Facile Synthesis and Characterization of TiO₂ Nanoparticles for Photovoltaic Applications

Facile Synthesis and Characterization of TiO₂ Nanoparticles for Photovoltaic Applications

M. Ganapathy*,

* M. Ganapathy Alpha College of Engineering Thirumazhisai,

Chennai, Tamilnadu, India-600124

M. Vimalan

Thirumalai Engineering College Kilambi Kanchipuram, Tamilnadu,India-631 551

R. Jeyasekaran

VHNSN College Virudhunagar Tamilnadu India-626001

P. Dennis Christy

1. ollege College of Engineering and Technology , Perungalathur Tamilnadu,India-600063

   Abstract TiO2 nanoparticles were synthesized by hydrothermal method. The as prepared nanoparticles were calcined at 400o C. The samples were characterized by XRD, SEM. Surface morphological studies obtained from SEM micrograph showed the particles with the spherical shapes are anatase in nature. The Crystalline size of TiO2 powder (TNP1 and TNP2) has obtained is ~39nm, 28 nm for anatase at 400 ºC by controlling the acidity.

   Keywords TiO2 particles; ;;

   1. INTRODUCTION

      TiO2-based nanostructured materials have been extensively studied over the past three decades due to its low toxicity/cost, chemical stability, excellent photocatalytic activity, and long lifetime of electron/hole pairs. However, one of the main disadvantages of TiO2 is its large band gap (about 3.2 eV) can absorb only the ultraviolet (UV) light contained in a solar spectrum and the inability to utilize visible light limits the efficiency of photocatalytic degradation of organic pollutants .TiO2 has three crystalline polymorphs: anatase, rutile and brookite. Anatase and brookite are in meta stable phase but can be transformed into rutile on heating. For photocatalytic and solar cell applications, anatase phase is more preferable.TiO2 nanostructures have been synthesized by coprecipitation, sol-gel , solvothermal and hydrothermal synthesis methods.(2-6) Among these methods, the alkaline hydrothermal method is relatively simple method and produce high yields of TiO2 nanoparticles. The higher photo conversion efficiency could be achieved by smaller TiO2 particles. However, the size, the morphology, and the structural properties of TiO2 nanostructures depend on the TiO2 precursors and reaction parameters, such as the reaction temperature, the reaction time, and the solution pH during the reaction. In this paper we prepared TiO2 nanoparticles by hydrothermal method with different pH values. It is found that the TiO2 crystallinity and morphology is improved when pH is varied.

   2. EXPERIMENTAL DETAILS

      TiO2 nano-powders were prepared via Hydrothermal method using titanium tetraisopropoxide, distilled water, ethyl alcohol and Citric acid as the starting materials. The

      drop wise addition of titanium isopropoxide (TTIP) was made to the solution of distilled water, ethyl alcohol and Citric acid under constant magnetic stirring. The mixture was further stirred for about 2 h and then transferred to the stainless steel auto clave with teflon lining. The sealed auto clave was subjected to thermal treatment at a steady temperature of 150 °C for 3 hour. The auto clave was removed from the oven and then naturally allowed to come to room temperature. The final product was filtered and dried in open atmosphere. Further it was calcined at 400o C for 2hour. The samples were prepared with two different pH values. The samples were named by TNP1 and TNP2.

   3. RESULTS AND DISCUSSION

1. Structural analysis

   The XRD measurement was performed under radiation of Cu K 1.54060 Ã…. Based on the diffractograms shown in figure 1 and 2, it can be seen that both TiO2 crystallites are in the anatase phase whereas the highest peaks of both TiO2 were found at (101) crystal plane.

   TNP 1

   TNP 1

   80

   intensity (a.u)

   intensity (a.u)

   60

   40

   20

   0

   0 20 40 60 80 100

   2 theta (degree)

   Fig. 1. XRD pattern of TNP 1 TiO2 particles

   70

   60

   50

   intensity (a.u)

   intensity (a.u)

   40

   30

   20

   10

   0

   -10

   TNP 3

   TNP 3

   10 20 30 40 50 60 70

   2 theta (degree)

   Fig. 2. XRD pattern of TNP 3 TiO2 particles

   Scherrer formula in the following equation was used to calculate the lower bound of the particle grain size.

   = —————— (1)

   Where is the mean size of the crystals, k is the shape factor (with typical value around 0.9), is the x-ray wavelength, is the line broadening at half the maximum intensity (FWHM), and is the Bragg angle. The crystallite size of TNP 1 and TNP 3 are calculated according to the Scherrer equation, was found to be 39 nm and 28 nm, respectively. The broadening of the peaks clearly shows the small size of the nanoparticles. Increasing the pH value the particle size was reduced; it was improve the efficiency of the device.

2. Surface morphological analysis

   As the surface to volume ratio is the dominant factor in nanostructured materials, probing the materials surface features is the prerequisite for many important applications. The surface morphology of the as prepared samples was examined by SEM. Figure 3, 4 shows the SEM images of the TiO2 nanoparticles with grain sizes 40-60 nm approximately. They reveal the formation of nanocrystalline materials and show the aggregates of nanoparticles with variable sizes. TiO2 nanoparticles are prone to aggregate due to the large surface area and high surface energy.

   Fig. 3. SEM image of TNP1 sample

   Fig. 4. SEM image of TNP 3 sample

   SEM images exhibits the inprovement of praticle grouth depends upon the pH value. Fig.4. Shows the uniform sherical paticles, Increasing the pH value, the morphology of the particles were changed.

3. UV Visible Spectroscopic analysis

   UV-vis spectrophotometer measurements were performed to compare the optical characterizations between two types of TiO2. Figure.5 and 6, shows the optical absorption of TiO2 nanoparticles (TNP 1 and TNP 3). It can be seen that TNP 3 generally has better absorption ability upon a broader wavelength interval. The blue shift in the exciton absorption clearly indicates the quantum confinement property of nanoparticles. In the quantum confinement range, the band gap of the particles increases resulting in the shift of the absorption edge to lower wavelength, as the particle size decrease.

   TNP 1

   TNP 1

   5.0

   4.5

   )

   R (

   F

   )

   R (

   F

   4.0

   Absorption

   Absorption

   3.5

   3.0

   2.5

   2.0

   1.5

   200 300 400 500 600 700 800

   wavelength (nm)

   Fig 6. Shows the absorption nspectra of TNP 1 TiO2 particles

   TNP 3

   TNP 3

   2.6

   2.4

   2.2

   Absorption

   Absorption

   2.0

   1.8

   1.6

   1.4

   Current is gradually increased with the voltage. The I-V Characterization and resistivity are depending upon the pH value of the TiO2 nano particles.

   IV. CONCLUSION

   TiO2 nanoparticles were synthesized by hydrothermal method at pH 1 and 3. The as prepared nanoparticles were

   o

   1.2 calcined at 400 C. The Crystalline size of TiO2 powder

   1.0

   0.8

   0.6

   200 300 400 500 600 700 800

   Wavelength (nm)

   (TNP1 and TNP 3) has obtained is ~39nm, 28 nm for anatase at

   400 ºC showed that crystallinity and morphology is improved when pH is increased. The optical band gaps of the prepared TiO2 particles were 3 and 3.1 eV. I V characterization results also reveals that the efficiency was improved when pH is increased.

   Fig 6. Shows th absorption nspectra of TNP 3 TiO2 particles

   —————— (2)

   Where A is constant and Eg is a band gap. The value of n is

   ½ or 2 depending on presence of the direct allowed and indirect allowed transitions. The prepared particles band gaps are 3.1 eV and 3.00 eV, respectively.

4. I-V Study

Room temperature resistivity of the TiO2 nanoparticles were determined by the standard and most widely using method the four probe technique.

Fig 7. Shows the I-V of Difffernt pH nano particles

REFERENCES

1. C. C. Wang and J. Y. Ying, Sol-gel synthesis and hydrothermalprocessing of anatase and rutile titaniananocrystals, Chemistryof Materials, vol. 11, no. 11, pp. 31133120, 1999.

2. T. L. R. Hewer, E. C. C. Souza, T. S. Martins, E. N. S. Muccillo,and R.

   S. Freire, Influence of neodymium ions on Photocatalyticactivity of TiO2 synthesized by sol-gel and precipitationmethods, Journal of Molecular Catalysis A, vol. 336, no. 1-2, pp.5863, 2011.

3. X.Chen and S. S. Mao, Titanium dioxide nanomaterials:synthesis, properties,modifications and applications,ChemicalReviews, vol. 107, no. 7, pp. 28912959, 2007.

4. S. Sreekantan and L. C. Wei, Study on the formation andphotocatalytic activity of titanate nanotubes synthesized Viahydrothermal method, Journal of Alloys and Compounds, vol.490, no.1-2, pp. 436442, 2010.

5. S. Mozia, E. Borowiak-Pale´n, J. Przepi´orski et al., Physicochemicalproperties and possible PhotocatalyticApplicationsoftitanate nanotubes synthesized via hydrothermal method,Journal of Physics andChemistry of Solids, vol. 71, no. 3, pp. 263272, 2010.

6. R.Ma, K. Fukuda,T.Sasaki,M.Osada, andY.Bando, Structuralfeatures of titanate nanotubes/nanobelts revealed byraman, Xrayabsorption fine structure and electron diffraction characterizations,Journal of Physical Chemistry B, vol. 109, no. 13, pp.6210 6214, 2005

7. A. Elsanousi, E. M. Elssfah, J. Zhang, J. Lin, H. S. Song,and C. Tang, Hydrothermal treatment duration effect on thetransformation of titanate nanotubes into nanoribbons, Journalof Physical Chemistry C, vol. 111, no. 39,pp. 1435314357, 2007.

8. Essick J, Mather R. Characterization of a bulk semiconductors band gap via near-absorption edge optical transmission experiment.Am J Phys 1993; 61: 646-9.