Ben Liu

Ligand‐Assisted Co‐Assembly Approach toward Mesoporous Hybrid Catalysts of Transition‐Metal Oxides and Noble Metals: Photochemical Water Splitting

A bottom-up synthetic approach was developed for the preparation of mesoporous transition-metal-oxide/noble-metal hybrid catalysts through ligand-assisted co-assembly of amphiphilic block-copolymer micelles and polymer-tethered noble-metal nanoparticles (NPs). The synthetic approach offers a general and straightforward method to precisely tune the sizes and loadings of noble-metal NPs in metal oxides. This system thus provides a solid platform to clearly understand the role of noble-metal NPs in photochemical water splitting. The presence of trace amounts of metal NPs (≈0.1 wt %) can enhance the photocatalytic activity for water splitting up to a factor of four. The findings can conceivably be applied to other semiconductors/noble-metal catalysts, which may stand out as a new methodology to build highly efficient solar energy conversion systems.

A facile synthesis of Fe3C@mesoporous carbon nitride nanospheres with superior electrocatalytic activity

We report a colloidal amphiphile-templating approach to preparing nanosized Fe3C encapsulated within mesoporous nitrogen-doped carbon nanospheres (Fe3C@mCN). The obtained Fe3C@mCN hybrids having a high surface area and ultrafine Fe3C nanocrystals exhibited superior activity and durability for oxygen reduction.

Synthesis of Mesoporous CoS2 and NixCo1–xS2 with Superior Supercapacitive Performance Using a Facile Solid-Phase Sulfurization

Synthesis of nanostructured transition metal sulfides is of particular interest in providing new methods to control their porosity and improve their surface area because those sulfides hold promising applications in high-energy density devices. Significant challenges remain currently to prepare metal sulfides having three-dimensional (3-D) continuous mesoporous structures, known to be critical for increasing their active surface sites and enhancing ion transport. We herein present a facile solid-phase sulfurization method to synthesize 3-D continuous mesoporous CoS2, NiS2, and their binary sulfides in a two-step nanocasting using bicontinuous KIT-6 as hard templates. The solid-phase sulfurization taking place at 400 °C yields mesoporous sulfides with highly crystalline frameworks and a stoichiometric ratio of metal-to-sulfur, 1:2 (mol), within 30 min. Elemental sulfur as an inexpensive sulfur source can be directly used for the solid-phase sulfurization of mesoporous oxides of Co3O4, NiO, and their binary oxides. This facile synthetic method is highly efficient to prepare mesoporous sulfides in the gram-scale production at a very low cost. Mesoporous sulfides are demonstrated to be superior electrode materials for pseudo-supercapacitors, given their high surface area and accessible bicontinuous mesopores, the suitable crystalline sizes, and the enhanced ion transport capability. The use of binary mesoporous sulfides presents interesting synergetic effect where the doping of metal ions can significantly enhance the capacitive performance of single-component sulfides. The binary sulfides of mNi0.32Co0.68S2 show a specific capacitance up to 1698 F g-1 at a current density of 2 A g-1. The supercapacitor device of mNi0.32Co0.68S2 has a high energy density of 37 Wh kg-1 at a power density of 800 W kg-1. We believe that the reported solid-phase synthesis offers a universal method toward the conversion of mesoporous oxides materials into various useful and functional forms for energy conversion and storage applications.

Electrocatalytic Oxidation of Alcohols, Tripropylamine, and DNA with Ligand-Free Gold Nanoclusters on Nitrided Carbon

Electrocatalytic properties of ligand-free gold nanoclusters (AuNCs, <2 nm) grown on nitrided carbon supports (denoted as AuNCs@N-C) were evaluated for the oxidation of representative organic molecules including alcohols, an amine, and deoxyguanosine in oligonucleotides. AuNCs@N-C catalysts were incorporated into films of architecture {PDDA/AuNCs@N-C} n by using layer-by-layer assembly with oppositely charged poly(diallyldimethylammonium) (PDDA) on pyrolytic graphite (PG) electrodes. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were used to survey the electrocatalytic properties of these AuNCs@N-C films. Ligand-free AuNCs in these films demonstrated excellent electrocatalytic oxidation activity with maximum peak currents and the lowest potentials for oxidizing ethanol, propanol, and tripropylamine (TprA) compared to controls with Au-surface capping agents or to larger sized Au nanocrystals on the nitrided carbon supports. EIS kinetic studies showed that ligand-free AuNCs films have the smallest charge-transfer resistance, largest electrochemically active surface area, and largest apparent standard rate constants, as compared to the control films for all compounds examined. DNA films on AuNCs@N-C were oxidized at deoxyguanosine moieties with good catalytic activity that depended on charge transport within the films.

Ultrafine and Ligand‐Free Precious Metal (Ru, Ag, Au, Rh and Pd) Nanoclusters Supported on Phosphorus‐Doped Carbon

We report the use of phosphorus-doped carbon (P-C) as support to grow ultrasmall (1-3 nm) and ligand-free precious metal nanocrystals (PMNCs) via chemical reduction. We show that the valence states of surface phosphorus species are critical in tuning the affinity between the carbon support and metal precursors, which rationally controls the loading size and uniformity of resultant PMNCs. Five kinds of PMNCs, including Ru, Ag, Au, Rh, and Pd, were grown in situ to demonstrate the key role of surface phosphorus sites on the P-C support. As a proof-of-concept application, Ru nanocatalysts with an average diameter of 1.0±0.2 nm supported on P-C were examined for the electrocatalytic hydrogen evolution reaction (HER). Ultrasmall and ligand-free Ru nanocatalysts exhibited superior HER activity and stability compared to its counterparts with surface agents or larger sizes. An overpotential of 27.6 mV (vs. reversible hydrogen electrode) for Ru nanocatalysts was achieved at a current density of 10 mA cm-2 . This novel method opens a new avenue to immobilize ligand-free and well-dispersed PMNCs on carbon; and, more importantly, it provides a new library of supported PMNCs with high catalytic activity.

Unconventional structural and morphological transitions of nanosheets, nanoflakes and nanorods of AuNP@MnO2

Two-dimensional (2-D) layered inorganic materials with ultra-high surface area and mechanical strength have shown impressive photo-/electro-catalytic activities. We herein report a facile synthetic strategy to grow monodispersed 2-D MnO2 nanosheet on an individual gold nanoparticle (AuNP@MnO2 nanosheet), and demonstrate that the strongly interacted AuNP and MnO2 nanosheet could greatly improve the electrocatalytic activity of the MnOx family for electrocatalytic oxygen reduction reactions (ORRs). AuNP@MnO2 nanosheets were prepared using a hydrothermal reduction of KMnO4 by citrate ligands capped on AuNPs. Because of the metastability of the layered MnO2 nanosheets, we observed unconventional structural and morphological transitions of amorphous MnO2 nanosheets to δ-MnO2 nanoflakes, and eventually to α-MnO2 nanorods under hydrothermal conditions. The layered MnO2 nanosheets underwent a structural expansion to nanoflakes before the curling and re-folding of layered MnO2 nanosheets occurred. The intermediate states and structural transitions via a “layer compression”, for the first time, were experimentally recorded at a single-NP scale using electron microscopy. Moreover, we found the electrocatalytic activity of AuNP@MnO2 nanosheets was enhanced roughly 30–40 times, compared to that of pure MnO2 nanosheets and AuNPs. The strong interaction of metal–oxide interfaces (MnO2 nanosheets and AuNPs) was likely responsible for the improved electrocatalytic activity. This interaction of MnO2 and AuNPs was weakened in the course of hydrothermal treatment where partially positively charged Au+ was reduced at elevated temperatures, accompanying with the decrease of ORR activity. This insight into the effect of topological nanostructures and metal–oxide interactions on the electrocatalytic performance of the MnOx family is believed to illustrate an alternative pathway to develop new efficient electrocatalysts.

“Enzymatic” Photoreduction of Carbon Dioxide using Polymeric Metallofoldamers Containing Nickel-Thiolate Cofactors

The photoreduction of CO2 by using enzyme‐mimicking polymeric metallofoldamers containing Ni–thiolate cofactors was explored. Metallofoldamers consisting of folded polymers incorporated with Ni–thiolate complexes were prepared by intramolecular Ni–thiolate coordination with thiol‐functionalized linear copolymers. The folded polymer backbone may resemble the protein framework to provide a second coordination environment to the active sites. We showed that Ni–metallofoldamers were superiorly active and selective for CO2 photoreduction. At 80 °C, the turnover frequency of the Ni–metallofoldamers could reach 0.69 s−1, which corresponds to 2500 turnovers per hour per Ni site. Our findings are expected to provide useful guidelines to investigate artificial enzymes and to understand the role of protein frameworks in photosynthesis.

Co-Template Directed Synthesis of Gold Nanoparticles in Mesoporous Titanium Dioxide

A bottom-up synthetic methodology to encapsulate pre-synthesized, well-defined gold nanoparticles (AuNPs) into mesoporous titanium dioxide framework (Au@mTiO2 ) is reported. This method employs two structurally and chemically similar templates of amphiphilic block copolymers as well as poly(ethylene oxide)-tethered AuNPs, which showed excellent stability during sol-gel transition and thermal annealing at elevated temperatures. Such synthesis enabled precise control of sizes and loading of AuNPs within the mesoporous TiO2 framework. In light-driven methanol dehydrogenation, the presence of AuNPs significantly enhanced the photocatalytic activity of mTiO2 . This co-template-directed synthesis presents new opportunities to understand the effect of AuNP size in photocatalysis using Au@mTiO2 materials.

Surface Engineering of Spherical Metal Nanoparticles with Polymers toward Selective Asymmetric Synthesis of Nanobowls and Janus-Type Dimers

New synthetic methods capable of controlling structural and compositional complexities of asymmetric nanoparticles (NPs) are very challenging but highly desired. A simple and general synthetic approach to designing sophisticated asymmetric NPs by anisotropically patterning the surface of isotropic metallic NPs with amphiphilic block copolymers (BCPs) is reported. The selective galvanic replacement and seed-mediated growth of a second metal can be achieved on the exposed surface of metal NPs, resulting in the formation of nanobowls and Janus-type metal-metal dimers, respectively. Using Ag and Au NPs tethered with amphiphilic block copolymers of poly(ethylene oxide)-block-polystyrene (PEO-b-PS), anisotropic surface patterning of metallic NPs (e.g., Ag and Au) is shown to be driven by thermodynamical phase segregation of BCP ligands on isotropic metal NPs. Two proof-of-concept experiments are given on, i) synthesis of Au nanobowls by a selective galvanic replacement reaction on Janus-type patched Ag/polymer NPs; and ii) preparation of Au-Pd heterodimers and Au-Au homodimers by a seed-mediated growth on Janus-type patched Au/polymer NPs. The method shows remarkable versatility; and it can be easily handled in aqueous solution. This synthetic strategy stands out as the new methodology to design and synthesis asymmetric metal NPs with sophisticated topologies.

Ultrasmall Ru Nanoclusters on Nitrogen-Enriched Hierarchically Porous Carbon Support as Remarkably Active Catalysts for Hydrolysis of Ammonia Borane

The development of the highly active nanocatalysts for effective hydrogen (H2) production is of great significance for its practical applications in fuel cells. Herein, we reported a facile and scale‐up synthetic methodology to grow in situ the remarkably active nanocatalysts of ultrasmall Ru nanoclusters on nitrogen (N)‐enriched hierarchically macroporous‐mesoporous carbon supports (Ru@hPCN). The resultant Ru@hPCN combines structural and chemical merits of well‐dispersed 0.7‐nm Ru nanoclusters, N‐enriched functional surface and 3D hierarchically porous framework, all of which synergistically boost the catalytic performance toward the hydrolysis of ammonia‐borane (AB). An unprecedented activity with a turnover frequency of 1850 min−1 was achieved for the Ru@hPCN, which was 6.0 folds over that of commercialized Ru/C catalyst. Mechanism studies showed that the remarkably enhanced activity can be ascribed to the easier dissociation of electropositive Hδ+ from H2O and the breakage of the B−N bonds as well as favorable mass transport in the Ru@hPCN during AB hydrolysis.