A SERS spectroelectrochemical investigation of the interaction of sulfide species with gold surfaces

Flexible Cation Exchange Environment via Ligand-Free Metal Chalcogenide Thin Films

Cation exchange (CE) has emerged as a premier postsynthetic method to carefully tune the chemical composition and properties of nanocrystals with excellent morphology retention. However, reaction conditions are typically dictated by the ubiquitous ligands bound to their surface, limiting their solubility and influencing the thermodynamics/kinetics of the reaction. To bypass these challenges, we report on CE reactions with Cu+, Ag+, Cu2+, Cd2+, Zn2+, and Mn2+ utilizing ligand-free CdS and Cu x Se y thin films as host templates. The exchange reactions could be performed sequentially or simultaneously (i.e., two guest cations) to access compositionally diverse products. The incorporation of cations on the host films was confirmed using SEM-EDS, XPS, and ICP-MS analyses, as well as tracking wavelength shifts in the UV-vis absorption spectra. The flexibility of this approach was demonstrated as reactions were carried out using an array of different guest precursor salts and solvents with a range of polarities. Moreover, the reactions were generalizable among selenide and sulfide films and proceeded under milder conditions in comparison with reported nanocrystal reactions. A ligand-free environment with flexible reaction conditions, as the work herein, could aid in deconvoluting the different factors involved in CE reactions and further expand its use for fundamental research and applications like photovoltaics, optoelectronics, and catalysis.

Engineered Au@CuO Nanoparticles for Wide-Range Quantitation of Sulfur Ions by Surface-Enhanced Raman Spectroscopy

Efficient detection of sulfide ions (S^2-), especially in a wide quantitative range, is of significance but faces challenges. This work strategizes and fabricates Au@CuO nanoparticles for quantitative surface-enhanced Raman spectroscopy (SERS) detection of the S^2- ions based on the S^2- concentration-dependent ion-solid interactions. We have achieved fast and quantitative S^2- detection in a wide range from 5 ppb to 64,000 ppm (saturation concentration of the S^2- source). We also demonstrated that the optimal CuO shell thickness for the detection is about 7 nm and that the detection can be further improved by prolonging the soaking duration. Moreover, this detection method has also shown the merits of reusable substrates (especially for low S^2- concentrations) and good anti-interference ability to many common anions (Cl^-, NO₃^-, OH^-, HCOO^-, CO₃^2-, and SO₄^2-). Finally, the high feasibility of this detection in actual water (tap water and pond water) has also been demonstrated. This work provides efficient S^2- detection with great potential in practical use and also inspires the design of quantifiable SERS substrates for detecting more small inorganic molecules and ions.

Probing the Surface Chemistry of Nanoporous Gold via Electrochemical Characterization and Atom Probe Tomography

Surface chemistry information is crucial in understanding catalytic and sensing mechanisms. However, resolving the outermost monolayer composition of metallic nanoporous materials is challenging due to the high tortuosity of their morphology. In this study, we first elaborate on the capabilities and limitations of atom probe tomography (APT) in resolving interfaces. Subsequently, an electrochemical approach is designed to characterize the surface composition of nanoporous gold (NPG), developed from dealloying an inexpensive precursor (95 at. % Ag, 5 at. % Au), by the means of aqueous electrochemical measurements of the selective electrosorption of sulfide ions, which react strongly with Ag, but to a significantly lesser extent with Au. Accordingly, cyclic voltammetry was performed at various scan rates on NPG in alkaline aqueous solutions (0.2 M NaOH; pH 13) in the presence and absence of 1 mM Na2S. Calibrations via similar voltammetric measurements on pure polycrystalline Ag and Au surfaces allowed for a quantitative estimation for the Ag surface coverage of NPG. The sensitivity threshold for the detection of the adsorbate-Ag interaction was assessed to be approximately 2% Ag surface coverage. As curves measured on NPG only showed featureless capacitive currents, no faradaic charge density associated with sulfide electrosorption could be detected. This study opens a new avenue to gain further insight into the monolayer surface coverage of metallic nanoporous materials and assists in enhancement of the interpretation of APT reconstructions.

Low optical dosage heating-reduced viscosity for fast and large-scale cleanup of spilled crude oil by reduced graphene oxide melamine nanocomposite adsorbents

Heating under low solar radiation intensity is demonstrated to facilitate the cleaning of crude oil by the hydrophobic nanocomposite adsorbents of reduced graphene oxide (RGO) melamine sponge (MS@RGO) foams. The heat generated by the irradiation reduces the viscosity of the crude oil, and consequently increases the oil-diffusion coefficient of the pores of the MS@RGO foams and speeds up the oil-sorption rate. Even under a solar radiation intensity as low as 2 kW m^-2, the temperature of crude oil rapidly rises to 68 °C or higher within 10 min. It only takes 29 s to completely absorb 6 g of crude oil at 60 °C by three tiny pieces of MS@RGO foam. This work makes better use of the excellent photothermal conversion characteristics of crude oil, and its photothermal conversion mechanism under simulated solar radiation is also discussed. This methodology can be adopted to clean up viscous crude oil or extract other chemicals effectively at a large scale, and provides a complete solution for the cleanup of crude oil in the sea or on the beach for actual engineering applications.

Photoelectrochemical Formation of Polysulfide at PbS QD-Sensitized Plasmonic Electrodes

Effective electron-hole separation is a key to enhance photoenergy conversion of semiconductor quantum dot (QD)-sensitized plasmonic solar cells. However, in contrast to intense studies on electron transfer, hole transfer from QDs and consequent chemical reactions with donors in electrolytes remain unclear. Herein, in situ electrochemical surface-enhanced Raman scattering (SERS) measurement on a PbS QD-sensitized TiO₂/Au/TiO₂ photoelectrode indicated formation of cyclo-octasulfur (α-S₈) via tuning the electrochemical potential. A photocurrent density of 100 nA/cm² was recorded simultaneously even with an extremely low QD loading. Two-dimensional correlation analysis of the SERS revealed subsequent formation of S₈^- and S₄^2- at -1.1 to -0.1 V (vs Ag/AgCl), S₈ from -0.3 V, and S₅^2- and S₆^2- at ≥0.2 V via complex disproportionation reactions. The sensitive detection is attributed to the enhanced electromagnetic field of localized surface plasmon resonance, which provides a better understanding of charge separation processes in QD-sensitized solar cells.

One-step facile synthesis and high H2-evolution activity of suspensible CdxZn1-xS nanocrystal photocatalysts in a S2-/SO32- system

For a CdS-based photocatalyst, both the photocorrosion resistance and the rapid H₂-production reaction are highly required for improving its photocatalytic H₂-production performance. In this study, a facile strategy was reported to simultaneously realize an improved photocorrosion resistance and rapid interfacial H₂-evolution reaction of Cd_xZn_1-xS solid-solution photocatalysts in a sulfur-rich S^2-/SO₃^2- solution. Here, the suspensible Cd_xZn_1-xS nanocrystal photocatalysts are prepared by a one-step co-precipitation route through the direct introduction of Zn²⁺/Cd²⁺ mixing ions in a sulfur-rich Na₂S-Na₂SO₃ solution, and the resultant Cd_xZn_1-xS nanocrystals (ca. 5 nm) display a suspensible structure owing to the numerous and selective adsorption of S^2-/SO₃^2- on the surface of these Cd_xZn_1-xS nanocrystals. It is found that the bandgap structure of Cd_xZn_1-xS (from 2.25 to 3.52 eV) nanocrystals can be easily controlled by adjusting the Cd²⁺/Zn²⁺ molar ratio. The photocatalytic experimental results suggested that the suspensible Cd_xZn_1-xS nanocrystal photocatalysts clearly displayed an excellent photocatalytic H₂-production performance, and the suspensible Cd_0.6Zn_0.4S nanocrystals exhibit the highest photocatalytic H₂-generation performance of 717.19 μmol h^-1, a value higher than that of the sole CdS (320.99 μmol h^-1) and ZnS (5.89 μmol h^-1) by a factor of 2.2 and 121.8 times, respectively. Based on the experimental results, a possible S^2- active site-mediated mechanism accounted for the high H₂-production activity of the suspensible Cd_xZn_1-xS nanocrystals, namely the numerous adsorbed S^2- ions not only function as efficient hole scavengers to rapidly consume the photogenerated holes, resulting in an improved photocorrosion resistance of suspensible Cd_xZn_1-xS nanocrystals, but also serve as effective H⁺-capturing active sites to accelerate the interfacial H₂-production reaction. Meanwhile, an optimum bandgap structure of suspensible Cd_xZn_1-xS nanocrystals is also extremely required for promoting the photocatalytic H₂-production activity. This research may provide advanced insights for developing stable and high-activity photocatalytic materials.

Detection of Sulfide Using Mercapto Tetrazine-Protected Fluorescent Gold Nanodots: Preparation of Paper-Based Testing Kit for On-Site Monitoring

This work demonstrates the development of a highly sensitive method to detect and quantify sulfide ions (S^2-) in water samples. First, we synthesized 6-mercapto-s-triazolo(4,3-b)-s-tetrazine (MTT) by the reaction between formaldehyde and 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole at room temperature. The synthetic MTT was used as a capping ligand for the synthesis of gold nanodots (AuNDs) via a one-pot green method at room temperature with only a 10 min reaction time. Transmission electron microscopy images exhibited that the MTT-AuNDs have an average particle size of 1.9 nm and an emission maximum at 672 nm upon excitation at 360 nm. The synthesized highly red emissive MTT-AuNDs are used as specific fluorescent probes for the detection of S^2-. The fluorescence of MTT-AuNDs was significantly and dose-dependently quenched by the addition of S^2-. The observed fluorescence quenching was ascribed to the formation of an Au₂S complex, which was determined by Raman and mass spectroscopy. A good linearity was achieved for the increasing concentration of S^2- from 870 nM to 16 μM, and the detection limit was found to be 2 nM (S/N = 3). The S^2- detection system that is described in this study was validated and agreed well with the standard methylene blue method. Furthermore, the present sensor was examined for its use in quantifying S^2- in real water samples obtained from lakes and rivers. In addition, the specificity was checked against the most likely ion interferences in real water. Moreover, a cost-effective and viable paper-based S^2- sensor was fabricated for environmental monitoring based on the use of MTT-AuNDs. The developed system would be an environmentally friendly and easy-to-use detection device for S^2- in water.