Field emission properties of the graphenated carbon nanotube electrode

Applied Surface Science 324 (2015) 174–178

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Field emission properties of the graphenated carbon nanotube electrode H. Zanin a,b,∗ , H.J. Ceragioli b , A.C. Peterlevitz b , Vitor Baranauskas b , F.R. Marciano c , A.O. Lobo c a

School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom Faculdade de Engenharia Elétrica e Computac¸ão, Departamento de Semicondutores, Instrumentos e Fotônica, Universidade Estadual de Campinas, UNICAMP, Av. Albert Einstein N. 400, CEP 13 083-852 Campinas, São Paulo, Brazil c Laboratory of Biomedical Nanotechnology/Institute of Research and Development at UNIVAP, Av. Shishima Hifumi, 2911, CEP 12244-000 Sao Jose dos Campos, SP, Brazil b

a r t i c l e

i n f o

Article history: Received 31 July 2014 Received in revised form 14 October 2014 Accepted 19 October 2014 Available online 24 October 2014 Keywords: Carbon–carbon composites Graphenated nanotubes Field emission Current stability

a b s t r a c t Reduced graphene oxide-coated carbon nanotubes (RGO-CNT) electrodes have been prepared by hot filament chemical vapour deposition system in one-step growth process. We studied RGO-CNT electrodes behaviour as cold cathode in field emission test. Our results show that RGO-CNT retain the low threshold voltage typical of CNTs, but with greatly improved emission current stability. The field emission enhancement value is significantly higher than that expected being caused by geometric effect (height divided by the radius of nanotube). This suggested that the field emission of this hybrid structure is not only from a single tip, but eventually it is from several tips with contribution of graphene nanosheets at CNT’s walls. This phenomenon explains why the graphenated carbon nanotubes do not burn out as quickly as CNT does until emission ceases completely. These preliminaries results make nanocarbon materials good candidates for applications as electron sources for several devices. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Field emission efficiency is determined by a combination of factors such as conductivity, surface work function and geometry of the emitter [1]. An ideal field emitter must be a good electrical conductor with low work function, high enhancement factor (ˇ) and be stable at fairly high emission current density. Carbon-based materials are suitable for use as cold cathode emitters, because of the low voltages required to extract electrons from its surface. Among carbon-based materials, diamond (mainly sp3 hybridized state of carbon), carbon nanotubes (CNTs) and graphene (both sp2 hybridized state of carbon) have low power consumption, potential for miniaturization, and outstanding field emission behaviour. All these properties make attractive candidates for applications as electron sources for several devices [2]. The emission of electrons from diamond in vacuum occurs readily as a result of the negative electron affinity (NEA) the hydrogenated surface due to features with nanoscale dimensions, which

∗ Corresponding author at: School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom. Tel.: +44 7462529137. E-mail address: [email protected] (H. Zanin). http://dx.doi.org/10.1016/j.apsusc.2014.10.102 0169-4332/© 2014 Elsevier B.V. All rights reserved.

can concentrate electric fields high enough to induce electron emission from them [3,4]. It has been direct measured by Chatterjee et al. [4] that the emission came from the grain boundaries (sp2 rich region) and not the protruding regions. In contrast, excellent electron emission behaviour of CNTs is related to high-aspectratio geometry [1]. To date, very good results from both carbon hybridizations have been reported with currents of 1 mA cm−2 for threshold fields as low as 2 V ␮m−1 [5]. The NEA and chemical stability of doped diamond and nanocrystalline diamond make them highly stable field emitter material [2], however with higher threshold field than high-aspect-ratio geometry emitters. The field-emission properties of diamond emitters depend upon the thickness, density, microstructure and sp3 /sp2 carbon ratio in the films with typical threshold field values ranging from 5 to 50 V ␮m−1 [7–10]. Lower threshold field from diamond electrodes were reported from samples with high ˇ values, which could require complex and expensive micro-fabrication [6,8]. High ˇ values are usually required to pattern and etch the diamond into suitable pyramids or needle shapes, although recently diamond nanocones or microstructured have been made using CVD process and show promising field-emission characteristics [7,9]. Xiao et al. reported low threshold field emission ∼2.5 V ␮m−1 to nested cones of graphitically bonded carbon (graphene sheets) covered with

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nitrogen-doped ultrananocrystalline diamond (N-UNCD), however no current transient was showed [8]. Zou et al. [9] and Zanin et al. [7] recently showed field emission studies of microstructured diamond films deposited onto a densely packed “forest” of vertically aligned multiwalled carbon nanotubes (VACNT). On both works, field-emission tests of diamond/carbon nanotube composite show the typical low threshold voltages for carbon nanotube structures (2 V ␮m−1 ) but with better stability and longer lifetime up to 35 h [9] and 75 h [7]. Getty et al. reported a field emission study of N-UNCD films grown on polished Si and contrast with N-UNCD films grown on sharpened Si tip arrays and Si ridges [2]. Authors showed the electron emission from planar N-UNCD films are fairly comparable to the emission from films grown on sharpened Si tip arrays and Si ridges, indicating that field emission in these materials is dominated by electron emission from nanoscale grain boundaries [2]. However, it was not clear why the planar N-UNCD film exhibits lower field emission threshold voltages compared to sharp emitters tested, which is unexpected. It is remarkable the life testing of electron emission from N-UNCD films, which showed the emission with low current degradation over 1000 h. Although with considerable fluctuation, emission stability of these diamond electrodes has been proved better than CNTs or graphene. The field emission of CNTs is problematic because they burn out during emission, ceasing the emission completely. Significant research efforts have been performed to improve field emission stability of carbon nanotubes. Chen et al. [10] improved field emission stability of thin film of MWCNT using a tip sonication treatment. Authors showed field emission current of thin-MWCNT film at high current density was quite stable up to 19 h and after tip sonication treatment that stability showed to be longer ∼30 h. Field emission stability was significantly improved during a long period of operation probably because many shortened thin-MWCNTs could then participate of the emission after the treatment. Pandey et al. [11] showed strontium titanate coated carbon nanotube matrices with low emission thresholds of 0.8 V ␮m−1 and very stable electron field emission for 40 h. Authors claimed low emission threshold was obtained due to tunnelling width as a function of the SrTiO3 thickness. Recently, significant research efforts have been centred on graphene due its electronic properties, which are very similar or even better than CNTs [12–15]. Consequently, much effort has been made to prepare and to apply reduced graphene as a field emission material. The electric field emission from graphene is challenging especially when the sheets lay on the substrate surface, resulting in low field enhancement [16]. Nevertheless, Malesevic et al. [17] showed that graphene could be deposited vertically aligned to substrate, which increased the field enhancement factor. Quian et al. [18] showed that the field emission from graphene oxide (GO) nanosheets prepared by Hummer method have excellent and stable field emission properties with a low threshold field (∼1.5 V ␮m−1 ). Huang et al. [19] showed the field emission properties of GO are found to be a non-monotonic function of the C/O ratio in a wide range of 2.06–14.80. Samples with C/O ratio of 6.98 show the lowest turn-on (∼1.8 V ␮m−1 ) and longest-time (10 h) current stability. In agreement with Huang, Kung et al. showed the emission current of vertically aligned (VA) CNT array increased ∼800% along with a decrease of the onset field emission voltage from 0.8 to 0.6 V/␮m, when treated by oxygen or ozone compared to as-grown samples [20]. Mathur et al. showed that oxygen plasma treatment opens VACNTs’ tips enhancing its field emission behaviour [22]. Authors measured the turn-on electric field ∼0.80 V ␮m−1 from untreated VACNTs and ∼0.60 V ␮m−1 from open ended VACNTs samples. These open VACNTs have more defects sites such as edge defects from the edge planes of the graphene sheet [21]. Kurt et al.

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[22] reported, for the first time, carbon nanotubes decorated with graphene nanosheets growing radial to the tube axis and Parker et al. [23] proposed a model that involves the build-up of residual strain followed by buckling to nucleate a graphitic-edge protrusion from the CNTs’ sidewall. All these literature have inspired us to prepare and study field emission behaviour of graphenated CNTs samples. Herein we demonstrate that the major advantage of these hybrid structures is their significant improvement in emission current stability due to multiple-graphene edges covering CNTs. 2. Experimental Reduced graphene oxide-coated CNTs (RGO-CNT) samples were prepared on titanium 10 mm × 10 mm × 1 mm substrates by hot filament chemical deposition vapour system (HFCVD) reactor. Prior to the deposition, 250 mg of nickel nitrate were diluted in 80 ml of acetone and 0.2 ml were dropped on the substrate with polyaniline and dried at room temperature for 2 h. For sample growth, the HFCVD chamber is maintained at 10 Torr with a constant flow of nitrogen (65 sccm) and oxygen (20 sccm). The carbon source was a mixture of propanone (acetone), camphor, and citric acid. This carbon source is dragged to HFCVD by hydrogen gas flux (15 sccm). A hot spiral tungsten filament heated to ∼1500 ◦ C dissociates gases and vapours into radicals, depositing RGO-CNT thin films on Ti substrates over a 30 min period. The MWCNT were prepared using a mixture of camphor (90% w) and ferrocene in a thermal chemical vapour deposition (CVD) furnace [24] for use as control samples. The mixture was vapourized at 220 ◦ C in an antechamber, and then the vapour was carried by an argon gas flow at atmospheric pressure to the chamber of the CVD furnace set at 850 ◦ C. The CVD growth took only a few minutes and produced black powder, which consisted of CNT with 20–50 nm diameter and up to 40 ␮m length [25]. 5 mg of this black powder was dispersed in ethanol and dropped on 10 mm × 10 mm × 1 mm titanium substrates to form a cold emission electrode. Micrographs were performed by high-resolution scanning and transmission electron microscopy (HR-SEM, FEI Inspect F50; and HR-TEM, JEOL 3010, respectively) to characterize the morphological and structural properties of samples. Raman spectrum was carried out using a Renishaw 2000 system with  = 514.5 nm. X-ray photoelectron spectra were performed using VSW H100 system to identify oxygen-functional groups on the sample surface. The wettability of samples was evaluated by the sessile-drop method using a Krüss Easy-Drop system to measure the contact angle (CA) of water on the samples. A parallel-plates electrodes configuration for field emission measurements with prepared sample acting as the cathode and a phosphor screen acting as the anode was employed [7]. Briefly, a silica spacer kept a fixed separation of d = 500 ␮m between the two electrodes. The vacuum chamber pressure ranges from 5 × 10−7 to 1 × 10−7 Torr. To normalize the data, we plotted emission current density, J (mA cm−2 ), versus electric field, E (V ␮m−1 ), as well as the form of a Fowler–Nordheim (F–N) plot (ln(J/E2 ) versus 1/E). 3. Results and discussion Fig. 1 shows the morphology and structure of the RGO-CNT onto titanium substrates. Fig. 1(a) shows top view SEM image, revealing porous microstructure composed of entangled tubes. Fig. 1(b)–(d) shows details of reduced graphene oxide nanosheets coating CNT radially to the tube axis. From these images, we also can see the outer diameter range from 200 nm to 600 nm. Fig. 1(e) presents a transmission electron micrograph of RGO-CNT, revealing

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Fig. 1. Scanning electron micrographs of RGO-CNTs (a–d). Transmission electron micrographs of (e) tubular structure and (f) reduced graphene nanosheets grown onto CNT.

its tubular structure. Fig. 1(f) shows details of the reduced graphene interplanar spacing of ∼0.35 nm and graphene sheet height of ∼100 nm. Fig. 2(a) shows first and second-orders Raman scattering spectrum of RGO-CNTs. We performed the deconvolutions using Lorentzian shapes for the 1350 cm−1 (D band) and 1570 cm−1 (G band), and Gaussian shape for bands around 1250, 1480–1520 and

1622 cm−1 (D band) [26,27]. The D and D band is assigned to the disorder and imperfection of the carbon crystallites, which is expected due to large amount of edges. The G band is assigned to one of the two E2g modes corresponding to stretching vibrations in the basal plane (sp2 domains) of graphene [28]. The G band is derived from their (2D) second-order harmonic. Relatively high intensity of G band reveals high crystallinity [29].

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Fig. 2. (a) First and second-order Raman spectrum and (b) C 1s and (c) O 1s XPS spectra of RGO-CNTs samples. All spectra were deconvoluted by Fityk software using Lorentzian and Guassian functions (solid lines).

Table 1 Evaluation of elements using EDS. Elements (weight%)

C

O

Ni

Sample 1 Sample 2 Sample 3

92.48 93.13 93.11

6.37 5.76 5.89

0.23 0.17 0.20

Mean

92.11

6.01

0.20

Fig. 2 shows (b) C 1s and (c) O 1s fitted XPS spectra recorded from as-grown RGO-CNTs samples. The C 1s curve has been deconvoluted into five peaks at around 284.7, 285.6, 288.7, 291 and 292.5 eV [30]. The peaks correspond to sp2 hybridization, defects, carbon atoms with C O, C O, carbonates and shake-up peak (␲–␲* transitions) respectively [24]. The O 1s curve has been deconvoluted into three peaks at around 535, 533, and 531 eV, which correspond to C O, COOH , and C O, respectively [24]. This infers the formation of strong C O bonds from the oxygen-containing groups that are situated along the RGO-CNTs [31]. The oxygen atom content per nanotube is up to 6%. All this amount of oxygen makes the as-grown samples exhibit superhydrophilic behaviour with contact angle of 0◦ with a drop of water. The energy dispersive spectra were carried out to identify the chemical elements into as-grown RGO-CNT samples. The results in terms of weight percentage are presented in Table 1. As expect, most part is constituted of carbon (∼92%) and oxygen (∼6%). The oxygen introduced during growth attaches oxygen-containing functional groups on carbon nanosheets and tubes surfaces. The atomic hydrogen formed during the growth; reduced the oxidized nanosheets, forming what we have been calling RGO-CNT

micro-porous material. These structures have enhanced capacitive properties [27] and combined with hydroxyapatite showed mesenchymal stem cell adhesion with active formation of membrane projections [31]. This hybrid material has outstanding field emission behaviour as showed below. Fig. 3 shows the data from the field emission measurements performed on RGO-CNTs and compared to CNT samples. The threshold field (Eth ) was defined as the field that corresponding to an electron emission density of 0.01 mA cm−2 . The Eth observed value was ∼2 V ␮m−1 , consistent with previously published values for RGOCNT arrays by Liu et al. [32]. These data are all consistent with the F–N model for electron emission via quantum-mechanical tunnelling through a potential barrier [33]. From that we calculated the field enhancement factor ˇ considering the work function of ∼4.7 eV for carbon electrodes [8]. The field enhancement factor was then estimated by fitting the slope of the F–N plot and the average field enhancement factor was ˇ ∼ 2780. This ˇ value is significantly higher than that expected being caused by geometric effect (height divided by the radius of nanotube). This suggested that the field emission is not only from a single tip, but eventually it is from several tips with contribution of graphene nanosheets at CNT’s walls. This phenomenon explains why the graphenated carbon nanotubes do not burn out as CNT does until emission ceases completely. In other to check how stable these samples are, the current transient measurements were performed. Stability tests were performed on the same samples measuring the emission current as a function of time maintaining at 3.2 V/␮m, as shown in Fig. 3(b). Emission current from RGO-CNT showed to be much more stable than that from CNT. The emission current from RGO-CNT showed to be stable form up to 50 h. Visual

Fig. 3. Field emission characteristics of the RGO-CNTs sample, showing curves (a) of current density, J, versus electric field, E, and (b) current density, J, versus time. Inset: (a) Fowler–Nordheim plots from the RGO-CNTs and (b) optical image of the field emission on phosphor screen during field emission current transient.

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observation of the bright area on the phosphor screen allowed the uniformity of the emission current within the emission area to be estimated (Fig. 3(b) (inset)). For all samples tested, the emissions were relatively uniform, considering the brightness on the phosphor screen. 4. Conclusions We have described method to produce highly porous RGOCNTs by HFCVD technique. HR-SEM and HR-TEM showed graphene nanosheet covering CNT, which were confirmed by Raman. Raman and XPS spectra point out well-crystalline oxide carbon material. The oxygen-containing groups were identified by XPS along the CNTs, which may contribute to enhance field emission properties compared to typical CNTs. Although there is no observation of improvement in field emission threshold voltage, the major advantage of these hybrid structures is their significant improvement in the stability of current emitted. This novel hybrid material combines the excellent electrical conductivity and transport properties of multiple CNTs with RGO. The good electrical contact between the RGO-CNT and the titanium substrate may also contribute to the high currents achieved. Although preliminary, these experiments have shown RGO-CNTs more stable electron field emitters than CNTs under same experimental conditions. However, more studies are necessary to understand the fundamentals regarding the field emission behaviour of this material considering a diagnostic on oxygen content, pores size, and tubes diameter. These experiments are underway in our group and we are also combining RGO-CNT with N-UNCD to improve even more the life time of these electrodes. Acknowledgements We gratefully acknowledge the LNNano - CNPEM-Campinas for microscopy support and mainly to Brazilian agencies CNPq (202439/2012-7) and FAPESP (2014/02163-7), (2011/17877-7), (2011/20345-7) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2014.10.102. References [1] O. Groning, O.M. Kuttel, C. Emmenegger, P. Groning, L. Schlapbach, Field emission properties of carbon nanotubes, J. Vac. Sci. Technol. B 18 (2) (2000) 665–678. [2] S.A. Getty, O. Auciello, A.V. Sumant, X. Wang, D.P. Glavin, P.R. Mahaffy, Characterization of Nitrogen-Incorporated Ultrananocrystalline Diamond as a Robust Cold Cathode Material, Conference on Micro- and Nanotechnology Sensors, Systems, and Applications II, Orlando, FL, 2010. [3] J.B. Cui, J. Ristein, L. Ley, Low-threshold electron emission from diamond, Phys. Rev. B 60 (23) (1999) 16135–16142. [4] V. Chatterjee, R. Harniman, P.W. May, P.K. Barhai, Direct observation of electron emission from the grain boundaries of chemical vapour deposition diamond films by tunneling atomic force microscopy, Appl. Phys. Lett. 104 (17) (2014). [5] W.I. Milne, K.B.K. Teo, E. Minoux, O. Groening, L. Gangloff, L. Hudanski, J.P. Schnell, D. Dieumegard, F. Peauger, I.Y.Y. Bu, M.S. Bell, P. Legagneux, G. Hasko, G.A.J. Amaratunga, Aligned carbon nanotubes/fibers for applications in vacuum microwave amplifiers, J. Vac. Sci. Technol. B 24 (1) (2006) 345–348. [6] S. Orlanducci, V. Guglielmotti, I. Cianchetta, V. Sessa, E. Tamburri, F. Toschi, M.L. Terranova, M. Rossi, One-step growth and shaping by a dual-plasma reactor of diamond nanocones arrays for the assembling of stable cold cathodes, Nanosci. Nanotechnol. Lett. 4 (3) (2012) 338–343.

[7] H. Zanin, P.W. May, M.H.M.O. Hamanaka, E.J. Corat, Field emission from hybrid diamond-like carbon and carbon nanotube composite structures, ACS Appl. Mater. Interfaces 5 (23) (2013) 12238–12243. [8] X. Xiao, O. Auciello, H. Cui, D.H. Lowndes, V.L. Merkulov, J. Carlisle, Synthesis and field emission properties of hybrid structures of ultrananocrystalline diamond and vertically aligned carbon nanofibers, Diamond Relat. Mater. 15 (2–3) (2006) 244–247. [9] Y. Zou, P.W. May, S.M.C. Vieira, N.A. Fox, Field emission from diamond-coated multiwalled carbon nanotube “teepee” structures, J. Appl. Phys. 112 (4) (2012). [10] G. Chen, D.H. Shin, S. Kim, S. Roth, C.J. Lee, Improved field emission stability of thin multiwalled carbon nanotube emitters, Nanotechnology 21 (1) (2010). [11] A. Pandey, A. Prasad, J.P. Moscatello, M. Engelhard, C. Wang, Y.K. Yap, Very stable electron field emission from strontium titanate coated carbon nanotube matrices with low emission thresholds, ACS Nano 7 (1) (2013) 117–125. [12] L.M. Hollanda, A.O. Lobo, M. Lancellotti, E. Berni, E.J. Corat, H. Zanin, Graphene and carbon nanotube nanocomposite for gene transfection, Mater. Sci. Eng. C: Mater. Biol. Appl. 39 (2014) 288–298. [13] L. Jiao, L. Zhang, X. Wang, G. Diankov, H. Dai, Narrow graphene nanoribbons from carbon nanotubes, Nature 458 (2009) 877–880. [14] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183–191. [15] T. Gokus, R.R. Nair, A. Bonetti, M. Boehmler, A. Lombardo, K.S. Novoselov, A.K. Geim, A.C. Ferrari, A. Hartschuh, Making graphene luminescent by oxygen plasma treatment, ACS Nano 3 (12) (2009) 3963–3968. [16] G. Eda, H.E. Unalan, N. Rupesinghe, G.A.J. Amaratunga, M. Chhowalla, Field emission from graphene based composite thin films, Appl. Phys. Lett. 93 (23) (2008). [17] A. Malesevic, R. Kemps, A. Vanhulsel, M.P. Chowdhury, A. Volodin, C. Van Haesendonck, Field emission from vertically aligned few-layer graphene, J. Appl. Phys. 104 (8) (2008). [18] M. Qian, T. Feng, H. Ding, L. Lin, H. Li, Y. Chen, Z. Sun, Electron field emission from screen-printed graphene films, Nanotechnology 20 (42) (2009). [19] Y. Huang, W. Wang, J. She, Z. Li, S. Deng, Correlation between carbon-oxygen atomic ratio and field emission performance of few-layer reduced graphite oxide, Carbon 50 (7) (2012) 2657–2665. [20] S.C. Kung, K.C. Hwang, I.N. Lin, Oxygen and ozone oxidation-enhanced field emission of carbon nanotubes, Appl. Phys. Lett. 80 (25) (2002) 4819–4821. [21] A. Mathur, S.S. Roy, K.S. Hazra, S. Wadhwa, S.C. Ray, S.K. Mitra, D.S. Misra, J.A. McLaughlin, Oxygen plasma assisted end-opening and field emission enhancement in vertically aligned multiwall carbon nanotubes, Mater. Chem. Phys. 134 (1) (2012) 425–429. [22] R. Kurt, C. Klinke, J.M. Bonard, K. Kern, A. Karimi, Tailoring the diameter of decorated C–N nanotubes by temperature variations using HF-CVD, Carbon 39 (14) (2001) 2163–2172. [23] C.B. Parker, A.S. Raut, B. Brown, B.R. Stoner, J.T. Glass, Three-dimensional arrays of graphenated carbon nanotubes, J. Mater. Res. 27 (7) (2012) 1046–1053. [24] H. Zanin, A. Margraf-Ferreira, N.S. da Silva, F.R. Marciano, E.J. Corat, A.O. Lobo, Graphene and carbon nanotube composite enabling a new prospective treatment for trichomoniasis disease, Mater. Sci. Eng. C: Mater. Biol. Appl. 41 (2014) 65–69. [25] H. Zanin, P.W. May, A.O. Lobo, E. Saito, J.P.B. Machado, G. Martins, V.J. TravaAiroldi, E.J. Corat, Effect of multi-walled carbon nanotubes incorporation on the structure, optical and electrochemical properties of diamond-like carbon thin films, J. Electrochem. Soc. 161 (5) (2014) H290–H295. [26] J. Tsukada, H. Zanin, L.C.A. Barbosa, G.A. da Silva, H.J. Ceragioli, A.C. Peterlevitz, R.F. Teofilo, V. Baranauskas, Electro-deposition of carbon structures at mid voltage and room temperature using ethanol/aqueous solutions, J. Electrochem. Soc. 159 (3) (2012) D159–D161. [27] H. Zanin, E. Saito, H.J. Ceragioli, V. Baranauskas, E.J. Corat, Reduced graphene oxide and vertically aligned carbon nanotubes superhydrophilic films for supercapacitors devices, Mater. Res. Bull. 49 (48) (2014) 7–493. [28] E.F. Antunes, A.O. Lobo, E.J. Corat, V.J. Trava-Airoldi, A.A. Martin, C. Verissimo, Comparative study of first- and second-order Raman spectra of MWCNT at visible and infrared laser excitation, Carbon 44 (11) (2006) 2202–2211. [29] M.A.V.M. Grinet, H. Zanin, A.E. Campos Granata, M. Porcionatto, F.R. Marciano, A.O. Lobo, Fast preparation of free-standing nanohydroxyapatite-vertically aligned carbon nanotube scaffolds, J. Mater. Chem. B 2 (9) (2014) 1196–1204. [30] S. Kundu, T.C. Nagaiah, W. Xia, Y. Wang, S. Van Dommele, J.H. Bitter, M. Santa, G. Grundmeier, M. Bron, W. Schuhmann, MuhlerF M., Electrocatalytic activity and stability of nitrogen-containing carbon nanotubes in the oxygen reduction reaction, J. Phys. Chem. C 113 (32) (2009) 14302–14310. [31] H. Zanin, E. Saito, F.R. Marciano, H.J. Ceragioli, A.E. Campos Granato, M. Porcionatto, A.O. Lobo, Fast preparation of nano-hydroxyapatite/superhydrophilic reduced graphene oxide composites for bioactive applications, J. Mater. Chem. B 1 (38) (2013) 4947–4955. [32] J. Liu, B. Zeng, X. Wang, W. Wang, H. Shi, One-step growth of vertical graphene sheets on carbon nanotubes and their field emission properties, Appl. Phys. Lett. 103 (5) (2013). [33] G. Chen, D.N. Futaba, H. Kimura, S. Sakurai, M. Yumura, K. Hata, Absence of an ideal single-walled carbon nanotube forest structure for thermal and electrical conductivities, ACS Nano 7 (11) (2013) 10218–10224.