Dunwei Wang, Boston College
Song Jin, University of Wisconsin-Madison
Juan Bisquert, Universitat Jaume I
Joel W. Ager III, Lawrence Berkeley National Laboratory
D2: Metal Oxides for Solar Fuels - TiO2
Tuesday PM, April 22, 2014
Westin, 2nd Floor, Metropolitan II
2:30 AM - *D2.01
Semiconductor Nanowires for Artificial Photosynthesis
Peidong Yang 1
1UC, Berkeley Berkeley USAShow Abstract
Nanowires, with their unique capability to bridge the nanoscopic and macroscopic worlds, have already been demonstrated as important materials for different energy conversion. One emerging and exciting direction is their application for solar to fuel conversion. The generation of fuels by the direct conversion of solar energy in a fully integrated system is an attractive goal, but no such system has been demonstrated that shows the required efficiency, is sufficiently durable, or can be manufactured at reasonable cost. One of the most critical issues in solar water splitting is the development of suitable photoelectrodes with high efficiency and long-term durability in an aqueous environment. Semiconductor nanowires represent an important class of nanostructure building block for direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. Nanowires can be readily designed and synthesized to deterministically incorporate heterojunctions with improved light absorption, charge separation and vectorial transport. Meanwhile, it is also possible to selectively decorate different oxidation or reduction catalysts onto specific segments of the nanowires to mimic the compartmentalized reactions in natural photosynthesis.
Recently, We have developed a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting. Similar to the photosynthetic system in a chloroplast, the artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. Under simulated sunlight, a 0.12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural photosynthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual blueprint of modular design that allows incorporation of newly discovered components for improved performance.
3:00 AM - D2.02
Observation and Alternation of Surface States on Metal Oxide Photoelectrodes
Chun Du 1 Ming Zhang 1 Ji-Wook Jang 1 Yang Liu 1 Gang-Yu Liu 1 Dunwei Wang 1
1Boston College Chestnut Hill USAShow Abstract
3:15 AM - *D2.03
TiO2 Nanotube Array Based Photoelectrochemical Water Splitting
Peng Wang 1 Zhonghai Zhang 1
1KAUST Thuwal Saudi ArabiaShow Abstract
In this presentation, we show that by varying the voltages during two-step anodization the morphology of the hierarchical top-layer/bottom-tube arrays TiO2 (TiO2 NTs) can be finely tuned between nanoring/nanotube, nanopore/nanotube, and nanohole-nanocave/nanotube morphologies, which allows us to optimize the photoelectrochemical (PEC) water splitting performance on the hierarchical TiO2 NTs. The optimized photocurrent density and photoconversion efficiency of the hierarchical TiO2 NTs were 1.59 mA cmminus;2 at 1.23 V vs. RHE and 0.84% respectively, which are the highest values ever reported on pristine TiO2 materials under illumination of AM 1.5G. The top porous layer of the hierarchical TiO2 NTs was found to have characteristics of photonic crystal, which was utilized to combine with plasmonic Au nanocrystals to produce visible-light active composite material. The selection of the Au nanocrystals is so that their surface plasmonic resonance (SPR) wavelength matches the photonic band gap of the photonic crystal and thus the SPR of the Au receives remarkable assistance from the photonic crystal substrate. Under visible light illumination (>420nm), the designed material produced a photocurrent density of ~150 mu;A cm-2, which is the highest value ever reported in any plasmonic Au/TiO2 system under visible light irradiation. Additionally, palladium nanocrystals were deposited onto the TiO2 NTs (Pd/TiO2 NTs) and, because of formation Schottky junctions between TiO2 and Pd, the Pd/TiO2 NTs showed significantly higher water contaminant decompsotiion activities than the TiO2 NTs.
3:45 AM - D2.04
Nitrogen and Transition Metal Codoped Titania Nanotube Arrays for Visible Light Sensitive Photoelectrochemical Water Oxidation
Tomiko M Suzuki 1 Gaku Kitahara 1 Takeo Arai 1 Yoriko Matsuoka 1 Takeshi Morikawa 1
1Toyota Central Ramp;D Labs, Inc. Nagakute, Aichi JapanShow Abstract
The anodization of a titanium metal sheet to form aligned titanium dioxide nanotube (TNT) arrays are of considerable research interest in field such as photocatalysts, solar cells, and sensors . Due to its wide band gap (3.2 eV), only small fraction of the solar light can be absorbed, so that it is an important issue to develop new TNT arrays with enhanced photocatalytic activities under visible light irradiation. In the research field of TiO2 particles, impurity doping such as nitrogen is one of typical approaches to extend spectral response of TiO2 to visible light region . Moreover, codoping of nitrogen and metal ion possesses potential to induce formation of new states which are close to the valence band and conduction band edges, respectively . The codoping approach is an efficient way to absorb wider spectrum of solar irradiation by TiO2 for photoelectrochemical water splitting for solar hydrogen generation and CO2 reduction . In this work, we report on the fabrication of titania nanotube arrays codoped with nitrogen and transition metals such as Fe, V, Cr, and Co (N,M-TNT) for the visible light-driven photoelectrochemical water oxidation.
Vertically aligned N,M-TNT were successfully prepared for the first time via an anodization process using low concentration transition metal (0.05-0.13 at%)-Ti alloys and a subsequent nitridation process.
Photoelectrochemical measurements were performed in a 3-electrode cell containing 0.1 M KOH with a N,M-TNT photoanode, a platinum cathode, and an Ag/AgCl reference electrode. The codoping of nitrogen and transition metal substantially improved the photocurrent of the TNT photoanode under visible light irradiation. The rate of increase in the photocurrent was dependent on transition metal species and it was found that codoping of iron showed the highest enhancement. N, Fe(0.13at%) codoped TNT photoanode yielded a visible-light-induced water oxidation with a photocurrent density of 0.76 mA/cm2 at 0.6 V (vs. Ag/AgCl) under visible light irradiation, which was 13 times and 5 times higher than that of Fe(0.13 at%)-TNT and N-TNT, respectively. Incident photon to current conversion efficiency (IPCE) in the visible light region of 400-700 nm was enhanced by the codoping of N and 0.13 at%-Fe, and it was measured to be 2.6% at 400 nm (at 0.6 V vs. Ag/AgCl). Oxygen detection was also conducted by a fluorescence measurement system.
The photoelectrochemical water oxidation activity of codoped TNT could be further improved by optimizing the amount of doping, kind of dopant, and nanotube structure. This scalable method for codoping to TNT can also be extended to other metal oxide nanotubes.
 P. Roy, et. al., Angew. Chem. Int. Ed., 50 (2011) 2904.  R. Asahi, T. Morikawa, et al., Science, 293 (2001) 269.  Y. Gai, J. Li, et al., Phys. Rev. Lett., 102 (2009) 036402., W. Zhu, et al., Phys. Rev. Lett., 103 (2009) 226401.  S. Sato, T. Arai, et al., J. Am. Chem. Soc., 133, (2011) 15240.
4:30 AM - *D2.05
Rapid Flame Processing of Metal Oxides Photoanodes for Enhanced Solar Water-Splitting
In Sun Cho 1 Lili Cai 1 Manca Logar 1 Pratap M Rao 1 Chi Hwan Lee 1 Robert Sinclair 2 Fritz B. Prinz 1 Xiaolin Zheng 1
1Stanford University Stanford USA2Stanford University Stanford USAShow Abstract
Photoelectrochemical (PEC) water-splitting is the simplest and cleanest route that directly converts sun light to hydrogen and potentially it will enable a low-cost production of hydrogen. One of the biggest challenges for the realization of the PEC water-splitting is to develop an efficient photoanode having a good light absorption, fast charge transport and transfer properties simultaneously. Typically metal oxides are considered to be good candidates because of their excellent photochemical stability and low-cost. However, their poor material quality such as large amount of defects, low surface area, low charge carrier&’s mobility/conductivity, which largely originated from the preparation method, limits the charge transport and transfer properties.
In this talk, i will present two novel flame processing techniques, i.e., flame reduction and doping, for metal-oxide photoanodes that greatly improve the charge transport and transfer properties, hence enhancing the PEC water-splitting performance. First, we developed a rapid flame reduction method to generate controllable amount of oxygen vacancies in TiO2 nanowires (NWs) that leads to nearly three times improvement in the PEC water-splitting performance. The flame reduction method has unique advantages of a high temperature (>1000 oC), ultra-fast heating rate, tunable reduction environment, and open-atmosphere operation, so it enables rapid formation of oxygen vacancies (<1min) near the surface region without damaging the nanowire morphology and crystallinity, and even applicable to various metal oxides. Second, we designed an ex-situ novel doping method which combines versatile solution phase chemistry and rapid flame annealing process (i.e., Sol-Flame) to dope TiO2 NWs with cobalt (Co). The sol-flame doping method not only preserves the morphology and crystallinity of the TiO2 NWs, but also allows fine control over the Co dopant profile by varying the concentration of Co precursor solution. In addition, the sol-flame doping is a general method to dope metal dopants into the metal oxides NWs regardless of their synthesis method. Finally, we extended the sol-flame doping method to codope TiO2 NWs with tungsten and carbon (W, C) by sequentially annealing W-precursor coated TiO2 nanowires in flame and CO gas. This is the first experimental demonstration that codoped TiO2:(W, C) nanowires outperform monodoped TiO2:W and TiO2:C and double the saturation photocurrent of undoped TiO2 for PEC water-splitting. Given the good controllability and versatility of the flame processing methods, it can be applied to other metal oxide photoanodes such as Fe2O3, WO3 and BiVO4 to further improve their PEC water-splitting performance.
5:00 AM - D2.06
Atomic Level In-Situ Characterization of Metal/TiO2 Photocatalysts Under Light Irradiation in Water Vapor
Liuxian Zhang 1 Peter A. Crozier 1
1Arizona State University Tempe USAShow Abstract
TiO2 is a semiconducting oxide used as a UV-light photocatalyst with potential applications to degradation of organics and solar fuel generation. The photocatalytic activity can be significantly enhanced via the deposition of metal particles onto the oxide surface. Photogenerated electrons are transferred to the metal while the holes remain in the TiO2 valence band thus suppressing electron-hole pair recombination. It is now recognized that atomic level in situ observations of catalytic nanomaterials are critical for understanding structure-reactivity relations because the active form of the material may exist only under reaction conditions. We have undertaken a series of in situ TEM experiments to develop a fundamental understanding of metal particle/TiO2 structure changes in reaction conditions. Such an analysis is performed under in situ conditions in the presence of light and reactants in an environmental transmission electron microscope (ETEM). Here we employ a modified ETEM with a broadband light source to study the behavior of metal particles on TiO2 semiconductor surfaces under photoreaction conditions. Insights from these experiments can help in the design of photocatalysts with better performance and stability. Preliminary experiments showed that the surfaces of anatase nano particles becomes disordered in water vapor under light exposure in the electron microscopes.  In this study we investigate the changes that occur in a variety of supported metal systems including Pt/TiO2, one of the most efficient metal/TiO2. Pt coupled anatase nanoparticles were prepared by photodeposition. Light induced surface and interface changes will be presented. Catalytical properties before and after structure change are tested by measuring H2 production under Xenon lamp using gas chromatography. Structure-reactivity relationships will also be discussed for the Pt system and a number of transition metals.
. Miller, B.K.; Crozier, P.A. Microscopy and Microanalysis., 2013 DOI: 10.1017/S1431927612014122.
. Zhang, L.; Miller, B.K.; Crozier, P.A. Nanoletter 2013, DOI: 10.1021/nl304333h
5:15 AM - D2.07
Cold Gas Sprayed Semiconductor-Based Electrodes for the Photo-Induced Water Oxidation
Iris Herrmann-Geppert 1 2 Thomas Emmler 2 Henning Gutzmann 1 Peter Bogdanoff 3 Thomas Dittrich 3 Thomas Klassen 1 2
1Helmut-Schmidt-University Hamburg Germany2Helmholtz-Zentrum Geesthacht Geesthacht Germany3Helmholtz-Zentrum Berlin Berlin GermanyShow Abstract
One of the most challenging tasks in photo assisted water splitting for hydrogen generation is the development of low cost, but highly efficient photoelectrodes. Identifying suitable catalysts and processes opens up the way to build photoelectrochemical cells for large-scale hydrogen production.
In this contribution the potential of cold gas spraying for the production of photoelectrodes employing semiconductors for the water oxidation reaction (OER) is presented. Conventional methods of coating usually employ wet chemical methods with subsequent calcination steps to obtain strong binding between the catalyst particles and the substrate. In the cold gas spraying process particles are accelerated to high velocities by a pressurized gas. The nitrogen used as process gas is preheated and then expanded in a De Laval type nozzle. By impact on the substrate the particles deform and break up and thus can build an efficient interface to the back contact (analyzed by cross-section SEM).
Cold gas spraying is a method for the direct coating of surfaces and does not require additives that have to be removed afterwards e.g. by a calcination step but allows the direct formation of a working electrode ensemble.
For the coating process only particles in the µm-range can be utilized. First investigation were performed with P25 TiO2 which was agglomerated to particles with a size of approximately 20 µm. The films yielded seven times higher photocurrents than comparable doctor blade references. This approach was extended to WO3 which shows high activity in the photo-induced water oxidation. Due to the impact on the substrate during the cold gas spraying the particles break up which form a porous film. Furthermore the substrate is deformed so that a caldera-type substrate structure is formed which enables an embedment of the TiO2 particles in the substrate.
Interestingly, in the physical-chemical analysis (Raman, XRD, UPS, XPS) indications were found that the catalyst surface is changed due to the cold gas spray process. Spray parameters and the film thickness on the substrate were varied in order to investigate the influence of the operation properties on the photoelectrocatalytic properties of the TiO2 and WO3 coatings. These findings are compared to films obtained from the established wet-chemical deposition methods.
5:30 AM - D2.08
Highly-Efficient Capillary Photoelectrochemical Water Splitting Using Cellulose Nanofiber-Templated TiO2 Photoanodes
Zhaodong Li 1 Chunhua Yao 2 Yanhao Yu 1 Zhiyong Cai 2 Xudong Wang 1
1University of Wisconsin-Madison Madison USA2USDA Forest Service Madison USAShow Abstract
High porosity three dimensional (3D) nanofiber networks for PEC photoanode development, offer extremely large surface area, excellent charge transport properties, as well as long optical paths for efficient light absorption. 3D cellulose nanofiber networks have been attracting increasing attention in nanomanufacturing owing to their great abundance, low-cost, degradability and bio-compatibility. Here, we used 3D cellulose nanofiber as templates for fabricating PEC photoanode via atomic layer deposition of TiO2. After annealing the cellulose-TiO2 core-shell nanostructure, anatase TiO2 nanotube 3D network was achieved, which offers tremendous surface area for PEC water splitting. Annealing the core-shell structure in vacuum can preserve the carbon from cellulose in TiO2 and make the TiO2 network into “black” so that realizes photoactivity in visible light region. Furthermore, based on the excellent hydrophilic property of cellulose, A novel capillary PEC setup is created as well. Such low-cost and large-area technique for creating “out-of-water” PEC electrode materials might have a potential value for solar energy application for their interaction that between reaction sites and light is not limited by the volume, surface and depth of electrolyte (water).
D1: Overview of Solar Hydrogen Production by PEC
Tuesday AM, April 22, 2014
Westin, 2nd Floor, Metropolitan II
9:00 AM - *D1.01
III-V Surface Treatments and Catalysis for Photoelectrochemical Water Splitting
John A Turner 1
1National Renewable Energy Lab Golden USAShow Abstract
The GaAs/GaInP2 PV/PEC tandem cell has shown to be a high-efficiency water splitting system, but this material system has not shown the necessary long-term stability and interfacial catalysis and energetics are still an issue. Stabilizing the system using surface treatments or solution additives has improved the stability but band-edge energetics and surface catalysis are still important challenges.
This report will discuss our recent results in modifying the band-edge energetics and attaching homogenous catalysts for hydrogen evolution.
9:30 AM - *D1.02
Critical Metrics and Fundamental Materials Challenges for Renewable Hydrogen Production Technologies
Eric Miller 1 Sara Dillich 1 Erika Sutherland 1 Katie Radolph 2 David Peterson 2 Chris Ainscough 2 Sarah Studer 1
1US Department of Energy Washington USA2US Department of Energy Golden USAShow Abstract
The US Department of Energy&’s (DOE) Fuel Cell Technologies Office has made significant progress in fuel cell technology advancement and cost reduction. Encouragingly, rollouts of fuel-cell vehicles by major automotive manufacturers are scheduled over the next several years. With these rollouts, enabling technologies for the widespread production of affordable renewable hydrogen become increasingly important. Near-term utilization of current reforming and electrolytic processes is necessary for early hydrogen markets, but transitioning to industrial-scale renewable hydrogen production remains essential to the longer term. Central to the long term vision is a portfolio of renewable hydrogen conversion processes, including, for example, the direct photoelectrochemical and thermochemical routes, as well as photo-assisted electrochemical routes. DOE utilizes technoeconomic analyses to assess the long-term viability of these emerging hydrogen production pathways and to help identify key materials- and system-level cost drivers. Sensitivity analysis from the technoeconomic studies will be discussed in connection with the metrics and fundamental materials properties that have direct impact on hydrogen cost. It is clear that innovations in macro-, meso- and nano-scale materials are all needed for pushing forward the state-of-the-art. These innovations, along with specific research and development pathways for advancing materials systems for the renewable hydrogen conversion technologies are discussed.
10:00 AM - *D1.03
Materials for Efficient Photoelectrochemical Water Splitting: The U.S. Department of Energy PEC Working Group
Heli Wang 1 Eric L Miller 2
1NREL Golden USA2US Department of Energy Washington USAShow Abstract
Development of durable photoelectrochemical (PEC) water splitting devices with high solar-to-hydrogen (STH) conversion efficiency has been a significant materials challenge for decades. Critical requirements on semiconductor materials&’ band gap, band edge, optoelectronic efficiency, and stability must be satisfied simultaneously. While earth-abundant metal oxide semiconductors can be stable, STH efficiencies have been limited by issues related to the wide band gap, band-edge mismatch and the poor opotoelectronic quality in these materials. Tandem cell configurations have been developed to address the band-edges mismatch, but more focus is needed on overcoming the efficiency limitations due to absorption, charge mobility, recombination, interfacial kinetics, etc.
Crystalline III-V materials offer an alternative pathway to efficient STH conversion. Over a range of compositions, these materials have suitable band gaps and optoelectronic quality. In addition, the band edge mismatch has been successfully addressed using monolithic PEC/PV tandem cell designs. Stability, however, remains a key issue. NREL, working with other members of the U.S Department of Energy PEC Working Group (including the Lawrence Livermore National Laboratory, and the University of Nevada at Las Vegas) have been investigating the corrosion of III-V materials and interfaces with a goal to develop surface modification methods for mitigating corrosion. Approaches have included coatings, ion bombardment, surface nitridation as well as electrolyte treatments.
Significant materials challenges remain, and effective usage of resources is needed. The PEC Working Gr