Effect of Chemical Treatment with Talwen on CuPcTs/PEDOT:PSS Thin Films Gas sensing.
Organic gas sensors based on variation of resistivity due to gas molecules adsorbed on films surfaces have high attention for their applications in environmental monitoring. In this work, we examine the effect of chemical treatment with talwen at different immersion time (40, 60 and 80 min) on structural properties and surface morphology of the blend (copper phthalocyanine tetrasulfonic acid tetrasodium salt/poly dioxyethylenethienylene doped with polystyrenesulphonic acid) (CuPcTs/PEDOT:PSS) thin films created by spin coating, and there effect on N[Osub.2] gas sensitivity to know the best properties for used as gas sensing. The produced thin films were examined by X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD measurement shows that the CuPcTs/PEDOT:PSS blend have one peak with [d.sub.hkl] values less than CuPcTs values. The crystallinety increase with increasing immersing time with talwen at 60 min then decrease. The grain size decrease at 40 min immersing time then increase with more immersing time with talwen. Also, the AFM measurement shows that the CuPcTs/PEDOT:PSS blends particles diameter decrease at 40 min treated time with talwen then increase with more treated time. It's found that the best chemical treatment time, for using samples as gas sensor, at 40 min and the best operating temperature 375 K.
KEYWORDS: Organic semiconductors, N[O.sub.2] gas sensor, X-ray diffraction
Organic behave as semiconductors due to the existence of alternating single and double carbon-carbon bonds. There are many thesis procedure depend on using organic semiconductors in many field due to the very promising applications of organic electronics  such as in solar cells, electronic devices and gas sensing . The p-type semiconducting CuPcTs was reported to be sensitive to many gases, such as N[O.sub.2]. phthalocyanine compound can be improved by chemical treatment with low solubility solvents .
The previous researches show that most of gas sensors based on principle of detection based on the changes of electrical resistance depending on the electronic nature of the target gas due to the interaction of gas molecules (chemi or physisorption) with the surface of sample . In addition, such sensors need to be operated at elevated temperatures to aid desorption of the gas . The resistance variation |[DELTA]R| = |[R.sub.air]-[R.sub.gas]| with N[O.sub.2] gas introduction, where [R.sub.air] the resistance of the sensor in air and [R.sub.gas] is sample resistance with existence of test gas. The gas sensitivity (S) of the sensor obtained using relationship: S = |[DELTA]R/[R.sub.air]|. After the exposure to testing gas the sensor was kept for a recovering period in air to calculate the recovery time. 
In this work, CuPcTs (from Ossila. UK) dissolved by magnetic stirrer (24 hour) in deionized water at 310 K. Then to remove any unsolved material, the solution was filtered using (0.45) [mu]m filter. The prepared CuPcTs solutions were used to make CuPcTs/PEDOT:PSS by mix it with same amount from PEDOT:PSS (1:2.5) with low resistivity (<0.0012 [OMEGA].cm) (from Ossila. UK) by magnetic stirrer in about 32 hour at 310 K.
2.5X2.5 cm glass substrate substrates were cleaned by cleaning them with a detergent solution and rinsed them by water. Three stage of Then cleaned by ultrasonic three times, 20 min for each one, using water, alcohol and acetone, and then dried by blowing air and by wiped soft papers. The blend were deposited on the glass substrate by spin coating with 1500 rev/min speed about 2 minute time. The films were treated with talwen organic solvent at different treated time (40, 60 and 80) min to study the optimum conditions which improve the blend films properties to use as N[O.sub.2] gas sensor.
All films were examined by XRD, from Shimadzu company, to study the effect of chemical treatment on structural properties, and by AFM using CSPM contact mode spectrometer, to study the effect of immersing time with talwen on surface morphology.
Then aluminum electrodes as mesh were deposited by thermal evaporation using Edward coating unit model (Auto 306) under high vacuum ([10.sup.-5] mbar), which was provided by rotary and diffusion pump, on its surface to increase the electrode area as shown in Fig (1).
Fig. (2) shows the schematic for arrangement used for gas sensing measurements. Copper pieces and HN[O.sub.3] acid where put in glass container to produce N[O.sub.2] gas from the chemical reaction
Cu + 4 HN[O.sub.3] [right arrow] Cu(N[O.sub.3])[.sub.2] + 2 N[O.sub.2] + 2 [H.sub.2]O
N[O.sub.2] gas was dried by special filter. The amount of testing gas controlled by two flow- meters to be 1:10 of incident air, and time of passing gas controlled by timer. The sample temperature was adjusted by heater attached by thermocouple which connected with thermometer control the power supplied to heater. Rotary pump was used to vacuum the chamber. Multimetre connected with computer used to draw the resistance variation with time.
RESULTS AND DISCUSSIONS
Fig. (3) illustrates the XRD patterns for as deposited CuPcTs/PEDOT:PSS blend thin films on glass substrate and chemically treated with talwen at different immersing time(40, 60 and 80) min. This figure shows that as deposited film and chemically treated at 40 and 60 minute have one peak arround 2[degrees] =4.5[degrees] which has shifted toward less values from standard CuPcTs peak in the direction (001) , i.e. increasing in [d.sub.hkl] values as a result of mixing PEDOT:PSS with CuPcTs. Peak intensity increase with increasing immersing time from 0 to 60 min then vanished at 80 min.
Fig. (4) shows the fitting for observed peaks in XRD patterns for CuPcTs/PEDOT:PSS thin films chemically treated with talwen at different time. The calculated FWHM and the calculated grain size using Scherrer equation were shown in Table (1). This table shows that the grain size decrease at 40 min immersing time from 22.5 nm to 21.8 nm then increase to 26.1 nm at 80 min.
Fig. (5) shows the AFM images for as deposited CuPcTs/PEDOT:PSS thin films on glass substrate and chemically treated with talwen at different time (40, 60 and 80 min).
Table (2) Shows the AFM parameters for CuPcTs/PEDOT:PSS thin films at different treated time with talwen. This table shows that the average particles diameter decrease at 40 min immersing time from 100.65 nm to 83.67 nm then increase to 89.14 nm and 102.61 nm at 60 min and 80 min respectively. The maximum roughness observed at 60 min treated time.
Fig. (6) shows the variation of resistance with time for CuPCTS/PEDOT:PSS thin films in two cases (gas-on and gas-off) with different operating temperature (RT, 323, 373 and 423) K and for samples chemically treated with different times (as deposited, 40, 60 and 80) min with talwen.
It can be seen from this figure that the sample resistance decrease with temperature. Also, Its found the resistance decrease exponentially when sample exposure to NO2 gas (oxidation gas), i.e. the performance of sample as p-type, where oxidation gas adsorbed on sample surface capture of some electrons from the lattice caused to produce more holes contribute in electrical conductivity cause to reduce sample resistance . Also, it's found that the ratio between the variation in sample resistance to original resistance (sensitivity), response time and recovery time varies with operating temperature and with chemical treatment time.
Figs. (7 to 9) show the variation of N[O.sub.2] gas sensitivity, response time and recovery time versus operating temperature for CuPCTS/PEDOT:PSS thin films samples to N[O.sub.2] gas with different chemically treatment time. These figures show that the sensitivity increase reaching maximum values with increasing operating temperature to 373 K then decrease at 423 K for all samples, also the minimum values of response time and recovery time at 373 K. The sensitivity increase with increasing chemical treatment time to 40 min then decrease at more immersing time with talwen, and the minimum values of response and recovery times at 40 min treated time. as a result of decreasing particle size and increasing surface roughness (i.e. increase surface area exposure to gas target) as shown in XRD and AFM measurements .
Table (3) shows all calculated values for gas sensitivity, response time and recovery time for CuPCTS/PEDOT:PSS thin films samples to NO2 gas with different operating temperature and different chemically treatment time.
The effect of chemically treatment with talwen on structural properties, surface morphology and gas sensing of CuPcTs/ PEDOT:PSS blend films show many points as follows:
* XRD measurement shows that the crystalline size decrease at 40 min immersing time from 22.5 nm to 21.8 nm then increase to 26.1 nm at 80 min immersing time with talwen.
* AFM measurements shows that the average particles diameter decrease at 40 min immersing time from 100.65 nm to 83.67 nm then increase to 89.14 nm and 102.61 nm at 60 min and 80 min respectively. Also the maximum roughness were found at 40 min immersing time.
* From gas sensing measurements to N[O.sub.2] gas. The sensitivity increase with increasing chemical treatment time to 40 min then decrease at more immersing time with talwen, and the minimum values of response and recovery times at 40 min treated time, as a result of decreasing particle size and increasing surface roughness, and the best operating temperature was found at 375 K.
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Table 1: The structural parameters of CuPcTs/PEDOT:PSS thin films chemically treated with Talwen at different time. time (min) 2[theta] (Deg.) FWHM (Deg.) [d.sub.hkl] Exp.(A) As deposited 4.635 0.354 19.0500 40 4.479 0.365 19.7140 60 4.457 0.328 19.8119 80 4.304 0.304 20.5156 time (min) G.S (nm) [d.sub.hkl] Std.(A) phase hkl As deposited 22.5 12.640 CuPcTs (001) 40 21.8 12.640 CuPcTs (001) 60 24.3 12.640 CuPcTs (001) 80 26.1 12.640 CuPcTs (001) Table 2: AFM parameters for CuPcTs/PEDOT:PSS thin films chemically treated with talwen at different time. Time with Tal. (min) Average Diameter (nm) RMS roughness (nm) As deposited 100.65 1.09 40 83.67 5.28 60 89.14 0.736 80 102.61 1.32 Time with Tal. (min) Peak-peak (nm) As deposited 4.39 40 20.6 60 2.96 80 5.11 Table 3: N[O.sub.2] sensitivity, response time and recovery time for CuPCTS/PEDOT:PSS thin films for different operating temperature and different chemical treatment time Time (min) Operating temp (K) Sesitivity (%) response time (s) 300 4.5 38.0 AS deposited 323 7.2 37.0 373 9.9 24.0 423 2.8 33.0 300 5.6 34.0 40 323 9.0 33.0 373 12.3 21.0 423 3.4 30.0 300 4.2 37.0 60 323 6.2 35.0 373 7.5 30.0 423 2.6 34.0 300 3.4 38.0 80 323 5.2 37.0 373 5.7 34.0 423 2.0 36.0 Time (min) recover time (s) 42.0 AS deposited 37.0 29.0 39.0 41.0 40 35.0 26.0 30.0 41.0 60 36.0 30.0 32.0 45.0 80 43.0 37.0 41.0
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|Author:||Ramadan, Amer Abbas; Suhail, Mahdi H.; Abdulameer, Ameer F.; Muhy, Izzat Mahmood|
|Publication:||Advances in Environmental Biology|
|Date:||Feb 1, 2017|
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