Hydrogen Generation from Irradiated Semiconductor-Liquid Interfaces Krishnan Rajeshwar Center for Renewable Energy Science & Technology (CREST) Department of Chemistry and Biochemistry The University of Texas at Arlington Arlington, TX 76019-0065 E-mail: [email protected] Chapter 7: Hydrogen Generation from Irradiated Semiconductor-Liquid Interfaces 7.1 Introduction and Scope ..........................................................................................3 7.2 Types of Approaches .............................................................................................7 7.3 More on Nomenclature and the Water Splitting Reaction Requirements ...............................................................12 7.4 Efficiency of Photoelectrolysis ............................................................................19 7.5 Theoretical Aspects ..............................................................................................25 7.6 Oxide Semiconductors .........................................................................................29 7.7 Nitrides, Oxynitrides and Oxysulfides .................................................................54 7.8 Metal Chalcogenide Semiconductors ..................................................................57 7.9 Group III-V Compound Semiconductors .............................................................61 7.10 Germanium and Silicon .......................................................................................65 7.11 Silver Halides .......................................................................................................66 7.12 Semiconductor Alloys and Mixed Semiconductor Composites ......................................................................68 7.13 Photochemical Diodes and Twin-Photosystem Configurations for Water Splitting ......................................................................71 7.14 Other Miscellaneous Approaches and Hydrogen Generation from Media Other than Water ..........................................72 7.15 Concluding Remarks ............................................................................................75 References ........................................................................................................................77 2 7.1 Introduction and Scope This chapter explores the possibility of using sunlight in conjunction with semiconductor/ electrolyte interfaces for the production of hydrogen from water and other suitable solvents. The underlying principles of solar energy conversion using semiconductor/electrolyte interfaces have been discussed in several review articles, book chapters and books,1-46 and will not be repeated here. This field of "photoelectrochemistry" had its early origins in attempts to use inorganic semiconductor/electrolyte interfaces in electronic devices.47-52 Subsequently, it was found that an electrochemical cell made from a n-TiO2 photoanode and a Pt counterelectrode evolved H2 53-57 and O2 from water under UV irradiation or even sunlight. In a historical sense, it is worth noting that H2 evolution had been observed at semiconductor (e.g., Ge)/electrolyte interfaces several years prior to the Japanese discovery, although the hydrogen evolution reaction (HER) occurred as a result of semiconductor photocorrosion.58 A flurry of activity ensued in the 1970s and 1980s on the photoelectrolysis of water; indeed, attempts to split water using sunlight and inorganic semiconductor(s) have continued in unabated manner to the present time. Table 1 contains a chronological listing of review articles summarizing the developments on this topic. Several books contain discussions on the photoelectrolysis of water4-8 and the reader also is referred to entire journal issues or book chapters (e.g., Refs. 57, 81-96) devoted to this subject. The present discussion builds on this body of literature by summarizing the state-of- the-art and future challenges. Every attempt is made here to cite the majority of articles that have appeared in the literature on this topic, although, inevitably, space constraints preclude an all-inclusive compilation. The interested reader can gain entry into the specialized literature using either the books, the reviews, or the selected articles cited below to highlight a particular 3 Table 1. Chronological listing of review articles on the photoelectrolysis of water. Entry number Title of review article Comments Reference Period 1975-1985 1 Photocatalytic Hydrogen This early review classifies solar 59 Production: A Solar Energy energy conversion methods according Conversion Alternative? to photosensitizer type. Review concludes that photoelectrolysis of water is the most promising scheme meriting further consideration. 2 p-n Photoelectrolysis Cells The concept of combining both n- and 60 p-type semiconductor electrodes for water splitting first discussed. 3 Solar Energy-Assisted Kinetic, energetic, and solid-state 61 Electrochemical Splitting of considerations in the search for Water suitable electrodes for water splitting elaborated. 4 Semiconducting Oxide A variety of binary and ternary oxide 62 Anodes in Photoassisted anodes for the photoassisted OER Electrolysis of Water discussed. 5 Photoelectrolysis of Water Appears to be the first comprehensive 63 with Semiconductors review article on the topic. 6 Design and Evaluation of Review in a similar vein as Ref. 62 64 New Oxide Photoanodes for (Entry 4 above) but with new data the Photoelectrolysis of from the authors' laboratory. Water with Solar Energy 7 Oxide Semiconductors in Once again, oxide electrodes are 65 Photoelectrochemical examined but with applicability Conversion of Solar Energy directed toward the conversion of solar energy into either electrical power or H2. 8 Conversion of Sunlight into Advances in the development of 29 Electrical Power and efficient regenerative Photoassisted Electrolysis of photoelectrochemical cells reviewed Water in with a brief discussion of p-type Photoelectrochemical Cells semiconductor photocathodes for the HER. 9 Artificial Photosynthesis: Review deals mostly with 30 Water Cleavage into microheterogeneous (particulate) Hydrogen and Oxygen by systems for water splitting. Visible Light 10 Hydrogen Evolving Solar Principles in the design of 66 Cells semiconductor electrodes, surface modification strategies, p-n junction cells, and photoelectrolysis by suspended semiconductor particles, discussed. 11 The Energetics of p/n The interfacial aspects of combining 67 Photoelectrolysis Cells both photoanodes and photocathodes discussed using both theory and experiment. 4 Period 1985-2005 12 Solar Hydrogen Production A general review on the subject with a 68 through Photobiological, section on semiconductor- and dye- Photochemical and based systems. Photoelectrochemical Assemblies 13 Overall Photodecomposition Review mostly dealing with 69 of Water on a Layered Niobate developments in the authors' laboratory Catalyst on particulate systems. 14 Artificial Photosynthesis: Various approaches based on the use of 40 Solar Splitting of Water to semiconductors discussed with a look Hydrogen and Oxygen at future prospects. 15 Solar Photoproduction of Review mainly addresses potential and 70 Hydrogen experimental efficiencies for four types of systems of which one comprises photoelectrolysis cells with one or more semiconductor electrodes. 16 Recent Progress of See Entry 13 above. 71 Photocatalysts for Overall Water Splitting 17 Photocatalytic Decomposition Principles of water-splitting reviewed 72 of Water including a section on semiconductor- based approaches. 18 Photo- and Mechano- See Entry 13 above. 73 Catalytic Overall Water Splitting Reactions to Form Hydrogen and Oxygen on Heterogeneous Catalysts 19 Multiple Band Gap The strategy of stacking 44 Semiconductor/Electrolyte semiconductors with variant Eg's Solar Energy Conversion discussed with a goal to enhance the overall process efficiency. 20 Development of Photocatalyst See Entry 13 above. 74 Materials for Water Splitting with the Aim at Photon Energy Conversion 21 New Aspects of See Entry 13 above. 75 Heterogeneous Photocatalysts for Water Decomposition 22 Photoelectrochemical As in Entry 5, a comprehensive review 76 Hydrogen Generation from which outlines the principles, R&D Water Using Solar Energy. progress, impact on hydrogen Materials-Related Aspects economy, and cost issues. 23 Photocatalytic Materials for See Entry 13 above. 77 Water Splitting 24 Photocatalytic Water Splitting See Entry 13 above. 78 into H2 and O2 over Various Tantalate Photocatalysts 5 25 Strategies for the See Entry 13 above. 79 Development of Visible- Light-Driven Photocatalysts for Water Splitting 26 Metal Oxide Photoelectrodes Topics reviewed include preparation of 80 for Hydrogen Generation oxide electrodes, sensitization of wide Using Solar Radiation Driven band gap oxides, tandem cells, solid Water Splitting solutions of oxides and porous/nano- crystalline materials. 6 development. An annotated bibliography does exist for the period, 1975-1983, for photoelectrochemical cell studies with semiconductor electrodes.97 7.2 Types of Approaches Figure 1 illustrates the interfacial energetics involved in the photoelectrochemical evolution of H2. Thus, the electronic energy levels in the semiconductor and in the contacting solution are shown on a common diagram. In a semiconductor, the filled electronic levels (valence band or VB) and the empty levels (conduction band or CB) are separated by a 98-100 "forbidden" gap, namely,