The Use of Tunable Optical Absorption Plasmonic Au and Ag Decorated TiO2 Structures as Efficient Visible Light Photocatalysts
, Yan Wang 2, Lingtong Zhang 2, Haitao Fu 1,2,*, Peng He 2, Dezhi Han 4, Tom Lawson 5 and Xizhong An 2,*Round 1
Reviewer 1 Report
The manuscript entitled “Tunable Optical Absorption of Plamonic Au-Ag codecorated TiO2 Nanostructures as Efficient Visible Light Photocatalysts” deals with the synthesis, characterization and increased photo-adsorption of UV and visible light of TiO2 crystals decorated with both Au and Ag noble metals. The photodegradation of methylene blue under visible light was demonstrated.
This study falls in the general field represented by the tuning of the photo-adsorption of lights of semiconducting materials useful as photocatalysts.
I found some important problems in this manuscript that must be addressed prior to publication as detailed in the following:
1) The Reference Paragraph misses many fundamental papers of this very subject and, therefore, bust be improved; see for examples (Catalysis Today, 2019, 321-322, 146-157; ACS Omega, 2018, 3, 11270-11277). I strongly recommend to add these papers in the References and discuss them in the Introduction.
2) The Composition analysis was reported in the text as wt. % of Ag and/or Au. Then the authors show the chemical formula of the materials indicating the same wt.% as stoichiometric composition (AuxAg(1-x)/TiO2，x=0.5) and this is totally wrong. The authors must transform the weights in moles and then show the corresponding stoichiometry in the formulas.
3) From TEM analyses it emerged that the obtained TiO2 crystals had about a 200 nm diameter and this is far away from what nowadays is believed to be nano. Therefore, I suggest to swap to word nano with micro throughout the manuscript.
4) The following sentence (page 3) related to EDS data is totally unclear “It can be found that Ti and O elements show similar distribution, which means that the two elements constitute the main particles”. The chemical formula of anatase is TiO2 and this implies that oxygen must be the double of Ti.
5) Figure 4f shows the XPS Au 4f spin-orbit components having similar intensities and this is impossible since they must show a 8:6 ratio as it emerges from the 2j+1 multiplicity (when J = 5/2 the multiplicity is 6; when J is 7/2 the multiplicity is 8). In addition, once more the authors do not cite at all any papers to justify the measured XPS binding energies. Again I must suggest the authors to read, add in the Reference list and comment their XPS results on the basis of the following appropriate related papers (J. Phys. Chem. C, 2015, 119, 23743−23751; Chem. Commun. 2014, 50, 4635-4638; Appl. Surf. Sci., 1995, 90, 383-387).
6) Also the photocatalytic experiments should be discussed by comparing their data with those already existing in the literature and partially suggested above.
7) Finally, the manuscript is written in a poor English starting from the title and should be revised by a native English speaker.
Author Response
1) The Reference Paragraph misses many fundamental papers of this very subject and, therefore, must be improved; see for examples (Catalysis Today, 2019, 321-322, 146-157; ACS Omega, 2018, 3, 11270-11277). I strongly recommend to add these papers in the References and discuss them in the Introduction.
As suggested, some highly related papers have been cited and discussed in the introduction, including the recommended papers.
2) The Composition analysis was reported in the text as wt. % of Ag and/or Au. Then the authors show the chemical formula of the materials indicating the same wt.% as stoichiometric composition (AuxAg(1-x)/TiO2，x=0.5) and this is totally wrong. The authors must transform the weights in moles and then show the corresponding stoichiometry in the formulas.
The authors thank for reviewer’s valuable comment. The value of stoichiometric composition is definitely different from the value in wt%. In order to tune the molar ratio between Au and Ag in the Au-Ag/TiO2 composites, the total amount of Au-Ag to TiO2 was fixed to the optimized value (2.25wt %) according to our preliminary optical study. AuxAg (1-x)/TiO2, (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) was applied to represent the different molar ratio between Au and Ag. After calculation, AuxAg (1-x)/TiO2 , (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) can separately represent the nanocomposites of TiO2 only doped with 2.25wt% Ag (x=0), TiO2 doped with 0.35wt% Au and 1.9 wt % Ag (x=0.2), TiO2 doped with 0.74wt% Au and 1.51wt% Ag(x=0.4), TiO2 doped with 1.18 wt% Au and 1.07wt% Ag (x=0.6), TiO2 doped with 1.67wt% Au and 0.61wt% Ag (x=0.8), and TiO2 only doped with 2.25wt% Au(x=1).
Above sentence have been added in paragraph 1 page 3 and error correction has been done through the text.
3) From TEM analyses it emerged that the obtained TiO2 crystals had about a 200 nm diameter and this is far away from what nowadays is believed to be nano. Therefore, I suggest to swap to word nano with micro throughout the manuscript.
As suggested, the word nano-related to TiO2 particles has been deleted or replaced by micro throughout the text. But the size of Au or Ag is in the range of nano size.
4) The following sentence (page 3) related to EDS data is totally unclear “It can be found that Ti and O elements show similar distribution, which means that the two elements constitute the main particles”. The chemical formula of anatase is TiO2 and this implies that oxygen must be the double of Ti.
Thanks for these comments, the sentence has been modified as follows:
It was found that Ti and O elements shared and constituted the TiO2 particles, but the intensity of O element looks stronger than that of Ti element since their number is double that of the Ti atoms.
5) Figure 4f shows the XPS Au 4f spin-orbit components having similar intensities and this is impossible since they must show a 8:6 ratio as it emerges from the 2j+1 multiplicity (when J = 5/2 the multiplicity is 6; when J is 7/2 the multiplicity is 8). In addition, once more the authors do not cite at all any papers to justify the measured XPS binding energies. Again I must suggest the authors to read, add in the Reference list and comment their XPS results on the basis of the following appropriate related papers (J. Phys. Chem. C, 2015, 119, 23743−23751; Chem. Commun. 2014, 50, 4635-4638; Appl. Surf. Sci., 1995, 90, 383-387).
The authors thank for reviewer’s helpful comment. The experimental results show the similar intensities between Au 4f5/2 and Au 4f7/2, this is probably due to the interaction and coexistence between Ag and Au. Similar results can be found in other literature. / Journal of Catalysis 233 (2005) 186–197. As suggested, the detailed discussion of XPS results has been conducted and suggested papers have been cited. The following discussion has been added in paragraph 1 page 6.
The binding energy of Au 4f7/2 is 83.7 eV, which had a negative shift of -0.3eV compared with bulk metallic gold（84.0 eV）. Similar results have also been proved by other studies [1,2]. Such a shift can be explained by the effect of TiO2 support. Arrii et al. reported the role of different kinds of support materials on the binding energy shifts of Au 4f7/2 and found that up to -1.1 eV negative shift occurred for TiO2 host particles [3]. The similar intensity between Au 4f7/2 and Au 4f 5/2 probably due to the interaction and coexistence between Ag and Au. Au-Ag bi-metal seems to provide Au or Ag a slightly greater possibility to lose an electron. In the meanwhile, Ag 3d3/2 and Ag 3d5/2 also shift to lower value (373.4 eV and 367.5 eV) compared with monometallic Ag (374.2eV and 368.3eV)[4].
6) Also the photocatalytic experiments should be discussed by comparing their data with those already existing in the literature and partially suggested above.
According to the suggestion, the photocatalytic performance of as-prepared Au-Ag/TiO2 has been compared with those existing in the literature. The following discussion has been added in paragraph 1 page 8.
The superior photocatalytic activity would benefit from the suitable ratio of Au and Ag, which could help to reduce the photogenerated electron-hole recombination via modifying the bandgap and trapping the charge carrier [5]. Similarly, Mohsen et al. synthesized the Ag-Au modified TiO2 nanotube and investigated the photocatalytic degradation performance towards the MB under visible light, the results reveal that Au-Ag/TiO2 nanotubes show remarkable enhancement of photocatalytic activity due to more visible light absorption[6]. Jaspal explored the photocatalytic performance of Ag-doped TiO2 nanoparticle fabricated by the CTAB-assisted facile wet chemical method and concluded that 100 mL 8uM aqueous solution of MB can be fully degraded under artificial sunlight within 60 min by using 5mg Ag-TiO2 Nanoparticles, while 40% degradation was achieved by using the same amount of pure TiO2 nanoparticles [7].Reinis et al. also reported a microwave-assisted strategy to prepare Au, Pt, Pd and Ag-doped TiO2 nanofibers, the high intensity LED lamp was applied to photodegrade the MB, and the noble metal-doped TiO2 showed double photocatalytic efficiency in comparison with pure TiO2 nanofibers. All the above works are in agreement with the trend of our results[8].
7) Finally, the manuscript is written in a poor English starting from the title and should be revised by a native English speaker.
English writing has been revised by a native English speaker.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors describes the preparation and characterization of bimetallic Au-Ag/TiO2 photocatalyst with visible light activity. The paper also contains the discussion about the role of Au-Ag co-decoration for the photocatalytic activity. The paper could be probably publishable, but needs a major revision before any further consideration for publication. The main concerns include:
What was the reason to use methylene blue (MB)? The use of MB (or other dyes) as a model compound for testing photocatalytic activity under visible light is questionable because MB absorbs visible light by itself. Therefore it is strongly recommended to use transparent organic compound (e.g. phenol) in the region of working wavelengths. Please consider the Ohtani's paper: Chem. Phys. Lett. 429 (2006), 606 and Rochkind et al., Molecules 2015, 20, 88, doi:10.3390/molecules20010088. Therefore, it would be better to change the model compund into non-dye organics and then perform a discussion. There are some language problems in the text such as singular or plural e.g. 'optical property', 'synthesis of TiO2 nanoparticle'. Furthermore, the term 'surface area' should be expressed more precisely as 'specific surface area'.Author Response
The authors describes the preparation and characterization of bimetallic Au-Ag/TiO2 photocatalyst with visible light activity. The paper also contains the discussion about the role of Au-Ag co-decoration for the photocatalytic activity. The paper could be probably publishable, but needs a major revision before any further consideration for publication. The main concerns include:
What was the reason to use methylene blue (MB)? The use of MB (or other dyes) as a model compound for testing photocatalytic activity under visible light is questionable because MB absorbs visible light by itself. Therefore it is strongly recommended to use transparent organic compound (e.g. phenol) in the region of working wavelengths.
Please consider the Ohtani's paper: Chem. Phys. Lett. 429 (2006), 606 and Rochkind et al., Molecules 2015, 20, 88, doi:10.3390/molecules20010088. Therefore, it would be better to change the model compund into non-dye organics and then perform a discussion. There are some language problems in the text such as singular or plural e.g. 'optical property', 'synthesis of TiO2 nanoparticle'. Furthermore, the term 'surface area' should be expressed more precisely as 'specific surface area'.
The authors appreciate the reviewer’s valuable comments, after reading the suggested references, we agreed that it needs to be further investigated if methylene blue is a suitable targeting organic model used in the visible light photocatalytic test. MB is usually found in the papermaking industries causing severe contamination on the environment. So far, there were a large number of published papers related to visible light degradation of MB using modified TiO2 based photocatalysts [9-14].
Since MB can also absorb visible light, this may accelerate the reaction process. Even so, because we compared the photocatalytic activities of a series of Au-Ag/TiO2 composites towards the degradation of MB in the same condition, the results should be convincing. As for pure TiO2, even MB can help absorb the visible light, the photodegradation efficiency was still quite low. It is notable that doping Au/Ag on the surface of TiO2 boosted the photocatalytic efficiency. As suggested, we did the photocatalytic test using phenol as targeting model (shown in Fig 1), at exactly the same condition, but photodegradation efficiency is very low, no obvious degradation of phenol was observed after 240 min photocatalytic reaction. With this in mind, the photocatalytic degradation towards different organic compounds will be systematically studied in future.
Figure 1 Time-dependent degradation curves of 30mg/L phenol with 20mg different photocatalysts under visible irradiation, (a) Au doped TiO2; (b) Ag doped TiO2;(c) Au0.6 Ag0.4/TiO2. （The figure and reference was shown in the word file)
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Please send the manuscript to a native English speaker to amend the English style since there continue to be errors also in the title of the paper (e.g.: PLAMONIC as to be PLASMONIC).
Author Response
Point 1: Please send the manuscript to a native English speaker to amend the English style since there continue to be errors also in the title of the paper (e.g.: PLAMONIC as to be PLASMONIC).
Response 1: Please provide your response for Point 1. (in red)
The authors thank reviewer’s comments. The manuscript has been modified again by another native English speaking colleague, changes were kept in tracking format. The typo error in the title has been corrected accordingly.
Reviewer 2 Report
Thank you for the reply and corrections.
The manuscript is publishable in its present form.
