In situ UV–vis investigation of growth of gold nanoparticles prepared by solution plasma sputtering in NaCl solution

•In situ UV–vis measurement reveals diameters of gold nanoparticles and concentrations of gold.•Ostwald ripening of gold nanoparticles occurs in NaCl solution.•We estimate equilibrium diameters of gold nanoparticles in various concentrations of NaCl solution. Gold nanoparticles are prepared in various concentrations of NaCl solutions by solution plasma sputtering. The absorption spectra of these solutions during and after the plasma process are measured by in situ ultraviolet–visible (UV–vis) spectroscopy to estimate the particle diameters and concentrations of gold. The distributions of particle diameters are obtained by transmission electron microscope (TEM) observations. These experiments indicate the gold nanoparticles with about 2.2nm are directly formed by plasma phase and the diameters are increasing over time. These increases of particle diameters are caused by Ostwald ripening of gold nanoparticles in NaCl solution. We estimate the equilibrium diameter at which the gold nanoparticles are not solved in NaCl solution using in situ UV–vis spectroscopy. These diameters are about 5, 7 and 10nm in 3, 5 and 10mM NaCl solution, respectively. We make it possible to control the diameter of gold nanoparticles prepared by solution plasma sputtering in NaCl solution.