Electrochemical polymerization in the presence of ZnO nano-particles was performed to produce PANI (CSA)- ZnO composites, Films 1 (b-d) and 2 (b-d). An electrolyte was prepared by addition of the aniline and (+)-cam- phor sulfonic acid (or (−)-camphor sulfonic acid) as chiral dopants and nano-ZnO particles to distilled water (Table 1). Polymerization was performed by applying direct voltage (DC) at 3.0 V. Then, the resultant film was washed with distilled water and methanol to remove low molecular weight fractions and residual ZnO on the surface. The structure is illustrated in Figure 1(a).

PANI doped with CSA film (Films 1a and 2a) without ZnO was prepared with the same procedure as preparation of Films 1 (b-d) and 2 (b-d).

Electrochemical polymerization in without ZnO nano-particles was performed. The resultant film was immersed into a solution of dispersed ZnO in water/PVA (PVA = polyvinyl alcohol) producing a film with double-layer structure consisting of PANI (CSA) and ZnO/PVA.

First, chiral-PANI films were prepared on the anode side of ITO glass electrodes by electrochemical polymerization with no ZnO electrolyte solution. ZnO was dispersed in 0.1 wt% PVA solution. Here, PVA is employed as a matrix for ZnO. Then, the film was immersed into the dispersion. After drying, the chiral-PANI/ZnO double-layer film (Film 3a and 3b) was obtained. Washing was performed with the same procedure as preparation of Films 1(a-d). The structure is indicated in Figure 1.

a) Camphor sulfonic acid (CSA). (+) and (−) represent optical activities. b), c) Abbreviations “1” and “2” correspond to (+)-CSA and (−)-CSA employed in the electrochemical polymerization, respectively. b) Electrochemical polymerization in the presence of ZnO nano-particles. c) Electrochemical polymerization with no ZnO nano-particles. Resultant films were immersed in the ZnO dispersed PVA in the water solution to deposit ZnO/PVA layer on the surface of the optically active PANI films. PVA = polyvinyl alcohol. d) Concentration of ZnO dispersion solution for immersion of PANI films. e) Dispersion concentration in 0.1 wt% PVA/water solution (M).

Circular dichroism (CD) absorption spectroscopy measurements of the films were carried out to confirm optical activity. All of the films show Cotton effect in the CD. The signal intensities increase with increase of composition of ZnO in the PANI. Especially, Film 1d and Film 2d display clear and intense complementary mirror-image Cotton effect, shown in Figure 2. These films show intense CD signals compared with those of pure optically active polyanilines. This can be due to interaction between ZnO and the optically active main-chain (PANI-CSA) that amplify the chirality. Addition of large amounts of ZnO is required in the polymerization reaction because the resultant composite films cannot absorb all of the ZnO in the electrochemical polymerization. In other words, the yielding percentage of nano-ZnO in the PANI/ZnO composite can be low.

These film show optical absorption bands at 430 nm and at around 850 nm due to π-π* transition of benzenoid segments and formation of polarons in quinoid segments respectively [20] .

As for PANI-ZnO/PVA (Films 3a and 3b, double-layer film), an interface oxidation reaction between PANI and ZnO/PVA layers may occur; however, interfacial contact between ZnO and PANI via the PVA matrix may be low. So, photocatalytic doping of PANI with ZnO could not occur upon irradiation of UV light in this study.

Scanning electron microscopy (SEM) images for Film 1d, 3a, and 3b are shown in Figure 3. In general, PANI- doped with CSA films show interconnected networks [21] . The films thus prepared display characteristic morphologies. Energy-dispersive X-ray spectroscopy (EDS) measurement result for Film 3a confirms the form of the film surface, because Film 3a has large amounts of Zn from ZnO (39.77 counts (a.u) in the component, Table 2). The EDS reveals that Film 1d contains small amounts of ZnO. This result implies that the PANI incorporates a small amount of ZnO during the electrochemical polymerization process.

The FTIR spectra of Film 2a after UV irradiation in water were recorded as shown in Figure 4. The incorporation of CSA into the polyaniline chain was confirmed with FTIR. The characteristic IR absorption peak at 1574, 1497, 1302, 1249, 1148, and 817 cm⁻¹ were found in all the spectra. These peaks are attributed to quinonoid (Q) C = C stretching, benzenoid (B) C = C stretching, v_QBQ C-N stretching, v_BBB C-N stretching, aromatic C-H in plane bending vibration, and C-H out-of-plane bending vibration of PANI, respectively [21] . Protonated polymer was confirmed with the peak at 1734 cm⁻¹ (C = O stretching vibration) and 1047 cm⁻¹ (SO³⁻ vibration) derived from CSA. Spectra of the PANI layer are not changed upon the UV irradiation, indicating that the chemical structure of the PANI is not changed upon UV light irradiation.

Figure 5 shows thermographic images of Film 3b on the ITO substrate upon infrared (IR, left) and ultraviolet (UV, right) light irradiation. The thermographic image upon irradiation of the IR light demonstrates that the film (Film 3b) absorbs visible range light and surface temperature is increased (∆T = +5˚C). On the other hand, the film absorbs UV light upon irradiation of UV light, and the surface does not reflect UV light, resulting no change of the surface temperature (∆T = −5˚C), although surrounding temperature is increased. PANI absorbs both the IR and the UV ranges, and ZnO absorbs UV light. So, ZnO enhances absorption of the UV light for the film.