Crystallization Engineering of CuNi 2 S 4 Ultra‐Fine Nanocrystals with Optimized Band Structures for Efficient Photocatalytic Pollutant Degradation and Hydrogen Production

The mono-dispersed cubic siegenite CuNi₂ S₄ ultra-fine (≈5 nm) nanocrystals are fabricated through crystallization engineering under hot injection. The strong hydroxylation on mostly exposed CuNi₂ S₄ (220) surface leads to the formation of multi-valence (Cu⁺ , Cu²⁺ , Ni²⁺ , Ni³⁺ ) species with unsaturated hybridization and coordination micro-environments, which can induce rich redox reactions to optimize interfacial kinetics for the adsorbed reaction intermediates. The as-synthesized CuNi₂ S₄ nanocrystals with ultra-small particle size and the characteristics of being highly dispersed can increase specific surface area and hydroxylated active sites, which considerably contribute to the improvement of photocatalytic activities. Experimental and theoretical studies indicate that the CuNi₂ S₄ with unique surface condition can properly modulate the charge density distribution and the electronic band structure, thus achieving an optimal band gap for enhancing visible light absorption. Additionally, the strong hydroxylation on CuNi₂ S₄ (220) surface can not only make the photocatalytic process stable in alkaline environment but also bring about an impurity level between conduction and valence band, which facilitates the separation of photo-induced charge carriers by suppressing the rapid re-combination of exited electrons and holes. The optimization of band structure should be the intrinsic reason for the efficient photocatalytic pollutant degradation and hydrogen production under visible light illumination.