Selamun Aleykum, Depending on the dispersion (the second metal) and hence the coverage of the active sites, photocatalytic activity might have decreased. Second metal might have worked as combination sites too. There are several reasons of degradation in activity. Why have you impregnated with second metal on the first hand?

Metal dopants acts as recombination centres which is reason for low photocatalytic activity.

A lot of literatures have reported that moderate doping will be helpful for photocatalytic activity, but excess doping will cause more defect site where will become the center of electron and hole recombination. So, i think you should modulate the amount of metal doping.

I want to ask how can you know the second metal doping can help improve the activity

Yes, I completely agree with what Dominique has suggested. There are metals as suggested earlier by someone if doped in appropriate amount enhances the photocatalytic activity. However, there are few metal which decrese the activity. This may be either due to recombination or also due to the in appropriate electronic str. of the doped metal.

There are different reasons for your question: first, this is depend on the type of metal loaded, second, the percentage of this metal, third, the type of loaded or deposition of this metal on the oxide surface, time of irradiation "in your photocatalytic run"all these reasons may affect on the decrease in photocatalytic activity, but in general i am agree with our researchers, almost type of metals reduce the recombination process, so will enhancement the photocatalytic activity.....not decrease.

Photocatalytic activity is completely based on the band gap of the material. higher the band of the material, excitation energy required will be high. When the electron jumps back to ground state, the energy released in the transition will be very high.

When metals are doped, there will be secondary energy states formed between the valance and conduction bands, reducing the band gap. This way we can tune the band gap of materials to desired excitation energies. But when the band gap is reduced way too much, the energy released form the electron transition is insignificant. This might be the reason for the loss in photocatalytic activity you suffered. As all the researchers pointed out, check your metal & metal % doped. I believe this can solve the issue

Good luck

May be, poisoning, structural instability of the oxides. It can be the quantity of doping, test 0.1, 0.5 and 1% of dopant.

Increasing amount of doped metals may reduce the photocatalic activity. Finer the size surface area will increase. nano particles have higher photocatalic activity.

Photocatalytic activity of a material depends upon its band gap energy and recombination factor. Firstly you did not clearly mentioned about the second metal which you doped after first doping. Please give the details of your material. Analyze your samples by PL, UV-vis DRS and photo-current measurement. You have to find out the band gap energy of all the samples: neat, 1st doping and 2nd doping material. Then you can find the reason for the deactivation of your material. Just find out whether the the band gap energy decreases after the second metal doping or not? Recombination factors may be the major reason for this deactivation. Ok, please give details of your materials, we will find the solutions...............

Good Luck

Thank you very much to all for their valuable suggestions. It is really a great help to me.

There could be many reasons.1) the morphology, i.e. the surface area of the double doped sample may be smaller because of repeated calcinations. 2) metal impurity may lead to lattice defects in a semiconducting material and hence certain closely spaced inter-bandgap charge trapping states. These localized trapping states may promote trapping and recombination of rediation generated e-/h+ charges.

Metal doping can make a positive/negative effect in ZnO by different physical effects. Restrincting to positive effects, some metals alter band levels (band gap energy) while others are mostly at surface positions and, typically, produce localized levels to allow more efficent oxygen capture. Positive alternation of charge recombination is relative complex as M doping always generates vancancies and those are typically recombination centers (still there are some reports claining positive effects). The effect of the metal itself in recombiantion is also a question whethen it can localize charge (or not) and be at the surface/bulk. In addition, other than electronic effects would be important and reports presenting positive photocatalytic effects through modification of surface acidity has been presented.

So, single M doping usually lead to several (positive/negative) effects and the photocatalytic results is typically a trade-off among them. The situation is more complex with co-doped samples. You can have an additive effect in some cases and thus a net negative/postive dominated by one of the metals or sinergistic (typically when the 2 metals are located at interacting distances, below 5 A) and thus more complex to analyze.

There may be some alternation/ disorder (deffects) in the lattice of the parent material after doping. This could affect the recombination factor which affects the photocatalytic activity.

You can find here a good analysis:

M. Radecka, M. Rekas, A. Trenczek-Zajac, K. Zakrzewska. “Importance of the band gap energy and flat band potential for application of modified TiO2 photoanodes in water photolysis,” Journal of Power Sources, vol.181, pp. 46–55. 2008.

Once again I would like to thank you all for your kindness.

I hope the article mentioned below can help you.

I hope the article mentioned below can help you.

degradation efficiency of of a material is exclusively attributed to the balance of the (high/low) crystallinity to reduce photo-excited electron–hole recombination rate and the (large/small) surface area for adsorbing substrates.Doping generates energy levels located in the forbidden band. Indeed, the exciton may be trapped by these energy levels at low temperature. These localized levels and absorb lower than that of the free exciton energy photons.

degradation efficiency of of a material is exclusively attributed to the balance of the (high/low) crystallinity to reduce photo-excited electron–hole recombination rate and the (large/small) surface area for adsorbing substrates.Doping generates energy levels located in the forbidden band. Indeed, the exciton may be trapped by these energy levels at low temperature. Thus the increment of photocatalytic activity was restricted

There may many reason. Particle size, purity of material are some of the reason for decrease in photo kinetic activities.

Doping with transition metals such as Cu promotes separation of charges and decreases the electron and hole recombination rate and thereby increasing the quantum yield of the photocatalytic process, thus favouring the photocatalytic activity [1]. In photocatalysis exists optimum dopant percentage varies according to the dopant species and its application, as to add a percentage larger than the optimum will have the opposite effect and photocatalytic activity will decrease [2].

1.-Linsebigler A. L., Lu G., Yates J. T. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735-758.

2.- Li G., Dimitrijevic N. M., L. Chen,† Tijana Rajh,‡ and Kimberly A. Gray. Role of Surface/Interfacial Cu2+ Sites in the Photocatalytic Activity of Coupled CuO-TiO2. J. Phys. Chem. C 2008, 112, 19040–19044.

It not easy to answer clearly this question, but you can begin with some test like:

1. Adsorption before and after doping

2. Band gap modification

3. E conduction band and E covalent band before and after doping

Even if this is not a precise answer to the question, please consult also:

Visible-light-responsive photoactive coating, coated article, and method of making same

US 7096692 B2/August 2006