Charge Transfer Dynamics in Organic-Inorganic Hybrid Heterostructures-Insights by Vibrational-Sum Frequency Generation Spectroscopy.

Organic-inorganic heterostructures play a pivotal role in modern electronic and optoelectronic applications including photodetectors and field effect transistors, as well as in solar energy conversion such as photoelectrodes of dye-sensitized solar cells, photoelectrochemical cells, and in organic photovoltaics. To a large extent, performance of such devices is controlled by charge transfer dynamics at and across (inner) interfaces, e.g., between a wide band gap semiconductor and molecular sensitizers and/or catalysts. Hence, a detailed understanding of the structure-dynamics-function relationship of such functional interfaces is necessary to rationalize possible performance limitations of these materials and devices on a molecular level. Vibrational sum-frequency generation (VSFG) spectroscopy, as an interface-sensitive spectroscopic technique, allows to obtain chemically specific information from interfaces and combines such chemical insights with ultrafast time resolution, when integrated as a spectroscopic probe into a pump-probe scheme. Thus, this minireview discusses the advantages and potential of VSFG spectroscopy for investigating interfacial charge transfer dynamics and structural changes at inner interfaces. A critical perspective of the unique spectroscopic view of otherwise inaccessible interfaces is presented, which we hope opens new opportunities for an improved understanding of function-determining processes in complex materials, and brings together communities who are devoted to designing materials and devices with spectroscopists.

Studying Molecular Rearrangement of P1 Dye at a Passivating Alumina Surface Using Vibrational Sum-Frequency Generation Spectroscopy: Effect of Atomic-Level Roughness.

The effect of roughness and thickness of alumina layers, mimicking the passivation layer commonly used in dye-sensitized photoelectrodes, on the molecular adsorption of P1 dye, 4-(bi(4-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl]-aminobenzoic acid) has been studied using surface-sensitive vibrational sum frequency generation(VSFG) spectroscopy. The VSFG spectra reveal the formation of poorly ordered dye layers on relatively rough surfaces where XPS measures a higher dye loading. Furthermore, these poorly ordered dye molecules are responsible for the generation of trapped electronic states as probed by successive photoluminescence (PL) measurements. Surface sensitive VSFG spectroscopy in combination with XPS and PL measurements provide complementary spectral information on ordering of the adsorbed dyes, their density on the surface and electronic states of the adsorbed monolayer which are prerequisite for improving our understanding of molecularly functionalized photoelectrodes and their further development.

A Happy Get-Together - Probing Electrochemical Interfaces by Non-Linear Vibrational Spectroscopy.

Electrochemical interfaces are key structures in energy storage and catalysis. Hence, a molecular understanding of the active sites at these interfaces, their solvation, the structure of adsorbates, and the formation of solid-electrolyte interfaces are crucial for an in-depth mechanistic understanding of their function. Vibrational sum-frequency generation (VSFG) spectroscopy has emerged as an operando spectroscopic technique to monitor complex electrochemical interfaces due to its intrinsic interface sensitivity and chemical specificity. Thus, this review discusses the happy get-together between VSFG spectroscopy and electrochemical interfaces. Methodological approaches for answering core issues associated with the behavior of adsorbates on electrodes, the structure of solvent adlayers, the transient formation of reaction intermediates, and the emergence of solid electrolyte interphase in battery research are assessed to provide a critical inventory of highly promising avenues to bring optical spectroscopy to use in modern material research in energy conversion and storage.

Not that innocent - ammonium ions boost homogeneous light-driven hydrogen evolution.

We report that the homogeneous light-driven hydrogen evolution reaction (HER) can be significantly enhanced by the presence of seemingly innocent ammonium (NH4+) cations. Experimental studies with different catalysts, photosensitizers and electron donors show this to be a general effect. Preliminary photophysical and mechanistic studies provide initial suggestions regarding the role of ammonium in the HER enhancement.

An Anthracene-Based Metal-Organic Framework for Selective Photo-Reduction of Carbon Dioxide to Formic Acid Coupled with Water Oxidation.

A Zr-based metal-organic framework has been synthesized and employed as a catalyst for photochemical carbon dioxide reduction coupled with water oxidation. The catalyst shows significant carbon dioxide reduction property with concomitant water oxidation. The catalyst has broad visible light as well as UV light absorption property, which is further confirmed from electronic absorption spectroscopy. Formic acid was the only reduced product from carbon dioxide with a turn-over frequency (TOF) of 0.69 h-1 in addition to oxygen, which was produced with a TOF of 0.54 h-1 . No external photosensitizer is used and the ligand itself acts as the light harvester. The efficient and selective photochemical carbon dioxide reduction to formic acid with concomitant water oxidation using Zr-based MOF as catalyst is thus demonstrated here.

Electrocatalytic Reduction of CO2 to Acetic Acid by a Molecular Manganese Corrole Complex.

The controlled electrochemical reduction of carbon dioxide to value added chemicals is an important strategy in terms of renewable energy technologies. Therefore, the development of efficient and stable catalysts in an aqueous environment is of great importance. In this context, we focused on synthesizing and studying a molecular MnIII -corrole complex, which is modified on the three meso-positions with polyethylene glycol moieties for direct and selective production of acetic acid from CO2 . Electrochemical reduction of MnIII leads to an electroactive MnII species, which binds CO2 and stabilizes the reduced intermediates. This catalyst allows to electrochemically reduce CO2 to acetic acid in a moderate acidic aqueous medium (pH 6) with a selectivity of 63 % and a turn over frequency (TOF) of 8.25 h-1 , when immobilized on a carbon paper (CP) electrode. In terms of high selectivity towards acetate, we propose the formation and reduction of an oxalate type intermediate, stabilized at the MnIII -corrole center.

Molecular cobalt corrole complex for the heterogeneous electrocatalytic reduction of carbon dioxide.

Electrochemical conversion of CO2 to alcohols is one of the most challenging methods of conversion and storage of electrical energy in the form of high-energy fuels. The challenge lies in the catalyst design to enable its real-life implementation. Herein, we demonstrate the synthesis and characterization of a cobalt(III) triphenylphosphine corrole complex, which contains three polyethylene glycol residues attached at the meso-phenyl groups. Electron-donation and therefore reduction of the cobalt from cobalt(III) to cobalt(I) is accompanied by removal of the axial ligand, thus resulting in a square-planar cobalt(I) complex. The cobalt(I) as an electron-rich supernucleophilic d8-configurated metal centre, where two electrons occupy and fill up the antibonding dz2 orbital. This orbital possesses high affinity towards electrophiles, allowing for such electronically configurated metals reactions with carbon dioxide. Herein, we report the potential dependent heterogeneous electroreduction of CO2 to ethanol or methanol of an immobilized cobalt A3-corrole catalyst system. In moderately acidic aqueous medium (pH = 6.0), the cobalt corrole modified carbon paper electrode exhibits a Faradaic Efficiency (FE%) of 48 % towards ethanol production.