Phase tuning of a thermal plasma synthesized cobalt oxide catalyst and understanding of its surface modification during the hydrolysis of NaBH4

NaBH4 is an attractive candidate for closed-loop hydrogen generation in small practical applications owing to its ambient condition hydrogen release mechanism, non-toxic byproduct, ability to regenerate, and stability at ambient conditions. The hydrolysis of NaBH4 requires a catalyst to accelerate the hydrogen generation process and cobalt oxide is one such promising catalyst in this reaction. The surface species and crystalline phases of cobalt oxide catalysts play an important role in determining the hydrogen generation rate and overall hydrolysis process. In this study, cobalt oxide nanoparticles are synthesized by a thermal plasma route. The two crystalline phases, namely c-CoO and Co3O4, are tuned using thermal plasma operating conditions. The catalysts so obtained have been thoroughly characterized using analytical techniques like XRD, XPS, HR-TEM, etc. Furthermore, the catalyst was used for hydrogen production in the hydrolysis process of NaBH4. The ex situ X-ray photoelectron spectra recorded at different stages of the hydrolysis process have been extensively used to understand surface modifications occurring at the surface of the catalyst. The Co+3/Co+2 ratio and attachment of other species during hydrolysis analyzed using XPS are correlated with the overall hydrolysis reaction before and after catalysis. It was concluded that the presence of the c-CoO (i.e. initial Co+2 species presence) phase brings stability to hydrogen production in that cycle.

Multi-spectroscopic study of electrochemically-formed oxide-derived gold electrodes

Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H3PO4, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au2O3 or Au(OH)3. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

Surface-Plasmon-Mediated Alloying for Monodisperse Au-Ag Alloy Nanoparticles in Liquid

Plasmonic noble-metal nanoparticles with broadly tunable optical properties and catalytically active surfaces offer a unique opportunity for photochemistry. Resonant optical excitation of surface-plasmon generates high-energy hot carriers, which can participate in photochemical reactions. Although the surface-plasmon-driven catalysis on molecules has been extensively studied, surface-plasmon-mediated synthesis of bimetallic nanomaterials is less reported. Herein, we perform a detailed investigation on the formation mechanism and colloidal stability of monodisperse Au-Ag alloy nanoparticles synthesized through irradiating the intermixture of Au nanochains and AgNO₃ solution with a nanosecond pulsed laser. It is revealed that the Ag atoms can be extracted from AgNO₃ solution by surface-plasmon-generated hot electrons and alloy with Au atoms. Particularly, the obtained Au-Ag alloy nanoparticles without any surfactants or ligands exhibit superior stability that is confirmed by experiments as well as DLVO-based theoretical simulation. Our work would provide novel insights into the synthesis of potentially useful bimetallic nanoparticles via surface-plasmon-medicated alloying.

LptD-antigen system on gold nanoparticles: an innovative strategy in the nanovaccine development

Nanovaccine development is a growing research field in which the development of new carriers and bioconjugation approaches is a priority. In this sense, this report describes for the first time, the development of a novel conjugate that consists of gold nanoparticles (AuNPs) obtained by a one-step synthesis using an immunogenic peptide of the Lipopolysaccharide-assembly protein LptD fromVibrio parahaemolyticusbacteria as a reducing and capping agent. The resultingLptD@AuNPscompounds were fully characterized and the results showed the high capacity of the peptide to form complexes and reduce gold ions. The reaction yield estimated was higher than 83% and the chemical integrity of the peptide on the NP surface revealed a tyrosine amino acid bonding on the AuNP surface. Furthermore, theLptD@AuNPsystem showed high colloidal stability in a wide pH range (3-11 pH values), where the hydrodynamic diameter and Zeta potential behavior were strongly influenced by the functional groups of the antigenic peptide. The cytotoxicity assays showed that the obtained system is safe for mouse leukocytes, while immunized mice withLptD@AuNPsproduced specific IgG antibodies. These encouraging results revealed the efficacy of some antigenic peptides as reducers and capping agents, in addition, opening the path to determine immunogenicity and immunoprotective efficacy of theLptD@AuNPsystem against the disease induced byVibrio parahaemolyticus.

Melt intercalation technique for synthesis of hetero-metallic@chitin bio-composite as recyclable catalyst for prothiofos hydrolysis

The compatibility of homo-metallic and hetero-metallic bio-composite was promisingly investigated as recyclable catalyst for prothiofos hydrolysis. Chitin as water insoluble biopolymer was functionalized as a template for generation of homo-metallic (Ag@chitin, Au@chitin and Pd@chitin) and hetero-metallic (Au@Ag@chitin, Pd@Ag@chitin and Pd@Au@Ag@chitin) composites, by using melt intercalation technique. Investigation of the compatibility of the synthesized homo-metallic and hetero-metallic bio-composites in hydrolysis of prothiofos was performed and affirmed via HPLC results. Immobilization of Pd in the composites showed perfection in the catalytic performance for prothiofos hydrolysis. Pd@Au@Ag@chitin exhibited the highest hydrolysis result of 99% for prothiofos was hydrolyzed within 150 min with rate constant (k₁) of 24.48 min^-1. After five recycles for Pd@Au@Ag@chitin, the hydrolysis of prothiofos was lowered from 346 mg/g to 269 mg/g with reduction percentage of 22%. The synthesized bio-composite was highly effective as recyclable catalyst and can be easily served in the remediation of pesticides.

Characterization of Gold Oxides Prepared by an Oxygen-dc Glow Discharge from Gold Films Using a Gold Discharge Ring by X-ray Photoelectron Spectroscopy

Gold oxides (0.1 - 2.0 nm thick) prepared from gold films by an oxygen-dc glow discharge using a gold discharge ring for 0.17 - 30 min at room temperature were characterized by X-ray photoelectron spectroscopy. The oxide thickness increased with increasing discharge time in contrast to the use of an aluminum ring, and thicker oxide films were obtained. The O 1s spectra show four components: I, II, III, and IV. Components I, II, and IV appear during the early formation periods (≤0.5 min). Components I and II are stable and assigned to hydroxyl groups on the surfaces. Component IV changes into component III (gold oxide) after longer discharge times (≥1 min). The gold oxides (2.0 nm thick) decompose after 15 d at room temperature and decompose immediately at temperatures exceeding 117°C. They also decompose under ultraviolet light irradiation (254, 302, and 365 nm) and decompose more rapidly in water vapor at the shorter wavelengths. The thicker nature of the gold oxides is advantageous for their preservation, and they were preserved in their oxidized state for 196 d in anhydrous dodecane in a dark atmosphere.

Reaction Monitoring of Gold Oxides Prepared by an Oxygen-dc Glow Discharge from Gold Films in Various Aqueous Solutions by a Surface Plasmon Resonance-based Optical Waveguide Sensing System and X-ray Photoelectron Spectroscopy

The chemical properties of thin (∼0.6 nm) gold oxide layers prepared by an oxygen-dc glow discharge from gold films at room temperature in various solvents and solutions were studied using a surface plasmon resonance (SPR)-based optical waveguide sensing system and X-ray photoelectron spectroscopy (XPS). New insights into the stability and reactivity of gold oxides under these conditions were obtained. The O 1s XPS spectra show three oxygen species, comprising components I, II, and III in the gold oxides, and components I and II are present in this order from the top surface of the oxide (component III). The gold oxide is stable in various solvents (water, methanol, ethanol, acetone, acetonitrile, diethyl ether, chloroform, hydrocarbons) for 10 - 30 min at room temperature. The gold oxide decomposes in aqueous 10^-2 M solutions of acetaldehyde, hydrochloric acid, and sodium hydroxide during these periods. Gold oxide decomposition in these solutions was monitored by SPR and the decomposition rates were obtained. Decomposition of the gold oxides was confirmed and their surfaces were characterized after decomposition by XPS.