Structure-correlated excitation wavelength-dependent optical properties of ZnO nanostructures for multifunctional applications

Abstract

The origin of visible emissions in ZnO is often attributed to the different intrinsic defect states, and is still highly debatable. In contrast, the origin of excitation wavelength-dependent tunable emission from ZnO remains largely unexplored. Therefore, we have measured the photoluminescence emissions from different ZnO nanostructures as a function of excitation wavelengths. We have also reported how the growth of the prepared low-dimensional nanostructures (nanoflakes, nanothorns, and nanorods) can be tuned from a thermodynamically controlled to a kinetically controlled process beside the investigation of the origin of excitation wavelength-dependent tunable emissions from them. Nanoflakes with a large exposed (0001) plane exhibited below band gap excitation and excitation wavelength-independent dominant green emission. Besides this, nanoflakes displayed a new phenomenon of oxygen vacancy (V_O) induced band gap narrowing, which can help us to tune their band gap. In contrast, the nanothorn (orange-red emitter) and nanorod (yellow emitter) nanostructures displayed a blue-shift in the emission peaks to the green region with an increase in excitation wavelengths. The results thus showed that green emission is attainable at below band gap excitation energies, but yellow and orange-red emissions are possible only at or above bandgap excitation energies. The origin of yellow and orange-red emissions has been attributed to the generated peroxide-like (O₂²⁻) surface species and superoxide charge transfer states () over the nanostructures, respectively, through controlled experiments. However, the origin of green emission has been attributed to the presence of sub-band-gap states and oxygen vacancies (V_O) below the nanostructure surface. Thus, we concluded that the excitation wavelength-dependent tunable visible emission properties originated from structure-correlated oxygen-related species instead of defect states. The application prospects of the nanostructures have also been explored. Nanothorns (orange-red) displayed the highest PLQY of 6.54%, whereas nanorods displayed the highest colour purity (yellow) of 95%. This makes them suitable for tuneable LED applications with low turn-on voltage in the range of 2–2.36 V. The nanoflakes and nanorods also displayed high photocatalytic dye degradation efficiencies (81% and 75.17%, respectively) and recyclability, which demonstrated their suitability as a photocatalyst. Thus, this work outlines the nanostructure design perspectives for achieving high-efficiency tunable emissions and photocatalytic degradation properties in metal-oxides.