INTRODUCTION:

In the recent years, metals (Ag, Au, Zr etc. ) and metal oxides thick film materials²⁵ (TiO₂, ZnO, CaO and CuO) and also their nanocomposites¹⁸, have been extensively used in the various types of applications such as gas sensing^2,17,24, photocatalytic degradation^3,6,12, solar cells¹⁴, semiconductor²⁵, antimicrobial activity⁷, Antifungal activity²¹, electrochemical cell^2,4 and catalytic applications^8,15,19. Among of them the thick film of TiO₂ is very stable and has been used in various applications^3,7,11 and it is suitable for gas sensing². Nowadays there is general opinion in the scientific era urgent need of development of reliable sensor which is an electronic device gives input signals for the control of critical problems.

For the expansion of sensor interest has been rising for the study transduction principal reproduction of system and structure investigation of the most excited materials and preferable technology. From the recent past in the atmosphere the pollution is continuously increases and become a critical issue. There are many sources of chemicals and toxic gases which are responsible for the pollution like automobiles, chemical industries, aero industries etc. So, it is important to identify and observing those kinds of harmful gases and ions in water and try to reduce their presence in the atmosphere. There is need of gas observing in for the major categories. The observing of amount of oxygen in the atmosphere which is responsible to influence the combustion process. Oxygen fixation in the area of 20% in the former and 5% in the last should be monitored. For combustible gases like LPG noticeable all around so as to ensure against the undesirable event of fire or blast. For harmful gases noticeable all around where need of is to screen fixation around its introduction limits which extend from short of what one ppm to a few hundred ppm. Most of the understand kinds of gas sensor are solid state gadgets¹⁰. Which consolidate rugged development with adequately low buy cost to permit boundless organization. There are three types of solid-state gas sensor currently in use on a large scale. They are based on solid electrolyte, catalytic combustion, and resistance modulation of semiconducting device oxides⁶. Solid electrolyte sensor on the basis of electrochemical process²² or ionic conductance of a solid electrolyte matter in the existing of a gas^4,11. The basic capacity of the strong electrolyte is to isolate two regions of different action of the species to be checked and permit high versatility of particles of that species between the two regions²⁰. Catalytic combustion sensor consists of catalyst sensor materials¹⁶, in this process the combustible gas is exposed towards the catalyst sensor materials, in which it reacts and burns as a fuel gas, which influence the resistance and correlated with concentration of the combustible fuel gas⁸. Semiconducting devices utilizing metal oxides, it is bases of gas sensor can be identify several gases with the support of conductivity changes of their surface due to the absorption, and removal of gases, here it is based on the semiconducting metal oxide and of adsorption phenomenon^5,10,23. Small size of aggregated dye (Cocktail) molecules give high potential of adsorption towards the TiO₂ film¹. Ag, Au and Titanium Nanoparticles extensively used in pharmaceuticals fields²¹. The electrical property of the surface is changes in the presence of adsorption of unwanted ions on the surface of the semiconducting material^3,5. The technology has been used for to make a sensor Pallet, thick film and thin film¹³. The thick film has been giving remarkable results comparing with other technology due to the case of fabrication low cost and easy perform hybridization.

MATERIAL AND METHODS:

Chemicals and reagents⁹:

TiO₂Powder, (10 M and 15 M)⁹ NaOH, 1 M HCl, ethyl cellulose and butyl cellulose

Synthesis of TiO₂ nanocrystals:

Hydrothermal method was used to synthesized TiO₂ nanocrystals by using the precursor (TiO₂). At room temperature, the mixture of TiO₂ powder (1.6 g) and 10 M NaOH (64 ml) solution was stirring continuously up to 20 min. The mixture was heated at 180 ⁰C for 12 hrs. in the Teflon stainless autoclave instrument., After heating, the white crystal was cooled at room temperature and washed by using de-ionized water for lowering the concentration of sodium hydroxide (NaOH), then it was filtered and washed with 1 M HCl solution. After treating with HCl solution, the mixture was again filtered and washed by de-ionized water. This experimental procedure is also same for changing the molarities of NaOH such as 15 M and 20 M.

Fig. 1. flow chart of synthesis of TiO₂ nanocrystals

Preparation of thick film:

The thixotropic paste was formed using the fine powder of titanium dioxide (TiO₂) mixing with the little amount of ethyl cellulose in a mixture of organic solvents, for example, butyl cellulose and terpinol. The proportion of inorganic part of was kept at 75:25 in figuring the paste. The paste was screen printed on the glass substrate in the ideal pattern. The film was terminated at 500 ⁰C for 30 min. silver contact was made for the electrical estimations.

RESULTS AND DISCUSSION:

I-V characteristics:

Fig. 2 represent I-V characteristics of the titanium dioxide (TiO₂) thick film at 27 ⁰C. It was clear according to the I-V characteristics, the contact fabricated on the film were ohmic in nature and the voltage was applied 1 V – 30 V in the range.

Fig. 2. I - V characteristics titanium dioxide (TiO₂) thick film

Electrical Conductivity:

Fig. 3: Electrical conductivity Vs temperature

Fig.3. Indicates the changes of conductivity with the change in temperature for the samples for LPG. The graph shows the linear variation of conductivity with temperature. The conductivity particularly depends upon the exposure of the gas greater than air.

UV Visible spectra:

Fig. 4. UV Visible spectra for 10 M NaOH

Fig. 5. UV Visible Spectra for 15 M NaOH

Fig. 4 and 5 shows that the band width is increases as molarities of NaOH is increases from the graph of variation of absorbance with the wave length, we found that as the absorption increases the band gap is also increases which are one of the most reliable evidence for the preparation of Nano material.

Hydrothermal process which is carried out for

Molarity	10M	15M
Band gap	3.75 eV	3.79 eV

Surface Morphology of the film:

Fig. 6. SEM images of titanium dioxide (TiO₂) thick film 10 M and 15 M NaOH

Fig. 6 indicates the scanning electron microscope (SEM) image of an unchanged TiO₂ thick film fired at 500 ⁰C, the film consists of empty spaces and to the full extent range of particles with size ranging from 100 nm or less distributed non-uniformly.

Composition of Ti and O in TiO2 thick film:

Sample	% Weight of Ti	% Weight of O
TiO₂	92.51 %	7.49%

The energy dispersive X-ray spectroscopy analysis were examined the composition of the deposited materials and it was indicated that the maximum peaks are of Titanium (Ti) and Oxygen (O) and absence of the other impurity. Stoichiometric mass % of Ti is 59.93% and O is 40.07% in TiO2 are respectively. The mass % of Ti and O in our sample is 92.51% and 7.49%, not as per the stoichiometric proportion showing oxygen deficiency, indicating to the semiconducting nature of TiO₂.

Sensitivity of TiO₂ films to LGP with operating temperature:

Fig. 7. Effect of temperature on the sensitivity of TiO₂ thick films of LPG gas

For examination of LPG sensing properties of the titanium dioxide (TiO₂) thick film at various temperature were concentrated under perfect experimental conditions. The temperature is huge factor for gas sensing materials and in planning of sensors, materials may be the various temperatures to accomplish crystallization and structural development. An adequate level of crystallinity is required to accomplish the ideal electronic properties essential for gas sensor application

Selectivity of TiO₂ thick films for various gases:

Fig. 8. Selectivity of TiO₂ thick films towards several gases

The values of selectivity coefficients of LPG gas against the other gases. It was observed that the figure of the TiO₂ thick films give maximum sensitivity to LPG at 250 ⁰C for 10 M and to H₂S at 250 ⁰C for 15 M.

Gas Sensing mechanism for LPG (10 M):

Fig. 9. Gas sensing mechanism of LPG

Physical Adsorption:

It is due to Vander Waal’s force of attraction between molecules of oxygen and surface of solid.

Chemical Adsorption:

It is due to the oxygen adsorbed by exchange of electrons or ions and covalent or ionic bonds are formed.

O₂ ---à O₂^- (near about 200 ⁰C) --------à 2O^- (near about

400 ⁰C) --------à 2O^2- (> 400 ⁰C)

O₂ + 2e^- ---à 2O^-_ads

On contact with the gas (G) which is being sensed

G + O^-_ads -----à GO _des + e^-

Gas sensing mechanism as shown in figure 9, belongs to the surface-controlled sort depends on the influence of the electrical conductivity of the semiconducting material due to exposure to LPG gas. The change in the gas sensitivity with the change of grain size, surface state and oxygen adsorption. The surface zone generally provides more adsorption as well as desorption sites and gives high potential towards sensitivity. The LPG sensing system depends on the adjustment in conductance of TiO₂ thick film, which is constrained by LPG species and the measure of chemical adsorbed oxygen superficially. It is predictable that the atmospheric oxygen molecules are adsorbed towards the surface of semiconductor oxides such as oxo, superoxide and peroxide form.

Composition of LGP gas:

Composition

(Vol %)

CH₄

C₂H₆

C₃H₈

C₄H₈

C₄H₁₀

C₅H₁₂

11.5

4.5

Gas sensing mechanism for H₂S gas (15 M):

Fig. 10, shows that adsorption of oxygen species on the surface of TiO₂ abstracting the electrons thus producing higher in potential barrier at the grain’s boundaries. If reductant gas like H₂S comes in contact with grains of TiO₂ the potential barrier will be lower as a result of oxidative conversion of hydrogen Sulphide (H₂S) gas and desorption of oxygen (O₂). The reaction of H₂S with an adsorbed oxygen ion as,

2H₂S + 3O₂ ---------à 2H₂O + 2SO₂ + 6e^-

Fig. 10. Gas sensing mechanism of H₂S gas

CONCLUSION:

The nanometer TiO₂ thick film was synthesized by hydrothermal and screen-printing technology and the LPG sensing properties were investigated. It was characterized by UV and SEM. The band gap values are 3.7 eV, and 3.79 eV for 12 hrs. obtained from absorption spectra. The results of the LPG and H₂S sensing studies reveal that the TiO₂ thick film prepared by hydrothermal method is useful for the fabrication of the LPG and H₂S for 10 M and 15 M respectively. The sensor also shows good selectivity against H₂, Cl₂, CO, NH₃ and CO₂gases.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

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Received on 22.06.2020 Modified on 17.07.2020

Asian J. Research Chem. 2020; 13(5):360-364.

DOI: 10.5958/0974-4150.2020.00068.1