Performance of Polyaniline Thin Film as a Functional Material of Acid Vapor Sensors

Polyaniline (PANI) is a conductive polymer that can be produced by the electrodeposition process. The purpose of this research is to evaluate the performance of PANI sensors on hydrochloric acid (HCl) and acetic acid (CH3COOH) vapor using a scan rate of 100 mV/s and ten cycles, a thin PANI layer was created on the surface of the ITO substrate. The PANI spectroscopic test results before and after CH3COOH vapor exposure revealed no differences and remained within the PANI functional group range. A four-point probe (FPP) test was performed to evaluate the sensing performance of the PANI thin film against analyte gas. The results of recovery time, response time, and sensitivity tended to increase as analyte concentration increased. The quickest CH3COOH sensing (1 ppm) has a reaction time of 29.7 seconds, a recovery time of 21.9 seconds, and a sensitivity of 5.11%. The greatest CH3COOH sensing (10 ppm) resulted in a reaction time of 50.3 seconds, a recovery time of 39.7 seconds, and a sensitivity of 13.64%. The reaction time for the lowest HCl sensing (1 ppm) was 42.6 s, the recovery time was 32.4 s, and the sensitivity was 7.82%. The greatest level of CH3COOH sensing (10 ppm) resulted in a response time of 60.6 s, a recovery time of 55.3 s, and a sensitivity of 16.31%. As a result, the PANI thin film is a functional material for acid gas sensors.


Introduction
Acid vapor is formed due to the reaction of toxic gases with water vapor in the atmosphere, which often comes from industrial activities and the burning of fossil fuels.These acidic vapors can cause air pollution, various health problems, and damage to the environment as a whole.Acid vapor contains toxic compounds such as sulfuric acid and nitric acid.Long-term exposure to acidic fumes can cause breathing problems, skin and eye irritation, and worsen health conditions for people with respiratory disorders such as asthma.An acid vapor detection device is needed, given the danger of acid vapor.An acid vapor sensor is required to monitor air quality in general.Information on acid vapor concentrations helps evaluate the level of air pollution in a particular area and plan mitigation actions.Research on the development of acid vapor sensors has been carried out a lot.The material used as a sensor must have specific requirements according to operating conditions, such as selectivity, sensitivity, kinetic response, stability, and long-term storage [1].Metal oxide material was the first to be developed as a conductometric chemical sensor (transduction type based on changes in electrical conductivity) in the form of a thick film more stable than sensors made from organic chemicals [2].Metal oxide materials such as WO3, ZnO, SnO2, and TiO2 are attracting attention because of their use as chemical sensors because they have good electronic properties and high thermal and chemical stability [3].Metal oxidebased sensors can be developed for various applications but have drawbacks, namely low selectivity, cannot be used for a long time, relatively high operating temperature, and requiring high power consumption [4].
Of the several materials that can be applied as active materials for acid vapor sensors, this research discusses the application of conductive polymers into acid vapor sensors to overcome industrial and laboratory workers' health hazards.One of the materials suitable for use as an active ingredient in acid vapor sensors is polyaniline (PANI).PANI is one type of conductive polymer that attracts attention.Based on the stability aspect in air, PANI has good stability among other conductive polymeric materials such as polyacetylene (PA), polydiacetylene (PdA), polythiophene (PT), and polypyrrole (PPy) [5].PANI offers several advantages because chemical and electrochemical methods efficiently synthesize it, have good environmental stability, and adjustable electrical conductivity [6].
PANI can be synthesized by several methods, including chemical methods and electrochemical methods.The electrochemical method (electrodeposition) is excellent for forming PANI thin films.Various kinds of substrates are used in the electrodeposition process, one of which is indium titanium oxide (ITO).ITO is a suitable substrate because it has a large surface area, good resistance to high temperatures, and is conductive.Several studies that have successfully applied PANI as an acid vapor sensor material, including Kondawar [7] have used PANI as an active ingredient in detecting HCl.The results showed that the detection of HCl gas had a good performance.Furthermore, Turemis [8] applied PANI/ZnO for gas sensing CH3COOH has good reversibility properties and has good performance in responding to CH3COOH gas.In this research, the performance test of PANI thin films against acid vapor (CH3COOH and HCl) was carried out in a non-vacuum chamber.The PANI thin film synthesis method used is electrodeposition using an ITO substrate.The performance studied includes sensitivity, reversibility, response, and recovery time.

Synthesis of PANI Thin Film
PANI synthesis was carried out by the electrodeposition method using a 797 VA Computrace.The substrate used in the electrodepositing process is Indium Tin Oxide (ITO).The electrolyte solutions used were 0.25 M aniline and 0.5 M HCl at a potential range of -0.6 V to +1.0 V with a scan rate of 100 mV/s and a cycle of 10.The PANI thin film on the ITO surface was produced from an electrodeposition process using Cyclic Voltammetry.

Characterization
Cyclic Voltammetry (CV) characteristics were obtained in the electrochemical PANI polymerization process on ITO substrates.FTIR testing to determine the functional groups of a compound where the resulting waves are used to identify the functional groups of a compound from the analysis of the bonds formed.Measurement of sensitivity, reversibility, response time, and recovery time for strong and weak acids with five different concentrations was carried out using a Four Point Probe (FPP).The acids used were CH3COOH and HCl with varying concentrations of 1 ppm, three ppm, five ppm, seven ppm, and ten ppm.The interaction scheme of the PANI layer with acid gas is shown in Figure 1.

Figure 1. Schematic of interaction between PANI thin film and acid vapor
The sensing process is carried out by connecting the PANI-coated ITO with the Four Point Probe (FPP) pin and inserting it into the testing chamber at room temperature equipped with a fan to transfer the evaporated gas.Measurement of gas concentration is calculated using equation (1).
Given that C: acid concentration (ppm), ܸ : volume of acid, ߩ : density of acid, ∅: volume fraction of acid needed, M: molecular weight of target acid, and V: volume of room [9].The response of the sample in detecting acid gas can be observed by the response time and sensor sensitivity.Exposure to acid gas in the test chamber with the resistance of the sample can be calculated by Equation 2. 3 With known S: sensor sensitivity, R1: resistance after flowing compound vapor (Ω), and R0 is resistance before flowing compound vapor (Ω) [7].

Cyclic Voltammetry
The electropolymerization process produces a thin green PANI layer deposited on the ITO's surface, as shown in Figure 2. Based on the oxidation-reduction state, the green color indicates the emeraldine salt (ES) phase, where the emeraldine salt phase is conductive.From the results of the CV characterization, the voltammogram curve is obtained, as shown in Figure 3.The voltammogram graph shows oxidation peaks marked with numbers 1, 2, and 3 and reduction peaks marked with numbers 4 and 5.An increase in the oxidation-reduction (redox) value on the voltammogram curve is evidence of the PANI layer's growth on the ITO substrate's surface.It can be seen that there are three oxidation peaks at +0.14, +0.48, and +0.89 V, which are changes in the facade of the leukomeraldine (LB) phase to emeraldine salt (ES) due to the removal of anions and the change of the ES phase to pernigraniline (PB) due to delocalized protons.Two reduction peaks at +0.08 and -0.24 V are the phase changes of PB back to ES and ES back to LB due to deported anions [9].This peak indicates the presence of electroactive regions formed in the coating and explains the presence of aniline monomers undergoing deprotonation reactions [10].

FTIR Characterization Results
The characterization results using Fourier transform Infrared Spectroscopy (FTIR) obtained a graph that can be seen in Figure 4.The presence of benzenoid and quinoid groups indicates the PANI functional group.It can be seen that there are spectral peaks at wave numbers 1483.28 and 1576.57cm-1, which indicate the presence of C=C stretching of the benzenoid chain and C=C stretching of quinoid bonds.Both types of bonds are functional groups that exist in all PANI phases.The figure shows the spectrum of PANI with different treatments before and after exposure to CH3COOH gas.The FTIR spectrum's peaks experience a slight wave shift, but this shift does not affect the functional groups present in PANI because it is still in the PANI range.It can also be seen that no functional groups were lost or added as a result of exposure to CH3COOH gas.There was no bond between CH3COOH and the PANI layer.The results of the FTIR test on CH3COOH gas indicated that there was no trapping and no interaction between CH3COOH and the PANI layer during the gas detection process, and there was no need to carry out another FTIR test after being exposed to HCl gas.The comparison of the results of the FTIR spectroscopy test, it was then matched with the results of previous studies shown in Table 1.

Sensing performance
To determine the performance of PANI thin films as acid vapor sensor materials can be observed through changes in response time, recovery time, reversibility, and sensitivity.The sensing process is carried out at room temperature.It is equipped with a fan, which transfers the gas resulting from the evaporation of the analyte solution to the PANI thin layer.The analyte solution is injected into the testing chamber and will quickly change into gas and mix with air.Before being exposed to the analyte gas, the PANI layer was given an input voltage (Vin) of 4 V and a measured current (Iin) of 1.33 A, and an R0 value of 11.12 Ω was obtained.Then, the PANI layer interacts with the analyte gas and causes changes in the values of Vout and Iout.This change occurs until the Vout and Iout values are constant (R1); this indicates that these values have reached the saturation point.After reaching the saturation point of 20 seconds, the gas will be removed (gas out), and at the same time, the Vout and Iout values will change back to the starting point.
(a) (b) Figure 5. Performance of thin films against acid gas sensing: (a) Changes in resistance of PANI thin films at different acid gas concentrations and (b) Stability of PANI thin films at one ppm after sensing acid gas three times.
Figure 5 (a) shows the change in resistance of the PANI film after sensing sour gas.Changes in resistance between CH3COOH and HCl tend to be linear.This is because the more analyte gas concentration is exposed, the more gas molecules interact with the PANI thin film [8]. Figure 5 (b) shows the process of sensing CH3COOH and HCl gas at a concentration of 1 ppm after repeated exposure to analyte gas three times the resistance value (R1) can return to the initial condition (R0).This phenomenon shows that PANI thin films have good reversibility.
When gas is injected into the chamber (gas in), the value of R0 will decrease to R1; this is the HCl gas detection process.When it reaches R1, it is called the response time; in this condition, the resistance value will be constant or reach the saturation point for up to 20 s, and then the gas is removed (gas out).At the time of gas out, the value of R1 will rise to the initial condition, namely R0, from R1 to R0, which is the recovery time (Figure 6).At increasing ppm, response time and recovery time value tends to increase.This is due to the slow rate of diffusion and desorption from the sensing surface.It can be compared that the measured response time values between the CH3COOH and HCl gas sensing results have a significant difference.On sensing one ppm CH3COOH, a response time of 29.7 s was obtained; on sensing one ppm HCl, a response time of 42.6 s was obtained.This is because the vapor pressure of the analyte gas is different.According to the Material Safety Data Sheet (MSDT), the CH3COOH vapor pressure is 15.7 mmHg, and the HCl vapor pressure is 11.97 mmHg.The greater the value of the vapor pressure of an element, the faster its evaporation rate.The sensitivity value is taken from the resistance value obtained from sensing CH3COOH and HCl gases.Figure 7 shows the sensitivity value of the PANI thin film to CH3COOH and HCl gas sensing; the sensitivity value increases with the number of gas concentrations exposed.There is a difference in the sensitivity value between CH3COOH gas and HCl due to differences in the type of acid gas, pH, molecular weight, and binding energy [11].
The highest sensitivity value was found at ten ppm, 13.64% and 16.31%.It can be seen that the higher the concentration of analyte gas exposed, the higher the sensitivity value of the PANI layer.Very few analyte gas molecules are available at the lowest sensitivity conditions for the PANI layer to interact at low concentrations.At the highest sensitivity, the number of analyte gas molecules available is more for the sensor to interact with.The increase in the sensitivity value with an increasing concentration on the CH3COOH gas sensor follows the research of Turemis [8] and the increase in the sensitivity value of HCl gas with an increasing concentration on the HCl gas sensor according to the research of Kondawar [7].Thus, the PANI sample can be used as a sensor material in detecting acid gas at low concentrations with a relatively high sensitivity value.

Conclusion
Based on the research results, it can be concluded that the PANI thin film was successfully synthesized using the potentiostat electrodeposition method on the surface of the ITO substrate.The sensing performance shows that the PANI film can detect acid gas at a concentration as low as one ppm.PANI thin films have suitable reversibility properties to acid gas sensing.The sensitivity value of HCl gas sensing is better than CH3COOH gas.

Figure 2 .
Figure 2. PANI thin films deposited on the ITO surface.

Figure 3 .
Figure 3. PANI Cyclic Voltammogram curve with a scan rate of 100 mV/s.

Figure 4 .
Figure 4. PANI FTIR characterization results before and after exposure to CH3COOH vapor.

Figure 6 .
Response time and recovery time of HCl gas in various concentrations of ppm: (a) CH3COOH and (b) HCl.

Figure 7 .
Figure 7. Sensitivity of PANI thin films on acid gas sensing.

Table 1 .
Identification of PANI bonds before and after exposure to CH3COOH vapor.