Electrostatic self-assembly of CdS nanowires-nitrogen doped graphene nanocomposites for enhanced visible light photocatalysis

Journal of Energy Chemistry 24(2015)145–156

Electrostatic self-assembly of CdS nanowires-nitrogen doped graphene nanocomposites for enhanced visible light photocatalysis Bin Hana ,

Siqi Liua,b , Zi-Rong Tanga∗ ,

Yi-Jun Xua,b

a. College of Chemistry, New Campus, Fuzhou University, Fuzhou 350108, Fujian, China; b. State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, Fujian, China [ Manuscript received September 3, 2014; revised October 10, 2014 ]

Abstract CdS nanowires-nitrogen doped graphene (CdS NWs-NGR) nanocomposites have been fabricated by an electrostatic self-assembly strategy followed by a hydrothermal reduction. The CdS NWs-NGR exhibits higher photoactivity for selective reduction of aromatic nitro organics in water under visible light irradiation than blank CdS nanowires (CdS NWs) and CdS nanowires-reduced graphene oxide (CdS NWs-RGO) nanocomposites. The enhanced photoactivity of CdS NWs-NGR can be attributed to the improved electronic conductivity due to the introduction of nitrogen atoms, which thus enhances the separation and transfer of charge carriers photogenerated from CdS NWs. Our work could provide a facile method to synthesize NGR based one-dimensional (1D) semiconductor composites for selective organic transformations, and broaden the potential applications for NGR as a cocatalyst. Key words nitrogen doping; graphene; CdS nanowire; photocatalytic organic synthesis; visible light

1. Introduction Graphene has attracted significant attention and interest since its discovery [1], due to its outstanding structural and electronic properties. In particular, the excellent mobility of charge carriers endows graphene with prominent capability to accept/transport electrons photogenerated from band gap photoexcitation of semiconductors upon light irradiation [2−11]. Thus, it is of great interest to adopt graphene as cocatalyst to fabricate diverse graphene-semiconductors composites with the aim to enhance the photocatalytic performance of semiconductors for solar energy conversion [12−15]. To date, graphene is mainly prepared by the reduction of exfoliated graphene oxide (GO) owing to its low cost and massive scalability, which is also called reduced graphene oxide (RGO) [16−20]. Notably, the inherent nature to undergo aggregation and presence of diverse defects for RGO unavoidably deteriorate the electrical properties and carrier mobility of graphene [21,22]. As a result, considerable efforts have been focused on improving the electrical properties of graphene by various strategies [23−30].

It has been demonstrated that doping the carbon network of graphene with heteroatoms (e.g., N, B and P) is able to introduce electrocatalytic active sites on graphene and increase the electrical conductivity of graphene [31−33]. Among various possible dopants, nitrogen possesses the advantages in several aspects. The size of nitrogen atom is similar to that of carbon atom and it contains five valence electrons available to form strong valence bonds with carbon atom [34]. Hence, the doping of nitrogen atoms into graphene network can be easily obtained, which can enhance the electronic conductivity due to the partial recovery of sp2 graphene network and the decrease of defects within the plane associated with nitrogen incorporation [35−37]. In fact, nitrogen doped graphene (NGR) has been utilized in diverse areas, such as molecular sensing [38], lithium battery [39−41], fuel cells [42,43] and photocatalysis [44−50]. However, the utilization of NGRsemiconductor composites for photocatalytic selective organic transformation under ambient conditions has been unavailable so far. Herein, taking one-dimensional (1D) CdS nanowires (NWs) as an example of semiconductor, we report a simple, electrostatic assembly method to fabricate CdS NWs-NGR

∗

Corresponding author. Tel/Fax: +86-591-22866126; E-mail: [email protected] This work was supported by the National Natural Science Foundation of China (NSFC) (20903022, 20903023, 21173045), the Award Program for Minjiang Scholar Professorship, the Science and Technology Development of Foundation of Fuzhou University (2009-XQ-10), the Open Fund of Photocatalysis of Fuzhou University (0380038004), and the Program for Returned High-Level Overseas Chinese Scholars of Fujian Province. Copyright©2015, Science Press and Dalian Institute of Chemical Physics. All rights reserved. doi: 10.1016/S2095-4956(15)60295-9

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nanocomposites with improved photocatalytic activity toward selective reduction of nitro organics in water under visible light irradiation as compared with blank CdS NWs and CdS NWs-RGO without nitrogen doping. Through a moderate refluxing process with ammonia solution that acts as nitrogen source, nitrogen can be readily introduced into the matrix of GO, giving partly reduced NGR [51]. Then, an electrostatic self-assembly approach is employed to fabricate CdS NWsNGR nanocomposites, which is afforded by a substantial electrostatic attractive interaction between positively charged CdS NWs and negatively charged partly reduced NGR in an aqueous solution, followed by a hydrothermal treatment to transform partly reduced NGR to NGR with sufficient removal of oxygenated functional groups. The use of NGR with improved electrical conductivity contributes to more efficient charge carriers separation and transfer than that of RGO, thereby resulting in the photoactivity enhancement of CdS NWs-NGR as compared with CdS NWs-RGO. It is hoped that this work could provide a facile approach to synthesize NGR based 1D semiconductor composites with enhanced photoactivity for selective organic transformations under ambient conditions.

static self-assembly of CdS NWs on the framework of partly reduced NGR. In a typical synthesis, CdS NWs (0.2 g) were firstly dispersed in 100.0 mL deionized water by ultrasonication. Then, a certain amount of negatively charged partly reduced NGR suspension was added into positively charged CdS NWs dispersion under vigorous stirring. After being stirred for 30 min, the mixture was centrifuged and washed with deionized water. For sufficient reduction of partly reduced NGR to NGR, a hydrothermal process was conducted as follows. CdS NWs-partly reduced NGR nanocomposites (0.2 g) were firstly dispersed in deionized water (80.0 mL), then autoclaved in a Teflonlined stainless steel vessel at 393 K for 12 h. The as-obtained dark green precipitates were collected, washed thoroughly with deionized water, and then dried in an oven at 333 K. The added amounts of NGR in CdS NWs-NGR nanocomposites were controlled to be 0.5, 1.0, 2.0, 5.0 and 10.0 wt%, respectively. For comparison, CdS NWs-RGO nanocomposites were also fabricated by the same method except using GO instead of partly reduced NGR.

2. Experimental 2.1. Chemicals Cadmium chloride (CdCl2 ·2.5H2O), sodium diethyldithiocarbamate trihydrate (C5 H10 NNaS2 ·3H2 O), ethylenediamine (C2 H8 N2 ), graphite powder, sulfuric acid (H2 SO4 ), hydrochloric acid (HCl), potassium persulfate (K2 S2 O8 ), hydrogen peroxide (30.0 wt%), potassium permanganate (KMnO4 ), ammonia solution (25.0 wt%), ethanol (C2 H6 O), ammonium formate (HCOONH4 ) and N,Ndimethylflormamide (DMF) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the materials were analytical grade and used as received without further purification. Deionized water used in the synthesis was from local sources. 2.2. Catalyst preparation The fabrication process towards NGR and CdS NWsNGR heterostructures via a moderate refluxing process combined with a nanoscale self-assembly strategy is depicted in Scheme 1. (I) Fabrication of graphene oxide. GO was synthesized from natural graphite powder by a modified Hummers method [52−54]. (II) Synthesis of CdS NWs. Uniform CdS NWs were prepared by a previously reported method [55,56]. (III) Preparation of partly reduced NGR. The partly reduced NGR was synthesized through a moderate refluxing process, with ammonia solution acting as the nitrogen resource [51]. In a typical way, ammonia solution (25.0 wt%, 0.3 mL) was mixed with GO aqueous solution (0.2 wt%, 30.0 mL) by magnetic stirring at 353 K for 8 h to form partly reduced NGR. (IV) Synthesis of CdS NWs-NGR nanocomposites by electro-

Scheme 1. Schematic illustration for preparing 1D CdS NWs-NGR nanocomposites by a simple electrostatic self-assembly method, followed by a hydrothermal reduction process

2.3. Characterization Zeta potentials (ξ) of CdS NWs, GO and partly reduced NGR were determined by dynamic light-scattering analysis (Malvern Zetasizer Nano-ZS90) at 298 K. In brief, 5.0 mg sample was dispersed in 10.0 mL of deionized water to obtain a concentration of ca. 500 mg/L in an aqueous solution. The crystal phase properties of the samples were analyzed with a Bruker D8 Advance X-ray diffractometer (XRD) using Nifiltered Cu Kα radiation at 40 kV and 40 mA in 2θ range from 10o to 75o with a scan rate of 0.02 o /s. The optical properties of the samples were characterized by a Cary 500 UV-visible ultraviolet/visible diffuse reflectance spectrophotometer (DRS), during which BaSO4 was employed as the internal reflectance standard. The Fourier transformed infrared spectroscopy (FT-IR) was performed on a Nicolet Nexus 670 FT-IR spectrophotometer at a resolution of 4 cm−1 . Field-emission scanning electron microscopy (FE-SEM) was used to determine the morphology of the samples on an FEI Nova NANOSEM 230 spectrophotometer. X-ray photoelec-

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tron spectroscopy (XPS) measurements were performed using a Thermo Scientific ESCA Lab250 spectrometer, which consists of monochromatic Al Kα as the X-ray source, a hemispherical analyzer, and sample stage with multiaxial adjustability to obtain the surface composition of the samples. The Brunauer-Emmett-Teller (BET) specific surface areas of the samples were analyzed by nitrogen (N2 ) adsorptiondesorption in a Micromeritics ASAP 2020 apparatus. The photoluminescence (PL) spectra for solid samples were investigated on an Edinburgh FL/FS900 spectrophotometer with an excitation wavelength of 380 nm. Photoelectrochemical measurements were performed in a homemade three-electrode quartz cell with a PAR VMP3Multib Potentiotat apparatus. A Pt plate was used as counter electrode, and Ag/AgCl electrodes were used as reference electrodes, while the working electrode was prepared on fluoride tin oxide (FTO) conductor glass. The sample powder (5.0 mg) was ultrasonicated in 1.0 mL of DMF to disperse it evenly to get slurry. The slurry was spread onto FTO glass, whose side part was previously protected using Scotch tape. The working electrode was dried overnight under ambient conditions. A copper wire was connected to the side part of the working electrode using conductive tape. Uncoated parts of the electrode were isolated with epoxy resin. The electrolyte was 0.2 mol/L aqueous Na2 SO4 solution (pH = 6.8) without additive. The electrochemical impedance spectroscopy (EIS) measurements were performed in the presence of 10.0 mmol/L K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] by applying an AC voltage with 5 mV amplitude in a frequency range from 0.001 Hz to 100 kHz under open circuit potential conditions. The cyclic voltammograms (CV) were measured in the same solution of three electrode cells as that of the EIS measurement. The visible light irradiation source was a 300 W Xe arc lamp system equipped with a UV-CUT filter (λ>420 nm). 2.4. Photocatalytic activity In a typical photocatalytic activity test, a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.) with a UV-CUT filter to cut off light with a wavelength λ<420 nm was used as the light irradiation source. A 6.0 mg portion of photocatalyst and 30.0 mg ammonium formate as a radical scavenger for photogenerated holes were added into 30.0 mL aromatic nitro organics solution (10 mg/L) in a quartz vial. The whole experimental process was conducted under N2 bubbling at a flow rate of 80 mL/min. Before visible light illumination, the suspension was stirred in the dark for 1 h to ensure the establishment of an adsorption-desorption equilibrium. During the reaction process, 3.0 mL sample solution was collected at a certain time interval and centrifuged to remove the catalyst completely at 12000 rmp. The solution was analyzed on a Varian ultraviolet-visible light (UV-vis) spectrophotometer (Cary-50, Varian Co.). 3. Results and discussion X-ray photoelectron spectroscopy (XPS) was carried out

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to confirm the chemical compositions of the as-prepared partly reduced NGR, RGO and their precursor GO. According to the high-resolution spectra of C 1s as shown in Figure 1(a) and 1(b), RGO showed a significant loss of oxygen-containing functional groups as compared with GO, which indicates the sufficient reduction of GO to RGO via the hydrothermal reduction treatment [5,12]. As shown in Figure 1(c), the obviously decreased but still existence of oxygenated functional groups, indicate that GO is partly reduced by nitrogen doping process [51,57]. Notably, a new peak at 285.85 eV appeared, which can be ascribed to C–N bonds [51,58,59], implying that partly reduced NGR can be successfully prepared through a moderate refluxing process with ammonia solution acting as the nitrogen resource. In addition, N 1s peak was also observed in XPS spectrum of partly reduced NGR, which can be fitted into three peaks. As displayed in Figure 1(d), the chemical state of N was similar to those of pyridinic N (398.9 eV), pyrrolic N (399.9 eV) and quaternary N (401.3 eV), the three common bonding configurations in NGR [36,47,51], indicating the successful doping of nitrogen into the skeleton of graphene [36,49,51,60]. The successful reduction of GO to RGO and partly reduced NGR to NGR after the hydrothermal treatment can be further confirmed by Fourier transformed infrared spectra (FT-IR) analysis. As shown in Figure 2, for both CdS NWS-NGR and CdS NWs-RGO, various oxygenated functional groups were obviously decreased after the hydrothermal treatment as compared with GO, indicating the efficient reduction through the hydrothermal treatment [61]. Raman spectroscopy is employed to characterize the structural properties of graphene-based materials, particularly to determine the defects and disordered structures [62]. Figure 3(a) shows the Raman spectra of GO sheets, CdS NWsRGO and CdS NWs-NGR. All of the samples displayed two prominent peaks at 1350 and 1600 cm−1 approximately, corresponding to the typical D and G bands of graphene [3−8,63], respectively. The G band is related to the vibration of sp2 -bonded carbon atoms, while the D band is related to structural defects on the graphitic plane. The value of ID /IG is a measure of the relative concentration of local defects or disorders compared with the sp2 -hybridized graphene domains [3−8,63]. The ID /IG ratio was 0.92 for GO and increased to 1.05 for CdS NWs-RGO after the hydrothermal reaction. The ID /IG ratio of CdS NWs-NGR was 1.02, which is between that of GO and CdS NWs-RGO. The increased ID /IG ratio of CdS NWs-RGO and CdS NWs-NGR compared with GO probably results from the generation of smaller nanocrystalline graphene domains and the loss of carbon atoms by the decomposition of oxygen-containing groups during the reduction process [64,65]. The lower ID /IG ratio of CdS NWs-NGR than CdS NWs-RGO indicates that partial sp2 domains are restored to a certain degree, which is attributed to the incorporation of N heteroatoms [37,66]. In Figure 3(b), the two peaks located at about 295 and 595 cm−1 were ascribed to the resonantly excited longitudinal optical (LO) phonon and the first (2LO) overtones of CdS NWs, respectively [67,68], which certify the existence of Cd-S in the nanocomposites.

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Figure 1. High-resolution XPS spectra of C 1s for GO (a), RGO (b), partly reduced NGR (c) and N 1s for partly reduced NGR (d)

Figure 2. Fourier transformed infrared spectra (FT-IR) of original GO (1), CdS NWs-RGO (2) and CdS NWs-NGR nanocomposites (3)

The good interfacial contact between CdS NWs and carbon materials was accomplished by the electrostatic self-

assembly process of CdS NWs with GO or partly reduced NGR, which can be demonstrated by the zeta potential (ξ) measurements. The CdS NWs exhibited a zeta potential value of +11.5 mV. In contrast, both GO and partly reduced NGR displayed significantly negatively charged surface, and the zeta potentials were −20.4 mV and −12.7 mV, respectively. Therefore, in an aqueous solution, the positively charged CdS NWs and negatively charged GO or partly reduced NGR could spontaneously establish a solid basis of electrostatic attraction for fabricating CdS NWs-GO and CdS NWs-partly reduced NGR hybrids. The morphology and structure of the samples were characterized by the field-emission scanning electron microscopy (FE-SEM). As displayed in Figure 4(a), the blank CdS NWs exhibited highly uniform 1D morphology with an average diameters of ca. 50−100 nm. Figure 4(b) shows that the NGR nanosheets and 1D semiconductor CdS NWs were integrated with a good interfacial contact, giving rise to CdS NWs-NGR. This good interfacial contact originated from the electrostatic attractive interaction between positively charged CdS NWs and negatively charged NGR. Since the intimate

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interfacial contact between graphene and semiconductors is a key factor for the efficient utilization of electron conductivity of graphene [12], it could be inferred that good interfacial

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contact between CdS NWs and NGR sheets is beneficial for the charge carriers transfer.

Figure 3. Raman spectra of GO, CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR showing the presence of typical G and D bands (a) and raman spectra of CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR showing the existence of Cd–S bond (b)

Figure 4. Typical FE-SEM images of CdS NWs (a) and CdS NWs-NGR (b), and the insets of (a) and (b) are the corresponding schematic models, respectively

Figure 5 shows the powder X-ray diffraction (XRD) analyses on the samples of CdS NWs, CdS NWs-RGO and CdS NWs-NGR. The XRD pattern of CdS NWs was ascribed to the hexagonal phase of CdS (JCPDS No. 41-1049). The peaks at 2θ values of 24.8o , 26.5o, 28.2o, 36.6o, 43.7o, 47.9o, 50.9o , 51.8o, 52.8o , 66.8o and 70.9o for CdS NWs were indexed to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (203), and (211) crystal planes of greenokite structure CdS (JCPDS No. 41-1049), respectively. The XRD pattern of CdS NWs-RGO was similar to CdS-NGR nanocomposites, as shown in Figure 5(b). There were no diffraction peaks of NGR and RGO as detected in CdS NWs-NGR and CdS NWsRGO. This may result from the low amount and relatively low diffraction intensity of NGR and RGO [48,50,69]. Ultraviolet-visible light diffuse reflectance spectra (UVvis DRS) were adopted to study the optical properties of the samples. As shown in Figure 6, all the samples of CdS NWs,

CdS NWs-RGO and CdS NWs-NGR exhibited pronounced adsorption bands in the visible light region spanning from 500 to 700 nm. In comparison with CdS NWs, both CdS NWsRGO and CdS NWs-NGR nanocomposites showed enhanced absorption intensity in the range of 500−700 nm with the increase of NGR or RGO contents, which is ascribed to the broad background absorption of NGR or RGO in the visible light region. Such an analogous phenomenon has also been widely observed in previous semiconductor-carbon nanocomposites [61,70]. The photocatalytic performance of CdS NWs-NGR was evaluated by selective reduction of nitro organics to corresponding amino organics in water [61,68]. Figure 7 shows the photocatalytic activities of blank CdS NWs, CdS NWsRGO and CdS NWs-NGR toward selective reduction of 4nitroaniline (4-NA) under visible light irradiation. The conversion of 4-NA to p-phenylenediamine (PPD) was analyzed

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by the ultraviolet-visible light adsorption spectra. It is clear to see from Figure 7(a) that introducing NGR into CdS NWs improved the photocatalytic activity obviously. When a small amount of NGR (e.g., 0.5 wt%, 1 wt% and 2 wt%) was added, the photocatalytic activity was improved, and the optimal photocatalytic activity was obtained with 2 wt% NGR. Further increasing the addition ratio of NGR would deteriorate the activity. This is because that the relatively high weight addition of carbon materials may block the light absorption of semiconductor CdS NWs. Such a “light-shielding” effect would result in the decrease of photoactivity [71,72]. For compar-

ison purpose, the photocatalytic activity of CdS NWs-RGO was also investigated under identical reaction conditions. As shown in Figure 7(b), the photocatalytic activity of CdS NWsRGO showed a similar trend to that of CdS NWs-NGR, and CdS NWs-2 wt% RGO exhibited the optimal photocatalytic activity. However, the photocatalytic activity of CdS NWsNGR was much higher than that of CdS NWs-RGO counterparts, suggesting that the integration of CdS NWs with NGR is more advantageous than non-doped RGO for improving the photocatalytic activity of semiconductor 1D CdS NWs.

Figure 5. XRD patterns of a series of CdS NWs-NGR (a) and CdS NWs-RGO (b) nanocomposites

Figure 6. UV-vis diffuse reflectance spectra (UV-vis DRS) of a series of CdS NWs-NGR (a) and CdS NWs-RGO (b) hybrids

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To determine whether such a photoactivity enhancement of CdS NWs based nanocomposites toward reduction of nitro organics is general, we have further investigated the photocatalytic activities of blank CdS NWs, CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR toward a variety of substituted nitroaromatics. As displayed in Figure 8, due to the hybridization of CdS NWs with NGR or RGO, both CdS NWs-2 wt%

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NGR and CdS NWs-2 wt% RGO possessed higher photoactivities than blank CdS NWs. In particular, CdS NWs-2 wt% NGR showed higher photoactivity than CdS NWs-2 wt% RGO. In the next, to disclose the origin accounting for the enhanced photoactivity of CdS NWs-NGR, we have then comparatively characterized the optimal samples of CdS NWs2 wt% NGR, CdS NWs-2 wt% RGO and blank CdS NWs.

Figure 7. Photocatalytic selective reduction of 4-NA to PPD over blank CdS NWs, CdS NWs-NGR (a) and CdS NWs-RGO (b) nanocomposites under visible light irradiation (λ>420 nm) with the addition of ammonium formate as a quencher for photogenerated holes and N2 purge in water at room temperature

The photoelectrochemical analysis is able to reflect the fate and transfer of photogenerated charge carriers, which are the key factors determining the overall photoactivity associated with semiconductor-based materials [12,56]. Figure 9 shows that CdS NWs-2 wt% NGR electrode displayed obviously higher transient photocurrent than those of blank CdS NWs and CdS NWs-2 wt% RGO under visible light irradiation, suggesting more efficient separation of photoexcited electron-hole pairs. In addition, photoluminescence (PL) spectrum is another effective method to qualitatively reflect the separation of photoexcited electron-hole pairs [61,70]. As shown in Figure 10, under an excitation of 380 nm, the PL intensity obtained over CdS NWs-2 wt% NGR was much weaker than those of blank CdS NWs and CdS NWs-2 wt% RGO. This suggests that the presence of NGR in the nanocomposites is able to retard the recombination of electron-hole pairs from semiconductor CdS NWs. The weaker intensity of CdS NWs-2 wt% NGR than CdS NWs-2 wt% RGO can be ascribed to the higher efficiency of NGR in inhibiting the charge carriers recombination than RGO. Electrochemical impedance spectra (EIS) Nyquist plot, a powerful instrument to characterize the charge-carrier migration [2,73], was adopted to further determine the prepon-

derance of CdS NWs-2 wt% NGR over CdS NWs and CdS NWs-2 wt% RGO in improving the charge carriers transfer. It is known that the high frequency arc corresponds to the charge transfer limiting process, which can be attributed to the charge transfer resistance at the interfacial contact between the electrode and the electrolyte solution [73]. As shown in Figure 11(a), the Nyquist plot of CdS NWs-2 wt%N GR electrode exhibited the most depressed semicircle at high frequency, manifesting that more efficient transfer of charge carriers was obtained over CdS NWs-2 wt% NGR than CdS NWs-2 wt% RGO and CdS NWs. The increased fate and transfer of photogenerated charge carriers were in accordance with the higher photoactivity of CdS NWs-2 wt% NGR as observed above. Figure 11(b) shows the cyclic voltammgrams (CV) of blank CdS NWs, CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR, which displayed the obvious anodic and cathodic peaks. The peak at positive potentials on the anodic sweep represents the oxidation of ferrocyanide to ferricyanide with the loss of one electron [74]. CdS NWs-2 wt% NGR showed the largest anodic current density as compared with blank CdS NWs and CdS NWs-2 wt% RGO, indicating the realization of enhanced electron transfer rate due to the incorporation of NGR.

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Figure 8. Photocatalytic performance of CdS NWs, CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR in the reduction of nitro compounds with various substituted groups in water

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the absence of ammonium formate or N2 , indicating that the inert atmosphere and the addition of ammonium formate as a hole scavenger are both indispensable for photocatalytic selective reduction of 4-NA over CdS NWs-2 wt% NGR. When a radical scavenger potassium persulfate for photogenerated electrons was added into the reaction system (Figure 13),

Figure 9. Transient photocurrent-time (I-t) curves of blank CdS NWs, CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR electrodes

Figure 10. Photoluminescence (PL) spectra of blank CdS NWs, CdS NWs2 wt% RGO and CdS NWs-2 wt% NGR with an excitation wavelength of 380 nm

To further understand the role of NGR on enhancing the photocatalytic activity of CdS NWs-2 wt% NGR as compared with blank CdS NWs and CdS NWs-2 wt% RGO, the surface area of the samples have been analyzed. As shown in Figure 12, we could see that the introduction of NGR or RGO could increase the surface area of the composites. Nevertheless, there was no significant difference between the surface areas of CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR. This result strongly suggests that the distinct improvement of photocatalytic activity of CdS NWs-2 wt% NGR as compared with CdS NWs-2 wt% RGO should not result from the difference of surface area. To explore the influence of reaction conditions on the photocatalytic selective reduction of nitro organics to corresponding amino organics over the samples, control experiments have been performed. As shown in Figure 13, without CdS NWs-2 wt% NGR photocatalyst or illumination, no conversion of 4-NA was obtained, which ensures that the reaction is really driven by a photocatalytic process. The conversion of 4-NA over CdS NWs-2 wt% NGR was strongly inhibited in

Figure 11. Electrochemical impedance spectroscopy (EIS) Nyquist plots (a) and cyclic voltammograms (b) of blank CdS NWs, CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR

Figure 12. Nitrogen adsorption-desorption isotherms of blank CdS NWs, CdS NWs-2 wt% RGO and CdS NWs-2 wt% NGR

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quenching positive holes can guarantee the fact that nitro organics would not undergo the oxidation reaction. The introduced NGR to CdS NWs matrix serves as an electron collector and transporter, thus facilitating the separation of photongenerated carriers and leading to an enhanced photocatalytic activity. Meanwhile, the presence of NGR increases the accumulating concentration of nitro organics over the surface of CdS NWs-NGR nanocomposites. As a result, the adsorbed nitro organics can be effectively reduced to amino organics by accepting the photogenerated electrons.

Figure 13. Control experiments for photocatalytic reduction of 4-nitroaniline over CdS NWs-2 wt% NGR with 3 min of irradiation time under visible light (λ>420 nm). (a) Reaction with the addition of ammonium formate in a N2 atmosphere, (b) Reaction without a photocatalyst, (c) Reaction without ammonium formate (HCOONH4 ), (d) Reaction without the purge of N2 , (e) Reaction without irradiation, (f) Reaction with potassium persulfate (K2 S2 O8 ) as a scavenger for electrons

no obvious conversion of 4-NA could be observed, demonstrating that photogenerated electrons play a decisive role in driving the photocatalytic reduction of nitroaromatics. In addition, these results also suggest that photocatalytic selective reduction of nitroaromatics over CdS NWs-2 wt% NGR requires an appropriate control of reaction conditions to ensure that the photoreduction process driven by photogenerated electrons is able to proceed efficiently. It has been established that CdS generally suffers from photocorrosion due to the oxidation of CdS by its own photogenerated holes, especially in aqueous solution [68,72]. Therefore, successive recycling experiments were conducted to investigate the photostability of the samples. Figure 14 displays the data of cycling reduction experiments of 4-NA over CdS NWs-2 wt% NGR photocatalyst under irradiation of visible light in aqueous solution. As seen, no noticeable activity change was observed during four successive recycles, suggesting that CdS NWs-2 wt% NGR photocatalyst is stable during the present reaction conditions. The good photostability of CdS NWs-2 wt% NGR can be ascribed to the fact that photogenerated holes are efficiently quenched by the addition of ammonium formate, which thus prevents the photocorrosion of CdS NWs during the photocatalytic reaction [61]. Clearly then, the proper control of reaction conditions guarantees the good photostability of CdS NWs-2 wt% NGR toward the photocatalytic reduction of nitro organics in an aqueous phase. Based on the above discussion, a tentative reaction mechanism is proposed as illustrated in Scheme 2. Under visible light irradiation (λ>420 nm), the electrons (e− ) are excited from the valence bands (VB) to their conduction bands (CB) of 1D CdS NWs, thereby forming electron-hole pairs. Simultaneously, the photogenerated holes, which generally contribute to the oxidation reaction, are effectively trapped by the quencher of ammonium formate. The reaction performed under N2 purge along with ammonium formate for

Figure 14. Recycling photocatalytic reduction of 4-NA over CdS NWs2 wt% NGR under visible light irradiation (λ>420 nm) with the addition of ammonium formate as a quencher for photogenerated holes and N2 purge in water at room temperature

Scheme 2. Schematic diagram illustrating the photocatalytic reduction of nitro organics to amino organics over CdS NWs-NGR under visible light irradiation (λ>420 nm) with the addition of ammonium formate as a quencher for photogenerated holes and N2 purge in water

4. Conclusions To sum up, through a moderate refluxing process, partly reduced nitrogen doped graphene is successfully synthesized with ammonia solution acting as the nitrogen resource. Then, a simple and efficient electrostatic self-assembly approach is

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employed to fabricate CdS NWs-NGR nanocomposites, followed by a hydrothermal reduction treatment. NGR with higher electrical conductivity results in more efficient charge carriers separation and transfer in CdS NWs-NGR nanocomposites. Thereby, CdS NWs-NGR hybrids demonstrate higher photocatalytic activity for selective organic transformation. It is hoped that this work could provide a facile method to synthesize NGR based 1D semiconductor composites for selective organic transformations, and broaden the potential applications for NGR as a cocatalyst. Acknowledgements The support by the National Natural Science Foundation of China (NSFC) (20903022, 20903023, 21173045), the Award Program for Minjiang Scholar Professorship, the Science and Technology Development of Foundation of Fuzhou University (2009XQ-10), the Open Fund of Photocatalysis of Fuzhou University (0380038004), and the Program for Returned High-Level Overseas Chinese Scholars of Fujian province is gratefully acknowledged.

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