Beyond MoS2 as the first transition metal dichalcogenide (TMD) to have gained recognition as an efficient catalyst for the hydrogen evolution reaction (HER), interest in other TMD nanomaterials is ...steadily beginning to proliferate. This is particularly true in the field of electrochemistry, with a myriad of emerging applications ranging from catalysis to supercapacitors and solar cells. Despite this rise, current understanding of their electrochemical characteristics is especially lacking. We therefore examine the inherent electroactivities of various chemically exfoliated TMDs (MoSe2, WS2, WSe2) and their implications for sensing and catalysis of the hydrogen evolution and oxygen reduction reactions (ORR). The TMDs studied are found to possess distinctive inherent electroactivities and together with their catalytic effects for the HER are revealed to strongly depend on the chemical exfoliation route and metal-to-chalcogen composition particularly in MoSe2. Despite its inherent activity exhibiting large variations depending on the exfoliation procedure, it is also the most efficient HER catalyst with a low overpotential of −0.36 V vs RHE (at 10 mA cm–2 current density) and fairly low Tafel slope of ∼65 mV/dec after BuLi exfoliation. In addition, it demonstrates a fast heterogeneous electron transfer rate with a k 0 obs of 9.17 × 10–4 cm s–1 toward ferrocyanide, better than that seen for conventional glassy carbon electrodes. Knowledge of TMD electrochemistry is essential for the rational development of future applications; inherent TMD activity may potentially limit certain purposes, but intended objectives can nonetheless be achieved by careful selection of TMD compositions and exfoliation methods.
Transition-metal dichalcogenides (TMDs) are at the forefront of research for their promising catalytic abilities and unique materials properties. With great interest in the study of mono- or ...few-layered TMDs, we seek to fundamentally explore the effects of doping on bulk TMDs, particularly MoS2 and WS2 for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) with p-dopants Nb and Ta. Despite promises reported in the computational studies of doped TMDs on the HER, our results show otherwise. Doped bulk TMDs display less catalytic activity in comparison to their undoped counterparts for the HER. A similar effect is observed for ORR catalysis. Characterization was done to shed light on its surface elemental composition, properties, and morphologies. It was found that doped WS2 has a high percentage of 1T phase but this does not correlate with a lower overpotential for the HER at −10 mA cm–2, which contradicts the general consensus. We therefore show that p-dopants have a negative electrocatalytic effect on the HER. These findings are of high importance for the field of TMD electrochemistry, as they challenge the current consensus that doping always improves the electrocatalysis of TMDs.
Presently, research in layered transition metal dichalcogenides (TMDs) for numerous electrochemical applications have largely focused on Group 6 TMDs, especially MoS2 and WS2, whereas TMDs belonging ...to other groups are relatively unexplored. This work unravels the electrochemistry of Group 10 TMDs: specifically PtS2, PtSe2, and PtTe2. Here, the inherent electroactivities of these Pt dichalcogenides and the effectiveness of electrochemical activation on their charge transfer and electrocatalytic properties are thoroughly examined. By performing density functional theory (DFT) calculations, the electrochemical and electrocatalytic behaviors of the Pt dichalcogenides are elucidated. The charge transfer and electrocatalytic attributes of the Pt dichalcogenides are strongly associated with their electronic structures. In terms of charge transfer, electrochemical activation has been successful for all Pt dichalcogenides as evident in the faster heterogeneous electron transfer (HET) rates observed in electrochemically reduced Pt dichalcogenides. Interestingly, the hydrogen evolution reaction (HER) performance of the Pt dichalcogenides adheres to a trend of PtTe2 > PtSe2 > PtS2 whereby the HER catalytic property increases down the chalcogen group. Importantly, the DFT study shows this correlation to their electronic property in which PtS2 is semiconducting, PtSe2is semimetallic, and PtTe2 is metallic. Furthermore, Pt dichalcogenides are effectively activated for HER. Distinct electronic structures of Pt dichalcogenides account for their different responses to electrochemical activation. Among all activated Pt dichalcogenides, PtS2 shows most accentuated improvement as a HER electrocatalyst with an exceptional 50% decline in HER overpotential. Knowledge on Pt dichalcogenides provides valuable insights in the field of TMD electrochemistry, in particular, for the currently underrepresented Group 10 TMDs.
Noble transition metal dichalcogenides (TMDs) such as layered PtS2, PtSe2, and PtTe2 are studied for their charge transfer and electrocatalytic hydrogen evolution reaction (HER) properties. These properties correlate to their electronic structures, and trends down the chalcogen group are explored. Electrochemical activation is effective for all platinum dichalcogenides. Amidst all activated materials, PtS2 showcases most enhanced HER efficiency.
Beyond graphene, transitional metal dichalcogenides, and black phosphorus, there are other layered materials called metal thiophosphites (MPS x ), which are recently attracting the attention of ...scientists. Here we present the synthesis, structural and morphological characterization, magnetic properties, electrochemical performance, and the calculated density of states of different layered metal thiophosphite materials with a general formula MPS x , and as a result of varying the metal component, we obtain CrPS4, MnPS3, FePS3, CoPS3, NiPS3, ZnPS3, CdPS3, GaPS4, SnPS3, and BiPS4. SnPS3, ZnPS3, CdPS3, GaPS4, and BiPS4 exhibit only diamagnetic behavior due to core electrons. By contrast, trisulfides with M = Mn, Fe, Co, and Ni, as well as CrPS4, are paramagnetic at high temperatures and undergo a transition to antiferromagnetic state on cooling. Within the trisulfides series the Néel temperature characterizing the transition from paramagnetic to antiferromagnetic phase increases with the increasing atomic number and the orbital component enhancing the total effective magnetic moment. Interestingly, in terms of catalysis NiPS3, CoPS3, and BiPS4 show the highest efficiency for hydrogen evolution reaction (HER), while for the oxygen evolution reaction (OER) the highest performance is observed for CoPS3. Finally, MnPS3 presents the highest oxygen reduction reaction (ORR) activity compared to the other MPS x studied here. This great catalytic performance reported for these MPS x demonstrates their promising capabilities in energy applications.
Graphene quantum dots is a class of graphene nanomaterials with exceptional luminescence properties. Precise dimension control of graphene quantum dots produced by chemical synthesis methods is ...currently difficult to achieve and usually provides a range of sizes from 3 to 25 nm. In this work, fullerene C60 is used as starting material, due to its well-defined dimension, to produce very small graphene quantum dots (∼2–3 nm). Treatment of fullerene C60 with a mixture of strong acid and chemical oxidant induced the oxidation, cage-opening, and fragmentation processes of fullerene C60. The synthesized quantum dots were characterized and supported by LDI-TOF MS, TEM, XRD, XPS, AFM, STM, FTIR, DLS, Raman spectroscopy, and luminescence analyses. The quantum dots remained fully dispersed in aqueous suspension and exhibited strong luminescence properties, with the highest intensity at 460 nm under a 340 nm excitation wavelength. Further chemical treatments with hydrazine hydrate and hydroxylamine resulted in red- and blue-shift of the luminescence, respectively.
Two-dimensional materials attract enormous attention across several scientific fields. The current demands in nano- and optoelectronics, semiconductors, or in catalysis have been accelerating the ...research process in the field of 2D materials. Among the 14th group 2D materials besides graphene and silicene, layered germanium represents a promising candidate for another class of materials, and its functionalization represents a way to tune either its electronic or optical properties. Here, the exfoliation and functionalization of germanane surface is achieved via abstraction of hydrogen from Ge–H bond and its subsequent alkylation utilizing n-alkyl halides or trifluoromethyl (CF3) group containing benzyl halides. Composition of materials is confirmed by several methods including FT-IR, Raman, X-ray photoelectron, and energy-dispersive X-ray spectroscopy as well as X-ray powder diffraction. Scanning and transmission electron spectroscopy is used to reveal the layered morphology of functionalized germananes.
Precisely engineered changes in Fermi levels of graphene-based materials are of high importance for their applications in electronic and electrochemical devices. Such applications include ...photoelectrochemical reactions or enhanced electrochemical performance toward reduction of oxygen. Here we describe a method for scalable and tunable boron doping of graphene via thermal exfoliation of graphite oxide in BF3 atmosphere at different temperatures. The temperature and atmospheric composition during exfoliation influences the kinetics of decomposition of the reactants and levels of doping, which range from 23 to 590 ppm. The resulting materials were characterized by prompt γ-ray analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning electron microscopy. Recent claims on enhanced catalytic properties of boron-doped graphenes toward the reduction of oxygen were addressed, as well as similar claims on enhanced capacitance.
Two-dimensional transition-metal dichalcogenides (TMDs) are lately in the scope within the scientific community owing to their exploitation as affordable catalysts for next-generation energy devices. ...Undoubtedly, only precise tailoring and control over the catalytic properties can ensure high efficiency and successful implementation of such devices in day-to-day practical utilization. However, contrary to theoretical predictions, systematic experimental work dealing with the doped materials and their impact to electrocatalysis are relatively underrated despite the considerable effect that it could bring into this field. Herein, we investigate the effect of four different dopants (i.e., Ti, V, Mn, and Fe) incorporated to both layered MoS2 and WS2 as solid-state solution toward their electrocatalytic performance through their evaluation as catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Our results pointed out that doping by Mn and Fe can enhance the electrocatalytic performance toward ORR, whereas doping by Ti and V revealed poor electrocatalytic effects (inhibition) compared to both undoped MoS2 and WS2. Surprisingly, none of the dopants contributed to the improvement of either MoS2 or WS2 toward HER activity. Therefore, in addition to the experimental data, density functional theory calculations were performed to further investigate the role of the dopants in the performance of MoS2 toward HER. According to these calculations, all dopants preferably occupied the edges of the crystal structure and thus could affect the electrocatalytic properties of the initial material. However, the observed ΔG values for hydrogen adsorption revealed that MoS2 is the best catalyst with a subsequent trend for doped materials following the less negative binding energies V < Ti < Mn < Fe, which was in good agreement with experimentally obtained overpotentials of the respective samples. This study thus elucidates the reasons for negative effects of doping in TMDs. This study brings an insight that not all dopants are beneficial and not all reactions are affected in the same way by dopants in TMDs.
Layered elemental materials, such as black phosphorus, exhibit unique properties originating from their highly anisotropic layered structure. The results presented herein demonstrate an anomalous ...anisotropy for the electrical, magnetic, and electrochemical properties of black phosphorus. It is shown that heterogeneous electron transfer from black phosphorus to outer‐ and inner‐sphere molecular probes is highly anisotropic. The electron‐transfer rates differ at the basal and edge planes. These unusual properties were interpreted by means of calculations, manifesting the metallic character of the edge planes as compared to the semiconducting properties of the basal plane. This indicates that black phosphorus belongs to a group of materials known as topological insulators. Consequently, these effects render the magnetic properties highly anisotropic, as both diamagnetic and paramagnetic behavior can be observed depending on the orientation in the magnetic field.
A shift in direction: The electrical, magnetic, and electrochemical properties of black phosphorus display an anomalous anisotropy. These unusual observations were interpreted by means of calculations, which manifested the metallic character of the edge plane and the semiconductivity of the basal plane, indicating that black phosphorus belongs to a group of materials known as topological insulators.
Fully hydrogenated graphene (graphane) and partially hydrogenated graphene materials are expected to possess various fundamentally different properties from graphene. We have prepared highly ...hydrogenated graphene containing 5% wt of hydrogen via Birch reduction of graphite oxide using elemental sodium in liquid NH3 as electron donor and methanol as proton donor in the reduction. We also investigate the influence of preparation method of graphite oxide, such as the Staudenmaier, Hofmann or Hummers methods on the hydrogenation rate. A control experiment involving NaNH2 instead of elemental Na was also performed. The materials were characterized in detail by electron microscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy both at room and low temperatures, X-ray fluorescence spectroscopy, inductively coupled plasma optical emission spectroscopy, combustible elemental analysis and electrical resistivity measurements. Magnetic measurements are provided of bulk quantities of highly hydrogenated graphene. In the whole temperature range up to room temperature, the hydrogenated graphene exhibits a weak ferromagnetism in addition to a contribution proportional to field that is caused not only by diamagnetism but also likely by an antiferromagnetic influence. The origin of the magnetism is also determined to arise from the hydrogenated graphene itself, and not as a result of any metallic impurities.
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