Design and Fabrication of Nanostructure TiO2 Doped NiO as A Gas Sensor for NO2 Detection

Abstract In this paper, thin films of undoped and nickel oxide (NiO) doped titanium dioxide (TiO2) were prepared using the chemical spray pyrolysis deposition (CSP) technique, with different concentrations of nickel oxide (NiO) in the range (3-9) wt%. The morphological, structural, electrical, and sensing properties of a gas of the prepared thin films were examined. XRD measurements showed that TiO2 films have a polycrystalline structure. AFM analysis showed that these films have a regular structure both before and after doping . The roughness of these films decreased after adding impurities but then the opposite of that took place. The electrical and gas sensing properties of titanium dioxide was also affected after doping. The highest sensitivity value was obtained at doping concentration of 5wt% and working temperature 473oK.


Introduction
Due to their low cost, ease of manufacture and use, flexibility in detecting a wide variety of toxic / flammable gases, and durability in harsh environments, semiconducting metal ISSN: 0067-2904

Al-Taa'y and Hasan
Iraqi Journal of Science, 2021, Vol.62, No.11(Special Issue), pp: 4385-4396 4386 oxides (SMO) have been applied to solid state gas sensors. These sensors depend on reversible changes in electrical conductivity of gas molecules to be adsorbed/ desorbed on their surfaces [1]. Titanium dioxide (TiO 2 ) is one of the most common semiconductor metal oxides for the production of conductometric gas sensors, due to its non-toxic nature, chemical stability, and commercial availability at low cost, robust, and general reactivity [2,3]. TiO 2 is a semiconductor that belongs to the group of transparent oxidized semiconductors and has high transparency in the visible region, high absorption in ultraviolet radiation, and high conductivity [4]. Doping opens up the possibility of modifying the electronic structure, chemical composition, and optical properties of TiO 2 nanoparticles. A great deal of work has been made to introduce metal ion doping, such as nickel, chromium, iron, vanadium, and zinc [5]. Many efforts have been made within this area, and it was presented by Nan Zhang et al., WO 3 -TiO 2 heterocyclic nanofibers (HNFs) were prepared using a simple two-step process. The mechanism that appeared in the gas stock characteristics was discussed [6]. Sheini et al. used Titanium dioxide TiO 2 to produce ceramic bodies sensitive to oxygen. Silver (Ag)-doped TiO 2 was prepared to obtained a gas sensor [7]. Eadi et al., (2017) prepared Iron (Fe) doped nanoparticles of TiO 2 for gas sensor application and photocatalytic degradation [5]. In this work, structural, morphological and electrical properties for TiO 2 pure and doped with NiO were measured and determined to prepare a gas sensor device for NO 2 gas sensitivity measurement.

Experiment
In this work, a undoped and doped TiO 2 were prepared with different concentrations (3-9wt%) of inorganic material NiO. All TiO 2 films were prepared with an aqueous solution of titanium (III) chloride (TiCl 3 ) and NiCl from Merck KGaA, Germany using chemical spray pyrolysis (CSP) technology. These films were fabricated with molar (0.1M) for all concentrations.
To prepare nanostructure thin films by the chemical spray pyrolysis, many parameters such as the nozzle-substrate distance, flow rate, solution deposition time, concentration and deposition temperature of the films were considered used for good. The nozzle was used to spray onto a glass substrate heated at 300°C. These thin films were annealed at 600°C for 1 hour. The thickness of the prepared thin films was measured using optical technique which was equal to 275nm. For films, structural, morphological, electrical properties and measurements of gas sensing were studied. Sensing measurements of all films were performed on n-type silicon wafers (111) substrate and sensitivity measurement was done toward NO2 as oxidizing gas.

Results and discussion
Structural properties of TiO 2 films before and after adding NiO were performed by the XRD diffraction studding. The structure of as deposited thin films of TiO 2 , when checked by x-ray diffraction was amorphous, thus all as deposited TiO 2 thin films, undoped and doped were annealed at 600 o C to transform the structure to polycrystalline. Figure 1 and Table 1 illustrates the XRD analysis of these materials, as it is shown pure TiO 2 has many diffraction peaks and a polycrystalline structure. This result is consistent with [8][9][10][11]. Crystal structure phase is found to be the anatase phase, this agrees with (ASTM) card no.
[96-900-9087]. (101) orientation was along the plane at diffraction angle of (2θ = 25.2119°) and (d = 3.5169A°) according to card no. [96-900-9087]. After adding different concentrations (3, 5, 7, and 9)wt% of NiO, the peaks intensity decreased. The structure was converted from polycrystalline to amorphous at 7wt% and the peaks intensity started to increase again at 9wt% percentage. There was an increase in full width of half maximum (FWHM) to more peaks but was happened a decreased crystallite size with increased doping percentage.  The chart of surface morphology at three dimensions for undoped and doped (with different concentrations) TiO 2 are shown in AFM images as declared in Figure 2. undoped and doped films displayed a granular structure through images of AFM. The roughness of films decreased after doping with NiO but increases at 9wt% concentration of NiO ( see Table 2). Thin films with high roughness are ideal for interaction between film and gas in gas sensors and results in higher sensitivity [12]. The average diameter shows significant reduction as NiO is introduced to the host material but then the average diameter gets to grow in a nonregular manner with the increase of nickel oxide concentration.  Hall effect measurements of the undoped and NiO doped TiO 2 thin films were performed with the van der Pauw method at room temperature. Electrical properties such as free carrier concentrations (n H ), mobility (µ H ), Hall coefficient (R H ) and conductivity (σ RT ) of all anatase phase thin films nanoparticles with different concentrations are exhibited in a Table 3. In this work, carrier concentrations of TiO2 films e decreased after doping with NiO but increased at 9wt% concentration of NiO. At low carrier concentrations, the grain may be seriously depleted by the free carriers due to the presence of trap states at the grain boundaries. So, the depletion region becomes very thin located at the grain boundary when the carrier concentration is sufficiently high [13]. From Table 3, it is observed that the conductivity (σR.T) is high or low according to the carrier concentration i.e. low carrier concentration results in low conductivity and visa versa.. Hall measurements resulted in negative hall coefficient for all films indicating n-type charge carriers. The gas sensing properties have been defined as a function of operating temperature in terms of sensitivity response, response time and recovery time. Sensing properties of nanoparticles TiO 2 undoped and doped with different concentrations of NiO were studied by exposure to NiO 2 gas. This was performed as a function of time and temperature(starting at room temperature up to 473°K), as shown in Figures 3 to 7. Figure 3 shows the sensitivity changes of pure TiO 2 thin films when exposed to NO 2 gas. Table 4 shows the sensitivity of the thin films (undoped and doped) at different working temperatures upon exposure to NO 2 gas.The highest sensitivity for the undoped TiO2 was at 423ºK. While the highest sensitivity for the doped TiO2 was for 5wt% doping concentration at 473ºK working temperature All the undoped and doped gas sensor cells acted as n-type semiconductor since the resistance get toincrease under exposure to NO 2 oxidizing gas except for TiO 2 :7%NiO/c-Si. However, gas sensor cells at working temperatures 423ºK and 473ºK acted as a p-type semiconductor since the resistance was reduced when exposed to NO 2 gas. This result is in agreement with that of Yüce and Saruhan [14].

Al-Taa'y and Hasan
Iraqi Journal of Science, 2021, Vol.62, No.11(Special Issue)     The response time values as a function of temperature of pure and doped samples increased but in a non-regular manner with the rising of working temperature except for theTiO 2 : 5%NiO/c-Si sample i.e. the response time was reduced.( as shown in Table 5). The recovery time of (pure, 3 and 9)wt.% increased by rising of working temperature while the recovery time gets to reduce by rising of working temperature for residual gas sensors cells i.e.TiO 2 :5% and 7% NiO/c-Si.

Conclusions
Polycrystalline structure of TiO 2 , undoped and doped with NiO up to 9wt%, was obtained using chemical spray pyrolysis technique subjected to annealing at 600 o C for one hour. Further increase of doping ratio leads to amorphous thin films. The degree of crystallinity as well as crystal size were growing with a further increase of doping ratio. The grain size as measured by from AFM affirms the same results. The charge carriers concentration as well as the conductivity was reduced by the addition of NiO to the host oxide in the first stage and then increased. The mobility exhibit to change in opposite to that. Maximum sensitivity obtained needs relative high adsorption sites which are provided from moderate crystal size and good surface roughness.