Preparation, characterization of Nb-TiO2 and its application in color degradation study

Introduction

Medicinal plants have greatly gained importance in the management and treatment of human Titanium dioxide, known as titania, is a naturally occurring oxide of titanium. Its chemical formula is TiO 2 and molecular mass is 79.88. It occurs in three mineral forms namely rutile, anatase and brookite. TiO 2 is produced either in anatase or in rutile form. Most of the anatase is produced in white powder where as some of the rutile crystals are off white in color. Out of the forms of anatase and rutile, rutile is more stable and anatase to rutile transformation takes place at a temperature higher than 600 0 . TiO 2 is most widely used in white pigment products such as paints, coatings, plastics etc. because of its brightness and high refractive index i.e. >2.4 which determines the opacity. Titania absorbs light only in ultra violet region. But to make it more effective and usable in normal solar light doping in TiO 2 required. The selection of doping element depends on the oxygen affinity of that element. There can be many elements which can be used as dopant. In this particular project work, Nb is used as doping element. It introduces a new level of electronic state into the band structure of the parent material. Nb could form a solid solution in the crystal structure of TiO 2 due to the fact the size of Nb 5+ (0.64A 0 ) is similar to that of Ti 4+ (0.605A 0 ), thus the particle size in the doping samples were not affected by Nb atoms. Nb 4d orbital is strongly hybridized to TiO 2 3d orbital to form a d nature conduction band without impurity state in the in gap region resulting in high carrier density exceeding 10 -21 cm -3 and excellent optical transparency in visible region. Nb doping in TiO 2 enhances photo catalytic activity. In 2000, D. Morris et al [1] studied the photoemission and STM of of the electronic structure of Nb-TiO 2 . High resolution core and valence level photoemission spectra of Nb-TiO 2 ceramics have been measured. Nb doping produces a well-defined photoemission peak in the bulk band gap of rutile, whose intensity increases with increase in doping level. Core level spectroscopy and resonant photoemission demonstrate that at low doping level the extra electrons associated with Nb doping are localized mainly on Ti rather than Nb. So, the highly doped material is better formulated as Ti(IV) 1-2x Ti(III) x Nb(V) x O 2 . A comparison of present result with V doped TiO 2 revels a simple quantitative difference between the two systems, in V doped TiO 2 material electrons are trapped on the dopant atom rather than on host cation sites. In 2008, Taro Hitosugi [2] studied the properties of TiO 2 based transparent conducting oxides, which exhibit transparent conducting properties comparable to those of indium tin oxide (ITO), the most widely used TCO. Epitaxial thin film growth of anatase Ti 1-x Nb x O 2 (TNO) with 0.03 < x <0.06 exhibit a resistivity of 2-3*10 -4 Ωcm and internal transmittance of ~95% in the visible light region. Furthermore, polycrystalline films deposited by the pulsed laser deposition and sputtering methods showed a resistivity of 4.6*10 -4 and 6*10 -4 Ωcm, respectively, at room temperature. It has also been highlighted that characteristics that are unique to TNO which distinguish it from ITO such as high refractive index, high transmittance in infrared and high stability in reducing atmosphere. Yujing Liu [3] studied Nb-TiO2 in which crystalline Nb-TiO 2 nanoparticles were synthesized via solvothermal procedures using tertiary-butyl alcohol as a novel reaction medium and their assembly into mesoporous films was investigated. The solvothermal procedure enables the preparation of crystalline doped and undoped non agglomerated titania nano particles whose size can be controlled from 4-15 nm by changing the reaction temperature and time. Substitution doping with niobium ion drastically increases the electrical conductivity of the titania particles. In contrast to the titania particles prepared by solvothermal synthesis in benzyl alcohol, those of similar size and crystallinity synthesized in tertiary-butyl alcohol can easily be dispersed at high concentration. The Nb doped titania nanoparticles can be assembled into regular three dimensional mesoporous structures with a narrow pore size distribution and high surface area. In 2012, Chandrakant Dighavkar et al [4] studied the gas pollutant detection by using doped titania thick films. Thick film of pure titania and various concentration (1 wt%, 3 wt%, 5 wt%, 7 wt% and 10 wt%) of Cr 2 O 3 and Nb were prepared on the alumina substrates using a screen printing technique. The doped films were fired at a temperature of 800 o C for 2h in an air atmosphere. Various oxidising and reducing gases were tested for these metal oxide doped films. Among these NH 3 and H 2 S gas sensing properties for these thick films were investigated for Cr 2 O 3 and Nb doped TiO 2 respectively at different operating temperatures. Pure TiO 2 was found to be insensitive to these gases. At 250 o C, 5 wt% Cr 2 O 3 doped films have shown significant sensitivity (88.23%) to NH 3 gas whereas 1 wt% Nb doped films have shown highest sensitivity (98.78%) to H 2 S gas at 200 °C [5] Flame spray pyrolysis has been successfully used for preparation of undoped and 1-5% Nb-TiO 2 , nano powders for application to acetone and ethanol gas sensing. The particle size of the doped TiO 2 and Nb doped TiO 2 were in the range of 10-20 nm. In this journal the sensing properties for acetone was studied at operating temperatures of 300-400 °C and for ethanol the operating temperature was 250-400 °C. It was observed that out of all samples 3 at% Nb was dispersed on TiO 2 sensing films. And this particular doping concentration showed a good response in both acetone and ethanol and it was observed that the best response occurred at a temperature of 350 °C and 400 °C. There are many work has been done to study the properties of TiO 2 and doped TiO 2 which we can read from the references [5][6][7][8][9][10] .

Experimental Method Sample preparation

Powders such as Titanium dioxide and niobium pentoxide were used are of analytical grade. Nb-TiO 2 in different mol% of Nb in it were prepared by solid state method. A typical preparation method is given below. Nb doped in TiO 2 in different mole concentration is calculated as below:

Where, x is the mole concentration of Nb, η is the ratio of weight of substance(W) to molecular mass(M) of the substance(W/M).

Example 1

Preparation of 1mol% of Nb 2 O 5 -TiO 2 considering the total mass of the sample to be 20 g. These powders (TiO 2 and Nb 2 O 5 ) are mixed in a mortar pestle and to it few drops of acetone was added and this mixture was mixed thoroughly for 3-4 hours. After that the sample was placed in air tight polythene. Similarly, the other samples with concentration 0.01%, 0.03%, 0.06%, 0.5%, 1%, 2%, 4%, 10%, 30% were prepared using the same formula and same procedure.

Pellet making: From the TG/DSC data the temperature at which it has to be heated was fixed. Considering the same data pellets were prepared (using the procedure given below) from the powder sample for calcinations study. To the powder samples 2% PVA (poly vinyl alcohol) was added. The powder was mixed thoroughly and was placed in the die. The rod was inserted inside an hollow cylindrical pipe, from the other side another rod was inserted and this set was placed on a hydraulic press. Then the pressure was applied (nearly 1500 ton). Due to this pressure and the binder present in the powder sample was formed as the small pellets.

Thermo gravimetric (TG) analysis: TG stands for thermo gravimetric analysis. This is the method in which changes in chemical and physical properties of a material can be measured as a function of increasing temperature and time. It can provide information about physical phenomena such as second order phase transition including vaporization, sublimation, absorption, adsorption etc. Similarly, it can also provide information about chemical phenomena like dissolving, decomposition and solid gas reactions. TG is commonly used to determine selected characteristics of materials that exhibit either mass loss or gain during these processes. Here, four samples i.e. 0.5%, 1%, 2% & 4% were given for TG analysis. Results are given under result and discussion part (vide infra).

Calcination study: Further Samples were heated using a 1700 °C horizontal tube furnace. A tube furnace is an electric heating device used to conduct syntheses and purifications of inorganic compounds and occasionally in organic synthesis.

Results and Discussion

Fig. 1 shows the photograph of the powders in mixed form, pellet after heating at 960 °C and 1400 o C After heating the sample it is been seen that at 960 °C there is no change in the physical appearance of the pellet i.e. the color of the pellet remains colorless. But when the pellet is heated at 1400 °C for 4 hours it is clearly seen that there is a change in the color of the pellet. The color is due to the formation of a new compound of Nb-TiO 2 i.e. niobium titanate [11] . From the TG curve we observe that as the temperature increases there is a very negligible loss of mass and then it remains constant over total period of time. The mass loss may be due to some impurities, water vapor and acetone (used while mixing of powder sample). Differential scanning calorimetry (DSC) curve shows that as the temperature increases there is a loss in the heat and after some time there is absorbance of heat (at a temperature of 962 °C in 2(a) and 1040 °C 2(b)). And after 1400 °C there is a change in phase. For other samples XRD patterns were taken but not shown here. In each case anatase grade titanium dioxide phases were obtained except for 30% Nb.TiO 2 sample. A comparative XRD patterns for three powders are shown here [Fig. 6]. Where, n is an integer, λ is the wavelength, d is the spacing between the planes in the atomic lattice, ө is the angle between the incident light and scattered plan Since, TiO2 is tetragonal in shape, lattice constant can be calculated as

TG analysis

Rel. Inte

From this equation we can find the lattice constant using the relevant hkl values.

Where, hkl are the miller indices which can be found from the JCPDF file, a and c are the lattice constants. Since, the shape of TiO2 is tetragonal Therefore, a=b=/=c Ex: hkl= 004

For this hkl value d= 2.37150 A 0

Using the formula we have c=9.4860 A 0

Similarly, for hkl= 020 d=1.8880A 0

We found that a= 3.7760A 0

Determination of crystallite size

Similarly, crystallite size was calculated from the Scherrer equation (given below).

Where τ is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size; K is a dimensionless shape factor, with a value close to unity. The shape factor has a typical value of about 0.9, but varies with the actual shape of the crystallite; λ is the Xray wavelength; β is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians. This quantity is also sometimes denoted as Δ (2θ); ө is the Bragg angle. Thus the crystal size was found to be 126.05824 A 0 A 0 for 0.06 mole% Nb-TiO2. Similarly, for TiO2 and 1mole% Nb-TiO2 the crystal size were found and data are given in Table 5. Color degradation of Methylene blue using Nb-TiO2 [12] Nb-TiO

Absorption spectra

Absoption spectra of MB before and after the addition of the powder at different time [ Fig 8]. It can be seen from the absorption graph that there are two main peaks at 663 nm and second at around 290 nm. The absorbation maxima was found to be 0.611567 at 663 nm. Absoption data with time at 663 nm is given in Table 5. It has been observed that MB absoption decreases with respect to time. From table it can be seen that absorption value decreases as there is increase in time and if further time is taken than it would seen that absorption is totally zero. In this report we have studies the properties of TiO2 and Nb-TiO2. From the XRD data it is clear that there is no change in the phase of Nb-TiO2.

References

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