Size/shape control of gold nanoparticles synthesized by alternating current glow discharge over liquid: the role of pH

In this study, we report the influence of pH level in HAuCl4 solution on the size and shape of gold nanoparticles synthesized by an alternating current glow discharge on liquid. The results indicate there have three distinct groups in the size/shape of gold nanoparticles due to varying pH, one at pH <= 8, one at pH 9 and the other at higher than 9. At pH <= 8, single spherical gold nanoparticles with an average size of 5.7 nm and coral-shaped clusters aggregated from these gold nanoparticles were formed after 10 min of alternating current glow discharge on liquids. In the case of pH 9, spherical AuNPs with the size of 120 nm and coral-shaped clusters with the size up to 100 nm were generated after 10 min of discharge. At pH >= 10, roughly spherical gold nanoparticles with an average size of 100 nm (at pH 10) to 30.6 nm (at pH 13) were generated after 10 min of discharge. There has a decrease in the average size of gold nanoparticles when pH increases from 10 to 13. Our findings open a new way to control the size/shape of gold nanoparticles synthesized by plasma over liquid via pH control in the precursor solution.