Symposium S.EN06—Rational Designed Hierarchical Nanostructures for Photocatalytic System

A cost effective synthesis route for tunable shapes and sizes of large scale monodisperse nanocrystals functionalized with copper-loaded dendrimer through a low-temperature thermal decomposition from iron (III) oleate has been demonstrated for the first time as a recoverable and reusable heterogeneous magnetic-nanoinitiator for polymerization of adhesive system. Herein, the colloidal system comprised of methacrylate ester monomers, peroxide and core-shell magnetic nanoparticles functionalized with copper-loaded dendrimer are used for the polymerization of adhesives with the nanoinitiators. The effect of various morphologies of the magnetic particles such as spherical, cubic, hexagonal shape as a core of the nanoinitiators are investigated through the polymerization of anaerobic adhesive formulation. To enable the recoverability of nanoinitiators and achieve simultaneously control of the loading of Cu(II) of nanoinitiators, core-shell magnetic nanoparticles and dendrimer have been designed to initiate highly efficient and safe polymerization through redox radical initiated cationic polymerization reaction under mild condition. Moreover, core-shell magnetic nanoparticles functionalized copper-loaded dendrimer can be separated simply by either magnetic separation or centrifugation and reused for several times without any significant mass loss[LPS(1] . The core-shell magnetic nanoparticles functionalized with copper-loaded dendrimer were characterized using transmission electron microscopy (TEM), Scanning electron microscopy (SEM-EDX), X-Ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR).

Even though the potential of hematite thin films for water splitting applications are widely accepted, researchers are still tackling the ‘rust challenge’. We report here on the influence of external magnetic fields applied parallel or perpendicular to the substrate during plasma enhanced chemical vapor deposition (PECVD) of hematite (α-Fe₂O₃) nanostructures and the accessibility of the very same through magnetic field assisted CVD of Fe(CO)₅. Hematite films grown from iron precursors showed pronounced changes in crystallographic textures depending upon whether PECVD was performed with or without the influence of external magnetic field and anisotropic film growth with higher exposure to water in case of mfCVD. In all cases, a superior photoelectrochemical (PEC) performance was obtained for hematite photoanodes synthesized under applied magnetic fields. Our experimental data on microstructure and functional properties of hematite films showed that magnetic fields have a significant effect on the crystallite size and texture with preferred growth and/or suppression of grains with specific texture in α-Fe₂O₃ films. Thereby, we are able to demonstrate magnetic field assisted gas phase depositions as new experimental playground for the ‘rust challenge’.

Solar photovoltaic (PV) and photoelectrochemical (PEC) energy conversion share the same fundamental requirements of photon absorption, charge separation, and selective carrier collection [1]. In addition, in order to produce storable chemical energy, PEC systems must drive multi-electron transfer reactions, i.e. water oxidation (OER, a 4 electron process) at the anode and hydrogen evolution (HER) or carbon diode reduction (CO₂R) at the cathode (2-18 electron processes) [2]. In a small number of cases it has been possible to have a single material perform all of the requisite steps and remain stable under operation: water oxidation photoanodes composed of TiO₂ or Fe₂O₃ are examples. However, in the majority of cases, it has been beneficial to construct multi-material PEC systems, with different components chosen for optimal light absorption, selective carrier collection/passivation, and catalysis.
In this context, the design of PEC systems for CO₂R will be discussed. While it is attractive to design semiconductor absorbers based on the position of their conduction band relative to the standard state redox potentials of a given CO₂ reaction, e.g. CO₂/CO, CO₂/HCOOH, CO₂/CH₄, etc., this analysis neglects the fact that these multi-electron reactions occur via a series of elementary proton coupled electron transfers, with each step having its own redox potential [3]. For this reason, either solid state or molecular CO₂R co-catalysts are usually integrated with the absorber in PEC CO₂R studies [4,5].
These concepts will be illustrated through the design and operation of Si-based CO₂R photocathodes. Adroit use of charge selective contacts allows for considerable flexibility in design. For example, in contrast to conventional photocathode designs which employ p-type absorbers, we used a back illumination geometry with an n-type Si absorber to permit the use of absorbing metallic catalysts which would otherwise block the light. Back and front interfaces were configured by ion implantation and by surface passivation to achieve carrier selectivity. Selectivity to C-C coupled products was achieved by using hierarchical Au- Ag-Cu nanostructures as electrocatalysts. The photovoltage, 550- 600 mV under simulated 1-sun illumination, and photocurrents exceeding 30 mA cm^-2 confirm the carrier selectivity and passivation of the front and back interfaces. Under simulated diurnal illumination conditions, over 60% faradaic efficiency to C₂₊ hydrocarbon and oxygenate products (mainly ethylene, ethanol, propanol) is maintained for several days. By coupling photocathodes to series-connected semi-transparent halide perovskite solar cells, we demonstrated stand-alone, “no-bias,” CO₂ reduction with a 1.5% conversion efficiency to hydrocarbons and oxygenates [6].
As further examples of the PEC design space for CO₂R, use of nanowire geometries which would permit the use of front illumination and the prospects of achieving cascade CO₂R [7] on the PEC surface will be discussed.

1. Wurfel, U.; Cuevas, A.; Wurfel, P. Charge Carrier Separation in Solar Cells. IEEE J. Photovoltaics 2015, 5, 461–469.
2. Osterloh, F. E. ACS Energy Lett. 2017, 2, 445–453.
3. Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. Energy Environ. Sci. 2010, 3, 1311
4. Arai, T.; Tajima, S.; Sato, S.; Uemura, K.; Morikawa, T.; Kajino, T. Chem. Commun. 2011, 47, 12664.
5. Qiu, J.; Zeng, G.; Ha, M.-A.; Ge, M.; Lin, Y.; Hettick, M.; Hou, B.; Alexandrova, A. N.; Javey, A.; Cronin, S. B. Nano Lett. 2015, 15, 6177–6181.
6. Gurudayal; Beeman, J. W.; Bullock, J.; Wang, H.; Eichhorn, J.; Towle, C.; Javey, A.; Toma, F. M.; Mathews, N.; Ager, J. W. Energy Environ. Sci. 2019, 12, 1068–1077.
7. Lum, Y.; Ager, J. W. Energy Environ. Sci. 2018, 11, 2935–2944.

Titanium dioxide (TiO₂), or titania, is a suitable choice for environmental remediation due to its photocatalytic advanced oxidation ability which can inactivate pathogens and decompose dyes. Current researches have been focusing on titania nanoparticles impregnated in polymers, due to higher active sites for contaminant adsorption, pore size control, and easy manufacturability. Low throughput rates, however, have far limited the adoption of titania-based water-treatment systems. Our research explored a scalable manufacturing technique and achieved morphology control of core-shell fibers incorporating titania nanoparticles. The novelty of this research is that it encompasses the benefits of remote ultraviolet (UV) light accessibility and porosity-enabled enhanced adsorption sites. The fibre core was composed of UV light transparent, flexible, and oxidation-resistant polymethyl methacrylate (PMMA)that served as an optical channel for UV light transmission and was prepared by melt spinning technique. The fibre shell was composed of a porous PMMA-titania nanoparticle composite prepared by dip-coating in colloidal solution with dispersed titania nanoparticles followed by coagulation through wet phase inversion. Titania in fiber shell was photo-activated by light radially refracted from the fiber core. Photocatalytic performance was optimized by controlling the pore-size, titania concentration and the thickness of the coated shell. Pore-sizes ranging from 50 nm up to 5 µm were achieved by varying the porogen concentration and selecting different solvent and non-solvent combinations. The core-shell interfacial interactions were also characterized using static mechanical tests and dynamic mechanical analysis. Finally, the photocatalytic performance of the fibers was characterized by monitoring the decomposition of methylene blue using UV-vis spectrophotometry. This research provided an understanding of the processing-structure-property relationships in the complex, multiphase core-shell structured fibers.

Recently, photocatalysis has emerged as a potential solution for growing energy crisis and environment pollution issues. However, higher charge recombination, slow electron transfer rate and large band-gap of photocatalytic materials remain key challenges in photocatalysis.

Here, we report a systemic study of the photocatalytic performance of K_0.5Na_0.5NbO₃ (KNN)-BaBiO₃ (BBO) perovskite heterostrcuture. KNN and BBO are synthesized by solid state reaction method and the composite is prepared by ball-milling technique with appropriate stoichiometric ratio of KNN and BBO. The materials are characterized using XRD, SEM, XPS, and RAMAN spectroscopy techniques. Photocatalytic degradation of Rhodamine 6G is monitored through UV-Vis absorption spectroscopy. We observed higher photocatalytic efficiency with KNN-BBO heterostrcuture than single junction KNN. This was attributed to the reduced band gap (approx1.92 eV) and fast electron transfer rate in heterostrcuture under visible light irradiation. Moreover, the improved performance of the composite (KNN-BO) material may be attributed to heterojunction band alignment and higher ferroelectric polarization of the composite material. [3] We have also examined the effect of calcination temperature on the photocatalytic dye degradation activity of KNN-BO heterostrcuture. Transient absorption spectroscopy confirms the energetics and electron transfer kinetics in the composite system. Our study opens up the possibility of designing novel perovskite based materials for efficient photocatalysis.
References:
1.Park, S., Lee, C. W., Kang, M. G., Kim, S., Kim, H. J., Kwon, J. E.,… & Nam, K. T. (2014). Physical Chemistry Chemical Physics, 16(22), 10408-10413.
2.Wang, Z., Gu, H., Hu, Y., Yang, K., Hu, M., Zhou, D., & Guan, J. (2010). CrystEngComm, 12(10), 3157-3162.
3.Cui, Y., Briscoe, J., Wang, Y., Tarakina, N. V., & Dunn, S. (2017). ACS applied materials & interfaces, 9(29), 24518-24526.

The role of non-stoichiometry in a hierarchically structured WO_3−x electrode, constituted from nanoscale fuzziness as well as microscale wire morphology, on the photoelectrochemical response is investigated. Through X-ray photoelectron spectroscopy (XPS) studies, the relative amounts of the various oxidation states of the constituent W are probed with respect to the observed response. It is concluded that an intermediate/optimal number of vacancies, yielding a W⁶⁺/(W⁵⁺ + W⁴⁺) ratio of around 2, would be beneficial for increasing the photocurrent. It is posited that defect engineering combined with optimized band structure modulation could be used for enhanced photocurrent density as well as electrode stability. The work would help considerably elucidate the role of defects as well as charge carriers for oxygen evolution reaction (OER) efficiency increase.

Combining a plasmonic metal, such as gold, with other popular catalysts, such as Ni or Pt, can extend its benefit to many energy-extensive reactions catalyzed by those metals. The efficiency of a plasmon-enhanced the catalytic reaction is mainly determined by the light absorption cross-section and the photoexcited charge carrier relaxation dynamics of the nanoparticles. We have investigated the charge carrier relaxation dynamics of gold/nickel (Au/Ni) and gold/platinum (Au/Pt) bimetallic nanoparticles. We found that electron-phonon and phonon-phonon coupling rates in Au/Ag, Au/Ni, and Au/Pt nanoparticles are significantly different than that of pure Au nanoparticles. This is due to the large differences in their band structures and the electron-phonon coupling rate. Our findings thus provide important insights towards the rational design of bimetallic plasmonic catalysts.

Thermal based desalination systems involve heating water and collecting the condensate. The use of solar energy to drive the desalination phenomena, utilizing the heat for enhancing water evaporation (steam generation) is attracting wide-spread attention. The process involves absorbing the spectral intensity of solar light and converting it to heat. This photo-thermal conversion can be enhanced by using a strong solar absorber diluted in water, which is a poor absorber of sun light. Among the solar absorbers, carbon materials have proven to enhance water desalination rate while being economical as compared to their plasmonic counterparts. Activated carbon is a carbon-rich source which has many applications such as toxic gas adsorption and industrial effluent treatment. Considering its optical quality, it has the potential to be a strong solar absorber for enhancing water evaporation rate. A series of characterization tests were performed on activated carbon. It showed 94.17% (granular) carbon composition with trace presence of hydrogen and oxygen through ultimate analysis. FT-IR analysis performed showed mostly C=C and aromatric C-H rings. Geometric (envelope) density of the sample was almost 710 kg/m³ with theoretical porosity of 0.663. The skeleton (true) density measured was 2106.9 kg/m³. The density measurements showed that activated carbon can be used as a solid dispersion in water. The granular particles sink to the bottom of the container and behave like a black bottom in enhancing water evaporation. The apparent zeta potential for nanoparticles was measured to be -47.9 mV, showing a good dispersion stability. Optical measurements showed that the absorptance of the activated carbon was 96.35%. The strength of absorbance increases with increasing concentration and can be attributed to increased extinction coefficient of the solar spectral intensity. This increases the fraction of solar absorption in the fluid dispersion system, leading to enhanced heat localization at higher concentrations. Outdoor experiments were performed to measure the mass loss of water utilizing activated carbon nanoparticle and granular activated carbon particles dispersed in DI water. The strong absorptance of the activated carbon led to increased water evaporation rate. Activated carbon particles are inexpensive compared to expensive plasmons such as copper or gold, and hence offer an economically viable solution for augmenting desalination of water.

Plasmonic nanoparticles that effectively convert light into nanoscale localized heat with low radiative losses are promising photothermal materials. A lot of plasmonic-enhanced interfacial solar steam generation devices have been developed based on a broad variety of floating substrates, showing improved evaporation rates and high solar-thermal efficiency. In particular, plasmonic wood-based devices are considered as one of the ideal platform for steam generation due to the attractive cost-effectiveness. However, up to now, the efficiency of the plasmonic wood-based device is still below 80% (evaporation rate ~1.0 kg m-2h-1) due to the considerable heat losses. In this study, by taking the full advantage of plasmonic nanoparticles in heat localization, we created nanoscale hot spots with huge light absorption and negligible heat losses in a TiN nanoparticles/wood device. Specifically, we constructed an ultrathin uniform plasmonic nanoparticle film on the micro-channels of wood using solution-processed technique. For the first time, water boiling is observed on the surface of the device under the illumination of only one sun. As a result, the 1-sun evaporation rate of such a system reaches the theoretic limit of wood-based steam generation devices (~1.5 kg m-2h-1), indicating an extremely high solar-thermal efficiency.

Today, much research on water-splitting and fuel synthesis is directed at improving the efficiency of catalysts and their immobilisation on (photo)electrodes in order to be electroactive under aqueous conditions in photoelectrochemical (PEC) devices. In such hybrid configurations, metal oxide (MO) semiconductor materials hold the key to high efficiencies, as their facile preparation, stability in water and modular high surface-area architecture render them promising platforms. By greatly expanding in this area of research, our recent studies show that achieving a synergistic size parity between macrocatalysts and the substrate is a promising strategy to improve the electrocatalytic performance.
In this invited talk, I will present our rational development of macro-meso-porous inverse opal (IO) titanium dioxide and indium tin oxide electrodes^[1], designed to host macromolecules – enzymes and polymeric catalysts – towards water-splitting and CO₂ reduction. Our recent efforts allowed the development of semi-artificial photosynthetic systems that demonstrate how, using a dye-sensitised approach, natural archetypes – photosystem II, hydrogenase and formate dehydrogenase – can be interfaced with a synthetic organic dye to achieve unassisted solar-driven water-splitting into H₂ and O₂, and CO₂-to-formate splitting.^[2,3] We took further advantage of the wide pore architecture to develop catalytic copolymeric architectures capable of electrocatalysing the reduction of CO₂-to-CO.^[4] Capitalising on the multifunctional capabilities intrinsically delivered by copolymer scaffolds, this work also provided a seminal example of enhanced product selectivity achieved only by tuning the environmental surroundings of the molecular catalyst. The ability of IO electrodes to host and drive large, electrocatalytically active polymers was further demonstrated on silicon-based photoelectrodes, where selective, precious metal-free, solar-driven CO₂ reduction was achieved in hydro-organic media.

^[1] D. Mersch, C-Y Lee, J. Z. Zhang, K. Brinkert, J. C. Fontecilla-Camps, A. W. Rutherford & E. Reisner J. Am. Chem. Soc. 2015, 137, 8541.
^[2] K. P. Sokol, W. E. Robinson, A. R. Olivera, J. Warnan, M. M. Nowaczyk, A. Ruff, I. A. Pereira & E. Reisner J. Am. Chem. Soc. 2018, 140, 16418.
^[3] K. P. Sokol, W. E. Robinson, J. Warnan, N. Kornienko, M. M. Nowaczyk, A. Ruff, J. Z. Zhang & E. Reisner Nat. Energy, 2018, 3, 944.
^[4] J. J. Leung, J. A. Vigil, J. Warnan, E. Edwardes Moore & E. Reisner Angew. Chem. Int. Ed. 2019, 58, 7697.

Mixed-anion compounds that consist of more than two anionic species in a single-phase have attracted attention as visible-light-absorbing semiconductors, since as compared to oxygen 2p orbital, p orbitals of less electronegative anion can form a valence band that possesses more negative potential. In this regard, oxyfluorides (oxide-fluoride) are obviously unsuitable as visible-light-responsive photocatalysts because of the highest electronegativity of fluorine.
However, we found that an oxyfluoride Pb₂Ti₂O_5.4F_1.2 possessed an unprecedented small band gap of ca. 2.4 eV (corresponding to ca. 510 nm absorption edge), and functioned as a stable photocatalyst for visible-light water reduction/oxidation when modified with suitable promoters. Photoelectrochemical water oxidation to O₂ was also achieved with a modified Pb₂Ti₂O_5.4F_1.2 electrode under simulated sunlight. Density functional theory calculations showed that the unprecedented visible-light-response of Pb₂Ti₂O_5.4F_1.2 arises from strong interaction between Pb-6s and O-2p orbitals, which is caused by a short Pb–O bond in the pyrochlore lattice due to the fluorine substitution.

Significant efforts have been made to develop a photocatalyst that functions under visible light from the viewpoint of solar energy utilization. Nitrogen/fluorine codoping into anatase TiO₂ allowed introduction of more nitrogen compared to only nitrogen-doped analogue, resulting in enhanced absorption and photocatalytic activity under visible light irradiation [K. Nukumizu et al. Chem. Lett. 2003, 32, 196; K. Maeda et al. J. Phys. Chem. C 2007, 111, 18264]. On the other hand, rutile TiO₂ (R-TiO₂), another representative polymorph of TiO₂, is known to possess high activity for O₂ evolution via water oxidation and used in Z-scheme water splitting in combination with a suitable H₂ evolution photocatalyst [A. Miyoshi et al. Chem. Eur. J. 2018, 24,18204]. In this work, we succeeded in introducing visible light response into R-TiO₂ by N/F codoping and evaluated its photocatalytic O₂ evolution activity.
N/F codoped R-TiO₂ (R-TiO₂:N,F) was synthesized by nitriding a mixture of R-TiO₂ and (NH₄)₂TiF₆. R-TiO₂ and (NH₄)₂TiF₆ were mixed using an agate mortar and a pestle with various molar ratios. The mixture was heated at 773 K for 1 h under NH₃ flow. For convenience, the nominal concentration of (NH₄)₂TiF₆ in the mixture is denoted as value C in the unit of mol%. X-ray diffraction (XRD), scanning transmission electron microscope, and X-ray absorption fine structure (XAFS) measurements revealed that the synthesized materials mainly show R-TiO₂ structure. Especially, C ≤ 15 samples did not show any impurity phases in XRD. Presence of the anions was confirmed by elemental analysis. Shift of diffraction peaks in XRD and increase in the pre-edge peak intensity in XAFS measurements compared to pristine rutile TiO₂ was observed, suggesting the incorporation of nitrogen and fluorine anions into R-TiO₂ causing lattice distortion.
Diffuse reflectance spectra showed more pronounced absorption in visible light region for materials synthesized using (NH₄)₂TiF₆ (C ≥ 10) compared to the sample synthesized without (NH₄)₂TiF₆ (C = 0). Furthermore, the visible light absorption became more pronounced with increasing C value. The enhanced visible light absorption in the R-TiO₂:N,F is attributed to increased nitrogen content, which was confirmed by combustion elemental analysis.
Photocatalytic activity of R-TiO₂:N,F for water oxidation was tested using AgNO₃ as a sacrificial electron acceptor. The C = 15 sample marked the highest activity among the synthesized R-TiO₂:N,F. The increase in activity up to C = 15 can be explained by increase in the visible light absorption. On the other hand the decrease in the activity for C > 15 was suspected to be caused by defect formation. To make this point clear, transient absorption spectroscopy was employed. The result revealed difference of carrier dynamics among the different C value samples. The C = 50 sample showed signals corresponding to deeply trapped electrons (~5000 cm^-1) and no free electrons (< 2000 cm^-1). On the other hand, the C = 15 sample had less deeply trapped electrons, but more free electrons. Since free electrons generally show higher reactivity compared to the trapped electrons [A. Yamakata et al. J. Photochem. Photobiol. A 2015, 313, 168], the decrease in the activity for C ≥ 20 is most likely attributed to the smaller population of free electrons available for reaction and/or the presence of higher concentration of trap states. This suggests enhancing visible light absorption and suppressing the defect formation with appropriate nitrogen/fluorine codoping is important for achieving high photocatalytic activity.
Finally, the C = 15 sample was loaded with RuO₂ and applied in Z-scheme water splitting system in combination with Ru loaded Rh doped SrTiO₃ (H₂ evolution photocatalyst) and [Co(bpy)₃]^3+/2+ (redox mediator). Stoichiometric water splitting was achieved upon visible light and simulated sunlight irradiation.

The clean production of hydrogen from non-fossil energy sources is highly desirable for future energy needs and realization of hydrogen fuel cell vehicles. One way of producing green hydrogen is via photoelectrochemical splitting water into hydrogen by utilizing solar energy and/or highly active catalytic materials, both subjects of much research interests.^1-3 Particularly, the atomic scale mechanisms of the photocatalysts that facilitate the water splitting for efficient hydrogen generation are currently poorly understood. Further understanding of the role of various photocatalysts and the physics governing the active hydrogen evolution sites will allow for better and efficient design of photoelectrochemical devices.

We have developed a unique in situ/ operando liquid cell transmission electron microscope (TEM) holder with photo-capable light source⁴ which can be used to study photocatalytic reactions in real time at nanometer length scales. Using some of the model materials such as Au nanoprisms and MoS₂ flakes, we will present the correlation of I-V characteristics with water splitting and simultaneous structural changes at the catalytically active sites. The following photocatalysts used were chosen because the proposed mechanism for each exhibits a spatial dependence: plasmonically enhanced catalysis for Au nanoprisms and catalytically active edge sites for MoS2 flakes. For accurate quantitative information and reliability of materials interface in liquid, we will also present strategies for site-specific deposition/loading of sample materials in the desired electrodes of liquid cell chips for electroanalytical characterization and TEM imaging. The availability and utilization of photo stimuli in liquid cell TEM can provide important fundamental insights into the understanding of several other photoelectrochemical systems.

References:
¹ Nowotny, J.; Sorrell, C. C.; Sheppard, L. R.; Bak, T. Int. J. Hydrogen Energy 2005, 30, 521
² Walter, M. G. et al. Solar water splitting cells. Chem. Rev. 110, 6446–6473 (2010)
³ Maeda, K and Domen, K., J. Phys. Chem. Lett., 1(18), 2655-2661 (2010)
⁴ Funding for the development of the sample holder was provided by the Department of Energy,
Office of Basic Energy Sciences, through SBIR Grant # DE-SC0015213

For many semiconductors the large mismatch between their small charge carrier diffusion length and their much larger light penetration depth is one of the most critical parameters for their photocatalytic and photoelectrochemical performance. Preparation of nanofibers by electrospinning is one way to overcome this mismatch. We use this technique to prepare fibers of the (111)-layered perovskite Ba₅Ta₄O₁₅.[1] The nanofiber diameter can be tailored by adjustment of the electrospinning parameters. We could show that the photocatalytic activity depends on the fiber diameter.[2] The fiber diameter for the optimum photocatalytic activity is ~160 nm. We could prepare composite nanofibers of Ba₅Ta₄O₁₅−Ba₃Ta₅O₁₅ and showed that this composite gives rise to strongly enhanced photocatalytic hydrogen production and water splitting performance compared to pure Ba₅Ta₄O₁₅ fibers with comparable diameter due to an improved charge carrier separation. The photocurrent doubling effect with methanol as sacrificial agent with this composite could be confirmed.[3]

References
[1] N. C. Hildebrandt, J. Soldat, and R. Marschall, Small 2015, 11, 2051–2057.
[2] A. Bloesser, P. Voepel, M. O. Loeh, A. Beyer, K. Volz, and R. Marschall, J. Mater. Chem. A 2018, 6,
1971–1978.
[3] A. Bloesser, R. Marschall, ACS Appl. Energy Mater. 2018, 1, 2520−2525.

Metal-organic frameworks (MOFs) are porous materials with various applications such as gas seperation and photocatalysis. Currently, most MOFs have micro-pores, which hamper diffusion within the framework and limit the potential applications. The pore size of MOFs can be enlarged through linker elimination, in which labile linkers in MOFs can be removed to generate hierarchically porous frameworks with enhanced mass transfer. Here we report a facile method to remove photosensitive linkers selectively through photolysis. The formation of mesopores is confirmed by N₂ sorption test and the photolytic mechanism is studied through ¹H NMR, thermogravimetry (TGA) and Raman spectroscopy. The photolytic approach can engineer MOF particles in a small domain and fabricate complicated patterns on MOFs, paving a way for programmable design of MOF-based devices.

The field of catalysis has been traditionally divided into subdisciplines of homogeneous and heterogeneous catalysis, where the former primarily studies catalytic processes by molecular structures and the latter chiefly deals with surface catalysis processes. Each subdiscipline enjoys a wealth of knowledge that underpins its own respective applications. For instance, researches on homogeneous catalysis have generated a wealth of knowledge on how to tailor the reaction selectivity and specificity at the atomic level for the production of high-value chemicals such as targeted drugs. The ease of separation and processing renders heterogeneous catalysts ideal for commodity chemical synthesis. However, how to take advantage of the two orthogonal fields and apply knowledge learned in one field to accelerate the advancements of both has remained a critical challenge. It is within this context that recent years have witnessed a surge of research activities on atomically dispersed catalysts. A prominent example is the single atom catalysts (SACs). On the one hand, such catalysts can maximize the atomic efficiency of heterogeneous catalyst materials by minimizing the amount of materials, especially precious metals such as Pt or Pd, used; on the other hand, such catalysts are reminiscent of homogeneous catalysts in that their structures tend to be better defined than conventional heterogeneous catalysts. It, therefore, opens up an opportunity to bridge the gap between the two traditional subfields. Within this context, we have focused on developing and understanding heterogeneous catalysts with dinuclear metal centers. They are similar to SACs in that the catalytically active centers are well-defined; they present a critical difference by featuring two metal atoms at the active site. This key feature enables the studies of chemical reactions that may benefit from the presence of the second metal centers. Examples of such reactions include H₂O oxidation, CH₄ activation, and preferential CO oxidation. In all cases, we have observed a clear benefit of having a second metal center at the active site. For instance, in water oxidation, we observed that the presence of the second Ir atom facilitates the proton-coupled electron transfer process so as to lower the activation energies during the rate-determining step of O-O bond formation. Similarly, for CH₄ activation, we found that the presence of the second Ir center promotes C-C bond formation for the synthesis of C₂ products. The same effect was observed for PROX, as well, where the presence of the 2^nd Ir metal center was found to enable both CO adsorption and O₂ activation. Our work opens a new door to the study of atomically dispersed catalysts for the chemical transformations of significant industrial relevance. We envision that similar studies will be readily expanded to other chemical processes or other catalysts systems or both.

The biological use of solar energy for the synthesis of fuels from water and carbon dioxide inspires researchers and engineers in their efforts to replace existing exhaustible energy sources with renewable energy technologies. Environmentally friendly schemes of photocatalytic visible-light hydrogen production known as artificial photosynthesis along with inorganic semiconductor material also utilize biological structures, such as enzymes, machineries of whole microorganism, capable of light-harvesting, water splitting, carbon dioxide and proton reduction. A natural membrane complex of retinal-containing proton pump bacteriorhodopsin (also known as purple membranes, PM) from the extremophile microbe has been attracting an attention of researchers owing to its minimalistic structure, exceptional robustness, excellent photophysical properties, and functional sophistication. This biological material can be assembled with inorganic nanomaterials, such as semiconductor or photonic particles, into artificial hierarchical structures and demonstrate functionality that spans far beyond its natural property. This talk will provide examples of catalytic systems based on natural proton pump for light-driven H2 production, ATP synthesis and CO2 reduction.

In this paper, we present the preparation of multicrystalline silicon ingots with different boron-gallium doping ratio, resistivity distribution, minority carrier lifetime, photoelectric conversion efficiency and light-induced degradation ratio were analyzed and contrasted . The result shows that with the increase of gallium doping ratio, resistivity and minority carrier lifetime of ingot, as well as conversion efficiency of silicon wafer were both in a decreasing trend, the light-induced degradation ratio also tended to decrease as a whole, but fluctuated locally. By comparing elements tested data, it was preliminarily concluded that the fluctuation may be caused by copper,iron and other impurities . Based on the performance of resistivity, conversion efficiency and light-induced degradation ratios of gallium-doped ingots, the optimal gallium-doped ratio of multicrystalline ingots was determined to be about 25%.

Due to the rapid depletion of fossil fuels and related environmental issues, developing technologies for producing renewable, clean fuels for our future is of great importance. Artificial photosynthesis via water splitting or CO 2 reduction offers an attractive and cost-effective route to achieve this goal. 1 However, existing photoelectrocatalytic materials generally suffer from low activity or instability, thus greatly impeding the feasibility of artificial photosynthesis technologies. 2, 3 Herein, we present design strategies for the synthesis and integration of photoelectrocatalytic materials.

References
1. Concepcion, J. J.; House, R. L.; Papanikolas, J. M.; Meyer, T. J., Chemical approaches to artificial photosynthesis. Proceedings of the National Academy of Sciences 2012, 109 (39), 15560-15564.
2. Liu, G.; Eichhorn, J.; Jiang, C.-M.; Scott, M. C.; Hess, L. H.; Gregoire, J. M.; Haber, J. A.; Sharp, I. D.; Toma, F. M., Interface engineering for light-driven water oxidation: unravelling the passivating and catalytic mechanism in BiVO4 overlayers. Sustainable Energy & Fuels 2019, 3 (1), 127-135.
3. Eichhorn, J.; Liu, G.; Toma, F. M., Chapter 8 Degradation of Semiconductor Electrodes in Photoelectrochemical Devices: Principles and Case Studies. In Integrated Solar Fuel Generators, The Royal Society of Chemistry: 2019; pp 281-303.

Photoelectrochemical (PEC) oxidation of glycerol is an attractive biomass feedstock for the production of renewable energy such as hydrogen and biofuel, such as glyceric acid, dihydroxyacetone, and formic acid. Recently, ZnS has been considered as a promising material for the production of biofuel because of its inexpensive cost and suitable potential for the oxidation of organic compounds. However, ZnS absorbs only ultraviolet region owing to a wide bandgap (~3.6 eV) and suffer from fast recombination of electron-hole pair.
In this study, we have synthesized Fe doped ZnS / ZnO photoanodes (FZZ) using all solution process for the efficient photo-oxidation of glycerol. By optimizing Fe doping concentration, FZZ samples have enhancement of light-harvesting at the visible region and high conversion efficiency of glycerol. In addition, the charge separation is enhanced by the formation of the heterostructure, providing a direct pathway for the electrons. As a result, the efficiency of photo-reforming of the FZZ is greatly improved over the pristine photoanode. Also, we can confirm the selectively oxidized product according to the various applied potential in PEC cells.

This study investigated the adsorption and photocatalytic activity of TiO2-boron nitride (BN) nanocomposites for the removal of contaminants of emerging concern in water using ibuprofen as a model compound. TiO2 nanofibers wrapped by BN nanosheets were synthesized by electrospinning method. Characterization of the nanocomposite photocatalysts indicated the BN nanosheets improved the light absorbance and reduced the recombination of the photoexcited charge carriers (e− and h+). The photocatalytic oxidation products and mechanisms of ibuprofen by the TiO2-BN catalysts were elucidated using a multiple analysis approach by high performance liquid chromatography, ultraviolet absorbance, dissolved organic carbon, fluorescence excitation-emission matrices, and electrospray ionization–liquid chromatography–tandem mass spectrometry. The experimental results revealed that the photocatalytic oxidation by the TiO2-BN nanocomposites is a multi-step process and the interactions between ibuprofen molecules and the TiO2-BN nanocomposites govern the adsorption process. The increasing BN nanosheet content in the TiO2 nanofibers facilitated the breakdown of ibuprofen degradation intermediates (hydroxyibuprofen, carboxyibuprofen, and oxypropyl ibuprofen). Kinetic modeling indicated both adsorption and photocatalytic oxidation of ibuprofen by the TiO2-BN nanocomposites followed the first-order kinetic model. The photocatalytic oxidation rate increased with the increasing BN content in the nanocomposite catalysts, which was attributed to the enhanced light absorption capacity and the separation efficiency of the photoexcited electron (e−)-hole (h+) pairs. Multiple photocatalytic cycles were conducted to investigate the reusability and regeneration of the nanofibers for ibuprofen degradation.

Copper-based transition metal chalcogenide (S, Se, Te) nanocrystals provide one of the most attractive platforms for both nanophotonic and photovoltaic applications as a vast range of material properties can be realized through compositional control of these nanocrystals consisting of earth-abundant elements. In transition metal Cu-chalcogenides, the strong p-d hybridization and the small variations in size and electronegativity between the transition metal cations and the chalcogens facilitate an extraordinary range of spectral and electroluminescence properties. Additionally, the defect concentrations and the possibility of non-stoichiometric and antisite defects in the Cu-chalcogenides can be used as key parameters to further tailor optoelectronic properties of these compounds. From the perspective of photovoltaic applications, the quest for a sustainable and affordable energy material still remains. A broad spectrum of electronic band gap can be realized through compositional control in the mixed metal Cu-chalcogenides. To this end, multinary Cu-chalcogenides offer further flexibility in band gap engineering and spectral properties through compositional variation of the different elements within the compound. However, it is synthetically challenging to realize the more complex multinary compositions of the Cu-chalcogenides. To date, several binary compositions of transition metal Cu-chalcogenides have been stabilized, but few reports of ternary, quaternary, and penternary complexes are available. What if we can synthetically realize a new gamut of higher order transition metal Cu-chalcogenides for a wide range of novel optoelectronic properties?
In this study, we report a new library of penternary Cu₂ZnAS_xSe_4-x (0 < x < 4, A = Al, In, Ga) nanocrystals and thin films. The nanocrystals are synthesized via a hot injection solution process to facilitate compositional control in these complex structures. These nanocrystals with multiple chalcogens and different cross-row elements within the structure are unique from the perspective of structural and material properties. The morphology and chemical composition of the new penternary nanocrystals are investigated via scanning and transmission electron microscopy equipped with energy dispersive x-ray spectroscopy. These materials exhibit a wide range of band gap, largely influenced by the Se content and cation composition, as determined through the experimental Tauc plots. In addition, a wide range of composition-controlled photoluminescence is obtained with this group of penternary Cu-chalcogenides. Thin films of these novel chalcogenide nanocrystals were deposited on Si wafer substrates using a spin coating technique. Transport properties (resistivity and Hall effect) of the complex chalcogenide thin films were explored. This new class of semiconductor materials from readily available elements will be transformative in terms of device applications for photovoltaics and quantum dot light emitting diodes.

Harvesting solar energy for carbon-free hydrogen production via photoelectrochemical (PEC) water splitting is a promising pathway to meet the increased energy demands and deal with severe environmental pollution including climate change. Silicon is the second earth-abundant element and a narrow bandgap material which is found to be a prime candidate for water splitting photoelectrodes due to ease of high-quality wafers fabrication. Si can be doped with elements of different valence states to fabricate n-type or p-type semiconductors for photoanode and photocathode respectively. However, high light reflection, low photovoltage output due to its narrow bandgap, and severe photocorrosion in the electrolyte remain as serious limitations to manufacture efficient Si-based photoelectrodes.
In this talk, I will discuss the fabrication of Si-based photoelectrodes coupled with earth-abundant transition metal-based catalysts for efficient water splitting. We introduced two-dimensional (2D) transition metal dichalcogenides catalysts on p-type Si-photocathode for highly efficient PEC hydrogen production. 2D transition metal dichalcogenides such as MoS₂ and WS₂ are a promising materials to replace noble metal catalysts due to their high density of catalytically active edge sites and large surface-to-volume ratio. Wafer-scale transferable transition metal dichalcogenides thin-film catalysts can provide a low onset potential and high photocurrent density. I will also talk about n-type Si photoanodes with high catalytic activity in transition metal-based catalysts synthesized via facile electrodeposition method for efficient water oxidation. Si photoanodes covered with water oxidation catalysts such as layered-double hydroxides, metal nanoparticles, and thin films exhibited high photoelectrochemical property and long-term stability.

Heterogeneous catalysts are used to reduce energy barrier for chemical reactions by promoting gas adsorption. Such energy barrier is commonly overcome by heating a reactor filled with gaseous reactants. On plasmonic nanostructures, optically-excited localized surface plasmon (LSP) resonances were found to induce H₂ or O₂ dissociation at a reduced temperature, mimicking photocatalysis. Energy harnessed by plasmonic nanostructures and transferred to adsorbed reactants during the dephasing of LSP resonances is theorized to initiate such reactions by compensating for the thermal energy required otherwise. Therefore, resolving sub-nanoparticle information of surface plasmon induced reactions, including (1) reactant adsorption, (2) LSP resonance energy that overcomes reaction barrier, and (3) product desorption, could elucidate design rules of plasmonic catalysts. However, such information has not been obtained under reactive environments by variants of optical spectroscopy, since their spatial resolution is diffraction-limited at best and the excitation of LSP modes by light is polarization-sensitive. Herein, we demonstrate that sub-nanoparticle information gained from electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) can be used to measure these reaction details and drive selected reactions.
Electron-beam-excited LSP resonances on shape- and size- controlled nanoparticles are exploited to drive chemical reactions in an environmental scanning transmission electron microscope equipped with a monochromated electron gun operated at 80 kV with an energy resolution of 0.08 eV. Low-loss EELS hyperspectral imaging and CL maps, combined with theoretical predictions based on electromagnetic boundary element method, reveal the spatial distribution of coupling efficiency of the convergent electron beam to the LSP modes. Coupling efficiency maps, derived by applying the non-negative matrix factorization (NMF) to an EELS hyperspectral image, shows specific LSP mode can be preferentially excited by positioning the electron beam with a nanometer-level precision. In situ core-loss EELS data is employed to measure local chemical compositions and oxidation states of the nanostructure in vacuum and the gas environments. For the first time, gas adsorption maps, were obtained using in situ EELS spectrum imaging. For shape-controlled Au nanoparticles on TiO₂ support in a CO environment, the adsorption maps indicate that CO is adsorbed on selective nanoparticle edges but not on the entire surfaces. Under the same CO environment, the LSP dipole mode of an Au nanoparticle is preferentially exited using electron beam to drive CO disproportionation: 2CO_(g) → CO_2(g) + C_(s), at room temperature (commonly initiated above 400 °C). One of the reaction products − amorphous carbon − is deposited only at the cantilevered corner of Au nanoparticles, indicating that the active sites for the reaction are located where preferable gas adsorption sites and locations of maximum plasmonic field amplitude superimpose. Energy transferred to the adsorbed reactants is further identified based on the spectral difference between EELS and CL, acquired during the reaction. For size-controlled Al nanoparticles, elemental analysis using EELS shows a core-shell structure of Al-Al₂O₃, where a crystalline Al core is enclosed in an oxide shell. The oxide thickness modulates the LSP resonance energy and can be used to match the energy barrier for selected reactions. In one of the examples, the oxtupole mode on core-shell Al nanoparticles are utilized to initiate carbon etching in a CO₂ environmmt through the reverse Boudouard reaction (CO_2(g) + C_(s) → 2CO_(g)) without any additional heat in an endothermic reaction.
Our methodology provides unprecedented information on LSP-induced chemical reactions at the nanometer scale, shedding light on the design principles for new hybrid plasmonic catalysts that will enable low-temperature, industry-relevant chemical processes.

This work, Ca(ZrYCeCrAg)O), modified from pristine CaZrO3, owning inner heterostructure, which was sonicated with visible light has been confirmed it can enhance piezo-photo catalytic activity for organic dye degradation. CaZrO3(CZO) demonstrate remarkable optical information storage application due to possessing numerous intrinsic traps and ferroelectricity, which imply long recombination lifetime. However, the large band gap (3.3eV) restrict its photo-catalytic efficiency for dye degradation. Herein introduction of several transition metal elements into CZO nanoparticles has been synthesized by a sol-gel process and forms heterojunction nanostructure(fluorite-((Zr,Y,Ce)O2) & perovskite-CaZrO3) The time-resolved photoluminescence (TRPL) reveals that recombination lifetime have already improved up to 620% compared to pristine CZO. CZO, which belong to Pnma group of perovskite structure, displays unique ferroelectricity which has spontaneous polarization for prolonging lifetime and enhance piezo-photo activity. Besides, modified-CZO surrounded by silver nanoparticles, demonstrating localized surface plasmon resonance (LSPR) effect as observed by surface enhanced raman spectrum(SERS). Through ultraviolet-visible (UV-Vis) spectrophotometer measurement, modified-CZO is achieving almost 200% degradation performance compared to pristine CZO. The vast enhancement arises from piezo-photo activity, which was combined through ferroelectricity, charge transfer mechanism for prolonging recombination lifetime, and induced local electric field for generating numerous electron-hole pair . The COMSOL software was employed to simulate the charge distribution. The mechanism on LSPR enhanced piezo-photo catalytic dye degradation will be investigated.
Keywords: CaZrO3, Ag, Ferroelectricity, Lifetime, Surface Plasmon Resonance, Dye Degradation

The use of zinc oxide (ZnO) nanostructures as a photocatalyst for the degradation of organic pollutants in water has received significant attention over the recent years. However, synthesis methods for producing ZnO nanostructures are generally costly, complicated, and hazardous to the environment. In this work, we demonstrate the synthesis of zinc oxide nanostructures by a simple hot water treatment (HWT) method and the photocatalytic activity of the hence produced nanostructures. HWT is a one-step, low-cost, eco-friendly, and scalable nanostructure growth method. By HWT, various metal oxide nanostructures can be produced simply by the interaction of metals with hot water without the need for any chemical additives in the solution. Growth of metal oxide nanostructures by HWT involves the formation of metal oxides and their release from the surface of the metal into water, the migration of the metal oxides in water and their re-deposition at a different part of the metallic surface, which initiates the growth of nanostructures. In this study, we used zinc powder and plates for producing the ZnO nanostructures by HWT in DI water at 75 ^oC. Scanning electron microscopy images and X-ray diffraction were utilized to verify the formation of ZnO nanostructures. Zinc plates produced a suspension of ZnO nanostructures in water, while on the other hand Zn powder resulted in a predominantly core-shell geometry of Zn particle-core and ZnO nanostructures-shell also mixed in water. We used these nanostructures + water suspensions for our photocatalytic degradation studies. Methylene blue was used as a model organic pollutant. We mixed the ZnO nanostructure suspension with methylene blue and exposed it to UV light. The degradation of methylene blue was observed by measuring its absorbance values using a UV-Visible spectrophotometer over a period of four hours. We observed a 20% decrease in the concentration of methylene blue in four hours when nanostructured Zn/ZnO core-shell powder suspension was used, and a 30% decrease was achieved when ZnO nanostructure-only suspension was used. Methylene blue alone was also exposed to UV light for the same period as a control experiment and we did not observe any significant decrease in its concentration. These preliminary results indicate that hot water treatment method presents a very simple, cost-effective, scalable, and eco–friendly alternative for the synthesis of zinc oxide nanostructures for photocatalytic water treatment applications.

Mitigating atmospheric CO₂ concentration is a great challenge for the twenty-first century. It can be done only by capturing and converting CO₂ into useful fuels with the help of catalysts. Knowledge of the reaction mechanism in CO₂ hydrogenation reactions is key to the rational design of highly efficient catalysts. Here, we investigate the CO₂ conversion into methanol on Rh intercalated Graphene/SiC using density functional theory calculations and kinetic Monte Carlo simulations. The adsorption energy, bonding configuration, bond lengths, and vibrational frequencies of all the surface intermediates involved in the reaction mechanisms are studied. Present results on methanol production suggest that the hydrogenation of CO₂ to CO occurs via the reverse-water-gas shift (CO₂ + H₂ → CO + H₂O) reaction rather than formate (HCOO^-) or direct C−O bond cleavage pathways. The produced CO can either desorb or further react with hydrogen to produce CH₄ or CH₃OH since the adsorption energy of CO is low. The intermediates of the CO₂ pathway for methanol production includes *OCOH, CO, *CHO, *CHOH, and *H₂COH. The reaction barrier energy for each elementary steps involved in CO₂ hydrogenation will be discussed, and that can provide further insights into the reaction mechanism. Finally, we will also discuss how the energetics affects the kinetics of the reaction, such as the overall rates for CO₂ conversion and the production of CO, CH₃OH, and CH₄ under reaction condition.

Plasmonic photocatalysis offers the promise of using light as the energy source to drive a variety of thermal energy-intensive phonon-driven chemical reactions close to room temperature [1]. Industrially relevant heterogeneous catalytic reactions targeted by plasmonic photocatalysis include CO₂ reduction, water-splitting to generate H₂, hydrocarbon reforming, Fischer-Tropsch reactions, synthesis of azo dyes, nitrogen fixation, hydrogenation reactions, and the oxidative degradation of organic compounds. Plasmonic catalysis also offers unique chemical bond activation mechanisms that differentiate hot carrier-driven chemical reactions from routine carrier-driven catalysis [2].

Plasmonic photocatalysis utilizes nanophotonics in the service of catalysis. Another nanophotonic architecture, namely photonic crystals (PhCs) which allow the propagation of photons of selected frequency bands, has been explored much less for catalysis. The primary reason is the impracticality of using top-down fabrication techniques such as electron beam lithography to mass-produce PhC catalysts. Bottom-up fabrication techniques for PhCs are rare, and suffer from a lack of adequate control over the optical properties of the resulting PhCs.

Herein, we present the bottom-up synthesis of large area PhCs consisting of arrays of self-organized vertically oriented TiO₂ nanotubes (TNTAs) with periodically modulated diameters, grown by a charge controlled pulsed electrochemical anodization (CCPEA) technique. The CCPEA technique enables superior control over the optical resonances exhibited by TNTA PhCs, compared with galvanostatic & potentiostatic pulsed anodization processes. When TNTA PhCs with well-defined photonic stop-bands are decorated or embedded with nanoparticles (NPs) of Au, Ag, CuPt, TiN, etc, plasmonic photonic crystals result.

TNTAs partially embedded with Au NPs are shown to be excellent photoanodes for visible light-driven photoelectrochemical water-splitting in alkaline electrolytes. Embedding Au NPs in the nanotube walls as opposed to decorating them results in high extinction coefficient, high Q-factor, red-shifted plasmon resonances [3] that facilitate the injection of hot electrons formed by plasmon dephasing, into the conduction band of TiO₂ while residual hot holes oxidize water to O₂. The observation of photoelectrochemical quantum yields approaching 30 % for visible wavelengths indicates surprisingly efficient hot carrier harvesting. Likewise, TNTA PhCs decorated with Au NPs are also excellent photocatalysts for the vapor phase reduction of CO₂ into light hydrocarbons. TNTAs conformally coated with titanium nitride exhibit near-infrared resonances and appreciable photocatalytic activity [4]. The TNTA structure provides a high density of active surface sites for reactant adsorption, and its own optical properties (PhC and Mie scattering effects) [5] interact with the plasmons to influence the photocatalytic behavior.

REFERENCES
1. AP Manuel, A Kirkey, N Mahdi & K Shankar, "Plexcitonics – fundamental principles and optoelectronic applications", Journal of Materials Chemistry C 2019, 7, 1821
2. P Christopher, H Xin, A Marimuthu & S Linic, "Singular characteristics and unique chemical bond activation mechanisms of photocatalytic reactions on plasmonic nanostructures", Nat. Mater. 2012, 11, 1044
3. S Farsinezhad, SP Banerjee, B Bangalore Rajeeva, BD Wiltshire, H Sharma, A Sura, A Mohammadpour, P Kar, R Fedosejevs & K Shankar, "Reduced Ensemble Plasmon Line Widths and Enhanced Two-Photon Luminescence in Anodically Formed High Surface Area Au–TiO2 3D Nanocomposites", ACS Appl. Mater. Interfaces 2016, 9, 741
4. S Farsinezhad, T Shanavas, N Mahdi, AM Askar, P Kar, H Sharma and K Shankar, "Core-shell titanium dioxide - titanium nitride nanotube arrays with near-infrared plasmon resonances", Nanotechnology 2018, 29, 154006
5. P Kar, S Zeng et al. "High rate CO2 photoreduction using flame annealed TiO2 nanotubes", Appl. Catal. B-Environ. 2019, 243, 522

The photocatalytic and photoelectrochemical reduction of water or CO₂ is an intriguing approach to producing sustainable solar fuels, and has attracted growing and intense interest. Nanostructuring of photocatalysts and photoelectrodes has been proven to be a strong strategy to dramatically improve overall solar-to-fuel conversion efficiencies. Another technological barrier for the practical implementation of solar fuel production is long-term material durability, which has recently been well addressed by using conformal coatings of protective layers onto the narrow band-gap semiconductors that are suitable for efficient solar-to-fuel conversions but photoelectrochemically unstable. These significant progresses may lead us to the practical implementation of solar fuel production. We focused on the exciting progresses achieved by using nanostructuring strategies, specifically regarding how the nanostructure influences the charge transport and separation. Special attention was paid to investigate how a nanoscale coating (overlayer) passivates the surface states, thereby reducing the surface electron hole recombination, and how a nanoscale coating (protective layer) prevents the photocorrosion or photopassivation of the semiconductors with optimal band gaps. We hope that the design strategies using these nanostructures will offer new and greater opportunities for efficient solar fuel production to existing photocatalytic and photoelectrochemical systems.

Conventional photocatalysts based on metals and semiconductors are sometimes rigid and possesses stability issues when applied in the treatment of industrial wastewater. Thus, novel photocatalysts based on polymer/nanomaterial composites are attracting more interest. Although much effort has been devoted to acquire catalysts with high efficiency and long-term durability, this work puts forward a simple and green three-steps procedure for synthesis of diethylenediamine-modified reduced graphene oxide (DD-rGO) grafted on a magnesio-thermically produced silicon carbide (SiC) to achieve one-vessel conversion of organic pollutants into useful fuels such as methanol and ethanol under simulated solar irradiation. The mechanism behind the distinct reactivity and stability of the as-prepared photocatalyst was proposed. We expect that these entirely new nanocomposite structures will not only pave way for the development of highly efficient and stable photocatalyst systems but also have a wider impact in applications (such as water treatment) where catalysts stability is essential.

The utilization of fossil fuel subsidiaries, for example, coal, heavy oil, natural gas and petrol have transformed into global issue of environment concern. The use of energy is rapidly expanding with rising population in view of which there is a need of utilizing efficient green sustainable energy sources to fulfil the requirements of energy supply [1]. The solar light based energy can be converted and stored in the form of fuels, for instance, generation of hydrogen (H₂) by photoelectrochemical (PEC) water splitting appears to be a potential alternative to the fossil fuels [2]. PEC water splitting is one of the most attractive technology for H₂ generation using a semiconductor nanocomposite materials [3, 4].
In the present work, hydrothermally grown sodium niobate nanorods (NaNbO₃-NRs) has been successfully coupled with reduced graphene oxide (RGO) nanosheets by using reflux method. Furthermore, the potentiality of the synthesized nanostructures has been investigated for photoelectrochemical (PEC) water splitting. A ~4 fold enhancement in the photocurrent density is demonstrated by fabricated RGO/NaNbO₃-NRs nanocomposite photoanode than that of NaNbO₃-NRs photoanode under UV-visible light illumination. The enhancement in the PEC activity of RGO/NaNbO₃-NRs nanocomposite is attributed to the more absorption of visible light of the solar spectra and increase the mobility of photogenerated charge carriers which improves the separation efficiency and reduces the recombination of charges. The possible mechanism responsible for the PEC water splitting have been proposed.
References
[1] W. Wang, M. Xu, X. Xu, W. Zhou, and Z. Shao, Angew. Chem. Int. Ed. 59, 136 (2020).
[2] J. Lee, J. Yun, S. R. Kwon, W. J. Chang, K. T. Nam, and T. D. Chung. Sci. Rep. 7, 12281-12290 (2017).
[3] S. Singh and N. Khare, RSC Advances. 5, 96562 (2015)
[4] D. Kumar, S. Singh and N. Khare, Int. J. Hydrogen Energy. 43, 8198 (2018).

Plasmonic nanoparticles have attracted great interest in harvesting solar energy due to their excellent capability in photothermal conversion. However, noble-metal-based plasmonic nanoparticles typically exhibit a narrow absorption peak, which limits their ability to fully utilize solar energy. Compared with conventional noble metal nanoparticles, transition metal nanoparticles show relatively broad resonance band, allowing absorption of a larger fraction of solar radiation. However, the application of these nanoparticles is limited due to their poor stability, especially in the presence of oxygen and water. In this presentation, we report the synthesis of Ni@C@SiO₂ core-shell nanoparticles that can be used as light absorbers. The stability of Ni nanoparticles is greatly enhanced by a layer of densified SiO₂. Moreover, by adding a carbon layer between Ni and SiO₂, the absorption peak of Ni can be expanded and red-shifted to maximize the overlapping between the absorption spectra of core-shell nanoparticles and the solar radiation spectra. The colloidal nature of the nanoparticles allows their easy processing into a thin film that can be attached to a floating melamine foam, producing an integrated solar steam generator. The three-layered core-shell nanoparticles exhibit an excellent photothermal property and high steam generation efficiency under solar illumination.

Photoelectrochemistry provides a strategy for creating fuels from sunlight.^1-5 In this poster presentation, I will describe the chemistry of immobilizing polypyridyl coatings containing cobaloxime-type catalysts onto either GaP or GaAs surfaces. When wired in a three-electrode configuration with appropriate counter and reference electrodes, the hybrid constructs use light to power the production of hydrogen gas with near unity faradaic efficiency from aqueous solutions in the absence of sacrificial chemical reductants and with no electrochemical forward biasing. When operating at the equilibrium potential of the hydrogen evolution reaction under simulated 1-sun illumination, the modified- GaP and GaAs assemblies produce current densities of 1 and 10 mA cm⁻², respectively. Analyses of the wavelength-resolved external and internal quantum efficiencies show the modified-GaP assemblies are active over wavelengths ranging from 300 to 550 nm, and the modified-GaAs assemblies are active over wavelengths ranging from 300 to 870 nm. The cobaloxime-modified GaAs thus gives a higher rate of photoelectrochemical fuel production due to the improved spectral overlap of the semiconductor's absorption properties with the solar flux spectrum.

[1] T. A. Faunce, S. Styring, M. R. Wasielewski, G. W. Brudvig, A. W. Rutherford, J. Messinger, A. F. Lee, C. L. Hill, H. DeGroot, M. Fontecave, D. R. MacFarlane, B. Hankamer, D. G. Nocera, D. M. Tiede, H. Dau, W. Hillier, L. Wang, R. Amal, Energy Environ. Sci., 6, 1074−1076 (2013); DOI: 10.1039/C3EE40534F.

[2] S. Ardo, D. F. Fernandez Rivas, M. A. Modestino, V. S. Schulze Greiving, F. F. Abdi, E. A. Alarcon Llado, V. Artero, K. Ayers, C. Battaglia, J.-P. Becker, D. Bederak, A. Berger, F. Buda, E. Chinello, B. Dam, V. Di Palma, T. Edvinsson, K. Fujii, H. Gardeniers, H. Geerlings, S. M. H. Hashemi, S. Haussener, F. Houle, J. Huskens, B. D. James, K. Konrad, A. Kudo, P. P. Kunturu, D. Lohse, B. Mei, E. L. Miller, G. F. Moore, J. Muller, K. L. Orchard, T. E. Rosser, F. H. Saadi, J.-W. Schuttauf, B. Seger, S. W. Sheehan, W. A. Smith, J. Spurgeon, M. H. Tang, R. van de Krol, P. C. K. Vesborg, P. Westerik, Energy Environ. Sci., 11, 2768−2783 (2018); DOI: 10.1039/C7EE03639F.

[3] A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, Ind. Eng. Chem. Res., 55, 5306-5314 (2016); DOI: 10.1021/acs.iecr.6b00478.

[4] B. L. Wadsworth, A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, ACS Catal., 6, 8048−8057 (2016); DOI: 10.1021/acscatal.6b02194.

[5] B. L. Wadsworth, A. M. Beiler, D. Khusnutdinova, E. A. Reyes Cruz, G. F. Moore, J. Am. Chem. Soc., 40, 15932−15941 (2019); DOI: 10.1021/jacs.9b07295.

Human-engineered materials capable of generating fuels from sustainable energy sources provide an approach to satiating modern societies' energy demands, with minimal environmental impact.¹ In this poster presentation, I will describe hybrid photoelectrosynthetic assemblies composed of gallium phosphide (GaP) functionalized with metalloporphyrin complexes of five different transition metal ions: Fe(III), Co(II), Ni(II), Cu(II), and Zn(II). Surface attachment of these metalloporphyrin complexes was achieved using a previously reported UV-induced photochemical grafting method.² The photoelectrochemical properties of these heterogeneous-homogeneous materials will be contrasted with electrochemical properties of the isolated, homogeneous metalloporphyrins and other previously reported heterogeneous-homogeneous constructs.^3,4

[1] (a) T. A. Funce, S. Styring, M. R. Wasielewski, G. W. Brudvig, A. W. Rutherford, J. Messinger, A. F. Lee, C. L. Hill, H. DeGroot, M. Fontecave, D. R. MacFarlane, B. Hankamer, D. G. Nocera, D. M. Tiede, H. Dau, W. Hillier, L. Wang, R. Amal, Energy Environ. Sci., 6, 1074−1076 (2013); DOI: 10.1039/c3ee40534f. (b) S. Ardo, D. F. Fernandez Rivas, M. A. Modestino, V. S. Schulze Greiving, F. F. Abdi, E. A. Alarcon Llado, V. Artero, K. Ayers, C. Battaglia, J.-P. Becker, D. Bederak, A. Berger, F. Buda, E. Chinello, B. Dam, V. Di Palma, T. Edvinsson, K. Fujii, H. Gardeniers, H. Geerlings, S. M. H. Hashemi, S. Haussener, F. Houle, J. Huskens, B. D. James, K. Konrad, A. Kudo, P. P. Kunturu, D. Lohse, B. Mei, E. L. Miller, G. F. Moore, J. Muller, K. L. Orchard, T. E. Rosser, F. H. Saadi, J.-W. Schüttauf, B. Seger, S. W. Sheehan, W. A. Smith, J. Spurgeon, M. H. Tang, R. van de Krol, P. C. K. Vesborg, P. Westerik, Energy Environ. Sci., 11, 2768−2783 (2018); DOI: 10.1039/c7ee03639f.

[2] D. Khusnutdinova, A. M. Beiler, B. L. Wadsworth, S. I. Jacob, G. F. Moore, Chem. Sci., 8, 253–259 (2017); DOI: 10.1039/c6sc02664h.

[3] B. L. Wadsworth, D. Khusnutdinova, J. M. Urbine, A. S. Reyes, G. F. Moore, ACS Appl. Mater. Interfaces, (2019); DOI: 10.1021/acsami.9b15286.

[4] B. L. Wadsworth, A. M. Beiler, D. Khusnutdinova, E. A. Reyes Cruz, G. F. Moore, J. Am. Chem. Soc., 141, 15932–15941 (2019); DOI: 10.1021/jacs.9b07295.

Thermal based desalination systems involve heating water and collecting the condensate. The use of solar energy to drive the desalination phenomena, utilizing the heat for enhancing water evaporation (steam generation) is attracting wide-spread attention. The process involves absorbing the spectral intensity of solar light and converting it to heat. This photo-thermal conversion can be enhanced by using a strong solar absorber diluted in water, which is a poor absorber of sun light. Among the solar absorbers, carbon materials have proven to enhance water desalination rate while being economical as compared to their plasmonic counterparts. Activated carbon is a carbon-rich source which has many applications such as toxic gas adsorption and industrial effluent treatment. Considering its optical quality, it has the potential to be a strong solar absorber for enhancing water evaporation rate. A series of characterization tests were performed on activated carbon. It showed 94.17% (granular) carbon composition with trace presence of hydrogen and oxygen through ultimate analysis. FT-IR analysis performed showed mostly C=C and aromatric C-H rings. Geometric (envelope) density of the sample was almost 710 kg/m³ with theoretical porosity of 0.663. The skeleton (true) density measured was 2106.9 kg/m³. The density measurements showed that activated carbon can be used as a solid dispersion in water. The granular particles sink to the bottom of the container and behave like a black bottom in enhancing water evaporation. The apparent zeta potential for nanoparticles was measured to be -47.9 mV, showing a good dispersion stability. Optical measurements showed that the absorptance of the activated carbon was 96.35%. The strength of absorbance increases with increasing concentration and can be attributed to increased extinction coefficient of the solar spectral intensity. This increases the fraction of solar absorption in the fluid dispersion system, leading to enhanced heat localization at higher concentrations. Outdoor experiments were performed to measure the mass loss of water utilizing activated carbon nanoparticle and granular activated carbon particles dispersed in DI water. The strong absorptance of the activated carbon led to increased water evaporation rate. Activated carbon particles are inexpensive compared to expensive plasmons such as copper or gold, and hence offer an economically viable solution for augmenting desalination of water.

The field of catalysis has been traditionally divided into subdisciplines of homogeneous and heterogeneous catalysis, where the former primarily studies catalytic processes by molecular structures and the latter chiefly deals with surface catalysis processes. Each subdiscipline enjoys a wealth of knowledge that underpins its own respective applications. For instance, researches on homogeneous catalysis have generated a wealth of knowledge on how to tailor the reaction selectivity and specificity at the atomic level for the production of high-value chemicals such as targeted drugs. The ease of separation and processing renders heterogeneous catalysts ideal for commodity chemical synthesis. However, how to take advantage of the two orthogonal fields and apply knowledge learned in one field to accelerate the advancements of both has remained a critical challenge. It is within this context that recent years have witnessed a surge of research activities on atomically dispersed catalysts. A prominent example is the single atom catalysts (SACs). On the one hand, such catalysts can maximize the atomic efficiency of heterogeneous catalyst materials by minimizing the amount of materials, especially precious metals such as Pt or Pd, used; on the other hand, such catalysts are reminiscent of homogeneous catalysts in that their structures tend to be better defined than conventional heterogeneous catalysts. It, therefore, opens up an opportunity to bridge the gap between the two traditional subfields. Within this context, we have focused on developing and understanding heterogeneous catalysts with dinuclear metal centers. They are similar to SACs in that the catalytically active centers are well-defined; they present a critical difference by featuring two metal atoms at the active site. This key feature enables the studies of chemical reactions that may benefit from the presence of the second metal centers. Examples of such reactions include H₂O oxidation, CH₄ activation, and preferential CO oxidation. In all cases, we have observed a clear benefit of having a second metal center at the active site. For instance, in water oxidation, we observed that the presence of the second Ir atom facilitates the proton-coupled electron transfer process so as to lower the activation energies during the rate-determining step of O-O bond formation. Similarly, for CH₄ activation, we found that the presence of the second Ir center promotes C-C bond formation for the synthesis of C₂ products. The same effect was observed for PROX, as well, where the presence of the 2^nd Ir metal center was found to enable both CO adsorption and O₂ activation. Our work opens a new door to the study of atomically dispersed catalysts for the chemical transformations of significant industrial relevance. We envision that similar studies will be readily expanded to other chemical processes or other catalysts systems or both.

The biological use of solar energy for the synthesis of fuels from water and carbon dioxide inspires researchers and engineers in their efforts to replace existing exhaustible energy sources with renewable energy technologies. Environmentally friendly schemes of photocatalytic visible-light hydrogen production known as artificial photosynthesis along with inorganic semiconductor material also utilize biological structures, such as enzymes, machineries of whole microorganism, capable of light-harvesting, water splitting, carbon dioxide and proton reduction. A natural membrane complex of retinal-containing proton pump bacteriorhodopsin (also known as purple membranes, PM) from the extremophile microbe has been attracting an attention of researchers owing to its minimalistic structure, exceptional robustness, excellent photophysical properties, and functional sophistication. This biological material can be assembled with inorganic nanomaterials, such as semiconductor or photonic particles, into artificial hierarchical structures and demonstrate functionality that spans far beyond its natural property. This talk will provide examples of catalytic systems based on natural proton pump for light-driven H2 production, ATP synthesis and CO2 reduction.

Due to the rapid depletion of fossil fuels and related environmental issues, developing technologies for producing renewable, clean fuels for our future is of great importance. Artificial photosynthesis via water splitting or CO 2 reduction offers an attractive and cost-effective route to achieve this goal. 1 However, existing photoelectrocatalytic materials generally suffer from low activity or instability, thus greatly impeding the feasibility of artificial photosynthesis technologies. 2, 3 Herein, we present design strategies for the synthesis and integration of photoelectrocatalytic materials.

References
1. Concepcion, J. J.; House, R. L.; Papanikolas, J. M.; Meyer, T. J., Chemical approaches to artificial photosynthesis. Proceedings of the National Academy of Sciences 2012, 109 (39), 15560-15564.
2. Liu, G.; Eichhorn, J.; Jiang, C.-M.; Scott, M. C.; Hess, L. H.; Gregoire, J. M.; Haber, J. A.; Sharp, I. D.; Toma, F. M., Interface engineering for light-driven water oxidation: unravelling the passivating and catalytic mechanism in BiVO4 overlayers. Sustainable Energy & Fuels 2019, 3 (1), 127-135.
3. Eichhorn, J.; Liu, G.; Toma, F. M., Chapter 8 Degradation of Semiconductor Electrodes in Photoelectrochemical Devices: Principles and Case Studies. In Integrated Solar Fuel Generators, The Royal Society of Chemistry: 2019; pp 281-303.

The clean production of hydrogen from non-fossil energy sources is highly desirable for future energy needs and realization of hydrogen fuel cell vehicles. One way of producing green hydrogen is via photoelectrochemical splitting water into hydrogen by utilizing solar energy and/or highly active catalytic materials, both subjects of much research interests.^1-3 Particularly, the atomic scale mechanisms of the photocatalysts that facilitate the water splitting for efficient hydrogen generation are currently poorly understood. Further understanding of the role of various photocatalysts and the physics governing the active hydrogen evolution sites will allow for better and efficient design of photoelectrochemical devices.

We have developed a unique in situ/ operando liquid cell transmission electron microscope (TEM) holder with photo-capable light source⁴ which can be used to study photocatalytic reactions in real time at nanometer length scales. Using some of the model materials such as Au nanoprisms and MoS₂ flakes, we will present the correlation of I-V characteristics with water splitting and simultaneous structural changes at the catalytically active sites. The following photocatalysts used were chosen because the proposed mechanism for each exhibits a spatial dependence: plasmonically enhanced catalysis for Au nanoprisms and catalytically active edge sites for MoS2 flakes. For accurate quantitative information and reliability of materials interface in liquid, we will also present strategies for site-specific deposition/loading of sample materials in the desired electrodes of liquid cell chips for electroanalytical characterization and TEM imaging. The availability and utilization of photo stimuli in liquid cell TEM can provide important fundamental insights into the understanding of several other photoelectrochemical systems.

References:
¹ Nowotny, J.; Sorrell, C. C.; Sheppard, L. R.; Bak, T. Int. J. Hydrogen Energy 2005, 30, 521
² Walter, M. G. et al. Solar water splitting cells. Chem. Rev. 110, 6446–6473 (2010)
³ Maeda, K and Domen, K., J. Phys. Chem. Lett., 1(18), 2655-2661 (2010)
⁴ Funding for the development of the sample holder was provided by the Department of Energy,
Office of Basic Energy Sciences, through SBIR Grant # DE-SC0015213

Harvesting solar energy for carbon-free hydrogen production via photoelectrochemical (PEC) water splitting is a promising pathway to meet the increased energy demands and deal with severe environmental pollution including climate change. Silicon is the second earth-abundant element and a narrow bandgap material which is found to be a prime candidate for water splitting photoelectrodes due to ease of high-quality wafers fabrication. Si can be doped with elements of different valence states to fabricate n-type or p-type semiconductors for photoanode and photocathode respectively. However, high light reflection, low photovoltage output due to its narrow bandgap, and severe photocorrosion in the electrolyte remain as serious limitations to manufacture efficient Si-based photoelectrodes.
In this talk, I will discuss the fabrication of Si-based photoelectrodes coupled with earth-abundant transition metal-based catalysts for efficient water splitting. We introduced two-dimensional (2D) transition metal dichalcogenides catalysts on p-type Si-photocathode for highly efficient PEC hydrogen production. 2D transition metal dichalcogenides such as MoS₂ and WS₂ are a promising materials to replace noble metal catalysts due to their high density of catalytically active edge sites and large surface-to-volume ratio. Wafer-scale transferable transition metal dichalcogenides thin-film catalysts can provide a low onset potential and high photocurrent density. I will also talk about n-type Si photoanodes with high catalytic activity in transition metal-based catalysts synthesized via facile electrodeposition method for efficient water oxidation. Si photoanodes covered with water oxidation catalysts such as layered-double hydroxides, metal nanoparticles, and thin films exhibited high photoelectrochemical property and long-term stability.

Solar photovoltaic (PV) and photoelectrochemical (PEC) energy conversion share the same fundamental requirements of photon absorption, charge separation, and selective carrier collection [1]. In addition, in order to produce storable chemical energy, PEC systems must drive multi-electron transfer reactions, i.e. water oxidation (OER, a 4 electron process) at the anode and hydrogen evolution (HER) or carbon diode reduction (CO₂R) at the cathode (2-18 electron processes) [2]. In a small number of cases it has been possible to have a single material perform all of the requisite steps and remain stable under operation: water oxidation photoanodes composed of TiO₂ or Fe₂O₃ are examples. However, in the majority of cases, it has been beneficial to construct multi-material PEC systems, with different components chosen for optimal light absorption, selective carrier collection/passivation, and catalysis.
In this context, the design of PEC systems for CO₂R will be discussed. While it is attractive to design semiconductor absorbers based on the position of their conduction band relative to the standard state redox potentials of a given CO₂ reaction, e.g. CO₂/CO, CO₂/HCOOH, CO₂/CH₄, etc., this analysis neglects the fact that these multi-electron reactions occur via a series of elementary proton coupled electron transfers, with each step having its own redox potential [3]. For this reason, either solid state or molecular CO₂R co-catalysts are usually integrated with the absorber in PEC CO₂R studies [4,5].
These concepts will be illustrated through the design and operation of Si-based CO₂R photocathodes. Adroit use of charge selective contacts allows for considerable flexibility in design. For example, in contrast to conventional photocathode designs which employ p-type absorbers, we used a back illumination geometry with an n-type Si absorber to permit the use of absorbing metallic catalysts which would otherwise block the light. Back and front interfaces were configured by ion implantation and by surface passivation to achieve carrier selectivity. Selectivity to C-C coupled products was achieved by using hierarchical Au- Ag-Cu nanostructures as electrocatalysts. The photovoltage, 550- 600 mV under simulated 1-sun illumination, and photocurrents exceeding 30 mA cm^-2 confirm the carrier selectivity and passivation of the front and back interfaces. Under simulated diurnal illumination conditions, over 60% faradaic efficiency to C₂₊ hydrocarbon and oxygenate products (mainly ethylene, ethanol, propanol) is maintained for several days. By coupling photocathodes to series-connected semi-transparent halide perovskite solar cells, we demonstrated stand-alone, “no-bias,” CO₂ reduction with a 1.5% conversion efficiency to hydrocarbons and oxygenates [6].
As further examples of the PEC design space for CO₂R, use of nanowire geometries which would permit the use of front illumination and the prospects of achieving cascade CO₂R [7] on the PEC surface will be discussed.

1. Wurfel, U.; Cuevas, A.; Wurfel, P. Charge Carrier Separation in Solar Cells. IEEE J. Photovoltaics 2015, 5, 461–469.
2. Osterloh, F. E. ACS Energy Lett. 2017, 2, 445–453.
3. Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Nørskov, J. K. Energy Environ. Sci. 2010, 3, 1311
4. Arai, T.; Tajima, S.; Sato, S.; Uemura, K.; Morikawa, T.; Kajino, T. Chem. Commun. 2011, 47, 12664.
5. Qiu, J.; Zeng, G.; Ha, M.-A.; Ge, M.; Lin, Y.; Hettick, M.; Hou, B.; Alexandrova, A. N.; Javey, A.; Cronin, S. B. Nano Lett. 2015, 15, 6177–6181.
6. Gurudayal; Beeman, J. W.; Bullock, J.; Wang, H.; Eichhorn, J.; Towle, C.; Javey, A.; Toma, F. M.; Mathews, N.; Ager, J. W. Energy Environ. Sci. 2019, 12, 1068–1077.
7. Lum, Y.; Ager, J. W. Energy Environ. Sci. 2018, 11, 2935–2944.

In this paper, we present the preparation of multicrystalline silicon ingots with different boron-gallium doping ratio, resistivity distribution, minority carrier lifetime, photoelectric conversion efficiency and light-induced degradation ratio were analyzed and contrasted . The result shows that with the increase of gallium doping ratio, resistivity and minority carrier lifetime of ingot, as well as conversion efficiency of silicon wafer were both in a decreasing trend, the light-induced degradation ratio also tended to decrease as a whole, but fluctuated locally. By comparing elements tested data, it was preliminarily concluded that the fluctuation may be caused by copper,iron and other impurities . Based on the performance of resistivity, conversion efficiency and light-induced degradation ratios of gallium-doped ingots, the optimal gallium-doped ratio of multicrystalline ingots was determined to be about 25%.

Today, much research on water-splitting and fuel synthesis is directed at improving the efficiency of catalysts and their immobilisation on (photo)electrodes in order to be electroactive under aqueous conditions in photoelectrochemical (PEC) devices. In such hybrid configurations, metal oxide (MO) semiconductor materials hold the key to high efficiencies, as their facile preparation, stability in water and modular high surface-area architecture render them promising platforms. By greatly expanding in this area of research, our recent studies show that achieving a synergistic size parity between macrocatalysts and the substrate is a promising strategy to improve the electrocatalytic performance.
In this invited talk, I will present our rational development of macro-meso-porous inverse opal (IO) titanium dioxide and indium tin oxide electrodes^[1], designed to host macromolecules – enzymes and polymeric catalysts – towards water-splitting and CO₂ reduction. Our recent efforts allowed the development of semi-artificial photosynthetic systems that demonstrate how, using a dye-sensitised approach, natural archetypes – photosystem II, hydrogenase and formate dehydrogenase – can be interfaced with a synthetic organic dye to achieve unassisted solar-driven water-splitting into H₂ and O₂, and CO₂-to-formate splitting.^[2,3] We took further advantage of the wide pore architecture to develop catalytic copolymeric architectures capable of electrocatalysing the reduction of CO₂-to-CO.^[4] Capitalising on the multifunctional capabilities intrinsically delivered by copolymer scaffolds, this work also provided a seminal example of enhanced product selectivity achieved only by tuning the environmental surroundings of the molecular catalyst. The ability of IO electrodes to host and drive large, electrocatalytically active polymers was further demonstrated on silicon-based photoelectrodes, where selective, precious metal-free, solar-driven CO₂ reduction was achieved in hydro-organic media.

^[1] D. Mersch, C-Y Lee, J. Z. Zhang, K. Brinkert, J. C. Fontecilla-Camps, A. W. Rutherford & E. Reisner J. Am. Chem. Soc. 2015, 137, 8541.
^[2] K. P. Sokol, W. E. Robinson, A. R. Olivera, J. Warnan, M. M. Nowaczyk, A. Ruff, I. A. Pereira & E. Reisner J. Am. Chem. Soc. 2018, 140, 16418.
^[3] K. P. Sokol, W. E. Robinson, J. Warnan, N. Kornienko, M. M. Nowaczyk, A. Ruff, J. Z. Zhang & E. Reisner Nat. Energy, 2018, 3, 944.
^[4] J. J. Leung, J. A. Vigil, J. Warnan, E. Edwardes Moore & E. Reisner Angew. Chem. Int. Ed. 2019, 58, 7697.