21 research outputs found
Heteroepitaxial Growth Of Colloidal Nanocrystals Onto Substrate Films Via Hot-injection Routes
Hot-injection synthesis of colloidal nanocrystals (NCs) in a substrate-bound form is demonstrated. We show that polycrystalline films submerged into hot organic solvents can nucleate the heteroepitaxial growth of semiconductor NCs, for which the ensuing lattice quality and size distribution are on the par with those of isolated colloidal nanoparticles. This strategy is demonstrated by growing lead chalcogenide NCs directly onto solvent-submerged TiO(2) substrates. The resulting PbX/VTiO(2) (X = S, Se, Te) nanocomposites exhibit heteroepitaxial interfaces between lead chalcogenide and oxide domains and show an efficient separation of photoinduced charges, deployable for light-harvesting applications. The extendibility of the present method to other material systems was demonstrated through the synthesis of CdS/TiO(2) and Cu(2)S/TiO(2) heterostructures, fabricated from PbS/TiO(2) composites via cation exchange. The photovoltaic performance of nanocrystal/substrate composites comprising PbS NCs was evaluated by incorporating PbS/TiO2 films Into prototype solar cells
Photocatalytic Hydrogen Production at Titania-Supported Pt Nanoclusters that are Derived from Surface-Anchored Molecular Precursors
Degussa P-25 TiO2 bearing surface-anchored Pt(dcbpy)Cl-2 [dcbpy = 4,4\u27-dicarboxylic acid-2,2\u27-bipyridine] prepared with systematically varied surface coverage produced Pt-0 nanoparticles under bandgap illumination in the presence of methanol hole scavengers. Energy-dispersive X-ray spectroscopy confirmed the presence of elemental platinum in the newly formed nanoparticles during scanning transmission electron microscopy (STEM) eleriments. According to the statistical analysis of numerous STEM images, the Pt-0 nanoclusters were distributed in a segregated manner throughout the titania surface, ranging in size from 1 to 3 nm in diameter. The final achieved nanoparticle size and net hydrogen production were determined as a function of the Pt(dcbpy)Cl-2 surface coverage as well as other systematically varied experimental parameters. The hybrid Pt/TiO2 nanomaterials obtained upon complete decomposition of the Pt(dcbpy)Cl-2 precursor displayed higher photocatalytic activity (300 mu mol/h) for hydrogen evolution in aqueous suspensions when compared with platinized TiO2 derived from H2PtCl6 precursors (130 mu mol/h), as ascertained through gas chromatographic analysis of the photoreactor headspace under identical experimental conditions. The conclusion that H-2 was evolved from Pt-0 sites rather than from molecular Pt(dcbpy)Cl-2 entities was independently supported by Hg and CO poisoning experiments. The formation of small Pt nanopartides (1.5 nm in diameter) prevail at low surface coverage of Pt(dcbpy)Cl-2 on TiO2 (0.5 to 2% by mass) that exhibit enhanced turnover frequencies with respect to all other materials investigated, induding those produced from the in situ photochemical reduction of H2PtCl6 center dot Pt-II precursor absorption in the ultraviolet region appeared to be partially responsible for attenuation of the H-2 evolution rate at higher Pt(dcbpy)Cl-2 surface coverage. The nanoparticle size and hydrogen evolution characteristics of the surface-anchored materials generated through photodeposition were directly compared with those derived from chemical reduction using NaBH4. Finally, Degussa P-25 thin films deposited on FTO substrates enabled electrochemically induced (-1.0 V vs Ag/AgCl, pH 7.0, phosphate buffer) electron trapping (TiO2(e(-))) throughout the titania. After removal of the applied bias and the anaerobic introduction of Pt(dcbpy)Cl-2, the accumulated electrons reduce this molecular species to Pt-0 nanoparticles on the titania electrode surface, as confirmed by TEM measurements, with the concomitant production of H-2 gas. The combined experiments illustrate that TiO2(e(-)) generated with bandgap excitation or via electrochemical bias affords the reduction of Pt(dcbpy)Cl-2 to Pt-0 nanoparticles that in turn are responsible for heterogeneous hydrogen gas evolution
Improving The Catalytic Activity Of Semiconductor Nanocrystals Through Selective Domain Etching
Colloidal chemistry offers an assortment of synthetic tools for tuning the shape of semiconductor nanocrystals. While many nanocrystal architectures can be obtained directly via colloidal growth, other nanoparticle morphologies require alternative processing strategies. Here, we show that chemical etching of colloidal nanoparticles can facilitate the realization of nanocrystal shapes that are topologically inaccessible by hot-injection techniques alone. The present methodology is demonstrated by synthesizing a two-component CdSe/CdS nanoparticle dimer, constructed in a way that both CdSe and CdS semiconductor domains are exposed to the external environment. This structural morphology is highly desirable for catalytic applications as it enables both reductive and oxidative reactions to occur simultaneously on dissimilar nanoparticle surfaces. Hydrogen production tests confirmed the improved catalytic activity of CdSe/CdS dimers, which was enhanced 3-4 times upon etching treatment. We expect that the demonstrated application of etching to shaping of colloidal heteronanocrystals can become a common methodology in the synthesis of charge-separating nanocrystals, leading to advanced nanoparticles architectures for applications in areas of photocatalysis, photovoltaics, and light detection
The Role of Hole Localization in Sacrificial Hydrogen Production by Semiconductor-Metal Heterostructured Nanocrystals
The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor-ligand interface, we demonstrate that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the degraded nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands
Risk Assessment of Phthalates and Their Metabolites in Hospitalized Patients: A Focus on Di- and Mono-(2-ethylhexyl) Phthalates Exposure from Intravenous Plastic Bags
Phthalate esters (PAEs) are plasticizers associated with multiple toxicities; however, no strict regulations have been implemented to restrict their use in medical applications in Lebanon. Our study aimed at assessing the potential risks correlated with phthalate exposure from IV bags manufactured in Lebanon. GC–MS analysis showed that di-(2-ethylhexyl) phthalate (DEHP) is the predominant phthalate found in almost all samples tested with values ranging from 32.8 to 39.7% w/w of plastic. DEHP concentrations in the IV solutions reached up to 148 µg/L, as measured by SPME-GC–MS/MS, thus resulting in hazard quotients greater than 1, specifically in neonates. The toxicity of DEHP is mainly attributed to its metabolites, most importantly mono-(2-ethylhexyl) phthalate (MEHP). The IV bag solution with the highest content in DEHP was therefore used to extrapolate the amounts of urinary MEHP. The highest concentrations were found in neonates having the lowest body weight, which is concerning, knowing the adverse effects of MEHP in infants. Our study suggests that the use of IV bags manufactured in Lebanon could pose a significant risk in hospitalized patients, especially infants in neonatal care. Therefore, Lebanon, as well as other countries, should start imposing laws that restrict the use of phthalates in medical IV bags and substitute them with less toxic plasticizers
Getting to the (Square) Root of the Problem: How to Make Noncoherent Pumped Upconversion Linear
We present experimental data illustrating that photochemical
upconversion
based on sensitized triplet–triplet annihilation can exhibit
anti-Stokes emissions whose intensities with respect to the excitation
power can vary between quadratic and linear using a noncoherent polychromatic
light source. The benchmark upconverting composition consisting of
Pd(II) octaethylporphyrin (PdOEP) sensitizers and 9,10-diphenylanthracene
(DPA) acceptors/annihilators in toluene was selected to generate quadratic,
intermediate, and linear behavior under both coherent and noncoherent
excitation conditions. Each of these power laws was traversed in a
single sample in one contiguous experiment through selective pumping
of the sensitizer using an Ar<sup>+</sup> laser. Wavelength-dependent
responses ranging from quadratic to pseudolinear were also recorded
from the identical sample composition when excited by Xe lamp/monochromator
output in a conventional fluorimeter, where the optical density at
λ<sub>ex</sub> dictates the observed incident power dependence.
Finally, pure linear behavior was derived from noncoherent excitation
for the first time at higher sensitizer concentrations
Structure and Activity of Photochemically Deposited “CoPi” Oxygen Evolving Catalyst on Titania
The cobalt phosphate “CoPi” oxygen evolving
catalyst
(OEC) was photochemically grown on the surface of TiO<sub>2</sub> photoanodes
short-circuited to a Pt wire under bandgap illumination in the presence
of Co(NO<sub>3</sub>)<sub>2</sub> and sodium phosphate (NaPi) buffer.
Extended photodeposition (15 h) using a hand-held UV lamp readily
permitted quantitative structural and electrochemical characterization
of the photochemically deposited CoPi OEC on titania. The formed catalytic
material was characterized by scanning electron microscopy (SEM) and
energy dispersive X-ray (EDX) spectroscopy experiments, illustrating
the production of easily visualized micrometer scale clusters throughout
the titania surface containing both cobalt and phosphate. X-ray absorption
fine structure (XAFS) and X-ray absorption near edge structure (XANES)
studies indicated that the newly formed material was structurally
consistent with the production of molecular cobaltate clusters composed
of a cobalt oxide core that is most likely terminated by phosphate
ions. The oxidation state, structure, and the oxygen evolution activity
of this CoPi catalyst photochemically grown on titania were quantitatively
similar to the analogous electrodeposited materials on titania as
well as those produced on other electroactive substrates. From pH-dependent
electrochemical measurements, proton-coupled electron transfer was
shown to be an important step in the oxygen evolution mechanism from
the photodeposited OEC clusters on TiO<sub>2</sub> in agreement with
previous reports on other materials. Similarly, the utilization of
NaClO<sub>4</sub> as electrolyte during the controlled potential electrolysis
experiments failed to maintain an appreciable current density, indicating
that the catalyst was rendered inactive with respect to the one immersed
in NaPi. The requirement of having phosphate present for long-term
catalytic activity implied that the same “repair” mechanism
might be invoked for the hybrid materials investigated here. The OEC
catalyst operated at Faradaic efficiencies close to 100% in controlled
potential electrolysis experiments, indicating that the holes relayed
to the photodeposited CoPi are indeed selective for promoting water
oxidation on titania