碳化聚合物点、碳纳米点和石墨烯碳纳米点降解RhB 的研究

赫付涛 刘晓惠 白 静 孟献瑞 徐元清 蔡永红 房晓敏 张文凯

（河南大学 化学化工学院， 河南 开封475004）

Carbon dots （CDs） possess an array of desirable properties， such as high photoluminescence，broadband optical absorption， low toxicity， high photostability and chemical stability， and good water solubility［1-8］. Therefore， CDs can be applied in biological imaging［9-11］， sensing［12］， energy storage fields［13-14］and information encryption［15-20］. CDs refer to the size of less than 20 nanometer carbon particles with fluorescence properties. The chemical structure of carbon dots can be the hybrid carbon structure of sp2and sp3，with single or multilayer graphite structure，or the aggregate particles of polymer［21］. Owing to the wide range of raw materials and various synthesis methods， various types of carbon dots are rapidly synthesized. However， the naming and classification of carbon dots are widely debated， which hinders the further study of carbon dots［22-23］. In CDs prepared at a low reaction temperature， the molecule／oligomer is dominant， while in CDs prepared at a high reaction temperature， the carbon nuclear state is dominant［24］.Carbon dots dominated by molecules／oligomers are called carbonized polymer dots （CPDs）. Carbon dots dominated by carbon nuclei are called carbon nanometer dots （CNDs）. we divided carbon dots into carbonized polymer dots （CPDs）， carbon nanodots（CNDs） and graphene carbon nanodots （GNDs）.

Graphene quantum dots generally have a single layer of graphene flake structure， while the surface and edge are passivated functional groups. Moreover， in graphene quantum dots， there is a significant quantum limiting effect， and the surface groups also affect the energy level structure of graphene quantum dots. In one⁃pot synthesis of carbon dots， there is a chemical equilibrium between reactants and small molecule fluorescence， polymer clusters， and carbon cores. In the synthesis process of CDs， chemical equilibrium was tuned by varying the starting material ratio， initial pH value and temperature. The equilibrium was largely influenced by the synthesis temperature. Under different temperature conditions， the percentages of the different components changed. Carbon dots have excellent light absorption capacity， up⁃conversion fluorescence properties and enhanced photoelectron／hole pair transport and transfer capacity， which make carbon dots or carbon dot complexes have a broad application prospect in the field of photocatalysis.Compared with other photocatalysts such as ZnO， CdS and TiO2， carbon dots have unique properties， such as good water solubility， low toxicity and stable chemical properties［25］. Many researchers used carbon dot composites to catalyze the degradation of antibiotics［26］，organic industrial dyes［27-29］，methoxypropionic acid［30］and tetracycline［31］. In this paper， with citric acid and ethylenediamine as precursors， the carbonized polymer points and carbon nanometer points were synthesized by solvent thermal synthesis at 180 ℃ and 300 ℃， respectively. Using GONDs aqueous solution and ethylenediamine as precursors， GNDs was obtained by solvent method at 180 ℃for 6 h（Scheme 1）. CPDs， CNDs and GNDs were characterized by AFM， UV， Raman and FT⁃IR.We studied the degradation of RhB by CPDs， CNDs and GNDs.

Scheme 1 Synthesis route of CPDs， CNDs and GNDs

1 Experimental section

1.1 Materials

CA were purchased from Sigma⁃Aldrich. Carbon fibers were purchased from the Beijing Carbon Institute of Engineering ＆ Technology Co.， Ltd. Rhodamine B（RhB），EDA and other analytic reagents were purchased from the Sinopharm. Chemical Reagent Co.，Ltd.

1.2 Characterization

The atomic force microscope （AFM） images were measured with Dimension Icon （Bruker Instruments Inc.） on new cleaved mica surface in tapping mode in air. UV⁃vis absorption spectra were recorded with a Hitachi U4100 Spectrometer. High resolution microscopy measurements were performed using a JEM1200EX TEM with an operating voltage of 120 kV.Fourier transform infrared spectroscopy （ FT⁃IR）characterization was carried out on a Bruker Vertex 70 FTIR spectrometer. Raman spectra were obtained using Renishaw RW1000 （Renishaw， Wotton⁃under⁃Edge，UK） confocal microscope with a 514 nm line of Argon⁃ion laser as exciting light.

1.3 Synthesis of CPDs and CNDs

18.6 g monohydrate citric acid （80 mmol） and ethylenediamine （5.36 mL） were dissolved in 300 mL water， transfer the solution to a three⁃neck flask with a mechanical agitator，reflux condenser and thermometer， CPDs and CNDs were synthesized by heating the mixture to 180 ℃and 300 ℃for one hour.The reaction liquid was cooled to room temperature and the product was washed with ethanol and acetone， and then was dissolved by ultrasound after the addition of distilled water. CPDs and CNDs were obtained after 4 d of dialysis （MW ＝3 500 Da）.

1.4 Synthesis of GNDs

0.30 g of resin⁃based carbon fiber was weighed and added to the mixture of H2SO4（60 mL） and HNO3（20 mL）， The solution was sonicated for two hours and stirred for 24 h at 120 ℃. Na2CO3was used to adjust the pH of the solution to 8. Product solution dialysis for one week （retained molecular weight：8 000-14 000 Da）. Black powder was obtained by rotating vacuum evaporation of GNDs solution. Using GONDs aqueous solution and ethylenediamine as precursors， GNDs was obtained by solvent method at 180 ℃for 6 h （Scheme 1）.

2 Results and discussion

2.1 Morphology characterization

Three carbon dots were characterized by atomic force microscope. We found the average height of 40 points in each AFM chart. The three carbon dots were evenly dispersed without aggregation. The average height of CPDs， CNDs and GNDs was 2.9 nm，5.2 nm and 1.2 nm（Fig. 1）.

Fig.1 AFM images of CPDs （a）， CNDs （b） and GNDs （c）

2.2 Structure of CPDs， CNDs and GNDs

The following conclusions can be drawn from the FT⁃IR spectra. The presence of a band at 3 300 cm-1indicates the presence of O⁃H／N⁃H groups. The presence of a band at 1 600 cm-1indicates the presence of C ＝C groups （Fig.2a）. In order to study the molecular structure of carbon dots， we used Raman spectroscopy to characterize three carbon dots （Fig.2b）. Typical carbon Raman spectra will have two broad peaks at 1 354 cm-1and 1 581 cm-1，corresponding to the disorder （ D band ） and graphitization degree （G band） of carbon materials，respectively.ID／IGrepresents the graphitization degree of the carbon material and sp3／sp2. All three carbon dots have peaks at 1 354 cm-1and 1 581 cm-1. TheID／IGratios of CPDs， CNDs and GNDs were 0.94， 0.83 and 0.80， respectively. The graphitization degree of CPDs， CNDs and GNDs increased successively.

Fig.2 FT-IR spectra （a） and Raman spectra （b） of CPDs （black）， CNDs （red） and GNDs （blue）

2.3 Optical characterization

We used ultraviolet to characterize CPDs，CNDs and GNDs. The CPDs has two UV peaks of 240 nm and 340 nm，and these can be attributed to π-π∗and n-π∗（C＝C／C＝N bonds） transitions，respectively.The CNDs has two UV peaks of 225 nm and 300 nm，which can also be attributed to π-π∗and n-π∗（C ＝C／C ＝N bonds）transitions，respectively. GNDs has two UV peaks at 230 nm and 335 nm. The UV peak at 230 nm is caused by the π-π∗transition of C＝C in the carbon six⁃member ring in the GO layer . The weak peak at 3 3 5 nm is attributed to the n-π∗transition of C ＝O （Fig.3a）. Under the excitation wavelength of 365 nm， the fluorescence peaks of CPDs， CNDs and GNDs were 450， 420 and 440 nm， respectively （ Fig. 3b ）. Fluorescence measurements of CPDs， CNDs and GNDs were performed at different excitation wavelengths. The fluorescence peak position of CPDs remained unchanged under different excitation wavelengths. With the increase of excitation wavelength， the fluorescence peak of CNDs and GNDs is red⁃shifted（Fig.4）.

Fig.3 Absorption （a） and emission （b， λex ＝365 nm） spectra of CPDs （black）， CNDs （red） and GNDs （blue）

Fig.4 Emission spectra with different excitation wavelengths of CPDs， CNDs and GNDs

2.4 The degradation of RhB

We first explored the light source power in the factors affecting the degradation rate of RhB. Without CDs， the degradation rate of RhB under three different power sources was studied. After 90 min， the degradation rates of RhB under three different power sources were 38.1%， 51.2% and 95.8%， respectively.When the power of the light source is 32 W， the power of the light source is too small to effectively induce the excitation of the photocatalyst. When the light source power is 96 W， RhB can degrade to more than 95%，but the degradation rate of dyes by CDs cannot be well compared. Therefore， 64 W ultraviolet mercury lamp should be selected as the light source（Fig.5a）.

Fig.5 （a） Degradation rate of RhB without carbon dots at three different light source powers； （b） Degradation rate of RhB by CPDs （black）， CNDs （red） and GNDs （blue）. The degradation rate of Rhb without carbon dots （gray）

3 mL （100 mg／L） dye solution， 3 mL （100 mg／L） CDs and 24 mL distilled water were poured into 50 mL quartz reaction tube for dye degradation experiment. 3 mL solution was taken every 10 min for UV⁃visible spectrophotometer measurement（Fig.6）. At 20 min， the degradation rate of RhB by CPDs was 45%， the degradation rate of RhB by CNDs was 31%，the degradation rate of RhB by GNDs was 29.5%， and the degradation rate of RhB in the absence of carbon dots was 6.3%. At 90 min，the degradation rate of RhB by CPDs was 84.3%， the degradation rate of RhB by CNDs was 57.1%， and the degradation rate of RhB by GNDs was 53.5%， and the degradation rate of RhB in the absence of carbon dots was 51.2%. The results show that the addition of three carbon dots was beneficial to the degradation of RhB. The degradation rate of RhB increased by three carbon dots is CPDs＞CNDs＞GNDs. CPDs is rich in surface groups， and its oxidizing group oxidizes and degrades RhB（Fig.5b）.

3 Conclusion

In summary， carbon dots dominated by molecules／oligomers are called carbonized polymer dots（CPDs） and carbon dots dominated by carbon nuclei are called carbon nanometer dots （CNDs）. we divided carbon dots into carbonized polymer dots （CPDs），carbon nanodots （ CNDs） and graphene carbon nanodots （ GNDs ）. With citric acid and ethylenediamine as precursors， the carbonized polymer points and carbon nanometer points were synthesized by solvent thermal synthesis at 180 ℃ and 300 ℃，respectively. Using GONDs aqueous solution and ethylenediamine as precursors， GNDs was obtained by solvent method at 180 ℃ for 6 h. CPDs， CNDs and GNDs were characterized by AFM， UV， Raman and FT⁃IR. The average height of CPDs， CNDs and GNDs was 2.9， 5.2 and 1.2 nm， respectively. The graphitization degree of CPDs， CNDs and GNDs increased successively. The fluorescence peak position of CPDs remained unchanged under different excitation wavelengths. With the increase of excitation wavelength， the fluorescence peaks of CNDs and GNDs are red⁃shifted. The surface of the three carbon dots contains carbonyl group and hydroxyl group.Degradation of RhB was performed by using CPDs，CNDs and GNDs. The degradation rate of RhB by three carbon dots is in the order of CPDs＞CNDs＞GNDs.