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SAW sensor based on highly sensitive nanoporous palladium thin film for hydrogen detection
Microelectronic Engineering 108 (2013) 218–221
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
SAW sensor based on highly sensitive nanoporous palladium thin film for hydrogen detection Cristian Viespe, Constantin Grigoriu ⇑ National Institute for Laser, Plasma and Radiation Physics, Atomistilor 409, Bucharest-Magurele 077125, Romania
a r t i c l e
i n f o
Article history: Available online 14 December 2012 Keywords: SAW sensor Hydrogen sensor Nanoporous film Palladium Pulsed laser deposition
a b s t r a c t Surface acoustic wave sensors (SAWSs) with nanoporous palladium (Pd) sensing material for hydrogen (H2) detection are reported. We fabricated sensors with a stable and fast response and with a high sensitivity for detecting H2 even at room temperature (RT). The fabricated sensors were ‘delay line’ type (quartz substrate, 70 MHz central frequency). The nanoporous sensitive layer was directly deposited onto a quartz substrate using the picosecond laser ablation method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to investigate the influence of different experimental conditions, such as laser power density and inert gas pressure, on the morphological proprieties of Pd thin films. The sensor performance (sensitivity, detection limit, and response time) at RT, for H2 concentrations in synthetic air between 0.008% and 2% were studied. We obtained a sensitivity and detection limit of 0.31 Hz/ppm and 48 ppm, respectively, for RT operation. The response time was between 15 and 44 s for H2 concentrations between 0.2% and 0.8% in synthetic air. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The European Union (EU) announced plans in October 2008 to designate hydrogen (H2) fuel cells as one of Europe’s leading strategic energy technologies of the future. Projects worth one billion Euros (over 6 years) were launched to enable the EU to take a leading global position in the race to develop and commercialise H2 fuel cells. However, the increasing use of H2 gas has drawbacks. Mixtures of H2 and air are highly explosive in ratios higher than 4.6 vol.% [1] and highly flammable at concentrations lower than 4% [2]. For this reason, it is very important to develop a highly sensitive H2 detector with a fast response and that is capable of continuously monitoring the gas concentration. Various types of sensor technologies, such as electrochemical [3,4], conductometric [5], Schottky junction [6], field effect [7], optical [8], surface acoustic wave (SAW) [9–11], and bulk acoustic wave [12], have been developed and used for H2 detection. However, the fast and precise detection of the presence of H2, particularly at concentrations lower than the explosive concentration at room temperature (RT), remains difficult [13]. Surface acoustic wave sensors (SAWs) are one of the most promising detection systems due to their smaller size, lower weight, minimal power requirements, and high sensitivity. The sensitivity and selectivity of SAWS are determined largely by the nature of the sensing film. Over the past few years, most SAWS re-
⇑ Corresponding author. Tel.: +40 214574027; fax: +40 214574243. E-mail address: grigoriu@ifin.nipne.ro (C. Grigoriu). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.12.001
search has focused on improving the performance of the films by using different nanostructured materials, such as palladium (Pd) nanoparticles [14,15], tungsten oxide (WO3) nanorods and nanoparticles [16], WO3 and Pd nanostructures [13,17] or polyaniline nanofibres [2]. In this study, the picosecond laser ablation method was used for the first time, to the best of our knowledge, to deposit a sensitive layer composed of nanoporous Pd. Using lasers with picosecond pulses or with a high repetition rate leads to major changes in the ablation process and, thus, implicitly, in the deposited film structure. In this case, nanoscale particles were ejected in the ablation process, in relatively large quantities, and deposited at very high rates. Using such a regime, at a certain ambient gas pressure a nanoporous film can be directly obtained. Nanoporous films are highly advantageous in SAWS because they facilitate rapid diffusion of gases in and out of the sensor, resulting in faster response and recovery times; furthermore, the higher surface areas of nanoporous films lead to considerably improved sensitivity. In this study, we investigated the influence of pressure and power density, which are the two experimental parameters with the greatest influence on the particle size and film structure [18]. Our goal was to obtain a high specific surface area nanoporous film with nanoscale particles. Pd was chosen for the sensitive layer material for its good H2 detection performance [12–15,17,19–22]. Sensing occurs via absorption of gas molecules on the functional H2–Pd layer. Gas absorption perturbs the proprieties of SAWs that travel on a piezoelectric substrate, resulting in frequency changes. The nanoporous sensitive layer was directly deposited onto a quartz substrate using the picosecond laser ablation method.
C. Viespe, C. Grigoriu / Microelectronic Engineering 108 (2013) 218–221
2. Experimental A layered SAW device was used as the transducing platform. The SAWS delay line was formed onto an ST-X quartz substrate using standard photolithographic methods. The SAWS operation frequency was 70 MHz, and the acoustic wavelength was 45.11 lm [23, 24]. A DHPVA-100 FEMTO (10–60 dB, 100 MHz) amplifier was used, and the frequency shift of the system was obtained using a CNT-90 Pendulum counter analyser using Time View 2.1 software with a resolution of 12 digits/s. The impedance was matched to the external circuit (50 O) by adding appropriate inductances. A network/ spectrum/impedance analyser (Agilent 4396B) with a transmission/reflection test kit (Agilent 87512A/B) was used to determine the optimum value of the inductors, as well as for signal attenuation and phase measurement. The Pd layer was directly deposited onto the quartz substrate using the laser ablation method with an neodymium-doped yttrium orthovanadate (Nd:YVO4) picosecond laser (10 ps pulse duration; 50 kHz repetition frequency, 2 h deposition time, 0.8 W average power, and 532 nm wavelength) in a continuous argon (Ar) flow of 0.5 sccm (Fig. 1). Pd films were obtained by ablation of a pure Pd target (99.95%) in an Ar atmosphere in the range of 50–300 mTorr with 0.5 sccm gas flow rates. The target–substrate distance was 4 cm. Before deposition, the chamber was evacuated to a base pressure of 10 5 Torr. The Ar flow rate and chamber pressure were controlled using an MKS monitoring system. To deposit a high-quality nanoporous sensitive layer, it is very important to have the same target roughness across the entire ablated surface during the deposition process. Therefore, special software was developed to correlate the target movement (XY axis), the laser repetition rate, and the size of the laser spot (moving the lens). The influence of various experimental conditions on the morphological proprieties of the Pd thin film was investigated, including power density (160 MW/cm2 and 250 MW/cm2) and inert gas pressure (50, 150, and 300 mTorr). For sensor testing, a mixture of H2 and synthetic air was prepared using a mass flow controller system (MKS Systems) connected to two cylinders: one with a mixture of H2 and synthetic air (2% H2/98% synthetic air) and the other one with 100% synthetic air. During testing, the total flow rate (H2 & synthetic air) was kept constant at 0.5 L/min to eliminate the influence of the flow rate on the results. Flow rate can influence the sensor baseline signal and
Fig. 1. System for deposition of a nanoporous Pd thin film.
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the response time, because higher flow rates can cause cooling, which results in a small frequency shift [25]. The morphological proprieties of the Pd thin film samples were investigated by X-ray diffraction (XRD, Panalytical X’Pert) and Scanning Electron Microscopy (SEM, FEI QUANTA).
3. Results and discussion SEM results (Fig. 2) reveal no notable differences between the Pd film structures deposited at a power density of 160 MW/cm2 and pressures of: (a) 50 mTorr; (b) 150 mTorr; and (c) 300 mTorr. However, an important difference was observed between sensors deposited at 160 MW/cm2 (Fig. 2c) and 250 MW/cm2 (Fig. 2d) at the same deposition pressure of 150 mTorr. At a power density of 160 MW/cm2, the film is nanoporous, while at 250 MW/cm2, the film is dense. The density of the Pd film depends on the kinetic energy of the ablated species [26–28]. As the laser power density is increased, the nanoparticle clusters are ejected from the target at higher velocities towards substrate (higher kinetic energy). The thermalisation and diffusion of the species on the substrate are essential to for determining the film structure. If the Pd nanoparticle clusters impinging on the substrate have a kinetic energy greater than surface diffusion, their mobility will be increased at the surface and they will rearrange so that a dense nonporous film will grow. Fig. 3 presents the XRD patterns of the films deposited at RT and various Ar pressures. The patterns reveal the formation of the pure polycrystalline face-centred cubic Pd phase. The Scherrer mean crystallite size values were in the range of 10–13 nm for all Pd cases. These figures reveal that the formation of the Pd phase is minimally affected by the Ar pressure in the deposition chamber and strongly affected by the laser power density. Films deposited at a power density of 160 MW/cm2 (50, 150, and 300 mTorr) exhibited crystalline structure corresponding to Pd diffraction peaks (1 1 1), (2 0 0), (2 2 0) and (3 3 1). In contrast, no peaks corresponding to the crystalline phase of Pd were observed in the spectrum for the film deposited at 250 MW/cm2. The structure of the SAWS sensitive layer is a critical parameter influencing performance. Nanostructure porosity facilitates rapid diffusion of gases in and out of the material, and a greater surface area will improve the sensitivity. Taking into account the results presented above, the following experimental conditions were chosen for depositing the H2-sensitive layers: Ar pressure of 300 mTorr, power density of 160 MW/cm2, repetition rate of 50 kHz, and 2 h deposition time. All of the results presented below are based on sensors with sensitive layers prepared under these conditions. Fig. 4a presents the Pd nanoparticle size distribution. Size distribution was determined from SEM images (for example, Fig. 2c) by measurement of approximately 1000 nanoparticles (image area 25 lm2). The particle distribution was found to be in the range of 25–160 nm, with more than 60% of the particles in the range of 60–90 nm. No notable differences were observed between the distributions of nanoparticles located in different parts of the film. The thickness of the sensitive layer, as indicated by the SEM image (Fig. 4b), was 1.6 lm. Fig. 5 plots the frequency shifts of the nanoporous Pd-based SAWS upon exposure to different H2 gas concentrations. The frequency shift was proportional to the concentration for H2 concentrations in the range of 0.008–2%. The dynamic response of SAWS to H2 gas (Fig. 6) revealed that the sensor response is proportional to the H2 concentration and is reversible. The response times were 15, 33, and 44 s for H2 concentrations of 0.2%, 0.4%, and 0.8%, respectively. The response times were longer at higher concentration because more time was
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Fig. 2. SEM images of the surface layer at: (a) 50 mTorr, 160 MW/cm2; (b) 150 mTorr, 160 MW/cm2; (c) 300 mTorr, 160 MW/cm2; and (d) 150 mTorr, 250 MW/cm2.
Fig. 4. Sensitive layer of Pd (300 mTorr, 160 MW/cm2): (a) nanoparticle size distribution and (b) cross section SEM image.
Fig. 3. XRD patterns of the Pd film samples: (1) 50 mTorr, 160 MW/cm2; (2) 150 mTorr, 160 MW/cm2; (3) 300 mTorr, 160 MW/cm2; and (4) 150 mTorr, 250 MW/cm2.
required to reach sensor stability. Moreover, the absorption time (33 s) at H2 concentrations in the range of 0.2–0.4% is approximately equal to the desorption time (34 s) for the same gas concentrations. Jakubik [9] and Joshi et al. [14] reported response times between 100 and 300 s, approximately 10 times longer than the response times of the sensors reported herein. Sunil et al. [15] reported a better response (10 s) using Pd nanoparticles mixed
with a Nafion (a membrane materials with a high surface area) as the sensitive layer, but these results were obtained by measuring resistance instead of frequency. Abe et al. [21] reported a new type of sensor with a response time of 1 s, but this sensor was more complicated to fabricate because it employed a piezoelectric ball as a substrate. The sensitivity at 0.31 Hz/ppm, defined as the frequency shift in Hz per unit analyte concentration in ppm, was determined from the slope of a linear curve-fit of the data, as shown in Fig. 5. The noise assessment was performed in air (without gas) by measuring the frequency fluctuation over 10 min; frequency fluctuation represents the maximum frequency deviation from the trend line (best-fit line). The detection limit (defined as three times the noise level divided by the sensitivity) was 48 ppm. Nonporous layers of Pd, even when employed in a multilayer combination with another material like WO3, did not exhibit sufficient sensitivity for H2 detection [9,13]. For example, Jakubik [9] obtained a detection
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160 nm. Response times of 15–44 s were obtained for concentrations between 0.2% and 0.8% H2 in synthetic air. The sensitivity and detection limit at RT were 0.31 Hz/ppm and 48 ppm, respectively, for sensors based on films deposited at 300 mTorr, 160 MW/cm2, and 50 kHz for 2 h. These results indicated that high sensitivities can be achieved utilising a nanoporous layer structure for gas sensing applications. Acknowledgements This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PN-II-RU-PD-2011-3-0141. References Fig. 5. Frequency shift for Pd layer-based SAWS, to H2 gas at RT.
Fig. 6. Dynamic response of Pd layer-based SAWS, to H2 gas at RT.
limit of 2% H2 in air at 25 °C for a single Pd layer. The same researchers, using a bilayer of WO3 and Pd, detected H2 in a concentration range from 1% to 4% in air [17]. Yamanaka et al. [21,22] obtained a better result (10 ppm) using a new type of sensor based on a ball SAW device. 4. Conclusions SAWS with Pd nanoporous sensing material were produced and tested for H2 sensitivity in synthetic air at RT. Good results were obtained even at concentrations less than 0.01%. The nanoporous film was made of Pd nanoparticles with sizes in the range of 25–
[1] C. Christofides, A. Mandelis, J. Appl. Phys. 68 (1990) R1–R30. [2] R. Arsat, X.F. Yu, Y.X. Li, W. Wlodarski, K. Kalantar-zadeh, Sens. Actuators B 137 (2009) 529–532. [3] A.K. Yagati, T. Lee, J. Min, J.W. Choi, Colloid Surf. B Biointerfaces 92 (2012) 161– 167. [4] K. Nakamoto, R. Kurita, O. Niwa, Anal. Chem. 84 (2012) 3187–3191. [5] A. Moafi, J.G. Partridge, A.Z. Sadek, D.W.M. Lau, K. Kalantar-zadeh, D.G. McCulloch, IEEE Sens. J. 11 (9) (2011) 1913–1916. [6] F. Hossein-Babaei, M. Purahmad, in: Proceedings of the Third International Conference on Sensing Technology, 2008, pp. 72–75. [7] E. Becker, M. Andersson, M. Eriksson, A.L. Spetz, M. Skoglundh, IEEE Sens. J. 11 (7) (2011) 1527–1534. [8] R.J. Westerwaal, N. Duim, I. Nieuwenhuijse, C. Perrotton, A. Dabirian, J.M. van Leeuwen, V. Palmisano, B. Dam, Sens. Actuators B 165 (2012) 88–96. [9] W.P. Jakubik, Thin Solid Films 515 (2007) 8345–8350. [10] Z. Sadek, W. Wlodarski, Y.X. Li, W. Yu, X. Li, X. Yu, K. Kalantar-zadeh, Thin Solid Films 515 (24) (2007) 8705–8708. [11] M.Z. Atashbar, A.Z. Sadek, W. Wlodarski, S. Sriram, M. Bhaskaran, C.J. Cheng, R.B. Kaner, K. Kalantar-zadeh, Sens. Actuators B 138 (2009) 85–89. [12] D. Chen, J.J. Wang, D.H. Li, Y. Xu, Sens. Actuators B 159 (2011) 234–237. [13] W.P. Jakubik, Thin Solid Films 517 (2009) 6188–6191. [14] R.K. Joshi, S. Krishnan, M. Yoshimura, A. Kumar, Nanoscale Res. Lett. 4 (2009) 1191–1196. [15] K.A. Sunil, S. Krishnan, K. McGrath, F. Rinaldi, S. Bhansali, Procedia Eng. 5 (2010) 168–171. [16] A. Penzoni, V. Russo, A. Bailini, C.S. Casari, M. Ferroni, A. Li Bassi, A. Migliori, V. Morandi, L. Ortolani, G. Sberveglieri, C.E. Bottani, Sens. Actuators B 153 (2011) 340–346. [17] W. Jakubik, M. Urbanczyk, E. Maciuk, Procedia Chem. 1 (2009) 200–203. [18] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [19] T. Hejczyk, M. Urbanczyk, Acta Phys. Pol. A 120 (4) (2011) 616–620. [20] J.S. Noh, J.M. Lee, W. Lee, Sensors 11 (2011) 825–851. [21] T. Abe, N. Iwata, T. Tsuji, T. Mihara, S. Akao, K. Noguchi, N. Nakaso, D. Sim, Y. Fukiura, H. Tanaka, K. Yamanaka, Jpn. J. Appl. Phys. 46 (2007) 4726–4728. [22] K. Yamanaka, S. Ishikawa, N. Nakaso, N. Takeda, D.Y. Sim, T. Mihara, A. Mizukami, I. Satoh, S. Akao, Y. Tsukahara, IEEE Trans. UFFC 53 (2006) 793–801. [23] C. Viespe, C. Grigoriu, Sens. Actuators B 147 (2010) 43–47. [24] I. Nicolae, C. Viespe, C. Grigoriu, Sens. Actuators B 158 (2011) 418–422. [25] D.S. Ballantine, R.M. White, S.J. Martin, A.J. Ricco, E.T. Zellers, G.C. Frye, H. Wohtjen, Acoustic Wave Sensors, Theory, Design, and Physico-Chemical Applications, Academic Press, 1997. [26] D. Riabinina, M. Chaker, F. Rosei, Appl. Phys. Lett. 89 (2006) 131501. [27] D. Riabinina, E. Irissou, B.L. Drogoff, M. Chaker, D. Guay, J. Appl. Phys. 108 (2010) 034322. [28] D. Cattaneo, N. Righetti, C.S. Casari, A. Li Bassi, C.E. Bottani, Appl. Surf. Sci. 253 (2007) 7917–7921.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
SAW sensor based on highly sensitive nanoporous palladium thin film for hydrogen detection Cristian Viespe, Constantin Grigoriu ⇑ National Institute for Laser, Plasma and Radiation Physics, Atomistilor 409, Bucharest-Magurele 077125, Romania
a r t i c l e
i n f o
Article history: Available online 14 December 2012 Keywords: SAW sensor Hydrogen sensor Nanoporous film Palladium Pulsed laser deposition
a b s t r a c t Surface acoustic wave sensors (SAWSs) with nanoporous palladium (Pd) sensing material for hydrogen (H2) detection are reported. We fabricated sensors with a stable and fast response and with a high sensitivity for detecting H2 even at room temperature (RT). The fabricated sensors were ‘delay line’ type (quartz substrate, 70 MHz central frequency). The nanoporous sensitive layer was directly deposited onto a quartz substrate using the picosecond laser ablation method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to investigate the influence of different experimental conditions, such as laser power density and inert gas pressure, on the morphological proprieties of Pd thin films. The sensor performance (sensitivity, detection limit, and response time) at RT, for H2 concentrations in synthetic air between 0.008% and 2% were studied. We obtained a sensitivity and detection limit of 0.31 Hz/ppm and 48 ppm, respectively, for RT operation. The response time was between 15 and 44 s for H2 concentrations between 0.2% and 0.8% in synthetic air. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The European Union (EU) announced plans in October 2008 to designate hydrogen (H2) fuel cells as one of Europe’s leading strategic energy technologies of the future. Projects worth one billion Euros (over 6 years) were launched to enable the EU to take a leading global position in the race to develop and commercialise H2 fuel cells. However, the increasing use of H2 gas has drawbacks. Mixtures of H2 and air are highly explosive in ratios higher than 4.6 vol.% [1] and highly flammable at concentrations lower than 4% [2]. For this reason, it is very important to develop a highly sensitive H2 detector with a fast response and that is capable of continuously monitoring the gas concentration. Various types of sensor technologies, such as electrochemical [3,4], conductometric [5], Schottky junction [6], field effect [7], optical [8], surface acoustic wave (SAW) [9–11], and bulk acoustic wave [12], have been developed and used for H2 detection. However, the fast and precise detection of the presence of H2, particularly at concentrations lower than the explosive concentration at room temperature (RT), remains difficult [13]. Surface acoustic wave sensors (SAWs) are one of the most promising detection systems due to their smaller size, lower weight, minimal power requirements, and high sensitivity. The sensitivity and selectivity of SAWS are determined largely by the nature of the sensing film. Over the past few years, most SAWS re-
⇑ Corresponding author. Tel.: +40 214574027; fax: +40 214574243. E-mail address: grigoriu@ifin.nipne.ro (C. Grigoriu). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.12.001
search has focused on improving the performance of the films by using different nanostructured materials, such as palladium (Pd) nanoparticles [14,15], tungsten oxide (WO3) nanorods and nanoparticles [16], WO3 and Pd nanostructures [13,17] or polyaniline nanofibres [2]. In this study, the picosecond laser ablation method was used for the first time, to the best of our knowledge, to deposit a sensitive layer composed of nanoporous Pd. Using lasers with picosecond pulses or with a high repetition rate leads to major changes in the ablation process and, thus, implicitly, in the deposited film structure. In this case, nanoscale particles were ejected in the ablation process, in relatively large quantities, and deposited at very high rates. Using such a regime, at a certain ambient gas pressure a nanoporous film can be directly obtained. Nanoporous films are highly advantageous in SAWS because they facilitate rapid diffusion of gases in and out of the sensor, resulting in faster response and recovery times; furthermore, the higher surface areas of nanoporous films lead to considerably improved sensitivity. In this study, we investigated the influence of pressure and power density, which are the two experimental parameters with the greatest influence on the particle size and film structure [18]. Our goal was to obtain a high specific surface area nanoporous film with nanoscale particles. Pd was chosen for the sensitive layer material for its good H2 detection performance [12–15,17,19–22]. Sensing occurs via absorption of gas molecules on the functional H2–Pd layer. Gas absorption perturbs the proprieties of SAWs that travel on a piezoelectric substrate, resulting in frequency changes. The nanoporous sensitive layer was directly deposited onto a quartz substrate using the picosecond laser ablation method.
C. Viespe, C. Grigoriu / Microelectronic Engineering 108 (2013) 218–221
2. Experimental A layered SAW device was used as the transducing platform. The SAWS delay line was formed onto an ST-X quartz substrate using standard photolithographic methods. The SAWS operation frequency was 70 MHz, and the acoustic wavelength was 45.11 lm [23, 24]. A DHPVA-100 FEMTO (10–60 dB, 100 MHz) amplifier was used, and the frequency shift of the system was obtained using a CNT-90 Pendulum counter analyser using Time View 2.1 software with a resolution of 12 digits/s. The impedance was matched to the external circuit (50 O) by adding appropriate inductances. A network/ spectrum/impedance analyser (Agilent 4396B) with a transmission/reflection test kit (Agilent 87512A/B) was used to determine the optimum value of the inductors, as well as for signal attenuation and phase measurement. The Pd layer was directly deposited onto the quartz substrate using the laser ablation method with an neodymium-doped yttrium orthovanadate (Nd:YVO4) picosecond laser (10 ps pulse duration; 50 kHz repetition frequency, 2 h deposition time, 0.8 W average power, and 532 nm wavelength) in a continuous argon (Ar) flow of 0.5 sccm (Fig. 1). Pd films were obtained by ablation of a pure Pd target (99.95%) in an Ar atmosphere in the range of 50–300 mTorr with 0.5 sccm gas flow rates. The target–substrate distance was 4 cm. Before deposition, the chamber was evacuated to a base pressure of 10 5 Torr. The Ar flow rate and chamber pressure were controlled using an MKS monitoring system. To deposit a high-quality nanoporous sensitive layer, it is very important to have the same target roughness across the entire ablated surface during the deposition process. Therefore, special software was developed to correlate the target movement (XY axis), the laser repetition rate, and the size of the laser spot (moving the lens). The influence of various experimental conditions on the morphological proprieties of the Pd thin film was investigated, including power density (160 MW/cm2 and 250 MW/cm2) and inert gas pressure (50, 150, and 300 mTorr). For sensor testing, a mixture of H2 and synthetic air was prepared using a mass flow controller system (MKS Systems) connected to two cylinders: one with a mixture of H2 and synthetic air (2% H2/98% synthetic air) and the other one with 100% synthetic air. During testing, the total flow rate (H2 & synthetic air) was kept constant at 0.5 L/min to eliminate the influence of the flow rate on the results. Flow rate can influence the sensor baseline signal and
Fig. 1. System for deposition of a nanoporous Pd thin film.
219
the response time, because higher flow rates can cause cooling, which results in a small frequency shift [25]. The morphological proprieties of the Pd thin film samples were investigated by X-ray diffraction (XRD, Panalytical X’Pert) and Scanning Electron Microscopy (SEM, FEI QUANTA).
3. Results and discussion SEM results (Fig. 2) reveal no notable differences between the Pd film structures deposited at a power density of 160 MW/cm2 and pressures of: (a) 50 mTorr; (b) 150 mTorr; and (c) 300 mTorr. However, an important difference was observed between sensors deposited at 160 MW/cm2 (Fig. 2c) and 250 MW/cm2 (Fig. 2d) at the same deposition pressure of 150 mTorr. At a power density of 160 MW/cm2, the film is nanoporous, while at 250 MW/cm2, the film is dense. The density of the Pd film depends on the kinetic energy of the ablated species [26–28]. As the laser power density is increased, the nanoparticle clusters are ejected from the target at higher velocities towards substrate (higher kinetic energy). The thermalisation and diffusion of the species on the substrate are essential to for determining the film structure. If the Pd nanoparticle clusters impinging on the substrate have a kinetic energy greater than surface diffusion, their mobility will be increased at the surface and they will rearrange so that a dense nonporous film will grow. Fig. 3 presents the XRD patterns of the films deposited at RT and various Ar pressures. The patterns reveal the formation of the pure polycrystalline face-centred cubic Pd phase. The Scherrer mean crystallite size values were in the range of 10–13 nm for all Pd cases. These figures reveal that the formation of the Pd phase is minimally affected by the Ar pressure in the deposition chamber and strongly affected by the laser power density. Films deposited at a power density of 160 MW/cm2 (50, 150, and 300 mTorr) exhibited crystalline structure corresponding to Pd diffraction peaks (1 1 1), (2 0 0), (2 2 0) and (3 3 1). In contrast, no peaks corresponding to the crystalline phase of Pd were observed in the spectrum for the film deposited at 250 MW/cm2. The structure of the SAWS sensitive layer is a critical parameter influencing performance. Nanostructure porosity facilitates rapid diffusion of gases in and out of the material, and a greater surface area will improve the sensitivity. Taking into account the results presented above, the following experimental conditions were chosen for depositing the H2-sensitive layers: Ar pressure of 300 mTorr, power density of 160 MW/cm2, repetition rate of 50 kHz, and 2 h deposition time. All of the results presented below are based on sensors with sensitive layers prepared under these conditions. Fig. 4a presents the Pd nanoparticle size distribution. Size distribution was determined from SEM images (for example, Fig. 2c) by measurement of approximately 1000 nanoparticles (image area 25 lm2). The particle distribution was found to be in the range of 25–160 nm, with more than 60% of the particles in the range of 60–90 nm. No notable differences were observed between the distributions of nanoparticles located in different parts of the film. The thickness of the sensitive layer, as indicated by the SEM image (Fig. 4b), was 1.6 lm. Fig. 5 plots the frequency shifts of the nanoporous Pd-based SAWS upon exposure to different H2 gas concentrations. The frequency shift was proportional to the concentration for H2 concentrations in the range of 0.008–2%. The dynamic response of SAWS to H2 gas (Fig. 6) revealed that the sensor response is proportional to the H2 concentration and is reversible. The response times were 15, 33, and 44 s for H2 concentrations of 0.2%, 0.4%, and 0.8%, respectively. The response times were longer at higher concentration because more time was
220
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Fig. 2. SEM images of the surface layer at: (a) 50 mTorr, 160 MW/cm2; (b) 150 mTorr, 160 MW/cm2; (c) 300 mTorr, 160 MW/cm2; and (d) 150 mTorr, 250 MW/cm2.
Fig. 4. Sensitive layer of Pd (300 mTorr, 160 MW/cm2): (a) nanoparticle size distribution and (b) cross section SEM image.
Fig. 3. XRD patterns of the Pd film samples: (1) 50 mTorr, 160 MW/cm2; (2) 150 mTorr, 160 MW/cm2; (3) 300 mTorr, 160 MW/cm2; and (4) 150 mTorr, 250 MW/cm2.
required to reach sensor stability. Moreover, the absorption time (33 s) at H2 concentrations in the range of 0.2–0.4% is approximately equal to the desorption time (34 s) for the same gas concentrations. Jakubik [9] and Joshi et al. [14] reported response times between 100 and 300 s, approximately 10 times longer than the response times of the sensors reported herein. Sunil et al. [15] reported a better response (10 s) using Pd nanoparticles mixed
with a Nafion (a membrane materials with a high surface area) as the sensitive layer, but these results were obtained by measuring resistance instead of frequency. Abe et al. [21] reported a new type of sensor with a response time of 1 s, but this sensor was more complicated to fabricate because it employed a piezoelectric ball as a substrate. The sensitivity at 0.31 Hz/ppm, defined as the frequency shift in Hz per unit analyte concentration in ppm, was determined from the slope of a linear curve-fit of the data, as shown in Fig. 5. The noise assessment was performed in air (without gas) by measuring the frequency fluctuation over 10 min; frequency fluctuation represents the maximum frequency deviation from the trend line (best-fit line). The detection limit (defined as three times the noise level divided by the sensitivity) was 48 ppm. Nonporous layers of Pd, even when employed in a multilayer combination with another material like WO3, did not exhibit sufficient sensitivity for H2 detection [9,13]. For example, Jakubik [9] obtained a detection
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160 nm. Response times of 15–44 s were obtained for concentrations between 0.2% and 0.8% H2 in synthetic air. The sensitivity and detection limit at RT were 0.31 Hz/ppm and 48 ppm, respectively, for sensors based on films deposited at 300 mTorr, 160 MW/cm2, and 50 kHz for 2 h. These results indicated that high sensitivities can be achieved utilising a nanoporous layer structure for gas sensing applications. Acknowledgements This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PN-II-RU-PD-2011-3-0141. References Fig. 5. Frequency shift for Pd layer-based SAWS, to H2 gas at RT.
Fig. 6. Dynamic response of Pd layer-based SAWS, to H2 gas at RT.
limit of 2% H2 in air at 25 °C for a single Pd layer. The same researchers, using a bilayer of WO3 and Pd, detected H2 in a concentration range from 1% to 4% in air [17]. Yamanaka et al. [21,22] obtained a better result (10 ppm) using a new type of sensor based on a ball SAW device. 4. Conclusions SAWS with Pd nanoporous sensing material were produced and tested for H2 sensitivity in synthetic air at RT. Good results were obtained even at concentrations less than 0.01%. The nanoporous film was made of Pd nanoparticles with sizes in the range of 25–
[1] C. Christofides, A. Mandelis, J. Appl. Phys. 68 (1990) R1–R30. [2] R. Arsat, X.F. Yu, Y.X. Li, W. Wlodarski, K. Kalantar-zadeh, Sens. Actuators B 137 (2009) 529–532. [3] A.K. Yagati, T. Lee, J. Min, J.W. Choi, Colloid Surf. B Biointerfaces 92 (2012) 161– 167. [4] K. Nakamoto, R. Kurita, O. Niwa, Anal. Chem. 84 (2012) 3187–3191. [5] A. Moafi, J.G. Partridge, A.Z. Sadek, D.W.M. Lau, K. Kalantar-zadeh, D.G. McCulloch, IEEE Sens. J. 11 (9) (2011) 1913–1916. [6] F. Hossein-Babaei, M. Purahmad, in: Proceedings of the Third International Conference on Sensing Technology, 2008, pp. 72–75. [7] E. Becker, M. Andersson, M. Eriksson, A.L. Spetz, M. Skoglundh, IEEE Sens. J. 11 (7) (2011) 1527–1534. [8] R.J. Westerwaal, N. Duim, I. Nieuwenhuijse, C. Perrotton, A. Dabirian, J.M. van Leeuwen, V. Palmisano, B. Dam, Sens. Actuators B 165 (2012) 88–96. [9] W.P. Jakubik, Thin Solid Films 515 (2007) 8345–8350. [10] Z. Sadek, W. Wlodarski, Y.X. Li, W. Yu, X. Li, X. Yu, K. Kalantar-zadeh, Thin Solid Films 515 (24) (2007) 8705–8708. [11] M.Z. Atashbar, A.Z. Sadek, W. Wlodarski, S. Sriram, M. Bhaskaran, C.J. Cheng, R.B. Kaner, K. Kalantar-zadeh, Sens. Actuators B 138 (2009) 85–89. [12] D. Chen, J.J. Wang, D.H. Li, Y. Xu, Sens. Actuators B 159 (2011) 234–237. [13] W.P. Jakubik, Thin Solid Films 517 (2009) 6188–6191. [14] R.K. Joshi, S. Krishnan, M. Yoshimura, A. Kumar, Nanoscale Res. Lett. 4 (2009) 1191–1196. [15] K.A. Sunil, S. Krishnan, K. McGrath, F. Rinaldi, S. Bhansali, Procedia Eng. 5 (2010) 168–171. [16] A. Penzoni, V. Russo, A. Bailini, C.S. Casari, M. Ferroni, A. Li Bassi, A. Migliori, V. Morandi, L. Ortolani, G. Sberveglieri, C.E. Bottani, Sens. Actuators B 153 (2011) 340–346. [17] W. Jakubik, M. Urbanczyk, E. Maciuk, Procedia Chem. 1 (2009) 200–203. [18] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [19] T. Hejczyk, M. Urbanczyk, Acta Phys. Pol. A 120 (4) (2011) 616–620. [20] J.S. Noh, J.M. Lee, W. Lee, Sensors 11 (2011) 825–851. [21] T. Abe, N. Iwata, T. Tsuji, T. Mihara, S. Akao, K. Noguchi, N. Nakaso, D. Sim, Y. Fukiura, H. Tanaka, K. Yamanaka, Jpn. J. Appl. Phys. 46 (2007) 4726–4728. [22] K. Yamanaka, S. Ishikawa, N. Nakaso, N. Takeda, D.Y. Sim, T. Mihara, A. Mizukami, I. Satoh, S. Akao, Y. Tsukahara, IEEE Trans. UFFC 53 (2006) 793–801. [23] C. Viespe, C. Grigoriu, Sens. Actuators B 147 (2010) 43–47. [24] I. Nicolae, C. Viespe, C. Grigoriu, Sens. Actuators B 158 (2011) 418–422. [25] D.S. Ballantine, R.M. White, S.J. Martin, A.J. Ricco, E.T. Zellers, G.C. Frye, H. Wohtjen, Acoustic Wave Sensors, Theory, Design, and Physico-Chemical Applications, Academic Press, 1997. [26] D. Riabinina, M. Chaker, F. Rosei, Appl. Phys. Lett. 89 (2006) 131501. [27] D. Riabinina, E. Irissou, B.L. Drogoff, M. Chaker, D. Guay, J. Appl. Phys. 108 (2010) 034322. [28] D. Cattaneo, N. Righetti, C.S. Casari, A. Li Bassi, C.E. Bottani, Appl. Surf. Sci. 253 (2007) 7917–7921.