Intrinsic photoanode band engineering: enhanced solar water splitting efficiency mediated by surface segregation in Ti-doped hematite nanorods


Band engineering is thoroughly employed nowadays targeting technologically scalable photoanodes for solar water splitting applications. Most often complex and costly recipes are necessary, for average performances. Here we report very simple photoanode growth and thermal annealing, with effective band engineering results. Strongly enhanced photocurrent, of more than 200 %, is measured for Ti-doped hematite nanorods grown from aqueous solutions and annealed under Nitrogen atmosphere, compared to air annealed ones. Oxidized surface states and increased density of charge carriers are found responsible for the enhanced photoelectrochemical activity, as shown by electrochemical impedance spectroscopy and synchrotron X-rays spectromicroscopies. They are found related to oxygen vacancies, acting as n-dopants, and the formation of pseudo-brookite clusters by surface Ti segregation. Spectro-ptychography is used for the first time at Ti L3 absorption edge to isolate Ti chemical coordination arising from pseudo-brookite clusters contribution. Correlated with electron microscopy investigation and Density Functional Theory (DFT) calculations, our data unambiguously prove the origin of the enhanced photoelectrochemical activity of N2-annealed Ti-doped hematite nanorods. Finally, we present here a handy and cheap surface engineering method beyond the known oxygen vacancy doping, allowing a net gain in the photoelectrochemical activity for the hematite-based photoanodes.


Supplementary material

Intrinsic photoanode band engineering: enhanced solar water splitting efficiency mediated by surface segregation in Ti-doped hematite nanorods
This document gives supplementary materials to support the main manuscript. Details are presented for the photoelectrochemical impedance spectroscopy studies. Synchrotron X-rays spectromicroscopy methods are described and explained.
Hyperspectral shadow XPEEM data
Hyperspectral shadow XPEEM data to exemplify the three different spectral regions exploited in this approach: surface, bulk and substrate. The overall spectral range is shown, from 450 to 750 eV, covering Ti L, O K, and Fe L absorption edges. XPEEM allow to isolate surface spectral component based on its high surface sensitivity, as in the case of the XPS experiments. Space charge effects, responsible for the loss of spatial resolution, are visible, related to the strong 3D character of the sample.
Spectro-ptychography hyperspectral data
Hyperspectral spectro-ptychography data recorded from the Nitrogen annealed sample. Narrow spectral regions were recorded across the Ti L3 t2g and O K absorption resonances. The first is sensitive to the chemical coordination difference between pseudo-brookite and Ti-doped hematite (or even ilmenite) and characterized by 0.1 eV energy shift, also reproduced by the DFT calculations. The second exhibits additional contrast around 533.5 eV, associated to the surface states. In both cases, but especially in the first one, the chemical contrast enhance the spatial resolution obtained by spectro-ptychography.