Expanding the one-dimensional CdS-CdSe composition landscape: Axially anisotropic CdS1- xSex nanorods

We report the synthesis and characterization of CdS1-xSex nanorods with axial anisotropy. These nanorods were synthesized via single injection of a mixture of trioctylphosphine sulfur and selenium precursors to a cadmium-phosphonate complex at high temperature. Transmission electron microscopy shows nanoparticle morphology changes with relative sulfur and selenium loading. When the synthetic selenium loading is between 5% and 10% of total chalcogenides, the nanorods exhibit pronounced axial anisotropy characterized by a thick head and a thin tail . The nanorods\u27 band gap red shifts with increasing selenium loading. X-ray diffraction reveals that CdS1-xSex nanorods have a wurtzite crystal structure with a certain degree of alloying. High-resolution and energy-filtered transmission electron microscopy and energy-dispersive X-ray spectroscopy confirm the head of the anisotropic nanorods is rich in selenium, whereas the tail is rich in sulfur. Time evolution and mechanistic studies confirm the nanorods form by quick growth of the CdSe-rich head, followed by slow growth of the CdS-rich tail. Metal photodeposition reactions with 575 nm irradiation, which is mostly absorbed by the CdSe-rich segment, show effective electronic communication between the nanorod head and tail segments

Controlled fabrication of colloidal semiconductor-metal hybrid heterostructures: Site selective metal photo deposition

Reliable synthesis of semiconductor-metal heterostructures would increase their availability for fundamental studies and applications in catalytic, magnetic, and opto-electronic devices. Here, we demonstrate there are three main pathways for the formation of Pt and Pd nanoparticles on CdS and CdS04Se0.6 nanorods. A thermal pathway and photochemical pathway occur when the metal precursor is heated or irradiated directly in the presence of an electron donor, leading to homogeneous nucleation and formation of freestanding metal nanoparticles. A separate photochemical pathway occurs in the presence of semiconductor nanorods, leading to exciton formation and quenching by electron trapping at surface defect sites. The localized electrons act as seeding points, leading to heterogeneous nucleation and formation of surface-bound metal nanoparticles. Careful selection of synthetic conditions allows deposition of Pt and Pd particles on CdS and CdS0.4Se0.6 nanorods with a high degree of selectivity (90-95% surface-bound obtained photochemically) over the formation of freestanding metal particles (70-94% unattached under thermal conditions). In addition, metal photo deposition occurs on specific segments of CdS0.4Se0.6 nanorods with compositional anisotropy by taking advantage of the band gap differential between different nanodomains. Irradiation at short wavelengths favors formation of Pd nanoparticles on the large band gap CdS-rich region of the nanorods (57% and 55% at 350 and 420 nm, respectively), while irradiation at longer wavelengths favors the formation of Pd nanoparticles on the small band gap CdSe-rich region of the nanorods (83% at 575 nm). The ability to tune the spatial composition of these and similar heterostructures will impact the ability to engineer and direct energy flows at the nanoscale

Chalcogenide and Pnictide Nanocrystals from the Silylative Deoxygenationof Metal Oxides

Transition metal chalcogenide and pnictide nanocrystals are of interest for optoelectronic and catalytic applications. Here, we present a generalized route to the synthesis of these materials from the silylative deoxygenation of metal oxides with trimethylsilyl reagents. Specific nanophases produced in this way include Ni3S2, Ni5Se5, Ni2P, Co9S8, Co3Se4, CoP, Co2P, and heterobimetallic (Ni/Co)9S8. The resulting chalcogenide nanocrystals are hollow, likely due to differential rates of ion diffusion during the interfacial phase transformation reaction (Kirkendall-type effect). In contrast, the phosphide nanocrystals are solid, likely because they form at higher reaction temperatures. In all cases, simultaneous partial decomposition of the deoxygenating silyl reagent produces a coating of amorphous silica around the newly formed nanocrystals, which could impact their stability and recyclability

Microstructure Effects on the Water Oxidation Activity of Co3O4/ Porous Silica Nanocomposites

We investigate the effect of microstructuring on the water oxidation (oxygen evolution) activity of two types of Co3O4/porous silica composites: Co3O4/porous SiO2 core/shell nanoparticles with varying shell thicknesses and surface areas, and Co3O4/mesoporous silica nanocomposites with various surface functionalities. Catalytic tests in the presence of Ru(bpy)3 2+ as a photosensitizer and S2O8 2- as a sacrificial electron acceptor show that porous silica shells of up to -20 nm in thickness lead to increased water oxidation activity. We attribute this effect to either (1) a combination of an effective increase in catalyst active area or consequent higher local concentration of Ru(bpy)3 2+; (2) a decrease in the permittivity of the medium surrounding the catalyst surface and a consequent increase in the rate of charge transfer; or both. Functionalized Co3O4/mesoporous silica nanocomposites show lower water oxidation activity compared with the parent nonfunctionalized catalyst, likely because of partial pore blocking of the silica support upon surface grafting. A more thorough understanding of the effects of microstructure and permittivity on water oxidation ability will enable the construction of next generation catalysts possessing optimal configuration and better efficiency for water splitting

Shape-Programmed Nanofabrication: Understanding the Reactivity of Dichalcogenide Precursors

Dialkyl and diaryl dichalcogenides are highly versatile and modular precursors for the synthesis of colloidal chalcogenide nanocrystals. We have used a series of commercially available dichalcogenide precursors to unveil the molecular basis for the outcome of nanocrystal preparations, more specifically, how precursor molecular structure and reactivity affect the final shape and size of II-VI semiconductor nanocrystals. Dichalcogenide precursors used were diallyl, dibenzyl, di-tert-butyl, diisopropyl, diethyl, dimethyl, and diphenyl disulfides and diethyl, dimethyl, and diphenyl diselenides. We find that the presence of two distinctively reactive C-E and E-E bonds makes the chemistry of these precursors much richer and interesting than that of other conventional precursors such as the more common phosphine chalcogenides. Computational studies (DFT) reveal that the dissociation energy of carbon-chalcogen (C-E) bonds in dichalcogenide precursors (R-E-E-R, E = S or Se) increases in the order (R): diallyl \u3c dibenzyl \u3c di-tert-butyl \u3c diisopropyl \u3c diethyl \u3c dimethyl \u3c diphenyl. The dissociation energy of chalcogen-chalcogen (E-E) bonds remains relatively constant across the series. The only exceptions are diphenyl dichalcogenides, which have a much lower E-E bond dissociation energy. An increase in C-E bond dissociation energy results in a decrease in R-E-E-R precursor reactivity, leading to progressively slower nucleation and higher selectivity for anisotropic growth, all the way from dots to pods to tetrapods. Under identical experimental conditions, we obtain CdS and CdSe nanocrystals with spherical, elongated, or tetrapodal morphology by simply varying the identity and reactivity of the dichalcogenide precursor. Interestingly, we find that precursors with strong C-E and weak E-E bond dissociation energies such as Ph-S-S-Ph serve as a ready source of thiol radicals that appear to stabilize small CdE nuclei, facilitating anisotropic growth. These CdS and CdSe nanocrystals have been characterized using structural and spectroscopic methods. An intimate understanding of how molecular structure affects the chemical reactivity of molecular precursors enables highly predictable and reproducible synthesis of colloidal nanocrystals with specific sizes, shapes, and optoelectronic properties for customized applications

Polytypism and Unique Site Preference in LiZnSb: A Superior Thermoelectric Reveals Its True Colors

The first example of polytypism in the I–II–V semiconductors has been demonstrated with the synthesis of cubic LiZnSb by a low-temperature solution-phase method. This phase exhibits a unique coloring pattern that is novel for this class of compounds. The choice of site configuration has a considerable impact on the band structure of these materials, which in turn affects the transport properties. While the hexagonal polytype has been suggested as a promising n-type and extremely poor p-type thermoelectric material, the cubic analogue is calculated to have high efficiencies for both the n- and p-type derivatives (1.64 and 1.43, respectively, at 600 K). Furthermore, the cubic phase is found to be the energetically favored polytype. This surprising result provides a rationale for the lack of success in synthesizing the hexagonal polytype in either stoichiometric or n-type compositions

Multishell Au/Ag/SiO2 Nanorods with Tunable Optical Properties as Single Particle Orientation and Rotational Tracking Probes

Three-layer core-shell plasmonic nanorods (Au/Ag/SiO2-NRs), consisting of a gold nanorod core, a thin silver shell, and a thin silica layer, were synthesized and used as optical imaging probes under a differential interference contrast microscope for single particle orientation and rotational tracking. The localized surface plasmon resonance modes were enhanced upon the addition of the silver shell, and the anisotropic optical properties of gold nanorods were maintained. The silica coating enables surface functionalization with silane coupling agents and provides enhanced stability and biocompatibility. Taking advantage of the longitudinal LSPR enhancement, the orientation and rotational information of the hybrid nanorods on synthetic lipid bilayers and on live cell membranes were obtained with millisecond temporal resolution using a scientific complementary metal-oxide-semiconductor camera. The results demonstrate that the as-synthesized hybrid nanorods are promising imaging probes with improved sensitivity and good biocompatibility for single plasmonic particle tracking experiments in biological systems

Cu2ZnSnS4−Au Heterostructures: Toward Greener Chalcogenide- Based Photocatalysts

Chalcogenide-based semiconductor-metal heterostructures are interesting catalysts for solar-to-chemical energy conversion, but current compositions are impractical due to the relative toxicity and/or scarcity of their constituent elements. To address these concerns, Cu2ZnSnS4 (CZTS) emerged as an interesting alternative to other chalcogenide-based semiconductors; however, the fabrication of CZTS metal heterostructures remains unexplored. In this paper, we systematically explore four methods of synthesizing CZTS-Au heterostructures, specifically: reaction of CZTS nanorods with either a soluble molecular gold precursor (AuCl3) or preformed gold (Au) nanoparticles, each under thermal (heating in the dark) or photochemical reaction conditions (350 nm lamp illumination at room temperature). We find that using AuCl3 under thermal deposition conditions results in the most well-defined CZTS-Au heterostructures, containing \u3e99% surface-bound 2.1 ± 0.5 nm Au islands along the whole length of the nanorod. These CZTS-Au heterostructures are photocatalytically active, reducing the model compound methylene blue upon irradiation much more effectively than bare CZTS nanorods. We also demonstrate the removal of Au from the CZTS-Au heterostructures by amalgamation. These results open up a new area of greener, CZTS-based photocatalysts for solar-to-chemical energy conversion

Templated Synthesis and Chemical Behavior of Nickel Nanoparticles within High Aspect Ratio Silica Capsules

One-dimensional transition metal nanostructures are of interest in many magnetic and catalytic applications. Using a combination of wet chemical synthesis, optical (infrared), and structural characterization methods (powder X-ray diffraction, scanning and transmission electron microscopy), we have investigated four paths to access 1D nickel nanostructures: (1) direct chemical reduction of a self-assembled nickel-hydrazine coordination complex, (2) thermal decomposition of the silica encapsulated nickel-hydrazine complex, (3) treatment of the silica encapsulated nickel-hydrazine complex with sodium borohydride followed by thermal annealing, and (4) electroless nickel plating using silica encapsulated nickel seed particles. We find that only route 1, which does not require a silica template, results in the formation of nickel nanorods, albeit some particle aggregation is observed. Routes 2 and 3 result in the formation of isotropic nickel structures under a reducing atmosphere. Route 4 results in heterogeneous nucleation and growth of existing particles only when partial etching of the silica capsule occurs. Detailed examination of the encapsulated nickel particles allows studying the effect of silica surface silanols on the oxidation of encapsulated nickel particles, the presence of nanoparticle-silica support interactions, the sintering mechanism of nickel and nickel oxide particles, and the fate of boride impurities. Nickel/silica nanostructures are strongly magnetic at room temperature