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Enhancing the hydrogen storage capacity of Pd-functionalized multi-walled carbon nanotubes
Applied Surface Science 258 (2012) 3405–3409
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhancing the hydrogen storage capacity of Pd-functionalized multi-walled carbon nanotubes Priyanka Singh a , Mukta V. Kulkarni a , Suresh P. Gokhale a , Samir H. Chikkali b , Chandrashekhar V. Kulkarni c,∗ a
Physical & Materials Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India Department of Chemistry, University of Konstanz, Universitätstrasse-10, Konstanz D-78457, Germany c Experimental Physics I, University of Bayreuth, Bayreuth 95440, Germany b
a r t i c l e
i n f o
Article history: Received 16 September 2011 Received in revised form 8 November 2011 Accepted 16 November 2011 Available online 11 December 2011 Keywords: Hydrogen based energy sources Hydrogen storage capacity Multiwalled carbon nanotubes Pd functionalization PVP capping
a b s t r a c t We demonstrate that the hydrogen storage capacity of multi-walled carbon nanotubes can be enhanced by polyvinylpyrrolidone functionalization. The polyvinylpyrrolidone acts as a stabilizing agent for Pd-nanoparticles, reduces their size and facilitates their uniform and enhanced loading onto multi-walled carbon nanotubes. According to sorption studies, the polyvinylpyrrolidone capping and consequent nanostructural modification enables 2.3 times more hydrogen adsorption than mere Pdfunctionalization of multi-walled carbon nanotubes. Corresponding morphological changes before and after polyvinylpyrrolidone capping, characterized using Raman Spectroscopy, X-ray diffraction, TEM and thermal analysis techniques, are also presented. The results contribute towards increasing the efficiency of hydrogen based sustainable energy sources. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is one of the major sustainable energy sources, which offers a clean and efficient energy system for both mobile and stationary power generation. Nevertheless, its use has remained marginal, despite of its abundance and environmental friendly nature, demanding more technological developments before launching the hydrogen-based fuel systems for real applications [1]. There are several physico-chemical treatments for isolation and storage of hydrogen for its efficient use in some later times [1,2]. Among all known hydrogen storage pathways, its physisorption on solid substrates appears to be very adaptive because of obvious reasons, for instance, the adsorbed gas can be released reversibly [3]. The carbon-based nanostructures [4–10], especially single [11–13] and multiwalled carbon nanotubes (MWCNTs) [14], have been extensively studied for their role as solid substrates, mainly due to the accessibility of enormous surface area that can be readily functionalized [15]. Thushow treated surfaces can be decorated with metals such as Pt, Pd, Li, Ni [16–18], etc. which equip with the contact points for ballistic transport
[19,20] and consequently adsorb more hydrogen onto the carbon nanostructures [21–23]. Theoretical predictions reveal that such transition metal atoms stabilized on sp2 -hybridized carbons (from MWCNTs) induce multiple bonding [24] of molecular hydrogen with adsorption energies intermediate between physisorption and chemisorption [4,25] and the adsorption, in this case, occurs via spillover mechanism [21,26]. Some of the crucial parameters that determine the effective storage capacity of hydrogen onto metal decorated CNTs include, the nature of metal–hydrogen bond [27], strength of binding [28], surface coverage and relative binding energies [18]. The hydrogen adsorption capacity can be modulated by controlling one or more of the aforementioned parameters. For example, the hydrogen storage performance of Pd functionalized carbon structures was measured to be ∼8 times higher compared to that without functionaliation [29,30]. In this paper, we show that this capacity is further increased by capping of Pd nanoparticles by polyvinylpyrrolidone (PVP). The latter acts as a stabilizing agent for the formed metal nanoparticles. 2. Materials and methods 2.1. Materials
Abbreviations: MWCNT, multi-walled carbon nanotubes; PVP, polyvinylpyrrolidone; Pd-nanoparticles, palladium nanoparticles. ∗ Corresponding author. Tel.: +49 921 55 2503; fax: +49 921 55 2521. E-mail address: [email protected] (C.V. Kulkarni). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.11.075
CVD (chemical vapor deposition) synthesized MWNTs of purity >95% were procured from Chemapol, India. Sodium borohydride (NaBH4 ), palladium chloride (PdCl2 ) and PVP were purchased from
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Aldrich, India. The Nitric acid (HNO3 , 69% GR) and polycarbonate membrane filters (8.0 m, 47 mm diameter) were obtained from Merck-India and Merck-Germany respectively. 2.2. Purification and acid treatment of MWCNTs As-procured MWNTs were first purified by air-oxidation at 450 ◦ C for 30 min to remove amorphous carbon and then treated with conc. HNO3 for carboxylation. For this, ∼200 mg of the purified MWNTs were added to 50 mL of conc. HNO3 (69%) and the mixture was refluxed at 70 ◦ C for 24 h with continuous magnetic stirring. After acid-treatment, the MWNTs were filtered through the polycarbonate membrane, washed several times with distilled deionized (Millipore Co.) water till the neutral pH was reached and finally dried under a high-wattage (Infra red) lamp for 1 h. The carboxylation of MWNTs with HNO3 enabled a good dispersion of the nanotubes in aqueous medium and improved their processability for further derivatization. 2.3. Pd functionalization and PVP capping The carboxylated MWNTs were further treated with PdCl2 so as to anchor Pd nanoparticles onto the nanotube surface. The stock solution of PdCl2 containing 5 wt% of Pd was prepared by dissolving 8.3 mg of PdCl2 in 0.1 M aqueous HCl solution with stirring at 80 ◦ C, cooling the solution to room temperature and finally diluting it to 100 mL. The Pd-functionalization was carried out with and without capping agent PVP. For this, 95 mg of the carboxylated MWNTs were ultrasonicated in 100 mL of PdCl2 (5 wt% Pd content) solution. After 1 h of ultrasonication, the NaBH4 solution was added slowly until the solution became colorless indicating the complete reduction of Pd2+ to Pd0 (Pd2+ → Pd0 ). The resulting reaction mixture of Pd-functionalized MWNTs was filtered through the polycarbonate membrane, washed with distilled deionized water and dried under a lamp. For PVP capping, the aqueous solution of 0.45 mol PVP was prepared and added drop wise to Pd-MWNTs solution prior to reduction with NaBH4 . 2.4. Characterization and hydrogen adsorption/desorption The samples of Pd-functionalized (with and without PVP capping) MWNTs were characterized for morphological and thermal analysis. Raman spectra were recorded using a Horiba JY LabRAM HR 800 micro-Raman spectrometer with 17 mW, 632.8 nm excitation. The TGA measurements were carried out using a Seiko TG/DTA 32 machine. TEM images were taken using a FEI, Tecnai F30 electron microscope operating at an accelerating voltage of 300 kV FEG. EDX measurements were carried out using a Phoenix EDAX module. XRD patterns were recorded on a Philips Xpert Pro powder Xray diffractometer using Ni filtered Cu K␣ radiation ( = 0.154 nm, current = 30 mA, voltage = 40 kV). The adsorption and desorption isotherms for hydrogen storage on the Pd-functionalized MWNTs were obtained using a Quadrasorb apparatus at 77 K. The isotherms were reproducible within the error of measurements. Also the material was recyclable when checked (twice) for hydrogen storage.
Fig. 1. Raman spectra of Pd-MWNTs (solid black line) and PdPVP-MWNTs (red dotted line): shift in peaks indicates increased disorder whereas increase in the intensity ratio of D-band to G-band (ID /IG ) corresponds to greater defect levels in MWCNTs due to PVP capping. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
be acid (HNO3 ) treated followed by the surface anchoring of the Pd-nanoparticles (for details see section 2.2 and 2.3). Subsequent PVP treatment further stabilizes the nanoparticles by forming a coordinate bond with Pd2+ . These after reduction provide more free sites for the physical adsorption at Pd(0). Representative coordination of PVP to palladium nanoparticles is shown in Supplementary information available online. Raman spectroscopy offers to access the defects in the structure and crystallinity of carbon nanotubes [33]. Raman Spectra of Pd-MWNTs and PdPVP-MWNTs scanned in the frequency range of 1000–3000 cm−1 are shown in Fig. 1. It showed characteristic G-band and Dband peaks observed for carbon nanotubes. The strong peaks at 1578.8 cm−1 (Pd-MWNTs) and 1562.4 cm−1 (PdPVP-MWNTs) correspond to the graphite-related G-band due to the in-plane oscillations of sp2 carbon atoms [34]. The peaks at 1326.8 cm−1 (Pd-MWNTs) and 1318.0 cm−1 (PdPVP-MWNTs) correspond to the disorder-induced D-band for graphitic carbon [35]. The peaks at 2650.8 cm−1 (Pd-MWNTs) and 2634.4 cm−1 (PdPVP-MWNTs) correspond to the first overtone of D-band, which is referred as G -band [35]. A slight movement of the peaks of G-band, D-band and G band towards lower Raman Shift values could be attributed to the PdPVP functionalization of carbon nanotubes. The intensity ratio of D-band to G-band (ID /IG ) indicates the extent of distortion or
3. Results and discussion 3.1. Morphological changes – defect formation due to PVP capping High degree of defects or distortions among the carbon nanostructures facilitates adsorption of more hydrogen [31,32]. Given the lack of such defects, the pristine MWCNTs are less susceptible for hydrogen adsorption. Therefore first they need to
Fig. 2. The PVP capped Pd-MWCNTs show higher thermal stability as observed from the TGA graph shown here; corresponding heats associated with both thermograms are shown on the right hand side (second y-axis).
P. Singh et al. / Applied Surface Science 258 (2012) 3405–3409
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Fig. 3. TEM images of (a) Pd-MWNTs and (b) PdPVP-MWNTs. Dark solid circles indicate spherical Pd-nanoparticles whereas long tubes represent MWCNTs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
defects in carbon nanotubes. The smaller the value of ID /IG lesser would be the distortion or defects. We observed the values of ID /IG as 0.92 and 1.07 for Pd-MWNTs and PdPVP-MWNTs respectively, which is ascribed to the higher degree of defects due to PVP capping. Although PVP capping increases the disorder in MWCNTs, the latter show higher stability for thermal changes (Fig. 2). Thermogravimetric analysis (TGA) can reveal various functional elements and impurities present in a sample; it also shows the change in thermal stability for a chemical treatment to the sample. The Pd-MWNTs started decomposing near 520.0 ◦ C and burnt off completely at 564.8 ◦ C while the PdPVP-MWNTs started decomposing near 686.3 ◦ C and burnt off completely at 707.0 ◦ C showing high thermal stability of the nanotubes. Thus, Pd-functionalized MWNTs with PVP capping offer better thermal stability via coordination bonding than that without PVP capping. This depicts the improved efficiency of MWCNTs to retain the metal naoparticles on their surface. 3.2. Enhanced and uniform metal loading It was seen from the TEM images (Fig. 3) that in Pd-MWNTs, the palladium nanoparticles were not well distributed on the nanotube surface due to agglomeration while in the case of PdPVPMWNTs, the spherical palladium nanoparticles were found to be uniformly distributed covering large area of the nanotube network. The most evident reasoning appears from the fact that the carbonyl group of the PVP can coordinates to the metal to stabilize the nanoparticles and thus overcomes the limit of aggregation observed without capping as shown in Fig. 3a. Moreover, the size of the Pd nanoparticles was reduced to ∼10–15 nm from 18 to 20 nm observed without PVP. The image of Pd lattice is also shown at the right-bottom corner in Fig. 3b. In both cases, there were numerous metal nanoparticles anchored onto the external walls of the nanotubes, however, EDX analysis (shown in SI available online) confirmed slight increase (from 4.39% to 4.61%) in Pd uptake when the nanoparticles were treated with PVP. From the elemental quantification based on EDX measurements and ICP analysis, the oxygen content was found to be reduced to almost half (15.81–7.9%), resulting into increase in carbon content (75.13–83.58%) because of PVP capping. The X-ray diffraction (XRD) studies at wide angles (10–80◦ ) also corroborate the metal enrichment of MWCNTs (Fig. 4). Characteristic peaks of graphitic carbon in Pd-MWNTs/PdPVP-MWNTs were observed at 2 values of 25.7◦ /25.7◦ , 42.9◦ /43.5◦ and 53.3◦ /53.2◦ corresponding to diffraction of (0 0 2), (1 0 0) and (0 0 4) planes [36]. The peaks at 2 values of 40.2◦ /39.2◦ , 46.6◦ /45.1◦ and 68.1◦ /66.4◦
in Pd-MWNTs/PdPVP-MWNTs can be assigned to respective crystalline planes (1 1 1), (1 1 0) and (1 0 0) appearing from the loaded Pd metal [37]. The decrease in peak intensities and their broadning (especially for higher order peaks) indicates increase in disorder while the shifting of the Pd(1 1 1) peak at lower 2 elucidates enhanced loading, i.e., larger lattice spacings. 3.3. Hydrogen adsorption measurements The hydrogen storage measurements were carried out at 77 K. The profiles of hydrogen adsorption/desorption isotherms for PdMWNTs and PdPVP-MWNTs are shown in Fig. 5. From the results, it was observed that the hydrogen adsorption in PdPVP-MWNTs is much higher (volume at STP) than that in Pd-MWNTs. The PVP capped Pd functionalized MWNTs adsorbed approximately 2.3 times more hydrogen than Pd functionalized MWNTs without PVP capping. The hydrogen storage on PdPVP-MWNTs and Pd-MWNTs were found to be 4 wt% and 1.7 wt% respectively at 77 K and ambient pressure. The adrorption experiments were reproducible with minimal error (5%). Both materials were recyclable for hydrogen storage [2]. It was checked qualitatively by removing the stored hydrogen by decreasing pressure [13] (Fig. 5) and increasing the temperature (>500–700 K) [38] followed by adsorbing again under
Fig. 4. Wide angle XRD patterns for Pd-MWNTs (solid black curve) and for PdPVPMWNTs (red dotted curve). Curved are shifted vertically for convenience. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 5. Hydrogen adsorption and desorption isotherms for (a) Pd-MWNTs and (b) PdPVP-MWNTs. Horizontal arrows indicate that the hydrogen is retained longer (wide pressure range) among the PVP capped MWCNTs.
above conditions. Here, note that PVP capped material had an extended stability as shown by thermal analysis (Fig. 2). The improved hydrogen adsorption performance of PdPVPMWNTs can therefore be attributed to the PVP capping, which, as mentioned earlier, stabilizes the interactions and prevents agglomeration of the nanoparticles whose reduced dimensions (more surface area) also contribute in hydrogen adsorption. 4. Conclusions We have shown that the PVP capping offers improved and uniform surface coverage of Pd-nanoparticle onto MWCNTs. As a consequence, the ensemble exhibited better thermal stability and more nanostructural defects. More specifically and importantly the PVP capping considerably enhanced the hydrogen storage capacity of Pd functionalized MWNTs which adsorbed 2.3 times more hydrogen than before. This work could prove useful for the development of efficient and green-hydrogen storage systems for their wide range of applications. Acknowledgments Authors gratefully acknowledge Council of Scientific and Industrial Research (CSIR), New Delhi, India for the financial support. SHC is thankful for the Postdoctoral Fellowship from Alexander von Humboldt foundation. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2011.11.075. References [1] L. Zhou, Progress and problems in hydrogen storage methods, Renewable and Sustainable Energy Reviews 9 (4) (2005) 395–408. [2] A. Züttel, Hydrogen storage methods, Naturwissenschaften 91 (4) (2004) 157–172. [3] S.-H. Jhi, et al., Hydrogen storage by physisorption: beyond carbon, Solid State Communications 129 (12) (2004) 769–773. [4] S. Dag, et al., Adsorption and dissociation of hydrogen molecules on bare and functionalized carbon nanotubes, Physical Review B 72 (15) (2005) 155404. [5] P. Bénard, R. Chahine, Storage of hydrogen by physisorption on carbon and nanostructured materials, Scripta Materialia 56 (10) (2007) 803–808. [6] R. Ströbel, et al., Hydrogen storage by carbon materials, Journal of Power Sources 159 (2) (2006) 781–801. [7] R. Ströbel, et al., Hydrogen adsorption on carbon materials, Journal of Power Sources 84 (2) (1999) 221–224. [8] A. Bachmatiuk, et al., Study on hydrogen uptake of functionalized carbon nanotubes, Physica Status Solidi B 243 (13) (2006) 3226–3229.
[9] M. Hirscher, et al., Hydrogen storage in sonicated carbon materials, Applied Physics A: Materials Science & Processing 72 (2) (2001) 129–132. [10] Z. Sun, et al., Decoration carbon nanotubes with Pd and Ru nanocrystals via an inorganic reaction route in supercritical carbon dioxide-methanol solution, Journal of Colloid and Interface Science 304 (2) (2006) 323–328. [11] A. Ansón, et al., Hydrogen adsorption studies on single wall carbon nanotubes, Carbon 42 (7) (2004) 1243–1248. [12] S.M. Lee, et al., A hydrogen storage mechanism in single-walled carbon nanotubes, Journal of the American Chemical Society 123 (21) (2001) 5059–5063. [13] C. Liu, et al., Hydrogen storage in single-walled carbon nanotubes at room temperature, Science 286 (1999) 1127–1129, 5442. [14] G.Q. Ning, et al., Hydrogen storage in multi-wall carbon nanotubes using samples up to 85 g, Applied Physics A: Materials Science & Processing 78 (7) (2004) 955–959. [15] M. Endo, et al., Applications of carbon nanotubes in the twenty-first century, Philosophical Transactions of the Royal Society A 362 (2004) 2223–2238, 1823. [16] A.D. Franklin, et al., Controlled decoration of single-walled carbon nanotubes with pd nanocubes, The Journal of Physical Chemistry C 111 (37) (2007) 13756–13762. [17] N. Karousis, et al., Carbon nanotubes decorated with palladium nanoparticles: synthesis, characterization, and catalytic activity, The Journal of Physical Chemistry C 112 (35) (2008) 13463–13469. [18] V.J. Surya, et al., The role of Li and Ni metals in the adsorbate complex and their effect on the hydrogen storage capacity of single walled carbon nanotubes coated with metal hydrides: LiH and NiH2, International Journal of Hydrogen Energy 35 (6) (2010) 2368–2376. [19] A. Felten, et al., Electronic structure of Pd nanoparticles on carbon nanotubes, Micron 40 (1) (2009) 74–79. [20] A. Javey, et al., Ballistic carbon nanotube field-effect transistors, Nature 424 (2003) 654–657, 6949. [21] R. Zacharia, et al., Enhancement of hydrogen storage capacity of carbon nanotubes via spill-over from vanadium and palladium nanoparticles, Chemical Physics Letters 412 (4–6) (2005) 369–375. [22] A. Ansón, et al., Hydrogen capacity of palladium-loaded carbon materials, The Journal of Physical Chemistry B 110 (13) (2006) 6643–6648. [23] S.-C. Mu, et al., Hydrogen storage in carbon nanotubes modified by microwave plasma etching and Pd decoration, Carbon 44 (4) (2006) 762–767. [24] A. Nikitin, et al., Hydrogen storage in carbon nanotubes through the formation of stable C H bonds, Nano Letters 8 (1) (2007) 162–167. [25] T. Yildirim, S. Ciraci, Titanium-decorated carbon nanotubes as a potential highcapacity hydrogen storage medium, Physical Review Letters 94 (17) (2005) 175501. [26] A.D. Lueking, R.T. Yang, Hydrogen spillover onto carbon: inference of spiltover hydrogen on optimization for hydrogen storage, Applied Catalysis A: General 265 (2004) 259–268. [27] N.S. Venkataramanan, et al., First-principles study of hydrogen storage over Ni and Rh doped BN sheets, Chemical Physics 359 (1–3) (2009) 173–178. [28] K. Iyakutti, et al., Clustering of functional molecules on a single-walled carbonnanotube surface and its effect on hydrogen storage, Physica Status Solidi B 246 (10) (2009) 2248–2256. [29] R. Campesi, et al., Hydrogen storage properties of Pd nanoparticle/carbon template composites, Carbon 46 (2) (2008) 206–214. [30] S.-C. Mu, et al., Performance of hydrogen storage of carbon nanotubes decorated with palladium, The Transactions of Nonferrous Metals Society of China 14 (5) (2004). [31] C.-H. Chen, C.-C. Huang, Hydrogen adsorption in defective carbon nanotubes, Separation and Purification Technology 65 (3) (2009) 305–310. [32] V. Gayathri, R. Geetha, Hydrogen adsorption in defected carbon nanotubes, Adsorption 13 (1) (2007) 53–59. [33] H. Hiura, et al., Raman studies of carbon nanotubes, Chemical Physics Letters 202 (6) (1993) 509–512.
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[37] Y. Yazawa, et al., Oxidation state of palladium as a factor controlling catalytic activity of Pd/SiO2 –Al2 O3 in propane combustion, Applied Catalysis B: Environmental 19 (3–4) (1998) 261–266. [38] Y. Eunjoo, T. Habe, J. Nakamura, Possibilities of atomic hydrogen storage by carbon nanotubes or graphite materials, Science and Technology of Advanced Materials 6 (6) (2005) 615.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhancing the hydrogen storage capacity of Pd-functionalized multi-walled carbon nanotubes Priyanka Singh a , Mukta V. Kulkarni a , Suresh P. Gokhale a , Samir H. Chikkali b , Chandrashekhar V. Kulkarni c,∗ a
Physical & Materials Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India Department of Chemistry, University of Konstanz, Universitätstrasse-10, Konstanz D-78457, Germany c Experimental Physics I, University of Bayreuth, Bayreuth 95440, Germany b
a r t i c l e
i n f o
Article history: Received 16 September 2011 Received in revised form 8 November 2011 Accepted 16 November 2011 Available online 11 December 2011 Keywords: Hydrogen based energy sources Hydrogen storage capacity Multiwalled carbon nanotubes Pd functionalization PVP capping
a b s t r a c t We demonstrate that the hydrogen storage capacity of multi-walled carbon nanotubes can be enhanced by polyvinylpyrrolidone functionalization. The polyvinylpyrrolidone acts as a stabilizing agent for Pd-nanoparticles, reduces their size and facilitates their uniform and enhanced loading onto multi-walled carbon nanotubes. According to sorption studies, the polyvinylpyrrolidone capping and consequent nanostructural modification enables 2.3 times more hydrogen adsorption than mere Pdfunctionalization of multi-walled carbon nanotubes. Corresponding morphological changes before and after polyvinylpyrrolidone capping, characterized using Raman Spectroscopy, X-ray diffraction, TEM and thermal analysis techniques, are also presented. The results contribute towards increasing the efficiency of hydrogen based sustainable energy sources. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is one of the major sustainable energy sources, which offers a clean and efficient energy system for both mobile and stationary power generation. Nevertheless, its use has remained marginal, despite of its abundance and environmental friendly nature, demanding more technological developments before launching the hydrogen-based fuel systems for real applications [1]. There are several physico-chemical treatments for isolation and storage of hydrogen for its efficient use in some later times [1,2]. Among all known hydrogen storage pathways, its physisorption on solid substrates appears to be very adaptive because of obvious reasons, for instance, the adsorbed gas can be released reversibly [3]. The carbon-based nanostructures [4–10], especially single [11–13] and multiwalled carbon nanotubes (MWCNTs) [14], have been extensively studied for their role as solid substrates, mainly due to the accessibility of enormous surface area that can be readily functionalized [15]. Thushow treated surfaces can be decorated with metals such as Pt, Pd, Li, Ni [16–18], etc. which equip with the contact points for ballistic transport
[19,20] and consequently adsorb more hydrogen onto the carbon nanostructures [21–23]. Theoretical predictions reveal that such transition metal atoms stabilized on sp2 -hybridized carbons (from MWCNTs) induce multiple bonding [24] of molecular hydrogen with adsorption energies intermediate between physisorption and chemisorption [4,25] and the adsorption, in this case, occurs via spillover mechanism [21,26]. Some of the crucial parameters that determine the effective storage capacity of hydrogen onto metal decorated CNTs include, the nature of metal–hydrogen bond [27], strength of binding [28], surface coverage and relative binding energies [18]. The hydrogen adsorption capacity can be modulated by controlling one or more of the aforementioned parameters. For example, the hydrogen storage performance of Pd functionalized carbon structures was measured to be ∼8 times higher compared to that without functionaliation [29,30]. In this paper, we show that this capacity is further increased by capping of Pd nanoparticles by polyvinylpyrrolidone (PVP). The latter acts as a stabilizing agent for the formed metal nanoparticles. 2. Materials and methods 2.1. Materials
Abbreviations: MWCNT, multi-walled carbon nanotubes; PVP, polyvinylpyrrolidone; Pd-nanoparticles, palladium nanoparticles. ∗ Corresponding author. Tel.: +49 921 55 2503; fax: +49 921 55 2521. E-mail address: [email protected] (C.V. Kulkarni). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.11.075
CVD (chemical vapor deposition) synthesized MWNTs of purity >95% were procured from Chemapol, India. Sodium borohydride (NaBH4 ), palladium chloride (PdCl2 ) and PVP were purchased from
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Aldrich, India. The Nitric acid (HNO3 , 69% GR) and polycarbonate membrane filters (8.0 m, 47 mm diameter) were obtained from Merck-India and Merck-Germany respectively. 2.2. Purification and acid treatment of MWCNTs As-procured MWNTs were first purified by air-oxidation at 450 ◦ C for 30 min to remove amorphous carbon and then treated with conc. HNO3 for carboxylation. For this, ∼200 mg of the purified MWNTs were added to 50 mL of conc. HNO3 (69%) and the mixture was refluxed at 70 ◦ C for 24 h with continuous magnetic stirring. After acid-treatment, the MWNTs were filtered through the polycarbonate membrane, washed several times with distilled deionized (Millipore Co.) water till the neutral pH was reached and finally dried under a high-wattage (Infra red) lamp for 1 h. The carboxylation of MWNTs with HNO3 enabled a good dispersion of the nanotubes in aqueous medium and improved their processability for further derivatization. 2.3. Pd functionalization and PVP capping The carboxylated MWNTs were further treated with PdCl2 so as to anchor Pd nanoparticles onto the nanotube surface. The stock solution of PdCl2 containing 5 wt% of Pd was prepared by dissolving 8.3 mg of PdCl2 in 0.1 M aqueous HCl solution with stirring at 80 ◦ C, cooling the solution to room temperature and finally diluting it to 100 mL. The Pd-functionalization was carried out with and without capping agent PVP. For this, 95 mg of the carboxylated MWNTs were ultrasonicated in 100 mL of PdCl2 (5 wt% Pd content) solution. After 1 h of ultrasonication, the NaBH4 solution was added slowly until the solution became colorless indicating the complete reduction of Pd2+ to Pd0 (Pd2+ → Pd0 ). The resulting reaction mixture of Pd-functionalized MWNTs was filtered through the polycarbonate membrane, washed with distilled deionized water and dried under a lamp. For PVP capping, the aqueous solution of 0.45 mol PVP was prepared and added drop wise to Pd-MWNTs solution prior to reduction with NaBH4 . 2.4. Characterization and hydrogen adsorption/desorption The samples of Pd-functionalized (with and without PVP capping) MWNTs were characterized for morphological and thermal analysis. Raman spectra were recorded using a Horiba JY LabRAM HR 800 micro-Raman spectrometer with 17 mW, 632.8 nm excitation. The TGA measurements were carried out using a Seiko TG/DTA 32 machine. TEM images were taken using a FEI, Tecnai F30 electron microscope operating at an accelerating voltage of 300 kV FEG. EDX measurements were carried out using a Phoenix EDAX module. XRD patterns were recorded on a Philips Xpert Pro powder Xray diffractometer using Ni filtered Cu K␣ radiation ( = 0.154 nm, current = 30 mA, voltage = 40 kV). The adsorption and desorption isotherms for hydrogen storage on the Pd-functionalized MWNTs were obtained using a Quadrasorb apparatus at 77 K. The isotherms were reproducible within the error of measurements. Also the material was recyclable when checked (twice) for hydrogen storage.
Fig. 1. Raman spectra of Pd-MWNTs (solid black line) and PdPVP-MWNTs (red dotted line): shift in peaks indicates increased disorder whereas increase in the intensity ratio of D-band to G-band (ID /IG ) corresponds to greater defect levels in MWCNTs due to PVP capping. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
be acid (HNO3 ) treated followed by the surface anchoring of the Pd-nanoparticles (for details see section 2.2 and 2.3). Subsequent PVP treatment further stabilizes the nanoparticles by forming a coordinate bond with Pd2+ . These after reduction provide more free sites for the physical adsorption at Pd(0). Representative coordination of PVP to palladium nanoparticles is shown in Supplementary information available online. Raman spectroscopy offers to access the defects in the structure and crystallinity of carbon nanotubes [33]. Raman Spectra of Pd-MWNTs and PdPVP-MWNTs scanned in the frequency range of 1000–3000 cm−1 are shown in Fig. 1. It showed characteristic G-band and Dband peaks observed for carbon nanotubes. The strong peaks at 1578.8 cm−1 (Pd-MWNTs) and 1562.4 cm−1 (PdPVP-MWNTs) correspond to the graphite-related G-band due to the in-plane oscillations of sp2 carbon atoms [34]. The peaks at 1326.8 cm−1 (Pd-MWNTs) and 1318.0 cm−1 (PdPVP-MWNTs) correspond to the disorder-induced D-band for graphitic carbon [35]. The peaks at 2650.8 cm−1 (Pd-MWNTs) and 2634.4 cm−1 (PdPVP-MWNTs) correspond to the first overtone of D-band, which is referred as G -band [35]. A slight movement of the peaks of G-band, D-band and G band towards lower Raman Shift values could be attributed to the PdPVP functionalization of carbon nanotubes. The intensity ratio of D-band to G-band (ID /IG ) indicates the extent of distortion or
3. Results and discussion 3.1. Morphological changes – defect formation due to PVP capping High degree of defects or distortions among the carbon nanostructures facilitates adsorption of more hydrogen [31,32]. Given the lack of such defects, the pristine MWCNTs are less susceptible for hydrogen adsorption. Therefore first they need to
Fig. 2. The PVP capped Pd-MWCNTs show higher thermal stability as observed from the TGA graph shown here; corresponding heats associated with both thermograms are shown on the right hand side (second y-axis).
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Fig. 3. TEM images of (a) Pd-MWNTs and (b) PdPVP-MWNTs. Dark solid circles indicate spherical Pd-nanoparticles whereas long tubes represent MWCNTs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
defects in carbon nanotubes. The smaller the value of ID /IG lesser would be the distortion or defects. We observed the values of ID /IG as 0.92 and 1.07 for Pd-MWNTs and PdPVP-MWNTs respectively, which is ascribed to the higher degree of defects due to PVP capping. Although PVP capping increases the disorder in MWCNTs, the latter show higher stability for thermal changes (Fig. 2). Thermogravimetric analysis (TGA) can reveal various functional elements and impurities present in a sample; it also shows the change in thermal stability for a chemical treatment to the sample. The Pd-MWNTs started decomposing near 520.0 ◦ C and burnt off completely at 564.8 ◦ C while the PdPVP-MWNTs started decomposing near 686.3 ◦ C and burnt off completely at 707.0 ◦ C showing high thermal stability of the nanotubes. Thus, Pd-functionalized MWNTs with PVP capping offer better thermal stability via coordination bonding than that without PVP capping. This depicts the improved efficiency of MWCNTs to retain the metal naoparticles on their surface. 3.2. Enhanced and uniform metal loading It was seen from the TEM images (Fig. 3) that in Pd-MWNTs, the palladium nanoparticles were not well distributed on the nanotube surface due to agglomeration while in the case of PdPVPMWNTs, the spherical palladium nanoparticles were found to be uniformly distributed covering large area of the nanotube network. The most evident reasoning appears from the fact that the carbonyl group of the PVP can coordinates to the metal to stabilize the nanoparticles and thus overcomes the limit of aggregation observed without capping as shown in Fig. 3a. Moreover, the size of the Pd nanoparticles was reduced to ∼10–15 nm from 18 to 20 nm observed without PVP. The image of Pd lattice is also shown at the right-bottom corner in Fig. 3b. In both cases, there were numerous metal nanoparticles anchored onto the external walls of the nanotubes, however, EDX analysis (shown in SI available online) confirmed slight increase (from 4.39% to 4.61%) in Pd uptake when the nanoparticles were treated with PVP. From the elemental quantification based on EDX measurements and ICP analysis, the oxygen content was found to be reduced to almost half (15.81–7.9%), resulting into increase in carbon content (75.13–83.58%) because of PVP capping. The X-ray diffraction (XRD) studies at wide angles (10–80◦ ) also corroborate the metal enrichment of MWCNTs (Fig. 4). Characteristic peaks of graphitic carbon in Pd-MWNTs/PdPVP-MWNTs were observed at 2 values of 25.7◦ /25.7◦ , 42.9◦ /43.5◦ and 53.3◦ /53.2◦ corresponding to diffraction of (0 0 2), (1 0 0) and (0 0 4) planes [36]. The peaks at 2 values of 40.2◦ /39.2◦ , 46.6◦ /45.1◦ and 68.1◦ /66.4◦
in Pd-MWNTs/PdPVP-MWNTs can be assigned to respective crystalline planes (1 1 1), (1 1 0) and (1 0 0) appearing from the loaded Pd metal [37]. The decrease in peak intensities and their broadning (especially for higher order peaks) indicates increase in disorder while the shifting of the Pd(1 1 1) peak at lower 2 elucidates enhanced loading, i.e., larger lattice spacings. 3.3. Hydrogen adsorption measurements The hydrogen storage measurements were carried out at 77 K. The profiles of hydrogen adsorption/desorption isotherms for PdMWNTs and PdPVP-MWNTs are shown in Fig. 5. From the results, it was observed that the hydrogen adsorption in PdPVP-MWNTs is much higher (volume at STP) than that in Pd-MWNTs. The PVP capped Pd functionalized MWNTs adsorbed approximately 2.3 times more hydrogen than Pd functionalized MWNTs without PVP capping. The hydrogen storage on PdPVP-MWNTs and Pd-MWNTs were found to be 4 wt% and 1.7 wt% respectively at 77 K and ambient pressure. The adrorption experiments were reproducible with minimal error (5%). Both materials were recyclable for hydrogen storage [2]. It was checked qualitatively by removing the stored hydrogen by decreasing pressure [13] (Fig. 5) and increasing the temperature (>500–700 K) [38] followed by adsorbing again under
Fig. 4. Wide angle XRD patterns for Pd-MWNTs (solid black curve) and for PdPVPMWNTs (red dotted curve). Curved are shifted vertically for convenience. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 5. Hydrogen adsorption and desorption isotherms for (a) Pd-MWNTs and (b) PdPVP-MWNTs. Horizontal arrows indicate that the hydrogen is retained longer (wide pressure range) among the PVP capped MWCNTs.
above conditions. Here, note that PVP capped material had an extended stability as shown by thermal analysis (Fig. 2). The improved hydrogen adsorption performance of PdPVPMWNTs can therefore be attributed to the PVP capping, which, as mentioned earlier, stabilizes the interactions and prevents agglomeration of the nanoparticles whose reduced dimensions (more surface area) also contribute in hydrogen adsorption. 4. Conclusions We have shown that the PVP capping offers improved and uniform surface coverage of Pd-nanoparticle onto MWCNTs. As a consequence, the ensemble exhibited better thermal stability and more nanostructural defects. More specifically and importantly the PVP capping considerably enhanced the hydrogen storage capacity of Pd functionalized MWNTs which adsorbed 2.3 times more hydrogen than before. This work could prove useful for the development of efficient and green-hydrogen storage systems for their wide range of applications. Acknowledgments Authors gratefully acknowledge Council of Scientific and Industrial Research (CSIR), New Delhi, India for the financial support. SHC is thankful for the Postdoctoral Fellowship from Alexander von Humboldt foundation. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2011.11.075. References [1] L. Zhou, Progress and problems in hydrogen storage methods, Renewable and Sustainable Energy Reviews 9 (4) (2005) 395–408. [2] A. Züttel, Hydrogen storage methods, Naturwissenschaften 91 (4) (2004) 157–172. [3] S.-H. Jhi, et al., Hydrogen storage by physisorption: beyond carbon, Solid State Communications 129 (12) (2004) 769–773. [4] S. Dag, et al., Adsorption and dissociation of hydrogen molecules on bare and functionalized carbon nanotubes, Physical Review B 72 (15) (2005) 155404. [5] P. Bénard, R. Chahine, Storage of hydrogen by physisorption on carbon and nanostructured materials, Scripta Materialia 56 (10) (2007) 803–808. [6] R. Ströbel, et al., Hydrogen storage by carbon materials, Journal of Power Sources 159 (2) (2006) 781–801. [7] R. Ströbel, et al., Hydrogen adsorption on carbon materials, Journal of Power Sources 84 (2) (1999) 221–224. [8] A. Bachmatiuk, et al., Study on hydrogen uptake of functionalized carbon nanotubes, Physica Status Solidi B 243 (13) (2006) 3226–3229.
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