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nanostructures using aerosol assisted chemical vapour deposition
Materials Letters 62 (2008) 4582–4584
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 ev i e r. c o m / l o c a t e / m a t l e t
Templated growth of tungsten oxide micro/nanostructures using aerosol assisted chemical vapour deposition C.S. Blackman a, X. Correig b,⁎, V. Katko b, A. Mozalev c, I.P. Parkin a, R. Alcubilla d, T. Trifonov d a
Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK Department d’Enginyeria Electronica, Universitat Rovira i Virgili, 43007 Tarragona, Spain Department of Micro- and Nanoelectronics, Belarusian State University of Informatics and Radioelectronics, 220013 Minsk, Belarus d Department d’Enginyeria Electronica, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain b c
a r t i c l e
i n f o
Article history: Received 11 July 2008 Accepted 21 August 2008 Available online 24 August 2008 Keywords: Chemical vapour deposition Electron microscopy Microstructure Tungsten oxide
a b s t r a c t Porous anodized alumina (PAA) and macroporous silicon (MS) substrates have been used to template the growth of tungsten oxide via aerosol assisted chemical vapour deposition from the precursor tungsten hexaphenoxide. The results show that thin PAA substrates have potential as templates for growing microstructured tungsten oxide films and MS substrates cause the growth of ‘grids’ of polycrystalline tungsten oxide. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The synthesis and functionalisation of micro- and nanostructured materials is a key research area in materials science and has potential application in a diverse range of functional devices, with one such example being gas-sensing [1]. Sensor performance is dependent on percolation paths of electrons through intergranular regions and research has focussed on replacing thick-film sensors manufactured from powders with those utilising thin-film techniques. Thin-film sensors offer higher reproducibility compared to their thick-film congeners but suffer reduced sensing performance due to lower porosity. Micro- and nanostructured materials can overcome this problem by having high surface area forms, and in the case of nanomaterials the massively enhanced surface/volume ratio augments the role of surface states in the sensor response. Therefore highly structured materials can bring benefits to the three ‘S’ of sensor technology (sensitivity, selectivity and stability). However successful exploitation of these materials requires they be produced in a controlled and ordered manner. One method that has been successfully exploited is template mediated growth using zeolites, membranes or nanotubes [2], where the size, shape and structural properties of the material is controlled by the template. Porous anodic alumina (PAA) is a suitable template for the synthesis of ordered structures because it has controllable pore diameters ranging from 5 to 300 nm, narrow pore size distribution and good mechanical and thermal stability [3]. Macroporous silicon (MS), prepared by ⁎ Corresponding author. Tel.: +34 977559623; fax: +34 977559605. E-mail address: [email protected] (X. Correig). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.08.027
photochemical etching, is composed of an ordered array of pores with diameters in the range 2–5 µm and depths of 200 µm and also has potential as a template [4]. Chemical vapour deposition (CVD) is applicable to producing highly structured materials and MetalOrganic (MO) CVD has been used to grow In2O3 [5] and CdS [6] nanotube arrays using PAA templates, resulting in well-aligned nanotubes within the pores, and it has also been used to infiltrate mesoporous TiO2 with CdS, leading to filling throughout the pore volume [7]. In the same report Aerosol-Assisted (AA) CVD was used to infiltrate CuInS2 into identical mesoporous substrates. Tungsten oxide is a well-known gas-sensor material and previously tungsten hexaphenoxide ([W(OC6H5)6]) has been used to deposit tungsten oxide films using AACVD [8]. Here we report the use of tungsten hexaphenoxide to prepare micro- and nanostructured films of
Fig. 1. SEM of tungsten oxide deposition at 450 °C on thin PAA substrate.
C.S. Blackman et al. / Materials Letters 62 (2008) 4582–4584
4583
between 0.2 and 2.0 dm3/min. The deposition time varied depending on solvent volume and carrier flow but was typically 20–60 min. PAA templates were prepared by anodizing a thin aluminium layer, sputter deposited on a silicon wafer, in 0.4 mol/dm3 tartaric acid electrolyte [9]. The average pore diameter in the PAA template was 300 nm, with an aspect ratio (pore depth:pore diameter) of 2:1. MS substrates with an aspect ratio of 100:1 were prepared by photo-assisted electrochemical etching of n-type silicon wafers in hydrofluoric acid (HF) solution [4]. XRD, Raman, EDX and SEM were used to analyse the deposited films [10]. 3. Results and discussion
Fig. 2. Side-on SEM of tungsten oxide deposition on MS template at 450 °C (inset, topdown image with scale bar representing 10 µm).
Fig. 3. Side-on SEM of tungsten oxide deposition on MS templates at 450 °C for 2 h (inset, top-down image).
tungsten oxide using PAA and MS templates, for potential use in gassensing applications. 2. Experimental The synthesis of ([W(OC6H5)6]) and details of the AACVD apparatus have been reported previously [8]. Depositions were carried out with typically 0.25 g [W(OC6H6)6] dissolved in 50 cm3 of toluene, at reactor temperatures in the range 300–600 °C with carrier N2 flow rates
Deposition using thin PAA templates at temperatures below 600 °C deposited dense, polycrystalline WO2.9 films (determined by XRD, Raman and EDX), as reported previously [8], and complete infiltration of thin PAA templates was observed (Fig. 1). We are currently investigating the removal of the alumina support to provide a high surface area structured tungsten oxide monolith, for use in gas-sensing. Deposition below 600 °C on MS templates yielded dense polycrystalline WO2.9 (XRD, Raman, EDX), which followed the geometry of the substrate closely, with very little deposition in the voids of the structure. Fig. 2 shows a side-on image of a cleaved MS template after deposition at 450 °C, with the differentiation between the template and the overlaying coating clearly contrasting. Inset a top-down image of the coated MS template is shown. It is clear from Fig. 2 that although some deposition is seen in the voids of the structure the film thickness diminished rapidly. Increasing four-fold the amount of precursor transported led to preferential deposition on the surface of the existing tungsten oxide layer as opposed to infilling, and although the pores diameter were narrowed they were not closed (Fig. 3). Under these conditions the film thickness was ~ 2 µm, which suggests that the ‘film’ could be self-supporting. The removal of the silicon substrate ‘template’ would provide a porous tungsten oxide grid and research into achieving this is ongoing. At 600 °C rather than a dense polycrystalline film, as obtained at lower temperatures, the deposition had a completely different morphology composed of nanowires of crystalline WO2.9 (XRD, Raman, EDX), and this was observed on both PAA and MS templates. The left hand image of Fig. 4 shows a SEM image of deposition on a PAA template, with the nanowires extending down into the pores, and also a side-on image of a cleaved MS template where the hair-like nanowire growth is clearly visible within the void. In the furthest right hand image this growth is clearly visible down their length. The deposition in the voids of the MS templates penetrated to at least a depth of 50 µm from the surface, with little sign of decreasing nanowire density at this depth. Further imaging was not possible because the cleave plane was not clean, but it is assumed that deposition continued beyond 50 µm. The nanowires originate from the wall surface, which combined with the depth at which growth was observed suggests that the growth is a surface phenomenon and not simply gas phase reaction, in which case we would expect material to have simply landed on the substrate surface. The difference between the low and high temperature growth mechanisms of tungsten oxide from [W(OC6H5)6], whilst not previously identified, has been observed in the literature, with depositions carried out at 550 °C [8] giving dense polycrystalline material, whilst those at 600 °C were composed of nanowires [11]. Previously the nanowire growth at 600 °C was attributed to the presence of an electric field, however whilst the electric field has clearly influenced the orientation of the growth in that work our results show that the temperature of reaction is the most important factor in determining the gross morphology.
Fig. 4. SEM of tungsten oxide deposition at 600 °C on PAA and MS templates.
4584
C.S. Blackman et al. / Materials Letters 62 (2008) 4582–4584
4. Conclusions
Appendix A. Supplementary data
We have shown below a critical temperature thin PAA substrates can be completely infiltrated with tungsten oxide, indicating their potential for growing highly structured films. Similar experiments on MS substrates demonstrated growth is localised principally at the top surface of the substrate, with little penetration of the voids within the substrate structure. However this causes the growth of grids of tungsten oxide, which could have potential for application in gassensing. Further, a dramatic change in the deposition mechanism of tungsten oxide from the precursor [W(OC6H5)6] has been identified between 550 and 600 °C, which leads to a dramatically different nanowire morphology at high deposition temperature.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2008.08.027. References [1] [2] [3] [4] [5] [6] [7] [8]
Acknowledgements X. Correig acknowledges the Ministerio de Ciencia y Tecnologia for the mobility grant PR-2006-0273. I.P. Parkin thanks the Royal Society/ Wolfson Trust for a merit award.
[9] [10] [11]
Comini E. Anal Chim Acta 2006;568:28. Hernández-Vélez M. Thin Solid Films 2006;495:51. Martin CR. Science 1994;266:1961. Trifonov T, Garin M, Rodríguez A, Marsal LF, Alcubilla R. Phys Stat Sol (a) 2007;204:3237. Shen XP, Liu HJ, Fan X, Jiang Y, Hong JM, Xu Z. J Cryst Growth 2005;276:471. Shen XP, Yuan AH, Wang F, Hogn JM, Xu Z. Solid State Commun 2005;133:19. Waters JP, Smyth-Boyle D, Govender K, Green A, Durrant J, O’Brien P. Chem Vap Deposition 2005;11:254. Cross WB, O’Neill SA, Parkin IP, Williams PA, Mahon MF, Molloy KC. Chem Mater 2003;15:2786. Khatko V, Mozalev A, Gorokh G, Solovei D, Guirado F, Llobet E, et al. J Electrochem Soc 2008;155:K116. Ashraf S, Binions R, Blackman CS, Parkin IP. Polyhedron 2007;26:1493. Parkin IP, Pratt KFE, Shaw G, Williams DE. J Mater Chem 2004;15:149.
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 ev i e r. c o m / l o c a t e / m a t l e t
Templated growth of tungsten oxide micro/nanostructures using aerosol assisted chemical vapour deposition C.S. Blackman a, X. Correig b,⁎, V. Katko b, A. Mozalev c, I.P. Parkin a, R. Alcubilla d, T. Trifonov d a
Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK Department d’Enginyeria Electronica, Universitat Rovira i Virgili, 43007 Tarragona, Spain Department of Micro- and Nanoelectronics, Belarusian State University of Informatics and Radioelectronics, 220013 Minsk, Belarus d Department d’Enginyeria Electronica, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain b c
a r t i c l e
i n f o
Article history: Received 11 July 2008 Accepted 21 August 2008 Available online 24 August 2008 Keywords: Chemical vapour deposition Electron microscopy Microstructure Tungsten oxide
a b s t r a c t Porous anodized alumina (PAA) and macroporous silicon (MS) substrates have been used to template the growth of tungsten oxide via aerosol assisted chemical vapour deposition from the precursor tungsten hexaphenoxide. The results show that thin PAA substrates have potential as templates for growing microstructured tungsten oxide films and MS substrates cause the growth of ‘grids’ of polycrystalline tungsten oxide. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The synthesis and functionalisation of micro- and nanostructured materials is a key research area in materials science and has potential application in a diverse range of functional devices, with one such example being gas-sensing [1]. Sensor performance is dependent on percolation paths of electrons through intergranular regions and research has focussed on replacing thick-film sensors manufactured from powders with those utilising thin-film techniques. Thin-film sensors offer higher reproducibility compared to their thick-film congeners but suffer reduced sensing performance due to lower porosity. Micro- and nanostructured materials can overcome this problem by having high surface area forms, and in the case of nanomaterials the massively enhanced surface/volume ratio augments the role of surface states in the sensor response. Therefore highly structured materials can bring benefits to the three ‘S’ of sensor technology (sensitivity, selectivity and stability). However successful exploitation of these materials requires they be produced in a controlled and ordered manner. One method that has been successfully exploited is template mediated growth using zeolites, membranes or nanotubes [2], where the size, shape and structural properties of the material is controlled by the template. Porous anodic alumina (PAA) is a suitable template for the synthesis of ordered structures because it has controllable pore diameters ranging from 5 to 300 nm, narrow pore size distribution and good mechanical and thermal stability [3]. Macroporous silicon (MS), prepared by ⁎ Corresponding author. Tel.: +34 977559623; fax: +34 977559605. E-mail address: [email protected] (X. Correig). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.08.027
photochemical etching, is composed of an ordered array of pores with diameters in the range 2–5 µm and depths of 200 µm and also has potential as a template [4]. Chemical vapour deposition (CVD) is applicable to producing highly structured materials and MetalOrganic (MO) CVD has been used to grow In2O3 [5] and CdS [6] nanotube arrays using PAA templates, resulting in well-aligned nanotubes within the pores, and it has also been used to infiltrate mesoporous TiO2 with CdS, leading to filling throughout the pore volume [7]. In the same report Aerosol-Assisted (AA) CVD was used to infiltrate CuInS2 into identical mesoporous substrates. Tungsten oxide is a well-known gas-sensor material and previously tungsten hexaphenoxide ([W(OC6H5)6]) has been used to deposit tungsten oxide films using AACVD [8]. Here we report the use of tungsten hexaphenoxide to prepare micro- and nanostructured films of
Fig. 1. SEM of tungsten oxide deposition at 450 °C on thin PAA substrate.
C.S. Blackman et al. / Materials Letters 62 (2008) 4582–4584
4583
between 0.2 and 2.0 dm3/min. The deposition time varied depending on solvent volume and carrier flow but was typically 20–60 min. PAA templates were prepared by anodizing a thin aluminium layer, sputter deposited on a silicon wafer, in 0.4 mol/dm3 tartaric acid electrolyte [9]. The average pore diameter in the PAA template was 300 nm, with an aspect ratio (pore depth:pore diameter) of 2:1. MS substrates with an aspect ratio of 100:1 were prepared by photo-assisted electrochemical etching of n-type silicon wafers in hydrofluoric acid (HF) solution [4]. XRD, Raman, EDX and SEM were used to analyse the deposited films [10]. 3. Results and discussion
Fig. 2. Side-on SEM of tungsten oxide deposition on MS template at 450 °C (inset, topdown image with scale bar representing 10 µm).
Fig. 3. Side-on SEM of tungsten oxide deposition on MS templates at 450 °C for 2 h (inset, top-down image).
tungsten oxide using PAA and MS templates, for potential use in gassensing applications. 2. Experimental The synthesis of ([W(OC6H5)6]) and details of the AACVD apparatus have been reported previously [8]. Depositions were carried out with typically 0.25 g [W(OC6H6)6] dissolved in 50 cm3 of toluene, at reactor temperatures in the range 300–600 °C with carrier N2 flow rates
Deposition using thin PAA templates at temperatures below 600 °C deposited dense, polycrystalline WO2.9 films (determined by XRD, Raman and EDX), as reported previously [8], and complete infiltration of thin PAA templates was observed (Fig. 1). We are currently investigating the removal of the alumina support to provide a high surface area structured tungsten oxide monolith, for use in gas-sensing. Deposition below 600 °C on MS templates yielded dense polycrystalline WO2.9 (XRD, Raman, EDX), which followed the geometry of the substrate closely, with very little deposition in the voids of the structure. Fig. 2 shows a side-on image of a cleaved MS template after deposition at 450 °C, with the differentiation between the template and the overlaying coating clearly contrasting. Inset a top-down image of the coated MS template is shown. It is clear from Fig. 2 that although some deposition is seen in the voids of the structure the film thickness diminished rapidly. Increasing four-fold the amount of precursor transported led to preferential deposition on the surface of the existing tungsten oxide layer as opposed to infilling, and although the pores diameter were narrowed they were not closed (Fig. 3). Under these conditions the film thickness was ~ 2 µm, which suggests that the ‘film’ could be self-supporting. The removal of the silicon substrate ‘template’ would provide a porous tungsten oxide grid and research into achieving this is ongoing. At 600 °C rather than a dense polycrystalline film, as obtained at lower temperatures, the deposition had a completely different morphology composed of nanowires of crystalline WO2.9 (XRD, Raman, EDX), and this was observed on both PAA and MS templates. The left hand image of Fig. 4 shows a SEM image of deposition on a PAA template, with the nanowires extending down into the pores, and also a side-on image of a cleaved MS template where the hair-like nanowire growth is clearly visible within the void. In the furthest right hand image this growth is clearly visible down their length. The deposition in the voids of the MS templates penetrated to at least a depth of 50 µm from the surface, with little sign of decreasing nanowire density at this depth. Further imaging was not possible because the cleave plane was not clean, but it is assumed that deposition continued beyond 50 µm. The nanowires originate from the wall surface, which combined with the depth at which growth was observed suggests that the growth is a surface phenomenon and not simply gas phase reaction, in which case we would expect material to have simply landed on the substrate surface. The difference between the low and high temperature growth mechanisms of tungsten oxide from [W(OC6H5)6], whilst not previously identified, has been observed in the literature, with depositions carried out at 550 °C [8] giving dense polycrystalline material, whilst those at 600 °C were composed of nanowires [11]. Previously the nanowire growth at 600 °C was attributed to the presence of an electric field, however whilst the electric field has clearly influenced the orientation of the growth in that work our results show that the temperature of reaction is the most important factor in determining the gross morphology.
Fig. 4. SEM of tungsten oxide deposition at 600 °C on PAA and MS templates.
4584
C.S. Blackman et al. / Materials Letters 62 (2008) 4582–4584
4. Conclusions
Appendix A. Supplementary data
We have shown below a critical temperature thin PAA substrates can be completely infiltrated with tungsten oxide, indicating their potential for growing highly structured films. Similar experiments on MS substrates demonstrated growth is localised principally at the top surface of the substrate, with little penetration of the voids within the substrate structure. However this causes the growth of grids of tungsten oxide, which could have potential for application in gassensing. Further, a dramatic change in the deposition mechanism of tungsten oxide from the precursor [W(OC6H5)6] has been identified between 550 and 600 °C, which leads to a dramatically different nanowire morphology at high deposition temperature.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matlet.2008.08.027. References [1] [2] [3] [4] [5] [6] [7] [8]
Acknowledgements X. Correig acknowledges the Ministerio de Ciencia y Tecnologia for the mobility grant PR-2006-0273. I.P. Parkin thanks the Royal Society/ Wolfson Trust for a merit award.
[9] [10] [11]
Comini E. Anal Chim Acta 2006;568:28. Hernández-Vélez M. Thin Solid Films 2006;495:51. Martin CR. Science 1994;266:1961. Trifonov T, Garin M, Rodríguez A, Marsal LF, Alcubilla R. Phys Stat Sol (a) 2007;204:3237. Shen XP, Liu HJ, Fan X, Jiang Y, Hong JM, Xu Z. J Cryst Growth 2005;276:471. Shen XP, Yuan AH, Wang F, Hogn JM, Xu Z. Solid State Commun 2005;133:19. Waters JP, Smyth-Boyle D, Govender K, Green A, Durrant J, O’Brien P. Chem Vap Deposition 2005;11:254. Cross WB, O’Neill SA, Parkin IP, Williams PA, Mahon MF, Molloy KC. Chem Mater 2003;15:2786. Khatko V, Mozalev A, Gorokh G, Solovei D, Guirado F, Llobet E, et al. J Electrochem Soc 2008;155:K116. Ashraf S, Binions R, Blackman CS, Parkin IP. Polyhedron 2007;26:1493. Parkin IP, Pratt KFE, Shaw G, Williams DE. J Mater Chem 2004;15:149.