Surface Morphology-Dependent Functionality of Titanium Dioxide-Nickel Oxide Nanocomposite Semiconductors

Abstract

In this study, TiO₂-NiO heterostructures were synthesized by combining hydrothermal and chemical bath deposition methods. The post-annealing temperature was varied to control the surface features of the TiO₂-NiO heterostructures. TiO₂-NiO heterostructures annealed at 350 °C comprised NiO-nanosheet-decorated TiO₂ nanostructures (NST), whereas those annealed at 500 °C comprised NiO-nanoparticle-decorated TiO₂ nanostructures (NPT). The NPT exhibited higher photodegradation activity than the NST in terms of methylene blue (MB) degradation under irradiation. Structural analyses demonstrated that the NPT had a higher surface adsorption capability for MB dyes and superior light-harvesting ability; thus, they exhibited greater photodegradation ability toward MB dyes. In addition, the NST showed high gas-sensing responses compared with the NPT when exposed to acetone vapor. This result was attributable to the higher number of oxygen-deficient regions on the surfaces of the NST, which increased the amount of surface-chemisorbed oxygen species. This resulted in a relatively large resistance variation for the NST when exposed to acetone vapor.

Keywords: functionality; morphology; semconductors; surface.

Figure 1

(a,b) Low- and high-magnification SEM images of TiO₂ nanostructures. (c,d) Low- and high-magnification SEM images of TiO₂–NiO composites formed at 350 °C. (e,f) Low- and high-magnification SEM images of TiO₂–NiO composites formed at 500 °C.

Figure 2

XRD patterns of TiO₂–NiO composites formed at (a) 350 °C and (b) 500 °C. The arrow represents the position of NiO Bragg reflection.

Figure 3

TEM analyses of TiO₂–NiO composites formed at 350 °C: (a) Low-magnification TEM image of a single TiO₂–NiO composite. (b) High-magnification TEM image taken from the TiO₂–NiO composite. (c) High-resolution transmission electron microscopy (HRTEM) image taken from the local region of the TiO₂–NiO composite. (d) Selected area electron diffraction (SAED) pattern of several TiO₂–NiO composites. (e) Energy dispersive X-ray spectroscopy (EDS) line-scanning profiles of the TiO₂–NiO composite. TEM analyses of TiO₂–NiO composites formed at 500 °C. (f,g) Low- and high-magnification TEM images of a single TiO₂–NiO composite, respectively. (h) HRTEM image taken from the local region of the TiO₂–NiO composite. (i) SAED pattern of several TiO₂–NiO composites. (j,k) EDS spectra of the NiO-nanoparticle-decorated TiO₂ nanostructures (NPT) and NiO-nanosheet-decorated TiO₂ nanostructures (NST) composites, respectively.

Figure 4

High-resolution XPS spectra in Ni 2p region of TiO₂–NiO composites formed at various temperatures: (a) 350 °C and (b) 500 °C. High-resolution XPS spectra in O 1s region of TiO₂–NiO composites formed at various temperatures: (c) 350 °C and (d) 500 °C.

Figure 5

Optical absorbance spectra of the pristine TiO₂ (black), NST (blue), and NPT (red).

Figure 6

Intensity variation of absorbance spectra of the methylene blue (MB) solution vs. irradiation duration containing various TiO₂–NiO composites under solar light irradiation: (a) NST. (b) NPT. (c) Schematic of photodegradation process of TiO₂–NiO composites toward MB. (d) The ratio of the remaining MB concentration after light irradiation (C) and the initial MB concentration without light irradiation (C_o) vs. irradiation time curves for the MB solution containing various TiO₂–NiO composites in dark conditions and under light irradiation. (e) Plot of ln (C_o/C) vs. reaction time for MB solution containing various TiO₂–NiO composites under irradiation. (f,g) Recycled performances of photodegradation of MB solution in the presence of the NST and NPT, respectively.

Figure 7

Gas-sensing response curves of various TiO₂–NiO composites on exposure to various acetone vapor concentrations (50–750 ppm): (a) NST. (b) NPT. (c) Gas-sensing responses vs. acetone vapor concentrations for various TiO₂–NiO composites. The gas sensing responses of the pristine TiO₂ flowers to 50 and 100 ppm acetone vapor are also shown for a comparison. (d,e) Cyclic gas-sensing response curves of the NST and NPT on exposure to 250 ppm acetone vapor, respectively. (f) Gas-sensing selectivity of NST.