CdS-graphene Nanocomposite for Efficient Visible-light-driven Photocatalytic and Photoelectrochemical Applications

Accepted Manuscript CdS-graphene Nanocomposite for Efficient Visible-light-driven Photocatalytic and Photoelectrochemical Applications Mohammad Ehtisham Khan, Mohammad Mansoob Khan, Moo Hwan Cho PII: DOI: Reference:

S0021-9797(16)30537-9 http://dx.doi.org/10.1016/j.jcis.2016.07.070 YJCIS 21456

To appear in:

Journal of Colloid and Interface Science

Received Date:

30 June 2016

Please cite this article as: M.E. Khan, M.M. Khan, M.H. Cho, CdS-graphene Nanocomposite for Efficient Visiblelight-driven Photocatalytic and Photoelectrochemical Applications, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.07.070

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CdS-graphene Nanocomposite for Efficient Visible-light-driven Photocatalytic and Photoelectrochemical Applications † Mohammad Ehtisham Khan1, Mohammad Mansoob Khan2*, and Moo Hwan Cho 1* 1

School of Chemical Engineering, Yeungnam University, Gyeongsan-si, Gyeongbuk 38541, South Korea. Phone: +82-53-810-2517, Fax: +82-53- 810-4631. 2 Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong, BE1410, Brunei Darussalam. *Email: [email protected] and [email protected] *Corresponding author. Fax: +82 53 810 4631. E-mail address: [email protected] (M.H. Cho)

ABSTRACT This paper reports cadmium sulphide nanoparticles-(CdS NPs)-graphene nanocomposite (CdS-Graphene), prepared by a simple method, in which CdS NPs were anchored/decorated successfully onto graphene sheets. The as-synthesized nanocomposite was characterized using standard characterization techniques. A combination of CdS NPs with the optimal amount of two-dimensional graphene sheets had a profound influence on the properties of the resulting hybrid nanocomposite, such as enhanced optical, photocatalytic, and photoelectronic properties. The photocatalytic degradation ability of the CdS-Graphene nanocomposite was evaluated by degrading different types of dyes in the dark and under visible light irradiation. Furthermore, the photoelectrode performance of the nanocomposite was evaluated by different electrochemical techniques. The results showed that the CdSGraphene nanocomposite can serve as an efficient visible-light-driven photocatalyst as well as photoelectrochemical performance for optoelectronic applications. The significantly enhanced photocatalytic and photoelectrochemical performance of the CdS-Graphene nanocomposite was attributed to the synergistic effects of the enhanced light absorption behavior and high electron conductivity of the CdS NPs and graphene sheets, which facilitates charge separation and lengthens the lifetime of photogenerated electron–hole pairs

by reducing the recombination rate. The as-synthesized narrow band gap CdS-graphene nanocomposite can be used for wide range of visible light-induced photocatalytic and photoelectrochemical based applications.

Keywords: CdS NPs, Graphene, CdS-Graphene nanocomposite, Photocatalytic degradation, photoelectrochemical

1. Introduction In recent years, emergent alarms about energy and environmental problems have encouraged extensive research on solar energy utilization [1,2]. Dyes are used widely in a range of fields, but their discharge into water can cause environmental pollution. In addition, most dyes are toxic, carcinogenic and harmful, resulting in adverse impacts on human and animal health [3]. Dyes find considerable applications in several industries, including textile, plastic, rubber, paper, concrete, and medicine with the textile industry as the main user. Unfortunately, approximately 10% of dyes used in industry are discharged directly into the environment as a harmful pollutant, which is environmentally unsafe and aesthetically unacceptable [3,4]. Heterogeneous photocatalysis involves the utilization of a semiconductor catalyst (such as, TiO2, ZnO, ZnS, and CdS) irradiated with light of an appropriate wavelength to generate highly reactive transitory oxidative species (i.e. •OH, •O2-, and HO2) for the mineralization of organic impurities and pollutants [5,6]. Therefore, a range of strategies have been explored for the photocatalytic degradation of organic dyes using semiconductor photocatalysts [2-7]. Generally, semiconductor catalysts show relatively low quantum degradation efficiency because of the high recombination rate of light-induced electron–hole pairs at or near the surface of the photocatalysts, which is considered one of the major limitations to obstructing the photocatalytic efficiency [8-10]. Therefore, many efforts

have been made to improve the characteristics of semiconductor-based photocatalysts for improving their activity. Among the many modification strategies, coupling semiconductors with carbonaceous nanomaterials has proven to be an effective way of enhancing the performance of photocatalysts [11,12]. In particular, the carbon-based nanostructures, acting as outstanding electron acceptors and highly conducting scaffolds, have found applications in photocatalysis [13]. In this regard, more recently, increasing interest has been invested in developing novel visible light-induced, graphene-based nanocomposites for photocatalysis and photoelectrochemical applications [14]. As an attractive native candidate for visible light driven photocatalysis, cadmium sulphide (CdS) is an essential II-VI group semiconductor that has been studied widely owing to its suitable band gap (E g = ~2.42 eV) [14,15] at 300 K, high absorption coefficient >104, and size dependent electronic and optical properties at room temperature, which allows for the more efficient absorption of visible light [15,16]. Therefore, CdS NPs have versatile applications in a range of fields, such as light emitting diodes, thin film transistors, solar cells, and photocatalytic degradation of organic pollutants and optoelectronic devices [15-17]. Nonetheless, there are still two prime drawbacks that need to be addressed to improve the photocatalytic performance of bare CdS NPs, such as the high recombination rate of photoinduced electron–hole pairs and photocorrosion. To solve these problems, many strategies have been developed to enhance the photocatalytic activity of CdS, including CdS quantum dots [18,19], deposition of noble metallic nanoparticles [20,21], formation of heterogeneous semiconductors, and modification with carbon nanomaterials [20-22]. Among the various carbon materials available, graphene, a single layer of carbon atoms densely packed in a honeycomb two dimensional (2D) lattice, has recently emerged as a nano building block for the fabrication/decoration of various metals and metal oxide materials owing to its unique sheet like morphology, ultrahigh electron conductivity, and mobility [23,24]. More recently, graphene-metal/metal oxide nanocomposites have been

found to be an efficient photocatalysts on account of the imperative role of the graphene sheet as an electron acceptor/transporter, which reduces the recombination rate of photoexcited electron–hole pairs remarkably [25-27]. Among these, CdS-Graphene nanocomposite have attracted increasing attention for a wide range of applications, such as the degradation of organic pollutants under visible light irradiation [28,29]. Moreover, previous studies of CdSGraphene nanocomposites focus mainly on the liquid phase degradation of organic pollutants or hydrogen production [30]. Furthermore, both the photocatalytic degradation of organic model pollutant dyes and detailed photoelectrochemical performance of the single step synthesized CdS-Graphene nanocomposite has not been investigated and reported yet. This paper reports a simple, one pot synthesis and detailed characterization of CdS-Graphene nanocomposite using standard techniques. The practical potential applications were examined by the degradation ability of the aqueous model dye pollutants and the improved photoelectrochemical performance of the CdS-Graphene nanocomposite. As indicated previously, organic dyes are used widely in many fields, which are the main organic pollutant source in water. This paper reports the effects of both the visible light absorption of CdS NPs and the electronic effects of graphene on the photocatalytic activity for the degradation of methylene blue (MB), methyl orange (MO), and Rhodamine B (RhB), which are well known water pollutants [31-33]. The efficient photodegradation of the pollutants in water was achieved in the presence of the as-synthesized CdS-Graphene nanocomposite as a photocatalyst. The CdS-Graphene nanocomposite was also used as a photoelectrode to measure the improved photoelectrochemical performance, such as cyclic voltammetry (CV), differential pulse voltammetry (DPV), linear sweep voltammetry (LSV), and electrochemical impedance spectrometry (EIS) under visible light irradiation [34]. To the best of the authors’ knowledge, there are few reports on CdS-Graphene nanocomposite materials available [3134]. Furthermore, together the photocatalytic and photoelectrochemical behaviour of this

nanostructured material has not been reported. Therefore, the present study assessed the single step synthesis and application of CdS-Graphene nanocomposite to develop a photocatalyst for the degradation of organic pollutants in water and to improve the photoelectrochemical performance for optoelectronic devices. 2. Experimental section 2.1 Materials Cadmium sulphate (CdSO4, 99.9%) and ammonium hydroxide (NH4OH, 98.5%) were purchased from Sigma Aldrich. Pure-graphene sheets were purchased from Iljin Nano Tech, Seoul, Korea (7-8 layer graphene sheets with a mean length of 500 nm). Sodium acetate (CH3COONa) and sodium sulphate (Na2SO4) were supplied by Duksan Pure Chemicals Co. Ltd., South Korea. Thiourea (98.0%), ethyl cellulose, and α-terpineol (C10H18O) were acquired from KANTO Chemical Co., Japan, whereas fluorine-doped transparent conducting oxide glass (FTO; F-doped SnO2 glass; 7 Ω sq-1) was supplied by Pilkington, USA. All the above reagents used in this study were of analytical grade and used as received. All the solutions were prepared from DI water obtained using a PURE ROUP 30 water purification system. 2.2 Methods X-ray diffraction (XRD, PANalytical, X’pert PRO-MPD, The Netherlands) was carried out using Cu Kα radiation (λ = 0.15405 nm). The XRD peaks of the crystalline phases matched the standard compounds reported in the JCPDS data file. Raman spectroscopy (Lab Ram HR800 UV Raman microscope; Horiba Jobin-Yvon, France) was performed to confirm the synthesis of the CdS-Graphene nanocomposite. The optical properties of the CdSGraphene nanocomposite were examined by UV-VIS-NIR diffuse absorbance/ reflectance spectrophotometer (VARIAN, Cary 5000, USA) and the photoluminescence (PL, Kimon, 1 K, Japan) of the samples was recorded over the scanning range, 200-800 nm, using an excitation

wavelength of 325 nm. The microstructures were observed by transmission electron microscopy (FE-TEM, Tecnai G2 F20, FEI, USA) operating at an accelerating voltage of 200 kV. Selected-area electron diffraction (SAED) was carried out by TEM. The elemental mapping of the sample containing the phases with different valences was obtained by TEM. Quantitative analysis was performed by energy dispersive spectrometry (EDS). X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XPS System, Thermo Fisher Scientific U.K.) was conducted using the following X-ray source: monochromatic Al Kα, hν = 1486.6eV, X-ray binding energy (BE) 15 kV, 150 W and spot size: 500 μm, Take-off angle: 90°, Pass energy: 20 eV, BE resolution: 0.6 eV. All the BE were calibrated to the BE of C 1s (284.6 eV). XPS peak fitting was performed using ‘‘AVANTAGE’’ software with a Shirley subtraction and the shape of the peaks used for the deconvolution was Gaussian–Lorentzian. XPS was conducted at the Korea Basic Science Institute (KBSI), South Korea. The photocatalytic and photoelectrochemical experiments were carried out using a 400 W lamp (3 M, USA) with λ > 500 nm and an irradiating intensity of 31 mW cm-2. The photocatalytic degradation ability of organic pollutants process was monitored by measuring the absorption of the organic model pollutants by UV–vis spectrophotometry (OPTIZEN 2120UV). Photoelectrochemical studies, such as cyclic voltammetry (CV), differential pulse voltammetry (DPV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS), were performed using a potentiostat (Versa STAT 3, Princeton Research, USA) comprised of a standard three-electrode system. Ag/AgCl (3 M KCl), a Pt gauge and FTO glass coated with the pure-graphene and CdS-Graphene nanocomposite were used as the reference, counter and working photoelectrodes, respectively. The experiment CV and DPV were performed in 0.2 M phosphate buffer solution (pH 7; 0.2% PBS), LSV, and EIS were performed in a 0.2 M sodium sulphate (Na 2SO4) solution as the supporting electrolyte at room temperature. The projection area of the photoelectrode was 1

cm2. The working electrodes were prepared as follows; 100 mg of each sample was mixed thoroughly by adding ethyl cellulose as a binder and α-terpineol as the solvent. The mixture was stirred and heated on a hot plate with a magnetic stirrer until a thick paste was obtained. The paste obtained was then coated on the FTO glass using the doctor-blade method and kept drying under a 60 W lamp overnight; the electrode was later used as a photoelectrode for photoelectrochemical measurements. 2.3 Facile synthesis steps of CdS-Graphene nanocomposite CdS-Graphene nanocomposite was prepared by a simple, one pot chemical precipitation method using CdSO4, thiourea, and NH4OH. In this simple, one step synthesis, an optimal amount of 0.20 g graphene sheets were added to 100 mL of DI water and subjected to a low sonication system (Branson 2800, low power sonication instrument) for 1 h. Subsequently, a 0.1 M aqueous CdSO4 solution and 0.02 M thiourea solution were added slowly to the above graphene dispersion. The pH of the reaction mixture was kept at 10 by adding NH4OH. The reaction mixture stirred for 2 h at room temperature. The color of the reaction mixture changed from a yellowish-black to dark yellowish-black and precipitation occurred. After the reaction was complete, the product was centrifuged, washed several times with de-ionized water and absolute ethanol, and then dried at 60 ˚C for 24 h. For the comparative study, CdS NPs were also prepared according to the same protocol. The asprepared samples were designated as CdS NPs and CdS-Graphene nanocomposite for further characterization and applications. 2.4 Photocatalytic degradation of MB, MO and RhB using the CdS-Graphene nanocomposite as a photocatalyst The photocatalytic degradation performance of the as-synthesized CdS-Graphene nanocomposite, CdS NPs, and pure-graphene samples were tested for the photodegradation of MB, MO, and RhB as model pollutant dyes and the rates of degradation were calculated. A

2 mg sample of each photocatalyst was suspended in 25 mL of an aqueous dye solution (concentration = 10 mgL-1) and each solution was sonicated for 10 min. The solutions were stirred in the dark for 30 min to complete the adsorption and desorption equilibrium of the specific substrate on the CdS-Graphene nanocomposite, CdS NPs, and pure-graphene. Visible light irradiation of the solutions was performed using a 400 W lamp (λ > 500 nm). The three sets of experiments were observed over a 6 h period. The rate of dye degradation was monitored by taking 2 mL of the samples from each set at every 1 h, centrifuging, removing the catalyst, and recording the UV-vis spectrum. As a control experiment, puregraphene was used and considered the reference photocatalyst. Each experiment was performed in triplicate to ensure the photocatalytic activities of the CdS-Graphene nanocomposite. 2.5 Stability and reusability tests of the CdS-Graphene nanocomposite The initial tests for the stability and efficiency were performed by suspending CdSGraphene nanocomposite in water and sonicating it for 1 h. The CdS NPs leaching in the solution were observed using a UV-vis spectrophotometer, which confirmed the stability of the CdS-Graphene nanocomposite and the possibility of using them as a catalyst (Fig. S1). A triplicate test of the reusability of the CdS-Graphene nanocomposite was tested after centrifuging the catalyst from the dye solution. The recovered catalyst was washed with DI water, dried in an oven at 60 ˚C, and reused for a second and third run to assess its catalytic ability with the dye solution under the same conditions (Fig. S2 (a,b, and c)).

2.6 Photoelectrochemical studies (CV, DPV, LSV, and EIS) of CdS-Graphene nanocomposite as a photoelectrode The photoelectrochemical performance of the CdS-Graphene nanocomposite and pure-graphene were examined by CV, DPV, LSV, and EIS under ambient conditions in the

dark and under visible light irradiation. The CV and DPV experiments were performed in 50 mL of 0.1 M PBS in the dark and under visible light irradiation. CV was performed at a scan rate of 50 mV s-1, whereas DPV was performed at a pulse height, pulse width, and scan rate of 50 mV, 0.005 s, and 4 mV s-1, respectively. The LSV and EIS experiments were performed in 50 mL of an aqueous 0.2 M Na2SO4 solution in the dark and under visible light irradiation at room temperature. The photocurrent response was examined by LSV in the dark and under visible light irradiation at a scan rate of 50 mV s -1 over the potential range, -1.0 to 1.0 V. EIS was performed in the dark and later under visible light irradiation (λ > 500 nm) with frequencies ranging from 1 to 104 Hz at 0.0 V vs. Ag/AgCl in potentiostatic mode.

3. Results and discussion 3.1 Proposed synthesis mechanism for the formation of CdS-Graphene nanocomposite CdS NPs were synthesized using simple, single step method, in which cadmium sulphate (CdSO4) and thiourea CS(NH2)2 were used as the cadmium and sulphur sources, respectively. Ammonia (NH4OH) was used to adjust the pH of the reaction mixture [35]. The cadmium sulphate and thiourea were stirred and sonicated individually for 1 h to detach the Cd2+ ions and S2- ions from the respective chemical precursor. The slow addition of Cd 2+ ions and S2- ions into the dispersed graphene solution allowed easy solubility, uniform dispersion, and maximum substitution of the ions on to the surface of the graphene sheets. Suitable reaction conditions and the strong affinity favor the anchoring of CdS NPs to the surface of the graphene sheets.

NH4OH CdSO4

Cd2+ H2O

Graphene dispersion

Stirring CdS NPs

Separately added to Graphene dispersion

Cd2+ S2-

SC(NH2)2

CdS-Graphene Nanocomposite

S2H2O

Stirring

Scheme 1. Proposed reaction mechanism steps for the formation of the CdS-Graphene nanocomposite. 3.2 Characterization of CdS NPs and CdS-Graphene nanocomposite 3.2.1 Structural and phase confirmation of CdS NPs and CdS-Graphene nanocomposite Fig. 1(a) presents the XRD pattern of the bare CdS NPs synthesized by a simple one step chemical precipitation method. The peaks were indexed, matched, and compared with the standard JCPDS data card no. 80-0006 (Hexagonal) [36]. The as-synthesized bare CdS NPs exhibited a hexagonal phase crystal structure with the lattice parameter, a = 4.121 Å, b = c = 6.682 Å. The most intense reflection peak was observed at 26.62 ° 2θ corresponding to the hexagonal (002) plane. The other peaks were indexed to 25.06 °, 28.24°, 36.82°, 43.94°, 48.06°, 52.16°, 67.19°, 71.26°, and 75.97° 2θ and planes were marked as (100), (101), (102), (110), (103), (112), (203), (211), and (105), respectively. The mean crystallite size of the CdS NPs was calculated to be 11.8 nm using the well-known Scherer’s formula, D = κλ/βcosθ,

….. (1)

where κ is the shape factor and has a typical value of ~ 0.9, λ is the wavelength (Cu Kα = 0.15405 nm), β is the full width at half maximum of the most intense peak (in radians), and θ is the main peak of CdS NPs at 26.62° 2θ. Fig 1(b) shows the XRD pattern of the CdS-

Graphene nanocomposite, which was well matched to the JCPDS data card no. 890440(Cubic) [36]. The most intense peak was observed at 26.53° 2θ (002), which matched the intense peak of graphene; the other peaks were matched to the CdS NPs. The inset in Fig 1(b) shows the XRD pattern of pure-graphene, where two main peaks can be seen clearly at 26.53 ° (002) and 54.70° (100). These two main peaks were also observed in XRD pattern of the CdS-Graphene nanocomposite, which further confirmed the presence and purity of graphene. The graphene peaks were matched and compared with the standard JCPDS No. 01-0646 [37].

Pure-Graphene

(b) CdS-Graphene

40 50 60 2 Theta (Degree)

(100)

(1

(103)

(101)

30

(102)

70

80

(105)

00

(211)

20

(103)

(110)

(100) 10

)

(100)

(112)

(002)

(110)

(101)

Intensity (a.u.)

Intensity (a.u.)

(002)

Intensity (a.u.)

(002)

(112)

Bare CdS NPs

(a)

(203) (105) 10

20

30

40 50 60 2 Theta (degree)

70

80

10

20

30

40 50 60 2 Theta (degree)

70

80

Fig. 1. (a) XRD pattern of bare CdS NPs, and (b) CdS-Graphene nanocomposite, and inset shows the pure-graphene pattern.

3.2.2. Raman analysis of pure-graphene and CdS-Graphene nanocomposite Raman spectroscopy provides valuable evidence of the electronic and structural properties of carbon materials, such as graphene [38]. Fig. 2 shows the Raman spectra of pure-graphene before and after the CdS NPs were anchored/decorated to its surface. The Raman spectrum of graphene consists of three peaks for the D and G band at 1350 cm -1, 1590 cm-1, and for the 2D band peak at 2729 cm-1 [24]. Similar characteristic peaks of the D and G bands at approximately 1350 cm-1 and 1590 cm-1 were observed for the CdS-Graphene nanocomposite [24]. The intensity of the D (ID) band provides information on the breathing

mode of the k-point, and the G band (ID) is related to the tangential stretching mode of the E2g phonon of the sp2 carbon atoms [24,38]. The inset in Fig. 2 shows the zoomed G band area of pure-graphene and the CdS-Graphene nanocomposite, which shows a slight shift towards a higher wavenumber and the intensity of the G band was enhanced dramatically after anchoring the CdS NPs to the surface of the graphene sheets, which further proves the presence of CdS NPs onto the graphene sheets. In the case of the 2D band, after the anchoring/decoration of CdS NPs, a broad 2D band was observed at 2729 cm-1 for the CdSGraphene nanocomposite with a slight increase in peak intensity. This confirms the anchoring and decoration of the CdS NPs over the graphene sheets, and also revealed the presence of few layer graphene sheets in the as-synthesized CdS-Graphene nanocomposite [38,39]. G band

Intensity (a.u.)

-1

G band (1595 cm )

Intensity (a.u.)

-1

D band (1350 cm )

Pure-G raphene C dS-G raphene

1520

1540

1560 1580 1600 1620 -1 W avenum ber (cm )

1640

-1

2D band (2729 cm ) Pure-Graphene CdS-Graphene

1200

1600 2000 2400 -1 Wavenumber (cm )

2800

Fig. 2. Raman spectra of pure-graphene and CdS-Graphene nanocomposite. The inset shows the zoomed region of the G band of pure-graphene and CdS-Graphene nanocomposite.

3.2.3. Optical analysis of the pure-graphene and CdS-Graphene nanocomposite The optical diffuse absorbance spectra of the as-synthesized CdS-Graphene nanocomposite and pure-graphene were measured at room temperature over the wavelength range of 200–800 nm. In Fig. 3(a) the optical absorption edge of the as-synthesized CdS-

Graphene nanocomposite was observed in the range, 400-500 nm, which corresponds to the shift of the valence band of S2- ions to the conduction band of Cd2+ ions [36]. Pure–graphene does not exhibit any absorption peak in the visible region of the solar spectrum. After the anchoring/decoration of CdS NPs onto the graphene sheets, the optical absorption of the CdSGraphene nanocomposite was observed in the visible region and the absorption edge was tremendously red shifted towards a higher wavelength compared to that of pure-graphene. The reduced crystallite size, the homogeneous distribution of CdS NPs onto the graphene sheet, and the strong interaction between graphene and CdS lead to an obvious red shift in the absorption peak of the CdS-Graphene nanocomposite [36]. The optical absorption in the visible region was enhanced by the anchoring of small size of CdS NPs onto the graphene sheets, which serves as a promising material for photocatalysis and optoelectronic-based applications. The reflectance peak (Fig. S3) between 500-600 nm in the case of the CdSGraphene nanocomposite was assigned to electronic conjugation between the CdS NPs and graphene sheets, which is similar to previous studies [31,36,40]. 11

Pure-Graphene CdS-Graphene

(a)

10

CdS-Graphene

8 7

*

(F(R) hν)

1/2

Absorbance (a.u.)

9

CdS-Graphene

6 5

Pure-Graphene

4 3

200

(b)

300

Fig. 3. (a) UV-

400 500 600 Wavelength (nm)

700

800

Eg = 2.25 eV

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 hv (eV) Vis, DRS

absorption spectra of pure-graphene and CdS-Graphene, (b) optical band gap of CdSGraphene nanocomposite.

In addition, the optical band gap was estimated from the Kubleka-Munk function, as shown in Fig. 3 (b) [40]. This is represented as

where F(R∞) was the K-M function or re-emission function, R∞ was the diffuse reflectance of an infinitely thick sample, K(λ) was the absorption coefficient, s(λ) was the scattering coefficient, and hν was the photon energy. Optical band gap (Eg) was determined by extrapolating the linear portion (denoted by dotted line in Fig. 3(b)) of the plot obtained between [(F(R∞)hν)1/2] versus hν. The calculated Eg of the as-synthesized CdS-Graphene was 2.25 eV. A photoluminescence study of the material is closely related to the surface states and stoichiometry, and is used to determine the efficiency of trapping, migration, and transfer of a charge carrier, as well as to understand the fate of the electron-hole pairs in semiconductors. Therefore, the luminescence properties were examined to further clarify the optical properties of the CdS-Graphene nanocomposite and its potential applications as a photonic material. Fig. 4 presents the room temperature photoluminescence spectra of CdS NPs, CdS-Graphene, and pure-graphene excited at 350 nm; the spectra were recorded over the wavelength range, 500 nm to 900 nm. Pure-graphene shows much less emission peak, whereas the CdS NPs shows a broad emission peak covering the maximum visible region of the spectrum (480 nm to 800 nm) with two distinct peaks centered at 514 nm and 732 nm. The near band edge emission peak at 514 nm and the other peak at 732 nm were assigned to the recombination of free electrons of CdS NPs. The main peak at 732 nm was attributed to the recombination of an electron in a sulphur vacancy with a hole in the valence band of CdS [41]. The anchoring/decoration of CdS NPs onto the graphene sheets resulted in PL spectra with lower intensity, which further proved the interaction of CdS NPs and graphene sheets. Therefore,

the fluorescence emission was quenched, indicating the high surface interaction of CdS NPs and graphene sheet as well as efficient electron transfer from the CdS NPs to graphene sheets. The overall PL studies of the CdS-Graphene nanocomposite clearly showed higher charge transfer ability, which could be responsible for improved photocatalytic activity and improved photoelectrochemical performance [24,41].

Intensity (a.u.)

Bare CdS NPs CdS-Graphene Pure-Graphene

500

600

700 800 Wavelength (nm)

900

Fig. 4. Room temperature-dependent photoluminescence spectra of bare CdS NPs, puregraphene, and CdS-Graphene nanocomposite.

3.2.4. XPS exploration of pure-graphene and CdS-Graphene nanocomposite X-ray photoelectron spectroscopy (XPS) was used to characterize and understand the chemical composition of the CdS-Graphene nanocomposite and pure-graphene. Fig. 5(a) shows the XPS survey scan spectrum of the as-synthesized CdS-Graphene nanocomposite, in which the major peaks were assigned to Cd, S, and C elements and the small O peak could be attributed to the presence of H2O from the atmosphere adsorbed on the sample surface. This indicates the successful preparation and anchoring of CdS NPs onto the graphene sheets. Fig. 5(b) shows the combined C 1s peaks of pure-graphene and CdS-Graphene nanocomposite at 284.6 eV, which is shifted slightly towards a higher binding energy in case of the CdSGraphene nanocomposite, which further supports the successful anchoring of CdS NPs onto

the graphene sheets. The shift of the ~284.8 eV peak towards a higher binding energy also supports the interaction between the CdS NPs and graphene sheets [42]. Fig 5(c) shows the S 2p3/2 spectrum, in which the S 2p peak appeared at 161.4 eV [42]. Slight shifts were observed in the Cd 3d and S 2p peaks to lower binding energies compared to the standard values reported in the literature for CdS NPs [43]. A possible reason could be that the Cd atoms accept electrons from graphene sheets, which causes an increase in electron density around the Cd atoms and a decrease in Cd-S bond length. Therefore, the binding energies of Cd 3d and S 2p decreased, which agrees well with the value reported for the CdS NPs [44]. (b)

Survey scan spectrum CdS-Graphene C 1s

Intensity (a.u.)

C 1s Pure-Graphene CdS-Graphene

Intensity (a.u.)

(a)

Cd 3d S 2p

281

1100 990 880 770 660 550 440 330 220 110 Binding energy (eV)

282

283 284 285 286 Binding energy (eV)

287

(c) S 2p3/2 161.4 eV

Intensity (a.u.)

S 2p spectrum

153

(d)

420

417

159 162 165 Binding energy (eV)

Cd 3d3/2 412.0 eV

414 411 408 405 Binding energy (eV)

402

168

(e)

Cd 3d5/2 405.3 eV

Intensity (a.u.)

Intensity (a.u.)

Fitted spectrum Cd 3d

156

Fitted spectrum C 1s

399 276

279

171

C 1s 284.6 eV

282 285 288 Binding energy (eV)

291

294

Fig. 5. (a) Survey scan spectrum, (b) Combined C 1s spectrum, (c) S 2p spectrum, and, (e and f) Fitted spectrum of Cd 3d and C1s of CdS-Graphene nanocomposite and pure-graphene.

Fig. 5(d and e) shows the high resolution XP spectra of the Cd 3d and C 1s peaks of CdS-Graphene nanocomposite, respectively. To further examine the surface analysis of the CdS NPs in the CdS-Graphene nanocomposite, there were two peaks for the Cd 3d 3/2 and Cd 3d5/2 core levels observed, which were cantered at 412.0 eV and 405.3 eV respectively. In the C 1s spectrum (Fig. 5(e)), the main peak was observed at 284.6 eV, which indicated the state of pure-graphene [26]. The binding energies of these photoelectron peaks (412.0, 405.3 and 284.6 eV) are characteristic of the Cd2+ ions and C 1s, which are indications of the successful anchoring/decoration of CdS NPs onto the graphene sheets. Overall XPS analysis revealed the effective and strong interaction between the CdS NPs and graphene sheets, which leads to the formation of CdS-Graphene nanocomposite.

3.2.5. Morphological exploration of the CdS-Graphene nanocomposite The morphology and composition of the CdS-Graphene nanocomposite was confirmed by TEM and EDS. Fig. 6(a) presents a TEM image of the CdS-Graphene nanocomposite formed. The 2D sheet like surface of the graphene was decorated successfully and densely packed with spherically CdS NPs. Furthermore, most of the synthesized CdS NPs were almost spherical in shape and dispersed over the graphene sheets, and they displayed a good interfacial interaction between the CdS NPs and graphene sheets. No free CdS NPs were observed outside of the graphene sheets. Opaque and few layer graphene sheets were observed from the background in Fig 6(a). A close observation from the HRTEM image of the CdS-Graphene nanocomposite displayed in Fig. 6(b) showed that the CdS NPs were distributed homogeneously over the graphene sheets. The interface between the

graphene sheets and CdS NPs can be clearly seen from the indicated arrow, which further proved the good interactions between them. Fig. 6(c) shows a HR-TEM image of CdS NPs and graphene sheets, in which the size of CdS NPs was ~10-12 nm and decorated on graphene sheets. The inset in Fig 6(c) clearly shows the interactions between the CdS NPs and graphene sheets.

(a)

(c)

(b)

Graphene Sheet 20 nm

CdS NPs CdS-Graphene Nanocomposite

0.2 µm

5 nm

10 nm

(d)

C (e)

(g)

16

(110) (102)

Percentage (%)

14

Cd

0.33 nm (100)

(f)

S

(h)

(h)

12 10 8 6 4

(100),(002), (101)

5 1 / nm

2 0 2

4

6 8 10 12 14 16 Particle diameter (nm)

18

20

Fig. 6. TEM images of the CdS-Graphene nanocomposite (a) Presence of CdS NPs onto the graphene sheets, (b) HR-TEM image showing the interaction between the CdS NPs and graphene sheets, (c) HR-TEM showing the particle size of the CdS NPs and graphene sheets, inset showing the interaction between the CdS NPs and graphene sheets, (d, e, and, f)

showing elemental mapping of pure-graphene (red color) and Cd (yellow color) and S (green color), (g) SAED pattern, and, (h) Particle size distribution graph of CdS NPs.

The elemental map presented in Fig. 6(d, e and f) shows the C, Cd, and S which provides strong evidence of the coexistence of the CdS NPs onto the graphene sheets. In Fig. 6(g) shows the SAED reflection patterns of the CdS-Graphene nanocomposite, whereas the colored dotted reflections show the crystalline nature of the CdS NPs, which was random in nature and attributed to the presence of a mixed phase (cubic and hexagonal) of CdS NPs, which is in accordance with the XRD pattern (Fig.1(b)). Fig. 6(h) presents the size distribution histogram of the CdS NPs, which clearly shows the mean particle diameter (~1012 nm). This was attributed to an increase in the active surface area of the CdS NPs with decreasing size of the catalyst. In addition, EDS shows that the product contained Cd, S, and C (Fig. S4), and elemental weight %, which further confirmed the presence/ decoration of the CdS NPs on the graphene sheets.

4. Applications of the CdS-Graphene nanocomposite 4.1 Estimation of the photocatalytic degradation of organic model pollutant dyes using puregraphene, CdS NPs and the CdS-Graphene nanocomposite From the viewpoint semiconductor photocatalysis, the role of a photocatalyst is to initiate or accelerate the specific reduction and oxidation (redox) reactions in the presence of visible light-irradiated semiconductors [1,2]. When the semiconductor catalyst was irradiated with photons, whose energy is equal to or greater than their band-gap energy (Eg), an electron (e--cb) is promoted from the valence band (VB) into the conduction band (CB), leaving a hole (h+-vb). Secondly, the excited electrons and holes migrate to the surface [45]. The rate of recombination is often inhibited by a scavenger or crystalline defects, which can easily trap

electrons or holes. Therefore, better crystallinity with few defects can usually minimize the trapping states and recombination sites, resulting in increased efficiency in the usage of the photogenerated carriers for the desired photoreactions [46]. For higher photocatalytic efficiency, the electron–hole pairs should be separated efficiently, and charges should be transferred rapidly across the surface/interface to restrain the recombination [46,47]. Here, the CdS-Graphene nanocomposite was used in practical applications as the photocatalyst, which showed highly improved performance compared with the bare CdS NPs, because the superior electron mobility and high specific surface area of graphene makes it more efficient. Graphene plays an important role as an efficient electron acceptor to enhance photoinduced charge transfer for improved photocatalytic performance [47]. The photocatalytic degradation performance of the CdS-Graphene nanocomposite was evaluated by degrading MB, MO, and RhB as a model dye pollutants under visible light irradiation. Fig. 7(a, b, and c) shows the photocatalytic degradation kinetics of MB, MO, and RhB as a function of the irradiation time. Here, C is the absorption of the MB, MO, and RhB solutions at each time interval of irradiation, and C0 is the absorption of the initial concentration (time 0). Fig. 7(a, b, and c) presents the photocatalytic degradation performance of the organic model dye pollutant, MB, MO, and RhB, were chosen to investigate the photocatalytic degradation ability of pure-graphene, bare CdS NPs, and CdS-Graphene nanocomposite under visible light irradiation using a 400 W lamp (λ> 500 nm). The kinetics of the reaction was plotted as the photodegradation constant of the model dye ln(C/C 0) vs. time (t). In Fig. 7(a) the rate of MB degradation was negligible in the absence of CdS NPs, which is shown by control analysis. The pure-graphene shows almost negligible degradation performance of ~16% and bare CdS NPs shows ~ 40% in 5 h under visible light irradiation. CdS NPs exhibits relatively low photodegradation efficiency because of the rapid recombination of electronhole pairs [48]. In contrast, the photodegradation performance of the CdS-Graphene

nanocomposite as a photocatalyst for MB was ~94.5% after 5 h under visible irradiation; almost complete degradation occurred. Fig. 7(b) shows the degradation performance of MO using pure-graphene, bare CdS NPs, and the CdS-Graphene nanocomposite. The performance of the CdS-Graphene nanocomposite as a photocatalyst for the photodegradation of MO was improved significantly compared to the pure-graphene and bare CdS NPs. The degradation performance for MO was ~80% in 5 h under visible light irradiation. 1.0

1.0

MB concentration C/C0

Light ON Dark

Dark

0.8

MO Concentration C/C0

0.8

Light ON

(a)

0.6

(b)

0.6

MB

0.4

MO

0.4

0.2

0.2

Pure-Graphene Bare CdS NPs CdS-Graphene

0.0 1

2

3

4 Time (h)

0.0 5

6

7

1.0

RhB Concentration C/C0

Pure-Graphene Bare CdS NPs CdS-Graphene 1

2

3

4 Time (h)

5

6

7

Light ON Dark

0.8

(c) RhB

0.6 0.4 0.2

Pure-Graphene Bare CdS NPs CdS-Graphene

0.0 1

2

3

4 Time (h)

5

6

7

Fig. 7. C/C0 versus time (h) plot for the photodegradation/ decolorization of (a) MB, (b) MO, and, (c) RhB using pure-graphene, bare CdS NPs, and CdS-Graphene nanocomposite in the dark and under visible light irradiation. Fig. 7(c) shows a profile of the photocatalytic degradation ability of the model pollutant dye RhB using pure-graphene, bare CdS NPs, and CdS-Graphene nanocomposite under visible light irradiation. A small decrease in the rate of degradation of RhB was

observed in the presence of pure-graphene after visible light irradiation, which suggests that RhB is quite stable under visible light [49,50]. In the case of the CdS-Graphene nanocomposite, much better photocatalytic performance was observed compared to the puregraphene and bare CdS NPs, which suggest that the graphene sheet can improve the photodegradation ability of CdS NPs. The CdS-Graphene shows high photodegradation efficiency up to ~91% in 5 h under visible light irradiation. This was attributed to the high surface-to-volume ratio of small CdS NPs decorated over the graphene sheets, which helps increase the photocatalytic sites and graphene helps reduce the recombination rate [48]. The degradation performance was significantly higher than that of the CdS NPs alone. This further proves that graphene is highly efficient for enhancing the catalytic performance of CdS NPs and behaves as an electron sink, which increases the separation of the photogenerated electron–hole pairs significantly and inhibits their recombination in the presence of the CdS-Graphene nanocomposite as a photocatalyst. Based on the above enhanced visible light-induced photocatalytic abilities shown by the CdS-Graphene nanocomposite, the visible light-induced photoelectrochemical performance using CV, DPV, LSV, and EIS, was performed in the presence of the CdS-Graphene nanocomposite as a photoelectrode.

4.2 Photoelectrochemical studies of pure-graphene and CdS-Graphene nanocomposite In the field of electrochemistry, CV is an influential tool for quantifying the redox behaviour and electron transfer kinetics as well as determining the electron stoichiometry of any system [51]. CV is used broadly to characterize the performance of a range of electrical energy storage devices, such as electrochemical capacitors, batteries, and fuel cells [47]. Fig. 8(a) shows the CV trace of the pure-graphene and CdS-Graphene nanocomposite in the dark and under visible light irradiation, which was performed at a scan rate of 50 mVs -1 over the

range, -1.0 to +1.0 V. The pure-graphene and CdS-Graphene nanocomposite as a photoelectrode in the dark exhibited a significantly lower anodic and cathodic peak, whereas the CdS-Graphene nanocomposite as a photoelectrode showed well resolved anodic and cathodic peaks under visible light irradiation. This shows the improved redox behaviour, which could be due to the presence/ decoration of CdS NPs onto the graphene surface. These observations confirm the enhanced electrochemical behaviour of the CdS-Graphene nanocomposite. The increased anodic and cathodic peak proves the improved current transfer ability of the CdS-Graphene nanocomposite under visible light irradiation, which also reveals the increased capacitive performance of the as-synthesized nanocomposite. Therefore, the enhanced capacitive performance of the CdS-Graphene nanocomposite can be attributed to its improved charge loading ability under visible light irradiation, which is the result of the anchoring/decoration of CdS NPs onto the graphene sheets. Differential pulse voltammetry (DPV) provides significant refinement against the charging current and produces an ideal and peak-shaped curve. DPV was performed on pure-graphene and the CdS-Graphene nanocomposite as a photoelectrode in the dark and under visible light irradiation to check the charging current behaviour [52,53]. Fig. 8(b) presents the well-defined quantized capacitance charging behaviour peaks in the dark and under visible light irradiation for the CdS-Graphene nanocomposite. The CdS-Graphene nanocomposite under visible light irradiation displayed improved and tremendous charging behaviour compared to pure-graphene. This was attributed to the anchoring/decoration of CdS NPs onto the graphene sheets. These stored electrons within the nanocomposite could be used for different photoelectrochemical performance-based devices.

0.0009

0.0006

(a)

0.0008

0.0003

(b)

Pure-Graphene in Dark Pure-Graphene in Light CdS-Graphene in Dark CdS-Graphene in Light

-0.0003 -0.0006 Pure-Graphene in Dark Pure-Graphene in Light CdS-Graphene in Dark CdS-Graphene in Light

-0.0009 -0.0012 -0.0015 -0.9 0.016

(c)

0.012 0.010

0.0006 0.0005 0.0004 0.0003 0.0002 0.0001 0.0000 -0.9

0.9

-0.6 -0.3 0.0 0.3 0.6 Potential (V vs. Ag/AgCl)

0.9

Pure-Graphene in Dark Pure-Graphene in Light CdS-Graphene in Dark CdS-Graphene in Light

(d)

Pure-Graphene in Dark Pure-Graphene in Light CdS-Graphene in Dark CdS-Graphene in Light

200

150

zimag/ohm

Photocurrent (A)

0.014

-0.6 -0.3 0.0 0.3 0.6 Potential (V vs. Ag/AgCl)

Photocurrent (A)

Photocurrent (A)

0.0007

0.0000

0.008 0.006 0.004

100

50

0.002 0.000

0

-0.002 -0.9

-0.6 -0.3 0.0 0.3 0.6 Potential V (V vs. Ag/AgCl)

0.9

50

100

150 zreal/ohm

200

250

Fig. 8. Visible light-induced performance of (a) CV, (b) DPV, (c) photocurrent measurement by LSV, and, (d) EIS Nyquist plot for the pure-graphene and CdS-Graphene nanocomposite as a photoelectrodes in the dark and under visible light irradiation. Linear sweep voltammetry (LSV) is a voltammetry method, where the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly with time [54]. LSV was performed in the dark and under visible light irradiation to provide evidence of the visible light-induced performance of pure-graphene and the CdS-Graphene nanocomposite [54,55]. Fig 8(c) shows LSV plots of pure-graphene and the CdS-Graphene nanocomposite in the dark and under visible light irradiation. The CdS-Graphene nanocomposite shows the improved performance of the photocurrent compared to pure-graphene. The CdS NPs anchored/decorated onto the surface of the graphene sheet can absorb visible light and generate more photoelectrons and graphene nanosheets play an effective role as an electron acceptor to enhance photoinduced charge

transfer and inhibit the backward reaction by separating the evolution sites of hydrogen and oxygen for the improved photocurrent performance of CdS-Graphene nanocomposite under visible light irradiation. Finally, these results showed that the CdS-Graphene nanocomposite can be used as an active material for photocurrent, capacitance, and optoelectronic devices. The interface charge separation of the photoelectrons and holes is an acute factor for the improved photocatalytic and photoelectrochemical performance. Electrochemical impedance spectroscopy (EIS) was performed in the dark and under visible light irradiation to understand the charge separation process and transport properties of the pure-graphene and CdS-Graphene nanocomposite as a photoelectrode, as shown in Fig. 8(d) [54]. In general, the complex impedance plot is normally presented as Z real vs. Zimaginary, which originates from the resistance and capacitance component of the electrochemical cell. A typical Nyquist plot includes one or more semicircular arcs with the diameter along the Zreal axis [56]. The semicircular arcs observed in the high and low frequency regions correspond to an electron transfer process, and its diameter represents electron transfer or charge transfer resistance [57,58]. In the present study, a semicircle arc with a smaller diameter for the CdS-Graphene nanocomposite was obtained compared to pure-graphene, which indicates a rapid electron-transfer process in the case of the CdS-Graphene nanocomposite under visible light irradiation. Overall, the small radius of the arc in the EIS spectra indicated lower electron transfer resistance at the surface of the photoelectrode, which is normally related to faster interfacial charge transfer. EIS spectrum clearly shows the smaller radius of the CdS-Graphene nanocomposite under visible light irradiation. The performance of the as-synthesized nanocomposite was better than other nanocomposites reported previously [50]. These EIS findings further confirmed that the CdS-Graphene nanocomposite could be used as an effective material for visible light active photoelectrodes. The boosted visible light-induced photoelectrochemical (CV, DPV, LSV, and EIS)

performance using CdS-Graphene nanocomposite confirmed the anchoring/decoration of CdS NPs onto graphene sheets. These improved performance results revealed an the interfacial interaction and charge transfer between the CdS NPs and graphene sheets, which could be responsible for the improved photoelectrochemical performance of the CdS-Graphene nanocomposite.

4.3 Schematic projected mechanism for photocatalytic activity of the CdS-Graphene nanocomposite Based on the above photocatalytic analysis, a probable photocatalytic mechanism for the CdS-Graphene nanocomposite was proposed and presented in Fig. 9. Under visible light irradiation, electrons (e-) are excited from the valence band (VB) of CdS NPs to its conduction band (CB), leaving holes in the VB, thereby forming the electron-hole pairs [59]. The photogenerated electrons can transfer rapidly to the graphene matrix due to the intimate interfacial contact between the CdS NPs and graphene sheets, resulting in a significantly improved lifetime of the photogenerated electron-hole charge carrier.

Visible light e- e eCdS-Graphene nanocomposite

ee-e--e-e-

O2

CdS NPs h+hh++h+h+

10 nm

Graphene Sheets

OH•OH

•O 2

H2O •OH

Mineralized products

CdS Nanoparticles

Fig. 9. Schematic presentation of the charge separation and transfer of electrons in the CdSGraphene system under visible light illumination. The photoexcited electrons transfer from

the conduction band of semiconductor CdS to the carbon atoms of the graphene sheets, which are accessible to mineralize/degrade the products. Fig. 9 shows a schematic diagram of the possible process for photocatalytic degradation under visible light irradiation (hν ≥ 500 nm). Charge separation in the CdS NPs is initiated and electron–hole pairs are generated. For the high charge carrier mobility in graphene, this could act as an electron acceptor and transporter to efficiently hinder the recombination rate of the photogenerated electron-hole pairs [32,48,50]. The negative charge then activates dissolved oxygen to form superoxide anion radicals (O 2•-). At the same time, the holes can react with adsorbed water to produce hydroxyl radicals ( •OH). Finally, the active species, such as holes, superoxide anion radicals, and hydroxyl radical, all with strong oxidizing ability, mineralize the dye molecules to CO2, H2O or some other smaller compounds [24,26]. The proposed reaction chain for the degradation of dye molecules is follows: CdS + hν → CdS (e– + h+)

(1)

CdS (e–) + graphene → CdS + graphene (e–)

(2)

e– + O2 → O2•–

(3)

OH– + h+ → •OH

(4)

Dye molecules (MB, MO and RhB) + •OH + O2•– + h+ + H2O  mineralized products 5. Conclusions This paper reports well-defined CdS-Graphene nanocomposite synthesized using a facile and single step method, through which intimate interfacial contact between CdS NPs and graphene sheets are achieved. Compared to the bare CdS NPs, this 2-D flat structure of the

CdS-Graphene

nanocomposite

provides

remarkably

improved

separation

of

photogenerated electron-hole pairs, in which the optimal amount of graphene sheets serve as an efficient electron collector and transporter. Moreover, the as-synthesized CdS-Graphene

nanocomposite exhibited much better photocatalytic performance towards the photocatalytic degradation of organic dye pollutants, such as methylene blue, methyl orange, and Rhodamine B under visible light irradiation. Furthermore, thorough studies for photoelectrochemical analysis of the CdS-Graphene nanocomposite under visible light irradiation showed improved performance compared to pure-graphene. CV, DPV, LSV, and EIS

confirmed

the

improved

optoelectronic

performance

of the

CdS-Graphene

nanocomposite under visible light irradiation. Overall, the as-synthesized CdS-Graphene nanocomposite can be used as an effective material for visible light-induced optoelectronicbased devices and for the degradation of organic dyes. The current work provides new insights into the synthesis of graphene-based semiconductor nanocomposites in the aqueous phase for different photo-based applications.

Acknowledgement

This study was supported by Priority Research Centres Program (NRF Grant No: 2014R1A6A1031189), and by Basic Science Research Program (NRF Grant No: 2015R1D1A3A03018029) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.

Appendix A. Supplementary material †Electronic Supplementary Information (ESI) available: [UV-vis spectra of CdS-Graphene for CdS leaching, Reusability test spectra of CdS-Graphene, UV-vis DRS spectra of CdSGraphene and EDS and % percentage of CdS-Graphene nanocomposite] See DOI: 10.1039/b000000x/

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Graphical abstract Visible light e- e eCdS-Graphene nanocomposite

ee-e--e-e-

O2

CdS NPs h+hh++h+h+

10 nm

Graphene Sheets

OH•OH

•O 2

H2O •OH

Mineralized products

CdS Nanoparticles

CdS-Graphene nanocomposite for visible light-driven photocatalytic degradation of organic model dye pollutants.