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New synthesis of Cu2O and Cu nanoparticles on multi-wall carbon nanotubes
Materials Research Bulletin 43 (2008) 1492–1496 www.elsevier.com/locate/matresbu
New synthesis of Cu2O and Cu nanoparticles on multi-wall carbon nanotubes A. Martı´nez-Ruiz a, G. Alonso-Nun˜ez b,* a
Facultad de Ciencias, Universidad Auto´noma de Baja California, Km 106 carretera Tijuana-Ensenada, C.P. 22800 Ensenada, B.C. Mexico b Departamento de Quı´mica de Materiales, Centro de Investigacio´n en Materiales Avanzados, Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31109 Chihuahua, Mexico Received 3 February 2007; received in revised form 21 April 2007; accepted 7 June 2007 Available online 14 June 2007
Abstract Cu2O and Cu nanoparticles were deposited on the surface of multi-walled carbon nanotubes with a diameter range of 15–90 nm by the impregnate method. Multi-wall carbon nanotubes with a length of 200 mm and a diameter range of 70–110 nm were grown inside of quartz tubing by the spray pyrolysis method using ferrocene/benzene under argon flow. The nanotubes were then treated with nitric acid to clean the surface and generate carboxylic groups. The copper was impregnated on multi-walled carbon nanotubes using a xylene solution of copper(I) phenylacetylide as the precursor. Copper and cuprous oxide nanoparticles were obtained during thermal treatment. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; C. Electron microscopy
1. Introduction Carbon nanotubes (CNTs) have been extensively studied because of their unique properties which include their electrical behavior and mechanical strength [1–7]. Chemical vapor deposition (CVD) is a well-known technique for the preparation of multi-walled carbon nanotubes (MWCNTs) [8]. The CVD method has many advantages over other methods such as producing a large quantity of MWCNTs, easily adjusting synthetic parameters such as the carbon source and the catalytic metal. Synthesis involves spraying a metallocence/hydrocarbon solution into a furnace under an inert to a slightly reductive gas flow [9]. Recently, studies of CNTs have focused on depositing metal or metal oxide nanoparticles onto the nanotube’s surface [10–14]. The physical and chemical properties of nanotube deposited nanoparticles may be changed to obtain desired properties. Currently, there is interest in using clean and renewable non-fossil energy sources; solar energy is the most easily obtainable and inexhaustible source; therefore, solar energy converters have attracted increased attention. Because of this, it is not surprising that intensive research has been done to find new low-cost materials that can be used in the fabrication of solar conversion devices [15,16]. Cuprous oxide (Cu2O) has attracted interest as a promising material for generating cheap photovoltaic power because of its theoretical
* Corresponding author. Tel.: +52 614 4391130; fax: +52 614 4391130. E-mail address: [email protected] (G. Alonso-Nun˜ez). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.06.026
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solar cell efficiency, material abundance, and simplicity in forming a semiconducting layer. Cu2O is believed to be a promising material for the fabrication of photo-electrochemical cells (PECs) [13,17,18]. PECs convert solar energy into storable chemical energy as hydrogen by water photo-electrolysis. In this work, we report the preparation of stable cuprous oxide nanocrystals on the MWCNT’s surface. 2. Experimental 2.1. Multi-wall carbon nanotubes (MWCNTs) MWCNTs were prepared by the spray pyrolysis method described in our previous report [19,20]. MWCNTs were grown inside of quartz tubing by spray pyrolysis of ferrocene/bencene under an argon flow. The quartz tubing was heated by a Thermolyne 1200 cylindrical furnace(c) model equipped with a very precise temperature control of 1 8C. The reaction temperature was 900 8C. CNTs were produced with reaction time of 10 min. Afterwards, a black film of MWCNTs was formed in the inner surface of the vycor tubing and was mechanically removed with a brush. The CNTs were then exposed to an acid solution of HNO3 and seethed for 6 h. This treatment purified the surface of the MWCNTs by removing the iron particles used as the catalyst for nanotube growth; with this acidic treatment the oxidation surface of the carbon nanotubes was also prepared. After the acid treatment, the CNTs were subsequently washed with distilled water many times until the pH of the CNT solution approached 7. The resulting CNTs were then filtered and dried. The characterization of the MWCNTs was carried out by a JEOL JSM5800 LV scanning electron microscope (SEM) to perform a morphological analysis. Transmission electron micrographs were obtained by a Philips CM-200 analytical transmission electron microscope (TEM) operating at 200 kV. The TEM specimens were prepared by dispersing them in acetone and an ultrasonic bath for 2 min; a suspension drop was placed onto a perforated carbon coated Cu grid, and was allowed to dry. X-ray diffraction analyses were carried out by a Philips XPert MPD Diffractometer equipped with a curved graphite diffracted beam monochromator, using Cu Ka radiation ˚ ) and operated at 43 kV and 30 mA. Thermogravimetric analysis was carried out by a SDT 2960 TGA(l = 1.54184 A DTA TA Instrument under nitrogen flow from 20 to 800 8C at 10 8C/min. 2.2. Cuprous oxide and copper nanoparticles on multi-wall carbon nanotubes The cuprous oxide and copper nanoparticles on MWCNTs were prepared by the following procedure: 100 mg of synthesized MWCNTs, as described previously, was sonicated in 30 ml of xylene for a few minutes to form a suspension. This suspension was mixed with 30 ml of copper(I) phenylacetylide (8.1 10 3 M)/xylene solution. Then
Fig. 1. SEM micrograph showing clean MWCNTs formed by spray pyrolysis of ferrocene/bencene. The diameters are about 110 nm and have a length of up to several hundred micrometers.
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Fig. 2. The SEM image of the copper and cuprite nanoparticles deposited on the surface of the MWCNTs.
the mixture was refluxed in a round volumetric flask at 90 8C and stirred with a magnetic agitator for about 12 h. Afterwards, the mixture was allowed to cool down to room temperature; and subsequently, the suspension was filtered and washed with acetone. The copper(I) phenylacetylide/MWCNTs obtained were dried to room temperature. The sample was placed into the furnace tube and then heated at 300 8C for 2 h with nitrogen flow to eliminate the organic part of the precursor of copper(I) phenylacetylide and generate Cu2O/MWNTs. Analysis of Cu2O–Cu/MWNTs were carried out by a SEM equipped with an electron dispersive X-ray analyzer (EDS) in a JEOL SEM model JSM-5800 LV. Additional studies of the MWCNTs were carried out with a transmission electronic microscope (TEM) model Philips CM200. A Philips CM200 and a Tecnai G2 analytical TEM operated at 200 kV were also used to study the microstructures of the materials. The Tecnai G2 analytical TEM has a high angle annular dark field detector (HAADF). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) in a Perkin-Elmer model Pyris was used to corroborate the formation of nanoparticles on the MWCNTs.
Fig. 3. STEM micrograph obtained by HAADF. Showing cuprous oxide nanocrystals with a size on the order of 15 nm deposited on a multi-wall carbon nanotube.
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Fig. 4. XRD confirms that the nanoparticles procured on the MWCNT’s substrate (graphite) are cuprite nanocrystal (Cu2O) and Cu nanoparticles.
3. Results and discussion MWCNTs are synthesized by the spray pyrolysis method using ferrocene as a catalyst to grow the nanotubes and the iron particles on the outside surface of MWCNTs are cleaned with nitric acid. As shown in the SEM micrograph of Fig. 1, the diameter of the CNTs are around 70–110 nm and have a length of up to several hundred micrometers. Fig. 2 shows the SEM image of the nanosize particles deposited on the surface of the MWCNTs. The sizes of the particles are observed to be around 90 nm. On the other hand, as shown in Fig. 3, the STEM micrograph shows the cuprous oxide and copper nanoparticles deposited on the multi-wall carbon nanotube as having a size on the order of 15 nm. The same figure shows an enlarged CNT with a nanoparticle on the outside surface and an iron catalyst inside of the CNT. We suggest that the Fe on the inside of the MWCNT because it presents a cylindrical form indicative of the interior shape of the carbon nanotube, while the nanoparticle of Cu and Cu2O are on the outside surface of the carbon nanotubes and present a roundish shape. The XRD micrograph in Fig. 4 confirms that the nanoparticles procured on
Fig. 5. X-ray photoelectron spectroscopy (XPS) shows that the Cu and CuO2 deposited on the MWCNTs.
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the MWCNT’s substrate are copper and cuprite nanocrystals (Cu2O). The oxidation of copper is formed by the reaction between Cu and the carboxylate functionalities of the nanotube surface. These groups are formed during oxidation with nitric acid and used to clean the CNTs [19]. The reaction occurs during thermal treatment at 300 8C, however not all of the Cu nanoparticles are oxidized to Cu2O as observed by spectroscopic analysis. The X-ray photoelectron spectroscopy (XPS) in Fig. 5 shows that the Cu and Cu2O are deposited on the MWCNTs. Thermogravimetric analysis (TGA) of the (Cu and Cu2O/MWCNT) sample also indicates that this occurs. It was found that the percentage of observed copper (13.49%) corresponded to the initial stechiometric reaction equation from copper(I) phenylacetylide and the MWCNTs. 4. Conclusions MWCNTs were produced by spray pyrolysis from ferrocene/bencene. SEM images show the length of the MWCNTs to be around 200 mm and to have a diameter range of 70–110 nm. The copper nanoparticles were deposited on the MWCNTs by solution impregnation with copper(I) phenylacetylide as the precursor. The cuprite and copper nanostructures were prepared in situ on the CNTs during thermal treatment at 300 8C; the temperature at which the reaction occurred between the Cu and carboxylate functionalities of the nanotube surface. The nanosizes of the particles were observed in the SEM and TEM micrographs. The structure of particles was determined by XRD and XPS. The evenly distributed nanoparticles on the surface of MWCNTs have an average size of approximately 15– 90 nm. Acknowledgements We would like to thank the Universidad Auto´noma de Baja California Programa de Movilidad Acade´mica 2006, and the financial support of CONACYT-UT and the International Center for Nanotechnology and Advanced Materials (ICNAM) under grant no. 07. We would also like to thank F. Paraguay-Delgado, C. Ornelas, E. Aparicio, I. Gradilla Martı´nez, and W. de la Cruz for their technical support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
S. Iijima, Nature 354 (1991) 56. R. Saito, M. Fujita, G. Dresselhaus, M.S. Dresselhaus, Appl. Phys. Lett. 60 (18) (1992) 2204. S. Iijima, C. Brabec, A. Maiti, J. Bernholc, J. Chem. Phys. 104 (5) (1996) 2089. R.S. Ruoff, D.C. Lorents, Carbon 33 (7) (1995) 925. M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 381 (1996) 678. A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. H.W. Zhu, A. Chen, Z.Q. Mao, C.L. Xu, X. Xiao, B.Q. Wei, J. Liang, D.H. Wu, J. Mater. Sci. Lett. 19 (14) (2000) 1237. H.W. Zhu, C.L. Xu, D.H. Wu, Q.B. Wei, R. Vajtai, P.M. Ajayan, Science 296 (2002) 884. S. Charanjeet, M.S.P. Shaffer, K.K. Kozoil, I.A. Klinloch, A.H. Windle, Chem. Phys. Lett. 372 (2003) 860. G.R. Chen, C.L. Xu, Z.Q. Mao, Y.B. Li, J. Zhu, L.J. Ci, B.Q. Wei, J. Liang, D.H. Wu, Chin. Sci. Bull. 45 (2) (2000) 134. Y. Zhang, N.W. Franklin, R.J. Chen, H. Dai, Chem. Phys. Lett. 331 (1) (2000) 35. K. Ota, A. Kawabata, H. Murakami, E. Kita, Mater. Trans. 42 (8) (2001) 1684. Y. Li, J. Ding, J. Chen, C. Xu, B. Wei, J. Liang, D. Wu, Mater. Res. Bull. 37 (2) (2002) 313. C. Xu, G. Wu, Z. Liu, D. Wu, T.T. Meek, Q. Han, Mater. Res. Bull. 39 (2004) 1499. C.A.N. Fernando, T.M.W.J. Bandara, S.K. Wethasingha, Solar Energy Mater. Solar Cells 70 (2001) 121. K.H. Yoon, W.J. Choi, D.H. Kang, Thin Solid Films 372 (2000) 250. A. Martı´nez-Ruiz, M.G. Moreno, N. Takeuchi, Solid State Sci. 5 (2003) 291. Z. Yang, Z. Li, H. Wu, B. Simard, Mater. Lett. 57 (2003) 3160. D.H. Galva´n, A. Aguilar-Elgue´zabal, G. Alonso, Opt. Mater. 29 (2006) 140. A. Aguilar-Elgue´zabal, W. Antu´nez, G. Alonso, F. Paraguay Delgado, F. Espinosa, M. Miki-Yoshida, Diamond Relat. Mater. 15 (2006) 1329.
New synthesis of Cu2O and Cu nanoparticles on multi-wall carbon nanotubes A. Martı´nez-Ruiz a, G. Alonso-Nun˜ez b,* a
Facultad de Ciencias, Universidad Auto´noma de Baja California, Km 106 carretera Tijuana-Ensenada, C.P. 22800 Ensenada, B.C. Mexico b Departamento de Quı´mica de Materiales, Centro de Investigacio´n en Materiales Avanzados, Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31109 Chihuahua, Mexico Received 3 February 2007; received in revised form 21 April 2007; accepted 7 June 2007 Available online 14 June 2007
Abstract Cu2O and Cu nanoparticles were deposited on the surface of multi-walled carbon nanotubes with a diameter range of 15–90 nm by the impregnate method. Multi-wall carbon nanotubes with a length of 200 mm and a diameter range of 70–110 nm were grown inside of quartz tubing by the spray pyrolysis method using ferrocene/benzene under argon flow. The nanotubes were then treated with nitric acid to clean the surface and generate carboxylic groups. The copper was impregnated on multi-walled carbon nanotubes using a xylene solution of copper(I) phenylacetylide as the precursor. Copper and cuprous oxide nanoparticles were obtained during thermal treatment. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; C. Electron microscopy
1. Introduction Carbon nanotubes (CNTs) have been extensively studied because of their unique properties which include their electrical behavior and mechanical strength [1–7]. Chemical vapor deposition (CVD) is a well-known technique for the preparation of multi-walled carbon nanotubes (MWCNTs) [8]. The CVD method has many advantages over other methods such as producing a large quantity of MWCNTs, easily adjusting synthetic parameters such as the carbon source and the catalytic metal. Synthesis involves spraying a metallocence/hydrocarbon solution into a furnace under an inert to a slightly reductive gas flow [9]. Recently, studies of CNTs have focused on depositing metal or metal oxide nanoparticles onto the nanotube’s surface [10–14]. The physical and chemical properties of nanotube deposited nanoparticles may be changed to obtain desired properties. Currently, there is interest in using clean and renewable non-fossil energy sources; solar energy is the most easily obtainable and inexhaustible source; therefore, solar energy converters have attracted increased attention. Because of this, it is not surprising that intensive research has been done to find new low-cost materials that can be used in the fabrication of solar conversion devices [15,16]. Cuprous oxide (Cu2O) has attracted interest as a promising material for generating cheap photovoltaic power because of its theoretical
* Corresponding author. Tel.: +52 614 4391130; fax: +52 614 4391130. E-mail address: [email protected] (G. Alonso-Nun˜ez). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.06.026
A. Martı´nez-Ruiz, G. Alonso-Nun˜ez / Materials Research Bulletin 43 (2008) 1492–1496
1493
solar cell efficiency, material abundance, and simplicity in forming a semiconducting layer. Cu2O is believed to be a promising material for the fabrication of photo-electrochemical cells (PECs) [13,17,18]. PECs convert solar energy into storable chemical energy as hydrogen by water photo-electrolysis. In this work, we report the preparation of stable cuprous oxide nanocrystals on the MWCNT’s surface. 2. Experimental 2.1. Multi-wall carbon nanotubes (MWCNTs) MWCNTs were prepared by the spray pyrolysis method described in our previous report [19,20]. MWCNTs were grown inside of quartz tubing by spray pyrolysis of ferrocene/bencene under an argon flow. The quartz tubing was heated by a Thermolyne 1200 cylindrical furnace(c) model equipped with a very precise temperature control of 1 8C. The reaction temperature was 900 8C. CNTs were produced with reaction time of 10 min. Afterwards, a black film of MWCNTs was formed in the inner surface of the vycor tubing and was mechanically removed with a brush. The CNTs were then exposed to an acid solution of HNO3 and seethed for 6 h. This treatment purified the surface of the MWCNTs by removing the iron particles used as the catalyst for nanotube growth; with this acidic treatment the oxidation surface of the carbon nanotubes was also prepared. After the acid treatment, the CNTs were subsequently washed with distilled water many times until the pH of the CNT solution approached 7. The resulting CNTs were then filtered and dried. The characterization of the MWCNTs was carried out by a JEOL JSM5800 LV scanning electron microscope (SEM) to perform a morphological analysis. Transmission electron micrographs were obtained by a Philips CM-200 analytical transmission electron microscope (TEM) operating at 200 kV. The TEM specimens were prepared by dispersing them in acetone and an ultrasonic bath for 2 min; a suspension drop was placed onto a perforated carbon coated Cu grid, and was allowed to dry. X-ray diffraction analyses were carried out by a Philips XPert MPD Diffractometer equipped with a curved graphite diffracted beam monochromator, using Cu Ka radiation ˚ ) and operated at 43 kV and 30 mA. Thermogravimetric analysis was carried out by a SDT 2960 TGA(l = 1.54184 A DTA TA Instrument under nitrogen flow from 20 to 800 8C at 10 8C/min. 2.2. Cuprous oxide and copper nanoparticles on multi-wall carbon nanotubes The cuprous oxide and copper nanoparticles on MWCNTs were prepared by the following procedure: 100 mg of synthesized MWCNTs, as described previously, was sonicated in 30 ml of xylene for a few minutes to form a suspension. This suspension was mixed with 30 ml of copper(I) phenylacetylide (8.1 10 3 M)/xylene solution. Then
Fig. 1. SEM micrograph showing clean MWCNTs formed by spray pyrolysis of ferrocene/bencene. The diameters are about 110 nm and have a length of up to several hundred micrometers.
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Fig. 2. The SEM image of the copper and cuprite nanoparticles deposited on the surface of the MWCNTs.
the mixture was refluxed in a round volumetric flask at 90 8C and stirred with a magnetic agitator for about 12 h. Afterwards, the mixture was allowed to cool down to room temperature; and subsequently, the suspension was filtered and washed with acetone. The copper(I) phenylacetylide/MWCNTs obtained were dried to room temperature. The sample was placed into the furnace tube and then heated at 300 8C for 2 h with nitrogen flow to eliminate the organic part of the precursor of copper(I) phenylacetylide and generate Cu2O/MWNTs. Analysis of Cu2O–Cu/MWNTs were carried out by a SEM equipped with an electron dispersive X-ray analyzer (EDS) in a JEOL SEM model JSM-5800 LV. Additional studies of the MWCNTs were carried out with a transmission electronic microscope (TEM) model Philips CM200. A Philips CM200 and a Tecnai G2 analytical TEM operated at 200 kV were also used to study the microstructures of the materials. The Tecnai G2 analytical TEM has a high angle annular dark field detector (HAADF). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) in a Perkin-Elmer model Pyris was used to corroborate the formation of nanoparticles on the MWCNTs.
Fig. 3. STEM micrograph obtained by HAADF. Showing cuprous oxide nanocrystals with a size on the order of 15 nm deposited on a multi-wall carbon nanotube.
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Fig. 4. XRD confirms that the nanoparticles procured on the MWCNT’s substrate (graphite) are cuprite nanocrystal (Cu2O) and Cu nanoparticles.
3. Results and discussion MWCNTs are synthesized by the spray pyrolysis method using ferrocene as a catalyst to grow the nanotubes and the iron particles on the outside surface of MWCNTs are cleaned with nitric acid. As shown in the SEM micrograph of Fig. 1, the diameter of the CNTs are around 70–110 nm and have a length of up to several hundred micrometers. Fig. 2 shows the SEM image of the nanosize particles deposited on the surface of the MWCNTs. The sizes of the particles are observed to be around 90 nm. On the other hand, as shown in Fig. 3, the STEM micrograph shows the cuprous oxide and copper nanoparticles deposited on the multi-wall carbon nanotube as having a size on the order of 15 nm. The same figure shows an enlarged CNT with a nanoparticle on the outside surface and an iron catalyst inside of the CNT. We suggest that the Fe on the inside of the MWCNT because it presents a cylindrical form indicative of the interior shape of the carbon nanotube, while the nanoparticle of Cu and Cu2O are on the outside surface of the carbon nanotubes and present a roundish shape. The XRD micrograph in Fig. 4 confirms that the nanoparticles procured on
Fig. 5. X-ray photoelectron spectroscopy (XPS) shows that the Cu and CuO2 deposited on the MWCNTs.
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the MWCNT’s substrate are copper and cuprite nanocrystals (Cu2O). The oxidation of copper is formed by the reaction between Cu and the carboxylate functionalities of the nanotube surface. These groups are formed during oxidation with nitric acid and used to clean the CNTs [19]. The reaction occurs during thermal treatment at 300 8C, however not all of the Cu nanoparticles are oxidized to Cu2O as observed by spectroscopic analysis. The X-ray photoelectron spectroscopy (XPS) in Fig. 5 shows that the Cu and Cu2O are deposited on the MWCNTs. Thermogravimetric analysis (TGA) of the (Cu and Cu2O/MWCNT) sample also indicates that this occurs. It was found that the percentage of observed copper (13.49%) corresponded to the initial stechiometric reaction equation from copper(I) phenylacetylide and the MWCNTs. 4. Conclusions MWCNTs were produced by spray pyrolysis from ferrocene/bencene. SEM images show the length of the MWCNTs to be around 200 mm and to have a diameter range of 70–110 nm. The copper nanoparticles were deposited on the MWCNTs by solution impregnation with copper(I) phenylacetylide as the precursor. The cuprite and copper nanostructures were prepared in situ on the CNTs during thermal treatment at 300 8C; the temperature at which the reaction occurred between the Cu and carboxylate functionalities of the nanotube surface. The nanosizes of the particles were observed in the SEM and TEM micrographs. The structure of particles was determined by XRD and XPS. The evenly distributed nanoparticles on the surface of MWCNTs have an average size of approximately 15– 90 nm. Acknowledgements We would like to thank the Universidad Auto´noma de Baja California Programa de Movilidad Acade´mica 2006, and the financial support of CONACYT-UT and the International Center for Nanotechnology and Advanced Materials (ICNAM) under grant no. 07. We would also like to thank F. Paraguay-Delgado, C. Ornelas, E. Aparicio, I. Gradilla Martı´nez, and W. de la Cruz for their technical support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
S. Iijima, Nature 354 (1991) 56. R. Saito, M. Fujita, G. Dresselhaus, M.S. Dresselhaus, Appl. Phys. Lett. 60 (18) (1992) 2204. S. Iijima, C. Brabec, A. Maiti, J. Bernholc, J. Chem. Phys. 104 (5) (1996) 2089. R.S. Ruoff, D.C. Lorents, Carbon 33 (7) (1995) 925. M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 381 (1996) 678. A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. H.W. Zhu, A. Chen, Z.Q. Mao, C.L. Xu, X. Xiao, B.Q. Wei, J. Liang, D.H. Wu, J. Mater. Sci. Lett. 19 (14) (2000) 1237. H.W. Zhu, C.L. Xu, D.H. Wu, Q.B. Wei, R. Vajtai, P.M. Ajayan, Science 296 (2002) 884. S. Charanjeet, M.S.P. Shaffer, K.K. Kozoil, I.A. Klinloch, A.H. Windle, Chem. Phys. Lett. 372 (2003) 860. G.R. Chen, C.L. Xu, Z.Q. Mao, Y.B. Li, J. Zhu, L.J. Ci, B.Q. Wei, J. Liang, D.H. Wu, Chin. Sci. Bull. 45 (2) (2000) 134. Y. Zhang, N.W. Franklin, R.J. Chen, H. Dai, Chem. Phys. Lett. 331 (1) (2000) 35. K. Ota, A. Kawabata, H. Murakami, E. Kita, Mater. Trans. 42 (8) (2001) 1684. Y. Li, J. Ding, J. Chen, C. Xu, B. Wei, J. Liang, D. Wu, Mater. Res. Bull. 37 (2) (2002) 313. C. Xu, G. Wu, Z. Liu, D. Wu, T.T. Meek, Q. Han, Mater. Res. Bull. 39 (2004) 1499. C.A.N. Fernando, T.M.W.J. Bandara, S.K. Wethasingha, Solar Energy Mater. Solar Cells 70 (2001) 121. K.H. Yoon, W.J. Choi, D.H. Kang, Thin Solid Films 372 (2000) 250. A. Martı´nez-Ruiz, M.G. Moreno, N. Takeuchi, Solid State Sci. 5 (2003) 291. Z. Yang, Z. Li, H. Wu, B. Simard, Mater. Lett. 57 (2003) 3160. D.H. Galva´n, A. Aguilar-Elgue´zabal, G. Alonso, Opt. Mater. 29 (2006) 140. A. Aguilar-Elgue´zabal, W. Antu´nez, G. Alonso, F. Paraguay Delgado, F. Espinosa, M. Miki-Yoshida, Diamond Relat. Mater. 15 (2006) 1329.