Hematite Nanorods Photoanodes Decorated by Cobalt Hexacyanoferrate: The Role of Mixed Oxidized States on the Enhancement of Photoelectrochemical Performance

In this work, we report on the incorporation of cobalt hexacyanoferrate (Co-NC-Fe) with different iron oxidation states (FeII and FeIII) on hematite (α-Fe2O3) nanorod photoanodes with a systematic investigation of enhanced photoelectrochemical performance under a neutral pH. A detailed investigation using in situ techniques was carried out to understand the fundamental aspects of chemical reactions during photoelectrochemical characterization. The α-Fe2O3/Co-NC-Fe photoanodes showed a higher photoelectrochemical performance compared to the individual materials. Despite the higher achieved performance of α-Fe2O3/Co-NC-Fe, the elucidation of the main contribution upon Co-NC-Fe incorporation was found to be dependent on the different charge transfer mechanism under light illumination. Therefore, given the complexity of the redox processes that take place in Co-NC-Fe under an applied bias, we have systematically investigated the dark- and light-induced effects on α-Fe2O3/Co-NC-Fe by in situ UV–vis and Fourier transform infrared spectroscopies. These techniques allowed us to monitor the effect of the electro and photoelectrochemical potential in several redox processes. The insightful results indicate that, in addition to surface state modification effects, the improvement of the photocurrent density may also result from long-lived holes at the electrode surface as a consequence of the electron transfer from the conduction band α-Fe2O3 to Co-NC-Fe with a FeIII oxidation state. This indicates a second electron pathway during charge separation, thereby contributing to the increase in the electron–hole pair lifetime and the accumulation of photogenerated holes at the α-Fe2O3/Co-NC-Fe photoanode surface to drive water oxidation.