Significant elemental segregation is shown to exist within individual hollow silver–gold (Ag–Au) bimetallic nanoparticles obtained from the galvanic reaction between Ag particles and AuCl4 –. ...Three-dimensional compositional mapping using energy dispersive X-ray (EDX) tomography within the scanning transmission electron microscope (STEM) reveals that nanoparticle surface segregation inverts from Au-rich to Ag-rich as Au content increases. Maximum Au surface coverage was observed for nanoparticles with approximately 25 atom % Au, which correlates to the optimal catalytic performance in a three-component coupling reaction among cyclohexane carboxyaldehyde, piperidine, and phenylacetylene.
In this study, aberration-corrected scanning transmission electron microscopy is employed to investigate the morphology of Au clusters formed from the aggregation of single atoms sputtered onto an ...amorphous carbon surface. The morphologies of surface-assembled clusters of
N
> 100 atoms are referenced against the morphologies of size-selected clusters determined from previously published results. We observe that surface-assembled clusters (at the conditions employed here) are approximately spherical in shape. The structural isomers of the imaged clusters have also been identified, and the distribution of structural types is broadly in agreement with those from size-selected cluster deposition sources. For clusters of approximately 147 atoms, we find a preference for icosahedra over decahedra and truncated octahedra, but at this size there is a high proportion of unidentified/amorphous structures. At around 309 atoms, we find a preference for decahedra over icosahedra and truncated octahedra, but over half the structures remain unidentifiable/amorphous. For sizes above approximately 561 atoms we are able to identify most of the structures, and find decahedra are still the most favoured, although in competition with single-crystal fcc morphologies. The similarity in structure between surface-assembled and size-selected clusters from a cluster source provides evidence of the relevance of size-selected cluster studies to clusters synthesised by other, industrially relevant, methodologies.
Au nanoparticles over 100 atoms in size formed
via
surface-growth of sputtered atoms on amorphous carbon are approximately spherical in morphology. The distribution of particle structural isomers was found to match those formed
via
gas-condensation.
The rise in atmospheric CO2 concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO2 as a feedstock. ...Prime among these is the hydrogenation of CO2 to make methanol, which is a key commodity chemical intermediate, a hydrogen storage molecule, and a possible future fuel for transport sectors that cannot be electrified. Pd/ZnO has been identified as an effective candidate as a catalyst for this reaction, yet there has been no attempt to gain a fundamental understanding of how this catalyst works and more importantly to establish specific design criteria for CO2 hydrogenation catalysts. Here, we show that Pd/ZnO catalysts have the same metal particle composition, irrespective of the different synthesis procedures and types of ZnO used here. We demonstrate that all of these Pd/ZnO catalysts exhibit the same activity trend. In all cases, the β-PdZn 1:1 alloy is produced and dictates the catalysis. This conclusion is further supported by the relationship between conversion and selectivity and their small variation with ZnO surface area in the range 6–80 m2g–1. Without alloying with Zn, Pd is a reverse water-gas shift catalyst and when supported on alumina and silica is much less active for CO2 conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO2 hydrogenation and will aid future catalyst discovery.
This paper presents an investigation of the limitations and optimisation of energy dispersive X-ray (EDX) tomography within the scanning transmission electron microscope, focussing on application of ...the technique to characterising the 3D elemental distribution of bimetallic AgAu nanoparticles. The detector collection efficiency when using a standard tomography holder is characterised using a tomographic data set from a single nanoparticle and compared to a standard low background double tilt holder. Optical depth profiling is used to investigate the angles and origin of detector shadowing as a function of specimen field of view. A novel time-varied acquisition scheme is described to compensate for variations in the intensity of spectrum images at each sample tilt. Finally, the ability of EDX spectrum images to satisfy the projection requirement for nanoparticle samples is discussed, with consideration of the effect of absorption and shadowing variations.
•We investigate the methodology of STEM-EDX tomography of nanoparticles.•We present a time-varied acquisition scheme to compensate for detector shadowing.•The ability of STEM-EDX tomography to meet the projection requirement is discussed.
We report a rapid solution-phase strategy to synthesize alloyed PtNi nanoparticles which demonstrate outstanding functionality for the oxygen reduction reaction (ORR). This one-pot coreduction ...colloidal synthesis results in a monodisperse population of single-crystal nanoparticles of rhombic dodecahedral morphology with Pt-enriched edges and compositions close to Pt1Ni2. We use nanoscale 3D compositional analysis to reveal for the first time that oleylamine (OAm)-aging of the rhombic dodecahedral Pt1Ni2 particles results in Ni leaching from surface facets, producing aged particles with concave faceting, an exceptionally high surface area, and a composition of Pt2Ni1. We show that the modified atomic nanostructures catalytically outperform the original PtNi rhombic dodecahedral particles by more than two-fold and also yield improved cycling durability. Their functionality for the ORR far exceeds commercially available Pt/C nanoparticle electrocatalysts, both in terms of mass-specific activities (up to a 25-fold increase) and intrinsic area-specific activities (up to a 27-fold increase).
The microstructure and mechanisms of reinforcement have been investigated in nanocomposites consisting of graphene nanoplatelets (GNPs) in natural rubber (NR). Nanocomposites with four different ...loadings of three different sized GNPs were prepared and were bench-marked against nanocomposites loaded with N330 carbon black. The microstructure of the nanocomposites was characterised through a combination of scanning electron microscopy, polarised Raman spectroscopy and X-ray computed tomography (CT), where it was shown that the GNPs were well dispersed with a preferred orientation parallel to the surface of the nanocomposite sheets. The mechanical properties of the nanocomposites were evaluated using tensile testing, and it was shown that, for a given loading, there was a three times greater increase in stiffness for the GNPs than for the carbon black. Stress transfer from the NR to the GNPs was evaluated from stress-induced Raman bands shifts indicating that the effective Young’s modulus of the GNPs in the NR was of the order of 100 MPa, similar to the value evaluated using the rule of mixtures from the stress–strain data.
Despite extensive efforts to develop high‐performance H2 evolution catalysts, this remains a major challenge. Here, we demonstrate the use of Cd/Pt precursor solutions for significant photocatalytic ...H2 production (154.7 mmol g−1 h−1), removing the need for a pre‐synthesized photocatalyst. In addition, we also report simultaneous in situ synthesis of Pt single‐atoms anchored CdS nanoparticles (PtSA‐CdSIS) during photoirradiation. The highly dispersed in situ incorporation of extensive Pt single atoms on CdSIS enables the enhancement of active sites and suppresses charge recombination, which results in exceptionally high solar‐to‐hydrogen conversion efficiency of ≈1 % and an apparent quantum yield of over 91 % (365 nm) for H2 production. Our work not only provides a promising strategy for maximising H2 production efficiency but also provides a green process for H2 production and the synthesis of highly photoactive PtSA‐CdSIS nanoparticles.
This work reports the use of Cd/Pt precursor solutions for substantially higher photocatalytic H2 production (154.7 mmol g−1 h−1), thereby eliminating the need for pre‐synthesized photocatalysts for H2 production. Furthermore, in parallel to H2 production, a simplified solar light‐assisted in situ synthesis of Pt single‐atom anchored CdS nanoparticles (PtSA‐CdSIS) has also been demonstrated.
The anomalous cyclic coarsening behaviour of γ′ precipitates after ageing at 1073 K has been investigated for the low misfit commercial powder metallurgy (PM) Ni-based superalloy RR1000. Using ...scanning transmission electron microscope (STEM) imaging combined with absorption-corrected energy-dispersive X-ray (EDX) spectroscopy, the elemental segregation as a function of coarsening behaviour has been experimentally observed for secondary γ′ precipitates. Elemental EDX spectrum imaging has revealed nanoscale enrichment of Co and Cr and a depletion of Al and Ti within the γ matrix close to the γ-γʹ interface. Our experimental results, coupled with complementary modelling and synchrotron X-ray diffraction analysis, demonstrate the importance of elastic strain energy resulting from local compositional variations for influencing precipitate morphology. In particular, elemental inhomogeneities, as a result of complex diffusive interactions within both matrix and precipitates, play a crucial role in determining the rate of coarsening. Our findings provide important new evidence for understanding the microstructural evolution observed for advanced superalloys when they are exposed to different heat treatment regimes.
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Tailoring nanoparticles’ composition and morphology is of particular interest for improving their performance for catalysis. A challenge of this approach is that the nanoparticles’ optimized initial ...structure often changes during use. Visualizing the three dimensional (3D) structural transformation in situ is therefore critical, but often prohibitively difficult experimentally. Although electron tomography provides opportunities for 3D imaging, restrictions in the tilt range of in situ holders together with electron dose considerations limit the possibilities for in situ electron tomography studies. Here, an in situ 3D imaging methodology is presented using single particle reconstruction (SPR) that allows 3D reconstruction of nanoparticles with controlled electron dose and without tilting the microscope stage. This in situ SPR methodology is employed to investigate the restructuring and elemental redistribution within a population of PtNi nanoparticles at elevated temperatures. The atomic structure of PtNi is further examined and a heat‐induced transition is found from a disordered to an ordered phase. Changes in structure and elemental distribution are linked to a loss of catalytic activity in the oxygen reduction reaction. The in situ SPR methodology employed here can be extended to a wide range of in situ studies employing not only heating, but gaseous, aqueous, or electrochemical environments to reveal in‐operando nanoparticle evolution in 3D.
An in situ 3D reconstruction methodology based on scanning transmission electron microscopy is presented to characterize the morphology and composition of bimetallic nanoparticle catalysts, requiring no tilting and using a low electron dose. PtNi nanoparticles are found to lose their Pt‐rich surface regions on heating and become increasingly homogenous, which is correlated with a decrease in the oxygen reduction activity.
State of the art electrocatalysts for the hydrogen evolution reaction (HER) are based on metal nanoparticles (NPs). It has been shown that the localized surface plasmon resonance (LSPR) excitation in ...plasmonic NPs can be harvested to accelerate a variety of molecular transformations. This enables the utilization of visible light as an energy input to enhance HER performances. However, most metals that are active toward the HER do not support LSPR excitation in the visible or near-IR ranges. We describe herein the synthesis of gold–rhodium core–shell nanoflowers (Au@Rh NFs) that are composed of a core made up of spherical Au NPs and shells containing Rh branches. The Au@Rh NFs were employed as a model system to probe how the LSPR excitation from Au NPs can lead to an enhancement in the HER performance for Rh. Our data demonstrate that the LSPR excitation at 533 nm (and 405 nm) leads to an improvement in the HER performance of Rh, which depends on the morphological features of the Au@Rh NFs, offering opportunities for optimization of the catalytic performance. Control experiments indicate that this improvement originates from the stronger interaction of Au@Rh NFs with H2O molecules at the surface, leading to an icelike configuration, which facilitated the HER under LSPR excitation.
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