Electrocatalytic water splitting to produce hydrogen comprises the hydrogen and oxygen evolution half reactions (HER and OER), with the latter as the bottleneck process. Thus, enhancing the OER performance and understanding the mechanism are critically important. Herein, we report a strategy for OER enhancement by utilizing gold nanoclusters to form cluster/CoSe composites; the latter exhibit largely enhanced OER activity in alkaline solutions. The Au/CoSe composite affords a current density of 10 mA cm at small overpotential of ∼0.43 V (cf. CoSe: ∼0.52 V). The ligand and gold cluster size can also tune the catalytic performance of the composites. Based upon XPS analysis and DFT simulations, we attribute the activity enhancement to electronic interactions between nanocluster and CoSe, which favors the formation of the important intermediate (OOH) as well as the desorption of oxygen molecules over Au/CoSe composites in the process of water oxidation. Such an atomic level understanding may provide some guidelines for design of OER catalysts.
An electrochemically mediated reversible addition-fragmentation chain-transfer polymerization (eRAFT) of (meth)acrylates was successfully carried out via electroreduction of either benzoyl peroxide (BPO) or 4-bromobenzenediazonium tetrafluoroborate (BrPhN2+) which formed aryl radicals, acting as initiators for RAFT polymerization. Direct electroreduction of chain transfer agents was unsuccessful since it resulted in the formation of carbanions by a two-electron transfer process. Reduction of BrPhN2+ under a fixed potential showed acceptable control, but limited conversion due to the generation of a passivating organic layer grafted on the working electrode surface. However, using fixed current conditions, easier to implement than fixed potential conditions, conversions > 80% were achieved. Well-defined homopolymers and block copolymers with a broad range of targeted degrees of polymerization were prepared.
Atomically precise alloying and de-alloying processes for the formation of Ag-Au and Cu-Au nanoparticles of 25-metal-atom composition (referred to as Ag(x)Au(25-x)(SR)18 and Cu(x)Au(25-x)(SR)18 , in which R = CH2CH2Ph) are reported. The identities of the particles were determined by matrix-assisted laser desorption ionization mass spectroscopy (MALDI-MS). Their structures were probed by fragmentation analysis in MALDI-MS and comparison with the icosahedral structure of the homogold Au25(SR)18 nanoparticles (an icosahedral Au13 core protected by a shell of Au12(SR)18). The Cu and Ag atoms were found to preferentially occupy the 13-atom icosahedral sites, instead of the exterior shell. The number of Ag atoms in Ag(x)Au(25-x)(SR)18 (x = 0-8) was dependent on the molar ratio of Ag(I)/Au(III) precursors in the synthesis, whereas the number of Cu atoms in Cu(x)Au(25-x)(SR)18 (x = 0-4) was independent of the molar ratio of Cu(II)/Au(III) precursors applied. Interestingly, the Cu(x)Au(25-x)(SR)18 nanoparticles show a spontaneous de-alloying process over time, and the initially formed Cu(x)Au(25-x)(SR)18 nanoparticles were converted to pure Au25(SR)18. This de-alloying process was not observed in the case of alloyed Ag(x)Au(25-x)(SR)18 nanoparticles. This contrast can be attributed to the stability difference between Cu(x)Au(25-x)(SR)18 and Ag(x)Au(25-x)(SR)18 nanoparticles. These alloyed nanoparticles are promising candidates for applications such as catalysis.
We report the use of a nickel-thiolate hexameric cluster, Ni6(SC2H4Ph)12, for photocatalytic hydrogen production from water. The nickel cluster was synthesized ex-situ and characterized by various techniques. Single crystal X-ray analysis, (1)H NMR, 2D COSY, ESI-MS, UV-visible spectroscopy, and TGA provided insight into the structure and confirmed the purity and stability of the cluster. Cyclic voltammetry helped confirm hydrogen evolution reaction (HER) activity of this catalyst. Photoreactions carried out using an iridium photosensitizer, Ir(F-mppy)2(dtbbpy)[PF6], and TEA as the sacrificial reductant revealed the high activity of the Ni6 cluster as a water reducing catalyst. High TONs (3750) and TOFs (970 h(-1)) were obtained at optimum catalyst concentration (0.025 mM), with low concentrations of catalyst yielding up to 30,000 turnovers. Quenching studies, along with the evidence obtained from the electrochemical analysis, showed that this water reduction system proceeds through a reductive quenching mechanism. Mercury poisoning studies confirmed that no active, metallic colloids were formed during the photocatalytic reaction.
Structurally tailored and engineered macromolecular (STEM) gels constitute part of an emerging field of smart materials. STEM gels are polymer networks containing latent initiator sites available for postsynthesis modification. STEM gels synthesized by controlled radical polymerization (CRP) are presented. First, reversible addition–fragmentation chain transfer (RAFT) polymerization was used to copolymerize (meth)acrylate monomer, di(meth)acrylate cross-linker, and inimer for the subsequent atom transfer radical polymerization (ATRP) grafting-from process. The resulting STEM gels were infiltrated with a second monomer, which formed side chains grafted from the inimer sites by photoactivated ATRP. This approach permits significant spatial and temporal control over the structure of the resulting material. Here, the technique was used to transform primary STEM gels into single-piece amphiphilic and hard/soft materials.
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