Observation of charge transfer cascade in α-Fe2O3/IrO2 photoanodes by in-operando X-rays absorption spectroscopy

In this work we show the direct observation, by means of spectro-photoelectrochemical experiments, of charge transfer between a semiconductor (-Fe2O3) and a metal oxide overlayer (hydrous IrOx) as a photoanode architecture in photoelectrochemical water splitting.1 The aim is to clarify the ambiguous role of oxygen evolving catalysts used as overlayers on top of photoanodes in photoelectrochemical water splitting cells. Previous literature suggested that the real benefit of covering hematite with overlayers like iridium or cobalt oxides is not due to an increase of reaction kinetics but the decrease of the electron density in the hematite2 or the storage of photogenerates holes.3 These effects are likely more important when hydrous overlayer, that can act as adapting catalysts,4 are considered. All these hypothesis can explain the observed improved hole lifetime and reduce recombination with electrons. The present experimental approach is similar to the one that allowed our recent disclosure of the oxidation states assumed by hydrous IrOx as catalyst for water oxidation.5 In the present case, FEXRAV6 and XANES have been used to probe changes in the charge state of Ir while the hematite was illuminated with 410nm radiation. Thanks to this in-operando setup, we were able to observe an increase of the density of empty Ir 5d states during hematite illumination and in correspondence of water spitting in the photoelectrochemical cell. The main conclusion is that a charge (hole) transfer between hematite and iridium occurs only when the hematite is illuminated. Hydrous iridium oxide is therefore capable of withdrawing holes from the semiconductor thus increasing the probability of interface reaction rather than charge recombination. 1 Minguzzi A., Lugaresi O., Achilli E., D'Acapito F., Naldoni A., Malara F., Locatelli C., Vertova A., Rondinini S., Ghigna P., In preparation 2 Badia-Bou L., Mas-Marza E., Rodenas P., M. Barea E., Fabregat-Santiago F., Gimenez S., Peris E., Bisquert J., J. Phys. Chem. C, 2013, 117, 3826−3833 3 Lin F., Boettcher S.W. Nature Materials, 2014, 13, 81-86 4 Barroso M., Mesa C.A., Pendlebury S.R. , Cowana A.J., Hisatomi T., Sivula K., Grätzel M., Klug D.R., Durrant J.R. PNAS, 2012, 109, 15640–15645 5 Minguzzi A., Lugaresi O., Achilli E., Locatelli C., Vertova A., Ghigna P., Rondinini S., Chem. Sci., 2014, 5, 3591-3597 6 Minguzzi, A.; Lugaresi, O.; Locatelli, C.; Rondinini S.; d'Acapito, F.; Achilli, E.; Ghigna, P. Anal. Chem. 2013, 85, 7009-7013.