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Improving water splitting performance of Cu2O through a synergistic ‘‘two-way transfer’’ process of Cu and graphene† Dingkun Zhang, Ding Wei, Zhentao Cui, Shanshan Wang, Song Yang, Minhua Cao* and Changwen Hu* H2 evolution catalysis has drawn great consideration and effective separation and delivery of the photoelectrons are particularly crucial during the whole process. In this paper, we fabricate porous Cu–Cu2O–graphene nanocomposites via a simple reflux synthesis route, which possess porous structure and excellent catalytic performance for water splitting. With Cu species being added into Cu2O–graphene, the resultant catalyst exhibits improved activity for H2 evolution reaction as compared to Cu2O, Cu–Cu2O and Cu2O–graphene, indicating excellent catalytic performance and potential practical use. We attribute this performance to the synergistic
Received 3rd July 2014, Accepted 13th October 2014
effect of Cu component and graphene, which features: (i) a broader range of light absorption; (ii) faster electron
DOI: 10.1039/c4cp02904f
species and graphene both contribute greatly to this catalysis process, in which Cu can cooperate with graphene
transfer; and (iii) lower recombination possibility of photogenerated electrons and holes. We believe that the Cu support to extract electrons and pass them to the Pt cocatalyst to form a ‘‘two-way transfer’’ process. It is also
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believed that this strategy can be extended to other catalysts based on Cu–Cu2O–graphene composites.
1. Introduction As a widely used and powerful energy source, H2 has drawn considerable attention due to its advantages including high energy density, environmental friendliness, long cycle life and so on. Among all the methods developed for producing H2, photocatalytic water splitting1 has attracted considerable research interest2 and made encouraging progress,3 in which inorganic nanostructures,4 especially oxide semiconductors,5,6 are widely studied. Besides conventional TiO2, MoS2, CdS and ZnO, Cu doped Cu2O has also been found to be an efficient photocatalyst. Working as a p-type semiconductor with band gap of 2.17 eV combined with Cu conductive species, the Cu–Cu2O hybrid has been viewed as green catalyst for photocatalysis due to its low cost, high activity and stability. For example, Cu@Cu2O,7,8 Cu–Cu2O–TiO29,10 and Fe3O4@Cu2O/Cu11 have been investigated as potential visiblelight photocatalysts. As is well known, for efficient photocatalysis, timely photoelectron transfer in Cu2O particles by Cu species is the key issue for decreasing photoelectron–hole recombination and increasing delivery speed.
Graphene is a type of carbon material formed by tightly piled single layer carbon atoms to form a two-dimensional (2D) honeycomb-like structure.12 Owing to its marvellous properties, including high specific surface area, superior conductivity and fast electronic mobility, graphene has been widely used in composites with a series of inorganic semiconductors to form various hybrids for H2 production catalysis,13,14 such as TiO2,15 MoS2,16 CdS,17 Sr2Ta2O7 xNx18 and Ru(dcbpy)3.19 It is well understood that a multifunctional graphene support could disperse effectively the main catalysts and promote electron transfer so that a higher catalytic H2 evolution rate can be attained. Herein, we introduce the catalytic performance of H2 evolution by Cu–Cu2O–graphene nanocomposites. The added Cu species serves as a promoter to enhance the catalytic performance of H2 evolution by modifying the route of photoelectron delivery. It cooperates with the graphene support by extracting electrons and passing them to the Pt cocatalyst,20 thus a ‘‘two-way transfer’’ process can be established.
2. Experimental Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-10-68912631; Tel: +86-10-68918468 † Electronic supplementary information (ESI) available: SEM images and XRD pattern. See DOI: 10.1039/c4cp02904f
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2.1
Materials
Natural graphite (NG) was purchased from Qingdao Baichuan Graphite Co., Ltd. Cu(NO3)26H2O was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Hydrazine hydrate was purchased from Beijing Chemical Works. All chemical reagents
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used in this experiment were analytical grade and used without further purification.
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2.2
Preparation of GO
Graphene oxide (GO) is synthesized from NG flakes according to the improved Hummer method.25,26 Expandable graphite is placed in a 200 mL beaker. Subsequently, 75 mL of H2SO4 is added, with stirring, in an ice-water bath and 4.5 g KMnO4 is slowly added over 1 h. After stirring for 4 h, the beaker is transferred to an oil bath at 40 1C for 5 days with vigorous electric stirring. Then, 3 mL of H2O2 (30 wt% aqueous solution) is added, and the mixture is stirred for another 2 h at room temperature. Finally, the mixture is washed thoroughly with a mixed aqueous solution of 3 wt% H2SO4/0.5 wt% H2O2. The concentration of the GO suspension was calibrated as 7 mg mL 1. 2.3
Preparation of Cu–Cu2O–graphene
Cu–Cu2O–graphene nanocomposites are prepared by a one-pot reflux treatment based on a three-step route. A certain amount of GO is dispersed into 200 mL ethanol by ultrasonication for 1 h. With stirring, 200 mL of 1.5 M Cu(NO3)26H2O aqueous solution is added dropwise into above solution. Then, the PH of the mixture is tuned to 10 by NH3H2O. Subsequently, 0.6 mL N2H4H2O is added into the solution. The resulting mixture is stirred and transferred to a round-bottom flask with reflux at 100 1C for 4 h. The resultant precipitate was centrifuged and washed three times with deionized water. After freeze-drying for 24 h, the final powder is dark red in color.27 2.4
and loaded onto the surface of Cu–Cu2O–graphene. The concentration of H2 evolved was determined by chromatography (N2000, Zhejiang University) with a thermal conductivity detector (N2 carrier) connected to the gas circulating line.
3. Results and discussion Scheme 1 depicts a general synthesis process, in which Cu–Cu2O– graphene nanocomposites are prepared by a one-pot reflux treatment based on a three-step route. Fig. 1a shows X-ray diffraction (XRD) patterns of three samples, which are obtained by varying the volume of N2H4H2O. For the sample obtained with 0.6 mL of N2H4H2O, all diffraction peaks can be indexed to Cu2O (JCPDS No. 65-3288) and Cu (JCPDS No. 65-9743). Besides, the diffraction peaks of the Cu2O species are relatively broad, indicating a small particle size. Moreover, the typical diffraction peak of graphene oxide (GO) appearing at 11.21 vanishes in this XRD pattern,21 which indicates effective reduction of GO to graphene during the reflux treatment. The resultant Cu–Cu2O– graphene nanocomposites have a metal content of 81.24 wt% confirmed by inductively coupled plasma spectroscopy (ICP) measurement and the mass ratio of Cu to Cu2O in the nanocomposites is 66.3(5)% : 33.7(5)% by Rietveld Refinement analysis. If the usage of N2H4H2O is adjusted to 0.2 and 0.7 mL, Cu2O– graphene and Cu–graphene are obtained, respectively, as shown in Fig. 1a.
Characterization
The composition and phase purity of all samples are analyzed by a Shimadzu XRD-6000 X-ray diffractometer using Cu-Ka (l = 1.54178 Å) irradiation with a scan rate of 61 per minute, operated at 40 kV voltage and 50 mA current. The morphologies of the samples are characterized by a field-emission scanning electron microscope (JEOL S-4800). Transmission electron microscope (TEM), high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) images are taken on a H-8100 TEM operating at 200 kV accelerating voltage. X-Ray photoelectron spectra (XPS) are recorded on an ESCALAB 250 spectrometer (Perkin-Elmer) to characterize the surface composition. The Brunauer–Emmett–Teller (BET) surface area is measured on a Belsorp-max surface area detecting instrument by N2 physisorption at 77 K. Raman spectra are recorded on an Invia Raman spectrometer, with an excitation laser wavelength of 514.5 nm. UV-vis diffuse reflectance spectra were achieved using Shimadzu UV-vis spectrophotometer (UV-3600). The impedance spectra were recorded on a CHI-660D potentiostat. 2.5
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Scheme 1 General synthesis process of Cu–Cu2O–graphene nanocomposites.
Catalytic test
The H2 evolution catalytic tests are performed in a closed circulation system under irradiation of simulated sunlight using an xenon arc lamp (300 W, CEL-HXF300, Jinyuan) with 0.1 g catalyst suspended in 100 mL solution (20 mL methanol, 79 mL H2O and 1 mL 1 mg mL 1 Pt2+). Methanol acts as a sacrificial agent while H2PtCl6 is photoreduced to form Pt nanoparticles
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Fig. 1 (a) XRD pattern of Cu–Cu2O–graphene and (b) Raman spectra of GO and Cu–Cu2O–graphene.
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Fig. 3 SEM image (a); TEM image (b); magnified TEM image (c); HRTEM image (d), inset shows relevant SAED pattern of Cu–Cu2O–graphene.
Fig. 2 Typical XPS survey spectra of Cu–Cu2O–graphene (a); C 1s XPS spectra of GO (b) and Cu–Cu2O–graphene (c); Cu 2p XPS spectra of Cu–Cu2O–graphene nanocomposites (d).
Raman spectra (Fig. 1b) show a comparison between GO and Cu–Cu2O–graphene. It can be clearly seen that two vibrations appearing in each curve can be attributed to the G band (1600 cm 1) and D band (1350 cm 1), respectively. In detail, the G band refers to sp2 carbon atoms in scattering of the E2g mode, while the D band represents internal scattering. The corresponding intensity ratio referred as ID/IG symbolizes disorder level and the average size of sp2 domains.22 Obviously, ID/IG decreases from 1.158 for GO to 1.084 for Cu–Cu2O–graphene, revealing that the reflux process indeed increases the size of graphene domains for the reason of graphitic ‘‘self-healing’’ in the nanocomposites.23 To investigate further the surface composition of the nanocomposites, X-ray photoelectron spectroscopy (XPS) measurement is carried out. Fig. 2a shows an XPS survey spectrum of the Cu–Cu2O–graphene, which clearly displays the presence of C (284.0 eV), O (533.2 eV) and Cu (928.2–945.1 eV). Fig. 2b shows the fitted high-resolution C 1s spectra of GO, which display four types of carbon with different chemical states, 284.0 eV for C–C in the carbon, 287.0 eV for C–O, 287.8 eV for CQO, and 288.5 eV for O–CQO, respectively. Fig. 2c displays the C 1s spectra of the Cu–Cu2O–graphene, which clearly show that the intensity of C–O species decreases significantly, whereas that of C–C/CQC species obviously increases, indicating that oxygen-contained species have been almost eliminated while the sp3/sp2-like carbon structure has been restored simultaneously. As for the Cu component, the Cu 2p XPS spectrum is shown in Fig. 2d, which exhibits one strong peak with a binding energy of 932.5 eV, corresponding to the Cu 2p3/2 spin–orbit peak of Cu+. The above XPS spectra and previous XRD result firmly support the composition of the resultant Cu–Cu2O–graphene and feasibility of the preparation method. The typical morphology and microstructure of the Cu–Cu2O– graphene nanocomposites are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM image in Fig. 3a clearly shows that Cu–Cu2O spheres
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with diameters ranging from 220 to 250 nm are dispersed on the graphene sheets. Moreover, spherical Cu–Cu2O particles are composed of nanoparticles (NPs) with average diameter of 60 nm. The TEM images of this sample shown in Fig. 3b and c further confirmed that porous Cu–Cu2O spheres consisting of NPs are wrapped in graphene sheets (marked by an arrow) and these are in agreement with the SEM results (Fig. 3a). On the contrary, for Cu2O–graphene and Cu–graphene samples, solid nonuniform Cu2O or Cu particles anchored on grapheme sheets were observed (Fig. S1 and S2, ESI†). The high-resolution TEM (HRTEM) image (Fig. 3d) reveals two lattice spacings of 0.25 and 0.21 nm, which match the (111) plane of cubic Cu2O and (111) plane of cubic Cu, respectively. Thus it can be deduced that the spherical Cu–Cu2O particles are composed of mixed nanocrystals with each component penetrating into another to form homogeneous porous nanospheres. The selected area electron diffraction (SAED) confirmed that the Cu–Cu2O–graphene is polycrystalline (inset in Fig. 2d). Owing to their high conductivity and support function, Cu–Cu2O–graphene nanocomposites are expected to be able to catalyze the targeted reaction easily. Hence their marvellous catalytic activity can be achieved due to contributions from each effective function and the mutual cooperation of each component. To investigate further the porous nature of the Cu–Cu2O– graphene nanocomposites, N2 adsorption–desorption isotherm measurements were carried out. Fig. 4a shows N2 adsorption–desorption isotherms and corresponding Barrett–Joyner–Halenda (BJH) pore size distribution plots. The isotherms exhibit type IV, i.e. a hysteresis loop at p/p0 between 0.4–0.9, indicating the presence of the mesopores in this sample. The corresponding pore size distribution curve also confirms the mesoporous structure (the inset in Fig. 4a). The mesopore size is mainly centered at 2.2 nm by the BJH method. Such a porous structure gives rise to a Brunauer–Emmett–Teller (BET) surface area as high as 50.18 m2 g 1, which is far higher than that of Cu2O–graphene and Cu–graphene samples (Fig. S3, ESI†). The high surface area may provide a large number of active sites for catalytic performance. At the same time, the pore structure is beneficial for the diffusion of reactants and the exposure of active sites, both of which contribute to an improved catalytic performance.
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Fig. 4 Nitrogen adsorption–desorption isotherm of Cu–Cu2O–graphene, inset shows the pore size distribution curves (a) and relevant UV-vis diffuse reflectance spectra (b).
A typical UV-vis diffuse reflectance spectrum (DRS) of the porous Cu–Cu2O–graphene is shown in Fig. 4b and for comparison, those of Cu2O–graphene and Cu2O are also provided. All samples show both ultraviolet and visible light absorption, but the porous Cu–Cu2O– graphene sample has a stronger UV-vis adsorption than the others and the relevant band gap is calculated to be 1.63 eV according to the Kubelka–Munk method.24 The maximum absorbance peak centers at 450 nm. The as-synthesized nanocomposites are then evaluated as photocatalysts for H2 generation within a 100 mL reaction solution. Different kinds of Cu-based/graphene nanocomposites (Cu–graphene, CuO–graphene, Cu2O–graphene and Cu–Cu2O– graphene) and relevant single-component catalysts (Cu–Cu2O, Cu2O, Cu and graphene) are tested under the same conditions. As shown in Fig. 5a, the catalytic performance of different catalyst systems varies significantly and the H2 evolution rate has a close relationship to the composition of the catalysts used. The system without addition of any catalyst (referred to as blank) shows little catalytic activity with a trace amount of H2 and therefore the background factors that may interfere with the final result can be neglected. Cu, graphene, Cu–graphene and CuO–graphene samples also produce negligible amount of H2 because of their photoexcitation-inert nature. The Cu2O and Cu–Cu2O samples still exhibit a low H2 evolution rate (0.27 and 0.43 mmol h 1 g 1). Despite a slight increase in H2 evolution in the presence of Cu, bare Cu2O will be rapidly photocorroded on exposure to light and its catalytic activity will vanish. On the contrary, the Cu–Cu2O– graphene sample displays an evidently improved catalytic activity with an H2 evolution rate of 16.2 mmol h 1 g 1. This catalytic performance confirms the fact that inevitable electron transfer limitation and photoelectron–hole recombination restrain strongly their activity. By comparing H2 evolution rates of different nanocomposites (Fig. 5b), we find that Cu–Cu2O–graphene possesses the best catalytic efficiency owning to the doped Cu component, which cooperates with graphene to extract and pass photoelectrons to Pt NPs so that the highest catalytic activity can be attained. More specifically, the added Cu component facilitates rapid electron separation and transfer, which is crucial for whole catalytic process. In addition, relevant electrochemical impedance tests (Fig. 6) indicate that Cu–Cu2O–graphene facilitates a lower passing barrier and faster delivery speed of photoelectrons. This result accords with the previous catalytic performance (Fig. 5a and b).
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Fig. 5 H2 evolution curves (a) and relevant H2 evolution rate (b) under simulated sunlight irradiation with different kinds of Cun+-based/graphene nanocomposites (Cu–graphene, CuO–graphene, Cu2O–graphene and Cu–Cu2O–graphene) and relevant single-component catalysts (Cu–Cu2O, Cu, Cu2O and graphene).
Fig. 6 Electrochemical impedance spectroscopy of Cu2O, Cu2O–graphene, Cu–Cu2O–graphene.
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Fig. 7 H2 evolution curves (a) and relevant H2 evolution rate (b) under simulated sunlight irradiation with addition of different amount of GO added (0.1% GN, 1.0% GN, 4.6% GN and 8.0% GN, respectively).
Compared with Cu2O–graphene, Cu–Cu2O–graphene possesses higher conductivity and faster transfer of photoexcited electrons, which finally dominate H2 evolution efficiency. Furthermore, we also compared comprehensively Cu–Cu2O– graphene samples with different graphene contents (0.1%, 1.0%, 4.6% and 8.0%) in terms of their catalytic activity (Fig. S4, ESI†). As is clearly shown in Fig. 7a and b, the Cu–Cu2O–graphene (1.0%) has a faster H2 evolution rate (16.2 mmol h 1 g 1) than those with a high or low content of graphene. This fact can be explained by following two aspects. On the one hand, insufficient graphene makes electron transfer between Cu2O and Pt particles harder and prevents Cu2O spheres from dispersing well onto the graphene sheets so that unnecessary aggregations of particles shadow most active sites for harvesting light. On the other hand, excess graphene inevitably covers most Cu2O active species so that energy transformation is cut irreversibly. Therefore, an appropriate amount of graphene is necessary to ensure excellent catalytic effect. The stability of the as-prepared porous Cu–Cu2O–graphene has also been investigated since it is very important for practical applications. Here, we choose the sample with the best catalytic efficiency [Cu–Cu2O–graphene (1.0%)] for testing. Keeping the reaction environment unchanged, we test the H2 generation rate as a function of time. As shown in Fig. 8a, under such conditions, except for slight fluctuations, our catalyst exhibits excellent stability along with activity during six cycles and after that the evolution speed decreases. In addition, the recovered Cu–Cu2O–graphene catalyst after a long photocatalysis time has a slight change in color (dark red), crystal structure and composition (by XRD, Fig. S5, ESI†). However, the mass ratio of Cu to Cu2O changes greatly
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Fig. 8 Stability ofn H2 evolution under simulated sunlight irradiation (a) and proposed ‘‘two-way transfer’’ mechanism for hydrogen production with Cu–Cu2O–graphene (b).
[before: Cu 66.3(5)%, Cu2O 33.7(5)%; and after: Cu 17.9(5)%, Cu2O 82.1(5)%]. Therefore, we attribute the stable evolution speed to the increase of the Cu2O active component. We deduce that graphene facilitates particle dispersion effectively to prevent aggregation effects during long time reaction, while Cu–Cu2O particle also acts as brace to prevent graphene from curling, crumpling and aggregating. Owning to excellent synergy and mutual dispersion effects between Cu–Cu2O and graphene, long term stability is gained with maintenance of activity, which confirms the suitability of this system for further modification and practical use. Based on above experimental results, a ‘‘two-way transfer’’ mechanism can be proposed, as shown in Fig. 8b. When Cu2O was excited by simulated sunlight, it can produce photoelectrons and holes simultaneously and those photoelectrons jump to the conductive band. With the help of the highly conductive Cu component, photoelectrons are delivered faster and separate from holes entirely, thus recombination of electrons and holes can be avoided effectively.28,29 Some of these photoelectrons are rapidly captured by Pt co-catalyst to participate further in H2 evolution catalysis, while others pass through the graphene support to the Pt particles loaded on it and then take part in hydrogen production as well. This kind of ‘‘two-way transfer’’ could separate and pass photoelectrons efficiently so that faster delivery speeds and higher H2 evolution rates can be obtained. It is believed that the Cu and graphene in Cu–Cu2O–graphene nanocomposites possess three features: broadening the range of light absorption; promoting electron transfer speed; and preventing effectively photoelectron–hole recombination. In addition, we infer that
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the generation of Cu species inevitably decreases the concentration of active Cu2O because of Cu2+ consumption, but definite efficiency acceleration reveals the importance of a Cu component coexisting at the cost of some Cu2O in the catalysts. Significant improvement in catalysis performance (Fig. 5) owing to the addition of Cu species firmly supports this deduction.
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Conclusions In summary, we have demonstrated a facile and simple method to prepare mesoporous Cu–Cu2O–graphene nanocomposites via a reflux synthesis route. Compared with the widely studied Cu2O–graphene, the novel Cu–Cu2O–graphene exhibits superior catalytic H2 evolution performance in terms of catalytic activity and stability. This excellent catalytic performance can be attributed to a ‘‘two-way’’ mechanism of Cu and graphene, which features: (i) a higher excitation activity; (ii) faster electron transfer; and (iii) lower recombination possibility of photo-generated electrons and holes, all of which do great help to improve catalysis performance. It is also believed that further modification and improvement can be applied to study the Cu–Cu2O–graphene system.
Acknowledgements This work was financially supported by the NNSFC (No. 21173021, 21231002, 21276026, 21471016 and 21271023), the 111 Project (B07012), 973 Program (2014CB932103), and the Institute of Chemical Materials, China Academy of Engineering Physics (No. 20121941006).
Notes and references 1 K. Honda and A. Fujishima, Nature, 1972, 238, 37–38. 2 D. Ravelli, D. Dondi, M. Fagnoni and A. Albini, Chem. Soc. Rev., 2009, 38, 1999–2011. 3 K. Maeda and K. Domen, J. Phys. Chem. Lett., 2010, 1, 2655–2661. 4 F. E. Osterloh, Chem. Soc. Rev., 2013, 42, 2294–2320. 5 G. R. Patzke, Y. Zhou, R. Kontic and F. Conrad, Angew. Chem., Int. Ed., 2010, 49, 2–36. 6 Z. G. Zou, J. H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414, 625–627. 7 Z. H. Ai, L. Z. Zhang, S. C. Lee and W. K. Ho, J. Phys. Chem. C, 2009, 113, 20896–20902.
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8 T. Y. Kou, C. H. Jin, C. Zhang, J. Z. Suna and Z. H. Zhang, RSC Adv., 2012, 2, 12636–12643. 9 Z. H. Li, J. W. Liu, D. J. Wang, Y. Gao and J. Shen, Int. J. Hydrogen Energy, 2012, 37, 6431–6437. 10 J. Xing, Z. P. Chen, F. Y. Xiao, X. Y. Ma, C. Z. Wen, Z. Li and H. G. Yang, Chem. – Asian J., 2013, 8, 1265–1270. 11 H. S. Wang, Y. G. Hu, Y. Jiang, L. G. Qiu, H. B. Wu, B. Guo, Y. H. Shen, Y. Wang, L. Zhu and A. J. Xie, Dalton Trans., 2013, 42, 4915–4921. 12 M. J. Allen, V. C. Tung and R. B. Kaner, Chem. Rev., 2010, 110, 132–145. 13 S. Bai and X. P. Shen, RSC Adv., 2012, 2, 64–98. 14 Q. J. Xiang, J. G. Yu and M. Jaroniec, Chem. Soc. Rev., 2012, 41, 782–796. 15 Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575–6578. 16 S. X. Min and G. X. Lu, J. Phys. Chem. C, 2012, 116, 25415–25424. 17 Q. Li, B. D. Guo, J. G. Yu, J. R. Ran, B. H. Zhang, H. J. Yan and J. R. Gong, J. Am. Chem. Soc., 2011, 133, 10878–10884. 18 A. Mukherji, B. Seger, G. Q. (Max) Lu and L. Z. Wang, ACS Nano, 2011, 5, 3483–3492. 19 M. S. Zhu, Y. P. Dong, B. Xiao, Y. K. Du, P. Yang and X. M. Wang, J. Mater. Chem., 2012, 22, 23773–23779. 20 P. D. Tran, S. K. Batabyal, S. S. Pramana, J. Barber, L. H. Wong and S. C. J. Loo, Nanoscale, 2012, 4, 3875–3878. 21 R. J. Liu, S. W. Li, X. L. Yu, G. J. Zhang, S. J. Zhang, J. N. Yao and L. J. Zhi, J. Mater. Chem., 2012, 22, 3319–3322. 22 S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565. 23 K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prud’homme, I. A. Aksay and R. Car, Nano Lett., 2008, 8, 36–41. 24 A. Mukherji, B. Seger, G. Q. Lu and L. Z. Wang, ACS Nano, 2011, 5, 3483–3492. 25 W. S. Hummers and R. J. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. 26 S. M. Paek, D. J. Yoo and I. Honma, Nano Lett., 2009, 9, 72–75. 27 X. X. Xu, C. Randorn, P. Efstathiou and J. T. S. Irvine, Nat. Mater., 2012, 11, 595–598. 28 Z. H. Ai, L. Z. Zhang, S. Lee and W. Ho, J. Phys. Chem. C, 2009, 113, 20896–20902. 29 T. Y. Kou, C. H. Jin, C. Zhang, J. Z. Sun and Z. H. Zhang, RSC Adv., 2012, 2, 12636–12643.
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Improving water splitting performance of Cu2O through a synergistic ‘‘two-way transfer’’ process of Cu and graphene† Dingkun Zhang, Ding Wei, Zhentao Cui, Shanshan Wang, Song Yang, Minhua Cao* and Changwen Hu* H2 evolution catalysis has drawn great consideration and effective separation and delivery of the photoelectrons are particularly crucial during the whole process. In this paper, we fabricate porous Cu–Cu2O–graphene nanocomposites via a simple reflux synthesis route, which possess porous structure and excellent catalytic performance for water splitting. With Cu species being added into Cu2O–graphene, the resultant catalyst exhibits improved activity for H2 evolution reaction as compared to Cu2O, Cu–Cu2O and Cu2O–graphene, indicating excellent catalytic performance and potential practical use. We attribute this performance to the synergistic
Received 3rd July 2014, Accepted 13th October 2014
effect of Cu component and graphene, which features: (i) a broader range of light absorption; (ii) faster electron
DOI: 10.1039/c4cp02904f
species and graphene both contribute greatly to this catalysis process, in which Cu can cooperate with graphene
transfer; and (iii) lower recombination possibility of photogenerated electrons and holes. We believe that the Cu support to extract electrons and pass them to the Pt cocatalyst to form a ‘‘two-way transfer’’ process. It is also
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believed that this strategy can be extended to other catalysts based on Cu–Cu2O–graphene composites.
1. Introduction As a widely used and powerful energy source, H2 has drawn considerable attention due to its advantages including high energy density, environmental friendliness, long cycle life and so on. Among all the methods developed for producing H2, photocatalytic water splitting1 has attracted considerable research interest2 and made encouraging progress,3 in which inorganic nanostructures,4 especially oxide semiconductors,5,6 are widely studied. Besides conventional TiO2, MoS2, CdS and ZnO, Cu doped Cu2O has also been found to be an efficient photocatalyst. Working as a p-type semiconductor with band gap of 2.17 eV combined with Cu conductive species, the Cu–Cu2O hybrid has been viewed as green catalyst for photocatalysis due to its low cost, high activity and stability. For example, Cu@Cu2O,7,8 Cu–Cu2O–TiO29,10 and Fe3O4@Cu2O/Cu11 have been investigated as potential visiblelight photocatalysts. As is well known, for efficient photocatalysis, timely photoelectron transfer in Cu2O particles by Cu species is the key issue for decreasing photoelectron–hole recombination and increasing delivery speed.
Graphene is a type of carbon material formed by tightly piled single layer carbon atoms to form a two-dimensional (2D) honeycomb-like structure.12 Owing to its marvellous properties, including high specific surface area, superior conductivity and fast electronic mobility, graphene has been widely used in composites with a series of inorganic semiconductors to form various hybrids for H2 production catalysis,13,14 such as TiO2,15 MoS2,16 CdS,17 Sr2Ta2O7 xNx18 and Ru(dcbpy)3.19 It is well understood that a multifunctional graphene support could disperse effectively the main catalysts and promote electron transfer so that a higher catalytic H2 evolution rate can be attained. Herein, we introduce the catalytic performance of H2 evolution by Cu–Cu2O–graphene nanocomposites. The added Cu species serves as a promoter to enhance the catalytic performance of H2 evolution by modifying the route of photoelectron delivery. It cooperates with the graphene support by extracting electrons and passing them to the Pt cocatalyst,20 thus a ‘‘two-way transfer’’ process can be established.
2. Experimental Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-10-68912631; Tel: +86-10-68918468 † Electronic supplementary information (ESI) available: SEM images and XRD pattern. See DOI: 10.1039/c4cp02904f
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2.1
Materials
Natural graphite (NG) was purchased from Qingdao Baichuan Graphite Co., Ltd. Cu(NO3)26H2O was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Hydrazine hydrate was purchased from Beijing Chemical Works. All chemical reagents
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used in this experiment were analytical grade and used without further purification.
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2.2
Preparation of GO
Graphene oxide (GO) is synthesized from NG flakes according to the improved Hummer method.25,26 Expandable graphite is placed in a 200 mL beaker. Subsequently, 75 mL of H2SO4 is added, with stirring, in an ice-water bath and 4.5 g KMnO4 is slowly added over 1 h. After stirring for 4 h, the beaker is transferred to an oil bath at 40 1C for 5 days with vigorous electric stirring. Then, 3 mL of H2O2 (30 wt% aqueous solution) is added, and the mixture is stirred for another 2 h at room temperature. Finally, the mixture is washed thoroughly with a mixed aqueous solution of 3 wt% H2SO4/0.5 wt% H2O2. The concentration of the GO suspension was calibrated as 7 mg mL 1. 2.3
Preparation of Cu–Cu2O–graphene
Cu–Cu2O–graphene nanocomposites are prepared by a one-pot reflux treatment based on a three-step route. A certain amount of GO is dispersed into 200 mL ethanol by ultrasonication for 1 h. With stirring, 200 mL of 1.5 M Cu(NO3)26H2O aqueous solution is added dropwise into above solution. Then, the PH of the mixture is tuned to 10 by NH3H2O. Subsequently, 0.6 mL N2H4H2O is added into the solution. The resulting mixture is stirred and transferred to a round-bottom flask with reflux at 100 1C for 4 h. The resultant precipitate was centrifuged and washed three times with deionized water. After freeze-drying for 24 h, the final powder is dark red in color.27 2.4
and loaded onto the surface of Cu–Cu2O–graphene. The concentration of H2 evolved was determined by chromatography (N2000, Zhejiang University) with a thermal conductivity detector (N2 carrier) connected to the gas circulating line.
3. Results and discussion Scheme 1 depicts a general synthesis process, in which Cu–Cu2O– graphene nanocomposites are prepared by a one-pot reflux treatment based on a three-step route. Fig. 1a shows X-ray diffraction (XRD) patterns of three samples, which are obtained by varying the volume of N2H4H2O. For the sample obtained with 0.6 mL of N2H4H2O, all diffraction peaks can be indexed to Cu2O (JCPDS No. 65-3288) and Cu (JCPDS No. 65-9743). Besides, the diffraction peaks of the Cu2O species are relatively broad, indicating a small particle size. Moreover, the typical diffraction peak of graphene oxide (GO) appearing at 11.21 vanishes in this XRD pattern,21 which indicates effective reduction of GO to graphene during the reflux treatment. The resultant Cu–Cu2O– graphene nanocomposites have a metal content of 81.24 wt% confirmed by inductively coupled plasma spectroscopy (ICP) measurement and the mass ratio of Cu to Cu2O in the nanocomposites is 66.3(5)% : 33.7(5)% by Rietveld Refinement analysis. If the usage of N2H4H2O is adjusted to 0.2 and 0.7 mL, Cu2O– graphene and Cu–graphene are obtained, respectively, as shown in Fig. 1a.
Characterization
The composition and phase purity of all samples are analyzed by a Shimadzu XRD-6000 X-ray diffractometer using Cu-Ka (l = 1.54178 Å) irradiation with a scan rate of 61 per minute, operated at 40 kV voltage and 50 mA current. The morphologies of the samples are characterized by a field-emission scanning electron microscope (JEOL S-4800). Transmission electron microscope (TEM), high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) images are taken on a H-8100 TEM operating at 200 kV accelerating voltage. X-Ray photoelectron spectra (XPS) are recorded on an ESCALAB 250 spectrometer (Perkin-Elmer) to characterize the surface composition. The Brunauer–Emmett–Teller (BET) surface area is measured on a Belsorp-max surface area detecting instrument by N2 physisorption at 77 K. Raman spectra are recorded on an Invia Raman spectrometer, with an excitation laser wavelength of 514.5 nm. UV-vis diffuse reflectance spectra were achieved using Shimadzu UV-vis spectrophotometer (UV-3600). The impedance spectra were recorded on a CHI-660D potentiostat. 2.5
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Scheme 1 General synthesis process of Cu–Cu2O–graphene nanocomposites.
Catalytic test
The H2 evolution catalytic tests are performed in a closed circulation system under irradiation of simulated sunlight using an xenon arc lamp (300 W, CEL-HXF300, Jinyuan) with 0.1 g catalyst suspended in 100 mL solution (20 mL methanol, 79 mL H2O and 1 mL 1 mg mL 1 Pt2+). Methanol acts as a sacrificial agent while H2PtCl6 is photoreduced to form Pt nanoparticles
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Fig. 1 (a) XRD pattern of Cu–Cu2O–graphene and (b) Raman spectra of GO and Cu–Cu2O–graphene.
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Fig. 3 SEM image (a); TEM image (b); magnified TEM image (c); HRTEM image (d), inset shows relevant SAED pattern of Cu–Cu2O–graphene.
Fig. 2 Typical XPS survey spectra of Cu–Cu2O–graphene (a); C 1s XPS spectra of GO (b) and Cu–Cu2O–graphene (c); Cu 2p XPS spectra of Cu–Cu2O–graphene nanocomposites (d).
Raman spectra (Fig. 1b) show a comparison between GO and Cu–Cu2O–graphene. It can be clearly seen that two vibrations appearing in each curve can be attributed to the G band (1600 cm 1) and D band (1350 cm 1), respectively. In detail, the G band refers to sp2 carbon atoms in scattering of the E2g mode, while the D band represents internal scattering. The corresponding intensity ratio referred as ID/IG symbolizes disorder level and the average size of sp2 domains.22 Obviously, ID/IG decreases from 1.158 for GO to 1.084 for Cu–Cu2O–graphene, revealing that the reflux process indeed increases the size of graphene domains for the reason of graphitic ‘‘self-healing’’ in the nanocomposites.23 To investigate further the surface composition of the nanocomposites, X-ray photoelectron spectroscopy (XPS) measurement is carried out. Fig. 2a shows an XPS survey spectrum of the Cu–Cu2O–graphene, which clearly displays the presence of C (284.0 eV), O (533.2 eV) and Cu (928.2–945.1 eV). Fig. 2b shows the fitted high-resolution C 1s spectra of GO, which display four types of carbon with different chemical states, 284.0 eV for C–C in the carbon, 287.0 eV for C–O, 287.8 eV for CQO, and 288.5 eV for O–CQO, respectively. Fig. 2c displays the C 1s spectra of the Cu–Cu2O–graphene, which clearly show that the intensity of C–O species decreases significantly, whereas that of C–C/CQC species obviously increases, indicating that oxygen-contained species have been almost eliminated while the sp3/sp2-like carbon structure has been restored simultaneously. As for the Cu component, the Cu 2p XPS spectrum is shown in Fig. 2d, which exhibits one strong peak with a binding energy of 932.5 eV, corresponding to the Cu 2p3/2 spin–orbit peak of Cu+. The above XPS spectra and previous XRD result firmly support the composition of the resultant Cu–Cu2O–graphene and feasibility of the preparation method. The typical morphology and microstructure of the Cu–Cu2O– graphene nanocomposites are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM image in Fig. 3a clearly shows that Cu–Cu2O spheres
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with diameters ranging from 220 to 250 nm are dispersed on the graphene sheets. Moreover, spherical Cu–Cu2O particles are composed of nanoparticles (NPs) with average diameter of 60 nm. The TEM images of this sample shown in Fig. 3b and c further confirmed that porous Cu–Cu2O spheres consisting of NPs are wrapped in graphene sheets (marked by an arrow) and these are in agreement with the SEM results (Fig. 3a). On the contrary, for Cu2O–graphene and Cu–graphene samples, solid nonuniform Cu2O or Cu particles anchored on grapheme sheets were observed (Fig. S1 and S2, ESI†). The high-resolution TEM (HRTEM) image (Fig. 3d) reveals two lattice spacings of 0.25 and 0.21 nm, which match the (111) plane of cubic Cu2O and (111) plane of cubic Cu, respectively. Thus it can be deduced that the spherical Cu–Cu2O particles are composed of mixed nanocrystals with each component penetrating into another to form homogeneous porous nanospheres. The selected area electron diffraction (SAED) confirmed that the Cu–Cu2O–graphene is polycrystalline (inset in Fig. 2d). Owing to their high conductivity and support function, Cu–Cu2O–graphene nanocomposites are expected to be able to catalyze the targeted reaction easily. Hence their marvellous catalytic activity can be achieved due to contributions from each effective function and the mutual cooperation of each component. To investigate further the porous nature of the Cu–Cu2O– graphene nanocomposites, N2 adsorption–desorption isotherm measurements were carried out. Fig. 4a shows N2 adsorption–desorption isotherms and corresponding Barrett–Joyner–Halenda (BJH) pore size distribution plots. The isotherms exhibit type IV, i.e. a hysteresis loop at p/p0 between 0.4–0.9, indicating the presence of the mesopores in this sample. The corresponding pore size distribution curve also confirms the mesoporous structure (the inset in Fig. 4a). The mesopore size is mainly centered at 2.2 nm by the BJH method. Such a porous structure gives rise to a Brunauer–Emmett–Teller (BET) surface area as high as 50.18 m2 g 1, which is far higher than that of Cu2O–graphene and Cu–graphene samples (Fig. S3, ESI†). The high surface area may provide a large number of active sites for catalytic performance. At the same time, the pore structure is beneficial for the diffusion of reactants and the exposure of active sites, both of which contribute to an improved catalytic performance.
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Fig. 4 Nitrogen adsorption–desorption isotherm of Cu–Cu2O–graphene, inset shows the pore size distribution curves (a) and relevant UV-vis diffuse reflectance spectra (b).
A typical UV-vis diffuse reflectance spectrum (DRS) of the porous Cu–Cu2O–graphene is shown in Fig. 4b and for comparison, those of Cu2O–graphene and Cu2O are also provided. All samples show both ultraviolet and visible light absorption, but the porous Cu–Cu2O– graphene sample has a stronger UV-vis adsorption than the others and the relevant band gap is calculated to be 1.63 eV according to the Kubelka–Munk method.24 The maximum absorbance peak centers at 450 nm. The as-synthesized nanocomposites are then evaluated as photocatalysts for H2 generation within a 100 mL reaction solution. Different kinds of Cu-based/graphene nanocomposites (Cu–graphene, CuO–graphene, Cu2O–graphene and Cu–Cu2O– graphene) and relevant single-component catalysts (Cu–Cu2O, Cu2O, Cu and graphene) are tested under the same conditions. As shown in Fig. 5a, the catalytic performance of different catalyst systems varies significantly and the H2 evolution rate has a close relationship to the composition of the catalysts used. The system without addition of any catalyst (referred to as blank) shows little catalytic activity with a trace amount of H2 and therefore the background factors that may interfere with the final result can be neglected. Cu, graphene, Cu–graphene and CuO–graphene samples also produce negligible amount of H2 because of their photoexcitation-inert nature. The Cu2O and Cu–Cu2O samples still exhibit a low H2 evolution rate (0.27 and 0.43 mmol h 1 g 1). Despite a slight increase in H2 evolution in the presence of Cu, bare Cu2O will be rapidly photocorroded on exposure to light and its catalytic activity will vanish. On the contrary, the Cu–Cu2O– graphene sample displays an evidently improved catalytic activity with an H2 evolution rate of 16.2 mmol h 1 g 1. This catalytic performance confirms the fact that inevitable electron transfer limitation and photoelectron–hole recombination restrain strongly their activity. By comparing H2 evolution rates of different nanocomposites (Fig. 5b), we find that Cu–Cu2O–graphene possesses the best catalytic efficiency owning to the doped Cu component, which cooperates with graphene to extract and pass photoelectrons to Pt NPs so that the highest catalytic activity can be attained. More specifically, the added Cu component facilitates rapid electron separation and transfer, which is crucial for whole catalytic process. In addition, relevant electrochemical impedance tests (Fig. 6) indicate that Cu–Cu2O–graphene facilitates a lower passing barrier and faster delivery speed of photoelectrons. This result accords with the previous catalytic performance (Fig. 5a and b).
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Fig. 5 H2 evolution curves (a) and relevant H2 evolution rate (b) under simulated sunlight irradiation with different kinds of Cun+-based/graphene nanocomposites (Cu–graphene, CuO–graphene, Cu2O–graphene and Cu–Cu2O–graphene) and relevant single-component catalysts (Cu–Cu2O, Cu, Cu2O and graphene).
Fig. 6 Electrochemical impedance spectroscopy of Cu2O, Cu2O–graphene, Cu–Cu2O–graphene.
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Fig. 7 H2 evolution curves (a) and relevant H2 evolution rate (b) under simulated sunlight irradiation with addition of different amount of GO added (0.1% GN, 1.0% GN, 4.6% GN and 8.0% GN, respectively).
Compared with Cu2O–graphene, Cu–Cu2O–graphene possesses higher conductivity and faster transfer of photoexcited electrons, which finally dominate H2 evolution efficiency. Furthermore, we also compared comprehensively Cu–Cu2O– graphene samples with different graphene contents (0.1%, 1.0%, 4.6% and 8.0%) in terms of their catalytic activity (Fig. S4, ESI†). As is clearly shown in Fig. 7a and b, the Cu–Cu2O–graphene (1.0%) has a faster H2 evolution rate (16.2 mmol h 1 g 1) than those with a high or low content of graphene. This fact can be explained by following two aspects. On the one hand, insufficient graphene makes electron transfer between Cu2O and Pt particles harder and prevents Cu2O spheres from dispersing well onto the graphene sheets so that unnecessary aggregations of particles shadow most active sites for harvesting light. On the other hand, excess graphene inevitably covers most Cu2O active species so that energy transformation is cut irreversibly. Therefore, an appropriate amount of graphene is necessary to ensure excellent catalytic effect. The stability of the as-prepared porous Cu–Cu2O–graphene has also been investigated since it is very important for practical applications. Here, we choose the sample with the best catalytic efficiency [Cu–Cu2O–graphene (1.0%)] for testing. Keeping the reaction environment unchanged, we test the H2 generation rate as a function of time. As shown in Fig. 8a, under such conditions, except for slight fluctuations, our catalyst exhibits excellent stability along with activity during six cycles and after that the evolution speed decreases. In addition, the recovered Cu–Cu2O–graphene catalyst after a long photocatalysis time has a slight change in color (dark red), crystal structure and composition (by XRD, Fig. S5, ESI†). However, the mass ratio of Cu to Cu2O changes greatly
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Fig. 8 Stability ofn H2 evolution under simulated sunlight irradiation (a) and proposed ‘‘two-way transfer’’ mechanism for hydrogen production with Cu–Cu2O–graphene (b).
[before: Cu 66.3(5)%, Cu2O 33.7(5)%; and after: Cu 17.9(5)%, Cu2O 82.1(5)%]. Therefore, we attribute the stable evolution speed to the increase of the Cu2O active component. We deduce that graphene facilitates particle dispersion effectively to prevent aggregation effects during long time reaction, while Cu–Cu2O particle also acts as brace to prevent graphene from curling, crumpling and aggregating. Owning to excellent synergy and mutual dispersion effects between Cu–Cu2O and graphene, long term stability is gained with maintenance of activity, which confirms the suitability of this system for further modification and practical use. Based on above experimental results, a ‘‘two-way transfer’’ mechanism can be proposed, as shown in Fig. 8b. When Cu2O was excited by simulated sunlight, it can produce photoelectrons and holes simultaneously and those photoelectrons jump to the conductive band. With the help of the highly conductive Cu component, photoelectrons are delivered faster and separate from holes entirely, thus recombination of electrons and holes can be avoided effectively.28,29 Some of these photoelectrons are rapidly captured by Pt co-catalyst to participate further in H2 evolution catalysis, while others pass through the graphene support to the Pt particles loaded on it and then take part in hydrogen production as well. This kind of ‘‘two-way transfer’’ could separate and pass photoelectrons efficiently so that faster delivery speeds and higher H2 evolution rates can be obtained. It is believed that the Cu and graphene in Cu–Cu2O–graphene nanocomposites possess three features: broadening the range of light absorption; promoting electron transfer speed; and preventing effectively photoelectron–hole recombination. In addition, we infer that
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the generation of Cu species inevitably decreases the concentration of active Cu2O because of Cu2+ consumption, but definite efficiency acceleration reveals the importance of a Cu component coexisting at the cost of some Cu2O in the catalysts. Significant improvement in catalysis performance (Fig. 5) owing to the addition of Cu species firmly supports this deduction.
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Conclusions In summary, we have demonstrated a facile and simple method to prepare mesoporous Cu–Cu2O–graphene nanocomposites via a reflux synthesis route. Compared with the widely studied Cu2O–graphene, the novel Cu–Cu2O–graphene exhibits superior catalytic H2 evolution performance in terms of catalytic activity and stability. This excellent catalytic performance can be attributed to a ‘‘two-way’’ mechanism of Cu and graphene, which features: (i) a higher excitation activity; (ii) faster electron transfer; and (iii) lower recombination possibility of photo-generated electrons and holes, all of which do great help to improve catalysis performance. It is also believed that further modification and improvement can be applied to study the Cu–Cu2O–graphene system.
Acknowledgements This work was financially supported by the NNSFC (No. 21173021, 21231002, 21276026, 21471016 and 21271023), the 111 Project (B07012), 973 Program (2014CB932103), and the Institute of Chemical Materials, China Academy of Engineering Physics (No. 20121941006).
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