A highly crystalline modified nitrogen-rich carbon nitride material and its preparation method

Abstract

The present invention discloses a highly crystalline modified nitrogen-rich carbon nitride material and a preparation method thereof. The modified nitrogen-rich carbon nitride material is HC-C3N5 with a nanosheet structure, which is composed of interlaced stacks of highly crystalline nanosheets. The preparation method of the above material is simple, and a highly crystalline modified nitrogen-rich carbon nitride material can be obtained by an alkaline potassium salt-assisted molten salt method. The material has a large surface area and a complete lattice structure, improves the utilization of the solar spectrum, accelerates the transfer of photogenerated carriers and catalytic reactions, and has high photocatalytic hydrogen production ability and good stability in the field of photocatalytic water splitting for hydrogen production; at the same time, in the field of photochromic materials, the material can achieve faster color change efficiency and more saturated color change depth, and realize a highly reversible color change.

Description

Technical Field

[0001] The present invention relates to a modified nitrogen-rich carbon nitride material, in particular to a highly crystalline modified nitrogen-rich carbon nitride material, and also to a preparation method of the modified nitrogen-rich carbon nitride material. Background Art

[0002] Photocatalytic hydrogen production has attracted much attention as a sustainable energy conversion technology. Photocatalytic hydrogen production uses visible light or ultraviolet light to excite catalysts to decompose water into hydrogen and oxygen, providing a potential solution for clean energy. Nitrogen-rich carbon nitride (C3N5) has a wider light absorption range, lower electron transport resistance, more active sites and higher chemical stability in photocatalytic hydrogen production. These advantages make C3N5 a potential photocatalytic hydrogen production material and are expected to play an important role in the field of sustainable energy. However, further research and optimization are still needed. Currently, most research focuses on constructing C3N5 into composite materials with other semiconductor materials (such as TiO2, ZnO, etc.) or catalysts (such as precious metals) to improve the separation efficiency and catalytic activity of photogenerated carriers. However, the preparation and interface engineering of composite materials may be complicated, and there are challenges in long-term stability. Therefore, a new modification method is needed to give full play to the advantages of C3N5 in photocatalytic hydrogen production and improve its catalytic performance. Summary of the invention

[0003] Purpose of the invention: The present invention aims to provide a highly crystalline modified nitrogen-rich carbon nitride material with excellent photocatalytic and photochromic properties.

[0004] Technical solution: The highly crystalline modified nitrogen-rich carbon nitride material described in the present invention is HC-C3N5 with a nanosheet structure, which is composed of staggered layers of highly crystalline nanosheets.

[0005] Preferably, the thickness of the nanosheet is 1 to 2 nanometers.

[0006] The method for preparing the above-mentioned highly crystalline modified nitrogen-rich carbon nitride material is characterized by comprising the following steps:

[0007] (1) taking potassium chloride, potassium hydroxide and 3-amino-1,2,4-triazole as raw materials, and grinding them thoroughly to obtain a mixed powder;

[0008] (2) calcining the mixed powder, cooling to room temperature to obtain a solid, and washing with water by centrifugation to obtain a solid aqueous solution;

[0009] (3) The solid aqueous solution is centrifuged, washed, dried and ground to obtain a highly crystalline modified nitrogen-rich carbon nitride material.

[0010] Preferably, in step (1), the mass ratio of potassium chloride, potassium hydroxide and 3-amino-1, 2, 4-triazole is 8-10:0.5-1:5-6.

[0011] Preferably, in step (1), the grinding time is 10-15 min.

[0012] Preferably, in step (2), the calcination treatment is specifically heating to 550-600 °C and then holding for calcination for 2-4 h, and the heating rate is 2-3 °C / min.

[0013] Preferably, in step (2), the water washing is carried out at a temperature of 80-90 °C.

[0014] Preferably, in step (2), the centrifugation is carried out at a rotation speed of 8000-9000 r / min.

[0015] Principle of the invention: The crystallinity of the highly crystalline modified nitrogen-rich carbon nitride material of the present invention has been greatly improved. High crystallinity means that the material has a more complete lattice structure. Therefore, HC-C3N5 has better electron transport characteristics, and electrons are transported more quickly and effectively inside the material, which helps to reduce the recombination of electrons and holes, improve the utilization rate of photo-generated carriers, enhance the utilization degree of the spectrum, facilitate the absorption and utilization of light, and make the photon energy more efficiently converted into electrons and holes, thereby enhancing the efficiency of photocatalytic hydrogen production. First, the thinner nanosheets can reduce the distance for carriers to transport inside the material, thus reducing the recombination loss of carriers. This helps to improve the effective separation of carriers and increases the efficiency of photocatalytic hydrogen production. Second, the larger surface area can expose more active sites, which can provide additional active reaction centers and is beneficial to the progress of the catalytic reaction. Third, high crystallinity can reduce lattice defects and lower the recombination rate of carriers. In this way, photo-generated carriers will be more easily separated and transported inside the material, thereby increasing the efficiency of the photocatalytic reaction.

[0016] Meanwhile, when the highly crystalline modified nitrogen-rich carbon nitride material of the present invention is heat-treated with an alkaline potassium salt, a cyano group is introduced. This cyano group can be oxidized by photo-generated holes to form a highly active enamide triazine radical cation, leaving photo-generated electrons stored in the material. The electron storage process is accompanied by a color change from yellow to blue, thus realizing photochromism, overcoming the problem that the photochromism of current carbon nitride materials is mainly achieved under anoxic conditions, achieving the effect of reversible photochromism in air with breakthrough of anoxic limitation, and still showing good stability after multiple cycles of color change.

[0017] The preparation method of the modified nitrogen-rich carbon nitride material of the present invention adopts a convenient one-step calcination method. First, the molten salt method provides a rapid crystal growth rate. Since the molten salt has a low viscosity and surface tension, the reactant molecules can freely diffuse and rearrange therein, thereby accelerating the crystal growth rate and helping HC-C3N5 to release cyanide (CN-) ions. The cyanide ions will react with the carbon nitride precursor, introducing cyanide groups into the carbon nitride structure; finally, a highly crystalline modified nitrogen-rich carbon nitride material modified with cyanide groups is obtained.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: (1) The ability of photocatalytic reaction to produce hydrogen is improved, about 3 times that of the prior art; (2) It has better chemical and thermal stability, can resist corrosion and thermal decomposition during the catalytic process, and shows better persistence and stability in the catalytic reaction; (3) The highly crystalline modified nitrogen-rich carbon nitride material enables more efficient conversion of photon energy into electrons and holes, thereby enhancing the efficiency of storing electrons. In the field of photochromic materials, it realizes a faster color change efficiency and a more saturated color change depth; (4) During the photochromic process, the color change of the material is highly reversible, and the metastable charge and discharge of photoelectrons will not cause any permanent molecular or geometric structure transformation. Description of the Drawings

[0019] Figure 1 Transmission electron microscope images of HC-C3N5 in Example 1 and B-C3N5 in Comparative Example 1, where a is the transmission electron microscope (TEM) image of B-C3N5 in Comparative Example 1, b is the high-magnification transmission electron microscope (HRTEM) image of B-C3N5 in Comparative Example 1, c is the partial enlarged view of Figure b, d is the transmission electron microscope (AFM) and corresponding height image of B-C3N5 in Comparative Example 1, e is the transmission electron microscope (TEM) image of HC-C3N5 in Example 1, f is the high-magnification transmission electron microscope (TEM) image of HC-C3N5 in Example 1; Figure g is the transmission electron microscope (AFM) and corresponding height image of HC-C3N5 in Example 1;

[0020] Figure 2 X-ray diffraction (XRD) patterns of HC-C3N5 in Example 1 and B-C3N5 in Comparative Example 1; where Figure b is the partial enlarged view of Figure a;

[0021] Figure 3 X-ray photoelectron spectroscopy (XPS) spectra of HC-C3N5 in Example 1 and B-C3N5 in Comparative Example 1;

[0022] Figure 4 UV-visible diffuse reflectance spectra (UV-vis DRS) of HC-C3N5 in Example 1 and B-C3N5 in Comparative Example 1;

[0023] Figure 5 Hydrogen evolution reaction (HER) graphs for 4 hours of Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 2, and Comparative Example 3;

[0024] Figure 6 Hydrogen evolution rate graphs for 4 hours of Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 2, and Comparative Example 3;

[0025] Figure 7 Graph of the experiment for the recycling of H2 by HC-C3N5 in Example HC-C3N5.

[0026] Figure 8 Fourier near-infrared (FT-IR), X-ray diffraction analysis pattern (XRD), and UV-visible diffuse reflectance spectrum (UV-vis DRS) of HC-C3N5 in Example 1 before and after the hydrogen evolution reaction;

[0027] Figure 9 Fluorescence (PL) spectra of HC-C3N5 in Example 1 and B-C3N5 in Comparative Example 1;

[0028] Figure 10 Transient fluorescence (TRPL) spectra of HC-C3N5 in Example 1 and B-C3N5 in Comparative Example 1;

[0029] Figure 11 Transient photocurrent response curve (PL) graphs of HC-C3N5 in Example 1 and B-C3N5 in Comparative Example 1;

[0030] Figure 12 Impedance curve (Zis) graphs of HC-C3N5 in Example 1 and B-C3N5 in Comparative Example 1;

[0031] Figure 13 X-ray diffraction (XRD) patterns of HC-C3N5 in Example 1 and C3N5 in Comparative Example 1;

[0032] Figure 14 Fourier infrared test (FT-IR) patterns of HC-C3N5 in Example 1 and C3N5 in Comparative Example 1;

[0033] Figure 15 Unbiased current test after irradiation of HC-C3N5 in Example 1 and C3N5 in Comparative Example 1;

[0034] Figure 16 Unpaired electrons recorded for HC-C3N5 in Example 1 under dark / irradiation conditions;

[0035] Figure 17UV-Vis absorption spectra of Example 1 and Comparative Example 1 under different irradiation times, where Figure a is the C3N5 suspension of Comparative Example 1 and Figure b is the HC-C3N5 of Example 1;

[0036] Figure 18 A / A0 irradiation diagram of HC-C3N5 of Example 1 HC-C3N5 under light-switching conditions;

[0037] Figure 19 Cyclic experiment on the photochromic response of HC-C3N5 in Example 1;

[0038] Figure 20 Digital photos of HC-C3N5 in Example 1 at different times of color change and recovery;

[0039] Figure 21 X-ray diffraction (XRD) patterns of HC-C3N5 in Example 1 before and after photochromism;

[0040] Figure 22 Digital photos of HC-C3N5 in Example 1 restoring the original color at different temperatures;

[0041] Figure 23 Digital photos of HC-C3N5 in Example 1 showing photochromism under different gas atmospheres. Detailed implementation manners

[0042] The technical solutions of the present invention will be further described below with reference to the accompanying drawings.

[0043] Example 1

[0044] Preparation of highly crystalline modified nitrogen-rich carbon nitride material HC-C3N5:

[0045] (1) Potassium chloride, potassium hydroxide and 3-amino-1,2,4-triazole were used as raw materials and mixed in a mortar in a ratio of 9:0.5:5, and ground thoroughly for 12 min;

[0046] (2) The mixed powder was put into a muffle furnace, heated at a rate of 2.5 °C / min to 550 °C, and then held for 2 h. After the calcination was completed, it was cooled to room temperature to obtain a solid, and the solid aqueous solution was obtained by centrifugation and washing with water;

[0047] (3) The solid aqueous solution was centrifuged at 8500 r / min, washed with deionized water at 90 °C, dried, and ground to obtain the highly crystalline modified nitrogen-rich carbon nitride material HC-C3N5.

[0048] Example 2

[0049] Compared with Example 1, the proportion of potassium chloride was changed:

[0050] (1) Mix potassium chloride, potassium hydroxide, and 3-amino-1,2,4-triazole as raw materials in a mortar in a ratio of 8:0.5:5, and grind thoroughly for 12 min;

[0051] (2) Conduct a calcination treatment on the mixed powder. After the calcination is completed, cool it to room temperature to obtain a solid, and centrifuge and wash it with water to obtain a solid aqueous solution;

[0052] (3) Centrifuge, wash, dry, and grind the solid aqueous solution to obtain a highly crystalline modified nitrogen-rich carbon nitride material HC-C3N5-2.

[0053] Example 3

[0054] Compared with Example 1, change the ratio of potassium chloride:

[0055] (1) Mix potassium chloride, potassium hydroxide, and 3-amino-1,2,4-triazole as raw materials in a mortar in a ratio of 10:0.5:5 and grind thoroughly;

[0056] (2) Conduct a calcination treatment on the mixed powder. After the calcination is completed, cool it to room temperature to obtain a solid, and centrifuge and wash it with water to obtain a solid aqueous solution;

[0057] (3) Centrifuge, wash, dry, and grind the solid aqueous solution to obtain a highly crystalline modified nitrogen-rich carbon nitride material HC-C3N5-3.

[0058] Comparative Example 1

[0059] Preparation of B-C3N5:

[0060] Synthesize bulk C3N5 through a simple thermal polymerization route. Transfer 3-amino-1,2,4-triazole (6 g) to a coffee roaster, with a heating rate of 5 °C / min, heat to 500 °C, and then hold for 3 h. After cooling to room temperature, the sample is placed in a beaker with 100 mL of distilled water and sonicated for 3 h. Next, the sample is washed with deionized water and ethanol to remove the residual precursor, and finally the sample is dried to obtain B-C3N5 powder.

[0061] Comparative Example 2

[0062] Compared with Example 1, change the ratio of potassium chloride:

[0063] (1) Mix potassium hydroxide and 3-amino-1,2,4-triazole as raw materials in a mortar in a ratio of 0.5:5 and grind thoroughly;

[0064] (2) Conduct a calcination treatment on the mixed powder. After the calcination is completed, cool it to room temperature to obtain a solid, and centrifuge and wash it with water to obtain a solid aqueous solution;

[0065] (3) Centrifuge, wash, dry, and grind the solid aqueous solution to obtain a solid material.

[0066] Comparative Example 3

[0067] Compared with Example 3, the proportion of potassium chloride was changed:

[0068] (1) Potassium chloride, potassium hydroxide and 3-amino-1,2,4-triazole were used as raw materials and thoroughly ground in a mortar in a ratio of 12:0.5:5;

[0069] (2) The mixed powder was calcined. After the calcination was completed, it was cooled to room temperature to obtain a solid, and then centrifuged and washed with water to obtain an aqueous solution of the solid;

[0070] (3) The aqueous solution of the solid was centrifuged, washed, dried and ground to obtain a solid material.

[0071] As Figure 1 shown, the as-prepared 1HC-C3N5 and Comparative Example 1 B-C3N5 were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). The TEM images clearly showed that B-C3N5 was a layered stacked structure composed of a large number of two-dimensional nanosheets. From AFM, the average thickness of B-C3N5 was about 90.86 nm. It is worth noting that HC-C3N5 also existed as 2D nanosheets, but had better dispersion and thickness than B-C3N5. The average thickness of HC-C3N5 was about 1.95 nm (only about 4 C-N layers)[], which confirmed that HC-C3N5 was successfully exfoliated from the stacked bulk into few-layer nanosheets. In addition, it can be seen from HRTEM that no obvious lattice fringes were found in B-C3N5, indicating that the as-prepared B-C3N5 was amorphous. Obvious lattice fringes appeared in the HC-C3N5 nanosheets, and the lattice spacings of the corresponding surfaces (100) and (002) were 1.02 and 0.32 nm, respectively, improving the crystallinity of the sample.

[0072] As Figure 2As shown, XRD characterization was performed on Example 1 and Comparative Example 1. The powder X-ray diffraction patterns of the samples were recorded using a Prometheus XRD diffractometer with CuKα radiation, where λ is 0.1540558 nm, and data collection was carried out in the θ / 2θ scanning mode. According to X-ray diffraction (XRD) analysis, both B-C3N5 and HC-C3N5 clearly showed the presence of two main peaks indexing the extended distance between repeat units of the (100) diffraction plane of the plane and the superimposed π-π conjugated aromatic ring (002) diffraction plane. By further magnifying the local XRD pattern, it can be seen that the (100) peak of HC-C3N5 shows a 7.9° left shift compared to B-C3N5 (13.2°), and the change in the diffraction peak is due to the integration of K ions into the heptazine unit of HC-C3N5 by molten salt treatment. At the same time, compared with B-C3N5 (27.5°), the (002) peak of HC-C3N5 shifts to 28.3° to the right, indicating a tighter interlayer stacking distance, which is due to stronger π-π interactions between adjacent heptazine layers. In addition, the full width at half maximum (FWHM) of the main peak in HC-C3N5 becomes narrower, which also indicates a higher crystallinity with the extension of the repeat unit distance.

[0073] As Figure 3 shown, XPS characterization was performed on HC-C3N5 of Example 1 and B-C3N5 of Comparative Example 1. The XPS measurement spectra show that B-C3N5 and HC-C3N5 mainly contain C, N, and O elements, while K is visible as an additional element in the HC-C3N5 sample.

[0074] As Figure 4 shown, UV-visible diffuse reflectance spectroscopy (UV-vis DRS) characterization was performed on HC-C3N5 of Example 1 and B-C3N5 of Comparative Example 1 to determine the light absorption and specific bandgap energy of B-C3N5 and HC-C3N5. Compared with B-C3N5, an obvious blue shift occurred in the absorption edge of HC-C3N5, which is due to the reduction in sample thickness, resulting in a significant quantum size effect.

[0075] As Figure 5 shown, a 4-hour hydrogen evolution reaction (HER) experiment was carried out on Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 2, and Comparative Example 3. Comparative Example 1, Comparative Example 2, and the comparative example had low activity in decomposing water to produce hydrogen under light, while the photocatalysts of Example 1, Example 2, and Example 3 showed higher photocatalytic activity than all comparative examples, with the best yield in Example 1. It is shown that the synthesized highly crystalline HC-C3N5 nanosheet photocatalyst is more conducive to the separation of photoinduced carriers, thereby promoting the hydrogen evolution reaction.

[0076] As Figure 6As shown, the photocatalytic hydrogen evolution rates of Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 2 and Comparative Example 3 were measured. After the thermal polymerization of alkaline potassium salts, Example 1, Example 2 and Example 3 all showed excellent hydrogen production performance, among which the hydrogen evolution rate of the sample in Example 1 was the best, being 3 mmol h -1 g -1 , approximately 3.27 times that of Comparative Example 1 (0.92 mmol -1 g -1 ), indicating that the low-layer nanosheet structure can provide more active sites and shorten the diffusion distance of photo-generated carriers, promoting the photocatalytic hydrogen evolution activity.

[0077] As Figure 7 shown, the cyclic stability of Example 1 HC-C3N5 was measured to explore the stability of the photocatalyst. The performance degradation of Example HC-C3N5 was negligible during 5 cyclic tests, indicating its good photocatalytic stability

[0078] As Figure 8 shown, the Fourier transform near-infrared (FT-IR), X-ray diffraction analysis pattern (XRD) and UV-visible diffuse reflectance spectrum (UV-vis DRS) of the photocatalyst of Example 1 HC-C3N5 before and after the hydrogen evolution reaction were measured. The results showed that there were no changes in the FT-IR, XRD and DRS spectra before and after the reaction, further confirming its superior stability.

[0079] As Figure 9 shown, the fluorescence (PL) spectra of the photocatalyst of Example 1 HC-C3N5 and Comparative Example 1 B-C3N5 were measured to explore the influence of the increased crystallinity and reduced thickness of the photocatalyst on the photogenerated carrier dynamics. B-C3N5 showed obvious spontaneous carrier recombination, while the weakening of the emission intensity in HC-C3N5 indicated enhanced carrier separation, thus indicating a reduced recombination rate of photogenerated electrons

[0080] As Figure 10 shown, time-resolved photoluminescence (TRPL) was performed on the photocatalyst of Example 1 HC-C3N5 and Comparative Example 1 B-C3N5. By directly evaluating the decay rate of the fluorescence peak and fitting it with a biexponential kinetic function, insights into the exciton lifetime were provided. The kinetic decay curves showed that the average lifetimes (τ ave ) of B-C3N5 and HC-C3N5 were 1.73 ns and 5.27 ns respectively. The extended average lifetime of the latter indicated that HC-C3N5 had better electron transport characteristics after high crystallization, and the transport of electrons inside the material was more rapid and effective. This helped to reduce the recombination of electrons and holes, improve the utilization rate of photogenerated carriers, and thus enhance the efficiency of photocatalytic hydrogen production.

[0081] As Figure 11As shown, transient photocurrent response analysis was carried out on the photocatalyst of Example 1 HC-C3N5 and Comparative Example 1 B-C3N5. The transient photocurrent of the photocatalyst in the 200 s light switch cycle was shown. Under dark conditions, the photocurrent density of both samples was close to zero, while under xenon lamp irradiation, the transient photocurrent density of HC-C3N5 was significantly higher than that of B-C3N5, indicating a higher density of photo-generated carriers and an increase in the charge availability for current transport.

[0082] As Figure 12 shown, electrochemical impedance spectroscopy (EIS) analysis was carried out on the photocatalyst of Example 1 HC-C3N5 and Comparative Example 1 B-C3N5 to explore the stability of the photocatalyst. From the pictures, the arc radius of HC-C3N5 was smaller than that of B-C3N5, indicating that the crystallization within the HC-C3N5 material led to a reduction in internal transport resistance.

[0083] According to Figure 13 shown, X-ray diffraction (XRD) characterization was carried out on Example 1 and Comparative Example 1 using an advanced X-ray diffractometer with CuKα radiation having a wavelength of 0.1540558 nm. The powder X-ray diffraction patterns of the samples were collected through a θ / 2θ scanning mode. According to the XRD analysis results, the diffraction patterns of C3N5 and HC-C3N5 clearly showed two main peaks, corresponding to the diffraction plane of the repeating unit (100) between extended distance planes and the diffraction plane of the stacked π-π conjugated aromatic rings (002), respectively. Further magnifying the local XRD pattern, it can be seen that compared with C3N5, the (100) peak of HC-C3N5 shifted 7.9° to the left (compared with 13.2°), which was due to the change in the diffraction peak caused by the integration of K ions into the heptazine unit of HC-C3N5 during molten salt treatment. At the same time, compared with the (002) peak of C3N5 (27.5°), the (002) peak of HC-C3N5 shifted to 28.3° to the right, indicating a smaller interlayer stacking distance, which was due to the stronger π-π interaction between adjacent heptazine layers. In addition, the full width at half maximum (FWHM) of the main peak in HC-C3N5 became narrower, which also indicated a higher crystallinity with the increase in the distance of the repeating unit.

[0084] As Figure 14 shown, FT-IR characterization was carried out on Example 1 HC-C3N5 and Comparative Example 1 C3N5. Both C3N5 and HC-C3N5 showed several characteristic peaks of CN polymers, namely the out-of-plane bending vibration of the heptazine unit (C-N-C) at 808 cm -1 and the stretching vibration of aromatic CN impurities at 1700 - 1200 cm -1 . It is worth noting that at 2179 cm -1A new peak was found at [specific location] and was classified as the asymmetric stretching vibration of the cyanide group (C≡N). After molten salt treatment, the intensity of this peak increased.

[0085] As Figure 15 shown, an unbiased current test was conducted on the reversible photochromic nanosheets of Example 1 HC-C3N5 and Comparative Example 1 C3N5 after irradiation. There was no current signal in the C3N5 suspension, indicating its weak electron capture ability and fewer bound electrons. In contrast, an obvious current signal could be observed in the HC-C3N5 suspension due to its significant electron capture ability. The presence of bound electrons in the suspension generates a potential difference between the working electrode and the counter electrode. As the number of bound electrons increases, the potential difference increases, thereby enhancing the obtained photocurrent response. In addition, the photocurrent intensity in the HC-C3N5 suspension decays due to the atmospheric oxygen consumption of the captured electrons during the electron transfer process or recombination with positively charged radicals.

[0086] As Figure 16 shown, the unpaired electrons of the reversible photochromic nanosheets of Example 1 HC-C3N5 were recorded by ESR under dark / irradiation conditions. Due to the presence of unpaired electrons in the aromatic ring of the CN moiety, both samples showed obvious resonance peaks at g = 2.0049. Notably, under irradiation conditions, the HC-C3N5 suspension showed a stronger paramagnetic signal than its dark state, indicating the presence of additional unpaired electrons in the material. These electrons are considered to be photoexcited and subsequently captured in the HC-C3N5 suspension accompanied by a color change.

[0087] As Figure 17 shown, the UV-visible absorption spectra of the suspension of Comparative Example 1 C3N5 and the reversible photochromic nanosheets of Example 1 HC-C3N5 at different irradiation times were measured. The UV-visible absorption spectrum of the suspension of Comparative Example 1 C3N5 showed little color change after irradiation. The detailed absorption spectrum of the HC-C3N5 suspension gradually broadened in the range of 500 - 800 nm, and there was an obvious broadening effect on the main absorption peak in the range of 400 - 500 nm, indicating a significant color change during the photochromic process.

[0088] As Figure 18As shown, the irradiation graph of A / A0 of HC-C3N5 under the condition of the reversible photochromic nanosheet optical switch in Example 1. The lower and upper parts respectively represent the changes in the HC-C3N5 suspension at a random wavelength of 660 nm under on / off light irradiation, where A / A0 represents the relative absorption intensity obtained within 60 s of irradiation. It can be seen that when the irradiation time reaches 60 s, the A / A0 value reaches 100%, and the determination coefficient (R2) of the linear regression fitting is 0.991. During the recovery process, the reversibility of the color change also shows a strong linear relationship (R2 = 0.992).

[0089] As Figure 19 shown, a cyclic experiment on the photochromic response of the reversible photochromic nanosheet of HC-C3N5 in Example 1 was measured. As shown in the figure, even after 10 repeated cycles, the absorbance of the HC-C3N5 suspension only decreased slightly, indicating that during continuous cycling, the color transition has high reversibility.

[0090] As Figure 20 shown, digital photos of the reversible photochromic nanosheet of HC-C3N5 in Example 1 at different times during color change (upper) and recovery (lower) were recorded. As the light irradiation duration increased, HC-C3N5 changed from yellow to blue within 60 seconds, and after the light stopped, as the contact time of the photochromic film with air increased, the accumulated electrons were gradually consumed by oxygen or recombined with holes, and the color of the film gradually returned to its original color.

[0091] As Figure 21 shown, X-ray diffraction analysis spectra (XRD) of the reversible photochromic nanosheet of HC-C3N5 in Example 1 before and after color switching were measured. The results show that there is no change in the XRD spectra before and after the reaction, confirming its excellent stability.

[0092] As Figure 22 shown, digital photos of the reversible photochromic nanosheet of HC-C3N5 in Example 1 recovering to its original color at different ambient temperatures were recorded. The results show that as the temperature rises, the rate of color recovery is faster.

[0093] As Figure 23 shown, digital photos of the reversible photochromic nanosheet of HC-C3N5 in Example 1 recovering to its original color under different gas atmospheres were recorded. The results show that as the oxygen content decreases, the rate of color switching is greater and the final color is darker. It is worth noting that the materials of this project can exhibit excellent photochromic ability even in an oxygen atmosphere.

Claims

1. A highly crystalline modified nitrogen-rich carbon nitride material, characterized in that, The modified nitrogen-rich carbon nitride material is a nitrogen-rich carbon nitride material with a nanosheet structure, which is composed of highly crystalline nanosheets stacked in an interlaced manner. The highly crystalline modified nitrogen-rich carbon nitride material is prepared by the following method: (1) Take potassium chloride, potassium hydroxide and 3-amino-1,2,4-triazole as raw materials and grind them thoroughly to obtain a mixed powder; (2) calcining the mixed powder, cooling it to room temperature to obtain a solid, and washing it with water by centrifugation to obtain a solid aqueous solution; (3) The solid aqueous solution is centrifuged, washed, dried and ground to obtain a highly crystalline modified nitrogen-rich carbon nitride material.

2. The modified nitrogen-rich carbon nitride material according to claim 1, characterized in that, The thickness of the nanosheet is 1 to 2 nanometers.

3. The modified nitrogen-rich carbon nitride material according to claim 1, characterized in that, In step (1), the mass ratio of potassium chloride, potassium hydroxide and 3-amino-1,2,4-triazole is 8-10:0.5-1:5-6.

4. The modified nitrogen-rich carbon nitride material according to claim 1, characterized in that, In step (1), the grinding time is 10 to 15 min.

5. The modified nitrogen-rich carbon nitride material according to claim 1, characterized in that, In step (2), the calcination treatment is specifically to heat to 550-600 °C and then keep the temperature to calcine for 2-4 h.

6. The modified nitrogen-rich carbon nitride material according to claim 5, characterized in that, The heating rate is 2-3°C / min.

7. The modified nitrogen-rich carbon nitride material according to claim 1, wherein In step (3), the washing temperature is 80-90°C.

8. The modified nitrogen-rich carbon nitride material according to claim 1, wherein In step (3), the centrifugation speed is 8000-9000 r / min.

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