Michael Volokh

Studying the chemical, optical and catalytic properties of noble metal (Pt, Pd, Ag, Au)–Cu2O core–shell nanostructures grown via a general approach

We studied the chemical, optical and catalytic properties of metal (Pt, Pd, Ag, Au)–Cu2O core–shell nanoparticles grown via a simple and reproducible approach which utilizes aqueous-phase reactions at room temperature. We were able to control the thickness of the Cu2O shell and examine the effect of the cores shape and size on the structure and properties of the hybrid nanocrystals. We also studied the optical properties of the hybrid nanocrystals, in particular the effect of the Cu2O shell thickness on the frequency of the plasmon of gold nanorods. In addition, the catalytic activity of the hybrid nanostructures was examined by testing the reduction reaction of 4-nitrophenol with NaBH4. Finally, the hybrid metal–Cu2O nanostructures were used as templates to form the yolk–shell of metal–Cu2S materials. The interface and the crystalline structures of the four hybrid nanostructures were extensively characterized by high resolution transmission electron microscopy (HRTEM), energy-filtered TEM (EFTRM) and X-ray diffraction (XRD).

Coating and enhanced photocurrent of vertically aligned zinc oxide nanowire arrays with metal sulfide materials.

Hybrid nanostructures combining zinc oxide (ZnO) and a metal sulfide (MS) semiconductor are highly important for energy-related applications. Controlled filling and coating of vertically aligned ZnO nanowire arrays with different MS materials was achieved via the thermal decomposition approach of single-source precursors in the gas phase by using a simple atmospheric-pressure chemical vapor deposition system. Using different precursors allowed us to synthesize multicomponent structures such as nanowires coated with alloy shell or multishell structures. Herein, we present the synthesis and structural characterization of the different structures, as well as an electrochemical characterization and a photovoltaic response of the ZnO-CdS system, in which the resulting photocurrent upon illumination indicates charge separation at the interface.

A General Synthesis of Porous Carbon Nitride Films with Tunable Surface Area and Photophysical Properties

Graphitic carbon nitride (g-CN) has emerged as a promising material for energy-related applications. However, exploitation of g-CN in practical devices is still limited owing to difficulties in fabricating g-CN films with adjustable properties and high surface area. A general and simple pathway is reported to grow highly porous and large-scale g-CN films with controllable chemical and photophysical properties on various substrates using the doctor blade technique. The growth of g-CN films, ascribed to the formation of a supramolecular paste, comprises g-CN monomers in ethylene glycol, which can be cast on different substrates. The g-CN composition, porosity, and optical properties can be tuned by the design of the supramolecular paste, which upon calcination results in a continuous porous g-CN network. The strength of the porous structure is demonstrated by high electrochemically active surface area, excellent dye adsorption and photoelectrochemical and photodegradation properties.

Highly luminescent CuGaxIn1−xSySe2−y nanocrystals from organometallic single-source precursors

The solution-based thermolysis of the organometallic single-source precursors [(iPr3PCu)4(MeGa)4S6] (1) and [(iPr3PCu)4(MeIn)4Se6] (2) is investigated. Multinary semiconductor nanocrystals with sizes in the range of 3–10 nm are obtained by a simple heating-up process in a solvent mixture of long-chain alkyl thiols and amines. The thiols also serve as a sulfur source and capping ligand for the nanoparticles. By mixing 1 and 2, nanocrystal compositions in the range from CuGaS2 to CuInSSe are accessible. Surface passivation of the nanocrystals with ZnS results in high stability and bright photoluminescence (PL). PL maxima are observed in the spectral range between 600 and 800 nm, depending on the size and composition of the nanocrystals. The highest PL quantum yields exceed 50% and are observed for a gallium to indium ratio of 1 : 5.

Carbon and Nitrogen Based Nanosheets as Fluorescent Probes with Tunable Emission

2D carbon and nitrogen based semiconductors (CN) have attracted widespread attention for their possible use as low-cost and environmentally friendly materials for various applications. However, their limited solution-dispersibility and the difficulty in preparing exfoliated sheets with tunable photophysical properties restrain their exploitation in imaging-related applications. Here, the synthesis of carbon and nitrogen organic scaffolds with highly tunable optical properties, excellent dispersion in water and DMSO, and good bioimaging properties is reported. Tailored photophysical and chemical properties are acquired by the synthesis of new starting monomers containing different substituent chemical groups with varying electronic properties. Upon monomer condensation at moderate temperature, 350 °C, the starting chemical groups are fully preserved in the final CN. The low condensation temperature and the effective molecular-level modification of the CN scaffold lead to well-dispersed photoluminescent CN thin sheets with a wide range of emission wavelengths. The good bioimaging properties and the tunable fluorescence properties are exemplified by in situ visualization of giant unilamellar vesicles in a buffered aqueous solution as a model system. This approach opens the possibility for the design of tailor-made CN materials with tunable photophysical and chemical properties toward their exploitation in various fields, such as photocatalysis, bioimaging, and sensing.

Electrophoretic deposition of single-source precursors as a general approach for the formation of hybrid nanorod array heterostructures

HYPOTHESIS Subjecting colloids to electric fields often results in (electrophoretic) deposition on conductive substrates. Dispersing a single-source precursor (SSP) of choice in an appropriate solvent, should allow its deposition on different substrates. The SSP-solvent interaction might play a role in the deposition (e.g., direction, rate, coverage). After thermal decomposition, the SSPs convert to the designed material, thus allowing formation of thin films or hybrid nanostructures. EXPERIMENTS Electrophoretic deposition (EPD) was applied on two representative SSPs in different solvents. These SSPs were deposited onto substrates covered with vertically-aligned ZnO nanorod (NR) arrays. After thermal decomposition, hybrid nanostructures were obtained and their morphology and interfaces were characterized by electron microscopy, X-ray diffraction, UV-vis, and electrochemistry. FINDINGS Tuning the organic dispersant-SSP interaction allows control over the final film morphology, which can result in coating and filling of NRs with metal-sulfides or metal-oxides after thermal decomposition of the SSP. These findings introduce a new facile method for a fast and large-scale uniform deposition of different (nanostructured) thin film semiconductors on a variety of substrates. We discuss the influence of the dispersant medium on the deposition of metallo-organic SSPs. As an example, the formed ZnO-CdS interface supports charge transfer upon illumination.

Layered Boron-Nitrogen-Carbon-Oxygen Materials with Tunable Composition as Lithium-Ion Battery Anodes

The insertion of heteroatoms with different electronegativity into carbon materials can tune their chemical, electronic, and optical properties. However, in traditional solid-state synthesis, it is challenging to control the reactivity of monomers, and therefore, the amount and position of heteroatoms in the final materials. Herein, a simple, scalable, and general molten-state route to synthesize boron-nitrogen-carbon-oxygen (BNCO) materials with tunable boron-nitrogen-carbon composition, as well as electronic and optical properties, is reported. The new synthetic approach consists of polycyclic aromatic hydrocarbons (PAHs) and ammonia-borane as reactants that form a clear liquid-state stage spanning a wide temperature range, before the solid-state reaction. The molten-state stage enhances the control over the synthetic intermediates and final materials, owing to improved monomer miscibility and reactivity. The BNCO composition and optical properties are tuned by the PAH selection and final reaction temperature. The advantages of this method are demonstrated herein through the tunable optical properties, excellent stability to oxidization, facile deposition on substrates, and good activity as an anode material in lithium-ion batteries.

Insight into the formation mechanism of PtCu alloy nanoparticles

Different compositions of PtCu alloy nanoparticles were formed using one-pot synthesis. The effects of different reaction parameters (precursor type, precursor ratio, stabilizing agent concentration, and temperature) were studied. To achieve an insight into the formation mechanism, a kinetic study of time-evolution of the system was conducted, followed by a study of the influence of the precursor addition step. Studying the formation mechanism of alloy nanoparticles enables a better design of next-generation hybrid metal nano-catalysts.