Volume 234, Issue 4 - Special Issue: DFG Priority Program SPP 1613 “Fuels Produced Regeneratively Through Light-Driven Water Splitting” / Guest Editors: Bernhard Kaiser and Wolfram Jaegermann

As is obvious from previous work on semiconductor photoelectrochemistry, single junction semiconductors do not provide either the required maximum photovoltage or a high photocurrent for solar water splitting, which is required for efficient stand-alone devices. From these experiences we conclude, that multi-junction devices must be developed for bias-free water splitting. In this article we present our design considerations needed for the development of efficient photo-electro-synthetic cells, which have guided us during the DFG priority program 1613. At first, we discuss the fundamental requirements, which must be fulfilled to lead to effective solar water splitting devices. Buried junction and photoelectrochemical arrangements are compared. It will become clear, that the photovoltaic (PV) and electrochemical (EC) components can be optimized separately, but that maximized conversion efficiencies need photovoltages produced in the photovoltaic part of the device, which are adapted to the electrochemical performance of the electrolyzer components without energetic losses in their coupling across the involved interfaces. Therefore, in part 2 we will present the needs to develop appropriate interface engineering layers for proper chemical and electronic surface passivation. In addition, highly efficient electrocatalysts, either for the hydrogen or oxygen evolution reaction (HER, OER), must be adjusted in their energetic coupling to the semiconductor band edges and to the redox potentials in the electrolyte with minimized losses in the chemical potentials. The third part of our paper describes at first the demands and achievements on developing multijunction thin-film silicon solar cells. With different arrangements of silicon stacks a wide range of photovoltages and photocurrents can be provided. These solar cells are applied as photocathodes in integrated directly coupled PV-EC devices. For this purpose thin Pt and Ni catalyst layers are used on top of the solar cells for the HER and a wire connected RuO 2 counter electrode is used for the OER. Electrochemical stability has been successfully tested for up to 10,000 s in 0.1 M KOH. Furthermore, we will illustrate our experimental results on interface engineering strategies using TiO 2 as buffer layer and Pt nanostructures as HER catalyst. Based on the obtained results the observed improvements, but also the still given limitations, can be related to clearly identified non-idealities in surface engineering either related to recombination losses at the semiconductor surface reducing photocurrents or due to not properly-aligned energy states leading to potential losses across the interfaces.

Chemical surface segregation is a design variable in the optimization of phocathodes but has largely been investigated through surface passivation or decoration. In this study a long charge carrier lifetime material, Al–Cr–Fe–O, exhibiting strong photocurrent recombination is investigated for its atomic scale crystallographic and chemical inhomogeneity. Combined scanning transmission electron microscopy and atom probe tomography unveils that insulating Al- and Cr-rich surface layers form during processing. These are discussed to be the primary reason for experimentally observed charge carrier recombination. This study highlights the importance of processing in the design, discovery and optimization of new light absorber materials for photoelectrochemical water splitting.

Photo-electrochemical (PEC) water splitting (WS) using metal oxide semiconductors is regarded as a promising approach for the renewable production of fuels and energy vectors such as hydrogen (H 2 ). Among metal oxide semiconductors, iron oxide in the form of hematite (α-Fe 2 O 3 ) is one of the most researched photo-anode materials, mainly due to its ability to absorb photons up to 600 nm combined to a set of desirable properties such as high photocorrosion resistance, environmental friendliness, large abundance and relatively low production costs. However, hematite main disadvantages are a low electrical conductivity and a high rate of charge recombination; both these shortcomings drastically limit functionality and efficiency of hematite-based photo-anodes in PEC devices. One-dimensional (1D) nanostructuring is a powerful tool to tackle such disadvantages as it provides the photoelectrode material with increased surface area along with directional charge transport properties and short charge diffusion distances to the electrolyte – these features can improve the lifetime of photo-generated charges and/or enhance the charge transfer efficiency, and can consequently lead to a superior photo-electrochemical performance. At the same time, chemical/physical modification can also compensate natural weaknesses of hematite in water photoelectolysis. The present mini-review outlines a series of most effective strategies for the fabrication of 1D hematite nanostructures as well as for their physicochemical modification, mainly by doping or co-catalyst decoration, to achieve superior PEC activity.

The oxygen evolution reaction (OER) is a thermodynamically and kinetically demanding process, therefore it requires the use of catalysts enabling to meet technologically relevant conditions. Here, we realized efficient OER catalysts fabricated using relatively cheap precursors consisting of earth-abundant metal oxides, i.e. nickel, iron, and chromium, and a simple one-step preparation method. It is shown that the catalyst is activated by anodic polarization under irradiation with visible light, which allows decreasing the overpotential necessary for the OER.

Removal of drug residuals in waste water by easy-separable catalysts is of fundamental interest for clean water provision by photocatalytic water remediation. Here, we present our studies to use the ferrimagnetic earth-abundant absorber MgFe 2 O 4 , prepared by microwave-assisted synthesis, for the photocatalytic removal of methylene blue and tetracycline from model solutions under dark and sunlight conditions. The surface of MgFe 2 O 4 has a strong influence on the tetracycline removal, while more than 75% tetracycline degradation can already be achieved in 2 h with unmodified MgFe 2 O 4 . Using surface-modified MgFe 2 O 4 nanoparticles, the adsorption of tetracycline is even more pronounced, however goes in line with release of the surface capping agent.

BiVO 4 is one of the most investigated and most promising metal oxide based photoanode materials for photoelectrochemical (PEC) water oxidation. Although it has several advantages (suitable band gap around 2.4 eV, suitable valence-band position for water oxidation, low toxicity, high abundance), it suffers from slow charge-carrier transport properties, high surface recombination, and limited water-oxidation activity. In the present work, we review the synthesis and doping strategies that we developed in the last years to improve the PEC performance of BiVO 4 photoanodes. Strategies ranging from single anion doping or cation doping to anion and cation co-doping will be presented for fluoride and molybdenum as anion and cation dopants, respectively. One major result is that co-doping allows combining the most important PEC specific benefits of each type of dopant, i.e. an increased charge-injection efficiency in case of fluoride as well as an increased charge-separation efficiency in case of molybdenum.

Hematite-based photoanodes have been intensively studied for photoelectrochemical water oxidation. The n -type dopant Sn has been shown to benefit the activity of hematite anodes. We demonstrate in this study that Sn-doped hematite thin films grown by atomic layer deposition can achieve uniform doping across the film thickness up to at least 32 mol%, far exceeding the equilibrium solubility limit of less than 1 mol%. On the other hand, with the introduction of Sn doping, the hematite crystallite size decreases and many twin boundaries form in the film, which may contribute to the low photocurrent observed in these films. Density functional theory calculations with a Hubbard U term show that Sn doping has multiple effects on the hematite properties. With increasing Sn 4+ content, the Fe 2+ concentration increases, leading to a reduction of the band gap and finally to a metallic state. This goes hand in hand with an increase of the lattice constant.

The temporal evolution of photogenerated carriers in CuWO 4 , CuO and WO 3 thin films deposited via a direct chemical vapor deposition approach was studied using time-resolved microwave conductivity and terahertz spectroscopy to obtain the photocarrier lifetime, mobility and diffusion length. The carrier transport properties of the films prepared by varying the copper-to-tungsten stoichiometry were compared and the results related to the performance of the compositions built into respective photoelectrochemical cells. Superior carrier mobility was observed for CuWO 4 under frontside illumination.

This paper gives an overview about recent theoretical and experimental work on electronic and optical properties of spinel ferrites M Fe 2 O 4 . These compounds have come into focus of research due to their possible application as photocatalyst material for photoelectrochemical water splitting. The theoretical background of state-of-the-art quantum-chemical approaches applied for predicting electronic and optical band gaps, absolute band positions, optical absorption spectra, dielectric functions and Raman spectra, is briefly reviewed. Recent applications of first-principles methods on magnetic and electronic properties of ferrites with M = Mg and the first row of subgroup elements Sc to Zn are presented, where it is shown that the fundamental band gap is strongly dependent on the spin state and the degree of inversion of the spinel structure. The observed variation of electronic properties may serve as an explanation for the large scattering of experimental results. The exchange of M and Fe cations has also a pronounced effect on the Raman spectra of ferrites, which is analyzed at atomic scale from first principles. Calculated optical absorption spectra of ferrites are compared to experimental spectra. The electronic nature of the first excitations and the role of oxygen vacancies are discussed. For the calculation of absolute band positions, which have a significant impact on the photoelectrochemical activity of the ferrites, models of the most stable ferrite surfaces are developed that take into account their polar nature and the interaction with the solvent. Theoretically predicted valence and conduction band edges are compared to results from electrochemical measurements. The role of cation exchange on the surface electronic structure is investigated both theoretically and experimentally.