TiO2 thin films by ultrasonic spray pyrolysis as photocatalytic material for air purification

In this study, we showed that the TiO2 thin films deposited onto window glass are practicable for air purification and self-cleaning applications. TiO2 films were deposited onto window glass by ultrasonic spray pyrolysis method. Different deposition temperatures were used in the range of 250–450°C. The structural, morphological, optical properties and surface chemical composition were investigated to understand probable factors affecting photocatalytic performance and wettability of the TiO2 thin films. The TiO2 thin films were smooth, compacted and adhered adequately on the substrate with a thickness in the range of 100–240 nm. X-ray diffraction patterns revealed that all the TiO2 thin films consisted of anatase phase structure with the mean crystallite size in the range of 13–35 nm. The optical measurements showed that the deposited films were highly transparent (approx. 85%). The wettability test results showed that the TiO2 thin films sprayed at 350°C and 450°C and annealed at 500°C for 1 h were superhydrophilic. The photocatalytic activity of the films was tested for the degradation of methyl tert-butyl ether (MTBE) in multi-section plug-flow reactor. The TiO2 film deposited at 350°C exhibited the highest amount of conversion of MTBE, approximately 80%.

they must tell this difference clearly. 2-What is the novelty and new contribution of this paper? 3-Authors should absolutely specify the new results reported in this work compared to other results. 4-More and proper discussion about the results is necessary. 5-Explain the increase and the decrease of grain size. 6-How much the thickness values of the films? 7-The English grammar should be reviewed.
Decision letter (RSOS-181578.R0) 21-Nov-2018 Dear Mr DUNDAR: Title: TiO<sub>2</sub> thin films by ultrasonic spray pyrolysis as photocatalytic material for air purification Manuscript ID: RSOS-181578 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The editor assigned to your manuscript has now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Subject Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
Please submit your revised paper before 14-Dec-2018. Please note that the revision deadline will expire at 00.00am on this date. If we do not hear from you within this time then it will be assumed that the paper has been withdrawn. In exceptional circumstances, extensions may be possible if agreed with the Editorial Office in advance. We do not allow multiple rounds of revision so we urge you to make every effort to fully address all of the comments at this stage. If deemed necessary by the Editors, your manuscript will be sent back to one or more of the original reviewers for assessment. If the original reviewers are not available we may invite new reviewers.
To revise your manuscript, log into http://mc.manuscriptcentral.com/rsos and enter your Author Centre, where you will find your manuscript title listed under "Manuscripts with Decisions." Under "Actions," click on "Create a Revision." Your manuscript number has been appended to denote a revision. Revise your manuscript and upload a new version through your Author Centre.
When submitting your revised manuscript, you must respond to the comments made by the referees and upload a file "Response to Referees" in "Section 6 -File Upload". Please use this to document how you have responded to the comments, and the adjustments you have made. In order to expedite the processing of the revised manuscript, please be as specific as possible in your response.
Please also include the following statements alongside the other end statements. As we cannot publish your manuscript without these end statements included, if you feel that a given heading is not relevant to your paper, please nevertheless include the heading and explicitly state that it is not relevant to your work.
• Ethics statement Please clarify whether you received ethical approval from a local ethics committee to carry out your study. If so please include details of this, including the name of the committee that gave consent in a Research Ethics section after your main text. Please also clarify whether you received informed consent for the participants to participate in the study and state this in your Research Ethics section. *OR* Please clarify whether you obtained the necessary licences and approvals from your institutional animal ethics committee before conducting your research. Please provide details of these licences and approvals in an Animal Ethics section after your main text. *OR* Please clarify whether you obtained the appropriate permissions and licences to conduct the fieldwork detailed in your study. Please provide details of these in your methods section.
Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The authors of the paper entitled: "TiO2 thin films by ultrasonic spray pyrolysis as photocatalytic material for air purification", presented the study of deposited TiO2 thin films by ultrasonic spray pyrolysis method. The scientific motivation has been presented in a satisfactory way. The structural, morphological, optical properties and surface chemical composition were investigated to understand probable factors affecting photocatalytic performance for the degradation of methyl tert-butyl ether in multi-section plug-flow reactor and wettability of the TiO2 thin films. The novelty of the investigation was the study of chemical composition and wettability of the films, as is not usual in reports on chemically sprayed TiO2 thin films. The Introduction is clearly writing, as well as the Experimental Part. Results were clearly discussed showing consistency with other reports. The paper is very interesting to me as well as found novelty of research finding. The manuscript is also well organized and I think it's suitable for the publication on Royal Society Open Science.

Reviewer: 2
Comments to the Author(s) Authors, the paper requires major changes with the better explanation. Here are my suggestions, questions, and confusion regarding your submission. I recommend accepting the paper after a major revision. 1-In the introduction of the manuscript, the authors have not given any information about the studies carried by the other researchers which are similar with their study and also they have not mentioned at all about the difference of their study compared to the other similar studies. So, they must tell this difference clearly. 2-What is the novelty and new contribution of this paper? 3-Authors should absolutely specify the new results reported in this work compared to other results. 4-More and proper discussion about the results is necessary. 5-Explain the increase and the decrease of grain size. 6-How much the thickness values of the films? 7-The English grammar should be reviewed. Comments to the Author(s) The authors of the paper entitled: "TiO 2 thin films by ultrasonic spray pyrolysis as photocatalytic material for air purification", presented the study of deposited TiO 2 thin films by ultrasonic spray pyrolysis method. The scientific motivation has been presented in a satisfactory way. The structural, morphological, optical properties and surface chemical composition were investigated to understand probable factors affecting photocatalytic performance for the degradation of methyl tert-butyl ether in multi-section plug-flow reactor and wettability of the TiO 2 thin films. The novelty of the investigation was the study of chemical composition and wettability of the films, as is not usual in reports on chemically sprayed TiO 2 thin films. The Introduction is clearly writing, as well as the Experimental Part. Results were clearly discussed showing consistency with other reports. The paper is very interesting to me as well as found novelty of research finding. The manuscript is also well organized and I think it's suitable for the publication on Royal Society Open Science.
Review report: 2 Comments to the Author(s) Authors, the paper requires major changes with the better explanation. Here are my suggestions, questions, and confusion regarding your submission. I recommend accepting the paper after a major revision.

Comment 1:
In the introduction of the manuscript, the authors have not given any information about the studies carried by the other researchers which are similar with their study.
Answer: The authors would like to thank the referee for his/her valuable remark. The introduction of the manuscript was improved accordingly. The photocatalytic activity of coatings was studied by the degradation of methylene blue in aqueous phase under UV or visible light."

Comment 2:
The authors would like to thank the referee for his/her valuable remark. The introduction of the manuscript was improved accordingly.

Answer:
In the present study, TiO 2 thin films were composed by sols derived from titanium tetraisopropoxide precursor. Films showed high transparency in visible spectral region. The study of the photocatalytic activity of thin films was carried out in a multi-section plug-flow photocatalytic reactor. Besides, the TiO 2 thin films were deposited by the ultrasonic spray pyrolysis method, which was used before only to test the photocatalytic activity of TiO 2 thin film for dye degradation in water treatment applications. On the other hand, we investigated wettability behavior of TiO 2 thin films, which is an important property of the window coatings for outdoor air treatment.
Added text: It now reads in the revised version on: Page 2, 9 th paragraph: "The present paper is a comprehensive study of unmodified TiO2 thin film synthesized by ultrasonic spray pyrolysis and applied for the abatement of air pollutants. No publications on the decomposition of VOC MTBE on transparent TiO 2 thin films fabricated at different temperatures reporting their photocatalytic activity regarding the materials characteristics were found available, thus this study would supply more insights into this topic." Comment 3: What is the novelty and new contribution of this paper? Answer: The combination of following items is considered as novel and original:  The first paper on transparent TiO 2 thin films (<200nm) for photocatalytic degradation of MTBE.  The method of film deposition, which is ultrasonic spray pyrolysis.  The unique reactor for photocatalytic activity test, which is multi-section plug-flow photocatalytic reactor.

Comment 4:
Authors should absolutely specify the new results reported in this work compared to other results.

Answer:
The new results compared to TiO 2 thin films deposited by wet-chemical deposition methods are; a) Ultrasonic spray pyrolysis proved, as a promising technique for the deposition of TiO 2 thin films for air purification applications. b) Multi-section plug-flow reactor is highly convenient to test photocatalytic activity of TiO 2 coatings by following the degradation of gaseous organic pollutants. c) The results of photocatalytic activity study showed the high influence of the deposition temperature on the specific surface properties of the TiO 2 thin films deposited by ultrasonic spray pyrolysis. d) The wettability results of TiO 2 thin films, which exhibited superhydrophlic behaviour even for the aged samples.

Comment 5:
More and proper discussion about the results is necessary.

Answer:
The authors have made the following amendments to the results and discussion section: Added texts: It now reads in the revised version on:

Surface Morphology:
Page 4, 2 nd paragraph: "The increase in the film thickness with deposition temperature has been found also in other studies, e.g. in case of ZrO 2 and TiO 2 deposition by spray pyrolysis [21,22]."

Structural Properties
Page 5, 3 rd paragraph: "Similar behaviour, where the crystallinity of the film was controlled by both the deposition and annealing temperature, has been observed also for TiO 2 thin films deposited by pulsed pneumatic spray pyrolysis method [22]."

Chemical Composition and Wettability
Page 7, 4 rd paragraph: "A decrease in the amount of OHgroups was observed in TiO 2 thin films deposited by sol-gel methods: Simonsen et al. [30] reported a decrease in OHgroups on the microwave assisted sol-gel TiO 2 thin films with the increase in calcination temperature. Chen et al.
[28] studied TiO 2 thin film grown by solgel spin coating and reported an increase of in the number of hydroxyl groups with an increase of film thickness; but with a further growth of thickness a decrease in the amount of OHgroups on the film surface was observed. Ennaceri et al. [31] reported gradually increase in the amount of surface OHgroups with the increase in the thickness of the TiO 2 thin films deposited by ultrasonic spray pyrolysis.  (Table 2). It was found that the highest amount of oxygen vacancies on the TiO 2 film surface belongs to the film fabricated at 350 o C ( Table 2). The lower number of oxygen vacancies defects on TiO 2 thin film deposited at 450 o C could be attributed to the higher deposition temperature: the intensive heat treatment could repair the oxygen vacancy defects. Liu et al.
[32] studied TiO 2 thin films deposited by ultrasonic spray pyrolysis and annealed at different temperatures; a decrease in oxygen surface defects on TiO 2 thin film with an increase in annealing temperature was reported." Page 7, 5 th paragraph: "Thus, XPS results showed that the film deposited at 350 o C has the highest amount of OHgroups and oxygen vacancy defects and less carbon content on the surface of the film ( Table 2). The OHon TiO 2 surface are considered very effective for photocatalytic degradation of organic pollutants as they are generally the precursors of hydroxyl radicals. In addition, the surface oxygen vacancy defects are crucial because water can dissociate on oxygen vacancies with the formation of two bridging OH-groups creating more OH-groups on the film surface, which is beneficial for adsorbing VOCs by forming hydrogen bonds with functional groups [28,33]. The carbon impurities were found giving an inhibiting effect to the active sites on the surface of TiO 2 thin films, and thus decreasing the photocatalytic activity [14]." "Additionally, the presence of the surface oxygen defects enhance the wetting properties, leading to the trapping of OHgroups [28]. Simonsen et al. [30] studied the effect of OHgroups on superhydrophilic properties of TiO 2 thin films with different characteristics: a linear correlation between the OHamount on the surface of the film and the water CA was observed. On the other hand, Chen et al. reported that the TiO 2 thin film which has the lowest level of OHand Vo, also demonstrated the superhydrophilic property under UV light [28]."

Photocatalytic Activity
Page 9, 7 th paragraph: "In the present study, the thickness of TiO 2 thin films increased with an increase to the deposition temperature ( It has been found that the highest reaction rate (0.85 µmol min -1 ) was obtained with the TiO 2 (P25) coating of 0.6 mg cm -2 and that the reaction rate stayed constant even after the increase in the specific catalyst quantity." Comment 6: Explain the increase and the decrease of grain size.

Answer:
The authors would like to thank the referee for his/her valuable remark. The grain size is influenced by the deposition temperature of the TiO 2 films. As seen from SEM images Fig 1, the grain size is increasing with increasing deposition temperature. While the surface of the film deposited at 250 o C and annealed at 500 o C consists of grains with a size of ca 20 nm, the film deposited at 450 o C and annealed at 500 o C consists of grains with a size of ca 50 nm. However, an increase and a decrease were observed in the mean crystallite size. Even though the annealing temperature (500 o C) is same, the crystallite size are different regarding to the differences in growing temperature. There is an increase in the mean crystallite size from 13 nm to 35 nm when the deposition temperature increases from 250 to 350 o C, while the TiO 2 film deposited at 450 o C showed 32 nm. The slight decrease of ca. 3 nm could be attributed to the fitting error, which is ca 10%.
Comment 7: How much the thickness values of the films?
Answer: Thank you for the question. We agree with referee that thickness optimization is needed. In our studies, we kept all deposition parameters constant, except deposition temperature. Therefore, we can relate film thickness and specific surface properties to the specific deposition temperature.
It now reads in the revised version on: Page 9, 8 th paragraph: "In the present study, the thickness of TiO 2 thin films increased with an increase to the deposition temperature (Table 1). It has been reported [44] that higher photocatalytic activity is attributed to a rougher surface of the film with higher thickness. However, in several studies, it has been reported that with the further increase in the thickness of films, the length of the migration path of the carriers to the surface of the catalyst increases while their generation rate remains constant; the charge carriers experience higher recombination rates, resulting in an overall decrease in the photocatalytic activity [44]. This could be another reason of lower photocatalytic activity of the TiO 2 thin film deposited at 450 o C if compared to the TiO 2 film deposited at 350 o C."