We have optimized the antimicrobial activity of these two oxides by size, doping and surface sensitization with metals, Ag, or oxides, CuO.
Optimum activity under visible visible light seems inversely proportional to size (below 30 nm) and strongly boosted by surface sensitization. Doping is not as good. Apparently, in absence of light activity seems to increase with the lowering of particle size.
My question is which of these issues plays the main role?
Surface sensitization with noble metals, small bandgap oxide and QDs can enhance the visible light adsorption and improve the catalytic activity of ZnO. However, Doping sometime can't do.
to B. Huang: thanks for your response. I think you are right and doping is a complex phenomenon (interstitial vs substitutional, nature of the vacancy to obtain charge neutrality, surface vs. bulk location, etc.) which is not easy to predict. In ZnO enhancing visible light activity by surface sensitization is probably the safest way to proceed as the morphology way is certainly complex as primary particle size and shape are known to be important. I wonder if there are good works analyzing surface sensitization as a function of these two variables.
You are right. The shape and size is important to properties of oxide nanoparticle. Small size sometime can result poor crystalline quality and lower the absorption of long wavelength light due to quantum size effect, although large surface area of small particles can improve the adsorption of substances. If we can get photocatalysts with high crystalline quality and high surface area, we have succeeded.
It may be useful to think about tadem systems (n-type/n-type semiconductors) with suitable alignment of the band gaps or to consider diode-type systems (n-type/p-type semiconductors). We got VIS activity (quite good) in a micro CuxS/nano TiO2 composite system. So getting VIS-active photocatalysts by senzitation should be actually band gaps engineering . Afterwards, surface (and more important interface) should be considered, along with surface charge and wettability, BET surface and all the good pieces of advice above mentioned.
to A. Duta: many thanks for the info. We have studied surface sensitization of TiO2 by some n/p oxides (a summary can be found in https://www.researchgate.net/publication/51820731_Advanced_nanoarchitectures_for_solar_photocatalytic_applications). While we (and many others) noted that visible and solar light activity are strongly dependent on the primary particle size of the oxides (e.g. QSE on band(s) position(s)/gap), other less known variables, related to defect structure and interfacial contact, were also important in a significant number of nanocomposite systems. However, we did not have (and I think is not in the literature) a general framework which may interpret the main physico-chemical properties governing the visible/solar light activity.
Article Advanced Nanoarchitectures for Solar Photocatalytic Applications
Yes you are right. And thank you for the very god synthesis paper you mentioned. Perhaps the significant knowledge from solid state solar cells (ETA, 3D) could be also translated to photocatalysis; this is something we are trying (and I am sure that we are not the only ones)
To S. Kumar; many thanks for the complete reference list. I was not aware of some of them (refs. 2 and 5) and will try to read soon. The use of polymer substrates (as well as other materials as C-containing materials) is certainly a hot topic which, I agree, would drive to rather interesting fotocatalysts in the near future.
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