Strategies and Methods of Modulating Nitrogen-Incorporated Oxide Photocatalysts for Promoted Water Splitting

Abstract

Conspectus The conversion of solar energy to chemicals and the storage of this energy is one of the most promising routes to realizing a “carbon zero” society, for which particulate photocatalytic water splitting to produce hydrogen has been considered to be a clean potential route. For this purpose, many d0 and d10 metal-based nitrogen-incorporated oxide (including nitrogen-doped oxide and oxynitride) semiconductor photocatalysts have been developed as good candidates to harvest major visible regions of solar light and to exhibit suitable energy levels to drive water splitting. Compared to the oxide-type photocatalysts, valence bands of the nitrogen-incorporated oxide can be upshifted in consideration of the lower electronegativity of nitrogen atoms concerning the oxygen atoms. However, a high-temperature nitridation process was needed to introduce nitrogen atoms into the metal oxide lattice. Because of the poor charge balance of the N3– substitution for O2– and the strong reducing effect of the active nitrogen species at high temperature, the nitriding process tends to introduce both anionic vacancy defects as well as low-valent metal ion defects. In addition, the crystal size of (oxy)­nitrides synthesized through high-temperature nitridation is generally large, and there is a lack of an effective charge-separation driving force within the bulk of the (oxy)­nitrides. Because of the scarcity of surface catalytic sites, the photocatalytic activity of the (oxy)­nitrides is rather low. Recently, our group has been devoted to addressing the above key challenges, especially for the d0-metal-based (oxy)­nitrides. The new preparation routes and/or methods were carried out to enhance the nitration kinetics for the synthesis of (oxy)­nitrides with low-defect concentrations and further fabricate some novel metal (oxy)­nitrides. The doping strategies were developed to suppress the generation of low-valent metal ion defects. A one-pot nitridation strategy was developed to construct the type II heterostructures of two metal oxynitrides to enhance their charge separation, and a strategy for the surface modification of metal oxynitrides with inert metal oxides was developed to promote the surface hydrophilicity and the cocatalyst dispersion. In addition, an intimate and strengthened interface between the cocatalyst and the metal oxynitride photocatalyst was achieved by a sequential cocatalyst decoration method, and it was demonstrated that the strengthened interface is beneficial to enhancing the surface charge separation efficiency. Benefitting from these methods and strategies, some novel visible-light-responsive materials have been exploited for promising water splitting, and remarkably improved water splitting performances have been achieved on some typical d0-metal-based (oxy)­nitrides regardless of H2/O2-evolving half-reactions and Z-scheme overall water splitting (Z-OWS) reactions. In this Account, we will give a concise summary of these methods and strategies, and the prospects of nitrogen-incorporated oxide photocatal...

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“…As far as the oxynitrides are concerned, plenty of defect states are generally formed, as mainly results from the charge imbalance between O 2− and N 3− and the facile reduction of the d 0 ‐metallic ions (especially for Ti 4+ and Nb 5+ ) under the widely‐adopted high‐temperature reductive ammonia atmosphere [8] . Generally, these defect states are unfavorable for charge separation as well as photocatalytic performances, so extended efforts on developing novel preparative methodologies or modifying ways have been made to address the issue of defects during the past decades [2c, 8b] . Even though continuous promotion and improvement on performances of the (oxy)nitride photocatalysts have been made, the material featured with both wide‐visible‐light absorption and low defect density is still in shortage due to the unavoidable charge imbalance during the substitution of N 3− to O 2− , as has greatly plagued the efficiency of solar‐to‐chemical energy conversion.…”

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“…The introduction of N atom narrows bandgaps of oxides by shifting the VB maximum upwards due to the more positive level of N 2p than that of O 2p states. The nitrogen-incorporated metal oxides are generally synthesized through thermal (873-1273 K) treatment of the precursors under ammonia flow [68]. However, the reductive ammonia treatment tends to reduce the metallic cations and produce defects of low-valent metal cations.…”

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