Good reasons for increasing the activity may be several.

We need to know the initial conditions and methods of measurement.

Trang,  please describe a technique for measuring a photocatalytic activity and characteristics of titanium dioxide and N-TiO2 (impurity, the surface area, pore size, phase composition, and size distribution of the nanoparticles).

Some issues of   N-doping titanium dioxide are shown in my joint article with your colleagues from VAST.

Article Photocatalytic equipment with Nitrogen-doped titanium dioxid...

Thanks EN Kabachkov for your article.

 In our experiment, N-TiO2 catalyst was synthesized by solvothermal with urea as N source and atomic ratio Ti:N = 1:2, surface area: 50 m2/g, particle size: 10 – 15 nm, anatase phase is prominent and XPS results showed that Ti and N contents were 30% and 1,2%, respectively. The photocatalytic activity of N doped TiO2 was 2 and 3 times higher than undoped TiO2 in the degradation methyl orange and methylene blue, respectively.

Most of the articles on N doped TiO2 referred to the improvement of the photocatalytic activity of TiO2 catalyst in the visible region by doping N due to the formation of new band gap level and thus electron-hole pairs were easily produced even under visible light irradiation. However, in our study, N doped TiO2 just showed the good activity in the UV region but no activity in the visible light region. And I have not found any document explain the similar issue.

I would appreciate your help in this matter.

Trang, from your description of a lot of uncertainties for the complete and objective answer to your question. Graphs, tables and properties of titanium dioxide are required. The dependence of the photocatalytic activity of TiO2 from titania synthesis parameters is also needed.

causes:

1. photocatalytic activity (quantum yield ) of TiO2 depends from pH sedimentation of N-TiO2 by solvothermal with urea as N source

2. specific surface area and phase composition of N-TiO2 depends from pH

3. dissolved oxygen in the water is required to be monitored. Reducing the concentration of dissolved oxygen in the reaction mixture during the oxidation process leads to the cessation of photocatalytic oxidation, whereas at a sufficiently high level of dissolved oxygen mineralization of organic pollutants is complete.

More details in my unpublished article:

Research High activity photocatalyst for water purification

Perhaps you should consider the reasons why efficiency can increase: (a) a larger amount of useful radiation to activate your process or (b) a better use of the existing radiation.

Now, (a) seems not to be the cause as you don't have a band gap shift. Thus, (b) is the obvious reason and should be linked with reduced recombination. Doping (if real) is done to develop a controlled amount of lattice defects, and considering the charge balance there are two likely (out of more) lattice reactions, that can induce defects able to limit the electron-hole recombination.

There is some relevant literature on this....

N could substitute the lattice oxygen and narrow the band gap energy

Dear Anca Duta,

Could you list name of some articles on defect caused by doping N leads to limit the e/h+ recombination?

Thank you so much !

It is important to see/find out which defects can limit e-h recombination and to understand the thermodynamics (even at a very simple level, for the beginning); then it would be perhaps wise  to see if N or another dopant will act according to the way you want and then to design/control your material.

I am not sure which exactly is your background (physics / chemistry; modelling/experimental R&I) so I chose two recent papers which are interesting. But, searching the databases and using some keywords like "TiO2, lattice defects" will bring you plenty of info and you have to "filter" it according to your needs and interest: Most of the time, personal decision is essential in research and can make the difference. So, here are the two titles:

• Xiaoye Xin, Tao Xu, Jiao Yin, Lan Wang, Chuanyi Wang, Management on the location and concentration of Ti3+ in anatase TiO2 for defects-induced visible-light photocatalysis, Applied Catalysis B: Environmental, Volumes 176–177, October 2015, Pages 354-362

• Zengxia Pei, Sunxian Weng, Ping Liu, Enhanced photocatalytic activity by bulk trapping and spatial separation of charge carriers: A case study of defect and facet mediated TiO2, Applied Catalysis B: Environmental, Volume 180, January 2016, Pages 463-470

This discussion is really interesting and helpful. In our study, we found enhanced results, in visible as well as in UV region. The reason behind the increased photocatalytic activity may be the decrease in recombination time of the hole - electron pair. They recombine at a faster rate than as compare to undoped too2. As more the pairs recombine and that too fast, more the pairs will be available for further dissociation in the same time. The reasons for not getting the same results in visible region can be,

1) Have used fresh catalyst in both the experiments? ( Sometimes the N from TiO2 wears out after one or two batches).

2) N is more active or absorbs more in the ultraviolet region than as compared to the visible region.

pH is very crucial.

All answers were really interested, just i want add one one factor which is very important with the mechanism of absorption shifting to visible. The factor is surface area of the catalyst which the doping generally reduce almost the particle size of the catalyst as well as the optical preppies . However, this reduction in size led to the increasing of surface area if agglomeration less which increase the reaction rate at surface.

Finally overall have to optimize all parameters of photo catalysis process which the doping not only affect on final performance also concentration of doping , loading of catalyst ,pH and concentration of pollutants.....

plz LOOK to this paper :

Visible light photocatalytic activity of Fe 3+-doped ZnO nanoparticle prepared via sol–gel technique

MM Ba-Abbad, AAH Kadhum, AB Mohamad, MS Takriff, K Sopian, Chemosphere 91 (11), 1604-1611

Dear Trang Nguyen,

This is my project on TiO2 doping, hope to be useful, 

Article Hydrothermal-assisted sol–gel synthesis of Cd-doped TiO2 nan...

The reason may be due to the increase in oxygen deficiency and other related defects. You can try to analyze O1s and Ti 2P XPS spectra to see the change in defects.