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The commercialization of fuel cells, especially for direct formic acid fuel cells (DFAFCs) and proton-exchange membrane fuel cells (PEMFCs), is significantly restrained by the high cost, poor stability, and sluggish kinetics of platinum... more
The commercialization of fuel cells, especially for direct formic acid fuel cells (DFAFCs) and proton-exchange membrane fuel cells (PEMFCs), is significantly restrained by the high cost, poor stability, and sluggish kinetics of platinum group metals (PGM) catalysts for both the anodic formic acid oxidation reaction (FAOR) and the cathodic oxygen reduction reaction (ORR). Currently, it has confronted with challenges, including exploring highly active, cost-effective, and stable catalysts to replace PGM for DFAFCs and PEMFCs. Recently, the increasing investigation has been focused on the single-atom catalysts (SACs) to enhance the catalytic performance owing to the maximum atom utilization and highly exposed active sites. The aim of this review is to present the recent research activities on carbon supported SACs. At the beginning of the review, metal-based SACs supported on different carbon supports, and the typical characterization methods are introduced. Subsequently, recent advances in metal-based SACs for FAOR and ORR catalysis are scientifically summarized. Particularly, some representative metal-based SACs for ORR activity are further exemplified with a deeper understanding of structure-activity relationships. Finally, the challenges and opportunities of SACs are prospected, such as the mechanism understanding and commercial applications.
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Metallic tungsten disulfide (WS2) monolayers have been demonstrated as promising electrocatalysts for hydrogen evolution reaction (HER) induced by the high intrinsic conductivity, however, the key challenges to maximize the catalytic... more
Metallic tungsten disulfide (WS2) monolayers have been demonstrated as promising electrocatalysts for hydrogen evolution reaction (HER) induced by the high intrinsic conductivity, however, the key challenges to maximize the catalytic activity are achieving the metallic WS2 with high concentration and increasing the density of the active sites. In this work, single-atom-V catalysts (V SACs) substitutions in 1T-WS2 monolayers (91% phase purity) are fabricated to significantly enhance the HER performance via a one-step chemical vapor deposition strategy. Atomic-resolution scanning transmission electron microscopy (STEM) imaging together with Raman spectroscopy confirm the atomic dispersion of V species on the 1T-WS2 monolayers instead of energetically favorable 2H-WS2 monolayers. The growth mechanism of V SACs@1T-WS2 monolayers is experimentally and theoretically demonstrated. Density functional theory (DFT) calculations demonstrate that the activated V-atom sites play vital important ...
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Significance Defects in two-dimensional (2D) transition-metal dichalcogenides play a crucial role in controlling the spatiotemporal dynamics of photogenerated charge carriers, which remain poorly understood to date. In this paper, the... more
Significance Defects in two-dimensional (2D) transition-metal dichalcogenides play a crucial role in controlling the spatiotemporal dynamics of photogenerated charge carriers, which remain poorly understood to date. In this paper, the defect-mediated carrier diffusion and recombination in WS 2 monolayers are quantitatively investigated by laser-illuminated microwave impedance microscopy. Surprisingly, the photoresponse is in general stronger in the more disordered regions and samples. Such counterintuitive observations are reconciled by spatiotemporally resolved experiments, which indicate that the electron lifetime is prolonged due to the slow release of holes from the trap states. The results reveal the intrinsic time and length scales of photocarriers in van der Waals materials, providing the guidance for implementing nanooptoelectronic devices based on 2D semiconductors.
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Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application in (opto-)electronic devices. Doping by molecular electron acceptors and donors has been... more
Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application in (opto-)electronic devices. Doping by molecular electron acceptors and donors has been suggested as a promising method to achieve EF-adjustment. Here, we demonstrate that the charge transfer (CT) mechanism between TMDC and molecular dopant depends critically on the electrical nature of the substrate as well as its electronic coupling with the TMDC. Using angle-resolved ultraviolet and X-ray photoelectron spectroscopy, we reveal three fundamentally different, substrate-dependent CT mechanisms between the molecular electron acceptor 1,3,4,5,7,8-hexafluoro-tetracyano-naphthoquinodimethane (F6TCNNQ) and a MoS2 monolayer. Our results demonstrate that any substrate that acts as charge reservoir for dopant molecules can prohibit factual doping of a TMDC monolayer. On the other hand, the three different CT mechanisms can be exploited for the desi...
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Bilayer WSe2 nuclei were initially grown along the atomic steps of a sapphire substrate, resembling a “graphoepitaxial mechanism” and gradually formed into overlapped 2H stacked WSe2 bilayers.
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Piezoelectric materials have been widely used for sensors, actuators, electronics, and energy conversion. Two-dimensional (2D) ultrathin semiconductors, such as monolayer h-BN and MoS with their atom-level geometry, are currently emerging... more
Piezoelectric materials have been widely used for sensors, actuators, electronics, and energy conversion. Two-dimensional (2D) ultrathin semiconductors, such as monolayer h-BN and MoS with their atom-level geometry, are currently emerging as new and attractive members of the piezoelectric family. However, their piezoelectric polarization is commonly limited to the in-plane direction of odd-number ultrathin layers, largely restricting their application in integrated nanoelectromechanical systems. Recently, theoretical calculations have predicted the existence of out-of-plane and in-plane piezoelectricity in monolayer α-InSe. Here, we experimentally report the coexistence of out-of-plane and in-plane piezoelectricity in monolayer to bulk α-InSe, attributed to their noncentrosymmetry originating from the hexagonal stacking. Specifically, the corresponding d piezoelectric coefficient of α-InSe increases from 0.34 pm/V (monolayer) to 5.6 pm/V (bulk) without any odd-even effect. In additi...
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A novel ternary cobalt–nickel phosphide nanosheet with nanowire edges on 3D nickel foam (CoNiP@NF) was synthesized and used as an excellent cathode for the HER over a wide pH range from 0 to 14.
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Amorphous thin film oxygen evolving catalysts, OECs, of first-row transition metals show promise to serve as self-assembling photoanode materials in solar-driven, photoelectrochemical `artificial leaf' devices. This report... more
Amorphous thin film oxygen evolving catalysts, OECs, of first-row transition metals show promise to serve as self-assembling photoanode materials in solar-driven, photoelectrochemical `artificial leaf' devices. This report demonstrates the ability to use high-energy X-ray scattering and atomic pair distribution function analysis, PDF, to resolve structure in amorphous metal oxide catalyst films. The analysis is applied here to resolve domain structure differences induced by oxyanion substitution during the electrochemical assembly of amorphous cobalt oxide catalyst films, Co-OEC. PDF patterns for Co-OEC films formed using phosphate, Pi, methylphosphate, MPi, and borate, Bi, electrolyte buffers show that the resulting domains vary in size following the sequence Pi < MPi < Bi. The increases in domain size for CoMPi and CoBi were found to be correlated with increases in the contributions from bilayer and trilayer stacked domains having structures intermediate between those of...
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The first realization of a tunable band gap in monolayer WS2(1-x) Se2x is demonstrated. The tuning of the band gap exhibits a strong dependence of S and Se contents, as proven by PL spectroscopy. Because of its remarkable electronic... more
The first realization of a tunable band gap in monolayer WS2(1-x) Se2x is demonstrated. The tuning of the band gap exhibits a strong dependence of S and Se contents, as proven by PL spectroscopy. Because of its remarkable electronic structure, monolayer WS2(1-x) Se2x exhibits novel electrochemical catalytic activity and offers long-term electrocatalytic stability for the hydrogen evolution reaction.
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Water oxidation is an important half-reaction to achieve overall water splitting. In this present study, we show that a series of molecular cobalt–salen complexes can serve as catalyst precursors to form nanostructured and amorphous... more
Water oxidation is an important half-reaction to achieve overall water splitting. In this present study, we show that a series of molecular cobalt–salen complexes can serve as catalyst precursors to form nanostructured and amorphous cobalt-based thin films during electrodeposition, which can catalyze the water oxidation reaction at low overpotentials. Cyclic voltammetry and bulk electrolysis using the cobalt-based film electrodes demonstrated obvious catalytic currents in 0.1 M KBi solution at pH 9.2. The onset catalytic potentials of the catalyst films are at ∼0.84 V (vs Ag/AgCl) with a film made by electrodeposition of cobalt–salen complex 2 on FTO and at ∼0.85 V for complex 4. Oxygen gas bubbles were clearly seen on the FTO electrode when the applied potential was above the onset potential. The Tafel plots using a catalyst film made of complex 4 showed that appreciable catalytic current was observed starting at η = 0.26 V for the film (a current density of 0.01 mA/cm2 required η = 290 mV), accompanied ...
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For the first time, noble-metal-free nickel phosphide (Ni2P) was used as an excellent catalyst precursor for water oxidation catalysis.
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In this study, we report for the first time on the use of a water-soluble BF2-annulated cobaloxime, Co(dmgBF2)2(OH2)2 (Co-DMB, dmgBF2 = difluoroboryl-dimethylglyoxime), as a catalyst precursor for electrocatalytic water oxidation. Oxygen... more
In this study, we report for the first time on the use of a water-soluble BF2-annulated cobaloxime, Co(dmgBF2)2(OH2)2 (Co-DMB, dmgBF2 = difluoroboryl-dimethylglyoxime), as a catalyst precursor for electrocatalytic water oxidation. Oxygen gas bubbles were clearly produced on the FTO electrode at a low overpotential under neutral pH conditions containing Co-DMB. Interestingly, stable green films were produced under these conditions. The current densities can reach to >5 mA/cm(2) at 1.1 V and >10 mA/cm(2) at 1.5 V (vs Ag/AgCl). The morphologies of the films showed nanoribbon structures, which were characterized by scanning electron microscope (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS).
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Two water-insoluble cobalt porphyrin complexes were synthesized and used to prepare electrode films on FTO. The films showed good activities and possible recyclability for water oxidation catalysis under benign conditions. No... more
Two water-insoluble cobalt porphyrin complexes were synthesized and used to prepare electrode films on FTO. The films showed good activities and possible recyclability for water oxidation catalysis under benign conditions. No heterogeneous cobalt oxides were observed in the reaction system, as have been confirmed by UV-vis spectroscopy, mass spectrometry, SEM, and EDX measurements.