Heterogeneous Wet Chemical Synthesis of Superlattice-Type Hierarchical ZnO Architectures for Concurrent H₂Production and N₂Reduction

Two wet chemical methods, namely, wet etching (heterogeneous) and chemical precipitation (homogeneous), have been exploited for the formation of hierarchical ZnO architectures without any specific templates, catalysts, or capping agents. As-synthesized ZnO have been extensively characterized by X-ray diffraction, UV-visible absorbance studies, Fourier transform infrared analysis, scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive analysis of X-rays, selected area electron diffraction, photoluminescence (PL), and textural analysis. Wet etching resulted in the formation of self-assembled hexagonal plates with a disordered superlattice-type texture and ultimately ended up in crystallites with high-defect concentration showing a red PL emission. Mechanistic aspects of the growth process in both the methods have been analyzed, and a rational explanation is presented for their observed morphologies. ZnO prepared by both methods have been tested for their photocatalytic water splitting abilities in producing H-2. Crystalline, spectral, textural, and activity analyses have been carried out for commercial ZnO for a comparison. Pt loaded ZnO was used for the concurrent hydrogen generation and dinitrogen fixation yielding a maximum of 86 mu mol of ammonia/(h/0.1 g of catalyst), thus achieving the activation of N-2 at room temperature and atmospheric pressure.