Enhanced Plasmonic Photocatalysis of Au-Decorated ZnO Nanocomposites

Round 1

Reviewer 1 Report

Paper with title Enhanced plasmonic photocatalysis of Au decorated ZnO 2 nanocomposites, need serious changes 

1. why author choose  Au decorated ZnO 2 nanocomposites, as it is expensive.

2. abstract need to be modified, like problem, aims, methodsm results and applicatiom. all in numerical form.

3. Introduction is very poor, no research gap. why author choose zinc and gold there are many other metal as well. 

4. what is main difference in Figure 2. X-ray diffraction data characteristic to the ZnO nanoparticles and the ZnO-Au nanocom- 201 posites.

5. Figure 3. TEM images representing the ZnO nanoparticles. veyy poor 

6. all figures are low quality 

Author Response

Reviewer 1:

Paper with title Enhanced plasmonic photocatalysis of Au decorated ZnO 2 nanocomposites, need serious changes 

1. Why author choose Au decorated ZnO 2 nanocomposites, as it is expensive.

Response The motivation for choosing Au decorated ZnO nanocomposites is strongly related to their unique properties. The properties resulting from this combination are not found in the individual components and this is a major point that overcomes the problems related to the high cost of gold. Moreover, gold is used in very small quantities and is added to boost the optical properties of ZnO. The localized surface plasmon resonance (LSPR) induced by the Au nanoparticles decorating the ZnO surface is one of the main strategies to modify the spectral response of ZnO photocatalysts toward visible light (solar light). It has to be noted that ZnO nanoparticles have specificity for UV light, which covers only 5% of the solar spectrum. In addition, in such nanostructures, the gold acts as a trap for photo-generated charge carriers, delaying their recombination with increased photocatalytic performance. Therefore, the advantage of using sunlight as a green and inexhaustible energy source for the degradation of harmful pollutants in wastewater and the increased photocatalytic performance of the ZnO-Au nanocomposites can exceed the disadvantage posed by the price of gold.

The following paragraph was introduced in the manuscript in the Introduction part:

The motivation for choosing Au decorated ZnO nanocomposites is strongly related to their unique properties. The localized surface plasmon resonance (LSPR) induced by the Au nanoparticles decorating the ZnO surface is one of the main strategies to modify the spectral response of ZnO photocatalysts toward visible light (solar light). Therefore, the advantage of using sunlight as a green and inexhaustible energy source for the degradation of harmful pollutants in wastewater and the increased photocatalytic performance of the ZnO-Au nanocomposites can exceed the disadvantage posed by the price of gold.

2. abstract need to be modified, like problem, aims, methodsm results and applicatiom. all in numerical form.

Response: The Abstract was modified accordingly in order to better evidence the problem, aim, results and application. Additionally, numerical details were added.

The rapid development of technological processes in various industrial fields has led to surface water pollution with different organic pollutants, such as dyes, pesticides, and antibiotics. In this context, it is necessary to find modern, environmentally friendly solutions to avoid the haz-ardous effects to the aquatic environment. The aim of this paper is to improve the photocatalytic performance of zinc oxide (ZnO) nanoparticles by using the plasmonic resonance induced by covering them with gold (Au) nanoparticles. Therefore, we evaluate the charge carriers’ behavior in terms of optical properties and reactive oxygen species (ROS) generation. The ZnO-Au nanocomposites were synthesized through a simple chemical protocol in multiple steps. ZnO nanoparticles (NPs) of approximately 20 nm in diameter were prepared by chemical precipitation. ZnO-Au nanocomposites were obtained by decorating the ZnO NPs with Au in different molar ratios, through a reduction process. X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) confirmed the simultaneous presence of hexagonal ZnO and cubic Au phases. The optical investigations evidenced the existence of the band-gap absorption peak of ZnO at 372 nm, as well as the surface plasmonic band of Au nanoparticles at 573 nm. The photocatalytic tests indicated increased photocatalytic degradation of Rhodamine B (RhB) and oxytetracycline (OTC) pollutants under visible light irradiation, in the presence of ZnO-Au nanocomposites (60-85%) compared to ZnO NPs (43%). This behavior can be assigned to the plasmonic resonance and the synergetic effects of the individual constituents in the composite nanostructures. The spin-trapping experiments showed the production of reactive oxygen species (ROS) when the nanostructures are in contact with the pollutants. This study begins new strategies to adjust the efficiency of photocatalytic devices by the combination of two types of nanostructures having synergistic functionalities in one single entity. ZnO-Au nanocomposites can be used as stable photocatalysts with excellent reusability, showing possible industrial applications.

3. Introduction is very poor, no research gap. why author choose zinc and gold there are many other metal as well. 

Response  The introduction part was modified . new paragraphs and new references were introduced in the manuscript.

The rapid development of technological processes in various industrial fields has led to surface water pollution with different organic pollutants ranging from dyes, pesticides to antibiotics or other organic compounds [1 2,3,4,5,6,7]. Numerous studies have suggested hazardous effect on human health and environment [6,7]. In this context, it is necessary to find modern, environmentally friendly solutions to avoid the hazardous effects to the aquatic environment [7].

Literature data reported the photocatalytic properties of different metal oxides interfaced with noble metals nanocomposites for the removal of various dyes, such as rhodamine B, methylene blue or rhodamine 6G [1,2,3], as well as other pollutants including 4-nitrophenol [4], nitrogen oxides [5] or antibiotics [6-8]. Other studies related to the photocatalytic removal of dyes from aqueous environment have shown efficiencies of degradation ranging from 45% to 100% depending on the time of visible light irradiation [43-46]. It is worth mentioning here that most papers related the general assets of the photocatalytic process without any details about ROS, which are critical factors in the photocatalytic mechanism.

Within the above context, this study presents a complex investigation of ZnO-Au photo-catalysts for the photo-degradation of RhB and OTC pollutants. The novelty of this paper refers to the fabrication of ZnO-Au nanocomposites using a simple and fast chemical synthesis and to the improved photocatalytic activity obtained for ZnO NPs by modulat-ing their spectral response through Au decoration. Detailed investigations into the pho-tocatalytic properties of the obtained nanocomposites were analyzed regarding the ROS generation, which lead to the degradation of the organic molecules, RhB and OTC. The motivation for choosing Au decorated ZnO nanocomposites is strongly related to their unique properties. The LSPR induced by the Au nanoparticles decorating the ZnO surface is one of the main strategies to modify the spectral response of ZnO photocatalysts toward visible light (solar light). Therefore, the advantage of using sunlight as a green and inexhaustible energy source for the degradation of harmful pollutants in wastewater and the increased photocatalytic performance of the ZnO-Au nanocomposites can exceed the disadvantage posed by the price of gold.

4. what is main difference in Figure 2. X-ray diffraction data characteristic to the ZnO nanoparticles and the ZnO-Au nanocomposites.

Response The main difference in the diffractograms of ZnO nanoparticles and ZnO-Au nanocomposites consists in the presence of specific lines characteristic to gold phase in the ZnO-Au1 and ZnO-Au2 samples additional to the lines specific to the ZnO nanoparticles. As explained in the manuscript text, the diffraction planes (111), (200), (311) of cubic Au are observed, that were not present in the XRD spectrum of ZnO nanoparticles.

5. Figure 3. TEM images representing the ZnO nanoparticles. veyy poor 

Response The TEM images were replaced with higher quality ones (see fig. 3 and 4)

6. all figures are low quality 

Response The figures were re-exported from Origin using 600 dpi resolution, in the format requested by this journal.

Reviewer 2 Report

Manuscript ID: Inorganics-2282961

General comments:

In the manuscript entitled "Enhanced plasmonic photocatalysis of Au decorated ZnO nanocomposites", authors have the claimed that photocatalytic activity of ZnO nanoparticles is enhanced due to the plasmon resonance induced by covering them with Au nanoparticles. The photocatalytic tests indicated that the ZnO-Au nanocomposites exhibit good photocatalytic activity for degrading Rhodamine B and oxytetracycline pollutants. This work seems to be adequate to be publish in Inorganics after minor revisions. The following issues should be addressed:

1. Comment:

Explain the novelty of the current work. Abstract didn’t begin with a brief but precise statement of the problem or issue only it has a description of the research.           

2. Comment:  

 The introduction and results and discussion method may be enriched by citing some recent works.

3. Comment:

  In synthesis description the authors have described “different amounts of HAuCl4 were added”. Write down the exact amounts with proper SI units.

4. Comment:

 The volume of the pollutants i.e RhB/OTC solution (10ml) for degradation experiment taken is very small?

5. Comment:

 Provide references for the support of XRD results.

5. Comment:

There should be a comparison of the current findings with previous reports.

6. Comment:

The resolution of images in Fig.3 and 4 is very low, should be improved.

7. Comment:

The photocatalytic degradation graphs for OTC are absent. These should be included with complete study details as for RhB dye.

8. Comment:

The structural stability test of the ZnO-Au nanocomposites after recycling test should be provided.

9. Comment:

In mechanism line 408 “This behavior was evidence by PL  spectra (Fig. 8),” it is either PL or Fluorescence?

10. Comment:

The stated mechanism is mostly based on theoretical. Some supporting experiment and studies should be added.

Comments for author File: Comments.pdf

Author Response

Reviewer 2:

General comments:

In the manuscript entitled "Enhanced plasmonic photocatalysis of Au decorated ZnO nanocomposites", authors have the claimed that photocatalytic activity of ZnO nanoparticles is enhanced due to the plasmon resonance induced by covering them with Au nanoparticles. The photocatalytic tests indicated that the ZnO-Au nanocomposites exhibit good photocatalytic activity for degrading Rhodamine B and oxytetracycline pollutants. This work seems to be adequate to be publish in Inorganics after minor revisions. The following issues should be addressed:

1. Comment: Explain the novelty of the current work. Abstract didn’t begin with a brief but precise statement of the problem or issue only it has a description of the research.     

Response  New paragraphs clearly stating the novelty of this work were added to the manuscript.

The rapid development of technological processes in various industrial fields has led to surface water pollution with different organic pollutants, such as dyes, pesticides, and antibiotics or other organic compounds [1-8]. Numerous studies suggested hazardous effects on human health and the environment [7, 8]. In this context, it is necessary to find modern, environmentally friendly solutions to avoid the hazardous effects to the aquatic environment [8].

Within the above context, this study presents a complex investigation of ZnO-Au photo-catalysts for the photo-degradation of RhB and OTC pollutants. The novelty of this paper refers to the fabrication of ZnO-Au nanocomposites using a simple and fast chemical synthesis and to the improved photocatalytic activity obtained for ZnO NPs by modulating their spectral response through Au decoration. Detailed investigations into the photocatalytic properties of the obtained nanocomposites were analyzed regarding the ROS generation, which lead to the degradation of the organic molecules, RhB and OTC. The motivation for choosing Au decorated ZnO nanocomposites is strongly related to their unique properties. The LSPR induced by the Au nanoparticles decorating the ZnO surface is one of the main strategies to modify the spectral response of ZnO photocatalysts toward visible light (solar light). Therefore, the advantage of using sunlight as a green and inexhaustible energy source for the degradation of harmful pollutants in wastewater and the increased photocatalytic performance of the ZnO-Au nanocomposites can exceed the disadvantage posed by the price of gold.

Additionally, the abstract was modified to present the issue addressed in this research.

The following text was introduced: The rapid development of technological processes in various industrial fields has led to surface water pollution with different organic pollutants, such as dyes, pesticides, and antibiotics. In this context, it is necessary to find modern, environmentally friendly solutions to avoid the hazardous effects to the aquatic environment. The aim of this paper is to improve the photocatalytic performance of zinc oxide (ZnO) nanoparticles by using the plasmonic resonance induced by covering them with gold (Au) nanoparticles.

2. Comment:  The introduction and results and discussion method may be enriched by citing some recent works.

Response  Additional recently published works were introduced in the manuscript, such as the following ones:

2. Yao, C.; Chen, W.; Li, L.; Jiang, K.; Hu, Z.; J. Lina, Xu, N.;Sun J.; Wu J. ZnO:Au nanocomposites with high photocatalytic activity prepared by liquid-phase pulsed laser ablation, Optics and Laser Technol.2021, 133, 106533
3. Lux, K.C.; Hot, J.; Fau, P.; Bertron, A.; Kahn, M.L.; Ringot, E.; Fajerwerg, K. Nano-gold decorated ZnO: An alternative photocatalyst promising for NOx degradation Chem. Eng. Sci. 2023, 267(5), 118377
4. Das, T. K.; Ghosh, S. K.; Das, N.Ch. Green synthesis of a reduced graphene oxide/silver nanoparticles-based catalyst for degradation of a wide range of organic pollutants,Nano -Structures & Nano-Objects. 2023, 34, 100960. https://doi.org/10.1016 /j.nanoso. 2023. 10096042. Gebreslassie, G.; Gebrezgiabher, M.; Lin, B.; Thomas, M.; Linert, W. Direct Z-Scheme CoFe₂O₄-Loaded g-C₃N₄Photocatalyst with High Degradation Efficiency of Methylene Blue under Visible-Light Irradiation. Inorganics2023, 11, 119.
5. Gebreslassie, G.; Gebrezgiabher, M.; Lin, B.; Thomas, M.; Linert, W. Direct Z-Scheme CoFe2O4-Loaded g-C3N4 Photocatalyst with High Degradation Efficiency of Methylene Blue under Visible-Light Ir-radiation. Inorganics 2023, 11, 119.
6. Chebanenko, M.I.; Tikhanova, S.M.; Nevedomskiy, V.N.; Popkov, V.I. Synthesis and Structure of ZnO-Decorated Graphitic Carbon Nitride (g-C3N4) with Improved Photocatalytic Activity under Visible Light. Inorganics 2022, 10, 249.
7. Gupta, N.K.; Gha, Y.; Kim, S.; Bae, J.; Kim, K.S. Photocatalytic Degradation of Organic Pollutants over MFe2O4 (M = Co, Ni, Cu, Zn) Nanoparticles at Neutral pH. Sci. Rep. 2020, 10, 4942.3.

See references section for a complete list of works that were newly added.

3. Comment: In synthesis description the authors have described “different amounts of HAuCl4 were added”. Write down the exact amounts with proper SI units.

Response: The synthesis description was modified with the following paragraph:

Further, ZnO nanoparticles were sonicated in 10 ml of water for 30 min, after which, 10 ^-2 mol/l of HAuCl₄ solution were added to the mixture as following: 110 ml (ZnOAu1) and 150 ml (ZnOAu2) under continuous stirring for another 2 h.

4. Comment: The volume of the pollutants i.e RhB/OTC solution (10ml) for degradation experiment taken is very small?

 Response:  The volume of degraded solution is one often used at laboratory level as can be seen in previous papers[65, 66].

65. Madanu, T.L.; Mouchet, S. R.; Deparis, O.; Liu, J.; Li, Y. ; Su,B.L. Tuning and transferring slow photons from TiO2 photonic crystals to BiVO4 nanoparticles for unprecedented visible light photocatalysis, J.Colloid and Interf. Sci.2023,634,290- 299, https://doi.org/10.1016/j.jcis.2022.12.033.
66. Guo, X.; Zhu, H.; Li, Q. Visible-light-driven photocatalyticpropertiesofZnO/ZnFe2O4 core/shell nanocablearrays, Appl. Cat.B: Env. 2014,160-161, 408-414. https://doi.org/10.1016/j.apcatb.2014.05.047

5. Comment: Provide references for the support of XRD results.

Response: The following sentence and references were introduced in the manuscript “The XRD data are in good agreement with results obtained by others [38,41]. “

38. Fageria P.; Gangopadhyay S.; Pande S. Synthesis of ZnO/Au and ZnO/Ag nanoparticles and their photocatalytic application using UV and visible light.RSC Adv. 2014, 4, 294262.
39. Raji R.; Gopchandran K.G. Plasmonic photocatalytic activity of ZnO:Au nanostructures: Tailoring the plasmon absorption and interfacial charge transfer mechanism, J. Hazard. Mat. 2019, 368, 345-357 https://doi.org/10.1016/j.jhazmat.2019.01.052

5. Comment: There should be a comparison of the current findings with previous reports.

Response: The new paragraph was introduced in the manuscript, introduction section

Literature data reported the photocatalytic properties of different metal oxides interfaced with noble metals nanocomposites for the removal of various dyes, such as rhodamine B, methylene blue or rhodamine 6G [1,2,3], as well as other pollutants including 4-nitrophenol [4], nitrogen oxides [5] or antibiotics [6-8]. Other studies related to the photocatalytic removal of dyes from aqueous environment have shown efficiencies of degradation ranging from 45% to 100% depending on the time of visible light irradiation [43-46]. It is worth mentioning here that most papers related the general assets of the photocatalytic process without any details about ROS, which are critical factors in the photocatalytic mechanism.

6. Comment: The resolution of images in Fig.3 and 4 is very low, should be improved.

Response  The TEM images were replaced with higher quality ones(see fig.3 an fig.4)

7. Comment: The photocatalytic degradation graphs for OTC are absent. These should be included with complete study details as for RhB dye.

Response  The photocatalytic degradation of OTC was performed. New figures and a new paragraph were introduced in the manuscript.

In the case of OTC the photocatalytic activity was calculated on the absorption band centered at 375 nm and the results show that ZnO-Au1 exhibits the best degradation (63%) followed by ZnO-Au2 (37%) and ZnO (29%). All nanocomposites samples have an increased photocatalytic activity compared with ZnO nanoparticles. 

The following figures were also added.

Figure 11. Photocatalytic degradation of  (a)RhB and (b) OTC in the presence of ZnO nanoparticles, respectively ZnO-Au1 and ZnO-Au2 nanocomposites

Figure 12. Evaluation of the photodegradation kinetic of  (a)RhB and (b) OTC in the presence of ZnO nanoparticles, respectively ZnO-Au1 and ZnO-Au2 nanocomposites

8. Comment: The structural stability test of the ZnO-Au nanocomposites after recycling test should be provided.

Response  The stability was tested by performing an additional XRD on the sample after the recycling test. No structural modifications were observed as it can be observed in the figure presented below (not introduced in the manuscript).

9. Comment: In mechanism line 408 “This behavior was evidence by PL  spectra (Fig. 8),” it is either PL or Fluorescence?

Response  The PL was replaced by fluorescence

10. Comment: The stated mechanism is mostly based on theoretical. Some supporting experiment and studies should be added.

Response  In order to describe the photocatalytic mechanism we relied here on both experimental and theoretical investigations. All the optical properties experiments, the EPR measurements, the photocatalytic tests, and the reactive oxygen species generation obtained based on the Electron Spin Resonance (ESR) investigations coupled with the spin-trapping technique stand as proof for the stated photocatalytic mechanism. We believe that the explained photocatalytic mechanism is well documented at this moment. 

Unfortunately, we do not have access at the moment to ultraviolet photoelectron spectroscopy and/or time-resolved fluorescence investigations, but we do plan to perform such investigations on our samples and other photocatalysts in the future. 

Additionally, we added a scavengers’ test, as it follows:

To evidence the active species involved in the degradation of RhB molecules, scavengers’ tests were performed. Two scavengers were used: vitamin C for O₂^•- and isopropyl alcohol (IPA). The degradation rate after addition of scavengers (5 mM) in the pollutant solution was determined and compared with that obtained without its use. The results are presented in Fig 15. A decrease in photocatalytic activity is observed in the presence of both scavengers, the decrease being more pronounced in the case of IPA addition. The behavior is similar for both pollutants. These results show that both species are involved in pollutant degradation but •OH radicals have a major role in the photodegradation process.

Figure 15. The effect of scavengers on the RhB and OTC photodegradation.

Reviewer 3 Report

Comments to the Authors:

In this manuscript authors studied photocatalytic performance of ZnO-Au nanocomposites regarding their optical properties and the behaviour of charge carriers. This research has value for the researchers in the related areas. However, the paper needs improvement before acceptance for publication. My detailed comments are as follow:

1.      In the introduction authors should include following relevant dye related articles with the discussion about why their degradation  is important :

a.       doi.org/10.1016/j.enmm.2020.100411

b.      doi.org/10.1016/j.nanoso.2023.100960

2.      Authors should marked the AuNPs in the TEM images

3.      The writing of objective section should be improved.

4.      There are few typos and grammatical errors

5.      Authors should provide % of Au obtained from EDS study.

Author Response

Reviewer 3:

Comments to the Authors:

In this manuscript authors studied photocatalytic performance of ZnO-Au nanocomposites regarding their optical properties and the behaviour of charge carriers. This research has value for the researchers in the related areas. However, the paper needs improvement before acceptance for publication. My detailed comments are as follow:

1. In the introduction authors should include following relevant dye related articles with the discussion about why their degradation  is important :
2. doi.org/10.1016/j.enmm.2020.100411
3. doi.org/10.1016/j.nanoso.2023.100960

Response  The above references were added to the reference list.

2. Authors should marked the AuNPs in the TEM images

Response  The TEM images were modified accordingly

3. The writing of objective section should be improved.

Response  The following paragraphs were added to the manuscript:

The rapid development of technological processes in various industrial fields has led to surface water pollution with different organic pollutants, such as dyes, pesticides, and antibiotics or other organic compounds [1-8]. Numerous studies suggested hazardous effects on human health and the environment [7, 8]. In this context, it is necessary to find modern, environmentally friendly solutions to avoid the hazardous effects to the aquatic environment [8].

Within the above context, this study presents a complex investigation of ZnO-Au photo-catalysts for the photo-degradation of RhB and OTC pollutants. The novelty of this paper refers to the fabrication of ZnO-Au nanocomposites using a simple and fast chemical synthesis and to the improved photocatalytic activity obtained for ZnO NPs by modulating their spectral response through Au decoration. Detailed investigations into the photocatalytic properties of the obtained nanocomposites were analyzed regarding the ROS generation, which lead to the degradation of the organic molecules, RhB and OTC. The motivation for choosing Au decorated ZnO nanocomposites is strongly related to their unique properties. The LSPR induced by the Au nanoparticles decorating the ZnO surface is one of the main strategies to modify the spectral response of ZnO photocatalysts toward visible light (solar light). Therefore, the advantage of using sunlight as a green and inexhaustible energy source for the degradation of harmful pollutants in wastewater and the increased photocatalytic performance of the ZnO-Au nanocomposites can exceed the disadvantage posed by the price of gold.

4. There are few typos and grammatical errors

Response  Some parts of the manuscript were rewritten. The grammatical errors were corrected.

5. Authors should provide % of Au obtained from EDS study.

Response  The percentage of each element in the ZnO-Au nanocomposites was calculated from the EDS investigations and a table with the obtained values was added to the manuscript.

Round 2

Reviewer 1 Report

The author has reported work but there are so many similar reportes, how you justified it

1. https://doi.org/10.1016/j.physe.2015.11.035

2. Bio-inspired Spinach-leaf-based Au/ZnO Nanocomposites as Photocatalyst

3. Two Hybrid Au-ZnO Heterostructures with Different Hierarchical Structures: Towards Highly Efficient Photo-catalysts

4. How your work is differet from above reports??

5. what is novelity of your work?

Author Response

Reviewer 2

Comments and Suggestions for Authors

The author has reported work but there are so many similar reports, how you justified it

1. https://doi.org/10.1016/j.physe.2015.11.035
2. Bio-inspired Spinach-leaf-based Au/ZnO Nanocomposites as Photocatalyst
3. Two Hybrid Au-ZnO Heterostructures with Different Hierarchical Structures: Towards Highly Efficient Photo-catalysts
4. How your work is different from above reports??

Response: The difference between our work and those listed above refers to the preparation method of the photocatalysts, their morphology, as well as to the amount of photocatalyst/pollutant used for photo-degradation testing, parameters that influence the photocatalytic efficiency. It is known that a large amount of photocatalyst, as well as a high concentration of pollutant leads to an increased photocatalytic efficiency. Our samples are able to degrade even small amounts of photocatalyst (6 mg) and low pollutant concentrations (1.0×10⁻⁵ mol/L). The new paragraphs and references were introduced in the manuscript:

Thus, Lu et al. [60] obtained Au/ZnO nanorods using the hydrothermal method at 800C and used them for methyl orange dye (5 mg/L) degradation. The morphology related phenomena, like photo-corrosion, were produced along the c-axis of pure ZnO nanorods, while Au decoration inhibited the occurrence of photo-corrosion and improved the photocatalytic activity. In addition, the same synthesis method led to the fabrication of honeycomb and porous cylindrical Au-ZnO heterostructures [39]. The photocatalytic performance toward the degradation of rhodamine B (RhB) was approximately 90% under 200-1100 nm irradiation [61]. The high photocatalytic efficiency, even after 30 minutes evidenced in the above studies, is mainly due to the morphology of the nanoparticles, and then to the composition and amount of photocatalyst, respectively pollutant used in the experiments.

In our experimental conditions, the small amount of photocatalyst (6 mg), as well as the low pollutant (1.0×10⁻⁵ mol/L) concentration could contribute to the long duration of the photocatalytic process.

5. what is novelty of your work?

Response: Some paragraphs were introduced in the manuscript to evidence the novelty of the work, according to the reviewer comments.

It is worth mentioning here that most papers related the general assets of the photocatalytic process using high concentration of pollutant without any details about ROS or core–shell defects, which are critical factors in the photocatalytic mechanism.

Within the above context, this study presents a complex investigation of ZnO-Au photocatalysts for the photo-degradation of RhB and OTC pollutants. The novelty of this paper refers to the fabrication of ZnO-Au nanocomposites using a simple and fast chemical synthesis route and to the improved photocatalytic activity obtained for ZnO NPs by modulating their spectral response through Au decoration. Detailed investigations into the photocatalytic properties of the obtained nanocomposites were analyzed regarding the ROS generation, which lead to the degradation of the organic molecules, RhB and OTC. Furthermore, the presence of core-shell defects revealed by EPR spectroscopy can tailor the e−/h+ recombination as a function of the Au content.

The motivation for choosing Au decorated ZnO nanocomposites is strongly related to their unique properties. The LSPR induced by the Au nanoparticles decorating the ZnO surface is one of the main strategies to modify the spectral response of ZnO photocatalysts toward visible light (solar light). Therefore, the advantage of using sunlight as a green and inexhaustible energy source for the degradation of harmful pollutants in wastewater and the increased photocatalytic performance of the ZnO-Au nanocomposites can exceed the disadvantage posed by the price of gold. Considering the above, the proposed nanocomposites aim to increase the photocatalytic performance using low doses of photocatalysts, low pollutant concentrations and low energy consumption.

Round 3

Reviewer 1 Report

well revised and accepted