The Effect of SDS on Pt particle dispersion

According to the previous results, SDS was chosen to add in the following experiment groups to increase the dispersion of Pt on CNTs. The experiment conditions were the same as previously mentioned, but SDS was added in all samples (SDS/PVP=1:1). PVP used in the experiment groups were 8000, 58000, 1300000 in molecular weights. Besides, the mixtures of PVP of different molecular weight (8000/58000, 58000/1300000, 8000/1300000, at the ratio of 1:1) were also used to verify the effect on Pt particle size or distribution.

The SEM images were shown in Fig. 5-16 of (a) PVP MW=8000, (b) 58000 (c) 1300000. It is apparent that monolayer of Pt particles of small size and uniform size distribution are highly dispersed on each CNT, no matter what the diameter of CNTs

is and the PVP molecule weight is.

The results of XRD analysis are shown in Fig. 5-17. Strong Pt characteristic peaks appear in all samples to indicate that PVP can help Pt precipitation on CNTs when SDS was added in these samples. The mean particle size shown in Fig. 5-16 are 4.4 nm, 4.3 nm, and 4.3nm respectively, which were calculated by Scherrer equation based on the Pt (111) peak of all samples in Fig. 5-17. Figure 5-18 shows the HRTEM image of highly Pt-dispersed CNTs. Pt particles with uniform size are evidently attached on CNTs side by side even without multilayer stacking.

Figure 5-19 shows the SEM analysis results when PVP of different molecule weights were mixed together and added into solutions with SDS in the ratio of (a) 8000:58000=1:1, (b) 58000:1300000=1:1 (c) 1300000:8000=1:1. It can be realized that all CNTs are covered by monolayer of uniform-sized Pt particles, no matter what the diameter of CNTs is. The mean particle size calculated by equation and XRD peak width of Pt (111) shown in Fig. 5-20 are 4.0nm, 4.2nm, and 4.3nm respectively.

The HRTEM images are shown in Fig. 5-21, and it is clear that Pt particles with uniform size can be dispersed on each CNT, even though the diameters of CNTs are different from each other. The results indicate that molecule weight of PVP is not an effective factor to control the particle size or dispersion of Pt particles in the microwave dielectric heating system. On the other hand, SDS shows an effective way to disperse Pt particles on CNTs for PVP of high and low molecular weights during these experiments.

Fig. 5-16 SEM images of Pt nanoparticles synthesized on MWCNTs with PVPs of different molecular weights and the addition of SDS. (a) PVP MW=8000, (b) 58000 (c) 1300000.

Fig. 5-17 XRD spectrums of Pt nanoparticles synthesized on MWCNTs with PVPs of different molecular weights and the addition of SDS. (a) PVP MW=8000, (b) 58000 (c) 1300000.

Fig. 5-18 TEM images of Pt nanoparticles synthesized on MWCNTs with PVPs of different molecular weights and the addition of SDS. (a) PVP MW=8000, (b) 58000 (c) 1300000.

Fig. 5-19 TEM images of Pt nanoparticles synthesized on MWCNTs with PVPs of different molecular weights and the addition of SDS. (a) 8000:58000=1:1, (b) 58000:1300000=1:1 (c) 1300000:8000=1:1.

Fig. 5-20 XRD spectrums of Pt nanoparticles synthesized on MWCNTs with PVPs of different molecular weights and the addition of SDS. (a) 8000:58000=1:1, (b) 58000:1300000=1:1 (c) 1300000:8000=1:1.

Fig. 5-21 TEM images of Pt nanoparticles synthesized on MWCNTs with PVPs of different molecular weights and the addition of SDS. (a) 8000:58000=1:1, (b) 58000:1300000=1:1 (c) 1300000:8000=1:1.

It was reported that sputtered Pt nanoparticles could also reach high dispersion of Pt on CNTs, but the disadvantage is that only several layers of CNTs on the top of surfaces can be sputtered on. Chemical methods do not have such restriction and can disperse nanoparticles on CNTs in any direction but have never been reported to reach the same high dispersion as this work. This work shows that SDS-addition can combine advantages of both physical and chemical methods to highly disperse Pt particles in any direction on CNTs. These results showed a successful way to synthesize Pt catalyst of suitable size, uniform size distribution and high dispersion on CNTs by using microwave-assisted heating method with the addition of SDS.

The mechanism to explain the role of SDS in Pt particle distribution and the loading amount can be considered in two parts shown in Fig. 5-22. First, it is well known that PVP in the solution can be adsorbed on the surface of Pt cluster to influence the growth of particle size or the shape of particles; that is, during the process a layer of PVP covers Pt particles. In the others, it has also been reported that PVP adsorption on a particle surface is strongly enhanced to about 40 times in the presence of SDS [204]. Pure PVP, in the absence of SDS, absorbs at levels in the order of only 0.02 mg/m², and to the order of 0.78 mg/m² as the addition of SDS. The surfactant adsorbs at the particle surface. Then PVP in turn forms a complex with the SDS micelles and the surface-adsorbed SDS will complex with the PVP molecules.

This is the reason for the enhanced PVP adsorption. Next, SDS is known to be able to wrap monolayer PVP around single-walled carbon nanotubes (SWNTs) without covalent modification by chemical treatment, which could change favorable properties of tubes [205]. This means that SDS has excellent ability to attach PVP on CNTs. The reason is that SDS can adsorb chemically on CNTs, and attract PVP molecules by the electrostatic attraction between the surfactant head-group of SDS and the PVP nitrogen atom of side group.

According to these discussions, Pt nanoparticles are covered by PVP in solution and SDS wraps PVP around CNTs. It is believed that the added-SDS in this work could improve the dispersion and loading of Pt on tubes by bounding Pt-covering PVP and tubes together.

Fig. 5-22 Mechanism amoung Pt particle, SDS, and PVP.