Different compositions of TiO2-polyaniline composites were prepared via chemical oxidative polymerisation of aniline over TiO2 nanoparticles followed by hydrothermal treatment. The prepared composites were characterised by various physicochemical techniques. From UV-VIS digital replay system (DRS) analysis, the shifts of the absorption maximum towards the visible region are indicated and transmission electron microscopy analysis showed that the particle size of TiO2 is reduced after incorporation of polyaniline. The photocatalytic activity of visible light responsive polyaniline modified TiO2 in the degradation of 4-nitrophenol was studied. The percentage degradation was monitored using a UV-VIS spectrophotometer. About 92% degradation was obtained with polyaniline modified TiO2.
The pollution of drinking water reservoirs and the aquatic environment by chemicals is a serious problem nowadays. Nitro aromatic compounds are recognized as environmentally hazardous. Nitrophenols are some of the most refractory substances present in industrial wastewaters because of their high stability and solubility in water (Paola et al. 2003). As a waste product, polynitrophenols (PNP) are hazardous and are a priority toxic pollutant used in pharmaceutical manufacturing, fungicides, insecticides and dyes. 4-Nitrophenol (4-NP) can be released into the soil as a result of hydrolysis of several organophosphates, including pesticides such as parathion and methyl parathion. Animal studies suggest that PNP may cause blood disorders. It is difficult to purify PNP-contaminated wastewater as a result of it being stable towards chemical and biological degradation (Yang et al. 2010). There is convincing evidence that 4-NP is one of the secondary pollutants formed in the tropospheric transformation of monoaromatic chemicals with NOx and ozone. Purification of wastewater contaminated with 4-NP is very difficult indeed. The presence of a nitro substituent is known to render benzenoid compounds more resistant to microbial degradation. 4-NP is very photostable, a phenomenon believed to be due to the charge transfer character of its triplet state. In aqueous aerated solutions, 4-NP is very reluctant to undergo photochemical transformations (Kochany 1991). Nitrophenols are involved in the synthesis of many products and appear in the degradation of pesticides like parathion (Meallier et al. 1977) and nitrofen (Nakagawa & Crosby 1974).
From a catalytic point of view, TiO2 possesses a unique type of surface involving both redox and acid–base sites. In addition to high thermal stability, its amphoteric character makes titania a promising catalytic material. The textural and acid–base properties of titania depend greatly on the method of preparation. Loading of sulphate using sulphuric acid makes TiO2 more acidic as its surface is positively charged due to protonation. Krishnakumar in 2011 reported the green synthesis of N-formylation of amines at room temperature using nano-TiO2-SO42− (Krishnakumar & Swaminathan 2011a). They also reported an efficient protocol for the green synthesis of quinoxaline and dipyridophenazine derivatives at room temperature using sulfated titania (Krishnakumar & Swaminathan 2011a, b). The photocatalytic degradation rate of the different nitrophenols depends on various parameters, such as temperature (Chen & Ray 1998), pH (Augugliaro et al. 1993; Chen & Ray 1998) and initial concentration of the pollutant (Nakagawa & Crosby 1974; Meallier et al. 1977; Augugliaro et al. 1991; Chen & Ray 1998). Here in this work we have tried to degrade nitrophenol using visible light responsive polyaniline-modified TiO2 systems and achieved around 92% degradation.
Preparation of catalysts
Pure TiO2 (T)
Titanium isopropoxide and acetic acid in a volume ratio 1:2 were taken in a beaker and magnetically stirred with dropwise addition of distilled water from a dropping funnel. The clear solution obtained was mixed with the solution of the surfactant P123 in water and the mixed solution was then sonicated for 3 hours. It was then transferred to an autoclave and kept overnight at 110 °C. The resultant mixture was then filtered and dried in an air oven at 110 °C followed by calcination at 500 °C for 5 hours.
Ammonium peroxodisulphate solution (12.5 g in 100 ml) was added dropwise to a mixture of 2.5 g TiO2 in 100 ml 1 M HCl solution and 5 ml of distilled aniline kept in an ice bath under constant stirring. The mixture, after sonication for about 3 hours, was transferred to an autoclave and kept overnight at 110 °C. The mixture was then filtered, washed with water and acetone to remove unreacted aniline, and dried in an oven at 110 °C. Three different compositions of the nanocomposites were made by varying the molar ratio of titanium dioxide and aniline and they were named as TP1(4:1), TP2(2:1) and TP3(1:1).
X-ray diffraction (XRD) patterns were recorded in a Bruker AXS D8 Advance X-Ray diffractometer using Ni filtered Cu Kα radiation (λ = 1.5406 Å) in the range 5–70° at a scan rate 2°/min. The IR spectra of the samples were recorded using a Thermo NICOLET 380 Fourier transform infrared spectroscopy (FTIR) Spectrometer by means of KBr pellet procedure. UV-digital replay system (DRS) spectra were taken in a UV-VIS double beam UVD-3500, Labomed Inc. spectrophotometer. Determination of surface area of the samples was achieved in a Micromeritics Tristar 3000 surface area analyzer. Prior to the measurements, the samples were degassed at 110 °C for 6 h. TG/DTA analysis was done on a PerkinElmer Pyris Diamond thermogravimetric/differential thermal analyzer instrument under a nitrogen atmosphere at a heating rate of 5–10°/min from room temperature to 1,000 °C. Morphology of all samples was observed on a PHILIPS, CM200, operating voltages: 20–200 kv; resolution: 2.4 Å. Raman spectra were recorded on a Horiba Jobin Yvon Lab Ram HR system at a spatial resolution of 2 mm in a backscattering configuration. The 514.5 nm line of Argon ion laser was used for excitation. X-ray photoelectron spectroscopy (XPS) measurement was performed using SPECS XPS system with 150 W achromatic Al Kα X-ray source at 1,486.6 eV energy. The survey scans were obtained at 70 eV pass energy and composition was obtained from survey scan. Core level spectra of C1 s was obtained at 25 eV pass energy and Ti 2p and O1 s core levels at 40 eV pass energies. C1 s and Ti 2p core levels were deconvoluted with Gaussian-Lorenztian to get component peaks.
The photocatalytic activities of the samples were evaluated by the degradation of 4-NP. Photocatalytic activities of the prepared systems were scanned using Oriel Uniform illuminator (Newport Model 66901). For a typical reaction, 10 ml of 10−4M aqueous 4-NP solution was used. A high pressure ozone free xenon lamp served as the visible light source; a glass filter was added to allow visible light (>400 nm) to pass through. The light source is a 150 W Xe lamp with an irradiation intensity of 96.8 mW cm−2. Before illumination, the suspensions were stirred for 30 min in the dark in order to reach adsorption–desorption equilibrium between the photocatalyst and 4-NP solution and the irradiation was performed for about 1 hour. After irradiation the percentage degradation was monitored using a UV-VIS spectrophotometer.
RESULTS AND DISCUSSION
Wide angle XRD
The XRD pattern of TiO2 and polyaniline modified TiO2 are shown in Figure 1. Pure titania showed the presence of only the photocatalytically active anatase phase with 2θ = 25.5 (1,0,1 plane), 38 (0,0,4) plane, 48.3 (2,0,0) plane, 54 (1,0,5) plane, 55.2 (2,1,1) plane, 63 (2,0,4) plane, 68.9 (1,1,6) plane. No characteristic peaks of rutile or brookite phases were observed indicating high purity of the system. When PANI is adsorbed onto the surface of TiO2, the molecular chain of adsorbed PANI is tethered and the degree of crystallinity decreases (Zhang et al. 2006).
Titania polyaniline composites exhibited the characteristic peaks of anatase phase with a slight lowering of intensity. PANI incorporation had no significant effect on the crystallization and phase characteristics of TiO2. Increase in the amount of polyaniline resulted in a further lowering of intensity of the diffraction peaks.
Optical absorption spectra
Figure 2 shows the UV-VIS absorption spectra of pure TiO2 and polyaniline modified TiO2. Clearly, the resulting PANI-TiO2 composite can strongly absorb not only the near ultraviolet light but also the visible light, whereas the TiO2 can absorb light with wavelengths below 250 nm only (Guo et al. 2012).
Polyaniline incorporation extended the absorption of TiO2 to the visible region as far as 800 nm compared with the bare TiO2 (Liao et al. 2011). Above 400 nm, only the composite shows absorption and a broad peak in the range 500–750 nm, indicating the presence of the PANI on the surface of the TiO2 nanoparticles (Salem et al. 2009).
The N2 adsorption/desorption isotherms and Barrett–Joyner–Halenda pore size distributions of the prepared sample is displayed in Figure 3. All the prepared samples show type IV isotherm with an H1 type hysteresis loop. Pure TiO2 shows a surface area around 56.87 m2g−1. In the case of 4:1 composite (TP1) surface area is 45.14 m2g−1, 2:1 composite (TP2) it is 46.04 m2g−1 and finally for 1:1 composite (TP1) it is 39.32 m2g−1. Polyaniline incorporation results in a drastic decrease in pore volume and an increase in pore diameter compared with TiO2. As we incorporate the conducting polymer in to the titania, the surface area and pore volume decreases and the pore diameter increases. The change in pore diameter is drastic when we incorporate lower amounts of the polymer and the change is not so great as the amount of polymer in the composite increases. This may be due to the fact that at lower monomer concentration the rate of diffusion of monomer in to the pore channels of the titania is comparable to that of the rate of polymerisation. So there is a chance of polymer formation inside the pore channels. This results in a pore volume decrease and may cause the expansion of the pores which results in an increase in pore diameter. But as the concentration or amount of monomer increases the rate of polymerisation is high compared to the rate of diffusion. So the polymerisation occurs at the surface before the monomer diffuses in to the pore channels. This may cause pore blockage which results in a decrease in surface area and pore volume but the change in pore diameter is not so prominent.
IR spectra of pure TiO2 and polyaniline, along with the composites are shown in Figure 4. Pure TiO2 shows peaks around 456, 1,623, 3,441 cm−1, which may be attributed to the Ti-O-Ti stretching, O-H bending, and O-H stretching, respectively.
Peaks for pure polyaniline appears at around 503 cm−1 (C-H out of plane bending vibration), 808 cm−1 (para-substituted aromatic rings), 1,133 cm−1 (C-H in plane bending) and 1,294 cm−1 (C-N stretching). Two bands in the range 1,450–1,600 cm−1 (1,491, 1,581 cm−1) can be assigned to the nonsymmetric C6 ring stretching modes. Higher frequency mode has a higher contribution from the quinoid ring and a lower one from the benzenoid ring. Peaks around 3,445 cm−1 corresponds to N-H stretching whereas aromatic C-H stretching results in a band in the range 3,000–2,500 cm−1 (Ganesan & Gedanken 2008).
PANI-TiO2 composites retained almost all the peaks of PANI and TiO2. PANI is believed to undergo polymerization on the surface of TiO2. Peaks at 1,645 cm−1 (bending mode of water) and 3,448 cm−1 (stretching mode of water) suggest the presence of water even after calcination. This is quite important since surface hydroxyl groups provide higher capacity for oxygen adsorption which is essential for the photocatalytic activity of anatase (Ohtani et al. 1997).
Raman spectra recorded for TiO2 and TiO2-PANI nanocomposites are shown in Figure 5. In the Raman spectrum of the anatase TiO2 the following modes can be assigned: ∼144 (Eg), 197 (Eg), 399 (B1g), 513 (A1g), 519 (B1g) and 639 cm−1 (Eg).
Raman spectra of TiO2-polyaniline composites showed two additional peaks at 1,337 and 1,573 cm−1 corresponding to C-N and C = C stretching, respectively, of the polymer chain. This confirmed the incorporation of polyaniline into titania (Huyen et al. 2011).
The TG-derivative thermogravimetric (DTG) curves of pure TiO2 and polyaniline/TiO2 nanocomposites with various weight percentages are shown in Figure 6. In the case of TiO2 the weight loss around 100 °C is due to the elimination of water and a major weight loss around 100–250 °C is due to the volatilization of water of hydration, and at around 300–400 °C is due to combustion of organic species such as P123 and CH3COOH (Liu et al. 2009).
In the case of the composite, three major weight losses were observed: the first around 100 °C due to the elimination of water, the second around 260 °C corresponds to the loss of dopant HCl and the final weight loss at high temperature around 450 °C is due to the breaking of the polymer backbone (Danielle et al. 2003).
X-ray photoelectron spectroscopy
XPS spectra of both pure and modified TiO2 are represented in Figure 7. In the X-ray photoelectron spectra of PANI modified TiO2, the elements C, O, Ti and N can be detected and their binding energies are 284.8, 531, 458.5, 401.3 eV, respectively. We also measured C1s and O1s core levels. The results imply that three peaks are observed at binding energies of 284.5, 286.1, and 288.2 eV for C1s and 531 eV for O1s.
The peak (284.8 eV) of C1s indicates the presence of 1C and the peak (286.3 eV) is attributed to 2C in the structure of PANI. The binding energy peak of N1s at 399.2 eV can be assigned to the benzenoid aniline (–NH–) structure. The binding energy (401 eV) of N1s in the nanocomposites, which is higher than that of PANI, also indicates strong interaction (e.g. hydrogen bonding) between TiO2 and PANI (Li et al. 2008).
Transmission electron microscopy (TEM)
Figure 8 shows TEM images of TiO2 and polyaniline/TiO2 nanocomposites. The TEM image of polyaniline/TiO2 nanocomposites shows the granular structures. The incorporation of polyaniline into the TiO2 decreases the particle size. Bare TiO2 shows a particle of 18 nm while the composite shows a particle size of 14 nm.
Photocatalytic activity studies
The photocatalytic activity TiO2 and PANI/TiO2 with different molar ratios under visible light illumination (>400 nm) was evaluated by comparing the photodegradation efficiency of 4-NP. The amount of catalyst and time was optimised by taking different amounts of catalyst and performing the reaction at different time periods. It was observed that maximum degradation efficiency was obtained at a catalyst loading of 20 g l−1 for a time period of 1 hour. Compared to pure TiO2, the modified TiO2 systems exhibit better activity under visible light irradiation.
Effect of catalyst loading
Subsequent experiments were accomplished to study the effect of the amount of TP composites on 4-NP removal efficiency at pH 6.4 and degradation time of 1 h.
The removal efficiency increases as the catalyst amount increases from 0.005 to 0.04 g, as shown in Figure 9(a). From the graph it is evident that, at first, the percentage degradation increases with the increase in amount of catalyst and beyond a certain limit the percentage degradation decreases. This may be due to a light scattering effect and reduction in light penetration through the effluent due to the obstruction of a large number of solid particles. Therefore, the optimal amount of catalyst was selected as 20 g l−1.
Effect of time
To show the influence of time upon the degradation of 4-NP, the reaction was carried out over a time period of 1 hour. It is seen from Figure 9(b) that as time increases the degradation increases and maximum degradation efficiency was obtained at 60 min. The initial degradation rate is higher and that may be due to more contact between the photocatalyst surface and p-nitrophenol. Later on the degradation rate is slower this may be due to the availability of less surface active sites on the photocatalyst surface.
Effect of pH
The degree of photocatalytic degradation of 4-NP was found to be affected by a change in pH. In order to study the effect of pH on photocatalytic degradation of 4-NP, a pH range from 2–10 was selected. Figure 9(c) shows the effect of varying the pH, from 2 to 10, on the degradation of 4-NP in the presence of TiO2 polyaniline under visible light irradiation.
Increasing the pH of the 4-NP solution from 2 to 6, increases the degradation from 10 to 83% at 60 min. Degradation of 4-NP was high between pH 2 and 6, while the degradation efficiency was lower in the alkaline environment above pH 7. Also there is a red shift in λmax as we move from acidic to alkaline pH. This is represented in Figure 9(d). Maximum degradation efficiency was obtained at pH 6. So the optimum pH for the reaction is fixed as 6 and further experiments were carried out at this pH.
Effect of polyaniline content
The polyaniline modified TiO2 nanocomposites shows superior activity compared with the bare TiO2. As the amount of polyaniline in the composite increases, the degradation first shows an increase and then a further increase in the polyaniline content decreases the degradation slightly. This may be because there is an optimum concentration for high catalytic activity. Figure 9(e) shows the effect of polyaniline content on the photocatalytic degradation of 4-NP.
In order to check the efficiency of the catalyst the recycling study was carried out. In the first cycle, the catalyst shows a degradation of 83% and, in the second and third cycles, it decreases to about 75–73% and after that it shows a drastic decrease. Therefore, it can maintain activity only up to the third cycle. Figure 9(f) shows the recycling studies of the catalyst.
In this study, a polyaniline modified TiO2 photocatalyst which was highly responsive to visible light was prepared. 4-NP is successfully degraded by TiO2-polyaniline nanocomposite-assisted photocatalysis in an aqueous dispersion under visible light. Photocatalytic degradation of 4-NP depends on several things including the amount of catalyst, pH and polyaniline content. The prepared catalyst shows a maximum of 91% degradation within a time period of 60 min. The visible light assisted photocatalysis with TiO2-polyaniline nanocomposites can be used as a viable technique for the treatment of 4-NP (an organic pollutant).
The authors are grateful to CSIR, New Delhi, for financial assistance and to Dr C. Anandan, Scientist, National Aerospace Laboratory Bangalore, for XPS analysis and Dr M. K. Jayaraj, Department of Physics, Cochin University of Science and Technology, for Raman analysis.