Effect of thiourea on grain refinement and defect structure of the pulsed electrodeposited nanocrystalline copper

The present investigation reports an extensive study on the grain refinement and the defect structure in the
pulsed electrodeposited nanocrystalline copper. Copper is deposited on the stainless steel substrate using
aqueous acidic copper sulfate solution prepared from 0.25 M CuSO4.5H2O and 0.5 M H2SO4 with addition
of varying amounts of thiourea. Copper deposits are made using 0, 9, 18, 27 and 36 mg/L addition of thiourea
in the electrolyte. The results indicate formation of nanocrystalline copper grains by even small addition of
thiourea with three orders of magnitude reduction in the grain size as compared to the sample deposited
without thiourea. The defects formed in the copper deposits are found to change predominantly from dislocations
to the twins with addition of thiourea. The grain refinement in the thiourea containing samples is
deemed to be due to change in the nucleation kinetics, growth as well as the kind of defect structure developed
in the samples. The transient studies at the initial stage of deposition using chronoamperometry indicate
the change in nucleation mode from instantaneous to progressive by addition of thiourea. EFTEM
investigation conclusively proves the preferential segregation of thiourea at the grain boundaries of copper
during pulsed electrodeposition. Detailed microscopic analysis reveals the grain refinement affected by the
defect structure.