Sulfuric Acid Treated g-CN as a Precursor to Generate High-Efficient g-CN for Hydrogen Evolution from Water under Visible Light Irradiation

Modifying the physical, chemical structures of graphitic carbon nitride (g-CN) to improve its optoelectronic properties is the most efficient way to meet a high photoactivity for clean and sustainable energy production. Herein, a higher monomeric precursor for synthesizing improved micro-and electronic structure possessing g-CN was prepared by high-concentrated sulfuric acid (SA) treatment of bulk type g-CN (BCN). Several structural analyses show that after the SA treatment of BCN, the polymeric melon-based structure is torn down to cyameluric or cyanuric acid-based material. After re-polycondensation of this material as a precursor, the resulting g-CN has more condensed microstructure, carbon and oxygen contents than BCN, indicating that C, O co-doping by corrosive acid of SA. This g-CN shows a much better visible light absorption and diminished radiative charge recombination by the charge localization effect induced by heteroatoms. As a result, this condensed C, O co-doped g-CN shows the enhanced photocatalytic hydrogen evolution rate of 4.57 μmol/h from water under the visible light (>420 nm) by almost two times higher than that of BCN (2.37 μmol/h). This study highlights the enhanced photocatalytic water splitting performance as well as the provision of the higher monomeric precursor for improved g-CN.


Introduction
Carbon (IV) nitride (C 3 N 4 ) is a binary compound consisting of alternating C and N atoms [1]. It is classified into three allotropes (molecular, graphitic, and crystal), among which the graphitic phase (g-C 3 N 4 ) has been computationally calculated to be the most stable allotrope under ambient conditions and of particular importance due to its unique chemical and electronic properties [2]. As an analogue (and complement) of carbon-based graphite, the ideal g-C 3 N 4 consists of tri-s-triazine (or triazine) molecules connected by trigonal N atoms being further extended into 2D graphene-like layers and stacked in a sorption analysis, and the chemical environment was unveiled with Fourier-transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). To identify the physicochemical-optoelectronic property relationship, UV/Vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) emission spectroscopy, flat band potential analysis (Mott-Schottky equation), electrochemical impedance spectroscopy (EIS) were conducted. Finally, for the evaluation of photocatalytic activity, catalyst (1 mg/mL) was irradiated with the visible light (>d420 nm) within water containing 10 vol.% of triethanolamine (TEOA) and chloroplatinic acid (H2PtCl6).

Sulfuric Acid Treatment of Bulk Type Graphitic Carbon Nitride (BCN/SA)
Protonation, exfoliation, and dissolution of BCN can be achieved by highly oxidative, high-concentrated sulfuric acid (SA) [20]. BCN (500 mg) is dissolved into SA (1 mL) at 100 °C. The slurry solution initially becomes a gel at room temperature due to the chemical bonding between g-CN mesogenic units and is subsequently turned into a clear solution at 100 °C (Figure 1a). After SA treatment for 2 h at 100 °C, BCN/SA particles are recovered by adding methanol (MeOH) into the BCN/SA solution. Dissolution of BCN in SA at 100 °C induces both protonation and oxidation, which renders it harsher to BCN than individual protonation with aqueous HCl solution or oxidation with aqueous KOH solution under reflux. Similarly to the scheme in Figure 1b, SA converts -NHx into -OH sites in melon or tri-s-triazine unit at high temperatures: a new band corresponding to -OH stretching mode is emerged at 3000~3500 cm −1 , while C-NH-C IR bending mode at 1310~1230 cm −1 is disappeared in FT-IR spectrum of BCN/SA (Figure 2a). The same trend can also be found in both elemental analysis (EA) and XPS: the atomic percentage of oxygen atom increase both from 13.88% to 17.87% and from 3.1% to 22.1%, while that of nitrogen atom decreases both from 47.18% to 35.31% and from 51.6% to 36.9%, respectively (Table 1). Oxidation and subsequent protonation yield monomeric Dissolution of BCN in SA at 100 • C induces both protonation and oxidation, which renders it harsher to BCN than individual protonation with aqueous HCl solution or oxidation with aqueous KOH solution under reflux. Similarly to the scheme in Figure 1b, SA converts -NH x into -OH sites in melon or tri-s-triazine unit at high temperatures: a new band corresponding to -OH stretching mode is emerged at 3000~3500 cm −1 , while C-NH-C IR bending mode at 1310~1230 cm −1 is disappeared in FT-IR spectrum of BCN/SA (Figure 2a). The same trend can also be found in both elemental analysis (EA) and XPS: the atomic percentage of oxygen atom increase both from 13.88% to 17.87% and from 3.1% to 22.1%, while that of nitrogen atom decreases both from 47.18% to 35.31% and from 51.6% to 36.9%, respectively (Table 1). Oxidation and subsequent protonation yield monomeric cyameluric (or cyanuric) acid derivatives, shifting the stretching vibration mode of triazine ring from 810 cm −1 to 788 cm −1 in FT-IR spectra, losing light absorption ability in the Catalysts 2021, 11, 37 4 of 13 region between 300 and 500 nm in solution UV/Vis absorption spectra and emerging a new peak at 150.3 ppm in liquid-state 13 C NMR spectrum attributed to the corner carbon of cyameluric (or cyanuric) acid (Figure 2b-d). An additional peak is accompanied at 141.5 ppm in liquid-state 13 C NMR spectrum of BCN/SA-24 h. We attribute this to the bay carbon of cyameluric acid as it is far by 9~10 ppm from that of the corner carbon [21]. Other peaks above 150.3 ppm are, in general, attributed to carbon in R1CONR2, which proves the presence of diverse oxidation products. These are well matched with high-resolution C 1s and N 1s XPS of BCN and BCN/SA. In those spectra, the N-C=N peak is shifted from 279.9 eV to 288.6 eV with much reduced areal contribution (from 80% to 68%) and also the counter C-N=C peak from 398.3 eV to 398.9 eV, respectively ( Figure 2e). This distinct shift can be regarded as evidence that significant changes occur in CN heterocycles such as the transformation of terminal C-NH-C to C-OH-C and/or tri-s-triazine ring-opening reactions [22]. 2021, 11, x FOR PEER REVIEW 4 of 13 cyameluric (or cyanuric) acid derivatives, shifting the stretching vibration mode of triazine ring from 810 cm −1 to 788 cm −1 in FT-IR spectra, losing light absorption ability in the region between 300 and 500 nm in solution UV/Vis absorption spectra and emerging a new peak at 150.3 ppm in liquid-state 13 C NMR spectrum attributed to the corner carbon of cyameluric (or cyanuric) acid (Figure 2b-d). An additional peak is accompanied at 141.5 ppm in liquid-state 13 C NMR spectrum of BCN/SA-24 h. We attribute this to the bay carbon of cyameluric acid as it is far by 9~10 ppm from that of the corner carbon [21]. Other peaks above 150.3 ppm are, in general, attributed to carbon in R1CONR2, which proves the presence of diverse oxidation products. These are well matched with high-resolution C 1s and N 1s XPS of BCN and BCN/SA. In those spectra, the N-C=N peak is shifted from 279.9 eV to 288.6 eV with much reduced areal contribution (from 80% to 68%) and also the counter C-N=C peak from 398.3 eV to 398.9 eV, respectively ( Figure 2e). This distinct shift can be regarded as evidence that significant changes occur in CN heterocycles such as the transformation of terminal C-NH-C to C-OH-C and/or tri-s-triazine ring-opening reactions [22].   The cyameluric (or cyanuric) acid derivatives form rectangular particles in SA as is shown in the polarized optical microscopy (POM) image ( Figure 3a). However, monomeric residues are unlikely to survive upon the addition of SA at high temperatures. It was indeed found that mass loss (53%) is severe, and the morphology of g-CN particles are much damaged after dissolution in SA and subsequent precipitation with methanol ( Figure 3b). Fiber-or plate-like particles of BCN are absent in SEM images of BCN/SA where aggregates of spherical particles are prevailing. On the other hand, Kroke et al. utilized solution UV/Vis spectroscopy in the pH range between 11.5 and 0.5 in order to observe the step-wise protonation from potassium cyamelurate to cyameluric acid as shown in Figure 1b, glimpsing that tri-striazine derivatives are stable in the pH range [21]. Moreover, FT-IR and liquid-state 13 C NMR spectroscopy mentioned above prove the presence of both cyameluric and cyanuric acid derivatives, referred to as higher monomeric residues, in BCN dissolved in SA. We thus assume that the rectangular particles are crystalline material originated essentially from cyameluric or cyanuric acid derivatives in which polymeric melon may coexist before further oxidation (Figure 3a). X-ray diffraction pattern of BCN dissolved in SA shows multiple sharp peaks in the 2θ range between 10 • and 30 • . These peaks are well matched with those of cyameluric acid with a space group of P2 1 2 1 2 1 , cell lengths (a = 6.4701 Å, b = 9.9340 Å and c = 12.0985 Å), cell angles (α = 90 • , β = 90 • and γ = 90 • ) and cell volume of 777.619 Å 3 , supporting our assumption (Figure 3c,d) [23]. Such ordering is disappeared after mixing with methanol for precipitation, which indicates the instability of the crystal. In addition to the presence of SA, this limits further studies of XRD, TGA, and so on.
The possibility of further decomposition of tri-s-triazine into triazine moieties (or lower), i.e., ring-opening reaction, is also evidenced by TGA under a protective gas atmosphere. Proving the presence of cyameluric core N atoms might be a better way, which is, however, highly limited by the fact that protonation of g-CN poses a significant effect on local symmetries by altering graphene-like conformation in solution [20]. Thus, 15 N NMR spectroscopy is unable to distinguish the core N atom. To this end, we rely on thermal analysis like TGA based on thermal stability difference in distinguishing tri-s-triazine and triazine derivatives similar to what Cheetham et al. did [24]. The thermal stability of the former is known to be above 500 • C and the latter around 350 • C, which gets slightly lower with the degree of oxidation (or content of oxygen atoms) [8,24]. It is clear from Figure 4a that BCN/SA contains triazine derivatives (or lower), indicating that BCN undergoes ringopening reaction in the presence of SA at high temperatures. The mass loss resulted from thermal decomposition is accelerated above 300 • C, while BCN without triazine moieties is indeed robust up to 500 • C. It is well known that triazine derivatives are condensed into melem by evolving ammonia at similar temperature ranges [16,25]. This thermal behavior is well reproduced in our system with DCDA, supporting the reliability of the results.

Re-Polycondensation of Sulfuric Acid-Treated Graphitic Carbon Nitride Precursor
After re-polycondensation of BCN/SA at 550 °C, the general chemical characteristics of BCN are well recovered in BCN/SA-CN: (1) characteristic CN heterocycles and triazine ring stretching vibration mode of BCN/SA, which is similar to cyanuric acid, turned similarly into those in BCN in the range between 1200 and 1600 cm −1 and at 810 cm −1 in FT-IR spectra, (2) characteristic two peaks at 157 ppm and 165 ppm appear at the similar position in solid-state 13 C cross polarization-magic angle spinning (CP-MAS) NMR spectra of both BCN and BCN/SA-CN, (3) N-C=N or C-N=C peaks in high-resolution C 1s and N 1s XPS spectra of BCN/SA is shifted back to the lower binding energies similar to those of BCN

Re-Polycondensation of Sulfuric Acid-Treated Graphitic Carbon Nitride Precursor
After re-polycondensation of BCN/SA at 550 °C, the general chemical characteristics of BCN are well recovered in BCN/SA-CN: (1) characteristic CN heterocycles and triazine ring stretching vibration mode of BCN/SA, which is similar to cyanuric acid, turned similarly into those in BCN in the range between 1200 and 1600 cm −1 and at 810 cm −1 in FT-IR spectra, (2) characteristic two peaks at 157 ppm and 165 ppm appear at the similar position in solid-state 13 C cross polarization-magic angle spinning (CP-MAS) NMR spectra of both BCN and BCN/SA-CN, (3) N-C=N or C-N=C peaks in high-resolution C 1s and N 1s XPS spectra of BCN/SA is shifted back to the lower binding energies similar to those of BCN

Re-Polycondensation of Sulfuric Acid-Treated Graphitic Carbon Nitride Precursor
After re-polycondensation of BCN/SA at 550 • C, the general chemical characteristics of BCN are well recovered in BCN/SA-CN: (1) characteristic CN heterocycles and triazine ring stretching vibration mode of BCN/SA, which is similar to cyanuric acid, turned similarly into those in BCN in the range between 1200 and 1600 cm −1 and at 810 cm −1 in FT-IR spectra, (2) characteristic two peaks at 157 ppm and 165 ppm appear at the similar position in solid-state 13 C cross polarization-magic angle spinning (CP-MAS) NMR spectra of both BCN and BCN/SA-CN, (3) N-C=N or C-N=C peaks in high-resolution C 1s and N 1s XPS spectra of BCN/SA is shifted back to the lower binding energies similar to those of BCN after re-polycondensation (Figure 5a-c). On the other hand, there is a significant difference between BCN and BCN/SA-CN in terms of atomic contents obtained from EA and XPS analyses: both C, O contents are increased by 0.30%, 0.18% in EA, and 1.60%, 0.96% in XPS, respectively, implying the possibility of C and/or O doping in BCN/SA-CN (Table  2). Such C and O co-doped g-CN was recently achieved with corrosive acids such as H 2 SO 4 and HNO 3 by thermal treatment of melamine-acid complex synthesis where C, O co-doping is rather stable than C-doping only [27]. In addition, the powder XRD pattern of BCN/SA-CN shows typical peaks of g-CN at 13.0 • and 27.5 • corresponding to (100) (Table  2). Such C and O co-doped g-CN was recently achieved with corrosive acids such as H2SO4 and HNO3 by thermal treatment of melamine-acid complex synthesis where C, O co-doping is rather stable than C-doping only [27]. In addition, the powder XRD pattern of BCN/SA-CN shows typical peaks of g-CN at 13.0° and 27.5° corresponding to (100)    The effect of the high degree of polycondensation as well as C, O co-doping on optoelectronic properties of BCN/SA-CN was investigated by UV/Vis DRS and PL emission spectroscopy, Mott-Schottky, EIS, photocurrent, and so on. In UV/Vis DRS spectra, BCN/SA-CN shows much enhanced n to π* transition at the wavelengths over 450 nm region with approximately identical π to π* transition compared to the pristine BCN due to the much narrower bandgap (2.59 eV vs. 2.84 eV) (Figure 6a-c). In addition to the superior visible light absorption (λ > 450 nm), BCN/SA-CN shows a much reduced emission spectrum centered on 456 nm, indicating the reduced charge recombination than BCN (Figure 6d). The high degree of polycondensation reduces the population of surface defect sites such as -NH x (x = 1 or 2) which induces charge recombination. In addition, heteroatoms can localize charges on BCN/SA-CN structure, and thus act as disturbing spots for the recombination of photoexcited electron hole pairs (EHPs) [28]. This is also well shown in electron spin resonance (ESR) spectroscopy in that the localized conjugation system induced by C, O-doping of BCN/SA-CN enable to distribute more lone pair electrons on the LUMO position over the tris-triazine molecules, thus intensify the resonance signal (Figure 6e). Sequentially conducted photocurrent under AM1.5G shows that the BCN/SA-CN exhibits a much larger photocurrent of 2.5 µA/cm 2 than that of BCN (1.5 µA/cm 2 ) (Figure 6f). This is clearly originated from the better absorption of visible light and reduced charge recombination of photogenerated EHPs by charge localization with the presence of heteroatoms, C and O, in BCN/SA-CN structure.  The effect of the high degree of polycondensation as well as C, O co-doping on optoelectronic properties of BCN/SA-CN was investigated by UV/Vis DRS and PL emission spectroscopy, Mott-Schottky, EIS, photocurrent, and so on. In UV/Vis DRS spectra, BCN/SA-CN shows much enhanced n to π* transition at the wavelengths over 450 nm region with approximately identical π to π* transition compared to the pristine BCN due to the much narrower bandgap (2.59 eV vs. 2.84 eV) (Figure 6a-c). In addition to the superior visible light absorption (λ > 450 nm), BCN/SA-CN shows a much reduced emission spectrum centered on 456 nm, indicating the reduced charge recombination than BCN (Figure 6d). The high degree of polycondensation reduces the population of surface defect sites such as -NHx (x = 1 or 2) which induces charge recombination. In addition, heteroatoms can localize charges on BCN/SA-CN structure, and thus act as disturbing spots for the recombination of photoexcited electron hole pairs (EHPs) [28]. This is also well shown in electron spin resonance (ESR) spectroscopy in that the localized conjugation system induced by C, O-doping of BCN/SA-CN enable to distribute more lone pair electrons on the LUMO position over the tris-triazine molecules, thus intensify the resonance signal (Figure 6e). Sequentially conducted photocurrent under AM1.5G shows that the BCN/SA-CN exhibits a much larger photocurrent of 2.5 µA/cm 2 than that of BCN (1.5 µA/cm 2 ) (Figure 6f). This is clearly originated from the better absorption of visible light and reduced charge recombination of photogenerated EHPs by charge localization with the presence of heteroatoms, C and O, in BCN/SA-CN structure.  Finally, photocatalytic hydrogen evolution reaction (HER) from the water was conducted to evaluate photocatalytic activity of BCN/SA-CN under the visible light (>420 nm) with the sacrificial electron donor, TEOA. BCN/SA-CN generates an almost two times higher amount of hydrogen during 4 h of photoreaction under the visible light (BCN/SA-CN = 4.57 µmol/h, BCN = 2.37 µmol/h), which is superior or comparable to those of hetero-atom doped g-CNs (Figure 7a, Table 3). In addition, this catalyst maintains its photocatalytic activity until 16 h of photoreaction ( Figure 7b). As shown in Figure 7c, BCN/SA-CN has a more positive HOMO level of −1.44 eV than BCN (−1.81 eV). Interestingly, BCN/SA-CN shows much larger resistance than BCN in electrochemical impedance spectroscopy (EIS), meaning that the charge transfer efficiency of BCN/SA-CN is rather lower than that of BCN (Figure 7d). This is due to the conflict effect of charge localization as explained above, where those heteroatoms also hinder not only the recombination of photogenerated EHPs, but also the efficient charge transfer of them to the surface of photocatalyst. A schematic diagram of overall band structure for each sample was illustrated in Figure 7e. Note that all of the g-CN samples used in this work produce only a negligible amount of hydrogen without TEOA. Finally, photocatalytic hydrogen evolution reaction (HER) from the water was conducted to evaluate photocatalytic activity of BCN/SA-CN under the visible light (>420 nm) with the sacrificial electron donor, TEOA. BCN/SA-CN generates an almost two times higher amount of hydrogen during 4 h of photoreaction under the visible light (BCN/SA-CN = 4.57 µmol/h, BCN = 2.37 µmol/h), which is superior or comparable to those of heteroatom doped g-CNs (Figure 7a, Table 3). In addition, this catalyst maintains its photocatalytic activity until 16 h of photoreaction ( Figure 7b). As shown in Figure 7c, BCN/SA-CN has a more positive HOMO level of −1.44 eV than BCN (−1.81 eV). Interestingly, BCN/SA-CN shows much larger resistance than BCN in electrochemical impedance spectroscopy (EIS), meaning that the charge transfer efficiency of BCN/SA-CN is rather lower than that of BCN (Figure 7d). This is due to the conflict effect of charge localization as explained above, where those heteroatoms also hinder not only the recombination of photogenerated EHPs, but also the efficient charge transfer of them to the surface of photocatalyst. A schematic diagram of overall band structure for each sample was illustrated in Figure 7e. Note that all of the g-CN samples used in this work produce only a negligible amount of hydrogen without TEOA.

Synthesis of BCN
An amount of 20 g of dicyandiamide (DCDA) was calcined at 550 • C on the glass container cover a glass lid in the box furnace in the air for 4 h. The heating rate was 2.3 • C/min. After that, yellow-colored bulk-type graphitic carbon nitride was collected and labeled as 'BCN'.

Synthesis of BCN/SA and BCN/SA-CN
An amount of 500 mg of bulk graphitic carbon nitride (BCN) was added to 1 mL of high concentrated H 2 SO 4 (95~98%) and heated to 100 • C for 2 h in an oil bath with a stirring. After that, 40 mL of methanol was added to the solution mixture and jerk well to get a white precipitate. The precipitate was filtered and washed with methanol two times and dried in a vacuum oven at 60 • C overnight. Finally, the white powder was obtained and labeled as 'BNC/SA'. Then, 500 mg of acid-treated bulk g-CN (BCN/SA) was kept on a borosilicate glass vial and heated to 550 • C for 4 h with a ramping rate of 2.3 • C/min. After cooled down, a brownish sample was collected and labeled as 'BCN/SA-CN'.

Photoelectrochemical Analysis
A three-electrode cell set up consists of a working electrode, a Pt wire as a counter electrode, and an Ag/AgCl in 3 M KCl as a reference electrode was used to measure the electrochemical properties (photocurrent, electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis) using an electrochemical analyzer (VSP, Seyssinet-Pariset, France, BioLogic). The working electrode was fabricated with FTO (fluorine-doped tin oxide) glass substrate. At first, 50 mg photocatalyst was dispersed in 1.8 mL ethanol and 0.2 mL Nafion solution (5 wt.% in ethanol) by stirring overnight. Then the slurry was drop cast on the substrate and dried in a vacuum oven at 60 • C for overnight (electrode active materials area = 0.5 cm 2 ). The working electrode was engrossed on the supporting electrolyte (0.2 M Na 2 SO 4 ). The cell was purged with nitrogen gas for 30 min to eliminate the air. The perturbation signal is maintained for Mott-Schottky (from −0.3 V to 1.2 V vs. Ag/AgCl, frequency: 100 Hz−200 kHz), EIS (0.5 V vs. Ag/AgCl, frequency: 500 kHz to 4 mHz). Light (100 mW/cm 2 , AM1.5G condition) was irradiated from the rear side of the semiconductor/conductive FTO interface for photocurrent measurement at the steady potential of 0.5 V vs. Ag/AgCl.

Photocatalytic Activity
In a 50 mL quartz reactor, 25 mg catalyst was dispersed in 25 mL of 3rd DI-water containing 10 vol.% of triethanolamine (TEOA) and 7 wt.% of Pt (H 2 PtCl 6 , deposition of metallic Pt on the catalyst was done by in-situ photodeposition method). The reaction mixture was kept on stirring for 4 h after that sonication for 1 h and purged with nitrogen for 30 min. The dispersion was irradiated under simulated solar light using 10 suns solar simulator (66902, Newport, Irvine, CA, USA) equipped with 300 W Xenon lamp (6258, Newport, Irvine, CA, USA) as a light source and 420 nm cut-on filter (FSQ-GG420, Newport, Irvine, CA, USA) while keeping the stirring 700 rpm. Finally, a gas chromatogram instrument was used to measure the evolved hydrogen gas (Donam Instruments Inc., Sungnam, Korea, thermal conductive detector, 5 Å molecular sieve column, nitrogen carrier gas).

Conclusions
Cyameluric or cyanuric acid-based higher monomeric precursor (BCN/SA) and their derivative g-CN (BCN/SA-CN) was successfully synthesized by high-concentrated sulfuric acid (SA) treatment of BCN and re-polycondensation. Results of structural and optical analyses show that polymeric melon is oxidized and protonated by corrosive acid, H 2 SO 4 , and the leaved structure is revealed as cyameluric or cyanuric acid-based higher monomeric material. With this material as a precursor, the resulting BCN/SA-CN has a more condensed microstructure and a larger content of carbon and oxygen, indicating the C, O co-doping. These enhance the visible light absorption by reduced bandgap and quenches radiative charge recombination by charge localization effect induced from the presence of heteroatoms in sp 2 -hybridized conjugation system. However, this also intensifies the charge transfer resistance at the same time, where the localization of charge hinders the efficient charge transfer of photogenerated EHPs to the surface of photocatalyst for water splitting reaction. As a result, BCN/SA-CN shows a higher photocatalytic hydrogen evolution rate of 4.57 µmol/h under the visible light irradiation (>420 nm) by almost two times than pristine BCN (2.37 µmol/h).

Data Availability Statement:
The data presented in this study are available in this article.