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1.
Langmuir ; 2020 Jul 02.
Artigo em Inglês | MEDLINE | ID: mdl-32568544

RESUMO

The injection of electrons from the scanning tunneling microscope tip can be used to perform nanoscale chemistry and study hot electron transport through surfaces. While nonlocal manipulation has been demonstrated primarily for aromatic adsorbates, here we confirm that oxygen atoms bonded to the Si(111) surface can also be nonlocally manipulated, and we fit the measured manipulation data to a single channel decay model. Unlike aromatic adsorption systems, oxygen atoms also insert below the surface of silicon. Although the inserted oxygen can be manipulated when the tip is directly over the relevant silicon adatom, it is not possible to induce nonlocal manipulation of inserted oxygen atoms at the same bias. We attribute this to the electrons injected at +4 eV initially relaxing to couple to the highest available surface state at +3.4 eV before laterally transporting through the surface. With a manipulation threshold of 3.8 eV for oxygen inserted into silicon, once carriers have undergone lateral transport, they do not possess enough energy to manipulate and remove oxygen atoms inserted beneath the surface of silicon. This result confirms that nonlocal nanoscale chemistry using the scanning tunneling microscope tip is dependent not only on the energy required for atomic manipulation, but also on the energy of the available surface states to carry the electrons to the manipulation site.

2.
Artigo em Inglês | MEDLINE | ID: mdl-32391685

RESUMO

The deposition of precisely controlled clusters from the beam onto suitable supports represents a novel method to prepare advanced cluster-based catalysts. In principle, cluster size, composition, and morphology can be tuned or selected prior to deposition. The newly invented matrix assembly cluster source (MACS) offers one solution to the long-standing problem of low cluster deposition rate. Demonstrations of the cluster activities under realistic reaction conditions are now needed. We deposited elemental silver (Ag) and gold (Au) clusters onto gram-scale powders of commercial titanium dioxide (TiO2) to investigate the catalytic oxidation of nitrophenol (a representative pollutant in water) by ozone in aqueous solution, as relevant to the removal of waste drugs from the water supply. A range of techniques, including scanning transmission electron microscopy (STEM), Brunauer-Emmett-Teller (BET) surface area test, and X-ray photoelectron spectroscopy (XPS), were employed to reveal the catalyst size, morphology, surface area, and oxidation state. Both the Ag and Au cluster catalysts proved active for the nitrophenol ozonation. The cluster catalysts showed activities at least comparable to those of catalysts made by traditional chemical methods in the literature, demonstrating the potential applications of the cluster beam deposition method for practical heterogeneous catalysis in solution.

3.
Nanoscale ; 2020 Mar 04.
Artigo em Inglês | MEDLINE | ID: mdl-32129392

RESUMO

We report a new approach to design flexible functional material platforms based on electropolymerized polyaniline (PANI) polymer nanofilms modified with bimetallic nanoclusters (NCs) for efficient electro-oxidation of small organic molecules. Composition defined ligand free Pt0.75Ni0.25 NCs were synthesized in the gas phase using the Cluster Beam Deposition (CBD) technology and characterized using RToF, HAADF-STEM, XAFS and XPS. NCs were then directly deposited on PANI coated templates to construct electrodes. Dopamine (DP) molecules were used as a representative organic analyte and the influence of the NC-PANI hybrid atomistic structure on the electrochemical and electrocatalytic performance was investigated. The as prepared, nearly monodispersed, Pt0.75Ni0.25 NCs of ca. 2 nm diameter featuring a PtOx surface combined with a shallow platelet-like Ni-O(OH) phase formed a densely packed active surface on PANI at ultralow metal coverages. Electrochemical measurements (EIS and CV) show a 2.5 times decrease in charge transfer resistance and a remarkable 6-fold increase at lower potential in the mass activity for Pt0.75Ni0.25 NCs in comparison with their pure Pt counterparts. The enhanced electrochemical performance of the Pt0.75Ni0.25 NC hybrid's interface is ascribed to the formation of mixed Pt metal and Ni-O(OH) phases at the surface of the alloyed PtNi cores of the bimetallic NCs under electrochemical conditions combined with an efficient charge conduction pathway between NCs.

4.
Nanoscale ; 12(14): 7688-7699, 2020 Apr 09.
Artigo em Inglês | MEDLINE | ID: mdl-32211622

RESUMO

The atomic configuration of nanoparticles is key to their properties, in a way that resembles the structure-function correlation of biomolecules, and depends on the growth mechanism. Coalescence is often an important step in the growth of nanoparticles, both in liquid and in the gas phase. In coalescence, two preformed clusters collide and merge to form a larger aggregate, whose shape evolves from an initial configuration, which is strongly out of equilibrium, towards more compact structures. Here the coalescence in the gas phase of gold clusters of size ∼2 nm is simulated by the Molecular Dynamics (MD) method. Our simulations reveal a persistent influence of the structure and relative orientation of the initial colliding clusters on the final coalesced aggregates, even after more than 1 µs at temperature T = 500 K. This result is interpreted in terms of a special type of kinetic trapping, in which the choice of the structural motif takes place in the initial stages of the coalescence process. In the subsequent evolution, the coalescing aggregate may equilibrate within that motif, while transformations between different motifs are not observed, so that inter-motif equilibration is not achieved. Inter-motif equilibration is only achieved at 550 K and above. The simulations also show that aggregate reshaping occurs by a variety of specific transformation pathways, which often involve the concerted displacements of many atoms.

5.
Nanoscale ; 12(7): 4459-4472, 2020 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-32030382

RESUMO

The development of non-platinum group metal catalysts for the hydrogen evolution reaction (HER) in water electrolyser devices is essential for their widespread and sustainable deployment. In recent years, molybdenum disulfide (MoS2) catalysts have received significant attention as they not only exhibit good electrocatalytic HER activity but also, crucially, acid-stability. However, further performance enhancement is required for these materials to be competitive with Pt and to that end transition metal doping of MoS2 has been explored as a route to further increasing its catalytic activity. In this work, cluster beam deposition was employed to produce controlled cobalt-doped MoS2 clusters (MoS2-Co). We demonstrate that, in contrast to previous observations of performance enhancement in MoS2 resulting from nickel doping (MoS2-Ni), the introduction of Co has a detrimental effect on HER activity. The contrasting behaviours of Ni and Co doping are rationalized by density functional theory (DFT) calculations, which suggest that HER-active surface vacancies are deactivated by combination with Co dopant atoms, whilst their activity is retained, or even partially enhanced, by combination with Ni dopant atoms. Furthermore, the adatom dopant-vacancy combination kinetics appear to be more than three orders of magnitude faster in MoS2-Co than for MoS2-Ni. These findings highlight a fundamental difference in the influence of transition metal dopants on the HER performance of MoS2 electrocatalysts and stress the importance of considering surface atomic defects when predicting their behaviour.

6.
PLoS One ; 14(6): e0217483, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31173616

RESUMO

The potential hazard posed by nanomaterials can be significantly influenced by transformations which these materials undergo during their lifecycle, from manufacturing through to disposal. The transformations may depend on the nanomaterials' own physicochemical properties as well as the environment they are exposed to. This study focuses on the mechanisms of transformation of cerium oxide nanoparticles (CeO2 NPs) in laboratory experiments which simulate potential scenarios in which the NPs are exposed to phosphate-bearing media. We have experimented with the transformation of four different kinds of CeO2 NPs, in order to investigate the effects of nanoparticle size, capping agent (three were uncapped and one was PVP capped) and oxidation state (two consisted mostly of Ce4+ and two were a mix of Ce3+/Ce4+). They were exposed to a reaction solution containing KH2PO4, citric acid and ascorbic acid at pH values of 2.3, 5.5 and 12.3, and concentrations of 1mM and 5mM. The transformations were followed by UV-vis, zeta potential and XRD measurements, which were taken after 7 and 21 days, and by transmission electron microscopy after 21 days. X-ray photoelectron spectroscopy was measured at 5mM concentration after 21 days for some samples. Results show that for pH 5 and 5mM phosphate concentration, CePO4 NPs were formed. Nanoparticles that were mostly Ce4+ did not dissolve at 1mM reagent concentration, and did not produce CePO4 NPs. When PVP was present as a capping agent it proved to be an extra reducing agent, and CePO4 was found under all conditions used. This is the first paper where the transformation of CeO2 NPs in the presence of phosphate has been studied for particles with different size, shapes and capping agents, in a range of different conditions and using many different characterisation methods.


Assuntos
Cério/química , Nanopartículas/química , Fosfatos/química , Concentração de Íons de Hidrogênio , Nanopartículas/ultraestrutura , Oxirredução , Tamanho da Partícula
7.
Acc Chem Res ; 51(9): 2296-2304, 2018 09 18.
Artigo em Inglês | MEDLINE | ID: mdl-30188111

RESUMO

It is hard to predict the future of science. For example, when C60 and its structure were identified from the mass spectra of gas phase carbon clusters, few could have predicted the era of carbon nanotechnology which the discovery introduced. The solubilization and functionalization of C60, the identification and then synthesis of carbon nanotubes, and the generation and physics of graphene have made a scale of impact on the international R&D (and to some extent industrial) landscape which could not have been foreseen. Technology emerged from a search for molecules of astrochemical interest in the interstellar gas. This little sketch provides the authors with the confidence to present here a status report on progress toward another radical future-the synthesis of nanoparticles (typically metals) on an industrial scale without solvents and consequently effluents, without salts and their sometimes accompanying toxicity, with minimal prospects for unwanted nanoparticle escape into the environment, with a high degree of precision in the control of the size, shape and composition of the nanoparticles produced and with applications from catalysts and sensors to photonics, electronics and theranostics. In fact, our story begins in exactly the same place as the origin of the nanocarbon era-the generation and mass selection of free atomic clusters in a vacuum chamber. The steps along the path so far include deposition of such beams of clusters onto surfaces in vacuum, elucidation of the key elements of the cluster-surface interaction, and demonstrations of the potential applications of deposited clusters. The principal present challenges, formidable but solvable, are the necessary scale-up of cluster beam deposition from the nanogram to the gram scale and beyond, and the processing and integration of the nanoclusters into appropriate functional architectures, such as powders for heterogeneous catalysis, i.e., the formulation engineering problem. The research which is addressing these challenges is illustrated in this Account by examples of cluster production (on the traditional nanogram scale), emphasizing self-selection of size, controlled generation of nonspherical shapes, and nonspherical binary nanoparticles; by the scale-up of cluster beam production by orders of magnitude with the magnetron sputtering, gas condensation cluster source, and especially the Matrix Assembly Cluster Source (MACS); and by promising demonstrations of deposited clusters in gas sensing and in heterogeneous catalysis (this on the gram scale) in relevant environments (both liquid and vapor phases). The impact on manufacturing engineering of the new paradigm described here is undoubtedly radical; the prospects for economic success are, as usual, full of uncertainties. Let the readers form their own judgements.

8.
ACS Nano ; 12(8): 7710-7720, 2018 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-29995378

RESUMO

We study the ultrafast structural dynamics, in response to electronic excitations, in heterostructures composed of size-selected Au nanoclusters on thin-film substrates with the use of femtosecond electron diffraction. Various forms of atomic motion, such as thermal vibrations, thermal expansion, and lattice disordering, manifest as distinct and quantifiable reciprocal-space observables. In photoexcited supported nanoclusters, thermal equilibration proceeds through intrinsic heat flow between their electrons and their lattice and extrinsic heat flow between the nanoclusters and their substrate. For an in-depth understanding of this process, we have extended the two-temperature model to the case of 0D/2D heterostructures and used it to describe energy flow among the various subsystems, to quantify interfacial coupling constants and to elucidate the role of the optical and thermal substrate properties. When lattice heating of Au nanoclusters is dominated by intrinsic heat flow, a reversible disordering of atomic positions occurs, which is absent when heat is injected as hot substrate phonons. The present analysis indicates that hot electrons can distort the lattice of nanoclusters, even if the lattice temperature is below the equilibrium threshold for surface premelting. Based on simple considerations, the effect is interpreted as activation of surface diffusion due to modifications of the potential energy surface at high electronic temperatures. We discuss the implications of such a process in structural changes during surface chemical reactions.

9.
Phys Chem Chem Phys ; 20(29): 19486-19491, 2018 Jul 25.
Artigo em Inglês | MEDLINE | ID: mdl-29998269

RESUMO

We report the discovery of bridge-bonded methylthiolate, SCH3, along the step edges of the Au(111) surface. Real-space imaging with a scanning tunnelling microscope reveals the presence of bridge-bonded SCH3 along both the [11[combining macron]0] and the [112[combining macron]] oriented step edges. The nearest neighbour distances of SCH3 along these steps are 2a and , respectively. The Au(111) terrace is covered with the usual CH3SAuSCH3 staples. The bridge-bonded alkanethiolate is expected to play a rather significant role in the formation of thiol-passivated Au nanoclusters because of the high fraction of atoms in similar low-coordination sites.

10.
Nano Lett ; 18(8): 5257-5261, 2018 08 08.
Artigo em Inglês | MEDLINE | ID: mdl-30001140

RESUMO

The shape of individual building blocks is an important parameter in bottom-up self-assembly of nanostructured materials. A simple shape change from sphere to spheroid can significantly affect the assembly process due to the modification to the orientational degrees of freedom. When a layer of spheres is placed upon a layer of spheroids, the strain at the interface can be minimized by the spheroid taking a special orientation. C70 fullerenes represent the smallest spheroids, and their interaction with a sphere-like C60 is investigated. We find that the orientation of the C70 within a close-packed C70 layer can be steered by contacting a layer of C60. This orientational steering phenomenon is potentially useful for epitaxial growth of multilayer van der Waals molecular heterostructures.

11.
Langmuir ; 34(28): 8388-8392, 2018 07 17.
Artigo em Inglês | MEDLINE | ID: mdl-29924615

RESUMO

We report controlled manipulation of magic number gold-fullerene clusters, (C60) m-(Au) n, on a Au(111) substrate at 110 K using scanning tunneling microscopy (STM). Each cluster consists of a two-dimensional gold island of nAu atoms confined by a frame of mC60 molecules. Using STM, C60 molecules are extracted from the molecular frame one at a time. The extraction is conducted by driving the STM tip into the cluster, leading to one of the molecules being squeezed out of the frame. Unlike at room temperature, the extracted molecules do not move away from the cluster because of the lack of thermal energy at 110 K; they are found to be attached to the outside of the frame. Reversible manipulation is also possible by pushing an extracted molecule back into the frame. This reversible manipulation is possible only for molecules from the edge of the cluster.

12.
Nanoscale ; 10(14): 6684-6694, 2018 Apr 05.
Artigo em Inglês | MEDLINE | ID: mdl-29589035

RESUMO

The structure and atomic ordering of Au-Ag nanoparticles grown in the gas phase are determined by a combination of HAADF-STEM, XPS and Refl-XAFS techniques as a function of composition. It is shown consistently from all the techniques that an inversion of chemical ordering takes place by going from Au-rich to Ag-rich compositions, with the minority element always occupying the nanoparticle core, and the majority element enriching the shell. With the aid of DFT calculations, this composition-tunable chemical arrangement is rationalized in terms of a four-step growth process in which the very first stage of cluster nucleation plays a crucial role. The four-step growth mechanism is based on mechanisms of a general character, likely to be applicable to a variety of binary systems besides Au-Ag.

13.
Small ; 14(13): e1703734, 2018 03.
Artigo em Inglês | MEDLINE | ID: mdl-29412512

RESUMO

The deposition of preformed nanocluster beams onto suitable supports represents a new paradigm for the precise preparation of heterogeneous catalysts. The performance of the new materials must be validated in model catalytic reactions. It is shown that gold/copper (Au/Cu) nanoalloy clusters (nanoparticles) of variable composition, created by sputtering and gas phase condensation before deposition onto magnesium oxide powders, are highly active for the catalytic reduction of 4-nitrophenol in solution at room temperature. Au/Cu bimetallic clusters offer decreased catalyst cost compared with pure Au and the prospect of beneficial synergistic effects. Energy-dispersive X-ray spectroscopy coupled with aberration-corrected scanning transmission electron microscopy imaging confirms that the Au/Cu bimetallic clusters have an alloy structure with Au and Cu atoms randomly located. Reaction rate analysis shows that catalysts with approximately equal amounts of Au and Cu are much more active than Au-rich or Cu-rich clusters. Thus, the interplay between the Au and Cu atoms at the cluster surface appears to enhance the catalytic activity substantially, consistent with model density functional theory calculations of molecular binding energies. Moreover, the physically deposited clusters with Au/Cu ratio close to 1 show a 25-fold higher activity than an Au/Cu reference sample made by chemical impregnation.

14.
Nanoscale ; 10(5): 2363-2370, 2018 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-29328339

RESUMO

Au nanoparticles represent the most remarkable example of a size effect in heterogeneous catalysis. However, a major issue hindering the use of Au nanoparticles in technological applications is their rapid sintering. We explore the potential of stabilizing Au nanoclusters on SiO2 by alloying them with a reactive metal, Ti. Mass-selected Au/Ti clusters (400 000 amu) and Au2057 clusters (405 229 amu) were produced with a magnetron sputtering, gas condensation cluster beam source in conjunction with a lateral time-of-flight mass filter, deposited onto a silica support and characterised by XPS and LEIS. The sintering dynamics of mass-selected Au and Au/Ti alloy nanoclusters were investigated in real space and real time with atomic resolution aberration-corrected HAADF-STEM imaging, supported by model DFT calculations. A strong anchoring effect was revealed in the case of the Au/Ti clusters, because of a much increased local interaction with the support (by a factor 5 in the simulations), which strongly inhibits sintering, especially when the clusters are more than ∼0.60 nm apart. Heating the clusters at 100 °C for 1 h in a mixture of O2 and CO, to simulate CO oxidation conditions, led to some segregation in the Au/Ti clusters, but in line with the model computational investigation, Au atoms were still present on the surface. Thus size-selected, deposited nanoalloy Au/Ti clusters appear to be promising candidates for sustainable gold-based nanocatalysis.

15.
Nano Lett ; 17(10): 6171-6176, 2017 10 11.
Artigo em Inglês | MEDLINE | ID: mdl-28906122

RESUMO

We demonstrate cascade manipulation between magic number gold-fullerene hybrid clusters by channelling thermal energy into a specific reaction pathway with a trigger from the tip of a scanning tunnelling microscope (STM). The (C60)m-Aun clusters, formed via self-assembly on the Au(111) surface, consist of n Au atoms and m C60 molecules; the three smallest stable clusters are (C60)7-Au19, (C60)10-Au35, and (C60)12-Au49. The manipulation cascade was initiated by driving the STM tip into the cluster followed by tip retraction. Temporary, partial fragmentation of the cluster was followed by reorganization. Self-selection of the correct numbers of Au atoms and C60 molecules led to the formation of the next magic number cluster. This cascade manipulation is efficient and facile with an extremely high selectivity. It offers a way to perform on-surface tailoring of atomic and molecular clusters by harnessing thermal energy, which is known as the principal enemy of the quest to achieve ultimate structural control with the STM.

16.
Nanotechnology ; 28(32): 325601, 2017 Aug 11.
Artigo em Inglês | MEDLINE | ID: mdl-28718457

RESUMO

A multilayer deposition method has been developed with the potential to capture and process atomic clusters generated by a high flux cluster beam source. In this deposition mode a series of sandwich structures each consisting of three layers-a carbon support layer, cluster layer and polymer release layer-is sequentially deposited to form a stack of isolated cluster layers, as confirmed by through-focal aberration-corrected HAADF STEM analysis. The stack can then be diced into small pieces by a mechanical saw. The diced pieces are immersed in solvent to dissolve the polymer release layer and form small platelets of supported clusters.

17.
Environ Sci Technol ; 51(14): 8010-8016, 2017 Jul 18.
Artigo em Inglês | MEDLINE | ID: mdl-28618231

RESUMO

Ceria nanoparticles (NPs) rapidly and easily cycle between Ce(III) and Ce(IV) oxidation states, making them prime candidates for commercial and other applications. Increased commercial use has resulted in increased discharge to the environment and increased associated risk. Once in complex media such as environmental waters or toxicology exposure media, the same redox transformations can occur, causing altered behavior and effects compared to the pristine NPs. This study used high resolution scanning transmission electron microscopy and electron energy loss spectroscopy to investigate changes in structure and oxidation state of small, polymer-coated ceria suspensions in complex media. NPs initially in either the III or IV oxidation states, but otherwise identical, were used. Ce(IV) NPs were changed to mixed (III, IV) NPs at high ionic strengths, while the presence of natural organic macromolecules (NOM) stabilized the oxidation state and increased crystallinity. The Ce(III) NPs remained as Ce(III) at high ionic strengths, but were modified by the presence of NOM, causing reduced crystallinity and degradation of the NPs. Subtle changes to NP properties upon addition to environmental or ecotoxicology media suggest that there may be small but important effects on fate and effects of NPs compared to their pristine form.


Assuntos
Cério , Nanopartículas , Ecotoxicologia , Espectroscopia de Perda de Energia de Elétrons , Suspensões
18.
Ultramicroscopy ; 176: 146-150, 2017 05.
Artigo em Inglês | MEDLINE | ID: mdl-28342572

RESUMO

Monolayer-protected (MP) Au clusters present attractive quantum systems with a range of potential applications e.g. in catalysis. Knowledge of the atomic structure is needed to obtain a full understanding of their intriguing physical and chemical properties. Here we employed aberration-corrected scanning transmission electron microscopy (ac-STEM), combined with multislice simulations, to make a round-robin investigation of the atomic structure of chemically synthesised clusters with nominal composition Au144(SCH2CH2Ph)60 provided by two different research groups. The MP Au clusters were "weighed" by the atom counting method, based on their integrated intensities in the high angle annular dark field (HAADF) regime and calibrated exponent of the Z dependence. For atomic structure analysis, we compared experimental images of hundreds of clusters, with atomic resolution, against a variety of structural models. Across the size range 123-151 atoms, only 3% of clusters matched the theoretically predicted Au144(SR)60 structure, while a large proportion of the clusters were amorphous (i.e. did not match any model structure). However, a distinct ring-dot feature, characteristic of local icosahedral symmetry, was observed in about 20% of the clusters.

19.
Nanotechnology ; 28(10): 105711, 2017 Mar 10.
Artigo em Inglês | MEDLINE | ID: mdl-28082730

RESUMO

A compact proximal retarding field analyzer for scanning probe energy loss spectroscopy measurements is described. Using the scanning tunneling microscope (STM) tip as a field emission (FE) electron source in conjunction with this analyzer, which is placed at a glancing angle to the surface plane, FE sample current and electron reflectivity imaging may be performed simultaneously. This is demonstrated in measurements of Ag nanostructures prepared on graphite by electron-beam lithography, where a material contrast of 13% is observed, with a lateral resolution of 25 nm, between the silver and graphite in electron reflectivity images. Topological contrast mechanisms such as edge enhancement and shadowing are also observed, giving rise to additional features in the electron reflectivity images. The same instrument configuration has been used to measure electron energy loss spectra on bare graphite, where the zero loss peak, π band plasmon loss peak and secondary electron peaks are observed. Using this simple and compact analyzer an STM, with sufficient open access to the tip-sample junction, may easily be augmented to provide simultaneous elemental and topographic mapping, supplementing STM image measurements with FE sample current and electron reflectivity images, as well as electron energy loss spectroscopy measurements, in the same instrument.

20.
ACS Catal ; 6(9): 6008-6017, 2016 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-27818842

RESUMO

This report focuses on a novel strategy for the preparation of transition metal-MoS2 hybrid nanoclusters based on a one-step, dual-target magnetron sputtering, and gas condensation process demonstrated for Ni-MoS2. Aberration-corrected STEM images coupled with EDX analysis confirms the presence of Ni and MoS2 in the hybrid nanoclusters (average diameter = 5.0 nm, Mo:S ratio = 1:1.8 ± 0.1). The Ni-MoS2 nanoclusters display a 100 mV shift in the hydrogen evolution reaction (HER) onset potential and an almost 3-fold increase in exchange current density compared with the undoped MoS2 nanoclusters, the latter effect in agreement with reported DFT calculations. This activity is only reached after air exposure of the Ni-MoS2 hybrid nanoclusters, suggested by XPS measurements to originate from a Ni dopant atoms oxidation state conversion from metallic to 2+ characteristic of the NiO species active to the HER. Anodic stripping voltammetry (ASV) experiments on the Ni-MoS2 hybrid nanoclusters confirm the presence of Ni-doped edge sites and reveal distinctive electrochemical features associated with both doped Mo-edge and doped S-edge sites which correlate with both their thermodynamic stability and relative abundance.

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