Effects of Visible Light on Gas Sensors: From Inorganic Resistors to Molecular Material-Based Heterojunctions

Abstract

In the last two decades, many research works have been focused on enhancing the properties of gas sensors by utilising external triggers like temperature and light. Most interestingly, the light-activated gas sensors show promising results, particularly using visible light as an external trigger that lowers the power consumption as well as improves the stability, sensitivity and safety of the sensors. It effectively eliminates the possible damage to sensing material caused by high operating temperature or high energy light. This review summarises the effect of visible light illumination on both chemoresistors and heterostructure gas sensors based on inorganic and organic materials and provides a clear understanding of the involved phenomena. Finally, the fascinating concept of ambipolar gas sensors is presented, which utilised visible light as an external trigger for inversion in the nature of majority charge carriers in devices. This review should offer insight into the current technologies and offer a new perspective towards future development utilising visible light in light-assisted gas sensors.

Keywords: adsorption–desorption mechanism; conductometric transducers; heterojunctions; light effect; resistors.

Figure 1

Schematic view of light-induced charge carrier generation in an inorganic semiconductor (E_V and E_C are the top of the valence band and the bottom of the conduction band, respectively) (a) and in a dye/inorganic semiconductor heterojunction device (HOMO and LUMO are the highest occupied and lowest unoccupied molecular orbitals, respectively) (b).

Figure 2

Sensing responses of a ZnO sensor to different concentrations of ethylene in the air as a function of time in the dark or under visible light irradiations (a) and diagram illustrating the visible light-activated gas sensing mechanism of ZnO sensor at room temperature showing the reaction of O₂ with electrons () and of target gas with holes (), both ${\mathrm{e}}^{-}$ and ${\mathrm{h}}^{+}$ being photo-generated (b) (modified from [47]).

Figure 3

Relative response of CdS_xSe_1−x nanoribbon-based sensors upon exposure to 2 ppm CH₃COOH under different conditions (adapted from [65]).

Figure 4

Molecular structures of MWCNTs (a) and PTL (b). Typical sensor response of PTL functionalised MWCNTs with 20% TEA exposure under dark and light conditions (c) (modified from [67]).

Figure 5

Dynamic response curves of the sensors based on ZnO, OV ZnO, ZnO/Pd and OV ZnO/Pd samples to 0.1% CH₄ in the dark (a) and under 590 nm light illumination (6 mW cm⁻²) (b); (c) the corresponding gas sensing responses (adapted from [73]).

Figure 6

Dynamic response curve (a) and fitted responses (b) of the OV ZnO/Pd sensor to 0.01–1% CH₄ under 590 nm light illumination (6 mW cm⁻²), the inset confirms the Langmuir’s law; (c) selectivity of the OV ZnO/Pd sensor to 0.1% CH₄ over CO and H₂S under 590 nm light illumination (6 mW cm⁻²) (adapted from [73]).

Figure 7

Schematic model for ZnO/CdSe heterostructure-based sensors when exposed to ethanol. E_F indicate Fermi levels of both materials (modified from [76]).

Figure 8

The amplification factor of sensitivity (a) and relative response time (b) under light illumination of different wavelengths and power densities. The band alignment diagram of the light-assisted SnS₂/rGO sensors (c). The light-induced electron–hole pairs are generated and separated in the conduction and valence bands of SnS₂ (adapted from [79]).

Figure 9

Electrical resistance responses of the WO₃-GO sensors to 0.9 ppm NO₂ gas performed under light illumination at different wavelengths at room temperature (adapted from [83]).

Figure 10

Chemical structure of copper phthalocyanine with different degrees of halogenation (a) and lutetium bis-phthalocyanine (LuPc₂) (b). The charge transport scheme between frontier orbitals and trap states in Cu(F₈Pc) and LuPc₂ under visible light exposure (c) (modified from [18]).

Figure 11

Response of CuCl₈Pc/LuPc₂- (a), CuF₈Pc/LuPc₂- (b) and CuF₁₆Pc/LuPc₂- (c) based heterojunction sensors towards 20 ppm of NH₃ under dark and light illumination (right sides of each curve) at room temperature. The inset of (c) represents a simplified device scheme (adapted from [18]).