The effect of time on Pt loading amount

Besides the temperature effect, the effects of treatment time under constant temperature of 140°C are investigated and shown isothermally in Fig. 5-27 with time intervals of(a)1.5, (b)2, (c)5, (d)10, (e)30, and (f)90min. From the SEM images, it is apparent that Pt particles are highly dispersed on CNTs in all samples of different reaction time, even though the reaction time is as short as 1.5min. These analysis results clearly indicate that temperature might be more important than reaction time to control the dispersion of Pt particles on CNTs. Fig. 5-28 shows the HRTEM images. It is obvious that all CNTs are highly dispersed on by Pt particles in all samples, even the sample of shortest treatment. However, for too long treatment time of about 90 min, the disadvantages are not only time and energy consuming but also the double layer or multilayer of Pt particles stacking on the first layer that results in the Pt peeling off from tube surfaces by internal stress, which can be found occasionally and is shown in Fig. 5-28(f).

Figure 5-29 shows the XRD results of samples of different reaction time. Pt signals are equally intense in all samples of different time and the mean particle size calculated by the equation and Pt (111) peak in all samples are 4.1nm, 4.4nm, 4.5nm, 4.5nm, 4.5nm, and 4.3nm respectively. The results evidently point out that the particle size are almost the same in spite of reaction time variation under constant temperature without coarsening and are not affected by increasing reaction time. Comparing to the temperature effect discussed previously, XRD results show again that reaction temperature not only dominates the dispersion of Pt particles, but also the particle size of Pt.

Fig. 5-27 SEM images of Pt particles synthesized on MWCNTs of different reaction time at 140°C. (a) 1.5min (b) 2min (c) 5min.

(a)

(b)

(c)

Fig. 5-27 SEM images of Pt particles synthesized on MWCNTs of different reaction time at 140°C. (d) 10 min (e) 30min (f) 90min.

(d)

(e)

(f)

Fig. 5-28 TEM images of Pt particles synthesized on MWCNTs of different reaction time at 140°C. (a) 1.5min (b) 2min (c) 5min.

(b)

(c)

(a)

Fig. 5-28 TEM images of Pt particles synthesized on MWCNTs of different reaction time at 140°C. (d) 10 min (e) 30min (f) 90min.

(d)

(e)

(f)

Fig. 5-29 XRD spectrums of Pt particles synthesized on MWCNTs of different reaction time at 140°C. (a) 1.5min (b) 2min (c) 5min.

(a)

(b)

(c)

Fig. 5-29 XRD spectrums of Pt particles synthesized on MWCNTs of different reaction time at 140°C. (d) 10 min (e) 30min (f) 90min.

(f) (e) (d)

The TGA analysis results of Pt loading amount with different reaction time are shown in Fig. 5-30. This curve shows that the loading amount can achieve 52.1 wt % as soon as the temperature reaches 140°C for 1.5 min of heating time. As the isothermal time increases, the loading amount of Pt on CNTs slightly increases. When the isothermal time is set up to 90 min, the loading amount can rise to 60 wt%.

However, it has been known that the disadvantages of long time treatment are not only time and energy consuming but also the appearance of double layers or triple-layers of Pt particles stacking layer by layer.

Fig. 5-30 Pt loading amount of different reaction time.

Figure 5-31 is the histogram of Pt particle size distribution obtained from the enlarged HRTEM images of well-dispersed Pt particles on CNTs. The total sampling amount is 2868 particles. This diagram shows the narrow size distribution of Pt clusters supported on CNTs in which diameter distribution of about 47% particles ranges between 4 and 5 nm and only a few particles are larger than 6nm or smaller than 3 nm. The average particle size is 4.3 nm by statistic calculation.

Fig. 5-31 Particle size distribution of Pt nanoparticles.

Fig. 5-32 is the high magnification SEM image of Pt-dispersed spiral MWCNTs.

CNTs were synthesized on Si substrate by thermal CVD method with Fe catalyst. It can be seen that Pt particles are uniformly and highly dispersed on straight and spiral MWCNTs.

Fig. 5-32 High magnification SEM image of Pt-dispersed spiral CNTs.

Fig. 5-33 shows the cross-sectional image of MWCNTs with sputtered Pt particles.

This cross-section was cut by FIB, as shown in Fig. (a). In Fig. (b), it is clearly shown that the sputtered Pt can be found in the upper layers of CNTs; however, in the inner layers marked by red line, CNTs are shelled by upper layers and can’t be dispersed on Pt particles by sputtering method. Fig. 5-34 is the cross-sectional image of CNTs cut by FIB. Pt particles were dispersed by microwave-assisted polyol method. CNTs were dried and dispersed on carbon tape and cut for cross-section in the edge. It is evident that each tube, compared to that of sputtering method, is fully covered by Pt particles in all directions, regardless of upper or inner layers.

Fig. 5-33 Cross sectional images of Pt-sputtered MWCNTs.

Fig. 5-34 Cross sectional images of highly Pt-dispersed MWCNTs by microwave assisted polyol method.

Figure 5-35 shows the XPS spectrum of Pt Pt 4f peak of the microwave synthesized Pt particles on MWCNTs. The Pt 4f peak consisted of two pairs of signals and the most intense peak (4f7/2 at 71.2 and 4f5/2 at 74.5 eV) can be assigned to metallic Pt.

A weak peak appeared at 72.5 eV could be regarded as the chemical state of Pt( ) in Ⅱ Pt(OH)2, and the other peak at 76.0eV might be halides. Chen et al. [206] reported a method to synthesize Pt catalyst on MWCNTs by polyol processes but without the addition of PVP, which has been reported to stabilize Pt particles preventing oxidation during synthesis. The XPS analysis result in their report showed an intense Pt oxidation state of PtO in the Pt catalyst; however, there is no such oxidation state found in our XPS analysis spectrum. This result indicates that surfaces of Pt particles during synthesis still need protection to prevent nanoparticles from oxidation.

Fig. 5-35 XPS spectrum of Pt Pt 4f peak of the microwave synthesized Pt particles on MWCNTs.