ABSTRACT Graphene quantum dots GQDs have emerged as promising nanomaterials due to their unique electronic, optical, and chemical properties, making them suitable for a range of applicat
Introduction to graphene quantum dots
Graphene and graphene oxide
Although the term "graphene" was introduced in 1987 by Mouras [1] to describe single sheets of graphite, it was not until 2004 that a groundbreaking experiment on graphene was conducted at the University of Manchester Specifically, Novoselov et al [2] were able to obtain monolayer graphene using Scotch tape This publication sparked a series of notable scientific research on 2D materials in general and graphene materials in particular In 2010, Geim and Novoselov were awarded the Nobel Prize for their research on graphene
Graphene exhibits a unique structure, composed of a single layer of carbon atoms arranged in a hexagonal lattice This structure allows graphene to adopt various configurations, including 3D graphite, 1D nanotubes, and 0D fullerenes Graphene's exceptional properties, including its exceptional electrical conductivity, mechanical strength, and thermal conductivity, are attributed to its π-conjugation Notably, graphene outperforms most materials in terms of current density and thermal conductivity, making it a promising material for various technological applications.
2630 m 2 /g, comparable to activated carbon but exceeding that of carbon black (less than 900 m 2 /g) and carbon nanotubes (CNTs, ranging from 100 to 1000 m 2 /g), graphene finds extensive applications in nanosystems and nanodevices [6]
Graphene oxide (GO), derived from graphite exfoliation, exhibits exceptional properties such as mechanical stability, high surface area, and tunable electrical and optical characteristics Its functionalized surface, with hydroxyl, carboxyl, and epoxy groups, enables interactions with other molecules and materials Due to its versatility, GO and its composites play pivotal roles in energy storage, environmental protection, and various applications like water splitting, water purification, and air pollutant removal.
Graphene quantum dots (GQDs)
Along with new publications on graphene, researchers gradually realized the limitations of graphene, including zero bandgap and low absorption ability Therefore, research related to the structural transformation of graphene has been carried out to compensate for its inherent limitations Among these, the successful synthesis of GQDs by Ponomarenko’s group in 2008 must be mentioned [14] Compared to carbon quantum dots (CQDs), which are typically quasi-spherical carbon nanoparticles smaller than 10 nm, GQDs possess the structure of the graphene network inside each dot and typically have sizes below 100 nm with less than 10 layers (in terms of thickness) [15] Kumar et al [16] reported that the size of these nanometer fragments of graphene typically ranged from 1.5 to 60 nm To be precise, ideal GQDs have only one layer and are made up of only carbon atoms However, in reality, synthesized GQDs still contain other elements, such as oxygen and hydrogen, and they usually have a few layers [17]
The unique structural characteristics of graphene quantum dots (GQDs) endow them with novel properties, such as unique fluorescence due to the quantum confinement effect Unlike carbon nanotubes (CNTs), GQDs possess enhanced solubility because of their edge effects, which can be modulated by functional groups The transition from two-dimensional (2D) graphene to zero-dimensional (0D) GQDs alters the electron distribution, resulting in a bandgap formation that ranges from 0 to 6 eV This transformation, from the zero-bandgap nature of graphene to semiconductor or insulator behavior, is attributed to changes in size or surface chemistry Given the significance of tailoring these properties for practical applications, various methods, including surface chemical modifications, have been explored to adjust the physical and chemical properties of GQDs.
Light absorption ability of GQDs
The optical characteristics of QGDs are crucial when these dots are used in applications such as solar cells, bioimaging, and photocatalysis GQDs’ optical properties are influenced by many factors like defects, doping, and the presence of surface functional groups GQDs exhibit strong light absorption in the ultraviolet region and can also absorb in the visible region Kim et al [26] reported that the size of GQDs significantly affects their ultraviolet-visible (UV-Vis) absorption spectrum due to the quantum confinement effect Accordingly, as the size of GQDs decreases, their absorption peak shifts toward shorter wavelengths, or "blue-shifts," as shown in Figure 1.1 The inset in that figure illustrates the relationship between the absorption peak energy and the average diameter of GQDs As the diameter of GQDs increases from 5 to 35 nm, the peak energy steadily decreases from approximately 6.2 to about 4.6 eV, approaching that of a single graphene sheet
Figure 1.1 UV-Vis absorption spectra of GQDs dispersed in water, featuring an average size of 12, 17, and 22 nm, alongside a graphene sheet for comparison The filled circles on the absorption spectra represent the positions of the absorption peaks for GQDs The inset graph presents the relationship between the absorption peak energy and the average size of GQDs According to Kim et al [26]
GQDs' UV-Vis absorption spectra vary based on synthesis methods due to distinct surface states Hydrothermal synthesis yields GQDs with a 320 nm UV absorption peak, while solvothermal synthesis using DMF solvent produces GQDs with absorption peaks at 227 nm Electrochemically exfoliated graphite GQDs exhibit a characteristic π-π٭ absorption peak at 227 nm Additionally, carbon fiber-derived GQDs show varying absorption spectra with increasing reaction temperatures (80, 100, and 120 °C).
"blue-shifts" in their UV absorption spectra (Figure 1.2) [29] The light emission of the three samples also shifted from yellow to blue hues under a 365 nm UV light as rising reaction temperature By adjusting alkali hydroxide concentration (0.15 – 0.4 M) in the electrolyte, Ahirwar et al [30] produced 4 GQD samples with the π-π٭ absorption peaks between 253 and 279 nm
Figure 1.2 UV−Vis spectra of GQDs A, B, and C, corresponding to synthesized reaction temperature at 120, 100, and 80°C, respectively The inset is a photograph of these GQDs under the excitation of 365 nm UV light According to Peng et al [29].
Photoluminescence (PL) properties of GQDs
The photoluminescence (PL) of graphene quantum dots (GQDs) is a key characteristic, influenced by quantum confinement effects PL emissions of GQDs span a wide spectrum (ultraviolet to red), and their size plays a crucial role in determining the band gap and PL behavior As GQD size increases, the energy band gap widens, causing emission shifts towards the red region Moreover, shape factor impacts PL emissions, as different shapes (circular, polygonal, etc.) and edge states (zigzag, armchair) can lead to size-dependent variations in visible PL emissions.
PL behaviors of GQDs when their sizes increase from 5 to 35 nm, although the corresponding absorption peak in UV-Vis spectra monotonically shifts to red light region
Figure 1.3 Illustrative scheme demonstrating the measured results and analysis of GQDs based on size and morphology According to Tian et al [31]
Nevertheless, it is essential to note that the PL properties of GQDs are not solely determined by their size and shape, as they are also influenced by external factors, including experimental conditions and the microenvironment For instance, Li et al [32] conducted a comprehensive investigation into the temperature's impact on PL properties at various excitation wavelengths They observed a significant reduction (over 50%) in the normalized PL intensity when excited at wavelengths of 310 nm,
340 nm, and 365 nm within a temperature range below 80 °C Interactions between GQDs and their surroundings play a crucial role in their PL spectra When GQDs were encapsulated within zeolitic imidazolate framework nanocrystals, their PL spectra showed a 32 nm redshift, primarily attributed to interactions between GQDs and zeolitic imidazolate framework nanocrystals [33] Furthermore, it has been documented that the pH of the solution can influence the fluorescent spectra of GQDs For instance, Fan et al electrochemically synthesized GQDs that exhibit a PL emission peak at 480 nm at pH 6.7, while this peak shifts to 440 nm at pH 6.8 [34] Yang et al also observed pH-dependent fluorescence in GQDs ranging from 2 to
5 nm [35], where an increase in pH correlated with an increase in GQDs' fluorescence intensity The underlying mechanisms behind these pH-dependent fluorescent properties of GQDs have been described elsewhere [36, 37]
The optical properties of GQDs hold significant importance in various applications, including biosensors, bioimaging, and photovoltaics To effectively utilize the PL properties of GQDs, it is not only their fluorescence that matters, but also their quantum yield, which signifies the fraction of molecules emitting photons upon direct excitation by a source The calculation of quantum yield with an indirect approach has been described in many works [38-40] Although each word presented a slightly different way to call physical quantities, the nature of the formula is unchanged Take the work of Tian et al as an example [38], the quantum yield of GQDs in water was measured by:
𝐺 𝑠 2 ) (1.1) where Y is the quantum yield, F refers to the integrated emission intensity (obtained from PL spectra), A refers to UV-Vis absorbance in the detective wavelength, and G refers to the refractive index of the solvent The subscript “u” means the sample, and
The reference standard (s) used for determining quantum yield (QY) is measured at the same wavelength as the excitation wavelength In this case, quinine sulfate in 0.1M sulfuric acid (YsW.7%) was chosen as the standard for an excitation wavelength of 350 nm To eliminate multiple absorption effects, the absorbance of both the graphene quantum dot (GQD) solution and the quinine sulfate standard was maintained below 0.1, with some studies preferring an even lower value (0.05).
Table 1.1 Demonstrating example of quantum yield calculation According to Tian et al
Band gap of GQDs
The optical characteristics of GQDs find their explanation in alterations to the electronic structure of these nanomaterials The inherent zero band gap of pristine graphene hinders its applications in most electronic devices However, the band gap of graphene can be adjusted through techniques such as introducing heteroatoms (doping), modifying size and altering surface chemistry Similarly, GQDs' band gap can be controlled by incorporating appropriate dopants and surface functional groups
The tunability of graphene quantum dots (GQDs) enables customization of their electrical properties This adaptability has been demonstrated by Feng et al., who altered the electrical properties of GQDs via selective boronization Moreover, Chen et al established a correlation between the size of GQDs and their band gap using computational methods Their findings revealed that as the size of GQDs increases, their band gap widens.
GQDs 1833250875 0.0917 1.33 15.2 decreases, showcasing the direct relationship between these two factors Tailoring GQDs' band gap has been explored not only theoretically but also experimentally Ye et al published an interesting work confirming the relationship between bandgap and size or molecular weight cutoff [43] They prepared 04 GQDs samples with the size distribution as depicted in Figure 1.4.a As can be seen from Figure 1.4.b, when the average GQDs size drops from 70 to 4.5 nm, these quantum dots solutions emit light across the majority of the visible spectrum from orange-red (∼1.9 eV) to green (∼2.4 eV) regions under a 365 nm UV light At the same time, the emission peak shows
"blue-shifts" from ∼620 to ∼520 nm, which is in accordance with the quantum confinement effect
Figure 1.4 (a) Chart of size distributions of 4 GQDs samples determined by TEM (b) The plots depicting band gap enlargement with the shrink of the size of GQDs; the inset shows the PL colors corresponding to the size of GQDs According to Ye et al [43]
In a separate study, Mandal and co-workers theoretically investigated the morphology-dependent band gap of GQDs [44] They introduced 4 distinct types of GQDs with optimized structures obtained by the conjugate gradient algorithm, as presented in Figure 1.5 Their analysis revealed a shift in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to higher and lower energy levels, respectively, when the dot sizes grow In other words, increasing the number of carbon atoms in all 4 types of GQDs leads to the reduction of HOMO–LUMO gap The author’s group also delved into the impact of organic functional groups on GQDs' band gap and found that different functional groups (either electron-withdrawing or electron-donating) have an obvious effect on the HOMO, LUMO levels, but no significant alteration in the HOMO–LUMO gap is observed Numerous authors have conducted in-depth examinations of the electronic properties of GQDs, as detailed in previous publications [45-48]
Figure 1.5 Optimized structure of the smallest unit of four GQDs types (left) The graph demonstrating the energy gap as a function of the total number of conjugated carbon atoms corresponding to 4 types of morphologies (right) According to Mandal et al [44].
Applications of GQDs
Bioimaging and biosensing applications
Graphene quantum dots (GQDs) have attracted significant research attention due to their exceptional optical, electrical, and structural properties Unlike heavy metal-based quantum dots, GQDs are environmentally friendly and exhibit low toxicity in vitro and in vivo, as demonstrated by their rapid clearance from major organs like kidneys Experimental and simulation studies suggest that the size of GQDs plays a crucial role in their toxicity, with smaller dots showing low cytotoxicity and larger dots rupturing lipid membranes.
Due to their exceptional characteristics, GQDs emerge as promising candidates for various applications, including sensor materials, bioimaging tools, and drug delivery systems Furthermore, they are widely employed as fluorescent probes for a variety of analytes due to their remarkable attributes, which include resistance to photobleaching, low cytotoxicity, strong biocompatibility, stable PL, and high solubility in aqueous environment [82] Notably, the most intriguing aspect of GQDs is their PL, which can be customized by managing factors such as size, surface modifications, and chemical doping [83] The adaptable and robust PL, coupled with the confirmed eco-friendly and low toxicity profile, positions GQDs as promising candidates for bioimaging field Moreover, surface functionalization enhances the biocompatibility of GQDs, representing a burgeoning area of research with diverse applications in the biomedical field [84, 85] Additionally, edge functionalization of GQDs with functional groups like carboxyl, carbonyl, epoxy, and hydroxyl, enables interactions with various biological molecules such as proteins, antibodies, and enzymes [86] Researchers have reported that graphene surface of GQDs can covalently link and conjugate with antibodies, proteins, nucleic acids and polymers via π−π interaction, facilitating highly selective and sensitive biosensors capable of identifying biomarkers associated with numerous types of cancers [87, 88].
Drug delivery
As mentioned above, GQDs have found applications in biomedicine and pharmaceuticals, such as photothermal therapy, biosensors, drug delivery, and molecular imaging They are emerging as strong contenders in the quest to create efficient drug delivery systems, including theragnostic agents, because GQDs possess a range of desirable qualities, such as low cytotoxicity, an abundance of peripheral - COOH groups, and stable PL properties under various chemical and thermal conditions However, an inherent challenge of GQDs applications is their hydrophobic nature induced by carbon structure In order to address this hydrophobicity, various strategies have been explored to mitigate hydrophobic surface interactions and enhance GQDs' biocompatibility These strategies include chemical PEGylation, oxidation, reducing the size of the QDs, and so on [16]
Figure 1.7 Proposed explanation for the roles of DOX/GQD conjugates in DOX delivery According to Wang et al [89]
GQDs have demonstrated promising applications in drug delivery, as evidenced by the work of Wang et al [89] They successfully utilized GQDs to deliver the anticancer drug doxorubicin (DOX) and enhance its effectiveness against breast cancer cells The GQDs-DOX conjugate showcased distinct cellular internalization pathways, resulting in improved drug delivery efficiency Notably, these conjugates exhibited increased nuclear uptake and cytotoxicity of DOX in drug-resistant cancer cells, suggesting the potential of GQDs to improve chemotherapeutic efficacy, even in cases of resistance.
Figure 1.8 Scheme showing the synthesis of DOX-loaded HER-labeled GQD-based nanocarriers and its drug release via cellular uptake for active targeting of breast cancer cells According to Ko et al [90]
In a related study, Ko et al [90] cleverly designed GQDs-based nanocarriers for breast cancer diagnosis and treatment, and labeled them with Herceptin (HER) and β-cyclodextrin (β-CD) Within these nanovesicles, HER served as an active targeting agent for HER2-overexpressed breast cancer, β-CD provided a hydrophobic site for loading DOX via host-guest chemistry, and GQDs functioned as diagnostic agents due to their pH-sensitive blue emission in the acidic environment of cancer cells
(Figure 1.8) These GQDs-based multifunctional drug delivery carriers resulted in an enhanced anticancer approach with both diagnostic and treatment capabilities Additionally, Dong et al [91] designed DOX-loaded amino acid (AA)-conjugated
GQDs with stable fluorescence properties and utilized them for dye-free imaging, as well as for tracking and monitoring drug delivery The inherent fluorescence properties of GQDs allowed for efficient real-time monitoring of the cellular uptake of DOX-GQDs-AA nanocarriers and subsequent drug release Furthermore, the nano- assemblies exhibited pH-dependent drug release characteristics.
Photocatalysis
Photocatalysis is a process in which the chemical reaction speed is significantly enhanced thanks to the presence of a catalyst under light exposure This field of science holds great significance for a sustainable future, particularly in the context of transitioning to renewable energy sources as alternatives to traditional fossil fuels However, the primary challenge of widely employing photocatalysis is the requirement of a catalyst capable of efficiently harnessing sunlight to drive enhanced chemical reactions A secondary challenge is to produce an efficient photocatalyst through a cost-effective and large-scale production process The effectiveness of a material for catalytic applications is determined by key factors such as its electronic structure, morphology, and atomic arrangement [16]
GQDs are considered promising for photocatalysis and electrocatalysis applications since they possess distinctive qualities, including high stability, solubility, an enlarged surface area, good conductivity, and non-toxicity Furthermore, GQDs offer the ability to adjust their band gap by varying particle size and introducing heteroatom dopants, chemical groups with defects, and edge configurations As for photocatalysis, the introduction of heteroatoms like N, P, and
S allows researchers to modify the solar-absorption properties of GQDs, optimizing their performance GQDs, either alone or in conjunction with other inorganic materials, have been employed in various photocatalytic reactions, including H2 generation [92-94], organic pollutants degradation [95-101], CO2 reduction [102, 103], and more
Yeh and colleagues [104] successfully developed GQDs with a narrow band gap, thereby improving their ability to absorb visible light, which in turn enhances their capacity for generating hydrogen and reducing carbon dioxide The group achieved this via chemical modifications to the GQDs The presence of electron- withdrawing groups, such as -COOH, and electron-donating groups, such as -NH2, results in the GQDs with p-type and n-type conductivity, respectively The author group explained the noticeable efficiency of producing H2 and reducing CO2 by the efficient p-n type intramolecular photochemical reaction, resulting from ohmic contact between n-type and p-type GQDs
Lei et al [105] exemplified the utilization of GQD composites in the case of
CdS/GQDs nanohybrid, which has proven to be a highly effective, strongly coupled photocatalyst with notable photocatalytic capabilities for hydrogen production Compared to CdS alone, the CdS/GQDs nanohybrid composites, containing 1.0 wt % of GQDs, displayed a 2.7-fold increase in photocatalytic efficiency This enhanced performance was supposed to originate from the augmented light absorption facilitated by GQDs Besides this work, several research groups have also employed the remarkable ability of GQDs to significantly enhance the response to visible light in photocatalytic processes Many GQDs-based composites with materials like
Bi2MoO6 [106], ZnO [107], and TiO2 [97, 101] have been synthesized and confirmed that they displayed superior photocatalytic activity compared to the bare oxides
Research teams have explored graphene quantum dots (GQDs)-based composites with organic polymers or organic/inorganic polymers for diverse photocatalytic purposes For instance, Fan et al created a GQDs-polyvinyl pyrrolidone-CdS nanocomposite that effectively degraded methyl orange with enhanced activity due to GQDs' upconversion and charge transfer between CdS and GQDs Additionally, GQDs modified with polymers have been engineered to degrade methylene blue under visible light using polyethylenimine and polyethylene glycol These studies demonstrate that the type of polymer influences the absorption properties of GQDs/polymer composites, making them applicable for the degradation of various organic compounds.
Energy harvesting
Utilizing GQDs presents a straightforward and efficient method to significantly enhance the power conversion efficiencies of various solar cells GQDs can serve as selective carrier window layers in solar cells, and when incorporated into traditional window layers, they can enhance their performance GQDs can contribute to increased light absorption within the active layer/material of solar cells and simultaneously improve hole extraction efficiency Importantly, by engineering the functional groups, the band gap, light transmission/absorption, and electrical properties of GQDs can be tailored over a broad range to meet specific performance requirements [16] In practice, the usage of GQDs has led to performance enhancements in various types of solar cells, including organic photovoltaics (OPVs) [111, 112], dye-sensitized solar cells (DSSCs) [113, 114], perovskite solar cells (PSCs) [115, 116], silicon solar cells (SSCs) [117, 118], and other innovative heterojunction solar cell designs [119, 120]
Lately, nanogenerators have emerged as a means to transform mechanical energy from various sources, such as vibrations, ultrasonic waves, air/liquid pressures, and sound waves, into electricity These nanogenerators find application in diverse fields, including energy generation, transducers, sensors, and actuators Flexible nanogenerators capable of scavenging different forms of mechanical energy show tremendous promise for powering nanosystems and low-power portable devices Efforts to create transparent and flexible nanogenerators have led to the use of carbon-based materials like graphene, CNTs, and GQDs as fillers or conducting substrates [121-123] A notable work by Lu’s group [123] introduced a straightforward method for constructing a piezoelectric nanogenerator using GQDs and poly(vinylidene fluoride) (PVDF) They achieved a self-powered piezoelectric composite that efficiently converts mechanical, vibrational, and hydraulic energy into electricity without the need for electrical poling The findings clearly indicate that both the voltage and current increased almost proportionally as the GQD loadings were raised This underscores the GQDs' remarkable capacity to finely adjust the electrical output of GQD/PVDF composites Notably, a GQD/PVDF sample with a
Utilizing 3 wt% graphene quantum dots (GQD) in the PVDF matrix significantly enhanced the photovoltaic performance of the composite Notably, the open-circuit voltage reached 0.75 V, while the short-circuit current increased to 25.7 nA, both over four times higher than the values obtained using pure PVDF.
In addition to piezoelectric nanogenerators, a new category of mechanical energy harvesting devices, triboelectric nanogenerators, has emerged based on the triboelectric and electrostatic effects These devices convert electric energy through contact or rubbing between two materials with different triboelectric polarities [124, 125] In these devices, nanoparticles are often distributed on materials’ surface (metal or polymer) to strengthen the contact surface area and energy conversion process Typically, Xu et al fabricated a triboelectric electronic skin system based on GQDs, towards the use in self-powered, smart artificial fingers [126] Their design created micro-gaps and rendered the electronic skin highly responsive to a range of mechanical stimuli, such as pressing, stretching, folding, and twisting To achieve this, they employed Ag nanowires coated with GQDs, serving as the electrode for the triboelectric nanogenerator and the friction layer The resulting electronic skin, which is transparent and lightweight, has an ultra-lightweight quality with an areal density of around 130 g/m 2 , rendering it satisfactory for portable and wearable electronic gadgets When subjected to a pressure of 10 N from the electronic skin, the GQDs- based nanogenerator yielded an output short-circuit current density of around 10 mA/cm², nearly 20 times higher than that of the reference device without GQDs It was also revealed that the output performance showed enhancement with an increase in load and GQDs content.
Synthesis strategies of GQDs
Basically, there are two main approaches for obtaining GQDs, as illustrated in Figure 1.9 True to their names, top-down methods involve breaking down carbon materials, including bulk forms, graphene, fullerenes, and CNTs, into nano-sized GQDs using various tools such as oxidative cleavage, hydrothermal or solvothermal processes, electrochemical oxidation, ultrasonic-assisted or microwave-assisted processes, chemical vapor deposition (CVD), and laser ablation [127] This approach has the advantage of using abundant raw materials and often results in oxygen- containing functional groups at the edges of GQDs, increasing their solubility and functionality However, some limitations of the top-down approach include low yield, high defect density, and difficulty controlling the size and shape of the products
Figure 1.9 Raw materials and two major approaches for producing GQDs [31]
In contrast, the bottom-up approach, sometimes referred to as carbonization, utilizes small molecules or suitable polymers as precursors Through processes like dehydration and carbonization, GQDs are formed and tend to have fewer defects compared to the top-down approach However, they often have poor solubility, low uniformity (high polydispersity), and may encounter aggregation issues with small- sized dots [31]
Tables 1.2 and 1.3 were extracted from a detailed review of Kumar et al [16] to briefly summarize typical publications on top-down and bottom-up methods
Table 1.2 Overview of top-down synthesis methods for GQDs and their applications According to Kumar et al [16]
GO 2–5 23.8 Sensing of metal ions and bio- imaging
Hydrothermal GO 5–13 6.9 Optoelectronics and biolabeling
Hydrothermal GO 5–19 7.4 Bioscience and energy
Electrochemical MWCNTs 3–8.2 5.1–6.3 Biomarkers and nanoelectronic devices
Electrochemical Graphite rod 5–10 14 Stem cell labeling [50]
Electrochemical Graphite rod ∼20 18.95 Detection of
Table 1.3 Overview of bottom-up synthesis routes for GQDs and their applications According to Kumar et al [16]
Pyrolysis Citric acid ∼15 9.0 Photovoltaic devices [140]
Pyrolysis L-glutamic acid ∼4.66 54.5 In vivo and in vitro cell imaging and detection of H2O2
Pyrolysis Citric acid ∼12.7 6.91 Bioimaging and biosensing
Pyrolysis Citric acid ∼2.0 62.8 In vitro cellular imaging
Doping heteroatoms into GQDs
Reasons for using heteroatoms to modify GQDs
Besides the mentioned advantages, GQDs typically have several limitations for practical applications, and functionalization is one of the directions to modify this type of material for specialized applications Specific methods include doping with heteroatoms, creating composites with inorganic compounds or polymers, as well as controlling the size and shape of quantum dots Modifying the optical, electrical, and chemical properties of the material is expected to address some of the challenges GQDs currently face Therefore, the functionalization of GQDs has always been considered an attractive topic in the field of carbon nanomaterials in recent years [31]
In fact, the enhancement of catalytic performance in oxidation-reduction reactions, which is crucial for energy storage and conversion applications, has been achieved by incorporating heteroatoms into the structures of GQDs Introducing heteroatoms, such as boron, nitrogen, oxygen, phosphorous, and sulfur, into the carbon lattice leads to significant disorientation of the electron network, which is otherwise uniformly conjugated This disruption plays a crucial role in controlling the surface characteristics by altering the charge distribution and the configuration of doped regions [146].
Impacts of doping nitrogen into GQDs
The doping of nitrogen atoms into the crystal lattice of graphene quantum dots (GQDs) affects the charge and spin distribution of electrons within the doped domains This alteration modifies the properties of the GQDs due to the electronegativity (3.04) and atomic radius (0.092 nm) of nitrogen, which is similar to that of carbon.
(0.0914 nm), nitrogen atoms easily integrate into the graphene lattice by substituting carbon atoms [148] The result is the fundamental transformation of the electronic states of the material and a shift towards specific reactivity and catalysis [147, 149, 150]
There are three main types of nitrogen-doped GQDs (N-GQDs) bonds, including graphitic N (nitrogen atoms replacing C in the hexagonal ring), pyrollic N (sp 3 hybridization, N at the outer edge of the dot), and pyridinic (N also at the outer edge of the dot but sp 2 hybridization) Illustrations for these bonding types are shown in Figure 1.10 (adapted from [151]).
The photodegradation of methylene blue (MB)
General information about MB
Methylene blue (MB) is an aromatic heterocyclic basic dye with a molecular weight of 319.85 g/mol It has the formula C16H18N3ClS and forms a blue aqueous solution MB is a solid, odorless, dark green powder that is highly water-soluble and stable at room temperature With a pKa of 3.8, it is a cationic thiazine dye with a peak absorption wavelength (λmax) at 663-664 nm and a melting point (Tm) of 100–110 °C.
Nitrogen-graphene quantum dots (N-GQDs) exhibit diverse bonding types of nitrogen atoms, contributing to their remarkable properties N-GQDs hold significant potential for applications in textiles, pharmaceuticals, food, and the production of paper, dyes, paints, and medicine Notably, their widespread use in the textile industry as a dominant dye showcases their versatility and value in coloring fabrics.
Figure 1.11 The model and the structure of MB molecule [153]
Nevertheless, textile industries typically discharge significant quantities of MB dyes into natural water sources, posing health risks to both humans and microorganisms [163] MB dye, when present at a certain concentration, is known to be hazardous to human health due to its considerable toxicity [164, 165] MB is characterized as toxic, carcinogenic, non-biodegradable, and poses a substantial threat to human health and the environment [159] Exposure to MB can result in various health risks for humans, including respiratory and abdominal disorders, blindness, gastrointestinal complications, and neurological disorders It can also lead to a wide range of adverse symptoms such as nausea, diarrhea, vomiting, cyanosis, shock, gastritis, jaundice, methemoglobinemia, and increased heart rate In addition,
MB was reported to cause the premature death of cells in tissues as well as skin and eye irritations [165-168].
Photocatalysis for decomposing MB
Photodegradation refers to the breakdown of a photodegradable substance into simpler and smaller fragments due to the absorption of photons, particularly those within sunlight, including infrared, visible light, and ultraviolet rays [169] Oxidation is a prevalent photodegradation reaction When the process involves a photocatalyst, typically a semiconductor material, the photocatalyst is activated by the absorption of photons and is capable of accelerating a reaction without being consumed [170] This technology shows promise for treating waste effluents by efficiently decolorizing and breaking down dye molecules into harmless inorganic species like
CO2 and H2O [171] Compared with other solutions, this approach is cost-effective, environmentally friendly, and straightforward for treating wastewater containing hazardous pollutants [172-174] A crucial issue in this context is the oxidation of MB into H2O and CO2 using a photocatalyst, which plays a pivotal role in eliminating dyes from industrial wastewater [175]
Methylene blue (MB) is a renowned organic dye known for its stability under visible light exposure However, relying solely on photolysis or catalysis proves inadequate for efficient MB degradation due to its inherent resilience Studies have shown that photolysis alone, even after prolonged exposure of 10 hours, results in a mere 7.9% degradation of the dye.
MB degraded after 24 hours in the presence of a catalyst without light exposure [178] Additionally, no or minimal decomposition was observed without a catalyst under visible light [179, 180] Similarly, no degradation occurred in acidic and neutral environments in the absence of light, as well as under sunlight exposure without employing a catalyst [181] In a basic medium, photolysis rate is rapid due to the formation of hydroxyl ions, a key radical for dye degradation Nevertheless, increasing the temperature only has a negligible effect, and under an argon atmosphere, degradation completely ceases [181] The efficient photodegradation of
MB is attributed to its potential role as a photocatalyst sensitizer [182] The limited or partial degradation of MB without catalysts may be result from the photosensitized phenomenon of MB molecules which can observed after exposure to various light sources [183, 184].
Research status and urgency of research
Research status
Ghosh et al [185] utilized the Scopus database to obtain interesting statistics about publications related to CQDs and GQDs As shown in Figure 1.12.a, the number of publications grew constantly from 2000 to 2019 Although this value dropped to less than 1500 in 2020, this decline may originate from the adverse effects of COVID-19 on the research community The pattern for the total citations during an 11-year period from 2010 is similar, with a peak of more than 84000 citations in
In 2019, CQDs and GQDs gained significant research interest, as evidenced by the high number of publications (Figure 1.12.b) The primary application sectors for these materials are chemical science and material science, accounting for a combined 42.4% of publications (Figure 1.12.c).
Figure 1.12 The overall statistics (based on the Scopus database) for CQD/GQD-based research in terms of (a) total number of publications (2000-2020), (b) citations of these published works (2010-2020), and (c) application fields for CQDs/GQDs (July 26, 2020)
Also relying on the Scopus database (26th Aug 2022), Wijayaratne et al [186] reported the chart (Figure 1.13) about the number of publications in the period 2001-
2021 containing the word “graphene quantum dots” in the title, abstract, or keywords There was nearly no work satisfying this constraint in 2001, but more than 1500 publications met this condition in 2020
Figure 1.13 The number of publications involving the word “graphene quantum dots” in the title, abstract, or keywords (from 2000 to 2021) [186] Source: Scopus database
As for doping nitrogen, two main challenges in the doping process are accurately controlling the amount of nitrogen and the type of bonds formed in N-GQDs because these two factors directly affect the material's reactivity, catalysis after doping, or the phenomenon of up-conversion PL [151] Therefore, many recent studies on N-GQDs focus on addressing these two issues with various approaches, depending on the material synthesis method.
Urgency of research
Until now, the academic community believes that research on GQDs is still in its early stages, with many challenges that must be overcome Among these, critical issues that need improvement include synthesis efficiency (product yield), quantum yield efficiency, precise control of properties (structure, size, etc.), unclear PL mechanisms, and narrow emission spectra [31]
To industrialize GQDs, they need to be mass-produced at a relatively low cost, but the product yield with many current methods is quite low, often below 10% [187] Therefore, new methods such as photo-Fenton reactions (with the highest efficiency of up to 45%) [187] and simple processes from coal [188] are expected to improve efficiency and gradually meet industrial requirements Additionally, the electrical and optical properties of GQDs, including bandgap structure and fluorescence, also need further enhancement For example, the reported quantum yield is typically in the range of 2% to 22.9% [187], much lower than traditional semiconductors' corresponding values Since these properties are directly related to the size, shape, and chemical composition of GQDs [189, 190], solutions are proposed similarly to those in Section 1.4 Furthermore, precise control of the size and shape of GQDs is also crucial, and adjusting the synthesis process parameters is considered a key solution for this
Understanding the photoluminescence (PL) mechanisms of graphene quantum dots (GQD) is crucial, but its complexity has led to inconsistent results Current explanations based on optical properties can be unreliable across varying sample conditions GQDs typically emit light in the blue-yellow range, which limits their applications in optoelectronic devices Researchers are exploring methods to expand the emission range, such as nitrogen doping, polyethyleneimine coating, and controlling citric acid carbonization These approaches aim to achieve broader PL spectra extending into the near-infrared region.
Based on the hurdles and solutions above, the thesis " Study on synthesis of nitrogen-doped graphene quantum dots and their optical properties" is essential as it can contribute to the advances in insights of GQDs.
Objectives and Contents
Objectives
In general, this thesis aimed to successfully synthesize N-GQDs with controlled input conditions and study their optical behaviors The basic targets of this research are listed below
Proposing a solvothermal process to produce N-GQDs and confirming its viability under adjusted conditions
Comparing and drawing evaluations about the correlation between reaction parameters and the characteristics of the synthesized materials
Surveying the catalysis of MB photodegradation under the presence of GQDs.
Research contents
Preparing GO suspension which is used as a precursor to N-GQDs synthesis
Preparing N-GQD suspensions according to the proposed method with an increment of reaction time or temperature
Replicating such a process for reference samples where DMF is substituted by double distilled water (DDW)
Analyzing the GQDs samples to gather information on their structure, functional groups, and optical behaviors
Exposing MB solution with the addition of each GQD suspension under a
400 nm LED panel and studying the decline in MB concentration over time.
Approaches for material synthesis
Employing a modified Hummers' method, graphene oxide (GO) was synthesized from graphite The specific procedures and conditions were adapted from prior studies [193, 194] with minor modifications to align with laboratory conditions.
On the other hand, the hydrothermal method is chosen for synthesizing GQDs because it is considered simple and cost-effective It can be scaled up in the future
Furthermore, the selected precursors are common and have high purity in commercial products.
Novelty and major contributions
Recent research has explored diverse methods for GQDs synthesis using GO Solvent-based approaches involving GO dispersion in dimethylformamide (DMF) with solvothermal processing have gained popularity Additionally, hydrogen peroxide addition to aqueous GO suspensions or coal tar has been employed to enhance graphene sheet cutting Notably, a novel approach combining GO suspension, hydrogen peroxide solution, and DMF has emerged.
This study explores the synthesis of graphene quantum dots (GQDs) using a specific combination of graphene oxide suspension, dimethylformamide, and hydrogen peroxide By varying the hydrothermal reaction parameters (time and temperature), the researchers aimed to establish correlations between synthesis conditions and the resulting GQD characteristics, including optical properties The objective is to gain insights into the influence of these parameters on the properties of the synthesized GQDs, fostering a deeper understanding of their behavior and facilitating the development of specialized applications that require tailored GQD properties.
Chemicals
Chemicals employed in this study were obtained from renowned suppliers: graphite (