Structural and Functional Behaviour of Ce-Doped Wide-Bandgap Semiconductors for Photo-Catalytic Applications

Round 1

Reviewer 1 Report

The authors synthesized CZ20, CCZ20, and CT20 and examined the structural and optical properties of the samples. The authors described different characterizations of the systems in the manuscript. Main purposes of the study were evidences and possibility of the systems to photo-catalytic applications. However, there was no improvement of C/C0 with Ce-added ZnO. The molar ratios of Zn:Ce and Ti:Ce were 4:1. There was a large amount of Ce in systems. The contribution of additional Ce atoms to catalysis was insignificant. The Authors described the synthesis methods and the structural and optical characterizations of the systems in details. This is insufficient to be published on a journal of catalysts.
Unfortunately, I cannot recommend the current manuscript for the publication on MDPI Catalysts.

1. The introduction part should clearly describe the purpose of the study.
2. The structural characterization parts are needed to be well-organized. 
3.  XAFS measurements at Ce absorption edge are recommended.

4. It is not clear the relation between the structural and optical properties and the catalysis of the Ce-added system.
5. The organization of the manuscript should be improved. 

Author Response

The reviewer correctly claimed the lack of increase of activity on the photocatalytic activity of the CeO₂-based composite materials. However, as clarified by the aim in the introduction, we were able to explain the decrease of photocatalytic activity by means of the characterizations performed. Moreover, we were able to derive the importance of both defects and surface chemistry, as well as the optical behaviour and the synthetic method, in the catalytic performance. We therefore believe our results to be useful to the successful selection and design of effective photocatalysts.

We replied with the following considerations:

1. We accept the suggestion and we provided to express the aim in a more straightforward way.
2. We agreed and provided to re-arrenge the structural part.
3. We agree that this measure could be useful but unfortunately, we are not able to perform such analysis in one week.
4. We provided to stress and clarify the role of structural and optical behaviours.
5. The comment of the reviewer is not specific, so we could not understand which modification needed to be applied. In fact, we followed the structure suggested by the journal and we already provided to rearrange the organization of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

In review, I received a manuscript entitled "Structural and Functional Behaviour of Ce-doped Wide-bandgap Semiconductors for Photo-Catalytic Applications" considered for publication in MDPI journal "Catalysts". The manuscript is describing the preparation of ZnO and TiO₂ powders doped with low contents of Ce. 

I consider the manuscript sufficient for publication. Here are some suggestions for further improvement of the manuscript:

1. The pH is a quite important parameter, what was the pH of the reaction mixtures in catalytic experiments?
2. The catalyst surface analyzes presented by the authors were carried out for powders, how does this relate to the aquatic environment (catalytic tests are conducted in the aquatic environment)?
3. Considering that the work was directed to the Catalysts journal, I believe that more space should be devoted to the discussion of catalytic experiments.
4. Were the functional and optical properties of the catalysts tested before or after the catalytic process? If before catalytic process please explain from where on the surface of the catalyst CO₂, CH_x?

Author Response

After considering the comments and suggestions, our replies are presented as follow:

1. We agree and provided to mention the information of pH in the manuscript.
2. We tested the photocatalyitic activity in different conditions, includint both aqueous and gas phase, with the purpose to correlate the results with the physico-chemical properties Nevertheless, we provided to re-consider surface properties with respect to the aquatic medium as suggested.
3. We agree.
4. Functional and optical properties were measured before the catalytic tests and we provided to modify the manuscript about the origin of C-based compounds. In fact, due to low loading of the catalyst we were not able to recover the sample after the reaction.

Author Response File: Author Response.docx