Review Reports - Supported Cu/W/Mo/Ni—Liquid Metal Catalyst with Core-Shell Structure for Photocatalytic Degradation

Round 1

Reviewer 1 Report

Comments to Catalysts-1452384

Title: Study on photocatalytic degradation of supported Cu/W/Mo/Ni—liquid metal catalyst with score-shell structure

Minor comments

The word “study” in the title is not attractive, all investigations are studies. I suggest change this word in the title.
Improve the quality of some imagines of the Figure 1.
Improve the quality of imagen of the presented graphics on Figure 5.
Some XPS spectra presented in the Figure 6 needs to improve the quality of image.

Major comments

¿Why did you use Methylene Blue and Red Congo for your experiments? The photocatalytic efficiency should be more interesting if you apply these new catalyst in the degradation of real substances or real wastewater.
¿What was the used photo-reactor (geometry, configuration, type, feed-flow, number of lamps or LEDs etc.)?.
¿What type of lamps and configuration in the reactor were used?
Methodology for degradation experiments are not clear. I suggest to include more details about this procedures as: initial concentration of dyes, times, catalyst concentration, samples, etc.
¿What is the time for results of photocatalytic degradation shown in the Figure 4?
The authors affirmed in the Abstract and the results of the study that they research in the rates and mechanism of the degradation using the new materials. However, theses aspects were not presented in explicit form. ¿What is the rate of photodegradation? It could be represented in kinetic or standard times of photodegradation. ¿What is the mechanism? It is not clear in the manuscript. I suggest to include the kinetic analysis of the degradation.
Finally, your study is a “PHOTO-catalytic system” but you did not include the quantifications of the light or photon-flux, actinometry or photonic efficiency; These parameters are mandatory in photocatalytic studies.

Author Response

Responses to referee comments

Dear editor and reviewers:

For respected referee, we are very thankful for your valuable time and comments on our manuscript in order to upgrade its quality and meet journal’s standards. Thank you for your comments concerning our manuscript entitled “Study on photocatalytic degradation of supported Cu/W/Mo/Ni—liquid metal catalyst with score-shell structure”, article reference: Catalysts-1452384. We appreciate for your warm work earnestly, and hope that the correction will meet with approval. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewers’ comments are as flowing:

Once again, thank you very much for your comments and suggestions.

Referee 1: The word “study” in the title is not attractive, all investigations are studies. I suggest change this word in the title. Improve the quality of some imagines of the Figure 1. Improve the quality of imagen of the presented graphics on Figure 5. Some XPS spectra presented in the Figure 6 needs to improve the quality of image. Reply: Thank you very much for your suggestions. We have revised the title. At the same time, we increased the resolution of Figures 1, 5, 6 and 7. (1) Original: Study on Photocatalytic Degradation of Supported Cu/W/Mo/Ni--liquid metal Catalyst with core-shell structure (1) Modified: Supported Cu/W/Mo/Ni--liquid metal Catalyst with core-shell structure for Photocatalytic Degradation

Why did you use Methylene Blue and Red Congo for your experiments? The photocatalytic efficiency should be more interesting if you apply this new catalyst in the degradation of real substances or real wastewater.

Reply: Thanks for your kind advice. Your suggestion was very perfect and reasonable. We have conducted preliminary experiments to determine that the liquid metal catalyst with core-shell structure could perform degradation of organic dyes. In the subsequent laboratory, we would try to use this catalyst to degrade other substances and real wastewater, and continue to study. Many thanks to reviewers for their valuable suggestions.

What was the used photo-reactor (geometry, configuration, type, feed-flow, number of lamps or LEDs etc.) ?

Reply: In this work, the photo-reactor was an ultraviolet light lamp with a wide wave of 340~400 nm, and a peak of 365 nm. The structure was ultraviolet lamp with T5-BL/BLB model. The power was 10 W, and the length of the lamp was 328 mm, the diameter of the ultraviolet lamp was 15 mm, and the whole length of the lamp frame was 350mm. This UV lamp has a single tube, and only one lamp, with external 220V power supply. We have added this description of the photo-reactor in the experimental part.

Original: Page 3 in manuscript Xenon lamp (simulated sunlight) was used as the light source. The distance between the reactor and light source was 5 cm, and the reaction time was 2~18 hours.

Modified: Page 3 in revised paper Ultraviolet light was used as the light source. The photo-reactor was an ultraviolet light lamp with a wide wave of 340~400 nm, and a peak of 365 nm. The structure was ultraviolet lamp with T5-BL/BLB model. The power was 10 W, and the length of the lamp was 328 mm, the diameter of the ultraviolet lamp was 15 mm, and the whole length of the lamp frame was 350mm. This UV lamp has a single tube, and only one lamp, with external 220V power supply. The distance between the reactor and light source was 5 cm, and the reaction time was 1~65 hours.

What type of lamps and configuration in the reactor were used? Methodology for degradation experiments is not clear. I suggest to include more details about these procedures as: initial concentration of dyes, times, catalyst concentration, samples, etc.

Reply: Thanks for your comments. In this work, the photo-reactor was an ultraviolet light lamp with a wide wave of 340~400 nm, and a peak of 365 nm. The same answer as in question 2.

We have added more details about these procedures of degradation experiments: The initial concentration of dyes was 20 mg/L, and the volume was 20 ml. The catalytic reaction time ranges from 0 to 65 hours, and the maximum reaction time is up to 65 hours. The concentration of catalyst was 0.1 g/ml. We have added this description of this degradation procedure to the experiment part.

Original: Page 3 in manuscript Ultrasound for 30 minutes without light to make the surface of catalyst reach the adsorption-desorption equilibrium. Modified: Page 3 in revised paper Ultrasound for 30 minutes without light to make the surface of catalyst reach the adsorption-desorption equilibrium. The initial concentration of dyes was 20 mg/L, and the volume was 100 ml. The catalytic reaction time ranges from 0 to 65 hours, and the maximum reaction time is up to 65 hours. The concentration of catalyst was 0.1 g/ml.

What is the time for results of photocatalytic degradation shown in the Figure 4? The authors affirmed in the Abstract and the results of the study that they research in the rates and mechanism of the degradation using the new materials. However, theses aspects were not presented in explicit form. What is the rate of photodegradation? It could be represented in kinetic or standard times of photodegradation.

Reply: In general, the time for results of photocatalytic degradation in Figure 4 was 20 hours. We have added the time, rate of photodegradation, and kinetic of photodegradation in our revised paper.

Original: Page 9-10 in manuscript The degradation rate of methylene blue was calculated according to the formula (2).

Modified: Page 9-10 in revised paper

The chromophore is the group that has no ultraviolet absorption, but the chromophore absorption peak can be strengthened. The methylene blue solution with the addition of four catalysts had different degrees of degradation effect, which reduced the peak strength associated with methylene blue respectively. It should be noted that under the same conditions, the rate of these significant catalytic degradation reactions depends on the type of nano-metal powder added to the catalyst.

Where the degradation rate formula is shown in (2); Then the degradation rate of methylene blue was calculated. The degradation rate of methylene blue was calculated according to the formula (2):

degradation rate D= (C₀-C_t)/C₀ ×100% = (A₀-A_t)/A₀ ×100%

(2)

D was the degradation rate. After degradation, C_t was the concentration of methylene blue at time T= t; C₀ was the initial concentration of methylene blue. The absorbency rate (A%) was one indicator to characterize the decrease of organic dye concentration. Where A₀ is the absorbance of solution at reaction time t=0, and A_t is the absorbance of solution A_t reaction time T.

……

Figure 7(e)(f) shows the degradation curve of methylene blue after adding Ni-LM for 5 hours and 18 hours, respectively. The reaction kinetics was analyzed in Figure 7(i), it shows the degradation rate In(C_t/C₀) and time of methylene blue for 5 hours. Where the kinetic analysis of the degradation formula is shown in (3); Then the degradation rate of methylene blue was calculated.

The kinetic equation was calculated according to the formula (3):

In(C_t/C₀)= -1.5396*10^-1t, k₁= -1.5396*10^-1s^-1

(3)

Where C_t was the concentration of methylene blue at time T= t; C₀ was the initial concentration of methylene blue. k₁= -1.5396*10^-1 s^-1. The linear results show that the degradation reaction conforms to first-order kinetics.

What is the mechanism? It is not clear in the manuscript. I suggest to include the kinetic analysis of the degradation.

Reply: Thanks for your kind advice. We have cited the relevant kinetic analysis of the degradation in revised manuscript. The same answer as in question 4. The kinetic analysis of the degradation has been described in the paper.

Modified: Page 9-10 in revised paper

The kinetic equation was calculated according to the formula (3):

In(C_t/C₀)= -1.5396*10^-1t, k₁= -1.5396*10^-1s^-1

(3)

Finally, your study is a “PHOTO-catalytic system” but you did not include the quantifications of the light or photon-flux, actinometry or photonic efficiency; These parameters are mandatory in photocatalytic studies.

Reply: We are very sorry. We discuss this part again. Photon efficiency refers to the ratio of the rate of light reaction to the rate of incident photon, in a specified time.

Number of molecules removed from organic dyes per unit time:

R= -Vdc/dt = V kc N_A (1)

C was the concentration of organic dye at time t= T, V was the reactor volume, N_A is Avogadro constant, 6.023*10²³ mol^-1, k is the reaction rate constant.

Number of photons entering the reactor per unit time:

Ne = P/ hD (2)

Where P was the radiation power of the light source, h was the Planck constant, 6.626*10^-34J*s, and D is the speed of light in vacuum, 3*10⁸ m/s.

Photon efficiency is the number of molecules decomposed per photon entering the reaction system:

PE = h c V k D N_A/ P λ = h c V k D N_A/ P λ ξ (3)

ξ was the transmittance of the light source radiation through the reaction quartz container.

The all photon efficiency is:

ξ _g = (ξ_d + (ξ_m)^1/5) / 2 (4)

ξ_d was remove photon efficiency; and ξ_m was the mineralized photon efficiency.

From equation (1-4), we could calculate the photon efficiency in our system.

Author Response File: Author Response.pdf

Reviewer 2 Report

This work details with the fabrication of mixed metal based photocatalyst combined with Ga2O3 towards the oxidation of dyes. My comments are listed below:

To carry out this photocatalytic reaction, the author have used 2 g of catalyst in only 20 mL of dye solution. In the review’ opinion, the efficiency of this catalyst is very low compared to the common catalysts. Such a huge report of catalyst/solution volume (100g/L) does not make any sense in terms of photocatalysis for water purification and technology transfer. Simply, 0.5 g/L of TiO2 can degradate MB under the mentioned conditions.
On top of that, in page 8, the author stated: ‘during the catalytic process, the LM changed from its original binding state to GaOOH’ does this change in the structure affect the photocatalytic efficiency? Especially if the catalyst is supposed to be reused.

Author Response

Referee 2: This work details with the fabrication of mixed metal based photocatalyst combined with Ga₂O₃ towards the oxidation of dyes. My comments are listed below:

1. To carry out this photocatalytic reaction, the author have used 2 g of catalyst in only 20 mL of dye solution. In the review’ opinion, the efficiency of this catalyst is very low compared to the common catalysts. Such a huge report of catalyst/solution volume (100g/L) does not make any sense in terms of photocatalysis for water purification and technology transfer. Simply, 0.5 g/L of TiO2 can degradate MB under the mentioned conditions.

Reply: Many thanks to reviewer for their very professional advice. We are sorry that it is not clearly explained and hope that the corrections will meet with approval. We apologize for our poor data recording.

In fact, at the beginning of the experiment, we added 2g liquid metal catalyst to 20ml solution. After the ultrasound, we found that a part of liquid metal became into micron droplets which suspended in the solution, and a large amount of liquid metal precipitated at the bottom of the solution. Therefore, we reduced the quality of liquid metal in subsequent experiments. We re-added 0.1g liquid metal catalyst to 100ml solution for catalytic experiment. The results showed that organic dyes could also be degraded.

We have corrected the inadequacies in the paper.

Original: Page 3 in manuscript

Take 20 ml methylene blue/Congo red solution, and adding 2 g of Ni/Cu/W/Mo-LM composite catalyst to the reactor. Ultrasound for 30 minutes without light to make the surface of catalyst reach the adsorption-desorption equilibrium.

Modified: Page 3 in revised paper

Take 100 ml methylene blue/Congo red solution, and adding 0.1 g of Ni/Cu/W/Mo-LM composite catalyst to the reactor. Ultrasound for 30 minutes without light to make the surface of catalyst reach the adsorption-desorption equilibrium.

Original: Page 9 in manuscript

In 20 mL methylene blue solution (20 mg/L), 2g liquid metal catalyst was ultrasonic treated to obtain micro/nano catalyst, to make the catalyst have as much area as possible in the solution.

Modified: Page 9 in revised paper

In 100 mL methylene blue and Congo red solution (20 mg/L), 0.1 g liquid metal catalyst was ultrasonic treated to obtain micro/nano catalyst, to make the catalyst have as much area as possible in the solution.

2. On top of that, in page 8, the author stated: ‘during the catalytic process, the LM changed from its original binding state to GaOOH’ does this change in the structure affect the photocatalytic efficiency? Especially if the catalyst is supposed to be reused.

Reply: Thanks again to reviewer for their professional advice. Regarding your questions, we answer as follows.

In page 8, a part of the LM changed from its original binding state to GaOOH during the catalytic process, which caused the loss of part of LM. In fact, this experimental phenomenon has also been reported in other literature of liquid metal catalysts [1]. However, after catalysis, most liquid metal catalysts still maintain the existence of liquid metal alloy, and could be transformed into nano-state again by ultrasound. Therefore, it could be further recycled and not affect the reuse of most catalysts.

The revised version is as follows:

Original: Page 8 in manuscript

It can be concluded that during the catalytic process, the LM changed from its original binding state to GaOOH due to the corresponding chemical reaction between Ga and water, resulting in the formation of spongy sediments.

Modified: Page 8 in revised paper

It can be concluded that during the catalytic process, a part of the LM changed from its original binding state to GaOOH due to the corresponding chemical reaction between Ga and water, which caused the loss of part of LM, and resulting in the formation of spongy sediments.

In fact, this experimental phenomenon has also been reported in other literature [29]. However, after 20 hours of catalysis, most liquid metal catalysts still maintain the existence of liquid metal alloy, and could be transformed into nano-state again by ultrasound.

[1] Yue Lu, Yiliang Lin, Zhaowei Chen, Quanyin Hu, Yang Liu, Shuangjiang Yu, Wei Gao, Michael D. Dickey, and Zhen Gu, Enhanced Endosomal Escape by Light-Fueled Liquid-Metal Transformer, Nano letter, 2017, 17, 2138–2145.

Author Response File: Author Response.pdf

Reviewer 3 Report

The introduction should be improved with citation of previously published works on photocatalytic liquid metals and the indication of novelty of this work.

The structure and surfaces characteristics of materials should be described in more detailed. The average particles size of Me (nanoparticles as in the text) after catalytic performance should be calculated. The time of photocatalytic reactions should be indicated (figure 5 b).

The surface properties or the physicspectral properties of dopants influence the photocatalytic performance? This should be explained and proven.

Description in the experimental section (methods) mismatches with a text especially in part of photocatalytic experiment

Time dependent degradation cures (C/Co or ln dependence as shown in figure 7 i) show be presented.

Reaction times in the experimental are 2-18 h, figure 7 says about 65 h under UV?

Figure 2 capture is not informative, figure 7 is hard to understand (English editing would probably help).

The degradation efficiency of prepared samples should be compared to published data

Overall view is like the manuscript was uncarefully prepared.

Author Response

1. The introduction should be improved with citation of previously published works on photocatalytic liquid metals and the indication of novelty of this work.

Reply: Many thanks to reviewer for their professional advice. In general, we have rewritten and improved the introduction part, and added previously published works (references [1-34]) on photocatalytic liquid metals, and the novelty of this work.

Original: Page 1-2 in manuscript With the rapid development of society, energy crisis and environmental pollution have become increasingly prominent. Photocatalytic energy could directly convert solar energy into chemical energy; besides, the reaction conditions were mild. It shows great application prospects in the field of energy and environmental protection. With growing global attention to environmental remediation, the use of light driven metal composite photocatalysts to decompose pollutants has attracted significant interest. …… Modified: Page 1-2 in revised paper

With the rapid development of society, energy crisis and environmental pollution have become increasingly prominent. Photocatalytic energy could directly convert solar energy into chemical energy; besides, the reaction conditions were mild. It shows great application prospects in the field of energy and environmental protection. With growing global attention to environmental remediation, the use of light driven metal composite photocatalysts to decompose pollutants has attracted significant interest. Liquid metals are metals that are liquid at room temperature [1]. They are represented by Gallium and its alloys (GaIn alloy or GaInSn Eutectic alloy) [2-3]. They are usually low melting point alloys (-19 ℃), and have high flexibility, variable shape, high electrical conductivity, high thermal conductivity, liquidity, high surface tension and flexibility, non-toxicity, and other characteristics [4]. Liquid metal photocatalysts were based on non-toxic gallium-based alloys [5, 6]. Catalysts prepared by liquid metals and their compounds have great potential in starting, accelerating chemical reactions, and improving product yield [7, 8]. Unlike molecular liquids and ionic liquids, liquid metals exhibited mobile metal cations in free electrons at room temperature. Thus, the catalytic reaction of liquid metals was different from traditional solid-phase catalytic reactions [9, 10]. Catalysts based on liquid metal system could increase the selectivity and stability of reaction, which could significantly improve the catalytic performance [11-13]. …… To the best of our knowledge, in the past, most liquid metal catalysts were studied by single catalysts, bimetallic and polymetallic liquid metal catalysts were rarely reported. While the catalytic performance of polymetallic catalysts was usually better than that of mono-metal catalysts. Further functionalization, green and high efficiency of liquid metal catalysts were a key challenge in this field. By doping different metals particles into liquid metal, the activity of the catalyst maybe improved. Nevertheless, none of the studies have investigated how different metal nanoparticles affect the structure and basic charge of liquid metals. Which metal particles have the best effect on liquid metal catalytic activity, has not been intensive studied. Herein, therefore, in this paper, a new type of liquid metal multiphase composite photocatalyst was developed, which provided a new idea for the development of traditional LM photocatalyst. This shell-core structure was used as photocatalyst for photocatalytic degradation of pollutants. Herein, liquid metal catalysts are attempted to be loaded with different metal nanoparticles (Cu/W/Mo/Ni) and combined with Ga₂O₃ to improve functionalization. The synthesis and photocatalytic reaction details of liquid metal-Ga₂O₃ composite catalysts were studied. The methods for improving its catalytic activity were studied, and the basic mechanism of liquid metal catalysts and their applications as photocatalysts were discussed. The effect of the composite liquid metal catalyst on the degradation of pollutants was analyzed.

2. The structure and surfaces characteristics of materials should be described in more detailed. The average particles size of Me (nanoparticles as in the text) after catalytic performance should be calculated. The time of photocatalytic reactions should be indicated (figure 5 b).

Reply: Thanks for your kind advice. (1) The structure and surfaces characteristics of materials were re-described and rewritten in the manuscript. The revised version is as follows:

Original: Page 4-5 in manuscript The surface of the catalysts showed regular rough cluster structures with different sizes (Figure 2 (a-c)). In all the samples (Cu/W/Mo/Ni-LM), there were granular mate-rials having rough and uneven patch covering a subspherical surface (Figure 2 (d, e)). When the oxide particles were large, the oxides on the catalyst surface could accumulate (Figure 2 (e)). The results show that there were different degrees of gallium oxides (Ga₂O₃) on the surface of the catalysts. The Ga₂O₃ was the shell layer, and the pure liquid metal was the core of the sphere. ……

Modified: Page 4-5 in revised paper Figure 2 illustrated the scanning micrographs and morphology of core-shell Cu/W/Mo/Ni-LM catalyst samples. In all the Cu/W/Mo/Ni-LM samples, these samples were mainly spherical structures, and have micro/nano sizes with sizes distribution ranging from 5 to 20 μm. Figure 2 (a) was the SEM image of Ni-LM micro-particle, on the spherical surface of the Ni-LM catalyst, there are small particles of nano Ni. Figure 2 (b) was The SEM images of Mo-LM particles. The figure show that there were different degrees of gallium oxides (Ga₂O₃) on the surface of the catalysts. The gray rough shell layer was the Ga₂O₃, and the white and smooth core of the sphere was pure liquid metal. Figure 2 (c-g, i) was the SEM of W-LM catalyst with a draped and viscoelastic oxide layer. The surface of the W-LM catalysts showed regular rough cluster structures with different sizes. There were also granular materials (Ga₂O₃) which having rough and uneven patch covered a subspherical surface. When the oxide particles were large, the oxides on the catalyst surface could accumulate (Figure 2 (d, e)). Figure 2 (h) was the SEM images of Cu-LM catalysts. There are also small Cu particles on the spherical surface of the Cu-LM catalyst. Moreover, under prolonged exposure to ultrasonic waves, the scale of some liquid metal catalysts with core-shell structure could reach to nanoscale (about 250 nm), as shown in Figure 3(a).The composition of micro/nano LM catalysts was determined by energy-dispersive spectroscopy (EDS). Figure 3(a-d) shows the element distribution of Cu, W, Mo, Ni, Ga, In and O in the Cu/W/Mo/Ni -LM sample, respectively. It can be clearly seen that in the micron scale, Cu, W, Mo and Ni existed on the surface of the spherical catalyst, and the central part of the catalyst were also occupied by Cu, W, Mo and Ni, respectively. The core of the particle is also occupied by all elements of the liquid metal catalyst (Ga and In). EDS data indicate that particles usually have core-shell structures with different element distributions. It shows that the four catalysts all have core-shell heterostructure.

(2) After photocatalysis of 20 hours, brown solid precipitates were obtained by the photocatalysis of methylene blue solution. These brown sponges were obtained at the bottom of solution. Thus, the average particles size of LM after catalytic performance was difficult to obtain. It could be concluded that during the catalytic process, a part of the LM changed from its original binding state to GaOOH and resulting in the formation of spongy sediments. However, after 20 hours of catalysis, most liquid metal catalysts still maintain the existence of liquid metal alloy, and could be transformed into nano-state again by ultra-sound.

(3) The time of photocatalytic reactions in figure 5 b was 20 hours. And we have added this information into the manuscript (Page 7).

3. The surface properties or the physic spectral properties of dopants influence the photocatalytic performance? This should be explained and proven.

Reply: Many thanks to reviewer for their professional advice. The surface properties or the physic spectral properties of dopants exact influence the photocatalytic performance.

According to literature [1], the physic spectral properties of Ag-doped liquid metal (BiInSn alloy) were different with pure liquid metal (BiInSn alloy), such as XPS valence band and UV-Vis spectra. The Ag-doped liquid metal does not change the conduction band (CB), but their valence band (VB). The valence band of the Ag-doped liquid metal favors generating OH radicals (2.72 V) and therefore those samples may exhibit higher activity if the dyes are degraded through this mechanism. In comparison to the pure liquid metal particles, metals (such as Ag, Ni) show a higher photocatalytic degradation activity towards Congo red. Thus, metal-doped liquid metal improves the catalytic activity of metal.

The revised version is as follows:

Modified: Page 11 in revised paper

By adding other noble metals (Cu/W/Mo/Ni) to liquid metals and modify or dissolve them, intermetallic compounds were formed. The electronic structure of supported liquid metal catalysts is changed, so that their catalytic performance in reduction or oxidation reactions is significantly improved. According to literature [31], the physic spectral properties of Ag-doped liquid metal (BiInSn alloy) were different with pure liquid metal. The Ag-doped liquid metal does not change the conduction band (CB), but their valence band (VB). The valence band of the Ag-doped liquid metal favors generating OH radicals (2.72 V) and therefore those samples may exhibit higher activity if the dyes are degraded through this mechanism. In comparison to the pure liquid metal particles, metallic nanoparticles, such as Ni, Cu, W, Mo, Ag, etc., could utilize surface plasma effect to increase visible light absorption, metals (such as Ag) show a higher photocatalytic degradation activity towards Congo red. Thus, metal-doped liquid metal improves the catalytic activity of metal.

The mechanism of liquid metal photocatalytic decomposition of organic pollutants is mainly caused by Ga₂O₃ semiconductor on its surface. According to KK Z et al., gallium oxide has the advantage of providing photogenerated charge, and the band gap is reported to be 4.2~4.9 eV [29]. As the outer skin of the catalyst has solid Ga₂O₃ semiconductor, and the inner core is a reductive liquid metal environment (Cu/W/Mo/Ni mixed Ga metal), it has high density oxygen vacancy. Under UV irradiation, a considerable number of electron-hole pairs are generated in the Ga₂O₃ semiconductor, and electrons can be transferred to the core of Cu/W/Mo/Ni mixed Ga liquid metal to achieve effective charge separation, thus accelerating photocatalysis and promoting the decomposition of organic pollutants.

[1] Shuhada A. Idrus-Saidi, Jianbo Tang, Mohammad B. Ghasemian, Jiong Yang, Jialuo Han, Nitu Syed, Torben Daeneke, Roozbeh Abbasi, Pramod Koshy, Anthony P. O'Mullane and Kourosh Kalantar-Zadeh, Liquid metal core-shell structures functionalized via mechanical agitation: The example of Field's metal, J. Mater. Chem. A, 2019, 7, 17876-17887.

4. Description in the experimental section (methods) mismatches with a text especially in part of photocatalytic experiment.

Reply: Thanks for your comments. We have rewritten the experimental section (methods), and make methods matches with the text of photocatalytic part. The revised version is as follows:

Original: Page 3 in manuscript

The composite LM photocatalyst was used for degradation of organic dyes. The methylene blue/Congo red solution (20 mg/L) were prepared. Take 20 ml methylene blue/Congo red solution, and adding 2 g of Ni/Cu/W/Mo-LM composite catalyst to the reactor. Ultrasound for 30 minutes without light to make the surface of catalyst reach the adsorption-desorption equilibrium. Xenon lamp (simulated sunlight) was used as the light source. The distance between the reactor and light source was 5 cm, and the reaction time was 2~18 hours.

Modified: Page 3 in revised paper

The composite LM photocatalyst was used for degradation of organic dyes. The methylene blue/Congo red solution (20 mg/L) were prepared. Take 100 ml methylene blue/Congo red solution, and adding 0.1 g of Ni/Cu/W/Mo-LM composite catalyst to the reactor. Ultrasound for 30 minutes without light to make the surface of catalyst reach the adsorption-desorption equilibrium. The initial concentration of dyes was 20 mg/L, and the volume was 20 ml. The catalytic reaction time ranges from 0 to 65 hours, and the maximum reaction time is up to 65 hours. The concentration of catalyst was 0.1 g/ml.

Ultraviolet light was used as the light source. The photo-reactor was an ultraviolet light lamp with a wide wave of 340~400 nm, and a peak of 365 nm. The structure was ultraviolet lamp with T5-BL/BLB model. The power was 10 W, and the length of the lamp was 328 mm, the diameter of the ultraviolet lamp was 15 mm, and the whole length of the lamp frame was 350 mm. This UV lamp has a single tube, and only one lamp, with external 220V power supply. The distance between the reactor and light source was 5 cm, and the reaction time was 0~65 hours.

5. Time dependent degradation cures (C/Co or ln dependence as shown in figure 7 i) show be presented.

Reply: Thanks for your kind advice. We have modified the figure 7 i and show the C/Co and ln dependence.

As four liquid metal catalysts Cu-LM, W-LM, Mo-LM, Ni-LM, the degradation efficiency of methylene blue was 91.378 %, 85.608 %, 53.769 %, and 92.000 %, respectively. Since Ni-LM catalyst has the best degradation effect on methylene blue, we focused on the degradation process of Ni-LM for many times at different degradation times, and carried out kinetic analysis. The time dependent degradation cures of other Cu/W/Mo-LM catalysts were not the focus of our study. We have shown the Ct/Co and ln dependence.

6. Reaction times in the experimental are 2-18 h, figure 7 says about 65 h under UV?

Reply: Thanks for your kind advice. We have modified the reaction time to make them consistent. The revised version is as follows:

Original: Page 3 in manuscript

Xenon lamp (simulated sunlight) was used as the light source. The distance between the reactor and light source was 5 cm, and the reaction time was 2~18 hours.

Modified: Page 3 in revised paper

7. Figure 2 capture is not informative, figure 7 is hard to understand (English editing would probably help).

Reply: Thanks for your comments. We have modified the figure capture of figure 2. And we have rewritten the description of the experimental results for figure 7. The revised version is as follows:

Original: Figure 2 Page 5 in manuscript Figure 2. (a-c) The SEM of oxidized LM with a draped and viscoelastic oxide layer; (d-i) SEM images of different Cu/W/Mo/Ni-LM particles prepared by the ultrasonication method. Modified: Figure 2 Page 5 in revised paper

Figure 2. SEM images of different Cu/W/Mo/Ni-LM particles prepared by the ultrasonication method: (a) The SEM images of Ni-LM particles; (b) The SEM images of Mo-LM particles; (c-g, i) The SEM of oxidized W-LM with a draped and viscoelastic oxide layer; (h) The SEM images of Cu-LM particles.

Original: Figure 7 Page 10 in manuscript Figure 7. The degradation rates of Cu/W/Mo/Ni-LM catalysts: (a) Catalytic degradation of meth-ylene blue by adding Cu/W/Mo/Ni-LM for 65 h; (b) Degradation efficiency of Cu/W/Mo/Ni-LM; (c)(d) Degradation curve and degradation efficiency of congo red by adding Cu/W/Mo/Ni-LM for 20 h; (e)(f)(g)(h)(i) degradation rate after adding Ni-LM for 5 h.

Modified: Figure 7 Page 10 in revised paper Figure 7. The degradation rates of Cu/W/Mo/Ni-LM catalysts: (a-b) The Degradation curve and catalytic degradation efficiency of methylene blue by adding Cu/W/Mo/Ni-LM for 65 h; (c-d) Degradation curve and degradation efficiency of congo red by adding Cu/W/Mo/Ni-LM for 20 h; (e-f) Degradation curve of methylene blue after adding Ni-LM for 5 hours and 18 hours, respectively; (g-h) Degradation curve of methylene blue after 2h photocatalytic degradation with Ni-LM and Mo-LM catalyst, respectively; (i) The kinetic of degradation rate of Ni-LM for 5 hours.

8. The degradation efficiency of prepared samples should be compared to published data

Reply: Thanks for your comments. In our paper, the degradation efficiency of methylene blue by the four liquid metal catalysts Cu-LM, W-LM, Mo-LM, Ni-LM were 91.378 %, 85.608 %, 53.769 %, and 92.000 %, respectively. And the degradation efficiency of Congo red by the four liquid metal catalysts were 79.4%~86.2 %. Compared with the reported literatures [1], the degradation rate of Ag-doped liquid metal (BiInSn) to Congo red and methylene blue were only 44% and 65%, respectively. Compared to published data, the degradation efficiency of prepared samples has better degradation efficiency.

The revised version is as follows:

Original: Page 9-10 in manuscript The Cu-LM catalyst has the best catalytic degradation effect on Congo red, and the degradation rate of Cu-LM catalyst can reach 86.2%. ……

Modified: Page 9-10 in revised paper

The Cu-LM catalyst has the best catalytic degradation effect on Congo red, and the degradation rate of Cu-LM catalyst can reach 86.2%. Compared with the reported literatures [31], the degradation rate of Ag-doped liquid metal (BiInSn) to Congo red and methylene blue were only 44% and 65%, respectively. Compared to published data the degradation efficiency of prepared samples has better degradation efficiency.

9. Overall view is like the manuscript was uncarefully prepared.

Reply: Thank you very much for your kind advice. According to your suggestion, we have made the modification to our manuscript. Regarding your kindhearted and significant suggestion, we have modified the language and content in this manuscript, which we hope meet with approval.

Author Response File: Author Response.pdf

Reviewer 4 Report

1) Enter more references and citations, because the field is very wide, and you have a lack of references in the paper.

2) Enter newer references from the last 4 years, 2018-2021, because they are almost completely missing (only three new quotes).

3) I could not find the citations of the references [5-7].

4) The introduction should be developed.

5) In order to increase the quality of the work, figure 1 needs an increase in resolution.

6) Write the equation (1) balanced: Ga + 2H2O → 3H + GaOOH

7) Enter a section “4-Discussions” before the conclusions, in which you briefly relate everything that the new work brings, compare the new results obtained with those already known from the already existing works.

Author Response

1. Enter more references and citations, because the field is very wide, and you have a lack of references in the paper.

Reply: Thanks for your kind advice. We have added more recent references [1-34]. And we have cited the relevant references in revised manuscript. And we have revised the order of the references.

Original: Page 12 in manuscript

Jin C, Zhang J, Li X K, Yang X Y, Li J J and Liu J Sci. Rep. 2013 3 3442.
Liang S T, Liu J Chem Eur J. 2018 24 17616.
Zhang Q, Gao Y and Liu J Appl. Phys. 2014 116 1091.

……

Lu Y, Lin Y, Chen Z W, Hu Q Y, Liu Y, Yu S J, Gao Wei, Dickey M D and Gu Z Nano let. 2017 17 2138-2145.
Lu Y, Lin Y, Pacardo D B, Wang C, Sun W, Ligler F S, Dickey M D and Gu Z Nat. Commun. 2015 6 10066.

Modified: Page 12 in revised paper

1. Ren Y., Sun X. Y., and Liu J. Micromachines 2020 11 20. 2. Elbourne A., Cheeseman S., Zavabeti A., Dickey M. D., Zadeh K. K., Truong V. K. ACS Nano 2020 14 802-817. 3. Choi Y Y, Dong H H, and Cho J H ACS Applied Materials & Interfaces 2020 8 9824-9832. 4. He S, Zhou C X, Chen H L, Su X and Hanc T 2020 8 3553-3561. 5. Yuan T B, Hu Z, Zhao Y X, Fang J J, Lv J, Zhang Q H, Zhuang Z B, Gu L, Hu S Nano Lett. 2020 20 29166-2922. 6. Wang L, Gao Y F, Wang X J, Cai R X, Chung C C, lftikhar S, Wang W, Li F X ACS Catal. 2021 11 10228-10238. 7. Jin C, Zhang J, Li X K, Yang X Y, Li J J and Liu J Sci. Rep. 2013 3 3442. …… 30. Lu Y, Lin Y, Pacardo D B, Wang C, Sun W, Ligler F S, Dickey M D and Gu Z Nat. Commun. 2015 6 10066. 31. Idrus-Saidi S A, Tang J B, Ghasemian M B, Yang J, Han J L, Syed N, Daeneke T, Abbasi R, Koshy P, O'Mullane A P, and Zadeh K K, J. Mater. Chem. A 2019 7 17876-17887.32. Yang N L, Gong F, Zhou Y K, Hao Y, Dong Z L, Lei H, Zhong L P, Yang X Y, Wang X W, Zhao Y , Liu Z, Cheng L, Biomaterials 2021 277 121125. 33. Zeng M Q, Li L Y, Zhu X H, Fu L Acc. Mater. Res. 2021 2 669–680. 34. Zou Z X, Liang J W, Zhang X H, Ma C, Xu P, Yang X, Zeng Z X S, Sun X X, Zhu C G, Liang D L, Zhuang X J, Li D, and Pan A L, ACS Nano 2021 15 10039−10047.

2. Enter newer references from the last 4 years, 2018-2021, because they are almost completely missing (only three new quotes).

Reply: Thanks for your kind advice. We have added newer references from the last 4 years, 2018-2021. The words are marked red in the modified versions. The same answer in question 1.

3. I could not find the citations of the references [5-7].

Reply: Thanks for your comments. Please forgive our mistake, we have checked the manuscript carefully and thoroughly in this paper to avoid such mistakes. Regarding your good suggestion, we have checked and adjusted the references. We have added the references [5-7] in our revised paper. And we also revised the order of the references.

4. The introduction should be developed.

Reply: Thank you for your carefully review. Regarding your kindhearted and significant suggestion, we have modified the language and content in introduction section as shown below. In general, we have rewritten and improved the introduction part, and added previously published works on photocatalytic liquid metals, and the novelty of this work.

To the best of our knowledge, in the past, most liquid metal catalysts were studied by single catalysts, bimetallic and polymetallic liquid metal catalysts were rarely reported. While the catalytic performance of polymetallic catalysts was usually better than that of mono-metal catalysts. Further functionalization, green and high efficiency of liquid metal catalysts were a key challenge in this field. By doping different metals particles into liquid metal, the activity of the catalyst maybe improved. Nevertheless, none of the studies have investigated how different metal nanoparticles affect the structure and basic charge of liquid metals. Which metal particles have the best effect on liquid metal catalytic activity, has not been intensive studied. Herein, therefore, in this paper, a new type of liquid metal multiphase composite photocatalyst was developed, which provided a new idea for the development of traditional LM photocatalyst. This shell-core structure was used as photocatalyst for photocatalytic degradation of pollutants. Herein, liquid metal catalysts are attempted to be loaded with different metal nanoparticles (Cu/W/Mo/Ni) and combined with Ga₂O₃ to improve functionalization. The synthesis and photocatalytic reaction details of liquid metal-Ga₂O₃ composite catalysts were studied. The methods for improving its catalytic activity were studied, and the basic mechanism of liquid metal catalysts and their applications as photocatalysts were discussed. The effect of the composite liquid metal catalyst on the degradation of pollutants was analyzed.

5. In order to increase the quality of the work, figure 1 needs an increase in resolution.

Reply: Thanks for your kind advice. We have increased the resolution of Figure 1.

6. Write the equation (1) balanced: Ga + 2H2O → 3H + GaOOH

Reply: Please forgive our small mistake, we have rewritten the equation. We have checked the manuscript carefully and thoroughly in this paper to avoid such mistakes and mistypes.

Original: Page 8 in manuscript Ga+H₂O=GaOOH

(1)Modified: Page 8 in revised paper

2Ga+4H₂O=2GaOOH+3H₂ (1)

7. Enter a section “4-Discussions” before the conclusions, in which you briefly relate everything that the new work brings, compare the new results obtained with those already known from the already existing works.

Reply: Thank you very much for your suggestion, we are sorry that it is not clearly explained. We have added the 4-Discussion section in the revised paper.

Original: Page 11 in manuscript _________

Modified: Page 11 in revised paper

Discussions

By adding other noble metals (Cu/W/Mo/Ni) to liquid metals and modify or dissolve them, intermetallic compounds were formed. The electronic structure of supported liquid metal catalysts is changed, so that their catalytic performance in reduction or oxidation reactions is significantly improved. According to literature [31], the physic spectral properties of Ag-doped liquid metal were different with pure liquid metal. The Ag-doped liquid metal does not change the conduction band (CB), but their valence band (VB). The valence band of the Ag-doped LM favors generating OH radicals (2.72 V) and therefore those samples may exhibit higher activity if the dyes are degraded through this mechanism. In comparison to the pure liquid metal particles, metallic nanoparticles, such as Ni, Cu, W, Mo, Ag, etc., could utilize surface plasma effect to increase visible light absorption, metals (such as Ag) show a higher photocatalytic degradation activity towards Congo red. Thus, metal-doped liquid metal improves the catalytic activity of metal.

The mechanism of liquid metal photocatalytic decomposition of organic pollutants is mainly caused by Ga₂O₃ semiconductor on its surface. According to KK Z et al., gallium oxide has the advantage of providing photogenerated charge, and the band gap is reported to be 4.2~4.9 eV [23]. As the outer skin of the catalyst has solid Ga₂O₃ semiconductor, and the inner core is a reductive liquid metal environment (Cu/W/Mo/Ni mixed Ga metal), it has high density oxygen vacancy. Under UV irradiation, a considerable number of electron-hole pairs are generated in the Ga₂O₃ semiconductor, and electrons can be transferred to the core of Cu/W/Mo/Ni mixed Ga liquid metal to achieve effective charge separation, thus accelerating photocatalysis and promoting the decomposition of organic pollutants.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I have reviewed the modified manuscript and in my opinion the authors answered with total criteria my observations.

The manuscript was improved and I think that it can be published.

Author Response

Referee 1: I have reviewed the modified manuscript and in my opinion the authors answered with total criteria my observations.The manuscript was improved and I think that it can be published.

Reply: Thanks so much for your support and reviewers’ valuable comments on our paper. We are grateful for the professional and kind suggestions of the reviewer. These professional suggestions helped us clarify the logic and improve the quality of the article. Please help us convey our respect to the reviewers. Thanks them again.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have addressed the comments properly. However, the manuscript should carefully checked to reduce the grammatical errors.

Author Response

Referee 2: The authors have addressed the comments properly. However, the manuscript should carefully check to reduce the grammatical errors.

Reply: We would like to thank the reviewer for carefully and kindly commenting the manuscript. Thank you very much for your time and positive feedback and valuable comments.

We have closely followed your kind reminding and suggestions to further improve the grammar of our paper. We have checked the whole manuscript and improved the language of the manuscript. Thanks for your suggestions.

Author Response File: Author Response.docx

Reviewer 4 Report

Revised version of the paper is suitable for production, after its final publisher check and arrangements.

Author Response

Referee 4: Revised version of the paper is suitable for production, after its final publisher check and arrangements.

Reply: Thanks for the carefully review and valuable suggestion.We have checked the manuscript carefully and thoroughly in this paper to avoid mistakes. We have also checked the whole manuscript and improved the language of the manuscript. Thanks for your suggestions.

Author Response File: Author Response.docx