Lipeng Wan,Feng-Qiang Xiong,Bingxue Zhang,Ruxin Che,Yue Li,Minghui Yang
LaNbON 2 has narrow bandgap and wide visible-light absorption band, yet no photocatalytic water oxidation on LaNbON 2 has been reported. By a post-annealing treatment in Ar, anion vacancies were brought into LaNbON 2 as shown by EPR spectroscopy. These could act as donors in the semiconductor. And consequently the oxidative power of holes was enhanced as indicated by the difference between fermi level and valence band maximum ( E F - E VBM ) evaluated from valence band XPS . The annealed LaNbON 2 photocatalyst acquired water oxidation ability for the first time, which was improved by combining CoO x as cocatalyst . Annealed LaNbON 2 derived from La 3 NbO 7 had smaller particle size, higher concentration of anion vacancies, bigger E F - E VBM and better performance for photocatalytic oxygen evolution reaction than LaNbON 2 derived from LaNbO 4 .
Combining structural coherence and large porosity, semiconducting porous single crystals (PSCs) warrant a great potential as photocatalysts. High photocarrier mobility can be realized in PSCs because of a deficiency in grain boundaries, which in turn greatly alleviates photocarriers’ recombination. The large porosity of PSCs not only provides plenty of accessible surfaces to accommodate catalytic reaction sites but also fortifies photon-matter interplay by reflecting and scattering photons. In this work, taking LaNbON 2 PSCs as an example, we demonstrate the importance of PSCs in activating otherwise almost inactive photocatalyst. Compared to conventional inert LaNbON 2 , LaNbON 2 PSCs exhibit exceptional activity for photocatalytic water redox reactions, delivering a high apparent quantum efficiency, i.e. 6.3% and 10.2% at 420 ± 20 nm for H 2 - and O 2 -evolution reactions, respectively. A Z-scheme system containing LaNbON 2 PSCs has succeeded in overall water splitting (H 2 /O 2 molar ratio 2:1) with high stability under visible light illumination (λ ≥ 420 nm).
g-C 3 N 4 usually acts as the oxidation counterpart for overall water splitting reactions in semiconductor heterojunctions. Due to the poor water oxidation ability of g-C 3 N 4 , it is difficult to achieve efficient photocatalytic overall water splitting over g-C 3 N 4 -based heterojunctions. In this work, one-dimensional (1D) LaNbON 2 with strong water oxidation ability is prepared for the first time, and combined with two-dimensional (2D) g-C 3 N 4 to form semiconductor heterojunction by electrostatic self-assembly. The results of surface states, theoretical calculations, band structures and chemical probes indicate that the 2D g-C 3 N 4 and the 1D LaNbON 2 are tightly bonded through the electrostatic attraction and form a S-scheme type semiconductor heterojunction. In the heterojunction, the 1D LaNbON 2 and the 2D g-C 3 N 4 work as oxidation and reduction counterpart, respectively. The built-in electric field at their heterointerfaces effectively accelerates carrier separation. Thereby the solar-driven photocatalytic overall water splitting rate of the optimal 2D g-C 3 N 4 /1D LaNbON 2 heterojunction is ∼ 3.3 times as much as that of the 2D g-C 3 N 4 .
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