Synthesis and Characterization of Strontium Carbonate Nanostructures via Simple Hydrothermal Method

Materials and physical measurements

All the chemical reagents used in this experiment such as Sr(NO₃)₂, ethylenediamine (C₂H₈N₂) and hydrazine (N₂H₄.H₂O) were of analytical grade and used as received without further purification. XRD patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Kα radiation. Scanning electron microscopy (SEM) images were obtained on Philips XL-30ESEM equipped with an energy-dispersive X-ray spectroscopy (EDX). FTIR spectra were recorded on a Shimadzu Varian 4300 spectrophotometer in KBr pellets.

Preparation of SrCO₃ nanostructures

Different mole ratios of Sr(NO₃)₂ and ethylenediamine were dissolved at water and mixed together. Specified value of hydrazine was added to obtain solution and the resulting solution was transferred to autoclave. The reaction was done at 140–180°C for 6–24 h and then the autoclave was gradually cooled down to room temperature. The obtained powder was centrifuged and washed several times with water and absolute ethanol for removing probably byproducts. Finally, the product was dried at 80°C for 10 h. Experimental condition of SrCO₃ formation was shown in Table 1.

X-ray diffraction pattern

Figure 1 shows XRD spectra of the obtained product. As shown in Figure 1, all of the peaks are related to SrCO₃ with orthorhombic (JCPDS = 05–0418). There are not any other peaks in this spectrum and it has been concluded that the as-synthesized product has high purity. The calculated crystal size by deby-sherrer formula was about 14.8 nm.

Figure 1:

XRD of sample no. 11.

EDAX spectroscopy

Chemical purity of synthesized product was examined with EDAX spectroscopy. As shown in Figure 2, there are Sr, C and O peaks that verified the presence of SrCO₃ in product. Si peak is related to the used substrate for EDAX analysis.

Figure 2:

EDAX spectroscopy of sample no. 11.

FT-IR spectroscopy

Figure 3(a) shows FT-IR of sample no. 7. Related peaks to SrCO₃ are seen in 1,771.34, 1,467.48, 1,070.34, 858.01 and 702.88 cm⁻¹ [18,19]. Broad peak at 3,727.42 cm⁻¹ is related to stretching vibration frequency of OH group at H₂O molecule that has been adsorbed on the sample surface [2,3]. Strong absorption peak at 1,467.48 cm⁻¹ is allocated to asymmetric stretching vibration of C–O band at CO₃^2– and weak peak at 1,070.34 cm⁻¹ is related to symmetric stretching vibration of C–O band at CO₃^2–. Also two absorption peaks at 702.88 and 858.01 cm⁻¹ can be assigned to the bending out-of-plane vibrations and in-plane vibrations [20]. Absorption peak at 1,771.34 cm⁻¹ is related to bond stretching vibrations of C = O [21]. Figure 3(b) shows FT-IR of sample no. 11. The observed peaks in this spectrum are exactly similar to FT-IR of sample no. 7, and related peaks of SrCO₃ are clearly seen at 1,771.59, 1,446.46, 170.35, 857.80 and 702.90 cm⁻¹. Figure 3(c) shows the FT-IR spectra of purified sample no. 11. The purified sample was obtained as follows: as-synthesized sample no.11 was added to water and was heated on the heater. Then the reaction was done in autoclave at 140°C for 24 h. With the purification, the peak’s intensity of prototype was increased. Broad peak at 3,428.54 cm⁻¹ is related to H₂O molecule adsorption on the sample surface.

Figure 3:

FT-IR of (a) sample no. 7, (b) sample no. 11 and (c) purified sample 11.

SEM analysis

Ethylenediamine concentration effect

Figure 4 shows Sr:en mole ratio effect on product size and morphology. When the mole ratio was selected to 1:1 (Figure 4(a)), microstructures that have been composed from microrods, irregular plates, small and large particles were obtained. By increasing the mole ratio to 1:2 (Figure 4(b)), regular rods and irregular particles were achieved. With further increase of concentration (1:3), some of the rods were broken and irregular particles, and plates have been formed (Figure 4(c)). In the mole ratio of 1:4, some of the rods were aggregated together and thick rods were obtained (Figure 4(d)). By increasing the mole ratio to 1:5 (Figure 4(e)) and 1:6 (Figure 4(f)), absence and presence of SrCO₃ rods can be seen. So in specified concentration, one dimension growth of SrCO₃ can be done.

Figure 4:

SEM image of (a–f) sample no. 1–6, respectively.

Time effect

For studying time effect on product size and morphology, the mole ratio and reaction temperature were fixed at 1:4 and 180°C, respectively. Then the reaction was done at different times. When the reaction time was selected to 6 h (Figure 5(a)), aggregated nanoparticles were obtained that by increasing the time to 12 h (Figure 5(b)), this nanoparticle became larger and lump-like masses were achieved. By increasing the time to 18 h (Figure 5(c)), the growth process has been continued and microrods were formed. When the reaction was done at 24 h (Figure 5(d)), the rods aggregated together and thicker rods were obtained. Finally with processing at 30 h (Figure 5(e)), aggregated nanoparticles were collected to micro-plates and microrod formation is still continued.

Figure 5:

SEM image of (a–d) sample no. 7–10, respectively.

Temperature effect

For investigation of temperature effect on product size and morphology, the Sr²⁺:en mole ratio and reaction time were considered 1:4 and 24 h, respectively. When the reaction was proceeded at 140°C (Figure 6(a)), small and uniform particles were obtained. By increasing the temperature to 150°C (Figure 6(b)), the growth rate became dominant in comparison to nucleation rate and, therefore, the nanoparticles have been grown one dimensional and uniform microrods were formed. When the reaction was done at 160°C (Figure 6(c)), length and diameter of the rods have been increased and aggregated together. But the temperature was selected to 170°C (Figure 6(d)), the rods were broken and finally rods, particles and plates were achieved. The morphology change indicates that morphology and reaction temperature have close relationship.

Figure 6:

SEM image of sample no. 11–14.