Light-Activated Chemoresistive and Plasmonic-Resonant Optical Sensors for NO2 and H2 Sensing Based on ZnO Doped Nanoparticles
Authors: Paolucci Valentina , Rigon Michele , Martucci Alessandro , Cantalini Carlo 
Published: May 2020

Doping Zinc oxide with trivalent cations such as Ga3+ or Al3+ has been shown to inﬂuence the optical and electrical properties of ZnO, with beneﬁcial eﬀects for its catalytic and sensing performances.
These aliovalent dopants provide the positive effect to both enhance ZnO conductivity turning highly resistive, undoped ZnO into a transparent conductive oxide (TCO), giving rise to localized surface-plasmon-resonance response (LSPR) in the near infrared, arising from the increased free electron concentration.
Aim of this work is to demonstrate the application of Gallium, Aluminum, Silicon and Germanium doped zinc oxide as both low temperature visible light-activated chemoresistive and plasmonic-resonant sensor for the detection of hazardous gases including hydrogen (H2) and nitrogen dioxide (NO2). Zinc Oxide nanoparticles (NPs) were synthesized through heat-up colloidal method [1,2], using Zn stearate (ZnSt2) as ZnO precursor. To obtain doped nanoparticles, a certain amount of gallium acetylacetonate, Aluminum acetylacetonate, Tetraethylorthosilicate and Germanium isopropoxide were added to the mixture to obtain Gallium (GZO), Aluminum (AZO), Silicon (SZO) and Germanium (GeZO) doping respectively. Spin-coated films were prepared and then stabilized on a hot plate at 150 °C in air. This procedure was repeated to obtain desired film thickness. Thin films were then treated in a 5% H2−95% Ar gas mixture at 450°C.
Changes of the electrical resistance of ZnO-based chemoresistive metal oxides to NO2 have been extensively reported in the literature by activating the sensor response by thermal and visible light irradiation in order to decrease the operating temperature toward mild (T < 100 °C) or even ambient (25 °C) conditions. In this study we explored the effect of temperature and visible light irradiation using purple–blue (PB) LED source (λ = 430 nm and 770 μW/cm2 intensity) on the electrical gas sensing properties of doped ZnO films for the detection of 400 ppb of NO2 at two different operating temperatures, 25°C and 75°C, and the results are shown in Figure 1 a-d in terms of normalized resistance changes.
As shown, both the presence of trivalent and tetravalent dopants influences the chemoresistive response of ZnO. A lot of literature reports on the effect of Gallium and Aluminum [2,3], but almost nothing, to the best of our knowledge, have been reported regarding the effect of tetravalent ions. By observing the results, Aluminum doped and Silicon doped Zinc Oxide NPs, in particular, gave outstanding results in terms of relative responses, possibly due to the increase of carrier density induced by doping. In fact, while at room temperature (RT) no response is observed, at 75°C OT in dark conditions Relative Response (RR) defined as Rgas/Rair is 10.93 and 5.62 for GeZO and GZO respectively, while it is 19.98 for AZO sample and 32.77 for SZO, with comparable adsorption/desorption times for all the samples.
Another evidence coming out from the reported results is the positive effect given by light activation. In particular, the enhancement given by Purple Blue light is confirmed, and the main improvement of sensor’s performances is observed at low temperatures [4,5]. At room temperature, while in dark conditions all the samples show no response to 400 ppb NO2, in case of light activation a slight but clear response is measured, together with a certain recovery of the baseline after gas exposure, even if no equilibrium conditions are achieved both in adsorption and in desorption, within the time scale of the experiments. Notwithstanding the effect of light to suppress in a certain way sensors’ Relative response, as already observed [2,3], the main advantage of UV-activation is the enhancement in terms of gas desorption, i.e. base line recovery, after gas exposure and this effect is evident both at 25°C and 75°C. Actually, at higher temperatures, thermal activation is the predominant phenomenon but the presence of light triggers the achievement of equilibrium conditions, giving desorption times almost halved in the case of light irradiation at 75°C OT.
In addition to chemoresistive gas sensing, as such dopants induce the generation of a localized surface plasmon resonance (LSPR) so that they can be used coupled with metal oxide as optical probes, the synthesized nanoparticles have been tested also as optical gas sensors. Optical gas sensing tests were performed by making optical absorption measurements in the 350–2500 nm wavelength range on films deposited on SiO2 substrates using a Harrick gas flow cell. Details are reported in ref. [2]. Figure 1e reports the optical absorption change (OAC) of doped ZnO exposed to 1000 ppm NO2 and Figure 1f shows the comparison of the Dynamic absorption change for two cycles Air/NO2 1000ppm at 2400nm at 150°C. Figure 1f reveals that even if the operating conditions are different as compared with the previous results, it seems that all the materials investigated are suitable also for optical gas sensing. Moreover, it turns out that the highest the resistance variation in chemoresistive tests, the lowest the absorption change in case of optical sensing. A question to be resolved is the particular influence of the single dopant species in the sensing behaviour and this, together with the study of different amounts of doping and the way to possibly enhance selectivity by using different techniques represent a future challenge for this particular work.

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4. Giancaterini, L.; et al, Sensors Actuators, B Chem.
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5. Paolucci, V. et al, Sensors (Basel).
2019, 19.

Figure 1