Optical Properties, Electronic Structures, and Photocatalytic Performances of Bandgap-Tailored SrBi2Nb2−xVxO9 Compounds
Jesus Diaz-Real
Round 1
Reviewer 1 Report
The submitted article is well written, presents a new material and is a fitting contribution to research on photocatalytic materials as well as science in general. It does have some (minor) shortcomings, though, and needs some clarification in certain parts. Here are my comments and suggestions:
Line 46: SBN shows higher activity than what?
Line 68 and others: specify the city and country of origin of machines used.
Line 102 and others: when writing down equation variables, a geometric sign or sign for a certain value, use italic so as to distinguish it from the rest of the text.
Figure 1.c) the unit is Angstrom not anstrom.
Line 116: Please include a table showing the specific surface area of all the samples.
Figure 2: Please describe the method of determining particle size if it was done with a program. How did you account for depth (a particle in the backgroound will appear smaller than one in front)? If you used multiple micrographs for particle size determination, include them in Supplementary info. Are there any differences in elemental composition between large and small particles in V containing samples (do a comparative EDS analysis)?
Line 131 and Figure 3: The graphs and the calculations look like you used the Kubelka-Munk transformation. Please write down the equation used clearly. Please include a table of band-gap values, at least in Supplementary information. I suggest you include the experimental and calculated band gap values.
Line 161: Equations should be appropriately numbered and written in italic. When variables are used in text, they should also be written in italic.
Line 172: What is the value of conduction band edge? If I understand correctly, the SBNV06 is visible light active, but cannot split water. Is that correct? Please clarify.
Line 182: What is the typical error in the photocatalytic measurement method? Could the difference in degradation between samples SBNV06 and SBNV08 be within the error margin?
Line 195-197: Refer to the table with a list of specific surface area of the samples (see comment 5) and draw conclusion based on both band gap values and specific surface area.
Line 201: BET is a measuring technique, not something you can deduce from particle size. You can say that smaller particles have a larger specific surface area (usually, if there is no porosity etc. included), but not BET specific surface area.
Line 209-211: Why did you compare your samples with WO3 and not TiO2? You discuss TiO2 in the introduction and there are a few industrial benchmarks available (Evonik P25, Hombikat, Kronos). Please cite a reference for WO3 used. Considering you used specific surface area and band gap values to explain the higher photocatalytic activity of SBNV06, you should also write down those values for WO3. It also looks like SBNV06 and WO3 have different plots (linear for SBNV06 and what seems like e^(-kt)). Is it possible the mechanism or at least rate limiting factor of photocatalytic activity is different? Please check and correct if necessary.
As stated above, I believe the article is sound and should be published if the listed shortcomings are corrected.
Comments for author File: 
Comments.pdf
Author Response
Author’s response to Reviewer #1
Comment 1. The submitted article is well written, presents a new material and is a fitting contribution to research on photocatalytic materials as well as science in general. It does have some (minor) shortcomings, though, and needs some clarification in certain parts. Here are my comments and suggestions: Line 46: SBN shows higher activity than what?
Response: We do appreciate the reviewer’s correction.
It was higher than the other two compounds, i.e., CaBi2Nb2O9 and BaBi2Nb2O9.
Revision: We have revised the sentence as follows.
“SBN exhibited much highest activity than the other two compounds (i.e., CaBi2Nb2O9 and BaBi2Nb2O9) because…”
Comment 2. Line 68 and others: specify the city and country of origin of machines used.
Response: We do appreciate the reviewer’s correction.
Revision: We have added the city and country of origin for all the equipment as follows.
(line 63) “The crystal structures and morphologies of the powders were determined by using an X-ray powder diffractometer (XRD) (D8 Advance, Bruker AXS, Karlsruhe, Germany) and a scanning electron microscope (SEM) (JSM-6330F, Jeol, Tokyo, Japan), respectively. The diffuse reflectance spectra were collected with a UV–VIS–near infrared (NIR) spectrophotometer (U-4001, Hitachi, Tokyo, Japan). The specific surface area measurements were conducted using a Brunauer–Emmett–Teller (BET) surface area analyzer (ASAP 2010, Micromeitics, Norcross, USA).”
Comment 3. Line 102 and others: when writing down equation variables, a geometric sign or sign for a certain value, use italic so as to distinguish it from the rest of the text.
Response: We do appreciate the reviewer’s correction.
Revision: As the reviewer pointed out, we have corrected all the errors.
Comment 4. Figure 1.c) the unit is Angstrom not anstrom.
Response: We do appreciate the reviewer’s kind correction.
Revision: As the reviewer pointed out, we have corrected all the errors in the Figure 1c as follows.
Figure 1. (a) Crystal structure of a layered perovskite compound of SrBi2Nb2O9. (b) X-ray diffraction patterns and (c) lattice parameter variation of SrBi2Nb2–xVxO9 compounds (x = 0, 0.02, 0.04, 0.06, and 0.08, denoted as SBNV0, SBNV02, SBNV04, SBNV06 and SBNV08, respectively).
Comment 4. Line 116: Please include a table showing the specific surface area of all the samples.
Response: We do appreciate the reviewer’s suggestion.
Revision: We have added a table showing the specific surface area of all the samples in the revised supplementary material.
Comment 5. Figure 2: Please describe the method of determining particle size if it was done with a program. How did you account for depth (a particle in the background will appear smaller than one in front)? If you used multiple micrographs for particle size determination, include them in Supplementary info. Are there any differences in elemental composition between large and small particles in V containing samples (do a comparative EDS analysis)?
Response & Revision: We do appreciate the reviewer’s comment.
We used a software ‘imageJ’ to obtain the distribution of the particle size briefly. As the reviewer pointed out, the depth is a critical issue that should be accounted for. In our data, however, we didn’t accounted for the depth issue and used one SEM image to obtain the distribution. Therefore, the obtained particle size-distribution data is very brief.
As the reviewer suggested, we also have check the elemental composition differences in the SBNV06 sample. Based on the EDS data, there was little differences in their elemental composition between large and small particles.
Comment 6. Line 131 and Figure 3: The graphs and the calculations look like you used the Kubelka-Munk transformation. Please write down the equation used clearly. Please include a table of band-gap values, at least in Supplementary information. I suggest you include the experimental and calculated band gap values.
Response: We do appreciate the reviewer’s correction.
Revision: We have added the Kubelka-Munk transformation in the revised manuscript clearly. And the experimental and calculated bandgap values are also added in the revised Supplementary information as follows.
(Line 131) “…(Tauc plots, here α is absorption coefficient obtained from Kubelka-Munk function (α/S)=(1-R)2/(2R), S is scattering coefficient, R is diffuse reflectance, h is Plank constant, and ν is a frequency of light).”
Comment 7. Line 161: Equations should be appropriately numbered and written in italic. When variables are used in text, they should also be written in italic.
Response: We do appreciate the reviewer’s kind correction.
Revision: We have corrected all those typos in the revised manuscript.
Comment 8. Line 172: What is the value of the conduction band edge? If I understand correctly, the SBNV06 is visible light active, but cannot split water. Is that correct? Please clarify.
Response: We do appreciate the reviewer’s insightful correction.
There were some mistakes for the calculation of conduction and valence band edge positions. We carefully re-calculated the values for the SNBV06 sample. According to the results, the conduction band edge of the SBNV06 is located at -0.04 V vs. RHE (i.e., higher than the water reduction potential) and the valence band edge is located at +2.71 V vs. RHE. Therefore, the SBNV06 is visible light active (because of its small band gap value of 2.72 eV) and it can split water but its reduction power is small because the energy difference between the conduction band edge and water-reduction potential (0 V vs. RHE) is very small.
Revision: We have corrected and re-draw the band structure and position (Figure 5) and added clearly explanations as follows.
(Line 169) “…the conduction-band edge was located at a lower energy level compared to SBNV0 (i.e., -0.02 V versus RHE), while the valence-band edge was located at +2.73 V versus RHE. This result indicates that the SBNV06 is visible light active and it can split water thermodynamically.”
Figure 5. Schematic band structure and positions (on the electrochemical scale) of SrBi2Nb2O9 and SrBi2Nb2–xVxO9 (x = 0.06).
Comment 9. Line 182: What is the typical error in the photocatalytic measurement method? Could the difference in degradation between samples SBNV06 and SBNV08 be within the error margin?
Response: We do appreciate the reviewer’s insightful comment.
As far as we know, there was no report about the typical error ranges of the photocatalytic measurement. So we repeated the photocatalytic measurements three times for the SBNV06 and SBNV08 samples. As shown below, we can find an error range of ~±1.5% for both SBNV06 and SBNV08. Therefore the difference in degradation % (i.e., ~3%) between SBNV06 and SBNV08 is not within the error margin. Nevertheless, it should be noted that the difference in degradation% is very small, so it can be reversed if the particle size (or surface area) of the SBNV08 decreases.
Revision: We have added error bars in Figure 6c in the revised manuscript as follows.
Figure 6. Visible-light-induced photocatalytic activity of SrBi2Nb2–xVxO9 powders with x = 0, 0.02, 0.04, 0.06, and 0.08 (SBNV0, SBNV02, SBNV04, SBNV06, and SBNV08, respectively), under irradiation with a halogen lamp having a 420 nm cut-off filter: (a) Concentration change profiles with the irradiation time. (b) Variation of the absorbance of the RhB dye solution with time for the SBNV06 (See Figure S2 for the others). (c) Comparison of the RhB dye decomposition efficiency after 5 h. The error bar was obtained from three times measurements of each samples.
Comment 10. Line 195-197: Refer to the table with a list of specific surface area of the samples (see comment 5) and draw conclusion based on both band gap values and specific surface area.
Line 201: BET is a measuring technique, not something you can deduce from particle size. You can say that smaller particles have a larger specific surface area (usually, if there is no porosity etc. included), but not BET specific surface area.
Response: We do appreciate the reviewer’s correction.
Response: As the reviewer pointed out, we have added the Table S1 and removed the ‘BET’ in the sentence.
“…therefore, the specific surface area of SBNV06 was larger than that of SBNV0 (Table S1), corresponding to a higher charge transfer efficiency….”
Comment 11. Line 209-211: Why did you compare your samples with WO3 and not TiO2? You discuss TiO2 in the introduction and there are a few industrial benchmarks available (Evonik P25, Hombikat, Kronos). Please cite a reference for WO3 used. Considering you used specific surface area and band gap values to explain the higher photocatalytic activity of SBNV06, you should also write down those values for WO3.
Response & Revision: We do appreciate the reviewer’s comments.
We used WO3 instead of TiO2 because it has a similar bandgap value with the SBNV06. We have cited a few references for WO3 photocatalysts. Also, the specific surface area and band gap values are added in Figure S4b as the reviewer suggested.
Comment 12. It also looks like SBNV06 and WO3 have different plots (linear for SBNV06 and what seems like e^(-kt)). Is it possible the mechanism or at least rate limiting factor of photocatalytic activity is different? Please check and correct if necessary.
Response: We do appreciate the reviewer’s suggestion.
As the reviewer pointed out, the photocatalytic mechanism of the SBNV06 and WO3 is indeed different. We have checked the rate constant (k, assuming a pseudo-first-order reaction) in both samples. As shown in Figure S4a, the k of the WO3 was 2.58 x 10-3 min-1, which is higher than that of the SBNV06 in the time range of 0-150 min. However, at 200-300 range, the SBNV06 shows higher k value than the WO3. These results indicate that the initial photocatalytic reaction of the SBNV06 is slow, implying the adsorption of RhB dye molecules on the surface of the SBNV06 is a rate-limiting factor (possibly because of surface property and/or smaller surface area than the WO3) at the initial stage of photocatalytic reaction. Actually, we observed that the adsorption of RhB of the SBNV06 (just after adsorption/desorption equilibrium step (1 h in the dark)) was much smaller than that of the WO3.
Revision: We have added the Figure S4 in the revised Supplementary material and also discussed them in the revised manuscript as follows.
“… Also, as shown in Figure S4, the reaction rate constant (k, assuming a pseudo first order reaction kinetics) of the WO3 (2.58 x 10-3 min-1) was higher than that of the SBNV06 at the time range of 0-150 min. However, at 200-300 min, the SBNV06 shows higher k value than the WO3. These results indicate that the initial photocatalytic reaction of the SBNV06 is slow, implying the adsorption of RhB dye molecules on the surface of the SBNV06 is a rate-limiting factor (possibly because of surface property and/or smaller surface area than the WO3) at the initial stage of photocatalytic reaction…”
Author Response File: Author Response.pdf
Reviewer 2 Report
1. In line 110, the particle size distribution is discussed from the Figure 2, where only the samples SBNV0 and SBNV06 are presented. Why only these two and not the rest of the samples? There should be an explanation given in the discussion, as this breaks the thread of discussion. Moreover, only the value for the SBNV06 is mentioned (2.9 m2/g), but the rest is not. Therefore, it is highly suggested that the authors mention the rest for a better comparison. Additionally, the BET isotherms are not presented, but it is highly suggested that the results are supported by the experimental measurements.
2. In line 122, the information for the optical properties is presented and stated as diffuse reflectance (DR) measurements. This is correct for the type of materials (solids), but the plot from Figure 3a presents Absorbance. From DR measurements, an estimation of the Absorbance for the materials can be given by the Kubelka-Munk modulus and it is usually regarded as such in the literature. So, it is interesting that the authors present this information naming it “Absorbance”. The authors should either, correct for the right type of measurement performed, or explain this discrepancy of the terms.
3. In line 144, the bandgap from the DFT calculations is compared to that obtained from optical measurements. An argument is made for the discrepancy from the one experimentally obtained, by saying that it is “slighty underestimated”, yielding a difference of ~0.8 eV (about 25% compared to 3.2 eV). The authors are invited to provide insights to support this deviation and rephrase the sentence (25% is a large deviation). Besides, and similar to the previous figures, some explanation should be given to mention why only the results for the SBNV0 and SBNV06 samples are discussed/presented.
4. In line 154, a calculation to estimate the position of valence band is presented. The references provided for the used equation are outdated and this analysis represents a rather too simplified way to estimate these values, particularly for perovskite materials which have such complicated chemistries. In this regard, the authors are invited to discuss the potential differences that arise from using such approach (limitations and cases where it fails), and also propose alternatives to experimentally obtain these values (e.g using photoelectrochemistry). Nevertheless, the discussion in line 167 deals only with SBNV06 vs SBNV0 and the change of the valence band is regarded as “little change”. Proper language and quantitative accuracy should be used to describe these changes, especially when data is available.
5. In line 191, some aspects that control the efficiency of the charge transfer are mentioned. The following phrase mentions that the charge transfer is affected by the surface area, where these two properties are not related. However, it is logic that with higher surface area there is a higher number of active sites and that his manifested as larger oxidation/degradation rates, but it is not related to the charge transfer kinetics. The authors should change this statement accordingly. A similar sentence is presented in line 198, explaining an increased charge transfer efficiency. The actual measurement is a change of degradation/oxidation degree which is associated to a larger surface area, and therefore the measurements from BET analysis agree. This sentence should also be changed. If done accordingly, the partial conclusion given in line 202 agrees with the previous discussion.
6. In line 205 a comparative analysis for the degradation RhB using SBNV06 and WO3 is presented. How was the WO3 obtained? Was it synthesized in-house or purchased as a chemical reagent? Proper information should be provided to understand the origin of the chemical if it was purchased (vendor/model/purity) or synthesized (described in the methodology), so that readers can compare with their own experiments. Moreover, the only information presented for these comparative analyses is for SBNV06, and again, the authors should present the data obtained from the rest of the samples. Showing partial information from experiments can be interpreted as a bad practice of biased selection of the data. Thus, the authors have to present consistent experimental data to avoid this problem.
Author Response
Author’s response to Reviewer #2
Comment 1. In line 110, the particle size distribution is discussed from the Figure 2, where only the samples SBNV0 and SBNV06 are presented. Why only these two and not the rest of the samples? There should be an explanation given in the discussion, as this breaks the thread of discussion. Moreover, only the value for the SBNV06 is mentioned (2.9 m2/g), but the rest is not. Therefore, it is highly suggested that the authors mention the rest for a better comparison. Additionally, the BET isotherms are not presented, but it is highly suggested that the results are supported by the experimental measurements.
Response: We do agree with the reviewer.
As the reviewer suggested, we have added all the SEM images and BET surface area of the rest of the samples. The SBNV0 and SBNV06 are two distinct samples for comparison and generally, a solid-state reaction method gives large and irregular particles (normally BET surface area is comparable). So we didn’t check the rest of the samples (specific surface area and SEM images) previously.
Revision: We have taken and added the SEM images and specific surface area in the revised supplementary material as follows.
Comment 2. In line 122, the information for the optical properties is presented and stated as diffuse reflectance (DR) measurements. This is correct for the type of materials (solids), but the plot from Figure 3a presents Absorbance. From DR measurements, an estimation of the Absorbance for the materials can be given by the Kubelka-Munk modulus and it is usually regarded as such in the literature. So, it is interesting that the authors present this information naming it “Absorbance”. The authors should either, correct for the right type of measurement performed, or explain this discrepancy of the terms.
Response & Revision: We do appreciate the reviewer’s correction.
There was a mistake during the drawing the DR data via origin software. We had added the “absorbance” accidentally. As the reviewer corrected, we have changed the ‘Absorbance’ to ‘Kubelka-Munk’ in Figure 3.
Comment 3. In line 144, the bandgap from the DFT calculations is compared to that obtained from optical measurements. An argument is made for the discrepancy from the one experimentally obtained, by saying that it is “slightly underestimated”, yielding a difference of ~0.8 eV (about 25% compared to 3.2 eV). The authors are invited to provide insights to support this deviation and rephrase the sentence (25% is a large deviation). Besides, and similar to the previous figures, some explanation should be given to mention why only the results for the SBNV0 and SBNV06 samples are discussed/presented.
Response: We do agree with the reviewer.
Revision: As the reviewer suggested, we have rephrased the sentence to ‘underestimated (~25%)’ and added references to explain the deviation and cited a reference paper dealing with the underestimation issue from DFT calculations.
(line 151) “..the calculated bandgap value was 2.4 eV (~25% underestimated compared with the experimentally obtained value…[31]”
[31] Perdew, J.P. Density functional theory and the band gap problem. International Journal of Quantum Chemistry 1985, 28, 497-523.
Comment 4. In line 154, a calculation to estimate the position of the valence band is presented. The references provided for the used equation are outdated and this analysis represents a rather too simplified way to estimate these values, particularly for perovskite materials which have such complicated chemistries. In this regard, the authors are invited to discuss the potential differences that arise from using such approach (limitations and cases where it fails), and also propose alternatives to experimentally obtain these values (e.g using photoelectrochemistry). Nevertheless, the discussion in line 167 deals only with SBNV06 vs SBNV0 and the change of the valence band is regarded as “little change”. Proper language and quantitative accuracy should be used to describe these changes, especially when data is available.
Response: We do appreciate the reviewer’s valuable suggestions.
We definitely agree with the reviewer. As the reviewer pointed out, the equation is a little outdated and may contain large deviations from the experimentally obtained band position values. However, it is very simple and quite useful to roughly estimate the relative band position of different materials. Since our case deals with bulk powders synthesized via a solid-state reaction method, it is very difficult to prepare a film electrode from the powder with irregular, aggregated and large-sized particles. Even if we prepare the film electrode, the morphology, surface, and interface will be different from the powder because of high-temperature annealing that applied for adhesion and necking formation. In that case, the Mott-Schottky measurement (to determine the flat band potential, i.e., band positions) will also contain some errors as well.
Revision: As the reviewer suggested, we have mentioned that the equation may have large deviations from the experimentally obtained values. For the second comments, reviewer #1 also gave similar questions.
(line 167) “. It should be noted that the conduction band position obtained from this equation may have a large deviation with the values obtained from other experimental techniques. Therefore, ultraviolet photoelectron spectroscopy (UPS)[34] or Mott-Schottky measurements should be combined for more exact values [35].”
We have corrected and re-draw the band structure and position (Figure 5) and added clearly explanations as follows.
(Line 169) “…the conduction-band edge was located at a lower energy level compared to SBNV0 (i.e., -0.02 V versus RHE), while the valence-band edge was located at +2.73 V versus RHE. This result indicates that the SBNV06 is visible light active and it can split water thermodynamically.”
Comment 5. In line 191, some aspects that control the efficiency of the charge transfer are mentioned. The following phrase mentions that the charge transfer is affected by the surface area, where these two properties are not related. However, it is logic that with the higher surface area there is a higher number of active sites and that his manifested as larger oxidation/degradation rates, but it is not related to the charge transfer kinetics. The authors should change this statement accordingly. A similar sentence is presented in line 198, explaining an increased charge transfer efficiency. The actual measurement is a change of degradation/oxidation degree which is associated with a larger surface area, and therefore the measurements from BET analysis agree. This sentence should also be changed. If done accordingly, the partial conclusion given in line 202 agrees with the previous discussion.
Response: We do agree with the reviewer.
Revision: We have changed the statements as the reviewer suggested: ‘charge transfer efficiency à a number of active sites’.
(line 191) “…photocatalytic activity of photocatalysts are light absorption (charge generation), charge transport, and a number of active sites [33,34]. As regards the light absorption efficiency, smaller bandgap values, i.e., efficient light absorption, facilitate the generation of charge carriers in the photocatalyst. On the other hand, the charge transport efficiency is greatly affected by electron/hole mobilities, defects and particles size [35]. In the case of the number of active sites, a large surface area is preferred because it provides a greater number of surface active sites for the adsorption of organic molecules…”
(line 198) “…therefore, the specific surface area of SBNV06 was larger than that of SBNV0 (Table S1), corresponding to a large number of surface active sites.”
Comment 6. In line 205 a comparative analysis for the degradation RhB using SBNV06 and WO3 is presented. How was the WO3 obtained? Was it synthesized in-house or purchased as a chemical reagent? Proper information should be provided to understand the origin of the chemical if it was purchased (vendor/model/purity) or synthesized (described in the methodology) so that readers can compare with their own experiments. Moreover, the only information presented for these comparative analyses is for SBNV06, and again, the authors should present the data obtained from the rest of the samples. Showing partial information from experiments can be interpreted as a bad practice of biased selection of the data. Thus, the authors have to present consistent experimental data to avoid this problem.
Response & Revision: We do agree with the reviewer.
The WO3 presented was synthesized (in-house) via a precipitation method. As the reviewer suggested, we have added all the detailed information including the origin of chemicals and preparation method in the revised manuscript. All of the photocatalytic tests are done with identical conditions including the WO3. That’s the reason why we compare only the SBNV06 (best-performing sample) and WO3 in Figure 7. The photocatalytic activities of the others (SBNV0 SBNV02, SBNV04, and SBNV08) are already shown in Figure 6 and Figure S2.
(line 59-67) 2.1. Preparation of SrBi2Nb2–xVxO9 and WO3 powders
The SrBi2Nb2-xVxO9 powders were synthesized via a solid-state method [24]. Stoichiometric mixtures of the starting materials (SrCO3, Bi2O3, Nb2O5, and V2O5, all 99.9%) were ball-milled with ZrO2 balls and ethanol for 48 h. Then, the resulting mixtures were dried and successively calcined at 850 oC for 6 h (the samples calcined below 850 oC exhibited impurity phases, see Figure S1). The WO3 powder was synthesized by a precipitation method. Briefly, 0.02 mole of sodium tungstate dihydrate (Na2WO4-2H2O, >99%, Sigma-Aldrich) were dissolved in deionized water (150 ml) and pH value was adjusted to 10 by using 0.1 M NaOH (98%, Alfa Aesar) aqueous solution. The resultant mixture was heated to 50 oC for 6 h. After washing and drying, the yellowish powder was annealing at 350 oC for 2 h.”
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors have studied the effect of V incorporation into SrBi2Nb2–xVxO9 layered perovskite compounds on the structural and optical features and its implication on the photocatalytic activity. In general, the text is well written, with a coherent sequence and well-designed experiments, and the conclusions are well supported by the results.
I will
emphasize the combination of theoretical calculations and experimental
characterizations to provide a datailed information about the band
structure of the developed compounds. In fact, provided the band
positions, it will be interesting to include some perspectives of
employment of these compounds for aplications in degradations of some
specific organic pollutants.
In my opinion, only minor issues should be addresses:
1. From the DFT calculation, the estimated band gap values of SBNV0 and SBNV06 are different from that calculated through optical measurements. Might the authors provide a brief description about such difference?
2. The authors describe the effect of V 3d
orbitals by lowering the conduction band position in SBNV compounds and
hence, the band gap is descreased with V incorporation. But, might they
explain a bit more about the "little change in the valence-band edge",
referred in line 169? At least from Figure 5 is not that clear such
change.
Author Response
Author’s response to Reviewer #3
Comment 1. The authors have studied the effect of V incorporation into SrBi2Nb2–xVxO9 layered perovskite compounds on the structural and optical features and its implication on the photocatalytic activity. In general, the text is well written, with a coherent sequence and well-designed experiments, and the conclusions are well supported by the results. I will emphasize the combination of theoretical calculations and experimental characterizations to provide detailed information about the band structure of the developed compounds. In fact, provided the band positions, it will be interesting to include some perspectives of employment of these compounds for applications in degradations of some specific organic pollutants. In my opinion, only minor issues should be addresses:
Response: We do appreciate the reviewer’s positive comments.
Revision: We have already added some perspectives of the compounds in the manuscript.
(line 226) “Therefore, when considering the small bandgap value of 2.75 eV and favorable band positions, the photocatalytic activity of SBNV06 could be further enhanced by controlling its particle size and morphology, via a solution synthesis method [38].”
Comment 2. From the DFT calculation, the estimated band gap values of SBNV0 and SBNV06 are different from those calculated through optical measurements. Might the authors provide a brief description of such difference?
Response & Revision: We do appreciate the reviewer’s comments.
As the reviewer #1 also raised the same question (Reviewer #1_comment 3), we have added a sentence and cited a reference to clarify the large deviation of band gap values between calculated by DFT and experimentally obtained values.
(line 151) “..the calculated bandgap value was 2.4 eV (~25% underestimated compared with the experimentally obtained value…[31]”
[31] Perdew, J.P. Density functional theory and the band gap problem. International Journal of Quantum Chemistry 1985, 28, 497-523.
Comment 3. The authors describe the effect of V 3d orbitals by lowering the conduction band position in SBNV compounds and hence, the band gap is decreased with V incorporation. But, might they explain a bit more about the "little change in the valence-band edge", referred in line 169? At least from Figure 5 is not that clear such change.
Response & Revision: We do appreciate the reviewer’s suggestion.
As reviewer #1 also asked the same question (comment 8), we have corrected a few errors from previous calculations and redraw the band structure/positions in Figure 5.
(Line 169) “…the conduction-band edge was located at a lower energy level compared to SBNV0 (i.e., -0.02 V versus RHE), while the valence-band edge was located at +2.73 V versus RHE. This result indicates that the SBNV06 is visible light active and it can split water thermodynamically.”
Author Response File: Author Response.pdf
