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

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

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

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.

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

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.