Surface Modification of Nanotitania Using High Energy Electron Beam Irradiation

International Journal of Emerging Technology and Advanced Engineering

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Surface Modification of Nanotitania Using High Energy

Electron Beam Irradiation

Priyanka KP

, Sunny Joseph

, Anu Tresa Sunny

, Jaseentha OP

, Thomas Varghese

1,2,4,5

Nanoscience Research Centre (NSRC), Department of Physics, Nirmala College, Muvattupuzha – 686 661, Kerala, India

3_{School of Chemical Sciences, Mahatma Gandhi University, Kottayam, 686 560, Kerala, India}

Abstract - Nanocrystalline TiO2 particles were prepared

using a low temperature sol-gel synthesis route, and were characterized by X-ray diffraction, transmission electron microscopy, UV-Visible spectroscopy and photoluminescence spectroscopy. Using high energy electron beam the surface modification of TiO2 was carried out. The optical properties

of the synthesized nanocrystalline TiO2 samples and the

electron beam irradiated samples were investigated in context of the band gap. The band gap obtained from the absorption spectra of irradiated sample shows a slight increase as compared to the pure samples, which might occur due to size reduction. The photoluminescence intensity of the irradiated sample was much larger as compared to the non-irradiated samples, which has been attributed to defects and particle size variation. The present investigation found that the beam irradiation is a new efficient method to enhance the optical absorption performance and photo-activity of TiO2

nanoparticles.

Keywords - Band gap, Electron beam irradiation, Nanotitania, Optical properties, Photoluminescence.

I. INTRODUCTION

Nanocrystalline TiO2 particles are of great interest due to

their unique optical and electronic properties, and several potential technological applications such as photocatalysis,

sensors, solar cells and memory devices [1-4]. TiO2 exists

in three polymorphic phases: rutile (tetragonal, density =

4.25 g/cm3), anatase (tetragonal, 3.894 g/cm3) and brookite

(orthorhombic, 4.12 g/cm3). The low-density solid phases

are less stable and undergo transition to rutile in the solid state. The transformation is accelerated by heat treatment and occurs at temperatures between 450 and 1200oC. This transformation is dependent on several parameters such as initial particle size, initial phase, dopant concentration, reaction atmosphere and annealing temperature [5-9].

Anatase phase is metastable and exhibits large

photocatalytic activity; rutile has a high chemical stability but is less active. The absorption spectrum of a semiconductor defines its possible uses. The useful semiconductors for photocatalysis have a band gap comparable to the energy of the photons of visible or ultraviolet light, having a value of Eg < 3.5 eV.

It has been reported in the literature that the value of Eg ranges from 2.86 to 3.34 eV for the anatase phase. The variations may be due to differences in the stoichiometric of the synthesis, the impurities content, the crystallite size and the type of electronic transition [10-15].

Control of the size, shape and structure of the colloidal precursor is an important factor in determining the properties of the nanomaterial. Titania powders can be synthesized using various methods such as sulfate process [16], chloride process [17], impregnation [18], co-precipitation [19], hydrothermal method [20-22], direct

oxidation of TiCl4 [23] and metal organic chemical vapor

deposition method [24]. Sol-gel method [25-27] is one of the most suitable routes to synthesize various metal oxides due to low cost, ease of fabrication and low reaction

temperatures. This method is widely used to fabricate TiO2

for films and particles. In general, the sol gel process involves the transition of a system from a liquid “sol” (mostly colloidal) into a solid “gel” phase. The homogeneity of the gels depends on the solubility of reagents in the solvent, the sequence of addition of reactants, the temperature and the pH. The optimum fabrication conditions are provided for the narrow size

distribution of the nanocrystalline TiO2.

In the present investigation, as-prepared TiO2

nanoparticles are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Visible absorption spectroscopy and photoluminescence (PL) spectroscopy. In addition, absorption and PL spectra of the electron beam irradiated sample are evaluated in context of the band gap.

II. EXPERIMENTAL

Titanium (IV)-n-butoxide (Ti(OBu)4) (98%, Alfa Aesar),

isopropyl alcohol (99%, Fisher Scientific) and HNO3

(Merck) were used for the synthesis of nanocrystalline

TiO2. Distilled water was used in all synthesis process.

A. Preparation of the Samples

In this work, the precursor solution was a mixture of 5 cc

titanium (IV)-n-butoxide (Ti(OBu)4) and 10 cc isopropyl

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The mixture solution was then added drop wise into another solution whose pH value was suitably adjusted to

2.0 by adding HNO3. The reaction was performed at room

temperature while stirring for 3 hours. After complete

hydrolysis of Ti(OBu)4, the solution was refluxed at 700C

for 20 hours and a sol was formed. Finally, the as-prepared

sol was dried at 600C for 36 hours to obtain TiO2 powders.

The obtained powder samples were calcined for 3 hours in

a furnace at temperature ranging from 150 to 400 0C in an

ambient atmosphere.

The as-prepared TiO2 nanoparticles were irradiated with

an electron beam of 8 MeV energy obtained from a variable energy Microtron, Department of Physics,

Mangalore University, Mangalore. Samples of TiO2

nanoparticles were irradiated at a dose rate of 2.0, 2.5, 3.0 and 3.5 kGy, and characterized by absorption and PL spectra. However, the samples with 3.5 kGy dose rate only have made significant modifications on the measured spectra.

B. Characterization

The structural characteristics of the synthesized

titania nanoparticles have been studied by X-ray

powder diffraction using Bruker D8 Advance

X-ray diffractometer (



= 1.5406 Å) with Cu-Kα

radiation in 2θ range from 20º to 70º. From the

X-ray diffraction patterns, it is observed that all the

X-ray peaks of the pure TiO

are well matching

with the X-ray pattern of JCPDS Card No.

21-1272. X-ray diffraction analyses were carried out

for the identification of the crystal phase and the

estimation of the average crystallite size. The

crystallite size was estimated from the Debye–

Scherrer equation [28]. The Debye–Scherrer

equation is given by

t= 0.9 λ/β cosθ, (1)

Where λ is the X-ray wavelength, β is the full width at half maximum (FWHM) of the peak and θ is the Bragg’s angle.

Transmission electron microscopy is a unique tool to get direct image of nanoparticles. It can give a real space image on the atom distribution in the nanocrystal and on its surface.

High magnification imaging with lattice contrast allows the determination of individual crystal morphology. For

TEM studies, the TiO2 powder was dispersed in ethanol

using an ultrasonic bath. A drop of suspension was placed on a copper grid coated with carbon film. After drying, the copper grid containing nanoparticles was placed on the holder for the imaging process. TEM photographs of the as

prepared nanocrystalline anatase TiO2 powder samples

were taken from a Tecnai 30 G2 S-twin (model), FEI make

300 KV High Resolution Transmission Electron

Microscope (HRTEM). The TEM images provide good reviews of the sample surface.

The absorption spectra of the pure and irradiated samples were recorded with a Shimadzu UV-Visible Spectrophotometer-UV 2600 model with ISR. For the measurement, the nanoparticles were pressed into thick pellet, and placed at the entrance port of the integrating sphere. Calibration of the absorption scale was done by standard reference material. The wavelength varies from 200 to 800 nm.

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Fig. 2. TEM photograph of TiO2 nanoparticles calcined at 400 0C.

Photoluminescence spectrum is an effective way to investigate the electronic structure and optic characteristics of semiconductor nanomaterials, by which the information such as surface oxygen vacancies, defects, separation and recombination of photo-induced charge carriers can be obtained [29]. Photoluminescence spectra were measured at room temperature by Fluoromax-3 Spectrophotometer. The effect of electron beam irradiation on PL spectra was investigated.

III. RESULTS AND DISCUSSION

Figure 1 shows the X-ray diffraction patterns of as

prepared and calcined TiO2 samples. The XRD pattern of

the sample calcinated at 300 0C revealed the presence of

the anatase phase and was most complete at 400 0C.

The crystallite size obtained using equation (1) was 6 nm

for TiO2 calcined at 400 0C. Figure 1 also shows that the

diffraction peaks become intense and their FWHM

gradually decreases with increasing calcinations

temperature, verifying the increase in particle size and increase in pertinent phase.

TEM was used to further examine the particle size, crystallinity and morphology of samples. TEM bright field

images of TiO2 nanoparticles in anatase phase are shown in

figure 2. The TEM image reveals that particles are spherical or going to be spherical in shape. As can be seen from the TEM image that the average particle size is less than 8 nm, which is in agreement with the crystallite size obtained from XRD pattern.

Figure 3 shows the absorbance spectra of pure and

irradiated samples of anatase TiO2 nanoparticles. Although

the curve shape of the absorbance spectrum of TiO2

nanoparticles did not show large change after the electron beam irradiation, it could be found that the absorbance spectrum shifted to the blue region, attributed to the quantum size effect [30]. The band gap obtained from the

graph for pure and irradiated anatase TiO2 samples were

2.85 eV and 2.91 eV respectively. This increase of band gap might occur due to size reduction of the nanoparticles caused by the electron beam irradiation. An additional advantage due to size reduction is that the large surface area to volume ratio makes possible the timely utilization of photogenerated carriers in interfacial processes [31]. Moreover, it could be seen that beam irradiation improved

the optical absorption performance of TiO2 nanoparticles.

Hence, they can be better suited for the fabrication of efficient solar cells.

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Figure 4 shows the room temperature photolum-inescence emission spectra of pure and irradiated samples

of anatase TiO2 nanoparticles. The TiO2 nanoparticles

could exhibit a strong and wide PL signal at the range from 400 to 500 nm with the excited wavelength of 300 nm and has two obvious PL peaks at about 420 and 480 nm respectively, possibly the former mainly resulting from band edge free excitons and the latter mainly resulting from

binding excitons [31,32]. In addition, it is found that pure

and irradiated TiO2 nanoparticles both could exhibit

obvious PL signal with similar curve shape. However, there was a small blue shift in the case of irradiated sample because the electron beam irradiation might result a slight reduction in particle size. It is obvious that decrease in particle size increase the surface energy, which can be attributed to the quantum confinement [33]. As a result, a slight blue shift (≈0.015 eV) in emission peak was observed. Besides, it is noted that the intensity of PL spectra of irradiated samples are much larger as compared to the non-irradiated samples. However, Jun and his co-workers reported that PL intensity decreased with beam irradiation [34].

The increase in PL intensity for irradiated sample is attributed to the self-trapped excitons recombination which is a combined effect of defect centers generated from oxygen vacancies, formation of small size particle, growth of the brookite phase and increased absorption over the UV and visible range [35]. The increase in PL intensity is an indication of higher photocatalytic activity.

Fig. 4.Room temperature PL spectra of TiO2 samples.

IV. CONCLUSION

Nanocrystalline titania was synthesized by sol-gel

synthesis route, and was characterized by XRD, TEM, UV-Visible and PL spectroscopy. The optical properties of the pure and the beam irradiated samples were investigated in context of the band gap. The absorbance spectra of irradiated samples shifted to the blue region, attributed to the quantum size effect. The band gap obtained for

irradiated TiO2 samples were larger as compared to the

pure sample. This increase of band gap might occur due to size reduction of the nanoparticles caused by the beam irradiation. The PL intensity of the irradiated sample was more, which has been attributed to defects and particle size variation. Moreover, the electron beam irradiation is found to be a new efficient method for the enhancement of the photo-response and improvement of the photo-activity of

TiO2 nanoparticles.

Acknowledgments

The authors acknowledge their thanks to Nirmala College, Muvattupuzha and Microtron Centre, Mangalore University, Mangalore, Karnataka State for providing the opportunity to undertake this study. The financial support by KSCSTE, Thiruvananthapuram, Kerala is gratefully acknowledged.

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