I. INTRODUCTION
Recently, Graphene-based hybrids have attracted much attention in the materials research field because of their great usefulness in electronics (Kim et al., Reference Kim, Parvez and Chhowalla2009), photocatalysis (Zhang et al., Reference Zhang, Lv, Li, Wang and Li2010), and photovoltaic devices (Kamat, Reference Kamat2010). As a one-atom-thick and two-dimensional honeycomb lattice structure, graphene has high specific surface area and remarkable electrical (Wang et al., Reference Wang, Robinson, Diankov and Dai2010), thermal (Si and Samulski, Reference Si and Samulski2008), optical and mechanical properties (Lee et al., Reference Lee, Kim, Han, Hwang, Jeon, Choi, Hong, Lee, Ruoff and Kim2010), which makes it a novel material for the preparation of high performance composites with other functional materials. CeO2 as one of the inexpensive and relatively harmless rare earth materials, has been extensively studied for many technological applications such as catalyst (Zhong et al., Reference Zhong, Hu, Cao, Liu, Song and Wan2007), fuel, or solar cell (Corma et al., Reference Corma, Atienzar, Garcia and Chane-Ching2004; Deluga et al., Reference Deluga, Salge, Schmidt and Verykios2004), sensor (Liao et al., Reference Liao, Mai, Yuan, Lu, Li, Liu, Yan, Shen and Yu2008), luminescent and UV shielding (Wang et al., Reference Wang, Li, Ou, Feng, Qu and Tong2011). Moreover, CeO2 can be considered as an n-type semiconductor with a wide band gap of 2.9–3.2 eV, and it has some properties similar to commonly used photocatalyst TiO2 such as nontoxicity, high stability, and strong absorption in the UV region (Ji et al., Reference Ji, Zhang, Chen and Anpo2009). Thus, CeO2 can be regarded as a potential photocatalyst for hydrogen gas generation by splitting water and photodegradation of organic pollutants. However, It has been reported that CeO2 exhibits a relatively low photocatalytic activity for the oxidation of pollutants in water compared with TiO2 under UV irradiation (Khalil et al., Reference Khalil, Mourad and Rophael1998; Subramanian et al., Reference Subramanian, Wolf and Kamat2001; Miyauchi et al., Reference Miyauchi, Nakajima, Watanabe and Hashi-moto2002). Moreover, Mauro et al. (Reference Mauro, Heloisa, Eduardo, Mario and Paulo2012) have successfully synthesized CeO2–graphene composite using a two-step route and this composite has also shown remarkable thermal stability up to 1400 °C. Zhang et al. (Reference Zhang, Yuan, Chai, Wang and Wu2013) have also developed a CeO2–graphene composites-based ECL biosensor for detection of cholesterol, which shows outstanding reproducibility, long-term stability, and selectivity. Ling et al. (Reference Ling, Yang, Rao, Yang, Zhang, Liu and Zhang2013) have reported a facile synthesis of layered CeO2–graphene hybrid with superior catalytic performance in dehydrogenation of ethylbenzene. Here, we developed a novel CeO2 nanowires–reduced graphene oxide (CeO2 NWs–RGO) hybrid photocatalyst through a green and simple hydrothermal process, during which the coupling between graphene oxide (GO) and CeO2 NWs and the subsequent in situ reduction of GO to RGO was achieved. The photocatalytic performance and adsorption ability for dyes were also investigated. Furthermore, it is demonstrated that all the CeO2 NWs–RGO hybrids exhibit higher photocatalytic activity than pristine CeO2 NWs because of the improved adsorption ability for dyes and the efficient separation of photogenerated electron/hole pairs by RGO.
II. EXPERIMENTAL
A. Synthesis of CeO2 NWs-RGO hybrid
All of the chemical reagents in this paper were of analytical grade and used as received without further purification. Before the synthesis of CeO2 NWs–RGO hybrids, GO was synthesized from natural graphite powder (99.9%) by the reported method with slight modification (Xu et al., Reference Xu, Bai, Lu, Li and Shi2008; Pant et al., Reference Pant, Park, Pokharel, Tijing, Lee and Kim2013). CeO2 nanowires (CeO2 NWs) were synthesized by a typical hydrothermal method according to the previous reported work (Fu et al., Reference Fu, Wang, Yu, Wang and Wang2007) and then annealed at 350 °C for 4 h in order to obtain a clean CeO2 surface after the removal of surface adsorbed substances. The CeO2 NWs–RGO hybrids were obtained via a one-step hydrothermal method using supercritical water as green reductant. Briefly, 0.2 g of CeO2 NWs were first added into the calculated amount of GO solution followed by ultrasonic dispersion for 30 min and vigorous stirring for another 2 h to disperse CeO2 NWs sufficiently. Then, the mixing solution was transferred into a 50 ml Teflon-sealed autoclave at 150 °C for 5 h and cooled down to room temperature naturally. The resulting hybrids were recovered by centrifugation, washed with DI water and alcohol several times, and fully dried in vacuum at 60 °C for 12 h. The amount of graphene in the CeO2 NWs–RGO hybrid was controlled to be 0, 0.5, 1.0, 2.0, 4.0, 6.0, and 8.0 wt%, and the corresponding samples were labeled as CG-0, CG-0.5, CG-1.0, CG-2.0, CG-4.0, CG-6.0, CG-8.0, and CG-10.0, respectively.
B. Photocatalytic activity measurements
The evaluation of photocatalytic activity of the prepared samples for photocatalytic decomposition of Rhodamine B (RhB) dyes was performed at room temperature and observed based on the absorption spectroscopic technique. Typically, 100 ml aqueous solution of RhB dyes (4.79 g l−1) and 50-mg as-prepared samples were dispersed in a 200-ml breaker. Then, the mixture was kept in the dark under stirring for 1 h to evaluate the dyes adsorptivity of various samples before the photocatalytic reaction was started by exposing the vessel under the UV irradiation (produced by a 500 W long arc mercury lamp with the main wave crest at 365 nm and intensity at the mixture surface was 25 W cm−2). At given time intervals of 15 min, 3 ml aliquots were sampled, at once, centrifuged at 4000 rpm for 5 min to deposit the as-prepared photocatalysts during both stages of dark stirring and UV irradiation.
C. Characterization
X-ray diffraction (XRD) patterns were obtained on an X'pert PRO x-ray diffractometer (PANalytical, The Netherlands) with Cu Kα as radiation source (λ = 0.154 06 nm). Morphological analysis was performed with an S-4800 field emission scanning electron microscope (FESEM, Hitachi, Japan) with an acceleration voltage of 10 kV. UV–vis absorption spectra were acquired using a UV–visible spectrophotometer (UV-2800, Shanghai Sunny Hengping Scientific Instrument Co., LTD) and the irradiation intensity was estimated with a radiometer, Model: UV-A, made in Photoelectric Instrument Factory of Beijing Normal University. UV–vis absorption spectra were acquired using a UV–Vis spectrophotometer (Varian Cary 5000, USA).
III. RESULTS AND DISCUSSION
As shown in Figure 1(a), the synthesis mechanism of CeO2 NWs–RGO hybrids can be interpreted as follows. The presence of oxygen-containing functional groups in GO provides the opportunities to attach the one-dimensional metal oxide nanostructure and facilitates the initial formation of coupling between GO and CeO2 NWs, as reported by Zhou et al. (Reference Zhou, Shi and Zhou2012) and Nguyen-Phan et al. (Reference Nguyen-Phan, Pham, Kweon, Chung, Kim, Hur and Shin2012). With increasing temperature and pressure, supercritical water can exhibit strong reducing power during the hydrothermal process, resulting in the cleavage reactions of various heterolytic bonds on the surface of GO, which is similar to the preparation of carbon-based nanostructures via the deoxygenation reaction of carbohydrates by Yao et al. (Reference Yao, Shin, Wang, Windisch, Samuels, Arey, Wang, Risen and Exarhos2007) and Sun and Li (Reference Sun and Li2004). It is clear that the GO shows a characteristic diffraction peak at 2θ = 11.3° [Figure (2i)] whereas two broad peaks at 22.7 and 42.8° are observed after the hydrothermal process, which can be ascribed to RGO (002) and (100) planes (Ahmad et al., Reference Ahmad, Ahmed, Hong, Xu, Khalid, Elhissi and Ahmed2013), suggesting that GO sheet is effectively reduced to RGO though the green hydrothermal treatment. As for the CeO2 NWs–RGO hybrid, most of the diffraction peaks can be indexed to the fluorite cubic phase of CeO2 (JCPDF No. 65-2975) with lattice constants a = 0.5411 nm and average crystalline size of 14 nm as calculated by the Scherrer formula, and it is clear that there is a visible cover of RGO sheets on the surface of CeO2 NWs [Figure (3c)]. However, no characteristic diffraction peaks corresponding to RGO can be observed [Figures 2(a)–2(g)], which is because of the limited amount and the destroyed regular stacks of RGO in the hybrids as proposed by Li et al. (Reference Li, Guo, Yu, Ran, Zhang, Yan and Gong2011). In addition, the morphology of the synthesized CeO2 NWs can be obviously characterized with the average diameter of about 30–80 nm and the length of several micrometers, as shown in Figure 3(b).
(Color online) Schematic illustration of (a) the formation of RGO–ZnO NRs composites and (b) the measurement of photocatalytic degradation performance for various samples.
(Color online) XRD Patterns of the (a) CG-0, (b) CG-0.5, (c) CG-1.0, (d) CG-2.0, (e) CG-4.0, (f) CG-6.0, (g) CG-8.0, (h) CG-8.0, (i) RGO, and (j) GO.
FESEM images of the (a) GO, (b) CeO2 NRs, and (c) CG-8.0.
The photocatalytic activity of the CeO2 NWs and CeO2 NWs–RGO hybrids was evaluated by the photocatalytic decomposition of RhB aqueous solution under UV-light irradiation. It is well known that the photocatalytic decomposition of organic dyes usually takes place on the surface of a photocatalyst, sufficient contact between organic dyes and photocatalysts is an key factor for achieving higher photocatalytic performance (Wang et al., Reference Wang, Yu, Xiang and Cheng2012), the adsorption ability for RhB dyes of various samples was investigated before UV-light irradiation to further illustrate the photocatalytic mechanism of CeO2 NWs–RGO hybrids, as shown in Figures 1(b) and 4. It is notable that the adsorption ability for RhB is significantly improved with increasing amount of RGO in the CeO2 NWs–RGO hybrids and the absorptance of CG-10.0 are as high as 89% (Figure 5), suggesting the feasible transfer for dyes molecules from the solution to the surface of the photocatalyst via the π–π conjugation between RhB dyes and RGO nanosheets (Perera et al., Reference Perera, Mariano, Vu, Nour, Seitz, Chabal and Balkus2012).
(Color online) Images of the degradation effect for (a) CG-0, (b) CG-0.5, (c) CG-1.0, (d) CG-2.0, (e) CG-4.0, (f) CG-6.0, (g) CG-8.0, and (h) CG-10.0.
(Color online) Reaction rate k and dye absorptivity of various samples.
As for the photodecomposition of RhB dyes, the photocatalytic degradation reaction can be assumed to follow a pseudo-first-order expression: ln(C 0 /C) = kt, where C 0/C is the normalized organic compounds concentration and k is the apparent reaction rate (min−1), as shown in Figures 5 and 6. It is clear that all the CeO2 NWs–RGO hybrids exhibit a higher photocatalytic activity than the pure CeO2 NWs (k = 0.000 96 min−1) and CG-8.0 even shows a highly enhanced photocatalytic performance with a k value of 0.0197 min−1, which is about 20 times that of pure CeO2 NWs. Moreover, the k values also show an obvious enhancement with the increasing amount of RGO in the CeO2 NWs–RGO hybrids, and the photoreacted solution of CG-6.0 and CG-8.0 is almost transparent under irradiation for 1 h. However, the photocatalytic activity of CG-10.0 exhibits an obvious decrease compared with that of CG-6.0 and CG-8.0 as shown in Figures 5 and 6, and we attribute this lower activity to the lower excitation efficiency of CeO2 in the hybrid because of the increased absorbance or scattering of irradiation through excess RGO in the reaction solution, according to the previous report by Wang and Zhang (Reference Wang and Zhang2011). To further confirm this reasonable interpretation, the typical UV–vis absorption spectra of various samples are shown in Figure 7. It is noted that all of the spectra exhibit a strong absorption below 400 nm with an absorption peak of about 355 nm and the prepared CeO2 NWs–RGO hybrids show an obviously enhanced adsorption compared with CeO2 NWs, and the enhancements of absorption increase with RGO contents of the CeO2 NWs–RGO hybrids, suggesting that the increased absorbance of irradiation through excess RGO really reduces the excitation efficiency of CeO2 in the hybrid. Moreover, the absorption edge of CeO2 NWs–RGO hybrids also displays a slight red-shift to higher wavelength with the increase of RGO content, which we attribute to the chemical bonding between CeO2 and RGO as similar reports obtained for TiO2–RGO composite materials (Lee et al., Reference Lee, You and Park2012).
(Color online) Photocatalytic activity for RhB degradation under UV-light irradiation of (a) CG-0, (b) CG-0.5, (c) CG-1.0, (d) CG-2.0, (e) CG-10.0, (f) CG-4.0, (g) CG-6.0, and (h) CG-8.0.
(Color online) UV-vis absorption spectra of (a) CG-0, (b) CG-0.5, (c) CG-1.0, (d) CG-2.0, (e) CG-4.0, (f) CG-6.0, (g) CG-8.0, and (h) CG-10.0.
As per the discussion above, the enhancement of photocatalytic activity can be attributed to the elevated absorption ability for dyes molecules as well as the effect of the effective inhibition of the recombination of photogenerated electron–hole pairs, as shown in Figure 8. Possible major photocatalytic reaction steps can be summarized as follows:
$$\hbox{CeO}_2+h \upsilon \to \hbox{CeO}_2 \lpar e^{-} +h^{+}\rpar$$
$$\hbox{CeO}_2 \lpar e^ - \rpar +\hbox{RGO} \to \hbox{CeO}_2+\hbox{RGO}\lpar e^ - \rpar$$
$$\hbox{O}_2+\hbox{RGO}\lpar e^ - \rpar \to \hbox{RGO}+\hbox{O}_2^-$$
$$\hbox{Dyes}+\hbox{RGO} \to \hbox{RGO}\lpar \hbox{Dyes}\rpar$$
$$\hbox{CeO}_2 \lpar e^ - \rpar + \hbox{RGO}\lpar \hbox{Dyes}\rpar \to \hbox{CeO}_2+\hbox{RGO} + OX \hbox{Prod}.$$
$$\hbox{O}_2^- + \hbox{RGO}\lpar \hbox{Dyes}\rpar \to \hbox{RGO}+OX \hbox{Prod}.$$
(Color online) Photocatalytic degradation mechanism for dyes over CeO2 NWs–RGO hybrids under UV-light irradiation.
Photogenerated electrons can quickly transfer to the RGO nanosheets and then react with the ubiquitously dissolved oxygen molecule to yield reactive oxygen species. In this case, the RGO nanosheets function as an effective cocatalyst and an electron sink to accept photogenerated electrons from excited CeO2 NWs, resulting in a rapid transfer of photogenerated electrons and thus a lower recombination rate of photogenerated electron/hole pairs. Meanwhile, the resultant hole in the valence band of CeO2 NWs can decompose the RhB dyes by direct oxidation because of the sufficient adsorbance of RhB dyes molecules on the surface of RGO and the strong coupling of CeO2 NWs and RGO, which has been demonstrated by Xu et al. (Reference Xu, Zhang, Chen and Zhu2011). Furthermore, once the RhB dyes molecules adsorbed on the surface of RGO are degraded, the RhB dyes molecules in solution can immediately diffuse onto the surface of RGO via the π–π conjugation to be further decomposed.
IV. CONCLUSION
In summary, CeO2 NWs–RGO hybrids with different RGO content were successfully synthesized by a green hydrothermal method without using any additive and surfactant. The possible formation mechanism can be attributed to the initial binding of the CeO2 NWs with GO sheets through carboxyl and oxygen linkages and subsequent in situ reduction of GO to RGO by supercritical water during hydrothermal process. Owing to the introduction of RGO, both the adsorption ability for dyes and the separation ratio of photogenerated electron–hole pairs have been greatly enhanced. The photodegradation results demonstrate that the content of RGO in hybrids has a great influence on the photocatalytic activity, obviously increased to almost 20 times when loaded with 8.0 wt% of RGO. This work has demonstrated that the novel CeO2–graphene hybrid can be used as an alternate photocatalyst for water environment protection.
Acknowledgements
This work was financially supported by The National Basic Research Program of China (Grant no. 2010CB923200), Program for New Century Excellent Talents in University, Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications, Peoples Republic of China), and National Natural Science Foundation of China (Grants numbers 51102019, 61177085, 60937003, and 51272031).
