Capping nanoparticles with graphene quantum dots for enhanced thermoelectric performance

Graphene-based thermoelectric materials: toward sustainable energy-harvesting systems

Among sustainable energy-harvesting systems, thermoelectric technology has attracted considerable attention because of its ability to directly convert heat into electricity and diverse applications. Graphene, with its exceptional electrical conductivity and mechanical properties, is a promising candidate for thermoelectric materials. However, efficient thermoelectric applications require materials with a high Seebeck coefficient and low thermal conductivity-criteria that graphene does not inherently satisfy, owing to its gapless energy band structure and ballistic thermal conduction. This review examines the thermoelectric properties of graphene, optimization strategies, and the potential of graphene hybridization for thermoelectric applications. To overcome the intrinsic limitations of graphene for thermoelectric utilization, nanostructuring strategies based on its synthesis methods are discussed. Furthermore, strategies for graphene hybridization are introduced, with a focus on maximizing thermoelectric efficiency through interactions with nanostructured materials of various dimensions. Finally, the potential of graphene-based thermoelectric materials and future research directions are discussed.

In Situ Synthesis of a Bi2Te3-Nanosheet/Reduced-Graphene-Oxide Nanocomposite for Non-Enzymatic Electrochemical Dopamine Sensing

Dopamine is a neurotransmitter that helps cells to transmit pulsed chemicals. Therefore, dopamine detection is crucial from the viewpoint of human health. Dopamine determination is typically achieved via chromatography, fluorescence, electrochemiluminescence, colorimetry, and enzyme-linked methods. However, most of these methods employ specific biological enzymes or involve complex detection processes. Therefore, non-enzymatic electrochemical sensors are attracting attention owing to their high sensitivity, speed, and simplicity. In this study, a simple one-step fabrication of a Bi₂Te₃-nanosheet/reduced-graphene-oxide (BT/rGO) nanocomposite was achieved using a hydrothermal method to modify electrodes for electrochemical dopamine detection. The combination of the BT nanosheets with the rGO surface was investigated by X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Fourier-transform infrared spectroscopy. Electrochemical impedance spectroscopy, cyclic voltammetry, and differential pulse voltammetry were performed to analyze the electrochemical-dopamine-detection characteristics of the BT/rGO nanocomposite. The BT/rGO-modified electrode exhibited higher catalytic activity for electrocatalytic oxidation of 100 µM dopamine (94.91 µA, 0.24 V) than that of the BT-modified (4.55 µA, 0.26 V), rGO-modified (13.24 µA, 0.23 V), and bare glassy carbon electrode (2.86 µA, 0.35 V); this was attributed to the synergistic effect of the electron transfer promoted by the highly conductive rGO and the large specific surface area/high charge-carrier mobility of the two-dimensional BT nanosheets. The BT/rGO-modified electrode showed a detection limit of 0.06 µM for dopamine in a linear range of 10–1000 µM. Additionally, it exhibited satisfactory reproducibility, stability, selectivity, and acceptable recovery in real samples.

Two-dimensional heterostructures built from ultrathin CeO2 nanosheet surface-coordinated and confined metal-organic frameworks with enhanced stability and catalytic performance

Two-dimensional (2D) metal-organic framework (MOF) based heterostructures will be greatly advantageous to enhance catalytic performance because they increase the contact surface and charge transfer. Herein, a novel 2D heterostructure named CeO2@NiFe-MOFs, in which monolayer NiFe-MOFs is coordinated with ceria (CeO2) to improve catalytic and stability performance, is successfully constructed by the strategy of in situ growth on the surface of ultrathin CeO2 nanosheets being functionalized with monolayer carboxylic acid groups. The 2D heterostructure possesses a sandwich structure, where monolayer NiFe-MOFs are coordinated to both the top and bottom surface of CeO2 nanosheets via joining carboxylic acid groups. In particular, CeO2 with robust coordination plays a significant role in the anchoring of carboxylic acid groups and binding strength of heterostructures. The 2D CeO2@NiFe-MOF heterostructure with a joint effect of metal-ligand coordination not only presents good structural stability but also significantly enhances the oxygen evolution reaction (OER) efficiencies in comparison to bare NiFe-MOFs, achieving a current density of 20 mA cm-2 at a low overpotential of 248 mV as well as durability for at least 40 h. Meanwhile, the electronics, optics, band gap energy and local strains of CeO2 decorated with 2D NiFe-MOFs are different to the properties of bare CeO2. Our study on the construction of an ultrathin CeO2 surface-coordinated and confined MOF layer may pave a new way for novel 2D MOF composites/heterostructures or multi-functional 2D CeO2 materials to be used in energy conversion or other fields.

Demonstration of Controlled Hydrogen Release Using Rh@GQDs during Hydrolysis of NH3BH3

Achieving the controlled release of H₂ through an effective approach still faces many challenges. Herein, high-quality graphene quantum dots (GQDs) are synthesized from a new precursor, 1,2,4-trihydroxy benzene, and a multifunctional platform of Rh@GQDs is further developed for the controlled H₂ evolution upon the hydrolysis of NH₃BH₃ (AB). More importantly, the designing concepts of multistep and stepless speed controls have been introduced in the domains of both H₂ evolution for the first time. Through a novel designing protocol, the rate of H₂ evolution can be freely regulated and constantly varied on demand by means of chelation between Zn²⁺ and ethylene diamine tetraacetic acid (EDTA). The density functional theory calculation indicates that Zn²⁺ has the priority to be adsorbed onto Rh(100) due to its larger adsorption energy (107.98 kcal·mol^-1) than that of AB (36.36 kcal·mol^-1). A controlling mechanism is presented such that Zn²⁺ will cover the active sites of the nanocatalyst to prevent the H₂ evolution, and EDTA can chelate Zn²⁺ to reactivate the nanocatalyst for the production of H₂, greatly facilitating use of this strategy in other catalytic reactions. Moreover, it is demonstrated that the protocol is equally valid for diverse hydrogen storage materials. Therefore, this work not only establishes whole new concepts for the controlled production of H₂ but also explains their mechanism, thus remarkably advancing the utilization of H₂ energy and significantly enlightening the controlled process of catalysis.

Photoluminescent Carbon Quantum Dots: Synthetic Approaches and Photophysical Properties

A number of synthetic methodologies and applications of carbon quantum dots (CQDs) have been reported since they were first discovered nearly two decades ago. Unlike metal-based or semiconductor-based (e. g., metal chalcogenides) quantum dots (MSQDs), CQDs have the unique feature of being prepared through a variety of synthetic protocols, which are typically understood from considerations of reaction models and photoluminescence mechanisms. Consequently, this brief review article describes quantum dots, in general, and CQDs, in particular, from various viewpoints: (i) their definition, (ii) their photophysical properties, and (iii) the superiority of CQDs over MSQDs. Where possible, comparisons are made between CQDs and MSQDs. First, however, the review begins with a general brief description of quantum dots (QDs) as nanomaterials (sizes≤10 nm), followed by a short description of MSQDs and CQDs. Described subsequently are the various top-down and bottom-up approaches to synthesize CQDs followed by their distinctive photophysical properties (emission spectra; quantum yields, Φs).

Nanocomposite of CuInS/ZnS and Nitrogen-Doped Graphene Quantum Dots for Cholesterol Sensing

In this paper, nitrogen graphene quantum dots (N-GQDs) and copper indium sulfide/zinc sulfide (CIS/ZnS) QDs were synthesized via facile hydrothermal and aqueous solution routes, respectively. Herein, a fluorescent nanocomposite has been synthesized between N-GQDs and CIS/ZnS QDs in an aqueous phase. This nanocomposite was characterized by photoluminescence, Raman, and ultraviolet-visible (UV-vis) spectroscopies, high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). This fluorescent nanocomposite was developed as a highly sensitive, selective nonenzymatic cholesterol optical biosensor in 0.312-5 mM cholesterol. HRTEM micrographs confirmed the preparation of CIS/ZnS QDs and N-GQDs with average diameters of 3 and 5 nm, respectively. The as-prepared NG/CIS/ZnS QD nanocomposite had a high sensitivity for cholesterol with a wide linear range of concentration of 0.312-5 mM with an excellent correlation coefficient (R 2) of 0.9688 and limit of detection (LOD) of 0.222 mM.

Stimulus Response of GQD-Sensitized Tb/GMP ICP Nanoparticles with Dual-Responsive Ratiometric Fluorescence: Toward Point-of-Use Analysis of Acetylcholinesterase and Organophosphorus Pesticide Poisoning with Acetylcholinesterase as a Biomarker

In this study, by rationally designing the stimulus response of graphene quantum dot (GQD)-sensitized terbium/guanine monophosphate (Tb/GMP) infinite coordination polymer (ICP) nanoparticles, we have constructed a smartphone-based colorimetric assay with ratiometric fluorescence, which could be applied for the detection of acetylcholinesterase (AChE) and organophosphorus pesticides (OPs) directly. First, GQDs with abundant functional groups were chosen as the guest, which not only could be used as one of the signal readouts but also served as the antenna ligand to further sensitize the fluorescence of the host Tb/GMP. Upon being excited at 330 nm, the green fluorescence of the Tb/GMP host is highly enhanced, while the blue fluorescence of GQDs is suppressed due to the confinement of the ICP host. With the presence of thiocholine (TCh), an enzymatic product hydrolyzed from acetylthiocholine (ATCh) by AChE, the competitive coordination of Tb³⁺ between GMP and TCh results in the collapse of the ICP network and thereby the release of GQDs into the solution; thus, the fluorescence of Tb/GMP turns off and the fluorescence of GQDs turns on. The dual-responsive ratiometric fluorescent intensity change leads to the corresponding green-to-blue fluorescent color change obviously, which constitutes a novel mechanism for the colorimetric analysis of AChE. Moreover, when OPs are subsequently introduced, the activity of AChE is blocked, thus preventing the stimulus response of GQD@Tb/GMP ICP nanoparticles, leading to the fluorescent color change from greenish-blue to green, which could also be employed for OP detection. Benefitting from the high sensitivity, good reliability, and the obvious color changes, the method demonstrated here is a promising candidate to realize smartphone-based point-of-use applications, which is of great importance for timely clinical diagnosis and treatment of OPs related to health issues with AChE as an exposure biomarker in less industrialized countries, in remote settings, or even in home care services.

Enhanced Thermoelectric Properties of Bilayer-Like Structural Graphene Quantum Dots/Single-Walled Carbon Nanotubes Hybrids

In order to improve the thermoelectric properties of single-walled carbon nanotubes (SWCNTs), bilayer-like structures of graphene quantum dots (GQDs) and SWCNTs films (b-GQDs/SWCNTs) were prepared by directly coating GQDs on the surface of SWCNTs films. Compared to pristine SWCNT films (p-SWCNTs), the electrical conductivity of b-GQDs/SWCNTs increased while their Seebeck coefficient decreased. The special interface structure of GQDs and SWCNTs can not only improve carrier transport to increase electrical conductivity but also scatter phonons to reduce thermal conductivity. A maximum power factor (PF) of 51.2 μW·m^-1·K^-2 is obtained at 298 K for the b-GQDs/SWCNTs (2:100), which is higher than the PF of 40.9 μW·m^-1·K^-2 by p-SWCNTs. Incorporation of GQDs shows an obvious improvement in power factor and a significant reduction in the thermal conductivity for SWCNTs, and thus, preparation of b-GQDs/SWCNTs provides a new strategy to enhance the thermoelectric properties of SWCNTs-based materials.

Recent Advances on Graphene Quantum Dots: From Chemistry and Physics to Applications

Graphene quantum dots (GQDs) that are flat 0D nanomaterials have attracted increasing interest because of their exceptional chemicophysical properties and novel applications in energy conversion and storage, electro/photo/chemical catalysis, flexible devices, sensing, display, imaging, and theranostics. The significant advances in the recent years are summarized with comparative and balanced discussion. The differences between GQDs and other nanomaterials, including their nanocarbon cousins, are emphasized, and the unique advantages of GQDs for specific applications are highlighted. The current challenges and outlook of this growing field are also discussed.

A guard to reduce the accidental oxidation of PbTe nanocrystals

In the synthesis of lead telluride nanocrystals (PbTe NCs), oxidized PbTe is commonly regarded as a waste material as this will reduce the performance of pure PbTe NCs. The waste is normally thrown away, leading to potential environment risks and is less economical in terms of atom usage. Conventional anti-oxidation methods such as inert gas flow or sealed systems cannot deal with leaking or accidental contamination. To solve this problem, by simulating accidental oxidation, we utilized a cheap and easily-performed strategy to reduce the oxidation to a very low level. Further analysis indicates that this anti-oxidation effect should be due to interactions between the double bonds from the coating ligands and the extended π bonds from the benzene rings. This strategy increases the synthesis efficiency of the reactants and reduces the environmental pollution risk.

One-step solvothermal synthesis of high-emissive amphiphilic carbon dots via rigidity derivation

In nanoscience, amphiphilic carbon dots (ACDs) are of great importance due to their excellent transferability for application in biological sensing, imaging and labelling. However, facile synthetic strategies are still limited, especially for obtaining high-emissive ACDs. Since the development of a high-emissive feature is strongly desired for improving the practical resolution in vivo, here we report a chemical strategy that uses rigid molecules to straightforwardly construct amphiphilic carbon dots (ACDs) with high luminescence quantum yields (QYs). By using 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate (HATU), a typical coplanar compound, as the only precursor, well-defined ACDs were prepared via a one-step solvothermal process which exhibited a superior QY of up to 29%, largely superior to those prepared from precursors with less rigid structures. The effect can be mainly attributed to a significant suppression of the competition of non-radiative decay through rigidity derivation. Metal ionic doping during the synthesis resulted in a further improvement of the crystallinity and monodispersity of the materials, with retention of the high-emissive ability. This high-emissive photoluminescence behavior of the ACDs is accompanied with an excitation-wavelength dependence, a high biocompatibility and a low toxicity, which together make the ACDs advantageous for application in multi-channel bioimaging.

Biochemical mechanisms of dose-dependent cytotoxicity and ROS-mediated apoptosis induced by lead sulfide/graphene oxide quantum dots for potential bioimaging applications

Colloidal quantum dots (CQD) have attracted considerable attention for biomedical diagnosis and imaging as well as biochemical analysis and stem cell tracking. In this study, quasi core/shell lead sulfide/reduced graphene oxide CQD with near infrared emission (1100 nm) were prepared for potential bioimaging applications. The nanocrystals had an average diameter of ~4 nm, a hydrodynamic size of ~8 nm, and a high quantum efficiency of 28%. Toxicity assay of the hybrid CQD in the cultured human mononuclear blood cells does not show cytotoxicity up to 200 µg/ml. At high concentrations, damage to mitochondrial activity and mitochondrial membrane potential (MMP) due to the formation of uncontrollable amounts of intracellular oxygen radicals (ROS) was observed. Cell membrane and Lysosome damage or a transition in mitochondrial permeability were also noticed. Understanding of cell-nanoparticle interaction at the molecular level is useful for the development of new fluorophores for biomedical imaging.

Graphene Quantum Dots Embedded in Bi2Te3 Nanosheets To Enhance Thermoelectric Performance

Novel Bi₂Te₃/graphene quantum dots (Bi₂Te₃/GQDs) hybrid nanosheets with a unique structure that GQDs are homogeneously embedded in the Bi₂Te₃ nanosheet matrix have been synthesized by a simple solution-based synthesis strategy. A significantly reduced thermal conductivity and enhanced powder factor are observed in the Bi₂Te₃/GQDs hybrid nanosheets, which is ascribed to the optimized thermoelectric transport properties of the Bi₂Te₃/GQDs interface. Furthermore, by varying the size of the GQDs, the thermoelectric performance of Bi₂Te₃/GQDs hybrid nanostructures could be further enhanced, which could be attributed to the optimization of the density and dispersion manner of the GQDs in the Bi₂Te₃ matrix. A maximum ZT of 0.55 is obtained at 425 K for the Bi₂Te₃/GQDs-20 nm, which is higher than that of Bi₂Te₃ without hybrid nanostrucure. This work provides insights for the structural design and synthesis of Bi₂Te₃-based hybrid thermoelectric materials, which will be important for future development of broadly functional material systems.

Interface Engineering in Solution-Processed Nanocrystal Thin Films for Improved Thermoelectric Performance

Solution-processed PbTe nanocrystal (NC) thin films are ligand exchanged by ethylenediamine and then annealed at 400 °C for enhancement of NC coupling. To further improve the performance, heterostructures are introduced into the PbTe/PbS films. Significantly, an optimized ZT of ≈0.30 is successfully achieved at 405 K. This method opens an avenue toward thermoelectric thin film devices with high performance.

Paper Thermoelectrics: Merging Nanotechnology with Naturally Abundant Fibrous Material

The development of paper-based sensors, antennas, and energy-harvesting devices can transform the way electronic devices are manufactured and used. Herein we describe an approach to fabricate paper thermoelectric generators for the first time by directly impregnating naturally abundant cellulose materials with p- or n-type colloidal semiconductor quantum dots. We investigate Seebeck coefficients and electrical conductivities as a function of temperature between 300 and 400 K as well as in-plane thermal conductivities using Angstrom's method. We further demonstrate equipment-free fabrication of flexible thermoelectric modules using p- and n-type paper strips. Leveraged by paper's inherently low thermal conductivity and high flexibility, these paper modules have the potential to efficiently utilize heat available in natural and man-made environments by maximizing the thermal contact to heat sources of arbitrary geometry.

Carbogenic nanodots derived from organo-templated zeolites with modulated full-color luminescence

Dual modulated luminescence of carbogenic nanodots derived from zeolites has been acquired by controlling the concentration and pH value of CND aqueous dispersions.

Hydrophilic N-doped carbogenic nanodots (denoted as CNDs) have been prepared from a N-methylpiperazine-templated zeolite precursor by calcination and NaOH treatment. The isolated CNDs exhibit tunable photoluminescence according to the concentration and pH value of aqueous dispersions of the CNDs. Fine-tuning of the fluorescence emission wavelength across the entire visible spectrum can be easily achieved by varying the concentration of the CND dispersions. Meanwhile, both the emission wavelength and intensity of the photoluminescence can be tuned by controlling the pH value of the CND dispersion. The pyrolysis of organic templates confined in nanoporous zeolites represents a new approach to controlling the optical properties of CNDs, which may open more opportunities in applications such as multimodal sensing and full-color displays.

Mechanistic insight into the nucleation and growth of oleic acid capped lead sulphide quantum dots

In this study, we present a detailed understanding on synthesis mechanism of PbS QDs so as to provide guidance for future QDs synthesis.

Morphology-tunable ultrafine metal oxide nanostructures uniformly grown on graphene and their applications in the photo-Fenton system

Hybrid nanostructures of low-dimensional metal oxide (MO) semiconductors based on two-dimensional (2D) graphene nanosheets have been considered as one of the most promising nanomaterials for an extensive variety of applications. Unfortunately, it is still challenging to rationally design and fabricate MO/graphene hybrids with highly controllable nanostructures and desirable properties, which are of paramount importance for practical applications. Here, we report a novel, facile and "green" glycerol-mediated self-assembly method, using α-Fe2O3 semiconductor as an illustrative example, for the controlled growth of MO with a well-defined nanostructure on 2D graphene nanosheets. Based on this new method, we first demonstrate the ability to exquisitely tune the α-Fe2O3 nanostructure from zero-dimensional quantum dots (∼3.2 nm) to one-dimensional mesoporous nanorods, and eventually to 2D mesoporous nanosheets over the entire surface of graphene nanosheets. A possible formation mechanism has been proposed based on the systematic investigation of the morphological evolution and growth processes of α-Fe2O3 on graphene. The as-synthesized samples exhibit excellent performance for the photo-Fenton treatment of polluted water at neutral pH under visible light irradiation. Moreover, TiO2 and Fe3O4 quantum dots (∼5.2 and 3.3 nm, respectively) ultradispersed on graphene are also successfully synthesized by this method, demonstrating its versatility for the rational fabrication of novel MO/graphene hybrids with huge potential applications.