I prepared Cu2O thin films by electrodeposition at various potentials. The XRD revealed the presence of Cu4O3 and Cu64O as secondary phases at given potentials. I found that the presence of these secondary phases decreases the crystallite size, enlarges slightly the lattice parameter (cubic) and increases the bandgap. What would be the reason? Is there any evidence of occupation states in band gap to blue shift absorption edge by sub-stoichiometric copper oxide? Thanks in advance.
Hello Mohamed Benaicha ,
This is not my field, but I was puzzled by the statement of your question. In the title of your question, you ask, "Why bandgap decreases and lattice parameters increase when the crystallite size decrease?" But in the body of your question, you ask, "I found that the presence of these secondary phases decreases the crystallite size, enlarges slightly the lattice parameter (cubic) and increases the bandgap. What would be the reason?" Are you not asking two different questions? In the title, the bandgap decreases, but in the body of the question it increases under the same condition, i.e., decrease in crystallite size.
You also state that x-ray diffraction reveals two sub-phases: Cu4O3 and Cu64O. Is the chemical formula of the second sub-phase correct, and how are both the sub-phases "sub-stoichiometric", or do you mean non-stoichiometric? If sub-stoichiometric and non-stoichiometric are different concepts, then how do you define each of them? I thought that a compound such as CuxOy is non-stoichiometric if x and y are not integers.
I was also curious as to how you electrodeposite Cu2O. Is this an electrolytic process, a sputtering process in the presence of oxygen, a Joule evaporation process in the presence of oxygen, or what? If it is electrolytic, then can you explain how the cuprous oxide is deposited instead of just copper metal at the cathode? Or does your process take place at a copper anode where atomic oxygen evolves?
I think a more detailed explanation by you may make it easier for other RG members to fully understand your question, and hopefully provide a satisfactory answer. Of course, it is also possible that since I am a non-expert, I am simply confused.
Thanks for an interesting question.
Regards,
Tom Cuff
Dear Mohamed. Please, check this ref. that might help:
Diehm, P. M.; Ágoston, P.; Albe, K. Size-Dependent Lattice Expansion in Nanoparticles: Reality or Anomaly? ChemPhysChem 2012, 13, 2443–2454.
DOI: 10.1002/cphc.201200257
Good luck.
E.-
This behaviour can be better understood if one considers that the interatomic spacing increases when the amplitude of the atomic vibrations increases due to the increased thermal energy. This effect is quantified by the linear expansion coefficient of a material. An increased interatomic spacing decreases the potential seen by the electrons in the material, which in turn reduces the size of the energy bandgap.
Dear Mohamed Benaicha
The Introduction section of the article "Evolution of crystallite size, lattice parameter and internal strain in Al precipitates during high energy ball milling of partly amorphous Al87Ni8La5 alloy" very interesting and useful information regarding your your question. Please read it via the following link:
https://www.helmholtz-berlin.de/pubbin/oai_publication?VT=1&ID=80507
Hi Dr.Thomas Cuff,
Thank for your answer. In fact, that was a mistake in the question title, I just made the correction. About the secondary phases, sub-stoechiometric, I meant that is so when compared to Cu2O compound. The Cu64O phase exists. The electrodeposition is the electrolytic technique (as you said) and Cu2O is obtained from alkaline lactic acid -based solution.
Thank you again
Mohamed
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