Physicochemical characterisation of the nanocomposites
4.2 Dimensions of MWCNTs in the nanocomposites and in their pristine state
E.T. Mombeshora Page 91 synthetic method on the ultimate morphology of the nanocomposites, e.g. the use of an ultrasonic water bath during the sol-gel method. Also, it is noted that at a 1:1 ratio of MWCNT:titania the nanocomposites were more spaghetti-like (see Figure 4.4), i.e. more randomly oriented in the sol-gel method than the CVD method.20
Figure 4.4: A comparison of the morphology at a 1:1 wt.% ratio of MWCNTs:titania for the nanocomposites synthesised by the (A) CVD and (B) sol-gel methods
4.2 Dimensions of MWCNTs in the nanocomposites and in their
E.T. Mombeshora Page 92 Figure 4.5: Representative TEM image for pristine MWCNTs
The results showed that a large percentage (39%) of the pristine MWCNTs had an outer diameter (OD) between 16-25 nm (see Figure 4.6).
Figure 4.6: A comparison of the outer diameter distribution for pristine and acid-treated MWCNTs
A fairly good uniformity of diameters was observed in the pristine MWCNTs. The average OD of pristine MWCNTs was 30 ± 15 nm. On acid treatment (see Figure 4.6), the fraction of MWCNTs with ODs between 36 and 45 nm increased from 15% to 30%, and in general the tubes increased in OD. The average OD after acid treatment was 34 ± 16 nm. This is best
5-15 16-25 26-35 36-45 46-55 56-65 66-75 76-85 0
5 10 15 20 25 30 35 40
Population (%)
Outer diameter (nm)
- Pristine MWCNTs - Acid treated MWCNTs
E.T. Mombeshora Page 93 explained by the hydrochloric acid purifying mechanism reported by Fan et al.2 which states that hydrochloric acid attack MWCNT defective sites causing expansion of tube walls followed by peeling off. Therefore, since hydrochloric acid was part of the acid treatment mixture that means the ultrasonic treatment was too mild to cause peeling off after expansion. Furthermore, the nitric acid treatment also debundled the MWCNTs so that a greater variety of MWCNT diameters were obtained causing a decline in homogeneity of the MWCNTs.22 TEM results correlated well with both the crystalline quality and thermal stability results discussed in the subsequent sections (see sections 4.4.1 and 4.5). For example, R (𝐼𝐷
𝐼𝐺, i.e. the ratio of the Raman D band to G band, see section 2.7.2) increases while thermal stability decreases on acid treatment. This may be explained by slight roughening of the tube walls (see Figure 4.7). This observation is in agreement with the work of Scalese et al.23 However, not all tube walls were roughened.
Figure 4.7: Representative TEM image for acid-treated MWCNTs showing the resulting roughening of the tube walls
All the nanocomposites from the sol-gel method (see Figures 4.8 and 4.10) showed particulate titania on the tubular MWCNT walls although the degree of coating was strongly influenced by wt.% ratios.12 With an increase in titania, the coating became more uniform
E.T. Mombeshora Page 94 and the coverage of the MWCNT tube walls more continuous. This observation was in strong agreement with the report by Vincent et al.24 in that the MWCNT surface is a preferred site of titania crystallisation.
Figure 4.8: Representative TEM images for MWCNT-titania nanocomposites synthesised by the sol-gel method at (A) 5 and (B) 10 wt.% of MWCNTs
In the nanocomposites with low wt.% of MWCNTs, i.e. high titania wt.% (a representative sample is 10 wt.% MWCNTs) the most frequent OD range shifted to 26-35 nm (see Figure 4.9) for both methods. The CVD method seems to be coating smaller diameter MWCNTs pretty well since the OD range of 5-15 nm and that of 16-25 nm decreased in frequency from 17% to 0% and 27% to 5% respectively (see Figure 4.9C). A similar trend is seen in the sol-gel method where the 5-15 nm frequency decreased from 17% to 3%, although the most significant drop was in the 36-45 nm range, i.e. from 29% to 12% (see Figure 4.9C). CVD nanocomposites were thicker than those by the sol-gel method.
However, the CVD method gave some tubes with ODs above 95 nm, i.e. 22%, which was far more than twice the OD of uncoated MWCNTs (see Figure 4.9). This was due to more than one tube being clustered together as a bundle, i.e. two MWCNT tubes were coated together with titania in some instances (see additional information in Figure G2, Appendix G). The changes in population of diameter ranges showed that the CVD method gave better coatings than the sol-gel method although the coating thickness was spread over a wider range (see Figure 4.9B and C).
E.T. Mombeshora Page 95 Figure 4.9: Distribution of MWCNT outer diameters in MWCNT-titania nanocomposites containing 10 wt.% of MWCNTs. Comparison of: (A) sol-gel and CVD methods, (B) before and after sol-gel coating, and (C) before and after CVD coating
At high wt.% of MWCNTs, i.e. low titania wt.%, represented by the 90 wt.% MWCNT samples (see Figures 4.10 and 4.12) from both methods, the dominant OD ranges were the 26-35 nm (25%) and 36-45 nm (35%) ranges for the CVD method and 16-25 nm range for the sol-gel method (see Figure 4.11). According to these observations, it may suggest that the sol-gel method is a better method of loading low wt.% titania onto MWCNTs because it gave a narrower distribution of MWCNT outer diameters.
E.T. Mombeshora Page 96 Figure 4.10: Representative TEM images for MWCNT-titania nanocomposites synthesised
by the sol-gel method at (A) 90 and (B) 80 wt.% of MWCNTs
Figure 4.11: A comparison of the MWCNT outer diameter distribution for MWCNT-titania nanocomposites at 90 wt.% of MWCNTs by the sol-gel and CVD synthetic methods
5-15 16-25 26-35 36-45 46-55 56-65 66-75 76-85 86-950ver 100 0
10 20 30 40 50
Population (%)
Outer diameter (nm) - sol-gel - CVD
E.T. Mombeshora Page 97 Figure 4.12: Representative TEM images for MWCNT-titania nanocomposites synthesised
by the CVD method at (A) 90 and (B) 80 wt.% of MWCNTs
The coating on the nanocomposites with low wt.% of MWCNTs from the CVD method had more uniform coatings than those from the sol-gel method (see Figures 4.8 and 4.13).
Furthermore, some tubes from CVD method were completely covered. The nanocomposites from CVD method tend to move towards a honeycomb structure25 on crossing over from high to low MWCNT wt.% (see Figures 4.12 and 4.13).
Figure 4.13: Representative TEM images for nanocomposite synthesised by CVD method at (A) 5 wt.% and (B) 10 wt.% of MWCNTs
While most researchers, such as Gao et al.15 used surfactants, for example sodium dodecylsulfate, to coat MWCNTs, this work shows that a comparable coating can be
E.T. Mombeshora Page 98 achieved without the use of surfactants. TEM images from similar nanocomposites reported in literature (see Figure 2.8 in Chapter Two, section 2.7.4) were comparable to the CVD-loaded nanocomposites reported were especially at high wt.% of MWCNTs. At high wt.% of MWCNTs a thin layer on the tube walls was observed in both methods.27 HRTEM images for the CVD and sol-gel methods at 5 wt.% MWCNTs are shown in Figure 4.14A and B respectively at different magnifications to show the topography of the individual MWCNTs in the MWCNT-titania nanocomposites.22
Figure 4.14: Representative HRTEM images at different magnifications for MWCNT-titania nanocomposites containing 5 wt.% of MWCNTs synthesised by the CVD (A1 and A2) and sol-gel methods (B1 and B2)
The titania lattice fringes (ca. d spacing of 0.35 nm) on the MWCNT walls for both synthetic methods are observed. However, the coating from the CVD method (see Figures 4.14, A1
E.T. Mombeshora Page 99 and A2) was better because it completely covered the MWCNTs. This is in concordance with the TEM observations. In the sol-gel method some parts of the MWCNTs were bare whilst some were thickly covered (see Figures 4.14, B1 and B2) and this is in agreement with observations by Korbély et al.17 This further suggests that the CVD method is better than the sol-gel method. These results can be attributed to the vacuum effect on titania deposition in the CVD method. Furthermore, it can be seen that the MWCNTs exhibit a curved texture due to defective graphene sheets1 in the nanocomposites from both methods (see Figures 4.14A2 and G1 in appendix G).22 From the TEM analysis it can be seen that titania caused changes in the morphology and diameters of the MWCNTs.14 Also, the mass ratio influenced the ultimate morphology of the nanocomposite, i.e. a change from isolated titania particulates to a uniform coating of titania on the MWCNTs with an increase in the titania wt.% (see SEM, TEM and HRTEM in Figures 4.2, 4.3, 4.8, 4.10, 4.13 and 4.14).
This is in agreement with work reported by authors such as Aman et al.12