Tuning the oxidation degree in sub-10 nm silver-oxide nanoparticles: From Ag2O monoxide to AgO (x> 1) superoxide

Plasmonic-mediated SC arylation and SS coupling on nanostructured silver electrodes monitored by in situ surface-enhanced Raman spectroscopy

Plasmon-mediated chemical reaction (PMCR) is a highly attractive field of research. Here we report in situ surface-enhanced Raman spectroscopic (SERS) monitoring of plasmonic-mediated SS bond-forming reaction. The reaction is thought to be a self-coupling reaction proceeding by photoinduced aromatic SC bond arylation. Surprisingly, the SC arylation and SS coupling are found to be occurred on both partially oxidized silver and silver nanoparticles. The results demonstrated that silver oxide or hydroxide and small molecule donor sacrifice agent played a crucial role in the reaction. This work facilitates the in-situ manipulation and characterization of the active silver electrode interface in conjunction with electrochemistry, and also establishes a promising new guideline for surface plasmon resonance photocatalytic reactions on metal nanostructures with high efficiency.

Free silver nanoparticles doped by potassium: Work-function change in experiment and theory

The composition-dependent change in the work-function (WF) of binary silver-potassium nanoparticles has been studied experimentally by synchrotron-based x-ray photoelectron spectroscopy (PES) and theoretically using a microscopic jellium model of metals. The Ag-K particles with different K fractions were produced by letting a beam of preformed Ag particles pass through a volume with K vapor. The PES on a beam of individual non-supported Ag-K nanoparticles created in this way allowed a direct absolute measurement of their WF, avoiding several usual shortcomings of the method. Experimentally, the WF has been found to be very sensitive to K concentration: Already at low exposure, it decreased down to ≈2 eV-below the value of pure K. In the jellium modeling, considered for Ag-K nanoparticles, two principally different adsorption patterns were tested: without and with K diffusion. The experimental and calculation results together suggest that only efficient surface alloying of two metals, whose immiscibility was long-term textbook knowledge, could lead to the observed WF values.

Tin-oxide nanoparticles deposited from a beam: what happens to the composition?

The debate around the oxidation states occurring in laboratory-prepared tin-oxide samples has been for a long time an obstacle for an unambiguous assignment of characterization studies performed on such samples. In particular the changes in the Sn core-level energies caused by oxidation - i.e. the chemical shifts - as measured by photoelectron spectroscopy (PES) have been under discussion. The assignment problem is especially pronounced for nanoscale structures, which are important for photovoltaics, electronics, catalysis, and gas sensing. The reasons for the difficulties lie both in the natural properties of tin oxides, which can have substantial deficiencies of oxygen and tin in the lattice, and in the shortcomings of the fabrication and PES-characterization procedures themselves. Our recent PES study on tin-oxide nanoparticles fabricated by vapour-aggregation gave a chemical shift two times larger than earlier reported for Sn(iv) oxide for the Sn 4d level. The implemented fabrication technique forms an in-vacuum beam of particles whose composition can be both controlled and characterized by PES. In the present work SnO and SnO2 nanoparticles fabricated this way were deposited from the beam and probed by PES directly, as well as after exposure to air. The deposited nanoparticle films were also imaged by TEM (Transmission Electron Microscopy). The effects of the deposition process and exposure to air on the chemical composition were studied. The PES study of deposited SnO2 nanoparticles in the Sn 4d and Sn 3d core-level regions revealed the same core level shift as for unsupported nanoparticles, indicating that the chemical composition is preserved in the deposition process. The TEM study demonstrated a crystalline structure of separate SnO2 particles with lattice constants close to the macroscopic Sn(iv)-oxide. The PES study on the particles exposed to air showed changes in the composition. For the film of initially SnO particles a higher intermediate oxide was created. For the SnO2 nanoparticle film a lower, but strong, intermediate oxide was observed, likely at the surface.

Ag-Cu oxide nanoparticles with high oxidation states: towards new high Tc materials

In Ag-Cu oxides possible to fabricate so far, superconductivity has not been detected, but high conductivity was. In the quest for superconductivity the demand is to create a high and peculiar copper-oxygen coordination. Such coordination makes it non-trivial to determine Cu oxidation states, which may be several and co-existing. Another reason for uncertainty is in oxygen deficiency typical for superconducting crystals. Finally, Cu oxidation is influenced by the other metals in the substance. For chemical fabrication the difficulty is to tune the relative abundances of elements in a fine way. Ag-Cu oxides have been also produced by reactive co-sputtering of Cu and Ag, but the composition with high Cu oxidation states necessary for high conductivity has not been realized. In the present work we have fabricated Ag-Cu-oxide nanoparticles containing Cu and Ag in high oxidation states actual for superconductivity. The fabrication includes reactive sputtering of Ag and Cu metals, their vapour oxidation and aggregation into nanoparticles. The ability to create different and high oxidation states, also co-existing, is demonstrated. The fabrication approach also allows overcoming the poor miscibility of Cu and Ag. The nanoparticle composition and the oxidation states could be determined due to an experimental arrangement in which photoelectron spectroscopy is applied to free nanoparticles in a beam in vacuum, what allows avoiding any contact of the particles to a substrate or atmosphere. The combination of the fabrication and characterization methods has proven to be a powerful approach when fine composition tuning and control are desirable.

Improvement of photocatalytic activity of silver nanoparticles by radio frequency oxygen plasma irradiation

Photocatalytic activity (PA) of silver nanoparticles (AgNPs) induced by radio frequency (RF) oxygen plasma irradiation (OPI) is investigated in this paper. An improvement in PA by 365% and 181% has been achieved when 15 nm AgNPs irradiated by oxygen plasma for 2 s were used to degrade 10(-5) M Rhodamine 6 G (R6G) under ultraviolet (UV) and visible lights, respectively. The PA caused by OPI is better than that induced by the localized surface plasma resonance (LSPR) of AgNPs. The mechanism for the improvement was explored by scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectra. The OPI-induced formation of AgO/AgNP and Ag2O/AgNP-heterogeneous photocatalysts and electrophilic oxygen are considered to be responsible for the PA improvement. This investigation deepens our understanding of oxygen-assisted photocatalysis of AgNPs and provides a practical approach using solar light for broad spectra photocatalysis with high efficiency.