Wide Dynamic Range Hydrogen Sensing Device Made of Hexagonal Nanoplates of AgInO2 with Exposed Facets
Authors: V. Beatriceveena T. , Gnanasekar K. I. 
Published: May 2020

Introduction
With the increasing consumption of fossil fuels and rapid growth of industrialization, the emission of various greenhouse, hazardous and explosive gases have increased dramatically in the modern years. Due to its potential application, hydrogen is widely used in petrochemicals, food, refining, ammonia production, electronics manufacturing, nuclear industry and recently as a fuel [1-3]. However, it is a colorless, odorless gas has many potential risks such as a wide explosion concentration range (4 - 75%), low ignition energy (0.02 mJ) and large flame propagation velocity [1]. Therefore, its detection and leakage control is challenging. Semiconductor-based delafossite oxides like AgInO2 have received wide attention especially as a light emitting diode, touch display panels, solar cells, heaters, defrosters, etc [4]. Herein AgInO2 nanoplates are suggested as a new delafossite-based gas sensing material with high response, wide dynamic range and selectivity to hydrogen gas as well as negligible humidity-dependent gas sensing characteristics.
Experimental
Crystalline powders of AgInO2 has been prepared by mixing the solutions of stoichiometric AgNO3 and In(NO3)3 with 4 M KOH and heated at 180˚C for 24 h in an autoclave using hydrothermal technique. When the dwell time was reached, the autoclave was cooled to room temperature naturally, and the resultant was washed thoroughly and then dried in air. The material was characterized using various techniques and described in our previous publication [5]. For gas sensing studies, AgInO2 powder was mixed with isopropyl alcohol and screen printed on an alumina substrate to form a sensing layer. The gas sensing studies of AgInO2 was carried out towards different gases at various temperatures (200-400˚C). At the optimum temperature of sensing the material was calibrated towards different concentrations of H2. Gas sensing response for a given concentration of gas, defined as Ra/Rg, where Ra & Rg are resistance of the material in air and in presence of gas respectively.

Results and Conclusions

The XRD pattern of AgInO2 is in agreement with the Rhombhohedral (166) system of AgInO2. It is worth noting that the relative intensity of (006) to (012) facets (i.e, I(006)/I(012) = 0.40, ICDD # 96-150-9398), is increased to 3.26 indicating texturing along c-axis [5]. FE-SEM image consists of hexagonal-shaped platelike morphology with an average width of ~ 300 nm and thickness of ~ 70 nm [5].
The response of AgInO2 nanoplates towards 100 ppm of H2, PG, NH3, NOx, CH4, butane and formaldehyde gases/vapours at different working temperatures were recorded to find the optimum temperature for each gas. The response towards 100 ppm of H2 increases with the increase of temperature and reaches a maximum value of 6.5 at 360˚C. The response to petroleum gas which consists of hydrocarbons increases with increase in temperature reaching a maximum value of 1.7 only at 400˚C. Almost the sensor is insensitive to NH3, NOx, CH, C4H10 and HCHO at all these temperatures which makes this material highly selective to hydrogen at 360˚C (Fig. 1(a)). After fixing the optimum temperature of sensing, the responses of AgInO2 towards different concentrations of H2 from 1 ppm to 10000 ppm at 360 ˚C were obtained. The response of the material increases in a good linear relationship from 1 ppm to 1000 ppm with R2 of 0.9949; thereafter it slowly attains the saturation limit. To the best of author’s knowledge, AgInO2 is one of the unique material which has the ability to sense hydrogen in the wide concentration range (i.e.) 1 ppm to 1 %. At the same time, the material also displays fast response of 36, 28, 26 & 62 s and better recovery of 19, 88, 465 & 1110 s for 1, 100, 1000 & 10000 ppm of H2 respectively.
Regarding the practical use, repeatability and stability of the material is another serious concern. 10 consecutive tests were performed to obtain the short-term stability and long-term stability tests were carried out for six months at an interval of 10 days towards 50 ppm of H2 at 360 ˚C. As shown in Fig. 1(b), the resistance of the material undergoes a sharp decrease upon exposure to H2 gas and reaches its initial baseline resistance while equilibrating the surface with clean air shows an n-type semiconductor-like behaviour. The average response (Ra/Rg) was derived to be 2.6 and there was no significant change in the response pattern even after 10 cycles of repeatable exposure. Similarly, the measurements maintain the average response value of 2.64 after 180 days cycle. This shows that the AgInO2 nanoplates exhibit a better gas sensing property towards hydrogen over a wide concentration range, fast response and recovery with better stability and repeatability.
The gas sensing mechanism of n-type semicondutor is based on conductivity changes caused by the adsorption and desorption of gas molecules on the surface. Moreover, the design and control of the surface state is another possible way to obtain good gas sensitivity and selectivity [2]. In the present case the material is predominantly exposed with high energy (00l) facets which has more dangling bonds than the material with (012) facet. This probably attributes to the high selectivity and sensitivity towards hydrogen. Therefore, the present material could be utilized as a sensor from ppm to % level detection of hydrogen. The salient outcomes of these experiments along with mechanism of sensing will be discussed.

Figure 1