Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Langmuir-Schaefer films of regioregular polythiophene derivatives as VOCs sensors Vinicius J.R. Oliveira, L.V.L. Citolino, S.A. Camacho, P. Alessio, C.A. Olivati∗ Faculdade de Ciências e Tecnologia, UNESP, Presidente Prudente, SP, 19060-900, Brazil H I G H L I G H T S • The P3HT and P3OT polymers were scattered on a water-air surface of a Langmuir trough. • Through isotherms (π-A) one can identify the organization phases of the molecules. • The thin films presented linear growth through of Langmuir Schaefer tech- nique. • The thin films on the interdigitated electrode quickly detected the organic vapor. • Raman mapping detected the physical interactions between organic poly- mers and vapors. G R A P H I C A L A B S T R A C T A R T I C L E I N F O Keywords: Polythiophene Langmuir-Schaefer VOCs sensors Spectroscopy measurements A B S T R A C T Conjugated polymers have been extensively studied in recent years, with high technological potential for ap- plication as an active layer of organic devices. In this context, polythiophene derivatives have attracted attention due to properties as stability and easy processability. In this work, poly(3-hexylthiophene) and poly(3-oc- tylthiophene) were studied in the form of nanostructured films fabricated by the Langmuir-Schaefer deposition technique. The films were characterized by optical (UV–Vis absorption spectroscopy and Raman scattering) and electrical measurements allowing the evaluation of the material concerning the technological potential to the manufacture of VOCs electric sensors. 1. Introduction Conductive polymers have remarkable properties in the solid state, such as high processability and stability. They have been used in the form of thin films with wide application in organic electronic devices as OLEDs, photo- voltaic [1–5]. These organic devices are frequently based on thin films [6]. Therefore, the Langmuir-Schaefer (LS) technique is known to provide nanostructured films with the great organization at the molecular level and control in the thickness and uniformity. Thus, the LS technique is suitable for the manufacture of sensors based the polymeric thin films [7,8]. In another way, concern about possible damages caused by volatile organic compounds (VOCs) in the atmosphere has increased, encoura- ging research on systems for the detection of these harmful compounds in the air [9,10]. https://doi.org/10.1016/j.matchemphys.2018.06.070 Received 19 April 2018; Accepted 25 June 2018 ∗ Corresponding author. E-mail addresses: vinijro@gmail.com (V.J.R. Oliveira), lucasvinicius_blink@hotmail.com (L.V.L. Citolino), sabrina.alessio@gmail.com (S.A. Camacho), priscila@fct.unesp.br (P. Alessio), olivati@fct.unesp.br (C.A. Olivati). Materials Chemistry and Physics 217 (2018) 421–426 Available online 28 June 2018 0254-0584/ © 2018 Elsevier B.V. All rights reserved. T http://www.sciencedirect.com/science/journal/02540584 https://www.elsevier.com/locate/matchemphys https://doi.org/10.1016/j.matchemphys.2018.06.070 https://doi.org/10.1016/j.matchemphys.2018.06.070 mailto:vinijro@gmail.com mailto:lucasvinicius_blink@hotmail.com mailto:sabrina.alessio@gmail.com mailto:priscila@fct.unesp.br mailto:olivati@fct.unesp.br https://doi.org/10.1016/j.matchemphys.2018.06.070 http://crossmark.crossref.org/dialog/?doi=10.1016/j.matchemphys.2018.06.070&domain=pdf For instance, dichloromethane is a colorless and highly volatile substance, as well as tetrahydrofuran; characteristics make them almost imperceptible in the atmosphere. However, these VOCs can be found in home environments, in products such as aerosols, pesticides, spray paints, or resin solvents. In fact, breathing large amounts of di- chloromethane leads to symptoms such as dizziness, nausea, numbness in the fingers, or in more severe cases result in respiratory and cardiac insufficiency [11]. The Tetrahydrofuran produces similar symptoms, in addition to respiratory tract irritation and headaches, besides being considered a carcinogenic agent to humans [12]. Thus, the main idea is to control these substances in the atmosphere, developing alternative portable sensors with low cost facilitates the popularization, resulting in greater health conditions to the environ- ments [13]. In this sense, sensors based on conductive organic poly- mers, for example, regioregular polythiophenes, as the active layer are an interesting alternative device. The conventional detectors available in the market, such as Hydrogen detectors, fuel gasses, and solvents, has a cost of sale of al- most US $ 1000 [14]. In contrast, companies such as ABTECH Scien- tific, Inc. Advanced Biochip Technologies, produce sensors based on organic conducting polymers with a maximum price of US $ 230 [15]. Sensors based on organic chemosensing polymers interact with the analyte of interest that changes their optical, chemical or electrical prop- erties [16]. The changes can be detected and translated into a response of the sensor to the interaction of the sensor with the analytes [4,17]. In this work, the polythiophene regioregular derivatives, Poly(3- hexylthiophene) (P3HT) and Poly (3-octylthiophene) (P3OT), were processed through the Langmuir-Schaefer (LS) technique, and char- acterized by optical (UV–Vis absorption spectroscopy and Raman scattering) and electrical measurements (direct current – DC). The re- sults reveal the device manufactured has technological potential to the application as VOCs electric sensor. 2. Materials and methods 2.1. Materials The regioregular derivatives of polythiophene, poly(3-hex- ylthiophene) (P3HT, (C10H14S)n) and poly(3-octylthiophene) (P3OT, (C12H18S)n), were acquired from Sigma-Aldrich (product numbers: 445703 and 445711, respectively, regiospecific≥ 90%). The solvents used for measures as sensors of volatile organic compounds (VOCs) were dichloromethane (CH2Cl2, MW=84.93 g/mol) and tetra- hydrofuran (C4H8O, MW=72.11 g/mol) also purchased from Sigma- Aldrich. The ultrapure water (18.2 MΩ cm) was acquired from a Millipore system of water purification. 2.2. Langmuir and Langmuir-Schaefer films of P3HT and P3OT Langmuir and Langmuir-Schaefer (LS) films of P3HT and P3OT were fabricated using a Langmuir trough KSV-NIMA model 5000. Langmuir films were produced by spreading 600 μL of P3HT and P3OT solutions (0.20 mg/mL dissolved in chloroform) onto air/water interface. The Langmuir monolayers were characterized by π-A isotherms at 23 °C using the Wilhelmy method. The monolayers were symmetrically compressed under a constant barrier speed at 10mm/min with the subphase containing ultrapure water. LS films of P3HT and P3OT were obtained by transferring the Langmuir monolayers from the air/water interface onto different solid substrates depending on the character- ization technique: quartz substrates (13 layers) for UV–Vis absorption and Raman spectroscopy) and interdigitated gold electrodes (IDE) (25 layers) for electrical characterization and sensor measures. The inter- digitated gold electrodes (IDEs) were fabricated using glass as a sub- strate by the technique of photolithography. The depositions were carried out approaching manually and horizontally the substrates to the air/water interface at a constant surface pressure of 20mN/m. 2.2.1. Films characterization The UV–Vis absorption spectroscopy characterization of P3HT and P3OT solutions (0.2 mg/mL, chloroform solution) and LS films were performed using a Varian spectrophotometer, model Cary 100, from 400 to 800 nm. The growth of P3HT and P3OT LS films deposited onto quartz substrates was monitored every 2 layers up to 13 layers. Raman analysis of P3HT and P3OT LS films were performed before and after THF gas exposure. For that, the LB films were exposed to THF gas atmosphere for 30min and then brought to Raman analysis. Raman spectra and morphological features (at micrometer scale) of P3HT and P3OT LS films (before and after gas exposure) were investigated using a Renishaw in-Via micro-Raman system equipped with a Leica micro- scope. A 50× microscope objective long lens allows collecting the spectra with ca. 1 μm2 spatial resolution, a CCD detection, and a com- puter-controlled three-axis-encoded (XYZ) motorized stage to allow Raman spectra to be recorded with a minimum step of 0.1 μm. Raman spectra and Raman mapping were carried out with the laser line at 633 nm, 1800 grooves/mm gratings and edge filters. Electrical characterizations of LS films of P3HT and P3OT, deposited onto IDE, was performed by current vs. voltage measurements (I-V), using a source of voltage Keysight, model B2901A. 2.3. VOCs sensor measures Electrical characterization of sensors built using LS films of P3HT and P3OT deposited onto IDE was carried out by alternating periods of exposure to continuous flow between, volatile organic compounds and nitrogen flow. The volatile organic compounds dichloromethane and tetrahydrofuran were used in the current versus time (I-t) measure- ments by applying a constant voltage of 5 V. The sensors (LS films of P3HT and P3OT) were submitted to the measures in a system con- taining a constant gas flow of 60 NL/h, where NL is a mass unit for gasses equal to the mass of 1 L, under a pressure of 1 atm at standard temperature. This flow value is kept during all measurement period, which may be accompanied by a flow meter. First of all, nitrogen flow is applied for 30min, to establish a baseline with this inert gas. After that, the nitrogen is released to drag the volatile organic compound to the sensor for 15min, and then, more 15min back to the nitrogen flow to return to the baseline. The process of alternating cycles between ni- trogen flow and the organic solvent is repeated until a standard beha- vior of the sensor's electrical signal is observed when in contact with the volatile organic compound. The schematic representation of the elec- trical measurements setup for VOCs sensors is shown in Fig. 1. Fig. 1. Schematic representation of the system for electrical conductivity and gas flow measurements. V.J.R. Oliveira et al. Materials Chemistry and Physics 217 (2018) 421–426 422 3. Results and discussion 3.1. Langmuir films of P3OT and P3HT The π-A isotherms of P3HT and P3OT Langmuir films are displayed in Fig. 2a and b, respectively. The occupied area per P3HT and P3OT molecules are estimated to be ca. 6.7Å2 and 5.2Å2, respectively, by extrapolating the linear region of the isotherms, at the phase condensed (20mN/m), to zero pressures. These values agree with those found in isotherms of the alkyl-substituted polythiophene derivative (P3AT). According to the authors [18], the low values of occupied area per molecule of these polythiophenes are associated with the angle between the aromatic ring (from P3HT and P3OT molecules) and the aqueous subphase at the Langmuir trough. Also, it is observed that the number of carbons in the lateral chain of these polythiophenes affects the sur- face pressure of the collapse region. For instance, the P3HT, which has the lower number of carbons in the lateral chain, exhibits the higher surface pressure at the collapse region, while the P3OT, containing the higher number of carbons in the lateral chain, shows the lower surface pressure at the collapse region. The rigidity of these regioregular polymers [19] forming Langmuir films at the interface air/water allows the deposition on solid substrates at a surface pressure of 20mN/m, fabricating Langmuir-Schaefer (LS) films [20]. 3.2. P3OT and P3HT LS films growth The growth of P3HT and P3OT LS films monitored by UV–Vis ab- sorption spectroscopy are presented in Fig. 3a and b, respectively, where the UV–Vis spectra were recorded every 2 layers up to 13 layers. The insets in Fig. 3a and b shows the absorbance at 520 nm and 524 nm for different numbers of deposited layers for P3HT and P3OT LS films, respectively. The linear growth of absorbance indicates that similar amounts of polythiophene derivates are transferred per deposited layer, revealing a controlled growth of the LS films. The three adsorption bands observed in the P3HT LS film (13 layers) at 520, 551 and 598 nm, and in the P3OT LS film (13 layers) at 524, 554 and 602 nm, are as- signed to vibronic peaks A0-2, A0-1, and A0-0, respectively [21,22]. The first two bands (A0-2 and A0-1) have contributions from an intrachain exciton [23], while the lowest energy band (A0-0) is attributed to in- terchain adsorption of the regioregular polythiophenes [24], char- acteristic of conjugated polymers, which are associated with π–π* transition [25,26] from carbon atoms linked by double bonds in the aromatic ring of the polythiophenes structures [27,28]. The intensity and the red-shift of the vibronic transition A0-0, at∼600 nm, are related with the ordering degree of the P3HT and P3OT chains [29], while the intensity of the amplitude ratio (A0–0/A0–1) is associated with the charge carrier mobility [24,30]. For a better understanding of how the molecular ordering and charge carrier mobility of the LS films changes throughout the depositions the parameters mentioned above are de- picted for 1 and 13 layers of each LS film in Table 1. The red-shift and the relative increase of absorption intensity of the band A0-0, noticed for the LS film of P3HT (from 1 to 13 layers), in- dicates an increase in the molecular ordering of this film [29]. Besides, the band A0-0 becomes more pronounced as the layers are deposited, which is another indication of the higher molecular ordering for the LS film of P3HT 13 layers [31]. When compared the amplitude ratio (A0–0/ A0–1) of the LS film of P3HT 13 layers with that of 1 layer, a higher value is found for the 13 layers, which favors the charge mobility of this film [31]. Similar behavior is observed for the LS film of P3OT (from 1 to 13 layers). Although a red-shift of the band A0-0 is not noticed, the relative increase of absorption intensity indicates an increase in the molecular ordering, from 1 to 13 layers [29]. Also, the increase in the amplitude ratio (A0–0/A0–1) for this film (from 1 to 13 layers) is higher than for the LS film of P3HT (from 1 to 13 layers). The latter suggests the charge carrier mobility increases more for the deposition of the LS film of P3OT than P3HT [31]. 3.3. DC electrical characterization The DC conductivity measurements were performed on an inter- digitated chromium-gold array for P3HT and P3OT LS films. From linear fit of I vs. V curves (Fig. 4), it is possible to estimate the con- ductivity (σ) and the resistance (R) of the material using Equation (1), the parameters presented in detail elsewhere [32]. = =σ R L A R k1 1 ( ). (1) where, the electrical conductivity is calculated from the linear adjust- ment of the presented experimental curve, and since the slope of the line is the inverse of R (in Ω), the electrical conductivity σ (S/m) is the inverse of the resistivity ρ, which can be determined by relating L as conductor length and A (cross-sectional area in m). Thus, the electrical conductivity σ can be obtained through the model of Olthuis et al. [32], and inserting the cell constant value К=5.1m-1. The constant K is equivalent to constant ratio L/A, obtained through the geometry of the conductor and considerer the spacing, height, and length of digits of the IDE used. The conductivity value measured in the dark was in the order of 10−6 S/m, which is in agreement with the values of neutral Fig. 2. π-A isotherms of (a) P3HT and (b) P3OT recorded for the ultrapure water subphase at room temperature (23 °C). The occupied area per molecules and the collapse regions are highlighted on the isotherms. Fig. 3. UV–Vis absorption for LS films of (a) P3HT and (b) P3OT. The insets show the absorbance at 520 nm and 524 nm of P3HT and P3OT LS films for different numbers of layers. Table 1 Parameters associated with the molecular ordering and charge carrier mobility for the LS films of P3HT and P3OT. LS Film Number of layers λA0-0 (nm) A0-0 (arb. unit.) A0-1 (arb. unit.) A0–0/A0–1 P3HT 1 layer 596 0.034 0.055 0.62 13 layers 598 0.409 0.616 0.66 P3OT 1 layer 602 0.032 0.054 0.60 13 layers 602 0.388 0.575 0.67 V.J.R. Oliveira et al. Materials Chemistry and Physics 217 (2018) 421–426 423 regioregular polythiophenes [18]. Figs. 5 and 6 show the electrical responses of the sensors upon the exposures to different VOCs. It was observed a negative and fast re- sponse in all cases, indicating that the conductivity of the materials is decreasing in the presence of the gases [33]. For P3HT/P3OT di- chloromethane sensors the current values return to initial ones showing that the devices are reversible [34]. On the other hand, when the mi- crosensors were exposed to THF, it was observed that the current did not return to the initial value being partially reversible. The negative electrical response of the sensor can be explained since the organic VOCs used are solvents of the polythiophenes, where the passage of this vapor on the polymer sample in the form of a thin film can alter the conformation of its chains. This effect caused by swelling of the LS films can increase the distance between the chains. Since the transport mechanism in these materials is due to polaron hopping conduction, the swelling effects cause a decrease in the electrical con- ductivity [4,35]. This swelling effect depends on the polymer-vapor interactions and the polymer solubility. The partial reversibility for THF sensors probably is due to the higher degree of solubility of this com- pound when compared to dichloromethane [36,37]. The P3HT and P3OT sensors show a rapid response to gas exposure. In the presence of dichloromethane, the sensors present a response within the first 3 s with an abrupt change of the electrical current. After exposure of the sample in nitrogen flow, the devices exhibited total recovery in approximately 3min. While in the presence of THF, the response time is around 5min and a return to a certain level of stabilization shortly after 13min of ex- posure in an inert atmosphere. The response and recovery times of the samples studied are in the same range to the recovery and detection time of organic vapors reported in the literature for polythiophene derivatives detecting volatile organic compounds [38–40]. 3.4. Morphology of P3HT and P3OT LS films before and after gas exposure The spatial distribution of the P3HT and P3OT in the LS films were evaluated, at the micrometer scale, using the micro-Raman technique, which combines morphological and chemical information by coupling an optical microscope to a Raman spectrograph. Fig. 7 shows point-by- point area Raman mappings superimposed to optical images of the P3HT and P3OT LS films before and after THF gas exposure. The Raman mappings were obtained recording spectra from areas of 100 μm×70 μm with 1 μm of step (three of them are shown in Fig. 7a, b, c, and d), where brighter spots indicate the higher relative intensity of the band at 1445 cm−1, assigned to symmetric C=C stretching [23,41]. The others two bands around 1381 and 728 cm−1 are attrib- uted to C―C intra-ring stretching and C―S―C ring skeleton deforma- tion, respectively [23,41,42]. To investigate how the THF gas atmosphere affects the morphology of the LS films, the molecular ordering of P3HT and P3OT was analyzed before and after gas exposure. For that, three important parameters were investigated to identify the molecular ordering of these polymers: i) peak position of C=C stretching mode (1445 cm−1) shifts to lower wavenumbers; ii) the relative intensity of C―C mode (1381 cm−1) to C=C mode (1445 cm−1) (IC―C/IC=C) becomes larger; and iii) full width at half maximum (FWHM) of C=C stretching mode (1445 cm−1) be- comes narrower [41,43]. These three parameters are presented in Table 2 for the LS films of P3HT and P3OT before and after THF gas exposure. Fig. 4. DC electrical characterization (I vs. V) for the P3HT and P3OT films deposited by Langmuir-Schaefer technique onto gold IDE. Fig. 5. Measures (I x t) with the LS film of P3HT in the presence of vapors of (a) dichloromethane and (b) tetrahydrofuran (THF). Fig. 6. Measures (I x t) with the LS film of P3OT in the presence of vapors of (a) dichloromethane and (b) tetrahydrofuran (THF). Fig. 7. Raman spectra and area Raman mappings superimposed to optical images of P3HT LS film (a) before and (b) after gas exposure and P3OT LS film (c) before and (d) after gas exposure. V.J.R. Oliveira et al. Materials Chemistry and Physics 217 (2018) 421–426 424 The Raman mappings superimposed to the optical images (insets of Fig. 7) reveal non-homogeneous film surfaces at the microscale, even for the LS films of P3HT and P3OT before THF gas exposure. Although the Raman spectra recorded from distinct spots of the mappings show discrepancies regarding the relative intensities of the band around 1445 cm−1, the relative intensities for the LS films after gas exposure are smaller than before gas exposure. For instance, in the case of P3HT LS film, the relative intensities of the band around 1445 cm−1 after gas exposure (Fig. 7b) are at least two times smaller than before gas ex- posure (Fig. 7a). Also, Raman spectra recorded from the region 3 (Fig. 7b) come from the substrate (glass Raman signal), indicating that the exposure to THF gas atmosphere affects the morphology of this film physically, forming cracked domains. These results are in agreement with the parameters of molecular ordering of the P3HT and P3OT LS films before and after gas exposure (Table 2). Differences in the values of IC―C/IC=C and FWHM of C=C mode found for the LS film of P3HT before and after gas exposure confirm that the morphology of this film was more affected than the LS film of P3OT, resulting in higher mole- cular disorder [44] which represents a limitation to the carrier mobility in polymer matrices [45,46]. 4. Conclusion LS nanostructured films of polythiophene derivatives (P3HT and P3OT) were produced. The films show controlled growth behavior and irregular morphology. Both derivatives were deposited onto solid sub- strates (IDE), presenting good optical and electrical properties, with reproducible values of electrical conductivity. The VOCs sensors pre- sent an electrical response for THF and dichloromethane detection, decreasing the electrical current. Complementary, Raman analysis of P3HT and P3OT LS films before and after THF exposure, confirmed that the morphology of P3HT LS film was more affected than the P3OT LS film, resulting in higher molecular disorder. The Raman mappings re- vealed the sensing mechanisms involves mainly physical interactions between the polymers and VOCs, forming cracked domains on the LS films after gas exposure. Thus, the device proposed here is promising for use in VOCs sensors. Acknowledgments The authors are grateful for the financial support of the agencies CAPES, FAPESP, INEO/CNPq and CNPEM/LNNano. References [1] S.R. 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