Inorganic semiconductor photocatalysts such as TiO2, ZnO and CdS have attracted extensive attention in the field of environmental chemistry for decomposition of organic pollutants in water and air from industries and households [1, 2, 3]. TiO2 is an important technological material that can be used for photocatalytic applications such as self-cleaning and water treatment because it is an effective catalyst that is photostable, reusable, inexpensive, nontoxic, and readily available [4, 5, 6]. However, the large band gap of TiO2 (3.2 eV), which is involved in the electron-hole recombination process under visible light in photocatalysis, limits its utilization of solar light [7]. To obtain photocatalysts that are active under visible light, many researchers have focused on the modification of the surface or bulk properties of TiO2 materials by polymer hybridization [8], ion doping [9], and semiconductor coupling [10]. In particular, hybrid structures consisting of a metal oxide and conjugated polymer are an advantageous combination to improve the photo-response of TiO2 in the visible region. Recently, it has been found that Sn4+ doped with TiO2 is stable, and Sn ions can replace Ti ions in the substitutional sites of TiO2, which alters the properties, electronic structure and light absorption properties of the resulting material [11]. Meanwhile, combining conjugated polymers with TiO2 has resulted in materials with interesting properties and applications like solar cells [12], photocatalysis [13, 14, 15], microbial fuel cells [16], and gas sensors [17]. Recently, TiO2/polymer nanohybrid photocatalysts have become a research focus. Song et al. [14] prepared a visible light-induced H-PVA/TiO2 composite loaded on glass via the sol-gel method and polythiophene (PTh)/TiO2 composite particles by a photoinduced polymerization process [15]. These catalysts photocatalytically degraded organic pollutants under visible-light irradiation.
Conjugated polymers such as polyaniline, polypyrrole, PTh and their derivates are promising materials for use as light-absorbing and charge-transporting components because of their suitable properties; for example, narrow, controllable bandgap, easy chemical tailoring, high charge carrier mobility, excellent stability and low cost [18, 19]. Moreover, conjugated polymers can also be used in photocatalysis. In recent years, there has been increasing interest in using PTh [20], polyaniline [21], polypyrrole [22], and their derivates [23] to sensitize TiO2 by forming photocatalytic TiO2/polymer hybrid materials. Among conjugated polymers, PTh is a well-known example that has wide-ranging applications in light-emitting diodes, photovoltaics and sensors [24]. PTh exhibits high charge carrier mobility, solubility, processability, long-term stability and strong absorptivity in the visible region [25]. Therefore, it can be considered a good candidate for sensitization of TiO2.
To evaluate the photocatalytic activity of a nanohybrid under visible-light irradiation, nitrobenzene (NB) and malachite green (MG) are often used as target organic pollutants. Nitroaromatic compounds are widely used as pesticides, explosives, solvents, and intermediates in the synthesis of dyes, plastics and other chemicals [26]. Most of these compounds are of global toxicological concern. Exposure of the skin or eyes to very small amounts of NB can cause mild irritation, vomiting, and headache, while continuous exposure may cause liver damage. Relatively high levels of MG are harmful to humans and other organisms because of its genotoxic and carcinogenic properties [27]. MG accumulates in fat tissue, which further increases its toxicity. Therefore, it is necessary to remove NB and MG when they are present as pollutants.
In the present work, we add PTh as a surface photosensitizer and Sn as a dopant to TiO2 through an in-situ oxidative polymerization technique at low temperature. The photocatalytic performance of the resulting hybrids in the degradation of toxic pollutants NB and MG in aqueous media is assessed. The role of the PTh layer on the Sn-doped TiO2 nanoparticles in the nanohybrid is investigated. Recycling experiments are performed to evaluate the photostability of the nanohybrid under visible-light irradiation.
Titanium tetra-n-butoxide and SnCl4 were used as titanium and tin sources, respectively, to prepare the nanohybrid catalyst. Thiophene monomer (99%), Milli-Q water, (NH4)2S2O8, and HCl were used without further purification to prepare solutions. NB and MG were used as organic pollutants in photodegradation experiments. All chemicals were reagent grade and obtained from Aldrich. Distilled water was used for all catalyst syntheses and to prepare NB and MG solutions.
The Sn-doped TiO2/PTh nanohybrid (SPNH) was prepared by a modified sol-gel process at low temperature. In a round-bottomed flask, SnCl4 (0.85 g, 0.75 wt%) was stirred with 2-propanol (12.5 mL) for 20 min followed by addition of titanium tetra-n-butoxide (12.5 mL). A suitable weight percentage (1.0 wt%) of distilled monomer thiophene determined from our previous work [28] was added to the above solution. The resulting mixture was stirred for 20 min and then added Milli-Q water (60 mL). A white precipitate formed immediately, which was then peptized with concentrated HCl at 60 °C for 8 h in a reflux system. To polymerize the thiophene, aqueous (NH4)2S2O8 solution (30 mL) containing 1.0 mol/L of HCl was added to the reaction mixture, which was subsequently stirred at room temperature for 3 h. The resulting brown solid was separated by centrifugation and then washed five times with distilled water. The resulting hybrid material denoted SPNH was dried at 40 °C for a week and stored in desiccator in the dark until further characterization. Sn-doped TiO2 nanoparticles (STN) were formed using the same experimental procedure without adding thiophene and (NH4)2S2O8.
X-ray diffraction (XRD) patterns of the SPNH and STN catalysts were acquired by a Panlytical X-Ray diffractometer using Cu Kα radiation at 45 kV and 40 mA with a scan rate of 0.2o. Fourier transform infrared (FT-IR) spectra of the samples in KBr pellets were recorded on a Shimadzu spectrometer in the range of 400 to 4000 cm-1. Band gap energies E and absorption edges of the samples were determined using UV-Vis diffuse reflectance spectroscopy (UV-DRS) (S-3100, Scinco Co., Ltd.) with BaSO4 as a reference. Spectra were recorded at room temperature in the wavelength range of 200−800 nm. The band gap energies (E) of the synthesized samples were calculated according to Eq. (1):
To evaluate the adsorption capacity of the synthesized catalysts, a solution of NB or MG (10 mg/L, 250 mL) was combined with the SPNH or STN (100 mg) under continuous stirring at 25 °C in a 1000-mL conical flask covered with aluminum foil to avoid light. Aliquots (5 mL) were withdrawn every 15 min and centrifuged to separate the catalyst from the supernatant. The concentration of NB and MG in the solution was then determined using UV-Vis spectrophotometry. The adsorption ratio (R) of NB and MG was calculated as follows:
The photocatalytic activities of the samples were evaluated by the decomposition of NB and MG under visible light (λ > 450 nm). Visible light was obtained by a high-pressure 400-W mercury vapor lamp with a 450-nm cutoff filter to ensure the desired irradiation light [29]. SPNH or STN photocatalyst (0.1 g) was added to an aqueous suspension of NB or MG (150 mL, 10 mg/L). To ensure establishment of an adsorption-desorption equilibrium of dye on the catalyst surface, each solution was stirred for 45 min in the dark. The suspensions were kept under constant air-equilibrated conditions before and during illumination. To determine the change in NB and MG concentration in each solution during irradiation, aliquots (5 mL) of the solution were withdrawn using a Millipore syringe (0.45 µm) at certain time intervals, and the absorbance of NB or MG remaining in the solution was measured on a UV-Vis spectrophotometer at 269 or 617 nm, respectively. The concentration of dye solutions during degradation was calculated using ratio C/C0, where C is the absorbance at time t and C0 is the initial dye concentration before degradation.
The production of hydroxyl radicals (•OH) on the surface of the samples was detected by a PL method using coumarin as a probe molecule. Coumarin readily reacts with •OH to produce the highly fluorescent product 7-hydroxycoumarin (7HC) [30]. The experimental procedure was similar to the measurement of photocatalytic activity except that the dye solution was replaced by aqueous coumarin solution (150 mL, 10 mg/L). The PL spectra of generated 7HC were measured on a Hitachi F-7000 fluorescence spectrophotometer. After visible-light irradiation for 1 h, the reaction solution was filtered and the increase in the PL intensity around 456 nm excited by 332-nm light was measured.
The XRD patterns of the SPNH and STN are shown in Fig. 1(a). Rutile phase (JCPDS: 01-088-1172) with preferred (110) orientation along with (101), (111), (211), (002) and (301) reflections were mainly detected in the prepared samples; no anatase phase was observed. The XRD data reveals that there was no change in the structure of the TiO2 diffraction pattern caused by the addition of PTh and Sn because there was no decrease in the intensity of the peak corresponding to the rutile phase at 2θ = 27.3° for SPNH compared with that for STN. This indicates that PTh is not incorporated in the TiO2 lattice. In addition, the broad XRD peaks of STN and SPNH indicate small crystallite sizes of about 18 and 14 nm, respectively (Table 1). The crystallite size is smaller for SPNH than STN possibly because of the presence of thiophene residue, which can control the growth and nucleation of metal oxide.
Figure 1(b) depicts the FT-IR spectra of STN, SPNH and PTh. The pure TiO2 has one major band at 585 cm-1 associated with the Ti−O−Ti stretching mode of TiO2 [28]. Two peaks at ~3404 and 1635 cm-1, which are characteristic of stretching and bending vibrations of water molecules, reveal that there is molecularly adsorbed water on the dried samples. After polymerization of thiophene, the characteristic peaks of PTh are observed, confirming the formation of PTh on the surface of STN. The bands at 1458 and 1543 cm-1 are attributed to the symmetric C−C stretching modes of the thiophene ring. The bands at 1450, 1440, 1135, and 1052 cm-1 correspond to symmetric C=C stretching, C−H stretching in the thiophene ring, C−H bending, and C−S stretching in the thiophene ring, respectively [31]. The band at 985 cm-1 is consistent with the symmetric S-O stretching vibration [32]. The FT-IR spectrum of SPNH showed absorption peaks attributed to Cα−Cα conjugation at 1450, 1135, and 1052 cm−1, which confirms that the thiophene monomer was successfully polymerized to PTh. Comparison of the spectrum of PTh with those of STN and SPNH (Fig. 1(c)) indicated that the nanoparticles were incorporated into PTh to form a core-shell structure, and, interaction exist between PTh and metal oxide in SPNH.
The UV-DRS of STN and SPNH are shown in Fig. 1(d). STN and SPNH both exhibit an apparent absorption in the visible region between 400 and 500 nm. Interestingly, a monotonic increase in the UV-DRS spectra of STN and SPNH was detected; there are evident red shifts in the absorbance peak edges of these samples, which should be caused by the presence of Sn and PTh. The extension of the absorption edge to longer wavelength for SPNH indicates there is a strong interaction between the metal oxide and PTh in the hybrid material. This interaction may accelerate photoinduced charge transfer from PTh to STN in SPNH. These results indicate that the nanohybrid can be excited by visible light. The band gap energies of the samples were calculated from the reflectance spectra using equation (1) and are presented in Table1.
XPS analysis of the SPNH sample was performed; the survey spectrum and high-resolution scans are provided in Fig. 2. The elements Ti, Sn ,O, C, and S were detected on the surface of the nanohybrid. Ti, Sn and O can be assigned to metal oxide in the hybrid, while C and S originate from PTh. The Ti 2p peaks are narrow with slight asymmetry and have binding energies of 459.408 and 465.123 eV, attributable to Ti 2p3/2 and Ti 2p1/2, respectively. These values are consistent with those reported for TiO2 [33]. The doublet peaks at 487.84 and 496.768 eV in the Sn 3d spectrum are ascribed to Sn 3d5/2 and Sn 3d3/2 of the substitutional Sn4+ dopants in the lattice, respectively (Fig. 2(e)). The radii of Sn4+ and Ti4+ ions are 0.071 and 0.068 nm, respectively. These values are so close that it is easy for Sn4+ to replace Ti4+ in the lattice of TiO2 [34]. These observations confirm the formation of Ti−O−Sn structure in the Sn-doped TiO2 induced by the substitution of Ti with Sn. The presence of an S 2p signal located at a binding energy of 168.617 eV (Fig. 2(f)) is associated with the formation of positively charged sulfur (Sδ+) [35]. This positive charge is consistent with the formation and transport of polarons and bipolarons in the conjugated chains, which indicates that presence of polymer in the sample. These results are consistent with the energy-dispersive X-ray spectroscopy (EDX) data and the elemental composition of STN and SPNH shown in Fig. 3. The XPS analysis confirmed the presence of all of the intended elements and respective compositions of the nanohybrid.
SEM images of the samples are presented in Fig. 4. The SEM image of STN showed an uneven distribution of agglomerated spherical particles, while SPNH possessed a well-dispersed uniform spherical structure with micro- and mesopores at the interface of the particles. The spheres in SPNH were more uniform and smaller than those in STN. The smaller particle size is consistent with an increase of surface area, which correlates with the BET surface areas of SPNH and STN (Table 1). These results show that the polymer produced uniform particles with controlled size and morphology.
The morphology and size of STN and SPNH were further studied by TEM and high-resolution (HR) TEM (Fig. 5). The TEM images in Fig. 5(a, c) reveal that both SPNH and STN were spherical with diameters of 14±2 and 20±2 nm, respectively. These values are consistent with the XRD results. Compared with STN, SPNH possessed smaller crystallites that were better dispersed. This indicates that polymerization of thiophene on the surface of metal oxide may allow the growth and nucleation of metal oxide to be controlled by the polymer residue. Figure 5(d) shows that SPNH consisted of particles with a core-shell structure coated with a polymer layer with a thickness of approximately 3 nm. In these images, the dark areas are the metal oxide and lighter regions are the polymer. The lattice spacing between adjacent lattice planes of metal oxide particles in SPNH is about 0.325 Å (Fig. 5(e)), corresponding to the distance between (110) crystal planes of the rutile phase of TiO2.
The adsorption of dye molecules on STN and SPNH was determined spectrophotometrically. To determine the surface adsorption of dye by the catalysts, the NB/MG concentration and catalyst loadings used were the same as those in the photocatalysis experiments. The catalysts were dried for 24 h at 60 °C to remove adsorbed water, then the dried catalyst was added to NB solution (10 mg/L, 250 mL) and kept in the dark for 2 h. Aliquots were removed every 15 min, centrifuged, and analyzed by UV-Vis spectrophotometry to determine the NB concentration. STN and SPNH adsorbed 14% and 24% NB, respectively, and 13% and 21% MG, respectively (Fig. 6). SPNH adsorbed more dye than STN, which may be caused by its higher surface area than that of STN. These results confirmed that adsorption of NB and MG on the catalyst surface was increased by the presence of PTh on the metal oxide. Similar enhancement in the adsorption capability of another photocatalyst for methyl orange was reported by Xu et al. However, although it is well known that the ability of a catalyst to adsorb organic compounds does not affect its photocatalytic activity, it does help to bring the organic molecule close to the photoactive site.
The photocatalytic activity of the samples was evaluated by their ability to degrade NB and MG in aqueous suspensions under visible-light irradiation at room temperature. Figure 7 shows the evolution of UV-Vis spectra of NB degradation by STN and SPNH during simulated visible-light irradiation over time. In both spectra, the intensity of the absorption peak from NB decreased gradually. The decrease in absorbance is consistent with the efficient decomposition of NB by both catalysts. This indicates that the degradation of NB proceeded under oxidative reaction conditions and electrophilic •OH are the predominant reactive species. •OH can produce isomeric nitrophenols, isomeric diphenols by denitration of nitrophenol isomers, and phenol, and these by-products can be oxidized to CO [36]. Compared with SPNH, STN exhibited a lower degradation capacity (Fig. 7(b)), probably because of the low adsorption capacity of STN caused by its small surface area (Table 1). When more NB molecules are adsorbed on the photocatalyst surface, the degradation of NB occurs more quickly. Figure 7(c) illustrates the degradation of NB over the catalyst samples under visible-light irradiation. When the photocatalyst was absent (blank in Fig. 7(c)), the photodegradation of NB under visible-light irradiation was very slow, with only 8.9% of NB removed after 105 min. In contrast, after 105 min of visible-light irradiation, 99.4% and 72.5% of NB was degraded in the presence of SPNH and STN, respectively. These results suggest that SPNH exhibited higher photocatalytic activity than STN under visible-light irradiation.
In the degradation of MG using STN and SPNH as photocatalysts, the color of MG solution decreased gradually (Fig. 8). The photocatalytic removal of MG differed from that of NB. While the intensity of the absorption peak of NB decreased, that of MG exhibited a hypsochromic shift. Because MG is a cationic triphenylmethane dye, the conjugated system made up of aryl rings is the main chromophore of MG. Thus, the complete photodegradation of MG involves five processes: cleavage of the central carbon reaction, N-demethylation, adduct reaction of •OH on MG, removal of benzene, and opening of phenyl rings [37, 38]. In the spectra, the absorbance of MG solution decreased over time. The main absorption peak almost disappeared after irradiation for 2 h (Fig. 8(a)), indicating the complete degradation of MG. SPNH removed 98.6% of MG after 120 min of irradiation, while STN removed 70.5%. When the photocatalyst was absent, with only 6.4% of MG was removed after 2 h of irradiation (blank in Fig. 8(c)).
PL measurements were carried out to examine the production of •OH on the surface of the samples using coumarin as a probe molecule, as shown in Fig. 9. •OH is an important reactive species in photocatalytic reactions and is responsible for oxidative decomposition of pollutants. The results confirmed that the SPNH and STN enhanced the formation of •OH in 1 h. SPNH induced a higher PL intensity than STN, which confirms that the rate of formation of •OH was higher for SPNH than STN. Overall, SPNH exhibited higher photocatalytic activity under visible-light irradiation than STN.
The morphology and surface area of a photocatalyst greatly influence its photocatalytic activity for the degradation of dyes under visible-light irradiation [39]. The presence of PTh in SPNH controlled the growth and nucleation of metal oxide, sensitized the rutile Sn-TiO2 surface to visible light, and improved the adsorption capacity of the photocatalyst. SPNH has a larger surface area than STN (Table 1), which resulted in an extremely high adsorption capacity. This is beneficial to the collection of organic pollutant molecules like NB and MG, because it leads to strong adsorption of organic contaminants, which contributes to high photodegradation efficiency. In SPNH, the polymer bound to Sn-TiO2 can actively harvest the visible light matching the semiconductor energy levels, and then inject electrons into the conduction band (CB) of TiO2 [40]. A schematic diagram of the charge transfer processes of PTh and Sn-TiO2 is illustrated in Fig. 10. When the conjugated PTh harvests visible light, an absorbed photon promotes an electron from the ground state of the polymer located in the semiconductor energy gap to an excited state that is in resonance with the CB of TiO2. The polymer p-orbital becomes the highest occupied molecular orbital (HOMO) in the combined system. Because the lowest unoccupied molecular orbital (LUMO) levels of the polymer are energetically higher than the CB edge of TiO2 [41, 42], the electron-transfer pathways in Fig. 10 are energetically feasible [43]. Meanwhile, the ejected electrons from the valence band of TiO2 get trapped by the Sn4+ ions doped in TiO2. The trapped electrons can move immediately to the HOMO of PTh, preventing electron-hole recombination. To achieve photosensitization, interfacial charge transfer between the photosensitizer and doped semiconductor needs to occur to make the catalyst capable of responding to visible light. However, there are few reports on the photocatalysis of TiO2 sensitized by such a polymer under visible-light irradiation.
The recyclability of a photocatalyst is a very important parameter to assess its practicability. Photocatalysts should maintain high photocatalytic activity during long-term use for their practical application. Moreover, the catalyst should be easily separated from solution to facilitate recycling [42]. SPNH can be easily recovered and separated from the reaction system simply through sedimentation or centrifugation.
Recycling experiments were carried out to evaluate the photostability of SPNH under visible-light irradiation. After 105/120 min of photocatalytic reaction with NB/MG, the photocatalyst was separated from the reaction solution and then washed with distilled water. The concentrations of NB or MG and photocatalyst were maintained as constant. As illustrated in Fig. 7(d) and 8(d), after five cycles, the catalytic activity of the catalyst towards the two pollutants decreased slightly but it was still high, revealing the good repeatability of the photocatalytic activity of the nanohybrid. The slight decrease in activity may be attributed to the coverage of the photocatalyst surface by NB or MG molecules. The photocatalyst surface is very difficult to clean thoroughly, so it influences the surface properties and photocatalytic efficiency of SPNH. Finally, the washed catalyst was examined by FT-IR spectroscopy (Fig. 11). All corresponding C=C, C−C, C-H and C-S stretching peaksof PTh in SPNH were observed in the spectrum, which indicates that there was no depletion or degradation of PTh by •OH generated in solution during visible-light irradiation. Moreover, PTh and its derivatives are not degraded under UV radiation in the presence of TiO2. The organic and inorganic components of SPNH before and after the recycling tests were examined by EDX analysis. All of the respective elements and the composition of SPNH remained unchanged after the recycling test (Fig. 12). This indicates that both the organic and the inorganic components of SPNH did not degrade during photocatalytic recycling.The good stability, reusability and high photocatalytic activity of SPNH make it an attractive candidate as a photocatalyst for industrial water purification.
In this study, a reusable Sn-doped TiO2/PTh nanohybrid was synthesized by a modified sol-gel method at low temperature without alteration of the crystalline structure of TiO2. The structural and physical studies of the resulting SPNH indicated that there was efficient interaction between PTh and Sn-doped TiO2 nanoparticles. In photocatalytic experiments, 99.4% and 72.5% of NB was removed in 105 min by SPNH and STN, respectively, under visible-light irradiation. Meanwhile, 98.6% and 70.5% of MG was removed after 120 min by SPNH and STN, respectively, under visible-light irradiation. The results show that PTh increased the photocatalytic activity of Sn-doped TiO2 nanoparticles under visible light. The catalyst showed no marked decrease in photocatalytic activity after five cycles, indicating good reusability. Based on its remarkable photocatalytic performance and reusability, SPNH shows promise in the field of photocatalysis and environmental remediation for industrial applications.
We are thankful to the DST-PURSE Programme for the financial assistance and Advanced Analytical Laboratory, Andhra University for their support in carrying out in this research work regarding SEM-EDX and XRD analysis.
2015, Vol. 36 Issue (10): 1668-1678
PDF (1047 KB)
2015, Vol. 36 Issue (10): 1668-1678
PDF (1047 KB)