Distilled pyrrole (Py, Sinopharm Chemical Reagent Research Institute Co., Ltd.) was dissolved under reduced pressure into acetonitrile (Shanghai Chemical Reagent Co., Ltd.), and KClO₄ (Shanghai Chemical Reagent Co., Ltd.) was dissolved in deionized water. The final solution is the mixture of the aforementioned solvents.

The morphology and the size of the as-prepared PPy films were characterized using SEM with the acceleration voltage of 10 kV (Sirion 200, manufactured by FEI, Holland).

FT-IR was applied to evaluate the functional group of PPy films. The FT-IR transmission spectra were obtained by using a FT-IR spectrophotometer (VERTEX 70, made by Bruker Co., Germany) in the range of 500 - 4000 cm⁻¹.

Figure 1 shows the schematic of the fabrications process. The ceramic substrates (7 mm × 6 mm printed interdigital gold electrodes) were cleaned by ultrasound for 10 min each in deionized water, ethanol and acetone, respectively, to remove surface dirt and oil stains; then the substrate was kept in deionized water and dried before being used [Figure 1(a)].

Initially, monomer pyrrole was added to acetonitrile to form a 0.1 M solution. Meanwhile, KClO₄ was dissolved into deionized water to form a 0.1 M solution. After 10 min ultrasonic dispersion for each solution, they were mixed together.

The potentiostatic polarization method was used in the polymerization process. The polarization voltage was 0.8 V and the polarization time was 2 hr. The working electrode and the auxiliary electrode were exchanged every 20 sec.

The sensitivities of PPy films were tested in different concentrations of ammonia. A 30 L container with a removable lid served as a test chamber, as the sensor could be set within. The films were statically exposed in the gas after the liquid evaporated. The sensitivity was defined as (R₀/R₁) × 100% (R₀ = the initial resistance and R₁ = the tested resistance).

Figure 2(a) shows the SEM micrograph of the PPy films produced by the electrochemical in-situ polymerization method. It is apparent that flower-like particles are obtained. Figure 2(b) shows the image of the PPy films, which is alternately deposited on the interdigitated golden electrodes. It can be concluded that the films firmly adhere to the golden electrodes. After desorption of the ammonia, the morphology of the PPy films is changed. Holes appear among the initial compacted thin films [Figure 2(c)]. It can be presumed that a reaction between PPy films and ammonia occurs.

The sensitivity feature of the sensor in 800 ppm ammonia is shown in Figure 3. The resistance of PPy films decrease after ammonia fills the container. The sensitivity time is less than 15 s and the recovery time is about 100 s. When the ammonia comes in contact with the PPy films, physical adsorption occurs between them in a complex chemical reaction, which results in a decrease in the resistance of the film. When PPy films were placed in ammonia, ammonia could be adsorbed and diffused on the films. Ammonia is a kind of reducible gas which is able to provide electrons, and increases the electron concentration of PPy films. Therefore when ammonia contacted the PPy films, the resistance of the PPy films decreased. As the result of incomplete reversible reaction, the resistance of PPy films did not recover to the original value [30] .

With the increase of ammonia concentration, the resistance of the films decreased, as shown in Figure 4. As the result of limited recovery time and incomplete reversible reaction, the resistance of the PPy films did not return to the original value. As the concentration of ammonia increased the sensitivity of the films increased, as shown in Figure 5, it can be seen that the sensitivity of PPy films is only 120% in 100 ppm ammonia. When the gas concentration changed from 300 ppm to 500 ppm, the sensitivity of the PPy films increased from 180% to 250%. From the data in Figure 5, it can be concluded that the relation between the sensitivity and the concentration is almost linear.

Figure 6 shows a typical sensitivity-recovery curve obtained for 500 ppm ammonia. To investigate the recovery ability of PPy films in ammonia, eight continuous sensitivity tests were conducted in 500 ppm ammonia. The resistance of the films gets back to the initial value, which leads to the conclusion that this material has enough stability for continuous detecting in ammonia.

Figure 7 shows the sensitivities of PPy films 8 trials in 500 ppm ammonia. By comparing the eight sensitivities, it can be deduced that the test has good reliability, and that this material also possesses competitive gas sensing characteristics, thus PPy films can be used to test ammonia for commercial purposes.

In order to discuss the sensitivity mechanism between PPy films and ammonia, we designed the experiment that the process was in-situ monitored by FT-IR spectroscopy. PPy films were placed in the ammonia flow for 2 min, during which FT-IR spectrums were used to scan at intervals of 10 sec.

In Figure 8, we present the FT-IR plural spectra of PPy films before, during and after the detection process. In Figure 8(a), the peak in 1650 to 1150 cm⁻¹ can be assigned to the symmetric/asymmetric C=C/C-C vibrations. From the band at 960 cm⁻¹, we conclude that PPy films are doped with ClO 4 − . The band at 1300 cm⁻¹ is attributed to the C-N stretching vibration modes. These would account for the presence of the pyrrole ring after electrochemical polymerization.

Figure 8(b) shows the FT-IR spectrum of PPy films after the adsorption of ammonia. Compared with Figure 8(a), two broad adsorption bands appear when PPy films are exposed to ammonia flow. The first band at 3340 cm⁻¹ is assigned to ammonia. After the adsorption of ammonia, PPy chemically reacted with the ammonia. As the band at 1600 cm⁻¹ is assigned to amino, we proposed that by losing electrons, ammonia becomes amino, attaching to the main chain of PPy.

Figure 8(c) illustrates the FT-IR spectrum of PPy films after desorption of the ammonia. There is no band at 3340 cm⁻¹, indicating that the reaction between PPy films and ammonia is reversible. Compared with Figure 8(a), the main groups of infrared characteristic absorption peaks have returned to their original positions, but some bias still exists, which indicates that the reaction is not fully reversible. In addition, a lot of infrared absorption peaks disappear in the band at 1600 cm⁻¹ due to the disappearance of the amino.

In order to further illustrate the sensitivity mechanism between ammonia and PPy films, the adsorption process was investigated by real-time FT-IR spectrometer. Figure 9 shows the FT-IR plural spectra of PPy films in ammonia with

different times. When ammonia contacted PPy films, physical adsorption occurred first. A tiny peak at 3340 cm⁻¹ appears in Figure 9(b). As result of the reducibility of ammonia, PPy films receive the electrons. The lost electrons, which do not belong to any atoms, attach themselves to the π-conjugated carbon chains. This leads to the change in the film conductivity, with the resistance of PPy films decreasing. The obvious band at 1600 cm⁻¹ appears in Figure 9(c) and Figure 9(d). The reaction process is shown in Figure 10.

There were two kinds of reaction mechanism between PPy films and ammonia. The cases are shown in 1-1 and 1-2.

P P y + C l O 4 − + N H 3 → P P y 0 − N H 3 + ⋅ C l O 4 − (1-1)

P P y + C l O 4 − + N H 3 → [ P P y ( − H 1 + ) ] 0 + N H 4 + C l O 4 − (1-2)

As shown in Figure 8(b), the band at 1600 cm⁻¹ is assigned to amino. By losing electrons, ammonia becomes amino, attaching to the main chain of PPy. We proposed that there are the simple compensation effects involving electron

transfer. In electron transfer compensation, ammonia molecules compensated the PPy films charge, which caused changes in the electrical conductivity of the PPy films. The results proved that there is direct evidence for the first case.