Generation of oxygen vacancies in ZnO nanorods/films and their effects on gas sensing properties

Abstract

The ethanol gas sensing properties of ZnO films and ZnO nanorod arrays that had been pretreated with H₂O₂ and annealed were examined. ZnO films were grown by sputtering, and ZnO nanorod arrays were synthesized on the sputtered ZnO films through a simple hydrothermal method. The ZnO nanorod gas sensors exhibited highly enhanced responses compared to the ZnO films due to the higher surface areas of the former. H₂O₂ pretreatment and annealing generated oxygen vacancies on the ZnO surface. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and diffused reflectance spectroscopy (DRS) methods were used to characterize the oxygen vacancies on the ZnO surfaces. The induced surface oxygen vacancies enhanced the gas sensing responses. The gas sensing mechanism in the ZnO-based gas sensor was discussed in terms of the effects of the oxygen vacancies.

Introduction

Gas sensors based on metal oxides, such as SnO₂, TiO₂, and ZnO, have attracted much attention due to their high sensitivity, low cost, and simple fabrication [1], [2], [3]. Since Seiyama et al. [4] discovered that the adsorption and desorption of gases produced rapid and remarkable changes in the electrical conductivities of semiconductors, several studies of the utility of semiconducting metal oxide materials as gas sensors have been reported [5], [6], [7], [8]. ZnO is one of the most promising metal oxide materials for this purpose due to its thermal and chemical stability under standard operating environments, its high conductivity, and its non-toxic properties [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. ZnO gas sensors have been intensively investigated recently, and films have been fabricated in a variety of structures, including as single crystals, thick/thin amorphous films, nanorod arrays, and nanotube assemblies [14], [15], [16], [17]. The general gas sensing mechanism underlying metal oxide sensor function involves a resistance change through a reaction between the oxygen species (${\text{O}}_2^{-},{\text{O}}_2^{2-},{\text{\hspace{0.17em}O}}^{2-}$) chemisorbed onto the metal oxide surface and the target gas molecules. Under ambient air conditions, the surface of a metal oxide material is covered with charged oxygen species that accept charge carriers (electrons) to form a depletion region on the surface. As these species react with reducing agents, oxygen species are desorbed from the surface and release the trapped electrons, which reduces the film thickness in the depletion region and, consequently, the resistance. As the surface coverage of adsorbed oxygen species increases, the gas sensing response is expected to increase as well. Although the surface oxygen species density usually varies with the surface area, however, the main factors that affect the adsorption of oxygen species are the surface atomic structures related to the exposed facets and the surface defect densities, including zinc and oxygen vacancies [9], [14], [18].

Oxygen vacancies are the predominant native defect present on metal oxide surfaces. Oxygen vacancies play an important role in metal oxide function, including ZnO, because they act as shallow donors with substitutional hydrogen atoms that exhibit n-type conductivity [19]. In addition, surface oxygen vacancies can present strong adsorption sites for oxidizing molecules. The surface oxygen vacancy density depends on the type of exposed facet of material [20]. For example, wurtzite ZnO nanorods have exposed surfaces with a top hexagonal plane corresponding to the polar (0 0 0 1) surface and side planes corresponding to the non-polar $\left\{1\overline{1}00\right\}$ surfaces. In general, the top (0 0 0 1) surface has more oxygen vacancies than the side $\left\{1\overline{1}00\right\}$ surfaces; hence the reactivity on the (0 0 0 1) surface is higher than on the side $\left\{1\overline{1}00\right\}$ surfaces [21]. Recent gas sensing studies reported that varying the exposed facets, surface areas, or aspect ratios may be used to control the surface oxygen vacancies [9], [18], [22]. It is difficult, however, to claim that oxygen vacancies alone contributed to the enhanced sensing properties; possibly other factors could be involved during changing structures or surface areas, and thus a systematic analysis of the effects of oxygen vacancies is needed. Few studies have attempted to control the oxygen vacancies alone as a means for enhancing the sensing properties. The introduction of stable oxygen vacancies at a surface poses several challenges.

Nanomaterials, especially one-dimensional (1D) nanostructures, provide good sensing properties because of their high surface-to-volume ratio (resulting from a high nanostructure aspect ratio), which facilitates the adsorption of oxygen groups and rapid transport of charge carriers. Many reports have attempted to improve the gas sensing properties of a device by introducing 1D nanostructures [8], [9], [10], [11], [12], [13], [14], [15]. The effects of the nanostructures, however, are not yet fully understood because the gas sensing properties can vary with the crystal size, morphology, fabrication method, and applied processing steps. In most cases, the synthesis of nanostructures requires high temperatures or complicated processes. The reproducibility of nanostructure synthesis tends to be poor.

In this work, we fabricated ZnO nanorod arrays using a simple hydrothermal method and investigated the gas sensing properties of these arrays in comparison with the corresponding properties of a ZnO film. The responses of the ZnO gas sensors were found to improve upon introduction of the ZnO nanorod arrays, and the surface preparation was highly reproducible. The target gas tested here was ethanol, which reacts with oxygen to form CO₂ and H₂O. The ZnO structures were used to develop oxygen vacancy-containing ZnO gas sensors through application of a post-treatment process with H₂O₂ and annealing. These studies revealed the mechanism underlying the contributions of oxygen vacancies to the gas sensing properties. The generation of surface oxygen vacancies was confirmed by performing a variety of analyses. The analytic results and gas sensing properties were compared to establish the relationship between the oxygen vacancies and the responses of the ZnO gas sensors.