Photophysics and electrochemistry relevant to photocatalytic water splitting involved at solid–electrolyte interfaces

Direct photon to chemical energy conversion using semiconductor–electrocatalyst–electrolyte interfaces has been extensively investigated for more than a half century. Many studies have focused on screening materials for efficient photocatalysis. Photocatalytic efficiency has been improved during this period but is not sufficient for industrial commercialization. Detailed elucidation on the photocatalytic water splitting process leads to consecutive six reaction steps with the fundamental parameters involved: The photocatalysis is initiated involving photophysics derived from various semiconductor properties(1: photon absorption, 2: exciton separation). The generated charge carriers need to be transferred to surfaces effectively utilizing the interfaces(3: carrier diffusion, 4: carrier transport). Consequently, electrocatalysis finishes the process by producing products on the surface(5: catalytic efficiency, 6: mass transfer of reactants and products). Successful photocatalytic water splitting requires the enhancement of efficiency at each stage. Most critically, a fundamental understanding of the interfacial phenomena is highly desired for establishing "photocatalysis by design" concepts, where the kinetic bottleneck within a process is identified by further improving the specific properties of photocatalytic materials as opposed to blind material screening. Theoretical modeling using the identified quantitative parameters can effectively predict the theoretically attainable photon-conversion yields. This article provides an overview of the state-of-the-art theoretical understanding of interfacial problems mainly developed in our laboratory.Photocatalytic water splitting(especially hydrogen evolution on metal surfaces) was selected as a topic,and the photophysical and electrochemical processes that occur at semiconductor–metal, semiconductor–electrolyte and metal–electrolyte interfaces are discussed.     

Electrochemically fabricated molecule–electrode contacts for molecular electronics

Connecting molecules to electrodes is key for a range of applications. Conventional methods typically involve a spontaneous reaction of thiol/disulfide-terminated molecules with metal surfaces. Although modifying metal surfaces with thiol chemistry is simple, it is limited to forming a specific S–metal bonding, which is labile and hence there are concerns regarding its mechanical instability. In addition, spontaneous grafting requires long processing times to achieve high molecular coverages on the surface, which adds challenges for manufacturing devices comprising molecular films. Electrochemical methods for forming molecular films on surfaces offer powerful advantages over traditional methods, including reaction acceleration, molecular coverage control, and guiding the chemical bonding at the molecule?electrode interface. Electrochemical grafting enables connecting molecules to various types of electrodes including those that cannot be functionalized by other methods. More recently, electrochemical approaches were expanded to enable connecting 2D materials to electrodes, opening a realm of possibilities for hybrid technologies. In this opinion, we survey the recent progress in electrochemical methods for connecting (bio) molecules to electrodes for advancing molecular and bioelectronics.     

Fat crystallisation at oil–water interfaces

This review focuses on recent advances in the understanding of lipid crystallisation at or in the vicinity of an interface in emulsified systems and the consequences regarding stability, structure and thermal behaviour. Amphiphilic molecules such as emulsifiers are preferably adsorbed at the interface. Such molecules are known for their ability to interact with triglycerides under certain conditions. In the same manner that inorganic crystals grown on an organic matrix see their nucleation, morphology and structure controlled by the underlying matrix, recent studies report a templating effect linked to the presence of emulsifiers at the oil/water interface. Emulsifiers affect fat crystallisation and fat crystal behaviour in numerous ways, acting as impurities seeding nucleation and, in some cases, retarding or enhancing polymorphic transitions towards more stable forms. This understanding is of crucial importance for the design of stable structures within emulsions, regardless of whether the system is oil or water continuous. In this paper, crystallisation mechanisms are briefly described, as well as recent technical advances that allow the study of crystallisation and crystal forms. Indeed, the study of the interface and of its effect on lipid crystallisation in emulsions has been limited for a long time by the lack of in-situ investigative techniques. This review also highlights reported interfacial effects in food and pharmaceutical emulsion systems. These effects are strongly linked to the presence of emulsifiers at the interface and their effects on crystallisation kinetics, and crystal morphology and stability.     

Molecular simulation of protein adsorption and conformation at gas-liquid,liquid–liquid and solid–liquid interfaces

The adsorption of proteins at surfaces and interfaces is important in a wide range of industries. Understanding and controlling the conformation of adsorbed proteins at surfaces is critical to stability and function in many technological applications including foods and biomedical testing kits or sensors. Studying adsorbed protein conformation is difficult experimentally and so over the past few decades researchers have turned to computer simulation methods to give information at the atomic level on this important area. In this review we summarize some of the significant simulation work over the past four years at both fluid (liquid–liquid and gas–liquid interfaces) and solid–liquid interfaces. Of particular significance is the work on surfactant proteins such as fungal hydrophobins, ranspumin-2 from the túngara frog and the bacteria protein BslA. These have evolved unique structures impart very high surface-active properties to the molecules. A highlight is the elucidation of the clam-shell unhinging mechanism of ranspumin-2 adsorption to the gas–liquid interface that is responsible for its adsorption to and stabilization of the air bubbles in túngara frog foam nests.     

Molecular electronics—the future of bioelectronics

In the last decade, molecular electronics, as an active frontier of interdisciplinary research areas, has become one of the most rapidly developing fields, and attracted worldwide interests. The fundamental element of molecular electronics is a molecular device or a supramolecular device, which is an organized molecular system constructed mainly by organic molecules or biomolecules that have some specific functions in signal detection, process, storage, and transmission through chemical or physical interactions at molecular or supramolecular levels. A molecular device (MD) can involve chemical information processes, and be relatively easy to realize a large number of links between the molecules. The links can be controlled by the external signals. These are the expected features of molecular computing and directly involve chemical and biological processes. MD may overcome some limitations of the solid-state chips, and can be directly applied to chemical and biological processes. Molecular electronics is a part of bioelectronics. It will play an important and revolutionary role in the next century. This paper intends to review the research activities of molecular electronics in China, particularly in LMBE.     

Enhanced CO2 electrolysis at metal–oxide interfaces

Journal of Solid State Electrochemistry - The catalytic reduction of CO2 to CO using solid oxide electrolytic cells (SOECs) is considered as a sustainable solution to simultaneously remove...     

Novel technique to form electrode–electrolyte nanointerface in all-solid-state rechargeable lithium batteries

The electrode–electrolyte nanocomposites, where the nano-sized NiS electrode with large capacity was embedded in the 80Li₂S · 20P₂S₅ electrolyte with high Li⁺ conductivity, were successfully prepared by the mechanochemical method. Contact area of solid–solid interface between the electrode and the electrolyte was remarkably increased in the nanocomposites. All-solid-state cell using the nanocomposites as a working electrode exhibited larger capacity and better cycling performance than the cell using the electrode obtained by conventional hand-mixing of powders. The mechanochemical technique sheds light on a new formation process of electrode–electrolyte interfaces endowing solid-state batteries with high power density and high energy density.     

Monolayers at solid–solid interfaces probed with infrared spectroscopy

The sensitivities of infrared spectra of thin adsorbate layers measured in either transmission, internal reflection or external reflection can be greatly increased if a light incidence medium with a high refractive index such as an IR-transparent solid material is used. This increase in sensitivity is due to the strong enhancement of the perpendicular electric field in a thin layer of low refractive index sandwiched between two high refractive index materials. Based on model calculations of a hypothetical sample layer, the influence and optimization of experimental parameters such as incidence angle, sample layer thickness and optical contact between layers are investigated. Under optimized conditions, this enhancement can exceed a factor of 100 when compared to conventional surface IR techniques. In addition, the spectra of sandwiched sample layers are governed by a uniform surface selection rule, such that only the perpendicular vibrational components are enhanced, and they permit a straightforward, substrate-independent analysis of surface orientations. Experimental examples of monolayer spectra of long-chain hydrocarbon compounds adsorbed onto gold and silicon substrates and contacted with a germanium crystal used as the incidence medium demonstrate the simple experimental realization and unprecedented sensitivity of this sandwich technique, and they offer novel insights into the chemistry and structure of monolayers confined and compressed between two solid surfaces. Figure IR reflection spectrum of a monolayer of a fatty acid methyl ester sandwiched between silicon and germanium.     

Ovalbumin at oil–water interfaces: Adsorption and emulsification

In this article, we study the adsorption of protein ovalbumin (OVA) at corn oil (CO), soybean oil (SBO), olive oil (OO), and water interfaces along with the emulsification of these oils in water. The dynamic interfacial tension (IFT) measurements show a reduction in IFT in the order SBO–water?～?CO–water?>?OO–water, with OVA adsorption being dominated by the free diffusion of OVA at the interfaces. CO–water, OO–water, and SBO–water emulsions cream with time. The cream phase consists of jammed closed-packed oil droplets due to depletion-induced inter-droplet attractions with higher G′ and G″ (～700?Pa) for emulsions with 1?wt% OVA.     

Metal—solvent interaction at electrode/solution interfaces reactivity scale in different solvents and photoemission threshold

A reactivity scale is built up for sp-metals on the basis of the relationship between the potential of zero charge and the electron work function in a vacuum. The latter is derived “in situ” from charge-potential curves as obtained by integration of different capacity data. Increase in solvophilicity with decreasing work function is observed. The reactivity scale is qualitatively preserved in different solvents. It is shown that the rate of solvent reorientation with field and the solvophilicity run parallel so that a quantitative relationship can be established in different solvents between the reciprocal of the experimental differential capacity at the potential of zero charge and the solvophilicity as expressed by the enthalpy of oxide formation. The relation between the potential of zero charge and the electron work function in polar solvents is also discussed and the concepts are applied to literature data for aqueous solutions. From these, the energy of delocalized electrons in water is derived and its quantitative significance discussed in relation to the procedure of obtainment of the photoemission data.     

Measuring the optical chirality of molecular aggregates at liquid–liquid interfaces

Some new experimental methods for measuring the optical chirality of molecular aggregates formed at liquid–liquid interfaces have been reviewed. Chirality measurements of interfacial aggregates are highly important not only in analytical spectroscopy but also in biochemistry and surface nanochemistry. Among these methods, a centrifugal liquid membrane method was shown to be a highly versatile method for measuring the optical chirality of the liquid–liquid interface when used in combination with a commercially available circular dichroism (CD) spectropolarimeter, provided that the interfacial aggregate exhibited a large molar absorptivity. Therefore, porphyrin and phthalocyanine were used as chromophoric probes of the chirality of itself or guest molecules at the interface. A microscopic CD method was also demonstrated for the measurement of a small region of a film or a sheet sample. In addition, second-harmonic generation and Raman scattering methods were reviewed as promising methods for detecting interfacial optical molecules and measuring bond distortions of chiral molecules, respectively.     

Size-dependent electrophoretic motion of polystyrene particles at polyethylene glycol–dextran interfaces

Currently, there is very limited information on the electrophoretic behavior of particles at a liquid–liquid interface formed by two conducting liquid solutions. Here, electrophoretic velocities of polystyrene particles at a polyethylene glycol (PEG)–dextran (DEX) interface were investigated in this paper. Experimental results show that the particle at the interface moves in the opposite direction to the applied electric field, with a velocity much lower than that in the PEG-rich phase and a litter larger than that in the DEX-rich phase. Similarly to the movement in Newtonian fluids, the velocity increases linearly with the increase in the applied electric field. Different to particle electrophoresis in Newtonian fluids, the velocities of the particles at the PEG–DEX interface increase linearly with the decrease in particle's diameters, implying a possible size-based particle differentiation at an interface.     

Molecular and colloidal self-assembly at the oil–water interface

Self-assembly is a versatile bottom-up approach for fabricating novel supramolecular materials with well-defined nano- or micro-structures associated with functionalities. The oil-water interface provides an ideal venue for molecular and colloidal self-assembly. This paper gives an overview of various self-assembled materials, including nanoparticles, polymers, proteins, and lipids, at the oil-water interface. Focus has been given to fundamental principles and strategies for engineering the self-assembly process, such as control of pH, ionic strength and use of external fields, to achieve complex soft materials with desired functionalities, such as nanoparticle surfactants, structured liquids, and proteinosomes. It has been shown that self-assembly at the oil-water interface holds great promise for developing well-structured complex materials useful for many research and industrial applications.     

Voltammetry at a liquid–liquid interface supported on a metallic electrode

A liquid–liquid interface supported on a metallic electrode has been used to study ion transfer (IT) and electron transfer (ET) reactions by cyclic voltammetry. The system is composed of an aqueous droplet supported on a platinum disc electrode and immersed into an organic electrolyte solution. Depending on the nature of the dissolved species present in the aqueous solution, and in the organic electrolyte solution, different electrochemical coupled reactions can be observed. This method enables a fast and convenient method to measure standard transfer potentials for example of ionised drug molecules.     

A bottom-up approach to understanding protein layer formation at solid–liquid interfaces

A common goal across different fields (e.g. separations, biosensors, biomaterials, pharmaceuticals) is to understand how protein behavior at solid–liquid interfaces is affected by environmental conditions. Temperature, pH, ionic strength, and the chemical and physical properties of the solid surface, among many factors, can control microscopic protein dynamics (e.g. adsorption, desorption, diffusion, aggregation) that contribute to macroscopic properties like time-dependent total protein surface coverage and protein structure. These relationships are typically studied through a top-down approach in which macroscopic observations are explained using analytical models that are based upon reasonable, but not universally true, simplifying assumptions about microscopic protein dynamics. Conclusions connecting microscopic dynamics to environmental factors can be heavily biased by potentially incorrect assumptions. In contrast, more complicated models avoid several of the common assumptions but require many parameters that have overlapping effects on predictions of macroscopic, average protein properties. Consequently, these models are poorly suited for the top-down approach. Because the sophistication incorporated into these models may ultimately prove essential to understanding interfacial protein behavior, this article proposes a bottom-up approach in which direct observations of microscopic protein dynamics specify parameters in complicated models, which then generate macroscopic predictions to compare with experiment. In this framework, single-molecule tracking has proven capable of making direct measurements of microscopic protein dynamics, but must be complemented by modeling to combine and extrapolate many independent microscopic observations to the macro-scale. The bottom-up approach is expected to better connect environmental factors to macroscopic protein behavior, thereby guiding rational choices that promote desirable protein behaviors.     

Electrodeposition of Zn–Mn alloys at high current densities from chloride electrolyte

The Zn–Mn alloy electrodeposition on a steel electrode in chloride electrolyte was investigated with the aim of obtaining deposits with as high as possible Mn percent. It was found that the deposition current density and concentration of Mn²⁺ ion in the chloride electrolyte significantly affect the Mn content in the alloy coating as well as the coating surface morphology. There was a transition from dendritic and spongy to smooth, bright, and amorphous structure of Zn–Mn deposits, when some critical deposition current density was reached, probably due to the metal oxyhydroxide inclusion in the coatings. Several plating additives were tested in order to decrease the hydroxide content and to improve surface appearance of the deposits. The 4-hydroxy-benzaldehyde was found to decrease oxygen and increase Mn percent in the coatings, and to significantly improve their surface morphology.