I think P25 is the best commercial photocatalyst in laboratory-scale. I suggest to read some review articles in which some comparisons were made..

P25 is more suitable than ZnO.

P-25 is non toxic, inert, cheap and its a mixed phase semiconductor oxide is better photocatalysts  thats why people prefer it for water purification 

Dear Prof Z Wang, Dr N. Biswal, Dr S. Gupta

Thanks for the comments. Some literature suggest that ZnO is relatively better while some literature and people including your-self say TiO2 is better. Both of these materials in their nano and micro particles forms are commencially available at almost same price and  ZnO is relatively little cheaper.

Also I am not sure whether TiO2 is available in micro size partiles, because nano size particles of TiO2 are very difficult to remove and reuse after their photochemical reaction. Moreover, if TiO2 size is increased its performance should also be reduced as surface area to volume ratio would decrease.

On the other hand, both the TiO2 and the ZnO are also used in tooth paste and other products. Also, little amount of Zn is said to be needed for human body functioning? Therefore, I will appreciate if some review articles are introduced to conclude which one is preferable any why?

If the TiO2 and the ZnO  performance is comparative than why ZnO is not recommended as a preferred photocatalyst, whileas, it can have useful nano/micro structure like tetrapod structure? Please be more specific for your any expert comments in this field. Thanks in anticipation.

as photocatalytic activity TiO2 is more active than ZnO in waste water treatment and industrial application but in bio medical applications ZnO is better due to its less toxicity as i think  

Dear Prof El-Molla,

Thank you very much for your comment. However, the following ref shows the photocatalytic effect of ZnO nanotetrapods is more than TiO2.  Wan et al, APL, 87, 083105 (2005). Would you kindly comment on this and oblige. On the other hand what is the reason that makes TiO2 less toxic than ZnO. Is there any proof to understand their comparative toxicity level for human or other animals or plants? Many thanks in anticipation. Best regards,

In photocatalysis, TiO2 has been studied extensively because of its high activity, desirable physical and chemical properties, non toxic,low cost, and availability.

Dear Jamal Alizadeh,

Thanks for the comments. You are absolutely correct that TiO2 has been studied extensively. It may be due to the fact that TiO2 has longer known history for its photocatalysis properties and ZnO came in light much later and is relatively new in the field. But now both are easily available now in their large surface area to volume ratio for nano and micro particles at almost the same cost. One reason for its less attraction could be its comparatively large solubility in acidic medium. Would this reason be enough to prevent its use in water purification application? If yes then why there is lots of research work being directed recently towards ZnO water purification technology development? Any justified reply will be highly appreciated to make a choice between TiO2 and ZnO for water purification applications. If TiO2 and ZnO both are safe and good with acceptable proof for their water purification applications with proof and without any worry for health issues, then the recycles process of the photocatalysts should be able to help in making a right choice? If ZnO has any particular known/proven reason which prevents (or not) its use in water purification application, it will be greatly appreciated to have any expert comments from the experts in the field. Many thanks in anticipation.

As some literature suggest that ZnO is better than TiO2. which is contrary to some of the above comments.

In fact TiO2 does not get desolved in water but the TiO2 get less affective for its photo catalyst activity due to the fact that its surface gets some layer grown on it?

Interestingly, however,

1. Both the TiO2 and the ZnO are already in use in tooth paste and many other products. 

2. A little amount of Zn is said to be needed for human body functioning? Some reports say that body has no biological system to store the necessary Zn for body functioning and hence requires to consume on daily basis. 

Therefore, it will be appreciated if some review articles are introduced to conclude which one or both are preferable or not and why? (considering cost matter is neglected).

If there is any expert in the field who may be specific for reliable information & comments on this field.

I specially want to know whether TiO2 and/or ZnO are permitted or not for their use to realize the photo catalyst property based water purification system?

Are there any national or international norms for letting these photo catalysts use or not to realize a photo catalyst based water purifier?.

Thanks a lot in anticipation.

It's maybe too late to look at my answer, but I suggest others to read this mini review of your compatriot colleagues:

Photocatalytic Oxidation of Pollutant Dyes in Wastewater by TiO2 and ZnO nano-materials – A Mini-review

KUNAL MONDAL, and ASHUTOSH SHARMA

According to this mini review, Photocatalytic decomposition with ZnO and TiO2 catalysts is mainly applied for treating organic contaminants dyes in wastewater because of their ability to attain fully mineralization of the organic contaminants under mild reaction conditions such as ambient pressure and temperature. Most recently, photocatalytic degradation of organic molecules using TiO2 and ZnO have been studied extensively due to the degradation of persistent organic pollutants, PAH (Polycyclic Aromatic Hydrocarbons) dyes and other organic chemicals which are known to be endocrine disruptors.

Particularly, titanium dioxide is largely used for many applications such as air and water purification, as an agent for antifogging and self-cleaning surface, whereas, zinc oxide has not been used like TiO2 for such applications. TiO2 has better photochemical stability than ZnO in aqueous solution; however, zinc-oxide is photochemically more active (although its surface to volume ratio was less). TiO2 and ZnO have been recommended to be efficient photocatalysts for the degradation and mineralisation of various toxic organic pollutants such as azo dyes in wastewater water.

Chapter Photocatalytic Oxidation of Pollutant Dyes in Wastewater by ...

P25 is more suitable for organic dye and if you want to degrade some other dyes like acid black 5 etc than ZnO is more efficient

Dear Mr Ali Johar, Thank you for your valuable comment. Warm regards, B.P.Singh

agree with @Zhaohui Wang

regards

Agree with Sanjeev Gupta

regards

I think ZnO NPs is too difficult to dissolving in water but in case of QD ZnO it can be dissolving easily using ultrasonication

Thank you very much.

Recently, finally I considered coating Panatetra ZnO with TiO2 nano layer to solve ZnO dissolution problem. This will not only maintain photocatalist activity but also making the final photocatalist composite material ZnO/TiO2 recovery process easier.

I think ZnO is more efficient and cost effective. But I think you have to think in doping for ZnO especially using Silver . Silver is plasmonic element it you want and additional discussion you can call me