Room Temperature Resistive Volatile Organic Compound Sensing

Jan 27, 2016 - Room temperature resistive volatile organic compound (VOC) sensing materials fabricated with vertically aligned-carbon nanotubes (VA-CN...
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Room Temperature Resistive Volatile Organic Compound Sensing Materials Based on a Hybrid Structure of Vertically Aligned Carbon Nanotubes and Conformal oCVD/iCVD Polymer Coatings Xiaoxue Wang,† Asli Ugur,† Hilal Goktas,† Nan Chen,† Minghui Wang,† Noa Lachman,‡ Estelle Kalfon-Cohen,‡ Wenjing Fang,§ Brian L. Wardle,‡ and Karen K. Gleason*,† †

Department of Chemical Engineering, ‡Department of Aeronautics and Astronautics, and §Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: Room temperature resistive volatile organic compound (VOC) sensing materials fabricated with vertically aligned-carbon nanotubes (VA-CNT) demonstrated 10-fold improved sensitivity upon application of a thin conformal layer of the conducting polymer coating ((poly(3,4-ethylenedioxythiophene) (PEDOT)). The PEDOT was directly synthesized on the VA-CNTs via oxidative chemical vapor deposition (oCVD). Conformal PEDOT coatings with thickness of 8 and 17 nm were easily achievable by oCVD. The hybrid VA-CNT/ oCVD PEDOT sensing materials exhibited excellent response to low concentrations of analyte gases of different polarity. The projected detection limit for n-pentane is as low as ∼50 ppm. A second polymer layer, nonconducting polystyrene (PS, ∼6 nm), was further conformally coated on the VA-CNT/PEDOT via initiative chemical vapor deposition (iCVD) to enhance the gas selectivity. The iCVD PS enhanced the selectivity of n-pentane over methanol by 2.7-fold and toluene by 4.4-fold. Several unique advantages of these sensing materials include the following: (1) detection of nonpolar hydrocarbon molecule n-pentane at room temperature; (2) high signal quality (signal-to-noise ratio typically ∼30 dB); (3) solvent-free facile fabrication method that preserves the accessible high-surface-area morphology of the VA-CNTs; (4) good reversibility and short response time (∼400 s). Our results indicate that both the polarity of the analyte molecule and the carrier transport regime of the PEDOT layer are important in sensing behavior. Furthermore, this versatile selective layer design is potentially useful for selectivity enhancement for other important target analytes. KEYWORDS: carbon nanotubes, oCVD, iCVD, polymer, gas sensing material

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considerations above, promising materials for a room temperature chemiresistor to detect nonpolar species include carbon nanotubes (CNT), gold nanoparticles, and graphene.10 Among them, CNT sensing materials stand out because of their high surface area and feasibility to be surface modified. Desirable characteristics of a chemiresistive sensing material include high sensitivity, fast response, and recovery with good reversibility; high signal-to-noise ratio; high selectivity for the target chemicals; low fabrication cost; and room temperature operation. Chemical sensing materials based on carbon nanotubes have been investigated widely.11,12 The main sensing mechanisms for carbon nanotubes include the following: the intrananotube mechanism, in which the Fermi level of CNTs is modulated by adsorption of analyte molecules;3 the internanotube mecha-

olatile organic compound (VOC) sensing has drawn great attention recently because of its significance in safety, environment protection, and health care.1,2 In particular, VOCs are among the most common sources of explosions and contamination in refineries and chemical plants.2 Some of the most important VOCs to detect in industry are nonpolar molecules, such as alkanes.2,3 Since nonpolar species do not donate or extract charge easily, they are difficult to detect using traditional semiconductor sensors.4,5 Another challenge is the operational temperature required for the sensing materials. Many traditional metal-oxide sensing materials require high operation temperature,6,7 which increases the heating and equipment costs. Currently, the most common detectors for alkanes are optical sensing materials,2,8 which are often expensive and cumbersome. A promising alternative sensor is the chemiresistor because of its small area, low energy consumption, and simplicity in measurement.9 The challenge is to find a material with an electrical resistance change when exposed to nonpolar alkanes at room temperature. With all the © XXXX American Chemical Society

Received: November 5, 2015 Accepted: January 27, 2016

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DOI: 10.1021/acssensors.5b00208 ACS Sens. XXXX, XXX, XXX−XXX

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Figure 1. Schematic of the hybrid structure of VA-CNT with conformal CVD polymers and the resistance measurement method. Two alligator clips were applied as electrical contact to measure the overall resistance of the hybrid structure. The scheme is not in real scale. In actuality, the VA-CNT forest is much denser and opaque, the height of VA-CNTs is ∼170 μm, the inter-CNT spacing is ∼80 nm.

nism, in which the intertube electron hopping3 is influenced by analyte molecules; and a dipole interaction mechanism based on molecular binding on active sites.3,11−14 Most of the CNT sensing materials work for highly polar or even redox active analyte gases, such as NH3, NO2, acetone, and methanol, mainly via the first and third mechanisms. However, growing interest was drawn to improving CNT sensing materials to detect nonpolar chemicals recently. Li et al. reported detection of nonpolar benzene via the second mechanism using CNT sensors.3,15 Peng et al. fabricated CNT sensing materials for nonpolar molecules.16 Wang et al. reported chemiresistors for aliphatic hydrocarbons with chemically modified CNTs.17 Using a CNT network structure, Slobodian et al. fabricated chemiresistor sensors responding to saturated iso-pentane vapor.11 Another related type of gas sensor is the conducting polymer nanowire sensor. The primary detection mechanisms for these sensing materials are the electron hopping rate decrease induced by analyte molecules and the induced partial charge transfer.5,18−21 Conducting polymers contain islands with high crystallinity and high conductivity embedded in insulating matrix.18,22 Electrons hop among the high conducting islands.18,22 The adsorption of gas molecules will decrease the electron hopping rate, and thereby increase resistance. Additionally, partial charge transfer with adsorbed donor or acceptor molecules leads to modulation of the Fermi level of the conducting polymers.18−20 Thus, similar to the CNT sensing materials, the polarity of the gas molecule is important for the sensing behavior, as polarity determines the degree of charge transfer. However, Cao et al. demonstrated that in certain transport regimes, polar molecules are also able to induce the influence on electron hopping rate like nonpolar molecules, and show an unusual sensing behavior independent of the gas polarity.18 Luo et al. also reported conducting polymer nanowire sensing materials to detect nonpolar chemicals.23 In order to enhance the response to hydrocarbons, we propose a novel hybrid sensing material structure that combines the CNT sensing materials and conducting polymer nanowire sensing materials in a single compact device. As an active sensing layer, a conducting polymer (PEDOT) was synthesized by oCVD technique conformally on the VA-CNTs which were produced by the thermal catalytic CVD method.24 The thickness of the conformal coating is controllable and

uniform, with the vertically aligned structure preserved. The conformality provides better control of the sample properties, and lowers the uncertainty of the synthesis process. The fabricated hybrid sensing material provides a response to low concentrations of nonpolar n-pentane gases. Moreover, to enhance the selectivity by excluding the unwanted gas molecules, a nonconductive PS thin film was deposited on top of the PEDOT via iCVD technique. Recently, similar carbon nanotubes−polymer composite sensing materials have been fabricated.15,25 However, these reports are mainly focused on detecting redox active analytes such as NH3 or NO2. The polymer selective layer was also not reported. Additionally, the polymer layer is applied using solution based methods in which surface tension effects must be controlled in order to maintain the aligned structure of carbon nanotubes. Here we demonstrate a high-signal-quality sensing, fast responding and recovering, highly selective, and easy-to-fabricate chemiresistive sensing material based on VA-CNT and oCVD/iCVD technology for the detection of the nonpolar VOC n-pentane.

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EXPERIMENTAL PROCEDURES

The design of the hybrid sensing material is shown in Figure 1. First, the VA-CNTs are grown on Si wafers by thermal catalytic CVD method. Then, conformal coatings of 8−17 nm PEDOT and ∼6 nm PS are synthesized by oCVD and iCVD techniques, respectively, on the CNTs. Growth of VA-CNTs. VA-CNT arrays were grown on 1 cm2 silicon wafers in a 44-mm-diameter quartz tube furnace at atmospheric pressure via thermal catalytic CVD with C2H4 gas as carbon feedstock, and H2 and He as gas carrier. Prior to the CVD process, Fe/Al2O3 thin film catalyst (1/10 nm) was deposited by electron beam evaporation on a bare silicon wafer and cut into 1 cm2 pieces. The wafers were then introduced into the quartz tube and the furnace was heated up to the growth temperature set at 740 °C under a flow of H2 and water vapor (flow set as 1040 and 15 sccm respectively). When the furnace reached the target temperature, C2H4 is flowed at a rate of 400 sccm for 5 min during which 170-μm-tall CNTs are formed. As-grown CNT arrays have 1% volume fraction (density of 109−1010 CNTs cm−2) corresponding to ∼80 nm inter-CNT spacing and average outer diameter of 8 nm, 3−7 circumferential walls.26 Growing the nanotubes in an aligned structure yields a well-defined carbon nanotube surface that can be modified with various functional materials to effectively enhance device sensitivity and detect a broader range of analytes. Vapor Deposition of Conformal Polymers. The conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) thin films conformally coated on VA-CNTs via oCVD technique24,27 by flowing B

DOI: 10.1021/acssensors.5b00208 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors monomer vapors simultaneously with sublimating an oxidizing agent which reacts at the substrate via oxidative step growth polymerization. The oxidant, FeCl3 (purchased from Sigma-Aldrich, CAS 7705−08− 0), was heated up to 160 °C in a crucible in a vacuum chamber. The monomer, 3,4-ethylenedioxythiophene (EDOT, purchased from Sigma-Aldrich, CAS 126213−50−1), was heated in a monomer jar to 140 °C, and was introduced into the vacuum chamber through vapor phase. The polymerization reaction took place and resulted in the formation of a conformal PEDOT film on the CNT array substrates, which were heated to 140 °C. The iCVD technology28 was utilized to synthesize the polystyrene (PS) and polyhydroxyethyl methacrylate (PHEMA). The monomer, styrene (purchased from Sigma-Aldrich, CAS 100−42−5) or 2hydroxyethyl methacrylate (HEMA, purchased from Sigma-Aldrich, CAS 868−77−9), was heated to 50 °C in a monomer jar. The initiator tert-butyl peroxide (TBPO) was kept at room temperature. Later, the vapors of the monomer and the initiator were introduced into the vacuum reactor chamber simultaneously. The filament temperature and chamber pressure were controlled at ∼200 °C and ∼800 mTorr, respectively. Flow rates of TBPO, HEMA, and styrene were controlled at ∼1.2 sccm, ∼ 0.15 sccm, and ∼3 sccm, respectively. The total deposition time is ∼1 h for the synthesis of PS or PHEMA.28 Materials Characterization. Scanning electron micrographs (SEM) were taken by a cold field-emission gun scanning electron microscope (FEG-SEM) JEOL 6700F under 5−15 kV. Transmission electron micrographs (TEM) were taken by JEOL 2011 high contrast digital TEM under 80 kV. A Thermo Scientific Nicolet iS50 FT-IR spectrometer was used to characterize the polymer films. Raman spectra were taken with Horiba LabRam HR (Model 800) at a wavelength of 513 nm to characterize the aligned carbon nanotubes and the composite of carbon nanotubes and polymers. Measurement of Sensing Response. We investigated the resistive response of different samples to a variety of gases. We chose n-pentane (dipole moment: 0 D)29 as a representative of nonpolar chemical, methanol (dipole moment: 2.87 D at 20 °C)29 as a polar chemical; toluene (dipole moment: 0.31 D at 20 °C)29 has polarity in between them. All the sensing responses were evaluated by measuring the resistance change across the two alligator clips on both ends of the samples when exposed to analytes (as shown in Figure 1). The response is defined as

response (%) =

R gas − R 0 R0

Figure 2. Scanning electron micrographs of samples: VA-CNT (a), VA-CNT/8 nm PEDOT (b), VA-CNT/17 nm PEDOT (c), and VACNT/17 nm PEDOT/6 nm PS (d), showing conformal coating of the polymer around VA-CNTs. The insets are SEM images of the same samples with lower magnification, showing the vertically aligned structure. All the black scale bars in the main SEM images are 100 nm, and the scale bars in the insets are 1 μm.

researchers have also reported and studied the wavy structure.31,32 This phenomenon is known to be due to the van der Waals interactions between nanotubes during the CNT growth.33,34 The polymer coating penetrates into the vertically aligned structure of the VA-CNT matrix, and results in the increase of the overall diameters. The ability to penetrate into nanostructures and form a conformal coating is a unique advantage of the oCVD and iCVD technologies.28 In all four samples, there is still sufficient void space for the analyte molecules to penetrate in, since the diameter of the analyte molecules is 10 nm) is beyond the diameter of CNTs, and thus the nanoscale intertube distance induced by waviness and entanglement provides a conduction pathway which can be described by 3D-VRH theory. Based on the transverse direction electronic conduction path, we propose different sensing mechanisms for the detection of nonpolar and polar analytes. Both of the mechanisms combine CNT sensor and PEDOT nanowire sensor mechanism. For the detection of nonpolar n-pentane, we propose that the inter nanotube mechanism3 of CNT sensors and the electronhopping hypothesis for PEDOT nanowire sensors18 play an important role (Figure 10a). The fact that the CNTs are in electrical contact instead of physical contact due to van der Waals force even when the percolation conductive network dominates52 is important in gas sensing. Li et al.3 suggested an inter nanotube mechanism in gas sensing which refers to the modulation of intertube electron transfer when exposed to nonpolar chemical molecules.3 The nonpolar n-pentane molecules form CNT-molecule-CNT junctions, hindering the inter nanotube electron transfer as shown in Figure 10a, and further increasing the overall resistance. Also, with the threshold distance below 1.8 nm,52 the length scale of two adjacent CNTs is larger than a gas molecule’s diameter (∼9 Å).35This effect can happen in both bare VA-CNT sensing materials and polymer coated VA-CNT sensing materials (Figure 10a). Additionally, oCVD PEDOT contains high conductivity regions with high crystallinity surrounded by amorphous regions. One of the critical carrier transport mechanisms is the electron hopping among the high conductivity domains.22 As a consequence of the gas molecule adsorption, the electron hopping rate between crystalline is decreased, and therefore the resistance is increased.18 This phenomenon is illustrated in Figure 10a as well. Due to the mechanism illustrated in Figure 10a, the resistance of the hybrid CNT/polymer structure increases after exposure to npentane as shown by the previous experimental results. With low polarity, the response to toluene is similar to n-pentane in accordance with the literature.23

For the detection of the polar molecule methanol, the intra nanotube mechanism for CNT sensing materials and partial charge transfer in PEDOT nanowire sensors dominates.5,18,19 The intra nanotube mechanism refers to the Fermi level modulation of carbon nanotubes upon polar VOC exposure, leading to the overall resistance change.3,12 The weak response of bare CNT can be explained by this mechanism. The partial charge transfer20 effect relates to the polarity of the gas molecule and the transport regime of the PEDOT layer. Polar molecules donate or extract electrons from the conducting polymer, i.e., modulate the Fermi level in the conducting polymer layer, changing the overall resistance. The resistance decrease of the VA-CNT-polymer structure after methanol exposure can be explained by partial charge transfer: methanol vapor is an acceptor, and it undergoes a partial electron transfer from the cationic polymer; thus, the carrier density increases in the p-type polymer and the resistance decreases.18 A larger resistive response to water vapor than to methanol vapor (Figure S5) verifies this assumption, since water provides more protons than methanol. This phenomenon is also in agreement with the literature.18 The partial charge transfer mechanism is illustrated in Figure 10b. These two mechanisms may have contrary effects on each other: a decrease in the electron hopping rate within the conducting polymer leads to higher resistance, but partial charge transfer of acceptor from the polymer leads to lower resistance. When adsorbing polar gas molecules, these two phenomena may happen simultaneously. Cao et al. reported that in this case, the transport regime is of crucial importance in deciding which effect dominates. In order to probe into the transport regime of the conducting polymer, the reduced activation energy18 W is defined as W = d lnσ(T)/d ln(T). The W−T curve of the oCVD PEDOT polymer film is shown in Figure S6. A zero W−T slope indicates that the transport regime of the polymer film is in the critical regime between metallic and insulator regimes where the sensing mechanism of methanol vapor is dominated by partial charge transfer.18 Therefore, in the experimental results, the resistance decreases after exposure to methanol. This result is also in accordance with the result from Cao et al.18 In order to perform a selective sensing, an iCVD nonconducting polymer layer was deposited on the PEDOT/CNT composite. The iCVD PS layer successfully enhanced the selectivity of n-pentane over methanol and toluene, and the iCVD PHEMA layer does not have the same function. We propose that the mechanism of this selectivity is related to the nonpolar nature of PS. In general, Hildebrand solubility parameters60 (δ) are used for predicting the solubility of nonpolar or slightly polar polymers by solvents. Similar values of δ indicate good miscibility. The δ values (cal(1/2) cm(−3/2)) of n-pentane, methanol, and poly(styrene) are 7,60 14.5,61 and 9.1,62 respectively. The difference of δ between methanol and PS is much greater than that of n-pentane and PS, which indicates a better miscibility of the latter two. At the same time, the difference of δ between n-pentane and PS is not too small as to destroy PS layer after n-pentane exposure. Qualitatively, this analysis provides an explanation of how the PS layer excludes the polar component. By excluding polar molecules, the PS layer enhanced the selectivity of this sensing material. In addition, given the mean intertube space of 80 nm and the wavy and entangled nature of CNTs, we suggest the coating thickness of PS layer to be less than 10 nm on the 17 nm PEDOT coating, in order to allow for gas molecule penetration G

DOI: 10.1021/acssensors.5b00208 ACS Sens. XXXX, XXX, XXX−XXX

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National Science Foundation under Grant No. CMMI1130437. We would like to thank Professor Jing Kong and Dr. Xi Ling from the Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, for their support and assistance in TEM images and Raman spectroscopy. One of the authors, Hilal Goktas, on leave from Physics Department of Canakkale Onsekiz Mart University, was supported by FDA ORISE Fellowship.

and avoid mass transfer distance that is too long for analyte molecules. Potentially, a specially designed iCVD polymer layer is able to exclude specific comparing gas molecules in order to enhance the selectivity for other target analytes based on their chemical or physical properties. Compared with the existing PEDOT nanowire sensors,18,63 our sensing material has the same order of magnitude resistive responses and better S/N ratios. Furthermore, the operational temperature (25 °C) is more preferable compared to traditional metal oxide sensors which requires an operational temperature ranging from 200 to 400 °C.6,64−67 A unique gas selecting iCVD layer is also an advantage of our device.

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ABBREVIATIONS iCVD, initiative chemical vapor deposition; oCVD, oxidative chemical vapor deposition; PEDOT, poly(3,4-ethylenedioxythiophene; PS, polystyrene; VA-CNT, vertically aligned carbon nanotubes

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CONCLUSIONS In conclusion, a room temperature chemiresistor based on VACNT array/polymer hybrids has been synthesized and characterized. The oCVD conducting polymer, PEDOT, enhanced the sensitivity to a variety of gases. The iCVD selective polymer, PS, enhanced the selectivity of n-pentane over methanol and toluene. The resistor sensing material has high signal quality, fast response and recovery, and high selectivity. Since it is solvent free, it also preserves vertically aligned structure and excludes the interference of solvent. In addition, the oCVD/iCVD provides a convenient way to tailor the mechanical and physical properties of the nanostructure. The radial direction of electrical conduction in the hybrid structure is explained. We also investigated the sensing mechanism, and proposed that the polarity of the gas molecule and transport regime of PEDOT layer play an important role. Furthermore, the versatile iCVD selective layer provides a platform for the selectivity enhancement of other target analytes, with proper polymer design based on the different properties of the analytes and comparing gas, such as hydrophobicity and solubility.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.5b00208. Structure of the sensing test system; SEM images showing the CNTs growing perpendicular to the Si wafers; SEM images with high magnitude; TEM images of VA-CNT-CVD polymer hybrid structure; the statistics of the coating thickness; comparison of the response of water vapor and other analytes; reduced activation energy measurement; definition of selectivity; the micromechanics model (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Royal Dutch Shell plc. for providing funding to this project. The work was partially supported by the H

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