Stable and Efficient Photoinduced Charge Transfer of MnFe₂O₄/Polyaniline Photoelectrode in Highly Acidic Solution

Round 1

Reviewer 1 Report

In this manuscript, an important problem in photocatalysis is tried to solve, that is, photoinduced electron-hole generation, which affect the photoinduced electron-hole generation.  For inorganic photocatalytic system, according to add MnFe₂O₄ nanoparticles to form heterogeneous structure with high stability, the photoelectric conversion efficiency is effectively improved. The work is practical and logical. However, there are some problems to be further improved as well:

1. The annotation in Fig. 5 should be a calibration error. According to the manuscript, A-Fe3 is the best sample.  
2. Fig. 7 shows that pH=2 is the peak, but there are not the data under other acidic conditions. Should you express acidic environment is better or other data are not listed? If possible, please provide more abundant results in supporting information, because acidity stability is a significant aspect in this work
3. The reaction time shorter than the catalyst recombination time is very important for catalytic reaction. This work mainly explains it from the perspective of carriers transport. However, for the heterojunction structure, the separation and recombination of electron holes are the main aspects, authors should give a more in-depth explanation.
4. Enhanced light absorption should be discussed in detail. The authors can explain it from the perspective of energy bands and properties of nanoparticles.

Author Response

Reviewer 1

In this manuscript, an important problem in photocatalysis is tried to solve, that is, photoinduced electron-hole generation, which affect the photoinduced electron-hole generation.  For inorganic photocatalytic system, according to add MnFe₂O₄ nanoparticles to form heterogeneous structure with high stability, the photoelectric conversion efficiency is effectively improved. The work is practical and logical. However, there are some problems to be further improved as well:

1. The annotation in Fig. 5 should be a calibration error. According to the manuscript, A-Fe3 is the best sample.  

Thank you for your comment, there is an error in Figure 5. The A-Fe3 should have been the best sample, we have corrected this error.

1. Fig. 7 shows that pH=2 is the peak, but there are not the data under other acidic conditions. Should you express acidic environment is better or other data are not listed? If possible, please provide more abundant results in supporting information, because acidity stability is a significant aspect in this work

Indeed, this work studied photoelectrochemical performance with different  basic/ neutral/acidic solutions; therefore, we chose pH=2 for acidic, pH=7 for neutral and pH=14 for base solution. A low pH value like pH=0 causes hazardous corrosion on the overall electrochemical system and has, therefore, been removed from our plan.

1. The reaction time shorter than the catalyst recombination time is very important for catalytic reaction. This work mainly explains it from the perspective of carriers transport. However, for the heterojunction structure, the separation and recombination of electron holes are the main aspects, authors should give a more in-depth explanation.

In response, we have added the following new text:

“Recently, conductive polymers have been examined as a hole transport layer (HTL) for photocatalyst, solar cell and light-emitting-diode. Because PANi has high electric conductivity and is a p-type semiconductor, it has previously been demonstrated that PANi exhibits excellent hole extraction ability and, therefore, hole transport leading to reaction in a time shorter than the time needed for recombination of the electron-hole^{54, 55}”

Page: 9, 303-307.

1. Enhanced light absorption should be discussed in detail. The authors can explain it from the perspective of energy bands and properties of nanoparticles.

In response, we have added the following new text:

“The bandgap energy has been calculated from Tauc relation,⁴⁸ with results shown in the inset of Figure 3,”

and

“The bandgap of the heterojunction was reduced by 0.15 eV leading to higher light harvested compared with separate samples and thus enhancement the photoelectrochemical activity. The PANi@MnFe₂O₄ interface or conductive polymer/metal oxide interface can be considered to involve a strong interaction and a weak interaction. In the strong interaction, a chemical reaction has occurred at the interface region leading to reforming of chemical bonds, where some sites contain oxygen or metal vacancies and non-symmetry structures. The weak interaction is a less effective and can lead to the creation of a defect state within the interface region, which can form additional occupied or unoccupied states within the bandgap. Thus, for this reason, there is an extended bandgap by 0.15 eV”

Page: 8, lines 199-200, 217-226

Reviewer 2 Report

Colloids and Interfaces

Reviewer’s Comments and Suggestions

The paper entitled “Stable and efficient photoinduced charge transfer of MnFe₂O₄/Polyaniline photoelectrode in highly acid solution” approaches the use of PANi@MnFe₂O₄ as a catalyst in the water-splitting process. The results presented are interesting, so the notes are to clarify queries and supplement the original text.

First of all, it will be interesting to improve the quality of the XRD diffractogram (Figure 1a). In the Polianniline-MnFe₂O₄ diffractogram, the authors want to show the peak regarding PANi, however, the blue curve above the diffractogram does confirm the presence of PANi. An inset like in the Uv-visible spectra will better represent the presence of the PANi.

About the UV-visible spectra, the authors affirm that the combination of PANi and ceramic material results in a wide range of activity between 300-820 nm and, its behavior is a result f higher light harvesting. Besides that, the band gap value is the lowest for PANi@MnFe2O4. To improve the discussion about the spectroscopy results, the authors should insert an explanation of the results based on the presence of defects, such as interface defects.

The results indicated that PANi accelerated photoinduced charges in the conduction and valence bands of MnFe₂O₄ to the surface of the electrode. What is the contribution of the band gap presented by PANi@MnFe₂O₄ for this electronic transition?

The authors prepared the particles by the hydrothermal method. What are the advantages to use hydrothermal synthesis to produce MnFe₂O₄ particles? Insert an excerpt about the synthesis method in the introduction section.

At last, what is the major contribution of this research to the field of colloids and interfaces as well as catalyst aiming energy generation? Insert an excerpt comparing the research reported here from the already reported in the literature.

Author Response

Reviewer 2

Reviewer’s Comments and Suggestions

The paper entitled “Stable and efficient photoinduced charge transfer of MnFe₂O₄/Polyaniline photoelectrode in highly acid solution” approaches the use of PANi@MnFe₂O₄ as a catalyst in the water-splitting process. The results presented are interesting, so the notes are to clarify queries and supplement the original text.

First of all, it will be interesting to improve the quality of the XRD diffractogram (Figure 1a). In the Polianniline-MnFe₂O₄ diffractogram, the authors want to show the peak regarding PANi, however, the blue curve above the diffractogram does confirm the presence of PANi. An inset like in the Uv-visible spectra will better represent the presence of the PANi.

We thank the reviewer for their comment. The PANi is too low an intensity compare with MnFe₂O₄. We have added a curve representing PANi above the blue line of composite (MnFe₂O₄/Polyaniline). We have added an inset plot in Figure 1a following the reviewer’s suggestion.

About the UV-visible spectra, the authors affirm that the combination of PANi and ceramic material results in a wide range of activity between 300-820 nm and, its behavior is a result f higher light harvesting. Besides that, the bandgap value is the lowest for PANi@MnFe2O4. To improve the discussion about the spectroscopy results, the authors should insert an explanation of the results based on the presence of defects, such as interface defects.

In response, the following text has been added:

“The bandgap energy has been calculated from Tauc relation,⁴⁸ with results shown in the inset of Figure 3,”

and

“The bandgap of the heterojunction was reduced by 0.15 eV leading to higher light harvested compared with separate samples and thus enhancement the photoelectrochemical activity. The PANi@MnFe₂O₄ interface or conductive polymer/metal oxide interface can be considered to involve a strong interaction and a weak interaction. In the strong interaction, a chemical reaction has occurred at the interface region leading to reforming of chemical bonds, where some sites contain oxygen or metal vacancies and non-symmetry structures. The weak interaction is a less effective and can lead to the creation of a defect state within the interface region, which can form additional occupied or unoccupied states within the bandgap. Thus, for this reason, there is an extended bandgap by 0.15 eV”

Page: 8, lines 199-200, 217-226

The results indicated that PANi accelerated photoinduced charges in the conduction and valence bands of MnFe₂O₄ to the surface of the electrode. What is the contribution of the band gap presented by PANi@MnFe₂O₄ for this electronic transition?

This comment has been addressed by above explanation.

The authors prepared the particles by the hydrothermal method. What are the advantages to use hydrothermal synthesis to produce MnFe₂O₄ particles? Insert an excerpt about the synthesis method in the introduction section.

We thank the reviewer for their comment. We have added a brief discussion about this method in the Supporting Information:

The hydrothermal method is commonly used to prepare nanostructured materials such as nanowires, spherical nanoparticles, nanoflowers and the like. This method produces highly uniform morphology and single crystals compared with other wet chemical method such as an ion-exchange precipitation method. Growth involves a small nucleus in water solution starting to self-assembly and became solid. The process is normally carried out in an apparatus consisting of a steel pressure vessel called autoclave¹.

At last, what is the major contribution of this research to the field of colloids and interfaces as well as catalyst aiming energy generation? Insert an excerpt comparing the research reported here from the already reported in the literature.

We have added a table that compares common materials with polyaniline:

Table 2. comparison of our result with previous results.

Page: 11-12, line 327-333

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript reported to use MnFe₂O₄/polyaniline composite as photoelectrode to give good photoelectrochemical performance. The manuscript needs major revision before acceptance.

1. Why MnFe₂O₄ combines with polyaniline to give better performance? Synergy is a too general explanation. The authors should give a more insightful discussion on function of two components of the prepared composite.
2. Purity of reagents are missing.
3. It is suggested to describe the preparation steps of MnFe₂O₄, instead of citing references.
4. Typically, amount of (NH₄)₂S₂O₈ used is more than that of aniline during polymerization. In this work, why use the same molar ratio of (NH₄)₂S₂O₈ and aniline?
5. No information about FTO used is provided.
6. SEM image of MnFe₂O₄/polyaniline in Figure 2d is not very clear. A better image should be given.
7. Line 178, d-space of 0.275 nm is stated. But in Figure 2b, d₃₁₁=0.257 nm?
8. Band gap results, which are shown in the inset of Figure 3, are not clear. A clearer image should be given.
9. The pH has effects on materials’ performance. In Figure 7, only three testing solutions with different pH values were optimized, which were few. More comparison is needed.
10. Comparison with current literature work on composite materials’ photoelectrochemical performance should be provided in a separate table.

11. References should be carefully checked: some are missing journal names. Please also correct subscript errors.

Author Response

Comments and Suggestions for Authors

The manuscript reported to use MnFe₂O₄/polyaniline composite as photoelectrode to give good photoelectrochemical performance. The manuscript needs major revision before acceptance.

1. Why MnFe₂O₄ combines with polyaniline to give better performance? Synergy is a too general explanation. The authors should give a more insightful discussion on function of two components of the prepared composite.

We thank the reviewer for their comment. We have added:

“The conductive polymer plays a remarkable role in the photoelectrode, creating high charge carrier mobility, a strong interfacial interconnection between the spinel ferrite magnetic nanoparticles and the conductive polymer, thus enhancing charge transport as well as increasing stabilization of photoelectrode toward high acidic solution^{29, 30}. “

Page:3, lines 67-71

1. Purity of reagents are missing.

We thank the reviewer for their comment. We have added:

Aniline (99%), sulfuric acid, ammonium persulfate ((NH₄)₂S₂O₈) (99%), dimethylformamide (DMF) (99%), and hydrochloric acid were provided by Sigma-Aldrich. All chemicals were used without further purification. Fluorine-doped tin oxide (FTO) coated glass were purchased from Sigma Aldrich (thickness: 2 mm, sheet resistance: 15 Ω/cm²) and used as transparent conducting oxide substrate.

1. It is suggested to describe the preparation steps of MnFe₂O₄, instead of citing references.

We thank the reviewer for their comment. We have added:

“It has been prepared by hydrothermal method. Firstly 1 mmol of MnCl₂⋅4H₂O and 2 mmol of FeCl₃⋅6H₂O were dissolved in distilled water, followed by dropwise addition of a diluted NaOH solution until the color became a brownish-black suspension and the pH was adjusted to 11. The above suspension was poured into a stainless-steel autoclave 100 mL, heated to 180 ℃ for 10 h, with the resulting product washed several times with ethanol and distilled water, and thereafter, dried overnight at 60 ℃.”

1. Typically, amount of (NH₄)₂S₂O₈ used is more than that of aniline during polymerization. In this work, why use the same molar ratio of (NH₄)₂S₂O₈ and aniline?

There is no need to add more (NH₄)₂S₂O₈ which works only as a strong oxidizer for Aniline

1. No information about FTO used is provided.

We thank the reviewer for their comment. We have added:

Fluorine-doped tin oxide (FTO) coated glass were purchased from Sigma Aldrich (thickness: 2 mm, sheet resistance: 15 Ω/cm²) and used as transparent conducting oxide substrate.

1. SEM image of MnFe₂O₄/polyaniline in Figure 2d is not very clear. A better image should be given.

Figure 2d has been improved. We think the SEM image is now more clear than before.

1. Line 178, d-space of 0.275 nm is stated. But in Figure 2b, d₃₁₁=0.257 nm?

We thank the reviewer for their comment. The correct value is d₃₁₁=0.257 nm in the figure. A correction has been made in the text.

1. Band gap results, which are shown in the inset of Figure 3, are not clear. A clearer image should be given.

We have separated into Figures a and b.

1. The pH has effects on materials’ performance. In Figure 7, only three testing solutions with different pH values were optimized, which were few. More comparison is needed.

Indeed, this work studied photoelectrochemical performance with different  basic/ neutral/acidic solutions; therefore, we chose pH=2 for acidic, pH=7 for neutral and pH=14 for base solution. A low pH value like pH=0 causes hazardous corrosion on the overall electrochemical system and has, therefore, been removed from our plan.

1. Comparison with current literature work on composite materials’ photoelectrochemical performance should be provided in a separate table.

We have added a table that compares common materials with polyaniline:

Table 2. comparison of our result with previous results.

Page: 11-12, line 327-333

11. References should be carefully checked: some are missing journal names. Please also correct subscript errors.

We thank the reviewer for their comment. Corrections have been made.

Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript discussed MnFe2O4/polyaniline hybrid photoelectrode and its enhanced photoelectrochemical activity and stability. The work is interesting and there are also some characterizations to support the research work. But there are multiple questions that need to be further explained. I will suggest publishing after all these questions have been answered.

(1) There are a lot of typos in the manuscript, like Line 91 “Sanning” Scanning, line 92 “Jeol”, JEOL, line 109 “electrolye”, line 114 ”reproduciblity” , line 115” illunimination” ,line 171 photoelectode, line 177 investigted, line 179 brightensess, line 307 photocurent, line 308 accelarated, line 309 recombintion… Author needs to carefully check.

(2) For figures, there are also some typos. Figure 1 line 155, wave number should be consistent with main text wavenumber. Figure 8, it is IPCE not IPEC. Also, I will suggest figures can be consistent with same size.

(3) The title is “in highly acidic solution”. How do you define highly? Why do you want to achieve the test in an acidic environment? Is it the major findings in the manuscript? There is no explanation related to acid solution environment in the introduction.

(4) It also comes out questions in the experimental section, starting from line 273. Why do you choose pH=2,7,14? Have you tested photocurrent in other pH? Will the photocurrent peak keeps increasing at pH lower than 2? Why lower pH will have this result, it needs explanation or assumption.

(5) In the introduction part starting from line 60, PANi has been discussed but I feel only one sentence “have demonstrated enhanced photoelectrochemical (PEC) performance due to synergy between the conductive polymer and inorganic photocatalyst.” is not clear enough to explain the hybrid structure functionality, should elaborate if possible.

(6) In the experimental section, MnFe2O4 synthesis method should be explained more in detail. You mentioned it is a hydrothermal method in the abstract and conclusion, but did not show in experimental section. Two literatures 30 and 31 are not clear enough for explaining your synthesis.

(7) Why put Table 1 between SEM and XRD section? And why do you want to choose these 13,26,52mg as the samples? What will the results be if there are different ratios other than these 3?

(8) For figure 2a, does the morphology have effect on photoelectrochemical activity? Line 182 is figure 2d not 2c. What do you want to share for figure 2d?

(9) Why in figure 6, there is only comparison with A0 and A-Fe3? What do you think if adding A-Fe2 and A-Fe1?

(10) There are multiple literatures working on similar photoelectrodes. This manuscript should compare the results with literatures in introduction on results section.

Author Response

Comments and Suggestions for Authors

The manuscript discussed MnFe2O4/polyaniline hybrid photoelectrode and its enhanced photoelectrochemical activity and stability. The work is interesting and there are also some characterizations to support the research work. But there are multiple questions that need to be further explained. I will suggest publishing after all these questions have been answered.

• There are a lot of typos in the manuscript, like Line 91 “Sanning” Scanning, line 92 “Jeol”, JEOL, line 109 “electrolye”, line 114 ”reproduciblity” , line 115” illunimination” ,line 171 photoelectode, line 177 investigted, line 179 brightensess, line 307 photocurent, line 308 accelarated, line 309 recombintion… Author needs to carefully check.

We thank the reviewer for their comment. We have made corrections and highlighted the changes in the manuscript with a yellow color.

• For figures, there are also some typos. Figure 1 line 155, wave number should be consistent with main text wavenumber. Figure 8, it is IPCE not IPEC. Also, I will suggest figures can be consistent with same size.

We have been corrected the Figures.

• The title is “in highly acidic solution”. How do you define highly? Why do you want to achieve the test in an acidic environment? Is it the major findings in the manuscript? There is no explanation related to acid solution environment in the introduction.

That is a great comment. We were missed highlighting this point. We have added in the manuscript:

  “under highly acidic conditions (a high efficiency hydrogen evolution reaction normally carried out under highly acidic conditions, as represented by the proton-reduction reaction”

Page:2, lines 52-54.

“the acidic electrolyte enhances the proton transport rate and H⁺ represented by the proton-reduction reaction.”

Page:11, lines: 306-308.

• It also comes out questions in the experimental section, starting from line 273. Why do you choose pH=2,7,14? Have you tested photocurrent in other pH? Will the photocurrent peak keeps increasing at pH lower than 2? Why lower pH will have this result, it needs explanation or assumption.

Indeed, this work studied photoelectrochemical performance with different  basic/ neutral/acidic solutions; therefore, we chose pH=2 for acidic, pH=7 for neutral and pH=14 for base solution. A low pH value like pH=0 causes hazardous corrosion on the overall electrochemical system and has, therefore, been removed from our plan.

• In the introduction part starting from line 60, PANi has been discussed but I feel only one sentence “have demonstrated enhanced photoelectrochemical (PEC) performance due to synergy between the conductive polymer and inorganic photocatalyst.” is not clear enough to explain the hybrid structure functionality, should elaborate if possible.

We have elaborated in greater details and added the text below with one more citation:

“The conductive polymer plays a remarkable role in the photoelectrode, creating high charge carrier mobility, a strong interfacial interconnection between the spinel ferrite magnetic nanoparticles and the conductive polymer, thus enhancing charge transport as well as increasing stabilization of photoelectrode toward high acidic solution^{29, 30}. “

Page:3, lines 67-71

• In the experimental section, MnFe2O4 synthesis method should be explained more in detail. You mentioned it is a hydrothermal method in the abstract and conclusion, but did not show in experimental section. Two literatures 30 and 31 are not clear enough for explaining your synthesis.

For this point we have added the experimental part:

“It has been prepared by hydrothermal method. Firstly 1 mmol of MnCl₂⋅4H₂O and 2 mmol of FeCl₃⋅6H₂O were dissolved in distilled water, followed by dropwise addition of a diluted NaOH solution until the color became a brownish-black suspension and the pH was adjusted to 11. The above suspension was poured into a stainless-steel autoclave 100 mL, heated to 180 ℃ for 10 h, with the resulting product washed several times with ethanol and distilled water, and thereafter, dried overnight at 60 ℃.”

• Why put Table 1 between SEM and XRD section? And why do you want to choose these 13,26,52mg as the samples? What will the results be if there are different ratios other than these 3?

We thank the reviewer for the comment.The table was moved up to below the preparation sample. The weights of Mn₂Fe₂O₄ were chosen according to the increase in photocurrent. At lower than 13 mg of Mn₂Fe₂O₄, the low photocurrent was obtained so it was hard to track the change in photocurrent compared with pure sample (i.e without Mn₂Fe₂O₄) . At higher than 52 mg of Mn₂Fe₂O₄ it is hard to prepare stable and smooth photoelectrode (see Figure S1). Therefore these ratios were adjusted according to prior experiments.

We have added in the text:

Higher ratios of causes the Mn₂Fe₂O₄/PANi to become unstable and rough as shown in Figure S1.

Page: 9, lines:191-192

• For figure 2a, does the morphology have effect on photoelectrochemical activity? Line 182 is figure 2d not 2c. What do you want to share for figure 2d?

The photoelectrochemical activity is highly affected by morphology, the particle size and shape, which can increase the interface between the two composites. Therefore, small particles can increase the surface area and enhance the interface between the two composites leading to improved photoelectrochemical activity, while the shape of the particles is also important for photoelectrochemical activity. For example, nanowires could provide higher surface areas than spherical nanoparticles.

Line 182 is Figure 2d not 2c, which has now been corrected. Thank you for your note.

We have been shown that our photoelectrode is smooth and has no cracks and cohesion.

• Why in figure 6, there is only comparison with A0 and A-Fe3? What do you think if adding A-Fe2 and A-Fe1?

The comparison was carried out to study stability or durability between pure PANi with the highest Mn₂Fe₂O₄ composite, so those two samples are critical while A-Fe2 and A-Fe1 are normally stable if the above samples are stable. 

• There are multiple literatures working on similar photoelectrodes. This manuscript should compare the results with literatures in introduction on results section.

We have added a table that compares common materials with polyaniline:

Table 2. comparison of our result with previous results.

Page: 11-12, line 327-333

Round 2

Reviewer 3 Report

The authors made some improvements on the manuscript but the following issues still remain:

1. Only two references are compared in table 2, which are too few.

2. References should be carefully checked. Please correct subscript errors (15, 16, 24 27, 29 etc.).

Author Response

We thank again reviewer 3 for his notes

 Comments and Suggestions for Authors

The authors made some improvements on the manuscript but the following issues still remain:

1. Only two references are compared in table 2, which are too few.

We have added more references in table 2 that related to PANi.

2. References should be carefully checked. Please correct subscript errors (15, 16, 24 27, 29 etc.).

Thank you for your valuable comment, we have corrected the chemicals formula

Author Response File: Author Response.pdf

Reviewer 4 Report

The author has addressed all the questions taken from the reviewer. I recommend accepting this manuscript after the Figure 1 problem. Figure 1 still needs to be revised due to 2 figures overlapping.

Author Response

Reviewer 4

We thank reviewer 4 for his notes

Comments and Suggestions for Authors

The author has addressed all the questions taken from the reviewer. I recommend accepting this manuscript after the Figure 1 problem. Figure 1 still needs to be revised due to 2 figures overlapping.

Thank you for your note, we have separated the figure as shown in figure 1

Author Response File: Author Response.pdf