Semiconductor clusters: Synthetic precursors for colloidal quantum dots

Semiconductor clusters have been implicated as reaction intermediates between molecular precursors and colloidal quantum dots (CQDs). The success of isolation of semiconductor clusters have enabled detailed investigation of the atomic information of semiconductor clusters. The identification of atomic information has emerged as an important topic because knowledge of the structure-function relationship of intermediate clusters has been helpful to reveal the synthetic mechanism of CQDs. Recently, they have been utilized as the synthetic precursors for CQDs, which was not readily achieved using conventional molecular precursors. This mini review briefly introduces the current understanding of their atomic information such as the composition, structure, and surface. We then discuss advantages, limitations, and the perspective of semiconductor clusters as a precursor for synthesis of CQDs

Reducing the Photodegradation of Perovskite Quantum Dots to Enhance Photocatalysis in CO2 Reduction

© 1996-2021 MDPI (Basel, Switzerland) unless otherwise stated. Solution-processed perovskite quantum dots (QDs) have been intensively researched as next-generation photocatalysts owing to their outstanding optical properties. Even though the intrinsic physical properties of perovskite QDs have been significantly improved, the chemical stability of these materials remains questionable. Their low long-term chemical stability limits their commercial applicability in photocatalysis. In this study, we investigated the photodegradation mechanisms of perovskite QDs and their hybrids via photoluminescence (PL) by varying the excitation power and the ultraviolet (UV) exposure power. Defects in perovskite QDs and the interface between the perovskite QD and the co-catalyst influence the photo-stability of perovskite QDs. Consequently, we designed a stable perovskite QD film via an in-situ cross-linking reaction with amine-based silane materials. The surface ligand comprising 2,6-bis(N-pyrazolyl)pyridine nickel(II) bromide (Ni(ppy)) and 5-hexynoic acid improved the interface between the Ni co-catalyst and the perovskite QD. Then, ultrathin SiO2 was fabricated using 3-aminopropyltriethoxy silane (APTES) to harness the strong surface binding energy of the amine functional group of APTES with the perovskite QDs. The Ni co-catalyst content was further increased through Ni doping during purification using a short surface ligand (3-butynoic acid). As a result, stable perovskite QDs with rapid charge separation were successfully fabricated. Time-correlated single photon counting (TCSPC) PL study demonstrated that the modified perovskite QD film exhibited slow photodegradation owing to defect passivation and the enhanced interface between the Ni co-catalyst and the perovskite QD. This interface impeded the generation of hot carriers, which are a critical factor in photodegradation. Finally, a stable red perovskite QD was synthesized by applying the same strategy and the mixture between red and green QD/Ni(ppy)/SiO2 displayed an CO2 reduction capacity for CO (0.56 mu mol/(g center dot h)).11Nsciescopu

Carbon-based asymmetric capacitor for high-performance energy storage devices

Carbon-based materials are widely used in energy storage research, as attractive materials with high conductivity, low cost, and high availability. However, a relatively low performance (e.g., energy and power densities) compared with metal oxides is an obstacle to use for commercial applications. Herein, we report on high-performance metal oxide-free asymmetric capacitors (ASCs) using n-type and p-type graphene films which are doped by nitrogen and boron atoms, respectively, exhibiting high energy and power densities with excellent stability. The enhanced performances of the ASCs arises from the synergistic effect of the non-faradaic capacitance and pseudocapacitance, which are confirmed with new analysis using cyclic voltammetry and electrochemical impedance spectroscopy for a pseudocapacitance effect of intercalation/deintercalation and galvanostatic charge-discharge profiles for and non-faradaic capacitance. The new ASC in an ionic liquid electrolyte (e.g., pure EMIMBF4) shows the high energy density of 77.41 Wh kg−1 in 3.0 V of the operating potential window with the excellent retention stability of ∼87% after 10,000 cycles. The carbon-based asymmetric capacitor of semiconducting graphene electrodes can offer the promise of exploiting both non-faradaic capacitance and intercalation/deintercalation pseudocapacitance to obtain a high-performance energy storage device. © 2019 Elsevier Lt

Graphene-based composite electrodes for electrochemical energy storage devices: Recent progress and challenges

As the importance of applications depending on electrical energy storage devices (EESDs), including portable electronics, electric vehicles, and devices for renewable energy storage, has gradually increased, research has focused more and more on innovative energy systems for advanced EESDs in order to achieve enhanced performance. Over the past two decades, graphene-based materials have been considered as ideal electrode materials for lithium-ion, sodium-ion, and lithium/sulfur batteries, as well as supercapacitors, due to their promising applications for advanced electrodes. In this review, we will demonstrate the issues and challenges of each type of EESD, with an emphasis placed on the use of graphene-based electrodes. Recent trends related to research into graphene-based composite materials as electrodes in Korea will also be shown and a summary of the overall strategies and future perspectives will be given. © 2017 Elsevier B.V

A molecular approach to an electrocatalytic hydrogen evolution reaction on single-layer graphene

A major challenge in the development of electrocatalysts is to determine a detailed catalysis mechanism on a molecular level for enhancing catalytic activity. Here, we present bottom-up studies for an electrocatalytic hydrogen evolution reaction (HER) process through molecular activation to systematically control surface catalytic activity corresponding to an interfacial charge transfer in a porphyrin monolayer on inactive graphene. The two-dimensional (2D) assembly of porphyrins that create homogeneous active sites (e.g., electronegative tetrapyrroles (N4)) on graphene showed structural stability against electrocatalytic reactions and enhanced charge transfer at the graphene-liquid interface. Performance operations of the graphene field effect transistor (FET) were an effective method to analyse the interfacial charge transfer process associated with information about the chemical nature of the catalytic components. Electronegative pristine porphyrin or Pt-porphyrin networks, where intermolecular hydrogen bonding functioned, showed larger interfacial charge transfers and higher HER performance than Ni-, or Zn-porphyrin. A process to create surface electronegativity by either central N-4 or metal (M)-N-4 played an important role in the electrocatalytic reaction. These findings will contribute to an in-depth understanding at the molecular level for the synergetic effects of molecular structures on the active sites of electrocatalysts toward HER1441sciescopu

Porosity-Engineering of MXene as a Support Material for a Highly Efficient Electrocatalyst toward Overall Water Splitting

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimThe use of 2 D transition metal carbide MXenes as support materials to incorporate catalytically active compounds is of interest because of their unique properties. However, the preparation of well-dispersed catalytic phases on the inter-connected porous MXene network is challenging and has been rarely explored. This work focuses on the synthesis of basal-plane-porous titanium carbide MXene (ac-Ti3C2) that is used subsequently as an effective host for the incorporation of a known catalytically active phase (IrCo) as an effective bifunctional electrocatalyst toward water splitting. The porous ac-Ti3C2 with abundant macro/meso/micropores is prepared by a wet chemical method at room temperature and provides ideal anchor sites for intimate chemical bonding with alien compounds. The resulting IrCo@ac-Ti3C2 electrocatalyst exhibits an excellent reactivity (220 mV at 10 mA cm−2) towards the oxygen evolution reaction in 1.0 m KOH, which surpasses that of the benchmark RuO2, a low voltage cell of 1.57 V (@ 10 mA cm−2) and good long-term durability. Our work demonstrates the effectiveness of porosity engineering in MXene nanosheets as a support material to shorten ion migration pathways, to increase electrolyte accessibility between inter-sheets and to overcome inherited re-stacking and aggregation issue

Layer-Dependent Band Structure of Ternary Metal Chalcogenides: Thickness-Controlled Hexagonal FeIn2S4

Two-dimensional (2D) transition metal dichalcogenides have received considerable attention due to their exotic electrical, chemical, and physical properties. Here, we report a layer-dependent band structure of a 2D semiconducting ternary metal chalcogenide (TMC), hexagonal FeIn2S4 (hFIS), which is prepared through thickness-controlled colloidal solution synthesis. The controlled dissociation rate of chalcogen precursors caused the growth of the different thicknesses of hFIS, which is coincident with mechanisms established in conventional 2D nanomaterial colloidal synthesis. The various thickness-dependent band structures of hFIS were investigated from the corresponding optical band gap and redox potentials. The unveiled layerdependent band structure of hFIS showed band gaps of approximately 1.02, 1.26, and 1.52 eV, corresponding to synthesis of the 7-8, 5-6, and 2-3 layers, respectively. This study will contribute to the exploration of other layer-dependent TMCs (MIn2X4, M = Fe, Co, Mn, and Zn and X = S, Se, and Te) for new optical and electronic device applications.11Nsciescopu

Low Iridium Content Confined inside a Co3O4 Hollow Sphere for Superior Acidic Water Oxidation

Noble-metal-oxide support catalysts have been demonstrated to be unique for electrocatalytic water oxidation in acidic media. Highly porous three-dimensional oxide supported can serve as an ideal platform to confine ultrasmall metal catalysts on specific sites and modulate their reactivity, resulting in the reduction of noble metal content in the catalyst by boosting the mass activity. However, due to poor control over the support morphology, geometric-driven shifts in mass activity of metal-oxide support catalysts for the oxygen evolution reaction in acidic media have not been realized. Herein, a nanoscale Kirkendall effect is exploited to produce and control a structural evolution yielding an oxygen-evolving catalyst that is highly efficient and robust in acidic medium. By selective reaction-diffusion under oxidizing conditions, the starting solid CoIr NC is directly transformed into an unprecedented Ir-Co3O4@Co3O4 porous-core@shell hollow nanospheres (ICO PCSHS), in which an ultrasmall Ir catalyst is spatially isolated within a porous Co3O4-backbone core, encapsulated by a hollow Co3O4 outer shell. With a low Ir content of 14 wt %, the iridium mass activity exhibited by ICO PCSHS-400 catalyst is 24 times higher than that of benchmark RuO2, substantially exceeding the known oxide-supported metal catalysts. More importantly, the electrocatalyst shows high stability during 8 h of continuous testing in acidic medium. © 2019 American Chemical Society11sciescopu

Uncovering the Role of Countercations in Ligand Exchange of WSe2: Tuning the d-Band Center toward Improved Hydrogen Desorption

Copyright © 2021 American Chemical Society. The role of countercations that do not bind to core nanocrystals (NCs) but rather ensure charge balance on ligand-exchanged NC surfaces has been rarely studied and even neglected. Such a scenario is unfortunate, as an understanding of surface chemistry has emerged as a key factor in overcoming colloidal NC limitations as catalysts. In this work, we report on the unprecedented role of countercations in ligand exchange for a colloidal transition metal dichalcogenide (TMD), WSe2, to tune the d-band center toward the Fermi level for enhanced hydrogen desorption. Conventional long-chain organic ligands, oleylamine, of WSe2 NCs are exchanged with short atomic S2- ligands having countercations to preserve the charge balance (WSe2/S2-/M+, M = Li, Na, K). Upon exchange with S2- ligands, the charge-balancing countercations are intercalated between WSe2 layers, thereby serving a unique function as an electrochemical hydrogen evolution reaction (HER) catalyst. The HER activity of ligand-exchanged colloidal WSe2 NCs shows a decrease in overpotential by down-shift of d-band center to induce more electron-filling in antibonding orbital and an increase in the electrochemical active surface area (ECSA). Exchanging surface functionalities with S2- anionic ligands enhances HER kinetics, while the existence of intercalated countercations improves charge transfer with the electrolyte. The obtained results suggest that both anionic ligands and countercationic species in ligand exchange must be considered to enhance the overall catalytic activity of colloidal TMDs.11Nsciescopu