Evolution of ZnO microstructures from hexagonal disk to prismoid, prism and pyramid and their crystal facet-dependent gas sensing properties

Abstract

Herein, the evolution of ZnO structures from hexagonal disk to prismoid, prism and pyramid was found via a facile two-step low temperature hydrothermal reaction, and the evolution was achieved by only adjusting the pH value of the reactive solution without the assistance of a template or a surfactant. The characterization results showed that the precursor (hexagonal Zn 5 (OH) 8 Cl 2 •2H 2 O disk) played a key role in the morphology evolution of ZnO during the early stage of the growth process and that the disks tended to stack together layer by layer in both directions (up and down) to form prismoid, prism and pyramid structures with the increase in pH value from 7 to 10. After calcination, the corresponding hexagonal ZnO microstructures were obtained. This structure evolution resulted in the weakening dominance of the (0001) plane in the total exposed crystal facets. Furthermore, despite the similar specific surface areas of the four hexagonal ZnO microstructures, the gas sensing properties of the sensors based on these microstructures deteriorated sequentially. At a working temperature of 330 °C, the ZnO disk with the most exposed (0001) plane showed the highest gas response toward ethanol, which was nearly 2, 3, and 6 times higher than those of the prismoid, prism and pyramid structures, respectively. This superior gas sensing performance strongly depends on the predominantly exposed polar facets (0001), which can provide more active sites for oxygen adsorption and subsequent reaction with the detected gas than other apolar facets. It demonstrates that the (0001) crystal facet plays a significant role in the gas sensing behavior of ZnO. This research will bring some inspiration to researchers for the fabrication of a high performance ZnO gas sensor as well as other metal oxides.

Surface-Controlled Photocatalysis and Chemical Sensing of TiO2, α-Fe2O3, and Cu2O Nanocrystals

“…that cannot be made explicit in the case of randomly oriented nanomaterials but that can be elucidated and exploited in the case of shape-controlled nanocrystals. Thus, recently substantial research has been initiated to take advantage of the selective exposure of high-energy facets of numerous metal oxide nanocrystals such as SnO2 [197][198][199][200], ZnO [198,[201][202][203][204][205], WO3 [198,206] Furthermore, the grain boundaries barriers are independent of the grain size, therefore, the sensitivity is independent of the average crystallite size.…”

“…that cannot be made explicit in the case of randomly oriented nanomaterials but that can be elucidated and exploited in the case of shape-controlled nanocrystals. Thus, recently substantial research has been initiated to take advantage of the selective exposure of high-energy facets of numerous metal oxide nanocrystals such as SnO 2 [197][198][199][200], ZnO [198,[201][202][203][204][205], WO 3 [198,206], anatase TiO 2 [9,106,112], α-Fe 2 O 3 [129,132,134,135], Cu 2 O [27,76,78,139,144,148] and their anisotropic properties to improve gas sensing behavior. The chosen examples that follow, illustrate this new trend.…”

Design and Synthesis of Novel 2D Porous Zinc Oxide-Nickel Oxide Composite Nanosheets for Detecting Ethanol Vapor

“…Among various binary metal oxides, ZnO shows distinct gas-sensing properties towards various harmful gases. ZnO in a form of nanostructure is of potential interest for the gas-sensing material application because nanostructured ZnO has a high specific surface area that can improve its gas-sensing performance towards target gases [ 5 , 6 , 7 ]. Several variables, such as size and morphology, have been shown to markedly affect ZnO nanomaterial’s sensing performance.…”

NH 4 F-assisted one-pot solution synthesis of hexagonal ZnO microdiscs for efficient ultraviolet photodetection

“…Owing to the high Q-factors, however, ZnO nanostructures such as micro-/nano-discs or plates, with suppressed c-axis orientation, are emerging and regarded as important building blocks for nanoscale optoelectronic devices [10]. Various fabrication methods have been reported to fabricate ZnO microdisc structure [11–13]. In order to obtain high-quality ZnO microdiscs, vapour phase methods, such as vapour phase transport (VPT) and chemical vapour deposition (CVD), have been widely used.…”