Physicochemical characterisation of the nanocomposites
4.5 Thermal stability
E.T. Mombeshora Page 107 The diffractogram at high wt.% of MWCNT shows that nanocomposites from sol-gel method had less symmetrical peaks and broader peaks. Furthermore, the nanocomposites with high titania wt.% by the sol-gel synthetic method had less intensity values, broader and less symmetrical peaks than CVD. In addition, if TEM images from Figure 4.13 and 4.8 from similar MWCNT:titania ratios are compared, it is seen that titania aggregates on MWCNT wall are bigger in nanocomposites by the CVD than sol-gel method. According to the diffractogram obtained, it can be suggested that nanocomposites synthesised by the sol-gel synthetic method are less crystalline than those by the CVD (see additional information in Appendix B, Figures B2-B3).37 This observation concurred with the deductions from the smaller values of R in CVD method nanocomposites except at 95 wt.% MWCNTs relative to sol-gel method (see Figure 4.16). This is due to the heat treatment17,22,23 involved in the CVD synthetic method even though nanocomposites from sol-gel also involved calcining, the period was shorter. Li et al.14 reported intensity of anatase diffraction to increase with an increase in titania coating ratio and this was in agreement with the observations in this work from both synthetic methods.
E.T. Mombeshora Page 108 MWCNTs. This correlated with the results presented on TEM images, i.e. encapsulated iron in MWCNTs (see Figure 4.5) and the qualitative analysis (see section 4.7.1).
A decrease in the thermal stability of pristine MWCNTs was observed after acid treatment (see Figure 4.18). The slight decrease in thermal stability was due to compromise of the MWCNT walls from the introduction of oxygen containing functional groups and slight defects observed in typical TEM images (see Figure 4.7) which decrease oxidative stability.32 This observation is in agreement with the report by Lehman et al.33 that states that carboxylic acid functional groups on MWCNTs reduce their thermal stability.
Figure 4.18: Comparison of the TG thermograms for pristine and acid-treated MWCNTs
Generally, in this work the decomposition temperature range for MWCNTs was between 450 and 620 C. This range was within the range reported by Li et al.26 Nanocomposites with low wt.% of MWCNTs, i.e. high titania wt.% ratio, showed a weight loss below 200 C even though the nanocomposites were dried overnight in an oven prior to the analysis and weight loss increased with increase in titania wt.% (see Figure 4.19). This correlated with the structural water27 peak observed in FTIR spectroscopy. The residual amount in Figure 4.19 was fairly close to the wt.% of titanium obtained by ICP-OES analysis of the composite.
200 400 600 800 1000
0 20 40 60 80 100
Weight %
Temperature (degrees celcius)
- Pristine MWCNTs - Acid-treated MWCNTs
E.T. Mombeshora Page 109 The slight variation could possibly arise from the residual iron introduced as a catalyst in the MWCNT synthesis.
Thermogram at high wt.% of MWCNTs ratio were steeper than at lower ratios and this may indicate a higher purity level of MWCNTs in the nanocomposites at high MWCNTs ratios (see Figure 4.19 and Table 4.4). Furthermore, at low wt.% of MWCNTs, i.e. higher wt.% of titania, thermal stability of MWCNTs decreased (see Table 4.4). Titania may act as a catalyst aiding thermal destruction of MWCNTs.34
Figure 4.19: Comparison of thermogram for MWCNT-titania nanocomposites synthesised by sol-gel method at (A) high and (B) low MWCNTs wt.%
No clear trend in thermal stability was observed (see Table 4.4) even though a decrease was expected since Raman spectroscopy showed that MWCNTs defects decreased with increase in titania wt.% (see section 4.4.1). MWCNTs in the nanocomposites with high titania wt.%
ratios synthesised by CVD method were more thermally stable than sol-gel method (see Table 4.4). This implies a higher graphitic nature of MWCNTs in the nanocomposites.
However, thermal stability was seen to decrease with increase in wt.% of MWCNTs ratio.
On the contrary, MWCNTs thermal stability was seen to decline from that of acid-treated MWCNTs (see Table 4.4) on increasing titania wt.% in the nanocomposites with high MWCNTs ratio.
E.T. Mombeshora Page 110 Table 4.4: Thermal stability temperatures for nanocomposites by the sol-gel and CVD
methods
MWCNTs wt.%
Maximum decomposition temperature /C Sol-gel CVD
5 607.5 683.7
10 558.5 635.5
20 596.9 630.6
80 610.0 597.7
90 608.7 599.0
95 604.4 624.2
Similar thermogram was obtained for nanocomposite synthesised by CVD method (see Figure 4.20). However, no weight loss was observed below 200 C. This can be attributed to synthetic method which eliminated water using the vacuum system and high temperatures involved.
Figure 4.20: Comparison of thermogram for MWCNT-titania nanocomposites by the CVD synthetic method at (A) high (B) low MWCNTs wt.%
E.T. Mombeshora Page 111 A comparison of nanocomposites from the sol-gel and CVD methods at 20 wt.% of titania (see Figure 4.21) shows that nanocomposite by the CVD method is more thermally stable and had no loss of water below 200 C. A similar comparison at 20 wt.% of MWCNTs (see Figure 4.21), also shows that nanocomposites by the sol-gel synthetic method are less thermally stable but the difference was largely pronounced. According to these results, it is seen that the method of synthesis and ratios of components of the nanocomposites has an effect on overall thermal stability of the MWCNTs in nanocomposites. It is also seen in this work that the nanocomposites from CVD method were more thermally stable than those from sol-gel method despite the wt.% ratios of components. The results on thermal stability are explained well with reference to Figures 4.2, 4.3, 4.8 and 4.13. In these Figures, it is seen that the titania coat was less uniform with more bare MWCNTs in nanocomposites by the sol-gel than by CVD method. The titania coat on MWCNTs could have limited movement of oxygen to the surface of the MWCNT and thus slow down the MWCNT decomposition. From the thermograms, a correlation with ICP-OES titanium values is seen on the overall titania wt.% ratios in the nanocomposites. According to the thermogram (see Figure 4.21A and B), titania wt.% in nanocomposites by the CVD synthetic method were closer to the targeted wt.% than those by sol-gel.
Figure 4.21: Comparison of the TG thermograms for MWCNT-titania nanocomposites by the CVD and sol-gel synthetic methods at MWCNTs wt.% of (A) 80 and (B) 20
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