Salt‐melt synthesis of poly(heptazine imide) in binary alkali metal bromides for enhanced visible‐light photocatalytic hydrogen production

Poly(heptazine imide) (PHI), a semicrystalline version of carbon nitride photocatalyst based on heptazine units, has gained significant attention for solar H2 production benefiting from its advantages including molecular synthetic versatility, excellent physicochemical stability and suitable energy band structure to capture visible photons. Typically, PHI is obtained in salt‐melt synthesis in the presence of alkali metal chlorides. Herein, we examined the role of binary alkali metal bromides (LiBr/NaBr) with diverse compositions and melting points to rationally modulate the polymerization process, structure, and properties of PHI. Solid characterizations revealed that semicrystalline PHI with a condensed π‐conjugated system and rapid charge separation rates were obtained in the presence of LiBr/NaBr. Accordingly, the apparent quantum yield of hydrogen using the optimized PHI reaches up to 62.3% at 420 nm. The density functional theory calculation shows that the dehydrogenation of the ethylene glycol has a lower energy barrier than the dehydrogenation of the other alcohols from the thermodynamic point of view. This study holds great promise for rational modulation of the structure and properties of conjugated polymeric materials.


| INTRODUCTION
[28][29] Recently, an external quantum efficiency of approximately 100% was achieved by selective deposition of Rh/Cr 2 O 3 and CoOOH as spatially separated hydrogen and oxygen evolution cocatalysts, respectively, on a single crystal Al:SrTiO 3 photocatalysts featuring anisotropic crystal facets. [30]This achievement in photocatalytic overall water splitting indicates nearly no energy loss in the process of the photogenerated electron-hole pairs separation/migration and provides a new horizon for rational design and development of more promising semiconductor photocatalysts.
[36] However, the slow rate of mass transfer of the reactants and incomplete polymerization of the intermediates were the major issues in the traditional solid-phase polymerization process, which generate oligomers (e.g., melem and melon) with abundant structural defects as the main products.These amorphous versions of PCN were extremely restrained by the rapid charge recombination at the structural defects. [37,38]To further enhance the separation and migration of charge carriers, it is necessary to promote the polymerization process and decrease the number of structural defects.By using a eutectic salt mixture having a lower melting point compared to the individual components and strong solvation ability of nitrides and oligomers, synthesis in salt-melt has shown significant advantages in improving the crystallinity of polymers.[41][42][43] For instance, methods based on binary alkali metal chlorides (e.g., LiCl/NaCl, NaCl/KCl, LiCl/KCl) have been extensively developed to synthesize new promising poly(heptazine imide) (PHI) catalysts. [44,45]It is proposed that the different contents and melting points of the binary metal chlorides significantly affect the polymerization process. [46]Note that when the polymerization was performed in the presence of LiCl/NaCl with a melting point of 552°C, which is quite close to the polymerization temperature of 550°C, fully condensed PHI with a lower number of structural defects was obtained, which greatly restrained the undesired charge recombination. [47]Thus, it is highly desired to investigate whether more promising PHIs with excellent structure and properties could be obtained, when the polymerization is conducted in the presence of diverse eutectic salts with different contents.
The binary metal bromides eutectic mixture (e.g., LiBr/KBr, LiBr/NaBr, and KBr/NaBr) are also used for the salt-melt synthesis of carbides, nitrides, and other materials. [48,49]For instance, when binary LiBr/KBr salts were selected as eutectic solvent for the synthesis of PCN, poly(triazine imide) (PTI) with high crystallinity was obtained. [50]The as-obtained PTI presents a larger layer stacking distance than that of PTI prepared in LiCl/KCl eutectic mixture, which is probably due to the incorporation of bromide between the triazine-based layers.However, the PTI single crystals obtained from LiBr/KBr eutectic mixture exhibit very poor photocatalytic activity in comparison with the PTI prepared in the alkali metal chlorides.These results indicate that the chemical structure and properties of PHI are influenced by both metal cations and halide ions. [51,52]Further investigations on the ionothermal synthesis of PCN are highly desired and should aim at revealing the relationship between the structure and properties of the materials.
In this study, PHI with higher crystallinity was synthesized in the presence of alkali metal bromides as the eutectic mixture.Typically, binary salts (LiBr/NaBr) were selected for the rational modulation of the polymerization process.By adjusting the reaction conditions such as the molar ratio of binary salts, reaction temperature, and holding time/temperature, we carefully explored their impact on the photocatalytic performance of the obtained materials.The proposed synthesis process is depicted in Scheme 1 (see more details in Section 4).Note that PHI prepared in LiBr/NaBr eutectic salts shows excellent photocatalytic activity.Furthermore, by optimizing the NaBr to LiBr molar ratio, reaction temperature and holding time, we obtained PHI, which exhibits an apparent quantum yield (AQY) value of 62.3% at 420 nm in photocatalytic hydrogen production.

| Structure analysis
The chemical structure of PHI/Na x Li y was first examined using powder X-ray diffraction (XRD).Samples synthesized at different Na:Li molar ratios display similar XRD diffraction peaks, indicating a consistent PHI core structure framework.As shown in Figure 1A, all samples present two distinct peaks at 8.3°and 28.1°, which correspond to the in-plane repeating of heptazine units (100) and the interlayer stacking mode of graphite-phase lamellar structure (002), respectively. [43,53]Note that a clear shift of (002) diffraction peak to a lower degree was observed when decreasing the molar ratio of Na + /Li + to 0/10, indicating that the interlayer distance increases.This is probably because the single LiCl salt can merely melt the carbon nitride oligomers and generate PHI with not fully condensed stacking mode.Besides, the peak at 8.3°becomes pronounced as the molar ratio of Na + /Li + decreases, which is attributed to the improvement of its in-plane packing order.
The structure information of PHI/Na x Li y can be further examined by Fourier-transform infrared (FTIR) spectra (Figure 1B).The absorption peaks at 809 cm −1 indicate the out-of-plane bending mode of heptazine rings.The broad absorption peaks at 1060-1710 cm −1 are related to the stretching vibration of heptazine derivatives.In addition, it can be seen that there is a relatively weak absorption peak located at 2180 cm −1 , which is attributed to the terminal cyanogroup. [54]The formation of the cyanogroups may be explained by the decomposition of heptazine rings during the calcination of crystalline carbon nitride. [37,55,56]The absorption peaks at 3000-3700 cm −1 are derived from the vibration of uncondensed amino groups and water molecules on the catalyst surface. [57]he Raman spectra (Figure 1C, recorded upon excitation at 325 nm) were also obtained to gain information about the chemical structure of the asprepared samples.As shown in Figure 1C, all samples exhibit similar spectra, indicating that modifying the Na:Li molar ratio of molten salt has minimal effect on the chemical structure of the heptazine ring.The broadband in the 1200-1700 cm −1 region is related to C─N stretching vibrations, which are the same as the D and G bands of a typical graphitic structure. [58,59]herefore, the formation of the graphitic structure is confirmed by this technique.The sharp peaks located at around 990 cm −1 are attributed to the symmetric N-breathing mode of heptazine units.Besides, clearly observed two peaks at 725 and 780 cm −1 are due to inplane bending vibrations of the C─N═C connected heptazine linkages.
As shown in Figure 1D, the solid-state 13 C nuclear magnetic resonance (NMR) spectrum of PHI/Na 1 Li 9 was obtained to further confirm the chemical structure.Two obvious dominant 13 C NMR signals are observed, which are in line with the previous reports.The peak at 157 ppm is assigned to the central carbon atom (C 1 ) in the heptazine unit in the form of N═C─N 2 (C), while the peak at 167 ppm represents the neighboring heptazine carbon atom (C 2 ). [44,60]No new peaks are observed in the spectra, indicating the robust stability of the PHI by using the alkali metal bromides as eutectic salts.
The microstructure and morphology of the samples were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).As shown in Figure 2A, PHI/Na x Li y displays a dense, irregular, disordered fragment stacked structure when polymerization of the precursors was performed at the eutectic point.It is remarkable, however, that the morphology of PHI/Na x Li y changes depending on the Na:Li molar ratio.Besides, further increase the content of NaBr, the irregular flaky fragments gradually grow larger and the ordered stacking between the layers is also destroyed (Figures 2B and S1).Interestingly, when the molar ratio of Na + to Li + is 1:9, a house tile-like stacked structure is found.Conversely, in the absence of Na + , the stacked structure is once again disrupted, leading to the enlargement and irregularity of the lamellar particles.This is in agreement with the XRD result.This also suggests that excess or absence of Na + salts leads to a significant change of morphology, which is also consistent with photocatalytic hydrogen production activity, as discussed below.The high-resolution TEM image was also used to examine the microstructure of the PHI/Na 1 Li 9 .The fringe with a lattice spacing of about 0.98 nm is clearly observed in Figure 2C, which is consistent with the peak of 8.3°in the XRD patterns.As shown in Figure 2D, a clear lattice fringe with a distance of 0.224 nm is discovered, corresponding to (111) crystal planes of the Pt nanoparticles, which were in situ deposited as cocatalysts for photocatalytic H 2 production as discussed below.The (111) crystal plane is usually considered as the active plane of metal Pt. [45,61] Besides, the energy-dispersive X-ray elemental mapping confirms that C, N, Na, and Pt elements are uniformly distributed in PHI/Na 1 Li 9 .Simultaneously, the low-temperature nitrogen adsorption-desorption isotherms (Figure S2) revealed that the specific surface area of PHI/Na 1 Li 9 and PHI/Na 1 Li 3 is 7.3 and 18.9 m 2 /g, respectively.It can be observed that increasing in-plane crystallinity results in a decrease in the specific surface area, as supported by the XRD patterns.
Elemental analysis, inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS) were carried out to further investigate the chemical composition of the samples (Tables S1 and S2, and Figure 3).As shown in Figure 3A ring (or the surface-absorbed CO 2 ), respectively.The peak located at 284.8 eV corresponds to the adventitious carbon in the PHI-Na x Li y . [62,63]In addition, the N 1s spectrum can be deconvoluted into three peaks (Figure 3C).The peaks at 398.9 and 400.6 eV are attributed to C─N═C and N─C 3 moieties, respectively, which derive from the tri-s-triazine rings.The peak at 401.5 eV can be assigned to the surface residual amino group (C─NH x ).Furthermore, the peak at 404 eV is considered to be a charge effect or positive charge localization in the heterocycles.As shown in Figure 3D, residual Na + ions were observed, which is in line with the ICP analysis (Table S1) and previous reports. [45,52]nterestingly, by comparing PHI/Na 1 Li 9 with PHI/ Na 1 Li 3 , it is observed that when the content of Na + cations increase, both C 1s and N 1s peaks are shifted towards lower binding energies.This is probably due to the presence of the cyanogroups, which cause undesired defects and also confirm that in-plane crystallinity increases at the molar ratio of Na + to Li + of 1:9.The O 1s spectra of all samples are shown in Figure S3.The presence of a large amount of oxygen is commonly attributed to the surface adsorbed species, such as H 2 O, O 2 , and CO 2 .No Br 3d and Li 1s signal was found in the samples (Figure S4), which is consistent with previous studies.

| Analysis of photoelectric properties
The UV−vis diffuse reflectance spectra show that the absorption edges of the PHI/Na x Li y powders were almost identical at different Na:Li molar ratios (Figure S5).Subsequently, the separation and recombination behavior of photogenerated carriers was evaluated by roomtemperature photoluminescence (PL) (Figure 4A) and time-resolved PL spectroscopy (Figure S6).The results show that PHI/Na 1 Li 9 has the weakest emission peak and almost the same lifetime as PHI/Na 1 Li 3 , suggesting that the recombination of the photogenerated carriers is hindered and the charge transfer rate is fast. [64]Furthermore, the Tauc plot (Figure S7) shows that the band gap of PHI/Na 1 Li 9 is 2.70 eV, slightly lower than that of the material prepared using the mixture of salts in 1:3 molar ratio corresponding to the eutectic point (2.74 eV), which provides a higher potential for harnessing visible photons and enhances the photocatalytic activity.The position of the valence band was determined by ultraviolet photoelectron spectroscopy (Figure S8).The valence band position of PHI/Na 1 Li 9 is 2.38 V and the conduction band is then calculated to be −0.32V by considering the band gap energy (Table S3).The corresponding conduction band and valence band energies are shown in Figure 4B.In Figure 4C, the photocurrent measurements performed in a threeelectrode photoelectrochemical cell show that the current density of PHI/Na 1 Li 9 is significantly higher than that of PHI/Na 1 Li 3 , indicating its exceptional charge transport properties. [65]Furthermore, the charge migration properties were further confirmed by the electrochemical impedance spectroscopy.According to the Nyquist plot, PHI/Na 1 Li 9 exhibits a significantly lower impedance than PHI/Na 1 Li 3 , indicating more efficient charge diffusion, which contributes to the exceptional photocatalytic performance of PHI/Na 1 Li 9 .

| Performance in photocatalytic hydrogen production
Photocatalytic hydrogen evolution experiments were performed to evaluate the activity of the materials.As shown in Figure 5A, the activity varies significantly depending on the Na:Li molar ratio that was used to obtain the catalysts.When the molar ratio of Na + to Li + is 1:9, the optimum activity is 438 µmol h −1 with triethanolamine as the sacrificial agent at λ > 420 nm.However, as the Na + molar ratio increases, the hydrogen production performance gradually decreases.Interestingly, in the absence of Na + , the catalyst activity is still low.It is proposed that the content of sodium salt affects the structural order of PHI/Na x Li y , as confirmed by XRD and SEM.In addition, the AQY of PHI/Na 1 Li 9 depending on the wavelength of incident photons was examined with triethanolamine as the sacrificial agent (Figure S9).The spectrum of the light source used to determine AQY is shown in Figure S10.As shown in Figure 5B, the activity of the catalyst did not significantly decrease after 10 runs (50 h) of cyclic reactions, which indicates the robustness of the sample.XRD, FTIR, and XPS (Figure S11-S14) analyses proved that PHI/Na 1 Li 9 did not undergo structural changes during the reaction.Besides, the effect of various alcohols, which were used as sacrificial agents instead of triethanolamine in photocatalytic H 2 evolution was also explored.The results show that the highest hydrogen production activity is achieved with ethylene glycol (EG) as the sacrificial agents.In Figure 5C, PHI/ Na 1 Li 9 shows remarkable activity with a H 2 yield of 520 μmol after 1 h of illumination.As shown in Figure 5D, the AQY at different wavelengths correlates well with the optical absorption of PHI/Na 1 Li 9 .The AQY reaches up to 62.3% with EG as the sacrificial agent at 420 nm.
Furthermore, under the same conditions, two other sets of molten salts (NaBr/KBr and LiBr/KBr) were selected for comparison.XRD patterns and FTIR spectra (Figure S15) confirmed the successful synthesis of the PHI structure.It can be viewed from the UV−vis diffuse reflectance spectra (Figure S16) that the samples synthesized using different molten salts show strong light absorption.The morphologies of samples prepared using different molten salts are shown in Figure S16.Additionally, the hydrogen production performance under different conditions was tested.As shown in Figure S18, PHI prepared using NaBr/LiBr salt exhibits the highest photocatalytic performance.The influence of synthesis temperature (Figures S19-S21) and holding times (Figures S22-S24) on the performance of materials in photocatalysis was further investigated.Results revealed that the catalyst shows the highest activity when prepared at a temperature of 550°C and a holding time of 4 h.

| Theoretical calculation
To further investigate the significant differences of H 2 production activity of PHI/Na 1 Li 9 using EG and methanol (MeOH) as sacrificial agents, density functional theory (DFT) calculations were carried out.The energy diagram of the dehydrogenation process of MeOH/PHI and EG/PHI composites is shown in Figure 6.Initially, MeOH and EG can both be stably adsorbed on Pt-loaded PHI/Na 1 Li 9 by forming one Pt-O adsorption bond, and the adsorption energies are predicted to be −1.29 and −1.41 eV, respectively.Then, the O─H bond of adsorbed MeOH and EG undergoes a cleavage and the dissociated H atom was adsorbed with Pt atom on the catalyst.The energy barrier of O─H cleavage over the MeOH/PHI is 0.72 eV, which is evidently higher than the barrier of EG/PHI (E a = 0.36 eV).This lower energy barrier of the EG/PHI system indicates the more feasibility of dehydrogenation process, and thus contributes to the significantly enhanced activity of photocatalytic H 2 production.Furthermore, it is noted that the dehydrogenation process of the EG/PHI system has a lower reaction energy (E r = −1.17eV), comparing with the MeOH/PHI system (E r = −0.75eV).Thereby, this process is easier to occur on EG/PHI from a thermodynamic perspective.In summary, the alkali metal bromides eutectic mixtures (NaBr/LiBr) were selected to synthesize PHI, by optimizing and adjusting the content of NaBr and LiBr.When the Na + :Li + molar ratio is 1:9, the in-plane crystallinity of heptazine-based carbon nitride is enhanced.The reduced number of defects facilitates charge separation and accelerates in-plane charge carrier transport.The obtained PHIs were tested as photocatalysts in hydrogen production.Accordingly, the optimized sample, PHI/ Na 1 Li 9 , displays an AQY of 62.3% in the presence of EG as the sacrificial agent at 420 nm.The DFT calculation shows that the dehydrogenation of the EG/PHI system is more likely than the dehydrogenation of the MeOH/PHI system from the thermodynamic point of view.This study emphasizes the significance of different molten salts to modulate the structure and thus to improve the photocatalytic performance.

| Synthesis of melon-based PCN catalysts
Typically, 8 g of melamine (C 3 H 6 N 6 ) was placed in a 50 mL porcelain crucible, covered with a lid, heated from room temperature to 550°C with a ramp 12°C/min, and maintained at 550°C for 4 h.After cooling down to room temperature, the yellow sample was ground into powder and collected for subsequent use, which is labeled as melon.

| Synthesis of PHI/Na x Li y catalysts
The PHI/Na x Li y was synthesized by salt-melt method.In general, 0.6 g melon and 6 g NaBr/LiBr with different molar ratios (Na:Li = 0:10, 1:9, 1:3, 7:3, and 9:1, with melting points of 540°C, 510°C, 506°C, 540°C, and 627°C), where 1:3 corresponds to the molar ratio of Na to Li at the eutectic point, were manually mixed in a mortar in a glovebox.Then, the mixture was transferred into a crucible, covered with a lid, heated from room temperature to 550°C with a ramp of 2°C/min, and maintained at 550°C for 4 h in the N 2 atmosphere.After cooling naturally, the samples were thoroughly washed with deionized water and dried at 60°C in a vacuum oven overnight.The final products are labeled as PHI/Na x Li y , while x and y represent the molar ratio of NaBr to LiBr.

| Synthesis of Pt/PHI/Na x Li y catalysts
A 50 mg photocatalyst was dispersed in 100 mL distilled water containing 10 mL of the 10 vol% sacrificial agents as an electron donor.The 3 wt% Pt cocatalysts were deposited onto the catalysts by an in-situ photodeposition method using H 2 PtCl 6 dissolved in the reactant solution.The photodeposition was conducted in a glass-closed gas circulation system that used a 300-W Xe light with a 420-nm cut-off filter for 1 h.The modified catalysts were then washed with distilled water and dried at 60°C under vacuum.

S C H E M E 1
Schematic illustration of the synthetic process of PHI/Na x Li y .PCN, polymeric carbon nitride; PHI, poly(heptazine imide).
, the elements C, N, O, and Na in PHI/Na 1 Li 9 and PHI/Na 1 Li 3 are confirmed by the XPS survey spectra.The C 1s spectrum can be deconvoluted into three components with peaks at 288.4, 286.4,and 284.8 eV.The two main peaks at 288.4 and 286.4 eV represent the sp 2hybridized carbon of N─C═N in heptazine units and C─NH x (or C─O) derived from the edge of the heptazine F I G U R E 3 XPS results.(A) The survey spectrum, (B) C 1s, (C) N 1s, and (D) Na 1s of PHI/Na 1 Li 9 and PHI/Na 1 Li 3 .PHI, poly(heptazine imide); XPS, X-ray photoelectron spectroscopy.

F
I G U R E 5 (A) Photocatalytic H 2 evolution rates of PHI/Na x Li y samples under visible-light (λ > 420 nm) irradiation, (B) cycle experiment of PHI/Na 1 Li 9 , (C) H 2 production activity of PHI/Na 1 Li 9 using various alcohols as sacrificial agents, and (D) dependence of AQYs on the wavelength of incident light for PHI/Na 1 Li 9 .AQY, apparent quantum yield; EG, ethylene glycol; EtOH, ethanol; HER, hydrogen evolution rates; IPA, isopropanol; MeOH, methanol; PHI, poly(heptazine imide); TEOA, triethanolamine.F I G U R E 6 The energy diagram and optimized configurations of (A) MeOH and (B) EG dehydrogenation on PHI/Na 1 Li 9 by DFT calculations.Atoms in brown, blue, gray, red and pink represent C, N, Pt, O, and H, respectively.DFT, density functional theory; EG, ethylene glycol; MeOH, methanol; PHI, poly(heptazine imide).