Alcohol Solvent Effect on Fluorescence Properties in the Solvothermal Synthesis of Carbon Quantum Dots

DENG Yafeng, ZHOU Yihua＊, YE Shuangli, QIAN Jun, CAO Sheng

(1. School of Printing and Packaging, Wuhan University, Wuhan 430079, China; 2. Wuhan Donghu University, Wuhan 430079, China)

Abstract: Highly monodisperse carbon quantum dots (CQDs) were synthesized by a solvothermal method using L-ascorbic acid as carbon source and different simple alcohols (methanol, ethanol, ethylene glycol,and isopropanol) as reaction solvents at 180 ℃ for 4 hours. The performance of CQDs was characterized by transmission electron microscope (TEM), Fourier infrared spectrometer (FTIR), UV-visible spectrophotometer,and fluorescence spectrophotometer. The results show that the prepared CQDs are wavelength-dependent,and have good hydrophilicity and similar surface compositions. However, there are more carbon and oxygencontaining functional groups on the surface of CQDs prepared with ethanol (CQDs-ET), and the type and number of functional groups will directly affect the fluorescence emission of CQDs. Also, it is found that the luminescence mechanisms of CQDs prepared by this solvothermal method are mainly based on the defect state of the oxygen group surface. And alcohol solvents do not directly participate in the formation of carbon nuclei during the reaction process, but it will affect the number and type of surface groups. Therefore, the influence of surface groups on the CQDs performance is greater than that of carbon nuclei in this experiment.

Key words: carbon quantum dots; solvothermal method; reaction solvent; luminescence mechanisms

1 Introduction

The emergence of CQDs is a breakthrough in the field of nanomaterials[1-4]. Since Xuet al[5]discovered carbon nanomaterials with fluorescence properties during the separation and purification of single-walled carbon nanotubes, and their fluorescence properties had also been studied. CQDs,especially fluorescent CQDs, has attracted the attention of a growing number of scientists. At present, there are two ways to prepare the CQDs: topdown method (Top-down) and bottom-up method(Bottom-up)[6]. Among them, the solvothermal synthesis can produce more uniform and stable CQDs, the particle size and the hydrophilicity and hydrophobicity of the CQDs can be controlled,and high yield of product and the carbon source can be easy to get. It can be said that this method is the most effective way to prepare multicolor and efficient CQDs at present. In the hydrothermal and solvothermal methods of CQDs, alcohols are commonly used, such as glycerol[7,8], methanol[9], and others. Thus, alcohol can not only be used as a carbon source to prepare CQDs but also can be used as solvents to participate in the reaction process.Some researchers[10]synthesized the fluorescent CQDs solution with L- ascorbic acid as the carbon source, through the solvothermal method, then the extraction by organic solvent. It is similar to most other methods and can get blue fluorescence CQDs.However, green fluorescent CQDs were synthesized by the solvothermal method using L- ascorbic acid as a carbon source and ethylene glycol aqueous solution as solvent[11]. Therefore, when the carbon source is the same, different solvents can change the fluorescence emission. Many researchers have focused on the influence of the carbon source and preparation condition in this process,whereas solvents have usually been neglected.Thus, finding out the relation between solvents and the optical properties of CQDs becomes very important to control this method and understand the luminescence mechanism.

Herein, we synthesized CQDs by a facile solvothermal process using L-ascorbic acid as a carbon source, and different simple alcohols (methanol, ethanol, ethylene glycol, and isopropanol)as reaction solvents. In this process, we focused on adjusting the alcohols because of its simple composition and structure, which makes it easy to explore the influence of CQDs chemical structures on fluorescence properties and mechanisms. The synthesized CQDs show blue luminescence at 365 nm under UV light. Also, the physicochemical and optical properties of the CQDs have been investigated. The results show that the emission of CQDs prepared by this method is mainly due to the defect luminescence of oxygen groups on the surface,and the reaction solvent has no influence on the formation of the carbon nucleus, but it will affect the surface groups, which determines the properties of the CQDs. Therefore, we obtained CQDs with different luminescence properties by adjusting the reaction solvent.

2 Experimental

2.1 Materials

L- ascorbic acid, anhydrous ethanol, methanol, isopropanol, ethylene glycol, acetone, and tetrahydrofuran were purchased from Chemical Reagent of China National Pharmaceutical Group.All other reagents were of analytical grade and used without further purification.

2.2 Synthesis of hydrophilic CQDs

Fig.1 shows the preparation process of carbon quantum dots. First, a small amount of L-ascorbic acid was dissolved in deionized water, and an alcohol solvent was added and continued stirring for 2 h to obtain a uniformity solution. 50 mL of the solution was transferred to a Teflon-lined stainless-steel autoclave and maintained at 180 ℃ for 4 h, then cooled to room temperature. The dark brown product was extracted with dichloromethane, and the water-phase solution was dialyzed to remove impurities for 2 days through a dialysis membrane (retained molecular weight: 3 000 Da). Finally, a yellow aqueous solution containing CQDs was obtained for further use. The solid powder was obtained from freeze-drying for dissolving in different solvents. The preparation methods of other samples were similar as described above,but the solvents used were different, which were a mixed solution of alcohol (methanol, ethanol,ethylene glycol, and isopropanol, respectively) and water.

2.3 Equipment and characterization

The morphologies of the samples were obtained by high-resolution transmission electron microscopy(HRTEM) using a JEM 2100 instrument at an accelerating voltage of 200 kV. Fourier transform infrared spectroscopy (FTIR) spectra were obtained on a Nicolet 5700 FTIR spectrometer. UV-Vis absorption spectra measurements were carried out using a Shimadzu 2550 spectrophotometer. The luminescence spectra were recorded on a Hitachi F-4600 5J2-0004 spectrophotometer. All the measurements were performed at room temperature.

3 Results and discussion

3.1 Preparation of hydrophilic CQDs

The diameter of CQDs was measured using TEM and HRTEM, which displays that the highly monodispersed CQDs with about 2-3 nm have been successfully fabricated in Fig.2. The lattice structure of the CQDs can be seen clearly and the dispersion of CQDs is uniform. Those whose size is too large may be due to the agglomeration of CQDs. Its lattice spacing is about 0.2 nm, corresponding to the graphite on the (100) surface, and it shows that its chemical structure is graphite structure.

Fig.3 is the UV-vis absorption spectrum of CQDs prepared by L-ascorbic acid with different alcohols.The result is consistent with the report[12]: The typical absorption of CQDs appears in the ultraviolet light region, which is a banded absorption, and there is a small trailing point to the visible region[13]. Besides, these four CQDs have obvious absorption peaks between 230-280 nm. This represents the π-π transition of the C=C bond in the CQDs[14].

3.2 Preparation of CQDs with different solvents

To investigate the surface functional groups of CQDs prepared using different alcohols, Fourier transform infrared (FTIR) measurements were carried out(Fig.4). (The CQDs in methanol or ethanol or isopropanol and deionized water are abbreviated as CQDs-MT,CQDs-ET, CQDs-IPA). The infrared spectra of different CQDs show great similarity, indicating that they have similar surface composition. Although the proportion of the surface groups of each CQD is different, the species are very similar. This demonstrates that the formation of CQDs mainly depends on the precursor, and the reaction solvent does not directly participate in the formation of CQDs. With the increase of the number of the carbon atom, the carbon content on the surface of the CQDs also increased significantly, so that the solvent was successfully combined with the CQDs. Compared with the other two alcohols, the surface groups of CQDs-MT are not obvious, indicating that methanol is not suitable for reaction solvents. In the other two curves, we can see the vibration of O-H, C=O, and C-O,respectively. After the formation of CQDs, many same groups are enriched on the same particle, and the absorption of the C-H vibrational band and C=O bond is decreasing and widening. Therefore, the surface group of CQDs-ET is the most abundant.

Fig.4(b) is the FTIR of CQDs prepared by different alcohols. (The CQDs in ethanol or ethylene glycol and deionized water are abbreviated as CQDs-ET, CQDs-EG). Similar to Fig.4(a), CQDs-ET and CQDs-EG also have obvious vibration of chemical bonds such as O-H,C-O, C=O, and C-H, and the covalent bonds O-H and C=O increase the hydrophilicity of CQDs, that is, the prepared CQDs have good hydrophilicity. However, in contrast, the vibration of C=C and C=O in the CQDs-ET surface group is more obvious, and this will affect the luminescence effect of the CQDs, so CQDs-ET is more suitable for the fluorescence application.

To further study the optical properties of CQDs prepared with different alcohols, PL spectra of four kinds of CQDs under 365 nm excitation were shown in Fig.5. In addition to CQDs-EG, the other three CQDs emitted visible blue light while CQDs-EG emitted blue-violet light. It is shown that in the hydrophilic and strong polar solvents, the formation of oxygen-containing groups is beneficial to blue light emission, and the type and quantity of functional groups will directly affect the emission light. This is consistent with the previous FTIR spectrum, that is, the number of oxygen groups on the surface of CQDs-EG is not as good as that of the other three CQDs.

Fig.6 shows PL of CQDs prepared by different solvents under 320nm-440nm excitation light (each interval is 20nm). All of them exhibit excitation wavelength dependence and the redshift phenomenon. The intensity of the emission wavelength increased first and then decreased, the color changed from purple to blue,and the amplitude of the redshift was larger. Especially in Fig.6(a), (b) and (d), the CQDs have strong emission light under the excitation wavelength of 360-380 nm,and the emission wavelength is about 450nm; when the excitation wavelength is 320 nm, there is almost no fluorescence. However, the redshift of CQDs-EG is in Fig.6(c). Under 320 nm-360 nm excitation, there is a purple emission of about 400 nm, and there is almost no fluorescence in other emitting wavelengths. The carbon-carbon bond content of CQDs surface groups does not have much effect on the excitation light dependence (i e, The redshift of emission light), but the hydroxyl group on the surface of the CQDs has a great influence on its luminescence performance, which is in agreement with the previous conclusion. There is a competitive process in the emission of different surface states of CQDs. Oxygen dominated surface states produce more defects, mainly corresponding to the blue and green light region. This indirectly indicates that the CQDs prepared in this experiment are defective state luminescence.

4 Conclusions

According to the results of the FTIR and PL spectra,the vibration of C=C and C=O in the CQDs-ET surface group is more obvious, and CQDs-ET is more suitable for the fluorescence application. Besides, it is found that alcohol solvents do not directly participate in the formation of carbon nuclei during the reaction process, but it affects the number and type of surface groups. Moreover, the surface groups play a more important role in the luminescence properties and the oxygen functional groups dominate the hydrophilicity. The luminescence mechanisms of CQDs prepared by this solvothermal method are mainly based on the defect state of the oxygen group surface. In this experiment, CQDs with different emission wavelengths under the same excitation light were prepared by changing the reaction solvent.The good hydrophilicity and fluorescent properties of these CQDs lay the foundation for their application in fluorescent anti-counterfeiting and other fields.