# Supercritical CH3OH-triggered isotype heterojunction and groups in g-C3N4 for enhanced photocatalytic H2 evolution

Liuhao Mao, Binjiang Zhai, Jinwen Shi, Xing Kang, Bingru Lu, Yanbing Liu, Cheng Cheng, Hui Jin, Eric Lichtfouse, Liejin Guo

► To cite this version:

Liuhao Mao, Binjiang Zhai, Jinwen Shi, Xing Kang, Bingru Lu, et al.. Supercritical CH3OH-triggered isotype heterojunction and groups in g-C3N4 for enhanced photocatalytic H2 evolution. ACS Nano, 2024, 10.1021/acsnano.4c03922 . hal-04577732

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Submitted on 22 May 2024

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# Supercritical CH₃OH-Triggered Isotype Heterojunction and Groups in g-C₃N₄ for Enhanced Photocatalytic H₂Evolution

Liuhao Mao,† Binjiang Zhai,† Jinwen Shi,* Xing Kang, Bingru Lu, Yanbing Liu, Cheng Cheng, Hui Jin, Eric Lichtfouse, and Liejin Guo

**ABSTRACT:** The structure tuning of bulk graphitic carbon nitride (g-C₃N₄) is a critical way to promote the charge carriers dynamics for enhancing photocatalytic H₂-evolution activity. Exploring feasible post-treatment strategies can lead to effective structure tuning, but it still remains a great challenge. Herein, a supercritical CH₃OH (ScMeOH) post-treatment strategy (250–300 °C, 8.1–11.8 MPa) is developed for the structure tuning of bulk g-C₃N₄. This strategy presented advantages of time-saving (less than 10 min), high yield (over 80%), and scalability due to the enhanced mass transfer and high reactivity of ScMeOH. During the ScMeOH post-treatment process, CH₃OH molecules diffused into the interlayers of g-C₃N₄ and subsequently participated in N-methylation and hydroxylation reactions with the intralayers, resulting in a partial phase transformation from g-C₃N₄ into carbon nitride with a poly(heptazine imide)-like structure (Q-PHI) as well as abundant methyl and hydroxyl groups. The modified g-C₃N₄ showed enhanced photocatalytic activity with an H₂-evolution rate 7.2 times that of pristine g-C₃N₄, which was attributed to the synergistic effects of the g-C₃N₄/Q-PHI isotype heterojunction construction, group modulation, and surface area increase. This work presents a post-treatment strategy for structure tuning of bulk g-C₃N₄ and serves as a case for the application of supercritical fluid technology in photocatalyst synthesis.

**KEYWORDS:** graphitic carbon nitride, hydrogen, poly(heptazine imide), supercritical methanol, solvothermal treatment

<figure><img src="image_1.png" /></figure>

The development of hydrogen (H₂) energy technology is instrumental in achieving the objective of carbon neutrality.¹ Among various H₂-production technologies, photocatalytic H₂ production from water is an effective, green, and promising route.²,³ The performance of photocatalytic H₂ production hinges on the photocatalyst, and the structure tuning of the photocatalyst by post-treatment is a commonly used strategy to enhance the performance of the photocatalyst.⁴ Comprehensive evaluation in various aspects including photocatalytic activity, stability, light-response range, toxicity, preparation difficulty, cost and so on is crucial in determining excellent photocatalysts.⁵

Graphitic carbon nitride (g-C₃N₄) emerges as a promising photocatalyst for H₂ evolution. First, it consists of earth-abundant nonmetallic elements and is usually synthesized by a facile thermal polymerization process using common precursors such as dicyandiamide, melamine, and urea, which makes it cheap and environmentally friendly. Additionally, it is responsive to visible light and exhibits good stability. However,

the practical applications of g-C₃N₄ in photocatalytic H₂ production are still limited by the inadequate specific surface area and the severe recombination rate of the charge carrier.⁶⁻⁸ To overcome these inherent deficiencies of bulk g-C₃N₄, structure tuning by post-treatment such as thermal treatment,⁹ plasma treatment,¹⁰ ionothermal treatment,¹¹,¹² and hydrothermal/solvothermal treatment¹³ have been developed. In addition to the photocatalytic performance of modified g-C₃N₄ after post-treatment, some issues such as yield, required time, equipment requirement, and operation process should also be considered for the modification strategy. Therefore, it is
# Supercritical CH₃OH-Triggered Isotype Heterojunction and Groups in g-C₃N₄ for Enhanced Photocatalytic H₂Evolution

Liuhao Mao,† Binjiang Zhai,† Jinwen Shi,* Xing Kang, Bingru Lu, Yanbing Liu, Cheng Cheng, Hui Jin, Eric Lichtfouse, and Liejin Guo

**ABSTRACT:** The structure tuning of bulk graphitic carbon nitride (g-C₃N₄) is a critical way to promote the charge carriers dynamics for enhancing photocatalytic H₂-evolution activity. Exploring feasible post-treatment strategies can lead to effective structure tuning, but it still remains a great challenge. Herein, a supercritical CH₃OH (ScMeOH) post-treatment strategy (250–300 °C, 8.1–11.8 MPa) is developed for the structure tuning of bulk g-C₃N₄. This strategy presented advantages of time-saving (less than 10 min), high yield (over 80%), and scalability due to the enhanced mass transfer and high reactivity of ScMeOH. During the ScMeOH post-treatment process, CH₃OH molecules diffused into the interlayers of g-C₃N₄ and subsequently participated in N-methylation and hydroxylation reactions

with the intralayers, resulting in a partial phase transformation from g-C₃N₄ into carbon nitride with a poly(heptazine imide)-like structure (Q-PHI) as well as abundant methyl and hydroxyl groups. The modified g-C₃N₄ showed enhanced photocatalytic activity with an H₂-evolution rate 7.2 times that of pristine g-C₃N₄, which was attributed to the synergistic effects of the g-C₃N₄/Q-PHI isotype heterojunction construction, group modulation, and surface area increase. This work presents a post-treatment strategy for structure tuning of bulk g-C₃N₄ and serves as a case for the application of supercritical fluid technology in photocatalyst synthesis.

**KEYWORDS:** graphitic carbon nitride, hydrogen, poly(heptazine imide), supercritical methanol, solvothermal treatment

<figure><img src="image_1.png" /></figure>

The development of hydrogen (H₂) energy technology is instrumental in achieving the objective of carbon neutrality.¹ Among various H₂-production technologies, photocatalytic H₂ production from water is an effective, green, and promising route.²,³ The performance of photocatalytic H₂ production hinges on the photocatalyst, and the structure tuning of the photocatalyst by post-treatment is a commonly used strategy to enhance the performance of the photocatalyst.⁴ Comprehensive evaluation in various aspects including photocatalytic activity, stability, light-response range, toxicity, preparation difficulty, cost and so on is crucial in determining excellent photocatalysts.⁵

Graphitic carbon nitride (g-C₃N₄) emerges as a promising photocatalyst for H₂ evolution. First, it consists of earth-abundant nonmetallic elements and is usually synthesized by a facile thermal polymerization process using common precursors such as dicyandiamide, melamine, and urea, which makes it cheap and environmentally friendly. Additionally, it is responsive to visible light and exhibits good stability. However,

the practical applications of g-C₃N₄ in photocatalytic H₂ production are still limited by the inadequate specific surface area and the severe recombination rate of the charge carrier.⁶⁻⁸ To overcome these inherent deficiencies of bulk g-C₃N₄, structure tuning by post-treatment such as thermal treatment,⁹ plasma treatment,¹⁰ ionothermal treatment,¹¹,¹² and hydrothermal/solvothermal treatment¹³ have been developed. In addition to the photocatalytic performance of modified g-C₃N₄ after post-treatment, some issues such as yield, required time, equipment requirement, and operation process should also be considered for the modification strategy. Therefore, it is
<figure><img src="image_2.png" /><figcaption>Figure 1. (a) Photocatalytic H₂-evolution rates (left axis) of CN-T-t samples by different post-treatment temperatures (horizontal axis) and the corresponding residence time (right axis). (b) Contour map between photocatalytic H₂-evolution rate, post-treatment temperature, and residence time. Panel (b) was plotted based on the data from Table S1. (c) Time courses and (d) action spectra of photocatalytic H₂ evolution for CN, CN-230-30, and CN-300-0. Left axis: wavelength-dependent AQYs; right axis: UV-vis absorbance. (e) Stability test of the photocatalytic H₂ evolution for CN-300-0. (f) Photocatalytic H₂-evolution rates of modified g-C₃N₄ by different alcohols (MeOH, EtOH, IPA, and NPA) post-treatment with a residence time of 0 min at different post-treatment temperatures.</figcaption></figure>

meaningful to develop an approach for g-C₃N₄ modification with the benefits of being effective, green, high-yielding, and scalable.

Supercritical fluids (SCFs) display unique physicochemical properties such as low viscosity, zero surface tension, and high diffusivity.¹⁴ The SCF technology has received much attention in the fields of nanomaterial synthesis ranging from scientific research to industrial application.¹⁵⁻¹⁹ Owing to the solvation behavior and enhanced mass transfer ability of SCFs, a homogeneous reaction environment could be formed by dissolving chemicals (including metal precursor, organic solvent, surfactant, redox agent, and so on) in SCFs, realizing a rapid SCF-assisted hydrothermal/solvothermal synthesis of metals, metal hydroxides, metal oxides, metal sulfides, metal-organic frameworks, and composites.²⁰⁻²² On the other hand, structure tuning of nanomaterials such as exfoliation,²³⁻²⁶ surface reduction and oxidation,²⁷ phase engineering,²⁸⁻³⁰ and elemental doping³¹⁻³³ could be realized by the post-treatment strategy utilizing SCFs. Among numerous SCFs, supercritical H₂O (ScH₂O) and supercritical CO₂ (ScCO₂) are extensively employed in the preparation and modification of nanomaterials.³⁴,³⁵ Recently, supercritical alcohols have become another important SCF following ScH₂O and ScCO₂.³⁶ Supercritical alcohols represented by supercritical CH₃OH (ScMeOH) have been used to synthesize various functional nanomaterials. For instance, reduced graphene oxide³⁷,³⁸ and TiO₂-graphene composite³⁹ have been prepared using supercritical alcohols. Anthracite coal has been converted into ribbon-like graphitic materials using supercritical ethanol with a yield of 6.4 wt %.⁴⁰ The critical condition of ScMeOH (239.4 °C, 8.1 MPa) is

more moderate than that of ScH₂O (374.1 °C, 22.1 MPa), while its critical pressure is approximate to that of ScCO₂ (7.4 MPa). Additionally, the low boiling point of CH₃OH (64.7 °C) facilitates its separation from the product. Compared with conventional CH₃OH, ScMeOH has lower hydrogen bonding, dielectric coefficient, surface tension, and viscosity and a much higher diffusion coefficient, rendering ScMeOH as an excellent reaction medium. ScMeOH is also electronegative and nucleophilic, thus allowing reactions such as alkylation, alcoholysis, and polymer degradation.⁴¹,⁴² In addition, ScMeOH decomposition produces a reductive medium containing H₂ and CO and hydroxyl ions.⁴³

In our previous attempt, it was found that g-C₃N₄ decomposed completely by the ScH₂O post-treatment due to excessive oxidation of ScH₂O.⁴⁴ Herein, a ScMeOH post-treatment strategy was proposed for structure tuning of g-C₃N₄, and the optimal post-treatment conditions as well as the critical role of the supercritical state of MeOH for g-C₃N₄ modification were explored. Through detailed characterizations of modified g-C₃N₄ and analysis of liquid/gas products after the ScMeOH post-treatment, the structural evolution of g-C₃N₄ and the mechanism of enhanced photocatalytic H₂-evolution activity for modified g-C₃N₄ were also investigated.

## RESULTS AND DISCUSSION

Evaluation of the Photocatalytic H₂-Evolution Performance. In this study, a series of modified g-C₃N₄ (CN-T-t, where *T* was the post-treatment temperature and *t* was the residence time at the target post-treatment temperature) were
<figure><img src="image_2.png" /><figcaption>Figure 1. (a) Photocatalytic H₂-evolution rates (left axis) of CN-T-t samples by different post-treatment temperatures (horizontal axis) and the corresponding residence time (right axis). (b) Contour map between photocatalytic H₂-evolution rate, post-treatment temperature, and residence time. Panel (b) was plotted based on the data from Table S1. (c) Time courses and (d) action spectra of photocatalytic H₂ evolution for CN, CN-230-30, and CN-300-0. Left axis: wavelength-dependent AQYs; right axis: UV-vis absorbance. (e) Stability test of the photocatalytic H₂ evolution for CN-300-0. (f) Photocatalytic H₂-evolution rates of modified g-C₃N₄ by different alcohols (MeOH, EtOH, IPA, and NPA) post-treatment with a residence time of 0 min at different post-treatment temperatures.</figcaption></figure>

meaningful to develop an approach for g-C₃N₄ modification with the benefits of being effective, green, high-yielding, and scalable.

Supercritical fluids (SCFs) display unique physicochemical properties such as low viscosity, zero surface tension, and high diffusivity.¹⁴ The SCF technology has received much attention in the fields of nanomaterial synthesis ranging from scientific research to industrial application.¹⁵⁻¹⁹ Owing to the solvation behavior and enhanced mass transfer ability of SCFs, a homogeneous reaction environment could be formed by dissolving chemicals (including metal precursor, organic solvent, surfactant, redox agent, and so on) in SCFs, realizing a rapid SCF-assisted hydrothermal/solvothermal synthesis of metals, metal hydroxides, metal oxides, metal sulfides, metal-organic frameworks, and composites.²⁰⁻²² On the other hand, structure tuning of nanomaterials such as exfoliation,²³⁻²⁶ surface reduction and oxidation,²⁷ phase engineering,²⁸⁻³⁰ and elemental doping³¹⁻³³ could be realized by the post-treatment strategy utilizing SCFs. Among numerous SCFs, supercritical H₂O (ScH₂O) and supercritical CO₂ (ScCO₂) are extensively employed in the preparation and modification of nanomaterials.³⁴,³⁵ Recently, supercritical alcohols have become another important SCF following ScH₂O and ScCO₂.³⁶ Supercritical alcohols represented by supercritical CH₃OH (ScMeOH) have been used to synthesize various functional nanomaterials. For instance, reduced graphene oxide³⁷,³⁸ and TiO₂-graphene composite³⁹ have been prepared using supercritical alcohols. Anthracite coal has been converted into ribbon-like graphitic materials using supercritical ethanol with a yield of 6.4 wt %.⁴⁰ The critical condition of ScMeOH (239.4 °C, 8.1 MPa) is

more moderate than that of ScH₂O (374.1 °C, 22.1 MPa), while its critical pressure is approximate to that of ScCO₂ (7.4 MPa). Additionally, the low boiling point of CH₃OH (64.7 °C) facilitates its separation from the product. Compared with conventional CH₃OH, ScMeOH has lower hydrogen bonding, dielectric coefficient, surface tension, and viscosity and a much higher diffusion coefficient, rendering ScMeOH as an excellent reaction medium. ScMeOH is also electronegative and nucleophilic, thus allowing reactions such as alkylation, alcoholysis, and polymer degradation.⁴¹,⁴² In addition, ScMeOH decomposition produces a reductive medium containing H₂ and CO and hydroxyl ions.⁴³

In our previous attempt, it was found that g-C₃N₄ decomposed completely by the ScH₂O post-treatment due to excessive oxidation of ScH₂O.⁴⁴ Herein, a ScMeOH post-treatment strategy was proposed for structure tuning of g-C₃N₄, and the optimal post-treatment conditions as well as the critical role of the supercritical state of MeOH for g-C₃N₄ modification were explored. Through detailed characterizations of modified g-C₃N₄ and analysis of liquid/gas products after the ScMeOH post-treatment, the structural evolution of g-C₃N₄ and the mechanism of enhanced photocatalytic H₂-evolution activity for modified g-C₃N₄ were also investigated.

## RESULTS AND DISCUSSION

Evaluation of the Photocatalytic H₂-Evolution Performance. In this study, a series of modified g-C₃N₄ (CN-T-t, where *T* was the post-treatment temperature and *t* was the residence time at the target post-treatment temperature) were
<figure><img src="image_3.png" /><figcaption>Figure 2. (a) Schematic illustration of the structural evolution of g-C₃N₄ by the ScMeOH post-treatment. (b) Three kinds of Q-PHI models with different groups and the corresponding exfoliated units. (c) Gibbs free energies calculated by molecular dynamics simulations for the formation pathways of Q-PHI with one methyl and one hydroxyl group. (d) Composition analysis of gas products from MeOH-230-30, CN-230-30, MeOH-300-0, and CN-300-0. (e) Composition analysis of CH₃OH and the liquid products from MeOH-230-30, CN-230-30, MeOH-300-0, and CN-300-0. MeOH-230-30 and MeOH-300-0 were blank runs in the absence of g-C₃N₄. (f) Element contents for CN, CN-230-30, and CN-300-0.</figcaption></figure>

prepared by the solvothermal post-treatment of g-C₃N₄ in subcritical or supercritical CH₃OH. Throughout the post-treatment process, temperature and pressure data were continuously recorded in real-time, as shown in Figure S1. The photocatalytic H₂-evolution activities of modified g-C₃N₄ are shown in Figure 1a,b. As for the modified g-C₃N₄ by subcritical CH₃OH (SubMeOH) post-treatment at 230 °C for 10–40 min, the optimal sample CN-230-30 showed a photocatalytic H₂-evolution rate of 18.0 µmol·h⁻¹. Interestingly, when the post-treatment temperature exceeded the critical point of CH₃OH (239.4 °C), the photocatalytic H₂-evolution activities of modified g-C₃N₄ by the ScMeOH post-treatment underwent a sudden enhancement compared with

the modified g-C₃N₄ by the SubMeOH post-treatment, simultaneously the corresponding required residence time decreased sharply from tens of minutes to a few minutes. The apparent colors of modified g-C₃N₄ could also be reflected by reactivity differences between SubMeOH and ScMeOH post-treatments, and the result is shown in Figure S2. The apparent colors of modified g-C₃N₄ by the SubMeOH post-treatment (CN-230-x, x = 5, 10, 20, 30, 40) were light yellow, similar to CN, and the structure of modified g-C₃N₄ by the ScMeOH post-treatment (such as CN-270-5, CN-280-3, and CN-290-2) was susceptible to damage and manifested as blackening with increased post-treatment temperature and residence time. It was found that ScMeOH was more effective and efficient for g-
<figure><img src="image_3.png" /><figcaption>Figure 2. (a) Schematic illustration of the structural evolution of g-C₃N₄ by the ScMeOH post-treatment. (b) Three kinds of Q-PHI models with different groups and the corresponding exfoliated units. (c) Gibbs free energies calculated by molecular dynamics simulations for the formation pathways of Q-PHI with one methyl and one hydroxyl group. (d) Composition analysis of gas products from MeOH-230-30, CN-230-30, MeOH-300-0, and CN-300-0. (e) Composition analysis of CH₃OH and the liquid products from MeOH-230-30, CN-230-30, MeOH-300-0, and CN-300-0. MeOH-230-30 and MeOH-300-0 were blank runs in the absence of g-C₃N₄. (f) Element contents for CN, CN-230-30, and CN-300-0.</figcaption></figure>

prepared by the solvothermal post-treatment of g-C₃N₄ in subcritical or supercritical CH₃OH. Throughout the post-treatment process, temperature and pressure data were continuously recorded in real-time, as shown in Figure S1. The photocatalytic H₂-evolution activities of modified g-C₃N₄ are shown in Figure 1a,b. As for the modified g-C₃N₄ by subcritical CH₃OH (SubMeOH) post-treatment at 230 °C for 10–40 min, the optimal sample CN-230-30 showed a photocatalytic H₂-evolution rate of 18.0 µmol·h⁻¹. Interestingly, when the post-treatment temperature exceeded the critical point of CH₃OH (239.4 °C), the photocatalytic H₂-evolution activities of modified g-C₃N₄ by the ScMeOH post-treatment underwent a sudden enhancement compared with

the modified g-C₃N₄ by the SubMeOH post-treatment, simultaneously the corresponding required residence time decreased sharply from tens of minutes to a few minutes. The apparent colors of modified g-C₃N₄ could also be reflected by reactivity differences between SubMeOH and ScMeOH post-treatments, and the result is shown in Figure S2. The apparent colors of modified g-C₃N₄ by the SubMeOH post-treatment (CN-230-x, x = 5, 10, 20, 30, 40) were light yellow, similar to CN, and the structure of modified g-C₃N₄ by the ScMeOH post-treatment (such as CN-270-5, CN-280-3, and CN-290-2) was susceptible to damage and manifested as blackening with increased post-treatment temperature and residence time. It was found that ScMeOH was more effective and efficient for g-
$C_3N_4$ modification compared with SubMeOH, and the high reactivity of ScMeOH determined that the residence time was very crucial and sensitive for $g-C_3N_4$ modification.

Three typical samples were selected for further analysis, including a sample of pristine $g-C_3N_4$ (CN), a modified $g-C_3N_4$ by prolonged the SubMeOH post-treatment (CN-230-30) and a modified $g-C_3N_4$ by a rapid ScMeOH post-treatment (CN-300-0). As shown in Figure 1c,d, CN-300-0 showed the best photocatalytic $H_2$-evolution rate of 50.1 $\mu mol \cdot h^{-1}$, which was 2.8 times that of CN-230-30 and 7.2 times that of CN, and it showed obvious higher apparent quantum yield (AQY) values as expected based on the photocatalytic $H_2$-evolution activity. Specifically, CN-300-0 showed a certain AQY values of 1.55% (450 nm), 0.78% (475 nm), and 0.25% (500 nm), while the AQY values of CN and CN-230-30 were not detected at the wavelength higher than 450 nm, and these results were in agreement with the UV-vis spectra. In addition, Figure 1e presents the cyclic photocatalytic $H_2$-evolution test for CN-300-0, and there was little decay after three cyclic tests (15 h), indicating good photocatalytic stability of the modified $g-C_3N_4$ by the ScMeOH post-treatment. It was worth mentioning that $g-C_3N_4$ was not only modified by the ScMeOH post-treatment effectively and efficiently compared with conventional solvothermal post-treatment (Table S2) but also exhibited an advantage of high yield, and the yield of CN-300-0 could reach 80.1% with the initial dispersion concentration as 100 mg·mL$^{-1}$ (Figure S3). Moreover, when $g-C_3N_4$ was modified by the SCF post-treatment using other alcohols including ethanol (EtOH), 1-propanol (NPA), and isopropanol (IPA), the photocatalytic activities of modified $g-C_3N_4$ could also be improved (Figure 1f).

**Structure Tuning of $g-C_3N_4$ by the ScMeOH Post-Treatment.** Ideal $g-C_3N_4$ is composed of few-layer 2D nanosheets bonded by van der Waals forces, and the in-plane structure of the nanosheet is composed of repeating units of continuous heptazine (Figure S4a). However, pristine $g-C_3N_4$ prepared from the thermal condensation of melamine is usually amorphous (Figure S4b).⁴⁵ In this work, structure tuning of $g-C_3N_4$ was realized by a ScMeOH post-treatment strategy, as depicted in Figure 2a. In this process, the $CH_3OH$ molecules in the supercritical state diffused into the interlayers of $g-C_3N_4$ and contacted with the intralayers. The high reactivity of ScMeOH further led to breakages of C-N bonds between NH-bridged melem monomers, and the amino group was expected to be electrophilically attacked by the methyl group, while the heptazine ring was expected to be nucleophilically attacked by the hydroxyl group, resulting in N-methylation and hydroxylation reactions, respectively (Figure S5). The ordered breakage of C-N bridging bonds led to the regular knocking out of heptazine rings from melon polymer; simultaneously, N-methylation and hydroxylation reactions occurred in the intralayers of $g-C_3N_4$, forming a carbon nitride with a poly(heptazine imide) (PHI)-like structure as well as abundant methyl and hydroxyl groups. In general, PHI refers to the structure with high crystallinity, which is typically synthesized by the ionthermal treatment of melon polymers,⁴⁶ thereby the PHI-like structure transformed from amorphous $g-C_3N_4$ in this work was termed as quasi-poly(heptazine imide) (Q-PHI). Figure S6 displays the basic structure unit of PHI and Q-PHI. According to the substitution positions of methyl and hydroxyl groups, there were three kinds of potential Q-PHI models, as shown in Figure 2b, and the corresponding chemical formulas of exfoliated units are listed in Table S2. In addition,

theoretical calculations were conducted to analyze the feasibility of the reactions, and the result is shown in Figure 2c. Specifically, the Gibbs free energy pathways for N-methylation and hydroxylation reactions in $g-C_3N_4$ were analyzed. The reaction of Q-PHI formation was spontaneous in thermodynamics due to the negative value of the Gibbs free energy change ($\Delta G = -105.1 \text{ kcal} \cdot \text{mol}^{-1}$). In addition, more $CH_3OH$ molecules enabled to overcome the energy barrier (40.7 kcal·mol$^{-1}$) at high temperature and pressure in chemical kinetics, which was in good accordance with the experimental results that the Q-PHI formation reaction occurred spontaneously and quickly under high temperature and pressure.

Analysis of the gas and liquid products after solvothermal post-treatment and the corresponding blank runs (in the absence of $g-C_3N_4$) was conducive to understanding the structural evolution of $g-C_3N_4$ by the ScMeOH post-treatment. The temperature and pressure curves for the post-treatment process of CN-230-30, CN-300-0, and the corresponding blank runs are shown in Figure S7. The composition of gas products was detected by gas chromatography (GC), and the results are shown in Figures 2d and S8, and the volume of gas products was measured using a graduated syringe, as shown in Figure S9. There was almost no $CH_3OH$ decomposition from MeOH-230-30, since only a trace of $H_2$ was detected, while certain amounts of $H_2$ (11.4 mL), CO (4.9 mL), and $CH_4$ (0.49 mL) were produced from MeOH-300-0. The proposed relevant reactions are provided in eqs 1 and 2.

$$CH_3OH \rightarrow CO + 2H_2 \quad (1)$$

$$CO + 3H_2 \rightarrow CH_4 + H_2O \quad (2)$$

It was found that neither MeOH-230-30 nor MeOH-300-0 had any detectable $CO_2$, while a significant amount of $CO_2$ was produced when $g-C_3N_4$ participated in the solvothermal reaction, simultaneously reducing gases ($H_2$, CO) almost disappeared, resulting in the gas products from CN-230-30 and CN-300-0 being mainly composed of $CO_2$. Specifically, the $CO_2$ amounts in the gas phase were 2.7 mL from CN-230-30 and 8.2 mL from CN-300-0, whose proportions were calculated as 88.0 and 91.8%, respectively. Furthermore, the composition of liquid products was measured by the combination of gas chromatography and mass spectrometry (GC-MS), and the results are shown in Figures 2e and S10. The GC-MS analysis of $CH_3OH$ could be regarded as a baseline, and the GC-MS analysis of liquid products from MeOH-230-30 and MeOH-300-0 exhibited similarity with $CH_3OH$. In contrast, several significant peaks appeared in the GC-MS signals for the liquid products from CN-230-30 and CN-300-0, which were identified as a series of triazine-derived compounds utilizing mass spectrometry. The formation of these compounds resulted from the reaction between ScMeOH and the exfoliated heptazine rings, and the formed short-chain organic compounds were further transformed into $CO_2$ and $NH_3$ with the help of ScMeOH and reducing gases. It was also consistent with the result of the appearance of $CO_2$ and the disappearance of $H_2$ in the gas products from CN-230-30 and CN-300-0. Moreover, the preparation of CN-300-0 produced a greater amount of $CO_2$ in gas products and a higher concentration of triazine-derived compounds in liquid products than those of CN-230-30, indicating that ScMeOH was more effective and efficient for structure tuning of $g-C_3N_4$ than SubMeOH. Figure 2f shows the element contents for samples measured by an elemental analyzer. According to the
$C_3N_4$ modification compared with SubMeOH, and the high reactivity of ScMeOH determined that the residence time was very crucial and sensitive for $g-C_3N_4$ modification.

Three typical samples were selected for further analysis, including a sample of pristine $g-C_3N_4$ (CN), a modified $g-C_3N_4$ by prolonged the SubMeOH post-treatment (CN-230-30) and a modified $g-C_3N_4$ by a rapid ScMeOH post-treatment (CN-300-0). As shown in Figure 1c,d, CN-300-0 showed the best photocatalytic $H_2$-evolution rate of 50.1 $\mu mol \cdot h^{-1}$, which was 2.8 times that of CN-230-30 and 7.2 times that of CN, and it showed obvious higher apparent quantum yield (AQY) values as expected based on the photocatalytic $H_2$-evolution activity. Specifically, CN-300-0 showed a certain AQY values of 1.55% (450 nm), 0.78% (475 nm), and 0.25% (500 nm), while the AQY values of CN and CN-230-30 were not detected at the wavelength higher than 450 nm, and these results were in agreement with the UV-vis spectra. In addition, Figure 1e presents the cyclic photocatalytic $H_2$-evolution test for CN-300-0, and there was little decay after three cyclic tests (15 h), indicating good photocatalytic stability of the modified $g-C_3N_4$ by the ScMeOH post-treatment. It was worth mentioning that $g-C_3N_4$ was not only modified by the ScMeOH post-treatment effectively and efficiently compared with conventional solvothermal post-treatment (Table S2) but also exhibited an advantage of high yield, and the yield of CN-300-0 could reach 80.1% with the initial dispersion concentration as 100 mg·mL$^{-1}$ (Figure S3). Moreover, when $g-C_3N_4$ was modified by the SCF post-treatment using other alcohols including ethanol (EtOH), 1-propanol (NPA), and isopropanol (IPA), the photocatalytic activities of modified $g-C_3N_4$ could also be improved (Figure 1f).

**Structure Tuning of $g-C_3N_4$ by the ScMeOH Post-Treatment.** Ideal $g-C_3N_4$ is composed of few-layer 2D nanosheets bonded by van der Waals forces, and the in-plane structure of the nanosheet is composed of repeating units of continuous heptazine (Figure S4a). However, pristine $g-C_3N_4$ prepared from the thermal condensation of melamine is usually amorphous (Figure S4b).⁴⁵ In this work, structure tuning of $g-C_3N_4$ was realized by a ScMeOH post-treatment strategy, as depicted in Figure 2a. In this process, the $CH_3OH$ molecules in the supercritical state diffused into the interlayers of $g-C_3N_4$ and contacted with the intralayers. The high reactivity of ScMeOH further led to breakages of C-N bonds between NH-bridged melem monomers, and the amino group was expected to be electrophilically attacked by the methyl group, while the heptazine ring was expected to be nucleophilically attacked by the hydroxyl group, resulting in N-methylation and hydroxylation reactions, respectively (Figure S5). The ordered breakage of C-N bridging bonds led to the regular knocking out of heptazine rings from melon polymer; simultaneously, N-methylation and hydroxylation reactions occurred in the intralayers of $g-C_3N_4$, forming a carbon nitride with a poly(heptazine imide) (PHI)-like structure as well as abundant methyl and hydroxyl groups. In general, PHI refers to the structure with high crystallinity, which is typically synthesized by the ionthermal treatment of melon polymers,⁴⁶ thereby the PHI-like structure transformed from amorphous $g-C_3N_4$ in this work was termed as quasi-poly(heptazine imide) (Q-PHI). Figure S6 displays the basic structure unit of PHI and Q-PHI. According to the substitution positions of methyl and hydroxyl groups, there were three kinds of potential Q-PHI models, as shown in Figure 2b, and the corresponding chemical formulas of exfoliated units are listed in Table S2. In addition,

theoretical calculations were conducted to analyze the feasibility of the reactions, and the result is shown in Figure 2c. Specifically, the Gibbs free energy pathways for N-methylation and hydroxylation reactions in $g-C_3N_4$ were analyzed. The reaction of Q-PHI formation was spontaneous in thermodynamics due to the negative value of the Gibbs free energy change ($\Delta G = -105.1 \text{ kcal} \cdot \text{mol}^{-1}$). In addition, more $CH_3OH$ molecules enabled to overcome the energy barrier (40.7 kcal·mol$^{-1}$) at high temperature and pressure in chemical kinetics, which was in good accordance with the experimental results that the Q-PHI formation reaction occurred spontaneously and quickly under high temperature and pressure.

Analysis of the gas and liquid products after solvothermal post-treatment and the corresponding blank runs (in the absence of $g-C_3N_4$) was conducive to understanding the structural evolution of $g-C_3N_4$ by the ScMeOH post-treatment. The temperature and pressure curves for the post-treatment process of CN-230-30, CN-300-0, and the corresponding blank runs are shown in Figure S7. The composition of gas products was detected by gas chromatography (GC), and the results are shown in Figures 2d and S8, and the volume of gas products was measured using a graduated syringe, as shown in Figure S9. There was almost no $CH_3OH$ decomposition from MeOH-230-30, since only a trace of $H_2$ was detected, while certain amounts of $H_2$ (11.4 mL), CO (4.9 mL), and $CH_4$ (0.49 mL) were produced from MeOH-300-0. The proposed relevant reactions are provided in eqs 1 and 2.

$$CH_3OH \rightarrow CO + 2H_2 \quad (1)$$

$$CO + 3H_2 \rightarrow CH_4 + H_2O \quad (2)$$

It was found that neither MeOH-230-30 nor MeOH-300-0 had any detectable $CO_2$, while a significant amount of $CO_2$ was produced when $g-C_3N_4$ participated in the solvothermal reaction, simultaneously reducing gases ($H_2$, CO) almost disappeared, resulting in the gas products from CN-230-30 and CN-300-0 being mainly composed of $CO_2$. Specifically, the $CO_2$ amounts in the gas phase were 2.7 mL from CN-230-30 and 8.2 mL from CN-300-0, whose proportions were calculated as 88.0 and 91.8%, respectively. Furthermore, the composition of liquid products was measured by the combination of gas chromatography and mass spectrometry (GC-MS), and the results are shown in Figures 2e and S10. The GC-MS analysis of $CH_3OH$ could be regarded as a baseline, and the GC-MS analysis of liquid products from MeOH-230-30 and MeOH-300-0 exhibited similarity with $CH_3OH$. In contrast, several significant peaks appeared in the GC-MS signals for the liquid products from CN-230-30 and CN-300-0, which were identified as a series of triazine-derived compounds utilizing mass spectrometry. The formation of these compounds resulted from the reaction between ScMeOH and the exfoliated heptazine rings, and the formed short-chain organic compounds were further transformed into $CO_2$ and $NH_3$ with the help of ScMeOH and reducing gases. It was also consistent with the result of the appearance of $CO_2$ and the disappearance of $H_2$ in the gas products from CN-230-30 and CN-300-0. Moreover, the preparation of CN-300-0 produced a greater amount of $CO_2$ in gas products and a higher concentration of triazine-derived compounds in liquid products than those of CN-230-30, indicating that ScMeOH was more effective and efficient for structure tuning of $g-C_3N_4$ than SubMeOH. Figure 2f shows the element contents for samples measured by an elemental analyzer. According to the
<figure><img src="image_4.png" /><figcaption>Figure 3. (a) XRD patterns, (b, c) partially magnified details of XRD patterns in (a), (d) FTIR spectra, (e) solid-state ¹³C NMR spectra, (f–h) high-resolution XPS of (f) C 1s, (g) N 1s, and (h) O 1s orbitals, (i) TG curves, and (j) dark solid-state EPR spectra for CN, CN-230-30, and CN-300-0.</figcaption></figure>

Q-PHI models with different grafted groups and the corresponding chemical formulas of exfoliated units in Table S3, the formation of Q-PHI and the grafting of methyl and hydroxyl groups comprehensively increased the C/N atomic ratio and also increased the H and O content in modified g-C₃N₄.

As shown in Figure 3a, two characteristic peaks at around 12.8 and 27.3°, which were attributed to (100) and (002) diffraction facets of g-C₃N₄, were found in CN, CN-230-30, and CN-300-0. Two new dominant characteristic diffraction peaks at 7.2 and 7.6°, which were ascribed to the (100) diffraction facet of the Q-PHI structure, appeared in both CN-230-30 and CN-300-0 (Figure 3b).⁴⁶ The in-planar arrangement of Q-PHI was asymmetric due to the introduction of methyl and hydroxylation groups, thus forming two independent X-ray diffraction (XRD) characteristic peaks of the Q-PHI phase. In addition, the overlapping (002) XRD

peak in CN-300-0 could be deconvoluted into three XRD peaks located at 25.6, 27.1, and 27.3°, and the corresponding interlayer spacing values were 3.48, 3.29, and 3.26 Å, respectively (Figure 3c). The grafted methyl and hydroxyl groups in the Q-PHI component expanded the interlayer spacing;⁴⁷ consequently, the three different values mentioned above represented the interlayer spacings of Q-PHI/Q-PHI, g-C₃N₄/Q-PHI, and g-C₃N₄/g-C₃N₄, respectively. Interestingly, when other alcohols including EtOH, NPA, and IPA were employed as SCFs for the g-C₃N₄ post-treatment, there was no Q-PHI component in the optimally modified g-C₃N₄ according to the XRD patterns (Figure S11). It was deduced that the CH₃OH molecule was more matching to penetrate the interlayers of g-C₃N₄ than longer-chain alcohols due to its smaller molecular size. Additionally, the free energy barrier of reaction for Q-PHI formation caused by CH₃OH was lower compared to that of longer-chain alcohols such as C₂H₅OH
<figure><img src="image_4.png" /><figcaption>Figure 3. (a) XRD patterns, (b, c) partially magnified details of XRD patterns in (a), (d) FTIR spectra, (e) solid-state ¹³C NMR spectra, (f–h) high-resolution XPS of (f) C 1s, (g) N 1s, and (h) O 1s orbitals, (i) TG curves, and (j) dark solid-state EPR spectra for CN, CN-230-30, and CN-300-0.</figcaption></figure>

Q-PHI models with different grafted groups and the corresponding chemical formulas of exfoliated units in Table S3, the formation of Q-PHI and the grafting of methyl and hydroxyl groups comprehensively increased the C/N atomic ratio and also increased the H and O content in modified g-C₃N₄.

As shown in Figure 3a, two characteristic peaks at around 12.8 and 27.3°, which were attributed to (100) and (002) diffraction facets of g-C₃N₄, were found in CN, CN-230-30, and CN-300-0. Two new dominant characteristic diffraction peaks at 7.2 and 7.6°, which were ascribed to the (100) diffraction facet of the Q-PHI structure, appeared in both CN-230-30 and CN-300-0 (Figure 3b).⁴⁶ The in-planar arrangement of Q-PHI was asymmetric due to the introduction of methyl and hydroxylation groups, thus forming two independent X-ray diffraction (XRD) characteristic peaks of the Q-PHI phase. In addition, the overlapping (002) XRD

peak in CN-300-0 could be deconvoluted into three XRD peaks located at 25.6, 27.1, and 27.3°, and the corresponding interlayer spacing values were 3.48, 3.29, and 3.26 Å, respectively (Figure 3c). The grafted methyl and hydroxyl groups in the Q-PHI component expanded the interlayer spacing;⁴⁷ consequently, the three different values mentioned above represented the interlayer spacings of Q-PHI/Q-PHI, g-C₃N₄/Q-PHI, and g-C₃N₄/g-C₃N₄, respectively. Interestingly, when other alcohols including EtOH, NPA, and IPA were employed as SCFs for the g-C₃N₄ post-treatment, there was no Q-PHI component in the optimally modified g-C₃N₄ according to the XRD patterns (Figure S11). It was deduced that the CH₃OH molecule was more matching to penetrate the interlayers of g-C₃N₄ than longer-chain alcohols due to its smaller molecular size. Additionally, the free energy barrier of reaction for Q-PHI formation caused by CH₃OH was lower compared to that of longer-chain alcohols such as C₂H₅OH
<figure><img src="image_5.png" /><figcaption>Figure 4. (a) TEM image, (b) high-resolution TEM image, and (c) magnified image of selected area in (b) for CN-300-0. Scale bar: (a) 1 µm, (b) 20 nm, and (c) 5 nm. (d) Dynamic light scattering spectra and (inset) the average sizes and (e) N₂ adsorption–desorption isotherms for CN, CN-230-30, and CN-300-0.</figcaption></figure>

(Figure S12). Taking IPA-340-0 as an example, the main structure tuning occurred on the surface of g-C₃N₄ through supercritical IPA post-treatment, leading to abundant isopropyl groups being grafted onto the surface of g-C₃N₄. The modification of the surface isopropyl groups contributed to the enhanced photocatalytic activity, as shown in Figure 1f.

Fourier transform infrared (FTIR) spectra were recorded to investigate the molecular structure and chemical bonds in the samples. As shown in Figure 3d, for all three samples, a sequence of representative absorption peaks that were attributed to the bending or stretching vibration modes of C-N heterocyclic rings was found at 1641, 1571, 1410, 1321, and 1241 cm⁻¹, and the peak at 807 cm⁻¹ was identified as the breathing modes of s-triazine ring units. On account of both g-C₃N₄ and Q-PHI being composed of heptazine rings, CN, CN-230-30, and CN-300-0 exhibited similar FTIR patterns at the above-mentioned positions. Furthermore, CN-230-30 and CN-300-0 displayed a broad band in the range 3350-3650 cm⁻¹, which was attributed to the mode of vibration of hydroxyl groups. Distinct peaks including a peak at 2930 cm⁻¹, corresponding to the stretching mode of C-H, a peak at 1386 cm⁻¹, corresponding to the bending mode of C-H, and a peak at 1135 cm⁻¹, corresponding to the stretching mode of C-O, were observed in CN-300-0. The variations of peaks that were attributed to the O-H, C-H, and C-O bonds indicated the appearance of more hydroxyl and methyl groups in the modified g-C₃N₄ by the ScMeOH post-treatment. Furthermore, Figure 3e shows the solid-state ¹³C nuclear magnetic resonance (NMR) spectra of CN, CN-230-30, and CN-300-0 that could reveal the chemical state of C atoms. Two strong peaks at 167.1 and 157.7 ppm were observed for all samples, which could be attributed to the N₂-C-NHₓ (C1

atoms) and C-N₃ (C2 atoms) in the tri-s-triazine ring, respectively.⁴⁸ Another new peak with a chemical shift of approximately 30.4 ppm appeared in the ¹³C NMR spectrum of CN-300-0, and a weak peak was observed at the same location in the spectrum of CN-230-30. It could be ascribed to the methyl groups grafted into the Q-PHI skeleton.⁴⁹

X-ray photoelectron spectra (XPS) were measured to investigate more structural information, and the results are shown in Figure 3f-h and Table S4. The C 1s XPS of CN could be fitted into three peaks indexed to standard carbon (284.8 eV), sp²-hybridized C in C-NHₓ (286.0 eV), and sp²-hybridized C in N-C≡N (288.2 eV). The N 1s XPS of CN could be fitted into four peaks indexed to sp²-hybridized N in C-N≡C (398.4 eV), sp³-hybridized in N-(C)₃ (398.9 eV), amino functions (400.5 eV), and charging effect (404.0 eV). The C 1s and N 1s XPS of CN-230-30 and CN-300-0 could also be fitted in a similar fitting rule. Compared with CN, another C 1s peak appeared in CN-230-30 (291.4 eV) and CN-300-0 (290.2 eV), which could be ascribed to sp²-hybridization in C-OH, and a new N 1s peak appeared in CN-300-0 (403.3 eV), which could be ascribed to N species in N-C-OH. The binding energies of C 1s in N≡C-N and N 1s in C≡N-C for all samples were positioned at similar locations without a significant shift, since the basic heptazine heterocyclic rings of modified g-C₃N₄ were not destroyed. Moreover, the C 1s peak of C-N for CN-300-0 (285.7 eV) shifted to lower binding energy in comparison with those for CN (286.0 eV) and CN-230-30 (286.0 eV), while the N 1s peak of N-(C)₃ for CN-300-0 (399.2 eV) shifted to higher binding energy in comparison with those for CN (398.9 eV) and CN-230-30 (399.1 eV). It should be attributed to the breaking of bridge linkages in heptazine rings, leading to the
<figure><img src="image_5.png" /><figcaption>Figure 4. (a) TEM image, (b) high-resolution TEM image, and (c) magnified image of selected area in (b) for CN-300-0. Scale bar: (a) 1 µm, (b) 20 nm, and (c) 5 nm. (d) Dynamic light scattering spectra and (inset) the average sizes and (e) N₂ adsorption–desorption isotherms for CN, CN-230-30, and CN-300-0.</figcaption></figure>

(Figure S12). Taking IPA-340-0 as an example, the main structure tuning occurred on the surface of g-C₃N₄ through supercritical IPA post-treatment, leading to abundant isopropyl groups being grafted onto the surface of g-C₃N₄. The modification of the surface isopropyl groups contributed to the enhanced photocatalytic activity, as shown in Figure 1f.

Fourier transform infrared (FTIR) spectra were recorded to investigate the molecular structure and chemical bonds in the samples. As shown in Figure 3d, for all three samples, a sequence of representative absorption peaks that were attributed to the bending or stretching vibration modes of C-N heterocyclic rings was found at 1641, 1571, 1410, 1321, and 1241 cm⁻¹, and the peak at 807 cm⁻¹ was identified as the breathing modes of s-triazine ring units. On account of both g-C₃N₄ and Q-PHI being composed of heptazine rings, CN, CN-230-30, and CN-300-0 exhibited similar FTIR patterns at the above-mentioned positions. Furthermore, CN-230-30 and CN-300-0 displayed a broad band in the range 3350-3650 cm⁻¹, which was attributed to the mode of vibration of hydroxyl groups. Distinct peaks including a peak at 2930 cm⁻¹, corresponding to the stretching mode of C-H, a peak at 1386 cm⁻¹, corresponding to the bending mode of C-H, and a peak at 1135 cm⁻¹, corresponding to the stretching mode of C-O, were observed in CN-300-0. The variations of peaks that were attributed to the O-H, C-H, and C-O bonds indicated the appearance of more hydroxyl and methyl groups in the modified g-C₃N₄ by the ScMeOH post-treatment. Furthermore, Figure 3e shows the solid-state ¹³C nuclear magnetic resonance (NMR) spectra of CN, CN-230-30, and CN-300-0 that could reveal the chemical state of C atoms. Two strong peaks at 167.1 and 157.7 ppm were observed for all samples, which could be attributed to the N₂-C-NHₓ (C1

atoms) and C-N₃ (C2 atoms) in the tri-s-triazine ring, respectively.⁴⁸ Another new peak with a chemical shift of approximately 30.4 ppm appeared in the ¹³C NMR spectrum of CN-300-0, and a weak peak was observed at the same location in the spectrum of CN-230-30. It could be ascribed to the methyl groups grafted into the Q-PHI skeleton.⁴⁹

X-ray photoelectron spectra (XPS) were measured to investigate more structural information, and the results are shown in Figure 3f-h and Table S4. The C 1s XPS of CN could be fitted into three peaks indexed to standard carbon (284.8 eV), sp²-hybridized C in C-NHₓ (286.0 eV), and sp²-hybridized C in N-C≡N (288.2 eV). The N 1s XPS of CN could be fitted into four peaks indexed to sp²-hybridized N in C-N≡C (398.4 eV), sp³-hybridized in N-(C)₃ (398.9 eV), amino functions (400.5 eV), and charging effect (404.0 eV). The C 1s and N 1s XPS of CN-230-30 and CN-300-0 could also be fitted in a similar fitting rule. Compared with CN, another C 1s peak appeared in CN-230-30 (291.4 eV) and CN-300-0 (290.2 eV), which could be ascribed to sp²-hybridization in C-OH, and a new N 1s peak appeared in CN-300-0 (403.3 eV), which could be ascribed to N species in N-C-OH. The binding energies of C 1s in N≡C-N and N 1s in C≡N-C for all samples were positioned at similar locations without a significant shift, since the basic heptazine heterocyclic rings of modified g-C₃N₄ were not destroyed. Moreover, the C 1s peak of C-N for CN-300-0 (285.7 eV) shifted to lower binding energy in comparison with those for CN (286.0 eV) and CN-230-30 (286.0 eV), while the N 1s peak of N-(C)₃ for CN-300-0 (399.2 eV) shifted to higher binding energy in comparison with those for CN (398.9 eV) and CN-230-30 (399.1 eV). It should be attributed to the breaking of bridge linkages in heptazine rings, leading to the
<figure><img src="image_6.png" /><figcaption>Figure 5. (a) UV–vis spectra, (b) steady-state PL spectra, (c) time-resolved PL decay spectra, (d) EIS results, (e) transient photocurrent curves under chopped light irradiation, and (f) solid-state EPR spectra for CN, CN-230-30, and CN-300-0 in the dark and under light irradiation. The inset in (d) shows an equivalent circuit. R<sub>s</sub>: electrolyte resistance; R<sub>ct</sub>: charge-transfer resistance from the electrode to the electrolyte; and C<sub>PE</sub>: constant phase element.</figcaption></figure>

surrounding chemical environment change. In addition, the O 1s spectrum of CN was fitted into two peaks belonging to the surface-adsorbed hydroxyl group (531.9 eV) and doped O (533.3 eV). Another O 1s XPS peak of the grafted hydroxyl (534.5 eV) was detected in CN-300-0. In addition, the survey-scan XPS of all samples as well as the Ni 2p and Cr 2p high-resolution XPS of CN-300-0 (Figure S13). There was no XPS signal attributed to metallic elements (including Ni, Cr, and Mo) detected in CN-300-0, thereby excluding the impacts of detached alloy particles in this study.

The thermogravimetric (TG) curves of the samples are shown in Figure 3i. CN remained stable up to 570 °C and decomposed completely up to 750 °C, and the TG curve of CN-230-30 revealed the analogous characteristics as CN. Different from CN and CN-230-30, CN-300-0 exhibited two significant decomposition stages. The first stage from 350 to 550 °C represented the decomposition of the Q-PHI component (~16.9%), and the second stage from 550 to 750 °C corresponded to the decomposition of the g-C₃N₄ component (~80.2%).⁵⁰ In the range of 25–350 °C, there was an obvious weight loss (~2.9%) in CN-300-0, which was attributed to the presence of hydroxyl and methyl groups (Figure S14).⁵¹ Figure 3j shows the electron paramagnetic resonance (EPR) spectra of samples in the dark, where CN, CN-230-30, and CN-300-0 all exhibited a single Lorentzian line with a g-factor of 2.004, which originated from the unpaired electrons on the C atoms in heptazine rings. The intensity of the EPR signal for CN-300-0 was much higher than that for CN-230-30 and CN, which was attributed to the increased delocalization degree of the π-conjugated system

caused by structure tuning.⁵² It was also found that CN-300-0 exhibited more prominent signals corresponding to Q-PHI and methyl/hydroxyl groups compared with CN-230-30 in the aforementioned characterizations, indicating that ScMeOH was more effective and efficient for structure tuning g-C₃N₄ than SubMeOH. As discussed above, the modified g-C₃N₄ by the ScMeOH post-treatment was a kind of composite composed of g-C₃N₄ and Q-PHI, while abundant methyl and hydroxyl groups were grafted into modified g-C₃N₄.

The SCFs are commonly applied in nanosheet production by exfoliation of layered materials, and some phenomena concerning the exfoliation of aggregated g-C₃N₄ in ScMeOH were discovered. CN and CN-230-30 displayed a thick bulk with the aggregation of 2D sheets (Figure S15a,b), whereas some 2D thin sheets with wrinkles appeared in CN-300-0 (Figures 4a and S15c). Notably, Figure S15d shows the energy-dispersive X-ray (EDX) spectrum scanned from the scanning electron microscopy (SEM) image of CN-300-0 as well as the corresponding result of elemental content, and the inexistence of metal elements in modified g-C₃N₄ was consistent with the XPS results. Figure 4b,c presents the high-resolution transmission electron microscopy (TEM) image of CN-300-0, where only the lattice fringe of approximately 0.69 nm corresponding to the (100) plane of g-C₃N₄ was observed. It revealed the limited distribution range of the Q-PHI component compared with the g-C₃N₄ component. Figure 4d shows the size-distribution curves of all samples, and the average hydrodynamic diameters of CN, CN-230-30, and CN-300-0 were 1.86, 1.11, and 0.76 µm, respectively. The larger-size pristine g-C₃N₄ particles disinte-
<figure><img src="image_6.png" /><figcaption>Figure 5. (a) UV–vis spectra, (b) steady-state PL spectra, (c) time-resolved PL decay spectra, (d) EIS results, (e) transient photocurrent curves under chopped light irradiation, and (f) solid-state EPR spectra for CN, CN-230-30, and CN-300-0 in the dark and under light irradiation. The inset in (d) shows an equivalent circuit. R<sub>s</sub>: electrolyte resistance; R<sub>ct</sub>: charge-transfer resistance from the electrode to the electrolyte; and C<sub>PE</sub>: constant phase element.</figcaption></figure>

surrounding chemical environment change. In addition, the O 1s spectrum of CN was fitted into two peaks belonging to the surface-adsorbed hydroxyl group (531.9 eV) and doped O (533.3 eV). Another O 1s XPS peak of the grafted hydroxyl (534.5 eV) was detected in CN-300-0. In addition, the survey-scan XPS of all samples as well as the Ni 2p and Cr 2p high-resolution XPS of CN-300-0 (Figure S13). There was no XPS signal attributed to metallic elements (including Ni, Cr, and Mo) detected in CN-300-0, thereby excluding the impacts of detached alloy particles in this study.

The thermogravimetric (TG) curves of the samples are shown in Figure 3i. CN remained stable up to 570 °C and decomposed completely up to 750 °C, and the TG curve of CN-230-30 revealed the analogous characteristics as CN. Different from CN and CN-230-30, CN-300-0 exhibited two significant decomposition stages. The first stage from 350 to 550 °C represented the decomposition of the Q-PHI component (~16.9%), and the second stage from 550 to 750 °C corresponded to the decomposition of the g-C₃N₄ component (~80.2%).⁵⁰ In the range of 25–350 °C, there was an obvious weight loss (~2.9%) in CN-300-0, which was attributed to the presence of hydroxyl and methyl groups (Figure S14).⁵¹ Figure 3j shows the electron paramagnetic resonance (EPR) spectra of samples in the dark, where CN, CN-230-30, and CN-300-0 all exhibited a single Lorentzian line with a g-factor of 2.004, which originated from the unpaired electrons on the C atoms in heptazine rings. The intensity of the EPR signal for CN-300-0 was much higher than that for CN-230-30 and CN, which was attributed to the increased delocalization degree of the π-conjugated system

caused by structure tuning.⁵² It was also found that CN-300-0 exhibited more prominent signals corresponding to Q-PHI and methyl/hydroxyl groups compared with CN-230-30 in the aforementioned characterizations, indicating that ScMeOH was more effective and efficient for structure tuning g-C₃N₄ than SubMeOH. As discussed above, the modified g-C₃N₄ by the ScMeOH post-treatment was a kind of composite composed of g-C₃N₄ and Q-PHI, while abundant methyl and hydroxyl groups were grafted into modified g-C₃N₄.

The SCFs are commonly applied in nanosheet production by exfoliation of layered materials, and some phenomena concerning the exfoliation of aggregated g-C₃N₄ in ScMeOH were discovered. CN and CN-230-30 displayed a thick bulk with the aggregation of 2D sheets (Figure S15a,b), whereas some 2D thin sheets with wrinkles appeared in CN-300-0 (Figures 4a and S15c). Notably, Figure S15d shows the energy-dispersive X-ray (EDX) spectrum scanned from the scanning electron microscopy (SEM) image of CN-300-0 as well as the corresponding result of elemental content, and the inexistence of metal elements in modified g-C₃N₄ was consistent with the XPS results. Figure 4b,c presents the high-resolution transmission electron microscopy (TEM) image of CN-300-0, where only the lattice fringe of approximately 0.69 nm corresponding to the (100) plane of g-C₃N₄ was observed. It revealed the limited distribution range of the Q-PHI component compared with the g-C₃N₄ component. Figure 4d shows the size-distribution curves of all samples, and the average hydrodynamic diameters of CN, CN-230-30, and CN-300-0 were 1.86, 1.11, and 0.76 µm, respectively. The larger-size pristine g-C₃N₄ particles disinte-
grated into smaller-size segments, causing the intensity and position variations in size-distribution peak curves, and the ScMeOH displayed enhanced breakage and modification effect compared with SubMeOH, as repeatedly demonstrated by the aforementioned various characteristic results. Figure 4e shows the N₂ adsorption–desorption isotherms and the corresponding pore size-distribution curves of CN, CN-230-30, and CN-300-0. Due to the shearing and exfoliation effects of high-pressure and high-temperature fluids, the solvothermal post-treatment enlarged the Brunauer–Emmette–Teller (BET) surface area and generated more mesopores (Figure S16), which could increase the reactive sites and enhance the mass transfer in photocatalytic reaction. However, the BET surface area of CN-230-30 (24.1 m²·g⁻¹) was larger than that of CN-300-0 (20.4 m²·g⁻¹), indicating that there are more reasons for the enhanced photocatalytic activity in addition to the enlarged specific surface area of modified g-C₃N₄. The zeta-potential of the CN aqueous dispersion was negative, -22.5 mV, which became positive, 16.3 and 29.6 mV, for CN-230-30 and CN-300-0, respectively (Figure S17). In addition, the CN, CN-230-30, and CN-300-0 showed water contact angles of 32.3, 47.6, and 44.1°, respectively (Figure S18). The significant change of zeta potentials and water contact angles resulted from the appearance of abundant groups on the particle surface after solvothermal post-treatment. The ScMeOH post-treatment resulted in the exfoliation and phase transformation of g-C₃N₄, as well as causing methylation and hydroxylation reactions within g-C₃N₄, leading to a g-C₃N₄/Q-PHI isotype heterojunction photocatalyst with grafted methyl and hydroxyl groups.

**Mechanism of Enhanced Photocatalytic H₂-Evolution Activity.** The ScMeOH post-treatment successfully realized structure tuning for g-C₃N₄, and further analysis of optical properties, band alignment, and photogenerated charge carrier dynamics for samples could help understand the mechanism for enhanced photocatalytic H₂-evolution activity. Figure 5a shows the ultraviolet–visible (UV–vis) spectra of CN, CN-230-30, and CN-300-0. The CN exhibited an intrinsic absorption band below 450 nm due to a π → π* electron transition, and the absorption edge of CN-230-30 red shifted slightly compared with that of CN. Moreover, an obvious new absorption band formed in the region from 450 to 700 nm in CN-300-0, which was ascribed to the n → π* electron transition caused by long pairs of the grafted hydroxyl group in the Q-PHI component.<sup>53,54</sup>

Based on the atomic structure models constructed for g-C₃N₄ and Q-PHI, density functional theory (DFT) calculations have provided theoretical evidence that g-C₃N₄ and Q-PHI formed a type II alignment (Figures S19 and S20). In addition, the Q-PHI component was *in situ* derived from g-C₃N₄, thus forming an intimate type II isotype heterojunction between g-C₃N₄ and Q-PHI, which led to efficient charge separation within the modified g-C₃N₄.<sup>55</sup> As shown in Figure 5b, CN displayed a strong band-to-band emission peak at 463 nm, and the photoluminescence (PL) intensity for CN-230-30 significantly decreased. In contrast, CN-300-0 showed the weakest PL intensity, with two emission peaks located at 442 and 464 nm, corresponding to Q-PHI and g-C₃N₄. Figure 5c shows the time-resolved PL decay spectra and the calculated average PL lifetime (τ<sub>ave</sub>). CN-300-0 had the shortest τ<sub>ave</sub> (3.79 ns) compared with CN-230-30 (4.27 ns) and CN (5.64 ns). Both the decreased steady-state PL intensity and the shortened PL lifetime demonstrated that the recombination of photo-

generated charge carrier was effectively suppressed.<sup>56</sup> In addition, from the electrochemical impedance spectra (EIS) results in Figure 5d, CN-300-0 had the smallest arc radius in its EIS results and CN-230-30 had a smaller arc radius than CN. These results suggested a lower charge-transfer impedance as well as a significant improvement in electron–hole separation and transfer rate of the modified g-C₃N₄ after the ScMeOH post-treatment. As seen in Figure 5e, the photocurrent of CN-300-0 and CN-230-30 were approximate and both were much higher than that of CN, implying an efficient separation and migration for the photogenerated charge carrier under light irradiation. The light irradiation could further enhance the EPR intensities of the photocatalyst owing to the presence of photogenerated electrons; the EPR spectra for samples in the dark and under light irradiation are shown in Figure 5f. Compared with the EPR signals in the dark, the EPR intensity enhancements under light irradiation in CN-230-30 and CN-300-0 were much higher than that of CN, indicating efficient generation of charge carrier in modified g-C₃N₄, which largely contributed to the enhanced photocatalytic activity.<sup>57</sup>

Based on the above analysis, a mechanism for the enhanced photocatalytic H₂-evolution activity for modified g-C₃N₄ by the ScMeOH post-treatment could be proposed and illustrated in Figure 6. According to the band structures of g-C₃N₄ and Q-

<figure><img src="image_7.png" /><figcaption>Figure 6. Schematic mechanism of the g-C₃N₄/Q-PHI isotype heterojunction with abundant grafted groups for photocatalytic H₂ evolution.</figcaption></figure>

PHI, a type II isotype heterojunction could be constructed between g-C₃N₄ and Q-PHI. The ordered and partial *in situ* phase transformation from g-C₃N₄ to Q-PHI formed a g-C₃N₄/Q-PHI composite coupled with chemical bonding, leading to an intimate heterojunction with a strong interfacial electric field that could effectively promote the separation and migration of charge carrier. In addition, the introduced abundant methyl and hydroxyl groups into the bulk structure of carbon nitride contributed to enhancing light absorption capacity, modulating the carrier dynamics, and increasing the active sites.<sup>58</sup> Furthermore, the exfoliation and linkage
grated into smaller-size segments, causing the intensity and position variations in size-distribution peak curves, and the ScMeOH displayed enhanced breakage and modification effect compared with SubMeOH, as repeatedly demonstrated by the aforementioned various characteristic results. Figure 4e shows the N₂ adsorption–desorption isotherms and the corresponding pore size-distribution curves of CN, CN-230-30, and CN-300-0. Due to the shearing and exfoliation effects of high-pressure and high-temperature fluids, the solvothermal post-treatment enlarged the Brunauer–Emmette–Teller (BET) surface area and generated more mesopores (Figure S16), which could increase the reactive sites and enhance the mass transfer in photocatalytic reaction. However, the BET surface area of CN-230-30 (24.1 m²·g⁻¹) was larger than that of CN-300-0 (20.4 m²·g⁻¹), indicating that there are more reasons for the enhanced photocatalytic activity in addition to the enlarged specific surface area of modified g-C₃N₄. The zeta-potential of the CN aqueous dispersion was negative, -22.5 mV, which became positive, 16.3 and 29.6 mV, for CN-230-30 and CN-300-0, respectively (Figure S17). In addition, the CN, CN-230-30, and CN-300-0 showed water contact angles of 32.3, 47.6, and 44.1°, respectively (Figure S18). The significant change of zeta potentials and water contact angles resulted from the appearance of abundant groups on the particle surface after solvothermal post-treatment. The ScMeOH post-treatment resulted in the exfoliation and phase transformation of g-C₃N₄, as well as causing methylation and hydroxylation reactions within g-C₃N₄, leading to a g-C₃N₄/Q-PHI isotype heterojunction photocatalyst with grafted methyl and hydroxyl groups.

**Mechanism of Enhanced Photocatalytic H₂-Evolution Activity.** The ScMeOH post-treatment successfully realized structure tuning for g-C₃N₄, and further analysis of optical properties, band alignment, and photogenerated charge carrier dynamics for samples could help understand the mechanism for enhanced photocatalytic H₂-evolution activity. Figure 5a shows the ultraviolet–visible (UV–vis) spectra of CN, CN-230-30, and CN-300-0. The CN exhibited an intrinsic absorption band below 450 nm due to a π → π* electron transition, and the absorption edge of CN-230-30 red shifted slightly compared with that of CN. Moreover, an obvious new absorption band formed in the region from 450 to 700 nm in CN-300-0, which was ascribed to the n → π* electron transition caused by long pairs of the grafted hydroxyl group in the Q-PHI component.<sup>53,54</sup>

Based on the atomic structure models constructed for g-C₃N₄ and Q-PHI, density functional theory (DFT) calculations have provided theoretical evidence that g-C₃N₄ and Q-PHI formed a type II alignment (Figures S19 and S20). In addition, the Q-PHI component was *in situ* derived from g-C₃N₄, thus forming an intimate type II isotype heterojunction between g-C₃N₄ and Q-PHI, which led to efficient charge separation within the modified g-C₃N₄.<sup>55</sup> As shown in Figure 5b, CN displayed a strong band-to-band emission peak at 463 nm, and the photoluminescence (PL) intensity for CN-230-30 significantly decreased. In contrast, CN-300-0 showed the weakest PL intensity, with two emission peaks located at 442 and 464 nm, corresponding to Q-PHI and g-C₃N₄. Figure 5c shows the time-resolved PL decay spectra and the calculated average PL lifetime (τ<sub>ave</sub>). CN-300-0 had the shortest τ<sub>ave</sub> (3.79 ns) compared with CN-230-30 (4.27 ns) and CN (5.64 ns). Both the decreased steady-state PL intensity and the shortened PL lifetime demonstrated that the recombination of photo-

generated charge carrier was effectively suppressed.<sup>56</sup> In addition, from the electrochemical impedance spectra (EIS) results in Figure 5d, CN-300-0 had the smallest arc radius in its EIS results and CN-230-30 had a smaller arc radius than CN. These results suggested a lower charge-transfer impedance as well as a significant improvement in electron–hole separation and transfer rate of the modified g-C₃N₄ after the ScMeOH post-treatment. As seen in Figure 5e, the photocurrent of CN-300-0 and CN-230-30 were approximate and both were much higher than that of CN, implying an efficient separation and migration for the photogenerated charge carrier under light irradiation. The light irradiation could further enhance the EPR intensities of the photocatalyst owing to the presence of photogenerated electrons; the EPR spectra for samples in the dark and under light irradiation are shown in Figure 5f. Compared with the EPR signals in the dark, the EPR intensity enhancements under light irradiation in CN-230-30 and CN-300-0 were much higher than that of CN, indicating efficient generation of charge carrier in modified g-C₃N₄, which largely contributed to the enhanced photocatalytic activity.<sup>57</sup>

Based on the above analysis, a mechanism for the enhanced photocatalytic H₂-evolution activity for modified g-C₃N₄ by the ScMeOH post-treatment could be proposed and illustrated in Figure 6. According to the band structures of g-C₃N₄ and Q-

<figure><img src="image_7.png" /><figcaption>Figure 6. Schematic mechanism of the g-C₃N₄/Q-PHI isotype heterojunction with abundant grafted groups for photocatalytic H₂ evolution.</figcaption></figure>

PHI, a type II isotype heterojunction could be constructed between g-C₃N₄ and Q-PHI. The ordered and partial *in situ* phase transformation from g-C₃N₄ to Q-PHI formed a g-C₃N₄/Q-PHI composite coupled with chemical bonding, leading to an intimate heterojunction with a strong interfacial electric field that could effectively promote the separation and migration of charge carrier. In addition, the introduced abundant methyl and hydroxyl groups into the bulk structure of carbon nitride contributed to enhancing light absorption capacity, modulating the carrier dynamics, and increasing the active sites.<sup>58</sup> Furthermore, the exfoliation and linkage
breakage effects of the S cMeOH post-treatment could also enlarge the surface area, thus enhancing the active sites. As a result, the synergistic promotions contributed to the excellent photocatalytic H₂-evolution activity of modified g-C₃N₄ by the ScMeOH post-treatment.

## CONCLUSION

A ScMeOH post-treatment strategy was developed for structure tuning of bulk g-C₃N₄. The structural evolution of g-C₃N₄ and the mechanism of enhanced photocatalytic H₂-evolution activity for modified g-C₃N₄ were investigated. This strategy presented advantages of time-saving (less than 10 min), high yield (over 80%), and scalability due to the enhanced mass transfer and high reactivity of ScMeOH. It was found that ScMeOH was much more effective and efficient for the structure tuning of g-C₃N₄ compared with SubMeOH. During the ScMeOH post-treatment process, CH₃OH molecules diffused into the interlayers of g-C₃N₄ and underwent N-methylation and hydroxylation reactions with the intralayers, resulting in a partial phase transformation from g-C₃N₄ to Q-PHI with abundant methyl and hydroxyl groups. The modified g-C₃N₄ showed enhanced photocatalytic activity with an H₂-evolution rate 7.2 times that of pristine g-C₃N₄, which was attributed to the synergistic effects of isotype heterojunction construction, group modulation, and surface area increase. This work presents a post-treatment strategy for structure tuning of bulk g-C₃N₄ and serves as a practical case to demonstrate the unique properties of SCFs, showing great potential for large-scale synthesis of catalysts by SCF-based technology.

## METHODS AND EXPERIMENTAL

To prepare pristine g-C₃N₄, melamine (5 g) was added in a covered ceramic crucible and calcined in a furnace under an air atmosphere at 550 °C for 4 h with a ramp rate of 5 °C min⁻¹ from room temperature to 550 °C. The obtained powders were ground and denoted as CN.

The solvothermal post-treatment of g-C₃N₄ was carried out using a Hastelloy C276 tube reactor (11 mL). Pristine g-C₃N₄ (500 mg) was dispersed in CH₃OH (5 mL) in the reactor and then ultrasonically treated for 5 min. Subsequently, the reactor was tightly sealed. In order to replace the internal atmosphere of the reactor, high-pressure Ar (around 5 MPa) from a gas cylinder was injected into the reactor through an inlet gas valve until the internal pressure reached around 4 MPa, and then the gas in the reactor was released through an outlet gas valve. The internal atmosphere of the reactor was replaced three times based on the above operation; at the end, the internal pressure was controlled at around 2 MPa for the following reaction to avoid solvent vaporization and thus enhance heat transfer. Afterward, the reactor was heated in an electric furnace and held at a target temperature for a certain time and then removed from the heating furnace and quenched in a water bath (25 °C). After the gas was released in a fume hood, the solid product was separated from the obtained dispersion by vacuum filtration through a microfiltration membrane (Nylon66, 0.2 µm), then washed with water three times and ethanol one time in sequence, and finally dried at 60 °C for 6 h. The obtained modified g-C₃N₄ powders were designated as CN-T-t, where *T* was the post-treatment temperature and *t* was the residence time at the target post-treatment temperature.

For comparison, a series of control samples were prepared by the same procedure except that different alcohols, including EtOH, NPA, and IPA, were used in the solvothermal post-treatment of g-C₃N₄. The obtained modified g-C₃N₄ powders were designated as F-T-t, where *F* was the kind of alcohol, *T* was the post-treatment temperature, and *t* was the residence time at the target post-treatment temperature.

# ASSOCIATED CONTENT

<figure><img src="image_8.png" /></figure>

## Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.4c03922.

Experimental sections on chemicals, sample characterization, gas and liquid products analysis, photocatalytic performance measurement, photoelectrochemical measurement, and computation details. Temperature and pressure curves, optical images, photocatalytic H₂-evolution rates and yields results, schematic diagram of structures, schematic diagram of N-methylation and hydroxylation reactions, temperature and pressure curves, contents of gas products results, schematic diagram of volume measurement using a graduated syringe, GC-MS results, XRD patterns, Gibbs free energies results, XPS results, TG curves, SEM images, BJH pore size-distribution curves, zeta-potential curves, photographs of water contact measurement, DFT results, and electronic band structure alignments. Preparation conditions and photocatalytic H₂-evolution rates, comparison of represented solvothermal post-treatment strategies, Q-PHI model description, and XPS data (PDF)

## AUTHOR INFORMATION

### Corresponding Author

Jinwen Shi - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China; orcid.org/0000-0001-7291-2840; Email: jinwen_shi@mail.xjtu.edu.cn

### Authors

Liuhao Mao - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Binjiang Zhai - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Xing Kang - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Bingru Lu - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Yanbing Liu - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Cheng Cheng - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Hui Jin - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China; orcid.org/0000-0001-9216-7921
breakage effects of the Sc cMeOH post-treatment could also enlarge the surface area, thus enhancing the active sites. As a result, the synergistic promotions contributed to the excellent photocatalytic H₂-evolution activity of modified g-C₃N₄ by the ScMeOH post-treatment.

## CONCLUSION

A ScMeOH post-treatment strategy was developed for structure tuning of bulk g-C₃N₄. The structural evolution of g-C₃N₄ and the mechanism of enhanced photocatalytic H₂-evolution activity for modified g-C₃N₄ were investigated. This strategy presented advantages of time-saving (less than 10 min), high yield (over 80%), and scalability due to the enhanced mass transfer and high reactivity of ScMeOH. It was found that ScMeOH was much more effective and efficient for the structure tuning of g-C₃N₄ compared with SubMeOH. During the ScMeOH post-treatment process, CH₃OH molecules diffused into the interlayers of g-C₃N₄ and underwent N-methylation and hydroxylation reactions with the intralayers, resulting in a partial phase transformation from g-C₃N₄ to Q-PHI with abundant methyl and hydroxyl groups. The modified g-C₃N₄ showed enhanced photocatalytic activity with an H₂-evolution rate 7.2 times that of pristine g-C₃N₄, which was attributed to the synergistic effects of isotype heterojunction construction, group modulation, and surface area increase. This work presents a post-treatment strategy for structure tuning of bulk g-C₃N₄ and serves as a practical case to demonstrate the unique properties of SCFs, showing great potential for large-scale synthesis of catalysts by SCF-based technology.

## METHODS AND EXPERIMENTAL

To prepare pristine g-C₃N₄, melamine (5 g) was added in a covered ceramic crucible and calcined in a furnace under an air atmosphere at 550 °C for 4 h with a ramp rate of 5 °C min⁻¹ from room temperature to 550 °C. The obtained powders were ground and denoted as CN.

The solvothermal post-treatment of g-C₃N₄ was carried out using a Hastelloy C276 tube reactor (11 mL). Pristine g-C₃N₄ (500 mg) was dispersed in CH₃OH (5 mL) in the reactor and then ultrasonically treated for 5 min. Subsequently, the reactor was tightly sealed. In order to replace the internal atmosphere of the reactor, high-pressure Ar (around 5 MPa) from a gas cylinder was injected into the reactor through an inlet gas valve until the internal pressure reached around 4 MPa, and then the gas in the reactor was released through an outlet gas valve. The internal atmosphere of the reactor was replaced three times based on the above operation; at the end, the internal pressure was controlled at around 2 MPa for the following reaction to avoid solvent vaporization and thus enhance heat transfer. Afterward, the reactor was heated in an electric furnace and held at a target temperature for a certain time and then removed from the heating furnace and quenched in a water bath (25 °C). After the gas was released in a fume hood, the solid product was separated from the obtained dispersion by vacuum filtration through a microfiltration membrane (Nylon66, 0.2 µm), then washed with water three times and ethanol one time in sequence, and finally dried at 60 °C for 6 h. The obtained modified g-C₃N₄ powders were designated as CN-T-t, where *T* was the post-treatment temperature and *t* was the residence time at the target post-treatment temperature.

For comparison, a series of control samples were prepared by the same procedure except that different alcohols, including EtOH, NPA, and IPA, were used in the solvothermal post-treatment of g-C₃N₄. The obtained modified g-C₃N₄ powders were designated as F-T-t, where *F* was the kind of alcohol, *T* was the post-treatment temperature, and *t* was the residence time at the target post-treatment temperature.

# ASSOCIATED CONTENT

<figure><img src="image_8.png" /></figure>

## Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.4c03922.

Experimental sections on chemicals, sample characterization, gas and liquid products analysis, photocatalytic performance measurement, photoelectrochemical measurement, and computation details. Temperature and pressure curves, optical images, photocatalytic H₂-evolution rates and yields results, schematic diagram of structures, schematic diagram of N-methylation and hydroxylation reactions, temperature and pressure curves, contents of gas products results, schematic diagram of volume measurement using a graduated syringe, GC-MS results, XRD patterns, Gibbs free energies results, XPS results, TG curves, SEM images, BJH pore size-distribution curves, zeta-potential curves, photographs of water contact measurement, DFT results, and electronic band structure alignments. Preparation conditions and photocatalytic H₂-evolution rates, comparison of represented solvothermal post-treatment strategies, Q-PHI model description, and XPS data (PDF)

## AUTHOR INFORMATION

### Corresponding Author

Jinwen Shi - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China; orcid.org/0000-0001-7291-2840; Email: jinwen_shi@mail.xjtu.edu.cn

### Authors

Liuhao Mao - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Binjiang Zhai - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Xing Kang - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Bingru Lu - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Yanbing Liu - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Cheng Cheng - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China

Hui Jin - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China; orcid.org/0000-0001-9216-7921
Eric Lichtfouse - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China; [orcid.org/0000-0002-8535-8073](orcid.org/0000-0002-8535-8073)

Liejin Guo - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China; [orcid.org/0000-0001-9412-3260](orcid.org/0000-0001-9412-3260)

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsnano.4c03922

## Author Contributions

†L.M. and B.Z. contributed equally to this work. The manuscript was written through contributions of all authors. All authors discussed the results and contributed to this manuscript.

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work is supported by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (No. 52488201), the National Natural Science Foundation of China (Nos. 52276213 and 52206276), the China Postdoctoral Science Foundation (No. 2022M712527), the Science and Technology Innovation Team Project of Shaanxi Province (No. S2023-ZC-TD-0071), and the Fundamental Research Funds for the Central Universities. The authors thank Jiamei Liu, Chuansheng Ma, Jiaxiang Sun, and Hang Guo from the Instrument Analysis Center of Xi'an Jiaotong University for their assistance with characterizations and corresponding analysis.

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(12) Zheng, D.; Wang, Q.; Pan, Z.; Wang, S.; Hou, Y.; Zhang, G. Poly(triazine imide) Nanospheres with Spatially Exposed Prismatic Facets for Photocatalytic Overall Water Splitting. *Sci. China Mater.* **2024**, *1–7*, DOI: 10.1007/s40843-023-2752-9.

(13) Yu, S.; Li, J.; Zhang, Y.; Li, M.; Dong, F.; Zhang, T.; Huang, H. Local Spatial Charge Separation and Proton Activation Induced by Surface Hydroxylation Promoting Photocatalytic Hydrogen Evolution of Polymeric Carbon Nitride. *Nano Energy* **2018**, *50*, 383–392.

(14) Liu, Y.; Shi, J.; Jin, H.; Guo, L. Chemical Recycling Methods for Managing Waste Plastics: A Review. *Environ. Chem. Lett.* **2024**, *22*, 149–169.

(15) Padmajan Sasikala, S.; Poulin, P.; Aymonier, C. Prospects of Supercritical Fluids in Realizing Graphene-Based Functional Materials. *Adv. Mater.* **2016**, *28*, 2663–2691.

(16) Hanabata, S.; Kusada, K.; Yamamoto, T.; Toriyama, T.; Matsumura, S.; Kawaguchi, S.; Kubota, Y.; Nishida, Y.; Haneda, M.; Kitagawa, H. Denary High-Entropy Oxide Nanoparticles Synthesized by a Continuous Supercritical Hydrothermal Flow Process. *J. Am. Chem. Soc.* **2024**, *146*, 181–186.

(17) Zhang, B.; Wang, S.; Li, Y.; Sun, P.; Yang, C.; Wang, D.; Liu, L. Review: Phase Transition Mechanism and Supercritical Hydrothermal Synthesis of Nano Lithium Iron Phosphate. *Ceram. Int.* **2020**, *46*, 27922–27939.

(18) Darr, J. A.; Zhang, J.; Makwana, N. M.; Weng, X. Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions. *Chem. Rev.* **2017**, *117*, 11125–11238.

(19) Gong, J.; Zhao, G.; Feng, J.; An, Y.; Li, T.; Zhang, L.; Li, B.; Qian, Z. Controllable Phosphorylation Strategy for Free-Standing Phosphorus/Nitrogen Cofunctionalized Porous Carbon Monoliths as High-Performance Potassium Ion Battery Anodes. *ACS Nano* **2020**, *14*, 14057–14069.

(20) Sun, Z.; Liu, Z.; Han, B.; Wang, Y.; Du, J.; Xie, Z.; Han, G. Fabrication of Ruthenium-Carbon Nanotube Nanocomposites in Supercritical Water. *Adv. Mater.* **2005**, *17*, 928–932.

(21) Liu, Y.; Zhang, M.; Zhang, Y.; Liu, Y.; Wang, L.; Li, X.; Xue, M.; Li, B.; Tao, X. Li₄Ti₅O₁₂/TiO₂ Dual-Phase Anode Materials Synthesized in Supercritical Water–Methanol System and Investigations on Its Superior Electrochemical Performance for Lithium-Ion Batteries. *J. Supercrit. Fluids* **2019**, *154*, No. 104596.

(22) Li, S.; Zhu, B.; Wang, W.; Zhang, H.; Li, Q. Efficient and Stable Supercritical-Water-Synthesized Ni-Based Catalysts for Supercritical Water Gasification. *J. Supercrit. Fluids* **2020**, *160*, No. 104790.

(23) Li, L.; Zheng, X.; Wang, J.; Sun, Q.; Xu, Q. Solvent-Exfoliated and Functionalized Graphene with Assistance of Supercritical Carbon Dioxide. *ACS Sustainable Chem. Eng.* **2013**, *1*, 144–151.

(24) Wang, N.; Wei, F.; Qi, Y.; Li, H.; Lu, X.; Zhao, G.; Xu, Q. Synthesis of Strongly Fluorescent Molybdenum Disulfide Nanosheets for Cell-Targeted Labeling. *ACS Appl. Mater. Interfaces* **2014**, *6*, 19888–19894.

(25) Zhou, Y.; Xu, Q.; Ge, T.; Zheng, X.; Zhang, L.; Yan, P. Accurate Control of VS₂ Nanosheets for Coexisting High Photoluminescence and Photothermal Conversion Efficiency. *Angew. Chem., Int. Ed.* **2020**, *59*, 3322–3328.

(26) Tong, X.; Du, L.; Xu, Q. Tough, Adhesive and Self-Healing Conductive 3D Network Hydrogel of Physically Linked Function-
Eric Lichtfouse - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China; orcid.org/0000-0002-8535-8073

Liejin Guo - State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), Xi'an 710049, China; orcid.org/0000-0001-9412-3260

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsnano.4c03922

## Author Contributions

†L.M. and B.Z. contributed equally to this work. The manuscript was written through contributions of all authors. All authors discussed the results and contributed to this manuscript.

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work is supported by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (No. 52488201), the National Natural Science Foundation of China (Nos. 52276213 and 52206276), the China Postdoctoral Science Foundation (No. 2022M712527), the Science and Technology Innovation Team Project of Shaanxi Province (No. S2023-ZC-TD-0071), and the Fundamental Research Funds for the Central Universities. The authors thank Jiamei Liu, Chuansheng Ma, Jiaxiang Sun, and Hang Guo from the Instrument Analysis Center of Xi'an Jiaotong University for their assistance with characterizations and corresponding analysis.

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(24) Wang, N.; Wei, F.; Qi, Y.; Li, H.; Lu, X.; Zhao, G.; Xu, Q. Synthesis of Strongly Fluorescent Molybdenum Disulfide Nanosheets for Cell-Targeted Labeling. *ACS Appl. Mater. Interfaces* **2014**, *6*, 19888–19894.

(25) Zhou, Y.; Xu, Q.; Ge, T.; Zheng, X.; Zhang, L.; Yan, P. Accurate Control of VS₂ Nanosheets for Coexisting High Photoluminescence and Photothermal Conversion Efficiency. *Angew. Chem., Int. Ed.* **2020**, *59*, 3322–3328.

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(27) Tayyebi, A.; Akhavan, O.; Lee, B.-K.; Outokesh, M. Supercritical Water in Top-Down Formation of Tunable-Sized Graphene Quantum Dots Applicable in Effective Photothermal Treatments of Tissues. *Carbon* **2018**, *130*, 267–272.

(28) Liu, W.; Xu, Q.; Cui, W.; Zhu, C.; Qi, Y. CO₂-Assisted Fabrication of Two-Dimensional Amorphous Molybdenum Oxide Nanosheets for Enhanced Plasmon Resonances. *Angew. Chem. 2017*, *129*, 1622–1626.

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(30) Zheng, X.; Guo, Z.; Zhang, G.; Li, H.; Zhang, J.; Xu, Q. Building a Lateral/Vertical 1T-2H MoS₂/Au Heterostructure for Enhanced Photoelectrocatalysis and Surface Enhanced Raman Scattering. J. Mater. Chem. A **2019**, *7*, 19922–19928.

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(32) Sandhiya, M.; Veerappan, U. K. K.; Sathish, M. Crumpled B, F Co-Doped Graphene Nanosheets for the Fabrication of All-Solid-State Flexible Supercapacitors. *Chem. Commun.* **2021**, *57*, 8336–8339.

(33) Suresh Balaji, S.; Sandhiya, M.; Sathish, M. Enhanced Electrochemical Performance of Supercritical Fluid Aided P-Doped Graphene Nanoflakes by I³⁻/I⁻ Redox Couple. *J. Energy Storage* **2021**, *33*, No. 102085.

(34) Kang, X.; Mao, L.; Shi, J.; Liu, Y.; Zhai, B.; Xu, J.; Jiang, Y.; Lichtfouse, E.; Jin, H.; Guo, L. Supercritical Carbon Dioxide Systems for Sustainable and Efficient Dissolution of Solutes: A Review. *Environ. Chem. Lett.* **2024**, *22*, 815–839.

(35) Shi, J.; Kang, X.; Mao, L.; Jiang, Y.; Zhao, S.; Liu, Y.; Zhai, B.; Jin, H.; Guo, L. Supercritical CO₂-Applied Equipment for Chemical Synthesis and Transformation: Current Status and Perspectives. *Chem. Eng. J.* **2023**, *459*, No. 141608.

(36) Abdulagatov, I. M.; Polikhronidi, N. G.; Abdurashidova, A.; Kiselev, S. B.; Ely, J. F. Thermodynamic Properties of Methanol in the Critical and Supercritical Regions. *Int. J. Thermophys.* **2005**, *26*, 1327–1368.

(37) Budi Nursanto, E.; Nugroho, A.; Hong, S.-A.; Kim, S. J.; Yoon Chung, K.; Kim, J. Facile Synthesis of Reduced Graphene Oxide in Supercritical Alcohols and its Lithium Storage Capacity. *Green Chem.* **2011**, *13*, 2714–2718.

(38) Seo, M.; Yoon, D.; Hwang, K. S.; Kang, J. W.; Kim, J. Supercritical Alcohols as Solvents and Reducing Agents for the Synthesis of Reduced Graphene Oxide. *Carbon* **2013**, *64*, 207–218.

(39) Leng, Y.; Gao, Y.; Wang, W.; Zhao, Y. Continuous Supercritical Solvothermal Synthesis of TiO₂-Pristine-Graphene Hybrid as the Enhanced Photocatalyst. *J. Supercrit. Fluids* **2015**, *103*, 115–121.

(40) Sasikala, S. P.; Henry, L.; Yesilbag Tonga, G.; Huang, K.; Das, R.; Giroire, B.; Marre, S.; Rotello, V. M.; Penicaud, A.; Poulin, P.; Aymonier, C. High Yield Synthesis of Aspect Ratio Controlled Graphenic Materials from Anthracite Coal in Supercritical Fluids. *ACS Nano* **2016**, *10*, 5293–5303.

(41) Li, S.; Cheng, W.; Liu, X.; Wang, C.; Li, W.; Yu, S. Supercritical Methanol Synthesis, Phase Evolution and Formation Mechanism of Cu₁.₈S and Cu₉S₅/CuS Complex Microcrystal. *J. Supercrit. Fluids* **2018**, *133*, 429–436.

(42) Wang, L.; Xu, W.; Yang, J. Applications of Supercritical Methanol in Chemical Reactions. *Prog. Chem. 2010*, *22*, 796–802.

(43) Xu, Y.; Musumeci, V.; Aymonier, C. Chemistry in Supercritical Fluids for the Synthesis of Metal Nanomaterials. *React. Chem. Eng.* **2019**, *4*, 2030–2054.

(44) Mao, L.; Lu, B.; Shi, J.; Zhang, Y.; Kang, X.; Chen, Y.; Jin, H.; Guo, L. Rapid High-Temperature Hydrothermal Post Treatment on Graphitic Carbon Nitride for Enhanced Photocatalytic H₂ Evolution. *Catal. Today* **2023**, *409*, 94–102.

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# Supercritical CH$_3$OH-triggered isotype heterojunction and groups in g-C$_3$N$_4$ for enhanced photocatalytic H$_2$ evolution

Liuhao Mao†, Binjiang Zhai†, Jinwen Shi*, Xing Kang, Bingru Lu, Yanbing Liu, Cheng Cheng, Hui Jin, Eric Lichtfouse, Liejin Guo

State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), 28 West Xianning Road, Xi'an 710049, China

† These authors contribute equally.

* Corresponding author. E-mail addresses: jinwen_shi@mail.xjtu.edu.cn (J. Shi)
# Supercritical CH₃OH-triggered isotype heterojunction and groups in g-C₃N₄ for enhanced photocatalytic H₂ evolution

Liuhao Mao†, Binjiang Zhai†, Jinwen Shi*, Xing Kang, Bingru Lu, Yanbing Liu, Cheng
Cheng, Hui Jin, Eric Lichtfouse, Liejin Guo

State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), International Research
Center for Renewable Energy (IRCRE), Xi'an Jiaotong University (XJTU), 28 West Xianning Road,
Xi'an 710049, China

† These authors contribute equally.

* Corresponding author. E-mail addresses: jinwen_shi@mail.xjtu.edu.cn (J. Shi)
# Experimental section

## Chemicals

All reagents were of analytical grade and were used without further purification. Melamine ($C_3H_6N_6$), methanol (MeOH, $CH_3OH$), ethanol (EtOH, $C_2H_5OH$), n-propanol (NPA, $C_3H_7OH$), isopropanol (IPA, $C_3H_7OH$) and triethanolamine (TEOA, $C_6H_{15}NO_3$) were purchased from Sinopharm Chemical Reagent Co., Ltd. Chloroplatinic acid hexahydrate ($H_2PtCl_6·6H_2O$) was purchased from Sigma-Aldrich Co., Ltd., and was made into aqueous solution with an equivalent Pt content of 0.775 $mg·mL^{-1}$. Argon gas (Ar, 99.999% purity) and Helium gas (He, 99.9999% purity) was purchased from Shaanxi Xinghua Group Co., Ltd. Deionized water with a resistivity of 18.25 $MΩ·cm$ was used in experiments.

## Characterizations

X-ray diffraction (XRD) patterns were obtained by a diffractometer (PANalytical, X'pert MPD Pro, Netherlands) by a scan rate of $10°·min^{-1}$ in the $2\theta$ range from 5 to 80° under Ni-filtered Cu $K\alpha$ irradiation ($\lambda = 1.5406 Å$). Fourier transform infrared (FTIR) spectra were recorded by an FTIR spectrophotometer (Bruker, Vertex 70, Germany) with using the KBr pellet technique. $N_2$ adsorption-desorption isotherms were recorded by an accelerated surface area and porosimetry analyzer (Micromeritics, ASAP 2460, USA) for Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) porosity analysis, and all materials were degassed in vacuum at 423 K for 12 h before $N_2$ adsorption analysis. X-ray photoelectron spectra (XPS) were recorded by an X-ray photoelectron spectroscope (Kratos, Axis Ultra DLD, Japan) using a monochromatic Al $K\alpha$ line source ($h\nu = 1486.69 eV$), and the adventitious C 1s peak at 284.8 eV was applied as reference. Ultraviolet-visible (UV-Vis) spectra were collected by an ultraviolet-visible-near-infrared spectrophotometer (Hitachi, U-4100, Japan) with $BaSO_4$ reference and diffuse reflectance mode in the range from 300 to 800 nm. Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) spectra were recorded by a field-emission scanning electron microscope (JEOL, JSM-7800F, Japan). Transmission electron microscopy (TEM) images were obtained from a
# Experimental section

## Chemicals

All reagents were of analytical grade and were used without further purification. Melamine ($C_3H_6N_6$), methanol (MeOH, $CH_3OH$), ethanol (EtOH, $C_2H_5OH$), n-propanol (NPA, $C_3H_7OH$), isopropanol (IPA, $C_3H_7OH$) and triethanolamine (TEOA, $C_6H_{15}NO_3$) were purchased from Sinopharm Chemical Reagent Co., Ltd. Chloroplatinic acid hexahydrate ($H_2PtCl_6·6H_2O$) was purchased from Sigma-Aldrich Co., Ltd., and was made into aqueous solution with an equivalent Pt content of 0.775 $mg·mL^{-1}$. Argon gas (Ar, 99.999% purity) and Helium gas (He, 99.9999% purity) was purchased from Shaanxi Xinghua Group Co., Ltd. Deionized water with a resistivity of 18.25 $MΩ·cm$ was used in experiments.

## Characterizations

X-ray diffraction (XRD) patterns were obtained by a diffractometer (PANalytical, X'pert MPD Pro, Netherlands) by a scan rate of $10°·min^{-1}$ in the $2\theta$ range from 5 to 80° under Ni-filtered Cu $K\alpha$ irradiation ($\lambda = 1.5406 Å$). Fourier transform infrared (FTIR) spectra were recorded by an FTIR spectrophotometer (Bruker, Vertex 70, Germany) with using the KBr pellet technique. $N_2$ adsorption-desorption isotherms were recorded by an accelerated surface area and porosimetry analyzer (Micromeritics, ASAP 2460, USA) for Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) porosity analysis, and all materials were degassed in vacuum at 423 K for 12 h before $N_2$ adsorption analysis. X-ray photoelectron spectra (XPS) were recorded by an X-ray photoelectron spectroscope (Kratos, Axis Ultra DLD, Japan) using a monochromatic Al $K\alpha$ line source ($h\nu = 1486.69 eV$), and the adventitious C 1s peak at 284.8 eV was applied as reference. Ultraviolet-visible (UV-Vis) spectra were collected by an ultraviolet-visible-near-infrared spectrophotometer (Hitachi, U-4100, Japan) with $BaSO_4$ reference and diffuse reflectance mode in the range from 300 to 800 nm. Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) spectra were recorded by a field-emission scanning electron microscope (JEOL, JSM-7800F, Japan). Transmission electron microscopy (TEM) images were obtained from a
transmission electron microscope (Thermo Scientific, Talos L120C G2, USA) with an
accelerating voltage of 120 kV. Zeta potentials and dynamic light scattering (DLS) sizes
were recorded by zeta potentials instrument (Malvern Instruments, Zetasizer Nano ZS,
England), and the powders were dispersed in pure water (c = 0.5 mg·mL⁻¹, pH = 7) for
measurement. Element contents were obtained using an elemental analyzer (EA, Euro
Vector, EA3000, Italy) with He as carrier gas and O₂ as reaction gas.
Photoluminescence (PL) spectra were obtained from a fluorescence spectrophotometer
(Edinburg Instruments, FLS1000, England) at room temperature, and the steady-state
and time-resolved PL signals were obtained under the excitation wavelengths of 370
and 405 nm, respectively. Thermogravimetric (TG) curves were recorded by a thermal
analyzer (Mettler Toledo, TGA/DSC3+, Switzerland) in the range from 25 to 800°C.
Solid-state ¹³C nuclear magnetic resonance (NMR) spectra were taken on an NMR
spectrometer JEOL, JNM-ECZ400R/S1, Japan). Electron paramagnetic resonance
(EPR) test was performed on an EPR spectrometer (Bruker, A300-9.5/12/S/W,
Germany) at room temperature both in the dark and under light irradiation. The
measurement of contact angle was performed on a commercial contact angle system
(Shanghai Zhongchen, SL200B China) with 6 µL of water droplet as the indicator, and
the samples (50 mg) were prepared as tablets with flat surface by a preforming machine.

Analysis of gas and liquid products from solvothermal posttreatment

The gas products were collected by a gas pocket for further analysis, and the
compositions of gas products were analyzed by gas chromatography (GC) using a gas
chromatograph (Agilent, 7890A, USA) equipped with a thermal conductivity detector
and a C-2000 capillary column, and Ar was used as carrier gas.

The liquid products were filtrated and collected through a filter (0.22 µm) to remove
the particle photocatalysts. The compositions of liquid products were analyzed by the
combination of gas chromatograph (GC, Agilent, HP 6980GC, USA)—mass
spectrometer (MS, Agilent, HP 5973MSD, USA), and He was used as carrier gas.

Photocatalytic performance measurements

Photocatalytic H₂-evolution measurements were carried out in a Pyrex glass reactor
(105 mL), and an Xe lamp (300 W) was used as the light source to provide visible-light
transmission electron microscope (Thermo Scientific, Talos L120C G2, USA) with an
accelerating voltage of 120 kV. Zeta potentials and dynamic light scattering (DLS) sizes
were recorded by zeta potentials instrument (Malvern Instruments, Zetasizer Nano ZS,
England), and the powders were dispersed in pure water (c = 0.5 mg·mL⁻¹, pH = 7) for
measurement. Element contents were obtained using an elemental analyzer (EA, Euro
Vector, EA3000, Italy) with He as carrier gas and O₂ as reaction gas.
Photoluminescence (PL) spectra were obtained from a fluorescence spectrophotometer
(Edinburg Instruments, FLS1000, England) at room temperature, and the steady-state
and time-resolved PL signals were obtained under the excitation wavelengths of 370
and 405 nm, respectively. Thermogravimetric (TG) curves were recorded by a thermal
analyzer (Mettler Toledo, TGA/DSC3+, Switzerland) in the range from 25 to 800°C.
Solid-state ¹³C nuclear magnetic resonance (NMR) spectra were taken on an NMR
spectrometer JEOL, JNM-ECZ400R/S1, Japan). Electron paramagnetic resonance
(EPR) test was performed on an EPR spectrometer (Bruker, A300-9.5/12/S/W,
Germany) at room temperature both in the dark and under light irradiation. The
measurement of contact angle was performed on a commercial contact angle system
(Shanghai Zhongchen, SL200B China) with 6 µL of water droplet as the indicator, and
the samples (50 mg) were prepared as tablets with flat surface by a preforming machine.

Analysis of gas and liquid products from solvothermal posttreatment

The gas products were collected by a gas pocket for further analysis, and the
compositions of gas products were analyzed by gas chromatography (GC) using a gas
chromatograph (Agilent, 7890A, USA) equipped with a thermal conductivity detector
and a C-2000 capillary column, and Ar was used as carrier gas.

The liquid products were filtrated and collected through a filter (0.22 µm) to remove
the particle photocatalysts. The compositions of liquid products were analyzed by the
combination of gas chromatograph (GC, Agilent, HP 6980GC, USA)—mass
spectrometer (MS, Agilent, HP 5973MSD, USA), and He was used as carrier gas.

Photocatalytic performance measurements

Photocatalytic H₂-evolution measurements were carried out in a Pyrex glass reactor
(105 mL), and an Xe lamp (300 W) was used as the light source to provide visible-light
irradiation after being equipped with a 400 nm cut-off filter. Photocatalyst (50 mg) was dispersed in TEOA aqueous solution (10 vol%, 80 mL), and then H₂PtCl₆ aqueous solution (0.645 mL, corresponding to 0.5 mg Pt) was added into the dispersion. The reactor was purged with Ar for 30 min to eliminate O₂ before the photocatalytic reaction. During the measurement, the solution was maintained at 35°C by circulating water and was stirred constantly. The evolved gas was detected by GC using a gas chromatograph (North Point, NP-GC-901A, China) with a thermal conductivity detector and a TDX-01 column, and Ar as carrier gas.

The measurement of apparent quantum yield (AQY) was carried out under the irradiation of light with different wavelengths, i.e., 400, 420, 450 475 and 500 nm, realized by band-pass filters. The light intensities were recorded by a photoradiometer (PerfectLight, PL-MW 2000, China).

The AQY values were obtained as follows.

$$AQY (\%) = \frac{\text{number of evolved hydrogen molecules} \times 2}{\text{number of incident photons}} \times 100\%$$

## Photoelectrochemical measurements

Photoelectrochemical measurements were carried out in a three-electrode chemical cell, in which the electrolyte, the counter electrode and the reference electrode were Na₂SO₄ aqueous solution (0.5 M), Pt foil and Ag/AgCl, respectively. The working electrode was fabricated by dropping the sample dispersion onto a glassy carbon electrode (0.2 mg·cm⁻²). Transient photocurrent curves were recorded under chopped light irradiation at an applied voltage of +0.6 V vs Ag/AgCl. Electrochemical impedance spectra (EIS) were measured from 100 kHz to 1 Hz at an applied voltage of +0.6 V vs Ag/AgCl.

## Computational details

Molecular dynamics simulations, including geometry optimization and transition state search, and frequency calculation, were realized with the Dmol3 program in Materials Studio using the m-GGA/M06-L method with DND basis.

Density functional theory (DFT) calculations were accomplished by the Vienna ab-initio Simulation Package (VASP) codes. The core electrons were described by
irradiation after being equipped with a 400 nm cut-off filter. Photocatalyst (50 mg) was dispersed in TEOA aqueous solution (10 vol%, 80 mL), and then H₂PtCl₆ aqueous solution (0.645 mL, corresponding to 0.5 mg Pt) was added into the dispersion. The reactor was purged with Ar for 30 min to eliminate O₂ before the photocatalytic reaction. During the measurement, the solution was maintained at 35°C by circulating water and was stirred constantly. The evolved gas was detected by GC using a gas chromatograph (North Point, NP-GC-901A, China) with a thermal conductivity detector and a TDX-01 column, and Ar as carrier gas.

The measurement of apparent quantum yield (AQY) was carried out under the irradiation of light with different wavelengths, i.e., 400, 420, 450 475 and 500 nm, realized by band-pass filters. The light intensities were recorded by a photoradiometer (PerfectLight, PL-MW 2000, China).

The AQY values were obtained as follows.

$$AQY (\%) = \frac{\text{number of evolved hydrogen molecules} \times 2}{\text{number of incident photons}} \times 100\%$$

## Photoelectrochemical measurements

Photoelectrochemical measurements were carried out in a three-electrode chemical cell, in which the electrolyte, the counter electrode and the reference electrode were Na₂SO₄ aqueous solution (0.5 M), Pt foil and Ag/AgCl, respectively. The working electrode was fabricated by dropping the sample dispersion onto a glassy carbon electrode (0.2 mg·cm⁻²). Transient photocurrent curves were recorded under chopped light irradiation at an applied voltage of +0.6 V vs Ag/AgCl. Electrochemical impedance spectra (EIS) were measured from 100 kHz to 1 Hz at an applied voltage of +0.6 V vs Ag/AgCl.

## Computational details

Molecular dynamics simulations, including geometry optimization and transition state search, and frequency calculation, were realized with the Dmol3 program in Materials Studio using the m-GGA/M06-L method with DND basis.

Density functional theory (DFT) calculations were accomplished by the Vienna ab-initio Simulation Package (VASP) codes. The core electrons were described by
projected augmented wave (PAW) pseudo-potentials. The exchange and correlation
effects of valence electrons were treated by the generalized gradient approximation
(GGA) using the functional developed by Perdew-Burke-Ernzerhof (PBE). The g-
C₃N₄ and Q-PHI were fully relaxed by the method of a conjugate-gradient algorithm
with the criterion of force convergence in 0.02 eV Å⁻¹ and total energy convergence
within 10⁻⁵ eV. The basis sets of plane waves were cut-off by 500 eV, and k-points
mesh of g-C₃N₄ and Q-PHI were generated by the Gamma center method for structure
relaxation. As for the band structure computation, due to the ripple of the g-C₃N₄ and
Q-PHI, the k path was changed from G-X-H₁-C-H-Y-G of the Q-PHI to the G-X-S-Y-
G of the g-C₃N₄
projected augmented wave (PAW) pseudo-potentials. The exchange and correlation
effects of valence electrons were treated by the generalized gradient approximation
(GGA) using the functional developed by Perdew-Burke-Ernzerhof (PBE). The g-
C₃N₄ and Q-PHI were fully relaxed by the method of a conjugate-gradient algorithm
with the criterion of force convergence in 0.02 eV Å⁻¹ and total energy convergence
within 10⁻⁵ eV. The basis sets of plane waves were cut-off by 500 eV, and k-points
mesh of g-C₃N₄ and Q-PHI were generated by the Gamma center method for structure
relaxation. As for the band structure computation, due to the ripple of the g-C₃N₄ and
Q-PHI, the k path was changed from G-X-H₁-C-H-Y-G of the Q-PHI to the G-X-S-Y-
G of the g-C₃N₄
Supplementary figures

<figure><img src="image_9.png" /><figcaption>**Figure S1.** Temperature (left axis) and pressure (right axis) curves for solvothermal posttreatment process.</figcaption></figure>
Supplementary figures

<figure><img src="image_9.png" /><figcaption>**Figure S1.** Temperature (left axis) and pressure (right axis) curves for solvothermal posttreatment process.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure S2.** Optical images for CN-*T*-t samples. The numbers above the sample images represented the values of *t* (unit: min).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure S2.** Optical images for CN-*T*-t samples. The numbers above the sample images represented the values of *t* (unit: min).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure S3.** Photocatalytic H₂-evolution rates (left axis) and yields (right axis) for modified g-C₃N₄ obtained from ScMeOH solvothermal posttreatment (treatment temperature: 300°C, residence time: 0 min) with different dispersion concentrations.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure S3.** Photocatalytic H₂-evolution rates (left axis) and yields (right axis) for modified g-C₃N₄ obtained from ScMeOH solvothermal posttreatment (treatment temperature: 300°C, residence time: 0 min) with different dispersion concentrations.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure S4.** Schematic diagram about the basic structure for (a) ideal g-C₃N₄ and (b) amorphous g-C₃N₄.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure S4.** Schematic diagram about the basic structure for (a) ideal g-C₃N₄ and (b) amorphous g-C₃N₄.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure S5.** Proposed mechanism about the breakage of C–N bonds between NH-bridged melem monomers through *N*-methylation and hydroxylation reactions by ScMeOH posttreatment.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure S5.** Proposed mechanism about the breakage of C–N bonds between NH-bridged melem monomers through *N*-methylation and hydroxylation reactions by ScMeOH posttreatment.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure S6.** Schematic diagram about the basic structure for (a) PHI and (b) Q-PHI.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure S6.** Schematic diagram about the basic structure for (a) PHI and (b) Q-PHI.</figcaption></figure>
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>**Figure S7.** Temperature (left axis) and pressure (right axis) curves for the posttreatment process of (a) MeOH-230-30, (b) CN-230-30, (c) MeOH-300-0 and (d) CN-300-0. MeOH-230-30 and MeOH-300-0 were blank runs in the absence of g-C₃N₄ while other conditions were the same.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Figure S7.** Temperature (left axis) and pressure (right axis) curves for the posttreatment process of (a) MeOH-230-30, (b) CN-230-30, (c) MeOH-300-0 and (d) CN-300-0. MeOH-230-30 and MeOH-300-0 were blank runs in the absence of g-C₃N₄ while other conditions were the same.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure S8.** Contents of gas products from MeOH-230-30, MeOH-300-0, CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure S8.** Contents of gas products from MeOH-230-30, MeOH-300-0, CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure S9.** Volume measurement using a graduated syringe.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Figure S9.** Volume measurement using a graduated syringe.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Figure S10.** GC-MS results of the main compounds in liquid products from CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Figure S10.** GC-MS results of the main compounds in liquid products from CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Figure S11.** XRD patterns for MeOH-300-0, EtOH-300-0, IPA-340-0 and NPA-320-0.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure S11.** XRD patterns for MeOH-300-0, EtOH-300-0, IPA-340-0 and NPA-320-0.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Figure S12.** Gibbs free energies calculated by molecular dynamics simulations for the formation pathways of Q-PHI by reactions with MeOH and EtOH.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure S12.** Gibbs free energies calculated by molecular dynamics simulations for the formation pathways of Q-PHI by reactions with MeOH and EtOH.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>**Figure S13.** (a) Survey-scan XPS for CN, CN-230-30 and CN-300-0. High-resolution XPS of (b) Ni 2p and (c) Cr 2p orbitals for CN-300-0.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Figure S13.** (a) Survey-scan XPS for CN, CN-230-30 and CN-300-0. High-resolution XPS of (b) Ni 2p and (c) Cr 2p orbitals for CN-300-0.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Figure S14.** TG curves for CN, CN-230-30 and CN-300-0 from 25 to 350°C.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Figure S14.** TG curves for CN, CN-230-30 and CN-300-0 from 25 to 350°C.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>**Figure S15.** SEM images for (a) CN, (b) CN-230-30, (c) CN-300-0. (d) EDX spectrum and (table from the inset) corresponding result of elemental content detected from the selected area in (figure from the inset) SEM image for CN-300-0. Scale bar: (a–c) 1 µm, (d) 10 µm.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Figure S15.** SEM images for (a) CN, (b) CN-230-30, (c) CN-300-0. (d) EDX spectrum and (table from the inset) corresponding result of elemental content detected from the selected area in (figure from the inset) SEM image for CN-300-0. Scale bar: (a–c) 1 µm, (d) 10 µm.</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S16.** BJH pore size distribution from 2–100 nm based on the desorption isotherms for CN, CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>**Figure S16.** BJH pore size distribution from 2–100 nm based on the desorption isotherms for CN, CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>**Figure S17.** Zeta-potential curves for CN, CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Figure S17.** Zeta-potential curves for CN, CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_30.png" /><figcaption>**Figure S18.** Photos of water contact measurement (equilibrium time of 9.0 s) for CN, CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>**Figure S18.** Photos of water contact measurement (equilibrium time of 9.0 s) for CN, CN-230-30 and CN-300-0.</figcaption></figure>
<figure><img src="image_31.png" /><figcaption>**Figure S19.** Structure models of (a) g-C₃N₄ and (b) Q-PHI. Calculated electronic band structures and corresponding density of states of (c) g-C₃N₄ and (d) Q-PHI. The Q-PHI model with one methyl and one hydroxyl group is selected for calculation in (b, d).</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S19.** Structure models of (a) g-C₃N₄ and (b) Q-PHI. Calculated electronic band structures and corresponding density of states of (c) g-C₃N₄ and (d) Q-PHI. The Q-PHI model with one methyl and one hydroxyl group is selected for calculation in (b, d).</figcaption></figure>
<figure><img src="image_32.png" /><figcaption>**Figure S20.** Electronic band structure alignments of g-C₃N₄ and Q-PHI.</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>**Figure S20.** Electronic band structure alignments of g-C₃N₄ and Q-PHI.</figcaption></figure>
Supplementary tables

<table><caption>**Table S1.** Preparation conditions and photocatalytic H₂-evolution rates of modified g-C₃N₄ by MeOH solvothermal posttreatment.</caption><thead><tr><th>Run</th><th>Sample</th><th>Temperature (°C)</th><th>Residence time (min)</th><th>H₂-evolution rate (µmol·h⁻¹)</th><th>Run</th><th>Sample</th><th>Temperature (°C)</th><th>Residence time (min)</th><th>H₂-evolution rate (µmol·h⁻¹)</th></tr></thead><tbody><tr><td>1</td><td>CN-230-5</td><td>230</td><td>5</td><td>9.4</td><td>22</td><td>CN-260-3</td><td>260</td><td>3</td><td>46.0</td></tr><tr><td>2</td><td>CN-230-10</td><td>230</td><td>10</td><td>9.2</td><td>23</td><td>CN-260-4</td><td>260</td><td>4</td><td>26.9</td></tr><tr><td>3</td><td>CN-230-20</td><td>230</td><td>20</td><td>17.1</td><td>24</td><td>CN-260-5</td><td>260</td><td>5</td><td>23.0</td></tr><tr><td>4</td><td>CN-230-30</td><td>230</td><td>30</td><td>18.0</td><td>25</td><td>CN-260-7</td><td>260</td><td>7</td><td>13.1</td></tr><tr><td>5</td><td>CN-230-40</td><td>230</td><td>40</td><td>11.4</td><td>26</td><td>CN-270-0</td><td>270</td><td>0</td><td>23.6</td></tr><tr><td>6</td><td>CN-240-3</td><td>240</td><td>3</td><td>28.7</td><td>27</td><td>CN-270-1</td><td>270</td><td>1</td><td>47.4</td></tr><tr><td>7</td><td>CN-240-5</td><td>240</td><td>5</td><td>36.3</td><td>28</td><td>CN-270-2</td><td>270</td><td>2</td><td>30.8</td></tr><tr><td>8</td><td>CN-240-7</td><td>240</td><td>7</td><td>29.5</td><td>29</td><td>CN-270-3</td><td>270</td><td>3</td><td>13.2</td></tr><tr><td>9</td><td>CN-240-10</td><td>240</td><td>10</td><td>23.8</td><td>30</td><td>CN-270-5</td><td>270</td><td>5</td><td>0.0</td></tr><tr><td>10</td><td>CN-250-0</td><td>250</td><td>0</td><td>12.3</td><td>31</td><td>CN-280-0</td><td>280</td><td>0</td><td>42.3</td></tr><tr><td>11</td><td>CN-250-1</td><td>250</td><td>1</td><td>32.5</td><td>32</td><td>CN-280-1</td><td>280</td><td>1</td><td>40.9</td></tr><tr><td>12</td><td>CN-250-2</td><td>250</td><td>2</td><td>35.8</td><td>33</td><td>CN-280-2</td><td>280</td><td>2</td><td>5.7</td></tr><tr><td>13</td><td>CN-250-3</td><td>250</td><td>3</td><td>24.7</td><td>34</td><td>CN-280-3</td><td>280</td><td>3</td><td>0.0</td></tr><tr><td>14</td><td>CN-250-4</td><td>250</td><td>4</td><td>35.8</td><td>35</td><td>CN-290-0</td><td>290</td><td>0</td><td>37.2</td></tr><tr><td>15</td><td>CN-250-5</td><td>250</td><td>5</td><td>38.8</td><td>36</td><td>CN-290-1</td><td>290</td><td>1</td><td>36.8</td></tr><tr><td>16</td><td>CN-250-6</td><td>250</td><td>6</td><td>34.7</td><td>37</td><td>CN-290-2</td><td>290</td><td>2</td><td>0.1</td></tr><tr><td>17</td><td>CN-250-7</td><td>250</td><td>7</td><td>37.3</td><td>38</td><td>CN-300-0</td><td>300</td><td>0</td><td>50.1</td></tr><tr><td>18</td><td>CN-250-10</td><td>250</td><td>10</td><td>14.9</td><td>39</td><td>CN-300-1</td><td>300</td><td>1</td><td>17.1</td></tr><tr><td>19</td><td>CN-260-0</td><td>260</td><td>0</td><td>8.7</td><td>40</td><td>CN-320-0</td><td>320</td><td>0</td><td>27.9</td></tr><tr><td>20</td><td>CN-260-1</td><td>260</td><td>1</td><td>38.9</td><td>41</td><td>CN-340-0</td><td>340</td><td>0</td><td>0.0</td></tr><tr><td>21</td><td>CN-260-2</td><td>260</td><td>2</td><td>42.4</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
Supplementary tables

<table><caption>**Table S1.** Preparation conditions and photocatalytic H₂-evolution rates of modified g-C₃N₄ by MeOH solvothermal posttreatment.</caption><thead><tr><th>Run</th><th>Sample</th><th>Temperature (°C)</th><th>Residence time (min)</th><th>H₂-evolution rate (µmol·h⁻¹)</th><th>Run</th><th>Sample</th><th>Temperature (°C)</th><th>Residence time (min)</th><th>H₂-evolution rate (µmol·h⁻¹)</th></tr></thead><tbody><tr><td>1</td><td>CN-230-5</td><td>230</td><td>5</td><td>9.4</td><td>22</td><td>CN-260-3</td><td>260</td><td>3</td><td>46.0</td></tr><tr><td>2</td><td>CN-230-10</td><td>230</td><td>10</td><td>9.2</td><td>23</td><td>CN-260-4</td><td>260</td><td>4</td><td>26.9</td></tr><tr><td>3</td><td>CN-230-20</td><td>230</td><td>20</td><td>17.1</td><td>24</td><td>CN-260-5</td><td>260</td><td>5</td><td>23.0</td></tr><tr><td>4</td><td>CN-230-30</td><td>230</td><td>30</td><td>18.0</td><td>25</td><td>CN-260-7</td><td>260</td><td>7</td><td>13.1</td></tr><tr><td>5</td><td>CN-230-40</td><td>230</td><td>40</td><td>11.4</td><td>26</td><td>CN-270-0</td><td>270</td><td>0</td><td>23.6</td></tr><tr><td>6</td><td>CN-240-3</td><td>240</td><td>3</td><td>28.7</td><td>27</td><td>CN-270-1</td><td>270</td><td>1</td><td>47.4</td></tr><tr><td>7</td><td>CN-240-5</td><td>240</td><td>5</td><td>36.3</td><td>28</td><td>CN-270-2</td><td>270</td><td>2</td><td>30.8</td></tr><tr><td>8</td><td>CN-240-7</td><td>240</td><td>7</td><td>29.5</td><td>29</td><td>CN-270-3</td><td>270</td><td>3</td><td>13.2</td></tr><tr><td>9</td><td>CN-240-10</td><td>240</td><td>10</td><td>23.8</td><td>30</td><td>CN-270-5</td><td>270</td><td>5</td><td>0.0</td></tr><tr><td>10</td><td>CN-250-0</td><td>250</td><td>0</td><td>12.3</td><td>31</td><td>CN-280-0</td><td>280</td><td>0</td><td>42.3</td></tr><tr><td>11</td><td>CN-250-1</td><td>250</td><td>1</td><td>32.5</td><td>32</td><td>CN-280-1</td><td>280</td><td>1</td><td>40.9</td></tr><tr><td>12</td><td>CN-250-2</td><td>250</td><td>2</td><td>35.8</td><td>33</td><td>CN-280-2</td><td>280</td><td>2</td><td>5.7</td></tr><tr><td>13</td><td>CN-250-3</td><td>250</td><td>3</td><td>24.7</td><td>34</td><td>CN-280-3</td><td>280</td><td>3</td><td>0.0</td></tr><tr><td>14</td><td>CN-250-4</td><td>250</td><td>4</td><td>35.8</td><td>35</td><td>CN-290-0</td><td>290</td><td>0</td><td>37.2</td></tr><tr><td>15</td><td>CN-250-5</td><td>250</td><td>5</td><td>38.8</td><td>36</td><td>CN-290-1</td><td>290</td><td>1</td><td>36.8</td></tr><tr><td>16</td><td>CN-250-6</td><td>250</td><td>6</td><td>34.7</td><td>37</td><td>CN-290-2</td><td>290</td><td>2</td><td>0.1</td></tr><tr><td>17</td><td>CN-250-7</td><td>250</td><td>7</td><td>37.3</td><td>38</td><td>CN-300-0</td><td>300</td><td>0</td><td>50.1</td></tr><tr><td>18</td><td>CN-250-10</td><td>250</td><td>10</td><td>14.9</td><td>39</td><td>CN-300-1</td><td>300</td><td>1</td><td>17.1</td></tr><tr><td>19</td><td>CN-260-0</td><td>260</td><td>0</td><td>8.7</td><td>40</td><td>CN-320-0</td><td>320</td><td>0</td><td>27.9</td></tr><tr><td>20</td><td>CN-260-1</td><td>260</td><td>1</td><td>38.9</td><td>41</td><td>CN-340-0</td><td>340</td><td>0</td><td>0.0</td></tr><tr><td>21</td><td>CN-260-2</td><td>260</td><td>2</td><td>42.4</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>**Table S2.** Comparison of represented solvothermal posttreatment strategies for g-C<sub>3</sub>N<sub>4</sub> modification towards photocatalytic H<sub>2</sub> evolution.</caption><thead><tr><th>Photocatalyst</th><th>Amount of g-C<sub>3</sub>N<sub>4</sub> and chemical</th><th>Solvothermal condition</th><th>Multiple<sup>a)</sup></th><th>Reference</th></tr></thead><tbody><tr><td>g-C<sub>3</sub>N<sub>4</sub>/Q-PHI isotype composite</td><td>500 mg g-C<sub>3</sub>N<sub>4</sub>, 5 mL methanol</td><td>300°C, 0 min</td><td>7.2</td><td>This work</td></tr><tr><td>Amino-modified g-C<sub>3</sub>N<sub>4</sub></td><td>50 mg g-C<sub>3</sub>N<sub>4</sub>, 30 mL ethylene glycol, 20 mg urea</td><td>160°C, 6 h</td><td>5.5</td><td>[1]</td></tr><tr><td>Fluorine modified g-C<sub>3</sub>N<sub>4</sub> nanosheets</td><td>250 mg g-C<sub>3</sub>N<sub>4</sub> nanosheets, 500 g HF (&gt;40%), 30 mL ethanol</td><td>140°C, 6 h</td><td>1.3</td><td>[2]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub> modified by sugar</td><td>400 mg bulk g-C<sub>3</sub>N<sub>4</sub>, 40 mg fructose, 60 mL water/ethanol (v:v = 1:1)</td><td>180°C, 6 h</td><td>13</td><td>[3]</td></tr><tr><td>Monolayer g-C<sub>3</sub>N<sub>4</sub> nanomesh</td><td>100 mg porous g-C<sub>3</sub>N<sub>4</sub>, 300 mL water/isopropanol (v:v = 2:3)</td><td>120°C, 24 h</td><td>5.5</td><td>[4]</td></tr><tr><td rowspan="4">Holey defect g-C<sub>3</sub>N<sub>4</sub></td><td>1000 mg g-C<sub>3</sub>N<sub>4</sub>, 70 mL methyl alcohol</td><td>200°C, 6 h</td><td>4.9</td><td rowspan="4">[5]</td></tr><tr><td>1000 mg g-C<sub>3</sub>N<sub>4</sub>, 70 mL isopropyl alcohol</td><td>200°C, 6 h</td><td>7.4</td></tr><tr><td>1000 mg g-C<sub>3</sub>N<sub>4</sub>, 70 mL tetrahydrofuran</td><td>200°C, 6 h</td><td>13.9</td></tr><tr><td>1000 mg g-C<sub>3</sub>N<sub>4</sub>, 70 mL N,N-dimethylformamide</td><td>200°C, 6 h</td><td>5.6</td></tr><tr><td>Fish-scale structured g-C<sub>3</sub>N<sub>4</sub> nanosheet</td><td>200 mg g-C<sub>3</sub>N<sub>4</sub> nanosheet, 30 mL ethylene glycol, a certain amount of PVP/CTAB</td><td>80°C, 12 h</td><td>2.9</td><td>[6]</td></tr><tr><td>(Na, O)-g-C<sub>3</sub>N<sub>4</sub></td><td>500 mg bulk g-C<sub>3</sub>N<sub>4</sub>, 500 mg NaOH, 60 mL ethanol</td><td>160°C, 10 h</td><td>6.9</td><td>[7]</td></tr></tbody></table>

$$ a] \text{Multiple} = \frac{\text{Photocatalytic H}_2 - \text{evolution activity of modified g-C}_3\text{N}_4}{\text{Photocatalytic H}_2 - \text{evolution activity of original g-C}_3\text{N}_4} $$
<table><caption>**Table S2.** Comparison of represented solvothermal posttreatment strategies for g-C<sub>3</sub>N<sub>4</sub> modification towards photocatalytic H<sub>2</sub> evolution.</caption><thead><tr><th>Photocatalyst</th><th>Amount of g-C<sub>3</sub>N<sub>4</sub> and chemical</th><th>Solvothermal condition</th><th>Multiple<sup>a)</sup></th><th>Reference</th></tr></thead><tbody><tr><td>g-C<sub>3</sub>N<sub>4</sub>/Q-PHI isotype composite</td><td>500 mg g-C<sub>3</sub>N<sub>4</sub>, 5 mL methanol</td><td>300°C, 0 min</td><td>7.2</td><td>This work</td></tr><tr><td>Amino-modified g-C<sub>3</sub>N<sub>4</sub></td><td>50 mg g-C<sub>3</sub>N<sub>4</sub>, 30 mL ethylene glycol, 20 mg urea</td><td>160°C, 6 h</td><td>5.5</td><td>[1]</td></tr><tr><td>Fluorine modified g-C<sub>3</sub>N<sub>4</sub> nanosheets</td><td>250 mg g-C<sub>3</sub>N<sub>4</sub> nanosheets, 500 g HF (&gt;40%), 30 mL ethanol</td><td>140°C, 6 h</td><td>1.3</td><td>[2]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub> modified by sugar</td><td>400 mg bulk g-C<sub>3</sub>N<sub>4</sub>, 40 mg fructose, 60 mL water/ethanol (v:v = 1:1)</td><td>180°C, 6 h</td><td>13</td><td>[3]</td></tr><tr><td>Monolayer g-C<sub>3</sub>N<sub>4</sub> nanomesh</td><td>100 mg porous g-C<sub>3</sub>N<sub>4</sub>, 300 mL water/isopropanol (v:v = 2:3)</td><td>120°C, 24 h</td><td>5.5</td><td>[4]</td></tr><tr><td rowspan="4">Holey defect g-C<sub>3</sub>N<sub>4</sub></td><td>1000 mg g-C<sub>3</sub>N<sub>4</sub>, 70 mL methyl alcohol</td><td>200°C, 6 h</td><td>4.9</td><td rowspan="4">[5]</td></tr><tr><td>1000 mg g-C<sub>3</sub>N<sub>4</sub>, 70 mL isopropyl alcohol</td><td>200°C, 6 h</td><td>7.4</td></tr><tr><td>1000 mg g-C<sub>3</sub>N<sub>4</sub>, 70 mL tetrahydrofuran</td><td>200°C, 6 h</td><td>13.9</td></tr><tr><td>1000 mg g-C<sub>3</sub>N<sub>4</sub>, 70 mL N,N-dimethylformamide</td><td>200°C, 6 h</td><td>5.6</td></tr><tr><td>Fish-scale structured g-C<sub>3</sub>N<sub>4</sub> nanosheet</td><td>200 mg g-C<sub>3</sub>N<sub>4</sub> nanosheet, 30 mL ethylene glycol, a certain amount of PVP/CTAB</td><td>80°C, 12 h</td><td>2.9</td><td>[6]</td></tr><tr><td>(Na, O)-g-C<sub>3</sub>N<sub>4</sub></td><td>500 mg bulk g-C<sub>3</sub>N<sub>4</sub>, 500 mg NaOH, 60 mL ethanol</td><td>160°C, 10 h</td><td>6.9</td><td>[7]</td></tr></tbody></table>

$$a] \text{ Multiple} = \frac{\text{Photocatalytic H}_2 - \text{evolution activity of modified g-C}_3\text{N}_4}{\text{Photocatalytic H}_2 - \text{evolution activity of original g-C}_3\text{N}_4}$$
<table><caption>**Table S3.** Q-PHI models with different grafted groups and corresponding chemical formulas of exfoliated units.</caption><thead><tr><td>Q-PHI model</td><td>Grafted group</td><td>Chemical formula of exfoliated unit</td></tr></thead><tbody><tr><td>Two methyl groups</td><td>(-CH<sub>3</sub>)<sub>2</sub></td><td>(C<sub>6</sub>N<sub>7</sub>)(NH<sub>2</sub>)(OH)<sub>2</sub></td></tr><tr><td>One methyl group and one hydroxyl group</td><td>-CH<sub>3</sub>, -OH</td><td>(C<sub>6</sub>N<sub>7</sub>)(NH<sub>2</sub>)(OH)(NH)(CH<sub>3</sub>)</td></tr><tr><td>Two hydroxyl groups</td><td>(-OH)<sub>2</sub></td><td>(C<sub>6</sub>N<sub>7</sub>)NH<sub>2</sub>(NH)<sub>2</sub>(CH<sub>3</sub>)<sub>2</sub></td></tr></tbody></table>
<table><caption>**Table S3.** Q-PHI models with different grafted groups and corresponding chemical formulas of exfoliated units.</caption><thead><tr><td>Q-PHI model</td><td>Grafted group</td><td>Chemical formula of exfoliated unit</td></tr></thead><tbody><tr><td>Two methyl groups</td><td>(-CH<sub>3</sub>)<sub>2</sub></td><td>(C<sub>6</sub>N<sub>7</sub>)(NH<sub>2</sub>)(OH)<sub>2</sub></td></tr><tr><td>One methyl group and one hydroxyl group</td><td>-CH<sub>3</sub>, -OH</td><td>(C<sub>6</sub>N<sub>7</sub>)(NH<sub>2</sub>)(OH)(NH)(CH<sub>3</sub>)</td></tr><tr><td>Two hydroxyl groups</td><td>(-OH)<sub>2</sub></td><td>(C<sub>6</sub>N<sub>7</sub>)NH<sub>2</sub>(NH)<sub>2</sub>(CH<sub>3</sub>)<sub>2</sub></td></tr></tbody></table>
<table><caption>**Table S4.** C 1s, N 1s and O 1s peak-area percentages (%) of different C species, N species and O species for CN, CN-230-30 and CN-300-0.</caption><thead><tr><th rowspan="2">Sample</th><th colspan="3">C 1s</th><th colspan="4">N 1s</th><th colspan="3">O 1s</th></tr><tr><th>C-C</th><th>C-N</th><th>N=C-N</th><th>C-OH</th><th>C-N=C</th><th>N-(C)<sub>3</sub></th><th>N-H</th><th>N-C-OH</th><th>π charging</th><th>Adsorbed -OH</th><th>Doped O</th><th>C-OH</th></tr></thead><tbody><tr><td>CN</td><td>64.7</td><td>25.0</td><td>10.2</td><td>0</td><td>37.5</td><td>31.5</td><td>23.6</td><td>0</td><td>7.4</td><td>47.6</td><td>52.4</td><td>0</td></tr><tr><td>CN-230-30</td><td>42.0</td><td>18.8</td><td>35.8</td><td>3.4</td><td>30.3</td><td>22.0</td><td>35.5</td><td>0</td><td>14.3</td><td>38.2</td><td>61.8</td><td>0</td></tr><tr><td>CN-300-0</td><td>27.3</td><td>21.0</td><td>31.1</td><td>20.6</td><td>26.0</td><td>23.0</td><td>21.6</td><td>28.4</td><td>1.1</td><td>30.6</td><td>60.7</td><td>8.8</td></tr></tbody></table>
<table><caption>**Table S4.** C 1s, N 1s and O 1s peak-area percentages (%) of different C species, N species and O species for CN, CN-230-30 and CN-300-0.</caption><thead><tr><th rowspan="2">Sample</th><th colspan="3">C 1s</th><th colspan="3">N 1s</th><th colspan="3">O 1s</th></tr><tr><th>C-C</th><th>C-N</th><th>N=C-N</th><th>C-OH</th><th>C-N=C</th><th>N-(C)<sub>3</sub></th><th>N-H</th><th>N-C-OH</th><th>π charging</th><th>Adsorbed -OH</th><th>Doped O</th><th>C-OH</th></tr></thead><tbody><tr><td>CN</td><td>64.7</td><td>25.0</td><td>10.2</td><td>0</td><td>37.5</td><td>31.5</td><td>23.6</td><td>0</td><td>7.4</td><td>47.6</td><td>52.4</td><td>0</td></tr><tr><td>CN-230-30</td><td>42.0</td><td>18.8</td><td>35.8</td><td>3.4</td><td>30.3</td><td>22.0</td><td>35.5</td><td>0</td><td>14.3</td><td>38.2</td><td>61.8</td><td>0</td></tr><tr><td>CN-300-0</td><td>27.3</td><td>21.0</td><td>31.1</td><td>20.6</td><td>26.0</td><td>23.0</td><td>21.6</td><td>28.4</td><td>1.1</td><td>30.6</td><td>60.7</td><td>8.8</td></tr></tbody></table>
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(3) Baranowska, D.; Zielinkiewicz, K.; Mijowska, E.; Zielinska, B. Sugars Induced Exfoliation of Porous Graphitic Carbon Nitride for Efficient Hydrogen Evolution in Photocatalytic Water-Splitting Reaction. *Sci. Rep.* **2024**, *14* (1), 1998.

(4) Han, Q.; Wang, B.; Gao, J.; Cheng, Z.; Zhao, Y.; Zhang, Z.; Qu, L. Atomically Thin Mesoporous Nanomesh of Graphitic C₃N₄ for High-Efficiency Photocatalytic Hydrogen Evolution. *ACS Nano* **2016**, *10* (2), 2745–2751.

(5) Dang, T. T.; Nguyen, T. K. A.; Bhamu, K. C.; Mahvelati-Shamsabadi, T.; Van, V. K. H.; Shin, E. W.; Chung, K.-H.; Hur, S. H.; Choi, W. M.; Kang, S. G.; Chung, J. S. Engineering Holey Defects on 2D Graphitic Carbon Nitride Nanosheets by Solvolysis in Organic Solvents. *ACS Catal.* **2022**, *12* (21), 13763–13780.

(6) Lin, B.; An, H.; Yan, X.; Zhang, T.; Wei, J.; Yang, G. Fish-Scale Structured g-C₃N₄ Nanosheet with Unusual Spatial Electron Transfer Property for High-Efficiency Photocatalytic Hydrogen Evolution. *Appl. Catal. B* **2017**, *210*, 173–183.

(7) Fang, W.; Liu, J.; Yu, L.; Jiang, Z.; Shangguan, W. Novel (Na, O) Co-Doped g-C₃N₄ with Simultaneously Enhanced Absorption and Narrowed Bandgap for Highly Efficient Hydrogen Evolution. *Appl. Catal. B* **2017**, *209*, 631–636.
References

(1) Yang, H.; Zhang, A.; Ding, J.; Hu, R.; Gong, Y.; Li, X.; Chen, L.; Chen, P.; Tian, X. Amino Modulation on the Surface of Graphitic Carbon Nitride for Enhanced Photocatalytic Hydrogen Production. *Carbon* **2024**, *219*, 118841.

(2) Lin, W.; Lu, K.; Zhou, S.; Wang, J.; Mu, F.; Wang, Y.; Wu, Y.; Kong, Y. Defects Remodeling of g-C₃N₄ Nanosheets by Fluorine-Containing Solvothermal Treatment to Enhance Their Photocatalytic Activities. *Appl. Surf. Sci.* **2019**, *474*, 194–202.

(3) Baranowska, D.; Zielinkiewicz, K.; Mijowska, E.; Zielinska, B. Sugars Induced Exfoliation of Porous Graphitic Carbon Nitride for Efficient Hydrogen Evolution in Photocatalytic Water-Splitting Reaction. *Sci. Rep.* **2024**, *14* (1), 1998.

(4) Han, Q.; Wang, B.; Gao, J.; Cheng, Z.; Zhao, Y.; Zhang, Z.; Qu, L. Atomically Thin Mesoporous Nanomesh of Graphitic C₃N₄ for High-Efficiency Photocatalytic Hydrogen Evolution. *ACS Nano* **2016**, *10* (2), 2745–2751.

(5) Dang, T. T.; Nguyen, T. K. A.; Bhamu, K. C.; Mahvelati-Shamsabadi, T.; Van, V. K. H.; Shin, E. W.; Chung, K.-H.; Hur, S. H.; Choi, W. M.; Kang, S. G.; Chung, J. S. Engineering Holey Defects on 2D Graphitic Carbon Nitride Nanosheets by Solvolysis in Organic Solvents. *ACS Catal.* **2022**, *12* (21), 13763–13780.

(6) Lin, B.; An, H.; Yan, X.; Zhang, T.; Wei, J.; Yang, G. Fish-Scale Structured g-C₃N₄ Nanosheet with Unusual Spatial Electron Transfer Property for High-Efficiency Photocatalytic Hydrogen Evolution. *Appl. Catal. B* **2017**, *210*, 173–183.

(7) Fang, W.; Liu, J.; Yu, L.; Jiang, Z.; Shangguan, W. Novel (Na, O) Co-Doped g-C₃N₄ with Simultaneously Enhanced Absorption and Narrowed Bandgap for Highly Efficient Hydrogen Evolution. *Appl. Catal. B* **2017**, *209*, 631–636.