Abstract
The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO4 . By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V2 O5 at grain boundaries of Mo-BiVO4 thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO4 . After photo-electrochemical operation, the degraded Mo-BiVO4 films show similar grain center and grain boundary spectra indicating V2 O5 dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.
Highlights
(PEC) water splitting is a promising variations of the material properties due to the exposure of different crystal approach to convert sunlight directly into facets and the presence of grain/domain boundaries
We have chosen Mo-BiVO4 thin films as a model system to understand the correlation between chemical composition, electronic structure, and nanoscale morphology
We utilized the correlation between the morphology of polycrystalline thin films and the contrast within the X-ray transmission maps to extract the X-ray absorption spectra at the grain center and at the grain boundaries from the measured scanning transmission X-ray microscopy (STXM) energy stacks
Summary
We have chosen Mo-BiVO4 thin films as a model system to understand the correlation between chemical composition, electronic structure, and nanoscale morphology. Electron small polarons, which are strongly localized charges at individual sites that cause lattice distortions, have been often correlated with materials limitations in metal oxides.[22] after doping, the photo-electrochemical performance of undoped BiVO4 is typically improved.[16,23] This improvement is commonly assigned to increased charge carrier concentration rather than to increased mobility While both charge carrier concentration and mobility would lead to higher conductivity, the latter is rather related to electron small polarons.[9] Capturing formation of polarons experimentally is challenging[12] and requires minimum concentration of dopants, here we propose first principles DFT simulations to predict the experimental X-ray absorption spectral feature as a function of both polaron and/or dopant concentration, and calculate the formation and hopping energies of polarons in Mo-doped BiVO4 (see Figure 5 and Figures S12 and S13, Supporting Information). These predictions indicate that it is possible to identify polaronic features in the X-ray absorption spectrum of Mo-BiVO4, whereas the high concentration of Mo which is required may at the same time exacerbate the phase segregation of V2O5
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