Controlling the Size, Shape, Phase, Band Gap, and Localized Surface Plasmon Resonance of Cu_2–<i>x</i>S and Cu_<i>x</i>In_<i>y</i>S Nanocrystals

We show how indium incorporation can effect the size, morphology, crystal structure, and optical properties of CIS nanocrystals (NCs) and demonstrate a robust methodology for preparing monodisperse plasmonic and nonplasmonic CIS NCs. In contrast with previous methods of producing CIS nanocrystals exhibiting localized surface plasmon resonance (LSPR), which employed expensive and hazardous bis­(trimethylsilyl) sulfide, we employ sulfur dissolved in oleic acid as the sulfur donor in combination with appropriate cation precursors to allow production of monodisperse CIS NCs with broad variation of the Cu:In ratio in the products. The crystal phase of the CIS NCs is determined not only by the reactivity of the ligands but also by the cation composition. The LSPR shifted to longer wavelengths with increasing In content until it vanished, while the band gap of the CIS NCs decreased linearly from 2.1 to 1.2 eV. The manipulation of optical properties of CIS NCs with controlled and well-defined size, shape, and phase may open up new possibilities for applying I–III–VI materials in solution-processed optoelectronic devices

Ultrathin Palladium Nanowires for Fast and Hysteresis-Free H₂ Sensing

One-dimensional nanomaterials are of great interest for gas-sensing applications due to their high surface-to-volume ratio and effective electron transport pathways. Palladium nanowires (PdNWs) form hydrides at room temperature, altering their electrical resistance and making them useful in H2 sensors. Reducing the PdNW diameter would improve sensor sensitivity and response speed, but conventional lithography cannot fabricate PdNWs <10 nm in diameter and is relatively expensive. We report the colloidal solvothermal synthesis of ultrathin PdNWs (diameter <5 nm). UV–ozone treatment was used to degrade surface ligands on the PdNWs before fabricating a sensor by simple drop-casting. The sensor showed a response of 1.7% and response and recovery times of 3.4 and 11 s to 1% H2 in air. It displayed hysteresis-free behavior and was stable under repeated exposure to 1% H2 in air while maintaining high selectivity for H2 relative to CO, CO2, and CH4. We attribute the observed fast response and recovery times and outstanding stability to a combination of factors, including ultrathin nanowire diameter, the formation of nanowire networks, and the presence of highly catalytically active facets on the surface that combine to make this one of the fastest reported chemiresistive Pd-based sensors for room-temperature detection of H2

Composition-Dependent Crystal Phase, Optical Properties, and Self-Assembly of Cu–Sn–S Colloidal Nanocrystals

We demonstrate a robust protocol for preparing monodisperse copper–tin–sulfide (CTS) nanocrystals (NCs) of tunable composition and controlled crystal phase. We show that the crystal phase of CTS NCs is determined not only by the reactivity of chalcogenide precursors but also by the cation composition (Cu:Sn ratio) and identity of ligands bound to the NC surface. In contrast to previous studies, we demonstrate broad variation of the Cu:Sn ratio in the product NCs, in both directions from the stoichiometric Cu₂SnS₃ compound. Localized surface plasmon resonance in the CTS NCs was tuned by varying the Cu:Sn elemental ratio. This demonstrates that the doping level of such alloy semiconductor NCs can be manipulated by varying the cation composition. This result opens new possibilities for applying CTS and related materials for solution-processed photovoltaic devices, by taking advantage of controllable and variable doping. In addition, reversible gelation of colloidal CTS NCs in nonpolar solvents was observed and is discussed

Efficient Surface Grafting of Luminescent Silicon Quantum Dots by Photoinitiated Hydrosilylation

We suggest a method for efficient (high-coverage) grafting of organic molecules onto photoluminescent silicon nanoparticles. High coverage grafting was enabled by use of a modified etching process that produces a hydrogen-terminated surface on the nanoparticles with very little residual oxygen and by carefully excluding oxygen during the grafting process. It had not previously been possible to produce such a clean H-terminated surface on free silicon nanoparticles or, subsequently, to produce grafted particles without significant surface oxygen. This allowed us to (1) prepare air-stable green-emitting silicon nanoparticles, (2) prepare stable dispersions of grafted silicon nanoparticles in a variety of organic solvents from which particles can readily be precipitated by addition of nonsolvent, dried, and redispersed, (3) separate these nanoparticles by size (and therefore emission color) using conventional chromatographic methods, (4) protect the particles from chemical attack and photoluminescence quenching, and (5) provide functional groups on the particle surface for further derivatization. We also show, using 1H NMR, that the photoinitiated hydrosilylation reaction does not specifically graft the terminal carbon atom to the surface but that attachment at both the first and second atom occurs

Valence Selectivity of Cation Incorporation into Covellite CuS Nanoplatelets

Synthesis of copper sulfide-based nanomaterials by cation incorporation into copper deficient copper sulfide (Cu_2–<i>x</i>S) is of interest as a powerful means to obtain nanostructures with otherwise inaccessible combinations of size, shape, composition, and crystal phase. Incorporation of a heterocation (M) may produce heterogeneous Cu_2–<i>x</i>S-MS nanocrystals (NCs) or homogeneous Cu-M-S alloys. However, the factors determining whether heterogeneous NCs or homogeneous alloy NCs are produced have not been fully elucidated. In this report, we incorporate diverse cations into covellite CuS nanoplatelet (NPl) templates in the presence of dodecanethiol (DDT). These cations are categorized by their valencies. We demonstrate that trivalent and tetravalent cations can be incorporated into reduced CuS NPls to produce homogeneous ternary alloy NPls, while the divalent cations cannot coexist with Cu⁺ ions in the Cu_2–<i>x</i>S phase. In turn, the incorporation of divalent cations leads to formation of heterogeneous NPls and finally produces copper-free metal sulfide NPls. The cation valence selectivity arises from conflicts between charge balance and coordination between Cu⁺ and divalent cations. This study not only provides better understanding of the relationship among the composition, morphology, and crystal structure of copper sulfide-based nanomaterials but also provides a pathway to controllable synthesis of complex nanostructures

Cu_2–<i>x</i>S_1–<i>y</i>Se_<i>y</i> Alloy Nanocrystals with Broadly Tunable Near-Infrared Localized Surface Plasmon Resonance

We report facile methods for synthesizing monodisperse Cu_2–<i>x</i>S_1–<i>y</i>Se_<i>y</i> alloy nanocrystals (NCs) with tunable composition and size. The near-infrared (NIR) localized surface plasmon resonance (LSPR) in these self-doped Cu_2–<i>x</i>S_1–<i>y</i>Se_<i>y</i> alloy NCs can be tuned over a broad range from 975 to 1650 nm. The LSPR shifted to longer wavelength with increasing sulfur content and with increased concentration of oleic acid used in the synthesis. This method provides new possibilities for broadly tuning LSPR wavelength by controlling the anion composition, cationic vacancy concentration, and surface ligands in heavily doped Cu_2–<i>x</i>S_1–<i>y</i>Se_<i>y</i> alloy NCs. This opens up access to LSPR absorbance across a broad portion of the near-IR using small colloidal quasi-isotropic NCs

Thermochemistry of C−C and C−H Bond Breaking in Fatty Acid Methyl Esters

Density functional theory quantum chemical calculations corrected with empirical atomic increments have been used to determine the gas-phase standard enthalpy of formation at 298.15 K of more than 80 oxygenated radicals potentially formed during thermal decomposition and oxidation of fatty acid methyl esters (FAMEs) at combustion temperatures. Neither experimental nor theoretical data have been found in the literature for these radicals. C−H and C−C bond scissions in FAMEs are examined and mechanistic information is obtained based on thermochemical considerations

Paper-Based Hydrogen Sensors Using Ultrathin Palladium Nanowires

Hydrogen (H2), as a chemical energy carrier, is a cleaner alternative to conventional fossil fuels with zero carbon emission and high energy density. The development of fast, low-cost, and sensitive H2 detection systems is important for the widespread adoption of H2 technologies. Paper is an environment-friendly, porous, and flexible material with great potential for use in sustainable electronics. Here, we report a paper-based sensor for room-temperature H2 detection using ultrathin palladium nanowires (PdNWs). To elucidate the sensing mechanism, we compare the performance of polycrystalline and quasi-single-crystalline PdNWs. The polycrystalline PdNWs showed a response of 4.3% to 1 vol % H2 with response and recovery times of 4.9 and 10.6 s, while quasi-single-crystalline PdNWs showed a response of 8% to 1 vol % H2 with response and recovery times of 9.3 and 13.0 s, respectively. The polycrystalline PdNWs show excellent selectivity, stability, and sensitivity, with a limit of detection of 10 ppm H2 in air. The fast response of ultrathin polycrystalline PdNW paper-based sensors arises from the synergistic effects of their ultrasmall diameter, high-index surface facets, strain-coupled grain boundaries, and porous paper substrate. This paper-based sensor is one of the fastest chemiresistive H2 sensors reported and is potentially orders of magnitude less expensive than current state-of-the-art H2-sensing solutions. This brings low-cost, room-temperature chemiresistive H2 sensing closer to the performance of ultrafast optical sensors and high-temperature metal oxide-based sensors