Prominent COF, g-C₃N₄, and Their Heterojunction Materials for Selective Photocatalytic CO₂ Reduction

Round 1

Reviewer 1 Report

Recommendation: Reject

Panagiota et al. synthesized several heterojunction materials composed of g-C3N4 and COF for photocatalytic CO2 reduction. Even though lots of characterizations were done, this manuscript cannot be accepted due to the low novelty. No new strategy for materials synthesis or new phenomena was observed. New interesting conclusion can be obtained. Compared with heterojunction materials, g-C3N4 even has better activity. The mechanism is not clear. The QE and isotope experiments were also missed. This is a routine work and cannot attract researchers. Besides of those questions, several concerns were also showed as following:

1.        Subscript and unit problems in Figure S10.

2.        Liquid products were not detected in this experiment.

3.        The structures of materials are not clear.

4.        Figures are not beautiful.

5.        The manuscript was not well written, and the Key points are not prominent.

6.        More important evidences should be provided to prove the successful synthesis of heterojunction materials.

7.        The active sites of materials and electron pathway are not clear.

 Extensive editing of English language required.

Author Response

Reviewer #1

Panagiota et al. synthesized several heterojunction materials composed of g-C₃N₄ and COF for photocatalytic CO₂ reduction. Even though lots of characterizations were done, this manuscript cannot be accepted due to the low novelty. No new strategy for materials synthesis or new phenomena was observed. New interesting conclusion can be obtained. Compared with heterojunction materials, g-C₃N₄ even has better activity. The mechanism is not clear. The QE and isotope experiments were also missed. This is a routine work and cannot attract researchers.

 

Response: New heterojunction materials were synthesized based on a route suggested by [10.1021/acsami.0c15780]. Crucially, both the exfoliated g-C₃N₄ and this specific COF have not been previously employed for the synthesis of heterojunction materials. Alterations in the synthesis also exist, as the substitution reaction proposed in this study, is quick (24 h), without the formation of any by products and a good yield. Compared with the heterojunction materials, g-C₃N₄ has indeed better photocatalytic activity for the 1^st day. However, the sustainability experiments showed that the 2^nd day, the hydrogen evolution reaction is preferable. A way to hinder this phenomenon is its combination with the COFs. A verification comes from the physical mixture of the two materials where H₂ production is more prominent than the CO formation. Last but not least, most of the HJ referred to in the literature, are used for H₂ production [references in the manuscript numbered 24,31,36,39] rather than CO₂ to CO conversion with H₂O.

Furthermore, this study was conducted not only to prove that g-C₃N₄ is an excellent photocatalyst, but also the framework was ensured to be active and proven to exhibit a 100% selectivity towards CO₂ to CO conversion. A new way to increase the activity of the COF is to be combined with g-C₃N₄, avoiding the introduction of heavy metals as co-catalysts and any other sacrificial agents. The mechanism will be re-written in the manuscript and explained with more details provided in question 7.

The reviewer is correct that ΑQE and isotope experiments are important, however they were not able to be accomplished as these characterization techniques are not accessible in our laboratory. Instead, control experiments were conducted to exclude the decomposition of the organic materials and to ensure the conversion and the source of CO. Even though QE is absent, the calculations of the production rates based on a batch reactor for a heterogeneous catalytic reaction in parallel with the theory and other reports, have justified results for the conversion.

 

Besides of those questions, several concerns were also showed as following:

1. Subscript and unit problems in Figure S10.

Response: The suggested corrections have been made in both Figure S9 and S10.

 

2. Liquid products were not detected in this experiment.

Response. Indeed, it is possible that other products that can be detected with liquid chromatography were formed. However, our work focuses exclusively on the formation of gas products by employing a GC directly connected to the custom made reactor.

Preliminary experiments with Liquid chromatography demonstrated the presence of a minor percent of adventitious carbon units.

 

3. The structures of materials are not clear.

Response: The structure of g-C₃N₄ as also its synthesis is presented in a previous article by Papailias and al [10.1016/j.apcatb.2018.07.078]. More information about the COF and its detailed structure can be found in the work by Bika et al [10.1039/d1tc02985a]. The heterojunction materials are based on the formation of a covalent bond between the precursor of cyanuric chloride of the COF part and the g-C₃N₄.  A repetitive unit of the HJ structure and specifically the new covalent bond was designed and displayed in Scheme 1a.

 

4. Figures are not beautiful.

Response: Some figures were changed in order to improve their quality and their representation; new indices were provided in the FTIR and XRD Figures.

 

5. The manuscript was not well written, and the Key points are not prominent.

Response: The key points were added at the end and the beginning of some sections in the manuscript, to ameliorate the clarity of the explanations and the flow of the research, as reviewer 1 indicated.

 

6. More important evidences should be provided to prove the successful synthesis of heterojunction materials.

       Response: As the reviewer explained in the beginning, lots of techniques were used to characterize the synthesis of the new heterojunction materials. Our research evaluates the photocatalytic activity and verifies the synthesis of the new materials. In lines 77-79, a chemical identification of the covalent bond and in lines 117-122, its structural identification are presented.

A morphological coherence is also in section 2.2., though the differences are not that distinguishable as the ones detected from FTIR and XRD. Moreover, the electronic and optical properties (Section 2.3.) by UV-Visible, PL, EI and the CB, VB evaluation displayed a clear pattern for the behavior of the three different weight ratios of HJ materials that fluctuate among the properties of bCOF and g-C₃N₄.

A supplementary evidence can be the increasing production rate from HJ(10:1) to HJ(1:1) and finally to HJ(1:10) indicating also the synergetic effect of COF and g-C₃N₄ depending on their ratio of the HJ materials (Section 2.4.2).

 

7. The active sites of materials and electron pathway are not clear.

 

According to theoretical DFT studies on carbon nitride materials [1,2,3], it is suggested that the nitrogen atoms are the preferred oxidation sites, whereas the carbon atoms provide the reduction sites for hydrogen production processes and water oxidation. In parallel, researchers found out that the specific reductions sites of the melem structure of g-C₃N₄ [[4] contain both carbon and nitrogen sites. Specifically, Shiraishi et al. [3,4] mention that formed e⁻ are localized at the C1 and N4 positions of the triazine ring, whereas the h⁺ are localized at the N2 and N6 positions. Recently, extended investigations of CO₂ adsorption on carbon nitride materials [5], highlight that the most preferable photocatalytic active sites for CO₂ reduction are the nitrogen atom.  However, as our heterojunction materials form new delocalized π-conjugated systems, it would be hard to separate the reduction from oxidation sites. DRIFT and DFT experiments are desired to be done to elucidate the specific active centers on the HJ catalysts’ surface responsible for the photocatalytic reaction. Such experiments could be the subject of a separate investigation that is planned to be performed in the nearest future.

The HJ catalysts are capable for the capture of CO₂ chemically and the adsorption of H₂O physically or via hydrogen bonding interaction between H₂O and N of triazine moieties [6]. With the assistance of H⁺, adsorption energy of CO₂ molecules is also enhanced [7].

The migration direction of photo-induced electrons in the heterojunction does not happen directly to the CO₂ molecules. We assume that electrons photoexcited from the HOMO covering primarily the carbon nitride part migrate to the N-triazine moieties of the COF part (LUMO) through ICT and then transfer to the adsorbed CO₂ molecules. In the meantime, holes transport to the active sites of carbon nitride and oxidize the adsorbed H₂O to form O₂.

 

 

[1] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen and M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76-80.

[2] J. Chen, S. Shen, P. Guo, P. Wu, L. Guo, Spatial engineering of photo-active sites on g-C₃N₄ for efficient solar hydrogen generation, J. Mater. Chem. A 2 (2014) 4605-4612.

[3] Y. Shiraishi, S. Kanazawa, Y. Sugano, D. Tsukamoto, H. Sakamoto, S. Ichikawa, T. Hirai, Highly Selective Production of Hydrogen Peroxide on Graphitic nCarbon Nitride (g‑C₃N₄) Photocatalyst Activated by Visible Light, ACS Catal. 4 (2014) 774-780.

[4] Y. Shiraishi, S. Kanazawa, Y. Kofuji, H. Sakamoto, S. Ichikawa, S. Tanaka, and T. Hirai, Sunlight-Driven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal-Free Photocatalysts, Angew. Chem. Int. Ed. 2014, 53, 1 – 7

[5] B. Zhu, L. Zhang, D. Xu, B. Cheng, J. Yu, Adsorption investigation of CO₂ on g-C₃N₄ surface by DFT calculation, Journal of CO₂ Utilization 21 (2017) 327–335

[6] K. Lei, Di Wang, Mi. Kou, Z. Ma, L. Wang, L. Ye and Y. Kong A Metal-free Donor-Acceptor Covalent Organic Framework Photocatalyst for Visible-Light-Driven Reduction of CO₂ with H₂O, ChemSusChem 2020, 13, 725

[7] J. Zhou, J-X. Cui, M. Dong, C-Y. Sun, S-Q You, X-L Wang, Z-Y. Zhou and Z-M. Su Synergetic effect of H⁺ adsorption and ethylene functional groups of covalent organic frameworks on the CO₂ photoreduction in aqueous solution, Chem. Commun. 2020, 56, 7261--7264

Author Response File: Author Response.pdf

Reviewer 2 Report

1.      What is the y-axis of Figure 1a and b? Is that “Intensity (a.u.)”? Further, the resolution of Figure 1a and b seems different.

2.      The resolution of Figure 2 can be improved.

3.      Figure 4 shows that both bCOF and g-C3N4 are selective for CO, however, their physical mixture (PM(1:10)) produces a significant amount of H2. What is the reason for that? What are the chemical and physical differences between the HJ(1:10) and the PM(1:10) sample?

4.      Figure 6 shows a 3-day sustainability test; however, a good catalyst should last for months or at least tens of days. What happens if longer time test is performed?

5.      g-C3N4 have been insensitively applied in photocatalysis, it is recommend to compared the catalysts presented in this work with other reported g-C3N4 based materials.

6.      There are quite a few recent studies focusing on photo/electro conversion of CO2 (e.g., DOI: 10.1021/acs.langmuir.2c01887; 10.1002/ange.202217565; 10.1016/j.cej.2023.143941; 10.1016/j.xcrp.2022.100949;), the authors are recommended to update the reference of this manuscript.

English can be slightly polished.

Author Response

Reviewer #2

1. What is the y-axis of Figure 1a and b? Is that “Intensity (a.u.)”? Further, the resolution of Figure 1a and b seems different.

Response: Indeed, the reviewer, is correct that the y-axis units represent Intensity (a.u.), hence suitable changes were made in Figures 1a and 1b. The resolution of the two Figures was set to be the same.

 

2. The resolution of Figure 2 can be improved.

Response: The micrographs in Figure 2 were renewed and the resolution was improved under the suggestion of Reviewer 2.

 

3. Figure 4 shows that both bCOF and g-C₃N₄ are selective for CO, however, their physical mixture (PM(1:10)) produces a significant amount of H₂. What is the reason for that? What are the chemical and physical differences between the HJ(1:10) and the PM(1:10) sample?

Response: The synthesis of the g-C₃N₄ photocatalyst undergoes paths that include chemical exfoliation with H₂SO₄. As Papailias et al. suggested [10.1016/j.apcatb.2018.07.078], there are remaining functional groups in its networks that contain not only H but also OH which can promote H₂ production [10.3390/catal10101147]. By reacting g-C₃N₄ with cyanuric chloride and then 4,4’ bipyridine to create the heterojunction materials, the -N-H, -O-H of the g-C₃N₄ network are sacrificed in order to form covalent bonds with the COF part.

Furthermore, the photocatalytic activity of the b-C₃N₄ displayed rates of 177.6 μmol g^-1 h^-1 H₂ and at 1.44 μmol g^-1 h^-1 CO and the sustainability experiment of g-C₃N₄ showed the evolution of the competitive hydrogen reaction. When combined in a physical mixture with the COF, Van der Waals forces arise and p-p stacking interactions that prevent the active sites responsible for the CO₂ to CO conversion, as this happens when the layers are stacked together at the bulk C₃N₄.

Moreover, when the two semiconductors are mixed, they form a type II system, where e^- and h⁺ migrate at ease. Thermodynamically, the barrier of the competitive H₂ evolution reaction is preferable to overpass as its the redox potential of H₂ to H is -0.41 eV than the one of CO₂ to CO -0.51 eV.

 

4. Figure 6 shows a 3-day sustainability test; however, a good catalyst should last for months or at least tens of days. What happens if longer time test is performed?

      Response: Generally, in the photocatalytic studies to firstly characterize the endurance of the catalyst, sustainability tests are made for a period of time 3-5 days. We agree with the reviewer that photocatalytic cycles of at least tens of days should be tested, but the short experiment can show a hint of the stability of the materials. Indeed, it is possible that the catalysts will be deactivated after longer periods, but there are easy to be gathered with precipitation methods and new ways must be investigated in order to regenerate the photocatalysts.

 

5. g-C₃N₄ have been insensitively applied in photocatalysis, it is recommend to compared the catalysts presented in this work with other reported g-C₃N₄ based materials.

Response: A comparison of the photocatalytic activity of the prepared HJ was made based on reports referencing g-C₃N₄ based materials found in the literature. It is presented at the end of section 2.4.1.

 

6. There are quite a few recent studies focusing on photo/electro conversion of CO₂ (e.g., DOI: 10.1021/acs.langmuir.2c01887; 10.1002/ange.202217565; 10.1016/j.cej.2023.143941; 10.1016/j.xcrp.2022.100949;), the authors are recommended to update the reference of this manuscript.

Response: We thank the reviewer for this reminder. To that end, the recent studies proposed by reviewer 2 were included in the reference list, as Ref.3: 10.1021/acs.langmuir.2c01887, Ref.4: 10.1016/j.xcrp.2022.100949 and Ref.14: 10.1016/j.cej.2023.143941.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors have studied the CO2 conversion to CO using COF, g-C3N4 and their heterojunction. The methods and results are very well presented and discussed. 

Some corrections need to be provided before the acceptance of the paper.

- Figures 1a and 1b, provide Y axis units 

- Index the XR diffractograms

-The SEM micrographs quality could be improved

- Please compare the band gaps obtained with literature by giving references

- The control experiment to uncover the carbon source of CO show that CO exists, is there a possibility that it comes from the decomposition of g-C3N4?

- The authors did not discuss oxidation reactions. Possible candidate of oxidation product is oxygen, how is the O2 production with the time?

- The balance between the reduction and oxidation sides in the overall CO2 reduction is important.

 -All the photocatalysis conditions should be given such as the pressure, temperature…

-Authors can show a schematic figure of their photoreactor

- Was the GC directly connected to the reactor? How were the data obtained at the different time points?

- Compare the CO prouctions obtained with other similar materials reported in literature

 

Author Response

Reviewer #3

The authors have studied the CO₂ conversion to CO using COF, g-C₃N₄ and their heterojunction. The methods and results are very well presented and discussed. 

Some corrections need to be provided before the acceptance of the paper.

Response: We thank the reviewer for the positive feedback regarding our manuscript. Below please find a point-by-point analysis of changes made in the manuscript.

 

- Figures 1a and 1b, provide Y axis units 

Response: We amended the Y axis according to the suggestions of the reviewer.

 

- Index the XR diffractograms

Response: The X-Ray diffractograms and FTIR spectra have been changed to incorporate a proper numbering for each sample, upon recommendation of the reviewer.

 

-The SEM micrographs quality could be improved

Response. The quality of the SEM micrographs is improved. 

 

- Please compare the band gaps obtained with literature by giving references

Response: At the end of Section 2.3, a comparison was made based on the values reported by the literature and the band gaps of the studied semiconductors, as following: Compared to reported heterojunction materials, the band gap of the pristine and the new HJ materials are similar or even larger than that of COF modified g-C₃N₄ catalysts in [24,26,31,36,39,40]. As for their band edges, the conduction and valence band positions vary in the same potential range with the conductive band edge that has even more negative values compared to the catalysts in the mentioned works, depending on the ratio between the two components.

 

- The control experiment to uncover the carbon source of CO show that CO exists, is there a possibility that it comes from the decomposition of g-C₃N₄?

Response: XRD patterns presented in the supplementary material revealed no alterations on the crystal structure of the material. Papailias and al. reported that g-C₃N₄ is synthesized at harsh conditions (500 ^oC). Characterization techniques as BET involve heating at high temperatures however, they do not affect the structure of g-C₃N₄ neither structurally nor chemically. Τhe case is also discussed in lines 439-453: “As a control experiment to uncover the carbon source of CO, the organic pristine semiconductors were dispersed in H₂O following the typical concentrations and the batch reactor was filled with argon, sealed, and illuminated with UV light. The GC measurements were collected after 18 h, as usual. Both samples presented amounts of desorbed gaseous CO₂, H₂ and CO, as summarized in Table S3. The results either verify that the samples had already physisorbed CO₂ or that the continuous UV illumination led to the degradation of the -materials. The XRD patterns (Figure S8) have shown for the bCOF and g-C₃N₄ that after the reduction process, there are no significant alterations on their chemical or structural composition, indicating that there is no degradation of their networks. This leads to the conclusion that the photocatalysts had physisorbed CO₂ before their dispersion in water and maintained the physisorbed CO₂ quantity throughout the bubbling process with Ar. The UV irradiation provided the necessary energy for a certain release and a small percentage had been also converted to the products of the reduction. Thus, the CO quantities after the photoreduction process indeed originate from the initially provided CO₂ gas into the batch reactor through the photocatalysts’ activity.”

 

- The authors did not discuss oxidation reactions. Possible candidate of oxidation product is oxygen, how is the O2 production with the time?

Response: The GC-2010 Shimadzu gas chromatographer of Carboxen®-1010 PLOT can detect H₂, O₂, N₂, CO, CH₄ and CO₂. The chromatograms contain the peaks of the products and through calibration curves with reference gases we evaluated the yield of the photocatalysts.

During the CO₂ reduction, different reactions take place as suggested in Table S1. Among them, O₂ can indeed be produced from the dissociation of CO₂ and H₂O and from the remaining physisorbed molecules. All the chromatograms have displayed the presence in the outlet gas of O₂. Though, a clear result cannot be obtained for the exact rate, as the quantity of O₂ in the inlet gas and dispersion was not evaluated. A calibration with the respective gas is necessary for the quantification of oxygen production, however we can qualitatively state that the production rate of O₂ would have a similar behavior with CO depending on the photocatalyst.

 

- The balance between the reduction and oxidation sides in the overall CO₂ reduction is important.

Response: As the semiconductor is activated by light irradiation, the excitation leads to the formation of electrons in the conduction band and holes in the valence band. These sites are responsible for the photoreduction (e^-) and the photoxidation (h⁺), respectively. The catalysts should possess higher energy than the reduction potential of CO₂ or participate in the multiply step reactions presented in Table S1. Meanwhile, the holes should be able to oxidize water to oxygen and protons, which are essential for the photoreduction. All the synthesized photocatalysts have their valence band at more positive potentials than the oxidation of water, providing the oxygen and protons for the continuation of the CO₂ multiply step photoreduction.

A new column in Table S1 was added separating photoreduction reactions with photooxidation reactions.

 

 -All the photocatalysis conditions should be given such as the pressure, temperature…

Response: The conditions of the photocatalytic process are already presented in the materials and methods in section 3.5. in lines 546-550.

 

-Authors can show a schematic figure of their photoreactor

Response: The graphical abstract represents the photocatalytic system with schematic figures of the photoreactor. In the Scheme 1 b images of the reactor were incorporated in the main text.

 

- Was the GC directly connected to the reactor? How were the data obtained at the different time points?

Response: The cover representation was revised in order to point out the position of the GC chromatographer in the photocatalytic system, which is directly connected to the outlet gas of the photoreactor. The process followed to obtain the data points during the repeated cycles is now incorporated at the beginning of section 2.4.5.: “To assess a catalyst, the catalytic durability is an important parameter. Each photocatalytic cycle follows the same procedure as CO₂ reduction described in section 3.5. The dispersion undergoes purging of 20 min with Ar flow, before loading the reactor with CO₂ gas and allowing the photocatalytic reaction to proceed for 24h. The outlet gas is analyzed by the GC at the end of each cycle and then the process is re-initialized with Ar flow to exclude the impact of the products produced in the previous cycle.’

 

- Compare the CO productions obtained with other similar materials reported in literature

Response: The resulting CO production rates were compared to other reported g-C₃N₄, COFs, modified g-C₃N₄ and COFs and their heterojunction or VdW materials. In section 2.4.1., a paragraph was added for this purpose: ‘As the photocatalytic systems in other reports include a different reactor, process and calculations for the photocatalytic activity, the results can only be compared qualitatively. Hence, the g-C₃N₄ catalysts in absence of sacrificial agents, co-catalysts and under UV irradiation have a high CO production rate compared even to composite systems with TiO₂ [44,45]. For the bCOF, the CO production rate is lower than those reported in references [19,46], though higher than in Ref. [9]. It is however, similar to COFs with metal co-catalysts [20] and sacrificial agents [18] and to MOFS [47].

In section 2.4.2., alterations were made in the last sentence and more information was included about the relationship of the studied HJs materials with others reported in the literature. The challenge was to increase the activity of COF with another carbon-based material and to adjust the synthesized heterojunction materials to the needs of this CO₂ reduction study towards selective CO production. For most of the reports with heterojunction or VdW catalysts of carbon nitride and covalent organic frameworks, catalysts are used for H₂ production [24,31,36,39]. A conclusion can be made that the CO rate of the HJ catalysts is 100% selective, but slightly lower than in references [26,40].

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The novelty of this manuscript is not high enough. Besides, the questions of mine had not been well solved.

The language can be improved.