How does the exciton get separated under light irradiation? Prof. Kamat published one paper `Charge Separation and Catalytic Activity of Ag@TiO2 Core-Shell Composite Clusters under UV-Irradiation` which says about the charge transfer issue for a silver core/tio2 shell nanoparticle.
By irradiating the TiO2 with UV light, electron will go to metal core due to the lower fermi level(0.4 V) of metal, and leave the hole on TiO2 which can grab electron from solvent. Eventually Ti(IV) becomed Ti(III) and lead to absorbance spectrum difference.
Another paper by Prof. Tetsu Tatsuma `Mechanisms and Applications of Plasmon-Induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles` showed that by irradiating metal nanoparticle to cause plasmon induced charge transfer, electron was transfer from metal to TiO2. It was confirmed by bias potential followed by spectroscopy check.
I have got a core-shell/metal-semiconductor nanoparticle, and I want to know what really happened when I irradiated the nanoparticle by visible-NIR light, not UV.
Do the two papers conclude that
1 UV induced semiconductor exciton leads to electron donating to metal due to fermi level difference
2 VIS-NIR induced metal excitation of plasmon resonance leads to the electron go to semiconductor due to the fermi level arising of metal part
or 3? The nanoparticle is polarized anisotropically?
Can you please suggest any models to elucidate possible mechanisms or certain experiments I can do?
I think the reason for different mechanisms in both the mentioned cases is the plasmonic photocatalysis. Under UV light, metal nanoparticles are not in the LSPR state (Ag@TiO2 in this case) so electrons can not be transferred from metal into the semiconductor. However, metal NPs can act as sink for the excited electrons from the semicondutor. So electrons will flow from the semicondutor into the metal in this case.
In the case of Au/TiO2 under visible light, the electrons inside Au NPs are excited due to LSPR. These excited electrons will be transferred to TiO2.
I suggest following Review Article for reference:
Zhang, X.; Chen, Y. L.; Liu, R. S.; Tsai, D. P., Plasmonic photocatalysis. Rep Prog Phys 2013, 76 (4), 046401.
When GNPs irradiated with visible light ,It does not make any change,Electrons in GNPS get resonance with the incidant radiation.I tried with low power laser in visible range and the GNPs are stable after irradiation which is confirmed with absorption spectra.And NIR of low power also does not produce any damage in particles.
i guess the power of laser is probably lower than 200-300 mW/cm2. I tried similar experiments with overnight irradiations, and absorbance spectra show very slightly broaden, and in SEM i can found some melted, larger gold sphere coming... well, this is an issue other than charge transfer, but more like a troublesome disadvantage
Collado, Laura, Anna Reynal, Fernando Fresno, Mariam Barawi, Carlos Escudero, Virginia Perez-Dieste, Juan M. Coronado, David P. Serrano, James R. Durrant, and A. Víctor. "Unravelling the effect of charge dynamics at the plasmonic metal/semiconductor interface for CO 2 photoreduction." Nature communications 9, no. 1 (2018): 1-10.
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