Silver Nanocubes–Synthetic Factors and Surface Properties

• Main Authors: Liou, TIng, 劉庭
• Other Authors: Yang, Tzyy-Schiuan
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/bp3vt8

碩士 === 國立中正大學 === 化學暨生物化學研究所 === 103 === Nanoparticles made of noble metals are greatly used as substrates for surface-enhanced Raman scattering (SERS) measurements. This is possible because nanostructures create concentrated and localized electric near field under surface plasmon resonance (SPR), which in turn enhances the Raman scattering signals of adsorbed molecules on the surface of nanoparticles. The key factors affecting this SPR are the size, shape, and composition of metal nanoparticles. Controllable synthesis of particles of different sizes and shapes is prerequisite to systematically developing applications. For most applications, noble metal nanoparticles are made of gold, but silver holds great promise based on its two major benefits: stronger SPR and lower cost. Ag nanocubes (AgNCs) are particular interesting because the corners of a nanocube produce higher SPR than the spherical counterparts and because the shape symmetry simplifies the interpretation of SERS signals. Although there has been a steady stream of publications on AgNCs for over a decade, they originate from a single laboratory so the reproducibility of the results remains unverified. Therefore, the purpose of this research was to demonstrate reproducibility of the previous work, and further characterize AgNCs. In this study, we adapted the synthetic method in the literature by adjusting several experimental parameters, including the reaction temperature, reaction time, the presence and absence of oxygen, the amounts of the catalyst and silver precursor. Several optimal experimental parameters deviated from the values in the literature. Ethylene glycol (EG) is used both as solvent and as the reducing agent in the synthesis. EG was heated up to 160 ℃ for an hour first. While heating temperature and duration were said to be strict 150 ℃ and an hour in the literature, we found that we had to adjust either heating temperature by ±5 ℃ or heating duration by ±15 min in response to different ambient humidity. For instance, the heating temperature should be increased to 165℃ or heating duration increased to 1 hour and 15 min in the humid hot summer day but reduced to 155 ℃ or 45 min in the dry winter day. We, however, develop a simple way to determine the adjustment by observing the color change of the reaction solution after addition of catalyst Na2S, capping molecule polyvinylpyrrolidone (PVP) and AgNO3. If the color of the solution did not change to golden yellow in 30 sec after addition of all chemicals, we should start again by preparing a new EG solution either by increasing heating temperature by 5 ℃ for an hour or keep same temperature but increase heating duration by 15 min. If the color of the solution changed to golden yellow but went through a series of color change to gray with visible black suspension in 5 min, the new EG solution should be prepared with lower temperature by 5 ℃ or shorter duration by 15 min. For a successful reaction, the color of the reaction solution went through a series of change to the final green-ochre with dark-red meniscus on the top of solution surface. The reaction time ranges from half to an hour, depending on the ambient humidity, while it is 10 min in the literature. When heating up EG, the cap of the glass vial should be removed to expose the solvent to the air. If we closed the opening of the glass vial and purged EG with nitrogen, no AgNCs were formed. This demonstrated oxygen in air is essential to the reducing power of EG for the formation AgNCs. The size of AgNCs increases with the reaction time, the amount of catalyst, and the amount of silver nitrate, i.e. the silver precursor. The main absorption peak of AgNCs is red-shifted to longer wavelength with the increase of the size. The presence of catalyst Na2S facilitates the formation of AgNCs but the final concentration of the catalyst have to be limited in a small range of 3.31 × 10-5 to 4.04× 10-5 M. Sulfur ions from the catalyst combined with silver ions into Ag2S, which accelerates the growth of AgNCs. If the amount of Na2S is less than 3.31 × 10-5 M, the AgNCs cannot be produced. If the amount of Na2S excesses 4.04× 10-5 M, black Ag2S precipitate appear. These results confirm the literature values. The literature asserted that the amount of AgNO3 to be used is restricted to 0.5 mL of 0.28 M. We found that increasing the volume of AgNO3 solution to 0.55 mL, AgNCs can still be formed but with much broader SPR band, indicating a wider size distribution. When the volume increased to 0.6 mL, red edge of the SPR band rose up, implying the formation of aggregation. The capping molecule PVP induces silver nanoparticles grow along {100} facets by the adsorption of carbonyl group to surface atoms so that the surface energy of {100} facets is the lowest one. PVP molecules on the AgNCs can’t be removed completely by centrifugation. They may either interfere with the SERS signals of analysts, the hinder the adsorption of analysts to AgNCs, or complicate further surface modifications. We measured the amount of PVP consumed along the reaction process and estimated the coverage density and the thickness of PVP on AgNCs. When the sizes of AgNCs are 38 to 42 nm, the coverage density and thickness of PVP are 100 to180 monomers/nm2 and 10 to 15 nm, respectively. The literature value is 140 monomers/nm2 for truncated AgNCs of 40 nm and 120 nm in length. We found the coverage density and thickness of PVP are not constant with the reaction time. When the size of AgNCs increases with the reaction time, the coverage density and thickness of PVP also increase. For the sizes of AgNCs from 70 to 100 nm, the coverage density and thickness of PVP range 715 to 800 monomers/nm2 and 40 to 50 nm, respectively. To the reaction point that free PVP in solution is depleted, the coverage density decreases slightly with size. Finally, we etched AgNCs by Fe(NO3)3 into silver nanospheres to obtain homogenous of Ag nanospheres with diameter around 30 nm. Ag nanospheres with uniform size distribution around 30 nm are difficult to obtain by directly synthesis of Ag nanospheres. However, TEM images of nanospheres show lots of etching debris adhered around the nanospheres. Small species of debris are trapped in PVP capping layer and are too small to be removed by simple centrifugation.