The civilian, commercial, and defense sectors of most advanced industrialized nations are faced with a tremendous set of environmental problems related to the remediation of hazardous wastes, contaminated groundwaters, and the control of toxic air contaminants. For example, the slow pace of hazardous waste remediation at military installations around the world is causing a serious delay in conversion of many of these facilities to civilian uses. Over the last 10 years problems related to hazardous waste remediation have emerged as a high national and international priority.

Environmental Applications of Semiconductor Photocatalysis

Bismuth vanadate (BiVO4) has a band structure that is well-suited for potential use as a photoanode in solar water splitting, but it suffers from poor electron-hole separation. Here, we demonstrate that a nanoporous morphology (specific surface area of 31.8 square meters per gram) effectively suppresses bulk carrier recombination without additional doping, manifesting an electron-hole separation yield of 0.90 at 1.23 volts (V) versus the reversible hydrogen electrode (RHE). We enhanced the propensity for surface-reaching holes to instigate water-splitting chemistry by serially applying two different oxygen evolution catalyst (OEC) layers, FeOOH and NiOOH, which reduces interface recombination at the BiVO4/OEC junction while creating a more favorable Helmholtz layer potential drop at the OEC/electrolyte junction. The resulting BiVO4/FeOOH/NiOOH photoanode achieves a photocurrent density of 2.73 milliamps per square centimenter at a potential as low as 0.6 V versus RHE.

https://science.sciencemag.org/content/sci/early/2014/02/12/science.1246913.full.pdf

Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting

There is a growing interest in the conversion of water and solar energy into clean and renewable H2 fuels using earth-abundant materials due to the depletion of fossil fuel and its serious environmental impact. This critical review highlights some key factors influencing the efficiency of heterogeneous semiconductors for solar water splitting (i.e. improved charge separation and transfer, promoted optical absorption, optimized band gap position, lowered cost and toxicity, and enhanced stability and water splitting kinetics). Moreover, different engineering strategies, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering of heterogeneous semiconductors are summarized and discussed thoroughly. The synergistic effects of the different engineering strategies, especially for the combination of co-catalyst loading and other strategies seem to be more promising for the development of highly efficient photocatalysts. A thorough understanding of electron and hole transfer thermodynamics and kinetics at the fundamental level is also important for elucidating the key efficiency-limiting step and designing highly efficient solar-to-fuel conversion systems. In this review, we provide not only a summary of the recent progress in the different engineering strategies of heterogeneous semiconductors for solar water splitting, but also some potential opportunities for designing and optimizing solar cells, photocatalysts for the reduction of CO2 and pollutant degradation, and electrocatalysts for water splitting.

Engineering heterogeneous semiconductors for solar water splitting

Photo-electrochemical hydrogen generation from water using solar energy. Materials-related aspects

In the present review we try to give a comprehensive and most up to date view to the field, with an emphasis on the currently most investigated anodic TiO2 nanotube arrays. We will first give an overview of different synthesis approaches to produce TiO2 nanotubes and TiO2 nanotube arrays, and then deal with physical and chemical properties of TiO2 nanotubes and techniques to modify them. Finally, we will provide an overview of the most explored and prospective applications of nanotubular TiO2.

One-dimensional titanium dioxide nanomaterials: nanotubes.

1. The Solid and the Solution.- 1.1. The Solid.- 1.1.1. Donors, Acceptors, and Traps.- 1.1.2. Energy Levels at the Surface.- 1.1.3. Conductance in Solids.- 1.2. The Solution.- 1.2.1. Introduction.- 1.2.2. The Electrode Fermi Energy as a Function of the Redox Couples in Solution.- 1.2.3. The Relation between the Hydrogen and the Vacuum Scales of Energy.- 1.2.4. Fluctuating Energy Levels in Solution.- 1.2.5. The Energy Levels Associated with Two-Equivalent Ions.- 1.2.6. Conductance in Liquids.- 2. The Solid/Liquid Interface.- 2.1. Surface Ions and Their Energy Levels.- 2.1.1. Adsorption.- 2.1.2. Surface States at the Solid/Liquid Interface.- 2.2. Double Layers at the Solid/Liquid Interface.- 2.2.1. General.- 2.2.2. The Gouy Layer.- 2.2.3. The Helmholtz Double Layer.- 2.2.4. The Space Charge Double Layer in the Semiconductor.- 2.3. Theoretical Predictions of the Energy Levels of Band Edges.- 2.4. The Band Model of the Solid/Solution Interface.- 3. Theory of Electron and Hole Transfer.- 3.1. Introduction.- 3.1.1. General.- 3.1.2. The Activation Energy in Electrode Reactions.- 3.2. Classical Model.- 3.3. The Energy Level Model of Charge Transfer.- 3.3.1. General.- 3.3.2. The Metal Electrode.- 3.3.3. The Semiconductor Electrode.- 3.4. Qualitative Description of Electrode Processes Using the Band Model.- 3.4.1. The Behavior of the Metal Electrode.- 3.4.2. The Behavior of the Semiconductor Electrode.- 3.4.3. The Transition between Semiconductor and Metallic Behavior.- 4. Measurement Techniques.- 4.1. Capacity Measurements.- 4.1.1. Introduction.- 4.1.2. Measurement Theory.- 4.1.3. Analysis.- 4.1.4. Complex Mott-Schottky Plots.- 4.1.5. Determination of Band Edges.- 4.2. Measurements of the Current/Voltage Characteristics.- 4.2.1. General Techniques Voltammetry.- 4.2.2. Rotating Electrodes.- 4.2.3. Illumination.- 4.3. Other Techniques.- 4.3.1. Techniques for Vs Measurement.- 4.3.2. Techniques to Determine Surface Species or Phases.- 4.3.3. Techniques to Study Electrode Reactions.- 5. The Properties of the Electrode and Their Effect on Electrochemical Measurements.- 5.1. The Behavior of the Perfect Crystal.- 5.1.1. The Helmholtz Double Layer: The Surface Charges on the Electrode.- 5.1.2. The Space Charge Region of the Perfect Crystal.- 5.2. The Behavior of Electrode Defects.- 5.2.1. Introduction.- 5.2.2. Deviations of Mott-Schottky Plots Due to Bulk Flaws.- 5.2.3. Current Flow Associated with Bulk Flaws.- 5.3. Observed Flat Band Potentials for Various Semiconductors.- 6. Observations of Charge Transfer at an Inert Semiconductor Electrode.- 6.1. Introduction.- 6.2. Majority Carrier Capture.- 6.2.1. Direct Carrier Transfer to Ions in Solution.- 6.2.2. Indirect Electron Transfer to Ions in Solution.- 6.3. Minority Carrier Capture.- 6.3.1. Minority Carrier Capture on Two-Equivalent Species: Radical Formation and Current Doubling.- 6.3.2. Minority Carrier Capture by One-Equivalent Ions.- 6.3.3. Photocatalysis.- 6.4. Intrinsic Surface States and Recombination Centers.- 6.4.1. Intrinsic Surface States as Carrier Transfer Centers.- 6.4.2. Intrinsic Surface States and Ions in Solution as Recombination Centers.- 6.5. Carrier Injection.- 6.5.1. Direct Electron and Hole Injection.- 6.5.2. Injection by Tunneling.- 6.5.3. Injection by Optically Excited Ions: Dye Injection.- 6.6. High-Current, High-Voltage Processes.- 6.6.1. Introduction.- 6.6.2. High Currents with Accumulation Layers.- 6.6.3. Tunneling and Breakdown on Non-Transition-Metal Semiconductors.- 6.6.4. Practical Electrodes.- 6.7. Analysis of Complicated Electrode Reactions using the Tools of Semiconductor Electrochemistry.- 6.7.1. The Photocatalytic Oxidation of Formic Acid.- 6.7.2. Analysis of the Energy Levels of Two-Equivalent Species.- 6.7.3. The Reduction of Iodine on CdS.- 7. Chemical Transformation in the Electrode Reaction.- 7.1. Introduction.- 7.2. Inner Sphere Changes during Redox Reactions at an Inert Electrode.- 7.3. Adsorption onto and Absorption into the Electrode.- 7.3.1. Adsorption of Water, Hydrogen, and Oxygen.- 7.3.2. Adsorption of Electrolyte Ions.- 7.3.3. Action of Deposited Species.- 7.3.4. Movement of Impurities and Defects into the Electrode.- 7.4. Corrosion.- 7.4.1. Introduction.- 7.4.2. Theory and Observations of Semiconductor Corrosion.- 7.4.3. Stabilizing Agents to Prevent Corrosion.- 8. Coated Electrodes.- 8.1. Introduction.- 8.1.1. The Band Model for Oxide Films.- 8.1.2. Thin Films.- 8.1.3. The Structure of Thick Films.- 8.2. Current Transport through Oxide Films.- 8.2.1. Thin Oxide Layers.- 8.2.2. Model of Electronic Conduction through Thick Coherent Layers.- 8.2.3. Semiconducting Oxide Layers on Metal Electrodes.- 8.2.4. Insulating Layers on Metal and Semiconductor Electrodes.- 8.3. Deposition of Reaction Products on Semiconductor Electrodes.- 9. Applications of Semiconductor Electrodes.- 9.1. Solar Energy Conversion.- 9.1.1. Introduction.- 9.1.2. Photovoltaic Cells.- 9.1.3. Conversion of Optical to Chemical Energy.- 9.1.4. Corrosion of PEC Cells.- 9.1.5. The Future Potential of PEC Solar Cells.- 9.2. Electrocatalysis on Semiconductors.- 9.2.1. General.- 9.2.2. Surface State Additives and Narrow Bands in Electrocatalysis.- 9.3. New Devices.- 9.4. Electropolishing of Semiconductors.- References.- References to Review Articles and Books.- Author Index.

/pdf/electrochemistry-at-semiconductor-and-oxidized-metal-50my4z5ggs.pdf

S. Roy Morrison

Papers

Electrochemistry at Semiconductor and Oxidized Metal Electrodes