High-sensitive room-temperature NO2 sensor based on a soluble n-type phthalocyanine semiconductor

Inorganic Chemistry Communications 77 (2017) 18–22

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Short communication

High-sensitive room-temperature NO2 sensor based on a soluble n-type phthalocyanine semiconductor Zhen Dong a, Xia Kong b, Yanling Wu b, Jinfu Zhang b, Yanli Chen a,b,⁎ a b

Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China School of Science, China University of Petroleum (East China), Qingdao 266580, China

a r t i c l e

i n f o

Article history: Received 12 December 2016 Received in revised form 12 January 2017 Accepted 17 January 2017 Available online 21 January 2017 Keywords: Phthalocyanine Gas sensor Self-assemblies N-type semiconductor

a b s t r a c t 2(3),9(10),16(17),23(24)-tetrakis(2,2,2-trifluoroethoxy) phthalocyanine H2Pc(OCH2CF3)4(1) has been synthesized characterized by using MALDI-TOF MS, 1H NMR, UV–vis spectra and differential pulse voltammetry. Our work addresses this by introducing four electron-withdrawing trifluoroethoxy substituents at the periphery of the phthalocyanine ring to ensure the sufficient solubility and suitable LUMO energy level and thus successfully realize soluble n-type organic semiconductor. Furthermore, H2Pc(OCH2CF3)4 exhibited the high crystallinity and large specific surface area as well as good conductivity in the thin solid film fabricated by a simple solution-based quasi–Langmuir–Shäfer (QLS) method. Importantly, the highly sensitive, stable and reproducible responses to electron-accepting gas NO2 in 100–500 ppb range are observed for the QLS film of 1 at room temperature, implying the excellent potential of 1 as the NO2 sensor for applications in practical environments. In addition, the response of the QLS film of 1 is linearly correlated to the NO2 concentration. The interaction between the H2Pc(OCH2CF3)4 film and NO2 molecules with different concentrations was found to follow first-order kinetics. The present result represents not only the first example of n-type phthalocyanine-based NO2 sensor obtained by a solution-based method, but more importantly provides a new strategy for the molecular design to obtain soluble n-channel organic semiconductors in the field of sensing device fabrication. © 2017 Published by Elsevier B.V.

The increasing concerning over the air-quality control, environmental monitoring, health-care, and other applications has aroused a great interest in reliable gas detection of nitrogen dioxide (NO2) at room temperature. Phthalocyanines, as the representatives of molecular semiconducting materials with large conjugated molecular structures, are known for their ability to detect trace concentrations, typically in the parts per million (ppm) levels and above [1], particularly strongly oxidizing and reducing gases such as NO2 and NH3 via charge-transfer interactions between the sensor and chemisorbed species that modify the sensor's resistance [2]. Although the sensitivity of devices based on nanostructured semiconducting metal oxides is excellent such as SnO2, TiO2, ZnO, In2O3, and WO3, offering detection of NO2 down to ppb levels, very high working temperature (200–400 °C), cross-sensitivity to other gases and low long-term stability limit their practical application [3–5]. Phthalocyanines, which are mostly p-type organic semiconductors, are of sensitivity at low temperature, and low-cost and easy fabrication features, enabling them promising candidates for NO2 sensing in the practical environment [6]. However, low selectivity (sensitive to many gases) and less sensitive to a lower-level NO2 compared to nanostructured metal oxides-based sensors is now the main ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Chen).

http://dx.doi.org/10.1016/j.inoche.2017.01.023 1387-7003/© 2017 Published by Elsevier B.V.

obstacle for the practical use of sensor devices [7]. The broad physicochemical properties of phthalocyanines, such as their high thermal and chemical stabilities compared with other organic materials and a rich substitution chemistry, lead to a large flexibility in tuning molecular structures (or macroscopic properties), and therefore in their sensing performance [8–10]. For example, it has been reported that the p-type CoPc is sensitive to both NH3 and O3, while the n-type Co[Pc(SO3Na)2,3] with the electron-withdrawing sulfonated substituents at the periphery of Pc ring shows a good selectivity that is only sensitive to NH3 [11]. Furthermore, the sensitivity of phthalocyanines towards NO2 was found to be improved significantly by introducing crown-ether groups [12] and flexible alkyl chain substituents on the periphery of the phthalocyanine macrocycle [13] to optimize both the molecular ordering in the films and the intermolecular phthalocyanine-analyte interactions. However, it must be pointed out that controlling and tuning the intermolecular interaction through the combination of molecular design with morphology-controlled self-assembly to improve sensing performance still remains a great challenge for chemists and material scientists. On the other hand, soluble phthalocyanine molecules can be deposited by layer-by-layer assembly techniques such as the Langmuir–Blodgett (LB: vertical transfer), Langmuir-Shäfer (LS: horizontal transfer) and Quasi-Langmuir–Shäfer (QLS) protocols that allow obtaining films with good thickness control, offering the possibility to easily produce

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multilayer structured films at air–water interface [14–16]. In particular, QLS method developed by Bouvet's group requires no sophisticated setup with a lower cost relative to LS protocol, which has been employed to obtain the well-organized films of phthalocyanine-based molecules at air–water interface [14]. In this communication, a new 2(3),9(10),16(17),23(24)Tetrakis(2,2,2-trifluoroethoxy)phthalocyanine compound H2Pc(OCH2CF3)4(1) has been synthesized (Scheme 1) by the general cyclic tetramerization method according to the published procedures [17]. A mixture of tetrakis[2,2,2-trifluoroethoxy]-phthalocyaninato (0.22 g, 1.00 mmol), 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) (0.1 ml) and n-amyl alcohol (3 ml) was heated and stirred at 120 °C for 4 h in a nitrogen atmosphere in pressure-resistant glass tube of 10 ml. Repeated chromatography followed by recrystallization from CH2Cl2 gave pure compound as a dark-green powder in good yield of 50.8%. MS (MALDI-TOF, DHB):m/z 906.6. UV–vis (THF, 1.6 × 10−6 M): λmax(log ε) = 349 (4.70), 605 (4.35), 664 (5.07), 678 (5.07) nm. 1H NMR (400 MHz, (CD3)2SO): δ 7.80 (d, 4H, Pc-Hα), 7.44 (q, 4H, Pc-Hα), 7.42 (q, 4H, Pc-Hβ), 3.6, 3.0 (s, 2H, Pc-N-H), 4.75 (q, 8H, Pc-OCH2CF3) (Fig. S1, Fig. S2). The electrochemical behavior of 1 was investigated by differential pulse voltammetry (DPV) in DMF. As shown in Fig. 1, H2Pc(OCH2CF3)4 exhibited totally two quasi-reversible one-electron redox processes within the electrochemical window of DMF, similar to the electrochemical behavior for unsubstituted H2Pc in DMF reported in literature [18]. As compared with the half-wave potentials of the first oxidation and the first reduction at +0.64 V and −0.72 V (vs SCE) for unsubstituted analogue H2Pc [18], introduction of four electron-withdrawing trifluoroethoxy groups at the peripheral positions of the phthalocyanine ring induces obvious shift of the two quasi-reversible one-electron redox processes to positive direction for both the first oxidation (+0.93 V vs SCE) and the first reduction (−0.62 V vs SCE), thus enlarging the potential difference between these two redox processes, actually the HOMO-LUMO gap of H2Pc(OCH2CF3)4 (1.55 eV), relative to that of H2Pc (1.36 eV). This indicated that peripheral trifluoroethoxy substituents make the compound H2Pc(OCH2CF3)4 more easily reducible than unsubstituted H2Pc since the half-wave potential for the first reduction, which involves the LUMO of H2Pc(OCH2CF3)4, is shifted in the more positive direction relative to that of H2Pc. The HOMO and LUMO energy levels of 1 was estimated to be about − 5.37 and − 3.82 eV, based on DPV measurements [19]. Ordered thin film of 1 was prepared by the QLS method. Electronic absorption spectrum, X-ray diffraction (XRD), and atomic force microscopy (AFM) techniques were used to evaluate the quality of the QLS film. As shown in Fig. 2, H2Pc(OCH2CF3)4 in CHCl3 solution displays typical electronic absorption spectrum for the monomeric phthalocyanine with a Soret band at 335 nm and two intense Q bands at 658 nm and 695 nm with two shoulders at 603 and 632 nm, respectively [20]. After being fabricated into the QLS film, the main Q bands of 1 were broadened and red-shifted to about 676 and 712 nm, while the Soret band was blue-shifted to 323 nm. Kasha's point-dipole model provides a rationale for the observed phenomenon. The extreme cases are represented by a head-to-tail arrangement of the dipoles, which results in a red-shifted absorption band (J aggregate), and a parallel arrangement of the dipoles (H aggregate) with a blue-shifted absorption band [21]. The red and blue shifted bands observed in the present case for the

Scheme 1. Synthesis of H2Pc(OCH2CF3)4 (1).

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Fig. 1. Differential pulse voltammetry (DPV) of H2Pc(OCH2CF3)4 in DMF containing 0.1 mol dm−3 [Bu4N]+[ClO4]− at a scan rate of 20 mV/s vs SCE.

QLS film of 1 appear to represent an intermediate case that is conventionally thought of as a slipped cofacial stack with an “edge-on” configuration for the Pc chromophores of 1 relative to a substrate [22]. This phenomenon has also been observed previously for unsymmetrically substituted phthalocyaninato copper complexes [19] and porphyrin aggregates [23,24] with a slipped cofacial oriented structure, where two transition moments interact in face-to-face and parallel orientation to give rise to blue and red shifts, respectively. The quality of the QLS film deposited on SiO2/Si substrate is further assessed using X-ray diffraction (XRD) technique. As shown in Fig. 3,the XRD diagram of 1 shows one comparatively strong and sharp diffraction peak at 2θ = 5.41° (corresponding to 1.63 nm), which corresponds to the thickness of one layer for the QLS film, indicating a regular layered structure of this film. Judging from a geometry-optimized molecular length of 1 (2.10 nm), the tilt angle relative to the substrate of 50.9° is estimated for 1, which is consistent with the result of polarized UV–vis spectra (Fig. S3) in which the orientation angle of the phthalocyanine ring with respect to the substrate is about 54.2° according to a literature method (the uncertainty of the measurement method, ±5°) [25–27]. This further confirmed that the H2Pc(OCH2CF3)4 molecules take a slipped cofacial stacking mode with an “edge-on” configuration in the QLS film. In addition, the XRD pattern for the QLS film of 1 also presents one sharp diffraction at 0.31 nm, which is attributed to the π-π stacking distance between tetrapyrrole cores of neighboring phthalocyanine molecules along the direction perpendicular to the tetarpyrrole rings [28]. The morphologies of the self-assembled aggregates of 1 were observed by atomic force microscopy (AFM). As shown in Fig. 4, the

Fig. 2. Electronic absorption spectra of H2Pc(OCH2CF3)4 (1) in dilute chloroform solution (dash line); QLS films (solid line) on quartz substrate.

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Fig. 5. I\ \V curves measured on the QLS film of H2Pc(OCH2CF3)4 (1). Fig. 3. X-ray diffraction patterns of the QLS films of 1 on SiO2/Si substrate.

images for the QLS film of 1 show uniform small-grain crystallites, with approximately 60 ± 5 nm in diameter. These uniform particles increase the specific surface area of the membrane so that the QLS film has more active sites for adsorption of NO2. A root-mean-square (Rms) roughness value of 0.466 nm is revealed, suggesting that the QLS film of 1 possesses an excellent membrane structure, in line with the result from XRD analysis. The QLS film of 1 consisting of uniform nanoparticles would be promising candidate for applications in electronic devices. To demonstrate the potentials of the QLS film of 1, the I-V measurement was carried out for the QLS film deposited onto glass substrate with ITO interdigitated electrodes (IDEs). As shown in Fig. 5, the QLS film of 1exhibits excellent conductivity of 1.07 × 10−6 S·cm−1, depending on the highly ordered molecular arrangement and well-defined nanostructures for the QLS film of 1. In order to obtain the applicability for gas sensing, the QLS filmbased sensor of 1 was exposed to NO2 gas diluted in dry nitrogen with the different concentration of 100–500 ppb at room temperature. The sensing performances were studied with a duty cycle where the dynamic exposure period is fixed at only 1 min. As can be seen from Fig. 6A, the negative current response of the QLS film of 1 was obtained upon exposure to electron-accepting NO2 gas, which is consistent with the sensing behavior reported previously for n-type semiconductors [2,29,30]. The doping of NO2 will lead to trapping of carriers (here electrons), thus decreasing of current. Considering the crown-ether-substituted phthalocyanines as the p-type organic semiconductors have been reported by Wright and co-workers [1], the n-type sensing response to NO2 of the QLS film of H2Pc(OCH2CF3)4 unambiguously reveals the significant substituents effect on tuning the nature of phthalocyanine organic semiconductors. By introducing four electron-withdrawing trifluoroethoxy

substituents at the periphery of the phthalocyanine ring to lower LUMO energy value of H2Pc(OCH2CF3)4 (−3.82 eV) and reduce electron injection barriers, hence bringing n-channel property [31]. Furthermore, current change of the sensors clearly shows a good separation for the different NO2 concentration levels and good reversibility to NO2 at room temperature, Fig. 6A. In order to quantitatively analyze the sensor responses, the percent current change was calculated for each concentration, as follows: %currentchange= [(Io − If)/Io] × 100, where Io is the current value at the beginning of an exposure/recovery cycle and If is the current value at the end of the 1 min exposure period. The sensor responses of the QLS films of 1 are linear with respect to various concentrations of NO2 in the range of 100–500 ppb (Adj. R-square N 0.99), Fig. 6B. Thus, such sensor can be used to quantitatively analyze the concentration of NO2 at room temperature. The slope of the linear fit of the percent current change as a function of NO2 concentration is about 0.0075%·ppb−1 in the range of 100–500 ppb. The excellent sensitivity with low detection limit down to ppb levels benefits from high film-conductivity and richly active sites for adsorption of NO2 due to a densely packed molecular architecture and uniform granular nanostructure with large specific surface area for the QLS film of 1. Furthermore, a small mismatch between low-lying LUMO energy level of 1 (−3.82 eV) and the work function of the ITO electrode (−4.5 eV) [32] results a low energy barrier for electron injection and thus good n-type response to electronaccepting NO2. In addition, the stability of the sensor was assessed by repeated exposure to 500 ppb of NO2 for eight duty cycles, Fig. 6C. As can be seen, the relative current change degrades b1% by compared to that in the first duty cycle. With regard to practical use, selectivity of the sensor is also concern. As can be seen in Fig. S4, there is an obvious discrimination of the QLS film of 1 to the different concentrations of NO2 ranging from 500 to 100 ppb in ambient air, which is similar to that

Fig. 4. AFM phase (left) and 3D (right) images of the QLS films on SiO2/Si substrate of H2Pc(OCH2CF3)4 (1).

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Fig. 6. A) The time-dependent current plot of sensor as a function of the NO2 concentration in N2 atmosphere for the QLS film of H2Pc(OCH2CF3)4. B) Sensor response varies linearly with NO2 concentration of H2Pc(OCH2CF3)4 QLS films. C) Cyclic detection at 500 ppb concentration of NO2. D) The kinetics plots of QLS film of H2Pc(OCH2CF3)4: ln(It − If) versus time are predominantly linear for different concentrations of NO2. Arrows indicate the direction of change with decreasing concentration of NO2.

observed when the sensor operates in N2 (Fig. 6A). Apart from drying the air, no further precautions were taken, showing that the fabricated sensor is selective for NO2. We also performed the electrical measurements on H2S and NH3, and no obvious current response was obtained. It has been reported that organic thin film sensors rely on weak intermolecular interactions rather than redox chemistry [33]. For H2Pc (OCH2CF3)4 chemiresistive sensor, the hydrogen bonding of NO2 to the two interior NH protons should primarily govern sensor response [33]. There is the additional possibility of weak binding (physisorption) to the organic region of the H2Pc(OCH2CF3)4 molecule for NO2 molecules, which may be governed by weak hydrophobic and possibly charge-transfer interactions and therefore the response of H2Pc(OCH2CF3)4 chemiresistive sensor. On the other hand, no obvious current response of the H2Pc(OCH2CF3)4 to H2S and NH3 should be mainly attributed to weaker the hydrogen bonding of H2S and NH3 to the two interior NH protons of H2Pc(OCH2CF3)4 due like to the close packing of the fluoroalkyl chains from H2Pc(OCH2CF3)4 molecules having a slipped cofacial stack with an “edge-on” configuration on the surface of the substrate that acts as a kinetic barrier against H2S and NH3 with larger molecular sizes relative to NO2 [31,34], in addition to weaker van der Walls interactions between H2Pc(OCH2CF3)4 and reducing H2S or NH3 gases with a lower molecular weight than those of an oxidizing NO2 gas. Indeed, it should be pointed out that, among pollutants, NO2 needs to be measured in the 39–117 ppb range when detected in airquality control. It should be very promising strategies to improve the sensitivity as NO2 needs to be measured in the concentration range of air-quality control through the combination of molecular design and optimization of molecular packing mode. On the other hand, Fig. 6D shows plots of ln(It − If) versus t for the QLS film of 1, where It is the current at time t and If is the final current after an exposure period. It is clearly found that ln(It – If) versus time are linear for different concentrations of NO2, suggesting that the interaction between the H2Pc(OCH2CF3)4 and NO2 molecules follows the first-order kinetics according to gas-MPc kinetics analysis developed by Tongpool and coworks [35]. Examination of these plots shows that the slope variation as a function of NO2 concentration is similar to each other. Consequently, the surface adsorption mechanism can be confirmed.

In conclusion, a new n-type phthalocyanine semiconductor H2Pc(OCH2CF3)4 (1) was successfully synthesized. The self-assembled film consisting of uniform nanoparticles with regular layered structure was fabricated by means of a solution-based QLS method. The QLS film of 1 exhibited excellent sensitivities, low detection limit, good stability and selectivity at room temperature. The present work provides a new strategy for the preparation of high-performance Pc-based chemical sensors through the combination of molecular design and solutionbased self-assembled technique. Acknowledgment Financial support from the Natural Science Foundation of China (No. 21371073), Research Fund for Introduced Talents of China University of Petroleum (No·Y1510051), and the Fundamental Research Funds for the Central Universities (No. 16CX06022A). Appendix A. Supplementary material Synthesis and characterization details of H2Pc(OCH2CF3)4 (1), Experimental section. MALDI-TOF MS, 1H NMR, polarized UV–vis spectra and the time-dependent current plots and relative response of sensors as a function of the analyte concentration for the QLS film of H2Pc (OCH2CF3)4(1) exposed to NO2 at varied concentration from 500 to 100 ppb in air atmosphere. Supplementary data associated with this article can be found, in the online version, at doi: 10.1016/j.inoche.2017. 01.023. Reference [1] P. Roisin, J.D. Wright, R.J.M. Nolte, O.E. Sielcken, S.C. Thorpe, Gas-sensing properties of semiconducting films of crown-ether-substituted phthalocyanines, J. Mater. Chem. 2 (1992) 131–137. [2] X. Kong, Z. Dong, Y. Wu, X. Li, Y. Chen, J. Jiang, High sensitive ambipolar response towards oxidizing NO2 and reducing NH3 based on bis(phthalocyaninato) europium semiconductors, Chin. J. Chem. 34 (2016) 975–982. [3] I.-D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers, Nano Lett. 6 (2009–2013) 2006.

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