Synthesis and Optical Properties of Red Carbon@(NH₄)₃ZnCl₅ Hybrid Heterostructures

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Mesquita and coworkers reported the synthesis and characterization of hybrid heterostructures composed of red carbon and (NH4)3ZnCl5 perovskite. This reviewer finds the work well-presented, and the research could appeal to material science fields. Thus, this reviewer thinks the manuscript could be accepted after commenting on some minor points:

• Figure 1 is not clear, so this reviewer recommends that the authors include the step where red carbon is added. Furthermore, the figure needs to include section B) in the caption.
• On page 5, line 188. The manuscript says,” The observed profile presents three regions of intense diffractions, centered respectively at 19.5, 25.1 e 33.2º2𝜃. The diffraction peak observed at 44.1º 2𝜃 originates from the diffraction of the aluminium sample holder”. This reviewer does not understand what the symbol “°” is doing in the paragraph and the letter “e”. This mistake is all over the manuscript, and I recommend that the author correct that.
• I recommend the authors to use “[]” when referring to metal complexes
• The authors form an impurity during the synthesis of the material, (NH4)2ZnCl4. What can the authors comment on the role of this impurity in the luminescence?

Author Response

Mesquita and coworkers reported the synthesis and characterization of hybrid heterostructures composed of red carbon and (NH4)3ZnCl5 perovskite. This reviewer finds the work well-presented, and the research could appeal to material science fields. Thus, this reviewer thinks the manuscript could be accepted after commenting on some minor points:

• Figure 1 is not clear, so this reviewer recommends that the authors include the step where red carbon is added. Furthermore, the figure needs to include section B) in the caption.

R: We appreciate the reviewer’s observation. The scheme has been revised to include the step involving the addition of red carbon, as requested. We also apologize for the omission of the caption for section B of Figure 1, which has now been properly included in the revised version of the manuscript.

• On page 5, line 188. The manuscript says,” The observed profile presents three regions of intense diffractions, centered respectively at 19.5, 25.1 e 33.2º2?. The diffraction peak observed at 44.1º 2? originates from the diffraction of the aluminium sample holder”. This reviewer does not understand what the symbol “°” is doing in the paragraph and the letter “e”. This mistake is all over the manuscript, and I recommend that the author correct that.

R: We thank the reviewer for the valuable observation. We acknowledge that the use of the symbol “°” followed by the letter “e” in the expression “°2?” was incorrect and likely a result of formatting or typesetting issues. This error has been carefully reviewed and corrected throughout the manuscript to properly present the angle notation as simply “2θ”, depending on the context. We appreciate the reviewer’s attention to detail.

• I recommend the authors to use “[]” when referring to metal complexes

R: We appreciate the reviewer’s suggestion. However, we respectfully clarify that the systems investigated in this work do not involve metal complexes in the strict chemical sense. Therefore, the use of square brackets “[ ]” is not applicable to the nomenclature adopted in our study. (NH₄)₃ZnCl₅ is a double salt.

• The authors form an impurity during the synthesis of the material, (NH4)2ZnCl4. What can the authors comment on the role of this impurity in the luminescence?

R:  The presence of (NH₄)₂ZnCl₄ as an impurity may influence the material's luminescent properties, potentially acting as an emission center or modifying the local electronic environment. However, the refinement results indicated that the sample contained less than 1% of (NH₄)₂ZnCl₄.

Reviewer 2 Report

Comments and Suggestions for Authors

In this article authors synthesized Red Carbon from polymerization of carbon suboxide (C3O2) and its derivates that have wide range of applications in a next-generation optoelectronic devices including sensors and LEDs. The work is interesting however needs few improvements.

1. How fundamentally the red carbon is different from black carbon? Its just a different syntheis route or something technically different?
2. Authors must describe in the introduction about the various ways of Red carbon and compare the current synthesis method advantages and disadvantages.
3. The XRD should include the precursors diffraction patterns and explain.
4. FT-IT picture should also contain the functional groups responsible for the wavenumbers.
5. Authors should give a little more detailed information on the structural difference in Figure. 4 A,B. Why a distinct morphology is observed.
6. Why there is a huge shift in the UV-Vis spectra in Figure. 6a and there is a shoulder peak. Alo UV vis spectra for Red carbon alone needs to be incorporated and explained the band gap.

Author Response

Reviewer 2

In this article authors synthesized Red Carbon from polymerization of carbon suboxide (C3O2) and its derivates that have wide range of applications in a next-generation optoelectronic devices including sensors and LEDs. The work is interesting however needs few improvements.

1.How fundamentally the red carbon is different from black carbon? Its just a different syntheis route or something technically different?

R: We thank the reviewer for the important question. Red carbon and black carbon are fundamentally different in terms of synthesis, composition, and structure. Black carbon is a partially graphitic carbon-based material typically obtained via pyrolysis of organic compounds, consisting mainly of sp²-hybridized carbon networks. In contrast, red carbon is not a graphitic carbon material but rather an organic polymer with a highly ordered structure and a significant content of heteroatoms such as oxygen. These differences confer red carbon with distinct optical and electronic properties compared to black carbon.

2. Authors must describe in the introduction about the various ways of Red carbon and compare the current synthesis method advantages and disadvantages.

R: We appreciate the suggestion. We agree that a detailed description of the various forms of red carbon and a comparison of synthesis methods would be helpful to contextualize the study. However, since the primary focus of this work is on the application of red carbon in heterostructures for optoelectronic devices, we chose not to delve deeply into these aspects in the introduction. Thank you again for your consideration, and we may explore this discussion more thoroughly in future work. Carbon suboxide and other “oxycarbons” were first reported in 1873 by Brodie, during research on the effect of electric current on carbon monoxide [17].

3. The XRD should include the precursors diffraction patterns and explain.

R: We have added the diffraction patterns of the precursors in the supplementary material and the information indicated in the text “The diffraction patterns obtained for these precursor reagents are shown in Figure S2.”

4. FT-IT picture should also contain the functional groups responsible for the wavenumbers.

R: We appreciate the reviewer’s suggestion. However, we opted not to include the functional group assignments directly on the FT-IR spectrum image to avoid excessive visual clutter and maintain the clarity of the figure. Instead, the corresponding functional groups have been discussed in detail in the manuscript text to ensure proper interpretation of the spectra.

5. Authors should give a little more detailed information on the structural difference in Figure. 4 A,B. Why a distinct morphology is observed.

R: The distinct morphology can be attributed to the presence of red carbon under the specific synthesis conditions, which may influence nucleation and growth rates. Red carbon prevents the formation of needle-like crystals and promotes the development of sheet-like structures. Its dispersion in the solution and size affect the recrystallization process of chloride salts, leading to the formation of different structures and surface features, which in turn contribute to the observed morphological differences.

6. Why there is a huge shift in the UV-Vis spectra in Figure. 6a and there is a shoulder peak. Alo UV vis spectra for Red carbon alone needs to be incorporated and explained the band gap.

R: The spectrum of the (NH₄)₃ZnCl₅ sample, within the analyzed spectral range, displays a single absorption band centered at 315 nm. In contrast, the composite material, red carbon@(NH₄)₃ZnCl₅, exhibits two distinct absorption bands, centered at 386 nm and 285 nm. Given that red carbon shows strong absorption starting around 400 nm (see solid-state spectrum in Figure 2), the band at 386 nm (Figure 6a) is attributed to its electronic transitions, which may be influenced in both intensity (hyperchromic or hypochromic effects) and energy (blue- or red-shift) due to interactions with the (NH₄)₃ZnCl₅ structure. The second band, observed at 285 nm, likely arises from overlapping contributions of both components in the composite. We have updated the Figure as suggested.

“The optical properties of the (NH4)3ZnCl5 and (NH4)3ZnCl5@red carbon were investigated by UV–vis spectroscopy. Figure 6 shows the electronic spectra obtained for these materials. For the (NH4)3ZnCl5 sample, absorption begins at 400 nm with a direct band gap, estimated from the Tauc plot, of 3.6 eV. On the other hand, in the composite material, absorption begins at 700 nm, with a strong intensity from 500 nm with a maximum at approximately 386 nm. This result is similar to that observed for red carbon, but with small shifts in wavelengths due to the interaction with the (NH4)3ZnCl5 structure.  The  band gap was estimated at 2.7 eV. The intermediate value between 1.95 to 3.6 eV, observed in the composite band structure coupling, enabling electron excitation from the valence band of one material to the conduction band of the other.”

Reviewer 3 Report

Comments and Suggestions for Authors

compounds-3585010-peer-review-v1

Synthesis and Optical Properties of Red Carbon@(NH4)3Zn)Cl5 Hybrid Heterostructures

Development of novel perovskite-based heterostructures can advance the researches of high-performance perovskite-based optoelectronic devices. Here, the authors synthesize a hybrid heterostructure composed of red carbon and (NH4)3ZnCl5, and characterize its optical properties. The results show that the hybrid heterostructure owns a type-I heterojunction energy band and good optical properties. Below are a few comments.

1. In the Title, (NH4)3Zn)Cl5 should be corrected as (NH4)3ZnCl5.
2. In the Introduction, the authors state that, the ternary compounds A3BX5-based perovskite structures have rarely been studied although A3BX5 based materials will demonstrate superior properties for photonic applications. To my best knowledge, currently the most studied perovskite structure should be ABX3. Why do the authors here choose to synthesize A3BX5-based perovskite?
3. Following the second comment, what are the emergence and special demands to synthesize a hybrid heterostructure using A3BX5-based perovskite?
4. From Figure 4B, the synthesized hybrid heterostructure is crystallized. How to use it to fabricate optoelectronic devices, for example, photodetectors?
5. How do the authors calculate and determine the energy band diagram of the hybrid heterostructure, as shown in Figure 6B? Could the authors provide UPS characterization results?
6. For Figure 7, clearly the introduction of red carbon enhances the PL strength of (NH4)3ZnCl5. While, the PL of red carbon itself is relatively weak. Why could red carbon enhance the PL of (NH4)3ZnCl5?
7. I totally understand that, this work mainly focuses on the synthesis and characterization of a novel hybrid heterostructure. However, the authors are still recommended to include a short discussion on the potential utilization of it in practical applications.

Author Response

Development of novel perovskite-based heterostructures can advance the researches of high-performance perovskite-based optoelectronic devices. Here, the authors synthesize a hybrid heterostructure composed of red carbon and (NH4)3ZnCl5, and characterize its optical properties. The results show that the hybrid heterostructure owns a type-I heterojunction energy band and good optical properties. Below are a few comments.

R: We are truly grateful for the positive evaluation of the manuscript.

1. In the Title, (NH4)3Zn)Cl5 should be corrected as (NH4)3ZnCl5.

       R: We are grateful for the correction

2. In the Introduction, the authors state that, the ternary compounds A3BX5-based perovskite structures have rarely been studied although A3BX5 based materials will demonstrate superior properties for photonic applications. To my best knowledge, currently the most studied perovskite structure should be ABX3. Why do the authors here choose to synthesize A3BX5-based perovskite?

R: Thank you for your question. Our main focus is on red carbon, a “new” material, an organic semiconductor that we believe to be a very interesting material, easy to prepare and with optical properties that can be controlled through synthesis, not only by the size of the conjugated chain, but also by means of structural modifications. Since studies with this material are still rare, especially regarding its optical properties, we sought a perovskite that does not absorb in the visible region of the spectrum (wide band gap) and is easy to prepare and has relative stability under ambient conditions. The immediate options considered were tin and zinc halides, owing to their low cost, ease of synthesis, and low toxicity. In general A₃BX₅ compounds tend to exhibit higher chemical, and moisture stability than ABX₃, due to their more compact structure. In addition, A₃BX₅ structures incorporate fewer toxic cations (such as Zn²⁺ or Sn²⁺) and can be synthesized under milder conditions. Finally, A₃BX₅ compounds can adopt lower-dimensional structures (0D, 1D, or 2D), which leads to interesting quantum properties such as charge carrier confinement and strong photoluminescence.

3. Following the second comment, what are the emergence and special demands to synthesize a hybrid heterostructure using A3BX5-based perovskite?

R: Our interest is related to the development of sensors based on the optical properties of these hybrid materials, which can present tunable optical properties, better stability and greater interaction with the analytes of interest. The motivation to synthesize a hybrid heterostructure using A₃BX₅-based perovskite stems from the desire to combine the advantageous properties of both components to achieve synergistic optical or electronic behavior and improve the surface properties to increase the interaction efficiency.

4. From Figure 4B, the synthesized hybrid heterostructure is crystallized. How to use it to fabricate optoelectronic devices, for example, photodetectors?

R: The collaborating research group specializes in the development of sensors, focusing on both electrochemical mechanisms and changes in the optical properties of materials. Within this context, a wide range of applications is possible—from environmental monitoring of toxic substances such as herbicides, pesticides, and heavy metals to quality control in the food and beverage industries. There are numerous reports in the literature for the use of perovskite-based structures for sensor development. Depending on the solvent characteristics, the materials can be employed either as thin films or in suspension.

5. How do the authors calculate and determine the energy band diagram of the hybrid heterostructure, as shown in Figure 6B? Could the authors provide UPS characterization results?

R: According to the experimental procedure, first, the band gap is determined from the UV-Vis spectra obtained for the material. On the other hand, the energy of the valence band is estimated using the oxidation potential of the material. Basically, a leveling is made between the relative energy of the electron in vacuum (4.4 eV) and the standard potential of the hydrogen electrode (0 V). In our case, the value used was 4.64 eV since we used an Ag/AgCl electrode as a reference (0.2 V vs EPH)

The suggestion to use UPS is indeed valuable; however, none of our collaborating research groups currently have access to this equipment. Additionally, the resubmission deadline does not allow sufficient time to establish new collaborations or access external facilities for these analyses.

6. For Figure 7, clearly the introduction of red carbon enhances the PL strength of (NH4)3ZnCl5. While, the PL of red carbon itself is relatively weak. Why could red carbon enhance the PL of (NH4)3ZnCl5?

R: Our results indicate that the improvements in the optical properties of the material are associated with the coupling of its band structures, leading to the formation of a Type I heterostructure. A Type I heterostructure is particularly suitable for improving the photoluminescent properties of materials because it promotes the spatial confinement of electrons and holes in the same material — the one with the smallest band gap. This confinement favors radiative recombination, increasing the intensity of light emission. On the other hand, the coupling of the band structures helps to prevent carriers from dispersing to regions with effects or traps, reducing losses due to non-radiative recombination.

7. I totally understand that, this work mainly focuses on the synthesis and characterization of a novel hybrid heterostructure. However, the authors are still recommended to include a short discussion on the potential utilization of it in practical applications.

R: We have added a paragraph at the end of the manuscript to address this point: “Finally, in addition to the structural and optical characterization presented, the red carbon@(NH₄)₃ZnCl₅ hybrid heterostructure demonstrates properties that make it a promising candidate for practical applications, such as environmental monitoring, food quality control, and biomedical diagnostics. The formation of a Type I heterojunction and the observed enhancement in photoluminescence suggest strong potential for the development of optoelectronic devices—particularly sensors that rely on changes in the electrochemical or optical response of the material upon interaction with target analytes.”

Reviewer 4 Report

Comments and Suggestions for Authors

This paper need revision before acceptance. 

1. Why the authors used red carbon? How it is different from activated carbon, carbon black in terms of properties.
2. How red carbon acted as transportor for hole and electrons.
3. Chemicals purity need to be added and company name.
4. XRD is not clear. Please improve its quality. 
5. Can we use this perovskite for photovoltaic applications?
6. Authors need to provide more details about the structure of perovskite.
7. Conclusion can be refined with future perspective of this perovskite.
8. Is the perovskite stable at ambient conditions.
9. English grammar can be improved.

Author Response

1. Why the authors used red carbon? How it is different from activated carbon, carbon black in terms of properties.

R: We thank the reviewer for the important question. Red carbon and black carbon are fundamentally different in terms of synthesis, composition, and structure. Black carbon, such as activated carbon or carbon black, is a partially graphitic carbon-based material typically obtained via pyrolysis of organic compounds, consisting mainly of sp²-hybridized carbon networks. In contrast, red carbon is not a graphitic carbon material but rather an organic polymer with a highly ordered structure and a significant content of heteroatoms such as oxygen. These structural differences confer red carbon with distinct optical and electronic properties compared to conventional black carbon materials. These properties motivated its use in our study, as we aimed to explore materials with unique surface chemistry and potential for enhanced.

2. How red carbon acted as transportor for hole and electrons.

R: In a Type I heterojunction, both photogenerated charge carriers accumulate in the constituent with the narrower band gap. Following photoexcitation, holes generated in semiconductor 1 (the wider‑gap material) are filled by electrons that migrate from the valence band of semiconductor 2 (the narrower‑gap material), thereby suppressing recombination. The remaining electrons in the higher‑energy conduction band then relax into the conduction band of semiconductor 2 and, finally, into its valence band.

3. Chemicals purity need to be added and company name.

R: We thank the reviewer for the suggestion. The purity and supplier information of the chemical reagents have been added to the revised manuscript and are highlighted in yellow for easy identification.

4. XRD is not clear. Please improve its quality. 

R: We appreciate the reviewer’s comment. The XRD figure has been improved to enhance clarity and resolution. Additionally, all units in the graph have been standardized to ensure consistency and better visualization. 

5. Can we use this perovskite for photovoltaic applications?

R: No tests have been carried out to evaluate this perovskite for photovoltaic applications. At first, we are more interested in its use as light-emitting devices..

6. Authors need to provide more details about the structure of perovskite.

R: We have added an extended discussion of the perovskite studied in this manuscript.

“(NH₄)₃ZnCl₅ crystallizes in an orthorhombic system (space group Pnma) and is isostructural with various A₃BX₅-type halides, such as (NH₄)₃ZnBr₅ and CsZnI₅. Its structure features isolated [ZnCl₄]²⁻ tetrahedra and additional chloride ions, with NH₄⁺ ions occupying interstitial sites and stabilizing the framework via hydrogen bonding. The Zn²⁺ center is tetrahedrally coordinated by Cl⁻ ions. This structural arrangement is distinct from tetragonal variants like Cs₃CoCl₅. The compound forms preferentially in ammonium chloride-rich conditions, and X-ray diffraction is used to distinguish it from similar phases. Intermediate stoichiometries may yield mixed phases containing both (NH₄)₂ZnCl₄ and (NH₄)₃ZnCl₅ [30, 31].”

Asker, W., Scaife, D., & Watts, J.O. (1972). Halogen nuclear quadrupole resonance and structural relationships in some complex halides of zinc and copper. Australian Journal of Chemistry, 25, 2301-2309; Friese, K., Madariaga, G., & Breczewski, T. (1998). Tricaesium Tetraiodozincate(II) Iodide, Cs3ZnI5. Acta Crystallographica Section C-crystal Structure Communications, 54, 1737-1739; Gaffar, M.A., El-Fadl, A.A., & Anooz, S.B. (2006). Effects induced by chemical non‐stoichiometry and γ‐irradiation on the habit and unit cell parameters of ammonium tetrachlorozincate. Crystal Research and Technology, 41; Amit, M., Horowitz, A., Ron, E., & Makovský, J. (1973). Preparation and Crystal Structures of Some Compounds of the A3 BX5 Type (A = Cs, Tl, NH4, B = Mn, Fe, Co, X = Cl, Br). Israel Journal of Chemistry, 11, 749-764).

7. Conclusion can be refined with future perspective of this perovskite.

R: Thank you for the suggestion. The conclusion has been revised to include the future perspective of this perovskite, highlighting its potential in upcoming research and possible applications.

8. Is the perovskite stable at ambient conditions.

R: Perovskite is hygroscopic, but for our future studies for application in sensors, the impact is less significant compared to hybrid perovskites used in photovoltaic devices..

9. English grammar can be improved.

R: We have reviewed the entire manuscript

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Authors have satisfactorily addressed all the reviewers' comments, and hence the manuscript can be accepted for publication in its current form.

Reviewer 3 Report

Comments and Suggestions for Authors

compounds-3585010-peer-review-v2

Synthesis and Optical Properties of Red Carbon@(NH4)3ZnCl5 Hybrid Heterostructures

The authors have properly addressed the comments raised by the reviewers and therefore, the revised manuscript in its current form is now acceptable for publication. I would like to recommend it to publish as is.

Reviewer 4 Report

Comments and Suggestions for Authors

Accepted.