Structure-dependent gas detection ability of clustered ZnS crystallites with heterostructure and tube-like architecture
Introduction
Core–shell heterostructures and tube-like nanomaterials have attracted considerable attention because their versatility renders them suitable for use in many devices [1], [2], [3]. As a major II–VI group semiconductor, zinc oxide (ZnO) has a wide band gap of approximately 3.3 eV at room temperature [4]. ZnO nanostructures can be prepared on a large scale by using a facile hydrothermal method [5], and ZnO nanostructures with various morphologies have been extensively studied for use in gas sensors [6], [7]. By contrast, zinc sulfide (ZnS), another II–VI group semiconductor, has a wider band gap (3.7 eV) and has been used in gas and ultraviolet (UV) light sensors [8], [9]. According to the relevant research, oxide and sulfide semiconductors with one-dimensional (1D) structures can potentially be applied in highly efficient nanodevices. One-dimensional semiconductors with a three-dimensional architecture exhibit a high surface-to-volume ratio and excellent physical and chemical properties [2], [6]. Experimental results have revealed that combining two wide band gap semiconductors can yield a novel material featuring improved functions compared with the individual semiconductor material [10], [11], [12]. In addition to heterostructures, 1D materials with a porous structure or a tube-like morphology have been demonstrated to exhibit superior gas sensing response and sensitivity [13], [14].
Although several studies have shown the superior gas sensing properties of oxide-based heterostructures and tubes, reports on the gas sensing property of ZnS-based heterostructures and tubes are limited [15]. Therefore, the gas sensing behavior of ZnS-based heterostructure and tubes toward various target gases requires further investigation for enhancing and optimizing the gas sensing characteristics of these materials. Several facile chemical approaches have been proposed for synthesizing various ZnS-based heterostructures and porous structures [16], [17], [18]. These studies revealed that different chemical solution routes led to various morphologies of ZnS-based heterostructures and porous structures. The microstructure of nanomaterials affects their gas sensing properties [19]. Although detailed methodology dependent structural characterizations of the ZnS-based heterostructures and porous structures have been performed, no studies have investigated the applications of these materials in gas-sensing devices. This insufficient information has hindered development of ZnS-based heterostructures and porous structures for applications in gas-sensing devices. In this paper, we report the synthesis of high-density ZnS-based heterostructures and tubes using ZnO rods as the sacrificial template for various durations of sulfidation. The ZnS-based heterostructures and tubes synthesized were structurally characterized. Moreover, the capabilities of the heterostructures and tubes to sense reducing and oxidizing gases were compared. The mechanisms associated with the gas-sensing responses were examined to determine the origin of the superior gas-sensing response of the ZnS-based heterostructures and tubes.
Section snippets
Experiments
Hydrothermally synthesized high density ZnO rods on the 200 nm-thick SiO2/Si (1 0 0) substrates were used as templates for further sulfidation treatment to growth of ZnO–ZnS core–shell rods and ZnS nanotubes. The synthesis of vertically aligned ZnO rods consisted of two steps corresponding to the formation of ZnO seed layer and the growth of rods. The detailed experiment on the synthesis of hydrothermally synthesized ZnO rods has been described elsewhere [20]. The as-synthesized ZnO rods were
Structural characterization
Fig. 1 (a) shows a SEM image of the as-prepared ZnO rods; the ZnO rods were covered homogeneously over the substrate. The diameter of the ZnO rods ranged from 70 nm to 100 nm, and the surface of the ZnO rods was smooth. Fig. 1(b) shows the SEM image of the ZnO rods subjected to a 5-h sulfidation treatment. After sulfidation, the hexagonal faces of the rods became rounded. The surface of the ZnO rods was covered with many granular particles, resulting in a rough surface. Density of the irregular
Conclusions
ZnO–ZnS core–shell rods and ZnS tubes were synthesized using ZnO rods as the sacrificial template during sulfidation. The experimental results showed that the duration of sulfidation affected the formation of the ZnS crystallites required for a core–shell heterostructure (ZnO–ZnS core–shell rods) or tube-like architecture. The structural analyses revealed that the surface of the ZnO–ZnS core–shell rods and ZnS tubes was rough because many tiny ZnS particles aggregated to form the ZnS shell
Acknowledgement
This work is supported by the Ministry of Science and Technology of Taiwan (Grant No. NSC 102-2221-E-019-006-MY3).
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