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Preparation of CdWO4-deposited reduced graphene oxide and its enhanced photocatalytic properties Jingjing Xu,* Mindong Chen and Zhengmei Wang Reduced graphene oxide/CdWO4 (RGO-CdWO4) composite photocatalysts were prepared by a simple one-pot hydrothermal method. Namely, the reduction of graphene oxide and the growth of CdWO4 crystal occurred simultaneously in one single process. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, nitrogen adsorption, UV-vis reflection spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The photocatalytic activities of the as-prepared
Received 5th August 2013, Accepted 20th November 2013 DOI: 10.1039/c3dt52120f www.rsc.org/dalton
1.
samples were investigated by degradation of methylene blue under UV light irradiation. An enhancement in photocatalytic activity was observed with RGO-CdWO4 composites compared with pure CdWO4. We also investigated the effect of the amount of graphene on the photocatalytic activity of the as-prepared composite photocatalysts. The results showed that there was an optimal amount of 2%. The mechanism of enhancement of the photocatalytic activity was also discussed.
Introduction
Heterogeneous photocatalysis based on semiconductors is an example of advanced oxidation technologies. It has received extensive attention in the field of degradation of organic compounds from industry and households, especially in the last two decades.1–7 TiO2 has been widely studied in photocatalytic processes since the discovery of the photocatalytic splitting of water on titania electrodes.8 Nevertheless, it featured both an inherent slow reaction rate and poor solar efficiency.8 It has been found that metal tungstates possess special physicochemical properties due to their interesting self-trapped excitons.9 It was reported that Bi2WO6 was a good photocatalyst for the degradation of contaminants under visible light.10–12 Zhu and Fu et al. found ZnWO4 was an efficient catalyst for dye degradation.13,14 Cadmium tungstate (CdWO4) with a monoclinic wolframite structure is a popular functional material because of its high average refractive index, low radiation damage and excellent X-ray absorption coefficient. Ye et al. found that CdWO4 exhibits photocatalytic activity comparable to the known semiconductors ZnWO4 and TiO2 under UV light irradiation.15 CdWO4 nanorods and nanowires could be synthesized by a surfactantfree hydrothermal route16 and the starting materials played a key role in the synthesis process. CdWO4 has two polymorphs: Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Sciences and Engineering, Nanjing University of Information Sciences and Technology, Nanjing 210044, China. E-mail: [email protected]; Fax: +86 25 58731090; Tel: +86 25 58731090
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monoclinic wolframite and tetragonal scheelite structure. Wang reported that monoclinic CdWO4 exhibited a much higher photocatalytic activity than the tetragonal structure for degradation of methyl orange (MO).8 The photocatalytic activity of Eu3+-doped CdWO4 was higher than the undoped sample for decomposition of MO solution.15 The rate of photocatalytic H2 evolution over CdS-modified CdWO4 was 34 times higher than for pure CdWO4.9 Recently, materials with a conjugative π structure were investigated, and were proved to be effective for enhancement of photocatalytic activity. These materials with conjugative π structure included C60, polyaniline, graphene and so on.17 Owing to a unique sp2 hybrid carbon network, graphene had a very high thermal conductivity, presented excellent mobility of charge carriers and showed large specific surface area.18 Thus, graphene was suitable for use as an adsorbent and catalyst support to enhance the transfer and separation of photogenerated electrons and holes. There has been much work focused on the preparation and activity of graphene semiconductors in recent years.19–24 However, a recent study by Li et al. has revealed that the effect of graphene on the photocatalytic performance depends on its reduction degree.25 It would be helpful for the achievement of enhanced photocatalytic activity if CdWO4 was combined with graphene, whose reduction degree was controlled. In this paper, a facile one-step solvothermal route was developed to synthesize reduced graphene oxide (RGO) hybridized CdWO4 nanorods. In the preparation, the reduction of graphene oxide and the growth of CdWO4 crystal occurred simultaneously in one single process. The photocatalytic activities
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of as-prepared samples were investigated by degradation of methylene blue under UV light irradiation. Furthermore, the synergistic effect between CdWO4 and RGO was also discussed, and the results showed that samples with 2% graphene exhibited the highest photocatalytic performance.
2. Experimental 2.1
Sample preparation
The graphite oxide (GO) was prepared by a modified Hummers method as reported in other papers.26–28 The RGO-CdWO4 composite photocatalysts were prepared by a one-pot hydrothermal method as follows: first of all, a defined amount of GO was added to 40 mL ultrapure water and then it was sonicated for 1 h. Then, 0.44 g Cd(CH3COO)2·2H2O was added into the above solution and stirred for 1 h. Separately, 0.66 g Na2WO4·2H2O was dissolved in 40 mL ultrapure water. Afterwards, the latter solution was added to the former one under vigorous stirring. Then the admixture was stirred for another 2 h. Finally, the suspension was sealed into a Teflon-lined autoclave and maintained at 160 °C for 24 h. Subsequently, the autoclave was cooled to room temperature naturally. The resulting samples were washed with ultrapure water and ethanol several times and dried at 60 °C under vacuum. In our experiment, four samples with initial GO content of 0.5%, 1.0%, 2% and 5% were prepared. The four samples were defined as RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3 and RGO-CdWO4-4, respectively. For comparison, pure CdWO4 was also prepared by the same method without the addition of GO. 2.2
Characterization
The phase structure properties were determined by X-ray diffractometery (XD-3A, Shimadazu Corporation, Japan) using graphite monochromatic copper radiation (Cu-Kα) at 40 kV, 30 mA over the 2θ range 10–80°. The morphologies were characterized by scanning electron microscopy (SEM, Hitachi, S-4800). UV-Vis diffuse reflectance spectra (DRS) of the obtained samples were obtained by a UV-vis spectrophotometer (Shimadzu, UV3600). The Brunauer–Emmett–Teller (BET) surface areas of the samples were measured by nitrogen adsorption–desorption on a Micromeritics ASAP 2010 apparatus. Before the nitrogen adsorption, the samples were degassed at 473 K under vacuum. Raman spectra were recorded on a microscopic Raman spectrometer (Renishaw 1000 NR). The photoluminescence spectra were measured by a fluorescence spectrophotometer (Hitachi, F-7000). The X-ray photoelectron spectroscopy (XPS) measurement was carried out on VG ESCALAB MK-II electron spectrometer. The concentration of methylene blue (MB) in the irradiation process was also analyzed using a UV-vis spectrophotometer (Shimadzu, UV3600). 2.3
Photocatalytic experiment
In order to investigate the photocatalytic activity of the asprepared RGO-CdWO4 samples, degradation experiments of MB were performed under UV light. 0.1 g of sample was
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dispersed into a 200 mL MB solution (MB concentration: 5 mg L−1) and then irradiated by a mercury lamp under continuous stirring. Before the irradiation, the suspension was maintained in the dark for 1 h to reach complete adsorption–desorption equilibrium. The blank experiment without catalyst was also investigated, and the value can be neglected with less than 1% degradation after 90 min illumination. The formation of hydroxyl radicals (•OH) in the photocatalytic process was detected by photoluminescence (PL) spectroscopy using terephthalic acid as a probe molecule. The terephthalic acid can react with •OH to produce highly fluorescent 2-hydroxyterephthalic acid, which shows an absorption peak at 426 nm.29 The PL intensity of 2-hydroxyterephthalic acid is proportional to the amount of •OH produced in the photocatalytic process. The process was similar to that of photocatalytic degradation of MB, with MB solution replaced by terephthalic acid–NaOH solution. 2.4
Photoelectrochemical measurements
Photocurrents were measured to investigate the charge separation efficiency of the as-prepared samples. The experiment was carried out on an electrochemical analyzer (CHI660D, Shanghai Chenhua) in a standard three-electrode system. The working electrodes had an active area of about 1 cm2, which was prepared by the following procedure: 0.1 g of the asprepared samples were added to 1 mL ethanol and ultrasonicated for 5 min, then the slurry was coated onto an F-doped SnO2-coated glass by the doctor blade method. Afterwards, the electrodes were calcined at 300 °C for 30 min. The counter electrode and reference electrode were a Pt plate and Ag/AgCl (saturating KCl), respectively.
3. Results and discussion 3.1
Characterization of samples
Fig. 1 shows the typical X-ray diffraction (XRD) patterns of the as-prepared pure CdWO4 and RGO/CdWO4 composite (RGO-CdWO4-3). It can be seen that the samples can all be indexed to a pure monoclinic phase of well-crystallized CdWO4 with a wolframite structure (JCPDS NO. 14-0676). The distinctive peaks center at 23.3, 29.0, 29.6, 30.5, 35.4, 35.7 and 47.6° match well with (110), (−111), (111), (020), (002), (200) and (220) crystal planes of CdWO4, which is consistent with the results of Ye et al.30 The results also show that the addition of RGO has no distinct effect on the crystal structure of CdWO4. Fig. 2 shows the morphology of the different samples under a field-emission SEM microscope. From Fig. 2(a), we can see the pure CdWO4 are nanorods. From the images of the composite samples (Fig. 2(b)–(e)), we can see that CdWO4 nanorods are deposited on the surface of RGO sheets. The samples show some wrinkles both on the edge of the RGO sheets and on the interlayer sheets. The layered structure of the graphene sheets in the RGO-CdWO4 samples would provide more active points for the adsorption of dyes and help to enhance the photocatalytic activity of CdWO4. This intimate interaction of
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Fig. 1
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Typical XRD patterns of CdWO4 and RGO-CdWO4-3.
Fig. 3
Fig. 2 SEM images of different samples (a) CdWO4, (b) RGO-CdWO4-1, (c) RGO-CdWO4-2, (d) RGO-CdWO4-3, (e) RGO-CdWO4-4, (f ) enlarged image of RGO-CdWO4-4 (the arrows indicate the typical wrinkles of the graphene).
graphene and CdWO4 enables the electron transfer from photo-excited CdWO4 to graphene during the photocatalysis process. Furthermore, we can see that the exposed graphene sheets are enhanced as the initial GO amount increases. Fig. 3 shows typical TEM images of RGO-CdWO4-3 with different magnifications. It can be seen that the CdWO4 nanorods are distributed on the surface of the reduced graphene oxide. Furthermore, it can be seen that the reduced graphene oxide are less than 10 layers. From the high resolution TEM image of a single CdWO4 nanorod, we can see that the
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TEM images of RGO-CdWO4-3 with different magnifications.
interlayer spacing is about 0.3 nm, which corresponds to the (111) plane of CdWO4.30 The optical properties of the as-prepared RGO-CdWO4 samples were investigated by UV-vis diffuse reflection spectroscopy, and the obtained DRS results are shown in Fig. 4. From the DRS data we can see that the RGO-CdWO4 samples show similar absorption and similar band gap energy to pure CdWO4. However, the RGO-CdWO4 samples all show absorption in the visible light region. Furthermore, we find that the absorption intensity of as-prepared hybrid samples is enhanced with the increase of the RGO amount. This increase of absorption intensity can be ascribed to the addition of black graphite-like material. It well known that the optical absorption band gap Eg can be deduced by the following equation for a semiconductor: (αhν)n = A(hν − Eg), where α is the absorption coefficient, hν is the incident photo energy, A is a proportionality constant relative to the materials and Eg is the band gap energy of the semiconductor. The index n can be set as 2 or 1/2 depending on the type of electronic transition of a given semiconductor. In the present work, the index n equals 1/2 because CdWO4 is an indirect gap semiconductor. Thus, the Eg can be determined by extrapolation of the linear portion of the (αhν)n curve versus the photon energy hν to (αhν)n =
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Fig. 4
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(a) UV-visible DRS of different samples, (b) energy dependence of (αhν)1/2.
0. The results are also shown in Fig. 4(b). It can be seen that the composite samples all exhibit a slight red-shift compared to pure CdWO4. The obtained band gap values are 3.65, 3.50, 3.44, 3.37 and 3.26 eV for pure CdWO4, RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3, RGO-CdWO4-4, respectively. Raman spectroscopy is a very effective tool for the characterization of the structures of different materials. The obtained Raman spectra of different RGO-CdWO4 samples are shown in Fig. 5. From the plot of pure CdWO4, we can see one strong peak at about 895 cm−1, and several peaks at about 227, 267, 304, 348, 385, 514, 544, 687 and 771 cm−1. All these peaks are characteristic of the monoclinic structure, which is in good agreement with Yan et al.8 The results are in good agreement with the XRD data. Furthermore, the Raman spectra all display two prominent peaks at around 1590 and 1320 cm−1, which correspond to disordered sp2 carbon (D-band) and well ordered graphite carbon (G-band).31 We found an increase of D/G intensity ratio for all RGO-CdWO4 samples compared to that of GO. The data of D/G are 1.18, 1.30, 1.36, 1.40 and 1.32 for GO, RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3 and RGO-CdWO4-4, respectively. The increasing intensity ratio can be ascribed to the following reasons: the oxygen functional groups in GO sheets have been reduced in the preparation process and the conjugated graphene network (sp2 carbon) are re-established.
Fig. 6(a) displays the typical nitrogen adsorption–desorption isotherms of CdWO4 and RGO-CdWO4-4. The nitrogen sorption isotherm of RGO-CdWO4-4 is similar to that of CdWO4 and belongs to type IV with a clear hysteresis loop, indicating the formation of mesoporous structures. The obtained BET specific surface areas for CdWO4, RGO-CdWO41, RGO-CdWO4-2, RGO-CdWO4-3, RGO-CdWO4-4 are 15.14, 17.28, 19.95, 23.49 and 31.95 m2 g−1, respectively. It can be seen that the BET specific area increases slightly with increasing the initial GO amount. Furthermore, the pore diameter distributions of CdWO4 and RGO-CdWO4-4 are also shown in Fig. 6(b). It can be seen that the pure CdWO4 and RGO-CdWO4 composites have formed mesoporous structures. The obtained average pore diameters are 12.30, 30.20, 27.48, 25.56, 20.05 nm for CdWO4, RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3, RGO-CdWO4-4, respectively. The mesoporous structures would help the adsorption and transition of MB or its intermediates during the photocatalytic process. The chemical states of the typical composite sample RGO-CdWO4-3 was investigated by XPS. The results show that the sample contains Cd, W, O and C elements. The high resolution spectrum of C 1s for RGO-CdWO4-3 is shown in Fig. 7. It can be seen that the spectrum can be devolved into three peaks at 284.6, 286.8 and 288.0 eV. The peak located at 284.6 eV is usually assigned to adventitious carbon and graphitic carbon,32 while the other peaks are assigned to C–O and CvO, respectively.33 The results are similar to that of pure GO (data not shown here). However, the intensity of the peaks decrease remarkably, which indicates that most of the C–O and CvO groups are removed. It also indicates that the GO can be reduced to RGO in our experimental condition. The results are also in good agreement with the above analysis. 3.2
Fig. 5
Raman spectra of different samples.
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Photocatalytic properties
The photocatalytic activities of the as-prepared samples were investigated under UV light irradiation for the degradation of MB, and the corresponding results are shown in Fig. 8. The blank experiment without catalyst was also investigated and the value can be neglected with less than 2% conversion after 2 hours illumination. The adsorption data of MB by different samples are shown in the inset of Fig. 8(a). It can be seen that
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Fig. 6
(a) Typical nitrogen adsorption–desorption isotherms, and (b) pore diameter distribution (left) of CdWO4 and RGO-CdWO4-4.
Fig. 7
High-resolution XPS spectrum of C1s for RGO-CdWO4-3.
the complete adsorption–desorption equilibrium can be obtained in 30 min for all samples. Therefore, 1 h is enough to reach adsorption–desorption equilibrium in the experiment. Fig. 8(a) shows the variation of MB concentration against the irradiation time in the presence of different catalyst samples. From the figure, we can see that the concentration of MB decreases gradually as the exposure time increases for all samples. It can also be seen that all the RGO-CdWO4 samples show higher percentage degradation of MB than that of pure CdWO4. In addition, we investigated the kinetic process of MB degradation. It illustrates that photocatalytic degradation of MB as a function of the irradiation time in the presence of different samples follows an apparent first-order kinetic reaction. The apparent rate constant can be chosen as the basic kinetic parameter for the comparison of different photocatalysts’ photocatalytic activity.34,35 The obtained data are given in Fig. 8(b). From the figure, the values of apparent rate constant of different samples are 0.012, 0.014, 0.018, 0.023 and 0.013 min−1 for CdWO4, RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3 and RGO-CdWO4-4, respectively. It can be seen that all RGO-CdWO4 samples show enhanced activity compared to pure CdWO4. The enhancement can be ascribed to the introduction of RGO into the composite samples, which can induce higher adsorption activity for MB and higher
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Fig. 8 (a) Variation of residual MB concentration against irradiation time in the presence of different samples (the inset shows the adsorption of MB by different samples), (b) apparent rate constant for MB degradation by different samples.
migration efficiency of photo-generated electrons. However, there is an optimal content of RGO (2%) for obtaining the highest photocatalytic activity. 3.3
Photocatalytic mechanism
It is well known that there are several factors contributing to the photocatalytic properties of semiconductors: crystal structure, adsorption activity, charge separation efficiency, and so on. To understand the mechanism of the enhancement of the photocatalytic properties of the RGO-CdWO4 samples, we
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performed several series of experiments, which will be shown and discussed in detail in the following section. First of all, in order to ascertain the active species in the degradation process of MB in this experiment, tert-butanol (t-BuOH) and benzoquinone (BQ) were introduced into the photocatalytic system. In these experiments, t-BuOH served as a hydroxyl radical scavenger, BQ served as a O2−• radical scavenger, and the results are shown in Fig. 9. It can be seen that when the hydroxyl radical scavenger (t-BuOH) was added, an inactivation of the RGO-CdWO4 composite photocatalyst was observed with nearly no degradation of MB. When the O2−• radical scavenger (BQ) was added to the reaction system, no obvious effect on the photocatalytic activity of RGO-CdWO4 was observed. The above results illustrate that hydroxyl radicals contribute mostly to the high photocatalytic performance of RGO-CdWO4 composite photocatalyst. The formation of •OH upon UV irradiation was also measured, which was the major active species in the MB degradation by RGO-CdWO4 samples. The •OH can react with terephthalic acid (TA) to form 2-hydroxyterephthalic acid (TAOH), which can be measured at 426 nm by a fluorescence spectrophotometer. Therefore, the fluorescence intensity of TAOH indicates the amount of •OH, as shown in Fig. 10. In Fig. 10(a),
Fig. 9 The MB concentration during photodegradation as a function of irradiation time with the addition of different scavengers (benzoquinone and tert-butanol). The used sample is RGO-CdWO4-3.
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the results show the fluorescence intensity increased with the irradiation time. Fig. 10(b) shows the plot of PL intensity at 426 nm against irradiation time. It can be seen that the formation rates of •OH radicals with the RGO-CdWO4 samples are all greater than that of pure CdWO4. The results are in good agreement with the photocatalytic degradation results of MB by the different samples. These results also indicate that the deposition of CdWO4 onto RGO promotes the formation of • OH radicals. A possible explanation is that the RGO can act as an electron shuttle, which can effectively lead to efficient separation of the photo-generated electron–hole pairs thus decreasing their recombination rate. To further prove the above explanation, the transient photocurrent responses were recorded for photoelectrodes prepared with different samples ( pure CdWO4 and RGO-CdWO4), and the results are shown in Fig. 11. From the figure, we can see that the photocurrent rapidly increases to a certain value when the light is turned on, and the photocurrent comes back to nearly zero again. Furthermore, we can see that all RGO-CdWO4 hybrids exhibit a much higher photocurrent density than pure CdWO4. The enhanced photoresponse results from the introduction of RGO into the hybrids. RGO can accept the photoinduced electrons from CdWO4 and transfer them to the external circuit quickly, since the RGO has an extensive two-dimensional π–π
Fig. 11 Photocurrent transient responses of different samples under UV light irradiation.
Fig. 10 (a) The fluorescence intensity of TAOH for RGO-CdWO4-3 at different time and (b) time dependence of the fluorescence intensity at 426 nm in the presence of different samples.
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Acknowledgements We are grateful for grants from the National Natural Science Foundation of China (no. 51202114), Natural Science Foundation of Jiangsu province (BK2012464) and University Foundation of Jiangsu province (no. 11KJB610004). A project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
References
Fig. 12 Schematic presentation of the photocatalytic degradation of dye MB.
conjugation structure. Similarly, in the suspension system of RGO-CdWO4 and MB aqueous solution, the RGO can attract the photo-generated electrons and thus help to reduce the recombination rate of photo-generated charges. Therefore, more holes or •OH can take part in the photocatalytic degradation of MB. Furthermore, the electrons can also react with O2 molecules to produce •O2−, which can react with H2O to produce •OH. Both •O2−, holes and •OH can induce the degradation of dye MB molecules. The detailed process is shown in Fig. 12. However, there is an optimal RGO amount to obtain the highest photocatalytic activity. The value is 2% in the present work. The photocatalytic activity started to decrease when the RGO exceeded 2%. It is well known that graphene is a dark material which can absorb incident light. Therefore, superfluous graphene would reduce the absorption efficiency of the incident light by CdWO4 nanorods. The deleterious effect counteracts the activity enhancement by the enhanced adsorption activity and enhanced migration efficiency of photo-induced electrons.
4.
Conclusions
We reported here a study on the preparation and photocatalytic activity of a new hybrid photocatalyst: CdWO4-deposited RGO. The results showed that the reduction of GO and the growth of CdWO4 crystal occurred simultaneously in one single process. The photocatalysis experiment showed that such hybrid photocatalysts show much higher photocatalytic activity than pure CdWO4 for MB degradation. The effect of RGO amount on the photocatalytic activity of the as-prepared hybrid photocatalysts was also investigated. The results showed that there was an optimal amount of 2%. This study will motivate new developments in photocatalytic technology and promote their practical application in environmental problems.
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Preparation of CdWO4-deposited reduced graphene oxide and its enhanced photocatalytic properties Jingjing Xu,* Mindong Chen and Zhengmei Wang Reduced graphene oxide/CdWO4 (RGO-CdWO4) composite photocatalysts were prepared by a simple one-pot hydrothermal method. Namely, the reduction of graphene oxide and the growth of CdWO4 crystal occurred simultaneously in one single process. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, nitrogen adsorption, UV-vis reflection spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The photocatalytic activities of the as-prepared
Received 5th August 2013, Accepted 20th November 2013 DOI: 10.1039/c3dt52120f www.rsc.org/dalton
1.
samples were investigated by degradation of methylene blue under UV light irradiation. An enhancement in photocatalytic activity was observed with RGO-CdWO4 composites compared with pure CdWO4. We also investigated the effect of the amount of graphene on the photocatalytic activity of the as-prepared composite photocatalysts. The results showed that there was an optimal amount of 2%. The mechanism of enhancement of the photocatalytic activity was also discussed.
Introduction
Heterogeneous photocatalysis based on semiconductors is an example of advanced oxidation technologies. It has received extensive attention in the field of degradation of organic compounds from industry and households, especially in the last two decades.1–7 TiO2 has been widely studied in photocatalytic processes since the discovery of the photocatalytic splitting of water on titania electrodes.8 Nevertheless, it featured both an inherent slow reaction rate and poor solar efficiency.8 It has been found that metal tungstates possess special physicochemical properties due to their interesting self-trapped excitons.9 It was reported that Bi2WO6 was a good photocatalyst for the degradation of contaminants under visible light.10–12 Zhu and Fu et al. found ZnWO4 was an efficient catalyst for dye degradation.13,14 Cadmium tungstate (CdWO4) with a monoclinic wolframite structure is a popular functional material because of its high average refractive index, low radiation damage and excellent X-ray absorption coefficient. Ye et al. found that CdWO4 exhibits photocatalytic activity comparable to the known semiconductors ZnWO4 and TiO2 under UV light irradiation.15 CdWO4 nanorods and nanowires could be synthesized by a surfactantfree hydrothermal route16 and the starting materials played a key role in the synthesis process. CdWO4 has two polymorphs: Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Sciences and Engineering, Nanjing University of Information Sciences and Technology, Nanjing 210044, China. E-mail: [email protected]; Fax: +86 25 58731090; Tel: +86 25 58731090
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monoclinic wolframite and tetragonal scheelite structure. Wang reported that monoclinic CdWO4 exhibited a much higher photocatalytic activity than the tetragonal structure for degradation of methyl orange (MO).8 The photocatalytic activity of Eu3+-doped CdWO4 was higher than the undoped sample for decomposition of MO solution.15 The rate of photocatalytic H2 evolution over CdS-modified CdWO4 was 34 times higher than for pure CdWO4.9 Recently, materials with a conjugative π structure were investigated, and were proved to be effective for enhancement of photocatalytic activity. These materials with conjugative π structure included C60, polyaniline, graphene and so on.17 Owing to a unique sp2 hybrid carbon network, graphene had a very high thermal conductivity, presented excellent mobility of charge carriers and showed large specific surface area.18 Thus, graphene was suitable for use as an adsorbent and catalyst support to enhance the transfer and separation of photogenerated electrons and holes. There has been much work focused on the preparation and activity of graphene semiconductors in recent years.19–24 However, a recent study by Li et al. has revealed that the effect of graphene on the photocatalytic performance depends on its reduction degree.25 It would be helpful for the achievement of enhanced photocatalytic activity if CdWO4 was combined with graphene, whose reduction degree was controlled. In this paper, a facile one-step solvothermal route was developed to synthesize reduced graphene oxide (RGO) hybridized CdWO4 nanorods. In the preparation, the reduction of graphene oxide and the growth of CdWO4 crystal occurred simultaneously in one single process. The photocatalytic activities
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of as-prepared samples were investigated by degradation of methylene blue under UV light irradiation. Furthermore, the synergistic effect between CdWO4 and RGO was also discussed, and the results showed that samples with 2% graphene exhibited the highest photocatalytic performance.
2. Experimental 2.1
Sample preparation
The graphite oxide (GO) was prepared by a modified Hummers method as reported in other papers.26–28 The RGO-CdWO4 composite photocatalysts were prepared by a one-pot hydrothermal method as follows: first of all, a defined amount of GO was added to 40 mL ultrapure water and then it was sonicated for 1 h. Then, 0.44 g Cd(CH3COO)2·2H2O was added into the above solution and stirred for 1 h. Separately, 0.66 g Na2WO4·2H2O was dissolved in 40 mL ultrapure water. Afterwards, the latter solution was added to the former one under vigorous stirring. Then the admixture was stirred for another 2 h. Finally, the suspension was sealed into a Teflon-lined autoclave and maintained at 160 °C for 24 h. Subsequently, the autoclave was cooled to room temperature naturally. The resulting samples were washed with ultrapure water and ethanol several times and dried at 60 °C under vacuum. In our experiment, four samples with initial GO content of 0.5%, 1.0%, 2% and 5% were prepared. The four samples were defined as RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3 and RGO-CdWO4-4, respectively. For comparison, pure CdWO4 was also prepared by the same method without the addition of GO. 2.2
Characterization
The phase structure properties were determined by X-ray diffractometery (XD-3A, Shimadazu Corporation, Japan) using graphite monochromatic copper radiation (Cu-Kα) at 40 kV, 30 mA over the 2θ range 10–80°. The morphologies were characterized by scanning electron microscopy (SEM, Hitachi, S-4800). UV-Vis diffuse reflectance spectra (DRS) of the obtained samples were obtained by a UV-vis spectrophotometer (Shimadzu, UV3600). The Brunauer–Emmett–Teller (BET) surface areas of the samples were measured by nitrogen adsorption–desorption on a Micromeritics ASAP 2010 apparatus. Before the nitrogen adsorption, the samples were degassed at 473 K under vacuum. Raman spectra were recorded on a microscopic Raman spectrometer (Renishaw 1000 NR). The photoluminescence spectra were measured by a fluorescence spectrophotometer (Hitachi, F-7000). The X-ray photoelectron spectroscopy (XPS) measurement was carried out on VG ESCALAB MK-II electron spectrometer. The concentration of methylene blue (MB) in the irradiation process was also analyzed using a UV-vis spectrophotometer (Shimadzu, UV3600). 2.3
Photocatalytic experiment
In order to investigate the photocatalytic activity of the asprepared RGO-CdWO4 samples, degradation experiments of MB were performed under UV light. 0.1 g of sample was
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dispersed into a 200 mL MB solution (MB concentration: 5 mg L−1) and then irradiated by a mercury lamp under continuous stirring. Before the irradiation, the suspension was maintained in the dark for 1 h to reach complete adsorption–desorption equilibrium. The blank experiment without catalyst was also investigated, and the value can be neglected with less than 1% degradation after 90 min illumination. The formation of hydroxyl radicals (•OH) in the photocatalytic process was detected by photoluminescence (PL) spectroscopy using terephthalic acid as a probe molecule. The terephthalic acid can react with •OH to produce highly fluorescent 2-hydroxyterephthalic acid, which shows an absorption peak at 426 nm.29 The PL intensity of 2-hydroxyterephthalic acid is proportional to the amount of •OH produced in the photocatalytic process. The process was similar to that of photocatalytic degradation of MB, with MB solution replaced by terephthalic acid–NaOH solution. 2.4
Photoelectrochemical measurements
Photocurrents were measured to investigate the charge separation efficiency of the as-prepared samples. The experiment was carried out on an electrochemical analyzer (CHI660D, Shanghai Chenhua) in a standard three-electrode system. The working electrodes had an active area of about 1 cm2, which was prepared by the following procedure: 0.1 g of the asprepared samples were added to 1 mL ethanol and ultrasonicated for 5 min, then the slurry was coated onto an F-doped SnO2-coated glass by the doctor blade method. Afterwards, the electrodes were calcined at 300 °C for 30 min. The counter electrode and reference electrode were a Pt plate and Ag/AgCl (saturating KCl), respectively.
3. Results and discussion 3.1
Characterization of samples
Fig. 1 shows the typical X-ray diffraction (XRD) patterns of the as-prepared pure CdWO4 and RGO/CdWO4 composite (RGO-CdWO4-3). It can be seen that the samples can all be indexed to a pure monoclinic phase of well-crystallized CdWO4 with a wolframite structure (JCPDS NO. 14-0676). The distinctive peaks center at 23.3, 29.0, 29.6, 30.5, 35.4, 35.7 and 47.6° match well with (110), (−111), (111), (020), (002), (200) and (220) crystal planes of CdWO4, which is consistent with the results of Ye et al.30 The results also show that the addition of RGO has no distinct effect on the crystal structure of CdWO4. Fig. 2 shows the morphology of the different samples under a field-emission SEM microscope. From Fig. 2(a), we can see the pure CdWO4 are nanorods. From the images of the composite samples (Fig. 2(b)–(e)), we can see that CdWO4 nanorods are deposited on the surface of RGO sheets. The samples show some wrinkles both on the edge of the RGO sheets and on the interlayer sheets. The layered structure of the graphene sheets in the RGO-CdWO4 samples would provide more active points for the adsorption of dyes and help to enhance the photocatalytic activity of CdWO4. This intimate interaction of
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Fig. 1
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Typical XRD patterns of CdWO4 and RGO-CdWO4-3.
Fig. 3
Fig. 2 SEM images of different samples (a) CdWO4, (b) RGO-CdWO4-1, (c) RGO-CdWO4-2, (d) RGO-CdWO4-3, (e) RGO-CdWO4-4, (f ) enlarged image of RGO-CdWO4-4 (the arrows indicate the typical wrinkles of the graphene).
graphene and CdWO4 enables the electron transfer from photo-excited CdWO4 to graphene during the photocatalysis process. Furthermore, we can see that the exposed graphene sheets are enhanced as the initial GO amount increases. Fig. 3 shows typical TEM images of RGO-CdWO4-3 with different magnifications. It can be seen that the CdWO4 nanorods are distributed on the surface of the reduced graphene oxide. Furthermore, it can be seen that the reduced graphene oxide are less than 10 layers. From the high resolution TEM image of a single CdWO4 nanorod, we can see that the
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TEM images of RGO-CdWO4-3 with different magnifications.
interlayer spacing is about 0.3 nm, which corresponds to the (111) plane of CdWO4.30 The optical properties of the as-prepared RGO-CdWO4 samples were investigated by UV-vis diffuse reflection spectroscopy, and the obtained DRS results are shown in Fig. 4. From the DRS data we can see that the RGO-CdWO4 samples show similar absorption and similar band gap energy to pure CdWO4. However, the RGO-CdWO4 samples all show absorption in the visible light region. Furthermore, we find that the absorption intensity of as-prepared hybrid samples is enhanced with the increase of the RGO amount. This increase of absorption intensity can be ascribed to the addition of black graphite-like material. It well known that the optical absorption band gap Eg can be deduced by the following equation for a semiconductor: (αhν)n = A(hν − Eg), where α is the absorption coefficient, hν is the incident photo energy, A is a proportionality constant relative to the materials and Eg is the band gap energy of the semiconductor. The index n can be set as 2 or 1/2 depending on the type of electronic transition of a given semiconductor. In the present work, the index n equals 1/2 because CdWO4 is an indirect gap semiconductor. Thus, the Eg can be determined by extrapolation of the linear portion of the (αhν)n curve versus the photon energy hν to (αhν)n =
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Fig. 4
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(a) UV-visible DRS of different samples, (b) energy dependence of (αhν)1/2.
0. The results are also shown in Fig. 4(b). It can be seen that the composite samples all exhibit a slight red-shift compared to pure CdWO4. The obtained band gap values are 3.65, 3.50, 3.44, 3.37 and 3.26 eV for pure CdWO4, RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3, RGO-CdWO4-4, respectively. Raman spectroscopy is a very effective tool for the characterization of the structures of different materials. The obtained Raman spectra of different RGO-CdWO4 samples are shown in Fig. 5. From the plot of pure CdWO4, we can see one strong peak at about 895 cm−1, and several peaks at about 227, 267, 304, 348, 385, 514, 544, 687 and 771 cm−1. All these peaks are characteristic of the monoclinic structure, which is in good agreement with Yan et al.8 The results are in good agreement with the XRD data. Furthermore, the Raman spectra all display two prominent peaks at around 1590 and 1320 cm−1, which correspond to disordered sp2 carbon (D-band) and well ordered graphite carbon (G-band).31 We found an increase of D/G intensity ratio for all RGO-CdWO4 samples compared to that of GO. The data of D/G are 1.18, 1.30, 1.36, 1.40 and 1.32 for GO, RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3 and RGO-CdWO4-4, respectively. The increasing intensity ratio can be ascribed to the following reasons: the oxygen functional groups in GO sheets have been reduced in the preparation process and the conjugated graphene network (sp2 carbon) are re-established.
Fig. 6(a) displays the typical nitrogen adsorption–desorption isotherms of CdWO4 and RGO-CdWO4-4. The nitrogen sorption isotherm of RGO-CdWO4-4 is similar to that of CdWO4 and belongs to type IV with a clear hysteresis loop, indicating the formation of mesoporous structures. The obtained BET specific surface areas for CdWO4, RGO-CdWO41, RGO-CdWO4-2, RGO-CdWO4-3, RGO-CdWO4-4 are 15.14, 17.28, 19.95, 23.49 and 31.95 m2 g−1, respectively. It can be seen that the BET specific area increases slightly with increasing the initial GO amount. Furthermore, the pore diameter distributions of CdWO4 and RGO-CdWO4-4 are also shown in Fig. 6(b). It can be seen that the pure CdWO4 and RGO-CdWO4 composites have formed mesoporous structures. The obtained average pore diameters are 12.30, 30.20, 27.48, 25.56, 20.05 nm for CdWO4, RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3, RGO-CdWO4-4, respectively. The mesoporous structures would help the adsorption and transition of MB or its intermediates during the photocatalytic process. The chemical states of the typical composite sample RGO-CdWO4-3 was investigated by XPS. The results show that the sample contains Cd, W, O and C elements. The high resolution spectrum of C 1s for RGO-CdWO4-3 is shown in Fig. 7. It can be seen that the spectrum can be devolved into three peaks at 284.6, 286.8 and 288.0 eV. The peak located at 284.6 eV is usually assigned to adventitious carbon and graphitic carbon,32 while the other peaks are assigned to C–O and CvO, respectively.33 The results are similar to that of pure GO (data not shown here). However, the intensity of the peaks decrease remarkably, which indicates that most of the C–O and CvO groups are removed. It also indicates that the GO can be reduced to RGO in our experimental condition. The results are also in good agreement with the above analysis. 3.2
Fig. 5
Raman spectra of different samples.
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Photocatalytic properties
The photocatalytic activities of the as-prepared samples were investigated under UV light irradiation for the degradation of MB, and the corresponding results are shown in Fig. 8. The blank experiment without catalyst was also investigated and the value can be neglected with less than 2% conversion after 2 hours illumination. The adsorption data of MB by different samples are shown in the inset of Fig. 8(a). It can be seen that
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Fig. 6
(a) Typical nitrogen adsorption–desorption isotherms, and (b) pore diameter distribution (left) of CdWO4 and RGO-CdWO4-4.
Fig. 7
High-resolution XPS spectrum of C1s for RGO-CdWO4-3.
the complete adsorption–desorption equilibrium can be obtained in 30 min for all samples. Therefore, 1 h is enough to reach adsorption–desorption equilibrium in the experiment. Fig. 8(a) shows the variation of MB concentration against the irradiation time in the presence of different catalyst samples. From the figure, we can see that the concentration of MB decreases gradually as the exposure time increases for all samples. It can also be seen that all the RGO-CdWO4 samples show higher percentage degradation of MB than that of pure CdWO4. In addition, we investigated the kinetic process of MB degradation. It illustrates that photocatalytic degradation of MB as a function of the irradiation time in the presence of different samples follows an apparent first-order kinetic reaction. The apparent rate constant can be chosen as the basic kinetic parameter for the comparison of different photocatalysts’ photocatalytic activity.34,35 The obtained data are given in Fig. 8(b). From the figure, the values of apparent rate constant of different samples are 0.012, 0.014, 0.018, 0.023 and 0.013 min−1 for CdWO4, RGO-CdWO4-1, RGO-CdWO4-2, RGO-CdWO4-3 and RGO-CdWO4-4, respectively. It can be seen that all RGO-CdWO4 samples show enhanced activity compared to pure CdWO4. The enhancement can be ascribed to the introduction of RGO into the composite samples, which can induce higher adsorption activity for MB and higher
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Fig. 8 (a) Variation of residual MB concentration against irradiation time in the presence of different samples (the inset shows the adsorption of MB by different samples), (b) apparent rate constant for MB degradation by different samples.
migration efficiency of photo-generated electrons. However, there is an optimal content of RGO (2%) for obtaining the highest photocatalytic activity. 3.3
Photocatalytic mechanism
It is well known that there are several factors contributing to the photocatalytic properties of semiconductors: crystal structure, adsorption activity, charge separation efficiency, and so on. To understand the mechanism of the enhancement of the photocatalytic properties of the RGO-CdWO4 samples, we
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performed several series of experiments, which will be shown and discussed in detail in the following section. First of all, in order to ascertain the active species in the degradation process of MB in this experiment, tert-butanol (t-BuOH) and benzoquinone (BQ) were introduced into the photocatalytic system. In these experiments, t-BuOH served as a hydroxyl radical scavenger, BQ served as a O2−• radical scavenger, and the results are shown in Fig. 9. It can be seen that when the hydroxyl radical scavenger (t-BuOH) was added, an inactivation of the RGO-CdWO4 composite photocatalyst was observed with nearly no degradation of MB. When the O2−• radical scavenger (BQ) was added to the reaction system, no obvious effect on the photocatalytic activity of RGO-CdWO4 was observed. The above results illustrate that hydroxyl radicals contribute mostly to the high photocatalytic performance of RGO-CdWO4 composite photocatalyst. The formation of •OH upon UV irradiation was also measured, which was the major active species in the MB degradation by RGO-CdWO4 samples. The •OH can react with terephthalic acid (TA) to form 2-hydroxyterephthalic acid (TAOH), which can be measured at 426 nm by a fluorescence spectrophotometer. Therefore, the fluorescence intensity of TAOH indicates the amount of •OH, as shown in Fig. 10. In Fig. 10(a),
Fig. 9 The MB concentration during photodegradation as a function of irradiation time with the addition of different scavengers (benzoquinone and tert-butanol). The used sample is RGO-CdWO4-3.
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the results show the fluorescence intensity increased with the irradiation time. Fig. 10(b) shows the plot of PL intensity at 426 nm against irradiation time. It can be seen that the formation rates of •OH radicals with the RGO-CdWO4 samples are all greater than that of pure CdWO4. The results are in good agreement with the photocatalytic degradation results of MB by the different samples. These results also indicate that the deposition of CdWO4 onto RGO promotes the formation of • OH radicals. A possible explanation is that the RGO can act as an electron shuttle, which can effectively lead to efficient separation of the photo-generated electron–hole pairs thus decreasing their recombination rate. To further prove the above explanation, the transient photocurrent responses were recorded for photoelectrodes prepared with different samples ( pure CdWO4 and RGO-CdWO4), and the results are shown in Fig. 11. From the figure, we can see that the photocurrent rapidly increases to a certain value when the light is turned on, and the photocurrent comes back to nearly zero again. Furthermore, we can see that all RGO-CdWO4 hybrids exhibit a much higher photocurrent density than pure CdWO4. The enhanced photoresponse results from the introduction of RGO into the hybrids. RGO can accept the photoinduced electrons from CdWO4 and transfer them to the external circuit quickly, since the RGO has an extensive two-dimensional π–π
Fig. 11 Photocurrent transient responses of different samples under UV light irradiation.
Fig. 10 (a) The fluorescence intensity of TAOH for RGO-CdWO4-3 at different time and (b) time dependence of the fluorescence intensity at 426 nm in the presence of different samples.
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Acknowledgements We are grateful for grants from the National Natural Science Foundation of China (no. 51202114), Natural Science Foundation of Jiangsu province (BK2012464) and University Foundation of Jiangsu province (no. 11KJB610004). A project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
References
Fig. 12 Schematic presentation of the photocatalytic degradation of dye MB.
conjugation structure. Similarly, in the suspension system of RGO-CdWO4 and MB aqueous solution, the RGO can attract the photo-generated electrons and thus help to reduce the recombination rate of photo-generated charges. Therefore, more holes or •OH can take part in the photocatalytic degradation of MB. Furthermore, the electrons can also react with O2 molecules to produce •O2−, which can react with H2O to produce •OH. Both •O2−, holes and •OH can induce the degradation of dye MB molecules. The detailed process is shown in Fig. 12. However, there is an optimal RGO amount to obtain the highest photocatalytic activity. The value is 2% in the present work. The photocatalytic activity started to decrease when the RGO exceeded 2%. It is well known that graphene is a dark material which can absorb incident light. Therefore, superfluous graphene would reduce the absorption efficiency of the incident light by CdWO4 nanorods. The deleterious effect counteracts the activity enhancement by the enhanced adsorption activity and enhanced migration efficiency of photo-induced electrons.
4.
Conclusions
We reported here a study on the preparation and photocatalytic activity of a new hybrid photocatalyst: CdWO4-deposited RGO. The results showed that the reduction of GO and the growth of CdWO4 crystal occurred simultaneously in one single process. The photocatalysis experiment showed that such hybrid photocatalysts show much higher photocatalytic activity than pure CdWO4 for MB degradation. The effect of RGO amount on the photocatalytic activity of the as-prepared hybrid photocatalysts was also investigated. The results showed that there was an optimal amount of 2%. This study will motivate new developments in photocatalytic technology and promote their practical application in environmental problems.
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