Effect of Surface Heterogeneity, Heat of Adsorption and Surface Area on the Characteristics of PANI-SnO2 Based H2s Gas Sensor

The emeraldine salt form of polyaniline (PANI) is a p-type organic semiconductor with polarons as charge carrier species[4] (figure 1 (a)). The protonation level of PANI changes upon exposure to a reducing or an oxidizing gas, which results as change in conductivity of the material[2]. Composites of the PANI with several metal oxides have been used to improve the sensor characteristics like sensitivity, selectivity, response time and limit of detection. SnO_2, one such metal oxide, makes a co-ordination with the N in PANI which results in a p-n junction between p-type polyaniline and n-type SnO₂[3]. Also, co-ordination with SnO₂ affects the protonated sites of polyaniline which introduces surface heterogeneity. The sensor characteristics are affected by surface heterogeneity as well as electrical properties of the material. Apart from this the surface area and morphology of the material also plays a major role in assisting the adsorption of the gas molecules on the sensor surface. In this work the effect of mixing the different amounts of SnO₂ in polyaniline on the H₂S sensing characteristics has been studied. The fractional change in electrical resistance of sensing element upon exposure to H₂S gas was recorded as the sensor response, and the surface site kinetics were calculated according to nature of the sensor response. The surface site kinetics were studied with Langmuir and Freundlich isotherms and the kinetics parameters were compared with the material properties such as heat of adsorption, surface area, and band gap.

PANI-SnO₂ composites were synthesized by solution casting method[3]. Four different wt. % (0, 5, 10 and 15) of SnO₂ nanoparticles (Sigma Aldrich, particle size < 100 nm) were added to 20 kDa molecular weight PANI emeraldine base (Sigma Aldrich) in 1 molar solution of camphorsulfonic acid (CSA) (Sigma Aldrich). The mixture was kept on stirring for 24 hours, during mixing the PANI gets protonated by CSA and SnO₂ forms co-ordination with N of PANI as shown in Figure 1 (b). After this reaction mixture was filtered, washed with DI and acetone, and dried. The composite powders were labeled as P-00, P-05, P-10, and P-15 according to increasing order of the SnO₂ loading.

The surface area of PANI decreased by addition of SnO₂, but the pore size was unaffected (table 1). The UV-Vis spectra showed Ï€-Ï€* and polaron-Ï€* transitions at 350 nm and 450 nm respectively, and a free carrier tail which confirmed polaron form of polyaniline with expanded coil structure[5]. Band gap of the material was increased with 5 wt. % SnO₂ due to non-uniform dispersion of the n-type metal oxide. However, the band gap decreased on further addition of SnO₂ due to more co-ordination of Sn with PANI. The N1s XPS spectra confirmed amine, imine and N⁺ type surface nitrogen moieties[6]. The percentage of active sites for each composite were approximated from N1s spectra.

The sensing elements were fabricated by drop casting the 10 wt. % slurry of composite powders in DI water over thermally deposited silver interdigitated electrodes on PET substrate. The sensor was placed in a 125 mL cubical chamber and nitrogen gas was flushed to make the atmosphere inert. Calculated amounts of H₂S gas were injected to make 10, 50 and 100 ppm concentrations inside the sensing chamber, and the electrical resistance of the sensing element was recorded by a digital multimeter (Keysight 34465a). The response time was calculated as the time taken by resistance in reaching the 63.2% of its saturation value. And the sensitivity was calculated in ppm^-1 from the slope of response v/s H₂S concentration plot.

H₂S is a reducing gas and it also shows mild acidic behavior in presence of moisture[1], so it can protonate as well as deprotonate the PANI which result as change in the resistance. Since there was no moisture in sensing chamber, so the reducing behavior of H₂S caused deprotonation of the PANI, which increased resistance of the sensing element upon exposure to H₂S. For these sensors (P-00, P-05, and P-15) N⁺ sites were considered as active site for chemisorption and surface site kinetics was evaluated from equation (1). However, the sensor P-10 had more uniform dispersion of SnO₂ in PANI, so there was more co-ordination of SnO₂ with PANI. In this case acidic behavior of H₂S caused protonation of PANI, and imine N sites were considered as active site of chemisorption. The chemisorption kinetics of surface sites was calculated from equation (2) for this sensor.

The response time of PANI sensor was improved on adding SnO₂ but the sensitivity was decreased (Figure 2(a)). Sensor P-10 show fastest response, but the sensitivity was decreased to 0.003 ppm^-1. Also, the reaction chemistry for P-10 sensor was different because the decreasing resistance response indicated protonation of PANI showing the acidic behavior of H₂S. The shorter response time for acidic behavior of the H₂S has been reported previously[1]. For other sensors (P-00, P-05, and P-15), the response time decreased with increase in band gap of the material, such behavior shows the importance of electrical properties of the material. The surface site kinetics (figure 2 (b)) show exponentially increasing behavior fitted with Langmuir kinetics (equation 3) and the adsorption isotherm show Freundlich (equation 4) behavior indicating heterogeneity in the surface sites of the sensing material. The adsorption coefficient α decreased with heat of adsorption for PANI-SnO₂ composites (figure 2 (c)). However, pristine PANI does not follow this trend due to more surface area (Table 1).

A high surface area with more active surface sites, and high heat of adsorption is desired for good sensitivity. A high band gap will give better response time of the sensor. However, the nature of surface chemistry will also play an important role in the sensor response characteristics. To design a PANI-SnO₂ composite based H₂S sensor, the environmental conditions (humidity level) of the sensor application area and synthesis procedure for more co-ordination of SnO₂ with PANI should be focused to promote role of the surface chemical reaction.

References

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