Enhancement of NO2 gas sensing behavior for ZnS/PPy nanostructure by loading graphene

The pure ZnS and ZnS-Gr nanocomposite have been preparedsuccessfully by a novel method using chemical co-precipitation. Alsoconductive polymer PPy nanotubes and ZnS-PPy nanocompositehave been synthesized successfully by chemical route. The effect ofgraphene on the characterization of ZnS has been investigated. X-raydiffraction (XRD) study confirmed the formation of cubic andhexagonal structure of ZnS-Gr. Dc-conductivity proves that ZnS andZnS-Gr have semiconductor behavior. The SEM proved thatformation of PPy nanotubes and the Gr nanosheet. The sensingproperties of ZnS-PPy/ZnS-Gr for NO2 gas was investigated as afunction of operating temperature and time under optimal condition.The sensitivity, response time and recovery time were calculatedwith different operating temperatures.


Introduction
Recent studies have converge on sensing gases such as CO, CO 2 , SO 2 , O 3 , H 2 and NH 3 in the atmosphere. A sensor is a device that can measure a physical parameter and transform it to an electrical signal. Physical parameters include temperature, light, velocity, as well as biological and chemical concentrations. Sensors that are designed to detect harmful or toxic gases need to be designed such that they are accurate, sensitive and rapid. Gas sensors are important for detecting and/or monitoring hazardous gases that exist in small concentrations [1].
Zinc sulfide (ZnS) is a II-VI semiconductor. This material happen naturally in the two various crystal structures Zincblende and wurtzite, which are cubic and hexagonal structures, respectively [2]. The Zincblende structure is the most steady structure at room temperature, and it convert into wurtzite at 1020 o C [3]. In both structures each Zn atom is connected to four S atoms in a tetrahedral arrangement, with a mixed bonding of covalent and ionic binding forces between the atoms [4]. The rising amount of ionic bonds in the semiconductor would lead to an increase of the inter elemental forces, which in turn could command to shorter distances between the atoms. As a result, the crystal structure would alteration from Zincblende to wurtzite. Since the energy gap increases with the amount of ionic bonds in a semiconductor, this could clear why the band gap is crystal structure dependent [5]. Graphene nano sheet (GNS) is one layer of graphite. graphene possesses singular physical properties, its two dimensional planar structure, half metallicity, and high electron mobility. All these electronic and structural properties of the GNS make it a promising route to use it in several applications, such as nanoelectronics, solid state sensors, and spintronic [6][7][8]. More recently, graphene, has been known to be a promising sensing material as a result of its distinctive and wonderful electrical and mechanical properties [9]. Hyeun Joong Yoon et al., G. Ko et al., studied the graphene sensing against CO 2 and NO 2 gases [9,10]. Polypyrrole (PPy), one of the most extensively investigated conducting polymers, has attracted a great deal of interest because of its good electrical conductivity, environmental stability and easy synthesis. PPy has been performed in a number of implementation, such as batteries, supercapacitors, sensors, microwave shielding and corrosion protection [11]. Nowadays there is a great interest in making conducting polymer sensors. polypyrrole has been widely used as an effective material for the detection of toxic, hazardous and flammable gases.
In this work, the sensing properties for ZnS/PPy will be improved by loading Graphene to detect NO 2 toxic gas.

Experimental details Synthesis of ZnS and ZnS-Gr nanocomposite
ZnS nanoparticles were synthesized by co-precipitation chemical method. A stock solution of Zn 2+ was prepared by adding 0.1 M of Zn(NO 3 ) 2 .6H 2 O into 50mL of distilled water. A stock solution of S 2− was prepared by adding 0.2 M of Na 2 S into 50mL of distilled water. Then put the first solution on magnetic stirrer at temperature of 80 o C for 1 hr, then the second solution is added drop wise and the whole solution is stirred for 30 min. Milky precipitation will be observed that indicates ZnS formation, then filtered and washed several times using distilled water, ethanol and finally by acetone then dried using oven at 100 o C for 5 hr. For preparing Zinc sulfide with Graphene (ZnS-Gr), the different volume ratios (5, 10, 20)% of Graphene (Gr) was added to stock solution of Zn 2+ and stirred together at 80 o C for 1 hr, then solution of S 2− is added drop wise and the whole solution is stirred for 30 min. Grey color precipitation will be observed that indicates ZnS-Gr formation, then filtered and washed several times using distilled water, ethanol and finally by acetone then dried using oven at 100 o C for 5 hr.

Synthesis of polypyrrole (PPy) and ZnS-PPy nanocomposite
Polypyrrole (PPy) nanotubes were prepared by the following method reported by An et al. [13]. 2 mM of methyl orange and 20 mM of FeCl 3 were dissolved in 500 ml of distilled water, followed by the addition of 0.84 ml pyrrole monomer with stirring for 24 h. The as-prepared sample was filtered and washed with distilled water several times and then dried at 100 °C for 6 h.
ZnS-PPy composite was synthesized by chemical method. A stock solution of Zn 2+ was prepared by adding 0.5 mM of Zn(NO 3 ) 2 .6H2O into 50 mL of distilled water. A stock solution of S 2− was prepared by adding 10 mM of Na 2 S into 50mL of distilled water. Then adding 5mL of ppy to first solution and put them on magnetic stirrer at temperature of 80 °C for 1 hr, then the second solution is added drop wise and the whole solution is stirred for 30 min. Then filtered and washed several times using distilled water, ethanol and finally by acetone then dried using oven at 100 °C for 5 hr.

Preparation of ZnS-Gr-PPy
The preparation of ZnS-PPy/ZnS-Gr by using Chemical spray pyrolysis Method (c.s.p). Here are the basic steps to the process of the preparation of films: The solutions put on the magnetic stirrer for about 15 minutes to ensure that the mixture solutions are mixed properly. Then the solution must be put in sprayer container. The glass substrate was heated and left until they reach the desired temperature. The solution (ZnS-PPy) sprays on glass substrate, then the second solution (ZnS-Gr) spray above the first solution. After that the films Zns-PPy/ZnS-Gr were gotten. During the operation of spraying, the glass substrate must be moved regularly and the amount of spray should be controlled to get the best homogeneity of the films. After the spray process completed, then the hot plate will be shut down and the samples are left on the surface of the heater to reach the room temperatures, then the substrates can be raised. Fig.1 shows the x-ray diffraction of ZnS/Gr at different volume ratios of Gr which are (5, 10, 20) %. From figure (1a) it has been observed that pure ZnS has cubic zinc blende structure with comparison to the standard card ICDPDS (96-110-1051). The three main peaks are observed in the diffractrogram at 2Ѳ equal 28.3549, 47.5354, 56.195 corresponds to (111), (220) and (311) planes respectively. For the ZnS/Gr system with different volume addition ratios (5, 10, 20) % of Gr, ZnS is still with cubic structure at 5% Gr volume ratio as shown in figure (1b). More addition of graphene which are (10, 20) % as shown in Figure  (1c,d,), new phase for ZnS appeared that is hexagonal structure with comparison to the standard card ASTM (I2-688) because Gr sheet has Hexagon structure (honeycomb lattice), so the graphene sheet becomes as a template to ZnS growth on this sheet. In the same time, graphene peaks will appear at ZnS/10% Gr but very slight as in figure (1c) and more Gr addition, Gr peaks will be distinguished as shown in figure (1d), so high Gr addition activate the presence of ZnS hexagonal phase beside ZnS cubic phase.

Results and discussion 1-Structural properties 1-1 X-Ray diffraction analysis results
The data of peaks positions and miller indices for the diffracted planes, FWHM and crystallite size are shown in Table 1. The interplaner spacing d is calculated using the relation d hkl = nλ/ 2sinѲ where λ is the X-ray wavelength (here λ = 1.54184 Å) and θ is the Bragg angle. The average crystallite size is calculated from Scherer formula D=0.9λ/(β cosθ) where β is the full-width at halfmaximum (FWHM) measured in radian.

1-2 SEM analysis results
Scanning electron microscopy (SEM) is a versatile technique for studying morphology of materials. The images of pure ZnS, as shown in Fig.2(a) prepared by co-precipitation route without adding the Graphene (Gr) which has nano size with nanoparticle structure where ZnS nanoparticles tend to aggregate without the dispersion of Graphene. Fig.2 (b) illustrates the images of Gr which have nanosheet shape. For the polymer polypyrrole (PPy), SEM images prove the formation of nanotubes structure as shown in Fig.2 (c) with diameter less than 100 nm. ZnS with Gr nano powder are shown in Fig.2 (d) and it is clear that ZnS nanoparticles are loaded on Graphene nanosheet.

Electrical properties 2.1 DC-measurement of ZnS/Gr
The dc-conductivity (σ d.c ) for pure ZnS and ZnS/Gr at different volume ratios of Gr is studied as a function of temperature with the range of (303-473) K as shown in Fig.3(a). Dcconductivity proves that ZnS and ZnS-Gr have semiconductor behavior because these samples have negative temperature coefficient. One can see that after PPy addition, activation energy decreases, this may be because PPy conductive polymer creates localized states inside band gap for the composites which is ZnS-Gr as seen in Fig.3(b).

3-Gas sensing measurement against NO 2 toxic gas
Thin films specimens are examined for gas sensing using NO 2 toxic gas at different operation temperatures. The sensitivity factor (S%) for n-type material and oxidizing gas is calculated by equation [14] S=|(R g -R a ) / R g |*100% where S is the sensitivity, R a and R g are the electrical resistance of the film in the air and electrical resistance of the film in the presence of gas, respectively.
Figs. 4-9 include the sensitivity of ZnS, Gr and ZnS-PPy/ZnS-Gr against NO 2 gas. Loading conductive polymer PPy to ZnS and to ZnS-Gr system that also make thin films being sensitive for input gas because ZnS nanoparticles which acts as catalyst will spread along the polypyrrole nanotubes also the surface area of PPy nanotube makes good area to adsorb input gas so sensitivity has been improved. Also conductive polymer such as PPy nanotubes represent bath to carriers to follow and reach electrodes and record electric current and the values of sensitivity and response and recovery times are shown in Table 3 and 4.   In general, sensitivity increases slightly with increasing operating temperature but for ZnS / 20% Gr / PPy thin film, the sensitivity increases significantly as shown in Fig.10. Results show that the sensitivity of the ratio ZnS-PPy decreases with increasing of the operating temperature. The reason for this may be that the surface would be unable to oxidize the gas so intensively and the NO 2 gas may burn before reaching the surface of the film at higher temperature. Thus, the gas sensitivity decreases with increasing temperature [15]. The ratio of ZnS-PPy/ZnS-20% Gr the sensitivity increases with increasing of the operating temperature. It is obvious in Figures an increase in the operating temperature leads to an improvement of the films sensitivity which is attributed to increase in the rate of surface reaction of the target gas because the presence of graphene nanosheet make the exposed area very large to react with gas molecules. Also the appearance of ZnS hexagonal phase may improve the sensitivity to gases.

Conclusions
The pure ZnS and ZnS/Gr have been prepared successfully by a novel method using chemical coprecipitation.
X-ray diffraction patterns proved that crystalline system for ZnS is changed from cubic zinc blende structure to hexagonal structure after graphene addition. For gas sensor