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Photocatalytic Enhancement of Hybrid C3N4/TiO2 Prepared via Ball Milling Method Jianwei Zhou,a,b Mo Zhangband Yongfa Zhu*b 5
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Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x C3N4/TiO2 hybrid photocatalysts with highly enhanced photocatalytic performances were prepared by a facile ball milling way. The layered structure of g-C3N4 was formed on the surface of TiO2. The mechanochemical process can promote C3N4 dispersing on the surface of TiO2 particles to form single layer hybrid structure and multi-layer core-shell structure. The photocatalytic activities of C3N4 /TiO2 under visible and UV light irradiation were 3.0 and 1.3 times that of pure g-C3N4 and TiO2, respectively. Under the visible light and UV irradiation, the photocurrent response was up to 2.5 times and 1.5 times as high as that of the pure TiO2 and C3N4, respectively. The obvious performance enhancement of gC3N4/TiO2 was mainly attributed to high separation and migration efficient of electron–hole pairs. 45
Introduction 15
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The photocatalysis is considered to be one of the most promising techniques for the control of environmental pollutants. Owing to its high activity, low cost and non-toxicity, TiO2 is becoming one of the most popular photocatalysts used in the photodegradation of organic and inorganic compounds. However, the wide bandgap of 3.2 eV and inability of absorbing visible light are the major drawback in its practical applications. There have been many attempts to enhance its photocatalytic performance, such as doping [1,2], depositing noble metal [3,4] and coupling with semiconductor [5,6]. Although its visible light absorption could be improved via nonmetal doping, doped TiO2 suffer from decrease in thermal stability and catalytic activity. It has been reported that hybrid composite based on TiO2 could promote the separation of photogenerated charge carriers and improve the photocatalytic efficiency [7-9].Conjugated material is proposed to be a good candidate for improving the transportation of photocarriers in the photocatalysis process by forming electronic interaction with photocatalysts, which possesses unique properties in electron or hole transporting. The semiconductors hybridized by conjugative π structure materials, such as TiO2, ZnO, Bi2WO6 and BiPO4 modified by C3N4, PANI and RGO, exhibited enhanced photocatalytic activity and chemical stability [10-15]. Recently, graphite-like carbon nitride (g-C3N4) with its intrinsic characteristic structure units named tri-striazine units has attracted attentions in photocatalytic field. The band-gap of gC3N4 is 2.73 eV, indicating that the response wavelength of gC3N4 is up to 450 nm. The g-C3N4 exhibits an excellent photocatalytic performance for the splitting of water into hydrogen gas using solar energy [16~19] and degrading organic This journal is © The Royal Society of Chemistry [year]
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pollutants under visible light irradiation, making them valuable materials for photocatalysis driven applications [20~28]. However, the photocatalytic efficiency of g-C3N4 is still limited due to the high recombination rate of photogenerated electron– hole pairs. Several methods have been developed to improve the photocatalytic performance of g-C3N4. For example, dye sensitization [29], transition metal doping [30] and coupled semiconductor [31-34]. These catalysts were mostly prepared by complicated method in liquid phase [35-40], the solid phase synthesis is rarely reported [41~44]. In this work, a facial synthesis of g-C3N4/TiO2 hybrid photocatalysts by one-step milling was developed. A series of g-C3N4/TiO2 composites with different mass fraction of C3N4 were successfully synthesized and characterized by various techniques. Remarkably, the photodegradation activity of g-C3N4/TiO2 for methylene blue (MB) degradation was enhanced compared with pure g-C3N4 and TiO2, which is attributed to the improvement of separation efficiency of photogenerated electron–hole pairs in g-C3N4/TiO2 composites. This simple mechanical milling method could be used as a universal pathway to improve the activity of photocatalyst and applied in the environmental remediation.
Experimental section Chemicals
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TiO2 (99.5% anatase, analytically pure grade) was purchased from Nanjing High Technology Nano-material Co.Ltd. without further purification. Other chemicals used in the experiments were of analytically pure and used without further purification. Preparation of g-C3N4/TiO2 photocatalyst The g-C3N4 was synthesized by directly heating melamine. In a typical run, 5 g melamine powder was put into an alumina [journal], [year], [vol], 00–00 |1
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crucible and heated in a muffle furnace at 520 °C for 4 h with a heating rate of 5 °C·min-1. After cooling to room temperature, the product was collected and ground into powder. The preparation of g-C3N4/TiO2 photocatalyst was carried out in a ball miller (XQM-0.4, made in Changsha, China). The procedure of preparation is as follows: TiO2 powder and agate ball are mixed in the agate ball milling tank with a ratio of 1:10, and then a certain amount of C3N4 is added. After being milled for a certain time at the speed of 350 rpm, the final samples are used for the determination of photocatalytic activity and characterization.
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Results and discussion
Characterization
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The surface areas and pore size were measured from N2 sorptiondesorption isotherms at 77K with an adsorption apparatus (TriStar II 3020, USA). The surface characteristics and microcrystalline structure of the photocatalysts were observed by using a TEM (HT-7700, Hitachi) with 100 kV scanning voltages, and HRTEM (JEOL JEM-2011). DRS measurements were carried out using a U-3010 (Hitachi, Japan). UV–Vis spectrophotometer equipped with an integrating sphere. The analysis ranged from 200 to 800 nm, and BaSO4 was used as a reflectance standard. To determine the crystal phase of the photocatalysts, X-ray diffraction measurement was carried out at room temperature using a XRD spectroscope (BRUKER D8 Advance) with CuKα radiation and the scanning speed of 5o min-1. The accelerating voltage and emission current are 40 kV and 40 mA , respectively. XPS examination was carried out on the PHI Quantera SXM system. Raman spectra were obtained by using a HORIBA HR800 confocal microscope Raman spectrometer employing an Ar-ion laser. To investigate the transition of photogenerated electrons before and after g-C3N4 modification, TiO2 and C3N4/TiO2 electrodes were prepared as follows: 5 mg of photocatalyst was suspended in 10 mL ethanol to produce slurry, which was then spread on a 2cm×4cm indium-tin oxide (ITO) glass electrode. Electrodes were exposed to UV light for 1h to eliminate ethanol and subsequently calcined at 120°C for 5h. The photoelectric performances were measured on an electrochemical system (CHI660B, China). A standard three-electrode cell with a working electrode (as-prepared photocatalyst), a platinum wire counter electrode, and a standard calomel electrode (SCE) as reference electrode were used in photoelectric studies. And 0.1M Na2SO4 was used as electrolyte solution. Potentials are given with reference to the SCE. The photoresponses of the photocatalysts as UV light (or visible light) on and off were measured at 0.0 V. Electrochemical impedance spectra (EIS) were recorded in the open circuit potential mode.
ambient temperature, 25 mg of photocatalyst was totally dispersed in the MB aqueous solution (50 mL, 0.03 mM). Before irradiation, the suspensions were magnetically stirred in the dark for 60 min to get absorption-desorption equilibrium between the photocatalyst and MB. At certain time intervals, 3 mL aliquots were sampled and centrifuged to remove the particles. The concentration of MB was analyzed by recording the absorbance at the characteristic band of 664 nm using a Hitachi U-3010 UVvis spectrophotometer.
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The sizes and the morphologies of the representative samples were examined by TEM. Fig. S1a and b display the irregular and agglomerated TiO2 spheres and g-C3N4 sheets, respectively. The agglomeration of TiO2 particles can be observed. The TEM images of g-C3N4/TiO2 (Fig. S1c,d) reveal that the hybrid materials are mainly irregularly agglomerated elliptic particles with the average size of 20-40 nm. The HR-TEM images of the g-C3N4/TiO2 composite (Fig. 1) clearly reveal the interface between g-C3N4 and TiO2 nanoparticles and an interplanar distance of 0.351 nm which corresponds to the (101) crystal plane of TiO2. As can be seen from the images, the TiO2 particles are surrounded by C3N4. Figure 1b (3%-C3N4/TiO2) shows the thickness of C3N4 layer on the surface is about 0.468 nm, corresponding to the C3N4 monolayer thickness. Therefore, it can be estimated that TiO2 is surrounded by monolayer of C3N4 and a core-shell-like structure is developed. HR-TEM images of 5%C3N4/TiO2 (Fig. 1c,d) demonstrate that TiO2 particles are surrounded by multilayer of C3N4 with the thickness of 3 nm, indicating the hybrid structure of C3N4/TiO2 rather than physical mixture of two separate phases TiO2 and g-C3N4.
Photocatalytic experiments
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The photocatalytic activities of g-C3N4/TiO2 were evaluated by degradation of MB aqueous solution under UV light or visible light irradiation. A 500WXe-lamp with a 420 nm cutoff filter was used as the light source to provide visible light irradiation. A 15 W medium pressure mercury lamp was used as the UV light. A series of 50 mL quartz tubes of 2.0 cm diameter were used as the reaction vessel. The temperature of the reaction solution was maintained at approximately 25°C to avoid temperature effects in the reaction. Experimental procedures were as follows: At 2|Journal Name, [year], [vol], 00–00
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Fig. 1HR-TEM images of 3%-C3N4/TiO2 (a), (b) and 5%-C3N4/TiO2 (c), (d)
Crystal structure of the samples was determined by XRD, as shown in Figure 2a. TiO2 exhibits dominant anatase phase diffraction peaks, the 2θ values of25.3°, 37.8°, 48.0°, 53.9°, This journal is © The Royal Society of Chemistry [year]
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62.7°, and 75.0° are undisputedly indexed to (101), (004), (200), (105), (204), and (215) crystal planes of anatase TiO2 (JCPDF No. 21-1272). The XRD pattern of g-C3N4 was also displayed in Fig.1a, the peak at 13.10° corresponds to (100) plane of tri-striazine units[45], while the peak at 27.40° agrees with interlayer stacking of aromatic segments, which is indexed as (002) peak of the stacking of the conjugated aromatic system [46]. Besides, little obvious change of the peaks of g-C3N4/TiO2 can be observed with the increase of g-C3N4 content. This result indicates that the hybrid materials were indeed composed of gC3N4 and TiO2. As shown in Fig. 2b, the diffraction peaks of C3N4 are not observed in XRD patterns, which are caused by the much lower intensity of XRD peaks of C3N4 than that of TiO2. This may be related to the hybridized and dispersed state of C3N4 on TiO2 surface.
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Fig. 4XPS spectra of 3%-C3N4/TiO2.
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Fig. 3 Raman spectra of g-C3N4/TiO2 and pure TiO2 (a) and expanded spectra in the range 100–250 cm-1 (b).
Fig. 3 shows the Raman spectra of pure TiO2 and g-C3N4/TiO2 hybrid composites. Generally, the Ti-O-Ti network structure can be identified by the Raman peaks over 140–800 cm-1 range, wherein six characteristic Raman active fundamental modes at 142 (Eg), 197 (Eg), 397 (B1g),518 (A1g+B1g), and 640 cm-1(Eg) correspond to anatase TiO2. As is shown in Fig. 3(a), these peaks can be attributed to the typical anatase TiO2. However, Fig. 4(b) shows the Raman peaks of g-C3N4/TiO2 red-shift slightly compared with pure TiO2. The Eg mode at 142.0 cm-1 of hybrid g-C3N4/TiO2 shifts to 145 cm-1, implying the formation of gC3N4/TiO2 hybrid composite. Fig. 4A illustrates high-resolution XPS spectra of different elements. The peak position of different atoms is determined by internally referencing the adventitious carbon at a binding energy of 284.8 eV. The survey spectrum (Fig. 4A) shows that the gC3N4/TiO2 surface is composed of C, N, O and Ti. Fig. 4B shows the XPS spectrum of C1s.The two peaks can be distinguished to be centered at 284.87 eV and This journal is © The Royal Society of Chemistry [year]
288.47 eV, respectively. The peak at 284.87 eV is exclusively assigned to the adventitious hydrocarbon from the XPS instrument. Another one at 288.47 eV is identified as carbon atoms that have one double and two single bonds with three N neighbors [47,48]. Fig. 4C is the high-resolutionN1s spectrum of the g-C3N4/TiO2. The asymmetrical feature of the observed N1s peaks suggests the coexistence of distinguishable models, pointing out chemically different N species in the g-C3N4, with main N1s binding energy at 399.17 eV. From Fig. 4D and E, the binding energy of Ti2p can be observed at around 458.67 (Ti2p3/2) and 464.57 eV (Ti2p1/2), while O1s is about 530.07 eV. All of them are in good agreement with previous report [49]. In comparison with those of pure TiO2, the binding energies of Ti2p in g-C3N4/TiO2 shift to higher energies(TableS1), indicating the electron interaction between Ti and N in g-C3N4/TiO2, which results from the formation of the coordination bond between N and Ti. 1.5 1.2 0.9 0.6 0.3
C3N4 10%-C3N4/TiO2 7%-C3N4/TiO2 5%-C3N4/TiO2 3%-C3N4/TiO2 TiO2
0.0 210 280 350 420 490 560 630 700
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Fig.5 UV-Vis diffuse reflectance spectra of C3N4, TiO2 and C3N4/TiO2
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UV-vis diffuse reflectance spectroscopy (DRS) was carried out to investigate the optical properties of the C3N4/TiO2 materials. As shown in Fig. 5, the absorption edge of C3N4 is about 450 nm. The optical absorption of g-C3N4/TiO2 is enhanced distinctly in both UV and visible light region compared to pure TiO2. This Journal Name, [year], [vol], 00–00 |3
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Fig. 2XRD patterns (a) and the enlarged XRD patterns (b) of C3N4, TiO2 and C3N4/TiO2 hybrid composite.
Absorbance
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suggests that the introduction of the C3N4 can possibly cause modifications in the fundamental process of electron-hole pairs formation during irradiation. Influence of mechanical milling on visible light photocatalytic activity The effect of ball milling rate on the photocatalytic activity of C3N4/TiO2 photocatalyst is shown in Fig. 6a. It can be seen that the ball milling rate influences the photocatalytic activity efficiently. The photocatalytic efficiency increases gradually with the increase of milling rate. When the ball milling rate is 350 rpm, the MB degradation rate of C3N4/TiO2 is about 3 times higher than that of C3N4 under visible light, indicating the hybrid structure of C3N4/TiO2 photocatalyst. Fig. 6b shows transient photocurrent responses of 3%-C3N4/TiO2 under visible light irradiation (λ>420 nm). It is well known that photocurrent is originated from the diffusion of the photogenerated electrons to the back contact and meanwhile the photoinduced holes are taken by the hole acceptor in the electrolyte. It is found that photocurrent response of theC3N4/TiO2 (350 rpm) is nearly 3 times as that of pure C3N4, suggesting more efficient separation and longer lifetime of photo excited electron-hole pairs of C3N4/TiO2 compared with C3N4.
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Fig.6 Visible light photocatalytic degradation of MB (C0=1×10-5 mol/L) by g-C3N4/TiO2 with various ball milling rate (r = 250 rpm, 300 rpm and 350 rpm) under 500 W Xe lamp irradiation, 420 nm cutoff filter and the reaction rate constant of photocatalysis by g-C3N4/TiO2 (inset) (a) Transient photocurrent responses of different milling rate under Xe lamp irradiation, 420nm cutoff filter. (b)
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Photocatalytic activities of the C3N4/TiO2 composites with different loading percentage of g-C3N4 were also investigated. As shown in Fig.7a, with the increased mass fraction of C3N4, the photocatalytic activity of g-C3N4/TiO2 is enhanced gradually, which is attributed to the visible light response from g-C3N4 and the improved photogenerated charge separation driven by buildin electric field at the interface between g-C3N4 and TiO2. As shown in Fig. 7b, fast and steady photocurrent response was observed for each light-on and light-off on both electrodes. The photocurrent density of the g-C3N4/TiO2 is almost 2~3 times as that of the g-C3N4. The great enhancement of photocurrent can be ascribed to the wider absorption spectrum region and stronger absorption of the g-C3N4/TiO2 than TiO2 and the improved charge separation and transportation efficiency through the interaction between the g-C3N4 and TiO2.
Fig.8 Electrochemical impedance spectroscopy (EIS) Nynquist plot of C3N4, 3%-C3N4/TiO2 (under visible light, 420 nm)
It can be seen from Fig. 8, EIS result reflects the impedance arc radius of the C3N4/TiO2 is smaller than that of C3N4 under visible light, indicating that C3N4/TiO2 demonstrates enhanced separation efficiency of photo-excited charge carriers compared with pure C3N4. In this regard, transient photocurrent response and EIS results reveal analogous trend to the photocatalytic activities of the samples. Influence of mechanical milling on UV light photocatalytic activity The influences of ball milling rate on the photocatalytic activity under UV light irradiation is shown in Fig. 9. The photocatalytic activity is enhanced gradually with the increase of ball milling rate. The optimum milling rate is 350 rpm, which is consistent with the results of photocurrent responses in the Fig. 9b. The anodic photocurrent spike produced by the UV light irradiation was observed with good reproducibility.
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Fig.7 Photocatalytic degradation of MB (C0=1×10-5 mol/L) by gC3N4/TiO2 with various weight content of C3N4 (3%, 5%, 7, and 10%) under 500 W Xe lamp irradiation, 420 nm cutoff filterand the reaction rate constant of photocatalysis by g-C3N4/TiO2 (inset) (a) Photocurrent responses of different weight content of C3N4 under Xe lamp irradiation, 420 nm cutoff filter (b)
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Fig.9 Effects of ball milling rate(r = 250 rpm, 300 rpm and 350 rpm) on the photocatalytic activity under UV light irradiation (15 W medium pressure mercury, dominant wavelength 254 nm, C0=1×10-5 mol/L) and the reaction rate constant of photocatalysis by g-C3N4/TiO2 (inset) (a) Transient photocurrent responses of different ball milling rate (r = 250 rpm, 300 rpm and 350 rpm) under UV light (15 W medium pressure mercury, dominant wavelength 254 nm) (b)
C3N4/TiO2 hybrid photocatalysts were obtained via a facile mechanical milling method. The dispersion and hybrid of conjugated molecule g-C3N4 on the surface of TiO2 was improved by mechanical ball milling process. The photocatalytic activity of g-C3N4/TiO2 was enhanced under UV light and visible light. The enhanced photocatalytic activities of the composite can be primarily attributed to the formation of hybrid structure in the contact interface between TiO2 and g-C3N4 which greatly promoted the separation of photogenerated electron-hole charge carriers. This mechanical milling method may open up a new avenue for exploring novel composite photocatalysts for environmental remediation.
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Acknowledges
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This work was partly supported by National Basic Research Program of China (973 Program) (2013CB632403), National High Technology Research and Development Program of China (2012AA062701) and Chinese National Science Foundation (20925725 and 21373121).
Notes and references a
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Fig.10 Photocatalytic activity of g-C3N4/TiO2 with different contents of C3N4 (1%,3%,5%,7% and 10%) under UV light (15 W medium pressure mercury, dominant wavelength 254 nm) and the reaction rate constant of photocatalysis by g-C3N4/TiO2 (inset) (a) Photocurrent responses of different content of C3N4 (3%,5%)under UV light irradiation (15 W medium pressure mercury, dominant wavelength 254 nm) (b)
As shown in Fig. 10a. The photocatalytic activity is enhanced gradually with the increased C3N4 loading. The photocatalytic rate of 5%-C3N4/TiO2 is about 1.3 times higher than that of TiO2 under UV light. Therefore, compared with TiO2, the photocatalytic activity of C3N4/TiO2 is not only significantly improved under visible light but also under UV light by mechanical milling. Fig. 10b shows the influences of C3N4 loading on the photocurrent under UV light irradiation. The photocurrent density of the 5%-C3N4/TiO2 is almost 2 times as that of the TiO2. The obvious enhancement of photocurrent can be ascribed to the improved charge separation and transportation efficiency through the strong interaction between the g-C3N4 and TiO2. With the exited of UV, the photogenerated electrons of TiO2 could transfer to C3N4 and the holes in the valence band of TiO2 could directly transfer to the HOMO orbital of C3N4, which leads to more efficient charge separation and enhanced photocatalytic activity of C3N4/TiO2. Under visible light irradiation, the excited electron from HOMO to LUMO orbital of C3N4 could directly inject into the conduction band (CB) of TiO2, leading to the dramatic visible light photocatalytic activity of C3N4/TiO2.
Conclusions This journal is © The Royal Society of Chemistry [year]
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Institute of energy and fuel, Xinxiang University, Xinxiang, China 453003 b Department of Chemistry, Tsinghua University, Beijing, China 100084 † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/ 1 R. Asahi, T. Morikawa, T. Ohwaki, K.Aoki, Y. Taga, Science 2001, 293, 269-271. 2 P. V.Kamat,Pure Appl. Chem. 2002, 74, 1693-1706. 3 B. Kraeutler, A. J.Bard,J. Am. Chem. Soc. 1978, 100, 4317-4318. 4 V. Subramanian, E.E. Wolf, P. V.Kamat,J. Am. Chem. Soc. 2004, 126, 4943-4950. 5 H. G. Kim, P. H. Borse, W. Choi, J. S. Lee, Angew. Chem. Int. Edit. 2005, 44, 4585-4589. 6 X. Z. Fu, L. A. Clark, Q. Yang, M. A. Anderson, Environ. Sci. Technol. 1996, 30, 647-653. 7 Y. Tao, C. Y. Wu, D. W. Mazyck,Ind. Eng. Chem. Res. 2006, 45, 51105116. 8 H. Wang, X. Quan, H. T. Yu, S. Chen,Carbon2008, 46, 1126-1132. 9A. Kongkanand, R. M. Dominguez, P. V. Kamat,Nano Lett. 2007, 7, 676-680. 10X. F. Lu, Q. L. Wang, D. L. Cui,J. Mater. Sci. Technol. 2010, 26, 925930. 11Y. J.Wang, R. Shi, J. Lin, Y. F. Zhu, Appl. Catal. B 2010, 100, 179183. 12 L. W.Zhang, Y. J. Wang , T. G. Xu, S. B. Zhu, Y. F. Zhu, J. Mol. Catal. A: Chem. 2010, 331, 7-14. 13L. W. Zhang, H. B. Fu, Y. F.Zhu,Adv. Funct. Mater. 2008, 18, 21802189. 14 H. Zhang, R. L. Zong, J. C. Zhao, Y. F. Zhu,Environ. Sci. Technol. 2008, 42, 3803-3807. 15 J. Lin, R. L. Zong, M. Zhou, Y. F.Zhu,Appl. Catal. B 2009, 89, 425435. 16 X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M.Antonietti,Nat. Mater. 2009, 8, 76-80. 17 S. C. Yan, S. B. Lv, Z. S. Li, Z. G. Zou, Dalton Trans. 2010, 39, 1488-1491. 18 F. Goettmann, A. Fischer, M. Antonietti, Arne. Thomas,Chem. Commun. 2006, 43, 4530-4532. 19 F. Goettmann, A. Thomas, M.Antonietti,Angew. Chem. Int. Edit. 2007, 46, 2717-2720. 20 F. Dong, L. W. Wu, Y. J. Sun, M. Fu, Z. B. Wu, S. C. Lee,J. Mater. Chem. 2011, 21, 15171-15174.
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Photocatalytic Enhancement of Hybrid C3N4/TiO2 Prepared via Ball Milling Method Jianwei Zhou,a,b Mo Zhangband Yongfa Zhu*b 5
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Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x C3N4/TiO2 hybrid photocatalysts with highly enhanced photocatalytic performances were prepared by a facile ball milling way. The layered structure of g-C3N4 was formed on the surface of TiO2. The mechanochemical process can promote C3N4 dispersing on the surface of TiO2 particles to form single layer hybrid structure and multi-layer core-shell structure. The photocatalytic activities of C3N4 /TiO2 under visible and UV light irradiation were 3.0 and 1.3 times that of pure g-C3N4 and TiO2, respectively. Under the visible light and UV irradiation, the photocurrent response was up to 2.5 times and 1.5 times as high as that of the pure TiO2 and C3N4, respectively. The obvious performance enhancement of gC3N4/TiO2 was mainly attributed to high separation and migration efficient of electron–hole pairs. 45
Introduction 15
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The photocatalysis is considered to be one of the most promising techniques for the control of environmental pollutants. Owing to its high activity, low cost and non-toxicity, TiO2 is becoming one of the most popular photocatalysts used in the photodegradation of organic and inorganic compounds. However, the wide bandgap of 3.2 eV and inability of absorbing visible light are the major drawback in its practical applications. There have been many attempts to enhance its photocatalytic performance, such as doping [1,2], depositing noble metal [3,4] and coupling with semiconductor [5,6]. Although its visible light absorption could be improved via nonmetal doping, doped TiO2 suffer from decrease in thermal stability and catalytic activity. It has been reported that hybrid composite based on TiO2 could promote the separation of photogenerated charge carriers and improve the photocatalytic efficiency [7-9].Conjugated material is proposed to be a good candidate for improving the transportation of photocarriers in the photocatalysis process by forming electronic interaction with photocatalysts, which possesses unique properties in electron or hole transporting. The semiconductors hybridized by conjugative π structure materials, such as TiO2, ZnO, Bi2WO6 and BiPO4 modified by C3N4, PANI and RGO, exhibited enhanced photocatalytic activity and chemical stability [10-15]. Recently, graphite-like carbon nitride (g-C3N4) with its intrinsic characteristic structure units named tri-striazine units has attracted attentions in photocatalytic field. The band-gap of gC3N4 is 2.73 eV, indicating that the response wavelength of gC3N4 is up to 450 nm. The g-C3N4 exhibits an excellent photocatalytic performance for the splitting of water into hydrogen gas using solar energy [16~19] and degrading organic This journal is © The Royal Society of Chemistry [year]
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pollutants under visible light irradiation, making them valuable materials for photocatalysis driven applications [20~28]. However, the photocatalytic efficiency of g-C3N4 is still limited due to the high recombination rate of photogenerated electron– hole pairs. Several methods have been developed to improve the photocatalytic performance of g-C3N4. For example, dye sensitization [29], transition metal doping [30] and coupled semiconductor [31-34]. These catalysts were mostly prepared by complicated method in liquid phase [35-40], the solid phase synthesis is rarely reported [41~44]. In this work, a facial synthesis of g-C3N4/TiO2 hybrid photocatalysts by one-step milling was developed. A series of g-C3N4/TiO2 composites with different mass fraction of C3N4 were successfully synthesized and characterized by various techniques. Remarkably, the photodegradation activity of g-C3N4/TiO2 for methylene blue (MB) degradation was enhanced compared with pure g-C3N4 and TiO2, which is attributed to the improvement of separation efficiency of photogenerated electron–hole pairs in g-C3N4/TiO2 composites. This simple mechanical milling method could be used as a universal pathway to improve the activity of photocatalyst and applied in the environmental remediation.
Experimental section Chemicals
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TiO2 (99.5% anatase, analytically pure grade) was purchased from Nanjing High Technology Nano-material Co.Ltd. without further purification. Other chemicals used in the experiments were of analytically pure and used without further purification. Preparation of g-C3N4/TiO2 photocatalyst The g-C3N4 was synthesized by directly heating melamine. In a typical run, 5 g melamine powder was put into an alumina [journal], [year], [vol], 00–00 |1
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crucible and heated in a muffle furnace at 520 °C for 4 h with a heating rate of 5 °C·min-1. After cooling to room temperature, the product was collected and ground into powder. The preparation of g-C3N4/TiO2 photocatalyst was carried out in a ball miller (XQM-0.4, made in Changsha, China). The procedure of preparation is as follows: TiO2 powder and agate ball are mixed in the agate ball milling tank with a ratio of 1:10, and then a certain amount of C3N4 is added. After being milled for a certain time at the speed of 350 rpm, the final samples are used for the determination of photocatalytic activity and characterization.
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Results and discussion
Characterization
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The surface areas and pore size were measured from N2 sorptiondesorption isotherms at 77K with an adsorption apparatus (TriStar II 3020, USA). The surface characteristics and microcrystalline structure of the photocatalysts were observed by using a TEM (HT-7700, Hitachi) with 100 kV scanning voltages, and HRTEM (JEOL JEM-2011). DRS measurements were carried out using a U-3010 (Hitachi, Japan). UV–Vis spectrophotometer equipped with an integrating sphere. The analysis ranged from 200 to 800 nm, and BaSO4 was used as a reflectance standard. To determine the crystal phase of the photocatalysts, X-ray diffraction measurement was carried out at room temperature using a XRD spectroscope (BRUKER D8 Advance) with CuKα radiation and the scanning speed of 5o min-1. The accelerating voltage and emission current are 40 kV and 40 mA , respectively. XPS examination was carried out on the PHI Quantera SXM system. Raman spectra were obtained by using a HORIBA HR800 confocal microscope Raman spectrometer employing an Ar-ion laser. To investigate the transition of photogenerated electrons before and after g-C3N4 modification, TiO2 and C3N4/TiO2 electrodes were prepared as follows: 5 mg of photocatalyst was suspended in 10 mL ethanol to produce slurry, which was then spread on a 2cm×4cm indium-tin oxide (ITO) glass electrode. Electrodes were exposed to UV light for 1h to eliminate ethanol and subsequently calcined at 120°C for 5h. The photoelectric performances were measured on an electrochemical system (CHI660B, China). A standard three-electrode cell with a working electrode (as-prepared photocatalyst), a platinum wire counter electrode, and a standard calomel electrode (SCE) as reference electrode were used in photoelectric studies. And 0.1M Na2SO4 was used as electrolyte solution. Potentials are given with reference to the SCE. The photoresponses of the photocatalysts as UV light (or visible light) on and off were measured at 0.0 V. Electrochemical impedance spectra (EIS) were recorded in the open circuit potential mode.
ambient temperature, 25 mg of photocatalyst was totally dispersed in the MB aqueous solution (50 mL, 0.03 mM). Before irradiation, the suspensions were magnetically stirred in the dark for 60 min to get absorption-desorption equilibrium between the photocatalyst and MB. At certain time intervals, 3 mL aliquots were sampled and centrifuged to remove the particles. The concentration of MB was analyzed by recording the absorbance at the characteristic band of 664 nm using a Hitachi U-3010 UVvis spectrophotometer.
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The sizes and the morphologies of the representative samples were examined by TEM. Fig. S1a and b display the irregular and agglomerated TiO2 spheres and g-C3N4 sheets, respectively. The agglomeration of TiO2 particles can be observed. The TEM images of g-C3N4/TiO2 (Fig. S1c,d) reveal that the hybrid materials are mainly irregularly agglomerated elliptic particles with the average size of 20-40 nm. The HR-TEM images of the g-C3N4/TiO2 composite (Fig. 1) clearly reveal the interface between g-C3N4 and TiO2 nanoparticles and an interplanar distance of 0.351 nm which corresponds to the (101) crystal plane of TiO2. As can be seen from the images, the TiO2 particles are surrounded by C3N4. Figure 1b (3%-C3N4/TiO2) shows the thickness of C3N4 layer on the surface is about 0.468 nm, corresponding to the C3N4 monolayer thickness. Therefore, it can be estimated that TiO2 is surrounded by monolayer of C3N4 and a core-shell-like structure is developed. HR-TEM images of 5%C3N4/TiO2 (Fig. 1c,d) demonstrate that TiO2 particles are surrounded by multilayer of C3N4 with the thickness of 3 nm, indicating the hybrid structure of C3N4/TiO2 rather than physical mixture of two separate phases TiO2 and g-C3N4.
Photocatalytic experiments
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The photocatalytic activities of g-C3N4/TiO2 were evaluated by degradation of MB aqueous solution under UV light or visible light irradiation. A 500WXe-lamp with a 420 nm cutoff filter was used as the light source to provide visible light irradiation. A 15 W medium pressure mercury lamp was used as the UV light. A series of 50 mL quartz tubes of 2.0 cm diameter were used as the reaction vessel. The temperature of the reaction solution was maintained at approximately 25°C to avoid temperature effects in the reaction. Experimental procedures were as follows: At 2|Journal Name, [year], [vol], 00–00
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Fig. 1HR-TEM images of 3%-C3N4/TiO2 (a), (b) and 5%-C3N4/TiO2 (c), (d)
Crystal structure of the samples was determined by XRD, as shown in Figure 2a. TiO2 exhibits dominant anatase phase diffraction peaks, the 2θ values of25.3°, 37.8°, 48.0°, 53.9°, This journal is © The Royal Society of Chemistry [year]
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62.7°, and 75.0° are undisputedly indexed to (101), (004), (200), (105), (204), and (215) crystal planes of anatase TiO2 (JCPDF No. 21-1272). The XRD pattern of g-C3N4 was also displayed in Fig.1a, the peak at 13.10° corresponds to (100) plane of tri-striazine units[45], while the peak at 27.40° agrees with interlayer stacking of aromatic segments, which is indexed as (002) peak of the stacking of the conjugated aromatic system [46]. Besides, little obvious change of the peaks of g-C3N4/TiO2 can be observed with the increase of g-C3N4 content. This result indicates that the hybrid materials were indeed composed of gC3N4 and TiO2. As shown in Fig. 2b, the diffraction peaks of C3N4 are not observed in XRD patterns, which are caused by the much lower intensity of XRD peaks of C3N4 than that of TiO2. This may be related to the hybridized and dispersed state of C3N4 on TiO2 surface.
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Fig. 4XPS spectra of 3%-C3N4/TiO2.
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Fig. 3 Raman spectra of g-C3N4/TiO2 and pure TiO2 (a) and expanded spectra in the range 100–250 cm-1 (b).
Fig. 3 shows the Raman spectra of pure TiO2 and g-C3N4/TiO2 hybrid composites. Generally, the Ti-O-Ti network structure can be identified by the Raman peaks over 140–800 cm-1 range, wherein six characteristic Raman active fundamental modes at 142 (Eg), 197 (Eg), 397 (B1g),518 (A1g+B1g), and 640 cm-1(Eg) correspond to anatase TiO2. As is shown in Fig. 3(a), these peaks can be attributed to the typical anatase TiO2. However, Fig. 4(b) shows the Raman peaks of g-C3N4/TiO2 red-shift slightly compared with pure TiO2. The Eg mode at 142.0 cm-1 of hybrid g-C3N4/TiO2 shifts to 145 cm-1, implying the formation of gC3N4/TiO2 hybrid composite. Fig. 4A illustrates high-resolution XPS spectra of different elements. The peak position of different atoms is determined by internally referencing the adventitious carbon at a binding energy of 284.8 eV. The survey spectrum (Fig. 4A) shows that the gC3N4/TiO2 surface is composed of C, N, O and Ti. Fig. 4B shows the XPS spectrum of C1s.The two peaks can be distinguished to be centered at 284.87 eV and This journal is © The Royal Society of Chemistry [year]
288.47 eV, respectively. The peak at 284.87 eV is exclusively assigned to the adventitious hydrocarbon from the XPS instrument. Another one at 288.47 eV is identified as carbon atoms that have one double and two single bonds with three N neighbors [47,48]. Fig. 4C is the high-resolutionN1s spectrum of the g-C3N4/TiO2. The asymmetrical feature of the observed N1s peaks suggests the coexistence of distinguishable models, pointing out chemically different N species in the g-C3N4, with main N1s binding energy at 399.17 eV. From Fig. 4D and E, the binding energy of Ti2p can be observed at around 458.67 (Ti2p3/2) and 464.57 eV (Ti2p1/2), while O1s is about 530.07 eV. All of them are in good agreement with previous report [49]. In comparison with those of pure TiO2, the binding energies of Ti2p in g-C3N4/TiO2 shift to higher energies(TableS1), indicating the electron interaction between Ti and N in g-C3N4/TiO2, which results from the formation of the coordination bond between N and Ti. 1.5 1.2 0.9 0.6 0.3
C3N4 10%-C3N4/TiO2 7%-C3N4/TiO2 5%-C3N4/TiO2 3%-C3N4/TiO2 TiO2
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Fig.5 UV-Vis diffuse reflectance spectra of C3N4, TiO2 and C3N4/TiO2
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UV-vis diffuse reflectance spectroscopy (DRS) was carried out to investigate the optical properties of the C3N4/TiO2 materials. As shown in Fig. 5, the absorption edge of C3N4 is about 450 nm. The optical absorption of g-C3N4/TiO2 is enhanced distinctly in both UV and visible light region compared to pure TiO2. This Journal Name, [year], [vol], 00–00 |3
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Fig. 2XRD patterns (a) and the enlarged XRD patterns (b) of C3N4, TiO2 and C3N4/TiO2 hybrid composite.
Absorbance
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suggests that the introduction of the C3N4 can possibly cause modifications in the fundamental process of electron-hole pairs formation during irradiation. Influence of mechanical milling on visible light photocatalytic activity The effect of ball milling rate on the photocatalytic activity of C3N4/TiO2 photocatalyst is shown in Fig. 6a. It can be seen that the ball milling rate influences the photocatalytic activity efficiently. The photocatalytic efficiency increases gradually with the increase of milling rate. When the ball milling rate is 350 rpm, the MB degradation rate of C3N4/TiO2 is about 3 times higher than that of C3N4 under visible light, indicating the hybrid structure of C3N4/TiO2 photocatalyst. Fig. 6b shows transient photocurrent responses of 3%-C3N4/TiO2 under visible light irradiation (λ>420 nm). It is well known that photocurrent is originated from the diffusion of the photogenerated electrons to the back contact and meanwhile the photoinduced holes are taken by the hole acceptor in the electrolyte. It is found that photocurrent response of theC3N4/TiO2 (350 rpm) is nearly 3 times as that of pure C3N4, suggesting more efficient separation and longer lifetime of photo excited electron-hole pairs of C3N4/TiO2 compared with C3N4.
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Fig.6 Visible light photocatalytic degradation of MB (C0=1×10-5 mol/L) by g-C3N4/TiO2 with various ball milling rate (r = 250 rpm, 300 rpm and 350 rpm) under 500 W Xe lamp irradiation, 420 nm cutoff filter and the reaction rate constant of photocatalysis by g-C3N4/TiO2 (inset) (a) Transient photocurrent responses of different milling rate under Xe lamp irradiation, 420nm cutoff filter. (b)
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Photocatalytic activities of the C3N4/TiO2 composites with different loading percentage of g-C3N4 were also investigated. As shown in Fig.7a, with the increased mass fraction of C3N4, the photocatalytic activity of g-C3N4/TiO2 is enhanced gradually, which is attributed to the visible light response from g-C3N4 and the improved photogenerated charge separation driven by buildin electric field at the interface between g-C3N4 and TiO2. As shown in Fig. 7b, fast and steady photocurrent response was observed for each light-on and light-off on both electrodes. The photocurrent density of the g-C3N4/TiO2 is almost 2~3 times as that of the g-C3N4. The great enhancement of photocurrent can be ascribed to the wider absorption spectrum region and stronger absorption of the g-C3N4/TiO2 than TiO2 and the improved charge separation and transportation efficiency through the interaction between the g-C3N4 and TiO2.
Fig.8 Electrochemical impedance spectroscopy (EIS) Nynquist plot of C3N4, 3%-C3N4/TiO2 (under visible light, 420 nm)
It can be seen from Fig. 8, EIS result reflects the impedance arc radius of the C3N4/TiO2 is smaller than that of C3N4 under visible light, indicating that C3N4/TiO2 demonstrates enhanced separation efficiency of photo-excited charge carriers compared with pure C3N4. In this regard, transient photocurrent response and EIS results reveal analogous trend to the photocatalytic activities of the samples. Influence of mechanical milling on UV light photocatalytic activity The influences of ball milling rate on the photocatalytic activity under UV light irradiation is shown in Fig. 9. The photocatalytic activity is enhanced gradually with the increase of ball milling rate. The optimum milling rate is 350 rpm, which is consistent with the results of photocurrent responses in the Fig. 9b. The anodic photocurrent spike produced by the UV light irradiation was observed with good reproducibility.
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Fig.7 Photocatalytic degradation of MB (C0=1×10-5 mol/L) by gC3N4/TiO2 with various weight content of C3N4 (3%, 5%, 7, and 10%) under 500 W Xe lamp irradiation, 420 nm cutoff filterand the reaction rate constant of photocatalysis by g-C3N4/TiO2 (inset) (a) Photocurrent responses of different weight content of C3N4 under Xe lamp irradiation, 420 nm cutoff filter (b)
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Fig.9 Effects of ball milling rate(r = 250 rpm, 300 rpm and 350 rpm) on the photocatalytic activity under UV light irradiation (15 W medium pressure mercury, dominant wavelength 254 nm, C0=1×10-5 mol/L) and the reaction rate constant of photocatalysis by g-C3N4/TiO2 (inset) (a) Transient photocurrent responses of different ball milling rate (r = 250 rpm, 300 rpm and 350 rpm) under UV light (15 W medium pressure mercury, dominant wavelength 254 nm) (b)
C3N4/TiO2 hybrid photocatalysts were obtained via a facile mechanical milling method. The dispersion and hybrid of conjugated molecule g-C3N4 on the surface of TiO2 was improved by mechanical ball milling process. The photocatalytic activity of g-C3N4/TiO2 was enhanced under UV light and visible light. The enhanced photocatalytic activities of the composite can be primarily attributed to the formation of hybrid structure in the contact interface between TiO2 and g-C3N4 which greatly promoted the separation of photogenerated electron-hole charge carriers. This mechanical milling method may open up a new avenue for exploring novel composite photocatalysts for environmental remediation.
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Acknowledges
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This work was partly supported by National Basic Research Program of China (973 Program) (2013CB632403), National High Technology Research and Development Program of China (2012AA062701) and Chinese National Science Foundation (20925725 and 21373121).
Notes and references a
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Fig.10 Photocatalytic activity of g-C3N4/TiO2 with different contents of C3N4 (1%,3%,5%,7% and 10%) under UV light (15 W medium pressure mercury, dominant wavelength 254 nm) and the reaction rate constant of photocatalysis by g-C3N4/TiO2 (inset) (a) Photocurrent responses of different content of C3N4 (3%,5%)under UV light irradiation (15 W medium pressure mercury, dominant wavelength 254 nm) (b)
As shown in Fig. 10a. The photocatalytic activity is enhanced gradually with the increased C3N4 loading. The photocatalytic rate of 5%-C3N4/TiO2 is about 1.3 times higher than that of TiO2 under UV light. Therefore, compared with TiO2, the photocatalytic activity of C3N4/TiO2 is not only significantly improved under visible light but also under UV light by mechanical milling. Fig. 10b shows the influences of C3N4 loading on the photocurrent under UV light irradiation. The photocurrent density of the 5%-C3N4/TiO2 is almost 2 times as that of the TiO2. The obvious enhancement of photocurrent can be ascribed to the improved charge separation and transportation efficiency through the strong interaction between the g-C3N4 and TiO2. With the exited of UV, the photogenerated electrons of TiO2 could transfer to C3N4 and the holes in the valence band of TiO2 could directly transfer to the HOMO orbital of C3N4, which leads to more efficient charge separation and enhanced photocatalytic activity of C3N4/TiO2. Under visible light irradiation, the excited electron from HOMO to LUMO orbital of C3N4 could directly inject into the conduction band (CB) of TiO2, leading to the dramatic visible light photocatalytic activity of C3N4/TiO2.
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Institute of energy and fuel, Xinxiang University, Xinxiang, China 453003 b Department of Chemistry, Tsinghua University, Beijing, China 100084 † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/ 1 R. Asahi, T. Morikawa, T. Ohwaki, K.Aoki, Y. Taga, Science 2001, 293, 269-271. 2 P. V.Kamat,Pure Appl. Chem. 2002, 74, 1693-1706. 3 B. Kraeutler, A. J.Bard,J. Am. Chem. Soc. 1978, 100, 4317-4318. 4 V. Subramanian, E.E. Wolf, P. V.Kamat,J. Am. Chem. Soc. 2004, 126, 4943-4950. 5 H. G. Kim, P. H. Borse, W. Choi, J. S. Lee, Angew. Chem. Int. Edit. 2005, 44, 4585-4589. 6 X. Z. Fu, L. A. Clark, Q. Yang, M. A. Anderson, Environ. Sci. Technol. 1996, 30, 647-653. 7 Y. Tao, C. Y. Wu, D. W. Mazyck,Ind. Eng. Chem. Res. 2006, 45, 51105116. 8 H. Wang, X. Quan, H. T. Yu, S. Chen,Carbon2008, 46, 1126-1132. 9A. Kongkanand, R. M. Dominguez, P. V. Kamat,Nano Lett. 2007, 7, 676-680. 10X. F. Lu, Q. L. Wang, D. L. Cui,J. Mater. Sci. Technol. 2010, 26, 925930. 11Y. J.Wang, R. Shi, J. Lin, Y. F. Zhu, Appl. Catal. B 2010, 100, 179183. 12 L. W.Zhang, Y. J. Wang , T. G. Xu, S. B. Zhu, Y. F. Zhu, J. Mol. Catal. A: Chem. 2010, 331, 7-14. 13L. W. Zhang, H. B. Fu, Y. F.Zhu,Adv. Funct. Mater. 2008, 18, 21802189. 14 H. Zhang, R. L. Zong, J. C. Zhao, Y. F. Zhu,Environ. Sci. Technol. 2008, 42, 3803-3807. 15 J. Lin, R. L. Zong, M. Zhou, Y. F.Zhu,Appl. Catal. B 2009, 89, 425435. 16 X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M.Antonietti,Nat. Mater. 2009, 8, 76-80. 17 S. C. Yan, S. B. Lv, Z. S. Li, Z. G. Zou, Dalton Trans. 2010, 39, 1488-1491. 18 F. Goettmann, A. Fischer, M. Antonietti, Arne. Thomas,Chem. Commun. 2006, 43, 4530-4532. 19 F. Goettmann, A. Thomas, M.Antonietti,Angew. Chem. Int. Edit. 2007, 46, 2717-2720. 20 F. Dong, L. W. Wu, Y. J. Sun, M. Fu, Z. B. Wu, S. C. Lee,J. Mater. Chem. 2011, 21, 15171-15174.
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