Search

The structural and chemical modification of TiO2 nanotubes (NTs) by the deposition of a well-controlled Au deposit was investigated using a combination of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), Raman measurements, UV-Vis spectroscopy and photoelectrochemical investigations. The fabrication of the materials focused on two important factors: the deposition of Au nanoparticles (NPs) in UHV (ultra high vacuum) conditions (1–2 × 10−8 mbar) on TiO2 nanotubes (NTs) having a diameter of ∼110 nm, and modifying the electronic interaction between the TiO2 NTs and Au nanoparticles (NPs) with an average diameter of about 5 nm through the synergistic effects of SMSI (Strong Metal Support Interaction) and LSPR (Local Surface Plasmon Resonance). Due to the formation of unique places in the form of “hot spots”, the proposed nanostructures proved to be photoactive in the UV-Vis range, where a characteristic gold plasmonic peak was observed at a wavelength of 580 nm. The photocurrent density of Au deposited TiO2 NTs annealed at 650 °C was found to be much greater (14.7 μA cm−2) than the corresponding value (∼0.2 μA cm−2) for nanotubes in the as-received state. The IPCE (incident photon current efficiency) spectral evidence also indicates an enhancement of the photoconversion of TiO2 NTs due to Au NP deposition without any significant change in the band gap energy of the titanium dioxide (Eg ∼3.0 eV). This suggests that a plasmon-induced resonant energy transfer (PRET) was the dominant effect responsible for the photoactivity of the obtained materials.

The oxidative dearomatization of phenols with the addition of nucleophiles to the aromatic ring induced by hypervalent iodine(III) reagents and catalysts has emerged as a highly useful synthetic approach. However, experimental mechanistic studies of this important process have been extremely scarce. In this report, we describe systematic investigations of the dearomatizing hydroxylation of phenols using an array of experimental techniques. Kinetics, EPR spectroscopy, and reactions with radical probes demonstrate that the transformation proceeds by a radical-chain mechanism, with a phenoxyl radical being the key chain-carrying intermediate. Moreover, UV and NMR spectroscopy, high-resolution mass spectrometry, and cyclic voltammetry show that before reacting with the phenoxyl radical, the water molecule becomes activated by the interaction with the iodine(III) center, causing the Umpolung of this formally nucleophilic substrate. The radical-chain mechanism allows the rationalization of all existing observations regarding the iodine(III)-promoted oxidative dearomatization of phenols.

In the present work, we investigated the behavior of titanium dioxide photocatalysts decorated with plasmonic metal – gold and silver – nanoparicles (NPs) for the photooxidation of small organic molecules. The TiO2 samples were large surface area thin film electrodes formed either by thermal oxidation of the Ti metal (for the rutile films) or deposited on the same substrate by a sol-gel method (for the anatase films). The TiO2 films were subsequently decorated with Au nanostructures using either photodeposition or vacuum sputtering. The Ag nanostructures were formed on TiO2 by electrodeposition. Although both the Au-TiO2 and the Ag-TiO2 films yielded photoanodic currents under illumination with visible wavelengths, consistent with the resonant frequencies of plasmonic metal NPs, prolonged polarization resulted in the drastic drop of photoactivity. Such deactivation, that did not affect the range of near-UV wavelengths absorbed by the pristine TiO2, has been assigned to the surface oxidation of the plasmonic NPs progressively blocking the charge regeneration within the metal nanoparticles. These results raise questions in regard to the use of Au-TiO2 and Ag-TiO2 photocatalysts for sunlight-driven photodegradation of organic contaminants in water.

Further advancement in sunlight-driven splitting of water as a means of producing hydrogen and oxygen is mainly hampered by the availability of easy-to-prepare, inexpensive n-type semiconductor mat...

Photoelectrochemical reduction of CO 2 is explored here with respect to the possible production of simple organic fuels. While the photoelectrochemical system is based on p -type Cu 2 O semiconductor, special attention has been paid to its stabilization, intentional modification and fabrication, as well as to optimization of experimental conditions permitting control and diversification of the CO 2 reduction products. Utilization of the mixed oxides photoelectrochemical system consisting of the electrodeposited, onto transparent conductive oxide (TCO) coated glass support, Cu 2 O catalyst and next coupled to non-stoichiometric TiO 2 , acting as a charge transfer enhancing layer. Such a heterojunction allows, from one side, increase density of charge carries in the conduction band of the photocathode and, on the other side, improve conductivity of the whole system. Moreover, the outer-most over-layer of Nafion is expected to facilitate attraction of CO 2 (dissolved in the sodium sulfate electrolyte) at the photoelectrochemically active mixed-metal-oxides interface. Consequently, the overall photoelectrochemical efficiency is improved, and the cathodic photocurrent (related to the CO 2 -reduction) of ca. 1.2 mA cm−2 (at 0.1 V vs. RHE). The reported hierarchical (layered mixed-oxide) system consisting of copper(I) oxide and titania over-layer (covered with the Nafion outer-most film) exhibits remarkable stability with time.

Tungsten trioxide (WO3) is one of a few stable semiconductor materials liable to produce solar fuel by photoelectrochemical water splitting. To enhance its visible light conversion efficiency, we incorporated plasmonic gold nanoparticles (Au NPs) derivatized with polyoxometalate (H3PMo12O40) species into WO3. The combined plasmonic and catalytic effect of Au NPs anchored to the WO3 surface resulted in a large increase of water photooxidation currents. Shielding the Au NPs with polyoxometalates appears to be an effective means to avoid formation of recombination centers at the photoanode surface.

In the present study, we utilize phosphomolybdate (PMo12O40 3-) modified gold nanoparticle (diameters, 30-40 nm) deposits on mesoporous tungsten oxide photoanodes to study plasmon-assisted photoelectrochemical water splitting. Such n-type semiconducting oxide as WO3 (band gap, 2.5 eV) suffers from inadequate solar light absorption and, therefore, there is a need to use of a thin plasmon active layer of nanostructured Au of proper architecture to enhance the interfacial light trapping and conversion. A remarkable increase in the solar energy conversion efficiency as well as in the photooxidation current (oxygen evolution) are believed to be induced by the plasmonic excitation of Au nanoparticles occurring within WO3 absorption range. The polyoxometallate (PMo12O40 3-) monolayer-type nanostructures may play electrocatalytic role, improve charge distribution, in addition to the stabilization effect. In the other study, the PMo12O40 3- modified Au nanoparticles, when deposited on step-bunched silicon Si(111) surface have also acted as electrocatalytic centers during photoelectrochemically induced reduction of carbon oxide (IV). The applicability of phosphomolybdate-protected gold nanoparticles to create porous matrix for photoelectrochemically assisted reduction of carbon dioxide is even more pronounced when the system is further decorated with copper. We also present the result of decorating multi-walled carbon nanotubes with gold and copper nanoparticles (by chemical reduction method and through electrodeposition) acting as catalytic systems active towards CO2 electroreduction under voltammetric conditions. Carbon nanotubes have been chemically modified here with surface oxygen complexes using HNO3 and H3PMo12O40 solutions.

The AFM scratching technique was applied to determine the absolute thickness of the electrodeposited poly o -methoxyaniline films in dependence of an anion type ( SO 4 2 - ,Cl − , ClO 4 - ) and a monomer concentration (0.01–0.06 M). The measured thickness range varies from 10 to 2000 nm in dependence on the anion identity and concentration. The AFM data are compared with the thickness calculated from the oxidation charge coupled to the reaction of leucoemeraldine to emeraldine form of the polymer. The proper way to determine capacitance charge is discussed. The observed excess of charge under CV dependence is explained by the charge required for cross linking of the polymeric chains. The anionic influence on the thickness of electrodeposited films is explained by influence of an anion on film morphology, rather than dependence on the anionic radiuses. The results point out that the determination of the number of electrons taking part in the process studied on the basis of the integration of the current–potential (CV), hence the mechanism of the polymerization and oxidation of the polymeric films might be easily misleading only because the capacitance charge of the polymer cannot be appropriate determined.

The proper deposition bath composition for electrochemical synthesis of the CdSe deposit in the hexagonal structure of the right elemental stoichiometry, and photoreacting as an n-type semiconductor which can be used as a stable photoanode is investigated. The deposits were prepared by a cyclic potentiodynamic technique and the concentration of Cd2+ and SeO32− in the deposition baths varied from 10−4 M to 0.1 M, and from 10−5 M to 10−3 M, respectively. The electrochemical, the X-ray diffraction (EDS and XRD), and the photoactivity studies of a number of deposits have shown that application of the solution composition following Cd:Se = 5:1 results in deposition of the stoichiometric CdSe. The detected ratio of reagents is explained on the base of reaction mechanism and necessary excess of cadmium ions preventing CdSe deposit dissolution. The procedure of CdSe electrosynthesis was developed to yield of a direct semiconductor in the hexagonal structure. The necessity for cadmium cations excess is explained on the basis of the mixed electrochemical/chemical deposition mechanism.

The in situ atomic force microscopy and the electrochemical studies on electropolymerization of the o -methoxyaniline in the 0.0–0.8 V versus NHE range of the electrode potential are described. It is proved that in the 0.0–0.3 V versus NHE a redox process takes place, resulting in the formation of poly( o -methoxyaniline) in its reduced form, leucoemeraldine. The different morphologies are exhibited by poly( o -methoxyaniline) under different polymerization conditions. The microscopic results show that with the increase of the monomer concentration in the bulk of electrolyte solution the globular morphology, related to the coil like molecular structure, is replaced by the fibrilar one, related to the opened-up, more conductive extended coil structure. It is shown that oxidation of a leucoemeraldine state of polymer to its emeraldine state results in the change of the morphology from the chain like structure to the massive fibrilar like structure. The reduction of oxidized polymer results in its irreversible fragmentation.

Sequential doping of polyaniline base in the oxidation state of emeraldine (abbreviated as PANI) first with iron(III) chloride and then treated with hexafluoroacetylacetone (HFAA) is described. The results obtained by using a combination of spectroscopic techniques (UV–vis–NIR, mass spectroscopy, Mossbauer effect spectroscopy, EPR) unequivocally show that complete Lewis acid-type complexation of PANI with FeCl 3 occurs only in the solid state, i.e. after removal of the solvent. In the solution an equilibrium is established between PANI complexed with FeCl 3 and FeCl 3 complexed with nitromethane. The addition of HFAA to the solution, being in equilibrium, transforms Lewis acid doped PANI into mixed doped polymer, which upon casting and solvent removal gives a solid material of a general formula: PANI(FeCl 3 ) x (HCl) y (HFAA) z . In this compound FeCl 3 is complexed with amine nitrogens, imine nitrogens are protonated with HCl. HFAA, dispersed in the polymer matrix, serves as a plasticizer. The determined chemical constitution is a direct consequence of the reaction of HFAA with FeCl 3 complexed on PANI imine sites to give HCl and iron(III) hexafluoroacetylacetonate. The former protonates the imine sites whereas the latter is removed from the system by extended pumping as proved by mass spectroscopy. Mossbauer spectra unequivocally show that FeCl 3 complexed on amine sites remains intact. Films of PANI(FeCl 3 ) x (HCl) y (HFAA) z show room temperature conductivity of ca. 3 × 10 −3 S cm −1 and improved mechanical properties as compared with PANI complexed solely with FeCl 3 due to HFAA plasticizing effects.

We investigated the structure and photo-electrochemical properties for water splitting of tungsten trioxide-ferric oxide thin films formed by spray pyrolysis. While annealing at 600°C produces films consisting of a mixture of monoclinic WO 3 and hematite α- Fe 2 O 3, the heating at a temperature above 1000°C affords formation of ferric tungstate Fe 2 WO 6. Both kinds of films exhibit optical absorption range comparable or exceeding that of α- Fe 2 O 3. Another important feature is a decreased rate of charge recombination of the mixed-oxide Fe 2 O 3- WO 3 with respect to the ferric oxide photo-anodes.

Doping of polyaniline with Lewis acids is studied with the goal to improve their processibility and stability by an alternative to Bronsted acid doping route. Polymer doped with inorganic Lewis acids are characterized by elemental analysis, 27 Al NMR and Mossbauer spectroscopies.