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Paper-based Humidity Sensor Coated with ZnO Nanoparticles: The Influence of ZnO
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 325 – 328
30th Eurosensors Conference, EUROSENSORS 2016
Paper-based Humidity Sensor Coated with ZnO Nanoparticles: The Influence of ZnO G. Niarchosa,*, G. Dubourga, G. Afroudakisb, V. Tsoutib, E. Makaronab, J. Matoviüa, V. Crnojeviü-Bengina, C. Tsamisb b
a BioSense Institute, 1 Zorana Djindijca, Novi Sad 21000, Serbia Institute of Nanoscience and Nanotechnology, National Center for Scientific Research “Demokritos”, Patriarhou Gregoriou & Neapoleos, Aghia Paraskevi 15310, Athens, Greece
Abstract In this work, we investigate the influence of ZnO nanoparticles on the performance of paper-based humidity sensors. The sensors were developed on standard, commercial printing paper with laser-patterned gold interdigitated electrodes (IDE) and ZnO nanoparticles spin-coated by various sol-gels. Blank devices were used as reference sensors in an effort to elucidate the role of ZnO and the dependence of the device performance on the concentration of the sol-gels and the number of spin-coated layers. Resistive measurements were conducted at room temperature to evaluate the devices’ response on known relative humidity levels. Relatively fast rise and response times were observed even at room temperature, while all devices tested did not require refreshing procedures. The deposition of ZnO was found to affect the sensor response, resulting in reduced sensor signal, due to the blocking of water diffusion in the porous paper material. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Humidity sensing; paper sensors; ZnO nanostructures;
1. Introduction Paper, a widely-used naturally-occurring material has been used for centuries for writing, drawing, printing or packaging. During the past decade though, alongside the developments of nanotechnology, paper has found novel uses and has emerged as an alternative substrate for micro/nanoelectronic devices and applications [1]. Combined with micro/nanostructures and nanomaterials, it offers a new means towards lightweight and cost-efficient disposable analytical devices covering a wide spectrum of applications that may range from environmental monitoring to personalized biosensors [2].
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.207
326
G. Niarchos et al. / Procedia Engineering 168 (2016) 325 – 328
Paper in essence consists of moist wood cellulose fibers pressed together, and as a material it has some unique properties like passive liquid transport, chemocompatibility with various moieties and biodegradability, thus making it an excellent candidate for low-cost sensing applications. On the other hand, metal oxide semiconductor-based gas sensors, usually employing oxides such as ZnO, TiO2, CdO, SnO2, CuO or WO3, have become increasingly popular, mainly due to their low-cost fabrication techniques, high stability and increased sensitivity to a variety of gases [3-5]. One of the most promising materials due to its multi-faceted nature that combines excellent electrical and optical properties, high chemical and mechanical stability and low-cost processing methods is ZnO [6]. In this work, paper has been combined with ZnO nanostructures in an effort to exploit and merge the advantages of both materials, and to develop accurate, reliable and low-cost sensors. In particular, the role of ZnO nanoparticles on the performance of the paper-based sensors is investigated when the devices are employed as humidity sensors for relative humidity levels (rH) in the range of 10 to 70%. The study focuses on the influence of the sol-gel composition and the number of the ZnO coating layers. 2. Experimental The humidity sensors were fabricated on two different commercial paper substrates, a plain rough printing paper of 80grm-2 and a photographic quality glossy paper of 200grm-2 basis weight. Initially, a thin film of Au (100nm) was directly sputtered on the papers and the IDEs were patterned on the surface via laser ablation using a short pulse laser (Nd:YAG-1064 nm, Rofin). The process is schematically illustrated in Fig. 1a along with a characteristic photograph of the sensors. This sensor design has already been demonstrated to respond to humidity changes [7,8].
(a)
(b)
Fig. 1. (a) Schematic of the fabrication process of the humidity sensors, (b) photograph of the experimental setup used to measure the responses of the sensors to relative humidity changes.
40mM and 150mM of sol-gels were prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)2.2H2O, Merck) into analytical grade ethanol (C2H6O, Carlo Erba Reagents) under vigorous stirring for 1hr at 60oC [9]. After obtaining homogeneous solutions, the sols were left to cool to room temperature. Deposition of the ZnO nanoparticles on the paper substrates was performed through successive spin-coatings the number of which ranged from 1 to 30. Each deposition step was followed by a 10-minute annealing at 100ȠC in the presence of atmospheric oxygen, in order for the ethanol to completely evaporate and the nanoparticles to bind together and form a uniform film. Higher annealing temperatures were also tested but it was observed that the paper substrate and the IDEs underwent structural damage. Resistive measurements of the sensors were performed in a controllable environment using a custom-made experimental setup (Fig.1b). The sensors were mounted in a sealed Teflon chamber. The rH was continuously monitored via a hydrometer. In order to determine the flow rates and concentration of the gases, mass-flow meters and controllers (Brooks Instruments) were utilized and controlled via a custom-made automated Labview-based program, which was simultaneously recording the sensors’ responses as measured by an amperometer. Nitrogen gas was selected to act as both the carrier gas and to remove excess humidity after each measurement.
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G. Niarchos et al. / Procedia Engineering 168 (2016) 325 – 328
3. Results Initially, the blank papers were characterized under rH levels from 10% to 70%, in order to be used as references for the coated sensors. Fig. 2 shows the response times of the blank sensors for both 80grm-2 and 200grm-2 basis weight for humidity levels between 20% and 70%, while on the inset is a graph depicting the humidity response of the 80gr sensor at controlled rH levels and at fixed time intervals. We observe that the gradual increase of the relative humidity inside the Teflon chamber causes the electrical resistance between the IDEs to drop at relatively fast rates. This behavior is expected, since paper’s porous surface facilitates the absorption and diffusion of the water molecules through the air “pockets” between the cellulose fiber network. A percentage of that water attaches onto the fibers causing the observed resistance decrease with increasing humidity levels. In addition, differences in the surface morphology and porosity between the two substrates also contribute to this effect as depicted in the SEM images of Figs. 3a and 3b of the blank paper substrates. The extended fiber network of the plain rough 80gr paper (Fig. 3a) is visible and remains such even after being coated with the ZnO nanoparticles (Fig. 3c), indicative of its high porosity. On the other hand, the structural morphology of the 200gr glossy paper is quite different, as the only visible pattern is the surface of the initial glossy coating (Fig. 3b) on top of which the ZnO particles form an additional layer (Fig. 3d). 50 1E10
Resistance (Ohm)
plain paper 80 g m -2
Time(min)
40
30
20% 1E9
40% 60% 70%
1E8
0
20
40
20
60
80
100
Time (min)
-2 80 gm response time -2 200 gm response time
10
0 10
20
30
40
50
60
70
80
Relative Humidity (%)
Fig. 2. Comparison of the response times between the blank devices on both paper substrates under rH levels of 20% - 70%. Inset: humidity response of the 80grm-2 blank sensor at controlled rH levels.
(a)
(b)
(c)
(d)
Fig. 3.SEM images of (a) the blank 80grm-2 paper substrate (scale bar: 100ȝm), (b) the blank 200grm-2 paper substrate (scale bar: 100ȝm), (c) a sensor on the 80grm-2 paper substrate with 5 coatings of 40mM sol-gel (scale bar: 100ȝm), and (d) a sensor on the 200grm-2 paper substrate with 5 coatings of 40mM sol-gel (scale bar: 100ȝm). Notice the difference in the scale bars.
Fig. 4 shows the ratio of the electrical resistance (Ro/R), as it was measured for sensors fabricated on both paper substrates coated with 1, 5 and 10 layers of ZnO nanoparticles from sol-gel solution of 40mM and 150mM sol-gels. In comparison to the blank sensors, we notice that as the number of the ZnO increases, the sensor’s response to
328
G. Niarchos et al. / Procedia Engineering 168 (2016) 325 – 328
humidity is decreasing. The experimental results indicate that ZnO coating acts as a diffusion barrier to the diffusion of water molecules through paper’s cellulose porous structure. Such behavior is in accordance to recent literature where ZnO nanostructures were used as protective coatings for various types of wood [10].
14
14
Plain paper 80 grm-2 Blank 1 layer 5 layers 10 layers
8
10
Ro/R
Ro/R
10
Glossy Paper 200 grm-2
12
12
6
Blank 150mM, 3 layers 150mM, 30 layers 40mM, 5 layers 40mM, 10 layers
8 6 4
4
2
2
0
0 20
30
40
50
60
70
0
10
20
30
40
rH (%)
rH (%)
(a)
(b)
50
60
70
Fig. 4. Ratio of electrical resistance versus rH levels of 20%-70% for various numbers of coatings on the (a) 80grm-2 and (b) the 200grm-2 paper substrate. In (b) solid symbols: 150mM sol-gel; Open symbols: 40mM sol-gel.
4. Conclusions The sensing properties of paper-based sensors coated with ZnO nanoparticles under known relative humidity levels have been investigated. Sensors exhibit relatively fast response and recovery times, are able to operate at room temperature and no refreshing procedures are required after each measurement. The ZnO nanostructure film plays the role of a passivation layer towards humidity, gradually reducing the sensitivity of the coated sensors as the number of coated layers is increasing. References [1] D.Tobjörk and R.Österbacka, Paper Electronics, Adv. Mater. 2011, 23, 1935–1961. [2] Carvalhal, R.F.; Kfouri, M.S.; Piazetta, M.H.D.; Gobbi, A.L.; Kubota, L.T. Electrochemical detection in a paper-based separation device. Anal. Chem. 2010, 82, 1162–1165. [3] T. Hübert, L. Boon-Brett, G. Black, U. Banach, Hydrogen sensors–a review, Sensors Actuators B Chem. 157 (2011) 329–352. [4] E. Comini, G. Faglia, G. Sberveglieri, Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts, Appl. Phys. Lett. (2002). [5] S. Pearton, F. Ren, Y. Wang, B. Chu, Recent advances in wide bandgap semiconductor biological and gas sensors, Prog. Mater. Sci. 55 (2010) 1–59. [6] A. Mitra, R.K. Thareja, Photoluminescence and ultraviolet laser emission from nanocrystallineZnO thin films, Journal of Applied Physics 89 (2001) 2025. [7] G. Niarchos, G. Dubourg, G. Afroudakis, V. Tsouti, E. Makarona, J. Matoviü, V. Crnojeviü-Bengin, C. Tsamis, Low-cost paper-based humidity sensor based on ZnO nanoparticles, Poster Presentation at Eurosensors XXIX, 4-6 September 2015, Freiburg, Germany. [8] F. Gîder, A. Ainla, J. Redston, B. Mosadegh, A. Glavan, T. J. Martin, and G. M. Whitesides, Paper-based Electrical Respiration Sensor, Angew.Chem, Int.Ed. 2016, 55, 5727-5732. [9] E. Makarona, M.C. Skoulikidou, Th. Kyrasta, A. Smyrnakis, A. Zeniou, E. Gogolides, C. Tsamis, Controllable fabrication of bioinspired three-dimensional ZnO/Si nanoarchitectures, Materials Letters, Volume 142, 1 March 2015, Pages 211–216. [10] Ch. Koutzagioti, G. Ntalos, C. Tsamis, E. Makarona, Low-cost multi-functional bioinspired wood coatings based on ZnO nanostructures, EMRS 2015 Spring Meeting, May 11-15, 2015, Lille, France.
ScienceDirect Procedia Engineering 168 (2016) 325 – 328
30th Eurosensors Conference, EUROSENSORS 2016
Paper-based Humidity Sensor Coated with ZnO Nanoparticles: The Influence of ZnO G. Niarchosa,*, G. Dubourga, G. Afroudakisb, V. Tsoutib, E. Makaronab, J. Matoviüa, V. Crnojeviü-Bengina, C. Tsamisb b
a BioSense Institute, 1 Zorana Djindijca, Novi Sad 21000, Serbia Institute of Nanoscience and Nanotechnology, National Center for Scientific Research “Demokritos”, Patriarhou Gregoriou & Neapoleos, Aghia Paraskevi 15310, Athens, Greece
Abstract In this work, we investigate the influence of ZnO nanoparticles on the performance of paper-based humidity sensors. The sensors were developed on standard, commercial printing paper with laser-patterned gold interdigitated electrodes (IDE) and ZnO nanoparticles spin-coated by various sol-gels. Blank devices were used as reference sensors in an effort to elucidate the role of ZnO and the dependence of the device performance on the concentration of the sol-gels and the number of spin-coated layers. Resistive measurements were conducted at room temperature to evaluate the devices’ response on known relative humidity levels. Relatively fast rise and response times were observed even at room temperature, while all devices tested did not require refreshing procedures. The deposition of ZnO was found to affect the sensor response, resulting in reduced sensor signal, due to the blocking of water diffusion in the porous paper material. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Humidity sensing; paper sensors; ZnO nanostructures;
1. Introduction Paper, a widely-used naturally-occurring material has been used for centuries for writing, drawing, printing or packaging. During the past decade though, alongside the developments of nanotechnology, paper has found novel uses and has emerged as an alternative substrate for micro/nanoelectronic devices and applications [1]. Combined with micro/nanostructures and nanomaterials, it offers a new means towards lightweight and cost-efficient disposable analytical devices covering a wide spectrum of applications that may range from environmental monitoring to personalized biosensors [2].
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.207
326
G. Niarchos et al. / Procedia Engineering 168 (2016) 325 – 328
Paper in essence consists of moist wood cellulose fibers pressed together, and as a material it has some unique properties like passive liquid transport, chemocompatibility with various moieties and biodegradability, thus making it an excellent candidate for low-cost sensing applications. On the other hand, metal oxide semiconductor-based gas sensors, usually employing oxides such as ZnO, TiO2, CdO, SnO2, CuO or WO3, have become increasingly popular, mainly due to their low-cost fabrication techniques, high stability and increased sensitivity to a variety of gases [3-5]. One of the most promising materials due to its multi-faceted nature that combines excellent electrical and optical properties, high chemical and mechanical stability and low-cost processing methods is ZnO [6]. In this work, paper has been combined with ZnO nanostructures in an effort to exploit and merge the advantages of both materials, and to develop accurate, reliable and low-cost sensors. In particular, the role of ZnO nanoparticles on the performance of the paper-based sensors is investigated when the devices are employed as humidity sensors for relative humidity levels (rH) in the range of 10 to 70%. The study focuses on the influence of the sol-gel composition and the number of the ZnO coating layers. 2. Experimental The humidity sensors were fabricated on two different commercial paper substrates, a plain rough printing paper of 80grm-2 and a photographic quality glossy paper of 200grm-2 basis weight. Initially, a thin film of Au (100nm) was directly sputtered on the papers and the IDEs were patterned on the surface via laser ablation using a short pulse laser (Nd:YAG-1064 nm, Rofin). The process is schematically illustrated in Fig. 1a along with a characteristic photograph of the sensors. This sensor design has already been demonstrated to respond to humidity changes [7,8].
(a)
(b)
Fig. 1. (a) Schematic of the fabrication process of the humidity sensors, (b) photograph of the experimental setup used to measure the responses of the sensors to relative humidity changes.
40mM and 150mM of sol-gels were prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)2.2H2O, Merck) into analytical grade ethanol (C2H6O, Carlo Erba Reagents) under vigorous stirring for 1hr at 60oC [9]. After obtaining homogeneous solutions, the sols were left to cool to room temperature. Deposition of the ZnO nanoparticles on the paper substrates was performed through successive spin-coatings the number of which ranged from 1 to 30. Each deposition step was followed by a 10-minute annealing at 100ȠC in the presence of atmospheric oxygen, in order for the ethanol to completely evaporate and the nanoparticles to bind together and form a uniform film. Higher annealing temperatures were also tested but it was observed that the paper substrate and the IDEs underwent structural damage. Resistive measurements of the sensors were performed in a controllable environment using a custom-made experimental setup (Fig.1b). The sensors were mounted in a sealed Teflon chamber. The rH was continuously monitored via a hydrometer. In order to determine the flow rates and concentration of the gases, mass-flow meters and controllers (Brooks Instruments) were utilized and controlled via a custom-made automated Labview-based program, which was simultaneously recording the sensors’ responses as measured by an amperometer. Nitrogen gas was selected to act as both the carrier gas and to remove excess humidity after each measurement.
327
G. Niarchos et al. / Procedia Engineering 168 (2016) 325 – 328
3. Results Initially, the blank papers were characterized under rH levels from 10% to 70%, in order to be used as references for the coated sensors. Fig. 2 shows the response times of the blank sensors for both 80grm-2 and 200grm-2 basis weight for humidity levels between 20% and 70%, while on the inset is a graph depicting the humidity response of the 80gr sensor at controlled rH levels and at fixed time intervals. We observe that the gradual increase of the relative humidity inside the Teflon chamber causes the electrical resistance between the IDEs to drop at relatively fast rates. This behavior is expected, since paper’s porous surface facilitates the absorption and diffusion of the water molecules through the air “pockets” between the cellulose fiber network. A percentage of that water attaches onto the fibers causing the observed resistance decrease with increasing humidity levels. In addition, differences in the surface morphology and porosity between the two substrates also contribute to this effect as depicted in the SEM images of Figs. 3a and 3b of the blank paper substrates. The extended fiber network of the plain rough 80gr paper (Fig. 3a) is visible and remains such even after being coated with the ZnO nanoparticles (Fig. 3c), indicative of its high porosity. On the other hand, the structural morphology of the 200gr glossy paper is quite different, as the only visible pattern is the surface of the initial glossy coating (Fig. 3b) on top of which the ZnO particles form an additional layer (Fig. 3d). 50 1E10
Resistance (Ohm)
plain paper 80 g m -2
Time(min)
40
30
20% 1E9
40% 60% 70%
1E8
0
20
40
20
60
80
100
Time (min)
-2 80 gm response time -2 200 gm response time
10
0 10
20
30
40
50
60
70
80
Relative Humidity (%)
Fig. 2. Comparison of the response times between the blank devices on both paper substrates under rH levels of 20% - 70%. Inset: humidity response of the 80grm-2 blank sensor at controlled rH levels.
(a)
(b)
(c)
(d)
Fig. 3.SEM images of (a) the blank 80grm-2 paper substrate (scale bar: 100ȝm), (b) the blank 200grm-2 paper substrate (scale bar: 100ȝm), (c) a sensor on the 80grm-2 paper substrate with 5 coatings of 40mM sol-gel (scale bar: 100ȝm), and (d) a sensor on the 200grm-2 paper substrate with 5 coatings of 40mM sol-gel (scale bar: 100ȝm). Notice the difference in the scale bars.
Fig. 4 shows the ratio of the electrical resistance (Ro/R), as it was measured for sensors fabricated on both paper substrates coated with 1, 5 and 10 layers of ZnO nanoparticles from sol-gel solution of 40mM and 150mM sol-gels. In comparison to the blank sensors, we notice that as the number of the ZnO increases, the sensor’s response to
328
G. Niarchos et al. / Procedia Engineering 168 (2016) 325 – 328
humidity is decreasing. The experimental results indicate that ZnO coating acts as a diffusion barrier to the diffusion of water molecules through paper’s cellulose porous structure. Such behavior is in accordance to recent literature where ZnO nanostructures were used as protective coatings for various types of wood [10].
14
14
Plain paper 80 grm-2 Blank 1 layer 5 layers 10 layers
8
10
Ro/R
Ro/R
10
Glossy Paper 200 grm-2
12
12
6
Blank 150mM, 3 layers 150mM, 30 layers 40mM, 5 layers 40mM, 10 layers
8 6 4
4
2
2
0
0 20
30
40
50
60
70
0
10
20
30
40
rH (%)
rH (%)
(a)
(b)
50
60
70
Fig. 4. Ratio of electrical resistance versus rH levels of 20%-70% for various numbers of coatings on the (a) 80grm-2 and (b) the 200grm-2 paper substrate. In (b) solid symbols: 150mM sol-gel; Open symbols: 40mM sol-gel.
4. Conclusions The sensing properties of paper-based sensors coated with ZnO nanoparticles under known relative humidity levels have been investigated. Sensors exhibit relatively fast response and recovery times, are able to operate at room temperature and no refreshing procedures are required after each measurement. The ZnO nanostructure film plays the role of a passivation layer towards humidity, gradually reducing the sensitivity of the coated sensors as the number of coated layers is increasing. References [1] D.Tobjörk and R.Österbacka, Paper Electronics, Adv. Mater. 2011, 23, 1935–1961. [2] Carvalhal, R.F.; Kfouri, M.S.; Piazetta, M.H.D.; Gobbi, A.L.; Kubota, L.T. Electrochemical detection in a paper-based separation device. Anal. Chem. 2010, 82, 1162–1165. [3] T. Hübert, L. Boon-Brett, G. Black, U. Banach, Hydrogen sensors–a review, Sensors Actuators B Chem. 157 (2011) 329–352. [4] E. Comini, G. Faglia, G. Sberveglieri, Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts, Appl. Phys. Lett. (2002). [5] S. Pearton, F. Ren, Y. Wang, B. Chu, Recent advances in wide bandgap semiconductor biological and gas sensors, Prog. Mater. Sci. 55 (2010) 1–59. [6] A. Mitra, R.K. Thareja, Photoluminescence and ultraviolet laser emission from nanocrystallineZnO thin films, Journal of Applied Physics 89 (2001) 2025. [7] G. Niarchos, G. Dubourg, G. Afroudakis, V. Tsouti, E. Makarona, J. Matoviü, V. Crnojeviü-Bengin, C. Tsamis, Low-cost paper-based humidity sensor based on ZnO nanoparticles, Poster Presentation at Eurosensors XXIX, 4-6 September 2015, Freiburg, Germany. [8] F. Gîder, A. Ainla, J. Redston, B. Mosadegh, A. Glavan, T. J. Martin, and G. M. Whitesides, Paper-based Electrical Respiration Sensor, Angew.Chem, Int.Ed. 2016, 55, 5727-5732. [9] E. Makarona, M.C. Skoulikidou, Th. Kyrasta, A. Smyrnakis, A. Zeniou, E. Gogolides, C. Tsamis, Controllable fabrication of bioinspired three-dimensional ZnO/Si nanoarchitectures, Materials Letters, Volume 142, 1 March 2015, Pages 211–216. [10] Ch. Koutzagioti, G. Ntalos, C. Tsamis, E. Makarona, Low-cost multi-functional bioinspired wood coatings based on ZnO nanostructures, EMRS 2015 Spring Meeting, May 11-15, 2015, Lille, France.