1192
F.A. Morsy et al.
Figure 9 Zeta Potential versus pH values of CaCO 3 core particles (C), silica nano-particles prepared from thermally treated SBRSA and SiO 2 /CaCO 3 nanocomposites (CS1, CS2 and CS3).
Table 5 Average crystal size, particle size, particle morphology, silica content and surface charge of SiO2/CaCO3 nanocomposites.
Samples
SiO 2 : CaCO 3 molar ratio Results
Particle morphology SiO 2 content (%) Surface charge (mV)
Av. crystal size, nm
Particle size range (nm)
17.4 20.2 22.8 28.1
CaCO 3 Core
C –
57.4
30–70
Rhombohedral
0
SiO 2 /CaCO 3 Nano- composite
CS1 1:15 CS2 1:10 CS3 1:5
92.9 80.3 80.2
40–80 60–90
Rhombohedral
1.82 2.12 2.99
Sphere-like
70–110
Spherical
FT-IR spectra as shown in Fig. 1. The broad absorption band at 3455.8 cm 1 and the peak at 1635 cm 1 are due to silanol OH groups and adsorbed water, respectively. The predomi- nant absorbance peak at 1320 cm 1 is due to siloxane bonds (Si–O–Si). The peaks at 1093, 798.3, and 451.2 cm 1 are due to the asymmetric, symmetric and the bending modes of SiO 2 , respectively (Kamath and Proctor, 1998). Fig. 2 shows the FT-IR spectra of the CaCO 3 core nano- particles and the SiO 2 /CaCO 3 nanocomposites CS1, CS2 and CS3 with SiO 2 :CaCO 3 molar ratios 1:15, 1:10 and 1:5, respec- tively. There are significant absorption peaks due to CO 3 2 group in CaCO 3 core particles (C) at 1466.86, 875.13 and 713.66 cm 1 , respectively (Nyquist et al., 1997). Because there is no split occurring at the 1465.86 and 875.13 cm 1 , one can conclude that the CaCO 3 has a calcite structure (standard cal- cium carbonate infrared spectra). Spectra of SiO 2 /CaCO 3 nanocomposites CS1, CS2 and CS3 reveal that there are absorption peaks at 3441.8, 1638.87, 1077.01, 797.38 and 463.80 cm 1 which confirms the formation of silica (Martinez et al., 1998) besides the absorption peaks of CaCO 3 , which indicates the presence of both silica and CaCO. The bands at 3441.8 and 1638.87 cm 1 are related to the constitu- tional water and those at 1086.01, 959.72, 797.38 and 461.34 cm 1 are attributed to the Si–OH stretching, Si–O–Si asymmetric stretching, symmetric stretching and bending vibrations, respectively.
3.3. Morphology and XRD analysis
Fig. 3 shows the SEM images of thermally treated SBRSA. It is observed that the ash has black with grey particles as a result of different steps of carbon combustion during the rice straw burning. The micrographs show that the SBRSA has mainly needle-like particles. Fig. 4 shows TEM micrograph of the produced silica. The silica nano-particles have narrow size distribution (particle sizes are ranged from 20–30 nm) with a spherical shape and good dispersion. Fig. 5 shows TEM micrographs of nano- CaCO 3 core particles (C) and SiO 2 /CaCO 3 nanocomposites (CS1, CS2 and CS3). The nano-CaCO 3 core particles have rhombohedral structure with particles size range 30–70 nm. SiO 2 /CaCO 3 nanocomposites CS1, CS2 and CS3 have rhom- bohedral, sphere-like and spherical morphology, with particle sizes ranging from 40–80, 60–90 and 70–110 nm, respectively. It is noticed that the SiO 2 /CaCO 3 nanocomposites are brighter than those of the single CaCO 3 particles. Fig. 6 represents par- ticle size distribution of SiO 2 /CaCO 3 nanocomposites (CS1, CS2 and CS3). Fig. 7 represents X-ray diffraction patterns of silica nano- particles prepared from SBRSA. It is clear that the typical sil- ica is observed at a broad peak centred at 2 h =22.5 , which indicates that the sample is semi-crystalline phase. Fig. 8 shows XRD patterns of the CaCO 3 nano-particles and SiO 2 /CaCO 3
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