TiO₂-Based Nanoheterostructures for Promoting Gas Sensitivity Performance: Designs, Developments, and Prospects

Abstract

Gas sensors based on titanium dioxide (TiO₂) have attracted much public attention during the past decades due to their excellent potential for applications in environmental pollution remediation, transportation industries, personal safety, biology, and medicine. Numerous efforts have therefore been devoted to improving the sensing performance of TiO₂. In those effects, the construct of nanoheterostructures is a promising tactic in gas sensing modification, which shows superior sensing performance to that of the single component-based sensors. In this review, we briefly summarize and highlight the development of TiO₂-based heterostructure gas sensing materials with diverse models, including semiconductor/semiconductor nanoheterostructures, noble metal/semiconductor nanoheterostructures, carbon-group-materials/semiconductor nano- heterostructures, and organic/inorganic nanoheterostructures, which have been investigated for effective enhancement of gas sensing properties through the increase of sensitivity, selectivity, and stability, decrease of optimal work temperature and response/recovery time, and minimization of detectable levels.

Keywords: TiO2; gas sensor; nanoheterostructures.

Figure 1

Crystal structures of TiO₂: (a) Rutile; (b) Anatase; (c) Brookite; and (d) TiO₂(B), red spheres represent Ti atoms, and the grey spheres represent O atoms.

Figure 2

Schematic image of gas sensing at different modes, where L represents the depletion layer, R represents particle size, and D_N represents the diameter of the neck cross section.

Figure 3

(a–f) Scanning electron microscope (SEM) images of TiO₂ hollow fibers synthesized with 1000 ALD cycles (a); TiO₂/ZnO double-layer hollow fibers synthesized with 20 ALD cycles (b); 50 ALD cycles (c); 90 ALD cycles (d); 220 ALD cycles (e); and 350 ALD cycles (f); (g) Transmission electron microscope (TEM) image of a single TiO₂/ZnO DLHF; (h) High resolution transmission electron microscopy (HRTEM) image of the outer layer ZnO [72]. Copyright 2014 American Chemical Society.

Figure 4

(a–c) SEM images of (a) TiO₂ NTAs and (b) hydrothermally treated TiO₂ NTAs; (c) TEM image of a typical NiTiO₃/TiO₂ NTs, the inset is the corresponding energy dispersive spectrometer (EDS) spectrum [79]. Copyright 2015 Wiley. (d–i) SEM images of as-anodized TiO₂ NTs (d–f) and TiO₂ NTs filled by Co-precursor nanorods (g–i) in top view(d,g), cross-sectional view (e,h), and bottom view (f,i) [80]. Copyright 2013 Royal Society of Chemistry.

Figure 5

(A) TEM images of a-Fe₂O₃ nanorods; (B) HRTEM image of an individual a-Fe₂O₃ nanorod, the insets are the enlarged HRTEM image (C) and the corresponding fast Fourier transform (FFT) pattern (D) taken from the frame-marked region in (B); (E) TEM image of TiO₂/a-Fe₂O₃ nanoheterostructures; (F) HRTEM image of TiO₂/a-Fe₂O₃ nanoheterostructures, the insets are the enlarged HRTEM image (G) and the corresponding FFT pattern (H) taken from the frame-marked region in (F) [65]. Copyright 2014 Royal Society of Chemistry.

Figure 6

SEM images of branched α-Fe₂O₃/TiO₂ nanoheterostructures: (a,b) panoramic and (c,d) magnified [62]. Copyright 2013 American Chemical Society.

Figure 7

(a) A diagram of the electrospinning process; (b–e) SEM images of (b,c) TiO₂ nanofibers and (d,e) TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures; (f) N₂ adsorption/desorption isotherms and (g) XRD patterns of TiO₂ nanofibers and TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures [95]. Copyright 2016 Nature.

Figure 8

(a–d) Gas response of the TiO₂ nanobelts and the SnO₂-TiO₂ hybrid oxides based sensors to 400 ppm methanol (a); ethanol (b); formaldehyde (c); and acetone (d) gases at different operating temperatures; (e) Response/recovery characteristics of the TiO₂ nanobelts and the SnO₂-TiO₂ hybrid oxides based sensors operated at 593 K to 400 ppm methanol, ethanol, formaldehyde, and acetone [105]. Copyright 2012 Royal Society of Chemistry.

Figure 9

Gas responses of different sensors (S-15: SBA-15, TS-s: TiO₂/SnO₂ (soft template), TS-h: TiO₂/SnO₂ (hard template), ATS-h: Ag-(TiO₂/SnO₂)) to ethanol operated at 275 °C, the inset is the calibration curve within the concentration ranging from 1 ppm to 50 ppm [104]. Copyright 2016 Royal Society of Chemistry.

Figure 10

The proposed sensing mechanism diagram of TiO₂/Ag_0.35V₂O₅ nanoheterostructures. (a) Schematic band structure of TiO₂/Ag_0.35V₂O₅ heterojunction exposed in air and ethanol gases (qΦ: energy barrier); (b) Sensing model of the TiO₂/Ag_0.35V₂O₅ nanoheterostructured sensor in air (Steps 1–3) and in ethanol (Steps 4–5) [95]. Copyright 2016 Nature.

Figure 11

(a) Energy band diagram of In₂O₃ and TiO₂, E_C: conduction band, E_V: valence band; (b) I-V curves of In₂O₃ nanofibers (INFs) and In₂O₃ beads@TiO₂-In₂O₃ composite nanofibers (TINF2) thin film sensors in air at room temperature (the gate voltage Vg = 0.1); (c) The gas sensing reactions based on Schottky junction between Au electrode and In₂O₃ beads [61]. Copyright 2015 American Chemical Society.

Figure 12

(a–d) SEM images of TiO₂ nanofibers (a); ZnO nanorods (b); and ZnO-TiO₂ nanoheterostructures (c,d); (e,f) Schematic diagram of catalytic reactions (e) and ideal band structure (f) of ZnO-TiO₂ nanoheterostructures [110]. Copyright 2013 Elsevier.

Figure 13

(a) Sensing response of the Fe₂O₃/TiO₂ tube like nanoheterostructures to ethanol at different temperatures; (b) Time dependent sensing response of the Fe₂O₃/TiO₂ tube like nanoheterostructures to ethanol vapor at 270 °C [107]. Copyright 2012 American Chemical Society.

Figure 14

(a) Sensing response of TiO₂/ZnO double layer hollow fibers to CO gas as a function of ZnO outer layer thickness; (b,c) Schematic diagrams of sensing mechanism of (b) ZnO hollow fibers and (c) TiO₂/ZnO double-layer hollow fibers [72]. Copyright 2014 American Chemical Society.

Figure 14

(a) Sensing response of TiO₂/ZnO double layer hollow fibers to CO gas as a function of ZnO outer layer thickness; (b,c) Schematic diagrams of sensing mechanism of (b) ZnO hollow fibers and (c) TiO₂/ZnO double-layer hollow fibers [72]. Copyright 2014 American Chemical Society.

Figure 15

(a) response curves and (b) response values of pure rGO and rGO/TiO₂ layered films to 0.1–0.5 ppm CH₂O [75]. Copyright 2015 Elsevier.

Figure 16

(a,b) HRTEM images of CNT/TiO₂ nanocmposites; (c–e) Response to 10 ppm of O₂ in CO₂ flow at 450 °C for (a) a TiO₂/MWCNT sensor annealed at 500 °C; (b) a TiO₂/MWCNT sensor annealed at 600 °C; and (c) a Nb-doped TiO₂/MWCNT sensor annealed at 500 °C [154]. Copyright 2008 Institute of Physics.

Figure 17

(a) SEM image of MWCNTs/TiO₂ xerogel film; (b) CO sensing properties of pure TiO₂ xerogel film and MWCNTs/TiO₂ xerogel film to 50 ppm CO at 350 °C [151]. Copyright 2013 Elsevier.

Figure 18

(a,b) Atomic force microscope (AFM) images of the surface morphology of PAA25 (a) and PAA400 (b) deposited on TiO₂ gel-immobilized mica; (c) Dynamic responses of the quartz crystal microbalance (QCM) electrode coated with a (TiO₂/PAA400)₂₀ film to ammonia at different concentrations. The inset shows a comparison of the calibration curves with data taken at different times; (d) Calibration curves for (TiO₂/PAA)_n (n = 5, 10, and 20) films [94]. Copyright 2010 American Chemical Society.

Figure 19

Gas response of a PPy/TiO₂ heterojunction at a fixed voltage of +0.6 V at concentration of 1040 ppm of LPG [83]. Copyright 2013 Elsevier.

Figure 20

(a–f) SEM images of PANi (a); TiO₂ (b); and PANi–TiO₂ (20–50 wt %) films (c–f); (g) Response of the pure TiO₂, pure PANi, and nanocomposite of the PANi-TiO₂ film toward 100 ppm NH₃ gas at room temperature [74]. Copyright 2012 Wiley.