Synthesis of nanostructured TiO2 (anatase) and TiO2(B) in ionic liquids

doi:10.1016/j.cattod.2014.01.007

Catalysis Today

Volume 230, July 2014, Pages 85-90

https://doi.org/10.1016/j.cattod.2014.01.007 Get rights and content

Highlights

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TiO₂(B) and anatase were synthesized in the presence of ionic liquids.
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The growth temperature did not exceed 95 °C in any step of the synthesis.
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The content of TiO₂(B) was influenced by organic solvents.
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The nitrogen atmosphere promoted the formation of anatase.

Abstract

Sol–gel synthesis of TiO₂ (anatase) and TiO₂(B) was carried out in the presence of imidazolium based ionic liquids with tetrafluoroborate and chloride ions. The prepared nanoparticles were characterized by cyclic voltammetry, Raman spectroscopy and scanning electron microscopy. The influence of organic solvent type and synthesis atmosphere on the product's composition was evaluated. It was established that a nitrogen atmosphere favours the growth of anatase crystalline phase and inhibits the formation of TiO₂(B).

Graphical abstract

Introduction

As a low cost, stable and environmentally friendly material, titania has found applications in photoelectrochemical solar cells, lithium batteries [1], [2], gas sensors [3], electrochromics [4], catalysis and photocatalysis [5], [6]. In the last decade various titania nanomaterials were prepared with properties controlled by their structure [7], [8], [9], [10].

The synthesis of nanomaterials can substantially benefit from the utilization of ionic liquids. They exhibit low vapor pressure, high thermal stability and offer chemical stability over a wide window of electrochemical potentials [11], [12]. These properties are used in organic synthesis, catalysis and for preparation of doped TiO₂ crystals [13] and particular forms of titania [14], [15], [16].

There are a variety of applications for anatase and TiO₂(B). TiO₂(B) accommodates similar levels of lithium TiO₂ similar to anatase or rutile, but the fast pseudocapacitive mechanism of this Li-storage makes it highly attractive for lithium batteries application [17]. Furthermore, TiO₂(B) finds useful applications in catalysis and photocatalysis. It is interesting to note that the mixture of TiO₂(B) and anatase exhibits enhanced photocatalytic activity compared with phase pure anatase or TiO₂(B) [18]. The activity of anatase/TiO₂(B) mixture is even outperforming than that of the anatase/rutile mixture in the popular benchmark photocatalyst P25 [18]. The applications of mesoporous TiO₂(B) in catalysis were reviewed in [19]. However, for industrial applications, scalable synthesis of TiO₂(B) in large quantities is still a challenge. TiO₂(B) or its mixture with other TiO₂ polymorphs can be prepared by solid state or sol–gel techniques [20], [21]. Hydrolysis of a titanium precursor in the presence of ionic liquids represents a relatively mild way for preparation of titanium dioxide in various phases and/or their mixtures. The composition of the resulting material is determined by the synthesis time, temperature, pressure and by the choice of ionic liquid. Kaper et al. [21] presented a detailed study of different ionic liquids used to synthesize anatase and TiO₂(B) phases. Ionic liquids with tetrafluoroborate counterions favour the formation of phase pure TiO₂(B) [22]. Other additives [23] and working atmosphere [24], [25] during synthesis also influence the crystallization of oxides.

In this work, solutions of immidazolium based ionic liquids with tetrafluoroborate and chloride counterions were used as reaction medium. One of the initial motivations of this study was to test whether or not tetrafluoroborate ions are needed to obtain a pure TiO₂(B) phase [22]. The influence of solvent (ethanol, chloroform and acetonitrile) and synthesis atmosphere on the ratio of anatase and TiO₂(B) in a product was also studied. Synthesis parameters of different mixtures of TiO₂ polymorphs were correlated with quantitative phase analysis by cyclic voltammetry of Li-insertion, Raman and optical spectroscopy and by scanning electron microscopy (SEM).

Section snippets

Materials and methods

1-Hexadecyl-3-methylimidazolium chloride (C₁₆mimCl) (Alfa Aesar), 1-butyl-3-methylimidazolium tetrafluoroborate (C₄mimBF₄) (Sigma Aldrich), 1-methyl-3-octylimidazolium chloride (C₈mimCl) (Sigma Aldrich) and 1-methyl-3-octylimidazolium tetrafluoroborate (C₈mimBF₄) (Sigma Aldrich) were used without any further purification. During a typical synthesis, 400 mg of ionic liquid with chloride counter ion and 600 mg of ionic liquid with tetrafluoroborate counter ion were mixed in a glove box whilst

Results and discussion

Synthesis was carried out with different ionic liquids, organic solvents and additional synthesis variables to study their effects on the final product. Details are summarized in Table 1. The reference material is coded TB5. It was synthesized by using the method described by Wessel et al. [22]. Experimental conditions were varied systematically, and are described in the entry ‘Modification’ in the last column of Table 1. For phase analysis of TiO₂ polymorphs, cyclic voltammetry of Li insertion

Conclusions

Nanoparticles of titania were synthesized in the presence of ionic liquids. Characterization by electrochemical Li-insertion, Raman spectroscopy, optical spectroscopy and SEM confirmed the presence of two forms of titania, anatase and TiO₂(B). The TiO₂(B) concentration in samples was in the range from 0% to 55% depending on the synthesizing conditions. Evaluation of the influence of particular ionic liquids confirmed the importance of the presence of 1-hexadecyl-3-methylimidazolium chloride.

Acknowledgements

This work was supported by the EC 7th FP project Orion (Contract No. NMP-229036) and by the Grant Agency of the Czech Republic (Contract No. 13-07724S).

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Synthesis of nanostructured TiO2 (anatase) and TiO2(B) in ionic liquids

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusions

Acknowledgements

Electrochim. Acta

Catal. Today

Electrochim. Acta

Mater. Lett.

J. Power Sources

Chin. J. Catal.

Mater. Res. Bull.

J. Eur. Ceram. Soc.

J. Alloys Compd.

Int. J. Inorg. Mater.