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Synthesis and photocatalytic activity of hydroxyapatite modified nitrogen-doped TiO2
Materials Chemistry and Physics 129 (2011) 654–659
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and photocatalytic activity of hydroxyapatite modified nitrogen-doped TiO2 Y. Liu, Q. Yang, J.H. Wei ∗ , R. Xiong, C.X. Pan, J. Shi Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China
a r t i c l e
i n f o
Article history: Received 19 July 2010 Received in revised form 6 May 2011 Accepted 9 May 2011 Keywords: Composite materials Nanostructures Sol–gel growth Adsorption Electron microscopy
a b s t r a c t TiO2 is a promising material as a photocatalyst for photodecomposition of hazardous organic pollutants under illumination, however, the low adsorption abilities, quantum yields and the lack of visible-light utilization hinder its practical application. In an attempt to extend light absorption of the TiO2 -based photocatalyst towards the visible light range and improve its photocatalytic activity, a new photocatalyst, HAP-modified N-TiO2 (HAP-N-TiO2 ) was synthesized by a facile wet chemical method. The properties of HAP-N-TiO2 were investigated by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption isotherm measurement, UV–visible absorption spectroscopy, and photocurrent measurement. The photocatalytic activity of the samples was evaluated by the decomposition of gaseous acetone under visible light irradiation. The research results demonstrated that 10%-HAP-N-TiO2 sample shows the best photocatalytic activity, which is much superior to the other samples under visible light irradiation. The remarkable photocatalytic activity may arise from the synergism between adsorption on hydroxyapatite and photoactivity by titania. N-doping makes the composite also active under visible-light irradiation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide has been extensively studied as the most promising environment protective photocatalyst because of its nontoxicity, low cost, chemical stability, and high catalytic activity [1–3], but the wide band gap of TiO2 (3.2 eV for the anatase structure and 3.0 eV for the rutile structure) restricts its photocatalytic applications to the UV range (about 5% fraction of the solar spectrum). To ensure visible-light photocatalytic activity of TiO2 , it is therefore important to introduce visible-light absorption. Up to now, much effort has been made to enhance the photocatalytic efficiency and visible light utilization of TiO2 by impurity doping, sensitization, metallization [4–8], etc. In these methods, doping the TiO2 with metal or nonmetal represents a promising approach to reduce the absorption threshold of TiO2 and extends its optical absorption range from ultraviolet to visible region [9–13]. However, visible-light absorption does not always result in visible-light photocatalytic activity, the key to enhancing photocatalytic activity lies in effectively combining the excitation, bulk diffusion, and surface transfer of photoinduced charge-carriers in the photocatalyst. Metal doping system suffers from thermal instability, also, the metal ions may serve as recombination centers for electrons and holes, thus reducing the overall activity of the photocatalyst activ-
∗ Corresponding author. Tel.: +86 27 68754613; fax: +86 27 68752569. E-mail address: [email protected] (J.H. Wei). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.05.021
ity of the photocatalyst. Non-metal impurities can efficiently avoid these problems. Among the nonmetal doping systems, nitrogen doping has been shown to be a good candidate for narrowing the bandgap energy of TiO2 . Asahi et al. [4] first suggested that N doping reduces the band-gap due to the mixing of N 2p with the O 2p states. Since then, there have been a great number of publications describing enhanced visible light photochemistry in N-doped TiO2 although its doping mechanism is still debated [14–16]. However, the practical application of the N-doped TiO2 (N-TiO2 ) is hindered by its low adsorption abilities, which conduct to a usually low photodecomposition rate. As a solution to these drawbacks, the enhancement of the photodegradation rate was achieved by supporting TiO2 on adsorbents that, besides allowing an easy separation of the photocatalyst, could also cooperate to the success of the photocatalytic reaction by concentrating the target substances around active TiO2 centers [17–19]. Hydroxyapatite ((Ca10 (PO4 )6 (OH)2 ), HAP) is known to have excellent adsorption properties as well as biocompatibility and has been used for bone repair and substitute. Recently, considering the applications in catalysis, many interests have been aroused for constructing HAP-based composite nanostructures, including HAP/TiO2 [20–22], Ag-HAP/TiO2 [23,24], HAP/SiO2 [25], HAP/Ru [26], HAP/Au [27], etc. Though N-TiO2 and HAP-modified TiO2 have already been synthesized and exhibited enhanced photocatalytic activities as compared to the pure TiO2 , however, little research has been
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reported about HAP-modified N-TiO2 (HAP-N-TiO2 ) up to now. In this work, HAP-N-TiO2 was synthesized by a facile wet chemical method. Compared with the N-TiO2 samples, the HAP-N-TiO2 sample exhibits high surface area, improved quantum efficiency and superb visible-light photocatalytic activity. The mechanisms of enhanced visible-light activity and photocatalytic performance in N-doped and HAP-N-TiO2 were discussed in detail. 2. Experimental 2.1. Sample preparation To prepare HAP-N-TiO2 photocatalyst, the detailed procedure was as following. Firstly, 3 ml N (CH2 CH2 OH)3 and 1 ml oleic acid were dissolved in 10 ml ethanol. Afterwards, Ti(OC4 H9 )4 was added dropwise into the above solution under vigorous stirring for 1 h. Then, appropriate urea dissolved by ethanol was added into the solution. The molar ratio of urea to Ti(OC4 H9 )4 was controlled to be 1%. During the synthesis, the mixtures were stirred at 400–500 rpm for 2 h at room temperature. Where after, a variable amount of hydroxyapatite dispersed in n-hexane was added into the above solution. The mixed solution was continuously stirred until the gel was formed. Finally, the gel was dried at 110 ◦ C for half an hour, heat treated at 400 ◦ C in air for 4 h, and then ground to obtain the HAP-N-TiO2 nanoparticles. The HAP-doping concentration (X) was chosen as 2, 5, 8, 10, 12, 15, which was the mole percentage of HAP in the theoretical TiO2 powders. The corresponding concentrations in the as-obtained photocatalysts were denoted as X%-HAP-N-TiO2 . Unless otherwise specified, HAP-N-TiO2 in the work refers to 10%-HAP-N-TiO2 . For comparison, undoped TiO2 and N-TiO2 were prepared in the same way without the corresponding additives of urea or HAP solution, respectively. P25 powder (composing of anatase and rutile, is widely studied and well known to have good photocatalytic activity) was obtained from the Degussa Company.
Fig. 1. XRD patterns of (a) hydroxyapatite, (b) undoped TiO2 , (c) N-TiO2 , and (d) HAP-N-TiO2 (A, anatase; R, rutile; H, hydroxyapatite).
The adsorption capacity of the catalysts was measured in the similar way to that of photocatalytic activity measurements. The only difference is that the adsorption process was carried out without light irradiation.
2.2. Characterization The crystalline phases of the catalysts were evaluated by a Bruker D8 Advance diffractometer with Cu K␣ radiation. The microstructure and morphology of the catalysts were observed by transmission electron microscopy (TEM, JEM2010, Japan) and selected area electron diffraction (SAED). The surface compositions of the catalysts were evaluated by X-ray photoelectron spectra (XPS, ESCA-3400, Japan) with monochromatic Mg K␣ excitation and a charge neutralizer was used to investigate the surface electronic states of the N doped samples. The accuracy of the reported XPS binding energies (BE) can be estimated to be ±0.2 eV. The UV–vis diffuse reflectance spectra (DRS) of samples were recorded by a spectrophotometer (Varian Carry 500 Scan) equipped with an integrating sphere attachment. The specific surface area of the catalysts was determined by applying the Brunauer Emmett Teller (BET) method to the sorption of nitrogen at 77 K. 2.3. Photocurrent tests The photoelectrochemical property was measured with an electrochemical workstation (CHI660A, CH Instruments Co.). A three-electrode configuration was used in experiments. A high-pressure mercury lamp (160 W) was used as a light source. All the experiments were conducted at room temperature. Briefly, 5 mg of photocatalyst was dispersed in 5 ml of ethanol and ultrasonically vibrated for 30 min. The 0.25 ml resultant slurry was then dip-coated onto a 10 mm × 20 mm indium tin oxide (ITO) glass electrode and dried under high-pressure mercury lamp irradiation to eliminate ethanol. The prepared photocatalyst/ITO electrode, platinum electrode, and saturation calomel electrode (SCE) were used as the working electrode, counter electrode, and reference electrode, respectively. The electrolyte was the 0.5 mol/l Na2 SO4 aqueous solution. The working electrode was activated in the electrolyte for 2 h before measurement. 2.4. Adsorption/photocatalytic activity measurements Gaseous photocatalytic activity was evaluated by the removal of acetone in air with initial equilibrium concentration (2500 ppm) in an 8 l reactor at room temperature (20 ◦ C). The weight of photocatalysts used for each experiment was kept at 0.5 g a 125 W high pressure Hg lamp with a cutoff filter ( ≥ 400 nm) as a visible light source were passed into the reactor for 1 h. To measurement the adsorption and catalytic activity under the dark media, the experiment was carried out under a black box with similar condition for 2 h. The analysis of acetone, carbon dioxide, and water vapor concentration in the reactor was conducted on line with a Photoacoustic IR Multigas Monitor (INNOVA Air Tech Instruments Model 1312). The photocatalytic activity of the samples was quantitatively evaluated according to the equation: ln(C/C0 ) = −kt [28]. Here C0 and C represent the initial equilibrium concentration and reaction concentration of acetone, respectively, k represents the apparent reaction rate constant, and t represents reactive time. Photocatalytic oxidation of the acetone is based on the following reaction: CH3 COCH3 + 4O2 → 3CO2 + 3H2 O
(1)
3. Results and discussion 3.1. Phase structure and morphology The XRD patterns of the as-prepared samples are shown in Fig. 1. The crystallinity of the HAP (Fig. 1a) is confirmed by the reflections observed at 2 values of 25.8◦ , 31.7◦ , 32.17◦ , 33.0◦ , 39.8◦ , 46.6◦ and 49.5◦ (JCPDS cards no. 74-565). Both the anatase (JCPDS cards no. 21-1272) and rutile phases (JCPDS cards no.21-1276) are observed in pure TiO2 and its corresponding doping samples (Fig. 1b–d). According to the equation given by Spurr and Myers [29], the relative contents of anatase (compared with rutile phase) in the TiO2 , N-TiO2 and HAP-N-TiO2 were 69.47%, 71.08% and 72.81%, respectively. In general, the crystal structure of TiO2 greatly affects photocatalytic activity. P25 (TiO2 ) is considered one of the most active photocatalyst, and its activity is believed to arise from its phase composition (75% anatase, 25% rutile) and from the good particle contact between anatase and rutile. Obviously, our samples have similar phase structure with P25, which are expected to result in fine photocatalytic activity. From Fig. 1d, it is also found the HAP phase (at 2 values of 32.17◦ ) in the co doped catalysts. In addition, it is noteworthy that N-doping has almost no influence on the phase structure of TiO2 nanocrystals; the probable reason is that the concentration of N is too low to be detected by XRD. The transmittance electron microscopy (TEM) images of samples were shown in Fig. 2. As can be seen from the images, the HAP shows rod-like structure in the range of about 100–200 nm length and 30–50 nm width. For the HAP-N-TiO2 samples, the monodisperse N-TiO2 nanocrystals with diameter about 10 nm can be clearly visible in the surface of HAP nanorod. The N-TiO2 nanocrystals were identified as polycrystalline anatase and rutile mixed structure from SAED pattern (the inset picture in Fig. 2b). The dark field image showing in Fig. 2c was taken from the diffraction rings of TiO2 indicated on the SAED pattern, which further identified that the dark spots on the surface of HAP were TiO2 nanocrystals. Similarly, the concentration of N is too low to be detected by SAED pattern.
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Fig. 2. TEM images of (a) HAP, (b) bright field electron image of HAP-N-TiO2 , and (c) dark field electron image taken from the diffraction rings in the SAED pattern [inset of (b)].
3.2. XPS analysis Fig. 3. XPS spectra of (a) N 1s, (b) Ti 2p, and (c) O 1s.
The surface chemical compositions and chemical states of asprepared samples were investigated using XPS as shown in Fig. 3, where, the C element at 284.8 eV can be ascribed to the adventitious hydrocarbon from the XPS instrument. The relative atomic concentration of N element in N-TiO2 is 1.0 at% based on the XPS data. High-resolution N 1 s spectra of as-prepared samples are shown in Fig. 3a. For N-TiO2 samples, one peak at 397.1 eV is only observed which is generally known as the N atom replacing the oxygen atoms in the TiO2 crystal lattice to form an N-Ti–N bond [30,31]. For HAP-
N-TiO2 samples, the peak at 396.2 eV is attributed to the Ti–N-Ti linkage, indicating that N atom is substitutionally doped into the TiO2 lattice; the peak at 399.8 reveals the presence of oxynitride such as Ti–N–O and Ti–O–N on the surface, possibly in interstitial location [32–35]. The above results show that nitrogen atoms have been successfully introduced into N-TiO2 and HAP-N-TiO2 samples.
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Fig. 4. UV–vis adsorption spectra of different samples.
Fig. 5. Photocurrent response of different samples.
The high-resolution XPS spectra of Ti 2p and O 1s region are shown in Fig. 3b and c, respectively. The peaks of Ti 2p are always regular showing the expected doublet with a binding energy (BE) of Ti 2p3/2 at 458.3 ± 0.2 eV and Ti 2p1/2 at 464.2 ± 0.2 eV, which is typical of Ti (IV) in the oxide [35]. The XPS spectra of O 1s (Fig. 3c) are fitted as two peaks by multiple Gaussians. The peak at about 530.3 eV is due to oxygen in the TiO2 crystal lattice and the peak at about 532.1 eV is due to hydroxyl oxygen. The hydroxyl oxygen peak of the HAP-N-TiO2 is more intensive than that of the undoped TiO2 and N-TiO2 samples due to excess hydroxyl introduced by HAP. From all of the results we can conclude that nitrogen and HAP have been successful incorporated into the titania by our method.
and the intragap localized states of N together contribute to the red shift of the absorption edge. Table 2 shows the BET surface area of different catalysts. The order of BET specific surface areas was as follows: HAP-N-TiO2 > NTiO2 > P25 > TiO2 . For HAP-N-TiO2 sample, the BET specific surface areas increase with the increasing of HAP concentration. It is widely accepted that photocatalysts with high specific surface areas and porous structures are beneficial to the enhancement of photocatalytic performance, due to more surface active sites, easier mass transportation, and higher light-harvesting efficiency [41]. So, HAPN-TiO2 samples are expected to exhibit improved photocatalytic performance.
3.3. UV–vis spectra and BET surface area
3.4. Photocurrent measurement
Fig. 4 shows the UV–vis/DRS of HAP, pure TiO2 , P25, N-TiO2 , and HAP-N-TiO2 , respectively. From the figure it is clear that the HAP shows no obvious adsorption in the range of 250–800 nm. Compared to the absorption spectra of pure TiO2 and P25, the absorption edge of N-TiO2 and HAP-N-TiO2 was obviously shifted to the visible light range. The band gap energy (EB ) was calculated using the equation, g = 1240/EB , where g is the optical absorption threshold [36]. The values of band gap and absorption threshold were shown in Table 1. The optical bandgap is 2.75 eV for the N-TiO2 and 2.76 eV for the HAP-N-TiO2 sample, which are much smaller than that of P25 (3.15 eV) and undoped TiO2 (3.10 eV). Obviously, the introduce of nitrogen results in the adsorption edge’s red shift. However, the origin of the adsorption edge’s red shift is hitherto controversial. Some studies have proposed that it is caused by a narrowing band gap via a mixed N 2p state with O 2p state in the valence band, while others have suggested the appearance of intragap localized states of dopants [37–39]. Additionally, a point was raised about the formation of oxygen vacancies and the emerged color centers which could contribute to absorb visible light [40]. For our samples, on the basis of above-mentioned XPS analysis, it is possible that the narrowing band gap, resulted from mixed N 2p with O 2p states,
The photocurrent response measurement was carried out under visible light pulsed irradiation to investigate the photo-induced charges separation efficiency of different samples. The results were shown in Fig. 5. The working electrode potentials were located at 0 V to simulate the same working condition as photocatalysis reaction system. It was found that the photocurrent of the P25, undoped TiO2 , N-TiO2 , and HAP-N-TiO2 electrodes were 0.18, 0.16, 0.61 and 0.83 A, respectively. The photocurrent of the N-TiO2 electrode was about 3.3 as high as those of the P25 electrode, because N doping led to a red shift of the spectrum onset and higher absorption in the visible light in the 380–800 nm range, and the HAP-N-TiO2 sample increased the photocurrent further to be 4.6 times as high as that of the P25 electrode. The photocurrent was in the order of HAP-N-TiO2 > N-TiO2 > pure TiO2 . Compared with the N-TiO2 , the HAP-N-TiO2 has a similar absorption in visible light, but can generate much more photoinduced electrons and holes, correspondingly resulting in higher photocurrent. Based on the results of BET analysis and photocurrent measurement, the possible mechanism for HAP-N-TiO2 decreasing the carrier recombination and improving the separation efficiency of photoinduced electrons and holes, generating the maximum photocurrent can be due to the introduction of HAP. HAP is a common solid-phase adsorbent for air and water purification because of its large specific surface area, abundant micropores and a series of surface functional groups. For HAP-N-TiO2 samples, the HAP on the surfaces of N-TiO2 can attract more photogenerated electrons and holes, the photogenerated electrons and holes generated on the surface of HAP-N-TiO2 samples more inclined to remain adsorbed on its surface for its much stronger adsorption ability than the other samples. Therefore, there exists large amounts of separated
Table 1 The absorption thresholds and corresponding photon energies of different samples. Sample
Absorption threshold (nm)
Corresponding photon energy (eV)
TiO2 P25 N-TiO2 HAP-N-TiO2
400 394 451 449
3.10 3.15 2.75 2.76
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Table 2 The specific surface area for different samples. Sample
TiO2
P25
N-TiO2
BET surface area (m2 /g)
81.46
58.52
89.65
HAP-N-TiO2 2%
5%
8%
10%
12%
15%
113.44
148.29
169.27
177.67
189.22
207.48
Note: X% refers to X%-HAP-N-TiO2 sample. Table 3 Adsorptive ability of the catalysts for acetone. Sample
Initial concentration (%)
Concentration after 1 h in dark (%)
Concentration after 2 h in dark (%)
HAP TiO2 P25 N-TiO2 HAP-N-TiO2
100 100 100 100 100
77 100 100 99 94
76 100 100 99 93
electrons and holes on the surface of HAP-N-TiO2 samples, thus generates higher photocurrent. The excessive electrons and holes are accepted by surface adsorbed O2 and H2 O, and formed more • OH radicals to participate the photocatalytic reaction. 3.5. Visible light-induced photocatalytic activity To evaluate the photodegradation capability of different samples, we examined the decomposition of gaseous acetone under visible light irradiation. The dark adsorption study of acetone on supported catalysts is presented in Table 2. It can be seen that after 1 h in the dark, about 23% acetone were absorbed by HAP, 6% acetone were absorbed by HAP-N-TiO2 and 1% acetone were absorbed by N-TiO2 , respectively, but adsorption of acetone by the undoped TiO2 and P25 within the same time is approximately zero. Another 1 h in the dark, the concentration of acetone change little, this means that adsorption equilibrium can be reached after 1 h in the dark (Table 3). To evaluate and compare the photocatalytic activity of the HAP-N-TiO2 samples, the degradation reactions of gaseous acetone under visible light irradiation were performed as photoreaction probes. The kinetics process of photocatalytic degradation of acetone and the apparent reaction rate constant k of different samples are shown in Fig. 6, and Degussa P25 was used as the reference sample for comparison purpose. The undoped TiO2 and P25 sam-
Scheme 1. Proposed mechanism for the visible light photodegradation of gaseous pollution on a HAP-N-TiO2 catalyst.
ples show poor photocatalytic activities in visible light region. This is assigned to the large band gap of the titanium dioxide. The activity of N-TiO2 sample is higher than that of the undoped TiO2 and P25, its rate constant k reaches 0.0027 min−1 . For the HAP-N-TiO2 sample, the photocatalytic activity significantly increases. As shown in Fig. 6, most of the HAP-N-TiO2 samples demonstrated considerably high photoactivity under visible light irradiation and the 10%-HAP-N-TiO2 sample exhibited the best photocatalytic efficiency in degradation of acetone in all tested doped and undoped TiO2 samples. Its k value is 0.077 min−1 which is 3.9 times to that of N-TiO2 and 11 times to that of P25. The higher photocatalytic activity of the 10%-HAP-N-TiO2 sample is partially due to its high specific surface area and improved charge separation. When the addition value of HAP was higher than 10%, although the adsorption of the composite increases, the relative content of TiO2 in the composite decreases. As a result, the photo-induced electrons/hole pairs and the photocatalytic activity in a given area decrease. On the basis of the above experimental results, the photocatalytical mechanisms for HAP-N-TiO2 system under visible light irradiation were proposed and elucidated in Scheme 1. In general, anatase TiO2 with 3.2 eV banding energy irradiated by visible light cannot effectively produce enough photo-induced electrons and holes, which are considered to play an essential role in the photodegradation reactions. While the N doped TiO2 will form new states just above the valence band. The band gap between the new states and the conduction band is about 2.75 eV and thus the catalysts will be excited easily by visible light to produce the photo-induced electrons. Meanwhile, the improved adsorption of HAP-N-TiO2 system can adsorb more O2 and H2 O molecules which can capture more electronic-hole pairs to form abundant • OH radicals which decomposed acetone into CO2 and H2 O on the surface of TiO2 . 4. Conclusions
Fig. 6. Photocatalytic degradation of acetone by (a) TiO2 , (b) P25, (c) N-TiO2 , (d) 15%-HAP-N-TiO2 , (e) 12%-HAP-N-TiO2 , (f) 2%-HAP-N-TiO2 , (g) 5%-HAP-N-TiO2 , (h) 8%-HAP-N-TiO2 , and (i) 10%-HAP-N-TiO2 .
HAP-N-TiO2 photocatalysts were successfully prepared with a facile one-pot method. The novel photocatalysts showed obvious visible light photocatalytic activity in decomposition gaseous acetone, while the 10%-HAP-N-TiO2 sample gave the best photocatalytic activity and demonstrated to be far superior to that of the commercial Degussa P25 counterpart. The probable photocatalytical mechanism was proposed. The high photoactivity of
Y. Liu et al. / Materials Chemistry and Physics 129 (2011) 654–659
the HAP-N-TiO2 can be attributed to the results of the synergistic effect, the synergism arises from cooperation of HAP adsorbing the chemical compound and the TiO2 photocatalyst generating oxygen reactive species that diffuse and react with the molecules located on the HAP. The doped nitrogen specie plays a key role in narrowing the band gap and expanding the photoactivity to visible light region. Acknowledgments We are grateful for the financial support from the National Program on key Basic Research Project (973 Grant Nos. 2009CB939704 and 2009CB939705), and the National Natural Science Foundation of China (No. 10974148). References [1] G. Liu, L.Z. Wang, H.G. Yang, H.M. Cheng, G.Q. Lu, J. Mater. Chem. 20 (2010) 831–843. [2] A. Fujishima, X.T. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582. [3] X.G. Han, Q. Kuang, M.S. Jin, Z.X. Xie, L.S. Zheng, J. Am. Chem. Soc. 131 (2009) 3152–3153. [4] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [5] G.G. Nakhate, V.S. Nikam, K.G. Kanade, S. Arbuj, B.B. Kale, J.O. Baeg, Mater. Chem. Phys. 124 (2010) 976–981. [6] S. Shanmugam, A. Gedanken, Small 3 (2007) 1189–1192. [7] T.W. Kim, S.G. Hur, S.J. Hwang, H. Park, W.Y. Choi, J.H. Choy, Adv. Funct. Mater. 17 (2007) 307–314. [8] L.L. Zhang, F.J. Lv, W.G. Zhang, R.Q. Li, H. Zhong, Y.J. Zhao, Y. Zhang, X. Wang, J. Hazard. Mater. 171 (2009) 294–300. [9] M. Hamadanian, A. Reisi-Vanani, A. Majedi, Mater. Chem. Phys. 116 (2009) 376–382. [10] A. Bansal, S. Madhavi, T.T.Y. Tan, T.M. Lim, Catal. Today 131 (2008) 250–254. [11] Y.Q. Gai, J.B. Li, S.S. Li, J.B. Xia, S.H. Wei, Phys. Rev. Lett. 102 (2009) 036402. [12] G. Liu, Y.M. Zhao, C.H. Sun, F. Li, G.Q. Lu, H.M. Cheng, Angew. Chem. Int. Ed. 47 (2008) 4516–4520.
659
[13] B. Cojocaru, S. Neatu, V.I. Parvulescu, V. Somoghi, N. Petrea, G. Epure, M. Alvaro, H. Garcia, ChemSusChem 2 (2009) 427–436. [14] X.B. Chen, Y.B. Lou, A.C.S. Samia, C. Burda, J.L. Gole, Adv. Funct. Mater. 15 (2005) 41–49. [15] Y.H. Ao, J.J. Xu, S.H. Zhang, D.G. Fu, Appl. Surf. Sci. 256 (2010) 2754–2758. [16] K.X. Song, J.H. Zhou, J.C. Bao, Y.Y. Feng, J. Am. Ceram. Soc. 91 (2008) 1369–1371. [17] W. Zhang, L.D. Zou, L.Z. Wang, Appl. Catal. A: Gen. 371 (2009) 1–9. [18] J. Ryu, K.Y. Kim, B.D. Hahn, J.J. Choi, W.H. Yoon, B.K. Lee, D.S. Park, C. Park, Catal. Commun. 10 (2009) 596–599. [19] H. Kominami, K. Yukishita, T. Kimura, M. Matsubara, K. Hashimoto, Y. Kera, B. Ohtani, Top. Catal. 47 (2008) 155–161. [20] T. Nonami, H. Hase, K. Funakoshi, Catal. Today 96 (2004) 113–118. [21] A.M. Hu, M. Li, C.K. Chang, D.L. Mao, J. Mol. Catal. A: Chem. 267 (2007) 79–85. [22] K. Ozeki, J.M. Janurudin, H. Aoki, Y. Fukui, Appl. Surf. Sci. 253 (2007) 3397–3401. [23] M. Pratap Reddy, A. Venugopal, M. Subrahmanyam, Water Res. 41 (2007) 379–386. [24] M.R. Elahifard, S. Rahimnejad, S. Haghighi, M.R. Gholami, J. Am. Chem. Soc. 129 (2007) 9552–9553. [25] H. Kanai, M. Nakao, S. Imamura, Catal. Commun. 4 (2003) 405–409. [26] C.M. Ho, W.Y. Yu, C.M. Che, Angew. Chem. Int. Ed. 43 (2004) 3303–3307. [27] A. Venugopal, M.S. Scurrell, Appl. Catal. A: Gen. 245 (2003) 137–147. [28] J.G. Yu, Q.J. Xiang, M.H. Zhou, Appl. Catal. B: Environ. 90 (2009) 595–602. [29] R.A. Spurr, H. Myers, Anal. Chem. 29 (1957) 760–762. [30] X.P. Wang, T.T. Lim, Appl. Catal. B: Environ. 100 (2010) 355–364. [31] G.D. Yang, Z. Jiang, H.H. Shi, T.C. Xiao, Z.F. Yan, J. Mater. Chem. 20 (2010) 5301–5309. [32] M. Mrowetz, W. Balcerski, J. Colussi, M.R. Hoffmann, J. Phys. Chem. B 108 (2004) 17269–17273. [33] J. Ananpattarachai, P. Kajitvichyanukul, S. Seraphin, J. Hazard. Mater. 168 (2009) 253–261. [34] F. Spadavecchia, G. Cappelletti, S. Ardizzone, C.L. Bianchi, S. Cappelli, C. Oliva, P. Scardi, Matteo Leoni, P. Fermo, Appl. Catal. B: Environ. 96 (2010) 314–322. [35] S.H. Kang, H.S. Kim, J.Y. Kim, Y.E. Sung, Mater. Chem. Phys. 124 (2010) 422–426. [36] G. Wu, T. Nishikawa, B. Ohtani, A. Chen, Chem. Mater. 19 (2007) 4530–4537. [37] A.V. Emeline, V.N. Kuznetsov, V.K. Rybchuk, N. Serpone, Int. J. Photoenergy (2008) 1–19. [38] V.N. Kuznetsov, N. Serpone, J. Phys. Chem. C 113 (2009) 15110–15123. [39] M.D. Arienzo, N. Siedl, A. Sternig, R. Scotti, F. Morazzoni, J. Bernardi, O. Diwald, J. Phys. Chem. C 114 (2010) 18067–18072. [40] A.V. Emeline, N.V. Sheremetyeva, N.V. Khomchenko, V.K. Ryabchuk, N. Serpone, J. Phys. Chem. C 111 (2007) 11456–11462. [41] Q.H. Zhang, L. Gao, J.K. Guo, Appl. Catal. B: Environ. 26 (2000) 207–215.
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Synthesis and photocatalytic activity of hydroxyapatite modified nitrogen-doped TiO2 Y. Liu, Q. Yang, J.H. Wei ∗ , R. Xiong, C.X. Pan, J. Shi Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China
a r t i c l e
i n f o
Article history: Received 19 July 2010 Received in revised form 6 May 2011 Accepted 9 May 2011 Keywords: Composite materials Nanostructures Sol–gel growth Adsorption Electron microscopy
a b s t r a c t TiO2 is a promising material as a photocatalyst for photodecomposition of hazardous organic pollutants under illumination, however, the low adsorption abilities, quantum yields and the lack of visible-light utilization hinder its practical application. In an attempt to extend light absorption of the TiO2 -based photocatalyst towards the visible light range and improve its photocatalytic activity, a new photocatalyst, HAP-modified N-TiO2 (HAP-N-TiO2 ) was synthesized by a facile wet chemical method. The properties of HAP-N-TiO2 were investigated by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorption–desorption isotherm measurement, UV–visible absorption spectroscopy, and photocurrent measurement. The photocatalytic activity of the samples was evaluated by the decomposition of gaseous acetone under visible light irradiation. The research results demonstrated that 10%-HAP-N-TiO2 sample shows the best photocatalytic activity, which is much superior to the other samples under visible light irradiation. The remarkable photocatalytic activity may arise from the synergism between adsorption on hydroxyapatite and photoactivity by titania. N-doping makes the composite also active under visible-light irradiation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide has been extensively studied as the most promising environment protective photocatalyst because of its nontoxicity, low cost, chemical stability, and high catalytic activity [1–3], but the wide band gap of TiO2 (3.2 eV for the anatase structure and 3.0 eV for the rutile structure) restricts its photocatalytic applications to the UV range (about 5% fraction of the solar spectrum). To ensure visible-light photocatalytic activity of TiO2 , it is therefore important to introduce visible-light absorption. Up to now, much effort has been made to enhance the photocatalytic efficiency and visible light utilization of TiO2 by impurity doping, sensitization, metallization [4–8], etc. In these methods, doping the TiO2 with metal or nonmetal represents a promising approach to reduce the absorption threshold of TiO2 and extends its optical absorption range from ultraviolet to visible region [9–13]. However, visible-light absorption does not always result in visible-light photocatalytic activity, the key to enhancing photocatalytic activity lies in effectively combining the excitation, bulk diffusion, and surface transfer of photoinduced charge-carriers in the photocatalyst. Metal doping system suffers from thermal instability, also, the metal ions may serve as recombination centers for electrons and holes, thus reducing the overall activity of the photocatalyst activ-
∗ Corresponding author. Tel.: +86 27 68754613; fax: +86 27 68752569. E-mail address: [email protected] (J.H. Wei). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.05.021
ity of the photocatalyst. Non-metal impurities can efficiently avoid these problems. Among the nonmetal doping systems, nitrogen doping has been shown to be a good candidate for narrowing the bandgap energy of TiO2 . Asahi et al. [4] first suggested that N doping reduces the band-gap due to the mixing of N 2p with the O 2p states. Since then, there have been a great number of publications describing enhanced visible light photochemistry in N-doped TiO2 although its doping mechanism is still debated [14–16]. However, the practical application of the N-doped TiO2 (N-TiO2 ) is hindered by its low adsorption abilities, which conduct to a usually low photodecomposition rate. As a solution to these drawbacks, the enhancement of the photodegradation rate was achieved by supporting TiO2 on adsorbents that, besides allowing an easy separation of the photocatalyst, could also cooperate to the success of the photocatalytic reaction by concentrating the target substances around active TiO2 centers [17–19]. Hydroxyapatite ((Ca10 (PO4 )6 (OH)2 ), HAP) is known to have excellent adsorption properties as well as biocompatibility and has been used for bone repair and substitute. Recently, considering the applications in catalysis, many interests have been aroused for constructing HAP-based composite nanostructures, including HAP/TiO2 [20–22], Ag-HAP/TiO2 [23,24], HAP/SiO2 [25], HAP/Ru [26], HAP/Au [27], etc. Though N-TiO2 and HAP-modified TiO2 have already been synthesized and exhibited enhanced photocatalytic activities as compared to the pure TiO2 , however, little research has been
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reported about HAP-modified N-TiO2 (HAP-N-TiO2 ) up to now. In this work, HAP-N-TiO2 was synthesized by a facile wet chemical method. Compared with the N-TiO2 samples, the HAP-N-TiO2 sample exhibits high surface area, improved quantum efficiency and superb visible-light photocatalytic activity. The mechanisms of enhanced visible-light activity and photocatalytic performance in N-doped and HAP-N-TiO2 were discussed in detail. 2. Experimental 2.1. Sample preparation To prepare HAP-N-TiO2 photocatalyst, the detailed procedure was as following. Firstly, 3 ml N (CH2 CH2 OH)3 and 1 ml oleic acid were dissolved in 10 ml ethanol. Afterwards, Ti(OC4 H9 )4 was added dropwise into the above solution under vigorous stirring for 1 h. Then, appropriate urea dissolved by ethanol was added into the solution. The molar ratio of urea to Ti(OC4 H9 )4 was controlled to be 1%. During the synthesis, the mixtures were stirred at 400–500 rpm for 2 h at room temperature. Where after, a variable amount of hydroxyapatite dispersed in n-hexane was added into the above solution. The mixed solution was continuously stirred until the gel was formed. Finally, the gel was dried at 110 ◦ C for half an hour, heat treated at 400 ◦ C in air for 4 h, and then ground to obtain the HAP-N-TiO2 nanoparticles. The HAP-doping concentration (X) was chosen as 2, 5, 8, 10, 12, 15, which was the mole percentage of HAP in the theoretical TiO2 powders. The corresponding concentrations in the as-obtained photocatalysts were denoted as X%-HAP-N-TiO2 . Unless otherwise specified, HAP-N-TiO2 in the work refers to 10%-HAP-N-TiO2 . For comparison, undoped TiO2 and N-TiO2 were prepared in the same way without the corresponding additives of urea or HAP solution, respectively. P25 powder (composing of anatase and rutile, is widely studied and well known to have good photocatalytic activity) was obtained from the Degussa Company.
Fig. 1. XRD patterns of (a) hydroxyapatite, (b) undoped TiO2 , (c) N-TiO2 , and (d) HAP-N-TiO2 (A, anatase; R, rutile; H, hydroxyapatite).
The adsorption capacity of the catalysts was measured in the similar way to that of photocatalytic activity measurements. The only difference is that the adsorption process was carried out without light irradiation.
2.2. Characterization The crystalline phases of the catalysts were evaluated by a Bruker D8 Advance diffractometer with Cu K␣ radiation. The microstructure and morphology of the catalysts were observed by transmission electron microscopy (TEM, JEM2010, Japan) and selected area electron diffraction (SAED). The surface compositions of the catalysts were evaluated by X-ray photoelectron spectra (XPS, ESCA-3400, Japan) with monochromatic Mg K␣ excitation and a charge neutralizer was used to investigate the surface electronic states of the N doped samples. The accuracy of the reported XPS binding energies (BE) can be estimated to be ±0.2 eV. The UV–vis diffuse reflectance spectra (DRS) of samples were recorded by a spectrophotometer (Varian Carry 500 Scan) equipped with an integrating sphere attachment. The specific surface area of the catalysts was determined by applying the Brunauer Emmett Teller (BET) method to the sorption of nitrogen at 77 K. 2.3. Photocurrent tests The photoelectrochemical property was measured with an electrochemical workstation (CHI660A, CH Instruments Co.). A three-electrode configuration was used in experiments. A high-pressure mercury lamp (160 W) was used as a light source. All the experiments were conducted at room temperature. Briefly, 5 mg of photocatalyst was dispersed in 5 ml of ethanol and ultrasonically vibrated for 30 min. The 0.25 ml resultant slurry was then dip-coated onto a 10 mm × 20 mm indium tin oxide (ITO) glass electrode and dried under high-pressure mercury lamp irradiation to eliminate ethanol. The prepared photocatalyst/ITO electrode, platinum electrode, and saturation calomel electrode (SCE) were used as the working electrode, counter electrode, and reference electrode, respectively. The electrolyte was the 0.5 mol/l Na2 SO4 aqueous solution. The working electrode was activated in the electrolyte for 2 h before measurement. 2.4. Adsorption/photocatalytic activity measurements Gaseous photocatalytic activity was evaluated by the removal of acetone in air with initial equilibrium concentration (2500 ppm) in an 8 l reactor at room temperature (20 ◦ C). The weight of photocatalysts used for each experiment was kept at 0.5 g a 125 W high pressure Hg lamp with a cutoff filter ( ≥ 400 nm) as a visible light source were passed into the reactor for 1 h. To measurement the adsorption and catalytic activity under the dark media, the experiment was carried out under a black box with similar condition for 2 h. The analysis of acetone, carbon dioxide, and water vapor concentration in the reactor was conducted on line with a Photoacoustic IR Multigas Monitor (INNOVA Air Tech Instruments Model 1312). The photocatalytic activity of the samples was quantitatively evaluated according to the equation: ln(C/C0 ) = −kt [28]. Here C0 and C represent the initial equilibrium concentration and reaction concentration of acetone, respectively, k represents the apparent reaction rate constant, and t represents reactive time. Photocatalytic oxidation of the acetone is based on the following reaction: CH3 COCH3 + 4O2 → 3CO2 + 3H2 O
(1)
3. Results and discussion 3.1. Phase structure and morphology The XRD patterns of the as-prepared samples are shown in Fig. 1. The crystallinity of the HAP (Fig. 1a) is confirmed by the reflections observed at 2 values of 25.8◦ , 31.7◦ , 32.17◦ , 33.0◦ , 39.8◦ , 46.6◦ and 49.5◦ (JCPDS cards no. 74-565). Both the anatase (JCPDS cards no. 21-1272) and rutile phases (JCPDS cards no.21-1276) are observed in pure TiO2 and its corresponding doping samples (Fig. 1b–d). According to the equation given by Spurr and Myers [29], the relative contents of anatase (compared with rutile phase) in the TiO2 , N-TiO2 and HAP-N-TiO2 were 69.47%, 71.08% and 72.81%, respectively. In general, the crystal structure of TiO2 greatly affects photocatalytic activity. P25 (TiO2 ) is considered one of the most active photocatalyst, and its activity is believed to arise from its phase composition (75% anatase, 25% rutile) and from the good particle contact between anatase and rutile. Obviously, our samples have similar phase structure with P25, which are expected to result in fine photocatalytic activity. From Fig. 1d, it is also found the HAP phase (at 2 values of 32.17◦ ) in the co doped catalysts. In addition, it is noteworthy that N-doping has almost no influence on the phase structure of TiO2 nanocrystals; the probable reason is that the concentration of N is too low to be detected by XRD. The transmittance electron microscopy (TEM) images of samples were shown in Fig. 2. As can be seen from the images, the HAP shows rod-like structure in the range of about 100–200 nm length and 30–50 nm width. For the HAP-N-TiO2 samples, the monodisperse N-TiO2 nanocrystals with diameter about 10 nm can be clearly visible in the surface of HAP nanorod. The N-TiO2 nanocrystals were identified as polycrystalline anatase and rutile mixed structure from SAED pattern (the inset picture in Fig. 2b). The dark field image showing in Fig. 2c was taken from the diffraction rings of TiO2 indicated on the SAED pattern, which further identified that the dark spots on the surface of HAP were TiO2 nanocrystals. Similarly, the concentration of N is too low to be detected by SAED pattern.
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Fig. 2. TEM images of (a) HAP, (b) bright field electron image of HAP-N-TiO2 , and (c) dark field electron image taken from the diffraction rings in the SAED pattern [inset of (b)].
3.2. XPS analysis Fig. 3. XPS spectra of (a) N 1s, (b) Ti 2p, and (c) O 1s.
The surface chemical compositions and chemical states of asprepared samples were investigated using XPS as shown in Fig. 3, where, the C element at 284.8 eV can be ascribed to the adventitious hydrocarbon from the XPS instrument. The relative atomic concentration of N element in N-TiO2 is 1.0 at% based on the XPS data. High-resolution N 1 s spectra of as-prepared samples are shown in Fig. 3a. For N-TiO2 samples, one peak at 397.1 eV is only observed which is generally known as the N atom replacing the oxygen atoms in the TiO2 crystal lattice to form an N-Ti–N bond [30,31]. For HAP-
N-TiO2 samples, the peak at 396.2 eV is attributed to the Ti–N-Ti linkage, indicating that N atom is substitutionally doped into the TiO2 lattice; the peak at 399.8 reveals the presence of oxynitride such as Ti–N–O and Ti–O–N on the surface, possibly in interstitial location [32–35]. The above results show that nitrogen atoms have been successfully introduced into N-TiO2 and HAP-N-TiO2 samples.
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Fig. 4. UV–vis adsorption spectra of different samples.
Fig. 5. Photocurrent response of different samples.
The high-resolution XPS spectra of Ti 2p and O 1s region are shown in Fig. 3b and c, respectively. The peaks of Ti 2p are always regular showing the expected doublet with a binding energy (BE) of Ti 2p3/2 at 458.3 ± 0.2 eV and Ti 2p1/2 at 464.2 ± 0.2 eV, which is typical of Ti (IV) in the oxide [35]. The XPS spectra of O 1s (Fig. 3c) are fitted as two peaks by multiple Gaussians. The peak at about 530.3 eV is due to oxygen in the TiO2 crystal lattice and the peak at about 532.1 eV is due to hydroxyl oxygen. The hydroxyl oxygen peak of the HAP-N-TiO2 is more intensive than that of the undoped TiO2 and N-TiO2 samples due to excess hydroxyl introduced by HAP. From all of the results we can conclude that nitrogen and HAP have been successful incorporated into the titania by our method.
and the intragap localized states of N together contribute to the red shift of the absorption edge. Table 2 shows the BET surface area of different catalysts. The order of BET specific surface areas was as follows: HAP-N-TiO2 > NTiO2 > P25 > TiO2 . For HAP-N-TiO2 sample, the BET specific surface areas increase with the increasing of HAP concentration. It is widely accepted that photocatalysts with high specific surface areas and porous structures are beneficial to the enhancement of photocatalytic performance, due to more surface active sites, easier mass transportation, and higher light-harvesting efficiency [41]. So, HAPN-TiO2 samples are expected to exhibit improved photocatalytic performance.
3.3. UV–vis spectra and BET surface area
3.4. Photocurrent measurement
Fig. 4 shows the UV–vis/DRS of HAP, pure TiO2 , P25, N-TiO2 , and HAP-N-TiO2 , respectively. From the figure it is clear that the HAP shows no obvious adsorption in the range of 250–800 nm. Compared to the absorption spectra of pure TiO2 and P25, the absorption edge of N-TiO2 and HAP-N-TiO2 was obviously shifted to the visible light range. The band gap energy (EB ) was calculated using the equation, g = 1240/EB , where g is the optical absorption threshold [36]. The values of band gap and absorption threshold were shown in Table 1. The optical bandgap is 2.75 eV for the N-TiO2 and 2.76 eV for the HAP-N-TiO2 sample, which are much smaller than that of P25 (3.15 eV) and undoped TiO2 (3.10 eV). Obviously, the introduce of nitrogen results in the adsorption edge’s red shift. However, the origin of the adsorption edge’s red shift is hitherto controversial. Some studies have proposed that it is caused by a narrowing band gap via a mixed N 2p state with O 2p state in the valence band, while others have suggested the appearance of intragap localized states of dopants [37–39]. Additionally, a point was raised about the formation of oxygen vacancies and the emerged color centers which could contribute to absorb visible light [40]. For our samples, on the basis of above-mentioned XPS analysis, it is possible that the narrowing band gap, resulted from mixed N 2p with O 2p states,
The photocurrent response measurement was carried out under visible light pulsed irradiation to investigate the photo-induced charges separation efficiency of different samples. The results were shown in Fig. 5. The working electrode potentials were located at 0 V to simulate the same working condition as photocatalysis reaction system. It was found that the photocurrent of the P25, undoped TiO2 , N-TiO2 , and HAP-N-TiO2 electrodes were 0.18, 0.16, 0.61 and 0.83 A, respectively. The photocurrent of the N-TiO2 electrode was about 3.3 as high as those of the P25 electrode, because N doping led to a red shift of the spectrum onset and higher absorption in the visible light in the 380–800 nm range, and the HAP-N-TiO2 sample increased the photocurrent further to be 4.6 times as high as that of the P25 electrode. The photocurrent was in the order of HAP-N-TiO2 > N-TiO2 > pure TiO2 . Compared with the N-TiO2 , the HAP-N-TiO2 has a similar absorption in visible light, but can generate much more photoinduced electrons and holes, correspondingly resulting in higher photocurrent. Based on the results of BET analysis and photocurrent measurement, the possible mechanism for HAP-N-TiO2 decreasing the carrier recombination and improving the separation efficiency of photoinduced electrons and holes, generating the maximum photocurrent can be due to the introduction of HAP. HAP is a common solid-phase adsorbent for air and water purification because of its large specific surface area, abundant micropores and a series of surface functional groups. For HAP-N-TiO2 samples, the HAP on the surfaces of N-TiO2 can attract more photogenerated electrons and holes, the photogenerated electrons and holes generated on the surface of HAP-N-TiO2 samples more inclined to remain adsorbed on its surface for its much stronger adsorption ability than the other samples. Therefore, there exists large amounts of separated
Table 1 The absorption thresholds and corresponding photon energies of different samples. Sample
Absorption threshold (nm)
Corresponding photon energy (eV)
TiO2 P25 N-TiO2 HAP-N-TiO2
400 394 451 449
3.10 3.15 2.75 2.76
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Table 2 The specific surface area for different samples. Sample
TiO2
P25
N-TiO2
BET surface area (m2 /g)
81.46
58.52
89.65
HAP-N-TiO2 2%
5%
8%
10%
12%
15%
113.44
148.29
169.27
177.67
189.22
207.48
Note: X% refers to X%-HAP-N-TiO2 sample. Table 3 Adsorptive ability of the catalysts for acetone. Sample
Initial concentration (%)
Concentration after 1 h in dark (%)
Concentration after 2 h in dark (%)
HAP TiO2 P25 N-TiO2 HAP-N-TiO2
100 100 100 100 100
77 100 100 99 94
76 100 100 99 93
electrons and holes on the surface of HAP-N-TiO2 samples, thus generates higher photocurrent. The excessive electrons and holes are accepted by surface adsorbed O2 and H2 O, and formed more • OH radicals to participate the photocatalytic reaction. 3.5. Visible light-induced photocatalytic activity To evaluate the photodegradation capability of different samples, we examined the decomposition of gaseous acetone under visible light irradiation. The dark adsorption study of acetone on supported catalysts is presented in Table 2. It can be seen that after 1 h in the dark, about 23% acetone were absorbed by HAP, 6% acetone were absorbed by HAP-N-TiO2 and 1% acetone were absorbed by N-TiO2 , respectively, but adsorption of acetone by the undoped TiO2 and P25 within the same time is approximately zero. Another 1 h in the dark, the concentration of acetone change little, this means that adsorption equilibrium can be reached after 1 h in the dark (Table 3). To evaluate and compare the photocatalytic activity of the HAP-N-TiO2 samples, the degradation reactions of gaseous acetone under visible light irradiation were performed as photoreaction probes. The kinetics process of photocatalytic degradation of acetone and the apparent reaction rate constant k of different samples are shown in Fig. 6, and Degussa P25 was used as the reference sample for comparison purpose. The undoped TiO2 and P25 sam-
Scheme 1. Proposed mechanism for the visible light photodegradation of gaseous pollution on a HAP-N-TiO2 catalyst.
ples show poor photocatalytic activities in visible light region. This is assigned to the large band gap of the titanium dioxide. The activity of N-TiO2 sample is higher than that of the undoped TiO2 and P25, its rate constant k reaches 0.0027 min−1 . For the HAP-N-TiO2 sample, the photocatalytic activity significantly increases. As shown in Fig. 6, most of the HAP-N-TiO2 samples demonstrated considerably high photoactivity under visible light irradiation and the 10%-HAP-N-TiO2 sample exhibited the best photocatalytic efficiency in degradation of acetone in all tested doped and undoped TiO2 samples. Its k value is 0.077 min−1 which is 3.9 times to that of N-TiO2 and 11 times to that of P25. The higher photocatalytic activity of the 10%-HAP-N-TiO2 sample is partially due to its high specific surface area and improved charge separation. When the addition value of HAP was higher than 10%, although the adsorption of the composite increases, the relative content of TiO2 in the composite decreases. As a result, the photo-induced electrons/hole pairs and the photocatalytic activity in a given area decrease. On the basis of the above experimental results, the photocatalytical mechanisms for HAP-N-TiO2 system under visible light irradiation were proposed and elucidated in Scheme 1. In general, anatase TiO2 with 3.2 eV banding energy irradiated by visible light cannot effectively produce enough photo-induced electrons and holes, which are considered to play an essential role in the photodegradation reactions. While the N doped TiO2 will form new states just above the valence band. The band gap between the new states and the conduction band is about 2.75 eV and thus the catalysts will be excited easily by visible light to produce the photo-induced electrons. Meanwhile, the improved adsorption of HAP-N-TiO2 system can adsorb more O2 and H2 O molecules which can capture more electronic-hole pairs to form abundant • OH radicals which decomposed acetone into CO2 and H2 O on the surface of TiO2 . 4. Conclusions
Fig. 6. Photocatalytic degradation of acetone by (a) TiO2 , (b) P25, (c) N-TiO2 , (d) 15%-HAP-N-TiO2 , (e) 12%-HAP-N-TiO2 , (f) 2%-HAP-N-TiO2 , (g) 5%-HAP-N-TiO2 , (h) 8%-HAP-N-TiO2 , and (i) 10%-HAP-N-TiO2 .
HAP-N-TiO2 photocatalysts were successfully prepared with a facile one-pot method. The novel photocatalysts showed obvious visible light photocatalytic activity in decomposition gaseous acetone, while the 10%-HAP-N-TiO2 sample gave the best photocatalytic activity and demonstrated to be far superior to that of the commercial Degussa P25 counterpart. The probable photocatalytical mechanism was proposed. The high photoactivity of
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the HAP-N-TiO2 can be attributed to the results of the synergistic effect, the synergism arises from cooperation of HAP adsorbing the chemical compound and the TiO2 photocatalyst generating oxygen reactive species that diffuse and react with the molecules located on the HAP. The doped nitrogen specie plays a key role in narrowing the band gap and expanding the photoactivity to visible light region. Acknowledgments We are grateful for the financial support from the National Program on key Basic Research Project (973 Grant Nos. 2009CB939704 and 2009CB939705), and the National Natural Science Foundation of China (No. 10974148). References [1] G. Liu, L.Z. Wang, H.G. Yang, H.M. Cheng, G.Q. Lu, J. Mater. Chem. 20 (2010) 831–843. [2] A. Fujishima, X.T. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582. [3] X.G. Han, Q. Kuang, M.S. Jin, Z.X. Xie, L.S. Zheng, J. Am. Chem. Soc. 131 (2009) 3152–3153. [4] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [5] G.G. Nakhate, V.S. Nikam, K.G. Kanade, S. Arbuj, B.B. Kale, J.O. Baeg, Mater. Chem. Phys. 124 (2010) 976–981. [6] S. Shanmugam, A. Gedanken, Small 3 (2007) 1189–1192. [7] T.W. Kim, S.G. Hur, S.J. Hwang, H. Park, W.Y. Choi, J.H. Choy, Adv. Funct. Mater. 17 (2007) 307–314. [8] L.L. Zhang, F.J. Lv, W.G. Zhang, R.Q. Li, H. Zhong, Y.J. Zhao, Y. Zhang, X. Wang, J. Hazard. Mater. 171 (2009) 294–300. [9] M. Hamadanian, A. Reisi-Vanani, A. Majedi, Mater. Chem. Phys. 116 (2009) 376–382. [10] A. Bansal, S. Madhavi, T.T.Y. Tan, T.M. Lim, Catal. Today 131 (2008) 250–254. [11] Y.Q. Gai, J.B. Li, S.S. Li, J.B. Xia, S.H. Wei, Phys. Rev. Lett. 102 (2009) 036402. [12] G. Liu, Y.M. Zhao, C.H. Sun, F. Li, G.Q. Lu, H.M. Cheng, Angew. Chem. Int. Ed. 47 (2008) 4516–4520.
659
[13] B. Cojocaru, S. Neatu, V.I. Parvulescu, V. Somoghi, N. Petrea, G. Epure, M. Alvaro, H. Garcia, ChemSusChem 2 (2009) 427–436. [14] X.B. Chen, Y.B. Lou, A.C.S. Samia, C. Burda, J.L. Gole, Adv. Funct. Mater. 15 (2005) 41–49. [15] Y.H. Ao, J.J. Xu, S.H. Zhang, D.G. Fu, Appl. Surf. Sci. 256 (2010) 2754–2758. [16] K.X. Song, J.H. Zhou, J.C. Bao, Y.Y. Feng, J. Am. Ceram. Soc. 91 (2008) 1369–1371. [17] W. Zhang, L.D. Zou, L.Z. Wang, Appl. Catal. A: Gen. 371 (2009) 1–9. [18] J. Ryu, K.Y. Kim, B.D. Hahn, J.J. Choi, W.H. Yoon, B.K. Lee, D.S. Park, C. Park, Catal. Commun. 10 (2009) 596–599. [19] H. Kominami, K. Yukishita, T. Kimura, M. Matsubara, K. Hashimoto, Y. Kera, B. Ohtani, Top. Catal. 47 (2008) 155–161. [20] T. Nonami, H. Hase, K. Funakoshi, Catal. Today 96 (2004) 113–118. [21] A.M. Hu, M. Li, C.K. Chang, D.L. Mao, J. Mol. Catal. A: Chem. 267 (2007) 79–85. [22] K. Ozeki, J.M. Janurudin, H. Aoki, Y. Fukui, Appl. Surf. Sci. 253 (2007) 3397–3401. [23] M. Pratap Reddy, A. Venugopal, M. Subrahmanyam, Water Res. 41 (2007) 379–386. [24] M.R. Elahifard, S. Rahimnejad, S. Haghighi, M.R. Gholami, J. Am. Chem. Soc. 129 (2007) 9552–9553. [25] H. Kanai, M. Nakao, S. Imamura, Catal. Commun. 4 (2003) 405–409. [26] C.M. Ho, W.Y. Yu, C.M. Che, Angew. Chem. Int. Ed. 43 (2004) 3303–3307. [27] A. Venugopal, M.S. Scurrell, Appl. Catal. A: Gen. 245 (2003) 137–147. [28] J.G. Yu, Q.J. Xiang, M.H. Zhou, Appl. Catal. B: Environ. 90 (2009) 595–602. [29] R.A. Spurr, H. Myers, Anal. Chem. 29 (1957) 760–762. [30] X.P. Wang, T.T. Lim, Appl. Catal. B: Environ. 100 (2010) 355–364. [31] G.D. Yang, Z. Jiang, H.H. Shi, T.C. Xiao, Z.F. Yan, J. Mater. Chem. 20 (2010) 5301–5309. [32] M. Mrowetz, W. Balcerski, J. Colussi, M.R. Hoffmann, J. Phys. Chem. B 108 (2004) 17269–17273. [33] J. Ananpattarachai, P. Kajitvichyanukul, S. Seraphin, J. Hazard. Mater. 168 (2009) 253–261. [34] F. Spadavecchia, G. Cappelletti, S. Ardizzone, C.L. Bianchi, S. Cappelli, C. Oliva, P. Scardi, Matteo Leoni, P. Fermo, Appl. Catal. B: Environ. 96 (2010) 314–322. [35] S.H. Kang, H.S. Kim, J.Y. Kim, Y.E. Sung, Mater. Chem. Phys. 124 (2010) 422–426. [36] G. Wu, T. Nishikawa, B. Ohtani, A. Chen, Chem. Mater. 19 (2007) 4530–4537. [37] A.V. Emeline, V.N. Kuznetsov, V.K. Rybchuk, N. Serpone, Int. J. Photoenergy (2008) 1–19. [38] V.N. Kuznetsov, N. Serpone, J. Phys. Chem. C 113 (2009) 15110–15123. [39] M.D. Arienzo, N. Siedl, A. Sternig, R. Scotti, F. Morazzoni, J. Bernardi, O. Diwald, J. Phys. Chem. C 114 (2010) 18067–18072. [40] A.V. Emeline, N.V. Sheremetyeva, N.V. Khomchenko, V.K. Ryabchuk, N. Serpone, J. Phys. Chem. C 111 (2007) 11456–11462. [41] Q.H. Zhang, L. Gao, J.K. Guo, Appl. Catal. B: Environ. 26 (2000) 207–215.