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CVD growth of large-area graphene over Cu foil by atmospheric pressure and its application in H2 evolution
Solid State Sciences 46 (2015) 84e88
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
CVD growth of large-area graphene over Cu foil by atmospheric pressure and its application in H2 evolution Shu Ye a, Kefayat Ullah a, Lei Zhu a, Asghar Ali a, Won Kweon Jang b, Won-Chun Oh a, * a b
Department of Advanced Materials Science & Engineering, Hanseo University, Chungnam 356-706, Korea Division of Electronic, Computer and Communication Engineering, Hanseo University, Chungnam 356-706, Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2015 Received in revised form 22 May 2015 Accepted 24 May 2015 Available online 28 May 2015
This study reports that the atmospheric pressure chemical vapor deposition (CVD) growth of large area graphene (LAG) over a Cu foil. The obtained large area graphene was further decorated with TiO2 nanoparticles via ultrasonic method. The surface structure, crystal phase, and elemental identification of these obtained LAG/TiO2 composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and Raman spectra. The photocatalytic H2 evolution result illustrates Cu-LAG/TiO2 has been found to be a potential catalyst for conversion of solar energy to clean hydrogen energy under visible light-driven despite that the H2 evolution activity is not high enough in this stage. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: CVD Ultrasound Graphene Cu foil H2 evolution
1. Introduction The successful growth of large-area graphene [1e4] has the potential to revolutionize applications of graphene in electronic and mechanical devices. Recently, CVD growth has been used to realize such films on metal surfaces. Most of the CVD growth uses polycrystalline Ni [5e8], Fe [9], and Cu [10e14] films/foils, and it has been considered that Ni and Fe give inhomogeneous graphene films with multi-layer flakes. The LAG has also been obtained using a solid-phase layer-stacking approach with ethanol wetting [15]. However, these nanocarbon hybrids required multiple fabrication/ synthesis steps or used rGO as the non-ideal substitute of graphene. Titanium dioxide (TiO2) is the most widely used semiconductor in environmental pollution control, conversion and energy storage, sensors, photovoltaics, and Li batteries because of its unique photoelectric properties, high chemical stability, low cost, and low toxicity towards both humans and the environment [16e20]. The improvement and optimization of TiO2 as a photocatalyst is a major task for technical applications of heterogeneous photocatalysis in the future. In this sense, many investigations for the enhancement of photocatalytic activity either in the UV or visible region have
* Corresponding author. E-mail address: [email protected] (W.-C. Oh). http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.010 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
been carried out [21e24]. However, many problems remain unresolved for practical applications such as narrow light response range and low separation probability of the photoinduced electronehole pairs in the TiO2 photocatalytic system. In order to improve the photocatalytic activity and the response into the visible light region, TiO2 doping with transition metals has been widely investigated [25]. In this study，the LAG materials were grown on Cu foil by using a single step CVD method under atmospheric pressure. The hybrid material was further decorated with TiO2 via an ultrasound method at a low temperature. The catalysts were characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The catalytic efficiency of the Cu-LAG/TiO2 composites was evaluated by H2 evolution under visible light. 2. Experimental procedures 2.1. Materials and reagents Benzene used as a carbon precursor material was purchased from Dae-Jung Chemical and Metals Co. Ltd Korea. Titanium (IV) nbutoxide (TNB, C16H36O4Ti), which was used as a titanium precursor, was purchased from Samchun Pure Chemical Co. Ltd., Korea. The Cu foil (99.9%), annealed uncoated, was used as the substrate
S. Ye et al. / Solid State Sciences 46 (2015) 84e88
material to grow LAG. Argon and nitrogen gas were purchased from Samchun Pure Chemical Co. Ltd., Korea. A specially designed split Si tube furnace divided into two parts, the inner tube and the outer tube, was used. The dimensions of the tube were 30 cm length and 4 cm outer diameter, with two nozzles, one for the inlet of benzene vapors and the other for the inlet of Ar and N2 gas. The inner furnace, also called the heating center, having a 8-cm length and 5cm diameter, was used to grow LAG on Cu foil. Ethylene glycol was purchased from Dae-Jung Chemical and Metals Co. Ltd, Korea. All chemicals were used without further purification. 2.2. Preparation of LAG material Briefly, the benzene was first heated below its boiling temperature, and vapors were produced. The vapors were then transferred to the tube simultaneously with Ar gas, which also acted as a carrier for benzene vapors and prevented the Cu foil from reacting with water molecules. The vapor flow and the Ar gas ratio were controlled through the control valve. The furnace temperature was increased to 500 C at the first step, and Ar gas was released into the Si furnace. After reaching the temperature of around 500 C, a Cu foil was inserted into the inner tube by flowing a controlled amount of Ar gas and benzene vapors. After 5 min, the coated Cu foil was checked and scratched using a blade, and then the powder was scratched from the coated Cu foil before being dried at 100 C in a dry oven [26]. 2.3. Attachment of TiO2 on graphene sheet 2 mL of TNB (about 6.5 105 mol) was added to 10 mL of the ethanol solution, followed by magnetic stirring for 5 min; after that, 10 mL distilled water was dropwised added to the solution with constant stirring. Then, we added 0.4 g of as-prepared LAG and 30 mL ethylene glycol into the solution. This step was then followed by constant stirring and ultrasonication (using 750 W, Ultrasonic Processor VCX 750, Korea) for 4 h. After completion, the black solution was filtered, washed 3 times with deionized water and ethanol, and then dried at 873 K. Finally, the sample was heated at 873 K for 1 h. The weight ratios of LAG to TNB were approximately
85
2:10. These obtained composites were named Cu-LAG/TiO2. 2.4. Characterization The crystal structures and phases of the samples were obtained by XRD (Shimata XD-D1, Japan) with Cu Ka radiation (l ¼ 1.54056) in the range of 2q from 10 to 80 at a scan speed of 1.20 m1. The decomposition kinetics for the photocatalytic activity was measured in the range between 300 and 700 nm using a spectrometer (Optizen POP, Mecasys, Korea). Energy dispersive X-ray spectroscopy (EDX) was also employed for elemental analysis. High Resolution Transmission Electron Microscopy (HRTEM, JEOL, JEM2010, Japan) was used to observe the surface state and structure of the photocatalyst composites at an acceleration voltage of 200 kV. 3. Results and discussion 3.1. Characterization of Cu-LAG/TiO2 composite The XRD patterns of he Cu-LAG/TiO2 composite are shown in Fig. 1. The patterns clearly showed peaks of TiO2, namely, the planes (101), (004), (200), (211), (204), (220), and (215) at 2 q values of about 25.38, 37.82, 48.18, 54.4, 62.92, 69.92, and 74.91, respectively. According to JCPDS-21-1272, all patterns were assigned to the polycrystalline anatase phase structure of TiO2. The crystallite size of the samples was calculated from the full-width at half maxima of the (101) peak of anatase TiO2 by the DebyeeSherrer equation:
d¼kl ¼ bcosq
(1)
where d represents the crystallite size of; l represents the wavelength of the incident X-ray; b is the full-width at half maximum (FWHM) of the diffraction peak; and y represents the scattering angle. The average crystalline size calculated from the above equation for pure TiO2 was 9.5 nm, and the average crystalline size of Cu-LAG/TiO2 composite was 7.7 nm [27]. Furthermore, the absence of graphene peaks in the XRD patterns of composite catalysts showed that CVD-made graphene did not change the
Fig. 1. X-ray diffraction patterns of the Cu-LAG/TiO2 composite.
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Fig. 2. Energy dispersive X-ray elemental analysis of Cu-LAG/TiO2composite.
Table 1 Energy dispersive X-ray elemental microanalysis (wt%) of the Cu-LAG/TiO2 composites. Sample
Element C
O
Ti
Cu
Zn
Total
LAG/T2
26.38
17.87
9.68
45.43
0.54
100.00
structure of TiO2 [28]. The EDX analysis of the Cu-LAG/TiO2 composite indicated that the nanocomposites were synthesized successfully. Fig. 2 shows that the composite were rich in C, O, Cu, and Ti elements rather than any other metal elements [29]. Very low concentrations of impurities were observed, and these impurities may have been due to starting the experiment with CVD-made lager-area graphene [30]. The elemental contents of the Cu-LAG/TiO2 is listed in Table 1. The morphology of CVD-grown graphene on Cu foil was observed by HRTEM. Fig. 3a and b is a typical HRTEM image of 5-nm CVD-grown graphene on a copper foil substrate at 500 C. Fig. 3a and b gives a larger-area graphene with some dark region considered to be valleys between the two graphene domains. It is well known that the graphene films are easily grown on copper foil by CVD [31]. From Fig. 3c, d, and e, the TEM images show that the LAG in the nanocomposite were rippled and resembled crumpled silk veil waves, which were appropriate for immobilizing the nanoparticles [32]. Furthermore, a highly dense deposit of the TiO2 and
Cu2O or CuO nanoparticles on the LAG can be observed in Fig. 3d and e, which were consistent with the XRD patterns. This morphology was expected to be beneficial for the photoinduced electron transfer between the TiO2 and Cu2O or CuO nanoparticles to the LAG. In addition, the HRTEM images clearly showed the presence of smaller particles with diameters of approximately 15 nm as evidenced by Fig. 3e, which show the TiO2 nanoparticles derived from the TNB material. 3.2. Photochemical hydrogen production The 0.05 g Cu-LAG/TiO2 nanocomposites were well dispersed in 100 ml aqueous solution containing Na2S/Na2SO3 as a sacrificial reagent. The TiO2 nanoparticles on the LAG surface act as H2 evolution centers, as decorated by the microwave-assisted method. A 356-nm light source was adjusted so that the maximum area of the sealed container would be exposed. Quantum yield (QY) was observed using the equation
np ¼ t S Q
(2)
QYð%Þ ¼ nH np 100
(3)
where, np is the amount of incident photons, t is irradiation time, s is irradiation area in m2, and Q is photon flux of the incident light. The Q (Y) was calculated from the ratio of the number of reacted
Fig. 3. HRTEM images of (a), (b) LAG; TEM images of (c), (d) and (e) Cu-LAG/TiO2.
S. Ye et al. / Solid State Sciences 46 (2015) 84e88
Fig. 4. Photocatalytic H2 evolution of Cu-LAG/TiO2 and P-25 using Na2S/Na2SO3 as the sacrificial reagent.
electrons during hydrogen evolution to the number of incident photons according to Equation (3), where nH is the amount of photogenerated H2 [33e35]. After 9 h irradiation, the photocatalytic H2 evolution and quantum yield efficiency (QYs) for the Cu-LAG/TiO2 nanocomposites and P-25 are shown in Figs. 4 and 5, respectively. The QYs of individual Cu-LAG/TiO2 nanocomposite were 6.2%, using Na2S/Na2SO3 as the reagent. In contrast, H2 evolution and QYs for P-25 were smaller than those of the Cu-LAG/TiO2 nanocomposite under the same conditions. The decrease in H2 evolution and corresponding QY efficiency may have been attributed to the fast recombination rate of the excited electrons and hole pair in TiO2. These results highlight the importance of Cu-LAG, which is absent in P-25. The sacrificial reagent provided electrons to consume the photogenerated holes, and TiO2 acted as a reaction center to produce H2 from water. The H2 evolution plot for a 20% methanol solution with Cu-LAG/TiO2 and P-25 as a photocatalyst is shown in Fig. 6. Cyclic photocatalytic hydrogen evolution experiments were carried out to further demonstrate the photostability and cyclic performance of the Cu-LAG/TiO2 composite photocatalysts. As shown in Fig. 7, the photocatalysts exhibited a minor loss of photocatalytic activity for hydrogen evolution under the same conditions after three runs, indicating the photocatalytic stability of our nanocomposite. The reused catalyst did not show any noticeable change in QYs efficiency, which emphasizes the excellent chemical
87
Fig. 6. Quantum yields for hydrogen evolution by Cu-LAG/TiO2 and P-25 with 20% methanol.
Fig. 7. Cyclic test of the Cu-LAG/TiO2 nanocomposite using Na2S/Na2SO3 as the sacrificial reagent under UV light irradiation.
stability of the catalysts and their benefit in practical applications. The possible reasons for H2 evolution of composite catalyst under visible light irradiation are the following. Firstly, the photocatalytic activity of Cu-LAG/TiO2 composite catalyst might be attributed to extended light absorption into visible light region due to copper and LAG incorporation. Secondly, in Cu-LAG/TiO2 system, the excited electrons of TiO2 could transfer from the conduction band to LAG through percolation mechanism [36,37]. Thus, in the composite, LAG served as an acceptor of generated electrons of TiO2 and effectively suppressed the charge recombination, leaving more charge carriers to form reactive species and promote the H2 evolution [38]. 4. Conclusions
Fig. 5. Quantum yields for hydrogen evolution by Cu-LAG/TiO2 and P-25 with the Na2S/Na2SO3 aqueous solution.
In conclusion, we successfully investigated a new and short route to produce LAG. The LAG materials were grown on Cu foil by using a single step CVD method under atmospheric pressure. This large-area graphene can be produced in industries at a large scale by further controlling experimental parameters to optimum values. We demonstrated the production of large-area graphene at a very short reaction time in a nitrogen/argon environment using a chemical vapor deposition method. The HRTEM images showed the multi-layer graphene growth on Cu foil. The obtained large-area graphene was decorated with TiO2 nanoparticles successfully via an ultrasonic method. Cu-LAG/TiO2 is a stable efficient
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photocatalyst for photocatalytic H2 evolution from water. This high photocatalytic activity might be ascribed to the synergistic effect between Cu and LAG, and it has been found to be a potential catalyst for conversion of solar energy to clean hydrogen energy under visible light-driven despite that the H2 evolution activity is not high enough in this stage. References [1] V. Panchal, A. Lartsev, A. Manzin, et al., Visualisation of edge effects in sidegated graphene nanodevices, Sci. Rep. 4 (2014) 5881, http://dx.doi.org/ 10.1038/srep05881. [2] Patrick Maher, et al., Tunable fractional quantum Hall phases in bilayer graphene, Science 345 (2014) 61e64. [3] K.S. Kim, et al., Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706e710. [4] X. Li, et al., Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (2009) 1312e1314. [5] L. Gao, et al., Repeated growth and bubbling transfer of graphene with millimeter size single-crystal grains using platinum, Nat. Commun. 3 (2012) 699. [6] P.W. Sutter, J.I. Flege, E.A. Sutter, Epitaxial graphene on ruthenium, Nat. Mater. 7 (2008) 406e411. [7] Q. Yu, Jie Lian, Sujitra Siriponglert, et al., Graphene segregated on Ni surfaces and transferred to insulators, Appl. Phys. Lett. 93 (2008) 113103. [8] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, et al., Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706. [9] A. Reina, X. Jia, J. Ho, et al., Large Area, few-layer graphene films on arbitrary substrates by chemical vapor deposition, Nano Lett. 9 (2009) 30e35. [10] J. Coraux, A.T.N. Diaye, C. Busse, T. Michely, Structural coherency of graphene on Ir (111), Nano Lett. 8 (2008) 565e570. [11] C. Berger, Z. Song, T. Li, et al., Electronic confinement and coherence in patterned epitaxial graphene, Science 312 (2006) 1191. [12] K.V. Emtsev, A. Bostwick, K. Horn, et al., Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide, Nat. Mater. 8 (2009) 203e207. [13] H. Wang, et al., Controllable synthesis of submillimeter single-crystal monolayer graphene domains on copper foils by suppressing nucleation, J. Am. Chem. Soc. 134 (2012) 3627e3630. [14] P. Sutter, Epitaxial graphene: how silicon leaves the scene, Nat. Mater. 8 (2009) 171e172. [15] Min-Quan Yang, Nan Zhang, Mario Pagliaro, Yi-Jun Xu, Artificial photosynthesis over grapheneesemiconductor composites. Are we getting better? Chem. Soc. Rev. 43 (2014) 8240e8254. [16] K.A. Ritter, J.W. Lyding, The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons, Nat. Mater 8 (2009) 235e242. [17] C. Tao, et al., Spatially resolving edge states of chiral graphene nanoribbons, Nat. Phys. 7 (2011) 616e620. [18] M. Pan, et al., Topographic and spectroscopic characterization of electronic edge states in CVD grown graphene nanoribbons, Nano Lett. 12 (2012) 1928e1933.
[19] X. Zhang, et al., Experimentally engineering the edge termination of graphene nanoribbons, ACS Nano 7 (2013) 198e202. [20] A.C. Ferrari, D.M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nat. Nanotechnol. 8 (2013) 235e246. [21] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, et al., Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401/1e/4. [22] N. Zhang, Y.H. Zhang, Y.J. Xu, Recent progress on graphene-based photocatalysts: current status and future perspectives, Nanoscale 4 (2012) 5792e5813. [23] L. Zhu, S.B. Jo, S. Ye, K. Ullah, Z.D. Meng, W.C. Oh, A green and direct synthesis of photosensitized CoS2egraphene/TiO2 hybrid with high photocatalytic performance, J. Ind. Eng. Chem. 22 (2015) 264e271. [24] L. Gong, I.A. Kinloch, R.J. Young, I. Riaz, R. Jalil, K.S. Novoselov, Adv. Mater. 22 (2010) 2694e2697. [25] Y.H. Zhang, Z.R. Tang, X.Z. Fu, Y.J. Xu, Engineering the unique 2D Mat of graphene to achieve graphene-TiO2 nanocomposite for photocatalytic selective transformation: what advantage does graphene have over its forebear carbon nanotube? ACS Nano 5 (9) (2011) 7426e7435. [26] J.J. Tian, B.S. Hu, Z.D. Wei, Y. Jin, Z.T. Luo, M.R. Xia, Q.J. Pan, Y.L. Liu, Surface structure deduced differences of copper foil and film for graphene CVD growth, Appl. Surf. Sci. 300 (2014) 73e79. [27] S. Reich, C. Thomsen, Raman spectroscopy of graphite, Phil. Trans. R. Soc. Lond. A 362 (2004) 2271e2288. [28] K. Ullah, S. Ye, L. Zhu, Z.D. Meng, et al., Microwave assisted synthesis of a noble metal-graphene hybrid photocatalyst for high efficient decomposition of organic dyes under visible light, Mater. Sci. Eng. B 180 (2014) 20e26. [29] A. Das, S. Pisana, B. Chakraborty, Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (2008) 210e215. [30] M.Q. Yang, Y.J. Xu, Selective photoredox using graphene-based composite photocatalysts, Phys. Chem. Chem. Phys. 15 (2013) 19102e19118. [31] C. Han, M.Q. Yang, B. Weng, Y.J. Xu, Improving the photocatalytic activity and anti-photocorrosion of semiconductor ZnO by coupling with versatile carbon, Phys. Chem. Chem. Phys. 16 (2014) 16891e16903. [32] C. Burda, Y. Lou, X. Chen, A.C.S. Semia, J. Stout, J.L. Gole, Enhanced nitrogen doping in TiO2 nanoparticles, Nano Lett. 3 (2003) 1049e1051. [33] X. Lia, W. Caia, I. Junga, et al., Synthesis, Characterization, and properties of large-area graphene films, ECS Trans. 19 (5) (2009) 41e52. [34] D. Pan, et al., Li storage properties of disordered graphene nanosheets, Chem. Mater. 21 (2009) 3136e3142. [35] H. Ago, Y. Ito, N. Mizuta, K. Yoshida, B. Hu, C.M. Orofeo, et al., Epitaxial chemical vapor deposition growth of single-layer graphene over cobalt film crystallized on sapphire, ACS Nano 4 (2010) 7407e7414. [36] C.Y. Chen, D. Dai, G.X. Chen, J.H. Yu, K. Nishimura, C.T. Lin, N. Jiang, Z.L. Zhan, Rapid growth of single-layer graphene on the insulating substrates by thermal CVD, Appl. Surf. Sci. 346 (2015) 41e45. [37] R.K. de Castro, J.R. Araujo, R. Valaski, L.O.O. Costa, B.S. Archanjo, B. Fragneaud, M. Cremona, C.A. Achete, New transfer method of CVD-grown graphene using a flexible, transparent and conductive polyaniline-rubber thin film for organic electronic applications, Chem. Eng. J. 273 (2015) 509e518. [38] N.R. Khalid, E. Ahmed, Zhanglian Hong, M. Ahmad, Yuewei Zhang, Sadia Khalid, Ceram. Int. 39 (2013) 7107e7113.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
CVD growth of large-area graphene over Cu foil by atmospheric pressure and its application in H2 evolution Shu Ye a, Kefayat Ullah a, Lei Zhu a, Asghar Ali a, Won Kweon Jang b, Won-Chun Oh a, * a b
Department of Advanced Materials Science & Engineering, Hanseo University, Chungnam 356-706, Korea Division of Electronic, Computer and Communication Engineering, Hanseo University, Chungnam 356-706, Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2015 Received in revised form 22 May 2015 Accepted 24 May 2015 Available online 28 May 2015
This study reports that the atmospheric pressure chemical vapor deposition (CVD) growth of large area graphene (LAG) over a Cu foil. The obtained large area graphene was further decorated with TiO2 nanoparticles via ultrasonic method. The surface structure, crystal phase, and elemental identification of these obtained LAG/TiO2 composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and Raman spectra. The photocatalytic H2 evolution result illustrates Cu-LAG/TiO2 has been found to be a potential catalyst for conversion of solar energy to clean hydrogen energy under visible light-driven despite that the H2 evolution activity is not high enough in this stage. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: CVD Ultrasound Graphene Cu foil H2 evolution
1. Introduction The successful growth of large-area graphene [1e4] has the potential to revolutionize applications of graphene in electronic and mechanical devices. Recently, CVD growth has been used to realize such films on metal surfaces. Most of the CVD growth uses polycrystalline Ni [5e8], Fe [9], and Cu [10e14] films/foils, and it has been considered that Ni and Fe give inhomogeneous graphene films with multi-layer flakes. The LAG has also been obtained using a solid-phase layer-stacking approach with ethanol wetting [15]. However, these nanocarbon hybrids required multiple fabrication/ synthesis steps or used rGO as the non-ideal substitute of graphene. Titanium dioxide (TiO2) is the most widely used semiconductor in environmental pollution control, conversion and energy storage, sensors, photovoltaics, and Li batteries because of its unique photoelectric properties, high chemical stability, low cost, and low toxicity towards both humans and the environment [16e20]. The improvement and optimization of TiO2 as a photocatalyst is a major task for technical applications of heterogeneous photocatalysis in the future. In this sense, many investigations for the enhancement of photocatalytic activity either in the UV or visible region have
* Corresponding author. E-mail address: [email protected] (W.-C. Oh). http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.010 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
been carried out [21e24]. However, many problems remain unresolved for practical applications such as narrow light response range and low separation probability of the photoinduced electronehole pairs in the TiO2 photocatalytic system. In order to improve the photocatalytic activity and the response into the visible light region, TiO2 doping with transition metals has been widely investigated [25]. In this study，the LAG materials were grown on Cu foil by using a single step CVD method under atmospheric pressure. The hybrid material was further decorated with TiO2 via an ultrasound method at a low temperature. The catalysts were characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. The catalytic efficiency of the Cu-LAG/TiO2 composites was evaluated by H2 evolution under visible light. 2. Experimental procedures 2.1. Materials and reagents Benzene used as a carbon precursor material was purchased from Dae-Jung Chemical and Metals Co. Ltd Korea. Titanium (IV) nbutoxide (TNB, C16H36O4Ti), which was used as a titanium precursor, was purchased from Samchun Pure Chemical Co. Ltd., Korea. The Cu foil (99.9%), annealed uncoated, was used as the substrate
S. Ye et al. / Solid State Sciences 46 (2015) 84e88
material to grow LAG. Argon and nitrogen gas were purchased from Samchun Pure Chemical Co. Ltd., Korea. A specially designed split Si tube furnace divided into two parts, the inner tube and the outer tube, was used. The dimensions of the tube were 30 cm length and 4 cm outer diameter, with two nozzles, one for the inlet of benzene vapors and the other for the inlet of Ar and N2 gas. The inner furnace, also called the heating center, having a 8-cm length and 5cm diameter, was used to grow LAG on Cu foil. Ethylene glycol was purchased from Dae-Jung Chemical and Metals Co. Ltd, Korea. All chemicals were used without further purification. 2.2. Preparation of LAG material Briefly, the benzene was first heated below its boiling temperature, and vapors were produced. The vapors were then transferred to the tube simultaneously with Ar gas, which also acted as a carrier for benzene vapors and prevented the Cu foil from reacting with water molecules. The vapor flow and the Ar gas ratio were controlled through the control valve. The furnace temperature was increased to 500 C at the first step, and Ar gas was released into the Si furnace. After reaching the temperature of around 500 C, a Cu foil was inserted into the inner tube by flowing a controlled amount of Ar gas and benzene vapors. After 5 min, the coated Cu foil was checked and scratched using a blade, and then the powder was scratched from the coated Cu foil before being dried at 100 C in a dry oven [26]. 2.3. Attachment of TiO2 on graphene sheet 2 mL of TNB (about 6.5 105 mol) was added to 10 mL of the ethanol solution, followed by magnetic stirring for 5 min; after that, 10 mL distilled water was dropwised added to the solution with constant stirring. Then, we added 0.4 g of as-prepared LAG and 30 mL ethylene glycol into the solution. This step was then followed by constant stirring and ultrasonication (using 750 W, Ultrasonic Processor VCX 750, Korea) for 4 h. After completion, the black solution was filtered, washed 3 times with deionized water and ethanol, and then dried at 873 K. Finally, the sample was heated at 873 K for 1 h. The weight ratios of LAG to TNB were approximately
85
2:10. These obtained composites were named Cu-LAG/TiO2. 2.4. Characterization The crystal structures and phases of the samples were obtained by XRD (Shimata XD-D1, Japan) with Cu Ka radiation (l ¼ 1.54056) in the range of 2q from 10 to 80 at a scan speed of 1.20 m1. The decomposition kinetics for the photocatalytic activity was measured in the range between 300 and 700 nm using a spectrometer (Optizen POP, Mecasys, Korea). Energy dispersive X-ray spectroscopy (EDX) was also employed for elemental analysis. High Resolution Transmission Electron Microscopy (HRTEM, JEOL, JEM2010, Japan) was used to observe the surface state and structure of the photocatalyst composites at an acceleration voltage of 200 kV. 3. Results and discussion 3.1. Characterization of Cu-LAG/TiO2 composite The XRD patterns of he Cu-LAG/TiO2 composite are shown in Fig. 1. The patterns clearly showed peaks of TiO2, namely, the planes (101), (004), (200), (211), (204), (220), and (215) at 2 q values of about 25.38, 37.82, 48.18, 54.4, 62.92, 69.92, and 74.91, respectively. According to JCPDS-21-1272, all patterns were assigned to the polycrystalline anatase phase structure of TiO2. The crystallite size of the samples was calculated from the full-width at half maxima of the (101) peak of anatase TiO2 by the DebyeeSherrer equation:
d¼kl ¼ bcosq
(1)
where d represents the crystallite size of; l represents the wavelength of the incident X-ray; b is the full-width at half maximum (FWHM) of the diffraction peak; and y represents the scattering angle. The average crystalline size calculated from the above equation for pure TiO2 was 9.5 nm, and the average crystalline size of Cu-LAG/TiO2 composite was 7.7 nm [27]. Furthermore, the absence of graphene peaks in the XRD patterns of composite catalysts showed that CVD-made graphene did not change the
Fig. 1. X-ray diffraction patterns of the Cu-LAG/TiO2 composite.
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S. Ye et al. / Solid State Sciences 46 (2015) 84e88
Fig. 2. Energy dispersive X-ray elemental analysis of Cu-LAG/TiO2composite.
Table 1 Energy dispersive X-ray elemental microanalysis (wt%) of the Cu-LAG/TiO2 composites. Sample
Element C
O
Ti
Cu
Zn
Total
LAG/T2
26.38
17.87
9.68
45.43
0.54
100.00
structure of TiO2 [28]. The EDX analysis of the Cu-LAG/TiO2 composite indicated that the nanocomposites were synthesized successfully. Fig. 2 shows that the composite were rich in C, O, Cu, and Ti elements rather than any other metal elements [29]. Very low concentrations of impurities were observed, and these impurities may have been due to starting the experiment with CVD-made lager-area graphene [30]. The elemental contents of the Cu-LAG/TiO2 is listed in Table 1. The morphology of CVD-grown graphene on Cu foil was observed by HRTEM. Fig. 3a and b is a typical HRTEM image of 5-nm CVD-grown graphene on a copper foil substrate at 500 C. Fig. 3a and b gives a larger-area graphene with some dark region considered to be valleys between the two graphene domains. It is well known that the graphene films are easily grown on copper foil by CVD [31]. From Fig. 3c, d, and e, the TEM images show that the LAG in the nanocomposite were rippled and resembled crumpled silk veil waves, which were appropriate for immobilizing the nanoparticles [32]. Furthermore, a highly dense deposit of the TiO2 and
Cu2O or CuO nanoparticles on the LAG can be observed in Fig. 3d and e, which were consistent with the XRD patterns. This morphology was expected to be beneficial for the photoinduced electron transfer between the TiO2 and Cu2O or CuO nanoparticles to the LAG. In addition, the HRTEM images clearly showed the presence of smaller particles with diameters of approximately 15 nm as evidenced by Fig. 3e, which show the TiO2 nanoparticles derived from the TNB material. 3.2. Photochemical hydrogen production The 0.05 g Cu-LAG/TiO2 nanocomposites were well dispersed in 100 ml aqueous solution containing Na2S/Na2SO3 as a sacrificial reagent. The TiO2 nanoparticles on the LAG surface act as H2 evolution centers, as decorated by the microwave-assisted method. A 356-nm light source was adjusted so that the maximum area of the sealed container would be exposed. Quantum yield (QY) was observed using the equation
np ¼ t S Q
(2)
QYð%Þ ¼ nH np 100
(3)
where, np is the amount of incident photons, t is irradiation time, s is irradiation area in m2, and Q is photon flux of the incident light. The Q (Y) was calculated from the ratio of the number of reacted
Fig. 3. HRTEM images of (a), (b) LAG; TEM images of (c), (d) and (e) Cu-LAG/TiO2.
S. Ye et al. / Solid State Sciences 46 (2015) 84e88
Fig. 4. Photocatalytic H2 evolution of Cu-LAG/TiO2 and P-25 using Na2S/Na2SO3 as the sacrificial reagent.
electrons during hydrogen evolution to the number of incident photons according to Equation (3), where nH is the amount of photogenerated H2 [33e35]. After 9 h irradiation, the photocatalytic H2 evolution and quantum yield efficiency (QYs) for the Cu-LAG/TiO2 nanocomposites and P-25 are shown in Figs. 4 and 5, respectively. The QYs of individual Cu-LAG/TiO2 nanocomposite were 6.2%, using Na2S/Na2SO3 as the reagent. In contrast, H2 evolution and QYs for P-25 were smaller than those of the Cu-LAG/TiO2 nanocomposite under the same conditions. The decrease in H2 evolution and corresponding QY efficiency may have been attributed to the fast recombination rate of the excited electrons and hole pair in TiO2. These results highlight the importance of Cu-LAG, which is absent in P-25. The sacrificial reagent provided electrons to consume the photogenerated holes, and TiO2 acted as a reaction center to produce H2 from water. The H2 evolution plot for a 20% methanol solution with Cu-LAG/TiO2 and P-25 as a photocatalyst is shown in Fig. 6. Cyclic photocatalytic hydrogen evolution experiments were carried out to further demonstrate the photostability and cyclic performance of the Cu-LAG/TiO2 composite photocatalysts. As shown in Fig. 7, the photocatalysts exhibited a minor loss of photocatalytic activity for hydrogen evolution under the same conditions after three runs, indicating the photocatalytic stability of our nanocomposite. The reused catalyst did not show any noticeable change in QYs efficiency, which emphasizes the excellent chemical
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Fig. 6. Quantum yields for hydrogen evolution by Cu-LAG/TiO2 and P-25 with 20% methanol.
Fig. 7. Cyclic test of the Cu-LAG/TiO2 nanocomposite using Na2S/Na2SO3 as the sacrificial reagent under UV light irradiation.
stability of the catalysts and their benefit in practical applications. The possible reasons for H2 evolution of composite catalyst under visible light irradiation are the following. Firstly, the photocatalytic activity of Cu-LAG/TiO2 composite catalyst might be attributed to extended light absorption into visible light region due to copper and LAG incorporation. Secondly, in Cu-LAG/TiO2 system, the excited electrons of TiO2 could transfer from the conduction band to LAG through percolation mechanism [36,37]. Thus, in the composite, LAG served as an acceptor of generated electrons of TiO2 and effectively suppressed the charge recombination, leaving more charge carriers to form reactive species and promote the H2 evolution [38]. 4. Conclusions
Fig. 5. Quantum yields for hydrogen evolution by Cu-LAG/TiO2 and P-25 with the Na2S/Na2SO3 aqueous solution.
In conclusion, we successfully investigated a new and short route to produce LAG. The LAG materials were grown on Cu foil by using a single step CVD method under atmospheric pressure. This large-area graphene can be produced in industries at a large scale by further controlling experimental parameters to optimum values. We demonstrated the production of large-area graphene at a very short reaction time in a nitrogen/argon environment using a chemical vapor deposition method. The HRTEM images showed the multi-layer graphene growth on Cu foil. The obtained large-area graphene was decorated with TiO2 nanoparticles successfully via an ultrasonic method. Cu-LAG/TiO2 is a stable efficient
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photocatalyst for photocatalytic H2 evolution from water. This high photocatalytic activity might be ascribed to the synergistic effect between Cu and LAG, and it has been found to be a potential catalyst for conversion of solar energy to clean hydrogen energy under visible light-driven despite that the H2 evolution activity is not high enough in this stage. References [1] V. Panchal, A. Lartsev, A. Manzin, et al., Visualisation of edge effects in sidegated graphene nanodevices, Sci. Rep. 4 (2014) 5881, http://dx.doi.org/ 10.1038/srep05881. [2] Patrick Maher, et al., Tunable fractional quantum Hall phases in bilayer graphene, Science 345 (2014) 61e64. [3] K.S. Kim, et al., Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706e710. [4] X. Li, et al., Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (2009) 1312e1314. [5] L. Gao, et al., Repeated growth and bubbling transfer of graphene with millimeter size single-crystal grains using platinum, Nat. Commun. 3 (2012) 699. [6] P.W. Sutter, J.I. Flege, E.A. Sutter, Epitaxial graphene on ruthenium, Nat. Mater. 7 (2008) 406e411. [7] Q. Yu, Jie Lian, Sujitra Siriponglert, et al., Graphene segregated on Ni surfaces and transferred to insulators, Appl. Phys. Lett. 93 (2008) 113103. [8] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, et al., Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706. [9] A. Reina, X. Jia, J. Ho, et al., Large Area, few-layer graphene films on arbitrary substrates by chemical vapor deposition, Nano Lett. 9 (2009) 30e35. [10] J. Coraux, A.T.N. Diaye, C. Busse, T. Michely, Structural coherency of graphene on Ir (111), Nano Lett. 8 (2008) 565e570. [11] C. Berger, Z. Song, T. Li, et al., Electronic confinement and coherence in patterned epitaxial graphene, Science 312 (2006) 1191. [12] K.V. Emtsev, A. Bostwick, K. Horn, et al., Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide, Nat. Mater. 8 (2009) 203e207. [13] H. Wang, et al., Controllable synthesis of submillimeter single-crystal monolayer graphene domains on copper foils by suppressing nucleation, J. Am. Chem. Soc. 134 (2012) 3627e3630. [14] P. Sutter, Epitaxial graphene: how silicon leaves the scene, Nat. Mater. 8 (2009) 171e172. [15] Min-Quan Yang, Nan Zhang, Mario Pagliaro, Yi-Jun Xu, Artificial photosynthesis over grapheneesemiconductor composites. Are we getting better? Chem. Soc. Rev. 43 (2014) 8240e8254. [16] K.A. Ritter, J.W. Lyding, The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons, Nat. Mater 8 (2009) 235e242. [17] C. Tao, et al., Spatially resolving edge states of chiral graphene nanoribbons, Nat. Phys. 7 (2011) 616e620. [18] M. Pan, et al., Topographic and spectroscopic characterization of electronic edge states in CVD grown graphene nanoribbons, Nano Lett. 12 (2012) 1928e1933.
[19] X. Zhang, et al., Experimentally engineering the edge termination of graphene nanoribbons, ACS Nano 7 (2013) 198e202. [20] A.C. Ferrari, D.M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nat. Nanotechnol. 8 (2013) 235e246. [21] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, et al., Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401/1e/4. [22] N. Zhang, Y.H. Zhang, Y.J. Xu, Recent progress on graphene-based photocatalysts: current status and future perspectives, Nanoscale 4 (2012) 5792e5813. [23] L. Zhu, S.B. Jo, S. Ye, K. Ullah, Z.D. Meng, W.C. Oh, A green and direct synthesis of photosensitized CoS2egraphene/TiO2 hybrid with high photocatalytic performance, J. Ind. Eng. Chem. 22 (2015) 264e271. [24] L. Gong, I.A. Kinloch, R.J. Young, I. Riaz, R. Jalil, K.S. Novoselov, Adv. Mater. 22 (2010) 2694e2697. [25] Y.H. Zhang, Z.R. Tang, X.Z. Fu, Y.J. Xu, Engineering the unique 2D Mat of graphene to achieve graphene-TiO2 nanocomposite for photocatalytic selective transformation: what advantage does graphene have over its forebear carbon nanotube? ACS Nano 5 (9) (2011) 7426e7435. [26] J.J. Tian, B.S. Hu, Z.D. Wei, Y. Jin, Z.T. Luo, M.R. Xia, Q.J. Pan, Y.L. Liu, Surface structure deduced differences of copper foil and film for graphene CVD growth, Appl. Surf. Sci. 300 (2014) 73e79. [27] S. Reich, C. Thomsen, Raman spectroscopy of graphite, Phil. Trans. R. Soc. Lond. A 362 (2004) 2271e2288. [28] K. Ullah, S. Ye, L. Zhu, Z.D. Meng, et al., Microwave assisted synthesis of a noble metal-graphene hybrid photocatalyst for high efficient decomposition of organic dyes under visible light, Mater. Sci. Eng. B 180 (2014) 20e26. [29] A. Das, S. Pisana, B. Chakraborty, Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (2008) 210e215. [30] M.Q. Yang, Y.J. Xu, Selective photoredox using graphene-based composite photocatalysts, Phys. Chem. Chem. Phys. 15 (2013) 19102e19118. [31] C. Han, M.Q. Yang, B. Weng, Y.J. Xu, Improving the photocatalytic activity and anti-photocorrosion of semiconductor ZnO by coupling with versatile carbon, Phys. Chem. Chem. Phys. 16 (2014) 16891e16903. [32] C. Burda, Y. Lou, X. Chen, A.C.S. Semia, J. Stout, J.L. Gole, Enhanced nitrogen doping in TiO2 nanoparticles, Nano Lett. 3 (2003) 1049e1051. [33] X. Lia, W. Caia, I. Junga, et al., Synthesis, Characterization, and properties of large-area graphene films, ECS Trans. 19 (5) (2009) 41e52. [34] D. Pan, et al., Li storage properties of disordered graphene nanosheets, Chem. Mater. 21 (2009) 3136e3142. [35] H. Ago, Y. Ito, N. Mizuta, K. Yoshida, B. Hu, C.M. Orofeo, et al., Epitaxial chemical vapor deposition growth of single-layer graphene over cobalt film crystallized on sapphire, ACS Nano 4 (2010) 7407e7414. [36] C.Y. Chen, D. Dai, G.X. Chen, J.H. Yu, K. Nishimura, C.T. Lin, N. Jiang, Z.L. Zhan, Rapid growth of single-layer graphene on the insulating substrates by thermal CVD, Appl. Surf. Sci. 346 (2015) 41e45. [37] R.K. de Castro, J.R. Araujo, R. Valaski, L.O.O. Costa, B.S. Archanjo, B. Fragneaud, M. Cremona, C.A. Achete, New transfer method of CVD-grown graphene using a flexible, transparent and conductive polyaniline-rubber thin film for organic electronic applications, Chem. Eng. J. 273 (2015) 509e518. [38] N.R. Khalid, E. Ahmed, Zhanglian Hong, M. Ahmad, Yuewei Zhang, Sadia Khalid, Ceram. Int. 39 (2013) 7107e7113.