Influence of Bio-Based Surfactants on TiO₂ Thin Films as Photoanodes for Electro-Photocatalysis

Round 1

Reviewer 1 Report

Introduction:

• Proofread the sentence below and need to re-write this sentence to make it clear.

” First, the photocatalyst, usually a semiconductor, is irradiated with light with an energy higher than its band gap separating the vacant conduction band (CB) and the valence band (VB) filled.”

• Please provide relevant references on Biosurfactants section.

Start from “Recently, with the aim of focusing on environmental approach……..    to     The main classes of biosurfactants are glycolipids, phospholipids, polymeric biosurfactants and lipopeptides.”

• Any reason of choosing the three bio-source surfactants used in this work? Some literatures of the use of the three surfactants with influence of photocatalytic activity would be good, as the author are focus on the effect of surfactants on TiO₂ thin films.
• How does the three bio-source surfactants used in this work classified (e.g. glycolipids, phospholipids, polymeric biosurfactants and lipopeptides)? More information on the used bio-source surfactants is needed. Can the author conclude which class of bio-source surfactant is better for photocatalytic applications?

Materials and Methods:

• General information of the three surfactants should be provided to make the work reproducible.
• More experimental information needed in synthesis of TiO₂ solution, such as the amount of each surfactant added and how the TiO₂/surfactant ratio being decided? Is the molar ratio fixed and how the molar ratio of TiO₂/surfactant alter the photocatalytic performance?
• What is the wavelength for 75 W UV light (Eurosolar lamp)?
• TYPO-The binding energy calibration would be 284.8 eV.

Results:

• According to Figure 2 XRD, different FWHM of anatase and rutile peaks at 25.2° and 27.4° were observed, suggesting the crystallite size are different than as prepared TiO₂. Information about the change in crystallite size of different TiO₂ polymorph may be helpful.
• Does the films thickness (nm) from Table 2 dependent to the amount of surfactant added during synthesis?
• Could film thickness affect the activity of the catalysts?
• Format issue on 1^st line on page 7 out of 16
• The binding energy of XPS spectra (Figure 4-5) should starts from high BE to low BE (e.g. present Figure 4 from 800-0 eV, not 0-800 eV).
• Figure 5c only show range for Ti 2p, no In 3d peaks were reported. In 3d was observed at 444.52 eV according to Table 3, but not within the Figure 5c range.
• The peak fitting in Figure 5c need to be redo. Ti 2p appear as doublet, which can be assigned to Ti 2p3/2 and Ti 2p1/2 due to spin orbital splitting (with split of ~5.54 eV), and doesn’t exist as single peak for each species. If two species were reported (Ti-O₂ and Ti-Ox (x<2), 4 peaks should be fitted into the spectra. Please refer to the XPS handbook or other literatures and re-do the fitting. Many typical Ti 2p XPS spectra for TiO₂ should be found easily.
• Please add references to prove the XPS fitting bonds are at the right range. Are the binding energy of the report bonds close to the reported literatures? Is there any literature reported the position of Ti-Ox (x<2)?
• Format error on Table 3, 2 in Ti2p3 Ti-O (TiO”2”) need to be subscripted.
• A reference TiO2 film with surfactant should be prepared as comparison in order to examine the film quality with/without the surfactant, especially in SEM.
• Again, can the film thickness be controlled via dip-coating deposition conditions?
• Please add references to “which is representative of the optical characteristics of TiO₂.”.
• Symbols (x) is missing in “Plank’s constant (6.63 10-34 J.s) and the velocity of light (3 108 m.s-1)” on page 9, Table 4, and page 13. Please check the though the draft
• Does film thickness affect the Pilkington test? Does thinner films (e.g. GC) has better degradation rate? The effect of film thickness in relation to Pilkington test can be important. Does thicker TiO₂ film suggested more TiO₂ material involved in the testing?
• Also, does the TiO₂ polymorph percentage contribute to the Pilkington test? Which TiO₂ polymorph is playing the important role in Pilkington test?
• According to SEM, TiO2-BIO has larger cracks lower surface area and better conductivity, but TiO2-BIO is also thinner. Does TiO2-BIO is thinner, so the electron transfer path to the conductive ITO is shorter and being conductive? The film thickness is critical here.
• Does crystallinity affect the conductivity of TiO₂?

Reference:

• References format should be described as follows, according to the Reference List and Citations Guide of the Catalysts Journal.

e.g. Journal Articles:
1. Author 1, A.B.; Author 2, C.D. Title of the article. Abbreviated Journal Name Year, Volume, page range.
Please read Reference List and Citations Guide for more detailed information.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

In this paper, the authors have synthesised three types of mixed-phase TiO₂ using three different surfactants GBACoco, GBAC18:1, and Lansperse BIO868. Then, they characterised the sample for photocatalytic performance in photodegradation of stearic acid under UV light. The paper is of benefit to the materials science and clean energy communities and can be published subject to these minor revisions:

The surfactant types have to be included in the abstract.

The AFM images can be improved in quality.

On page 12, the authors say “density favorable to the circulation of electrons” when comparing the resistivity data. Do they mean “density of states”? Or simply materials density? Both can affect electron circulation.

By examining the XPS results, can the authors confirm the presence or the absence of Ti³⁺ on the surface?

In the introduction, when discussing the role of the surface modification TiO₂ for photocatalytic performance, the authors may want to refer to these two in-depth studies:

https://doi.org/10.1016/j.apsusc.2016.06.178

https://doi.org/10.1016/j.apcatb.2021.120380

there is no photocurrent measurement vs time and/or other parameters so that I cannot really judge if the synthesized materials have a practical improvement. 

In the XPS, the authors have tried to show that there are also other peaks related to the reduced Ti species, but not so clear which oxidation state of Ti. But they have not specified which oxidation states they are. 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Recommendation:  Publish with minor revisions as noted

Comments: Authors have conducted the study of the behavior of TiO₂ thin films as photoanodes for electro photocatalysis for the influence of bio-based surfactants employing various techniques such as XRD, SEM, AFM, EIS, CV, etc. Three commercial surfactants were involved in this study, GC, GB, and BIO. However, not much information regarding the chemical composition of the surfactants and why these surfactants perform differently in the sol-gel formation of the TiO₂were provided. I strongly suggest the authors add this information to make this study more systematic. Overall, I would recommend this work to be published in Catalysts after the authors address the questions listed below.

1) The authors state “Three different types of surfactants have been tested giving rise to three different materials being structurally and morphologically characterized by DRX, Rietveld refinement, BET, SEM, AFM and XPS, completed by light absorption, photocatalytic (Pilkington test), electronic (EIS and C-AFM) and photoelectrochemical (Cyclic voltammetry) measurements.” in the abstract, the “DRX” should be referring to “XRD” from the flow of the text. Such a typo should be avoided in an academic journal article. The authors should be cautious and check such errors in other parts of the manuscript too.

2) The authors mentioned “conductive AFM or C-AFM” on page 1, page 4, page 12, and page 14 with random exchanging of the term, which may cause confusion to non-experts. The abbreviation should be used in a parathesis right after the first appearance of a specific term, and then used independently thereafter. The authors should be cautious and check similar problems.

3) On page 3 of the manuscript regarding the experimental characterizations, the wavelength of the X-ray used in the XRD measurement should be provided.

4) In Figure 2, the authors used the two peaks from the anatase phase and rutile phase of TiO₂ to compare the proportions of the TiO₂ phases, which is a good indicator. Just wondering if the authors have done any X-ray absorption studies for these TiO₂ synthesized? If yes, are the results consistent with the information from XRD here?

5) In Table 2 (on page 6), and the paragraph right above Figure 6 (on page 8), the uncertainties should be provided for the average pore width and root mean square roughness. Additionally, the “rough mean square roughness” looks like a typo based on my understanding. The images for the “flat and homogeneous surface apart for cracks” should be provided.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Authors have answered my questions. 

Reviewer 2 Report

The paper is revised accordingly, and it can be published in the current format.