Doping MoS2 with Graphene Quantum Dots: Structural and Electrical Engineering towards Enhanced Electrochemical Hydrogen Evolution

High conversion continuous flow exfoliation of 2D MoS2

We report a low-cost and highly efficient process for exfoliating of MoS2 using an energy efficient vortex fluidic device (VFD). This method is high in green chemistry metrics in avoiding the use of auxiliary substances, and the process is scalable, with a conversion of as received MoS2 into 2D sheets at ∼73%.

Highly defective graphene quantum dots-doped 1T/2H-MoS2 as an efficient composite catalyst for the hydrogen evolution reaction

We present a new composite catalyst system of highly defective graphene quantum dots (HDGQDs)-doped 1T/2H-MoS2 for efficient hydrogen evolution reactions (HER). The high electrocatalytic activity, represented by an overpotential of 136.9 mV and a Tafel slope of 57.1 mV/decade, is due to improved conductivity, a larger number of active sites in 1T-MoS2 compared to that in 2H-MoS2, and additional defects introduced by HDGQDs. High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) were used to characterize both the 1T/2H-MoS2 and GQDs components while Fourier-transform infrared spectroscopy (FTIR) was employed to identify the functional groups on the edge and defect sites in the HDGQDs. The morphology of the composite catalyst was also examined by field emission scanning electron microscopy (FESEM). All experimental data demonstrated that each component contributes unique advantages that synergistically lead to the significantly improved electrocatalytic activity for HER in the composite catalyst system.

Emerging Trends of Carbon-Based Quantum Dots: Nanoarchitectonics and Applications

Carbon-based quantum dots (QDs) have emerged as a fascinating class of advanced materials with a unique combination of optoelectronic, biocompatible, and catalytic characteristics, apt for a plethora of applications ranging from electronic to photoelectrochemical devices. Recent research works have established carbon-based QDs for those frontline applications through improvements in materials design, processing, and device stability. This review broadly presents the recent progress in the synthesis of carbon-based QDs, including carbon QDs, graphene QDs, graphitic carbon nitride QDs and their heterostructures, as well as their salient applications. The synthesis methods of carbon-based QDs are first introduced, followed by an extensive discussion of the dependence of the device performance on the intrinsic properties and nanostructures of carbon-based QDs, aiming to present the general strategies for device designing with optimal performance. Furthermore, diverse applications of carbon-based QDs are presented, with an emphasis on the relationship between band alignment, charge transfer, and performance improvement. Among the applications discussed in this review, much focus is given to photo and electrocatalytic, energy storage and conversion, and bioapplications, which pose a grand challenge for rational materials and device designs. Finally, a summary is presented, and existing challenges and future directions are elaborated.

Electronic Properties and Electrocatalytic Water Splitting Activity for Precious-Metal-Adsorbed Silicene with Nonmetal Doping

Since nonmetal (NM)-doped two-dimensional (2D) materials can effectively modulate their physical properties and chemical activities, they have received a lot of attention from researchers. Therefore, the stability, electronic properties, and electrocatalytic water splitting activity of precious-metal (PM)-adsorbed silicene doped with two NM atoms are investigated based on density functional theory (DFT) in this paper. The results show that NM doping can effectively improve the stability of PM-adsorbed silicene and exhibit rich electronic properties. Meanwhile, by comparing the free energies of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) intermediates of 15 more stable NM-doped systems, it can be concluded that the electrocatalytic water splitting activity of the NM-doped systems is more influenced by the temperature. Moreover, the Si-S2-Ir-doped system exhibits good HER performance when the temperature is 300 K, while the Si-N2-Pt-doped system shows excellent OER activity. Our theoretical study shows that NM doping can effectively promote the stability and electrocatalytic water splitting of PM-adsorbed silicene, which can help in the application of silicene in electrocatalytic water splitting.

Oxygen-regulated carbon quantum dots as an efficient metal-free electrocatalyst for nitrogen reduction

An electrocatalytic nitrogen reduction reaction under ambient conditions provides a wonderful blueprint for the conversion of nitrogen to ammonia. However, current research on ammonia synthesis is mainly focused on metal-based catalysts. It is still a great challenge to realize the effective activation of N₂ on non-metallic catalysts. Herein, carbon quantum dots are reported to reduce dinitrogen to ammonia under ambient conditions. Benefiting from its numerous defect sites, this metal-free catalyst shows excellent catalytic performance in 0.1 M HCl with a faradaic efficiency of 17.59%. In addition, both experimental and theoretical results confirm that the catalytic performance of the catalyst can be improved by appropriately controlling the oxygen content of samples at different temperatures, and the utmost ammonia yield is 134.08 μg h^-1 mg^-1_cat., which is almost three times higher than that of a reported metal-free material. The proposed oxygen regulation provides a new method to optimize the surface properties of metal-free catalysts for ammonia synthesis.

Investigations on Structural, Electronic and Optical Properties of MoS2/CDs Heterostructure via First-Principles Study

Much effort has been made for MoS2/CDs heterostructure application in the field of photocatalysts. However, the impacts of functional groups of CDs on the properties of the heterostructure are ambiguous. Here, the impacts of hydroxyl, carbonyl, and carboxyl groups of CDs on the structural, electronic, and optical properties of MoS2/CDs’ heterostructure were investigated by conducting a first-principles study. The calculated energy band structure and band gap of monolayer MoS2 were consistent with the experimental values. The band gap of MoS2 was obviously decreased after the construction of MoS2/CDs and MoS2/CDs–hydroxyl/carboxyl, thus helping to improve the light adsorption range. However, the band gap of MoS2/CDs–carbonyl was slightly increased compared with that of monolayer MoS2. The CDs with functional groups can spontaneously bind on 2D-MoS2 and form a stable MoS2/CDs heterostructure. It was confirmed that the MoS2/CDs’ heterostructure belongs to the typical type-II band alignment, which contributes to the separation of photogenerated charge and hole. Notably, the carbonyl and carboxyl groups on the CDs obviously reduced the optical absorption intensity of the MoS2/CDs in the ultraviolet region. The hydroxyl groups have little effect on optical absorption intensity. Thus, the CDs with more hydroxyl groups are beneficial to produce a higher photocatalytic performance. This paper reveals the impacts of surface functional groups and provides a promising approach for designing the MoS2/CDs’ heterostructure to enhance the photocatalytic properties.

Simple preparation of Ag-BTC-modified Co3Mo7O24 mesoporous material for capacitance and H2O2-sensing performances

{Co3Mo7O24}@Ag-BTC-2 was synthesized by a grinding method, and it showed excellent performance in a supercapacitor and H2O2 sensing.

Electrogenerated chemiluminescence of Ru(bpy)32+ at MoS2 nanosheets modified electrode and its application in the sensitive detection of dopamine

Electrogenerated chemiluminescence (ECL) of Ru(bpy)₃²⁺ was studied at a MoS₂ nanosheets modified glassy carbon electrode (MoS₂NS/GCE) in neutral condition. Electrochemical results revealed that MoS₂ nanosheets could significantly catalyze the electrochemical oxidation of Ru(bpy)₃²⁺, as a result, strong anodic ECL was obtained. Several impact factors, such as the modified amount of MoS₂ nanosheets suspension, the pH value, and the concentration of Ru(bpy)₃²⁺, were investigated to obtain the optimal experimental condition. Dopamine exhibited apparent inhibiting effect on ECL intensity of Ru(bpy)₃²⁺-MoS₂ nanosheets through energy transfer process, and could be sensitively detected in the range of 1.0 × 10^-9 to 1.0 × 10^-4 mol L^-1. The linear equation between the decrease of ECL intensity and the logthium of dopamine concentration was determined as ΔI = 9965.02 + 1077.03lgC (C in mol L^-1), with the detection of 8.5 × 10^-10 mol L^-1 (3σ). The modified electrode exhibited satisfactory sensitivity, selectivity, and stability, which can be used to detect dopamine in real samples.

Electrochemistry in Carbon-based Quantum Dots

Electrochemistry belongs to an important branch of chemistry that deals with the chemical changes produced by electricity and the production of electricity by chemical changes. Therefore, it can not only act a powerful tool for materials synthesis, but also offer an effective platform for sensing and catalysis. As extraordinary zero-dimensional materials, carbon-based quantum dots (CQDs) have been attracting tremendous attention due to their excellent properties such as good chemical stability, environmental friendliness, nontoxicity and abundant resources. Compared with the traditional methods for the preparation of CQDs, electrochemical (EC) methods offer advantages of simple instrumentation, mild reaction conditions, low cost and mass production. In return, CQDs could provide cost-effective, environmentally friendly, biocompatible, stable and easily-functionalizable probes, modifiers and catalysts for EC sensing. However, no specific review has been presented to systematically summarize both aspects until now. In this review, the EC preparation methods of CQDs are critically discussed focusing on CQDs. We further emphasize the applications of CQDs in EC sensors, electrocatalysis, biofuel cells and EC flexible devices. This review will further the experimental and theoretical understanding of the challenges and future prospective in this field, open new directions on exploring new advanced CQDs in EC to meet the high demands in diverse applications.

Synthesis and characterization of graphene quantum dots

Conventional inorganic semiconductor quantum dots (QDs) have numerous applications ranging from energy harvesting to optoelectronic and bio-sensing devices primarily due to their unique size and shape tunable band-gap and also surface functionalization capability and consequently, have received significant interest in the last few decades. However, the high market cost of these QDs, on the order of thousands of USD/g and toxicity limit their practical utility in many industrial applications. In this context, graphene quantum dot (GQD), a nanocarbon material and a new entrant in the quantum-confined semiconductors could be a promising alternative to the conventional toxic QDs due to its potential tunability in optical and electronic properties and film processing capability for realizing many of the applications. Variation in optical as well as electronic properties as a function of size, shape, doping and functionalization would be discussed with relevant theoretical backgrounds along with available experimental results and limitations. The review deals with various methods available so far towards the synthesis of GQDs along with special emphasis on characterization techniques starting from spectroscopic, optical and microscopic techniques along with their the working principles, and advantages and limitations. Finally, we will comment on the environmental impact and toxicity limitations of these GQDs and their hybrid nanomaterials to facilitate their future prospects.

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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.

Highly Efficient Hydrogen Evolution from Seawater by Biofunctionalized Exfoliated MoS2 Quantum Dot Aerogel Electrocatalysts That Is Superior to Pt

As a source of clean and sustainable energy, reliable hydrogen production requires highly efficient and stable electrocatalysts. In recent years, molybdenum disulfide (MoS₂) has been demonstrated as a promising electrocatalyst for hydrogen evolution reactions (HERs). Here, we demonstrate that a three-dimensional (3D) MoS₂ quantum dot (MoS₂QD) aerogel is an efficient cathode electrocatalyst that can be used to enhance the HER in acid, neutral, and alkaline (e.g., real seawater) environments. In studying the effects of the exfoliated MoS₂ dimension for the HER, we found that the biofunctionalized exfoliated MoS₂QD shows much higher cathodic density, a more lower energy input, and a lower Tafel slope for the HER than the larger size of the chlorophyll-assisted exfoliated MoS₂, highlighting the importance of the size of the MoS₂ aerogel support for accelerating the HER performance. Moreover, the electrocatalytic activity of MoS₂QD-aerogel is superior to that of Pt in neutral conditions. In real seawater, the MoS₂QD-aerogel sample exhibits stable HER performance after consecutive scanning for 150 cycles, while the HER activity of the Pt dramatically decreases after 50 cycles. These results showed for the first time how the 3D MoS₂ configuration in MoS₂ aerogel can be used to effectively produce hydrogen for clean energy applications.

1T/2H multi-phase MoS2 heterostructures: synthesis, characterization and thermal catalysis decomposition of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate

Dependence of ln(β/T_p²) on 1/T_p for TKX-50 and mixtures with 10 wt% 2H-MoS₂ and 1T/2H-MoS₂.

Enhanced electrocatalytic hydrogen generation from water via cobalt-doped Cu2ZnSnS4 nanoparticles

Herein, we adopted a novel noble metal-free Co-doped CZTS-based electrocatalyst for the hydrogen evolution reaction (HER), which was fabricated using a facile, effective, and scalable strategy by employing a sonochemical method. The optimized Co-doped CZTS electrocatalyst shows a superior HER performance with a small overpotential of 200 and 298 mV at 2 and 10 mA-1, respectively, and Tafel slope of 73 mV dec-1, and also exhibits excellent stability up to 700 cycles with negligible loss of the cathodic current. The ease of synthesis and high activity of the Co-doped CZTS-based cost-effective catalytic system appear to be promising for HER catalysis.

Recent progress in two-dimensional inorganic quantum dots

This review critically summarizes recent progress in the categories, synthetic routes, properties, functionalization and applications of 2D materials-based quantum dots (QDs).

Enhanced hydrogen evolution of MoS2/RGO: vanadium, nitrogen dopants triggered new active sites and expanded interlayer

A vanadium and nitrogen co-doping technique has been demonstrated as a “killing two birds with one stone” solution for simultaneously increasing active sites, improving catalysis kinetics, and modulating electric structure to improve its HER performance.

Improved catalysts for hydrogen evolution reaction in alkaline solutions through the electrochemical formation of nickel-reduced graphene oxide interface

The hydrogen evolution reaction is significantly boosted at the Ni-reduced graphene oxide interface via spillover of discharged H adatoms.

Oxygen-Incorporated MoS2 Nanosheets with Expanded Interlayers for Hydrogen Evolution Reaction and Pseudocapacitor Applications

Two-dimensional transition-metal dichalcogenides (TMDs) nanosheets have attracted tremendous research interest. Engineering the structure of MoS₂ may result in desirable performance for energy applications. In this work, oxygen-incorporated MoS₂ nanosheets with expanded interlayers have been synthesized by a solvothermal reaction. The oxygen-incorporated MoS₂ nanosheets with rich defects demonstrate excellent hydrogen evolution reaction activity with a small Tafel slope of 42 mV decade^-1 as well as excellent long-term stability. Interestingly, a large expanded ∼8.40 Å interlayer of (002) faces can be achieved by controlling the reaction time. This material also shows excellent long-term cycling stability (up to 20 000 cycles) as well as high specific capacitance for pseudocapacitors. We believe that the structural modification strategy can be applied for other TMDs to further optimize the performance for various applications.