Properties of carbon and iron modified TiO2 photocatalyst synthesized at low temperature and photodegradation of acid orange 7 under visible light

Applied Surface Science 256 (2010) 4260–4268

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Properties of carbon and iron modified TiO2 photocatalyst synthesized at low temperature and photodegradation of acid orange 7 under visible light Yongmei Wu a, Jinlong Zhang a,b,*, Ling Xiao a, Feng Chen a a b

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China School of Chemistry and Materials Science, Guizhou Normal University, Guiyang 550001, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 October 2009 Received in revised form 19 January 2010 Accepted 2 February 2010 Available online 10 February 2010

The nanoparticles of TiO2 modified with carbon and iron were synthesized by sol–gel followed solvothermal method at low temperature. Its chemical composition and optical absorption were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence emission spectroscopy (PL), UV–vis absorption spectroscopy, and electron paramagnetic resonance (EPR). It was found that carbon and iron modification causes the absorption edge of TiO2 to shift the visible light region. Fe(III) cation could be doped into the matrix of TiO2, by which could hinder the recombination rate of excited electrons/holes. Superior photocatalytic activity of TiO2 modified with carbon and iron was observed for the decomposition of acid orange 7 (AO7) under visible light irradiation. The synergistic effects of carbon and iron in modified TiO2 nanoparticles were responsible for improving visible light photocatalytic activity. ß 2010 Elsevier B.V. All rights reserved.

Keywords: TiO2 Modification Visible light Photocatalyst Acid orange 7

1. Introduction Titanium dioxide as an important photocatalyst has already been attracted most interests due to its specific optical and electronic properties, low cost, chemical stability and non-toxicity. However, titania has a large band gap (3.2 eV) and therefore only a small fraction of solar light, about 5% in the UV region can be utilized [1,2]. Hence, many attempts have been devoted to prepare TiO2 photocatalyst that is capable of efficient utilization of visible light. Until now, several strategies including doping of TiO2 with transition metals [3,4], anchoring organic dyes onto the surface of TiO2 [5,6] and doping of TiO2 with anionic non-metals [7,8] have been investigated. Among them, doping of TiO2 with transition metal cations was reported as a good tool to improve photocatalytic properties for enhancement of visible light response. Iron doped TiO2 nanocrystalline particles showed better photocatalytic activities than pure TiO2 under visible light in many reports [9–12]. It was believed that Fe(III) cations could act as shallow traps in the lattice of titania, which was benefit to inhibit electron/hole recombination properties. In general terms, it appeared that optimum photocatalytic properties could be achieved upon doping iron at a relatively weak level [13]. Additionally, doping with non-

* Corresponding author at: Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China. Tel.: +86 21 064252062; fax: +86 21 64252062. E-mail address: [email protected] (J. Zhang). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.012

metallic species, such as N, C, S, P and halogen atoms also caused the photosensitization of TiO2 in the visible light region [14–19]. Among various non-metal modified titania, carbon-containing composites titania has been reported as a kind of promising photocatalyst. Kisch and co-workers [14,15] reported that carboncontaining titania prepared by a modified sol–gel process using different alkoxide precursors was able to photodegrade pchlorophenol under visible light (l > 400 nm). Treschev et al. found carbon-containing TiO2 nanoparticles prepared under calcinations at 200 8C exhibited high photocatalytic activity for decoloring of methylene blue and the removal of nitrogen monoxide under visible light illumination [16]. Kang et al. synthesized C-doped TiO2 powders by grinding TiO2 with ethanol and heating treatment and this C-doped TiO2 photocatalyst showed a good visible light activity for NO gas decomposition [20]. Chen and co-workers synthesized C-doped TiO2 micronanosphere and nanotubes via a chemical vapor deposition method and they claimed that carbon doping led to lower band gap and higher photocurrent than TiO2 P25 under visible light irradiation [21]. Choi and co-workers reported that carbon-doped TiO2 prepared from a conventional sol–gel synthesis using titanium alkoxide precursor without adding external carbon precursors and they claimed that the carbons species from titanium alkoxide precursor could be incorporated into the lattice of TiO2 by calcinations of low temperature at 200–250 8C [22]. Valentin et al. proved that carbon atom could replace oxygen atom or titania atom depending on the concentration of oxygen in the structure of TiO2 by DFT calculation of doping of TiO2 with carbon.

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Carbon impurities resulted in forming the new states in the band gap as well as oxygen vacancies in the bulk of TiO2, which could be responsible for the visible photoactivity of C-doped TiO2 [23]. More recently, the simultaneous doping of two kinds of atoms into TiO2 has attracted considerable interest, since it could result in a higher photocatalytic activity and peculiar characteristics compared with single element doping into TiO2. Some studies were reported on co-doped materials such as C and N [24,25], S and N [26,27], F and N [28,29], B and N [30] and N and a variety of metal ions [31,32], and in some cases authors underlined the synergistic effect of co-doping. For example, Cong et al. reported that cooperation of nitrogen and iron cations led to the much narrowing of the band gap and greatly improved the photocatalytic activity in visible light region [31]. Wei et al. found that co-doping TiO2 with lanthanum and nitrogen led to significant enhancement in photodegradation of methyl orange under visible light; the substitution of N for O was responsible for the band gap narrowing of TiO2, and La doping prevented the aggregation of powder in the process of preparation [32]. Sun et al. observed that carbon and sulfur co-doped TiO2 showed high photodegradation of 4chlorophenol under visible light irradiation [33]. Klabunde and co-workers synthesized C-doped and C and V co-doped TiO2 photocatalysts that were quite active for the degradation of astaldehyde [34]. Tryba et al. prepared TiO2 modified by carbon and iron photocatalyst by impregnating the powder TiO2 with FeC2O4 solution and heating it at 400–800 8C under flow of Ar gas [35]. They reported that the Fe–C–TiO2 sample showed the higher photoactivity for phenol decomposition and dyes degradation under UV light and H2O2 [36–38]. However, their Fe–C–TiO2 photocatalysts needed to be treated at high temperature. The sol–gel process and solvothermal method have been widely used to synthesize TiO2-based photocatalyst. The incorporation of metal ions (dopants) in the sol allows the ions to have direct interaction with the polycondensation of titanium alkoxide during the gelation stage. When the wet gel is treated by solvothermal method, metal ions could be doped into the lattice of TiO2. Additionally, crystallization of TiO2 could occur in the process of solvothermal treatment, by which could avoid the process of calcination of TiO2 at high temperature. To our best knowledge, carbon and iron co-modified TiO2 using sol–gel process followed with solvothermal method under moderate conditions have not yet been reported. In the present paper, carbon and iron modified TiO2 photocatalysts with enhanced photocatalytic activity for photodegradation of acid orange 7 (AO7) under visible light were obtained directly via the combination of sol–gel process and solvothermal treatment at low temperature. The resulting TiO2 photocatalysts were investigated using XRD, EPR, nitrogen physical adsorption, XPS, UV–vis absorption spectroscopy and photoluminescence emission spectra. The effect of amount of iron dopants on the properties of carbon modified TiO2 was also investigated. 2. Materials and methods

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2.2. Catalyst preparation The iron and carbon modified TiO2 nanoparticles were prepared by combining sol–gel method followed with solvothermal treatment. 6 mL tetrabutyl titanate was dissolved into 34 mL anhydrous ethanol (solution A), solution B consisted of 17 mL anhydrous ethanol, 0.4 mL concentrated nitric acid (68%), 1.6 mL distilled water and the required stoichiometric amount of Fe(NO3)39H2O. Then solution A was added drop-wise to solution B under magnetic stirring at ambient temperature. The resultant mixture was stirred at room temperature for 2 h until the transparent sol was obtained. The sol was then aged at ambient temperature for 2 days and the gel was obtained, which was then transferred into a 100 mL Teflon-inner-liner stainless steel autoclave. The autoclave was kept for 10 h under 180 8C for crystallization (the pressure in the autoclave was about 1.5 MPa). After this solvothermal treatment, the precipitate gained was washed by distilled water, dried at 100 8C for 24 h and ground to obtain iron and carbon modified TiO2 nanoparticles. Carbon modified TiO2 was also prepared by the same way in the absence of Fe(NO3)39H2O. The iron doping concentration (x) was chosen as 0.30, 0.50, 0.57, 0.70, and 0.90, which was the mole percentage of Fe(III) in the theoretical titania powder. The obtained photocatalysts with corresponding iron concentration were denoted as xFe/C–TiO2. The carbon modified TiO2 sample was also prepared by the same way in the absence of Fe(NO3)39H2O, denoted as C–TiO2. 2.3. Catalyst characterization XRD analysis of the as-prepared photocatalysts was carried out at room temperature with a Rigaku D/max 2550 VB/PC apparatus using Cu Ka radiation (l = 1.5406 A˚) and a graphite monochromator, operated at 40 kV and 30 mA. Diffraction patterns were recorded in the angular range of 10–808 with a stepwidth of 0.028 s1. The SBET of the samples was determined through nitrogen adsorption at 77 K (Micromeritics ASAP 2010). All the samples were degassed at 473 K before the measurement. The X-band EPR spectra were recorded at room temperature (Varian E-112). To analyze the light absorption of the photocatalysts, UV–vis absorption spectra (DRS) were obtained using a scan UV–vis spectrophotometer (Varian Cary 500) equipped with an integrating sphere assembly, while BaSO4 was used as a reference. To investigate the chemical states of the photocatalysts, X-ray photoelectron spectroscopy (XPS) was recorded with Perkin Elmer PHI 5000C ESCA System with Al Ka radiation operated at 250 W. The shift of binding energy due to relative surface charging was corrected using the C1s level at 284.6 eV as an internal standard. The photoluminescence (PL) emission spectra of the samples were measured using a Varian Cary Eclipse fluorescence with a 175 W Xe lamp as excitation source and a photomultiplier tube at 400 V. Each sample was dry-pressed into a 10-mmdiameter round disk containing about 100 mg of mass. The sample excitation was done at 280 nm at room temperature and the emission was scanned between 300 nm and 800 nm.

2.1. Chemicals 2.4. Photocatalytic activity test Tetrabutyl titanate (TBOT, 98%) and nitric acid (HNO3, 68%) were purchased from Shanghai Lingfeng chemical regent Co. Ltd, China. Ethanol (C2H5OH, 99.7%) and Iron(III) nitrate nonahydrate (Fe(NO3)39H2O, reagent grade) were produced by Sinapharm chemical reagent Co. Ltd, China. Acid orange 7 (AO7, Sigma– Aldrich) was received as used. All the chemicals were used without any further purification. Commercial pure anatase TiO2 (produced by Shanghai Kangyi Co. Ltd) with specific surface area of 120 m2/g and primary particle size of 10 nm were used for comparison purpose.

The photocatalytic activities of samples were evaluated in terms of the degradation of AO7 under visible light illumination. The chemical structure of AO7 is shown in Fig. 1. The photodegradation reactions were carried out with a homemade photoreactor. A 1000 W tungsten halogen lamp equipped with a UV cut-off filters (l > 420 nm) was used as a visible light source (the average light intensity was 60 mW cm2). The distance between the light and the reaction tube was fixed at 24 cm. The lamp was cooled with flowing water in a quartz cylindrical jacket

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Table 1 Effect of different amount of Fe on the physical properties of TiO2 samples and carbon content.

Fig. 1. The structure of the AO7 molecule.

around the lamp, and ambient temperature is maintained during the photocatalytic reaction because of good ventilation. The photocatalyst powder (0.08 g) was dispersed in a 100 mL quartz photoreactor containing 80 mL of a 50 mg L1 AO7 solution. The pH value of the AO7 solution is kept 5.00. The mixture was sonicated for 10 min and stirred for 30 min in the dark in order to reach the adsorption–desorption equilibrium. At the given time intervals, the sample of 3.5 mL was taken from the mixture and immediately centrifuged, then filtered through a 0.22 mm Millipore filter to remove photocatalysts. The concentration of the filtrate was analyzed by checking the absorbance at 484 nm with a UV–vis spectrophotometer (Varian Cary 100). The reproducibility was checked by repeating the measurements at least three times and was found to be within the acceptable limit (5%). 3. Results and discussion 3.1. XRD, Raman spectra and BET analysis Fig. 2 depicted the XRD patterns of the as-prepared samples. It was found that all diffraction peaks can be perfectly indexed as anatase phase of TiO2 [JCPDS no. 21-1272, spacegroup: I41/ amd(141)]. In the case of samples containing iron, no significant characteristic peaks for iron oxide were detected, which suggested that no significant iron segregation was produced in the samples. It may be attributed to the lower iron content in these samples below the detection limit of this technique or the substitution of Ti(IV) ions by Fe(III) ions and inserting into the crystal lattice of TiO2 due to the similar radii of Ti(IV) (0.76 A˚) and Fe(III) (0.79 A˚) [39,40]. The average size of crystallite was calculated using the Scherrer equation: D¼

Kl b cos u

(1)

Sample

Particle size (nm)a

SBET (m2/g)b

C content (at.%)c

C–TiO2 0.30Fe–C–TiO2 0.50Fe–C–TiO2 0.57Fe–C–TiO2 0.70Fe–C–TiO2 0.90Fe–C–TiO2

11.1 11.0 10.9 10.8 10.7 10.6

153 161 160 158 159 160

8.10 7.51 6.03 7.23 7.95 7.49

a b c

Determined by XRD results. Determined by N2 physical adsorption. Determined by XPS results.

The crystallite sizes of all the samples calculated for anatase (1 0 1) peak are shown in Table 1. As can be seen from Table 1, the crystallite sizes of iron and carbon modified TiO2 were slightly lower than that of C modified TiO2, which indicates the occurrence of a slight lattice distortion in the anatase structure. The dimension decrease in crystallite size may be caused by a number of defects in the anatase crystallites produced by the substitution of part of the Ti(IV) site by Fe(III) cations [12]. Raman spectroscopy confirms the presence of anatase form of titania (shown in Fig. 3), as evidenced by the presence of mainly five bands corresponding to the five active modes expected for this tetragonal structure [41] at 146 cm1 (Eg), 197 cm1 (Eg), 394 cm1 (B1g), 514 cm1 (B1g), 638 cm1(Eg). Compared to C– TiO2 sample, a slightly broadening of the Raman peaks of 0.57Fe/C– TiO2 were observed. This effect was explained by increased disorder due to the inclusion of iron ions in the titania network or by the lower size of the titania nanocrystals due to the presence of iron impurities at the grain boundaries, which slowed the growth of titania grains. The Raman result is consistant with XRD. The specific surface area of the samples is measured using the BET method by N2 adsorption and desorption at 77 K. The specific surface areas of all samples are summarized in Table 1. It can be seen that all of the carbon and iron modified TiO2 samples showed similar specific surface areas value ranging from 158 m2/g to 161 m2/g, which were slightly higher than that of C–TiO2 sample. 3.2. EPR studies

where b is the full width half maximum (FWHM) of the 2u peak, K is a shape of factor of the particles (it equals to 0.89), u and l are the incident of angle and the wavelength of the X-rays, respectively.

EPR characterization of iron has been done for pure anatase TiO2, C–TiO2 and 0.57Fe/C–TiO2 samples. The spectra recorded at room temperature are shown in Fig. 4. With respect to the pure

Fig. 2. XRD patterns of samples with different amount of Fe—a: C–TiO2; b: 0.30Fe/C– TiO2; c: 0.50Fe/C–TiO2; d: 0.57Fe/C–TiO2; e: 0.70Fe/C–TiO2; f: 0.90Fe/C–TiO2.

Fig. 3. Raman spectra of TiO2 sample—a: 0.57Fe/C–TiO2 and b: C–TiO2.

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iron and carbon modified TiO2 extended to 500 nm. The band gap was 2.95 eV for 0.30Fe/C–TiO2, 2.88 eV for 0.57Fe/C–TiO2 and 2.76 eV for 0.90Fe/C–TiO2, decreased by increasing the iron contents. This result is consistent with the changes in the color of the samples. Besides the effect of C, the origin of this visible light absorption was also due to the formation of a dopant energy level within the band gap of TiO2 with iron doping. The electrontransition from the dopant energy level (the excitation of 3d electrons of Fe(III)) to the conduction band could effectively redshift the band edge absorption threshold. Additionally, a band centered at ca. 500 nm was particularly apparent for 0.90Fe/C–TiO2 samples. This could be ascribed to the d–d transition of Fe(III) (2T2g ! 2A2g, 2T1g) or the charge transfer transition between interacting iron ions (Fe3+ + Fe3+ ! Fe4+ + Fe2+) [11].

Fig. 4. EPR spectra of samples—a: pure TiO2; b: C–TiO2; c: 0.57Fe/C–TiO2.

anatase TiO2 and C–TiO2 samples, no intensive signal could be observed, suggested the presence of Ti(IV) cations. While the 0.57Fe/C–TiO2 sample showed very intense signal at g = 1.99 and very weak signal at g = 4.22. Signal at g = 1.99 could be attributed to Fe(III) substituted for Ti(IV) in the lattice of anatase TiO2 [40]. Another signal at g = 4.22 could be assigned to iron oxide dispersed on the surface of titania [42]. The analysis of EPR results proved that a large number of iron cations could be incorporated into the crystal lattice of anatase TiO2 and few were located on the surface of TiO2. 3.3. The UV–vis absorption spectroscopy The as-prepared C–TiO2 was light yellow powder. However, the color of samples containing iron changed from light yellow to dark yellow or light brown by increasing the iron contents, which had a profound effect on their opticals response in the visible wavelength range. Fig. 5(A) showed the UV–vis absorption spectra of C–TiO2 and carbon and iron modified TiO2 samples compared with commercial pure anatase TiO2. The band gap energies can be calculated by a plot (hna)1/2 versus photon energy (hn). The absorption coefficient a and indirect band gap Eg are related through the following equation [43] 1=2

ðahvÞ

/ hn  Eg

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(2)

where n is the frequency and h is Planck’s constant. The Tauc plot (ahn)1/2 versus hn, was shown in Fig. 5(B). The pure anatase TiO2 had the band gap energy of 3.26 eV. The band gap for C modified TiO2 sample was 3.05 eV, which was lower than that of pure anatase TiO2. That indicated carbon species could act as a photosensitizer, which benefited to extend the light absorption of C-doped TiO2 to visible light region [12]. With Fe(III) cations introduced into TiO2, the absorption curve of

3.4. The photoluminescence emission studies The photoluminescence emission (PL) is useful to disclose the efficiency of charge carrier trapping, immigration and transfer, and to understand the fate of electron–hole pairs in semiconductor particles [44]. The PL emission spectra of all samples are shown in Fig. 6. One peak around 565 nm was observed for these samples, which was ascribed to the charge transfer from Ti3+ to the oxygen anion in a TiO68 complex [45]. It can be seen that the positions of the peaks were similar, while PL intensities were quite different among these samples. It is known that the PL emission is the result of the recombination of excited electrons and holes, the lower PL intensity may indicate the lower recombination rate of electron– holes under light irradiation [46]. The PL intensity of C–TiO2 was the highest among all the samples, indicates the increase of recombination of electron and hole. The emission intensities were significantly weakened in carbon and iron modified TiO2, which implied that the recombination of charge carriers was effectively suppressed because Fe(III) cations could act as shallow traps in the lattice of titania. It was reported that Fe(III) cations can act as photogenerated hole trappers, the trapped holes by Fe(III) cations can migrate to the surface adsorbed hydroxyl ion to produce hydroxyl radicals[4,12]. Moreover, the lowest intensity in the 0.57Fe/C–TiO2 sample was observed, implied that the charge carriers were separated more effectively, which reasonably led to a higher photocatalytic activity since the photodegradation reactions were evoked by these charge carriers. 3.5. XPS analysis Fig. 7 shows the XPS spectra of C–TiO2 (A) and 0.57Fe/C–TiO2 (B) samples. XPS peaks displayed that the C–TiO2 sample contained only Ti, O, C elements and the 0.57Fe/C–TiO2 sample included Ti, O, C, Fe elements. Although adding nitric acid during the preparation,

Fig. 5. (A) UV–vis absorption spectra of some samples and (B) plots of (ahn)1/2 versus photon energy(hn) for some samples—a: pure TiO2; b: C–TiO2; c: 0.30Fe/C–TiO2; d: 0.57 Fe/C–TiO2; e: 0.90 Fe/C–TiO2.

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Fig. 6. Photoluminescence spectra of all samples—a: C–TiO2; b: 0.90Fe/C–TiO2; c: 0.30Fe/C–TiO2; d: 0.50Fe/C–TiO2; e: 0.70Fe/C–TiO2; f: 0.57Fe/C–TiO2.

no traces of nitrogen element were detected. Ti 2p and Fe 2p XPS spectra of the as-prepared samples are shown in Fig. 8. With respect to the XPS peaks of Ti 2p, the binding energies of Ti 2p3/2 and Ti 2p1/2 for C–TiO2 and 0.57Fe/C–TiO2 are at 459.1 eV and 464.9 eV, which agreed with Ti(IV) in titanium oxide [47]. Binding energy of Fe 2p3/2 and 2p1/2 at 710.7 eV, 722.7 eV for 0.57Fe/C–TiO2 sample suggests that Fe species are present as Fe(III) oxidation

states [12]. The Fe 2p XPS spectra showed extremely low contents of Fe on the surface of 0.57Fe/C–TiO2, which are in consistent with EPR results. O1s XPS spectra of C–TiO2 and 0.57Fe/C–TiO2 samples are shown in Fig. 9(A) and (B), respectively. With respect to O1s of C– TiO2 and 0.57Fe/C–TiO2 samples, the asymmetric peaks can be fitted with two peaks at different positions. The major one, center at about 530.0 eV, could be attributed to lattice oxygen in TiO2, in agreement with the literature proposal [48]. The minor one, located at 532.0 eV, was ascribed to adsorbed OH groups on the surface of TiO2 [49]. The fitting data are summarized in Table 2. It can be seen that the O proportion belonging to surface OH group on the surface of 0.57Fe/C–TiO2 samples is larger than that of C–TiO2 sample, which would benefit the adsorption of organic compound or capture the photogenerated holes and the formation of hydroxyl radical. C1s XPS spectra of C–TiO2 and 0.57Fe/C–TiO2 samples are shown in Fig. 10. As can be seen in Fig. 10(A), there were three peaks around 284.6 eV, 287.2 eV and 289.2 eV for C–TiO2 sample, which could be contributed to three states of carbon species. The lower binding energy at 284.6 eV was associated with the adventitious elemental carbon [16,50] or the carbon absorbed on the surface of TiO2 as a contaminant (contamination of organic residues on their surfaces). The other two peaks at 287.2 eV and at 289.2 eV suggested the existence of C–O and C5 5O, respectively, indicating the formation of carbonated species [15]. A peak around 281 eV resulting from Ti–C bond [51] was not observed, so we assumed that carbon does not substitute oxygen atom in the lattice

Fig. 7. XPS spectra of (A) C–TiO2 and (B) 0.57 Fe/C–TiO2.

Fig. 8. XPS spectra of (A) Ti 2p and (B) Fe 2p.

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Fig. 9. O1s XPS spectra of (A) C–TiO2 and (B) 0.57Fe/C–TiO2.

of anatase TiO2. Fig. 10(B) shows XPS peak of C1s for 0.57Fe/C–TiO2 samples. Different with C–TiO2, there were only two peaks around 284.6 eV and 288.7 eV, which could be assigned to the adventitious elemental carbon and the existence of C5 5O, respectively [15]. This result indicated that carbonate species and the electronic properties of the surface of the 0.57Fe/C–TiO2 have been changed after iron doping. Kisch and co-workers reported that pyrolysis of the alcohols employed in the sol–gel process led to carbonaceous species embedded in the TiO2 matrix and a highly condensed, coke-like carbonaceous species formed during calcination at 250 8C was responsible for visible light photosensitization. If these C-doped TiO2 samples were heated at 400 8C, the carbon content totally eliminated from the catalyst and the photocatalysts were found to become completely inactive under visible light [14]. Choi and coworkers also reported that C-doped TiO2 was synthesized through a sol–gel route using titanium but oxide as a precursor of both Ti and C and an incomplete calcinations at low temperature. They suggested carbon species as an interstitial dopant were incorporated into the lattice of TiO2, which created midgap energy levels in TiO2 with inducing visible light activity [22]. In our cases, after the sol–gel process, the samples were solvothermally treated at a temperature of 180 8C. Therefore it can be speculated that the carbonaceous species were embedded in the TiO2 matrix during the process of solvothermal by employing alkoxide precursor and alcohol; thus the content of carbon in the as-prepared TiO2 sample would be affected by the condition of preparation. The carbon

contents of all of samples determined by XPS results were showed in Table 1. It can be seen that the carbon content is kept ranging from 6% to 8% under the solvothermal temperature at 180 8C. When carbon modified TiO2 sample was heated at a high temperature of 400 8C, color of carbon modified TiO2 changed from light yellow to pure white and it exhibited poor photocatalytic activity under visible light irradiation, in agreement with the result of Kisch’s group [14]. 3.6. Photocatalytic activity Fig. 11(A) shows the photocatalytic degradation curves of AO7 over the pure anatase TiO2, C–TiO2 and carbon and iron modified TiO2 photocatalysts with different iron doping concentration under visible light irradiation. The photocatalytic degradation of AO7 with reaction time is first order as confirmed by the linear transforms of ln(C0/C)  t shown in Fig. 11(B), from which the apparent rate constants were obtained. The apparent rate constants obtained with various catalyst samples with different iron content are shown in Fig. 11(C). Fig. 11(A) shows that no degradation of AO7 takes place in the absence of photocatalyst. The degradation rate of AO7 on the pure anatase TiO2 under visible light irradiation is very low (10%), which can be attributed to the self-sensitization of AO7. Obviously, the photocatalytic activity of C–TiO2 is superior to that of pure TiO2 for the degradation of AO7. Besides the self-sensitization of AO7, the carbonate species on the surface of C–TiO2 cause the absorption edge extended to visible light range and thus play an important role in

Fig. 10. C1s XPS spectra of (A) C–TiO2 and (B) 0.57Fe/C–TiO2.

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Table 2 The XPS fitting data of O1s electron of C–TiO2 and 0.57Fe/C–TiO2 sample. Sample

C–TiO2 0.57Fe/C–TiO2

O (lattice)

O (O–H)

BE (eV)

Area (%)

BE (eV)

Area (%)

530.0 530.1

70.7 63.2

532.0 532.1

29.3 36.8

improving visible light photoactivity, although C–TiO2 has slightly higher surface areas than pure anatase TiO2, as compared to the C– TiO2 sample, the photodegradation ratio of AO7 with carbon and iron modified TiO2 samples increased significantly (as shown in Fig. 10(B)). Over the 0.57Fe/C–TiO2 sample, the degradation ratio of AO7 is the highest (as shown in Fig. 10(C)). When iron dopant concentration exceeded 0.57 mol%, however, the degradation ratio significantly decrease. The higher photocatalytic activity of carbon and iron comodified TiO2 here observed may be attributed to the following reasons. Firstly, appropriate amount of the doped iron (0.57 mol%) in TiO2 can effectively capture the photoinduced electrons and holes, which inhibited the combination of photoinduced carriers and improved the photocatalytic activity as confirmed by PL spectra. Secondly, 0.57Fe/C–TiO2 has more amount of surface OH group than C–TiO2 sample, which would be beneficial for the adsorption of organic compound, capturing the photogenerated hole and the formation of hydroxyl radical. Thirdly, the specific surface area of 0.57Fe/C–TiO2 sample was slightly larger than that of C–TiO2 which may favor the adsorption of organic compound as well as provide more possibly accessible active sites. While iron dopant content exceeds 0.57 mol%, Fe(III) becomes the recombination centers of the photoinduced electrons and holes, which is detrimental to photocatalytic reactions. Finally, the most important reason is that iron and carbon have synergistic effects on improving the photocatalytic activity under visible light irradiation. Here we proposed a possible mechanism for the synergistic effects of iron and carbon, which is illustrated in Scheme 1.

Scheme 1. Mechanism for the synergistic effects of iron and carbon.

Under visible light irradiation, on the one hand, interstitial carbon dopant could create intra-band-gap states close to the valence band edges, Fe(III) present in the substitutional positions into the lattice of TiO2 would introduce a dopant energy level below the conduction band of TiO2, both carbon and iron comodification leads to a narrower band gap than C-doped TiO2. This conduces to more absorption of carbon and iron co-modified TiO2 catalysts in the visible light region. Consequently, more photons from visible irradiation are utilized to generate photogenerated electrons and holes. On the other hand, C and Fe species in the co-modified TiO2 systems may improve the separation efficiency of photogenerated electrons and holes. Under visible light irradiation, Fe ions act as shallow electron-trapping centers (Eq. (3)), Subsequently, Fe2+ could be oxided to Fe3+ by transferring electrons to absorbed O2 on the surface of TiO2 (Eq. (4)) and a neighboring surface Ti4+ (Eq. (5)), which then lead to interfacial

Fig. 11. (A) AO7 degradation under visible light illumination for 5 h in the presence of C–TiO2 with various iron doping, pure anatase TiO2 and without photocatalyst; (B) ln(C/ C0) versus time for C–TiO2 with various iron doping concentration; (C) rate constants over C–TiO2 with various iron doping concentration.

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electron transfer (Eq. (6)) [4,12]. Fe3þ þ e ! Fe2þ

(3)

Fe2þ þ O2ðadsÞ ! Fe3þ þ O2 

(4)

Fe3þ þ Ti4þ ! Fe2þ þ Ti3þ

(5)

Ti3þ þ O2ðadsÞ ! Ti4þ þ O2 

(6)

Meanwhile, the carbon species could trap the part of photogenerated holes [22]. Thus, the recombination of photogenerated electrons and holes is suppressed. As a consequence, the quantum efficiency of the photocatalytic reaction catalyzed by co-modified TiO2 is improved. In a word, the synergistic effects of carbon and iron may not only enhance the utilization efficiency of solar energy due to C and Fe comodification narrowing the band gap, but also sufficiently promote the separation of photogenerated holes and electrons and then lead to high photodegradation of AO7 under the visible light irradiation. 4. Conclusions The carbon and iron modified TiO2 catalysts were successfully synthesized by the combination of sol–gel process with solvothermal treatment at low temperature. The C and Fe modified TiO2 photocatalyst showed high specific surface areas, small crystallite size, as well as more surface adsorbed water and hydroxyl groups, which contribute to their high photocatalytic activity for the degradation of acid orange 7 under visible irradiation. It was found that Fe(III) cations and carbonate species are successfully incorporated into the crystal lattice of TiO2. Both carbon species and iron ions comodification could lead to narrowing the band gap of TiO2. Fe(III) cations can help the separation of photogenerated electrons by trapping them temporarily and shallowly. The synergistic effects of carbon and iron may efficiently promote the separation of photogenerated holes and electrons, and are responsible for high photodegradation of AO7 under visible light irradiation. Acknowledgments This work has been supported by National Nature Science Foundation of China (20773039, 20977030); National Basic Research Program of China (973 Program, 2007CB613301, 2010CB732306); the Ministry of Science and Technology of China (2006AA06Z379, 2006DFA52710) and ‘‘The Zhuoyue Project’’ of East China University of Science and Technology. References [1] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C 1 (2000) 1–21. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [3] X.H. Wang, J.G. Li, H. Kamiyama, Y. Moriyoshi, T. Ishigaki, Wavelength-sensitive photocatalytic degradation of methyl orange in aqueous suspension over iron(III)doped TiO2 nanopowders under UV and visible light irradiation, J. Phys. Chem. B 110 (2006) 6804–6809. [4] W. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantumsized TiO2: correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679. [5] D. Liu, P.V. Kamat, K.G. Thomas, K.J. Thomas, S. Das, M.V. George, Picosecond dynamics of an IR sensitive squaraine dye. Role of singlet and triplet excited states in the photosensitization of TiO2 nanoclusters, J. Chem. Phys. 106 (1997) 6404–6411. [6] A. Kay, R. Humphry-Baker, M. Gra¨tzel, Artificial photosynthesis. 2. Investigations on the mechanism of photosensitization of nanocrystalline TiO2 solar cells by chlorophyll derivatives, J. Phys. Chem. 98 (1994) 952–959. [7] R. Asahi, T. Morikawa, T. Ohwakl, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [8] C. Burda, Y. Lou, X. Chen, A.C. Samia, J. Stout, J.L. Gole, Enhanced nitrogen doping in TiO2 nanoparticles, Nano Lett. 3 (2003) 1049–1051.

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