Promoting CO2 Electroreduction to Multi-Carbon Products by Hydrophobicity-Induced Electro-Kinetic Retardation.

Advancing the performance of the Cu-catalyzed electrochemical CO2 reduction reaction (CO2 RR) is crucial for its practical applications. Still, the wettable pristine Cu surface often suffers from low exposure to CO2 , reducing the Faradaic efficiencies (FEs) and current densities for multi-carbon (C2+ ) products. Recent studies have proposed that increasing surface availability for CO2 by cation-exchange ionomers can enhance the C2+ product formation rates. However, due to the rapid formation and consumption of *CO, such promotion in reaction kinetics can shorten the residence of *CO whose adsorption determines C2+ selectivity, and thus the resulting C2+ FEs remain low. Herein, we discover that the electro-kinetic retardation caused by the strong hydrophobicity of quaternary ammonium group-functionalized polynorbornene ionomers can greatly prolong the *CO residence on Cu. This unconventional electro-kinetic effect is demonstrated by the increased Tafel slopes and the decreased sensitivity of *CO coverage change to potentials. As a result, the strongly hydrophobic Cu electrodes exhibit C2+ Faradaic efficiencies of ≈90 % at a partial current density of 223 mA cm-2 , more than twice of bare or hydrophilic Cu surfaces.

Oriented mesoporous nanopyramids as versatile plasmon-enhanced interfaces.

We developed a facile interfacial oriented growth and self-assembly process to fabricate three-dimensional (3D) aligned mesoporous iron oxide nanopyramid arrays (NPAs). The unique NPAs possess a 3D mesostructure with multiple features, including high surface area (~175 m(2)/g), large pore size (~20 nm), excellent flexibility (bent over 150 times), and scalability at the foot scale for practical applications. More importantly, these NPAs structures enable versatile enhancement of localized surface plasmon resonance and photoelectrochemical conversion. The integration of plasmonic gold with 3D NPAs remarkably improves the performance of photoelectrochemical conversion, leading to ~6- and 83-fold increases of the photocurrent under simulated solar and visible-light illumination, respectively. The fabrication and investigation of NPAs provide a new paradigm for preparing unconventional mesoporous oriented thin films and further suggest a new strategy for designing plasmonic metal/semiconductor systems for effective solar energy harvesting.

Unconventional 0-, 1-, and 2-dimensional single-crystalline copper sulfide nanostructures.

We report the synthesis of several unconventional 0-, 1- and 2-dimensional copper sulfide nanostrucutures by the chemical vapor deposition method. The key factor for morphology and structure control of a variety of copper sulfide products is the tuning of deposition and growth temperature to fit for the surface energy barriers and promote different growth directions. At a high growth temperature (480 °C) that provides enough thermal energy, a 0-D octahedral copper sulfide single crystal structure was synthesized. At a slightly lower growth temperature (460 °C), a new 1-D copper sulfide nanorod structure with a nanocrystal head was discovered for the first time. At a much lower growth temperature (150 °C), 2-D copper sulfide nanoflakes with a single crystal hexagonal structure were obtained. These novel structural varieties of copper sulfide can lead to discovering more unconventional material structures and growth mechanisms of other transitional metal chalcogenides, and may allow for new copper sulfide based devices and applications.

Frequency domain detection of biomolecules using silicon nanowire biosensors.

We demonstrate a new protein detection methodology based upon frequency domain electrical measurement using silicon nanowire field-effect transistor (SiNW FET) biosensors. The power spectral density of voltage from a current-biased SiNW FET shows 1/f-dependence in frequency domain for measurements of antibody functionalized SiNW devices in buffer solution or in the presence of protein not specific to the antibody receptor. In the presence of protein (antigen) recognized specifically by the antibody-functionalized SiNW FET, the frequency spectrum exhibits a Lorentzian shape with a characteristic frequency of several kilohertz. Frequency and conventional time domain measurements carried out with the same device as a function of antigen concentration show more than 10-fold increase in detection sensitivity in the frequency domain data. These concentration-dependent results together with studies of antibody receptor density effect further address possible origins of the Lorentzian frequency spectrum. Our results show that frequency domain measurements can be used as a complementary approach to conventional time domain measurements for ultrasensitive electrical detection of proteins and other biomolecules using nanoscale FETs.