A photocatalyst with high crystallinity CN homojunction, its preparation method and application

Abstract

This invention relates to a highly crystalline CN homojunction photocatalyst, its preparation method, and its applications. The method includes the following steps: melamine and 5-amino-1H-tetrazole are used as raw materials, and g-C3N4 and g-C3N5 are prepared by high-temperature calcination, respectively. g-C3N4 and g-C3N5 are then mixed and ground with KCl, followed by high-temperature calcination to obtain highly crystalline g-C3N4 and g-C3N5. Finally, the two highly crystalline g-C3N4 and g-C3N5 are subjected to solvent calcination to obtain a highly crystalline CN homojunction photocatalyst. The preparation method provided by this invention is simple, uses inexpensive raw materials, and effectively reduces product costs. The construction of the homojunction improves the separation efficiency of photogenerated carriers, broadens the absorption range of visible light, and thus enhances the activity of water splitting for hydrogen production under visible light, showing promising application prospects.

Description

Technical Field

[0001] This invention relates to the field of photocatalyst technology, and in particular to a photocatalyst with high crystallinity CN homojunction, its preparation method, and its application. Background Technology

[0002] The energy crisis and environmental pollution are widely recognized as two major challenges facing human society in the 21st century. Over the past fifty years, hydrogen energy, as a highly green and promising fuel, has attracted significant attention from scientists due to its high energy content, environmental friendliness, and recyclability. Among various hydrogen production technologies, solar photocatalytic water splitting based on semiconductor photocatalysts is considered one of the most attractive, promising, and economical methods.

[0003] Carbon nitride, as an organic semiconductor, has attracted widespread attention in photocatalysis and solar energy conversion due to its advantages such as simple preparation, low cost, visible light response, and good chemical stability. However, as an amorphous material, the large number of carrier recombination centers on its surface severely restricts its practical applications. In recent years, highly crystalline carbon nitride has been successfully synthesized and proven to have excellent photogenerated carrier separation capabilities. During ongoing research, it has been discovered that in addition to the common C3N4 structure, carbon nitride materials also have a C3N5 structure, both of which are graphite-like phases. Currently, many researchers have prepared "homogeneous junction" photocatalysts with excellent photogenerated carrier separation efficiency. This is mainly because the two components constituting the "junction" system have similar structures, and the constructed homojunction has an atomically matched interface, where photogenerated charges are easily separated and transported. Highly crystallizing two structurally similar carbon nitride materials, C3N4 and C3N5, and constructing a "homogeneous junction" system to improve their photocatalytic activity is a very attractive and promising research direction. Summary of the Invention

[0004] The purpose of this invention is to provide a highly crystalline g-C3N4 / g-C3N5(CN) homojunction photocatalyst, its preparation method, and its applications. This photocatalyst can promote efficient charge separation under visible light and exhibits high photocatalytic activity in water splitting for hydrogen production. It has broad application prospects in environmental pollution control, energy, and other fields.

[0005] The objective of this invention is achieved through the following technical solution.

[0006] A method for preparing a CN homojunction photocatalyst with high crystallinity includes the following steps:

[0007] Step 1, Preparation of highly crystalline g-C3N4: Melamine was calcined once in a muffle furnace to obtain g-C3N4; g-C3N4 was mixed with KCl, ground evenly, and then calcined a second time in a muffle furnace to obtain highly crystalline g-C3N4.

[0008] Step 2, Preparation of highly crystalline g-C3N5: 5-amino-1H-tetrazole was calcined once in a muffle furnace to obtain g-C3N5; g-C3N5 was mixed with KCl and ground evenly, and then calcined a second time in a muffle furnace to obtain highly crystalline g-C3N5.

[0009] Step 3: Preparation of high crystallinity CN homojunction: High crystallinity g-C3N4 and high crystallinity g-C3N5 are mixed in ethanol, ultrasonically dispersed, heated in a water bath until the ethanol is completely evaporated, and then calcined in a muffle furnace to obtain high crystallinity CN homojunction photocatalyst.

[0010] Furthermore, in step 1, the mass ratio of g-C3N4 to KCl is 1.5:0 to 10.

[0011] Furthermore, in step 2, the mass ratio of g-C3N5 to KCl is 1.5:0 to 4.

[0012] Furthermore, in step 3, the mass ratio of highly crystalline g-C3N4 to highly crystalline g-C3N5 is 1~0:0~1.

[0013] Further, in step 1, the primary roasting temperature is 520–620℃, and the primary roasting time is 2–6 h; in step 1, the secondary roasting temperature is 550–650℃, and the secondary roasting time is 1–3 h; in step 1, the roasting heating rate is 5–10℃ / min.

[0014] Furthermore, in step 2, the primary roasting temperature is 350–450℃, and the primary roasting time is 2–6 h; in step 2, the secondary roasting temperature is 550–650℃, and the secondary roasting time is 1–3 h; and in step 2, the roasting heating rate is 5–10℃ / min.

[0015] Furthermore, the amount of ethanol added in step 3 is 10-50 mL.

[0016] Furthermore, in step 3, after mixing the highly crystalline g-C3N4 and highly crystalline g-C3N5 in ethanol, ultrasonic dispersion is also included.

[0017] The ultrasonic dispersion time is 10–60 min.

[0018] Furthermore, after ultrasonic dispersion in step 3, water bath heating is also included:

[0019] The water bath heating temperature is 50–80°C.

[0020] Furthermore, in step 3, the calcination temperature is 350–450°C, the calcination time is 1–5 h, and the calcination heating rate is 5–10°C / min.

[0021] In this invention, unless otherwise specified, all raw material components are commercially available products well known to those skilled in the art.

[0022] The present invention does not impose any special limitations on the grinding process, and any process known to those skilled in the art can be used.

[0023] The present invention also provides the application of the photocatalyst described in the above technical solution or the photocatalyst prepared by the preparation method described in the above technical solution in the field of photocatalytic water splitting.

[0024] Compared with existing technologies, the present invention has the following advantages and beneficial effects:

[0025] (1) The preparation methods of high crystallinity g-C3N4, high crystallinity g-C3N5 and high crystallinity CN homojunction photocatalysts are simple and the raw materials are cheap. They have potential development space in large-scale production and preparation. They are a simple, efficient, economical process method that is easier to solve practical problems.

[0026] (2) The homojunction photocatalyst prepared in this invention can effectively promote the separation efficiency of photogenerated charges, thereby improving the photocatalytic activity. When this homojunction photocatalyst system was applied to the experiment of photocatalytic water splitting, it showed excellent photocatalytic ability under visible light conditions. Attached Figure Description

[0027] Figure 1 The photocurrent response diagrams are for the photocatalysts described in Example 2 and Comparative Examples 3-4.

[0028] Figure 2 The XRD patterns are of the photocatalysts described in Examples 1-3 and Comparative Examples 1-4.

[0029] Figure 3 The visible light hydrogen production activity diagrams are for the photocatalysts described in Examples 2 and Comparative Examples 3-4. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0032] Example 1

[0033] The highly crystalline CN homojunction photocatalyst prepared in this embodiment is prepared by the following steps:

[0034] Step 1: Preparation of highly crystalline g-C3N4: 4g of melamine was calcined in a muffle furnace at 570℃ for 4h to obtain g-C3N4; g-C3N4 and KCl were mixed and ground evenly in a mass ratio of 1:1 and then calcined in a muffle furnace at 600℃ for 2h to obtain highly crystalline g-C3N4.

[0035] Step 2, Preparation of highly crystalline g-C3N5: 4g of 5-amino-1H-tetrazole was calcined in a muffle furnace at 400℃ for 4h to obtain g-C3N5; g-C3N5 with KCl at a mass ratio of 1:1 was mixed and ground evenly, and then calcined in a muffle furnace at 600℃ for 2h to obtain highly crystalline g-C3N5.

[0036] Step 3: Preparation of highly crystalline CN homojunctions: Highly crystalline g-C3N4 and g-C3N5 at a mass ratio of 3:1 were mixed in 20 mL of ethanol and ultrasonically dispersed for 30 min. The mixture was then heated in a 65 °C water bath until the ethanol was completely evaporated, and subsequently calcined in a muffle furnace at 400 °C for 2 h to obtain the highly crystalline CN homojunction photocatalyst. The sample was labeled C3N4-C3N5-1.

[0037] Example 2

[0038] Other conditions are the same as in Example 1, except that in step 3, the mass ratio of high-crystallinity g-C3N4 to high-crystallinity g-C3N5 is 1:1. The sample is labeled C3N4-C3N5-2.

[0039] Example 3

[0040] Other conditions are the same as in Example 1, except that in step 3, the mass ratio of highly crystalline g-C3N4 to highly crystalline g-C3N5 is 1:5. The sample is labeled C3N4-C3N5-3.

[0041] Comparative Example 1

[0042] Other conditions are the same as steps 1 and 3 in Example 1, except that high-crystallinity g-C3N5 is not added in step 3. The sample is labeled C3N4.

[0043] Comparative Example 2

[0044] Other conditions are the same as steps 2 and 3 in Example 1, except that high-crystallinity g-C3N4 is not added in step 3. The sample is labeled C3N5.

[0045] Comparative Example 3

[0046] Other conditions were the same as in Comparative Example 1, except that KCl was not added, i.e., amorphous C3N4 was used. The sample was labeled as amorphous C3N4.

[0047] Comparative Example 4

[0048] Other conditions were the same as in Comparative Example 2, except that KCl was not added, resulting in amorphous C3N5. The sample was labeled as amorphous C3N5.

[0049] Test Example 1

[0050] Photocurrent response testing is an experimental method used to characterize the photoelectric response capability of materials. Under simulated sunlight excitation, electrons generated by a photocatalyst are transferred to form a photocurrent, the magnitude of which reflects the separation efficiency of photogenerated electron-hole pairs. Generally, the larger the photocurrent generated by the photocatalyst sample under illumination, the stronger the photoresponse capability of the material and the better the separation efficiency of photogenerated carriers. The photocatalysts described in Example 2 and Comparative Examples 3-4 were characterized for photocurrent response. The test results are as follows: Figure 1 As shown, the photocurrent intensity of the highly crystalline CN homojunction photocatalyst is significantly greater than that of the single-component C3N4 or C3N5 photocatalysts. Therefore, it can be inferred that, compared to single-component C3N4 or C3N5 materials, the highly crystalline CN homojunction photocatalyst exhibits higher separation efficiency of photogenerated carriers and stronger photoresponse capability under simulated sunlight irradiation.

[0051] Test Example 2

[0052] The photocatalysts described in Example 2 and Comparative Examples 1-2 were characterized by XPS semi-quantitative analysis. Table 1 presents the results of the XPS semi-quantitative analysis.

[0053] Table 1. Quantitative XPS analysis of different photocatalysts

[0054]

[0055] In Table 1, the C / N ratio of the C3N4 material is 0.76, close to the theoretical value of 0.75; the C / N ratio of the C3N5 material is 0.55, close to the theoretical value of 0.60; and the C / N ratio of the highly crystalline CN homojunction material is 0.68, falling between 0.60 and 0.75. Based on the above characterization results of the photocurrent response, it can be inferred that this invention synthesizes a highly crystalline CN homojunction photocatalyst formed by the composite of two single components, C3N4 and C3N5.

[0056] Test Example 3

[0057] XRD characterization is used to analyze the crystallinity of samples. The XRD spectra of different photocatalyst materials prepared in this experiment are shown below. Figure 2 As shown, compared with amorphous C3N4 and amorphous C3N5, the main diffraction peaks of C3N4, C3N5, and the highly crystalline CN homojunction sample are sharp at a position around 2θ = 27.4°, indicating that the addition of KCl during the preparation process increases the crystallinity of amorphous C3N4 and amorphous C3N5 materials, and the prepared CN homojunction photocatalytic material also has high crystallinity.

[0058] Application examples

[0059] The photocatalysts described in Examples 1-3 and Comparative Examples 1-4 were tested for catalytic activity.

[0060] 0.02 g of photocatalyst and 90 mL of deionized water were placed in a reactor and mixed under stirring to form a suspension. Then, 60 μL of chloroplatinic acid (concentration 10 mg / mL, calculated as Pt) and 10 mL of triethanolamine were added sequentially. The reactor was connected to a photocatalytic testing system, and the water was evacuated for 20 min to remove dissolved oxygen. A xenon lamp was then turned on for irradiation, and the photocatalytic water splitting reaction was carried out under the illumination of a 300 W xenon lamp (λ>420 nm) with a filter. Timing was started after the lamp was turned on, and samples were taken every 0.5 h until the second hour after timing began. The activity test results were displayed using a gas chromatograph connected to the photocatalytic testing system. The test results are shown in Table 2.

[0061] Table 2. Visible Light Hydrogen Production Activity of Different Photocatalysts

[0062] Serial Number Hydrogen production rate μmol / h <![CDATA[C3N4-C3N5-1]]> 120 <![CDATA[C3N4-C3N5-2]]> 143 <![CDATA[C3N4-C3N5-3]]> 130 <![CDATA[C3N4]]> 55 <![CDATA[C3N5]]> 61 <![CDATA[Amorphous C3N4]]> 9 <![CDATA[Amorphous C3N5]]> 0.87

[0063] As can be seen from the table, the photocatalyst provided by this invention, due to the formation of a highly crystalline CN homojunction, greatly promotes the separation efficiency of photogenerated electrons and holes in the catalyst, thereby significantly improving the photocatalytic activity of the photocatalyst. In photocatalytic water splitting, under visible light conditions with λ>420 nm, the hydrogen production rate of the highly crystalline CN homojunction was 120–143 μmol / h, while the hydrogen production rates of the photocatalysts prepared in Comparative Examples 1, 2, 3, and 4 were 55 μmol / h, 61 μmol / h, 9 μmol / h, and 0.87 μmol / h, respectively, all significantly lower than the photocatalyst products obtained in Examples 1–3. Furthermore, Table 2 clearly shows that compared to amorphous C3N4 and C3N5 photocatalysts, the visible light hydrogen production activity of the photocatalytic water splitting process increased from 9 μmol / h (amorphous C3N4) to 55 μmol / h (C3N4) and from 0.87 μmol / h (amorphous C3N5) to 61 μmol / h (C3N5) simply after KCl crystallization treatment. These activity tests confirm that constructing homojunctions or crystallization treatment can significantly improve the photocatalytic hydrogen production performance of water splitting.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, any changes and modifications made to the embodiments described herein based on the innovative concept of the present invention, or equivalent structural or procedural transformations made using the content of the present invention's specification and drawings, directly or indirectly applying the above technical solutions to other related technical fields, are all included within the scope of protection of this patent.

Claims

1. A method for preparing a CN homojunction photocatalyst with high crystallinity, characterized in that, Includes the following steps: Step 1, Preparation of highly crystalline g-C3N4: 4 g of melamine was calcined in a muffle furnace at 570 °C for 4 h to obtain g-C3N4; g-C3N4 and KCl were mixed and ground evenly in a 1:1 mass ratio and then calcined in a muffle furnace at 600 °C for 2 h to obtain highly crystalline g-C3N4. Step 2, Preparation of highly crystalline g-C3N5: 4 g of 5-amino-1H-tetrazole was calcined in a muffle furnace at 400 °C for 4 h to obtain g-C3N5; g-C3N5 with KCl at a mass ratio of 1:1 was mixed and ground evenly, and then calcined in a muffle furnace at 600 °C for 2 h to obtain highly crystalline g-C3N5; Step 3: Preparation of highly crystalline CN homojunction: Highly crystalline g-C3N4 and highly crystalline g-C3N5 with a mass ratio of 1:1 were mixed in 20 mL of ethanol and ultrasonically dispersed for 30 min. The mixture was then placed in a 65 ℃ water bath and heated until the ethanol was completely evaporated. After that, it was calcined in a muffle furnace at 400 ℃ for 2 h to obtain a highly crystalline CN homojunction photocatalyst. The sample was labeled as C3N4-C3N5-2.

2. The application of the highly crystalline CN homojunction photocatalyst prepared by the preparation method described in claim 1 in the field of photocatalytic water splitting.

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