Daniel M. Chevrier

Photochemical route for synthesizing atomically dispersed palladium catalysts

Lightly dispersed palladium Catalysts made from atomically dispersed metal atoms on oxide supports can exhibit very high per atom activity. However, the low loadings needed to prevent metal particle formation can limit overall performance. Liu et al. stably decorated titanium oxide nanosheets with relatively high loadings of single palladium atoms by reducing the ions with ultraviolet light and ethylene glycol. These catalysts cleaved H2 into atoms and were highly effective for hydrogenating alkenes and aldehydes. Science, this issue p. 797 Ultraviolet light and ethylene glycol enable decoration of titanium oxide nanosheets with high loading of palladium atoms. Atomically dispersed noble metal catalysts often exhibit high catalytic performances, but the metal loading density must be kept low (usually below 0.5%) to avoid the formation of metal nanoparticles through sintering. We report a photochemical strategy to fabricate a stable atomically dispersed palladium–titanium oxide catalyst (Pd1/TiO2) on ethylene glycolate (EG)–stabilized ultrathin TiO2 nanosheets containing Pd up to 1.5%. The Pd1/TiO2 catalyst exhibited high catalytic activity in hydrogenation of C=C bonds, exceeding that of surface Pd atoms on commercial Pd catalysts by a factor of 9. No decay in the activity was observed for 20 cycles. More important, the Pd1/TiO2-EG system could activate H2 in a heterolytic pathway, leading to a catalytic enhancement in hydrogenation of aldehydes by a factor of more than 55.

Identification of a Highly Luminescent Au22(SG)18 Nanocluster

The luminescence property of thiolated gold nanoclusters (Au NCs) is thought to involve the Au(I)-thiolate motifs on the NC surface; however, this hypothesis remains largely unexplored because of the lack of precise molecular composition and structural information of highly luminescent Au NCs. Here we report a new red-emitting thiolated Au NC, which has a precise molecular formula of Au22(SR)18 and exhibits intense luminescence. Interestingly, this new Au22(SR)18 species shows distinctively different absorption and emission features from the previously reported Au22(SR)16, Au22(SR)17, and Au25(SR)18. In stark contrast, Au22(SR)18 luminesces intensely at ∼665 nm with a high quantum yield of ∼8%, while the other three Au NCs show very weak luminescence. Our results indicate that the luminescence of Au22(SR)18 originates from the long Au(I)-thiolate motifs on the NC surface via the aggregation-induced emission pathway. Structure prediction by density functional theory suggests that Au22(SR)18 has two RS-[Au-SR]3 and two RS-[Au-SR]4 motifs, interlocked and capping on a prolate Au8 core. This predicted structure is further verified experimentally by Au L3-edge X-ray absorption fine structure analysis.

Properties and applications of protein-stabilized fluorescent gold nanoclusters: short review

Research is turning toward nanotechnology for solutions to current limitations in bio- medical imaging and analytical detection applications. New to fluorescent nanomaterials that could help advance such applications are protein-stabilized gold nanoclusters. They are potential candidates for imaging agents and sensitive fluorescence sensors because of their biocompat- ibility and intense photoluminescence. This review discusses the strategy for synthesizing fluorescent protein-gold nanoclusters and the characterization methods employed to study these systems. Optical properties and relevant light-emitting applications are reported to present the versatility of protein-gold nanoclusters. These new bio-nano hybrids are an exciting new system that remains to be explored in many aspects, especially regarding the determination of gold nanocluster local structure and the enhancement of quantum yields. Understanding how to finely tune the optical properties will be pivotal for improving fluorescence imaging and other nanocluster applications. There is a promising future for fluorescent protein-gold nanoclusters as long as research continues to uncover fundamental structure-property relation- ships.

X20CoCrWMo10-9//Co3O4: a metal–ceramic composite with unique efficiency values for water-splitting in the neutral regime

Water splitting allows the storage of solar energy into chemical bonds (H2 + O2) and will help to implement the urgently needed replacement of limited available fossil fuels. In particular, in a neutral environment electrochemically initiated water splitting suffers from low efficiency due to high overpotentials (η) caused by the anode. Electro-activation of X20CoCrWMo10-9, a Co-based tool steel resulted in a new composite material (X20CoCrWMo10-9//Co3O4) that catalyzes the anode half-cell reaction of water electrolysis with a so far, unequalled effectiveness. The current density achieved with this new anode in pH 7 corrected 0.1 M phosphate buffer is over a wide range of η around 10 times higher compared to recently developed, up-to-date electrocatalysts and represents the benchmark performance which advanced catalysts show in regimes that support water splitting significantly better than pH 7 medium. X20CoCrWMo10-9//Co3O4 exhibited electrocatalytic properties not only at pH 7, but also at pH 13, which are much superior to the ones of IrO2–RuO2, single-phase Co3O4- or Fe/Ni-based catalysts. Both XPS and FT-IR experiments unmasked Co3O4 as the dominating compound on the surface of the X20CoCrWMo10-9//Co3O4 composite. By performing a comprehensive dual beam FIB-SEM (focused ion beam-scanning electron microscopy) study, we could show that the new composite does not exhibit a classical substrate-layer structure due to the intrinsic formation of the Co-enriched outer zone. This structural particularity is basically responsible for the outstanding electrocatalytic OER performance.

Energy Migration Upconversion in Manganese(II)-Doped Nanoparticles

We report the synthesis and characterization of cubic NaGdF4:Yb/Tm@NaGdF4:Mn core-shell structures. By taking advantage of energy transfer through Yb→Tm→Gd→Mn in these core-shell nanoparticles, we have realized upconversion emission of Mn(2+) at room temperature in lanthanide tetrafluoride based host lattices. The upconverted Mn(2+) emission, enabled by trapping the excitation energy through a Gd(3+) lattice, was validated by the observation of a decreased lifetime from 941 to 532 μs in the emission of Gd(3+) at 310 nm ((6)P(7/2)→(8)S(7/2)). This multiphoton upconversion process can be further enhanced under pulsed laser excitation at high power densities. Both experimental and theoretical studies provide evidence for Mn(2+) doping in the lanthanide-based host lattice arising from the formation of F(-) vacancies around Mn(2+) ions to maintain charge neutrality in the shell layer.

A Segregated, Partially Oxidized, and Compact Ag10 Cluster within an Encapsulating DNA Host

Silver clusters develop within DNA strands and become optical chromophores with diverse electronic spectra and wide-ranging emission intensities. These studies consider a specific cluster that absorbs at 400 nm, has low emission, and exclusively develops with single-stranded oligonucleotides. It is also a chameleon-like chromophore that can be transformed into different highly emissive fluorophores. We describe four characteristics of this species and conclude that it is highly oxidized yet also metallic. One, the cluster size was determined via electrospray ionization mass spectrometry. A common silver mass is measured with different oligonucleotides and thereby supports a Ag10 cluster. Two, the cluster charge was determined by mass spectrometry and Ag L3-edge X-ray absorption near-edge structure spectroscopy. Respectively, the conjugate mass and the integrated white-line intensity support a partially oxidized cluster with a +6 and +6.5 charge, respectively. Three, the cluster chirality was gauged by circular dichroism spectroscopy. This chirality changes with the length and sequence of its DNA hosts, and these studies identified a dispersed binding site with ∼20 nucleobases. Four, the structure of this complex was investigated via Ag K-edge extended X-ray absorption fine structure spectroscopy. A multishell fitting analysis identified three unique scattering environments with corresponding bond lengths, coordination numbers, and Debye-Waller factors for each. Collectively, these findings support the following conclusion: a Ag10(+6) cluster develops within a 20-nucleobase DNA binding site, and this complex segregates into a compact, metal-like silver core that weakly links to an encapsulating silver-DNA shell. We consider different models that account for silver-silver coordination within the core.

Water as the Key to Proto-Aragonite Amorphous CaCO3

Temperature and pH value can affect the short-range order of proto-structured and additive-free amorphous calcium carbonates (ACCs). Whereas a distinct change occurs in proto-vaterite (pv) ACC above 45 °C at pH 9.80, proto-calcite (pc) ACC (pH 8.75) is unaffected within the investigated range of temperatures (7-65 °C). IR and NMR spectroscopic studies together with EXAFS analysis showed that the temperature-induced change is related to the formation of proto-aragonite (pa) ACC. The data strongly suggest that the binding of water molecules induces dipole moments across the carbonate ions in pa-ACC as in aragonite, where the dipole moments are due to the symmetry of the crystal structure. Altogether, a (pseudo-)phase diagram of the CaCO3 polyamorphism in which water plays a key role can be formulated based on variables of state, such as the temperature, and solution parameters, such as the pH value.

Description and Role of Bimetallic Prenucleation Species in the Formation of Small Nanoparticle Alloys

We report the identification, description, and role of multinuclear metal-thiolate complexes in aqueous Au-Cu nanoparticle syntheses. The structure of these species was characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy techniques. The observed structures were found to be in good agreement with thermodynamic growth trends predicted by first-principles calculations. The presence of metal-thiolate complexes is then shown to be critical for the formation of alloyed Au-Cu architectures in the small nanoparticle regime (diameter ∼2 nm). In the absence of mixed metal-thiolate precursors, nanoparticles form with a Cu-S shell and a Au-rich interior. Taken together, these results demonstrate that prenucleation species, which are discrete molecular precursors distinct from both initial reagents and final particle products, may provide an important new synthetic route to control final metal nanoparticle composition and composition architectures.

Bonding properties of thiolate-protected gold nanoclusters and structural analogs from X-ray absorption spectroscopy

Abstract Subnanometer, atomically precise thiolate-protected gold nanoclusters represent an important advancement in our understanding of thiolate-protected gold nanoparticles and thiolate-gold chemistry. Aside from being a link between larger gold nanoparticles and small gold complexes, gold nanoclusters exhibit extraordinary molecule-like optical, electronic, and physicochemical properties that are promising for next-generation imaging agents, sensing devices, or catalysts. The success in elucidating a number of unique thiolate-gold surface and gold core structures has greatly improved our understanding of thiolate-gold nanoclusters. Nevertheless, monitoring the structural and electronic behavior of thiolate-protected gold nanoclusters in a variety of media or environments is crucial for the next step in advancing this class of nanomaterials. Not to mention, there are a number of thiolate-protected gold nanoclusters with unknown structures or compositions that could reveal important insights on application-based properties such as luminescence or catalytic activity. This review summarizes some of the recent contributions from X-ray absorption spectroscopy (XAS) studies on the intriguing bonding properties of thiolate-protected gold nanoclusters and some structural analogs. Advantages from XAS include a local structural, site- and element-specific analysis, suitable for ultra-small particle sizes (1–2 nm), along with versatile experimental conditions.

Disordered amorphous calcium carbonate from direct precipitation

Amorphous calcium carbonate (ACC) is known to play a prominent role in biomineralization. Different studies on the structure of biogenic ACCs have illustrated that they can have distinct short-range orders. However, the origin of so-called proto-structures in synthetic and additive-free ACCs is not well understood. In the current work, ACC has been synthesised in iso-propanolic media by direct precipitation from ionic precursors, and analysed utilising a range of different techniques. The data suggest that this additive-free type of ACC does not resemble clear proto-structural motifs relating to any crystalline polymorph. This can be explained by the undefined pH value in iso-propanolic media, and the virtually instantaneous precipitation. Altogether, this work suggests that aqueous systems and pathways involving pre-nucleation clusters are required for the generation of clear proto-structural features in ACC. Experiments on the ACC-to-crystalline transformation in solution with and without ethanol highlight that polymorph selection is under kinetic control, while the presence of ethanol can control dissolution re-crystallisation pathways.