Various Applications of ZnO Thin Films Obtained by Chemical Routes in the Last Decade

Abstract

This review addresses the importance of Zn for obtaining multifunctional materials with interesting properties by following certain preparation strategies: choosing the appropriate synthesis route, doping and co-doping of ZnO films to achieve conductive oxide materials with p- or n-type conductivity, and finally adding polymers in the oxide systems for piezoelectricity enhancement. We mainly followed the results of studies of the last ten years through chemical routes, especially by sol-gel and hydrothermal synthesis. Zinc is an essential element that has a special importance for developing multifunctional materials with various applications. ZnO can be used for the deposition of thin films or for obtaining mixed layers by combining ZnO with other oxides (ZnO-SnO₂, ZnO-CuO). Also, composite films can be achieved by mixing ZnO with polymers. It can be doped with metals (Li, Na, Mg, Al) or non-metals (B, N, P). Zn is easily incorporated in a matrix and therefore it can be used as a dopant for other oxidic materials, such as: ITO, CuO, BiFeO_3, and NiO. ZnO can be very useful as a seed layer, for good adherence of the main layer to the substrate, generating nucleation sites for nanowires growth. Thanks to its interesting properties, ZnO is a material with multiple applications in various fields: sensing technology, piezoelectric devices, transparent conductive oxides, solar cells, and photoluminescence applications. Its versatility is the main message of this review.

Keywords: ZnO thin films; composite materials; doping; nanostructures; seed layer; sol-gel.

Figure 1

Evolution of the number of scientific papers related to the search of “ZnO” and “ZnO thin films” phrases published between 1980–2023. Inset: illustration of the last decade. Source: Scopus (accessed on 12 May 2023).

Figure 2

Graphic illustration of sol-gel solution preparation, ZnO seed layer deposition and ZnO nanorod growth. Reprinted from [94] with permission from Elsevier.

Figure 3

Schematic diagram of CBD technique used to obtain ZnO thin films. Reprinted from [113] with permission from Elsevier.

Figure 4

Graphical representation of SILAR method used to obtain ZnO thin films. Reprinted with permission from [116]. Copyright 2022, Yergaliuly et al.

Figure 5

BF TEM (a) and HAADF images from ZnO NWs (b) on Au substrate. Reprinted from [47] with permission from Elsevier.

Figure 6

The growth directions of ZnO wurtzite crystal and possible morphologies. Reprinted from [143]. Copyright 2017, Leonardi, S.

Figure 7

SEM images of (A) AZO, (B) AZO:rGO (2 wt.%), (C) AZO:rGO (4 wt.%), and (D) AZO:rGO (6 wt.%) composite thin films. Reprinted from [160] with permission from Elsevier.

Figure 8

Band structure (a) and partial density of states (b) of ZnO. Reprinted from [198] with permission from Elsevier.

Figure 9

Applications of ZnO in different fields.

Figure 10

(a) PL spectra for as-grown and annealed AZO samples, (b) AZO450 PL spectra before and after glucose exposure at different concentrations in the presence of GOx. Reprinted from [30] with permission from Elsevier.