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Synthesis and characterization of hexagonally packed mesoporous tantalum oxide thin film
December 2002
Materials Letters 57 (2002) 444 – 447 www.elsevier.com/locate/matlet
Synthesis and characterization of hexagonally packed mesoporous tantalum oxide thin film Bharat L. Newalkar a, Sridhar Komarneni a,*, Hiroaki Katsuki b a
Materials Research Laboratory, Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA b Saga Ceramics Research Laboratory, 3037-7, Arita-machi, Saga 844-0024, Japan Received 16 March 2002; accepted 19 March 2002
Abstract A mesoporous thin film form of tantalum oxide, a wide band gap and semiconductor ceramic, is prepared by templating a tantalum silica precursor (tantalum ethoxide) with pluronic P123 copolymer (EO20PO70EO20; M.W. 5800) using sol – gel-based spin casting approach. The obtained crystalline phase was characterized by means of X-ray diffraction, nitrogen adsorption – desorption measurements at 196 jC and transmission electron microscopy (TEM) techniques. Such form of tantalum oxide is believed to have potential applications in novel devices such as solar cells and photocatalysts. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Sol – gel synthesis; Mesoporous tantalum oxide; Thin film
1. Introduction Recently, tantalum oxide (Ta2O5) has attracted much attention due to its semiconducting and wide band gap properties. Its thin film form is extensively investigated as a component layer in optical coatings, solid state ion sensors, dynamic memory capacitor structures, and metal oxide semiconductor transistors [1– 8]. Of late, mesoporous tantalum oxide is also found to be of technological importance due to its ability to decompose water in the visible light [9,10]. In view of such high application potential of tantalum oxide, efforts need to be focused on the synthesis of
*
Corresponding author. Tel.: +1-814-865-1542; fax: +1-814865-2326. E-mail address: [email protected] (S. Komarneni).
its mesoporous form. However, very few reports have appeared in this regard. Until recently, the only reported efforts of mesoporous Ta2O5 materials were of powder materials with hexagonal structure [11,12]. These materials are usually prepared using ligandassisted and supramolecular templating methods [11,12]. However, to maximize the utility of the Ta2O5 material for photocatalytic or device applications, it is essential to obtain this oxide in the form of a thin film. However, it is believed to be difficult due to the strong tendency of tantalum precursors to precipitate and crystallize. Therefore, to accomplish the aforementioned goal, we herein propose a supramolecular templating approach using sol – gel methodology. Thus, in the present study, a thin film of mesoporous Ta2O5 is obtained by templating tantalum ethoxide with pluronic P123 copolymer (EO20PO70 EO20; M.W. 5800) in the pH range of 2.0– 2.5. Recent
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 0 8 - X
B.L. Newalkar et al. / Materials Letters 57 (2002) 444–447
445
to remove the polymer template. Thus obtained film was characterized by means of low angle X-ray diffraction (Philips X’pert powder diffractometer system), transmission electron microscopy (Jeol, JEM-2010, Japan) and nitrogen adsorption – desorption measurements (Autosorb, AS-1, Quantachrome, USA) at 196 jC. X-ray diffraction patterns were recorded using CuKa radiation with a 0.02j step size and 1-s step time over the range 0.5j<2h<10j.
3. Results and discussion Fig. 1 illustrates typical X-ray diffraction patterns observed for as-prepared and annealed tantalum oxide thin films. The as-synthesized tantalum oxide film ˚ displayed a primary diffraction peak with d=75 A (Fig. 1a), and was found to be similar for the annealed sample (Fig. 1b). Such pattern is believed to be of an ordered hexagonal phase as the used template is anticipated to form a liquid crystalline phase under
Fig. 1. X-ray diffraction patterns for (a) annealed, and (b) calcined tantalum oxide thin film.
studies have clearly established the templating potential of P123 to obtain mesoporous silicas [13].
2. Experimental Typically, 1.25 g of P123 (Aldrich) was dissolved in 14 g of 2-propanol (Aldrich) and mixed with a precursor solution. The precursor solution was obtained by mixing 3 g tantalum ethoxide (Aldrich), 0.53 g acetylacetone and 4 g of 2-propanol. The sol thus obtained was homogenized for 4 h under stirring and was hydrolyzed at a low pH of about 2.0 by the addition of dilute hydrochloric acid for 5 h under stirred conditions. The sol so obtained was then deposited on glass slides by spin coating at a speed of 2000 rpm for 10 s. The deposited film on glass slide was aged at 80 jC for 3 days and then annealed at 150 jC for 1 day and finally calcined at 350 jC for 4 h in order
Fig. 2. TEM micrograph for the calcined mesoporous tantalum oxide thin film.
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B.L. Newalkar et al. / Materials Letters 57 (2002) 444–447
the present experimental conditions [13]. Thus, the observed X-ray diffraction pattern is indexed with hexagonal symmetry with a unit cell parameter (a0) ˚ . However, the X-ray diffraction pattern of 86.6 A showed structural degradation upon calcination (Fig. 1b), as revealed by the transformation of an ordered mesoporous phase to a disordered phase. Thus, the obtained data seem to indicate that the optimum conditions for synthesizing highly ordered films exist at a pH value of about 2.0, but the calcination conditions employed in the present study are found to have detrimental effect on the structural uniformity of the film. Such effect is also seen from the microscopy study. A typical TEM micrograph obtained for the calcined film is shown in Fig. 2, wherein the presence of mesoporous network is noticed but showed a lack of ordered mesoporosity which is usually observed for
templated mesoporous silica obtained under similar conditions. The textural properties of mesoporous oxide framework after heating at 350 jC were also investigated. The measured nitrogen adsorption – desorption isotherm (Fig. 3) is of Type IV in nature but without a pronounced condensation step as per the IUPAC classification and exhibited a H1 hysteresis loop, which is typical of mesoporous solids [14]. However, the textural properties like surface area and pore volume for the tantalum oxide framework are found to be 162 m2/g and 0.14 cm3/g, respectively, which are lower than those observed for mesoporous silica obtained via similar approach. This trend is found to be consistent with those observed on the basis of Xray diffraction and transmission electron microscopy (TEM) analysis and further reinforced the structural degradation of the formed mesoporous framework
Fig. 3. Nitrogen adsorption – desorption isotherms for calcined mesoporous tantalum oxide. An inset shows a pore size distribution curve for the formed mesoporous framework based on BJH approach.
B.L. Newalkar et al. / Materials Letters 57 (2002) 444–447
under present calcination temperature. The pore size distribution (PSD) for the tantalum oxide framework was calculated by applying BJH approach [14] using adsorption branch of the measured isotherm and is also shown in Fig. 3. The PSD curve thus obtained showed a narrow pore size distribution with an ˚. average pore size of about 37 A In summary, a mesoprous tantalum oxide thin film is successfully fabricated using sol – gel approach. However, further studies with regard to calcination process seem to be important in order to retain the structural characteristics of the thin film upon calcination. Tantalum oxide films of high surface area need to be developed for potential applications in designing novel devices such as solar cells and photocatalysts.
Acknowledgements The authors gratefully acknowledge the support of this work by the NSF MRSEC program under grant number DMR-0080019.
447
References [1] S. Jakobs, A. Duparre, M. Huter, H.K. Pulker, Thin Solid Films 351 (1999) 141. [2] U. Teravaninthorn, Y. Miyahara, T. Moriizumi, Jpn. J. Appl. Phys. 26 (1987) 2116. [3] B.C. Lai, N. Kung, J.Y. Lee, J. Appl. Phys. 85 (1999) 4087. [4] R.A.B. Devine, C. Chaneliere, J.L. Autran, B. Balland, B. Baillet, J.L. Leray, Microelectron. Eng. 36 (1997) 61. [5] M. Houssa, R. Degraeve, P.W. Martens, M.M. Heyns, J.S. Jeon, A. Halliyal, B. Ogle, J. Appl. Phys. 86 (1999) 6462. [6] P.K. Roy, I.C. Kizilyalli, Appl. Phys. Lett. 72 (1998) 2835. [7] B.C. Lai, J.Y. Lee, J. Electrochem. Soc. 146 (1999) 266. [8] H. Treichel, A. Mitwalsky, N.P. Sandler, D. Tribula, W. Kern, A.P. Lane, Adv. Mater. Opt. Electron. 1 (1992) 299. [9] Y. Takahara, J.N. Kondo, T. Takata, D. Lu, K. Domen, Chem. Mater. 13 (2001) 1194. [10] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 615. [11] D.M. Antonelli, J.Y. Ying, Chem. Mater. 8 (1996) 874. [12] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater. 11 (1999) 2813. [13] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [14] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, 1982.
Materials Letters 57 (2002) 444 – 447 www.elsevier.com/locate/matlet
Synthesis and characterization of hexagonally packed mesoporous tantalum oxide thin film Bharat L. Newalkar a, Sridhar Komarneni a,*, Hiroaki Katsuki b a
Materials Research Laboratory, Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA b Saga Ceramics Research Laboratory, 3037-7, Arita-machi, Saga 844-0024, Japan Received 16 March 2002; accepted 19 March 2002
Abstract A mesoporous thin film form of tantalum oxide, a wide band gap and semiconductor ceramic, is prepared by templating a tantalum silica precursor (tantalum ethoxide) with pluronic P123 copolymer (EO20PO70EO20; M.W. 5800) using sol – gel-based spin casting approach. The obtained crystalline phase was characterized by means of X-ray diffraction, nitrogen adsorption – desorption measurements at 196 jC and transmission electron microscopy (TEM) techniques. Such form of tantalum oxide is believed to have potential applications in novel devices such as solar cells and photocatalysts. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Sol – gel synthesis; Mesoporous tantalum oxide; Thin film
1. Introduction Recently, tantalum oxide (Ta2O5) has attracted much attention due to its semiconducting and wide band gap properties. Its thin film form is extensively investigated as a component layer in optical coatings, solid state ion sensors, dynamic memory capacitor structures, and metal oxide semiconductor transistors [1– 8]. Of late, mesoporous tantalum oxide is also found to be of technological importance due to its ability to decompose water in the visible light [9,10]. In view of such high application potential of tantalum oxide, efforts need to be focused on the synthesis of
*
Corresponding author. Tel.: +1-814-865-1542; fax: +1-814865-2326. E-mail address: [email protected] (S. Komarneni).
its mesoporous form. However, very few reports have appeared in this regard. Until recently, the only reported efforts of mesoporous Ta2O5 materials were of powder materials with hexagonal structure [11,12]. These materials are usually prepared using ligandassisted and supramolecular templating methods [11,12]. However, to maximize the utility of the Ta2O5 material for photocatalytic or device applications, it is essential to obtain this oxide in the form of a thin film. However, it is believed to be difficult due to the strong tendency of tantalum precursors to precipitate and crystallize. Therefore, to accomplish the aforementioned goal, we herein propose a supramolecular templating approach using sol – gel methodology. Thus, in the present study, a thin film of mesoporous Ta2O5 is obtained by templating tantalum ethoxide with pluronic P123 copolymer (EO20PO70 EO20; M.W. 5800) in the pH range of 2.0– 2.5. Recent
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 0 8 - X
B.L. Newalkar et al. / Materials Letters 57 (2002) 444–447
445
to remove the polymer template. Thus obtained film was characterized by means of low angle X-ray diffraction (Philips X’pert powder diffractometer system), transmission electron microscopy (Jeol, JEM-2010, Japan) and nitrogen adsorption – desorption measurements (Autosorb, AS-1, Quantachrome, USA) at 196 jC. X-ray diffraction patterns were recorded using CuKa radiation with a 0.02j step size and 1-s step time over the range 0.5j<2h<10j.
3. Results and discussion Fig. 1 illustrates typical X-ray diffraction patterns observed for as-prepared and annealed tantalum oxide thin films. The as-synthesized tantalum oxide film ˚ displayed a primary diffraction peak with d=75 A (Fig. 1a), and was found to be similar for the annealed sample (Fig. 1b). Such pattern is believed to be of an ordered hexagonal phase as the used template is anticipated to form a liquid crystalline phase under
Fig. 1. X-ray diffraction patterns for (a) annealed, and (b) calcined tantalum oxide thin film.
studies have clearly established the templating potential of P123 to obtain mesoporous silicas [13].
2. Experimental Typically, 1.25 g of P123 (Aldrich) was dissolved in 14 g of 2-propanol (Aldrich) and mixed with a precursor solution. The precursor solution was obtained by mixing 3 g tantalum ethoxide (Aldrich), 0.53 g acetylacetone and 4 g of 2-propanol. The sol thus obtained was homogenized for 4 h under stirring and was hydrolyzed at a low pH of about 2.0 by the addition of dilute hydrochloric acid for 5 h under stirred conditions. The sol so obtained was then deposited on glass slides by spin coating at a speed of 2000 rpm for 10 s. The deposited film on glass slide was aged at 80 jC for 3 days and then annealed at 150 jC for 1 day and finally calcined at 350 jC for 4 h in order
Fig. 2. TEM micrograph for the calcined mesoporous tantalum oxide thin film.
446
B.L. Newalkar et al. / Materials Letters 57 (2002) 444–447
the present experimental conditions [13]. Thus, the observed X-ray diffraction pattern is indexed with hexagonal symmetry with a unit cell parameter (a0) ˚ . However, the X-ray diffraction pattern of 86.6 A showed structural degradation upon calcination (Fig. 1b), as revealed by the transformation of an ordered mesoporous phase to a disordered phase. Thus, the obtained data seem to indicate that the optimum conditions for synthesizing highly ordered films exist at a pH value of about 2.0, but the calcination conditions employed in the present study are found to have detrimental effect on the structural uniformity of the film. Such effect is also seen from the microscopy study. A typical TEM micrograph obtained for the calcined film is shown in Fig. 2, wherein the presence of mesoporous network is noticed but showed a lack of ordered mesoporosity which is usually observed for
templated mesoporous silica obtained under similar conditions. The textural properties of mesoporous oxide framework after heating at 350 jC were also investigated. The measured nitrogen adsorption – desorption isotherm (Fig. 3) is of Type IV in nature but without a pronounced condensation step as per the IUPAC classification and exhibited a H1 hysteresis loop, which is typical of mesoporous solids [14]. However, the textural properties like surface area and pore volume for the tantalum oxide framework are found to be 162 m2/g and 0.14 cm3/g, respectively, which are lower than those observed for mesoporous silica obtained via similar approach. This trend is found to be consistent with those observed on the basis of Xray diffraction and transmission electron microscopy (TEM) analysis and further reinforced the structural degradation of the formed mesoporous framework
Fig. 3. Nitrogen adsorption – desorption isotherms for calcined mesoporous tantalum oxide. An inset shows a pore size distribution curve for the formed mesoporous framework based on BJH approach.
B.L. Newalkar et al. / Materials Letters 57 (2002) 444–447
under present calcination temperature. The pore size distribution (PSD) for the tantalum oxide framework was calculated by applying BJH approach [14] using adsorption branch of the measured isotherm and is also shown in Fig. 3. The PSD curve thus obtained showed a narrow pore size distribution with an ˚. average pore size of about 37 A In summary, a mesoprous tantalum oxide thin film is successfully fabricated using sol – gel approach. However, further studies with regard to calcination process seem to be important in order to retain the structural characteristics of the thin film upon calcination. Tantalum oxide films of high surface area need to be developed for potential applications in designing novel devices such as solar cells and photocatalysts.
Acknowledgements The authors gratefully acknowledge the support of this work by the NSF MRSEC program under grant number DMR-0080019.
447
References [1] S. Jakobs, A. Duparre, M. Huter, H.K. Pulker, Thin Solid Films 351 (1999) 141. [2] U. Teravaninthorn, Y. Miyahara, T. Moriizumi, Jpn. J. Appl. Phys. 26 (1987) 2116. [3] B.C. Lai, N. Kung, J.Y. Lee, J. Appl. Phys. 85 (1999) 4087. [4] R.A.B. Devine, C. Chaneliere, J.L. Autran, B. Balland, B. Baillet, J.L. Leray, Microelectron. Eng. 36 (1997) 61. [5] M. Houssa, R. Degraeve, P.W. Martens, M.M. Heyns, J.S. Jeon, A. Halliyal, B. Ogle, J. Appl. Phys. 86 (1999) 6462. [6] P.K. Roy, I.C. Kizilyalli, Appl. Phys. Lett. 72 (1998) 2835. [7] B.C. Lai, J.Y. Lee, J. Electrochem. Soc. 146 (1999) 266. [8] H. Treichel, A. Mitwalsky, N.P. Sandler, D. Tribula, W. Kern, A.P. Lane, Adv. Mater. Opt. Electron. 1 (1992) 299. [9] Y. Takahara, J.N. Kondo, T. Takata, D. Lu, K. Domen, Chem. Mater. 13 (2001) 1194. [10] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 615. [11] D.M. Antonelli, J.Y. Ying, Chem. Mater. 8 (1996) 874. [12] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater. 11 (1999) 2813. [13] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [14] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, 1982.