Research Article www.acsami.org
Sensitive and Selective NO2 Sensing Based on Alkyl- and AlkylthioThiophene Polymer Conductance and Conductance Ratio Changes from Differential Chemical Doping Hui Li,† Jennifer Dailey,† Tejaswini Kale,† Kalpana Besar,† Kirsten Koehler,‡ and Howard E. Katz*,† †
Department of Materials Science and Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States ‡ Department of Environmental Health and Engineering, Johns Hopkins Bloomberg School of Public Health, 615 North Wolfe Street, Baltimore, Maryland 21205, United States S Supporting Information *
ABSTRACT: NO2-responsive polymer-based organic fieldeffect transistors (OFETs) are described, and room-temperature detection with high sensitivity entirely from the semiconductor was achieved. Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecylthioquaterthiophene) (PQT12 and PQTS12, respectively), were used as active layers to detect a concentration at least as low as 1 ppm of NO2. The proportional on-current change of OFETs using these polymers reached over 400% for PQTS12, which is among the highest sensitivities reported for a NO2-responsive device based on an organic semiconducting film. From measurements of cyclic voltammetry and the electronic characteristics, we found that the introduction of sulfurs into the side chains induces traps in films of the PQTS12 and also decreases domain sizes, both of which could contribute to the higher sensitivity of PQTS12 to NO2 gas compared with PQT12. The ratio of responses of PQTS12 and PQT12 is higher for exposures to lower concentrations, making this parameter a means of distinguishing responses to low concentrations for extended times from exposures to high concentrations from shorter times. The responses to nonoxidizing vapors were much lower, indicating good selectivity to NO2 of two polymers. This work demonstrates the capability of increasing selectivity and calibration of OFET sensors by modulating redox and aggregation properties of polymer semiconductors. KEYWORDS: NO2 sensing, field effect transistor, thiophene polymers, high sensitivity, ratiometric sensing
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conducting oxides are operated at high temperature,7,8 which causes power consumption and possibly safety issues and thus restricts the wide application in room-temperature detection. Very recently, Choi et al. reported porous RuO2 nanosheets with sensitivity of 1.124% at 20 ppm at a film temperature of 80.3 °C.9 Ratiometric sensing of NO2 was also achieved by using inorganic materials.10,11 For example, the concentration of NO2 can be determined by ratiometric dual-color quantum dots.11 The high power requirements, possible toxicity, sensitivity to temperature and relative humidity, and drift of existing sensors limit the applicability of existing NO2 sensors for long-term (weeks to months) deployment in homes or occupational environments. Less expensive and lower-power sensors that address these challenges, or provide early indications for when a shorter-term deployment of the highpower sensors would be needed, would allow a more complete
INTRODUCTION Nitrogen dioxide (NO2) is one of the most common toxic gases produced by combustion engines, and it is particularly relevant for health and safety in work and ambient environments when NO2 concentrations reach the 1−10 ppm range.1 The exposure to this air pollutant has been known to cause various lung diseases2,3 and contributes to smog formation.4−6 Because of the increasing attention to environmental issues in recent years and the recognition of the variability of NO2 concentrations across space and time, the sensing and detection of NO2 has become a research focus. Analytic methods like gas chromatography use passive badges to collect NO2 and, as such, they are cumulative samples, measuring a total accumulated amount of NO2 as a single output value, revealing little temporal evolution in concentrations. Furthermore, samples are expensive to analyze, limiting the number of samples that can be collected. Sensors have the potential to dramatically increase the resolution of NO2 measurements at greatly decreased cost and may be small enough for deployment in homes or even as wearable personal samplers. Traditional chemiresistive sensors for NO2 based on semi© XXXX American Chemical Society
Received: February 23, 2017 Accepted: May 30, 2017
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DOI: 10.1021/acsami.7b02721 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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OFET characteristics, we propose that the presence of sulfur increases the trap density and promotes redox interactions between the polymer and NO2 molecules. As a result, PQTS12 OFETs show higher sensitivity than those of PQT12 and satisfactory selectivity for NO2. Furthermore, the ratio of the responses of the two polymers varied inversely with the NO2 concentration, making this ratio an additional marker for determining concentration during a dosimetric exposure measurement. This is the first demonstration of designed ratiometric sensing by two different active OFETs based on polymer semiconductors to gauge a vapor concentration.
characterization of exposures for health assessments considering variability in NO2 concentrations. Solution-processed semiconductors have been widely investigated in thin film devices because of their low cost, flexibility, and room-temperature operation.12 A variety of solutionprocessed polymer materials have been explored for sensing gases such as the most reported ammonia (NH3),13−16 sulfur dioxide (SO2),17 and H2S,18 which are all based on thin-film transistors. Other efforts, including adjusting the thickness of the sensing film,19 changing device structures,20 and using composite materials by blending carbon nanotubes or different oxides21−23 with polymer semiconductors have been made to further improve the sensitivity. Although few solutionprocessed organic materials were reported to detect NO2,24,25 the sensitivity is far lower than for other typical toxic gas sensors. Using composite materials is still the main strategy to enhance the sensitivity of NO2 sensors26−29 because of the larger ratio of surface area to volume. Nevertheless, the problem of additive materials would also be brought into devices,30 and the interaction mechanism becomes complicated. Thiophene polymers are successfully used in NO2 sensors based on organic field-effect transistors (OFETs). For instance, regioregular polyhexylthiophene and its copolymer hybridized with SnO2 were reported as an active layer to detect NO2,27 but they are not tailored to be selective for NO2 gas. Although poly(3,4-ethylenedioxythiophene) (PEDOT) nanotube-based sensors showed good response toward sensing NO2 at room temperature, good sensitivity (>50%) could only be achieved under a relatively high concentration of NO2 (>50 ppm).31 Therefore, the development of simple and high-performance detectors sensing NO2 concentrations in the 1−10 ppm range remains a challenge and needs further development. Generally speaking, reducing the thickness of active layers or preparing nanostructures or porous structures in films enhances the interaction between semiconductor and analytes.32 These are morphological effects that can introduce binding sites or charge carrier traps with which analytes can interact. The covalent structural modification of semiconductors is another efficient strategy to achieve the favorable microstructure for sensing. In this work, we introduce functionality to a semiconducting polymer to increase its likelihood of complexing and becoming oxidized (doped) by NO2, thus increasing its current response relative to an unfunctionalized analogue. Two thiophene-based polymers, PQT12 and PQTS12, the latter containing sulfide groups adjacent to thiophene rings serving as a functionalized analogue, were synthesized and compared in OFET configurations (Figure 1). On the basis of cyclic voltammetry and
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RESULTS AND DISCUSSION The first step in the synthesis of PQTS12 is the formation of 3dodecylsulfanylthiophene by reaction of 3-bromothiophene with 1-dodecanethiol.33 After bromination via N-bromosuccinimide, 2-bromo-3-dodecylsulfanyl-thiophene was obtained. Monomer 1 (5,5′-dibromo-4,4′-didodecylsulfanyl-2,2′-dithiophene) was obtained via palladium-catalyzed C−H homocoupling of 2-bromo-3-dodecylsulfanylthiophene.34 The synthetic procedures are shown in the Supporting Information (Figure S1). The polymerization of monomer 1 and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene via Stille coupling reactions afforded crude dark red PQTS12. This synthetic method is different from the literature method reported by Paek et al.35 A typical PQT12 polymer was also synthesized through the same synthetic procedures from 3-bromothiophene. The two polymers were purified by Soxhlet extraction. The lowmolecular-weight portions and residual catalyst impurities were removed by dissolution in ethanol, acetone, and hexane. The chloroform extracts were concentrated and precipitated in ethanol, and then the solids were collected as a product. PQTS12 shows a number-average molecular weight (Mn) of 30 900 with a polydispersity index (PDI) of 1.8, and PQT12 has a lower molecular weight with Mn of 10 700 with a PDI of 1.2, as determined by gel permeation chromatography relative to polystyrene standards. The differential scanning calorimetry (DSC) curves show that the PQT12 and PQTS12 exhibit endothermic peaks at 118 and 190 °C, respectively, corresponding to the melting of the backbone (Figure S2). The electronic structures of the two polymers were investigated using cyclic voltammetry (CV) of thin films (Figure 2). Ferrocene/ferrocenium (Fc/Fc+) was used as an external standard, which was assigned an absolute energy of −4.8 eV versus vacuum level. There is reversible p-doping/ dedoping in the positive potential range for PQT12 and PQTS12. PQTS12 possesses a lower onset oxidation potential (0.40 V) than that of PQT12 (0.45 V), indicating a modest electron-donating ability of the dodecylsulfanyl side chain. From the electrochemical oxidation doping results, we can derive the highest occupied molecular orbital (HOMO) levels of the PQTS12 and PQT12 as 5.04 and 5.09 eV, respectively. The detailed calculation is shown in the Supporting Information. A sequence of five successive cyclic voltammograms of the two polymer films were taken to investigate film quality. We chose low potential (1.0 V) to avoid decomposing of polymers. We found for PQT12 that the first oxidation potentials (O1) are negatively shifted in later scans compared with the first scan. However, for PQTS12, the first oxidation potentials are positively shifted after the first scan, indicating that PQTS12 film has been oxidized. For PQT12 film, we obtained a stable electrochemical behavior after the second scan. However, there is a new peak (O2) that appears in the
Figure 1. (a) Schematic diagram of device structure. (b) Chemical structures of polymer semiconductors. B
DOI: 10.1021/acsami.7b02721 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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voltage, and V0 is the turn-on voltage. For pure PQT12 film with thickness of 100 nm, the surface trap density is about 7.1 × 1015 cm−2. For pure PQTS12 film, the value (1.2 × 1016 cm−2) is higher than that of PQT12 film.37 This further confirms that more traps exist in PQTS12 film which is consistent with the result of CV. Tapping mode atomic force microscopy (AFM) was employed to analyze the morphology of PQT12 and PQTS12 thin film. As shown in Figure 3, continuous thin film
Figure 3. AFM height images and phase images of (a, b) PQT12 film and (c, d) PQTS12 film after annealing at 120 °C for 10 min. The resolution is 2 μm × 2 μm.
Figure 2. Successive cyclic voltammograms of (a) PQT12 film and (b) PQTS12 film using dry CH3CN as solvent and 0.1 M n-Bu4NPF6 as a supporting electrolyte at a scan rate of 50 mV s−1.
formation with large domains was observed in the PQT12 thin film, which provided efficient channels for the carrier transport. Smaller grains and numerous grain boundaries were observed in the PQTS12 thin film, which presumably promoted the trap activity and decreased the mobility. Stability measurements of devices in air are presented in Figure S4. The current of PQT12 increases only negligibly (2.5%) after 20 min in air, while the devices based on PQTS12 are somewhat more sensitive to moisture and oxygen on the tens of minutes time scale, the current increasing 21.7% after 20 min. The initial increase of current indicates that there are some gases in air that facilitate the carrier transport in the short term. On the days time scale, there is a slower but more substantial drift toward lower currents for both devices that could be compensated for by the use of referencing, as described later. To measure responses to a given NO2 concentration, we prepared 12 devices simultaneously, six of them as control devices and the others as sensing devices. As NO2 gas was diluted by air that has the same humidity and components as the ambient atmosphere, the control devices were stored in air for the same time intervals as the exposure times for the sensing devices and were used to calculate the effect of air on the device current. According to the calculations for limit of blank (LOB) and limit of detection (LOD),38 we see that there is the possibility of up to about 45% sensitivity to blank for PQT12 film and 50% sensitivity to blank for PQTS12 film exposed in air, respectively (Table S1). This means that the sensitivity of air below the value of LOD cannot be exactly determined and
PQTS12 film after the second scan, and the intensity of O1 decreases with the scan times. This might be due to the formation of new species as an intermediate which indicates the possibility of side reactions in the PQTS12 film. Generally, the negative shift of the oxidation potential suggests the decreased possibility of side reactions that modify the polymer making it more difficult to oxidize.36 This indicates that the introduction of sulfur induces reactive sites in the PQTS12 film which are responsible for the positive shifts and side reactions during the electrochemical measurement. Top-contact, bottom-gate architecture OFETs were fabricated as sensor devices. The polymer semiconductors were spin coated from a 4 mg mL−1 chlorobenzene solution at 2000 rpm and then were annealed at 120 °C for 10 min in the glovebox. Au source/drain electrodes (W = 8000 μm, L = 250 μm) were used to measure the electrical performances of PQT12 and PQTS12-based devices. The field-effect mobilites were calculated from the transfer characteristics of more than seven devices in the saturation region (VDS = −60 V). The original devices without gas exposure show typical p-type transport (Figure S3). The transistors of PQT12 show a hole mobility of 0.027 ± 0.004 cm2 V−1 s−1, while PQTS12 films exhibit a hole mobility of 0.010 ± 0.002 cm2 V−1 s−1. The lower mobility of PQTS12 indicates that the sulfurs in side chains impact the carrier transport in films. We calculate the trap densities using Ntrap = Ci × |VT − V0|/e, where Ci is the capacitance per unit area (11.5 nF cm−2), VT is the threshold C
DOI: 10.1021/acsami.7b02721 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Current changes measured under different exposure times with different gate voltages with NO2 concentration being (a, b) 1 ppm and (c, d) 5 ppm. PQT12: a, c; PQTS12: b, d.
Table 1. Sensitivity of Sensors and Control Devices with Different Exposure Times (VG = −30 V) conc.
exposure time (min)
PQT12-control
1 ppm
5 10 15 20 60 120 3 5 10 15 20
14.5% 3.8% 2.0% 5.0% 27.0% 19.1% 29.2% 27.9% 19.8% 22.7% 32.6%
5 ppm
PQT12-NO2
PQTS12-control
± ± ± ± ± ± ± ± ± ± ±
24.3% 46.0% 36.0% 41.7% 40.6% 10.8% 43.9% 19.9% 40.3% 28.1% 33.6%
14.0 31.6 41.7 57.7 123.9 120.3 119.8 200.5 293.4 269.5 179.8
2.1% 1.6% 3.1% 5.8% 4.9% 1.1% 4.6% 6.3% 3.7% 4.2% 3.5%
PQTS12-NO2
R
± ± ± ± ± ± ± ± ± ± ±
3.1 3.2 2.7 3.6 2.9 2.6 1.3 1.1 1.2 1.5 1.7
42.7 100.4 114.1 208.1 363.6 310.8 161.2 314.5 349.1 411.3 315.2
4.8% 5.2% 3.7% 2.9% 1.9% 4.5% 5.6% 10.1% 1.8% 3.2% 6.1%
calculated by the formula 100% × (IDS,NO2 − IDS,air − ΔIreference)/IDS,air, where IDS,NO2 is the drain current after exposure of NO2, IDS,air is the drain current before exposure to NO2, and ΔIreference is the current increase of control device exposed in air with the same time intervals as sensing device. The responses of currents change with gate voltages. As shown in Figure S6, the sensitivities of both PQT12 and PQTS12 decrease with the increase of gate voltage. This means that these sensors are endowed with much higher sensitivity under lower gate voltage. Figure 4 displays the responses of OFETs with gate voltages of −30, −40, −50, and −60 V. We
that it is nonmonotonic with exposure time because it is not significantly larger than the noise. We take these effects into account for the calculation of all sensitivities mentioned later, as a means of referencing, as the current increase contributed by house air was deducted from the total current increase after gas exposure. The sensor devices were tested after exposure to NO2 gas, that is, OFETs were turned on after NO2 exposure ended. All transistors showed significantly increased drain current when exposed to NO2 gas. The transfer curves of optimized devices are shown in Figure S5. Change in drain current (ΔI) was used as a measure of the sensitivity of these devices, D
DOI: 10.1021/acsami.7b02721 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces choose these voltages to eliminate the noise from instability near the threshold voltage. All plots in the figure show the average values from four or five devices. Clearly, PQTS12 show higher response under the same exposure conditions. In addition, the time to saturation for PQT12 film is somewhat shorter, indicating weaker total response capability for PQT12. With gate voltage of −30 V, the sensors based on PQTS12 films exhibit quite high sensitivity up to 360% and 410% under 1 ppm for 60 min and 5 ppm for 15 min, respectively, which are unusually high responses among polymer semiconductors interacting with NO2. Meanwhile, the sensors based on PQT12 films show highest sensitivity of 120% and 270%, respectively, under the same conditions. We noted that the average ratio between the sensitivities of PQTS12 and PQT12 (R = sensitivity(PQTS12)/sensitivity(PQT12)) varies with the concentration of NO2. For 1 ppm of NO2 gas concentration (Figure 4a, b), the ratio is 3, while for higher NO2 gas concentration, 5 ppm, this value is about 1.5. The data are collected in Table 1. This suggests the possibility that the concentration of gas could be detected by the ratio of sensitivity used in conjunction with the proportional current changes. The resultant responses with gate voltages of −40 V and −50 V give satisfactory sensitivities, while the gate voltage of −60 V gives lowest sensitivity among the gate voltages that we examined. The average sensitivity is about 200% for PQTS12 film exposed to 1 ppm of NO2 for 120 min at this gate voltage, which is still a high response compared to previously reported results. The average sensitivity for PQT12 film is 60% under the same conditions. With 5 ppm exposure, the average sensitivity is up to 180% for PQTS12 film, but this value is reached after only 15 min exposure. NO2 can react with oxygen and water. We also tested the effect of humidity on the sensing of NO2 by a PQTS12 film (Figure S7, Table S3). The current slightly increases under wet air without NO2, but the sensitivity continuously increases under NO2 with higher humidity, which can be ascribed to the oxidizability of intermediates or the resultant HNO3 from the NO2/O2/H2O reaction. In practical use, a separate calibration of humidity level would add further precision to determinations of NO2 exposure. We also calculated the ratios between the sensitivities of PQTS12 and PQT12 when the gate voltage was set to be −40, −50, and −60 V. Again, we find that the results follow the rules mentioned earlier, that is, the ratios of responses of PQTS12 and PQT12 are 3 and 1.5 with 1 and 5 ppm, respectively, and insusceptible to gate voltages. The R values are collected in Figure 5, and the data are shown in the Supporting Information (Table S2). Besides 1 and 5 ppm exposure, we also exposed the devices with 10 ppm of NO2 for 5 min; the average sensitivity is 195% and 229% for PQT12-based devices and PQTS12-based devices, respectively. The ratio of responses of PQTS12 and PQT12 is about 1.2. These results further demonstrate that the ratio can be used as a marker for determining concentration during a dosimetric exposure measurement. Selectivity studies of PQT12 and PQTS12 films were carried out by exposure to common solvent vapors like acetone, chlorobenzene, hexane, ethyl acetate, methanol, 2-propanol, triethylamine, and chloroform, as shown in Figure 6. The transfer curves before and after vapor exposure are shown in Figures S8 and S9. The solvent vapor pressures were much higher than the NO2 vapor pressures used in this study. Acetone could be considered a more Lewis basic carbonyl compound than atmospheric CO2, while methanol is a carrier of OH groups at higher vapor pressure than atmospheric
Figure 5. Sensitivity ratio of PQTS12 and PQT12 with different gate voltages with NO2 concentration being 1 and 5 ppm.
Figure 6. Responses of IDS after exposure to common solvent vapors on (a) PQT12- and (b) PQTS12-based sensor. (VG = −30 V) acetone (16 200 ppm); chlorobenzene (1050 ppm); ethyl acetate (8300 ppm); hexane (10 400 ppm); 2-propanol (3800 ppm); methanol (11 100 ppm); triethylamine (4500 ppm); chloroform (14 000 ppm).
humidity. The data were obtained using a gate voltage of −30 V. For PQTS12 films, a large increase of IDS is observed after exposure to chlorobenzene and chloroform, which could E
DOI: 10.1021/acsami.7b02721 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Environmental Protection Agency to Yale University. It has not been formally reviewed by EPA. The views expressed in this document are solely those of Howard Katz and the coauthors listed on the title page and do not necessarily reflect those of the Agency. EPA does not endorse any products or commercial services mentioned in this publication. We warmly thank Professor Patty McGuiggan for AFM measurements.
possibly be attributed to the rearrangement of morphology analogous to saturated solvent vapor annealing treatment39−41 or to HCl impurity in the film, which could be prefiltered using a weakly basic powder in a real application. Only chloroform vapor significantly increases the conductivity of the PQT12 film, and this may have been because of the HCl contaminant ubiquitously present in chloroform. Triethylamine gives a large decrease of IDS because of its electron-rich, trapping character as we and others have reported for amines in the past.42 PQTS12 is a bit more sensitive to the alcohols methanol and 2-propanol compared with PQT12. Only minor changes to IDS are observed after exposure of this OFET to acetone, ethyl acetate, and hexane. Except for chloroform and triethylamine, we found that current decreases with high concentrations of other solvents and that current increases with low concentrations using PQT12 film. This indicates two possible mechanisms. At low concentrations, there is a synergistic interaction with oxygen or other impurities that slightly dopes the semiconductor or decreases grain boundary barriers. At higher (fully saturated) concentrations, the dipole quenching effect is more important, as many of the vapors caused current decreases.
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CONCLUSION In conclusion, two thiophene-polymers, PQT12 and PQTS12, are reported as NO2 sensing materials for room-temperature detection. Thin films of PQTS12 show relatively lower hole mobility because of traps in films and smaller domain morphology based on cyclic voltammetry and AFM characterizations, respectively. PQTS12 shows higher response to NO2 gas, with the highest observed sensitivity of 410% and satisfactory selectivity. Such high sensitivity and selectivity can be attributed to the incorporation of sulfurs in the side chains affecting both the electronic and morphological properties. The response ratio relative to PQT12 provides a means of distinguishing concentration level from exposure time effects on the responses.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02721. Materials and fabrication of devices, synthesis of monomers and polymers, stability of devices, transfer curves before and after exposure, effect of humidity, additional sensitivity data (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Howard E. Katz: 0000-0002-3190-2475 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Polymer synthesis and charge carrier generation and mobility studies for this work were supported by the Department of Energy, Office of Basic Energy Sciences, Grant Number DEFG02-07ER46465. Sensing analysis was developed under Assistance Agreement No. RD835871 awarded by the U.S. F
DOI: 10.1021/acsami.7b02721 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.7b02721 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
