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.

Hierarchical nanostructures with much increased surface-to-volume ratio have been of significant interest for prototypical gas sensors. Herein we report a novel resistive gas sensor based on TiO ₂ /V ₂ O ₅ branched nanoheterostructures fabricated by a facile one-step synthetic process, in which well-matched energy levels induced by the formation of effective heterojunctions between TiO ₂ and V ₂ O ₅ , a large Brunauer–Emmett–Teller surface area and complete electron depletion for the V ₂ O ₅ nanobranches induced by the branched-nanofiber structures are all beneficial to the change of resistance upon ethanol exposure. As a result, the ethanol sensing performance of this device shows a lower operating temperature, faster response/recovery behavior, better selectivity and about seven times higher sensitivity compared with pure TiO ₂ nanofibers. This study not only confirms the gas sensing mechanism for performing enhancement of branched nanoheterostructures, but also proposes a rational approach to the design of nanostructure-based chemical sensors with desirable performance.

Much greater surface-to-volume ratio of hierarchical nanostructures renders them attract considerable interest as prototypical gas sensors. In this work, a novel resistive gas sensor based on TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures is fabricated by a facile one-step synthetic process and the ethanol sensing performance of this device is characterized systematically, which shows faster response/recovery behavior, better selectivity, and higher sensitivity of about 9 times as compared to the pure TiO₂ nanofibers. The enhanced sensitivity of the TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures should be attributed to the extraordinary branched hierarchical structures and TiO₂/Ag_0.35V₂O₅ heterojunctions, which can eventually result in an obvious change of resistance upon ethanol exposure. This study not only indicates the gas sensing mechanism for performance enhancement of branched nanoheterostructures, but also proposes a rational approach to design nanostructure based chemical sensors with desirable performance.

Photocatalysis of TiO2 has recently drawn considerable attention, while the photoefficiency of TiO2 is limited by its large band-gap energy and usually fast electron-hole recombination. Here, we present an unconventional heterostructure of Ag nanoparticles modified TiO2 nanofibers synthesized by one-step electrospinning process, to improve the photoefficiency of TiO2 host. The efficient promotion of the visible light photocatalysis of Ag/TiO2 nanofiber heterostructures can be ascribed to the electronic excitation of Ag nanoparticles under visible light and the transfer of the electrons to TiO2 conduction band, which deeply depends on the number of Ag/TiO2 junctions and the height of Schottky barrier. The Ag/Ti molar ratio can be easily controlled by the electrospinning process and the Ag/TiO2 nanofibers with Ag/Ti molar ratio of 0.05 exhibit the highest photocatalytic activity. Simultaneously, the Ag/TiO2 nanofiber heterostructures show excellent photocatalytic stability.