Large Ultrathin Anatase TiO2 Nanosheets with Exposed

Sep 11, 2012 - characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRT...
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Large Ultrathin Anatase TiO2 Nanosheets with Exposed {001} Facets on Graphene for Enhanced Visible Light Photocatalytic Activity Wan-Sheng Wang, Dong-Hong Wang, Wen-Gang Qu, Li-Qiang Lu, and An-Wu Xu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: Tailored synthesis of well-defined anatase TiO2 nanocrystals with a high percentage of reactive facets has attracted widespread attention due to the scientific and technological importance. Here, high-quality nanosized anatase ultrathin TiO2 nanosheets, mainly dominated by {001} facets, were grown on graphene nanosheets by a simple one-pot solvothermal synthetic route. The obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), ultraviolet−visible (UV−vis) diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of as-prepared TiO2/graphene composites for degradation of methylene blue (MB) under visible-light irradiation at λ ≥ 400 nm was investigated. The results show that TiO2/graphene nanocomposites have a higher photocatalytic activity than pure TiO2 and P25. This enhanced photocatalytic activity suggests that the photoinduced electrons in TiO2 prefer transferring to the graphene efficiently. As a consequence, the electron transfer via Ti−O−C between TiO2 and C interaction greatly retards the recombination of photoinduced charge carriers and prolongs the carrier lifetime, thus contributing to the enhancement of photocatalytic performance. The amount of graphene is an important factor affecting the photocatalytic activity of TiO2/graphene nanocomposites. The optimum amount of graphene is ca. 1 wt %, at which the TiO2/ graphene sample displays the highest reactivity. Furthermore, the photodegradation rate does not show an obvious decrease during five successive cycles, indicating that our TiO2/graphene nanocomposites are stable visible-light photocatalysts. g−1), optical transparency, and chemical stability.9−13 Like single-walled carbon nanotubes, conducting/semiconducting fullerenes, graphene is also expected to be used as an electron efficient acceptor to enhance photoinduced charge transfer for improved catalytic activity.14−16 When decorated with other materials, graphene can slow the recombination of photo- or electrochemically generated electron−hole pairs.17−20 The effective combination of TiO2 and graphene is expected to be very effective in increasing the charge transfer rate of electrons and the adsorption of conjugated dye molecules through π−π interactions with graphene. Our group has recently reported the use of N-graphene sheets as electron transfer channels for reducing the recombination of the photogenerated electrons and holes to improve photoconversion efficiency of the Ngraphene/CdS composites.21 More recently, it is noted that nanocrystals with highly active facets usually exhibit superior surface-dependent properties, and favorable surface atomic structure such as high reactive facets is expected to effectively enhance the photocatalytic activity. Most available anatase TiO2 crystals are dominated by

1. INTRODUCTION Semiconductor photocatalysts, which have long been studied to alleviate the deterioration of natural environments created by pollutants, have attracted increasing attention.1 Titanium dioxide (TiO2), a very important oxide semiconductor, is regarded as a suitable material for extensively investigating in the photocatalytic field owing to its chemical stability, high chemical inertness, nontoxicity, and low cost.2 However, the large bandgap (3.2 eV) of anatase TiO2 restricts its use only to the narrow light-response range of ultraviolet (only about 3− 5% of total sunlight),3 and electron and hole can easily recombine before they emigrate to the photocatalyst surface, which significantly limits its photocatalytic applications. Recently, to improve the efficiency of TiO2 photocatalytic activity, researchers found that the introduction of carbon materials such as carbon nanotubes4 can enhance its efficiency of photocatalytic activity. In particular, graphene, an expeditiously rising star on the horizon of materials science in electronic, optical, and catalytic fields,5−8 has become one of the most exciting topics of research in recent years. This twodimensional (2D) material constitutes a new nanocarbon comprising layers of carbon atoms arranged in six-membered rings, which exhibits excellent mobility of charge carriers (200 000 cm2 V−1 s−1), large surface area (calculated value, 2630 m2 © 2012 American Chemical Society

Received: July 1, 2012 Revised: August 23, 2012 Published: September 11, 2012 19893

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Max-3A X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å), the operation voltage and current maintained at 40 kV and 40 mA, respectively. Transmission electron microscopic (TEM) images and high-resolution transmission electron microscopic (HRTEM) images were taken on a JEOL-2010 microscope with an accelerating voltage of 200 kV. All the TEM samples were prepared by depositing a drop of diluted suspensions in ethanol on a carbon-film-coated copper grid. Raman measurements were operated with a Perkin-Elmer 400F Raman spectrometer using a 514.5 nm laser beam. The X-ray photoelectron spectroscopy (XPS) was performed at a PerkinElmer RBD upgraded PHI-5000C ESCA system. A Shimadzu spectrophotometer (model 2501 PC) was used to record the UV−vis diffuse reflectance spectra of the samples in the region of 250 to 800 nm. Fourier transform infrared (FTIR) spectra were conducted on a Nicolet Nexus spectrometer with samples embedded in KBr pellets. 2.3. Photocatalytic Measurement. Photocatalytic activities of the samples were evaluated by the photocatalytic decomposition of methylene blue (MB). Typically, 20 mg of photocatalyst was dispersed in 40 mL of 10 mg/L MB aqueous solution in a reactor of double layer condensated by running water to keep the temperature unchanged. Prior to irradiation, the suspension was magnetically stirred in dark for 1 h to ensure the establishment of an adsorption/desorption equilibrium between the photocatalyst and MB. Then the suspension was illuminated by a Xe lamp (GX-500) combined with a UV cutoff filter (λ ≥ 400 nm) with stirring. At given time intervals, about 3 mL aliquots were sampled, centrifuged, and filtered through a 0.45 μm membrane filter to remove the remaining particles. The degradation of MB was monitored with a U3900/3900H UV−vis spectrophotometer (Hitachi). The MB concentration after adsorption equilibrium is regarded as the initial concentration (C0). For comparison, blank experiments without catalyst were also carried out. Additionally, the recycle experiments were performed for five consecutive cycles to test the durability. After each cycle, the catalyst was filtrated and washed thoroughly with ethanol several times to remove residual dye impurities, and then dried at 60 °C for the next test.

thermodynamically stable {101} facets, rather than the much more reactive {001} facets. The order of the average surface energies of anatase TiO2 reported in the literature is 0.90 J m−2 for {001} > 0.44 J m−2 for {101}.22,23 Since Lu and Qiao et al. reported the synthesis of micrometer-sized anatase TiO2 crystals with exposed {001} facets, increasing efforts have been focused on the synthesis, properties, modifications, and applications of anatase TiO2 with exposed {001} facets.24 Therefore, preparation of uniform and anatase single crystals with controllable crystallographic facets supported by graphene may significantly improve the photocatalytic efficiency. For example, the production of anatase TiO2 nanosheets with a high percentage of exposed {001} facets exhibit an excellent activity in the photocatalytic degradation of organic contaminants,25 and the synthesis of graphene-hybridized anatase TiO2 nanosheets showed perfect performance for lithium storage by Lou et al.26 However, to our knowledge, the study on the synthesis of desirable TiO2/graphene nanocomposites which involve ultrathin TiO2 nanosheets with a high percentage of exposed {001} facets on a graphene support is still limited. In this work, we report a facile strategy to prepare novel anatase TiO2/graphene nanosheets with a high percentage of {001} facets using hydrofluoric acid (HF) as a morphology control agent under solvothermal conditions. The results revealed that large ultrathin anatase TiO2 nanosheets with nearly 100% enwrapped {001} facets were uniformly dominant on the platform of graphene nanosheets. The morphology, the structure, and the influence of graphene oxide (GO) content in composites on property were investigated in detail. The obtained TiO2/graphene nanosheets exhibited a much higher photocatalytic activity compared with the pure TiO2 and commercial P25 under visible light. Simultaneously, the photocatalytic efficiency exhibited no significant loss after five successive cycles. How are these findings explained? A possible mechanism has been proposed. The effective separation of photoinduced charges triggered by graphene and large ultrathin anatase TiO2 nanosheets with exposed {001} facets plays a leading role in the high efficiency of the photocatalytic activity.

2. EXPERIMENTAL SECTION 2.1. Preparation. All regents were analytical grade and used without any further purification. For the synthesis of TiO2/ graphene hybrid catalysts, GO was prepared from natural graphite powder (99%, Shanghai Sinopharm Chemical Reagent Co., Ltd.) using a modified Hummers method.27 Briefly, 3.0 mg of GO was dissolved in 13 mL of 1-butanol solution and sonicated for 1 h; thereafter, 0.45 g of TiF4 powder was added into the above GO solution and ultrasonicated for another 30 min. Then, 0.25 mL of hydrofluoric acid (HF) was gradually added under stirring. Finally, the mixed solution was transferred into a Teflon-lined stainless steel autoclave with a capacity of 20 mL, and then heated at 200 °C for 20 h. After cooling naturally, the gray precipitates were collected by centrifugation, washed with ethanol and distilled water several times, and dried in a vacuum oven at 60 °C overnight. A series of TiO2/graphene nanocomposites were synthesized by varying the content of GO, and the detailed reaction conditions are listed in Table S1 in the Supporting Information for TiO2/0.5 wt % graphene, TiO2/1 wt % graphene, TiO2/3 wt % graphene, and TiO2/5 wt % graphene. Pure TiO2 was synthesized by a similar procedure except for the absence of GO. 2.2. Characterization. The X-ray powder diffraction (XRD) patterns of the samples were performed on a Rigaku/

3. RESULTS AND DISCUSSION The overall fabrication procedures of the graphene consisting of uniform TiO2 nanosheets are schematically illustrated in Scheme 1. It starts with the dispersion of graphene oxide (GO) in 1-butanol as the template, proceeding with the selfassembly of Ti4+ precursor on GO nanosheets. 1-Butanol was selected to act as a protective capping agent to promote the stabilization of the {001} facets28 and the dispersion of GO. HF may play an important role in the formation of TiO2 with exposed {001} surfaces, because first-principle calculations suggest that F− can reduce the surface energy of the {001} surface to a level lower than that of {101} surfaces.25a In this work, when adding GO, Ti4+ can be easily grafted on the GO surfaces via chemical bond because of abundant oxygen functional groups on the GO surfaces, which is more conducive to the formation of the {001} surfaces. In other words, GO as s template plays a decisive role in the formation of large ultrathin anatase TiO2 nanosheet enwrapped {001} facets, because graphene, providing a support membrane, can stabilize 2D materials.29 In addition, the percentage of the synthesized TiO2 nanosheets can be estimated to be about 92% based on the used weight of the precursors. 19894

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nanosheets are covered by graphene sheets. The rectangular TiO2 nanosheets have a typical thickness of 10 nm with average dimensions (length × width) of 1.2 × 1.1 μm, and a schematic illustration of a single TiO2 nanosheet is provided in the inset of Figure 2a. In contrast, pure TiO2 sheets without using a graphene template are shown in Figure S2b in the Supporting Information, and they have a typical thickness of 97 nm with average dimensions (length × width) of 520 × 493 nm. These results indicate that pure TiO2 sheets are much thicker than TiO2 nanosheets in TiO2/graphene composites, and pure TiO2 sheet dimensions are smaller than TiO2 nanosheets in TiO2/ graphene, suggesting that graphene nanosheets indeed act as the template for large ultrathin TiO 2 crystal growth. Rectangular TiO2 nanosheets of TiO2/graphene composites are semitransparent and cracked under electron beam, indicating that TiO2 nanosheets are very thin (Figure 2b and Figure S2a in the Supporting Information). A slab model (Figure S3 in the Supporting Information) can more clearly show that anatase TiO2 with (001) surfaces is well deposited on the surface of the graphene. To better study the interface structure between the two phases, the obtained samples were further examined with high-resolution transmission electron microscopy (HRTEM). Selected area electron diffraction (SAED) (inset Figure 2d) further confirms the nature of the single crystals. Additionally, the HRTEM image recorded from the white circled area in Figure 2d clearly shows the continuous (200) atomic planes of anatase TiO2 single crystals with a lattice spacing of 0.19 nm (Figure 2d). The SAED pattern indexed as [001] zone axis and the HRTEM image demonstrate that the top and bottom of a single nanosheet are bounded by {001} facets. Further, the percentage of the {001} facets can be estimated to be 98% statistically, based on TEM measurements. To investigate the crystalline phase of TiO2 and quality of graphene, Raman spectroscopy was applied as a powerful tool to detect the significant structural changes in GO during the solvothermal reaction process. As observed in Figure 3a, there are two typical Raman features of GO. The band at around 1352 cm−1 is common for disordered sp2 carbon and has been called the D-band; the other band, appearing at approximately 1594 cm−1, is often called the G-band.31 In comparison to that of pure GO, a decreased ID/IG intensity ratio reflects the removal of hydroxyl and epoxy groups and the restoration of sp2-hybridized carbon, confirming the existence of graphene sheets in the TiO2/graphene composites (Figure 3b).32 In addition, Figure 3b exhibits the typical optical modes of anatase, namely, Eg(1) peak (145 cm−1), B1g(1) peak (399 cm−1), Eg(2) peak (637 cm−1), and the A1g + B1g(2) modes centered around 516 cm−1, respectively.33 The results confirm that the TiO2 sheets are decorated with graphene, in agreement with XRD analysis. In order to further prove the reduction of graphene oxide after solvothermal process, Fourier transform infrared spectra (FT-IR) were measured. It can be observed from Figure 4a that GO exhibits several characteristic absorption bands of oxygencontaining groups. The IR absorption at 1733 cm−1 could be attributed to the CO stretching vibration, and the broad peak at the range of 3000−3500 cm−1 is attributed to the O−H stretching vibrations of the C−OH groups. The peaks at 1052 cm−1 and 1250 cm−1 are attributed to C−O and C−O−C stretching modes, respectively.34 The CC skeleton vibration peak could be observed around 1600 cm−1, whereas, for the TiO2 /graphene sample (Figure 4b), the intensities of absorption bands of oxygen-containing functional groups

Scheme 1. Synthetic Procedures for Preparation of TiO2/ Graphene Nanosheets

The crystallographic structure of the as-obtained products was determined by X-ray diffraction (XRD) measurements. XRD patterns of as-prepared GO, pure TiO2, and TiO2/ graphene nanosheets are shown in Figure 1, respectively. In

Figure 1. Representative XRD patterns of GO (a), TiO2/1 wt % graphene composites (b), and pure TiO2 (c).

Figure 1a, a {002} diffraction peak can be observed at 10.80° without other peaks, which indicates that the pristine graphite was oxidized into GO with a well ordered, lamellar structure.30 From Figure 1b, it can be seen that no peak ascribed to GO can be observed in the TiO2/1 wt % graphene composites; this is due to the fact that the graphene content is very low (1 wt %). Simultaneously, all the diffraction peaks in Figure 1b are the same as for the crystal structure of the pure TiO2 (Figure 1c), and match well with the crystal structure of the anatase-phase TiO2 (space group: I41/amd, JCPDS No. 21-1272), indicating the formation of TiO2/graphene nanocomposites. XRD patterns of the TiO2/graphene composites with different graphene contents are shown in Figure S1 in the Supporting Information. It is obvious that the composites with different weight ratios of GO exhibit similar XRD patterns, indicating all samples are composed of anatase-phase TiO2. The microstructures of the obtained samples were examined with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). In Figure 2a−c, it is observed that the crumpled layered structure is graphene sheets, and TiO2 19895

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Figure 2. SEM image of anatase TiO2 nanosheets on graphene (inset is a single nanosheet schematic morphology) (a). TEM image of anatase TiO2 nanosheets on graphene (b). Enlarged TEM image of the TiO2/graphene nanosheet (c). The HRTEM images of TiO2 nanosheet (the inset shows the corresponding SAED pattern) (d).

(O−H, CO, C−O−C, and C−O) are significantly decreased and carbonyl CO band (1733 cm−1) even disappeared for the TiO2/graphene, indicating that GO is reduced to graphene through the solvothermal protocol. In addition, the broad IR band at low frequency (below 1000 cm−1) reflects the stretching vibration of Ti−O−Ti bonds in crystalline TiO2;18b and the band at around 1570 cm−1 can be attributed to the skeletal vibration of the graphene sheets, due to the interactions between titanium dioxide and graphene.32,35 To investigate the chemical state of TiO2/graphene and the interactions between TiO2 and graphene in composites, we also carried out X-ray photoelectron spectroscopy (XPS) measurements, and the results are shown in Figure 5. The obvious peaks of C and O in the survey spectrum of GO can be clearly detected, as shown in Figure 5a. Compared with GO, the survey spectrum of TiO2/graphene shows the presence of Ti 2p originating from TiO2. The peak at the binding energy of 684.5 eV is assigned to F 1s, which is a typical value for the fluorinated TiO2 system such as the surface Ti−F species.24a,28b In addition, the oxidation state of the Ti in TiO2/graphene is shown (in inset of Figure 5b). Two bands located at 458.8 and 464 eV can be recognized, and assigned to the distinct Ti2p3/2 and Ti2p1/2 signals in the Ti4+ chemical state.24a The main C 1s XPS spectra for GO and TiO2/graphene nanosheets are shown in Figure 5c, and the C 1s peaks of TiO2/graphene nanosheets are clearly weaker in comparison with those of GO. The higher resolution XPS data of C 1s peaks of GO and TiO2/graphene nanosheets are shown in Figures 5d and 5e, respectively. From the C 1s XPS spectrum of GO (Figure 5d), a considerable degree of oxidation can be detected at peaks of 286.7 and 288.5 eV; for the peak at the binding energy of 286.7 eV is assigned to C−O and C−O−C, and the signal at 288.5 eV is attributed to the O−CO oxygen-containing carbonaceous band.28b,36 In addition, the deconvoluted peak centered at the binding energy of 284.6 eV is assigned to the C−C, CC, and C−H bonds (sp2). In contrast, the peak intensities of the oxygen functional groups substantially decreased in the C 1s XPS spectrum of TiO2/graphene (Figure 5e), confirming that

Figure 3. Raman spectra of GO (a) and TiO2/graphene nanosheets (b).

Figure 4. FTIR spectra of the GO (a) and TiO2/graphene composites (b).

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Figure 5. The survey XPS spectra of the GO (a) and TiO2/graphene nanosheets (b); C 1s XPS spectra of GO and TiO2/graphene nanosheets (c); fitted XPS spectra of C 1s of GO (d) and TiO2/graphene nanosheets (e); O 1s XPS spectra of GO and TiO2/graphene nanosheets (f).

the strong interaction between the Ti, O, and C after the solvothermal treatment. The UV−vis diffuse reflectance spectra of as-prepared TiO2 and TiO2/graphene photocatalysts are shown in Figure 6. A wide background absorption in the visible light region is observed for the TiO2/graphene photocatalysts. This can be attributed to the presence of carbon in the composites, reducing reflection of light.37,38 Compared with pure TiO2, we observe a red shift in the absorption edge and a strong absorption in the visible light range for TiO2/graphene samples. This phenomenon should occur due to the interaction between C and Ti atoms on the surface during solvothermal treatment.39

GO has been converted to graphene. It is worth noting that no peak assigned to C−F is detected, indicating no F-doped graphene produced during the solvothermal process. In addition, the O 1s XPS spectra of GO and TiO2/graphene nanosheets are presented in Figure 5f, which exhibit different peak shape. For GO, the O 1s peak at 532.5 eV is closely related to the significant hydroxyl groups on the surface of GO. What is more, the O 1s peak of TiO2/graphene at around 530.5 eV may be assigned to O in Ti−O−C bond. All these results further confirm GO reduction to graphene, the successful integration between TiO2 and graphene, and the presence of 19897

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irradiation. On the contrary, the photocatalytic activity of pure TiO2 and P25 is much lower. The degradation rate of pure TiO2 is about 35.5%, and for P25, almost 82.2% of MB remains in the solution after 1 h irradiation. In addition, the total organic carbon (TOC) removal curve is shown in Figure S4 in the Supporting Information, from which it can be clearly observed that the MB can be thoroughly mineralized, demonstrating the strong photooxidative ability of TiO2/ graphene nanosheets. Additional experiments were performed to evaluate the stability and reusability of the TiO2/graphene. As shown in Figure S5 in the Supporting Information, the photocatalytic activity exhibits only a slight decrease after running for five successive cycles, indicating that our TiO2/graphene nanocomposites are a stable visible-light photocatalyst. The molar ratio of TiF4/GO in solvothermal reaction system is an important factor affecting the photocatalytic activity of TiO2/ graphene. The effects of as-obtained samples with different contents of graphene on the photocatalytic activity were systematically studied (Figure S6 in the Supporting Information). With the graphene content in the nanocomposites increasing, the photocatalytic efficiency at first increased and then decreased. The 1 wt % graphene loading sample exhibits the highest degradation efficiency among different TiO2/ graphene composites. The results show that a suitable loading content of graphene is crucial for optimizing the photocatalytic activity of TiO2/graphene nanocomposites. The addition of an optimal amount of graphene into the matrix of TiO2 can endow the TiO2/graphene composites several excellent characteristics: increased adsorption of pollutants, an improved light absorbance and an extended light absorption range, and enhanced transfer of photogenerated charge carriers.10b,40 During the photocatalytic reaction process, both water and gaseous oxygen take part in the reaction, and a general simplified scheme of the photodegradation process of an organic chemical is given by eqs 1−9, where R represents the organic chemicals.

Figure 6. UV−vis spectra of pure TiO2 nanosheets and TiO2/1 wt % graphene nanosheets.

Meanwhile, it is noted that the powder color changed from white to gray after graphene addition (inset in Figure 6). Methylene blue (MB) is one of the representative organic dyes and has been widely applied in industrial production, which often contaminates environments. Thus, in the present work, photocatalytic degradation of MB over TiO2/graphene nanocomposites was conducted under visible-light irradiation (λ ≥ 400 nm), together with those on P25 and pure TiO2 for a comparison. Under visible-light irradiation, the blue color of the MB solution gradually diminished upon irradiation in the presence of TiO2/graphene nanocomposites, indicating that photocatalysis destroyed the chromophoric structure of the dye. Total concentrations of MB were simply determined from the maximum absorption (λ = 664 nm) measurements by UV− visible spectra. C/C0 was used to describe the degradation and stands for the concentration ratio after and before a certain reaction time. Figure 7 shows the photocatalytic evaluation of as-prepared samples and P25 under visible light illumination. It is clearly observed that the TiO2/graphene nanosheets exhibit the highest photocatalytic activity among tested photocatalysts: 82.5% of MB is degraded by TiO2/graphene within 1 h

hν

TiO2 → e− + h+

(1)

e− + h+ → energy

(2)

h+ + H 2O → •OH + H+

(3)

h+ + OH− → •OH

(4)

O2 + e− → •O2−

(5)

•

O2− + H+ → HO2•

(6)

•

2HO2 → H 2O2 + O2 •

−

•

(7) −

H 2O2 + O2 → OH + OH + O2 •

OH + R → CO2 + H 2O

(8)

(R = organic chemicals) (9)

Semiconductors are able to promote the formation of electron−hole (e−/ h+) pairs (eq 1), when the photon energy is equal to or higher than the band gap (Eg). Charge carriers can emigrate to the surface of the semiconductor under the action of electric field, or recombine (eq 2) with energy dissipation. Normally, hole may react with H2O or OH− in the solvent, generating •OH radicals adsorbed at the surface (eqs 3, 4), and electron reacts with oxygen molecule, producing •O2−

Figure 7. Photocatalytic degradation of MB by TiO2/1 wt % graphene nanosheets, pure TiO2 nanosheets, P25, and no photocatalyst. 19898

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Scheme 2. Schematic Illustration Showing the Reaction Mechanism for Photocatalytic Degradation of Organic Pollutants (Dyes) over the TiO2/Graphene Composites

radicals (eq 5). Two HO2• radicals will evolve into hydrogen peroxide (eq 7), which can be directly supplied to the system gaseous oxygen (eq 8). Hydroxyl radicals, due to their high oxidizing potential (2.80 V),15 can react with organic chemicals (R), until complete decomposition (eq 9). In view of the above mechanism, a number of factors which influence the photodegradation efficiency can be identified and discussed: (i) electron−hole pairs are efficiently produced; (ii) efficient emigration of electron−hole pairs to the surface and inhibition of recombination; (iii) anatase TiO2 nanocrystals with reactive facets. Therefore, the high photocatalytic activity of our products is due to the following factors: First, the light absorption ranges of the catalysts play a necessary role in visible-light catalytic decomposition. However, a large bandgap of anatase TiO2 restricts its use only to the narrow light-response range of ultraviolet. When integrated with graphene, the products exhibit an obvious red shift in the absorption edge and higher absorption intensity in the visible region (Figure 6), suggesting that the incorporation of graphene is crucial to the improvement of the absorption of visible light.18b,19,41 Second, considering the large bandgap of TiO2 (Eg = 3.2 eV) and the work function of graphene (ϕ = 4.42 eV), the barrier height is 1.22 eV. Actually, graphene has been reported to be an important candidate for a charge acceptor due to its 2D planarconjugation structure. The conducting band of TiO2 is more negative than the work function of graphene, such that the photogenerated electron transfer from TiO2 to graphene is energetically favorable.42 Thus, graphene as an acceptor of the generated electrons of TiO2/grapehene composites effectively inhibits the charge recombination, leaving more charge carriers to promote the degradation of dyes, as shown in Scheme 2. Additionally, graphene has exhibited unexpectedly excellent conductivity because of its 2D planar conjugated structure.43 Therefore, in TiO2/graphene nanosheets, the rapid transport of charge carriers can be achieved and an effective charge separation subsequently takes place. Overall, both the electron accepting and transporting properties of graphene in TiO2/ graphene composites can effectively suppress the electron−hole recombination and dramatically enhance the photoactivity.

Third, for anatase TiO2, both theoretical and experimental studies found that the {001} facets in the equilibrium state are especially reactive. On the flat anatase {001} facets, all of the titanium atoms are five-coordinate with a Ti−O−Ti angle of 146°, and the titanium atoms on the {001} facets are much more exposed, forming abundant oxygen deficiency. For the {001} facets, the preferable adsorption of dioxygen together with the low rate in producing photoinduced electrons may result in the formation of dioxygen-derived active species, which leads to more dioxygen-derived oxygen atoms being incorporated into the substrate on the {001} facets. The interfacial electron transfer is mediated by the surface defects, and the separation of photogenerated electron−hole pairs is accelerated by the {001} facets.44 Significantly, in TiO2/ graphene composites, ultrathin anatase TiO2 nanosheet enwrapped {001} facets can be produced due to a combination of GO nanosheets as templates and HF as a morphology controlling agent. Further work is underway to establish the roles of the cocatalyst and photocatalyst in photocatalytic degradation.

4. CONCLUSIONS In summary, we have synthesized a novel hybrid composite by in situ growing ultrathin anatase TiO2 nanosheets with dominating high-energy {001} facets onto graphene support though a one-step solvothermal method. The as-prepared TiO2/graphene nanocomposites have been demonstrated to be highly photocatalytically active under visible light irradiation, and the highest photocatalytic activity was found for the sample with a content of 1 wt % graphene. On the basis of TEM, XPS, Raman spectra, and FT-IR spectra analyses, the high photocatalytic activity can be attributed to three crucial factors: the formation of a chemical Ti−O−C bond extending the light absorption range into visible light, the high charge separation rate based on the electron transfer, and the effective exposure of highly reactive {001} facets of TiO2. This finding further implies that the performance of catalysts can be increased by the support of graphene, and GO works as template for forming ultrathin anatase TiO2 nanosheets with dominating high-energy {001} facets. The present study provides a facile approach to synthesize novel TiO2/graphene nanocomposites 19899

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and gives more direct evidence for the strong electronic interaction between the graphene and TiO2. It can be expected that our findings will provide new possibilities for graphene as a protective material and template not only for TiO2 nanosheets but also for other semiconductors to find their potential application in optoelectronic fields as well as water purification.

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ASSOCIATED CONTENT

S Supporting Information *

Samples and corresponding experimental conditions, XRD pattern, SEM images, TEM images, schematic illustration, TOC removal curve, and photocatalytic evaluation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-551-3600246. Tel: +86-551-3602346. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2011CB933700, 2010CB934700), the 100 Talents program of the Chinese Academy of Sciences, and the National Natural Science Foundation of China (21271165).

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REFERENCES

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