A hybrid electrochemical/chemical synthesis of semiconductor nanocrystals on graphite: a new role for electrodeposition in materials synthesis

Abstract

Metal (copper and cadmium) nanocrystallites —electrochemically deposited on the graphite basal plane— have been converted on a particle-by-particle basis to nanocrystals of a semiconducting metal salt (CuI and CdS). Deposition of the metal nanocrystals from a dilute ([M n+ ]≈1.0 mM) plating solution is accomplished using a potentiostatic pulse with an overpotential of −500 mV and a duration in the range 50–200 ms. Non-contact atomic force microscopy images of the surface show that defect sites, such as step edges, and apparently atomically smooth terraces on the graphite basal plane both are covered with metal particles at an areal density of 10 9–10 10 cm −2. Each of these metal nanocrystallites is then converted into a semiconducting salt particle in two subsequent steps: first, metal nanocrystals are chemically or electrochemically oxidized at the appropriate pH to yield either metal oxide or hydroxide nanocrystals; secondly, the oxygen or hydroxide is displaced with an anion to yield the semiconducting salt of interest. This final displacement step is always chemical in nature. The CuI and CdS nanocrystals prepared using this “E/C” approach possess an excellent lattice match to graphite and epitaxially oriented nanocrystals are obtained with both materials. The particle size distribution for E/C-synthesized particles is determined by that of the parent metal particle distribution. Both CdS and CuI nanocrystals prepared by the E/C method are strongly luminescent at the band edge.