- Email: [email protected]
Low temperature synthesis of delafossite (CuAlO2) using aluminum nitrate
Materials Letters 63 (2009) 587–588
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Low temperature synthesis of delafossite (CuAlO2) using aluminum nitrate L. Torkian a, M.M. Amini b,⁎ a b
Department of Applied Chemistry, Islamic Azad University, South Branch, Tehran, Iran Department of Chemistry, Shahid Beheshti University, G.C., Tehran Iran
a r t i c l e
i n f o
Article history: Received 24 July 2008 Accepted 22 November 2008 Available online 6 December 2008 Keywords: Heat treatment X-ray techniques Microwave Delafossite
a b s t r a c t Delafossite CuAlO2, a p-type semiconductor with novel morphology, has been synthesized at 300 °C by conventional heating and microwave irradiation. Both conventional heating and microwave irradiation of a mixture of aluminum nitrate and copper oxide in mole ratio of 1:1 resulted in the formation of delafossite CuAlO2. © 2008 Published by Elsevier B.V.
1. Introduction Delafossite CuAlO2 was synthesized by Friedel in 1873 from the mineral CuFeO2, and it is a transparent p-type semiconductor material with a band gap of more than 3 eV. This material has a wide variety of applications, including optoelectronics device technology [1], refrigeration of electronic devices [2], reduction of water with visible light (water photoelectrolysis), field emission technology [3], and light emitting diodes (LEDs). It is also used as a thermoelectric material in electricity generation [4], and it is superior to PbTe and Bi2Te3 when considering thermal and oxidizing resistance [2]. The field emission properties of delafossite make it a technologically important material, and, along with a new group of materials, it can create new possibilities in field emission technology [3]. Although various techniques have been used to prepare CuAlO2 thin films [1,5–8], there are few reports on the preparation of bulk CuAlO2. Polycrystalline CuAlO2 powder can be prepared by heating a stoichiometric mixture of Cu2O and Al2O3 at about 1100 °C for 24 h [3,9], by a prolonged solid state reaction between CuO and Al2O3 above 1000 °C [10], by ion exchange of α-LiAlO2 with CuCl at temperatures below 500 °C in a sealed tube with base pressure of 10− 6 mbar for 40– 50 h [11], and by the sol–gel method [12]. Almost all of the methods used in the preparation of CuAlO2 involve high temperature or prolonged heating processes. However, prolonged, high temperature processes change grain size and result in the formation of agglomerated powders, which significantly affect the thermoelectric properties. Therefore, processing CuAlO2 powder at lower temperatures should prove beneficial. In the present work, we report on the preparation of
⁎ Corresponding author. Tel.: +98 21 29903109; fax: +96 21 22431663. E-mail address: [email protected] (M.M. Amini). 0167-577X/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.matlet.2008.11.052
CuAlO2 by conventional and also by microwave heating of a mixture of copper oxide and aluminum nitrate. 2. Experimental Microwave heating was performed in a 1000-Watt, 2.45-GHz National model 3000 microwave oven without microwave suspector material. Conventional heating was performed in a box furnace. Aluminum nitrate (99.5%, Merck) and copper oxide (CuO, 99%, Merck) were mixed in mole ratio of 1:1 (total weight 5 g) as a precursor. For conventional processing, the precursor powder was heated at 300 °C in a furnace for 3 h. For microwave processing, the heat treatment was performed in a 1000-W microwave oven for 20 min. A maximum processing temperature of 800 °C has been estimated for the microwave processed samples. The XRD patterns for the powders were recorded with a Seifert 3003 diffractometer with Cu Kα radiation. Electron microscopy was performed on an Oxford LEO 4401 scanning electron microscope (SEM). 3. Results and discussion 3.1. Conventional processing of delafossite CuAlO2 The X-ray diffraction pattern of the powder synthesized by conventional heating of aluminum nitrate and copper oxide in a mole ratio of 1:1 is shown in Fig. 1. All of the peaks in the pattern are indexed for the space group R3m, lattice constants of a = 2.856 and c = 16.943 Å, and correspond to the delafossite phase [13]. The much lower processing temperature required in this study can be attributed to the use of aluminum nitrate because aluminum nitrate has a very low decomposition temperature (135 °C) and decomposes to aluminum hydroxide with an active surface. In a subsequent reaction with copper oxide, delafossite CuAlO2 is produced on the active surface. In comparison to the earlier reports in which the delafossite phase is prepared from CuO and Al2O3 above 1000 °C for 24 h [10], in this work, delafossite CuAlO2 was produced at only 300 °C for 3 h. Therefore, replacing aluminum oxide with aluminum nitrate reduces the reaction time and temperature significantly [10]. Notably, when aluminum
588
L. Torkian, M.M. Amini / Materials Letters 63 (2009) 587–588
Fig. 1. X-ray diffraction pattern of CuAlO2 prepared by conventional or microwave heating of aluminum nitrate and copper(II) oxide. sulfate is used instead of aluminum nitrate, delafossite phase was not formed. Considering the much higher decomposition temperature of aluminum sulfate (770 °C), the absence of delafossite CuAlO2 is reasonable. Preparation of CuAlO2 by solid state reaction of copper oxide and alumina requires much higher temperature and prolonged heating for the activation of the alumina surface [2,10]. The morphology of the delafossite CuAlO2 that was prepared by conventional heating at 300 °C for 3 h is shown in Fig. 2a. During conventional heating, large CuAlO2 crystals with not so well-defined natural crystalline faces are formed. Note that the morphology of the delafossite phase that was prepared by a solid state reaction of aluminum nitrate and copper oxide was similar to the morphology of crystals prepared by a solid state reaction of copper oxide and aluminum oxide above 1000 °C [2], but differs to a great extent from that of prepared by ion exchange from LiAlO2 [11], or by the sol–gel method [12]. 3.2. Microwave processing of delafossite CuAlO2 Microwave heating has potential for solid state reactions, especially when the precursor is a microwave-absorbing material such as CuO. For example, if 5–6 g CuO is exposed to 500 W of microwave radiation for 30 s, its temperature will rise to 800 °C within a few minutes [14]. In the majority of studies of the synthesis of ceramics and metal oxides by microwave radiation, special cells with high microwave absorber, such as SiC, have been employed in order to efficiently absorb microwave radiation [15]. In the present study, without using special cells made of a material susceptible to microwave heating, we prepared delafossite in a short period of time. The X-ray diffraction pattern of the product that we obtained from the microwave heating of aluminum nitrate and copper oxide in a mole ratio of 1:1 for 20 min is revealed formation of delafossite CuAlO2 (Fig. 1). The decrease in the reaction time for formation of the delafossite phase by microwave irradiation can be attributed to the high efficiency of copper oxide's absorption of microwave radiation [14,16]. Mingos et al. reported that the extremely high temperature effect of microwave radiation is associated with the presence of special ions [17]. It seems that, in the present case, the formation of delafossite in such a short time can be attributed to the presence of nitrate ions in proximity to the copper oxide, which is a strong absorber, because replacing nitrate ions with other anions such as sulfate failed to produce delafossite. Furthermore, the formation of delafossite by microwave irradiation in a short period of time compared to that required with the conventional method indicates that, in addition to the conventional heating effect, the non-heating effect of microwave radiation plays a significant role. The morphology of the delafossite CuAlO2 prepared by microwave heating is shown in Fig. 2b. The morphology of the microwave-processed material is, different to some extent, from that of the material prepared by conventional heating. Delafossite CuAlO2 crystals with well-defined natural crystalline faces are formed. Interestingly, the morphology of the microwave-processed material is not similar to the morphology found for delafossite prepared by other procedures.
4. Conclusion Delafossite CuAlO2 can be prepared at low temperatures by conventional or microwave heating of aluminum nitrate and copper oxide during a short period of time. It seems that the copper oxide acts
Fig. 2. SEM micrograph of CuAlO2, a) prepared by conventional heating of aluminum nitrate and copper(II) oxide, b) prepared by microwave heating.
as a microwave-absorbing material for preparation of delafossite, and that the aluminum nitrate also has a pivotal role in delafossite formation by both conventional and microwave heating. Acknowledgements We thank the South Branch of Azad University and Shahid Beheshti University for supporting this work. References [1] Kawazoe M, Yasukawa H, Hyodo, Kurita M, Yanagi H, Hosono H. Nature 1997;389:939–42. [2] Park K, Ko KY, Seo W-S. J Eur Ceram Soc 2005;25:2219–22. [3] Banerjee AN, Chatropadhyay KK. Appl Surf Sci 2004;225:243–9. [4] Park K, Ko KY, Kwon H-C, Nahm S. J Alloys Compd 2007;437:1–6. [5] Ohashi M, Iida Y, Morikawa. J Am Ceram Soc 2002;85:270–2. [6] Wang Y, Gong H. Chem Vap Depos 2000;6:285–8. [7] Ong CH, Gong H. Thin Solid Films 2003;445:299–303. [8] Neumann-Spallart M, Pai SP, Pinto R. Thin Solid Films 2007;515:8641–4. [9] Banerjee AN, Kundoo S, Chatropadhyay KK. Thin Solid Films 2004;440:5–10. [10] Koriche N, Bouguelia A, Aider A, Trari M. Int J Hydrogen Energy 2005;30:639–8649. [11] Dloczik L, Tomn Y, Konenkamp R, Lux-Steiner MC, Dittrich TH. Thin Solids Films 2004;451–452:116–9. [12] Deng Z, Zhu X, Tao R, Dong W, Fang X. Mater Lett 2007;61:686–9. [13] Joint Committee Powder Diffraction File, Card No. 77-2493. [14] Smart LL, Moore L. Solid state chemistry. 3rd ed. New York: Chapman Hall; 1999. [15] Janney MA, Calhoun CL, Kimrey HD. J Am Ceram Soc 1992;75:341–6. [16] Kato M, Sakakibara K, Koike Y. Appl Supercond 1997;5:33–9. [17] Mingos DMP, Michael D, Baghurst DR. Chem Soc Rev 1991;20:1–47.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Low temperature synthesis of delafossite (CuAlO2) using aluminum nitrate L. Torkian a, M.M. Amini b,⁎ a b
Department of Applied Chemistry, Islamic Azad University, South Branch, Tehran, Iran Department of Chemistry, Shahid Beheshti University, G.C., Tehran Iran
a r t i c l e
i n f o
Article history: Received 24 July 2008 Accepted 22 November 2008 Available online 6 December 2008 Keywords: Heat treatment X-ray techniques Microwave Delafossite
a b s t r a c t Delafossite CuAlO2, a p-type semiconductor with novel morphology, has been synthesized at 300 °C by conventional heating and microwave irradiation. Both conventional heating and microwave irradiation of a mixture of aluminum nitrate and copper oxide in mole ratio of 1:1 resulted in the formation of delafossite CuAlO2. © 2008 Published by Elsevier B.V.
1. Introduction Delafossite CuAlO2 was synthesized by Friedel in 1873 from the mineral CuFeO2, and it is a transparent p-type semiconductor material with a band gap of more than 3 eV. This material has a wide variety of applications, including optoelectronics device technology [1], refrigeration of electronic devices [2], reduction of water with visible light (water photoelectrolysis), field emission technology [3], and light emitting diodes (LEDs). It is also used as a thermoelectric material in electricity generation [4], and it is superior to PbTe and Bi2Te3 when considering thermal and oxidizing resistance [2]. The field emission properties of delafossite make it a technologically important material, and, along with a new group of materials, it can create new possibilities in field emission technology [3]. Although various techniques have been used to prepare CuAlO2 thin films [1,5–8], there are few reports on the preparation of bulk CuAlO2. Polycrystalline CuAlO2 powder can be prepared by heating a stoichiometric mixture of Cu2O and Al2O3 at about 1100 °C for 24 h [3,9], by a prolonged solid state reaction between CuO and Al2O3 above 1000 °C [10], by ion exchange of α-LiAlO2 with CuCl at temperatures below 500 °C in a sealed tube with base pressure of 10− 6 mbar for 40– 50 h [11], and by the sol–gel method [12]. Almost all of the methods used in the preparation of CuAlO2 involve high temperature or prolonged heating processes. However, prolonged, high temperature processes change grain size and result in the formation of agglomerated powders, which significantly affect the thermoelectric properties. Therefore, processing CuAlO2 powder at lower temperatures should prove beneficial. In the present work, we report on the preparation of
⁎ Corresponding author. Tel.: +98 21 29903109; fax: +96 21 22431663. E-mail address: [email protected] (M.M. Amini). 0167-577X/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.matlet.2008.11.052
CuAlO2 by conventional and also by microwave heating of a mixture of copper oxide and aluminum nitrate. 2. Experimental Microwave heating was performed in a 1000-Watt, 2.45-GHz National model 3000 microwave oven without microwave suspector material. Conventional heating was performed in a box furnace. Aluminum nitrate (99.5%, Merck) and copper oxide (CuO, 99%, Merck) were mixed in mole ratio of 1:1 (total weight 5 g) as a precursor. For conventional processing, the precursor powder was heated at 300 °C in a furnace for 3 h. For microwave processing, the heat treatment was performed in a 1000-W microwave oven for 20 min. A maximum processing temperature of 800 °C has been estimated for the microwave processed samples. The XRD patterns for the powders were recorded with a Seifert 3003 diffractometer with Cu Kα radiation. Electron microscopy was performed on an Oxford LEO 4401 scanning electron microscope (SEM). 3. Results and discussion 3.1. Conventional processing of delafossite CuAlO2 The X-ray diffraction pattern of the powder synthesized by conventional heating of aluminum nitrate and copper oxide in a mole ratio of 1:1 is shown in Fig. 1. All of the peaks in the pattern are indexed for the space group R3m, lattice constants of a = 2.856 and c = 16.943 Å, and correspond to the delafossite phase [13]. The much lower processing temperature required in this study can be attributed to the use of aluminum nitrate because aluminum nitrate has a very low decomposition temperature (135 °C) and decomposes to aluminum hydroxide with an active surface. In a subsequent reaction with copper oxide, delafossite CuAlO2 is produced on the active surface. In comparison to the earlier reports in which the delafossite phase is prepared from CuO and Al2O3 above 1000 °C for 24 h [10], in this work, delafossite CuAlO2 was produced at only 300 °C for 3 h. Therefore, replacing aluminum oxide with aluminum nitrate reduces the reaction time and temperature significantly [10]. Notably, when aluminum
588
L. Torkian, M.M. Amini / Materials Letters 63 (2009) 587–588
Fig. 1. X-ray diffraction pattern of CuAlO2 prepared by conventional or microwave heating of aluminum nitrate and copper(II) oxide. sulfate is used instead of aluminum nitrate, delafossite phase was not formed. Considering the much higher decomposition temperature of aluminum sulfate (770 °C), the absence of delafossite CuAlO2 is reasonable. Preparation of CuAlO2 by solid state reaction of copper oxide and alumina requires much higher temperature and prolonged heating for the activation of the alumina surface [2,10]. The morphology of the delafossite CuAlO2 that was prepared by conventional heating at 300 °C for 3 h is shown in Fig. 2a. During conventional heating, large CuAlO2 crystals with not so well-defined natural crystalline faces are formed. Note that the morphology of the delafossite phase that was prepared by a solid state reaction of aluminum nitrate and copper oxide was similar to the morphology of crystals prepared by a solid state reaction of copper oxide and aluminum oxide above 1000 °C [2], but differs to a great extent from that of prepared by ion exchange from LiAlO2 [11], or by the sol–gel method [12]. 3.2. Microwave processing of delafossite CuAlO2 Microwave heating has potential for solid state reactions, especially when the precursor is a microwave-absorbing material such as CuO. For example, if 5–6 g CuO is exposed to 500 W of microwave radiation for 30 s, its temperature will rise to 800 °C within a few minutes [14]. In the majority of studies of the synthesis of ceramics and metal oxides by microwave radiation, special cells with high microwave absorber, such as SiC, have been employed in order to efficiently absorb microwave radiation [15]. In the present study, without using special cells made of a material susceptible to microwave heating, we prepared delafossite in a short period of time. The X-ray diffraction pattern of the product that we obtained from the microwave heating of aluminum nitrate and copper oxide in a mole ratio of 1:1 for 20 min is revealed formation of delafossite CuAlO2 (Fig. 1). The decrease in the reaction time for formation of the delafossite phase by microwave irradiation can be attributed to the high efficiency of copper oxide's absorption of microwave radiation [14,16]. Mingos et al. reported that the extremely high temperature effect of microwave radiation is associated with the presence of special ions [17]. It seems that, in the present case, the formation of delafossite in such a short time can be attributed to the presence of nitrate ions in proximity to the copper oxide, which is a strong absorber, because replacing nitrate ions with other anions such as sulfate failed to produce delafossite. Furthermore, the formation of delafossite by microwave irradiation in a short period of time compared to that required with the conventional method indicates that, in addition to the conventional heating effect, the non-heating effect of microwave radiation plays a significant role. The morphology of the delafossite CuAlO2 prepared by microwave heating is shown in Fig. 2b. The morphology of the microwave-processed material is, different to some extent, from that of the material prepared by conventional heating. Delafossite CuAlO2 crystals with well-defined natural crystalline faces are formed. Interestingly, the morphology of the microwave-processed material is not similar to the morphology found for delafossite prepared by other procedures.
4. Conclusion Delafossite CuAlO2 can be prepared at low temperatures by conventional or microwave heating of aluminum nitrate and copper oxide during a short period of time. It seems that the copper oxide acts
Fig. 2. SEM micrograph of CuAlO2, a) prepared by conventional heating of aluminum nitrate and copper(II) oxide, b) prepared by microwave heating.
as a microwave-absorbing material for preparation of delafossite, and that the aluminum nitrate also has a pivotal role in delafossite formation by both conventional and microwave heating. Acknowledgements We thank the South Branch of Azad University and Shahid Beheshti University for supporting this work. References [1] Kawazoe M, Yasukawa H, Hyodo, Kurita M, Yanagi H, Hosono H. Nature 1997;389:939–42. [2] Park K, Ko KY, Seo W-S. J Eur Ceram Soc 2005;25:2219–22. [3] Banerjee AN, Chatropadhyay KK. Appl Surf Sci 2004;225:243–9. [4] Park K, Ko KY, Kwon H-C, Nahm S. J Alloys Compd 2007;437:1–6. [5] Ohashi M, Iida Y, Morikawa. J Am Ceram Soc 2002;85:270–2. [6] Wang Y, Gong H. Chem Vap Depos 2000;6:285–8. [7] Ong CH, Gong H. Thin Solid Films 2003;445:299–303. [8] Neumann-Spallart M, Pai SP, Pinto R. Thin Solid Films 2007;515:8641–4. [9] Banerjee AN, Kundoo S, Chatropadhyay KK. Thin Solid Films 2004;440:5–10. [10] Koriche N, Bouguelia A, Aider A, Trari M. Int J Hydrogen Energy 2005;30:639–8649. [11] Dloczik L, Tomn Y, Konenkamp R, Lux-Steiner MC, Dittrich TH. Thin Solids Films 2004;451–452:116–9. [12] Deng Z, Zhu X, Tao R, Dong W, Fang X. Mater Lett 2007;61:686–9. [13] Joint Committee Powder Diffraction File, Card No. 77-2493. [14] Smart LL, Moore L. Solid state chemistry. 3rd ed. New York: Chapman Hall; 1999. [15] Janney MA, Calhoun CL, Kimrey HD. J Am Ceram Soc 1992;75:341–6. [16] Kato M, Sakakibara K, Koike Y. Appl Supercond 1997;5:33–9. [17] Mingos DMP, Michael D, Baghurst DR. Chem Soc Rev 1991;20:1–47.