- Email: [email protected]
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres
Available online at www.sciencedirect.com
JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X
Journal of Environmental Sciences 2013, 25(10) 2150–2156
www.jesc.ac.cn
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres Xiaoxia Lin1, ∗, Degang Fu2 , Lingyun Hao1 , Zhen Ding3 1. School of Material Engineering, Jinling Institute of Technology, Nanjing 211169, China 2. State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China 3. Jiangsu Centers for Disease Control and Prevention, Nanjing 210009, China Received 23 January 2013; revised 26 April 2013; accepted 28 April 2013
Abstract C,N,S-tridoped TiO2 hollow spheres (labeled as C,N,S-THs) were synthesized using carbon spheres as template and C,N,S-tridoped TiO2 nanoparticles as building blocks. The structure and physicochemical properties of the catalysts were characterized by Xray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis diffuse reflectance spectrum (DRS), N2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS) and Photoluminescence emission spectroscopy (PL). The results showed that the hollow spheres had average diameter of about 200 nm and the shell thickness was about 20 nm. The tridoped TiO2 hollow spheres exhibited strong absorption in the visible-light region. C,N,S-tridoped could narrow the band gap of the THs by mixing the orbit O 2p with C 2p, N 2p and S 3p orbits and shift its optical response from ultraviolet (UV) to the visible-light region. PL analysis indicated that the electron-hole recombination rate of TiO2 hollow spheres had been effectively inhibited when doped with C, N and S elements. The photocatalytic activities of the samples were evaluated for the degradation of X-3B (Reactive Brilliant Red dye, C.I. Reactive Red 2) aqueous solution under visible-light (λ > 420 nm) irradiation. It was found that the C,N,S-tridoped TiO2 hollow spheres indicated higher photocatalytic activity than commercial P25 and the undoped counterpart photocatalyst. Key words: titania; C,N,S-tridoped; hollow spheres; visible-light; photocatalysis DOI: 10.1016/S1001-0742(13)60414-3
Introduction Titanium dioxide (TiO2 ) has been widely used in purification of water and air because of its chemical and biological inertness, strong oxidizing power, non-toxicity and long-term stability against photo and chemical corrosion (Li et al., 2011; Ghanbary et al., 2012). However, its commercial-scale application is hampered, due to its large band gap, TiO2 can only use ultraviolet (wavelength < 387 nm) as excitation source (Wang et al., 2011). Many researchers reported that doping TiO2 with nonmetal elements, such as carbon, sulfur, fluorine and nitrogen, could extend the absorption spectrum into the visible region (Wong et al., 2008; Periyat et al., 2008; Yu et al., 2002; Kang et al., 2008). More recently, the simultaneous doping of two or three kinds of nonmetal presents a promising strategy to further enhance the visible-light activity of TiO2 , because it could emerge a higher photocatalytic activity compared with single element doping into TiO2 . Zhou and Yu (2008) prepared C,N,S-tridoped * Corresponding author. E-mail: [email protected]
TiO2 powders with enhanced daylight-induced photocatalytic activity by calcining the mixture of TiO2 gel and thiourea at 500°C for 3 hr. Synthesized C,N,S-tridoped TiO2 nanoparticles with visible-light photocatalytic activity by biomolecule-controlled hydrothermal method (Wang et al., 2009). Xiao and Ouyang (2011) synthesized C,N,Stridoped TiO2 nanotubes via hydrothermal synthesis and post-treatment, they attributed to the high visible-light activity resulting from the balance between visible-light absorption and recombination of electron/hole pairs. Our previous research on synthesis of C,N,S-tridoped mesoporous TiO2 has also indicated that the catalyst exhibited excellent visible-light photocatalytic activity (Ao et al., 2009). On the other hand, the photocatalytic activity is tightly related to the structure of photocatalyst, such as particle size and surface area. Recently, hollow spheres structures have attracted enormous attention because of their high surface area, low bulk density, good surface permeability as well as large light-harvesting efficiencies (Subagio et al., 2010). Moreover, TiO2 hollow spheres can be separated
No. 10
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres
more easily compared to TiO2 nanoparticles for their relative large dimension. Therefore, if we combine visible responsive activity of C,N,S-tridoped TiO2 hollow structure, it would achieve higher visible photocatalytic activity. In the present work, C,N,S-tridoped TiO2 hollow spheres were synthesized by sol-gel process using carbon spheres as template. The photocatalytic activity of the asprepared C,N,S-THs was investigated under visible-light irradiation with X-3B (Reactive Brilliant Red dye, C.I. Reactive Red 2) as model pollutant. As expected, C,N,STHs exhibited higher visible photocatalytic activity than that of P25 and undoped THs.
1 Materials and methods 1.1 Preparation of carbon spheres The synthesized process of carbon spheres had been described in our previous work (Lin et al., 2011). According to the previous report, 9 g glucose was firstly dissolved in 70 mL water to form a clear solution. The solution was then sealed in a 100-mL Teflon-lined stainless autoclave and maintained at 180°C for 5 hr. The samples were then washed by ethanol and water for five cycles, respectively. The obtained carbon spheres were then dried at 80°C for 2 hr under vacuum. 1.2 Synthesis of C,N,S-tridoped TiO2 hollow spheres (labeled as C,N,S-THs) C,N,S-tridoped TiO2 nanoparticles was prepared by a modified sol-gel method. Thiourea was chosen as C, N and S precursor. Appropriate amount of thiourea (added to thiourea:TiO2 molar ratio of 0.5, 1 and 2) was dissolved into aqueous solution, whose acidity was adjusted with HNO3 to be 2.0. The 25 mL Ti(OBu)4 diluted with 8 mL (i-PrOH) was added dropwise into above acid aqueous solution. Then, the solution was kept under reflux condition at 75°C for 24 hr. Finally, C,N,S-tridoped TiO2 sol was obtained after n-butyl alcohol was removed from the solution in rotatory evaporator under vacuum. For preparation of C,N,S-THs, 0.3 g carbon spheres were added into 70 mL C,N,S-tridoped TiO2 sol prepared by the above mentioned method. Then, the solution was stirred rapidly at 75°C for 6 hr under vacuum condition. Finally the products were centrifuged, washed and redispersed in ethanol and water for three cycles. The corresponding samples were marked as C,N,S-THs-0.5, C,N,S-THs-1, and C,N,S-THs-2, respectively. In order to prepare C,N,S-THs, the TiO2 -carbon spheres composite particles were calcined at 500°C for 3 hr (ramped up at 5°C/min) to remove the carbon spheres cores. For comparison, undoped TiO2 hollow spheres (labeled as THs) were also prepared by the
2151
same way without adding thiourea. 1.3 Photocatalysts characterization The structure properties were determined by X-ray diffractometer (XRD, XD-3A, Shimadzu Corporation, Japan) using CuKα irradiation at 40 kV, 30 mA over the 2θ range 20–80◦ . The morphologies and size were observed by scanning electron micrographs (SEM, Sirion, FEI). The UV-Vis absorption spectra of the THs were observed with Shimadzu UV-8500 equipped with an integrating sphere. Nitrogen adsorption-desorption isotherms were collected at 77 K using an ASAP2020 instrument (BET and BJH models for specific surface area and porosity evaluation) for samples. Prior to measurement, all samples were degassed at 150°C for 5 hr. The binding energy was identified by X-ray photoelectron spectroscopy (XPS) with Al Kα radiation (ESCALB MK-II) with charge correction. XPS spectra were referenced to the C 1s peak (Eb = 284.8 eV) resulting from the adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. The photoluminescence (PL) spectra were measured with a Xe lamp as the excitation source at room temperature. 1.4 Photocatalytic reactions To investigate the photocatalytic activity of as-prepared C,N,S-THs, degradation experiments were studied under visible-light irradiation. A 250 W halogen lamp (Instrumental Corporation of Beijing Normal University) with a light filter cutting off the short wavelength below 420 nm was used as light source with an average irradiation intensity of 9 MW/cm2 at upper surface of the reactor. The distance between the light source and the reactor was 20 cm. All the photocatalytic experiments were performed at ambient temperature. In a typical experiment, 0.2 g catalyst was merged into the X-3B solution with a volume of 200 mL and an initial concentration of 50 mg/L. Prior to photoreaction, the solution was stirred in the dark for 1 hr to reach adsorption-desorption equilibrium. The concentration of X-3B was determined from the absorbance at the wavelength of 535 nm using UV-Vis spectrophotometer.
2 Results and discussion 2.1 Characterization of C,N,S-tridoped TiO2 hollow spheres Figure 1 shows the XRD patterns of THs and C,N,STHs phases formed at 500°C. The characteristic diffraction peaks at 25.4◦ , 37.9◦ , 48.2◦ , 54.7◦ , 62.8◦ , showing that the hollow spheres have formed anatase phase. The average crystal sizes of THs can be estimated by applying the Scherrer equation: D = kλ/βcosθ
(1)
where, D is the crystal size, λ is the wavelength of Xray irradiation, is the Scherrer constant (k = 0.9), θ is
Journal of Environmental Sciences 2013, 25(10) 2150–2156 / Xiaoxia Lin et al.
2152
Intensityθ(a.u.)
C,N,S-THs-2
C,N,S-THs-1 C,N,S-THs-0.5
THs
20
30
40
50 60 70 80 2θθ(degree) Fig. 1 XRD patterns of THs, C,N,S-THs-0.5, C,N,S-THs-1 and C,N,STHs-2.
the characteristic X-ray radiation (θ = 12.7◦ ) and β is the full-width-at-half-maximum of the (101) plane (in radians) (Xie and Yuan, 2003). The values are estimated to be 12.0, 11.9, 11.7, 11.2 nm for THs, C,N,S-THs-0.5, C,N,S-THs-1 and C,N,S-THs2, respectively. Therefore, the C,N,S-tridoped can slightly decrease average crystallite sizes of the samples. The SEM image of carbon spheres is depicted in Fig. 2a. It can be seen that diameter of carbon spheres ranges from 200 to 400 nm. Figure 2b shows SEM image of C,N,STHs-1 calcined at 500°C. The result shows that diameter of the hollow spheres is about 200 nm. From the broken sphere in the inset of Fig. 2b, we can clearly find that the hollow structure of TiO2 spheres has been formed and the thickness of the shell is about 20 nm. The UV-Vis spectra of the prepared samples are shown in Fig. 3. It can be seen that C, N and S doping can effectively shift the absorption spectra into the visible region. Yu et al. (2005) reported that modification of TiO2 by C, N and S could alter the crystal and electronic structures of TiO2 . The red shift was attributed to the fact that C,N,S-tridoped could narrow the band gap of the TiO2 by mixing the orbit O 2p with C 2p, N 2p and
S 3p orbits (Zhou and Yu, 2008). Additionally, further observation shows that the absorbance in the visible-light region increases with increasing molar ratio of thiourea to TiO2 . This may be with higher thiourea, more C, N and S could be adsorbed on THs or incorporated into the lattice of THs, resulting in large band gap narrowing. All the as-prepared TiO2 samples have similar nitrogen adsorption-desorption isotherms and pore size distribution curves. Figure 4 only presents the typical plot of nitrogen absorption-desorption isotherm and pore size distribution of C,N,S-THs-1. Figure 4a indicates that the samples show an isotherm of type IV with hysteresis loop (Tarafdar et al., 2006), which indicates the existence of mesoporous structure of THs. Figure 4b shows the sample has bimodal pore size distribution. The sample contains small mesopores (ca. 7.9 nm) and larger mesopores with a maximum pore diameter of ca. 17.7 nm. The smaller mesopores are related to finer intra-aggregated pores formed between primary particles, and the larger are associated with interaggregated pores produced by inter-aggregated secondary constructs. This bimodal mesopores size distribution is beneficial to light-harvesting and mass transport (Yu et al., 2007; Ao et al., 2008). Table 1 summarizes the results of different samples calculated based on N2 adsorptiondesorption isotherms. It shows that the BET and pore volume increase, with increasing molar ratio of thiourea to TiO2 . This can be ascribed to the decreasing of the crystallite size in accordance with XRD analysis. The high-resolution XPS spectrum of C 1s is shown in Fig. 5a. Three peaks are observed at the binding energies of 284.8, 286.4, and 288.7 eV. For XPS measurements, samples for XPS measurement are coated on carbon tape attached to the sample holder. Free carbon could not exist Table 1
Surface area measurement of the different samples
Sample
BET surface area (m2 /g)
Pore volume (cm3 /g)
THs C,N,S-THs-0.5 C,N,S-THs-1 C,N,S-THs-2
291.7 307.3 329.6 345.9
0.279 0.282 0.293 0.311
b
a
Fig. 2
Vol. 25
SEM images of carbon spheres (a) and C,N,S-THs-1 (b).
No. 10
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres
Intensity (a.u.)
Line a: THs Line b: C,N,S-THs-0.5 Line c: C,N,S-THs-1 Line d: C,N,S-THs-2
b
a
300
d c
400
500 600 700 Wavelength (nm) Fig. 3 UV-Vis spectra of THs, C,N,S-THs-0.5, C,N,S-THs-1 and C,N,STHs-2.
under high calcinations temperature (500°C), so it is confirmed that the peak of 284.8 eV came from carbon tape. The peaks at 286.4 and 288.7 eV indicate the existence of C–O bonds, and C element might substitute for some of the lattice Ti atoms to form a Ti–O–C structure, which can induce the narrowing of the band gap of the doped TiO2 (Chen et al., 2007; Wang et al., 2009). Figure 5b shows the high-resolution XPS spectra of the N 1s region. A pair of N 1s peaks is observed: one peak at around 396.0 eV, which confirms that N atoms incorporate into TiO2 , and may
2153
substitute the sites of oxygen atoms; the other at 399.9 eV may account for the presence of oxidized state of N or C– N bonds (Ao et al., 2009). Asahi et al. (2001) and Diwald et al. (2004) confirmed that nitrogen atoms undoubtedly incorporated into TiO2 and substituted the sites of oxygen atoms. The high-resolution S 2p XPS spectrum is shown in Fig. 5c. It can be seen that the strong peak at 168.8 eV can be assigned to S6+ (SO4 2− ) states, according to the previous studies (Periyat et al., 2008; Sayago et al., 2001). These SO4 2− ions can form S=O and O–S–O bonds on the surface of TiO2 , creating unbalanced charge on Ti and vacancies/defects in the TiO2 network (Jung and Grange, 2000; Wang et al., 2008). The peak around 160–163 eV corresponding to the Ti–S bond is important due to the fact that sulfur atoms replaced oxygen atoms in the TiO2 lattice (Hebenstreit et al., 2001). Photoluminescence (PL) emission spectra have been widely used to investigate the efficiency of charge carrier trapping, immigration, and transfer to understand the fate of electron-hole pairs in semiconductor particles (Xu et al., 2008). Figure 6 shows the PL spectra of C,N,S-THs and THs. The PL intensity of these samples increase as follows: C,N,S-THs-1 < C,N,S-THs-2 < C,N,S-THs-0.5 < THs. This indicates that TiO2 incorporating with appropriate amount of C, N, S may decline the recombination rate of photogenerated electrons and holes in TiO2 . The lower PL intensity means the lower recombination rate of electronb
3
Pore volume (cm3/g)
Adsorbed volume (cm /g)
a
0.4 0.6 0.8 Relative pressure (P/P0)
1.0
20 30 Pore diameter (nm) Fig. 4 Nitrogen adsorption-desorption isotherm (a) and pore size distribution (b) of C,N,S-THs-1. 284.8 eV
C1s
286.4 eV
b
0
10
399.9 eV
40
168.8 eV S2p
c
N1s
396.0 eV
Intensity (a.u.)
Intensity (a.u.)
a
0.2
Intensity (a.u.)
0.0
162.1 eV
288.7 eV
280
285 290 Binding energy (eV)
295 392 394 396 398 400 402 404 406 160 Binding energy (eV)
165 170 Binding energy (eV) Fig. 5 High-resolution XPS spectra of C 1s region (a), N 1s region (b) and S 2p region (c) of the C,N,S-THs-1.
175
Journal of Environmental Sciences 2013, 25(10) 2150–2156 / Xiaoxia Lin et al.
2154
Vol. 25
Table 2 Degradation parameter of X-3B by different samples
Line a: THs Line b: C,N,S-THs-0.5 Line c: C,N,S-THs-2 Line d: C,N,S-THs-1
Intensity (a.u.)
a
Sample
Degradation (%)
P25 THs C,N,S-THs-0.5 C,N,S-THs-1 C,N,S-THs-2
b
10−4
9.89 × 0.00422 0.0115 0.0152 0.0119
R 0.998 0.993 0.987 0.990 0.988
c d
500
600 650 700 Wavelength (nm) Fig. 6 Photoluminescence spectra of THs, C,N,S-THs-0.5, C,N,S-THs-1 and C,N,S-THs-2.
550
hole pairs and higher photocatalytic activity (Wu et al., 2009). 2.2 Photocatalytic activity The photocatalytic activities of C,N,S-THs were tested by degradation of X-3B at room temperature under visiblelight irradiation and results are shown in Fig. 7. For comparison, the activities of THs and P25 were also evaluated under the identical conditions. The blank experiment without catalysts (self-photosensitized process) indicates that the photolysis can be neglected as it is about 2.2% after 120 min irradiation. From Fig. 7, we can see that the degradation ratio of X-3B by C,N,S-THs is much higher than P25 and pure THs, indicating the C,N,S-THs is active under visible-light. The apparent firstorder kinetic equation (ln(C0 /C) = kapp t) is used to fit experiment data, where, kapp is apparent rate constant, C is the solution-phase concentration of X-3B, and C0 is the initial concentration at t = 0 (Ao et al., 2009). The obtained apparent rate constant kapp are listed in Table 2. It can be seen that the C,N,S-THs-1 has the highest photocatalytic
1.0 -
activity. The higher photocatalytic activity of C,N,S-THs can be attributed to the following factors. One possible reason might be that the high surface area and porous structure of the C,N,S-THs may play a role in the photocatalytic activity of the prepared samples. Large specific surface area can provide more surface active sites for the adsorption of contamination molecules. Furthermore, the hollow spherical structures allow multireflections of irradiation light within their interior cavities, enhancing their lightharvesting ability. The surface area values of C,N,S-THs-1 is higher than THs and lower than C,N,S-THs-2, but the activity of the C,N,S-THs-1 is the highest, which indicates surface area should not be the most important factor for the photocatalytic activity (Wang et al., 2009). It has been reported that electron-hole recombination on the particle surface could be enhanced with increasing surface area (Liu et al., 2009; Joo et al., 2010). The second reason may be co-doped with C, N and S can evidently narrow the band gap of THs and increase the absorption in the visible-light region. More electrons and holes can be generated and participate in the photocatalytic redox reactions because the prepared catalyst can be activated by visible-light. The sample C,N,S-THs-1 possesses the highest photocatalytic activity, when the C, N and S dopant is higher than this level, the photocatalytic activity declines instead. This phenomenon indicates that too much dopant would decrease the distance between trapping sites in a photocatalyst particle, and increase the recombination rate to decrease the photocatalytic activity (Cong et al., 2.0
a
b 1.6 -
0.9 -
P25 THs C,N,S-THs-0.5 C,N,S-THs-1 C,N,S-THs-2
0.8 1.2 ln(C/C0)
0.7 C/C0
12.1 37.9 74.4 83.1 75.6
kapp (min−1 )
0.6 0.4 0.3 0.2 0.1 00
Self-degradation of X-3B P25 THs C,N,S-THs-0.5 C,N,S-THs-1 C,N,S-THs-2
0.8 0.4 0.0
40 60 80 100 120 Irradiation time (min) Fig. 7 Photocatalytic activity of different samples (a) and variations in ln(C 0 /C) as a function of irradiation time and linear fits of different samples (b) C0 and C represent the initial concentration of X-3B and concentration at time t, respectively. 20
40
60 80 Irradiation time (min)
100
120
0
20
No. 10
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres
2007), which is consistent with the PL results. Besides the above reason, another possible reason is that the codoped TiO2 consists of several phases (undoped and doped TiO2 , anatase and rutile). As we all known, the composite of two kinds of semiconductors or two phases of the same semiconductor are in direct contact, the recombination of photo-generated electrons and holes will be reduced thus enhances photocatalytic activity (Zhou and Yu, 2008).
3 Conclusions In this article, we prepared C,N,S-tridoped TiO2 hollow spheres using carbon spheres as template. C,N,S-tridoped TiO2 hollow spheres exhibit obvious absorption in visiblelight region and cause a red-shift of adsorption spectrum for TiO2 hollow spheres. The photocatalytic activities of as-prepared TiO2 hollow spheres were investigated by degradation of X-3B under visible-light irradiation. It was found that C,N,S-tridoped TiO2 hollow spheres showed significantly higher photocatalytic activity than P25 and undoped TiO2 hollow spheres. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51172043), the Doctor Research Starting Fund of Jinling Institute of Technology (No. JITB-201307), and the Social Developing Program of Jiangsu Province (No. BE2011797).
References Ao Y H, Xu J J, Fu D G, Yuan C W, 2008. A simple method for the preparation of titania hollow sphere. Catalysis Communications, 9(15): 2574–2577. Ao Y H, Xu J J, Fu D G, Yuan C W, 2009. Synthesis of C,N,S-tridoped mesoporous titania with enhanced visible light-induced photocatalytic activity. Microporous and Mesoporous Materials, 122(1-3): 1–6. Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y, 2001. Visiblelight photocatalysis in nitrogen-doped titanium oxides. Science, 293(5528): 269–271. Chen D M, Jiang Z Y, Geng J Q, Wang Q, Yang D, 2007. Carbon and nitrogen co-doped TiO2 with enhanced visiblelight photocatalytic activity. Industrial and Engineering Chemistry Research, 46(9): 2741–2746. Cong Y, Zhang J L, Chen F, Anpo M, He D N, 2007. Preparation, photocatalytic activity, and mechanism of Nano-TiO2 codoped with nitrogen and iron(III). Journal of Physical Chemistry C, 111(28): 10618–10623. Diwald O, Thompson T L, Zubkov T, Goralski E G, Walck S D, Yates J T Jr, 2004. Photochemical activity of nitrogendoped rutile TiO2 (111) in visible light. Journal of Physical Chemistry B, 108(19): 6004–6008. Ghanbary F, Modirshahla N, Khosravi M, Behnajady M A, 2012. Synthesis of TiO2 nanoparticles in different thermal conditions and modeling its photocatalytic activity with artificial neural network. Journal of Environmental Sciences, 24(4):
2155
750–756. Hebenstreit E L D, Hebenstreit W, Geisler H, Thornburg S N, Ventrice C A Jr, Hite D A et al., 2001. Sulfur on TiO2 (110) studied with resonant photoemission. Physical Review B, 64(11): 115418. Joo J Y, Shim J M, Seo H J, Jung N, Wiesner U, Lee J et al., 2010. Enhanced photocatalytic activity of highly crystallized and ordered mesoporous titanium oxide measured by silicon resonators. Analytical Chemistry, 82(7): 3023–3037. Jung S M, Grange P, 2000. Characterization and reactivity of pure TiO2 -SO4 2− -SCR catalyst: influence of SO4 2− content. Catalysis Today, 59(3-4): 305–312. Kang I C, Zhang Q W, Yin S, Sato T, Saito F, 2008. Novel method for preparation of high visible active N-doped TiO2 photocatalyst with its grinding in solvent. Applied Catalysis B: Environmental, 84(3-4): 570–576. Li X H, Zhang H D, Zheng X X, Yin Z Y, Wei L, 2011. Visible light responsive N-F-codoped TiO2 photocatalysts for the degradation of 4-chlorophenol. Journal of Environmental Sciences, 23(11): 1919–1924. Lin X X, Rong F, Ji X, Fu D G, 2011. Visible light photocatalytic activity and photoelectrochemical property of Fe-doped TiO2 hollow spheres by sol-gel method. Journal of Sol-Gel Science and Technology, 59(2): 283–289. Liu G, Wang X W, Wang L Z, Chen Z G, Li F, Lu G Q et al., 2009. Drastically enhanced photocatalytic activity in nitrogen doped mesoporous TiO2 with abundant surface states. Journal of Colloid and Interface Science, 334(2): 171–175. Periyat P, Pillai S C, McCormack D E, Colreavy J, Hinder S J, 2008. Improved high-temperature stability and sun-lightdriven photocatalytic activity of sulfur-doped anatase TiO2 . Journal of Physical Chemistry C, 112(20): 7644–7652. Sayago D I, Serrano P, B¨ohme O, Goldoni A, Paolucci G, Rom´an E et al., 2001. Adsorption and desorption of SO2 on the TiO2 (110)-(1×1) surface: A photoemission study. Physical Review B, 64(20): 205402. Subagio D P, Srinivasan M, Lim M, Lim T T, 2010. Photocatalytic degradation of bisphenol-A by nitrogen-doped TiO2 hollow sphere in a vis-LED photoreactor. Applied Catalysis B: Environmental, 95(3-4): 414–422. Tarafdar A, Biswas S, Pramanik N K, Pramanik P, 2006. Synthesis of mesoporous chromium phosphate through an unconventional sol-gel route. Microporous and Mesoporous Materials, 89(1-3): 204–208. Wang H Q, Wu Z B, Liu Y, 2009. A simple two-step template approach for preparing carbon-doped mesoporous TiO2 hollow microspheres. Journal of Physical Chemistry C, 113(30): 13317–13324. Wang P H, Yap P S, Lim T T, 2011. C-N-S tridoped TiO2 for photocatalytic degradation of tetracycline under visiblelight irradiation. Applied Catalysis A: General, 39(1-2): 252–261. Wang Y W, Huang Y, Ho W K, Zhang L Z, Zou Z G, Lee S C, 2009. Biomolecule-controleed hydrothermal synthesis of C-N-S-tridoped TiO2 nanocrystalline photocatalysts for NO removal under simulated solar light irradiation. Journal of Hazardous Materials, 169(1-3): 77–87. Wang Y, Wang Y, Meng Y L, Ding H M, Shan Y K, Zhao X et al., 2008. A highly efficient visible-light-activated
2156
Journal of Environmental Sciences 2013, 25(10) 2150–2156 / Xiaoxia Lin et al.
photocatalyst based on bismuth- and sulfur-codoped TiO2 . Journal of Physical Chemistry C, 112(17): 6620–6626. Wong M S, Hsu S W, Rao K K, Kumar C P, 2008. Influence of crystallinity and carbon content on visible light photocatalysis of carbon doped titania thin films. Journal of Molecular Catalysis A: Chemical, 279(1): 20–26. Wu Y M, Liu H B, Zhang J L, Chen F, 2009. Enhanced photocatalytic activity of nitrogen-doped titania by deposited with gold. Journal of Physical Chemistry C, 113(33): 14689– 14695. Xiao Q, Ouyang L L, 2011. Photocatalytic photodegradation of xanthate over C, N, S-tridoped TiO2 nanotubes under visible light irradiation. Journal of Physics and Chemistry of Solids, 72(1): 39–44. Xie Y B, Yuan C W, 2003. Visible-light responsive cerium ion modified titania sol and nanocrystallites for X-3B dye photodegradation. Applied Catalysis B: Environmental, 46(2): 251–259. Xu J J, Ao Y H, Fu D G, Yuan C W, 2008. Low-temperature
Vol. 25
preparation of F-doped TiO2 film and its photocatalytic activity under solar light. Applied Surface Science, 254(10): 3033–3038. Yu J C, Ho W K, Yu J G, Yip H, Wong P K, Zhao J C, 2005. Efficient visible-light-induced photocatalytic disinfection on sulfur-doped nanocrystalline titania. Environmental Science and Technology, 39(4): 1175–1179. Yu J C, Yu J G, Ho W, Jiang Z T, Zhao L Z, 2002. Effect of F-doping on the photocatalytic activity and microstructures of nanocrystalline TiO2 powders. Chemistry of Materials, 14(9): 3808–3816. Yu J G, Liu S W, Yu H G, 2007. Microstructures and photoactivity of mesoporous anatase hollow microspheres fabricated by fluoride-mediated self-transformation. Journal of Catalysis, 249(1): 59–66. Zhou M H, Yu J G, 2008. Preparation and enhanced daylightinduced photocatalytic activity of C,N,S-tridoped titanium dioxide powders. Journal of Hazardous Materials, 152(3): 1229–1236.
JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X
Journal of Environmental Sciences 2013, 25(10) 2150–2156
www.jesc.ac.cn
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres Xiaoxia Lin1, ∗, Degang Fu2 , Lingyun Hao1 , Zhen Ding3 1. School of Material Engineering, Jinling Institute of Technology, Nanjing 211169, China 2. State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China 3. Jiangsu Centers for Disease Control and Prevention, Nanjing 210009, China Received 23 January 2013; revised 26 April 2013; accepted 28 April 2013
Abstract C,N,S-tridoped TiO2 hollow spheres (labeled as C,N,S-THs) were synthesized using carbon spheres as template and C,N,S-tridoped TiO2 nanoparticles as building blocks. The structure and physicochemical properties of the catalysts were characterized by Xray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis diffuse reflectance spectrum (DRS), N2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS) and Photoluminescence emission spectroscopy (PL). The results showed that the hollow spheres had average diameter of about 200 nm and the shell thickness was about 20 nm. The tridoped TiO2 hollow spheres exhibited strong absorption in the visible-light region. C,N,S-tridoped could narrow the band gap of the THs by mixing the orbit O 2p with C 2p, N 2p and S 3p orbits and shift its optical response from ultraviolet (UV) to the visible-light region. PL analysis indicated that the electron-hole recombination rate of TiO2 hollow spheres had been effectively inhibited when doped with C, N and S elements. The photocatalytic activities of the samples were evaluated for the degradation of X-3B (Reactive Brilliant Red dye, C.I. Reactive Red 2) aqueous solution under visible-light (λ > 420 nm) irradiation. It was found that the C,N,S-tridoped TiO2 hollow spheres indicated higher photocatalytic activity than commercial P25 and the undoped counterpart photocatalyst. Key words: titania; C,N,S-tridoped; hollow spheres; visible-light; photocatalysis DOI: 10.1016/S1001-0742(13)60414-3
Introduction Titanium dioxide (TiO2 ) has been widely used in purification of water and air because of its chemical and biological inertness, strong oxidizing power, non-toxicity and long-term stability against photo and chemical corrosion (Li et al., 2011; Ghanbary et al., 2012). However, its commercial-scale application is hampered, due to its large band gap, TiO2 can only use ultraviolet (wavelength < 387 nm) as excitation source (Wang et al., 2011). Many researchers reported that doping TiO2 with nonmetal elements, such as carbon, sulfur, fluorine and nitrogen, could extend the absorption spectrum into the visible region (Wong et al., 2008; Periyat et al., 2008; Yu et al., 2002; Kang et al., 2008). More recently, the simultaneous doping of two or three kinds of nonmetal presents a promising strategy to further enhance the visible-light activity of TiO2 , because it could emerge a higher photocatalytic activity compared with single element doping into TiO2 . Zhou and Yu (2008) prepared C,N,S-tridoped * Corresponding author. E-mail: [email protected]
TiO2 powders with enhanced daylight-induced photocatalytic activity by calcining the mixture of TiO2 gel and thiourea at 500°C for 3 hr. Synthesized C,N,S-tridoped TiO2 nanoparticles with visible-light photocatalytic activity by biomolecule-controlled hydrothermal method (Wang et al., 2009). Xiao and Ouyang (2011) synthesized C,N,Stridoped TiO2 nanotubes via hydrothermal synthesis and post-treatment, they attributed to the high visible-light activity resulting from the balance between visible-light absorption and recombination of electron/hole pairs. Our previous research on synthesis of C,N,S-tridoped mesoporous TiO2 has also indicated that the catalyst exhibited excellent visible-light photocatalytic activity (Ao et al., 2009). On the other hand, the photocatalytic activity is tightly related to the structure of photocatalyst, such as particle size and surface area. Recently, hollow spheres structures have attracted enormous attention because of their high surface area, low bulk density, good surface permeability as well as large light-harvesting efficiencies (Subagio et al., 2010). Moreover, TiO2 hollow spheres can be separated
No. 10
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres
more easily compared to TiO2 nanoparticles for their relative large dimension. Therefore, if we combine visible responsive activity of C,N,S-tridoped TiO2 hollow structure, it would achieve higher visible photocatalytic activity. In the present work, C,N,S-tridoped TiO2 hollow spheres were synthesized by sol-gel process using carbon spheres as template. The photocatalytic activity of the asprepared C,N,S-THs was investigated under visible-light irradiation with X-3B (Reactive Brilliant Red dye, C.I. Reactive Red 2) as model pollutant. As expected, C,N,STHs exhibited higher visible photocatalytic activity than that of P25 and undoped THs.
1 Materials and methods 1.1 Preparation of carbon spheres The synthesized process of carbon spheres had been described in our previous work (Lin et al., 2011). According to the previous report, 9 g glucose was firstly dissolved in 70 mL water to form a clear solution. The solution was then sealed in a 100-mL Teflon-lined stainless autoclave and maintained at 180°C for 5 hr. The samples were then washed by ethanol and water for five cycles, respectively. The obtained carbon spheres were then dried at 80°C for 2 hr under vacuum. 1.2 Synthesis of C,N,S-tridoped TiO2 hollow spheres (labeled as C,N,S-THs) C,N,S-tridoped TiO2 nanoparticles was prepared by a modified sol-gel method. Thiourea was chosen as C, N and S precursor. Appropriate amount of thiourea (added to thiourea:TiO2 molar ratio of 0.5, 1 and 2) was dissolved into aqueous solution, whose acidity was adjusted with HNO3 to be 2.0. The 25 mL Ti(OBu)4 diluted with 8 mL (i-PrOH) was added dropwise into above acid aqueous solution. Then, the solution was kept under reflux condition at 75°C for 24 hr. Finally, C,N,S-tridoped TiO2 sol was obtained after n-butyl alcohol was removed from the solution in rotatory evaporator under vacuum. For preparation of C,N,S-THs, 0.3 g carbon spheres were added into 70 mL C,N,S-tridoped TiO2 sol prepared by the above mentioned method. Then, the solution was stirred rapidly at 75°C for 6 hr under vacuum condition. Finally the products were centrifuged, washed and redispersed in ethanol and water for three cycles. The corresponding samples were marked as C,N,S-THs-0.5, C,N,S-THs-1, and C,N,S-THs-2, respectively. In order to prepare C,N,S-THs, the TiO2 -carbon spheres composite particles were calcined at 500°C for 3 hr (ramped up at 5°C/min) to remove the carbon spheres cores. For comparison, undoped TiO2 hollow spheres (labeled as THs) were also prepared by the
2151
same way without adding thiourea. 1.3 Photocatalysts characterization The structure properties were determined by X-ray diffractometer (XRD, XD-3A, Shimadzu Corporation, Japan) using CuKα irradiation at 40 kV, 30 mA over the 2θ range 20–80◦ . The morphologies and size were observed by scanning electron micrographs (SEM, Sirion, FEI). The UV-Vis absorption spectra of the THs were observed with Shimadzu UV-8500 equipped with an integrating sphere. Nitrogen adsorption-desorption isotherms were collected at 77 K using an ASAP2020 instrument (BET and BJH models for specific surface area and porosity evaluation) for samples. Prior to measurement, all samples were degassed at 150°C for 5 hr. The binding energy was identified by X-ray photoelectron spectroscopy (XPS) with Al Kα radiation (ESCALB MK-II) with charge correction. XPS spectra were referenced to the C 1s peak (Eb = 284.8 eV) resulting from the adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. The photoluminescence (PL) spectra were measured with a Xe lamp as the excitation source at room temperature. 1.4 Photocatalytic reactions To investigate the photocatalytic activity of as-prepared C,N,S-THs, degradation experiments were studied under visible-light irradiation. A 250 W halogen lamp (Instrumental Corporation of Beijing Normal University) with a light filter cutting off the short wavelength below 420 nm was used as light source with an average irradiation intensity of 9 MW/cm2 at upper surface of the reactor. The distance between the light source and the reactor was 20 cm. All the photocatalytic experiments were performed at ambient temperature. In a typical experiment, 0.2 g catalyst was merged into the X-3B solution with a volume of 200 mL and an initial concentration of 50 mg/L. Prior to photoreaction, the solution was stirred in the dark for 1 hr to reach adsorption-desorption equilibrium. The concentration of X-3B was determined from the absorbance at the wavelength of 535 nm using UV-Vis spectrophotometer.
2 Results and discussion 2.1 Characterization of C,N,S-tridoped TiO2 hollow spheres Figure 1 shows the XRD patterns of THs and C,N,STHs phases formed at 500°C. The characteristic diffraction peaks at 25.4◦ , 37.9◦ , 48.2◦ , 54.7◦ , 62.8◦ , showing that the hollow spheres have formed anatase phase. The average crystal sizes of THs can be estimated by applying the Scherrer equation: D = kλ/βcosθ
(1)
where, D is the crystal size, λ is the wavelength of Xray irradiation, is the Scherrer constant (k = 0.9), θ is
Journal of Environmental Sciences 2013, 25(10) 2150–2156 / Xiaoxia Lin et al.
2152
Intensityθ(a.u.)
C,N,S-THs-2
C,N,S-THs-1 C,N,S-THs-0.5
THs
20
30
40
50 60 70 80 2θθ(degree) Fig. 1 XRD patterns of THs, C,N,S-THs-0.5, C,N,S-THs-1 and C,N,STHs-2.
the characteristic X-ray radiation (θ = 12.7◦ ) and β is the full-width-at-half-maximum of the (101) plane (in radians) (Xie and Yuan, 2003). The values are estimated to be 12.0, 11.9, 11.7, 11.2 nm for THs, C,N,S-THs-0.5, C,N,S-THs-1 and C,N,S-THs2, respectively. Therefore, the C,N,S-tridoped can slightly decrease average crystallite sizes of the samples. The SEM image of carbon spheres is depicted in Fig. 2a. It can be seen that diameter of carbon spheres ranges from 200 to 400 nm. Figure 2b shows SEM image of C,N,STHs-1 calcined at 500°C. The result shows that diameter of the hollow spheres is about 200 nm. From the broken sphere in the inset of Fig. 2b, we can clearly find that the hollow structure of TiO2 spheres has been formed and the thickness of the shell is about 20 nm. The UV-Vis spectra of the prepared samples are shown in Fig. 3. It can be seen that C, N and S doping can effectively shift the absorption spectra into the visible region. Yu et al. (2005) reported that modification of TiO2 by C, N and S could alter the crystal and electronic structures of TiO2 . The red shift was attributed to the fact that C,N,S-tridoped could narrow the band gap of the TiO2 by mixing the orbit O 2p with C 2p, N 2p and
S 3p orbits (Zhou and Yu, 2008). Additionally, further observation shows that the absorbance in the visible-light region increases with increasing molar ratio of thiourea to TiO2 . This may be with higher thiourea, more C, N and S could be adsorbed on THs or incorporated into the lattice of THs, resulting in large band gap narrowing. All the as-prepared TiO2 samples have similar nitrogen adsorption-desorption isotherms and pore size distribution curves. Figure 4 only presents the typical plot of nitrogen absorption-desorption isotherm and pore size distribution of C,N,S-THs-1. Figure 4a indicates that the samples show an isotherm of type IV with hysteresis loop (Tarafdar et al., 2006), which indicates the existence of mesoporous structure of THs. Figure 4b shows the sample has bimodal pore size distribution. The sample contains small mesopores (ca. 7.9 nm) and larger mesopores with a maximum pore diameter of ca. 17.7 nm. The smaller mesopores are related to finer intra-aggregated pores formed between primary particles, and the larger are associated with interaggregated pores produced by inter-aggregated secondary constructs. This bimodal mesopores size distribution is beneficial to light-harvesting and mass transport (Yu et al., 2007; Ao et al., 2008). Table 1 summarizes the results of different samples calculated based on N2 adsorptiondesorption isotherms. It shows that the BET and pore volume increase, with increasing molar ratio of thiourea to TiO2 . This can be ascribed to the decreasing of the crystallite size in accordance with XRD analysis. The high-resolution XPS spectrum of C 1s is shown in Fig. 5a. Three peaks are observed at the binding energies of 284.8, 286.4, and 288.7 eV. For XPS measurements, samples for XPS measurement are coated on carbon tape attached to the sample holder. Free carbon could not exist Table 1
Surface area measurement of the different samples
Sample
BET surface area (m2 /g)
Pore volume (cm3 /g)
THs C,N,S-THs-0.5 C,N,S-THs-1 C,N,S-THs-2
291.7 307.3 329.6 345.9
0.279 0.282 0.293 0.311
b
a
Fig. 2
Vol. 25
SEM images of carbon spheres (a) and C,N,S-THs-1 (b).
No. 10
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres
Intensity (a.u.)
Line a: THs Line b: C,N,S-THs-0.5 Line c: C,N,S-THs-1 Line d: C,N,S-THs-2
b
a
300
d c
400
500 600 700 Wavelength (nm) Fig. 3 UV-Vis spectra of THs, C,N,S-THs-0.5, C,N,S-THs-1 and C,N,STHs-2.
under high calcinations temperature (500°C), so it is confirmed that the peak of 284.8 eV came from carbon tape. The peaks at 286.4 and 288.7 eV indicate the existence of C–O bonds, and C element might substitute for some of the lattice Ti atoms to form a Ti–O–C structure, which can induce the narrowing of the band gap of the doped TiO2 (Chen et al., 2007; Wang et al., 2009). Figure 5b shows the high-resolution XPS spectra of the N 1s region. A pair of N 1s peaks is observed: one peak at around 396.0 eV, which confirms that N atoms incorporate into TiO2 , and may
2153
substitute the sites of oxygen atoms; the other at 399.9 eV may account for the presence of oxidized state of N or C– N bonds (Ao et al., 2009). Asahi et al. (2001) and Diwald et al. (2004) confirmed that nitrogen atoms undoubtedly incorporated into TiO2 and substituted the sites of oxygen atoms. The high-resolution S 2p XPS spectrum is shown in Fig. 5c. It can be seen that the strong peak at 168.8 eV can be assigned to S6+ (SO4 2− ) states, according to the previous studies (Periyat et al., 2008; Sayago et al., 2001). These SO4 2− ions can form S=O and O–S–O bonds on the surface of TiO2 , creating unbalanced charge on Ti and vacancies/defects in the TiO2 network (Jung and Grange, 2000; Wang et al., 2008). The peak around 160–163 eV corresponding to the Ti–S bond is important due to the fact that sulfur atoms replaced oxygen atoms in the TiO2 lattice (Hebenstreit et al., 2001). Photoluminescence (PL) emission spectra have been widely used to investigate the efficiency of charge carrier trapping, immigration, and transfer to understand the fate of electron-hole pairs in semiconductor particles (Xu et al., 2008). Figure 6 shows the PL spectra of C,N,S-THs and THs. The PL intensity of these samples increase as follows: C,N,S-THs-1 < C,N,S-THs-2 < C,N,S-THs-0.5 < THs. This indicates that TiO2 incorporating with appropriate amount of C, N, S may decline the recombination rate of photogenerated electrons and holes in TiO2 . The lower PL intensity means the lower recombination rate of electronb
3
Pore volume (cm3/g)
Adsorbed volume (cm /g)
a
0.4 0.6 0.8 Relative pressure (P/P0)
1.0
20 30 Pore diameter (nm) Fig. 4 Nitrogen adsorption-desorption isotherm (a) and pore size distribution (b) of C,N,S-THs-1. 284.8 eV
C1s
286.4 eV
b
0
10
399.9 eV
40
168.8 eV S2p
c
N1s
396.0 eV
Intensity (a.u.)
Intensity (a.u.)
a
0.2
Intensity (a.u.)
0.0
162.1 eV
288.7 eV
280
285 290 Binding energy (eV)
295 392 394 396 398 400 402 404 406 160 Binding energy (eV)
165 170 Binding energy (eV) Fig. 5 High-resolution XPS spectra of C 1s region (a), N 1s region (b) and S 2p region (c) of the C,N,S-THs-1.
175
Journal of Environmental Sciences 2013, 25(10) 2150–2156 / Xiaoxia Lin et al.
2154
Vol. 25
Table 2 Degradation parameter of X-3B by different samples
Line a: THs Line b: C,N,S-THs-0.5 Line c: C,N,S-THs-2 Line d: C,N,S-THs-1
Intensity (a.u.)
a
Sample
Degradation (%)
P25 THs C,N,S-THs-0.5 C,N,S-THs-1 C,N,S-THs-2
b
10−4
9.89 × 0.00422 0.0115 0.0152 0.0119
R 0.998 0.993 0.987 0.990 0.988
c d
500
600 650 700 Wavelength (nm) Fig. 6 Photoluminescence spectra of THs, C,N,S-THs-0.5, C,N,S-THs-1 and C,N,S-THs-2.
550
hole pairs and higher photocatalytic activity (Wu et al., 2009). 2.2 Photocatalytic activity The photocatalytic activities of C,N,S-THs were tested by degradation of X-3B at room temperature under visiblelight irradiation and results are shown in Fig. 7. For comparison, the activities of THs and P25 were also evaluated under the identical conditions. The blank experiment without catalysts (self-photosensitized process) indicates that the photolysis can be neglected as it is about 2.2% after 120 min irradiation. From Fig. 7, we can see that the degradation ratio of X-3B by C,N,S-THs is much higher than P25 and pure THs, indicating the C,N,S-THs is active under visible-light. The apparent firstorder kinetic equation (ln(C0 /C) = kapp t) is used to fit experiment data, where, kapp is apparent rate constant, C is the solution-phase concentration of X-3B, and C0 is the initial concentration at t = 0 (Ao et al., 2009). The obtained apparent rate constant kapp are listed in Table 2. It can be seen that the C,N,S-THs-1 has the highest photocatalytic
1.0 -
activity. The higher photocatalytic activity of C,N,S-THs can be attributed to the following factors. One possible reason might be that the high surface area and porous structure of the C,N,S-THs may play a role in the photocatalytic activity of the prepared samples. Large specific surface area can provide more surface active sites for the adsorption of contamination molecules. Furthermore, the hollow spherical structures allow multireflections of irradiation light within their interior cavities, enhancing their lightharvesting ability. The surface area values of C,N,S-THs-1 is higher than THs and lower than C,N,S-THs-2, but the activity of the C,N,S-THs-1 is the highest, which indicates surface area should not be the most important factor for the photocatalytic activity (Wang et al., 2009). It has been reported that electron-hole recombination on the particle surface could be enhanced with increasing surface area (Liu et al., 2009; Joo et al., 2010). The second reason may be co-doped with C, N and S can evidently narrow the band gap of THs and increase the absorption in the visible-light region. More electrons and holes can be generated and participate in the photocatalytic redox reactions because the prepared catalyst can be activated by visible-light. The sample C,N,S-THs-1 possesses the highest photocatalytic activity, when the C, N and S dopant is higher than this level, the photocatalytic activity declines instead. This phenomenon indicates that too much dopant would decrease the distance between trapping sites in a photocatalyst particle, and increase the recombination rate to decrease the photocatalytic activity (Cong et al., 2.0
a
b 1.6 -
0.9 -
P25 THs C,N,S-THs-0.5 C,N,S-THs-1 C,N,S-THs-2
0.8 1.2 ln(C/C0)
0.7 C/C0
12.1 37.9 74.4 83.1 75.6
kapp (min−1 )
0.6 0.4 0.3 0.2 0.1 00
Self-degradation of X-3B P25 THs C,N,S-THs-0.5 C,N,S-THs-1 C,N,S-THs-2
0.8 0.4 0.0
40 60 80 100 120 Irradiation time (min) Fig. 7 Photocatalytic activity of different samples (a) and variations in ln(C 0 /C) as a function of irradiation time and linear fits of different samples (b) C0 and C represent the initial concentration of X-3B and concentration at time t, respectively. 20
40
60 80 Irradiation time (min)
100
120
0
20
No. 10
Synthesis and enhanced visible-light responsive of C,N,S-tridoped TiO2 hollow spheres
2007), which is consistent with the PL results. Besides the above reason, another possible reason is that the codoped TiO2 consists of several phases (undoped and doped TiO2 , anatase and rutile). As we all known, the composite of two kinds of semiconductors or two phases of the same semiconductor are in direct contact, the recombination of photo-generated electrons and holes will be reduced thus enhances photocatalytic activity (Zhou and Yu, 2008).
3 Conclusions In this article, we prepared C,N,S-tridoped TiO2 hollow spheres using carbon spheres as template. C,N,S-tridoped TiO2 hollow spheres exhibit obvious absorption in visiblelight region and cause a red-shift of adsorption spectrum for TiO2 hollow spheres. The photocatalytic activities of as-prepared TiO2 hollow spheres were investigated by degradation of X-3B under visible-light irradiation. It was found that C,N,S-tridoped TiO2 hollow spheres showed significantly higher photocatalytic activity than P25 and undoped TiO2 hollow spheres. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51172043), the Doctor Research Starting Fund of Jinling Institute of Technology (No. JITB-201307), and the Social Developing Program of Jiangsu Province (No. BE2011797).
References Ao Y H, Xu J J, Fu D G, Yuan C W, 2008. A simple method for the preparation of titania hollow sphere. Catalysis Communications, 9(15): 2574–2577. Ao Y H, Xu J J, Fu D G, Yuan C W, 2009. Synthesis of C,N,S-tridoped mesoporous titania with enhanced visible light-induced photocatalytic activity. Microporous and Mesoporous Materials, 122(1-3): 1–6. Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y, 2001. Visiblelight photocatalysis in nitrogen-doped titanium oxides. Science, 293(5528): 269–271. Chen D M, Jiang Z Y, Geng J Q, Wang Q, Yang D, 2007. Carbon and nitrogen co-doped TiO2 with enhanced visiblelight photocatalytic activity. Industrial and Engineering Chemistry Research, 46(9): 2741–2746. Cong Y, Zhang J L, Chen F, Anpo M, He D N, 2007. Preparation, photocatalytic activity, and mechanism of Nano-TiO2 codoped with nitrogen and iron(III). Journal of Physical Chemistry C, 111(28): 10618–10623. Diwald O, Thompson T L, Zubkov T, Goralski E G, Walck S D, Yates J T Jr, 2004. Photochemical activity of nitrogendoped rutile TiO2 (111) in visible light. Journal of Physical Chemistry B, 108(19): 6004–6008. Ghanbary F, Modirshahla N, Khosravi M, Behnajady M A, 2012. Synthesis of TiO2 nanoparticles in different thermal conditions and modeling its photocatalytic activity with artificial neural network. Journal of Environmental Sciences, 24(4):
2155
750–756. Hebenstreit E L D, Hebenstreit W, Geisler H, Thornburg S N, Ventrice C A Jr, Hite D A et al., 2001. Sulfur on TiO2 (110) studied with resonant photoemission. Physical Review B, 64(11): 115418. Joo J Y, Shim J M, Seo H J, Jung N, Wiesner U, Lee J et al., 2010. Enhanced photocatalytic activity of highly crystallized and ordered mesoporous titanium oxide measured by silicon resonators. Analytical Chemistry, 82(7): 3023–3037. Jung S M, Grange P, 2000. Characterization and reactivity of pure TiO2 -SO4 2− -SCR catalyst: influence of SO4 2− content. Catalysis Today, 59(3-4): 305–312. Kang I C, Zhang Q W, Yin S, Sato T, Saito F, 2008. Novel method for preparation of high visible active N-doped TiO2 photocatalyst with its grinding in solvent. Applied Catalysis B: Environmental, 84(3-4): 570–576. Li X H, Zhang H D, Zheng X X, Yin Z Y, Wei L, 2011. Visible light responsive N-F-codoped TiO2 photocatalysts for the degradation of 4-chlorophenol. Journal of Environmental Sciences, 23(11): 1919–1924. Lin X X, Rong F, Ji X, Fu D G, 2011. Visible light photocatalytic activity and photoelectrochemical property of Fe-doped TiO2 hollow spheres by sol-gel method. Journal of Sol-Gel Science and Technology, 59(2): 283–289. Liu G, Wang X W, Wang L Z, Chen Z G, Li F, Lu G Q et al., 2009. Drastically enhanced photocatalytic activity in nitrogen doped mesoporous TiO2 with abundant surface states. Journal of Colloid and Interface Science, 334(2): 171–175. Periyat P, Pillai S C, McCormack D E, Colreavy J, Hinder S J, 2008. Improved high-temperature stability and sun-lightdriven photocatalytic activity of sulfur-doped anatase TiO2 . Journal of Physical Chemistry C, 112(20): 7644–7652. Sayago D I, Serrano P, B¨ohme O, Goldoni A, Paolucci G, Rom´an E et al., 2001. Adsorption and desorption of SO2 on the TiO2 (110)-(1×1) surface: A photoemission study. Physical Review B, 64(20): 205402. Subagio D P, Srinivasan M, Lim M, Lim T T, 2010. Photocatalytic degradation of bisphenol-A by nitrogen-doped TiO2 hollow sphere in a vis-LED photoreactor. Applied Catalysis B: Environmental, 95(3-4): 414–422. Tarafdar A, Biswas S, Pramanik N K, Pramanik P, 2006. Synthesis of mesoporous chromium phosphate through an unconventional sol-gel route. Microporous and Mesoporous Materials, 89(1-3): 204–208. Wang H Q, Wu Z B, Liu Y, 2009. A simple two-step template approach for preparing carbon-doped mesoporous TiO2 hollow microspheres. Journal of Physical Chemistry C, 113(30): 13317–13324. Wang P H, Yap P S, Lim T T, 2011. C-N-S tridoped TiO2 for photocatalytic degradation of tetracycline under visiblelight irradiation. Applied Catalysis A: General, 39(1-2): 252–261. Wang Y W, Huang Y, Ho W K, Zhang L Z, Zou Z G, Lee S C, 2009. Biomolecule-controleed hydrothermal synthesis of C-N-S-tridoped TiO2 nanocrystalline photocatalysts for NO removal under simulated solar light irradiation. Journal of Hazardous Materials, 169(1-3): 77–87. Wang Y, Wang Y, Meng Y L, Ding H M, Shan Y K, Zhao X et al., 2008. A highly efficient visible-light-activated
2156
Journal of Environmental Sciences 2013, 25(10) 2150–2156 / Xiaoxia Lin et al.
photocatalyst based on bismuth- and sulfur-codoped TiO2 . Journal of Physical Chemistry C, 112(17): 6620–6626. Wong M S, Hsu S W, Rao K K, Kumar C P, 2008. Influence of crystallinity and carbon content on visible light photocatalysis of carbon doped titania thin films. Journal of Molecular Catalysis A: Chemical, 279(1): 20–26. Wu Y M, Liu H B, Zhang J L, Chen F, 2009. Enhanced photocatalytic activity of nitrogen-doped titania by deposited with gold. Journal of Physical Chemistry C, 113(33): 14689– 14695. Xiao Q, Ouyang L L, 2011. Photocatalytic photodegradation of xanthate over C, N, S-tridoped TiO2 nanotubes under visible light irradiation. Journal of Physics and Chemistry of Solids, 72(1): 39–44. Xie Y B, Yuan C W, 2003. Visible-light responsive cerium ion modified titania sol and nanocrystallites for X-3B dye photodegradation. Applied Catalysis B: Environmental, 46(2): 251–259. Xu J J, Ao Y H, Fu D G, Yuan C W, 2008. Low-temperature
Vol. 25
preparation of F-doped TiO2 film and its photocatalytic activity under solar light. Applied Surface Science, 254(10): 3033–3038. Yu J C, Ho W K, Yu J G, Yip H, Wong P K, Zhao J C, 2005. Efficient visible-light-induced photocatalytic disinfection on sulfur-doped nanocrystalline titania. Environmental Science and Technology, 39(4): 1175–1179. Yu J C, Yu J G, Ho W, Jiang Z T, Zhao L Z, 2002. Effect of F-doping on the photocatalytic activity and microstructures of nanocrystalline TiO2 powders. Chemistry of Materials, 14(9): 3808–3816. Yu J G, Liu S W, Yu H G, 2007. Microstructures and photoactivity of mesoporous anatase hollow microspheres fabricated by fluoride-mediated self-transformation. Journal of Catalysis, 249(1): 59–66. Zhou M H, Yu J G, 2008. Preparation and enhanced daylightinduced photocatalytic activity of C,N,S-tridoped titanium dioxide powders. Journal of Hazardous Materials, 152(3): 1229–1236.