Low-Temperature Preparation of SiO₂/Nb₂O₅/TiO₂–SiO₂ Broadband Antireflective Coating for the Visible via Acid-Catalyzed Sol–Gel Method

Round 1

Reviewer 1 Report

This manuscript by Xu et al. reports on the preparation of oxide thin films by the low-temperature sol-gel method as a broadband anti-reflective coating material. By tuning the synthesis conditions, the Authors have succeeded in addressing the porosity problems that limit the AR performance. Authors have nicely correlated theoretical modeling data with the experimentally determined optical data to obtain a comprehensive understanding of the AR properties of these oxide thin films. I think overall it is a nice work and can be accepted after the following points are addressed. I suggest a major revision. Please consider improving the clarity.   1. I feel that the Authors should identify the role of Nb2O5 in the triple layers and compare it with SiO2-TiO2 double layer. Such critical data is missing. I would like to see the transmission data of double-layer compared with a triple layer with Nb2O5. For example, similar to Fig. 6 data for triple layer, data for the double layer without Nb2O5 should be presented. TiO2-SiO2 hybrid systems are already reported earlier for AR application. The essence of this paper is to introduce Nb2O5, and its importance should be clearly discussed.   2. For AR application, the thin film needs to be transparent in the IR region, as stated by the Authors. Typically, the presence of free carriers (electrons) is related to absorption in the IR region. However, in the whole manuscript, there is no specific reference to this particular point, which plays a critical role in determining the broadband AR performance. Further, how free carrier concentration changes in triple layer, upon adding Nb2O5.   3. Followed by comment #2, the Authors employ Shimadzu, UV3600 plus for measuring transmission data, which has wavelength range till 3300 nm. However, essential data that conveys the central message of this paper (Figs. 6 and 8) is only shown until 1000 nm. Have authors measured beyond this wavelength? If such data is measured (for example, till 2500 nm), it would be better to show that further corroborates the broadband AR property of the developed thin films.    4. In the FT-IR data in Fig. 4, it is essential to show the data of pristine SiO2 and TiO2 where there is no hybridization, followed by comparing with SiO2-TiO2 hybrid. The formation of Si-O-Ti bonds , as supported by the feature at 939 cm-1 should be compared with its pristine counterparts.   5. The Authors propose that the triple layer oxide thin film has potential value in practical application. Have Authors compared such experimental data (transmission) with currently existing AR coating material, for example, TEC8, TEC15, and Asahi U. Showing this data will improve the quality and further enhance the importance of this work.   6. In the triple layer data shown in Fig. 6, is the bottom layer deposited directly using the TiO2-SiO2 hybrid or separately? Did not get the info?   7. I don’t understand the meaning of “T30S70 T33S67 T36S64” in Table 1 and Figure 3. Is it some kind of technical name chosen by the Author to depict the concentration/molar ratio of sol? if yes, which concentration is changing? It would help in comparison if Authors show the variable in these naming system. Perhaps, some accessible name with clearly defined sample parameters will help.   8. I do understand the importance of SAXS in determining the interaction of SiO2 and TiO2. However, please explain how such correlation/interaction will affect the AR and/or transmission properties. For instance, relate Table 1 data with Fig. 5 more clearly.

Author Response

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Reviewer 2 Report

The presented article demonstrates a new variant of triple-layered antireflective coatings, describes the process of the synthesis, and gives some theory and potentially may be interesting for readers, but several points must be addressed.

1. The article needs extensive grammar and style editing. 
2. The introduction section should be corrected. It is unclear why niobium oxide is better than titania. There are a lot of examples in the literature with highly refractive titania coatings which do not face named problems (lowering of RI due to organic dopants or porosity), as an example 10.1021/acsnano.5b06074. 
3. Figure 8a is unclear

Author Response

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Reviewer 3 Report

This is a good and well organized paper, the experimental design is clearly described and well-argued also on the basis of the literature. In my opinion, both research methods and results are of interest for the readers in the field.

Author Response

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Reviewer 4 Report

The article well describes the potential of the sol-gel process for the practical application of the manufacture of anti-reflective coatings. The theoretical background is given in the introduction and in connection with the discussion of the results.

The experiments are well described, the results are carefully evaluated and clearly presented in the discussion.

Only for the preparation of the TiO₂-SiO₂ mixed sol some important details are missing that should be added.

Lines 120/121: The following data should be added here: (a) volume of the mixed solution, (b) volume of anhydrous ethanol containing deionized water and hydrochloric acid, (c) molar ratio of the three components (ethanol, water, and hydrochloric acid) in the solution mentioned under (b).

Line 122: The molar ratio of anhydrous ethanol and acetic acid of the solvent for TiCl₄ is also missing and should be added.

Author Response

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Round 2

Reviewer 1 Report

The authors have addressed most comments and added additional control data in FTIR measurements and Figure without Nb2O5. Please address the below points before it can be accepted.   1. Authors have only changed the word from "broadband" to "visible" only in the title. However, in all parts of the manuscript (texts and figures), broadband AR coating is explained, which is not there. Please change throughout the manuscript to visible light transmission, not just in the title. Of course, you can foresee this material for broadband AR coating. However, as of now, the data is just not there to support the broadband AR coating claim.   2. Please rewrite the title in simpler way. Hard to understand. Authors can read it for themselves. Writing both broadband and visible together is confusing. There is no need to write complete details of the synthesis procedure like “acid-catalyzed” in the title.   3. In line #330, based on reference #53, the Authors state that their material has higher transmission than commercially available materials like TEC 8, TEC 15, and Asahi U. However, the #53 reference has data corresponding to TEC 15 only, not TEC 8 or Asahi samples.