Heterogeneous Wet Chemical Synthesis of Superlattice-Type Hierarchical ZnO Architectures for Concurrent H₂ Production and N₂ Reduction

Abstract

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 highdefect 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 µmol of ammonia/(h/0.1 g of catalyst), thus achieving the activation of N 2 at room temperature and atmospheric pressure.

Production of electricity and hydrogen by photocatalytic degradation of organic wastes in a photoelectrochemical cell

“…This discrepancy simply suggests that the performance of a photocatalyst depends, first of all, on some crucial parameters, for example, methods of preparation, nanoparticle size, crystallinity etc. ZnO is considered to be vulnerable to oxidation (ZnO + 2h + →Zn 2+ + 1/2O 2 ) [79,80] but this process is impeded in the presence of a sacrificial agent that consumes holes, as is the case of the PFCs. ZnO was frequently used for making photoanodes in the past [79,81,82] and it was the semiconductor with which the so called "current doubling effect" was originally observed (see Section 7.2).…”

“…ZnO has been synthesized in many different nanostructures like nanoneedles [88], nanorods [87], nanofibers [89], nanowires [90], etc. and it has an inherent ability to crystallize in various geometries resulting in hierarchical architectures [80]. For all these properties, ZnO has been broadly studied, being second studied oxide semiconductor after titania.…”

Quantum‐Dot‐Sensitized Nitrogen‐Doped ZnO for Efficient Photoelectrochemical Water Splitting

“…[14] However, the majority of studies on ZnO:N have focused on its PEC performance. [10,14,15] Reports on sensitized ZnO:N, which has more potential for efficient PEC water splitting, have been few.…”

Hierarchical Nitrogen‐Doped Flowerlike ZnO Nanostructure and Its Multifunctional Environmental Applications

“…, a characteristic strong ZnO stretching band around 470 cm −1 is observed 26. The absorption bands at 2900 and 2395 cm −1 are attributed to respective stretching vibration of CH modes and the absorption of atmospheric CO 2 on the samples.…”

“…). Obviously, the ZnO flowerlike nanostructure has a mesoporous nature arising from the interpetal pores 26. This merit would improve the specific surface area greatly.…”