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May 24, 2016 - Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Roa...
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Rational Design of a Unique Ternary Structure for Highly Photocatalytic Nitrobenzene Reduction Bocheng Qiu, Yuanxin Deng, Qiaoying Li, Bin Shen, Mingyang Xing,* and Jinlong Zhang* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China S Supporting Information *

ABSTRACT: The rational design and controllable synthesis of TiO2 and noble metal composite photocatalysts represent an unprecedented challenge for developing the solar-driven reduction of nitrobenzene (NB) to aminobenzene (AB), owing to the recombination over the interface between the noble metals and TiO2, which is harmful to the conversion efficiency of NB to AB. Here, we design a unique ternary structure (the high separation of TiO2 and Pt nanoparticles on the surface of reduced graphene oxide (RGO)) through the sol−gel and microwave-assisted strategies. The substrate of RGO can be used as an “electric wire” to effectively transfer the photogenerated electrons from the isolated TiO2 nanocrystals to the isolated Pt nanoparticles, which greatly decreases the interface recombination between TiO2 and Pt and further improves the conversion efficiency of NB to AB under the solar light irradiation. We anticipate our research provides a new way to overcome the interface recombination on the binary photocatalysts in the photocatalytic reaction.

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INTRODUCTION In the past decades, constant efforts have been devoted to design promising materials with advanced nanostructure for developing the solar-driven reduction reaction from nitrobenzene (NB) to aminobenzene (AB).1−4 The noble metals with small sizes and controllable morphologies have been widely investigated by many researchers for great potential application in the photoreduction of NB.5,6 However, the surface energy increases with the decreasing size of the noble metal nanoparticles, which easily results in serious aggregation of the noble metal nanoparticles, further leading to the decreasing of catalytic activity and selectivity.7−10 In much research, the method of oleic acid modified on the surface of noble metals has been extensively used to form the ultradispersed noble metal nanoparticles.11 However, the long-chain molecule on the surface of noble metal nanoparticles may lead to the weak ability to capture and transfer the electrons, thus decreasing the reduction ability. Therefore, in order to solve this problem, using the supporting substrate is a reliable approach to anchor and stable the noble metal particles, especially for the nanosized noble metal particles.12,13 In addition, it has been proved that the catalytic activity per the metal atom of noble metal nanoparticles greatly depends upon the nature of the supporting substrate.7 For this reason, extensive research work has been focused on the using of sp2 hybrid carbon materials as the supporting substrate to deposit noble metals. Carbon materials including carbon nanotube, carbon nanosphere, and carbon nanofiber are regarded as exceptionally promising supporting materials of noble metal nanoparticles for photoelectrocatalytic application due to their © XXXX American Chemical Society

high surface area and excellent electric conductivity. As a new carbon allotrope, graphene has been regarded as an ideal substrate to support Pt, Pd, or Au nanoparticles due to the high surface area, superior mechanical property, excellent charge carrier mobility, high thermal conductivity, and ultrathin thickness. So far, graphene-based substrates composited with noble metal nanoparticles have been studied for photocatalytic or electrocatalytic reaction.14,15 For instance, Zhao et al.15 prepared monodisperse Pt nanoparticle@graphene nanobox composites, and the as-prepared composites supplied excellent electrical conductivity for the fast transfer of electrons, leading to an excellent performance in the oxygen reduction reaction. Park et al.16 introduced RGO as recombination inhibitor into the Pt/TiO2 composite, thus improving hydrogen evolution activity. On the other hand, TiO2 is the key catalyst for photocatalysis.17,18 Nanosized TiO2 crystals with high surface area and low recombination rate of photogenerated electron−hole pairs are very effective for photocatalytic activity.19−21 In addition, TiO2 nanocrystals composited with noble metals have been regarded as a kind of fascinating nanocomposite because the loaded noble metal can further enhance the charge separation and transportation properties.22,23 However, the recombination over the interface between the loaded noble metals and TiO2, especially to the large sized or aggregated TiO2, is very harmful to the conversion efficiency of NB to Received: April 14, 2016 Revised: May 24, 2016

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DOI: 10.1021/acs.jpcc.6b03800 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C AB.24 Hence, it is urgent to design a rational nanostructure to overcome the interface recombination, thus improving the reduction ability of the catalyst. Herein, we demonstrate that reduced graphene oxide (RGO) can be used for deposition of TiO2 nanocrystals and Pt nanoparticles to constitute a unique ternary structure through the sol−gel and microwave-assisted strategies, respectively, in which RGO acts as the stabilizer of TiO2 nanocrystals and Pt nanoparticles, as well as the “electric wire” to prevent the recombination of electrons and holes. Compared with the traditional agglomerated ternary structure, the as-prepared Pt− TiO2−RGO composite gives rise to a high photocatalytic performance toward the reduction of NB with triethanolamine (TEOA) as a sacrificial agent. In addition, our findings exhibit that RGO and Pt can be used as the excellent electron transfer substrate and electron capture agent, respectively.

Preparation of Pt-TiO2-RGO. 0.2 g of TiO2-RGO powder was dispersed into 50 mL of ethylene glycol (EG) with the assistance of ultrasound. Subsequently, the mixture was kept stirring for 30 min, and then 5 mL 5 mg/mL EG solution of hexachloroplatinic acid was slowly added into the mixture drop by drop and continued magnetically stirring for 2 h. After stirring, 1 mL of 1 M NaOH/EG was dropped into the above mixture slowly. After stirring for 30 min, the above mixture was placed into a microwave oven (480 W) for 15 min irradiation. The solid sample was obtained after filtration with ethanol and water and finally dried at 40 °C in a vacuum drier, which was denoted as Pt-TiO2-RGO. For comparison, Pt−TiO2 was prepared by the following method. 0.2 g of anatase TiO2 powders was dispersed in a solution of EG. Then, 5 mL of 5 mg/mL EG solution of hexachloroplatinic acid was slowly added into the solution, and it continued magnetically stirring for 2 h. After adding 1 mL of 1 M NaOH/EG, the solution was transferred to the microwave oven (480 W) for 15 min irradiation. Finally, the precipitates were collected by washing with water and ethanol. The Pt-TiO2 composite was obtained by drying the precipitates at 40 °C overnight. Characterization. The crystal structure of all the samples was characterized by X-ray diffraction patterns (XRD, RigakuD/MAX 2550 diffractmeter). Transmission electron microscopy (TEM) images of samples were investigated on JEM-1400 and JEM-2100 microscopes (JEOL, Japan). The surface morphologies and structure of the samples were observed by field emission scanning electron microscope (FESEM) performed on a NOVA Nano SEM 450. The X-ray photoelectron spectroscopy (XPS) data were obtained from a PerkinElmer PHI 5000C ESCA system with a monochromatic Al Kα source operated at 250 W. All the binding energies can be corrected by using the C 1s level at 284.4 eV as an internal reference. Raman spectra were recorded at room temperature using Raman microscopes (Renishaw, UK) under an excitation laser wavelength of 532 nm. N2 adsorption−desorption isotherms were recorded at 77 K (ASAP2020). Simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) of samples were conducted on a Pyris Diamond (PerkinElmer) apparatus. The samples were heated from 40 to 800 °C at a heating rate of 20 K min−1 in a dynamic air flow. Photocatalytic Reduction of Nitrobenzene (NB). The reduction reaction was conducted under a 300 W Xe lamp irradiation (with AM 1.5 air mass filter) in the presence of TEOA. Acetonitrile was selected as reaction solvent for NB reduction due to its low-cost and less-toxic properties. Briefly, 50 mg of Pt-TiO2-RGO catalyst, 50 mL of NB solution (0.01 M), and 0.6 mL of TEOA (4.5 mmol) were added into a twonecked quartz flask under Ar atmosphere. The concentrations of AB and NB were investigated on a high-performance liquid chromatograph (HPLC) (Shimadzu SPD-M20A). The catalysts after photocatalytic reaction were collected by centrifugation and washed with ethanol and deionized water several times. The samples were dried at 298 K in a vacuum drier. The conversion rate of NB, the yield of AB, and reaction selectivity were calculated by the following equations:

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EXPERIMENTAL SECTION Materials. All chemical reagents used in this experiment including H2PtCl6·6H2O (AR), ethylene glycol (EG) (AR), NaOH (AR), titanium diisopropoxide bis(acetylacetonate) (TDA), H2SO4 (AR), NaNO3 (AR), KMnO4 (AR), H2O2 (AR), ethanol (AR), ultrapure water (H2O), concentrated ammonia solution (∼28 wt %), and acetonitrile (AR) were used as obtained without any further purification. Nitrobenzene (NB), aminobenzene (AB), and triethanolamine (TEOA) were purchased from Aladdin. Graphite powder (30 μm) was purchased from Sigma-Aldrich (St. Louis, MO). Preparation of Graphene Oxide (GO). GO was prepared by an improved Hummer method. In a typical procedure, 2.0 g of graphite powder was added into 50 mL of concentrated H2SO4 in a 500 mL three-necked and round-bottomed flask supplied with a mechanical stirrer under ice bath. After stirring 10 min, 6 g of KMnO4 powder was slowly added into the mixture in 30 min while the temperature was kept at below 5 °C; then the above mixture was heated to 35 °C and stirred vigorously for 2 h. Subsequently, 80 mL of water was added, and then the mixture was heated at 98 °C. Afterward, the slurry was continuously stirred for another 30 min before 80 mL of 30% H2O2 and 280 mL of water solution were added sequentially to dissolve insoluble manganese species. Finally, the dispersion was then repeatedly centrifuged at 4000 rpm and washed with 5% aqueous HCl solution to remove the residual salts. The washed graphene oxide solid was obtained by lyophilization. Preparation of TiO2-RGO. 75 mg of GO powder was dispersed into a mixture containing 25 mL of acetonitrile and 75 mL of ethanol with the assistance of ultrasound. After an ultrasonic treatment for 90 min, 0.1 mL of NH3 was dropped into the above suspension. After magnetic stirring for 30 min, 1 mL of TDA was slowly added into the above mixture and then continued stirring for another 30 min. The dispersion was heated to 60 °C and maintained at 60 °C for several times (2.5−24 h). Afterward, the resultant products were collected by centrifugation, washed three times with ethanol, and redispersed in 25 mL of water with the assistance of ultrasound. After ultrasonic treatment for 30 min, the suspension was then transferred into a 50 mL Teflon-lined stainless steel autoclave and reacted at 180 °C for 12 h. Finally, the resultant products were collected by a freeze-drying method and denoted as TiO2RGO. (The time of the sol−gel process includes 2.5, 6, 12, and 24 h.)

NB conversion (%) =

AB yield (%) = B

C0 − C1 × 100 C0

C2 × 100 C1 DOI: 10.1021/acs.jpcc.6b03800 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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C2 × 100 C0 − C1

modified Hummer method, which was then dispersed in the mixture containing acetonitrile and ethanol through a ultrasonic treatment, followed by adding the titanium diisopropoxide bis(acetylacetonate) (TDA) as the TiO2 precursor. Thereafter, 0.1 mL of concentrated ammonia (NH3) was slowly added into the above mixture to induce the hydrolysis of TDA and then continued stirring for 12 h, which can result in the formation of the amorphous TiO2 on the GO sheet, denoted as A-TiO2-GO (“A” represents “amorphous”). Followed by a hydrothermal treatment, the amorphous TiO2 nanoparticle can be crystallized into the anatase nanocrystals accompanied by the reduction of GO sheet, which is denoted as TiO2−RGO. Next, the highly dispersed Pt nanoparticle on TiO2-RGO is realized by the microwave reduction of hexachloroplatinic acid (H2PtCl6), which is named Pt-TiO2-RGO. TEM, XRD, and Raman Analysis. High-resolution transmission electron microscopy (HRTEM) image of the A-TiO2GO sheet exhibits highly dispersed TiO2 nanoparticles loaded on the GO sheet (Figure 2a). More importantly, no free GO sheet or free TiO2 nanoparticles can be observed, which indicates that the sol−gel process is successful to control the TiO2 nanoparticles in situ growth on the GO sheet. Meanwhile, no lattice fringes of TiO2 appeared (Figure 2a), verifying TiO2 nanoparticles deposited on the GO sheet as an amorphous form. X-ray diffraction (XRD) patterns (Figure 2b) show that the (002) reflection at 2θ = 11.2° of GO sheet for the A-TiO2GO composite can be still observed clearly, which suggests that the GO sheet cannot be reduced by the sol−gel treatment. After the hydrothermal treatment, the GO sheet is reduced into RGO sheet because the absence of characteristic peak of GO; meanwhile, the amorphous TiO2 nanoparticles have been highly crystallized into anatase nanocrystals. TEM (Figure S1a,b) images clearly show uniform loading of TiO2 nanocrystals onto the RGO sheet. Furthermore, the field emission scanning electron microscopy (FESEM) images disclose that the TiO2 nanocrystals are selectively loaded on the graphene rather than wrapped by the graphene (Figure S1c,d). More interestingly, the content of TiO2 on the graphene can be controlled by adjusting the time of the sol−gel process, as shown in Figure S2. With the sol−gel time increasing, the

Here, C0 is the initial concentration of NB; C1 and C2 are the concentrations of NB and AB, respectively, after the photocatalytic reactions. Photoelectrochemical Tests. Photocurrent response measurements of samples were conducted with an analyzer (Zahner, Zennium) under simulated solar light irradiation using a conventional three-electrode cell, using a Pt wire as the counter electrode, and a saturated calomel electrode as the reference electrode. The working electrode was prepared by spreading the 100 μL suspension of FTO (fluorine doped tin oxide) glass substrate. The suspension was prepared by dispersing catalyst (10 mg) into a ternary solution containing 100 μL of Nafion, 400 μL of ethanol, and 500 μL of H2O. The working electrode was dried at 40 °C. A 0.5 M solution of Na2SO4 was used as the electrolyte.

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RESULTS AND DISCUSSION Preparation Process of Pt-TiO2-RGO. The synthesis procedure of the Pt-TiO2-RGO composite is illustrated in Figure 1. First, graphene oxide (GO) was synthesized through a

Figure 1. Schematic formation process of the Pt-TiO2-RGO composite.

Figure 2. HRTEM images of the A-TiO2-GO sheet (a). The XRD patterns of GO, A-TiO2-GO, and TiO2-RGO sheet (b). TEM (c) and HRTEM (d) images of the Pt-TiO2-RGO composite. Inset of (d) is the corresponding fast Fourier transform (FFT) pattern taken at the relatively flat edge. C

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Figure 3. Photocatalytic reduction of nitrobenzene over different photocatalysts under Xe lamp (AM 1.5) for 8 h ([Catal.] = 50 mg, [NB] = 0.5 mmol, [TEOA] = 6 mmol, [CH3CN] = 50 mL) (a−d) and the corresponding transient photocurrent responses for different samples (e). Proposed mechanism for the photocatalytic reduction of NB to AB (f).

RGO, which is greatly superior to that of most previously reported TiO2-RGO composite materials.27,28 With the microwave deposition of Pt, the slight increase of the surface area from 375.38 to 382.78 m2/g can be obtained (Figure S6b), indicating the absence of graphene aggregation under the microwave process. Such a high surface area can endow the composite with more surface-active sites and make the chargecarrier transfer easier, which leads to an enhanced photoelectrocatalytic activity. The X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical state of the PtTiO2-RGO composite. The wide-survey spectrum (Figure S7a) exhibits the presence of Pt, Ti, C, and O elements in the composite. The peaks at 72.0 and 75.2 eV (Figure S7b) corresponded to 4f7/2 and 4f5/2 components of the metallic Pt, revealing the existence of metallic Pt(0) and thereby suggesting the successful microwave deposition of Pt on the RGO. The C 1s XPS spectrum of GO (Figure S7c) is divided into two peaks, which are assigned to the sp2-bonded carbon (C−C, 284.6 eV) and (C−O−C, 286.7 eV). In contrast, the decrease of the epoxy C−O−C bonds at 286.7 eV for the Pt-TiO2-RGO composite can be clearly observed, which also validates the reduction of GO.19 Photocatalytic Reduction of NB Tests. Inspired by the ultradispersed and surface-modifier-free nature of Pt nanoparticle, excellent photocatalytic activity of TiO2 nanocrystal, and the good conductivity of RGO, we selected the reduction of nitrobenzene as a model reaction to investigate the photocatalytic activity of Pt-TiO2-RGO composite. The photocatalytic reduction of NB with the presence of TEOA was carried out under simulated solar irradiation. Figure S8 shows the adsorption capacities of NB in the dark for Pt-RGO, TiO2-RGO, Pt-TiO2-RGO, and Pt-TiO2. It can be seen that these samples except Pt-TiO2 all show an excellent adsorption capacity due to the existence of RGO. Under simulated solar irradiation, the amount of NB for Pt-TiO2-RGO decreased linearly with light irradiation, while AB as the reduction product of nitrobenzene was formed (Figure 3a), which demonstrates the excellent reduction ability and high selectivity. Under the simulated solar irradiation, TiO2 nanocrystals are easily excited

content of TiO2 in the TiO2-RGO composite is correspondingly increasing from 33.6 to 48.6 wt %. However, the distribution of nanocrystals is almost keeping on ultradispersed state, although the mass fraction of TiO2 increases obviously to 43.2 wt % (Figure S3a−c). However, when the TiO2 content continues to increase to 48.6 wt %, it began to appear the aggregation phenomenon (Figure S3d). The obvious enhancement of the intensity ratio of D/G bands after the loading of TiO2 nanoparticles in the Raman spectra (Figure S4) confirms the reduction of GO,25 which is in good agreement with the XRD result. Furthermore, the Raman spectra indicate the characteristic peaks of TiO2 (anatase) at 154 cm−1. The weak intensity and the obvious shift toward high wavenumber compared to the Eg mode at 144 cm−1 of pure TiO2 (anatase) further suggest the ultradispersed and small nature of TiO2 nanoparticles and their strong coupling interaction with graphene. BET and XPS Analysis. After the microwave treatment, the Pt nanoparticles have been immobilized on the surface of the TiO2-RGO composite. As shown in Figure. 2c, the Pt nanoparticles and TiO2 nanocrystals are highly dispersed on the RGO sheet, and no free Pt nanoparticles appear. In addition, it is deserved to be mentioned that a small amount of Pt nanoparticles are loaded on the TiO2 nanocrystals. From the HRTEM image (Figure 2d), we can clearly observe two sets of lattice spacing. The lattice spacing of 0.196 nm correspond to the (200) planes of Pt, agreeing with the previously reported value.26 The HRTEM image also shows a lattice spacing of 0.350 nm corresponding to the (101) plane of the anatase TiO2. In addition, the corresponding FFT pattern (Figure 2d, inset) further indicates that TiO2 exists in the form of single crystal. Moreover, the edge of graphene (four layers) can be obviously seen as indicated by the arrow, which assures the good conductivity of composite. The size distribution curves of Pt nanoparticles (Figure S5a) display the average size centered at 2 nm, and the mean diameter of the TiO2 nanocrystals (Figure S5b) is determined to be ca. 9.5 nm from the analysis of the size distribution curve. The adsorption data (Figure S6a) reveal a remarkably high surface area of 375.38 m2/g for TiO2D

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Figure 4. HRTEM images for Pt-TiO2-RGO with different aging times: 2.5 (a), 6 (b), 12 (c), and 24 h (d). The photoreduction of NB for AB generation over the Pt-TiO2-RGO composites with different aging times (corresponding different TiO2 contents: 33.6, 34.7, 43.2, and 48.6 wt %) (e) and the corresponding samples of transient photocurrent responses (f).

Photocurrent Response Measurement. To further explain the photoelectrochemical property of Pt-TiO2-RGO, we used transient photocurrent response measurement to detect the lifetime and mobility rate of the photoexcited charge carriers. The photocurrent−time curves (Figure 3e) show the photocurrent of Pt-TiO2-RGO is much higher than that of the other three samples (800 nA/cm2 vs 550 nA/cm2, 100 nA/cm2 and 50 nA/cm2), revealing low recombination rate of the photogenerated electron−hole pairs, which is in consistent with the photoreduction activity. The excellent photocurrent property indicates that the introduction of RGO can be used as an “electric wire” to effectively transfer the photogenerated electrons from the isolated TiO2 nanocrystals to the isolated Pt nanoparticles under the solar light irradiation, which greatly decreases the interface recombination between TiO2 and Pt. The photoelectrons collected on the Pt surface trigger the photoreduction of NB at room temperature. It has been reported that the reduction of NB to AB is considered to proceed several reaction steps (Figure 3f).4 Further reactions will result in the formation of oligomers of AB as the minor products, thus decreasing the yield of AB in this reaction system.4 Effect of TiO2 Content in the Composite on Photocatalytic Activity. In order to further demonstrate the importance of high dispersion and separation of TiO2 and Pt nanoparticles on the surface of RGO in the photoreduction of NB, we prepared the composite with different TiO2 content by adjusting the reaction time of the sol−gel process. With the TiO2 content increasing, the Pt nanoparticles still keep the ultradispersed state (Figure 4a−c). However, when the reaction

to generate the electron and hole, and then RGO can quickly transfer the photogenerated electron to the Pt nanoparticles. After reaction time of 8 h, the conversion rate of NB and the selectivity of AB are up to 44% and 99.0%, respectively (Figure 3a). If we prolong the reaction time of 20 h (Figure S9), the conversion rate of NB and the selectivity of AB can be improved to 95% and 99%, respectively, which are very high and superior to that of some previous reported work.4,6 Moreover, the HRTEM image of Pt-TiO2-RGO (Figure S10) after photocatalyitc tests shows that the sample still remains the initial morphology, which reveals the structure stability of PtTiO2-RGO. In order to exclude the effect of surface plasmon resonance (SPR) caused by the Pt nanoparticles, Pt-RGO and Pt-TiO2 are selected to investigate the reduction ability. It also can be observed that the Pt nanoparticles are uniformly loaded on the surface of RGO or TiO2 (Figure S11), and the average size of the Pt nanoparticles is centered on 2 nm. However, we noted that the amount of NB decrease slowly, but the reduction product of AB has not been detected over Pt-RGO (Figure 3b), which indicates these ultrasmall Pt nanoparticles are quietly difficult to be excited to generate the electron by the solar light.29 Not surprisingly, with the absence of graphene, Pt-TiO2 displays much lower photoreduction ability of NB than PtTiO2-RGO (Figure 3c), indicating that the interface recombination between TiO2 and Pt is indeed harmful to the conversion efficiency of NB, and the introduction of graphene is like an “electric wire” to suppress this recombination. In addition, in the absence of Pt, TiO2-RGO also does not have the reduction ability of NB (Figure 3d). E

DOI: 10.1021/acs.jpcc.6b03800 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (14CG30) and the Fundamental Research Funds for the Central Universities (22A201514021).

time prolongs to 24 h (TiO2 content: 48.6 wt %), most of the Pt nanoparticles are loaded on the surface of the TiO2 nanocrystals (Figure 4d). That is because the RGO surface has been completely coated by the TiO2 nanocrystals before the microwave treatment (Figure S3d). As a result, the Pt-TiO2RGO (24 h) composite shows a lower photoreduction activity than Pt-TiO2-RGO (12 h) (Figure 4e), which may be caused by the high interface recombination rate between the aggregated TiO2 and Pt nanoparticles. This result indicates that the importance of separation of TiO2 and Pt nanoparticles in the NB photoreduction. On the other hand, when the sol− gel time increases from 2.5 to 12 h, a corresponding increase of photocatalytic activity can be observed, which indicates that the optimum time of the sol−gel process should be controlled at about 12 h to ensure the enough amount of TiO2. More direct evidence by the transient photocurrent responses on the transfer rate of the photogenerated electron−hole pairs are given in Figure 4f. Interestingly, the tendency of the photocurrent intensity is consistent with tendency of photoreduction activity. That is, the Pt-TiO2-RGO (12 h) is shown to produce electrons and exhibit the highest photocurrent response among all the Pt-TiO2-RGO composites under simulated solar light irradiation. The results reveal the design for the nanostructure of Pt-TiO2-RGO (12 h) is very rational and successful.

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(1) Gao, Y.; Ma, D.; Wang, C.; Guan, J.; Bao, X. Reduced Graphene Oxide as a Catalyst for Hydrogenation of Nitrobenzene at Room Temperature. Chem. Commun. 2011, 47, 2432−2434. (2) Mantha, R.; Taylor, K. E.; Biswas, N.; Bewtra, J. K. A Continuous System for Fe0 Reduction of Nitrobenzene in Synthetic Wastewater. Environ. Sci. Technol. 2001, 35, 3231−3236. (3) Fu, H.; Zhu, D. Graphene Oxide-Facilitated Reduction of Nitrobenzene in Sulfide-Containing Aqueous Solutions. Environ. Sci. Technol. 2013, 47, 4204−4210. (4) Kamegawa, T.; Seto, H.; Matsuura, S.; Yamashita, H. Preparation of Hydroxynaphthalene-Modified TiO2 Via Formation of Surface Complexes and Their Applications in the Photocatalytic Reduction of Nitrobenzene under Visible-Light Irradiation. ACS Appl. Mater. Interfaces 2012, 4, 6635−6639. (5) Tanaka, A.; Nishino, Y.; Sakaguchi, S.; Yoshikawa, T.; Imamura, K.; Hashimoto, K.; Kominami, H. Functionalization of a Plasmonic Au/TiO2 Photocatalyst with an Ag Co-Catalyst for Quantitative Reduction of Nitrobenzene to Aniline in 2-Propanol Suspensions under Irradiation of Visible Light. Chem. Commun. 2013, 49, 2551− 2553. (6) Tada, H.; Ishida, T.; Takao, A.; Ito, S. Drastic Enhancement of TiO2-Photocatalyzed Reduction of Nitrobenzene by Loading Ag Clusters. Langmuir 2004, 20, 7898−7900. (7) Qi, H.; Yu, P.; Wang, Y.; Han, G.; Liu, H.; Yi, Y.; Li, Y.; Mao, L. Graphdiyne Oxides as Excellent Substrate for Electroless Deposition of Pd Clusters with High Catalytic Activity. J. Am. Chem. Soc. 2015, 137, 5260−5263. (8) Fuku, K.; Hayashi, R.; Takakura, S.; Kamegawa, T.; Mori, K.; Yamashita, H. The Synthesis of Size-and Color-Controlled Silver Nanoparticles by Using Microwave Heating and Their Enhanced Catalytic Activity by Localized Surface Plasmon Resonance. Angew. Chem., Int. Ed. 2013, 52, 7446−7450. (9) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem., Int. Ed. 2009, 48, 60−103. (10) Wang, F.; Li, C.; Sun, L. D.; Xu, C. H.; Wang, J.; Yu, J. C.; Yan, C. H. Porous Single-Crystalline Palladium Nanoparticles with High Catalytic Activities. Angew. Chem., Int. Ed. 2012, 51, 4872−4876. (11) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. A General Strategy for Nanocrystal Synthesis. Nature 2005, 437, 121−124. (12) Xia, B. Y.; Wu, H. B.; Yan, Y.; Wang, H. B.; Wang, X. One-Pot Synthesis of Platinum Nanocubes on Reduced Graphene Oxide with Enhanced Electrocatalytic Activity. Small 2014, 10, 2336−2339. (13) Xi, G.; Ye, J.; Ma, Q.; Su, N.; Bai, H.; Wang, C. In Situ Growth of Metal Particles on 3d Urchin-Like WO3 Nanostructures. J. Am. Chem. Soc. 2012, 134, 6508−6511. (14) Min, S.; Lu, G. Dye-Cosensitized Graphene/Pt Photocatalyst for High Efficient Visible Light Hydrogen Evolution. Int. J. Hydrogen Energy 2012, 37, 10564−10574. (15) Lv, Y.; Fang, Y.; Wu, Z.; Qian, X.; Song, Y.; Che, R.; Asiri, A. M.; Xia, Y.; Tu, B.; Zhao, D. In-Situ Confined Growth of Monodisperse Pt Nanoparticle@Graphene Nanobox Composites as Electrocatalytic Nanoreactors. Small 2015, 11, 1003−1010. (16) Park, Y.; Kang, S.-H.; Choi, W. Exfoliated and Reorganized Graphite Oxide on Titania Nanoparticles as an Auxiliary Co-Catalyst for Photocatalytic Solar Conversion. Phys. Chem. Chem. Phys. 2011, 13, 9425−9431. (17) Qiu, B.; Zhong, C.; Xing, M.; Zhang, J. Facile Preparation of CModified TiO2 Supported on MCF for High Visible-Light-Driven Photocatalysis. RSC Adv. 2015, 5, 17802−17808. (18) Zhou, Y.; Liu, Y.; Liu, P.; Zhang, W.; Xing, M.; Zhang, J. A Facile Approach to Further Improve the Substitution of Nitrogen into

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CONCLUSIONS In summary, we have designed the scalable sol−gel method and the simple microwave approach to produce the highly separated TiO2 nanocrystals and Pt nanoparticles on the graphene, respectively. Our finding exhibits that the precise controlling and effective separation of the catalysts (involving the controlled content of TiO2 nanocrystals, the conductive “electric wire” of RGO, and the individual separation and deposition of Pt nanoparticles) allow the unprecedented photocatalytic reduction activity of NB under the simulated solar light irradiation. In additional, the rational design and controllable synthesis of graphene-based composite can be adopted to extend other functional materials and achieve promising application in the photocatalytic organics synthesis and other selective photoreduction reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03800. TEM and HRTEM images, SEM images, Raman spectra, XPS spectra, N2adsorption−desorption isotherms, size distribution, adsorption curves, photocatalytic tests (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (M.X.). *E-mail [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by National Nature Science Foundation of China (21577036, 21203062, 21377038, 21237003) and sponsored by “Chenguang Program” supported F

DOI: 10.1021/acs.jpcc.6b03800 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b03800 J. Phys. Chem. C XXXX, XXX, XXX−XXX