Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/Alq3 thin films Mahdi H Suhail a, Amer A Ramadan a, Shujahadeen B Aziz b, Omed Gh Abdullah b, * a b Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq Department of Physics, College of Science, University of Sulaimani, Kurdistan Region, Iraq a r t i c l e i n f o a b s t r a c t Article history: Received 27 May 2017 Received in revised form 29 June 2017 Accepted July 2017 Available online xxx The nitrogen dioxide (NO2) gas sensor based on the blend of copper phthalocyanine-tetrasulfonic acid tetrasodium/tris-(8-hydroxyquinoline)aluminum (CuPcTs/Alq3) thin films was fabricated The effect of chemical surface treatment with toluene on the structural, surface morphology and device sensitivity has been examined The X-ray diffraction (XRD) patterns for as-deposited and chemically treated with toluene films exhibit a broad hump peak at 2q ¼ 24 The atomic force microscopy (AFM) measurements show that the average particle diameter decreases with immersing time The needle like shapes can be seen from scanning electron microscopy (SEM) images for films treated at 60 immersing time with toluene Gas sensor characterizations demonstrate that all samples have superior NO2 gas sensitivity at 373 K operating temperature The increase of sensitivity with increasing chemical treatment time up to 60 was observed All films show a stable and repeatable response patterns © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Organic blend CuPcTs/Alq3 Chemical treatment Sensitivity NO2 gas sensor Introduction Organic donor and acceptor materials are widely considered to be the most promising candidates to develop inexpensive renewable energy sources based on donor/acceptor interface bilayered (heterojunction) and blended (bulk-heterojunction BHJ) photovoltaic cells [1,2] Compared to the bilayered systems, the BHJ provides a larger interfacial area between the donor and the acceptor material, which is essential for the formation of the charge-transfer state as well as charge separation [3,4] Copper (II) phthalocyanine (CuPc) is an organic semiconductor that extensively studied as an active layer for optoelectronic device applications [5,6] The structure of copper (II) phthalocyaninetetrasulfonic acid tetrasodium salt (CuPcTs) is very similar to CuPc except that polar SO3Na joined to the corners of the four benzene rings, that makes this compound water-soluble [7] Recent studies reveal that many research groups focused on the fabrication of highly efficient solar cells and gas sensors based CuTsPc molecule, due to their relatively simple synthesis, economically attractive, chemical stability and environmentally friendly [8,9] Increasing * Corresponding author E-mail address: omed.abdullah@univsul.edu.iq (O.Gh Abdullah) Peer review under responsibility of Vietnam National University, Hanoi interest in tris-(8-hydroxyquinoline)aluminum(III) (Alq3) for technical applications started after a report on using Alq3 as the active medium in efficient electroluminescent devices [10] The optical, electrical, and charge carriers transport mechanism for both the amorphous and crystalline Alq3 films are studied further to optimize the device performance [11] Based on its molecular structure the Alq3 can exist in two different geometric isomers: meridional and facial [12] The different highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels predicted for the two isomers are expected to influence the injection barrier and could act as traps for charge carriers [10] Polymer-phthalocyanine blend materials were already demonstrated to be less crystalline, higher conductivity, and more efficient for gas sensing than pure phthalocyanine [13,14] Furthermore, various methods have been proposed to enhance phthalocyanine blend properties via suitable solvent treatment by immersing in the selected solvents The selection of an ideal solvent requires a balance between surface modification of metal phthalocyanine and effectiveness in chemical dissolving [15,16] Toluene is one of the major organic solvents, has been extensively used to modify the surface morphology and optical behavior of the organic active layer by immersing sample in low solubility solvents of toluene [17] This chemical surface treatment with toluene caused the increment in the light absorption through an increment in charge transport, which leads to improving the device http://dx.doi.org/10.1016/j.jsamd.2017.07.003 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Please cite this article in press as: M.H Suhail, et al., Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/Alq3 thin films, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.003 M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 performance [18] The performance of a chemical gas sensor depends on several issues such as sensitivity, selectivity, stability and response/recovery times [19] Several works have revealed that the conductivity of different p-conjugated polymer films varies by exposing them to certain gaseous species A number of researchers have investigated nitrogen dioxide (NO2) gas sensors based on different organic and inorganic materials [20,21] Sensing properties are mostly determined by the adsorbed/desorbed gas molecules on the surface of the active layer resulting in decrease in the carriers density, thereby increasing the resistance of the films [22] In spite of a large number of already available sensing layers for NO2 gas sensors, they still have more or less disadvantages, such as low sensitivity, high operating temperature [23e27] Thus, new materials need to be tested to detect this harmful gas in higher sensitivity [28,29] The extensive survey of literature reveals that there is no any report on the effect of different immersed times in toluene on the efficiency of gas sensors Thus, the present study aims to use the synthesized CuPcTs/Alq3 blend material as the main sensing layer for NO2 gas sensing The effect of chemical treatment with toluene on the performance of NO2 gas sensing was also reported The CuPcTs/Alq3 film was characterized in terms of morphology and crystallinity using combination of scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) Experiment details 2.1 Fabrication of CuPcTs/Alq3 thin film All chemicals used in the present were of analytical grade The copper (II) phthalocyanine-tetrasulfonic acid tetrasodium salt (CuPcTs; MW: 984.25 g/mol) {C32H12CuN8O12S4Na4} and tris-(8hydroxyquinoline)aluminum salt (Alq3; MW: 459.43 g/mol) {C27H18AlN3O3} were purchased from SigmaeAldrich, and used without further purification The scheme of the molecular structure of CuPcTs and Alq3 are shown in Fig Thin films of CuPcTs/ Alq3 blend were prepared by taking ml from both CuPcTs and Alq3 solution in chloroform, with 15 mg/ml concentration The mixture were stirred using magnetic stirrer for 12 h with shaking vigorously at ambient temperature (310 K) The blend solution was then filtered using 0.45 mm filter to remove undissolved materials The prepared blend CuPcTs/Alq3 solutions were deposit on the glass substrate using spin-coating with spinning speed of 1500 rev/ for The films were dried at room temperature to form solid films Optical interferometer method was used to measure the thickness of the films and found to be in the range between (600e750 nm) The prepared thin films were then treated with toluene (C19H27NO; MW: 285.42378 g/mol) at a different immersed times (40, 60 and 80 min) to find the optimum treatment time to enhance film properties as a NO2 gas sensor The treated films with toluene are carefully dried under ambient conditions 2.2 Film characterization and property measurements The crystal structure of as-deposited CuPcTs/Alq3 blend thin film and chemically treated ones with toluene at a different time has been analysis using X-ray diffraction (Shimadzu 6000) technique The source of radiation was CuKa with wavelength l ¼ 1.5405 Å Scanning electron microscopy , type JSM-7600F produced by JEOL Ltd Japan, provides topographical information at magnifications of 10Â to 300,000Â, with virtually unlimited depth of field The changes in film surface morphology of CuPcTs/Alq3 blend films during the chemical surface treatment was recorded using CSPM contact mode atomic force microscopy which can provide enough information in 3D images 2.3 Gas sensor system and measurement Gas sensing performances were measured by a homemade sensor testing system shown in Fig The system consists of stainless steel cylindrical test chamber with a diameter of 16.3 cm and height 20 cm The rotary pump was used to evacuate the system It has an inlet for allowing the tested gas to flow in and an air admittance valve to allow the flow of atmospheric air after evacuation A multi-pin feedthrough at the base of the chamber allows the electrical connections to be established to the heater, thermocouple and sensor electrodes A hot plate heater controlled by the GEMO DT109 PID temperature controller and a K-type thermocouple inside the chamber were used to measure the operating temperature of the sensor A PC-interfaced digital multimeter (Vector 70C) connected to a personal computer is used to measure the variation of the sensor resistance when exposed to air-NO2 mixing through a flow-meters and needle valve arrangement The nitrogen dioxide (NO2) gas was produced by the reaction of copper pieces with concentrated HNO3 acid in a glass container The chemical reaction for production of NO2 gas is as follows: Cu ỵ HNO3 / Cu(NO3)2 ỵ NO2 ỵ H2O (1) Fig Molecular structure of: (a) copper phthalocyanine-tetrasulfonic acid tetrasodium (CuPcTs); (b) tris-(8-hydroxyquinoline)aluminum (Alq3) Please cite this article in press as: M.H Suhail, et al., Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/Alq3 thin films, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.003 M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 Fig Schematic diagram for the homemade NO2 gas sensor testing system Fig XRD for CuPcTs/Alq3 thin films chemically treated with toluene at a different time NO2 gas was dried by special filters with flow rate 2.5 Nm2/min The amount of testing gas controlled by two flow-meters to be 1:10 of incident air, and time of passing gas controlled by a timer The blend film sample was loaded into a closed chamber and the electrical resistance of the sensor was measured by a multimeter connected to the computer when the different ratio of target gas with air was flowing into the chamber in (on) and (off) case of the target gas Results and discussion 3.1 Structural properties Fig illustrates the X-ray diffraction (XRD) patterns for asdeposited CuPcTs/Alq3 blend film and chemically treated with toluene at different immersing times (40, 60 and 80 min) It is clear from Fig that the chemically treated samples are more amorphous compared to as-prepared CuPcTs/Alq3 blend film As can be seen in the figure all the samples exhibit a broad peak centered at 2q ¼ 24 Earlier studies on polymer electrolytes and composites confirmed the fact that the increase of broadness is an evidence for the increase of amorphous fraction [30e33] The top-view AFM images of as-deposited CuPcTs/Alq3 thin film and chemically treated with toluene at different times (40, 60 and 80 min) are shown in Fig From these images, the surface roughness and the average diameter of the randomly distributed particles on the film surface can be measured Indeed, the surface morphology is clearly affected by chemical treatment with toluene According to Yang et al [34], the modification of the film surface did Please cite this article in press as: M.H Suhail, et al., Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/Alq3 thin films, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.003 M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 Fig AFM micrographs and diameter distribution diagram for CuPcTs/Alq3 blend thin films chemically treated with toluene at a different times: (a) min, (b) 40 min, (c) 60 min, (d) 80 Please cite this article in press as: M.H Suhail, et al., Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/Alq3 thin films, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.003 M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 Table AFM parameters for CuPcTs/Alq3 thin films chemically treated with toluene at different times Treatment time with toluene (min) Average diameter (nm) RMS roughness (nm) Peakepeak (nm) 40 60 80 115.66 70.75 64.25 67.82 0.66 1.93 0.42 0.66 3.95 8.85 1.56 4.40 not occur during the immersion process Here, the film has started to aggregate and align during the evaporation of the solvent Table shows the AFM parameters for CuPcTs/Alq3 thin films at different treated times with toluene It is clear that the average particle diameter for as-deposited films is 115.66 nm, and decreased to 64.25 nm for 40 immersing time, and then increased to 67.82 nm for 80 (see Table 1) The decrease in average particle diameter enhances the absorption of NO2 gas on the sensor surface, which leads to an increase in sensor sensitivity, as will be shown later Fig shows the SEM images of as-deposited CuPcTs/Alq3 thin films on a glass substrate and chemically treated ones with toluene in different immersing times It seems that the aggregated particles on the film surface disappeared with increasing the time of immersion It is also obvious that some needle shaped particles appeared at 60 immersing time with toluene The Hall effect was determined at room temperature according to van der Pauw configuration Table shows Hall effect parameters for CuPcTs/Alq3 blend films The negative sign of the Hall coefficient RH confirms the n-type conductivity for all the prepared films The carrier mobility (m) and their conductivity (sRT) increase with increasing the surface treatment time with toluene up to 60 and then decrease with more treatment time, while the carrier concentration has a reverse behavior It is well understood that electrical conductivity depends on the carrier concentration (n) and carrier mobility (s ¼ nem, where n is the charge carrier concentration, e is electronic charge, and m is carrier mobility) [35,36] It is obvious from Table that the highest conductivity corresponds to Table Hall measurements for CuPcTs/Alq3 blend at different treatment times with toluene Treatment time with toluene (min) sRTÂ10À3 (U$cm)À1 RH ( U) nÂ1016 (cmÀ3) m 40 60 80 2.44 2.90 3.81 3.39 À510.5 À1286.3 À2286.8 À367.5 1.22 0.49 0.27 1.70 1.25 3.74 8.72 1.25 (cm2/V$sec) the lowest resistivity The increase in conductivity (sRT) upon increase immersed time can be attributed to the increase in carrier mobility The lowest value of resistance (maximum conductivity) after chemical treatment with toluene for 60 min, recommends that gas sensing responses will improve, as shown later 3.2 Gas sensors In this section, the ability of synthesized CuPcTs/Alq3 blend films before and after chemically treated with toluene for NO2 gas sensing are exhibited The performance of gas sensors generally characterized by three parameters: sensitivity, selectivity and response time [37,38] Sensitivity is the ability of the sensor to quantitatively recognize the gas under given conditions Selectivity is its ability to sense a particular gas free from interference, and response time is a measure of how quickly the maximum signal change is achieved with gas concentration changes [39,40] Because the response of a gas sensor highly depends on operating temperature, the relation between the response and temperature is firstly studied for CuPcTs/Alq3 thin films chemically treated with toluene at the same rate of gas exposes Figs 6e8 show the variation of resistance of CuPcTs/Alq3 thin films with operating time for two gas pulses at different operating temperatures (RT, 323, 373 and 423 K), for samples chemically treated with toluene at different immersing times (40, 60 and 80 min), respectively The moment at which the gas turn-on and turn-off is monitored on the figures It can be seen from these figures that the values of electrical resistance vary with increasing operating temperature and the Fig SEM images for CuPcTs/Alq3 thin films deposited on glass substrate treated with toluene in different immersing times Please cite this article in press as: M.H Suhail, et al., Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/Alq3 thin films, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.003 M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 Fig Resistance variation for CuPcTs/Alq3 thin film chemically treated with toluene at 40 min, for different operating temperatures Fig Resistance variation for CuPcTs/Alq3 thin film chemically treated with toluene at 60 min, for different operating temperatures Fig Resistance variation for CuPcTs/Alq3 thin film chemically treated with toluene at 80 min, for different operating temperatures Please cite this article in press as: M.H Suhail, et al., Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/Alq3 thin films, Journal of Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.003 M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 Fig NO2 gas sensitivity versus operating temperature for CuPcTs/Alq3 gas sensor with different chemical treatment time with toluene CuPcTs/Alq3 gas sensor decrease with increasing the operating temperature The observed minimum values of response time and recovery time at 373 K indicates that the best operating temperature for NO2 sensor based on CuPcTs/Alq3 thin film is around 373 K On the other hand, the sensitivity increase with increasing chemical treatment time up to 60 min, then decreases for more time of immersing in toluene The minimum values of response and recovery times (18 and 20 s) were observed for the sample chemically treated with toluene for 60 min, as a result of decreasing particle size and increasing surface roughness as confirmed by XRD and AFM measurements The significant reductions of the response and recovery times for chemically treated samples are sufficient for the practical application of CuPcTs/Alq3 blend film as NO2 gas sensor Conclusion Fig 10 NO2 response time versus operating temperature for CuPcTs/Alq3 gas sensor with different chemical treatment with toluene This work presents the impact of toluene surface treatment on the structure, surface morphology, and NO2 sensing properties of organic semiconductor based on the blend CuPcTs/Alq3 thin film prepared by a spin-coating technique AFM measurement revealed that the particles diameter of CuPcTs/Alq3 blends decreased with treated time up to 60 and then increased with more treated time, whereas SEM images show the disappearance of some aggregation of particles on the as-deposited CuPcTs/Alq3 film surface It has been shown that the increase in conductivity of chemically treated films is due to the increase of amorphous fraction as well the enhancement of the carrier mobility The sensitivity as well as the response time for the toluene-treated film has been enhanced in compared to film without toluene treatment The increase of sensitivity due to toluene treatment may hold great promise for further advancement in sensor technology Acknowledgement The authors would like to thank the Ministry of Science and Technology for the facility in their laboratories The authors gratefully acknowledge the University of Sulaimani for the financial support given to this work References Fig 11 NO2 recovery time versus operating temperature for CuPcTs/Alq3 gas sensor with different chemical treatment with toluene time of chemical surface treatment The sensor resistance value increased when the films were exposed to NO2 gas due to the oxidizing nature of NO2 The charge transfer occurs between the adsorbed NO2 gas on the surface of the sample and sensing element due to the electron-acceptor of NO2 molecules, resulting in the increase of resistance value upon exposure to NO2 [41e43] This result is well matched with obtained result for Hall Effect tests It can also be noted that the ratio of sample resistance to original resistance (sensitivity), response time and recovery time varies with operating temperature and with chemical surface treatment time Figs 9e11 show, respectively, the variety of NO2 gas sensitivity, response time and recovery time versus operating temperature for CuPcTs/Alq3 gas sensor samples to NO2 gas for different chemical surface treatment time with toluene The increase in sensitivity with 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treatment with toluene. .. article in press as: M.H Suhail, et al., Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/ Alq3 thin films, Journal of Science: Advanced Materials and... article in press as: M.H Suhail, et al., Chemical surface treatment with toluene to enhances sensitivity of NO2 gas sensor based on CuPcTs/ Alq3 thin films, Journal of Science: Advanced Materials and