Nucleation Controlled in the Aggregative Growth of Strontium Carbonate Microcrystals

The in fluence of PABA(p-aminobenzoicacid) and HEEDTA (N-(2-hydroxyethyl) ethylenediamineN, N, Ntriacetic acid) on Strontionite crystals via simple CO2 d iffusion route is described. The results showed that the experimental parameters have great influence on the shape evolution of products. The presence of templating species and varied pH are the key primary conditions for the g rowth morphology. Spike like crystals self assembled in the form of flower like and cauliflower shaped cluster with high crystallin ity were identified. The crystals undergo an interesting morphology changes and have been characterized by X-ray d iffraction (XRD), Scanning Electron Microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) techniques.


Introduction
The ability to man ipulate the morphogenesis of materials through chemical synthesis is an important requirement of modern materials [1] chemistry, due to the fact that shape, dimension, and size of materials have great influence on their physico-chemical properties and their related applicat ions. Among the variety of mo rphologies, self-assembled structures have been extensively studied due to their potential applicat ions in various nanodevices [2,3]. However, only a few groups have examined synthetic methods that lead to assembled SrCO 3 structures. Literature reports shows that, surfactants or templates, need to be disposed for obtaining pure sample [4,5]. Therefo re, it is still a challenge to develop simp le and reliable synthetic methods for the synthesis of self-assembled structures.
We are reporting the successful synthesis of SrCO 3 structures of spike like and cauliflower shaped morphology, efficiently achieved by using two organic additives-PABA and HEEDTA with simple CO 2 gas flo w d iffusion technique.

Characterization
X-ray diffraction measurements of the strontium carbonate hierarchial structures were recorded using a Rigaku diffracto meter (Cu rad iation, λ = 0.1546 n m) running at 40 kV and 40 mA (To kyo, Japan). FT-IR spectra of SrCO 3 structures were recorded with a Thermo Nicolet Ne xus (Washington, USA) 670 spectrophotometer. Crystals were collected on a round cover glass (1.2 cm), washed with deionized water and dried in a desiccator at room temperature. The cover g lass was then mounted on a SEM stub and coated with gold for SEM analysis.

Preparati on of SrCO3 Microcrystals
A typical procedure for preparat ion of crystalline SrCO 3 crystals was carried out as follows: In a g lass bottle 2.5 mmol SrCl 2 , 0.1mmol PA BA/HEEDTA were d issolved in 20 mL water and was stirred continuously to ensure complete solubility. Then the p H of the solution was adjusted to 7.0 and 10.0 by using 0.1M NaOH. A fter that the prepared solution was then covered with parafilm which was punched with three needle holes and placed in larger desiccator containing freshly crushed ammoniu m carbonate (20 g) at the bottom. After 24hr crystallizat ion, the parafilm was removed and the white precipitate deposited on the glass bottle centrifuged and washed thoroughly with distilled water, fo llo wed by ethanol and allowed to dry at room temperature fo r further crystallizat ion.

Effect of Additives on the Morpholog y of SrCO 3
Significant changes in the morphologies were observed when the pH of the reaction conditions varied between 7 and 10. Figure 1 and 2 shows the SEM images of SrCO 3 structures by adding the additives PABA and HEEDTA at different pH conditions. In general SrCO 3 dendrimers are obtained in the absence of additive [12], while in the presence of PABA at init ial p H 3.0, indiv idual spike like crystals were observed (Figure 1a, b). When the pH of the reaction mixtu re was increased to 7, the individual spikes aggregates to coniform like structures (Figure 1c, d). On further increasing the pH to 10.0, the spikes aggregates more closely and self assembled into flower like structures (Figure 1e, f). Figure 2a shows the SEM images of SrCO 3 structures in the presence of the additive HEEDTA. Remarkab le changes in the morphology was observed with this addit ive and the shape of SrCO 3 structures changed from dendrimeric to cauliflower bunch like structures with short spikes of subunits having smooth surface at in itial p H 3 (Figure 2 a, b). At neutral pH mixed phase of cau liflo wer bunches with rough surface and spadix shaped structures with fibre like units are identified (Figure 2c, d). At pH 10.0, mo rphology of the SrCO 3 structures appears similar to that observed at pH 3 with rough surface (Figure 2 e, f). The strontianite crystals clearly aggregates into cauliflower bunch like and coniform structures in CO 2 diffusion route. To the best of our knowledge, such close packed aggregates of strontionite crystals have not been observed in other bio mimet ic approaches to the growth of SrCO 3 crystals.

Structural Characterizati on of SrCO 3 Microcrystals
The XRD pattern of the isolated solids could be indexed to that orthorhombic structure of strontium carbonate and all the peaks are assigned by using JCPDS (05-418). The results implied that the aggregation did not alter the phase structure of SrCO 3 .To our expectation, the crystallinity nature of the product was sensitive to that pH conditions. At lower pH condition showed a greater crystallinity than the product prepared under higher pH condition, implying the influence of solution pH on the crystallinity. In Figure 3 (113), of pure orthorhombic strontionite respectively. It may also be seen that the peak (111) is the strongest, suggesting that SrCO 3 crystals obtained in aqueous solution grow main ly along with the (111) phase.

FTIR Studies
To identify the growth mechanism and the effect of PA BA and HEEDTA on SrCO 3 microstructures, the sample was analyzed by FT-IR spectroscopy. The sharp peaks at 856, 703 cm -1 (Figure 5 a)  The IR bands at 1476,1469 and 1465 cm -1 (Figure 5 a, b, c) correspond to the asymmetric stretching mode of C-O bond while the weak band at 1074 cm -1 (Figure 5 b, c ) is attributed to the symmetric C-O stretching vibration. The band at 3418 cm -1 (Figure 5 b) can be attributed to OH stretching vibration due to hydrogen bonding and or N-H stretch of the -NH 2 group fro m the functional groups present in additives. The band at 3423 cm -1 (Figure 5 c) can be attributed to OH stretching vibration due to hydrogen bonding and or N-H stretch of the -NH 2 group from the functional groups present in additive In comparison with Figures 5b, the C-O stretching vibration peak around 1465 cm -1 in Figure 5c, shifts to higher frequency by 4 cm -1 (1469 cm -1 ), suggesting that PABA and HEEDTA have different influence of SrCO 3 . This is probably due to the fact that the two organic mo lecules can adsorb onto the different planes of SrCO 3 nuclei and influence the mode of crystal growth, resulting in litt le change of microstructure.

Conclusions
In summary, un iform h ierarch ical SrCO 3 co mp lex structures in the form of spike like and cauliflower bunch like units were efficiently obtained by a facile ammoniu m carbonate method in the presence of PABA and HEEDTA as additives. The present uniform hierarchical SrCO 3 structures were self assembled by the related crystalline particles, assisted by the additives would enlarge the potential applications of strontium carbonate microstructures. This strategy may also be expected for the preparation of other meta l carbonates.