I have prepared silver coated silica sphere. After few days the color start to change from light brown to darker brown! is the silver getting oxidized? xrd shows fcc peaks for silver (~6nm) but there is a peak at 32.7 also..
Please have a look at a closely related question.
https://www.researchgate.net/post/Is_necessary_to_keep_silver_nanoparticles_protected_from_light_after_synthesized_it
The 32,7° peak (111 Ag2O) often shows up in literature and is attributed to oxide.
Yes, in air XPS (ESCA) shows the silver on the surface to be in the univalent oxidation sate. That is, the top 5 - 10 atomic layers consist of oxide. Some more material in (slides 30 - 38):
September 2nd, 2015 Silver colloids and invisible ink https://www.brainshark.com/malvern/vu?pi=560497681&b=1&tx=120486&c1=22
For general characterization see:
March 27th, 2014 Nanotechnology characterization techniques https://www.brainshark.com/malvern/vu?pi=zCTzgg96hz0z0
You can also be observing aggregaition of your core-shell nanoparticles which occurs slowly over time in your solution. Keep in mind that when metal nnanostructures become in electronic contact, new plasmonic bands appear. Esentially a doublet of spherical particles behaves like a rod-like particle, which has a longitudinal plasmon band red-shifted from the transverse plasmon band. In a sphere there is only one transverse plasmon excitation. However, for core shell particles it gets a bit more complex than this crude vision.But it is worth you explore this possibility.
Thank you very much for answering my question. I will try to do XPS on my sample to further confirm as suggested by Dr. Rawle and will update the results. @Jose Hoak- sir my samples are in powdered form, silver coating on silica micro spheres. The supernatant while separating the powder was dark red in color. UV measurement shows a single peak at 420nm. I was unable to do uv of my powder sample as i dont have access to an integrating sphere. I will update the UV results in case i am able to do UV measurement.
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