Colloidal Synthesis of 1T-WS₂ and 2H-WS₂ Nanosheets: Applications for Photocatalytic Hydrogen Evolution

In recent years, a lot of attention has been devoted to monolayer materials, in particular to transition-metal dichalcogenides (TMDCs). While their growth on a substrate and their exfoliation are well developed, the colloidal synthesis of monolayers in solution remains challenging. This paper describes the development of synthetic protocols for producing colloidal WS₂ monolayers, presenting not only the usual semiconducting prismatic 2H-WS₂ structure but also the less common distorted octahedral 1T-WS₂ structure, which exhibits metallic behavior. Modifications of the synthesis method allow for control over the crystal phase, enabling the formation of either 1T-WS₂ or 2H-WS₂ nanostructures. We study the factors influencing the formation of the two WS₂ nanostructures, using X-ray diffraction, microscopy, and spectroscopy analytical tools to characterize them. Finally, we investigate the integration of these two WS₂ nanostructured polymorphs into an efficient photocatalytic hydrogen evolution system to compare their behavior

European journal of cell biology : EJCB

A detailed investigation examines how the size of allylbenzene-capped silicon nanocrystals (ncSi:AB) affects their chemical reactivity with gaseous O₂, H₂O, and O₂/H₂O as probed by in situ luminescence spectroscopy. Specifically, changes in the photoluminescence (PL) of size-separated ncSi:AB are monitored through alterations of their PL absolute quantum yield (AQY) as well as the wavelength and intensity of their PL spectra over time. These experiments, conducted under both continuous and intermittent illumination, help elucidate the roles of O₂, H₂O, and mixtures of O₂/H₂O, with respect to oxidation of ncSi:AB as a function of their size, providing vital information for any perceived application in advanced materials and biomedical devices

Polymer-like Conformation and Growth Kinetics of Bi₂S₃ Nanowires

One-dimensional inorganic crystals (i.e., crystalline nanowires) are one of the most intensely investigated classes of materials of the past two decades. Despite this intense effort, an important question has yet to be answered: do nanowires display some of the unique characteristics of polymers as their diameter is progressively decreased? This work addresses this question with three remarkable findings on the growth and form of ultrathin Bi₂S₃ nanowires. (i) Their crystallization in solution is quantitatively describable as a form of living step-growth polymerization: an apparently exclusive combination of addition of “monomer” to the ends of the nanowires and coupling of fully formed nanowires “end-to-end”, with negligible termination and initiation. (ii) The rate constants of these two main processes are comparable to those of analogous processes found in polymerization. (iii) The conformation of these nanowires is quantitatively described as a worm-like conformation analytically analogous to that of semiflexible polymers and characterized by a persistence length of 17.5 nm (shorter than that of double-stranded DNA) and contour lengths of hundreds of micrometers (longer than those of most synthetic polymers). These findings do not prove a chemical analogy between crystals and polymers (it is unclear if the monomer is a molecular entity <i>tout court</i>) but demonstrate a physical analogy between crystallization and polymerization. Specifically, they (i) show that the crystallization of ensembles of nanoscale inorganic crystals can be conceptually analogous to polymerization and can be described quantitatively with the same experimental and mathematical tools, (ii) demonstrate that one-dimensional nanocrystals can display topological characteristics of polymers (e.g., worm-like conformation in solution), (iii) establish a unique experimental model system for the investigation of polymer-like topological properties in inorganic crystals, and (iv) provide new heuristic guidelines for the synthesis of polymer-like nanowires

Metadynamics-Biased ab Initio Molecular Dynamics Study of Heterogeneous CO₂ Reduction via Surface Frustrated Lewis Pairs

The recent discovery of frustrated Lewis pairs (FLPs) capable of heterolytically splitting hydrogen gas at the surface of hydroxylated indium oxide (In₂O_3–<i>x</i>(OH)_<i>y</i>) nanoparticles has led to interesting implications for heterogeneous catalytic reduction of CO₂. Although the role of surface FLPs in the reverse water-gas shift (RWGS) reaction (CO₂ + H₂ → CO + H₂O) has been experimentally and theoretically demonstrated, the interplay between surface FLPs and temperature and their consequences for the reaction mechanism have yet to be understood. Here we use well-tempered metadynamics-biased ab initio molecular dynamics to obtain the free energy landscape of the multistep RWGS reaction at finite temperatures. The reaction is simulated at 20 and 180 °C, and the minimum energy reaction pathways and energy barriers corresponding to H₂ dissociation and CO₂ reduction are obtained. The reduction of CO₂ at the surface FLP catalytically active site, where H₂ is heterolytically dissociated and bound, is found to be the rate-limiting step and is mostly unaffected by increased temperature conditions; however, at 180 °C the energetic barriers associated with the splitting of H₂ and the subsequent adsorption of CO₂ are reduced by 0.15 and 0.19 eV, respectively. It is suggested that increased thermal conditions may enhance reactivity by enabling the surface FLP to become further spatially separated. Product H₂O is found to favor dissociative adsorption over direct desorption from the surface of In₂O_3–<i>x</i>(OH)_<i>y</i> and may therefore impede sustained catalytic activity by blocking surface sites

Size-Dependent Absolute Quantum Yields for Size-Separated Colloidally-Stable Silicon Nanocrystals

Size-selective precipitation was used to successfully separate colloidally stable allylbenzene-capped silicon nanocrystals into several visible emitting monodisperse fractions traversing the quantum size effect range of 1–5 nm. This enabled the measurement of the absolute quantum yield and lifetime of photoluminescence of allylbenzene-capped silicon nanocrystals as a function of size. The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation. Visible emission absolute quantum yields as high as 43% speak well for the development of “green” silicon nanocrystal color-tunable light emitting diodes that can potentially match the performance of their toxic heavy metal chalcogenide counterparts

Effect of Precursor Selection on the Photocatalytic Performance of Indium Oxide Nanomaterials for Gas-Phase CO₂ Reduction

Nonstoichiometric indium oxide nanoparticles, In₂O_3–<i>x</i>(OH)_<i>y</i>, have been shown to function as active photocatalysts for gas-phase CO₂ reduction under simulated solar irradiation. Herein we demonstrate that the choice of starting material has a strong effect on the photocatalytic activity of indium oxide nanoparticles. We examine three indium oxide materials prepared via the thermal decomposition of either indium­(III) hydroxide or indium­(III) nitrate and correlate their stability and photocatalytic activity to the number and type of defect present in the material. Further, we use ¹³CO₂ isotope-tracing experiments to clearly identify the origins of the observed carbon-containing products. Significantly, we find that the oxidizing nature of the precursor anion has a substantial impact on the defect formation within the sample. This study demonstrates the importance of surface defects in designing an active heterogeneous photocatalyst and provides valuable insight into key parameters for the precursor design, selection, and performance optimization of materials for gas-phase CO₂ reduction

Nanostructured Indium Oxide Coated Silicon Nanowire Arrays: A Hybrid Photothermal/Photochemical Approach to Solar Fuels

The field of solar fuels seeks to harness abundant solar energy by driving useful molecular transformations. Of particular interest is the photodriven conversion of greenhouse gas CO₂ into carbon-based fuels and chemical feedstocks, with the ultimate goal of providing a sustainable alternative to traditional fossil fuels. Nonstoichiometric, hydroxylated indium oxide nanoparticles, denoted In₂O_3–<i>x</i>(OH)_<i>y</i>, have been shown to function as active photocatalysts for CO₂ reduction to CO <i>via</i> the reverse water gas shift reaction under simulated solar irradiation. However, the relatively wide band gap (2.9 eV) of indium oxide restricts the portion of the solar irradiance that can be utilized to ∼9%, and the elevated reaction temperatures required (150–190 °C) reduce the overall energy efficiency of the process. Herein we report a hybrid catalyst consisting of a vertically aligned silicon nanowire (SiNW) support evenly coated by In₂O_3–<i>x</i>(OH)_<i>y</i> nanoparticles that utilizes the vast majority of the solar irradiance to simultaneously produce both the photogenerated charge carriers and heat required to reduce CO₂ to CO at a rate of 22.0 μmol·g_cat^–1·h^–1. Further, improved light harvesting efficiency of the In₂O_3–<i>x</i>(OH)_<i>y</i>/SiNW films due to minimized reflection losses and enhanced light trapping within the SiNW support results in a ∼6-fold increase in photocatalytic conversion rates over identical In₂O_3–<i>x</i>(OH)_<i>y</i> films prepared on roughened glass substrates. The ability of this In₂O_3–<i>x</i>(OH)_<i>y</i>/SiNW hybrid catalyst to perform the dual function of utilizing both light and heat energy provided by the broad-band solar irradiance to drive CO₂ reduction reactions represents a general advance that is applicable to a wide range of catalysts in the field of solar fuels

Spatially Confined Redox Chemistry in Periodic Mesoporous Hydridosilica–Nanosilver Grown in Reducing Nanopores

Periodic mesoporous hydridosilica, PMHS, is shown for the first time to function as both a host and a mild reducing agent toward noble metal ions. In this archetypical study, PMHS microspheres react with aqueous Ag(I) solutions to form Ag(0) nanoparticles housed in different pore locations of the mesostructure. The dominant reductive nucleation and growth process involves SiH groups located within the pore walls and yields molecular scale Ag(0) nanoclusters trapped and stabilized in the pore walls of the PMHS microspheres that emit orange-red photoluminescence. Lesser processes initiated with pore surface SiH groups produce some larger spherical and worm-shaped Ag(0) nanoparticles within the pore voids and on the outer surfaces of the PMHS microspheres. The intrinsic reducing power demonstrated in this work for the pore walls of PMHS speaks well for a new genre of chemistry that benefits from the mesoscopic confinement of Si–H groups

Enhanced Hematite Water Electrolysis Using a 3D Antimony-Doped Tin Oxide Electrode

We present herein an example of nanocrystalline antimony-doped tin oxide (nc-ATO) disordered macroporous “inverse opal” 3D electrodes as efficient charge-collecting support structures for the electrolysis of water using a hematite surface catalyst. The 3D macroporous structures were created <i>via</i> templating of polystyrene spheres, followed by infiltration of the desired precursor solution and annealing at high temperature. Using cyclic voltammetry and electrochemical impedance spectroscopy, it was determined that the use of this 3D transparent conducting oxide with a hematite surface catalyst allowed for a 7-fold increase in active surface area for water splitting with respect to its 2D planar counterpart. This ratio of surface areas was evaluated based on the presence of oxidized trap states on the hematite surface, as determined from the equivalent circuit analysis of the Nyquist plots. Furthermore, the presence of nc-ATO 2D and 3D “underlayer” structures with hematite deposited on top resulted in decreased charge transfer resistances and an increase in the number of available active surface sites at the semiconductor–liquid junction when compared to hematite films lacking any nc-ATO substructures. Finally, absorption, transmission, and reflectance spectra of all of the tested films were measured, suggesting the feasibility of using 3D disordered structures in photoelectrochemical reactions, due to the high absorption of photons by the surface catalyst material and trapping of light within the structure

Looking Inside a Working SiLED

In this study, we investigate for the first time morphological and compositional changes of silicon quantum dot (SiQD) light-emitting diodes (SiLEDs) upon device operation. By means of advanced transmission electron microscopy (TEM) analysis including energy filtered TEM (EFTEM) and energy dispersive X-ray (EDX) spectroscopy, we observe drastic morphological changes and degradation for SiLEDs operated under high applied voltage ultimately leading to device failure. However, SiLEDs built from size-separated SiQDs operating under normal conditions show no morphological and compositional changes and the biexponential loss in electroluminescence seems to be correlated to chemical and physical degradation of the SiQDs. By contrast, we found that, for SiLEDs fabricated from polydisperse SiQDs, device degradation is more pronounced with three main modes of failure contributing to the reduced overall lifetime compared to those prepared from size-separated SiQDs. With this newfound knowledge, it is possible to devise ways to increase the lifetimes of SiLEDs