Layer-by-Layer Assembly of Polyoxometalates for Photoelectrochemical (PEC) Water Splitting: Toward Modular PEC Devices

"Double-Use" Strategy for Improving the Photoelectrochemical Performance of BiVO4 Photoanodes Using a Cobalt-Functionalized Polyoxotungstate

Doping and surface-modification are well-established strategies for the performance enhancement of bismuth vanadate (BiVO4) photoanodes in photoelectrochemical (PEC) water splitting devices. Herein, a "double-use" strategy for the development of high-performance BiVO4 photoanodes for solar water splitting is reported, where a molecular cobalt-phosphotungstate (CoPOM = Na10[Co4(H2O)2(PW9O34)2]) is used both as a bulk doping agent as well as a surface-deposited water oxidation cocatalyst. The use of CoPOM for bulk doping of BiVO4 is shown to enhance the electrical conductivity and improve the charge separation efficiency, resulting in the enhancement of the maximum applied-bias photoconversion efficiency (ABPE) by a factor of ∼18 to 0.54% at 0.87 V vs. RHE, as compared to pristine BiVO4 (0.03% at 1.04 V vs. RHE). The ratio of W/Co on the surface of the photoanode is related to the activity and stability. In addition, modification of CoPOM-doped BiVO4 with CoPOM as a surface cocatalyst enhances the hole extraction and improves the water oxidation kinetics, resulting in the overall enhancement of the ABPE to 0.79% (at 0.82 V vs. RHE), i.e., by a factor of ∼26 with respect to pristine BiVO4. This study establishes the "double-use" strategy involving CoPOMs as an effective, straightforward, and easily scalable approach for the development of high-quality photoanodes for solar water splitting and highlights the future potential of utilizing well-designed polyoxometalates as precursors for the synthesis of energy materials.

Crafting Insulating Polymer Mediated and Atomically Precise Metal Nanoclusters Photosensitized Photosystems Towards Solar Water Oxidization

Atomically precise metal nanoclusters (NCs) have been deemed as a new generation of metal nanomaterials because of their characteristic atomic stacking fashion, quantum confinement effect, and multitude of active sites. The discrete molecular-like energy band structure of metal NCs endows them with photosensitization capability for light harvesting and conversion. However, applications of metal NCs in photoelectrocatalysis are limited by the ultrafast charge recombination and unfavorable stability, impeding the construction of metal NC-based photosystems. In this work, we elaborately crafted multilayered metal oxide (MO)/(metal NCs/insulating polymer)n photoanodes by a facile layer-by-layer (LbL) assembly technique. In these well-defined heterostructured photoanodes, glutathione (GSH)-wrapped metal NCs (Agx@GSH, Ag9@GSH6, Ag16@GSH9, and Ag31@GSH19) and an insulating poly(allylamine hydrochloride) (PAH) layer are alternately deposited on the MO substrate in a highly ordered integration mode. We found that photoelectrons of metal NCs can be tunneled into the MO substrate via the intermediate ultrathin insulating polymer layer by stimulating the tandem charge transfer route, thus facilitating charge separation and boosting photoelectrochemical water oxidation performances. Our work would open a new frontier for judiciously regulating directional charge transport over atomically precise metal NCs for solar-to-hydrogen conversion.

Layer-by-Layer Construction of a Nanoarchitecture by Polyoxometalates and Polymers: Enhanced Electrochemical Hydrogen Evolution Reaction

In this contribution, a nanoarchitectural approach was employed to produce a nanolayer of polyoxometalate (POM) on the surface of a glassy carbon electrode (GCE) to achieve a higher surface area with higher electrocatalytic activity toward the electrochemical hydrogen evolution reaction (HER). To accomplish this, the well-known layer-by-layer (LbL) technique was employed, which involved the alternate adsorption of the POM, Na_0.3[N(C₄H₉)₄]_7.7 [(Mo₃O₈)₄(O₃PC(O)(C₃H₆NH₂CH₂C₄H₃S)PO₃)₄], abbreviated as [(TBA)Mo₁₂(AleThio)₄], and polyethyleneimine (PEI) polymer. This nanolayered electrode exhibited catalytic properties toward the HER in 0.5 M H₂SO₄ with the resulting polarization curves indicating an increase in the HER activity with the increasing number of POM layers, and the overpotential required for this reaction was lowered by 0.83 V when compared with a bare GCE. The eighth PEI/[(TBA)Mo₁₂(AleThio)₄] bilayer exhibited a significantly lower HER overpotential of -0.077 V at a current density of 10 mA cm^-2. Surface characterization of the LbL-assembled nanolayers was carried out using X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy. We believe that the synergetic effect of the positively charged PEI polymer and the catalytically active molybdate POM is the cause for the successful response to the electrochemical HER.

Towards Heterogeneous Catalysis: A Review on Recent Advances of Depositing Nanocatalysts in Continuous-Flow Microreactors

As a promising technology, microreactors have been regarded as a potential candidate for heterogeneous catalytic reactions as they inherently allow the superior advantages of precise flow control, efficient reactant transfer, flexible operation, etc. However, the wide market penetration of microreactors is still facing severe challenges. One of the most important reasons is the preparation of a high-performance catalytic layer in the microreactor because it can directly influence the catalytic activity and stability the reactor and thus the deployment the microreactor technology. Hence, significant progress in depositing nanocatalysts in microreactors has been made in the past decades. Herein, the methods, principles, recent advances, and challenges in the preparation of the catalyst layer in microreactors were presented. A general description of the physicochemical processes of heterogeneous catalytic reactions in microreactors were first introduced. Then, recent advances in catalyst layer preparation in microreactors were systematically summarized. Particular attention was focused on the most common sol-gel method and its latest developments. Some new strategies proposed recently, including bio-inspired electroless deposition and layer-by-layer self-assembly, were also comprehensively discussed. The remaining challenges and future directions of preparing the catalytic layer in microreactors with high performance and low cost were highlighted.

Immobilization of a [CoIIICoII(H2O)W11O39]7– Polyoxoanion for the Photocatalytic Oxygen Evolution Reaction

The ongoing transition to renewable energy sources and the implementation of artificial photosynthetic setups call for an efficient and stable water oxidation catalyst (WOC). Here, we heterogenize a molecular all-inorganic [Co^IIICo^II(H₂O)W₁₁O₃₉]^7– ({Co^IIICo^IIW₁₁}) Keggin-type polyoxometalate (POM) onto a model TiO₂ surface, employing a 3-aminopropyltriethoxysilane (APTES) linker to form a novel heterogeneous photosystem for light-driven water oxidation. The {Co^IIICo^IIW₁₁}-APTES-TiO₂ hybrid is characterized using a set of spectroscopic and microscopic techniques to reveal the POM integrity and dispersion to elucidate the POM/APTES and APTES/TiO₂ binding modes as well as to visualize the attachment of individual clusters. We conduct photocatalytic studies under heterogeneous and homogeneous conditions and show that {Co^IIICo^IIW₁₁}-APTES-TiO₂ performs as an active light-driven WOC, wherein {Co^IIICo^IIW₁₁} acts as a stable co-catalyst for water oxidation. In contrast to the homogeneous WOC performance of this POM, the heterogenized photosystem yields a constant WOC rate for at least 10 h without any apparent deactivation, demonstrating that TiO₂ not only stabilizes the POM but also acts as a photosensitizer. Complementary studies using photoluminescence (PL) emission spectroscopy elucidate the charge transfer mechanism and enhanced WOC activity. The {Co^IIICo^IIW₁₁}-APTES-TiO₂ photocatalyst serves as a prime example of a hybrid homogeneous–heterogeneous photosystem that combines the advantages of solid-state absorbers and well-defined molecular co-catalysts, which will be of interest to both scientific communities and applications in photoelectrocatalysis and CO₂ reduction.

Emerging Surface, Bulk, and Interface Engineering Strategies on BiVO4 for Photoelectrochemical Water Splitting

The photoelectrochemical (PEC) cell that collects and stores abundant sunlight to hydrogen fuel promises a clean and renewable pathway for future energy needs and challenges. Monoclinic bismuth vanadate (BiVO₄ ), having an earth-abundancy, nontoxicity, suitable optical absorption, and an ideal n-type band position, has been in the limelight for decades. BiVO₄ is a potential photoanode candidate due to its favorable outstanding features like moderate bandgap, visible light activity, better chemical stability, and cost-effective synthesis methods. However, BiVO₄ suffers from rapid recombination of photogenerated charge carriers that have impeded further improvements of its PEC performances and stability. This review presents a close look at the emerging surface, bulk, and interface engineering strategies on BiVO₄ photoanode. First, an effective approach of surface functionalization via different cocatalysts to improve the surface kinetics of BiVO₄ is discussed. Second, state-of-the-art methodologies such as nanostructuring, defect engineering, and doping to further enhance light absorption and photogenerated charge transport in bulk BiVO₄ are reviewed. Third, interface engineering via heterostructuring to improve charge separation is introduced. Lastly, perspectives on the foremost challenges and some motivating outlooks to encourage the future research progress in this emerging frontier are offered.

CuInS2 Photocathodes with Atomic Gradation-Controlled (Ta,Mo)x(O,S)y Passivation Layers for Efficient Photoelectrochemical H2 Production

An atomic gradient passivation layer, (Ta,Mo)_x(O,S)_y, is designed to improve the charge transportation and photoelectrochemical activity of CuInS₂-based photoelectrodes. We found that Mo spontaneously diffused to the a-TaO_x layer during e-beam evaporation. This result indicates that the gradient profile of MoO_x/TaO_x is formed in the sublayer of (Ta,Mo)_x(O,S)_y. To understand the atomic-gradation effects of the (Ta,Mo)_x(O,S)_y passive layer, the composition and (photo)electrochemical properties have been characterized in detail. When this atomic gradient-passive layer is applied to CuInS₂-based photocathodes, promising photocurrent and onset potential are seen without using Pt cocatalysts. This is one of the highest activities among reported CuInS₂ photocathodes, which are not combined with noble metal cocatalysts. Excellent photoelectrochemical activity of the photoelectrode can be mainly achieved by (1) the electron transient time improved due to the conductive Mo-incorporated TaO_x layer and (2) the boosted electrocatalytic activity by Mo_x(O,S)_y formation.

Accelerating charge transfer via nonconjugated polyelectrolyte interlayers toward efficient versatile photoredox catalysis

One of the challenges for high-efficiency single-component-based photoredox catalysts is the low charge transfer and extraction due to the high recombination rate. Here, we demonstrate a strategy to precisely control the charge separation and transport efficiency of the catalytic host by introducing electron or hole extraction interlayers to improve the catalytic efficiency. We use simple and easily available non-conjugated polyelectrolytes (NCPs) (i.e., polyethyleneimine, PEI; poly(allylamine hydrochloride), PAH) to form interlayers, wherein such NCPs consist of the nonconjugated backbone with charge transporting functional groups. Taking CdS as examples, it is shown that although PEI and PAH are insulators and therefore do not have the ability to conduct electricity, they can form good electron or hole transport extraction layers due to the higher charge-transfer kinetics of pendant groups along the backbones, thereby greatly improving the charge transfer capability of CdS. Consequently, the resultant PEI-/PAH-functionalized nanocomposites exhibit significantly enhanced and versatile photoredox catalysis.

Nanoarchitectonics: what's coming next after nanotechnology?

In science and technology today, the crucial importance of the regulation of nanoscale objects and structures is well recognized. The production of functional material systems using nanoscale units can be achieved via the fusion of nanotechnology with the other research disciplines. This task is a part of the emerging concept of nanoarchitectonics, which is a concept moving beyond the area of nanotechnology. The concept of nanoarchitectonics is supposed to involve the architecting of functional materials using nanoscale units based on the principles of nanotechnology. In this focus article, the essences of nanotechnology and nanoarchitectonics are first explained, together with their historical backgrounds. Then, several examples of material production based on the concept of nanoarchitectonics are introduced via several approaches: (i) from atomic switches to neuromorphic networks; (ii) from atomic nanostructure control to environmental and energy applications; (iii) from interfacial processes to devices; and (iv) from biomolecular assemblies to life science. Finally, perspectives relating to the final goals of the nanoarchitectonics approach are discussed.

Selective photocatalytic production of CH4 using Zn-based polyoxometalate as a nonconventional CO2 reduction catalyst

Efficient and selective production of CH₄ through the CO₂ reduction reaction (CO2RR) is a challenging task due to the high amount of energy consumption and various reaction pathways. Here, we report the synthesis of Zn-based polyoxometalate (ZnPOM) and its application in the photocatalytic CO2RR. Unlike conventional Zn-based catalysts that produce CO, ZnPOM can selectively catalyze the production of CH₄ in the presence of an Ir-based photosensitizer (TIr3) through the photocatalytic CO2RR. Photophysical and computation analyses suggest that selective photocatalytic production of CH₄ using ZnPOM and TIr3 can be attributed to (1) the exceptionally fast transfer of photogenerated electrons from TIr3 to ZnPOM through the strong molecular interactions between them and (2) effective transfer of electrons from ZnPOM to *CO intermediates due to significant hybridization of their molecular orbitals. This study provides insights into the design of novel CO2RR catalysts for CH₄ production beyond the limitations in conventional studies that focus on Cu-based materials.

Research Progress on Catalytic Water Splitting Based on Polyoxometalate/Semiconductor Composites

In recent years, due to the impact of global warming, environmental pollution, and the energy crisis, international attention and demand for clean energy are increasing. Hydrogen energy is recognized as one of the clean energy sources. Water is considered as the largest potential supplier of hydrogen energy. However, artificial catalytic water splitting for hydrogen and oxygen evolution has not been widely used due to its high energy consumption and high cost during catalytic cracking. Therefore, the exploitation of photocatalysts, electrocatalysts, and photo-electrocatalysts for rapid, cost effective, and reliable water splitting is essentially needed. Polyoxometalates (POMs) are regarded as the potential candidates for water splitting catalysis. In addition to their excellent catalytic properties and reversibly redox activities, POMs can also modify semiconductors to overcome their shortcomings, and improve photoelectric conversion efficiency and photocatalytic activity, which has attracted more and more attention in the field of photoelectric water splitting catalysis. In this review, we summarize the latest applications of POMs and semiconductor composites in the field of photo-electrocatalysis (PEC) for hydrogen and oxygen evolution by catalytic water splitting in recent years and take the latest applications of POMs and semiconductor composites in photocatalysis for water splitting. In the conclusion section, the challenges and strategies of photocatalytic and PEC water-splitting by POMs and semiconductor composites are discussed.

Electrochemistry of Multilayer Electrodes: From the Basics to Energy Applications

Growing environmental concern has increased the demand for clean energy, and various technologies have been developed to utilize renewable energy sources. With the development of highly efficient energy conversion and storage systems, fundamental studies on the electrochemistry of electrodes are critical because the functionality of most of these systems relies on interfacial electrochemical reactions that occur on the surfaces of the electrodes. In this context, efficient electrode design methods are required to study specific electrochemical principles and the mechanisms of interfacial reactions on the surface of electrodes.Compared with other electrode fabrication methods, layer-by-layer (LbL) assembly is a simple, inexpensive, and versatile process for producing highly ordered multilayer thin-film electrodes from a diverse array of materials. LbL-assembled multilayer electrodes exhibit distinct electrochemical properties compared with electrodes created via other fabrication methods because of the nanoscale control of the composition and structures of electrodes afforded by LbL assembly. LbL assembly can generate unique nanoarchitectures from a multiplicity of electroactive components to investigate the detailed electrochemical mechanisms within the electrode, allowing for investigations of the internal-architecture-dependent electrochemical properties within the electrodes. As electrochemical LbL research has progressed over the last 10 years, our group has performed pioneering studies on the fundamental electrochemical properties of multilayer electrodes fabricated via LbL assembly for diverse energy applications. In this Account, we aim to outline the fundamental electrochemistry occurring at the nanoscale level on multilayer thin-film LbL electrodes using our work to illustrate these concepts, including the dependence of the electrochemistry on the thickness and architecture of multilayer electrodes, competition between mass and charge transfer, and control over the ion-permeation selectivity and interfacial dipole moments in multilayer electrodes. We anticipate that our approach to LbL-assembled electrodes will be of great interest and provide an attractive platform for the investigation of fundamental multilayer thin-film electrochemistry. We also believe that it will provide guidelines for research efforts toward future electrode engineering.

Virtually Transparent TiO2/Polyelectrolyte Thin Multilayer Films as High-Efficiency Nanoporous Photocatalytic Coatings for Breaking Down Formic Acid and for Escherichia coli Removal

Virtually transparent photocatalytic multilayer films composed of TiO₂ nanoparticles and polyelectrolytes were built on model surfaces using layer-by-layer assembly and investigated as photocatalytic nanoporous coatings. Formic acid (HCOOH) and Escherichia coli were used as models for the degradation of gaseous pollutants and for studying antibacterial properties. Positively charged TiO₂ nanoparticles were coassembled with negatively charged poly(sodium 4-styrenesulfonate) (NaPSS) which leads to highly transparent nanoscale coatings in which the content of TiO₂ particles is controlled mainly by the number of deposition cycles and the enhanced translucency with respect to titania powders is likely due to the presence of the polyelectrolytes in the interstitial space between the particles. Build-up and structural properties of the films were determined by ellipsometry, quartz crystal microbalance (QCM-D, with dissipation monitoring), and UV-vis spectrophotometry in transmission and scanning electron microscopy. Complementary photophysical and activity tests of (PSS/TiO₂)_n multilayer films were performed in the gas-phase under UV-A light and revealed a peculiar dependence on the number of layer pairs (LPs), corresponding to a clear deviation from the usual observations in photocatalysis with increasing TiO₂ amounts. Most notably, a single LP film showed a strongly enhanced HCOOH mineralization and outperformed films with a higher number of LPs, with respect to the quantity of TiO₂ catalyst present in the films. It is believed that the high quantum yield (8.1%) of a coating consisting of a single TiO₂ layer which is 6-7 times higher than that of a 6-10 LP film could be due to the optimum accessibility of the TiO₂ crystallites toward both HCOOH and water molecules. In thicker films, while no detrimental light screening was observed with increasing the number of LPs, diffusion phenomena could cap the efficiency of the access of the pollutant and water to the catalytic surface. Unlike for HCOOH mineralization, three PSS/TiO₂ LPs were required for observing a maximum antibacterial activity of the nanocomposite coatings. This is likely due to the fact that micrometer-sized E. coli bacteria do not enter into the interstitial space between the TiO₂ particles and require a different surface morphology with respect to the number of active contact points for optimum degradation.

Tetraruthenium Polyoxometalate as an Atom-Efficient Bifunctional Oxygen Evolution Reaction/Oxygen Reduction Reaction Catalyst and Its Application in Seawater Batteries

Although development and utilization of efficient catalysts with earth-abundant and cheap elements are desired, precious noble metal-based catalysts are still widely used and studied due to the urgent need to address energy and environmental issues. Polyoxometalates (POMs) can be excellent candidates in this context. In this study, we found that oxo-bridged tetraruthenium polyoxometalate (RuPOM) exhibits excellent electrocatalytic activity for both oxygen evolution and reduction reactions (OER and ORR) with minimal use of noble metal elements and can be used for the development of efficient seawater batteries (SWBs). The deposition of RuPOM on a desired electrode with conducting carbon Ketjen black (KB) by the simple slurry coating method imparted bifunctional OER/ORR activity to the underlying electrode. Although the mass activity was similar, RuPOM/KB mixtures exhibited superior activity even compared to commercially available Pt/C when comparing the activity per noble metal element. Based on these findings, we employed RuPOM to develop efficient SWBs. RuPOM significantly lowered the charging potential and increased the discharging potential of SWBs, which are related to OER and ORR, respectively. This study can provide insights into the development of POM-based electrocatalysts and their application in energy storage and conversion devices.

Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives

Polyoxometalates (POMs) are molecular metal oxide clusters that feature a broad range of structures and functionalities, making them one of the most versatile classes of inorganic molecular materials. They have attracted widespread attention in homogeneous catalysis. Due to the challenges associated with their aggregation, precipitation, and degradation under operational conditions and to extend their scope of applications, various strategies of depositing POMs on heterogeneous substrates have been developed. Recent ground-breaking developments in the materials chemistry of supported POM composites are summarized and links between molecular-level understanding of POM-support interactions and macroscopic effects including new or optimized reactivities, improved stability, and novel function are established. Current limitations and future challenges in studying these complex composite materials are highlighted, and cutting-edge experimental and theoretical methods that will lead to an improved understanding of synergisms between POM and support material from the molecular through to the nano- and micrometer level are discussed. Future development in this fast-moving field is explored and emerging fields of research in POM heterogenization are identified.

Two new estertin modified tungstosilicates: synthesis, catalytic activity and photoelectrochemical property

Two estertin functionalized tungstosilicates were synthesized and analyzed by DFT. They exhibit good photoelectrocatalytic performance for oxidation of methanol.

Layer-by-Layer Assembly: Recent Progress from Layered Assemblies to Layered Nanoarchitectonics

As an emerging concept for the development of new materials with nanoscale features, nanoarchitectonics has received significant recent attention. Among the various approaches that have been developed in this area, the fixed-direction construction of functional materials, such as layered fabrication, offers a helpful starting point to demonstrate the huge potential of nanoarchitectonics. In particular, the combination of nanoarchitectonics with layer-by-layer (LbL) assembly and a large degree of freedom in component availability and technical applicability would offer significant benefits to the fabrication of functional materials. In this Minireview, recent progress in LbL assembly is briefly summarized. After introducing the basics of LbL assembly, recent advances in LbL research are discussed, categorized according to physical, chemical, and biological innovations, along with the fabrication of hierarchical structures. Examples of LbL assemblies with graphene oxide are also described to demonstrate the broad applicability of LbL assembly, even with a fixed material.

FeO x Derived from an Iron-Containing Polyoxometalate Boosting the Photocatalytic Water Oxidation Activity of Ti3+-Doped TiO2

The development of efficient and stable catalyst systems using low-cost, abundant, and nontoxic materials is the primary demand for photocatalytic water oxidation. Distinguishing the true active species in a heterogeneous catalytic system is important for construction of efficient catalytic systems. Herein, hydrothermally synthesized Ti³⁺ self-doped TiO₂, labeled as Ti³⁺/TiO₂, was first used as a light absorber in a powder visible light-driven photocatalytic water oxidation reaction. When an iron-containing polyoxometalate Na₂₇[Fe₁₁(H₂O)₁₄(OH)₂(W₃O₁₀)₂(α-SbW₉O₃₃)₆] (Fe11) was used as a cocatalyst, an amorphous layer of active species was wrapped outside the initial Ti³⁺/TiO₂ nanorod and the in situ formed composite was labeled as F/Ti³⁺/TiO₂. When the composite F/Ti³⁺/TiO₂ was tested as a photocatalytic water oxidation catalyst, dramatically improved oxygen evolution performance was achieved. The composite F/Ti³⁺/TiO₂ showed an oxygen evolution rate of 410 μmol/g/h, which was about 11-fold higher than that of prism Ti³⁺/TiO₂. After 24 h of illumination, an O₂ yield of 36.4% was achieved. The contrast experiments, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy characterization demonstrated that FeO _x is the true cocatalyst that enhanced the oxygen evolution activity of TiO₂. A recycling experiment proved that the composite F/Ti³⁺/TiO₂ has favorable stability in the oxygen production process.

Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO4 Photoanodes

We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO₄ modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy. In particular, we prepared BiVO₄ photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO₄ due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes.

Interface Engineering of Hematite with Nacre-like Catalytic Multilayers for Solar Water Oxidation

An efficient water oxidation photoanode based on hematite has been designed and fabricated by tailored assembly of graphene oxide (GO) nanosheets and cobalt polyoxometalate (Co-POM) water oxidation catalysts into a nacre-like multilayer architecture on a hematite photoanode. The deposition of catalytic multilayers provides a high photocatalytic efficiency and photoelectrochemical stability to underlying hematite photoanodes. Compared to the bare counterpart, the catalytic multilayer electrode exhibits a significantly higher photocurrent density and large cathodic shift in onset potential (∼369 mV) even at neutral pH conditions due to the improved charge transport and catalytic efficiency from the rational and precise assembly of GO and Co-POM. Unexpectedly, the polymeric base layer deposited prior to the catalytic multilayers improves the performance even more by facilitating the transfer of photogenerated holes for water oxidation through modification of the flat band potential of the underlying photoelectrode. This approach utilizing polymeric base and catalytic multilayers provides an insight into the design of highly efficient photoelectrodes and devices for artificial photosynthesis.

Semiconducting Synthetic Melanin-Based Organic/Inorganic Hybrid Photoanodes for Solar Water Oxidation

We report the development of semiconducting melanin-based organic/inorganic hybrid photoanodes for solar water oxidation. Synthetic melanin thin-film incorporating polyoxometalate (POM) water oxidation catalysts (WOCs) are readily deposited on the surface of various n-type inorganic semiconductors (e.g., Fe₂ O₃ , BiVO₄ , and TiO₂ ) by electropolymerization. The deposition of melanin and POM hybrid (MP) thin-film results in the remarkably improved performance of an underlying semiconductor photoanode for solar water oxidation with a significantly increased photocurrent density and decreased onset potential for water oxidation through the formation of a melanin-based p-n heterojunction structure. We believe that this study can provide insights into the design and fabrication of various melanin-based optoelectronic devices by utilizing its unique physicochemical properties.

Self-Assembled Supramolecular Hybrid of Carbon Nanodots and Polyoxometalates for Visible-Light-Driven Water Oxidation

Water splitting is considered the most attractive pursuit in the field of solar energy conversion. In this study, we report the synthesis and application of a supramolecular hybrid of carbon nanodot (CD) and cobalt polyoxometalate (Co-POM) to solar water oxidation. The self-assembly of the alginate-based CD and Co-POM led to the formation of a spherical hybrid of CD/Co-POM. Owing to the facile transfer of photogenerated holes from CD under visible light irradiation, the hybrid donor-acceptor type of CD/Co-POM enabled the rapid scavenging of holes and accumulation of a long-lived oxidation state of Co-POM for efficient solar water oxidation, outperforming conventional [Ru(bpy)₃]²⁺-based systems. We believe that this study offers new insights into the development of CD-based nanocomposites with various photocatalytic and optoelectronic applications.

Non-covalent polyhedral oligomeric silsesquioxane-polyoxometalates as inorganic–organic–inorganic hybrid materials for visible-light photocatalytic splitting of water

Hydrophilic to hydrophobic conversion of polyoxometalate by integration of POSS-NH₂via non-covalent interactions for sustainable photocatalytic hydrogen production reactions.