Physicochemical characterisation of the nanocomposites
4.4 Crystallinity and phases
E.T. Mombeshora Page 102
E.T. Mombeshora Page 103 0.9028. This result was contrary to the findings by Rinaldi et al.1 in that the R value decreased on ultrasonic treatment due to removal amorphous carbon.
Table 4.2: Comparison of D and G bands for pristine and acid-treated MWCNTs obtained from the Raman spectroscopy
Sample D band G band
ID/IG
Position Width Position Width
Pristine
MWCNTs 1347.2 60.996 1595.6 88.865 0.5112
Acid-treated
MWCNTs 1350.6 44.741 1558.0 10.740 0.9028
This trend is not expected in the MWCNTs used in this work because the TG thermogram presented in Figure 4.18 does not show presence of substantial amount of amorphous carbon. The decrease in R value may imply the decline in crystallinity and introduction of defects onto MWCNTs from acid treatment. The main impurity was the Fe catalyst as indicated by the TGA residue in correlation with EDX and TEM (see Figures 4.21 and 4.5).
The encapsulated catalyst was difficult to remove without compromising the multi-shell sp2 hybridised carbon structures constituting the walls.
General trend was observed to be upshift of the G band position by about 32 cm-1 on loading titania which is almost equal to the initial downshift by the acid treatment of MWCNTs (see additional information in Table D1 to D4 in Appendix D). This is due to titanium metal strain on C-C bonds.26 G band shift could also, have been introduced by a wide range of tube sizes, varying defect density, tube bundling and rough sample surface due to the coating process also observed by TEM and HRTEM images (see Figures 4.5- 4.19).35 No significant change on width of the D band (see additional information in Table D1 to D2, in Appendix D) may be deduced from the Raman data but G band generally tends to increase with an increase in titania wt.% in the sol-gel method. On the contrary, G band width tends to decrease with an increase in MWCNTs ratio in the nanocomposite by the CVD method (see additional information in Appendix D and Table D4) but no noticeable trend was observed on the D band. No noticeable effects were observed in nanocomposites
E.T. Mombeshora Page 104 with high MWCNTs wt.% by the CVD method, i.e. low titania wt.% ratios (see additional information in Appendix D, Table D3).
While authors such as Delhaes et al.23 reported that the G band of MWCNT is narrower than the D band, additional data in Appendix D shows an opposite trend. Peak broadening in Raman spectroscopy is influenced by temperature, excitation wavelength and crystalline nature of the nanomaterials. Hence, this was the source of the differences of this work and their work. The results from this work concurred with report by Stobonski et al.21 in that peak width of the G and D bands of the MWCNTs depends on their electrochemical environment and experimental parameters.
The spectrum was re-plotted, the bands were fit into Lorenztian curve and no other corrections were done using the Origin software. The ID/IG ratio (R) was calculated by using the area under the D- and G-band peaks respectively. The R value decreased from that of acid-treated MWCNTs in the nanocomposites (see Figure 4.16 and Table 4.2). This may imply that titania seats on the MWCNTs defects introduced during the acid treatment. This observation agreed with the work reported by Li et al.14 where reduction in R value was attributed to calcinations. Furthermore, R value (see Figure 4.16) for most nanocomposites was comparative to the range reported by Osswald et al.35 In Figure 4.16, low wt.%
MWCNTs (5-20 wt.% of MWCNTs) by the sol-gel and CVD methods, R values increased with an increase in wt.% of MWCNTs. A similar trend in R was also observed in nanocomposites with high MWCNTs wt.% (5-20 wt.% of MWCNTs). It is also seen in Figure 4.16 that nanocomposites by the CVD synthetic method had smaller R value than those by sol-gel except at 95 wt.% of MWCNTs. According to this observation it may be suggested that CVD method gave less defective and more crystalline nanocomposites than sol-gel method. This is in agreement with powder XRD results in section 4.4.2.
E.T. Mombeshora Page 105 Figure 4.16: Comparison of the ID/IG ratio of the nanocomposites by the sol-gel and CVD
synthetic methods
4.4.2 Crystal structures and phases
The XRD results are presented in Figure 4.17 and Table 4.3 gives representative spectra.
The XRD analysis shows that intensity of the main peaks of the MWCNTs increased by the acid treatment (see additional information in Appendix B). This may imply that acid treatment increased crystallinity degree of the MWCNTs.37 This is in agreement with a number of characterisation techniques, i.e. SEM, TEM and HRTEM results, TGA residual mass decrease and shape of the residual TG curve, Raman spectroscopy peaks that disappeared at 630, 702, 924 and 1149 cm-1 on acid treatment. The peaks observed from powder XRD analysis for pristine MWCNTs and after acid treatment include Miller indices (h k l) assigned to 002, 100 and 004 for 2 𝜃 of 25.9°, 42.5° and 53° respectively. MWCNT peaks were not observed in nanocomposites with high titania wt.% ratios (see Figure 4.17). The XRD results corroborate the views of many researchers such as Cong et al.37 in that anatase phase (with h k l index of 101) overlapped with the MWCNTs plane characteristic peak (with h k l index of 002). This has been explained due to similar crystal inter-planer spacing, d spacing reported to be 0.35 nm for the anatase phase titania and 0.34 nm for the MWCNTs.37,38 Table 4.3 gives the assignment of peaks at 2𝜃 angles to various Miller indices.
According to the peaks in the diffractogram, the only phase of titania in the nanocomposites was anatase.17 It should be noted that both methods gave similar XRD spectra.
E.T. Mombeshora Page 106 Figure 4.17: XRD spectra for MWCNT-titania nanocomposites with high titania wt.%
synthesised by CVD method
Table 4.3: The assigned 2𝜃 angles for anatase titania and MWCNTs in MWCNT-titania nanocomposites synthesised by the sol-gel and CVD synthetic methods
2𝜃/ (h k l)
26.2 (101) titania and MWCNTs overlap
42.1 (100) MWCNTs
44.1 (101) MWCNTs
54.1 (105) MWCNTs
38 (004) Anatase Titania 48.1 (200) Anatase Titania 55 (211) Anatase Titania 63 (204) Anatase Titania 68.8 (116) Anatase Titania 70.3 (220) Anatase Titania 75 (215) Anatase Titania
The nanocomposites by the CVD synthetic method at low wt.% of MWCNTs were observed to have sharper peaks than at high wt.%. This may imply that crystallinity of titania decreased with an increase in wt.% of MWCNTs in the CVD synthetic method.
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.