Application of PANI/TiO₂ Composite for Photocatalytic Degradation of Contaminants from Aqueous Solution

Abstract

Polyaniline (PANI) is a promising conducting polymer for surface modification of TiO₂ to achieve extended photoresponse to visible light and increased photocatalytic efficiency. In this study, we report the synthesis of a PANI/TiO₂ composite with different weight ratios of PANI, which was subsequently employed for photocatalytic degradation of methylene blue (MB), bisphenol A (BPA), and bacteriophage MS2 under visible-light irradiation. The functional groups, morphology, and light response of the composite were characterized by Fourier-transform infrared spectroscopy, field-emission transmission electron microscopy, and diffuse reflectance UV–visible spectroscopy, respectively. The PANI/TiO₂ composite containing 4% by weight ratio of PANI was most suitable for MB degradation, and this photocatalyst was very stable even after repeated use (four cycles). The degradation of BPA and bacteriophage MS2 by PANI/TiO₂ composite reached 80% in 360 min and 96.2% in 120 min, respectively, under visible-light irradiation. Therefore, the PANI/TiO₂ composite with enhanced visible-light photocatalytic efficiency and stability can be widely used for the degradation of water contaminants.

1. Introduction

Since the first reports on the photoelectrolytic water splitting of titanium dioxide (TiO₂) by Fujishima and Honda in 1972, photocatalysis has been recognized as an effective technique for the degradation of water contaminants over the past decades [1,2]. Among the semiconductors for photocatalysis, TiO₂-based nanomaterials are most widely used because of their high efficiency, low cost, mechanical robustness, chemical stability, and non-corrosive property [3,4]. However, the wide-band-gap energy (3.2 eV) that is only photoactive under ultraviolet (UV) light (λ < 366 nm) and the relatively high recombination rate of electron-hole pairs restrict the photocatalytic activity of TiO₂ [5].

Visible-light-response photocatalysts provide a popular way to solve environmental problems by directly harvesting energy from sunlight [6]. In this regard, much effort has been made to extend the photo-response region of TiO₂ to visible light and to increase its photocatalytic efficiency under UV and visible-light irradiation by surface modification, such as metal loading, impurity doping, inorganic adsorbates, polymer coating, dye sensitization, and charge-transfer complexation [7]. Among the surface modification methods for oxide-based semiconductors, conducting polymers have attracted much attention owing to their high conductivity and stability, simple synthesis, and excellent environmental compatibility [8]. With an extended π-conjugated electron system, the conducting polymer acts as a stable photosensitizer to sensitize TiO₂ by the absorption of a wide spectrum of UV and visible-light (190–800 nm) irradiation [5]. TiO₂ modified with conducting polymers, such as polyaniline (PANI), polypyrrole, and polythiophene, has demonstrated excellent photocatalytic activity for the degradation of contaminants under visible-light irradiation [5,9,10].

Especially, PANI has been widely used as a conducting polymer in semiconductor modification because of its high stability, low cost, and easy polymerization [11]. In addition, it has also been regarded as a promising material to overcome the inherent drawbacks of TiO₂ by facilitating charge (electron-hole) transport due to its well-matched energy level. TiO₂ is a typical n-type semiconductor, whereas PANI is generally considered a p-type semiconductor [5]. Thus, their strategic combination provides excellent stability and conductivity, as well as enhanced photocatalytic performance. There are several reports on the manufacture, properties, and applications of PANI/TiO₂ composite for photocatalytic degradation of water contaminants [5,12,13].

Here, we demonstrate the enhanced photocatalytic efficiency of PANI/TiO₂ composite under visible-light irradiation for degradation of water contaminants (methylene blue (MB), bisphenol A (BPA), and bacteriophage MS2). The PANI/TiO₂ composites were prepared using different PANI to TiO₂ weight ratios, and their performance was evaluated by photocatalytic dye degradation experiments. The properties of the prepared samples were analyzed. The applicability and reusability of PANI/TiO₂ composites were also evaluated.

2. Material and Methods

2.1. Chemicals

TiO₂ (≥99.5%), aniline (C₆H₅NH₂, ≥99%), BPA (C₁₅H₁₆O₂, ≥99%), and ammonium persulfate (H₈N₂O₈S₂, ≥98%) were purchased from Sigma–Aldrich Co., Ltd. (St. Louis, MO, USA). MB (C₁₆H₁₈ClN₃S, ≥96%) and hydrochloric acid (HCl, 36.5%) were bought from Samchun Pure Chemical Co., Ltd. (Pyeongtaek, Korea). Ethyl alcohol (C₂H₅OH, ≥99.5%) was sourced from Duksan Pure Chemicals Co., Ltd. (Ansan-si, Korea). Deionized water (DI, 18.2 MΩ of resistivity) was obtained from a Direct-Q 3 UV system (Millipore, Billerica, MA, USA) and used in the preparation of all solutions. All the chemicals were used as received without further purification.

2.2. Preparation of PANI/TiO₂ Photocatalyst

To prepare PANI (Figure 1) powder, 10 mL of 1 M HCl was added to 90 mL DI (Sol A). Sol B was prepared by dissolving 5 g of ammonium sulfate in 20 mL of Sol A. Sol C was prepared by adding 2 mL aniline to 80 mL of Sol A. Then, the mixture of Sol B and Sol C was stirred for 6 h. Afterward, the obtained precipitate was washed with 1000 mL DI and 100 mL ethanol. The washed PANI was dried in an oven (FTVO-701, Sci Finetech Co., Seoul, Korea) at 70 °C for 12 h. To prepare the PANI/TiO₂ composite, 2 g TiO₂ and 0.04, 0.08, 0.12, 0.16, and 0.20 g PANI (2%, 4%, 6%, 8%, and 10%, respectively) were added to 100 mL DI. After sonication of each mixture for 6 h, the PANI/TiO₂ composites were oven-dried at 70 °C overnight before use.

2.3. Characterizations

To confirm the composition of PANI, the Fourier-transform infrared (FTIR) spectrum of PANI was recorded using a Nicolet iS50 FTIR apparatus equipped with an attenuated total reflectance accessory (Thermo Scientific, Waltham, MA, USA). Diffuse reflectance spectra (DRS) of pristine TiO₂ and the PANI/TiO₂ composite were measured using a diffuse reflectance UV–visible spectrophotometer (S-4100, Scinco Co., Ltd., Seoul, Korea) at a wavelength range of 200–800 nm and BaSO₄ powder as reference material. The morphology of the prepared PANI/TiO₂ sample was observed under a field-emission transmission electron microscope (FE-TEM; Tecnai G2 F30 S-Twin, FEI, Hillsboro, OR, USA). X-ray diffraction (XRD) pattern of 4% PANI/TiO2 sample was obtained using high power X-ray diffractometer (D/max-2500V/PC, Rigaku, Tokyo, Japan).

2.4. Experimental Procedures

The photocatalysis experiments were conducted by degrading MB and BPA under visible-light irradiation. The photocatalyst concentration was adjusted to 0.8 g/L, under specified otherwise. The photocatalyst was sonicated in DI, and the suspension was spiked into 5 μM MB and 5 mg/L BPA solution, respectively. Aliquots of 1 mL were sampled and filtered through a 0.20-μm polytetrafluoroethylene (PTFE) filter (13JP020AN, Advantec, Tokyo, Japan) at a specific time. Six visible light lamps (4 W) were placed in a black acrylic box. A quartz reactor with a 420-nm cutoff filter was placed at 6 cm from the visible-light lamps. The light intensity was determined by Reinecke’s salt actinometry (1.03 mW/cm²). The adsorption and the photolysis experiment were conducted without visible light and photocatalyst, respectively. To observe the reusability of the photocatalyst, a reuse test was conducted for four cycles. After each cycle, the sampled amount of solution was replenished for the next test. The photocatalytic activity was further assessed through a bacteriophage inactivation test by adding 5 mL of 4% PANI/TiO₂ suspension (4 g/L) to 10 mL of bacteriophage MS2 suspension (10⁵ PFU/mL).

2.5. Analytical Methods

The residual MB concentration was analyzed using a UV–visible spectrophotometer (NEO-S2117, Neogen, Seoul, Korea) at a wavelength of 664 nm. BPA concentrations were analyzed using a YL 9100 high-performance liquid chromatography (HPLC) system (Young In Chromass, Anyang-si, Korea) equipped with a YL 9120 UV–visible detector, YL 9150 autosampler, and YL C18-4D column (4.6 mm × 150 mm, 5 μm). The ratio of the DI and acetonitrile in the mobile phase was 50:50 (v/v), which was eluted at a flow rate of 1 mL/min. The column temperature was 35 °C, and the BPA was detected at 230 nm. A plaque assay was used to measure the bacteriophage MS2 concentration, and the solution was 10,000-fold diluted in DI.

3. Results and Discussion

3.1. Characterization of PANI/TiO₂ Composite

The FTIR spectrum of prepared PANI and PANI/TiO₂ composite are shown in Figure 2. The peaks of PANI appeared at 788, 1292, 1481, and 1563/cm, which correspond to a 1,4 substitution pattern of the benzene ring, C–N stretching vibration of primary amines, C=C stretching vibrations of quinone and the benzene ring, and C=N stretching vibrations of quinone and the benzene ring, respectively [14,15]. This observation indicates that PANI was synthesized successfully. The broad band of the PANI/TiO₂ composite at 500/cm is attributed to the TiO₂ vibration [12]. Morphological observations of PANI/TiO₂ composite by TEM (Figure 3) revealed relatively small-sized TiO₂ particles dispersed and anchored on the PANI chain. The result of the TEM analysis suggests that PANI/TiO₂ composite was completely fabricated. XRD analyses were used to identify the crystalline structure of the PANI/TiO₂ composite (Figure 4). The characteristic peaks at 25.28, 37.80, 53.89 and 62.69 matched well with the (101), (004), (105) and (204) crystal planes of the synthetic anatase (JCPDS #21-1272) [16]. The crystal size estimated by the Scherrer equation was about 16.27 ± 3.13 nm [17,18], which was well matched with the size derived from the TEM image (16.59 ± 3.59 nm).

The capacity to absorb light is an important factor for the activation of the visible-light photocatalyst [19]. To compare the light absorbance of the pristine TiO₂ and 4% PANI/TiO₂ composite in the visible-light region, DRS analysis was conducted in the wavelength range of 200–800 nm. The reflectance results (Figure 5) indicated a weaker photo-reflectance of 4% PANI/TiO₂ composite than TiO₂ at wavelengths above 400 nm, which means the composite has a stronger absorption capacity of visible light and the possibility of photocatalytic activity under visible-light irradiation. The red-shift in the absorption edge of 4% PANI/TiO₂ in this study could be elucidated by a color change [20,21,22]. PANI/TiO₂ composites appeared blue and grayish, which had a positive effect on the visible-light absorption. By contrast, pristine TiO₂ powder was white. The band gap energies (E_g) of samples were estimated from the intersection point of the absorption onsets and the baseline of the absorption spectrum using Kubelka–Munk remission function [2,23]. The E_g of pristine TiO₂ and PANI/TiO₂ composites were 3.21 and 3.15 eV, respectively. A similar band gap energy reduction in the PANI/TiO₂ composite (3.07 eV) has also been reported in the literature [12], which could lead to higher photocatalytic activity.

3.2. Photocatalytic Activity

To compare the effect of PANI composition in the PANI/TiO₂ composites, different weight ratios of PANI (2%, 4%, 6%, 8%, 10%) were added when the catalyst was prepared. Plots of C/C₀ versus visible-light-irradiation time were constructed for the different catalysts (Figure 6a).

As the weight ratio of PANI increased from 0% to 4%, the degradation efficiency of MB increased from 38.65 ± 2.68% to 48.10 ± 4.27%, but with 10% PANI composition, it decreased to 30.33 ± 2.33% in 240 min. This phenomenon shows that the proper amount of PANI is needed to optimize the degradation efficiency of a contaminant owing to obstruction of the molecular structure of PANI [24]. As a result, 4% PANI/TiO₂ composite displayed enhanced photocatalytic activity compared with commercial TiO₂ under visible-light irradiation. This is due to the synergetic effect of the well-matched energy levels of TiO₂ and PANI, PANI/TiO₂ composite could interfere the recombination of holes and electrons, which can improve the production of the reactive oxygen species that degrades MB. In addition, since PANI is a photosensitizer with a narrow bandgap, PANI/TiO₂ composite could enhance the response to visible light by inserting the energy level of PANI into the energy level of TiO₂, which was consistent with the DRS result. These electron transfer between the PANI and TiO₂ semiconductors and the enhanced photocatalytic activity of the composites has already been reported experimentally in other literature [12,25]. Adsorption of MB by the 4% PANI/TiO₂ did not occur (Figure 6b), and the direct photolysis of MB by visible-light irradiation was relatively low (4.69 ± 1.67%, Figure 6c), indicating that MB degraded predominantly by photocatalytic activity.

Different amounts of 4% PANI/TiO₂ (0, 0.32, 0.8, 1.12, 1.6 g/L) were added to the MB solution to optimize the catalyst dose. Results of the removal ratio of MB by different photocatalyst doses (Figure 7) showed that MB degradation increased from 3.88 ± 1.51% to 67.37 ± 7.82% as the dose of the photocatalyst increased to 1.12 g/L but adversely decreased to 58.40 ± 0.38% at the dose of 1.6 g/L. This result could be explained in terms of photocatalyst agglomeration and visible-light penetration into the suspension [26]. As the photocatalyst concentration in the solution is increased, the photocatalyst tends to agglomerate, which will reduce the amount of photocatalyst surface exposed to the visible-light. Furthermore, the agglomeration and sedimentation of photocatalyst particles also decrease the penetration depth of visible light from the reactor wall, causing light scattering, which lowers the light intensity entering the suspension.

3.3. Application to BPA and Bacteriophage MS2 Degradation

When the optimized PANI/TiO₂ composite was applied to the photocatalytic degradation of 5 mg/L BPA, BPA was degraded by 80 ± 0.75% in 360 min (Figure 8a). To calculate the degradation rate (k_app), the pseudo-first-order degradation kinetics of BPA by PANI/TiO₂ were plotted (Figure 8b) to solve Equation (1): [27]

ln(C₀/C) = k_appt

(1)

BPA had a rate constant of 0.00375 ± 0.00018/min, and the correlation coefficient (R²) was 0.99, which indicates that BPA degradation by PANI/TiO₂ follows pseudo-first-order kinetics.

Bacteriophages are potential indicator organisms for the efficiency of treatment processes because they are characterized by their stability to sunlight, temperature, and chemical/physical treatments [28]. PANI/TiO₂ degraded 96.2% of bacteriophage MS2 in 120 min under visible-light irradiation (Figure 9). The first-order rate constant of bacteriophage MS2 was 0.02637 ± 0.0055/min. These results highlight the applicability of PANI/TiO₂ for the removal of recalcitrant organic compounds and bacteriophages.

3.4. Reusability

For practical applications, the reusability of photocatalysts is important as it is related to the stability of the photocatalyst [29]. The reuse test was conducted for four cycles under visible-light irradiation to investigate the stability of PANI/TiO₂. After the photocatalytic reaction, the catalyst was reused for the next cycle, and the degradation efficiency was determined by analyzing the concentrations of MB residue in the samples. After four reuses, more than 99% of MB was degraded by PANI/TiO₂ in 360 min under visible-light irradiation (Figure 10). PANI/TiO₂ did not show any notable loss of photocatalytic activity, suggesting that it remains stable during the degradation of MB. It also suggests that active sites of PANI/TiO₂ were not occupied by the adsorption of MB molecules because MB might be oxidized by the continuously generated reactive oxygen species on the surface of PANI/TiO₂ [30].

4. Conclusions

In this study, a PANI/TiO₂ composite with enhanced visible-light photocatalytic activity was prepared. The results indicated that the PANI ratio and the amount of PANI/TiO₂ dose were important factors in the photocatalytic performance of PANI/TiO₂. A 4% PANI ratio and 1.12 g/L PANI/TiO₂ dose were most suitable for MB degradation. The degradation of BPA and bacteriophage MS2 by PANI/TiO₂ reached 96.2% in 120 min and 80% in 360 min. The photocatalytic activity of PANI/TiO₂ did not show any noticeable decrease, indicating the high stability of PANI/TiO₂. In conclusion, PANI/TiO₂ is an enhanced visible-light-activated photocatalyst for the degradation and removal of contaminants.