In this study, we report the room-temperature synthesis, detailed characterization, and first-principles modeling of monoclinic AgCuO2, a p-type transparent conducting oxide (TCO) candidate. AgCuO2 was synthesized via an oxidative coprecipitation method using Ag(I) and Cu(II) precursors in the presence of potassium persulfate as an oxidizing agent. Structural and morphological analyses, including powder X-ray diffraction, Rietveld refinement, high-resolution transmission electron microscopy, and atomic force microscopy, confirm the formation of a highly crystalline crednerite-type phase with minimal CuO impurity. Optical spectroscopy, combined with Kelvin probe, surface photovoltage spectroscopy, and UV photoemission measurements, reveals a direct optical bandgap of similar to 2.5 eV and a Fermi level close to the valence band maximum, indicative of p-type semiconductor behavior. Importantly, the ambiguity of whether AgCuO2 is metallic or a semiconductor was resolved in the present study both by experiment and theory. Complementary electrochemical analyses demonstrated quasi-reversible redox activity and stability of the AgCuO2 surface, while conductivity measurements provided insight into the charge transport mechanism. Density functional theory (DFT + U) calculations corroborated the experimental findings, predicting a layered structure with Cu in a +3 oxidation state and revealing that Ag vacancy defects could enhance electrical conductivity without compromising optical transparency. This integrated experimental and computational approach establishes AgCuO2 as an electronically conductive, optically transparent, and electrochemically robust TCO candidate with potential for optoelectronic, photoelectrochemical, or solar photovoltaic applications.

This study explores the manifold consequences of introducing copper into an alkaline earth metal (A = Mg, Ca, or Sr) pyrovanadate compound (A2V2O7) framework. Thus, powder X-ray diffraction coupled with Rietveld refinement showed that phase pure alloys, namely, Mg0.67Cu1.33V2O7, CaCuV2O7, and SrCuV2O7 could be obtained via solution combustion synthesis. Local structure distortions from copper insertion into the A2V2O7 compound framework were revealed by Raman spectroscopy and X-ray photoelectron spectroscopy. Importantly, the Cu2-xAxV2O7 alloy framework is shown below to be an excellent platform for demonstrating the complementarity of the two outcomes of bandgap photon absorption, namely, photovoltaic or photoelectrochemical (PEC) activity versus photoluminescence (PL). Thus, PL from the parent pyrovanadate was quenched when copper was introduced; concomitantly, PEC activity emerged for the semiconductor alloys. Changes in the electronic band structures on copper introduction were experimentally probed by diffuse reflectance spectroscopy and Kelvin probe measurements. These data were complemented by density functional theory (DFT) calculations. Finally, the magnetic attributes of the three alloys are discussed via both experiment and theory.

A nano-structured photoelectrode made from Sb 2 Se 3 was prepared and was utilized for CO 2 reduction. Au nanoparticle catalysts were deposited to facilitate CO formation.

Gallium telluride (GaTe), a layered semiconductor, was investigated in the photoelectrochemical (PEC) hydrogen evolution reaction (HER). GaTe nanoflakes were prepared by both mechanical and liquid phase exfoliations, and were subsequently used to obtain micro-, and macroelectrodes. When studying them as photocathodes, the highest photocurrent density of similar to 6 mA cm(-2) was achieved with a 808 +/- 50 nm thick flake. Importantly, the GaTe microelectrodes presented a thickness dependent PEC activity. Macroscopic (1 cm(2)) GaTe electrodes were also prepared from the liquid phase exfoliated nanoflakes, by immobilizing them on glassy carbon electrodes. GaTe photocathodes showed loading and solar power flux dependent PEC activity and reaching a maximum photocurrent of similar to 4 mA cm(-2) with a good photostability. Overall, these results add another candidate to the pool of photoelectrodes applicable in different PEC processes.

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.

Here, we demonstrate a two-step electrosynthesis approach for the preparation of silver pyrovanadate, Ag4V2O7 in thin-film form. In the first, cathodic step, polycrystalline Ag was deposited on fluorine doped tin oxide (FTO) substrate from a non-aqueous bath. Aqueous pyrovanadate species were then generated by aging of a CO2-infused sodium orthovanadate (Na3VO4) solution for three weeks. Silver ions were subsequently generated in situ in this medium using anodic stripping of the Ag/ITO films from the first step. Interfacial precipitation of the Ag+ ions with the pyrovanadate species afforded the targeted product in phase pure form. The various stages of the electrosynthesis were monitored in situ via the combined use of voltammetry, electrochemical quartz crystal nanogravimetry (EQCN), and coulometry. The Ag4V2O7 thin films were characterized by a variety of experimental techniques, including X-ray diffraction, laser Raman spectroscopy, diffuse reflectance spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements afforded information on the energy band structure of the p-type Ag4V2O7 semiconductor. Finally, the electrochemical and photoelectrochemical properties of the electrosynthesized Ag4V2O7 thin films were studied in both aqueous and non-aqueous electrolytes.

Two-dimensional (2D) materials have unique band structure and show a great promise for optoelectronic and solar energy harvesting applications. Photoelectrochemical (PEC) processes are intensively studied employing these materials, due to their high specific surface area, and the possibility of surface modification by defect engineering/catalyst deposition. The PEC activity of different 2D and layered materials was scrutinized for water oxidation/reduction and for inorganic ion oxidation by a statistical analysis to reveal any specific trends. Furthermore, some frequently studied performance improvement strategies (i.e., heterojunctions, tunnelling, and co‒catalysts) are also discussed. Overall, exploring novel materials of 2D family, and new directions are both needed to initiate further discussions and additional research activity, which might enable to harness the full potential of these exciting materials.

Tin(II) selenide (SnSe) is an attractive photocathode candidate for performing photoelectrochemical (PEC) hydrogen evolution reaction (HER), because of its negative conduction band position relative to the HER redox level and a high absorption coefficient for efficiently harvesting solar energy. To prepare thinner layered SnSe flakes from larger size commercial SnSe crystals, liquid phase exfoliation (LPEx) was employed in isopropanol/ water mixtures (IPA/H2O) and pure IPA. Macroscopic (1 cm(2)) electrodes were prepared from the exfoliated SnSe flakes by immobilizing them on glassy carbon electrodes. These flakes obtained by exfoliating the as-received commercial SnSe in pure IPA exhibited 10 times higher PEC activity than those prepared in IPA/H2O. An additional size separation to make three different size fractions of SnSe crystals served to further optimize the LPEx process. Electrodes prepared from the largest flakes showed the highest photocurrent density of 2.44 +/- 0.65 mA cm(-2) at 0.74 V versus RHE under 1 Sun, and similar to 30% incident-photon-to-electron conversion efficiency at 900 nm. Decoration of the SnSe surface with Pt catalyst islands further improved the PEC activity to 4.39 +/- 0.15 mA cm(-2). This photocurrent density represents the highest value reported to date on macroscopic electrodes assembled from SnSe.

Abstract Photoelectrochemical (PEC) hydrogen evolution reaction (HER) was studied on exfoliated, pristine and Pt‐decorated tungsten diselenide (p‐WSe2) nanoflake samples, using a previously developed microdroplet PEC microscopy approach. The WSe2 nanoflakes had well‐defined thicknesses as measured by atomic force microscopy, and the Pt nanoparticles (NPs) were deposited by a variable number of atomic layer deposition (ALD) cycles. An exceptionally high photocurrent density of 49.6 mA cm−2 (under 220 mW cm−2 irradiation) and internal‐photon‐to‐electron‐conversion efficiency (∼90% at 550 nm) were demonstrated on these Pt‐decorated WSe2 (WSe2‐Pt) photocathodes. The Pt NP loading and thickness of WSe2 nanoflakes (in the 24–235 nm range) were used to fine‐tune their PEC activity for HER. We found similar charge transfer and surface recombination kinetics of pristine and WSe2‐Pt specimens (as assessed by intensity‐modulated photocurrent spectroscopy), which indicated significant differences in their bulk properties. X‐ray and ultraviolet photoelectron spectroscopies were performed to identify defect states and quantify the density of states around the valence band of WSe2. The elevated temperature of the ALD process and the evolving Pt NP phase conspired to passivate the sub‐surface (i.e., bulk) defects in the WSe2 nanoflakes, resulting in their vastly improved PEC performance.

Here, we describe a strategy for preparing CdS/MoS2 heterostructures using initially electrodeposited MoSx on a polycrystalline gold substrate. The excess sulfur intrinsic to the electrodeposited MoS3 surface was derivatized with Cd to form spherical CdS/MoS2 particles by judicious adjustment of the medium pH and interfacial electrochemistry. The progression of this conversion was monitored by a combination of cyclic/linear sweep voltammetry coupled with electrochemical quartz crystal nanogravimetry. The electrodeposited MoSx and CdS/MoS2 films were further characterized by scanning electron microscopy, energy-dispersive X-ray analysis, laser Raman spectroscopy, and X-ray photoelectron spectroscopy. Heterojunction formation between MoS2 and CdS particles was confirmed by high-resolution transmission electron microscopy as well as via Kelvin probe measurements of the contact potential differences, with and without the presence of CdS on the MoS2 surface. The nonoptimized CdS/MoS2 heterostructures showed improved photoelectrochemical response compared with CdS or MoS2 for oxidation of sulfite species.

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.

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 Ag 0 -TiO 2 /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.

A key requirement for the exploitation of two-dimensional (2D)-crystals in the field of composites relies on their large-scale production. In this respect, liquid phase exfoliation of layered-crystals is emerging as one of the most promising approaches for the scalable production of high-quality 2D-crystals. However, the dependence of the 2D crystal flakes morphology, i.e. thickness and lateral size, on the mechanical properties of the polymer composites is not fully understood yet. Herein, we tackle this issue by designing an environmentally friendly approach, based on the exfoliation of bulk hexagonal-boron nitride ( h -BN), widely used as filler in polymer composites for its high intrinsic stiffness, i.e. approaching 1 TPa, in a water/surfactant solution with controlled thickness and lateral size by using cascade ultra-centrifugation. Our approach allows us to obtain two populations of flakes with aspect ratio, i.e. lateral size over thickness, equal to 250 and 350, respectively. The h -BN flakes with tuned aspect ratio are subsequently used as filler in a polycarbonate (PC) matrix by exploiting solution blending in 1,3-dioxolane, a solvent with Hansen’s solubility parameters matching the ones of h -BN, thus enhancing the dispersion of the filler inside the matrix, as evaluated by Raman mapping. We tested the composite mechanical properties finding that flakes with higher aspect ratio show superior reinforcements in terms of both ultimate tensile strength and Young’s modulus, compared with their lower aspect ratio counterparts. As example, at 0.1 wt% of loading, the difference in reinforcement in terms of Young’s Modulus is of 56 MPa, being the increment, compared to pristine PC, of ∼22% for composites produced with higher aspect ratio fillers, whereas it is instead of only ∼17% for lower aspect ratio fillers.

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 approximate to 20 % in the oxygen permeability is obtained by using 0.1 wt. % of graphene flakes filler, compared to the unloaded matrix.

Interest in silver-based semiconductors dates to the early years of analog photography. On the other hand, the recent discovery of new silver oxide-based ternary semiconductors has caused a resurgence of interest in these compounds for new technologies. In this vein, the present study provides a counter example to the two perennial issues confronting the electro-synthesis of compound semiconductor films, namely, those of crystallinity and phase purity. It is shown here, for the first time, that a two-step electrosynthesis strategy yields crystalline and phase-pure alpha-Ag3VO4 films on transparent, conductive oxide substrates under very mild process (ambient temperature and pressure) and time-efficient (minutes synthesis duration) conditions. A complementary suite of experimental methods and thermodynamics/kinetics analyses facilitated a deep understanding of the electrosynthesis mechanism, optoelectronic behavior, semiconductor band structure, thermal and electrochemical stability, and electrochemical/photoelectrochemical attributes.

The interfacial adsorption of single-walled carbon nanotubes and few-layer graphene flakes, prepared by solution phase exfoliation, is compared. Strong adsorption of carbon nanotubes was observed at the water/1,2-dichloroethane interface, while a weaker adsorption of the graphene dispersion was seen. Addition of electrolyte to the organic phase was found to have a strong effect on the adsorption of graphene. A simple surface energy model does not fully explain these observations, rather residual charges and their distribution appears to be the key factor behind this difference in adsorptive behaviour. Carbon nanomaterials adsorbed at the liquid-liquid interface can function as bipolar electrodes: a preliminary investigation of the oxidation of the 1,2-dichlorobenzene by metal-modified graphene particles is performed.

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 microdisk electrodes, remain unverified. Herein, we present a study of electron tunneling across a hexagonal boron nitride barrier between a graphite electrode and redox levels in a liquid solution. This was achieved by the fabrication of microdisk electrodes with a typical diameter of 5 µm. Analysis of voltammetric measurements, using two common redox mediators, yielded several electrochemical parameters, including the electron transfer rate constant, limiting current, and transfer coefficient. They show a significant departure from the Butler-Volmer behavior in a clear manifestation of the Marcus-Hush theory of electron transfer. In addition, our system provides a novel 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.

The electrochemical activity of the basal plane and edge plane of graphite has long been a subject of an extensive debate. While significant advances have been made, several gaps still exist in our understanding of this issue, namely, the relative differences in the electrochemical activity of the perfect basal plane and perfect edge plane and the dependence of measurable electrochemical quantities on the edge/defect density of the basal plane. In this work, we employ a microdroplet electrochemical cell technique and atomic force microscopy to measure localized electrochemical properties of the graphitic surface with known edge coverage. The electron transfer rate, capacitance, and density of electronic states of the perfect basal plane and perfect edge plane are estimated, and a qualitative model is proposed for the dependence of the electrochemical quantities on the defect density of the basal plane.

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.

Layered transition metal dichalcogenides (TMDCs) have become promising candidates for photoelectrochemical (PEC) studies from the 80’s. Both n- and p-type MoSe2, and WSe2 materials were studied in photoelectrocatalytic reactions using model redox couples and hydrogen evolution reaction. (1-2) Exfoliation of the bulk, layered crystals to single- and few-layered two-dimensional (2D) nanosheets leads to new physical phenomena, different from their bulk counterparts. (3) The electrochemical properties of 2D nanoflakes −which define the performance of these materials in energy-related applications− depends on their structural properties. These structural features include the number of layers, the basal/edge planes, and the defect density. (4-5) Therefore, to employ 2D nanosheets for energy conversion and storage we need to understand their fundamental PEC properties, using an approach with spatial resolution. In our study, TMDCs samples, MoSe2, WSe2, and MoWSe2 nanosheets were mechanically exfoliated to get bulk, few-layered, and monolayer specimens. The number of layers, and defect density in the separated nanosheets were characterised by Raman spectroscopy and atomic force microscopy. Our recently developed custom-designed microdroplet cell-based photoelectrochemical approach was applied to identify the structural parts and to measure the PEC activity of the flakes (10-50 μm droplets in diameter). PEC measurements, including photovoltammetry, photocurrent transient analysis, and quantum efficiency were carried out to reveal the role of structural properties on the light harvesting, charge transport, and recombination properties of TMDCs. We have determined the band diagrams of these materials using bandgap values from PEC studies and work functions achieved from ambient pressure photoemission spectroscopy (APS) and surface photovoltage spectroscopy (SPS) measurements. Our photoelectrochemical microscopy technique also allowed to probe the PEC reactivity of novel member of 2D family, such as MoWSe2 mixed transition metal dichalcogenide. Finally, I demonstrate the use of model reversible redox species (K4[Fe(CN)6], (NH4)2[IrCl6]) to mimic photoelectrocatalytic processes. (1) F. R. F. Fan, H. S. White, B. L. Wheeler, A. J. Bard; J. Am. Chem. Soc. 1980, 102, 5142-5148 (2) W. Kautek, H. Gerischer; Ber. Bunsenges. Phys. Chem. 1980, 84, 645-653 (3) K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, A. K. Geim; PNAS, 2005, 102, 10451-10453 (4) P. S. Toth, A. T. Valota, M. Velický, I. A. Kinloch, K. S. Novoselov, E. W. Hill, R. A. W. Dryfe; Chem. Sci., 2014, 5, 582-589 (5) M. Velický, M. A. Bissett, C. R. Woods, P. S. Toth, T. Georgiou, I. A. Kinloch, K. S. Novoselov, R. A. W. Dryfe; Nano Lett., 2016, 16, 2023-2032