- Email: [email protected]
The role of water vapour in ZnO nanostructures for humidity sensing at room temperature
Accepted Manuscript Title: The role of water Vapour in zno nanostructures for humidity sensing at room temperature Author: J. Herr´an I. Fern´andez E. Ochoteco G. Caba˜nero H. Grande PII: DOI: Reference:
S0925-4005(14)00311-6 http://dx.doi.org/doi:10.1016/j.snb.2014.03.043 SNB 16694
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-12-2013 4-3-2014 11-3-2014
Please cite this article as: J. Herr´an, I. Fern´andez, E. Ochoteco, G. Caba˜nero, H. Grande, The role of water Vapour in zno nanostructures for humidity sensing at room temperature, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.03.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ip t
SHORT COMMUNICATION
cr
THE ROLE OF WATER VAPOUR IN ZnO NANOSTRUCTURES
us
FOR HUMIDITY SENSING AT ROOM TEMPERATURE
an
J. Herrán*, I. Fernández, E. Ochoteco, G. Cabañero, H. Grande
M
IK4-CIDETEC, Sensors Unit, Materials Division Pº Miramón 196, E-20009 San Sebastián, Spain
Ac ce p
te
d
Tel.+34 943 309 022 Fax.+34 943 309 136 e-mail: [email protected]
Page 1 of 22
Abstract
The role of water Vapour in ZnO nanostructures for humidity sensing at room
ip t
temperature is presented and discussed. Experimental and theoretical results demonstrate that ZnO nanoparticles and nanorods, show different physico-chemical
cr
behaviour under different relative humidity atmospheres. While electrical current density increases as RH does in the case of the ZnO nanoparticles, ZnO nanorods show inverse
us
behaviour. These facts are related to the capillary condensation and water electric dipole
an
moment effects, respectively. Additionally, a simultaneous validation between the sensor developed and a commercial device corroborates the potential application of this kind of
d
M
low-cost sensing nanostructures presented in this work.
Ac ce p
condensation
te
Keywords: humidity sensor, room temperature, ZnO nanostructures, capillary
2 Page 2 of 22
1. INTRODUCTION
Solid state humidity sensors are being developed for different purposes such as IAQ
ip t
(Indoor Air Quality), HVAC (Heating Ventilating Air Conditioning), process control, agriculture and instrumentation [1, 2]. Several sensing principles can be used to these
cr
purposes, but solid state sensors are an attractive choice due to their low cost and functionality. In this field, several materials (ceramics, semiconductors and polymers)
us
have been tested and have shown diverse results related to humidity sensing [3–11].
an
During the last years, some authors have reported humidity sensitivity of ZnO synthesized by different methods (Chemical Vapour deposition, Pulse Laser Deposition,
M
chemical synthesis and screen-printing technology) [12-18], and some developments have been carried out in order to study their performance as sensing material for water
d
vapour detection at room temperature. In this context, the nanotechnology has been
te
crucial to achieve the goal of room temperature operation, due to the physico-chemical properties of the ZnO nanomaterials. Moreover, it is well known the improved properties
Ac ce p
of these nanostructures sensing materials due to the high surface/volume ratio: kinetics, sensitivity, sensor response and, finally, the reduction of the device dimensions allows diminishing the power consumption of the system [12-13].
In this work, the role of water vapour in ZnO nanostructures for humidity sensing at room temperature is presented and discussed. Due to the fact the interaction between the water molecules and the semiconductor surface takes place at room temperature, the sensing reactions involved in the humidity monitoring are based on physiadsorption phenomena. Previous results based on ZnO nanorods presented by the authors [1] show how an electronic interchange between the dissociated water and the ZnO nanorods dominates the system, increasing the resistance as the RH (Relative Humidity)
3 Page 3 of 22
does. The electrons of the sensing material bulk (ZnO nanorod) are captured on the surface by the water molecules, and the current density of device diminishes. However, the present novel research carried out shows an inverse behaviour in a system based on
ip t
ZnO nanoparticles. Under this nanostructure scenario (boundary between water vapour and ZnO nanoparticles), capillary condensation phenomena of water vapour are
cr
observed on the surface of the nanoparticles, and a water conductive path is formed on the electrodes of the device. Due to this fact, the resistance of the system diminishes as
us
RH does. Theoretically, these results are according to the literature described about this
an
kind of devices: capillary condensation of water vapour is observed in nanoparticles between 2 and 100 nm [19], while an electronic interchange between the dissociated
M
water and the ZnO dominates the system in thin-films [18]. This theoretical behaviour exposed in the previous paragraphs has been validated by means of the experimental
Ac ce p
te
d
results of the sensing devices developed in this work.
4 Page 4 of 22
2. EXPERIMENTAL
The fabrication process of test samples was as follows: a nanoparticle dispersion was
ip t
prepared according to the Pacholski et al. method [20]. Briefly, 0.11 g of zinc acetate dehydrate was dissolved in 50 mL of methanol under vigorous stirring at 60°C.
cr
Subsequently, a 0.03 M solution of KOH (25 mL) in methanol was added dropwise at 60°C. The reaction mixture was stirred for 2 h at 60°C. The obtained colloidal
us
suspension, without any further purification treatment, was deposited by drop-coating on
an
the substrate, where a metallic interdigitated electrode has been previously patterned.
M
The final conductometric device was followed by a 5 minutes air annealing at 100°C.
Electrical characterization was performed inside a sealed plastic chamber by a keithley
d
2400 sourcemeter. Tests were carried out under different RH at room temperature (25 ±
te
1 ºC). The humidity target atmosphere is obtained by means of a mixing system consisting of mass flow controllers (MFCs) from Bronkhorst Hi-Tech controlled by a PC.
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A Dynamic Data Exchange communication is established between the computer and the MFCs to operate them by Labview©. Inside the chamber, a commercial humidity sensor (LinPicco™ A05 Basic, Capacitive Humidity Module) was used as a humidity indicator in order to control the different atmospheres.
5 Page 5 of 22
3. RESULTS AND DISCUSSION
ZnO nanoparticles as humidity sensor at room temperature
ip t
3.1
Regarding materials characterization, structural studies of the ZnO nanoparticles were in
previous
IK4-CIDETEC
publications
[21-22].
Optical
density
and
cr
reported
fluorescence spectra were performed for the nanoparticles suspension, and the reported
us
values for ZnO nanocrystals colloids were 3.4 nm in diameter. Moreover, those values
an
agree with the size of the nanocrystals detected through HRTEM. In fact, it was observed isolated crystalline particles with an average diameter of 3.5 nm and high
M
crystallinity. The diffraction rings are indexed using the typical wurtzite structure of ZnO. The average diameter of the ZnO nanoparticles is a crucial issue related to the capillary
te
the following paragraphs.
d
condensation of water vapour on the semiconductor surface, as it will be discussed in
Ac ce p
In order to get a stable sensor signal, the operation voltage has been fixed at 1 V in a pulse mode and 1 second of fixed duty cycle. It has been tested that a continuous operation voltage does not show stability of the sensor based line. The instability is related to temperature gradient produced within the interface between the water vapor molecules and the ZnO nanoparticles.
The joule effect associated to the electrical
current density is in relation to this phenomenon, and a pulse operation mode avoids the problem. A thermodynamical equilibrium is reached by this typical electrical technique very often implemented in sensors signal conditioning.
Figure 1 and figure 2 show the dynamic sensor response at different RH conditions of the commercial device and the ZnO nanoparticles sensor, respectively. The time
6 Page 6 of 22
necessary to fill/evacuate the chamber is estimated around one minute due to the step data acquisition (30 s) and the response time of the commercial device (lower than 5 s, datasheet parameters). In this context, the kinetics of the ZnO nanopaticles sensors
ip t
show excellent response times ( around 5 seconds) according to the commercial sensor datasheet parameters. The system kinetics is in good agreement to the reduction of
cr
diffusion effects between gas target and the bulk of the material: from classic ZnO coating systems to ZnO nanostrutured materials [1, 13-14]. As a valued point, the base
us
line of the ZnO nanoparticles sensor and the commercial one shows similar behaviour.
an
Due to this fact, it can be assumed that the ZnO nanoparticles sensor shows good stability and no drift effects are observed. Moreover, according to this excellent kinetics
M
behaviour, the possibility of sensing at room temperature allows the integration in mobile and portable systems due to the low power consumption of the device (around a
te
d
maximum of 16 μW pulsed operation voltage).
As it can be observed in the figure 2 (humidity dynamic response ZnO nanoparticles),
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the electrical current of the system increases as humidity does. This physico-chemical behaviour is related to the capillary condensation produced on the ZnO nanoparticles surface [1, 12-16, 19, 23]. A conductive water path dominates the equivalent resistance of the circuit and an increase of the current density is observed. This effect is not observed in ZnO nanorods systems [1, 9, 18], nevertheless an inverse behaviour is observed. Basically, the different physico-chemical behaviour is based on adsorption effects, and it will be discussed in depth in the following paragraphs (subsection 3.2).
In this context, figure 3 shows the ZnO nanoparticles sensor response at different RH values (3 different sensors and 8 measurements per humidity set value and sensor). Repeatability of the response was checked by means of 8 measurements for each RH
7 Page 7 of 22
value. As a representative value, the relative standard deviation (R.S.D.) at 42% RH is 6.2%. According reproducibility, 3 different sensors were tested at the different RH concentrations, and the R.S.D. at 42% RH is 8.6%. Moreover, the stability of the system
ip t
is represented by the error bar obtained from the dynamic curves: RH from the commercial sensor (x-axis) and measured electrical current from the ZnO nanoparticles
cr
(y-axis). It is worth to mention the precision and no-overlapping of the experimental points. Under a theoretical point of view, the sensor response can be fitted as an
us
exponential curve: I = I 0 exp( RH ) (1), as it is represented in the equation inserted in the
an
figure 3. This behaviour is related to the capillary condensation, which takes place in nanoparticles between 2 and 100 nm [19]. Additionally, it is well known that the depletion
M
place between the nanoparticales and the gas target is also modelled as an exponential curve, due to the potential barrier formed among the ZnO nanoparticles. All these facts
d
corroborate these theoretical approaches proposed to the experimental results obtained
Discussion: the role of water vapour in ZnO nanostructures for humidity
Ac ce p
3.2
te
(figure 3).
sensing at room temperature
As it has been presented in the section 3.1 and according to previous results reported by IK4-CIDETEC [1], ZnO nanorods and nanoparticles show different physico-chemical behaviour under water vapour atmospheres. In this context, the following graphical sketches represent the different nano-interaction scenarios, in relation to sensing principles and transduction parameters.
8 Page 8 of 22
Figure 4 shows the humidity sensing principle of ZnO nanorods. The sensing mechanism is based on the interaction between the semiconductor nanorod (ZnO) and the water vapor. The water electric dipole moment (oxygen δ- and hydrogen δ+) [9, 24]
ip t
allows to trap electrons from the conduction band of the semiconductor, and the current density of the system decreases (J2
cr
to physical parameters involved in the sensing process (space charge and the work
us
function) in the reference 1.
an
It is worth to mention that the humidity sensing principle of ZnO nanorods (J2
M
temperature, the physiadsorption dominates the sensing reactions; chemiadsorption phenomena are not observed due to the energy of the system is not high enough to
te
d
produce these effects [9, 24].
Regarding to the results presented in this work, related to ZnO nanoparticles humidity
Ac ce p
sensing, an inverse physical behavior can be observed. As it has been previously mentioned, the humidity sensing principle of ZnO nanoparticles (the electrical current of the system increases as humidity does) is associated to the electronic interchange produced by the capillary condensation of water vapour on the semiconductor nanoparticle interface (for nanoparticles size between 2 and 100 nm [19]). This electrical behaviour is not observed in this kind of Ag/ZnO Schotcky diode configuration.
Figure 5 shows the humidity sensing principle of ZnO nanoparticles. It can be graphically observed how the conductive water path formed on the electrodes increases the conductivity of the system. While RH increases, the water path will be formed by more water molecules and the conductivity of the system will increase. In absence of water,
9 Page 9 of 22
the resistive path is out of range due to the high resistance related to the potential barrier formed between each ZnO nanoparticle. Experimental results presented in figure 3, corroborate this sensing mechanism proposed: equation 1 fits the experimental data to
ip t
the theoretical exponential behaviour associated to the capillary condensation
cr
phenomena.
us
4. CONCLUSIONS
an
ZnO nanoparticles for relative humidity monitoring at room temperature are presented. The semiconductor sensing material is based on ZnO nanoparticles and it is synthesized
M
by Pacholski method. The response time is estimated around 5 seconds and the sensitivity shows an exponential relation in the humidity range measured (0 and 80%
te
d
RH).
The electrical changes are related to the depletion place formed between the ZnO
Ac ce p
nanoparticales and the water molecules, at different RH concentrations. The sensing process is based on capillary condensation of the water molecules on the ZnO nanoparticles surface.
Due to this fact, a conductive water path dominates the
equivalent resistance of the circuit, and the current density increases as the RH does. The experimental results fit the theoretical exponential curve, according to the models proposed.
Regarding to the role of water vapour in ZnO nanostructures for humidity sensing at room temperature, it has been experimentally and theoretically demonstrated that ZnO nanorods and nanoparticles show different physical-chemical behaviour. While electrical current density increases as RH does in the case of the ZnO nanoparticles, ZnO
10 Page 10 of 22
nanorods show inverse behaviour. These facts are related to the capillary condensation and water electric dipole moment effects, respectively.
ip t
Finally, in both sensing systems, a simultaneous validation between the sensor developed and a commercial device corroborates the potential application presented in
cr
this work. Moreover, the possibility of sensing at room temperature allows the integration in mobile and portable systems due to the low power consumption of the device (around
us
a few μW).
an
Acknowledgements
This work has been funded by the Spanish Government through the project Hyper
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(Hybrid Neuroprosthetic and Neurorobotic Devices for Functional Compensation and
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REFERENCES
te
(CSD2009-00067).
d
Rehabilitation of Motor Disorders) under program Consolider-Ingenio 2010-2014
1. J. Herrán, I. Fernández, R. Tena-Zaera, E. Ochoteco, G. Cabañero, H. Grande, Schottky diodes based on electrodeposited ZnO nanorod arrays for humidity sensing at room temperature, Sensors and Actuators B: Chemical, Volume 174, November 2012, Pages 274-278.
2. Zhi Chen and Chi Lu, Humidity Sensors: A Review of Materials and Mechanisms, Sensor Letters Vol. 3, 274-295, 2005. 3. S. Chakraborty, K. Nemoto, K. Hara, and P. T. Lai, Moisture sensitive field effect transistors using SiO2/Si3N4/Al2O3 gate structure, Smart Mater. Structure 8, 274 (1999).
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4.S. A. Krutovertsev, A. E. Tarasova, L. S. Krutovertseva, and A. V. Zorin, Integrated multifunctional humidity sensor, Sens. Actuators A 62, 582 (1997). 5.T. Nitta, Z. Terada, and S. Hayakawa, Humidity-sensitive electrical conduction of
ip t
MgCr2O4-TiO2 porous ceramics, J. Am. Ceram. Soc. 63, 295, (1980). 6. G. García-Belmonte, V. Kytin, T. Dittrich, J. Bisquert, Effect of humidity on the ac
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conductivity of nanoporous TiO2, JAP 94, 8 (2003).
7. J. F. Boyle and K. A. Jones, The effects of CO, water vapor and surface temperature
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on the conductivity of a SnO2 gas sensorJ. Electronic Mater. 6, 717 (1977).
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8.J. Kleperis, M. Kundzins, G. Vitins, V. Eglitis, G. Vaivars, and A. Lusis, Gas-sensitive gap formation by laser ablation in In2O3 layer: application as humidity sensor, Sens.
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Actuators B 28, 135 (1995).
9. Yun Wang Yeow, J.T.W. Liang-Yih Chen, Synthesis of aligned zinc oxide nanorods
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for humidity sensing, 3rd International Conference on Sensing Technology, 2008.
te
10. L. G. Wade, Jr., Organic Chemistry, Prentice Hall, NJ (2001). 11. I. Palacios, R. Castillo, and R. A. Vargas, Thermal and transport properties of the
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polymer electrolyte based on poly(vinylalcohol)–KOH–H2OElectrochim. Acta 48, 2195 (2003).
12. Q. Wan, Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, X. G. Gao, J. P Li, Positive temperature coefficient resistance and humidity sensing properties of Cd-doped ZnO nanowires, Appl. Phys. Lett. 84 (2004) 3085-3087. 13. Q. Qi, T. Zhang, Q. J. Yu, R. Wang, Y. Zeng, L. Liu, H. B. Yang, Properties of humidity sensing ZnO nanorods-based sensor fabricated by screen-printing, Sens. Actuators B 133 (2008) 638-643. 14. Jingbin Han, Fengru Fan, Chen Xu, Shisheng Lin, Min Wei, Xue Duan and Zhong Lin Wang, ZnO nanotube-based dye-sensitized solar cell and its application in selfpowered devices, Nanotechnology 21 (2010) 405203 (7pp).
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15. L. Gu, K. Zheng, Y. Zhou, J. Li, X. Mo, G.R. Patzke, G. Chen, Humidity sensors based on ZnO/TiO2 core/shell nanorod arrays with enhanced sensitivity, Sensors and Actuators B: Chemical (2010), doi:10.1016/j.snb.2010.12.024.
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16. Yongsheng Zhang, Ke Yu, Desheng Jiang, Ziqiang Zhu, Haoran Geng and Laiqiang Luo, Zinc oxide nanorod and nanowire for humidity sensor, Applied Surface Science
cr
Volume 242, Issues 1-2, 31 March 2005, Pages 212-217.
17. Xiaofeng Zhou, Jian Zhang, Tao Jiang, Xiaohua Wang and Ziqiang Zhu, Humidity
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detection by nanostructured ZnO: A wireless quartz crystal microbalance investigation,
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Sensors and Actuators A: Physical Volume 135, Issue 1, 30 March 2007, Pages 209214. Majumdar
and
P.
Banerji,
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sensitivity
of
p-ZnO/n-Si
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18. Sayanee
heterostructure, Sensors and Actuators B 140 (2009), 134-138.
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19. P.M. Faia, C.S. Furtado, A.J. Ferreira, Humidity sensing properties of a thick-film
190.
te
titania prepared by a slow spinning process, Sensors and Actuators B 101 (2004) 183–
Ac ce p
20. C. Pacholski, A. Kornowski and H. Weller, Angew. Chem., Int. Ed., 2002, 41, 1188. 21. J. Ajuria et al., Inverted ITO-free organic solar cells based on p and n semiconducting oxides. New designs for integration in tandem cells, top or bottom detecting devices, and photovoltaic windows, Energy Environ. Sci., 2011,4, 453-458 22. Michele Sessolo et al., Zinc oxide nanocrystals as electron injecting building blocks for plastic light sources, J. Mater. Chem., 2012,22, 4916-4920 23. R. Tena-Zaera, J. Elias, C. Levy-Clement, I. Mora-Sero, Y. Luo, and J. Bisquert, Electrodeposition and impedance spectroscopy characterization of ZnO nanowire arrays, Physica Status Solidi a-Applications and Materials Science, 2008, 205, 23452350.
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24.S. Peulon and D. Lincot, Mechanic study of cathodic electrodeposition of zinc oxide and zinc hydroxychloride films from oxigenated aqueous zinc chloride solutions, Journal
ip t
of The Electrochemical Society, 1998, 145, 864-874.
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BIOGRAPHIES
Dr. Jaime Herrán received his first class honours degree in Physics at the University of
us
Cantabria in 2004 and his PhD at the University of Navarra in 2008. He was working in
an
the communication engineering department of the University of Cantabria (2003-2004), the Microsystems unit of the CEIT and Tecnun, University of Navarra (2005-2010) and in
M
2008, he was working as a postdoc in the Microsystems Technology division of the CSEM in Neuchâtel (Switzerland) during 1 year. Since 2010, he is a researcher and a
d
project manager in the Sensors Unit of the Materials Division at IK4-CIDETEC, San
te
Sebastián, Spain. His research interests are solid-state Microsystems, nanotechnology
Ac ce p
and printed electronics. For further contact, his e-mail address is: [email protected]
Iván Fernández got his Electrónic Engineer Bachelor Degree by the University of País Vasco (UPV / EHU) in 2005 and the Automatic and Electronic Engineer Master Degree by the University of Mondragón (MU) in 2008. He has extensive experience in printing techniques of materials for different plastic electronics applications with Inkjet and Screen Printing. At present, its activity is focused on the design and manufacture of pressure sensors along with the development of signal conditioning circuits and connectivity systems thereof.
14 Page 14 of 22
Dr.
Estibalitz Ochoteco is Manager of Sensors Unit in IK4-CIDETEC. She is
specialized in the tailor made synthesis of electroactive materials for advanced electrochemical applications as sensors, biosensors, actuators, nanotechnologies. She
ip t
has published more than 40 articles in peer review journals, and is author of 9 patents. She has participated during the last 5 years in national projects, regional projects,
cr
contracts with industries and European Projects (FP6 and FP7). Since 2011, she is
us
member of the Scientific Committee of the "Smart System Integration Conference".
an
German Cabañero Sevillano. Degree in Material Science Eng., University of Navarra. He is a New Materials Department Manager at IK4-CIDETEC specialized in
M
Nanotechnology, Biomaterials, Sensors and Photonics. He generates project ideas and actively contributes to developments and strategy. Moreover, he contributes to the
d
maintaining of state of the art skill, facilities and infrastructure, e.g. evaluation and
te
implementation of new technologies and concepts. He has worked in a wide range of National and European projects (Transportation, Mining, Construction, Energy,…).
Ac ce p
Before working at IK4-CIDETEC, he worked as R & D Manager at Leading Enterprises Group and CTC.
Dr. Hans-Jürgen Grande is CTO at IK4-CIDETEC. He is PhD in Chemistry Science since 1998 (University of the Basque Country). He has managed during the last 12 years a large number of R&D projects at national and international level. In addition, he has published a large number of scientific articles in the field of electrochemistry. He is member of the Technical Committee at IK4 Research Alliance.
15 Page 15 of 22
FIGURE LEGENDS
cr
Figure 2. Humidity dynamic response ZnO nanoparticles
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Figure 1. Humidity dynamic response commercial sensor
an
Figure 4. Humidity sensing principle of ZnO nanorods
us
Figure 3. ZnO nanoparticles sensor response
Ac ce p
te
d
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Figure 5. Humidity sensing principle of ZnO nanoparticles
16 Page 16 of 22
Ac ce p
te
d
M
an
us
cr
ip t
Highlights: ZnO nanostructures (nanoparticles and nanorods) are evaluated as humidity sensors at room temperature Physico-chemical behaviour of the ZnO structures is exposed and discussed ZnO nanparticles shows inverse behaviour than ZnO nanorods under different RH atmospheres Promising results as RH sensor is observed for ZnO nanostructures
17 Page 17 of 22
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Figure(s)
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Figure(s)
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S0925-4005(14)00311-6 http://dx.doi.org/doi:10.1016/j.snb.2014.03.043 SNB 16694
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-12-2013 4-3-2014 11-3-2014
Please cite this article as: J. Herr´an, I. Fern´andez, E. Ochoteco, G. Caba˜nero, H. Grande, The role of water Vapour in zno nanostructures for humidity sensing at room temperature, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.03.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ip t
SHORT COMMUNICATION
cr
THE ROLE OF WATER VAPOUR IN ZnO NANOSTRUCTURES
us
FOR HUMIDITY SENSING AT ROOM TEMPERATURE
an
J. Herrán*, I. Fernández, E. Ochoteco, G. Cabañero, H. Grande
M
IK4-CIDETEC, Sensors Unit, Materials Division Pº Miramón 196, E-20009 San Sebastián, Spain
Ac ce p
te
d
Tel.+34 943 309 022 Fax.+34 943 309 136 e-mail: [email protected]
Page 1 of 22
Abstract
The role of water Vapour in ZnO nanostructures for humidity sensing at room
ip t
temperature is presented and discussed. Experimental and theoretical results demonstrate that ZnO nanoparticles and nanorods, show different physico-chemical
cr
behaviour under different relative humidity atmospheres. While electrical current density increases as RH does in the case of the ZnO nanoparticles, ZnO nanorods show inverse
us
behaviour. These facts are related to the capillary condensation and water electric dipole
an
moment effects, respectively. Additionally, a simultaneous validation between the sensor developed and a commercial device corroborates the potential application of this kind of
d
M
low-cost sensing nanostructures presented in this work.
Ac ce p
condensation
te
Keywords: humidity sensor, room temperature, ZnO nanostructures, capillary
2 Page 2 of 22
1. INTRODUCTION
Solid state humidity sensors are being developed for different purposes such as IAQ
ip t
(Indoor Air Quality), HVAC (Heating Ventilating Air Conditioning), process control, agriculture and instrumentation [1, 2]. Several sensing principles can be used to these
cr
purposes, but solid state sensors are an attractive choice due to their low cost and functionality. In this field, several materials (ceramics, semiconductors and polymers)
us
have been tested and have shown diverse results related to humidity sensing [3–11].
an
During the last years, some authors have reported humidity sensitivity of ZnO synthesized by different methods (Chemical Vapour deposition, Pulse Laser Deposition,
M
chemical synthesis and screen-printing technology) [12-18], and some developments have been carried out in order to study their performance as sensing material for water
d
vapour detection at room temperature. In this context, the nanotechnology has been
te
crucial to achieve the goal of room temperature operation, due to the physico-chemical properties of the ZnO nanomaterials. Moreover, it is well known the improved properties
Ac ce p
of these nanostructures sensing materials due to the high surface/volume ratio: kinetics, sensitivity, sensor response and, finally, the reduction of the device dimensions allows diminishing the power consumption of the system [12-13].
In this work, the role of water vapour in ZnO nanostructures for humidity sensing at room temperature is presented and discussed. Due to the fact the interaction between the water molecules and the semiconductor surface takes place at room temperature, the sensing reactions involved in the humidity monitoring are based on physiadsorption phenomena. Previous results based on ZnO nanorods presented by the authors [1] show how an electronic interchange between the dissociated water and the ZnO nanorods dominates the system, increasing the resistance as the RH (Relative Humidity)
3 Page 3 of 22
does. The electrons of the sensing material bulk (ZnO nanorod) are captured on the surface by the water molecules, and the current density of device diminishes. However, the present novel research carried out shows an inverse behaviour in a system based on
ip t
ZnO nanoparticles. Under this nanostructure scenario (boundary between water vapour and ZnO nanoparticles), capillary condensation phenomena of water vapour are
cr
observed on the surface of the nanoparticles, and a water conductive path is formed on the electrodes of the device. Due to this fact, the resistance of the system diminishes as
us
RH does. Theoretically, these results are according to the literature described about this
an
kind of devices: capillary condensation of water vapour is observed in nanoparticles between 2 and 100 nm [19], while an electronic interchange between the dissociated
M
water and the ZnO dominates the system in thin-films [18]. This theoretical behaviour exposed in the previous paragraphs has been validated by means of the experimental
Ac ce p
te
d
results of the sensing devices developed in this work.
4 Page 4 of 22
2. EXPERIMENTAL
The fabrication process of test samples was as follows: a nanoparticle dispersion was
ip t
prepared according to the Pacholski et al. method [20]. Briefly, 0.11 g of zinc acetate dehydrate was dissolved in 50 mL of methanol under vigorous stirring at 60°C.
cr
Subsequently, a 0.03 M solution of KOH (25 mL) in methanol was added dropwise at 60°C. The reaction mixture was stirred for 2 h at 60°C. The obtained colloidal
us
suspension, without any further purification treatment, was deposited by drop-coating on
an
the substrate, where a metallic interdigitated electrode has been previously patterned.
M
The final conductometric device was followed by a 5 minutes air annealing at 100°C.
Electrical characterization was performed inside a sealed plastic chamber by a keithley
d
2400 sourcemeter. Tests were carried out under different RH at room temperature (25 ±
te
1 ºC). The humidity target atmosphere is obtained by means of a mixing system consisting of mass flow controllers (MFCs) from Bronkhorst Hi-Tech controlled by a PC.
Ac ce p
A Dynamic Data Exchange communication is established between the computer and the MFCs to operate them by Labview©. Inside the chamber, a commercial humidity sensor (LinPicco™ A05 Basic, Capacitive Humidity Module) was used as a humidity indicator in order to control the different atmospheres.
5 Page 5 of 22
3. RESULTS AND DISCUSSION
ZnO nanoparticles as humidity sensor at room temperature
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3.1
Regarding materials characterization, structural studies of the ZnO nanoparticles were in
previous
IK4-CIDETEC
publications
[21-22].
Optical
density
and
cr
reported
fluorescence spectra were performed for the nanoparticles suspension, and the reported
us
values for ZnO nanocrystals colloids were 3.4 nm in diameter. Moreover, those values
an
agree with the size of the nanocrystals detected through HRTEM. In fact, it was observed isolated crystalline particles with an average diameter of 3.5 nm and high
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crystallinity. The diffraction rings are indexed using the typical wurtzite structure of ZnO. The average diameter of the ZnO nanoparticles is a crucial issue related to the capillary
te
the following paragraphs.
d
condensation of water vapour on the semiconductor surface, as it will be discussed in
Ac ce p
In order to get a stable sensor signal, the operation voltage has been fixed at 1 V in a pulse mode and 1 second of fixed duty cycle. It has been tested that a continuous operation voltage does not show stability of the sensor based line. The instability is related to temperature gradient produced within the interface between the water vapor molecules and the ZnO nanoparticles.
The joule effect associated to the electrical
current density is in relation to this phenomenon, and a pulse operation mode avoids the problem. A thermodynamical equilibrium is reached by this typical electrical technique very often implemented in sensors signal conditioning.
Figure 1 and figure 2 show the dynamic sensor response at different RH conditions of the commercial device and the ZnO nanoparticles sensor, respectively. The time
6 Page 6 of 22
necessary to fill/evacuate the chamber is estimated around one minute due to the step data acquisition (30 s) and the response time of the commercial device (lower than 5 s, datasheet parameters). In this context, the kinetics of the ZnO nanopaticles sensors
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show excellent response times ( around 5 seconds) according to the commercial sensor datasheet parameters. The system kinetics is in good agreement to the reduction of
cr
diffusion effects between gas target and the bulk of the material: from classic ZnO coating systems to ZnO nanostrutured materials [1, 13-14]. As a valued point, the base
us
line of the ZnO nanoparticles sensor and the commercial one shows similar behaviour.
an
Due to this fact, it can be assumed that the ZnO nanoparticles sensor shows good stability and no drift effects are observed. Moreover, according to this excellent kinetics
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behaviour, the possibility of sensing at room temperature allows the integration in mobile and portable systems due to the low power consumption of the device (around a
te
d
maximum of 16 μW pulsed operation voltage).
As it can be observed in the figure 2 (humidity dynamic response ZnO nanoparticles),
Ac ce p
the electrical current of the system increases as humidity does. This physico-chemical behaviour is related to the capillary condensation produced on the ZnO nanoparticles surface [1, 12-16, 19, 23]. A conductive water path dominates the equivalent resistance of the circuit and an increase of the current density is observed. This effect is not observed in ZnO nanorods systems [1, 9, 18], nevertheless an inverse behaviour is observed. Basically, the different physico-chemical behaviour is based on adsorption effects, and it will be discussed in depth in the following paragraphs (subsection 3.2).
In this context, figure 3 shows the ZnO nanoparticles sensor response at different RH values (3 different sensors and 8 measurements per humidity set value and sensor). Repeatability of the response was checked by means of 8 measurements for each RH
7 Page 7 of 22
value. As a representative value, the relative standard deviation (R.S.D.) at 42% RH is 6.2%. According reproducibility, 3 different sensors were tested at the different RH concentrations, and the R.S.D. at 42% RH is 8.6%. Moreover, the stability of the system
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is represented by the error bar obtained from the dynamic curves: RH from the commercial sensor (x-axis) and measured electrical current from the ZnO nanoparticles
cr
(y-axis). It is worth to mention the precision and no-overlapping of the experimental points. Under a theoretical point of view, the sensor response can be fitted as an
us
exponential curve: I = I 0 exp( RH ) (1), as it is represented in the equation inserted in the
an
figure 3. This behaviour is related to the capillary condensation, which takes place in nanoparticles between 2 and 100 nm [19]. Additionally, it is well known that the depletion
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place between the nanoparticales and the gas target is also modelled as an exponential curve, due to the potential barrier formed among the ZnO nanoparticles. All these facts
d
corroborate these theoretical approaches proposed to the experimental results obtained
Discussion: the role of water vapour in ZnO nanostructures for humidity
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3.2
te
(figure 3).
sensing at room temperature
As it has been presented in the section 3.1 and according to previous results reported by IK4-CIDETEC [1], ZnO nanorods and nanoparticles show different physico-chemical behaviour under water vapour atmospheres. In this context, the following graphical sketches represent the different nano-interaction scenarios, in relation to sensing principles and transduction parameters.
8 Page 8 of 22
Figure 4 shows the humidity sensing principle of ZnO nanorods. The sensing mechanism is based on the interaction between the semiconductor nanorod (ZnO) and the water vapor. The water electric dipole moment (oxygen δ- and hydrogen δ+) [9, 24]
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allows to trap electrons from the conduction band of the semiconductor, and the current density of the system decreases (J2
cr
to physical parameters involved in the sensing process (space charge and the work
us
function) in the reference 1.
an
It is worth to mention that the humidity sensing principle of ZnO nanorods (J2
M
temperature, the physiadsorption dominates the sensing reactions; chemiadsorption phenomena are not observed due to the energy of the system is not high enough to
te
d
produce these effects [9, 24].
Regarding to the results presented in this work, related to ZnO nanoparticles humidity
Ac ce p
sensing, an inverse physical behavior can be observed. As it has been previously mentioned, the humidity sensing principle of ZnO nanoparticles (the electrical current of the system increases as humidity does) is associated to the electronic interchange produced by the capillary condensation of water vapour on the semiconductor nanoparticle interface (for nanoparticles size between 2 and 100 nm [19]). This electrical behaviour is not observed in this kind of Ag/ZnO Schotcky diode configuration.
Figure 5 shows the humidity sensing principle of ZnO nanoparticles. It can be graphically observed how the conductive water path formed on the electrodes increases the conductivity of the system. While RH increases, the water path will be formed by more water molecules and the conductivity of the system will increase. In absence of water,
9 Page 9 of 22
the resistive path is out of range due to the high resistance related to the potential barrier formed between each ZnO nanoparticle. Experimental results presented in figure 3, corroborate this sensing mechanism proposed: equation 1 fits the experimental data to
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the theoretical exponential behaviour associated to the capillary condensation
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phenomena.
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4. CONCLUSIONS
an
ZnO nanoparticles for relative humidity monitoring at room temperature are presented. The semiconductor sensing material is based on ZnO nanoparticles and it is synthesized
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by Pacholski method. The response time is estimated around 5 seconds and the sensitivity shows an exponential relation in the humidity range measured (0 and 80%
te
d
RH).
The electrical changes are related to the depletion place formed between the ZnO
Ac ce p
nanoparticales and the water molecules, at different RH concentrations. The sensing process is based on capillary condensation of the water molecules on the ZnO nanoparticles surface.
Due to this fact, a conductive water path dominates the
equivalent resistance of the circuit, and the current density increases as the RH does. The experimental results fit the theoretical exponential curve, according to the models proposed.
Regarding to the role of water vapour in ZnO nanostructures for humidity sensing at room temperature, it has been experimentally and theoretically demonstrated that ZnO nanorods and nanoparticles show different physical-chemical behaviour. While electrical current density increases as RH does in the case of the ZnO nanoparticles, ZnO
10 Page 10 of 22
nanorods show inverse behaviour. These facts are related to the capillary condensation and water electric dipole moment effects, respectively.
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Finally, in both sensing systems, a simultaneous validation between the sensor developed and a commercial device corroborates the potential application presented in
cr
this work. Moreover, the possibility of sensing at room temperature allows the integration in mobile and portable systems due to the low power consumption of the device (around
us
a few μW).
an
Acknowledgements
This work has been funded by the Spanish Government through the project Hyper
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(Hybrid Neuroprosthetic and Neurorobotic Devices for Functional Compensation and
Ac ce p
REFERENCES
te
(CSD2009-00067).
d
Rehabilitation of Motor Disorders) under program Consolider-Ingenio 2010-2014
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2. Zhi Chen and Chi Lu, Humidity Sensors: A Review of Materials and Mechanisms, Sensor Letters Vol. 3, 274-295, 2005. 3. S. Chakraborty, K. Nemoto, K. Hara, and P. T. Lai, Moisture sensitive field effect transistors using SiO2/Si3N4/Al2O3 gate structure, Smart Mater. Structure 8, 274 (1999).
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12. Q. Wan, Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, X. G. Gao, J. P Li, Positive temperature coefficient resistance and humidity sensing properties of Cd-doped ZnO nanowires, Appl. Phys. Lett. 84 (2004) 3085-3087. 13. Q. Qi, T. Zhang, Q. J. Yu, R. Wang, Y. Zeng, L. Liu, H. B. Yang, Properties of humidity sensing ZnO nanorods-based sensor fabricated by screen-printing, Sens. Actuators B 133 (2008) 638-643. 14. Jingbin Han, Fengru Fan, Chen Xu, Shisheng Lin, Min Wei, Xue Duan and Zhong Lin Wang, ZnO nanotube-based dye-sensitized solar cell and its application in selfpowered devices, Nanotechnology 21 (2010) 405203 (7pp).
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16. Yongsheng Zhang, Ke Yu, Desheng Jiang, Ziqiang Zhu, Haoran Geng and Laiqiang Luo, Zinc oxide nanorod and nanowire for humidity sensor, Applied Surface Science
cr
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20. C. Pacholski, A. Kornowski and H. Weller, Angew. Chem., Int. Ed., 2002, 41, 1188. 21. J. Ajuria et al., Inverted ITO-free organic solar cells based on p and n semiconducting oxides. New designs for integration in tandem cells, top or bottom detecting devices, and photovoltaic windows, Energy Environ. Sci., 2011,4, 453-458 22. Michele Sessolo et al., Zinc oxide nanocrystals as electron injecting building blocks for plastic light sources, J. Mater. Chem., 2012,22, 4916-4920 23. R. Tena-Zaera, J. Elias, C. Levy-Clement, I. Mora-Sero, Y. Luo, and J. Bisquert, Electrodeposition and impedance spectroscopy characterization of ZnO nanowire arrays, Physica Status Solidi a-Applications and Materials Science, 2008, 205, 23452350.
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24.S. Peulon and D. Lincot, Mechanic study of cathodic electrodeposition of zinc oxide and zinc hydroxychloride films from oxigenated aqueous zinc chloride solutions, Journal
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BIOGRAPHIES
Dr. Jaime Herrán received his first class honours degree in Physics at the University of
us
Cantabria in 2004 and his PhD at the University of Navarra in 2008. He was working in
an
the communication engineering department of the University of Cantabria (2003-2004), the Microsystems unit of the CEIT and Tecnun, University of Navarra (2005-2010) and in
M
2008, he was working as a postdoc in the Microsystems Technology division of the CSEM in Neuchâtel (Switzerland) during 1 year. Since 2010, he is a researcher and a
d
project manager in the Sensors Unit of the Materials Division at IK4-CIDETEC, San
te
Sebastián, Spain. His research interests are solid-state Microsystems, nanotechnology
Ac ce p
and printed electronics. For further contact, his e-mail address is: [email protected]
Iván Fernández got his Electrónic Engineer Bachelor Degree by the University of País Vasco (UPV / EHU) in 2005 and the Automatic and Electronic Engineer Master Degree by the University of Mondragón (MU) in 2008. He has extensive experience in printing techniques of materials for different plastic electronics applications with Inkjet and Screen Printing. At present, its activity is focused on the design and manufacture of pressure sensors along with the development of signal conditioning circuits and connectivity systems thereof.
14 Page 14 of 22
Dr.
Estibalitz Ochoteco is Manager of Sensors Unit in IK4-CIDETEC. She is
specialized in the tailor made synthesis of electroactive materials for advanced electrochemical applications as sensors, biosensors, actuators, nanotechnologies. She
ip t
has published more than 40 articles in peer review journals, and is author of 9 patents. She has participated during the last 5 years in national projects, regional projects,
cr
contracts with industries and European Projects (FP6 and FP7). Since 2011, she is
us
member of the Scientific Committee of the "Smart System Integration Conference".
an
German Cabañero Sevillano. Degree in Material Science Eng., University of Navarra. He is a New Materials Department Manager at IK4-CIDETEC specialized in
M
Nanotechnology, Biomaterials, Sensors and Photonics. He generates project ideas and actively contributes to developments and strategy. Moreover, he contributes to the
d
maintaining of state of the art skill, facilities and infrastructure, e.g. evaluation and
te
implementation of new technologies and concepts. He has worked in a wide range of National and European projects (Transportation, Mining, Construction, Energy,…).
Ac ce p
Before working at IK4-CIDETEC, he worked as R & D Manager at Leading Enterprises Group and CTC.
Dr. Hans-Jürgen Grande is CTO at IK4-CIDETEC. He is PhD in Chemistry Science since 1998 (University of the Basque Country). He has managed during the last 12 years a large number of R&D projects at national and international level. In addition, he has published a large number of scientific articles in the field of electrochemistry. He is member of the Technical Committee at IK4 Research Alliance.
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FIGURE LEGENDS
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Figure 2. Humidity dynamic response ZnO nanoparticles
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Figure 1. Humidity dynamic response commercial sensor
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Figure 4. Humidity sensing principle of ZnO nanorods
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Figure 3. ZnO nanoparticles sensor response
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Figure 5. Humidity sensing principle of ZnO nanoparticles
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Ac ce p
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Highlights: ZnO nanostructures (nanoparticles and nanorods) are evaluated as humidity sensors at room temperature Physico-chemical behaviour of the ZnO structures is exposed and discussed ZnO nanparticles shows inverse behaviour than ZnO nanorods under different RH atmospheres Promising results as RH sensor is observed for ZnO nanostructures
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