热剥离法制备含缺陷g-C3N4纳米片及光催化性能

Preparation of Defective g-CN Nanosheets by Thermal Exfoliation and Its Photocatalytic Performance

以三聚氰胺为前驱体,采用热剥离法在不同温度制度下制备了g-C_(3)N_(4)纳米片,通过X射线衍射(XRD)、场发射扫描电镜(FESEM)、透射式电子显微镜(TEM)、紫外-可见漫反射(UV-Vis DRS)对其晶型结构、微观形貌、光吸收性能进行了表征。并以盐酸四环素(TC-HCl)和罗丹明B(RhB)为目标降解物,评价了材料的光催化活性。结果表明,当热剥离温度为550℃时,g-C_(3)N_(4)纳米片显示出最佳的降解性能。4 h对TC-HCl的降解效率为82.9%,2 h对RhB的降解效率为97%。这是由于,随着热剥离温度的升高,B-g-C_(3)N_(4)被逐渐剥离为g-C_(3)N_(4)纳米片,550℃剥离的纳米片厚度为9.346 nm,减少的厚度显著增大了光催化材料的比表面积。并且由于氧气的刻蚀,在g-C_(3)N_(4)纳米片表面会产生大量缺陷,为降解物提供活性位点。随着温度升高,g-C_(3)N_(4)纳米片的禁带宽度由2.47 eV拓宽至2.65 eV,抑制了光生电子-空穴的复合。其中还原电位由-1.37 eV移动至-1.46 eV,为光催化材料提供了更强的还原能力,氧化电位由1.10 eV移动至1.19 eV,为光催化材料提供了更强的氧化能力。

Tetracycline hydrochloride(TC-HCl)is widely used as a broad-spectrum antibiotic.The increase of TC-HCl dosage will pollute the water through various ways endanger human health.Semiconductor photocatalytic materials with strong redox ability,can decompose and mineralize TC-HCl,received extensive attention and research.However,the traditional photocatalytic materials have some disadvantages,such as low utilization of visible light,easy recombination of photogenerated electron-hole and less photocatalytic activity sites.Therefore,it is necessary to optimize the microstructure and band structure of semiconductor photocatalytic materials to improve their photocatalytic performance.The g-C_(3)N_(4) nanosheets were prepared by thermal exfoliation method under different temperature regimes.Firstly,melamine precursor was loaded into 60 mm× 90 mm ceramic ark and heated to 550 ℃ in muffle furnace.Secondly,the ceramic ark was taken out after natural cooling to room temperature in the muffle furnace,and the obtained massive g-C_(3)N_(4) was put into agate mortar and ground for 30 min.Finally,the ground photocatalyst was put into the ceramic ark,and the g-C_(3)N_(4) nanosheets containing defects were obtained after thermal stripping at 550 ℃.The crystal structure,micro-morphology and optical properties of obtained samples were characterized by X-ray diffraction(XRD),transmission electron microscope(TEM),field emission scanning electron microscope(FESEM)and UV-Vis diffuse reflectance spectrum(UV-Vis DRS).The photocatalytic activity of the material was evaluated by using tetracycline hydrochloride(TC-HCl)and rhodamine B(RhB)as the degradation target.The light source for photocatalytic performance test was 500 W long arc xenon lamp.The absorbances of TC-HCl and RhB were measured at 356 and 554 nm,respectively.The results showed that the photocatalytic performance of g-C_(3)N_(4) nanosheets increased with the increase of thermal exfoliation temperature.When the thermal exfoliation temperature was 550 ℃,g-C_(3)N_(4) nanosheets showed the best degradation performance.The degradation efficiency of TC-HCl at 4 h was 82.9%,and the degradation rate was 0.0074 min-1.The degradation efficiency of RhB at 4 h was 97%,and the degradation rate was 0.0267 min-1.The degradation rates of TC-HCl and RhB in g-C_(3)N_(4) nanosheets were 4.6 and 8 times higher than those in bulk g-C_(3)N_(4),respectively.The reasons for the improvement of photocatalytic performance were summarized as follows:During the thermal exfoliation process,the air between g-C_(3)N_(4) nanosheets expanded with the increase of temperature.The expanded air weakened van der Waals forces between the layers of g-C_(3)N_(4) nanosheets and gradually peeled the massive g-C_(3)N_(4) into flakes.In addition,oxygen in the air reacted with g-C_(3)N_(4) to destroy its skeleton structure.Defects were generated on the surface of gC_(3)N_(4) nanosheets.The formation of NH3 and CO_(2) in the reaction also promoted the stripping of massive g-C_(3)N_(4).The thickness of massive g-C_(3)N_(4) after stripping was 9.346 nm.The reduced thickness increased the specific surface area of the photocatalyst.The defects caused by oxygen etching provided more reactive sites for the photocatalyst and significantly enhance the photocatalytic activity of the materials.With the increase of thermal exfoliation temperature,the absorption wavelength of g-C_(3)N_(4) shifted from 486 to 457 nm.The band gap of g-C_(3)N_(4) nanosheets was broadened from 2.47 to 2.65 eV.The quantum size effect could be induced by the reduction of the nanosheet size after thermal exfoliation.The reduction potential was shifted from-1.37 to-1.46 eV.A more negative reduction potential meant a good charge carrier transport ability,it could reduce the recombination of photogenerated electrons and holes,providing a stronger reduction ability for photocatalytic materials.The oxidation potential moved from 1.10 to 1.19 eV.The more positive the oxidation potential,the higher the mobility of the generated holes,the stronger the photooxidation ability of holes,and the stronger the oxidation ability of photocatalytic material.The experimental results also showed that g-C_(3)N_(4) nanosheets with stronger redox ability could well degrade organic pollutants.This article considered from the perspective of microscopic morphology control and band structure optimization.Compared with bulk g-C_(3)N_(4),the photocatalytic performance of the defect-containing g-C_(3)N_(4) nanosheets prepared by the thermal exfoliation method was significantly improved,and it had a good degradation effect on TC-HCl and RhB.But there were still shortcomings:(1)The photocatalytic performance of the material still needs to be further improved.Materials that match the energy level of the g-C_(3)N_(4) nanosheets could be selected to recombine to form a heterojunction structure to improve the photo-generated electron-hole separation ability.(2)The recovery of photocatalytic materials needed to be further solved.Therefore,the magnetic materials could be loaded with g-C_(3)N_(4) nanosheets.(3)It should not be limited to the degradation of traditional organic pollutants.It should be applied in more fields,such as photocatalytic hydrogen evolution,CO_(2) reduction,organic synthesis and photocatalytic sterilization.