Ingenium 2016
to work and can be used only for specific systems. Our method is general and can be used to recover NPs that do not at all require salt recrystallization for their recovery, but would otherwise require expensive and time consuming recovery techniques. Salt recrystallization is the process by which the complementary ions of a salt dissolved in a solution begin to recompose themselves and recrystallize under the influence of temperature or the presence of a solution in which the salt is insoluble. These crystals then go on to “catch” the desired NPs, which allows for easier separation. We demonstrate that salt recrystallization can significantly simplify and shorten NP recovery procedures as well as the deposition of NPs onto supports by circumventing the required membrane centrifugation without affecting the yields and size distributions of the recovered NPs. Additionally, embedding NPs in thermally stable salts can mitigate particle growth and agglomeration even beyond the decomposition temperature of typical capping agents.
Methods
Synthesis of 6 nm silica Silica NPs, 6 nm in diameter, were synthesized using a two-phase variation of the Stöber method. Briefly, a catalyst stock solution was prepared by mixing ammonia with deionized (DI) water to a pH of 11.4. Next, 2.6 g of tetraethyl-orthosilicate (TEOS) was diluted in 5 ml ethanol, mixed thoroughly, and added to 34.75 g of catalyst stock solution. The solution was then stirred for 3 hours at 60 °C. Synthesis of Silica Support Materials Silica supports, approximately 120 nm in diameter, were synthesized using the conventional Stöber method. A solution containing 18 mL of TEOS, 99 mL of DI water, 36 mL of ammonium hydroxide (30% by volume), and 65 mL of ethanol (190 proof) was mixed at room temperature for one hour. The resulting white solid was separated from the solution via centrifugation. Synthesis of platinum nanoparticles Pt NPs (3–5 nm in diameter) were synthesized from a solution containing 1.25 mL of 10 mM chloroplatinic acid aqueous solution and 2.5 mL of 1.39 mM polyvinylpyrrolidone (PVP, ~ 10,000 molecular weight) aqueous solution at 0 °C, which was stirred until well mixed. Next, 1.25 mL of 0.1 M sodium borohydride aqueous solution was added rapidly. The solution was left to react for 30 minutes. When using
the conventional method of NP separation, the NPs were then recovered using membrane centrifugation (Centricon Ultracel-10K, Amicon Ultra Inc.). Conventional synthesis of Pt/SiO2 Pt NPs were synthesized as described above. After washing, the Pt NPs and 240 mg of (120 nm) silica support were dispersed in 2 ml of DI water. The solution was stirred for one1 hour at room temperature and then dried in vacuum at 100 °C. The resulting Pt/SiO2 sample was then calcined at 300 °C to remove PVP. Characterization of nanomaterials Transmission electron microscopy (TEM) images were taken on a JEOL JEM2100F with an accelerating voltage of 200 keV. Sizes of NPs were determined using ImageJ with TEM images from particular samples. From these, the mean and standard deviation were then calculated. Nanoparticle separation via salt recrystallization Salt was added to the solution containing NPs until the solution was saturated. Liquid in which the salt is insoluble was then rapidly added to the solution and agitated to accelerate precipitation. The resulting precipitate and NPs were then recovered via conventional centrifugation. Finally, the salt was removed via calcination in air. Ammonium chloride salt was used to recover silica NPs and ammonium bicarbonate and potassium chloride salts were used to recover platinum NPs. Results and Discussion Separation of 6 nm silica The presence of surface hydroxyl groups on the silica NP surface renders these particles highly stable in the basic synthesis solution environment, and hence makes their recovery difficult. The hydrogen ions on the hydroxyls dissociate, which results in the oxygens being negatively charged and strong electrostatic repulsive forces between particles. This repulsion between particles is what prevents them from collecting after conventional centrifugation. Their high stability in solution necessitates the use of membrane centrifugation for their recovery. Membrane centrifugation, while effective, is a tedious, time-consuming, and inefficient process. Membrane centrifuge tubes are typically very small. In our case, each tube only held 15 mL of solution, but the synthesis of 6 nm silica results in 43 mL solution and
Undergraduate Research at the Swanson School of Engineering
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