An Experimental Study on the Shape Changes of TiO 2 Nanocrystals Synthesized by Microemulsion-Solvothermal Method

Titanium dioxide (TiO2) nanocrystals of different shape were successfully synthesized in a new microemulsion system through a solvothermal process. The TiO2 nanocrystals were prepared from the reaction of tetrabutyl titanate (TBT), H2O, and oleic acid (OA), which were used as solvent and surfactant at 300◦C and 240◦C in a stainless steel autoclave. The sphere, polygon, and rhombus-shaped nanocrystals have been prepared at 300◦C and the dotandrod shaped nanocrystals have been synthesized at 240◦C. The effect of the reaction time on the shape and size of TiO2 nanocrystals in this method was studied in the present paper. The size distribution of TiO2 nanocrystals prepared at 300◦C for different hours is also studied. In addition, an attempt to describe the mechanism of shape change of TiO2 nanocrystals was presented in this paper.


Introduction
Nanosized metal oxide particles have attracted great attention because of their unusual size-dependent optical and electronic properties over the last two decades.Among all the metal oxide, TiO 2 , an important semiconductor, has been widely used in pigment, paints, additive, photovoltaic cell, gas sensors and photocatalysis.It is reported that nanoscale TiO 2 has peculiar properties which is not expected in bulk materials.TiO 2 is an anisotropic material with different phases, its properties are usually dependent on the size, shape, morphology, crystal structure, surface area and porosity [1].It is essential to study the shape changes in the TiO 2 nano-crystalline growth process.There are a lot of reports focusing on the synthesis of various shapes of TiO 2 nanoparticles, such as sphere [2], rod [3], needle [4],wire [5], cube [6], bullet [7], diamond [7].Several studies [3,7] have synthesized two shapes in just one kind of chemical process, while few of them have studies the shape change process of the nano-crystalline in the chemical reactions.
Although numerous techniques were successfully used to synthesize nano-crystalline TiO 2 , there were still several problems which were difficult to be solved.The TiO 2 nanomaterials prepared by the micelle method normally have amorphous structure, and calcination is usually necessary in order to induce high crystallinity, this process usually leads to the growth and agglomeration of TiO 2 nanoparticles [8].Poor stability of the microemulsion systems, large size of TiO 2 nanocrystals and poor dispersion stability also exist under solvothermal conditions.In recently years, a series of new ways for nanoparicle synthesis have been designed.Microemulsion-hydrothermal/solvothermal method is a kind of way of them.This new method combines microemulsion techniques with a hydrothermal/solvothermal process, and it has been explored for the preparation of lots of nanocrystals such as SrCO 3 nanostructures [9], Ca 10 (PO 4 ) 6 (OH) 2 nanofibers [10], BaF 2 whiskers [11], wollastonite nanowires [12], cobalt nanorods [13], aluminium orthophosphate nanocrystals [14], PbS nanocrystals [15] and CdS nanorods [16].Here we introduced a new microemulsion-solvothermal method.A kind of new microemulsion system was prepared and a higher solvothermal temperature was chosen to obtain a better crystalline state.It was found that four kinds of shape of TiO 2 nanocrystals including sphere, polygon, rhombus and rod were successfully synthesized.The shape change process of TiO 2 nanocrystals were studied based on the experimental analysis.

Experimental
2.1.Synthesis.The TiO 2 nanocrystals were prepared by a microemulsion-solvothermal method.TBT and OA were of analytical grade and purchased from Shanghai Chemical Reagent Co. without further purification.The formation of monodisperse TiO 2 nanocrystals could be divided into two steps.
Step 1: the synthesis of the microemulsion precursor.
Step 2: the manufacture of TiO 2 nanocrystals in solvothermal process.
Step 1. 1.7 mL of TBT was dissolved in 18 ml of OA under vigorous stirring at room temperature.After an hour, 2.7 mL of deionized water was added into this solution dropwise.All the mixture solution was kept stirring for another 1 hour at room temperature in a separate flask to obtain a microemulsion precursor.
Step 2. The microemulsion precursor was put into a stainless steel autoclave.The autoclave was kept at 300   was also studied.After reaction, the TiO 2 nanocrystals were harvested by centrifugation and washed with ethanol once.a JEM-200CX operating at 160 kV.The size distribution is obtained from TEM which contain thousands of nanocrystals using Scnimage, Photoshop and Origin software.For TEM and HRTEM specimen preparation, the products were redispersed in hexane and dried on the carbon-coated copper grid at room temperature before performance.

Result and Discussion
3.1.TiO 2 Nanocrystals Synthesis at 300 • C. Figure 1 represents the TEM image of the TiO 2 nanocrystals prepared by microemulsion-solvothermal method for different time at 300 • C. Several special shapes of TiO 2 nanocrystals including sphere, polygon and rhombus were obtained at this temperature.Figure 1(a) shows the TEM images of TiO 2 nanocrystals with sphere and smaller nanorods prepared at 300 • C for 0.5 hours.Figure 1(b) shows the TEM and HRTEM images for nanocrystals with sphere shape prepared at 300 • C for 2 hours.The TiO 2 nanocrystals are nearly closepacked and atomic lattice structure are clearly observed in HRTEM.Figures 1(c) and 1(d) shows the TEM and HRTEM images of TiO 2 nanocrystals with polygon and rhombus prepared at 300 • C for 4 hours and 17 hours, respectively.Anisotropic crystal growth for TiO 2 nanocrystals could be observed from the HRTEM image of Figure 1(d).The growth rate in the (001) direction is much faster than those in other directions.(200), ( 105), ( 211) and ( 204) are observed.The result is consistent with the XRD pattern of TiO 2 samples with 13 nm diameters reported by Niederberger [17].In order to study the homogeneity of the nanocrystalline, we assume that these nanocrystals are spherical.The software can be used to calculate area of each nanocrysatals, and then the distribution of nanocrystals can be geted by the corresponding diameter.The size distribution of TiO 2 nanocrystals prepared at 300 • C for different hours are also present in Figure 3.As the time increased, the average diameter of TiO 2 nanocrystals become larger and the particle size distribution become less uniform.4(a) shows a number of spherical nanocrystals prepared at 240 • C for 0.5 hours.The spherical nanocrystals were combined together.A similar phenomenon could be seen more clearly in Figure 4(b) which shows TEM images of TiO 2 nanocrystals prepared at 240 • C for 2 hours.Lots of nanorods rather than nanodots are also found in Figure 4(b) and a nanorod is chosen for HRTEM analysis.The nanorod is around 25 nm long and 6 nm wide and it can be seen as a combination of two smaller nanorods along (001) direction.Figures 4(c) and 4(d) shows the TEM images of TiO 2 nanorods at 240 • C for 4 hours and 17 hours, respectively.More nanorods could be observed as the time increased, sphere nanocrystals disappeared for 17 hours.

The Shape Change
Process of TiO 2 Nanocrystals.Figure 5 shows schematic process on the shape changes of the TiO 2 nanocrystals.It could be described as follows.In the presence of OA, reverse micelles are easily formed in nonaqueous media after a water drop.Moreover, TBT reacts with water in reverse micelles vigorously.As a result, spherical titanium dioxide sol micelles are formed at room temperature after water added.This process takes a long time along with the hydrolysis reaction.As the temperature is increased, the sol evolves will quickly transform into nanocrystalline, leading to the formation of small TiO 2 nanodots.When the reaction temperature is set at 300 • C, due to the rapid nucleation of reactive intermediates, the TiO 2 nanodots will grow in a diffusion-control pattern, leading to the formation of spherical TiO 2 nanocrystals.With the extension of reaction time, the crystal surface energy will play a more important role for the shape of TiO 2 nanocrystals.Due to high surface energy of the {001} face [18], the spherical TiO 2 nanocrystals generated at the initial reaction stage grow along (001) direction.As a result, the spherical nanocrystals are easily transformed into polygon prismatic nanocrystals.
As the nanocrystals grow along (001) direction, the specific surface area of TiO 2 nanocrystals along (001) direction is gradually lower, leading to two-dimensional growth of TiO 2 nanocrystals.Because the surface energy of the {100} face is higher than that of the {101} face, the shape of TiO 2 nanocrystals finally transform into rhombus with a further extension of time.When the reaction temperature is set at 240 • C, the reaction rate of reactive intermediates decreases.As a result, the TiO 2 nanodots are easy to assemble together to form nano-rods by a kind of oriented attachment mechanism.The oriented attachment mechanism has been reported for a variety of metal oxide system [19,20].It is said that the oriented attachment between several sphere shaped nanocrystals makes the reduction of surface energy (it is ususlly observed in (001) direction because of high surface energy along (001) direction).Therefore more stable rod shaped nanocrystals are formed at low temperature.These are our correlation analysis on the basis of experimental result, some detailed formation process of the TiO 2 nanocrystals still needs to be investigated further.

Conclusion
The anatase TiO 2 nanocrystals of different shapes have been successfully synthesized by microemulsion-solvothermal method using a new microemulsion system prepared by OA, TBT and H 2 O.The sphere, polygon and rhombus shaped nanocrystals have been prepared at 300 • C for 2 hours, 4 hours and 17 hours, respectively.and the dot and rod shaped nanocrystals has been synthesized at 240 • C for 0.5 hours and 17 hours.The result indicates that the shape of TiO 2 nanocrystals could change from dot to sphere, polygon, rhombus at 300 • C, and it could also change from dot to rod at 240 • C with the increase of reaction time.This microemulsion-solvothermal method could provide another new way for the synthesis and control of other nanocrystals with some special morphology.
Figure 2(b) shows the XRD pattern of this kind of TiO2 nanocrystals.Compared with the standard spectrum of anatase titanium dioxide shown in Figure 2(a), the main peaks corresponding to anatase TiO 2 including (101), (004),

Figure 5 :
Figure 5: Schematic process on the nucleation and growth of the TiO 2 nanocrystals.

Figure 2 (
c) presents the XRD pattern of the TiO 2 nanorods at 240 • C. The main peaks indicate the highly anatase1 crystalline structure of the TiO 2 .Compared with Figure 2(b), the main peaks is quite broad, That represents the value of degree of crystallization synthesized at 240 • C is much lower than that synthesized at 300 • C.

2
Nanocrystals Synthesis at 240 • C. Figure 4 represents the TEM image of the TiO 2 nanocrystals by microemulsion-solvothermal method for different time at 240 • C. Sphere and rod shapes of TiO 2 nanocrystals were obtained.Figure