Enhancement of Visible-Light Photocatalytic Efficiency of TiO₂ Nanopowder by Anatase/Rutile Dual Phase Formation

Round 1

Reviewer 1 Report

Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

In this paper Chen and Lin prepare TiO2, using acetylene flow rate and oxygen/C2H2 ratio to tune the composition and assess the activity of mixed phase TiO2.

The work was interesting and generally well presented and is useful, although I have a question about whether the composition can be controlled in this manner and if a reproducible composition (ratio of anatase to rutile) can be achieved over multiple synthesis attempts. The discussion of the rutile formation mechanism is well written and very nice and linked to the author's results.

Following are some more comments

• Line 93, text beginning "One mechanism is that.....environment" does not make sense, please rewrite
• - Line 103-105 text is repeated (rutile nucleation is described twice)
• Can authors compare their EPR to P25 from the Gray group work?
• It would help in figure 3 if the authors can also indicate the anatase:rutile ratio
• Line 215 "we have ever used" is not suitable English.
• Can authors be sure there is no carbon doping either into the lattice or onto the surface of particles?
• Line 239 "duel" should be "dual"
• Figure 6 should be redrawn to indicate the situation for those samples with no visible light absorption.
• Is methylene blue a suitable chemical for the degradation test, as it may degrade anyway under illumination.

Author Response

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Reviewer 3 Report

Rutile is only active when particle size below 10 nm. How do authors explain this?

Why does activity increase sharply after 4 h reaction time?

What is the ratio of anatase and rutile?

Authors should experimental details of photocatalysis; degradation of methylene blue

What is intensity of light used?

Does methylene undergo photolysis? And act as visible light sensitizer?

Poor referencing in paper. Author should cite reference or add experimental evidence their claims.

Author Response

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Reviewer 4 Report

1. Line 122-127 (page 4 of 9)
   - Why with the acetylene flow rate of 600 sccm, rutile forms with the lowest C2H2 : O2 ratio, and not form with more O2???
   - At the same C2H2 : O2 ratio (1/3), why rutile did not form with the acetylene flow rate of 800 sccm???
   - Have you try with the other acetylene flow rates??? only two different ratios are not enough to prove the rutile formation mechanism.
2. The data with the acetylene flow rate of 600 sccm obtained by XRD, UV-vis abs. spectra, EPR signal is not match each other.
3. Figure 5
   Please show the degradation of methylene blue data by the C/C0 curves and blank curve without photocatalyst.

Author Response

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Round 2

Reviewer 1 Report

The revised manuscript is carefully done and yields information that will be of interest to researchers working on TiO2.

Author Response

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Reviewer 3 Report

This is paper has a serious miss presentation of the data. authors must consider comments seriously and revise the manuscript accordingly. The title, abstract, results and conclusions are not supporting each other in the present form?

1. Rutile is only active when particle size below 10 nm. How do authors explain this?

A: The sample with acetylene:oxygen = 1:3 is the sample showing highest visible-light photocatalytic acivity among those prepared with 600 sccm acetylene flow rate. The sample with acetylene:oxygen = 1:3 happens to be the only sample having rutile with small grain size ~ 10 nm. The small rutile grain size is to echo the finding that rutile nucleation is feasible under oxygen-deficient environment. Under this rutile formation mechanism, the rutile forms along with the annihilation of the photonic center, such that the visible-light absorption is largely decreased. This finding allows us to argue that the higher photocatalytic for this sample is mainly due to anatase/rutile dual-phase effect compared with other samples with significant visible-light absorption but without anatase/rutile dual-phase.

Reviewer:  Authors say “The major role of mix-phased structure of TiO₂ is the enhancement of carrier separation and not of light harvest, for the powder produced in this study” If that’s true, you are opposing to the title and the conclusions of this paper. And what are the evidences for charge carrier separation?

Authors also claim that the visible-light absorption could be attributed to the formation of defect level related to the photonic center within the band gap of anatase. Is there any change in band gap? What are the experimental evidences for this claim? Are these defects are identified PL or XPS?

2. Why does activity increase sharply after 4 h reaction time?

A: The data for methylene blue photocatalytic degradation we presented is the genuine data intended to show the variation of photocatalytic activity of our powders. It is not intended to argue that the activity increases after 4 hours. Your point is well-taken. We have re-drawn Figure 4 (now Figure 5) and using straight lines assuming first-order reaction (even it is not necessarily first order) to differentiate their activities to avoid confusion. Thank you so much for your reminder.

We do not exclude the possibility that the reaction may not exactly first order, however. We would like to verify if the reaction does increase after some period of time or just because of experimental errors in the follow-up study.

Reviewer: Strange response from the authors. As a reviewer, I am not questioning your data rather it is about understanding your data to provide my comments. As there is sudden jump in the activity, its worth understanding the unusual trend in the activity.

3. What is the ratio of anatase and rutile?

A: Thank you for your suggestion that anatase-to-rutile ratios should be presented to facilitate comparison among effects by different ratios. We have added anatase-to-rutile ratios for power samples in Figure 4, which is very important to show our claim that visible light absorption does not correlate with photocatalytic activity.

Reviewer: Okay

4. Authors should provide experimental details of photocatalysis; degradation of methylene blue

A: We have added the paragraph. “Photocatalytic activity was accessed by degradation of methylene blue under visible light using PL lamp (PHILIPS 36W/865/4P). Methylene blue solution was prepared by dissolving 10 mg of methylene blue in 1 liter of deionized water. Then 200 ml of the methylene blue solution was taken in a 500 ml of beaker with the addition of 10 mg of the TiO2 powder under study. The beaker was then placed under the lamp with constant magnetic stirring for photocatalytic degradation of methylene blue. Each one hour, 3 ml of methylene blue solution was taken and measured UV-vis absorption spectroscopy.

  Reviewer: Okay

5. What is intensity of light used?

A: We specify fixed intensity of light using constant distance (20 cm) between lamp and the bottom of the beaker containing methylene blue solution. The PHILIPS 36W/865/4P consists of 2 parallel fluorescent tubes with 385 mm in illuminant length and 18 mm in diameter. Two tubes are connected in parallel with 2 mm in separation. The estimated intensity on sample is ~2W/m² assuming 10% of power conversion efficiency and 20 cm of distance between lamp and methylene solution.

Reviewer: 1 sun intensity is 1000 w/m2. Are the author sure that they only using 2 W/m2? (2/1000 = 0.002)

6. Does methylene undergo photolysis? And act as visible light sensitizer?

A: Methylene blue is commonly used in TiO₂ photocatalysis study. Methylene blue solution does not degrade photocatalytically during 5-hour period of time under our illumination condition when TiO₂ powder is not present. The photolysis is not likely.

When the methylene blue is attached to TiO₂, only the negatively charged electron is possible to move toward TiO₂ and the positive charge should accumulate on the methylene blue molecule, because the hole is not energetic enough to overcome the barrier imposed by valance band edge of TiO₂. The coulombic attractive force should pull back the electron and recombine with hole. Since the photocatalytic reaction cannot proceed without the separation of carriers, the methylene blue is not likely to be the sensitizer in this experiment.

Reviewer: Okay

7. Poor referencing in paper. Author should cite reference or add experimental evidence their claims.

A: We have added some more references to strengthen our claim. The papers we added are as follows.

․   Mo, S.-D.; Ching, W.Y. Electronic and optical properties of three phases of titanium dioxide: Rutile, anatase, and brookite. Physical Review B 1995, 51, 13023-13032. (ref. 6)

․   Hanaor, D.A.H.; Sorrell, C.C. Review of the anatase to rutile phase transformation. Journal of Materials Science 2011, 46, 855-874. (ref. 19)

․   Gouma, P.I.; Mills, M.J. Anatase-to-Rutile Transformation in Titania Powders. J. Am. Ceram. Soc. 2001, 84, 619-622. (ref. 20)

․   Penn, R.L.; Jillian, F.B. Formation of rutile nuclei at anatase {112} twin interfaces and the phase transformation mechanism in nanocrystalline titania. Am. Mineral. 1999, 84, 871-876. (ref. 21)

․     Zhang, H.; Banfield, J.F. Understanding Polymorphic Phase Transformation Behavior during Growth of Nanocrystalline Aggregates:  Insights from TiO2. The Journal of Physical Chemistry B 2000, 104, 3481-3487. (ref. 22)

However, due to time constraint, we may not be able to include all that are relevant to our claim. We would like to welcome your suggestion so we can add more in the revision to make this publication more meaningful.

Reviewer: Okay

Author Response

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Round 3

Reviewer 3 Report

Accept.