**1. Introduction**

Composite nanorod systems composed of various binary semiconductor oxides have been shown a promising approach to enhance the gas-sensing properties of the constituent compounds. For example, hydrothermally derived flower-like CeO2–SnO2 composites exhibit improved trimethylamine gas-sensing response than that of the pristine SnO2 [1]. Moreover, hydrogen-sensing properties of ZnO nanofibers are significantly enhanced through NiO loading in a composite structure [2]. SnO2/ZnO hetero-nanofibers demonstrate improved acetone gas-sensing responses in comparison with that of the pristine ZnO nanofibers [3]. By tuning the sputtering coated VOx morphology on the one-dimensional ZnO, ZnO–VOx composites demonstrate improved oxidizing gas-sensing responses than that of the pristine ZnO [4].

Among various binary oxides, ZnO is one of the most studied n-type semiconductor oxides which was widely used for gas-sensing material because of its low cost, high chemical stability, and versatile preparation methods. Furthermore, ZnO in a low-dimensional structure is of potential interest for gas sensor device applications because its high specific surface area enables efficient reaction between the oxide surface and target gas molecules [5,6]. However, developing high gas-sensing responses of the ZnO nanostructures toward various target gases is still highly desired and is technically challenging. Various ZnO-based composite systems incorporated with another binary oxide have been proposed to improve the ZnO gas-sensing properties based on the aforementioned demand [2,4,7]. By contrast, WO3 is another promising gas-sensing binary oxide. It also has advantages of low cost, high chemical stability, and excellent process-dependent reproducibility. Recent progress has shown that WO3 with various

morphologies is promising in applications of gas sensors to detect toxic gases [8–10]. Moreover, WO3 crystals can be synthesized through various physical and chemical methods and the crystalline quality and morphology can be easy controlled through varying the process conditions. [11,12]. Although various ZnO-based composite systems have been proposed to improve the gas-sensing responses to target gases, the reports on construction of one-dimensional ZnO–WO3 composite system are still limited in number. Moreover, thermal annealing of solid materials is an efficient method to modulate their microstructures and to control their physical and chemical properties [13,14]. The past research works show that conducting proper thermal annealing procedures causes the possible solid-state reaction between the constituent oxides in a nanoscale in low-dimensional oxide systems [15,16]. The existence of phase transformation between the binary oxides in a low-dimensional composite system modifies the electric properties of the original composite systems without thermal annealing procedures. This broadens the design of the functionality and changes property performance of the pristine oxide composite system.

#### **2. Materials and Methods**

In this study, ZnO-based composite nanorods coated with the WO3 and ZnWO4 shell layers were synthesized through a combinational methodology of hydrothermal growth and sputtering. Hydrothermally synthesized high-density ZnO nanorods were used as templates for growing the ZnO-based composite nanorods. The hydrothermal growth reactions of the ZnO nanorods were conducted at 95 ◦C for 9 h. The detailed process procedures were reported elsewhere [14]. During sputtering growth of the WO3 shell layer, the sputtering power of tungsten metallic target was fixed at 80 W. The thin-film growth temperature was maintained at 375 ◦C with an Ar/O2 ratio of 3:2. Then, the as-synthesized ZnO–WO3 composite nanorods were subsequently annealed in a 95% N2/5% H2 atmosphere for 20 min at the temperatures of 400~500 ◦C to induce a solid-state reaction between the ZnO surface and WO3 ultra-thin layer.

The scanning electron microscopy (SEM; Hitachi S-4800, Tokyo, Japan) was used to investigate the surface morphology of nanorod samples. X-ray diffraction (XRD; Bruker D2 PHASER, Karlsruhe, Germany) was further used to investigate crystallographic structures of the samples. Moreover, the detailed microstructures of the composite nanorods with and without the thermal annealing were characterized by high-resolution transmission electron microscopy (HRTEM; Philips Tecnai F20 G2, Amsterdam, The Netherland). X-ray photoelectron spectroscope (XPS; PHI 5000 VersaProbe, Chigasaki, Japan) analysis was performed to determine the chemical binding states of the constituent elements of the composite nanorods. Silver contact electrodes were formed on the surface of the nanorod samples for gas-sensing measurements. The ethanol vapor concentrations of 25–500 ppm were used as target gas. The dry air was used as carrier gas herein. The variation of sensor resistance before and after introducing ethanol vapor was recorded. The gas-sensing response of the sensors to ethanol vapor herein is defined as the Ra/Rg, which is the electric resistance ratio of the resistance of the gas sensor in the absence of ethanol vapor to the resistance of the sensor in ethanol vapor.
