1, The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
3, Walter Schottky Institut and Physik Department, Technische Universität München, Munich, , Germany
Photoelectrochemical (PEC) water splitting is a promising approach to provide carbon neutral power and renewable fuels. The chemical transformation of water into oxygen and hydrogen takes place at the surface of catalysts and photoactive materials. Consequently, the activity, efficiency, and reaction pathway are critically controlled by the material surface properties. Under PEC operating conditions, surface properties strongly depend on the surrounding environment, and they may be altered in the course of the reaction. Thereby, absorption of molecules can modify the chemistry at the surface, for example by influencing the kinetics of reactants, products, or reaction intermediates, but they can also directly impact the electronic transport properties of the photoactive material by effectively acting as surface trap states. In this context, improved understanding of these complex surface interactions will aid the development of highly efficient light absorbers as well as the integration of effective passivation and catalyst layers for these materials.
In this work, we elucidate the influence of chemical interactions of adsorbed oxygen and water on charge transport and interfacial charge transfer of photogenerated charge carriers in polycrystalline BiVO4 thin films – a promising material for solar water splitting. The charge transfer between adsorbates and BiVO4 is monitored by in situ Kelvin probe measurements under dry nitrogen, humid nitrogen, and oxygen environments at atmospheric pressure. To gain complementary insight into the relationship between surface interactions and interfacial charge transport characteristics, we employ photoconductive AFM under the respective in situ conditions. By combining these complementary techniques, we demonstrate that adsorbed oxygen acts as a surface trap state for electrons, which enhances the built-in potential and depletes the BiVO4 layer, resulting in an increase of the measured surface photovoltage. Furthermore, we have recently demonstrated that the low intrinsic bulk conductivity of BiVO4 limits the electron transport through the film, and that the transport mechanism can be attributed to space charge limited current (SCLC) in the presence of trap states. By analyzing the SCLC, we estimate that the contribution of surface adsorbed oxygen to the total number of shallow traps is as large as 40%. For humid environments, our results are consistent with the adsorption of water as an oriented dipole layer, which does not induce a surface charge transfer but partially inhibits the adsorption of oxygen at the surface. Disentangling the individual effects of oxygen and water on surface band alignment and charge carrier trapping underpins the importance of trap state passivation for efficient transport of photogenerated charge carriers in BiVO4.
 Eichhorn et al. Nanoscale imaging of charge carrier transport in water splitting photoanodes, Nat. Commun. (2018).