Abstract: In order to enable mass production of photocatalytically active electrodes for direct conversion of solar energy into hydrogen, abundant materials and inexpensive preparation methods are needed. BiVO 4 photoelectrodes prepared by aerosol deposition satisfy these requirements, in addition offering reasonable photocurrent densities, and can potentially contribute to the establishment of a prospective economy based on hydrogen as energy source. Nevertheless, in order to allow the economically feasible implementation of BiVO 4 electrodes for direct water splitting, there is need for further improvement of the electrodes efficiency. In particular, an increased understanding of factors limiting the photocurrent is needed. In this work, BiVO 4 photoelectrodes, prepared by aerosol deposition of BiVO 4 particles on FTO-substrates, were investigated by surface photovoltage (SPV) as well as photocurrent and Mott-Schottky measurements. Surface photovoltage spectroscopy under front and back illumination gives evidence of limitation of charge transport by a significant lower mobility of electrons compared to holes, leading to a nearly tenfold higher photocurrent, when illuminated from the back, compared to illumination from the front. Furthermore, the presence of traps for holes as well as for electrons on the BiVO 4 surface has been confirmed by transient surface photovoltage spectroscopy and their influence on the photocurrent is discussed. Figure 1

Abstract: Cold gas spraying (CGS) is presented as an innovative approach to deposit semiconductor particles onto substrates in order to form photoelectrodes for electrochemical applications. Thin layers of TiO 2 (P25-20 by Evonik Industries) are deposited onto titanium substrates (TiO 2 -CGS films) at different temperatures of the gas carrier within the CGS process (300 - 1000 °C). Structural characterization reveals unchanged bulk properties of the TiO 2 nanoparticles. Clearly, the short duration time of the CGS process hinders crystalline bulk changes of the TiO 2 particles in the hot gas stream. However, surface photovoltage measurements indicate that the CGS process modified defect states at the surface when exposed to different gas temperature. In photoelectrochemical measurements TiO 2 -CGS films yield seven times higher photocurrents and IPCE values than comparable films prepared by the well-established doctor blade technique. The increased efficiency might be due to an enhanced particle to substrate bonding caused by particles welding to the metallic substrate during the cold gas spray process.

Abstract: Photoassisted water splitting for hydrogen generation requires the development of low cost, but highly efficient photoelectrodes. For the industrial hydrogen production with photoelectrochemical cells suitable catalysts and feasible photoelectrode preparation processes still need to be identified. This contribution explores the potential of cold gas spraying (CGS) for the production of photoanodes employing semiconductors for the water oxidation reaction (OER). Conventional large area coating techniques usually employ wet chemical methods with subsequent calcination steps to obtain enhanced binding between the catalyst particles and the substrate. In the cold gas spraying process, particles are accelerated to high velocities by a pressurized gas. The nitrogen used as process gas is preheated and then expanded in a De Laval type nozzle. On impact with the substrate, the particles deform and break up and thus can build an efficient interface to the back contact. For a first demonstration TiO 2 aggregates delivered by EVONIK were probed for the preparation of TiO 2 photoelectrodes. In photoelectrochemical experiments these cold gas sprayed TiO 2 photoelectrodes showed seven times higher photocurrents in the photooxidation of water (at 1.23V(NHE)) than reference electrodes prepared by the established doctor blade technique. In systematic experiments it was observed that with increasing gas temperature in the coating process the obtained photocurrent is enhanced. The better performance can be mainly attributed to an improved bonding of the TiO 2 particles to the substrate (back contact) due to their increased impact energy. Although the bulk characteristics of the particles remained unchanged incident photon to current efficiency (IPCE) and surface photovoltage measurements reveal the formation of surface-localized interband transitions at the TiO 2 surface (1.2, 2 and 2.4 eV) during the CGS process. In order to study the influence of these defects on the photoreaction performance surface modification by plasma treatment is employed. For this structure-activity correlation analysis by Raman, XRD and XPS is performed. From these initial experiments the preparation is extended to other potential metal oxides (e.g. WO 3 , Fe 2 O 3 and BiVO 4 ) for the photooxidation of water. Besides the relevant particle-substrate bonding and defect chemistry of the semiconductor layer due to the process, morphology investigation by SEM, gas sorption and 3D topography are considered for the optimization of the coating technique.

Abstract: Thin layers of semiconductors with wide band gaps (wide gap SCs), such as TiO 2 , are of great interest for the passivation and/or specific activation of semiconductor surfaces in photoelectrochemical (PEC) systems [1]. Defect states in the bulk of wide gap SCs and at related semiconductor hetero-junctions play an important role for the electronic properties of PEC systems. Due to high activation energies in wide gap SCs, electronic states can be charged for relatively long times. This persistent charging of wide gap SCs can have large impact on photogeneration and modulated charge separation in modulated surface photovoltage (SPV) spectroscopy and thus opens a new perspective for the investigation of electronic defect states at/near interfaces with wide gap SCs. In this work, persistent charging is applied to the investigation of defect states in c-Si(n ++ )/TiO 2 (ALD) systems before and after conversion from amorphous TiO 2 to anatase by post annealing. Thin layers of amorphous TiO 2 with thicknesses of up to 100nm were deposited onto highly doped silicon wafers (c-Si(n ++ )) and converted into anatase by annealing for 30 min at 450°C. Modulated SPV measurements were performed at room temperature in the fixed capacitor arrangement [2] with a quartz prism monochromator and a xenon arc lamp for excitation over a range from 0.8 to 4.5 eV. Incidentally, the in-phase (X) or phase-shifted by 90° (Y) signals are much faster or slower, respectively, than the modulation period (see for details also [3] ). The X- and Y- signals of two modulated SPV spectra were measured subsequently within about 30 min. This kind of measurement regime opened the opportunity for the analysis of the influence of charging on electronic transitions of defect states in c-Si(n ++ )/TiO 2 (ALD) systems and it will be straight forward to compare those results with the optical properties of holes and electrons trapped in TiO 2 [4]. As an example, the figure shows modulated SPV spectra of an as prepared (a) and annealed (b) sample. The X-signals of the as-prepared sample where quenched with ongoing absorption in amorphous TiO 2 whereas the onset energy for quenching shifted by more than 0.1 eV towards lower energies for the second scan. In contrast, the Y-signals of the as-prepared sample became more negative between about 3 and 3.5 eV and changed to more positive and changed sign at higher photon energies. Electrons photogenerated in amorphous TiO 2 near the interface with c-Si were transferred into c-Si and partially trapped near the interface resulting in an increase of the SPV signals related to photogeneration in c-Si(n ++ ) and in quenching of charge transfer from amorphous TiO 2 into c-Si(n ++ ) in the second scan. For the annealed sample, the signals were much larger and modulated charge separation with opposite direction at the c-Si and TiO 2 surfaces became dominant. The Y-signals for the virgin scan were larger than for the second scan at photon energies up to about 2.5 eV, but smaller for higher photon energies, i.e. photogeneration by defects in anatase caused an increase of positive charge at the interface with c-Si(n ++ ) and of negative charge at the external surface whereas the signature of the hole traps nearly disappeared in the second scan shown in the figure. Furthermore, the influence of the specific spectrum of the photon flux could be widely eliminated by analyzing the spectra of the ratio of the amplitudes before and after charging (not shown) which allowed for the extraction of defect peaks induced by charging. Acknowledgement: The authors are grateful to A. Schwartzberg, D. Olynick and S. Cabrini for their insight and support for the preparation and coating of the samples by atomic layer deposition. The authors furthermore gratefully acknowledge financial support by the Federal Ministry of Education and Research of Germany, in the framework of the project "FocusH2" (No. 03SF0479A). The transient SPV spectroscopy instrumentation was developed at HZG with support from the Helmholtz Association of German Research Centres, within the "HEMF" platform (Helmholtz Energy Materials Foundry). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. [1] S. Hu et al., Science 344 (2014) 1005. [2] V. Duzhko et al., Phys. Rev. B 64 (2001) 075204. [3] Th. Dittrich, S. Fengler, Surface photovoltage analysis of photoactive materials, World Scientific. ISBN No. 978-1786347657 (2019). [4] D. Bahnemann et al., J. Phys. Chem. 88 (1984) 709; Bahnemann et al., J. Phys. Chem. B 101 (1997) 4265; R. Memming, Semiconductor Electrochemistry, Wiley-VCH (2001 2 nd ed. 2015); J. Schneider and D. Bahnemann, J. Phys. Chem. C 122 (2018) 13979. Figure 1

Abstract: Abstract not Available.

Abstract: Abstract not Available.

Abstract: Semiconducting photoabsorbers used in photoelectrochemical applications often exhibit low stability under operating conditions. One strategy implemented in recent years to prevent corrosion of photoelectrodes is the application of thin films of titanium dioxide (TiO2), coated by atomic layer deposition (ALD). However, the stability of these coatings under photoelectrochemical conditions is also limited. We are using spectroscopic ellipsometry (SE) and in-situ atomic force microscope (AFM) characterization[1, 2] in order to quantify the influence of the coating process parameters and of post-deposition treatments on the degradation of these protective layers. Here we show our recent investigations under anodic operation in acidic environment. For fully amorphous TiO2 layers, a significant contributor to the corrosion is a purely chemical process, with an activation energy of 57 kJ/mol. Furthermore, the degradation rate doubles upon illumination under simulated sunlight. With increasing pH, the stability increases significantly. Partially crystalline protective coatings exhibit improved stability under operating conditions in sulfuric acid. References: [1] Kriegel H. et al. J. Mater. Chem. A, 2020, 8, 18173. [2] Raudsepp R. et al. (in preparation). Figure 1

Abstract: One of the most environmentally friendly ways to produce hydrogen is by electrochemical splitting of water, using only sunlight as an energy source. In a photoelectrochemical cell with a semiconductor photoanode, water oxidation takes place on the surface of the electrode, at the interface between semiconductor and electrolyte. While photoelectrodes with structured surfaces tend to outperform planar ones, an increased surface area can be accompanied by enhanced surface recombination, which is deleterious to photoelectrode performance. Furthermore, the geometry of the surface structuring can influence light absorption and transport processes, and consequently the water splitting efficiency. In order to investigate the effect of surface structuring on water splitting performance, model photoelectrodes are needed, where the geometry and aspect ratio of the surface features can be controlled. In this work, we report on the use of laser interference lithography to generate precisely defined surface patterns on n+Si substrates. The geometry and aspect ratio of the surface features is determined during a reactive ion etching step. After etching, the samples are conformally coated with TiO 2 by atomic layer deposition. Photoelectrochemical characterization of the electrodes is complemented by extensive physicochemical studies in an effort to identify the structural parameters with highest impact on performance in the photodriven water oxidation reaction. Figure 1

Abstract: The photo-electrochemical (PEC) cell, which combines the technology of electrolyzers and photovoltaics, is a promising candidate for the electrolysis of hydrogen with reduced external energy supply. Multiple parameters of such cells have to be adjusted to achieve the most efficient assembly for given requirements. These parameters include the choice of photoactive material, type and kinetics of electrolyte and the cell geometry, which finally determine the photocurrent. As the dependency between these parameters and the cell performance is complex, numerical simulations are an indispensable tool for the optimization process. To guarantee a reliable reference model that applies for all possible parameter changes, iterative verification with experiments is necessary. Especially the precise modelling of the interfaces between photoactive substrate and electrolyte poses a significant challenge for the numerical simulations. Nevertheless, precise modelling of this region is crucial, as it largely determines the cell efficiency by controlling the reaction rate, gas saturation, electrode corrosion and thus the photocurrent. In this work we present results of numerical modelling of PEC cells with COMSOL Multiphysics, their benchmarking with experimental data measured with varying 3D-printed geometries, electrode substrates and electrolyte compositions as well as new approaches for optimizing the efficiency of PEC cells.

Abstract: The photo-electrochemical (PEC) cell, which combines the technology of electrolyzers and photovoltaics, is a promising candidate for the electrolysis of hydrogen with reduced external energy supply. Multiple parameters of such cells have to be adjusted to achieve the most efficient assembly for given requirements. These parameters include the choice of photoactive material, type and kinetics of electrolyte and the cell geometry, which finally determine the photocurrent. As the dependency between these parameters and the cell performance is complex, numerical simulations are an indispensable tool for the optimization process. To guarantee a reliable reference model that applies for all possible parameter changes, iterative verification with experiments is necessary. Especially the precise modelling of the interfaces between photoactive substrate and electrolyte poses a significant challenge for the numerical simulations. Nevertheless, precise modelling of this region is crucial, as it largely determines the cell efficiency by controlling the reaction rate, gas saturation, electrode corrosion and thus the photocurrent. In this work we present results of numerical modelling of PEC cells with COMSOL Multiphysics, their benchmarking with experimental data measured with varying 3D-printed geometries, electrode substrates and electrolyte compositions as well as new approaches for optimizing the efficiency of PEC cells.