Influence of Hole-Sequestering Ligands on the Photostability of CdSe Quantum Dots

Chalcogenide nanocrystals or quantum dots (QDs) such as CdSe and PbSe have great potential as absorbers for QD-sensitized solar cells, but their practical utility is limited by fast degradation when exposed to ambient environments. Here we present results showing that small organic molecules acting as hole-accepting ligands can be very effective in reducing photooxidation of CdSe QDs. The aromatic amine, 4-dimethylaminothiophenol (DMATP), is shown to be especially effective in enhancing stability of CdSe QDs when illuminated in air or in aqueous environments. Using photoluminescence and density functional theory (DFT) calculations, we show that the enhanced stability results from hole transfer from the QD to the ligand and delocalization of the resulting positive charge on the aromatic ring and amino group instead of the sulfur atom that links the molecule to the CdSe

Spectroelectrochemical Signatures of Capacitive Charging and Ion Insertion in Doped Anatase Titania Nanocrystals

Solution-processed films of colloidal aliovalent niobium-doped anatase TiO₂ nanocrystals exhibit modulation of optical transmittance in two spectral regionsnear-infrared (NIR) and visible lightas they undergo progressive and reversible charging in an electrochemical cell. The Nb-TiO₂ nanocrystal film supports a localized surface plasmon resonance in the NIR, which can be dynamically modulated via capacitive charging. When the nanocrystals are charged by insertion of lithium ions, inducing a well-known structural phase transition of the anatase lattice, strong modulation of visible transmittance is observed. Based on X-ray absorption near-edge spectroscopy, the conduction electrons localize only upon lithium ion insertion, thus rationalizing the two modes of optical switching observed in a single material. These multimodal electrochromic properties show promise for application in dynamic optical filters or smart windows

Synthesis and Properties of Semiconducting Iron Pyrite (FeS₂) Nanowires

We report the growth and structural, electrical, and optical characterization of vertically oriented single-crystalline iron pyrite (FeS₂) nanowires synthesized via thermal sulfidation of steel foil for the first time. The pyrite nanowires have diameters of 4–10 nm and lengths greater than 2 μm. Their crystal phase was identified as cubic iron pyrite using high-resolution transmission electron microscopy, Raman spectroscopy, and powder X-ray diffraction. Electrical transport measurements showed the pyrite nanowires to be highly p-doped, with an average resistivity of 0.18 ± 0.09 Ω cm and carrier concentrations on the order of 10²¹ cm^–3. These pyrite nanowires could provide a platform to further study and improve the physical properties of pyrite nanostructures toward solar energy conversion

Facile Solution Synthesis of α-FeF₃·3H₂O Nanowires and Their Conversion to α-Fe₂O₃ Nanowires for Photoelectrochemical Application

We report for the first time the facile solution growth of α-FeF₃·3H₂O nanowires (NWs) in large quantity at a low supersaturation level and their scalable conversion to porous semiconducting α-Fe₂O₃ (hematite) NWs of high aspect ratio via a simple thermal treatment in air. The structural characterization by transmission electron microscopy shows that thin α-FeF₃·3H₂O NWs (typically <100 nm in diameter) are converted to single-crystal α-Fe₂O₃ NWs with internal pores, while thick ones (typically >100 nm in diameter) become polycrystalline porous α-Fe₂O₃ NWs. We further demonstrated the photoelectrochemical (PEC) application of the nanostructured photoelectrodes prepared from these converted hematite NWs. The optimized photoelectrode with a ∼400 nm thick hematite NW film yielded a photocurrent density of 0.54 mA/cm² at 1.23 V vs reversible hydrogen electrode potential after modification with cobalt catalyst under standard conditions (AM 1.5 G, 100 mW/cm², pH = 13.6, 1 M NaOH). The low cost, large quantity, and high aspect ratio of the converted hematite NWs, together with the resulting simpler photoelectrode preparation, can be of great benefit for hematite-based PEC water splitting. Furthermore, the ease and scalability of the conversion from hydrated fluoride NWs to oxide NWs suggest a potentially versatile and low-cost strategy to make NWs of other useful iron-based compounds that may enable their large-scale renewable energy applications

Quantitative Determination of Ligand Densities on Nanomaterials by X‑ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is a nearly universal method for quantitative characterization of both organic and inorganic layers on surfaces. When applied to nanoparticles, the analysis is complicated by the strong curvature of the surface and by the fact that the electron attenuation length can be comparable to the diameter of the nanoparticles, making it necessary to explicitly include the shape of the nanoparticle to achieve quantitative analysis. We describe a combined experimental and computational analysis of XPS data for molecular ligands on gold nanoparticles. The analysis includes scattering in both Au core and organic shells and is valid even for nanoparticles having diameters comparable to the electron attenuation length (EAL). To test this model, we show experimentally how varying particle diameter from 1.3 to 6.3 nm leads to a change in the measured <i>A</i>_C/<i>A</i>_Au peak area ratio, changing by a factor of 15. By analyzing the data in a simple computational model, we demonstrate that ligand densities can be obtained, and, moreover, that the actual ligand densities for these nanoparticles are a constant value of 3.9 ± 0.2 molecules nm^–2. This model can be easily extended to a wide range of core–shell nanoparticles, providing a simple pathway to extend XPS quantitative analysis to a broader range of nanomaterials