Water-dispersible TiO2 nanoparticles via a biphasic solvothermal reaction method
© Mohan et al.; licensee Springer. 2013
Received: 17 September 2013
Accepted: 21 November 2013
Published: 1 December 2013
A biphasic solvothermal reaction method has been used for the synthesis of TiO2 nanoparticles (NPs). In this method, hydrolysis and nucleation occur at the interface of organic phase (titanium (IV) n-propoxide and stearic acid dissolved in toluene) and water phase (tert-butylamine dissolved in water) resulting in the nucleation of the stearic acid-capped TiO2 NPs. These NPs are hydrophilic due to hydrophobic stearic acid ligands and could be dispersed in toluene, but not in water. These stearic acid-capped TiO2 NPs were surface-modified with 2,3-dimercaptosuccinic acid (DMSA) in order to make them water soluble. The resultant TiO2 NPs were easily redispersed in water without any noticeable aggregation. The Rietveld profile fitting of X-ray diffraction (XRD) pattern of the TiO2 NPs revealed highly crystalline anatase structure. The average crystallite size of TiO2 NPs was calculated to be 6.89 nm, which agrees with TEM results. These results have important implications for the use of TiO2 in biomedical, environmental, and industrial applications.
TiO2 nanoparticles (NPs) have been widely investigated in the recent past due to their applications in a wide range of fields including solar cells , water photolysis for hydrogen production , sensors , and antireflective and photochromic devices . TiO2 has three well-known crystallographic phases in nature: anatase, rutile, and brookite. Among these, anatase has been proved to have excellent chemical and physical properties for environmental remediation  and many other uses [6–8]. Numerous methods for the synthesis of TiO2 NPs have been developed, such as hydrolytic sol-gel process , nonhydrolytic sol-gel process , hydrothermal methods , solvothermal methods , and so on. The synthesis of TiO2 nanoparticles generally involves hydrolysis and condensation of titanium precursors. The titanium precursors are extremely water sensitive; therefore, in conventional aqueous/alcohol-phase/sol-gel method in conventional solution-phase synthetic routes, small amount of water is used to inhibit the hydrolysis. However, prepared TiO2 NPs suffer from poor crystallinity and inferior material properties as compared to those prepared through high-temperature, nonhydrolytic methods. Furthermore, these methods of synthesis suffer from problems of aggregation, size, and low monodispersity and post-treatment procedures (for converting amorphous phase to crystalline TiO2 phase) which greatly affect the desired properties of the nanoparticles and restrict their large-scale production and applicability. The properties of TiO2 are highly dependent on surface area, crystalline phase, and single crystallinity. The high-quality TiO2 NPs prepared through nonhydrolytic methods are insoluble in aqueous medium, which make their utilization toward biological/biomedical applications impossible. At present, the synthesis methods for production of water-dispersible TiO2 NPs with a tunable size is challenging to the researchers.
In this letter, we present the preparation of water-soluble and biocompatible highly crystalline TiO2 NPs through biphasic solvothermal interface reaction method.
The following chemicals were used as purchased: titanium (IV) n-propoxide, tert-butylamine, 2,3-dimercaptosuccinic acid (DMSA) and stearic acid (SA) (Sigma-Aldrich, Steinheim, Germany) and toluene (Penta, Chrudim, Czech Republic). All the chemicals were of analytical grade purity. Deionized water (Millipore) was used to prepare aqueous solutions (≥18 MΩ). In biphasic solvothermal reaction method, the reaction occurs at the interface of water phase and organic phase at elevated temperature. In the synthesis procedure, the organic phase consists of 90 μL of titanium (IV) n-propoxide and 0.5 g of SA dissolved in 10 mL of toluene. The water phase contains 100 μL of tert-butylamine dissolved in 10 mL of deionized (DI) water. First, water phase was added to a Teflon-lined steel autoclave. Then, the organic phase was added slowly into the Teflon-lined steel autoclave without any stirring. The autoclave was sealed and heated to 170°C for 6 h. The reaction mixture was then cooled to room temperature, and methanol was added to precipitate the TiO2 NPs. TiO2 NP precipitates were recovered by centrifugation and washed several times with methanol to remove the excess of surfactant. This resulted in hydrophobic SA-coated TiO2 NPs, which are dispersible in toluene. The water dispersiblity of TiO2 NPs was achieved by treating the SA-coated TiO2 NPs in a solution of ethanol and toluene containing 2,3-DMSA for 24 h with vigorous stirring. This resulted in DMSA-coated TiO2 NPs which were recovered via centrifugation. Then, the final NPs were easily dispersed in water.
The crystal structure and morphology of as-synthesized nanoparticles were investigated with X-ray diffraction (XRD) using monochromatic Cu Kα radiation (λ = 1.5418 Å) and transmission electron microscope (TEM). The crystalline nature of the NPs was then examined by TEM measurements. The optical properties were investigated by UV-visible (UV-vis) absorption and fluorescence spectra at room temperature.
Results and discussion
A facile route for the synthesis of TiO2 NPs through biphasic solvothermal interface reaction method has been reported. The XRD pattern of TiO2 NPs revealed the anatase structure. The average XRD crystallite size was calculated as 6.89 nm using the Scherrer formula. The optical studies showed that the bandgap is 3.1 eV. The results show that synthesized nanoparticles are monodispersed with long-term stability. This synthesis method is simple with very high production yield and does not require any post-treatment procedure, and products can be collected from organic phase which effectively avoids TiO2 grain aggregation.
The work has been supported by the project ‘CEITEC - Central European Institute of Technology’ CZ.1.05/1.1.00/02.0068 from the European Regional Development Fund and by the NanoBioTECell GACR P102/11/1068 project for the conceptual development of research organization 00064203.
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