This is a review article for the synthesis and photochemical application of graphitic carbon nitride (g-C3N4) and its derivatives. The non-metal material g-C3N4 has been investigated as a photocatalyst due to its easy synthesis from readily accessible, affordable precursors, and the photochemical conversion varies from solar hydrogen production (Chem. Sci. 2024, 15, 6088-6094), CO2 reduction to hydrocarbons (Angew. Chem. Int. Ed. 2022, 61, e202210789), oxidation of organic molecules (Angew. Chem. Int. Ed. 2023, 62, e202301815), and H2O2 production (Angew. Chem. Int. Ed. 2024, 63, e202316346). As such, there are so many review papers already published at the same time. In this regard, it was not clear for the Reviewer what’s the specialized, remarkable, or superior aspects of the submitted review paper as compared with other already published Review Articles. In addition, RSC Advances is a journal of general chemistry, and therefore, questions from broad sections of the chemistry readers should be satisfied. Nevertheless, the discussion and contents of the submitted paper is too shallow, with no details, for example, of the photoinduced electron transfer (charge separation) within the materials. Detailed understanding of the photophysics of the materials will be indispensable as a "comprehensive review", but there was unfortunately no description regarding the "the-state-of-the-art" scientific aspects for deeper understanding of the photochemistry. The submitted paper indeed has a considerable volume, but the quantitative discussion was limited and not satisfactory as a writing, which will make the potential readers boring. In addition, there are several misunderstandings of the photochemical and electrochemical aspects even in the previous original works by the authors. As such, the Reviewer cannot recommend its publication in RSC Advances. The detailed comments from the Reviewer are shown below.

Major comments
1) Novelty as a Review Article
1-1) There are so many review papers already published, and it was not clear for the Reviewer what’s the specialized, remarkable, or superior aspects of the submitted review paper as compared with other already published review articles.
1-2) In 2022, a review article was published in the same journal (RSC Advances) with the same topic, although the review article was not cited in the submitted paper. The Reviewer cannot understand the necessity of publishing additional new review article with the same topic in the same journal.
1-3) Section “3.1.2 Effects of operational parameters:” The discussion is too shallow, while the contents were already partly discussed in the previous review article by the same authors in [165].
1-4) Section “3.2. Hydrogen production:” The authors cited two review papers, not original papers: [188,189] These (and other) reviews were well-written, while this review lacks an advanced perspective and survey as compared with these published papers. Therefore, the necessity of publication of the submitted paper was again not clear to the Reviewer.
2) Inclusivity as a “Comprehensive” Review Paper
2-1) Although the review title has a word “the state-of-the-art” and the authors stated this as “comprehensive” review, the fundamental of graphitic carbon nitride was almost unclear even after reading the review paper. For example, where and how the photoinduced charge separation occurs within the material? What’s the detailed structure of g-C3N4, including the stacking manner of the layers? The authors referred to the unique band structure of materials, but the details and origin are not well clear. Why is g-C3N4 structure selectively formed? (with any kinetic or thermodynamic fundamental?) Such an explanation will satisfy the potential readers as a "comprehensive review."
2-2) The first figure of the submitted paper is regarding the numbers of papers published previously, but the introduction of the materials itself, including chemical structure, electronic structure, excited state dynamics etc should be discussed first in Figure 1, if this paper appears as “comprehensive” review paper. In addition, the potential readers will be curious why the material is called as “graphitic carbon nitride.”
2-3) Figure 1 indicates there are more than 4000 papers published in the related area in 2023 and 2024. Nevertheless, this review cited at most 270 papers. Although the review title has a word “the state-of-the-art” and the authors mentioned this as “comprehensive,” there are so many latest papers not cited in the submitted review article. The followings are selected papers not cited but related to the contents, based on the brief literature surveys by the Reviewer: Angew. Chem. Int. Ed. 2024, 63, e202316346; Angew. Chem. Int. Ed. 2023, 62, e202301815; Angew. Chem. Int. Ed. 2022, 61, e202210789; Chem. Sci. 2024, 15, 6088-6094; Chem. Sci. 2023, 14, 6269-6277; Chem. Sci. 2022, 13, 9927-9939; Chem. Sci. 2021, 12, 3633-3643; Chem. Sci. 2017, 8, 4087-4092; Chem. Commun. 2023, 59, 7423-7426; Chem. Commun. 2022, 58, 10084-10087; RSC Adv. 2023, 13, 937-947; RSC Adv. 2023, 13, 6688-6698; RSC Adv. 2023, 13, 9168-9179; RSC Adv. 2022, 12, 33688-33695; RSC Adv. 2022, 12, 33598-33604.
3) Regarding the Accurate, Quantitative Discussion
3-1) Overall, quantitative discussion was limited in the paper, and explanations were somehow shallow and not satisfactory as a writing. For example, the authors mentioned the hole mobility of g-C3N4 in the early section of the submitted paper, but the discussion was too concise and there was no quantitative aspect in the discussion, almost no value as a content, which will make the potential readers boring.
3-2) Electrochemistry: The authors seem to be confused regarding the use of eV. For example, in page 4, the authors discussed “The optimal bandgap of g-C3N4 at 2.7 eV (460 nm), with VB and CB potentials at −1.09 and +1.56 eV, respectively, make g-C3N4 attractive material for overall water splitting [33, 34].” In this sentence, "the optical band gap of 2.7 eV" is okay, while "potential of 1.56 eV" should be "1.56 V" In this case, the authors also need to show the corresponding reference electrode. For example, “1.56 V vs NHE” or “1.56 V vs Ag/AgCl” or “1.56 V vs RHE”. Otherwise, the values are meaningless when discussing the water splitting. This is the same for Figure 11. Furthermore, the Reviewer cannot understand the experimental methods for determining the values. Why are these physicochemical values decisively determined irrespective of g-C3N4 conditions? In addition, "optimal bandgap" should be correctly "optical bandgap." The same problems were found in the paper cited as [152] by the same authors.
3-3) Photodynamics: In page 5, the authors discussed “the performance of photocatalytic techniques is further decreased by the quick coupling of photo-generated charge carriers in g-C3N4 [41].” The orders of time scale? How many quantum yields? Fundamental scientific works regarding the excited state physics based on, for example, transient absorption spectroscopy and time-resolved fluorescence spectroscopy will help readers understand the quantitative aspect of the photodynamics as "the-state-of-the-art" review, but this review paper lacks such a quantitative aspect with advanced technologies at all, although it says “comprehensive.” Section “3.2. Hydrogen production” mentioned the processes as “When excited by photons with energy equal to or higher than its bandgap, g-C3N4 can generate electron-hole pairs that can be involved in a series of reactions to produce hydrogen.” but the discussion was too shallow, while there was no suitable citations.
3-4) In the introduction part, the authors discussed “g-C3N4 as a photocatalyst because of its non-toxicity, high visible light harvesting,” but Figure 2c doesn’t indicate “high visible light harvesting,” the Reviewer thinks. Discussions in the submitted paper were frequently contradicted at different sections, and this will make the potential readers painful.

Minor comments
1) The review paper deals with the photodegradation of organic pollutants by g-C3N4, but the Reviewer cannot understand why this can be socially and economically established even with the poor longevity of the photocatalysis. For example, the authors mentioned the “excellent stability” of the materials, but Figure 11b indicates this material is not suitable for its practical applications. Discussion on the critical points will also be essential as a “comprehensive” review article.
2) Structural details and physicochemical properties of g-C3N4 were not clear to the Reviewer, including the porosity (SBET), photophysics, and so on. For example, in page 5, the sentence “Other limitations of g-C3N4 are its relatively low specific surface area” should accompany the representative values. If the paper were “comprehensive” review, the detailed description of the material itself would also be highly expected by the potential readers.
3) In the abstract, the authors mentioned the “environmental friendliness” of g-C3N4, but the Reviewer cannot understand the value.
4) In the abstract, the authors mentioned “shedding light on the key factors that influence their performance” but the Reviewer cannot find any new messages even after reading the review article including the final summary, unfortunately. This means the contents are not systematically arranged.
5) In the last section, the authors mentioned “The scale-up of synthesis methods and the development of cost-effective production techniques are essential for the practical application of g-C3N4-based composites” but this is obvious and clear. The next sentence discussed the improvement of the longevity, and this should accompany a strategy which is nice for guiding readers. Nevertheless, there is no such information, unfortunately.
6) The construction of the introduction part should be improved. In page 3, there was a discussion regarding hydrogen production starting from “In addition to these environmental concerns, …” and this paragraph was followed by again water purification section starting from “On the other hand, …” This order of writing is very confusing to the Reviewer, and probably making the potential readers annoyed.
7) In the introduction part, there was an explanation of photochemical degradation of organic pollutants. However, it was not clear to the Reviewer what’s the advantages of the method over other methods including simple bleaching?
8) In the introduction of clean energy production, the discussion lacks quantitative aspect of energy utilization and production, and therefore, the explanation is weak, the Reviewer felt. Referring to the following excellent paper will help the authors clearly discuss the energy issues: N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729-15735.
9) In page 3, there is a description “photocatalysis approach has emerged as a cost-effective, trustworthy,…” but the Reviewer cannot agree with the description even after reading the whole contents with the poor photocatalysis durability as previously mentioned, unfortunately. The Reviewer thinks the authors’ statements were sometimes exaggerated with poor basis.
10) In page 4, there is a description “These redox processes are responsible for generating reactive species, such as superoxide radicals (•O2-), and hydroxyl radicals (•OH) which play a key role in the overall photocatalytic process[23, 24]” Then, the potential readers will be curious about the oxidative durability of g-C3N4 against these reactive species. This discussion will also be highly related to the longevity of the material itself.
11) Again, For the non-specialists, it would be less clear why the chemical formula of graphitic carbon nitride was denoted as "g-C3N4." By showing the unit structure, brief introduction should be made, because this review is “comprehensive.” In addition, is there any basis including thermodynamical preference of the g-C3N4 formation irrespective of the synthetic procedure? More detailed discussion regarding this would be indispensable.
12) In page 5, there is a sentence “The presence of carbon and nitrogen atoms with distinct valence states results in the creation of multiple band structures” but the potential readers cannot understand well without figures.
13) In page 6, there is a sentence “This indicates a scarcity of dedicated review papers, which are essential for providing interested researchers and the scientific community with a comprehensive evaluation of g-C3N4-composites' application as photocatalysts.” but this doesn’t make sense to the Reviewer.
14) In page 7, there was a description that the review highlights the characterization techniques used to understand the crystal structure, morphology, surface area, nanoparticle distribution, and compositional properties of g-C3N4-based photocatalysts. However, they are very fundamental, while "a state-of-the-art review" is expected to show the advanced analytical methods including time-resolved spectroscopy to elucidate the photochemical and photophysical basis of the photocatalysis. These methods (crystal structure, surface area etc.) is again too fundamental, unfortunately.
15) In page 7, the authors started with the topic of “2. Modification of g-C3N4 for improved photocatalytic activity.” Why is "mdification" discussed first? The Reviewer thinks that, as a "comprehensive" review, this paper should first discuss the chemical and physical fundamental of g-C3N4, then typical synthesis methods and introduction of "why the C3N4 structure forms." These introductions of the materials will satisfy the general readers of RSC Advances.
If this review skipps these important introductions, the phrase "a state-of-the-art review" in the title and "comprehensive" in the abstract should be removed from the submitting manuscript.
16) In page 46, there is a sentence “Computational studies using density functional theory (DFT) simulations have provided valuable insights into the electronic structure, band alignment, and charge carrier dynamics at the g-heterostructure interfaces .[222]” This explanation doesn't make sense to the Reviewer, because the key message was not clear in the sentence. In addition, “DFT simulations” might be “DFT calculations.”
17) In Section 3, there is a sentence “As a solution to this issue, g-C3N4-based nanomaterials have emerged as highly researched photocatalysts for the treatment of wastewater contaminated with diverse pollutants.” For this sentence, suitable citations are needed. More importantly ,with the fragile, poor stability of g-C3N4 discussed in this paper, how to solve the environmental issues photochemically? That is, a substantial durability is indispensable for practical application of g-C3N4, as the authors could agree. Even though the performance is improved in terms of photon utilization and rate of reactions, it is completely meaningless if the photocatalysis is soon deactivated. Nevertheless, there was no discussion regarding them.
18) In Figure 12, “CO2 photoreduction” should be changed to “CO2 reduction coupled with water oxidation,” because this single process is not related to photochemistry.
19) Section “3.3. Carbon dioxide reduction:” The CO2RR performance seems to be limited to the choice of the co-catalysts. What's the great advantage of using g-C3N4 as the photocatalyst? In addition, the production of methanol upon CO2RR indicates the once formed methanol could be easily subjected to the photoanodic oxidation, leading to the apparent poor activity. This would be far away from the practical applications, unfortunately.
20) In page 5, the authors discussed the lack of stability under photocatalytic conditions, but other section referred to its “excellent stability” (for example in the abstract part). This kind of inconsistency makes the potential readers annoyed.

The authors revised the manuscript based on the comments raised by the Reviewers.
As a consequence, more accurate, quantitative and valuable discussions are realized.
As per my knowledge, the paper can be accepted in the present form.