World Journal Of Engineering
Shiv Kumar Gupta, Niranjan Ramgir, Manmeet Kaur, Niyanta Datta, Vibha Saxena Aditee Joshi,
Ajay Singh and Dinesh Kumar Aswal
Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
Introduction
Metal or metal oxide nanowires (NWs) dispersed in organic matrix is emerging as a new class of electronic materials [1]. Recent findings demonstrated that the intrinsic properties of
NWs can easily be retained in the organic matrix
[2]. This has lead to the upsurge in research towards novel hybrid nanocomposite materials.
The galore of applications includes batteries, electroluminescent devices, supercapacitors, photovoltaic cells and gas sensors. In the present work, we demonstrate ZnO NWs dispersed in polymer matrix namely of polypyrole (PPy) and poly(3)hexithiophene (P3HT) as room temperature operating Cl
2
and NO
2
sensors, respectively.
Experimental
ZnO NWs were synthesized using carbothermal method [3]. Nanocomposites films were prepared by drop casting method on quartz/glass substrates using colloidal solutions containing
ZnO NWs and PPy/P3HT in different weight percentage [1, 3]. For ZnO: PPy composites, colloidal solutions were prepared using 0.1 M aq. solution of pure distilled pyrole, 0.05 M aq. solution of ammonium persulphate
(polymerizing agent), and a suspension of NWs in ethanol. For ZnO: P3HT composites solutions were prepared by mixing P3HT in chloroform and NWs dispersed in ethanol. All the solutions were ultransonicated for 15 min prior to drop casting to assure uniform NW dispersion.
Surface morphologies were investigated using scanning electron microscope (SEM, VEGA
MV2300T/40). Gas sensing measurements were performed in a static set-up as described elsewhere [1]. Briefly, the sensor was mounted upside down in a stainless steel chamber
(volume 250 cm 3 ) and a measured concentration
Email: drgupta@barc.gov.in
of test gas was injected using syringe. The response data was acquired using a personal computer equipped with labview software.
Recovery of the sensor was recorded by exposing the sensor to fresh air. The sensitivity was defined as S = R a
/R g
where R a
and R g
are resistance in air and test gas, respectively.
Response and recovery times were defined as the time needed for 90% of total resistance change on exposure to gas and air, respectively.
Results and Discussion
Figure 1 shows the SEM images of PPy and
P3HT films before and after the formation of nanocomposites. Inset of Figure 1 shows as grown NWs having diameter of 80-100 nm and length of several µm. Both pure PPy and P3HT films exhibited a granular morphology while 1:1 composite films are uniform at the microscale.
NWs get coated with polymers and are randomly distributed in the matrix.
Fig 1. SEM images of pure polymer samples
and respective composite films.
Systematic investigation carried out by varying the NWs content in the host matrix indicated
379

World Journal Of Engineering that 0.5:1 and 1:1 mixing ratio are the best composition for composite films with PPy and
P3HT respectively. ZnO: PPy films are highly selective towards Cl
2
while ZnO: P3HT films are selective towards NO
2
at room temperature.
Composite films exhibited better sensing characteristics as compared to that of pure polymer and ZnO NW films. For example, figure 2 shows the response characteristics of pure PPy, NW and composite films towards 5 ppm of Cl
2
. Pure PPy and ZnO NW films suffers from the drawback of poor sensitivity and recovery (>24 h). On the other hand composite film detected Cl
2
with high sensitivity and fast response kinetics. The sensor films exhibited S =
15.8 with a response time of ~ 55 s towards 5 ppm of Cl
2
. modifying the electrical properties of the host matrix. In particular, O 2 terminated NW surface transfer electrons to polymers that are thereby reduced. In reduced state charges are strongly localized in PPy ring and films are insensitive towards most gases improving their selectivity.
In case of P3HT films, NWs besides reducing are also expected to form donar-acceptor like complexes thereby improving selectivity [3].
Fig 2. Response curves of (a) pure PPy films, (b)
ZnO NWs and (c) ZnO: PPy (1:1 wt.%) composite films towards 5 ppm Cl
2
.
Besides, room temperature operation composite films offer various advantages that include full recovery of base resistance, high reproducibility, selectivity (fig 3 (a)) and improved sensitivity values. Additionally, long term stability measurement indicated high stability. More specifically, the base line resistance stability of
ZnO: P3HT nanocomposite films was found to be better than 5% over a period of 30 days even after repeated exposure to test gas as shown in fig 3 (b). The improved sensing performance in composite films was investigated by FTIR, UV-
Vis, XPS and XRD measurements and is attributed mainly to the role of ZnO NWs in
380
Fig 3.
Selectivity histogram recorded for (a)
ZnO: PPy and (b) long term stability measurement of ZnO: P3HT composite films towards 8 ppm NO
2
.
Conclusions
Highly selective room temperature sensors towards Cl
2
and NO
2
gases have been demonstrated using inorganic/organic nanocomposite films. The nanocomposites films were obtained using ZnO NWs and polymers namely PPy and P3HT.
References
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