Thin‐Film Oxynitride Photocatalysts for Solar Hydrogen Generation: Separating Surface and Bulk Effects Using Synchrotron X‐Ray and Neutron‐Based Techniques

Abstract

The conversion of solar light into hydrogen by photoelectrochemical water splitting is one of the potential strategies that can allow the development of a carbon‐neutral energy cycle. Oxynitride semiconductors are promising materials for this application, although important limitations must still to be addressed. One of the most important issues is physicochemical degradation of the semiconductor, at the interface with water, where the electrochemical reactions occur. In this regard, thin films, with well‐defined and atomically flat surfaces, are invaluable tools for characterizing material properties and degradation mechanisms, while identifying strategies to mitigate detrimental effects. Thin oxynitride films may allow the use of complementary characterizations, not applicable to conventional powder samples. In particular, the study of the solid–liquid interface can benefit enormously from the use of thin films for synchrotron‐based surface‐sensitive X‐Ray scattering methods and neutron reflectometry. These investigation approaches promise to speed up the design and discovery of new materials for the production of solar fuels, while paving the way for similar applications in other research fields. This work aims at reviewing the literature contributions on oxynitride thin films for solar water splitting summarizing what is learnt so far and suggesting experimental strategies to unveil what is still not clear.

Stability and degradation of (oxy)nitride photocatalysts for solar water splitting

“…56 Moreover, with respect to ex situ and more importantly in situ characterization, thin lm photoelectrodes are advantageous due to their well-dened and atomically at surfaces allowing the use of dedicated surface characterization technques. 74 For instance, ex situ and in situ neutron reectometry and grazing-incidence Xray absorption spectroscopy are used to explore surface modi-cations of oxynitride thin lms. 109,113 The study on SrTaO x N y showed, that the oxynitride surface undergoes a compositional change due to the dissolution of SrO x into the electrolyte enriching Ta and lattice Sr at the surface and the slight loss of nitrogen from the structure.…”

“…Due to the roughness of particle based lms, the surface area being in contact with the electrolyte (solid-liquid interface) is signicantly enhanced resulting in higher photocurrents. 74,76 Although the syntheses of thin lm oxynitrides, using pulsed reactive crossed-beam laser ablation and radiofrequency magnetron sputtering, 76,127,128 are rather complex and the resulting photoelectrodes suffer from low photocurrents and limited scalability, epitaxially-grown lms can be very useful to investigate the activity of the exposed crystal facets on the performance for water splitting. For example, Burns et al found that (011) oriented LaTiO 2 N lms exhibited 30% higher photocurrents than those with (001) orientation, mostly due to a faster extraction of charges derived from photons in the visible region.…”

“…For example, Burns et al found that (011) oriented LaTiO 2 N lms exhibited 30% higher photocurrents than those with (001) orientation, mostly due to a faster extraction of charges derived from photons in the visible region. 129 The dilemma "particles based vs. thin lms" has been discussed to a great extent by Lippert et al 74,76 Briey, particlebased systems have 10 to 20 times higher surface area and better photon absorption due to their micro-scale features compared to the nano-scale thicknesses of thin lms. This results in particle-based photoelectrodes achieving photocurrents more than 10 times higher than their thin lm counterparts.…”

“…35 Moreover, for photoelectrode preparation, commonly a thin layer in a size range from a few nanometres to several micrometres is deposited on a conductive substrate. 12,35,73,74 The preparation method hereby inuences strongly the mechanical stability of the thin layer which can be for instance a compact thin lm or can compose of individual particles that are arranged randomly. 75,76 An intermediate format are precursor lms that form nanostructures during their transformation to the nal composition such as nanorods in the case of Ta 3 N 5 .…”

Perovskite BaTaO2N: From Materials Synthesis to Solar Water Splitting

“…One of the challenging issues of oxynitride-based semiconductors is physicochemical degradation at the interface with water, where the electrochemical reaction takes place. To understand the degradation mechanism and to find strategies to mitigate such detrimental effects, Pergolesi et al [152] suggested that the study of solid-liquid interface can benefit enormously from the use of thin films with well-defined and atomically flat surfaces for synchrotronbased surface-sensitive X-Ray scattering methods and neutron reflectometry. Particularly, synchrotron radiation-based soft X-ray angle-resolved photoemission spectroscopy (SX-APRES) enables the probing of the k-resolved electronic structure in the subsurface region despite atmospheric surface contamination and provides the opportunity to explore materials, surfaces, and interfaces as well as their reactivity and evolution.…”