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Effect of carbon coating on spontaneous C12A7 whisker formation
Applied Surface Science 444 (2018) 336–338
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Short Communication
Effect of carbon coating on spontaneous C12A7 whisker formation Vladimir I. Zaikovskii a,b, Alexander M. Volodin a, Vladimir O. Stoyanovskii a, Svetlana V. Cherepanova a,b, Aleksey A. Vedyagin a,c,⇑ a
Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia Novosibirsk State University, Novosibirsk 630090, Russia c National Research Tomsk Polytechnic University, Tomsk 634050, Russia b
a r t i c l e
i n f o
Article history: Received 18 October 2017 Revised 3 March 2018 Accepted 8 March 2018 Available online 9 March 2018 Keywords: Calcium aluminate Carbon nanoreactor Whiskers Structured carbon layers HRTEM
a b s t r a c t A carbon nanoreactor concept was applied to study the stabilization effect of carbon shell on phase composition and morphology of dodecacalcium hepta-aluminate Ca12Al14O33. The starting C12A7 powder was obtained using aluminum and calcium hydroxides as precursors. Carbon shell was formed by a chemical vapor deposition of divinyl at 550 °C. After the calcination at 1400 °C, the product was characterized by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM). It was observed for a first time that spontaneous formation of calcium aluminate whiskers take place under the conditions described. Each whisker consists of a ‘head’ (globular particle of 0.5 microns in diameter) and a ‘tail’ (prolonged whisker of few microns in length and 0.1–0.2 microns in diameter). According to HRTEM, the ‘head’ is characterized with microcrystal lattice of Ca12Al14O33 compound. XRD data show the presence of CaAl2O4 phase traces. The ‘head’ and ‘tail’ of the whisker are covered with structured graphene layers of 10 nm and 3 nm, correspondingly. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Formation of whiskers including their spontaneous growth is widely studied during the last decades [1–6]. The dominating amount of research works are dedicated to investigation of driving force and mechanism of metal whisker formation [1–3], but mostly in order to prevent this undesired phenomenon. Great potential for application of carbon whiskers attracts the special attention of the researchers to develop their preparation techniques [4,5]. Lu et al. [6] have fabricated the carbon titania composite whiskers for electrochemical applications. In this case, the carbon plays a role of stabilizing shell covering the titania. The similar approach named as ‘carbon nanoreactor concept’ was reported in our recent works [7–11] for stabilization of different oxide systems in dispersed state. On the other hand, unique properties of dodecacalcium heptaaluminate Ca12Al14O33 also known as C12A7 and materials based on it facilitate the appearance of a new wave of research activity including novel methods of its synthesis and characterization [12–18]. Among the variety of its application, the electron-donor supports for the precious metal based catalysts should be ⇑ Corresponding author at: Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia. E-mail address: [email protected] (A.A. Vedyagin). https://doi.org/10.1016/j.apsusc.2018.03.056 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.
mentioned especially [13,15,17,18]. As we have reported recently [9], nanocrystalline C12A7 also can be prepared and stabilized using the carbon nanoreactor concept. In the present work, conventionally used for the carbon shell formation dipping of the sample into polyvinyl alcohol with subsequent pyrolysis in an inert atmosphere was replaced with chemical vapor deposition of divinyl from a gas phase. High temperature treatment of the resulted material in argon at 1400 °C surprisingly led to spontaneous formation of calcium aluminate whiskers consisting of globular ‘head’ and prolonged ‘tail’. Thereby, the obtained results widen the synthetic routes for obtaining the nanocrystalline C12A7 system. The effect of spontaneous sub-structuring for studied material is reported for a first time. 2. Experimental Calcium aluminate C12A7 was prepared as described elsewhere [9]. Aluminum and calcium hydroxides were mixed in a required stoichiometry, thoroughly stirred in distilled water for 10 h, filtered and dried at 110 °C. Then it was calcined in a muffle furnace at 550 °C for 6 h. Obtained sample was used as a starting material for further synthesis. Deposition of the carbon shell was carried out by means of a chemical vapor deposition (CVD) method. The C12A7 powder was placed into quartz reactor, and then the mixture of 10 vol.%
V.I. Zaikovskii et al. / Applied Surface Science 444 (2018) 336–338
divinyl in an argon was purged through the reactor at 550 °C. The content of deposited carbon lies within a range of 4–8 wt.%. High temperature treatment of the samples was carried out in a special corundum ampoule. The sample (0.5–1 g) was placed into a graphite crucible which was inserted in the ampoule purged with argon. The ampoule was located inside the high-temperature tubular furnace manufactured using the StarbarÒ carbide-silicon heating element. The temperature control accuracy was 2 °C. The temperature ramping rate was 4 °C/min. The sample was maintained at 1400 °C for 6 h. High resolution transmission electron microscopy (HRTEM) images were obtained using a JEM-2010 electron microscope (JEOL, Japan) with a lattice-fringe resolution of 0.14 nm at accelerating voltage of 200 kV. The samples for HRTEM were prepared on a perforated carbon film mounted on a copper grid. X-ray diffraction (XRD) patterns of the samples were recorded using a Brucker D8 diffractometer.
3. Results and discussion As we have reported recently [9], the use of carbon coating helps to stabilize the calcium aluminate in a dispersed state preventing the sintering process up to 1450 °C. No morphological
Fig. 1. Low resolution TEM image of C12A7@C whisker spontaneously formed during the calcination in argon at 1400 °C.
337
changes of the material were observed to take place even at such high temperatures. In that case, polyvinyl alcohol (PVA) was used as a carbon source to realize the carbon nanoreactor concept. It should be emphasized that polyvinyl alcohol starts to exhibit the stabilizing effect already at temperatures of 250–300 °C (near the melting point for PVA). Finally, the calcination of PVA-stabilized C12A7 at 700 °C has resulted in appearance of the layer of amorphous carbon, which serves as a shell preventing the direct contact of the oxide particles and their agglomeration. Typical size of the C12A7 particles was found to be stabilized within a range of 100–150 nm. Surprisingly, when the divinyl was subjected to pyrolysis over C12A7, the further calcination in argon at 1400 °C has led to appearance of calcium aluminate whiskers. Note that the temperature of 1400 °C is a melting point for the dodecacalcium hepta-aluminate. The whiskers formed are of few microns in length and 0.1–0.2 microns in diameter. Fig. 1 demonstrates the TEM image of such whisker. At the beginning of each whisker a globular particle of larger diameter is well seen. Such different behavior of the similar carbon-stabilized C12A7 system is supposed to be connected with the following. The CVD of divinyl was performed at 550 °C, when the oxide particles are partly agglomerated. As a result, the average size of the stabilized particles is seemed to be near 500 nm. Further high temperature treatment of such C12A7@C particles near the melting point for C12A7 (1400 °C) leads to the restructuration of the most part of the carbon shell into a closed carbon tube filled with C12A7.
Fig. 3. TEM (A) and FFT (B) images of the edge of the globular C12A7@C particle.
Fig. 2. XRD patterns for C12A7@C whiskers after calcination in argon at 1400 °C.
338
V.I. Zaikovskii et al. / Applied Surface Science 444 (2018) 336–338
XRD pattern for the resulted C12A7@C whiskers is shown in Fig. 2. It is seen that the sample is not a single-phase calcium aluminate C12A7, while it contains traces of calcium monoaluminate CaAl2O4. The formation of latter is known to take place at 1300 °C under appropriate conditions [19]. As it was already mentioned, each whisker is connected with a larger globular particle. Study the sample with high resolution transmission electron microscopy revealed that the particle is characterized by a monocrystal lattice (Fig. 3A). The lattice parameters calculated from a fast Fourier transform (FFT) image shown in Fig. 3B correspond to Ca12Al14O33 compound (ICDD PDF database, card number 09-0413). The surface of the particle is covered with graphene multilayers of about 10 nm in thickness (Fig. 4). These carbon layers being in a close contact with calcium aluminate replicate the morphology of the surface. Fig. 5 illustrates the TEM images of the edge of whisker. The bright border of graphene layers is well seen in a dark field image (Fig. 5A). The amount of carbon layers covering the whisker is believed to be 8–10 with a total thickness of about 3 nm (Fig. 5B). Thus, it can be concluded that the deposition of carbon on the calcium aluminate surface in the form of easy-to-structure graphene packages followed by calcination in argon at temperature near the C12A7 melting point leads to spontaneous formation of C12A7 whiskers. No formation of such nanostructures was observed at lower temperatures or if polyvinyl alcohol was used as a carbon precursor.
Fig. 4. High resolution TEM image of the edge of the globular C12A7@C particle.
Fig. 5. Dark field (A) and bright field (B) TEM images of the whisker edge.
4. Conclusions Spontaneous formation of whiskers (metal, carbon, oxide, etc.) is a well-known and intensively studied phenomenon. Depending on nature and composition of the bulk matrix, the driving forces for the whisker formation process might be different. In present work we have reported for a first time the spontaneous formation of calcium aluminate whiskers, which take place in an argon atmosphere at 1400 °C under carbon nanoreactor conditions. Surprisingly, the nature of carbon precursor was found to be crucial for the phenomenon to take place. The driving forces of the process are believed to be the temperature near the melting point of C12A7 and the trend of nanostructuring of carbon deposits into multilayered pseudo-nanotubes. Acknowledgments Financial support from Russian Science Foundation (project No. 1613-10168) is acknowledged with gratitude. References [1] N. Furuta, K. Hamamura, Growth mechanism of proper tin-whisker, Jpn. J. Appl. Phys. 8 (1969) 1404–1410. [2] M.W. Barsoum, E.N. Hoffman, R.D. Doherty, S. Gupta, A. Zavaliangos, Driving force and mechanism for spontaneous metal whisker formation, Phys. Rev. Lett. 93 (2004) (206104–1–4). [3] Y. Liu, P. Zhang, Y.M. Zhang, J. Ding, J.J. Shi, Z.M. Sun, Spontaneous growth of Sn whiskers and a new formation mechanism, Mater. Lett. 178 (2016) 111–114. [4] J.J. Cuomo, J.M.E. Harper, Carbon whisker formation, IBM Tech. Disclos. Bull. 20 (1977) 775–776. [5] J.R. Rostrup-Nielsen, J. Sehested, Whisker carbon revisited, Stud. Surf. Sci. Catal. 139 (2001) 1–12. [6] L. Lu, Y. Zhu, F. Li, W. Zhuang, K.Y. Chan, X. Lu, Carbon titania mesoporous composite whisker as stable supercapacitor electrode material, J. Mater. Chem. 20 (2010) 7645–7651. [7] A.F. Bedilo, E.I. Shuvarakova, A.M. Volodin, E.V. Ilyina, I.V. Mishakov, A.A. Vedyagin, V.V. Chesnokov, D.S. Heroux, K.J. Klabunde, Effect of modification with vanadium or carbon on destructive sorption of halocarbons over nanocrystalline MgO: the role of active sites in initiation of the solid-state reaction, J. Phys. Chem. C 118 (2014) 13715–13725. [8] A.M. Volodin, A.F. Bedilo, I.V. Mishakov, V.I. Zaikovskii, A.A. Vedyagin, R.M. Kenzhin, V.O. Stoyanovskii, K.S. Golohvast, Carbon nanoreactor for the synthesis of nanocrystalline high-temperature oxide materials, Nanotechnol. Russ. 9 (2014) 700–706. [9] A.M. Volodin, V.I. Zaikovskii, R.M. Kenzhin, A.F. Bedilo, I.V. Mishakov, A.A. Vedyagin, Synthesis of nanocrystalline calcium aluminate C12A7 under carbon nanoreactor conditions, Mater. Lett. 189 (2017) 210–212. [10] Shan Liu, Yan-Hui Sun, Feng-Chen Zhou, Jun-Min Nan, Improved electrochemical performance of a-Fe2O3 nanorods and nanotubes confined in carbon nanoshells, Appl. Surf. Sci. 375 (2016) 101–109. [11] M.A. Mohamed, W.N.W. Salleh, J. Jaafar, M.S. Rosmi, Z.A.M. Hir, M.A. Mutalib, A.F. Ismail, M. Tanemura, Carbon as amorphous shell and interstitial dopant in mesoporous rutile TiO2: bio-template assisted sol-gel synthesis and photocatalytic activity, Appl. Surf. Sci. 393 (2017) 46–59. [12] B. Matovic´, M. Prekajski, J. Pantic´, T. Bräuniger, M. Rosic´, D. Zagorac, D. Milivojevic´, Synthesis and densification of single-phase mayenite (C12A7), J. Eur. Ceram. Soc. 36 (2016) 4237–4241. [13] S. Yang, J.N. Kondo, K. Hayashi, M. Hirano, K. Domen, H. Hosono, Partial oxidation of methane to syngas over promoted C12A7, Appl. Catal. A-Gen. 277 (2004) 239–246. [14] L. Gong, Z. Lin, S. Ning, J. Sun, J. Shen, Y. Torimoto, Q. Li, Synthesis and characteristics of the C12A7-O nanoparticles by citric acid sol–gel combustion method, Mater. Lett. 64 (2010) 1322–1324. [15] Z. Wang, Y. Pan, T. Dong, X. Zhu, T. Kan, L. Yuan, Y. Torimoto, M. Sadakata, Q. Li, Production of hydrogen from catalytic steam reforming of bio-oil using C12A7O -based catalysts, Appl. Catal. A-Gen. 320 (2007) 24–34. [16] M. Zahedi, N. Roohpour, A.K. Ray, Kinetic study of crystallisation of sol–gel derived calcia–alumina binary compounds, J. Alloys Compd. 582 (2014) 277–282. [17] J. Li, B. Yin, T. Fuchigami, S. Inagi, H. Hosono, S. Ito, Application of 12CaO7Al2O3 electride as a new electrode for superoxide ion generation and hydroxylation of an arylboronic acid, Electrochem. Commun. 17 (2012) 52–55. [18] J. Li, S. Inagi, T. Fuchigami, H. Hosono, S. Ito, Selective monocarboxylation of olefins at 12CaO7Al2O3 electride cathode, Electrochem. Commun. 44 (2014) 45– 48. [19] J.M. Rivas Mercury, A.H. De Aza, P. Pena, Synthesis of CaAl2O4 from powders: particle size effect, J. Eur. Ceram. Soc. 25 (2005) 3269–3279.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Short Communication
Effect of carbon coating on spontaneous C12A7 whisker formation Vladimir I. Zaikovskii a,b, Alexander M. Volodin a, Vladimir O. Stoyanovskii a, Svetlana V. Cherepanova a,b, Aleksey A. Vedyagin a,c,⇑ a
Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia Novosibirsk State University, Novosibirsk 630090, Russia c National Research Tomsk Polytechnic University, Tomsk 634050, Russia b
a r t i c l e
i n f o
Article history: Received 18 October 2017 Revised 3 March 2018 Accepted 8 March 2018 Available online 9 March 2018 Keywords: Calcium aluminate Carbon nanoreactor Whiskers Structured carbon layers HRTEM
a b s t r a c t A carbon nanoreactor concept was applied to study the stabilization effect of carbon shell on phase composition and morphology of dodecacalcium hepta-aluminate Ca12Al14O33. The starting C12A7 powder was obtained using aluminum and calcium hydroxides as precursors. Carbon shell was formed by a chemical vapor deposition of divinyl at 550 °C. After the calcination at 1400 °C, the product was characterized by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM). It was observed for a first time that spontaneous formation of calcium aluminate whiskers take place under the conditions described. Each whisker consists of a ‘head’ (globular particle of 0.5 microns in diameter) and a ‘tail’ (prolonged whisker of few microns in length and 0.1–0.2 microns in diameter). According to HRTEM, the ‘head’ is characterized with microcrystal lattice of Ca12Al14O33 compound. XRD data show the presence of CaAl2O4 phase traces. The ‘head’ and ‘tail’ of the whisker are covered with structured graphene layers of 10 nm and 3 nm, correspondingly. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Formation of whiskers including their spontaneous growth is widely studied during the last decades [1–6]. The dominating amount of research works are dedicated to investigation of driving force and mechanism of metal whisker formation [1–3], but mostly in order to prevent this undesired phenomenon. Great potential for application of carbon whiskers attracts the special attention of the researchers to develop their preparation techniques [4,5]. Lu et al. [6] have fabricated the carbon titania composite whiskers for electrochemical applications. In this case, the carbon plays a role of stabilizing shell covering the titania. The similar approach named as ‘carbon nanoreactor concept’ was reported in our recent works [7–11] for stabilization of different oxide systems in dispersed state. On the other hand, unique properties of dodecacalcium heptaaluminate Ca12Al14O33 also known as C12A7 and materials based on it facilitate the appearance of a new wave of research activity including novel methods of its synthesis and characterization [12–18]. Among the variety of its application, the electron-donor supports for the precious metal based catalysts should be ⇑ Corresponding author at: Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia. E-mail address: [email protected] (A.A. Vedyagin). https://doi.org/10.1016/j.apsusc.2018.03.056 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.
mentioned especially [13,15,17,18]. As we have reported recently [9], nanocrystalline C12A7 also can be prepared and stabilized using the carbon nanoreactor concept. In the present work, conventionally used for the carbon shell formation dipping of the sample into polyvinyl alcohol with subsequent pyrolysis in an inert atmosphere was replaced with chemical vapor deposition of divinyl from a gas phase. High temperature treatment of the resulted material in argon at 1400 °C surprisingly led to spontaneous formation of calcium aluminate whiskers consisting of globular ‘head’ and prolonged ‘tail’. Thereby, the obtained results widen the synthetic routes for obtaining the nanocrystalline C12A7 system. The effect of spontaneous sub-structuring for studied material is reported for a first time. 2. Experimental Calcium aluminate C12A7 was prepared as described elsewhere [9]. Aluminum and calcium hydroxides were mixed in a required stoichiometry, thoroughly stirred in distilled water for 10 h, filtered and dried at 110 °C. Then it was calcined in a muffle furnace at 550 °C for 6 h. Obtained sample was used as a starting material for further synthesis. Deposition of the carbon shell was carried out by means of a chemical vapor deposition (CVD) method. The C12A7 powder was placed into quartz reactor, and then the mixture of 10 vol.%
V.I. Zaikovskii et al. / Applied Surface Science 444 (2018) 336–338
divinyl in an argon was purged through the reactor at 550 °C. The content of deposited carbon lies within a range of 4–8 wt.%. High temperature treatment of the samples was carried out in a special corundum ampoule. The sample (0.5–1 g) was placed into a graphite crucible which was inserted in the ampoule purged with argon. The ampoule was located inside the high-temperature tubular furnace manufactured using the StarbarÒ carbide-silicon heating element. The temperature control accuracy was 2 °C. The temperature ramping rate was 4 °C/min. The sample was maintained at 1400 °C for 6 h. High resolution transmission electron microscopy (HRTEM) images were obtained using a JEM-2010 electron microscope (JEOL, Japan) with a lattice-fringe resolution of 0.14 nm at accelerating voltage of 200 kV. The samples for HRTEM were prepared on a perforated carbon film mounted on a copper grid. X-ray diffraction (XRD) patterns of the samples were recorded using a Brucker D8 diffractometer.
3. Results and discussion As we have reported recently [9], the use of carbon coating helps to stabilize the calcium aluminate in a dispersed state preventing the sintering process up to 1450 °C. No morphological
Fig. 1. Low resolution TEM image of C12A7@C whisker spontaneously formed during the calcination in argon at 1400 °C.
337
changes of the material were observed to take place even at such high temperatures. In that case, polyvinyl alcohol (PVA) was used as a carbon source to realize the carbon nanoreactor concept. It should be emphasized that polyvinyl alcohol starts to exhibit the stabilizing effect already at temperatures of 250–300 °C (near the melting point for PVA). Finally, the calcination of PVA-stabilized C12A7 at 700 °C has resulted in appearance of the layer of amorphous carbon, which serves as a shell preventing the direct contact of the oxide particles and their agglomeration. Typical size of the C12A7 particles was found to be stabilized within a range of 100–150 nm. Surprisingly, when the divinyl was subjected to pyrolysis over C12A7, the further calcination in argon at 1400 °C has led to appearance of calcium aluminate whiskers. Note that the temperature of 1400 °C is a melting point for the dodecacalcium hepta-aluminate. The whiskers formed are of few microns in length and 0.1–0.2 microns in diameter. Fig. 1 demonstrates the TEM image of such whisker. At the beginning of each whisker a globular particle of larger diameter is well seen. Such different behavior of the similar carbon-stabilized C12A7 system is supposed to be connected with the following. The CVD of divinyl was performed at 550 °C, when the oxide particles are partly agglomerated. As a result, the average size of the stabilized particles is seemed to be near 500 nm. Further high temperature treatment of such C12A7@C particles near the melting point for C12A7 (1400 °C) leads to the restructuration of the most part of the carbon shell into a closed carbon tube filled with C12A7.
Fig. 3. TEM (A) and FFT (B) images of the edge of the globular C12A7@C particle.
Fig. 2. XRD patterns for C12A7@C whiskers after calcination in argon at 1400 °C.
338
V.I. Zaikovskii et al. / Applied Surface Science 444 (2018) 336–338
XRD pattern for the resulted C12A7@C whiskers is shown in Fig. 2. It is seen that the sample is not a single-phase calcium aluminate C12A7, while it contains traces of calcium monoaluminate CaAl2O4. The formation of latter is known to take place at 1300 °C under appropriate conditions [19]. As it was already mentioned, each whisker is connected with a larger globular particle. Study the sample with high resolution transmission electron microscopy revealed that the particle is characterized by a monocrystal lattice (Fig. 3A). The lattice parameters calculated from a fast Fourier transform (FFT) image shown in Fig. 3B correspond to Ca12Al14O33 compound (ICDD PDF database, card number 09-0413). The surface of the particle is covered with graphene multilayers of about 10 nm in thickness (Fig. 4). These carbon layers being in a close contact with calcium aluminate replicate the morphology of the surface. Fig. 5 illustrates the TEM images of the edge of whisker. The bright border of graphene layers is well seen in a dark field image (Fig. 5A). The amount of carbon layers covering the whisker is believed to be 8–10 with a total thickness of about 3 nm (Fig. 5B). Thus, it can be concluded that the deposition of carbon on the calcium aluminate surface in the form of easy-to-structure graphene packages followed by calcination in argon at temperature near the C12A7 melting point leads to spontaneous formation of C12A7 whiskers. No formation of such nanostructures was observed at lower temperatures or if polyvinyl alcohol was used as a carbon precursor.
Fig. 4. High resolution TEM image of the edge of the globular C12A7@C particle.
Fig. 5. Dark field (A) and bright field (B) TEM images of the whisker edge.
4. Conclusions Spontaneous formation of whiskers (metal, carbon, oxide, etc.) is a well-known and intensively studied phenomenon. Depending on nature and composition of the bulk matrix, the driving forces for the whisker formation process might be different. In present work we have reported for a first time the spontaneous formation of calcium aluminate whiskers, which take place in an argon atmosphere at 1400 °C under carbon nanoreactor conditions. Surprisingly, the nature of carbon precursor was found to be crucial for the phenomenon to take place. The driving forces of the process are believed to be the temperature near the melting point of C12A7 and the trend of nanostructuring of carbon deposits into multilayered pseudo-nanotubes. Acknowledgments Financial support from Russian Science Foundation (project No. 1613-10168) is acknowledged with gratitude. References [1] N. Furuta, K. Hamamura, Growth mechanism of proper tin-whisker, Jpn. J. Appl. Phys. 8 (1969) 1404–1410. [2] M.W. Barsoum, E.N. Hoffman, R.D. Doherty, S. Gupta, A. Zavaliangos, Driving force and mechanism for spontaneous metal whisker formation, Phys. Rev. Lett. 93 (2004) (206104–1–4). [3] Y. Liu, P. Zhang, Y.M. Zhang, J. Ding, J.J. Shi, Z.M. Sun, Spontaneous growth of Sn whiskers and a new formation mechanism, Mater. Lett. 178 (2016) 111–114. [4] J.J. Cuomo, J.M.E. Harper, Carbon whisker formation, IBM Tech. Disclos. Bull. 20 (1977) 775–776. [5] J.R. Rostrup-Nielsen, J. Sehested, Whisker carbon revisited, Stud. Surf. Sci. Catal. 139 (2001) 1–12. [6] L. Lu, Y. Zhu, F. Li, W. Zhuang, K.Y. Chan, X. Lu, Carbon titania mesoporous composite whisker as stable supercapacitor electrode material, J. Mater. Chem. 20 (2010) 7645–7651. [7] A.F. Bedilo, E.I. Shuvarakova, A.M. Volodin, E.V. Ilyina, I.V. Mishakov, A.A. Vedyagin, V.V. Chesnokov, D.S. Heroux, K.J. Klabunde, Effect of modification with vanadium or carbon on destructive sorption of halocarbons over nanocrystalline MgO: the role of active sites in initiation of the solid-state reaction, J. Phys. Chem. C 118 (2014) 13715–13725. [8] A.M. Volodin, A.F. Bedilo, I.V. Mishakov, V.I. Zaikovskii, A.A. Vedyagin, R.M. Kenzhin, V.O. Stoyanovskii, K.S. Golohvast, Carbon nanoreactor for the synthesis of nanocrystalline high-temperature oxide materials, Nanotechnol. Russ. 9 (2014) 700–706. [9] A.M. Volodin, V.I. Zaikovskii, R.M. Kenzhin, A.F. Bedilo, I.V. Mishakov, A.A. Vedyagin, Synthesis of nanocrystalline calcium aluminate C12A7 under carbon nanoreactor conditions, Mater. Lett. 189 (2017) 210–212. [10] Shan Liu, Yan-Hui Sun, Feng-Chen Zhou, Jun-Min Nan, Improved electrochemical performance of a-Fe2O3 nanorods and nanotubes confined in carbon nanoshells, Appl. Surf. Sci. 375 (2016) 101–109. [11] M.A. Mohamed, W.N.W. Salleh, J. Jaafar, M.S. Rosmi, Z.A.M. Hir, M.A. Mutalib, A.F. Ismail, M. Tanemura, Carbon as amorphous shell and interstitial dopant in mesoporous rutile TiO2: bio-template assisted sol-gel synthesis and photocatalytic activity, Appl. Surf. Sci. 393 (2017) 46–59. [12] B. Matovic´, M. Prekajski, J. Pantic´, T. Bräuniger, M. Rosic´, D. Zagorac, D. Milivojevic´, Synthesis and densification of single-phase mayenite (C12A7), J. Eur. Ceram. Soc. 36 (2016) 4237–4241. [13] S. Yang, J.N. Kondo, K. Hayashi, M. Hirano, K. Domen, H. Hosono, Partial oxidation of methane to syngas over promoted C12A7, Appl. Catal. A-Gen. 277 (2004) 239–246. [14] L. Gong, Z. Lin, S. Ning, J. Sun, J. Shen, Y. Torimoto, Q. Li, Synthesis and characteristics of the C12A7-O nanoparticles by citric acid sol–gel combustion method, Mater. Lett. 64 (2010) 1322–1324. [15] Z. Wang, Y. Pan, T. Dong, X. Zhu, T. Kan, L. Yuan, Y. Torimoto, M. Sadakata, Q. Li, Production of hydrogen from catalytic steam reforming of bio-oil using C12A7O -based catalysts, Appl. Catal. A-Gen. 320 (2007) 24–34. [16] M. Zahedi, N. Roohpour, A.K. Ray, Kinetic study of crystallisation of sol–gel derived calcia–alumina binary compounds, J. Alloys Compd. 582 (2014) 277–282. [17] J. Li, B. Yin, T. Fuchigami, S. Inagi, H. Hosono, S. Ito, Application of 12CaO7Al2O3 electride as a new electrode for superoxide ion generation and hydroxylation of an arylboronic acid, Electrochem. Commun. 17 (2012) 52–55. [18] J. Li, S. Inagi, T. Fuchigami, H. Hosono, S. Ito, Selective monocarboxylation of olefins at 12CaO7Al2O3 electride cathode, Electrochem. Commun. 44 (2014) 45– 48. [19] J.M. Rivas Mercury, A.H. De Aza, P. Pena, Synthesis of CaAl2O4 from powders: particle size effect, J. Eur. Ceram. Soc. 25 (2005) 3269–3279.