Influence of the deposition parameters of graphene ...

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Influence of the deposition parameters of graphene oxide nanofilms on the kinetic characteristics of the SAW humidity sensor S.M. Balashov a,∗ , O.V. Balachova a , A.V.U. Braga a , A. Pavani Filho a , S. Moshkalev b a b

Department of Microsystems, Center for Information Technology Renato Archer (CTI), Campinas, SP, Brazil Center of Semiconductor Components, State University of Campinas (UNICAMP), Campinas, SP, Brazil

a r t i c l e

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Article history: Available online xxx Keywords: Graphene oxide film Atomizer Adsorption kinetic parameters Humidity sensor

a b s t r a c t Symmetric surface acoustic wave (SAW) atomizer was used to deposit graphene oxide (GO) nanofilms from microdroplets of the aqueous GO dispersion onto the surface of the SAW humidity sensors. The sensors with the deposited GO films were tested in a wide range of relative humidity and showed the amplitude of the response in the range of 1–40 kHz sufficient for the majority of applications. The kinetics of the adsorption of water molecules was modeled using the assumption that two independent first order adsorption processes take place. Saturation amplitudes and rate coefficients for both processes were experimentally evaluated using LSQ fitting of the sensor response in the time domain for the films obtained by different number of depositions. The mechanism of the adsorption of water molecules to the GO flakes is proposed and discussed. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction

2. Deposition of sensitive films

GO thin films are known to be very promising candidates for the sensitive elements of various types of fast and reliable humidity sensors [1–3]. The fast response and recovery of the films make possible to use GO-based sensors when relative humidity (RH) of the atmosphere changes significantly during a few seconds [4]. The simplest ways of deposition of such films are spray or spin coating [1–3]. Although these methods allow one to obtain the films with the thickness in nanometer range, it is not always convenient to use them because they can require a significant area of deposition or produce films with the thickness difficult to control. To overcome this problem, we propose a specially designed SAW atomizer which can be used to deposit the film in controllable way by atomizing GO dispersion on the droplet-by-droplet basis. Despite the intensive study of the GO films as the sensitive elements for various types of analytes, there are only few attempts to link the experimental results to adsorption models (see e.g. [5]). Also, the behavior of typical adsorption kinetic curves for the water molecules is not clear yet. In the preset work we developed a special SAW sensor for monitoring the kinetic curves of the adsorption of water molecules to the GO film in a wide range of RH. We explain the obtained kinetic curves using the multi-exponential adsorption model [6,7].

GO was synthesized from natural graphite (Nacional de Grafite, Brazil; Graflake 99580; 99.50% purity) using a modified Hummer’s method described by Marcano et al. [8]. Then, the obtained solid material was mixed with DI water with the initial concentration of 1.65 mg/ml, dispersed by ultrasonication for 1 h at room temperature and centrifuged for 10 min at 5000 rpm (1700 × g). The supernatant was aspirated and transferred to a clean vessel for further use. This method allows one to obtain very stable and homogeneous aqueous GO dispersions that can be stored for several months without changing their properties. Sensitive GO films were obtained at the active surface of the sensors by deposition of microsprays of these aqueous GO dispersions using the SAW atomizer (Fig. 1). To produce microspray, a microliter droplet of the GO dispersion (5) is placed in the center of symmetric SAW structure formed by two interdigital transducers (2) printed on the surface of a piezoelectric crystal (1). A continuous RF signal from the generator (3) through a matching circuit (8) is applied to the transducers where it is transformed into the SAW (4). The SAW propagates toward the droplet of dispersion, breaks it into the nanoliter droplets (6) forming a microspray that can be deposited onto the surface of the sample (7) positioned a few millimeters above the atomizer. More technical details of this process can be found elsewhere [9]. As far as the particles of the microspray contain the GO flakes from de dispersion, after their deposition onto the surface of the sample (the SAW quartz delay line (DL) in our case) and drying, the

∗ Corresponding author. Tel.: +55 1937466169. E-mail address: [email protected] (S.M. Balashov).

http://dx.doi.org/10.1016/j.snb.2014.11.050 0925-4005/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Please cite this article in press as: S.M. Balashov, et al., Influence of the deposition parameters of graphene oxide nanofilms on the kinetic characteristics of the SAW humidity sensor, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.11.050


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GO. It means that the final phase characteristic of the SAW DL is linear which is necessary for the precise measurements.

3. Experimental setup and RH measurements

Fig. 1. Thin film deposition using the SAW atomizer. 1 – piezoelectric crystal; 2 – SAW transducers; 3 – RF generator; 4 – SAW generated by the transducers; 5 – microliter droplet; 6 – spray of the atomized nanoliter droplets; 7 – sample/sensor; 8 – matching circuit.

material can form a very thin GO film on it. The thickness of the film is controlled by the number of microliter droplets which are atomized one-by-one. The profile and partial view of the GO film obtained by the deposition of a single nanoliter droplet from the microspray are shown in Fig. 2. As it can be seen, the final thickness of the GO layer can be as low as 7 nm. The process of formation of the sensitive GO film after consecutive 1, 2 and 10 atomizations of 0.2-␮l droplets is shown in Fig. 3. The deposition is done through the window only onto the active area of the sensor, which corresponds to the SAW propagation path of the delay line. Note that 10 atomized microdroplets form a quite uniform GO film, whereas a small number of atomizations (Fig. 3a and b) results in formation of the films with partial coverage of the surface. Although the films are not uniform in this case, the reflectivity of the SAW from each individual section of the film is low due to the low density of the

The SAW sensor used in this work consists of two channels. Each channel – the quartz oscillator – is stabilized by the SAW delay line with central frequency of 160 MHz. One DL is coated by the sensitive GO film obtained by the method described in the previous section. The details of construction of this sensor were described earlier [9]. The output differential frequency proportional to mass loading of the DL with the sensitive film is registered once a second. In order to study a process of desorption of water molecules from the surface of the liquid sample, we used a specially designed and constructed test chamber (see Fig. 4). To provide a stable relative humidity inside the chamber, we used oversaturated water solutions of different salts [3]. In Fig. 4, the liquid sample (1) and the SAW delay line coated by the GO film (2) are placed inside the chamber. The equilibrium in this case is determined by three different processes characterized by their own saturation times. The first process with saturation time 0 is the adsorption of water molecules from the gas phase (4) to the surface of the GO flakes of the film (5). This type of adsorption is determined by week van der Waals forces. The density of the adsorbed molecules in this case is proportional to the partial pressure of the water vapor in the gaseous phase. The second process with saturation time 1 corresponds to the desorption of water molecules from the surface of the solution (3) into the gas phase (4) and the third one with saturation time 2 corresponds to the adsorption of water molecules by the bulk of GO flakes (6). The GO film is known to have very small response time [4] and we assume

Fig. 2. AFM measurements of the deposited GO film. a – film profile; b – partial view.

Fig. 3. GO films obtained by the SAW atomization of 0.2-␮l droplets of aqueous GO dispersions: (a) 1 deposition; (b) 2 depositions; (c) 10 depositions.



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that it is determined by the first process. We assume also that saturation times of the two other processes are significantly higher and that the relation 2 > 1 0 is valid. This assumption means that the output frequency of the sensor is determined by kinetics of the second and the third processes and, considering them independent, it can be concluded that the resulting frequency is proportional to their sum. 4. Modeling of adsorption kinetics The kinetics of adsorption can be normally described by the first or second order diffusion equations [6]. Their solutions are the time-dependent first or second order exponential adsorption kinetic curves (AKC) which have characteristic saturation amplitude and time. When several first order processes take place simultaneously, the multi-exponential model [6,7] can be used. In this model it is possible to approximate the AKC as the sum of several processes each of which has its own saturation amplitude and time. The validity of the model to be used should be checked by the examination of experimental AKC and fitting it to a given model. Considering that for our case two distinct processes were indicated, the AKC in multi-exponential model can be expressed as f (t) = F0 + A1 (1 − exp(−˛1 t)) + A2 (1 − exp(−˛2 t))

(1)

where f(t) is a differential frequency response of the sensor; F0 is a baseline frequency; Ai and ˛i = 1/ i (i = 1, 2) are the saturation amplitude and rate coefficient of correspondent processes. This equation was LSQ-fitted to experimental AKCs with the minimum mean squared error (MSE) as a target and the correspondent parameters were extracted. The result of typical fitting process is shown in Fig. 5. Note that if one of the processes is slow, its exponent can be expanded into the Taylor series with only linear term left [9]. This approximation gives very similar results to the fitting with Eq. (1). Fitting with the first and second order AKCs gave the MSE several times bigger and was excluded from further analysis.

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Fig. 6. Saturation amplitude of the second process A1 . N is the number of atomizations (N = 0 corresponds to the free surface of quartz).

bar is not shown, it means that for a given point it happened to be smaller than the point marker. The second process saturation amplitude (Fig. 6) grows with the increasing of the surface coating (number of atomizations). Growth of the A1 is not proportional to N because of the partial overlap (see Fig. 3) of dried nanoliter droplets during the sequence of atomizations. It can be seen that the saturation amplitude of the second adsorption process is linear in a wide range of RH and can be used as the calibration curve for the sensor. The rate coefficient for this process (Fig. 8) has a week dependence on N confirming the assumption that the adsorption sites of the second process are situated on the surface of the flakes. The saturation amplitude for the third process has nonlinear behavior with slope change in the region of RH = 70–80% (Fig. 7). We explain this by the fact that, starting from these values, RH is high enough for the molecules of water to start to penetrate between the GO flakes. The rate coefficient for the third process (Fig. 9) has as week dependence on N as for the second process,

20 Frequency (KHz)

5. Experimental results and discussion The measurement procedure for each sample always started with the condition of RH = 0 (dry atmosphere), which was achieved by placing commercially available silica gel inside the test chamber. This was done in order to keep the same value of F0 for all measurements. The liquid sample was placed instantly inside the chamber and the measurements started from this moment (Fig. 5). The results of the extraction of saturation amplitudes for two processes are shown in Figs. 6 and 7. The values of rate factors are shown in Figs. 8 and 9. Each point in these curves has the standard deviation marked by the error bar which corresponds to three independent measurements of the same sample. If the correspondent error

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Fig. 5. Typical adsorption kinetic curve measured by the sensor (red curve) and the result of its fitting by Eq. (1) (blue curve). 1 is a moment at which the liquid sample was introduced. Discrepancy between experimental and fitting curve is less then 1.5%.

Frequency (KHz)

Fig. 4. Test chamber for the measurements of RH. 1 – liquid sample (solution); 2 – sensitive GO film; 3 – water molecules of the solution; 4 – water molecules desorbed from the surface of the solution; 5 – water molecules adsorbed to the surface of GO flakes; 6 – water molecules adsorbed to the bulk of GO flakes.

N=0 N=5 N = 11 N = 34

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Fig. 7. Saturation amplitude A2 of the third process. For N see capture for Fig. 6.


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Fig. 8. Rate coefficient ˛1 of the second process. For N see capture for Fig. 6.

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Fig. 9. Rate coefficient ˛2 of the third process. For N see capture for Fig. 6.

which means that the depth of penetration of water molecules into the film does not depend on the effective film thickness and, thus, is smaller then 7 nm (the thickness of the nanoliter droplet according to Fig. 2). It is possible that the GO flake itself has two types of adsorption sites. The first type corresponds to the sites situated on the inner surface of the flake (second process) and the second type corresponds to the sites situated near or directly on its border (third process). Due to the difference in the degree of oxidation, these sites have different energy and the GO flake has different number of them. The energy difference can explain the difference in rate coefficients (Figs. 8 and 9) and the difference in the number of sites explains the difference in saturation amplitudes (Figs. 6 and 7). To understand the details of the adsorption of water molecules by the GO flake further investigation will be done. 6. Conclusion The SAW digital atomizer developed for deposition of the GO films can be used for controlled nanofilm formation in small areas. It can be used for the sensitive film deposition onto the active surface of SAW sensors. The response of the GO humidity SAW sensor can be described by the use of two-exponential model, which corresponds to two adsorption processes. The first process is approximately ten times faster than the second one. The fastest adsorption process can be used for calibration of fast humidity SAW sensor based on the GO nanofilms. Acknowledgements This work has been sponsored by The National Council for Scientific and Technological Development (CNPq, project 590032/2011-9). Electron microscopy images have been taken at the Center for Nanoscience and Nanotechnology (C2NANO)/MCT (high resolution SEM-FEG Inspect). Atomic force microscopy

images have been taken at the State University of Campinas (UNICAMP). The authors are grateful to M.A. Canesqui (UNICAMP) and P.H. Nascimento (CTI) for technical assistance during AFM measurements and sample preparation. References [1] Y. Yao, X. Chen, H. Guo, Z. Wu, Graphene oxide thin film coated quartz crystal microbalance for humidity detection, Appl. Surf. Sci. 257 (2011) 7778–7782. [2] Y. Yao, X. Chen, H. Guo, Z. Wu, X. Li, Humidity sensing behaviors of graphene oxide-silicon bi-layer flexible structure, Sens. Actuators B B161 (2012) 1053–1058. [3] S.M. Balashov, O.V. Balachova, A. Pavani Filho, M.C.Q. Bazetto, M.G. Almeida, Surface acoustic wave humidity sensors based on graphene oxide thin films deposited with the surface acoustic wave atomizer, ECS Trans. 49 (2012) 445–450. [4] H. Bi, K. Yin, X. Xie, J. Ji, S. Wan, L. Sun, M. Terrones, M.S. Dresselhaus, Ultrahigh Humidity Sensitivity of Graphene Oxide, Scientific Reports v3, Article number: 2714, 2013, http://dx.doi.org/10.1038/srep02714. [5] F. Zhang, B. Wang, S. He, R. Man, Preparation of graphene-oxide/polyamidoamine dendrimers and their adsorption properties toward some heavy metal ions, J. Chem. Eng. Data 59 (5) (2014) 1719–1726, http://dx.doi.org/10.1021/je500219e. [6] A.W. Marczewski, Kinetics and equilibrium of adsorption of organic solutes on mesoporous carbon, Appl. Surf. Sci. 253 (2007) 5818–5826, http://dx.doi.org/ 10.1016/j.apsusc.2006.12.037. [7] A.W. Marczewski, Application of mixed order rate equations to adsorption of methylene blue on mesoporous carbons, Appl. Surf. Sci. 256 (2010) 5145–5152, http://dx.doi.org/10.1016/j.apsusc.2009.12.078. [8] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806–4814. [9] S.M. Balashov, O.V. Balachova, A.V.U. Braga, M.C.Q. Bazetto, A. Pavani Filho, The Optimized Saw Humidity Sensor With Nanofilms of Graphene Oxide, in: IEEE Sensors 2013 Conference Proceeding, 2013, http://dx.doi.org/ 10.1109/ICSENS.2013.6688308.

Biographies Sergey Balashov received his Ph.D. in 1987. He worked in the Institute of Radio Engineering in Moscow. In 1994 he joined the ITU as an expert in telecommunications in the research center CPqD TELEBRAS in Brazil. From 2000 to 2013 he worked as the designer of SAW devices for various companies in South Korea. Since 2013 he works in the Center for Information Technology Renato Archer – CTI as the technologist. His research interests are the design of various types of SAW sensors, SAW based microfluidics and 3D bioprinting based on the SAW technology. Olga Balachova graduated from the Department of Physics of the Moscow State University/Russia in 1982. She worked in the Russian Space Corporation “Energia” from 1982 to 1994 as an engineer. She received her Ph.D. in Electrical Engineering from the State University of Campinas/Brazil in 2001. Currently, she is a researcher in the Center for Information Technology Renato Archer – CTI (Campinas/Brazil). Her interests include microfluidics, methods of nanofilms deposition and their characterization, and development of SAW sensors. Ana Valeria Ulhano Braga graduated from the Biology Department of the Pontifical Catholic University of Campinas, Brazil in 2010. She had worked in the area of food microbiology for 2 years. In 2012, she joined the Center for Information Technology Renato Archer in Campinas. Currently, she is a master’s degree student at the Food Engineering Department of the University of Campinas (UNICAMP) and she works on the development of new types of water activity sensors for the food industry. Her research interests include the development of nanofabricated biosensors and their application in the food industry. Aristides Pavani Filho graduated from Electrical Engineering department of University of Campinas (1982) and received his master degree in Electrical Engineering at UNICAMP (1990). Currently he is senior technologist at Centro de Tecnologia da Informação Renato Archer – CTI, Coordinator of CSS and General Coordinator of CTI Northeast. He has experience in Electrical Engineering with emphasis in microlithography, mask making, microsystems, MEMS and SAW sensors. Stanislav Moshkalev received the B.S. and M.S. degree in Quantum Electronics from St. Petersburg Polytechnic University, St. Petersburg, Russia, in 1975 and the Ph.D. degree in physics from A.F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg, Russia, in 1984. His research interests include MEMS, NEMS, microfluidics, novel electron devices (memory, phototransistors, sensors) based on nanostructured carbon materials. Dr. Moshkalev is a member of IEEE, Brazilian Physical Society, Brazilian Microelectronics Society, and Brazilian Carbon Association. Dr. Moshkalev’s awards and honors include the Researcher Fellowship from CNPq and the Distinguished Visiting Researcher Fellowship from IRCEP (the Queen’s University of Belfast, UK).
