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Solvothermal synthesis of Fe–C codoped TiO2 nanoparticles for visible-light photocatalytic removal of emerging organic contaminants in water
Applied Catalysis A: General 409–410 (2011) 257–266
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Solvothermal synthesis of Fe–C codoped TiO2 nanoparticles for visible-light photocatalytic removal of emerging organic contaminants in water Xiaoping Wang a , Yuxin Tang b , Ming-Yian Leiw c , Teik-Thye Lim a,∗ a Division of Environmental and Water Resources Engineering, School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, 50 Nanyang Avenue, Singapore 639798, Singapore b School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore 639798, Singapore c School of Electrical and Electronic Engineering, Nanyang Technological University, Block S1, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 21 July 2011 Received in revised form 6 September 2011 Accepted 8 October 2011 Available online 14 October 2011 Keywords: Fe–C codoping Carbonate species Photocatalytic degradation Toxicity
a b s t r a c t Anatase Fe/C–TiO2 nanoparticles were synthesized by a facile solvothermal method. The results showed that Fe was incorporated into TiO2 lattice by substituting Ti4+ , while C was present in the form of carbonate species on the surface. Fe3+ dopants could introduce a new dopant energy level into TiO2 band gap while the carbonate species served as photosensitizer. Both of them were responsible for the visible-light photocatalytic activity of the Fe/C–TiO2 . In addition, the presence of Fe dopants and carbonate species could favor the formation of surface hydroxyl groups, and inhibit recombination of photo-generated electrons and holes. Moreover, the Fe/C–TiO2 displayed larger surface area than C–TiO2 and Fe–TiO2 . The synergistic effects of Fe and C codoping into TiO2 resulted in improved photocatalytic activities of Fe/C–TiO2 for degradation of bisphenol A (BPA) and clofibric acid (CA) as compared to C–TiO2 , Fe–TiO2 and P25 under visible light and simulated solar light irradiation. The toxicities of BPA solutions gradually decreased throughout BPA mineralization. The photocatalytic activity of the Fe/C–TiO2 was maintained effectively even after several cyclic experiments. Finally, the possible photocatalytic mechanisms over the Fe/C–TiO2 under visible light and UV irradiation were proposed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis have received great attention as viable treatment technology for removing recalcitrant pollutants including emerging organic contaminants (EOCs) [1]. However, currently practical application of TiO2 -based photocatalysis is still limited, since TiO2 has a large band gap (3.2 and 3.0 eV for anatase and rutile, respectively) and therefore only small fraction (5%) of solar light can be utilized for its photoexcitation. Hence, considerate efforts have been devoted to TiO2 modification in order to extend the absorption band-edge of TiO2 into visible light region. Until now, doping with metallic species such as V, Cr, W and Fe, has been studied for improved photocatalytic performance under visible light irradiation [2,3]. Among them, Fe doped TiO2 has been intensively investigated. Fe ions could be incorporated into TiO2 by substitution of Ti4+ , introducing a new energy level within the band gap of TiO2 which is responsible for the visible light photoactivity [2,4,5]. Furthermore, Fe dopants could act as hole and electron traps to inhibit recombination of the photogenerated electron and hole, leading to enhanced photoactivity [5]. It has been generally accepted that the
∗ Corresponding author. Tel.: +65 6790 6933; fax: +65 6791 0676. E-mail address: [email protected] (T.-T. Lim). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.10.011
effect of Fe doping is highly dependent on dopant amount as the doping sites could also serve as the charge recombination centers when present in high concentration [6,7]. The additional benefit of Fe doping is that the trace amount of Fe that might be leached out from photocatalyst is non toxic. Alternatively, TiO2 doping with non-metals such as N, C, S, P, is another promising approach to induce visible-light activity [3,8]. As reported in some studies, C doped TiO2 photocatalysts exhibited good visible-light photocatalytic activities on degradation of various organic pollutants [9,10]. Moreover, in comparison with other non-metal dopants, C modified TiO2 can be obtained without introducing additional dopant source, since the organic substituent from the Ti precursor (e.g. titanium alcoholate) and solvent could serve as carbon source [11], which normally leads to the formation of carbonate species on the TiO2 surface [11]. Most recently, it has been reported that simultaneously doping of two elements into TiO2 could result in higher visible-light responses as compared to the TiO2 doped with single element [12–14]. Many studies have concerned about modification of TiO2 with Fe and N dopants [15]. To our best knowledge, there have been relatively few studies on Fe–C codoped TiO2 for visible-light photocatalysis. Wu et al. prepared the C and Fe modified TiO2 by sol–gel followed solvothermal method at low temperature of 180 ◦ C. They found that synergistic effects of C and Fe were responsible for
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Table 1 Chemical structures and main properties of target pollutants. Pollutant
Class
Solubility in water (mg L−1 )
log KOW
pKa
Bisphenol A (C15 H16 O2 )
Endocrine disrupting chemical
120–300 (25 ◦ C)
3.32–4.16
9.59 (pKa1 ), 10.2 (pKa2 )
Clofibric acid (C10 H11 ClO3 )
Active metabolite (lipid regulator)
2.57
3.2
582.5 (20 ◦ C)
improving visible light photocatalytic performance for decomposition of acid orange 7 [9]. However, their synthesis involving sol-gel route and solvothermal treatment required long period of time. On the other hand, to date, the roles of Fe3+ impurities and C dopants in the form of carbonate species in Fe and/or C doped TiO2 photocatalysis is still under debate. In this study, Fe/C–TiO2 nanoparticles were synthesized via a facile solvothermal method. The synergistic effects of Fe and C codoping on crystallite size, surface area, formation of surface hydroxyl group and photoluminescence behavior were investigated. Aqueous EOCs can be generally divided into two groups, which are hydrophobic or hydrophilic molecules. The adsorption of hydrophilic pollutants on the hydrophilic surface of TiO2 will be favored in aqueous solution in contrast to hydrophobic organic compounds. It is well known that photocatalytic removal rate could depend on the ability to concentrate the target compounds on the TiO2 surface. Therefore, the hydrophobic compound (bisphenol A (BPA)) and hydrophilic compound (clofibric acid (CA)) were selected as target pollutants in this study. Photocatalytic degradation and mineralization of these two EOCs were carried out. BPA as an endocrine disruptor has been widely used as a raw material in synthesis of expoxy resins and polycarbonate plastics. CA is the bioactive metabolite of several fibrate drugs, which are widely used as blood lipid regulators. Several studies have revealed CA was resistant to conventional biodegradation in the aquatic environment; its accumulation would therefore induce potential adverse health effects on both humans and animals [16]. The chemical structures and main properties of BPA and CA are given in Table 1. The evolution of toxicity throughout the mineralization of pollutants was evaluated. Finally, the photocatalytic mechanisms over the Fe/C–TiO2 under visible light and UV irradiation were proposed.
2. Experimental 2.1. Synthesis and characterization The Fe/C–TiO2 photocatalysts were synthesized by a simple solvothermal method. Typically, 0.02 mol of titanium isopropoxide (TTIP, ≥98%, Merck) was dissolved in 85 mL of isopropanol (≥99%, Merck). 5 mL of oxalic acid (0.1 M) and a certain amount of FeCl3 ·6H2 O (≥98%, Sigma–Aldrich) were added. The resulting mixture was stirred vigorously for 1 h at room temperature and then transferred into a 125 mL of Telfon-lined autoclave. The autoclave was heated and maintained at 120 ◦ C for 4 h. The product was separated by centrifugation, thoroughly washed with deionized (DI) water, and then dried at 80 ◦ C overnight. Finally the samples were calcined at 300 ◦ C for 8 h (or 450, 600 ◦ C for 4 h) in a muffle furnace. The as-prepared photocatalysts were denoted as xFe/C–Ti(T), where x and T referred to the molar ratio of FeCl3 ·6H2 O to TTIP and calcination temperature respectively. For comparison, C–TiO2 photocatalysts (C–Ti(300) and C–Ti(450)) were prepared through the similar procedure without introducing FeCl3 ·6H2 O.
Chemical structure
X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer with monochromated high˚ in a 2 range of 5–80◦ . intensity Cu K␣ radiation ( = 1.5418 A) Morphologies of the particles were examined by the transmission electron microscopy (TEM, JEOL JEM-2010). N2 adsorption/desorption isotherms of samples at liquid-nitrogen temperature (77 K) were obtained using a Quantachrome Autosorb-1 system. The Brunauer–Emmett–Teller surface area (SBET ) and Barrett–Joyner–Halenda (BJH) pore size distribution were derived. Thermogravimetric analysis (TGA) was performed using thermogravimetric analyzer (Perkin Elmer) with a heating rate of 5 ◦ C min−1 in an air flow. X-ray photoelectron spectroscopy (XPS) studies were conducted on a Krato Axis Ultra spectrometer with a monochromatic Al K␣ excitation source (h = 1486.71 eV). The binding energies (BE) were calibrated using C 1s core level at 284.8 eV for the adventitious carbon as reference. The UV–vis spectroscopy measurements were carried out using a UV–vis spectrometer (Lambda 35, Perkin-Elmer). Photoluminescence (PL) emission spectra of the samples were recorded using Fluorolog3 spectrofluorometer equipped with a Nd:YAG laser system of excitation wavelength at 260 nm (Horiba Scientific, New Jersey, USA). Fourier transform infrared spectra (FT-IR) were obtained for samples embedded in KBr pellets using Perkin Elmer 2000 spectrophotometer. 2.2. Photocatalytic activity measurement Stock solutions of BPA (≥99%, Sigma–Aldrich) and CA (97%, Aldrich) were prepared by dissolving the chemicals in DI water. Photocatalytic experiments were carried out in a vis-LED photoreactor as described in our previous work [17]. White LED was employed in this study. The spectrum of white LED exhibits a dominant peak at ca. 450 nm and a lower broadband from 500 to 650 nm. For comparison, photocatalytic activities of synthesized samples under UV and simulated solar light irradiation were also investigated. The light sources were a 365 nm mercury UV lamp (UVP Pen-Ray, 8 W, light intensity measured at a distance of 0.75 in. is 1.2 mW cm−2 ) and a solar simulator (Newport, USA) equipped with a 150 W xenon arc lamp, respectively. The suspension containing photocatalyst (dosage = 0.5 g L−1 ) and pollutant (5 mg L−1 ) was stirred in the dark for 60 min before irradiation to establish adsorption/desorption equilibrium of the target pollutant on the photocatalyst surface. All the tests were carried out at natural pH. Aliquots of the solution were collected at appropriate time intervals and filtered using membrane filters (0.45 m pore size). 2.3. Analytical methods Concentrations of the pollutants were measured using highperformance liquid chromatograph (HPLC, Perkin-Elmer Series 200), which comprised a Series 200 UV/Vis detector and C18 column (5 m, 150 mm × 4.6 mm, Inertsil ODS-3 column). The
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Fig. 1. XRD patterns of synthesized photocatalysts.
mobile phase consisted of acetonitrile and water with a gradient concentration at a flow rate of 1.0 mL min−1 . The detection wavelengths were selected as 225 nm, 228 nm for BPA, CA respectively. The total organic carbon (TOC) remained in the solution was determined using a Shimadzu ASI-V TOC analyzer. Toxicity evaluation of the solutions prior to and after photocatalytic treatment was performed using a Microtox toxicity test system (Azur Microtox Analyzer Model 500). This system is based upon the use of bioluminescent marine bacterium (Vibrio fischeri) which produces luminescence as a by-product of normal metabolism. Any inhibition of normal metabolism, such as that caused by the presence of toxic compounds, would result in a decreased rate of luminescence. The inhibition of V. fischeri exposed to the samples for 15 min was measured. Microtox test for each sample was performed in triplicate at least. The inhibitory effect of the test sample was expressed in terms of inhibition percentage [17]. 3. Results and discussion 3.1. Characterization of prepared photocatalysts XRD patterns of as-synthesized photocatalysts are presented in Fig. 1. The characteristic peak at ca. 2 = 25.3◦ corresponding to (1 0 1) plane of anatase phase is observed for the samples calcined at 300 ◦ C. Moreover, for the Fe-containing samples, this peak is broader and weaker, and slightly shifts to a higher 2 degree as
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compared to that of reference C–Ti(300). A weak peak at 30.5◦ can be found in the patterns of 3%Fe/C–Ti(300) and 5%Fe/C–Ti(300), which suggests the presence of ferric pseudobrookite Fe2 TiO5 (JCPDS No. 01-087-1996). Based on the above observations, it is inferred that low content of Fe3+ ions might be incorporated into the TiO2 lattice by substitution of Ti4+ ions due to their close ionic radii (0.79 and 0.75 A˚ for Fe3+ , Ti4+ , respectively) [6], and the incorporation of Fe could result in the lattice distortion of TiO2 and decrease of crystalline size. Trace amount of isolated Fe-containing compounds could be also formed in the sample containing a high Fe concentration. Therefore there could be a solubility limit for Fe incorporated into the anatase structure [18]. On the other hand, with increase of calcination temperature, 2%Fe/C–Ti(T) shared the diffraction peak of anatase at 25.3◦ , which appears stronger and sharper but occurs at the same position. It is indicated that calcination of 2%Fe/C–Ti(300) with elevated temperatures led to increased size and improved crystallinity of anatase TiO2 , however might not induce the lattice distortion. The transformation of anatase to rutile occurred at the calcination temperature up to 600 ◦ C. The anatase crystallite sizes of all the samples determined using Scherrer equation are listed in Table 2. TEM images of selected samples are shown in Fig. 2. As observed in Fig. 2A, the primary grains were approximately uniform but with irregular shape, and the average particle size was about 8 nm which is similar to that estimated from XRD pattern. Lattice spacing between the (1 0 1) planes is found to be 0.352 nm (Fig. 2B) and diffraction rings are shown in the SAED pattern (inset of Fig. 2B). These results confirm that the photocatalyst were of polycrystalline anatase structure. In addition, generally xFe/C–Ti(300) showed smaller particle sizes than that of C–Ti(300), while increase of the Fe content caused insignificant influences on the morphologies and particle size distributions of the synthesized photocatalysts. This may be due to the relatively limited contents of Fe impurities in the solvothermally synthesized photocatalysts. According to IUPAC classification, all the prepared samples exhibited a type IV isotherm with a H2 type hysteresis loop (Fig. 3), indicating the presence of mesopores. It is proposed that the porosity of TiO2 photocatalysts prepared via such solvothermal route could arise from the network of dominant interparticle pores surrounding the particles. The derived SBET , pore diameter (dp ) and pore volume (Vp ) of photocatalysts are summarized in Table 2. Obviously, the xFe/C–Ti(300) displayed larger SBET (145.8–159.9 m2 g−1 ), but slightly smaller Vp (4.5–5.1 cm3 g−1 ) than those of C–Ti(300) (128.8 m2 g−1 and 6.2 cm3 g−1 , respectively). The reason may be that the addition of Fe could help to form well-dispersed and defined TiO2 particles as evidenced by TEM and XRD analyses. Additionally, although SBET of samples could be notably decreased with the increase of calcination temperature,
Table 2 Physicochemical properties of derived photocatalysts. Photocatalyst
5%Fe/C–Ti(300) 3%Fe/C–Ti(300) 2%Fe/C–Ti(300) 1%Fe/C–Ti(300) C–Ti(300) C–Ti(450) 2%Fe–Ti(450) 2%Fe–Ti(600) a b c d
Element content Ca (at%)
Cb (wt%)
Fea (at%)
Fec (wt%)
17.04 18.19 16.01 18.30 16.78 5.36 3.02 0.98
0.72 0.76 0.70 0.74 0.68 0.03 0.01 0
1.02 0.51 0.42 0.16 0 0 0.58 0.69
0.48 0.32 0.20 0.05 8.2 9.5 0.23 0.21
Anatase crystallite sized (nm)
SBET (m2 g−1 )
dp (nm)
Vp (cm3 g−1 )
6.6 5.9 6.3 7.1 128.8 95.6 8.9 22.9
151.4 145.8 149.5 159.9 6.2 7.3 101.4 67.7
4.5 4.6 4.8 5.1 0.19 0.17 6.2 14.8
0.17 0.17 0.18 0.20
Surface concentration induced by XPS results. Bulk content obtained by elemental analysis. Bulk concentration determined using microwave-assisted digestion followed by analysis with ICP-OES. Derived from XRD patterns.
0.16 0.04
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Fig. 2. TEM images of 2%Fe/C–Ti(300) (A), C–Ti(300) (C) and 5%Fe/C–Ti(300) (D), HRTEM image and SEAD pattern of 2%Fe/C–Ti(300) (B).
2%Fe–Ti(450) and C–Ti(450) still exhibited relatively high SBET of 101.4 and 95.6 m2 g−1 , respectively. Fig. 4 presents the TG/DTG profiles of 2%Fe/C–Ti(T) precursor dried at 80 ◦ C. The thermal decomposition during heating in the air involved three main steps. The first step with weight loss of 5.3% occurred at 30–125 ◦ C and two endothermic peaks at ca. 57 and 121 ◦ C are observed in the DTG curve, mainly can be ascribed to the removal of surface-adsorbed water and CO2 molecules. The second stage happened at 125–300 ◦ C resulting in 5.0% of weight loss, which could be due to the thermal decomposition of residual
organic species such as oxalic acid (as supported by the DTG peak around 218 ◦ C). The subsequent step of minor weight loss of 1.4% took place at 300–600 ◦ C, possibly because of the loss of carbon species on the surface of photocatalyst, which will be confirmed by the results of XPS, IR and elemental analysis latter. The weight of sample became stable at 600 ◦ C and above. The surface elemental compositions and chemical states of the prepared photocatalysts were investigated by the XPS measurements, as shown in Fig. 5. Fe 2p XPS spectra of all the samples with the introduction of Fe exhibit the doublet peaks located at ca.
Fig. 3. Nitrogen adsorption/desorption isotherms (A) and pore size distributions (B).
X. Wang et al. / Applied Catalysis A: General 409–410 (2011) 257–266
Fig. 4. TG/DTG profiles of 2%Fe/C–Ti(T) precursor dried at 80 ◦ C.
724.1–723.5 and 711.1–710.0 eV corresponding to Fe 2p1/2 and Fe 2p3/2 , respectively, indicating the presence of Fe3+ oxidation state [19]. The Fe 2p3/2 from these Fe-containing TiO2 showed slightly higher binding energies as compared to typical Fe 2p3/2 from Fe2 O3 (709.8 eV) [19], which may be associated with the fact that Fe3+ ions interacted strongly with TiO2 in the form of Ti–O–Fe bonds [20]. Moreover, generally the doublet peaks become more intense with increase of Fe loading. Fig. 5C presents the high-resolution XPS spectra of C 1s. A peak at 284.8 eV shared by all the samples is known as the adventitious carbon. For samples calcined at 300 ◦ C, the C 1s spectra display two additional peaks at around 286.0 eV and
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288.0 eV, which are assigned to C–O and C O bonds, respectively. Several studies have attributed the C dopants to carbonate species which was present on the TiO2 surface in the form of Ti–O–C or Ti–O C linkages [14,21]. The existence of two peaks indicates that there may not be only one kind of carbonate species. These conclusions are also supported by the FTIR measurements. As presented in Fig. 8, the above-mentioned samples showed the peaks with lowintensity at around 1065, 1336 and 1436 cm−1 , which are assigned as carbonate species (i.e. monodentate carbonate and bicarbonate, respectively) [10,22–24]. The peak at ca. 288.0 eV or 286.0 eV is extremely weak in the 2%Fe–Ti(450) and C–Ti(450) and absent in the 2%Fe–Ti(600). Two peaks observed in the Ti 2p XPS spectra of as-synthesized photocatalysts are around 464.5–463.4 eV for Ti 2p1/2 and 458.7–457.5 eV for Ti 2p3/2 , respectively [25]. As seen from the Ti 2p spectra of C–Ti(300), the peaks at 464.2 eV for Ti 2p1/2 and 458.7 eV for Ti 2p3/2 can be ascribed to the pristine TiO2 [25]. This result implies that the formation of carbonate species may not change the chemical states of Ti. Therefore, the presence of Ti 2p peaks with lower binding energies (e.g. 464.2 and 463.4 eV for Ti 2p1/2 , 458.4 and 457.5 eV for Ti 2p3/2 ) in the Fe-containing TiO2 , may be mainly attributed to Ti–O–Fe linkages induced by the incorporation of Fe3+ into TiO2 lattice at least at the surface [20]. The atomic concentrations of C and Fe on the surface of photocatalysts are summarized in Table 2. It is noted that C concentration, which excluded the adventitious carbon, remained quite stable at around 17% on the surface of these samples calcined at 300 ◦ C, while it drastically decreased with the increasing calcination temperatures and can be negligible for the 2%Fe–TiO2 (450) and 2%Fe–TiO2 (600). It reveals that carbon dopants in the form of carbonate species for the as-prepared photocatalysts might be resulted from the organic precursors (isopropanol or TTIP) and can be removed by calcination
Fig. 5. Survey scan XPS spectra (A), high-resolution XPS spectra of Fe 2p (B), C 1s (C) and Ti 2p (D) regions for derived photocatalysts.
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Fig. 7. PL spectra of derived photocatalysts.
Fig. 6. UV–vis diffuse reflectance spectra of the synthesized photocatalysts and P25.
at high temperatures. On the other hand, Fe atomic concentrations were gradually increased with the elevated addition of FeCl3 ·6H2 O. Therefore, it can be concluded that Fe and C were successfully introduced in TiO2 for samples xFe/C–Ti(300), while single dopant was presented in samples C–Ti(300), 2%Fe–Ti(450) and 2%Fe–Ti(600). The bulk C and Fe contents are also presented in Table 2, the former was determined by elemental analysis (CHNS/O analyzer, PerkinElmer 2400 Series II), and the latter was examined using inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer Optima 2000DV). Apparently, surface concentrations of Fe and C as induced by XPS were much higher than their bulk contents, especially in the case of C. The 2%Fe/C–Ti(300) contained 0.70 wt% of carbon according to elemental analysis, which is in good agreement with that predicted by TG analysis. Hence it is believed that both Fe dopant and carbonate species had higher concentrations at the exterior than at the interior of TiO2 nanoparticles, in particular carbonate species was predominantly present on the surface. This result can be understood based upon the mechanism of TiO2 doping with metal or non-mental species by solvothermal method [26]. In this process, Fe3+ ions firstly dispersed and were adsorbed on the surface of TiO2 gel due to strong electrostatic interaction. Since Fe3+ (0.064 nm) has a rather similar radius to that of Ti4+ ions (0.068 nm) and a strong affinity for O2− (or OH− ), Fe3+ could diffuse into the bulk of TiO2 by substituting Ti4+ or incorporating into Ti4+ vacancy during the process of TiO2 crystallization with solvothermal treatment. In the end, the wellformed polycrystalline anatase structure of TiO2 could suppress the Fe diffusion from surface to the interior of TiO2 . The external surface enrichment of Fe dopant was therefore formed in the resulting samples. On the other hand, few studies have reported C dopants were injected into the deep bulk of TiO2 via the wet chemical method, such as sol–gel and hydrothermal (solvothermal) method. UV–vis diffuse reflectance spectra (DRS) of the as-prepared photocatalysts are depicted in Fig. 6. C–Ti(300) exhibited the enhanced absorption of visible light as compared to P25, which could be attributed to the presence of carbonate species on the surface. The introduction of Fe for xFe/C–Ti(300) further caused a significant red shift of absorption edge to the visible light region in comparison to C–Ti(300), and the absorption of visible light (400–650 nm) increased with increasing Fe content in TiO2 . It has been widely accepted that Fe3+ doping in the TiO2 lattice can extend photoresponse of TiO2 into visible light range. However, the origin of the red shift of absorption edge addressed in the literature is controversial. Some studies have concluded that Fe doping would
introduce additional electronic states within the band gap, leading to effective red-shift of the absorption threshold [4,27,28]. Other researchers have reported that this red-shift is caused by narrowing of the band gap of pristine TiO2 [29]. In comparison with 2%Fe/C–Ti(300) and C–Ti(300), although 2%Fe–Ti(450) and C–Ti(450) showed decreased visible-light absorption due to the loss of carbonate species on the surface as discussed above, their DRS spectra at wavelength ≤380 nm are similar. It is thus suggested that C modification by formation of surface carbonate species may not cause significant change of TiO2 band gap. The similar phenomenon was observed in commercially available C modified TiO2 [11]. However, for 2%Fe–Ti(600), the higher calcination temperature of 600 ◦ C resulted in slightly stronger visible-light response extending to 700 nm. This may be because of possible interface states induced by the mixed anatase–rutile phase structure [30]. Fig. 7 shows the PL spectra of derived photocatalysts. One peak around 560 nm can be found in the PL emission spectra of all the samples, which may be induced by surface oxygen vacancies and intrinsic defects in the doped TiO2 [12,31], because of the presence of carbonate species and/or Fe dopants in these photocatalysts. The PL intensities rank in the following order: C–Ti(300) > 2%Fe–Ti(450) > 1%Fe/C–Ti(300) > 5%Fe/C–Ti(300) > 3% Fe/C–Ti(300) > 2%Fe/C–Ti(300). Since PL emission mainly arises from the recombination of photo-induced electrons and holes, the lower PL intensity may reflect the lower recombination rate of electrons and holes. Generally C–TiO2 and Fe–TiO2 exhibit the relatively higher intensities as compared to Fe/C–TiO2 , which suggests that TiO2 codoping with Fe and C may improve the separation of photo-induced electrons and holes. It has been reported that Fe3+ could act as electron sinks to facilitate the transfer of photo-induced electrons and suppress the recombination of electrons and holes [9,12]. It is therefore observed that PL intensities gradually decreased with introduction of small amount of Fe. However, 5%Fe/C–Ti(300) and 3%Fe/C–Ti(300) showed higher PL intensities than 2%Fe/C–Ti(300). The possible reason is that Fe impurities could serve as recombination centers when present in high concentrations, which may be associated with the trace amount of isolated ferric pseudobrookite Fe2 TiO5 formed in the 5%Fe/C–Ti(300) and 3%Fe/C–Ti(300), as evidenced by the XRD results. The effective inhibition of electron–hole recombination for the 2%Fe/C–Ti(300) could improve its visible-light photocatalytic performance. Besides the characteristic peaks for carbonate species, one peak at 1640 cm−1 corresponding to surface adsorbed hydroxyl groups [32], is also present in the FTIR spectra of samples calcined at 300 ◦ C (Fig. 8). It become weaker with the decreasing Fe content and disappear at the higher calcination temperatures. This result indicates
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Fig. 8. FTIR spectra of synthesized photocatalysts.
that the presence of carbonate species and Fe impurities on TiO2 surface could facilitate the formation of hydroxyl groups, since TiO2 doping with Fe ions and carbonate species could introduce oxygen vacancies in the crystal lattice or surface of TiO2 [33,34]. It is believed that the surface hydroxyl group is a crucial factor influencing the photocatalytic activity of TiO2 photocatalysts.
Fig. 9 shows the photocatalytic degradation (PCD) of BPA using as-prepared photocatalysts under white LED irradiation. Photolysis of BPA throughout 4 h of white LED irradiation was negligible. In general, the solvothermally synthesized Fe and/or C doped TiO2 showed enhanced visible-light photocatalytic performance towards BPA degradation compared to that of P25. It is suggested that the lattice defects induced by the Fe dopants and/or carbonate species were responsible for the visible-light photocatalytic activities of these photocatalysts. Apart from the lattice defects, it is postulated that the visible-light photocatalytic activity of doped TiO2 is dependent upon many other factors, such as crystalline structure, particle size, textural properties (i.e. surface area and pore structure), optical properties, surface hydroxyl density, etc. Firstly, xFe/C–Ti(300) with larger SBET than that of C–Ti(300) could provide more active sites, which play an important role in the heterogeneous photocatalytic process. Secondly, incorporation of Fe into TiO2 lattice not only enhanced the absorption ability of visible
light, but also suppressed recombination of photo-induced electrons and holes. Additionally, TiO2 doping with Fe ions facilitated formation of surface hydroxyls groups which is another crucial factor governing the photocatalytic activity. Therefore, it is reasonable that xFe/C–Ti(300) showed higher photocatalytic activities than C–Ti(300). Furthermore, 2%Fe/C–Ti(300), which demonstrated the most effective separation of photo-generated electrons and holes, exhibited the highest BPA removal rate among xFe/C–Ti(300). BPA was almost completely degraded by the 2%Fe/C–Ti(300) within 4 h of irradiation. PCD efficiencies of the Fe–TiO2 (2%Fe–Ti(450) and 2%Fe–Ti(600)) and C–Ti(450) were significantly lower than 2%Fe/C–Ti(300) and C–Ti(300) respectively, possibly due to their weakened visible-light absorption, as well as decreased SBET and surface hydroxyl groups. On the basis of the above results, it is proposed that the synergistic effects of Fe and C codoping into TiO2 could result in improved visible-light photocatalytic activity of Fe/C–TiO2 as compared to those of C–TiO2 and Fe–TiO2 . As seen from Fig. 10, CA can also be efficiently degraded by the Fe/C–TiO2 under white LED irradiation. The CA was degraded to 96% within the initial 2 h, and completely removed after 4 h of reaction. Photolysis caused insignificant removal of CA within 4 h. Similarly, Fe/C–TiO2 showed higher visible-light photocatalytic ability for CA degradation than C–TiO2 , Fe–TiO2 and P25. It is also found that CA removal rate was slightly faster than that of BPA, which may be related to its molecular structure and physicochemical properties. Removal efficiencies of BPA and CA with different light sources were compared, as shown in Fig. 11. It is evident that Fe/C–TiO2
Fig. 9. Photocatalytic activities of synthesized photocatalysts for BPA degradation under white LED irradiation.
Fig. 10. Photocatalytic degradation of CA under white LED irradiation.
3.2. Photocatalytic degradation of emerging organic contaminants
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Fig. 11. BPA and CA removal efficiencies under the irradiation of different light sources.
Fig. 12. Photocatalytic mineralization of BPA and CA using 2%Fe/C–Ti(300) under white LED irradiation, and evolution of toxicities upon BPA degradation. Fig. 13. Cycling runs in the photocatalytic degradation of BPA using 2%Fe/C–Ti(300) under white LED irradiation.
displayed enhanced photocatalytic activities as compared to C–TiO2 and Fe–TiO2 regardless of light sources. Under irradiation of white LED, 97% of BPA and 95% of CA were removed by the Fe/C–TiO2 within 2 h and 3 h, respectively as compared to 9% and 13% by P25. Since UV emission at 365 nm was not detected using Accumax XRP-3000 radiometer and photolysis induced negligible removal of BPA and CA, the activity of P25 under white LED irradiation may be attributed to formation of surface complexation between the phenolic compounds and P25 [35]. Also it can be found that Fe/C–TiO2 showed better or at least comparable photocatalytic activity for BPA and CA degradation in comparison with P25 under simulated solar light irradiation. However, the Fe/C–TiO2 similar to other synthesized TiO2 photocatalysts which have been reported in literature, was less active than P25 under UV irradiation. BPA and CA were completely removed within 4 h under white LED irradiation, while only 61% and 71% of TOC were mineralized, respectively, as shown in Fig. 12. The remaining TOC may be attributed to the formation of more persistent intermediates or aliphatic carboxylate ions that might be decomposed with longer irradiation [17]. In view of the formation of intermediates during PCD of these organic pollutants, which might be more toxic than their parent molecules [36], it is of great significance to examine the toxicity of the solutions at different irradiation times for better evaluation of the photocatalytic process. 15-min EC50 (concentration of
toxicant that produces a 50% reduction of luminescence) values of BPA and CA obtained in the present study were 2.23 ± 0.45 mg L−1 and 232 ± 28 mg L−1 , respectively. The inhibition of luminescence in V. fischeri bioassay of the untreated CA solution of 5 mg L−1 was not detectable. Moreover, no increase of inhibition was observed during the PCD of CA. For the BPA solutions, to maintain the luminescence reduction in the acceptable range of detection, all the samples were diluted from 100% to 58.4% with Microtox diluents, which was a specially prepared nontoxic 2% sodium chloride solution. The luminescence inhibition reflects the toxicity level of the samples. The greater inhibition indicates the higher level of toxicity, and vice versa. As displayed in Fig. 12, the toxicities of BPA solutions gradually decreased as the irradiation proceeded. Fig. 13 shows the durability of the Fe/C–TiO2 for BPA degradation under while LED irradiation. In a typical procedure, the photocatalyst was reused by simple filtration and washing with DI water without further treatment. The reduction of visible-light photocatalytic activity was insignificant even after five cycles. Fe ion was not detected in the BPA solutions throughout the photocatalytic process, indicating that no Fe was leached out from the Fe/C–TiO2 under the experimental conditions. It can be found that color of Fe/C–TiO2 remained yellow during PCD of BPA. Furthermore, the optical property and visible-light photocatalytic activity
X. Wang et al. / Applied Catalysis A: General 409–410 (2011) 257–266
265
Fig. 14. Proposed photocatalytic mechanisms over the Fe/C–TiO2 under visible light and UV irradiation.
of Fe/C–TiO2 almost did not change after the acidic or alkaline des˛ et al. [37]). It is suggeated orption treatments (described by Zabek that the carbon dopants in the Fe/C–TiO2 were strongly bound to TiO2 surface under such solvothermal condition. Therefore, the synthesized Fe/C–TiO2 was chemically stable to be applied in aqueous photocatalysis. 3.3. Proposed mechanism As concluded above, the Fe/C–TiO2 was modified with Fe and C by the formation of Ti–O–Fe linkages and carbonate species respectively, which were responsible for its visible-light photocatalytic activity. For the role of carbonate species in the visible-light photocatalytic ability of C–TiO2 , some studies have demonstrated that carbonate species assigned to interstitial carbon dopant could create the intragap localized states of C 2p above the valence band (VB) of TiO2 [9], while others have proposed that carbonate species could act as photosensitizer [10,14]. According to XRD, UV–vis and XPS measurements, the carbonate species in the synthesized Fe/C–TiO2 did not result in any TiO2 lattice distortion, change in band gap and chemical state of Ti. The carbonate species was predominantly present on the surface as revealed by large variations in the surface and bulk C contents (Table 2). The C content can be removed by calcination at high temperatures. Therefore, it is suggested that the carbonate species on the surface of Fe/C–TiO2 served as photosensitizer for the PCD of pollutants under visible light irradiation, rather than introduced additional intragap states. On the other hand, we hypothesized that Fe3+ doping into TiO2 lattice by substituting Ti4+ would introduce a new dopant energy level into TiO2 band gap. The photocatalytic mechanisms induced by the Fe/C–TiO2 under visible light and UV irradiation were proposed, as illustrated in Fig. 14. Under visible light irradiation, the excited photosensitizer (carbonate species on the TiO2 surface) injected an electron into the conduction band (CB) of TiO2 . The electron was subsequently transferred to oxygen adsorbed on the TiO2 surface producing superoxide radical (O2 •− ), which is one of highly reactive oxygen species that can degrade many organic compounds [10]. Two kinds of Fe impurity states located within the electronic structure of Fe-doped TiO2 have been reported: one is above the valence band (VB) due to the fact that t2g level of 3d orbital of Fe3+ ion lies above the VB of TiO2 [5,6], the other is below CB because the energy level of Fe 3d electrons is lower than that of Ti 3d at the bottom of CB [9,28,29]. Therefore two possible schemes for the mechanism of Fe and C codoping are proposed. Under visible light irradiation, an electron was photoexcited from Fe3+ dopant level to the CB or from the VB to this dopant level. As shown in Fig. 14A, Fe3+ was hence converted to Fe4+ according to Eq. (1) [6]. The photogenerated CB electron further reacted with adsorbed O2 to form O2 •− , while Fe4+ could react with surface hydroxyl group to produce hydroxyl radical (OH• ) (Eq. (2)). Meanwhile, since the energy level of Fe3+ /Fe2+ is below the CB edge of TiO2 [5,6], Fe3+ could act as a electron sink to trap photogenerated electron and produce O2 •− (Eqs. (3)–(6)) [6,38]. As illustrated by Fig. 14B, the photoexcited Fe/C–TiO2 generated a hole in the VB of TiO2 and an electron in the
Fe3+ dopant level. The hole could migrate to the surface leading to the formation of OH• , while Fe3+ accepted an electron to form Fe2+ (Eq. (3)). In this case, Fe3+ would act as a hole trap due to that the energy level of Fe4+ /Fe3+ is above the valence band edge of TiO2 (Eq. (7)) [5,6]. Additionally, it is possible that visible light response could be due to charge transfer transition between Fe ions (Eq. (8)) [33]. The photocatalytic reaction could involve the mechanistic steps of Eqs. (2) and (4)–(6). However, Fe3+ dopant with high concentration could serve as recombination centers for the photogenerated electron–hole (Eqs. (9) and (10)) [6,7], leading to the decrease of activity of the photocatalyst. hv
Fe3+ −→Fe4+ + e− 4+
Fe
3+
Fe
−
+ OH → Fe −
+ e → Fe
3+
(1) + OH•
(2)
2+
(3)
Fe2+ + O2 → Fe3+ + O2 •− 2+
Fe
3+
Ti
3+
Fe
+ Ti
4+
→ Fe
+ O2 → Ti
4+
+
+ h → Fe
3+
+ Ti
+ O2
(4)
3+
(5)
•−
(6)
3+
(7)
Fe3+ + Fe3+ → Fe2+ + Fe4+ 4+
Fe
2+
Fe
−
+ e → Fe +h
+
(8)
3+
(OH• )
→ Fe
(9) 3+
−
(+OH )
(10)
Under UV irradiation, the effects of dopants on photocatalytic process may be different, as shown in Fig. 14C. Fe3+ appeared to be both electron and hole traps, or it became recombination centers when present at a high content. 4. Conclusions Fe/C–TiO2 photocatalysts were synthesized by a facile solvothermal method. The derived photocatalysts were predominantly of anatase phase with mesoporous structure, exhibited large specific surface areas. Ti–O–Fe linkages and carbonate species were evidenced in the Fe/C–TiO2 photocatalysts. The presence of Fe dopants and carbonate species could favor the formation of surface hydroxyl groups, and inhibit recombination of photo-generated electrons and holes. However, when present at high concentrations, Fe dopant could act as charge recombination center. Generally Fe/C–TiO2 exhibited the higher efficiencies for degradation of BPA and CA as compared to C–TiO2 , Fe–TiO2 and P25 under visible light and simulated solar light irradiation. After 4 h of irradiation with visible light, BPA and CA can be completely removed by the Fe/C–TiO2 , concomitantly 61% and 71% of TOC were mineralized, respectively. The BPA solutions became less toxic after the photocatalytic treatment. Furthermore, visible-light photocatalytic activity of the Fe/C–TiO2 was maintained effectively after several cyclic experiments. This study suggests that the solvothermally synthesized Fe/C–TiO2 with high chemical stability
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could be a potential photocatalyst for degradation of aqueous recalcitrant organic contaminants under solar light. Acknowledgement The National Research Foundation (NRF) of Singapore is hereby acknowledged for the financial support through the project EWI RFP 0802-11, which was aimed to develop novel photocatalysts for water purification using solar light. References [1] O.K. Dalrymple, D.H. Yeh, M.A. Trotz, J. Chem. Technol. Biotechnol. 82 (2007) 121–134. [2] H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, K. Ikeue, M. Anpo, J. Photochem. Photobiol. A 148 (2002) 257–261. [3] A. Zaleska, Recent Patents on Engineering 2 (2008) 157–164. [4] Y. Yalc¸in, M. Kilic¸, Z. C¸inar, Appl. Catal. B 99 (2010) 469–477. [5] W. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669–13679. [6] M. Asiltürk, F. Sayilkan, E. Arpac¸, J. Photochem. Photobiol. A 203 (2009) 64–71. [7] Z. Li, W. Shen, W. He, X. Zu, J. Hazard. Mater. 155 (2008) 590–594. [8] H. Wang, J.P. Lewis, J. Phys. Condens. Matter 18 (2006) 421–434. [9] Y. Wu, J. Zhang, L. Xiao, F. Chen, Appl. Surf. Sci. 256 (2010) 4260–4268. [10] C. Lettmann, K. Hildenbrand, H. Kisch, W. Macyk, W.F. Maier, Appl. Catal. B 32 (2001) 215–227. ˛ J. Eberl, H. Kisch, Photochem. Photobiol. Sci. 8 (2009) 264–269. [11] P. Zabek, [12] D. Zhang, Transit. Met. Chem. 35 (2010) 933–938. [13] A. Kubacka, B. Bachiller-Baeza, G. Colón, M. Fernández-García, Appl. Catal. B 93 (2010) 274–281. [14] X. Yang, C. Cao, K. Hohn, L. Erickson, R. Maghirang, D. Hamal, K. Klabunde, J. Catal. 252 (2007) 296–302. [15] J. Zhang, Y. Wu, M. Xing, S.A.K. Leghari, S. Sajjad, Energy Environ. Sci. 3 (2010) 715–726.
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Solvothermal synthesis of Fe–C codoped TiO2 nanoparticles for visible-light photocatalytic removal of emerging organic contaminants in water Xiaoping Wang a , Yuxin Tang b , Ming-Yian Leiw c , Teik-Thye Lim a,∗ a Division of Environmental and Water Resources Engineering, School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, 50 Nanyang Avenue, Singapore 639798, Singapore b School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore 639798, Singapore c School of Electrical and Electronic Engineering, Nanyang Technological University, Block S1, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 21 July 2011 Received in revised form 6 September 2011 Accepted 8 October 2011 Available online 14 October 2011 Keywords: Fe–C codoping Carbonate species Photocatalytic degradation Toxicity
a b s t r a c t Anatase Fe/C–TiO2 nanoparticles were synthesized by a facile solvothermal method. The results showed that Fe was incorporated into TiO2 lattice by substituting Ti4+ , while C was present in the form of carbonate species on the surface. Fe3+ dopants could introduce a new dopant energy level into TiO2 band gap while the carbonate species served as photosensitizer. Both of them were responsible for the visible-light photocatalytic activity of the Fe/C–TiO2 . In addition, the presence of Fe dopants and carbonate species could favor the formation of surface hydroxyl groups, and inhibit recombination of photo-generated electrons and holes. Moreover, the Fe/C–TiO2 displayed larger surface area than C–TiO2 and Fe–TiO2 . The synergistic effects of Fe and C codoping into TiO2 resulted in improved photocatalytic activities of Fe/C–TiO2 for degradation of bisphenol A (BPA) and clofibric acid (CA) as compared to C–TiO2 , Fe–TiO2 and P25 under visible light and simulated solar light irradiation. The toxicities of BPA solutions gradually decreased throughout BPA mineralization. The photocatalytic activity of the Fe/C–TiO2 was maintained effectively even after several cyclic experiments. Finally, the possible photocatalytic mechanisms over the Fe/C–TiO2 under visible light and UV irradiation were proposed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis have received great attention as viable treatment technology for removing recalcitrant pollutants including emerging organic contaminants (EOCs) [1]. However, currently practical application of TiO2 -based photocatalysis is still limited, since TiO2 has a large band gap (3.2 and 3.0 eV for anatase and rutile, respectively) and therefore only small fraction (5%) of solar light can be utilized for its photoexcitation. Hence, considerate efforts have been devoted to TiO2 modification in order to extend the absorption band-edge of TiO2 into visible light region. Until now, doping with metallic species such as V, Cr, W and Fe, has been studied for improved photocatalytic performance under visible light irradiation [2,3]. Among them, Fe doped TiO2 has been intensively investigated. Fe ions could be incorporated into TiO2 by substitution of Ti4+ , introducing a new energy level within the band gap of TiO2 which is responsible for the visible light photoactivity [2,4,5]. Furthermore, Fe dopants could act as hole and electron traps to inhibit recombination of the photogenerated electron and hole, leading to enhanced photoactivity [5]. It has been generally accepted that the
∗ Corresponding author. Tel.: +65 6790 6933; fax: +65 6791 0676. E-mail address: [email protected] (T.-T. Lim). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.10.011
effect of Fe doping is highly dependent on dopant amount as the doping sites could also serve as the charge recombination centers when present in high concentration [6,7]. The additional benefit of Fe doping is that the trace amount of Fe that might be leached out from photocatalyst is non toxic. Alternatively, TiO2 doping with non-metals such as N, C, S, P, is another promising approach to induce visible-light activity [3,8]. As reported in some studies, C doped TiO2 photocatalysts exhibited good visible-light photocatalytic activities on degradation of various organic pollutants [9,10]. Moreover, in comparison with other non-metal dopants, C modified TiO2 can be obtained without introducing additional dopant source, since the organic substituent from the Ti precursor (e.g. titanium alcoholate) and solvent could serve as carbon source [11], which normally leads to the formation of carbonate species on the TiO2 surface [11]. Most recently, it has been reported that simultaneously doping of two elements into TiO2 could result in higher visible-light responses as compared to the TiO2 doped with single element [12–14]. Many studies have concerned about modification of TiO2 with Fe and N dopants [15]. To our best knowledge, there have been relatively few studies on Fe–C codoped TiO2 for visible-light photocatalysis. Wu et al. prepared the C and Fe modified TiO2 by sol–gel followed solvothermal method at low temperature of 180 ◦ C. They found that synergistic effects of C and Fe were responsible for
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Table 1 Chemical structures and main properties of target pollutants. Pollutant
Class
Solubility in water (mg L−1 )
log KOW
pKa
Bisphenol A (C15 H16 O2 )
Endocrine disrupting chemical
120–300 (25 ◦ C)
3.32–4.16
9.59 (pKa1 ), 10.2 (pKa2 )
Clofibric acid (C10 H11 ClO3 )
Active metabolite (lipid regulator)
2.57
3.2
582.5 (20 ◦ C)
improving visible light photocatalytic performance for decomposition of acid orange 7 [9]. However, their synthesis involving sol-gel route and solvothermal treatment required long period of time. On the other hand, to date, the roles of Fe3+ impurities and C dopants in the form of carbonate species in Fe and/or C doped TiO2 photocatalysis is still under debate. In this study, Fe/C–TiO2 nanoparticles were synthesized via a facile solvothermal method. The synergistic effects of Fe and C codoping on crystallite size, surface area, formation of surface hydroxyl group and photoluminescence behavior were investigated. Aqueous EOCs can be generally divided into two groups, which are hydrophobic or hydrophilic molecules. The adsorption of hydrophilic pollutants on the hydrophilic surface of TiO2 will be favored in aqueous solution in contrast to hydrophobic organic compounds. It is well known that photocatalytic removal rate could depend on the ability to concentrate the target compounds on the TiO2 surface. Therefore, the hydrophobic compound (bisphenol A (BPA)) and hydrophilic compound (clofibric acid (CA)) were selected as target pollutants in this study. Photocatalytic degradation and mineralization of these two EOCs were carried out. BPA as an endocrine disruptor has been widely used as a raw material in synthesis of expoxy resins and polycarbonate plastics. CA is the bioactive metabolite of several fibrate drugs, which are widely used as blood lipid regulators. Several studies have revealed CA was resistant to conventional biodegradation in the aquatic environment; its accumulation would therefore induce potential adverse health effects on both humans and animals [16]. The chemical structures and main properties of BPA and CA are given in Table 1. The evolution of toxicity throughout the mineralization of pollutants was evaluated. Finally, the photocatalytic mechanisms over the Fe/C–TiO2 under visible light and UV irradiation were proposed.
2. Experimental 2.1. Synthesis and characterization The Fe/C–TiO2 photocatalysts were synthesized by a simple solvothermal method. Typically, 0.02 mol of titanium isopropoxide (TTIP, ≥98%, Merck) was dissolved in 85 mL of isopropanol (≥99%, Merck). 5 mL of oxalic acid (0.1 M) and a certain amount of FeCl3 ·6H2 O (≥98%, Sigma–Aldrich) were added. The resulting mixture was stirred vigorously for 1 h at room temperature and then transferred into a 125 mL of Telfon-lined autoclave. The autoclave was heated and maintained at 120 ◦ C for 4 h. The product was separated by centrifugation, thoroughly washed with deionized (DI) water, and then dried at 80 ◦ C overnight. Finally the samples were calcined at 300 ◦ C for 8 h (or 450, 600 ◦ C for 4 h) in a muffle furnace. The as-prepared photocatalysts were denoted as xFe/C–Ti(T), where x and T referred to the molar ratio of FeCl3 ·6H2 O to TTIP and calcination temperature respectively. For comparison, C–TiO2 photocatalysts (C–Ti(300) and C–Ti(450)) were prepared through the similar procedure without introducing FeCl3 ·6H2 O.
Chemical structure
X-ray diffraction (XRD) patterns were collected using a Bruker D8 Advance X-ray diffractometer with monochromated high˚ in a 2 range of 5–80◦ . intensity Cu K␣ radiation ( = 1.5418 A) Morphologies of the particles were examined by the transmission electron microscopy (TEM, JEOL JEM-2010). N2 adsorption/desorption isotherms of samples at liquid-nitrogen temperature (77 K) were obtained using a Quantachrome Autosorb-1 system. The Brunauer–Emmett–Teller surface area (SBET ) and Barrett–Joyner–Halenda (BJH) pore size distribution were derived. Thermogravimetric analysis (TGA) was performed using thermogravimetric analyzer (Perkin Elmer) with a heating rate of 5 ◦ C min−1 in an air flow. X-ray photoelectron spectroscopy (XPS) studies were conducted on a Krato Axis Ultra spectrometer with a monochromatic Al K␣ excitation source (h = 1486.71 eV). The binding energies (BE) were calibrated using C 1s core level at 284.8 eV for the adventitious carbon as reference. The UV–vis spectroscopy measurements were carried out using a UV–vis spectrometer (Lambda 35, Perkin-Elmer). Photoluminescence (PL) emission spectra of the samples were recorded using Fluorolog3 spectrofluorometer equipped with a Nd:YAG laser system of excitation wavelength at 260 nm (Horiba Scientific, New Jersey, USA). Fourier transform infrared spectra (FT-IR) were obtained for samples embedded in KBr pellets using Perkin Elmer 2000 spectrophotometer. 2.2. Photocatalytic activity measurement Stock solutions of BPA (≥99%, Sigma–Aldrich) and CA (97%, Aldrich) were prepared by dissolving the chemicals in DI water. Photocatalytic experiments were carried out in a vis-LED photoreactor as described in our previous work [17]. White LED was employed in this study. The spectrum of white LED exhibits a dominant peak at ca. 450 nm and a lower broadband from 500 to 650 nm. For comparison, photocatalytic activities of synthesized samples under UV and simulated solar light irradiation were also investigated. The light sources were a 365 nm mercury UV lamp (UVP Pen-Ray, 8 W, light intensity measured at a distance of 0.75 in. is 1.2 mW cm−2 ) and a solar simulator (Newport, USA) equipped with a 150 W xenon arc lamp, respectively. The suspension containing photocatalyst (dosage = 0.5 g L−1 ) and pollutant (5 mg L−1 ) was stirred in the dark for 60 min before irradiation to establish adsorption/desorption equilibrium of the target pollutant on the photocatalyst surface. All the tests were carried out at natural pH. Aliquots of the solution were collected at appropriate time intervals and filtered using membrane filters (0.45 m pore size). 2.3. Analytical methods Concentrations of the pollutants were measured using highperformance liquid chromatograph (HPLC, Perkin-Elmer Series 200), which comprised a Series 200 UV/Vis detector and C18 column (5 m, 150 mm × 4.6 mm, Inertsil ODS-3 column). The
X. Wang et al. / Applied Catalysis A: General 409–410 (2011) 257–266
Fig. 1. XRD patterns of synthesized photocatalysts.
mobile phase consisted of acetonitrile and water with a gradient concentration at a flow rate of 1.0 mL min−1 . The detection wavelengths were selected as 225 nm, 228 nm for BPA, CA respectively. The total organic carbon (TOC) remained in the solution was determined using a Shimadzu ASI-V TOC analyzer. Toxicity evaluation of the solutions prior to and after photocatalytic treatment was performed using a Microtox toxicity test system (Azur Microtox Analyzer Model 500). This system is based upon the use of bioluminescent marine bacterium (Vibrio fischeri) which produces luminescence as a by-product of normal metabolism. Any inhibition of normal metabolism, such as that caused by the presence of toxic compounds, would result in a decreased rate of luminescence. The inhibition of V. fischeri exposed to the samples for 15 min was measured. Microtox test for each sample was performed in triplicate at least. The inhibitory effect of the test sample was expressed in terms of inhibition percentage [17]. 3. Results and discussion 3.1. Characterization of prepared photocatalysts XRD patterns of as-synthesized photocatalysts are presented in Fig. 1. The characteristic peak at ca. 2 = 25.3◦ corresponding to (1 0 1) plane of anatase phase is observed for the samples calcined at 300 ◦ C. Moreover, for the Fe-containing samples, this peak is broader and weaker, and slightly shifts to a higher 2 degree as
259
compared to that of reference C–Ti(300). A weak peak at 30.5◦ can be found in the patterns of 3%Fe/C–Ti(300) and 5%Fe/C–Ti(300), which suggests the presence of ferric pseudobrookite Fe2 TiO5 (JCPDS No. 01-087-1996). Based on the above observations, it is inferred that low content of Fe3+ ions might be incorporated into the TiO2 lattice by substitution of Ti4+ ions due to their close ionic radii (0.79 and 0.75 A˚ for Fe3+ , Ti4+ , respectively) [6], and the incorporation of Fe could result in the lattice distortion of TiO2 and decrease of crystalline size. Trace amount of isolated Fe-containing compounds could be also formed in the sample containing a high Fe concentration. Therefore there could be a solubility limit for Fe incorporated into the anatase structure [18]. On the other hand, with increase of calcination temperature, 2%Fe/C–Ti(T) shared the diffraction peak of anatase at 25.3◦ , which appears stronger and sharper but occurs at the same position. It is indicated that calcination of 2%Fe/C–Ti(300) with elevated temperatures led to increased size and improved crystallinity of anatase TiO2 , however might not induce the lattice distortion. The transformation of anatase to rutile occurred at the calcination temperature up to 600 ◦ C. The anatase crystallite sizes of all the samples determined using Scherrer equation are listed in Table 2. TEM images of selected samples are shown in Fig. 2. As observed in Fig. 2A, the primary grains were approximately uniform but with irregular shape, and the average particle size was about 8 nm which is similar to that estimated from XRD pattern. Lattice spacing between the (1 0 1) planes is found to be 0.352 nm (Fig. 2B) and diffraction rings are shown in the SAED pattern (inset of Fig. 2B). These results confirm that the photocatalyst were of polycrystalline anatase structure. In addition, generally xFe/C–Ti(300) showed smaller particle sizes than that of C–Ti(300), while increase of the Fe content caused insignificant influences on the morphologies and particle size distributions of the synthesized photocatalysts. This may be due to the relatively limited contents of Fe impurities in the solvothermally synthesized photocatalysts. According to IUPAC classification, all the prepared samples exhibited a type IV isotherm with a H2 type hysteresis loop (Fig. 3), indicating the presence of mesopores. It is proposed that the porosity of TiO2 photocatalysts prepared via such solvothermal route could arise from the network of dominant interparticle pores surrounding the particles. The derived SBET , pore diameter (dp ) and pore volume (Vp ) of photocatalysts are summarized in Table 2. Obviously, the xFe/C–Ti(300) displayed larger SBET (145.8–159.9 m2 g−1 ), but slightly smaller Vp (4.5–5.1 cm3 g−1 ) than those of C–Ti(300) (128.8 m2 g−1 and 6.2 cm3 g−1 , respectively). The reason may be that the addition of Fe could help to form well-dispersed and defined TiO2 particles as evidenced by TEM and XRD analyses. Additionally, although SBET of samples could be notably decreased with the increase of calcination temperature,
Table 2 Physicochemical properties of derived photocatalysts. Photocatalyst
5%Fe/C–Ti(300) 3%Fe/C–Ti(300) 2%Fe/C–Ti(300) 1%Fe/C–Ti(300) C–Ti(300) C–Ti(450) 2%Fe–Ti(450) 2%Fe–Ti(600) a b c d
Element content Ca (at%)
Cb (wt%)
Fea (at%)
Fec (wt%)
17.04 18.19 16.01 18.30 16.78 5.36 3.02 0.98
0.72 0.76 0.70 0.74 0.68 0.03 0.01 0
1.02 0.51 0.42 0.16 0 0 0.58 0.69
0.48 0.32 0.20 0.05 8.2 9.5 0.23 0.21
Anatase crystallite sized (nm)
SBET (m2 g−1 )
dp (nm)
Vp (cm3 g−1 )
6.6 5.9 6.3 7.1 128.8 95.6 8.9 22.9
151.4 145.8 149.5 159.9 6.2 7.3 101.4 67.7
4.5 4.6 4.8 5.1 0.19 0.17 6.2 14.8
0.17 0.17 0.18 0.20
Surface concentration induced by XPS results. Bulk content obtained by elemental analysis. Bulk concentration determined using microwave-assisted digestion followed by analysis with ICP-OES. Derived from XRD patterns.
0.16 0.04
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Fig. 2. TEM images of 2%Fe/C–Ti(300) (A), C–Ti(300) (C) and 5%Fe/C–Ti(300) (D), HRTEM image and SEAD pattern of 2%Fe/C–Ti(300) (B).
2%Fe–Ti(450) and C–Ti(450) still exhibited relatively high SBET of 101.4 and 95.6 m2 g−1 , respectively. Fig. 4 presents the TG/DTG profiles of 2%Fe/C–Ti(T) precursor dried at 80 ◦ C. The thermal decomposition during heating in the air involved three main steps. The first step with weight loss of 5.3% occurred at 30–125 ◦ C and two endothermic peaks at ca. 57 and 121 ◦ C are observed in the DTG curve, mainly can be ascribed to the removal of surface-adsorbed water and CO2 molecules. The second stage happened at 125–300 ◦ C resulting in 5.0% of weight loss, which could be due to the thermal decomposition of residual
organic species such as oxalic acid (as supported by the DTG peak around 218 ◦ C). The subsequent step of minor weight loss of 1.4% took place at 300–600 ◦ C, possibly because of the loss of carbon species on the surface of photocatalyst, which will be confirmed by the results of XPS, IR and elemental analysis latter. The weight of sample became stable at 600 ◦ C and above. The surface elemental compositions and chemical states of the prepared photocatalysts were investigated by the XPS measurements, as shown in Fig. 5. Fe 2p XPS spectra of all the samples with the introduction of Fe exhibit the doublet peaks located at ca.
Fig. 3. Nitrogen adsorption/desorption isotherms (A) and pore size distributions (B).
X. Wang et al. / Applied Catalysis A: General 409–410 (2011) 257–266
Fig. 4. TG/DTG profiles of 2%Fe/C–Ti(T) precursor dried at 80 ◦ C.
724.1–723.5 and 711.1–710.0 eV corresponding to Fe 2p1/2 and Fe 2p3/2 , respectively, indicating the presence of Fe3+ oxidation state [19]. The Fe 2p3/2 from these Fe-containing TiO2 showed slightly higher binding energies as compared to typical Fe 2p3/2 from Fe2 O3 (709.8 eV) [19], which may be associated with the fact that Fe3+ ions interacted strongly with TiO2 in the form of Ti–O–Fe bonds [20]. Moreover, generally the doublet peaks become more intense with increase of Fe loading. Fig. 5C presents the high-resolution XPS spectra of C 1s. A peak at 284.8 eV shared by all the samples is known as the adventitious carbon. For samples calcined at 300 ◦ C, the C 1s spectra display two additional peaks at around 286.0 eV and
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288.0 eV, which are assigned to C–O and C O bonds, respectively. Several studies have attributed the C dopants to carbonate species which was present on the TiO2 surface in the form of Ti–O–C or Ti–O C linkages [14,21]. The existence of two peaks indicates that there may not be only one kind of carbonate species. These conclusions are also supported by the FTIR measurements. As presented in Fig. 8, the above-mentioned samples showed the peaks with lowintensity at around 1065, 1336 and 1436 cm−1 , which are assigned as carbonate species (i.e. monodentate carbonate and bicarbonate, respectively) [10,22–24]. The peak at ca. 288.0 eV or 286.0 eV is extremely weak in the 2%Fe–Ti(450) and C–Ti(450) and absent in the 2%Fe–Ti(600). Two peaks observed in the Ti 2p XPS spectra of as-synthesized photocatalysts are around 464.5–463.4 eV for Ti 2p1/2 and 458.7–457.5 eV for Ti 2p3/2 , respectively [25]. As seen from the Ti 2p spectra of C–Ti(300), the peaks at 464.2 eV for Ti 2p1/2 and 458.7 eV for Ti 2p3/2 can be ascribed to the pristine TiO2 [25]. This result implies that the formation of carbonate species may not change the chemical states of Ti. Therefore, the presence of Ti 2p peaks with lower binding energies (e.g. 464.2 and 463.4 eV for Ti 2p1/2 , 458.4 and 457.5 eV for Ti 2p3/2 ) in the Fe-containing TiO2 , may be mainly attributed to Ti–O–Fe linkages induced by the incorporation of Fe3+ into TiO2 lattice at least at the surface [20]. The atomic concentrations of C and Fe on the surface of photocatalysts are summarized in Table 2. It is noted that C concentration, which excluded the adventitious carbon, remained quite stable at around 17% on the surface of these samples calcined at 300 ◦ C, while it drastically decreased with the increasing calcination temperatures and can be negligible for the 2%Fe–TiO2 (450) and 2%Fe–TiO2 (600). It reveals that carbon dopants in the form of carbonate species for the as-prepared photocatalysts might be resulted from the organic precursors (isopropanol or TTIP) and can be removed by calcination
Fig. 5. Survey scan XPS spectra (A), high-resolution XPS spectra of Fe 2p (B), C 1s (C) and Ti 2p (D) regions for derived photocatalysts.
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Fig. 7. PL spectra of derived photocatalysts.
Fig. 6. UV–vis diffuse reflectance spectra of the synthesized photocatalysts and P25.
at high temperatures. On the other hand, Fe atomic concentrations were gradually increased with the elevated addition of FeCl3 ·6H2 O. Therefore, it can be concluded that Fe and C were successfully introduced in TiO2 for samples xFe/C–Ti(300), while single dopant was presented in samples C–Ti(300), 2%Fe–Ti(450) and 2%Fe–Ti(600). The bulk C and Fe contents are also presented in Table 2, the former was determined by elemental analysis (CHNS/O analyzer, PerkinElmer 2400 Series II), and the latter was examined using inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer Optima 2000DV). Apparently, surface concentrations of Fe and C as induced by XPS were much higher than their bulk contents, especially in the case of C. The 2%Fe/C–Ti(300) contained 0.70 wt% of carbon according to elemental analysis, which is in good agreement with that predicted by TG analysis. Hence it is believed that both Fe dopant and carbonate species had higher concentrations at the exterior than at the interior of TiO2 nanoparticles, in particular carbonate species was predominantly present on the surface. This result can be understood based upon the mechanism of TiO2 doping with metal or non-mental species by solvothermal method [26]. In this process, Fe3+ ions firstly dispersed and were adsorbed on the surface of TiO2 gel due to strong electrostatic interaction. Since Fe3+ (0.064 nm) has a rather similar radius to that of Ti4+ ions (0.068 nm) and a strong affinity for O2− (or OH− ), Fe3+ could diffuse into the bulk of TiO2 by substituting Ti4+ or incorporating into Ti4+ vacancy during the process of TiO2 crystallization with solvothermal treatment. In the end, the wellformed polycrystalline anatase structure of TiO2 could suppress the Fe diffusion from surface to the interior of TiO2 . The external surface enrichment of Fe dopant was therefore formed in the resulting samples. On the other hand, few studies have reported C dopants were injected into the deep bulk of TiO2 via the wet chemical method, such as sol–gel and hydrothermal (solvothermal) method. UV–vis diffuse reflectance spectra (DRS) of the as-prepared photocatalysts are depicted in Fig. 6. C–Ti(300) exhibited the enhanced absorption of visible light as compared to P25, which could be attributed to the presence of carbonate species on the surface. The introduction of Fe for xFe/C–Ti(300) further caused a significant red shift of absorption edge to the visible light region in comparison to C–Ti(300), and the absorption of visible light (400–650 nm) increased with increasing Fe content in TiO2 . It has been widely accepted that Fe3+ doping in the TiO2 lattice can extend photoresponse of TiO2 into visible light range. However, the origin of the red shift of absorption edge addressed in the literature is controversial. Some studies have concluded that Fe doping would
introduce additional electronic states within the band gap, leading to effective red-shift of the absorption threshold [4,27,28]. Other researchers have reported that this red-shift is caused by narrowing of the band gap of pristine TiO2 [29]. In comparison with 2%Fe/C–Ti(300) and C–Ti(300), although 2%Fe–Ti(450) and C–Ti(450) showed decreased visible-light absorption due to the loss of carbonate species on the surface as discussed above, their DRS spectra at wavelength ≤380 nm are similar. It is thus suggested that C modification by formation of surface carbonate species may not cause significant change of TiO2 band gap. The similar phenomenon was observed in commercially available C modified TiO2 [11]. However, for 2%Fe–Ti(600), the higher calcination temperature of 600 ◦ C resulted in slightly stronger visible-light response extending to 700 nm. This may be because of possible interface states induced by the mixed anatase–rutile phase structure [30]. Fig. 7 shows the PL spectra of derived photocatalysts. One peak around 560 nm can be found in the PL emission spectra of all the samples, which may be induced by surface oxygen vacancies and intrinsic defects in the doped TiO2 [12,31], because of the presence of carbonate species and/or Fe dopants in these photocatalysts. The PL intensities rank in the following order: C–Ti(300) > 2%Fe–Ti(450) > 1%Fe/C–Ti(300) > 5%Fe/C–Ti(300) > 3% Fe/C–Ti(300) > 2%Fe/C–Ti(300). Since PL emission mainly arises from the recombination of photo-induced electrons and holes, the lower PL intensity may reflect the lower recombination rate of electrons and holes. Generally C–TiO2 and Fe–TiO2 exhibit the relatively higher intensities as compared to Fe/C–TiO2 , which suggests that TiO2 codoping with Fe and C may improve the separation of photo-induced electrons and holes. It has been reported that Fe3+ could act as electron sinks to facilitate the transfer of photo-induced electrons and suppress the recombination of electrons and holes [9,12]. It is therefore observed that PL intensities gradually decreased with introduction of small amount of Fe. However, 5%Fe/C–Ti(300) and 3%Fe/C–Ti(300) showed higher PL intensities than 2%Fe/C–Ti(300). The possible reason is that Fe impurities could serve as recombination centers when present in high concentrations, which may be associated with the trace amount of isolated ferric pseudobrookite Fe2 TiO5 formed in the 5%Fe/C–Ti(300) and 3%Fe/C–Ti(300), as evidenced by the XRD results. The effective inhibition of electron–hole recombination for the 2%Fe/C–Ti(300) could improve its visible-light photocatalytic performance. Besides the characteristic peaks for carbonate species, one peak at 1640 cm−1 corresponding to surface adsorbed hydroxyl groups [32], is also present in the FTIR spectra of samples calcined at 300 ◦ C (Fig. 8). It become weaker with the decreasing Fe content and disappear at the higher calcination temperatures. This result indicates
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Fig. 8. FTIR spectra of synthesized photocatalysts.
that the presence of carbonate species and Fe impurities on TiO2 surface could facilitate the formation of hydroxyl groups, since TiO2 doping with Fe ions and carbonate species could introduce oxygen vacancies in the crystal lattice or surface of TiO2 [33,34]. It is believed that the surface hydroxyl group is a crucial factor influencing the photocatalytic activity of TiO2 photocatalysts.
Fig. 9 shows the photocatalytic degradation (PCD) of BPA using as-prepared photocatalysts under white LED irradiation. Photolysis of BPA throughout 4 h of white LED irradiation was negligible. In general, the solvothermally synthesized Fe and/or C doped TiO2 showed enhanced visible-light photocatalytic performance towards BPA degradation compared to that of P25. It is suggested that the lattice defects induced by the Fe dopants and/or carbonate species were responsible for the visible-light photocatalytic activities of these photocatalysts. Apart from the lattice defects, it is postulated that the visible-light photocatalytic activity of doped TiO2 is dependent upon many other factors, such as crystalline structure, particle size, textural properties (i.e. surface area and pore structure), optical properties, surface hydroxyl density, etc. Firstly, xFe/C–Ti(300) with larger SBET than that of C–Ti(300) could provide more active sites, which play an important role in the heterogeneous photocatalytic process. Secondly, incorporation of Fe into TiO2 lattice not only enhanced the absorption ability of visible
light, but also suppressed recombination of photo-induced electrons and holes. Additionally, TiO2 doping with Fe ions facilitated formation of surface hydroxyls groups which is another crucial factor governing the photocatalytic activity. Therefore, it is reasonable that xFe/C–Ti(300) showed higher photocatalytic activities than C–Ti(300). Furthermore, 2%Fe/C–Ti(300), which demonstrated the most effective separation of photo-generated electrons and holes, exhibited the highest BPA removal rate among xFe/C–Ti(300). BPA was almost completely degraded by the 2%Fe/C–Ti(300) within 4 h of irradiation. PCD efficiencies of the Fe–TiO2 (2%Fe–Ti(450) and 2%Fe–Ti(600)) and C–Ti(450) were significantly lower than 2%Fe/C–Ti(300) and C–Ti(300) respectively, possibly due to their weakened visible-light absorption, as well as decreased SBET and surface hydroxyl groups. On the basis of the above results, it is proposed that the synergistic effects of Fe and C codoping into TiO2 could result in improved visible-light photocatalytic activity of Fe/C–TiO2 as compared to those of C–TiO2 and Fe–TiO2 . As seen from Fig. 10, CA can also be efficiently degraded by the Fe/C–TiO2 under white LED irradiation. The CA was degraded to 96% within the initial 2 h, and completely removed after 4 h of reaction. Photolysis caused insignificant removal of CA within 4 h. Similarly, Fe/C–TiO2 showed higher visible-light photocatalytic ability for CA degradation than C–TiO2 , Fe–TiO2 and P25. It is also found that CA removal rate was slightly faster than that of BPA, which may be related to its molecular structure and physicochemical properties. Removal efficiencies of BPA and CA with different light sources were compared, as shown in Fig. 11. It is evident that Fe/C–TiO2
Fig. 9. Photocatalytic activities of synthesized photocatalysts for BPA degradation under white LED irradiation.
Fig. 10. Photocatalytic degradation of CA under white LED irradiation.
3.2. Photocatalytic degradation of emerging organic contaminants
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Fig. 11. BPA and CA removal efficiencies under the irradiation of different light sources.
Fig. 12. Photocatalytic mineralization of BPA and CA using 2%Fe/C–Ti(300) under white LED irradiation, and evolution of toxicities upon BPA degradation. Fig. 13. Cycling runs in the photocatalytic degradation of BPA using 2%Fe/C–Ti(300) under white LED irradiation.
displayed enhanced photocatalytic activities as compared to C–TiO2 and Fe–TiO2 regardless of light sources. Under irradiation of white LED, 97% of BPA and 95% of CA were removed by the Fe/C–TiO2 within 2 h and 3 h, respectively as compared to 9% and 13% by P25. Since UV emission at 365 nm was not detected using Accumax XRP-3000 radiometer and photolysis induced negligible removal of BPA and CA, the activity of P25 under white LED irradiation may be attributed to formation of surface complexation between the phenolic compounds and P25 [35]. Also it can be found that Fe/C–TiO2 showed better or at least comparable photocatalytic activity for BPA and CA degradation in comparison with P25 under simulated solar light irradiation. However, the Fe/C–TiO2 similar to other synthesized TiO2 photocatalysts which have been reported in literature, was less active than P25 under UV irradiation. BPA and CA were completely removed within 4 h under white LED irradiation, while only 61% and 71% of TOC were mineralized, respectively, as shown in Fig. 12. The remaining TOC may be attributed to the formation of more persistent intermediates or aliphatic carboxylate ions that might be decomposed with longer irradiation [17]. In view of the formation of intermediates during PCD of these organic pollutants, which might be more toxic than their parent molecules [36], it is of great significance to examine the toxicity of the solutions at different irradiation times for better evaluation of the photocatalytic process. 15-min EC50 (concentration of
toxicant that produces a 50% reduction of luminescence) values of BPA and CA obtained in the present study were 2.23 ± 0.45 mg L−1 and 232 ± 28 mg L−1 , respectively. The inhibition of luminescence in V. fischeri bioassay of the untreated CA solution of 5 mg L−1 was not detectable. Moreover, no increase of inhibition was observed during the PCD of CA. For the BPA solutions, to maintain the luminescence reduction in the acceptable range of detection, all the samples were diluted from 100% to 58.4% with Microtox diluents, which was a specially prepared nontoxic 2% sodium chloride solution. The luminescence inhibition reflects the toxicity level of the samples. The greater inhibition indicates the higher level of toxicity, and vice versa. As displayed in Fig. 12, the toxicities of BPA solutions gradually decreased as the irradiation proceeded. Fig. 13 shows the durability of the Fe/C–TiO2 for BPA degradation under while LED irradiation. In a typical procedure, the photocatalyst was reused by simple filtration and washing with DI water without further treatment. The reduction of visible-light photocatalytic activity was insignificant even after five cycles. Fe ion was not detected in the BPA solutions throughout the photocatalytic process, indicating that no Fe was leached out from the Fe/C–TiO2 under the experimental conditions. It can be found that color of Fe/C–TiO2 remained yellow during PCD of BPA. Furthermore, the optical property and visible-light photocatalytic activity
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Fig. 14. Proposed photocatalytic mechanisms over the Fe/C–TiO2 under visible light and UV irradiation.
of Fe/C–TiO2 almost did not change after the acidic or alkaline des˛ et al. [37]). It is suggeated orption treatments (described by Zabek that the carbon dopants in the Fe/C–TiO2 were strongly bound to TiO2 surface under such solvothermal condition. Therefore, the synthesized Fe/C–TiO2 was chemically stable to be applied in aqueous photocatalysis. 3.3. Proposed mechanism As concluded above, the Fe/C–TiO2 was modified with Fe and C by the formation of Ti–O–Fe linkages and carbonate species respectively, which were responsible for its visible-light photocatalytic activity. For the role of carbonate species in the visible-light photocatalytic ability of C–TiO2 , some studies have demonstrated that carbonate species assigned to interstitial carbon dopant could create the intragap localized states of C 2p above the valence band (VB) of TiO2 [9], while others have proposed that carbonate species could act as photosensitizer [10,14]. According to XRD, UV–vis and XPS measurements, the carbonate species in the synthesized Fe/C–TiO2 did not result in any TiO2 lattice distortion, change in band gap and chemical state of Ti. The carbonate species was predominantly present on the surface as revealed by large variations in the surface and bulk C contents (Table 2). The C content can be removed by calcination at high temperatures. Therefore, it is suggested that the carbonate species on the surface of Fe/C–TiO2 served as photosensitizer for the PCD of pollutants under visible light irradiation, rather than introduced additional intragap states. On the other hand, we hypothesized that Fe3+ doping into TiO2 lattice by substituting Ti4+ would introduce a new dopant energy level into TiO2 band gap. The photocatalytic mechanisms induced by the Fe/C–TiO2 under visible light and UV irradiation were proposed, as illustrated in Fig. 14. Under visible light irradiation, the excited photosensitizer (carbonate species on the TiO2 surface) injected an electron into the conduction band (CB) of TiO2 . The electron was subsequently transferred to oxygen adsorbed on the TiO2 surface producing superoxide radical (O2 •− ), which is one of highly reactive oxygen species that can degrade many organic compounds [10]. Two kinds of Fe impurity states located within the electronic structure of Fe-doped TiO2 have been reported: one is above the valence band (VB) due to the fact that t2g level of 3d orbital of Fe3+ ion lies above the VB of TiO2 [5,6], the other is below CB because the energy level of Fe 3d electrons is lower than that of Ti 3d at the bottom of CB [9,28,29]. Therefore two possible schemes for the mechanism of Fe and C codoping are proposed. Under visible light irradiation, an electron was photoexcited from Fe3+ dopant level to the CB or from the VB to this dopant level. As shown in Fig. 14A, Fe3+ was hence converted to Fe4+ according to Eq. (1) [6]. The photogenerated CB electron further reacted with adsorbed O2 to form O2 •− , while Fe4+ could react with surface hydroxyl group to produce hydroxyl radical (OH• ) (Eq. (2)). Meanwhile, since the energy level of Fe3+ /Fe2+ is below the CB edge of TiO2 [5,6], Fe3+ could act as a electron sink to trap photogenerated electron and produce O2 •− (Eqs. (3)–(6)) [6,38]. As illustrated by Fig. 14B, the photoexcited Fe/C–TiO2 generated a hole in the VB of TiO2 and an electron in the
Fe3+ dopant level. The hole could migrate to the surface leading to the formation of OH• , while Fe3+ accepted an electron to form Fe2+ (Eq. (3)). In this case, Fe3+ would act as a hole trap due to that the energy level of Fe4+ /Fe3+ is above the valence band edge of TiO2 (Eq. (7)) [5,6]. Additionally, it is possible that visible light response could be due to charge transfer transition between Fe ions (Eq. (8)) [33]. The photocatalytic reaction could involve the mechanistic steps of Eqs. (2) and (4)–(6). However, Fe3+ dopant with high concentration could serve as recombination centers for the photogenerated electron–hole (Eqs. (9) and (10)) [6,7], leading to the decrease of activity of the photocatalyst. hv
Fe3+ −→Fe4+ + e− 4+
Fe
3+
Fe
−
+ OH → Fe −
+ e → Fe
3+
(1) + OH•
(2)
2+
(3)
Fe2+ + O2 → Fe3+ + O2 •− 2+
Fe
3+
Ti
3+
Fe
+ Ti
4+
→ Fe
+ O2 → Ti
4+
+
+ h → Fe
3+
+ Ti
+ O2
(4)
3+
(5)
•−
(6)
3+
(7)
Fe3+ + Fe3+ → Fe2+ + Fe4+ 4+
Fe
2+
Fe
−
+ e → Fe +h
+
(8)
3+
(OH• )
→ Fe
(9) 3+
−
(+OH )
(10)
Under UV irradiation, the effects of dopants on photocatalytic process may be different, as shown in Fig. 14C. Fe3+ appeared to be both electron and hole traps, or it became recombination centers when present at a high content. 4. Conclusions Fe/C–TiO2 photocatalysts were synthesized by a facile solvothermal method. The derived photocatalysts were predominantly of anatase phase with mesoporous structure, exhibited large specific surface areas. Ti–O–Fe linkages and carbonate species were evidenced in the Fe/C–TiO2 photocatalysts. The presence of Fe dopants and carbonate species could favor the formation of surface hydroxyl groups, and inhibit recombination of photo-generated electrons and holes. However, when present at high concentrations, Fe dopant could act as charge recombination center. Generally Fe/C–TiO2 exhibited the higher efficiencies for degradation of BPA and CA as compared to C–TiO2 , Fe–TiO2 and P25 under visible light and simulated solar light irradiation. After 4 h of irradiation with visible light, BPA and CA can be completely removed by the Fe/C–TiO2 , concomitantly 61% and 71% of TOC were mineralized, respectively. The BPA solutions became less toxic after the photocatalytic treatment. Furthermore, visible-light photocatalytic activity of the Fe/C–TiO2 was maintained effectively after several cyclic experiments. This study suggests that the solvothermally synthesized Fe/C–TiO2 with high chemical stability
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