Enhancement Study of the Photoactivity of TiO₂ Photocatalysts during the Increase of the WO₃ Ratio in the Presence of Ag Metal

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work is devoted to the synthesis of photocatalytic compositions combined from TiO₂, WO₃, and Ag nanoparticles for the photocatalytic processes (photoreduction of Cr(VI) and photooxidation of methylene blue) driven under visible light irradiation.

Unfortunately, the text is rather carelessly written and contains many typos, bad sentences and missing designation, etc. I will not point out all the shortcomings and focus on the main one. But authors should very carefully and thoughtfully check their paper.

1 The authors cited the papers on photocatalytic properties of TiO₂, Ag-TiO₂, WO₃-TiO₂. But there are published papers that show the results for triple photocatalytic systems consisted of TiO₂, WO₃, and Ag:

https://doi.org/10.1016/j.chemosphere.2021.131848

https://doi.org/10.1016/j.matlet.2016.05.082

https://doi.org/10.1039/C4RA02161D

https://doi.org/10.1039/C2NR31030A and etc.

These works should be discussed in Introduction.

2 Section of description of XRD. When estimating the average crystallite size (D), dislocation density (δ), and microstrain (ε), the hkl of peaks for which estimation was done should be indicated. What is the reason for placing WO₃ data in Table 1 when comparing the data for TiO₂ reflexes calculations? Why are no data for WO₃-TiO₂ samples given? In Fig. 1, the crystalline phases are not indicated.

3 Bad using the “nanocomposites (NCs) nanoparticles (NPs)” in the phrase on Lines 126-128.

4 Capture of Fig.3 is incomplete.

5 Lines 178-180. The authors wrote that “However, at a higher 4% Ag/7%WO3/TiO2 content, the surface area returned to increase due to the sediment of the small size of WO3 particles on the external surface of TiO2.” But, the data on the specific surface area in the paper are not presented. Only there is a Fig 4b with the pore size distribution, the description to which needs to be improved.

6 Strange caption of Fig.5 showing the UV-vis spectra. How did the graphs on Fig.5b be calculated?

7 Line 190. The term of “small band gap” should be clarified.

8 Why are no data for bang gap for WO₃-TiO₂ samples shown?

9 Part of discussion of catalytic experiments. The obtained rate constants on photoreduction of Cr(VI) and photooxidation of methylene blue should be compared with literature data. This comparison may be presented as a separate table.

10. “Materials and methods” section requires also a serios revision. Authors should check the description of sample preparation. All data on amounts, concentrations, volumes of reagents and etc should be pointed out. The description of catalytic experiments needs to be revised. “The suspension was stirred for 30 minutes in darkness to reach adsorption-desorption equilibrium. Subsequently, the solution was centrifuged, and 2 mL of the supernatant was collected to separate the photocatalyst.” is failed sequence in the description of experiments.

I think the authors will find other shortcomings themselves. Once again, the authors need to rework the paper very carefully for the next review in order to get closer to the average level of works in this area.

Comments on the Quality of English Language

Unfortunately, the text is rather carelessly written and contains many typos, bad sentences and missing designation, etc. 

Author Response

Reviewer #1: This work is devoted to the synthesis of photocatalytic compositions combined from TiO₂, WO₃, and Ag nanoparticles for the photocatalytic processes (photoreduction of Cr(VI) and photooxidation of methylene blue) driven under visible light irradiation.

Unfortunately, the text is rather carelessly written and contains many typos, bad sentences and missing designation, etc. I will not point out all the shortcomings and focus on the main one. But authors should very carefully and thoughtfully check their paper.

We thank the reviewer so much for the invaluable comment. As per the reviewer's comments, we have revised the typos.

1- The authors cited the papers on photocatalytic properties of TiO₂, Ag-TiO₂, WO₃-TiO₂. But there are published papers that show the results for triple photocatalytic systems consisted of TiO₂, WO₃, and Ag:

https://doi.org/10.1016/j.chemosphere.2021.131848

https://doi.org/10.1016/j.matlet.2016.05.082

https://doi.org/10.1039/C4RA02161D

https://doi.org/10.1039/C2NR31030A .

These works should be discussed in Introduction.

- We thank the reviewer for invaluable suggestion. We found these references beneficial and can enrich the introduction and we have added it .

2- Section of description of XRD. When estimating the average crystallite size (D), dislocation density (δ), and macrostrain (ε), the hkl of peaks for which estimation was done should be indicated. What is the reason for placing WO3 data in Table 1 when comparing the data for TiO2 reflexes calculations? Why are no data for WO3-TiO2 samples given? In Fig. 1, the crystalline phases are not indicated.

 - As per the reviewer’s comment, we have added (WO3/TiO2) and (hkl) to the Fig 1.

3- Bad using the “nanocomposites (NCs) nanoparticles (NPs)” in the phrase on Lines 126-128.

- As per the reviewer’s comment, we have modified

4- Capture of Fig.3 is incomplete.

- As per the reviewer’s comment, we have completed it.

5- Lines 178-180. The authors wrote that “However, at a higher 4% Ag/7%WO3/TiO2 content, the surface area returned to increase due to the sediment of the small size of WO3 particles on the external surface of TiO2.” But, the data on the specific surface area in the paper are not presented. Only there is a Fig 4b with the pore size distribution, the description to which needs to be improved.

- As per the reviewer’s comment, we have added the value of SA into table 2.

6- Strange caption of Fig.5 showing the UV-vis spectra. How did the graphs on Fig.5b be calculated?

- As per the reviewer’s comment, we have added the relation that calculate the Bandgap.

7- Line 190. The term of “small band gap” should be clarified.

- As per the reviewer’s comment, there is an error in the wording of this paragraph and we have modified it and highlighted it in yellow color.

8- Why are no data for bang gap for WO3-TiO2 samples shown?

- As per the reviewer’s comment, we have added the BG of WO3-TiO2 in the graph.

9- Part of discussion of catalytic experiments. The obtained rate constants on photoreduction of Cr (VI) and photooxidation of methylene blue should be compared with literature data. This comparison may be presented as a separate table.

- As per the reviewer’s comment, we have added the comparison in the supplementary Data

10- “Materials and methods” section requires also a serios revision. Authors should check the description of sample preparation. All data on amounts, concentrations, volumes of reagents and etc should be pointed out. The description of catalytic experiments needs to be revised. “The suspension was stirred for 30 minutes in darkness to reach adsorption-desorption

- As per the reviewer’s comment, we have modified it and highlighted it in yellow color

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Referee report on manuscript “Enhancement study of the photoactivity activity of TiO2 photo-  catalysts during the increase of WO3 ratio in the presence of Ag metal”

Although this is a rather interesting and important article that can be recommended for publication, however some comments/questions should be taken in to account.

1.     Line 23-26. Since TiO2 has several modifications, it was useful to specify it here/

2.     Line 29. It is important to note here that rare earth metals (Eu, Ce etc) are also important here. See, for example:

Matějová, Lenka, et al. "Preparation, characterization and photocatalytic properties of cerium doped TiO2: On the effect of Ce loading on the photocatalytic reduction of carbon dioxide." Applied Catalysis B: Environmental 152 (2014): 172-183.

 Serga, Vera, et al. "Study of phase composition, photocatalytic activity, and photoluminescence of TiO2 with Eu additive produced by the extraction-pyrolytic method." Journal of materials research and technology 13 (2021): 2350-2360.

3.     Table 1. Here the precision of determination of D is excessive, the measurement error is not indicated.

4.     Fig.5b. Straight lines are drawn arbitrarily and this is not justified in any way.

5.     Fig.6.  It is unclear, how non-elementary is this luminescence spectrum?  It would be useful to supplement this measurement with excitation spectra.

Author Response

Reviewer #2: Referee report on manuscript “Enhancement study of the photoactivity activity of TiO2 photo-  catalysts during the increase of WO3 ratio in the presence of Ag metal”

Although this is a rather interesting and important article that can be recommended for publication, however some comments/questions should be taken in to account.

1- Line 23-26. Since TiO2 has several modifications, it was useful to specify it here.

- As per the reviewer’s comment, we have modified it.

2- Line 29. It is important to note here that rare earth metals (Eu, Ce etc) are also important here. See, for example:

Matějová, Lenka, et al. "Preparation, characterization and photocatalytic properties of cerium doped TiO2: On the effect of Ce loading on the photocatalytic reduction of carbon dioxide." Applied Catalysis B: Environmental 152 (2014): 172-183.

 Serga, Vera, et al. "Study of phase composition, photocatalytic activity, and photoluminescence of TiO2 with Eu additive produced by the extraction-pyrolytic method." Journal of materials research and technology 13 (2021): 2350-2360.

- We thank the reviewer for invaluable suggestion. We found these references beneficial and can enrich the introduction and we have add it.

3- Table 1. Here the precision of determination of D is excessive, the measurement error is not indicated.

-         The XRD output data was taken from the XRD lab as its.

4- Fig.5b. Straight lines are drawn arbitrarily and this is not justified in any way.

 - As per the reviewer’s comment, we have removed it.

5-Fig.6.  It is unclear, how non-elementary is this luminescence spectrum?  It would be useful to supplement this measurement with excitation spectra.

- As per the reviewer’s comment, unfortunately, our device cannot show the excitation spectra.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Unfortunately, not all my comments were taken into account by the authors.

1   1) I asked to add in the Introduction that the triple systems consisted of TiO2, WO3, and Ag are already successfully used in photocatalysis. This has not done.

I    2) I asked to specified the estimating the average crystallite size (D), dislocation density (δ), and macrostrain (ε). Authors should clarify the reflex/reflexes used for such calculations. This has not happened.

I    3) I have not found the formula used for calculation of [F(R)E]2 on Fig 5b.

T   4) The English should be improved through all the text. For example, I point to the phrase marked yellow in the revised text: “The absorption of pure TiO2 at 380nm indicated that the sample has a high bandgap in the redshift absorption for the 4% Ag/x%WO3/TiO2 (with x = 1, 3, 5, and 7 wt.%) nanocomposites revealed that the bandgaps of the composite samples were enhanced.”

T   5) There are typos. For example, see new revised phrase “Additionally, The photocatalytic activity of prepared samples was illustrated via photoreduction of Cr(VI) to Cr(III) using visible light irradiation. (halogen lamp 1000 W, λ > 420 nm) Set at 20 cm distance above the reactor.”

Once again, the authors should find all shortcomings and revise the paper very carefully.

My request to pay more attention in marking the changes.

Comments on the Quality of English Language

The englsh should be improved. The typos should be corrected.

Author Response

Dear Editor,

We thanks you and the reviewers for the careful and critical reading of our paper. Please find below enumerated responses to the reviewers. Comments and a summary of modifications have been enclosed in the revised version of the manuscript following the addressed commentaries.

The modifications in the revised manuscript are indicated by a highlight (yellow color).

Comments and Suggestions for Authors

1. I asked to add in the Introduction that the triple systems consisted of TiO₂, WO₃, and Ag are already successfully used in photocatalysis. This has not done.

Please see page 2 (lines 56-75 ).

Li et al. prepared Ag/WO₃/TiO₂ nanowires via hydrothermal technique [22]. They reported that the as-prepared composite photocatalysts showed greatly improved absorbance even in the infrared regime and dramatically improved photocatalytic activities toward methyl orange degradation than that of pure TiO₂ nanowires. In addition, the as-prepared Ag/WO₃/TiO₂ composite exhibited excellent recyclability for pollutants degradation. It is suggested that, with the synergetic help of WO₃ and Ag nanoparticles, more photogenerated electrons and holes pairs can be produced, participate in the photodegradation reaction and enhance the photocatalytic activity dramatically [22]. Xu et al. used an aqueous sol-gel route for the preparation of nanostrucutred Ag-WO₃/TiO₂ [23]. Ag-WO₃/TiO₂ nanoparticles with a mixe phase (anatase/rutile) showed more excellent photocatalytic activity on the removal of MB than the single-doped TiO₂, owing to their small particle size distribution, larger surface area, and higher absorbance of visible light [23]. Ag@TiO₂/WO₃ synthesized by using the sol-gel method and the photocatalytic activity enhanced compared to its counterparts [24]. The rate of degradation of methylene blue (MB) using Ag@TiO₂/WO₃ was ~20 and ~25 times higher in contrast with pure TiO₂ and Degussa P25, respectively. This increase in the photocatalytic performance because of the enhancement of light-harvesting, a large number of charge carrier injection due to surface plasmon resonance effect exhibited by Ag nanoparticles under the irradiation of visible light, and lower recombination rate due to ease of charge carrier transfer through the junction between the two metal oxides [24].

22. Li, Y.; Wu, W.; Dai, P.; Zhang, L.; Sun, Z.; Li, G.; Wu, M.; Chen, X.; Chen, C. WO₃ and Ag Nanoparticle Co-Sensitized TiO₂ Nanowires: Preparation and the Enhancement of Photocatalytic Activity. RSC Adv. 2014, 4, 23831–23837, doi:10.1039/c4ra02161d.
23. Xu, J.Y.; Wen, C.; Jia, L.M.; Xiao, C.F. Ag/WO₃-Codoped TiO₂ Nanoparticles: Relation between Structure, Sorption, and Photocatalytic Activity. In Proceedings of the Second International Conference on Smart Materials and Nanotechnology in Engineering; SPIE, 2009; Vol. 7493, pp. 1522–1528.
24. Basumatary, B.; Basumatary, R.; Ramchiary, A.; Konwar, D. Evaluation of Ag@TiO₂/WO₃ Heterojunction Photocatalyst for Enhanced Photocatalytic Activity towards Methylene Blue Degradation. Chemosphere 2022, 286, 131848, doi:10.1016/j.chemosphere.2021.131848.

2. I asked to specify the estimating the average crystallite size (D), dislocation density (δ), and macrostrain (ε). Authors should clarify the reflex/reflexes used for such calculations. This has not happened.

We try to discuss it on page 3, however, we did not understand the question of the reviewer about what he means by reflex/reflexes???

It is important to investigate the microstructural parameters of any material synthesized at the nanoscale and to see the impact of doping on such parameters. Hence, herein we have investigated the impact of doping on the crystallinity of TiO_2, we have estimated the crystallite size (D) values of the prepared nanocomposites using the Scherrer equation:

where  denotes the full width at half maximum (FWHM),  is the shape factor with a value of 0.9, and  represents the wavelength of the XRD source. The calculated D values of the prepared pure WO₃, pure TiO₂ NPs, and 4% Ag/x%WO₃/TiO₂ (with x = 1, 3, 5, and 7 wt.% of WO₃) nanocomposites are presented in Table 1. The crystallite size values decrease systematically with the WO₃ dopant ratio increased in TiO₂. Also, the other parameters of the structure, such as the dislocation density (δ) and the microstrain , were calculated from the FWHM of the most intense peak () and its angular position for the prepared samples by [30,31]:

The estimated value of δ and  is also presented in Table 1. The given values of δ after doping WO₃/TiO₂ with 4%Ag samples increase as the D values decrease. The δ values are increasing with increasing the WO₃ content, whereas the ε values are varying between (11.2×10^-3 and 7.55×10^-3). These microstructural parameters indicate the impact of doping.

3. I have not found the formula used for calculation of [F(R)E]² on Fig 5b.

Please see page 4 (lines 172-180 ).

The optical energy gap of prepared samples is calculated during the Kubelka-Munk relation [44].

                                                                                   (4)

 where R is the reflectance. In terms of F(R) the Tauc’s equation adjusts to become [45]:

                                                                                    (5)

Eg refers to the band gap as mentioned above,  represents the energy of light, and              n=1/2 for direct allowed transition and n = 2 for indirect allowed transition.

44. Kubelka, P.; Munk, F.A Contribution to the Optics of Pigments. Z. Technol. Phys. 1931, 12, 593– 599.
45. Tauc, J.; Grigorovici, R.; Vancu, A. Optical Properties and Electronic Structure of Amorphous Germanium. Phys. Status Solidi B1966, 15, 627– 637,  doi: 10.1002/pssb.19660150224.
46. The English should be improved through all the text. For example, I point to the phrase marked yellow in the revised text: “The absorption of pure TiO₂ at 380nm indicated that the sample has a high bandgap in the redshift absorption for the 4% Ag/x%WO3/TiO2 (with x = 1, 3, 5, and 7 wt.%) nanocomposites revealed that the bandgaps of the composite samples were enhanced.”

         We have modified the above-mentioned sentence in revised version, and we have done the language corrections throughout the manuscript.

Please see page 4 (lines 161-163).

The absorption edge of pure TiO₂ was observed at 380 nm indicating that the sample has a high bandgap, however, for 4% Ag/3%WO₃/TiO₂ nanocomposites the bandgap was noticed to be lowest viz. 2.40 eV due to the red shift in absorption edge.

5. There are typos. For example, see new revised phrase “Additionally, The photocatalytic activity of prepared samples was illustrated via photoreduction of Cr(VI) to Cr(III) using visible light irradiation.(halogen lamp 1000 W, λ > 420 nm) Set at 20 cm distance above the reactor.”

          We have done the corrections accordingly

Please see page 6 (lines 272-276).

the photocatalytic activity of prepared samples was illustrated via photoreduction of Cr(VI) to Cr(III) using visible light irradiation (halogen lamp 1000 W, λ > 420 nm) set at 20 cm distance above the reactor. In detail, 50 mg of catalyst into 50 mL Cr(VI) solution (10 mg L⁻¹) was suspended and maintained a constant pH of 3.0 using 100μ of formic acid as a hole scavenger.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

After successful revision this manuscript can be recommended for publication.

Author Response

Thank you very much.