Tuning Surface Chemistry and Nanostructures in Carbon Dots and their Applications


Student thesis: Doctoral Thesis

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Awarding Institution
Award date5 Sep 2018


Fluorescent carbon-based materials have drawn broad interdisciplinary attention in recent years owing to exceptional advantages as low-cost, metal-free, high optical absorptivity, chemical stability, biocompatibility, and low toxicity. Among these materials (carbon dots, carbon nanotubes, nanodiamonds, graphene, and fullerene), carbon dots (CDs) have drawn the most extensive notice in the nanocarbon family. CDs are attracting great interest owing to their unique physical properties, which are closely related to the nanostructures (quantum confinement effect) and surface chemistry (functional groups) . The CDs are usually synthesized through controlled thermal pyrolysis of numerous carbon-based precursors .Excellent biocompatibility and superior photostablity distinguish them from traditional fluorescent materials (semiconductor quantum dots like CdSe,PbSe,HgTe and fluorescent dyes), and make them potential candidate to replace semiconductor quantum dots and dyes for many exciting applications.

The thesis begins with recent adances in the development of carbon-based nanomaterials and their applications including cell imaging, sensing, catalysis and LEDs. Related mechanisms are also explored and discussed which could open up new opportunities for applications.

In chapter 2, here we design and synthesize a type of nitrogen doped C-dots (N-CDs) which can be used for promotion of plant growth. Our findings reveal that the amount of bean sprouts after cultivating 70 g of mung beans in 0.2 mg/mL of N-CDs for 72 h can reach up to 646 g (wet weight), which, in comparison with that (i.e., 550 g) cultivated from the same amount of mung beans in pure water, indicates the positive impact of the N-CDs on growth and yield of the mung bean. Besides, the blue florescence which comes from the N-CDs can be observed during the growth of the bean sprouts, and it as the time prolongs can transfer from the neck of the bean sprouts to the root, implying the particles of the N-CDs are absorbed by the mung beans. This work reveals us that the N-CDs can not only serve as a new agricultural technique that promote the growth and yield of plant, but also present their ability of monitoring the growth progress of plant in dark and dynamically visualizing how the CDs particles transfer in the body of plant, opening up new possibilities for CDs and CDs-related members for future agricultural purposes.

In chapter 3, we reported an electrochemical sensor of paracetamol (PA) and H2O2 based on a Nitrogen doped carbon dots (NCDs) modified glassy carbon electrode (GCE). The oxygen groups on the surface of the NCDs, and the charge delocalization of the NCDs warrant an excellent electrocatalytic activity of the GCE toward paracetamol (PA) oxidation and H2O2 reduction. PA and H2O2 were detected at 0.34 V (vs. 3 M Ag/AgCl) and -0.4 V (vs. 3 M Ag/AgCl) using differential pulse voltammetry and amperometric I-T measurement, respectively. The modified GCE has a linear response to PA in the 0.5 to 600 μM concentration range, and to H2O2 in the 0.05 μM to 2.25 mM concentration range. The detection limits are 157 nM and 41 nM, respectively.

In chapter 4, 2D Reduced graphene oxide (RGO) coupled with 0D g-C3N4 nanodots (CNDs) as a 2D/0D type of heterojunction photocatalysts were successfully fabricated via a hydrothermal approach. The ultrasmall CNDs were uniformly dispersed in the RGO nanosheets which induced a point contact region in the heterojunction interface, resulting in quicker diffusion from the interior to the surface. The as-obtained nanocomposites of RGO-CNDs exhibited significantly enhanced visible-light-driven photocatalytic performance (4.5 fold times higher than pure CNDs) during the Methylene blue (MB) degradation. What is more, excellent photostablity could be achieved by the composite (nearly 90% after 5 cycles). The enhanced photocatalytic activity was attributed to the enhanced surface area (by RGO), increased active sites (2D/0D structure), enhanced visible light absorption and effective charge separation.

In chapter 5, hierarchical hybrid reduced graphene oxide (RGO)-MoS2 nanocomposites were successfully synthesized via a hydrothermal synthesis approach. The morphology, structure of the composites as well as its photocatalytic activities in the degradation of methylene blue (MB) were recorded with scanning electron microscopy, X-ray diffraction, Transmission electron microscopy and UV-Vis absorption. The results demonstrate that the RGO-MoS2 nanocomposites exhibit excellent photocatalytic performance in degradation of MB with a maximum degradation rate up to 80% under visible light irradiation for 30 min. To understand the reason of the excellent photocatalytic performance, first-principles calculation were carried out. It suggests that the excellent photocatalytic activity should be assign to the fact that there exist the potential electron transfer between conduction band maximum (CBM) of MoS2 and CBM of RGO. Meanwhile, the enhanced visible light absorption, reduced electron–hole pair recombination and enhanced surface area for absorption of dyes also could be the contributors for the excellent photocatalytic performance.

In chapter 6, a novel type of white light emitting diode (WLED) material (g-C3N4-silica-gels) fabricated using the facile one-step solvothermal reaction of citric acid, thiourea, and [3-(2-aminoethylamino)propyl]trimethoxysilane (AEATMS). The resulting g-C3N4-silica-gels feature high quantum yield (27%), high transparency and mechanical flexibility, which can be uniformly coated onto a UV-LED bulb (peak wavelength at 365 nm) by simple dip-coating, enabling bright white light emission. The metal-free g-C3N4-silica-gels reported here are amenable to low-cost large-scale production and, unlike the conventional WLED materials, do not contain any hazardous substances, holding great promise for low-cost environmental and biomedical friendly applications.

In chapter 7, a new type of RE-free material capable of white light-emission upon excitation at 365 nm, fabricated by conveniently compositing carbon dots (CDs) with Zr(IV)-based metal-organic-frameworks (Zr-MOFs). The WLED lamp constructed by coating a commercial UV LED chip with the CDs/Zr-MOF nanocomposite exhibits excellent white color-emitting properties, featuring a CIE chromaticity coordinate at (0.31, 0.34), high color rendering index (CRI) of 82, luminous efficiency of 1.7 lm/W , desirable fluorescence quantum yields (QYs) of 16% which is four times higher than pure Zr-MOFs, and no obvious luminous degradation after exposure to air for three months. This study opens a door for the rational design of efficient and practical RE free MOF based WLED materials.