Fixed Energy X‑ray Absorption Voltammetry

In this paper, the fixed energy X-ray absorption voltammetry (FEXRAV) is introduced. FEXRAV represents a novel in situ X-ray absorption technique for fast and easy preliminary characterization of electrode materials and consists of recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to an X-ray absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the original oxidation state determines a variation of the absorption coefficient. Although the information given by FEXRAV obviously does not supply the detailed information of X-ray absorption near edge structure (XANES) or extended X-ray absorption fine structure (EXAFS), it allows to quickly map the oxidation states of the element under consideration within the selected potential windows. This leads to the rapid screening of several systems under different experimental conditions (e.g., nature of the electrolyte, potential window) and is preliminary to more deep X-ray absorption spectroscopy (XAS) characterizations, like XANES or EXAFS. In addition, the time-length of the experiment is much shorter than a series of XAS spectra and opens the door to kinetic analysis

α- and γ‑FeOOH: Stability, Reversibility, and Nature of the Active Phase under Hydrogen Evolution

α-FeOOH (goethite) and γ-FeOOH (lepidocrocite) were found to be the main corrosion products of the steel cathode in the sodium chlorate process; the identification of the phases formed under reducing potentials, along with the study of the electrodes during the reoxidation, is fundamental to understanding their role in this process. In this work, FeOOH-based electrodes were investigated through in situ and in operando X-ray absorption spectroscopy (XAS), combined to electrochemical measurements (e.g., voltammetry and chronoamperometry). At sufficiently negative potentials (below −0.4 V vs RHE ca.) and under hydrogen evolution conditions an unknown iron­(II)-containing phase is formed. A comprehensive analysis of the whole XAS spectrum allowed proposing a structure bearing a relation with that of green rust (space group <i>P</i>3̅1<i>m</i>). This phase occurs independently of the nature of the starting electrode (α- or γ-FeOOH). During electrochemical reoxidation, however, the original phase is restored, meaning that the reduced phase brings some memory of the structure of the starting material. Spontaneous reoxidation in air suppresses the memory effect, producing a mixture of α and γ phases

Structure and Stability of a Copper(II) Lactate Complex in Alkaline Solution: a Case Study by Energy-Dispersive X‑ray Absorption Spectroscopy

Energy-dispersive X-ray absorption spectroscopy was applied, aimed at solving the problem of the structure and stability of a copper­(II) lactate complex in alkaline solution, used as a precursor for the electrodeposition of Cu₂O. The application of multiple scattering calculations to the simulation of the X-ray absorption near-edge structure part of the spectra allowed an accurate resolution of the structure: the copper­(II) cation is surrounded by four lactate ions in a distorted tetrahedral environment, with the lactate anions acting as monodentate ligands. This results in an atomic arrangement where copper is surrounded by four oxygen atoms located at quite a short distance (ca. 1.87 Å) and four oxygen atoms located quite far apart (ca. 3.1–3.2 Å). The complex was finally found to be stable in a wide range of applied potentials

α‑Fe₂O₃/NiOOH: An Effective Heterostructure for Photoelectrochemical Water Oxidation

The study of the semiconductor/electrocatalyst interface in electrodes for photoelectrochemical water splitting is of paramount importance to obtain enhanced solar-to-fuel efficiency. Here, we take into consideration the multiple effects that a thin layer of photodeposited amorphous Ni-oxyhydroxide (NiOOH) induces on hematite (α-Fe₂O₃) photoanodes. The reduction of overpotential produced a photocurrent onset potential advance of 150 mV and an increase of photocurrent of about 50% at 1.23 V vs RHE. To give an interpretation to these phenomena, we carried out deep electrochemical investigations by cyclic voltammetry and electrochemical impedance spectroscopy. The effective charge injection into the electrolyte due to the reduction of the charge transfer resistance at the electrode/electrolyte interface was observed and increased along with the amount of deposited NiOOH. The benefits of NiOOH deposition are ascribable to its ability to scavenge holes from hematite surface traps. This effect is mitigated at a potential higher than 1.25 V, since a fraction of photogenerated holes is consumed into the Ni redox cycle

Perfluorinated Zinc Porphyrin Sensitized Photoelectrosynthetic Cells for Enhanced TEMPO-Mediated Benzyl Alcohol Oxidation

This research introduces a novel series of perfluorinated Zn(II) porphyrins with positive oxidation potentials designed as sensitizers for photoelectrosynthetic cells, with a focus on promoting the oxidation of benzyl alcohol (BzOH) mediated by the 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO) organocatalyst. Three dyes, CLICK-3, CLICK-4, and BETA-4, are meticulously designed to explore the impact of substituents and their positions on the perfluorinated porphyrin ring in terms of redox potentials and energy level alignment when coupled with SnO2/TiO2-based photoanodes and TEMPO mediator. A comprehensive analysis utilizing spectroscopy, electrochemistry, photophysics, and computational techniques of the dyes in solution and sensitized thin films unveils an enhanced charge-separation character in the 4D−π–1A type BETA-4. Incorporating four dimethylamino donor groups at the periphery of the porphyrin ring and a BTD-accepting linker at the β-pyrrolic position equips the structure with a more efficient donor–acceptor system. This enhancement ensures improved light-harvesting capacity, resulting in a doubled incident photon-to-current conversion efficiency (IPCE% ≃30%) in the presence of LiI compared to meso-substituted dyes CLICK-3 and CLICK-4. Sensitizing SnO2/TiO2 thin films with BETA-4 successfully promotes the photooxidation of benzyl alcohol (BzOH) in the presence of the rapid TEMPO radical catalyst, yielding photocurrents of approximately 125 μA/cm2 in an optimized TBPy/LiClO4/ACN electrolyte. Notably, when lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) replaces TBPy as the base for TEMPO-catalyzed BzOH oxidation, a remarkable photocurrent of around 800 μA/cm2 is achieved, marking one of the highest values reported for this photoelectrochemical reaction to date. This study underscores that the proper functionalization of perfluorinated zinc porphyrins positions these dyes as ideal candidates for sensitizing SnO2/TiO2 in the photodriven oxidation of BzOH. It also highlights the crucial role of carefully tuning electrolyte composition based on the electronic properties of molecular sensitizers

Understanding solid-gas reaction mechanisms by operando soft X-ray absorption spectroscopy at ambient pressure

Ambient-pressure operando soft X-ray absorption spectroscopy (soft-XAS) was applied to study the reactivity of hydroxylated SnO2 nanoparticles toward reducing gases. H2 was first used as a test case, showing that the gas phase and surface states can be simultaneously probed: Soft-XAS at the O K-edge gains sensitivity toward the gas phase, while at the Sn M4,5-edges, tin surface states are explicitly probed. Results obtained by flowing hydrocarbons (CH4 and CH3CHCH2) unequivocally show that these gases react with surface hydroxyl groups to produce water without producing carbon oxides and release electrons that localize on Sn to eventually form SnO. The partially reduced SnO2 – x layer at the surface of SnO2 is readily reoxidized to SnO2 by treating the sample with O2 at mild temperatures (>200 °C), revealing the nature of “electron sponge” of tin oxide. The experiments, combined with DFT calculations, allowed devising of a mechanism for dissociative hydrocarbon adsorption on SnO2, involving direct reduction of Sn sites at the surface via cleavage of C–H bonds and the formation of methoxy- and/or methyl-tin species at the surface

Hierarchical Hematite Nanoplatelets for Photoelectrochemical Water Splitting

A new nanostructured α-Fe₂O₃ photoelectrode synthesized through plasma-enhanced chemical vapor deposition (PE-CVD) is presented. The α-Fe₂O₃ films consist of nanoplatelets with (001) crystallographic planes strongly oriented perpendicular to the conductive glass surface. This hematite morphology was never obtained before and is strictly linked to the method being used for its production. Structural, electronic, and photocurrent measurements are employed to disclose the nanoscale features of the photoanodes and their relationships with the generated photocurrent. α-Fe₂O₃ films have a hierarchical morphology consisting of nanobranches (width ∼10 nm, length ∼50 nm) that self-organize in plume-like nanoplatelets (350–700 nm in length). The amount of precursor used in the PE-CVD process mainly affects the nanoplatelets dimension, the platelets density, the roughness, and the photoelectrochemical (PEC) activity. The highest photocurrent (<i>j</i> = 1.39 mA/cm² at 1.55 V_RHE) is shown by the photoanodes with the best balance between the platelets density and roughness. The so obtained hematite hierarchical morphology assures good photocurrent performance and appears to be an ideal platform for the construction of customized multilayer architecture for PEC water splitting

An Efficient Cu_<i>x</i>O Photocathode for Hydrogen Production at Neutral pH: New Insights from Combined Spectroscopy and Electrochemistry

Light-driven water splitting is one of the most promising approaches for using solar energy in light of more sustainable development. In this paper, a highly efficient p-type copper­(II) oxide photocathode is studied. The material, prepared by thermal treatment of CuI nanoparticles, is initially partially reduced upon working conditions and soon reaches a stable form. Upon visible-light illumination, the material yields a photocurrent of 1.3 mA cm^–2 at a potential of 0.2 V vs a reversible hydrogen electrode at mild pH under illumination by AM 1.5 G and retains 30% of its photoactivity after 6 h. This represents an unprecedented result for a nonprotected Cu oxide photocathode at neutral pH. The photocurrent efficiency as a function of the applied potential was determined using scanning electrochemical microscopy. The material was characterized in terms of photoelectrochemical features; X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, fixed-energy X-ray absorption voltammetry, and extended X-ray absorption fine structure analyses were carried out on pristine and used samples, which were used to explain the photoelectrochemical behavior. The optical features of the oxide are evidenced by direct reflectance spectroscopy and fluorescence spectroscopy, and Mott–Schottky analysis at different pH values explains the exceptional activity at neutral pH

Easy Accommodation of Different Oxidation States in Iridium Oxide Nanoparticles with Different Hydration Degree as Water Oxidation Electrocatalysts

In this paper, we present a comprehensive study on low hydration Ir/IrO₂ electrodes, made of an Ir core and an IrO₂ shell, that are designed and synthesized with an innovative, green approach, in order to have a higher surface/bulk ratio of Ir–O active centers. Three materials with different hydration degrees have been deeply investigated in terms of structure and microstructure by means of transmission electron microscopy (TEM) and synchrotron radiation techniques such as high-resolution (HR) and pair distribution function (PDF) quality X-ray powder diffraction (XRPD), X-ray absorption spectroscopy (XAS), and for what concerns their electrochemical properties by means of cyclic voltammetry and steady-state <i>I</i>/<i>E</i> curves. The activity of these materials is compared and discussed in the light of our most recent results on hydrous IrO_<i>x</i>. The main conclusion of this study is that the Ir core is noninteracting with the IrO_<i>x</i> shell, the latter being able to easily accommodate Ir in different oxidation states, as previously suggested for the hydrated form, thus explaining the activity as electrocatalysts. In addition, in operando XAS experiments assessed that the catalytic cycle involves Ir­(III) and (V), as previously established for the highly hydrated IrO_<i>x</i> material