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DOI : 10.2240/azojomo0298

Effect of K+ Doping on the Phase Transformation of TiO2 Nanoparticles

R.Vijayalakshmi and K. V. Rajendran

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AZojomo (ISSN 1833-122X) Volume 6 November 2010

Topics Covered

Abstract
Keywords
Introduction
Experimental Procedure
Results And Discussion
Effect Mechanism Of Doping K+ On TiO2 Phase Transformation
Conclusion
Acknowledgment
Reference
Contact Details

Abstract

Pure and K doped TiO2 nanoparticles were prepared by the sol-gel process. The effect of K+ doping on TiO2 anatase to rutile phase transformation was investigated. It is found that the K doping shifted the phase transformation and has a stabilization effect on the anatase grain growth. With a suitable amount (ca. 1&3mol %) the K dopant reduces anatase grain size and increases the specific surface area of TiO2 powder.

Keywords

Nanoparticles, Sol-Gel, Doping, Grain Size, Surface Area, Powder

Introduction

Semiconductor photocatalysis has been investigated extensively for light- stimulated degradation of pollutants, particularly for complete destruction of toxic and non-biodegradable compounds to carbon dioxide and inorganic constituents. Several semiconductors exhibit band-gap energies suitable for photocatalytic degradation of contaminants. Among the photo catalysts applied, TiO2 is one of the most widely employed photocatalytic semiconducting materials because of the peculiarities of chemical inertness, non-photo corrosion, low cost and non-toxicity. TiO2 has three kinds of different crystal structure; anatase, brookite and rutile. Among them, anatase and brookite are meta-stable phases and irreversibly transform to the thermally stable rutile phase upon heat treatment in the temperature range from 450 to 900°C [1], in which better crystallinity and larger crystallite size can be obtained at the same time. Many studies have clarified that anatase exhibited greater photocatalytic property than rutile because the highly hydroxylate surface responsible for photocatalytic reaction is readily formed in the anatase structure. Therefore, the stabilization of anatase phase becomes a subject of interest to be studied [2, 3].

Doping the impurity ion is known as one of the effective ways to manipulate the internal properties of TiO2 such as crystalline structure and crystallite size [4, 5]. Research of the effect of impurity doping on the phase transition has already begun during 1960s. The similarities between dopant and host ions; electron valence, ionic radius and chemical property have been reported to significantly effect on the ion interchanging during crystal nucleation process, in which the dopant ion is allowed to enter in the host lattice as either substitutional or interstitial site [6-10]. It is therefore interesting and necessary to examine the effect of metal additives on the TiO2 phase transformation and grain growth.

In our work, using the sol-gel method, we prepared pure and K+ doped TiO2 powders and characterized by means of X-ray Diffraction (XRD), Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Energy Dispersive Spectroscope (EDS) and Ultra Violet – Visible (UV-Vis) absorption and also investigated the effect of K+ doped on the TiO2 anatase to rutile phase transformation and anatase grain growth [11].

Experimental Procedure

The sol-gel synthesized TiO2 was obtained from Titanium (IV) isopropoxide (TTIP) was dissolved in absolute ethanol and distilled water was added to the solution in terms of a molar ratio of Ti: H2O=1:4. Nitric acid was used to adjust the pH and for restrain the hydrolysis process of the solution. The solution was vigorously stirred for 30 min in order to form Sols. After aging for 24 hrs, the Sols were transformed into gels. In order to obtain nanoparticles, the gels were dried under 120°C for 2 hr to evaporate water and organic material to the maximum extent. Sintering processes at the various temperatures from 450 – 750°C were subsequently carried out to obtain desired TiO2 crystalline.

The alkaline ion doped TiO2 nanoparticles were synthesized with the same method mentioned above, except for the addition of the corresponding alkaline dopant (KNO3) in ethanol. The doping concentration was varied for the mol fraction; 1& 3mol % and it was designated as K1 and K3 respectively. The crystalline phases of TiO2 were determined using X-ray diffractometer Schimadzu model: XRD 6000 with CuKa radiation in the range 20-80° (λ=0.154nm). UV-Vis absorption spectra were recorded on a Varian Cary 5E spectrophotometer at room temperature in the range between 200 to 1000nm.The microstructure and grain size were analysed through a Scanning Electron Microscope Hitachi S-4500 and Transmission Electron Microscope (TEM) using a model JEOL-2010 microscope.

Results And Discussion

The sol samples synthesized by sol-gel method are amorphous, and gradually transform to crystal state during the sintering process. In order to analyze the relative ratio in the anatase-rutile mixed phases, XRD is a very effective procedure because each of the components in the mixture gives their characteristic pattern, independently [12]. The powder XRD clarified that the anatase phase was stabilized and the transition to rutile phase was suppressed with increasing the doping concentrations. The specific surface area & particle size of pure, K1 and K3- TiO2 samples calcined at 450, 550, 650 and 750°C are summarized in Table 1. It was observed from the table that the surface areas are in the decreasing order of TiO2>K1>K3 when the samples are calcined at 450, 550, 650 and 750°C respectively. Dopant K+ may disperse on or compound with TiO2, which prevents TiO2 agglomeration and reduces the diminishing rates of surface area with increasing calcination temperature, rendering K+ doped TiO2 more porous than plain TiO2 [13].

Table1. The specific surface area and particle size of pure, K1 and K3- TiO2 nanoparticles

Samples Particle Size (nm) Specific Surface Area (m2/g)
450°C  550°C  650°C  750° C 450°C  550°C  650°C  750° C
Pure TiO2 14  20  28  - 94.4  66.1  50.65  -
1% K doped TiO2 10  15  19  25 132.2  88.1  69.55  52.8
3% K doped TiO2 7  13  17  22 188.8  101.66  77.7  60.1

Fig.1 (a, b and c) showed the XRD patterns of pure and K doped TiO2 nanoparticles calcined at different temperature. The material shows a high degree of crystallinity and existence of fully anatase phase at 450°C. We choose 450°C as calcination temperature, as this temperature was found to have highest activity among samples calcined at different temperature. It could be seen that the rutile began to appear for pure TiO2 samples when the calcining temperature was 550°C, and the anatase phase disappeared when calcining temperature was 650°C, while a little rutile phase appeared for K-doped TiO2 samples when the calcining temperature was 550°C, and even most was still anatase phase at the calcining temperature of 650°C. These demonstrated that K+ dopant could greatly inhibit the phase transformation from anatase to rutile, and enhance the beginning temperature of phase transformation obviously [14]. The crystal sizes of anatase and rutile phases increases with increasing calcination temperature, and those in K+ doped TiO2 are smaller than those of pure TiO2 since particle agglomeration is retarded by doping K+.

Figure 1. XRD patterns of the as-prepared samples (a) pure TiO2 (b) 1% K-doped TiO2 (c) 3% K-doped TiO2

The surface morphologies of the pure & K+ doped TiO2 sample were evaluated by SEM analysis which clearly indicated a significant change in crystalline growth that effectively leads the reduction of crystalline size, as shown in Fig. (2 a, b and c). In addition, more uniform and homogeneous distribution of nanoparticles was obtained by doping K+ ion into the TiO2 nanoparticles. Although it is clear that alkaline ions could successfully suppress the nanoparticle size as well as stabilize the anatase phase of TiO2.

Figure 2. SEM micrographs of pure and doped TiO2 calcined at 450°C (a) SEM of pure TiO2 (b) SEM of 1% K in TiO2 (c) SEM of 3% K in TiO2

Effect Mechanism Of Doping K+ On TiO2 Phase Transformation

In general, the ionic radius and calcining temperature are two of the most important conditions, which can strongly influence the ability of the dopant to enter into TiO2 crystal lattice to form stable solid solution. If the ionic radius of the dopant is much bigger or smaller than that of Ti4+, the dopant substituting for TiO2 crystal lattice ions must result into Crystal Lattice Distortion (CLD). Thus, certain amount of energies can be accumulated so that the substitution process can be suppressed [15]. It could be seen from Fig.1 that the XRD peaks of pure and K-doped TiO2 nanoparticles had the same positions mostly demonstrating that K+ did not enter into TiO2 crystal lattice to substitute for Ti4+. This was because the radius of K+ (1.51Å) was much bigger than that of Ti4+ (0.64Å). In addition, the phase about K+ element could not be found in Fig.1, possibly demonstrating that K+ was dispersed uniformly onto TiO2 nanoparticle. Thus, the chemical bonds of Ti-O-K three elements around the anatase crystallites could easily occur during the process of thermal treatment, which possibly inhibited producing and growing of the crystal nucleus of rutile.

For semiconductor nanoparticles, the quantum confinement effect is expected, and the absorption edge will be shifted to a higher energy when the particle size decreases. The K+ doped TiO2 exhibited an absorption edge at 345 and 364 nm which is blue shift considerably compared with the pure TiO2 (376 nm) (shown in Fig.3). The absorption edge of doped TiO2 is stronger than that of the pure TiO2. Figure reveals that absorption intensities and the threshold wavelength decrease in order of TiO2 >K1 > K3. Due to a larger particle, size for samples treated at 550 and 650°C the absorption edge appears red shift to some extent [16]. Considering the blue shift of the absorption positions from the bulk TiO2, the absorption onsets of the present samples can be assigned to the direct transition of electron in the TiO2 nanoparticles.

Figure 3. DRS patterns of pure and doped TiO2 calcined at 450°C

Fig.4a showed the TEM photographs of pure and 3mol% K doped TiO2 nanoparticles calcined at 450°C. It could be found that the pure and K doped TiO2 nanoparticles both appeared similar sphere, with the average particle size of about 7 and 14 nm respectively, demonstrating that K dopant could inhibit the increase of TiO2 particle size. In fig. 5 the Selected Area Electron Diffraction (SAED) pattern of K+ doped TiO2 at 450°C is shown. The first four rings are assigned to the (101), (004), (200), (005) reflections of the anatase phase. The Selected Area Electron Diffraction (SAED) studies are in good agreement with the XRD measurements. Energy dispersive Spectrum (EDS) displayed in Fig. 6 and table 2 furnish the composition of various elements in the prepared sample.

Figure 4. TEM photomicrographs of pure and doped TiO2 calcined at 450°C (i) TEM of pure TiO2 (ii) TEM of 3% K in TiO2

Figure 5. SAED pattern of the K-doped TiO2

Figure 6. EDS spectrum of the K-doped TiO2

Table 2. The composition of various elements presents in K3-doped TiO2 nanoparticles.

Element Wt% At%
O K 91.60 96.36
K K 01.54 01.23
TiK 06.86 02.41

Conclusion

Investigation in the present study has revealed that nanoparticles of TiO2 doped with K+ prepared using the sol-gel method shows a synergistic effect, which shifted the transformation anatase-rutile to higher temperature. According to the XRD analysis, the K+ did not enter the TiO2 crystal lattice to substitute for Ti4+. Indeed the radius of K+ (1.51Å) is much larger than that of Ti4+ (0.64Å). They were probably dispersed uniformly onto TiO2 nanoparticles. The particle size of pure TiO2 (14nm) is larger than that of K1 & K3 doped TiO2 nanoparticles (10nm & 7nm), revealing that the introduction of K can effectively prevent TiO2 from further growing up in the process of calcination. The K doped TiO2 nanoparticles showed a concomitant blue shift in the absorption spectrum with a decrease in the particle size. The absorption edge of the K doped TiO2 nanoparticles calculated to be 345 and 364nm are slightly smaller than the value of 376nm for the pure TiO2.

Acknowledgment

The authors are grateful to the University Grant Commission, for extending financial assistance to carry out this work.

Reference

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Contact Details

R.Vijayalakshmi and K. V. Rajendran
Department of Physics, Presidency College
Chennai, TamilNadu, India

E-mail : [email protected]

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 12[1] (2010) 25-30.

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