Effect of T emperature on S tructure, Morphology, and Optical Properties of TiO₂ Nanoparticles

Sidra Jamil ?? ¹ , Muhammad Fasehullah ²

1 Department of Physics, NED University of Engineering and Technology, Karachi, 75270, Pakistan

2 State key laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing, 400044, China

Abstract

This paper reports synthesis of pure and mixed-phase TiO₂ nanoparticles using a sol-gel technique with titanium isopropoxide as a precursor. The prepared samples were then calcined at different temperatures. Variation in calcination temperature has been analyzed on particle size and crystalline phases, morphology, bandgap, and crystallinity using X-Ray Diffraction (XRD), Scanning Electron Microscope-Energy Dispersive X-Ray Spectroscopy (SEM-EDX), and UV-Vis spectroscopic analysis, respectively. Moreover, XRD data reveal anatase and rutile phases of TiO₂ depending on the calcination temperature. From this, one may observe that the calcination temperature greatly influences the presence of anatase and rutile phases of TiO₂. It can change the phase from the metastable anatase phase to the stable rutile phase. Also, increasing the calcination temperature can increase the particle size as estimated by the Scherrer equation, which was found to be 7 nm to 60 nm with a temperature from 400 °C to 1200 °C. SEM analysis shows the growth of spherical-shaped nanoclusters with irregular morphology, whereas EDX spectroscopic analysis confirms the purity of the samples. It has been observed that increasing the temperature reduces the bandgap through UV-Vis spectroscopic analysis. The photocatalytic degradation of Phenol Red was studied by using synthesized anatase TiO₂ nanoparticles.

Keywords

Titanium dioxide, Nanoparticles, Temperature, Optical properties

INTRODUCTION

Semiconducting materials have been extensively investigated to be used as photocatalysts for light-stimulated degradation of environmental pollutants due to their suitable bandgap energies, mainly for the destruction of toxic and non-biodegradable compounds.^1-3 Among various materials, Titanium dioxide or titania (TiO₂) is an eco-friendly, stable, and low-cost ceramic material that is widely recognized as an excellent model photocatalyst for environmental purification due to its strong oxidizing ability to create photogenerated holes, chemical stability, and non-toxicity.^4-6 TiO₂ usually exists in three crystalline phases, anatase, rutile, and brookite. Anatase and rutile are tetragonal, whereas brookite belongs to the orthorhombic crystalline system. Rutile is a thermodynamically stable phase possessing less photoactivity, enabling it to be used as a pigment and sunscreens⁷.In contrast, the brookite and anatase phases are metastable while anatase is more photoactive, having a bandgap of 3.2eV.^4,8,9 However, its large bandgap doesn't enable it to absorb light in the visible region. Its photocatalytic efficiencycan be enhanced by tuning the bandgap either by doping with transition metals or by strain.¹⁰ TiO₂ has many applications in the electronic industry, such as MOSFET and gas sensors at high temperatures. It has remarkable usage in the pigment industry due to its high refractive index (typically 3.87 for rutile and 2.5-3 for anatase). TiO₂, when synthesized at the nanoscale, has applications in dye-sensitized solar cells, production and storage of hydrogen, lithium-ion rechargeable batteries, antibacterial and self-cleaning ability.^3,11 The temperature calcination enables the anatase phase of TiO₂ to be transformed into brookite and rutile phases.¹²

The potential of titania is strongly based on its morphology, crystalline structure, and average particle size.¹³ The specific surface area is a critical requirement to improve the catalytic activity of TiO_2, which certainly depends on particle size.¹⁴ Several methods have been employed to prepare TiO₂ at the nanoscale¹⁵, such as precipitation¹⁶, microemulsion¹⁷, hydrothermal¹⁸, and sol-gel¹⁹. Amongst all, sol-gel is the most suitable technique to synthesize nanosized metal oxide materials at low temperatures with high photocatalytic activity, homogeneity, and high purity.⁶

The sol-gel process modifies a liquid into a solid phase, sol to gel¹⁹. The sol-gel process usually requires the assimilation of metal alkoxides (sol) solution into a three-dimensional polymer matrix (gel). Once the precursor is fully integrated into the matrix, its polymerization is initiated by water, which undergoes a hydrolysis reaction that forms the product upon aging. The final step is the calcination of colloids at different temperatures and durations, which causes the particle agglomeration and usually results in causing polydispersity. Since the reaction is the hydrolysis of metal alkoxides, tailoring various types of nanoparticles is possible, while the overall reaction is simplistic^14, 20,21.A.K. Tripathi et al. prepared pure and mixed-phase TiO₂ by sol-gel method calcined at temperature 400 °C to 700 °C having a particle size between 19 nm to 68 nm.²² Y.-F. Chen et al. obtained anatase powder possessing a diameter of 10nm and a specific surface area of 106.9 m²/g when calcined at 400 °C and formed a rutile phase at calcination temperatures above 600 °C.¹ H. Lin et al. synthesized nanocrystalline anatase TiO₂ of particle size ranged between 12 to 29 nm by the metal-organic chemical vapor deposition method (MOCVD). It showed that particle size plays a crucial role in changing the bandgap and specific surface area of photocatalysts.²³ Effect of pH was investigated to alter the bandgap of titania nanoparticles by the sol-gel method. The optical analysis reveals the decrease in bandgap with increasing pH.²⁴Recently, a hydrothermal synthesis route was adopted to synthesize titania and metal-doped titania nanoparticles. The Ni-doped titania exhibited excellent photocatalytic activity for neutral red and methylene blue dyes.²⁵Moreover, the gold-assisted titania Au/TiO₂ has also proven to be a photocatalyst for dye degradation as well as photocatalytic hydrogen generation.²⁶ The present work attempts to demonstrate the effect of calcination temperature on crystalline phases, morphology, and bandgap of TiO₂ nanoparticles via the sol-gel method. To have a clear understanding of the photocatalytic activity of anatase TiO_2, phenol red has been used under UV light for the degradation process. 

EXPERIMENTAL SETUP

Materials

All reagents, titanium tetra isopropoxide (TTIP), and isopropanol were obtained from Sigma-Aldrich, used without further purification. Deionized water was used as the standard solvent in all experiments.

Synthesis of TiO₂ nanoparticles

TiO₂ nanoparticles were synthesized using the most reliable sol-gel method by hydrolysis of 5 mL solution of Titanium Tetra Isopropoxide (TTIP), an organometallic precursor in a 5 mL solution of isopropanol at room temperature¹¹. The mixture was then added at a rate of 5 mL/min. in 200 mL of deionized water resulted in the formation of white colloidal solution and was stirred vigorously for 2 hours. The solution was then covered and aged at room temperature for 24 hours, filtered to get a white paste, and dried at 90 °C for 2 hours to evaporate water and other organic solvents. Figure 1 shows the flowchart of synthesis methodology adopted for the preparation of TiO₂ nanoparticles. To improve crystallization and observe the effect of calcination temperature on prepared samples, the final product was calcined at 400°C to 1200°C for 4 hours in a muffle furnace. The white crystalline powder was ground using ball milling to obtain fine particles of TiO₂.

Figure 1: Flow chart for the synthesis of TiO₂ nanoparticles by Sol-Gel route.

Characterization of TiO₂ nanoparticles

The XRD analysis of the obtained TiO₂ nanoparticles was recorded using Bruker D8 Advance X-ray diffractometer (Germany), CuKα (λ = 0.15406 nm) radiation operated at 30 kV and 30 mA at room temperature. Quanta 400 FEG Scanning Electron Microscope (USA) was operated at higher magnification to study the morphology of prepared samples. UV-1800, UV-Vis Spectrophotometer, Shimadzu scientific instruments were used in the range of 300 to 600 nm to study the optical behavior.

RESULTS AND DISCUSSION

X-Ray Diffraction Analysis

The XRD spectra of the obtained TiO₂ nanoparticles are shown in Figure 2 and 3. It can be found that the calcination temperatures greatly affect the occurrence of anatase and rutile phases of TiO₂. At 400 °C and 600 °C, the maximum intensity peak is observed at 2θ = 25.5° for (101) plane, which corresponds to the anatase phase of TiO₂ crystal with tetragonal structure (JCPDS 00-021-1272)¹⁷. Whereas at 800 °C, 1000°C, and 1200 °C, the XRD peak at 2θ = 27.7° for (110) plane gives evidence of the presence of tetragonal rutile phase of TiO₂ (JCPDS 00-021-1276)¹⁸. At 600 °C, the less intense peaks of the rutile phase appear, which shows the transformation from anatase to rutile and is said to be the transition state of TiO₂ nanoparticles. The average crystallite size 

D = kλβcosθ

Where k = 0.89 the shape factor, β is the full-width at half maxima (FWHM) in radians and 2dsinθ=nλ

The XRD analysis reveals that as temperature increases from 400 °C to 1200 °C, the crystallite size varies from 7 to 60 nm. The lattice parameters ( a=b≠c ) for tetragonal structure are calculated by;

1d2= h2+k2a2+l2c2

The XRD parameters, including crystallite size, inter planner spacing, lattice parameter, c/a ratio, and unit cell volume for tetragonal crystalline structure at various temperatures of the strongest peak, have been listed in Table.1

Figure 2: XRD peaks of TiO₂ sample calcinated at temperatures (a) 400 °C, (b) 600 °C , (c) 800 °C , (d) 1000 °C, and (e) 1200 °C. 

Table 1: Summary of XRD parameters of TiO₂ nanoparticles at different temperatures

The X-ray density for the samples can be calculated by the relation given below as;

ρ=nMNV

Where n = 4 for anatase and n = 2 for rutile, M is the molecular weight, N is the Avogadro's number, and V is the unit cell volume. The bulk density of the naoparticles increases with the increase of calcination temperature from 400 °C to 1200 °C (Table.1).

Broadening of XRD peaks occurs when particle size is less than 100 nm^27,28. X-ray diffraction peaks show broadening due to strain when the crystallites are small enough. The strain ε can be described by the relation;

ε =β4tanθ

At different calcination temperatures, the strain decreases as the particle size increases. It is already established that with a reduction in particle size, a tremendous increase in the surface area to volume ratio was observed¹¹. The higher specific surface area of the anatase phase enables TiO₂ to work efficiently as a photocatalyst⁹. In the present study, the specific surface area was calculated by the relation given below;

SSA=6 × 103D×ρ

Where, D is the crystallite size and r is the density of TiO₂.

Figure 3: Graph from XRD data illustrates the relation between temperature and (a) particle size, (b) strain, (c) SSA (d) dislocation density.

The length of dislocation lines per unit volume is defined as dislocation density which is irregularity as well as the defects in the crystalline structure. The larger dislocation density indicates the hardness of the material. Beyond 600 °C, the volume fraction of the anatase phase decreases upon heating, revealing the transformation of phase from anatase to rutile. From the calculation of dislocation densities, one can define the transformation of phase depending on size. As the size increases, the dislocation density decreases. The dislocation density is obtained by using the formula:

δ=1D2

Stoichiometry and morphology analysis

Energy Dispersive X-ray spectroscopy (EDX) pattern of TiO₂ nanoparticles is shown in Figure 4(a), and the chemical composition in wt.% is listed in Table 2. The results validate the high purity of nanoparticles being synthesized. 

Figure 4: (a) EDX pattern of TiO2 nanoparticles, SEM images of TiO2 samples calcinated at different temperatures (b) 400 °C (c) 600 °C (d) 800 °C (e) 1000 °C, and (f) 1200 °C 

SEM images of the sample were carried out to estimate the growth and surface morphology of the sample. The grains are spherically shaped with uniform size distribution and irregular distribution in nanoclusters for all temperatures. As the calcination temperature increases, the grain size increases as the particles agglomerate. Figure 4(b-f) shows the SEM images of TiO₂ nanoparticles at different temperatures.

Table 2: Energy Dispersive X-Ray (EDX) data of TiO₂ nanoparticles

Optical properties

The bandgap energy (E_g) of TiO₂ nanoparticles can be calculated using the following equation;

αhvn=Bhv-Eg

where h n is the photon energy, B is a constant, n depends on transition (n= 2/3 for directly forbidden gap, 2 for direct bandgap, and 1/2 for indirect bandgap), and a is the absorption coefficient. The absorption coefficient (a) was determined by Beer-Lambert's relation.

α=2.303Ad

Where, A is the absorbance from UV-Vis data and d is the path length of the cuvette. At higher calcination temperature, the sample illustrates redshifts due to induced oxygen vacancies which create defects. Absorbance curves of nanoparticles at various calcination temperatures are shown in Figure 5(a). The tauc plot of (αhν)1/2  versus hν  gives the band gap energy of TiO₂ nanoparticles (Figure 5(b)). The bandgap energy of the sample calcined at 400 °C is found to be 3.44 eV, whereas bulk anatase has a bandgap energy of 3.21 eV and is found to be 2.86 eV calcined as 600 °C, as displayed in Figure 5(c). In nanoscale, as the particle size reduces, the number of overlapping orbitals gets narrower; as a result, the width of the bands decreases. Ultimately, the gap between the valence and conduction bands increases, which increases the bandgap. As temperature increases, the particle size increases, and the bandgap decrease, as reported in literature^29,30. The bandgap energy of rutile is 2.38 eV, 2.17 eV, and 2.04 eV at 800 °C, 1000 °C, and 1200 °C, respectively. At higher calcination temperatures, the crystallites are larger in size; hence, their optical band gap decreases, supporting photo generation.³¹

Figure 5: (a) Absorbance curve of TiO₂ nanoparticles at different calcination temperatures, (b) Tauc plots of TiO₂ samples, (c) variation of bandgap with increasing temperature

Photocatalytic activity of anatase TiO2 synthesized at 400 °C

The photocatalytic activity was carried out in a cylindrical Pyrex-glass beaker with a 1.0 L capacity. A 125-W mercury lamp as an ultra-violet light source was placed just above the beaker, which was filled with 0.6 L of 6 mg/L of Phenol Red solution and 40 mg/L of nanoparticles of TiO₂-400 °C photocatalyst. The whole beaker was cooled with an electric fan from outside the beaker, and the test was carried out at room temperature while fresh air bubbles were introduced into the suspension using a pump. The degradation of model pollutant phenol Red was monitored by taking 5 ml of the suspension at the irradiation time intervals of 1 hour. Each time the suspension was centrifuged to separate the photocatalyst particles from the phenol red solution. Subsequently, the degradation rate was calculated according to the change in absorbance of the dye solution.

Figure 6: UV-Vis spectra changes of Phenol red (6 mg/L) in aqueous nano TiO₂ dispersion (40 mg/L) irradiated under UV light at a varying time at 0, 1, and 2 hours. Inset shows the normalized absorbance at 435 nm.

The absorption spectra of the samples were recorded by measuring the absorbance at 435 nm corresponding to the maximum absorption wavelength of Phenol red with a UV–Vis absorption spectrophotometer. The concentration of Phenol Red is proportional to the absorbance of Phenol Red according to the Beer-Lambert law, so the degradation efficiency of Phenol Red can be calculated by 

R=Co-CCo ×100%= Ao-AAo ×100%

Where A_o, A, and C_o, C are the absorbance and concentration of Phenol Red when the reaction time is 0 and t, respectively. Figure 6 shows gradual photo decolorization of the polluted solution; the initial concentration of dye 0.949 decreases to 0.304 after 1 hour of irradiation, which offers 68% degradation, and over one more hour of irradiation, it reduces to 0.249, which results in further degradation of 18%. Hence, TiO₂ sintered at 400 °C possesses an anatase phase with excellent photocatalytic activity.

CONCLUSIONS

Titanium dioxide nanoparticles were successfully prepared using a sol-gel technique with titanium tetraisopropoxide (TTIP) as a precursor. The effect of calcination temperature on the phase transformation of TiO₂ was observed. When the temperature is raised to 700 °C and 800 °C, the anatase phase begins to transform into a rutile phase. The increased calcination temperatures enhance the crystallite size from 7 nm to 60 nm, decreasing the strain and specific surface area to some extent. Tuning of bandgap as a function of calcination temperature was observed from tauc plots. The SEM-EDX results showed the purity, chemical composition, and morphology of TiO₂ nanoparticles. The photocatalytic test was carried out, which degraded the model pollutant Phenol Red up to a great extent. In summary, the sintering temperature dramatically influences the phase stability and transformation of titanium dioxide nanoparticles, and an anatase phase acts as an outstanding photocatalyst.

ACKNOWLEDGMENTS

The authors acknowledge NED University of Engineering and Technology for providing a platform to conduct this research work. 

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