Insights on the Formation of Nanoparticles Prepared by Magnetron Sputtering Onto Liquids: Gold Sputtered Onto Castor Oil as a Case Study

Abstract

Magnetron sputter deposition of metal targets over liquids allows producing colloidal solutions of small metal nanoparticles (NPs) without any additional reducing or stabilizing reagents. Despite that this synthetic approach is known for almost 15 years, the detailed mechanism of NP formation is still unclear. Detailed investigations must be carried out to better understand the growth mechanism and, ultimately, control the properties of the NPs. Here, the combination of the gold (Au) target and castor oil, a highly available green solvent, was chosen as a model system to investigate how different experimental parameters affect the growth of NPs. The effect of deposition time, applied sputter power, working gas pressure, and type of sputter plasma (direct current magnetron sputtering (DC-MS) vs. high-power impulse magnetron sputtering (HiPIMS)) on properties of Au NPs has been studied by UV-vis spectroscopy and transmission electron microscopy (TEM), and further supported by quantum-chemistry calculations and mass-spectrometry analysis. The mechanism of the Au NP formation includes the production of primary NPs and their subsequent aggregative growth limited by diffusion in the viscous castor oil medium. Final Au NPs have a narrow size distribution and a medium diameter of 2.4–3.2 nm when produced in DC-MS mode. The NP size can be increased up to 5.2 ± 0.8 nm by depositing in HiPIMS mode which, therefore, mimics energy and time-consuming post synthesis annealing.

Sputtering onto liquids: a critical review

“…An increase in temperature leads to a decrease in viscosity and hence to a higher number of collisions yielding larger NPs. Thus, depositions with a HiPIMS power supply [131,132] might be used as an alternative to the annealing process [174][175][176] for producing larger metallic NPs.…”

“…Yonezawa’s group [ 120 ] showed that only the absorbance of the Au NP solutions increases proportionally with the current (from 10 to 30 mA) but not the size of the NPs after sputtering onto PEG. It was also shown that the size of metal NPs does not depend on the sputter power in the case of sputtering iron (Fe) onto the surface of silicon oil [ 130 ] and in the case of sputtering of Ag or Au onto castor oil [ 131 – 132 ].…”

“…2.1.1.5 Gas pressure: The effect of changing the working pressure, while keeping the other parameters identical, was studied in [ 11 , 131 – 132 161 – 163 ]. Increasing the pressure from 0.067 Pa to 2 Pa during the sputtering of Ag onto castor oil led to a slight increase in the diameter of primary Ag NPs from 2.1 nm to 2.8 nm [ 131 ].…”

“…Depositions of Ag and Au onto castor oil with DC and HiPIMS power supplies have shown that the particle size is also affected by the plasma type. NPs obtained at the same working pressure in castor oil in HiPIMS mode were more stable and twice as large as those obtained in DC-MS mode: 2–3 nm for DC-MS vs 5–7 nm for HiPIMS with a Ag target [ 131 ] and 2.4 ± 0.9 nm for DC-MS vs 5.2 ± 0.8 nm for HiPIMS with a Au target [ 132 ]. The positive voltage applied to the cathode, in the bipolar HiPIMS mode, allowed for an additional increase of the size of primary Ag NPs and their aggregates.…”

“…Thus, the presence of functional groups having a high affinity to sputtered species or the ability to stabilize NPs via electrostatic repulsion and steric stabilization must be considered during the selection of a suitable host liquid. Here, theoretical chemistry studies such as quantum chemistry-based calculations and molecular dynamics simulations may provide useful insights, as shown in [ 131 – 132 163 , 174 – 175 179 ]. Below, we highlight some trends.…”