Size control of Au NPs supported by pH operation
Introduction
Au NPs are studied by many researchers [1], [2], [3], [4], [5] since the report of Au colloid by Faraday [6]. Au NPs are used as catalyst and stain in a huge field, in addition, submicron Au particles are now expected to become useful functional particles as particle gun used for plant gene transfer [7]. There are many reports about Au NPs but reports on submicron particles are rare. Almost all report used polyelectrolyte or surfactant as a stabilizer but such stabilizer affected the operation of particle gun. We have studied PSD (particle size distribution) control of not only Au NPs but also submicron particles by reduction crystallization without any stabilizer such as polyelectrolyte or surfactant. Previous study suggested importance of seeds policy and feeding profile for PSD control [8]. However, those methods take time and effort due to preparation of seeds or devices. Therefore, this study focused on effect of pH because it is easy to control pH of reducing agent in our experimental system. Batch and single-jet operation were chosen to obtain Au particles. In batch operation, the reaction is too fast to obtain larger particles. To obtain larger submicron sized particle, single-jet operation is necessary. In this paper, effect of pH in the reduction crystallization was investigated to clarify the possibility of Au NPs PSD control and their growth process by pH operation.
Section snippets
Experimental methods
Au seeds and Au particles were produced by mixing 0.01 M AsA (Ascorbic acid) aqueous solution and 0.001 M HAuCl4·4H2O (hydrogen tetrachloroaurate tetra-hydrate) aqueous solution. The formula of the reduction is shown below.2HAuCl4+3C6H8O6→2Au+C6H6O6+8HCl
All the resulting suspension including Au particles was dropped onto a copper grid covered with a thin film and dried under air to observe by Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM). PSD and coefficient of
Effect of pH in the reducing agent in batch operation
Reduction crystallization experiment was conducted in a batch operation, by mixing AsA (Ascorbic Acid) aqueous solution and Au ion solution (HAuCl4·4H2O sol.) at once, varying pH value of AsA aqueous solution [9]. In lower pH conditions, average size of the obtained Au nanoparticles was 120 nm with agglomerated shape and CV 0.2–0.3 (Fig. 2, Fig. 3). Agglomeration was found out to be prevented by electrostatic repulsion, increasing pH. This phenomena is understood by the surface zeta potential
Conclusion
Effect of pH in the reduction crystallization was investigated to clarify the possibility of Au NPs PSD control and their growth process by pH operation. Precipitation experiments to form Au NPs of size range 10–600 nm were performed in single-jet system using ascorbic acid (AsA) as a reducing agent with adjusting pH of AsA. Au NPs are found to grow in the process of nucleation, agglomeration, agglomeration growth and surface growth. Au NPs tend to grow by agglomeration and become larger size in
Acknowledgements
This research was supported by Waseda University (Grant no. 2015S-079) Grant for Special Research Projects.
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