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)₃²⁺ 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)₃²⁺; (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

Solution-Grown Sodium Bismuth Dichalcogenides: Toward Earth-Abundant, Biocompatible Semiconductors

Many technologically relevant semiconductors contain toxic, heavily regulated (Cd, Pb, As), or relatively scarce (Li, In) elements and often require high manufacturing costs. We report a facile, general, low-temperature, and size tunable (4–28 nm) solution phase synthesis of ternary APnE₂ semiconductors based on Earth-abundant and biocompatible elements (A = Na, Pn = Bi, E = S or Se). The observed experimental band gaps (1.20–1.45 eV) fall within the ideal range for solar cells. Computational investigation of the lowest energy superstructures that result from “coloring”, caused by mixed cation sites present in their rock salt lattice, agrees with other better-known members of this family of materials. Our synthesis unlocks a new class of low cost and environmentally friendly ternary semiconductors that show properties of interest for applications in energy conversion

How Robust are Semiconductor Nanorods? Investigating the Stability and Chemical Decomposition Pathways of Photoactive Nanocrystals

Anisotropic II–VI semiconductor nanostructures are important photoactive materials for various energy conversion and optical applications. However, aside from the many available surface chemistry studies and from their ubiquitous photodegradation under continuous illumination, the general chemical reactivity and thermal stability (phase and shape transformations) of these materials are poorly understood. Using CdSe and CdS nanorods as model systems, we have investigated the behavior of II–VI semiconductor nanorods against various conditions of extreme chemical and physical stress (acids, bases, oxidants, reductants, and heat). CdSe nanorods react rapidly with acids, becoming oxidized to Se or SeO₂. In contrast, CdSe nanorods remain mostly unreactive when treated with bases or strong oxidants, although bases do partially etch the tips of the nanorods (along their axis). Roasting (heating in air) of CdSe nanorods results in rock-salt CdO, but neither CdSe nor CdO is easily reduced by hydrogen (H₂). Another reductant, <i>n</i>-BuLi, reduces CdSe nanorods to metallic Cd. Variable temperature X-ray diffraction experiments show that axial annealing and selective axial melting of the nanorods precede particle coalescence. Furthermore, thermal analysis shows that the axial melting of II–VI nanorods is a ligand-dependent process. In agreement with chemical reactivity and thermal stability observations, silica-coating experiments show that the sharpest (most curved) II–VI surfaces are most active against heterogeneous nucleation of a silica shell. These results provide valuable insights into the fate and possible ways to enhance the stability and improve the use of II–VI semiconductor nanostructures in the fields of optics, magnetism, and energy conversion

Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II–VI Semiconductor Epitaxy

Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character <i>via</i> strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent II–VI, III–V and IV–VI semiconductor quantum dots. Here, we use relatively unexplored IV/II–VI epitaxy as a way to enhance the photoluminescence and improve the optical stability of colloidal Ge nanocrystals. Selected on the basis of their relatively small lattice mismatch compared with crystalline Ge, we explore the growth of epitaxial CdS and ZnS shells using the successive ion layer adsorption and reaction method. Powder X-ray diffraction and electron microscopy techniques, including energy dispersive X-ray spectroscopy and selected area electron diffraction, clearly show the controllable growth of as many as 20 epitaxial monolayers of CdS atop Ge cores. In contrast, Ge etching and/or replacement by ZnS result in relatively small Ge/ZnS nanocrystals. The presence of an epitaxial II–VI shell greatly enhances the near-infrared photoluminescence and improves the photoluminescence stability of Ge. Ge/II–VI nanocrystals are reproducibly 1–3 orders of magnitude brighter than the brightest Ge cores. Ge/4.9CdS core/shells show the highest photoluminescence quantum yield and longest radiative recombination lifetime. Thiol ligand exchange easily results in near-infrared active, water-soluble Ge/II–VI nanocrystals. We expect this synthetic IV/II–VI epitaxial approach will lead to further studies into the optoelectronic behavior and practical applications of Si and Ge-based nanomaterials

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-<i>tert</i>-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 < dibenzyl < di-<i>tert</i>-butyl < diisopropyl < diethyl < dimethyl < 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

High-Precision Tracking with Non-blinking Quantum Dots Resolves Nanoscale Vertical Displacement

Novel non-blinking quantum dots (NBQDs) were utilized in three-dimensional super-localization, high-precision tracking applications under an automated scanning-angle total internal reflection fluorescence microscope (SA-TIRFM). NBQDs were randomly attached to stationary microtubules along the radial axis under gliding assay conditions. By automatically scanning through a wide range of incident angles with different evanescent-field layer thicknesses, the fluorescence intensity decay curves were obtained. Fit with theoretical decay functions, the absolute vertical positions were determined with sub-10-nm localization precision. The emission intensity profile of the NBQDs attached to kinesin-propelled microtubules was used to resolve the self-rotation of gliding microtubules within a small vertical distance of ∼50 nm. We demonstrate the applicability of NBQDs in high-precision fluorescence imaging experiments

High-Precision Tracking with Non-blinking Quantum Dots Resolves Nanoscale Vertical Displacement

Novel non-blinking quantum dots (NBQDs) were utilized in three-dimensional super-localization, high-precision tracking applications under an automated scanning-angle total internal reflection fluorescence microscope (SA-TIRFM). NBQDs were randomly attached to stationary microtubules along the radial axis under gliding assay conditions. By automatically scanning through a wide range of incident angles with different evanescent-field layer thicknesses, the fluorescence intensity decay curves were obtained. Fit with theoretical decay functions, the absolute vertical positions were determined with sub-10-nm localization precision. The emission intensity profile of the NBQDs attached to kinesin-propelled microtubules was used to resolve the self-rotation of gliding microtubules within a small vertical distance of ∼50 nm. We demonstrate the applicability of NBQDs in high-precision fluorescence imaging experiments

Surface Doping Quantum Dots with Chemically Active Native Ligands: Controlling Valence without Ligand Exchange

One remaining challenge in the field of colloidal semiconductor nanocrystal quantum dots is learning to control the degree of functionalization or “valence” per nanocrystal. Current quantum dot surface modification strategies rely heavily on ligand exchange, which consists of replacing the nanocrystal’s native ligands with carboxylate- or amine-terminated thiols, usually added in excess. Removing the nanocrystal’s native ligands can cause etching and introduce surface defects, thus affecting the nanocrystal’s optical properties. More importantly, ligand exchange methods fail to control the extent of surface modification or number of functional groups introduced per nanocrystal. Here, we report a fundamentally new surface ligand modification or “doping” approach aimed at controlling the degree of functionalization or valence per nanocrystal while retaining the nanocrystal’s original colloidal and photostability. We show that surface-doped quantum dots capped with chemically active native ligands can be prepared directly from a mixture of ligands with similar chain lengths. Specifically, vinyl and azide-terminated carboxylic acid ligands survive the high temperatures needed for nanocrystal synthesis. The ratio between chemically active and inactive-terminated ligands is maintained on the nanocrystal surface, allowing to control the extent of surface modification by straightforward organic reactions. Using a combination of optical and structural characterization tools, including IR and 2D NMR, we show that carboxylates bind in a bidentate chelate fashion, forming a single monolayer of ligands that are perpendicular to the nanocrystal surface. Moreover, we show that mixtures of ligands with similar chain lengths homogeneously distribute themselves on the nanocrystal surface. We expect this new surface doping approach will be widely applicable to other nanocrystal compositions and morphologies, as well as to many specific applications in biology and materials science

Selective Alcohol Dehydrogenation and Hydrogenolysis with Semiconductor-Metal Photocatalysts: Toward Solar-to-Chemical Energy Conversion of Biomass-Relevant Substrates

Photocatalytic conversion of biomass is a potentially transformative concept in renewable energy. Dehydrogenation and hydrogenolysis of biomass-derived alcohols can produce renewable fuels such as H₂ and hydrocarbons, respectively. We have successfully used semiconductor-metal heterostructures for sunlight-driven dehydrogenation and hydrogenolysis of benzyl alcohol. The heterostructure composition dictates activity, product distribution, and turnovers. A few metal (M = Pt, Pd) islands on the semiconductor (SC) surface significantly enhance activity and selectivity and also greatly stabilize the SC against photoinduced etching and degradation. Under selected conditions, CdS-Pt favors dehydrogenation (H₂) over hydrogenolysis (toluene) 8:1, whereas CdS_0.4Se_0.6-Pd favors hydrogenolysis over dehydrogenation 3:1. Photochemically generated, surface-adsorbed hydrogen is useful in tandem catalysis, for example, via transfer hydrogenation. We expect this work will lead to new paradigms for sunlight-driven conversions of biomass-relevant substrates

High-Precision Tracking with Non-blinking Quantum Dots Resolves Nanoscale Vertical Displacement

Novel non-blinking quantum dots (NBQDs) were utilized in three-dimensional super-localization, high-precision tracking applications under an automated scanning-angle total internal reflection fluorescence microscope (SA-TIRFM). NBQDs were randomly attached to stationary microtubules along the radial axis under gliding assay conditions. By automatically scanning through a wide range of incident angles with different evanescent-field layer thicknesses, the fluorescence intensity decay curves were obtained. Fit with theoretical decay functions, the absolute vertical positions were determined with sub-10-nm localization precision. The emission intensity profile of the NBQDs attached to kinesin-propelled microtubules was used to resolve the self-rotation of gliding microtubules within a small vertical distance of ∼50 nm. We demonstrate the applicability of NBQDs in high-precision fluorescence imaging experiments