Chemical surface treatment with toluene to enhances sensitivity of no2 gas sensor based on cupcts alq3 thin films

6e8show the variation of resistance of CuPcTs/Alq3thinfilms with operating time for two gas pulses at different operating temperatures RT, 323, 373 and 423 K, for samples chemically treat

Original Article

Chemical surface treatment with toluene to enhances sensitivity of

a Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq

b Department of Physics, College of Science, University of Sulaimani, Kurdistan Region, Iraq

a r t i c l e i n f o

Article history:

Received 27 May 2017

Received in revised form

29 June 2017

Accepted 5 July 2017

Available online xxx

Keywords:

Organic blend

CuPcTs/Alq 3

Chemical treatment

Sensitivity

NO 2 gas sensor

a b s t r a c t

The nitrogen dioxide (NO2) gas sensor based on the blend of copper phthalocyanine-tetrasulfonic acid tetrasodium/tris-(8-hydroxyquinoline)aluminum (CuPcTs/Alq3) thinfilms 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 toluenefilms exhibit a broad hump peak at 2q¼ 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 forfilms treated at 60 min immersing time with toluene Gas sensor characterizations demonstrate that all samples have superior NO2gas sensitivity at

373 K operating temperature The increase of sensitivity with increasing chemical treatment time up to

60 min was observed Allfilms 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/)

1 Introduction

Organic donor and acceptor materials are widely considered to

be the most promising candidates to develop inexpensive

renew-able energy sources based on donor/acceptor interface bilayered

(heterojunction) and blended (bulk-heterojunction 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

exten-sively studied as an active layer for optoelectronic device

phthalocyanine-tetrasulfonic 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

interest in tris-(8-hydroxyquinoline)aluminum(III) (Alq3) for tech-nical applications started after a report on using Alq3as the active medium in efficient electroluminescent devices[10] The optical, electrical, and charge carriers transport mechanism for both the amorphous and crystalline Alq3films are studied further to opti-mize the device performance[11] Based on its molecular structure the Alq3can 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 injec-tion barrier and could act as traps for charge carriers[10] Polymer-phthalocyanine blend materials were already demon-strated 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

effectiveness in chemical dissolving[15,16] Toluene is one of the major organic solvents, has been exten-sively 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

* Corresponding author.

E-mail address: omed.abdullah@univsul.edu.iq (O.Gh Abdullah).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

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/ ).

performance [18] The performance of a chemical gas sensor

de-pends on several issues such as sensitivity, selectivity, stability and

response/recovery times[19]

Several works have revealed that the conductivity of different

gaseous species A number of researchers have investigated

ni-trogen dioxide (NO2) gas sensors based on different organic and

inorganic materials[20,21] Sensing properties are mostly

deter-mined 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 thefilms [22] In spite of a

large number of already available sensing layers for NO2gas

sen-sors, they still have more or less disadvantages, such as low

sensitivity, high operating temperature[23e27] Thus, new

ma-terials 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

terms of morphology and crystallinity using combination of

scanning electron microscopy (SEM), atomic force microscopy

(AFM) and X-ray diffraction (XRD)

2 Experiment details

2.1 Fabrication of CuPcTs/Alq3thinfilm

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

{C27H18AlN3O3} were purchased from SigmaeAldrich, and used

without further purification The scheme of the molecular

struc-ture of CuPcTs and Alq3are shown inFig 1 Thinfilms of CuPcTs/

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

materials

The prepared blend CuPcTs/Alq3solutions were deposit on the

glass substrate using spin-coating with spinning speed of 1500 rev/

min for 2 min Thefilms were dried at room temperature to form

the thickness of thefilms and found to be in the range between

conditions

2.2 Film characterization and property measurements The crystal structure of as-deposited CuPcTs/Alq3blend thinfilm 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 CuKawith wavelengthl¼ 1.5405 Å Scanning electron microscopy , type JSM-7600F produced by JEOL Ltd Japan, provides topographical information at magnifications of

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 2 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 sys-tem It has an inlet for allowing the tested gas toflow in and an air admittance valve to allow theflow of atmospheric air after evacu-ation A multi-pin feedthrough at the base of the chamber allows the electrical connections to be established to the heater, thermo-couple and sensor electrodes

A hot plate heater controlled by the GEMO DT109 PID temper-ature 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 per-sonal computer is used to measure the variation of the sensor resistance when exposed to air-NO2mixing through aflow-meters and needle valve arrangement

The nitrogen dioxide (NO2) gas was produced by the reaction of copper pieces with concentrated HNO3acid in a glass container The chemical reaction for production of NO2gas is as follows:

Fig 1 Molecular structure of: (a) copper phthalocyanine-tetrasulfonic acid tetrasodium (CuPcTs); (b) tris-(8-hydroxyquinoline)aluminum (Alq 3 ).

M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 2

NO2gas was dried by specialfilters with flow rate 2.5 Nm2/min.

The amount of testing gas controlled by twoflow-meters to be 1:10

of incident air, and time of passing gas controlled by a timer The

blendfilm sample was loaded into a closed chamber and the

elec-trical resistance of the sensor was measured by a multimeter

con-nected to the computer when the different ratio of target gas with air

wasflowing into the chamber in (on) and (off) case of the target gas

3 Results and discussion

3.1 Structural properties

Fig 3illustrates the X-ray diffraction (XRD) patterns for

toluene at different immersing times (40, 60 and 80 min) It is clear from Fig 3that the chemically treated samples are more amor-phous compared to as-prepared CuPcTs/Alq3blendfilm As can be seen in thefigure all the samples exhibit a broad peak centered at

2q¼ 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/Alq3thinfilm and chemically treated with toluene at different times (40, 60 and

roughness and the average diameter of the randomly distributed particles on thefilm 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 Fig 2 Schematic diagram for the homemade NO 2 gas sensor testing system.

Fig 3 XRD for CuPcTs/Alq 3 thin films chemically treated with toluene at a different time.

Fig 4 AFM micrographs and diameter distribution diagram for CuPcTs/Alq 3 blend thin films chemically treated with toluene at a different times: (a) 0 min, (b) 40 min, (c) 60 min, (d) 80 min.

M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 4

not occur during the immersion process Here, thefilm has started

to aggregate and align during the evaporation of the solvent

Table 1shows the AFM parameters for CuPcTs/Alq3thinfilms at

different treated times with toluene It is clear that the average

decreased to 64.25 nm for 40 min immersing time, and then

increased to 67.82 nm for 80 min (seeTable 1) The decrease in

average particle diameter enhances the absorption of NO2gas on

the sensor surface, which leads to an increase in sensor sensitivity,

as will be shown later

Fig 5shows the SEM images of as-deposited CuPcTs/Alq3thin

films on a glass substrate and chemically treated ones with toluene

in different immersing times It seems that the aggregated particles

on thefilm surface disappeared with increasing the time of

im-mersion It is also obvious that some needle shaped particles

appeared at 60 min immersing time with toluene

The Hall effect was determined at room temperature according

to van der Pauw configuration.Table 2shows Hall effect parameters

for CuPcTs/Alq3blendfilms The negative sign of the Hall coefficient

RHconfirms 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 min

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 (s¼ nem, where n is the charge carrier

concen-tration, e is electronic charge, andmis carrier mobility)[35,36] It is

obvious fromTable 2that the highest conductivity corresponds to

the lowest resistivity The increase in conductivity (sRT) upon in-crease immersed time can be attributed to the inin-crease 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/Alq3blendfilms

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 oper-ating temperature, the relation between the response and tem-perature is firstly studied for CuPcTs/Alq3 thin films chemically treated with toluene at the same rate of gas exposes.Figs 6e8show the variation of resistance of CuPcTs/Alq3thinfilms 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 thesefigures that the values of electrical resistance vary with increasing operating temperature and the

Table 1

AFM parameters for CuPcTs/Alq 3 thin films chemically treated with toluene at

different times.

Treatment time

with toluene (min)

Average diameter (nm)

RMS roughness (nm)

Peakepeak (nm)

Fig 5 SEM images for CuPcTs/Alq 3 thin films deposited on glass substrate treated with toluene in different immersing times.

Table 2 Hall measurements for CuPcTs/Alq 3 blend at different treatment times with toluene Treatment time

with toluene (min)

sRT 10 3

(U$cm) 1 R H

(U)

n10 16

(cm3)

m

(cm 2 /V$sec)

Fig 6 Resistance variation for CuPcTs/Alq 3 thin film chemically treated with toluene at 40 min, for different operating temperatures.

Fig 7 Resistance variation for CuPcTs/Alq 3 thin film chemically treated with toluene at 60 min, for different operating temperatures.

Fig 8 Resistance variation for CuPcTs/Alq 3 thin film chemically treated with toluene at 80 min, for different operating temperatures.

M.H Suhail et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e8 6

time of chemical surface treatment The sensor resistance value

oxidizing nature of NO2 The charge transfer occurs between the

element due to the electron-acceptor of NO2molecules, 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 9e11show, respectively, the variety of NO2gas sensitivity,

response time and recovery time versus operating temperature for

CuPcTs/Alq3gas sensor samples to NO2gas for different chemical

surface treatment time with toluene The increase in sensitivity

with increasing operating temperature is observed Indeed, the

sensitivity reaches maximum values at 373 K, then decreases at

423 K for all samples The response time and recovery time of the

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 tempera-ture for NO2sensor based on CuPcTs/Alq3thinfilm is around 373 K

On the other hand, the sensitivity increase with increasing chem-ical treatment time up to 60 min, then decreases for more time of immersing in toluene The minimum values of response and re-covery times (18 and 20 s) were observed for the sample chemically treated with toluene for 60 min, as a result of decreasing particle

AFM measurements The significant reductions of the response and recovery times for chemically treated samples are sufficient for the practical application of CuPcTs/Alq3blendfilm as NO2gas sensor

4 Conclusion This work presents the impact of toluene surface treatment on the structure, surface morphology, and NO2sensing properties of organic semiconductor based on the blend CuPcTs/Alq3thinfilm prepared by a spin-coating technique AFM measurement revealed that the particles diameter of CuPcTs/Alq3blends decreased with treated time up to 60 min and then increased with more treated time, whereas SEM images show the disappearance of some ag-gregation of particles on the as-deposited CuPcTs/Alq3film surface

It has been shown that the increase in conductivity of chemically treatedfilms 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-treatedfilm has been enhanced

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 thefinancial support given to this work

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