Biomimetic synthesis of nanostructured WO3 · H2O particles and subsequent thermal conversion to WO3

Nanostructured tungsten oxide (WO3) particles were prepared in aqueous solution by mimicking biomineralization. Precursor WO3 · H2O particles were generated by ageing a 60°C (NH4)10W12O41 · 5H2O solution containing gelatin. This was followed by heating to 600°C in air for thermal conversion to WO3. The addition of gelatin led to the formation of layered structures consisting of WO3 · H2O platy particles, which contained segmented, block-like nanoscale units. The macroscopic layered structure was preserved after thermal conversion to WO3, while the morphology of the block-like units changed to orthogonally crossed nanorods.


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Accept with minor revision (please list in comments)

Comments to the Author(s)
The present manuscript reports biomimetic synthesis of nanostructured WO3·H2O particles. The synthesis/characterization techniques are performed at a high technical standard, and discussion given in the manuscript are basically sound. In this context, this manuscript is potentially acceptable, according to the publication criteria of Royal Society Open Science. However, I think, further discussions should be added to show clearly the back ground of the present study, and give robustness to the conclusions.
I'd like to add following remarks to be further considered by the authors; 1) I wonder if the present manuscript is the first report on the biomimetic synthesis of WO3·H2O (and WO3) or not. Otherwise, what's a significant advance from the previous biomimetic synthesis of WO3-based materials? The advance of the present work should be clearly discussed in the introduction section; significant advance of scientific knowledge is a criteria of acceptance by RSOS.
2) The authors describe their previous works on the synthesis of CeO2 and SnO in the introduction section (the second paragraph in the introduction). The context somehow gives us an impression that the present work had been routinely-done. I suggest to add some comments to describe scientific challenges for the biomimetic synthesis of WO3, and how the present system is different from previous ones.
3) The crystallization in the present system is highly sensitive to the pH of the solution. pH after the reaction as well as before the reaction should be given to confirm robustness of the discussion; precipitation of crystals is usually accompanied by the change of pH of reaction mixture. 4) What is the aging time employed for samples appeared in Figs 1, 2, 3, 4, 5, 6, and 7. The authors describes in the experimental section that they tested aging time 1-7 days. Which aging time was chosen for the discussions in respective figures? 5) Reaction kinetics of the crystallization process should be discussed on at least one sample. I'm wondering how the crystallization occurs as a function of reaction time. Is there any induction period for the crystallization? How does the morphology develop with reaction time? 6) The weight ratio of gelatin/WO3 should be calculated from the TG data of the obtained product. This information gives us an idea on what kind of composite is indeed obtained. 7) Fig4: "the flat face of the layered structure" is ambiguous. It should be marked in Fig.4. 8) It might be interesting to compare TEM images and diffractions before and after the calcination. Is there any relevance of crystallographic orientation between them?
The inner structure of the particles should be analyzed. Are the size and shape of the pores, the crystalline size and orientation the same on the surface and inside of the particles? Surface area, pore volume, pore size distribution, true density, particle density, etc. are also importance to be measured for detailed characterization.
In the Introduction section, the authors mentioned that WO3 has been used in various devices. Thus, readers would be interested in what sort of advantages the newly prepared WO3 particles have or which application field they can be practically used in.
Compared with the other published works on biomimetic synthesis, the method and findings in the manuscript sound not quite new. Thus, a little more characterization or property measurement of the obtained particles would further demonstrate importance of the work.

29-Mar-2019
Dear Dr Uchiyama: Title: Biomimetic synthesis of nanostructured WO3·H2O particles and subsequent thermal conversion to WO3 Manuscript ID: RSOS-182137 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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I'd like to add following remarks to be further considered by the authors; 1) I wonder if the present manuscript is the first report on the biomimetic synthesis of WO3·H2O (and WO3) or not. Otherwise, what's a significant advance from the previous biomimetic synthesis of WO3-based materials? The advance of the present work should be clearly discussed in the introduction section; significant advance of scientific knowledge is a criteria of acceptance by RSOS.
2) The authors describe their previous works on the synthesis of CeO2 and SnO in the introduction section (the second paragraph in the introduction). The context somehow gives us an impression that the present work had been routinely-done. I suggest to add some comments to describe scientific challenges for the biomimetic synthesis of WO3, and how the present system is different from previous ones.
3) The crystallization in the present system is highly sensitive to the pH of the solution. pH after the reaction as well as before the reaction should be given to confirm robustness of the discussion; precipitation of crystals is usually accompanied by the change of pH of reaction mixture. 4) What is the aging time employed for samples appeared in Figs 1, 2, 3, 4, 5, 6, and 7. The authors describes in the experimental section that they tested aging time 1-7 days. Which aging time was chosen for the discussions in respective figures? 5) Reaction kinetics of the crystallization process should be discussed on at least one sample. I'm wondering how the crystallization occurs as a function of reaction time. Is there any induction period for the crystallization? How does the morphology develop with reaction time? 6) The weight ratio of gelatin/WO3 should be calculated from the TG data of the obtained product. This information gives us an idea on what kind of composite is indeed obtained. 7) Fig4: "the flat face of the layered structure" is ambiguous. It should be marked in Fig.4. 8) It might be interesting to compare TEM images and diffractions before and after the calcination. Is there any relevance of crystallographic orientation between them?
Reviewer: 2 Comments to the Author(s) The authors report the new biomimetic synthesis of WO3·H2O particles, which can further be converted to WO3 particles by heat treatment at higher temperature. The synthesis method and discussion on the mechanisms of the particle formation are technically sound, and the conclusions are supported by experimental data obtained from sufficient number of samples. Therefore the manuscript would be publishable in Royal Society Open Science after addressing minor issues shown below.
The inner structure of the particles should be analyzed. Are the size and shape of the pores, the crystalline size and orientation the same on the surface and inside of the particles? Surface area, pore volume, pore size distribution, true density, particle density, etc. are also importance to be measured for detailed characterization.
In the Introduction section, the authors mentioned that WO3 has been used in various devices. Thus, readers would be interested in what sort of advantages the newly prepared WO3 particles have or which application field they can be practically used in.
Compared with the other published works on biomimetic synthesis, the method and findings in the manuscript sound not quite new. Thus, a little more characterization or property measurement of the obtained particles would further demonstrate importance of the work.

Referee 1's original comment
The present manuscript reports biomimetic synthesis of nanostructured WO 3 ·H 2 O particles. The synthesis/characterization techniques are performed at a high technical standard, and discussion given in the manuscript are basically sound. In this context, this manuscript is potentially acceptable, according to the publication criteria of Royal Society Open Science. However, I think, further discussions should be added to show clearly the back ground of the present study, and give robustness to the conclusions.
I'd like to add following remarks to be further considered by the authors;

Referee 1's Question 1:
1) I wonder if the present manuscript is the first report on the biomimetic synthesis of WO 3 ·H 2 O (and WO 3 ) or not. Otherwise, what's a significant advance from the previous biomimetic synthesis of WO 3 -based materials? The advance of the present work should be clearly discussed in the introduction section; significant advance of scientific knowledge is a criteria of acceptance by RSOS.
Our response 1: Recently, many types of biomimetic synthesis of inorganic materials have been suggested, where the factors incorporated from biomineralization vary from work to work. Many of the biomimetic works of functional metal oxide materials focus on the similarity in the "resultant nanostructure" between the products and the real biominerals, and the resultant device performance.
On the other hand, our groups have studied "new aqueous technique" mimicking biomineralization for nanostructural control of materials. We think that the key factors of Appendix A biomineralization are (1) the interaction between inorganic crystals and biological polymers, and (2) the multistep synthetic procedure via metastable phases as the precursor materials, and have suggested new approaches containing the one or both factors for making nanostructures like biominerals. The "biomimetic synthetic route" would lead to the formation of "biomimetic nanostructure" which is the hierarchical structures consisting of oriented inorganic nanocrystallites.
In the present work, the biomimetic aqueous route with biological polymers has been suggested for preparing WO 3 materials. The synthetic method with a biological polymer, gelatin, is a new and original concept for WO 3 -based materials. Moreover, the WO 3 products of the present work had the hierarchical nanostructures like biominerals (the layered structure consisting of crystallographically-oriented nanorods).
We believe that these concept and results of the present work have a fully significant advance and this manuscript is worthy to be published in Royal Society Open Science.
In order to clarify the advance of the present work, in pp. 1, line 37, the introduction section has been modified as follows; "Biomimetic structures have been widely made from nanoscale inorganic units and biological polymers [11][12][13][18][19][20][21][22][23][24][25]. Many works about biomimetic synthesis of functional metal oxide materials mainly focused on the similarity in the resultant nanostructure between the products and the real biominerals, and the resultant device performance. On the other hand, we have focused on "biomimetic synthetic route", and attempted to construct novel aqueous techniques for making nanostructured materials. We think that the key factors of biomineralization are (1) the interaction between inorganic crystals and biological polymers, and (2)

Referee 1's Question 2:
2) The authors describe their previous works on the synthesis of CeO 2 and SnO in the introduction section (the second paragraph in the introduction). The context somehow gives us an impression that the present work had been routinely-done. I suggest to add some comments to describe scientific challenges for the biomimetic synthesis of WO 3 , and how the present system is different from previous ones.

Our response 2:
We have previously reported several works about the biomimetic synthesis of inorganic materials. As mentioned in the response 1, we think that the key factors of biomineralization are (1) the interaction between inorganic crystals and biological polymers, and (2) the multistep synthetic procedure via metastable phases as the precursor materials. The suitable route for making a biomimetic nanostructure varies from one compound to another, and thus we have to found the adequate method depending on the target materials. Thus, our papers always contain a new insight, and thus are not routine works.
The present work mainly focused on the nanostructural control of precursor materials (WO 3 ·H 2 O) with biological polymers and the thermal conversion to metal oxide materials (WO 3 ).
As mentioned in the introduction section, we have reported the biomimetic synthesis of nanostructured CeO 2 materials, which was done on the basis of a similar strategy. We achieved the preparation of nanostructured CeCO 3 OH particles with biological polymers (gelatin and agar) and the thermal conversion to CeO 2 , while the crystallographic orientation of inorganic units like biominerals were not observed in the CeO 2 products. Thus, in the present work, we selected WO 3 ·H 2 O and WO 3 for making hierarchical structures consisting of oriented inorganic nanocrystallites like biominerals. The topotactic transformation from WO 3 ·H 2 O to WO 3 crystals has been reported, which would allow us to keep the crystallographic orientation of inorganic units after the thermal conversion to metal oxide.
In order to more clearly show the concept of the present work, in pp. 1, line 54, the introduction section has been modified as follows;

Referee 1's Question 3:
3) The crystallization in the present system is highly sensitive to the pH of the solution. pH after the reaction as well as before the reaction should be given to confirm robustness of the discussion; precipitation of crystals is usually accompanied by the change of pH of reaction mixture.
Our response 3: We appreciate the referee's helpful suggestion. The precipitation of WO 3 ·H 2 O under an acidic condition seems to be as follows; WO 4 2-+ 2H + → WO 3 ·H 2 O This reaction consumes H + ions, resulting in the increase in the pH value.
About some conditions, we measured the pH value of the solutions before and after the reaction.
The pH value is shown as follows; Our response 4 and 5: The aging times which employed for the samples appeared in Figures 1-7 corresponds to those shown in Table 1.
In this work, at first, we performed preliminary experiments to know the reaction time to obtain sufficient amounts of products. We changed the aging time between 1-7 days at all pH and [gelatin] conditions, and knew the sufficient time that the increase in the sample yield stopped.
About the experimental condition with C ge = 2.0 g L -1 at pH 1.0, we investigated the effect of the reaction time on the morphology and crystal phase of the WO 3 ·H 2 O particles. No precipitation was observed for 1-3 days, while yellowish precipitates appeared after 4 days. The diffraction peaks attributed to WO 3 ·H 2 O were observed irrespective of the reaction times (4-7 days). The morphology of the 4-7 days WO 3 ·H 2 O products changed with reaction time. The samples obtained on 4 days was the mixture of layered plates and spherical particles. The spherical particles disappeared with increasing reaction times, and only layered plates were observed after 6 days.
The spherical particles found in the samples of 4-5 days were thought to be the composites of tungstate ions and gelatin. As described in the experimental section, in this work, the And, figure captions has been modified as follows; Figure 1. XRD patterns of WO 3 precursors prepared from (NH 4 ) 10 W 12 O 41 solutions with C ge =0-2.0 g L -1 and HCl at pH 0.6-1.0 (the aging time was as shown in Table 1). and HCl at pH 0.6 (a,b), pH 0.8 (c,d) and pH 1.0 (e,f) (the aging time was as shown in Table 1). Figure 3. SEM images of WO 3 precursors prepared from (NH 4 ) 10 W 12 O 41 solutions with C ge =0.2 g L -1 (a,b) and 2.0 g L -1 (c,d) and HCl at pH 0.6 (the aging time was as shown in Table 1). Figure 4. SEM images of WO 3 precursors prepared from (NH 4 ) 10 W 12 O 41 solutions with C ge =0.2 g L -1 (a) and 1.5 g L -1 (b-d) and HCl at pH 1.0 (the aging time was as shown in Table 1).  Table 1).   Figure 5 shows the XRD patterns of WO 3 precursors prepared by aging for 4-7 days. The diffraction peaks attributed to WO 3 ·H 2 O were observed irrespective of the aging times. Figure 6 shows the SEM images of the WO 3 precursors. And, the XRD patterns and SEM images WO 3 precursors prepared by aging for 4-7 days from (NH 4 ) 10 W 12 O 41 solutions with C ge = 2.0 g L -1 and HCl at pH 1.0 has been added as Figures 5 and 6.

Referee 1's Question 6:
6) The weight ratio of gelatin/WO 3 should be calculated from the TG data of the obtained product.
This information gives us an idea on what kind of composite is indeed obtained.
Our response 6: As mentioned in pp. 7, line 21, the weight loss attributed to the combustion of residual gelatin at 550 ºC was ca. 1 wt%. Moreover, we performed FT-IR analysis for the WO 3 ·H 2 O sample, where no absorption peaks due to gelatin were not seen. Thus, only a little amount of gelatin was thought to remain on the WO 3 ·H 2 O products.
FT-IR spectra has been added as Figure 8 into the main text as follows; Figure 8. FT-IR spectra for WO 3 precursors prepared from (NH 4 ) 10 W 12 O 41 solutions with C ge = 0 and 2.0 g L -1 and HCl at pH 1.0 (the aging time was as shown in Table 1).
And, in pp. 7, line 21, the description on the FT-IR analysis has been added as follows; "In addition, a slight weight loss of 1 wt% was detected at 550 °C, which could be attributed to combustion of residual gelatin. We also investigated the presence of residual gelatin on the WO 3 precursors by FT-IR analysis. Figure 8 shows FT-IR spectra for the WO 3 precursors (C ge = 0 and 2.0 g L -1 , pH 1.0). No absorption peaks due to gelatin were not detected, which also indicates that only a little amount of gelatin remained on the WO 3 ·H 2 O products."  Table 1).

Referee 1's Question 8:
8) It might be interesting to compare TEM images and diffractions before and after the calcination.
Is there any relevance of crystallographic orientation between them?
Our response 8: Thank you for a good suggestion. As the referee 1 mentioned, the comparison of TEM images between the samples before and after calcination is definitely an interesting issue.
In fact, we attempted to evaluate the crystallographic orientation of the WO 3 ·H 2 O precursor materials, and to discuss the relevance to the resultant WO 3 materials. However, the WO 3 ·H 2 O crystals were not stable toward an electron beam, and deformed during the TEM analysis. Thus, we have not yet obtained the adequate information about the crystallographic orientation.
On the other hand, as discussed in the XRD analysis part, the flat face of WO 3 ·H 2 O layered plates was found to be (010) plane of the WO 3 ·H 2 O crystal. Moreover, the electron diffraction pattern of WO 3 (Fig. 7d) shows that the flat face of heat-treated WO 3 layers is (001)  In order to clarify the crystallographic relevance between the samples before and after calcination, in pp. 8, line 56, the discussion part about the TEM analysis has been modified as follows; " Figure 10 shows FE-SEM and FE-TEM images of the heat-treated WO 3 products (C ge = 1.5 g-L -1 , pH 1.0). As shown in figure 10a,  Thank you for your careful reading and positive review.

Our response to Reviewer 2
Thank you for your careful reading and positive review.

Referee 2's original comment
The authors report the new biomimetic synthesis of WO 3 ·H 2 O particles, which can further be converted to WO 3 particles by heat treatment at higher temperature. The synthesis method and discussion on the mechanisms of the particle formation are technically sound, and the conclusions are supported by experimental data obtained from sufficient number of samples. Therefore the manuscript would be publishable in Royal Society Open Science after addressing minor issues shown below.

Referee 2's Question 1:
The inner structure of the particles should be analyzed. Are the size and shape of the pores, the crystalline size and orientation the same on the surface and inside of the particles? Surface area, pore volume, pore size distribution, true density, particle density, etc. are also importance to be measured for detailed characterization.
Our response 1: Thank you for a good suggestion. As the referee 1 mentioned, the detailed analysis of the porous structures of the WO 3 products is very important.
We attempted to observe the inside structure of the products. However, the unit crystallites were densely packed in the layered architectures, and thus the inner structure was hard to be evaluated by SEM and TEM analysis. The detailed analysis is our important remaining issue.
Although the direct observation of the pore and crystallite sizes was not achieved, we evaluated the BET surface area of the WO 3 products by N 2 adsorption method. The surface area of the layered structure obtained by an addition of gelatin (C ge = 2.0 g L -1 ) was 5.02 m 2 g -1 , which was larger than that of the random aggregates of C ge = 0 g L -1 (2.31 m 2 g -1 ). These shows that the synthesis route with biological polymer is effective for preparing nanostructured materials.
In pp. 9 line 5 the description on the surface area of the WO 3 products has been added as follows; "We evaluated the BET surface area of the WO 3 products by N 2 adsorption method. The surface area of the layered structure obtained by an addition of gelatin (C ge = 1.5 g-L -1 , pH 1.0) was 5.02 m 2 g -1 , which was larger than that of the random aggregates of C ge = 0 g-L -1 , pH 1.0 (2.31 m 2 g -1 ). "

Referee 2's Question 2:
In the Introduction section, the authors mentioned that WO 3 has been used in various devices.
Thus, readers would be interested in what sort of advantages the newly prepared WO 3 particles have or which application field they can be practically used in.
Our response 2: As mentioned in the response 1, the addition of gelatin resulted in the increase in the surface area of WO 3 particle materials. Such WO 3 materials with a larger surface area are thought to be suitable for the photoelectrodes and gas sensors. Since photoelectrochemical reactions and gas sensing occur on the surface of the electrode materials, a larger surface area of the nanostructured electrodes would result in better device performance.
In order to clarify the advantage of the WO 3 products, in pp. 9, line 3, the discussion part has been modified as follows; "These results suggested that the WO 3 layered structures prepared with gelatin have highly ordered nanostructures consisting of oriented inorganic nanoscale units like biominerals such as nacres, sea urchin spines, and eggshells [9][10][11][12][13][14][15][16][17]. We evaluated the BET surface area of the WO 3 products by N 2 adsorption method. The surface area of the layered structure obtained by an addition of gelatin (C ge = 1.5 g-L -1 , pH 1.0) was 5.02 m 2 g -1 , which was larger than that of the random aggregates of C ge = 0 g-L -1 , pH 1.0 (2.31 m 2 g -1 ). The hierarchical nanostructures are thought to be suitable for photoelectrode and gas sensor materials." Now, we are trying to prepare WO 3 film materials on the basis of the present work, and to evaluate the device performance. We would report the results in the near future.

Referee 2's Question 3:
Compared with the other published works on biomimetic synthesis, the method and findings in the manuscript sound not quite new. Thus, a little more characterization or property measurement of the obtained particles would further demonstrate importance of the work.
Our response 3: As the referee 2 mentioned, recently, many types of biomimetic synthesis of inorganic materials have been suggested. Many of them focus on the similarity in the "resultant nanostructure" between the products and the real biominerals. On the other hand, our groups have studied new "synthetic route" mimicking biomineralization. We think that the key factors of biomineralization are (1) the interaction between inorganic crystals and biological polymers, and (2) the multistep synthetic procedure via metastable phases as the precursor materials, and have suggested new approaches containing the one or both factors for making nanostructures like biominerals. The "biomimetic synthetic route" would lead to the formation of "biomimetic nanostructure" which is the hierarchical structures consisting of oriented inorganic nanocrystallites.
In the present work, the biomimetic aqueous route with biological polymers has been suggested for preparing WO 3 materials. The synthetic method with a biological polymer, gelatin, is a new and original concept for WO 3 -based materials. Moreover, the WO 3 products of the present work had the hierarchical nanostructures like biominerals (the layered structure consisting of crystallographically-oriented nanorods).
We believe that these concept and results of the present work have a fully significant advance and this manuscript is worthy to be published in Royal Society Open Science.