- Email: [email protected]
Preparation of flexible VOC sensor based on carbon nanotubes and gold nanoparticles
Sensors and Actuators B 194 (2014) 173–179
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Preparation of flexible VOC sensor based on carbon nanotubes and gold nanoparticles Cihat Tasaltin ∗ , Fevzihan Basarir Materials Institute, TÜBI˙ TAK Marmara Research Center, 41470 Gebze, Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 1 August 2013 Received in revised form 12 December 2013 Accepted 16 December 2013 Available online 24 December 2013 Keywords: Flexible sensor VOC sensing Multi-walled carbon nanotube Gold nanoparticle Polyimide
a b s t r a c t Novel flexible volatile organic compound (VOC) sensor was prepared by deposition of gold nanoparticle (AuNP) and amine modified multi-walled carbon nanotubes (MWCNT-NH2 ) on polyimide substrate via electrospraying technique. First, interdigitated electrodes (IDE) were fabricated on polyimide (Kapton® ) substrate by photolithography. Then, the substrates were subjected to oxygen plasma etching, followed by electrospraying of AuNP and MWCNT-NH2 solution on the substrate. The thin films were characterized by FE-SEM and AFM as well as electrical conductivity measurement. Chemical sensing behaviors of the sensors were analyzed against polar (water, propanol and ethanol) and nonpolar (hexane, toluene, trichloroethylene and chloroform) VOCs. In addition, effect of the AuNP/MWCNT-NH2 ratio on the conductivity and sensing was investigated. Bending test (100 times) resulted in negligible resistance change, demonstrating the successful preparation of flexible sensor. © 2013 Elsevier B.V. All rights reserved.
1. Introduction During the last decade, fabrication of sensors on flexible polymer substrates has attracted great interest, owing to their low-cost, lightness, easy processing, conformability and flexibility [1]. For flexible sensors, polymers including polyimide (PI) [2], polyethylene terephthalate (PET) [3], polydimethylsiloxane (PDMS) [4] and polyethylene naphthalate (PEN) [5] have been evaluated, with polyimide considered the most promising, due to its high thermal, mechanical and chemical stability. Besides, volatile organic compound (VOC) sensing is a current challenging issue because they have great influence on the indoor air quality [6]. Sensing materials have been known to have significant effect on the sensor performance; therefore, there has been great research effort on development of new materials for VOC sensing. Most of the commercial sensors in the current market are based on metal oxides. Although they have good performance, metal oxide based sensors have extreme power consumption and poor chemical selectivity. Mentioned problems have pushed researchers to develop new sensing materials based on organic and composite materials since they have lower power consumption and better chemical selectivity. Phthalocyanine derivatives have been widely studied in the early works, however, water sensitivity and slow response time as well as the problems in film formation have
∗ Corresponding author. Tel.: +90 262 677 3042; fax: +90 262 677 2309. E-mail address: [email protected] (C. Tasaltin). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.12.063
limited their use [7,8]. Consequently, various conducting polymers have been developed [9,10]; however, despite their better performance, they have stability problems which prevent their long-term use. Recent achievements in nanotechnology resulted in development of nano-scale material based sensors. VOC sensing with ligand-stabilized gold nanoparticles (AuNP) was first introduced in 1998 by the work of Wohltjen and Snow [11]. They have successfully demonstrated that the octanethiol coated AuNPs were highly sensitive to organic vapors and the response was fast and reversible. Since then, several research groups have shown VOC sensing by coating of AuNPs with various ligands [12–17]. When the VOC was exposed to the AuNP film, the VOC passes through the pores, which led to swelling of the film [18]. It has been demonstrated that the current between neighboring nanoparticles decreases exponentially with increasing width of the tunneling barrier, resulting in increased resistance of the films [13,14,18]. Conductivity of these materials has been discussed in the context of an activated tunneling model as defined in Eq. (1). ∝ exp(−ˇı) exp
−E a
kb T
(1)
where ˇ, ı, Ea and kb represent the tunneling decay constant, edgeto-edge separation of the metal cores, activation energy, Boltzmann constant, respectively. The first term defines the tunneling current between the neighboring AuNPs, which decreases with increasing particle distance [13]. The second term describes the thermal activation of charge carriers in which Arrhenius term is inversely
174
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
proportional to the permittivity of the organic matrix. According to the equation, conductivity is also dependent of the dielectric constant of the analyte which was ascribed to a change in permittivity of the matrix as the dominating component of sensing mechanism [14,15]. On the other hand, carbon nanotube (CNT), another interesting nanomaterial, was first observed by Iijima [19]. Particularly, multi-walled CNT (MWCNT) based gas sensors have attracted great interest, owing to their large surface area-volume ratio, high electron mobility and high capability of gas adsorption. In addition, the electronic properties suggest that MWCNTs might transfer the charges efficiently upon exposing to analytes [20,21]. Thus, up to date, MWCNTs have been frequently used to detect gases including NOx [22,23], NH3 [24,25] and SO2 [26]. These works exhibited that MWCNTs are highly sensitive to the mentioned gases with a low detection limit and short response time. However, VOC sensing with good performance was a real challenge with MWCNTs. CNT behaves electrically as a thin-film semiconductor with ptype conductivity with the advantage of the sensing properties of the individual nanotubes or their bundles/ropes incorporated in the network [23,27,28]. Increased surface coverage with CNTs forms “spaghetti like” structure which results in increased electron conducting paths between the electrodes [29]. This effect of CNTs in the sensing films can be explained by percolation theory and recent research shows that percolation effect must be considered for sensor applications [30,31]. From gas sensing point of view, the metallic conductive pathways of the CNTs are not desired. The electrical conductivity of the CNT network can be altered by modifying the surface and sidewalls of the nanotubes. Thus, a strategy of altering the electronic properties of nanotubes is by functionalizing with catalytically active materials for enhancing gas sensitivity and imparting selectivity at reduced cross-sensitivity. Several approaches have been attempted such as introducing functional groups on MWCNTs [32,33] as well as functionalization of MWCNTs with polymer [34,35] and metal nanoparticles [36,37]. These approaches have greatly enhanced the sensitivity of the MWCNT based sensors. Recently, Penza has demonstrated the fabrication of CNT–AuNP hybrid structure based gas sensors [38]. Enhanced gas response (NO2 , NH3 , CO, N2 O, H2 S and SO2 ) was exhibited upon deposition of AuNPs on the CNT network. However, CNTs and AuNPs were fabricated by high-cost and complex radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) and sputtering, respectively. Moreover, this hybrid structure has never been utilized for VOC sensing. In this work, therefore, it was attempted to prepare flexible VOC sensor on polyimide substrate by low-cost solution based deposition of amine modified multi-walled carbon nanotubes (MWCNT-NH2 ) and AuNPs simultaneously via electrospraying technique. First, interdigitated electrodes (IDE) were fabricated on polyimide (Kapton® ) substrate by photolithography. Then, the
substrates were subjected to oxygen plasma etching, followed by electrospraying of AuNP and MWCNT-NH2 solution on the substrate. Finally, sensor behaviors were measured against polar and nonpolar VOCs. 2. Materials and methods 2.1. Materials HAuCl4 ·3H2 O, (CH3 (CH2 )7 )4 NBr, C12 H26 S and NaBH4 were purchased from Sigma-Aldrich (St. Louis, USA) and used as received. Amine modified multi-walled carbon nanotubes (MWCNT-NH2 ) was obtained from NANOSHEL (USA) while the flexible polyimide film (Kapton® , 100 HN) with 25.4 m thickness was kindly donated by DuPont (USA). 2.2. Synthesis of AuNPs The AuNPs (∼2–3 nm) capped with dodecanethiol were synthesized by two-phase reduction method, as reported previously [39]. 2.3. Sensor fabrication and measurement The flexible polyimide substrates were equipped with lithographically defined interdigitated electrodes (IDE) in order to investigate the electronic and vapor-sensing properties. As demonstrated in Fig. 1a, the IDE consists of 16 finger pairs with the following dimensions; 5 mm electrode width, 100 m spacing and 100 nm electrode thickness). Prior to film deposition, the polyimide substrates were rinsed with acetone and ethanol, followed by oxygen plasma treatment (30 W, 40 mTorr and 30 s) to make the surface hydrophilic (contact angle less than 10◦ ) [40]. AuNP and MWCNT-NH2 were coated on the polyimide substrate via electrospraying technique. As demonstrated in Fig. 1b, the electrospraying equipment is basically a two-compartment setup with a sample holder that rotates at a velocity of 1.000 rpm, exposing the IDE to the positive electrospray mist and a negative discharge cloud. The coating voltages were adjusted to ∼+3.5 kV for the needle and ∼−1.5 kV for the tungsten tip [7,41]. The polyimide substrates were supported by glass back panels in order to prevent the curling of polyimide film during the coating process. In order to understand the effect of AuNP and MWCNT-NH2 concentration on sensing properties, four different coating compositions have been prepared (Table 1). Typically, 1.25 mg/ml AuNP and MWCNT-NH2 were dissolved in chloroform in different vials and subjected to sonication in order to increase the dispersion. Then, the sensing materials were mixed in different ratios and the total volume was adjusted to 2 ml by adding chloroform, followed by coating 400 l solution on the substrate. The morphology of
Fig. 1. (a) Image of the prepared IDE electrodes on the flexible substrate and (b) schematic diagram of the electrospraying system.
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
175
Table 1 Sensors and related compositions of sensing materials. Sensor number
AuNP (ml)
MWCNT-NH2 (ml)
Solvent (ml)
Conductivity of the sensors (S/cm)
I II III IV
1 1 1 0.5
– 0.5 1 1
1 0.5 – 0.5
1.2 × 10−7 4.2 × 10−3 3.1 × 10−2 1.2 × 10−1
Table 2 Properties of analytes: saturation vapor pressure at −10 ◦ C, the tested concentration range, the saturation vapor pressure at the environmental temperature of 22 ◦ C as calculated using Antoine’s equation. Analyte
Dielectric constant (ε)
Dipole moment ()
Hexane (HEX) Toluene (TOL) Trichloroethylene(TCE) Chloroform (CHL) Propanol (PROP) Ethanol (ETH) Water
1.89 2.38 3.40 4.81 17.90 24.50 80.10
0.08 0.26 0.81 1.15 1.66 1.69 1.85
the coated sensing materials was characterized by FE-SEM (JEOL 63335F JSM) and AFM (Quesant Ambios, Q-Scope).
Concentration (ppm) Min.
Max.
860 120 550 1,150 132 460 4,730
5,160 1,200 3,100 5,800 660 2,300 25,260
P◦ (−10 ◦ C) ppm
P◦ (22 ◦ C) ppm
34,600 4,920 15,800 46,200 2,650 9,200 –
177,130 33,126 87,000 220,000 22,300 66,200 31,578
is the actual analyte concentration and p0i is the saturation vapor pressure at the measurement temperature [41]. 2.5. Bending test
2.4. Sensor measurement Chemical sensing behaviors were analyzed against polar (water, propanol and ethanol) and nonpolar (hexane, toluene, trichloroethylene and chloroform) VOCs. The gas stream containing VOC vapor was generated from cooled bubblers that were immersed in a thermally controlled bath with synthetic air as the carrier gas. The gas stream saturated with the analyte was diluted with pure synthetic air to adjust the gas concentration to the desired amount by using computer driven mass flow controllers (MKS Instruments Inc., USA) at a constant flow rate of 200 ml/min. Typical experiments consisted of repeated exposure to the analyte gas (10 min) and a subsequent purging with pure air (10 min) to reset the baseline. The concentrations of each VOC were varied in the range of 100–5000 ppm (Table 2). The temperature of the sensor chip was kept at room temperature (22 ◦ C) with the help of temperature controller (Lake Shore, USA). Electrical resistances (DC) were measured with a programmable electrometer (Keithley 617). Instruments were controlled and read by computer using a GPIB interface. For a better comparison, the responses of different vapor pressures’ relative concentrations pi /p0i are used, where pi
To investigate the mechanical flexibility of the sensor, bending experiment was performed by directly bending the sensor to 90◦ manually by hand. The test was at least repeated 100 times and resistance change was investigated. 3. Results and discussion 3.1. Morphological and electrical characterizations It is well known that the sensing layer plays a crucial role in the sensor and its structural and chemical characteristics will affect directly the sensing performance. Sensing layers based on AuNP and MWCNT-NH2 has been analyzed by using AFM and SEM. According to Energy Dispersive Spectrum (EDS) and SEM analysis, it has been seen that MWCNT-NH2 and AuNPs were successfully coated on the substrate (Fig. 2a and b) and EDS analysis indicates that the coated film consists of Au and C atoms. The Sensor I consisting only AuNPs demonstrated very smooth surface and interconnected network structure but due to their small size, it was difficult to distinguish AuNPs (Fig. 3a). The coating thickness was
Fig. 2. (a) Energy dispersive spectrum, (b) SEM image for Sensor III.
176
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
Fig. 3. FE-SEM images of Sensors (I–IV), (a) I, (b) II, (c) III, (d) IV.
measured as ∼200 nm and we assume that AuNPs were homogeneously coated with MWCNT-NH2 on the substrate and the AuNPs wrapped the surface of CNTs. As the concentration of MWCNT-NH2 was increased in the coating solution (Table 1), the density of the nanotubes increased in the sensing layer, as evidenced by FE-SEM micrographs (Fig. 3b–d). This also correlates well the with conductivity increase with increased CNT concentration. In addition, the sensor surface was compared with FE-SEM before and after VOC treatment. However, as shown in Fig. S1, no change in the film morphology was observed. Current–voltage measurements were carried out in the range of ±1 V and steps of 0.05 V under dry air at RT. Conductivity of the sensing layers strictly depend on ratios of the components as indicated in Table 1. According to I–V curve (Fig. 4a), conductivity of layer increases with increasing MWCNT-NH2 concentration in the film which can be attributed to the increment in number of electron pathways. In addition, the film conductivity increase with CNT concentration is a clear proof of being under percolation threshold. Also, we believe that the MWCNT-NH2 behave as parallel resistors, which led to decrease in total resistance as shown in Fig. 4b.
The conductivity of the Sensor I is consistent with the previous reports [13,18]. However, the conductivity increase is not so high although there is significant increase in the concentration of MWCNT-NH2 . This can be explained by the formation of defects on the CNTs during the surface modification as well as the insulating property of the organic groups on CNTs. However, metallic bare AuNP and CNT as well as the insulating organic groups on them have strong influence on the total conductivity of device. The electrical conductivity in the Sensors II–IV occurs via tunneling between MWCNT-NH2 ’s and AuNPs have negligible effect. Unfortunately, the conductivity data of Sensor I was not included in Fig. 4a, due to the large difference between the others. The CNT added films are at least ∼100 times more conductive than Sensor I, which illustrates the dominant role of CNTs in total conductivity. 3.2. Sensor responses Hereby, sensor responses were indicated by resistance change between the response and baseline (R = Rresponse − Rbaseline ) in order to show the resistance change numerically. For Sensor I
Fig. 4. (a) I–V Curve of the Sensors (II–IV) and (b) equivalent circuit model (I–IV).
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
177
Fig. 5. Sensor responses at various concentrations (a) I, (b) II, (c) III and (d) IV.
(Fig. 5a) case, the film resistance increased upon analyte exposure. Nevertheless, the AuNP film response is more significant for nonpolar analytes (HEX, TOL, TCE, CHL) than polar counterparts (PROP). In addition, the films do not have any response to water and ethanol vapor. Consistent with the previous work, the analytes with low dielectric constant (nonpolar VOCs) seems to have a higher tendency to increase the resistance [18]. This can be attributed to the good interaction of nonpolar VOC vapors with alkyl chains on AuNPs. As mentioned in the Introduction part, the distance between the AuNPs increases during vapor sorption, which led to increase in resistance. Also it is known that length of the alkyl chain has strong influence on the electronic properties of the AuNP film [12,18]. The characteristics of the sensors changed significantly by the changing the ratio of the AuNP and MWCNT-NH2 . It is worthy to note that base-resistance (resistance values of without gases) values decreased and the magnitude of sensor response increased by increasing the concentration of MWCNT-NH2 . In addition, as shown in Fig. 5b–d, the direction of the polarity of the sensors has changed for Sensors II–IV by addition of MWCNT-NH2 . Hereby, we assume that sensing mechanism for the Sensors II–IV occurs in two stages. In the first stage; AuNPs that are believed to wrap the MWCNT-NH2 behaves as the filters and determine the gases that will reach to the surface of MWCNT-NH2 . In the second stage; interactions of the VOC’s and organic functional group on MWCNT-NH2 determine the sensing behavior. For instance, AuNPs are assumed to adsorb HEX vapor and do not allow it to reach to MWCNT-NH2 , thus very small resistance change was observed. Increasing the polarity of the VOCs (Table 2) has led to an increase in
resistance change, which is applicable for TOL, TCE and CHL. However, for the polar analytes with higher dielectric constant (Water, ETH, PROP) almost no sensor response was detected because AuNPs do not allow the passage of the said analytes. The assumption is also consistent with the sensor response, as shown in Fig. 5a. The humidity influences on the sensors are illustrated in Figs. S2 and S3, respectively. The VOCs that were allowed to reach MWCNT-NH2 surface are believed to inject electrons to the conductive path, thus, decreased the resistance of the devices since amine groups are known with their good electron-withdrawing property. For Sensors II–IV, the sensor responses are influenced by two facts; (1) swelling effect of the alkyl chains on AuNP and (2) electron injection to the conductive path (interaction between VOC and sensing layer). However, the magnitude of the CNT was much higher than the AuNP which has resulted in decreased resistance (see Fig. 5b to d). Previous report by Valentini demonstrated that CNTs containing defects exhibited a greater sensitivity towards analytes compared to defect free sensors [42]. This was explained by the interaction of the defect sites with the gases via weak charge, hydrogen bounding and – interaction and large charge transfer area. This was mainly attributed to the strong sp2 carbon–carbon molecular bonding within CNTs which renders defect free CNTs less sensitive to gas molecule bonding. In our report, we believe that the attached amine groups on the MWCNT produced some defects sites along the sidewall of the CNTs. Thus, NH2 groups and defected CNTs might have an action during the sensing of VOCs. For Sensor II, MWCNT-NH2 surface was supposed to be wrapped by the AuNPs and hence, VOC’s
178
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
Fig. 7. A biplot of loadings and scores in the PCA.
Fig. 8. Influence of bending on resistance of Sensor II. Fig. 6. (a) Schematic drawing of the proposed sensing mechanism and (b) response of Sensor II for TCE.
could not reach to the CNT surface. Therefore, there is no influence of defected surface on sensor signal (see Fig. 6a). However, when the concentration of MWCNT-NH2 was increased (concentration of AuNPs decreased), the possibility of VOCs to reach the defects was increased, which led to decreased response magnitude as proposed by Watts [43]. The sensor response decreases inversely proportional to the ratio of MWCNT-NH2 (see Fig. 6b). Several research groups have systematically evaluated CNT composites as sensing materials [35,36,44–46]. Sensing mechanism of those sensors was explained by the interaction between the gas and composite structures and influence of composite ratio was generally ignored. This is the fundamental difference that distinguishes this paper from the previous works. In addition, we have carried out PCA analysis to understate the influence of AuNP/MWCNT-NH2 ratio on the sensor characteristics.
All the VOCs except Hexane are localized in the PCA map as shown in Fig. 7. The sensors with metamorphosed structures of sensing coating formed by the same material generate different sensor responses. These results could help to create a sensor array prepared using the same sensing material but having different sensor profiles. On the other hand, the Hexane response is disposed by the negative effect of the MWCNT-NH2 (Fig. 5b–d). 3.4. Bending test Bending test was manually performed by hand at 90◦ up to 100 times and the I–V curve was obtained in order to understand the influence of bending on sensor resistance. Minor resistance increase was obtained for Sensors I–IV and only data for Sensor II is illustrated as representative (Fig. 8). The small difference between the resulting characteristics clearly indicates that bending of the sensor did not significantly affect its sensing performance.
3.3. Data analysis 4. Conclusions The similarity of the sensors was explored using the principal component analysis (PCA). PCA is a powerful method for analyzing, classifying and reduction the dimensionality of the numerical datasets in multi-vitiate problem [47,48]. This method allows the similarities or dissimilarities among the individual sensors to be explored according to their responses. The goal of the evaluation is to identify the possible variations in the sensor characteristics caused by the changes of ratios of the AuNP and MWCNT-NH2 on the IDE transducer. To obtain the best PCA classification, each sensor must have a different sensing profile compared to others. The PCA was performed with the help of MATLAB and the sensor response after normalization and centering. A biplot of the loadings and scores in the PCA is shown in Fig. 7. The first two principal components contain approximately 97% of the total variance.
Preparation of flexible VOC sensor based on MWCNT-NH2 and AuNPs via solution based process was successfully achieved in this work. Highest gas sensitivity to CHL and TCE has been observed by MWCNT-NH2 and AuNP composite films at RT. Sensing mechanisms was modeled for electrical charge transfer between MWCNT-NH2 networks and adsorbed VOC molecules. We have systematically investigated the effect of sensing layer composition on the sensing mechanism. Minor resistance increase was obtained for Sensors I–IV upon bending 100 times manually. Acknowledgement The authors would like to acknowledge the financial support from FP7 (Grant No: PEOPLE-2011-CIG-303779), TÜBI˙ TAK
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
(Grant No: 111M510) and Materials Institute of TÜBI˙ TAK Marmara Research Center. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.12.063. References [1] B. Hu, W. Chen, J. Zhou, High performance flexible sensor based on inorganic nanomaterials, Sensors and Actuators B: Chemical 176 (2013) 522–533. [2] T.-R. Rashid, D.-T. Phan, G.-S. Chung, A flexible hydrogen sensor based on Pd nanoparticles decorated ZnO nanorods grown on polyimide tape, Sensors and Actuators B: Chemical 185 (2013) 777–784. [3] P.-G. Su, C.-T. Lee, C.-Y. Chou, Flexible NH3 sensors fabricated by in situ selfassembly of polypyrrole, Talanta 80 (2009) 763–769. [4] L. Yuan, J. Dai, X. Fan, T. Song, Y.T. Tao, K. Wang, et al., Self-cleaning flexible infrared nanosensor based on carbon nanoparticles, ACS Nano 5 (2011) 4007–4013. [5] J. Soyoun, J. Taeksoo, V.K. Varadan, Pentacene-based low-voltage strain sensors with PVP/Ta2O5 hybrid gate dielectrics, IEEE Transactions on Electron Devices 57 (2010) 391–396. [6] T. Kenny, Chapter 7—chemical sensors, in: S.W. Jon (Ed.), Sensor Technology Handbook, Newnes, Burlington, 2005, pp. 181–191. [7] C. Tasaltin, I. Gurol, M. Harbeck, E. Musluoglu, V. Ahsen, Z.Z. Ozturk, Synthesis DMMP sensing properties of fluoroalkyloxy and fluoroaryloxy substituted phthalocyanines in acoustic sensors, Sensors and Actuators B: Chemical 150 (2010) 781–787. [8] T. Richardson, V.C. Smith, A. Topacli, J. Jiang, C.H. Huang, In situ visible spectroscopy of a gadolinium bisphthalocyanine LB film exposed to chlorine gas, Supramolecular Science 4 (1997) 465–470. [9] H. Yoon, J. Jang, Conducting-polymer nanomaterials for high-performance sensor applications: issues and challenges, Advanced Functional Materials 19 (2009) 1567–1576. [10] T. Rajesh, D. Kumar Ahuja, Recent progress in the development of nanostructured conducting polymers/nanocomposites for sensor applications, Sensors and Actuators B: Chemical 136 (2009) 275–286. [11] H. Wohltjen, A.W. Snow, Colloidal metal–insulator–metal ensemble chemiresistor sensor, Analytical Chemistry 70 (1998) 2856–2859. [12] Y. Joseph, I. Besnard, M. Rosenberger, B. Guse, H.-G. Nothofer, J.M. Wessels, et al., Self-assembled gold nanoparticle/alkanedithiol films: preparation, electron microscopy, XPS-analysis, charge transport, and vapor-sensing properties†, The Journal of Physical Chemistry B 107 (2003) 7406–7413. [13] Y. Joseph, B. Guse, T. Vossmeyer, A. Yasuda, Gold nanoparticle/organic networks as chemiresistor coatings: the effect of film morphology on vapor sensitivity, Journal of Physical Chemistry C 112 (2008) 12507–12514. ´ X. Chen, J. Michl, T. Vossmeyer, A. Yasuda, Vapor sensitivity [14] Y. Joseph, A. Peic, of networked gold nanoparticle chemiresistors: importance of flexibility and resistivity of the interlinkage, TheJournal of Physical Chemistry C 111 (2007) 12855–12859. [15] H.L. Zhang, S.D. Evans, J.R. Henderson, R.E. Miles, T.H. Shen, Vapour sensing using surface functionalized gold nanoparticles, Nanotechnology 13 (2002) 439–444. [16] N. Krasteva, I. Besnard, B. Guse, R.E. Bauer, K. Müllen, A. Yasuda, et al., Selfassembled gold nanoparticle/dendrimer composite films for vapor sensing applications, Nano Letters 2 (2002) 551–555. [17] F.P. Zamborini, M.C. Leopold, J.F. Hicks, P.J. Kulesza, M.A. Malik, R.W. Murray, Electron hopping conductivity and vapor sensing properties of flexible network polymer films of metal nanoparticles, Journal of the American Chemical Society 124 (2002) 8958–8964. [18] T. Vossmeyer, Y. Joseph, I. Besnard, O. Harnack, N. Krasteva, B. Guse, et al., Gold-nanoparticle/dithiol films as chemical sensors and first steps toward their integration on chip, Proceedings of SPIE 5513 (2004) 202–212. [19] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [20] T. Zhang, S. Mubeen, N.V. Myung, M.A. Deshusses, Recent progress in carbon nanotube-based gas sensors, Nanotechnology 19 (2008) 332001. [21] J. Li, Y. Lu, Q. Ye, M. Cinke, J. Han, M. Meyyappan, Carbon nanotube sensors for gas and organic vapor detection, Nano Letters 3 (2003) 929–933. [22] S.-I.L. Moon, K.-K. Paek, Y.-H. Lee, H.-K. Park, J.-K. Kim, S.-W. Kim, et al., Biasheating recovery of MWCNT gas sensor, Materials Letters 62 (2008) 2422–2425. [23] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi, S. Santucci, Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films, Applied Physics Letters 82 (2003) 961–963. [24] O.K. Varghese, P.D. Kichambre, D. Gong, K.G. Ong, E.C. Dickey, C.A. Grimes, Gas sensing characteristics of multi-wall carbon nanotubes, Sensors and Actuators B: Chemical 81 (2001) 32–41. [25] L.H. Nguyen, T.V. Phi, P.Q. Phan, H.N. Vu, C. Nguyen-Duc, F. Fossard, Synthesis of multi-walled carbon nanotubes for NH3 gas detection, Physica E: Low-dimensional Systems and Nanostructures 37 (2007) 54–57. [26] J. Suehiro, G. Zhou, M. Hara, Detection of partial discharge in SF6 gas using a carbon nanotube-based gas sensor, Sensors and Actuators B: Chemical 105 (2005) 164–169.
179
[27] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, et al., Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [28] M. Penza, G. Cassano, R. Rossi, A. Rizzo, M.A. Signore, M. Alvisi, et al., Effect of growth catalysts on gas sensitivity in carbon nanotube film based chemiresistive sensors, Applied Physics Letters 90 (2007) 103101–103103. [29] Y. Battie, O. Ducloux, P. Thobois, N. Dorval, J.S. Lauret, B. Attal-Trétout, et al., Gas sensors based on thick films of semi-conducting single walled carbon nanotubes, Carbon 49 (2011) 3544–3552. [30] Y. Battie, L. Gorintin, O. Ducloux, P. Thobois, P. Bondavalli, G. Feugnet, et al., Thickness dependent sensing mechanism in sorted semi-conducting single walled nanotube based sensors, Analyst 137 (2012) 2151–2157. [31] P. Bondavalli, P. Legagneux, D. Pribat, Carbon nanotubes based transistors as gas sensors: state of the art and critical review, Sensors and Actuators B: Chemical 140 (2009) 304–318. [32] H.-L. Hsu, J.-M. Jehng, Y. Sung, L.-C. Wang, S.-R. Yang, The synthesis, characterization of oxidized multi-walled carbon nanotubes, and application to surface acoustic wave quartz crystal gas sensor, Materials Chemistry and Physics 109 (2008) 148–155. [33] J. Mäklin, T. Mustonen, K. Kordás, S. Saukko, G. Tóth, J. Vähäkangas, Nitric oxide gas sensors with functionalized carbon nanotubes, Physica Status Solidi B Basic Solid State Physics 244 (2007) 4298–4302. [34] J.K. Abraham, B. Philip, A. Witchurch, V.K. Varadan, C.C. Reddy, A compact wireless gas sensor using a carbon nanotube/PMMA thin film chemiresistor, Smart Materials and Structures 13 (2004) 1045. [35] K.S.V. Santhanam, R. Sangoi, L. Fuller, A chemical sensor for chloromethanes using a nanocomposite of multiwalled carbon nanotubes with poly(3methylthiophene), Sensors and Actuators B: Chemical 106 (2005) 766– 771. [36] A. Star, V. Joshi, S. Skarupo, D. Thomas, J.-C.P. Gabriel, Gas sensor array based on metal-decorated carbon nanotubes, Journal of Physical Chemistry B 110 (2006) 21014–21020. [37] Y. Lu, C. Partridge, M. Meyyappan, J. Li, A carbon nanotube sensor array for sensitive gas discrimination using principal component analysis, Journal of Electroanalytical Chemistry 593 (2006) 105–110. [38] M. Penza, R. Rossi, M. Alvisi, G. Cassano, E. Serra, Functional characterization of carbon nanotube networked films functionalized with tuned loading of Au nanoclusters for gas sensing applications, Sensors and Actuators B: Chemical 140 (2009) 176–184. [39] M.J. Hostetler, J.E. Wingate, C.-J. Zhong, J.E. Harris, R.W. Vachet, M.R. Clark, et al., Alkanethiolate gold cluster molecules with core diameters from 1.5 to 5.2 nm: core and monolayer properties as a function of core size, Langmuir 14 (1998) 17–30. [40] F. Basarir, T.-H. Yoon, Preparation of gold patterns on polyimide coating via layer-by-layer deposition of gold nanoparticles, Journal of Colloid and Interface Science 352 (2010) 11–18. [41] C. Tasaltin, M.A. Ebeoglu, Z.Z. Ozturk, Acoustoelectric effect on the responses of SAW sensors coated with electrospun ZnO nanostructured thin film, Sensors 12 (2012) 12006–12015. [42] L. Valentini, F. Mercuri, I. Armentano, C. Cantalini, S. Picozzi, L. Lozzi, et al., Role of defects on the gas sensing properties of carbon nanotubes thin films: experiment and theory, Chemical Physics Letters 387 (2004) 356–361. [43] P.C.P. Watts, N. Mureau, Z. Tang, Y. Miyajima, J.D. Carey, S.R.P. Silva, The importance of oxygen-containing defects on carbon nanotubes for the detection of polar and non-polar vapours through hydrogen bond formation, Nanotechnology 18 (2007) 175701. [44] C. Wei, L. Dai, A. Roy, T.B. Tolle, Multifunctional chemical vapor sensors of aligned carbon nanotube and polymer composites, Journal of the American Chemical Society 128 (2006) 1412–1413. [45] D.R. Kauffman, A. Star, Carbon nanotube gas and vapor sensors, Angewandte Chemie International Edition 47 (2008) 6550–6570. [46] Z. Zanolli, R. Leghrib, A. Felten, J.-J. Pireaux, E. Llobet, J.-C. Charlier, Gas sensing with Au-decorated carbon nanotubes, ACS Nano 5 (2011) 4592–4599. [47] M. Penza, G. Cassano, Application of principal component analysis and artificial neural networks to recognize the individual VOCs of methanol/2-propanol in a binary mixture by SAW multi-sensor array, Sensors and Actuators B: Chemical 89 (2003) 269–284. [48] B. Mumyakmaz, A. Özmen, M.A. Ebeo˘glu, C. Tas¸altın, I˙ . Gürol, A study on the development of a compensation method for humidity effect in QCM sensor responses, Sensors and Actuators B: Chemical 147 (2010) 277–282.
Biographies Cihat Tasaltin received his Ph.D. degree from the Department of Physics of the Gebze Institute Technology in Turkey. He is a researcher at the TUBITAK Marmara Research Center, Materials Research Institute, Gebze, Turkey. His research activities are in the field of chemical gas sensors using acoustic transducers for health, safety and environmental applications as well as the investigation of sensing properties of organic and inorganic materials. Fevzihan Basarir is working as senior researcher in Materials Institute of TUBITAK Marmara Research Center. He has received his M.Sc. and Ph.D. degree in materials science from Gwangju Institute of Science and Technology (GIST) in 2004 and 2011, respectively. His research interest includes development and assembly of nanomaterials for electronics and optoelectronic applications.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Preparation of flexible VOC sensor based on carbon nanotubes and gold nanoparticles Cihat Tasaltin ∗ , Fevzihan Basarir Materials Institute, TÜBI˙ TAK Marmara Research Center, 41470 Gebze, Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 1 August 2013 Received in revised form 12 December 2013 Accepted 16 December 2013 Available online 24 December 2013 Keywords: Flexible sensor VOC sensing Multi-walled carbon nanotube Gold nanoparticle Polyimide
a b s t r a c t Novel flexible volatile organic compound (VOC) sensor was prepared by deposition of gold nanoparticle (AuNP) and amine modified multi-walled carbon nanotubes (MWCNT-NH2 ) on polyimide substrate via electrospraying technique. First, interdigitated electrodes (IDE) were fabricated on polyimide (Kapton® ) substrate by photolithography. Then, the substrates were subjected to oxygen plasma etching, followed by electrospraying of AuNP and MWCNT-NH2 solution on the substrate. The thin films were characterized by FE-SEM and AFM as well as electrical conductivity measurement. Chemical sensing behaviors of the sensors were analyzed against polar (water, propanol and ethanol) and nonpolar (hexane, toluene, trichloroethylene and chloroform) VOCs. In addition, effect of the AuNP/MWCNT-NH2 ratio on the conductivity and sensing was investigated. Bending test (100 times) resulted in negligible resistance change, demonstrating the successful preparation of flexible sensor. © 2013 Elsevier B.V. All rights reserved.
1. Introduction During the last decade, fabrication of sensors on flexible polymer substrates has attracted great interest, owing to their low-cost, lightness, easy processing, conformability and flexibility [1]. For flexible sensors, polymers including polyimide (PI) [2], polyethylene terephthalate (PET) [3], polydimethylsiloxane (PDMS) [4] and polyethylene naphthalate (PEN) [5] have been evaluated, with polyimide considered the most promising, due to its high thermal, mechanical and chemical stability. Besides, volatile organic compound (VOC) sensing is a current challenging issue because they have great influence on the indoor air quality [6]. Sensing materials have been known to have significant effect on the sensor performance; therefore, there has been great research effort on development of new materials for VOC sensing. Most of the commercial sensors in the current market are based on metal oxides. Although they have good performance, metal oxide based sensors have extreme power consumption and poor chemical selectivity. Mentioned problems have pushed researchers to develop new sensing materials based on organic and composite materials since they have lower power consumption and better chemical selectivity. Phthalocyanine derivatives have been widely studied in the early works, however, water sensitivity and slow response time as well as the problems in film formation have
∗ Corresponding author. Tel.: +90 262 677 3042; fax: +90 262 677 2309. E-mail address: [email protected] (C. Tasaltin). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.12.063
limited their use [7,8]. Consequently, various conducting polymers have been developed [9,10]; however, despite their better performance, they have stability problems which prevent their long-term use. Recent achievements in nanotechnology resulted in development of nano-scale material based sensors. VOC sensing with ligand-stabilized gold nanoparticles (AuNP) was first introduced in 1998 by the work of Wohltjen and Snow [11]. They have successfully demonstrated that the octanethiol coated AuNPs were highly sensitive to organic vapors and the response was fast and reversible. Since then, several research groups have shown VOC sensing by coating of AuNPs with various ligands [12–17]. When the VOC was exposed to the AuNP film, the VOC passes through the pores, which led to swelling of the film [18]. It has been demonstrated that the current between neighboring nanoparticles decreases exponentially with increasing width of the tunneling barrier, resulting in increased resistance of the films [13,14,18]. Conductivity of these materials has been discussed in the context of an activated tunneling model as defined in Eq. (1). ∝ exp(−ˇı) exp
−E a
kb T
(1)
where ˇ, ı, Ea and kb represent the tunneling decay constant, edgeto-edge separation of the metal cores, activation energy, Boltzmann constant, respectively. The first term defines the tunneling current between the neighboring AuNPs, which decreases with increasing particle distance [13]. The second term describes the thermal activation of charge carriers in which Arrhenius term is inversely
174
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
proportional to the permittivity of the organic matrix. According to the equation, conductivity is also dependent of the dielectric constant of the analyte which was ascribed to a change in permittivity of the matrix as the dominating component of sensing mechanism [14,15]. On the other hand, carbon nanotube (CNT), another interesting nanomaterial, was first observed by Iijima [19]. Particularly, multi-walled CNT (MWCNT) based gas sensors have attracted great interest, owing to their large surface area-volume ratio, high electron mobility and high capability of gas adsorption. In addition, the electronic properties suggest that MWCNTs might transfer the charges efficiently upon exposing to analytes [20,21]. Thus, up to date, MWCNTs have been frequently used to detect gases including NOx [22,23], NH3 [24,25] and SO2 [26]. These works exhibited that MWCNTs are highly sensitive to the mentioned gases with a low detection limit and short response time. However, VOC sensing with good performance was a real challenge with MWCNTs. CNT behaves electrically as a thin-film semiconductor with ptype conductivity with the advantage of the sensing properties of the individual nanotubes or their bundles/ropes incorporated in the network [23,27,28]. Increased surface coverage with CNTs forms “spaghetti like” structure which results in increased electron conducting paths between the electrodes [29]. This effect of CNTs in the sensing films can be explained by percolation theory and recent research shows that percolation effect must be considered for sensor applications [30,31]. From gas sensing point of view, the metallic conductive pathways of the CNTs are not desired. The electrical conductivity of the CNT network can be altered by modifying the surface and sidewalls of the nanotubes. Thus, a strategy of altering the electronic properties of nanotubes is by functionalizing with catalytically active materials for enhancing gas sensitivity and imparting selectivity at reduced cross-sensitivity. Several approaches have been attempted such as introducing functional groups on MWCNTs [32,33] as well as functionalization of MWCNTs with polymer [34,35] and metal nanoparticles [36,37]. These approaches have greatly enhanced the sensitivity of the MWCNT based sensors. Recently, Penza has demonstrated the fabrication of CNT–AuNP hybrid structure based gas sensors [38]. Enhanced gas response (NO2 , NH3 , CO, N2 O, H2 S and SO2 ) was exhibited upon deposition of AuNPs on the CNT network. However, CNTs and AuNPs were fabricated by high-cost and complex radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) and sputtering, respectively. Moreover, this hybrid structure has never been utilized for VOC sensing. In this work, therefore, it was attempted to prepare flexible VOC sensor on polyimide substrate by low-cost solution based deposition of amine modified multi-walled carbon nanotubes (MWCNT-NH2 ) and AuNPs simultaneously via electrospraying technique. First, interdigitated electrodes (IDE) were fabricated on polyimide (Kapton® ) substrate by photolithography. Then, the
substrates were subjected to oxygen plasma etching, followed by electrospraying of AuNP and MWCNT-NH2 solution on the substrate. Finally, sensor behaviors were measured against polar and nonpolar VOCs. 2. Materials and methods 2.1. Materials HAuCl4 ·3H2 O, (CH3 (CH2 )7 )4 NBr, C12 H26 S and NaBH4 were purchased from Sigma-Aldrich (St. Louis, USA) and used as received. Amine modified multi-walled carbon nanotubes (MWCNT-NH2 ) was obtained from NANOSHEL (USA) while the flexible polyimide film (Kapton® , 100 HN) with 25.4 m thickness was kindly donated by DuPont (USA). 2.2. Synthesis of AuNPs The AuNPs (∼2–3 nm) capped with dodecanethiol were synthesized by two-phase reduction method, as reported previously [39]. 2.3. Sensor fabrication and measurement The flexible polyimide substrates were equipped with lithographically defined interdigitated electrodes (IDE) in order to investigate the electronic and vapor-sensing properties. As demonstrated in Fig. 1a, the IDE consists of 16 finger pairs with the following dimensions; 5 mm electrode width, 100 m spacing and 100 nm electrode thickness). Prior to film deposition, the polyimide substrates were rinsed with acetone and ethanol, followed by oxygen plasma treatment (30 W, 40 mTorr and 30 s) to make the surface hydrophilic (contact angle less than 10◦ ) [40]. AuNP and MWCNT-NH2 were coated on the polyimide substrate via electrospraying technique. As demonstrated in Fig. 1b, the electrospraying equipment is basically a two-compartment setup with a sample holder that rotates at a velocity of 1.000 rpm, exposing the IDE to the positive electrospray mist and a negative discharge cloud. The coating voltages were adjusted to ∼+3.5 kV for the needle and ∼−1.5 kV for the tungsten tip [7,41]. The polyimide substrates were supported by glass back panels in order to prevent the curling of polyimide film during the coating process. In order to understand the effect of AuNP and MWCNT-NH2 concentration on sensing properties, four different coating compositions have been prepared (Table 1). Typically, 1.25 mg/ml AuNP and MWCNT-NH2 were dissolved in chloroform in different vials and subjected to sonication in order to increase the dispersion. Then, the sensing materials were mixed in different ratios and the total volume was adjusted to 2 ml by adding chloroform, followed by coating 400 l solution on the substrate. The morphology of
Fig. 1. (a) Image of the prepared IDE electrodes on the flexible substrate and (b) schematic diagram of the electrospraying system.
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
175
Table 1 Sensors and related compositions of sensing materials. Sensor number
AuNP (ml)
MWCNT-NH2 (ml)
Solvent (ml)
Conductivity of the sensors (S/cm)
I II III IV
1 1 1 0.5
– 0.5 1 1
1 0.5 – 0.5
1.2 × 10−7 4.2 × 10−3 3.1 × 10−2 1.2 × 10−1
Table 2 Properties of analytes: saturation vapor pressure at −10 ◦ C, the tested concentration range, the saturation vapor pressure at the environmental temperature of 22 ◦ C as calculated using Antoine’s equation. Analyte
Dielectric constant (ε)
Dipole moment ()
Hexane (HEX) Toluene (TOL) Trichloroethylene(TCE) Chloroform (CHL) Propanol (PROP) Ethanol (ETH) Water
1.89 2.38 3.40 4.81 17.90 24.50 80.10
0.08 0.26 0.81 1.15 1.66 1.69 1.85
the coated sensing materials was characterized by FE-SEM (JEOL 63335F JSM) and AFM (Quesant Ambios, Q-Scope).
Concentration (ppm) Min.
Max.
860 120 550 1,150 132 460 4,730
5,160 1,200 3,100 5,800 660 2,300 25,260
P◦ (−10 ◦ C) ppm
P◦ (22 ◦ C) ppm
34,600 4,920 15,800 46,200 2,650 9,200 –
177,130 33,126 87,000 220,000 22,300 66,200 31,578
is the actual analyte concentration and p0i is the saturation vapor pressure at the measurement temperature [41]. 2.5. Bending test
2.4. Sensor measurement Chemical sensing behaviors were analyzed against polar (water, propanol and ethanol) and nonpolar (hexane, toluene, trichloroethylene and chloroform) VOCs. The gas stream containing VOC vapor was generated from cooled bubblers that were immersed in a thermally controlled bath with synthetic air as the carrier gas. The gas stream saturated with the analyte was diluted with pure synthetic air to adjust the gas concentration to the desired amount by using computer driven mass flow controllers (MKS Instruments Inc., USA) at a constant flow rate of 200 ml/min. Typical experiments consisted of repeated exposure to the analyte gas (10 min) and a subsequent purging with pure air (10 min) to reset the baseline. The concentrations of each VOC were varied in the range of 100–5000 ppm (Table 2). The temperature of the sensor chip was kept at room temperature (22 ◦ C) with the help of temperature controller (Lake Shore, USA). Electrical resistances (DC) were measured with a programmable electrometer (Keithley 617). Instruments were controlled and read by computer using a GPIB interface. For a better comparison, the responses of different vapor pressures’ relative concentrations pi /p0i are used, where pi
To investigate the mechanical flexibility of the sensor, bending experiment was performed by directly bending the sensor to 90◦ manually by hand. The test was at least repeated 100 times and resistance change was investigated. 3. Results and discussion 3.1. Morphological and electrical characterizations It is well known that the sensing layer plays a crucial role in the sensor and its structural and chemical characteristics will affect directly the sensing performance. Sensing layers based on AuNP and MWCNT-NH2 has been analyzed by using AFM and SEM. According to Energy Dispersive Spectrum (EDS) and SEM analysis, it has been seen that MWCNT-NH2 and AuNPs were successfully coated on the substrate (Fig. 2a and b) and EDS analysis indicates that the coated film consists of Au and C atoms. The Sensor I consisting only AuNPs demonstrated very smooth surface and interconnected network structure but due to their small size, it was difficult to distinguish AuNPs (Fig. 3a). The coating thickness was
Fig. 2. (a) Energy dispersive spectrum, (b) SEM image for Sensor III.
176
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
Fig. 3. FE-SEM images of Sensors (I–IV), (a) I, (b) II, (c) III, (d) IV.
measured as ∼200 nm and we assume that AuNPs were homogeneously coated with MWCNT-NH2 on the substrate and the AuNPs wrapped the surface of CNTs. As the concentration of MWCNT-NH2 was increased in the coating solution (Table 1), the density of the nanotubes increased in the sensing layer, as evidenced by FE-SEM micrographs (Fig. 3b–d). This also correlates well the with conductivity increase with increased CNT concentration. In addition, the sensor surface was compared with FE-SEM before and after VOC treatment. However, as shown in Fig. S1, no change in the film morphology was observed. Current–voltage measurements were carried out in the range of ±1 V and steps of 0.05 V under dry air at RT. Conductivity of the sensing layers strictly depend on ratios of the components as indicated in Table 1. According to I–V curve (Fig. 4a), conductivity of layer increases with increasing MWCNT-NH2 concentration in the film which can be attributed to the increment in number of electron pathways. In addition, the film conductivity increase with CNT concentration is a clear proof of being under percolation threshold. Also, we believe that the MWCNT-NH2 behave as parallel resistors, which led to decrease in total resistance as shown in Fig. 4b.
The conductivity of the Sensor I is consistent with the previous reports [13,18]. However, the conductivity increase is not so high although there is significant increase in the concentration of MWCNT-NH2 . This can be explained by the formation of defects on the CNTs during the surface modification as well as the insulating property of the organic groups on CNTs. However, metallic bare AuNP and CNT as well as the insulating organic groups on them have strong influence on the total conductivity of device. The electrical conductivity in the Sensors II–IV occurs via tunneling between MWCNT-NH2 ’s and AuNPs have negligible effect. Unfortunately, the conductivity data of Sensor I was not included in Fig. 4a, due to the large difference between the others. The CNT added films are at least ∼100 times more conductive than Sensor I, which illustrates the dominant role of CNTs in total conductivity. 3.2. Sensor responses Hereby, sensor responses were indicated by resistance change between the response and baseline (R = Rresponse − Rbaseline ) in order to show the resistance change numerically. For Sensor I
Fig. 4. (a) I–V Curve of the Sensors (II–IV) and (b) equivalent circuit model (I–IV).
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
177
Fig. 5. Sensor responses at various concentrations (a) I, (b) II, (c) III and (d) IV.
(Fig. 5a) case, the film resistance increased upon analyte exposure. Nevertheless, the AuNP film response is more significant for nonpolar analytes (HEX, TOL, TCE, CHL) than polar counterparts (PROP). In addition, the films do not have any response to water and ethanol vapor. Consistent with the previous work, the analytes with low dielectric constant (nonpolar VOCs) seems to have a higher tendency to increase the resistance [18]. This can be attributed to the good interaction of nonpolar VOC vapors with alkyl chains on AuNPs. As mentioned in the Introduction part, the distance between the AuNPs increases during vapor sorption, which led to increase in resistance. Also it is known that length of the alkyl chain has strong influence on the electronic properties of the AuNP film [12,18]. The characteristics of the sensors changed significantly by the changing the ratio of the AuNP and MWCNT-NH2 . It is worthy to note that base-resistance (resistance values of without gases) values decreased and the magnitude of sensor response increased by increasing the concentration of MWCNT-NH2 . In addition, as shown in Fig. 5b–d, the direction of the polarity of the sensors has changed for Sensors II–IV by addition of MWCNT-NH2 . Hereby, we assume that sensing mechanism for the Sensors II–IV occurs in two stages. In the first stage; AuNPs that are believed to wrap the MWCNT-NH2 behaves as the filters and determine the gases that will reach to the surface of MWCNT-NH2 . In the second stage; interactions of the VOC’s and organic functional group on MWCNT-NH2 determine the sensing behavior. For instance, AuNPs are assumed to adsorb HEX vapor and do not allow it to reach to MWCNT-NH2 , thus very small resistance change was observed. Increasing the polarity of the VOCs (Table 2) has led to an increase in
resistance change, which is applicable for TOL, TCE and CHL. However, for the polar analytes with higher dielectric constant (Water, ETH, PROP) almost no sensor response was detected because AuNPs do not allow the passage of the said analytes. The assumption is also consistent with the sensor response, as shown in Fig. 5a. The humidity influences on the sensors are illustrated in Figs. S2 and S3, respectively. The VOCs that were allowed to reach MWCNT-NH2 surface are believed to inject electrons to the conductive path, thus, decreased the resistance of the devices since amine groups are known with their good electron-withdrawing property. For Sensors II–IV, the sensor responses are influenced by two facts; (1) swelling effect of the alkyl chains on AuNP and (2) electron injection to the conductive path (interaction between VOC and sensing layer). However, the magnitude of the CNT was much higher than the AuNP which has resulted in decreased resistance (see Fig. 5b to d). Previous report by Valentini demonstrated that CNTs containing defects exhibited a greater sensitivity towards analytes compared to defect free sensors [42]. This was explained by the interaction of the defect sites with the gases via weak charge, hydrogen bounding and – interaction and large charge transfer area. This was mainly attributed to the strong sp2 carbon–carbon molecular bonding within CNTs which renders defect free CNTs less sensitive to gas molecule bonding. In our report, we believe that the attached amine groups on the MWCNT produced some defects sites along the sidewall of the CNTs. Thus, NH2 groups and defected CNTs might have an action during the sensing of VOCs. For Sensor II, MWCNT-NH2 surface was supposed to be wrapped by the AuNPs and hence, VOC’s
178
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
Fig. 7. A biplot of loadings and scores in the PCA.
Fig. 8. Influence of bending on resistance of Sensor II. Fig. 6. (a) Schematic drawing of the proposed sensing mechanism and (b) response of Sensor II for TCE.
could not reach to the CNT surface. Therefore, there is no influence of defected surface on sensor signal (see Fig. 6a). However, when the concentration of MWCNT-NH2 was increased (concentration of AuNPs decreased), the possibility of VOCs to reach the defects was increased, which led to decreased response magnitude as proposed by Watts [43]. The sensor response decreases inversely proportional to the ratio of MWCNT-NH2 (see Fig. 6b). Several research groups have systematically evaluated CNT composites as sensing materials [35,36,44–46]. Sensing mechanism of those sensors was explained by the interaction between the gas and composite structures and influence of composite ratio was generally ignored. This is the fundamental difference that distinguishes this paper from the previous works. In addition, we have carried out PCA analysis to understate the influence of AuNP/MWCNT-NH2 ratio on the sensor characteristics.
All the VOCs except Hexane are localized in the PCA map as shown in Fig. 7. The sensors with metamorphosed structures of sensing coating formed by the same material generate different sensor responses. These results could help to create a sensor array prepared using the same sensing material but having different sensor profiles. On the other hand, the Hexane response is disposed by the negative effect of the MWCNT-NH2 (Fig. 5b–d). 3.4. Bending test Bending test was manually performed by hand at 90◦ up to 100 times and the I–V curve was obtained in order to understand the influence of bending on sensor resistance. Minor resistance increase was obtained for Sensors I–IV and only data for Sensor II is illustrated as representative (Fig. 8). The small difference between the resulting characteristics clearly indicates that bending of the sensor did not significantly affect its sensing performance.
3.3. Data analysis 4. Conclusions The similarity of the sensors was explored using the principal component analysis (PCA). PCA is a powerful method for analyzing, classifying and reduction the dimensionality of the numerical datasets in multi-vitiate problem [47,48]. This method allows the similarities or dissimilarities among the individual sensors to be explored according to their responses. The goal of the evaluation is to identify the possible variations in the sensor characteristics caused by the changes of ratios of the AuNP and MWCNT-NH2 on the IDE transducer. To obtain the best PCA classification, each sensor must have a different sensing profile compared to others. The PCA was performed with the help of MATLAB and the sensor response after normalization and centering. A biplot of the loadings and scores in the PCA is shown in Fig. 7. The first two principal components contain approximately 97% of the total variance.
Preparation of flexible VOC sensor based on MWCNT-NH2 and AuNPs via solution based process was successfully achieved in this work. Highest gas sensitivity to CHL and TCE has been observed by MWCNT-NH2 and AuNP composite films at RT. Sensing mechanisms was modeled for electrical charge transfer between MWCNT-NH2 networks and adsorbed VOC molecules. We have systematically investigated the effect of sensing layer composition on the sensing mechanism. Minor resistance increase was obtained for Sensors I–IV upon bending 100 times manually. Acknowledgement The authors would like to acknowledge the financial support from FP7 (Grant No: PEOPLE-2011-CIG-303779), TÜBI˙ TAK
C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173–179
(Grant No: 111M510) and Materials Institute of TÜBI˙ TAK Marmara Research Center. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.12.063. References [1] B. Hu, W. Chen, J. Zhou, High performance flexible sensor based on inorganic nanomaterials, Sensors and Actuators B: Chemical 176 (2013) 522–533. [2] T.-R. Rashid, D.-T. Phan, G.-S. Chung, A flexible hydrogen sensor based on Pd nanoparticles decorated ZnO nanorods grown on polyimide tape, Sensors and Actuators B: Chemical 185 (2013) 777–784. [3] P.-G. Su, C.-T. Lee, C.-Y. Chou, Flexible NH3 sensors fabricated by in situ selfassembly of polypyrrole, Talanta 80 (2009) 763–769. [4] L. Yuan, J. Dai, X. Fan, T. Song, Y.T. Tao, K. Wang, et al., Self-cleaning flexible infrared nanosensor based on carbon nanoparticles, ACS Nano 5 (2011) 4007–4013. [5] J. Soyoun, J. Taeksoo, V.K. Varadan, Pentacene-based low-voltage strain sensors with PVP/Ta2O5 hybrid gate dielectrics, IEEE Transactions on Electron Devices 57 (2010) 391–396. [6] T. Kenny, Chapter 7—chemical sensors, in: S.W. Jon (Ed.), Sensor Technology Handbook, Newnes, Burlington, 2005, pp. 181–191. [7] C. Tasaltin, I. Gurol, M. Harbeck, E. Musluoglu, V. Ahsen, Z.Z. Ozturk, Synthesis DMMP sensing properties of fluoroalkyloxy and fluoroaryloxy substituted phthalocyanines in acoustic sensors, Sensors and Actuators B: Chemical 150 (2010) 781–787. [8] T. Richardson, V.C. Smith, A. Topacli, J. Jiang, C.H. Huang, In situ visible spectroscopy of a gadolinium bisphthalocyanine LB film exposed to chlorine gas, Supramolecular Science 4 (1997) 465–470. [9] H. Yoon, J. Jang, Conducting-polymer nanomaterials for high-performance sensor applications: issues and challenges, Advanced Functional Materials 19 (2009) 1567–1576. [10] T. Rajesh, D. Kumar Ahuja, Recent progress in the development of nanostructured conducting polymers/nanocomposites for sensor applications, Sensors and Actuators B: Chemical 136 (2009) 275–286. [11] H. Wohltjen, A.W. Snow, Colloidal metal–insulator–metal ensemble chemiresistor sensor, Analytical Chemistry 70 (1998) 2856–2859. [12] Y. Joseph, I. Besnard, M. Rosenberger, B. Guse, H.-G. Nothofer, J.M. Wessels, et al., Self-assembled gold nanoparticle/alkanedithiol films: preparation, electron microscopy, XPS-analysis, charge transport, and vapor-sensing properties†, The Journal of Physical Chemistry B 107 (2003) 7406–7413. [13] Y. Joseph, B. Guse, T. Vossmeyer, A. Yasuda, Gold nanoparticle/organic networks as chemiresistor coatings: the effect of film morphology on vapor sensitivity, Journal of Physical Chemistry C 112 (2008) 12507–12514. ´ X. Chen, J. Michl, T. Vossmeyer, A. Yasuda, Vapor sensitivity [14] Y. Joseph, A. Peic, of networked gold nanoparticle chemiresistors: importance of flexibility and resistivity of the interlinkage, TheJournal of Physical Chemistry C 111 (2007) 12855–12859. [15] H.L. Zhang, S.D. Evans, J.R. Henderson, R.E. Miles, T.H. Shen, Vapour sensing using surface functionalized gold nanoparticles, Nanotechnology 13 (2002) 439–444. [16] N. Krasteva, I. Besnard, B. Guse, R.E. Bauer, K. Müllen, A. Yasuda, et al., Selfassembled gold nanoparticle/dendrimer composite films for vapor sensing applications, Nano Letters 2 (2002) 551–555. [17] F.P. Zamborini, M.C. Leopold, J.F. Hicks, P.J. Kulesza, M.A. Malik, R.W. Murray, Electron hopping conductivity and vapor sensing properties of flexible network polymer films of metal nanoparticles, Journal of the American Chemical Society 124 (2002) 8958–8964. [18] T. Vossmeyer, Y. Joseph, I. Besnard, O. Harnack, N. Krasteva, B. Guse, et al., Gold-nanoparticle/dithiol films as chemical sensors and first steps toward their integration on chip, Proceedings of SPIE 5513 (2004) 202–212. [19] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [20] T. Zhang, S. Mubeen, N.V. Myung, M.A. Deshusses, Recent progress in carbon nanotube-based gas sensors, Nanotechnology 19 (2008) 332001. [21] J. Li, Y. Lu, Q. Ye, M. Cinke, J. Han, M. Meyyappan, Carbon nanotube sensors for gas and organic vapor detection, Nano Letters 3 (2003) 929–933. [22] S.-I.L. Moon, K.-K. Paek, Y.-H. Lee, H.-K. Park, J.-K. Kim, S.-W. Kim, et al., Biasheating recovery of MWCNT gas sensor, Materials Letters 62 (2008) 2422–2425. [23] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi, S. Santucci, Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films, Applied Physics Letters 82 (2003) 961–963. [24] O.K. Varghese, P.D. Kichambre, D. Gong, K.G. Ong, E.C. Dickey, C.A. Grimes, Gas sensing characteristics of multi-wall carbon nanotubes, Sensors and Actuators B: Chemical 81 (2001) 32–41. [25] L.H. Nguyen, T.V. Phi, P.Q. Phan, H.N. Vu, C. Nguyen-Duc, F. Fossard, Synthesis of multi-walled carbon nanotubes for NH3 gas detection, Physica E: Low-dimensional Systems and Nanostructures 37 (2007) 54–57. [26] J. Suehiro, G. Zhou, M. Hara, Detection of partial discharge in SF6 gas using a carbon nanotube-based gas sensor, Sensors and Actuators B: Chemical 105 (2005) 164–169.
179
[27] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, et al., Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [28] M. Penza, G. Cassano, R. Rossi, A. Rizzo, M.A. Signore, M. Alvisi, et al., Effect of growth catalysts on gas sensitivity in carbon nanotube film based chemiresistive sensors, Applied Physics Letters 90 (2007) 103101–103103. [29] Y. Battie, O. Ducloux, P. Thobois, N. Dorval, J.S. Lauret, B. Attal-Trétout, et al., Gas sensors based on thick films of semi-conducting single walled carbon nanotubes, Carbon 49 (2011) 3544–3552. [30] Y. Battie, L. Gorintin, O. Ducloux, P. Thobois, P. Bondavalli, G. Feugnet, et al., Thickness dependent sensing mechanism in sorted semi-conducting single walled nanotube based sensors, Analyst 137 (2012) 2151–2157. [31] P. Bondavalli, P. Legagneux, D. Pribat, Carbon nanotubes based transistors as gas sensors: state of the art and critical review, Sensors and Actuators B: Chemical 140 (2009) 304–318. [32] H.-L. Hsu, J.-M. Jehng, Y. Sung, L.-C. Wang, S.-R. Yang, The synthesis, characterization of oxidized multi-walled carbon nanotubes, and application to surface acoustic wave quartz crystal gas sensor, Materials Chemistry and Physics 109 (2008) 148–155. [33] J. Mäklin, T. Mustonen, K. Kordás, S. Saukko, G. Tóth, J. Vähäkangas, Nitric oxide gas sensors with functionalized carbon nanotubes, Physica Status Solidi B Basic Solid State Physics 244 (2007) 4298–4302. [34] J.K. Abraham, B. Philip, A. Witchurch, V.K. Varadan, C.C. Reddy, A compact wireless gas sensor using a carbon nanotube/PMMA thin film chemiresistor, Smart Materials and Structures 13 (2004) 1045. [35] K.S.V. Santhanam, R. Sangoi, L. Fuller, A chemical sensor for chloromethanes using a nanocomposite of multiwalled carbon nanotubes with poly(3methylthiophene), Sensors and Actuators B: Chemical 106 (2005) 766– 771. [36] A. Star, V. Joshi, S. Skarupo, D. Thomas, J.-C.P. Gabriel, Gas sensor array based on metal-decorated carbon nanotubes, Journal of Physical Chemistry B 110 (2006) 21014–21020. [37] Y. Lu, C. Partridge, M. Meyyappan, J. Li, A carbon nanotube sensor array for sensitive gas discrimination using principal component analysis, Journal of Electroanalytical Chemistry 593 (2006) 105–110. [38] M. Penza, R. Rossi, M. Alvisi, G. Cassano, E. Serra, Functional characterization of carbon nanotube networked films functionalized with tuned loading of Au nanoclusters for gas sensing applications, Sensors and Actuators B: Chemical 140 (2009) 176–184. [39] M.J. Hostetler, J.E. Wingate, C.-J. Zhong, J.E. Harris, R.W. Vachet, M.R. Clark, et al., Alkanethiolate gold cluster molecules with core diameters from 1.5 to 5.2 nm: core and monolayer properties as a function of core size, Langmuir 14 (1998) 17–30. [40] F. Basarir, T.-H. Yoon, Preparation of gold patterns on polyimide coating via layer-by-layer deposition of gold nanoparticles, Journal of Colloid and Interface Science 352 (2010) 11–18. [41] C. Tasaltin, M.A. Ebeoglu, Z.Z. Ozturk, Acoustoelectric effect on the responses of SAW sensors coated with electrospun ZnO nanostructured thin film, Sensors 12 (2012) 12006–12015. [42] L. Valentini, F. Mercuri, I. Armentano, C. Cantalini, S. Picozzi, L. Lozzi, et al., Role of defects on the gas sensing properties of carbon nanotubes thin films: experiment and theory, Chemical Physics Letters 387 (2004) 356–361. [43] P.C.P. Watts, N. Mureau, Z. Tang, Y. Miyajima, J.D. Carey, S.R.P. Silva, The importance of oxygen-containing defects on carbon nanotubes for the detection of polar and non-polar vapours through hydrogen bond formation, Nanotechnology 18 (2007) 175701. [44] C. Wei, L. Dai, A. Roy, T.B. Tolle, Multifunctional chemical vapor sensors of aligned carbon nanotube and polymer composites, Journal of the American Chemical Society 128 (2006) 1412–1413. [45] D.R. Kauffman, A. Star, Carbon nanotube gas and vapor sensors, Angewandte Chemie International Edition 47 (2008) 6550–6570. [46] Z. Zanolli, R. Leghrib, A. Felten, J.-J. Pireaux, E. Llobet, J.-C. Charlier, Gas sensing with Au-decorated carbon nanotubes, ACS Nano 5 (2011) 4592–4599. [47] M. Penza, G. Cassano, Application of principal component analysis and artificial neural networks to recognize the individual VOCs of methanol/2-propanol in a binary mixture by SAW multi-sensor array, Sensors and Actuators B: Chemical 89 (2003) 269–284. [48] B. Mumyakmaz, A. Özmen, M.A. Ebeo˘glu, C. Tas¸altın, I˙ . Gürol, A study on the development of a compensation method for humidity effect in QCM sensor responses, Sensors and Actuators B: Chemical 147 (2010) 277–282.
Biographies Cihat Tasaltin received his Ph.D. degree from the Department of Physics of the Gebze Institute Technology in Turkey. He is a researcher at the TUBITAK Marmara Research Center, Materials Research Institute, Gebze, Turkey. His research activities are in the field of chemical gas sensors using acoustic transducers for health, safety and environmental applications as well as the investigation of sensing properties of organic and inorganic materials. Fevzihan Basarir is working as senior researcher in Materials Institute of TUBITAK Marmara Research Center. He has received his M.Sc. and Ph.D. degree in materials science from Gwangju Institute of Science and Technology (GIST) in 2004 and 2011, respectively. His research interest includes development and assembly of nanomaterials for electronics and optoelectronic applications.