What if the Electrical Conductivity of Graphene Is Significantly

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What if the Electrical Conductivity of Graphene Is Significantly Deteriorated for the Graphene-Semiconductor Composite based Photocatalysis? Bo Weng, and Yi-Jun Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10298 • Publication Date (Web): 01 Dec 2015 Downloaded from http://pubs.acs.org on December 4, 2015

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ACS Applied Materials & Interfaces

What if the Electrical Conductivity of Graphene Is Significantly Deteriorated for the GrapheneSemiconductor Composite based Photocatalysis? Bo Weng, †,‡ 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 *To whom correspondence should be addressed. Tel. /Fax: +86 591 83779326 E-mail: [email protected]

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Abstract

The extraordinary electrical conductivity of graphene has been widely regarded as the bible in literature to explain the activity enhancement of graphene-semiconductor composite photocatalysts. However, from the viewpoint of an entire composite based artificial photosynthetic system, the significant matter of photocatalytic performance of graphenesemiconductor composite system is not just a simple and only issue of excellent electrical conductivity of graphene. Herein, the intentional design of melamine resin monomers functionalized three-dimensional (3D) graphene (donated as MRGO) with significantly deteriorated electrical conductivity enables us to independently focus on studying the geometry effect of MRGO on the photocatalytic performance of graphene-semiconductor composite. By coupling semiconductor CdS with graphene, including MRGO and reduced graphene oxide (RGO), it has been found that the CdS-MRGO composites exhibit much higher visible light photoactivity than CdS-RGO composites although the electrical conductivity of MRGO is remarkably much lower than that of RGO. The comparison characterizations evidence that such photoactivity enhancement is predominantly attributed to the restacking-inhibited 3D architectural morphology of MRGO, by which the synergistic effects of boosted separation and transportation of photogenerated charge carriers and increased adsorption capacity can be achieved. Our work highlights that the significant matter of photocatalytic performance of graphene-semiconductor composite is not a simple issue on how to harness the electrical conductivity of graphene, but the rational ensemble design of graphene-semiconductor composite, which includes the integrative optimization of geometrical and electrical factors of individual component and the interface composition.


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Keywords: electrical conductivity, geometry effect, graphene, semiconductor, composite photocatalyst

1. Introduction Recently, there has been a dramatic increase of interest in fabrication of graphene-based semiconductor composite for multifarious photocatalytic applications owing to the unique twodimensional morphology, tantalizing electrical and physicochemical properties of graphene sheets.1-18 Regarding the preparation of graphene-semiconductor composite photocatalysts, the most common protocol is using graphene oxide (GO), fabricated from modified Hummers’ method,19 as the precursor of graphene.2, 5-8, 11, 20-25 The abundance of oxygen moieties spreading throughout GO surface imparts it with ample solution processability, providing an accessible platform to construct various graphene-semiconductor composites. However, frequently used GO-derived graphene (i.e., reduced graphene oxide, RGO) suffers from topological defects and readily forms aggregated structures, which inevitably affect its properties, such as electrical conductivity, surface area or optical transparency, and then the photocatalytic performance of RGO-semiconductor composite.2,

26, 27

Thus, chemical and/or physical modifications of RGO

sheets are of paramount significance to harness the intriguing characteristics of RGO for enhancing the photoactivity of RGO-semiconductor composite photocatalysts. Thus far, multifarious strategies, including doping with heteroatoms (e.g., B, S, N and P),28-31 decreasing its defect density32, 33 or adding a tiny amount of metal ions as “interfacial mediator”,5 have been developed in an attempt to modify RGO sheets toward achieving high efficient RGOsemiconductor photocatalysts. Despite tremendous research on this respect, almost all studies reported thus far have been focused on harnessing the electrical conductivity of RGO, whereas the other geometric property of RGO (e.g., unique and diverse processable morphology),


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especially its influence on the photocatalytic property of RGO-semiconductor system, has been overlooked. More recently, it is validated that the morphology of RGO plays a pivotal role in ameliorating the photoactivity of RGO-semiconductor composites.34, 35 For instance, Zhang et al. have shown that the growth of TiO2 nanocrystals on three-dimensional (3D) graphene aerogels (GAs) can impressively improve the photoactivity due to the high electrical conductivity of 3D GAs.35 In addition, Qi and co-authors34 have demonstrated that the 3D architecture of RGO with improved electrical conductivity would expedite the charge separation in CdS/P25/RGO system and thus lead to the enhancement of photoactivity. The main focus in those works regarding the construction of 3D graphene has been to ameliorate the electrical conductivity of graphene, thereby improving the photocatalytic efficiency of graphene-semiconductor composites. However, the correlation between the unique 3D geometry of graphene and its influence on photoactivity of graphene-semiconductor photocatalysts is still rather unclear. Actually, up to now, there have been no reports on solely focusing on studying the effect of geometrical parameter of graphene instead of the well-known electrical conductivity on the activity of graphene-semiconductor photocatalysts. With these motivations, we for the first time herein report the intentional design of melamine resin monomers functionalized 3D graphene (donated as MRGO) particularly with significantly deteriorated electrical conductivity to predominantly focus on studying the geometry effect of MRGO on the photocatalytic performance of graphene-semiconductor composites. By adopting a facile solvothermal method, the composites of CdS-RGO and CdS-MRGO with different addition rations of graphene (i.e., RGO and MRGO) have been obtained for the purpose of direct comparison study. The results have shown that though the remarkably lower electrical conductivity of MRGO than RGO, the CdS-MRGO composites still exhibit much higher visible


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light photoactivity than CdS-RGO composites. The comparative characterizations reveal that such photoactivity enhancement is determined to result from the restacking-inhibited 3D architectural morphology of MRGO instead of its poor electrical conductivity, by which the synergistic effects of boosted separation and transportation of photogenerated charge carriers and increased adsorption capacity can be achieved. This work highlights that the significant matter of photocatalytic performance of graphene-semiconductor composite system is not an only, simple issue on how to harness the electrical conductivity of graphene. Rather, it is necessary for designing graphene-semiconductor photocatalysts with high performance from a system-level consideration of such composite as an ensemble,1,2 which includes the integrative optimization of geometrical and electrical property of individual component (graphene or semiconductor) and the interface composition.

2. Experimental Section 2.1 Materials Graphite powder, hydrochloric acid (HCl), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), concentrated sulfuric acid (H2SO4, 98%), ethanol (C2H5OH), potassium permanganate (KMnO4), hydrogen peroxide (H2O2 30%), nitric acid (HNO3, 65%), cadmium acetate (Cd(CH3COO)2·2H2O), ammonium oxalate ((NH4)2C2O4, AO), dimethyl sulfoxide (C2H6OS2, DMSO) and benzyl alcohol (C7H8O) were all obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 4-methylbenzyl alcohol (C8H10O), 4-methoxybenzyl alcohol (C8H10O2), 4-chlorobenzyl alcohol (C7H7ClO), 4-fluorobenzyl alcohol (C7H7FO), 3methyl-2-buten-1-ol (C5H10O) and benzotrifluoride (BTF > 99%) were supplied by Alfa Aesar. Melamine and 37 wt % formaldehyde aqueous solution were purchased from Sigma Aldrich. All


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of the reagents were used as received without further purification. Deionized (DI) water was supplied by local sources. 2.2. Catalyst Preparation 2.2.1 Preparation of graphene oxide (GO): GO was prepared by using natural graphite powder as precursor via a modified Hummers method,23 details for the synthesis of GO has been presented in our previous works.1, 22, 33 2.2.2 Synthesis of melamine resin monomers functionalized 3D graphene oxide (MGO): The MGO was synthesized by a modified recipe.36 Firstly, to prepare melamine resin monomers (MRs) solution, 0.25 g of melamine and 4.425 mL of 37 wt % formaldehyde aqueous solution were added into 40 mL of DI water. A transparent solution was obtained after this solution was heat-treated at 343 K for 10 min, suggesting the good solubility of MRs in the aqueous solution. Then, the GO solution (10 mL, 10 mg/mL) was introduced into the MRs solution and stirred at 371 K for 3 h. During the stirring, the condensation reaction between GO sheets and MRs took place, and the MGO powder precipitated. The final product was collected, washed thoroughly with distilled water followed by a rinse in ethanol, and then dried in an oven at 333 K. 2.2.3 Fabrication of CdS-graphene (RGO and MRGO) composites: The given amount of as-prepared MGO and GO was ultrasonically dispersed in 40 mL of DMSO

in

a

cylindrical

vessel.

Then,

0.106

g

of

cadmium

acetate

dihydrate

(Cd(CH3COO)2·2H2O) was added into the above solution. After stirred vigorously for 60 min, the cylindrical vessel was sealed in a 50 mL Teflon-lined autoclave and treated at 453 K for 12 h. As the autoclave cooled to room temperature under ambient conditions, the products were


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separated by centrifugation and washed with acetone and absolute ethanol for three times, respectively. Followed by drying at 333 K, the target products were obtained. 2.3. Characterization Field-emission scanning electron microscopy (FESEM) was employed to determine the morphology of the samples on a Hitachi New Generation cold field emission SEM SU-8000 spectrophotometer. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were analyzed using a JEOL model JEM 2010 EX instrument at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was operated on a Thermo Scientific ESCA Lab 250 spectrometer which is made of a mono chromatic Al Kα as the X-ray source, a hemispherical analyzer, and sample stage with multiaxial adjustability to obtain the surface composition of the products. All the binding energies were calibrated by the C 1s peak at 284.6 eV. Micromeritics ASAP2010 equipment was used to obtain the nitrogen adsorption-desorption isotherms and the Brunauer-Emmett-Teller (BET) surface areas at 77 K. The powders were degassed at 413 K to remove all surface-adsorbed contaminants prior to measurements. The X-ray diffraction (XRD) patterns of the samples were measured on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation in the 2θ ranging from 5 to 80o at a scan rate of 0.02o s-1. The optical properties of the samples were characterized by UV-vis diffuse reflectance spectroscopy (DRS) using UV-vis spectrophotometer (Cary 500, Varian Co.) in which BaSO4 was employed as a reference. The photoluminescence spectra (PL) for powder samples were analyzed on an Edinburgh Analytical Instrument F900 spectrophotometer with an excitation wavelength of 420 nm. To ensure the comparability of the PL spectra, the experimental parameters, including the excitation wavelength, slit width, and the amount of the samples, were identical. The electrochemical analysis was conducted in a conventional three


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electrode cell, which uses a Pt plate as the counter electrode and an Ag/AgCl electrode as the reference electrode. The working electrode was prepared on fluorine doped tin oxide (FTO) glass that was cleaned by sonication in ethanol and dried at 355 K for 2 h. The boundary of FTO glass was protected using Scotch tape. The 5 mg sample was fully dispersed in 0.5 mL of N, NDimethylflormamide (DMF, supplied from Sinopharm Chemical Reagent Co., Ltd.) by sonication to get slurry. The slurry was spread onto the pre-treated FTO glass. After air drying, the working electrode was further dried at 393 K for 2 h to improve adhesion. Then, the Scotch tape was unstuck, and the uncoated part of the electrode was isolated with epoxy resin. The exposed area of the working electrode was 0.25 cm2. The electrochemical impedance spectroscopy (EIS) experiments were conducted on an electrochemical workstation (Autolab, PGSTAT204) in the electrolyte of 0.5 M KCl aqueous solution containing 0.01 M K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) by applying an AC voltage with 5 mV amplitude in a frequency range from 1 Hz to 85 kHz with the open circuit potential of 0.2 V under visible light irradiation (λ > 420 nm). The specific electrical resistivity of RGO and MRGO was determined using a four-point probe technique on a ST2722 (Suzhou Jingge Electronic Co., P. R. China) power resistivity tester under the pressure of 8 MPa. The radical species were analyzed by an electron spin resonance (ESR) spectrometer (Bruker EPR A300). In detail, the sample (3 mg) was dispersed in 0.5 mL of purified benzotrifluoride (BTF), into which 20 µL of 5,5-dimethyl-lpyrroline-N-oxide (DMPO)/benzyl alcohol solution (1:10, v/v) was added. The mixture was oscillated to obtain a well-blended suspension. The irradiation source (λ > 420 nm) was a 300 W Xe arc lamp system, the very light source for our photocatalytic selective oxidation of alcohols. The parameters for the ESR spectrometer were as follows: centre field = 3510 G, microwave frequency = 9.84 GHz, and power = 2.00 mW.


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2.4. Photocatalytic activity Photocatalytic selective oxidation of alcohols was performed according to the previous works.5 In detail, 0.1 mmol of alcohol and 8 mg of catalyst were added in 1.5 mL of benzotrifluoride (BTF), which was saturated with pure molecular oxygen (99.99%). Subsequently, the above mixture was transferred into a 10 mL Pyrex glass bottle, which was filled with molecular oxygen at a pressure of 0.1 MPa and stirred for 30 min for the establishment of adsorption-desorption equilibrium. The mixture was irradiated by a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfect light Co., Ltd.) with a UV-CUT filter to cut off light of wavelength less than 420 nm. The power density of the light source used in the measurement is ca. 220 mW cm-2. After that, the mixture was centrifuged at 12000 rpm for 10 min to remove the catalyst particles. The remaining solution was analyzed using an Agilent 7820 Gas Chromatograph (GC) system comprising a HP-FFAP analysis column (30m × 0.32 mm × 0.25 µm) and a flame-ionization detector (FID). Conversion of benzyl alcohol and selectivity of benzaldehyde were defined as follows: Conversion (%) = [(C 0 − C r ) / C 0 ] × 100

Selectivit y (%) = [C p /(C 0 − C r )] × 100

where C0 is the initial concentration of alcohol, Cr and Cp are the concentrations of the substrate alcohol and the corresponding product aldehyde, respectively, at a certain time after the photocatalytic reaction. The experimental errors of conversion and selectivity are less than 2%. The recycling test of photocatalytic activity on the used catalyst was done as follows. The used photocatalyst after the reaction was washed with deionized water for 3 times and fully dried at 333 K in an oven. Then, the fresh reaction solution of 1.5 mL BTF containing 0.1 mmol of benzyl alcohol was mixed with this used catalyst to subject to the second run photocatalytic


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activity test. By analogy, the recycled third and fourth photocatalytic activity testing was performed. The experimental apparatus is shown in Figure S1 (Supporting Information).

3. Results and Discussion 3.1 Preparation and Morphology of Restacking-Inhibited Melamine Resin Monomers Functionalized Three-Dimensional (3D) Graphene Oxide (MGO) Scheme 1 comparatively illustrates the synthetic routes of dried GO sheets and restackinginhibited 3D MGO. Both synthetic routes start from the GO aqueous solution (concentration: 10 mg/mL). During the formation process of dried GO, water molecules remain stuck in GO powder via hydrogen bonding interaction between water molecules and oxygen-containing functional groups of GO,36-39 which are often referred to as “intercalated” water molecules, as shown in Scheme 1. The presence of intercalated water molecules strengthens the interaction along the caxis perpendicular of GO sheets, thus leading to the severe restacking of dried GO sheets.37, 39 In contrast, at the initial stage, the sample of MGO is firstly treated with melamine resin monomers (MRs), which can eliminate the hydrogen bonding by a condensation reaction36,

40, 41

of the

hydroxyl or carboxylic acid groups on GO sheets with the hydroxyl end groups of MRs, as presented in Equation S1. Consequently, the restacking of MGO can be efficiently inhibited at the dried stage and the MGO sample possesses restacking-inhibited characteristic in solid state. The restacking-inhibited nature of MGO is reflected by scanning electron microscopy (SEM) analysis. Of particular note is that the obtained samples (dried GO and MGO) are directly measured in solid state without any redundant treatment, such as ultrasonic treatment in aqueous or other solvent media. The abundant thin MGO sheets with a fluffy and lamellar morphology scattering in random 3D orientations are observed in Figure 1A and B. Such transparent and


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flexible geometry suggests the unique restacking-inhibited 3D property of MGO.36,

42

By

contrast, the SEM images in Figure 1C and D reveal that the dried GO sheets appear as micrometer-scale particles with densely stacked structures in solid state, which is ascribed to the presence of intercalated water molecules in the dried GO powder as discussed above. The timedependent evolution experiments have been performed to unveil the formation process of restacking-inhibited MGO, as displayed in Figure S2 (Supporting Information), suggesting that the extensive aggregation of GO sheets can be successfully prevented by functionalization with MRs. In addition, the restacking-inhibited property of MGO can be tuned by varying the addition amounts of MRs, as shown in Figure S3 (Supporting Information).

3.2 Structure and Electronic Properties of Restacking-Inhibited 3D Graphene The powder X-ray diffraction (XRD) patterns of dried GO and MGO are shown in Figure 2A. It can be seen that the peak position of dried GO powder is located at ca. 10.3°,6, 8, 43 which is indicative of decent crystallinity of aligned GO layers along their stacks due to the oxygencontaining functional groups attached on both sides of GO sheet and intercalated water molecules.36 Notably, a dramatic peak shift to higher 2θ angles (24.6°) in the XRD pattern of MGO is discerned in Figure 2A, which corresponds to the moderately aligned graphitic arrays along the (002) direction36 and is attributed to the partial reduction of MGO during the reflux process at 371 K for 3 h. The partial reduction of MGO can be further evidenced by the X-ray photoelectron spectra (XPS). As revealed in Figure 2B, a loss of the oxygen-containing carbonaceous peaks of the C-OH and O=C-OH is observed based on the C 1s XPS spectrum of MGO sample as compared with that of dried GO sheets (Figure S4, Supporting Information), indicating the partial reduction of MGO. In addition, the new peak centered at 287.5 eV in


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Figure 2B is assigned to the triazine carbon (N-C=N) in the parent MRs (Figure S5, Supporting Information),44-46 which can be also confirmed by the Fourier transform infrared spectroscopy (FT-IR). As shown in Figure S6 (Supporting Information), the characteristic peak at 816 cm-1 is ascribed to the stretching vibration of N-C=N,47-49 which is in conformity with the XPS results. Nitrogen (N2) adsorption/desorption analysis has been performed to give quantitative sets of information on the surface area and pore structure of dried GO and MGO. As shown in Figure 2C, the isotherm of dried GO with significant hysteresis characteristic at relative pressure from 0.4 to 0.9 indicates the dominant presence of mesopores existing between the restacked GO sheets, which is verified by the pore size distribution of GO sample in Figure S7A (Supporting Information).50, 51 The specific surface area and pore volume of GO sample are determined to be 132.4 m2/g and 0.1 cm3/g, respectively. The rapid rise after P/P0 = 0.9 for the sample of MGO, as mirrored in Figure 2D, represents the presence of a large amount of macropores contributed by the interspaces between restacking-inhibited 3D MGO.50 The pore size distribution of MGO in Figure S7B (Supporting Information) further supports that a large portion of pores in MGO are macropores. The specific surface area and pore volume of MGO are calculated to be 150.8 m2/g and 0.47 cm3/g, respectively. It is obvious that the sample of MGO holds larger surface area and pore volume than dried GO owing to its restacking-inhibited 3D property, suggesting the potential applications in photocatalysis as they would offer abundant channels for the reactants into the interior void space of the sample. In addition, the dried GO and restacking-inhibited 3D MGO samples show dramatically different volume with same mass of 100 mg (Figure S8, Supporting Information), which further suggests the difference in the porosity between the samples and is another indication of the restacking-inhibited characteristic of MGO.


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Furthermore, the restacking-inhibited 3D characteristic of MGO can be well maintained even after the solvothermal reduction of MGO to reduced MGO (denoted as MRGO), as revealed in Figure S9 (Supporting Information). The four-point probe technique has been employed to investigate the electrical conductivity of graphene, including reduced GO sheets (RGO) and MRGO samples. The results are illustrated in Figure 3. It is clear that the resistivity of MRGO sample is obviously larger than that of RGO, indicating the rather poor electrical conductivity of MRGO sample. Specifically, the bulk resistivity of MRGO (ca. 1.0 × 105 Ω cm) is approximately five orders of magnitude higher than that of RGO sheets (ca. 1 Ω cm), which can be attributed to the introduction of insulated MRs. In the framework of significantly deteriorated electrical conductivity of graphene, such restacking-inhibited 3D MRGO would enable us to focus on solely studying the effect of geometrical parameter of graphene on the activity of graphene-semiconductor photocatalysts.

3.3 Structure and Optical Properties of CdS-RGO and CdS-MRGO Composites By adopting a simple solvothermal method, the composites of CdS-RGO and CdS-MRGO with different weight addition ratios of graphene have been fabricated,52, 53 during which the precursors of GO and MGO have been reduced to RGO and MRGO, respectively. The X-ray diffraction (XRD) results of CdS-RGO and CdS-MRGO composites in Figure S10 (Supporting Information) suggest that both of the composites possess analogous XRD patterns. The wellresolved diffraction peaks at 26.5°, 44.0° and 52.1° can be indexed to the (111), (220) and (311) crystal planes of cubic CdS (JCPDS Card No. 10-0454), respectively. Of note, the diffraction peaks of the samples are broad because the crystallite sizes of CdS nanoparticles in the composites are small, which can be attributed to the slow release of S2- ions from dimethyl


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sulfoxide (DMSO).4,

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Using the Scherrer formula, the average crystallite sizes of CdS in

different samples have been estimated, as presented in Table S1. The average crystallite sizes of CdS particles increase from 2.0 to 4.6 nm with the increase of graphene contents. It may be due to the fact that the presence of graphene could facilitate the crystallization of CdS nanoparticles to a certain extent.4 Furthermore, the crystallite size of CdS particles in CdS-RGO and CdSMRGO composites is similar when the weight ratio of graphene is identical. No typical diffraction peaks belonging to the separate graphene (RGO or MRGO) are observed, which could be attributed to the fact that the relatively weak diffraction intensity of graphene might be shielded by the main peak of semiconductor CdS.52, 53 The UV-vis diffuse reflectance spectra (DRS) are used to investigate the optical properties of the CdS-RGO and CdS-MRGO composites. As shown in Figure S11 (Supporting Information), both the CdS-RGO and CdS-MRGO composites show an absorption edge at about 520 nm, corresponding to the intrinsic band gap absorption of CdS (2.4 eV). The introduction of graphene (i.e., RGO and MRGO) induces the increased light absorption intensity in visible light region for CdS-RGO and CdS-MRGO composites, which is in accordance with the color change of the samples from yellow to dark green (Figure S12, Supporting Information). In addition, the absorption intensity of CdS-RGO and CdS-MRGO composites in visible light region is enhanced gradually with each incremental addition of RGO or MRGO owing to the intrinsic background absorption of black colored graphene. The similarity in XRD and DRS results indicates that there is no significant difference on the crystal phase, the crystallite size and the light absorption property for the CdS-RGO and CdS-MRGO composites prepared with different kinds of graphene (i.e., RGO and MRGO).


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3.4 Photocatalytic Performance of CdS-RGO and CdS-MRGO Composites In the following, we have performed the photocatalytic selective oxidation of alcohols under visible light illumination (λ > 420 nm) over the two series of samples, i.e., CdS-RGO and CdSMRGO composites (Equation S2, Supporting Information). The significant difference in photoactivity between the CdS-RGO and CdS-MRGO composites with the same addition amount of graphene (i.e., RGO and MRGO) is observed. As shown in Figure 4, in the case of CdS-RGO samples, the photocatalytic activity of the samples is first increased with the increasing RGO loading prior to an optimal value, and then decreased. The CdS-5%RGO composite shows the best photoactivity in all of the CdS-RGO composites. The conversion for oxidation of benzyl alcohol over CdS-5%RGO photocatalyst is ca. 33% under visible light irradiation for 2 h, which exhibits a moderate photoactivity enhancement as compared to that of blank CdS (ca. 26% conversion). Notably, the photocatalytic performance toward oxidation of benzyl alcohol over the samples of CdS-MRGO is dramatically enhanced as compared to that of CdS-RGO composites. For example, even with a small amount of MRGO, e.g., 1%, the conversion of benzyl alcohol over CdS-1%MRGO composite is increased to 57%. The photocatalytic performance of CdS-MRGO composites shows a tendency to increase with increasing MRGO contents, achieving a maximum conversion of 91% at the MRGO content of 5%. This value is over 3.5 times and 2.8 times as high as that of blank CdS and CdS-5%RGO, respectively. Similar to the case of CdS-RGO composites, the photocatalytic performance of CdS-MRGO will be decreased with the excessive addition of MRGO, suggesting that a synergistic interaction between CdS and MRGO is required in order to optimally enhance the photoactivity of semiconductor CdS. For instance, the photoactivity of CdS-30%MRGO composite is lowered


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greatly as compared to that of optimized CdS-5%MRGO composite. It has been well recognized that the excessive addition amount of graphene is detrimental to improve the photoactivity of semiconductor, which is a universal problem for graphene-semiconductor composites.1, 2, 5, 52 On one hand, the relatively high weight ratio of MRGO in CdS-MRGO composites would scatter the photons by surplus carbon and shield the light from being absorbed by the semiconductor CdS. On the other hand, the introduction of high addition amount of black MRGO leads to shielding of the active sites on the photocatalyst surface and also reduces the efficiency of light passing through the depth of the reaction medium, which is called a light “shielding effect”.5, 9, 52 In most cases, independent of what kind of probe reaction is taken over graphene-semiconductor composites, the weight addition ratio of graphene is generally lower than 5% in order to reach an optimal synergistic interaction between graphene and semiconductor for the improved photoactivity.1, 2, 9 To confirm if the remarkable enhanced photoactivity is general, the optimum samples, i.e., CdS-5%RGO and CdS-5%MRGO, have been chosen to evaluate the photocatalytic activity for selective oxidation of a range of alcohols (e.g., 4-methylbenzyl alcohol, 4-methoxybenzyl alcohol, 3-methyl-2-buten-1-ol, 4-chlorobenzyl alcohol, and 4-fluorobenzyl alcohol). The results are listed in Table S2. An overall view of the photoactivity steers us to the finding that the CdS5%MRGO composite exhibits dramatic photoactivity enhancement as compared to blank CdS and CdS-5%RGO in all selected reaction systems under identical reaction conditions. In addition, over these optimal CdS-5%RGO or CdS-5%MRGO photocatalysts, the primary product for oxidation of benzyl alcohol is benzaldehyde with a high selectivity (> 92%).

3.5 Origins of the Enhanced Photocatalytic Activity


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On the basis of the above activity data, it is clear that the photoactivity of CdS-MRGO composites is much higher than that of CdS-RGO composite samples. However, in this work, the common view of excellent electrical conductivity of graphene cannot be regarded as the main factor to explain the improved photoactivity of CdS-MRGO composites because, as mentioned above, the electrical conductivity of MRGO has been significantly deteriorated (Figure 3). Aiming to in-depth clarify the origins accounting for the improved photocatalytic activity of CdS-MRGO composites, we have then comparatively characterized the samples with optimal photoactivity (i.e., CdS-5%RGO and CdS-5%MRGO) using a combination of characterization tools. The morphology and microscopic structure information of CdS-5%RGO and CdS-5%MRGO composites are characterized by the joint SEM and transmission electron microscopy (TEM). The typical SEM and TEM images of CdS-5%RGO composite in Figure 5A and C disclose that CdS nanoparticles with small size uniformly carpet on the surface of RGO sheets, featuring an apparently large sheet-like structure with intimate interfacial contact.52,

53

However, the

morphology of CdS-5%MRGO composite is quite different from that of CdS-5%RGO, as shown in Figure 5B and D. It is clear that the semiconductor CdS is trended to aggregate into large particles and partial MRGO sheets are isolated without the decoration of CdS. In addition, the restacking-inhibited 3D morphology of MRGO can be directly observed (Figure 5B), indicating that the 3D morphology of MRGO is well preserved during the fabrication process of CdSMRGO composites. The selected area electron diffraction (SAED) patterns of the CdS-5%RGO and CdS-5%MRGO composite (in the inset of Figure 5C and D) reveal that both samples possess a polycrystalline structure, which is in agreement with the results of XRD analysis (Figure S10, Supporting Information). Furthermore, the high-resolution TEM (HR-TEM)


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images of the CdS-5%RGO (Figure 5E) and CdS-5%MRGO (Figure 5F) display the high crystallinity with clear lattice structure, which is notarized to be CdS phase by the interplanar spacing value (0.336 nm for CdS (111)). Both of SEM and TEM results suggest that the CdS5%MRGO composite with restacking-inhibited 3D MRGO shows distinctly different morphology and structure as compared with the sample of CdS-5%RGO. Since the “structure dictates function” is the basic concept in chemistry and has been widely recognized by the wellestablished morphology-dependent photocatalytic activity, therefore, we infer that the restacking-inhibited 3D morphology of MRGO may ameliorate the photoactivity of CdS-MRGO composites by providing multidimensional electrical transport pathways to promote the separation efficiency of photogenerated electron-hole pairs in the composite system. The above inference has been validated by the following analysis of photoluminescence (PL) and electrochemical impedance spectroscopy (EIS), which are often used to reflect the fate and transfer of photogenerated charge carriers from a photocatalyst. PL spectra can give useful information about the recombination of photoexcited electron-hole pairs and have also been used to study the fate of charge carriers successfully.5, 9, 22, 32, 54-57 As depicted in Figure 6A, after the introduction of graphene (RGO and MRGO) into semiconductor CdS, the PL intensity of composites (i.e., CdS-5%RGO and CdS-5%MRGO) is quenched as compared to bare CdS, which is attributed to the fact that the graphene supplies as additional nonradiative delay channel for the transfer of electrons from semiconductor to graphene. Therefore, the energy-wasteful recombination process of the photogenerated electron-hole pairs can be greatly retarded.58 The sample of CdS-5%MRGO shows more restrained PL intensity than CdS-5%RGO composite, suggesting that the recombination of electron-hole pairs is mostly suppressed among the three samples. To further reveal the lifetime of the charge carriers in the


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composites, the room-temperature PL decays of CdS-5%RGO and CdS-5%MRGO photocatalysts have been performed, as presented in Figure S13 (Supporting Information). It is observed clearly that the sample of CdS-5%MRGO exhibits a 22% enhancement of lifespan as compared to CdS-5%RGO composite (Table S3), which could be ascribed to restackinginhibited 3D morphology of MRGO that provides multidimensional 3D electron transport channels to separate the photoexcited electron-hole pairs, thus prolonging the lifetime of charge carriers. Based on the above results, the high photocatalytic performance of the CdS-5%MRGO composite toward selective oxidation of alcohols could be attributed to the utilization of restacking-inhibited 3D MRGO, thereby prolonging the lifetime of electron-hole pairs photogenerated from semiconductor CdS. Besides, the electrochemical impedance spectroscopy (EIS), as a method to monitor charge transfer process on the electrode and at the contact interface between electrode and electrolyte,5, 9, 35, 59, 60

has been performed under visible light illumination, and the results are shown in Figure

6B. It is clearly seen that the Nyquist diagrams of blank CdS, CdS-5%RGO and CdS-5%MRGO electrodes exhibit a typical semicircle at high frequency, which corresponds to the charge transfer limiting process and is ascribed to the double-layer capacitance in parallel with the charge transfer resistance at the contact interface between electrode and electrolyte solution.5, 52, 61-63

The CdS-5%MRGO electrode exhibits the most depressed semicircle at high frequency

among those three electrodes, suggesting that the smaller resistance and more efficient transfer of charge carriers between electrode and electrolyte solution are obtained over CdS-5%MRGO than those over blank CdS and CdS-5%RGO electrodes. This can be ascribed to the fact that, for the composite of CdS-5%MRGO, the restacking-inhibited MRGO with few-layer sheets scattered in random 3D orientations can promote the access of ferricyanide and ferrocyanide ions to the


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electrode surface, which increases the contact region and promotes the electron communication between electrode and electrolyte solution, thereby leading to the efficient transfer of charge carriers in CdS-5%MRGO composite.61, 62, 64 The enhanced transfer of photogenerated charge carriers is in accordance with the high photoactivity of CdS-5%MRGO composite as observed above. The BET surface area and pore structure of CdS-5%RGO and CdS-5%MRGO composites have been measured by nitrogen adsorption at 77 K. As displayed in Figure 6C, the surface area of CdS-5%MRGO (65.1 m2/g) is slightly higher than that of CdS-5%RGO composite (57.2 m2/g). The pore size distribution data of CdS-5%RGO sample ranged from 1 to 30 nm (Figure S14A, Supporting Information) indicate the richness of micro- and meso-pore in CdS-5%RGO composite. In contrast, the pore size distribution data of CdS-5%MRGO composite in Figure S14B (Supporting Information) reveal that the size of most of the pores is belonged to meso- and macro-pore, which is ascribed to the presence of restacking-inhibited 3D MRGO. As a result, the greater specific surface area and pore diameters of the CdS-5%MRGO composite can provide more surface active sites, leading to the improvement of photocatalytic activity. Furthermore, the adsorption experiment in Figure 6D shows that the CdS-5%MRGO composite exhibits stronger adsorption ability toward reactant than CdS-5%RGO sample owing to its high surface area. Since the heterogeneous photocatalysis is a surface-based process,65 the high adsorption capacity over the CdS-5%MRGO enables the charge carriers to effectively react with benzyl alcohol adsorbed on the surface of photocatalyst, which in turn contributes to the photoactivity enhancement of CdS-5%MRGO composite. Additionally, the CdS-5%MRGO composite shows moderately poorer adsorption capability toward the product, i.e., benzaldehyde, than CdS5%RGO composite, as revealed in Figure S15 (Supporting Information), which is beneficial for


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the desorption of benzaldehyde from the photocatalyst surface, thus facilitating the photoactivity of CdS-5%MRGO composite.

3.6 Photostability and Photocatalytic Mechanism The photostability of CdS-5%MRGO has been investigated via recycling photoactivity experiments, as displayed in Figure S16 (Supporting Information). Throughout the testing cycles, the CdS-5%MRGO composite exhibits persistent photocatalytic activity for aerobic selective oxidation of benzyl alcohol under visible light irradiation (λ > 420 nm), indicating the good recyclability of the CdS-5%MRGO composite. To learn the role of photogenerated active species in the photocatalytic process and explore the underlying photocatalytic mechanism of photocatalytic selective oxidation of benzyl alcohol over CdS, CdS-5%RGO and CdS-5%MRGO composites under visible light irradiation, a series of blank or controlled experiments with the addition of various radical scavengers has been performed (Figure 7). Initial blank experiments performed in the absence of catalysts and/or visible light show that no benzaldehyde is detected, confirming that the reaction is really driven by a photocatalytic process (Figure S17, Supporting Information). Controlled experiments taking ammonium oxalate (AO), K2S2O8 and benzoquinone (BQ) as the radical scavengers for photogenerated holes, electrons and superoxide radicals (O2•-), respectively, obviously suggest that, for photocatalytic oxidation of benzyl alcohol over CdS, CdS-5%RGO and CdS-5%MRGO, the primary active radical species are photogenerated holes, electrons, and activated oxygen (e.g., O2•-, which presence is further confirmed by the electron spin resonance (ESR) spectra analysis in Figure S18, Supporting Information). In particular, the highest intensity of O2•- as detected by ESR over CdS-5%MRGO is in agreement with its higher separation efficiency of charge carriers than CdS and CdS-


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5%RGO as verified by the PL and EIS analysis. Notably, although electrons cannot directly participate in the oxidation reaction, they can activate molecular oxygen (e.g., forming O2•-), which then takes part in the oxidation of alcohols. In contrast, a control experiment using tertbutyl alcohol (TBA) as the radical scavenger for hydroxyl radicals (•OH) shows no change for the conversion of benzyl alcohol. The observation is in agreement with the previous reports that no hydroxyl radicals are formed in the BTF solvent.7, 66 Based on the above analysis, a tentative reaction mechanism for selective oxidation of alcohols over CdS-MRGO composites is speculated as follow. Under visible light irradiation (λ > 420 nm), electrons in the valence band (VB) of CdS are photoexcited to its conduction band (CB), leaving the holes in the VB. The energetic photogenerated electrons in the CB of CdS are apt to transfer to MRGO owing to their matchable energy band position. The MRGO with restackinginhibited 3D morphology can provide multidimensional electron transport pathways to improve the separation efficiency of photoexcited electrons, as illustrated in Figure 8, thereby depressing the recombination of photogenerated electron-hole pairs and prolonging the lifespan of charge carriers. The photogenerated electrons can be trapped by molecular oxygen by which activated oxygen species can be obtained, for example, the formation of superoxide radical (O2•-) on the surface of the photocatalyst. The adsorbed reactants, i.e., benzyl alcohol, can then be oxidized by the holes and activated oxygen species, thereby leading to the product of benzaldehyde.52

4. Conclusion In summary, we have used the restacking-inhibited 3D MRGO with significantly deteriorated electrical conductivity to predominantly focus on studying the geometry effect of MRGO on the photocatalytic performance of graphene-semiconductor composites. The results show that the


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CdS-MRGO composites exhibit much higher visible light photoactivity than CdS-RGO composites although the electrical conductivity of MRGO is remarkably much lower than that of RGO. The characterizations evidence that such photoactivity enhancement is predominantly attributed to the restacking-inhibited 3D architectural morphology of MRGO, by which the synergistic effects of boosted separation and transportation of photogenerated charge carriers and increased adsorption capacity are obtained. This work highlights that the significant matter of photocatalytic performance of graphene-semiconductor composite system is not an only, simple issue on how to harness the electrical conductivity of graphene, but the rational design of graphene-semiconductor

photocatalysts

with

high

performance

from

a

system-level

consideration of such composite as an ensemble,1,2 which includes the integrative optimization of geometrical and electrical property of individual component (graphene or semiconductor) as well as the interface composition.

Supporting Information. List of acronyms and abbreviations, additional characterization and photoactivity results. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgement. The support from the Key Project of National Natural Science Foundation of China (U1463204), the National Natural Science Foundation of China (20903023 and 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (NO. 2014A05), the 1st Program of Fujian Province for Top Creative Young


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Talents, and the Program for Returned High-Level Overseas Chinese Scholars of Fujian province is gratefully acknowledged. References 1.

Zhang, N.; Yang, M.-Q.; Liu, S.; Sun, Y.; Xu, Y.-J., Waltzing with the Versatile Platform

of Graphene to Synthesize Composite Photocatalysts. Chem. Rev. 2015, 115, 10307-10377. 2.

Yang, M.-Q.; Zhang, N.; Pagliaro, M.; Xu, Y.-J., Artificial Photosynthesis over

Graphene-Semiconductor Composites. Are We Getting Better? Chem. Soc. Rev. 2014, 43, 82408254. 3.

Xiao, F.-X.; Miao, J.; Liu, B., Layer-by-Layer Self-Assembly of CdS Quantum

Dots/Graphene Nanosheets Hybrid Films for Photoelectrochemical and Photocatalytic Applications. J. Am. Chem. Soc. 2014, 136, 1559-1569. 4.

Li, Q.; Guo, B.; Yu, J.; Ran, J.; Zhang, B.; Yan, H.; Gong, J. R., Highly Efficient Visible-

Light-Driven Photocatalytic Hydrogen Production of CdS-Cluster-Decorated Graphene Nanosheets. J. Am. Chem. Soc. 2011, 133, 10878-10884. 5.

Zhang, N.; Yang, M.-Q.; Tang, Z.-R.; Xu, Y.-J., Toward Improving the Graphene-

Semiconductor Composite Photoactivity via the Addition of Metal Ions as Generic Interfacial Mediator. ACS Nano 2014, 8, 623-633. 6.

Zhang, Y.; Tang, Z.-R.; Fu, X.; Xu, Y.-J., TiO2−Graphene Nanocomposites for Gas-

Phase Photocatalytic Degradation of Volatile Aromatic Pollutant: Is TiO2−Graphene Truly Different from Other TiO2−Carbon Composite Materials? ACS Nano 2010, 4, 7303-7314.


24

Page 25 of 42

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


7.

Zhang, Y.; Tang, Z.-R.; Fu, X.; Xu, Y.-J., Engineering the Unique 2D Mat of Graphene

to Achieve Graphene-TiO2 Nanocomposite for Photocatalytic Selective Transformation: What Advantage does Graphene Have over Its Forebear Carbon Nanotube? ACS Nano 2011, 5, 74267435. 8.

Zhang, Y.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J., Graphene Transforms Wide Band Gap ZnS

to a Visible Light Photocatalyst. The New Role of Graphene as a Macromolecular Photosensitizer. ACS Nano 2012, 6, 9777-9789. 9.

Weng, B.; Liu, S.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J., A Simple yet Efficient Visible-

Light-Driven CdS Nanowires-Carbon Nanotube 1D–1D Nanocomposite Photocatalyst. J. Catal. 2014, 309, 146-155. 10. Weng, B.; Wu, J.; Zhang, N.; Xu, Y.-J., Observing the Role of Graphene in Boosting the Two-Electron Reduction of Oxygen in Graphene–WO3 Nanorod Photocatalysts. Langmuir 2014, 30, 5574-5584. 11. Yang, M.-Q.; Han, C.; Zhang, N.; Xu, Y.-J., Precusor Matters in Boosting Photoredox Activity of Graphene/Semiconductor Composites. Nanoscale 2015, 7, 18062-18070. 12. Akhavan, O., Graphene Nanomesh by ZnO Nanorod Photocatalysts. ACS Nano 2010, 4, 4174-4180. 13. Huang, X.; Qi, X.; Boey, F.; Zhang, H., Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666-686. 14. Kamat, P. V., Graphene-Based Nanoassemblies for Energy Conversion. J. Phys. Chem. Lett. 2011, 2, 242-251.


25


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

Page 26 of 42

15. Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R., Reduced Graphene Oxide as a Solid-State Electron Mediator in Z-Scheme Photocatalytic Water Splitting under Visible Light. J. Am. Chem. Soc. 2011, 133, 11054-11057. 16. Tu, W. G.; Zhou, Y.; Liu, Q.; Yan, S. C.; Bao, S. S.; Wang, X. Y.; Xiao, M.; Zou, Z. G., An In Situ Simultaneous Reduction-Hydrolysis Technique for Fabrication of TiO2-Graphene 2D Sandwich-Like Hybrid Nanosheets: Graphene-Promoted Selectivity of Photocatalytic-Driven Hydrogenation and Coupling of CO2 into Methane and Ethane. Adv. Funct. Mater. 2013, 23, 1743-1749. 17. Yuan, L.; Xu, Y.-J., Photocatalytic Conversion of CO2 into Value-Added and Renewable Fuels. Appl. Surf. Sci. 2015, 342, 154-167. 18. Feng, W.; Weng, S.; Zheng, Z.; Fang, Z.; Liu, P., Rational Design of a Charge Shunt: Modification upon Crystal Facet Engineering of Semiconductor Photocatalysts. Chem. Commun. 2015, 51, 11186-11189. 19. Offeman, R.; Hummers, W., Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339-1339. 20. Fan, W.; Zhang, Q.; Wang, Y., Semiconductor-Based Nanocomposites for Photocatalytic H2 Production and CO2 Conversion. Phys. Chem. Chem. Phys. 2013, 15, 2632-2649. 21. Zhang, N.; Xu, Y.-J.,, The Endeavour to Advance Graphene-Semiconductor CompositeBased Photocatalysis. CrystEngCommun 2015, DOI: 10.1039/c5ce01712b.


26

Page 27 of 42

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


22. Han, C.; Chen, Z.; Zhang, N.; Colmenares, J. C.; Xu, Y.-J., Hierarchically CdS Decorated 1D ZnO Nanorods-2D Graphene Hybrids: Low Temperature Synthesis and Enhanced Photocatalytic Performance. Adv. Funct. Mater. 2015, 25, 221-229. 23. Weng, B.; Yang, M.-Q.; Zhang, N.; Xu, Y.-J., Toward the Enhanced Photoactivity and Photostability of ZnO Nanospheres via Intimate Surface Coating with Reduced Graphene Oxide. J. Mater. Chem. A 2014, 2, 9380-9389. 24. Zhang, N.; Yang, M.-Q.; Tang, Z.-R.; Xu, Y.-J., CdS–Graphene Nanocomposites as Visible Light Photocatalyst for Redox Reactions in Water: A Green Route for Selective Transformation and Environmental Remediation. J. Catal. 2013, 303, 60-69. 25. Kamat, P. V., Graphene-Based Nanoarchitectures. Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Carbon Support. J. Phys. Chem. Lett. 2010, 1, 520-527. 26. Banhart, F.; Kotakoski, J.; Krasheninnikov, A. V., Structural Defects in Graphene. ACS Nano 2011, 5, 26-41. 27. Gómez-Navarro, C.; Meyer, J. C.; Sundaram, R. S.; Chuvilin, A.; Kurasch, S.; Burghard, M.; Kern, K.; Kaiser, U., Atomic Structure of Reduced Graphene Oxide. Nano Lett. 2010, 10, 1144-1148. 28. Tang, Z.-R.; Zhang, Y.; Zhang, N.; Xu, Y.-J., New Insight into the Enhanced Visible Light Photocatalytic Activity over Boron-Doped Reduced Graphene Oxide. Nanoscale 2015, 7, 7030-7034.


27


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

Page 28 of 42

29. Xue, Y.; Yu, D.; Dai, L.; Wang, R.; Li, D.; Roy, A.; Lu, F.; Chen, H.; Liu, Y.; Qu, J., Three-Dimensional B,N-Doped Graphene foam as a Metal-Free Catalyst for Oxygen Reduction Reaction. Phys. Chem. Chem. Phys. 2013, 15, 12220-12226. 30. Yang, Z.; Yao, Z.; Li, G.; Fang, G.; Nie, H.; Liu, Z.; Zhou, X.; Chen, X. a.; Huang, S., Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction. ACS Nano 2012, 6, 205-211. 31. Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y., Synthesis of Phosphorus‐Doped Graphene and its Multifunctional Applications for Oxygen Reduction Reaction and Lithium Ion Batteries. Adv. Mater. 2013, 25, 4932-4937. 32. Liang, Y. T.; Vijayan, B. K.; Gray, K. A.; Hersam, M. C., Minimizing Graphene Defects Enhances Titania Nanocomposite-Based Photocatalytic Reduction of CO2 for Improved Solar Fuel Production. Nano Lett. 2011, 11, 2865-2870. 33. Zhang, Y.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J., Improving the Photocatalytic Performance of Graphene-TiO2 Nanocomposites via a Combined Strategy of Decreasing Defects of Graphene and Increasing Interfacial Contact. Phys. Chem. Chem. Phys. 2012, 14, 9167-9175. 34. Han, W.; Ren, L.; Gong, L.; Qi, X.; Liu, Y.; Yang, L.; Wei, X.; Zhong, J., SelfAssembled Three-Dimensional Graphene-Based Aerogel with Embedded Multifarious Functional Nanoparticles and Its Excellent Photoelectrochemical Activities. ACS Sustainable Chem. Eng. 2014, 2, 741-748.


28

Page 29 of 42

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


35. Qiu, B.; Xing, M.; Zhang, J., Mesoporous TiO2 Nanocrystals Grown in Situ on Graphene Aerogels for High Photocatalysis and Lithium-Ion Batteries. J. Am. Chem. Soc. 2014, 136, 58525855. 36. Lee, J. H.; Park, N.; Kim, B. G.; Jung, D. S.; Im, K.; Hur, J.; Choi, J. W., RestackingInhibited 3D Reduced Graphene Oxide for High Performance Supercapacitor Electrodes. ACS Nano 2013, 7, 9366-9374. 37. Acik, M.; Mattevi, C.; Gong, C.; Lee, G.; Cho, K.; Chhowalla, M.; Chabal, Y. J., The Role of Intercalated Water in Multilayered Graphene Oxide. ACS Nano 2010, 4, 5861-5868. 38. Zhu, Y. W.; Murali, S.; Cai, W. W.; Li, X. S.; Suk, J. W.; Potts, J. R.; Ruoff, R. S., Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906-3924. 39. Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M., Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4, 4806-4814. 40. Hulicova, D.; Yamashita, J.; Soneda, Y.; Hatori, H.; Kodama, M., Supercapacitors Prepared from Melamine-Based Carbon. Chem. Mater. 2005, 17, 1241-1247. 41. Raymundo-Pinero, E.; Cazorla-Amoros, D.; Linares-Solano, A.; Find, J.; Wild, U.; Schlogl, R., Structural Characterization of N-Containing Activated Carbon Fibers Prepared from a Low Softening Point Petroleum Pitch and a Melamine Resin. Carbon 2002, 40, 597-608.


29


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

Page 30 of 42

42. Luo, J. Y.; Jang, H. D.; Sun, T.; Xiao, L.; He, Z.; Katsoulidis, A. P.; Kanatzidis, M. G.; Gibson, J. M.; Huang, J. X., Compression and Aggregation-Resistant Particles of Crumpled Soft Sheets. ACS Nano 2011, 5, 8943-8949. 43. Moon, I. K.; Lee, J.; Ruoff, R. S.; Lee, H., Reduced graphene oxide by chemical graphitization. Nat. Commun. 2010, 1, 73. 44. Pevida, C.; Drage, T. C.; Snape, C. E., Silica-Templated Melamine–Formaldehyde Resin Derived Adsorbents for CO2 Capture. Carbon 2008, 46, 1464-1474. 45. Derylo-Marczewska, A.; Goworek, J.; Pikus, S.; Kobylas, E.; Zgrajka, W., Characterization of Melamine-Formaldehyde Resins by XPS, SAXS, and Sorption Techniques. Langmuir 2002, 18, 7538-7543. 46. Coullerez, G.; Léonard, D.; Lundmark, S.; Mathieu, H. J., XPS and ToF-SIMS Study of Freeze-Dried and Thermally Cured Melamine–Formaldehyde Resins of Different Molar Ratios. Surf. Interface Anal. 2000, 29, 431-443. 47. Bledowski, M.; Wang, L.; Ramakrishnan, A.; Khavryuchenko, O. V.; Khavryuchenko, V. D.; Ricci, P. C.; Strunk, J.; Cremer, T.; Kolbeck, C.; Beranek, R., Visible-Light Photocurrent Response of TiO2-Polyheptazine Hybrids: Evidence for Interfacial Charge-Transfer Absorption. Phys. Chem. Chem. Phys. 2011, 13, 21511-21519. 48. Gao, C. Y.; Moya, S.; Lichtenfeld, H.; Casoli, A.; Fiedler, H.; Donath, E.; Mohwald, H., The Decomposition Process of Melamine Formaldehyde Cores: The Key Step in the Fabrication of Ultrathin Polyelectrolyte Multilayer Capsules. Macromol. Mater. Eng. 2001, 286, 355-361.


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49. Gindl, W.; Hansmann, C.; Gierlinger, N.; Schwanninger, M.; Hinterstoisser, B.; Jeronimidis, G., Using a Water-Soluble Melamine-Formaldehyde Resin to Improve the Hardness of Norway Spruce Wood. J. Appl. Polym. Sci. 2004, 93, 1900-1907. 50. Su, Y.-S.; Manthiram, A., Lithium–Sulphur Batteries with a Microporous Carbon Paper as a Bifunctional Interlayer. Nat. Commun. 2012, 3, 1166. 51. Kruk, M.; Jaroniec, M., Gas Adsorption Characterization of Ordered Organic−Inorganic Nanocomposite Materials. Chem. Mater. 2001, 13, 3169-3183. 52. Zhang, N.; Zhang, Y.; Pan, X.; Fu, X.; Liu, S.; Xu, Y.-J., Assembly of CdS Nanoparticles on the Two-Dimensional Graphene Scaffold as Visible-Light-Driven Photocatalyst for Selective Organic Transformation under Ambient Conditions. J. Phys. Chem. C 2011, 115, 23501-23511. 53. Cao, A.; Liu, Z.; Chu, S.; Wu, M.; Ye, Z.; Cai, Z.; Chang, Y.; Wang, S.; Gong, Q.; Liu, Y., A Facile One-step Method to Produce Graphene–CdS Quantum Dot Nanocomposites as Promising Optoelectronic Materials. Adv. Mater. 2010, 22, 103-106. 54. Tu, W. G.; Zhou, Y.; Liu, Q.; Tian, Z. P.; Gao, J.; Chen, X. Y.; Zhang, H. T.; Liu, J. G.; Zou, Z. G., Robust Hollow Spheres Consisting of Alternating Titania Nanosheets and Graphene Nanosheets with High Photocatalytic Activity for CO2 Conversion into Renewable Fuels. Adv. Funct. Mater. 2012, 22, 1215-1221. 55. Liang, Y. T.; Vijayan, B. K.; Lyandres, O.; Gray, K. A.; Hersam, M. C., Effect of Dimensionality on the Photocatalytic Behavior of Carbon–Titania Nanosheet Composites: Charge Transfer at Nanomaterial Interfaces. J. Phys. Chem. Lett. 2012, 3, 1760-1765.


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56. Xu, Y.-J.; Zhuang, Y.; Fu, X., New Insight for Enhanced Photocatalytic Activity of TiO2 by Doping Carbon Nanotubes: A Case Study on Degradation of Benzene and Methyl Orange. J. Phys. Chem. C 2010, 114, 2669-2676. 57. Liu, S.; Yang, M.-Q.; Xu, Y.-J., Surface Charge Promotes the Synthesis of Large, Flat Structured Graphene-(CdS Nanowire)-TiO2 Nanocomposites as Versatile Visible Light Photocatalysts. J. Mater. Chem. A 2014, 2, 430-440. 58. Tu, W. G.; Zhou, Y.; Zou, Z. G., Versatile Graphene-Promoting Photocatalytic Performance of Semiconductors: Basic Principles, Synthesis, Solar Energy Conversion, and Environmental Applications. Adv. Funct. Mater. 2013, 23, 4996-5008. 59. Zhang, K.; Han, P.; Gu, L.; Zhang, L.; Liu, Z.; Kong, Q.; Zhang, C.; Dong, S.; Zhang, Z.; Yao, J.; Xu, H.; Cui, G.; Chen, L., Synthesis of Nitrogen-Doped MnO/Graphene Nanosheets Hybrid Material for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2012, 4, 658-664. 60. Zhang, J.; Yu, J. G.; Jaroniec, M.; Gong, J. R., Noble Metal-Free Reduced Graphene Oxide-ZnxCd1-xS

Nanocomposite

with

Enhanced

Solar

Photocatalytic

H2-Production

Performance. Nano Lett. 2012, 12, 4584-4589. 61. Lu, T.; Zhang, Y.; Li, H.; Pan, L.; Li, Y.; Sun, Z., Electrochemical Behaviors of Graphene–ZnO and Graphene–SnO2 Composite Films for Supercapacitors. Electrochim. Acta 2010, 55, 4170-4173. 62. Guo, H.-L.; Wang, X.-F.; Qian, Q.-Y.; Wang, F.-B.; Xia, X.-H., A Green Approach to the Synthesis of Graphene Nanosheets. ACS Nano 2009, 3, 2653-2659.


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Page 33 of 42

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


63. Xiao, F.-X.; Hung, S.-F.; Miao, J.; Wang, H.-Y.; Yang, H.; Liu, B., Metal-ClusterDecorated TiO2 Nanotube Arrays: A Composite Heterostructure toward Versatile Photocatalytic and Photoelectrochemical Applications. Small 2015, 11, 554-567. 64. Wang, W. H.; Wang, X. D., Investigation of Ir-Modified Carbon Felt as the Positive Electrode of an All-Vanadium Redox Flow Battery. Electrochim. Acta 2007, 52, 6755-6762. 65. Weng, B.; Zhang, X.; Zhang, N.; Tang, Z. R.; Xu, Y. J., Two-Dimensional MoS2 Nanosheet-Coated Bi2S3 Discoids: Synthesis, Formation Mechanism, and Photocatalytic Application. Langmuir 2015, 31, 4314-4322. 66. Lang, X.; Ji, H.; Chen, C.; Ma, W.; Zhao, J., Selective Formation of Imines by Aerobic Photocatalytic Oxidation of Amines on TiO2. Angew. Chem., Int. Ed. 2011, 50, 3934-3937.


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Scheme 1. The schematic illustrations of the synthetic process of dried GO and MGO started from the GO solution. The black color of GO solution in the scheme is due to the high concentration (10 mg/mL). The MGO solution is obtained after refluxing and settles for 15 h. The mass of dried GO and MGO samples in Pyrex glass bottle is 100 mg


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Figure 1. SEM images of MGO (A, B) and dried GO (C, D) in solid state


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Figure 2. X-ray diffraction (XRD) spectra of GO and MGO (A); C 1s X-ray photoelectron spectra (XPS) of MGO (B); and the nitrogen adsorption-desorption isotherms of GO (C) and MGO (D)


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1.0X105

Resistivity (Ωcm)

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0.9X105

5 4 3 2 1

RGO

MRGO

Figure 3. Resistivity of RGO and MRGO measured by the four-probe electrical resistivity method

Figure 4. Photocatalytic performance of CdS, CdS-RGO and CdS-MRGO composites with different weight addition ratios of RGO or MRGO for photocatalytic selective oxidation of benzyl alcohol under visible light (λ > 420 nm) for 2 h


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Figure 5. Typical SEM, TEM and HR-TEM images of CdS-5%RGO (A, C, E) and CdS5%MRGO (B, D, F); insets in (C) and (D) are the corresponding selected area electron diffraction (SAED) patterns (Inset mages in E and F show the enlarged high-magnification images)


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Figure 6. The photoluminescence (PL) emission spectra of CdS, CdS-5%RGO and CdS5%MRGO composites (A); electrochemical impedance spectroscopy (EIS) Nyquist diagrams of CdS, CdS-5%RGO and CdS-5%MRGO composites in 0.5 M KCl aqueous solution containing 0.01 M K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) (B); N2 adsorption-desorption isotherms of CdS-5%RGO and CdS-5%MRGO (C) and bar plot showing the remaining benzyl alcohol (BA) in reaction solution after being kept in dark for 2 h to achieve the adsorption-desorption equilibrium over CdS-5%RGO and CdS-5%MRGO (D)


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100

Conversion (%)

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CdS CdS-5% RGO CdS-5% MRGO

80 60 40 20 0

Original

AO

K2S2O8

BQ

TBA

Figure 7. Control experiments using different radical scavengers for photocatalytic selective oxidation of benzyl alcohol over CdS, CdS-5%RGO and CdS-5%MRGO composites in the BTF solvent under visible light irradiation (λ > 420 nm) for 2 h; ammonium oxalate (AO) as scavenger for photogenerated holes, K2S2O8 as scavenger for photogenerated electrons, benzoquinone (BQ) as scavenger for superoxide radicals and tert-butyl alcohol (TBA) as scavenger for hydroxyl radicals


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Figure 8. Scheme illustrating the photogenerated electron transport pathways in CdS-RGO (a) and CdS-MRGO composites (b) under the visible-light illumination


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TOC Graphic


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What if the Electrical Conductivity of Graphene Is Significantly

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