Synthesis of TiO2-ZnS nanocomposites via sacrificial template sulfidation and their ethanol gas-sensing performance

Abstract

TiO₂-ZnS core-shell composite nanorods were synthesized by using ZnO as a sacrificial shell layer in a hydrothermal reaction. ZnO thin films of different thicknesses were sputter-deposited onto the surfaces of TiO₂ nanorods as templates for hydrothermally synthesizing TiO₂-ZnS core-shell nanorods. Structural analysis revealed that crystalline TiO₂-ZnS composite nanorods were formed without any residual ZnO phase after hydrothermal sulfidation in the composite nanorods. The thickness of the ZnO sacrificial shell layer affected the surface morphology and sulfur-related surface defect density in hydrothermally grown ZnS crystallites of TiO₂-ZnS composite nanorods. Due to the distinctive core-shell heterostructure and the heterojunction between the TiO₂ core and the ZnS shell, TiO₂-ZnS core-shell nanorods exhibited ethanol gas-sensing performance superior to that of pristine TiO₂ nanorods. An optimal ZnO sacrificial shell layer thickness of approximately 60 nm was found to enable the synthesis of TiO₂-ZnS composite nanorods with satisfactory gas-sensing performance through sulfidation. The results demonstrated that hydrothermally derived TiO₂-ZnS core-shell composite nanorods with a sputter-deposited ZnO sacrificial shell layer are promising for applications in gas sensors.

Fig. 1. SEM images of various nanorods: (a) TiO₂, (b) TiO₂–ZnS-1, (c) TiO₂–ZnS-2, and (d) TiO₂–ZnS-3.

Fig. 2. XRD patterns of various nanorods: (a) TiO₂, (b) TiO₂–ZnS-1, (c) TiO₂–ZnS-2, and (d) TiO₂–ZnS-3.

Fig. 3. TEM analyses of the TiO₂–ZnS-1 nanorod: (a) low-magnification TEM image, (b) and (c) HRTEM images taken from various regions of the nanorod (d) EDS line-scanning profiles across the composite nanorod.

Fig. 4. TEM analyses of the TiO₂–ZnS-2 nanorod: (a) low-magnification TEM of the TiO₂–ZnS-2 nanorod, (b) and (c) HRTEM images taken from various regions of the nanorod, (d) SAED pattern of several TiO₂–ZnS-2 nanorods, (e) EDS spectrum of the nanorod.

Fig. 5. TEM analyses of the TiO₂–ZnS-3 nanorod: (a) low-magnification TEM of the TiO₂–ZnS-3 nanorod, (b) and (c) HRTEM images taken from various regions of the TiO₂–ZnS-3 nanorod, (d) SAED patterns of several TiO₂–ZnS-3 nanorods, and (e) EDS spectrum of the TiO₂–ZnS-3 nanorod.

Fig. 6. High-resolution XPS spectra of various TiO₂–ZnS nanorods: Zn2p of (a) TiO₂–ZnS-1, (b) TiO₂–ZnS-2, (c) TiO₂–ZnS-3. S2p of (d) TiO₂–ZnS-1, (e) TiO₂–ZnS-2, and (f) TiO₂–ZnS-3.

Fig. 7. Temperature-dependent gas-sensing response of the TiO₂–ZnS-3 nanorods exposed to 250 ppm ethanol vapor.

Fig. 8. Dynamic gas-sensing response–recovery curves of the various sensors exposed to different concentrations of ethanol vapor (50–1000 ppm): (a) TiO₂–ZnS-1, (b) TiO₂–ZnS-2, and (c) TiO₂–ZnS-3. (d) Gas-sensing response values comparison of the sensors based on the TiO₂, TiO₂–ZnS-1, TiO₂–ZnS-2, and TiO₂–ZnS-3 toward to 50–1000 ppm ethanol vapor. (e) Cycling tests of the TiO₂–ZnS-3 sensor on exposure to 250 ppm ethanol vapor. (f) Dynamic gas-sensing response–recovery curve of the TiO₂–ZnS-3 sensor exposed to 5 ppm ethanol vapor.

Fig. 9. Schematic diagrams of the gas-sensing mechanisms of TiO₂–ZnS composite nanorods.

Fig. 10. (a) SEM image of TiO₂ nanorods with the 80 nm-thick ZnO sacrificial layer after sulfidation. (b) Dynamic gas-sensing response–recovery curves of TiO₂–ZnS nanorods. (c) Gas-sensing responses of TiO₂–ZnS nanorods made from 80 nm-thick ZnO sacrificed layer after sulfidation.