Graphite phase carbon nitride film for photoanode and forming method thereof

Abstract

A method for forming a graphite phase carbon nitride (g-CN) film by hot vapor condensation, the method comprising the steps of: a) providing a solid phase thiourea precursor and a solid phase melamine precursor in a vessel; b) covering the container with a first substrate; and c) thermally generating a gas-phase thiourea source and a gas-phase melamine source from the solid-phase thiourea precursor and the solid-phase melamine precursor in an air environment, thereby forming a g-CN layer on the first substrate. A g-CN film formed by the method is also presented.

Description

Technical Field

[0001] The present invention relates to a method for forming a graphite-phase carbon nitride film, and particularly, but not exclusively, to a method for forming a graphite-phase carbon nitride film by condensing hot vapor; and a graphite-phase carbon nitride film formed by the method. Background of the Invention

[0002] To address the global energy crisis and environmental problems, efforts have been made to find alternative pathways for converting solar energy into stable chemical fuels, especially carbon-free hydrogen fuels, via photocatalytic water splitting. In this regard, photoelectrochemical (PEC) water splitting has become an attractive pursuit for cost-effectively converting sunlight and water into chemical fuels.

[0003] Among the reported photocatalysts, graphitic carbon nitride (g-CN) is considered a promising candidate for PEC water splitting to generate hydrogen energy due to its suitable band gap (2.7 eV), low cost, and environmental friendliness. Generally, g-CN can be prepared by the thermal decomposition of nitrogen-rich precursors such as cyanamide, dicyandiamide, thiocyanate, and melamine. However, the photocatalytic efficiency of pristine g-CN is still hampered by the shortcomings of rapid charge recombination, limited visible light harvesting ability, and low surface area.

[0004] Although various modification methods such as metal or nonmetal doping, supramolecular preorganization, and construction of heterojunctions or homojunctions based on g-CN have been employed to enhance catalytic activity, it is believed that these approaches may have a detrimental effect on the compatibility and stability of g-CN due to its diverse physicochemical properties with different atomic ratios.

[0005] The present invention therefore seeks to obviate or at least mitigate such disadvantages by providing new or otherwise improved methods for preparing g-CN, in particular methods for preparing high PEC performance g-CN for use in solar energy conversion. Summary of the Invention

[0006] In a first aspect of the present invention, a method for forming a graphitic carbon nitride (g-CN) film by thermal vapor condensation (TVC) is provided, the method comprising the steps of: a) providing a solid-phase thiourea precursor and a solid-phase melamine precursor in a container; b) covering the container with a first substrate; and c) thermally generating a gaseous thiourea source and a gaseous melamine source from the solid-phase thiourea precursor and the solid-phase melamine precursor in an air environment, thereby forming a graphitic carbon nitride layer on the first substrate.

[0007] In an optional embodiment, the solid phase thiourea precursor and the solid phase melamine precursor are physically separated by a supporting substrate.

[0008] Optionally, the support substrate is positioned at an angle relative to a side of the container.Optionally, the support substrate comprises a solid phase thiourea precursor layer.

[0009] In an optional embodiment, the method further comprises the step of forming the solid-phase thiourea precursor layer.

[0010] Optionally, the method further comprises the steps of: heating thiourea solid in deionized water to form a hot saturated thiourea solution; forming a saturated thiourea layer on the supporting substrate by at least partially immersing the supporting substrate in the saturated thiourea solution; and drying the saturated thiourea layer to obtain the solid-phase thiourea precursor.

[0011] Optionally, the step of drying the saturated thiourea solution layer is performed under ambient conditions.

[0012] Optionally, the thiourea solid in deionized water is heated to a temperature of about 60°C to about 150°C.

[0013] In an optional embodiment, the support substrate is at least partially immersed in the saturated thiourea solution for about 2 to 15 seconds.

[0014] Optionally, the solid phase thiourea precursor layer has a thickness of about 1.5 mm.

[0015] Optionally, the weight of the solid thiourea layer is about 0.6 g.

[0016] In an optional embodiment, the weight of the solid phase melamine precursor is from about 1 g to about 5 g.

[0017] Optionally, step c) includes the steps of: annealing the solid-phase thiourea precursor and the solid-phase melamine precursor to produce the vapor-phase thiourea source and the vapor-phase melamine source containing activated carbodiimide species; and allowing the vapor-phase thiourea source and the vapor-phase melamine source to deposit and react to form the graphitic carbon nitride layer on the first substrate.

[0018] Optionally, the annealing step is performed in a muffle furnace at about 550° C. for about 3 hours, with a heating rate of about 3° C. / minute.

[0019] In an optional embodiment, the method further comprises step d): post-annealing the graphite carbon nitride layer formed in step c). Optionally, step d) is performed in a muffle furnace at about 300° C. for about 30 minutes, wherein the heating rate is about 3° C. / minute.

[0020] Optionally, the first substrate comprises FTO glass. Optionally, the support substrate comprises a glass strip.

[0021] In an optional embodiment, the container has been polished. Optionally, the method further comprises the step of continuously polishing the edge of the container with an abrasive, such as abrasive paper (sandpaper) of about 400 Cw to about 1200 Cw.

[0022] In an optional embodiment, the method further comprises the step of placing a load on top of the first substrate to minimize leakage of the gas-phase thiourea source and the gas-phase melamine source.

[0023] In an optional embodiment, each of the solid phase thiourea precursor and the solid phase melamine precursor is in powder form.

[0024] Optionally, the method further comprises the steps of preparing a homogeneous mixture of the solid-phase thiourea precursor and the solid-phase melamine precursor by grinding thiourea solid and melamine solid; and transferring the homogeneous mixture into the container for heat treatment.

[0025] Preferably, the weight ratio of the thiourea solids to the melamine solids is about 1 to 3:3.

[0026] Optionally, step c) includes the steps of: annealing the solid-phase thiourea precursor and the solid-phase melamine precursor to generate the gas-phase thiourea source and the gas-phase melamine source; and allowing the gas-phase thiourea source and the gas-phase melamine source to deposit and react to form the graphitic carbon nitride layer on the first substrate.

[0027] Optionally, the annealing step is performed in a muffle furnace at about 500° C. to about 550° C. for about 3 hours, with a heating rate of about 3° C. / minute.

[0028] In a second aspect of the present invention, there is provided a graphitic carbon nitride film formed by the method according to the first aspect, the graphitic carbon nitride film comprising a C:N:O atomic ratio of about 2.3 to 4.9:1.9 to 4.8:1.

[0029] Optionally, the graphitic carbon nitride film comprises a C:N atomic ratio of about 1.25 to about 1.45.

[0030] Optionally, the graphitic carbon nitride film comprises a plurality of nanoporous and mesoporous structures. Optionally, the graphitic carbon nitride film comprises a plurality of nanoflakes on the surface of the film.

[0031] Optionally, the plurality of nanosheets has a length of about 7 μm to about 13 μm.

[0032] In an optional embodiment, the graphitic carbon nitride film has a thickness of about 2.5 μm to about 4.3 μm.

[0033] In an optional embodiment, the graphite phase carbon nitride film has a transient photocurrent density of about 51.4 μA cm at 1.23 V vs. RHE in 0.1 M KOH electrolyte with 10% triethanolamine when subjected to intermittent simulated AM 1.5G illumination. -2 to about 404 μA cm -2 .

[0034] In a third aspect of the present invention, there is provided a graphite-phase carbon nitride film formed by the method according to the first aspect, the graphite-phase carbon nitride film comprising a C:N:O atomic ratio of about 3.9 to 7.5:4 to 7.9:1.

[0035] In an optional embodiment, the graphitic carbon nitride film comprises a plurality of nanosheets at least partially stacked upon one another.

[0036] Optionally, the plurality of nanosheets comprises a nanomesh structure or a sheet-like structure.

[0037] Optionally, the graphitic carbon nitride film comprises a plurality of nanoflowers on the surface of the film.

[0038] Optionally, the graphite-phase carbon nitride film has a thickness of about 2.81 μm to about 3.21 μm.

[0039] In an optional embodiment, the graphite phase carbon nitride film has a transient photocurrent density of about 298.6 μA cm at 1.23 V vs. RHE in 0.1 M Na2SO4 electrolyte with 10% triethanolamine when subjected to intermittent simulated AM 1.5G illumination. -2 to about 790.5 μA cm -2 .

[0040] In an optional embodiment, the graphitic carbon nitride film has a solar-to-hydrogen energy conversion efficiency of about 0.75% when operated in a 0.1 M Na2SO4 electrolyte with 10% triethanolamine when the film has a C:N:O atomic ratio of 7.48:7.92:1. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

[0042] Figure 1 shows a schematic diagram illustrating the preparation of g-CN membranes from melamine precursors and thiourea precursors by a hot vapor condensation (TVC) method according to an embodiment of the present invention;

[0043] Figure 2 shows a photograph showing a side view of FTO glass with a solid thiourea precursor deposited thereon by drying a saturated thiourea layer;

[0044] Figure 3 Photographs of g-CN membranes (CNM, CNMT1-500, CNMT1-550, CNMT2-500, CNMT2-550) produced according to embodiments of the present invention are shown;

[0045] Figure 4A shows a photograph of g-CN-M10 film deposited on FTO glass;

[0046] Figure 4B shows a photograph of g-CN-M3T0.6 film deposited on FTO glass;

[0047] Figure 4C shows a photograph of a g-CN-M4T0.6 film deposited on FTO glass;

[0048] Figure 4D shows a photograph of a g-CN-M5T0.6 film deposited on FTO glass;

[0049] Figure 4E A photograph of a g-CN-M3T0.6′ film (i.e., g-CN-M3T0.6 without post-annealing) deposited on FTO glass is shown;

[0050] Figure 5A SEM images of g-CN-M10 are shown, where the inset is a cross-sectional image;

[0051] Figure 5B SEM images of g-CN-M3T0.6 are shown, where the inset is a cross-sectional image;

[0052] Figure 5C SEM images of g-CN-M4T0.6 are shown, where the inset is a cross-sectional image;

[0053] Figure 5D SEM images of g-CN-M5T0.6 are shown, where the inset is a cross-sectional image;

[0054] Figure 6A Shown are the photographs and corresponding SEM images of g-CN films prepared from 10 g of melamine precursor at 550 °C for 3 h;

[0055] Figure 6B Shown are the photographs and corresponding SEM images of g-CN films prepared from 10 g of melamine precursor at 550 °C for 4 h;

[0056] Figure 6C Shown are the photographs and corresponding SEM images of g-CN films prepared from 10 g of melamine precursor at 550 °C for 5 h;

[0057] Figure 6D Shown are the photographs and corresponding SEM images of g-CN films prepared from 10 g of melamine precursor at 550 °C for 6 h;

[0058] Figure 6E Shows a photograph and corresponding SEM image of a g-CN film prepared from 20 g of melamine precursor at 550 °C for 3 h;

[0059] Figure 6F Shown Figures 6A to 6E Transient photocurrent density curve of the sample in 0.1M KOH solution;

[0060] Figure 7A SEM images of g-CN-M10 are shown, where the scale bar is 1 μm;

[0061] Figure 7B SEM images of g-CN-M3T0.6 are shown, where the scale bars are 10 μm and 5 μm;

[0062] Figure 7C SEM images of g-CN-M4T0.6 are shown, where the scale bar is 5 μm;

[0063] Figure 7D SEM images of g-CN-M5T0.6 are shown, where the scale bar is 5 μm;

[0064] Figure 8A TEM image of g-CN-M10 is shown;

[0065] Figure 8B TEM image of g-CN-M3T0.6 is shown;

[0066] Figure 8C TEM image of g-CN-M4T0.6 is shown, highlighting the nanoporous and mesoporous structures;

[0067] Figure 8D TEM image of g-CN-M5T0.6 is shown, highlighting the nanoporous and mesoporous structures;

[0068] Figure 8E HRTEM image of g-CN-M3T0.6 is shown, where the inset is a magnified image of the selected area (dashed square) and the attribution of the lattice fringes;

[0069] Figure 8F The fast Fourier transform image of g-CN-M3T0.6 is shown;

[0070] Figure 8GThe estimated average interlayer distance in g-CN-M3T0.6 is shown;

[0071] Figure 9A XRD patterns of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 samples are shown;

[0072] Figure 9B FTIR spectra of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 samples are shown;

[0073] Figure 10 C1s (top) and N1s (bottom) core-level XPS spectra of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 samples are shown;

[0074] Figure 11 is a table summarizing the chemical composition of C and N in g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 samples;

[0075] Figure 12 is a schematic diagram illustrating potential g-CN allotropes having conjugated structural units approaching graphite-like structures from small to large (i.e., with increasing degree of polymerization) derived from melamine and thiourea using a TVC method according to an embodiment of the present invention;

[0076] Figure 13A SEM images of CNMs are shown, where the inset is a cross-sectional image;

[0077] Figure 13B SEM images of CNMT1-500 are shown, wherein the inset is a cross-sectional image;

[0078] Figure 13C SEM images of CNMT1-550 are shown, wherein the inset is a cross-sectional image;

[0079] Figure 13D SEM images of CNMT2-500 are shown, wherein the inset is a cross-sectional image;

[0080] Figure 13E SEM images of CNMT2-550 are shown, wherein the inset is a cross-sectional image;

[0081] Figure 14 shows the element distribution map of CNMT1-500;

[0082] Figure 15ATEM images of CNMT1-500 are shown;

[0083] Figure 15B TEM images of CNMs are shown;

[0084] Figure 15C TEM images of CNMT1-550 are shown;

[0085] Figure 15D TEM images of CNMT2-500 are shown;

[0086] Figure 15E TEM images of CNMT2-550 are shown;

[0087] Figure 16A HRTEM image of CNMT1-500 is shown;

[0088] Figure 16B Shown Figure 16A Magnified HRTEM image of a selected area, wherein the inset is a magnified image of a circular selected area showing interlayer striations of CNMT1-500;

[0089] Figure 17 XRD patterns of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 are shown;

[0090] Figure 18 FTIR spectra of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 are shown;

[0091] Figure 19A Shown is the full spectrum analysis of XPS spectra of CNM and CNMT1-500;

[0092] Figure 19B The XPS spectra of C1s of CNM and CNMT1-500 are shown;

[0093] Figure 19C The XPS spectra of N1s of CNM and CNMT1-500 are shown;

[0094] Figure 19D XPS spectra of S2p of CNM and CNMT1-500 are shown;

[0095] Figure 20 is a summary from 19A to 19D Table of the chemical compositions (atomic %) of C, N, and O in CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 samples obtained from XPS spectra of ;

[0096] Figure 21A UV-Vis absorption spectra of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 are shown;

[0097] Figure 21B UV-Vis absorption spectra of g-CN-M3T0.6 and g-CN-M3T0.6′ are shown;

[0098] Figure 22A Valence band XPS spectra of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 are shown. The valence band top (VBM) values were measured by XPS valence spectrum. For g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6, the VBM values were 1.53, 1.32, 1.35, and 1.39 eV, respectively.

[0099] Figure 22B The energy band structures of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6 and g-CN-M5T0.6 are shown. g Value and general formula E CBM =E VBM -E g Combined, the conduction band bottom (CBM) values of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 were derived to be −1.17, −1.22, −1.26, and −1.29 eV, respectively;

[0100] Figure 23 The total density of states (TDOS) of trimers composed of three melamine, heptazine rings, or larger conjugated clusters are shown. They were calculated using CAM-B3LYP / 6-31G* and then analyzed using the versatile wave function analysis tool. The optimized structures of the three trimers are shown next to the TDOS curves. The energy gap (E) is determined by the size. g ) values are labeled and colored, respectively, as calculated based on the energy level difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital;

[0101] Figure 24A UV-Vis absorption spectra of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 are shown;

[0102] Figure 24BThe photoluminescence (PL) spectra of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 are shown;

[0103] Figure 25A The transient photocurrent densities of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M KOH-10% TEOA are shown;

[0104] Figure 25B The transient photocurrent density of g-CN-M3T0.6 and g-CN-M3T0.6′ (without post-annealing) in 0.1 M KOH-10% TEOA is shown;

[0105] Figure 25C The transient photocurrent densities of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M Na2SO4-10% TEOA are shown;

[0106] Figure 25D is a table summarizing the transient photocurrent densities of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M KOH-10% TEOA or 0.1 M Na2SO4-10% TEOA;

[0107] Figure 25E The discontinuous light LSV curves of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M KOH-10% TEOA are shown;

[0108] Figure 25F The discontinuous light LSV curves of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M Na2SO4-10% TEOA are shown;

[0109] Figure 25G The transient photocurrent density of g-CN-M1T0.6 film in 0.1 M KOH-10% TEOA or 0.1 M Na2SO4-10% TEOA is shown, with the film photograph inserted;

[0110] Figure 25H The transient photocurrent density of g-CN-M2T0.6 film in 0.1 M KOH-10% TEOA or 0.1 M Na2SO4-10% TEOA is shown, with the film photograph inserted;

[0111] Figure 26A The relationship between the photocurrent density and the thiourea:melamine mass ratio in the range of 0 to 0.6 is shown;

[0112] Figure 26B The relationship between the photocurrent density and film thickness of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 is shown;

[0113] Figure 27A Shown are photostability evaluations of g-CN-M3T0.6 with / without the sacrificial agent TEOA (inset: photostability of g-CN-M10 sample for comparison);

[0114] Figure 27B Photostability evaluation of g-CN-M3T0.6 / g-CN-M3T0.6′ with / without post-annealing is shown;

[0115] Figure 28A Nyquist plots of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M KOH-10% TEOA in the dark are shown;

[0116] Figure 28B Nyquist plots of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M KOH-10% TEOA under illumination are shown;

[0117] Figure 28C Nyquist plots of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M Na2SO4-10% TEOA in the dark are shown;

[0118] Figure 28D The Nyquist plots of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M Na2SO4-10% TEOA under illumination are shown;

[0119] Figure 28E Mott–Schottky plots of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M KOH-10% TEOA are shown;

[0120] Figure 28FMott-Schottky plots of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 in 0.1 M Na2SO4-10% TEOA are shown;

[0121] Figure 29A The discontinuous light LSV curves of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 in 0.1 M Na2SO4-10% TEOA are shown;

[0122] Figure 29B Transient photocurrent density curves of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 in 0.1 M Na2SO4-10% TEOA are shown;

[0123] Figure 29C The transient photocurrent density curves of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 in 0.1 M Na2SO4 solution are shown;

[0124] Figure 29D is a table summarizing the transient photocurrent densities of CNM, CNMT1-500, CNMT1-550, CNMT2-500, CNMT2-550, CNMT3-500, and CNMT3-550 in 0.1 M Na2SO4 solution with or without 10% TEOA;

[0125] Figure 30 is a table comparing the PEC performance of g-CN of the present invention with reported g-CN photoelectrodes prepared from melamine and thiourea precursors;

[0126] Figure 31 The photocurrent density-electrolyte relationship of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 is shown;

[0127] Figure 32A The photocurrent density-precursor weight relationship of CNMT1-500, CNMT2-500, and CNMT3-500 is shown;

[0128] Figure 32B A photograph showing g-CN film grown from thiourea powder on FTO glass.

[0129] Figure 33A The photocurrent density of CNMT1-500 and CNMT1-550 in 0.1 M Na2SO4 solution is shown;

[0130] Figure 33B The photocurrent density of CNMT1-500 and CNMT1-550 in 0.1 M Na2SO4-10% TEOA solution is shown;

[0131] Figure 33C The photocurrent density-annealing temperature relationship of CNMT1-500, CNMT1-550, CNMT2-500 and CNMT2-550 is shown;

[0132] Figure 34A A photograph of CNMT1-600 is shown;

[0133] Figure 34B A photograph of CNMT2-600 is shown;

[0134] Figure 35 The EIS patterns of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 under dark and light conditions are shown;

[0135] Figure 36 Mott-Schottky curves of CNM, CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 are shown;

[0136] Figure 37 It is summarized based on Figure 36 Table of flat-band potentials calculated from the Mott-Schottky diagram;

[0137] Figure 38 The results show that the irradiance of 1 sun at AM 1.5G (100mW / cm -2 ) Photostability measurements of CNM and CNMT1-500 using a three-electrode workstation;

[0138] Figure 39A Shown is a photograph of the CNMT1-500 membrane before all PEC tests;

[0139] Figure 39B Shown are photos of the CNMT1-500 membrane after all PEC tests;

[0140] Figure 39C Shown are SEM images of CNMT1-500 membranes before all PEC tests;

[0141] Figure 39D SEM images of CNMT1-500 membrane after all PEC tests are shown;

[0142] Figure 40AThe relative potential energy profiles calculated by DFT for the carbodiimide (HN=C=NH) involved in the growth of melamine into a heptazine ring via steps 1-6 are shown. The red and blue numbers are the activation energies (E) relative to each previous local minimum. a ) and heat of reaction (ΔH). Each step of the chemical reaction is listed in the lower left corner. Figures 41A to 41C The geometric structures of 16 transition states and 16 intermediates are shown in FIG;

[0143] Figure 40B The relative potential energy profiles calculated by DFT for the carbodiimide (HN=C=NH) involved in the growth of the heptazine ring into a larger conjugated cluster via steps 7-16 are shown. The red and blue numbers are the activation energies (E) relative to each previous local minimum. a ) and heat of reaction (ΔH). Each step of the chemical reaction is listed in the lower left corner. Figures 41A to 41C The geometric structures of 16 transition states and 16 intermediates are shown in FIG;

[0144] Figure 40C The growth of melamine into g-CN clusters (corresponding to Figure 40A and Figure 40B TS1-16 and IM1-16) shown in the table;

[0145] Figure 41A The geometries of the transition states (TSs) and intermediates (IMs) involved in the growth of melamine into large conjugated g-CN clusters, calculated using CAM-B3LYP / 6-31G* calculations, are shown: (a) TS1, (b) IM1 with NH3, (c) TS2, (d) IM2, (e) TS3, (f) IM3, (g) TS4, (h) IM4, (i) TS5, (j) IM5 with NH3, (k) TS6, and (l) heptazine ring (i.e., IM6). Gray, blue, and white spheres represent C, N, and H atoms.

[0146] Figure 41B The geometries of the transition states (TSs) and intermediates (IMs) involved in the growth of melamine into large conjugated g-CN clusters, calculated using CAM-B3LYP / 6-31G* calculations, are shown: (m) TS7, (n) IM7 with NH3, (o) TS8, (p) IM8, (q) TS9, (r) IM9, (s) TS10, (t) IM10 with NH3, (u) TS11, and (v) IM11. Gray, blue, and white spheres represent C, N, and H atoms.

[0147] Figure 41CShown are the geometries of the transition states (TSs) and intermediates (IMs) involved in the growth of melamine into large conjugated g-CN clusters, calculated using CAM-B3LYP / 6-31G* calculations: (w) TS12, (x) IM12, (y) TS13, (z) IM13, (aa) TS14, (ab) IM14, (ac) TS15, (ad) IM15, (ae) TS16, and (af) IM16 with NH3. Gray, blue, and white spheres represent C, N, and H atoms.

[0148] Figure 42A Shown is the relative potential energy profile of carbodiimides (HN=C=NH) derived from melamine or thiourea (TU) precursors, as calculated using DFT calculations: Melamine decomposes into three cyanamides (N≡C-NH2). The structures of the reactants, TS, and products are shown near the profile. Gray, blue, white, and yellow spheres represent C, N, H, and S atoms, respectively.

[0149] Figure 42B Shown is the relative potential energy profile of carbodiimide (HN=C=NH) derived from melamine or thiourea (TU) precursors, as calculated using DFT calculations: N≡C-NH2 isomerizes to HN=C=NH via H-transfer thermal isomerization. The structures of the reactants, TS, and products are shown adjacent to the profile. Gray, blue, white, and yellow spheres represent C, N, H, and S atoms, respectively.

[0150] Figure 42C Shown is the relative potential energy profile of carbodiimide (HN=C=NH) derived from melamine or thiourea (TU) precursors, as calculated by DFT calculations: TU decomposes into HN=C=S and NH3. The structures of the reactants, TS, and products are shown near the profile. Gray, blue, white, and yellow spheres represent C, N, H, and S atoms, respectively.

[0151] Figure 42D Shown is the relative potential energy profile of carbodiimides (HN=C=NH) derived from melamine or thiourea (TU) precursors, as calculated by DFT calculations: HN=C=S dimers condense to form HN=C=NH and CS2. The structures of the reactants, TS, and products are shown adjacent to the profile. Gray, blue, white, and yellow spheres represent C, N, H, and S atoms, respectively.

[0152] Figure 43AThe geometries of the transition states (TS) and intermediates (IMs) involved in the formation of the N-C3 coordination bond in g-CN membranes, calculated using CAM-B3LYP / 6-31G* calculations, are shown: (a) TS1, (b) IM1, (c) TS2, (d) IM2 with NH3 (dimer), (e) TS3, (f) IM3, (g) TS4, (h) IM4 with NH3 (trimer). Gray, blue, and white spheres represent C, N, and H atoms. TS1-4 and IM1-4 correspond to Figure 43B Steps 1-4 of the chemical reaction listed in;

[0153] Figure 43B This is a table summarizing the N-C3 coordination bond formation in g-CN by DFT calculation;

[0154] Figure 44A The spatial distribution of photoinduced holes and electrons in the first excited state of g-CN(H1) is shown. The distances between the centers of photoinduced holes and electrons (D h-e ,by = 0.002 arbitrary units, holes are pink and electrons are green;

[0155] Figure 44B The spatial distribution of photoinduced holes and electrons in the first excited state of g-CN(H3) is shown. The distance between the centers of photoinduced holes and electrons (D h-e ,by = 0.002 arbitrary units, holes are pink and electrons are green;

[0156] Figure 44C The spatial distribution of photoinduced holes and electrons in the first excited state of g-CN(H6) is shown. The distances between the centers of photoinduced holes and electrons (D h-e ,by = 0.002 arbitrary units, holes are pink and electrons are green;

[0157] Figure 44D g-CN(H 10 ) Spatial distribution of photoinduced holes and electrons in the first excited state. The distance between the centers of photoinduced holes and electrons (D h-e ,by = 0.002 arbitrary units, holes are pink and electrons are green;

[0158] Figure 45 is the fundamental energy gap (E g, in eV) and the electron-hole pair binding energy (E b , in eV);

[0159] Figure 46A The UV-vis absorption spectrum of g-CN(H1) is shown. The excited state with the maximum oscillator strength is marked on the spatial distribution of the photoinduced holes and electrons shown. Isovalue = 0.002 arbitrary units, holes are pink and electrons are green;

[0160] Figure 46B The UV-vis absorption spectrum of g-CN(H3) is shown. The excited state with the maximum oscillator strength is marked on the spatial distribution of the photoinduced holes and electrons shown. Isovalue = 0.002 arbitrary units, holes are pink and electrons are green;

[0161] Figure 46C The UV-vis absorption spectrum of g-CN(H6) is shown. The excited state with the maximum oscillator strength is marked on the spatial distribution of the photoinduced holes and electrons shown. Isovalue = 0.002 arbitrary units, holes are pink and electrons are green;

[0162] Figure 46D g-CN(H 10 ). The excited states with the maximum oscillator strength are marked in the spatial distribution of photoinduced holes and electrons. Isometric value = 0.002 arbitrary units, holes are pink and electrons are green;

[0163] Figure 47A A photograph of the experimental setup used for H2 emission testing under AM 1.5G 1 sun illumination (100 mW / cm -2 ) is performed via a three-electrode configuration system;

[0164] Figure 47B Shown are hydrogen measurements and the corresponding Faradaic efficiencies of CNM and CNMT1-500 photoanodes (squares);

[0165] Figure 47C Shown is a comparison of H2 evolution between CNM and CNMT1-500 under the same experimental conditions;

[0166] Figure 47D Shown are the solar-to-hydrogen (STH) efficiencies of CNM and CNMT1-500. Inset: Transient photocurrent density measurements of CNM and CNMT1-500 films via a three-electrode photoelectrochemical configuration.

[0167] Figure 48shows a performance comparison of the STH efficiency of some reported photoanodes and the g-CN membrane of the present invention for solar water splitting; and

[0168] Figure 49 The table summarizes the reported STH efficiencies of g-CN based photocatalysts and their comparison with the g-CN of the present invention. All work was done in the laboratory, except for the mp-CN work, which was performed in a large photoreactor under natural sunlight. DETAILED DESCRIPTION

[0169] As used herein, the forms "a", "an" and "the" are intended to include both the singular and the plural, unless the context clearly indicates otherwise.

[0170] As used herein, the words "example" or "exemplary" are intended to serve as examples, instances, or illustrations. Any aspect or design described in this disclosure as "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, the use of the words "example" or "exemplary" is intended to present concepts in a concrete manner. As used in this application, the term "or" is intended to mean an inclusive "or" rather than an exclusive "or." That is, unless specified otherwise or the context dictates otherwise, "X employs A or B" is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then "X employs A or B" holds true in any of the foregoing instances.

[0171] As used herein, the phrase "about" is intended to refer to values that deviate slightly from the values described herein. For example, "about 100° C." may mean any value from 100.2…100.5…101…101.2° C.; "about 5 seconds" may mean any value from 4.9…4.95…5…5.01…5.05…5.1 seconds; "about 1.5 mm" may mean any value from 1.48…1.49…1.5…1.51…1.52 mm.

[0172] Without intending to be limited by theory, the inventors have designed a method for forming graphite phase carbon nitride (g-CN) films, in particular by hot vapor condensation, through their own research, trials and experiments. It is believed that the method of the present disclosure will provide a low-cost and efficient way to make large, uniform g-CN films with desired thickness and surface morphology. For example, in embodiments, the amount of raw materials used for the production can be greatly reduced by a factor of 10. It is also believed that the g-CN films prepared by the method of the present invention will be promising candidates for photoelectrochemical (PEC) devices, green energy hydrogen fuel production via photocatalysis, electrocatalysis, lithium-ion batteries, light-emitting diodes (LEDs), chemical sensors, antimicrobial agents, and the like. For example, in some embodiments, the g-CN films prepared may have a conductivity of about 404.4 μA cm-2 to about 790.5 μA cm -2 In some other embodiments, the prepared g-CN film can have a solar-to-hydrogen efficiency of about 0.75%.

[0173] According to the present invention, a method for forming a g-CN film by thermal vapor condensation is provided. As used herein, the term "thermal vapor condensation (TVC)" or "thermal condensation" generally refers to a chemical vapor deposition method that involves easily heating (raw) precursor materials in an air atmosphere to produce a gaseous precursor to perform a multi-step gas phase polycondensation reaction. In particular, it should be understood that TVC as described herein should be distinguished from other thermal treatment processes that require heating raw materials and / or decomposing raw materials in an atmosphere of substantially inert gas (e.g., N2) or inert gas (e.g., Ar, He, etc.) to carry out subsequent reactions such as pyrolysis.

[0174] The method may include the following steps: a) providing a solid-phase thiourea precursor and a solid-phase melamine precursor in a container; b) covering the container with a first substrate; and c) thermally generating a gaseous thiourea source and a gaseous melamine source from the solid-phase thiourea precursor and the solid-phase melamine precursor in an air environment, thereby forming a g-CN layer on the first substrate.

[0175] In embodiments, the solid-phase thiourea precursor and the solid-phase melamine precursor can be physically separated by a supporting substrate. The supporting substrate can, for example, be positioned at an angle relative to one side of the container. In some embodiments, the supporting substrate can be positioned at an angle (such as from about 30° to about 160°) relative to the base (bottom) of the container. In some specific embodiments, the supporting substrate can be positioned at an angle less than 90° (such as from 30° to 60°) relative to the base (bottom) of the container (that is, the supporting substrate is in an inclined position relative to the bottom of the container) so that the solid-phase thiourea precursor can be positioned above the solid-phase melamine precursor, or vice versa. It is believed that the supporting substrate is positioned at an angle described herein, such as being positioned in an inclined position to avoid preventing the gaseous thiourea source or gaseous melamine source generated in step c) from reaching the first substrate, and to avoid the pressure buildup in the container caused by such prevention.

[0176] In certain embodiments, the support substrate may include a solid-phase thiourea precursor layer, such as a solid-phase thiourea precursor layer disposed on one of the sides / surfaces of the support substrate. In some more specific embodiments, the support substrate may include a solid-phase thiourea precursor layer formed / disposed on the top side and / or bottom side of the support substrate.

[0177] In an embodiment, a solid-phase thiourea precursor layer can be formed on a supporting substrate by heating thiourea solid in deionized water to form a hot saturated thiourea solution; forming a saturated thiourea layer on the supporting substrate by at least partially immersing the supporting substrate in the saturated thiourea solution; and drying the saturated thiourea layer to obtain a solid-phase thiourea precursor.

[0178] In an exemplary embodiment, a hot saturated thiourea solution can be prepared by heating, for example, 18 g of thiourea solid in 20 ml of deionized water to a temperature of about 60° C. to about 150° C., particularly about 120° C. The supporting substrate can then be at least partially immersed in the saturated thiourea for about 2-15 seconds to form a saturated thiourea layer thereon, followed by drying the saturated thiourea layer under ambient conditions, such as for 5 minutes, to obtain a solid-phase thiourea precursor. In particular, the inventors unexpectedly discovered that by at least partially immersing the supporting substrate in the saturated thiourea for about 5 seconds, a solid-phase thiourea precursor layer having a thickness of about 1.5 mm, or in other words, a weight of about 0.6 g, can be consistently obtained.

[0179] The support substrate can be any shape that can be placed in the container and can be made of any suitable material that does not react with the solid phase thiourea precursor and does not decompose during subsequent heat treatment of the method. For example, in a specific embodiment, the support substrate can be a glass strip.

[0180] In some embodiments, the amount (weight) of solid-phase melamine can be adjusted from about 1 g to about 5 g. That is, in some embodiments, the weight (or mass) ratio of solid-phase thiourea precursor to solid-phase melamine precursor can be about 0.12 to about 0.6 (or 1:1.67 to 1:8.33).

[0181] In some embodiments, the container may be polished to facilitate the capture of the gaseous thiourea source and the gaseous melamine source when g-CN is formed. In some specific embodiments, the container may be a ceramic crucible, and its edge may be polished using an abrasive, such as abrasive paper (sandpaper), particularly abrasive paper having electrochemically coated silicon carbide thereon. The mesh of the abrasive paper may be from about 400 Cw to about 1200 Cw. As used herein, "Cw" is the unit of mesh defined by sand per unit length.

[0182] Preferably, the abrasive paper can be applied continuously to polish the ceramic crucible. For example, in an embodiment, the edge can be polished with 400Cw, 800cW and 1200Cw sandpapers in succession.

[0183] In step b), the container can be covered with a first substrate. It should be understood that the first substrate can serve as a support, and the thermally generated gaseous thiourea source and gaseous melamine source can be deposited on the support to achieve g-CN formation. The first substrate can be any shape / size sufficient to cover the (top) opening of the container. Preferably, the first substrate can be made of any suitable material that does not react with the gaseous thiourea source and gaseous melamine source and / or does not decompose during heat treatment. Preferably, the first substrate can be a conductive material such as FTO glass.

[0184] Optionally or additionally, a load may be placed on top of the first substrate to minimize leakage of the gaseous thiourea source and the gaseous melamine source during heat treatment, thereby promoting the formation of a uniform and homogeneous g-CN film. For example, in an embodiment where the container is a ceramic crucible and the first substrate is FTO glass, a ceramic plate, such as about 90.5 g, may be placed on top of the FTO glass before starting the heat treatment in step c).

[0185] Step c) includes the steps of: annealing a solid-phase thiourea precursor and a solid-phase melamine precursor to generate a vapor-phase thiourea source and a vapor-phase melamine source containing an active carbodiimide species; and allowing the vapor-phase thiourea source and the vapor-phase melamine source to deposit and react to form a g-CN layer on the first substrate. Specifically, the annealing step can be performed in a muffle furnace in an air atmosphere at approximately 550°C for approximately 3 hours, with a heating rate of approximately 3°C / minute.

[0186] It is believed that performing the annealing step in an air atmosphere not only simplifies the equipment requirements for g-CN synthesis, but also promotes g-CN polymerization (g-CN film formation) compared to the case where a N2 atmosphere is required. In particular, the inventors unexpectedly discovered that the above-mentioned annealing step / conditions can lead to the production of smaller reactive carbodiimide species in a solid-phase thiourea precursor that is more preferred than a solid-phase melamine precursor, and that the carbodiimide species can be individually anchored to the gas-phase melamine source, thereby forming heptazine rings or even larger conjugated structures, which can further anchor more reactive carbodiimide species thereto, thereby forming a graphite-like structure.

[0187] In embodiments, the method may further include step d) post-annealing the g-CN layer formed in step c). It is believed that performing the post-annealing step can improve the film quality / performance of the g-CN layer formed in step c), such as uniformity, UV-Vis absorption, transient photocurrent density, photostability, etc. Details of the improved film quality / performance due to the post-annealing step will be discussed later in this disclosure. In a specific embodiment, step d) can be performed in a muffle furnace at approximately 300° C. for approximately 30 minutes, with a heating rate of approximately 3° C. / minute.

[0188] Optionally or additionally, in step d), a load may be placed on the first substrate, as described herein, on top of the first substrate.

[0189] In some alternative embodiments, each of the solid phase thiourea precursor and the solid phase melamine precursor may be in powder form.

[0190] In these embodiments, the method may include the steps of: preparing a homogeneous mixture of a solid thiourea precursor and a solid melamine precursor by grinding thiourea solid and melamine solid; and transferring the homogeneous mixture to a container for heat treatment. In particular, the weight (mass) ratio of thiourea solid to melamine solid may be about 1-3:3, such as 1:3, 1.2:3, 1.5:3, 1.8:3, 2:3, 2.1:3, 2.5:3, 3:3, etc.

[0191] In an embodiment where the weight ratio of thiourea solid to melamine solid is 2:3 (or equal to 1:1.5), the corresponding powder form of the precursor can be prepared by grinding 2 g of thiourea solid using an agate pestle, followed by mixing with 3 g of melamine solid and further grinding for, for example, 20 minutes to obtain a homogeneous mixture. This homogeneous mixture can then be transferred to a container, such as a ceramic crucible, particularly a ceramic crucible that has been polished as described herein and covered with a first substrate for heat treatment. As used herein, the term "mixture" generally refers to a material composed of two or more different chemical substances that are not chemically bonded (i.e., two or more different chemical substances are physically mixed to form a material). It should be understood that the term "mixture" should be distinguished from the term "composite" used in the art, which generally involves creating a new (different) material by combining two or more constituent materials.

[0192] Similarly, after covering the container with the first substrate in step b), optionally or additionally, a support such as a ceramic plate as described herein may be placed on top of the first substrate to minimize leakage of the gaseous thiourea source and the gaseous melamine source.

[0193] In these embodiments, step c) may include annealing the solid-phase thiourea precursor and the solid-phase melamine to produce a vapor-phase thiourea source and a vapor-phase melamine source; and allowing the vapor-phase thiourea source and the vapor-phase melamine source to deposit and react to form a g-CN layer on the first substrate. Specifically, the annealing step may be performed in a muffle furnace at about 500° C. to about 550° C. for about 3 hours, with a heating rate of about 3° C. / minute.

[0194] According to the present invention, it also relates to a g-CN membrane formed by the method as described herein. In some embodiments in which a post-annealing step is performed, the g-CN membrane may comprise a C:N:O atomic ratio of about 2.3-4.9:1.9-4.8:1, such as 2.31-4.89:1.92-4.79:1, 2.29:-4.91:1.89-4.81:1, etc. In particular, the g-CN membrane may have a C:N atomic ratio of about 1.25 to about 1.45. The g-CN membranes prepared in these embodiments may comprise a plurality of nanoporous and mesoporous structures. In particular, these g-CN membranes may also comprise a plurality of nanosheets on the membrane surface, which are considered to be the constituent units of the nanoporous and mesoporous structures. In some embodiments, the length of the plurality of nanosheets may be from about 7 μm to about 13 μm, such as 7.4 μm, 7.9 μm, 8.1 μm, 9.3 μm, 10.4 μm, 11.9 μm, 12.5 μm, 13.4 μm, etc. In addition, the thickness of the g-CN film prepared in these embodiments may be from about 2.5 μm to about 4.3 μm, such as 2.5 μm, 2.6 μm, 3 μm, 3.3 μm, 3.6 μm, 4 μm, 4.2 μm, 4.3 μm, etc.

[0195] In some other embodiments in which the g-CN film can be formed from respective powder forms of a solid-phase thiourea precursor and a solid-phase melamine precursor, the g-CN film can comprise a C:N:O atomic ratio of about 3.9-7.5:4-7.9:1, such as 3.91:4.01:1, 6.88:7.49:1, 6.68:7.03:1, 7.48:7.92:1, and the like. The g-CN film prepared in these embodiments can comprise a plurality of nanosheets at least partially stacked on top of one another. Optionally or additionally, the g-CN film prepared in these embodiments can comprise a plurality of nanoflowers on the surface of the film. In particular, the plurality of nanosheets can comprise a nanomesh structure or a sheet-like structure. The nanomesh structure is believed to promote the photocatalytic activity of the g-CN film because these structures provide readily accessible channels for reactant adsorption and / or provide active sites for the reaction. Additionally, the g-CN films prepared in these embodiments may have a thickness of about 2.81 μm to about 3.21 μm, such as 2.81 μm, 2.85 μm, 2.88 μm, 2.90 μm, 2.93 μm, 3.01 μm, 3.10 μm, 3.21 μm, and the like.

[0196] It is believed that the g-CN film prepared by the method described herein is capable of performing photoelectrochemical (PEC) reactions. In particular, it is believed that at a thickness in the micrometer range, the absorption of sunlight by the g-CN film may be promoted to generate more charge carriers, which may have better carrier mobility and PEC performance. For example, in some embodiments, the g-CN film can have a transient photocurrent density of about 51.4 μA cm at 1.23 V vs. RHE in 0.1 M KOH electrolyte with 10% triethanolamine when subjected to intermittent simulated AM 1.5G illumination. -2 to about 404 μA cm -2 In some other embodiments, the g-CN film can have a transient photocurrent density of about 298.6 μA cm at 1.23 V vs. RHE in 0.1 M Na2SO4 electrolyte with 10% triethanolamine under intermittent simulated AM 1.5G illumination. -2 to about 790.5 μA cm -2 In some further embodiments, the g-CN membrane may be capable of producing hydrogen via photoelectrocatalytic water splitting. For example, in embodiments where the g-CN membrane may have a C:N:O atomic ratio of 7.48:7.92:1, the membrane may have a solar-to-hydrogen energy conversion efficiency of approximately 0.75% when operated in a 0.1 M Na2SO4 electrolyte with 10% triethanolamine. Details of the PEC performance of the g-CN membranes of the present invention will be discussed later in this disclosure.

[0197] Hereinafter, the present invention is described in more detail by way of Examples, but the present invention is not limited thereto.

[0198] Example

[0199] Material

[0200] All chemicals were purchased from Sigma-Aldrich Co. LLC, including thiourea (CH4N2S), melamine (C3H6N6), sodium sulfate (Na2SO4), potassium hydroxide (KOH), triethanolamine (TEOA), acetone (C3H6O), ethanol (C2H6O), isopropanol (C3H8O) and FTO glass (50 × 50 × 2.2 mm 3 ,8Ωsq -1 ), all of which were used without further treatment. Before use, the FTO glass was cleaned by ultrasonic treatment with deionized water, acetone, ethanol, and isopropanol for 20 minutes, respectively. The edges of the ceramic crucibles were polished in advance with abrasive papers (400, 800, and 1200 Cw, 230 mm x 280 mm) electrochemically coated with silicon carbide to facilitate the deposition of chemical vapor during g-CN synthesis.

[0201] Characterization

[0202] In a Bruker D2 Phaser with a Lynxeye detector in Cu K α Radiation (λ = ) was used to study the structural analysis. In all samples, the XRD patterns were measured as 2θ varied from 10° to 80° with a step size of 0.02°. Fourier transform infrared spectroscopy (FTIR) was obtained using a Perkin Elmer spectrum II spectrometer in reflection mode. The surface morphology of the films was magnified using a JEOL scanning electron microscope (SEM). The morphology was further characterized using a JEOL JEM 2100F field emission scanning transmission electron microscope (TEM). The XRD patterns were obtained using a Thermo Fisher Scientific USA Model ESCALAB 250XI spectrometer in Al K α X-ray photoelectron spectroscopy (XPS) data and surface film composition were recorded under irradiation. The C 1s peak was calibrated at 284.5 eV. The XPS depth profile of Ar + Sputtering rate: 2.1 nm min -1 Ultraviolet-visible (UV-Vis) spectra were recorded on a PerkinElmer Lambda 1050+ UV / VIS / NIR broadband spectrometer. Computational UV-Vis spectrophotometry was used to elucidate the microstructure and energy level configuration of the fabricated g-CN films. Photoluminescence (PL) spectra were obtained from time-resolved fluorescence (FLUO) on an Edinburgh Instruments FLS980.

[0203] Photoelectrochemical performance measurements

[0204] The photoelectrochemical (PEC) performance of the fabricated g-CN film was characterized by using a three-electrode electrochemical workstation (CHI 760E, Shanghai Chenhua Limited, China) with the g-CN film as the working electrode, the Ag / AgCl electrode as the reference electrode, and the platinum foil as the counter electrode. 2 The intermittent light linear sweep voltammetry (LSV) curves were recorded under a 150 W Xe lamp light source (Newport) with an intensity of 100 nm. Before checking the PEC performance, the membrane was cut and sealed with epoxy resin so that the effective conductive area between the photoelectrode and the electrolyte was approximately 0.5 cm 2Four different electrolytes were used, including 0.1 M Na2SO4 with (pH 10.36) or without (pH 7.4) 10% triethanolamine (TEOA) (sacrificial reagent), and 0.1 M KOH with (pH 12.5) or without (pH 12.8) 10% TEOA. The reference potential vs. Ag / AgCl can be converted to the RHE scale at 25°C using the Nernst equation:

[0205] E RHE =E Ag / AgCl +0.059×pH+0.197 (1)

[0206] All based on 1cm 2 The g-CN photoanodes were all illuminated from the sample side (front illumination).

[0207] Hydrogen production from solar water splitting was measured by gas chromatography in 50 mL of 0.1 M Na2SO4 solution with 10% TEOA under the same system as the PEC measurement. An externally applied potential of approximately 0.42 V (calculated according to Equation (1)) was provided using an electrochemical workstation (CHI 760E).

[0208] The Nyquist plots were acquired under open circuit voltage conditions in the frequency range of 0.1 Hz to 100 kHz and at an AC voltage of 10 mV. The Mott-Schottky plots involve measuring the space charge layer capacitance (C) of the g-CN film when a bias voltage is applied according to the following equation: sc ):

[0209]

[0210] where ε r , ε0, e and A are the relative dielectric constant of the semiconductor, the vacuum dielectric constant, the charge of the electron and the effective surface area of the semiconductor, respectively, and N d is the free carrier density, E represents the applied potential and E fb is the flat band potential, k represents the Boltzmann constant and T is the absolute temperature. The Mott-Schottky plot was formed in the dark under sinusoidal modulation at a frequency of 3000 Hz.

[0211] The H2 gas production of the g-CN photoanode was measured in a three-electrode configuration using a gas chromatograph over a period of 2 h. In addition, the ideal gas production can be calculated based on Faraday's law of water electrolysis using the following equation (3):

[0212]

[0213] Where α is the number of electrons transferred (2 for H2), n is the actual amount of H2 (mol), and F is approximately 96485 C mol -1 is the Faraday constant, and Q is the total charge.

[0214] The solar-to-hydrogen (STH) efficiency can be calculated based on the current I passing through the system as follows:

[0215]

[0216] Where I is the current measured in the three-electrode configuration, FE H2 is the Faradaic efficiency of hydrogen production, and P 光 is the power density of the incident light (100 mA cm in this work) -2 ).

[0217] Example 1A

[0218] Preparation of g-CN membrane, g-CN-MxT0.6 (x = 1-5)

[0219] It is believed that melamine can be used as a substrate to anchor small species to grow into larger conjugated structures. In addition, it is believed that the chemical reaction between the two precursors: melamine and thiourea (TU) with different mass ratios may affect the polymerization of g-CN, which in turn affects the microstructure and PEC performance of g-CN films.

[0220] In this embodiment, the g-CN membrane g-CN-MxT0.6 (x=1-5) is prepared by Figure 1 Specifically, 18 g of TU was stirred in 20 ml of deionized water and then gradually heated to 120 °C to obtain a completely transparent hot saturated solution. A piece of glass strip was immersed in the hot saturated TU solution for 5 seconds to deposit an area of 1 × 1.5 cm 2 (equivalent to about 0.6g) of TU, and then dried at room temperature for 5 minutes. Figure 2 ) were placed obliquely in a 25 ml ceramic crucible containing 1, 2, 3, 4 or 5 g of melamine at the bottom. The mass ratio of melamine to TU was 3:0.6, 4:0.6 or 5:0.6. 2 The top of the crucible was covered with FTO glass to deposit the g-CN film. Optionally, a ceramic plate weighing about 90.5 g was placed on the FTO glass to minimize vapor loss during the subsequent calcination process. Next, the crucible was heated in a muffle furnace (Carbolite) at 3°C min in an air atmosphere with ambient pressure (i.e., not filled with any special gas such as nitrogen, argon, etc.). -1The calcination was performed from room temperature to 550°C at a heating rate of 100°C, maintained at 550°C for 3 hours, and then naturally cooled to room temperature. In particular, it is believed that the use of an air atmosphere instead of an atmosphere such as N2 or other inert gases and the use of a muffle furnace for calcination can simplify equipment requirements and reduce production costs when making / producing g-CN-MxT0.6 membranes on a large scale.

[0221] To improve film quality, the as-prepared g-CN films were post-annealed at 300°C for 30 minutes at the same heating rate and then naturally cooled to room temperature. Ultimately, a highly uniform, slightly yellowish, stable g-CN film was obtained. For comparison, a g-CN-M3T0.6 film without post-annealing (i.e., g-CN-M3T0.6′) was also prepared.

[0222] The prepared membranes were named g-CN-M1T0.6, g-CN-M2T0.6, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6, as they were derived from 1, 2, 3, 4, and 5 g of melamine and 0.6 g of TU, respectively. As a blank reference, a g-CN membrane prepared from 10 g of melamine in the absence of TU was named g-CN-M10.

[0223] Example 1B

[0224] Preparation of g-CN membrane, CNMT-xt (x = 1-3, t = 500 or 550)

[0225] In this embodiment, melamine is used as a precursor for growing (virgin) g-CN films (denoted as CNM) on FTO glass. Melamine / thiourea-g-CN (mela / thio-g-CN) (denoted as CNMT-xt, where x represents the precursor ratio and t represents the (annealing) temperature) films are produced from melamine and thiourea powders via a thermal vapor condensation (TVC) method.

[0226] Specifically, for the original g-CN, 10g of melamine was placed in a finely polished crucible and treated FTO glass was placed on it. Before the subsequent calcination process, a ceramic plate weighing about 90.5g was placed on the FTO glass. Afterwards, the crucible was calcined in a muffle furnace (Carbolite) at 550°C for 3 hours with a heating rate of 3°C / min. The resulting g-CN was named CNM. To prepare mela / thio-g-CN, three amounts of thiourea (1g, 2g or 3g) were ground using an agate pestle to obtain a fine powder. It was then mixed with 3g of melamine and continued to grind for an additional 20 minutes to obtain a homogeneous powder, which was then moved to a ceramic crucible and covered with FTO glass. Before the subsequent calcination process, a ceramic plate weighing about 90.5g was placed on the FTO glass. The prepared equipment was then calcined at two different temperatures (500°C and 550°C) for 3 hours (heating rate: 3°C / min). The corresponding membranes are designated CNMT1-500, CNMT1-550, CNMT2-500, CNMT2-550, CNMT3-500 and CNMT3-550. The weight ratio of thiourea to melamine is 1:1.5, representing 1, 1:1 representing 2, and 1:3 representing 3. Figure 3 The appearances of the prepared CNM, CNMT1-500, CNMT1-550, CNMT2-500 and CNMT2-550 are shown in FIG.

[0227] Example 2A

[0228] Structural characterization of g-CN-MxT0.6

[0229] Four membranes (g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, g-CN-M5T0.6) Figures 4A to 4D ) indicates a completely continuous and homogeneous surface property. In contrast, the pinhole-free and uniform color distribution on the g-CN-M3T0.6′ film ( Figure 4E ), indicating the importance of the post-annealing step in improving the quality of the g-CN-MxT0.6 film. The thinner g-CN-M10 does not appear as yellowish as the other films. From the SEM cross-sectional image ( 5A to 5D ), the thicknesses of g-CN-M10, g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 films were estimated to be 0.36, 4.3, 3.6, and 2.5 μm, respectively. The significant difference in film thickness of g-CN appears to be derived from the use of a single melamine precursor and a melamine-thiourea precursor.

[0230] In order to make a comprehensive comparison of film thickness, longer condensation times and higher amounts of precursors were further tried to obtain thicker g-CN films from melamine. Figures 6A to 6F As shown in Figure 2, it has been recognized that g-CN films derived from melamine cannot be prepared as thick as those derived from melamine-thiourea precursors. Among all g-CN films derived from melamine precursors, g-CN-M10 exhibits the best PEC performance ( Figure 6F This indicates that the non-covalent adsorption of g-CN-M10 on FTO is so poor that the g-CN film prepared using 10 g of melamine is much thinner than the g-CN films prepared using 3, 4, and 5 g of melamine and 0.6 g of TU.

[0231] g-CN-M10 membrane ( Figure 7A ) has a particle size of about 0.3 μm, which is much smaller than that of g-CN-M3T0.6( Figure 7B ) particle size, with 12-13 μm nanosheets well adhered to the FTO substrate. When a larger amount of melamine powder was reacted with 0.6 g TU, g-CN-M4T0.6 and g-CN-M5T0.6 ( Figure 7C and Figure 7D ) nanosheets are mostly less than 10 μm in length. It is believed that the poor polymerization of g-CN-M10 may lead to the smaller particle size. Therefore, the larger nanoparticles generate stronger non-covalent interactions (e.g., π-π stacking) between the polymer segments and between the segments and the FTO glass, preventing the deposited film from falling to the bottom of the crucible, thereby retaining a thicker film on the FTO glass. This significant difference in the morphology of g-CN films derived from melamine alone and melamine-TU precursors suggests that different reaction mechanisms may exist in these two cases.

[0232] As shown in TEM images ( Figures 8A to 8D ), the g-CN structures of the three g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 films mostly end with curled edges, which are actually part of the nanosheets observed in the SEM images. In particular, the g-CN-M3T0.6 ( Figure 8B ) has a rich surface of nanoporous and mesoporous structures, which is more than that of g-CN-M4T0.6 membrane ( Figure 8C ) and g-CN-M5T0.6( Figure 8D ) film. The nanopores are believed to be active sites that can enhance light capture and provide accessible channels for adsorption of external species to achieve chemical reactions that meet PEC capabilities. The thickest g-CN-M3T0.6 film is characterized by high-resolution TEM images ( Figure 8E) clearly shows the side view of the π-π stacking of the (002) crystal plane (i.e., the conjugated structure in g-CN), which corresponds to the fast Fourier transform map ( Figure 8F ) is given in the diffraction circles. The (002) interplanar spacing in the lattice fringes is estimated to be 0.324 nm ( Figure 8G ), which is consistent with previous studies.

[0233] For all samples, X-ray diffraction (XRD) patterns ( Figure 9A ) shows that the (002) peak is at about 27.8° and the (100) peak is at about 15.1°, which is attributed to the interlayer stacking of the conjugated structure and the repetition of the in-plane structural unit of the g-CN unit. The (100) and (002) peak intensities in the g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 samples are all comparable, demonstrating the presence of a large amount of g-CN polymer containing a large conjugated structure attached to the FTO glass. In contrast, the (100) and (002) peak intensities of g-CN-M10 are lower than those of the other three samples, indicating that there are fewer g-CN crystalline fragments in g-CN-M10. This may be attributed to the poor crystallinity and thinner film thickness of g-CN-M10 derived from the melamine precursor.

[0234] The chemical functional groups in the four g-CN samples were characterized by FTIR spectroscopy ( Figure 9B ) to characterize. At 805cm -1 The representative peak at 805 cm is attributed to the typical out-of-plane breathing vibration of the s-triazine unit, thus confirming the complete skeleton structure of g-CN. It is worth noting that the peaks at 805 cm of g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 -1 peak( Figure 9B , inset) is blue-shifted relative to g-CN-M10, which is attributed to the more s-triazine units in the three samples, resulting in stronger vibrational resonance. 1200-1620 cm -1 The prominent band at 2900–3300 cm -1 The peaks at are associated with OH stretching, NH and CH vibrations in the uncondensed –NH2 groups.

[0235] In the X-ray photoelectron spectroscopy (XPS) C1s ( Figure 10 , top), the two main peaks at 288.5 and 284.6 eV are attributed to the sp conjugated N=CN bonds and C=C bonds (e.g., N is missing in C=NC) in the g-CN backbone. 2 contamination, and the secondary peak at 285.3 eV is attributed to sp 3In contrast, the intensities of the N=CN peaks in g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 are significantly higher than those of their corresponding contaminant peaks, demonstrating the presence of a large number of conjugated structures in g-CN derived from melamine and TU precursors. In particular, the C=C and sp 3 The peak intensity of -C contamination is the weakest relative to its N=CN peak, indicating that its g-CN conjugated structure is the most ideal. The C=C bond in g-CN-M10 is stronger than its C=NC peak, indicating that the g-CN polymerization derived from melamine is the worst.

[0236] Figure 11 The chemical compositions of C and N in the four samples are shown, showing that all g-CN films are rich in C. g-CN-M3T0.6, derived from the least amount of melamine (only 3 g), has the highest N concentration, indicating the fewest internal N defects, which is consistent with the C1s peak in XPS (the weakest C=C peak). It is believed that the chemical reaction in the case of g-CN-M3T0.6 is most conducive to achieving a large-area conjugated structure, which can lead to optimal charge carrier mobility in PEC cells.

[0237] In the N1s spectrum ( Figure 10 , bottom), the three XPS peaks at 398.2, 399.4, and 400.7 eV are attributed to the sp 2 -N, tertiary -N in N–C3 groups, and amino groups (e.g., C–NH–C / C–NH2). The presence of C=N–C and N–C3 bonds generally suggests the presence of triazine and / or tri-s-triazine units. Again, among the four samples, g-CN-M3T0.6 has the largest difference between its C=N–C and N-C3 peaks, indicating that the largest area conjugated structure can be formed in g-CN-M3T0.6, with the highest degree of polymerization and closer to the graphite-like CN structure ( Figure 12 ).

[0238] Example 2B

[0239] Structural characterization of CNMT-xt

[0240] SEM images ( 13A to 13E ) shows the morphology of CNM and the fabricated CNMT film, which is uniform and pinhole-free. A large number of nanoflowers are uniformly implanted on the surface of the CNMT film ( 13B to 13E ), while CNM membranes are composed of continuous nanoparticles ( Figure 13A ). Compared with the nanoparticles of CNM membrane, the size of nanoflowers is significantly increased. It is believed that the large surface area can produce increased photocatalytic activity and light absorption. For CNMT2-550 ( Figure 13E ), it appears that there are fewer nanoflowers than in other CNMT films, and it is believed that this may affect the PEC performance.

[0241] Cross-sectional SEM images ( 13B to 13E , inset) shows the uniform thickness of the film attached to the FTO glass surface. The film thicknesses of CNM and CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 are 0.26 μm, 2.93 μm, 2.88 μm, 3.21 μm, and 2.81 μm, respectively. Obviously, the CNMT film is thicker, about 11 times that of the CNM film. On the one hand, it can be seen that the thickness values of the mela / thio-g-CN films synthesized from each source are almost the same, which means that the different weight ratios of the precursors and the temperature do not affect the film thickness. Element distribution diagram ( Figure 14 ) confirmed that the CNMT1-500 film was uniformly distributed with C, N and O elements.

[0242] TEM analysis was performed to further characterize the morphology and confirmed the formation of two g-CN films from different precursors. Figures 15A to 15E ) morphology ends with curled edges, which is consistent with the SEM results. Specifically, CNMT1-500 ( Figure 15A ) nanosheets are composed of a wrinkled nanomesh structure. This structure is believed to be beneficial for photocatalytic activity because it provides easily accessible channels for reactant adsorption and more surface active sites for the reaction, thereby providing enhanced photocurrent density (see discussion below). CNM membrane ( Figure 15B ) consists of denser and thicker layers, while CNMT1-550, CNMT2-500 and CNMT2-550 ( Figures 15C to 15E ) showed a thinner sheet structure. The obvious morphological difference is believed to be due to the different molecular structures of melamine and thiourea. It is also believed that the heteroatoms in melamine and the additional sulfur in thiourea may stimulate the carbon nitride condensation process and the formation of nanostructures. The appearance of the newly formed mela / thio-g-CN CNMT1-500 film is shown in the HRTEM image ( Figure 16A ), thus demonstrating the sheet stacking structure and confirming a very complete interfacial contact between the two g-CN layers. Clear lattice fringes with a d-spacing of 0.338 nm corresponding to the (002) plane were observed ( Figure 16B ).

[0243] The diffraction patterns of CNM, CNMT1-500, CNMT1-550, CNMT2-500 and CNMT2-550 films were characterized by XRD ( Figure 17 ) to characterize. The characteristic peaks of all mela / thio-g-CN samples showed a clear peak at 28.1°, which was attributed to the (002) crystal plane of g-CN. However, the peaks of CNM samples could not be observed due to the weak signal, which may be due to the SEM results ( Figure 13A , inset). In particular, the CNMT film has a stronger signal, indicating a higher crystallinity and a larger degree of polymerization from the reaction of melamine and thiourea. At the same time, CNMT1-500 shows the highest intensity peak among the candidates, indicating the highest degree of crystallinity. Another reflection peak at 14.9° is attributed to the in-plane heptazine ring repeating unit corresponding to the (100) crystal plane. Interestingly, the intensity of the (100) peak appears to be stronger than that of the (002) peak, indicating that the CNMT film may be composed of a higher percentage of tri-s-triazine units.

[0244] FTIR analysis was performed on all samples to reveal structural changes ( Figure 18 ). Specifically, 810cm -1 The peaks near the heptazine ring system are derived from the out-of-plane bending vibration. -1 to 1800cm -1 The second absorption band observed at 3000 cm is attributed to the typical CN(-C)-C vibration in heterocyclic or bridged C-NH-C units. -1 and 3500cm -1 The broad bands between the two precursors are attributed to stretching vibrations of amino (-NH) and / or hydroxyl (-OH) groups. All IR bands of the CNMs are in good agreement with those of the CNMT sample, indicating that the chemical structure of the mela / thio-g-CN candidates is nearly identical to that of the CNMs. Therefore, from XRD and FTIR studies, it can be concluded that the new g-CN films (CNMT films) prepared from both precursors retain the internal crystalline structure of g-CN from melamine.

[0245] X-ray photoelectron spectroscopy (XPS) was also studied to understand the elemental chemical composition and oxidation state of the fabricated films. The original synthesized films were mainly composed of carbon, nitrogen and oxygen elements ( Figure 19A To clearly understand the nature of the bonding in CNM and CNMT1-500 films, high-resolution C1s and N1s spectroscopy was performed. The C1s and N1s spectra were deconvoluted into various lines corresponding to different binding energies ( Figure 19B and Figure 19C). The C 1s spectrum shows two peaks at 288.1eV and 284.6eV, which are attributed to the N–C=N and C–C coordination of graphitic carbon. Compared with CNM, the signal at 284.6eV for the (CC) unit of the adventitious carbon species in the spectrum of CNMT1-500 has been reduced, indicating that a smaller portion of CC bonds with lower carbon content is observed, possibly in the form of C=C bonds, in addition to the presence of g-CN domains. In addition, the NC=N peak in CNMT1-500 shifts to higher binding energies from 288.1eV to 288.4eV, indicating a slight change in the local arrangement of C atoms. In the N1s spectrum ( Figure 19C ), three prominent peaks can be convoluted at 398.4eV, 399.3eV and 400.6eV. These peaks can be attributed to sp 2 The CN=C bond, tertiary nitrogen N-C3 group and amino group (C-NH-C and C-NH2), while the peak at 403.9eV is attributed to the charge effect, redistribution of additional electrons and carbon configuration transfer. For CNM and CNMT1-500, in the S2p spectrum ( Figure 19D ), no major sulfur (S) peak could be detected in the thiourea. This indicates that sulfur was completely released from thiourea during the heat treatment. Figure 20 The composition of each element in all synthesized samples is presented in Figure 2. A small amount of adventitious oxygen was also detected due to oxygen functional groups. The nitrogen peak is stronger in the CNMT1-500 spectrum than in the CNM ( Figure 19A and Figure 20 ). Therefore, based on the overall XPS results, it can be concluded that nitrogen-rich g-CN was formed in this work without doping with any elements.

[0246] Example 3A

[0247] Photophysical properties of g-CN-MxT0.6

[0248] In UV–vis absorption spectroscopy ( Figure 21A ), the absorption edges of g-CN-M10, g-CN-M5T0.6, g-CN-M4T0.6, and g-CN-M3T0.6 were estimated to be 419.84, 433.61, 445.65, and 455.89 nm, respectively, indicating that g-CN-M3T0.6 has the best photon capture ability. When the UV-Vis absorption spectra of g-CN-M3T0.6 and g-CN-M3T0.6' were further compared, it was found that the two samples responded to almost the same absorption edge, but g-CNM3T0.6 (after post-annealing) was able to absorb more light ( Figure 21B ). As shown in the Tauc diagram ( Figure 21A, inset), the polymer size-dependent band gaps of g-CN-M10, g-CN-M5T0.6, g-CN-M4T0.6, and g-CN-M3T0.6 films were estimated to be 2.70, 2.68, 2.61, and 2.54 eV, respectively.

[0249] Combined with valence band XPS spectra ( Figure 22A ), and the energy band arrangement diagrams of the four g-CN samples were derived ( Figure 22B ), which shows the relative oxidation (O2 / H2O) and reduction (H + / H2) potential, the valence band top and conduction band bottom of g-CN-M3T0.6 are most conducive to water decomposition. It is believed that adding TU as a precursor to melamine promotes the polymerization degree of g-CN, and the g-CN will have a larger area of conjugated structure to form a favorable electronic structure, that is, the size-dependent performance gap ( Figure 23 The 1:5 ratio of TU to melamine in g-CN-M3T0.6 is believed to promote a more efficient polymerization reaction under TVC conditions.

[0250] Example 3B

[0251] Photophysical properties of CNMT-xt

[0252] UV-vis spectra of the developed CNMT-xt photocatalyst sample ( Figure 24A ) revealed a long absorption tail extending over 700 nm. The light edge red-shifted with increasing optical density for all samples. In addition, CNMT1-500 showed the highest absorption intensity. The red-shift in light absorption may be caused by the enhanced condensation of the precursor, reflecting that the thermally induced nanosheet structure is thinner than that of the CNM sample, as is evident in the TEM images ( Figures 15A to 15E When the temperature is further increased from 500°C to 550°C, a blue shift in the absorption edge is observed, which is likely due to the strong quantum confinement effect. Therefore, the mela / thio-g-CN film prepared at 500°C has a higher ability to absorb visible light, resulting in better photoactivity than other samples.

[0253] In addition, the steady-state photoluminescence (PL) emission spectra of the samples were characterized to determine the migration and separation of photoexcited charge carriers. All candidates showed a luminescence peak centered around 450 nm ( Figure 24B), which is attributed to the radiative recombination rate of electrons and holes. Obviously, compared with the CNM film, the CNMT film undergoes obvious PL intensity quenching, which indicates that the intrinsic radiative recombination rate of photogenerated electron-hole pairs in these CNMT candidates has been significantly suppressed. In contrast, the higher fluorescence intensity represents the faster recombination rate of photocarriers. In addition, the PL emission peaks of CNMT1-550 and CNMT2-550 are weaker than those of the other two CNMT films because the production of cyanuric acid may change their chemical structure. Therefore, structural defects may promote electron transfer and delocalization, thereby reducing the intensity of the emission spectrum. Overall, the CNMT structures newly synthesized from two nitrogen-rich precursors by the TVC method exhibit much lower photocarrier recombination rates than CNMs.

[0254] Example 4A

[0255] Photoelectrochemical properties of g-CN-MxT0.6

[0256] The photocurrent, which is considered one of the main indicators of photoelectrochemical (PEC) properties, has been studied because it is believed to be related to the flow of charge carriers generated by the photocatalyst upon light irradiation. The transient photocurrent density ( Figure 25A ) reached 404.4 μA cm at 1.23 V vs. RHE. -2 , which is the photocurrent density in the g-CN-M10 film (25.2 μA cm -2 ) is 16 times that of g-CN-M4T0.6, and the photocurrent density (130.6 μA cm -2 ) and is 3 times higher than that in g-CN-M5T0.6 (94.8 μA cm -2 ). In addition, it has been found that the transient photocurrent density of g-CN-M3T0.6 is higher than that of g-CN-M3T0.6' ( Figure 25B ), the g-CN-M3T0.6′ was not post-annealed during the fabrication process.

[0257] When 0.1M Na2SO4-10% TEOA electrolyte is used instead, the transient photocurrent density of g-CN-M3T0.6 ( Figure 25C ) can reach 319.8μA cm -2 , which is still much higher than the other three samples. The specific transient photocurrent density of the sample can be seen in Figure 25D Intermittent light linear sweep voltammetry (LSV) curve ( Figure 25E and Figure 25F) also showed that among the samples, g-CN-M3T0.6 exhibited the best photoresponse properties in either 0.1M KOH-10% TEOA or 0.1M Na2SO4-10% TEOA. It is believed that the alkaline 0.1M KOH solution may promote higher photocurrent densities than the neutral 0.1M Na2SO4 solution. Overall, g-CN films derived from melamine and TU precursors exhibit significantly enhanced PEC performance compared to g-CN derived from melamine alone, particularly for g-CN-M3T0.6.

[0258] To investigate the effect of the precursor ratio between melamine and TU on the PEC performance of g-CN, the transient photocurrent density as discussed above was further compared in two g-CN samples (named g-CN-M1T0.6 and g-CN-M2T0.6) derived from 1 g and 2 g of melamine powder and the same amount of 0.6 g of TU, which were prepared under the same synthetic procedure as any of g-CN-M3T0.6, g-CN-M4T0.6, or g-CN-M5T0.6.

[0259] The transient photocurrent density of g-CN-M1T0.6 in 0.1 M KOH-10% TEOA and 0.1 M Na2SO4-10% TEOA reached as high as 68.9 and 51.4 μA cm -2 ( Figure 25D and Figure 25G ), while the transient photocurrent density of g-CN-M2T0.6 reached as high as 112.3 and 82.5 μA cm in the two electrolyte solutions. -2 ( Figure 25D and Figure 25H ). Trends of the photocurrent density of the six samples relative to the mass ratio of TU to melamine ( Figure 26A ) showed that the precursor ratio can clearly regulate the PEC performance of the prepared g-CN membrane, where a TU:melamine ratio of 1:5 (i.e., g-CN-M3T0.6) achieved the best PEC performance. The enhanced PEC performance may also be related to the membrane thickness ( Figure 26B ), because thicker films promote stronger light absorption and cause charge carriers to spread over a larger area.

[0260] The photostability of g-CN-M3T0.6 samples was investigated by monitoring the photocurrent density changes under 3600 s illumination using two working electrolytes with or without sacrificial reagents ( Figure 27A Notably, the photocurrent density of g-CN-M3T0.6 in 0.1M KOH without TEOA stabilizes at 60–70 μA cm within 1 h after a rapid initial decay. -2range, while in 0.1 M KOH-10% TEOA, it experiences a slight smooth decay within the first 500 s and remains above 150 μA cm -2 , which is much higher than the plateau value without TEOA. This means that the added TEOA is crucial as an effective hole scavenger to eliminate the kinetic barrier of the oxidation half-reaction to achieve the maximum photocurrent density and hinder the charge carrier recombination, which has been assumed that holes are released from the g-CN surface and transferred to the oxidized TEOA + Compared with the photostability of g-CN-M10 ( Figure 27A , inset), g-CN-M3T0.6 has an absolute advantage in PEC performance due to its superior microstructure derived from the optimal ratio of TU to melamine precursor. In addition, when comparing the photostability of g-CN-M3T0.6 and g-CN-M3T0.6', it was found that the photocurrent density of g-CN-M3T0.6 remained above 150 μA cm after the first 500 seconds. -2 , while g-CN-M3T0.6' continued to decay under the same experimental conditions ( Figure 27B ). Therefore, this indicates that the post-annealing step is crucial for the film quality / performance of g-CN-MxT0.6.

[0261] The charge transfer properties between the electrolyte and the photoelectrode containing g-CN-MxT0.6 under light or dark conditions were characterized by electrochemical impedance spectroscopy (EIS, also known as Nyquist plot). Generally speaking, a smaller semicircle diameter at lower resistance in the Nyquist plot indicates higher conductivity, faster charge carrier migration, and lower charge recombination rate.

[0262] It has been found that the arc diameter of g-CN-M10 ( 28A to 28D ) is always greater than that of g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 films, whether in 0.1M KOH-10% TEOA or in 0.1M Na2SO4-10% TEOA, under light or dark conditions, indicating that the charge separation and electron-hole pair transfer in g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 films are enhanced. At the same time, the g-CN-M3T0.6 sample achieves the minimum arc diameter of the semicircle most of the time, which supports the maximum suppression of the charge transfer resistance between the electrolyte and the electrode. Mott-Schottky (MS) diagram ( Figure 28E and Figure 28F) indicates that all samples are n-type semiconductors. The reduced flat band potentials of g-CN-M3T0.6, g-CN-M4T0.6, and g-CN-M5T0.6 reflect the greatly improved charge carrier density compared to the g-CN-M10 film. Therefore, it is believed that the g-CN film prepared based on the melamine-TU precursor can improve the bulk conductivity. Combined with the MS curve of g-CN-M10 ( Figure 28E and Figure 28F , inset), it can be seen that there is a left shift on the potential axis from g-CN-M10 to g-CN-M3T0.6, which promotes the separation of charge carriers and reduces the charge recombination rate, thereby enhancing the conductivity, which is consistent with the EIS Nyquist Figure 1 To.

[0263] Example 4B

[0264] Photoelectrochemical properties of CNMT-xt

[0265] The PEC performance of CNMT-xt membranes was achieved as described herein in a three-electrode configuration in two electrolytes with or without the addition of 10% TEOA in 0.1 M Na2SO4 solution. Linear sweep voltammetry (LSV) measurements of all membranes were performed at 1.23 V vs. RHE in 0.1 M Na2SO4-10% TEOA solution under intermittent simulated AM 1.5G illumination ( Figure 29A ). Figures 29B to 29D Figure 3 shows the transient photocurrent density of all samples under intermittent light illumination in 0.1 M Na2SO4 solution with or without 10% TEOA.

[0266] To estimate the hole extraction efficiency, 10% TEOA was added to 0.1 M Na2SO4 electrolyte as an effective hole scavenger. The photocurrent density of CNMT1-500, CNMT1-550, CNMT2-500, and CNMT2-550 increased to 790.5 μA cm -2 、447.5μA cm -2 、611.2μA cm -2 、393.9μA cm -2 , while CNM in this system reached 28.2 μA cm -2 ( Figures 29B to 29D As a result, the photocurrent of CNMT1-500 is enhanced by 28 times compared with that of CNM samples, which is much better than that of the reported graphitic carbon nitride photoanode produced by co-production of the two precursors ( Figure 30 ).

[0267] The enhanced photocurrent density is believed to be attributable to 1) TEOA oxidation; and / or 2) film quality and / or film thickness. For TEOA oxidation, most holes are believed to be extracted from layers that tend to lower the kinetic barrier of the oxidation half-reaction, thereby allowing for the maximum photocurrent density. Therefore, TEOA is considered a favorable choice for improving the photocurrent density in this work ( Figure 31 ). For film quality and / or film thickness, it is considered to be related to the precursor weight dependency and / or deposition temperature.

[0268] The effect of precursor weight dependence was first investigated by using CNMT3-500 and CNMT3-550, which were synthesized from 1 g of thiourea and 3 g of melamine at an annealing temperature of 500°C or 550°C according to the procedure disclosed herein. The PEC properties can be tuned by varying the amount of thiourea added to the melamine precursor ( Figure 29D and Figure 32A For comparison, we attempted to use thiourea powder alone to prepare a g-CN film on FTO glass. A porous and non-uniform g-CN film was formed ( Figure 32B ). Combined with SEM results ( 13A to 13E ), indicating that thiourea can actually help melamine to achieve uniform and thick (at the micrometer level) g-CN films on FTO glass.

[0269] As for the calcination temperature, all CNMT samples prepared at 500 °C showed better PEC performance than those prepared at 550 °C ( Figure 31 and Figures 33A to 33C ). It is believed that during heat treatment, the layer structure may peel off moderately and the conjugated polytriazine ring structure has a certain degree of destructiveness ( 13B to 13E Therefore, the surface structure of g-CN may be altered, such as the increase of surface defects (which can be confirmed by UV results ( Figure 24A and Figure 24B ), which may promote the recombination of photogenerated charge carriers as charge recombination centers, thereby reducing the photocatalytic activity.

[0270] Further increasing the treatment temperature to 600 °C changes the color of the g-CN film to brown ( Figure 34A and Figure 34B ), which does not seem to work. Therefore, it is believed that 550 °C will be the upper limit of the calcination temperature for making uniform g-CN films in this work.

[0271] Overall, it is believed that with an appropriate ratio of thiourea and melamine precursors and annealing temperature, a uniform g-CN film with micrometer-scale thickness will be produced, which can promote light penetration within the electrode (i.e., increase the amount of charge carriers) and reduce electron-hole recombination within the active layer before their reaction due to its sufficient electron diffusion length (i.e., better charge carrier mobility), thereby improving the PEC performance.

[0272] To elucidate the enhancement of photocurrent, the charge transport properties between the photoelectrode and the electrolyte were characterized by electrochemical impedance spectroscopy (EIS) measurements of the fabricated g-CN film. The Randles-Ershler (RE) circuit model ( Figure 35 , inset) is used as an equivalent circuit to simulate the PEC process. The resistor R s Represents the contact resistance between the electrolyte and the external circuit. R ct The resistance is related to the charge transfer process at the interface between the photoanode and the electrolyte. The resulting arc diameter of the semicircle in the Nyquist plot represents the corresponding R at the interface between the electrolyte and the electrode. ct The reduced arc diameter of the semicircle in the Nyquist plot under dark and light conditions demonstrates reduced charge transfer and transport resistance from the membrane to the electrolyte. CNMT1-500 exhibits the shortest arc diameter under both conditions; thus, the charge recombination rate in this candidate is suppressed, which correlates with the best PEC performance among the candidates.

[0273] The Mott-Schottky (MS) relationship is used to reveal the n-type characteristics of the g-CN film in terms of flat band potential and band alignment, and to explain the enhanced free carrier density. Figure 36 ) indicates that g-CN is an n-type semiconductor. The steepest slope of the CNM film indicates the lowest free carrier density. Generally, bulk conductivity depends on carrier density. After constructing the CNMT film on the FTO substrate, the introduced free carriers are guaranteed to improve the bulk conductivity. In addition, the intercept of the slope is shifted towards the negative direction, indicating significant band bending in the space charge region and the value of the flat band potential ( Figure 37 ). The increased band bending promotes the separation of charge carriers. Therefore, the fast recombination rate of charge carriers is hindered, which is consistent with the Nyquist Figure 1 To.

[0274] Based on the above findings, it is believed that the chemical structure, composition, morphology, and film thickness of the synthesized mela / thio-g-CN samples, obtained by introducing different weights of thiourea into melamine, are significantly different from those of g-CN derived from melamine alone. Due to the intimate contact between the film and the substrate, the contact resistance for charge transport and charge collection in PEC applications is reduced, revealing sufficient electron diffusion length to minimize electron-hole recombination. Consequently, efficient light harvesting is achieved, as discussed in the optical properties, which also indicate good PEC performance, including UV and PL spectroscopy.

[0275] The photostability of CNMT films was investigated. In particular, CNMT1-500 was selected as an exemplary embodiment and compared with CNM films in terms of actual long-term use in PEC applications.

[0276] It was found that CNMT1-500 has good stability after the initial decay, maintaining the photocurrent density at about 300 μA cm -2 ( Figure 38 In contrast, CNM membranes ( Figure 38 , inset) decayed continuously during the 1-hour irradiation treatment time, which may be due to enhanced charge separation and hole extraction. It is believed that the improved stability may be attributed to the film quality. For example, after long-term testing of all PEC properties, the film did not appear to have any damage ( Figure 39A and Figure 39B ), revealing a strong cohesive force between the film and the FTO glass. It is believed that such excellent contact between the film and the substrate will lead to lower resistance, thereby achieving higher photocurrent and better stability. In addition, the CNMT1-500 film was characterized after all photoelectrochemical evaluations were performed by SEM ( Figure 39C and Figure 39D Due to the excellent stability during PEC operation, the resulting nanoflowers were nearly identical. Furthermore, the CNMT1-500 sample overcomes common limitations regarding g-CN film thickness. The film is sufficiently thick, reaching the micrometer level as measured above, to effectively absorb light in the visible range; thus, the FTO film is well-suited to collecting charge carriers, which improves electronic conductivity. Therefore, this work offers the opportunity to implement PEC applications through skillful selection of starting precursors and processing temperatures.

[0277] Example 5A

[0278] Density functional theory (DFT) calculations of g-CN-MxT0.6

[0279] In order to reveal the fundamental reason why the g-CN polymerization of g-CN-MxT0.6 can be improved by using melamine and TU precursors, a series of DFT calculations were performed and the specific calculation details are as follows:

[0280] The gas-phase microscopic reaction of thiourea and melamine to g-CN was calculated at the CAM-B3LYP / 6-31G* level, including geometry optimization of reactants (R), intermediates (IM), products (P), and transition states (TS), and subsequent frequency calculations to confirm their nature as local minima or first-order saddle points. Intrinsic reaction coordinate (IRC) calculations were also performed using CAM-B3LYP / 6-31G* to demonstrate that the TS is related to the expected R / IM and IM / P and to calculate the activation energy (E) for each step of the reaction. a ) and reaction heat (ΔH). All calculations were performed using the Gaussian 09 program package.

[0281] Based on calculations, it is believed that small reactive carbodiimide (HN=C=NH) species, mainly generated from the TU precursor, are anchored one by one on melamine, leading to the formation of heptazine rings or even larger conjugated structures. The DFT calculated energy profile ( Figures 40A to 40C ) showed that the chemical reaction of melamine with HN=C=NH alternately undergoes polycondensation, cycloaddition / addition, and hydrogen transfer reactions to produce larger g-CN conjugated clusters and ammonia. This reaction pathway is feasible due to the activation barrier ranging from 6 to 60 kcal / mol, which can be easily overcome under TVC synthesis conditions (550°C for 3 hours). In particular, the heat of reaction during the growth of melamine into heptazine rings is mostly exothermic, which thermodynamically increases the driving force of the chemical reaction. Notably, the heptazine ring clusters are not the final product but can continue to anchor more HN=C=NH to form larger-area conjugated structures resembling a true graphite phase, which is likely present in g-CN-M3T0.6 samples with an appropriate ratio of TU to melamine.

[0282] To visualize how melamine grows into g-CN, Figures 41A to 41C The DFT-calculated geometries of the 16 transition states (TS) and 16 intermediates (IM) involved in the 16-step chemical reaction of melamine with HN=C=NH are shown. The -CNH2 group is primarily an active site that anchors the -N=C=NH group while releasing NH3 gas (i.e., the polycondensation reaction in steps 1, 7, and 10, or the self-condensation reaction in steps 5 and 16). When the -CNH group encounters HN=C=NH, an addition reaction occurs, in which the -N=C=NH group is attached to C to form an sp 3-C and -H are connected to -NH to form -NH2, such as in steps 4 and 14. Once HN=C=NH is connected to the substrate, another HN=C=NH may also combine with it to form a new six-membered ring, i.e., a cycloaddition reaction occurs, such as in steps 2, 8, and 11. When two H atoms are connected to two adjacent N atoms, one H can migrate from one N to the other, i.e., an H transfer reaction occurs, thereby forming a new -NH2 group to serve as an active site for the subsequent polycondensation reaction, such as in steps 3, 6, 9, 12, 13, and 15. In general, the cycloaddition reaction of HN=C=NH combined with the melamine substrate proceeds relatively easily due to a small energy barrier (about 20 kcal / mol), while the polycondensation reaction proceeds relatively difficult due to a large energy barrier (49-51 kcal / mol) and endothermic reaction heat. Therefore, the polycondensation reaction should be the rate-determining step. In contrast, the occurrence of steps 1–6 is more favorable than that of steps 7–16, indicating that the heptazine ring cluster may be the most likely unit structure for g-CN crystallization.

[0283] It is worth noting that there is a big difference between the chemical reactions starting from melamine and TU in terms of the production of HN=C=NH species. The process from melamine to HN=C=NH occurs in two steps ( Figure 42A and Figure 42B ): 1) Melamine decomposes into three cyanamides at an extremely high energy barrier of 124.9 kcal / mol and an unfavorable endothermic reaction heat of +73.4 kcal / mol; 2) A hydrogen atom migrates from one side of the cyanamide to the other to form HN=C=NH, accompanied by a still higher energy barrier of 91.6 kcal / mol and a reaction heat of +3.1 kcal / mol. In contrast, the following two steps of the chemical reaction proceed relatively easily due to the much lower energy barriers of melamine (40.8 and 39.5 kcal / mol): 1) TU decomposes into HN=C=S and NH3, and then 2) the HN=C=S dimer undergoes polycondensation to form HN=C=NH and CS2( Figure 42C and Figure 42D It was found that HN=C=NH species can be derived more easily from TU than from melamine precursors, leading to a significant improvement in the polymerization of g-CN once TU as a precursor was added to melamine in the experiments.

[0284] In addition to forming conjugated units, the N-C3 coordination bonds in the g-CN membrane can also be formed by hydrogen transfer and condensation between three g-CN conjugated clusters ( Figure 43A Considering melamine as the representative unit of g-CN, the whole process will go through four steps from monomer to dimer and trimer, accompanied by the release of two ammonia molecules. The barrier is in the range of 34-53 kcal / mol and the heat of reaction is almost endothermic (Figure 43B The chemical reaction between larger g-CN units may further increase the difficulty of N-C3 coordination bond formation due to the more serious steric effect. This indicates that N-C3 coordination bonds are not as easy to form as unit growth ( Figure 40A and Figure 40B This may be the reason why g-CN-M3T0.6 has the largest difference between its C=N–C and N-C3 XPS peaks among the four samples, because the growth of its conjugated units driven by more HN=C=NH is easier and faster than the connection between conjugated units.

[0285] Experimental microstructural characterization and the growth mechanism revealed by the calculations disclosed in this article consistently demonstrate that the unique polymer structure of g-CN-M3T0.6 derived from a TU to melamine mass ratio of 1:5 has the highest degree of polymerization, which means that g-CN-M3T0.6 has the best optical properties and PEC performance among the four samples, while g-CN-M10 is the worst.

[0286] Example 5B

[0287] Density functional theory (DFT) calculations of CNMT-xt

[0288] The electronic properties of the g-CN structure of CNMT-xt were studied using the DFT method. Specifically, pure g-CN is composed of seven heptazine rings. The simulations were performed using the Gaussian 09 program package. The geometry was optimized and the band gap was calculated at the B3LYP / 6-31G(d,p) level. Based on the supercell structure of g-CN, cluster models were constructed containing one (g-CN(H1)), three (g-CN(H3)), six (g-CN(H6)), and ten (g-CN(H 10 ))Heptazine ring to consider the effect of size on photochemical properties ( Figures 44A to 44D ). Basic energy gap (E g ) is calculated as the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) ( Figure 45 ). E g The value decreases from 5.00 eV to 3.50 eV, while the size increases from one to ten heptazine rings, showing the same trend as the reported DFT study results.

[0289] In addition, the system size also affects the UV–vis absorption spectrum, the electron-hole pair binding energy, and the spatial distribution of photoinduced electrons and holes. To calculate the UV–vis absorption spectrum, the first sixty excited states at the ωB97XD / 6-31G(d,p) level were considered. The spatial distribution of photoinduced holes and electrons was analyzed using the Multiwfn code and visualized using VMD software. For g-CN(H1) at 201.81 nm, for g-CN(H3) at 239.95 nm, for g-CN(H6) at 240.00 nm, and for g-CN(H 10 ) UV–vis absorption spectrum at 256.94 nm ( Figures 46A to 46D ) of the calculated maximum oscillator intensity red-shifts with the increase of g-CN size. This is consistent with the experimental UV-vis spectrum ( Figure 24A ) are similar to the trends observed in

[0290] After UV-vis absorption, the electronic state of the g-CN system will quickly decay to the first excited state according to Kasha's rule. Therefore, the spatial distribution of photoinduced holes and electrons in the first excited state is analyzed ( Figures 44A to 44D ). The photoinduced electrons are mainly located at the C atoms, but the holes are mainly located at the CN bonds where the nitrogen (N) atoms are least present. Due to the high symmetry of g-CN (H1) ( Figure 44A ), the centers of the photoinduced holes and electrons almost overlap in the first excited state (S1), thus exhibiting minimal spatial separation. In addition, the increased size of g-CN leads to a longer distance (D) between the photoinduced holes and electrons. h-e ) and lower electron-hole pair binding energy ( Figure 45 Finally, in g-CN(H 10 ) was obtained D h-e The results demonstrate that the larger size of g-CN promotes effective isolation of photoinduced holes and electrons, leading to reduced recombination between them. This allows for the presence of more persistent photoinduced holes and electrons to react with water, thereby achieving better photoelectrochemical performance.

[0291] Example 6

[0292] Photoelectrocatalytic H2 production via CNMT-xt

[0293] To investigate the water splitting activity of CNMT-xt membranes for H2 production, the same system as the photoelectrochemical measurements was used at 100 mW / cm 2 Under light, a three-electrode system is used to move about 1cm 2The g-CN film (CNMT1-500) was used as the working electrode, and Pt and Ag / AgCl electrodes were attached as the counter electrode and reference electrode, respectively ( Figure 47A After 2 hours of illumination, a total of 4.34 μmol of H2 gas was generated from the CNMT1-500 membrane in 50 mL of 0.1 M Na2SO4-10% TEOA electrolyte solution (pH = 10.36), which is 16 times that of the CNM membrane (i.e., about 0.27 μmol) ( Figure 47B In other words, the more time is consumed, the more hydrogen is released from the g-CN photoanode ( Figure 47C Then, for CNMT1-500, the Faradaic efficiency (FE) (defined as the ratio of actual evolved gas divided by the ideal evolved gas calculated using current density) was calculated to be about 77.3%, and the solar-to-hydrogen (STH) conversion efficiency was determined to be 0.75%, while the CNM membrane achieved a negligibly small value ( Figure 47D ). In addition, it should be noted that the g-CN membrane in this work generally outperforms other reported g-CN-based photocatalysts in terms of complete water splitting, except when combined with foreign elements such as CDot and metals ( Figure 48 and Figure 49 ).

[0294] The present invention has been presented by way of example only, and various other modifications and / or alterations to the described embodiments may be made by those skilled in the art without departing from the scope of the invention as specified in the appended claims.

Claims

1. A method for forming a graphite-phase carbon nitride film by condensing hot vapor, the method comprising the steps of: a) providing a solid phase thiourea precursor and a solid phase melamine precursor in a container; b) covering the container with a first substrate; as well as c) thermally generating a gaseous thiourea source and a gaseous melamine source from the solid-phase thiourea precursor and the solid-phase melamine precursor in an air environment, thereby forming a graphite-phase carbon nitride layer on the first substrate.

2. The method of claim 1, wherein the solid phase thiourea precursor and the solid phase melamine precursor are physically separated by a supporting substrate.

3. The method of claim 2, wherein the support base is positioned at an angle relative to a side of the container. The method of claim 3 , wherein the supporting substrate comprises a solid-phase thiourea precursor layer.

5. The method of claim 1, further comprising: The step of forming the solid-phase thiourea precursor layer.

6. The method of claim 5, further comprising the steps of: heating thiourea solid in deionized water to form a hot saturated thiourea solution; forming a saturated thiourea layer on the supporting substrate by at least partially immersing the supporting substrate in the saturated thiourea solution; and The saturated thiourea layer is dried to obtain the solid-phase thiourea precursor.

7. The method of claim 6, wherein the step of drying the saturated thiourea solution layer is performed under ambient conditions.

8. The method of claim 6, wherein the thiourea solid in deionized water is heated to a temperature of 60°C to 150°C.

9. The method of claim 6, wherein the support substrate is at least partially immersed in the saturated thiourea solution for 2 to 15 seconds.

10. The method of claim 6, wherein the thickness of the solid-phase thiourea precursor layer is 1.5 mm.

11. The method according to claim 6, wherein the weight of the solid thiourea layer is 0.6 g.

12. The method of claim 1, wherein the weight of the solid phase melamine precursor is 1 g to 5 g.

13. The method of claim 1, wherein step c) comprises the following steps: annealing the solid-phase thiourea precursor and the solid-phase melamine precursor to produce the vapor-phase thiourea source and the vapor-phase melamine source comprising an activated carbodiimide species; as well as The vapor-phase thiourea source and the vapor-phase melamine source are allowed to deposit and react to form the graphite-phase carbon nitride layer on the first substrate.

14. The method of claim 13, wherein the annealing step is performed in a muffle furnace at 550°C for 3 hours with a heating rate of 3°C / min.

15. The method according to claim 1, further comprising step d): post-annealing the graphite carbon nitride layer formed in step c).

16. The method of claim 13, wherein step d) is performed in a muffle furnace at 300°C for 30 minutes with a heating rate of 3°C / minute. The method of claim 1 , wherein the first substrate comprises FTO glass.

18. The method of claim 1, wherein the support substrate comprises a glass strip.

19. The method of claim 1, wherein the container has been polished.

20. The method according to claim 19, further comprising the step of continuously polishing the edge of the container with an abrasive of 400 Cw to 1200 Cw.

21. The method of claim 1, wherein the method further comprises the step of placing a load on top of the first substrate to minimize leakage of the vapor-phase thiourea source and the vapor-phase melamine source.

22. The method of claim 1, wherein each of the solid phase thiourea precursor and the solid phase melamine precursor is in powder form.

23. The method of claim 22, further comprising the steps of: preparing a homogeneous mixture of the solid-phase thiourea precursor and the solid-phase melamine precursor by grinding thiourea solid and melamine solid; as well as The homogenous mixture is transferred to the container for heat treatment.

24. The method of claim 23, wherein the weight ratio of the thiourea solids to the melamine solids is 1 to 3:

3.

25. The method of claim 23, wherein step c) comprises the steps of: annealing the solid-phase thiourea precursor and the solid-phase melamine precursor to generate the vapor-phase thiourea source and the vapor-phase melamine source; as well as The vapor-phase thiourea source and the vapor-phase melamine source are allowed to deposit and react to form the graphite-phase carbon nitride layer on the first substrate.

26. The method of claim 25, wherein the annealing step is performed in a muffle furnace at 500°C to 550°C for 3 hours with a heating rate of 3°C / min.

27. A graphite-phase carbon nitride film formed by the method of claim 15, wherein the graphite-phase carbon nitride film comprises a C:N:O atomic ratio of 2.3 to 4.9:1.9 to 4.8:

1.

28. The graphitic carbon nitride film of claim 27, wherein the graphitic carbon nitride film comprises a C:N atomic ratio of 1.25 to 1.

45.

29. The graphitic carbon nitride film of claim 27, wherein the graphitic carbon nitride film comprises a plurality of nanoporous and mesoporous structures.

30. The graphitic carbon nitride film of claim 27, wherein the graphitic carbon nitride film comprises a plurality of nanosheets on a surface of the film. 31 . The graphitic carbon nitride film of claim 30 , wherein the length of the plurality of nanosheets is 7 μm to 13 μm.

32. The graphite-phase carbon nitride film of claim 27, wherein the graphite-phase carbon nitride film has a thickness of 2.5 μm to 4.3 μm.

33. The graphitic carbon nitride film of claim 27, wherein the graphitic carbon nitride film has a transient photocurrent density of 51.4 μA cm at 1.23 V vs. RHE in a 0.1 M KOH electrolyte with 10% triethanolamine when subjected to intermittent simulated AM 1.5G illumination. -2 to 404 μA cm -2 .

34. A graphitic carbon nitride film formed by the method of claim 22, wherein the graphitic carbon nitride film comprises a C:N:O atomic ratio of 3.9 to 7.5:4 to 7.9:

1.

35. The graphitic carbon nitride film of claim 34, wherein the graphitic carbon nitride film comprises a plurality of nanosheets at least partially stacked on each other.

36. The graphitic carbon nitride film of claim 35, wherein the plurality of nanosheets comprises a nanomesh structure or a sheet structure.

37. The graphitic carbon nitride film of claim 34, wherein the graphitic carbon nitride film comprises a plurality of nanoflowers on a surface of the film.

38. The graphite-phase carbon nitride film of claim 34, wherein the graphite-phase carbon nitride film has a thickness of 2.81 μm to 3.21 μm.

39. The graphitic carbon nitride film of claim 34, wherein the graphitic carbon nitride film has a transient photocurrent density of 298.6 μA cm at 1.23 V vs. RHE in a 0.1 M Na2SO4 electrolyte with 10% triethanolamine when subjected to intermittent simulated AM 1.5G light. -2 to 790.5 μA cm -2 .

40. The graphitic carbon nitride film of claim 34, wherein the graphitic carbon nitride film has a solar-to-hydrogen energy conversion efficiency of 0.75% when operated in a 0.1 M Na2SO4 electrolyte with 10% triethanolamine when the film has a C:N:O atomic ratio of 7.48:7.92:1.

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