Chapter III Preparation and practical characterization of gold colloids
3.3 Influence of reaction conditions
3.3.1 The influence of reaction solvent
The protector- PVA (Mr = 50,000) is very hard to be dissolve in simple organic compound (e.g. ethanol & acetone), and can be totally dissolved in water only at temperature higher than 70 oC (70-90 oC, the physical property of PVA changes at T higher than 90 oC).
In this work, ethanol is also tested to be the reaction medium (colloid I.2-Et) to see the influence of reaction medium on Au-NPs growth. The UV-vis absorbance spectra of colloid I.2-Et 10 min and 2 weeks after preparation are shown in Figure 3.2. Differing from the colloid I.1-ref prepared with distill water, colloid I.2-Et prepared with ethanol displays very broad absorption peaks ranging from 490 nm to 750 nm, which is seemed to be consistent by two overlapped peaks locating at 530 nm and 700 nm, respectively. By the way, deposition of tiny blue powders is observed immediately after addition of NaBH4. The color of the suspension is black-blue as shown in the inset picture of Figure 3.2. Such color with two plasmon bonds in UV-vis spectra is always reported to be related to randomly arranged aggregations, pearl-like connected Au nanoparticles or Au nanorods (gold nanocrystals with both longitude and latitude
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diameters).[43-46]
Figure 3.2 UV-vis spectra of Au colloid I.2-Et prepared using ethanol as solvent 10 min (red line) and 2 weeks (blue dotted line) after preparation. [NaBH4] = 0.1 M, [Au] = 10-4 M, NaBH4: Au = 5: 1 (molar ratio), PVA: Au = 5: 1 (weight ratio), MW = 50,000 g/mol.
It is a worthy note that the PVA solution used here is dissolved in water (the PVA used in this work cannot directly dissolved in ethanol without water), whilst the addition of HAuCl4
solution is dissolved in EtOH. In this case, the gold precursor will not be ideally protected by PVA molecules as shown in step (1) of Schema 3.1. Besides, it is well-known that the reducibility of NaBH4 in ethanol will be weakened than in the water. In this case, after addition of NaBH4, the germs are disorderedly formed and more likely to be aggregated without suitable PVA protection. After 2 weeks, the SPR bonds nearly disappear due to the separation of Au-NPs from liquid phase. The nanoparticles in colloid I.2-Et by using EtOH as solvent are very unstable and totally separated from the liquid phase during less than two weeks, which is unavailable for preparing stable Au-NPs with homogeneous small particles.
3.3.2 The influence of reaction temperature
The impact of reaction temperature as a factor turning the sizes and shapes of Au-NPs is also
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Absorbance (a.u.)
Wavenumber (nm) After 2 weeks 530 nm
700 nm
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wondered. As can be seen from Figure 3.3, the widths of SPR bonds the colloids prepared at higher temperature (35 oC and 50 oC) are broader and the intensities are much weaker comparing with colloid I.1-ref prepared at 25 oC.
Figure 3.3 UV-vis spectra of Au colloids prepared at different temperature. [NaBH4] = 0.1 M, [Au] = 10-4 M, NaBH4: Au = 5: 1 (molar ratio), PVA: Au= 5: 1 (weight ratio), reaction temperature for I.1-ref, I.3-35, and I.4-50 is 25oC, 35oC, and 50oC, respectively.
The SPR bond maximum over colloid I.3-35 is red-shifted to about 530 nm indicating the formation of larger gold nanoparticles. Whilst, the UV-vis spectrum of colloid I.4-50 is a little weird comparing to the other spectra, which displays very weak SPR bond with a large trail until 750 nm. The much more weakened color and intensity of SPR bonds over colloids I.3-35 and I.4-50 suggest that only small amounts of Au-NPs exist in colloids I.3-35 and I.4-50 due to the formation of huge gold aggregates as deposition and cannot be reflected by the UV-vis spectrometer.
The color of colloids also changes from orange into blue and dark brown, respectively. After two weeks aging, the color of colloids is barely changed. However, the colloid VII and VIII seem unstable during the two weeks with largely reduced plasmon bond intensities. All the spectra are tested under room temperature (25oC), thus there is no effect from the temperature on the spectra.[47] The temperature of preparation does impact the formation of Au-NPs. It is a
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common knowledge that the gold nuclei are surrounded by PVA and thus separated from each other. Thus the gently raise of temperature (from 20oC to 35oC and 50oC) will not cause the further the further growth or aggregation of formed Au-NPs if the other operation conditions maintain the same. It was previously suggested that the reaction rate could be intensely enhanced under higher temperature and thus form heterogeneous particles.[48] Another problem that cannot be neglected is that, the viscosity of solution largely decreases along with the temperature (e.g. the viscosity of water decreases from 1.004 mPa·s at 25oC to 0.554 mPa·s at 50oC[49]). In the solution possessing lower viscosity under higher reaction temperature, the nuclei are facile to move and form large particles during preparation, making the gold particles unstable and easily deposited.
3.3.3 The influence of reactant concentration
For understanding the impacts of reactant concentration on the formation of Au-NPs, the gold colloids are synthesized using different concentration conditions of both gold source and NaBH4 reduce agent. The corresponding UV-vis absorbance spectra are shown in Figure 3.4.
The corresponding colors of gold colloids are displayed in the inset picture.
Figure 3.4 UV-vis spectra of Au colloids from different reactant concentration. NaBH4: Au= 5:
1 (molar ratio), PVA: Au= 5: 1 (weight ratio). The [Au] for I.5-GH, 1.6-GM, and I.1-ref is 10-2
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M, 10-3 M, and 10-4 M, respectively, and the [NaBH4] = 0.1 M. The I.7-SL is of the same condition of I.1-ref, except that the [NaBH4] = 0.01 M.
The higher concentration (10-2 M) of HAuCl4 solution of colloid I.5-GH results in a black-blue color. A very broad plasmon resonance bond ranging from 470 nm to about 800 nm with low intensity is observed, indicating that the distribution of Au-NPs in colloid I.5-GH is very heterogeneous from very small nanoparticles to large aggregations. The lower concentration of gold precursor receives colloid I.6-GM and I.1-ref with plasmon bonds locating around 510 nm. Although lower HAuCl4 concentration may be good to obtain smaller sizes of Au-NPs, it is not unlimited to reduce the concentration during the industrial production.
A highly diluted gold precursor will not only take excessive volume but also difficult for uniformly mixing of reactants during reaction. We suggest that the concentration of gold precursor between 10-3 and 10-4 M is more suitable for application.
One colloid I.7-SL prepared using a lower NaBH4 concentration (0.01 M) was also tested by the UV-vis measurement. The peak maximum is red-shifted to about 520 nm comparing to the colloid I.1-ref. The color also changes into rose-violet. Combining with the color and SPR bond, the Au-NPs in colloid I.7-SL are believed to be larger than colloid I.1-ref.
The concentration of reactant also largely affects the formation of Au-NPs. Lower concentration of gold precursor or higher concentration of reducer are more appropriate for preparing small nanoparticles with narrow size distribution. When the gold precursor is condensed, the reaction with NaBH4 is rather forceful that the nucleation is too fast and the formed gold nuclei might collide together to form larger gold crystals. When the NaBH4 solution is too diluted, the addition of NaBH4 cannot uniformly react with the gold precursor since the larger volume of reducer solution needs more time to be added and blended equally.
Larger nuclei will be formed during the addition of NaBH4. Upon the above two conditions, the formation of Au-NPs is uneven under the same reaction conditions (e.g. stirring speed and temperature). Large sizes or broad size distribution will be formed, which is not the requirement
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of general preparation of gold colloid for catalysis application.