Enhanced Photoelectrochemistry in CdS/Au Nanoparticle Bilayers

Three different configurations of Au‐nanoparticle/CdS‐nanoparticle arrays are organized on Au/quartz electrodes for enhanced photocurrent generation. In one configuration, Au‐nanoparticles are covalently linked to the electrode and the CdS‐nanoparticles are covalently linked to the bare Au‐nanoparticle assembly. The resulting photocurrent, φ = 7.5 %, is ca. 9‐fold higher than the photocurrent originating from a CdS‐nanoparticle layer that lacks the Au‐nanoparticles, φ = 0.8 %. The enhanced photocurrent in the Au/CdS nanoparticle array is attributed to effective charge separation of the electron–hole pair by the injection of conduction‐band electrons from the CdS‐ to the Au‐nanoparticles. Two other configurations involving electrostatically stabilized bipyridinium‐crosslinked Au/CdS or CdS/Au nanoparticle arrays were assembled on the Au/quartz crystal. The photocurrent quantum yields in the two systems are φ = 10 % and φ = 5 %, respectively. The photocurrents in control systems that include electrostatically bridged Au/CdS or CdS/Au nanoparticles by oligocationic units that lack electron‐acceptor units are substantially lower than the values observed in the analogous bipyridinium‐bridged systems. The enhanced photocurrents in the bipyridinium‐crosslinked systems is attributed to the stepwise electron transfer of conduction‐band electrons to the Au‐nanoparticles by the bipyridinium relay bridge, a process that stabilizes the electron–hole pair against recombination and leads to effective charge separation.     

Enhancement of photoelectrochemical properties with α–Fe2O3 on surface modified FTO substrates

Despite a number of favorable properties, several drawbacks need to be addressed before hematite can be considered a highly desirable material for applications in water-splitting photoelectrochemical (PEC) electrodes. In the present work, the PEC performance was improved by the synergetic effects of the preparation techniques and modification of the fluorine-doped tin oxide (FTO) glass. The hematite photoanodes deposited onto the pretreated FTO supports clearly decreased the overpotential by up to 100 mV with a higher photocurrent compared to the bare FTO glass. In particular, an electrochemical preparation method including moderate annealing resulted in a highly enhanced PEC performance by influencing the FTO glass during the electrode preparation process.     

Light induced electropolymerization of poly(3,4-ethylenedioxythiophene) on niobium oxide

The photoelectrochemical polymerization of poly(3,4-ethylenedioxythiophene), PEDOT, was successfully realized on anodic film grown to 50 V on magnetron sputtered niobium. Photocurrent Spectroscopy was employed to study the optical properties of Nb/Nb₂O₅/PEDOT/electrolyte interface in a large range of potential, and to get an estimate of the band gap and flat band potential of both the oxide and the polymer. Scanning Electron Microscopy was used to study the morphology of PEDOT. Both the optical and morphological features of the photoelectrochemically grown polymer were compared with those showed by PEDOT electropolymerized on gold conducting substrate.     

Photoelectrochemistry of poly(3-methylthiophene), I: Surface morphology and thickness effect

Photocyclovoltammetric experiments with films of poly(3-methylthiophene) under polychromatic light irradiation show a p-type semiconductor behavior in the reduced state, with a flat band potential of 0.18 V. The photocurrent depends on film thickness and surface morphology. Films with different thickness were characterized by scanning electron microscopy, indicating a globular morphology with globule sizes changing according to the charge density used during polymer deposition. The highest photocurrent is observed for the lowest globule diameter. These films were also irradiated with monochromatic light from the electrolyte side and from the substrate side. A higher photocurrent and a photocurrent spectrum matching the absorption spectrum of the reduced form of the polymer is observed on irradiation from the electrolyte side of a 0.7 μm thick film. In contrast, for irradiation from the electrode side, the photocurrent is lower and the spectrum shows a peak at lower energy. These results were interpreted in terms of a delocalized space charge zone, different kinetics for charge transfer and mass transport across the solvent swollen polymer film and different depth of penetration of the light as a function of wavelength.     

Wavelength-dependent switching of the photocurrent direction at the surface of molecular semiconductor electrodes based on orbital-confined excitation and transfer of charge carriers from higher excited states

Photoelectrochemical experiments are performed at evaporated thin films of unsubstituted phthalocyanines (Pc) and derivatives with electronegative substituent groups in the ligand. Light absorption in the B-band leads to occupation of a higher excited singlet state S₂ relative to the first excited singlet state S₁ that is populated by Q-band absorption. Charge carriers in S₂ can have sufficient lifetime to be transferred to adsorbed reactants at the electrode surface despite the competing relaxation into S₁ and to the ground state S₀. The assignment of the well-defined B- and Q-bands in the solid state to transitions between distinct molecular orbitals is thereby proven to be of practical relevance. If a material of a suitable position of energy levels is chosen, the direction of charge transfer can be switched by illumination with light of the two different wavelengths. According to the separate pathways of reaction starting from the S₂ and S₁ excited states different quantum efficiencies are obtained following absorption in either the B- or Q-band. Surface traps in the different reactions of charge transfer to the electrolyte are detected by charging and discharging photocurrent spikes. Implications of the present findings for the fundamental discussion of electrical and electrochemical properties involving illumination of molecular modified electrodes which are in part also of technical significance due to the related phenomena of photoconduction (photocopiers, laser printers), light emission (organic light emitting diodes) and charge transfer (solar energy conversion) are discussed.     

Synthesis and characterization of bifunctional β-MnO2-based Pt/C photoelectrochemical cell for hydrogen production

A manganese dioxide (β-MnO₂) photocatalyst for light-induced water splitting and hydrogen generation is studied via the impregnation and heat treatment method. The phase and fluorescence characterizations are examined by X-ray diffraction (XRD) and photoluminescence. A series of peaks for its (110) and (200) planes are identified as those for pure β-MnO₂ crystals with lattice spacings of 3.11 and 2.19 Å, respectively. The photoluminescence shows that the primary signals are located in the blue-violet spectral region, corresponding to the band-edge emission of β-MnO₂ (376 nm). Furthermore, two types of photoelectrochemical cell with added Pt are constructed to compare hydrogen production rates. A significant enhancement of light-induced hydrogen generation by water splitting is observed on Pt/β-MnO₂/C, with a hydrogen generation rate of 194.67 μmol cm⁻¹ h⁻¹, greater than that on a Pt/TiO₂/C photocatalyst, which can be attributed to the effective inhibiting of CO poisoning, thus maintaining the catalyst's surface area in methanol oxidation.     

A photoelectrochemical solar cell based on ZnO/dye/polypyrrole film electrode as photoanode

Nanostructured ZnO film electrodes were prepared. A preliminary PEC solar cell based on nanostructured Zno/dye/polypyrrole (PPy) film electrode was fabricated. A fill factor of 0.754 and a high overall light to electricity conversion efficiency of 1.3% for this PEC solar cell were obtained. The sensitization mechanisms of the nanostructured ZnO electrodes were also discussed.     

Photoelectrochemical study of nickel base alloys oxide films formed at high temperature and high pressure water

The oxide film formed on nickel base alloys at high temperature and high pressure water exhibits semi-conducting properties evidenced by photocurrent generation when exposed to monochromatic light. The use of macro- and micro-photoelectrochemical techniques (PEC and MPEC) aims to identify the different semiconductor phases and their distribution in the oxide film.Three different nickel base alloys were corroded in recirculation loop at 325 °C in pressurised water reactor primary coolant conditions for different exposition durations.PEC experiments on these materials enable to obtain macroscopic energy spectra showing three contributions. The first one, with a band gap around 2.2 eV, was attributed to the presence of nickel hydroxide and/or nickel ferrite. The second one, with a band gap around 3.5 eV, was attributed to Cr₂O₃. The last contribution, with a band gap in the range of 4.1-4.5 eV, was attributed to the spinel phase Ni_1−xFe_xCr₂O₄. In addition, macroscopic potential spectra recorded at different energies highlight n-type semi-conduction behaviours for both oxides, Cr₂O₃ and Ni_1−xFe_xCr₂O₄.Moreover, MPEC images recorded at different energies exhibit contrasted regions in photocurrent, describing the distribution of nickel hydroxide and/or nickel ferrite and Cr₂O₃ in the oxide film at a micron scale.It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in the oxide scale.     

Direct anodic growth of thick WO3 mesosponge layers and characterization of their photoelectrochemical response

Thick mesoporous tungsten oxide (WO₃) layers can be formed by anodization of tungsten in a 10 wt% K₂HPO₄/glycerol electrolyte, if the electrolyte temperature is around 80-100 °C. At 90 °C, a regular mesoporous WO₃ layer was grown up to a thickness of approximately 9 μm. This WO₃ mesosponge layer consists of typical feature sizes of 20-30 nm and pore widths of 10-30 nm. The photoresponse of different layer thicknesses and different annealing treatments was characterized in a photoelectrochemical cell. The highest photocurrents were observed with a 2.5 μm thick WO₃ layer annealed at 550 °C consisting of a mixture of orthorhombic, triclinic and monoclinic phases. Incident photon to current efficiencies (IPCEs) of the samples were 73.4% in a 1 M HClO₄ electrolyte and 167.5% for methanol photo-oxidation in 0.1 M CH₃OH/1 M HClO₄ electrolyte, at 1 V vs. Ag/AgCl under illumination at a wavelength of 420 nm.