Controllable Synthesis of [11-2-2] Faceted InN Nanopyramids on ZnO for Photoelectrochemical Water Splitting

Indium nitride (InN) is one of the promising narrow band gap semiconductors for utilizing solar energy in photoelectrochemical (PEC) water splitting. However, its widespread application is still hindered by the difficulties in growing high-quality InN samples. Here, high-quality InN nanopyramid arrays are synthesized via epitaxial growth on ZnO single-crystals. The as-prepared InN nanopyramids have well-defined exposed facets of [0001], [11-2-2], [1-212], and [-2112], which provide a possible routine for understanding water oxidation processes on the different facets of nanostructures in nanoscale. First-principles density functional calculations reveal that the nonpolar [11-2-2] face has the highest catalytic activity for water oxidation. PEC investigations demonstrate that the band positions of the InN nanopyramids are strongly altered by the ZnO substrate and a heterogeneous n-n junction is naturally formed at the InN/ZnO interface. The formation of the n-n junction and the built-in electric field is ascribed to the efficient separation of the photogenerated electron-hole pairs and the good PEC performance of the InN/ZnO. The InN/ZnO shows good photostability and the hydrogen evolution is about 0.56 µmol cm^-2 h^-1 , which is about 30 times higher than that of the ZnO substrate. This study demonstrates the potential application of the InN/ZnO photoanodes for PEC water splitting.