Evaluation of Thermal Behavior and Properties of Carbon Dots Prepared by Green Synthesis

Abstract

Carbon Quantum Dots are highly fluorescent, non-toxic, thermally stable and water-soluble novel carbon nanomaterials. The dominance of quantum behaviour below 10nm size has drawn scientists’ interest due to their good biocompatibility, low toxicity, high physicochemical and photochemical stability and ease of synthesis. Due to these properties, carbon dots are the best alternatives for semiconductor quantum dots. Carbon dots have shown applications in optoelectronics, photovoltaics, photocatalyst, biomedical such as biosensing, bioimaging, drug delivery and detection for heavy metal ions in food and water samples. The Bottom-Up or Top-Down approaches can be used to synthesis carbon dots. Hydrothermal and microwave irradiation assisted synthesis methods are the simplest and most efficient methods among many reported in the literature [1]. Recently, waste-derived synthesis methods were also developed by carbonizing the waste materials. For waste-derived methods, various types of waste resources such as fruit & vegetable waste, dairy waste, expired drugs, candle soot, chitosan and gelatin were used as a carbon source [2]. In this paper, we reported the microwave-assisted green synthesis of carbon dots using sweet lime as a carbon source and Ethyl diamine as the surface passivating agent. The change in the optical and thermal properties are studied and analysed with a change in the concentration of EDA in carbon dots. Synthesized samples are characterised by Dynamic Light Scattering and Zeta potential particle sizer to analyse the effect of EDA concentration on the distribution of particle size in the samples and change in surface charge of as-prepared carbon dots. The optical parameters are studied with UV- Visible absorption spectroscopy. Composition and molecular bonding are confirmed by Fourier Transform Infrared Spectroscopy. Thermal behaviour and kinematics are studied by simultaneous Thermogravimetric Analyser and Differential Scanning Calorimetry.

DLS study shows that the dots are monodispersed and the negative value of Zeta potential indicates the force of repulsion between adjacent like charges in the material [3]. Zeta potential becomes more negative due to changes in EDA concentration. Prominent peaks in UV- Visible spectra are found at 280nm and 284nm corresponds to the n-π* and π-π* transitions for C=O and C=C bonds [4]. Although a slight shift in absorption peak is observed due to a change in EDA concentration in carbon dots. The calculated value of the energy bandgap is 4.37eV. FTIR spectra confirm the presence of hydroxyl and carboxyl functional groups on the surface of the carbon dot. The thermal properties of Carbon Dots are analysed by simultaneous TGA/DSC technique. All major and minor weight losses reported in TG are in agreement with peaks in DTA and endothermic and exothermic peaks in DSC curves. A significant change in thermal degradation is observed with a change in EDA concentration. The results indicate the stability of the carbon dots is also the function of the anionic nature and nature of the surface passivation agent.