Growth of sputter-deposited gold nanoparticles in ionic liquids

Abstract

The growth of gold nanoparticles (NPs) synthesized by sputter deposition on an ionic liquid surface is studied in situ in the bulk phase of the ionic liquids (ILs) 1-butyl-3-methylimidazolium dicyanamide [C₁C₄Im][N(CN)₂], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [C₁C₄Im][Tf₂N], 1-butyl-3-methylimidazolium tetrafluoroborate [C₁C₄Im][BF₄], 1-butyl-3-methylimidazolium hexafluorophosphate [C₁C₄Im][PF₆] and 1-butyl-3-methylimidazolium triflate [C₁C₄Im][TfO]. It is found that primary nanoparticles with a diameter smaller than 2.5 nm are present in the sample immediately after sputtering. Growth of these primary particles proceeds after the end of the sputtering process and stops when the nanoparticles reach a certain size. Depending on the viscosity of the ionic liquid this growth process can proceed several hours to several days. The growth speed is fastest for the least viscous ionic liquid and follows the trend [C₁C₄Im][N(CN)₂] > [C₁C₄Im][Tf₂N] > [C₁C₄Im][TfO] > [C₁C₄Im][BF₄] > [C₁C₄Im][PF₆]. It is also found that a higher concentration of sputtered gold results in faster growth of the gold nanoparticles. A discussion on the growth mechanism of sputtered gold NPs is included.