Surface Analysis of Perovskite Oxynitride Thin Films as Photoelectrodes for Solar Water Splitting

Abstract

Perovskite oxynitride semiconductors have attracted huge interest recently as promising photoelectrode materials for photoelectrochemical (PEC) water splitting. Depicted by, the extensive studies of the PEC activity of oxynitride powder-based photoelectrodes and/or deposited thin-film electrodes. High-crystalline-quality, oxynitride thin films grown by physical vapor deposition are ideal model systems to study the fundamental physical and chemical properties of the surface of these materials, including their evolution. In this work, using a combination of high-sensitivity low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS), we monitor surface evolution of LaTiO x N y (LTON) and CaNbO x N y (CNON) thin films before and after the PEC characterizations. The as-prepared epitaxial LTON films show a preferential LaO termination at the surface layers, followed by a Ti-enriched subsurface. Whereas, the polycrystalline CNON thin films exhibit a non-uniform surface, with a mixed surface termination and a significant Ca-segregated subsurface. After the PEC characterizations, additional precipitated LaO species are found on the outer surface of the LTON epitaxial films. However, no significant surface change is observed on the polycrystalline CNON films by LEIS. The XPS analysis shows, an increase of the oxidized Ti and Nb cations (Ti4+ and Nb5+) after the PEC reaction in the LTON and CNON films, respectively. The initial drops in photocurrent for the LTON and CNON films are attributed to the changes in the surface chemical status. This work provides insight into the surface characteristics and evolution of LTON and CNON oxynitride thin films as photoelectrodes for PEC applications.

Charting Ba-Based Double Perovskite Oxides for Visible-Light-Driven Photocatalytic Water Splitting

“…In past decades, harvesting this intermittent but sustainable energy source has played a significant role in the transition from a high- to a low-carbon economy. − Of the different solar energy harvesting methods, the photocatalytic water splitting, dubbed “artificial photosynthesis”, that stores sunlight in H 2 bonds stands out by offering environmental benefits to the entire society . However, its efficiency is still too low to be used on large scales. − Such an enhancement is hard to realize as efficient photocatalysts must meet multiple formidable criteria. First, the band gap ( E gap ) of a photocatalyst has to exceed 1.23 eV (water-splitting free energy) but must be less than 2 eV, ensuring that the visible-light region (the major solar radiation) of the solar spectrum can be utilized.…”

“…More importantly, to achieve both oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) in a single material, the valence band (VB) and conduction band (CB) band edges should engulf the redox potentials of a water molecule. Thus, the candidate materials should be tailorable enough to enable exquisite control over its VB and CB via various modulation methods such as band , and facet engineering, ,− doping, − and quantum confinement , to name a few. Finding such “all-in-one” materials is a daunting task.…”

Iridium-Incorporated Strontium Tungsten Oxynitride Perovskite for Efficient Acidic Hydrogen Evolution

“…Considering the great importance of the electronic structure of an active site and its reaction intermediates’ binding energies in an electrochemical reaction, developing suitable oxynitride perovskites with desired metal active sites is a promising concept for rational electrocatalyst design. To date, there are no reports of such studies and current oxynitride perovskites have only been used in photoelectrochemical reactions and other applications. − …”