Metal excellent device performances due to their high

Metal oxides semiconductors have acquired a significant research interest due to its simple processing procedure and low cost and high potential for the optoelectronic and electronic applications 1–5. Among all the metal oxides, zinc oxide plays a vital role due to its abundant band gap energy, excellent chemical and thermal stability, large excitation energy and high sensitivity towards toxic gas sensors 6-8. Due to its extremely flexible properties, ZnO has been investigated extensively and employed in a variety of applications such as civil, medical, ceramic, rubber industry, electronic materials, glasses, paints, catalyst and pigment. Various forms of zinc oxide materials like heterostructures, single crystals, sintered pellets, thin films and thick films were synthesised and reported earlier 9. ZnO nanostructured materials have shown to be promising candidates for attaining excellent device performances due to their high surface to volume ratio and unique physical and chemical properties. So far, various methods have been employed to synthesise zinc oxide nanostructured thin films, such as RF magnetron sputtering, hydrothermal, chemical vapour deposition, molecular beam epitaxy, sol-gel, electrospinning and spray pyrolysis method 10-12. Among all these techniques, spray pyrolysis technique is widely used because of its simplicity, low cost as there is no need of vacuum system, multiple dopants may be added to the precursor solution, large area deposition, very good stoichiometry and uniform thin films.Detection of toxic volatile organic compounds in the environment has been a significant challenge in the contest of global warming and atmospheric pollution. The existence of VOCs in the atmosphere has been a severe impact on the health of human being and may lead to severe complications and even death. Among all the volatile organic compounds, benzene toluene and xylene (BTX) are colourless aromatic hydrocarbons, water-insoluble liquids and highly inflammable with complex structure. They may be carcinogenic, mutagenic and exhibit other adverse health issues 13. Therefore, the task to detect these gases as quickly and accurately as possible, especially in deficient concentrations, to alert people to the extent of in-door and out-door inhalation of noxious pollutants is of great importance. Many conventional technologies applied to detect volatile organic compounds have many shortcomings, viz. for gas chromatography-mass spectrometry, the disadvantages may be that the subsequent analytical procedures are time-consuming and very complicated, and the equipment is costly, complicated, bulky and also energy-consuming.  Currently, gas sensors are widely adopted to meet these requirements, because they facilitate detection of gas, possess very high sensitivity to certain target gases and obtain real-time detection results. Furthermore, the equipment is easily handled but is low-cost. So the appearance of gas sensors is destined to create novel avenues for the detection of BTX. In the present investigations, preparation of ZnO thin films at different substrate temperatures with optimised deposition parameters using spray pyrolysis method. Moreover, these deposited thin films are exposed to various concentrations of benzene, toluene and xylene to measure the sensitivity and selectivity of the sensor are reported. 2. Materials & methods2.1. Thin film depositionThe required amount of zinc acetate dihydrate (Zn(CH3COO)2.2H2O), (Sigma Aldrich) purity 99.99% is dissolved in 30ml of deionised water and stirred continuously for 30 minutes. To obtain clear solution two drops of acetic acid is added.  The glass substrates are cleaned for 15minutes in deionised water followed by acetone and ethanol using ultrasonicator. The precursor solution is loaded into the spray dispenser and sprayed using automated PC interfaced chemical spray pyrolysis technique at different substrate temperatures (275 °C – 425°C), and other optimised deposition parameters are shown in table 1. 2.2. Characterization techniquesStructural analysis of zinc oxide thin films was performed using an STOE X-ray diffractometer with Cu K? radiation in the glancing incidence configuration. The XRD patterns of the thin films were recorded in the 2? range of 20-80° under identical conditions with an incidence angle of 0.1°. The morphology of the thin films was examined by atomic force microscopy (AFM) (Solver, NT-MDT). The measurements were recorded in semi contact mode using a conical Si tip having a 10 nm radius of curvature with22 cone angle and 3:1 aspect ratio. Roughness prevailing in the films was calculated for 20×20 ?m scan area using an image analysis software regarding root mean square (RMS) value which is equal to 50% of the peak to valley height. The morphological features of thin films were examined using scanning electron microscope (Zeiss Evo18 special edition). Raman studies were analysed using an Argon laser excitation source emitting at 488 nm coupled to Labram – HR800 micro Raman spectrometer at room temperature in the backscattering geometry. The optical transmittance spectra were recorded by using a double beam Shimadzu UV-3100spectrometer in the wavelength range of 200–800nm at room temperature, with air as the reference medium with scan rate 50 nm/min.2.3. Gas sensing measurement setupTo investigate the changes in the sensing properties of zinc oxide thin films in the presence of a specific gas atmosphere, ohmic contacts were established on the thin film surface using highly conducting silver paste and copper wire.  The gas sensing setup consists of tetragonal metallic chamber and PC interfaced high resistance electrometer (Keithley 6517A, USA). Desired ppm quantity of benzene, toluene and xylene were injected into the chamber with the help of a chromatographic syringe. The resistance of the sensor was continuously measured while the specific ppm of test gas was injected into the chamber. 3.0 Results and discussion3.1. Structural studies3.1.1. X-Ray diffractionX-ray diffraction patterns of zinc oxide thin films confirmed that all the thin films were polycrystalline in nature with hexagonal wurtzite structure and all the planes are in good agreement with JCPDF file number 89-0510. The characteristic reflections corresponding to (100), (002), (101), (102), (110), (103),(112) and (103) as can be seen fig.1. The preferred orientation of crystal growth was changed from (100) to (002) reflection as the substrate temperature was increased from 275°C to 425°C. It is probably due to the heat treatment and the difference in the precursor chemistry 14.  To determine preferred orientation, texture coefficient was calculated 15 using the equation (1). Variation of texture coefficient for all reflections concerning the substrate temperature was shown in fig. 2. TC(hkl)=I(hkl)I0(hkl)1n1n(I(hkl)I0(hkl)-1———-(1)Where TC(hkl) is the texture coefficient of (hkl) plane, Where I(hkl) is the intensity of the XRD peak corresponding to the (hkl) planes, ‘n’ is the number of peaks, I0(hkl) denotes the standard intensity. TC(002) was considered as the preferred orientation in all the thin films. The intensity of (002) plane is increased with increasing substrate temperature. The average crystallite size was determined for (002) plane using Debye-Scherer’s formula is given in equation (2).Crystallite size(d) =0.94??cos?——(2)Where ? is the wavelength of X-rays (1.5406A° ), ? is the full width at half maximum (FWHM), and ? is the diffraction angle. Increase in crystallite size with increase in substrate temperature is due to the coalescence of the crystallite of the films.  The lattice constants ‘a’ and ‘c’ were calculated using the following equation 16.1d(hkl)2=43h2+k2+l2a2+l2c2 ………….(3)The calculated ‘c’ and ‘a’ values were well in agreement with standard values from the JCPDS card number 89-0510. Calculated lattices constants were tabulated in table2. Dislocation and strain were calculated from the following formulae (4).  Dislocation density (?), is defined the length of dislocation lines per unit volume of the crystal, it is found to decrease with increasing the substrate temperature. This may be due to the decrease in the internal micro-strain within the films and an increase in the grain size. The decrease in dislocation density indicates the formation of high-quality thin films. It may be due to the inhomogeneous strain component which is localized at the subgrain and subdomain level near grain boundaries 17.Strain (?) =   ?cos?4     ………………………(4) Dislocation density (?) = 1d2………(5)                   Fig.1. XRD patterns of ZnO thin films with different substrate temperatureTable.1. Deposition parameters of ZnO thin filmsParameterDescriptionPrecursorZinc acetate dihydrateSolventDi-ionized waterConcentration 0.1MCarrier gasairCarrier gas pressure0.8barSolution flow rate1ml/minDuration8 minSubstrateBlue star glassNozzle to substrate distance20 cmSubstrate temperature 275°C, 325°C, 375°C, 425°CTable 2. Structural properties of ZnO thin films deposited at various substrate temperatureS.NoSubstrate temperature(°C) Film thickness(nm)Lattice parameters a (nm)         c(nm)             c/aCrystallite size (nm)1234275325375425        940        550        500        3900.324             0.519           1.6010.324             0.521           1.6080.324             0.521           1.6080.323             0.519           1.60611.0712.8614.2018.28Fig.2. Texture coefficient of ZnO thin films deposited at different substrate temperatureFig.3. Variation of dislocation density and strain with substrate temperatureFig.4. SEM images of ZnO thin films deposited at(a)275°C (b) 325°C (c) 375°C (d) 425°C3.1.2. Scanning Electron Microscopy Fig. 4 depicts SEM micrographs of ZnO thin films deposited at different substrate temperatures. It is very clear that a significant morphology change is observed while the substrate temperature is increased. At lower substrate temperature the morphology of the thin films appeared like a wrinkled network whereas at a substrate temperature of 425°C changed to flower morphology with a characteristic dimension of 2µm.At this temperature an evident change in the grain morphologies can be observed. These changes in grain morphologies can be correlated with the changes of the growth rate control mechanism at that substrate temperature.  3.1.3. Raman spectroscopyFig.5. Shows the Raman spectra of ZnO thin films deposited at different substrate temperatures measured at room temperature. The hexagonal ZnO thin film has a wurtzite structure of space group of C6v. For all the samples, four prominent Raman modes are observed at 383, 395, 412 and 438cm-1. These are Raman actives modes in the Raman analysis of zinc oxide thin films. The phonon mode at 383cm-1 is corresponding to A1 (TO), 395 cm-1is corresponding to quasi A1(TO), 412 cm-1is due to E1(TO) and 438cm-1is related to E2(high) 18. E2(high)  is band characteristic of the wurtzite phase. Moreover, it is shifted to low frequency due to stress in the film at different substrate temperatures in agreement with x-ray diffraction studies.Fig.5. Raman spectra of ZnO thin films deposited at different substrate temperatures3.1.4. Topographical studiesAtomic force microscopy measurements were performed to investigate the effect of substrate temperature on surface morphology of zinc oxide thin films. 2 dimensional and three dimensional AFM images were taken by scanning over 20×20 µm2 surface area and have shown in fig.6. It is evident that the sample deposited at 425°C exhibits uniform grains in comparison to the other three samples. Therefore, it can be understood that the strain in the thin film deposited at 425°C can be attributed to the uniform distribution of the grains on the surface.  The root-mean-square average (RMS) as determined from atomic force microscopy measurements. It can be seen that the thin films deposited at various substrate temperatures show different surface roughness.  The surface roughness of all thin films had shown a distinct decrease when the substrate temperature increased from 275 to 425°C. It can be attributed due to the enhancement in adatom mobility for higher substrate temperature reduced the surface roughness. The reason is that the film deposited at higher substrate temperature has perfect crystals with bigger crystallite size119.  Calculated RMS roughness of the zinc oxide thin films has shown in table 3. Fig.6. 2D and 3D topography of ZnO thin films deposited at different substrate temperature (a) 275°C  (b) 325°C (c) 375°C (d) 425°C3.2. Optical studiesUV-Visible spectrophotometer records the optical properties of ZnO thin films deposited on glass substrate at various substrate temperatures (275-425°C). All the measurements are realised at room temperature.  The optical band gap is obtained from the transmittance data with the following formula 20.   ?h?=A (hv-Eg)n…………..(6) Where Eg is the energy band gap,  ‘hv’ is the energy of the incident photon, ‘A’ is a constant and ‘n’ is a constant. The value of ‘n’ depends upon the type of optical transition.The absorption coefficient (?) is determined from the relation ?=(1t) lnT                                                       Where, ‘T’ is the transmittance and ‘t’ is the thickness of the film.The variation of (?h?)2 with a photon energy of zinc oxide thin films deposited at different substrate temperatures have depicted in the fig.6. The optical band gap of zinc oxide thin films is decreased from 3.10eV to 2.96eV as substrate temperature increased from 275°C to 425°C. The optical band gaps estimated using Tauc’s plot were closer to the characteristic band gap of zinc oxide material. The decrease in the band gap values with substrate temperature also confirmed the presence of oxygen vacancies or more Zn2+ ions and the corresponding donor level placed below the conduction band 21. Variation of the optical band gap is tabulated in table 3.Fig.7. Tauc’s plot of zinc oxide thin films deposited deferent substrate temperaturesTable 3: variation of RMS roughness and optical band gap with substrate temperatureS.NoSubstrate temperature (oC)RMS roughness (nm)Optical Bandgap (eV)1234275325375425100.0697.1083.7420.803. Gas sensing propertiesZnO is an n-type semiconductor oxide with free electrons as the main charge carriers.  Its sensing mechanism could be explained through modulation of the depletion layer by oxygen adsorption. As shown in Fig 8. , when the zinc oxide sensors are exposed to ambient atmosphere, oxygen molecules in the ambient atmosphere is adsorbed on the surface of the zinc oxide and ionised to chemisorbed oxygen species (O2- , O- and O2-) by capturing free electrons from the conduction band of ZnO, increasing the electrical resistance of the sensors. As a consequence, electron depletion layer is formed on the surface area of ZnO and generates the barrier height at the boundary between crystallites. When the sensors are exposed to xylene, the target gas molecules can react with adsorbed oxygen species on the surface of ZnO. This process released the trapped electrons back to the conduction band and finally led to an increase of electron carrier density, which resulted in a decrease of resistances of the sensors. This reaction can be expressed as follows22.O2 +e-zno surface ?O-2(ads)C6H4CH3CH3 (g) ? (C6H4CH3CH3)ads (C6H4CH3CH3)ads + xO (ads) ? COx + H2O (g) + xeFig.8. Adsorption and reaction model of the sensing process on the ZnO sensor surfaceSelectivity is one of the major important concerns of the metal oxide based gas sensors. It is defined as the ratio of the response of the sensor towards the interfering gas to the response of the sensor towards the desired gas. Hence, the selectivity of the ZnO thin films was investigated by measuring the response towards xylene, benzene and toluene vapours.  The measured selectivity results are shown in Fig.9. It was found ZnO thin films have shown better response towards xylene than that of others. It is due to the low bond dissociation energy of xylene might have supported for enhanced response and selectivity 23. And also the sample which is deposited at a substrate temperature of 425°C is showing better response towards the interfacing gases due to its high crystallite size and microstructural properties of the thin films at that temperature.  Fig.9. Selectivity towards 75ppm of interfacing volatile organic compounds towards the ZnO thin films.The response of ZnO thin film which is deposited at an optimized substrate temperature of 425°C towards various concentrations of xylene ranging from 75 to 500ppm is shown in fig.10. The response(S) of ZnO thin film was calculated from the following equation                              Response (R) = RaRg   (Ra >>Rg)……………(7)Where Rathe resistance of the thin film is measured at ambient atmosphere and Rg is the resistance of the thin film measured in the presence of target gas. Fig.10. Response of ZnO thin film towards 75-500ppm of xyleneThe recovery and response times of the sensing element are calculated from the transient resistance response plot, which is defined as the time taken to attain 90% to the change in resistance from its baseline resistance respectively. The transient resistance response plot of the zinc oxide thin film is shown in Fig. 11. From this figure the response and recovery times are found to be 38 s and 44 s respectively.  Comparison between the xylene sensing performances of the developed ZnO sensors and literature is presented in table 4.Fig.11. Transient response of zinc oxide thin films exposed to 75ppm of xyleneTable 4: Response and recovery times of ZnO towards xyleneS.NoMaterialOperating temperature(°C)Concentration(ppm)ResponseTime (sec)RecoveryTime (sec)Ref123456ZnO nanoflowerZnO nanowireZnO nanoparticlesZnO porous nano solidZnO thin filmsZnO thin films20037037042542027200100300030001007592592       44       3838903283442425262627Present study4.0. ConclusionsThis study was able to show that good quality ZnO thin films can be deposited using low-cost method and material. Zinc acetate dihydrate precursor solution was sprayed on a glass substrate  at different substrate temperatures for 8minutes. The produced thin films were polycrystalline in nature with wurtzite structure. It was found that crystallite size was increased with increasing substrate temperature in the range of 275-425°C. The morphology of the thin films has changed from wrinkled network to flower morphology as substrate temperature increased.  The optical band gap is decreased with increasing substrate temperature. The film which was deposited at 425°C showed an excellent response value to xylene (75ppm) at room temperature with the response and recovery times 36 and 44s, respectively. Especially the sensor presents successful discrimination between benzene, toluene and   xylene. We believe that our sensor can be used in fabricating low cost and effective xylene sensor at room temperature in future.AcknowledgementThe authors express sincere gratitude to Ch. Gopal Reddy, Chairman, Dr. A. Raji Reddy, Director, CMR Technical Campus for their constant encouragement during the present work.  One of the author P. Nagaraju would like to thank DST-SERB, New Delhi for providing funds under early career research scheme (File number ECR/2016/000534) to carry out the present work.