Determining the Efficiency of Photoelectrode Materials by Coupling Cavity‐Microelectrode Tips and Scanning Electrochemical Microscopy

Photoelectrochemical water splitting (PEC-WS) is a promising route to obtain hydrogen (and oxygen) from sunlight and water. However, too many semiconductors show poor stability, due to photodegradation phenomena in aqueous solutions, thus loosing efficiency under operative conditions. Aim of this paper is to introduce a simple and fast method for screening different semiconductor materials and identify their efficiency in H2 (or O2) production with respect to photocorrosion. This method could be used with any finely dispersed semiconductor (powder) for a fast, preliminary evaluation of the material's behaviour without interferences from the supporting material (i. e. FTO) or any binder. The method is based on the combination of scanning electrochemical microscopy (SECM) in the tip generation/substrate collection (TG/SC) mode and of cavity microelectrodes as SECM tips. Here, we show results obtained on three powder materials, namely core-shell CuI/CuO, CuI and TiO2

Achieving efficient H2O2 production by a visible-light absorbing, highly stable photosensitized TiO2

In this paper we propose titanium dioxide modified with cobalt(II) 2-ethylhexanoate (Co@TiO2) as photoactive material for the efficient visible light driven production of H2O2. We demonstrate that visible light activity can be achieved thanks to a photoinduced electron transfer from the excited complex towards the TiO2 conduction band that corresponds to a cobalt-to-titanium-charge-transfer transition. H2O2 is synthetized by a combination of oxygen reduction and water oxidation, that is possible thanks to the correct band position of Co@TiO2. A mechanism of H2O2 formation is suggested on the basis of experimental evidences. Reactive oxygen intermediates, together with H2O2 are responsible for the photocatalytic degradation of a nonionic surfactant, methylene blue and phenol for wastewater treatments. Finally, and quite interestingly, Co@TiO2 can be also used in a photoelectrochemical setup, where it can be adopted both as photoanode and photocatode, and the switching potential corresponds to the redox potential of the adsorbed complex Co(III)/Co(II) couple

Influence of Strain on the Band Gap of Cu₂O

Cu\u2082O has been considered as a candidate material for transparent conducting oxides and photocatalytic water splitting. Both applications require suitably tuned band gaps. Here we explore the influence of compressive and tensile strain on the band gap by means of density functional theory (DFT) modeling. Our results indicate that the band gap decreases under tensile strain while it increases to a maximum under moderate compressive strain and decreases again under extreme compressive strain. This peculiar behavior is rationalized through a detailed analysis of the electronic structure by means of density of states (DOS), density overlap region indicators (DORI), and crystal overlap Hamilton populations (COHP). Contrary to previous studies we do not find any indications that the band gap is determined by d10-d10 interactions. Instead, our analysis clearly shows that both the conduction and the valence band edges are determined by Cu-O antibonding states. The band gap decrease under extreme compressive strain is associated with the appearance of Cu 4sp states in the conduction band region

Chlorine Dioxide Degradation Issues on Metal and Plastic Water Pipes Tested in Parallel in a Semi-Closed System

Chlorine dioxide (ClO2) has been widely used as a disinfectant in drinking water in the past but its effects on water pipes have not been investigated deeply, mainly due to the difficult experimental set-up required to simulate real-life water pipe conditions. In the present paper, four different kinds of water pipes, two based on plastics, namely random polypropylene (PPR) and polyethylene of raised temperature (PERT/aluminum multilayer), and two made of metals, i.e., copper and galvanized steel, were put in a semi-closed system where ClO2 was dosed continuously. The semi-closed system allowed for the simulation of real ClO2 concentrations in common water distribution systems and to simulate the presence of pipes made with different materials from the source of water to the tap. Results show that ClO2 has a deep effect on all the materials tested (plastics and metals) and that severe damage occurs due to its strong oxidizing power in terms of surface chemical modification of metals and progressive cracking of plastics. These phenomena could in turn become an issue for the health and safety of drinking water due to progressive leakage of degraded products in the water

Ad hoc tailored electrocatalytic MnO2nanorods for the oxygen reduction in aqueous and organic media

Metal-air batteries are one of the most promising electrochemical systems for energy storage and conversion. Herein we report promising results by exploiting manganese dioxide nanoparticles as ORR electrocatalysts. MnO2 nanorods were prepared through a hydrothermal synthesis, i.e. by varying both the salt precursors (Le. manganese sulphate or chloride) and the oxidizing agents (Le. ammonium persulfate or potassium permanganate). All the nanopowders were finely characterized on structural, morphological and surface points of view. Then, their electrocatalytic power was tested either in aqueous 0.1 M potassium hydroxide or in Tetra Ethylene Glycol Dimethyl Ether (TEGDME)/LiNO3 0.5 M electrolytes, by using Gas Diffusion Electrodes (GDEs) and Glassy Carbon (GC) as cathodes, respectively. All the nanoparticles promoted the ORR by causing a shift of the onset potential up to 100 mV in both solvents. Nevertheless, this shift was different according to the solvent/electrolyte used: in the case of the ether-based solvent, different values are obtained by adopting the synthesized MnO2 powders. Thus, we hypothesized that the structural/surface properties of MnO2 samples are leveled in the aqueous medium (Le. in a OH rich solvent, the hydroxyls can interact with the homologs on the MnO2 surface), contrary to what occurs in the organic solvent. Furthermore, a different behavior was observed also on the kinetic point of view thus leading to diverse interpretations of the oxygen reduction mechanism, especially in TEGDME

Electrodeposited cu thin layers as low cost and effective underlayers for Cu2O photocathodes in photoelectrochemical water electrolysis

Cu2O is one of the most studied semiconductors for photocathodes in photoelectrochemical water splitting (PEC-WS). Its low stability is counterbalanced by good activity, provided that a suitable underlayer/support is used. While Cu2O is mostly studied on Au underlayers, this paper proposes Cu(0) as a low-cost, easy to prepare and highly efficient alternative. Cu and Cu2O can be electrodeposited from the same bath, thus allowing in principle to tune the final material\u2019s physico-chemical properties with high precision with a scalable method. Electrodes and photoelectrodes are studied by means of electrochemical methods (cyclic voltammetry, Pb underpotential deposition) and by ex-situ X-ray absorption spectroscopy (XAS). While the potential applied for the deposition of Cu has no influence on the bulk structure and on the photocurrent displayed by the semiconductor, it plays a role on the dark currents, making this strategy promising for improving the material\u2019s stability. Au/Cu2O and Cu/Cu2O show similar performances, the latter having clear advantages in view of future use in practical applications. The influence of Cu underlayer thickness was also evaluated in terms of obtained photocurrent

Time-Resolved X-ray Absorption Spectroscopy in (Photo)Electrochemistry

This minireview aims at providing a complete survey concerning the use of X-ray absorption spectroscopy (XAS) for time-resolved studies of electrochemical and photoelectrochemical phenomena. We will see that time resolution can range from the femto-picosecond to the second (or more) scale and that this joins the valuable throughput typical of XAS, which allows for determining the oxidation state of the investigated element, together with its local structure. We will analyze four different techniques that use different approaches to exploit the in real time capabilities of XAS. These are quick-XAS, energy dispersive XAS, pump & probe XAS and fixed-energy X-ray absorption voltammetry. In the conclusions, we will analyze possible future perspectives for these techniques

The role of surface electrification on the growth and structural features of titania nanoparticles

TiO2 particles, prepared by following a sol-gel preparative route, were submitted to hydrothermal steps performed at solution pH values corresponding, respectively, to positive, zero and negative oxide surface charges. After the hydrothermal step all the samples were thermally treated at 300 and 600degreesC, for the same length of time (6 h). The powders, both precursors and calcined samples, were characterized for phase composition-crystallinity. particle morphology and surface electrification features. The role played by the particles electrification during the hydrothermal step in affecting the physico-chemical properties of the powders is discussed

Charge-transfer patterns for [Ru(NH3)6]3+/2+ at SAM modified gold electrodes : impact of the permeability of a redox probe

Electrochem. performance of a [Ru(NH3)6]3+/2+ redox couple at gold electrodes modified by alkanethiol self assembled monolayer (SAM) films of the type [-SH -(CH2)n - CH3] with different no. of methylene units (n = 2 to 10) in the presence and absence of glucose additives has been studied using fast scan cyclic and steady-state voltammetry. Specific scatter of measured rate consts. caused by enhanced sensitivity of this probe to minor defects of SAMs has been obsd. in a general agreement with the published data for thicker SAMs (n = 9 to 18). In addn., we have disclosed the anomalous viscosity-imposed drop of the heterogeneous rate const. for the case of Au electrodes modified by thinner n-alkanethiol SAMs (n = 2, 4). Taking into the account the fact of [Ru(NH3)6]3+/2+ couple's capability to penetrate into the SAM interior, we ascribe the obtained results to the manifestation of the solvent-friction mechanism under the condition where the redox species presumably together with a few of solvating water mols. reside in a SAM's peripheral interior marked by much higher local viscosity (slower dielec. relaxation) compared to the electrolyte soln