Au-decorated Sb2Se3 photocathodes for solar-driven CO2 reduction.

Photoelectrodes with FTO/Au/Sb2Se3/TiO2/Au architecture were studied in photoelectrochemical CO2 reduction reaction (PEC CO2RR). The preparation is based on a simple spin coating technique, where nanorod-like structures were obtained for Sb2Se3, as confirmed by SEM images. A thin conformal layer of TiO2 was coated on the Sb2Se3 nanorods via ALD, which acted as both an electron transfer layer and a protective coating. Au nanoparticles were deposited as co-catalysts via photo-assisted electrodeposition at different applied potentials to control their growth and morphology. The use of such architectures has not been explored in CO2RR yet. The photoelectrochemical performance for CO2RR was investigated with different Au catalyst loadings. A photocurrent density of ∼7.5 mA cm-2 at -0.57 V vs. RHE for syngas generation was achieved, with an average Faradaic efficiency of 25 ± 6% for CO and 63 ± 12% for H2. The presented results point toward the use of Sb2Se3-based photoelectrodes in solar CO2 conversion applications.

Solution Combustion Synthesis and Characterization of Magnesium Copper Vanadates.

Magnesium vanadate (MgV2O6) and its alloys with copper vanadate were synthesized via the solution combustion technique. Phase purity and solid solution formation were confirmed by a variety of experimental techniques, supported by electronic structure simulations based on density functional theory (DFT). Powder X-ray diffraction combined with Rietveld refinement, laser Raman spectroscopy, diffuse reflectance spectroscopy, and high-resolution transmission electron microscopy showed single-phase alloy formation despite the MgV2O6 and CuV2O6 end members exhibiting monoclinic and triclinic crystal systems, respectively. DFT-calculated optical band gaps showed close agreement in the computed optical bandgaps with experimentally derived values. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements confirmed a systematic variation in the optical bandgap modification and band alignment as a function of stoichiometry in the alloy composition. These data indicated n-type semiconductor behavior for all the samples which was confirmed by photoelectrochemical measurements.

Visible Light-Generated Antiviral Effect on Plasmonic Ag-TiO2-Based Reactive Nanocomposite Thin Film.

The recent coronavirus pandemic pointed out the vulnerability of humanity to new emerging infectious diseases. Experts warn that future pandemics may emerge more frequently with greater devastating effects on population health and the world economy. Although viruses are unable to propagate on lifeless surfaces, they can retain their infectivity and spread further on contact with these surfaces. The objective of our study is to analyze photoreactive composite films that exert antiviral effects upon illumination. Reactive plasmonic titanium dioxide-based polymeric nanocomposite film was prepared with a thickness of 1-1.5 µm, which produces reactive oxygen species (ROS) under visible light irradiation (λ ≥ 435 nm). These species are suitable for photooxidation of adsorbed organic molecules (e.g., benzoic acid) on the nanocomposite surface. Moreover, high molecular weight proteins are also degraded or partially oxidized in this process on the composite surface. Since the Ag0-TiO2/polymer composite film used showed excellent reactivity in the formation of OH• radicals, the photocatalytic effect on high molecular weight (M = ∼66.000 Da) bovine serum albumin (BSA) protein was investigated. Given that changes in the structure of the protein were observed upon exposure to light, we assumed virucidal effect of the illuminated photoreactive composite film. We tested this hypothesis using an airborne-transmitted herpesvirus. As a result, we obtained a drastic decrease in infection capability of the virus on the photoreactive surface compared to the control surface.

Structural Features Dictate the Photoelectrochemical Activities of Two-Dimensional MoSe2 and WSe2 Nanostructures.

The exfoliation of layered materials into two-dimensional (2D) semiconductors creates new structural domains, for example, basal planes, defect-rich in-planes, and edge sites. These surface species affect the photoelectrochemical (PEC) performance, which in turn determines their applicability in solar energy conversion technologies. In this study, a custom-designed microdroplet cell-based spatially resolved PEC approach was employed to identify the structural parts and to measure the PEC activity of the mechanically exfoliated MoSe2 and WSe2 nanosheets for bulk, few-layer, and monolayer specimens. The PEC performance decreased with the decreasing thickness of nanoflakes, and the relative PEC activity (photo/total current) reduced by introducing more defects to the 2D flakes: 1-3% loss was found for in-plane defects and 30-40% for edge defects. While edge sites act as charge carrier recombination centers, their electrocatalytic activity is higher than that of the basal planes. The comparison of PEC activity of micromechanically and liquid phase exfoliated bulk and few-layer MoSe2 and WSe2 flakes further confirmed that the PEC performance of 2D flakes decreases with an increasing number of edge sites.

Poly(methyl methacrylate)-Assisted Exfoliation of Graphite and Its Use in Acrylonitrile-Butadiene-Styrene Composites.

One of the applications of graphene in which its scalable production is of utmost importance is the development of polymer composites. Among the techniques used to produce graphene flakes, the liquid-phase exfoliation (LPE) of graphite stands out due to its versatility and scalability. However, solvents suitable for the LPE process are generally toxic and have a high boiling point, making the processing challenging. The use of low boiling point solvents could be convenient for the processing, due to the easiness of their removal. In this study, the use of poly(methyl methacrylate) (PMMA) as a stabilizing agent is proposed for the production of graphene flakes in a low boiling point solvent, that is, acetone. The graphene dispersions produced in the mixture acetone-PMMA have higher concentration, +175 %, and contain a higher percentage of few-layer graphene flakes (<5 layers), that is, +60 %, compared to the dispersions prepared in acetone. The as-produced graphene dispersions are used to develop graphene/acrylonitrile-butadiene-styrene composites. The mechanical properties of the pristine polymer are improved, that is, +22 % in the Young's modulus, by adding 0.01 wt. % of graphene flakes. Moreover, a decrease of ≈20 % in the oxygen permeability is obtained by using 0.1 wt. % of graphene flakes filler, compared to the unloaded matrix.

Electron Tunneling through Boron Nitride Confirms Marcus-Hush Theory Predictions for Ultramicroelectrodes.

Marcus-Hush theory of electron transfer is one of the pillars of modern electrochemistry with a large body of supporting experimental evidence presented to date. However, some predictions, such as the electrochemical behavior at disk ultramicroelectrodes, remain unverified. Herein, we present a study of electron tunneling across a hexagonal boron nitride acting as a barrier between a graphite electrode and redox mediators in a liquid solution. This was achieved by the fabrication of disk ultramicroelectrodes with a typical diameter of 5 µm. Analysis of voltammetric measurements, using two common outer-sphere redox mediators, yielded several electrochemical parameters, including the electron transfer rate constant, limiting current, and transfer coefficient. They depart significantly from the Butler-Volmer kinetics and instead show behavior previously predicted by the Marcus-Hush theory of electron transfer. In addition, our system provides a noteworthy experimental platform, which could be applied to address a number of scientific problems such as identification of reaction mechanisms, surface modification, or long-range electron transfer.

Liquid-Phase Exfoliated Indium-Selenide Flakes and Their Application in Hydrogen Evolution Reaction.

Single- and few-layered InSe flakes are produced by the liquid-phase exfoliation of ß-InSe single crystals in 2-propanol, obtaining stable dispersions with a concentration as high as 0.11 g L-1 . Ultracentrifugation is used to tune the morphology, i.e., the lateral size and thickness of the as-produced InSe flakes. It is demonstrated that the obtained InSe flakes have maximum lateral sizes ranging from 30 nm to a few micrometers, and thicknesses ranging from 1 to 20 nm, with a maximum population centered at ≈5 nm, corresponding to 4 Se-In-In-Se quaternary layers. It is also shown that no formation of further InSe-based compounds (such as In2 Se3 ) or oxides occurs during the exfoliation process. The potential of these exfoliated-InSe few-layer flakes as a catalyst for the hydrogen evolution reaction (HER) is tested in hybrid single-walled carbon nanotubes/InSe heterostructures. The dependence of the InSe flakes' morphologies, i.e., surface area and thickness, on the HER performances is highlighted, achieving the best efficiencies with small flakes offering predominant edge effects. The theoretical model unveils the origin of the catalytic efficiency of InSe flakes, and correlates the catalytic activity to the Se vacancies at the edge of the flakes.

Enhanced Photoelectrochemical Performance of Cuprous Oxide/Graphene Nanohybrids.

Combination of an oxide semiconductor with a highly conductive nanocarbon framework (such as graphene or carbon nanotubes) is an attractive avenue to assemble efficient photoelectrodes for solar fuel generation. To fully exploit the possible synergies of the hybrid formation, however, precise knowledge of these systems is required to allow rational design and morphological engineering. In this paper, we present the controlled electrochemical deposition of nanocrystalline p-Cu2O on the surface of different graphene substrates. The developed synthetic protocol allowed tuning of the morphological features of the hybrids as deduced from electron microscopy. (Photo)electrochemical measurements (including photovoltammetry, electrochemical impedance spectroscopy, photocurrent transient analysis) demonstrated better performance for the 2D graphene containing photoelectrodes, compared to the bare Cu2O films, the enhanced performance being rooted in suppressed charge carrier recombination. To elucidate the precise role of graphene, comparative studies were performed with carbon nanotube (CNT) films and 3D graphene foams. These studies revealed, after allowing for the effect of increased surface area, that the 3D graphene substrate outperformed the other two nanocarbons. Its interconnected structure facilitated effective charge separation and transport, leading to better harvesting of the generated photoelectrons. These hybrid assemblies are shown to be potentially attractive candidates in photoelectrochemical energy conversion schemes, namely CO2 reduction.

Exfoliation of natural van der Waals heterostructures to a single unit cell thickness.

Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeite's layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeite's electronic band structure to rationalize the experimental results.

Electrochemical Investigation of Adsorption of Single-Wall Carbon Nanotubes at a Liquid/Liquid Interface.

There is much interest in understanding the interfacial properties of carbon nanotubes, particularly at water/oil interfaces. Here, the adsorption of single-wall carbon nanotubes (SWCNTs) at the water/1,2-dichloroethane (DCE) interface, and the subsequent investigation of the influence of the adsorbed nanotube layer on interfacial ion transfer, is studied by using the voltammetric transfer of tetramethylammonium (TMA+) and hexafluorophosphate (PF6-) as probe ions. The presence of the interfacial SWCNT layer significantly suppresses the transfer of both ions across the interface, with a greater degree of selectivity towards the PF6- ion. This effect was attributed both to the partial blocking of the interface by the SWCNTs and to the potential dependant adsorption of background electrolyte ions on the surface of the SWCNTs, as confirmed by X-ray photoelectron spectroscopy, which is caused by an electrostatic interaction between the interfacial SWCNTs and the transferring ion.

Asymmetric MoS2 /Graphene/Metal Sandwiches: Preparation, Characterization, and Application.

The polarizable organic/water interface is used to construct MoS2 /graphene nanocomposites, and various asymmetrically dual-decorated graphene sandwiches are synthesized. High-resolution transmission electron microscopy and 3D electron tomography confirm their structure. These dual-decorated graphene-based hybrids show excellent hydrogen evolution activity and promising capacitance performance.

Photoelectrochemistry of Pristine Mono- and Few-Layer MoS2.

Two-dimensional crystals are promising building blocks for the new generation of energy materials due to their low volume, high surface area, and high transparency. Electrochemical behavior of these crystals determines their performance in applications such as energy storage/conversion, sensing, and catalysis. Nevertheless, the electrochemistry of an isolated monolayer of molybdenum disulfide, which is one of the most promising semiconducting crystals, has not been achieved to date. We report here on photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS2, namely the electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization. This enables a comparative qualitative correlation among the electrochemical data, MoS2 structure, and external illumination, although the absolute magnitudes of the electron transfer and capacitance are specific to the redox mediator and electrolyte used in these measurements ([Ru(NH3)6](3+/2+) and LiCl, respectively). Our work shows a strong dependence of the electrochemical properties on the number of MoS2 layers and illumination intensity and proves that an effective interlayer charge transport occurs in bulk MoS2. This highlights the exciting opportunities for tuning of the electrochemical performance of MoS2 through modification of its structure, external environment, and illumination.

Electron transfer kinetics on natural crystals of MoS2 and graphite.

Here, we evaluate the electrochemical performance of sparsely studied natural crystals of molybdenite and graphite, which have increasingly been used for fabrication of next generation monolayer molybdenum disulphide and graphene energy storage devices. Heterogeneous electron transfer kinetics of several redox mediators, including Fe(CN)6(3-/4-), Ru(NH3)6(3+/2+) and IrCl6(2-/3-) are determined using voltammetry in a micro-droplet cell. The kinetics on both materials are studied as a function of surface defectiveness, surface ageing, applied potential and illumination. We find that the basal planes of both natural MoS2 and graphite show significant electroactivity, but a large decrease in electron transfer kinetics is observed on atmosphere-aged surfaces in comparison to in situ freshly cleaved surfaces of both materials. This is attributed to surface oxidation and adsorption of airborne contaminants at the surface exposed to an ambient environment. In contrast to semimetallic graphite, the electrode kinetics on semiconducting MoS2 are strongly dependent on the surface illumination and applied potential. Furthermore, while visibly present defects/cracks do not significantly affect the response of graphite, the kinetics on MoS2 systematically accelerate with small increase in disorder. These findings have direct implications for use of MoS2 and graphene/graphite as electrode materials in electrochemistry-related applications.

Novel organic solvents for electrochemistry at the liquid/liquid interface.

Two-phase voltammetry has been carried out using a reverse cell configuration, i.e. with the lower density organic solvent on the top of the aqueous solution in the cell, where the organic solvents contain either nitrile or ketone functional groups. The transfer of tetraphenylarsonium TPAs(+), tetrabutylammonium TBA(+), tetrapropylammonium TPrA(+), and perchlorate CIO4(-) ions across these liquid/liquid interfaces has been observed. The standard Gibbs energies of ion partition from water to di-n-butyl ketone (5-nonanone) were calculated and compared with the previously reported 2-heptanone/water interface. Ion transfer (IT) and electron transfer (ET) were also investigated at the 5-nonanone/water interface. ET was exemplified using the ferri/ferrocyanide redox couple as the aqueous phase couple and the 7,7,8,8-tetracyanoquinodimethane (TCNQ) as the organic species.

Functionalization of graphene at the organic/water interface.

A simple method for the deposition of noble metal (Pd, Au) nanoparticles on a free-standing chemical vapour deposited graphene (CVD GR) monolayer is reported. The method consists of assembling the high purity CVD GR, by transfer from poly (methyl methacrylate) (PMMA), at the organic/water interface. Metal deposition can then proceed using either spontaneous or electrochemically-controlled processes. The resultant graphene-based metal nanoclusters are characterized using atomic force and electron microscopy techniques, and the location of the nanostructures underneath the graphene layer is determined from the position and the intensity changes of the Raman bands (D, G, 2D). This novel process for decoration of a single-layer graphene sheet with metal nanoparticles using liquid/liquid interfaces opens an alternative and useful way to prepare low dimensional carbon-based nanocomposites and electrode materials.

Electron transfer kinetics on mono- and multilayer graphene.

Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized. Here we use voltammetric determination of the electron transfer rate for three redox mediators, ferricyanide, hexaammineruthenium, and hexachloroiridate (Fe(CN)(6)(3-), Ru(NH3)(6)(3+), and IrCl(6)(2-), respectively), to measure the reactivity of graphene samples prepared by mechanical exfoliation of natural graphite. Electron transfer rates are measured for varied number of graphene layers (1 to ca. 1000 layers) using microscopic droplets. The basal planes of mono- and multilayer graphene, supported on an insulating Si/SiO(2) substrate, exhibit significant electron transfer activity and changes in kinetics are observed for all three mediators. No significant trend in kinetics with flake thickness is discernible for each mediator; however, a large variation in kinetics is observed across the basal plane of the same flakes, indicating that local surface conditions affect the electrochemical performance. This is confirmed by in situ graphite exfoliation, which reveals significant deterioration of initially, near-reversible kinetics for Ru(NH3)(6)(3+) when comparing the atmosphere-aged and freshly exfoliated graphite surfaces.

On the unexpected cation exchange behavior, caused by covalent bond formation between PEDOT and Cl- ions: extending the conception for the polymer-dopant interactions.

The ionic motion in connection with the redox transformation of poly(3,4-ethylenedioxythiophene) (PEDOT) conjugated polymer have been studied by both experimental-electrochemical (electrochemical quartz crystal nanobalance, EQCN) and spectroscopic (infrared spectroscopy, IR-ATR)-and theoretical methods. The observations have been completed by direct, semiquantitative analytical data, provided by energy dispersive X-ray (EDX) microanalysis. The EQCN results suggested an anomalous behavior, since only cationic movements have been observed for films deposited from chloride solutions. Chloride ions were proved to be immobile also when bulky tetrabutylammonium (Bu(4)N(+)) cations were substituted with even larger (hexadecyltrimethylammonium) cations. Since PEDOT films synthesized in the presence of other spherical, not too large anions-such as perchlorate and tetrafluoroborate-endowed mixed ion exchange behavior together with the Bu(4)N(+) cation, the possibility of a special interaction between chloride and the polymeric chain has been assumed. Semiempirical and DFT calculations indicated that chloride ions interact with the α carbon atoms of the thiophene rings of the oxidized EDOT oligomers, creating sp(3) type perturbations in the polymer chain. FTIR-ATR spectra evidenced the appearance of C-Cl bonds. Elementary analysis, performed by EDX spectroscopy with eight polymer samples at different doping levels clearly showed the permanent presence of constant amount of chlorine, independently of the oxidation state of the PEDOT layer. Finally, the presented observations call attention to the fact that unique dopant-polymer interactions during the electrochemical polymerization are of prime importance, being able to rule over conventions for the charge compensation of conjugated polymers, often solely based on steric parameters.