Physica E 60 (2014) 75–79
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Reversible hydrogen storage in functionalized single-walled carbon nanotubes D. Silambarasan a, V. Vasu a,n, K. Iyakutti b,nn, V.J. Surya c, T.R. Ravindran d a
School of Physics, Madurai Kamaraj University, Madurai-625021, Tamil Nadu, India Department of Physics & Nanotechnology, SRM University, Kattankulathur-603203, Tamil Nadu, India c New Industry Creation Hatchery Center, Tohoku University, Sendai-980-8579, Japan d Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, Tamil Nadu, India b
H I G H L I G H T S
G R A P H I C A L
Functionalized SWCNTs shows a storage capacity of 4.77 wt% at 50 1C. Entire (100%) stored hydrogen is released in the temperature range of 90–125 1C. Hydrogenation and dehydrogenation is stabilized and are reproducible. Deterioration level of the sample is only of 2.3%. Storage capacity achieved here is close to the US DOE target.
Functionalized SWCNTs shows a maximum hydrogen storage capacity of 4.77 wt% at 50 1C and the entire (100%) stored hydrogen is released in the temperature range of 90–125 1C
art ic l e i nf o
a b s t r a c t
Article history: Received 12 August 2013 Accepted 12 February 2014 Available online 20 February 2014
In this work, functionalized carbon nanotubes (CNTs) based hydrogen storage medium has been designed by the facile drop-casting method. Initially, the commercial single-walled carbon nanotubes (SWCNTs) were puriﬁed by standard methods and functionalized with borane (BH3). The morphology of SWCNTs was imaged by transmission electron microscopy (TEM). The energy dispersive spectroscopy (ED) shows that the puriﬁed SWCNTs are free from elemental impurities. The functional groups in the functionalized SWCNTs were analyzed by fourier transform infra-red spectroscopy (FTIR). Then, the functionalized SWCNTs were hydrogenated in a Seivert like hydrogenation setup for different time duration. Elemental analysis (CHN) combined with thermo gravimetric/thermal desorption spectroscopy (TG/TDS) measurements were used to quantify the amount of hydrogen stored in the functionalized SWCNTs. A maximum hydrogen storage capacity of 4.77 wt% is achieved at 50 1C and the entire (100%) stored hydrogen is released in the temperature range of 90–125 1C. The amount of hydrogen stored in functionalized SWCNTs increases with increasing hydrogenation duration. The entire hydrogenation and dehydrogenation process was probed by Raman and CHN-elemental analyses. The whole hydrogenation and dehydrogenation experiments were stabilized and they were repeatable. The achieved hydrogen storage capacity in this investigation is close to the US DOE target. & 2014 Elsevier B.V. All rights reserved.
Keywords: Single-walled carbon nanotubes Functionalization Borane Hydrogen Storage capacity Dehydrogenation
Corresponding author. Tel.: þ 91 94437 96898. Corresponding author. Tel.: þ 91 94434 58568. E-mail addresses: [email protected]
(V. Vasu), [email protected]
A B S T R A C T
http://dx.doi.org/10.1016/j.physe.2014.02.006 1386-9477 & 2014 Elsevier B.V. All rights reserved.
Hydrogen is emerging as a green fuel for transportation applications [1,2]. The storage and delivery of hydrogen remains
D. Silambarasan et al. / Physica E 60 (2014) 75–79
a subject of technological importance in recent times. Storage of hydrogen in the form of gas under high pressure and liquid at cryogenic temperatures has basic problems associated with leakage, safety and storage capacity. On the other hand, solid-state materials provide an alternative choice for hydrogen storage, but the interaction between hydrogen and host material involved either strong interaction (covalent or ionic) or weak (van der Waals forces) interaction. Materials like metal hydrides, metal organic frameworks, clathrates and other nanostructures could not provide the combined advantages of high gravimetric storage capacity, reliability, and suitable kinetics for applications [3–6]. The nanostructures based on carbon have attracted the scientiﬁc community as one of the possible materials for hydrogen storage [7–10]. In the group of various carbon nanostructures, carbon nanotubes (CNTs) are widely investigated potential hydrogen storage material [11–17]. The remarkable properties of CNTs such as hollowness, cylindrical shape, interstitial sites, and nanometer scale diameter and porosity make them as one of the interesting candidates for hydrogen storage . The initial hydrogen storage work of Dillon et al.  in SWCNT bundles led to extensive investigation of CNTs for hydrogen storage. Consequent investigations of hydrogen storage in CNTs indicated that, bare CNTs are not suitable material for the storage of hydrogen [19–23]. The reason is that, the nature of interaction between hydrogen molecules and CNTs involved physisorption (van der Waals interaction), and hence the stored hydrogen is stable only at lower temperatures. The modiﬁcation of CNTs by the addition of atoms or molecules could lead to enhanced interaction between hydrogen and CNTs which resulted in higher storage capacity [24–29]. Further, it is pointed out that, the functionalization of SWCNTs with transition metal atoms itself occupies more weight percentage on SWCNTs  and also they forms strong metal hydrides while hydrogenation [31,32]. In this work, a hydrogen storage medium (HSM) based on SWCNTs, capable of storing and releasing hydrogen in the temperature range suitable for fuel cell applications has been designed. Here, the modiﬁcation of SWCNTs is made by means of functionalizing them with BH3. The simulation studies  based on density functional theory (DFT) carried out by our group indicated that, functionalization of SWCNTs with BH3 enhances the binding energy of hydrogen molecules and thereby increases the storage capacity. Hence, we have chosen BH3 for the functionalization of SWCNTs. Moreover, it is decided to conduct the hydrogenation experiment just above room temperature, because hydrogen storage at very lower temperatures and pressure conditions is not viable for mobile applications. The functionalized SWCNTs were hydrogenated for different time duration. Further, the hydrogenated samples were annealed to check desorption of hydrogen. The amount of hydrogen uptake and desorption temperature range have been measured. The binding energy of hydrogen and the nature of hydrogen binding are estimated based on the characterization results. The whole hydrogenation and dehydrogenation experiments were repeated to examine the reproducibility.
Alumina substrates were cleaned with ethanol, acetone and distilled water by means of sonication for 30 min (alumina substrates were taken as it will not react while heating).
2.2. Methods The puriﬁed SWCNTs dispersed in 2-propanol (ultrasonicated for 1 h in the ratio of 5 mg/ml) were deposited on alumina substrates maintained at 70 1C using simple drop cast method. After deposition, the substrates were annealed at 300 1C for 1 h to remove any impurities. LiBH4 was used as the precursor for BH3. LiBH4 mixed with di-ethyl ether in a ratio of 25 mg/ml was drop casted over the surface of SWCNTs. Then the substrates were annealed at 275 1C (decomposition temperature of LiBH4) for 1 h which yields borane. The released BH3 reacts with SWCNTs and forms a complex, SWCNT þ BH3 . This complex acts as a hydrogen storage medium (HSM). The weight percentage of the hydrogen present in the functionalized sample was estimated using CHN-elemental analysis. This is one of the widely used standard techniques for the composition measurement of elements such as carbon, hydrogen and nitrogen as well as hydrogen storage capacity [33–37]. Then the functionalized samples were loaded in the Seivert like hydrogenation setup  and hydrogenated for different time duration by maintaining the substrate temperature at 50 1C and the hydrogen ﬂow rate of 0.5 l/min, and then the samples were left in the chamber to attain room temperature. After hydrogenation, the hydrogen content present in the sample was again estimated using CHN-elemental analysis. The storage capacities are calculated as the difference of hydrogen content in the samples before and after hydrogenation experiment and the results are presented in Table 1. Further, the hydrogenated samples were annealed to check desorption of hydrogen. In this process, the hydrogenated samples were annealed at 200 1C for 30 min in a furnace. The temperature was controlled by a digital PID (proportional–integral–derivative) controller. After annealing, the samples were left in the furnace to reach room temperature.
2.3. Characterization The morphology of SWCNTs was analyzed by transmission electron microscopy (TEM) using JEOL JEM 2100 model unit with an accelerating voltage of 200 kV. Energy dispersive X-ray spectrum (EDS) of SWCNTs was recorded using JEOL-MODEL 6390 unit with an accelerating voltage of 5 kV. FTIR spectra were recorded over the range 4000–450 cm 1 using Shimadzu model (FTIR8400S, CE) spectrometer at room temperature with a resolution of 1 cm 1. Raman measurements were carried out in Renishaw InVia model spectrometer with the laser excitation of 514 nm. CHN-elemental analysis was performed using Elementar Vario EL III model analyzer. The thermo gravimetric/thermal desorption spectroscopy (TG/TDS) measurements were carried out using Perkin Elmer-Diamond model unit over the temperature range, 40–800 1C at a scanning rate of 10 1C/min. Table 1 Hydrogen adsorption and desorption characteristic parameters.
2.1. Materials SWCNTs were purchased from Sigma Aldrich with the purity of 498%. The chemical reagents of Merck products with 99% purity were used for experiments. The expected amorphous carbon in the purchased SWCNTs was removed by heating them to 300 1C for 1 h. Then, the metal catalyst impurities were removed by washing with nitric acid and distilled water, and dried at 100 1C for 1 h.
H2 ﬂow duration (min)
H2 (wt %)
Ed (kJ/ mol)
CBH1 CBH2 CBH3 CBH4
30 35 40 45
3.277 3.785 4.345 4.770
121 113 111 107
20.66 20.11 19.97 19.69
0.310 0.302 0.300 0.287
D. Silambarasan et al. / Physica E 60 (2014) 75–79
3. Results and discussion 3.1. Morphology and functional group analyses The TEM image presented in Fig. 1 shows the good distribution and separation of SWCNTs with the average diameter of 2–4 nm. The ED spectrum of SWCNTs is shown in Fig. 2. The spectrum exhibits the characteristic peaks of the elements carbon (C), and oxygen (O) along with that of copper (Cu) from the supporting grid. The absence of any other peaks except those due to C and O is the evidence of the quality of SWCNTs without any elemental impurities. Fig. 3 shows the FTIR spectrum of SWCNTs functionalized with BH3. Absorption peaks at 1608 cm 1, 1027 cm 1, and 729 cm 1 correspond to C ¼ C stretching, C–C stretching and skeleton vibrations of carbon atoms in CNTs, respectively. The presence of BH3 group observed at 1260 cm 1 and 2350 cm 1 in the spectrum corresponds to deformation  and asymmetric stretching  modes of B–H bonds, respectively. The electrostatic interaction between BH3 and CNT leads to the elongation of B–H bonds in BH3 group, which resulted in an asymmetric stretching of B–H bond and this behavior was observed in our theoretical investigations too . These observed absorption modes indicate non-dissociative adsorption of BH3 on CNT. Further, absorption peaks appearing at 1430 cm 1 corresponds to C–H asymmetric deformation vibrations. During the process of functionalization there is a possibility that some of the BH3 molecules dissociate into BH2 and H. This H may be absorbed by CNT which results in the formation of C–H bonds. Therefore, the appeared C–H mode may be due to the dissociative adsorption of BH3 on CNT. It may be inferred that, though borane is unstable at room temperature, the (SWCNTþBH3) complex is stable and it is in agreement with our theoretical results .
Fig. 3. FTIR spectrum of SWCNTs functionalized with BH3.
Fig. 4. Raman spectra of SWCNTs, functionalized SWCNTs, hydrogenated and dehydrogenated SWCNTs.
3.2. Raman analysis
Fig. 1. TEM image of SWCNTs.
Fig. 2. ED spectrum of SWCNTs.
Fig. 4 shows the Raman spectra of SWCNTs (C), SWCNTs functionalized with BH3 (CB), and hydrogenated functionalized SWCNTs (CBH1, CBH2, CBH3 and CBH4). Generally, the disordered (D) and graphitic (G) bands are the two characterization bands for CNTs. The D band occurs in the range 1300–1400 cm 1 and is usually assigned to the presence of amorphous/disordered carbon in CNTs. The tangential (G) mode is the strong and most intensive high-energy mode of SWCNTs, which is typically observed in the range 1565–1595 cm 1. The G band originates from in-plane tangential stretching of C–C bonds in SWCNTs. In this mode, the displacement of carbon atoms occurs in the circumferential direction (opposite directions along the surface of the tube). Likely, the two bands namely D and G appear in the spectra of all the samples in their corresponding region. In the Raman spectra, absence of peak around 1800 cm 1, which corresponds to MWCNTs, indicates that the sample contains only SWCNTs . Generally in Raman spectrum, the intensity ratio of D to G band is a measure of defect/ amorphous carbon concentration in SWCNTs. The D/G ratio values
D. Silambarasan et al. / Physica E 60 (2014) 75–79
of all the samples are given in the ﬁgure. The low D/G intensity ratio (0.12) of SWCNTs indicates the presence of lesser amount of defect/amorphous carbon concentration in SWCNTs. The D/G ratio (0.21) of the functionalized sample CB is relatively more than SWCNTs. After hydrogenation, the intensity of D band has increased and the corresponding G band intensity has decreased. The corresponding D/G ratio has increased due to the increase in defect density in sample CB. The D/G intensity ratio has increased from 0.32 to 0.59 as the degree of hydrogenation increases from 3.277 wt% to 4.77 wt%. To check desorption of hydrogen from the hydrogenated sample, we have carried out thermal annealing. The sample CBH4 was annealed at 200 1C for 30 min (the dehydrogenated sample is designated as DCB) and the entire process was probed by Raman and CHNelemental analyses. It is noted that after annealing, the intensities of D and G bands have decreased and increased, respectively, which indicates hydrogen desorption from the sample. We have repeated the entire hydrogenation and dehydrogenation experiments a number of times. The D/G ratio of functionalized sample (CB) is 0.21 and for the dehydrogenated sample (DCB) the D/G ratio is 0.225. The change in D/ G ratio, from 0.21 (CB) to 0.225 (DCB) is 0.015 (7.1%) after the ﬁrst cycle. For the second cycle the difference is 0.02 ( 9.5%). In the third cycle it changes to 11.6%. There is an increase of 2.1–2.4% in the D/G ratio between two successive cycles. If we take the change in D/G ratio of the dehydrogenated SWCNTs after each cycle, it is about 2.3%. The entire hydrogenation and dehydrogenation cycles are independent events and this effect is not cumulative. The quality of CNTs deteriorates due to dehydrogenation is only about 2.3%. This indicates that the functionalized SWCNTs are restored to the original level after dehydrogenation. So at the end of any number of cycles the change in D/G ratio value and the deterioration in the sample are around 2.3%. This is the limitation in our method. The expected deviation in storage capacity is within 5% about the mean value. Zhang et al.  observed the percentage of change in D/G ratio and it was about 3%. It may be noted that, the Raman spectrum corresponding to DCB and CB are similar. This in turn conﬁrms desorption of hydrogen from the sample. On comparing the spectrum of samples DCB and CB, one can infer that the functionalized SWCNTs are recovered after dehydrogenation. 3.3. Thermal analysis Fig. 5 shows the thermo gravimetric spectra of all hydrogenated samples CBH1, CBH2, CBH3, CBH4 and inset shows the region associated with hydrogen desorption. For example, the spectrum corresponding to the sample CBH1 shows a weight loss of about 3.27 wt% in the temperature range 105–140 1C and it corresponds to the desorption of stored hydrogen from the sample. The corresponding inset ﬁgure shows that, the desorption starts at 105 1C, and ends
at 140 1C with desorption peak temperature of 121 1C. It may be noted that the second weight loss starts above 275 1C corresponds to desorption of borane. Hence, one can conﬁrm that the initial weight loss in the sample is due to the release of stored hydrogen (and not the hydrogen in the borane) which is in comparison with Raman results that conﬁrms desorption of hydrogen. The activation energy of desorption, Ed can be calculated from the desorption peak maximum using the following equation  ! T 2m E ln ¼ d β RT m where, Tm is the temperature at peak maximum (121 1C), β is the heating rate (10 1C/min) and R is the universal gas constant. The calculated Ed for our sample is 20.66 kJ/mol. The binding energy (EB) of hydrogen is calculated using van't Hoff equation  for the desorption temperature 121 1C and the value is 0.31 eV per H2, respectively. The similar calculation and discussion is opting for other samples namely CBH2, CBH3 and CBH4. Further, it is observed that as hydrogen storage capacity increases, the peak of desorption temperature is shifting toward lower temperatures. This kind of behavior is agreed with the observation of Ioannatos and Verykios . The various parameters involved in the discussion for all the samples are presented in Table 1. These binding energies of hydrogen released from the functionalized SWCNTs exist in the range of binding energy (0.2–0.4 eV) recommended for an ideal HSM . This recommended range lies between the physisorption and chemisorption limits, and in our case it is weak chemisorption. Generally in this range, the interaction between the functionalized SWCNTs and hydrogen molecules is primarily due to the combination of electrostatic, inductive and covalent charge transfer mechanisms [7,25,27]. Here, borane acts as a bridge between the hydrogen molecules and SWCNTs. On hydrogenation, it helps to hold the hydrogen molecules onto SWCNTs in the ideal binding energy limits. Since the binding energy of hydrogen molecules lies within the ideal range, it is supposed that the stored hydrogen binds sufﬁciently strong with the host and is desorbed in the temperature range feasible for mobile applications [7,42]. The storage capacity measured by CHNelemental analysis and the weight loss of the corresponding sample measured by thermo gravimetric studies is same. Hence, one can conclude that not only the thermo gravimetric results but also the CHN-elemental analysis provides evidence for the measurement of hydrogen storage in the designed system. From the thermo gravimetric results, it is noted that the quantity of hydrogen desorbed is equal to the quantity of hydrogen adsorbed. Thus, our system exhibits 100% desorption in the temperature range of 90–140 1C, which may be suitable for vehicular based fuel cells than the desorption temperature reported in Refs. [21,42]. This observation reveals that the adsorption sites on the CNTs surface are relatively uniform and that there are no sites which form very weak or very strong adsorption bonds . Nikitin et al.  reported the hydrogenation studies on SWCNT ﬁlms using atomic hydrogen. They observed the desorption of chemisorbed hydrogen in the temperature range between 200 1C and 300 1C. Here, the designed hydrogen storage system shows desorption of hydrogen in the temperature range of 90–140 1C, and hence the nature of binding should be weak chemisorption which may be feasible for vehicular applications. Based on the investigation, one can conclude that the SWCNTs functionalized with borane having the capability of higher hydrogen uptake. 4. Conclusion
Fig. 5. Thermogravimetric spectra of hydrogenated samples. Inset shows the desorption curve.
In this paper, we have presented the hydrogenation and dehydrogenation studies of functionalized SWCNTs. The SWCNTs
D. Silambarasan et al. / Physica E 60 (2014) 75–79
were successfully functionalized with BH3 using simple drop casting method. The functionalized samples were hydrogenated for different time duration. The amount of hydrogen stored in the functionalized SWCNTs increases with increasing hydrogen treatment duration. A maximum storage capacity of 4.77 wt% is achieved at 50 1C which is close to the US DOE target of 5.5 wt% by 2015 for a HSM to be used for on-board applications. The hydrogenated system is stable at room temperature and the entire (100%) stored hydrogen are released in the temperature range of 90–125 1C. Desorption studies suggest that the adsorption of hydrogen involves relatively weak chemisorption. The whole hydrogenation and dehydrogenation processes are stabilized and repeatable. In viewpoint of storage capacity, stability and desorption behavior of this designed HSM, one can conclude that the SWCNTs functionalized with borane can be a viable HSM for mobile applications. Acknowledgments Madurai Kamaraj University (MKU), University Grants Commission (UGC), Council of Scientiﬁc and Industrial Research (CSIR), and Sri Ramaswamy Memorial (SRM) University are acknowledged with thanks. References  G.E. Ioannatos, X.E. Verykios, Int. J. Hydrogen Energy 35 (2010) 622.  E. Durgun, S. Ciraci, T. Yildirim, Phys. Rev. B: Condens. Matter 77 (2008) 085405.  O. S-i, Y. Nakamori, J.R. Eliseo, A. Züttel, C.M. Jensen, Chem. Rev. 107 (2007) 4111.  J.Q. Zhang, Y.H. Hu, Int. J. Hydrogen Energy 37 (2012) 10467.  Y.H. Hu, L. Zhang, Adv. Mater. 22 (2010) E117.  L.J. Murray, M. Dinca, J.R. Long, Chem. Soc. Rev. 38 (2009) 1294.  R.C. Lochan, M. Head-Gordon, Phys. Chem. Chem. Phys. 8 (2006) 1357.  J. Li, F. Terumi, G. Hajime, O. Toshiyuki, F. Yoshiya, Y. Sidney, J. Chem. Phys. 119 (2003) 2376.  E.J. Duplock, M. Schefﬂer, P.J.D. Lindan, Phys. Rev. Lett. 92 (2004) (225502-1225502-4).  A. Zuttel, P. Sudan, P. Mauron, T. Kyobayashi, C. Emenenrgger, L. Schlapbach, Int. J. Hydrogen Energy 27 (2002) 203.  M.-W. Zhao, Y.-Y. Xia, Y.-C. Ma, M.-J. Ying, X.-D. Liu, L.-M. Mei, Chin. Phys. Lett. 19 (2002) 1498.
 A. Okati, A. Zolfaghari, F.S. Hashemi, N. Anousheh, H. Jooya, Fullerenes Nanotubes Carbon Nanostruct. 17 (2009) 324.  A.R. Muniz, M. Meyyappan, D. Maroudas, Appl. Phys. Lett. 95 (2009) (1631111-163111-3).  M. Rzepka, P. Lamp, M.A. de la Casa-Lillo, J. Phys. Chem. B 102 (1998) 10894.  S.M. Lee, K.H. An, Y.H. Lee, G. Seifert, T. Frauenheim, J. Am. Chem. Soc. 123 (2001) 5059.  F.L. Darkrim, D. Levesque, J. Chem. Phys. 109 (1998) 4981.  J.R. Cheng, X.H. Yuan, L. Zhao, D.C. Huang, M. Zhao, L. Dai, R. Ding, Carbon 42 (2004) 2019.  Z. He, S. Wang, X. Wang, Z. Iqbal, Int. J. Energy Res. 37 (2013) 754.  A.C. Dillon, K.M. Jones, T.A. Bekkendahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377.  Y. Ye, C.C. Ahn, C. Witham, B. Fultz, J. Liu, A.G. Rinzler, D. Colbert, K.A. Smith, R. E. Smalley, Appl. Phys. Lett. 74 (1999) 2307.  J.S. Arellano, L.M. Molina, A. Rubio, M.J. Lo´pez, J.A. Alonso, J. Chem. Phys. 117 (2002) 2281.  P. Sudan, A. Zuttel, P. Mauron, C. Emmenegger, P. Wenger, L. Schlapbach, Carbon 41 (2003) 2377.  G.G. Tibbetts, G.P. Meisner, C.H. Olk, Carbon 39 (2001) 2291.  X.M. Wu, Y. Wang, K.M. Dong, J.M. Zhou, G.D. Lin, H.B. Zhang, Acta Chim. Sin. 63 (2005) 484.  V.J. Surya, K. Iyakutti, M. Rajarajeswari, Y. Kawazoe, Physica E 41 (2009) 1340.  W. Liu, Y.H. Zhao, Y. Li, Q. Jiang, E.J. Lavernia, J. Phys. Chem. C 113 (2009) 2028.  V.J. Surya, K. Iyakutti, N.S. Venkataramanan, H. Mizuseki, Y. Kawazoe, Phys. Status Solidi B 248 (2011) 2147.  A. Reyhani, S.Z. Mortazavi, S. Mirershadi, A.Z. Moshfegh, P. Parvin, A. Nozad Golikand, J. Phys. Chem. C 115 (2011) 6994.  A.L.M. Reddy, S. Ramaprabhu, Int. J. Hydrogen Energy 33 (2008) 1028.  K. Iyakutti, Y. Kawazoe, M. Rajarajeswari, V.J. Surya, Int. J. Hydrogen Energy 34 (2009) 370.  T. Yildirim, S. Ciraci, Phys. Rev. Lett 94 (2005) (175501-1-175501-4).  R. Jianwei, L. Shijun, L. Junmin, Chin. Sci. Bull. 51 (2006) 2959.  D. Silambarasan, V.J. Surya, V. Vasu, K. Iyakutti, Int. J. Hydrogen Energy 36 (2011) 3574.  K.S. Subrahmanyam, Prashant Kumar, A. Urmimala Maitra, K.P.S.S. Govindaraj, U.V. Hembram, C.N.R.Rao Waghmare, Proc. Nat. Acad. Sci. 108 (2011) 2674.  M. Sankaran, B. Viswanathan, Indian J. Chem. 47 (2008) 808.  A. Badzian, T. Badzian, E. Breval, A. Piotrowski, Thin Solid Films 398–399 (2001) 170.  D. Silambarasan, V.J. Surya, V. Vasu, K. Iyakutti, Int. J. Hydrogen Energy 38 (2013) 4011.  A. Kaldor, R.F. Porter, J. Am. Chem. Soc. 93 (1971) 2140.  K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley and Sons, New York, 1978.  D. Silambarasan, V. Vasu, V.J. Surya, K. Iyakutti, IEEE Trans. Nanotechnol. 11 (2012) 1047.  G. Zhang, P. Qi, X. Wang, Y. Lu, D. Mann, H. Li, H. Dai, J. Am. Chem. Soc. 128 (2006) 6026.  H. Lee, M.C. Nguyen, J. Ihm, Solid State Commun 146 (2008) 431.  A. Nikitin, X. Li, Z. Zhang, H. Ogasawara, H. Dai, A. Nilsson, Nano Lett. 8 (2008) 162.