Mg-doped ZnO composites

Applied Surface Science 400 (2017) 129–138

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Full Length Article

Photocatalytic and electrochemical performance of three-Dimensional reduced graphene Oxide/WS2 /Mg-doped ZnO composites Weiwei Yu a , Xi’an Chen b , Wei Mei a , Chuansheng Chen a,∗ , Yuenhong Tsang c a b c

College of Materials Science and Engineering, Changsha University of Science and Technology, Changsha, 410114, China Zhejiang Key Laboratory of Carbon Materials, College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, 325027, China Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, 999077, China

a r t i c l e

i n f o

Article history: Received 27 September 2016 Received in revised form 18 November 2016 Accepted 17 December 2016 Available online 21 December 2016 Keyword: Reduced graphene oxide WS2 nanosheet Synergistic effect Photocatalytic properties Electrochemical performance

a b s t r a c t To improve the dispersion of reduced graphene oxide and enhance the photocatalytic property of reduced graphene oxide/Mg-doped ZnO composites (rGMZ), the reduced graphene oxide/WS2 /Mg-doped ZnO composites (rGWMZ) were prepared by electrostatic self-assembly and coprecipitation methods. The effects of mass ratio of WS2 nanosheets to reduced graphene oxide (WS2 /rGO wt.%) and calcination temperature on the photocatalytic and electrochemical property of rGWMZ composites were investigated. Experimental results showed that the photocatalytic efficiency of rGWMZ composites is three-fold compared with that of rGMZ composites when the WS2 /rGO wt.% is 20.8% and calcination temperature is 500 ◦ C, in which the degradation ratio Rhodamin B (RhB) can reach 95% within 15 min under the UV light and 90% within 90 min under simulated solar light. In addition, the rGWMZ show larger capacitance and smaller resistance than rGMZ. The enhancement for photocatalytic activity and electrochemical performance of rGWMZ is ascribed to improving the specific surface area, electrical conductivity and electronic storage capability because of the synergistic effect of rGO and WS2 nanosheets. © 2016 Elsevier B.V. All rights reserved.

1. Introduction ZnO materials as a semiconductor metal oxides have been widely applied in photocatalytic field due to its wide direct bandgap (∼3.37 eV), large exciton binding energy about 60 meV at room temperature, chemical stability and non-toxicity. However, the intrinsic ZnO materials can only absorb the ultraviolet light (about 5% of sunlight) and own high recombination rate of photogenerated carriers, thus limiting their wide practical application. Recently, many effective solutions have been applied to improve the photocatalytic efficiency of ZnO photocatalysts [1–6]. The photocatalytic activity of ZnO can be improved by Mg doping because of tuable electronic band gap of ZnO by varying the Mg2+ doping concentration and more oxygen vacancies or zinc vacancies due to ZnO atructural instability [7–9]. However, there still exists high recombination rate of photoelectron–hole pairs in Mg doping ZnO material due to the poor electrical conductivity and serious agglomeration. Graphene carrier is a very valid way to prevent the agglomeration of ZnO nanostructure and provide the conductive platform owing to its large surface area and

∗ Corresponding author. E-mail address: [email protected] (C. Chen). http://dx.doi.org/10.1016/j.apsusc.2016.12.138 0169-4332/© 2016 Elsevier B.V. All rights reserved.

high electrical conductivity [10–15]. However, graphene prefer to self-restack and poor compatibility with other nanomaterials. Hence, their intrinsically unique physical properties are not fully exploited as carriers for preparing high-performance photocatalysts [16]. Compared with graphene, reduced graphene oxide with many carboxyl, epoxy and hydroxyl groups reveals very good dispersion and strong interaction with nanocatalyst. Moreover, these functional groups disturb the surface structure of rGO offering more active sites [17,18]. But, this also reduce the conductivity of rGO due to the collapse of surface structure, resulting in decreasing the enhance effect [19]. In order to overcome this problem, composite catalytic carrier, assembled with rGO and other nanomaterial, has been widely researched.For example, 1D carbon nanotubes are deposited on rGO improving the dispersion and preventing the curling of rGO. Most importantly, the effective synergistic effect between rGO and nanostructures can enhance their electron transfer rate significantly [20,21]. Based on the 1D material/rGO composite catalytic carrier, 2D Layered material assembled with rGO has attracted widespread attention. Layered WS2 owns unique nature open bandgap, distinctive physicochemical properties, native vacancy defects and low electrical conductivity [22]. Constructing 3D reduced graphene oxide/WS2 sheet can alleviate the agglomeration of rGO and produce effective synergistic effect improving their electrical conductivity [23]. For example, Konda

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2.1. Prepartion of rGWMZ composites All the chemical used in synthesis have been used as received from chemical suppliers without any further purification and processing. The reduced graphene oxide was prepared by modified Hummers methods. The WS2 nanosheets solution was prepared by the mechanical shear exfoliation method, referring to the previous reports [26]. The rGWMZ composites were performed typically as follows: 15 mL rGO solution (0.8 g L−1 ) was sonicated for 15 mins, and then 2 mL WS2 solution (1.25 g/L) was slowly added to the above rGO solution, forming the WS2 /rGO solution. Under vigorous stirring, 5.49 g ZnC2 O4 ·2H2 O and 0.54 g MgC2 O4 ·2H2 O (the molar ratio of Mg to Zn is 1:10) were dissolved in the WS2 /rGO solution. Subsequently, 100 mL oxalic acid solution (0.25 mol/L) was dropwise added to the aforesaid solution, and the solution was evaporated under 80 ◦ C. After dried at 80 ◦ C for 24 h, the resultant product (rGWMZ20.8 precursor) was annealed at 500 ◦ C for 2 h under the protection of nitrogen gas. The achieved samples are recorded as rGWMZ20.8. When the mass ratio of WS2 /rGO are 10.4 wt.%, 31.2 wt.% and 41.6 wt.%, the resultant hybrids are named as rGWMZ10.4, rGWMZ31.2 and rGWMZ41.6, respectively. Furthermore, the rGWMZ20.8 precursor was calcined at 400 ◦ C and 600 ◦ C, respectively. As a comparison, the reduced graphene oxide/magnesiumdoped zinc oxide, the WS2 /magnesium-doped zinc oxide and the reduced graphene oxide/WS2 /zinc oxide calcined at 500 ◦ C under the same conditions, labeled as rGMZ, WMZ and rGWZ. 2.2. Characterization TG-DTA was performed by a NETZSCH STA 409 thermal analyzer under nitrogen atmosphere. X-ray diffraction (XRD) measurement was performed using Philips PW1710 diffractometer with Cu Ka1 radiation. Scanning electron microscopy (SEM) observations was carried out with S-4800 field emission scanning electron microscope. Transmission electron microscopy (TEM) analyses and element mapping images were conducted on a JEM-3010 transmission electron microscope. The Brunauer-Emmett-Teller (BET) surface areas were determined using a nitrogen adsorption analyzer (ASAP2020HD88). The Raman spectroscopy was executed by Renishaw inVia. Fluorescence spectra measurements and quantum yield were characterized on a Hitachi F4500 fluorescence spectrophotometer and the wavelength of excitation is 320 nm. UV–vis diffuse reflection spectra were recorded on TU-1901 spectrophotometer. RhB was used as model dye to evaluate the photocatalytic activity of rGWMZ composites. The typical experiments can refer to our previous reports [11]. The cyclic voltammetry (CV) and AC impedance (EIS) spectras were performed with CHI 660 elec-

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Shiva et al. employed synergistic interactions between reduced graphene oxide and layered WS2 to improve the overall performance of Li-ion batteries [24]. Ke-Jing Huang et al. prepared layer tungsten sulfide-graphene nanocomposite with excellent electrochemical performances [25]. Inspired by these features, we firstly constructed the reduced graphene oxide/WS2 catalyzer carrier by electrostatic attraction, then obtained the 3D rGWMZ by constant temperature coprecipitation method. The rGWMZ exhibits enhancement photocatalytic activity because of the effective synergistic effect between WS2 nanosheets and reduced graphene oxide. Moreover, the enhancing photocatalytic activity mechanism of rGWMZ has been discussed in detail.

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trochenmical workstation, with the scan voltage range from −0.1 V to 0.8 V and scan rate of 2 mV/s. The working electrode was glassy carbon electrod coated 10 ␮L mixed liquor, which is composed of 5 mg sample, 950 ␮L absolute ethyl alcohol and 50 ␮L Nafion. Carbon stick electrode as the counter electrode and saturated calomel electrode. The electrolyte of CV test was 0.5 mol/L dilute sulphuric acid solution and in EIS test was 1 mol/L sodium sulfate solution. The electron spin resonance (ESR) spectra were obtained by using X-band JEOL JES-FA200 spectrometer. The captured agent of hydroxyl radical (·OH) in water is 5, 5-dimethyl-1-pyrroline-Noxide (DMPO), and the microwave frequency is 9072.418 MHz. 3. Results and discussion 3.1. Structure and morphology of rGWMZ composites In order to consider the effect of annealed temperature on the structure of material, TG-DTA is used to analyse the thermal decomposition of rGWMZ20.8 precursor under nitrogen atmosphere, and the corresponding result is shown in Fig. 1. The DTA curve shows a strong endothermic peak is observed at around 150 ◦ C, and there is 20% mass loss in the TG curve, originating from the loss of crystal water in zinc oxalate dihydrate crystallization. The endothermic peak at 420 ◦ C, correspond to the weight loss of 40% in TG curve, is attributed to the thermal decomposition of zinc oxalate (ZnC2 O4 = ZnO + CO↑+ CO2 ↑). There was no sign change in TG curve above 500 ◦ C, while there is an exothermic peak at about 780 ◦ C, indicating that the crystalline phase of residue is highly stable from 500 ◦ C to 750 ◦ C. According to above result, we choose the annealed temperature are 400 ◦ C, 500 ◦ C and 600 ◦ C, respectively. Fig. 2a illustrates the effect of the mass ratio of WS2 to rGO on XRD patterns of resultant smaples. It is obvious that the diffraction peaks of all samples display in same position, which are attributed to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes of hexagonal wurtzite ZnO (JCPDS NO.36-1451), respectively. Fig. 2b is the XRD patterns of rGWMZ20.8 precursor annealed at different temperature. The diffraction peaks of rGWMZ20.8 precursor are in accordance with JCPDS card (NO. 25-1029) for zinc oxalate dihydrate. After annealed, the diffraction peaks of samples are corresponding hexagonal wurtzite ZnO, and the intensity of diffraction peaks increase as increasing the annealed temperature. No diffraction peaks of rGO, carbon and WS2 are observed, which may because their content is too low. The morphology of different samples was examined by using SEM. As shown in Fig. 3, we find that the rGO (Fig. 3a) and rGO/WS2 carriers (Fig. 3b) present very different morphologies. Pure rGO exhibits huge flake with seriously stacking, while the rGO/WS2 car-

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Fig. 2. XRD patterns of different samples: (a) the effect of the mass ratio of WS2 to rGO, (b) the effect of annealed temperature on rGWMZ20.8 precursor.

Fig. 3. SEM images of different sample: (a) rGO, (b) rGW, (c) rGWMZ20.8, (d) rGMZ. TEM studies further analyse the morphology of rGO/WS2 and rGWMZ20.8.

riers shows smooth membranes with wrinkles and some slices, attesting that the WS2 nanosheets can be deposited on reduced graphene oxide sheets and improve the dispersion of rGO significantly. The rGWMZ20.8 (Fig. 3c) is flat columnar with a large number of slit through holes inside, which are composed of a lot of small nanopaticles. Contrast with the image of rGMZ (shown in Fig. 3d), we can see that there are no obvious difference in image, but rGWMZ20.8 possess irregular and small columnar structure. Fig. 4a is the TEM image of 3D rGO/WS2 carrier. It is very evident that there are many nanoplates on the rGO sheets. The shadow spots are characterized by EDS image (inserting in Fig. 4a), and

reveal the presence of C, W and S elements, which can confirms WS2 insertion. The HRTEM image of shadow spots (Fig. 4b) show two characteristic lattice spacing of about 0.29 nm and 0.35 nm, corresponding to the interplanar spacing of the (110) planes and (004) planes of WS2 crystal, respectively, indicating the WS2 nanoplate are successfully introduced onto rGO sheets. Fig. 4c shows TEM of rGWMZ20.8. We observe that the flat columnar are composed of small nanopaticles, and the nanoparticles are loaded on rGO sheets (inserting in Fig. 4c), matched well with corresponding SEM result. The high magnification TEM of rGWMZ20.8 (Fig. 4d) demonstrate that the Mg-doped ZnO nanopaticles show clear fringers and the

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Fig. 4. TEM (a) and HRTEM image (b) of rGO/WS2 , TEM (c) and HRTEM image (d) of rGWMZ20.8. Table 1 The textural parameters of rGWMZ. Sample

Surface area (m2 /g)

Pore volume (cm3 /g)

Pore size (nm)

rGMZ rGWMZ10.4 rGWMZ20.8 rGWMZ31.2 rGWMZ40.8 rGWMZ20.8 400 ◦ C rGWMZ20.8 600 ◦ C

10.8255 15.6446 17.6808 17.3168 9.9073 59.4411 2.9441

0.106984 0.154696 0.158023 0.136951 0.119063 0.276764 0.067597

45.4256 35.3496 32.5693 29.5975 67.2982 16.7833 38.2155

atomic interspacing is about 0.25 nm and 0.29 nm, consistent with the (101) planes and (100) planes of ZnO crystal, respectively. The inserting in Fig. 4d shows the ZnO nanoparticles possesses a single crystalline structure. These results clearly reveal that the single crystalline ZnO nanoparticles are decorated on the 3D rGO/WS2 . In order to further explore the element composition and distribution in the rGWMZ20.8, EDS images are characterized and the correspongding results are shown in Fig. 5. It reveals the presence of C, O, Zn, Mg, W and S, which can confirms that the Mg-doped ZnO nanopaticles are decorated on the reduced graphene oxide sheet. According to the cumulative electron microscopy analysis, we can further deduce WS2 nanosheets successfully insert into the layers of rGO. The textural properties (BET surface area, pore volume and pore size) of rGWMZ samples with different WS2 /rGO wt.% and annealed temperature are presented in Table 1. The specific surface area of rGWMZ increase with increasing WS2 /rGO wt.%, and the maximum reach 17.68 m2 /g when the WS2 /rGO is 20.8 wt.%. Meanwhile, the rGWMZ20.8 owns smaller pore size of 32.57 nm belong to the mesoporous. The reason of increasing surface area

may be that the inserting WS2 nanosheets can improve the dispersion of reduced graphene oxide and effectively restrain GO stacking. The influence of annealed temperature to specific surface area is obvious, when annealed temperature rise from 400 ◦ C to 600 ◦ C, the specific surface area of rGWMZ20.8 reduce from 59.4411 m2 /g to 2.9441 m2 /g. Fig. 6 shows the Nitrogen adsorption-desorption isotherms of rGWMZ with different WS2 /rGO wt.% and annealed temperature. All the curves belong to type IV isotherms with a H3 type hysteresis loop and not reach adsorption saturation when approaching saturated vapor pressure. These phenomena indicate that the rGWMZ composites exist slit shape pores, and the pore size is continuous distribution from mesoporous and macroporous [26], which further conform the results of SEM and TEM. 3.2. Photocatalytic property of rGWMZ composites Fig. 7 depicts the photocatalytic degradation efficiency of different samples for RhB under UV light and visible light. Fig. 7a shows the effect of the mass ratio of WS2 to rGO on the photocatalytic performance of rGWMZ. Obviously, the photocatalytic activity of rGMZ is lowest than that of all rGWMZ, and the degradation amount only has 30% within 30 min. The photocatalytic activity of rGWZ is also weaker than that of all rGWMZ. While, the degradation amount of rGWMZ20.8 reaches 95% within 15 min and nearly 100% within 20 min is equal to commercial TiO2 . As far as we know, the catalytic efficiency of most reported rGO based system are weaker than commercial TiO2 , which implies that the photocatalytic activity of rGMZ is improved by inserting WS2 nanosheets into rGO sheet. The photocatalytic activity of rGWMZ is enhanced with the increasing WS2 /rGO wt.% but inverse when more than 20.8%. As shown in Fig. 7b, the calcined temperature can effect the photocatalytic activity of rGWMZ20.8, and the sample calcinated at 500 ◦ C

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Fig. 5. EDS image of rGWMZ20.8.

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Fig. 6. Nitrogen adsorption-desorption isotherms: (a) the effect of the WS2 /rGO wt.%, (b) the effect of annealed temperature on rGWMZ20.8 precursor.

shows the highest activity. To evaluate the photocatalytic stability of rGWMZ20.8, the sample is further detected by performing the recycling experiments, as shown in Fig. 7c. The rGWMZ20.8 has very high photocatalytic stability that the RhB degradation rates of seven all reach nearly 96% within 25 min. In addition, the photocatalytic activity of rGWMZ with different WS2 /rGO wt.% and calcined temperature were also evaluated under visible light (Fig. 7(d) and (e)). We can acquire the consistent conclusion with photocatalytic degradation experiment under UV light. The rGWMZ shows best photocatalytic degradation efficiency of comes up to about

90% within 90 min when WS2 /GO wt.% = 20.8% and the calcined at 500 ◦ C. These results imply that the photocatalytic performance of rGMZ is improved by inserting WS2 nanosheets into rGO no matter under UV light or visible light. 3.3. Enhancement mechanism The Raman spectrum of rGWMZ20.8 was tested and compared with pure rGO and rGMZ shown in Fig. 8. The positions and intensity of vibrating peaks of rGWMZ20.8 and rGMZ have no obvious

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Fig. 7. Photocatalytic performance of samples: (a) the effect of the mass ratio of WS2 to rGO, (b) the effect of annealed temperature on rGWMZ20.8, (c) cyclic degradation test of rGWMZ20.8, (a)–(c) under UV light; (d) the effect of the mass ratio of WS2 to rGO, (e) the effect of annealed temperature on rGWMZ20.8, (d)–(e) under visible light.

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Fig. 9. (a) Nyquist plots of rGMZ and rGWMZs with different WS2 /rGO wt.%. (b) Nyquist plots of rGWMZ20.8 with different annealed temperature.

change, proving that introduction of WS2 has no influence on the structure of samples. Two intensive peaks at around 324 cm−1 and 432 cm−1 , which originated from the characteristic peaks of hexagonal wurtzite ZnO. The wide peaks at around 1040 cm−1 originated from Zn complex [27]. Around 1335 cm−1 and 1601 cm−1 can observe two peaks, corresponding to the D band and G band of reduced graphene oxide. The D band is for sp3 defects or disorder in carbon, and the G band provides information on in-plane vibration of sp2 -bonded carbon atoms in a 2D hexagonal lattice [28]. The relative intensity ratio of the D-band to G-band (defined as ID /IG) represents the oxidation/reduction degree of reduced graphene oxide, and the lower value, the higher reduction degree. In order to further explore the reduction degree of reduced graphene oxide in different rGWMZs, the local Raman spectrum from 1250 cm−1 to 1800 cm−1 was tested and shown in Fig. 8(b). D-band peak and G-band peak appear in all samples at same postion demonstrating the existence of reduced graphene oxide, and the value of ID /IG of pure rGO, rGMZ and rGWMZ with increasing WS2 /rGO wt.% are 1.01, 0.86, 0.8 0.68, 0.81 and 0.8, respectively. This result shows that introduction of WS2 promoted rGO reduction. Because the reduction can lead to the enhancement conductivity of rGO sheet, it is more easily transfer the excited electrons and reduce the recombination of electron-hole pairs, thus improving the photodegradation efficiency of rGMZ composites [29,30]. As is known to all, impedance can illuminate the conductivity of material because of its inversely proportional to conductivity [31]. The smaller impedance, the better the conductivity of the material. EIS reveal the impedance of rGWMZs with different WS2 /rGO wt.% in comparison to that of rGMZ (shown in Fig. 9 (a)). It is obvious that the reaction resistance of rGWMZs is smaller than that of rGMZ, indicating that the conductivity of rGMZ is improved by inserting WS2 nanosheets into rGO sheet, which are attributed to the reduction of rGO and the excellent electrical properties of WS2 [32,33]. Moreover, the charge transferring in rGWMZs suffers less impedance than that in rGMZ composites during diffusion and reaction. In addition, we also explored the effect of annealed temperature on the impedance of rGWMZ20.8 shown in Fig. 9(b). We observe that the rGWMZ20.8 calcined at 500 ◦ C presents the smallest impedance than the samples at 400 ◦ C and 600 ◦ C. Fig. 10a shows the CV curves of samples with different WS2 /rGO wt.%. It is very evident that the rGWMZ20.8 owns the biggest capacitance than others. Fig. 10b displays the effect of annealed temperature on specific capacitance of rGWMZ20.8. We observe that the rGWMZ20.8 calcined at 500 ◦ C presents the biggest capacitance than the samples at 400 ◦ C and 600 ◦ C. The result manifest that introducing WS2 nanosheets improve the specific capacitance of rGMZ composites. Because larger capacitance illustrates

a strong charge storage ability, introducting WS2 nanosheets can more effective store photoproduction electron, leading to accelerating the separatation of electron-hole pairs. Furthermore, the I–V curves show similarly rectangular-shaped, and no pronounced reduction and oxidation peak, indicating that the charge-discharge of samples is in the form of electric double-layer mode and highly reversible [34]. Based on the mechanism of the formation of electric double layer capacitors, the specific capacitance of electric double layer is proportional to the surface area of the electrode. Hence, we think that the enhancement in charge storage come of the bigger specific surface area and good electronic storage capability of WS2 nanosheets. In order to evaluate the light absorbance of samples, UV–vis diffuse reflection spectra were tested. Fig. 11a shows the UV–vis diffuse reflection spectra of rGWMZ composites with various WS2 /rGO wt.%. Apparently, both visible-light and infrared-light absorption of rGWMZ is higher than that of rGMZ, which illustrate introducing WS2 can improve light absorption efficiency of rGWMZ. In addition, infrared-light absorption of rGWMZ is higher than that of ZnO. The UV–vis diffuse reflection spectra of rGWMZ samples calcined at different temperature are shown in Fig. 11b. The rGWMZ20.8 annealed at 500 ◦ C is superior than 400 ◦ C and 600 ◦ C for visible and infrared light absorption performance, which manifest the 500 ◦ C treatment temperature is good for light absorption ability of rGWMZ composites. Fig. 12 shows the fluorescence spectra of different samples at the room temperature, obtained at the excitation wavelength of 320 nm. As shown in Fig. 12a, the rGWMZ with different WS2 /rGO wt.% show two typical emission band. The UV band at around 390 nm is corresponding to the near band edge emission of ZnO due to exciton recombination by an exciton collision process. The green emission band at about 500 nm is attributed to the singly ionized oxygen vacancy in the ZnO nanostructures [35]. The spectra of different samples seem to be similar to each other, meaning that introduced WS2 do not alter the structure of rGMZ samples. However, the WS2 effect the intensity of emission peaks. The rGWMZ31.2 and rGWMZ20.8 show stronger green emission peaks than others. Fig. 12b shows the fluorescence spectra of rGWMZ samples calcined at different temperature. The rGWMZ20.8 annealed at 500 ◦ C show stronger FL peak intensity than those samples annealed at 400 ◦ C and 600 ◦ C. Normally, weaker fluorescence peak represents lower recombination rate of the electron-hole pairs. Nevertheless, there are many other factors influencing the intensity of fluorescence peak, like as quantum yield, surface active point, structure defects and so on [36,37]. So, the quantum yield of fluorescence is tested, and the corresponding results are shown in Table 2. We observe

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Fig. 10. CV curves of rGWMZ samples: (a) t different mass ratio of WS2 to rGO, (b) annealed temperature on rGWMZ20.8.

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(b) Wavelength (nm)

Fig. 12. The FL spectra of rGWMZ sample: (a) different mass ratio of WS2 to rGO,(b) annealed at different temperature.

that all rGWMZ composites are superior to rGMZ composites and rGWMZ31.2 possess highest quantum yield. Meanwhile, the quantum yield of rGWMZ20.8 samples calcined at 500 ◦ C is higher than those calcined at 400 ◦ C and 600 ◦ C. These results demonstrate that inserting WS2 nanosheets can improve the quantum yield of rGMZ composites, facilitating to forming abundant photoexcitation electron under visible light, meanwhile increasing the recombination rate of the electron-hole pairs. Therefore, WS2 nanosheets can enhance the intensity of green emission peaks.

In addition, the structure defects of rGMZ and rGWMZ20.8 are tested by ESR (Fig. 13). In the ESR spectra, there are four strong characteristic peaks of DMPO−·OH adducts are observed at about 321.9, 323.4, 324.9 and 326.4 mT, respectively. This result indicates that hydroxyl radicals are generated by both rGMZ and rGWMZ20.8 under light radiation. In light of formula (g = 0.07145 × /H,  is 9071.621 MHz), the g values of four resonance signal are 2.014, 2.004, 1.995 and 1.986, respectively, which are assigned to VZn − defect, (VZn − )2 - defect and Vo + defect of ZnO structure [38]. Obvi-

W. Yu et al. / Applied Surface Science 400 (2017) 129–138

137

Table 2 The quantum yield of rGWMZ samples. Sample

quantum yield

Sample

quantum yield

rGMZ rGWMZ10.4 rGWMZ20.8 rGWMZ31.2 rGWMZ41.6

1.8% 2.4% 3.2% 3.8% 2.9%

rGWMZ20.8 400 ◦ C rGWMZ20.8 500 ◦ C rGWMZ20.8 600 ◦ C

2.2% 3.2% 2.2%

rGMZ rGWMZ

Intensity(a.u.)

600 400

Fig. 14. The complete scheme of electron transfer in rGWMZ composites and photocatalytic degradation mechanism.

4. Conclusions

200 0 -200 -400 -600 320

322

324

326

328

We have demonstrated a facile method to prepare the 3D reduced graphene oxide/WS2 nanosheets/Mg-doped ZnO composites with high photocatalytic activity and stability. rGO and WS2 sheets can assemble into 3D rGO/WS2 carriers by electrostatic interaction. WS2 nanosheets can significantly improve the photocatalytic activity of rGMZ composites because of the synergistic effect of rGO and WS2 nanosheets, which the photocatalytic efficiency can increase two times. The enhancement is attributed to increasing the specific surface area, and heightening the electronic conductivity and the charge storage ability.

Magnetic Field (mT) Acknowledgements Fig. 13. ESR spectra of rGMZ and rGWMZ20.8.

ous, signals of the formed hydroxyl radical species in rGWMZ are stronger than those in rGMZ, elucidating that the rGWMZ sample owns more defects than rGMZ. As discussed above, it is concluded that the WS2 nanosheets can improve the photocatalytic activity of rGMZ composites significantly, and the enhancement is attributed to the following points: (1) the reunion of graphene oxide is inhibited by WS2 nanosheets, which increase the specific surface area of rGMZ composites. (2) WS2 nanosheets can facilitate the reduction of rGO, heighten the electrical conductivity. Furthermore, the synergistic effect between rGO and WS2 can provide a highway of electronic [39], improving the separation efficiency of electrons-holes. (3) the narrow bandgap of WS2 can enhance absorption ability for visible light, and the excellent capacitance can storage the photogenerated electrons, effectively preventing the electron-hole pairs combining. In the process of photocatalytic degradation, the RhB dispersed in the solution are firstly adsorbed onto the surface of catalysts. Under light irradiation, the electrons (e− ) are excited from the valence band to the conduction band of Mg-ZnO, leaving holes (h+ ) in the VB. 3D rGO/WS2 have high electrical conductivity and large electron-storage capacity, and it is easy to capture photogenerated electrons (e− ) from the CB of ZnO and quickly migrate photogenerated holes (h+ ) to the surface of rGWMZ composites, efficient separating and hindering the recombination of electronhole pairs. The h+ on the surface of catalysts can directly oxidize the adsorbed RhB into CO2 and H2 O or react with adsorbed water to form hydroxyl radical (·OH), which is available to promote the decomposition of RhB. Furthermore, e− can oxidize the oxygen adsorbed on the surface of ZnO also promote the formation of ·OH, resulting in degrading the RhB. The process of electron transport in rGWMZ composites and photocatalytic degradation are graphical represented in Fig. 14.

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