For photoelectrochemical (PEC) water splitting to become a viable route for hydrogen production, we need to develop efficient photoelectrodes with good long-term stability at low cost. Metal oxides are interesting photoelectrode materials due to their low cost and generally good stability in aqueous solutions. However, due to their relatively low charge injection efficiency, an additional catalyst layer is needed to be deposited on the surface of the metal oxides. While this has been proven to be an effective strategy, little is known about the charge carrier dynamics at the semiconductor/catalyst and catalyst/electrolyte interfaces and the parameters that are affecting these dynamics. Understanding the interactions at this interface will presumably lead to a rational design of the semiconductor/catalyst system and higher efficiency.In this work, we use BiVO4/MnOx photoelectrode as a model system. BiVO4 is currently the highest performing metal oxide photoanode [1], and MnOx is a promising catalyst material for electrochemical water oxidation [2,3]. To investigate the charge carrier dynamics at the BiVO4/MnOx interface, we attempted to control the band edge position of both BiVO4 and MnOx. In BiVO4, we introduced N via high temperature nitridation treatment. As a result, we observed an onset of absorption shift to >600 nm, as compared to the 520 nm onset of absorption for unmodified BiVO4. This corresponds to an increase of total absorbed AM1.5 current of ~40%. In the case of the MnOx, we plan to dope MnOx with Co or Ni. We utilize atomic layer deposition (ALD) to deposit thin films of MnOx with monolayer precision [4, 5], as evidenced by in situ spectroscopic analysis. Our in-line X-ray/ultraviolet photoelectron spectroscopy (XPS/UPS) system allows us to study the valence band and Mn oxidation state without exposing the samples to air. The latter is important since the ability of manganese to appear in different oxidation states, predominantly +3/+4, is crucial for the catalytic activity. XPS spectra showed the as-deposited films to consist of a MnO/Mn2O3 mixed phase. Post-deposition heat treatment allows us to control the Mn oxidation state. The effect of the oxidation state on the catalytic activity will be discussed.And we will present an outlook for semiconductor/catalyst electrode design for efficient solar water splitting.  

[1] Pihosh et al., Sci. Rep., 2015, 5, 11141-11151, DOI: 10.1038/srep11141

[2] A. Ramiréz et al. J. Phys. Chem. C, 2014, 118, 14073-14081, DOI: 0.1021/jp500939d

[3] P. Hillebrand et al. ECS Trans., 2014, 61, 9-20, DOI: 0.1149/06114.0009ecst

[4] K. L. Pickrahn et al. Adv. Energy Mater., 2012, 2, 1269-1277, DOI: 10.1002/aenm.201200230

[5] B. B. Burton et al. J. Phys. Chem. C, 2009, 113, 1939-1946, DOI: 10.1021/jp806088m