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Electron-irradiation induced changes in the phases and photocatalytic activity of TiO2 nanoparticles
Nuclear Instruments and Methods in Physics Research B 276 (2012) 7–13
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Electron-irradiation induced changes in the phases and photocatalytic activity of TiO2 nanoparticles K.B. Sapnar, S.D. Dhole, V.N. Bhoraskar ⇑ Department of Physics, University of Pune, Pune 411007, India
a r t i c l e
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Article history: Received 12 September 2011 Received in revised form 30 December 2011 Available online 18 January 2012 Keywords: Electron irradiation TiO2 nanoparticles Photocatalytic activity Brookite Electron–hole pairs
a b s t r a c t Samples of TiO2 nanoparticles, with mixed anatase and rutile phases, were irradiated with 6.5 MeV electrons at fluences, 1.5, 2.0, 2.5, 3.0, and 3.5 1015 e cm2 and characterized by several methods. With increasing electron fluence, a continuous decrease in the average particle size from 80 nm to around 30 nm were observed along with a decrease in the rutile and the anatase phases of TiO2, but at different rates, and growth of the TiO2 brookite phase at slow rate. The photocatalytic activities of different electron irradiated TiO2 samples, in the photodegradation of methylene blue, were studied by recording UV– Vis absorption spectra of the respective solutions. On electron irradiation, even though the rutile phase in the TiO2 was decreasing, the photocatalytic activity of the nanoparticles increased continuously with fluence up to 3.0 1015 e cm2, but decreased at 3.5 1015 e cm2. The energy levels introduced by the brookite phase and the electron induced defects in TiO2 could have effectively reduced the electron–hole recombination rate in the absence of the rutile phase. The observed enhancement in the photocatalytic activity of the irradiated TiO2 is attributed to the formation of small size particles, the introduction of the oxygen related vacancies and other defects, the growth of the brookite phase, and increased absorption of radiation over the ultraviolet and visible range. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Different types of semiconductor photocatalysts are widely used in the degradation of organic compounds, in both gas phase and aqueous systems, in addition to the field of solar energy storage [1–3]. Among different photocatalysts, zinc oxide (ZnO) and titanium dioxide (TiO2) are commonly used for the photodegradation of pollutants, due to their high efficiency, low cost, and band energies in the UV–Vis range. TiO2 nanoparticles are also used in a number of applications such as sensor devices, paints, dyesensitized solar cells [4,5], etc. TiO2 exists in three phases, namely anatase, brookite and rutile. For different applications, nanoscale brookite film and rutile rods have been synthesized by a hydrothermal process [6]. Anatase particles, synthesized by a chemical process, are normally smaller (< 30 nm). By increasing the processing period to obtain relatively larger sizes, the anatase phase can be transformed to brookite and rutile phases [7,8]. In the hydrothermal process, all the three phases of TiO2 anatase, rutile and brookite can be synthesized by adjusting the process parameters [9]. The crystal structure of TiO2 has three polymeric forms, in which anatase and rutile are tetragonal, whereas the brookite phase is orthorhombic. Even though there is disagreement over the direct and ⇑ Corresponding author. Tel.: +91 2025699072; fax: +91 2025691684. E-mail address: [email protected] (V.N. Bhoraskar). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2012.01.012
indirect band gap energies of the TiO2 brookite phase, the measured band gap energies of the anatase, brookite and rutile phases are, respectively 3.21, 3.13 and 3.00 eV [8]. The photodegradation of a chemical compound using a semiconductor photocatalyst is carried out under exposure to UV–Vis radiation. In this process, the electrons of the semiconductor are excited from the valence band to the conduction band. As a result positive holes in the valence band and electron in the conduction band exist during exposure. The efficiency of the photocatalyst is therefore governed by the rate of electron–hole recombination. Attempts have therefore been made to enhance the photocatalytic activity of TiO2 by reducing the rate of the electron–hole recombination. Among different methods, metallic doping in TiO2 is found effective in enhancing the photocatalytic activity. In the un-doped TiO2 compound, the relative efficiency for the photodegradation, is higher for the anatase phase as compared to the rutile phase, mainly attributed to the lower rate of electron–hole recombination, and higher degree of surface absorption for certain chemical species [10]. Earlier, the photocatalytic activity of the anatase phase of TiO2 was found higher as compared to that of the brookite and rutile phases [11,12]. However, later on it was observed that the mixed phases of TiO2, such as anatase–rutile, anatase–brookite, can exhibit relatively higher degree of photocatalytic activity, mainly due to the lower recombination rate of the photo-degenerated electron–hole pairs [11–14]. The observed higher degree of
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photocatalytic efficiency has been attributed due to the process of the (i) transfer of the electrons from anatase to a lower energy electron trapping sites in rutile, and (ii) transfer of the electrons produced in the conduction band of rutile to the electron trapping sites of anatase, under exposure of UV–Vis radiation [10]. These processes enhance the rate of separation of electrons from the holes, and therefore decrease the electron–hole recombination rate. Moreover, even though there is no general agreement on the effect of the particle size on the photocatalytic efficiency of these three polymorphic forms of TiO2 (anatase, brookite and rutile) [14], a number of workers have observed experimentally that the photocatalytic activity increases with a decrease in particle size [15]. This is supported by the fact that the surface area of the particles increases with decreasing size and therefore the total area illuminated by UV–Vis radiation is much higher for the smaller size particles. A few workers [16,17] have studied effects of electron irradiation on TiO2, mostly in the form of thin films. However, studies on the irradiation effects of 6.5 MeV electrons on the phases of TiO2 nanoparticles, and the related photocatalytic activity have not been reported. In the present work, samples of TiO2 powder, consisting of mixed anatase and rutile phases, were irradiated with 6.5 MeV electrons at different fluences. Variations in the anatase and rutile phases, and the growth of the brookite phase with electron fluence were studied. Moreover, in the degradation of methylene blue, the observed changes in the photocatalytic activity of TiO2 nanoparticles with electron fluences were investigated. The results of the present study show that the TiO2 nanoparticles, with mixed anatase, and brookite phases, have a relatively higher photocatalytic activity as compared to virgin TiO2 powder, consisting of anatase and rutile phases. In addition, the reduced crystallite size and surface defects also play an important role in enhancing the photocatalytic efficiency of TiO2. 2. Materials and methods 2.1. Synthesis of TiO2 nanoparticles TiO2 nanoparticles were synthesized in the laboratory using the sol–gel method. The solution was prepared by mixing titanium isopropoxide 10 ml, with ethanol 40 ml, and was stirred for about 30 min, at room temperature. This solution was then added drop wise to another solution, prepared by mixing 10 ml deionised water, 10 ml isopropoxide, and 5 ml of HNO3. A yellowish color gel was formed after stirring this solution mixture for about 1 h. After annealing the gel at 120 °C for 2 h, yellow color powder, containing amorphous TiO2 particles was obtained. The TiO2 powder was then annealed in steps, over the temperature range 250–600 °C, for different periods in the range 2–3 h, and then cooled to room temperature under atmospheric conditions. The results of XRD analysis revealed that the annealed powder consisted of TiO2 nanoparticles, with mixed anatase and rutile phases. These laboratory synthesized TiO2 nanoparticles were used in the present study.
Initially, a Faraday cup was positioned at a distance of 120 mm from the electron-beam extraction port of the Race-Track Microtron. For measuring the electron fluence, the Faraday-cup was connected to an electron-current integrator system. After mounting the first TiO2 sample on the Faraday cup, the electron accelerator, Microtron was switched on. The extracted 6.5 MeV electrons were made to be incident on the TiO2 sample, mounted on the Faraday cup. In order to irradiate the nanoparticles-powder of the sample with electrons uniformly, the size of the electron beam was made large enough to cover the entire TiO2 sample. The number of electrons falling on the TiO2 sample was continuously measured by the current integrator. The electron beam was turned off when the TiO2 sample received an electron fluence 1.5 1015 e cm2. Following the same experimental procedure, the second, the third, the forth, and the fifth samples of the TiO2 powder were irradiated with 6.5 MeV electrons, respectively at fluences 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2. The sixth TiO2 sample was not irradiated, and kept as the as-synthesized reference sample. The as-synthesized and the electron irradiated TiO2 samples were characterized by the (i) scanning electron microscopy (SEM), (ii) EDAX, (iii) X-ray Diffraction (XRD) and (iv) UV–Vis spectroscopy methods. For each of the TiO2 powder sample, the XRD spectrum was recorded using Cu Ka radiation (k = 1.5418 Å), at a scan rate of 1–20/min with 0.10 resolution. In each recorded spectrum, the positions and the relative intensities of the XRD peaks were obtained after comparing with the standard data (JCPDS card No. 36-1451). The XRD spectra recorded from one sample of the assynthesized, and five samples of the electron irradiated TiO2 nanoparticles are shown in Fig. 1. Similarly, the SEM images for these samples are shown in Fig. 2. The UV–Vis spectrum of each of these six TiO2 samples was recorded in the absorption mode, over 200–800 nm range, using a SHIMADZU make UV–Vis spectrophotometer. These UV–Vis absorption spectra are shown in Fig. 3. The EDAX spectra of one as-synthesized and a few electron irradiated TiO2 samples were recorded for observing the variations in the O/Ti ratio. After characterization, all these six TiO2 samples were used in the study of the photodegradation of methylene blue in aqueous solution.
2.2. Electron irradiation and characterization of TiO2 nanoparticles The electron beam of 6.5 MeV energy was obtained from the Race-Track Microtron of the Department of Physics, University of Pune, Pune (India). For the irradiation experiment, each sample was made by packing 60 mg powder of TiO2 in a polyethylene bag. By folding the bag, the size of the sample was made close to 4 10 10 mm. In this manner, such six samples of the TiO2 powder were made, numbered, and kept ready for the experiment.
Fig. 1. XRD spectra (a) of the as-synthesized TiO2 powder sample, and (b–f) of the five TiO2 powder samples irradiated with 6.5 MeV electrons at fluences: (b) 1.5 1015 e cm2, (c) 2.0 1015 e cm2, (d) 2.5 1015 e cm2, (e) 3.0 1015 e cm2, and (f) 3.5 1015 e cm2.
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Fig. 2. SEM images (a) of the as-synthesized TiO2 powder sample and (b–e) of the four TiO2 powder samples irradiated with 6.5 MeV electrons at different fluences: (b) 2.0 1015 e cm2, (c) 2.5 1015 e cm2, (d) 3.0 1015 e cm2, and (e) 3.5 1015 e cm2. The SEM image in (f) is of a typical agglomerated particle in (d), recorded at 0.2 lm scale.
2.3. Photocatalytic activity of the electron-irradiated TiO2 nanoparticles
Fig. 3. UV–Vis absorption spectra, over 200–800 nm range, recorded from (i) one as-synthesized TiO2 powder sample (ii) other five TiO2 powder samples irradiated with 6.5 MeV electrons, respectively at fluencies: 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2.
The photodegradation of methylene blue in aqueous solution was studied using both the as-synthesized and the electron irradiated TiO2 nanoparticle samples, as photocatalyst. In all the photodegradation experiments, a UV lamp, type Kr-85 OSRAM-400 W, was used as a source of UV radiation for exposing the methylene blue solution. In the first experiment, 5 mg of the as-synthesized TiO2 powder was added into a freshly prepared 45 ml of 105 molar methylene blue aqueous solution. This mixture-solution, after stirring for 30 min, was divided into nine equal parts by pouring 5 ml by volume of the solution in each of the nine numbers of small size quartz circular dishes. The solution of the first quart dish was exposed to UV radiation for 15 min. This UV exposed solution was then immediately characterized by the UV–Vis spectrophotometer (SHIMADZU) over the wavelength range 400–800 nm, in the absorption mode. The characteristic absorption peak of the methylene blue was obtained at 665 nm in the recorded spectrum. Similarly, under the identical experimental conditions, the solution of the second quartz dish was exposed to UV radiation for 30 min, and the absorption spectrum over the range 400–800 nm was recorded. Following the same experimental procedures, the solutions of the remaining six quartz dishes were also exposed to UV radiation sequentially.
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Fig. 5. Variations in the degradation D (%) with the period of UV exposure, for the six different methylene blue solutions prepared separately by mixing with six types of TiO2 powder samples: (i) one solution mixed with as-synthesized TiO2 powder, and (ii) other five solutions were prepared by mixing TiO2 powder irradiated with 6.5 MeV electrons, respectively at fluences: 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2.
Fig. 4. UV–Vis absorption spectra for the two sets of methylene blue solutions: (a) set of solutions was prepared by mixing with the as-synthesized TiO2 powder, and (b) set of solutions prepared by mixing with the TiO2 powder sample irradiated with 6.5 MeV electrons at a fluence of 2.5 1015 e cm2 and exposed separately to UV radiations for different periods, varying from 15 to 120 min.
However, the UV exposure period was increased in a step of 15 min from one solution sample to the other solution sample. In this manner, the UV exposure period for the solution of the first quartz dish and that of the eighth quartz dish was, respectively 15 and 120 min. The UV–Vis absorption spectrum of the ninth solution was recorded without exposing it to UV radiation. In this manner, all the nine solutions were subjected to UV–Vis spectroscopic measurements, over 400–800 nm wavelength range. In the second experiment, 5 mg powder of the TiO2, already irradiated with 6.5 MeV electrons at a fluence 1.5 1015 e cm2 was added into a freshly prepared 45 ml of 105 molar methylene blue aqueous solution. This mixture-solution after stirring for 30 min was divided into nine equal parts, by pouring 5 ml by volume of the solution in each of the nine quartz-circular dishes. Following the same experimental procedure as that adopted for the as-synthesized TiO2 powder, the solutions of the eight quartz dishes were exposed to UV radiations sequentially. In this experiment also, the UV exposure period was varied from 15 to
120 min, in a step of 15 min, from the first solution to the eighth solution. The solution of the ninth quartz container was not exposed to UV radiation. All these eight UV irradiated solutions, and the ninth solution TiO2, which was not exposed to UV radiation, were subjected to UV–Vis spectroscopic measurements, over 400–800-nm wavelength range, in the absorption mode. Following the same experimental procedure, the third, the fourth, the fifth, and the sixth experiments were carried out using the TiO2 powder samples irradiated with electrons at fluences, 2 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2, respectively. In this manner, the photodegradation of methylene blue in aqueous solution was studied by using one sample of the as-synthesized TiO2 nanoparticles, and five samples of TiO2 nanoparticles irradiated with 6.5 MeV electrons at different fluences. The changes in the absorption spectra with the UV exposure period, varied from 15 to 120 min, are shown (i) in Fig. 4(a) for the methylene blue solutions prepared by mixing as-synthesized TiO2 nanoparticles, and (ii) in Fig. 4(b) for the methylene solutions prepared by mixing TiO2 nanoparticles irradiated with 6.5 MeV electrons at a fluence of 2.5 1015 e cm2. In the recorded UV–Vis absorption spectra of the methylene blue aqueous solution, mixed with the as-synthesized or electron irradiated TiO2 nanoparticles, the intensity of the peaks before and after exposure to UV radiation are denoted by C0 and Ct, respectively. For different methylene blue aqueous solutions, mixed with the as-synthesized or the electron irradiated TiO2 powder, the percentage degradation D (%) is defined as follows:
D ð%Þ ¼
C0 Ct 100 C0
ð1Þ
The variations in the D (%) with the period of UV exposure for these six different methylene blue solutions are shown in Fig. 5. 3. Results 3.1. XRD spectra The crystal structure of TiO2 has three polymeric forms, in which anatase and rutile are of tetragonal and brookite is orthorhombic. Fig. 1 shows the recorded XRD spectra, in which the
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spectrum (a) is for the as-synthesized TiO2 powder, where as the XRD spectra (b–f) are of theTiO2 powder samples irradiated with 6.5 MeV electrons, respectively at fluences 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2. From the spectrum (a) in Fig. 1, it is observed that the main crystalline peaks of anatase h1 0 1i, and rutile h1 1 0i have appeared at 25.28°(2h) and 27.44°(2h), respectively. These results show that the as-synthesized TiO2 had crystalline structure, consisting of anatase and rutile phases, but without the brookite phase. In addition, the intensity of the XRD peak corresponding to the anatase h1 0 1i plane is relatively higher as compared to that of the rutile h1 1 0i plane. These results reveal that, in the as-synthesized TiO2 powder, the concentration of the anatase phase was higher as compared to that of the rutile phase. The irradiation effects of electrons on the TiO2 powder is clearly observed in the XRD spectra shown in Fig. 1. An analysis of the XRD spectra in Fig. 1 reveal that the width of each XRD peak corresponding to the anatase and rutile phases increased, however, the respective XRD peak height decreased with the increase in the electron fluence. Moreover, the rate of decrease in the peak height with the electron fluence was relatively higher for the rutile phase as compared to that of the anatase phase. In addition, the formation of the brookite phase in the electron irradiated TiO2 is revealed by the appearance of the XRD peak corresponding to the brookite h1 2 1i plane at 30.8°(2h) in each XRD spectrum (b–e), shown in Fig. 1. However, the intensity of the brookite peak was small, and increased marginally with the increase in the electron fluence from 1.5 1015 to 3.0 1015 e cm2. Moreover, both the brookite and the rutile phases appear to be converted to distorted phases at 3.5 1015 e cm2 electron fluence. From the XRD spectra in Fig. 1, the estimated average size of the particles is around 80 nm for the as-synthesized TiO2 powder and around 30 nm for the TiO2 powder irradiated with electrons at a fluence 2.5 1015 e cm2. The average particle size was estimated from the FWHM of the corresponding XRD peak, and using the Scherer’s formula. In addition to the crystallite size, the broadening of the XRD also depends on several other factors, such as (i) the instrumental broadening, (ii) the internal strain in the crystal, (iii) the temperature variations, and (iv) the nature of the sample. The instrumental broadening puts a limit on the estimation of the particle size on the higher side. In the present work, the XRD diffractometer used for recording the XRD spectra had the instrumental broadening limit 100 nm. All the XRD spectra were recorded at room temperature (24 °C), and therefore, the contributions due to the temperature variations in the peak broadening are negligible. The effects of internal strain can be qualified based on the Williamson–Hall analysis [18,19], in which the contributions from the crystallite size and the lattice strain can be related with the FWHM of the recorded XRD peak, through the following Eq. (2):
bcosh 1 gsinh ¼ þ k k e
ð2Þ
where h is the Bragg angle of the diffraction peak, e is the effective crystallite size, b is the measured FWHM of the XRD peak in radian, k is the X-ray wavelength, and g is the effective strain [18,19]. A plot of bcosh/k versus sinh/k can be used to estimate the crystallite size, considering the effect of strain. From the Scherer’s formula, the estimated average sizes of TiO2 particles were around (i) 80 nm for the unirradiated TiO2 nanoparticles, and (ii) 30 nm for the electron irradiated TiO2 nanoparticles. However, from the plots using equation Eq. (2), the estimated particle sizes were (i) 74 nm for the unirradiated TiO2 nanoparticles, and (ii) 28 nm for the electron irradiated TiO2 nanoparticles.
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3.2. Surface morphology The surface morphology of the as-synthesized and the electron irradiated TiO2 powder samples was studied by scanning electron microscopy. The SEM image recorded from one as-synthesized TiO2 powder is shown in Fig. 2(a). Similarly, the SEM images recorded from the TiO2 powder samples irradiated with 6.5 MeV electrons at 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2 fluences are, respectively shown in Fig. 2(b–e). It can be observed from the SEM images in Fig. 2 that due to the process of the particle agglomeration, the particle size distribution is wide, and a number of large size particles exist in all the electron irradiated TiO2 samples along with nanoparticles. A typical SEM image of an agglomerated particle in Fig. 2(d), recorded at 0.2 lm scale is shown in Fig. 2(f). The EDAX spectra of the as-synthesized and the electron irradiated TiO2 samples were recorded, to understand the variations in the O/Ti ratio with the electron fluence. However, these EDAX spectra are not shown in this paper. 3.3. UV–Vis absorption spectra The UV–Vis absorption spectra over the wavelength 200– 800 nm range recorded from the as-synthesized and the electron irradiated TiO2 powder samples are shown in Fig. 3. In these UV– Vis spectra, each spectrum is marked by a symbol, indicating the electron fluence used for irradiating the TiO2 powder sample. In this manner, out of these six plots, one spectrum is for the as-synthesized TiO2 powder, and the other five spectra are for the TiO2 powder samples irradiated with 6.5 MeV electrons, respectively at fluences 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2. In addition, it can be observed in Fig. 3 that (i) the peak position of each spectrum has shifted to higher wavelength, (ii) the peak width has increased, and (iii) the intensity of absorption over 450–800 nm range relatively increased, with the increase in the electron fluence. These results indicate that on electron irradiation new energy levels, related to the induced defects, have been introduced in the band gap of TiO2. As a result, ultra-band transitions could occur on absorption of higher wave length radiation. Moreover, the changes in the intensity and shifting of the peak position of the absorption spectrum towards higher wavelength can be attributed to the (i) formation of the brookite phase of TiO2, (ii) introduction of new energy levels related to induced defects and (iii) decrease in the anatase and rutile phases in the electron irradiated TiO2. The energy levels corresponding to brookite phase and induced defects can occupy positions below the conduction band energy level of the anatase phase. 3.4. Photocatalytic activity of TiO2 Fig. 4(a) shows the absorption spectra recorded from the nine methylene blue aqueous solutions prepared by mixing with the as-synthesized TiO2 powder, and exposed to UV radiations for different periods, varying from 15 to 120 min, in a step of 15 min. Similarly, the absorption spectra recorded from the other nine methylene blue solutions prepared by mixing TiO2 powder irradiated with 6.5 MeV electrons at fluence 2.5 1015 e cm2, and exposed to UV radiations for different periods, varying from 15 to 120 min are shown in Fig. 4(b). A comparison of the absorption spectra in Fig. 4(a) with those in Fig. 4(b), clearly show that the rate of degradation of methylene blue with the period of UV exposure is higher in the solution prepared by mixing with the electron irradiated TiO2 powder, as compared to the solution prepared by mixing with the as-synthesized TiO2 powder.
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Fig. 5 shows six plots, each showing variations in the degradation D (%) of the methylene blue solutions mixed with TiO2 powder with the period of UV exposure, varied from 15 to 120 min in a step of 15 min. In Fig. 5, out of these six plots, (i) one plot is for the methylene blue solution prepared by mixing with the as-prepared TiO2 powder, and (ii) other five plots are for the five different methylene solutions prepared by mixing TiO2 powder irradiated with 6.5 MeV electrons, respectively at fluences, 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2. The percentage degradation, D (%), for the methylene blue in the aqueous solution was obtained from expression Eq. (1). The peak intensity of the absorption spectra recorded from the methylene blue solution (i) but not exposed to UV radiation is denoted by C0, and (ii) of the same solution exposure to UV radiation for a period of t minutes is denoted by Ct. From the plots in Fig. 5, it is observed that for a given period of UV exposure, the percentage degradation, D (%), of the methylene blue in the solution initially increases with the electron fluences: 1.5 1015, 2.0 1015, 2.5 1015, and 3.0 1015 e cm2, used for irradiating the TiO2 powder samples. However, the D (%) of the methylene blue decreased substantially, when the solution was mixed with the TiO2 powder sample, irradiated with electrons at fluence 3.5 1015 e cm2. This decrease may be attributed to the formation of a large number of defects in TiO2 nanoparticles, and conversion of a fraction of the crystalline TiO2 into highly disordered phase. 4. Discussion The XRD spectra in Fig. 1 reveal that the width of some of the XRD peaks had increased gradually with the increase in the electron fluence. Similarly, from the SEM images in Fig. 2, it is observed that even though the TiO2 nanoparticles of different sizes exist in the as-synthesized and electron irradiated TiO2 samples, however, the average particle size is found to decrease with the increase in the electron fluence. These results of the present study show that with the increase in the electron fluence, the average particle size had decreased gradually from 80 nm for the unirradiated TiO2 nanoparticles to around 30 nm for the electron irradiated TiO2 nanoparticles. However, the particle sizes estimated from Eq. (2), based on Williamson–Hall analysis [18,19], show that the contributions of the strain is marginal in broadening the XRD peak of the electron irradiated TiO2 nanoparticles. Furthermore, from the XRD spectra shown in Fig. 1, it can be observed that the rate of decrease in the peak intensity with the electron fluence is higher for the rutile h1 1 0i XRD peak as compared to other TiO2 XRD peaks. These results indicate that during electron irradiation, a number of bridging oxygen atoms of the rutile h1 1 0i plane must have been displaced from the lattice positions, leading to formation of oxygen vacancies [20]. Similarly, in the recorded EDAX spectra of the electron irradiated TiO2 samples, a decrease in the O to Ti ratio with the increase in the electron fluence was observed. These results indicated that a number of oxygen atoms were displaced from the TiO2 during electron irradiation, and therefore possibly led to formation of the oxygen related vacancies. The excess electrons, produced due to the displacement of oxygen atoms, can interact with the oxygen atoms or molecules (O, O2) if present near the vacancies, and contribute in the process of formation of the negatively charged oxygen species, such as O, O 2 and O3 on the TiO2 surfaces, under UV irradiation. In this process, positively charged bridging oxygen vacancies (BOV+) can also be produced [21]. The oxygen vacancies and other induced defects can act as the electron capture centers, and therefore can effectively reduce the rate of recombination of the photogenerated electrons and holes. The oxygen vacancies in the surface region of TiO2 can therefore
contribute significantly in enhancing the photocatalytic activity of TiO2 under UV irradiation [22]. In addition, the vacancies produced in the TiO2 crystallites make the surface unbalanced in the charge neutrality. As a result, in the region of the positively charged vacancies, the OH species can be absorbed on the TiO2 surface, which in turn interact with the photogerated holes, and further reduce the electron–hole recombination rate. All these processes, therefore, in turn contribute toward enhancing the photocatalytic activity of the electron irradiated TiO2 nanoparticles, under the exposure of UV–Vis radiation. In addition, it is observed in Fig. 1 that the XRD peak of the brookite h1 2 1i phase has appeared in the XRD spectra of the TiO2 powder irradiated with electrons at three fluences: 1.5 1015, 2.0 1015, 2.5 1015, and 3.0 1015 e cm2. Moreover, the width of this XRD peak indicates that the brookite particles are of nanosizes. These XRD results reveal that the brookite phase was produced in the electron irradiated TiO2 powder, and therefore, could have led to formation of the mixed anatase–brookite phase. In the hydrothermal reaction [6], the anatase phase can be transformed into the brookite phase by adjusting the process parameters, such as chemical composition, solution pH value, temperature, etc. The earlier studies on the formation of the brookite phase have shown that the anatase h1 1 2i twin interfaces consist of two slabs of octahedral arranged in zigzag chains, with a unit cell of brookite [6,23]. In this manner, a number of such twin surfaces of anatase can serve as seeds for the growth of brookite phase, at elevated temperature. In the present work, it appears that the rise in temperature of the anatase TiO2 during electron irradiation must have initiated the process of transformation of a fraction of the anatase into the brookite phase. The formation of the brookite phase must have contributed in enhancing the photocatalytic activity of TiO2 nanoparticles. The density of states (DOS), and hence the number of electrons near the Fermi level in the brookite is higher as compared to that in the anatase and rutile phases [24]. The higher photocatalytic activity of the TiO2 nanoparticles, with anatase–brookite phases can be attributed to the relatively large number of electrons near the Fermi energy, which in turn contribute effectively in separating the photogenerated electrons from the holes [24]. Moreover, the reported band gap energy of the nanocrystalline brookite phase is higher than the bulk value, and can vary from 3.14 to 3.4 eV [25]. However, at an electron fluence of 3.5 1015 e cm2, it appears that the brookite phase was converted into highly disordered phase, because the brookite XRD peak intensity decreased substantially. In Fig. 3, the UV–Vis spectra indicate that in the respective absorption spectrum, (a) the peak position has shifted from around 310–360 nm, (b) the peak intensity decreased slowly, followed by a gradual increase in the peak width, and (c) the absorption over 400–800 nm has increased gradually, with the increase in the electron fluence used for irradiating the samples of TiO2 nanoparticles. Fig. 4(a and b) shows that the photocatalytic activity of the electron irradiated TiO2 is much higher as compared to that of the as-synthesized TiO2 nanoparticles. As observed in Fig. 5 the photocatalytic activity of the TiO2 powder irradiated with electrons at a fluence 3.0 1015 e cm2 was found to be maximum, but appreciably decreased at an electron fluence of 3.5 1015 e cm2. This observed decrease in the photocatalytic activity maybe due to (i) the increase in the defect density in the TiO2 nanoparticles above a critical value, and hence enhancement in the electron–hole recombination rate, and (ii) conversion of TiO2 crystalline phase in to disordered phase. The photocatalytic activity of the electron irradiated TiO2 nanoparticles gradually increased with electron fluence up to 3.0 1015 e cm2, even though in the corresponding TiO2 samples the anatase phase was reducing. This may be due to the formation of the (a) anatase–brookite phases, and (b) oxygen
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related vacancies and defects in the electron irradiated TiO2 nanoparticles. However, in this process, the overall contribution of the oxygen vacancies and the other defects must be large, as compared to that of the brookite phase. Acknowledgments One of the authors K.B.S. is grateful to the authorities of the D.E. Society, Pune and the Principal Fergusson College, Pune for sanctioning study leave for the Ph.D. work, and also to the U.G.C. New Delhi for the award of the Teacher Fellowship under the F.I.P., sanctioned to the college. The corresponding author V.N.B. is thankful to the C.S.I.R., New Delhi, for providing financial support under the scheme of Emeritus-Scientist. References [1] A. Hagfeldt, M. Gratzel, Acc. Chem. Res. 33 (2000) 269–277. [2] S. Ardizzone, C.L. Bianchi, G. Cappelletti, S. Gialanella, C. Pirola, V. Ragaini, J. Phys. Chem. C 111 (2007) 13222 (10pp). [3] H. Zhang, D. Chen, X. Lv, Y. Wang, H. Chang, J. Li, J. Environ. Sci. Technol. 44 (2010) 1107–1111. [4] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [5] A. Linsebigler, L. Gand, J. Yates, Chem. Rev. 95 (1995) 735–758. [6] X. Meng, J.H. Lee, M.H. Park, S.M. Yu, D.W. Shin, C. Yang, V.N. Bhoraskar, J.B. Yoo, Cryst. Eng. Commun. 13 (2011) 3983–3987. [7] A. Zaban, S.T. Aruna, B.A. Tirosh, B.A. Gregg, Y.J. Master, J. Phys. Chem. B 104 (2000) 4130–4133.
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[8] A. Navrotsk, Geochem. Trans. 4 (2003) 34–37. [9] D.R. Coronado, G.R. Gattorno, M.E. Pesqueria, C. Cab, R.D. Coss, G. Oskam, Nanotechnology 19 (2008) 45605–45614. [10] D.C. Hurum, A.G. Agrios, K.A. Gray, T. Rajli, M.C. Thurnaur, J. Phys. Chem. B 107 (2003) 4545–4549. [11] T. Miyagi, M. Kamei, T. Mitsuhashi, T. Ishigaki, A. Yamazaki, Chem. Phys. Lett. 390 (2004) 399–402. [12] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 6–11. [13] J. Yu, J.C. Yu, M. Leung, W. Cheng, B. Ho, X. Zhao, J. Zhao, J. Catal. 217 (2003) 69–78. [14] S.U. Bitao, W. Ke, B. Jie, M. Hongmei, T. Yongchun, M. Shixiong, S. Shixiong, L. Ziqiang, Front. Chem. China 2 (2007) 364–368. [15] H.K. Jun, H.N. Byung, D.L. Gun, S.H. Seong, Korean J. Chem. Eng. 22 (2005) 370–374. [16] X.G. Hou, A.D. Liu, Radiat. Phys. Chem. 77 (2008) 345–351. [17] K.D. Kim, W.S. Tai, Y.D. Kim, B.C. Lee, K.H. Yang, K. Park, Radiat. Phys. Chem. 79 (2010). 696-70. [18] S.B. Qadri, J.P. Yang, E.F. Skelton, B.R. Ratna, Appl. Phys. Lett. 70 (1997) 1020–1021. [19] M.L. Breen, A.D. Dinsmore, R.H. Pink, S.B. Qadri, B.R. Ratna, Langmuir 17 (2001) 903–907. [20] P. Kruger, S. Bourgeois, B. Domenichini, H. Magnan, D. Chandesris, P. LeFevre, A.M. Flank, J. Jupille, L. Floreano, A. Cossaro, A. Verdini, A. Morgante, Phys. Rev. Lett. 100 (2008) 055501–55504. [21] S.G. Wang, D. Wen, D.B. Cao, Y.W. Li, J. Wang, H. Jiao, Surf. Sci. 577 (2005) 69–76. [22] H. Einaga, A. Ogata, S. Futamura, T. Ibusuki, Chem. Phys. Lett. 338 (2001) 303–307. [23] R.L. Penn, J.F. Banfield, Am. Mineral. 84 (1999) 871–876. [24] J.Y. Park, C. Lee, K.W. Jung, D. Jung, Bull. Korean Chem. Soc. 30 (2009) 402–404. [25] A.D. Paola, G. Cutalo, M. Addamo, M. Bellardita, R. Campostrini, M. Ischia, R. Ceccato, L. Palmisamo, Colloids Surf. A 317 (2008) 366–376.
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Electron-irradiation induced changes in the phases and photocatalytic activity of TiO2 nanoparticles K.B. Sapnar, S.D. Dhole, V.N. Bhoraskar ⇑ Department of Physics, University of Pune, Pune 411007, India
a r t i c l e
i n f o
Article history: Received 12 September 2011 Received in revised form 30 December 2011 Available online 18 January 2012 Keywords: Electron irradiation TiO2 nanoparticles Photocatalytic activity Brookite Electron–hole pairs
a b s t r a c t Samples of TiO2 nanoparticles, with mixed anatase and rutile phases, were irradiated with 6.5 MeV electrons at fluences, 1.5, 2.0, 2.5, 3.0, and 3.5 1015 e cm2 and characterized by several methods. With increasing electron fluence, a continuous decrease in the average particle size from 80 nm to around 30 nm were observed along with a decrease in the rutile and the anatase phases of TiO2, but at different rates, and growth of the TiO2 brookite phase at slow rate. The photocatalytic activities of different electron irradiated TiO2 samples, in the photodegradation of methylene blue, were studied by recording UV– Vis absorption spectra of the respective solutions. On electron irradiation, even though the rutile phase in the TiO2 was decreasing, the photocatalytic activity of the nanoparticles increased continuously with fluence up to 3.0 1015 e cm2, but decreased at 3.5 1015 e cm2. The energy levels introduced by the brookite phase and the electron induced defects in TiO2 could have effectively reduced the electron–hole recombination rate in the absence of the rutile phase. The observed enhancement in the photocatalytic activity of the irradiated TiO2 is attributed to the formation of small size particles, the introduction of the oxygen related vacancies and other defects, the growth of the brookite phase, and increased absorption of radiation over the ultraviolet and visible range. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Different types of semiconductor photocatalysts are widely used in the degradation of organic compounds, in both gas phase and aqueous systems, in addition to the field of solar energy storage [1–3]. Among different photocatalysts, zinc oxide (ZnO) and titanium dioxide (TiO2) are commonly used for the photodegradation of pollutants, due to their high efficiency, low cost, and band energies in the UV–Vis range. TiO2 nanoparticles are also used in a number of applications such as sensor devices, paints, dyesensitized solar cells [4,5], etc. TiO2 exists in three phases, namely anatase, brookite and rutile. For different applications, nanoscale brookite film and rutile rods have been synthesized by a hydrothermal process [6]. Anatase particles, synthesized by a chemical process, are normally smaller (< 30 nm). By increasing the processing period to obtain relatively larger sizes, the anatase phase can be transformed to brookite and rutile phases [7,8]. In the hydrothermal process, all the three phases of TiO2 anatase, rutile and brookite can be synthesized by adjusting the process parameters [9]. The crystal structure of TiO2 has three polymeric forms, in which anatase and rutile are tetragonal, whereas the brookite phase is orthorhombic. Even though there is disagreement over the direct and ⇑ Corresponding author. Tel.: +91 2025699072; fax: +91 2025691684. E-mail address: [email protected] (V.N. Bhoraskar). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2012.01.012
indirect band gap energies of the TiO2 brookite phase, the measured band gap energies of the anatase, brookite and rutile phases are, respectively 3.21, 3.13 and 3.00 eV [8]. The photodegradation of a chemical compound using a semiconductor photocatalyst is carried out under exposure to UV–Vis radiation. In this process, the electrons of the semiconductor are excited from the valence band to the conduction band. As a result positive holes in the valence band and electron in the conduction band exist during exposure. The efficiency of the photocatalyst is therefore governed by the rate of electron–hole recombination. Attempts have therefore been made to enhance the photocatalytic activity of TiO2 by reducing the rate of the electron–hole recombination. Among different methods, metallic doping in TiO2 is found effective in enhancing the photocatalytic activity. In the un-doped TiO2 compound, the relative efficiency for the photodegradation, is higher for the anatase phase as compared to the rutile phase, mainly attributed to the lower rate of electron–hole recombination, and higher degree of surface absorption for certain chemical species [10]. Earlier, the photocatalytic activity of the anatase phase of TiO2 was found higher as compared to that of the brookite and rutile phases [11,12]. However, later on it was observed that the mixed phases of TiO2, such as anatase–rutile, anatase–brookite, can exhibit relatively higher degree of photocatalytic activity, mainly due to the lower recombination rate of the photo-degenerated electron–hole pairs [11–14]. The observed higher degree of
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photocatalytic efficiency has been attributed due to the process of the (i) transfer of the electrons from anatase to a lower energy electron trapping sites in rutile, and (ii) transfer of the electrons produced in the conduction band of rutile to the electron trapping sites of anatase, under exposure of UV–Vis radiation [10]. These processes enhance the rate of separation of electrons from the holes, and therefore decrease the electron–hole recombination rate. Moreover, even though there is no general agreement on the effect of the particle size on the photocatalytic efficiency of these three polymorphic forms of TiO2 (anatase, brookite and rutile) [14], a number of workers have observed experimentally that the photocatalytic activity increases with a decrease in particle size [15]. This is supported by the fact that the surface area of the particles increases with decreasing size and therefore the total area illuminated by UV–Vis radiation is much higher for the smaller size particles. A few workers [16,17] have studied effects of electron irradiation on TiO2, mostly in the form of thin films. However, studies on the irradiation effects of 6.5 MeV electrons on the phases of TiO2 nanoparticles, and the related photocatalytic activity have not been reported. In the present work, samples of TiO2 powder, consisting of mixed anatase and rutile phases, were irradiated with 6.5 MeV electrons at different fluences. Variations in the anatase and rutile phases, and the growth of the brookite phase with electron fluence were studied. Moreover, in the degradation of methylene blue, the observed changes in the photocatalytic activity of TiO2 nanoparticles with electron fluences were investigated. The results of the present study show that the TiO2 nanoparticles, with mixed anatase, and brookite phases, have a relatively higher photocatalytic activity as compared to virgin TiO2 powder, consisting of anatase and rutile phases. In addition, the reduced crystallite size and surface defects also play an important role in enhancing the photocatalytic efficiency of TiO2. 2. Materials and methods 2.1. Synthesis of TiO2 nanoparticles TiO2 nanoparticles were synthesized in the laboratory using the sol–gel method. The solution was prepared by mixing titanium isopropoxide 10 ml, with ethanol 40 ml, and was stirred for about 30 min, at room temperature. This solution was then added drop wise to another solution, prepared by mixing 10 ml deionised water, 10 ml isopropoxide, and 5 ml of HNO3. A yellowish color gel was formed after stirring this solution mixture for about 1 h. After annealing the gel at 120 °C for 2 h, yellow color powder, containing amorphous TiO2 particles was obtained. The TiO2 powder was then annealed in steps, over the temperature range 250–600 °C, for different periods in the range 2–3 h, and then cooled to room temperature under atmospheric conditions. The results of XRD analysis revealed that the annealed powder consisted of TiO2 nanoparticles, with mixed anatase and rutile phases. These laboratory synthesized TiO2 nanoparticles were used in the present study.
Initially, a Faraday cup was positioned at a distance of 120 mm from the electron-beam extraction port of the Race-Track Microtron. For measuring the electron fluence, the Faraday-cup was connected to an electron-current integrator system. After mounting the first TiO2 sample on the Faraday cup, the electron accelerator, Microtron was switched on. The extracted 6.5 MeV electrons were made to be incident on the TiO2 sample, mounted on the Faraday cup. In order to irradiate the nanoparticles-powder of the sample with electrons uniformly, the size of the electron beam was made large enough to cover the entire TiO2 sample. The number of electrons falling on the TiO2 sample was continuously measured by the current integrator. The electron beam was turned off when the TiO2 sample received an electron fluence 1.5 1015 e cm2. Following the same experimental procedure, the second, the third, the forth, and the fifth samples of the TiO2 powder were irradiated with 6.5 MeV electrons, respectively at fluences 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2. The sixth TiO2 sample was not irradiated, and kept as the as-synthesized reference sample. The as-synthesized and the electron irradiated TiO2 samples were characterized by the (i) scanning electron microscopy (SEM), (ii) EDAX, (iii) X-ray Diffraction (XRD) and (iv) UV–Vis spectroscopy methods. For each of the TiO2 powder sample, the XRD spectrum was recorded using Cu Ka radiation (k = 1.5418 Å), at a scan rate of 1–20/min with 0.10 resolution. In each recorded spectrum, the positions and the relative intensities of the XRD peaks were obtained after comparing with the standard data (JCPDS card No. 36-1451). The XRD spectra recorded from one sample of the assynthesized, and five samples of the electron irradiated TiO2 nanoparticles are shown in Fig. 1. Similarly, the SEM images for these samples are shown in Fig. 2. The UV–Vis spectrum of each of these six TiO2 samples was recorded in the absorption mode, over 200–800 nm range, using a SHIMADZU make UV–Vis spectrophotometer. These UV–Vis absorption spectra are shown in Fig. 3. The EDAX spectra of one as-synthesized and a few electron irradiated TiO2 samples were recorded for observing the variations in the O/Ti ratio. After characterization, all these six TiO2 samples were used in the study of the photodegradation of methylene blue in aqueous solution.
2.2. Electron irradiation and characterization of TiO2 nanoparticles The electron beam of 6.5 MeV energy was obtained from the Race-Track Microtron of the Department of Physics, University of Pune, Pune (India). For the irradiation experiment, each sample was made by packing 60 mg powder of TiO2 in a polyethylene bag. By folding the bag, the size of the sample was made close to 4 10 10 mm. In this manner, such six samples of the TiO2 powder were made, numbered, and kept ready for the experiment.
Fig. 1. XRD spectra (a) of the as-synthesized TiO2 powder sample, and (b–f) of the five TiO2 powder samples irradiated with 6.5 MeV electrons at fluences: (b) 1.5 1015 e cm2, (c) 2.0 1015 e cm2, (d) 2.5 1015 e cm2, (e) 3.0 1015 e cm2, and (f) 3.5 1015 e cm2.
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Fig. 2. SEM images (a) of the as-synthesized TiO2 powder sample and (b–e) of the four TiO2 powder samples irradiated with 6.5 MeV electrons at different fluences: (b) 2.0 1015 e cm2, (c) 2.5 1015 e cm2, (d) 3.0 1015 e cm2, and (e) 3.5 1015 e cm2. The SEM image in (f) is of a typical agglomerated particle in (d), recorded at 0.2 lm scale.
2.3. Photocatalytic activity of the electron-irradiated TiO2 nanoparticles
Fig. 3. UV–Vis absorption spectra, over 200–800 nm range, recorded from (i) one as-synthesized TiO2 powder sample (ii) other five TiO2 powder samples irradiated with 6.5 MeV electrons, respectively at fluencies: 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2.
The photodegradation of methylene blue in aqueous solution was studied using both the as-synthesized and the electron irradiated TiO2 nanoparticle samples, as photocatalyst. In all the photodegradation experiments, a UV lamp, type Kr-85 OSRAM-400 W, was used as a source of UV radiation for exposing the methylene blue solution. In the first experiment, 5 mg of the as-synthesized TiO2 powder was added into a freshly prepared 45 ml of 105 molar methylene blue aqueous solution. This mixture-solution, after stirring for 30 min, was divided into nine equal parts by pouring 5 ml by volume of the solution in each of the nine numbers of small size quartz circular dishes. The solution of the first quart dish was exposed to UV radiation for 15 min. This UV exposed solution was then immediately characterized by the UV–Vis spectrophotometer (SHIMADZU) over the wavelength range 400–800 nm, in the absorption mode. The characteristic absorption peak of the methylene blue was obtained at 665 nm in the recorded spectrum. Similarly, under the identical experimental conditions, the solution of the second quartz dish was exposed to UV radiation for 30 min, and the absorption spectrum over the range 400–800 nm was recorded. Following the same experimental procedures, the solutions of the remaining six quartz dishes were also exposed to UV radiation sequentially.
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Fig. 5. Variations in the degradation D (%) with the period of UV exposure, for the six different methylene blue solutions prepared separately by mixing with six types of TiO2 powder samples: (i) one solution mixed with as-synthesized TiO2 powder, and (ii) other five solutions were prepared by mixing TiO2 powder irradiated with 6.5 MeV electrons, respectively at fluences: 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2.
Fig. 4. UV–Vis absorption spectra for the two sets of methylene blue solutions: (a) set of solutions was prepared by mixing with the as-synthesized TiO2 powder, and (b) set of solutions prepared by mixing with the TiO2 powder sample irradiated with 6.5 MeV electrons at a fluence of 2.5 1015 e cm2 and exposed separately to UV radiations for different periods, varying from 15 to 120 min.
However, the UV exposure period was increased in a step of 15 min from one solution sample to the other solution sample. In this manner, the UV exposure period for the solution of the first quartz dish and that of the eighth quartz dish was, respectively 15 and 120 min. The UV–Vis absorption spectrum of the ninth solution was recorded without exposing it to UV radiation. In this manner, all the nine solutions were subjected to UV–Vis spectroscopic measurements, over 400–800 nm wavelength range. In the second experiment, 5 mg powder of the TiO2, already irradiated with 6.5 MeV electrons at a fluence 1.5 1015 e cm2 was added into a freshly prepared 45 ml of 105 molar methylene blue aqueous solution. This mixture-solution after stirring for 30 min was divided into nine equal parts, by pouring 5 ml by volume of the solution in each of the nine quartz-circular dishes. Following the same experimental procedure as that adopted for the as-synthesized TiO2 powder, the solutions of the eight quartz dishes were exposed to UV radiations sequentially. In this experiment also, the UV exposure period was varied from 15 to
120 min, in a step of 15 min, from the first solution to the eighth solution. The solution of the ninth quartz container was not exposed to UV radiation. All these eight UV irradiated solutions, and the ninth solution TiO2, which was not exposed to UV radiation, were subjected to UV–Vis spectroscopic measurements, over 400–800-nm wavelength range, in the absorption mode. Following the same experimental procedure, the third, the fourth, the fifth, and the sixth experiments were carried out using the TiO2 powder samples irradiated with electrons at fluences, 2 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2, respectively. In this manner, the photodegradation of methylene blue in aqueous solution was studied by using one sample of the as-synthesized TiO2 nanoparticles, and five samples of TiO2 nanoparticles irradiated with 6.5 MeV electrons at different fluences. The changes in the absorption spectra with the UV exposure period, varied from 15 to 120 min, are shown (i) in Fig. 4(a) for the methylene blue solutions prepared by mixing as-synthesized TiO2 nanoparticles, and (ii) in Fig. 4(b) for the methylene solutions prepared by mixing TiO2 nanoparticles irradiated with 6.5 MeV electrons at a fluence of 2.5 1015 e cm2. In the recorded UV–Vis absorption spectra of the methylene blue aqueous solution, mixed with the as-synthesized or electron irradiated TiO2 nanoparticles, the intensity of the peaks before and after exposure to UV radiation are denoted by C0 and Ct, respectively. For different methylene blue aqueous solutions, mixed with the as-synthesized or the electron irradiated TiO2 powder, the percentage degradation D (%) is defined as follows:
D ð%Þ ¼
C0 Ct 100 C0
ð1Þ
The variations in the D (%) with the period of UV exposure for these six different methylene blue solutions are shown in Fig. 5. 3. Results 3.1. XRD spectra The crystal structure of TiO2 has three polymeric forms, in which anatase and rutile are of tetragonal and brookite is orthorhombic. Fig. 1 shows the recorded XRD spectra, in which the
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spectrum (a) is for the as-synthesized TiO2 powder, where as the XRD spectra (b–f) are of theTiO2 powder samples irradiated with 6.5 MeV electrons, respectively at fluences 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2. From the spectrum (a) in Fig. 1, it is observed that the main crystalline peaks of anatase h1 0 1i, and rutile h1 1 0i have appeared at 25.28°(2h) and 27.44°(2h), respectively. These results show that the as-synthesized TiO2 had crystalline structure, consisting of anatase and rutile phases, but without the brookite phase. In addition, the intensity of the XRD peak corresponding to the anatase h1 0 1i plane is relatively higher as compared to that of the rutile h1 1 0i plane. These results reveal that, in the as-synthesized TiO2 powder, the concentration of the anatase phase was higher as compared to that of the rutile phase. The irradiation effects of electrons on the TiO2 powder is clearly observed in the XRD spectra shown in Fig. 1. An analysis of the XRD spectra in Fig. 1 reveal that the width of each XRD peak corresponding to the anatase and rutile phases increased, however, the respective XRD peak height decreased with the increase in the electron fluence. Moreover, the rate of decrease in the peak height with the electron fluence was relatively higher for the rutile phase as compared to that of the anatase phase. In addition, the formation of the brookite phase in the electron irradiated TiO2 is revealed by the appearance of the XRD peak corresponding to the brookite h1 2 1i plane at 30.8°(2h) in each XRD spectrum (b–e), shown in Fig. 1. However, the intensity of the brookite peak was small, and increased marginally with the increase in the electron fluence from 1.5 1015 to 3.0 1015 e cm2. Moreover, both the brookite and the rutile phases appear to be converted to distorted phases at 3.5 1015 e cm2 electron fluence. From the XRD spectra in Fig. 1, the estimated average size of the particles is around 80 nm for the as-synthesized TiO2 powder and around 30 nm for the TiO2 powder irradiated with electrons at a fluence 2.5 1015 e cm2. The average particle size was estimated from the FWHM of the corresponding XRD peak, and using the Scherer’s formula. In addition to the crystallite size, the broadening of the XRD also depends on several other factors, such as (i) the instrumental broadening, (ii) the internal strain in the crystal, (iii) the temperature variations, and (iv) the nature of the sample. The instrumental broadening puts a limit on the estimation of the particle size on the higher side. In the present work, the XRD diffractometer used for recording the XRD spectra had the instrumental broadening limit 100 nm. All the XRD spectra were recorded at room temperature (24 °C), and therefore, the contributions due to the temperature variations in the peak broadening are negligible. The effects of internal strain can be qualified based on the Williamson–Hall analysis [18,19], in which the contributions from the crystallite size and the lattice strain can be related with the FWHM of the recorded XRD peak, through the following Eq. (2):
bcosh 1 gsinh ¼ þ k k e
ð2Þ
where h is the Bragg angle of the diffraction peak, e is the effective crystallite size, b is the measured FWHM of the XRD peak in radian, k is the X-ray wavelength, and g is the effective strain [18,19]. A plot of bcosh/k versus sinh/k can be used to estimate the crystallite size, considering the effect of strain. From the Scherer’s formula, the estimated average sizes of TiO2 particles were around (i) 80 nm for the unirradiated TiO2 nanoparticles, and (ii) 30 nm for the electron irradiated TiO2 nanoparticles. However, from the plots using equation Eq. (2), the estimated particle sizes were (i) 74 nm for the unirradiated TiO2 nanoparticles, and (ii) 28 nm for the electron irradiated TiO2 nanoparticles.
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3.2. Surface morphology The surface morphology of the as-synthesized and the electron irradiated TiO2 powder samples was studied by scanning electron microscopy. The SEM image recorded from one as-synthesized TiO2 powder is shown in Fig. 2(a). Similarly, the SEM images recorded from the TiO2 powder samples irradiated with 6.5 MeV electrons at 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2 fluences are, respectively shown in Fig. 2(b–e). It can be observed from the SEM images in Fig. 2 that due to the process of the particle agglomeration, the particle size distribution is wide, and a number of large size particles exist in all the electron irradiated TiO2 samples along with nanoparticles. A typical SEM image of an agglomerated particle in Fig. 2(d), recorded at 0.2 lm scale is shown in Fig. 2(f). The EDAX spectra of the as-synthesized and the electron irradiated TiO2 samples were recorded, to understand the variations in the O/Ti ratio with the electron fluence. However, these EDAX spectra are not shown in this paper. 3.3. UV–Vis absorption spectra The UV–Vis absorption spectra over the wavelength 200– 800 nm range recorded from the as-synthesized and the electron irradiated TiO2 powder samples are shown in Fig. 3. In these UV– Vis spectra, each spectrum is marked by a symbol, indicating the electron fluence used for irradiating the TiO2 powder sample. In this manner, out of these six plots, one spectrum is for the as-synthesized TiO2 powder, and the other five spectra are for the TiO2 powder samples irradiated with 6.5 MeV electrons, respectively at fluences 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2. In addition, it can be observed in Fig. 3 that (i) the peak position of each spectrum has shifted to higher wavelength, (ii) the peak width has increased, and (iii) the intensity of absorption over 450–800 nm range relatively increased, with the increase in the electron fluence. These results indicate that on electron irradiation new energy levels, related to the induced defects, have been introduced in the band gap of TiO2. As a result, ultra-band transitions could occur on absorption of higher wave length radiation. Moreover, the changes in the intensity and shifting of the peak position of the absorption spectrum towards higher wavelength can be attributed to the (i) formation of the brookite phase of TiO2, (ii) introduction of new energy levels related to induced defects and (iii) decrease in the anatase and rutile phases in the electron irradiated TiO2. The energy levels corresponding to brookite phase and induced defects can occupy positions below the conduction band energy level of the anatase phase. 3.4. Photocatalytic activity of TiO2 Fig. 4(a) shows the absorption spectra recorded from the nine methylene blue aqueous solutions prepared by mixing with the as-synthesized TiO2 powder, and exposed to UV radiations for different periods, varying from 15 to 120 min, in a step of 15 min. Similarly, the absorption spectra recorded from the other nine methylene blue solutions prepared by mixing TiO2 powder irradiated with 6.5 MeV electrons at fluence 2.5 1015 e cm2, and exposed to UV radiations for different periods, varying from 15 to 120 min are shown in Fig. 4(b). A comparison of the absorption spectra in Fig. 4(a) with those in Fig. 4(b), clearly show that the rate of degradation of methylene blue with the period of UV exposure is higher in the solution prepared by mixing with the electron irradiated TiO2 powder, as compared to the solution prepared by mixing with the as-synthesized TiO2 powder.
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Fig. 5 shows six plots, each showing variations in the degradation D (%) of the methylene blue solutions mixed with TiO2 powder with the period of UV exposure, varied from 15 to 120 min in a step of 15 min. In Fig. 5, out of these six plots, (i) one plot is for the methylene blue solution prepared by mixing with the as-prepared TiO2 powder, and (ii) other five plots are for the five different methylene solutions prepared by mixing TiO2 powder irradiated with 6.5 MeV electrons, respectively at fluences, 1.5 1015, 2.0 1015, 2.5 1015, 3.0 1015, and 3.5 1015 e cm2. The percentage degradation, D (%), for the methylene blue in the aqueous solution was obtained from expression Eq. (1). The peak intensity of the absorption spectra recorded from the methylene blue solution (i) but not exposed to UV radiation is denoted by C0, and (ii) of the same solution exposure to UV radiation for a period of t minutes is denoted by Ct. From the plots in Fig. 5, it is observed that for a given period of UV exposure, the percentage degradation, D (%), of the methylene blue in the solution initially increases with the electron fluences: 1.5 1015, 2.0 1015, 2.5 1015, and 3.0 1015 e cm2, used for irradiating the TiO2 powder samples. However, the D (%) of the methylene blue decreased substantially, when the solution was mixed with the TiO2 powder sample, irradiated with electrons at fluence 3.5 1015 e cm2. This decrease may be attributed to the formation of a large number of defects in TiO2 nanoparticles, and conversion of a fraction of the crystalline TiO2 into highly disordered phase. 4. Discussion The XRD spectra in Fig. 1 reveal that the width of some of the XRD peaks had increased gradually with the increase in the electron fluence. Similarly, from the SEM images in Fig. 2, it is observed that even though the TiO2 nanoparticles of different sizes exist in the as-synthesized and electron irradiated TiO2 samples, however, the average particle size is found to decrease with the increase in the electron fluence. These results of the present study show that with the increase in the electron fluence, the average particle size had decreased gradually from 80 nm for the unirradiated TiO2 nanoparticles to around 30 nm for the electron irradiated TiO2 nanoparticles. However, the particle sizes estimated from Eq. (2), based on Williamson–Hall analysis [18,19], show that the contributions of the strain is marginal in broadening the XRD peak of the electron irradiated TiO2 nanoparticles. Furthermore, from the XRD spectra shown in Fig. 1, it can be observed that the rate of decrease in the peak intensity with the electron fluence is higher for the rutile h1 1 0i XRD peak as compared to other TiO2 XRD peaks. These results indicate that during electron irradiation, a number of bridging oxygen atoms of the rutile h1 1 0i plane must have been displaced from the lattice positions, leading to formation of oxygen vacancies [20]. Similarly, in the recorded EDAX spectra of the electron irradiated TiO2 samples, a decrease in the O to Ti ratio with the increase in the electron fluence was observed. These results indicated that a number of oxygen atoms were displaced from the TiO2 during electron irradiation, and therefore possibly led to formation of the oxygen related vacancies. The excess electrons, produced due to the displacement of oxygen atoms, can interact with the oxygen atoms or molecules (O, O2) if present near the vacancies, and contribute in the process of formation of the negatively charged oxygen species, such as O, O 2 and O3 on the TiO2 surfaces, under UV irradiation. In this process, positively charged bridging oxygen vacancies (BOV+) can also be produced [21]. The oxygen vacancies and other induced defects can act as the electron capture centers, and therefore can effectively reduce the rate of recombination of the photogenerated electrons and holes. The oxygen vacancies in the surface region of TiO2 can therefore
contribute significantly in enhancing the photocatalytic activity of TiO2 under UV irradiation [22]. In addition, the vacancies produced in the TiO2 crystallites make the surface unbalanced in the charge neutrality. As a result, in the region of the positively charged vacancies, the OH species can be absorbed on the TiO2 surface, which in turn interact with the photogerated holes, and further reduce the electron–hole recombination rate. All these processes, therefore, in turn contribute toward enhancing the photocatalytic activity of the electron irradiated TiO2 nanoparticles, under the exposure of UV–Vis radiation. In addition, it is observed in Fig. 1 that the XRD peak of the brookite h1 2 1i phase has appeared in the XRD spectra of the TiO2 powder irradiated with electrons at three fluences: 1.5 1015, 2.0 1015, 2.5 1015, and 3.0 1015 e cm2. Moreover, the width of this XRD peak indicates that the brookite particles are of nanosizes. These XRD results reveal that the brookite phase was produced in the electron irradiated TiO2 powder, and therefore, could have led to formation of the mixed anatase–brookite phase. In the hydrothermal reaction [6], the anatase phase can be transformed into the brookite phase by adjusting the process parameters, such as chemical composition, solution pH value, temperature, etc. The earlier studies on the formation of the brookite phase have shown that the anatase h1 1 2i twin interfaces consist of two slabs of octahedral arranged in zigzag chains, with a unit cell of brookite [6,23]. In this manner, a number of such twin surfaces of anatase can serve as seeds for the growth of brookite phase, at elevated temperature. In the present work, it appears that the rise in temperature of the anatase TiO2 during electron irradiation must have initiated the process of transformation of a fraction of the anatase into the brookite phase. The formation of the brookite phase must have contributed in enhancing the photocatalytic activity of TiO2 nanoparticles. The density of states (DOS), and hence the number of electrons near the Fermi level in the brookite is higher as compared to that in the anatase and rutile phases [24]. The higher photocatalytic activity of the TiO2 nanoparticles, with anatase–brookite phases can be attributed to the relatively large number of electrons near the Fermi energy, which in turn contribute effectively in separating the photogenerated electrons from the holes [24]. Moreover, the reported band gap energy of the nanocrystalline brookite phase is higher than the bulk value, and can vary from 3.14 to 3.4 eV [25]. However, at an electron fluence of 3.5 1015 e cm2, it appears that the brookite phase was converted into highly disordered phase, because the brookite XRD peak intensity decreased substantially. In Fig. 3, the UV–Vis spectra indicate that in the respective absorption spectrum, (a) the peak position has shifted from around 310–360 nm, (b) the peak intensity decreased slowly, followed by a gradual increase in the peak width, and (c) the absorption over 400–800 nm has increased gradually, with the increase in the electron fluence used for irradiating the samples of TiO2 nanoparticles. Fig. 4(a and b) shows that the photocatalytic activity of the electron irradiated TiO2 is much higher as compared to that of the as-synthesized TiO2 nanoparticles. As observed in Fig. 5 the photocatalytic activity of the TiO2 powder irradiated with electrons at a fluence 3.0 1015 e cm2 was found to be maximum, but appreciably decreased at an electron fluence of 3.5 1015 e cm2. This observed decrease in the photocatalytic activity maybe due to (i) the increase in the defect density in the TiO2 nanoparticles above a critical value, and hence enhancement in the electron–hole recombination rate, and (ii) conversion of TiO2 crystalline phase in to disordered phase. The photocatalytic activity of the electron irradiated TiO2 nanoparticles gradually increased with electron fluence up to 3.0 1015 e cm2, even though in the corresponding TiO2 samples the anatase phase was reducing. This may be due to the formation of the (a) anatase–brookite phases, and (b) oxygen
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related vacancies and defects in the electron irradiated TiO2 nanoparticles. However, in this process, the overall contribution of the oxygen vacancies and the other defects must be large, as compared to that of the brookite phase. Acknowledgments One of the authors K.B.S. is grateful to the authorities of the D.E. Society, Pune and the Principal Fergusson College, Pune for sanctioning study leave for the Ph.D. work, and also to the U.G.C. New Delhi for the award of the Teacher Fellowship under the F.I.P., sanctioned to the college. The corresponding author V.N.B. is thankful to the C.S.I.R., New Delhi, for providing financial support under the scheme of Emeritus-Scientist. References [1] A. Hagfeldt, M. Gratzel, Acc. Chem. Res. 33 (2000) 269–277. [2] S. Ardizzone, C.L. Bianchi, G. Cappelletti, S. Gialanella, C. Pirola, V. Ragaini, J. Phys. Chem. C 111 (2007) 13222 (10pp). [3] H. Zhang, D. Chen, X. Lv, Y. Wang, H. Chang, J. Li, J. Environ. Sci. Technol. 44 (2010) 1107–1111. [4] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [5] A. Linsebigler, L. Gand, J. Yates, Chem. Rev. 95 (1995) 735–758. [6] X. Meng, J.H. Lee, M.H. Park, S.M. Yu, D.W. Shin, C. Yang, V.N. Bhoraskar, J.B. Yoo, Cryst. Eng. Commun. 13 (2011) 3983–3987. [7] A. Zaban, S.T. Aruna, B.A. Tirosh, B.A. Gregg, Y.J. Master, J. Phys. Chem. B 104 (2000) 4130–4133.
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