Research on the Construction of Cu₂O Photonic Crystals on Different Textile Substrates and Their Mechanical Properties

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This article mainly concerted on structural color generating materials. This has always been the hot research topic. The authors herein prepared the Cu₂O microspheres onto different fabrics, and the structural color revealed some effect. They also did some research on the mechanical and colorfastness properties, just one question was remained, there need more specific on the size of the microspheres corresponds to color.

 

In this research, the Cu₂O microspheres were prepared on different types of fabric, such as cotton, silk, polyester and nylon fabrics, and they also showed some structural color effect and mechanical property and colorfastness. The description and illustration were clearly in this manuscript , and the analysis were reasonable, and it is worthy of published in the journal.

Author Response

Reviewer 1

Comments 1：There need more specific on the size of the microsphere corresponds to color.

Response 1：We sincerely thank you for your feedback which would help to improve the quality of our manuscript. When we prepare the Cu₂O microsphere, we added that the amount of other reagents remained the same and by varying the ratios of copper acetate and trisodium citrate (0.8, 1.0, 1.1, and 1.2), Cu₂O microspheres with different particle sizes could be obtained. The particles with different particle sizes were prepared in batches and not prepared and sorted together. In the review manuscript, the correspondence between the size and color of the microsphere has been explained more clearly in lines 263-272.

Lines 263-272：The reflection of the constructed structural color fabrics was tested, and the results are shown in Figure 7. It can be seen from Figure 7 that four different sizes of Cu₂O microspheres can be obtained by adjusting the ratio of citrate and Cu²⁺ in the process of synthesizing Cu₂O. The reflectance of the structural color patterns obtained by the self-assembly of microspheres with different particle sizes is high, and the reflection peak is sharp, which means that the structural color effect is good. However, due to the strong absorption of the Cu₂O microsphere in the blue-violet band of visible light, it presents a yellow-orange color, making it difficult to obtain blue structural color. Nevertheless, the orange fabrics can also be obtained in this work by rationally utilizing the color of the Cu₂O microsphere itself (orange-yellow).

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The paper describes a novel method of coloring four different kinds of fabrics by spraying  an alcoholic solution containing CuO2 Nano spheres onto the fabrics. The deposited nanoparticles self-organize forming photonic crystals that interact with the ambient white light to reflect different predetermined colors. The intended color can be selected by adjusting the nanoparticle size during wet chemical particle growth.
The paper is well presented in good English and supported by well-prepaed pictures. 

I therefore recommend the paper for publication in MDPI textiles.

Before publication, a few Minor points could be corrected:

- Fig.6: Reflectivity spectra. In subfigures a,b,c,d the average particle size could be mentioned that lead to the observiert color.

- Fig.1: Low-cost, environmentally friendly wet-chemical processing.  In the Main Text sputtering is mentioned which obviously dies not  play any role in the dying process.

 

 

 

Author Response

Reviewer 2

Comment 1: - Fig.6: Reflectivity spectra. In subfigures a,b,c,d the average particle size could be mentioned that lead to the observiert color.

Response 1：We sincerely thank you for your feedback which would help to improve the quality of our manuscript. We have annotated the figure. In addition，in the review manuscript, the correspondence between the size and color of the microsphere has been explained more clearly in lines 263-272.

Lines 263-272：The reflection of the constructed structural color fabrics was tested, and the results are shown in Figure 7. It can be seen from Figure 7 that four different sizes of Cu₂O microspheres can be obtained by adjusting the ratio of citrate and Cu²⁺ in the process of synthesizing Cu₂O. The reflectance of the structural color patterns obtained by the self-assembly of microspheres with different particle sizes is high, and the reflection peak is sharp, which means that the structural color effect is good. However, due to the strong absorption of the Cu₂O microsphere in the blue-violet band of visible light, it presents a yellow-orange color, making it difficult to obtain blue structural color. Nevertheless, the orange fabrics can also be obtained in this work by rationally utilizing the color of the Cu₂O microsphere itself (orange-yellow).

Comment 2: - Fig.1: Low-cost, environmentally friendly wet-chemical processing. In the Main Text sputtering is mentioned which obviously dies not play any role in the dyeing process.

Response 2: We thank the reviewer for the positive and constructive comments regarding our paper. The sputtering mentioned is to spray the high-concentration particles repeatedly on the fabric surface.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

• Lack of Novelty and Relevance: 

  

The paper discusses the construction of Cu2O photonic crystals on various textile substrates and examines their mechanical properties. Upon review, the paper does not appear to present a significant novelty. The synthesis of the compounds has been documented in multiple previous publications. Additionally, the application of Cu2O on textile substrates has already been reported. 

Several fundamental issues are present that render the manuscript not ready for publication in its current form. 

 

• Experimental Clarity: 

• On page 4, the author described the synthesis of Cu2O. However, the reaction process and the conversion of copper acetate monohydrate to copper oxide without specifying a calcination temperature are not clearly explained. The author indicated that NaOH is used in the synthesis of Cu2O. It is noted that NaOH can bind to the crystallite structure of copper oxide, particularly at a concentration of 0.4 M. 

 

• Figures and Data Presentation: 

• Arial font should be used in all tables and figures. The second line of the table should be drawn with a thinner size. 

• The FESEM images, particularly image 2f, appear to have issues. This may be due to inadequate sample preparation, leading to improper focus as the samples are stacked on each other. 

• In the EDS analysis, there is a noticeable peak around 2 eV. Could you please clarify what this peak represents? 

 

• Key Analyses Missing: 

•   

• It is important to include the XRD analysis to demonstrate the crystallite structure of the compounds at each step, particularly for Cu2O. 

• In section 3.3, the authors mention “Cu2O structural color microspheres with different particle sizes”. However, there are no procedures outlined for synthesizing Cu2O of different sizes. If the sizes are obtained from a single process, it is necessary to show the size distribution using DLS and provide a clearer explanation to avoid confusion. 

• There is no discussion provided for Figure 2, which contains FESEM images of the samples. The results are not explained adequately in this section. 

•  

 

• Scientific Reasoning: 

• Section 3.3 explains structural color microspheres and their size effects. Were the size variations intentional or incidental? 

 

• Practical Application and Testing: 

• Long-term durability testing, including UV resistance, wear resistance, and washing cycles, was not conducted. 

• Adhesion testing related to the Cu2O particle applied using PVA adhesion must be performed (e.g., peel or shear tests). 

• The authors should present the fixation test with the corresponding analysis. However, this test appears to be absent. 

• Fabric hand testing is required.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Comments on the Quality of English Language

Quality of English is generally good.

Author Response

Reviewer 3

Comment 1: Lack of Novelty and Relevance:

The paper discusses the construction of Cu₂O photonic crystals on various textile substrates and examines their mechanical properties. Upon review, the paper does not appear to present a significant novelty. The synthesis of the compounds has been documented in multiple previous publications. Additionally, the application of Cu2O on textile substrates has already been reported.

Response 1: Thank you very much for your professional comments on our article. I would like to clarify a few points. First, other articles have focused on the preparation of Cu₂O particles and the modulation of their color. Second, there is a lack of studies comparing the application and properties of structural colors on different fabrics. The focus of our article is based on the application of structural colors to different fabrics: cotton, silk, polyester and nylon, and also compares their mechanical properties.

Comment 2: Experimental Clarity: 

On page 4, the author described the synthesis of Cu2O. However, the reaction process and the conversion of copper acetate monohydrate to copper oxide without specifying a calcination temperature are not clearly explained. The author indicated that NaOH is used in the synthesis of Cu2O. It is noted that NaOH can bind to the crystallite structure of copper oxide, particularly at a concentration of 0.4 M.

Response 2: Thank you very much for your question. Copper acetate serves as a copper source providing Cu²⁺. After adding a PVP aqueous solution, the solution turns light blue. When dihydrate sodium citrate is added, the solution exhibits a deep blue transparent state. Since the reduction reaction designed in this experiment needs to be carried out under alkaline conditions, strong base NaOH is chosen as the donor of OH-. After adding the mixed solution containing dihydrate sodium citrate, PVP, and Cu²⁺ to a 0.4M NaOH solution, Cu²⁺ first forms a copper citrate complex with the carboxylate ions dissociated from dihydrate sodium citrate, resulting in a homogeneous and stable emulsion. This complex can control the concentration of free Cu²⁺ in the reaction solution, ensuring that the reduction system is a homogeneous reaction. Ascorbic acid is a common green reducing agent that can reduce divalent copper to monovalent copper at room temperature. Therefore, ascorbic acid is selected as the reducing agent, and after its addition, Cu²⁺ in the solution can be reduced to produce Cu₂O. The type and amount of ligands in the reaction system, as well as the mass transfer rate of the solvent, will all affect the final morphology of Cu₂O. Additionally, slowing down the reduction reaction rate is beneficial for obtaining uniformly sized Cu₂O microspheres.

Comment 3: Arial font should be used in all tables and figures. The second line of the table should be drawn with a thinner size.

Response 3: I really appreciate your suggestions. I've updated the formatting of the Figure and Table text as you asked, along with the second row of Table 1.

Comment 4: The FESEM images, particularly image 2f, appear to have issues. This may be due to inadequate sample preparation, leading to improper focus as the samples are stacked on each other.

Response 4: Thank you for pointing this out. In the review manuscript，Figure 2(f) has been modified.

Comment 5: In the EDS analysis, there is a noticeable peak around 2 eV. Could you please clarify what this peak represents?

Response 5: We sincerely thank you for your feedback which would help to improve the quality of our manuscript. In the EDS analysis, there is a noticeable peak around 2 eV. This peak represents the Pt element. The reason why this element appears is that the sample passes through the gold-plated before testing. Therefore, this element was not taken into account when analyzing EDS. See lines 216-219 for more information.

Lines 216-219: Since the color structured fibers were plated with gold before characterization, there is a peak of platinum element near 2 eV. In order to ensure the accuracy of the characterization, the platinum element is not considered in the analysis.

Comment 6: It is important to include the XRD analysis to demonstrate the crystallite structure of the compounds at each step, particularly for Cu₂O.

Response 6: We sincerely appreciate your positive and constructive comments on our papers, which will help to improve the quality of our manuscripts. According to your comments, we have added the content of XRD in the lines 243-250 of the manuscript.

Lines 243-250: The crystal structures of the prepared Cu₂O microspheres were characterized using XRD and the results are shown in Figure. 5(a). As shown in Figure. 5(a), the prepared microspheres showed characteristic diffraction peaks at 2θ = 30.1°, 36.9°, 42.8°, 61.9°, 74.0°, and 78.0°, which corresponded to the crystal planes (110), (111), (200), (220), (311), and (222), respectively. The XRD spectra of the cuprous oxide microspheres are consistent with the standard cubic cuprous oxide spectra in PDF No. 05-0667, and other phases, such as copper oxide, are not present in the XRD spectra. Therefore, the XRD spectra show that the prepared microspheres are Cu₂O microspheres.

Comment 7: In section 3.3, the authors mention “Cu₂O structural color microspheres with different particle sizes”. However, there are no procedures outlined for synthesizing Cu2O of different sizes. If the sizes are obtained from a single process, it is necessary to show the size distribution using DLS and provide a clearer explanation to avoid confusion.

Response 7: We sincerely appreciate your positive and constructive comments on our papers, which will help to improve the quality of our manuscripts. According to your comments, we have added the content of particle size analysis in the lines 251-253 of the manuscript.

Lines 251-253: The diameter distribution of Cu₂O microspheres was assessed with a particle size meter. Figure 5(b) illustrates that the particle size distribution is primarily focused between 190 nm and 275 nm, indicating that the microspheres are well-prepared.

Comment 8: There is no discussion provided for Figure 2, which contains FESEM images of the samples. The results are not explained adequately in this section.

Response 8: Thank you very much for pointing this out.. The detailed analysis of FESEM images appears in article lines 205-210.

Lines 205-210: The morphology of the fabric surface after Cu₂O spraying was observed using scanning electron microscope (SEM) as shown in Figure 2. The SEM images in Fig. 2(a), (c), (e) and (g) are the original images of cotton, silk, polyester and nylon fabrics, respectively. Fig. 2 (b), (d), (f) and (h) are the SEM images of cotton, silk, polyester and nylon fabrics after pretreatment and spraying of Cu₂O dispersion, respectively, and it can be seen that the Cu₂O microspheres are uniformly distributed on the surface of the fabric.

Comment 9: Section 3.3 explains structural color microspheres and their size effects. Were the size variations intentional or incidental?

Response 9: Thank you very much for your question. The Cu₂O microspheres size variations can be prepared by varying the molar ratio of Cit^3- and Cu²⁺. Meanwhile, different sizes of Cu₂O microspheres will correspond to different colors. The specifics of the Cu2O microsphere size change can be seen in the lines 263-272 of manuscript.

Lines 263-272：The reflection of the constructed structural color fabrics was tested, and the results are shown in Figure 7. It can be seen from Figure 7 that four different sizes of Cu₂O microspheres can be obtained by adjusting the ratio of citrate and Cu²⁺ in the process of synthesizing Cu₂O. The reflectance of the structural color patterns obtained by the self-assembly of microspheres with different particle sizes is high, and the reflection peak is sharp, which means that the structural color effect is good. However, due to the strong absorption of the Cu₂O microsphere in the blue-violet band of visible light, it presents a yellow-orange color, making it difficult to obtain blue structural color. Nevertheless, the orange fabrics can also be obtained in this work by rationally utilizing the color of the Cu₂O microsphere itself (orange-yellow).

Comment 10: Long-term durability testing, including UV resistance, wear resistance, and washing cycles, was not conducted.

Response 10: Thank you very much for your suggestions on this part of the content, we have refined the content in lines 337-350 of the manuscript.

Lines 339-352: The friction fastness test was conducted in accordance with the EU standard ISO 105-X12:2001 “Color fastness to rubbing for color fastness tests on textiles”[36], and a rubbing fastness tester was used to test the rubbing fastness of the fabric samples. As shown in Figure 12, after 5 rubbing tests, there is no obvious scratch on the surface of the fabric sample and the color remains unchanged. After identification, the rubbing level of the fabric sample is 5, which indicates that the rubbing fastness of the fabric sample basically meets the standard for daily use. No matter whether after several times high-speed rotary washing or rapid water washing, the fabric samples with structural color on the surface did not fade, indicating that the prepared fabric with structural color on the surface also possesses excellent washing resistance, as shown in Figure 13.

Comment 11: Adhesion testing related to the Cu₂O particle applied using PVA adhesion must be performed (e.g., peel or shear tests).

Response 11: We sincerely thank you for your feedback which would help to improve the quality of our manuscript. We performed adhesion tests related to Cu₂O particles using a PVA binder - shear tests - and added to the contents of the manuscript, as reflected in manuscript lines 325-336.

Lines 325-335: To further validate the color fastness of the synthesized structural color fabrics, we employed cotton fabric for assessments of shear color fastness, friction color fastness, and washing color fastness. Initially, the shear color fastness test was performed in accordance with GB/T 3917.2-2009, which outlines the methodology for determining the tear force of trouser-shaped test specimens using the single tear method. The fabric underwent shearing, and the performance of Cu₂O on the fabric surface adjacent to the shear was examined using scanning electron microscopy (SEM), as illustrated in Figure 11. The SEM images presented in Figure 11 depict the structured color cotton fabrics post-shearing, revealing that the Cu₂O microspheres are uniformly distributed in proximity to the fabric shear. This observation shows that the shearing process has a negligible impact on the color fastness of the fabrics.

Comment 12: The authors should present the fixation test with the corresponding analysis. However, this test appears to be absent.

Response 12: We think this is a good suggestion. However, we are not sure what the criteria are for fixing the test. Can you provide some standards from your side for our reference?

Comment 13: Fabric hand testing is required.

Response 13：We sincerely appreciate your feedback, which will help improve the quality of our manuscripts. However, we are not particularly clear about the testing criteria for fabric hand testing. Could you please point out the specific reference standard? Thank you!

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

The use of pigments and various dyes to impart color to fabric or create a pattern on it involves the use of a large number of solvents, including water. After use, contaminated water enters the environment as wastewater, thereby polluting it. Therefore, the search for environmentally friendly dyes and dyeing methods is an urgent task. Along with chemical compounds used to color substrates, it is possible to rely on physical approaches. For example, when using substances with a high refractive index.

In the presented work, it is proposed to use Cu2O to regulate the color of the substrate.

What is the size of Cu2O microspheres and emulsion droplets during their preparation? How do the authors characterize the emulsion?

Figure 1 is not a diagram.

In the heading 3.1., it is necessary to change the color for Cu2O.

I have a feeling that the micrographs presented in Figure 2 were obtained not on fabrics, but on substrates. Please provide micrographs with a lower magnification, where the structure of the fabric will be visible. Also, in microphotograph f, the distribution of microspheres is not quite uniform and some of them are embedded in the matrix. How did this embedding occur? How will the microspheres function in this case?

I do not quite agree with the statement in lines 220-222, the data obtained only indicate the presence of elements. Also, 33.55% carbon indicates that not the entire surface is covered with microspheres.

The spectra in Figure 4 are of low quality and are not correctly formatted.

From the presented mechanical results, it is not entirely clear to me how the deformation properties increase for some samples? I am also interested in the question of how the microspheres behave under repeated mechanical impact?

For Figure 11, the numbering is broken.

The work contains typos and minor errors, for example, in line 161, a period is missing at the end of the sentence. Therefore, I recommend additional editing work with the text of the manuscript. in some cases questions arise about the terminology used, for example in line 164 "dissolved" (167 - 2 wt% PVA adhesive), how is that? I don't understand what is happening with the system. the authors should carefully check the manuscript and exclude questionable use of terminology.

Author Response

Reviewer 4

Comment 1: What is the size of Cu₂O microspheres and emulsion droplets during their preparation? How do the authors characterize the emulsion?

Response 1: Thank you very much for your question. By varying the ratios of copper acetate and trisodium citrate (0.9, 1.0, 1.1, and 1.2), Cu₂O nano-microspheres with corresponding particle sizes of 190 nm, 210 nm, 245 nm, and 275 nm were finally prepared. The diameters of Cu₂O microspheres prepared in different ratios are different with a few nanometers deviation in the particle size. After the microspheres are prepared and dried, they become Cu₂O powder, and then the powder is proportionally dissolved in anhydrous ethanol to obtain a dispersion. We characterized it by measuring the particle size distribution of Cu₂O ethanol dispersions.

Comment 2: Figure 1 is not a diagram.

Response 2: Thank you very much for your suggestion. I have made changes to the content of Figure 1 in lines 176-177.

Lines 176-177: Figure 1. Preparation process (a) the preparation of Cu₂O microspheres (b) the preparation of structural color fabrics

Comment 3: In the heading 3.1., it is necessary to change the color for Cu₂O.

Response 3: We were sorry for our careless mistakes. Thank you for your reminder. As suggested by the reviewer, we have corrected the “Cu₂O” in “Cu₂O”.

Comment 4: I have a feeling that the micrographs presented in Figure 2 were obtained not on fabrics, but on substrates. Please provide micrographs with a lower magnification, where the structure of the fabric will be visible. Also, in microphotograph f, the distribution of microspheres is not quite uniform and some of them are embedded in the matrix. How did this embedding occur? How will the microspheres function in this case?

Response 4: Thanks for your careful checks. We have made modifications to the annotations of the figure. The embedding of microspheres is due to the presence of PVA, which better binds the microspheres to the substrate. PVA will fill the gaps between fibers on the fabric, forming a smooth surface, enhancing the reflection of microspheres, and making the fabric color more brighter.

Finally, in Figure. 2f, the less uniform distribution of the microspheres is due to inadequate sample preparation, resulting in improper focusing of the samples when stacked together. I have therefore retaken the fiber photograph of Figure. 2f and made a replacement for Figure. 2f in line 211 of the manuscript.

Comment 5: I do not quite agree with the statement in lines 220-222, the data obtained only indicate the presence of elements. Also, 33.55% carbon indicates that not the entire surface is covered with microspheres.

Response 5: Thank you very much for your valuable suggestions. In the statement lines 220-222, we express that the Cu₂O microsphere is successfully loaded to the fiber surface, not the entire fabric surface is loaded. The presence of 33.55% of the C element was due to the fact that the binder PVA was also detected.

Comment 6: The spectra in Figure 4 are of low quality and are not correctly formatted.

Response 6: Thank you very much for your careful scrutiny. We sincerely apologize for our mistakes. In the revised manuscript, we have included the correct spectra in line 235.

Comment 7: From the presented mechanical results, it is not entirely clear to me how the deformation properties increase for some samples? I am also interested in the question of how the microspheres behave under repeated mechanical impact?

Response 7: We express our gratitude to the reviewer for their positive and constructive feedback regarding our manuscript. The enhancement in the deformation properties observed in certain samples can be attributed to the influence of PVA adhesive on the mechanical characteristics of these samples. There is a lack of viable methods for testing microspheres at the nanoscale.

Comment 8: For Figure 11, the numbering is broken.

Response 8: We sincerely apologize for our careless mistakes. Thank you for your reminder. As suggested by the reviewer, we have corrected the “10” to “11” in line 325.

Comment 9: The work contains typos and minor errors, for example, in line 161, a period is missing at the end of the sentence. Therefore, I recommend additional editing work with the text of the manuscript. in some cases questions arise about the terminology used, for example in line 164 "dissolved" (167 - 2 wt% PVA adhesive), how is that? I don't understand what is happening with the system. the authors should carefully check the manuscript and exclude questionable use of terminology.

Response 9: Thank you very much for your valuable feedback. In the revised manuscript, the manuscript was once again scrutinized and corrections for spelling mistakes and some minor errors were completed. Regarding the use of terminology that appeared in line 164 "dissolved" (167 - 2 wt% PVA adhesive) of the manuscript, changes were also made in lines 165-168 of the revised manuscript, and the terminology throughout the text was carefully checked.

Lines 165-168: First, the synthesized Cu₂O single crystal spheres were fully dispersed in anhydrous ethanol to obtain 2 wt% Cu₂O dispersion. Secondly, polyvinyl alcohol particles（PVA） were dispersed in deionized water and ultrasonically shaken at 90℃ for 3 hours to obtain 2 wt% PVA binder.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The revised version is OK with me.

I recommend publication.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors answered all questions and made the appropriate edits to the manuscript. The manuscript may be considered for publication.