Chemical Engineering Journal 283 (2016) 816–825

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /ce j

Polymer supported graphene–CdS composite catalyst with enhancedphotocatalytic hydrogen production from water splitting under visiblelight

http://dx.doi.org/10.1016/j.cej.2015.08.0181385-8947/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 21 64252695.E-mail address: caoxj@ecust.edu.cn (X. Cao).

Juan Xu, Le Wang, Xuejun Cao ⇑State Key Laboratory of Bioreactor Engineering, Department of Bioengineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

h i g h l i g h t s

� A polymer supported graphene–CdScomposite catalyst was developed.

� The highest H2 production rate of thecomposite catalyst was 175 lmol h�1.

� The hydrogen production rate of thecomposite catalyst was 19.3 timeshigher than that of physical mixedsample.

� The combination of the photo-induced electrons and holes wassuppressed by loading CdS ongraphene.

� The aggregation between single-layergraphene sheets was inhibited by thestructure.

g r a p h i c a l a b s t r a c t

Polymer supported graphene–CdS composite catalyst was prepared and the photocatalytic activity of CdSwas improved by 19.3 times after being loaded on the surface of the graphene.

a r t i c l e i n f o

Article history:Received 17 April 2015Received in revised form 27 July 2015Accepted 7 August 2015Available online 10 August 2015

Keywords:GrapheneHydrogenPhotocatalysisHeterogeneous catalysisSolar energy

a b s t r a c t

A polymer supported graphene structure was synthesized using organic microspheres as the support ofthe graphene oxide. Graphene oxide was reduced to graphene and CdS was decorated on the surface ofthe graphene through a solvothermal process. The photocatalytic hydrogen production reaction was car-ried out using the composite catalyst under visible light irradiation (k > 400 nm). A series of compositecatalysts were synthesized with different reaction time. The maximal hydrogen production rate of175 lmol h�1 was obtained from the 3 h prepared sample (PSGM/rGO/CdS-3), which is 19.3 times higherthan the mechanical mixing sample with the same composition. The quantum efficiency of the PSGM/rGO/CdS-3 sample was 3.99% at 420 nm. Graphene served as electron acceptor and hydrogen evolutionsites to promote the separation of the photo-induced charge carriers. Quantum size effect was observedfrom ultraviolet–visible diffuse reflectance spectroscopy and PL measurements. The presence of quantumsize effect widened the bandgap of the catalyst and enhanced the photocatalytic hydrogen productionrate. The stability of the composite catalyst was tested and demonstrated that this graphene-based com-posite catalyst was a stable and potential catalyst for photocatalytic hydrogen production.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

As energy and environmental problems turn more and moreserious, searching a new energy resource that clean, low-carbonand high energy density is urgent. Hydrogen energy is one of the

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most suitable candidates to substitute for fossil energy [1].Semiconductor photocatalysts to convert solar energy to hydrogenenergy is attracting much attention [2]. This is an environmentallyfriendly way to produce hydrogen.

The semiconductors used for photocatalytic water splitting arerequired to possess a more negative conduction band than thereduction potential of H+ to H2 and a more positive valance bandthan the oxidation potential of H2O to O2 [3]. Various kinds ofsemiconductors like TiO2 [4], WO3 [5], CdS [6], ZnS [7], GaN [8]and BiVO4 [9] have been discovered to be suitable for photocat-alytic water splitting. Three key processes directly affect the quan-tum efficiency of the photocatalyst used for water splitting. Thefirst one is the ability for the catalyst to capture the photons inUV–vis–NIR range, which is closely related to the band gap. Thesecond one is the separation and transportation efficiency of thephoto-generated charge carriers. The third one is the catalytic effi-ciency on the surface. CdS with a band gap of 2.4 eV has beenextensively investigated in photocatalytic field, owing to its partic-ular visible light response ability and high catalytic efficiency.Compared with other photocatalyst, the high recombination ofthe photo-generated charge carriers and the photocorrosion hasbeen the main drawback of CdS catalyst [10]. High recombinationof charge carriers caused low activity and photocorrosion resultedin low photo-stability. Against these problems, researchers havefocused on combining CdS with other materials such as TiO2

[11], ZnO [12], ZnS [13], CdSe [14] and so on. The photocatalyticactivity and the photo-stability were improved through thismethod. Despite CdS composite catalyst displayed better activitythan pure CdS, photochemical instability and the low separationefficiency of the photo-generated electron and holes remain themajor problems for CdS-based photocatalysts.

Graphene is a single-layer carbon nanomaterial with two-dimensional honeycomb structure. It shows outstanding electricalproperty which offers excellent mobility of charge carriers at roomtemperature. Due to its large specific surface area, extraordinaryoptical, thermal and mechanical properties, graphene has beenwidely studied for its application [15]. Chemically reduced gra-phene oxide (rGO) is an excellent substitute material for pristinegraphene. As rGO not only possesses similar physicochemicalproperties as pristine graphene, but also can be produced in alow cost and large-scale way and can be easily used in synthesizinggraphene-based nanocomposites [16]. Graphene-based nanocom-posites have been used in various fields, such as solar cell [17],super capacitor [18], photocatalytic degradation and water split-ting [19,20]. In the field of photocatalysis, graphene has been usedas support for functional nanomaterials, which shows special syn-ergistic effects. Singh etc. synthesized graphene supported plas-monic photocatalyst for hydrogen production and proved thatgraphene acted as an electron acceptor to suppress the recombina-tion of charge carriers [21]. Dubale and co-workers reported thatgraphene exhibited synergetic effect with Cu2O in enhancing theabsorbance of the visible light, separating the charge carriers andsuppressing the photocorrosion [22]. Xian and co-workers pre-pared SrTiO3-graphene nanocomposites and suggested that gra-phene could capture the photogenerated electrons and increasedthe availability of electrons and holes for photocatalytic reaction[19]. As graphene-based composites exhibit such particular chem-ical and physical characters, CdS–graphene composite was investi-gated and showed enhanced activity in dye degradation andhydrogen evolution [23–25]. However, in graphene-basednanocomposites, strong p–p interaction always leads to aggrega-tion and stacking between graphene sheets. To conquer this prob-lem, new methods need to be proposed. Joonsuk and co-workers[26] had demonstrated that graphene oxide could be coated onamine-functionalized polymer microspheres. The polymer sup-ported graphene oxide structure was used in this work. To support

graphene nanosheets, amine-functionalized poly (styrene/glycidylmethacrylate) microspheres were synthesized. Graphene oxide,with abundant epoxide groups in its basal planes, can be easilywrapped on the surface of amine-functionalized microspheres.Then the loading of CdS and the reduction of graphene oxide wereaccomplished through a single solvothermal process. The CdS-decorated polymer supported graphene structure helps to not onlyprevent the stacking and aggregation between single-layer gra-phene sheets, but also inhibit the aggregation of CdS nanoparticles.

2. Materials and methods

2.1. Materials

Glycidyl methacrylate (GMA) was purchased from AdamasReagent Co., Ltd. Graphite powder, potassium permanganate, con-centrated sulfuric acid, phosphoric acid, hydrogen peroxide (30%),styrene, azodiisobutyronitrile (AIBN), polyvinyl pyrrolidone (PVP),dimethyl sulfoxide (DMSO), ethanol, cadmium acetate, sodium sul-fide, sodium sulfite and aluminum oxide (200–300FcP) were pur-chased from Shanghai Sinopharm Chemical Reagent Co., Ltd.AIBN was purified by recrystallization in ethanol. Styrene wasrefined using a basic alumina column to remove inhibitors.

2.2. Synthesis of amine-functionalized poly-(styrene/glycidylmethacrylate)/graphene oxide microspheres (PSGM/GO)

The organic polymer microspheres were synthesized by disper-sion polymerization method. Under nitrogen atmosphere, PVP(2.0 g), GMA (3.0 g) and styrene (12.0 g) were dissolved in ethanol(60 ml). Next, an AIBN solution (0.2 g AIBN was pre-dissolved in30 ml ethanol) was added to the mixture. Then the mixture wasdegassed with nitrogen for 10 min and carefully sealed. The reac-tion was maintained under 343 K for 12 h in a shaking bath. Theproduct was washed with ethanol and deionized water severaltimes, and dried under room temperature. 5.0 g as-prepared pro-duct was re-dispersed in 40 ml deionized water and 5.0 g ethidenediamine as coupling agent was added into the suspension to realizethe amination. Graphene oxide (GO) was prepared through oxida-tion of graphite according to the literature [27]. To coat 5 wt% GOon the surface of the microspheres, 200 mg amine-functionalizedmicrospheres was dispersed in 40 ml deionized water and 10 mgGO was added into the suspension. The mixture was stirred andkept at 343 K for 4 h in water bath. After cooling to room temper-ature, the mixture was centrifuged and washed with deionizedwater and ethanol, and then the product was dried under roomtemperature.

2.3. Synthesis of PSGM/rGO/CdS

To prepare PSGM/rGO/CdS, 100 mg amine-functionalizedmicrospheres and 150 mg Cd(CH3COO)2�2H2O was added into60 ml DMSO. The suspension was transferred into a 100 mlTeflon-lined stainless steel autoclave (Yanzheng Experimentinstrument Co., Ltd, Shanghai). Then it was heated to 453 K andmaintained for different hours (3 h, 6 h and 12 h). After the reac-tion was stopped and cooled, the product was filtered and washedwith ethanol several times. The product was dried under roomtemperature and labeled as PSGM/rGO/CdS-x (x = 3, 6 and 12).Pure CdS and PSGM/rGO was also prepared under the same exper-iment condition for 12 h.

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2.4. Photocatalytic hydrogen production reaction

In this study, an online photocatalytic hydrogen production sys-tem (CEL-SPH2N, AuLight, Beijing) was used to test the activity ofthe photocatalyst. In the typical process, 0.1 g photocatalyst wasdispersed into 100 ml 0.5 M Na2S/Na2SO3 aqueous solution andthe suspension was ultrasonic dispersed. A 300W Xenon lamp(CEL-HXF300, AuLight, Beijing) with an optical filter (k > 400 nm)to cut off the light in the ultraviolet region was used as light source.The solution was degassed before the reaction and an online gaschromatography (SP7800, TCD, nitrogen as carrier gas and 5Amolecular sieve column) was used to detect the evolution of thehydrogen.

2.5. Characterization

Atomic force microscopy (AFM) was performed on a Veeco/DIatomic force microscope instrument. The morphologies wereinvestigated by scanning electron microscopy (SEM, Hitachi, S-3400N) and transmission electron microscopy (TEM, JEOL, JEM-1400). Powder X-ray diffraction (XRD, RIGAKU, D/max2550) wasused to characterize the crystalline phase of products. Cu Ka radi-ation (k = 0.15405 nm, 40 kV, 100 mA) was used and 2h scanningrange was 10–80�. UV–vis diffuse reflectance spectra (DRS) wereobtained with a Scan UV–vis spectrophotometer (Varian, Cary500) within a range 200–800 nm. Raman spectra measurementswere recorded with an inVia Reflex Raman spectrometer with524.5 nm laser excitation from 100 to 3000 cm�1. The X-ray photo-electron spectra (XPS, Thermo Scientific, ESCALAB 250Xi) were car-ried out using Al Ka radiation. All binding energies were referred tothe C 1s peak of 284.8 eV. Elemental analysis was conducted usingAgilent 725ES inductively coupled plasma atomic emission spec-trometry (ICP-AES). Photoluminescence spectra were recordedusing Fluorolog-3-P (Jobin Yvon).

3. Results and discussion

3.1. Catalyst structure

The aqueous dispersion of graphene oxide was preparedthrough oxidation of graphite flake using KMnO4/H2SO4/H3PO4

according to the literature procedure. Fig. 1 showed the atomicforce microscopy (AFM) images of the graphene oxide sheets. Thethickness of the graphene sheets was measured as about 1.1 nm,

Fig. 1. AFM image of the graphene oxid

which is in agreement with the literature [24], indicating the for-mation of the single layered graphene oxide.

The morphology of the organic polymer microspheres, thePSGM/GO and PSGM/rGO/CdS was investigated using SEM, TEMand HRTEM instruments. Fig. 2a shows the SEM image of the orig-inal organic polymer microspheres. The synthesized microsphereswere micron-sized and its diameter was about 2.5 lm. When theaqueous dispersion of graphene oxide sheets was mixed with thesuspension of the amine-functionalized polymer microspheres,the single layered graphene oxide sheets with epoxy groupsreacted and formed CAN bond with the amine group on the surfaceof the polymer microspheres. The graphene oxide sheets werefirmly coated on the surface of the microspheres (Fig. 2b) and sep-arated from each other, resulting in less opportunity foragglomeration.

The PSGM/rGO/CdS catalyst was prepared through a solvother-mal method. DMSO was used as solvent, sulfur source and reduc-tant of graphene oxide. The temperature of the reaction was setas 453 K and the reaction time was changed from 3 h to 12 h afterthe reaction temperature increased to 453 K. During the solvother-mal process, graphene oxide was reduced and CdS was loaded onthe surface of PSGM/rGO microspheres (Fig. 2c). As shown inFig. 2(e and f), the surface of PSGM/rGO/CdS catalyst was coveredwith CdS. When the reaction time was set as 3 h, the surface ofthe microspheres was covered with a thin layer of CdS. It was seenfrom Fig. 2g that the diameter of CdS particles loaded on the sur-face of the PSGM/rGO microspheres was about 6–15 nm. Whenlooking into the high resolution HRTEM image of PSGM/rGO/CdS-3 (Fig. 2g), the lattice spacing of 0.336 nm corresponding to(111) facet of cubic phase CdS was observed on the surface ofthe microsphere, indicating the formation of heterojunctionbetween CdS and rGO. In addition, TEM–EDX measurement wasimplemented to investigate the chemical compositions at theinterface (Fig. 2h). Signals of C, S and Cd elements were observedfrom the spectra, which further confirmed the formation of hetero-junction. As the reaction time increased, the surface of the micro-spheres covered with more CdS and CdS gradually aggregated tolarger particles. It was worth noting that the size of the CdS parti-cles grown on the surface of the microspheres increased with thereaction time and the amount of CdS covered on the surfacechanged.

To determine the amount of CdS loaded on the surface of themicrospheres, ICP-AES measurement was carried out. The percent-age of CdS catalyst for the samples synthesized with different reac-tion time (3 h, 6 h and 12 h) were 15.46 wt%, 35.02 wt% and

e and the height profile of a layer.

Fig. 2. (a) SEM image of the original organic polymer microspheres, 6000�; (b) SEM image of graphene oxide wrapped organic polymer microspheres (PSGM/GO), 6000�,(inset with a higher magnification, 20,000�); (c) SEM image of solvothermal synthesized composite catalyst – sample PSGM/rGO/CdS-6, 20,000�, (inset with TEM image ofsample PSGM/rGO/CdS-6, scale bar: 0.5 lm); TEM image of the surface of sample (d) PSGM/rGO/CdS-3, scale bar: 100 nm; (e) PSGM/rGO/CdS-6, scale bar: 100 nm; (f) PSGM/rGO/CdS-12, scale bar: 100 nm; (g) HRTEM image of sample PSGM/rGO/CdS-3, scale bar: 2 nm; (h) EDS spectra of sample PSGM/rGO/CdS-3 with inset of HRTEM image ofsample PSGM/rGO/CdS-3 at the sampling interval (scale bar: 5 nm).

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Fig. 3. XRD diffraction patterns of sample PSGM/GO, PSGM/rGO/CdS-12 (12 h),PSGM/rGO/CdS-6 (6 h), PSGM/rGO/CdS-3 (3 h) and CdS.

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39.41 wt%, indicating different loading amount at different reac-tion time. It was observed that the weight percent of CdS increasedwith the extension of reaction time. It was deduced that in the ini-tial stage of the synthesis reaction, CdS was produced. As the reac-tion to proceeded, more CdS was synthesized and loaded on thesurface of the PSGM/rGO microspheres. As a result, the weight per-cent of CdS increased.

The XRD patterns of the composite catalysts synthesized withdifferent reaction time were shown in Fig. 3. The XRD pattern ofCdS was indexed on the basis of Joint Committee on PowderDiffraction Standards (JCPDS). Three obvious peaks observed at2h values of 26.5�, 44.0� and 52.1� were attributed to (111),(220) and (311) facets of cubic CdS, respectively. The XRD patternsof PSGM/rGO/CdS composite catalysts were consistent with CdS.The crystallinity of CdS loaded on the surface of the microsphereswas improved as the reaction time increased from 3 h to 12 h.Moreover, the XRD pattern of PSGM/GO was also included in thefigure. A broad peak corresponding to polymer PSGMwas observedat 2h = 19.5�, which was consistent with the reported article [28].However, the broad diffraction peak of PSGM become weak inthe patterns of the composite catalyst samples. That was becausethe signal of the CdS catalyst was much higher than that of thepolymer.

The specific surface area (Table 1) of CdS and the PSGM/rGO/CdS composite catalysts were investigated using a nitrogenadsorption–desorption process. The specific surface area of CdSwas 125.23 m2 g�1. However, the specific surface areas of the com-posite catalysts were much lower than that of CdS. That wasbecause after loading CdS on PSGM/rGO microspheres, the particlediameter of the composite catalyst was much larger than that ofthe pure CdS. When the reaction time increased from 3 h to 12 h,

Table 1Measured specific surface area (m2 g�1) and average pore diameter (Å) of theprepared samples.

Sample SABETa (m2 g�1) Average pore diameterb (Å)

CdS 125.23 56.381PSGM/rGO/CdS-3 5.67 408.793PSGM/rGO/CdS-6 29.21 70.374PSGM/rGO/CdS-12 36.49 75.153

a Specific surface area calculated from the nitrogen adsorption isotherm using theBET method.

b Calculated from nitrogen absorption branch using the BJH method.

the specific surface area increased from 5.67 m2 g�1 to36.49 m2 g�1. It was observed that the weight percent of CdS inthe composite catalyst increased with the increasing of the reac-tion time, which is consistent with the variation trend of the speci-fic surface area. While the specific surface area of CdS was muchhigher than that of PSGM microsphere, the specific surface areaof the composite catalyst was dominated by the weight percentof CdS.

UV–vis diffuse reflectance was applied to investigate the opticalabsorption of CdS and the composite catalyst samples, shown inFig. 4. The sample of PSGM/rGO just showed a bit of absorptionin ultraviolet region, as the graphene coating polymer microsphereexhibited poor light absorption ability. CdS and other PSGM/rGO/CdS-x samples (3, 6 and 12) showed high absorption in both ultra-violet and visible region. CdS showed an absorption edge of523 nm and the bandgap was calculated to be 2.37 eV. As shownin Fig. 4, the absorption for PSGM/rGO/CdS-x samples (3, 6 and12) were lower than CdS and the absorption edges for these sam-ples were estimated to be 496 nm, 509 nm and 512 nm. Theseabsorption edges were red-shifted from PSGM/rGO/CdS-3 toPSGM/rGO/CdS-12. The bandgap was determined to 2.50 eV, 2.44eV and 2.42 eV for PSGM/rGO/CdS-x samples (3, 6 and 12), respec-tively. The bandgap of CdS decreased when the solvothermal reac-tion time increased. This phenomenon could be attributed toquantum size effect which changed the band structure of CdS[29,30]. The diameter of CdS particles loaded on the surface ofthe PSGM/rGO microspheres was about 6–15 nm. As the reactiontime increased, the CdS particles grew into larger ones. In the ini-tial stage of the reaction, the change of the particle size was obvi-ous. As the reaction proceeded to the late stage, the change ofparticle size of CdS was tiny. So the particle size of CdS particlesloaded on PSGM microspheres for 3 and 6 h was smaller than thatof 12 h. According to quantum size effect, the band gap energychanged with the particle size of the material and material withsmaller particle size showed higher band gap energy. When thesize of CdS nanoparticles increased, the conduction band andvalance band of CdS changed and the bandgap increased [31].

When looking into the photoluminescence spectra of PSGM/rGO/CdS composite catalysts, weak emission bands centeredaround 500 nm were discovered (Fig. 5). This emission bandaround 500 nm was due to intrinsic emission for CdS, which wasrelated to the band gap energy of the material [32]. When the reac-tion time increased from 3 h to 12 h, redshift of the emission peak

Fig. 4. UV–vis diffuse reflectance spectra of different samples: (a) PSGM/rGO; (b)PSGM/rGO/CdS-3 (3 h); (c) PSGM/rGO/CdS-6 (6 h); (d) PSGM/rGO/CdS-12 (12 h); (e)PSGM/rGO/CdS-24 (24 h); (f) CdS in the wavelength range of 250–800 nm.

Fig. 5. Photoluminescence spectra of PSGM/rGO/CdS-3 (3 h); PSGM/rGO/CdS-6(6 h) and PSGM/rGO/CdS-12 (12 h).

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was observed. The redshift of the emission peak was caused by thedecrease of the band gap energy of CdS. This tendency was in keep-ing with UV–vis DRS absorption edge, which also indicated thepresence of quantum size effect on these samples [31,33,34].

Raman spectra of PSGM/GO and PSGM/rGO/CdS-6 were shownin Fig. 6. It can be seen that three strong signals at about619 cm�1, 1000 cm�1 and 1030 cm�1 originating from poly-styrene were discovered from the spectrum of PSGM/GO [35]. Forthis sample, a G peak at 1597 cm�1 and a D peak at 1341 cm�1

for graphene oxide were also observed, originating from the firstorder scattering of the tangential stretching (E2g) mode and the dis-order in the planar sp2-hybridized carbon network, respectively.The spectrum of PSGM/rGO/CdS showed three peaks at 297 cm�1,772 cm�1 and 887 cm�1 for CdS [36] and very weak G and D peakfor graphene sheet, indicating the formation of CdS on the gra-phene. The SEM figures shows that the surface of the graphenecoated microspheres is almost completely covered with CdS andthe surface was rugged. This morphology may cause the weak sig-nal of graphene and the absent of the PSGM signal.

X-ray photoelectron spectroscopy (XPS) was employed to inves-tigate the chemical state of the catalyst. The XPS spectra of PSGM/GO and PSGM/rGO/CdS-3 were displayed in Fig. 7. Fig. 7a showedthe spectrum of C1s from PSGM/GO and four peaks were observed

Fig. 6. Raman spectra of sample PSGM/rGO/CdS-6 (6 h) and PSGM/GO.

at 284.6 eV, 285.7 eV, 287.0 eV and 288.2 eV, corresponding to CAC(sp2 bonded carbon), CAOH (hydroxyls), CAO (carbonyls) andOAC@O (carboxyl) groups, respectively [37,38]. This spectrumindicated that PSGM/GO contained high percentage of oxygenfunctional group. After carried out the solvothermal process, theC1s spectrum of PSGM/rGO/CdS-3 (Fig. 7b) changed compared tothat of PSGM/GO. Three peaks were observed at 284.6 eV,286.0 eV and 288.6 eV, which were ascribed to CAC (sp2 bondedcarbon), CAOH (hydroxyls) and OAC@O (carboxyl) groups, respec-tively. The peak of CAO group almost disappeared and the inten-sity of the peaks for CAOH became weaker than that of PSGM/GO, indicating the reduction of GO by DMSO through thesolvothermal process. The spectra of Cd 3d and S 2p for CdS loadedon the surface of rGO were also shown in Fig. 7(c and d). The bind-ing energy peaks for Cd element were found at 405.2 eV (Cd 3d5/2)and 412.0 eV (Cd 3d3/2). The binding energy peaks for S elementwere found at 161.6 eV (S 2p3/2) and at 162.9 eV (S 2p1/2). Thiswas in accordance with the reported values in the literature [39].So through the solvothermal process, GO was reduced into rGOand CdS was synthesized and loaded on the surface of rGO.

3.2. The photocatalytic activity and the stability of the photocatalyst

The photocatalytic reaction was conducted in an aqueous solu-tion containing 0.5 M Na2SO3 and 0.5 M Na2S as sacrificial reagentsunder simulated solar light irradiation (k > 400 nm). Fig. 8 dis-played the reaction time courses for photocatalytic hydrogen pro-duction over the aforementioned catalysts. As seen from Fig. 8A,pure CdS exhibited a low hydrogen evolution rate of 35.8 lmol h�1

in 5 h under visible-light irradiation. To investigate the specialaffection of the synthesis method, the hydrogen production rateof a mechanical mixed sample (PSGM/rGO + CdS) composed of84.5 mg PSGM/rGO and 15.5 mg CdS was also tested under thesame reaction condition. A hydrogen evolution rate of8.62 lmol h�1 in 5 h was obtained from the PSGM/rGO + CdS sam-ple. In addition, the hydrogen production rate for the PSGM/rGO/CdS-x (x = 3, 6 and 12) samples are determined to be 175, 135and 126 lmol h�1 in 5 h, respectively. Moreover, the quantum effi-ciency (QE) of the PSGM/rGO/CdS-3 catalyst under different irradi-ation wavelength was measured (Fig. 8B). The QE of the catalystdecreases with the increasing of the incident wavelength, indicat-ing the significant influence of the light absorbance on the photo-catalytic activity. The apparent QE of PSGM/rGO/CdS-3 sample was3.99% at 420 nm, using 0.1 g catalyst in the reaction. It is worthnoticed that the PSGM/rGO/CdS-x (x = 3, 6 and 12) samples contain15.46 wt%, 35.02 wt% and 39.41 wt% of CdS, respectively. Thehydrogen production rate of PSGM/rGO/CdS-3 sample which onlycontains 15.46 wt% of CdS and without loading with any noblemetal is still 3.90 times higher than that of pure CdS sample. ThePSGM/rGO/CdS-3 sample possesses the same composition withthe PSGM/rGO + CdS sample. However, the PSGM/rGO/CdS-3 sam-ple shows a hydrogen production rate 19.3 times higher than thatof PSGM/rGO + CdS sample. This remarkable enhancement indi-cates that the intimate contact between CdS and graphene is indis-pensable to promote the electrons to transfer from CdS tographene. This is in accordance with the result of the recent studies[40,41] that the heterojunction formed between graphene and thephoto-sensitive material is of great importance for efficient separa-tion of charge carriers and high photocatalytic. It was reported byPeng and co-workers [37] that a hydrogen evolution rate of314 lmol h�1, 0.28 times higher than that of single CdS, wasobtained from GO/CdS composite catalyst. A hydrogen productionrate of 210 lmol h�1 was obtained from N-doped rGO/CdS com-posite catalyst using 0.2 g catalyst in the reaction, 4.25 timeshigher than that of CdS [40]. The result was attributed to the intro-duction of rGO to the composite catalysts, for the heterojunction

Fig. 7. XPS spectra of (a): C1s of PSGM/GO; (b): C1s of sample PSGM/rGO/CdS-3; (c): Cd 3d of sample PSGM/rGO/CdS-3, (d): S 2p of sample PSGM/rGO/CdS-3.

Fig. 8. (A) Photocatalytic hydrogen evolution with 0.5 M Na2SO3 and 0.5 M Na2S aqueous solution using 0.1 g of the catalysts of: (a) PSGM/rGO/CdS-3 (3 h), (b) PSGM/rGO/CdS-6 (6 h), (c) PSGM/rGO/CdS-12 (12 h), (d) CdS and (e) PSGM/rGO + CdS; (B) the apparent quantum efficiency of PSGM/rGO/CdS-3 under different irradiation wavelength.

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formed between CdS and rGO improved the separation efficiencyof the photo-induced charge carriers [42,43].

It can be seen that after loading CdS on the surface of PSGM/rGOmicrospheres, the photocatalytic activity of the composite cata-lysts were obviously improved. BET measurement showed thatthe specific surface area of CdS was much higher than that of all

the composite catalysts. However, the photocatalytic activity ofCdS was much lower than that of all the composite catalysts.When the specific surface area of the composite catalysts increasedfrom 5.67 m2 g�1 to 36.49 m2 g�1, the hydrogen production ratedecreased from 175 to 126 lmol h�1. So it was not the specific sur-face area of the catalyst which dominated the change of the

Fig. 9. (A) Cycling test of catalyst PSGM/rGO/CdS-3 under visible light irradiation (k > 400 nm) with 0.5 M Na2SO3 and 0.5 M Na2S aqueous solution for 20 h; (B) XRDdiffraction patterns of catalyst PSGM/rGO/CdS-3 before (a) and after (b) the cycling test.

Fig. 10. XPS spectra of C1s, Cd3d and S2p for sample PSGM/rGO/CdS-3 after the cycling test.

J. Xu et al. / Chemical Engineering Journal 283 (2016) 816–825 823

photocatalytic activity of the composite catalyst. In the other hand,the PSGM/rGO/CdS-3 sample shows a large average pore diametermuch higher than the other prepared samples. Since a larger porediameter is beneficial to the mass transfer. It is easier for the reac-tion solution to contact and react with the surface of the catalyst.Moreover the produced hydrogen can transfer to the solution moreeasily. This can be one of the reasons for the high photocatalyticactivity of the PSGM/rGO/CdS-3 composite catalyst.

It is observed that the photocatalytic hydrogen production ratedecreases with the increasing of the solvothermal process reactiontime. This can be ascribed to quantum size effect that affected thephotocatalytic activity of the composite catalysts [44,45].According to the UV–vis diffuse reflectance spectra, the absorptionedge of the catalysts shifts from 496 to 512 nm. The optical absorp-tion and the band-gap of the catalysts increase with the increasingof the reaction time, which is favorable for the improvement of thephotocatalytic activity. However, the photocatalytic hydrogen pro-duction rate shows an opposite trend. Gerischer [46] reported that,the electron transfer rate between the catalyst and the solutionwould increase when the bandgap of the semiconductor catalystwas widened. Michael et al. [45] reported that when the bandgap of the catalyst was broadened, the valence band edge mightobtain higher oxidizing ability to oxidize sacrificial agent, resultingin faster kinetics. According to the theory mentioned above, thecatalyst with broadened bandgap possesses higher thermodynamicdriving force for redox reaction to produce hydrogen and consume

hole-scavenger. This was why the hydrogen production rate of thecomposite catalyst decreased with the increase of the reactiontime.

To investigate the photostability of the composite catalyst,cycling test of sample PSGM/rGO/CdS-3 was performed. The pho-toreaction was repeated for four times with evacuation for eachtime. As shown in Fig. 9A, after 20 h reaction, the composite cata-lyst still kept high hydrogen production rate in each five hours. Aslight decrease in total hydrogen production was due to the con-sumption of S2�/SO3

2� during the reaction. The XRD measurementwas performed on sample PSGM/rGO/CdS-3 before and after thecycling test. It was seen that he XRD patterns (Fig. 9B) exhibitedno difference on the composite catalyst before and after reaction.XPS measurement was also applied to investigate the chemicalstate of the composite catalyst after the cycling test (Fig. 10). Thebinding energies of C1s, Cd3d and S2p remained unchanged fromthat of the catalyst before the test. The result indicated thatPSGM/rGO/CdS-3 composite catalyst is stable for photocatalytichydrogen production.

3.3. Proposed photocatalytic mechanism

In graphene-based composite catalyst, graphene plays a greatimportant role in receiving photo-induced electrons and providingthe reaction sites [40], inhibiting the combination of the photo-induced electrons and holes and improving the photocatalytic

Fig. 11. Proposed hydrogen evolving mechanism of PSGM/rGO/CdS composite catalyst.

824 J. Xu et al. / Chemical Engineering Journal 283 (2016) 816–825

activity. For the composite catalyst in this work, the photocatalytichydrogen evolving mechanism was proposed as follows (Fig. 11).Under the visible irradiation, photo-induced electrons and holesare generated in conduction band and valance band, respectively.On one hand, the photo-induced electrons are likely to transferto the surface of CdS or to graphene [47]. The protons that absorb-ing on the surface of the catalyst accept the photo-induced elec-trons and transform to hydrogen. On the other hand, photo-induced holes are consumed by S2�/SO3

2� sacrifice agent.

CdS!hv CdSþ e� þ hþ ð1Þ

2H2Oþ 2e�ðCdSÞ ! H2 þ 2OH� ð2Þ

2H2Oþ 2e�ðgrapheneÞ ! H2 þ 2OH� ð3Þ

SO2�3 þH2Oþ 2hþ ! SO2�

4 þ 2Hþ ð4Þ

2S2� þ 2hþ ! S2�2 ð5Þ

S2�2 þ 2SO2�3 þ 2hþ ! 2S2O

2�3 ð6Þ

In this work, the XRD analysis exhibited a cubic phase for CdSloaded on graphene. The crystal structure kept unchanged for dif-ferent samples, indicating that the phase structure did not affectthe photocatalytic activity of the composite catalyst. For PSGM/rGO/CdS composite catalyst prepared in this experiment, CdS cov-ered on the surface of graphene in different degree. Take PSGM/rGO/CdS-3 for example, graphene surface was not all covered withCdS, with some place exposing to the reaction solution. This dis-persion state was a significant factor for the high hydrogen produc-tion rate of this sample, because graphene was not only used aselectron acceptor, but also the active sites for hydrogen evolutionreaction in graphene-based composite catalyst [40]. Moreover,hydrogen evolution reaction preferred to proceed on the surfaceof graphene. In this case, photo-induced electrons can transferfrom CdS to graphene and the separation efficiency of the chargecarriers is improved. With more active sites and higher separationefficiency of the photo-induced electrons and holes, the

photocatalytic hydrogen production rate of the composite catalystwas enhanced. For sample PSGM/rGO/CdS-6, more CdS was cov-ered on the surface of graphene and the active sites on graphenewere shielded from reaction solution. The electrons’ transfer path-way to graphene was blocked and the separation efficiencydecreased. As a result, the hydrogen production rate of the catalystdropped.

Moreover, the results of UV–vis DRS and PL measurements indi-cated the presence of quantum size effect. The bandgap was broad-ened when the solvothermal reaction time decreased from 12 h to3 h. Quantum size effect of the composite catalysts caused thedecrease of the photocatalytic activity from PSGM/rGO/CdS-3 toPSGM/rGO/CdS-12. The above discussion explained the reason forthe improvement of the hydrogen evolving activity of the compos-ite catalyst.

4. Conclusion

A novel PSGM/rGO/CdS composite catalyst was synthesizedthrough a solvothermal process. Polymer supported graphenestructure was applied to support CdS. This structure efficientlyinhibited the aggregation of the graphene sheets. The PSGM/rGO/CdS composite catalyst showed high photocatalytic activity forhydrogen production under visible light irradiation. The highesthydrogen production rate (175 lmol h�1) was obtained from thesample PSGM/rGO/CdS-3, which was 19.3 times higher than thatof the physical mixed sample PSGM/rGO + CdS (8.62 lmol h�1).The quantum efficiency of the PSGM/rGO/CdS-3 composite catalystwas 3.99% at 420 nm. The heterojunction formed between CdS andgraphene promoted electrons to transfer from CdS to graphene andreacted with protons on the active sites on graphene. The recombi-nation of the charge carriers was suppressed as a result. The quan-tum size effect caused the widening of the bandgap of CdS and theincreasing of the thermodynamic driving force. Under the compre-hensive function of the above three factors, the photocatalytichydrogen production activity was improved. The cycling testshowed that PSGM/rGO/CdS composite catalyst was stable forhydrogen production. In conclusion, PSGM/rGO/CdS composite

J. Xu et al. / Chemical Engineering Journal 283 (2016) 816–825 825

catalyst can be considered a potential catalyst used for photocat-alytic hydrogen evolution.

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