Rhodium-Doped Barium Titanate Perovskite as a Stable p-Type Photocathode in Solar Water Splitting

Abstract

Solar water splitting from a p−n-conjugated photoelectrochemical (PEC) system is a promising way to produce hydrogen sustainably. At present, finding a compatible p-type photocathode material for the p−n system remains a great challenge in consideration of the photocurrent and stability. This paper highlighted a promising candidate, Rh/BaTiO 3 , by switching BaTiO 3 from an n-type photoanode to a p-type photocathode upon Rh doping. The dopant activated visible light absorption up to 550 nm and an onset potential as high as 1.0 V (vs RHE). Using surface photovoltage spectroscopy as a powerful characterization tool, the n-to p-type transition of the semiconductor was studied and explained microscopically by which we quantitatively isolated the cathodic contribution caused by the Rh dopant. Unbiased overall solar water splitting was accomplished by serially connecting the Pt/Rh/BaTiO 3 photocathode to a CoO x /Mo/BiVO 4 photoanode, which produced a solar to hydrogen conversion efficiency of 0.1% and an excellent stability over 100 h of operation at ambient pressure. This work revealed the key role that the Rh dopant played in the n-to p-type adjustment of titanate semiconductors and demonstrated its great potential for application in PEC water splitting.

Unveiling Valence State-Dependent Photocatalytic Water Splitting Activity and Photocathodic Behavior in Visible Light-Active Iridium-Doped BaTiO3

“…almost a decade ago . Recently, Shi et al investigated the effect of the Rh doping level on the Fermi level position and n- to p-type transition was studied by isolating the cathodic contribution by the Rh dopant . Despite years of research related to BTO as a H 2 evolution photocatalyst, there exists no report investigating the effect of the dopant’s valence state on the optoelectronic properties and subsequent photocatalytic and/or photoelectrochemical behavior of BTO.…”

A Visible Light-Responsive TiO2 Photocathode Achieved by a Rh Dopant

“…It has been reported that the transformation of an SrTiO 3 perovskite oxide from an n-type to a p-type semiconductor could be achieved by introducing acceptor energy levels above its valence band, thereby effectively lowering the Fermi level. The result was a high onset potential up to 1.0 V versus the RHE as well as extended light absorption from the ultraviolet (UV)-responsive oxide host to the visible region. − Similarly, our group has introduced a Rh dopant into BaTiO 3 , which created deep acceptor levels above the valence band and thus realized photocurrent switching from anodic to cathodic . An onset potential of 1.1 V versus the RHE was obtained on the Rh:BaTiO 3 photocathode, and its edge of light absorption was extended from 380 to 550 nm.…”

Orienting Charge Migration in TiO2 Photocathode through Directionally Distributed Rh Dopant

“…In this work, we employ the Rh-doped rutile TiO 2 electrode as an example to illustrate the great potential of gradient doping for enhancing photocurrent and transformation from photoanode to photocathode. Unlike many other reports that induce a full transformation of TiO 2 from n-type to p-type by heavy doping, ,,, a minimum amount of Rh dopant is used in this work to keep its n-type nature. First, the Rh dopants with a graded distribution create upward band bending along the direction of higher doping concentration near the rutile particle surface.…”

“…However, the poor stability of these materials, even with protection layers, remains a major challenge. Ti­(IV)-based oxides, such as TiO 2 , SrTiO 3 , and BaTiO 3 possess remarkable stability over a wide range of operating conditions, − and more importantly, they can act as p-type photocathodes upon doping with certain transition metal elements. − Rhodium is an effective dopant that can extend the light absorption of the UV­(ultraviolet)-responsive Ti­(IV)-based oxides to visible region by introducing intragap states. − Such intragap states also act as acceptor energy levels above the valence band of the titanate host (O 2p), and a heavier concentration of the Rh dopant leads to a lower Fermi level of the semiconductor. With a Rh doping level higher than 2 mol %, the Ti­(IV)-based oxides have frequently been switched from n-type to p-type, i.e.…”

“…With a Rh doping level higher than 2 mol %, the Ti­(IV)-based oxides have frequently been switched from n-type to p-type, i.e. from an anode to a cathode photoelectrochemically. ,,,, Apart from applications in photoelectrochemistry, Rh-doped Ti­(IV)-based oxides have also been researched as photocatalysts for hydrogen evolution from water. ,− The Rh-doped Ti­(IV)-based oxides are generally synthesized by calcination at temperatures higher than 1273 K to distribute the Rh element as uniformly as possible, while a study on gradient doping of Rh element has not been attempted to this day.…”