Photoinduced Crystallization and Activation of Amorphous Titanium Dioxide

Titanium dioxide (TiO₂) is one of the most common photosensitive materials used in photocatalysis, solar cells, self-cleaning coatings, and sunscreens. Although the crystalline TiO₂ phases such as anatase and rutile are well-known to be photoactive, whether amorphous TiO₂ is active in photocatalytic reactions is still controversial. Here we show that amorphous TiO₂ prepared by the commonly used sol–gel method of tetrabutyl titanate hydrolysis is active in photocatalytic water reduction and methylene blue oxidation under the irradiation of a xenon lamp. The amorphous TiO₂ gains photoactivity after an induction period of approximately an hour, suggesting that phase transition is involved. Using an extensive series of microscopic and spectroscopic analyses, we further show that the photoinduced crystallization by amorphous TiO₂ forms a nanometer-thin layer of rutile nanocrystallites under the irradiation in the middle ultraviolet range. The resulting core–shell nanoparticles have a bandgap of 3.3 eV and are enriched with surface-active sites including reduced titanium and oxygen vacancies. The revelation of photoinduced crystallization raises the possibility of preparing photosensitive TiO₂ using low-temperature radiation techniques that can not only save energy but also incorporate heat-sensitive components into manufacturing

Microwave-Assisted Solution–Liquid–Solid Synthesis of Single-Crystal Copper Indium Sulfide Nanowires

Chalcopyrite copper indium sulfide (CuInS₂) is an important semiconductor with a bandgap optimal for terrestrial solar energy conversion. Building photovoltaic and microelectronic devices using one-dimensional CuInS₂ nanowires can offer directional conduits for rapid and undisrupted charge transport. Currently, single-crystal CuInS₂ nanowires can be prepared only using vapor-based methods. Here, we report, for the first time, the synthesis of single-crystal CuInS₂ nanowires using a microwave-assisted solution–liquid–solid (MASLS) method. We show that CuInS₂ nanowires with diameters of less than 10 nm can be prepared at a rapid rate of 33 nm s^–1 to more than 10 μm long in less than 10 min, producing a high mass yield of 31%. We further show that the nanowires are free of structural defects and have a near-stoichiometric composition. The success of MASLS in preparing high-quality tertiary nanowires is explained by a eutectic growth mechanism involving an overheated alloy catalyst

Particle-Level Engineering of Thermal Conductivity in Matrix-Embedded Semiconductor Nanocrystals

Known manipulations of semiconductor thermal transport properties rely upon higher-order material organization. Here, using time-resolved optical signatures of phonon transport, we demonstrate a “bottom-up” means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. Growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap

Study of Nucleation and Growth Mechanism of the Metallic Nanodumbbells

We propose a general nucleation and growth model that can explain the mechanism of the formation of CoPt₃/Au, FePt/Au, and Pt/Au nanodumbbells. Thus, we found that the nucleation event occurs as a result of reduction of Au⁺ ions by partially oxidized surface Pt atoms. In cases when Au³⁺ is used as a gold precursor, the surface of seeds should be terminated by ions (e.g., Co²⁺, Pb²⁺) that can reduce Au³⁺ to Au⁺ ions, which can further participate in the nucleation of gold domain. Further growth of gold domain is a result of reduction of both Au³⁺ and Au⁺ by HDA at the surface of gold nuclei. We explain the different ability of CoPt₃, Pt, and FePt seeds to serve as a nucleation center for the reduction of gold and further growth of dumbbells. We report that the efficiency and reproducibility of the formation of CoPt₃/Au, FePt/Au, and Pt/Au dumbbells can be optimized by the concentration and oxidation states of the surface ions on metallic nanocrystals used as seeds as well as by the type of the gold precursor

Photocatalytic Hydrogen Generation Efficiencies in One-Dimensional CdSe Heterostructures

To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model one-dimensional (1D) heterostructures based on CdSe nanowires (NWs). Specifically, CdSe/CdS core/shell NWs and Au nanoparticle (NP)-decorated core and core/shell NWs have been produced using facile solution chemistries. These systems enable us to explore sources for efficient charge separation and enhanced carrier lifetimes important to photocatalytic processes. We find that visible light H₂ generation efficiencies in the produced hybrid 1D structures increase in the order CdSe < CdSe/Au NP < CdSe/CdS/Au NP < CdSe/CdS with a maximum H₂ generation rate of 58.06 ± 3.59 μmol h^–1 g^–1 for CdSe/CdS core/shell NWs. This is 30 times larger than the activity of bare CdSe NWs. Using femtosecond transient differential absorption spectroscopy, we subsequently provide mechanistic insight into the role nanoscale heterojunctions play by directly monitoring charge flow and accumulation in these hybrid systems. In turn, we explain the observed trend in H₂ generation rates with an important outcome being direct evidence for heterojunction-influenced charge transfer enhancements of relevant chemical reduction processes

How “Hollow” Are Hollow Nanoparticles?

Diamond anvil cell (DAC), synchrotron X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) techniques are used to probe the composition inside hollow γ-Fe₃O₄ nanoparticles (NPs). SAXS experiments on 5.2, 13.3, and 13.8 nm hollow-shell γ-Fe₃O₄ NPs, and 6 nm core/14.8 nm hollow-shell Au/Fe₃O₄ NPs, reveal the significantly high (higher than solvent) electron density of the void inside the hollow shell. In high-pressure DAC experiments using Ne as pressure-transmitting medium, formation of nanocrystalline Ne inside hollow NPs is not detected by XRD, indicating that the oxide shell is impenetrable. Also, FTIR analysis on solutions of hollow-shell γ-Fe₃O₄ NPs fragmented upon refluxing shows no evidence of organic molecules from the void inside, excluding the possibility that organic molecules get through the iron oxide shell during synthesis. High-pressure DAC experiments on Au/Fe₃O₄ core/hollow-shell NPs show good transmittance of the external pressure to the gold core, indicating the presence of the pressure-transmitting medium in the gap between the core and the hollow shell. Overall, our data reveal the presence of most likely small fragments of iron and/or iron oxide in the void of the hollow NPs. The iron oxide shell seems to be non-porous and impenetrable by gases and liquids

Capping Ligands as Selectivity Switchers in Hydrogenation Reactions

We systematically investigated the role of surface modification of nanoparticles catalyst in alkyne hydrogenation reactions and proposed the general explanation of effect of surface ligands on the selectivity and activity of Pt and Co/Pt nanoparticles (NPs) using experimental and computational approaches. We show that the proper balance between adsorption energetics of alkenes at the surface of NPs as compared to that of capping ligands defines the selectivity of the nanocatalyst for alkene in alkyne hydrogenation reaction. We report that addition of primary alkylamines to Pt and CoPt₃ NPs can drastically increase selectivity for alkene from 0 to more than 90% with ∼99.9% conversion. Increasing the primary alkylamine coverage on the NP surface leads to the decrease in the binding energy of octenes and eventual competition between octene and primary alkylamines for adsorption sites. At sufficiently high coverage of catalysts with primary alkylamine, the alkylamines win, which prevents further hydrogenation of alkenes into alkanes. Primary amines with different lengths of carbon chains have similar adsorption energies at the surface of catalysts and, consequently, the same effect on selectivity. When the adsorption energy of capping ligands at the catalytic surface is lower than adsorption energy of alkenes, the ligands do not affect the selectivity of hydrogenation of alkyne to alkene. On the other hand, capping ligands with adsorption energies at the catalytic surface higher than that of alkyne reduce its activity resulting in low conversion of alkynes