Introduction
Preface
In this form, carbon atoms are coiled into cylindrical tubes, discovered by Iijima 1, 2. The carbon atoms in fullerene (C60) are bonded by the pentagons and hexagons, with each C60 molecule consisting of 60 sp2 carbon atoms4.
Graphene Quantum Dots
The different hybridization and arrangement of carbon atoms in different allotropes enables the formation of hybrids with foreign materials, which can have interesting properties and play an important role in various applications. It is interesting to note that due to the ultrafine size of GQDs, the ratio of edge to basal plane carbon atoms is high, resulting in the predominance of edge states and edge oxygen functional groups 7-9.
Comparison of GQDs and Semiconductor QDs
When graphene is cut into small pieces, it exhibits interesting features such as edge sites (zigzag and armchair) and oxygenated functional groups. Another absorption tail extended from the UV to the visible region is assigned to the n–π* transition of the oxygen functional groups 6 , 10 .
Raman Spectroscopy of Carbon Nanomaterial
- Raman Spectrum of Graphene and GQDs
- Graphitic Band
- Disordered Band
The D band is strongly present in the defective graphene, while it is absent for graphene. It is well known that, except for the G band, all the defect bands and second-order Raman modes, including the combination modes, are dispersive in nature 15 , 22 , 23 .
Methodologies for Synthesis of GQDs
- Top down Method
- Bottom up Method
In general, the PL mechanism from GQDs is explained in terms of intrinsic states (blue PL) and emission of defect states (green PL). Intrinsic states arise from size/edge defects where electron-hole recombination occurs. These results indicate a strong interaction between GQDs and TiO2 through defects in TiO2. The shift in the Raman peak in the hybrid samples is probably due to the formation of the potential C-O-Ti- bond when GQD and TiO2 come into contact with each other. or due to strain.
Fabrication of GQDs Based Heterostructures
Applications of GQDs and Its Heterostructures
- Sensing Applications
- Metal Ions Detection
- Chemicals Sensing
- Detection of Bio-Molecules
- Bio-Imaging
- Solar Cells
- Supercapacitors
- Visible light Photocatalysis
Motivation and Focus of the Present Thesis
This thesis focuses on the controlled synthesis, elucidating the formation mechanism of GQDs and its heterostructures, tuning its optical properties and exploring its applications in bio-imaging and visible light photocatalysis. We propose a new formation mechanism for circular-shaped GQDs by studying the critical role of in-plane epoxy functional groups in the formation of GQDs.
Organization of the Thesis
The first column and the third column represent the images with bright files, while the second column and the fourth column represent the fluorescence images of the images of different GQDs. Bright blue fluorescence is observed inside cells, indicating that GQDs are taken up by A-375 and HeLa cells and are mainly localized in the cytoplasmic region. The inset shows the HRTEM lattice image of a TiO2 NP with the strained region marked by the oval-shaped dotted line. e-f) lattice TEM and HRTEM images of T0/GQD hybrid, respectively. g) TEM image of T16/GQD hybrid, (h) lattice HRTEM image of T16/GQD showing the simultaneous presence of TiO2 lattices and GQDs.
Experimental Techniques
Synthesis Techniques of GQDs and Its Heterostructures
- High Pressure Reactor for the
- High Temperature Furnace
- Planetary Ball Milling
- Spin Coater
- Microscopy Techniques
- Field Emission Scanning Electron
- High Resolution Transmission
- Spectroscopic Techniques
- Raman Spectroscopy
- Ultraviolet-Visible Absorption Spectroscopy
- Diffuse Reflectance Spectroscopy (DRS)
- Photoluminescence (PL) Spectroscopy
- Time Resolved Photoluminescence (TRPL) Spectroscopy
- Fourier Transform Infrared (FTIR) Spectroscopy
- Electron Paramagnetic Resonance (EPR) Spectroscopy
- X-ray Photoelectron Spectroscopy (XPS)
- Analytical Techniques
- X-ray Diffraction (XRD)
- Thermo Gravimetric Analysis (TGA)
The photograph of the TRPL instrument (LifeSpec II, Edinburgh Inst., UK) used in this study is shown in Fig.
Visible Light Photocatalysis
Bio-Imaging of Cancer Cells
- Culture of Cells
- Cytotoxicity
- Confocal Microscopy Imaging
In the present study, a confocal microscope (Zeiss LSM 880) is used for the fluorescence imaging of the GQD. The absorption tail appears due to the presence of disordered states (oxygenated functional groups and/or nitrogen vacancies) in the GQD-GCN.
Formation Mechanism of Graphene Quantum Dots During
Introduction
In general, the PL mechanism of GQDs is explained in terms of intrinsic states (blue PL) and defect states emission (green PL). We develop the PL and Raman spectra of GQDs to gain insight into the origin of PL emission from GQDs.
Experimental Details
- Preparation of GO and rGO
- Size reduction of GO
- Synthesis of GQDs
- Functionalization of GQDs
- Synthesis of GQDs from rGO
- Fabrication of GQDs Thin Films
In situ chemical functionalization of GQDs was performed to achieve edge modification with oxygen functional groups. To understand the mechanism of intriguing PL originating from the edge states and functional groups of GQDs, we performed thermal annealing of GQDs under different gas environments.
Morphology, Structural and Optical Properties of OGO and rGO
- TEM Imaging Analysis
- Optical Absorption and TGA Studies
The absorption spectrum appearing in the n-π* region (UV-vis wavelength) of OGO is due to the in-plane oxygen functional groups 28. The additional FTIR bands due to the in-plane epoxy in OGO strongly suggest the presence of.
TEM and AFM Imaging
3.3 (c) presents a TEM image of GQDs in S12, showing small fragments of GQDs separated from the OGO sheet after the hydrothermal reaction. This establishes the critical role of epoxy functional groups on the OGO basal plane in cutting OGO sheets to form GQDs.
Formation Mechanism of GQDs
In the first step, the epoxy functional groups in OGO tend to appear on the C-C bond and form a quasi-ring-like structure underlying the C-C bonds on the basal plane of OGO during the hydrothermal reaction (Fig. Formation of quasi-ring type epoxy). functional groups and porous structure of OGO are proposed based on the evidence from the TEM imaging (Fig.
Raman Study on the Edges and Functional Groups
Band D is derived from the law of conservation of momentum due to the formation of edge states (zigzag and armchair). As a result of OGO cleavage, the carbon atoms are redistributed and the attachment of oxygen functional groups takes place at the sp2-sp3 edge carbon sites.
Ultraviolet-visible Absorption Study
During the reaction, the COOH functional groups attach to the edges of the GQDs, resulting in the broad absorption band for the functionalized GQDs samples.
Photoluminescence Analysis
Similarly, a strong increase in P2 intensity after annealing implies an enrichment of armchair edges in GQDs. 3.8 (a) shows the change in the relative percentage of zigzag edges in the pristine and annealed samples, while Fig.
X-ray Photoelectron Spectroscopy Study
This is possibly due to the fact that ether functional group has a lower thermal stability than the other functional groups 56. This is consistent with the increased intensity of green PL (530 nm) and increased intensity of FTIR bands related to the oxygenated functional groups after The O2 annealing.
FTIR Analysis
The integrated intensity of the O1s spectrum for the S24O sample is 1.78 times higher than that of the pristine sample, which reveals the binding of oxygen atoms to the edge/in-plane sites of the GQDs. Note that the integrated intensity of the O1s spectrum for the S24H sample is 1.56 times higher than that of the pristine samples.
Summary & Conclusions
These results are consistent with the optical studies of the samples discussed in the section below. For further confirmation of the presence of Au in the hybrid system, we performed elemental mapping on Au@GQD-GCN.
Solvent Dependent Growth of Luminescent
Introduction
The effect of solvent on the synthesis of GQDs and understanding the PL mechanism of GQDs is important for optoelectronic, drug delivery and bio-imaging applications. In this chapter, we present the synthesis of GQDs using three different solvents and studied its PL mechanism when dispersed in liquid media.
Experimental Details
- Synthesis and Functionalization of GQDs
- Determination of PL Quantum Yields
- Cell Culture
- Procedure for the Cell Viability Test
- Preparation of Cells for Confocal Imaging
The structural changes of GQDs due to the binding of different oxygen functional groups are analyzed by TGA/DTG profiles. 86% degradation of MB in the presence of Au-decorated GQD-GCN hybrid catalyst even at ultra-low concn.
XRD and TEM Imaging Analyses
Raman Spectroscopy Analysis
Thermogravemetric Analysis (TGA) / Differential TG Analyses
4.3(a) represents the TGA/DTG profiles of GQD-DMF and the multiple DTG peaks arise due to the loss of different oxygenated functional groups and different forms of carbon in GQD. There are additional DTG peaks in the region resulting from the loss of several forms of very stable C=.
Optical Properties Study
For a quantitative assessment of the PL emission of GQDs, we measured the QY of GQDs by using the QS as a standard dye, which has a QY of 0.54. To understand the charge transfer process of GQDs in solvents, we measured the lifetime of carriers in GQDs using the TRPL measurements.
Cell Viability and Bio-imaging Studies
4.6 (e1 and e2) shows the bright field and fluorescence images of the HeLa cell without any addition of GQDs. The first column (a1-e1) and the third column (a3-e3) represent the bright images, while the second column (a2-e2) and the fourth column (a4-e4) represent the fluorescence images of the different GQDs.
Summary & Conclusions
In-Situ Decoration of Plasmonic Au Nanoparticles
Introduction
NPs-based hybrids have received tremendous attention in recent times due to the surface plasmon resonance (SPR) effect. Similar to the graphene analogy in sp2 hybridized graphitic structure, graphitic carbon nitride (GCN) has been considered as a promising material for photocatalysis due to possessing band gap 5, 6.
Experimental Details
- Synthesis of Au NPs
- Preparation of GQD-GCN Hybrid
- In-Situ Decoration of Au NPs on
- Visible Light Photocatalytic Measurements
- Photodegradation of MB
- Hydroxyl Radical Test
- Superoxide Radical Test
The large amount of water was taken for better dispersion of GQD-GCN and to prevent agglomeration of Au NPs over the hybrid nanosystem. 5.1(a) shows the steps involved in the synthesis of GQD, GCN and Au NPs decoration on GQD-GCN.
Morphology, Composition and Structural Analyses
Note that the lattice fringes are not evident at the Au decorated GQD-GCN junction. Along with the diffraction peaks of GQD and GCN, the reflection lines of face-centered-cubic (fcc) Au are clearly observed in the XRD pattern of Au@GQD-GCN.
Raman Studies
Integration of Au NPs on GQD-GCN hybrid facilitates the strong coupling between surface plasmon and fluorescent material18.
FTIR Studies
In GQD-GCN, the vibrational modes corresponding to C=O and C=C of GQD are incorporated with s-heptazine ring vibrational mode in C3N4 in the range 1100-1650 cm-1. In Au-decorated GQD-GCN sample, the FTIR vibrational modes of the GQD or GCN are merged with the FTIR spectrum of Au@GQD-GCN.
Optical Properties
- Optical Absorption
- Steady State Photoluminescence
- Time Resolved Photoluminescence
The change in the absorption characteristic of Au NPs in Au@GQD-GCN may be due to the different dielectric media and the formation of Au–C bonds in the hybrid system 25. The GQD-GCN hybrid exhibits an emission peak at 432 nm with reduction of the green emission region.
X-ray Photoelectron Spectroscopy Analysis
Note that after Au decoration, the average lifetime of the hybrid system decreases to 16.2 ns. There is a negative shift (−0.8 eV) of the doublets of Au in the hybrid system relative to the positions reported for pristine metallic Au, which are 83.8 and 87.4 eV, respectively, 37, 39.
Visible light Photocatalytic Degradation of MB
- Degradation and Reaction Kinetics
- Free Radical Detection and Degradation Mechanism
To understand the role of Au NPs with hybrid of GQD or GCN and its effect in PC activity, we performed photocatalysis of MB in the presence of Au-GQD and Au-GCN. 5.9(d), the value in parentheses represents the rate constant of MB in different catalyst samples.
Summary & Conclusions
The role of oxygen vacancies in the formation of TiO2-GQD hybrid is discussed later. These results clearly demonstrate the role of oxygen vacancies in TiO2 in the formation of TiO2/GQD hybrid.
Interfacial Charge Transfer in Oxygen Deficient
Introduction
Due to the ultrafine size and high ratio of carbon atoms in the edge and basal plane, GQDs possess the edge states and edge oxygen functional groups 7–10. In this regard, GQDs have meager in-plane oxygen functional groups than those of other graphene materials.
Experimental and Computational Details
- Sample Preparation
- Fabrication of GQDs and TiO 2 Nanoparticles
- Fabrication of GQDs and TiO 2 NPs Hybrids
- Visible Light Photocatalysis
- Photodegradation of methylene blue
- Free Radicals Scavenger Test
- Computational Methodology
Xing et al studied the graphene and TiO2 hybrid formation through the OH functional groups 5. The GQD contained nine hexagonal rings with two epoxy functional groups out of the GQD plane.
X-ray Diffraction and TEM Studies
So, we believe that the ultrasonication induced in-plane epoxy disorder enables the formation of the possible C-O-Ti bonds in TiO2-GQD hybrid. From this result, it is believed that the oxygen in GQDs can bind to the oxygen-deficient sites in TiO2-x, leading to the formation of possible C-O-Ti bonds in the T16/GQD hybrid samples.
Steady State and Time resolved Photoluminescence Studies
In particular, the intensity of green PL emission is significantly reduced compared to that of the blue PL in the hybrid sample. This extended lifetime value in the hybrid sample indicates the efficient charge separation at the TiO2-GQD interface.
Thermogravimetric Analysis Study
In the case of the hybrid prepared under stirring (T16/GQDM), the faster weight loss characteristics indicate that the GQDs may be loosely attached to the TiO2 surface, perhaps physisorbed on the TiO2 surface due to insufficient energy to change local structure (e.g., eg oxygen vacancies) in TiO2 and negligible rearrangement in the functional groups in GQDs. On the other hand, in the case of the hybrid prepared by ultrasonication (T16/GQD), the slower and low weight loss characteristic indicates a strong bond between TiO2 and the GQDs through some changes in the local structure (e.g. functional groups rearrangement in GQDs .
Electron Paramagnetic Resonance Study
It is likely that the in-plane epoxy (C-O) functional groups in GQDs transfer to the oxygen-deficient sites in TiO2 and form a hybrid through the possible C-O-Ti bond. In addition, the EPR signal intensity of T16/GQD is reduced, probably due to the passivation of the defects in TiO2 by the GQDs.
Computational Study on the Optical Spectra of TiO 2 /GQD Hybrid
6.11(e) shows the concentration ratio (final to initial concentration, C/C0) of MB and MB in various catalyst samples. On the other hand, the degradation of MB in T16 is increased to 44%, while it is extremely high (97%) for the T16/GQD hybrid.
Visible light Photocatalytic Degradation Studies of MB
- Degradation and Reaction Kinetics
- Free Radical Detection and Degradation Mechanism
Summary & Conclusions
We introduced the concept of the rearrangement of oxygen functional groups in GQDs and the consequence of in-plane epoxy disorder in the TiO2-GQD hybrid formation, for the first time. From these studies, we identified each of the state-specific PL bands of GQDs, for the first time.
Summary and Conclusions
Summary and Highlights of the Thesis Contribution
In this thesis, we presented a systematic study of the controlled synthesis of GQDs, an understanding of the formation mechanism of GQDs and their heterostructures, and an understanding of the origin of visible PL emission from GQDs. GQDs synthesized from water, DMF and DMSO solvents show good biocompatibility on A-375 and HeLa cell lines.
Recommendations for Future work
We proposed a mechanism for the formation of GQD-TiO2 NPs heterojunction based on controlled experiments and density functional theory (DFT) based simulation results. The evidence for the existence of C-O-Ti bonding in the hybrid of GQD-TiO2 was investigated by the FTIR, XPS and Raman results.
