Synthesis of Au/SS5-C

The synthesis method of Au/SS5-C is simple as shown in part 2.1.4 of Chapter II. The as-synthesized silica supported Au-NPs are named as Au/SS1 and Au/SS5. After calcination, the two samples are marked as Au/SS1-C and Au/SS5-C, respectively. In the traditional synthesize methods for preparing supported Au-NPs such as impregnation and deposition-precipitation method, the real variation and development of Au-NPs during deposition and calcination is generally not mentioned.^{[33, 34]} Different from the traditional methods, in this work both the Au-NPs and silica globules were already preformed before the coating process. Thus what happens during the coating process can be clearly understood.

Besides, the Stöber silica is non-porous globules unlike normal mesoporous supports. Thus

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under this circumstance, the Au-NPs can be totally dispersed on the surface of silica. During the calcination, there is no chance for the Au-NPs to be trapped into the pores of support, which makes the analysis of the growth mechanism much more facile.

The UV-vis measurement is a useful technique for preliminary understanding the sizes and shapes of Au-NPs before microscopy analysis. The illustration in Figure 4.3 displays the desired synthesis path in this work. Correspondingly, the UV-vis spectra of Au/SS1 and Au/SS5 during preparation are also shown.

Figure 4.3 Illustration of the expected synthesis progress (left) and UV-vis spectra (right) of liquid during the preparation of Au/SS1 and Au/SS5. The solid lines are taken from the solution stirred for 10 min after the pH value adjusted to 1; the hollow lines are taken from the supernatant solution after centrifugation.

The initial gold colloid displays one peak at about 511 nm that is not very intensive. It was previous reported that the colloid Au-NPs generally possessed the plasmon resonance absorbance at about 520 nm due to the collective oscillation of electrons ^[35]. The position of this peak will changed along with the shape and size of Au-NPs. The weak peak at 511 nm indicates the existence of very small spherical Au-NPs (only several nm). We have tested the impact of the pH modification. Without the adjustment of pH value, the color of the mother solution is barely changed after the coating procedure and even the centrifugation, the location and the

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intensity of the absorption peak perform no differences. Adjusting of pH value to 1 after the silica is added into gold colloid, the color of the supernatant liquid become nearly transparent and the support changes from white to reddish brown. The first UV-vis scanning from the supernatant of mixture solution (solid line in Figure 4.3) is taken 10 min after the pH is settled to 1. As we mentioned before, the as-prepared Stöber silica is non-porous spheres, the initial ten minutes is thus decisive for the adsorption of Au-NPs onto silica surface. The largely reduced intensity of the plasmon resonance absorbance bond of supernatant liquid also reveals that Au-NPs have been partially captured by the surface of silica. After centrifugation, the intensity of the peak further decreased. It is indicated that the Au-NPs coated onto non-porous Stöber silica should be fulfilled under low pH value. Besides, the coating process cannot be fulfilled only by stirring, in which the centrifugation process is indispensible for capturing the Au-NPs.

The coating process is more like a precipitation process with force. However the reason why these zero state Au nanoparticles loading onto silica favor the acid condition is still under discussion. It is also remarkable that the position of the absorption peaks over the initial colloid, the colloid after coating and centrifugation is barely shifted, demonstrating that the sizes of Au-NPs won’t change during the coating process.

Figure 4.4 SEM images of Au/SS1 before (a) and after (b) calcination in air at 300 ^oC for 4 h.

For the view of the morphologies of Au-NPs coated silica samples, the SEM technology of Au/SS1 before and after calcination (300 ^oC for 4 h in air) is operated (Figure 4.4). No Au-NPs can be seen in the Au/SS1 sample before and after calcination. As shown in Figure 4.1, the bulk silica is compromised by compact silica globules layer by layer, and intervals exist between

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Au/SS1-C Au/SS1

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spheres. After the Au-NPs coating on SS1, the globular morphology still maintains unless that the surface of the SS1 seems to be a little modified. There are still intervals between globules, but parts of the globules are connected by the glue-like stuff. After calcination, the Au/SS1-C is visualized to be more dispersed and with more regular globules comparing to the Au/SS1. The glue-like matters are disappeared. However, the images are lack of definition caused by the charge of silica. One cannot confirm that the glue-like matters are from PVA by the SEM images.

Figure 4.5 SEM images of Au/SS5 sample before (a) and after (b) calcination in air at 300 ^oC for 4 h. The bar in the inset picture of left image is 100 nm.

The SEM images of Au/SS5 before and after calcination are also carried out as shown in Figure 4.5. Different from the Au/SS1, the surface of each Au/SS5 globule is smooth without prominence. Glittery ring can be observed on the boundary of each silica sphere, which should be ascribed to the special light parameter when taking photos but not the gold. There is no clear observation of single Au-NPs before calcination. Since the theoretical loading of gold is rather low as 1 wt%, there is no chance for the gold to encapsulate the large SS5 spheres even as thin a layer. It is thus considered that the gold is dispersed on the surface. The inset picture in the right image of Figure 4.5 is a dark field microscopy of Au/SS5-C sample. Sporadic bright spots with size about 5-10 nm can be seen in the calcined Au/SS5-C, which is never found in the same sample before calcination, suggesting that some clustering of smaller Au-NPs occur during calcination. For better evidence, the TEM images are required.

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Figure 4.6 TEM images of Au/SS1, Au/SS1-C, and Au/SS5-C samples and the corresponding diameter distribution of three samples.

The TEM images of three samples based on SS1 and SS5 are shown in Figure 4.6. The revealed SS1 globules are mainly spheres with irregular boundaries. This is in accordance with the previous suggestion that the Stöber silica with smaller spherical diameter was more difficult to achieve, especially for those with diameters lower than 100 nm, the circular degree would also be decreased ^{[36, 37]}. The diameters of SS1 globules are mainly ranging from 40 nm to 60 nm, which is in accordance with the SEM. The Au-NPs with average size of 2.9 nm are highly dispersed on the surface of Stöber silica. During all the TEM images of Au/SS1, there is no observation of Au-NPs larger than 5 nm, most of which are around 3 nm. There are even black spots much smaller than 2 nm on the surface of the silica spheres. But since they are beyond the limitation of the microscope, it is difficult to declare that it is Au-NPs or not. They are not considered into the calculation of size distribution of Au-NPs. The image suggested that we have the first time successfully anchored the Au-NPs about 3 nm onto the surface of non-porous Stöber silica globules with a modified impregnation method. However, no legible observation of the PVA as layer encapsulating the surface is displayed. While the interesting part is that, during the characterization several Au-NPs was trying to leave the surface of silica spheres, nevertheless they were captured and pulled back by the glue-like substances on the surface

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(Figure 4.7), which ought to be from the polymer-PVA. After calcination at 300^oC for 4 h under air, the size distribution of Au-NPs becomes much broader with the average diameter of 4.2 nm.

Au-NPs larger than 5 nm and even of 15 nm are appeared. Some nanoparticles about 2 to 3 nm still maintain on the SS1 surface. It is indicated that there do exist some extremely small particles with melting-point lower than 300^oC (< 2 nm ^[38]), which are melted under calcination temperature and assembled into larger gold particles. In addition, small parts of the Au-NPs of 3 nm are also diminished. This is because of that during the calcination, the partial of the PVA are removed from the sample. The smaller Au-NPs without the protection of PVA begin to move for the sake of lower energy and formed clusters at higher temperature. No matter how, there are still considerable amounts of Au-NPs about 3 nm remaining on the surface of SS1, which is really interesting for some of the catalysis reaction ^[39].

Figure 4.7 TEM image of Au/SS1. In this sample, Au-NPs about 3 nm was successfully loaded on the surface of SS1 support without addition of any organic couplant. The nanoparticles in the red circle on the surface of support are captured by the glue-like stuff, which is inferred to be the PVA.

The colloidal Au-NPs used for coating SS5 are the same of SS1. The TEM image of Au/SS5-C is displayed in Figure 4.6. After calcination, the Au-NPs in Au/SS5-C are also grown larger. Different from the Au/SS1-C sample, there is no gold particles larger than 8 nm were observed in all the images of Au/SS5-C and large amounts of nanoparticles are around 3 nm

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which is in the ideal confine for some reactions as reported in previous work ^[40-42]. Since the single silica globule of SS5 possess much larger area than that of the SS1, the Au-NPs on the SS5 will be much separated from each other than on the SS1. During the calcinations process, the PVA was removed step by step, the smaller gold particles begin to move and have more chance to be clustered. It is inferred that the long distance between particles on SS5 makes the particles more difficult to move toward each other and partially protected the Au-NPs. Thus the Au-NPs in Au/SS5-C won’t grow into very large particles as in Au/SS1-C.