Gas sensing performance of Sol-gel grown NiO-doped Cr 2 O 3 nanoparticles

The sensors based on Nickel oxide doped chromic oxide (NiO: Cr2O3) nanoparticals were fabricated using thick-film screen printing of sol-gel grown powders. The structural, morphological investigations were carried out using XRD, AFM, and FESEM. Furthermore, the gas responsivity were evaluated towards the NH3 and NO2 gas. The NiO0.10: Cr2O3 nanoparticles exhibited excellent response of 95 % at 100oC and better selectivity towards NH3 with low response and recovery time as compared to pure Cr2O3 and can stand as reliable sensor element for NH3 sensor related applications.


Introduction
Metal oxides have wide band gaps because of significant contribution of ionic character to the chemical bonds between the metallic cations and oxide ions.In general, metal oxides are not electrically conducting.However, current interest in material science is in unraveling the fundamental aspects of transparent conducting oxides (TCO) and their applications as semiconducting and conducting transparent thin films.A transparent conducting oxide is a wide band-gap semiconductor that has a relatively high concentration of free electrons in its conduction band.These arise either from defects in the material or from DOI: 10.20723/ijp.16.37.15-22 extrinsic dopants, the impurity levels of which lie near the conduction band edge.As implicit in the name, transparent conductors must be simultaneously transparent and conducting, an unusual combination.The physics behind TCO materials as to why they possess both high conductivity and high transparency is important in attempting to improve our understanding of them and to develop new TCO materials [1].Metal oxides as nanoparticles can exhibit unique chemical properties due to their limited size and high density of surface atoms [2].Among metal oxides, special attention has been made on the formation and properties of Cr 2 O 3 .For nanoparticles of Cr 2 O 3 , though toxic [3] it can be widely used in fields such as catalyst [4], coating, wear and corrosion resistance [5], advanced colorant [6], H 2 absorption material [7] and so on.It is significant to find an economical process which can be used to prepare them on a large scale.

B. Preparation of Films
The mixture of 1g nanoparticles for each (NiO, Cr 2 O 3 , and doped NiO (0.01 , 0.06 , 0.10) :Cr 2 O 3 ) in 10 ml acetone was stirred and dispersed by ultrasonic method to give a translucent solution.The homogenous mixture as sensing layer was then screen printed onto its surface and dried at 120°C for three hours using an oven.

Characterizations
Recently, nanostructured semiconducting materials were synthesized by different physical and chemical methods.This search represents the various characterization techniques utilized in the present work and it also includes the basic principles of the characterization techniques in X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), field emission scanning electron microscopy (FE-SEM).

Results and discussion 1. XRD
The phase identification and structural changes were investigated with the help of X-ray diffraction (XRD) technique.Fig. 1 shows the typical XRD patterns recorded 2θ angle range 20-70 o for the all synthesized samples.The X-ray analysis of the prepared the thin films was studied according to the prepared method of NiO doped Cr 2 O 3 , when concentration of NiO (0.01, 0.06, and 0.10) was prepared at room temperature.Fig. 1  ( where D is the crystallite size, λ is wavelength of radiation used, β is the full width at the half maximum peak at diffraction angle 2θ.

AFM
The AFM image of undoped Cr 2 O 3 and doped NiO (0.01, 0.06, and 0.10) nanoparticles in two and three dimensions respectively, is shown in Fig. 2. The results of AFM image for the previous doped and undoped synthesized Cr 2 O 3 and NiO nanoparticles showed that the diameter of the particles was average of 59.3 nm and 48 nm receptively.The doped diameter of the particles was average of 74-48 nm respectively.

FESEM
The FESEM micrographs of undoped

Gas sensing properties
Thin films of Cr 2 O 3 and NiO doped Cr 2 O 3 nanoparticals were tested to various gases such as: NO 2 , NH 3 , at operating temperatures ranging from 35 o C to 300 o C. Fig. 4 shown the relationship between operating temperature and gas response of undoped and NiO doped Cr 2 O 3 nanoparticals sensors.The sensor response described in this paper was estimated with the following formula [9]: S= (R gas -R air /R air ) ×100% (2) where, R air is the resistance in air and R gas is the resistance in presence of test gas at a given temperature.For each sample, the response towards NH 3 increased with operating temperature, reaching its maximum and then decreased rapidly with the increase in operating temperature.Also, it is evident that the response was influenced by Ni addition.This behavior is mainly due to the influence of operating temperature on the amount of absorbed oxygen species on the surface of Cr 2 O 3 film [10].At low temperature, the amount of absorbed oxygen species is low so the sensor response is consequently small while at very high temperature, the progressive desorption of the previously adsorbed oxygen species occurs and, hence, the sensor response decreases.The NiO 0.10 sample showed maximum response of 95 % toward NH 3 gas at moderate operating temperature of 100ºC which was the highest among all the other samples Cr 2 O 3 (22 % at 100 °C), NiO 0.01 (58.4 % at 100 °C), and NiO 0.06 (61 % at 100 °C) respectively.Smaller crystallite size of NiO 0.10 sample provides larger specific surface area and higher surface activity for oxygen adsorption.The fast reaction of adsorbed oxygen with ethanol gives a large change in the electrical conductivity of the sensor and eventually a higher sensor response.When the Ni concentration was less than the optimum value the distribution was more discrete and this amount may not be sufficient to promote the reaction effectively whereas for excess Ni there was almost agglomeration of these particles which hinders the reactions.At the optimum concentration (0.10) there was an uniform distribution of these particles as a result of which not only the initial resistance of the sensor is high but this amount can promote the reaction most effectively leading to enhanced response.The fast reaction of adsorbed oxygen with NH 3 gives a large change in the electrical conductivity of the sensor and eventually a higher sensor response.
displays X-ray diffraction patterns of the as-prepared Cr 2 O 3 and NiO doped Cr 2 O 3 samples.The XRD spectra of NiO doped Cr 2 O 3 consist of (012) (104), (110), (113), (202), (024), and (116) peaks, and all the observed diffraction peaks can be indexed to Cr 2 O 3 rhombohedral structure.The strong (110) peak proves that Cr 2 O 3 with rhombohedral structure were obtained in both undoped and NiO doped Cr 2 O 3 samples.No diffraction peaks of other structures were detected in these samples, indicating that the NiO ion successfully occupied Cr 2 O 3 lattice site and there were no secondary phases or precipitates in the samples.The crystallites sizes of the Cr 2 O 3 and NiO doped Cr 2 O 3 are estimated using Debye-Scherrer equation.
Cr 2 O 3 and NiO (0.01, 0.06, and 0.10) doped Cr 2 O 3 thin films prepared by print screen at room temperature are shown in Fig.3 a, b,c and d respectively.It can be seen that the pure Cr 2 O 3 nanoparticles were nearly uniform spherical shapes and very small particles in evidently dispersed without large agglomerates.Excess addition of NiO into the Cr 2 O 3 caused the agglomeration of grains due to the grain growth events.The average diameter of particle 30-60 nm.

Fig. 4 :
Fig.4: The gas response of NH 3 as a function of operating temperature for undoped and NiO doped Cr 2 O 3 nanoparticles.

Fig. 5 :Cr2O3Fig. 6 :Fig. 7 :
Fig. 5: The gas response of NO 2 as a function of operating temperature for undoped and NiO doped Cr 2 O 3 nanoparticles.The transient response characteristics of all the samples toward the gases NO 2 and NH 3 are shown in Fig.6.These measurements were performed by injecting NO 2 and NH 3 gases into the chamber first and sensors resistance was measured in air and in the presence of NO 2 and NH 3 .All the samples respond rapidly as soon as NO 2 and NH 3 gases were injected into the chamber.Fig. 6 shown the response and recovery times for pure Cr 2 O 3 and NiO doped Cr 2 O 3 nanoparticles samples.It is seen that the Ni 0.10 sensor exhibits

procedure 1. Synthesis of pure Cr 2 O 3 and NiO doped Cr 2 O 3 nanoparticles sensors.
The mixture was heated to 80 °C to form a homogeneous sol solution.The obtained sol was slowly heated to evaporate the solvent and it forms a hard homogeneous gel.The Pyrolysis of the final gel was performed at a temperature of 400 °C for 4 hours.During the pyrolysis process the PVA polymeric network through the outer surface, nickel and chromic nitrate salts simultaneously calcinated and converted into NiO doped Cr 2 O 3 nanoparticles.The obtained samples were crushed to prepare a fine powder.