Robust Hierarchically Structured Biphasic Ambipolar Oxide Photoelectrodes for Light-Driven Chemical Regulation and Switchable Logic Applications

Actively Driven Light-Addressable Sensor/Actuator System for Automated pH Control for the Integration in Lab-On-A-Chip (LoC) Platforms

The miniaturization of microfluidic systems usually comes at the cost of more difficult integration of sensors and actuators inside the channel. As an alternative, this work demonstrates the embedding of semiconductor-based sensor and actuator technologies that can be spatially and temporally controlled from outside the channel using light. The first element is a light-addressable potentiometric sensor, consisting of an Al/Si/SiO2/Si3N4 structure, that can measure pH changes at the Si3N4/electrolyte interface. The pH value is a crucial factor in biological and chemical systems, and besides measuring, it is often important to bring the system out of equilibrium or to adjust and control precisely the surrounding medium. This can be done photoelectrocatalytically by utilizing light-addressable electrodes. These consist of a glass/SnO2:F/TiO2 structure, whereby direct charge transfer between the TiO2 and the electrolyte leads to a pH change upon irradiation. To complement the advantages of both, we integrated a light-addressable sensor with a pH sensitivity of 41.5 mV·pH-1 and a light-addressable electrode into a microfluidic setup. Here, we demonstrated a simultaneous operation with the ability to generate and record pH gradients inside a channel under static and dynamic flow conditions. The results show that dependent on the light-addressable electrode (LAE)-illumination conditions, pH changes up to ΔpH of 2.75 and of 3.52 under static and dynamic conditions, respectively, were spatially monitored by the light-addressable potentiometric sensor. After flushing with fresh buffer solution, the pH returned to its initial value. Depending on the LAE illumination, pH gradients with a maximum pH change of ΔpH of 1.42 were tailored perpendicular to the flow direction. In a final experiment, synchronous LAE illumination led to a stepwise increase in the pH inside the channel.

Electrolyte-Controlled Photoelectrochemical Photocurrent Switching Effect in High-Performance Self-Powered Broadband Photoelectrochemical-Type Photodetectors Based on MnPS3 Nanosheets

Photoelectric devices are extensively applied in optical logic systems, light communication, optical imaging, and so on. However, traditional photoelectric devices can only generate unidirectional photocurrent, which hinders the simplification and multifunctionality of devices. Recently, it has become a new research focus to achieve controllable reversal of the output photocurrent direction (bipolar current) in a photoelectric system. Considering that the device with bipolar current adds a reverse current operating state compared to traditional devices, the former is more suitable for developing new multifunctional photoelectric devices. Due to the existence of electrolytes, photoelectrochemical (PEC) systems contain chemical processes such as ion diffusion and migration and electrochemical reactions, which are unable to occur in solid-state transistor devices, and the effect of electrolyte pH on the performance of PEC systems is usually ignored. We prepared a MnPS3-based PEC-type photodetector and reversed photocurrents by adjusting the pH of electrolytes, i.e., the electrolyte-controlled photoelectrochemical photocurrent switching (PEPS) effect. We clarified the effect of pH values on the direction of photocurrent from the perspectives of electrolyte energy level rearrangement splitting and the kinetic theory of the semiconductor electrode. This work not only contributes to a deeper understanding of carrier transport in PEC processes but also inspires the development of advanced multifunctional photoelectric devices.

Sustainable Cauliflower-Patterned CuFe2O4 Electrode Production from Chalcopyrite for Supercapacitor Applications

The primary purpose of this study was to produce an ore-based high-capacity supercapacitor electrode. For this, chalcopyrite ore was first leached with nitric acid, and then metal oxide synthesis was carried out immediately on nickel foam using a hydrothermal technique from the solution. Cauliflower-patterned CuFe₂O₄ with a wall thickness of about 23 nm was synthesized on the Ni foam surface, characterized by XRD, FTIR, XPS, SEM, and TEM investigations. The produced electrode also displayed a feature of a battery-like charge storage mechanism with a specific capacity of 525 mF cm^-2 at 2 mA cm^-2 current density, energy of 8.9 mWh cm^-2, and a power density of 233 mW cm^-2. Additionally, even after 1350 cycles, this electrode still performed at 109% of its original capacity. The performance of this finding is 255% higher than that of the CuFe₂O₄ in our earlier investigation; despite being pure, it performs far better than some of its equivalents in the literature. Obtaining such performance from an electrode made from ore indicates that the use of ore has a lot of potential for supercapacitor production and property improvement.

Size-Independent Reconfigurable Logic Gate with Bismuth Oxide Based Photoelectrochemical Device

XOR gate, an important building block in computational circuits, is often constructed by combining other basic logic gates, and the hybridity inevitably leads to its complexity. A photoelectrochemical device could realize XOR function based on the current change of the photoelectrode; however, such signal is highly sensitive to photoelectrode size and therefore requires precise manufacturing at a high cost. Herein we developed a novel XOR gate based on the light-induced open-circuit potential (OCP) of the Bi₂O₃ photoelectrode. Surprisingly, the OCP of Bi₂O₃ does not increase with light intensity according to the traditional logarithmic relationship. Instead, an unusual decrease in OCP is observed at high light intensity, which is attributed to the dramatic light-induced increase in surface states that can be easily regulated by varying the oxygen partial pressure during reactive magnetron sputtering. Based on such a nonmonotonic variation of OCP, a facile Bi₂O₃-based gate is designed to realize the XOR function. Unlike the commonly used current signal, OCP is size independent, and therefore, the Bi₂O₃-based gate does not require high manufacturing accuracy. Moreover, in addition to XOR, the Bi₂O₃-based PEC gate also demonstrates great versatility in realizing other logic functions including AND, OR, NOT, NIH, NAND, and NOR. The strategy of modulating and applying nonmonotonic OCP signal opens a new avenue for designing size-independent reconfigurable logic gates at low manufacturing cost.

Epitaxial III-V/Si Vertical Heterostructures with Hybrid 2D-Semimetal/Semiconductor Ambipolar and Photoactive Properties

Hybrid materials taking advantage of the different physical properties of materials are highly attractive for numerous applications in today's science and technology. Here, it is demonstrated that epitaxial bi-domain III-V/Si are hybrid structures, composed of bulk photo-active semiconductors with 2D topological semi-metallic vertical inclusions, endowed with ambipolar properties. By combining structural, transport, and photoelectrochemical characterizations with first-principle calculations, it is shown that the bi-domain III-V/Si materials are able within the same layer to absorb light efficiently, separate laterally the photo-generated carriers, transfer them to semimetal singularities, and ease extraction of both electrons and holes vertically, leading to efficient carrier collection. Besides, the original topological properties of the 2D semi-metallic inclusions are also discussed. This comb-like heterostructure not only merges the superior optical properties of semiconductors with good transport properties of metallic materials, but also combines the high efficiency and tunability afforded by III-V inorganic bulk materials with the flexible management of nano-scale charge carriers usually offered by blends of organic materials. Physical properties of these novel hybrid heterostructures can be of great interest for energy harvesting, photonic, electronic or computing devices.

Plasmonic Photoelectrochemistry: In View of Hot Carriers

Utilizing plasmon-generated hot carriers to drive chemical reactions has emerged as a popular topic in solar photocatalysis. However, a complete description of the underlying mechanism of hot-carrier transfer in photochemical processes remains elusive, particularly for those involving hot holes. Photoelectrochemistry enables to localize hot holes on photoanodes and hot electrons on photocathodes and thus offers an approach to separately explore the hole-transfer dynamics and electron-transfer dynamics. This review summarizes a comprehensive understanding of both hot-hole and hot-electron transfers from photoelectrochemical studies on plasmonic electrodes. Additionally, working principles and applications of spectroelectrochemistry are discussed for plasmonic materials. It is concluded that photoelectrochemistry provides a powerful toolbox to gain mechanistic insights into plasmonic photocatalysis.

Nanoscale Heterogeneities and Composition-Reactivity Relationships in Copper Vanadate Photoanodes

The photoelectrochemical performance of thin film photoelectrodes can be impacted by deviations from the stoichiometric composition, both at the macroscale and at the nanoscale. This issue is especially pronounced for the class of ternary compounds that are currently investigated for simultaneously achieving the optoelectronic characteristics and chemical stability required for solar fuel generation. Here, we combine macroscopic photoelectrochemical testing with atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM) to reveal relationships between photoelectrochemical activity, nanoscale morphology, and local chemical composition in copper vanadate (CVO) thin films as a model system. For films with varying Cu/(Cu + V) ratios around the ideal stoichiometry of stoiberite Cu₅V₂O₁₀, AFM resolves submicrometer morphology variations, which correlate with variations of the Cu content resolved by STXM. Both stoichiometric and Cu-deficient films exhibit a clear photoresponse, which indicates electronic tolerance to reduced Cu content. While both films exhibit homogeneous O and V content, they are also characterized by local regions of Cu enrichment and depletion that extend beyond individual grains. By contrast, Cu-rich photoelectrodes exhibit a tendency toward CuO secondary phase formation and a significantly reduced photoelectrochemical activity, indicating a significantly poor electronic tolerance to Cu-enrichment. These findings highlight that the average film composition at the macroscale is insufficient for defining structure-function relationships in complex ternary compounds. Rather, correlating microscopic variations in chemical composition to macroscopic photoelectrochemical performance provides insights into photocatalytic activity and stability that are otherwise not apparent from pure macroscopic characterization.

Fabrication of One-Dimensional Organic Nanofiber Networks via Electrophoretic Deposition for a Nonvolatile Memory Device

Despite many advanced growth methodologies for organic nanofibers (ONFs), the lack of efficient and scalable ONF-based film preparation technologies has long been a hindrance in their practical application in organic electronic devices. Here, a typical cathode electrophoretic deposition (C-EPD) technology was developed to controllably produce ONFs and their corresponding thin films. Using the solvent effect and an external electric field force during the C-EPD process, a one-dimensional ONF network was formed, which exhibits compact molecular packing and superior optoelectronic properties in the thin-film state. Prototype sandwich-structure memory devices based on these ONF films exhibited a binary nonvolatile memory performance significantly superior than that of the bulk materials. This study provides an efficient and scalable ONF fabrication technology for high-performance electronic devices in various potential applications.

Hydrogenation of ZnFe2O4 Flat Films: Effects of the Pre-Annealing Temperature on the Photoanodes Efficiency for Water Oxidation

The effects induced by post-synthesis hydrogenation on ZnFe2O4 flat films in terms of photoelectrochemical (PEC) performance of photoanodes for water oxidation have been deeply investigated as a function of the pre-annealing temperature of the materials. The structure and morphology of the films greatly affect the efficacy of the post synthesis treatment. In fact, highly compact films are obtained upon pre-annealing at high temperatures, and this limits the exposure of the material bulk to the reductive H2 atmosphere, making the treatment largely ineffective. On the other hand, a mild hydrogen treatment greatly enhances the separation of photoproduced charges in films pre-annealed at lower temperatures, as a result of the introduction of oxygen vacancies with n-type character. A comparison between present results and those obtained with ZnFe2O4 nanorods clearly demonstrates that specific structural and/or surface properties, together with the initial film morphology, differently affect the overall contribution of post-synthesis hydrogenation on the efficiency of zinc ferrite-based photoanodes.

Buckling and twisting of advanced materials into morphable 3D mesostructures

Recently developed methods in mechanically guided assembly provide deterministic access to wide-ranging classes of complex, 3D structures in high-performance functional materials, with characteristic length scales that can range from nanometers to centimeters. These processes exploit stress relaxation in prestretched elastomeric platforms to affect transformation of 2D precursors into 3D shapes by in- and out-of-plane translational displacements. This paper introduces a scheme for introducing local twisting deformations into this process, thereby providing access to 3D mesostructures that have strong, local levels of chirality and other previously inaccessible geometrical features. Here, elastomeric assembly platforms segmented into interconnected, rotatable units generate in-plane torques imposed through bonding sites at engineered locations across the 2D precursors during the process of stress relaxation. Nearly 2 dozen examples illustrate the ideas through a diverse variety of 3D structures, including those with designs inspired by the ancient arts of origami/kirigami and with layouts that can morph into different shapes. A mechanically tunable, multilayered chiral 3D metamaterial configured for operation in the terahertz regime serves as an application example guided by finite-element analysis and electromagnetic modeling.

Recent Advances in Earth-Abundant Photocathodes for Photoelectrochemical Water Splitting

The conversion of solar energy into hydrogen through photoelectrochemical (PEC) water splitting is an attractive way to store renewable energy. Despite the intriguing concept of solar hydrogen production, efficient PEC devices based on earth-abundant semiconductors should be realized to compete economically with conventional steam reforming processes. Herein, recent milestones in photocathode development for PEC water splitting, particularly in earth-abundant semiconductors, in terms of new techniques for enhancing performance, as well as theoretical aspects, are highlighted. In addition, recent research into newly emerging low-cost p-type semiconductors in the PEC field, such as Cu₂ BaSn(S,Se)₄ and Sb₂ Se₃ , are scrutinized and the advantages and disadvantages of each material assessed.

Copper ferrites@reduced graphene oxide anode materials for advanced lithium storage applications

Copper ferrites are emerging transition metal oxides that have potential applications in energy storage devices. However, it still lacks in-depth designing of copper ferrites based anode architectures with enhanced electroactivity for lithium-ion batteries. Here, we report a facile synthesis technology of copper ferrites anchored on reduced graphene oxide (CuFeO₂@rGO and Cu/CuFe₂O₄@rGO) as the high-performance electrodes. In the resulting configuration, reduced graphene offers continuous conductive channels for electron/ion transfer and high specific surface area to accommodate the volume expansion of copper ferrites. Consequently, the sheet-on-sheet CuFeO₂@rGO electrode exhibits a high reversible capacity (587 mAh g⁻¹ after 100 cycles at 200 mA g⁻¹). In particular, Cu/CuFe₂O₄@rGO hybrid, which combines the advantages of nano-copper and reduced graphene, manifests a significant enhancement in lithium storage properties. It reveals superior rate capability (723 mAh g⁻¹ at 800 mA g⁻¹; 560 mAh g⁻¹ at 3200 mA g⁻¹) and robust cycling capability (1102 mAh g⁻¹ after 250 cycles at 800 mA g⁻¹). This unique structure design provides a strategy for the development of multivalent metal oxides in lithium storage device applications.