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.

Hybrid metal nanoparticles can allow separate reaction steps to occur in close proximity at different metal sites and accelerate catalysis. We synthesized iron-nickel hydroxide-platinum (transition metal-OH-Pt) nanoparticles with diameters below 5 nanometers and showed that they are highly efficient for carbon monoxide (CO) oxidation catalysis at room temperature. We characterized the composition and structure of the transition metal-OH-Pt interface and showed that Ni(2+) plays a key role in stabilizing the interface against dehydration. Density functional theory and isotope-labeling experiments revealed that the OH groups at the Fe(3+)-OH-Pt interfaces readily react with CO adsorbed nearby to directly yield carbon dioxide (CO2) and simultaneously produce coordinatively unsaturated Fe sites for O2 activation. The oxide-supported PtFeNi nanocatalyst rapidly and fully removed CO from humid air without decay in activity for 1 month.

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.

The applications of lanthanide-doped upconversion nanocrystals in biological imaging, photonics, photovoltaics and therapeutics have fuelled a growing demand for rational control over the emission profiles of the nanocrystals. A common strategy for tuning upconversion luminescence is to control the doping concentration of lanthanide ions. However, the phenomenon of concentration quenching of the excited state at high doping levels poses a significant constraint. Thus, the lanthanide ions have to be stringently kept at relatively low concentrations to minimize luminescence quenching. Here we describe a new class of upconversion nanocrystals adopting an orthorhombic crystallographic structure in which the lanthanide ions are distributed in arrays of tetrad clusters. Importantly, this unique arrangement enables the preservation of excitation energy within the sublattice domain and effectively minimizes the migration of excitation energy to defects, even in stoichiometric compounds with a high Yb(3+) content (calculated as 98 mol%). This allows us to generate an unusual four-photon-promoted violet upconversion emission from Er(3+) with an intensity that is more than eight times higher than previously reported. Our results highlight that the approach to enhancing upconversion through energy clustering at the sublattice level may provide new opportunities for light-triggered biological reactions and photodynamic therapy.

Zhang and Sham Reply: In Moriarty's Comment [1], he raised an issue with our interpretation of the XPS binding energy shifts in terms of the initial state effects [2].Let us first note that our paper reported the preparation and electronic structure of a series of thiol-capped Au nanoparticles using a number of techniques [2] including XAFS, TEM, XRD, UV-visible, and XPS. Our interpretation of the Au binding energy was not based solely on initial state effects although the notion of a positively charged nanoparticle in the final state when the cluster was supported on a poorly conducting substrate [3-6] was not explicitly addressed. This will not affect the main conclusions of our Letter although the issues are valid and timely. We will address them below.It is recognized that the core level binding energy depends on initial and final state effects. The final state effect in this context arises from the relaxation of the core hole, which is screened by conduction electrons in metals and polarized charge in nonconductors. Although it is common to assume that the final state effect in similar metallic systems is the same, its details are still being pursued [7]. The final state effect Moriarty alluded to in his Comment deals with how fast a small cluster can be neutralized in the spirit of the work by Wertheim et al. [3] and others [4 -6].Charging is an experimental problem for nonconductors and results in a shift of the spectrum to higher binding energy. This is accompanied by a linewidth broadening and a skew line shape. The issue here is whether or not the nanoparticle (NP) is neutralized fast enough upon photoemission (NP-substrate transport). It has been reported [3-6] that the Fermi level shifts to above zero binding energy in the XPS spectrum if the final state of the cluster is not neutralized within the photoemission time scale. Moriarty's comments suggested that this could be the case in the XPS of our thiol-capped Au NPs deposited on a conducting carbon substrate. We argue that this is not necessarily the case. The experimental results show only a small 4f shift of 0.36 eV for the smallest NP (1.6 nm), where charging should be the most severe. Also, the 4f peak does not show any abnormal line shape [8]. Furthermore, the valence band exhibits the trend of Au d bandwidth narrowing and band centroid shift expected for a reduction in NP size. Charging in these nanoparticles would have adversely affected the trend.The biggest observed shift of 0.36 eV is comparable in magnitude to the surface core level shift in Au metal. The

We report in this article a detailed study on how to stabilize a first-row transition metal (M) in an intermetallic L1-MPt alloy nanoparticle (NP) structure and how to surround the L1-MPt with an atomic layer of Pt to enhance the electrocatalysis of Pt for oxygen reduction reaction (ORR) in fuel cell operation conditions. Using 8 nm FePt NPs as an example, we demonstrate that Fe can be stabilized more efficiently in a core/shell structured L1-FePt/Pt with a 5 Å Pt shell. The presence of Fe in the alloy core induces the desired compression of the thin Pt shell, especially the two atomic layers of Pt shell, further improving the ORR catalysis. This leads to much enhanced Pt catalysis for ORR in 0.1 M HClO solution (at both room temperature and 60 °C) and in the membrane electrode assembly (MEA) at 80 °C. The L1-FePt/Pt catalyst has a mass activity of 0.7 A/mg from the half-cell ORR test and shows no obvious mass activity loss after 30 000 potential cycles between 0.6 and 0.95 V at 80 °C in the MEA, meeting the DOE 2020 target (<40% loss in mass activity). We are extending the concept and preparing other L1-MPt/Pt NPs, such as L1-CoPt/Pt NPs, with reduced NP size as a highly efficient ORR catalyst for automotive fuel cell applications.

Active and durable electrocatalysts for methanol oxidation reaction are of critical importance to the commercial viability of direct methanol fuel cell technology. Unfortunately, current methanol oxidation electrocatalysts fall far short of expectations and suffer from rapid activity degradation. Here we report platinum–nickel hydroxide–graphene ternary hybrids as a possible solution to this long-standing issue. The incorporation of highly defective nickel hydroxide nanostructures is believed to play the decisive role in promoting the dissociative adsorption of water molecules and subsequent oxidative removal of carbonaceous poison on neighbouring platinum sites. As a result, the ternary hybrids exhibit exceptional activity and durability towards efficient methanol oxidation reaction. Under periodic reactivations, the hybrids can endure at least 500,000 s with negligible activity loss, which is, to the best of our knowledge, two to three orders of magnitude longer than all available electrocatalysts.

We report a study of the structure and electronic properties of a series of thiol-capped Au nanoparticles (NP) of nominal sizes of 1.6, 2.4, and 4.0 nm. Transmission electron microscopy, x-ray powder diffraction, x-ray absorption fine structure, and x-ray photoemission spectroscopy have been used to investigate the size-dependent systematics of lattice contraction and charge redistribution of these NPs. It is found that the lattice contracts and the d charge at the Au atom site depletes relative to bulk Au as the size of the NP decreases. The implication of these observations is discussed in terms of the interplay of quantum-size and surface effect.