ZnO nanoparticles are known to show slow photocurrent decay. It is often related with oxygen desorption at the surface in UV illumination and adsorbing once the light turned off. Although the energetics are not much different in TiO2, it does not show the slow decay. I am looking to get an explanation why oxygen desorption will occur at ZnO but not at TiO2 NP.
The phenomenon of persistent photoconductivity in ZnO and related slow photoconductivity decay when UV light is switched off is often related to the existence of defect traps due to oxygen vacancies.
- When UV light is shined on, the electrons are excited from these trap levels into the conduction band of ZnO.
- When light is switched off these electrons are not able to recombine back instantly on their parent defect levels because there is a potential barrier separating conduction band states (where the photoexcited electrons are ) and the defect level states. Because of this potential barrier, the decay is very slow, lasting for minutes, and even for hours.
- When the same photoconductivity experiment are performed on ZnO samples under an oxygen rich atmosphere, then the persisitent photoconductivity (PPC) phenomenon strongly decreases and even disappears. This gives evidence that this PPC is directly related to oxygen vacancies. Increasing the oxygen pressure of the atmosphere induces a decreases of the PPC effect indicating a decrease of active oxygen-vacancy related defect densities as a result of oxygen adsorption on these oxygen vacancies.
- For ZnO PPC effect explanation see the following references :
1. Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires, by Jiming Bao, et al. ,in Nanoscale Research Letters volume 6, Article number: 404 (2011)
2. Reduction of persistent photoconductivity in ZnO thin film transistor-based UV photodetector by Pavel Ivanoff Reyes et al. in APPLIED PHYSICS LETTERS 101, 031118 (2012)
3. Persistent Photoconductivity in ZnO Nanowires in Different Atmospheres
by Davide Cammi et al. in Advances in Condensed Matter Physics / 2014 Volume 2014 |Article ID 184120 | https://doi.org/10.1155/2014/184120
As to TiO2 material: there are also several evidences for PPC effects as well, in TiO2 various materials : in nanocrystalline thin films as well as in nanotubes fact that the same phenomenon is not observed in TiO2 is likely to be due to the absence of oxygen-vacancies in this material. See following links:
4. Power-law photoconductivity time decay in nanocrystalline TiO2 thin films
by D Comedi et al. in Journal of Physics: Condensed Matter, Volume 19, Number 48
5. Relaxation of Photoconductivity and Persistent Photoconductivity in TiO2 nanotubes by M. Enachi et al. in Conference: 2nd International Conference on Nanotechnologies and Biomedical Engineering, April 18-20, 2013 At: Chisinau, Moldova Volume: 1
Best regards,
Professor A. Kadri
Hi Abderrahmane Kadri , Thank you for your response. This is very helpful. However, although some work reported evidence of PPC in TiO2, this is very uncommon. On the other hand, ZnO and ITO showed PPC in most of the work. However, should it be considered as a property of the metal-oxides (as oxigen defects can be deep level defects) Or should we consider it as a property of the nanoparticles (due to the surface defects)?
Hi Sudip Saha,
The matter of the real origin of the defects is extremely complicated, however, at least in the case of ZnO, most people, basing on experimental evidence (see papers above), attribute the defect states in this material to O-vacancies. And by deep defects it is meant that the activation energy is large (hence deep levels). This does not mean defects deep in the material's bulk volume. These kind of O-vacancies-related defects are likely to be surface defects in ZnO not too much volume defects, because of adsorption and desorption also observed phenomena.
In general, PPC depends not only on the defects involved in a given material (its chemical origin) but also on the nature (3-dimensional, 2-dimensional, thin film layers, nanoparticles, nanocrystallines, nanotubes, ...) of the material because defect are not only related to some chemical species as O in ZnO but also on several other parameters as symmetry, strain, growth process and so on..
But there is experimental evidence that PPC indeed exists in TiO2, maybe just to a lesser extent than in ZnO. Here in TiO2, PPC effects were attributed to inhomogeneities (disorder) at least in one paper.
Best regards,
Professor A. Kadri
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