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Crystalline titania nanoparticles synthesized in nonpolar Lα lecithin liquid-crystalline media in one stage at ambient conditions
Colloids and Surfaces B: Biointerfaces 87 (2011) 203–208
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Crystalline titania nanoparticles synthesized in nonpolar L␣ lecithin liquid-crystalline media in one stage at ambient conditions Yury Shchipunov a,b,∗ , Anna Krekoten a a
Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Division of Chemical Engineering, Pusan National University, San 30, Jangjun Dong, GeumjungGu, Busan 609-735 Republic of Korea b
a r t i c l e
i n f o
Article history: Received 26 February 2011 Accepted 30 April 2011 Available online 6 June 2011 Keywords: Lecithin Bimolecular layer Template synthesis Titania nanoparticles
a b s t r a c t High-temperature modification of titania in the form of nanoplatelets is synthesized fast in one step at ambient conditions without any additional treatment like aging or calcination. Lecithin, which is the main component of lipid matrix of biological membranes, is first used as a structure-driven template. It is demonstrated that this natural surfactant can self-organize into lamellar L␣ mesophase when small amounts of water are admixed in its solution in nonpolar solvent. The water locating mainly in lecithin polar region as hydration shell at this concentration triggers the hydrolysis–condensation reactions after the precursor addition that results in instantaneous titania formation in the form of crystalline nanoparticles. Planar lamellar sheets serve as the template specifying its crystallinity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide TiO2 or titania nanoparticles (TNPs) well attract particular interest because of their great potentialities for developing very effective photocatalysts, solar-energy converter and sensors, characterizing high chemical stability and low toxicity. This activity is obvious from some recent reviews [1–8]. The efficacy of materials and devices on the basis of TNPs is highly dependable on their dimension, crystallinity and crystalline form of TiO2 [2,3,7,8]. As an example, it is a P25 photocatalyst developed by the Degussa. At present it is the best commercial product [9], using as a standard. The highest photocatalytic activity occurs in a mixture of 85–70% anatase and 15–30% rutile. They both are the main crystalline form of TiO2 . Their structure can be presented by an octahedron in which six oxygen atoms are at the corners, surrounding a titanium atom [10]. The difference between these crystalline forms is in the distortion of octahedra of oxygen atoms in anatase. The absence of distortion makes the rutile form most dense and stable. The problem is that titania synthesized at ambient conditions is amorphous. To transfer it into a crystalline form, TiO2 is heat treated. A transitions to anatase is observed in the range
∗ Corresponding author at: Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia. Tel.: +7 4232 314481; fax: +7 4232 311889. E-mail addresses: [email protected], [email protected] (Y. Shchipunov). 0927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.04.038
of 500–800 ◦ C and then to rutile at 915 ◦ C [11]. There are quite abundant efforts to obviate the need for the high-temperature calcination, aiming to find an effective low-temperature route for TNPs fabrication. Methods applied to fabricate TNPs in solutions can be separated into two main groups, non-hydrolytic and hydrolytic [5,12–14]. They can be carried out through a sol–gel, hydrothermal, solvothermal, sonochemical, microwave or electrochemical synthesis that enables one to prepare nanosized crystalline titania of different shape and dimensionality. The hydrothermal methods hold much favour. They are usually performed at 150–200 ◦ C or even lower temperature around 50–60 ◦ C [15,16]. The TiO2 is synthesized fast, especially in hydrolytic routes, but the degree of its crystallinity is low. To promote further crystallizationof the product, it is put through more or less prolonged additional treatment(s). For instance, it can be left in the same hydrothermal solution under refluxing at increased temperature up to few days. To our knowledge, there are few published articles in which crystalline titania was prepared at ambient conditions. Zhang et al. [17] synthesized rutile through the sol–gel processing by using reverse micelles as a template. It seems that the crystallinity of as-prepared TiO2 was poor. The product was left for aging for 20 days. A faster preparation procedure was suggested by Lu et al. [18]. They used a template consisting in polysterene latex surrounded by covalently attached anionic polyelectrolyte brushes. TNPs of 4–12 nm synthesized by controllable precursor feed rate at room temperature were mainly precipitated on the brushes. As shown by means of X-ray diffraction, they were crystalline, consisting of anatase. Kaper et al. [19] used an ionic liquid. They could synthesize
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a rutile by means of the sol–gel processing at ambient conditions. It is of interest that as-prepared titania was amorphous. A transition to crystalline form occurred in the course of extraction of ionic liquid. When recently repeating the synthesis by adding surfactants, these authors carried out it at 100 ◦ C like typical hydrothermal route [20]. Bansal et al. [21] prepared crystalline TiO2 from K2 TiF6 by rather uncommon way. They performed syntheses in the presence of fungus Fusarium oxysporum. It is the author’s opinion that the TNP formation was mediated by a low-molecular weight extracellular protein. The application of surfactant templates to control and regulate the formation of any nanoparticles, not only TNPs is practised widely [5,14,22–26]. Furthermore, surfactants serve as a stabilizing agent, preventing TNPs against the merging and agglomeration. In their absence, this role should be fulfilled by molecules of organic solvents in nonaqueous syntheses [14,27]. Here we performed a template synthesis of TNPs at ambient conditions by using lecithin. Lecithin is one of the main lipid components of biological membranes [28], using widely as a natural surfactant [29]. It can self-organize into reversed polymer-like micelles in organic media in the presence of trace amounts of water [30,31]. We expected the formation of rod-like TNPs in the micellar core by regulating the sol–gel processing by means of controlled amount of H2 O. This effective way was previously realized with polysaccharides dissolved in ethylene glycol [32]. Unexpectedly, we found nanosized platelets of highly crystalline titania that was in its high-temperature modification, rutile. As demonstrating in the article, the structure-directing synthesis was caused by templating the TiO2 on lecithin L␣ -mesophase. Its formation is brought about by a previously non-described transition of polymer-like micelles into multilamellar state after adding a critical amount of water in a lecithin solution. Conditions, at which the fast direct synthesis of crystalline titania occurs at ambient conditions, its examination by X-ray diffraction and high-resolution transmission electron microscopy as well as lecithin template by 2 H NMR spectroscopy, polarizing optical microscopy and dynamic rheology are reported. 2. Materials and methods 2.1. Chemicals Soybean lecithin Epikuron 200 purchased from Lukas Meyer, AG (Germany) and titanium(IV) isopropoxide, from ABCR were used as supplied. n-Heptane was of puriss quality, water, doubly distilled. 2.2. Lecithin organogel L˛ mesophase preparation It is detailed in our previous publications [33,34]. Weighed amounts of lecithin and water were placed in appropriate volume of n-heptane and stirred by magnetic stirrer at 40–50 ◦ C as long as a homogeneous mixture had not been formed. After that it was allowed to stay at the ambient temperature at least for 3 days to reach a homogeneous distribution of H2 O in the sample. L␣ mesophase was prepared by the same protocol, except that more water was added than needed for the organogel. 2.3. Titania synthesis Lecithin organogel or L␣ mesophase prepared as described above were taken within three days after the preparation. A weighted appropriate amount of titania precursor was introduced and vigorously stirred to reach its homogeneous distribution in the mixture as fast as possible. It was performed at ambient temperature or slight heating (up to 40 ◦ C) to decrease the viscosity. The precursor addition resulted is a drastic change of solution properties. First of all the viscosity was sharply decreased. Further-
Fig. 1. The zero-shear viscosity vs. the water to lecithin molar ratio. The lecithin concentration is 28 mg/ml. The vertical lines show rough boundaries between various types of systems formed with changing the water/lecithin molar ratio, but the transitions from low-viscous solution to the viscoelastic state and then into the liquid-crystalline mesophaseoccur smooth.
more, there was a transition of transparent organogel or mesophase into a turbid, nontransparent system. The following transformation depended on the amount of admixed precursor. The viscosity could sharply increased that accomplished by a transition into solid-like state or it could unchanged or solid precipitate could appear in the mixture. 2.4. Material characterization Powder X-ray diffraction analysis was performed by using a ˚ Samples for TEM observations Rigaku Miniflex, CuK␣ ( = 1.5418 A). were prepared by dispersing ultrasonically a synthesized material in ethanol and then drying a putted drop of solution on a copper grid at room temperature. Pictures were taken by a JEM-2010 transmission electron microscope (JEOL) at an accelerating voltage of 200 kV. Samples for deuteron NMR experiments were prepared with deuterated water. 2 H NMR spectra were taken by a Bruker MSL 300 NMR spectrometer at a deuterium resonance frequency of 46.07 MHz. Rheological measurements were performed only with lecithin organogels by means of a Rotovisco RT 20 stress-controlled rheometer (Haake). The rheometer was run in either an oscillation or creep regime as described in [35]. A cell had a cone and plate geometry. Optical microscopy observations were carried out with a polarizing microscope (Zeiss). Organogel samples were placed in a rectangular capillary and left to equilibrate for a day at ambient temperature before the examination. 3. Results 3.1. Characterization of templating system used for the syntheses Lecithin is readily soluble in organic solvents including nheptane used as a reaction media here. Its solutions are low-viscous, demonstrating Newtonian behavior. Solubilized lecithin forms small reversed spherical micelles from firth molecules in nonpolar media [36]. As soon as trace amounts of water are introduced, one can find a sharp increase in the solution viscosity. This is illustrated by Fig. 1 in which there is a dependence of the zero-shear viscosity on the water to lecithin molar ratio. The sharp increase in viscosity looks like a jellification. This jellylike solution was termed the organogel [37], whereas it is a not a real gel, being a viscoelastic system with a Maxwell behavior (see Refs. [34,38,39]). The viscosity increase is brought about by a transformation of the spherical micelles into cylindrical aggregates [31]. Their contour length can reach 1 mm, while the diameter is of about
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Fig. 2. TEM images of nanoparticles synthesized in the lecithin-containing solutions. Nanocrystal outlines in B are shown by light brown. Sample compositions: (A) molar ratio of water to the lecithin was 7, lecithin concentration, 11 wt.%, titanium(IV) isopropoxide, 8.2 wt.%; (B) molar ratio of water to the lecithin was 5, lecithin concentration, 11 wt.%, titanium(IV) isopropoxide, 6.7 wt.%.
5–7 nm. These lengthy micelles are called the polymer-like ones because of their entanglement like linear polymer macromolecules do it, making up a temporal three-dimensional network in the bulk [40,41]. The lecithin organogel phase exists in a rather narrow range of the water/lecithin molar ratios. Rough boundaries can be seen in Fig. 1. The real structural organization of the system adjacent from the right side in the plot was not revealed before this study. It was mainly examined in details by means of the dynamic rheology. It was inferred from obtained results that there is a transformation (see Refs. [39,42,43]), but the nature of restructuring has been unclear. On study on lecithin solutions with increased water/lecithin molar ratios by using 2 H NMR spectroscopy and polarizing microscopy, we came to a conclusion about a transition from the polymer-like micelles to the multilamellar liquid-crystalline state. This is considered in needed details in the following section. 3.2. Titania synthesis and characterization Syntheses have been performed in the whole range of water/lecithin molar ratios seen in the plot in Fig. 1. It means that various lecithin structures were examined as the template. The admixing of titania precursor into a lecithin solution without added water did not cause a notable change in it. When trace amount of H2 O had been added before the precursor, its introduction resulted in a change of optical and mechanical properties. The initial organogel was completely transparent. It became turbid and then the transparency could returned with few minutes or the organogel remained nontransparent. These transformations depended on the precursor amount added. In parallel with the change of optical properties there was also a change in the viscosity. With admixing the precursor, sharp thinning was first observed. Next changes depended on the precursor amount added. The viscosity could increase somewhat, transfer into the solid-like state or separate into solid and liquid phases. In case the separation was absent, the transformations were accomplished by the formation of a monolithic, optically transparent or opalescent material. The transparent or slightly opalescent samples were of chief interest. Their examination under the transmission electron microscopy (TEM) revealed planar nanosized particles instead of expected rod-like or fibrillar ones. Representative images are given in Fig. 2. Nanoparticles were of two main types, differing in shape and dimensions. As seen in Fig. 2, they were rectangle or hexagonal. The particle dimensions were around 50 nm (Fig. 2A) or 15 nm (Fig. 2B).
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Fig. 3. High resolution TEM images of nanoparticles with crystalline lattices of two types found in the samples. The corresponding Fast Fourier transform images are inset. Lecithin organogel compositions of (A) and (B) correspond to that in Fig. 2.
The following unexpected result was that these nanoparticles were crystalline. Representative images of lattices observed by means of the high-resolution TEM are shown in Fig. 3. As obvious from measured spacings indexed in the pictures, they are of two types equal to ca. 0.3 and 0.6 nm. The crystallinity of samples was further confirmed by X-ray diffraction (XRD). Fig. 4 presents X-ray diffractograms of three samples of various compositions. One can see well-resolved sharp reflections. The XRD peaks may be assigned to the (0 0 4) crystal face of anatase, (2 1 0), (3 1 0) and (3 2 1) of rutile. It is worthy mentioning that this XRD pattern was well reproducible. The same number of the same reflections was obtained with samples prepared in various syntheses. The surprising thing is that the high-temperature modification was synthesized at ambient conditions without heating or following heat treatment. It is obvious from Fig. 4 that the variation of precursor amount in a rather narrow range occurred a profound effect on the crystallinity of product. The water and lecithin also influenced much the TNP formation. Fig. 5 shows how it depended on the lecithin concentration at a fixed molar ratio of water to titania precursor in the reaction system. The TiO fabrication in crystalline form is regulated by all three components that present a severe difficulty with revealing a certain correlation. Our observation showed that it took place only in the area in which solidified optically transparent material was formed.
Fig. 4. Powder XRD analysis of titania samples as-synthesized at ambient conditions. Reaction composition: molar ratio of water to the lecithin was 7.5, lecithin concentration, 11 wt.%, titanium(IV) isopropoxide concentrations expressed in wt.% are shown alongside the corresponding diffractogram. The A and R mark, respectively, anatase and rutile.
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hPeak (a.u.)
206
10
3
10
2
0.0004
0.0008
0.0012
CLecithin (mol) Fig. 5. The peak height at 37.9◦ in diffractograms (see Fig. 4) vs. the lecithin concentration in the reaction media. Water to titania precursor molar ratio was 1.05.
4. Discussion The TiO2 synthesis was performed in the common hydrolytic route. As soon as the titania precursor comes into contact with water, it triggers instant hydrolysis [12,13]: Ti(–OR)4 + nH2 O → (HO–)n Ti(–OR)4−n + nHO–R and condensation reactions: (RO–)4−n Ti(–OH)n + (HO–)n Ti(–OR)4−n → (OH)n−1 (RO–)4−n Ti–O–Ti(–OR)4−n (OH)n−1 + H2 O or Ti(–OR)4 + (HO–)n Ti(–OR)4−n → (RO–)3 Ti–O–Ti(–OR)4−n (OH)n−1 + HO–R where R is a hydrocarbon radical and n ≤ 4. The reactions proceed with very high rate. Therefore, the titania if formed instantly where the water is localized. As shown in [32], this enables one to template TiO2 , tailoring its structural organization. Here it was realized through templating on lecithin polar region. A structural formula of lecithin may be seen in Fig. 6A. It is a diacyl derivative of 3-glycerophosphoric acid, containing also choline attached to the phosphoric acid residue [28,44,45]. This 1,2-diacylsn-glycero-3-phosphocholine is called often phosphatidylcholine or lecithin. It is soluble in organic solvents and swells in water,
Fig. 6. Structural formula (A) and space filling model (B) of lecithin molecule. Area of preferential location of water molecules as hydration shell is shown by blue. It is anticipated that it serves as a nucleating center for titania.
forming lamellar mesophase L␣ [29,44,46]. When being in nonpolar media, lecithin absorbs or solubilized water that resulted in transformation of small reversed spherical micelles into cylindrical aggregates called the polymer-like ones [30,31]. The water introduced is involved into the hydration of lecithin polar region. This phospholipid shows a strong affinity for the water owing to extended polar region. It can absorb up to 21H2 O molecules per lecithin molecule from vapor, while its whole hydration shell can include 33 to 39 water molecules [29,30,46,47]. Crystalline titania was found in systems in which molar ratio of H2 O/lecithin varied between 5 and 8 (see Figs. 2 and 4). It means that the major portion of introduced water was in the hydration shells, not in the solution bulk that, among other things, is nonpolar. As revealed in [33,48], the H2 O molecules, which induced the organogel formation, are mainly attached to the phosphate and carbonyl groups through hydrogen boundings (Fig. 6B). They serve as a linking bridge between them, providing the stabilization of huge cylindrical micelles [33,49]. Their core consists of linear networks from alternating sequence of water molecules and phosphate groups of lecithin molecules. When beginning with the titania synthesis in the organogel phase, we followed the common knowledge that there are the polymer-like micelles. It was reasonable to expect the formation of rod-like or fibrillar TNP. The fabrication of plate nanoparticles (Figs. 2 and 3) poses the problem of real structural organization of lecithin. The polymer-like micelles exist in a rather narrow range of water to lecithin molar ratios [31,34]. The optimal ratio is around 3.2 where is the maximum in Fig. 1. Crystalline titania was found at larger values being between 4.5 and 8. This is a region of significant change in the rheological properties of lecithin organogel [31,39,43,50]. The used dynamic rheology could not provide an insight into the reasons for observed changes and real structural organization of lecithin in the region of increased water to lecithin molar ratios. It became apparent after an application ofsuch techniques as 2 H NMR spectroscopy and polarizing optical microscopy. Results thus obtained showed unambiguously that the micellar system transformed into a liquid crystalline state which can be assigned to the L␣ mesophase. It consists of planar bimolecular lecithin films stacked into a multilamellar structure [29,44]. Corresponding experimental data (I–VII) with schematic drawings(A–D) of possible self-organized lecithin structures and their transformations with increasing the water amount are summarized in Fig. 7. The initial state is represented by reversed spherical micelles (A). Their solution is low-viscous. As followed from frequency dependencies of the complex viscosity |*|, storage modulus G and loss modulus G in Fig. 7I, it demonstrates the characteristic rheological behavior that is typical of systems called Newtonian liquids [51]. The water addition in trace amounts brings into transition to viscoelastic state. This is obvious from the frequency dependences of rheological parameters (Fig. 7II) of the same solution after the addition of 3.0 molecules per lecithin molecule. This type obeys the Maxwell behavior that is indicative of a system made up of a three-dimensional network of entangled polymer-like micelles (see Refs. [31,34]). A micellar segment is schematically presented by drawing in Fig. 7B. As followed from 2 H NMR spectrum (Fig. 7III), the organogel is isotropic. The further added water results also in a change of rheological properties (not shown, see Refs. [39,50]). Examination under the polarizing optical microscope revealed a transition from isotropic to anisotropic system. The texture seen in the picture in Fig. 7IV is called “oily streaks” [52,53]. The anisotropy is also confirmed by means of 2 H NMR spectroscopy. One may see the quadrupolar splitting of a line in spectrum in Fig. 7V. Both these methods point
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Fig. 7. Schematic drawings (A)–(D). They present water-induced transformations of initial reversed spherical micelles (A), into cylindrical one (B), bimolecular layer (C) as the main structural block of L␣ mesophase and this bimolecular layer with titania after the addition of TiO2 precursor (D). Experimental data I–VII. (I) The complex viscosity |*|, storage modulus G and loss modulus G vs. the oscillation frequency of the initial solution with 28 mg/ml lecithin. (II) The similar frequency dependences of rheological parameters of the same solution after addition of 3.0 molecules per lecithin molecule. (III) 2 H NMR spectrum of isotropic organogel consisting of 50 mg/ml of lecithin and 3.2 water molecules per lecithin one. (IV) The texture observed under the polarizing optical microscope. Lecithin concentration was 60 mg/ml, water to lecithin molar ratio 5.5. (V) Thequadrupolar splitting of a line occurred in 2 H NMR spectrum. Molar ratio of H2 O to lecithin 4.5. (VI) A TEM image of titania nanoparticles synthesized in liquid-crystalline media. Lecithin amount 11 wt.%, water/lecithin molar ratio 5. (VII) X-ray diffractogram of TiO2 nanoparticles shown above.
to the formation of a well-organized lamellar L␣ phase consisting of stacked planar bimolecular layers as shown by schematic drawing in Fig. 7C. More details concerning the mechanism of polymer-like micelles transformation into lamellar mesophase is in an article that is under the preparation. It seems that the planar bimolecular films serve as a template for titania. The precursor introduced will not be involved into hydrolysis-condensation reactions before its contact with H2 O. Water molecules are in the form of a hydrating shell mainly of phosphate groups in this concentration range (Fig. 6B) [33,48,49,54]. When reaching it, the precursor molecule will experience the instant hydrolysis, resulting in the nucleation of reaction product on lecithin bimolecular layer (Fig. 7D). The negatively charged phosphate groups (Fig. 6A) can serve as nucleating center for titania because of positive charge both of titanium atom and its oxide [12]. The planar alignment of lecithin molecules in bimolecular layers results in the formation of inorganic particle (Fig. 7VI) with crystallographically determined lattice (Fig. 7VII). To our knowledge, this is a first application of lecithin as the template for metal oxide preparation. We met only an article by Yoon et al. [55] in which lecithin vesicles were used to fabricate a titania nanocomposites, not TNPs. Nanoparticles were synthesized separately. Thereupon they were introduced into lecithin-containing system. Here it was demonstrated that lecithin can manipulate the synthesis of nanosized titania. An important point is that the TNPs synthesized on lecithin template are in crystalline state although they were prepared at the ambient conditions. This is unordinary result. The common practice is to perform a synthesis at increased temperature (around 200 ◦ C in the hydrothermal routes [5,14]) or treat the initially fabricated amorphous titania at increased temperature (400–1000 ◦ C [5,32]). The use of lecithin simplifies the synthesis, enabling us to prepare nanosized TiO2 in one step at mild conditions. Preliminary experiments have demonstrated that our approach can be extended for other metal oxides. It allows us considering it as a versatile synthetic route for metal oxide nanomaterials with unprecedented control on their structure, developing rational pathways for nanosized particle preparation.
Formation of crystalline TNPs at ambient conditions was also found in [17–19,21], though there are plenty publications devoted to the problem of low-temperature synthesis. The preparation of rutile in [17] included long-aging procedure continuing for 20 days. As-synthesized titania in [19] was not-crystalline. It was transferred in rutile after the extraction of ionic liquid applied as the reaction media. The following article reported about the synthesis of brookite, another crystalline form, was performed at 100 ◦ C like typical hydrothermal route [20]. A low-molecular weight extracellular protein of fungus Fusarium oxysporum mediated the rutile formation [21], but this is the unordinary templating agent which, furthermore, is expensive. Lecithin offers several advantages. This is the cheap natural surfactant manufactured commercially because of its wide applications in food, pharmaceuticals and cosmetics [29]. The TNPs are synthesized fast at room temperature. As-prepared titania is in its high-temperature modification. An additional treatment or aging is not needed. 5. Conclusions Lecithin, which is the natural surfactant separated from lipid matrix of biological membranes, was applied first for templating synthesis of nanosized titania. We have found unexpectedly the formation of crystalline TiO2 nanoplatelets when trying to prepare rod-like or fibrillar TNPs through hydrolytic route in media with reversed cylindrical (polymer-like) micelles. The surprising thing was that the high-temperature modification was synthesized at room temperature. Any additional treatment like aging or calcination was not carried out. Crystalline TNPs were formed fast in one stage without the pH control that was demonstrated by means of high-resolution TEM and X-ray diffraction. By attempting to gain an insight into the structure-driven synthesis, we revealed a previously unknown transformation of reversed polymer-like lecithin micelles into planar bimolecular layers constituting lamellar L␣ mesophase. It was induced by small amounts of water admixed into nonaqueous solution. The water at these concentrations localized mainly in the lecithin polar region as the
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hydration shell. It triggered the hydrolysis–condensation reactions after the titania precursor addition that results in instantaneous titania nucleation. Planar lamellar lecithin sheets specified the formation of thin crystalline nanoparticles. Acknowledgements This work was partially supported by grants from the Presidium of Far East Department, Russian Academy of Sciences and a grant-in-aid for the World Class University Program from the Ministry of Education, Science & Technology of Republic Korea (No. R32-2008-000-10174-0). We appreciate the help of Prof. D.-W. Park with the X-ray diffraction measurements, Dr. D. Burgemeister and Prof. C. Schmidt with the NMR-spectroscopy measurements and Ms. C. Thunig with the polarizing microscopy as well as for fruitful discussions with all them. References [1] K. Kabra, R. Chaudhary, R.L. Sawhney, Ind. Eng. Chem. Res. 43 (2004) 7683. [2] S. Mori, S. Yanagida, in: T. Sogo (Ed.), Nanostructured Materials for Solar Energy Conversion, Elsevier, Amsterdam, 2006 (Chapter 7). [3] J. Bisquert, in: J.A. Rodriguez, M. Fernandez-Garcia (Eds.), Synthesis, Properties, and Applications of Oxide Nanomaterials, John Wiley & Sons, Hoboken, NJ, 2007 (Chapter 16). [4] G. Colon-Ibanez, C. Belver-Colderia, M. Fernandez-Garcia, in: J.A. Rodriguez, M. Fernandez-Garcia (Eds.), Synthesis, Properties, and Applications of Oxide Nanomaterials, John Wiley & Sons, Hoboken, NJ, 2007 (Chapter 17). [5] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891. [6] U.I. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C Photchem. Rev. 9 (2008) 1. [7] G. Liu, L. Wang, H.G. Yang, H.M. Cheng, G.Q. Lu, J. Mater. Chem. 20 (2010) 831. [8] F.E. Osterloh, Chem. Mater. 20 (2007) 35. [9] J. Ryu, W. Choi, Environ. Sci. Technol. 42 (2007) 294. [10] R.J.D. Tilley, Crystals and Crystal Structures, John Wiley, Chippenham, 2006. [11] Physico-Chemical Properties of Oxides, Metallurgy, Moscow, 1978. [12] C.J. Brinker, G.W. Scherer, Sol–Gel Science. The Physics and Chemistry of Sol–Gel Processing, Academic Press, Boston, 1990. [13] A.C. Pierre, Introduction to Sol–Gel Processing, Kluwer, Boston, 1998. [14] M. Niederberger, N. Pinna, Metal Oxide Nanoparticles in Organic Solvents. Synthesis, Formation, Assembly and Application, Springer, London, 2009. [15] X.Q. Chen, W.H. Shen, Chem. Eng. Technol. 32 (2009) 1061. [16] P. Zhang, S. Yin, T. Sato, Mater. Res. Bull. 45 (2010) 275. [17] D. Zhang, L. Qi, J. Ma, H. Cheng, J. Mater. Chem. 12 (2002) 3677. [18] Y. Lu, M. Hoffmann, R.S. Yelamanchili, A. Terrenoire, M. Schrinner, M. Drechsler, M.W. Moller, J. Breu, M. Ballauff, Macromol. Chem. Physics 210 (2009) 377. [19] H. Kaper, F. Endres, I. Djerdj, M. Antonietti, B.M. Smarsly, J. Maier, Y.S. Hu, Small 3 (2007) 1753.
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Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Crystalline titania nanoparticles synthesized in nonpolar L␣ lecithin liquid-crystalline media in one stage at ambient conditions Yury Shchipunov a,b,∗ , Anna Krekoten a a
Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Division of Chemical Engineering, Pusan National University, San 30, Jangjun Dong, GeumjungGu, Busan 609-735 Republic of Korea b
a r t i c l e
i n f o
Article history: Received 26 February 2011 Accepted 30 April 2011 Available online 6 June 2011 Keywords: Lecithin Bimolecular layer Template synthesis Titania nanoparticles
a b s t r a c t High-temperature modification of titania in the form of nanoplatelets is synthesized fast in one step at ambient conditions without any additional treatment like aging or calcination. Lecithin, which is the main component of lipid matrix of biological membranes, is first used as a structure-driven template. It is demonstrated that this natural surfactant can self-organize into lamellar L␣ mesophase when small amounts of water are admixed in its solution in nonpolar solvent. The water locating mainly in lecithin polar region as hydration shell at this concentration triggers the hydrolysis–condensation reactions after the precursor addition that results in instantaneous titania formation in the form of crystalline nanoparticles. Planar lamellar sheets serve as the template specifying its crystallinity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide TiO2 or titania nanoparticles (TNPs) well attract particular interest because of their great potentialities for developing very effective photocatalysts, solar-energy converter and sensors, characterizing high chemical stability and low toxicity. This activity is obvious from some recent reviews [1–8]. The efficacy of materials and devices on the basis of TNPs is highly dependable on their dimension, crystallinity and crystalline form of TiO2 [2,3,7,8]. As an example, it is a P25 photocatalyst developed by the Degussa. At present it is the best commercial product [9], using as a standard. The highest photocatalytic activity occurs in a mixture of 85–70% anatase and 15–30% rutile. They both are the main crystalline form of TiO2 . Their structure can be presented by an octahedron in which six oxygen atoms are at the corners, surrounding a titanium atom [10]. The difference between these crystalline forms is in the distortion of octahedra of oxygen atoms in anatase. The absence of distortion makes the rutile form most dense and stable. The problem is that titania synthesized at ambient conditions is amorphous. To transfer it into a crystalline form, TiO2 is heat treated. A transitions to anatase is observed in the range
∗ Corresponding author at: Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia. Tel.: +7 4232 314481; fax: +7 4232 311889. E-mail addresses: [email protected], [email protected] (Y. Shchipunov). 0927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.04.038
of 500–800 ◦ C and then to rutile at 915 ◦ C [11]. There are quite abundant efforts to obviate the need for the high-temperature calcination, aiming to find an effective low-temperature route for TNPs fabrication. Methods applied to fabricate TNPs in solutions can be separated into two main groups, non-hydrolytic and hydrolytic [5,12–14]. They can be carried out through a sol–gel, hydrothermal, solvothermal, sonochemical, microwave or electrochemical synthesis that enables one to prepare nanosized crystalline titania of different shape and dimensionality. The hydrothermal methods hold much favour. They are usually performed at 150–200 ◦ C or even lower temperature around 50–60 ◦ C [15,16]. The TiO2 is synthesized fast, especially in hydrolytic routes, but the degree of its crystallinity is low. To promote further crystallizationof the product, it is put through more or less prolonged additional treatment(s). For instance, it can be left in the same hydrothermal solution under refluxing at increased temperature up to few days. To our knowledge, there are few published articles in which crystalline titania was prepared at ambient conditions. Zhang et al. [17] synthesized rutile through the sol–gel processing by using reverse micelles as a template. It seems that the crystallinity of as-prepared TiO2 was poor. The product was left for aging for 20 days. A faster preparation procedure was suggested by Lu et al. [18]. They used a template consisting in polysterene latex surrounded by covalently attached anionic polyelectrolyte brushes. TNPs of 4–12 nm synthesized by controllable precursor feed rate at room temperature were mainly precipitated on the brushes. As shown by means of X-ray diffraction, they were crystalline, consisting of anatase. Kaper et al. [19] used an ionic liquid. They could synthesize
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a rutile by means of the sol–gel processing at ambient conditions. It is of interest that as-prepared titania was amorphous. A transition to crystalline form occurred in the course of extraction of ionic liquid. When recently repeating the synthesis by adding surfactants, these authors carried out it at 100 ◦ C like typical hydrothermal route [20]. Bansal et al. [21] prepared crystalline TiO2 from K2 TiF6 by rather uncommon way. They performed syntheses in the presence of fungus Fusarium oxysporum. It is the author’s opinion that the TNP formation was mediated by a low-molecular weight extracellular protein. The application of surfactant templates to control and regulate the formation of any nanoparticles, not only TNPs is practised widely [5,14,22–26]. Furthermore, surfactants serve as a stabilizing agent, preventing TNPs against the merging and agglomeration. In their absence, this role should be fulfilled by molecules of organic solvents in nonaqueous syntheses [14,27]. Here we performed a template synthesis of TNPs at ambient conditions by using lecithin. Lecithin is one of the main lipid components of biological membranes [28], using widely as a natural surfactant [29]. It can self-organize into reversed polymer-like micelles in organic media in the presence of trace amounts of water [30,31]. We expected the formation of rod-like TNPs in the micellar core by regulating the sol–gel processing by means of controlled amount of H2 O. This effective way was previously realized with polysaccharides dissolved in ethylene glycol [32]. Unexpectedly, we found nanosized platelets of highly crystalline titania that was in its high-temperature modification, rutile. As demonstrating in the article, the structure-directing synthesis was caused by templating the TiO2 on lecithin L␣ -mesophase. Its formation is brought about by a previously non-described transition of polymer-like micelles into multilamellar state after adding a critical amount of water in a lecithin solution. Conditions, at which the fast direct synthesis of crystalline titania occurs at ambient conditions, its examination by X-ray diffraction and high-resolution transmission electron microscopy as well as lecithin template by 2 H NMR spectroscopy, polarizing optical microscopy and dynamic rheology are reported. 2. Materials and methods 2.1. Chemicals Soybean lecithin Epikuron 200 purchased from Lukas Meyer, AG (Germany) and titanium(IV) isopropoxide, from ABCR were used as supplied. n-Heptane was of puriss quality, water, doubly distilled. 2.2. Lecithin organogel L˛ mesophase preparation It is detailed in our previous publications [33,34]. Weighed amounts of lecithin and water were placed in appropriate volume of n-heptane and stirred by magnetic stirrer at 40–50 ◦ C as long as a homogeneous mixture had not been formed. After that it was allowed to stay at the ambient temperature at least for 3 days to reach a homogeneous distribution of H2 O in the sample. L␣ mesophase was prepared by the same protocol, except that more water was added than needed for the organogel. 2.3. Titania synthesis Lecithin organogel or L␣ mesophase prepared as described above were taken within three days after the preparation. A weighted appropriate amount of titania precursor was introduced and vigorously stirred to reach its homogeneous distribution in the mixture as fast as possible. It was performed at ambient temperature or slight heating (up to 40 ◦ C) to decrease the viscosity. The precursor addition resulted is a drastic change of solution properties. First of all the viscosity was sharply decreased. Further-
Fig. 1. The zero-shear viscosity vs. the water to lecithin molar ratio. The lecithin concentration is 28 mg/ml. The vertical lines show rough boundaries between various types of systems formed with changing the water/lecithin molar ratio, but the transitions from low-viscous solution to the viscoelastic state and then into the liquid-crystalline mesophaseoccur smooth.
more, there was a transition of transparent organogel or mesophase into a turbid, nontransparent system. The following transformation depended on the amount of admixed precursor. The viscosity could sharply increased that accomplished by a transition into solid-like state or it could unchanged or solid precipitate could appear in the mixture. 2.4. Material characterization Powder X-ray diffraction analysis was performed by using a ˚ Samples for TEM observations Rigaku Miniflex, CuK␣ ( = 1.5418 A). were prepared by dispersing ultrasonically a synthesized material in ethanol and then drying a putted drop of solution on a copper grid at room temperature. Pictures were taken by a JEM-2010 transmission electron microscope (JEOL) at an accelerating voltage of 200 kV. Samples for deuteron NMR experiments were prepared with deuterated water. 2 H NMR spectra were taken by a Bruker MSL 300 NMR spectrometer at a deuterium resonance frequency of 46.07 MHz. Rheological measurements were performed only with lecithin organogels by means of a Rotovisco RT 20 stress-controlled rheometer (Haake). The rheometer was run in either an oscillation or creep regime as described in [35]. A cell had a cone and plate geometry. Optical microscopy observations were carried out with a polarizing microscope (Zeiss). Organogel samples were placed in a rectangular capillary and left to equilibrate for a day at ambient temperature before the examination. 3. Results 3.1. Characterization of templating system used for the syntheses Lecithin is readily soluble in organic solvents including nheptane used as a reaction media here. Its solutions are low-viscous, demonstrating Newtonian behavior. Solubilized lecithin forms small reversed spherical micelles from firth molecules in nonpolar media [36]. As soon as trace amounts of water are introduced, one can find a sharp increase in the solution viscosity. This is illustrated by Fig. 1 in which there is a dependence of the zero-shear viscosity on the water to lecithin molar ratio. The sharp increase in viscosity looks like a jellification. This jellylike solution was termed the organogel [37], whereas it is a not a real gel, being a viscoelastic system with a Maxwell behavior (see Refs. [34,38,39]). The viscosity increase is brought about by a transformation of the spherical micelles into cylindrical aggregates [31]. Their contour length can reach 1 mm, while the diameter is of about
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Fig. 2. TEM images of nanoparticles synthesized in the lecithin-containing solutions. Nanocrystal outlines in B are shown by light brown. Sample compositions: (A) molar ratio of water to the lecithin was 7, lecithin concentration, 11 wt.%, titanium(IV) isopropoxide, 8.2 wt.%; (B) molar ratio of water to the lecithin was 5, lecithin concentration, 11 wt.%, titanium(IV) isopropoxide, 6.7 wt.%.
5–7 nm. These lengthy micelles are called the polymer-like ones because of their entanglement like linear polymer macromolecules do it, making up a temporal three-dimensional network in the bulk [40,41]. The lecithin organogel phase exists in a rather narrow range of the water/lecithin molar ratios. Rough boundaries can be seen in Fig. 1. The real structural organization of the system adjacent from the right side in the plot was not revealed before this study. It was mainly examined in details by means of the dynamic rheology. It was inferred from obtained results that there is a transformation (see Refs. [39,42,43]), but the nature of restructuring has been unclear. On study on lecithin solutions with increased water/lecithin molar ratios by using 2 H NMR spectroscopy and polarizing microscopy, we came to a conclusion about a transition from the polymer-like micelles to the multilamellar liquid-crystalline state. This is considered in needed details in the following section. 3.2. Titania synthesis and characterization Syntheses have been performed in the whole range of water/lecithin molar ratios seen in the plot in Fig. 1. It means that various lecithin structures were examined as the template. The admixing of titania precursor into a lecithin solution without added water did not cause a notable change in it. When trace amount of H2 O had been added before the precursor, its introduction resulted in a change of optical and mechanical properties. The initial organogel was completely transparent. It became turbid and then the transparency could returned with few minutes or the organogel remained nontransparent. These transformations depended on the precursor amount added. In parallel with the change of optical properties there was also a change in the viscosity. With admixing the precursor, sharp thinning was first observed. Next changes depended on the precursor amount added. The viscosity could increase somewhat, transfer into the solid-like state or separate into solid and liquid phases. In case the separation was absent, the transformations were accomplished by the formation of a monolithic, optically transparent or opalescent material. The transparent or slightly opalescent samples were of chief interest. Their examination under the transmission electron microscopy (TEM) revealed planar nanosized particles instead of expected rod-like or fibrillar ones. Representative images are given in Fig. 2. Nanoparticles were of two main types, differing in shape and dimensions. As seen in Fig. 2, they were rectangle or hexagonal. The particle dimensions were around 50 nm (Fig. 2A) or 15 nm (Fig. 2B).
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Fig. 3. High resolution TEM images of nanoparticles with crystalline lattices of two types found in the samples. The corresponding Fast Fourier transform images are inset. Lecithin organogel compositions of (A) and (B) correspond to that in Fig. 2.
The following unexpected result was that these nanoparticles were crystalline. Representative images of lattices observed by means of the high-resolution TEM are shown in Fig. 3. As obvious from measured spacings indexed in the pictures, they are of two types equal to ca. 0.3 and 0.6 nm. The crystallinity of samples was further confirmed by X-ray diffraction (XRD). Fig. 4 presents X-ray diffractograms of three samples of various compositions. One can see well-resolved sharp reflections. The XRD peaks may be assigned to the (0 0 4) crystal face of anatase, (2 1 0), (3 1 0) and (3 2 1) of rutile. It is worthy mentioning that this XRD pattern was well reproducible. The same number of the same reflections was obtained with samples prepared in various syntheses. The surprising thing is that the high-temperature modification was synthesized at ambient conditions without heating or following heat treatment. It is obvious from Fig. 4 that the variation of precursor amount in a rather narrow range occurred a profound effect on the crystallinity of product. The water and lecithin also influenced much the TNP formation. Fig. 5 shows how it depended on the lecithin concentration at a fixed molar ratio of water to titania precursor in the reaction system. The TiO fabrication in crystalline form is regulated by all three components that present a severe difficulty with revealing a certain correlation. Our observation showed that it took place only in the area in which solidified optically transparent material was formed.
Fig. 4. Powder XRD analysis of titania samples as-synthesized at ambient conditions. Reaction composition: molar ratio of water to the lecithin was 7.5, lecithin concentration, 11 wt.%, titanium(IV) isopropoxide concentrations expressed in wt.% are shown alongside the corresponding diffractogram. The A and R mark, respectively, anatase and rutile.
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hPeak (a.u.)
206
10
3
10
2
0.0004
0.0008
0.0012
CLecithin (mol) Fig. 5. The peak height at 37.9◦ in diffractograms (see Fig. 4) vs. the lecithin concentration in the reaction media. Water to titania precursor molar ratio was 1.05.
4. Discussion The TiO2 synthesis was performed in the common hydrolytic route. As soon as the titania precursor comes into contact with water, it triggers instant hydrolysis [12,13]: Ti(–OR)4 + nH2 O → (HO–)n Ti(–OR)4−n + nHO–R and condensation reactions: (RO–)4−n Ti(–OH)n + (HO–)n Ti(–OR)4−n → (OH)n−1 (RO–)4−n Ti–O–Ti(–OR)4−n (OH)n−1 + H2 O or Ti(–OR)4 + (HO–)n Ti(–OR)4−n → (RO–)3 Ti–O–Ti(–OR)4−n (OH)n−1 + HO–R where R is a hydrocarbon radical and n ≤ 4. The reactions proceed with very high rate. Therefore, the titania if formed instantly where the water is localized. As shown in [32], this enables one to template TiO2 , tailoring its structural organization. Here it was realized through templating on lecithin polar region. A structural formula of lecithin may be seen in Fig. 6A. It is a diacyl derivative of 3-glycerophosphoric acid, containing also choline attached to the phosphoric acid residue [28,44,45]. This 1,2-diacylsn-glycero-3-phosphocholine is called often phosphatidylcholine or lecithin. It is soluble in organic solvents and swells in water,
Fig. 6. Structural formula (A) and space filling model (B) of lecithin molecule. Area of preferential location of water molecules as hydration shell is shown by blue. It is anticipated that it serves as a nucleating center for titania.
forming lamellar mesophase L␣ [29,44,46]. When being in nonpolar media, lecithin absorbs or solubilized water that resulted in transformation of small reversed spherical micelles into cylindrical aggregates called the polymer-like ones [30,31]. The water introduced is involved into the hydration of lecithin polar region. This phospholipid shows a strong affinity for the water owing to extended polar region. It can absorb up to 21H2 O molecules per lecithin molecule from vapor, while its whole hydration shell can include 33 to 39 water molecules [29,30,46,47]. Crystalline titania was found in systems in which molar ratio of H2 O/lecithin varied between 5 and 8 (see Figs. 2 and 4). It means that the major portion of introduced water was in the hydration shells, not in the solution bulk that, among other things, is nonpolar. As revealed in [33,48], the H2 O molecules, which induced the organogel formation, are mainly attached to the phosphate and carbonyl groups through hydrogen boundings (Fig. 6B). They serve as a linking bridge between them, providing the stabilization of huge cylindrical micelles [33,49]. Their core consists of linear networks from alternating sequence of water molecules and phosphate groups of lecithin molecules. When beginning with the titania synthesis in the organogel phase, we followed the common knowledge that there are the polymer-like micelles. It was reasonable to expect the formation of rod-like or fibrillar TNP. The fabrication of plate nanoparticles (Figs. 2 and 3) poses the problem of real structural organization of lecithin. The polymer-like micelles exist in a rather narrow range of water to lecithin molar ratios [31,34]. The optimal ratio is around 3.2 where is the maximum in Fig. 1. Crystalline titania was found at larger values being between 4.5 and 8. This is a region of significant change in the rheological properties of lecithin organogel [31,39,43,50]. The used dynamic rheology could not provide an insight into the reasons for observed changes and real structural organization of lecithin in the region of increased water to lecithin molar ratios. It became apparent after an application ofsuch techniques as 2 H NMR spectroscopy and polarizing optical microscopy. Results thus obtained showed unambiguously that the micellar system transformed into a liquid crystalline state which can be assigned to the L␣ mesophase. It consists of planar bimolecular lecithin films stacked into a multilamellar structure [29,44]. Corresponding experimental data (I–VII) with schematic drawings(A–D) of possible self-organized lecithin structures and their transformations with increasing the water amount are summarized in Fig. 7. The initial state is represented by reversed spherical micelles (A). Their solution is low-viscous. As followed from frequency dependencies of the complex viscosity |*|, storage modulus G and loss modulus G in Fig. 7I, it demonstrates the characteristic rheological behavior that is typical of systems called Newtonian liquids [51]. The water addition in trace amounts brings into transition to viscoelastic state. This is obvious from the frequency dependences of rheological parameters (Fig. 7II) of the same solution after the addition of 3.0 molecules per lecithin molecule. This type obeys the Maxwell behavior that is indicative of a system made up of a three-dimensional network of entangled polymer-like micelles (see Refs. [31,34]). A micellar segment is schematically presented by drawing in Fig. 7B. As followed from 2 H NMR spectrum (Fig. 7III), the organogel is isotropic. The further added water results also in a change of rheological properties (not shown, see Refs. [39,50]). Examination under the polarizing optical microscope revealed a transition from isotropic to anisotropic system. The texture seen in the picture in Fig. 7IV is called “oily streaks” [52,53]. The anisotropy is also confirmed by means of 2 H NMR spectroscopy. One may see the quadrupolar splitting of a line in spectrum in Fig. 7V. Both these methods point
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Fig. 7. Schematic drawings (A)–(D). They present water-induced transformations of initial reversed spherical micelles (A), into cylindrical one (B), bimolecular layer (C) as the main structural block of L␣ mesophase and this bimolecular layer with titania after the addition of TiO2 precursor (D). Experimental data I–VII. (I) The complex viscosity |*|, storage modulus G and loss modulus G vs. the oscillation frequency of the initial solution with 28 mg/ml lecithin. (II) The similar frequency dependences of rheological parameters of the same solution after addition of 3.0 molecules per lecithin molecule. (III) 2 H NMR spectrum of isotropic organogel consisting of 50 mg/ml of lecithin and 3.2 water molecules per lecithin one. (IV) The texture observed under the polarizing optical microscope. Lecithin concentration was 60 mg/ml, water to lecithin molar ratio 5.5. (V) Thequadrupolar splitting of a line occurred in 2 H NMR spectrum. Molar ratio of H2 O to lecithin 4.5. (VI) A TEM image of titania nanoparticles synthesized in liquid-crystalline media. Lecithin amount 11 wt.%, water/lecithin molar ratio 5. (VII) X-ray diffractogram of TiO2 nanoparticles shown above.
to the formation of a well-organized lamellar L␣ phase consisting of stacked planar bimolecular layers as shown by schematic drawing in Fig. 7C. More details concerning the mechanism of polymer-like micelles transformation into lamellar mesophase is in an article that is under the preparation. It seems that the planar bimolecular films serve as a template for titania. The precursor introduced will not be involved into hydrolysis-condensation reactions before its contact with H2 O. Water molecules are in the form of a hydrating shell mainly of phosphate groups in this concentration range (Fig. 6B) [33,48,49,54]. When reaching it, the precursor molecule will experience the instant hydrolysis, resulting in the nucleation of reaction product on lecithin bimolecular layer (Fig. 7D). The negatively charged phosphate groups (Fig. 6A) can serve as nucleating center for titania because of positive charge both of titanium atom and its oxide [12]. The planar alignment of lecithin molecules in bimolecular layers results in the formation of inorganic particle (Fig. 7VI) with crystallographically determined lattice (Fig. 7VII). To our knowledge, this is a first application of lecithin as the template for metal oxide preparation. We met only an article by Yoon et al. [55] in which lecithin vesicles were used to fabricate a titania nanocomposites, not TNPs. Nanoparticles were synthesized separately. Thereupon they were introduced into lecithin-containing system. Here it was demonstrated that lecithin can manipulate the synthesis of nanosized titania. An important point is that the TNPs synthesized on lecithin template are in crystalline state although they were prepared at the ambient conditions. This is unordinary result. The common practice is to perform a synthesis at increased temperature (around 200 ◦ C in the hydrothermal routes [5,14]) or treat the initially fabricated amorphous titania at increased temperature (400–1000 ◦ C [5,32]). The use of lecithin simplifies the synthesis, enabling us to prepare nanosized TiO2 in one step at mild conditions. Preliminary experiments have demonstrated that our approach can be extended for other metal oxides. It allows us considering it as a versatile synthetic route for metal oxide nanomaterials with unprecedented control on their structure, developing rational pathways for nanosized particle preparation.
Formation of crystalline TNPs at ambient conditions was also found in [17–19,21], though there are plenty publications devoted to the problem of low-temperature synthesis. The preparation of rutile in [17] included long-aging procedure continuing for 20 days. As-synthesized titania in [19] was not-crystalline. It was transferred in rutile after the extraction of ionic liquid applied as the reaction media. The following article reported about the synthesis of brookite, another crystalline form, was performed at 100 ◦ C like typical hydrothermal route [20]. A low-molecular weight extracellular protein of fungus Fusarium oxysporum mediated the rutile formation [21], but this is the unordinary templating agent which, furthermore, is expensive. Lecithin offers several advantages. This is the cheap natural surfactant manufactured commercially because of its wide applications in food, pharmaceuticals and cosmetics [29]. The TNPs are synthesized fast at room temperature. As-prepared titania is in its high-temperature modification. An additional treatment or aging is not needed. 5. Conclusions Lecithin, which is the natural surfactant separated from lipid matrix of biological membranes, was applied first for templating synthesis of nanosized titania. We have found unexpectedly the formation of crystalline TiO2 nanoplatelets when trying to prepare rod-like or fibrillar TNPs through hydrolytic route in media with reversed cylindrical (polymer-like) micelles. The surprising thing was that the high-temperature modification was synthesized at room temperature. Any additional treatment like aging or calcination was not carried out. Crystalline TNPs were formed fast in one stage without the pH control that was demonstrated by means of high-resolution TEM and X-ray diffraction. By attempting to gain an insight into the structure-driven synthesis, we revealed a previously unknown transformation of reversed polymer-like lecithin micelles into planar bimolecular layers constituting lamellar L␣ mesophase. It was induced by small amounts of water admixed into nonaqueous solution. The water at these concentrations localized mainly in the lecithin polar region as the
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