Influence of Surfactants on the Synthesis of TiO₂ Nanoparticles

R.Vijayalakshmi and K.V. Rajendran

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

Topics Covered

Abstract

TiO₂ nanocrystalline powders with an average diameter of 8-18nm have been successfully synthesized using different surfactants cetyltrimethyl ammonium bromide (CTAB), poly ethylene glycol (PEG) and sodium dodecyl sulphate (SDS) via sol-gel method. The as-synthesized TiO₂ nanopowders were characterized using X-ray diffraction (XRD), UV-absorption spectroscopy, Scanning electron microscope (SEM) and Transmission electron microscope (TEM). XRD pattern reveals that high crystalline anatase TiO₂ nanoparticles have been synthesized. The spherical morphology of TiO₂ nanoparticles were observed in SEM and TEM analysis. The UV absorption edges exhibited a blue shift, which can be ascribed to the quantum confinement effect. The probable growth mechanisms of TiO₂ are discussed.

Keywords

Nanoparticles, Grain Size, Surface Area, Powder

Introduction

Semiconductor nanoparticles have been extensively studied from both experimental and theoretical viewpoints, owing to their potential application in solar energy conservation, photo catalysis and in the field of optoelectronics [1-4]. Metal oxide nanoparticles play an important role in the selective surface modification of various substrates in the form of coating. Titanium dioxide (TiO₂) is a direct band gap n-type semiconductor (E_g =3.2 eV) and has been the most strategic material used in many applications such as photocatalysts, solar cells, transparent electrodes and ultraviolet laser diodes etc [5-7]. Recent studies have shown that many fundamental physical or chemical properties of semiconductor materials strongly depend on the size and morphology of the materials. Several physical and chemical synthetic methods are available for the fabrication of this material including sol-gel [8], CVD [9], micro emulsion [10], hydrothermal process [11] etc. Among these a single source sol-gel route is one of the most extensively used, better control over stoichiometric composition, easy route of synthesis, better homogeneity, production of high purity powder and cost effective method for the preparation of nanomaterials. In this study, we report the synthesis of TiO₂ nanoparticles and the influence of surfactants such as CTAB (cationic), PEG (non-ionic) and SDS (anionic) on the size, morphology and optical properties are discussed for the first time via sol-gel process.

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 above solution was stirred vigorously until a clear solution was obtained, then 0.01 mol of cetyltrimethyl ammonium bromide (CTAB) was added to the above solution. After stirring the sol was aged for 24 h, it was transformed into gels. In order to obtain nanoparticles, the gels were dried under 120°C for 2 h to evaporate water and organic material to the maximum extent. Then the dry gels were ground and sintered at 450°C to obtain desired TiO₂ nanocrystalline powders. The same procedure was followed for the preparation of TiO₂ nanoparticles using poly ethylene glycol (PEG) and sodium dodecyl sulphate (SDS).

The prepared samples were subjected to different characterization including X-ray diffraction (XRD), UV-Vis absorption spectroscopy (UV), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The crystalline structure of materials was analyzed by XRD (XPERT PRO with CuKα radiation λ=1.5406 Å) at scanning speed of 2°/min from 20° to 80°. The surface morphology was analyzed by SEM (JEOL, JSM-67001). TEM was carried out using a model JEOL-2010 microscope. The absorption spectra were carried out in the range of 200 - 800 nm by using SHIMDZU UV 310PC.

Results and Discussion

Fig.1. shows the XRD patterns of as synthesized TiO₂ at 450°C, they exhibit[s] varied intense peaks that are easily distinguishable. The peaks were indexed as (101), (004), (200), (105), (211), (204), (220) and are in good agreement with the reported values of TiO₂ (JCPDS 21-1272). No extra peaks were observed in the XRD pattern due to either titanium metal or surfactants, which imply the formation of pure and single-phase of titanium oxide. From the Fig.1 (a) (CTAB), it is noted that the peak intensity of the samples was notably higher than that of the intensities in Fig.1 (b) (PEG) and 1(c) (SDS) due to larger particle size. Using Debye Scherrer’s formula the crystallite sizes of TiO₂ nanoparticles can be determined [12]. The results obtained are tabulated in table1.

The absorption spectra of the TiO₂ obtained from different surfactant were shown in Fig.2. exhibits blue shift in the absorption band edge, which could be attributed to well known quantum size effect of semiconductor [13]. As the small size of nanoparticles, result in spatial confinement of the charge carrier wave function, this is termed as quantum size effect. The quantum size effects not only include blue shift of the absorption edge and exciton energy but it also covers the increase in exciton association strength and binding energy [14]. In addition, the blue shift of the absorption position from the bulk TiO₂, the absorption onsets of the present samples could be assigned to the direct transition of electron in TiO₂ nanocrystals. The absorption band edges observed at 310, 325 and 349 nm indicate a blue shift from that of the bulk TiO₂ (385 nm). A prominent blue shift occurred in the case of the CTAB samples (Fig. 2(a)) when compared to that of PEG and SDS (Fig. 2(b) and 2(c)).

Figure 1. XRD patterns of the as-prepared samples
a) CTAB TiO₂, b) PEG TiO₂, c) SDS TiO₂

Table 1.The size and morphology of different surfactant

The surface morphology of the samples obtained using CTAB, PEG and SDS as examined by SEM is shown in the Fig.3 (a)-(c). It shows that the prepared TiO₂ nanoparticles using different surfactant are spherical and well dispersed with average crystallite size of about 8-20 nm. In the case of cationic surfactant CTAB, low agglomeration, high dispersive and uniform crystal size due to the electrostatic interaction takes place between CTA⁺ cations and Ti (OH)₆²- anions, the cation CTA⁺ condense into aggregates in which counter ions Ti (OH)₆²- are interrelated in the interfaces between the head group to form CTA⁺ - Ti (OH)₆²- pair. With PEG, being a non-ionic surfactant TiO₂ formation was not possible due to the electrostatic interaction and was due to weak Van Der Wall’s interaction, and the acidic solution could provide protons binding to PEG molecules via hydrogen bonding [15]. In the case of anionic surfactant SDS, assisted TiO₂ nanoparticles there were no distinct changes in the morphology.

The TEM images of CTAB, PEG and SDS assisted TiO₂ products were shown in Fig.4 (a)-(c). As shown in Fig.4(a), it is evident that CTAB assisted product consists of spherical nanoparticles with the diameter range of 8-12 nm. Most of the nanoparticles were well separated although some of them partially aggregated. In our studies the surfactants possibly acts as a soft template, we believe that the nanoparticles fabricated through our approach grow mainly by Oswald ripening mechanism. Initially many nanoparticles with different sizes appear in the solution with the reaction proceedings, the nanoparicles with the larger size grow at the cost of smaller ones due to higher surface free energy [16]. The TiO₂ particles diffused and aggregated oriently by polar forces and then pearl necklace aggregation further recrystalised to form nanoparticles. It is noted from Fig.4(b) shows that the product mainly consist of agglomeration nanoparticles with an average particle size about 11 nm which is comparatively small to that of the value reported by Wang et.al [17]. Fig. 4(c) exhibits slightly elongated spherical particles with size ranging from 12-18 nm. Moreover, the particle sizes of all the samples obtained from TEM patterns are comparable to those calculated from Scherrer’s equation. Particle formation is a very complex process. It involves nucleation, growth, and coagulation, flocculation, all which may be influenced significantly by the surfactant assemblies. The addition of surfactant can influence particle growth, coagulation, and flocculation. Therefore, surfactants play an important role in the preparation of other metal oxide nanoparticles. Further, in depth study about the formation of TiO₂ nanoparticles is in progress to extend our understanding.

The associated SAED pattern (Fig.5) taken from the particle shown in Fig. 4(a) can be indexed as a tetragonal anatase TiO₂ single crystal. The first six rings are assigned to the (101), (004), (200), (005), (211), (204) reflections of the anatase phase.

Figure 2. DRS UV-absorption spectra of different surfactant TiO₂

Figure 3. SEM micrographs of different surfactant TiO₂
a) CTAB SEM TiO₂
b) PEG SEM TiO₂
c) SDS SEM TiO₂

Figure 4. TEM photomicrographs of different surfactant TiO₂
(a) CTAB TEM TiO₂
(b) PEG TEM TiO₂
(c) SDS TEM TiO₂

Figure 5. SAED pattern of the CTAB surfactant TiO₂

Conclusions

TiO₂ nanoparticles having particle sizes from 8-20nm were productively synthesized via a sol-gel method. XRD results showed that the TiO₂ nanoparticles were single crystalline with tetragonal anatase phase. SEM and TEM images of TiO₂ nanoparticles using different surfactant are well dispersed, uniform and spherical with crystal size of about 8-18nm. Blue shifts occurred in all the cases (310, 325, 349 nm) from that of bulk (385 nm), but a prominent shift was observed for CTAB assisted samples. From the above discussion it can be concluded that TiO₂ nanoparticles prepared using CTAB a cationic surfactant showed a lesser particle size compared to the samples prepared from PEG a non-ionic surfactant and SDS an anionic surfactant. Our results suggested that surfactants have a great influence to reducing the size of the nanoparicles. This convenient synthesis strategy can be applied as general approach for the preparation of other metal oxide nanoparticles.

Acknowledgment

The authors thank the University Grant Commission (UGC) for their financial support.

References

Contact Details

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

E-mail: [email protected]

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 11[2] (2009) 63-68.