Srinivas Thanneeru

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.

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.

Multiblock thermoplastic elastomers via one-pot thiol–ene reaction

We report a facile approach to designing multiblock thermoplastic elastomers using a two-step thiol–ene polyaddition reaction. It is based on the utilization of intermolecular hydrogen bonding of widely available and cost-effective monomer of N,N′-methylenebis(acrylamide) (MBAm) as physical cross-links. Thiol-terminated “soft” prepolymers were first prepared using ethylene glycol dimethacrylate (EGDMA) and an excess of 1,6-hexanedithiol (HDT); subsequently, the thiol-terminated prepolymers were further reacted with MBAm as a chain-extension reaction to yield the multiblock thermoplastic elastomers. The prepolymers with oligo(ethylene glycol) segments had a low glass-transition temperature, acting as elastic “soft” blocks; while MBAm units could form up to 4 hydrogen bonds that serve as physical networks to endow the elasticity to multiblock polymers. Proton nuclear magnetic resonance spectroscopy and gel permeation chromatography indicated the occurrence of the two-step thiol–ene reactions. The reaction kinetics of thiol–ene reactions was found to be highly dependent on the molecular weights of monomers. The first thiol–ene reaction of EGDMA and HDT could reach >90% conversion of both monomers within 5 min; while the kinetics of the second chain extension reaction was relatively slow and it took approximately 7 h to reach 90% conversion. The formation of the intermolecular hydrogen bonding between amide groups of MBAm units was confirmed by variable-temperature Fourier transform infrared spectroscopy and differential scanning calorimetry. The viscoelasticity and elasticity of the thermoplastic elastomers were found to be largely determined by the content of MBAm. With a molar ratio of 15% MBAm relative to EGDMA, the maximum elongation at break of elastomers reached >400%. Our synthetic method has the advantages of mild reaction conditions, high conversion and adjustable mechanical properties of elastomers; additionally, it does not involve heavy syntheses and expensive monomers/catalysts. Our findings conceivably stand out as a new tool to synthesize and engineer thermoplastic elastomers using the combination of thiol–ene chemistry and supramolecular interaction.

Dynamic Coordination of Eu–Iminodiacetate to Control Fluorochromic Response of Polymer Hydrogels to Multistimuli

New fluorochromic materials that reversibly change their emission properties in response to their environment are of interest for the development of sensors and light-emitting materials. A new design of Eu-containing polymer hydrogels showing fast self-healing and tunable fluorochromic properties in response to five different stimuli, including pH, temperature, metal ions, sonication, and force, is reported. The polymer hydrogels are fabricated using Eu-iminodiacetate (IDA) coordination in a hydrophilic poly(N,N-dimethylacrylamide) matrix. Dynamic metal-ligand coordination allows reversible formation and disruption of hydrogel networks under various stimuli which makes hydrogels self-healable and injectable. Such hydrogels show interesting switchable ON/OFF luminescence along with the sol-gel transition through the reversible formation and dissociation of Eu-IDA complexes upon various stimuli. It is demonstrated that Eu-containing hydrogels display fast and reversible mechanochromic response as well in hydrogels having interpenetrating polymer network. Those multistimuli responsive fluorochromic hydrogels illustrate a new pathway to make smart optical materials, particularly for biological sensors where multistimuli response is required.

“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.

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.