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EFFECT OF HIGH ENERGY ELECTRON BEAM IRRADIATION ON THE OPTICAL PROPERTIES OF NANOCRYSTALLINE TiO2
K. P. Priyanka1, Sunny Joseph1, AnuTresa Sunny2, Thomas Varghese1*
1Nanoscience Research Centre (NSRC), Dept. of Physics, Nirmala College, Muvattupuzha - 686 661, Kerala, India 2School of Chemical Sciences, Mahatma Gandhi University, Kottayam - 686 560, Kerala, India
The effect of high energy electron beam irradiation on the optical properties of TiO2 nanoparticles was studied in order to improve the optical absorption performance and photo-activity. Electron beam irradiation may have resulted in size reduction, which in turn caused an increase of the optical band gap and photoluminescence intensity. Irradiation at a suitable dose rate was found to enhance the optical absorption performance and photo-activity of the tested TiO2 nanoparticles.
Keywords: TiO2, electron beam irradiation, optical properties, photoluminescence. 1. Introduction
Nanocrystalline TiO2 is a well-known semiconductor with photocatalytic activities, having great potential as photoelectric converters with applications in solar cells, environmental purification and for the generation of hydrogen gas. TiO2 exists in three polymorphic phases: rutile (tetragonal, density = 4.25 g/cm3), anatase (tetragonal, 3.894 g/cm3) and brookite (or-thorhombic, 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 thermal treatment and occurs at temperatures between 450-1200 °C. This transformation is dependent on several parameters, such as initial particle size, initial phase, dopant concentration, reaction atmosphere and annealing temperature [1-5].
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 light, having a value of Eg < 3.0 eV. Eg value ranges from 2.86 to 3.34 eV have been reported in the literature for the anatase phase. Thus, TiO2 would prove to be the most efficient if it would exhibits the unique ability to absorb visible light significantly. Unfortunately, however, TiO2 is a poor visible light absorber, requiring a large band gap energy (3.2 eV) for the excitation of electrons from the filled valence band to the vacant conduction band. In order to enhance the photo-response and the photo-activity of TiO2, a variety of approaches have been employed: e.g. (i) doping with transition metals or nonmetals into TiO2, (ii) reducing TiO2 using hydrogen plasma, (iii) adsorbing metal complexes] or organic dyes onto the surfaces of TiO2 [6-9] and (iv) high energy electron beam irradiation of TiO2 nanoparticles [10].
The sol-gel method [11] 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, particles or monoliths. The optimum fabrication conditions are provided for the narrow size distribution of the nanocrystalline TiO2. In this work, the sol-gel synthesis route has been employed for the preparation of TiO2 nanoparticles.
The prepared TiO2 nanoparticles were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Visible and photoluminescence (PL) spectroscopy. In addition, the optical properties of the synthesized nanocrystalline TiO2 samples and the electron beam irradiated samples were evaluated in context of the band gap.
2. Materials and Methods
Titanium (IV)-n-butoxide (Ti(OBu)4) (98%, Alfa Aesar), isopropyl alcohol (99%, Fisher Scientific) and HNO3 (Merck) were used without further purification for the synthesis of nanocrystalline TiO2. Distilled water was used in all synthesis procedures .
The scheme for preparing the nanocrystalline TiO2 used in this work has been presented schematically as shown in fig. 1. The precursor solution was a mixture of 5 ml titanium (IV)-n-butoxide (Ti(OBu)4) and 10 ml isopropyl alcohol. This solution was then added dropwise to a pH 2 solution of HNO3. The hydrolysis was performed at room temperature while stirring for 3 hours. After complete consumption of Ti(OBu)4, the solution was refluxed at 70 °C for 20 hours and a sol was formed. Finally, the formed sol was dried at 60 °C for 36 hours to obtain TiO2 powders. The obtained powder samples were calcined for 3 hours in a furnace at 300-800 °C in an aerobic atmosphere.
Fig. 1. The scheme of preparation of nanotitania by sol-gel method
The 800 °C calcined TiO2 nanoparticles were irradiated with an 8 MeV electron beam obtained from a variable energy Microtron, (Department of Physics, Mangalore University, Mangalagangotri, Karnataka State). The rutile samples of TiO2 nanoparticles were irradiated at a dose rate of 0.5, 1.5, 2.5 and 3.5 kGy, and characterized by UV-Visible absorption spectroscopy and PL spectroscopy. It is of interest to note that the samples with 2.5 and 3.5 kGy dose rates had significant modifications on both their UV and PL spectra.
The structural characteristics of the synthesized titania nanoparticles were also studied by X-ray powder diffraction using a Bruker D8 Advance X-ray diffractometer (A = 1.5406 A) with Cu-Ka radiation in 20 range from 20 ° to 70 °. From the X-ray diffraction analysis all the peaks of pure TiO2, were well matchedwith the X-ray pattern of JCPDS Card No. 21-1272. These analyses were performed to determine both the crystal phase and the average crystallite size.
The latter value was estimated from the Debye-Scherrer equation [11]. The Debye-Scherrer equation is given by
t = 0.9A/P cos 9, (1)
where A is the X-ray wavelength, p is the full width at half maximum (FWHM) of the peak and 9 is the Bragg's angle.
Transmission electron microscopy is a unique tool to get a direct image of the nanoparti-cles, revealing the 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 the suspension was placed on a copper grid coated with carbon film. After drying, the copper grid containing the nanoparticles was placed on a holder for the imaging process. TEM photographs of the prepared nanocrystalline rutile TiO2 powder samples were taken using a Tecnai 30 G2 S-twin (model), FEI make 300 KV High Resolution Transmission Electron Microscope (HRTEM). The TEM images in fig. 2 provide good reviews of the sample surface.
Fig. 2. TEM images of TiO2 nanoparticles calcined at 800 °C
The absorption spectra of the pure and irradiated samples were recorded with a Shimadzu UV-Visible Spectrophotometer-UV 2600 model with ISR attachment. For measurement, the nanoparticles were pressed into a thick pellet, and placed at the entrance port of the integrating sphere using a sample holder. Calibration of the absorbance scale was done using standard reference materials. The wavelengths tested ranged from 200-800 nm.
Photoluminescence spectroscopy is an effective way to investigate the electronic structure and optical characteristics of semiconductor nanomaterials. This analytical technique reveals information such as surface defects and oxygen vacancies as well as the separation and recombination of photo-induced charge carriers [12]. Photoluminescence spectra were measured at room temperature by a Fluoromax-3 spectrophotometer. The effect of electron beam irradiation on PL spectra was investigated. The photo activity of electron beam irradiated TiO2 samples can be evaluated by the measurement of PL spectra, since stronger PL intensities were associated with higher photo activities.
20 (degree)
Fig. 3. The XRD spectra of TiO2 nanoparticles calcined at different temperatures
(A — Anatase; R — Rutile)
3. Results and Discussion
Figure 3 shows the X-ray diffraction patterns of uncalcined and calcined TiO2 samples (A — anatase, R — rutile). The XRD pattern of the sample calcined at 300 °C revealed the presence of the anatase phase. Above 400 ° C rutile peaks started to appear and a mixture of both the anatase and rutile phases of TiO2 existed up to 800 °C. When the calcination temperature was 600 °C, sharp peaks associated with the rutile phase were observed. This suggests that there was a phase transition from anatase to rutile at temperatures lower than 600 ° C. The residual anatase phase still exists in the samples up to 800 °C. At 800 °C, the transition to the rutile phase is almost complete. Fig. 3 shows that the diffraction peaks become intense and their FWHM gradually decreases with increasing calcination temperatures, verifying the increase in both the particle size and pertinent phase. The crystallite size obtained using equation (1) was 6 nm in the case of anatase TiO2 calcined at 400 ° C and 33 nm in the case of rutile TiO2 calcined at 800 °C.
TEM was used to further examine the particle size, crystallinity and morphology of titania samples. TEM bright field images of TiO2 nanoparticles in rutile (calcined at 800 °C) phase are shown in fig. 2. The TEM image revealed that particles were spherical or near-spherical in shape. Selected area electron diffraction (SAED) is shown in the inset of fig. 2, which clearly indicates that the TiO2 nanoparticles were highly crystalline in nature. As can be seen from the TEM image, the average particle size is about 30 nm, which is in agreement with the crystallite size obtained from XRD data.
Figure 4 shows the UV-Visible absorbance spectra of pure and irradiated samples of rutile TiO2 nanoparticles. S1 is pure sample, S2 and S3 are irradiated rutile samples at a dose rate of 2.5 kGy and 3.5 kGy respectively. The figure shows that the absorbance spectrum of rutile TiO2 is shifted to the blue region, attributed to the quantum size effect [9]. Similar observations were reported for anatase TiO2 samples [10]. The band gap values obtained for rutile samples
1.4
200 300 400 500 600
WAVE LENGTH (n.m.)
Fig. 4. UV-Vis absorption spectra of rutile TiO2 samples
were 2.81 eV (S1), 2.85 eV (S2) and 2.88 eV (S3) respectively. This increase in the band gap might occur due to size reduction of the nanoparticles resulting from electron beam irradiation. An additional advantage of size reduction is that the large surface area to volume ratio makes possible timely utilization of photogenerated carriers in interfacial processes [10]. Moreover, beam irradiation appeared to improve the optical absorption performance of TiO2 nanoparticles. In addition, both non- and irradiated TiO2 nanoparticles were useful for photocatalytic activities because their band gap energy values were comparable to the energy of visible or UV light photons. As a result, these materials may be well-suited for the fabrication of efficient solar cells.
Figure 5 shows the room temperature photoluminescence emission spectra of pure and irradiated samples of rutile TiO2 nanoparticles. S1 is pure rutile TiO2, S2 and S3 are irradiated rutile samples with dose rates of 2.5 kGy and 3.5 kGy respectively. The rutile TiO2 nanoparticles exhibited a strong and wide PL signal at 400-500 nm with the excited wavelength of 300 nm as shown in fig. 5. The figure shows two obvious PL peaks at about 420 and 485 nm respectively. The former peak will be mainly resulting from band edge free excitons and the latter a result of binding excitons [13]. Both pure and irradiated TiO2 nanoparticles could exhibit obvious PL signal with similar curve shapes. However, there was a small blue shift in the case of irradiated samples because the electron beam irradiation might result in a slight reduction in particle size. The decrease in particle size had an obvious effect, increasing the surface energy, which can be attributed to the quantum confinement [14,15]. As a result, a slight blue shift (^ 0.0139 eV) in the emission peak was observed. The PL spectra of irradiated samples were shown to have much larger intensities relative to non-irradiated samples. Sample S3, which was irradiated at 3.5 kGy, showed a large intensity PL spectrum. However, Jun and co-workers reported that the PL intensity decreased with beam irradiation [16]. The increase in PL intensity for an irradiated sample is attributed to the recombination of self-trapped excitons, which is a combined effect of defect centers generated by oxygen vacancies, small particle size, growth of the brookite phase and increased absorption over the UV and visible range [17]. The increase in PL intensity is an
250 300 350 400 450 500 550 600 650 WAVE LENGTH (n.m.)
Fig. 5. Room temperature PL spectra of rutile TiO2 samples
indication of higher photocatalytic activity. Moreover, the beam irradiation with suitable dose rate can help to enhance the photo-activity of TiO2 nanoparticles. Similar observations have been reported for anatase TiO2 nanoparticles [10].
4. Conclusion
TiO2 nanoparticles were synthesized by the sol-gel method and then irradiated with high energy electron beams. The optical properties of the pure and the beam-irradiated samples of rutile TiO2 were investigated in context of the band gap. The absorption spectra of the irradiated samples were shifted to the blue region, which was attributed to the quantum size effect. The band gap obtained for the irradiated rutile TiO2 samples were larger as compared to their pure samples. 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 samples was more, which was thought to arise from defects and particle size variation. Furthermore, the data obtained here would seem to show that the high energy electron beam irradiation at a suitable dose rate helps to enhance the optical response and 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 of this work by the KSCSTE, Thiruvananthapuram is gratefully acknowledged.
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