Preface
Finding new ways to power the future by making cleaner and safer energy without creating additional CO2 in the atmosphere is one, if not simply the most important, problem facing humanity today. Indeed, meeting the current and future demand for energy, which according to various sources should double by 2050, while managing the environmental consequences of energy production and consumption, is of crucial importance nowadays, with the industrial and economic rise of highly populated countries...
Layered Structures
In studies on nanostructured thin films, layered structures have also attracted the attention of researchers and are known to offer considerably higher phototoelectrochemical and photo-catalytic activity. A combination of small- and large-bandgap material deposited one over the other, as shown in Figure 13.11, may absorb the full solar spectrum more efficiently. If energy-band edges match at the junction, better and efficient separation of photogenerated electrons and holes is possible. Liou et...
Theoretical Calculations ofTiO2 Surfaces and Adsorbents
Among the possible rutile surfaces, the surface designated with a Miller index of 110 is the most stable 29 . This surface can assume either stoichiometric or reduced forms. The stoichiometric TiO2 110 surface has oxygen atoms placed on the titanium atoms protruding from the outermost layer. The oxygen atoms bridge two titanium atoms and thus are called bridging oxygen atoms. The bridging oxygen atoms are loosely bonded to the surface and tend to be missing, depending on the external...
Macrocrystalline WO3 Films
In those early experiments, WO3 photoanodes were illuminated either by the large spectral range output of xenon lamps or, in order to determine the spectral photoresponse, with monochromatic light of varying wavelengths whilst being polarized against a metal cathode in aqueous solutions of different pH. This resulted in photoelectrolysis of water assisted by an external bias, with hydrogen generated at the cathode and oxygen formed at the photoanode. The bandgap energies, derived from...
Nanostructured Films
It had been the search for semitransparent photoanodes, which could be employed in a photoelectrolysis cell PEC , in conjunction with a photovoltaic PV cell to form a tandem device 5,15 , that led to the development of nanostructured WO3 films. Such films consist of a network of WO3 particles with sizes in the range of tens of nanometers, as shown in a typical scanning electron micrograph SEM in Figure 12.2. Due to their relatively large and evenly distributed porosity, the whole films, which...
Choice of Electrolytes
The electrolyte composition and its pH play an important role in affecting the PEC response, onset potential and flatband potential of the semiconductor electrode in PEC cells. The rate of transfer of carriers at the junction is enhanced by proper band-edge matching of the semiconductor with the redox level of water. However, the positions of the conduction and valence bands of semiconductors are dependent on pH. An increase in pH results in a shift in the conduction- and valence-band positions...
Crystal Structure of TiO2
TiO2 exists as three polymorphs, brookite, anatase and rutile. Among these, rutile is the most stable phase. Both the anatase and rutile structures are commonly used in photocatalysis applications. Figure 10.1 illustrates the crystal structures of TiO2 in the form of TiO6 octahedrons. Each octahedron can be considered as one Ti4 cation surrounded by six O2 anions. In rutile, each distorted octahedron is in contact with ten neighbor octahedrons two sharing edge oxygen pairs and eight sharing...
Potentiostatic Anodization
Prakasam et al. first synthesized a self-organized nanoporous iron III oxide thin film by potentiostatic anodization of iron foil at different anodization potentials 58 . For this purpose, 0.25 mm thick pure iron foil was used and anodized in a two-electrode system with iron foil as the anode and platinum foil as the counter-electrode. The electrolytic medium used in the study consisted of 1 HF 0.5 NH4F 0.2 0.1 M HNO3 in glycerol pH3 at 10 C. The current time behavior during the potentiostatic...
Photoelectrochemical and Photovoltaic Properties
11.7.1 Dye-Sensitized Solar Cells DSSCs The photoelectrochemical properties of electrodeposited porous ZnO films have been especially investigated in view of the use of these films in DSSCs and similar solar-cell concepts. This is due to the very similar bandgap and band positions of ZnO compared to TiO2, which was used in the original concept for DSSCs. Electrodeposition of porous ZnO for ZnO films has the advantage of film preparation at relatively low temperatures, which allows film...
Structural and ElectricalElectronic Properties
a-Fe2O3, known as hematite, has been a material of interest in PEC splitting of water for a long time. It is isostructural with FeTiO3, a-Al2O3, Cr2O3 and V2O3. The crystal structure of a-Fe2O3 comprises distorted hexagonal close packing of oxygen atoms with 2 3 octahedral interstices occupied by Fe. Thus, each Fe atom has six oxygen neighbors and each oxygen atom has four Fe neighbors. In 1954, Morin 1 presented the electrical properties of a-Fe2O3, and suggested two alternative models. The...
Importance of Theoretical Studies on TiO2 Systems
Since the discovery of the Fujishima Honda effect 1 , the surface chemistry of TiO2 has been expected to play a central role in solar hydrogen generation processes. In addition, strong photocatalytic activity 2 and photoinduced hydrophilic conversion phenomena 3 have been observed on TiO2 surfaces. A dye-sensitized TiO2 surface can be used as a building block for solar-cell devices 4 . Introduction of some impurities in TiO2 thin films induces significant changes in their magnetic 5 and...
Speculated Active Species on TiO2 Superoxide Anion O2 and the Hydroxyl Radical
Complete understanding of the photocatalytic reaction process on a TiO2 surface has not been established. Initially, the reaction was conjectured to involve several reactive intermediates 2,59 61 a superoxide anion O2 , hydroxyl radicals OH' , and surface-trapped electron holes h . The OH' radical was considered to be a product of the reaction between a surface hydroxyl group and a photogenerated hole However, several researchers denied the direct production of hydroxyl radicals. In reports,...
List of Contributors
Bruce D. Alexander, University of Greenwich, UK. Shamindri M. Arachchige, Virginia Polytechnic Institute and State University, USA. Jan Augustynski, Warsaw University, Poland. Email Jan.Augustynski unige.ch Marcus Bar, Helmholtz-Zentrum Berlin fur Materialien und Energie, Berlin, Germany. Email marcus.baer helmholtz-berlin.de Flemming Besenbacher, Aarhus University, Denmark. Email fbe inano.dk Karen J. Brewer, Virginia Polytechnic Institute and State University, USA. Email kbrewer vt.edu Lin X....
Contents
PART ONE FUNDAMENTALS, MODELING, AND EXPERIMENTAL INVESTIGATION OF PHOTOCATALYTIC REACTIONS FOR DIRECT SOLAR HYDROGEN GENERATION 1 Solar Hydrogen Production by Photoelectrochemical Water Splitting The Promise and Challenge 3 1.3 Solar Pathways to Hydrogen 5 1.3.1 The Solar Resource 5 1.3.2 Converting Sunlight 6 1.3.3 Solar-Thermal Conversion 7 1.3.4 Solar-Potential Conversion 8 1.3.5 Pathways to Hydrogen 9 1.4 Photoelectrochemical Water-Splitting 10 1.4.2 PEC Water-Splitting Reactions 10 1.4.3...