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transition metal oxide composites and its application for hydrogen storage
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 7 5 9 4 e7 5 9 9
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Agent-free synthesis of graphene oxide/transition metal oxide composites and its application for hydrogen storage Won G. Hong a,1, Byung Hoon Kim a,c,1, Sang Moon Lee a, Han Young Yu b, Yong Ju Yun b, Yongseok Jun c, Jin Bae Lee a, Hae Jin Kim a,* a
Division of Materials Science, Korea Basic Science Institute, 52 Yeoeun-dong, Yuseong-gu, Daejeon 305-333, Republic of Korea Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea c Interdisciplinary School of Green Energy, KIER-UNIST Advanced Center for Energy, UNIST, Ulsan 689-798, Republic of Korea b
article info
abstract
Article history:
Graphene oxide (GO) wrapped transition metal oxide composite materials were synthesized
Received 6 December 2011
by a very simple route without any additional agents and the hydrogen adsorption prop-
Received in revised form
erties of the materials were investigated. The morphologies of GO/V2O5 and GO/TiO2 were
27 January 2012
examined by scanning electron microscopy (SEM) and transmission electron microscopy
Accepted 3 February 2012
(TEM). The results show that single- or few-layered GO sheets wrapped throughout the V2O5
Available online 3 March 2012
and TiO2 particles. According to X-ray photoelectron spectroscopy (XPS), the CeOH species of GO and the surface-adsorbed oxygen of the transition metal oxide bond together via
Keywords:
a dehydration reaction. The wrapping phenomenon of GO causes the enhancement of
Graphene oxide
hydrogen storage capacity at liquid nitrogen temperature (77 K) compared with those of the
Wrapping
pristine transition metal oxides and GO. The enhancement of hydrogen storage capacity of
Hydrogen storage
GO-wrapped transition metal oxide composite materials results from the existence of
Transition metal oxide
interspaces between the transition metal oxide particles and the thin GO layers.
Dehydration reaction
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Recently, graphene, a single atomic layer sheet of sp2 bonded carbon, has attracted considerable attention because of its unique properties and potential for various applications [1]. Many methods have been developed to produce large quantities of processable graphene sheets that are separated well from each [2e4]. One of the most promising methods is to use graphene oxide (GO) synthesized by the oxidation of graphite. With the reduction of GO, most of the properties of graphene are reportedly recovered. Furthermore, GO itself is considered to be a good candidate for use in composite materials that exhibit good electrical and gas-adsorption properties [5e7].
GO has various oxygen functional groups (hydroxyl, epoxide, and carbonyl groups) on its basal plane and edge [8e11]. The oxygen functional groups of GO can interact with many different functional groups or particles [12e14]. Recently, GO-based composite materials have been introduced for various applications such as optoelectronic devices and lithium ion batteries [15,16]. In addition, GO itself and GO-based composites have been considered as new hydrogen storage media due to their large surface-to-volume ratio and intrinsically lightweight. For example, it was proposed that GO has the potential to become an ideal substrate for hydrogen storage [17] and that GO layers can be used as building blocks for potentially useful hydrogen storage
* Corresponding author. Tel.: þ82 42 865 3953; fax: þ82 42 865 3610. E-mail address: [email protected] (H.J. Kim). 1 These authors contributed equally to this work. 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.02.010
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 7 5 9 4 e7 5 9 9
materials [18]. In addition, Wang et al. studied Pd-doped graphite oxide, which showed an obvious enhancement of hydrogen storage ability in comparison with that of pristine graphite oxide [19]. However, the synthesis of the GO-based composites mentioned above contains a somewhat tiresome process. Additional heat treatment or chemical modifications of the transition metal oxide are needed to achieve the composite. Herein, we report a very simple route to synthesize GO/V2O5 and GO/TiO2 composites without any additional agents. Using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), we show that single- or few-layered GO sheets wrapped throughout the transition metal oxide by the interaction between the oxygen functional groups of GO and the outermost oxygen of transition metal oxide particles. Additionally, we found that the wrapping of GO causes the enhancement of hydrogen storage capacity compared with those of the pristine transition metal oxides, from 0.16 wt% for V2O5 (0.58 wt% for TiO2) to 1.36 wt% for GO/V2O5 (1.26 wt% for GO/TiO2).
2.
GO were characterized using tapping-mode atomic force microscopy (AFM, Veeco, DI 3100).
2.3.
High-pressure H2 uptake measurement
Hydrogen adsorption with high-pressure of up to 70 bar was measured volumetrically with the pressure-composition isotherm (PCT, HIZEN). 99.9999% hydrogen gas was used in all H2 sorption measurements. In the case of PCT measurement, the system was calibrated with LaNi5 at room temperature and activated carbon (surface area w3000 m2/g) at 77 K, respectively. Before all measurements, the samples were degassed and heated at 120 C until a pressure of 4e8 107 mbar was reached. Then the desired hydrogen pressure was introduced in the thermo-stat chamber and after equilibrium was reached, the gas was permitted to expand in the sample holder. The measured pressure drop was caused by the gas expansion in the sample holder and hydrogen adsorption in the sample. After no more pressure change was observed, we waited for an additional 300 s to be sure that the thermal equilibrium was reached.
Experimental section 3.
2.1. Preparation of GO/transition metal oxide composites GO was synthesized by a modified Hummers method [20], starting from graphite powder of 450 nm size. The graphite was treated with concentrated H2SO4, K2S2O8 and P2O5. After filtering, washing, and drying, the product was re-suspended in concentrated H2SO4 and oxidized further with KMnO4 and H2O2; the result was a thick, brownish yellow GO suspension. The GO suspension was centrifuged and washed with 10% HCl and DI water, and then the suspension was dried at 50 C for three days to obtain GO. GO-wrapped transition metal oxides, GO/V2O5 and GO/TiO2, were prepared by simple mixing in aqueous dispersion. V2O5 (Aldrich, 99.6þ%) and TiO2 (Alfa Aeser, 99.6%, anatase) were commercially purchased and were used as received without further purification. Approximately 0.15 g of the GO was dispersed in 200 ml of DI water through sonication for 2 h. Concurrently, 0.5 g of transition metal oxide powder was suspended in 200 ml of DI water through sonication for 5 min. This suspension was mixed with the dispersed GO and stirred for 24 h. The mixture was then washed with DI water using a centrifuge to remove residual GO.
2.2.
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Characterization
The structural characterization of the samples was carried out with an XRD (Philips X’pert) with graphite monochromatized ˚ ). The morphologies of the Cu-Ka radiation (l ¼ 1.54178 A samples were obtained a using field-emission SEM (FEI Sirion 200) and a field-emission TEM (JEOL JEM2100F). The Raman spectra were obtained using a Renishaw InVia micro-Raman spectrometer (Renishaw, Wotton-under-Edge) with a 514 nm Argon laser. The surface states were analyzed by XPS (AXISNOVA, Kratos Inc.) using monochromatic Al Ka radiation (1486.6 eV). The binding energy was corrected using the C 1s peak at 284.6 eV. The size and thickness of the single-layered
Results and discussion
Single-layered GO was demonstrated by tapping-mode AFM (Fig. 1a). It was found that the thickness of the GO sheet is w1.0 nm (Fig. 1b). From the analysis of the AFM image and line profile, we could conclude that the exfoliated GO with w500 nm in size is a single-layered sheet. The morphologies of V2O5, GO/V2O5, TiO2, and GO/TiO2 were examined by SEM and TEM. SEM images of V2O5 and TiO2 are also presented (Fig. 1c and d). Fig. 1e and f shows the SEM images of GO/V2O5, and GO/TiO2, respectively. Transparent GO sheets were uniformly wrapped around V2O5 and TiO2 particles. Fig. 1g and h shows the TEM images of GO/V2O5 and GO/TiO2. The thicknesses of the GO sheets (marked with arrows) are approximately 2.10 nm for GO/V2O5 and 1.42 nm for GO/TiO2, which indicate that the GO sheets have one or a few layers. More importantly, even after the lengthy sonication during the preparation of the TEM specimen, the GO sheets still wrap around the V2O5 and TiO2 particles. Since GO contains a wide range of oxygen functional groups, it is expected to lead to direct interaction such as hydrogen-bonding or a dehydration reaction between the surface oxygen of the transition metal oxide and the GO. Furthermore, the suitably small size (w500 nm) and flexible nature of GO facilitate the sticking of GO on the surface of V2O5 and TiO2 particles. The crystal structures of the GO, V2O5, GO/V2O5, TiO2, and GO/TiO2 were studied by XRD as shown in Fig. 2. The V2O5 powder has a single phase with an orthorhombic structure (Pmmn), as reported previously (Fig. 2a) [21]. The anatase structure (space group I41/amd ) of TiO2 is shown in Fig. 2b. The inset of Fig. 2a shows the XRD patterns for GO. The most intensive peak of GO sheets appears at 2q ¼ 10.72 corre˚ . This is attributed sponding to the interlayer spacing of 8.25 A to the introduction of oxygen functional groups on the GO ˚ indicates that some sheets; the interlayer spacing of 8.25 A water has been absorbed between the GO sheets via hydrogenbonding [22,23]. XRD patterns of the GO-wrapped transition
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Fig. 1 e (a) AFM image of GO on SiO2/Si substrate. (b) Line profile for the image shown in (a). The thickness of the GO sheet is w1.0 nm. SEM images of (c) V2O5, (d) TiO2, (e) GO/V2O5, (f) GO/TiO2, TEM images of (g) GO/V2O5 and (h) GO/TiO2. It can be seen that thin layers of GO wrap V2O5 and TiO2. Arrows indicate GO layers.
metal oxide are quite similar to that of pristine transition metal oxide. Table S1 in the supplementary information shows the differences of lattice parameters and unit cell volume between transition metal oxide (V2O5 and TiO2) and GO-wrapped transition metal oxide. As a result of the
interaction between oxygen functional groups in GO and the outermost oxygen in V2O5 and TiO2, the lattice parameters of the transition metal oxide slightly decrease. This result implies that the wrapping phenomenon of GO has little effect on the crystalline structure of V2O5 and TiO2. On the Contrary,
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 7 5 9 4 e7 5 9 9
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Fig. 2 e XRD patterns of (a) V2O5, GO/V2O5, (b) TiO2 and GO/TiO2. The inset is the XRD spectrum of GO. Asterisk shows the peak corresponding to GO.
after the formation of composites, the peak originated from GO is shifted to higher angle, 2q ¼ 11.72 for GO/V2O5 and 2q ¼ 12.39 for GO/TiO2 (Fig. S1 in the supplementary information). Accordingly, the interlayer spacing of GO in GO/V2O5 ˚ and 7.28 A ˚ , respectively. The and GO/TiO2 decreases 7.59 A decrease of the interlayer spacing of GO in GO/V2O5 and GO/ TiO2 indicates that there exists a chemical interaction between GO and transition metal oxide. Raman spectroscopy has been used extensively to demonstrate the degree of disorder in graphene-based materials [24]. Fig. 3 presents the micro-Raman spectra in the region of 250e2000 cm1 for GO, V2O5, GO/V2O5, TiO2, and GO/TiO2. In the inset of Fig. 3a, the two prominent peaks of GO appearing at around 1350 and 1590 cm1 are attributed to the D and G bands, respectively. In the spectra of GO/V2O5 and GO/TiO2, the D and G bands of GO still exist. The G bands for GO/V2O5 (1599 cm1) and GO/TiO2 (1600 cm1) were shifted to higher frequencies compared with that of GO (1590 cm1). The significant blueshifted G band is indicative of an increased disorder induced in the sp2 carbon lattice [25] and of the compressive strain effect [26] caused by the bonding between GO and the surfaceadsorbed oxygen of the transition metal oxide. The Raman spectra of GO/V2O5 and GO/TiO2 in the region below 1000 cm1 display analogous Raman bands for V2O5 and TiO2. The position and relative intensity of the Raman bands are in good agreement with those for pristine V2O5 and TiO2. The change of chemical state of the composites was characterized by XPS. The carbon and oxygen 1s core level spectra of GO, GO/V2O5, and GO/TiO2 were normalized to the
graphite carbon peak at 284.6 eV (Fig. 4). Curve fitting of the XPS spectra was performed using a GaussianeLorentzian peak shape after performing a linear background correction. The XPS spectra of O 1s in GO/V2O5 and GO/TiO2 are decomposed into four components as shown in Fig. 4a and b. Two peaks correspond to CeO (P1) and C]O (P2) species that originated from GO, as shown in the middle of Fig. 4. The other peaks are related to the lattice oxygen (P3) and the surfaceadsorbed oxygen (P4) that originated from V2O5 and TiO2 (lower panels of Fig. 4). After the reaction is completed, P3 increases, while P4 weakens (see Table S2 in the supplementary information). This result shows that the oxygen functional groups of GO react with the surface-adsorbed oxygen of the transition metal oxide. The C 1s XPS spectra of GO, GO/ V2O5 and GO/TiO2 consist of different chemically shifted components, as can be seen in Fig. 4c. The C 1s spectrum of GO shows binding energies at 284.6 (C]C), 285.48 (CeOH), 286.68 (CeOeC), 287.39 (C]O), 288.55 eV (O]CeO). These values are in agreement with those of previous works [27]. After the formation of composites, the peak associated with the CeOH species that originated from GO disappears (GO/V2O5) or is greatly weakened (GO/TiO2), while the peak related to the CeOeC species significantly increases. Taken together, these XPS analyses indicate that the CeOH species of GO react with the surface-adsorbed oxygen of the transition metal oxide via the dehydration reaction and thus that GO sheets and transition metal oxide particles bond together. In addition, it is possible that any remaining oxygen functional groups of GO are used for hydrogen-bonding with the remaining surface
Fig. 3 e Raman spectra of (a) V2O5, GO/V2O5, (b) TiO2 and GO/TiO2. The inset is the Raman spectrum of GO. The prominent D (w1350 cmL1) and G (w1600 cmL1) bands were observed in GO-wrapped composites.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 7 5 9 4 e7 5 9 9
Fig. 4 e The O 1s core level spectra of (a) GO/V2O5 and (b) GO/TiO2. (c) The C 1s core level spectra of GO (bottom), GO/TiO2 (middle) and GO/V2O5 (top). The open black circles are experimental values and the red solid lines are fitting lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
oxygen groups of the transition metal oxide. Thermogravimetric analysis (TGA) supports this argument. It shows higher thermal stability of GO/V2O5 and GO/TiO2 compared with that of GO even at high temperatures (Fig. S2 in the supplementary information). It results from the chemical bonding between GO and transition metal oxide. Fig. 5 shows the high-pressure H2 adsorption isotherms at 77 K for GO, V2O5, GO/V2O5, TiO2 and GO/TiO2. The pristine transition metal oxides absorb a small amount of molecular H2 (0.16 wt% for V2O5 and 0.58 wt% for TiO2 at 70 bar). However, a significant increase of H2 uptake was observed in GO/V2O5 and GO/TiO2: the values for this increase were w1.36 wt% and 1.26 wt%, respectively, at 70 bar. As can be seen in Fig. 5, H2 storage capacity of GO could be obtained by w0.95 wt%, whose adsorption isotherm was similar to those of
GO/V2O5 and GO/TiO2. There are three possibilities for the reason why H2 uptake of the composites is comparable to that of GO: i) adsorption of GO itself, ii) catalytic adsorption due to transition metal oxides, and iii) the existence of the interspaces between transition metal oxides and GO. However, H2 adsorption of GO itself cannot support the increase of the H2 storage capacity of the composites compared with that in transition metal oxides because the GO-wrapped composites have a quite small amount of single- or few-layered GO sheets. Moreover, since transition metal oxides do not adsorb large quantity of hydrogen, catalytic H2 adsorption can be excluded. In this light, we suggest that H2 molecules are stored at the surface of the outermost GO sheets [28], between the GO layers as already observed in previous studies [18,19], and at the interspaces between the transition metal oxide particles
Fig. 5 e High-pressure H2 adsorption of (a) V2O5, GO, GO/V2O5, (b) TiO2, GO and GO/TiO2.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 7 5 9 4 e7 5 9 9
and the GO layers. The last one, the existence of the interspaces may result in the enhancement of H2 storage capacity of GO-wrapped composites.
4.
Conclusions
In summary, GO-wrapped transition metal oxide composite materials have been prepared using a very simple route without any additional agents. The results show that singleor few-layered GO sheets wrapped throughout the V2O5 and TiO2 by the dehydration reaction and the hydrogen-bonding between the CeOH species of GO and the adsorbed oxygen on the surface of the transition metal oxide particles. On the other hand, if the GO sheets are large or if the transition metal oxide particles are too small to be mixed together, the transition metal oxide particles will surround the GO sheets. Volumetric hydrogen storage measurement of the GO-wrapped transition metal oxide composite materials was performed at 77 K. The wrapping of GO causes an enhancement of hydrogen storage capacity compared to those of the pristine transition metal oxides, from 0.16 wt% for V2O5 (0.58 wt% for TiO2) to 1.36 wt% for GO/V2O5 (1.26 wt% for GO/ TiO2). This agent-free synthesis method may be adapted to other applications. For example, it provides an easy way to replace the carbon coating method for lithium ion batteries synthesized with transition metal oxides.
Acknowledgement This work was supported by the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Program, funded by the Ministry of Science and Technology of Korea.
Appendix. Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.ijhydene.2012.02.010.
references
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[7] Petit C, Bandosz TJ. Graphite oxide/polyoxometalate nanocomposites as adsorbents of ammonia. J Phys Chem C 2009;113:3800e9. [8] Lerf A, He H, Forster M, Klinowski J. Structure of graphite oxide revisited. J Phys Chem B 1998;102:4477e82. [9] He H, Klinowski J, Forster M, Lerf A. A new structural model for graphite oxide. Chem Phys Lett 1998;287:53e6. [10] Xu Y, Bai H, Lu G, Li C, Shi G. Flexible graphene films via the filtration of water-soluble noncovalent functionalized graphene sheets. J Am Chem Soc 2008;130:5856e7. [11] Si Y, Samulski ET. Synthesis of water soluble graphene. Nano Lett 2008;8:1679e82. [12] Xu C, Wang X. Fabrication of flexible metal-nanoparticle films using graphene oxide sheets as substrates. Small 2009; 5:2212e7. [13] Williams G, Seger B, Kamat PV. TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide. ACS Nano 2008;2:1487e91. [14] Compton OC, Dikin DA, Putz KW, Brinson LC, Nguyen ST. Electrically conductive “alkylated” graphene paper via chemical reduction of amine-functionalized graphene oxide paper. Adv Mater 2010;22:892e6. [15] Yin Z, Sun S, Salim T, Wu S, Huang X, He Q, et al. Organic photovoltaic devices using highly flexible reduced graphene oxide films as transparent electrodes. ACS Nano 2010;4: 5263e8. [16] Zhu J, Zhu T, Zhou X, Zhang Y, Lou XW, Chen X, et al. Facile synthesis of metal oxide/reduced graphene oxide hybrids with high lithium storage capacity and stable cyclability. Nanoscale 2011;3:1084e9. [17] Wang L, Lee K, Sun YY, Lucking M, Chen Z, Zhao JJ, et al. Graphene oxide as an ideal substrate for hydrogen storage. ACS Nano 2009;3:2995e3000. [18] Burress JW, Gadipelli S, Ford J, Simmons JM, Zhou W, Yildirim T. Graphene oxide framework materials: theoretical predictions and experimental results. Angew Chem Int Ed 2010;49:8902e4. [19] Wang L, Yang FH, Yang RT, Miller MA. Effect of surface oxygen groups in carbons on hydrogen storage by spillover. Ind Eng Chem Res 2009;48:2920e6. [20] Hummers WS, Offeman RE. Preparation of graphite oxide. J Am Chem Soc 1958;80:1339. [21] Lao ZJ, Konstantinov K, Tournaire Y, Ng SH, Wang GX, Liu HK. Synthesis of vanadium pentoxide powders with enhanced surface-area for electrochemical capacitors. J Power Sourc 2006;162:1451e4. [22] Buchsteiner A, Lerf A, Pieper J. Water dynamics in graphite oxide investigated with neutron scattering. J Phys Chem B 2006;110:22328e38. [23] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, et al. Preparation and characterization of graphene oxide paper. Nature 2007;448:457e60. [24] Pimenta MA, Dresselhaus G, Dresselhaus MS, Cancado LG, Jorio A, Saito R. Studying disorder in graphite-based system by Raman spectroscopy. Phys Chem Chem Phys 2007;9: 1276e91. [25] Kudin KN, Ozbas B, Schniepp HC, Prud’homme RK, Aksay IA, Car R. Raman spectra of graphite oxide and functionalized graphene sheets. Nano Lett 2008;8:36e41. [26] Wang YY, Ni ZH, Yu T, Shen ZX, Wang HM, Wu YH, et al. Raman studies of monolayer graphene: the substrate effect. J Phys Chem C 2008;112:10637e40. [27] Park S, An J, Piner RD, Jung I, Yang D, Velamakanni A, et al. Aqueous suspension and characterization of chemically modified graphene sheets. Chem Mater 2008;20:6592e4. [28] Ghosh A, Subrahmanyam KS, Krishna KS, Datta S, Govindaraj A, Pati SK, et al. Uptake of H2 and CO2 by graphene. J Phys Chem C 2008;112:15704e7.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Agent-free synthesis of graphene oxide/transition metal oxide composites and its application for hydrogen storage Won G. Hong a,1, Byung Hoon Kim a,c,1, Sang Moon Lee a, Han Young Yu b, Yong Ju Yun b, Yongseok Jun c, Jin Bae Lee a, Hae Jin Kim a,* a
Division of Materials Science, Korea Basic Science Institute, 52 Yeoeun-dong, Yuseong-gu, Daejeon 305-333, Republic of Korea Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea c Interdisciplinary School of Green Energy, KIER-UNIST Advanced Center for Energy, UNIST, Ulsan 689-798, Republic of Korea b
article info
abstract
Article history:
Graphene oxide (GO) wrapped transition metal oxide composite materials were synthesized
Received 6 December 2011
by a very simple route without any additional agents and the hydrogen adsorption prop-
Received in revised form
erties of the materials were investigated. The morphologies of GO/V2O5 and GO/TiO2 were
27 January 2012
examined by scanning electron microscopy (SEM) and transmission electron microscopy
Accepted 3 February 2012
(TEM). The results show that single- or few-layered GO sheets wrapped throughout the V2O5
Available online 3 March 2012
and TiO2 particles. According to X-ray photoelectron spectroscopy (XPS), the CeOH species of GO and the surface-adsorbed oxygen of the transition metal oxide bond together via
Keywords:
a dehydration reaction. The wrapping phenomenon of GO causes the enhancement of
Graphene oxide
hydrogen storage capacity at liquid nitrogen temperature (77 K) compared with those of the
Wrapping
pristine transition metal oxides and GO. The enhancement of hydrogen storage capacity of
Hydrogen storage
GO-wrapped transition metal oxide composite materials results from the existence of
Transition metal oxide
interspaces between the transition metal oxide particles and the thin GO layers.
Dehydration reaction
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Recently, graphene, a single atomic layer sheet of sp2 bonded carbon, has attracted considerable attention because of its unique properties and potential for various applications [1]. Many methods have been developed to produce large quantities of processable graphene sheets that are separated well from each [2e4]. One of the most promising methods is to use graphene oxide (GO) synthesized by the oxidation of graphite. With the reduction of GO, most of the properties of graphene are reportedly recovered. Furthermore, GO itself is considered to be a good candidate for use in composite materials that exhibit good electrical and gas-adsorption properties [5e7].
GO has various oxygen functional groups (hydroxyl, epoxide, and carbonyl groups) on its basal plane and edge [8e11]. The oxygen functional groups of GO can interact with many different functional groups or particles [12e14]. Recently, GO-based composite materials have been introduced for various applications such as optoelectronic devices and lithium ion batteries [15,16]. In addition, GO itself and GO-based composites have been considered as new hydrogen storage media due to their large surface-to-volume ratio and intrinsically lightweight. For example, it was proposed that GO has the potential to become an ideal substrate for hydrogen storage [17] and that GO layers can be used as building blocks for potentially useful hydrogen storage
* Corresponding author. Tel.: þ82 42 865 3953; fax: þ82 42 865 3610. E-mail address: [email protected] (H.J. Kim). 1 These authors contributed equally to this work. 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.02.010
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 7 5 9 4 e7 5 9 9
materials [18]. In addition, Wang et al. studied Pd-doped graphite oxide, which showed an obvious enhancement of hydrogen storage ability in comparison with that of pristine graphite oxide [19]. However, the synthesis of the GO-based composites mentioned above contains a somewhat tiresome process. Additional heat treatment or chemical modifications of the transition metal oxide are needed to achieve the composite. Herein, we report a very simple route to synthesize GO/V2O5 and GO/TiO2 composites without any additional agents. Using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), we show that single- or few-layered GO sheets wrapped throughout the transition metal oxide by the interaction between the oxygen functional groups of GO and the outermost oxygen of transition metal oxide particles. Additionally, we found that the wrapping of GO causes the enhancement of hydrogen storage capacity compared with those of the pristine transition metal oxides, from 0.16 wt% for V2O5 (0.58 wt% for TiO2) to 1.36 wt% for GO/V2O5 (1.26 wt% for GO/TiO2).
2.
GO were characterized using tapping-mode atomic force microscopy (AFM, Veeco, DI 3100).
2.3.
High-pressure H2 uptake measurement
Hydrogen adsorption with high-pressure of up to 70 bar was measured volumetrically with the pressure-composition isotherm (PCT, HIZEN). 99.9999% hydrogen gas was used in all H2 sorption measurements. In the case of PCT measurement, the system was calibrated with LaNi5 at room temperature and activated carbon (surface area w3000 m2/g) at 77 K, respectively. Before all measurements, the samples were degassed and heated at 120 C until a pressure of 4e8 107 mbar was reached. Then the desired hydrogen pressure was introduced in the thermo-stat chamber and after equilibrium was reached, the gas was permitted to expand in the sample holder. The measured pressure drop was caused by the gas expansion in the sample holder and hydrogen adsorption in the sample. After no more pressure change was observed, we waited for an additional 300 s to be sure that the thermal equilibrium was reached.
Experimental section 3.
2.1. Preparation of GO/transition metal oxide composites GO was synthesized by a modified Hummers method [20], starting from graphite powder of 450 nm size. The graphite was treated with concentrated H2SO4, K2S2O8 and P2O5. After filtering, washing, and drying, the product was re-suspended in concentrated H2SO4 and oxidized further with KMnO4 and H2O2; the result was a thick, brownish yellow GO suspension. The GO suspension was centrifuged and washed with 10% HCl and DI water, and then the suspension was dried at 50 C for three days to obtain GO. GO-wrapped transition metal oxides, GO/V2O5 and GO/TiO2, were prepared by simple mixing in aqueous dispersion. V2O5 (Aldrich, 99.6þ%) and TiO2 (Alfa Aeser, 99.6%, anatase) were commercially purchased and were used as received without further purification. Approximately 0.15 g of the GO was dispersed in 200 ml of DI water through sonication for 2 h. Concurrently, 0.5 g of transition metal oxide powder was suspended in 200 ml of DI water through sonication for 5 min. This suspension was mixed with the dispersed GO and stirred for 24 h. The mixture was then washed with DI water using a centrifuge to remove residual GO.
2.2.
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Characterization
The structural characterization of the samples was carried out with an XRD (Philips X’pert) with graphite monochromatized ˚ ). The morphologies of the Cu-Ka radiation (l ¼ 1.54178 A samples were obtained a using field-emission SEM (FEI Sirion 200) and a field-emission TEM (JEOL JEM2100F). The Raman spectra were obtained using a Renishaw InVia micro-Raman spectrometer (Renishaw, Wotton-under-Edge) with a 514 nm Argon laser. The surface states were analyzed by XPS (AXISNOVA, Kratos Inc.) using monochromatic Al Ka radiation (1486.6 eV). The binding energy was corrected using the C 1s peak at 284.6 eV. The size and thickness of the single-layered
Results and discussion
Single-layered GO was demonstrated by tapping-mode AFM (Fig. 1a). It was found that the thickness of the GO sheet is w1.0 nm (Fig. 1b). From the analysis of the AFM image and line profile, we could conclude that the exfoliated GO with w500 nm in size is a single-layered sheet. The morphologies of V2O5, GO/V2O5, TiO2, and GO/TiO2 were examined by SEM and TEM. SEM images of V2O5 and TiO2 are also presented (Fig. 1c and d). Fig. 1e and f shows the SEM images of GO/V2O5, and GO/TiO2, respectively. Transparent GO sheets were uniformly wrapped around V2O5 and TiO2 particles. Fig. 1g and h shows the TEM images of GO/V2O5 and GO/TiO2. The thicknesses of the GO sheets (marked with arrows) are approximately 2.10 nm for GO/V2O5 and 1.42 nm for GO/TiO2, which indicate that the GO sheets have one or a few layers. More importantly, even after the lengthy sonication during the preparation of the TEM specimen, the GO sheets still wrap around the V2O5 and TiO2 particles. Since GO contains a wide range of oxygen functional groups, it is expected to lead to direct interaction such as hydrogen-bonding or a dehydration reaction between the surface oxygen of the transition metal oxide and the GO. Furthermore, the suitably small size (w500 nm) and flexible nature of GO facilitate the sticking of GO on the surface of V2O5 and TiO2 particles. The crystal structures of the GO, V2O5, GO/V2O5, TiO2, and GO/TiO2 were studied by XRD as shown in Fig. 2. The V2O5 powder has a single phase with an orthorhombic structure (Pmmn), as reported previously (Fig. 2a) [21]. The anatase structure (space group I41/amd ) of TiO2 is shown in Fig. 2b. The inset of Fig. 2a shows the XRD patterns for GO. The most intensive peak of GO sheets appears at 2q ¼ 10.72 corre˚ . This is attributed sponding to the interlayer spacing of 8.25 A to the introduction of oxygen functional groups on the GO ˚ indicates that some sheets; the interlayer spacing of 8.25 A water has been absorbed between the GO sheets via hydrogenbonding [22,23]. XRD patterns of the GO-wrapped transition
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Fig. 1 e (a) AFM image of GO on SiO2/Si substrate. (b) Line profile for the image shown in (a). The thickness of the GO sheet is w1.0 nm. SEM images of (c) V2O5, (d) TiO2, (e) GO/V2O5, (f) GO/TiO2, TEM images of (g) GO/V2O5 and (h) GO/TiO2. It can be seen that thin layers of GO wrap V2O5 and TiO2. Arrows indicate GO layers.
metal oxide are quite similar to that of pristine transition metal oxide. Table S1 in the supplementary information shows the differences of lattice parameters and unit cell volume between transition metal oxide (V2O5 and TiO2) and GO-wrapped transition metal oxide. As a result of the
interaction between oxygen functional groups in GO and the outermost oxygen in V2O5 and TiO2, the lattice parameters of the transition metal oxide slightly decrease. This result implies that the wrapping phenomenon of GO has little effect on the crystalline structure of V2O5 and TiO2. On the Contrary,
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Fig. 2 e XRD patterns of (a) V2O5, GO/V2O5, (b) TiO2 and GO/TiO2. The inset is the XRD spectrum of GO. Asterisk shows the peak corresponding to GO.
after the formation of composites, the peak originated from GO is shifted to higher angle, 2q ¼ 11.72 for GO/V2O5 and 2q ¼ 12.39 for GO/TiO2 (Fig. S1 in the supplementary information). Accordingly, the interlayer spacing of GO in GO/V2O5 ˚ and 7.28 A ˚ , respectively. The and GO/TiO2 decreases 7.59 A decrease of the interlayer spacing of GO in GO/V2O5 and GO/ TiO2 indicates that there exists a chemical interaction between GO and transition metal oxide. Raman spectroscopy has been used extensively to demonstrate the degree of disorder in graphene-based materials [24]. Fig. 3 presents the micro-Raman spectra in the region of 250e2000 cm1 for GO, V2O5, GO/V2O5, TiO2, and GO/TiO2. In the inset of Fig. 3a, the two prominent peaks of GO appearing at around 1350 and 1590 cm1 are attributed to the D and G bands, respectively. In the spectra of GO/V2O5 and GO/TiO2, the D and G bands of GO still exist. The G bands for GO/V2O5 (1599 cm1) and GO/TiO2 (1600 cm1) were shifted to higher frequencies compared with that of GO (1590 cm1). The significant blueshifted G band is indicative of an increased disorder induced in the sp2 carbon lattice [25] and of the compressive strain effect [26] caused by the bonding between GO and the surfaceadsorbed oxygen of the transition metal oxide. The Raman spectra of GO/V2O5 and GO/TiO2 in the region below 1000 cm1 display analogous Raman bands for V2O5 and TiO2. The position and relative intensity of the Raman bands are in good agreement with those for pristine V2O5 and TiO2. The change of chemical state of the composites was characterized by XPS. The carbon and oxygen 1s core level spectra of GO, GO/V2O5, and GO/TiO2 were normalized to the
graphite carbon peak at 284.6 eV (Fig. 4). Curve fitting of the XPS spectra was performed using a GaussianeLorentzian peak shape after performing a linear background correction. The XPS spectra of O 1s in GO/V2O5 and GO/TiO2 are decomposed into four components as shown in Fig. 4a and b. Two peaks correspond to CeO (P1) and C]O (P2) species that originated from GO, as shown in the middle of Fig. 4. The other peaks are related to the lattice oxygen (P3) and the surfaceadsorbed oxygen (P4) that originated from V2O5 and TiO2 (lower panels of Fig. 4). After the reaction is completed, P3 increases, while P4 weakens (see Table S2 in the supplementary information). This result shows that the oxygen functional groups of GO react with the surface-adsorbed oxygen of the transition metal oxide. The C 1s XPS spectra of GO, GO/ V2O5 and GO/TiO2 consist of different chemically shifted components, as can be seen in Fig. 4c. The C 1s spectrum of GO shows binding energies at 284.6 (C]C), 285.48 (CeOH), 286.68 (CeOeC), 287.39 (C]O), 288.55 eV (O]CeO). These values are in agreement with those of previous works [27]. After the formation of composites, the peak associated with the CeOH species that originated from GO disappears (GO/V2O5) or is greatly weakened (GO/TiO2), while the peak related to the CeOeC species significantly increases. Taken together, these XPS analyses indicate that the CeOH species of GO react with the surface-adsorbed oxygen of the transition metal oxide via the dehydration reaction and thus that GO sheets and transition metal oxide particles bond together. In addition, it is possible that any remaining oxygen functional groups of GO are used for hydrogen-bonding with the remaining surface
Fig. 3 e Raman spectra of (a) V2O5, GO/V2O5, (b) TiO2 and GO/TiO2. The inset is the Raman spectrum of GO. The prominent D (w1350 cmL1) and G (w1600 cmL1) bands were observed in GO-wrapped composites.
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Fig. 4 e The O 1s core level spectra of (a) GO/V2O5 and (b) GO/TiO2. (c) The C 1s core level spectra of GO (bottom), GO/TiO2 (middle) and GO/V2O5 (top). The open black circles are experimental values and the red solid lines are fitting lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
oxygen groups of the transition metal oxide. Thermogravimetric analysis (TGA) supports this argument. It shows higher thermal stability of GO/V2O5 and GO/TiO2 compared with that of GO even at high temperatures (Fig. S2 in the supplementary information). It results from the chemical bonding between GO and transition metal oxide. Fig. 5 shows the high-pressure H2 adsorption isotherms at 77 K for GO, V2O5, GO/V2O5, TiO2 and GO/TiO2. The pristine transition metal oxides absorb a small amount of molecular H2 (0.16 wt% for V2O5 and 0.58 wt% for TiO2 at 70 bar). However, a significant increase of H2 uptake was observed in GO/V2O5 and GO/TiO2: the values for this increase were w1.36 wt% and 1.26 wt%, respectively, at 70 bar. As can be seen in Fig. 5, H2 storage capacity of GO could be obtained by w0.95 wt%, whose adsorption isotherm was similar to those of
GO/V2O5 and GO/TiO2. There are three possibilities for the reason why H2 uptake of the composites is comparable to that of GO: i) adsorption of GO itself, ii) catalytic adsorption due to transition metal oxides, and iii) the existence of the interspaces between transition metal oxides and GO. However, H2 adsorption of GO itself cannot support the increase of the H2 storage capacity of the composites compared with that in transition metal oxides because the GO-wrapped composites have a quite small amount of single- or few-layered GO sheets. Moreover, since transition metal oxides do not adsorb large quantity of hydrogen, catalytic H2 adsorption can be excluded. In this light, we suggest that H2 molecules are stored at the surface of the outermost GO sheets [28], between the GO layers as already observed in previous studies [18,19], and at the interspaces between the transition metal oxide particles
Fig. 5 e High-pressure H2 adsorption of (a) V2O5, GO, GO/V2O5, (b) TiO2, GO and GO/TiO2.
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and the GO layers. The last one, the existence of the interspaces may result in the enhancement of H2 storage capacity of GO-wrapped composites.
4.
Conclusions
In summary, GO-wrapped transition metal oxide composite materials have been prepared using a very simple route without any additional agents. The results show that singleor few-layered GO sheets wrapped throughout the V2O5 and TiO2 by the dehydration reaction and the hydrogen-bonding between the CeOH species of GO and the adsorbed oxygen on the surface of the transition metal oxide particles. On the other hand, if the GO sheets are large or if the transition metal oxide particles are too small to be mixed together, the transition metal oxide particles will surround the GO sheets. Volumetric hydrogen storage measurement of the GO-wrapped transition metal oxide composite materials was performed at 77 K. The wrapping of GO causes an enhancement of hydrogen storage capacity compared to those of the pristine transition metal oxides, from 0.16 wt% for V2O5 (0.58 wt% for TiO2) to 1.36 wt% for GO/V2O5 (1.26 wt% for GO/ TiO2). This agent-free synthesis method may be adapted to other applications. For example, it provides an easy way to replace the carbon coating method for lithium ion batteries synthesized with transition metal oxides.
Acknowledgement This work was supported by the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Program, funded by the Ministry of Science and Technology of Korea.
Appendix. Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.ijhydene.2012.02.010.
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