ARTICLE pubs.acs.org/JPCC
Enhanced Photocatalytic Activity and Electron Transfer Mechanisms of Graphene/TiO2 with Exposed {001} Facets Baojiang Jiang,† Chungui Tian,† Qingjiang Pan,† Zheng Jiang,‡ Jian-Qiang Wang,*,‡ Wensheng Yan,§ and Honggang Fu*,† †
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080 People's Republic of China ‡ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, People's Republic of China § University of Science and Technology of China, People's Republic of China
bS Supporting Information ABSTRACT: We present a simple and feasible strategy to synthesize the novel anatase TiO2/graphene composites with exposed TiO2 {001} high-energy facets by the hydrofluoric acid and methanol joint assisted solvothermal reactions. During the synthesis process, graphene was uniformly covered with a large number of anatase TiO2 nanoparticles (20 25 nm), exposing the {001} facets. The X-ray photoelectron spectroscopy and X-ray absorption measurements show the presence of electron transfer between TiO2 and graphene. Furthermore, transient photovoltage spectra of the composite also exhibits prolonged mean lifetime of electron hole pairs compared with pure TiO2. The electron transfer between Ti and C will greatly retard the recombination of photoinduced charge carriers and prolong electron lifetime, which contribute to the enhancement of photocatalytic performance. During the photocatalysis measurement, the TiO2/graphene composites have high photocatalytic activity compared with the P25 under UV light, likely due to the effective separation of photoinduced charge and exposure of high reactive {001} facets.
1. INTRODUCTION Semiconductor photocatalysis is a promising approach for solving worldwide environmental pollution issues.1 Titanium dioxide (TiO2), as an important semiconductor, has been extensively investigated in the photocatalytic field, owing to its peculiar chemical and physical behaviors.2 4 However, the high charge recombination rate in TiO2 significantly restricts its photocatalytic application. Recently, researchers found that the introduction of carbon materials can enhance the charge separation rate of TiO2@C composites.5 Graphene, an inexpensive and novel carbon material, was found to serve as a good candidate to composite TiO2 to enhance photoinduced charge separation of composites because of its excellent electronic property and unique two-dimensional nanostructure.6 For examples, a P25graphene nanocomposite has been reported to be preferable to degradating methylene blue rather than the bare P25, for the former has a higher charge separation rate.7 The graphene/TiO2 nanocrystals hybrid structure was also prepared by directly growing TiO2 nanocrystals on graphene oxide (GO) sheets.8 Some mechanisms have been proposed to explain the interaction between semiconductor and carbon materials.9,10 However, because of the lack of sufficient evidence, the convictive power r 2011 American Chemical Society
of related research for the interaction and the electron transfer process in the composites is still not strong enough. Additionally, it is noted that the photocatalytic efficiency of TiO2 also intrinsically depends on the surface atomic structure and the crystallinity. In particular, the favorable surface atomic structure such as high reactive facets is expected to effectively enhance photocatalytic property.11 Since the successful synthesis of anatase TiO2 sheets with exposed {001} facets by Lu and Qiao et al., increasing attention has focused on the exposed high reactive facets,12 for example, the production of high reactive TiO2 micro sheets with a high percentage of {001} facets and different self-assembled structures from TiO2 micro/nanosheets for the photocatalytic decomposition of pollutants.13 Lou et al. reported the synthesis of graphene-supported anatase TiO2 nanosheets which show good performance for lithium storge.5 But, to our knowledge, it is still missing direct evidence for the novel TiO2/graphene composites consisting of high quality graphene and high reactive anatase TiO2 with exposed {001} facets, which have not been reported yet. The high electron conductivity Received: August 8, 2011 Revised: October 25, 2011 Published: October 28, 2011 23718
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Table 1. Sample Numbers and Corresponding Experimental Conditions sample
a
Ti(OBu)4
GOa
methanol
HF
solvothermal time
solvothermal temp 180 °C
TGCS-1
1.7 mL
0.3 mg
11 mL
0.5 mL
24 h
TGCS-2
1.7 mL
1.5 mg
11 mL
0.5 mL
24 h
180 °C
TGCS-3
1.7 mL
0.1 mg
11 mL
0.5 mL
24 h
180 °C
Graphite oxide.
of the graphene probably influences the reactivity properties of the TiO2 nanoparticles through electronic interaction between the graphene and semiconductor nanoparticles. Furthermore, the synergistic effect of high reactive facets and graphene may significantly enhance the photocatalytic activity of composites. Thus, we expect our detail studies on novel TiO2/graphene can help to gain deeper insights into the electron transfer process of semiconductor-carbon for further improving their catalytic activity. In this work, we present a feasible strategy to synthesize the novel anatase TiO2/graphene composites (TGCS) with exposed TiO2 {001} facets by the hydrofluoric acid (HF) and methanol joint assisted solvothermal reactions. The results revealed that graphene were uniformly covered with a large number anatase TiO2 nanoparticles, exposing the {001} facets. The X-ray photoelectron spectroscopy (XPS) measurement exhibit the strong interactions between titania and graphene. X-ray absorption spectroscopy (XAS) further shows the presence of electron transfer from TiO2 to graphene, which is beneficial to enhancing the charge separation rate of TiO2. Importantly, the TGCS exhibit high photocatalytic activity compared with the P25 under UV light, likely due to the effective separation of photoinduced charge and exposure of high reactive {001} facets.
2. EXPERIMENTAL SECTION 2.1. Synthesis of TGCS. Graphite oxide (GO) was prepared through Hummers’ method, by reacting graphite powder in a mixture of H2SO4 and KMnO4. After completion of the reaction, H2O2 was added to the reaction vessel. The GO was filtered and washed severial times with HCl and DI water. The final products were dried. In a typical experimental procedure, briefly, 0.3 mg of GO was dissolved in 11 mL of methanol and sonicated for 30 min to produce solution of graphene oxide sheets. 1.7 mL of Ti(OBu)4 was added to the solution of graphene oxide under stirring. Then, 0.5 mL of hydrofluoric acid (HF) was added dropwise into the suspension with low speed. Finally, the mixed solution was placed in a dried Teflon autoclave with a capacity of 14 mL, and then kept at 180 °C for 24 h. After being cooled to room temperature, the gray powder (TGCS-1) was separated by high-speed centrifugation, washed with ethanol and distilled water several times, and dried under 80 °C. A series of TiO2/graphene composites including TGCS-1, 2, 3 were synthesized by varying the volume of GO. The detailed reaction conditions were listed in Table 1. 2.2. Synthesis of Highly Reactive TiO2 (HR-TiO2). 1.7 mL of Ti(OBu)4 and 0.5 mL of HF were added in 11 mL of methanol under sonicating to produce homogeneous solution, and then the solution was transferred into a dried Teflon autoclave with a capacity of 14 mL and kept at 180 °C for 24 h. After being cooled to room temperature, the white powder (HR-TiO2) was separated by high-speed centrifugation and washed with ethanol and distilled water several times.
2.3. Synthesis of TGCS without HF. 0.3 mg of GO was dissolved in 11 mL of methanol and then sonicated for 30 min to produce solution of graphene oxide. 1.7 mL of Ti (OBu) 4 was added to the solution of graphene oxide under stirring. Then, the mixed solution was placed in a dried Teflon autoclave with a capacity of 14 mL and then kept at 180 °C for 24 h. After being cooled to room temperature, the sample was separated by highspeed centrifugation, washed with ethanol and distilled water for several times, and dried under 80 °C. 2.4. Characterization of TGCS. The as-prepared products were measured by the powder X-ray diffraction (XRD) pattern using Rigaku D/max-IIIB with Cu Kα radiation. The morphology and structure of as-prepared products were observed by highresolution transmission electron microscopy (HRTEM, JEM2100) with an acceleration voltage of 200 kV. Carbon-coated copper grids were used as the sample holders. Raman measurements were performed with a Jobin Yvon HR800 micro-Raman spectrometer at 457.9 nm. The Fourier transform infrared spectra (FT-IR) of the samples were collected with a PE Spectrum One B IR spectrometer. X-ray photoelectron spectroscopy (XPS) spectrum was recorded with PHI 5700. Thermogravimetric analysis was performed on a TG (TA, Q600) thermal analyzer under air with a heating rate of 10 K/min. Nitrogen adsorption desorption isotherms at 77 K were collected on an AUTOSORB-1 (Quantachrome Instruments) nitrogen adsorption apparatus. The Brunauer Emmett Teller (BET) equation was used to calculate the specific surface area. The surface photovoltage (SPV) measurements of the samples were carried out with a home-built apparatus. The powder samples were sandwiched between two indium tin oxide (ITO) glass electrodes, and the change of surface potential barrier between in the presence of light and in the dark was SPV signal. The raw SPV data were normalized with a Model Zolix UOM-1S illuminometer made in China. As for the transient photovoltage (TPV) measurement, the sample chamber consisted of an FTO electrode, a 10 mm thick mica spacer as electron isolator (to prevent the photoinduced electrons in the semiconductor from being directly injected to the electrode), and a platinum wire gauze electrode (with a transparency of about 50%). The construction was a sandwich like structure of FTO electrode/sample/mica/gauze platinum electrode, and the components of sandwich like structure were cascaded up directly by sequence without further treatment. During the measurement, the gauze platinum electrode was connected to the core of a BNC cable, which provided the input signals to the oscilloscope. The samples were excited from platinum wire gauze electrode with a laser radiation pulse (wavelength of 355 nm (50 mJ/cm2) and pulse width of 5 ns) from a thirdharmonic Nd:YAG laser (Polaris II, New Wave Research). The intensity of the pulse was determined with an EM500 singlechannel oulemeter (Molectron)). The TPV signals were registered with a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix). 23719
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Figure 1. (a) TEM images of the TGCS-1, showing the thin graphene sheet and uniform distribution of high reactive TiO2. (b) Part of enlarged TEM images of TGCS-1 is corresponding to the black circle part in (a). The HRTEM images of TiO2 (inset of the corresponding FFT image) (c), HRTEM image from the side face of TiO2 nanoparticle (d) and the thin edge of graphene (e) in TGCS-1 are corresponding to the black circle parts in (b), respectively.
The XAS data at the Ti K-edge were measured at room temperature in transmission mode at beamline BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF), China. The station was operated with a Si (111) double-crystal monochromator. During the measurement, the synchrotron was operated at energy of 3.5GeV and a current between 150 and 210 mA. The photon energy was calibrated with Ti metal foil. Data processing was performed using the program ATHENA. All fits to the extended X-ray absorption fine structure (EXAFS) data were performed using the program ARTEMIS. The XAS data at Ti L-edge, O K-edge, and C K-edge were recorded in total electron yield mode at beamline (U18) of the National Synchrotron Radiation Laboratory (NSRL), China. The beamline covered an energy range from 100 to 1000 eV, with an energy resolution at 0.2 eV. 2.5. Photocatalytic Experiments of TGCS. In a typical process, aqueous solution of the MB dyes (10 mg/L, 50 mL) and the photocatalysts (P25, HR-TiO 2, and TGCS-1, 2, 3, 0.1 g) were placed in a 100 mL cylindrical quartz vessel, and the light was provided by a 40-W tubelike UV lamp with a maximum emission at approximately 365 nm, which was placed at about 10 cm from the reactor above. Prior to irradiation, the suspension was kept in the dark under stirring for 60 min to ensure the establishing of an adsorption/desorption equilibrium. At given time intervals, 2 mL aliquots were collected from the suspension and immediately filtrated and analyzed by recording variations of the absorption band maximum (660 nm) of MB using a UV visible
spectrophotometer (UV 2550, Shimadzu). In the durability test of catalyst in the photodegradation of methylene blue (MB) under UV light, six consecutive cycles were tested, each lasting for 60 min. After each cycle, the catalyst was filtrated and washed thoroughly with water, and then fresh MB solution was added to the catalytic system.
3. RESULTS AND DISCCUSSION We make use of the transmission electron microscopy (TEM) to analyze the morphology and structure of typical sample. Serials of sample (TGCS-1, 2, 3) and corresponding experimental conditions are listed in Table 1. It is observed from the image in Figure S1 of Supporting Information) that GO has a crumpled layered structure with thin thickness. After solvothermal synthesis for 24 h at 180 °C, graphene in TGCS-1 is covered with TiO2 nanoparticles as shown in Figure 1a. TGCS-2 (Figure S2a of Supporting Information) and TGCS-3 (Figure S3a of Supporting Information) exhibit similar results. As a partial picture of Figure 1a, Figure 1b shows an enlarged image. It is clear to see that about 20 25 nm TiO2 nanoparticles uniformly cover the surface of graphene without obvious aggregation relative to the pure high reactive TiO2 (HR-TiO2) nanoparticles from the solvothermal method (Figure S4 of Supporting Information). These results demonstrate graphenes inhibit the aggregation of TiO2 nanoparticles. Additionally, in the high-resolution TEM image (Figure 1c) of the top of TiO2 nanoparticle, the visible 23720
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The Journal of Physical Chemistry C lattice fringes and its corresponding FFT image (Figure 1c, inset) indicate that the top facets of TiO2 nanoparticle are the (001) facet.12,14 The HRTEM in Figure 1d shows that the lattice spacing parallel to the top facets of TiO2 is 0.235 nm, corresponding to the (001) planes of anatase TiO2,14 which can also further confirm the above assignment. It is noting that the thickness (two layers) of the obtained graphene in TGCS-1 could be easily observed at the edge of graphene as shown in Figure 1e. The TiO2/graphene composites without HF were also characterized by XRD and TEM. The result of XRD (Figure S5 of Supporting Information) shows the composite is of typical anatase TiO2 structure. Moreover, the TEM images (Figure S6a of Supporting Information) exhibit many big particles including sphere and irregular particles on the surface of graphene sheet. The HRTEM image (the inset in Figure S6b of Supporting Information) illustrates that the lattice spacing of particles is 0.35 nm, corresponding to the (101) planes of anatase TiO2, which can also further confirm the anatase structure. By analysis of the HRTEM images, we could not find any TiO2 nanoparticle with exposure of {001} surface. Therefore, during the synthesis procedure for TGCS-1, the HF plays a key role in the formation of the high reactive TiO2 by F bounding the {001} surface. Additionally, methanol is selected as the solvent take place of the water in our work, which is in favor of the slow alcoholysis of Ti(OBu)4 and the efficient composite between the TiO2 and the graphene. Finally, the fine graphene/ high reactive TiO2 with exposed {001} facets have been prepared by our facile solvothermal method. Then, the XRD technology is employed for further analyzing the crystalline phase of different products (Figure 2). For Figure 2a, only the diffraction peak of GO (2θ = 10°) can be observed without other peaks.15 Notably, in the diffraction pattern of TGCS-1 (Figure 2c), the sharp peak of GO is of lack, suggesting the disruption of the GO layers leads to the formation of graphene. Furthermore, it can be found that the XRD data for the TGCS-1 exhibit the clear peaks of anatase TiO2 (JCPDS No. 21 1272), which are similar to the diffraction pattern of pure TiO2 (Figure 2b). This indicates that the anatase phase is exclusively formed in samples. Interestingly, the composites (TGCS-1) possess much larger surface area (88.4 m2/g) than HR-TiO2 (32.2 m2/g), which is attributable to introducing graphene into composites. The GO’s surface area is about 20.1 m2/g because the serious aggregation of GO sheets obviously decreases the surface area after drying process. However, TiO2 nanoparticles uniformly grow on the surface of graphene sheet, which will not only prompt the segregation of graphene sheets, but also enhance the surface area of composites. The crystalline phase of TiO2 and quality of graphene in the composites were further confirmed by our Raman spectral determination. We find four apparent Raman bands at 151, 396, 514, and 638 cm 1 in Figure S7 of Supporting Information, which can be assigned to the characteristic peaks of pure anatase TiO2.16 In addition, the intensity of D peak (1372 cm 1)17 of graphene is becoming lower in TGCS-1. This indicates that methanol is in favor of the reduction of GO for the fabrication of composites under solvothermal. The result can also be further corroborated by the analysis of IR spectra. Figure S8 of Supporting Information shows the FT-IR spectra of GO, HR-TiO2, and TGCS-1, respectively. For TGCS-1 (Figure S8c of Supporting Information), the carbonyl CdO band (1719 cm 1)18 significantly decreases and even disappears compared to that of GO (Figure S4a), also showing that the ability of methanol is sufficient to reduce GO to graphene. The IR absorption band (1635 cm 1) of TGCS-1 may
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Figure 2. XRD patterns of (a) GO, (b) HR-TiO2, and (c) TiO2/ graphene composites (TGCS-1).
be attributed to the skeleton vibration of the graphene sheets and deformation vibration of hydroxyl.18 In addition, the peaks below 1000 cm 1 are wider than the corresponding ones in pure TiO2 and shifting toward high wavenumber. This is related to the TiO2 and graphenes interaction.18,19 In the solvothermal process, the graphenes are reduced from GO and then firmly interact with TiO2, eventually forming composite structure. To investigate the surface composition and the TiO2-graphene interaction in composites, we also carried out X-ray photoelectron spectroscopy (XPS) measurement for different products, and the results are shown in Figure 3. In Figure 3a, the main C 1s peak for GO shows the presence of abundant C O and C(O)O chemical binding states,17,20 corresponding to the peak at 287.3 eV. After the solvothermal, the peaks of C O and C (O) O of TGCS-1 visibly weaken. For Figure 3b, two peaks of HR-TiO2 at 458.8 and 464.7 eV are assigned to the Ti (2p3/2) and Ti (2p1/2) spin orbital splitting photoelectrons in the Ti4+ chemical state, respectively. Interestingly, for TGCS-1, Ti 2p slightly shift toward higher binding energy compared to that of the HR-TiO2. Similar phenomenon can be obtained in TGCS-2 (Figure S2b of Supporting Information) and TGCS-3 (Figure S3b of Supporting Information). Normally, this kind of shift is induced by the change of chemical state or coordinated environment.9,10 Therefore, we performed X-ray absorption fine spectra measurements (XAFS), which are sensitive to valence and local structures of investigated elements. Through the X-ray absorption near edge structure (XANES) and EXAFS analysis at Ti K-edge of HR-TiO2 and TGCS-1, there are no visible structure discrepancies between them (Figure S9 of Supporting Information). Compared with the bulky TiO2, the features above 4980 eV of HR-TiO2 and TGCS-1 broaden and decrease significantly. This can be ascribed to the sizes of the particles decrease, which increase the fraction of surface TiO2 and the inhomogeneity in the lattice structures in particles.21 Transmissionmode XAS measurements normally can give materials’ bulk structure information, while XPS measurements give materials’ surface structure information. Thus, this kind of shift in XPS measurement can be attributed to the presence of strong interactions at interfaces between titania and graphene. The intense interaction may result in the formation of electron 23721
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Figure 3. XPS spectra of C 1s (a), Ti 2p (b), and O 1s (c) of different samples including GO, HR-TiO2, and TGCS-1.
Figure 4. (a) Ti L-edge XAS spectra and (b) O K-edge XAS spectra for HR-TiO2 and TGCS-1.
transfer channel, which is beneficial to the improvement of photoinduced charge separation rate during the photocatalytic process. In addition, the O 1s XPS spectra (Figure 3c) of three samples exhibit different peak shape. For GO, the O 1s peak at 531.9 eV is closely related to the significant hydroxyl groups on the surface of GO.17,19 The O 1s peak at 529.9 eV in HR-TiO2 is mainly attributed to the oxygen of TiO2 crystal lattice, agreeing with previous reports.22 However, the crystal lattice oxygen and hydroxyl oxygen are all present in TGCS-1. All these results can further confirm the successful incorporation of titania and graphene, and the presence of the intense interaction between the Ti and C. Further electronic structural information of surface and interfaces was obtained by the soft-XAS measurments, which has higher probing depth than XPS.23 Compared with the Ti K edge
XAS, the Ti L edge XAS mainly give complementary electronic properties of Ti compounds, reflect the transition from Ti 2p orbitals into Ti 3d and 4s orbitals in the conduction band. As shown in Figure 4, the Ti L edge XAS spectra in the energy range of 454 470 eV are consist of two sets of peaks (L3 and L2), due to spin orbit coupling splitting of the initial 2p states into 2p3/2 and 2p1/2. Both of the L3 and L2 features are further split into t2g (formed by dxy, dxz, dyz orbitals) and eg (formed by dx2 y2 and dz2 orbitals) features because of the low symmetry of the Oh ligand field compared to the spherical field.24 The L3 eg feature splits further into a double peaked structure centered at 461 eV has been attributed to the distortions from octahedral symmetry. We observed that the first two peaks (L3 t2g and L3 eg) shift to high energy in the presence of graphene (Figure 4a), which is in accord with results from XPS measurements. Moreover, the 23722
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Figure 5. SPV (a) and TPV responses (b) of different samples including TGCS-1, HR-TiO2, and GO.
relative peak intensities of the fours peaks (from 454 to 468 eV) increase from HR-TiO2 to TGCS-1. This kind of shift is normally induced by change of metal’s chemical state or oxygen vacancies.10,25 However, there is no according change in O K edge for TGCS-1 (Figure 4b). This shows the presence of electron transfer from Ti 3d orbitals in the conduction band of TiO2 to the C 2s orbitals (graphene), which is beneficial to enhance the charge separation rate. Additionally, the O K edge XAS spectra (Figure 4b) reflect the O 2p orbitals coupled with the Ti 2p, 3d, and 4s and Ti 4p orbitals. The O K edge is almost identical for HR-TiO2 and TGCS-1. The double peaks in the range of 528 535 eV corresponds to the transitions from the O 1s orbital to mixed state of O 2p and Ti 3d. The O 2p orbital is coupled with the split Ti 3d and of t2g and eg, respectively. The region above 535 eV is related to the antibonding O 2p orbital coupling with Ti 4s and T 4p orbitals, which is sensitive to long-range order.26 The C K edge XAS spectrum of TGCS-1 is shown in Figure S10 of Supporting Information with two main features at 285 eV and at 290 293 eV, respectively, corresponding to the C 1s f π* and C 1s f σ* transitions of core electrons into π* and σ* orbital.27 The surface photovoltage (SPV) technology can effectively reflect the information about the separation and recombination of photoinduced charge carriers (electron hole pairs). The transient photovoltage (TPV) technique is also a very promising method for the investigation of dynamic properties of the photoinduced charge carriers in semiconductor materials.28 Figure 5 shows the SPV and TPV spectra of TGCS-1, HR-TiO2, and GO. For HRTiO2 in Figure 5a, it can be seen that an obvious SPV response occurs at 300 400 nm. We related it to the electron transitions from the valence band to the conduction band of TiO2.29 Noticeably, photovoltaic response of TGCS-1 significantly increases from 0.0045 mV to 0.044 mV, which is about 10 times higher than that of HR-TiO2. This demonstrates that once the TiO2 is excited the photoinduced electrons will transfer from TiO2 into graphene, while the holes were left in TiO2, which is proved by the positive signal of photovoltaic response. Through this electron-transfer process, the electron hole pairs generated in the excited TiO2 could be efficiently separated, thus resulting
in the improved photovoltaic response of TGCS-1. This demonstrates that the photoinduced electron hole pairs in the TGCS-1 could be separated more efficiently according to the SPV principle.28 Figure 5b shows the TPV spectra of different samples. Of interest is that TGCS-1 exhibits prolonged mean lifetime of electron hole pairs with respect to HR-TiO2, which is further evidence that the integration of graphene and TiO2 will greatly retard the recombination of electron hole pairs in the excited TiO2. Therefore, it further confirms the existence of electron transfer between C 2s of graphene and the Ti 3d in the conduction band of TiO2 in the excited TiO2/ Graphene composites. It is well-known that the methylene blue (MB) is one of the most important representative organic dye substance and has been widely applied in the industrial production, which often contaminates environment. Thus, in the work we choose it represents organic substances to evaluate photocatalytic activity of the samples. As a reference, the photocatalytic behavior of Degussa P25 was also measured. Before the photocatalysis, the solution including MB and catalytic is stirred in dark for one hour for the adsorption equilibrium. The MB concentration after adsorption equilibrium is regarded as the initial concentration (C0). Under UV light, the photocatalytic evaluation of as-prepared samples and P25 are shown in Figure 6a. The results show that the TGCS1 exhibits the highest photocatalytic activity; the average degradation rate of MB is 85.2% within 60 min. In contrast, the photocatalytic activity of Degussa P25 is low; making almost 59.2% MB remain in the solution within the same 60 min. The degradation yield of HR-TiO2 is about 65.5%. In addition, the photodegradation of MB was monitored for six cycles (each cycle is 60 min). After each cycle, TGCS-1 was filtrated and dried thoroughly, and then fresh MB solution was added. The photodegradation rate retains constantly during the six consecutive cycles (Figure 6b), indicating as-prepared photocatalyst TGCS-1 is stable under UV light irradiation. The TiO2 content in TGCS1 was determined by TG-DSC in air (Figure S11 of Supporting Information). It is shown that TGCS-1 loses about 6% total weight, ascribed to the loss of the graphene. The photocatalytic evaluation of different products such as TGCS-2 and TGCS-3 is 23723
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Figure 6. (a) The variation of normalized C/C0 of MB concentration in different time under UV light irradiation (C0, the initial MB concentration after adsorption equilibrium). (b) The cycling degradation rate for MB of TGCS-1 under UV irradiation.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures depicting the representative TEM image of GO from Hummer’s method, TEM, XPS and TG-DSC for TGCS-2 and TGCS-3, typical TEM image of pure HR-TiO2 by solvothermal methods, XRD patterns of TiO2/graphene without HF during preparation process, TEM images of TiO2/graphene without HF during preparation process, Raman spectra of GO, HR-TiO2, and TGCS-1, FTIR spectra of GO, HR-TiO2, and TGCS-1 by solvothermal methods, Ti K-edge XANES spectra and Fourier transforms of the EXAFS spectra, C K-edge XAS for TGCS-1, and TG-DSC curves recorded for the as-prepared samples (TGCS-1) at 1000 °C in air. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 7. Photocatalytic degradation rates of MB solution on the different samples under UV light.
’ AUTHOR INFORMATION Corresponding Author
listed in Figure 7. TGCS-2 and TGCS-3 also exhibit significant enhanced photocatalytic property relative to P25 under UV light.
*E-mail:
[email protected] (H.F.);
[email protected] (J.-Q.W.). Fax: (+86)451 8666 1259 (H.F.); (+86)21 3393 3212 (J.-Q.W.). Phone: (+86) 451 8660 9115 (H.F.); (+86)21 3393 3212 (J.-Q.W.).
4. CONCLUSIONS In all, a high-activity TiO2/graphene with exposed {001} facets composite photocatalyst was synthesized by the HF and methanol joint assisted solvothermal reactions. The TiO2/graphene composites exhibit high photocatalytic activity compared with the P25 under UV light. On the basis of TEM, XPS, XAS, and TPV analyses, the high photocatalytic activity can be attributable to two crucial factors: the high charge separation rate based on the electron transfer, the effective exposure of high reactive {001}facts of TiO2. This work provides a facile approach to synthesize novel graphene-based photocatalyst with high photocatalytic activity and give more directly evidence for the intensified electronic interaction between the graphene and TiO2 nanoparticles.
’ ACKNOWLEDGMENT This work was supported by the Key Program Projects of the National Natural Science Foundation of China (Grant No. 21031001), the NSFC (Grant Nos. 20971040 and 21001112), New Century Excellent Talents in Heilongjiang Provincial University (Grant No. 1154-NCET-010), National Basic Research Program of China (Grant No. 2010CB934501), Youth Foundation of Heilongjiang Province of China (Grant No. QC2010021), Special Fund of Technological Innovation Talents in Harbin City (Grant No. 2011RFQXG013). The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) and U18 (National Synchrotron Radiation Laboratory) for providing the beam time. The authors also thank Tengfei Jiang of Jilin University for TPV measurements. 23724
dx.doi.org/10.1021/jp207624x |J. Phys. Chem. C 2011, 115, 23718–23725
The Journal of Physical Chemistry C
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