I synthesized Au deposited on graphite electrochemically by using chronopotentiometry. How much ever I tried I couldn't get UV absorbance peak of AuNPs in any number of solutions I tried? But when I did a XRD I find AuNP peaks at proper diffraction angles i.e. 38 deg, 44.2 deg, 64.2 deg, 77.11 deg. Why this is so?
You should give some details about the measurement setup. For example, assuming you have performed a reflectance measurement, have you used an integration sphere to collect most of the diffused light?
And you should also check for the thickness and uniformity of the NP film deposited on the graphite substrate, just to be sure you catch enaugh reflected signal from gold.
In any case, Au NPs have not absortion resonances in the UV region, but in the visible one in the 500-600 nm wavelength range. I suppose you are referring to such peak. I suspect that the sample volume probed in a single measurement contains a too small amount of NPs to have a significant Au spectrum, and the intensity of the reflected signal is further limited by isotropic diiffuson (Reyleigh) or, even worse, anisotropic Mie diffusion of the light.
Zhu, Huaiyong, Xi Chen, Zhanfeng Zheng, Xuebin Ke, Esa Jaatinen, Jincai Zhao, Cheng Guo, Tengfeng Xie, and Dejun Wang. "Mechanism of supported gold nanoparticles as photocatalysts under ultraviolet and visible light irradiation." Chemical Communications 48 (2009): 7524-7526.
Dear Sai Prasad Nayak , normally Au spherical NP´s have absorbption maximum around 520nm. The absorbption is size and shape dependent. As the NP diameter grows the maximum shows a redshift. If the gold NPs are anisotropic, rod-like shape, then you can observe two maxima. These maxima are called Localized Surface Plasmon Resonances (LSPRs).
What you are obtaining from the diffractogram is information relative to the atomic arrangement of Au atoms in the NPs you have, in particular those angles are attributed to the (111), (200), (220), (311) and (222) planes, respectively, which are characteristic of the fcc structure of Au. It means your Au NPs have this structure fcc. It doesn´t tell you anything about the size or shape of your NP´s.
Often, to get UV absorption with Au NP´s it is necessary caping them with some chemicals.
Hope it helps.
Thanks Dr Azeez Barzinjy by such a well explained paper. It is worth reading. I was aware of the UV tail in the spectra of Au NPs but, honestly I didn´t know what was its origin.
Thanks Dr Azeez, Manuel Gomez and Giuseppe curo for showing the right direction. I also think that the sample volume probed in a single measurement contains a too small amount of NPs to have a significant Au spectrum.
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