Irradiation-catalysed degradation of methyl orange using BaF2–TiO2 nanocomposite catalysts prepared by a sol–gel method

BaF2–TiO2 nanocomposite material (hereinafter called the composite) was prepared by a sol–gel method. The composite surface area, morphology and structure were characterized by Brunauer–Emmett–Teller method, X-ray diffraction analysis and a scanning electron microscopy. The results showed that BaF2 and TiO2 form a PN-like structure on the surface of the composite. Composites were used to catalyse the degradation of methyl orange by irradiation with ultraviolet light, γ-rays and an electron beam (EB). It was demonstrated that the composite is found to be more efficient than the prepared TiO2 and commercial P25 in the degradation of methyl orange under γ-irradiation. Increasing the composite catalyst concentration within a certain range can effectively improve the decolorization rate of the methyl orange solution. However, when the composite material is used to catalyse the degradation of organic matter in the presence of ultraviolet light or 10 MeV EB irradiation, the catalytic effect is poor or substantially ineffective. In addition, a hybrid mechanism is proposed; BaF2 absorbs γ-rays to generate radioluminescence and further excites TiO2 to generate photo-charges. Due to the heterojunction effect, the resulting photo-charge will produce more active particles. This seems to be a possible mechanism to explain γ-irradiation's catalytic behaviour.

from Aladdin Chemical Reagent -this will facilitate the reading and comprehension of the manuscript.
-Please check Figure 1, some letters are missing. Also it legend should be changed, ex: Schematic representation of the preparation… Page 5: -Please indicate the version of software program ImagePro Plus. -Attention to section 3.4, please clarify: units of dose rate, why different dose rates and methyl orange concentrations were used at gamma irradiation and electron beam irradiations? This is important information that is not present, someone who are not familiar with the techniques might not understand it. Title: Irradiation-catalyzed degradation of methyl orange using BaF2-TiO2 nanocomposite catalysts prepared by a sol-gel method Manuscript ID: RSOS-191156 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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-Line 30: a hyphen is missing -Line 49: "Like traditional gamma-ray irradiation technology" at the beginning of the sentence does not make sense. Sentence may start at "However…" Page 4: Attention should be given to the 3.2 section. -Line30-31: "(5 mL concentrated hydrochloric acid)" is not needed and induce confusion. -Line 42: Last sentence must include the explanation about the way authors refer to the pristine and obtained materials. For instance, it must be said here, and not just at page 9, that synthesized TiO2 will be called "TiO2" while "P25" will be used to designate the commercial TiO2 obtained from Aladdin Chemical Reagent -this will facilitate the reading and comprehension of the manuscript.
-Please check Figure 1, some letters are missing. Also it legend should be changed, ex: Schematic representation of the preparation…   1. In page 4, in introduction part, the author seems describe the shortcoming for ionization irradiation. The author seems to solve this problem, wheras the description is in short. I think the author should try more to explain their innovation.
.Response: Thank you very much. I have re-written this part, as following: "In the series of processes, the yield of active particles plays an important role. However, like conventional ultraviolet radiation and ionizing radiation, there are problems requiring large doses and long reaction times before producing sufficient active particles [9][10][11]. To this end, a catalyst is added to increase the yield of the active particles during the irradiation, thereby reducing the irradiation time and dosage.    Figure 2 shows the adsorption-desorption isotherm of the composite catalyst. It can be seen from Fig. 2 that all the samples have a typical type IV isotherm, indicating that the composite catalyst forms a mesoporous structure. From the results, the adsorption capacity of 0.75-BaF 2 -TiO 2 and 0.35-BaF 2 -TiO 2 is similar. In Table 1, the results from BET surface area measurements for the composite catalysts are given. As shown in the table 1, specific surface area and pore size of TiO 2 were 59.3 m2/g and 19.3 nm, while these values were 46.04-13.83 (m 2 g -1 ) and 23.1-30.17 nm for the composite catalyst，changing with the initial BaF 2 to TiO 2 ratios from 0.35 to 1.5. Overall there is a decrease of the surface area when compared to that of the neat TiO 2 . This indicated that nanophase TiO 2 particles were only embedded onto the surface of the BaF2 substrates and introduction of TiO 2 onto the surface of BaF 2 results in certain reduction of specific surface area. The interface between these TiO 2 and BaF 2 should be the major reaction sites to catalytic reaction.  The morphologies of BaF 2 ,TiO 2 and composite catalysts were revealed by SEM investigation, and SEM images of BaF 2 ,TiO 2 and representative composite catalyst (0.75-BaF 2 -TiO 2 ) shown in Figures 4a-d. As seen, the BaF 2 -TiO 2 composite particles are more regular and have a more defined shape than the prepared BaF 2 and TiO 2 , the TiO 2 and BaF 2 particles are layered. And the SEM image was processed using ImagePro Plus 6.0 software, which showed that the particle size of BaF 2 -TiO 2 composite is 175200 nm, which is smaller than the 256312 nm size of BaF 2 . Therefore, the TiO 2 particles hinder aggregation of the BaF 2 particles. In addition, due to the difference in band gap between TiO 2 and BaF 2 , the synthesized composite forms a PN-like structure (similar to a bridge) between BaF 2 and TiO 2 compared to pure TiO 2 crystal. The synthesized composite can induce specific electron transfer processes, improve charge separation efficiency, produce more active particles, and achieve better catalytic effect.  Figure 5e shows the specific content of the surface of composite catalyst; the relative content of elemental Ti is 44.3% and that of Ba is 19.7%.In addition, the SEM image has indicated that the surface of the composite catalyst forms a composite structure consisting of two phases. In order to explore the composition of the two phases, two points (A and B) were selected in Figure 4d for SEM mapping analysis. The SEM mapping spectrum is presented in Figure 5f-g, the point A consists mainly of Ba and F elements, and the point B consists mainly of Ti and O elements. And the elemental composition of each point is almost the same as the elemental mass ratio of TiO 2 or BaF 2 .Combined with the XRD patterns, the two phases that make up the composite catalyst are TiO 2 and BaF 2 . As for other composite catalysts, their spectra are similar to those of 0.75-BaF 2 -TiO 2 , but the peak intensities are different due to different BaF 2 contents. conditions. In addition, in the revised manuscript, I added a description of the methyl orange adsorption experiment with different catalyst. As following: "The 1 h adsorption capacity of the different catalysts for methyl orange was tested in the dark using a 20 mg / L methyl orange solution as a solvent. The experimental results are shown in Fig.  6 (for the sake of clearer images, Figure 6 only shows the UV-visible spectrum of 0.75-BaF 2 -TiO 2 ), P25 has the strongest adsorption capacity, and BaF2 has almost no adsorption capacity as a catalyst. As for the composite catalyst, its adsorption capacity is between BaF2 and TiO 2 . Among them, 0.35-BaF 2 -TiO 2 has the strongest adsorption capacity, and 1.5-BaF 2 -TiO 2 has the lowest adsorption capacity. This is consistent with the N 2 adsorption results of Figure 3. In general, the adsorption of methyl orange solution by the catalyst was small within 1 h. However,  Since the concentration of the catalyst in the next catalytic experiment is based on the concentration of TiO 2 (i.e., the concentration of TiO 2 in the composite catalyst added the same as the concentration of pure TiO 2 ), it is necessary to know how much composite catalyst is required per gram of TiO 2 . Response: Thank you very much. I explained this problem in the revised manuscript, as following: "Since the concentration of the catalyst in the next catalytic experiment is based on the concentration of TiO 2 (i.e., the concentration of TiO 2 in the composite catalyst added the same as the concentration of pure TiO 2 ), it is necessary to know how much composite catalyst is required per gram of TiO 2 . Table 2 lists the total mass required when different composite catalysts contain 1g of TiO 2 ." "In addition, as the content of BaF 2 in the composite catalyst increases, the specific surface decreases, which may be one of the reasons for the decrease in the catalytic ability of 1.5-BaF 2 -TiO 2 ." 7. Generally, the author give the result in catalysis efficiency. However, main mechanism is unknown. I think mechanism is more important, the author should try to explain it clearly.
Response: thank you very much for your suggestion. I described a possible mechanism in the revised manuscript, as following: "The mechanism of high energy radiation used to induce TiO 2 -BaF 2 composite catalyst γ-radiation catalysis is not yet clear. It is true that composite catalyst can be excited by 60 Co irradiation source, thus interior UV from radioluminescence(RL) by radio-sensitive BaF 2 should be a possible route. When the composite catalyst was irradiated with γ-irradiation, Ba 2+ (5p) was excited to Ba 2+ (5p*) leaving a hole, and then electrons from F -(2p)valence band to the cation Ba 2+ (5p) level with the release of 5.6 eV, 6.4 eV radiation(As shown in Equation 11 and Equation 12) [36]. In addition, an interior electric field developed in BaF 2 -TiO 2 depletion layer is another probably scheme. It is believed that the formed TiO 2 particles are probably combined with the BaF 2 surface via the Ti-O-Ba structural units .Since the energy band structures of TiO 2 and BaF 2 are different from each other, a typical "hetero-junction"(HJ) would be formed between TiO 2 and BaF 2 . Due to the energy bands of TiO 2 and BaF 2 will bend into each other within this hetero-junction that benefit charge separation within composite catalysts. An inner electronic fields thus established in the hetero-junction directed from BaF 2 to TiO 2 . Under this inner electric field, radiationinduced electrons in the TiO 2 will drift into the BaF 2 to endow the composite with irradiation -catalytic activity(As shown in Equation 13) [20][21][22]. We believe that the γ-radiation catalytic mechanism of composite catalyst happens by near BaF 2 and TiO 2 and seemed to hybridizing of γ-irradiation and UV as illustrated in Figure 15.
Response: Thank you very much. My goal is to degrade methyl orange at a lower dose by using a composite catalyst, so we used a low dose. The description was revised in the introduction part.
As following: "However, like conventional ultraviolet radiation and ionizing radiation, there are problems requiring large doses and long reaction times before producing sufficient active particles. To this end, the addition of a catalyst during irradiation has been proposed to increase the yield of active particles, thereby reducing the irradiation time and doses." 9. The atmosphere during irradiation is very important, the author should be care of this item.
Response: Thank you very much for your suggestion. In the follow-up work, I will study the effect of atmosphere on the catalytic effect. But due to time and some technical reasons, I can't complete this experiment now. I sincerely hope to get your understanding.
Special thanks to you for your good comments.
I tried my best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper.
I appreciate for editors and reviewers' warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Thank you and best regards.
To reviewers 2 Firstly, I wish to clarify a question in the manuscript. In the originally submitted manuscript, the doping ratio of BaF 2 to TiO 2 was 0.5, 1 and 2. This was described based on the addition amount of Response: Thank you very much. The reason for using methyl orange as the target pollutant is to study a method for degrading azo wastewater, and methyl orange is very representative. In addition, in the manuscript, I also made a similar description. As following: "With ongoing the textile printing and dyeing industry industrial development, a large amount of toxic and not easily degradable wastewater is continually being discharged into the environment. And the textile printing and dyeing industry mainly produces organic wastewater, with azo dyes as a typical pollutant [1,2]. Therefore, the proper treatment of wastewater containing azo dyes(e.g., methyl orange ) is currently a central area of research in water treatment." 2. In page no-2, line no-14, 1. Summary must be change to Abstract.
Response: Thank you very much. Done as suggested.
3. The introduction part is not well-written. Also, there are few explanations of the rationale for the study design, line no-42 and 51. The novelty of the work was not highlighted in this manuscript?
Response: Thank you very much for your suggestion. I have written the introduction section. In the revised manuscript, a more detailed explanation of the purpose and principle of the experiment, as following: "With ongoing the textile printing and dyeing industry industrial development, a large amount of toxic and not easily degradable wastewater is continually being discharged into the environment.
And the textile printing and dyeing industry mainly produces organic wastewater, with azo dyes as a typical pollutant. Therefore, the proper treatment of wastewater containing azo dyes(e.g., methyl orange ) is currently a central area of research in water treatment.
In the field of wastewater treatment, UV degradation and ionizing radiation degradation are  The heading modify to chemicals and instruments to write elaborately the instruments utilized for present manuscript.
Response: Thank you very much. Done as suggested. As following: Response: Thank you very much. Heating the composite again to 105 o C is just to dry the material.
Because we have washed the material before.
Response: Thanks you for pointing this out. Inspired by you, I renamed the composite catalyst. Now I can express them very clearly. As following: "And composite catalysts are named as X-BaF 2 -TiO 2 , where X (0.35, 0.75 and 1.5) corresponds to the weight ratio of BaF 2 to TiO 2 in the material." 7. In Fig.1  "The crystallite size of the TiO 2 and BaF 2 was calculated using Scherrer's equation (9) [33,34]: In addition, according to Equation (1) O or Ba, F is essentially the same. Figure 5e shows the specific content of the surface of composite catalyst; the relative content of elemental Ti is 44.3% and that of Ba is 19.7%.In addition, the SEM image has indicated that the surface of the composite catalyst forms a composite structure consisting of two phases. In order to explore the composition of the two phases, two points (A and B) were selected in Figure 4d for SEM mapping analysis. The SEM mapping spectrum is presented in Figure 5f beam, but BaF2-TiO 2 showed 10% higher decolourization efficiency why?
Response: Thank you very much. In response to this question, I have made some explanations in the revised manuscript , as following: "The possible reason is that BaF 2 is present in the composite catalyst, and BAF 2 as a detector material has a higher absorption cross section for γ-rays than TiO 2 . BaF 2 can effectively absorb high-energy radiation and emit ultraviolet light of 220 and 315 nm, which is then used to excite TiO 2 for photocatalysis. therefore, the prepared composite catalyst material can be more effectively used for degrading methyl orange solutions with γ-radiation." 11. In some places the author mentioned UV scanning spectrum. The author modify UV-visible spectrum.
Response: Thank you very much. Done as suggested.
12. The author reported reusability study only second run, in general, at least repeat five times for reusability study and how the catalyst was reused write clearly?
Response: Thanks you for pointing this out. Since the composite catalyst was attached to the quartz wool in the experiment, only a part of the composite catalyst could be recovered. So I didn't have enough catalyst to complete five experiments. I am very sorry that I can't answer your question. I have removed this part of the manuscript. 15. The authors reported decolourization rate increased 173% to 79.38% in some places, how 173% possible justify? or modify that one.
Response: Thank you very much. Done as suggested.
"At this concentration, the solution was irradiated for 1 h under the same conditions, and the decolorization rate of the methyl orange solution was increased from 29.1% to 79.38%, compared with the same dose using only γ-irradiation." 16. The authors can also report adsorption isotherms studies of methyl orange dye for BaF 2 -TiO 2 catalyst to know the monolayer sorption capacity.
Response: Thank you very much. Done as suggested.
"Catalyst adsorption of methyl orange The 1 h adsorption capacity of the different catalysts for methyl orange was tested in the dark using a 20 mg / L methyl orange solution as a solvent. The experimental results are shown in Fig.   6 (for the sake of clearer images, Figure 6 only shows the UV-visible spectrum of 0.75-BaF 2 -TiO 2 ), P25 has the strongest adsorption capacity, and BaF 2 has almost no adsorption capacity as a catalyst. As for the composite catalyst, its adsorption capacity is between BaF 2 and TiO 2 . Among them, 0.35-BaF 2 -TiO 2 has the strongest adsorption capacity, and 1.5-BaF 2 -TiO 2 has the lowest adsorption capacity. This is consistent with the N 2 adsorption results of Figure 3. In general, the adsorption of methyl orange solution by the catalyst was small within 1 h. However, for the correctness of the data, the results of all experiments have deducted the adsorption of methyl orange by the catalyst. Special thanks to you for your good comments.
I tried my best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper.
I appreciate for editors and reviewers' warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Thank you and best regards.

To reviewers 3
Firstly, I wish to clarify a question in the manuscript. In the originally submitted manuscript, the doping ratio of BaF 2 to TiO 2 was 0.5, 1 and 2. This was described based on the addition amount of The results shown represent all the ratios?).
Response: Thank you very much. Because the experiment is to test the effect of the composite catalyst on the degradation of methyl orange. In order to make the image more concise, in each experiment I only selected the composite catalyst with the best catalytic effect for explanation.
However, in the revised manuscript, other proportions of the composite catalyst are also described in the text. As following  Combined with the above characterization results, these results further demonstrate that our synthesized TiO 2 is our desired crystal form. When the 0.35-BaF 2 -TiO 2 composite was added as a catalyst, the decolorization rate of methyl orange solution was only 19.3%, which is lower than that of pure TiO 2 . A possible reason for this is limited penetration of the ultraviolet light into the solution. And the presence of BaF 2 hinders UV absorption by TiO 2 , resulting in some catalytic ability of the prepared composite sample but a rather weak photocatalytic ability. The theory is also supported by the catalytic ability of different composite catalysts in the experiment (The higher the BaF 2 content, the lower the catalytic effect). "Since the concentration of the catalyst in the next catalytic experiment is based on the concentration of TiO 2 (i.e., the concentration of TiO 2 in the composite catalyst added the same as the concentration of pure TiO 2 ), it is necessary to know how much composite catalyst is required per gram of TiO 2 . Table 2  Response: Thank you very much. Done as suggested. As following: "Section 4.2: it is clear that the prepared composite catalyst has almost no catalytic ability for electron beam degradation of organic matter. The possible reason for this is that the energy of the electron beam is too high, beyond the absorption range of BaF 2 and TiO 2 . Therefore, the addition of the catalyst in this experiment has almost no effect. concentration in the composite catalyst is 1 g L -1 , the decolorization rate of the methyl orange solution is 52.24%, and when its concentration reaches the optimum value of 3 g L -1 , there are more catalysts in the solution, which will produce more active particles, and the decolorization rate increased to 79.38%. At this point, the catalyst concentration in the solution has reached saturation. Continued increase in concentration will hinder the absorption of gamma rays by the methyl orange solution, further affecting the yield of active particles produced by water radiolysis, and resulting in a decrease in catalytic effect.

Mechanism of composite catalyst
The mechanism of high energy radiation used to induce TiO 2 -BaF 2 composite catalyst γ-radiation catalysis is not yet clear. It is true that composite catalyst can be excited by 60 Co irradiation source, thus interior UV from radioluminescence(RL) by radio-sensitive BaF 2 should be a possible route.
When the composite catalyst was irradiated with γ-irradiation, Ba 2+ (5p) was excited to Ba 2+ (5p*) leaving a hole, and then electrons from F -(2p)valence band to the cation Ba 2+ (5p) level with the release of 5.6 eV, 6.4 eV radiation(As shown in Equation 11 and Equation 12) [17]. In addition, an interior electric field developed in BaF 2 -TiO 2 depletion layer is another probably scheme. It is believed that the formed TiO 2 particles are probably combined with the BaF 2 surface via the Ti-O-Ba structural units .Since the energy band structures of TiO 2 and BaF 2 are different from each other, a typical "hetero-junction"(HJ) would be formed between TiO 2 and BaF 2 . Due to the energy bands of TiO 2 and BaF 2 will bend into each other within this hetero-junction that benefit charge separation within composite catalysts. An inner electronic fields thus established in the hetero-junction directed from BaF 2 to TiO2. Under this inner electric field, radiationinduced electrons in the TiO 2 will drift into the BaF 2 toendow the composite with irradiation -catalytic activity(As shown in Equation 13) [20][21][22]. We believe that the γ-radiation catalytic mechanism of composite catalyst happens by near BaF 2 and TiO 2 and seemed to hybridizing of γ-irradiation and UV as illustrated in Figure 15.
Response: Thank you very much. Done as suggested. In addition, due to the addition of some content, the number of the picture has also changed. As following: " Fig. 7 Ultraviolet-visible spectra before and after ultraviolet light irradiation of methyl orange solution with different catalysts (C BaF2 , C TiO2 , C P25 : 1 g L -1 ; C 0.35-BaF2-TiO2 : 1.35 g L -1 ) " " Fig. 9 Ultraviolet-visible spectra before and after γ-ray irradiation of methyl orange solutions with different catalysts (C BaF2 , C TiO2 , C P25 : 1 g L -1 ; C 0.75-BaF2-TiO2 : 1.75 g L -1 ) " Response: Thanks you for pointing this out. I made a mistake while processing the data. I have corrected this error in the revised manuscript. In addition, since I added some pictures, the number of the original picture has also changed. As following: Response: Thank you very much. Done as suggested.
18. Authors may not generalize the catalytic activity for organic matter since they have just tested methyl orange solutions. Please correct.
Response: Thank you very much. Done as suggested.
"Therefore, more active particles capable of degrading methyl orange are produced. " "Overall, the BaF 2 -TiO 2 composite material prepared herein is an excellent γ-irradiation degradation methyl orange catalyst. " Special thanks to you for your good comments.
I tried my best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper.
I appreciate for editors and reviewers' warm work earnestly, and hope that the correction will meet with approval.
Once again, thank you very much for your comments and suggestions.
Thank you and best regards.