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Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn

The transition metal dichalcogenides are about 60 in number. Two-thirds of these assume layer structures. Crystals of such materials can be cleaved down to less than 1000 A and are then transparent in the region of direct band-to-band transitions. The transmission spectra of the family have been correlated group by group with the wide range of electrical and structural data available to yield useful working band models that are in accord with a molecular orbital approach. Several special topics have arisen; these include exciton screening, d-band formation, and the metal/insulator transition; also magnetism and superconductivity in such compounds. High pressure work seems to offer the possibility for testing the recent theory of excitonic insulators.

The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties

Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe(2)O(3)) has emerged as a promising photo-electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion.

Solar Water Splitting: Progress Using Hematite (α‐Fe2O3) Photoelectrodes

The behavior of semiconducting electrodes for photoelectrolysis of water is examined in terms of the physical properties of the semiconductor. The semiconductor‐electrolyte junction is treated as a simple Schottky barrier, and the photocurrent is described using this model. The approach is appropriate since large‐band‐gap semiconductors have an intrinsic oxygen overpotential which removes the electrode reaction kinetics as the rate‐limiting step. The model is successful in describing the wavelength and potential dependence of the photocurrent in WO3 and allows a determination of the band gap, optical absorption depth, minority‐carrier diffusion length, flat‐band potential, and the nature of the fundamental optical transition (direct or indirect). It is shown for WO3 that minority‐carrier diffusion plays a limited role in determining the photoresponse of the semiconductor‐electrolyte junction. There are indications that the diffusion length in this low carrier mobility material is determined by diffusion‐controlled bulk recombination processes rather than the more common trap‐limited recombination. It is also shown that the fundamental optical transition is indirect and that the band‐gap energy depends relatively strongly on applied potential and electrolyte. This effect seems to be the result of field‐induced crystallographic distortions in antiferroelectric WO3.

Photoelectrolysis and physical properties of the semiconducting electrode WO2

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

Tungsten trioxide as an electrode for photoelectrolysis of water

Photoassisted electrolysis of water using single crystal α-Fe2O3 anodes

Small-Polaron Hopping Motion in Some Chalcogenide Glasses

Auger and X-ray photoelectron spectroscopic and electrochemical characterization of titanium thin film electrodes

A variety of compositions of glass in the As-Te-I, As-Te-Ge, and As-Te chalcogenide series have been prepared and structurally analyzed The analytical techniques included x-ray powder diffraction, density determinations, and differential-scanning-calorimetry experiments The temperature dependence of the Hall mobility, Seebeck coefficient, and dc conductivity was determined for six representative alloy compositions The Hall mobility was found to be $n$-type, low, and activated with nearly equal activation energies in each case The conductivity mobility was deduced from the $p$-type Seebeck coefficients and the conductivity measurements to have an activated character with a characteristic energy of 019 \ifmmode\pm\else\textpm\fi{} 002 eV for all the measured compositions These results appear to be inconsistent with the familar Cohen-Fritzsche-Ovshinsky (CFO) picture for transport in amorphous semiconductors, and a new model for conduction in these glasses is proposed This model involves the hopping of holelike small polarons between sites associated with a common As-Te structural entity which is postulated to be present in all the alloys measured here The sign, magnitude, and temperature dependence of the dc conductivity, Seebeck coefficient, and Hall mobility are all consistent with this model

Rod K. Quinn

Papers

Tungsten trioxide as an electrode for photoelectrolysis of water

Photoassisted electrolysis of water using single crystal α-Fe2O3 anodes

Small-Polaron Hopping Motion in Some Chalcogenide Glasses

Auger and X-ray photoelectron spectroscopic and electrochemical characterization of titanium thin film electrodes

Electrical Transport and Structural Properties of Bulk As-Te-I, As-Te-Ge, and As-Te Chalcogenide Glasses