Cr2O3:TiO2 Nanostructure Thin Film Prepared by Pulsed Laser Deposition Technique as NO2 Gas Sensor

Pulsed laser deposition (PLD) technique was applied to prepared Chromium oxide (Cr2O3) nanostructure doped with Titanium oxide (TiO2) thin films at different concentration ratios 3,5,7 and 9 wt % of TiO2. The effect of TiO2 dopant on the average size of crystallite of the synthesized nanostructures was examined by X-ray diffraction. The morphological properties were discussed using atomic force microscopy(AFM). Observed optical band gap value ranged from 2.68 eV to 2.55 eV by ultraviolet visible(UV-Vis.) absorption spectroscopy with longer wave length shifted in comparison with that of the bulk Cr2O3 ~3eV. This indicated that the synthesized samples are attributed to the enhancement of the quantum confinement effect. Gas response sensitivity, and recovery times of the sensor in the presence of NO2 gas were studied and discussed. In this work it is found that, the sensitivity increases when doping ratio increases from 3wt% to 5wt% of TiO2 and return to decrease over that. The optimum concentrations ratio for NO2 gas sensitivity is 5wt% of TiO2 and sensitivity is 168.75% at 200 o C.


Introduction:
Semiconductor materials oxide have a great deal of attention in recent years, because of their unique optical, electrical, magnetic and chemical properties (1). Among various semiconductor materials, oxide Cr 2 O 3 is one of the successful semiconductor metals oxide and broadly studied compounds due to its wide band gap ∼3eV (2). This kind of p-type broad optical band gap semiconductors metal oxide may be a decent candidate for many applications in gas sensors, optical storage system (3), coating materials for thermal protection, wear resistant materials (4).The above applications of Cr 2 O 3 material depends not only on their addition of suitable dopants but also on their structure, phase, shape, size, and synthesizing techniques. As a result, the synthesis of nanomaterial with large surface area to volume and high chemical activities has been the substance of active research (5). Pulse laser deposition has been used for the growth of many nanomaterial oxides, which could be useful with gas sensors.
The sensing properties of this metal oxide has been proved for some flammable or deadly gases such as NO 2, . Advances in nanotechnology provide increasing in the response of semiconductor metals oxide; generally because of the increasing of the surface area exposed to gas. Thin films of nanostructure provide high sensitivity and also faster response times (6). The average crystallite size D was estimated from the Debye-Scherrer's equation (7). D = Kλ / (β cosθ )…………….1 K = 0.9 Scherer's constant, λ = 1.54056 Å, X-ray radiation's wavelength, β represent FWHM, peak full width at half maximum in radians and θ is the Bragg diffraction angle at which FWHM measure. The optical band gap energy Eg of the assynthesized nanoparticles is obtained from the UV-Vis spectra by using a well-known Tauc's relation (8). αhʋ = A(hʋ -E g ) n ……………. 2 where, (α) represents the absorption coefficient, A is constant and exponent n = ½ for direct transition (8).
Sensing measurements is carried out by measuring the R a resistance of thin film in air, R g resistance in the presence NO 2 gas and evaluate the gas sensitivity S% for reducing gas from bellow equation (9). S% =( R a) /R g) )%……………3 In this work, the results of a organized study of Cr 2 O 3 doped with TiO 2 are extant to find probability, optimal conditions for pulsed laser deposition PLD of Nano-crystalline films on glass substrate. The crystallite size, morphology, and optical properties of the as-synthesized Cr 2 O 3 :TiO 2 nanoparticles are investigated and discussed. The thin films prepared are studies for sensing properties to NO 2 gas.

Materials and Methods:
Powder of Cr 2 O 3 was mixed with different doping concentrations ratio (1-x) when x=3,5,7,9 %wt of TiO 2 with high purity 99.99% pressing under 6 to 8Ton to form a target with 2cm diameter and 0.5cm thickness. The films were deposited by Nd:YAG laser, Second Harmonic Generation SHG. The laser beam was focused on the target inside an UHV chamber in 45 o angle. Pulse laser depositions energy was 700mJ frequency of laser pulse was 6Hz and the space between the target and the substrate about ~2cm. The glass substrates were cleaned by ethanol and thereafter rinsed with distilled water in an ultrasonic. The crystalline structures of the assynthesized nanoparticles were characterized through XRD X-ray diffraction diffractrometer model D2 PHASER BRUKER made by Germany powder diffraction system with Cu Kα-radiation λ=1.54056Å,Voltage=40 kV, Current = 30mA and the scanning 2θ range from 20 o to 80º. Morphological images of thin film surfaces were measured using Atomic Force Microscope AFM, (SPM, Model AA3000, Inc,), made by Angstrom Advanced which can provide information about average diameter root mean roughness RMS and average roughness. The wavelength range of optical absorbance spectrum300nm-1100nm was noted at room temperature by spectrophotometer (SHIMADZU UV-1800, Japan). The aim of this work is to prepare a sensor of Cr 2 O 3 :TiO 2 thin film which can detect hazards gas such us NO 2 gas.

Results and Discussions: Structural and Morphological Properties
The X-ray diffraction patterns of the assynthesized nanoparticles are shown in Fig.1. The characteristic peaks observed in the spectrum of films are good crystalline nature, and the rhombohedral lattice may be referred to hexagonal axes (10). It is observed that the crystalline size in planes 012, 104 and 110 increases with the increasing concentration from 3 to 7wt% and decreased after that. The intensities of the diffraction peak in pattern were higher at 3wt% concentration in plane 012 and 2θ=24.41 indicating that the addition of TiO 2 dopant as well as the increasing concentration of dopant in Cr 2 O 3 matrix slightly increases the crystal size of the thin films, the broadening of the dominant peaks also slightly increased. The decreasing peak intensity of Cr 2 O 3 nanoparticle as an effect of TiO 2 doping may be due to the fact that a restrained quantity of TiO 2 atoms exist as interstitials sharing the oxygen with Cr atoms and hence increase the crystallinity size, this agreement with Mohanapandian (11).
The behavior was observed when increasing concentration from 3 to 9 wt% the area under the curve decreased also peak intensity. New peaks for a composite were observed which returned to the doping metal Titanium dioxide in the XRD image only with 9wt% concentration ratio, in phase Brookite at plane 210, 211 as show in Fig.1. Three-dimensional AFM figures and the chart of grain density distribution for Cr 2 O 3 :TiO 2 at different doping ratio 3, 5, 7, 9wt% of TiO 2 deposition on glass substrate with lengths of 2.5×2.5 cm 2 and thickness of film about 200nm are observed in Fig.2. AFM images were taken in order to determine the average diameter, root mean square roughness RMS and average roughness. The images of AFM displayed all specimen were granular structure, the granular films show greater surface area, which is decent for thin film gas interaction and results in upper sensitivity, where the sensitivity of gas has a proportional relationship with the thin film roughness, it is in agreement with Deshpande (12). The grain size increased with the increasing of concentration ratio from 3 to 9wt% of TiO 2 as shown in Table.1. Maximum RMS roughness and average roughness are 9.93nm and 7.53nm respectively at 5wt% doping ratio and 9.33nm and 7.94nm at 7wt% doping ratio. Thin films roughness increases due to the presence of several hillocks, which are faceted and distributed randomly on the relatively smooth structure densification of the deposition processes (13).

Optical Properties
The UV-Vis absorption coefficient spectra of different concentration of TiO 2 doped Cr 2 O 3 nanoparticles, it can show that the absorption coefficient increase with increasing concentrations ratio of TiO 2 and higher absorption coefficient occur at wavelength 360 nm as shown in Fig.3. It is clearly observed that the absorption peaks showed a longer wavelength shift as the concentration of dopant TiO 2 in the host Cr 2 O 3 matrices increase from 3 to 9 wt% and the absorption coefficient decrease with increase wavelength it is agreement with Abdullah M.M (14).  The curves are plotted between (αhυ) 2 versus (hυ) and extrapolating of the linear portions of the curves to the hυ axis gives E g , which is shown in Fig.5.

Figure 5. Optical energy gap for Cr 2 O 3 thin films at different ratio TiO 2
The estimated optical band gaps(E g ), absorption coefficient(α) and transmissions(T) are given in Table. 2. The E g gradually decreased from 2.68 eV at 3wt% to 2.55eV at 9wt% of TiO 2 . An obvious strong longer wavelength shift was observed for higher 3,5,7and 9wt% of Cr 2 O 3 :TiO 2 films when compared with the bulk system ~3eV (15). The variation of E g and crystallite size with respect to various concentrations ratio of TiO 2 in Cr 2 O 3 system. The observed E g value of ratio doping 3wt% TiO 2 with Cr 2 O 3 nanoparticle ~2.68eV is in agreement with the reported values and marginally less than that of the sample prepared by thin film techniques (16,17).

Properties of Gas Sensors
Figures from 6,7,8,9 shown the decrease and increase of resistance for Cr 2 O 3 :TiO 2 thin film with time after gas on and gas off respectively. This behavior can be explained as the target gas interaction with the surface of the semiconductor metal oxide thin film, normally when surface adsorbed oxygen ions, the results is change in charge carriers concentration of the metal oxide. This change in charge carries concentration assists to change the resistivity or conductivity of the metal oxide. The majority charge carriers of Ptype semiconductor are holes and upon interaction with a oxidizing gas such as NO 2 will reduce in resistivity or increase in conductivity occurs. A resistance decrease with a oxidizing gas is showed, where the negative charge introduction in with metal oxide increases the positive hole charge carriers concentration (18,19).The sensitivity (S%) was estimated from eq 3.  Table.3 and Fig.10 show that the sensitivity increases with the increase of concentration ratio of TiO 2 in Cr 2 O 3 from 3 to 5wt%. The maximum sensitivity was found 168.75.% and 133.33% at 5wt% concentration at 200 o C and 250 o C it is agreement with Fine (20). The other gas sensors properties shown in Table 3and Fig.11, which display the variation of response time of Cr 2 O 3 :TiO 2 film with different ratio of TiO 2 and different temperature at 700mJ laser energy, the response time decrease with increasing concentration ratio of TiO 2 in Cr 2 O 3 from 3 to 5wt%.  Conclusions: /This work, studies the characterization of structural, morphological and optical for thin films of Cr 2 O 3 doped with TiO 2 nanoparticles manufactured by simple cost effective pulse laser deposition technique. Also it studies the gas sensor properties by calculated the sensitivity, response and recover time. Concerning the structural properties, a systematic increase in the crystallite size, and a decrease in the FWHM parameter were shown with increasing the concentrations ratio of TiO 2 in the prepared nanoparticles. The as-prepared nanoparticles are high purity, composition and produced with minimal agglomeration. The crystallite size calculated from XRD data shows good agreement with those particle size achieved from FAM. The optical properties of the current study confirm the role of TiO 2 in improving the optical transparency in the visible region and observed that the optical band gap was decreasing when increased doping ratio of TiO 2 . From gas sensor measurement, it is found that the sensitivity increased with increasing the concentrations ratio from 3 to 5wt% of TiO 2 and the sensor showed very rapid response to NO 2 gas.