A Fe2N / g-C3N4 heterojunction material and its preparation method and use

Abstract

The present invention relates to an Fe2N / g-C3N4 heterojunction material, a preparation method thereof and uses thereof; by introducing a new heterojunction structure, the charge transfer rate and photocatalytic efficiency are significantly improved. A two-step calcination method capable of precisely constructing an electron transfer channel is invented. During the first calcination process, oxygen (O) replaces the pyridine nitrogen (N) sites on g-C3N4 and is successfully introduced into the g-C3N4 structure. During the second calcination, a large number of oxygen (O) sites are exposed in a hot steam atmosphere, forming Fe-O bonds with nano-iron powder. Ammonia is generated by the deamination of g-C3N4, forming Fe2N with nano-iron powder and in-situ generating an Fe2N / g-C3N4 heterojunction, which is named Fe-OCN. The newly formed Fe-O bonds serve as atomic-level interfacial electron channels, accelerating the rapid transfer of photo-generated electrons to Fe2N. The high conductivity and charge storage capacity of Fe2N greatly improve the charge transfer efficiency and photocatalytic hydrogen production activity. The modified Fe-OCN has the highest photocatalytic activity, with a hydrogen production rate of not less than 5 mmol g ‑1 h ‑1 , and at the same time its quantum efficiency at a wavelength of 405 nm is not less than 19%.

Description

Technical Field

[0001] The present invention relates to a catalytic material, and in particular to a heterojunction material based on g-C3N4 and a preparation method and application thereof. Background Art

[0002] As a clean energy, hydrogen energy has attracted much attention due to its high energy density and the fact that it only produces water without pollutants during use. At present, hydrogen (H2) production technologies are mainly concentrated in steam methane reforming, water electrolysis and biological methods, but these technologies have disadvantages such as high energy consumption, high cost and low efficiency. Photocatalytic hydrogen production, as an emerging technology, uses sunlight as energy to decompose water into H2, which has the advantages of simple operation, low cost and environmental friendliness. However, the efficiency of photocatalytic hydrogen production is limited by the performance of photocatalysts, and the development of new and efficient photocatalyst materials has become a technical difficulty in this field. At present, researchers are committed to developing photocatalysts with high light absorption capacity, excellent charge separation efficiency and good stability. Among them, semiconductor materials have become a research hotspot in the field of photocatalytic hydrogen production. Carbon nitride (g-C3N4) is a stable, economical, environmentally friendly, visible light responsive organic semiconductor photocatalyst, which has attracted the interest of scientists due to its high photocatalytic activity.

[0003] However, g-C3N4 also has defects such as fewer active sites, high recombination rate of photogenerated electron-hole pairs, and limited light absorption in the visible light range. Improving the separation efficiency of photogenerated electron-hole pairs is crucial for photocatalytic water splitting. Therefore, extensive modification studies have been carried out, including heterostructure construction, electronic structure modification, metal and non-metal doping, etc. Among these strategies, constructing heterojunctions has been proven to be an effective method for spatial (directional) separation of photogenerated electrons and holes. However, the interface mismatch in traditional heterojunction structures leads to unstable interactions between the two semiconductors. In addition, poor electron transfer efficiency also limits the effective photocatalytic activity of heterojunctions.

[0004] Inspired by the above factors, forming chemical bonds at the heterojunction interface can effectively solve the above problems. However, the precise construction of chemically bonded interfacial heterojunction catalysts and interfacial charge behaviors to improve the efficiency of photocatalytic hydrogen production remains unclear. Summary of the invention

[0005] In order to solve the problems existing in the above-mentioned technologies, the present invention provides a chemically bonded interfacial heterojunction catalyst for improving the efficiency of photocatalytic hydrogen production, and a preparation method and application thereof.

[0006] The present invention provides a Fe2N / g-C3N4 heterojunction material, which comprises Fe2N, g-C3N4 and a Fe-O bond bridge.

[0007] The oxygen -O in the air replaces the pyridine nitrogen -N part of the g-C3N4, forming oxygen-O-mixed g-C3N4, namely OCN.

[0008] Fe2N is formed by ammonia produced by deamination of g-C3N4 and nano-iron powder, and Fe2N and OCN are connected by Fe-O bonds to form a heterostructure.

[0009] The preparation method of the Fe2N / g-C3N4 heterojunction material provided by the present invention comprises the following steps:

[0010] In the first step of calcination, oxygen-O replaces the pyridine nitrogen-N part of g-C3N4 during the calcination process.

[0011] It was successfully introduced into the g-C3N4 structure to produce OCN;

[0012] In the second step of calcination, the oxygen-O sites in OCN are exposed in large quantities under the hot steam atmosphere, forming Fe-O bonds with nano-iron powders. At the same time, g-C3N4 deaminates to produce ammonia, forms Fe2N with nano-iron powders, and generates Fe2N / g-C3N4 heterojunction in situ, named Fe-OCN.

[0013] Preferably, the method comprises the following steps: mixing OCN and nano iron powder and placing them into a square porcelain boat I; then taking anhydrous sodium dihydrogen phosphate and placing them into a square porcelain boat II; placing the square porcelain boat I in the downwind direction and the square porcelain boat II in the upwind direction, and calcining them in a tubular furnace; before calcining, passing N2 to maintain an inert environment in the tubular furnace; and obtaining a sample.

[0014] Preferably, 0.5 g of OCN and nano iron powder accounting for 5% of the weight of OCN are ground and mixed in a mortar, and then placed in a square porcelain boat I; 3 g of anhydrous sodium dihydrogen phosphate is placed in a square porcelain boat II. Square porcelain boat I is placed in the downwind direction, and square porcelain boat II is placed in the upwind direction, and calcined in a tube furnace. Before calcination, nitrogen is passed for 30 minutes to maintain an inert environment in the tube furnace, and nitrogen is continuously passed at a flow rate of 100 mL / min. At 5 ° C min -1 The calcination rate was increased to 600 °C for 1 h and then cooled naturally. The obtained sample did not need to be ground and was named Fe-OCN.

[0015] Preferably, the amount of nano iron powder used accounts for 2 to 10% of the weight of OCN, the flow rate of nitrogen is 50 to 200 mL / min; the calcination rate ranges from 2 to 10 °C min -1 , calcination temperature is 450 to 650°C, and time is 0.5 to 3h.

[0016] The Fe2N / g-C3N4 heterojunction material of the present invention is used for photocatalytic hydrogen production.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The interface mismatch and lack of precise electron transfer channels in traditional heterojunction structures limit the photocatalytic efficiency. This application study significantly improves the charge transfer rate and photocatalytic efficiency by introducing a new heterojunction structure. Specifically, a two-step calcination method that can accurately construct an electron transfer channel was invented. During the first calcination process, oxygen (O) replaced some of the pyridine nitrogen (N) sites on g-C3N4 and was successfully introduced into the g-C3N4 structure. During the second calcination, a large number of oxygen (O) sites were exposed in a hot steam atmosphere, forming Fe-O bonds with nano-iron powders. At the same time, g-C3N4 deaminated to produce ammonia, forming Fe2N with nano-iron powders and in-situ generating Fe2N / g-C3N4 heterojunctions, which were named Fe-OCN. The newly formed Fe-O bonds act as atomic-level interface electron channels, accelerating the rapid transfer of photogenerated electrons to Fe2N. The high conductivity and charge storage capacity of Fe2N greatly improve the charge transfer efficiency and photocatalytic hydrogen production activity. The modified Fe-OCN has the highest photocatalytic activity, and the hydrogen production rate under visible light is not less than 5 mmol g -1 h -1 , and its quantum efficiency at a wavelength of 405nm is not less than 19%.

[0019] We directly utilize the abundant nitrogen in g-C3N4 to generate Fe2N with the provided iron, saving time and cost, while forming Fe-O bond electron channels, thereby controlling the precise establishment of direct electron transfer channels. g-C3N4 is effectively connected to Fe2N via Fe-O bonds, and the high-quality interfacial Fe-O bonds act as atomic-level "bridges", greatly improving the charge transfer rate, thereby improving the electron utilization rate and the photocatalytic performance of g-C3N4. This work provides a constructive reference for the design and synthesis of organic-inorganic heterojunctions with chemically bonded interfaces, the establishment of fast electron transfer channels, and the realization of targeted electron transfer. This approach also provides new insights for exploring new low-cost materials as effective photocatalysts and developing more efficient photocatalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the synthesis steps of the sample of Example 1 of the present invention;

[0021] Figure 2 TEM image of the sample of Example 1 of the present invention;

[0022] Figure 3 The XRD pattern of the sample of Example 1 of the present invention;

[0023] Figure 4 This is the XPS spectrum of the sample of Example 1 of the present invention;

[0024] Figure 5 The photoelectric response of the sample of Example 1 of the present invention;

[0025] Figure 6 This is a photocatalytic hydrogen production performance diagram in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] Example 1

[0027] like Figure 1 As shown, the synthesis process of in-situ construction of Fe2N / g-C3N4 heterojunction and control includes the following steps:

[0028] Synthesis Example 1: Synthesis of OCN:

[0029] 0.01 mol of melamine (MA) and 0.01 mol of thiocyanate (TA) were ground evenly and placed in a crucible and calcined in a muffle furnace. -1 The temperature was raised to 550°C at a rate of 1.50 °C and maintained at this temperature for 4 h. The sample was then cooled naturally and ground into fine powder to obtain a sample named OCN.

[0030] Synthesis Example 2: Synthesis of Fe-OCN:

[0031] 0.5g OCN and 5% nano iron powder of OCN weight were ground and mixed in a mortar, and then placed in a square porcelain boat I. 3g anhydrous sodium dihydrogen phosphate was placed in a square porcelain boat II. Square porcelain boat I was placed in the downwind direction, and square porcelain boat II was placed in the upwind direction, and calcined in a tube furnace. Before calcination, N2 was passed for 30 minutes to maintain an inert environment in the tube furnace, and nitrogen was continuously passed at a flow rate of 50-200mL / min. -1 The calcination rate is increased to 600℃ for 1h, and then cooled naturally. The obtained sample does not need to be ground and is named Fe-OCN. The square porcelain boat refers to an existing chemical container suitable for calcination.

[0032] Synthesis Example 3: Synthesis of Fe-CN:

[0033] In the steps of Synthesis Example 1, TA was not added during the synthesis of OCN, and the steps of Synthesis Example 2 were continued. The obtained material was named Fe-CN.

[0034] Synthesis Example 4: Synthesis of FeOCN:

[0035] In the step of Synthesis Example 2, the material obtained by not adding anhydrous sodium dihydrogen phosphate during the synthesis of Fe-OCN is named FeOCN.

[0036] Test example:

[0037] The properties of OCN, Fe-OCN, Fe-CN and FeOCN prepared in Example 1 of the present invention were tested below.

[0038] 1. Analysis of the synthesis of OCN, Fe-OCN, Fe-CN and FeOCN materials prepared in Example 1

[0039] Figure 1 The material synthesis routes of i.) Fe-CN; ii.) FeOCN and iii.) Fe-OCN are summarized. Figure 1 As shown in Figure 1, when the precursor is only MA and calcined at high temperature in a muffle furnace, the MA molecules undergo deamination and cyclization reactions to generate g-C3N4 (named CN). In the second step of the synthesis, Fe is added to CN and NaH2PO4 is placed upwind of it and calcined together in a tubular furnace. NaH2PO4 decomposes under heat to produce water vapor, which forms hot steam under high-temperature calcination conditions, causing the structure of CN to be strongly destroyed during the secondary calcination and deaminated and condensed again. The ammonia generated in this process reacts with Fe at high temperature to generate Fe2N. However, under these conditions, CN and Fe2N cannot form an effective electron transmission channel due to physical contact (called Fe-CN).

[0040] When the precursor is mixed and calcined by MA and TA in the first step, TA is pyrolyzed at high temperature to produce gas to form a porous structure. A small amount of sulfur-S element enters the skeleton of g-C3N4 and replaces the pyridine nitrogen-N site on g-C3N4. As the temperature continues to rise, the unstable sulfur-S is detached, and the oxygen-O in the air is easily introduced into the vacant site after the sulfur-S is detached, forming an oxygen-O doped g-C3N4 structure OCN. If NaH2PO4 is not added during the second step of calcination, there is no hot steam in the calcination environment, and only the second high temperature causes a low degree of deamination condensation of OCN. A small amount of ammonia is generated in this process to react with Fe to generate a small amount of Fe2N. And hot steam will destroy the OCN structure and cause it to be peeled off to expose the internal oxygen-O site, but only high temperature conditions can cause the OCN structure to be less damaged, and the internal oxygen-O site cannot be exposed, so Fe-O bonds cannot be generated. In this case, OCN and Fe2N also only exist physically adsorbed, such as Figure 1 ii. FeOCN shown in FIG.

[0041] If NaH2PO4 is added during the second step of calcination, there will be a hot steam environment during the reaction, causing the OCN structure to be strongly stripped and destroyed. In this case, the deamination condensation process of OCN will be more intense, thereby producing more ammonia to react with Fe at high temperature to form Fe2N. At the same time, the oxygen-O sites inside OCN will be fully exposed to combine with Fe to form Fe-O bonds, thereby forming a Fe2N / OCN heterojunction structure Fe-OCN (such as Figure 1 ⅲ as shown).

[0042] 2. Characterization of OCN, Fe-OCN, Fe-CN and FeOCN materials prepared in Example 1

[0043] 2.1. Material characterization: Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used to characterize the materials. TEM is used to analyze the morphology and microstructure of the samples; XRD is used to analyze the chemical structure of the samples; and XPS spectroscopy is used to further analyze and confirm the component elements and element states.

[0044] 2.2 Results and Discussion

[0045] (1) Transmission electron microscope (TEM)

[0046] Depend on Figure 2 Transmission electron microscope TEM images show that OCN has become a few-layer carbon nitride ( Figure 2 a), the high temperature steam environment of the second calcination makes the layered structure of the original carbon nitride more exfoliated, and the structure of Fe-OCN is more stretched and thinner, forming a curled sheet structure ( Figure 2 b), which further proves that the carbon nitride structure is strongly destroyed during the second calcination step, thus generating enough ammonia to react with Fe to form Fe2N and expose oxygen-O sites to form Fe-O bonds to connect Fe2N.

[0047] (2) XRD was used to analyze the physical structure of the sample, with Cu target Kα radiation and a scanning speed of 5°min -1 , scanning angle 10°-80°, scanning step 0.02°.

[0048] Depend on Figure 3 a As can be seen, all samples have two typical characteristic peaks of g-C3N4, which well maintain the basic crystal structure of g-C3N4, where the (100) crystal plane is the repeating unit of the carbon nitride heptazine ring plane, and the (002) crystal plane represents the interlayer stacking of carbon nitride. Figure 3 b is Figure 3Figure a is an enlargement of 35-70°. For Fe-OCN, the three peaks at 40.9°, 42.9° and 56.7° represent Fe2N (PDF#97-002-0390), which are derived from the (002), (011) and (012) crystal planes of the ICDD powder diffraction database, indicating that Fe was successfully introduced into OCN and exists in the form of compound Fe2N. In contrast, for Fe-CN, the precursor CN was synthesized without the addition of TA, so the g-C3N4 structure in CN is relatively complete and has a high amino content. During the second calcination, CN is stripped by hot steam, forming a large amount of ammonia that reacts with Fe to generate more Fe2N, so the Fe2N diffraction peak is more obvious in the XRD results of Fe-CN. For FeOCN, there is only thermal stripping during the second calcination, resulting in a slow deamination rate of carbon nitride, so the Fe2N diffraction peak is weak.

[0049] (3) X-ray photoelectron spectroscopy XPS

[0050] Depend on Figure 4 The XPS spectrum of the sample of Example 1 of the present invention, wherein the N1s and O1s spectra are as follows Figure 4 As shown in a and 4b, the peaks at 398.7 eV, 400.2 eV and 401.2 eV of the N 1s spectrum correspond to the CN=C bond, N-(C)3 and C-NH x and π excitation peaks. After Fe doping, C-NH x The peak weakened, indicating that during the second calcination, OCN was stripped by hot steam from multiple layers to thin layers, and formed Fe2N after intense deammoniation.

[0051] The 532.2 and 533.2 eV peaks of OCN in the O 1s spectrum are CO and OH of surface water, respectively. For Fe-OCN material, the 531.3 eV peak is CO-Fe. Compared with OCN, the CO bond environment changes after the introduction of Fe, Fe coordinates with O to form Fe-O bonds, and the CO bond gains electrons and transfers to a lower binding energy.

[0052] Based on the results of XRD and XPS, we demonstrated the successful construction of the Fe-O electron transfer channel and the successful synthesis of the Fe2N reactive active center.

[0053] 3. Photoelectric response test of OCN, Fe-OCN, Fe-CN and FeOCN materials prepared in Example 1

[0054] 3.1 Photoluminescence spectrum PL test:

[0055] We used photoluminescence (PL) to study the recombination / separation of photogenerated carriers in Fe-OCN, FeOCN, Fe-CN, and OCN at an excitation wavelength of 310 nm to confirm the effect of Fe-O bond formation on the improved photocatalytic performance of the Fe2N / g-C3N4 heterojunction system. After the formation of Fe-O bonds between OCN and Fe2N, the luminescence intensity of Fe-OCN was weaker than that of FeOCN, Fe-CN, and OCN, showing a significantly lower recombination probability of photogenerated electrons and holes ( Figure 5 a), indicating that the formation of Fe-O bonds can effectively accelerate electron migration.

[0056] 3.2 Fluorescence lifetime FL test

[0057] Figure 5 b shows that the average lifetime τ of Fe-OCN is 498.4 μs, which is much higher than 173.1 μs of FeOCN, 304.5 μs of Fe-CN and 243.4 μs of OCN. Therefore, the presence of Fe-O bonds can effectively accelerate the transfer of electrons from OCN to Fe2N and inhibit the radiative charge recombination in OCN and Fe2N, thus exhibiting higher photocatalytic activity.

[0058] 3.3 Transient photocurrent response and electrochemical impedance spectroscopy (EIS) test

[0059] Transient photocurrent response ( Figure 5 c) and electrochemical impedance spectroscopy ( Figure 5 d) also shows that the Fe-O electron transfer channel can significantly enhance the separation and migration of photogenerated electrons. Compared with FeOCN (0.02μA), Fe-CN (0.05μA) and OCN (0.16μA), Fe-OCN has a higher photocurrent response (0.53μA), and Fe-OCN has the smallest arc in the EIS spectrum, indicating that it has the lowest charge transfer resistance. The formation of Fe-O bonds can effectively accelerate the transfer of electrons from OCN to Fe2N.

[0060] 4. Photocatalytic hydrogen production performance test of OCN, Fe-OCN, Fe-CN and FeOCN materials prepared in Example 1

[0061] Based on the XRD and XPS results, we have demonstrated the successful construction of the Fe-O electron transfer channel and the synthesis of the Fe2N reaction active center. The photoelectric response test further proved that the combined effect of the two led to enhanced interfacial electron migration, and Fe-OCN has excellent photocatalytic performance. In order to verify the great advantages of Fe-OCN in the photocatalytic hydrogen production process, we tested the photocatalytic hydrogen production reaction under visible light irradiation of λ≥420nm in the presence of Pt co-catalyst and triethanolamine sacrificial agent. Figure 6As shown in a, Fe-OCN with both Fe2N reactive active centers and Fe-O fast photogenerated electron transfer channels has the highest visible light photocatalytic hydrogen production activity of 5986.29 μmol g -1 h -1 , is OCN (445.30 μmol g -1 h -1 ) is 13.44 times that of FeOCN and Fe-CN materials. However, there is no Fe-O electron transfer channel in FeOCN and Fe-CN materials, and Fe2N itself cannot become a reaction active center, and the catalytic activity is much lower than that of Fe-OCN. At the same time, the apparent quantum efficiency of Fe-OCN in visible light ( Figure 6 b), the quantum efficiency at a wavelength of 405 nm is 19.09%.

Claims

1. A Fe2N / g-C3N4 heterojunction material, characterized in that: The Fe2N / g-C3N4 heterojunction material comprises Fe2N, g-C3N4 and Fe-O bond bridges. The preparation method of the Fe2N / g-C3N4 heterojunction material comprises the following steps: In the first step of calcination, melamine and thiocyanate are mixed and calcined. During the calcination process, oxygen-O in the air replaces the pyridine nitrogen-N part of the g-C3N4 and is successfully introduced into the g-C3N4 structure to obtain oxygen-O mixed g-C3N4, namely OCN. In the second step of calcination, the oxygen-O sites in OCN are exposed in large quantities under the hot steam atmosphere, forming Fe-O bonds with nano-iron powder. At the same time, the oxygen-O-doped g-C3N4 deaminates to produce ammonia, which forms Fe2N with nano-iron powder. Fe2N and OCN are connected by Fe-O bonds, and the Fe2N / g-C3N4 heterojunction is generated in situ, named Fe-OCN.

2. The Fe2N / g-C3N4 heterojunction material according to claim 1, characterized in that: The second step of calcination includes the following steps: mixing OCN with nano iron powder and placing them in a square porcelain boat I; then taking anhydrous sodium dihydrogen phosphate and placing it in a square porcelain boat II; placing the square porcelain boat I in the downwind direction and the square porcelain boat II in the upwind direction, and calcining them in a tubular furnace; passing nitrogen before calcination to maintain an inert environment in the tubular furnace; and obtaining a sample.

3. The Fe2N / g-C3N4 heterojunction material according to claim 1, characterized in that: During the second calcination process, 0.5 g of OCN and nano iron powder accounting for 5% of the weight of OCN were ground and mixed in a mortar, and then placed in a square porcelain boat I; 3 g of anhydrous sodium dihydrogen phosphate was placed in a square porcelain boat II; the square porcelain boat I was placed in the downwind direction, and the square porcelain boat II was placed in the upwind direction, and calcined in a tube furnace; N2 was passed for 30 min before calcination to maintain an inert environment in the tube furnace, and nitrogen was continuously passed at a flow rate of 50 to 200 mL / min; the temperature was set at 5 ° C min -1 The calcination rate was increased to 600 °C for 1 h and then cooled naturally. The obtained sample did not need to be ground and was named Fe-OCN.

4. The Fe2N / g-C3N4 heterojunction material according to claim 2, characterized in that: The amount of nano iron powder used is 2 to 10% of the weight of OCN, the flow rate of nitrogen is 50 to 200 mL / min; the calcination rate ranges from 2 to 10 °C min -1 , calcination temperature is 450 to 650°C, and time is 0.5 to 3h.

5. The Fe2N / g-C3N4 heterojunction material according to claim 1, characterized in that: Used as photocatalytic hydrogen production.

6. The Fe2N / g-C3N4 heterojunction material according to claim 5, characterized in that: Fe-OCN with Fe2N active center and Fe-O fast photogenerated electron transfer channel has visible light photocatalytic hydrogen production activity, and the hydrogen production capacity is not less than 5 mmol g - 1 h -1 .

7. The Fe2N / g-C3N4 heterojunction material according to claim 5, characterized in that: The apparent quantum efficiency of Fe-OCN in visible light is not less than 19% at a wavelength of 405 nm.

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