Shao-Ming Fang

Homogeneous Pd nanoparticles produced in direct reactions: green synthesis, formation mechanism and catalysis properties

The direct reactions between ascorbic acid (AA) and palladium salts can produce homogeneous Pd nanoparticles on a large scale by grinding or shaking the two reactants together for ca. 2 minutes at room temperature. The size of homogeneous Pd nanoparticles can be tuned conveniently by using different palladium salts as precursors in the reactions without adding any solvent and organic protectors. Compared to Pd nanoparticles with much smaller sizes of ca. 2.7 nm produced by grinding a solid mixture of AA and Pd(CH3COO)2, homogeneous Pd nanoparticles of 35.6 ± 5 nm can be generated in the direct reaction between AA and Pd(NO3)2·2H2O. It was found that AA can reduce Pd2+ to Pd0 to form Pd nanoparticles directly, accompanied by its oxidation to 2,3-diketogulonic acid (2,3-DKG) and a series of fragment species of 2,3-DKG simultaneously. The small amount of crystalline water in Pd(NO3)2·2H2O can promote the formation of Pd nanoparticles dramatically, compared to the reactions conducted with Pd(CH3COO)2, Pd(NO3)2 and PdCl2 without crystalline water. Based on the experimental results, a two-step reaction mechanism is proposed to understand the formation of Pd nanoparticles. The quasi solid-state features of the direct reactions could lead to defects of high concentration on the surface of the as-prepared Pd nanoparticles, which can be applied to the catalysis of the Suzuki reaction immediately after their formation in the reactions.

Tunable electronic and magnetic properties of graphene-like ZnO monolayer upon doping and CO adsorption: a first-principles study

Graphene-like zinc oxide monolayer (g-ZnO) is a new class of two-dimensional nanomaterials with unique new properties that is still largely unknown. This work studies the tunability of the electronic and magnetic properties of g-ZnO upon chemical doping (with B, N and C) and CO adsorption by using first-principles calculations. Both electronic and magnetic properties of g-ZnO exhibit strong dependency on its structural change and molecular adsorption. The g-ZnO with oxygen atom substituted by a C or N atom (one atom per supercell) are ferromagnetic (FM) half metal (HM), while that substituted by a B atom is an FM semiconductor. The doped g-ZnO shows strong chemisorption to CO molecule by forming A–CO bond (A = B, N or C), in contrast to the weak physisorption of the intrinsic g-ZnO. Furthermore, CO adsorption converts the N- and C-doped g-ZnO to n-type semiconductor with nonmagnetic (NM) ground states, while B-doped g-ZnO becomes a ferromagnetic half metal (FM-HM). The mechanism for property change has been investigated by analyzing their partial density of states (PDOS) upon different conditions. This study provides insights in the physical properties and chemical reactivity of g-ZnO, which could help in realizing their diverse potentials in electronic and magnetic devices.

Reverse Adhesion of a Gecko-Inspired Synthetic Adhesive Switched by an Ion-Exchange Polymer–Metal Composite Actuator

Inspired by how geckos abduct, rotate, and adduct their setal foot toes to adhere to different surfaces, we have developed an artificial muscle material called ion-exchange polymer-metal composite (IPMC), which, as a synthetic adhesive, is capable of changing its adhesion properties. The synthetic adhesive was cast from a Si template through a sticky colloid precursor of poly(methylvinylsiloxane) (PMVS). The PMVS array of setal micropillars had a high density of pillars (3.8 × 10(3) pillars/mm(2)) with a mean diameter of 3 μm and a pore thickness of 10 μm. A graphene oxide monolayer containing Ag globular nanoparticles (GO/Ag NPs) with diameters of 5-30 nm was fabricated and doped in an ion-exchanging Nafion membrane to improve its carrier transfer, water-saving, and ion-exchange capabilities, which thus enhanced the electromechanical response of IPMC. After being attached to PMVS micropillars, IPMC was actuated by square wave inputs at 1.0, 1.5, or 2.0 V to bend back and forth, driving the micropillars to actively grip or release the surface. To determine the adhesion of the micropillars, the normal adsorption and desorption forces were measured as the IPMC drives the setal micropillars to grip and release, respectively. Adhesion results demonstrated that the normal adsorption forces were 5.54-, 14.20-, and 23.13-fold higher than the normal desorption forces under 1.0, 1.5, or 2.0 V, respectively. In addition, shear adhesion or friction increased by 98, 219, and 245%, respectively. Our new technique provides advanced design strategies for reversible gecko-inspired synthetic adhesives, which might be used for spiderman-like wall-climbing devices with unprecedented performance.

Effect of Pd doping on the acetone-sensing properties of NdFeO3

The acetone-sensing properties of the undoped and Pd doped perovskite-type oxides NdFeO3 were investigated from room temperature to 400°C. The perovskite-type NdFeO3 was synthesized by a sol-gel method, and the dopants Pd with the content from 1wt% to 5wt% were implanted into NdFeO3 nanoparticles by thermal diffusion. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques show that NdFeO3 is an orthorhombic structure with the average particle size of about 40 nm. A giant acetone-sensing response of 675.7 is observed when the Pd content in NdFeO3 powders is about 3wt%. The response and recovery time of the sensor to the 5×10−4 acetone gas are 16 and 1 s, respectively. At the same time, it performs a good selectivity to acetone gas and may be a new promising material candidate for the acetone-sensor development.

Copper nanoparticles spaced 3D graphene films for binder-free lithium-storing electrodes

Copper nanoparticles (Cu NPs) spaced reduced graphene oxide (rGO) films are fabricated by integrating electrophoresis deposition (EPD) with thermal H2 annealing. The Cu NPs are formed through thermal aggregation and reduction of Cu(II) ions which are counter cations of GO. By tuning the annealing temperature from 400 to 850 °C, the mean diameter of the Cu NPs increases from 35 to 124 nm. Chemical characterizations reveal that the EPD technique may partially remove the O-containing groups of the GO film, while complete removal of the O-containing groups and effective repair of the graphene defects are achieved by thermal H2 treatment. With the implantation of Cu NPs, the resultant rGO/Cu NP films exhibit high porosity and amazing electric conductivities, enabling their direct use as lithium-ion battery (LIB) electrodes (vs. Li/Li+) without a general current collector, binder, and other additives. This novel LIB has a high charge/discharge capacity (>463 mA h g−1), an excellent reversibility, and a high coulombic efficiency (nearly 100%) for 300 cycles at a current density of 0.2 A g−1. It also exhibits good rate capacity: 849 mA h g−1@0.1 A g−1, 618 mA h g−1@0.2 A g−1, 516 mA h g−1@0.3 A g−1, and 470 mA h g−1@0.5 A g−1. Our new technique provides advanced design strategies for high performance LIBs and ultracapacitors, which might be used for mobile phone and electrical vehicles with unprecedented performances.

Fabrication and adhesion of a bio-inspired microarray: capillarity-induced casting using porous silicon mold

Inspired by the setal microstructure found on the geckos toe-pads, a highly dense array of high-aspect-ratio (HAR) artificial setae has been developed with a novel mold-casting technique using a porous silicon (PSi) template. To overcome the high fluid resistance in the HAR capillary pores, the PSi template surface is modified with a monolayer coating of dimethylsilane. The coating exhibits similar chemical composition and surface energy to the precursor of the poly(dimethylsiloxane) (PDMS) replica. The compatibility between the template and the replica addresses the major challenge of molding HAR microstructures, resulting in high-resolution replicas of artificial PDMS microsetae with complicated geometry resembling a real geckos setae. The artificial setae are characterized by a mean radius of 1.3 μm, an aspect ratio of 35.1, and a density of ~4.7 × 105 per mm2. Results from adhesion characterizations reveal that with increasing preload, the shear adhesion of micro-setae continually increases while the normal adhesion decreases. The unique adhesion performance is caused by both van der Waals forces and the elastic resistance of PDMS setae. With further structural optimizations and the addition of an actuation mechanism, artificial setal arrays might eventually demonstrate the fascinating adhesion performances of the gecko for mimetic devices such as wall-climbing devices.