**5. Conclusions**

A combinational methodology of hydrothermal growth and sputtering was used to synthesize ZnO–WO3 composite nanorods. Furthermore, a thermal annealing procedure was conducted in a hydrogen-contained atmosphere to induce a microstructural modification of the composite nanorods. The structural analysis revealed that the ZnO nanorods sputtering coated with the ultra-thin WO3

thin film formed well crystalline ZnO–WO3 composite nanorods. The thermal annealing procedure at 400 ◦C further engendered the formation of ternary ZnWO4 phase and deteriorated WO3 phase on the surfaces of the ZnO nanorods. The ethanol gas-sensing test results demonstrated that the construction of the ZnO–WO3 composite nanorods is advantageous for improving the gas-sensing response of the ZnO nanorods to ethanol vapor. The formation of the heterogeneous junction between the ZnO and WO3 contributed to the enhanced ethanol gas-sensing responses. Moreover, an increase of potential barrier number in the ZnO–WO3 composite nanorods annealed at 400 ◦C improved the gas-sensing responses of the composite nanorods without a thermal annealing. The composite nanorods annealed at 400 ◦C exhibited a strong response of 16.2 at the gas concentration of 50 ppm, while the pristine ZnO–WO3 could only reach 7.3 at the identical gas concentration. Such intriguing ethanol gas-sensing response enhancement could be ascribed to the existence of heterogeneous junctions at interfaces of ZnO/ZnWO4, ZnO/WO3, and ZnWO4/WO3 in the composite nanorods after annealing at 400 ◦C. The local structural modification of the composite nanorods through a proper thermal annealing condition is feasible to control the gas-sensing behavior of the oxide composite nanorods. Moreover, the ZnO–WO3 nanorods annealed at 400 ◦C exhibited high selectivity to ethanol vapor among the various target gases of NH3, H2, and NO2. This composite nanorod system is of potential to e ffectively detect ethanol vapor in an open environment.

**Author Contributions:** Methodology, C.W.C.; formal analysis, C.W.C.; Writing—original draft preparation, Y.C.L. and C.W.C.; supervision, Y.C.L.; Writing—Review and Editing, Y.C.L.

**Funding:** This research was funded by Ministry of Science and Technology of Taiwan. Grant No. MOST 105-2628-E-019-001-MY3 & 107-2813-C-019-020-E.

**Conflicts of Interest:** The authors declare no conflict of interest.
