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WO3 nano-needles by Aerosol Assisted CVD for optical sensing
Available online at www.sciencedirect.com
ProcediaProcedia Engineering 00 (2011) 000–000 Engineering 25 (2011) 761 – 764
Procedia Engineering www.elsevier.com/locate/procedia
Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece
WO3 nano-needles by Aerosol Assisted CVD for optical sensing Muhammad U.Qadria,ba*, Toni Stoychevab, Maria Cinta Pujola, Eduard Llobetb, Xavier Correigb, Josep Ferre Borullb, Magdalena Aguilóa and Francesc Díaza a
Física i Cristal·lografía de Materials i Nanomaterials (FiCMA-FiCNA-EMAS), Universitat Rovira i Virgili, (URV), Campus Sescelades, c/Marcel·lí Domingo s/n, 43007 Tarragona, Spain. b Dept. d’Enginyeria Electronica i Automatica, EMAS, Universitat Rovira i Virgili (URV), Campus Sescelades, c/Marcel·lí Domingo s/n, 43007 Tarragona, Spain.
Abstract
A new method of synthesising nano-particle-functionalised nanostructured materials via Aerosol Assisted Chemical Vapour Deposition (AACVD) has been used to grow WO3 nano-needles. Co-deposition of Gold (Au) nanoparticles with WO3 nano-needles has been implemented to deposit a sensing layer, which has shown surface plasmon resonance. The performed optical and structural studies had demonstrated that these materials have a high potential for an optical gas sensing. © 2011 Published by Elsevier Ltd. Keywords:WO3; AACVD; WO3 Nano-needles; Optical sensing
1. Introduction With the growing of industry, the emission of different pollutant gases like CO2, NH3, H2 is increasing. In order to control the emissions from industry and other similar sources, we need sensors that can effectively detect these gases. Nowadays, existing gas sensors have certain issues related to their sensing properties like low selectivity, slow response and non reproducibility. This increases the demand to develop a gas sensor with faster response and higher selectivity. One solution is when the sensor operated on the principles of optics and photonics, this way the response time can be enhanced. Furthermore, by optimization of the growth and structural conditions we can overcome the selectivity issue. The problem is to have materials working on these principles and then to grow them. The applications of tungsten oxide (TO), especially in gas sensing, are in focus of the researchers for more than a decade. Moreover, * Corresponding author. Tel.: +34-977-558791; fax: +34977559605. E-mail address: [email protected].
1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.12.187
762
Muhammad U.Qadri et al. /Engineering Procedia Engineering 25 (2011) 761 – 764 Author name / Procedia 00 (2011) 000–000
TO has been of a great interest in the last few years due to its promising electrical and optical applications for gas sensing. Its functional properties can significantly be enhanced by lowering the feature size to nano-scale, structuring the film morphology and improving crystallographic texture of the film grains [1,2]. TO nanostructures (nanorods nanowires...etc) can offer a high surface to volume ratio, which could promote its sensitivity towards the gaseous components. The new growth techniques and state of the art characterization open more doors to pave the way for developing new devices with TO. Herein, we demonstrate the deposition of thin film by AACVD, which is a recently developed new method for deposition of TO nano-needles. AACVD is a variant of the traditional CVD process involving the use of liquid–gas aerosols to transport soluble precursors to a heated substrate. The method was traditionally been used when in a conventional atmospheric pressure CVD (APCVD), the precursor proves involatile or is thermally unstable. However, by designing precursors specifically for AACVD, the restrictions of volatility and thermal stability are eliminated and the range of precursors suitable for deposition is broadened. 1.1. Method of preparation A new method of synthesising nano-particle-functionalised nanostructured materials via AACVD has been used for deposition of TO nano-needles as a sensing layer on quartz substrate. Recently, this method has been used in the department of Electrical and Electronics Engineering of the University Rovira I Virgili (URV) to deposit WO3 nano-needles on alumina gas sensor substrates [3]. Considering that, in the series of our experiments we grow the TO nano-needles applying following procedure. A piezoelectric ultrasonic atomizer was used to generate an aerosol from a precursor W(OPh) 6 synthesised according to the literature [4], dissolved in a 50:50 (volume) mixer of acetone and toluene. For the synthesis of Aufunctionalised TO nano-needles a precursor mixer (5 mg HAuCl4. 3H2O (Sigma-Aldrich, 99.9%) in 2.5 cm3 methanol (Sigma-Aldrich, Z99.6%) and 75 mg W(OPh) 6 in 12.5 cm3 acetone (Sigma-Aldrich, min. 99.8%)) was prepared. Moreover, for all the experiments the concentration of the solution was kept 6.7Mmol and the precursor aerosol was transported to the heated substrate by a nitrogen (Carburos Metàlicos, N2 Premier) gas flow (0.5 Lmin-1). Under these conditions the time taken to transport the entire volume of the solution, i.e. the deposition time, was typically 45 min. The substrates used were 10 mm x10 mm x 1 mm Quartz with inter-digitated Pt electrodes (gap: 300 mm, thickness: 9 mm) on the top side of the surface and a Pt heater on the back side. The first experiment was made by applying the same deposition conditions used and described in the reference [3]. It was found, that when applying the same method on a quartz substrate and using the same conditions of deposition and solvents, a nano-crystalline film of TO nano-grains was grown. This result was different from the initial experiment where TO nanoneedles were grown. In the second part of the experiments the solvents were changed and the substrate temperature of the deposition was increased. This way we were able to grown TO nano-needles onto quartz substrate. The experimental conditions used are described in table 1. Table 1. The different samples and the experimental conditions used. Sample
Substrate Temperature T [ºC]
Deposition Time [minutes]
Precursor W(OPh)6 [mg]
Solvent
Morphology
1
500
60
100
Acetone +Toluene
Oriented Nano-grains
2
400
25
75 + Au(5)
Acetone+Methanol
Nano-grains
3
500
50
100
Acetone+Methanol
Nano-needles
763
Muhammad U.Qadri et al.name / Procedia Engineering 25 (2011) 761 – 764 Author / Procedia Engineering 00 (20111) 000–000
1.2. Structural characterization In order to understand the crystalline structure of the samples XRD measurements was made. A crystalline phase has been observed clearly for the pure TO samples, with only two peaks assigned to the crystallographic planes (020) and (040) of the P21/n space group belonging to the monoclinic system. XRD patterns are shown in fig. 1 (a). The grown structure is highly oriented in the (010) direction. This preferred orientation is similarly reported in [5]. Regarding to the XRD of the film with Au nano particles we did not observed any significant peaks neither from TO nor from Au. Furthermore, annealing procedure was conducted for these crystalline films. However, not significant changes in the phases and structure were observed.
(020)
2500 Sample 1 Sample 2 Sample 3 ICDD 83-0950
1500
1000
(040)
Intensity[arb.u]
2000
500
0
10
20
30
40
50
2 (a)
60
70
(b)
(c)
Fig 1. (a) XRD graph of the three different samples deposited according to the conditions described in the table 1; (b) ESEM image of the sample 1; (c) ESEM image of the sample 3; in (c) the nano-needles can be seen clearly.
1.3. Morphological characterization The environmental scanning electron microscopy (ESEM) was used to analyze the surface morphology of the TO nano-needles. The ESEM images taken are shown in the fig. 1 (b, c). We can see that the image of the sample 1 (fig. 1 b) it comprises of only nano-grains. In fig. 1 (c) is the image of the sample 3 and it comprises of the nano-needles however not crystalline. The size of the nano-needles is about 2-3 m in length and 90 -100 nm in diameter. Due to the very low dimension of the grains found in sample 2, it was not possible to take any image using ESEM. 1.4. Transmittance measurements The transmittance spectra of these nano-needles and nano-grains grown via AACVD have been performed using Varian cary scan 500 spectrometer. The obtained graphs are shown below in the fig. 2. The presence of Au nanoparticles has been confirmed by the existence of surface plasmon resonance (SPR) in the optical transmittance. As it was expected, the SPR band is located in the visible range, around 550 nm (seen in fig. 2). This evidence makes this material very promising for room temperature
764
Muhammad U.Qadri et al. /Engineering Procedia Engineering 25 (2011) 761 – 764 Author name / Procedia 00 (2011) 000–000
optical gas sensing. However, the film of the sample composite of oriented nano-grains was not enough transparent for the transmittance measurements.
Transmittance [%]
100
80
60
40
Sample 3 Sample 2
20
200
250
300
350
400
450
500
550
600
650
700
750
800
Wavelength [nm] Fig 2. Transmittance of the sample 2 and 3. SPR is present in sample 3 where the sample was decorated with Au nano-particles.
Conclusion Herein, we have demonstrated the fabrication of TO nano-needles via AACVD for optical sensing devices. The structural and morphological properties of newly grown TO films via AACVD have been investigated. The crystalline structure has been identified using XRD measurements, which show monoclinic phase. The performed transmittance measurement on these films suggests that it is an excellent future material for the optical sensing devices. Furthermore, for the Au decorated TO films we were able to see SPR in the visible green region around 550 nm. Acknowledgements This work was supported by the Spanish Government under projects MAT2008-06729-C0202/NAN,TEC2010-21574-C02-02, TEC2009-07107, PI09/90527 as well as by the Catalan Authority under projects 2009SGR235, 2009SGR 789 and by the Research Centre on Engineering of Materials and Systems (EMaS) of the URV. Muhammad U. Qadri thanks the Catalan Government for the funds provided through the fellowship 2011FI_B100069. References [1] Blackman C S, Parkin I P. Atmospheric Pressure Chemical Vapor Deposition of Crystalline Monoclinic WO3 and WO3-x Thin films from Reaction of WCl6 with O-containing Solvents and Their Photochromic and Electrochromic Properties. Chem. Mater. 2005; 17:1583-1590. [2] Ashraf S. The APCVD of tungsten oxide thin films from reaction of WCl6 with ethanol and results on the gas-sensing properties. Polyhedron 2008; 26:1493-1498. [3] Vallejos S, Stoycheva T, Umek P, Navio C, Snyders R, Bittencourt C, Llobet E, Blackman C, Moniz S, Correig X. Au nanoparticlefunctionalised WO3 nanoneedles and their application in high sensitivity gas sensor devices. Chem Comm 2010; DOI: 10.1039/c0cc02398a. [4] Warren B. Cross, Ivan P. Parkin, and Shane A. O’Neill. Tungsten Oxide Coatings from the Aerosol-Assisted Chemical Vapor Deposition of W(OAr)6 (Ar ) C6H5, C6H4F-4, C6H3F2-3,4); Photocatalytically Active γ-WO3 Films. Chem. Mater. 2003, 15, 2786-2796. [5] Palgrave R G. Parkin I P. Aerosol Assisted Chemical Vapor Deposition Using Nanoparticle Precursors: A Route to Nanocomposite Thin Films. J. Am. Chem. Soc. 2006;128: 1587-1597.
ProcediaProcedia Engineering 00 (2011) 000–000 Engineering 25 (2011) 761 – 764
Procedia Engineering www.elsevier.com/locate/procedia
Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece
WO3 nano-needles by Aerosol Assisted CVD for optical sensing Muhammad U.Qadria,ba*, Toni Stoychevab, Maria Cinta Pujola, Eduard Llobetb, Xavier Correigb, Josep Ferre Borullb, Magdalena Aguilóa and Francesc Díaza a
Física i Cristal·lografía de Materials i Nanomaterials (FiCMA-FiCNA-EMAS), Universitat Rovira i Virgili, (URV), Campus Sescelades, c/Marcel·lí Domingo s/n, 43007 Tarragona, Spain. b Dept. d’Enginyeria Electronica i Automatica, EMAS, Universitat Rovira i Virgili (URV), Campus Sescelades, c/Marcel·lí Domingo s/n, 43007 Tarragona, Spain.
Abstract
A new method of synthesising nano-particle-functionalised nanostructured materials via Aerosol Assisted Chemical Vapour Deposition (AACVD) has been used to grow WO3 nano-needles. Co-deposition of Gold (Au) nanoparticles with WO3 nano-needles has been implemented to deposit a sensing layer, which has shown surface plasmon resonance. The performed optical and structural studies had demonstrated that these materials have a high potential for an optical gas sensing. © 2011 Published by Elsevier Ltd. Keywords:WO3; AACVD; WO3 Nano-needles; Optical sensing
1. Introduction With the growing of industry, the emission of different pollutant gases like CO2, NH3, H2 is increasing. In order to control the emissions from industry and other similar sources, we need sensors that can effectively detect these gases. Nowadays, existing gas sensors have certain issues related to their sensing properties like low selectivity, slow response and non reproducibility. This increases the demand to develop a gas sensor with faster response and higher selectivity. One solution is when the sensor operated on the principles of optics and photonics, this way the response time can be enhanced. Furthermore, by optimization of the growth and structural conditions we can overcome the selectivity issue. The problem is to have materials working on these principles and then to grow them. The applications of tungsten oxide (TO), especially in gas sensing, are in focus of the researchers for more than a decade. Moreover, * Corresponding author. Tel.: +34-977-558791; fax: +34977559605. E-mail address: [email protected].
1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.12.187
762
Muhammad U.Qadri et al. /Engineering Procedia Engineering 25 (2011) 761 – 764 Author name / Procedia 00 (2011) 000–000
TO has been of a great interest in the last few years due to its promising electrical and optical applications for gas sensing. Its functional properties can significantly be enhanced by lowering the feature size to nano-scale, structuring the film morphology and improving crystallographic texture of the film grains [1,2]. TO nanostructures (nanorods nanowires...etc) can offer a high surface to volume ratio, which could promote its sensitivity towards the gaseous components. The new growth techniques and state of the art characterization open more doors to pave the way for developing new devices with TO. Herein, we demonstrate the deposition of thin film by AACVD, which is a recently developed new method for deposition of TO nano-needles. AACVD is a variant of the traditional CVD process involving the use of liquid–gas aerosols to transport soluble precursors to a heated substrate. The method was traditionally been used when in a conventional atmospheric pressure CVD (APCVD), the precursor proves involatile or is thermally unstable. However, by designing precursors specifically for AACVD, the restrictions of volatility and thermal stability are eliminated and the range of precursors suitable for deposition is broadened. 1.1. Method of preparation A new method of synthesising nano-particle-functionalised nanostructured materials via AACVD has been used for deposition of TO nano-needles as a sensing layer on quartz substrate. Recently, this method has been used in the department of Electrical and Electronics Engineering of the University Rovira I Virgili (URV) to deposit WO3 nano-needles on alumina gas sensor substrates [3]. Considering that, in the series of our experiments we grow the TO nano-needles applying following procedure. A piezoelectric ultrasonic atomizer was used to generate an aerosol from a precursor W(OPh) 6 synthesised according to the literature [4], dissolved in a 50:50 (volume) mixer of acetone and toluene. For the synthesis of Aufunctionalised TO nano-needles a precursor mixer (5 mg HAuCl4. 3H2O (Sigma-Aldrich, 99.9%) in 2.5 cm3 methanol (Sigma-Aldrich, Z99.6%) and 75 mg W(OPh) 6 in 12.5 cm3 acetone (Sigma-Aldrich, min. 99.8%)) was prepared. Moreover, for all the experiments the concentration of the solution was kept 6.7Mmol and the precursor aerosol was transported to the heated substrate by a nitrogen (Carburos Metàlicos, N2 Premier) gas flow (0.5 Lmin-1). Under these conditions the time taken to transport the entire volume of the solution, i.e. the deposition time, was typically 45 min. The substrates used were 10 mm x10 mm x 1 mm Quartz with inter-digitated Pt electrodes (gap: 300 mm, thickness: 9 mm) on the top side of the surface and a Pt heater on the back side. The first experiment was made by applying the same deposition conditions used and described in the reference [3]. It was found, that when applying the same method on a quartz substrate and using the same conditions of deposition and solvents, a nano-crystalline film of TO nano-grains was grown. This result was different from the initial experiment where TO nanoneedles were grown. In the second part of the experiments the solvents were changed and the substrate temperature of the deposition was increased. This way we were able to grown TO nano-needles onto quartz substrate. The experimental conditions used are described in table 1. Table 1. The different samples and the experimental conditions used. Sample
Substrate Temperature T [ºC]
Deposition Time [minutes]
Precursor W(OPh)6 [mg]
Solvent
Morphology
1
500
60
100
Acetone +Toluene
Oriented Nano-grains
2
400
25
75 + Au(5)
Acetone+Methanol
Nano-grains
3
500
50
100
Acetone+Methanol
Nano-needles
763
Muhammad U.Qadri et al.name / Procedia Engineering 25 (2011) 761 – 764 Author / Procedia Engineering 00 (20111) 000–000
1.2. Structural characterization In order to understand the crystalline structure of the samples XRD measurements was made. A crystalline phase has been observed clearly for the pure TO samples, with only two peaks assigned to the crystallographic planes (020) and (040) of the P21/n space group belonging to the monoclinic system. XRD patterns are shown in fig. 1 (a). The grown structure is highly oriented in the (010) direction. This preferred orientation is similarly reported in [5]. Regarding to the XRD of the film with Au nano particles we did not observed any significant peaks neither from TO nor from Au. Furthermore, annealing procedure was conducted for these crystalline films. However, not significant changes in the phases and structure were observed.
(020)
2500 Sample 1 Sample 2 Sample 3 ICDD 83-0950
1500
1000
(040)
Intensity[arb.u]
2000
500
0
10
20
30
40
50
2 (a)
60
70
(b)
(c)
Fig 1. (a) XRD graph of the three different samples deposited according to the conditions described in the table 1; (b) ESEM image of the sample 1; (c) ESEM image of the sample 3; in (c) the nano-needles can be seen clearly.
1.3. Morphological characterization The environmental scanning electron microscopy (ESEM) was used to analyze the surface morphology of the TO nano-needles. The ESEM images taken are shown in the fig. 1 (b, c). We can see that the image of the sample 1 (fig. 1 b) it comprises of only nano-grains. In fig. 1 (c) is the image of the sample 3 and it comprises of the nano-needles however not crystalline. The size of the nano-needles is about 2-3 m in length and 90 -100 nm in diameter. Due to the very low dimension of the grains found in sample 2, it was not possible to take any image using ESEM. 1.4. Transmittance measurements The transmittance spectra of these nano-needles and nano-grains grown via AACVD have been performed using Varian cary scan 500 spectrometer. The obtained graphs are shown below in the fig. 2. The presence of Au nanoparticles has been confirmed by the existence of surface plasmon resonance (SPR) in the optical transmittance. As it was expected, the SPR band is located in the visible range, around 550 nm (seen in fig. 2). This evidence makes this material very promising for room temperature
764
Muhammad U.Qadri et al. /Engineering Procedia Engineering 25 (2011) 761 – 764 Author name / Procedia 00 (2011) 000–000
optical gas sensing. However, the film of the sample composite of oriented nano-grains was not enough transparent for the transmittance measurements.
Transmittance [%]
100
80
60
40
Sample 3 Sample 2
20
200
250
300
350
400
450
500
550
600
650
700
750
800
Wavelength [nm] Fig 2. Transmittance of the sample 2 and 3. SPR is present in sample 3 where the sample was decorated with Au nano-particles.
Conclusion Herein, we have demonstrated the fabrication of TO nano-needles via AACVD for optical sensing devices. The structural and morphological properties of newly grown TO films via AACVD have been investigated. The crystalline structure has been identified using XRD measurements, which show monoclinic phase. The performed transmittance measurement on these films suggests that it is an excellent future material for the optical sensing devices. Furthermore, for the Au decorated TO films we were able to see SPR in the visible green region around 550 nm. Acknowledgements This work was supported by the Spanish Government under projects MAT2008-06729-C0202/NAN,TEC2010-21574-C02-02, TEC2009-07107, PI09/90527 as well as by the Catalan Authority under projects 2009SGR235, 2009SGR 789 and by the Research Centre on Engineering of Materials and Systems (EMaS) of the URV. Muhammad U. Qadri thanks the Catalan Government for the funds provided through the fellowship 2011FI_B100069. References [1] Blackman C S, Parkin I P. Atmospheric Pressure Chemical Vapor Deposition of Crystalline Monoclinic WO3 and WO3-x Thin films from Reaction of WCl6 with O-containing Solvents and Their Photochromic and Electrochromic Properties. Chem. Mater. 2005; 17:1583-1590. [2] Ashraf S. The APCVD of tungsten oxide thin films from reaction of WCl6 with ethanol and results on the gas-sensing properties. Polyhedron 2008; 26:1493-1498. [3] Vallejos S, Stoycheva T, Umek P, Navio C, Snyders R, Bittencourt C, Llobet E, Blackman C, Moniz S, Correig X. Au nanoparticlefunctionalised WO3 nanoneedles and their application in high sensitivity gas sensor devices. Chem Comm 2010; DOI: 10.1039/c0cc02398a. [4] Warren B. Cross, Ivan P. Parkin, and Shane A. O’Neill. Tungsten Oxide Coatings from the Aerosol-Assisted Chemical Vapor Deposition of W(OAr)6 (Ar ) C6H5, C6H4F-4, C6H3F2-3,4); Photocatalytically Active γ-WO3 Films. Chem. Mater. 2003, 15, 2786-2796. [5] Palgrave R G. Parkin I P. Aerosol Assisted Chemical Vapor Deposition Using Nanoparticle Precursors: A Route to Nanocomposite Thin Films. J. Am. Chem. Soc. 2006;128: 1587-1597.