Synthesis of string-bean-like anisotropic titania nanoparticles with basic amino acids

Abstract

An “assembly–aggregation–peptization” approach is reported for the preparation of colloidal dispersions of string-bean-like anisotropic titania nanoparticles using arginine. These titania nanoparticles have a single-crystalline anatase structure and possess high surface area, which is advantageous for potential functional materials.

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Titania (TiO₂) nanomaterials have been of great interest for both fundamental and technological aspects in photovoltaics,¹ photocatalysis,² sensing,³ and surface coating,⁴ because of their wide band gap, nontoxicity, and inexpensive fabrication. Among TiO₂ nanomaterials with multiple, distinct morphologies, one-dimensional (1D) TiO₂ nanostructures are of critical significance owing to their unique physicochemical properties. Various wet-chemical methods have been developed to synthesize 1D TiO₂ nanostructures, such as nanorods,⁵ nanowires,⁶ nanotubes,⁷ and nanochains.⁸ Despite these successes, the obtained anisotropic TiO₂ nanostructures usually present smooth surfaces and lack high surface area that provide numerous active sites for surface reactions.⁹ Anisotropic TiO₂ NPs directly dispersed in water is essential, because most widely used photocatalysts are applied in an aqueous environment such as water splitting.¹⁰ Synthesis of aqueous colloidal dispersion of 1D TiO₂ nanostructures with high surface area via a wet-chemistry process still remains a major challenge.

Recently, we prepared worm-like anisotropic silica nanoparticles (NPs) by controlled 1D self-assembly of presynthesized spherical silica seeds in alcohol–water mixed media with the aid of the basic amino acid arginine.¹¹ The binding of basic amino acids to the surfaces of silica NPs via electrostatic force induces an electronic change on the surface that is essential for 1D self-assembly. This suggests that basic amino acids are able to change interparticle interactions in the liquid phase. This capability could potentially be exploited to prepare anisotropic colloidal metal oxide NPs in aqueous system.

Here we describe an “assembly–aggregation–peptization” approach for the synthesis of colloidal dispersions of bean-like anisotropic TiO₂ NPs having a single-crystalline anatase structure. Triethanolamine is used as a complexing agent to retard the hydrolysis rate and change the reaction mode of titanium alkoxide precursors,¹² allowing the formation of small TiO₂ nuclei. Similar to the silica nanospheres system,¹¹ anisotropic assembly of TiO₂ nuclei can be achieved with the aid of arginine and alcohol that is generated by hydrolysis of titanium alkoxides. After growth and aggregation of linearly assembled TiO₂ seeds to form precipitates, highly dispersed bean-like TiO₂ NPs are obtained by a peptization process¹³ in which aggregates break down into colloidal particles under electrostatic repulsion by particle charging.

In a typical synthesis, a pale yellow stock solution of titanium (0.05 M) was prepared by mixing titanium isopropoxide (TTIP) and triethanolamine (TEOA) with a molar ratio of TTIP : TEOA = 1 : 2, followed by the addition of distilled water. Arginine (10 wt%) was added to the prepared stock solution. The reaction was carried out in an autoclave at 150 °C for 24 h. Precipitates were formed after reaction. TiO₂ was collected by centrifugation, washed with 0.01 M NaOH and 2 M HNO₃, and distilled water for several times. A stable TiO₂ sol was obtained after washing.

The morphology and structure of anisotropic TiO₂ NPs were characterized by field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Fig. 1(A) shows the SEM image of the product which consists of bean-like anisotropic TiO₂ NPs. They were spin-coated onto Si substrates from their colloidal system to give a good dispersion state. The TEM image of an individual TiO₂ NP (Fig. 1 (B)) reveals that it exhibits high single-crystallinity, preferentially grown along the c-axis direction, and a typical length of ca. 30 nm and diameter of ca. 10 nm. The formation of single crystalline bean-like TiO₂ NPs without domain boundary is favourable from the thermodynamic viewpoint, in which the elimination of high energy surfaces by oriented attachment leads to a substantial reduction in the surface free energy.

Fig. 1 (A) FE-SEM image of bean-like anisotropic TiO₂ NPs synthesized from titanium tetraisopropoxide with 10 wt% of arginine at 150 °C. (B) HRTEM image of a single bean-like anisotropic TiO₂.

Nitrogen adsorption–desorption isotherms were measured to determine the specific surface areas and pore size distribution of the bean-like anisotropic TiO₂ NPs. The powder sample for nitrogen adsorption was obtained by drying of the TiO₂ dispersion at 60 °C. The pore diameter distribution determined using the BJH method is quite narrow and centered at ca. 24 nm (Fig. S1, ESI†), which can be attributed to interparticle mesopores. The BET specific surface area of the product is ca. 117 m² g⁻¹, which is much larger than that of Degussa P25 (55 m² g⁻¹),¹⁴ which has been widely used as a highly active photocatalyst. Bean-like anisotropic TiO₂ NPs have rough surfaces thus possess higher surface area. The total pore volume is 0.38 cm³ g⁻¹.

The effect of the arginine concentration on the morphology of the TiO₂ NPs was examined under otherwise identical conditions. In the absence of arginine, almost isotropic TiO₂ NPs were formed (Fig. S2 (A), ESI†). When 1 wt% of arginine was used, the formation of anisotropic TiO₂ NPs with low aspect ratio started to form (Fig. S2 (B), ESI†). Further increase in arginine concentration to 5 wt% resulted in formation of anisotropic TiO₂ NPs with higher aspect ratio (Fig. S2 (C), ESI†).

When titanium tetraisopropoxide was replaced with titanium tetraethoxide or titanium tetrabutoxide, similar anisotropic TiO₂ NPs were observed (Fig. S3, ESI†). However, the diameter of the obtained anisotropic TiO₂ NPs was slightly changed. The different sizes of TiO₂ NPs were possibly related to different kinds of alcohol that was generated by hydrolysis of titanium alkoxides. These imply that anisotropic self-assembly of TiO₂ nuclei could be achieved in different alcohol–water media in the presence of arginine, which was similar to silica NPs system.¹¹ Preparation of other types of bean-like metal oxide NPs becomes possible when appropriate metal alkoxides are used.

We have tested whether anisotropic TiO₂ NPs can be prepared by using other types of basic amino acids besides arginine. Lysine, which is zwitterionic when dissolved in water, was chosen for comparison and was successfully used to prepare anisotropic TiO₂ NPs. It was found that the type of basic amino acids affects the aspect ratio of anisotropic NPs. Anisotropic TiO₂ NPs with low aspect ratio were obtained in the presence of 10 wt% of lysine (Fig. S4, ESI†). The pH value of the reaction solution varies depending on the types of basic amino acids, particularly at very high concentration, which may modify the surface electronic potential of TiO₂ nuclei and hence affect the final morphology.

It is notable that the morphology of the TiO₂ NPs can be controlled simply by adjusting the reaction temperature. When the temperature increased from 150 °C to 190 °C at fixed reaction duration of 24 h, only TiO₂ NPs with pyramid-like shape or irregular shape are observed (Fig. S5, ESI†). Lower temperature thus favors the formation of bean-like anisotropic TiO₂ NPs, which is likely due to amino acid decomposition at elevated temperatures (e.g., 190 °C, Fig. S6, ESI†). This suggests that basic amino acids with their structural integrity are indispensable to the preparation of bean-like anisotropic TiO₂ NPs. TiO₂ seed nuclei carry a negative charge at the basic condition. The amino acid arginine is able to bind the surface of TiO₂ seed nuclei and modify the surface electrical dipole via the zwitterionic head group of the residue. Surface segregation of the negative and positive charges might occur after the binding of zwitterionic arginine to TiO₂ surfaces. This segregation effect¹⁵ might produce a dipole across TiO₂ surfaces to drive 1D assembly. Additionally, the higher concentration of arginine leads to higher ionic strength of the medium, which weakens the electrostatic repulsion between the TiO₂ nuclei. Linearly assembled TiO₂ seeds are generated through arginine-induced electric dipolar interaction in the presence of alcohol, which finally leads to the formation of bean-like TiO₂ NPs. The formation of pyramid-like TiO₂ NPs is favored at relatively high temperature (190 °C), which is likely due to inherent crystal property of TiO₂.

To help trace the growth process of bean-like anisotropic TiO₂ NPs and further understand the related formation mechanism, a series of experiments was carried out with different reaction periods to monitor the evolution of the TiO₂ NP shapes. By simply adjusting the reaction period, the shape and morphology of the TiO₂ NPs could be controlled, as demonstrated through FE-SEM imaging. The SEM images (Fig. 2(A)–(C)) were recorded for the samples obtained after 3 h, 8 h, and 24 h, respectively. As shown in Fig. 2(A), mainly linearly assembled TiO₂ seeds were obtained at the early stage (3 h) of the reaction. When the reaction time was prolonged to 8 h, individual TiO₂ NPs are observed clearly in a single nanochain (Fig. 2(B)), indicating that bean-like TiO₂ NPs are formed through self-assembly of small TiO₂ NPs. After 24 h, bean-like anisotropic TiO₂ NPs with increased length and larger diameter were formed (Fig. 2(C)). The final product was a light-blue suspension, showing good colloidal stability (Fig. 2(D)).

Fig. 2 SEM images of anisotropic TiO₂ NPs synthesized at different reaction times: (A) 3, (B) 8, (C) 24 h at 150 °C. (D) Photograph of a suspension of bean-like TiO₂ NPs obtained after 24 h reaction time.

The possible formation scheme of bean-like anisotropic TiO₂ NPs is illustrated in Scheme 1. It involves four steps: (1) triethanolamine is mixed with titanium alkoxides to suppress their rapid hydrolysis, thus the formation of small TiO₂ nuclei and their self-assembly with the aid of basic amino acid could be achieved simultaneously, (2) bean-like TiO₂ NPs forms after regrowth of the linearly assembled TiO₂ seeds under hydrothermal condition, (3) intermediate TiO₂ NPs aggregate at very high ionic strength, and (4) highly dispersed bean-like TiO₂ NPs are achieved by a peptization process. Therefore, it is distinguishable from the conventional “precipitation–peptization” route.¹⁶ We define it as an “assembly–aggregation–peptization” route. Basic amino acids play a vital role in the preparation of bean-like anisotropic TiO₂ NPs.

Scheme 1 Proposed scheme for the formation of bean-like anisotropic TiO₂ NPs with basic amino acids.

In summary, we demonstrated a facile approach for preparing bean-like, single crystalline anatase TiO₂ NPs with high quality. The anisotropic TiO₂ NPs possessing high surface area are promising candidates for achieving highly efficient photovoltaic and photocatalysis devices. Our approach will provide a versatile route for the preparation of anisotropic NPs of other semiconducting metal oxides.

This work was supported by a Grant-in-Aid for Scientific Research (B) (23350098) from the Japan Society for the Promotion of Science. A part of this work was conducted in the Research Hub for Advanced Nano Characterization, The University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, the nitrogen adsorption isotherm, and SEM images. See DOI: 10.1039/c3ra47170e

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