ChemPhysChem

The cover picture shows a sub-monolayer of ssDNA-stabilised reduced graphene oxide assembled by electrostatic adsorption. The reduced graphene sheets are capable of mediating electron transfer from redox species in solution to electrode surfaces modified by an insulating self-assembled monolayer. This behaviour contrasts with the insulating properties of the graphene oxide precursor. Voltammetric and impedance responses provide information on density of state and electrochemical reactivity of these carbon-based nanostructures. Details can be found in the article by D. J. Fermín et al. on p. 2956 ff.

Mismatched dye solar cells in modules can be forced to operate in reverse bias (RB). As shown on p. 2964 ff. by T. M. Brown et al., 0.4 V in RB with time has little effect on cell efficiency whereas harsher but real tests leads to increasing overpotentials at electrodes causing H₂ evolution, dye degradation and breakdown.

Ubiquitous titania electrodes: The electrochemistry of nanostructured titanium dioxide electrodes is reviewed with a focus on the fundamental aspects that determine their behavior both in the dark and under illumination (see picture). Some applications in the fields of environmental remediation (heterogeneous photocatalysis), and energy saving (electrochromism), generation (artificial photosynthesis) and accumulation (Li-ion batteries) are also dealt with.

Electron-transfer reaction dynamics at electrodes (see picture) have a long history of both theoretical modeling and experimental efforts to test such theories with contributions by many researchers. This Minireview reports on the most recent fundamental experimental data which have solved some of the long-standing issues and opened up the field to further experimental and theoretical progress.

Catch the sun: Dye-sensitized photoelectrosynthesis cells (DSPECs) represent a promising approach to solar fuels with solar-energy storage in chemical bonds. The targets are water splitting and carbon dioxide reduction by water to CO, other oxygenates, or hydrocarbons (see picture). A systematic basis for DSPECs comprises a modular approach with light absorption/excited-state electron injection, and catalyst activation assembled in integrated structures.

Indoline dyes with additional anchor groups are studied as sensitizers for ZnO in view of potential applications in photovoltaics. Improved performance is observed, as well as a better adsorption of the dye on ZnO—even in the presence of competing coadsorbates—which leads to an increased stability of the dye-sensitized solar cells.

Built for two: InP(100)-based half-cells show significant solar-to-hydrogen efficiencies, but require a bias due to insufficient voltage. Tandem absorber structures may provide both adequate potential and efficient utilization of the solar spectrum. The picture shows contour lines of calculated limiting conversion efficiencies for ideal monolithic tandem solar cells.

The other side counts: The charge accumulated on the metal/semiconductor (M/SC) junction interfaced with electrolyte (El) gives rise to an induced bound charge density, σ, which influences the ac response of the semiconductor in solution (see picture).

Bo(u)nd to act: We study the electrochemical behavior of the redox metalloenzyme copper nitrite reductase immobilized on a Au(111)-electrode surface modified by a self-assembled cysteamine molecular monolayer using a combination of cyclic voltammetry and electrochemically-controlled atomic force microscopy. The enzyme shows no voltammetric signals in the absence of nitrite substrate, whereas a strong reductive electrocatalytic signal appears in the presence of nitrite.

The time of their life: The long-term lifetime of large-area dye solar cell devices was tested under outdoor conditions and indoors with continuous light soaking. Different degradation rates were found in outdoor samples with horizontally and vertically oriented cells, and in cells working at open-circuit voltage and maximum power point (see picture). Correlation between electrolyte triiodide depletion and degradation phenomena is analyzed.

Changing potential: Anodization of α-Fe₂O₃ (hematite) electrodes in alkaline electrolyte under constant potential conditions the electrode surface in a way that the properties of the material are altered, as determined by cyclic voltammetry, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy (see picture).

Silver bismuth tungstate nanoparticles are prepared by solution combustion synthesis, modified with a Pt co-catalyst, and finally used for the photogeneration of syngas (CO+H₂). Formic acid is used to produce CO₂ in situ and subsequently reduce it to CO. The photocatalytic activity of these nanoparticles is superior to that of nanoparticles prepared by solid-state synthesis.

Electron-launching nanopads: Chemically reduced graphene oxide mediates the electron-transfer responses across insulating molecular layers at electrode surfaces. The significant contrast in electrochemical reactivity (see picture) with graphene-oxide sheets provides information on changes of the density of states brought about by the chemical reduction step.

Hydrogen evolution in reverse biased cell: Subjecting a dye solar cell to prolonged reverse bias causes degradation of its charge transfer mechanisms, ascribed to partial irreversible oxidations of the dye molecules, and significant decrease of the diffusion limited current. As a result, the potential across the reverse biased cell increases in time, causing gas, identified as hydrogen, to evolve at the counter electrode, leading to cell failure.

At the limit: It is illustrated how diffusion limitations of cobalt redox couples in dye-sensitized solar cells (see picture) can be eliminated by tuning the porosity, particle and pore sizes of the mesoporous TiO₂ layers through simple TiCl₄ post-treatment. Thus, thicker TiO₂ films required for higher photocurrent densities can be used without encountering mass transport limitations, leading to power conversion efficiencies of 10 %.

The bends: The surface recombination of light-induced charge carriers during the etching of Si is investigated by means of photoluminescence measurements (see picture). The surface recombination velocity is strongly reduced during the first stages of etching, pointing to a interface well passivated by the etching process.

High and low: In the oscillatory electrooxidation of p-type Si, two types of current oscillations with high (red) and low (black and blue) amplitudes (see picture), exist, which follow different mechanisms. This observation clarifies apparently contradictory observations reported in the literature.

Differential pulse voltammetry and cyclic voltammetry measurements show that it is possible to distinguish quantized double-layer charging events of the core from electron transfer to ferrocene (Fc) groups attached to a nanoparticle surface in Fc–monolayer-protected clusters (Fc-MPCs). The voltammetric features depend on the number of redox centres incorporated in the cluster and an electrostatic switching mechanism can be observed at the nanoscale.

Voltammetric signals related to grain boundary traps can be unequivocally identified when working with TiO₂ nanostructured electrodes of different morphologies. Various ways of controlling these traps—and thus the electrochemical behavior of TiO₂—are discussed.

A visible difference: TiO₂–polyheptazine (TiO₂–PH) hybrid photoanodes loaded with photodeposited cobalt oxide-based oxygen-evolving cocatalyst (Co-Pi) exhibit visible-light-driven photooxidation of water to oxygen at moderate bias potentials (see picture).

Light capture: Anodically grown WO₃ electrodes show interesting properties displaying both a partial visible absorption and good charge-transport abilities. They exhibit typical crystalline oxide nanostructures and they reach 70 % of photon-to-current conversion in 1 M H₂SO₄ at 1.5 V versus SCE under illumination (see picture).

The solar-cell materials Cu₂ZnSnSe₄ and Cu(In,Ga)Se₂ (CZTS and CIGS) are taken as a case study to show how thermal stability at surfaces and interfaces can be predicted and rationalized. The authors describe how the annealing conditions can be controlled to maximize the stability—and therefore the quality—of the material.

It is all about adsorptivity: During benzene oxidation at a boron-doped diamond electrode, adsorption of a reaction intermediate occurs, which partially blocks the electrode surface and lowers the anodic current (see picture, blue curve). However, using a nanocrystalline diamond electrode, benzene is oxidized concurrently with oxygen evolution, and a (quinoid) intermediate is adsorbed at the electrode.

The photoelectrochemical conversion of sunlight into chemical fuels such as hydrogen requires careful interface engineering of semiconductor materials to achieve high conversion efficiencies. Recent results on the optimization of p-GaP semiconductor electrodes for solar hydrogen generation are summarized.