Noncovalently Functionalized Graphene-Directed Synthesis of

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Non-Covalently Functionalized Graphene-Directed Synthesis of Ultra-Large Graphene-Based TiO Nanosheet Composites: Tunable Morphology and Photocatalytic Applications 2

Xiaoyang Pan, Min-Quan Yang, Zi-Rong Tang, and Yi-Jun Xu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 10 Sep 2014 Downloaded from http://pubs.acs.org on September 16, 2014

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The Journal of Physical Chemistry

Non-Covalently Functionalized Graphene-Directed Synthesis of Ultra-large Graphene-Based TiO2 Nanosheet Composites: Tunable Morphology and Photocatalytic Applications Xiaoyang Pan†,‡, Min-Quan Yang†,‡, Zi-Rong Tang‡, and Yi-Jun Xu†,‡,* †

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350002, P.R. China ‡ College of Chemistry, New Campus, Fuzhou University, Fuzhou, 350108, P. R. China * Corresponding author. E-mail: [email protected] Tel. +86 591 83779326

Abstract Ultra-large graphene-based TiO2 nanosheet composites are successfully fabricated by a non-covalent functionalization approach using benzyl alcohol as linking agent. In the synthetic procedure, the aromatic molecules of benzyl alcohol direct themselves onto graphene (GR) surface via π-π interaction. Therefore, the basal planes of GR nanosheets are uniformly functionalized with hydroxyl groups derived from benzyl alcohol, which not only improves the dispersion of GR in solution but also induces finely homogeneous coating of TiO2 nanocrystals onto the surface of GR nanosheets. The resulting GR@TiO2 nanocomposites, which feature unique ultra-large 2D sheet-like morphology with the lateral size far larger than the original GR and densely interfacial contact, are able to act as highly active photocatalysts toward selective reduction of aromatic nitro compounds to amines in water under ambient conditions. The higher photoactivity of GR@TiO2 than blank TiO2 is attributed to the efficient charge carriers separation and transfer by the GR platform. It is hoped that the facile synthesis strategy in this work could contribute to fabricating other ultra-large functional GR-based 2D sheet-onto-sheet composites with tunable morphology toward target photocatalytic applications. Keywords: graphene; nanosheets; tunable morphology; photocatalysis; semiconductor TiO2.

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Introduction Graphene (GR), a truly two-dimensional (2D) honeycomb carbon network, has received considerable attention due to its high surface area, high chemical stability and unique electronic and mechanical properties.1-8 These unique properties endow GR with prominent ability to act as a 2D nanoscale building block for developing novel nanoarchitectured hybrid materials,3, 5, 9-10 which has been demonstrated as an effective strategy to enhance the functionality of materials. The integration of nanomaterials with GR potentially paves a new way to enhance their electronic, chemical, and photoelectrochemical properties.2, 11-14 To date, GR-based hybrid materials have been utilized in many technological fields, such as photocatalysis, batteries, supercapacitors, nanoelectronics and sensors, etc.1-27 Among various topics, the application of GR-semiconductor hybrid materials in the field of photocatalysis has been attracting particular interest.1-2, 7-8, 10, 17-27 For the synthesis of GR-semiconductor composite photocatalysts, graphene oxide (GO) with abundant oxygenation functional groups covalently attached to the surface has often been utilized as the precursor for GR.8-10, 21 During the synthesis of such hybrid composite materials, the role of GO can be generally classified into two categories.1-8 One is that GO merely acts as the precursor of GR without utilizing the unique properties of functional groups on GO for tailoring the size and microstructure of semiconductor ingredients.17, 22-25 The other is that GO acts not only as the precursor for GR but also as a 2D substrate for structurally directing the growth and formation of semiconductor nanocystals.23-24, 26-27 For example, during the latter process, the negatively charged oxygenation groups on the GO sheet favorably interact with the positively charged precursors of semiconductor (metal ions, etc.), and thus serve as nucleation sites for in-situ growth of semiconductor on the GR sheet.16, 28-29 Therefore, the resulting GR-semiconductor composites generally have the intimately interfacial contact, which is proved to be beneficial to effectively use the structural and electronic merit of GR for achieving improved photocatalytic performance.15, 22-23, 27, 30-31 Notably, the hydrophilic oxygenation functional groups coupled with the hydrophobic carbon network allow GO to act as an amphiphilic macromolecular surfactant with dual molecular-colloid properties.32-33 Considering that amphiphilic surfactants are often used as structure-directing agents to fabricate nanostructures,34-35 the use of GO as 2D surfactant for morphology control is easy to imagine. Indeed, several pioneering works have elucidated that GO can be utilized to manipulate the morphology and microstructure of the nanomaterials during the synthesis process.29, 36-39 For example, our group has revealed that GO with abundant oxygenation groups can promote the formation and loading of alloyed Au-Pd bimetallic nanoparticles onto the GR sheet.39 GO can also be used as a novel, macromolecular surfactant to realize the synthesis of reduced GO (RGO) wrapped ZnO nanocrystals with tunable structure, morphology, and photocatalytic activity.29 In these cases, the oxygenation functional groups on GO play a key role on promoting synthesis of GR-based nanocomposites with controllable shape and microstructure.29, 36-39 In distinct contrast, due to the absence of abundant oxygenation functional groups on the 2


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GR surface, GR generally is not able to play a structure-directing role as GO does during the solution-phase synthesis of GR-based hybrid nanocomposites. One promising strategy to resolve this issue is to adopt appropriate linker molecules to selective pre-functionalization of the GR surface in a controlled homogeneous manner,40-41 by which the functionalized GR would be endowed with the capability to finely tuning the structure and morphology of GR-based hybrid composite materials. Herein, taking the mostly investigated TiO2 semiconductor as an example, we provide an alternatively non-covalent functionalization route for controlled synthesis of ultra-large sandwich-like GR-based TiO2 nanosheets composites, which involves the use of benzyl alcohol functionalized GR nanosheets as 2D template and structural scaffold. During the growth process, the oxygenation functional groups covalently functionalized on the GO surface are firstly removed by chemical reduction. As a result, the GR nanosheets with significantly less oxygenation functionalities are obtained. Then, benzyl alcohol is attached to the GR surface through the π-π interaction, which allows the basal planes of GR sheets to be uniformly functionalized by hydroxyl groups, which can attract the TiO2 precursor in the solution sol-gel process. Consequently, benzyl alcohol enables the use of GR as a structural template for fabricating ultra-large GR@TiO2 nanosheets composites instead of the use of covalent functionalization. The photocatalytic performances of the resulting samples are evaluated by selective reduction of aromatic nitro compounds. It is demonstrated that the GR@TiO2 nanocomposites exhibit obviously enhanced photocatalytic activities than blank TiO2. The improved photocatalytic activities are attributed to the efficient photogenerated charge carriers separation and transfer by the GR platform. Experimental section 2.1 Materials Tetrabutyl-orthotitanate (TBOT), benzyl alcohol (BA), anhydrous ethanol (EtOH), graphite powder, hydrochloric acid (HCl), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), hydrogen peroxide (30%H2O2), potassium permanganate (KMnO4), 2-nitroaniline (C6H6N2O2), 3-nitroaniline (C6H6N2O2), 4-nitroaniline (C6H6N2O2), 2-nitrophenol (C6H5NO3), 3-nitrophenol (C6H5NO3), 4-nitrophenol (C6H5NO3), 4-nitrotoluene (C7H7NO2) and 4-nitroanisole (C7H7NO3) were purchased from Sinopharm chemical regent Co., Ltd. (Shanghai, China). Deionized water and ultrapure water were supplied from local sources. All of the materials were used as received without further purification. 2.2 Synthesis (a) Synthesis of graphene (GR) nanosheets Graphene oxide (GO) was synthesized from graphite powder according to the modified Hummers method as shown in supporting information. The reduction of GO to GR was typically performed as follows. 40 mg of GO powder was dispersed into 100 ml of deionized water completely by ultrasonication. Then 20 ml of 50 mM NaBH4 aqueous solution was added and the suspension was stirred at room temperature for 5 hours. Finally, the suspension was vacuum-filtered and washed in ethanol and dried in air at room temperature. (b) Synthesis of GR@TiO2 nanocomposites In a typical experiment, a given amount of GR was dispersed in ethanol (EtOH) with the 3



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aid of ultrasonication for 30 minutes. Benzyl alcohol (BA) and water (H2O) were then added and the suspension was stirred at 0 oC. Tetrabutyl-orthotitanate (TBOT) was dissolved in ethanol and the resulting solution was slowly dropped into the above-mentioned GR suspension, so that the final molar ratio of a mixture of TBOT:BA:EtOH:H2O was 1:5:100:5. After two hours stirring, the precipitates were vacuum-filtered, washed in ethanol and dried in air at room temperature followed by calcination in nitrogen atmosphere at 450 oC for 2 hours. Blank TiO2 was prepared using the same procedure for GR@TiO2 nanocomposites without adding the GR scaffold. 2.3 Characterization The crystal structures of the as-prepared catalysts were recorded on a Bruker D8 Advance X-ray diffractometer (XRD) with Cu Kα radiation. The optical properties of the samples were characterized by a Cary 500 UV-visible ultraviolet/visible diffuse reflectance spectrophotometer (DRS), during which BaSO4 was employed as the internal reflectance standard. A field-emission scanning electron microscopy (FSEM, FEI Nova NANOSEM 230) and transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN) were used to determine the morphology and microscopic structure of the as-synthesized samples. Tapping-mode atomic force microscopy (AFM) measurement was performed on a Nanoscope IIIA system. The sample for AFM imaging was prepared by depositing suspensions of GO in ethanol on a freshly cleaved mica surface. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Scientific ESCA Lab250 spectrometer which consists of a monochromatic Al Kα as the X-ray source a hemispherical analyzer and sample stage with multi-axial adjustability to obtain the composition on the surface of samples. All the binding energies were calibrated by the C 1s peak of the surface adventitious carbon at 284.6 eV. Raman spectroscopy was conduced on a Renishaw Inva Raman System 1000 with a 532 nm Nd:YAG excitation source at room temperature. The photoluminescence spectra for the samples were investigated on an Edinburgh FL/FS900 spectrophotometer, and the excitation wavelength was set at 365 nm. The electrochemical analysis was carried out in a conventional three-electrode cell. Ag/AgCl electrode was used as reference electrode and Pt electrode acted as the counter electrode. Fluoride-tin oxide (FTO) glass was used to prepare the working electrode, which was firstly cleaned by sonification in ethanol for 30 min and dried at 80 oC. The sample powder (10 mg) was ultrasonicated in 1 mL anhydrous ethanol to disperse it evenly to get slurry. The slurry was spreading onto FTO glass whose side part was previously protected using Scotch tape. The working electrode was dried overnight under ambient conditions. A copper wire was connected to the side part of the working electrode using a conductive tape. Uncoated parts of the electrode were isolated with epoxy resin. The exposed area of the working electrode was 0.25 cm2. The irradiation source was a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) system equipped with a UV band-pass filter (λ=365±15 nm), the same light source as that for photoactivity test in the following. The photocurrent measurements were performed in a home made three electrode quartz cells with a PAR VMP3 Multi Potentiotat apparatus. The electrolyte was 0.2 M aqueous Na2SO4 solution (pH=6.8) without additive. The electrochemical impedance spectroscopy (EIS) measurements were 4


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performed in the presence of 0.2 M Na2SO4 (pH=6.8) solution by applying an AC voltage with 5 mV amplitude in a frequency range from 1 Hz to 100 kHz under open circuit potential conditions. 2.4 Photocatalytic activity For photocatalytic selective reduction of aromatic nitro compounds, a 30 mg portion of the photocatalyst was added into 60 ml of aqueous solution which contained 10 mg·L-1 aromatic nitro compounds and 30 mg HCOONH4 in a quartz vial. The resulting suspension was stirred in the dark for 1 h to ensure the establishment of an adsorption-desorption equilibrium between the sample and reactant. Then the reaction system was irradiated by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) with band-pass filter (λ=365±15 nm). As the reactions proceed, 3 ml of the suspension was taken at a certain time interval and was centrifuged to remove the catalyst. Afterward, the solution was analyzed on a Varian UV-vis spectrophotometer (Cary-50, Varian Co.). The whole experimental process was conducted under N2 bubbling at the flow rate of 60 mL/min. In order to determine the conversion and selectivity of the reaction, the reaction solution was analyzed with a Shimadzu High Performance Liquid Chromatograph (HPLC–LC20AT equipped with a C18 column and SPD-M20A photo diode array detector). Conversion and selectivity for reduction of nitro compounds were defined as the follows: Conversion(%) = [(C0 − Cr ) / C0 ] × 100

Selectivity (%) = [C p /(C0 − Cr )] × 100 Where C0 is the initial concentration of nitro compound, Cr and Cp are the concentration of reactant nitro compound and product amine, respectively, at a certain time after the catalytic reaction. Result and discussion The fabrication of GR@TiO2 nanocomposites is schematically illustrated in Scheme 1. The graphene (GR) nanosheets are firstly obtained via the reduction of graphene oxide (GO) using NaBH4 (Figure S1 and S2). Then benzyl alcohol is non-covalently functionalized on the surface of GR nanosheets to provide anchoring groups (hydroxyl groups) for the subsequent TiO2 coating. The aromatic molecules of benzyl alcohol direct themselves onto the surface of GR through π-π stacking interactions whereas the hydroxyl groups on benzyl alcohol improve the solubility of GR and thus produce stable dispersion of GR nanosheets in ethanol.42-43 In addition, benzyl alcohol functionalized on the surface of GR could also contribute to the hydrolysis of tetrabutyl-orthotitanate (TBOT, the precursor of TiO2) at low water concentrations, and hence provide anchoring groups for a TiO2 coating.43-46 During this process, an ethanolic solution of TBOT at 0 oC is employed to deposit titania on the GR nanosheets. Typically, a small amount of water is employed for promoting the TBOT hydrolysis. Following that, further pyrolysis is carried out for the crystallization of titania at 450 oC under N2 atmosphere, by which GR@TiO2 nanocomposites are obtained.

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The morphology of the as-prepared GR@TiO2 nanocomposites is initially investigated via field-emission scanning electron microscopy (FE-SEM). It is clearly seen that irregular aggregated TiO2 nanoparticles are formed for blank TiO2 (Figure 1a, b). When benzyl alcohol functionalized GR is added during the synthetic procedure, the morphology of as-formed TiO2 can be chemically tuned, which is quite different from blank TiO2 (Figure 1c-f). For example, for 10% and 20%GR@TiO2, the much smaller TiO2 nanoparticles are obtained which spread uniformly on the surface of GR nanosheets, as displayed in Figure 1e, f. Furthermore, these GR@TiO2 nanocomposites still well maintain the 2D sheet-like structure of GR. However, when the weight content of GR is decreased (2% and 5%), a considerable amount of free TiO2 particles appear which are not coated onto the surface of the GR nanosheets (Figure 1c, d). The SEM results indicate that GR nanosheets functionalized with benzyl alcohol have a great influence on the morphology of as-synthesized GR@TiO2 samples. On one hand, benzyl alcohol functionalized GR nanosheets can act as a 2D structural template for TiO2 coating, which can effectively prevent the aggregation of TiO2 nanoparticles. On the other hand, the amount of GR has a great effect on the morphology of the GR@TiO2 nanocomposites. To further obtain the microscopic structure information of GR@TiO2 nanocomposite, FE-SEM coupled with elemental mapping analysis and transmission electron microscopy (TEM) analysis have been performed. As displayed in Figure 2, many free standing GR@TiO2 nanosheets with morphology similar to that of GR nanosheets are observed. It is very interesting to note that the lateral size of these GR@TiO2 samples are even in the range of a few tens of micrometres, which is significantly much larger than that of initial GO nanosheets with the average lateral size ranging from 0.3 to 1.3 µm, as shown in Figure S3 and S4. This suggests that the use of benzyl alcohol functionalized GR contributes to the cross-linking of separated single GR nanosheets, thereby forming the ultra-large GR@TiO2 nanosheets. The nature of the structure can be further unraveled by the element mapping images of titanium (Ti), oxygen (O) and carbon (C) in the 20%GR@TiO2. As shown in the right panels of Figure 2, the elements of Ti, O and C are homogeneously distributed in the testing area, indicating that the TiO2 nanocrystals are homogenously distributed on the GR nanosheets to form ultra-large GR-based TiO2 nanosheets. The TEM images (Figure 3a, b) further reveal that uniform TiO2 nanocrystals are homogeneously coated on the GR nanosheets with a smooth surface. In particular, the distinct, contrast cracks identified from Figure 3a provide direct evidence to support the cross-linking of separated single GR nanosheets to form the ultra-large GR@TiO2 nanosheets as discussed in the above SEM observation. Additionally, the high-resolution TEM (HRTEM) images display that TiO2 nanocrystals coated on GR nanosheets are highly crystallized with a lattice spacing of 0.352 nm, which can be ascribed to the (101) crystal facet of anatase TiO2 (Figure 3c, d).47 The ring-like mode in the selected-area electron diffraction (SAED) pattern (inset of Figure 3d) demonstrates the poly-crystalline structure of the sample. To investigate the effect of benzyl alcohol (BA) on the morphology control of GR@TiO2 nanocomposites, we use the same reaction conditions (TBOT:BA:EtOH:H2O in the molar ratio of 1:x:100:5), but change the BA content x from 0 to 40. Here, the weight content of GR is 20% with respect to the expect mass of the produced TiO2 coating. The SEM images shown 6


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in Figure 4 reveal a strong influence of the BA concentration on the morphology of the TiO2 coating. In the absence of BA, the TiO2 does not significantly interact with the GR nanosheets but rather precipitate as large irregular nanoparticles which randomly decorate on the surface of GR (Figure 4a). With adding a small amount of BA (BA:TBOT=2), more TiO2 nanoparticles with smaller particle size are decorated on the surface of GR nanosheets (Figure 4b). At appropriate amount of BA (BA:TBOT=5), the GR nanosheets are fully covered with TiO2 nanoparticles and no apparent aggregation can be observed, as shown in Figure 4c. At the highest BA concentration, the GR nanosheets are densely coated by TiO2 nanoparticles (Figure 4d), but the excess benzyl alcohol can be hardly removed from the suspension by filtration, which leads to the presence of an organic layer at the surface of TiO2. Therefore, it can be concluded that the optimal coating of TiO2 onto GR nanosheets is obtained at a BA:Ti molar ratio of 5. From the above results, it is clearly seen that benzyl alcohol functionalization is of crucial importance for directing the synthesis of GR@TiO2 nanocomposites with desirable ultra-large sheet morphology. That is, the benzyl alcohol, noncovalently functionalized on the surface of GR, provides uniform hydroxyl functional groups for the growth and anchoring of the TiO2 nanocrystals, leading to synthesis of sandwich-like GR-based TiO2 composites with ultra-large sheet morphology. To further confirm the role of benzyl alcohol functionalization on morphology control of GR@TiO2 composites, control experiment is performed with GO to see if similar sandwich-like sheet morphology could be obtained (20wt%GO in the GO-TiO2 nanocomposite). As shown in Figure S5, although the addition of GO could reduce the particle size of TiO2, no sandwich-like sheet morphology in a uniform manner can be obtained in a controllable way. This proves that the sandwich-like sheet morphology obtained in GR@TiO2 (20wt%GR) is unique to the structure-directing ability of benzyl alcohol functionalized GR. The above results demonstrate the significant effect of benzyl alcohol functionalized GR on the morphology of GR@TiO2 nanocomposite. It is well known that the nucleation and growth of crystals are strongly affected by the intrinsic crystal structure and the external conditions including the temperature, time and surfactants, and so on.48-49 It has been demonstrated that benzyl alcohol can act as a weak surfactant which can adsorb on the π-conjugated aromatic network of carbon materials through π-π interaction and provide anchoring groups (hydroxyl groups) for TiO2 nucleation and growth at low water concentrations.43-44 For the formation of GR@TiO2 nanosheets, the benzyl alcohol concentrated on the surface of GR nanosheets through the π-π stacking interaction is beneficial for the heterogeneous nucleation process of TiO2 nanocrystals around the surface of GR nanosheets rather than a homogeneous nucleation process in ethanolic solution.43-44 During this process, the hydroxyl groups of benzyl alcohol interact with the titanium precursors and contribute to the hydrolysis of TBOT at low water concentrations.43-44 Once the TiO2 nucleation occurs, the residual TBOT can further hydrolyze in the presence of water and grow around the titania nucleus on the surface of GR nanosheets according to the well-known liquid-crystal template mechanism.50-51 As a result, TiO2 is uniformly coated onto the surface of GR nanosheets to form a sandwich-like GR based TiO2 nanosheets. 7



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Figure 5 shows the XRD patterns of the blank TiO2 and GR@TiO2 nanocomposites. It is seen that all the indentified peaks of these samples can be perfectly indexed to anatase TiO2 (JCPDS card no. 21-1272).52-53 The peaks at 2θ values of 25.5, 37.9, 48.1, 54.0, 55.1, 62.8, 68.9, 70.3 and 75.2o can be assigned to (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal facets of anatase TiO2, respectively.47 With the increase of GR content, the diffraction peaks of TiO2 become weaker and the widths of the peaks become slightly wide, implying that the average crystalline size of TiO2 nanoparticles is decreased (Table S1) in the presence of GR. It may be due to the fact that the introduction of GR can serve as 2D “mat” to anchor the forming TiO2 nanoparticles and thus restrain TiO2 from growing into large particles.52-53 The same phenomenon has also been observed in the previous reports.52-53 However, no diffraction peak at about 25.0o appears for the GR in the GR@TiO2 nanocomposites. This can be attributed to two probable reasons. Firstly, the relatively low diffraction intensity of GR at 25.0o might be shielded by the main peak of anatase TiO2 at 25.5o. Secondly, the intercalation of TiO2 particles may destroy the regular stack of the GR.52 To further investigate the structural properties of the GR@TiO2 nanocomposite, the Raman analysis is performed. As shown in Figure 6, there are two typical Raman features of GR nanosheets. The D band at around 1352 cm-1 is attributed to the disordered sp2 carbon; the other hand, the peak appearing at approximately 1594 cm-1, is often called the G-band.54 As compared to GO, the intensity ratio of the D band to G band, ID/IG, changes from 1.02 to 0.97, indicating the removal of hydroxyl and epoxy groups and the restoring of sp2-hydridized carbon (the inset of Figure 6).14, 55 This result also confirms the existence of GR nanosheets in the GR@TiO2 composites. Below 1000 cm-1, typical optical modes of anatase TiO2 can be clearly observed at 144 cm-1 (Eg(1)), 399 cm-1 (B1g), 516 cm-1 (A1g) and 637 cm-1 (Eg(2)).54 The optical properties of the samples, determined by UV-vis diffuse reflectance spectroscopy (DRS), are mirrored in Figure 7. Obviously, the introduction of GR has a significant effect on the light absorption of GR@TiO2 nanocomposites. With the increase of GR content, there is an enhanced absorbance in the visible-light region ranging from 400-800 nm (Figure 7). Furthermore, the optical absorption band edge of the GR@TiO2 nanocomposites undergoes the red-shift as compared with that of blank TiO2, revealing a band gap narrowing of the semiconductor TiO2 due to the introduction of GR into the matrix of TiO2 (Figure 7). Such an analogous phenomenon is also observed on other GR-based semiconductor nanocomposites, resulting from the electronic interaction between GR and the semiconductor.13-14, 31, 55-56 The photocatalytic activities of the resulting nanocomposites are firstly evaluated by selective reduction of 4-nitroaniline (4-NA) to p-phenylenediamine (PPD) in the aqueous phase with the addition of ammonium formate as quencher for photogenerated holes and N2 purge under UV light irradiation. As shown in Figure 8, the GR@TiO2 nanocomposites exhibit obviously enhanced photoactivities as compared with that of the blank TiO2. The photocatalytic reduction efficiency of 4-NA follows the order: 5%GR@TiO2 > 10%GR@TiO2 > 20%GR@TiO2 > 2%GR@TiO2 > blank TiO2. Namely, the 5%GR@TiO2 shows the best photocatalytic performance toward the reduction of 4-NA among these 8


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samples. Within 8 minutes, the 4-NA is completely reduced to p-phenylenediamine (Figure S6) with a high selectivity (99%, determined by high performance liquid chromatograph). It should be particularly noted that the higher addition ratio of GR would result in the deterioration of photocatalytic activities of the GR@TiO2 nanocomposites owing to the decreased amount of primary photoactive ingredient TiO2 and the light “shielding effect” of black color GR.10, 26, 57 These results indicate a synergistic effect between GR nanosheets and TiO2. Thus, in order to achieve an optimal photocatalytic performance, it is crucial to control the composition ratio in the nanocomposite of [email protected], 26, 57 We further examine the efficacy of GR@TiO2 nanocomposite under the same reaction conditions for a series of substituted aromatic nitro compounds. As shown in Table 1, it is clearly seen that the 5%GR@TiO2 exhibits obviously enhanced photoactivities than that of the blank TiO2. The reaction proceeds very fast over 5%GR@TiO2, which is over within 8 minutes in most cases. Moreover, the reaction is highly selective for the synthesis of a series of aromatic amines. These results illustrate the highly active and selective of GR@TiO2 nanocomposite photocatalyst for aromatic nitro reduction under ambient conditions. In addition, the stability of the 5%GR@TiO2 has also been evaluated. As displayed in Figure 9, after five times recycling photoactivity test for the selective reduction of 4-NA, the photocatalytic performance of the used 5%GR@TiO2 is similar to that over its fresh counterpart. Thus we can conclude that the 5%GR@TiO2 nanocomposite is stable for nitro reduction under the reaction conditions. To understand the role of the GR on promoting photocatalytic activities, photoluminescence (PL) analysis of the samples is conducted, as shown in Figure 10a. The blank TiO2 exhibits a broad emission peak around 500 nm under band-gap excitation, which is attributed to the charge recombination on the defect sites of TiO2.52 The presence of GR in GR@TiO2 nanocomposites obviously reduces the intensity of the PL emission of TiO2, indicating the reduced charge carriers recombination of GR@TiO2 nanocomposites in comparison to the blank TiO2.22, 58 This is mainly due to the fact that GR can accept the photogenerated electrons from TiO2 and thus improve the charge carriers’ separation (Figure 10b).15, 59 This is also supported by the photoelectrochemical results. As shown in Figure 10c, the transient photocurrent response for 5%GR@TiO2 is much higher than that for the blank TiO2 under intermittent UV light irradiation. That is, the addition of GR is able to enhance the photocurrent significantly, which indicates more efficient transfer of photogenerated electrons over the 5%GR@TiO2 than that over the blank TiO2. This is in accordance with the above PL analysis. In addition, electrochemical impedance spectroscopy (EIS) Nyquist plots have also been carried out. As displayed in Figure 10d, the Nyquist plots of the 5%GR@TiO2 and blank TiO2 both show semicycles at high frequency, which correspond to the charge transfer limiting process and is attributed to the double-layer capacitance (Cdl) in parallel with the charge-transfer resistance (Rct) at the contact interface between electrode and electrolyte solution.12-13, 56 As shown in Figure 10d, the introduction of GR leads to obvious decrease of the arc as compared to blank TiO2, indicating that the 5%GR@TiO2 has much smaller Rct than the blank TiO2.12-13, 56 This result suggests that the transfer of charge carriers over 5%GR@TiO2 is much more efficient than that over the blank TiO2. Therefore, the enhanced 9



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photoactivity of GR@TiO2 nanocomposite is attributed to the efficient charge separation by GR. To confirm that the lifetime of photo-excited electron-hole pairs is the primary factor determining the photoactivity of GR@TiO2 nanocomposite for nitro reduction, the surface area and porosity of the blank TiO2 and 5%GR@TiO2 have been investigated. As shown in Figure S7, both of these two samples have adsorption-desorption isotherms of type IV according to the IUPAC classification.12-13 This result indicates that these two samples both possess porous structure which is attributed to well-defined and order inter-particle spaces, as shown in Figure S8.12-13, 60 The Brunauer-Emmett-Teller (BET) surface areas of the blank TiO2 and 5%GR@TiO2 are ca. 102 m2/g and 61 m2/g, respectively. It can be seen that the BET surface of the 5%GR@TiO2 is smaller than that of blank TiO2. This is attributed to the fact that the pore size and pore volume of the 5%GR@TiO2 is much smaller than that of the blank TiO2, as shown in Table S2. The decreased BET surface area of 5%GR@TiO2 is mainly due to the fact that TiO2 nanocrystals of GR@TiO2 nanocomposite are densely coated on the surface of GR (Figure 1c-f) and thus the inter-particle spaces of TiO2 nanoparticles are much smaller than that of the blank TiO2 (Figure S8), in which the TiO2 nanocrystals are aggregated and connected to form a porous network (Figure 1a, b). Although the surface area of blank TiO2 is higher than that of 5%GR@TiO2 nanocomposite, the photoactivity of the blank TiO2 is obviously lower than that of the 5%GR@TiO2 nanocomposite. This result strongly suggests that the obviously enhanced photoactivity of GR@TiO2 nanocomposite is mainly attributed to the efficient charge carrier separation by GR. On the basis of the above results, a possible reaction mechanism can be proposed as the following, which is schematically shown in Figure 11. Under UV light irradiation, electron-hole pairs are generated over the semiconductor TiO2 in water. Since the conduction band of TiO2 is more negative than the work function of GR, the photogenerated electrons can transfer from TiO2 to the GR nanosheets, thus promoting the charge carriers’ separation. Simultaneously, the photogenerated holes are trapped by ammonium formate. In addition, the N2 atmosphere provides an anaerobic condition for nitro reduction. Therefore, the photogenerated electrons can be transferred to the adsorbed nitro compounds to give the target products. Conclusion In conclusion, we have described a non-covalent functionalization process to coat GR with TiO2 by using benzyl alcohol as linking agent. The π-π interactions of its benzene ring with the aromatic network of GR enable the use of GR nanosheets as structural scaffold for fabrication of GR-based TiO2 (GR@TiO2) composites, which features ultra-large sheet morphology with the lateral size far larger than the original GR nanosheets. During the growth process of GR@TiO2 composites, benzyl alcohol adsorbed on the surface GR nanosheets not only promotes the hydrophilicity of GR in the solution but also provides anchoring groups (hydroxyl groups) for the nucleation and growth of TiO2 nanocrystals, and simultaneously promotes the cross-linking of the separated, single GR, thereby forming the 10


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ultra-large sheet structure of GR@TiO2. The GR@TiO2 composites exhibit higher photoactivities than blank TiO2 toward selective reduction of aromatic nitro compounds. The enhanced photoactivities of GR@TiO2 are ascribed to the efficient charge carrier separation by GR. It is hoped that our method could provide a facile and straightforward approach for the facile synthesis of GR-semiconductor composite materials with desirable ultra-large sheet structural morphology for targeted artificial photocatalytic applications. Acknowledgement. The support by the National Natural Science Foundation of China (21173045, 20903023, 20903022), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), Program for Returned High-level Overseas Chinese Scholars of Fujian province, and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged. Supporting Information Available: Experimental details; XRD patterns of graphene oxide (GO) and graphene (GR); X-ray photoelectron spectra (XPS) of C (1s) and its peak deconvolution of GO (a) and GR (b); the atomic force microscopy (AFM) images and height profiles taken along the yellow line of the initial GO nanosheets; the lateral size distribution of the initial GO nanosheets determined by AFM analysis; SEM image of the 20%GO-TiO2 nanocomposite; the average crystallite sizes of the TiO2 nanoparticles in the blank TiO2 and the GR@TiO2 nanocomposites calculated from the (101) crystal plane of anatase TiO2 on the basis of the Scherrer formula; time-dependent UV-vis spectral changes of the photocatalytic reduction of 4-nitroaniline (4-NA) to p-phenylenediamine (PPD) over 5% GR@TiO2 nanocomposite under UV light irradiation (λ=365±15 nm); N2 adsorption-desorption isotherms of the blank TiO2 and 5%GR@TiO2; the pore size distributions of the blank TiO2 and 5%GR@TiO2; the surface area and porosity of the blank TiO2 and 5%GR@TiO2. This material is available free of charge via the Internet at http://pubs.acs.org. References 1. Zhang, N.; Zhang, Y.; Xu, Y.-J., Recent Progress on Graphene-Based Photocatalysts: Current Status and Future Perspectives. Nanoscale 2012, 4, 5792-5813. 2. Chen, D.; Zhang, H.; Liu, Y.; Li, J., Graphene and Its Derivatives for the Development of Solar Cells, Photoelectrochemical, and Photocatalytic Applications. Energy Environ. Sci. 2013, 6, 1362-1387. 3. Guo, S.; Dong, S., Graphene Nanosheet: Synthesis, Molecular Engineering, Thin Film, Hybrids, and Energy and Analytical Applications. Chem. Soc. Rev. 2011, 40, 2644-2672. 4. Han, C.; Yang, M.-Q.; Weng, B.; Xu, Y.-J. Phys. Chem. Chem. Phys. 2014, 16, 16891-16903. 5. Sun, Y.; Wu, Q.; Shi, G., Graphene Based New Energy Materials. Energy Environ. Sci. 2011, 4, 1113-1132. 6. Wang, H.; Dai, H., Strongly Coupled Inorganic-Nano-Carbon Hybrid Materials for Energy Storage. Chem. Soc. Rev. 2013, 42, 3088-3113. 11



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Scheme 1. Flowchart for the synthesis of GR@TiO2 nanocomposites using non-covalently functionalized GR nanosheets as template.

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Figure 1. SEM images of blank TiO2 (a, b) and GR@TiO2 nanocomposites: 2%GR@TiO2 (c), 5%GR@TiO2 (d), 10%GR@TiO2 (e) and 20%GR@TiO2 (f).

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Figure 2. SEM images of 20%GR@TiO2 nanocomposite (a) and elemental mapping patterns (b, c and d) of the boxed area in the inset of panel a.

Figure 3. TEM (a, b) and HRTEM (c, d) images of 20%GR@TiO2 nanocomposite; the inset of panel d is the image of SAED pattern.

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A(004) A(200)

A(204)

A(116) A(220)

A(101)

A(105) A(211)

Figure 4. SEM images of GR coated with TiO2 using a reaction mixture of TBOT:BA:EtOH:H2O in the molar ratio of 1:x:100:5, with x=0 (a), 2 (b), 5 (c) and 40 (d).

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A(215)

TiO2 2% GR@TiO2 5% GR@TiO2 10% GR@TiO2 20% GR@TiO2

10

20

30 40 50 60 2-Theta (degree)

70

80

Figure 5. XRD patterns of the blank TiO2 and GR@TiO2 nanocomposites.

19



5% GR@TiO2 GO

D

Intensity (a.u.)

Intensity (a.u.)

Egg(1)

G

ID/IG

0.97

1.02 1000

1200 1400 1600 -1 Raman shift (cm )

B1g A1g Eg(2)

400

1800

5% GR@TiO2

800 1200 -1 Raman shift (cm )

1600

Figure 6. Raman spectra of the 5%GR@TiO2 nanocomposite; the inset is the partial Raman spectra of the 5% GR@TiO2 and GO between 1000 and 1800 cm-1.

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.2

TiO2 2% GR@TiO2

1.0

5% GR@TiO2 10% GR@TiO2

0.8

20% GR@TiO2

0.6 0.4 0.2 0.0

300

400 500 600 700 Wavelength (nm)

800

Figure 7. UV-vis diffuse reflectance spectra (DRS) of the blank TiO2 and GR@TiO2 nanocomposites.

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100 Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

TiO2 2% GR@TiO2

60

10% GR@TiO2

5% GR@TiO2 20% GR@TiO2

40 20 0

0

2 4 6 Irradiation Time (min)

8

Figure 8. Photocatalytic reduction of 4-nitroaniline (4-NA) to p-phenylenediamine (PPD) over the blank TiO2 and GR@TiO2 nanocomposites under UV light irradiation (λ=365±15 nm).

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Table 1. Photocatalytic reduction of various aromatic nitro compounds in water over 5%GR@TiO2 for 8 minutes under UV light irradiation (λ=365±15 nm). a Substrate Product Conversion (%) Selectivity (%) H 2N

NO2

H2N

NH2

99 (40)

98 (98)

94 (56)

99 (98)

79 (45)

96 (98)

97 (60)

98 (96)

98 (58)

99 (97)

99 (41)

97 (99)

H2N

H2 N NO2

NH2

NH2

NH2 NH2

NO2

HO

NO2

HO

HO

NH2

HO NH2

NO2

OH

OH NH2

NO2

H 3C

NO2

H3C

NH2

83 (54)

99 (97)

H3CO

NO2

H3CO

NH2

91 (38)

97 (98)

a

The data in parenthesis are conversion and selectivity over the blank TiO2 under identical reaction conditions.

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st

100 Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nd

rd

1 run 2 run 3 run 4th run 5th run

80 60 40 20 0 0

8 16 24 32 Irradiation Time (min)

40

Figure 9. Recycled photocatalytic reduction of 4-NA over 5%GR@TiO2 under UV light irradiation (λ=365±15 nm).

Figure 10. Photoluminescence spectra of the blank TiO2 and 5%GR@TiO2 nanocomposite (a); scheme illustrating the transfer of charge carriers in GR@TiO2 nanocomposites under UV light irradiation (λ=365±15 nm) (b); transient photocurrent response of the sample electrodes of the blank TiO2 and 5%GR@TiO2 under UV light irradiation (λ=365±15 nm) (c); electrochemical impedance spectroscopy (EIS) Nyquist plots of the sample electrodes of the blank TiO2 and 5%GR@TiO2 under UV light irradiation (λ=365±15 nm) (d).

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Figure 11. Schematic diagram of the proposed mechanism for photocatalytic reduction of aromatic nitro compounds over the GR@TiO2 nanocomposites under the UV light irradiation (λ=365±15 nm).

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Table of Contents (TOC) Graphic

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Noncovalently Functionalized Graphene-Directed Synthesis of

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