There may be several factors that contribute to the enhanced activity. First of all the ZnO may have altogether different vacancy states when it was synthesized and kept for years. It could easily come across the atmospheric oxygen, moisture and other gases to have some chemistry which can modify the properties easily. Did you check the XPS of the sample just after synthesis and after keeping it for 2  years? That may provide some fruitful information.

The bottom line is that you do not have the same structure anymore, although, you didn't do any chemistry but the nature did some chemistry with your sample. 

what was your particle size

Its primary particle size, according to TEM, is about 50 nm 

There may be several factors that contribute to the enhanced activity. First of all the ZnO may have altogether different vacancy states when it was synthesized and kept for years. It could easily come across the atmospheric oxygen, moisture and other gases to have some chemistry which can modify the properties easily. Did you check the XPS of the sample just after synthesis and after keeping it for 2  years? That may provide some fruitful information.

The bottom line is that you do not have the same structure anymore, although, you didn't do any chemistry but the nature did some chemistry with your sample. 

Thanks Paritosh. I checked the XPS and XANES of the sample just after synthesis. I may also check the properties of the sample which is kept for 2 yrs, if my PI is interested.

Dear Dr. Sudrajct:

Operational parameters can be shortened in a partial way or significantly when subject of long time service or severe service ambient. In this sense, the deterioration of operational parameters can be thermal, mechanical or chemical.

In major part, some kind of combination of previous factors is most common. The previous factor can be further considered at catalyst disperse in a medium fluid or immobilized in a support each case with particular and specific route of degradation. Some events can be mechanical or chemical depending on the approach. Also, decreasing of overall activity can be classified as chemical or mechanical or both.

Chemical: contaminants react with catalyst. Ionic species, salts, organic molecules or groups, etc…etc, active sites undergoes gradually deactivation. Also, presence in the liquid of effluent of specific chemical poisons for the selected catalyst. Some salts can mechanically cover the catalyst surface. Some gases can be adsorbed.

Physico-chemical: specific by-products or contaminants are adsorbed on the catalyst surface in a gradual way gives formation of flocculate structures. In a minor extension, this phenomenon can also leads to clogging of the surface of the catalyst.

Mechanical: during draining of reactor some fraction of particles of catalyst are gradually leached or are lost with treated effluent.

Best regards

Marcos Nobre

Have you performed a surface analysis? I guess that the acetate is attached to the surface, so that a Photo-Kolbe reaction is likely. Hence the system changes with time (illumination). It seem to me that due to the stabilization of this radical may influence the reactivity (reactivity over time). The resulting radical is less stable so that it react non-selectively.

Article ZnO Nanoparticle Induced Photo-Kolbe Reaction, Fragment Stab...

Article Synthesis and testing of ZnO nanoparticles for photo-initiat...

Thanks Marcos, and Michael also for your important works

I would suggest you re-characterize the 'ZnO' powder. You mentioned that it was in the lab. I don't know if the powder was stored under vacuum conditions or not. Considering that you have quite fine nanopowder, it could be that exposure to CO2 and O2 in the lab over the course of 2 yrs could have converted some of the ZnO to ZnCO3, so there could be a chance that you now have a ZnO + ZnCO3 mixture rather than pure ZnO. A quick XRD scan could check if that is the case. 

If you have confirmed that you still have phase pure ZnO, I'll be curious as to Micheal's suggestions, since it is expected that you would have lost any acetate induced reactivity during thermal decomposition synthesis.

I am of the opinion that it would be a good thing if you could re-characterize the powder, you may find clues to the changes in photocatalytic behaviour of the sample

You are right but only if the reaction takes place until all ZnAc is decomposed. Normally such procedure leads to strongly asymmetric particles (wires). This ZnO surface will absorb nearly everything. From my point of view a measurable transition (XRD) of ZnO to ZnCO3 (at RT) will not take place due e.g. to the differences in crystal structure. Note that XRD is normally not used to analyse non-crystalline phases.

Article Synthesis of ZnO Nanowires by Thermal Decomposition of Zinc ...

15% performance enhancement may be originated from the different operation conditions, especially the actual optical irradiance at the catalyst . 

You should replicate your experiment to be sure that this result is reliable and accurate. If there is some doubt, you could use a new sample of the same dye. If the result is clear, the reason could be a completed thermal decomposition or some recristallization of the Zn compounds... 

Crystallinity of particles improves with aging, and therefore their photophysical and optical  properties also. At least, I know this fact about TiO2 particles as an example. This might be an answer.

Dear Elena,

Thanks for your information. Could you please let me know your paper about those findings ?

Regards

Here are some articles. One is our work published in JACS2009 ( and references wherein citing older works from the same authors) and more recent work from other group. I am quite sure you can find even more articles. Good luck!

re you sure that it is pure ZnO? otherwise there might be surface changes due to defects migration, that can actually change the chemistry and narrow the band gap

Thanks Micheal, I made the comment based on the premise that he had high purity ZnO at the initial stage. If there happened to be some occurrence of incomplete conversion of ZnAc to ZnO during initial synthesis, then I guess we may have found a clue to changes in catalytic performance after 2 years.

With regards ZnO to ZnCO3 conversion at RT, thanks for pointing out the thermodynamics of the process.

So I guess like others have suggested, Hanggara might want to convince himself about the purity of the sample before banging his head on too many walls trying to understand enhancement in photocatalytic activity 

Crystalline zinc oxide maybe chang when after keeping it for long time The change is caused by a small loss of oxygen to the environment at enough temperatures to form the non-stoichiometric Zn1+xO. The molecular structures of the some metal oxide species are changed in the terminal M=O and bridging M-O-M.  Due to oxygen vacancies or zinc interstitials different type show enhanced photocatalytic activity. 

Dear All. I thank you for all your comments.

I find a paper which may be close to my answer. 

Visible–Light–Induced Photodegradation of Rhodamine B over Hierarchical TiO2: Effects of Storage Period and Water-Mediated Adsorption Switch. Ind. Eng. Chem. Res., 2012, 51 (39), pp 12782–12786.

The differences between mine and theirs are the dye, catalyst, and radiation source used. I used Rhodamine 6G and ZnO under UV light. I need your expertise in this matter.

Regards !

Have you done any structural, optical analysis after synthesis and studied the correlation of those with photocatalytic properties? After 2 years of aging, you may repeat the process t find the reason behind it. Properties of ZnO definitely affect with aging.