bluuebunny/crossref_metadata_2025 huggingface dataset:
Vanadium Nitride Thin Films and Nanoclusters: Growth and Electrochemical/XPS Characterization
Authors: Bondarchuk Oleksandr , Goikolea Eider , Rojo Teofilo , Mysyk Roman
Published: February 2020

Pseudocapacitive or electrochemically active materials can achieve capacitance values up to 10-100 times greater than those obtained by materials based only on an electric double layer (EDL) mechanism. The pseudocapacitive behavior of transition metal oxides has been extensively studied in the last decades and assigned to redox reactions occurring at the surface of the material. Very recently, transition metal nitrides such as MoxN, VN or TiN have emerged as promising electrode materials for electrochemical capacitors. These materials are inexpensive, have a high molar density good chemical resistance and, most importantly, in contrast to oxides they exhibit very high electronic conductivity value. Extremely high specific capacitance values of 1340 F g-1 was reported for nanostructured VN [1]. To shed some light on the mechanism of such huge capacitance we have carried out a comparative study of VN thin films and VN nanoclusters grown and characterized in-situ to relate electrochemical properties with the structure and composition of the surface of vanadium nitride.

An UHV system (SPECS) equipped with tools for XPS, e-beam assisted evaporation of metals, high pressure cell and electrochemical cell for I-V cycling was used for in-situ synthesis and characterization.   

A recipe to grow VN thin films and nanoclusters via direct nitridation of thin vanadium film in atmosphere of 1 bar N2 at temperature 800 oC has been developped. Stoichiometry of the VNxOy compounds  can be controoled by post oxidation of VN in oxygen atmosphere at elevated temperature. Growth of VN nanoclusters using the same procedure has been performed on HOPG surface.

Electrochemical characterization of VN and VNxOy thin films demostrated impressive areal capacitance of ~2000 µFcm-2 in 1M KOH electrolyte at scan rate 1Vs-1. XPS characterizationwas applied to elucidate electrochemical reactions occurring on the surface of VN films.

[1] D. Choi et al., Adv. Mater., 18, 1178 (2006)
Understanding the Influence of Structure on Activity and Stability in the Catalysis of the Oxygen Evolution Reaction (OER) Using Crystalline Oxides As a Platform
Authors: Gardner Graeme , Smith Paul , Kaplan Christopher , Al-Sharab Jafar F. , Go Yong Bok , Greenblatt Martha , Dismukes Gerard Charles
Published: February 2020

Oxides of manganese and cobalt have proved to be effective (electro)catalysts for the water oxidation half-reaction in water splitting. Recently, the research focus has mostly been on developing high surface area morphologies of catalysts with high specific activity that are potentially easily adaptable to photoelectrochemical cells for water splitting. However, it is still not completely understood why these catalysts show such high intrinsic activity, and many have amorphous structures, making it difficult to elucidate structure-function relationships. Using crystalline oxides as an experimental framework, we have elucidated the effects of structure on the activity and stability of cobalt and manganese oxides for the oxygen evolution reaction (OER). For instance, in two distinct polymorphs of lithium cobalt oxide (LiCoO2), we have found the cubic structure (Fd-3m, spinel-like), with a Co4O4 oxo-metallic cube motif, is highly active for water oxidation, whereas the layered polymorph (R-3m) with slabs of CoO2 (used for rechargeable Li ion batteries) is not. In fact, upon electrochemical processing, the layered polymorph undergoes a surface restructuring that we can monitor using high-resolution transmission electron microscopy (HRTEM). This has been shown to occur in both aqueous and non-aqueous electrolyte. After the transformation occurs, which can also be monitored by the electrochemical profile and corrosion of lithium, the OER activity of the layered material mirrors that of the cubic LiCoO2. However, it is unstable and eventually diminishes with repeated electrochemical cycling or prolonged electrolysis. In this way, we show that the underlying bulk crystal structure has influence over not just activity, but also stability, both equally important for commercial applications. Parallel to this work, we isolated discrete Co2O2, Co3O3, and Co4O4 molecular clusters in the same ligand sets, and found only the complete Co4O4 cubanes were catalytically active for water oxidation. This difference in activity correlates to the accessibility of a formal Co4+ oxidation state, which occurs at modest potentials only for the cubane, which compensates by delocalizing the hole across all cobalt and bridging oxos. The singly oxidized cubane is sufficient to produce O2 just in reaction with hydroxide. We believe this phenomenon relates back to the solid-state, and is in part the reason for the high activity in the cubic LiCoO2.
Screening of Oxygen Reduction and Evolution Electrocatalysts By Scanning Electrochemical Microscopy
Authors: Huang Sin-Xian , Xiong Wei-Fen , Weng Yu-Ching
Published: February 2020

A rapid screening technique based on scanning electrochemical microscopy (SECM) in a tip-generation–substrate-collection (TG–SC) mode was applied to identify potential electrocatalysts for both oxidation reduction reaction (ORR) and oxidation evolution reaction (OER). Ternary ruthenium based chalcogenide and ruthenium based oxide electrocatalyst arrays were fabricated and screened for ORR and OER, respectively. The potential electrocatalysts were then characterized by SEM, EDX, XRD and XPS. The catalytic activity of potential electrocatalysts were further examined during cyclic voltammetric scans of the substrate with a tip close to the substrate. The quantitative rate of ORR on the candidate substrates was determined for different substrate potentials from SECM approach curves by fitting to a theoretical model. The electron transfer number of OER was calculated rotating ring-disk electrode (RRDE) technique. The best ruthenium based electrocatalysts for ORR and OER would be identified in this work.
Nickel Oxide Nanoclusters for Electrocatalytic Water Oxidation
Authors: Kim Dong Wook , Avila Jason Ryan , Baik sung-Il , Beyzavi M. Hassan , Emery Jonathan , Seidman David , Martinson Alex , Farha Omar K. , Hupp Joseph T.
Published: February 2020

The growth and/or deposition of uniformly dispersed nanoclusters is a significant challenge for the field of electrocatalysis science. Here we employ ALD technique to obtain ultrafine nickel oxide nanoclusters uniformly dispersed on flat electrodes. Atomic layer deposition (ALD) is a versatile technique to the precise growth of a variety of materials. Specifically, the key point in ALD process is that precursor molecules evaporated at an appropriate temperature react only with chemically reactive surface sites (e.g. hydroxyl groups on surfaces), and these reactions are self-limiting, i.e. controllable at the atomic level. Thus, we utilize a self-limiting nature of ALD for nanocluster formation. Meso-tetra(4-carboxyphenyl)porphyrin (TCPP) monolayer is adsorbed parallel to the electrode surface, and subsequently metallated with Mn (or Fe)-OH (denoted by MnTCPP or FeTCPP) as reactive sites for nickel oxide growth. The nickel oxide nanoclusters, then, are grown on the -OH functional groups of MnTCPPs (or FeTCPPs) via ALD. The growth of nickel oxide nanoclusters is demonstrated in detail by transmission electron microscopy images and grazing incidence small angle X-ray scattering analysis, as well as by in-situ monitoring mass gain during deposition. Additionally, to evaluate the electrocatalytic water oxidation activity of nanoclusters, TCPP molecules are removed from nanocluster films through post-ozone-treatments at a relatively low temperature. Based on electrochemical analysis, 1-2 nm sized-nanocluster films show a significantly higher water oxidation activity (i.e. higher water oxidation currents for unit number of nickel atoms deposited on films) compared with that of a very thin film. Consequently, this shows that the uniformly dispersed nanoclusters should be very useful for many applications including electrocatalysis, catalysis, and supercapacitors.
Monolayer-Precision Synthesis of Molybdenum Sulfide Nanoparticles and Their Nanoscale Size Effects in Hydrogen Evolution Reaction
Authors: Seo Bora , Jung Gwan Yeong , Sa Young Jin , Cheon Jae Yeong , Kwak Sang Kyu , Joo Sang Hoon
Published: February 2020

Metal sulfide-based nanostructured materials have emerged as promising catalysts for hydrogen evolution reaction (HER), and significant progress has been achieved in enhancing their activity and durability for the HER. The understanding of nanoscale size-dependent catalytic activities can suggest critical information regarding catalytic reactivity, providing the scientific basis for the design of advanced catalysts. However, nanoscale size effects in metal sulfide-based HER catalysts have not yet been established fully, due to the synthetic difficulty in precisely size-controlled metal sulfide nanoparticles. In this work, we prepared molybdenum sulfide (MoS2) nanoparticles with monolayer precision from one to four layers with the nearly constant basal plane size of 5 nm, and explored their size-dependent catalytic activity in the HER. Using density functional theory (DFT) calculations, we identified the most favorable single-, double-, and triple-layer MoS2 model structures for the HER, and calculated elementary step energetics of the HER over these three model structures. Combining HER activity measurements and the DFT calculation results, we establish that the turnover frequency of MoS2 nanoparticles in the HER increases in a quasi-linear manner with decreased layer numbers. Cobalt-promoted MoS2 nanoparticles also exhibited similar HER activity trend. We attribute the higher HER activity of smaller metal sulfide nanoparticles to the higher degree of oxidation, higher sulfur coordination number, formation of the 1T phase, and lower activation energy at the rate-determining step. This insight into the nanoscale size-dependent HER activity trend will facilitate the design of advanced HER catalysts as well as other hydrotreating catalysts.
Effect of Graphene/MoS2 interface on Hydrogen Evolution Reaction
Authors: Rabelo Adriano Cesar , Firmiano Edney G. S. , Leite Edson Roberto
Published: February 2020

In a previous work, we demonstrated the performance of MoS2 synthesized on Reduced Graphene Oxide as a electrocatalyst to hydrogen evolution reaction (HER) and the role of graphene to avoid 3D growth of MoS2.1 The present work shows the improved performance as a electrocatalyst to HER of Molybdenum Disulfide (MoS2) synthesized on Mechanical Exfoliated Graphene (MoS2/GRA) instead of chemically Exfoliated Graphene. By X-ray Photoelectron Spectroscopy of the composites, we detected that the first layers of MoS2 are bonded to oxygen of the RGO by chemically covalent bond (Mo-O-C) and absence of this bands to MoS2 when synthesized on mechanical exfoliated graphene, as illustrated in figure 1a and 1b. The structure and morphology were characterized by Scanning Electron Microscopy and Transmission Electron Microscopy. The electrochemical characterizations shows that overpotential was significantly reduced when mechanically exfoliated graphene was used in the synthesis of composite (MoS2/GRA), (see Figure 1c). A synergetic effect was observed, once pure materials were employed a poor performance was reported. Moreover, the interface structure is a key parameter on the acting of composite, taking account that the simple mix of both, chemically exfoliated graphene with MoS2 and mechanically exfoliated Graphene with MoS2, exhibited different on set values.

Figure 1
Modeling of Bio-Electrochemical and Mechanical Interactions in a Photosynthetic Cell
Authors: Hemanth Kumar Tanneru , Resmi Suresh M.P , Ramanan Aravind Vyas , Mehdi Shahparnia , Muthukumaran Packirisamy , Pillay Pragesan , Williamson Sheldon S , Rengaswamy Raghunathan
Published: February 2020

Micro photosynthetic cell (µPSC) is an electrochemical device, which generates electricity, by harnessing the electrons from photosynthesis and respiration processes of the photoautotrophs.

Till date, focus has been mostly on experimental aspects of  and very little work has been pursued on the development of mathematical models for µPSC.  Modeling a system like µPSC is complex due to the fact that the device operation depends on interactions of microorganisms with several operational parameters such as light intensity, quantum yield and so on. Further, the electrode structure and the electrochemical interactions at the surface of the electrodes affect the device performance.

 Modeling of µPSC could help in understanding the performance limiting step(s) in the series of processes that occur during the operation of device. The performance of the device can be improved by focusing on the rate of limiting steps. Modeling could also help in determining optimal design and operational parameters which can maximize the device performance.

 A simple mathematical model based on first principles is proposed to predict the performance of the µPSC. Sensitivity analysis is performed to obtain the most sensitive rate parameters of the model. The optimal values of sensitive rate constants are obtained from the experimental data through optimization. The developed model is validated by comparing the predictions of the model with the experimental data obtained from the response of the system to step changes in load. Figures 1 and 2 show the model performance in terms of i-V characteristics and comparison of experimental and estimated voltage profiles for step changes in external loads.

 Keywords:, First principles model, Parameter estimation, and Optimization and Sensitivity analysis.

Figure 1
Benchmarking Components for Photoelectrochemical Water-Splitting Devices
Authors: Chatman Shawn M , McCrory Charles C L , Peters Jonas C , Jaramillo Thomas F
Published: February 2020

Objective evaluation of solar energy conversion  efficiencies is of fundamental importance to  the development of solar energy driven photoelectrochemical (PEC) water-splitting devices.  Unfortunately there is no current standard methodology for evaluating the efficiency of fuel-forming PEC components and devices.  Consequently, in the literature, there is a wide array of testing procedures and calculations being used to report device and component efficiency.  Here we will highlight some of the common testing procedures and  efficiency calculations being used, pointing out important areas where there is no universal agreement.  In addition, we will present the testing procedure used within the Benchmarking facility of the Joint Center for Artificial Photosynthesis (JCAP).  We illustrate our procedure by presenting pertinent photoelectrochemical data collected from measurements taken on well-known PEC water-splitting components and devices (e.g. Pt/p-Si, Ni/BVO, triple junction Si, p-GaP, p-InP, n-TiO2)  and explain how this data can be translated into real world device-pertinent figures-of-merit.
IrO2 Surface and Nanostructure Stability from First Principles and Variable Charge Force Field Calculations
Authors: Sen Fatih G. , Kinaci Alper , Narayanan Badri , Davis Michael J. , Gray Stephen K. , Sankaranarayanan Subramanian K. R. S. , Chan Maria K. Y.
Published: February 2020

IrO2 is one of the most efficient water-oxidation electrocatalysts and can also act as a photocatalyst for the water splitting reaction when used in the form of nano-sized clusters. The structure and surface properties of nanoclusters greatly influence the photocatalytic properties of IrO2, and atomistic scale understanding of these properties is essential to elucidate the photocatalytic mechanisms of IrO2. Here, we carried out first principles calculations based on spin polarized density functional theory (DFT) including spin-orbit coupling and the Hubbard U correction on the bulk and surface structures of IrO2. The stability and electronic structure of low index rutile (100), (001), (110) and (101) surfaces of IrO2, were studied. The relative surface energies were obtained as (110) < (101) < (100) < (001). The equilibrium shape of IrO2 nanoparticles was deduced using a Wulff construction. In order to study the structural stability of IrO2 nanoclusters and the long time-scale dynamics of IrO2 polymorphs larger than about 2 nm in diameter, we developed the first empirical interatomic potential (force field) for IrO2 based on Morse functional form coupled with a variable charge method (QEq). The Morse+QEq parameters were optimized using an evolutionary algorithm with respect to a DFT training set, and was shown to be successful in predicting bulk and surface properties of rutile IrO2 and various polymorphs including anatase, brookite, columbite and pyrite derived from DFT calculations. Pressure induced phase transformations of bulk IrO2 polymorphs were reported and thermodynamically stabile phases of IrO2 at nano-scale were obtained using the surface energies of stable polymorphs. Our results will shed light on the development of stable nanoscale IrO2 electrocatalysts and photocatalysts that can efficiently utilize solar energy for water splitting reaction.

Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
Thin Film Photovoltaics with Organic Metal Halide Peroviskites
Authors: Kamat Prashant V
Published: February 2020

Organic metal halide perovskites have now emerged as new and promising candidates for developing low cost thin film solar cells with efficiencies in the range of 20.1% certified efficiency.  The recent advances of CH3NH3PbI3 based thin film solar cells have manly focused on the design architecture and improving the stability. Both TiO2 and Al2O3 mesoscopic architectures provide robust support to design high efficiency thin film solar cells. The excited state dynamics leading to the charge separation remains to be fully understood. Through comparative spectral analysis between the absorption features of charge transfer complexes in the solution phase and the final solid-state perovskite films, we are able to fully classify the absorption features in the excited state of CH3NH3PbI3 across the transient absorption spectrum recorded following laser pulse excitation. The excited state of CH3NH3PbI3 characterized through transient absorption spectroscopy show dual behavior, viz., charge transfer complex state and charge separated state.1,2 The interplay between these two different excited states leads to efficient charge separation. Insights into the charge recombination kinetics and band filling aspects of CH3NH3PbI3 provide new ways to tune the efficiency of perovskite solar cells. These spectroscopic studies provide further insight into the optoelectronic behavior of methyl ammonium lead iodide films.  Excited state properties that elucidate the exceptional photovoltaic properties will be discussed.

Acknowledgments

The research described herein is supported by the Office of Basic Energy Sciences, U.S. Department of Energy.

 

Selected Publications

Christians, J. A.; Fung, R.; Kamat, P. V., An Inorganic Hole Conductor for Organo-Lead Halide Perovskite Solar Cells. Improved Hole Conductivity with Copper Iodide. J. Am. Chem. Soc.
2014,
136, 758–764.

Manser, J. S.; Kamat, P. V., Band Filling with Charge Carriers in Organometal Halide Perovskites. Nature Photonics
2014, DOI: 10.1038/NPHOTON.2014.1171.
Novel Photoelectrodes and Noble Metal Free Catalysts for Light Driven Water Electrolysis
Authors: Fiechter Sebastian
Published: February 2020

The generation of fuels from sunlight and water is considered as a task of paramount importance for a sustainable energy supply in the future. Decomposition of water by a photoelectrochemical process is a possibility to harvest solar energy in form of hydrogen in a large scale. For this purpose, a membra­ne can be used which immersed in an aqueous electrolyte and illuminated by sunlight is able to split water into hydrogen and oxygen. Such a device is frequently addressed as artificial leaf. From an eco­nomic and environmental point of view this device should consist of cheap, abundant and non-toxic elements featuring a Solar-to-Hydro­gen (STH) efficiency > 10% and generating hydrogen at a price of ~3 €/kg H2.

In our lab two approaches have been pursued in the last time to realize artificial leaf type structures:

triple junction thin film a-Si/µc-Si solar cells in superstrate geometry with integrated catalysts [1],

tandem junction solar cells combined with novel semiconducting ternary metal oxide anodes [2].

As photoanode materials Fe2WO6 and BiVO4 are under investigation and have been improved with re­spect to their photovoltage and their photocurrent densities using sol-gel and spray pyrolysis as prepa­ration technique. To lower the costs of the device platinum was replaced by carbon supported MoS2 nanoparticles as hydrogen evolving catalyst. Deposited as a blend with PEDOT:PSS on the backside of the solar cell an SHE of 3.7% was yielded. RuO2 was replaced by manganese and cobalt oxide thin films, respectively.

References:

[1]      Diana Stellmach, Peter Bogdanoff, Onno Gabriel, Bernd Stannowski, Rutger Schlatmann,

          Roel van de Krol, Sebastian Fiechter; FORMATEX 2013, 880-886.

[2]      Fatwa F. Abdi, Lihao Han, Arno H.M. Smets, Miro Zeman, Bernard Dam, Roel van de Krol,          NATURE COMM. 2013, 4:2195, DOI: 10.1038/ncomms3195.

Figure 1
Enabling Solar Fuels Technology By High Throughput Discovery of Earth Abundant Oxygen Evolution Reaction Catalysts
Authors: Haber Joel A. , Gregoire John M. , Guevarra Dan , Jones Ryan , Shinde Aniketa , Becerra-Stasiewicz Natalie , Xiang Chengxiang , Slobodan Mitrovic , Jung Suho , Kisielowski Christian , Yano Junko , Jin Jian
Published: February 2020

The High Throughput Experimentation (HTE) project of the Joint Center for Artificial Photosynthesis performs accelerated discovery of new earth-abundant photoabsorbers and electrocatalysts.  We will describe several new screening instruments for high throughput (photo-)electrochemical measurements and summarize the discovery pipelines. This approach will be illustrated using the high throughput discovery, follow-on verification, and device implementation of a new quaternary metal oxide catalyst. Discovering improved electrocatalysts for the oxygen evolution reaction (OER) is of great importance for efficient solar fuels generation, regenerative fuel cells, and recharging metal air batteries. We report a new Ce-rich family of active catalysts composed of earth abundant elements, which was discovered using high-throughput methods to produce 5456 discrete compositions in the (Ni-Fe-Co-Ce)Ox composition space. The activity and stability of this new OER catalyst was verified by re-synthesis and extensive electrochemical testing of samples in a standard format in 1.0 M NaOH. Characterization of selected compositions by XRD, XPS, SEM, TEM, EDS, XRF mapping, and EXAFS both as-synthesized and after electrochemical testing, reveal the important of nanostructure to the observed electrochemical performance.  The discovery of additional electrocatalysts by expansion of this composition space investigated and of new composition spaces tested for OER activity and stability under acidic conditions will be reported.
High-Efficiency Tandem Absorbers for Economical Solar Hydrogen Production
Authors: Deutsch Todd G , Young James L. , Döscher Henning , Wang Heli , Turner John A
Published: February 2020

In this talk I will present an overview of our research strategy to develop a semiconductor-based device capable of over 20% solar-to-hydrogen efficiency with several thousand hours of stability under operating conditions. Our goal is to improve solar-to-hydrogen (STH) efficiency from 12.4% to over 20% by developing novel tandem semiconductor materials and configurations. Our approach focuses on classes of materials that have either demonstrated exceptionally high efficiency or theoretically can produce highly efficient materials. The four tandem material sets in our research portfolio are III-Vs on GaAs, InGaN on Si, III-V-N on Si, and dual photoelectrode pairings. We are evaluating catalytic nitride, oxide, and sulfide-based semiconductor surface modifications to extend durability from a few hundred hours to nearly 1000 hours, with a stretch target of 3500 hours. Cost reductions in the synthesis of high-efficiency III-V photoelectrochemical devices could be realized from emerging epitaxial synthesis technologies such as spalling, epitaxial lift-off, close-space vapor transport, or hydride vapor phase epitaxy to yield economical hydrogen production. Our group addresses synthesis cost, in a limited capacity, by investigating novel material configurations.

Within the next few years, we plan to demonstrate a prototype photoreactor that produces 3 L of standard hydrogen within an 8-hour period under moderate solar concentration (10x), which can be accomplished using only 6 cm2 of a 20% STH material.
Microfluidic Photocatalytic Device Utilizing Anodized Titania Nanotube Arrays: Application and Simulation Validation
Authors: Smith York R. , Jayamohan Harikrishnan , Hansen Lauryn , Mohanty Swomitra K. , Gale Bruce K. , Misra Mano
Published: February 2020

Microfluidic photocatalytic reactors have advantages over conventional bulk reactors such as large surface-area-to-volume ratio and high control of fluid flow. Although titania nanotubular arrays (TNA) have shown enhanced photocatalytic degradation compared to nanoparticle films in a batch reactor configuration, their application in a microfluidic format has yet to be fully explored. The photocatalytic performance of a microfluidic reactor with TNA catalyst was compared with the performance of microfluidic format with TiO2 nanoparticulate (commercial P25) catalyst. The microfluidic device was fabricated using non-cleanroom based soft lithography, making it suitable for economical large-scale manufacturing. The photocatalytic performance was evaluated at different flow rates ranging from 25 to 200 μL/min. The TNA microfluidic system demonstrated enhanced photocatalytic performance over microfluidic TiO2 nanoparticulate layers, especially at higher flow rates (50-200 μL/min).

A computational model of the microfluidic format was developed to evaluate the effect of diffusion coefficient and rate constant on the photocatalytic performance. The improved performance of the TNA photocatalyst over the nanoparticle film can be attributed to higher generation of, and improved diffusion of oxidizing species. The model and microfluidic platform developed can easily be modified to suit other channel geometries and pollutants. A comparison of similar microfluidic photocatalytic platforms was made on the basis of dimensionless Peclet number. The system described in this study demonstrates high advective flow compared to otherwise similar microfluidic photocatalytic systems.
Enhanced Water Splitting at Thin Film WO3 Photoanodes Modified with the Electro-Catalysts
Authors: Solarska Renata , Bienkowski Krzysztof , Augustynski Jan , Kulesza Pawel J
Published: February 2020

An effective solar light-driven water splitting in photo-electrochemical devices requires use of materials which combine high photo-conversion efficiency and long term stability. For the latter reason the choice of the suitable systems is restricted to the semiconducting oxides that, in most cases, do not undergo photo-corrosion in aqueous solutions and are able to absorb efficiently the visible light. However, although the nanostructured scaffold of the semiconducting photo-electrodes contributes generally to decrease the bulk charge recombination by reducing the minority charge carriers pathway to reach the solution species, at the same time, it tends to enhance the recombination of charges at the surface due to the increased contact area with the electrolyte. Common approach to reduce recombination losses, involves deposition at the photo-electrode surface of electrocatalysts allowing to overcome slow kinetics of oxygen evolution. There are only few, precious metal (Pt, IrO2), oxygen-evolution electrocatalysts that can be used in pH acidc in which WO3 operates effectively. In this regard, we will present an increase of water photo-oxidation currents at WO3 photo-anodes, caused by presence of polyoxometalates (POMs), such as phosphomolybdate (PMoO40
3-) Keggin type ions. This class of compounds is known to undergo reversible, stepwise multi-electron transfer reactions that are combined with their excellent proton conductivity. What is more, the POMs might act as a buffer layer when form a cover for a metallic nano - particles largely used as a regular catalysts. The POMs may act either as relays for photogenerated electrons or as catalysts directly involved in the hole transfer reactions. In particular, they may affect the pathway of water oxidation by modifying the stability of the intermediates (e.g., peroxides) involved in this processes. Ongoing efforts devoted to minimize the bias voltage required to perform visible light-driven photo-oxidation of water at NS WO3 photoanodes also include the use of dopants or incorporation of plasmonic metal nanostructures allowing to enhance the light capture by the WO3 films1.

1. R. Solarska, K. Bienkowski, S. Zoladek, A. Majcher, T. Stefaniuk, P. J. Kulesza and J. Augustynski, Angew. Chem. Int. Ed.
2014, 53: DOI: 10.1002/anie.201408374
Effective Charge Extraction in the Heterogeneous Interfaces with TiO2 Nanoparticles/Nanotubes and Conducting Polymers
Authors: Park Taiho
Published: February 2020

The properties of interfaces between organic and inorganic materials in heterojunctions are relevant to a variety of electronic applications. The performances of such devices strongly depend on the kinetics of the charge transfer reactions and the thermodynamic properties (e.g., energetic difference at the junctions) at the heterogeneous interfaces. Heterogeneous interfaces can be controlled by modifying the interfacial properties to improve the photovoltaic performance. After electron injection into the conduction band (CB) from a dye excited by photon absorption, an electron relaxes to the lowest energy level of the CB and is subsequently trapped in the intra-band states. Trapping then increases the electron collection time through the TiO2 particles. The photoinduced electrons in the trap states can recombine with dye cations or the cationic hole transporting material. These recombination reactions are rapid (the characteristic electron recombination time, t
rec, is short) if many trap states in the lower-lying quasi-Fermi level of the TiO2 electrode exist. In this talk, I present how to control such recombination reactions in a molecular levels toward zero electron loss in organic based photocells.
Bismuth-Based Multimetal Oxides: Evaluation of Their Photoelectrochemical and Photocatalytic Properties
Authors: Subramanian Vaidyanathan
Published: February 2020

One of the approaches used to improve the photocatalytic activity of single metal oxides is to use a foreign atom as a dopant to facilitate improved light absorbance/charge separation. Unlike, single metal oxides, using multimetal oxides offer the option to include an additional element as a defacto part of the structure to aid in tuning the band edge positions, improving light absorbance, and enhancing photocatalysis.

Of particular interest is the use of bismuth which allows the shifting of the valence band closer to the conduction band to reduce bandgap. In this presentation, bismuth titanates will be examined as a case study on multimetal oxides. BTO will be evaluated for its photocatalytic and photoelectrochemical properties. The role of 3d elements in altering the photocatalytic properties of the BTO is presented. Photoelectrochemical techniques can aid in the fundamental understanding of the charge generation, separation, and transport in such oxides. To this end, the electrochemical responses of the bismuth titanate films and the effects of a third element inclusion will be discussed.

The insights drawn from this work can aid in designing new formulations that represent other oxide family groups such as scheelites, delafossites, or perovskites.
Theoretical and Experimental Study of Cobalt Spinel Oxides for Solar Driven Hydrogen Production
Authors: Yan Yanfa
Published: February 2020

Using sunlight and semiconductors to split water in a photoelectrochemical (PEC) process to generate hydrogen provides a viable way to produce renewable fuel. Metal oxide photoelectrodes are of particular interest due to their low cost and relatively high stability in aqueous media. Spinel Co-Fe-Al oxide system has shown the potential for PEC hydrogen production.  The electronic and optical properties of Co based spinel oxides, CoX2O4  (X =Al, Ga, In) are calculated using first-principle density functional theory. We show that the desirable properties for solar photoconversion can be obtained by controlling the alloy compositions. The ability to manipulate the electronic and optical properties is attributed to the different s-orbital energies and sizes of the cations. Our calculated bandgaps as function of composition provide detailed practical guidance for synthesizing Co based spinel oxides with electronic and optical properties necessary to achieve high efficiency PEC decomposition of water for hydrogen production by sunlight. Our experimental work demonstrated the tunable band gaps and the PEC responses of the Co based spinel oxides.
Gold-Based Nanomaterials for Catalytic CO2 Conversion Applications
Authors: Matranga Christopher
Published: February 2020

Managing carbon emissions is one of the most pressing issues currently faced by the energy sector.  In addition to technical challenges associated with the capture and storage of CO2, are cost issues that make implementation of these technologies impractical.  One interesting approach for dealing with these issues is to catalytically convert CO2 into liquid fuels, olefins, aromatics, and industrial chemicals that can be sold to offset carbon management costs.  This approach requires the development of novel catalysts capable of utilizing carbon-friendly forms of energy to activate CO2 and drive a chemical reaction.  My talk will focus on two Au-based catalyst currently being developed for CO2 applications.  The first system is based on atomically precise Au25

q
clusters (q = -1, 0, +1) where we have manipulated the electronic structure and ground-state charge of the cluster to tailor its interaction with CO2, H+, OH-, O2 and other key species in order to improve electrocatalytic activity and direct chemical reaction pathways.  The second system is based on Au-ZnO heterostructures where visible-light plasmonic excitations in Au nanoparticles are used to generate heat and drive chemical reactions on the ZnO substrate.  These two systems demonstrate how the size of Au nanoparticles can be altered to create dramatically different electronic structures (bound vs. free electrons), modulate catalytic activity, and impact how we finally utilize these materials for different catalysis applications.Managing carbon emissions is one of the most pressing issues currently faced by the energy sector.  In addition to technical challenges associated with the capture and storage of CO2, are cost issues that make implementation of these technologies impractical.  One interesting approach for dealing with these issues is to catalytically convert CO2 into liquid fuels, olefins, aromatics, and industrial chemicals that can be sold to offset carbon management costs.  This approach requires the development of novel catalysts capable of utilizing carbon-friendly forms of energy to activate CO2 and drive a chemical reaction.  My talk will focus on two Au-based catalyst currently being developed for CO2 applications.  The first system is based on atomically precise Au25

q
clusters (q = -1, 0, +1) where we have manipulated the electronic structure and ground-state charge of the cluster to tailor its interaction with CO2, H+, OH-, O2 and other key species in order to improve electrocatalytic activity and direct chemical reaction pathways.  The second system is based on Au-ZnO heterostructures where visible-light plasmonic excitations in Au nanoparticles are used to generate heat and drive chemical reactions on the ZnO substrate.  These two systems demonstrate how the size of Au nanoparticles can be altered to create dramatically different electronic structures (bound vs. free electrons), modulate catalytic activity, and impact how we finally utilize these materials for different catalysis applications.
Corrosion and Passivation of p-GaInP2 Photocathodes at Light-Dark Boundaries
Authors: Young James L , Döscher Henning , Deutsch Todd G , Turner John A
Published: February 2020

The tandem GaAs:GaInP2 PV:PEC (photovoltaic:photoelectrochemical) cell has achieved a record 12.4% solar-to-hydrogen water splitting efficiency, but the instability of the p-GaInP2 PEC junction greatly limits device lifetime. We have developed surface modifications to stabilize p-GaInP2 for 300+ hours, but failure can often be associated with corrosion that initiates at photoelectrode edges, beneath stationary bubbles, and at defects.  Common to these conditions that initiate corrosion, is the presence of sharp light-dark boundaries, which may accelerate corrosion. In this work, we investigate photo-corrosion behavior of p-GaInP2 at light-dark boundaries. A light-dark boundary was simulated by illuminating a ~2 mm diameter region at the center of a 6 mm diameter photoelectode and a set of durability tests were performed in 3M sulfuric acid at constant current.  After each test, samples were analyzed with optical microscopy, profilometry, and SEM with EDS and the corrosion products in solution were quantified with ICP-MS.  We observe a corrosion trench at the outer perimeter of the illuminated region and spatially-dependent corrosion features that correspond with the placement of the counter electrode as well as a region where corrosion is inhibited that corresponds with the direction of evolving H2 bubbles.  To explain these observations, we will present a set of experiments and measurements to show how spatial variation in surface band bending and H2 saturation can explain corrosion characteristics at light-dark boundaries.
Photoelectrochemical Performance of Composition Tuned BiOCl Nanosheet Arrays
Authors: Myung Yoon , Banerjee Sriya , Wu Fei , Banerjee Parag
Published: February 2020

BiOCl nanosheet arrays were successfully synthesized by successive ionic layer adsorption and reaction method onto transparent conducting electrode (TCO). The composition and band structure have been controlled by vacuum assisted annealing. The as-prepared materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, UV-Vis spectroscopy, Cyclic Voltammetry (CV) and Electrochemical Impedance spectroscopy (EIS). Our result indicate that 30 cycle deposited BiOCl nanosheet arrays have effectively covered surface of TCO electrode. Their composition and band structures were gradually changed from BiOCl to Bi12O15Cl6 with band gap range 3.5 eV to 2.7 eV by increasing annealing temperature. We investigated their phoelectrochemical performance and the photoelectrodes have an optimum photocurrent density of 100 μA/cm2 when annealed at 500 oC. The excellent photoelectrochemical properties of our photoelectrodes suggest that the BiOCl nanosheet arrays have potential applications in Solar energy harvesting.
Efficient Photocatalytic Hydrogen Production By Ta2O5 Nanotube Powders Sputtered Decorated with Ni Nanoparticles
Authors: Gonçalves Rentao , Wender Heberton
Published: February 2020

Modern life requires abundant and economical energy, and the sun is the best source of energy on earth. Research into materials and techniques which enable the transformation of energy from the sun has increased in recent decades due to the environmental problems associated with the use of fossil fuels. In this context, after the pioneering work of Honda and Fujishima using semiconductor photoanodes for water splitting,[1] photocatalytic hydrogen production has emerged as a low-cost alternative process for producing clean and renewable fuels. A number of oxide semiconductor photocatalysts, such as TiO2, NaTaO3 and Ta2O5, have been studied. They show reasonable photolysis efficiency, especially in Ta-based photocatalysts under UV light irradiation.[2-4] The photocatalytic activity can be drastically improved by loading NiO-Ni nanoparticle (NP) co-catalysts onto the surface of these Ta-based materials. In this respect, loading NiO-Ni NP co-catalysts onto Ta-based oxides has been reported as an important strategy for enhancing the overall activity of photocatalytic water splitting. Here we prepared very small Ni(0) nanoparticles (NPs) supported onto Ta2O5 nanotubes by magnetron sputtering deposition method. Ta2O5 nanotubes with 0.126 wt% of Ni showed superior photocatalytic activity for hydrogen production by water splitting when compared to pure Ta2O5 nanotubes, with a photonic efficiency estimated at 7.41%. In addition, insights into the role of Ni co-catalysts were obtained using in situ X-ray absorption spectroscopy (XAS) experiments performed at the Ta L3-edge and the Ni K-edge, both with and without simulated solar irradiation.

REFERENCES 

1.            A. Fujishima and K. Honda, Nature, 1972, 238, 37-38.

2.            P. Zhang, J. Zhang and J. Gong, Chem. Soc. Rev., 2014.

3.            Z. Zou, et al, Nature, 2001, 414, 625-627.

4.            R. V. Gonçalves, et al, J. Phys. Chem. C., 2012, 116, 14022-14030.
Facile Synthesis of Ultrafine Nanoparticles of Iron Based Oxides and Their Application in Solar Cells
Authors: Ahmmad Bashir , Hayasaka Shogo , Kanomata Kensaku , Basith M. a. , Kubota Shigeru , Hirose Fumihiko
Published: February 2020

Facile and eco-friendly methods have been developed for the fabrication of ultrafine nanoparticles of iron based oxides. Initially, mesoporous α-Fe2O3 nanoparticles were prepared by the green tea mediated hydrothermal method. The nanoparticles of α-Fe2O3 were highly pure and well crystallized with an average particle size of 60 nm. For the fabrication of ultrafine nanoparticles, the as-prepared  α-Fe2O3 were dispersed in propanol and sonicated for 1.5 h. TEM analysis confirmed the formation of single-crystalline nanoparticles with a mean size of 15 nm. The ultrafine nanoparticles showed improved performance when applied in a solar cell. We also prepared Gd and Ti doped BiFeO3 microparticles by solid state reaction technique, which was then subjected to ultrasonication for the fabrication of ultrafine nanoparticles. The average particles size of the sample was found to be 12 nm. The ultrafine nanoparticles and their bulk counterparts were successfully applied to a solar cell.
Photochemical Deposition of Co-Ac Catalyst on ZnO for Solar Water Oxidation
Authors: Irshad Ahamed , Munichandraiah N
Published: February 2020

Photo assisted electrochemical splitting of water is a promising method for solar energy storage as hydrogen fuel. However, the overall solar to fuel conversion efficiency is limited by the rapid recombination of photogenerated charge carriers and sluggish kinetics of water oxidation at the photoanode. Hence, the design of an efficient and cost effective co-catalyst for water oxidation is a challenging task. In the present study, a novel Co-based catalyst (Co-Ac) is prepared photochemically and used for photoelectrochemical water oxidation reaction in a neutral medium. Arrays of ZnO nanorods are deposited on a conducting glass substrate by an electrochemical method. By exposing ZnO in a neutral acetate solution containing Co(NO3)2 to UV radiation, Co-Ac is deposited on the surface of ZnO. Co-Ac acts as a co-catalyst to enhance the water oxidation kinetics and to reduce the charge carrier recombination at the semiconductor electrode. The on-set potential for oxygen evolution reaction is lower at the Co-Ac-ZnO photoanode in relation to the bare ZnO. There is a three to four fold increase in the photooxidation current due to the presence of Co-Ac on ZnO, indicating a high activity of the catalyst. This material is remarkable owing to its ease of formation, high catalytic activity and stability under operating conditions. Results of these studies will be presented.

Figure 1
Controlling Active Surface Area of TiO2 Nanotube Imprinted Ti Foils for Efficient Dye-Sensitized Solar Cells
Authors: Lin Lu-Yin , Yeh Min-Hsin , Chen WeiChieh , Ho KuoChuan , Ramamurthy Vittal
Published: February 2020

The counter electrode (CE) of dye–sensitized solar cells (DSSCs) plays an important role for transferring electrons and catalyzing the I−/I3
− reduction. Active surface area of the substrate determines the reduction sites of the deposited catalyst as well as the catalytic ability of the CE. A simple and effective method for enhancing and controlling the active surface area of metal foils is provided in this study. The Ti foils are imprinted by TiO2 nanotubes (TNT) via anodization and ultrasonic vibration techniques. The active surface area of imprinted Ti foils is controlled by varying the anodization voltage to produce TNT imprints with different diameters and depths. A solar–to–electricity conversion efficiency (η) of 9.35% was obtained for the DSSC with a TNT–imprinted Ti foil as the CE substrate, while the cell with an imprints–free Ti foil shows an η of 7.81%. The enhanced η is due to the improved electrocatalytic ability of the CE by using the TNT–imprinted Ti foil as the substrate with higher active surface area.