Fabrication of nano/micro-structures of cuprous oxide by electrodeposition

Abstract

Nanoparticles/nanocrystals have been recognized by their remarkable and technologically attractive properties which are different from those of bulk materials due to their ultra large surface area and extremely fine nanostructure. Highly sophisticated properties such as optical, magnetic, electronic, catalytic, mechanical, chemical and tribological properties can be obtained by advanced nanostructured coatings, making them desirable for industrial applications. This thesis encompasses the fabrication of nano/micro-structures of cuprous oxide (Cu2O) including nanocrystals, nanowires, nanocrystalline coatings and co-deposition of Cu2O/Cu by electrodeposition. The investigation in this thesis involved a systematic study by using a simple two-electrode system with copper sulphate as the electrolyte at pH 4.0 at room temperature and without the aids of any additive. The substrates under investigation included silicon wafer, stainless steel plate, highly oriented pyrolytic graphite (HOPG) and silver wire. By changing the kinds of the substrates, deposition modes, and deposition potentials, different types of nano/micro-structures of Cu2O were yielded.

Nanometer-to-micron sized Cu2O single crystals were fabricated on an Au/Pd sputter-coated silicon wafer and stainless steel cathode. Different morphologies of the crystals were studied and their structural characterization was performed. The Cu2O crystals were generally of an octahedral shape. A growth mechanism was proposed to explain the morphologies of the observed nano/micro-structures. The effect of the electrodeposition parameters such as deposition voltage and deposition time, on the size of the crystals and their coverage on the substrates was examined. Highly aligned Cu2O nano/microwires were fabricated on the step edges of the HOPG substrate. With the same deposition potentials, longer deposition time would increase the diameter of the wires. Various crystal morphologies including flower-like and butterfly-like structures, and dendrites and truncated octahedra were observed on the working electrode of HOPG. Some of the morphologies were revealed for the first time by the one-step electrodeposition.

To investigate the differences between yield by DC and pulsed electrodeposition respectively, the sizes and coverages of the deposited crystals on the substrate by DC and pulsed electrodeposition were compared. Two completely different forms of Cu2O, namely uniform nanocrystalline coatings and isolated single crystals, were fabricated on Ag microwires by pulsed and DC electrodeposition respectively. The results illustrated a very different effect on crystal nucleation between DC and pulsed electrodeposition, and suggested the possibility of using different voltage waveforms for electrodeposition in order to fabricate coatings with different nano/micro-structures on substrates. Gradual transition in the deposition products from pure Cu2O to pure Cu during electrodeposition with various DC potential was investigated. The percentage of Cu content was studied in depth for a full picture of the relationship between the depositing potentials and the compositions of the deposits. This thesis provides a method to fabricate nanocrystalline Cu2O, Cu and Cu/Cu2O on substrates in a single step without the use of additives.