Combining Mesoporosity and Ti-Doping in Hematite Films for Water Splitting

Abstract

In this study, we report the synthesis of Ti-doped mesoporous hematite films by soft-templating for application as photoanodes in the photoelectrolysis of water (water splitting). Because the activation of the dopant requires a heat treatment at high temperature (≥ 800°C), it usually results in the collapse of the mesostructure. We have overcome this obstacle by using a temporary SiO 2 scaffold to hinder crystallite growth and thereby maintain the mesoporosity. The beneficial effect of the activated dopant has been confirmed by comparing the photocurrent of doped and undoped films treated at different temperatures. The role of the mesostructure was investigated by comparing dense, collapsed and mesoporous films heated at different temperatures and characterized under front and back illumination. It turns out that the preservation of the mesotructure enables a better penetration of the electrolyte into the film and therefore, reduces the distance that the photogenerated holes have to travel to reach the electrolyte. As a result, we found that mesoporous films with dopant activation at 850°C perform better than comparable dense and collapsed films.

Controlled Aqueous Growth of Hematite Nanoplate Arrays Directly on Transparent Conductive Substrates and Their Photoelectrochemical Properties

“…For more detailed analysis, the Nyquist plots for all samples were fitted to an equivalent circuit model including one resistor and three RC circuits (shown in the Supporting Information). Three RC circuits can be assigned to the electrode–electrolyte interface including the Helmholtz layer and depletion layer, and the bulk electrode as reported in previous studies . The fitted impedance parameter values including resistances ( R ) and constant‐phase elements (CPE) for backside illumination (listed in Table ) indicate that intentional Sn doping significantly decreases the resistance at both the bulk photoelectrode and hematite–electrolyte interface in accordance with the enhanced photocurrent and negative shift of the onset potential.…”

“…However, its photocatalytic efficiency is restricted by poor electrical conductivity, short hole diffusion length (2–4 nm), and slow oxygen‐evolution kinetics . Enormous efforts focusing on tailoring the morphology, impurity doping, and fabricating hematite‐based composites have been made to address these limitations . One potential solution to address the short hole diffusion length associated with bulk α‐Fe 2 O 3 is to use two‐dimensional (2D) nanostructuring for the photoelectrodes .…”

Enhancement of Water Oxidation Photocurrent for Hematite Thin Films Electrodeposited with Polyvinylpyrrolidone

“…However, for thicker films made without PVP, the corrected back-side EQY is significantly lower than for front-side at wavelengths below 450 nm, indicating these photons are absorbed too far from the electrolyte interface to be collected efficiently. Use of PVP increases porosity, allowing the electrolyte to penetrate more fully into the hematite film, and thus improves collection near the FTO by decreasing the length charge carriers must diffuse before reaching the interface, 29,36 such that optical transmission losses from the substrate entirely account for the difference in front-versus back-side photocurrents for all but the thickest films.…”

Local vs Nonlocal States in FeTiO3 Probed with 1s2pRIXS: Implications for Photochemistry

“… 1 Various dopings have been investigated to improve its low charge-carrier transport properties and to optimize its optical absorption, among which Ti-doping appeared as a promising solution. 2 Alternatively, efficient Fe–Ti associations have also been considered in various oxides 3 including heterojunction assemblies of iron and titanium oxides, 4 in dye sensitized solar cells, 5 and in heterobinuclear complexes. 6 In all of these, the photoinduced metal–metal charge transfer (MMCT) is expected to be the main mechanism that enables harvesting solar light energy with multivalent transition metal ions in both solid state and molecular complexes.…”