical water splitting is a promising method to directly convert renewable, abundant solar energy into hydrogen fuel. For this technology to be commercially viable, devices for water splitting must be efficient, cheap, and stable. Most of the top-performing devices for unassisted water splitting, however, utilize expensive III-V materials such as InGaP₂ and GaAs as the absorber material, thus limiting their scalability. Additionally, most efficient semiconductor absorbers exhibit instability under water splitting conditions, leading many to employ a separated photovoltaic + electrolyzer system design which may incur higher balance of systems costs. We present an efficient, integrated solar water splitting device using silicon-only photovoltaic junctions. By connecting three silicon Heterojunction with Intrinsic Thin layer (HIT) solar cells in series and using an atomic layer deposited (ALD) TiO₂ protection layer, we are able to demonstrate unassisted water splitting with over 10% solar-to-hydrogen efficiency in both concentrated acid and base electrolyte under one sun illumination. Enhanced (>60h) chemical stability is achieved by incorporating the ALD-TiO₂ layer in these compact, integrated devices. The TiO₂ is able to protect the underlying HIT cell because of its excellent stability over a range of pH and potentials, and does not limit the efficiency of the HIT cell due to good band alignment with the conduction band of silicon. The device design, in which the illumination occurs on the side of the solar cell opposite the electrochemical reaction, avoids both parasitic losses due to light absorption and reflection and the instability of silicon at anodic potentials. We use electron beam deposited Pt as the HER catalyst. Iridium oxide or NiFe prepared by chemical solution deposition or by electrodeposition, respectively, act as the OER catalyst. These layers are deposited on porous Ti or Ni substrates, achieving an OER overpotential of 270 mV at 10 mAcm^-2.

Lithium iron phosphates (LiFePO₄ and Li₃Fe₂(PO₄)₃) are a group of structurally different cathode materials for lithium-ion batteries (LIBs). In particular, LiFePO₄ was reported and proposed as possible alternatives to the LIB cathode material LiCoO₂, which is the most popular cathode material for rechargeable consumer applications, such as phones, laptops, and electric vehicles. LiCoO₂-based LIBs are however unsustainable; the raw materials are not widely available and, despite improvements in battery design, the batteries are prone to suffer from dangerous oxygen release upon overcharge, which can lead to fire hazards. LiFePO₄ has a high theoretical energy density of 170 mAh/g, good chemical and physical stability and a discharge potential of 3.4-3.5 V vs. Li/Li⁺. However, LiFePO₄ cathode suffer from poor electronic conductivity and low ionic diffusivity, which can lead to losses of high initial capacity and poor rate capabilities which can hinder its practical applications. Multi-walled carbon nanotubes (MWCNTs) providing tridimensional networks have been proven to be the most effective in reducing the resistance and improving the electrochemical performance of the nanocomposites composed of MWCNTs and LiFePO₄ nanomaterials. In this work, nanocomposite of LeFePO₄/MWCNTs was prepared as cathode materials for LIBs. First, a sheet of MWCNTs, so called ‘Buckypaper’ with uniform thickness was prepared by surface-engineered tape-casting (SETC) technique. SETC enables fabrication of strong stand-alone structured carbon nanotube (CNT) sheet with tunable thickness and composition. Secondly, nanometer-scale LiFePO₄ particles were grown on the surface of the MWCNTs by hydrothermal process. A sheet of buckypaper was treated by O₂ plasma to enhance hydrophilicity and at the same time generate nucleation sites in the MWCNT networks for growth of LiFePO₄ particles. The precursors include stoichiometric amounts of LiCl, FeCl₂, H₃PO₄ with ascorbic acid as reducing agent and Deionized water. The resulting mixture was introduced into a sealed Teflon-lined autoclave and heated at 160°C for 3 h. It was found that nanometer-scale LiFePO₄ particles with good crystal quality were grown on the MWCNTs sheet and uniformly distributed. Size and distribution density of the LiFePO₄ particles were controlled by the experimental conditions such as mixing ratio of precursor chemicals, temperature and time of hydrothermal process as well as pre-treatment of MWCNT sheets with O₂ plasma. X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Raman spectroscopy were used to characterize the nanocomposite composed of LiFePO₄ nanoparticles on MWCNT sheet. The electrochemical characterization of the cathode of LiFePO₄/MWCNTs is in progress using a CR2025-type coin cell with lithium foil and anode. This process could pave the way of enabling fabrication of ‘Flexible’ lithium-ion batteries with light-weight, high energy density and improved safety issue.

Although Li-ion batteries are very promising for energy storage applications, their areal capacity is quite low (2.09 mAh cm^-2 for conventional LiFePO₄ on aluminum foil). The aim of this work is to enhance the areal capacity of LiFePO₄ cathode via Surface-Engineered Tape-Casting (SETC) technique. SETC technique enables fabrication of strong stand-alone structured carbon nanotube (CNT) sheet with tunable thickness and composition. The CNTs form electrically conductive networks that also result in enhancing the charge transport of Li-ions. In addition, areal capacity will be increased by using the following approaches: increasing the thickness of CNT-LiFePO₄ sheet and changing the CNT/LiFePO₄ ratio. We will determine the optimum thickness and composition of the CNT-LiFePO₄ sheet. Batteries will be assembled in a glove box and characterized (CV, charge-discharge, EIS) using a potentiostat/galvanostat. Investigation of the multi-layered CNT-LiFePO₄ sheets by scanning electron microscopy, Raman scattering spectroscopy and X-ray diffraction is also in progress. Preliminary results show 0.4 mAh cm^-2 for 2 layers of CNT-LiFePO₄ (CNT:LiFePO₄ = 1:1, w/w ratio). Fabrication of the electrodes with more than 10 layers of CNT-LiFePO4 is in progress. Such thick CNT-LiFePO₄ electrodes is very good candidate for applications which is very sensitive to weight but not sensitive to size, such as space applications and electrical vehicles.

Using TiO₂ nanostructures in photoelectrochemical water-splitting have attracted intensive research as one of the most promising methods for Hydrogen production. Unfortunately, the challenge to overcome the large band gas is still persisting. Alloying, annealing in a reducing atmosphere, and multipodal nanotubes have proven to be effective pathways owing to structure modification, band gap tuning, graded refractive index, and easier charge transport. Herein, we make use of alloyed single and multipodal Ti-Nb-Zr-O nanotubes (MPNTs) annealed under different atmospheres: Air, O₂, and H₂ to enhance TiO₂ behavior. SEM was used to confirm the formation of MPNTs. Structural analysis using XRD, Raman Spectroscopy, and XPS confirmed the formation of a single mixed oxide Ti-Nb-Zr-O in a strained-anatase crystal structure in both Air and Oxygen atmosphere, while on the other hand, ZrTiO₄ appeared in the Hydrogen annealed samples. XPS analysis showed the formation of valence band tail states causing a band-gap reduction in the hydrogen annealed samples, which in turn reflected on the absorption spectra, which showed a similar tail, reaching NIR/Vis. region. Mott-Schottky analysis showed 4 orders of magnitude increase in the carrier density when compared to other annealing atmospheres. These synergistic effects resulted in almost 25-fold enhancement in the photocurrent compared to Oxygen and Air annealing. Multinary metal oxides would act as a promising candidate for engineering the required material properties for efficient PEC because of the vast number of possible material combinations.

Growing concerns about environmental hazards of current fossil fuel-dependent society have led to development of technologies for sustainable energy cycle. Water electrolyzer plays key role in converting renewable electricity to chemical energy by fabricating fuel in this cycle. The bottleneck in efficient electrolyzer is oxygen evolution reaction(OER), which requires significant amount of overpotential. As conventional precious metal based OER catalysts have shown limitations such low stability and high cost, earth-abundant 3d transition metal oxides have been explored as alternative OER catalyst with high activity and cost-effectiveness. A variety of materials have been reported, including (oxy)hydroxides, spinels, perovskites and organometallics. Among them, Ni-Fe (oxy)hydroxide particularly shows promising activity in alkaline electrolysis. Although the origin of high activity and reaction mechanism of Ni-Fe (oxy)hydroxide is still on debate, recent progress reveals that active sites of Ni-Fe (oxy)hydroxide are located at the edge or defect sites of (oxy)hydroxide sheet. This show the limitation of (oxy)hydroxide material fabricated by conventional electrodeposition or hydrothermal method, because only limited number of active sites are exposed and most of the metal sites remain inactive.
In this study, we adopted a novel approach to electrochemically activate Ni-Fe alloy substrate to generate highly active OER catalyst. In contrast to conventional electrodeposition or hydrothermal method in which precursor ions in solution are deposited as catalyst, our method turns the elements in substrate to active sites with optimal structure and composition. The specific OER activity of activated Ni-Fe is significantly higher than previously reported (oxy)hydroxide catalyst. Voltammetric analysis of Ni redox behavior reveals that activated Ni-Fe shows distinct feature from conventional (oxy)hydroxide sheet and has certainly higher density of active sites. Moreover, easy tuneability of substrate composition and microstructure makes it possible to add other elements and change structure, suggesting strategies to enhance activity and stability of the catalyst. This study provides a novel platform for designing electrocatalysts not only for water oxidation but also for other electrochemical applications.

Strong electric field enhancement in localised surface plasmon resonance (LSPR) modes provides a useful way to channel energy transfer between light and matter in nanostructured metals. Shaped metallic nanoparticles exhibit controllable red shifts of absorption potentially useable in solar cells and near infrared detectors, though usually with spread out absorption response. Several methods of fabricating shaped metallic nanostructures exist, including electron beam lithography, and nanosphere lithography, besides solution-processed techniques. Large area solar cells require low cost absorbing materials accessible through solution-phase deposition using environmentally safe inks. We report a refinement of solution-processed methods using green synthesis under controlled temperature, and illumination conditions, involving two-generation seeded growth and evolution of seeded spherical silver nanoparticles into triangular nanoparticles of controlled shape and size. Initial synthesis of silver nanoparticles using a standard citrate method was unremarkable. Measured absorbance at 392 nm was correlated with spherical silver particles with a size distribution centered at 8 nm measured using transmission electron microscopy (TEM). Addition of Ag(I) ions under 590 nm illumination evolved the aqueous ink over a period of 72 hours to triangular nanoparticles with edge lengths centered at 65 nm and a redshift of the absorption to the 600-800 nm band. Longer illuminated period synthesis is found to extend the band tail of absorption to 1200 nm, thereby degrading the sharpness of the absorption. A second generation seeded growth using these triangular nanoparticles under dark at 25°C results in an absorption peak at 720 nm with full width at half maximum (FWHM) of 170 nm. This absorption response correlates well with numerical first-principles finite difference time domain (FDTD) modelling (absorption peak ~815 nm) based on a uniform distribution with concentration levels and particle dimensions matched with the experimental TEM data. Dynamic light scattering (DLS) measurements for these triangular nanoparticles yield a hydrodynamic diameter estimate of 86±8 nm, in excellent agreement with TEM measurements with edge lengths of 72±16 nm. This synthetic pathway demonstrates the possibility of narrow tuning of the absorption under controlled illumination and temperature conditions for large volume ink production at industrial scale for spectrally selective solar absorbers.

The addition of transition metals, even in a trace amount, into heteroatom-doped carbon (M-N/C) is intensively investigated to further enhance oxygen reduction reaction (ORR) activity. However, the influence of metal decoration on the electrolysis of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), is seldom reported. In addition, it remains a big challenge to incorporate multiple dopants into the carbon framework. In this work, NiFe-decorated nitrogen, phosphorus, sulfur co-doped carbon nanoribbons (NiFe-N,P,S/C) derived from biomass is reported as a highly-active electrocatalyst for overall water splitting. Alfafa is chosen as the carbon source due to its abundance in C, N, P and S. The addition of NiFe bimetal during the one-step template-free synthesis is found to contribute to much longer and more robust carbon ribbons in contrast to the monometal-decorated or metal-free carbon (Ni-N,P,S/C,Fe-N,P,S/C, N,P,S/C).The pyrolysis temperature, and molar ratio between Ni and Fe are optimized. Ni0.75Fe0.25-N,P,S/C prepared at 900°C stands out from all the investigated materials and exbihtes a significantly enhanced OER, HER, as well as overall water splitting activity and stablity. The OER activity of Ni0.75Fe0.25-N,P,S/C exceeds that of the benchmark RuO2, and it also achieves a much better overall water splitting activity and stability in comparison to individual Pt/C and RuO2, making it one of the best carbon-based water splitting electrocatalysts.

Solar driven water splitting is a promising technology to produce hydrogen fuels. Efficient oxygen evolution reaction (OER) catalysts were required for water splitting, since hydrogen evolution is severely constrained by sluggish kinetics of OER. Thus, main challenge for water splitting is to lower OER overpotential and integrating OER catalysts with solar cells. Most of previously reported OER catalysts are particles coated on conductive substrates as glassy carbon or carbon paper with polymeric binders. However, there are distinct limitations like slow charge transfer and poor stability under high current density. Thus, enormous efforts have been recently devoted to develop binder-free OER catalysts by using solution based process such as electrodeposition or hydrothermal growth on the porous metallic substrates. However, the porous substrates are not proper to deposit the semiconductor layers uniformly to fabricate solar cells. To solve such problems, Ni-Fe metal foil was electrodeposited, followed by anodization. The Ni-Fe foil has enough flatness to identify the correlation between the crystallinity and the catalytic performance of Ni-Fe OER catalysts by thermal annealing in the high vacuum. Moreover, the Ni-Fe foil is able to deposit solar cells, so it could be used as the catalytic substrates of the solar cells.
Here, we fabricated Ni-Fe oxyhydroxide film by anodizing Ni-Fe film, which exhibits low OER overpotential of 0.251 V to deliver substantial current density of 10mA/cm² and excellent stability for 36 h in 1 M KOH solution. We also integrated the catalytic substrate with an amorphous silicon (a-Si:H) solar cell to demonstrate a monolithic photo-assisted water splitting device with a structure of Ni-Fe foil / Ag (120 nm) / Al-doped ZnO (60 nm) / p-i-n a-Si:H (250 nm) / ITO (60 nm). When the device was illuminated under 100mW/cm² AM 1.5G solar irradiation in the alkaline electrolyte, the photocurrent generated from the amorphous silicon layer significantly lowered OER overpotential by 0.8 V. It was the first demonstration of the monolithic device which has a significant impact on large-scalable and highly efficient solar-driven water splitting devices.

Hydrogen, as a next generation clean energy source, can be obtained from the electrolysis of water using a variety of catalysts such as Co₃O₄, Ni, and MoS₂. Photoelectrochemical (PEC) water splitting is an electrolysis process that produces hydrogen and oxygen by the light-assisted catalysis of water. An excited electron and hole generated by the light absorption of a photocatalyst material are the catalyst for a redox reaction. ZnO can be used for photoelectrochemical water splitting devices due to its high photocatalytic activity. In its more occurring natural state ZnO has a hexagonal wurtzite crystal structure. This crystal structure prefers to grow toward the [001] direction (along c-axis). The characteristic of easily achieving 1-dimensional structure is one of the main reasons why ZnO has been intensively researched for low-dimensional applications. In 1-dimensional ZnO nanostructures, while the plane surface of (001) perpendicular to the c-axis is a polar surface terminated by Zn or O, the largest surfaces are non-polar ({100} and {110}). In PEC water splitting, surface polarity is critical because it greatly affects wettability at the interface.
In this work, an effective technique to control the reaction kinetics which extensively affects the product morphology is introduced. In low-temperature chemical vapor deposition, Zn vapor supply was controlled to obtain two different morphologies: hexagonal nanopyramid and hexagonal prism. The surfaces of pyramidal ZnO are semi-polar {112} family of planes while the prismatic ZnO is covered with polar (001) plane with non-polar side planes. The effect of surface polarity of synthesized ZnO was characterized by cyclic voltammetry. The morphology, crystal structure and defects, PEC performance, and operation mechanism are discussed.

There has been much interest lately in the preparation of TiO₂ in 2D morphologies due to its potential for improved properties for photocatalytic applications. Materials with layered (or lamellar) structures can be more easily prepared with platelet or sheet-like morphologies due to the natural bonding anisotropy in the intra- and interlayer directions, but anatase does not exhibit such a layered structure. Platelet-type nanocrystals of anatase have been successfully prepared using hydrothermal methods, but fluoride anions, typically from toxic reagents such as hydrofluoric acid, must be used as a structure-directing agent. Nanosheet-type structures with larger lateral dimensions have also been demonstrated by converting nanosheets of layered, lepidocrocite-type titanate compounds to anatase using hydrothermal or calcination treatments. However, this is a multi-step process that involves ion-exchange and intercalation of bulky ammonium cations, which may become strongly adsorbed on the nanosheets surfaces and interfere with their photocatalytic properites.

Here we report the preparation of TiO₂ nanosheets by chemical conversion of TiS₂ nanosheets. This approach is attractive, as the preparation of 2D nanosheets using chemical and electrochemical reduction methods has been demonstrated for a large number of layered, transition metal dichalcogenide materials and hence provides the opportunity for a large number of layered metal oxides through conversion of sulfide nanosheet starting materials. Additionally, using TiS₂ as the precursor can enable sulfur-doped TiO₂, which has a smaller bandgap than pure anatase and enhanced photocatalytic properties.
The synthesis of TiO₂ nanosheets was achieved as follows: (1) The first step involves the reduction of bulk TiS₂ material through electrochemical lithiation to form Li_xTiS₂. The lithiation weakens the van der Waals interaction between the TiS₂ interlayers and enables exfoliation via ultrasonication. The exfoliated TiS₂ nanosheets are uniformly dispersed in water. (2) The as-obtained TiS₂ nanosheet dispersion is oxidized using a hydrothermal treatment to obtain TiO₂ nanosheets. Detailed structural characterization using atomic force microscopy, electron microscopy, and X-ray photoelectron spectroscopy was performed to understand the conversion process. It was found that the synthesized materials adopted the anatase structure and were polycrystalline, but the nanosheet morphology of the original TiS₂ nanosheets was maintained. Photocatalytic tests using methylene blue as a model organic compound showed that the TiO₂ nanosheets displayed good activity. The results show that this approach may be a potentially general method to obtain metal oxide nanosheets by conversion from the corresponding layered sulfide nanosheets.

Solar thermal technologies, which convert solar energy into heat, have received increasing interest during the past few decades and are considered to be a promising candidate because of their high energy storage density and high energy conversion efficiency in many emerging applications such as solar collectors for heating and cooling systems, solar cookers, solar-heated clothes and steam generators. In this study, we fabricate the metal/dielectric based solar absorber on various substrate such as glass, fabric, thermoelectric etc. The sample, consisting of a layered titanium (Ti) and magnesium fluoride (MgF₂) film multi-layers structure, were fabricated using electron beam evaporator at room temperature. The absorbers showed a high absorption of approximately 85% over a wavelength range of 0.2−4.0 μm, and the selective absorption can be tuned by the thickness of each layer. Under 1 sun illumination, the light absorber on various stretchable substrates increased the substrate temperature to approximately 60°C. The thermoelectric generator (TEG) with the absorber on the top surface also showed an enhanced output power of 60%, compared with that without the absorber. A solar water heating system was fabricated using a solar absorber with this absorption characteristic. Under 40 sun condensation, the surface temperature of the absorber on aluminum (Al) substrate of 1.5 cm x 1.5 cm area increased up to 92°C and the water temperature increased by 5°C. Although absorber is small area, water temperature increase. Moreover, the heat transfer and antireflection technology of absorber is also investigated for optimized solar absorption. This work should play an important role distillation water purification technology using a multi-layer thin-film photo-thermal conversion material for decentralized water supply.

We have recently shown that localized photothermal heating achieved by solar illumination of absorber- coated membranes is a highly promising strategy for converting conventional membrane distillation, an energy-intensive process, into an entirely solar-driven process. Transforming this approach into a solar technology radically alters the scaling laws that govern this process. We have examined several strategies for boosting performance that involve light management and modification of heat transfer within the NESMD approach.

Water splitting and rechargeable metal–air batteries are highly competitive options for a sustainable energy future, but their commercialization is hindered by the absence of cost-effective, highly efficient and stable catalysts for the oxygen evolution reaction (OER). Here we report the rational design and synthesis of a double perovskite PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ nanofiber as a highly efficient and robust catalyst for OER [1]. Co-doping of strontium and iron into PrBaCo₂O_5+δ (PBC) is found to be very effective in enhancing intrinsic activity (normalized by the geometrical surface area, ~4.7 times), as validated by electrochemical measurements and first-principles calculations. Further, the nanofiber morphology enhances its mass activity remarkably (by ~20 times) as the diameter is reduced to ~20 nm, attributed to the increased surface area and an unexpected intrinsic activity enhancement due possibly to a favourable e_g electron filling associated with partial surface reduction, as unravelled from chemical titration and electron energy-loss spectroscopy. Markedly enhanced oxygen evolution reaction capability is shown when compared to existing catalysts. The mass activity of the ultrafine nanofiber is ~72 times greater than the initial PBC powder catalyst, and 2.5 times greater than the state-of-the-art noble metal-based catalyst (i.e., IrO₂). This work not only results in a highly efficient and durable electrocatalyst for OER, which may have important technological implications, but also offers new insight into the development of advanced materials by nanostructure engineering for other applications of energy storage and conversion.

[1] Zhao, B. T., Zhang, L., Zhen, D. X., Yoo, S., Ding, Y., Chen, D. C., Chen, Y., Zhang, Q. B., Doyle, B., Xiong, X. H., Liu, M. L. A tailored double perovskite nanofiber catalyst enables ultrafast oxygen evolution. Nat. Commun. 8, 14586 (2017).

Atomically thin transition metal dichalcogenides have attracted significant interest for technological use due to a wide diversity of their electronic and optical properties. Recently, nanostructured MoSe₂ and WSe₂ have been identified as suitable catalysts for electrochemical water splitting, considering their band edge energies with respect to the redox potential of hydrogen evolution reaction (HER) and good stability in aqueous solutions.¹ Single- and few-layered nanosheets of MoSe₂ and WSe₂ have been mainly achieved via exfoliation of bulk powders in solution, although this approach generally yields to dispersions that are heterogeneous in flake size and thickness. On the other hand, colloidal synthesis can be significantly advantageous as it presents fine morphological and compositional control over free-standing nanostructures processed at low temperatures in liquid phase allowing for designing the material with multiplied number of active sites and desired electronic properties respectively. In this work, we report a colloidal synthesis of MoSe₂, WSe₂ and alloyed W_xMo_1-xSe₂ branched nanosheets reaching 500 nm in lateral size. The materials present high crystal quality under high-resolution TEM characterization. The growth was achieved upon a reaction between transition metal carbonyls and trioctylphosphine selenide in a coordinating solvent at 300^oC in inert atmosphere. The electrocatalytic activity of MoSe₂, WSe₂ and alloyed W_xMo_1-xSe₂ nanostructures towards hydrogen evolution reaction was assessed in three-electrode electrochemical cell in 1M H₂SO₄. Alloyed W_xMo_1-xSe₂ branched nanosheets require a smaller HER overpotential at the cathodic current density of -10 mA/cm² (-360 mV vs RHE) than the MoSe₂ branched nanosheets (-390 mV vs RHE, this work) and outperform the exfoliated MoSe₂ and WSe₂ counterparts.²

References:
1. Zhuang et al, J. Phys. Chem. C, 2013, 117, 20440–20445.
2. Chia et al, ACS Nano, 2015, 9, 5164–5179.

Inspired by the functions and structures of the root and stalk of a turnip, a novel BiVO₄/CuSCN heterojunction was constructed for photoelectrochemical water splitting, by initially fabricating bulky BiVO₄ film (the root) and subsequently depositing p-type CuSCN nanorods (the stalk) on top. This BiVO₄/CuSCN photoanode produced an enhanced photocurrent density of 1.78 mA cm^-2 and hole injection efficiency of 82% at the potential 1.23 V vs. RHE. About 40% increase in photocurrent density coupled with a dramatic cathodic shift (~220 mV) in onset potential compared with bare BiVO₄. The heterojunction also possesses external quantum efficiency of approximately 33% in the range from 350-450 nm with fairly high solar energy conversion efficiency (0.5%). The unique electrode architecture design favors the facile water splitting process over conventionally fabricated electrode by providing the more active sites and facilitates transportation and consumption of photoinduced holes, open a new route for the high-efficiency photoanodes.

I will present my group work in the past 10 years in designing nanomaterials and devices for applications in water. Examples include: 1) We developed three-dimensional nanostructured electrodes to interface with microbes effectively for high efficiency microbial fuel cells and invented microbial batteries where air cathodes are replaced by solid-state battery cathodes, resulting in multiple times of efficiency improvements. 2) We invented a low-cost disinfection technology based on nanoscale electroporation and tunable bandgap of MoS₂ layered materials for photoelectrochemical disinfection. 3) We also originated a concept of seawater battery which can do both desalination and electricity generation. 4) We developed new electrochemical devices for uranium extraction from seawater and heavy metal extraction.

Power-to-Gas (P2G) technologies that convert renewable energies such as solar and wind to chemical fuels have been the focus of many scientific endeavors recently. Specifically, H₂ production by water electrolysis (2H₂O à 2H₂ + O₂) is a promising P2G technology. The emerging market of Fuel Cell Vehicles (FCV) and fast-growing Hydrogen Refueling Stations (HRS) increase the demand for clean, pure, high-pressure hydrogen at a competitive price. The state-of-the-art electrolyzers operating today include Alkaline Electrolyzers (AEL) and Proton Exchange Membrane Electrolyzers (PEMEL). Alkaline electrolysis represents a commercial technology that produces high purity H₂ using non-precious metal catalysts. However, it has several shortcomings that limit its potential integration as a P2G technology, including limited partial load and high-pressure capabilities, and a low tolerance to electrolyte impurities. These limitations stem from the required diaphragm and single-cell configuration. PEM electrolysis, on the other hand, can operate at higher differential pressures, but also faces dangerous gas crossover at partial load, as well as low tolerance to impurities and membrane degradation. It also suffers from the high cost of catalysts and membranes [1], [2]. In addition, both technologies work in corrosive high/low pH environments, requiring expensive construction materials, safety measures and maintenance. In a recent publication in Nature Materials [3] we suggested a new Photoelectrochemical (PEC) water splitting technology wherein hydrogen and oxygen are produced in two completely separate cells, connected to each other externally only by electrical wires. This is achieved by introducing an additional set of Ni(OH)₂/NiOOH electrodes, called the auxiliary electrodes (AEs). Nickel hydroxide is commonly used in rechargeable alkaline batteries, and can be electrochemically cycled (charged/discharged) many times with minimal energy loss. By placing a "charged" (NiOOH) auxiliary electrode in the oxygen cell, and electrically connecting it to a "discharged" (Ni(OH)₂) auxiliary electrode in the hydrogen cell, electrolysis can be performed in two separate cells. During electrolysis, one auxiliary electrode charges while the other discharges. Thereafter, the process can be repeated by cycling the auxiliary electrodes between the charged/discharged states. The presentation will describe the operation of our membraneless electrolysis system, its benefits and potential.
[1] A. Buttler and H. Spliethoff, “Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids : A review,” Renew. Sustain. Energy Rev., no. September, 2017
[2] D. V Esposito, “Membraneless Electrolyzers for Low-Cost Hydrogen Production in a Renewable Energy Future,” Joule, pp. 1–8, 2017
[3] A. Landman et al., “Photoelectrochemical water splitting in separate oxygen and hydrogen cells,” Nat. Mater., vol. 16, pp. 646–651, 2017

Astronaut crews aboard the International Space Station (ISS) rely on recycled water from cabin condensation and urine to sustain their daily water intake. A Water Recovery System (WRS) uses a series of sedimentation, multifiltration beds, and a thermal catalytic oxidation reactor (COR) to reclaim nearly all crewmembers’ daily water supply. However, operation of the COR is an energy intensive process requiring high pressure and temperature to eliminate remaining volatile organics. The ISS and deep space travel represent an environment that is sensitive to the water-energy nexus and could benefit from a low-cost, low-energy alternative process.

Interest in photocatalytic TiO₂ microfluidic reactors for applications in volatile organic compound removal has grown considerably over the past decade. Advantages of microfluidic devices over traditional packed-bed thermal oxidation systems include increased surface area to volume ratios and greatly reduced mass diffusion lengths, which can translate to enhanced performance on a miniature scale. TiO₂-based photocatalysts operate at standard pressure and temperature and have been functionalized for novel use in water purification, water-splitting, and photochemistry. However, low volumetric throughput remains a critical limitation in many applications, as does the difficulty associated with integrating TiO₂ uniformly within complex microfluidic device geometries.

Herein, we present our recent efforts to optimize growth of nanoporous TiO₂ (NPT) for use within Ti-based microfluidic devices. NPT is grown in situ, directly from the Ti channel surfaces, using a H₂O₂-based oxidation process. Advantages of this approach include: a) conformal catalyst coverage of complex geometries; b) high porosity yielding increased surface area and fluidic accessibility; and c) potential for scalable fabrication of large area photocatalytic devices with increased volumetric throughput. Additionally, we propose that incorporating a novel high-density, high-aspect-ratio micropillar array within the reaction chamber to serve as a scaffold for NPT oxidation will yield significant performance enhancements to overcome the majority of microreactor limitations.

Using a multi-criteria Taguchi design of experiments study, optimal NPT growth conditions were determined based upon methylene blue degradation efficiency and NPT film crack area. Our studies identified the key oxidation parameters responsible for photocatalytic performance. These ideal growth conditions were applied to a Ti deep reactive ion etched (DRIE) micropillar array to form NPT in situ and demonstrated a notable improvement in photocatalytic response compared flat NPT films. Collectively, these results represent important steps towards our goal of developing robust, high-performance multi-scale photocatalytic microreactors with complex channel geometries for increased mass and photon transfer efficiency.

Electrochemical and chemical tuning of catalytic materials have shown its efficacy in significantly modifying the electronic strcutures of catalysts, and thus improving their activities in electrocatalysis. Here we present a few examples including Li intercalation tuning of MoS₂ for improved hydrogen evolution reaction, Li extraction tuning of LiCoO₂ for improved oxygen evolution reaction, and Li ion battery cycling of metal oxides for overall water-splitting catalysis. We successfully scaled up to more than 50 A of H₂ production current in a unit cell for large-scale deployment in real applications.

TiO₂ phtotocatalyst has been attracting great research attentions due to its potential in solar energy conversion/storage for various applications. When TiO₂ is excited, both reduction and oxidation reactions could happen. Photocatalytic reduction could provide promising solutions to hydrogen production from water splitting, CO₂ reduction for fuel production, and the removal of various environmental pollutants. To enhance the photocatalytic reduction efficiency, sacrificial agents are usually needed to deplete the photogenerated holes. However, it could increase the complexity and cost of the operation, and may not be appropriate for some applications (for example, drinking water treatment) due to the addition of substances with potential hazard.

For the enhancement of the photocatalytic efficiency, noble/transition metal modification is widely used as the electron trapping center to enhance the photogenerated electron-hole pair separation. However, this material design strategy could not solve problems associated with the addition of sacrificial agents for photocatalytic reduction. Thus, it would be most desirable to design a photocatalyst system for photocatalytic reduction in which the charge carrier recombination could be minimized by modifications with hole trapping and consumption capability. As a semimetal element, bismuth may provide the hole trapping and consumption capability. Unlike most metals, bulk Bi has a relatively low work function of ~ 4.22 eV, close to that of TiO₂ at ~ 4.20 eV. With its size decrease into the nano range, a transition from metal to semiconductor could occur with the moving up of its conduction subbands and moving down of its valence subbands. So photogenerated electrons could not transfer from TiO₂ to Bi quantum dots anymore, while photogenerated holes could transfer from TiO₂ to Bi quantum dots and be consumed by oxidizing Bi⁰ to Bi³⁺. Thus, it could enhance the lifetime of photogenerated electrons for an efficient reduction process, while no sacrificial agents are needed to deplete holes.

In this study, Bi quantum dots were deposited on rutile TiO₂ nanoparticle surface to create the Bi/TiO₂ heterojunction photocatalyst. In this Bi/TiO₂ photocatalyst, rutile TiO₂ served as the main visible light absorber, while Bi quantum dots served as the hole trapping centers to enhance the charge carrier separation and eliminate the need of sacrificial agents to consume photogenerated holes in photocatalytic reduction process. Thus, an efficient photocatalytic bromate reduction under visible light illumination was achieved by this Bi/TiO₂ photocatalyst without the addition of sacrificial agents in the reaction solution, and it demonstrated a good regeneration capability and reusability. This study demonstrated a novel strategy for the design of photocatalysts with strong photocatalytic reduction capabilities for a broad range of technical applications.

Iron oxide has a long electroanalytical history as photoanode in photoelectrochemical water splitting cells (PEC) for solar hydrogen production ¹. It is considered an environmentally benign material. In the last decades, steady progress was made in the photocurrent enhancement of iron oxide based photoanodes². We present a PEC reactor which is based on hematite as basic electrode material. With various doping, nanostructure and heterostructure approaches we can steadily improve the photocurrent and efficiency of the system and arrive at a photocurrent of 3.3 mA/cm^{2 3}. The reactor is made from repeating units with 10cm x 10cm large panes ⁴ and needs no electrolyte membrane and is operated outside in field tests. We discuss its use in decentralized hydrogen supply systems⁵ and its potential use in bio-based PEC systems⁶.

1. K.L. Hardee and A.J. Bard: SEMICONDUCTOR ELECTRODES .5. APPLICATION OF CHEMICALLY VAPOR-DEPOSITED IRON-OXIDE FILMS TO PHOTOSENSITIZED ELECTROLYSIS. Journal of the Electrochemical Society 123, 1024 (1976).
2. A. Kay, I. Cesar and M. Gratzel: New benchmark for water photooxidation by nanostructured alpha-Fe2O3 films. J Am Chem Soc 128, 15714 (2006).
3. J.-J. Wang, Y. Hu, R. Toth, G. Fortunato and A. Braun: A facile nonpolar organic solution process of a nanostructured hematite photoanode with high efficiency and stability for water splitting. J. Mater. Chem. A 4, 2821 (2016).
4. Y. Hu, D.K. Bora, F. Boudoire, F. Häussler, M. Graetzel, E.C. Constable and A. Braun: A dip coating process for large area silicon-doped high performance hematite photoanodes. Journal of Renewable and Sustainable Energy 5, 043109 (2013).
5. A. Braun, M.M. Diale, K.D. Maabong and R. Toth: Safe And Decentralised Hydrogen Fuel Production And Storage For Residential Building And Mobility Applications, in 6th International Disaster and Risk Conference 2016 - "Integrative Risk Management - towards resilient cities", edited by M. Stal, D. Sigrist, S. Wahlen, J. Portmann, J. Glover, N. Bernabe, D. M. d. Vitry and W. J. Ammann (Global Risk Forum GRF Davos, Davos, Switzerland, Davos, Switzerland, 2016), pp. 102.
6. D.K. Bora, A. Braun and E.C. Constable: "In rust we trust". Hematite - the prospective inorganic backbone for artificial photosynthesis. Energy & Environmental Science 6, 407 (2013).

Solar electrochemical energy conversion devices including dye-sensitized solar cells and photoelectrochemical (PEC) water splitting systems are crucial tools to mitigate the future environmental impact from fossil fuel consumption. In PEC systems, composed of a cathode performing the hydrogen evolution reaction (HER), and a photoanode where oxygen is evolved, the water-oxidation reaction requires significantly more energy (four electron-hole pairs compared to two), and hence is more challenging compared to the HER [1].
2D layered materials, including transition metal dichalcogenides, are of great interest for opto-electronic devices due to their layer dependant electronic properties. Recent theoretical work has shown that both MoS₂ and WS₂ at mono-layer have the potential to function as a photoanode for water oxidation [2]. Experimentally, we have recently realized this in the form of thin films of chemically exfoliated MoS₂/WS₂ heterojunctions for water oxidation, which demonstrate a synergistic effect beyond either individual components performance. This effect arises from efficient charge transfer between the two van der Waals stacked components leading to electron-hole separation and increased reaction time at the surface [3]. Whilst these heterostructures are excellent as proof-of-concept devices, the photocurrents and incident photon-to-current efficiency values are quite low compared to state-of-the-art materials.
Here, we present a high crystal quality MoS₂/WS₂ heterostructures [4] grown by chemical vapour deposition for salt water oxidation. These heterostructures demonstrate photocurrents densities up to 0.8 mA/cm² (at 1-sun, +0.7V vs Ag/AgCl) and IPCE peaking at 1.6% in 3.5% NaCl. This performance is superior to both liquid phase processed heterostructures for water oxidation and WSe₂ for photo catalytic hydrogen evolution. These heterostructures can be grown over a cm² area with a high electrochemically active surface area (100 m²/g) and can be transferred onto a variety of flexible substrates for device specific requirements.
These results pave the way for the use of 2D crystal heterostructures in water splitting devices and provide a viable option for the energetically challenging water oxidation reaction.
References
[1] M. Gratzel, Nature 2001, 414, 338.
[2] J. Kang, S. Tongay, J. Zhou, J. Li, J. Wu, Applied Physics Letters 2013, 102, 012111.
[3] F. M. Pesci, M. S. Sokolikova, C. Grotta, P. C. Sherrell, F. Reale, K. Sharda, N. Ni, P. Palczynski, C. Mattevi, ACS Catalysis 2017, 4990.
[4] P. C. Sherrell, M. S. Sokolikova, P. Palczynski, F. Reale, F. M. Pesci, C. Mattevi, Submitted: 2017.

Layered double perovskite (LnBaM₂O_5+δ, Ln = lanthanides, M = Mn or Co) has recently caused intensive attentions in water electrolysis area. Here a series of Mn-based layered perovskite oxides are investigated as enhanced bifunctional catalysts for water electrolysis. First, we found that compared with the pristine Pr_0.5Ba_0.5MnO_3−δ, the layered PrBaMn₂O_5+δ (H-PBM) is characterized by boosted oxygen reduction/evolution reaction activities. The improvement can be ascribed to the introduction of additional oxygen vacancies, an optimized e_g filling of Mn ions, and the facile incorporation of oxygen into layered H-PBM. In addition, the effect of oxygen vacancies present in layered perovskites on water electrolysis activity is further investigated with neodymium barium manganese oxides. We found that by facilely controlling annealing conditions, the caused small oxygen composition changes could markedly alter the crystallographic structure, electronic configuration and atomic arrangements (structural distortion) of the layered perovskite oxides, and thus the oxygen and hydrogen evolution reaction (OER and HER) activities. The large family of layered perovskite is for the first time applied in overall water splitting area. Layered NdBaMn₂O_5.5 demonstrates significant improvement in catalyzing OER and HER, in contrast to its counterparts including disordered Nd_0.5Ba_0.5MnO_3-δ as well as NdBaMn₂O_5.5-δ and NdBaMn₂O_5.5+δ (δ < 0.5). The substantially enhanced performance is attributed to the approximately half-filled eg orbit occupancy, optimized O p-band center location, as well as distorted structure. Interestingly, we found that for the investigated perovskite oxides, their OER and HER activity seem correlated, i.e., the material achieving a higher OER activity is also more active in catalyzing HER. As a result, the previously developed OER activity descriptors such e_g filling and Op band center location, which determines the charge transfer between the surface cation and adsorbed reaction intermediates, may potentially apply to HER activity prediction as well.

The National Academy of Engineering identified “providing access to clean water” as one of the top 10 grand challenges for engineering in the 21^st century. A central requirement for safe drinking water is the availability of low-cost and real-time water quality monitoring. Current detection methods for critical analytes in water are often too expensive or unsuitable for in-situ and real-time detection (an unmet need). As a result, there is a lack of water quality monitoring along the water distribution line and at the point of use, which is inadequate because of potential deterioration in water quality within water distribution systems (e.g., Flint Water Crisis). This talk will unveil a powerful approach to real-time water sensors through a graphene-based field-effect transistor platform. The working principle of the sensor is that the conductivity of 2D nanomaterial channel (usually measured in resistance) changes upon binding of chemical or biological species to molecular probes anchored on the graphene surface. As such, the presence and the concentration of analytes, such as heavy metals, bacteria, and nutrients, can be rapidly determined by measuring the sensor resistance change. The talk will introduce the performance of the sensor for detection of various water contaminants and focus on the molecular engineering aspects of the sensor device through both theoretical and experimental approaches. The talk will end with a brief introduction on the translation of the platform technology from concept to prototype product through partnership with industries.

Although desalination technologies have been widely adopted as a means to produce freshwater, many of them require large installations and access to advanced infrastructure. Recently, floating structures for solar evaporation have been proposed employing the concept of interfacial solar heat localization as a high-efficiency approach to desalination. However, the challenge remains to prevent salt accumulation while simultaneously maintaining heat localization. We present experimental demonstration of a salt-rejecting evaporation structure that can operate continuously under sunlight to generate clean vapor while floating in a saline body of water such as ocean. The evaporation structure is coupled with a low-cost polymer film condensation cover to produce freshwater at a rate of 2.5 Lm^-2day^-1, enough to satisfy individual drinking needs. The entire system costs $3 m^-2 – over an order of magnitude lower than conventional solar stills, does not require energy infrastructure, and can provide cheap drinking water to water-stressed and disaster-stricken communities.

Efficient utilization of solar energy to generate clean water from seawater or contaminated water has attracted more and more attention to deal with water scarcity. Some of the previous works focused on the water evaporation using synthetic ( 3.5% NaCl) seawater. It is reported that salt crystal accumulation on material surface is easily to be removed in DI water. However, in a practical solar-driven water evaporation, contents in water are more complicated than NaCl. As a result, fouling of the material is one of the biggest concerns, which would reduce the evaporation efficiency and shorten the material’s service lifetime.Therefore, fouling control strategies should be employed including water pretreatment and periodical cleaning.

However, most of the photothermal materials developed previously are too fragile to stand a strong shear force for washing, which is due to the porous structure or physical properties of the materials. Generally, porous structure is designed in order to enhance light absorption by reducing light reflection, as well as to accelerate the steam transport. Therefore, porous photothermal materials with high mechanical strength are needed for washing to control the corrosion and fouling.

Recently, porous SiC ceramics have been increasingly studied due to its excellent mechanical strength and chemical stability, controlled permeability and excellent corrosion resistance at high temperature. Moreover, fortunately, coming with the preparation process of SiC ceramic is the by-product: free carbon. An excess quantity of carbon is distributed uniformly in SiC ceramic, which makes SiC into SiC-C composite. Carbon-based material is one of the most popular photothermal materials, which is providing the possibility for SiC-C composite to be a candidate as photothermal material.

Herein, we rationally designed a tandem-structured SiC-C composite monolith as the photothermal material. Free carbon plays as the light absorber and light to thermal convertor and SiC plays as a strong structure support and heat conductor. The tandem structure is designed in order to reduce the light reflection, and at the same time keep the support with good mechanical property. It is demonstrated that the tandem-structured SiC-C composite monolith can meet the requirements expected for an efficient photothermal material, such as hydrophilic surfaces, high light absorption in the full solar spectrum range and, especially, the high mechanical strength for physical cleaning. More importantly, it is the first time to notice and investigate the difference of the fouling behavior between the synthetic seawater and the natural seawater (Red Sea) on the surface of photothermal material. Furthermore, wastewater collected from WWTP in KAUST is also investigated to see the fouling behavior. All of these results indicated the importance of mechanical properties to design a photothermal material for durable fouling control in solar-driven water evaporation applications.

Photoelectrochemical (PEC) cells are a promising approach for solar energy storage. In traditional PEC cells, solar energy drives water oxidation at the anode and proton reduction at the cathode, “splitting” water to form O₂ and H₂ gases. The efficiency, however, is limited by the high overpotentials required for water oxidation.¹ Splitting seawater to generate H₂, NaOH, and Cl₂ (equivalently H₂ and NaOCl) is a promising alternative to overcome the efficiency limits of traditional PEC cells. In this case, chloride oxidation occurs at the anode to generate Cl₂, and proton reduction occurs at the cathode to generate hydroxide ions and H₂. While chloride oxidation occurs at similarly positive potentials to water oxidation, its overpotential is significantly lower.² Moreover, NaOCl is a valuable chemical oxidant used extensively for water purification,³ which remains a significant worldwide problem.⁴

Alloy films of TiO₂-RuO₂ or TiO₂-IrO_x synthesized by atomic layer deposition (ALD) represent an ultra-thin analogue to the dimensionally stable anode (DSA), the current industry standard for chloride oxidation. While the DSA is 10s of microns thick, we deposited alloy films ranging from 10-45 nm, thus reducing noble metal usage by a factor of 100 to 1000 per unit area. An all-ALD process for alloy deposition also enables precise control of the noble metal content in the film by altering the ratio of TiO₂ to RuO₂ or IrO_x cycles. Despite using less noble metal, ALD alloys performed competitively with a commercial DSA tested under the same conditions. For example, a 45 nm TiO₂-RuO₂ alloy composed with 46:54 Ru:Ti ratio had a slightly higher overpotential (74 mV vs. 58 mV at 1 mA cm^-2) but an equivalent Tafel slope of 52 mV dec^-1 and a higher Cl₂ yield than the DSA (93% vs 87%). This 46% Ru alloy also maintained a higher current density than the DSA over a 10 hour stability test. The overpotentials for TiO₂-IrO_x alloys with 49% Ir varied from 41 mV to 111 mV at 1 mA cm^-2, but adding an additional ALD IrO_x coating did not reduce the overpotentials further. Similar experiments with PVD Ir coatings showed comparable Tafel slopes of 55mV dec^-1 despite slightly lower overpotentials for Cl₂ evolution (30 mV). This suggests that we do not sacrifice catalytic activity by diluting noble metal ions in a TiO₂ matrix. Finally, ALD TiO₂-RuO₂ alloys possess a work function > 5 eV, capable of generating photovoltages > 500 mV on n-type silicon in a photoelectrochemical cell.[5] Overall, this work demonstrates the potential of ALD to tune the composition of electrode materials for practical application in solar-driven saltwater splitting.

[1] M.G. Walter, et al. Chem. Rev. 110, 6446–73 (2010).
[2] N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. 103, 15729–35 (2006).
[3] http://www.eurochlor.org/the-chlorine-universe/what-is-chlorine-used-for.aspx.
[4] http://www.sodis.ch/index_EN
[5] O. L. Hendricks et al. ACS Applied Materials & Interfaces, 2016, 8, 23763-23773

Fresh water withdrawal for thermoelectric power generation accounts for approximately 41% of all fresh water withdrawal in the US while 3% of cooling tower water load is evaporated and dissipated. Dry-cooling systems suffer from two major drawbacks: 1) the low air-side heat transfer coefficient, and 2) the performance penalty when ambient temperature is high. Radiative cooling has been proposed as supplemental cooling, which has no water dissipation to the atmosphere and no loss of power plant efficiency. We have developed a scalable manufactured metamaterial—an engineered material with extraordinary properties not found in nature -- a 50-micron thick polymer film randomly filled with glass microspheres of 8-micron diameter, and backed with a 100nm silver coating. While the film reflects almost all sunlight, the microspheres interact strongly with infrared radiation to emit heat at high enough rates to achieve a net cooling effect. We have demonstrated an average radiative cooling flux greater than 110 W/m² in a continuous three-day field test and more than 90 W/m² cooling power was observed under direct sunshine. However, due to the intrinsic low density of radiative cooling flux, cold collection becomes crucial for many engineering applications. With the ability demonstrated for radiative cooling to cool down water 10°C below ambient temperature at noon time under the sun. We propose a radiative cooled-cold collection and storage (RadiCold) system with operation scheduling to produce cold water at a constant sub-ambient temperature with reduced operational cost. The RadiCold system can be either used directly for air conditioning in buildings, or coupled to other energy systems for energy saving. The integration of the RadiCold system with a dry-cooling power plant can significantly increase the thermoelectric power conversion efficiency while reducing water consumption.

In this study, a spontaneous charge spatial separation (SCSS) and simple hot press process (HPP) has been adopted to enhance the efficiency of photocatalytic water splitting. Originally, the photocatalytic activity of hematite is limited by its relatively poor absorptivity, very short excited-state lifetime, and a short hole diffusion length. To address these issues, pseudocubic polyhedral α-Fe₂O₃ phtoelectrode was fabricated and achieved the spontaneous charge spatial separation in water splitting process. The intrinsic charge spatial distribution has to be taken into account when selecting the facets, as it results in accumulation of photoexcited electrons and holes on certain semiconductor facets. Furthermore, we develop a new technology of using a simple hot press process to improve carrier transport, charge separation and longtime stability for photocatalytic water splitting. Without complicated nanostructure fabrication, this hot press process demonstrated a 2-fold enhanced photocurrent under A.M. 1.5 solar simulator irradiation. The enhancement is attributed to the improvement of carrier transport properties in TiO₂-Fe₂O₃ matrix due to significant reduction of the film thickness after the hot press process. In addition, Tin (Sn) from the FTO substrate was diffused and doped into these polyhedral α-Fe₂O₃ and hot press TiO₂-Fe₂O₃ during the sintering process to serves as an electron donor and increases the carrier density. In addition, plasmonic gold nanoparticles were incorporated into this two system, which would provide the strong field and hot electrons, resulting in the enhancement of visible light absorption efficiency and inhibit charge recombination, this leads the photocatalytic water splitting to achieve a further stage.

There is a growing interest in quasi-one-dimensional TiO₂ nanostructures (e.g. nanorods, nanowires, nanobelts and nanotubes). These structures offer superior charge transfer properties and high stability, making them highly suitable for application as photocatalysts with improved water splitting efficiency. Nanotubes and nanowires in particular offer direct electrical pathways for photogenerated electrons and could increase the electron transport rate, which in turn may improve the performance of photovoltaic devices such as solar-to-hydrogen cells. However, so far, low overall solar-to-hydrogen efficiencies has been achieved. This challenge has been attributed to the unsuitable band gap of TiO₂, amongst others reasons. Hence, different deposition methods have been suggested as a route to engineer the band gap of TiO₂ nanotubes and nanowires via doping, including molecular (and atomic) diffusion through in-situ anodic oxidation and chemical bath deposition. As this is the subject of ongoing research, we report on the preparation of TiO₂ nanotubes on FTO (F: SnO₂) glass substrates using a gelation step, which offers the possibility for doping prior to calcination, as an alternative preparation method for band gap engineered TiO₂ nanostructures.

First, a template made of tall ZnO nanorods is prepared by chemical bath deposition. These rods are then coated with a Ti-based acidic gel prepared from a solution made of ammonium hexafluorotitanate and boric acid. Finally, the gel is calcinated at 550 ^oC into TiO₂ nanotubes. Detailed structural, morphological and optical investigations are reported in this paper.

Ammonia is synthesized directly from N₂ and H₂O at room temperature and atmospheric pressure over nanoparticles decorated FeNi₃ alloy in-situ anchored on the porous graphene (Fe₃O₄/FeNi₃/G). An ammonia yield rate of and Faradaic efficiency of 14 µg h⁻¹ mg_cat⁻¹ and 8.9 %, respectively, were obtained under -0.1 V vs. RHE at room temperature, which is higher than those of Au and Fe₂O₃-CNT. The high intrinsic activity can be mainly attributed to synergistic effects of FeNi₃ alloy, Fe₃O₄, and G, showing a potential of being one of the most effective non-precious metal-based electrocatalysts for ammonia electrochemical synthesis.

The oxygen evolution reaction (OER) has drawn significant interest in the field of renewable and sustainable energy in recent years, with potential applications for hybrid electric vehicles (HEV) among other areas. Perhaps its most important feature is the fact that it is a necessary ‘step’ in the evolution of H₂ gas by water electrolysis, bringing with it a significant potential (E ≈ 1.23 V). To overcome this potential barrier with minimum overpoetential η, an effective electrocatalyst is required to facilitate the reaction. Ni_xFe_y Layered Double Hydroxide (LDH) has been shown to exhibit efficient catalysis of the OER, demonstrating a more competitive overpotential than previously studied electrocatalysts based on rare earth metals such as iridium^[1]. Ni_xFe_y LDH has other advantages over these catalysts such as earth abundance, cost and stability (in operating conditions). Downsizing of the Ni_xFe_y LDH can result in an even smaller value of η. This enhanced catalytic activity is a result of an increase in edge-site density in dispersions with smaller average size^[2]. The aim here is to improve Ni_xFe_y LDH catalysis based on this relationship.

In this work, synthesis of high quality, planar Ni_xFe_y LDH platelets with regular hexagonal morphology was achieved using a wet chemistry method at low temperature (100 ^oC). Using platelets synthesized in this way, OER catalysis has been demonstrated with η = 0.36 V, a competitive value when compared to many state-of-the-art OER electrocatalysts in identical conditions^[3] (5 mVs^-1, quoted at current density j = 10 mAcm^-2). Next, the project concentrated on the post-synthetic treatment of Ni_xFe_y LDH dispersions to reduce lateral platelet dimensions and further improve edge-site density for electrocatalytic optimization. This study comes in accordance with the growing number of high-performance electrocatalysts being studied for OER.

Two primary techniques are employed here to improve Ni_xFe_y LDH catalytic activity. Firstly, size selection of the material using centrifugation. Operating in the relatively low centrifuge rate range of 500 – 3000 rpm, it is possible to isolate hexagon dispersions with average lateral platelet size from 0.8 μm down as low as 0.4 μm. Secondly, tip sonication of dispersions is employed, with the aim of breaking LDH platelets and hence exposing more edge sites. In contrast to the centrifugation method, this will essentially destroy the materials hexagonal morphology but with an expected trade off of enhanced catalysis with respect to OER.

[1] Gong M.; Li Y.; Wang H.; Liang Y.; Wu J. Z.; Zhou J.; Wang J.; Regier T.; Wei F.; Dai H. Journal of the American Chemical Society 2013 135 (23), 8452-8455.
[2] Song F.; X. Hu, Nat. Comm, 2014 Volume 5, 447.
[3] Tahir M.; Pan L.; Idrees F.; Zhang X.; Wang L.; Zou J.; Lin Wang Z. Nano Energy, Volume 37, 2017, Pages 136-157.

Scalable photocatalytic and antifouling applications for thin films can be attained by extending the light absorption ability of titanium dioxide (TiO₂) from ultra violet to the visible range through hydrogenation. TiO₂ thin films and TiO₂ nanostructures grown on titanium substrate were subjected to hydrogenation in high temperature low pressure CVD chamber. The absorption enhancement after hydrogenation was produced without the use of any conventional diatomic hydrogen dissociation catalyst such as platinum. In our study, the titanium substrate itself was the active component in driving the hydrogen dissociation and diffusion. In this way, photocatalytic thin films are scalable and cost-effective. Using hydrogenation, a 50% enhancement in chemically-deposited films and a 21% enhancement in ebeam-deposited films for methylene blue photocatalytic degradation is achieved. As the TiO₂ deposition with simple chemical-etching is shown to be highly effective, the TiO₂ nanostructures grown on titanium mesh performs as a scalable and highly effective photocatalytic device. Finally, fundamental physical insight of the TiO₂ hydrogenation dynamics is provided to distinguish the optical effects of various processing conditions.

Layered double hydroxides (LDH) have a general stoichiometry of A_xMO₂, where M is a first-row transition metal and A is an alkali intercalated metal or a proton. LDHs were originally discovered as battery electrode materials, but presently are also the most active oxygen evolution reaction (OER) catalysts in alkaline media ^[1]. However, their activities for OER and for oxygen reduction reaction (ORR) in full range of A_x and M stoichiometry is largely unexplored.

In this work, we perform a systematic high-throughput screening of A_xMO₂ materials as bifunctional catalysts for OER and ORR using a simplified model. Our model is based on observation that the electro-catalytic activity to a large degree is defined by the surface coordination of the active site (M-atom) and is almost independent of the type of the A-atom or the layer-to-layer spacing. Using this model, we have found that our screening correctly detects the known active LDHs, but also identifies new very active OER and also ORR catalysts. The obtained systematic trends will also help to establish a principle to rationally design other materials as bifunctional catalysts for OER and ORR.

[1] S. Barwe, C. Andronescu, J. Masa, and W. Schuhmann, Current Opinion in Electrochemistry (2017).

I will discuss our recent results on multifunctional structures based natural wood materials toward sustainability. I will briefly discuss transparent wood as a replacement of glass for energy efficient building, transparent paper with advanced light management as a replacement of plastics for flexible, biodegradable electronics, and super strong nanopaper as a replacement of steel for automobile and building applications.

I will then focus on wood nanostructures toward energy-water nexus including the following technological areas:
(1) Solar steam generation for water desalination;
(2) 3D wood membrane for waste water filtration (removal of heavy metals and organic waste);
(3) Biodegradable wood-based batteries;
(4) Microbial fuel cells using waster water.

References:
(1) Mesoporous, Three-Dimensional Wood Membrane Decorated with Nanoparticles for Highly Efficient Water Treatment, ACS Nano, 2017, online.
(2) Wood Composite as an Energy Efficient Building Material: Guided Sunlight Transmittance and Effective Thermal Insulation, Advanced Energy Materials, 2016, 6, 1601122.
(3) Anisotropic Transparent Wood-Composites, Advanced Materials, 2016, 28, 5181.

The fundamental and serious problem of solar desalination, one of the most promising technique to solve the global water shortage, is poor efficiency. To improve the solar-to-vapor efficiency, a new device based on three-dimensional graphene networks (3DGNs) with a high solar absorption property and a mesoporous structure has been developed. The device contains 3DGN-photoabsorbers and a wood post that serves to deliver water and prevent heat loss to bulk water by a capillary force and thermal insulation property, respectively. It has shown greatly enhanced solar-to-vapor conversion efficiency of about 91.8% under one sun illumination and excellent desalination efficiency of five orders salinity decrement. Since this highly efficient solar desalination device is made by mass-producible 3DGNs and the earth-abundant wood piece, it provides a straightforward way to efficiently supply worldwide clean water.

The enhancement of the efficiency of evaporation of liquid water, for water evaporation and steam generation, is of major technological as well as scientific interest, with applications ranging from water heaters to distillation and desalination. The use of solar radiation for this purpose amounts to a better utilization of an abundant energy resource.

The relative influence of the capillary, Marangoni, and hydrophobic forces in mediating the evaporation of water from carbon foam based porous media, in response to incident solar radiation, are investigated. It is indicated that inducing hydrophilic interactions on the surface, through nitric acid treatment of the foams, has a similar effect to reduced pore diameter and the ensuing capillary forces. The efficiency of water evaporation may be parameterized through the Capillary number (Ca), with a lower Ca being preferred. The proposed study is of much relevance to efficient solar energy utilization.

We specifically indicate that enhanced capillary pressures, either due to a decreased pore size or the chemical modification of the surfaces would serve to increase the evaporative efficiency of water through light absorbing (where it is indicated that the absorption could be of the order of 93%-95%) porous media. The proposed media may also be used for desalination where the larger electrical conductivity may be of advantage; additionally, a decreased interfacial thermal resistance in ionic solutions may also be beneficial.

New technologies for water desalination and purification is critical to resolve the global water scarcity^1,2. Conventional water desalination is large scale and energy intensive. The small-scale point of use water desalination powered by sustainable energy is desired. Solar to thermal energy conversion is the most efficient way to harvest solar energy (more than 90% conversion efficiency)³. Traditional electrical power source can be eliminated by harvesting thermal energy from solar and applied on water desalination. Natural wood as one of abundant substance can be turned into black carbonized woods and worked as solar absorbers by simple carbonization process⁴. Black carbonized wood can convert solar irradiation to heat. By floating the thin black carbonized wood on the surface of seawater, solar heating is created on the air water interface. seawater is lifted inside the naturally aligned channels by capillary force and evaporated by solar heating. By introducing phase change material (a class of materials that can store a large amount of heat by melting/ freezing) into the black carbonized wood, longer working time and higher evaporation rate is expected. This desalination process required no electricity compared to the traditional method for water desalination like reverse osmosis.

Reference
1. Addams L, Boccaletti G, Kerlin M, S. M. Charting our water future: economic frameworks to inform decision-making. McKinsey & Company (2009).
2. Ni, G. et al. Steam generation under one sun enabled by a floating structure with thermal concentration. Nat. Energy 1, 16126 (2016).
3. Yang, J. et al. Hybrid graphene aerogels/phase change material composites: Thermal conductivity, shape-stabilization and light-to-thermal energy storage. Carbon N. Y. 100, 693–702 (2016).
4. Zhu, M. et al. Tree-Inspired Design for High-Efficiency Water Extraction. Adv. Mater. 1704107, 1704107 (2017).

Recent advances in two-dimensional (2D) nanomaterials offer unprecedented opportunities to fabricate a new class of materials that can potentially revolutionize desalination technology. In this talk, I will first discuss the promise of 2D nanomaterials (e.g., graphene oxide/GO and MoS₂) as building blocks to ingeniously make new nanostructured membranes with exceptional mechanical, electrical, chemical, and biological properties, thereby significantly improving the separation efficiency in desalination. In addition, I will introduce our recent study on synthetic GO leaf and solar-powered artificial tree for desalination. The potential of the artificial tree for treating high salinity water will be discussed with a focus on achieving zero liquid discharge (ZLD).

Nanofibrous materials, which are at the forefront of advanced fibrous materials, hold great promise in improving the performance and extending capabilities of products used in different fields. Electrospinning, a primarily route towards such objects, has become one of the most effective and versatile technologies for the large-scale production of nanofibrous materials. This report focus on the progress of electrospun nanofibrous materials for the application of water treatment achieved by our group in recent years, which are summarized as follows: (1) the two-dimensional nanonets with an extremely small diameter (< 20 nm) and small pore size were controllably fabricated via the novel electro-netting technique, and achieved its application in the fields of water purification, individual protection and health care. (2) In contrast to the brittle nature of ceramic nanofibers, various ceramic fibrous membranes (eg. SiO₂, TiO₂, ZrO₂, etc) with unexpected softness have successfully been fabricated by synergistically optimizing the crystalline and porous structure of single fiber, which could be potentially used for the photocalytic degradation of water contaminant or the water treatment in extreme situations. (3) The hierarchical nanofibrous membranes with micro/nano roughness, regulable porous structure, and differentiated wettability have been prepared for the separation of the emulsified oil/water effluent with high efficiency and high flux. (4) The superamphiphobic and compact structured nanofibrous membranes have been fabricated through simultaneously regulating the tortuous pore structure and surface wettability, and the resultant membranes that is capable of selectively transporting aqueous water and water vapor exhibited promising application in the seawater desalination and water purification. Such progress might afford the possibility of moving beyond our current alternative to ensure sustainable lives for the future.

Impregnating water treatment membranes with nanomaterials can reduce fouling, increase water permeability, and add extra functions; all of which potentially lead to increased energy efficiency in the membrane treatment process. We developed a new spray coating method to modify the surface of water treatment membranes with nanomaterials. We evaluated this method by coating PVDF ultrafiltration (UF) membranes, using varying deposition parameters, with gold and silver nanoparticles having various surface functionalizations. We will discuss our results in terms of (1) deposition efficiency, (2) characteristics of the nanoparticle coatings (e.g. uniformity, nanoparticle integrity, repeatability of coating), (3) nanoparticle loading as a function of coating method (e.g. nanoparticle choice, deposition time), (4) membrane performance (i.e. water permeability), and (5) nanoparticle stability on membrane surfaces. We will identify ideas to improve the spray coating method for various membrane applications, and areas that need to be further investigated. The nanoparticle spraycoating method has advantages over some other nanoparticle impregnation methods, including applicability to multiple membrane substrates, ability to directly coat the surface where the nanoparticles are most effective, the ability to coat with varying nanomaterials or cocktails of nanomaterials, and no additional chemicals are required aside from the nanoparticles themselves. Furthermore, this method can easily be scaled-up and integrated into current membrane manufacturing to coat membranes at a rate of m² per minute. Overall, the spray coating method is a flexible and reliable method for nano-enabling water treatment membranes and can potentially improve energy efficiency of the membrane treatment process.

1D SiC materials have attracted much attention because of the high strength and modulus, low density, excellent oxidation resistance and chemical stability. In this work, flexible SiC nanofiber mats with designable components and structure were successfully fabricated by electrospinning and high temperature treatment using polycarbosilane (PCS) as precursor. In the process of high temperature pyrolysis, SiC nanowires with the diameter of 100 ~ 200 nm were in-stu grown on the surface of continuous SiC nanofiber (~ 500 nm) without any catalyst. The hierarchical structure dramatically improved the specific surface area of SiC nanofiber mat from 131.1 m2/g to 396. 8 m2/g. After bending over 1000 times, the SiC nanofiber mat remained undamaged, showing good flexibility and bending resistance. Moreover, the SiC nanofiber mat shows super hydrophobic and oleophilic properties. The SiC nanofiber mat with multi-scale structure exhibits great potential to be applied in harsh environment as catalyst support and water treatment template material.

Membrane technology is recognized as an energy saving option for separation and purification. While already well-established for water desalination, it has great perspective to further expand its application in the purification of aqueous and organic streams. In addition to the most common processes such as ultra- (UF), nanofiltration (NF) and reverse osmosis, by choosing the right combination of chemical composition, morphology and conductivity, membranes can be coupled as key components in energy conversion processes such as fuel cell, electrolysis, pressure retarded osmosis (PRO). Polymeric membranes are typically multilayered. The design of the material to be used in each part and the layer morphologies are crucial for the successful performance in each operating process. The chemistry of the top layer and the porosity and shape of the substrate are important to guarantee high flux, selectivity and fouling resistance. We have been exploring different chemical modifications for the membrane top layer and recent approaches will be discussed in the presentation. A recent approach for the formation of the top layer of membranes to be used in NF or PRO has been to promote the interfacial polymerization of a mixture of (5,10,15,20-(tetra-4-aminophenyl)porphyrin) and m-phenylene diamine with trimesoyl chloride. Porphyrin is a photosensitizer molecule. When the membrane is exposed to visible light, singlet oxygen is generated and its antimicrobial activity is stimulated, minimizing biofouling. Permeation as high as 35 Lm^-2h^-1bar^-1 with 99 % rejection of Brilliant Blue (826 g/mol). Other modifications of the top layer, such as the incorporation of amine-terminated dendrimers and zwitterionic building blocks, are under investigation. Apart from the chemistry itself, adding electron conductivity to the membrane surface layer by incorporating carbon nanotubes or depositing thin metallic layers has been previously demonstrated to increase fouling resistance and extend the use of membranes to bioelectrochemical systems (e. g. microbial fuel cell or electrolysis). The morphology and geometry of the porous substrate is as important for the membrane performance as flat-sheet and hollow fibers. By controlling the bore fluid composition and the spinning conditions of hollow fibers we have been able to systematically tune the porosity and the geometry of the fiber lumen, changing its cross section from triangle to square, pentagon, hexagon and circle.

We analyze the acceleration of water condensation by using radiative passive cooling. We show that water production can be enhanced by using novel photonic coating that suppresses sunlight absorption while maintaining a high emissivity in the mid-infrared spectrum regime.

Wearable energy harvesting devices are greatly attractive and receive intensive research efforts in recent years, aiming at powering various emerging flexible and wearable electronics, such as smart clothing, eye glasses, wristwatches, and even healthcare sensors. So far, inflexible rigid plate is the typical form of energy harvesting and storage, although some thin film-shaped power systems have been designed to achieve flexible or stretchable energy devices, the film structure could not be twisted freely or deformed extensively to fully satisfy the requirements of wearable electronic devices. To this end, a lot of efforts were made to explore the fiber/fabric based device, attempting to promise wearable energy devices. Rainwater, ocean waves and waterfalls are sources of clean energy which is almost inexhaustible, renewable and not limited by daylight. Moreover, the flowing water not only carries mechanical energy, but also produces triboelectricity due to contact electrification process with air or other substrates.
Realizing energy harvesting from water flow using triboelectric generators (TEGs) based on our daily wearable fabric or textile has practical significance. Challenges remain on the methods to fabricate conformable TEGs that can be easily incorporated into waterproof textile, or directly harvest energy from water using hydrophobic textiles. In this work, we developed for the first time a wearable all-fabric-based TEGs for water energy harvesting, with additional self-cleaning and antifouling properties. Hydrophobic cellulose oleoyl ester nanoparticles (HCOENPs) were prepared from microcrystalline cellulose (MCC), as a low-cost and nontoxic coating material to achieve superhydrophobic coating on fabrics, including cotton, silk, flax, polyethylene terephthalate (PET), polyamide (Nylon) and polyurethane (PU). The resultant PET fabric based water-TEG (WTEG) can generate an instantaneous output power density of 0.14 W m^-2 at a load resistance of 100 MΩ. An all-fabric-based dual-mode TEG (DMTEG) was further realized to harvest both the electrostatic energy and mechanical energy of water, achieving the maximum instantaneous output power density of 0.30 W m^-2. The HCOENPs coated fabric provides excellent breathability, washability and environmentally friendly fabric-based TEGs, making it a promising wearable self-powered system.

Understanding and controlling ice nucleation and accretion on surfaces is an important area of research due to its prevalence in nature and technology. Ice formation can negatively affect numerous activities and applications, including aviation, power transmission, road transportation, solar energy harvesting, and shipping. To prevent ice accretion, active anti-icing protection systems are employed, such as chemical treatment or heating, and their expenditures are expected to be >$10 billion by 2021 ^[1]. Both approaches have economic drawbacks as well as environmental (chemicals) and energy concerns, the latter (heating) generally consuming non-renewable sources. Here we show that by leveraging sunlight (renewable energy source, ~1 kW m^-2), one can use rationally engineered solar metamaterial absorbers to induce localized heating and perform anti-icing (significant nucleation delay of water droplets) and de-icing (partially melting and removing adhering ice). An important point of this approach is that the surfaces can be partially transparent while also absorbing a significant fraction of the incident solar irradiation. A ~10 ^oC temperature increase, rapid deicing and 10s’ orders of magnitude boosted inhibition of water condensate and frost formation have been experimentally achieved for semi-transparent films, towards surfaces with significant performance against icing even under nominal sun power (~1 kW m^-2). Guided by thermoplasmonics and percolation theory, the films are fabricated as gold–dielectric nanocomposites with a broadband absorption in the visible light regime, owing to localized surface plasmon oscillations in the nanometer-scale, which then dissipate into heat. The films retain the same level of performance for several freezing cycles. In comparison with electrically-heated de-icing surfaces, energy efficiency, transparency and immunity to photodegradation are three important aspects where these nanomaterials are superior. Our research into solar metamaterial absorbers represents a feasible, environmentally friendly approach to anti-icing and de-icing. We expect its relevance to extend beyond environmentally-friendly applications, wherever transparency or ultra-low thickness are a must, for instance in windshields and optical elements.


[1] MarketWatch. 2017. Ice Protection Systems Market Worth 10.17 Billion USD by 2021. 10 March. Accessed October 30, 2017. https://www.marketwatch.com/story/ice-protection-systems-market-worth-1017-billion-usd-by-2021-2017-03-10-10203310.

The materials for fog harvesting have been paid much more attention as water scarcity is facing mankind. Interestingly, biological surfaces create the enigmatical reality that can be contributed to learning of human beings. Such biological surfaces with multi-gradient micro- and nanostructures display unique wetting functions in nature via evolvement, e.g., water collection or repellency, which have inspired researchers to design originality of materials for promising future applications.
In nature, a combination of multiple gradients in a periodic spindle-knot structure take on surface of spider silk after wet-rebuilding process in mist. This structure drives tiny water droplets directionally toward the spindle-knots for highly efficient water collection. Inspired by gradient MN of wet spider silk, artificial bioinspired fibers with water collecting properties can be fabricated to integrate the gradients of curvature and roughness by means of designing humps or spindle-knots, with features of multiple gradients (e.g., roughness, smooth, temperature-respond, photo-triggering, etc.,) via improved techniques such as dip-coating, fluid-coating, tilt-angle coating, electrospun and self-assembly, to combine the Rayleigh instability theory. The geometrically-engineered thin fibers display a strong water capturing ability than previously thought. The bead-on-string heterostructured fibers are capable of intelligently responding to environmental changes in humidity. Also a long-range gradient-step spindle-knotted fiber can be driven droplet directionally in a long range. An electrospun fiber at micro-level can be fabricated by the self-assembly wet-rebuilt process, thus the fiber displays strong hanging-droplet ability. The temperature or photo or roughness-responsive fibers can achieve a controlling on droplet driving in directions, which contribute to water collection in efficiency. In addition, a conical spine can be designed to combine with periodic roughness gradient, which can be used to integrate an array for high efficiency of fog harvesting. Inspired by desert beetles, strategies have been explored to fabricate hydrophobic−hydrophilic patterned surfaces for high-efficient fog collection. The surfaces with star-shape hydrophilic patterns are designed to raise the efficiency of water collection.
These bioinspired as-designed materials are significant to develop novel functional materials for fog harvesting tasks.
References:
1. Y. Zheng, et al., Nature 2010, 463, 640-643.
2. H. Bai, L. Wang, J. Ju, R. Sun, Y. Zheng*, L. Jiang. Adv. Mater. 2014, 26, 5025-5030.
3. T. Xu, Y. Lin, M. Zhang, W. Shi, Y. Zheng*. ACS Nano, 2016, 10, 10681−10688.
4. Y. Zheng, Bio-inspired Wettability Surfaces: Developments in Micro- and Nanostructures. Pan Stanford Publishing. 2015, 0-216.

In many regions of the world collection of dew could provide an alternative or supplementary source of potable water.¹ While engineering of surface topography and chemistry to enhance the rate of dew collection has been extensively studied, the effects of mechanical properties of the surface have barely been explored. This topic is highly relevant, since most materials that are commercially viable for dew and fog collectors are polymers, which are substantially softer than typical metallic condenser surfaces. Encouragingly, Sokuler et al.² have showed that softer substrates promote higher water droplet nucleation density. Wang et al.³ also recently showed that use of an elastic collector that is stretched by wind can increase water collection efficiency. Motivated by these results, here we discuss the effect of substrate’s mechanical properties on all processes relevant to droplet condensation including nucleation, growth, and shedding. Specifically, we experimentally quantify the effect of hydrophobic silicone substrate’s shear modulus on droplet nucleation density and shedding diameter under vertical orientation. By combining analytical model of substrate’s deformation by drop’s surface tension and Laplace pressure with finite element modeling, we theoretically quantify how soft surfaces affect heat transfer across the droplets. Finally, we substitute these results into overall dropwise condensation heat transfer model.⁴ Our results indicate that despite the increase in droplet nucleation density, decreasing substrate’s shear modulus below 500 kPa decreases the overall condensation rate. This effect is primarily due to additional thermal resistances posed by liquid within substrate depression caused by capillary forces of micro-droplets. Consequently, our work indicates that attention must be paid to mechanical properties of the material when selecting polymeric materials for dew collectors.

1.Tomaszkiewicz et al. Env. Rev. 24, 2015.
2. Sokuler et al., Langmuir 26, 2010.
3. Wang et al. ACS App. Mater. Inter. 9, 2017.
4. Phadnis and Rykaczewski, Langmuir, 2017.

Instead of removing salts and pollutants from water, atmospheric water capture (AWC) removes clean water from air. Treating unconventional water supplies (e.g., oceans, brackish groundwater, wastewater, stormwater) to drinking water quality is more expensive than treating water supplies developed decades ago (e.g., pristine lakes or rivers). Therefore, in a paradigm shift, water can be obtained from an alternate freshwater reservoir – the atmosphere. An AWC system operates in a three step process: i) desiccant materials adsorb water vapor in the surrounding air; ii) heat drives desorption of vapor form the desiccant onto a cool surface, where iii) clean water is condensed and collected. Current desiccant-based AWC systems such as electrical dehumidifiers and outdoor solar heat driven systems are limited in their capabilities (1-2.5 L/m²/day). Assuming $0.10/kWh, electrical systems are energy intensive (0.29 L/kWh), and the cost of water production (~$0.35/L) is still 50x-100x more than ocean desalination.
We reduced energy demands of bulk heating of desiccants (step ii) through application of photothermal nanomaterials, thus almost eliminating energy operating costs for AWC systems. We have developed nanomaterial-enabled desiccants that are light-active, producing localized centers of heat directly on their surface in the presence of solar light. Heat transfer from the activated nanomaterial to the adjacent adsorbed water vapor molecule will increase kinetics of desorption. The regenerated desiccant can thus be cycled back as an adsorbent (step i) more quickly, allowing for a larger quantity of water to be harvested daily.
Commercially available silica gel desiccants (SiO₂) were made light-active through silanization with 3-aminopropyltriethoxysilane followed by nanoparticle coating. Inexpensive Cabot Emperor 2000 carbon black (CBNP) was compared to gold (AuNP), a model plasmonic photothermal nanomaterial. UV-Vis measurements showed CBNP coated SiO₂ to absorb light in the full visible spectrum, while AuNP coated SiO₂ only absorbed light at resonant wavelengths correlated to nanoparticle size. Visible light spectra utilization efficiency correlates with differences in surface temperature and rate of heating under 1-sun of simulated solar irradiation (Xe/Hg lamp). In under 5 minutes, desiccant surface temperature increased 8x with CBNP monolayer vs 4x with AuNP monolayer, in comparison to bare SiO₂. Thermodynamic and heat transfer models were developed to quantify heat generation by nanoparticles and calculate theoretical water vapor desorption potential. These results motivate development of an optimized, energy-efficient solar light and heat driven AWC system which can at least double quantity of water produced per day in comparison to commercial alternatives.

Cloud seeding materials as a promising water augmentation technology have drawn more attention recently. We designed and synthesized a novel type of core/shell NaCl/TiO₂ (CSNT) particles with controlled particle size, which successfully adsorbed more water vapor at low relative humidity, (from 20 % RH) than that of pure NaCl, deliquesced at lower environmental RH of 62 - 66 % than the hygroscopic point (h_g.p., 75 % RH) of NaCl, and formed larger water droplets ～ 6 - 10 times of its original measured size area, whereas the pure NaCl still remained as crystal at the same condition. These benefits of CSNT particles were observed visually through in-situ observation under Environmental - Scanning Electron Microscope (E-SEM). The enhanced performance was attributed to the synergistic effect of the hydrophilic TiO₂ shell and hygroscopic NaCl core microstructure. Specifically, as for pure NaCl crystal, the condensation of water vapor molecules occurred at the interface of NaCl crystal and the water vapor, due to the weak hydrophilic property of the NaCl surface, their deliquescence was totally dependent on the environment RH conditions, ie, if the environmental RH value was below 75%, no interaction will happen, only when the environmental RH value was above 75%, the NaCl crystal can deliquesce. Whereas for the CSNT particles, this process was enhanced by the core/shell structure, the hydrophilic TiO₂ shell accumulated water molecules around the core, and increased significantly the local RH value near the NaCl to saturation or supersaturation conditions, this promoted the deliquescence process to occur even when the environmental RH value was lower than 75%. Moreover, the critical particle size of CSNT particles (0.4 - 10 μm) as cloud-seeding materials was predicted via Kelvin equation based on their surface hydrophilicity. Finally, CSNT particles were added in the cloud chamber to evaluate their water droplet formation in a three-dimensional environment. It was found that both the water-droplet concentration and the droplet size increased greatly across all size ranges. Especially, at 100 % RH, the concentration of water droplet size between 10 - 25 μm (which is very crucial to the rainfall) caused by CSNT particles was up to 290% more than that by NaCl. These excellent results were highly consistent and positively confirmed that CSNT particles can be a type of effective cloud-seeding materials. This strategy was the first effort to design nano/micro-structured materials for rain enhancement and broad water augmentation technology.

Electrodialysis is an industrial-scale technique used to purify water. Here the application of a DC electric field drives dissolved ions across ion-selective membranes to decrease the salt concentration and create a purified water stream. Due to the large electricity costs for operating such a facility, only a fraction of water purification plants are based on electrodialysis techniques, with the majority employing reverse osmosis. One of the key factors influencing the cost of electrodialysis is the conductivity of the ion-selective membranes. Here we present recent results from studies leveraging layer by layer (LbL) deposition of polyelectrolytes of poly(ethyleneimine) and poly(acrylic acid) onto nanoporous polymer support layers in order to increase their ionic conductivity and ionic selectivity. Use of crosslinking agents creates mechanically stable polyelectrolyte films on and in the nanopores, though choice of various crosslinking agents is shown to either enhance or hinder ionic selectivity. Variation of LbL polyelectrolyte deposition parameters and starting nanoporous supporting polymer support is demonstrated to control the ionic selectivity and conductivity, adhesion, and durability of the coated membranes. The performance of these membranes is compared to commercial electrodialysis membranes using a small-scale electrodialysis test bed.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Contamination of the water supply by arsenic is a serious threat to more than 100 million people around the world. In addition to the very important negative health impact, arsenic contamination poses an important challenge for the sustainability of the communities in the areas contaminated, since in many cases the contaminated water supply is a critical drinking water resource. Furthermore, arsenic poisoning has a great social cost due to their negative effects in children and adults alike particularly in impoverished communities. Due to the toxicity of arsenic, various technologies have been developed for its removal, among which, adsorption stands out due to its simplicity of operation and high efficiency. However, a problem faced by adsorbent materials during the adsorption process is the presence of natural salts in the water, such as sulfates, phosphates and carbonates, which can considerably reduce the adsorption capacity of nanomaterials. In this work we have functionalized graphene oxide (GO) with molecules whose structure containing sulfur increase its selectivity towards the adsorption of arsenic regardless of the presence of secondary salts. Graphene oxide was functionalized using sulfuric acid, cysteamine and sulfanilic acid. Samples were characterized with FTIR, XPS, Raman spectroscopy, SEM and TEM. Adsorption tests were performed for As (III) and the effect of the three salts was evaluated. We have observed that GO by its own has an adsorption capacity of 35 mg/g, however in the presence of carbonates it agglomerates inhibiting any arsenic adsorption. When using the functionalized material GO showed to be able to maintain to some level its adsorption capacity.

Effective oil removal from rock surface highly depends on the surface wettability and interfacial interaction among water, gas, oil and reservoir rock. Utilization of surface engineering, including morphology control, surface chemistry alteration and fluid infusion, has led to great advances in enhancing oil mobility. When a nanotextured surface is infused with a low surface tension oil (positive spreading coefficient), intrusive water droplets will be cloaked by the light oil layer and drag the oil along the injection fluid flow. In order to taking advantage of this cloaking-driving oil drainage mechanism, properly designed nanotextured surfaces are used in lab experiments to mimic reservoir rock and study microscopic oil displacement process. In this work, we investigate the effect of solid surface wettability and oil properties, including surface tension and viscosity, on water-oil-solid interfacial interactions through experiments in both optical microscopy and environmental scanning electron microscopy (ESEM). Different types of nanoparticles, such as calcined carbon-silica and calcium carbonate nanoparticles, are deposited on surfaces to tailor the wettability. These nanotextured surfaces are further infused by oil with different surface tensions and thus different spreading coefficients, causing different level of oil cloaking on water droplets. Thickness of the cloaking oil layer and emulsion flow rate are measured to quantify oil removal effectiveness. This study provides some fundamental insights into interfacial interaction among oil, gas, water and solid towards revealing pore-scale oil displacement mechanism.

About a decade ago, researchers started investigating the use of nanotechnology towards water processing and purification. The general size of the nanotechnology based water filtration products are smaller than traditional devices which leads to concerns regarding use and maintenance. Recently, graphene and plasma based devices and have also been investigated in a variety of areas of water purification. Research by Nobel Prize winner Andrei Geim with Rahul Nair has indicated that defects in graphene oxide result in a barrier that is highly impermeable to everything except water vapor. Additionally, plasma has been used to chemically treat water to reduce the number of harsh chemicals as well as destroy bacterial and viral organisms. The combination of these two technologies can be useful in water filtration applications. The following work deals with the development of a low cost graphene and plasma based hybrid water purification systems towards removal of organic water and salt based contaminants.

It is well known that sand bed has numerous voids inside and the voids are much smaller than the sand particles. Therefore, the sand bed has been utilized as filtration media for a long time and it is still used for pre-treatment of water in modern industries. This study provides a new principle for fabrication of nano voids using nanoparticles, in the similar way of conventional sand bed. Polystyrene nanoparticles with various sizes of 150, 60 and 24 nm have been embedded in the pores of the anodic aluminum oxide (AAO) template, so that voids between the nanoparticles could be made in nano scale. These voids can be considered as effective pores for filtration or separation. By this method, a lot of nano voids could be fabricated on a large area (diameter of template : 25 mm) with a simple process. Since overall size of the voids is expected to be much smaller than that of the nanoparticles, this principle is expected to have effective pore size of a few nanometers. And as long as the sizes of the nanoparticles are uniform, size of the voids could be quite uniform also. Furthermore, we treated the surfaces of the nanoparticles with arginine/phenylalanine peptide ((RF)4), so that the particles could be electrically charged. In order to evaluate embedment of the nanoparticles, hydraulic experiment was performed with the substrate and pressure drops were measured. The pressure drops became higher as smaller nanoparticles were embedded, which means that smaller voids were made inside.

Oxygen is most prominent terminal electron acceptor for microbial fuel cells (MFCs) and oxygen reduction reaction (ORR) governs the performance of this system. However, high cathode overpotential losses during ORR cause serious performance depletion in MFC in short and long run. Therefore, it is necessary to develop low cost sustainable cathode catalysts to improve ORR in order to enhance power generation from MFCs. The present study demonstrates synthesis of a ferromagnetic low-cost cathode catalyst of family metal co-doped ferrite, CoZnFe₂O₄, using a simple one-pot high temperature redox process. The results obtained from X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) revealed successful synthesis of CoZnFe₂O₄. The electrochemical results carried out under air saturated 1 N KCl as neutral solution for evaluating the ORR kinetics supported with this catalyst showed excellent redox current of 34.2 mA for cathode and 5.9 mA for anode with less charge transfer resistance of 0.7 mΩ. Under nitrogen purged 1 N KCl solution, these redox peaks were unnoticeable, conforming that the major reaction was ORR supported by CoZnFe₂O₄ catalyst. Thus, the MFC using this catalyst in cathode could generate an excellent power density of 9.6 W/m³ which was found about 5 times higher than the power density of 1.96 W/m³ obtained from a MFC using cathode having no catalyst and was noted surprisingly higher than the power density of MFC with Pt-C cathode (8.1 W/m³). The chemical oxygen demand (COD) removal efficiency was about 83 to 88 % from all test MFCs, revealing bifunctional capabilities of MFCs for treatment of sanitary wastewater along with bioenergy recovery. The results obtained from this experiment set the synthesized catalyst in a position to replace mostly used Pt-C catalyst for MFCs.

Introduction
A problem derived to the increasing emergence of microorganisms resistant to antibacterial agents of such phenomenon is related with water quality and the diseases provoked by the presence of bacteria and parasites. The advanced oxidation processes (AOP’s) have attracted attention as processes that imply the formation of hydroxyl radicals (OH) with a high oxidation potential. In the case of microorganisms, this radicals attack the lipid bilayer that conforms the external cell wall, generating reactions of lipid peroxidation that are lethal for microorganisms. The heterogeneous photocatalysis, involves the absorption of light by a catalyst (semiconductor). During the process, oxidation-reduction reactions occur, that can degrade organic compounds and reduce inorganic ions.
Bismuth oxide is a semiconductor that in last years has reached importance as a photocatalyst, in particular when it is tetragonal beta phase which presents a low bandgap (~2.7 eV). The intention of using the bismuth oxide as photocatalyst is because Mexico is the second worldwide bismuth producer and it would be attractive to give this mineral a higher value. The objective of this research is to use bismuth oxide films as antibacterial agent for water treatment. The antibacterial effect was tested by the inhibition of E. coli in presence of Bi₂O₃ under UV and visible light.
Experimental Methods
Bi₂O₃ films were deposited by the spray pyrolysis technique using Bismuth acetate (CCH₃CO₂)₃Bi) as precursor in a solution 0.05 M dissolved in 25%v acetic acid and 75%v deionized water. The films were grown on Corning glass substrates of 1.5 cm x 1.5 cm. The deposition temperature and time were 450°C and 20 min. respectively. The antibacterial effect of Bi₂O₃ films was determined by counting the E. coli colony-forming units (CFU). 2 mL of E. coli culture with a concentration of 1x10⁷ cell/mL were put in each well with a Bi₂O₃ sample and irradiated with UV light. A similar sample was irradiated with white light.. After two hours of irradiation the samples were poured into a Petri dish with agar and the bacteria were incubated during 24 hours. Afterwards the CFU were counted. All the experiments were performed by triplicate.
Results
The XRD pattern of the Bi₂O₃ film shows that the material presents a beta phase, with a tetragonal crystalline structure. After the experimental process it’s possible to observe a decrease in the bacteria proliferation when the culture is in contact with the photocatalytic Bi₂O₃ film. It was found a lower bacteria proliferation when the semiconductor is irradiated with UV light.
Discussion and conclusion
The results demonstrate that the bismuth oxide present antibacterial activity under both white light and UV light after two hours of exposition. It is necessary to perform more experiments to optimize the deposition conditions as well as the concentration of the bacterial culture in order to enhance the bacteria annihilation by the photocatalyst.

Electrospinning of silica nanofibers without organic polymer addiditon is regarded as a highly promising methodology for the production of thermal and chemical resistant nanofibrous materials applicable in various advanced applications. The combination of sol-gel technology and the electrospinning process allows for the production of ceramic nanofibrous membranes. By proper control of the sols rheology, addition of an organic polymer can be skipped in the nanofibres preparation procedure. This broadens their applicability since an additional heat treatment, typically used to remove the organic polymer, can be avoided. It allows for functionalisation of these nanofibrous membranes with various components that would otherwise not withstand the heat treatment.
The influence on viscosity, concentration and degree of cross linking will be described (1,2). The use of these nanofibrous structures coated with titania are very well suited for water purification (3) while a post-functionalisation with a dye can open the possibility of using this fibres as colorimetric sensors (4).

1. Geltmeyer, J., De Roo, J., Van den Broeck, F., Martins, J., De Buysser, K. and De Clerck, K., The influence of tetraethoxysilane sol preparation on the electrospinning of silica nanofibers, Journal of Sol-Gel Science and Technology 1-10 (2016)
2. Geltmeyer, J., Van der Schueren, L., Goethals, F., De Buysser, K. and De Clerck, K., Optimum sol viscosity for stable electrospinning of silica nanofibres, Journal of Sol-Gel Science and Technology 67 188-195 (2013)
3. Geltmeyer, J., Teixido, H., Meire, M., Van Acker, T., Deventer, K., Vanhaecke, F., Van Hulle, S., De Buysser, K. and De Clerck, K., TiO₂ functionalized nanofibrous membranes for removal of organic (micro)pollutants from water, SEPARATION AND PURIFICATION TECHNOLOGY 1-37 (2017)
4. Geltmeyer, J., Vancoillie, G., Steyaert, I., Breyne, B., Cousins, G., Lava, K., Hoogenboom, R., De Buysser, K. and De Clerck, K., Dye Modification of Nanofibrous Silicon Oxide Membranes for Colorimetric HCl and NH₃ Sensing, Advanced Functional materials 26 5987-5996 (2016)

Modern industrial activities, involving petroleum production/transportation, pharmaceutical, and food industries, have been producing copious oily wastewater which poses a great threat to the ecosystems. With the growing demand for clean water as well as the limited resources water supply, treatment of the oily wastewater is an inevitable worldwide challenge. Since the oil/water mixture (a free mixture, a dispersion or an emulsion) is a multiphase system, its separation is essentially an interfacial issue. Recent research has focused on designing and producing novel materials with special wettability to address this problem. In particular, these separation materials generally show contrasting affinities towards oil or water, i.e., superhydrophobicity/superoleophilicity, superhydrophilicity/underwater superoleophobicity, superhydrophilicity/in air superoleophobicity, asymmetric superwettability and switchable superwettability. According to the separation methods, these super wettable materials could be further divided into filtration materials (e.g., metal meshes, fabrics, membranes and liquid-infused membranes) and absorption materials (e.g., foams, aerogels, and sponges). Among them, membranes technologies are in rapid growth due to its low cost, ease of operation and high separation efficiency.
Herein, an effective method to manipulate the surface chemistry and morphology of glass fiber by exploiting the strong attractive interactions between poly(dimethylsiloxane) (PDMS) and GF is reported. The effects of PDMS molar mass, annealing temperature, fumed silica and poly(N, N-dimethylaminoethyl methacrylate) (PDMAEMA) on GF’s surface modification were investigated and the mechanisms behind were elucidated. Using PDMS as a molecular binder, the SiO₂ nanoparticles can be firmly anchored onto the GF surfaces to produce superhydrophobic/superoleophilic GF membranes with “oil selectivity”. Besides, PDMAEMA can readily functionalize GF surfaces through block copolymerization of PDMS, resulting in pH-responsive/underwater superoleophobic GF membranes with “water selectivity”. The superwetting membranes give excellent oil/water separation performances with high separation efficiency (99%), good organic solvent resistance, and long-term stability. This approach is scalable and expected to be applied to any PDMS/glass substrates pairs to achieve customized functionalities.

Reverse osmosis (RO) is one of the most widely employed technologies for water desalination. A drawback of this technology is high consumption of electricity by electric motors used for high-pressure water pumping. Energy consumption can account for up to 70% of the desalination costs. Due to the high energy intensity the carbon footprint of desalination processes is substantial. Modern seawater desalination RO plants emit between 1.4 and 1.8 kg CO2 per cubic meter of produced water. High capital costs due to expensive high-pressure water pumps and concentrate water energy recovery systems such as pressure exchanges, or Pelton turbines is another drawback. Most of pumps and compressors including those used in RO desalination plants are driven by electric motors or internal combustion engines (diesels, gas turbines). Therefore compression and pumping are always associated with multiple energy transformations. Pumping systems account for nearly 20% of the world’s electrical energy demand and range from 25-50% of the energy usage in certain industrial plant operations. To improve economics of water desalination plants we propose innovative energy efficient, inexpensive, robust water pump powered by heat. The basic principle of the pump (or engine) is the same as that of regenerative type external combustion engines with closed cycle – working fluid expands when it is heated and contracts when it is cooled. Regeneration of heat makes pumps of this type very energy efficient. The novelty of the pump comprises a new working cycle in combination with the use of a dense working fluid which is liquid in the cold space of the pump and gas or supercritical fluid in the hot space of the pump. The working fluid of the engine has very high thermal expansion, yet low compressibility when it is in liquid phase. Carbon dioxide, water-alcohols mixtures or mixtures hydrocarbons can be used as the working fluids. A distinguishable feature of the pump proposed is its simplicity. It does not have any high precision parts and even does not require super alloys and any other expensive materials. The working fluid in the engine is compressed by heat and the energy of the working fluid is directly transmitted to a liquid to be pumped. The pump can generate very high pressures (hundreds bars) and therefore can be used instead of modern plunger pumps driven by electric motors, thus eliminating completely the consumption of electricity for pumping in RO processes. Another important feature of the pumps is that no high temperature heat sources are needed. Heat sources with temperature of 200 – 300 0C could be sufficient to create pressure drops typical of RO processes. Therefore different sustainable and renewable energy sources such as solar radiation, waste heat, and geothermal energy can be used to power the pump.

Irrigation of farmland consumed approximately 80% of total water usage in California. The costs associated with groundwater monitoring and cleansing have cost the state more than $33 millions of dollars. This research project has significant economic impact to the California agricultural industry and consumers by helping clean up surface and ground water contaminated by pesticides.
Contamination of California ground and surface water by pesticides is an important environmental issue that affects California welfare. It has been reported that a total of 192 million pounds and 186 million pounds of pesticides were used for agricultural practice in California in 2011 and 2012, respectively, among them Propanil is in the top 100 pesticides used list. Because pesticides are relatively mobile and persistent in soil they can penetrate into ground water following their soil application by farmers. As a result, Propanil has been detected in surface and ground water in many areas of California with various concentrations. Crops including vegetables and fruit could be contaminated if the water containing pesticides is reused for irrigation. It is important to remove pesticides from the irrigated water prior to its reuse.
Propanil water samples with concentration at ppm (part per million) level are placed into a beaker containing trace amount of hydrogen peroxide and a glass plate coated with nanoparticles. The samples are then exposed to visible light radiations coming from either regular light bulb or the Sun, and the change of the Propanil concentration was monitored using ultraviolet-visible spectroscopy. Cataphotolysis of Propanil molecules takes place at the surface of nanoparticles, on which the reduction-oxidation reactions occur, leading to oxidative degradation of the Propanil molecules. The Propanil degradation is indicated by the absorbance decrease of the ultraviolet-visible spectrum of Propanil at 248 nm. The product of cataphotolysis is probed using liquid chromatography coupled with mass spectrometry (LC-MS), and the acidity of the water sample was measured using a pH meter. Kinetics information is acquired by monitoring the decay of Propanil as a function of time during its cataphotolysis.
Propanil molecules were found to undergo decomposition during cataphotolysis when the water samples are radiated by visible light either from light bulb or from the Sun. The sunlight was found to increase the degradation rate in comparison to the visible light from the light bulb. The cataphotolytic degradation of Propanil in water follows the first order chemical kinetics, with a rate constant of k = (4.29±0.49) x 10-3 s-1. No detectable organic products were found from cataphotolysis of Propanil in the LC-MS examination of the water samples. The acidity of the water sample increases, suggesting that the Propanil molecules are oxidized into carbon dioxide, which dissolved in water to form carbonic acid, giving rise to lower pH value than the neutral water.

It is known that pristine graphene is impermeable to any gases or liquids. As the most derivatives of graphene, graphene oxide with variable C/O ratio can be potentially applied in separation and filtration applications. Recent reports suggest that graphene oxide can also be used to remove chemical contaminations from solution. In this work, we have successfully established the molecule sponge behaviour of graphene oxide and observed the adsorption/desorption process within the graphene oxide membrane. This thorough understanding of mass transport in the graphene oxide laminates helps our further research on filtrating natural organic matters (NOMs). We report a laboratory scale innovative use of graphene oxide membranes to remove NOMs from water that had been treated and still contained 5 mg/L dissolved organic carbon. Our study shows that graphene oxide based membranes can reject nearly 100% of NOM while maintaining high water flux of 65 L m^-2 h^-1 bar^-1 at atmospheric pressure. Our results indicate that it is possible to develop a graphene oxide-based water filtration technology.

One of the most challenging problems when trying to recycle urine for different purposes is the removal of urea. In this project we elaborated an ureolysis system using the bacterium Proteus vulgaris for the transformation of urea to ammonia and its subsequent oxidation to nitrogen at Pt electrodes. Our system was tested under different conditions of pH, time and urea and bacteria concentrations. Our results indicate that a pH 8 is optimal for the Proteus vulgaris urease activity and the ammonia oxidation reaction. A reaction time and concentration dependence on the ammonia oxidation reaction current density was obtained from the system. Results also showed limited ammonia oxidation under high urea concentrations in ~ 2.5 x 10⁹ cfu/mL Proteus vulgaris in synthetic urine.

As environmental problems such as global warming and air pollution arise due to fossil fuel-based energy production, the need for alternative energy development is emphasized. Especially, hydrogen energy has much attention to the ideal next generation alternative energy. Recently, solar water splitting, which exploits the reaction phenomenon of photoelectrode material and water under solar irradiance, has been one of promising candidates for generating hydrogen ecofriendly. The efficiency of solar water splitting can be improved by enhancing the reaction between the light and photoelectrode material based on following strategies: 1) widening a surface area of photoelectrode material and 2) reducing the strong surface reflection at the interface between photoelectrode material and water. These two goals can be achieved by introducing surface nanostructures such as nanowire, nanopore, and tapered nanostructure.
Meanwhile, various photoelectrode materials such as TiO₂, ZnO, WO₃, and so forth are used for solar water splitting. However, these conventional materials exhibit weak points such as wide energy bandgap that absorbs only the ultraviolet region of the solar spectrum, low charge carrier mobility, and chemical corrosion in the electrolyte. On the other hand, GaN can overcome these limitations due to tunable band gap, an outstanding electrical feature, and chemical stability. Also, other photoelectrode materials are difficult to manipulate surface texture, whereas GaN can be easily fabricated in nanostructure by using conventional semiconductor processing.
Thus, in this study, we investigate the water splitting characteristics of photo-electrochemicals by fabricating GaN anti-reflective nanostructures and measuring current density in bulk structures and nanostructures. After the deposition of SiO₂ and Ag on GaN, a thermal dewetting technique of Ag thin film and SiO₂ etching are utilized to form a nanoparticle mask, and a nanostructure is formed by using an ICP RIE etching process. The measurement of reflectance is performed by using ultraviolet-visible spectrometer for the optical analysis of the nanostructures. This result demonstrates that the reflectance of the nanostructure is 20% lower than that of the bulk structure. The photocurrent density of GaN with anti-reflective nanostructures is measured under the condition of artificial solar light with an intensity of 100 mW /cm². As a result, the planar GaN is 0.1 mA / cm² at a voltage of 0.8 V (versus Ag / AgCl), the photocurrent density of 0.15 mA /cm² and 0.2 mA/cm² is measured respectively, and the photocurrent density is improved 1.5 to 2 times in the GaN nanostructure compare to the planar GaN.

Among commercial polymers, Polybenzimidazol (PBI) is known to have high heat resistance and chemical resistance. In addition, the polymer has excellent mechanical properties and chemical properties and is utilized in diverse fields besides material engineering, nanotechnology and optics. This study produced a polybenzimidazole membrane using the non-solvent induced phase separation. Observe the morphology, which alters various conditions during manufacturing and manufactured a morphology-controllable Polybenzimidazole membrane. DMAc, THF were used as a solvent, co-solvent and membrane was manufactured by knife casting method. Morphology was observed by surface, cross-section using scanning electron microscope(SEM).

The degradation of organic pollutants in the aqueous environment by using semiconductor photocatalyst has become an attractive process. Photocatalysis as a green, feasible and sustainable technology has received growing attention owing to its potential to solve energy and environmental problems. Currently, the major focus in photocatalysis is the design and development of highly efficient and low-cost photocatalysts. Therefore, various semiconductor metal oxide has been designed and developed for the efficient removal of organic pollutants from wastewater. As a robust semiconductor ZnO has been widely used for wastewater treatment due to its charge carrier generation upon excitation by light and reactive oxygen species formation in aqueous suspension. However, fast recombination of massive charge carrier and low solar energy conversion efficiency limits their large scale applications. Therefore, design of visible light responsive photocatalyst is of great concern from commercial point of view. Herein, we report the strategy for the suppression of electron−hole pair recombination rate, extended the absorption edge in the visible region and enhanced the photocatalytic efficiency by introducing rare earth metal as dopant. The doping of rare earth metals is an effective way for enhancing photocatalytic performance of a semiconductor under visible light irradiation. Such modifications not only reduced massive charge carrier also serving for harvesting of visible light. The photocatalytic activity of undoped and Er-doped ZnO nanoparticles (NPs) was investigated by studying the degradation of two different organic dyes as a function of irradiation time. The results indicate that the photocatalytic activity of doped ZnO was found to be higher than undoped ZnO for degradation of