Physicochemical characterisation of the nanocomposites
4.1 Morphology
A number of MWCNT-titania nanocomposites were synthesised by varying the wt.% of MWCNTs, i.e. 2, 5, 10, 15, 20, 40, 50, 60, 80, 90 and 95 wt.% by means of the sol-gel and CVD methods. In this work, these nanocomposites are discussed by their targeted wt.%
rather than the actual experimental loadings.
The first step in the synthesis of the nanocomposites was to functionalise the commercially purchased MWCNTs. The acid treatment using mild ultrasonic bath was principally chosen because it has been reported to cause both minimal damage to MWCNTs and to open the ends of the MWCNTs to enable the removal of impurities.1 Common impurities in MWCNTs are metal oxide particles arising from the catalyst used in their synthesis and soot-like amorphous carbon pyrolysis formed as a by-product. Such defects on MWCNTs alter their physicochemical properties such as lowering the thermal decomposition stability.1 Most importantly, amorphous carbon can interfere with MWCNT/titania interactions in electrochemistry and disperseability in ethanol during the synthetic sol-gel method.1,2 The acid treatment of MWCNTs was done to improve their dispersion in ethanol and thereby
E.T. Mombeshora Page 87 enhance chemical interactions during MWCNT-titania nanocomposite synthesis. Similar observations have been reported in the literature.1,3 In these reports the acid treatment of MWCNTs introduces oxygen-containing surface groups. This improve the overall chemical reactivity of the MWCNTs and the dispersion of metal or metal oxides onto the surfaces of the MWCNTs. Ultrasonic treatment detaches loosely attached amorphous carbon via acoustic streaming and jet pulses.1
SEM was used to check the morphology of the MWCNTs after acid treatment and in the MWCNT-titania nanocomposites. A comparison of the SEM images of the as-received pristine MWCNTs (see Figure 4.1A) and the acid-treated MWCNTs (see Figure 4.1B) indicates fewer agglomerates in the acid treated-MWCNTs. However, this is not conclusive.
The key findings from SEM analysis are that the acid treatment did not drastically alter the surface morphology of the MWCNTs. The lengths and diameters are not significantly affected by acid treatment. Several authors have noted that a mixture of concentrated H2SO4 and HNO3 does not severely damage MWCNTs. Our results using a mixture of HCl and HNO3 show a similar trend and corroborate those earlier reports.3-5
Figure 4.1: Morphology of (A) pristine MWCNTs and (B) MWCNTs treated with a mixture of nitric and hydrochloric acids in the ratio 3:1 in an ultrasonic water bath
Typical electron microscopy images of the morphology of the MWCNT-titania nanocomposites are shown in Figures 4.2 and 4.3. The SEM images generally show that titania coated the MWCNTs. With the sol-gel samples, at low titania loadings (designated 90
E.T. Mombeshora Page 88 wt.% and 80 wt.% MWCNTs) the metal oxide forms isolated particulate-like structures on the walls of the MWCNTs. With increasing amounts of titania, the sol-gel method results in more coating of material along the walls of the MWCNTs. However, the SEM images do show that some tubes are not uniformly coated, and that spherical titania agglomerates form that are not associated with the MWCNTs. The morphology of MWCNT-titania nanocomposites observed was similar to the structures reported by several authors,6-10 especially at high MWCNT ratios.11 The use of surfactants to detangle MWCNTs and thereby cause their separation and flexibility has been reported.13 In this study no surfactants were used, therefore it must be concluded that the ratios of the components in the nanocomposites influenced the distribution of titania on the MWCNTs tube walls. As can be seen in Figures 4.2 and 4.3 there was a shift from isolated titania particulates to a uniform coating of the MWCNTs with an increase in the titania wt.%.14 The increase in appearance of titania nanoparticle aggregates in the MWCNTs walls with an increase in the wt.% ratios of titania (see Figure 4.2C) shows that MWCNTs are centres of deposition.15 A comparison of the morphology at 10 and 20 wt.% MWCNTs by the sol-gel and CVD methods (see Figures 4.2 and 4.3), shows that the CVD approach gave a more uniform coating of titania on the MWCNTs than the sol-gel.
The observations by SEM of smaller titania particulates on acid-treated MWCNTs (shown in Figure 4.2B and C) concurred with the observations by Zhao et al.16 in that the MWCNTs were homogenously covered. However, in some instances more than one MWCNT was coated together in a cluster in the CVD method (see additional information in Appendix G, Figure G2). This was not observed in the sol-gel method because the synthesis procedure involved the use of an ultrasonic water bath and stirring which reduces the chances of agglomeration. MWCNTs were completely coated at 2 and 10 wt.% of MWCNTs (low MWCNTs wt.%) in the nanocomposites synthesised by CVD method (see Figure 4.3). At 10 wt.% of MWCNTs most MWCNTs were fully coated although some bare MWCNTs were visible. Furthermore, Fana et al.2 reported that defects such as roughened surfaces on the MWCNT walls are vital nucleation centres for metal oxide deposition. In another report, explanation by Aman et al.12 basis was the mismatch of particle size of Ti4+ cations (6.8 nm) and MWCNTs diameter. The MWCNT size promote particle growth on the surface and thus titania is deposited onto the MWCNTs walls since it cannot fit inside the MWCNTs.12 Also,
E.T. Mombeshora Page 89 the MWCNT interlayer spacing, i.e. 0.34 nm, is similar to the titania d-spacing, i.e. 0.35 nm, and therefore titania particles cannot fit in-between layers.18,19
Figure 4.2: Morphology of MWCNT-titania nanocomposites synthesised by the sol-gel method at MWCNT wt.% of: (A) 90, (B) 80, (C) 50, (D) 20 (E) 10 and (F) 2
E.T. Mombeshora Page 90 Figure 4.3: Typical morphology of MWCNT-titania nanocomposites synthesised by the CVD
method at MWCNT wt.% of: (A) 2, (B) 10, (C) 20, (D) 50, (E) 80 and (F) 90
MWCNTs are covered homogeneously via preferential heterogeneous nucleation on hydroxyl, carbonyl and carboxyl groups on the MWCNT surfaces.16 In the nanocomposites with a 1:1 titania:MWCNT ratio, nanocomposites synthesized by the CVD method had more agglomerated titania than those prepared by the sol-gel method (see Figure 4.4) even though the component ratios were the same. Again this suggest the influence of the
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