A compendium of all-in-one solar-driven water splitting using ZnIn2S4-based photocatalysts: guiding the path from the past to the limitless future

Photocatalytic water splitting represents a leading approach to harness the abundant solar energy, producing hydrogen as a clean and sustainable energy carrier. Zinc indium sulfide (ZIS) emerges as one of the most captivating candidates attributed to its unique physicochemical and photophysical properties, attracting much interest and holding significant promise in this domain. To develop a highly efficient ZIS-based photocatalytic system for green energy production, it is paramount to comprehensively understand the strengths and limitations of ZIS, particularly within the framework of solar-driven water splitting. This review elucidates the three sequential steps that govern the overall efficiency of ZIS with a sharp focus on the mechanisms and inherent drawbacks associated with each phase, including commonly overlooked aspects such as the jeopardising photocorrosion issue, the neglected oxidative counter surface reaction kinetics in overall water splitting, the sluggish photocarrier dynamics and the undesired side redox reactions. Multifarious material design strategies are discussed to specifically mitigate the formidable limitations and bottleneck issues. This review concludes with the current state of ZIS-based photocatalytic water splitting systems, followed by personal perspectives aimed at elevating the field to practical consideration for future endeavours towards sustainable hydrogen production through solar-driven water splitting.

Regulating Excess Electrons in Reducible Metal Oxides for Enhanced Oxygen Evolution Reaction Activity: A Mini-Review

Identifying a universal activity descriptor for metal oxides, akin to the d-band center for transition metals, remains a significant challenge in catalyst design, largely due to the intricate electronic structures of metal oxides. This review highlights a major advancement in formulating the number of excess electrons (NEE) as an activity descriptor for oxygen evolution reaction (OER) on reducible metal oxide surfaces. We elaborate on the quantitative relationship between NEE and the adsorption properties of OER intermediates, and unveil the decisive role of the octet rule on the OER performance of these oxides. This insight provides a robust theoretical basis for designing effective OER catalysts. Moreover, we discuss critical experimental evidence supporting this theory and summarize recent advances in employing NEE as a guiding principle for developing highly efficient OER catalysts experimentally.

Adsorption Energy in Oxygen Electrocatalysis

Adsorption energy (AE) of reactive intermediate is currently the most important descriptor for electrochemical reactions (e.g., water electrolysis, hydrogen fuel cell, electrochemical nitrogen fixation, electrochemical carbon dioxide reduction, etc.), which can bridge the gap between catalyst's structure and activity. Tracing the history and evolution of AE can help to understand electrocatalysis and design optimal electrocatalysts. Focusing on oxygen electrocatalysis, this review aims to provide a comprehensive introduction on how AE is selected as the activity descriptor, the intrinsic and empirical relationships related to AE, how AE links the structure and electrocatalytic performance, the approaches to obtain AE, the strategies to improve catalytic activity by modulating AE, the extrinsic influences on AE from the environment, and the methods in circumventing linear scaling relations of AE. An outlook is provided at the end with emphasis on possible future investigation related to the obstacles existing between adsorption energy and electrocatalytic performance.

Electrodeposition of nickel–iron on stainless steel as an efficient electrocatalyst coating for the oxygen evolution reaction in alkaline conditions

Significant amount of effort has been devoted in the development of water electrolysis technology as the prime technology for green hydrogen production. In this paper, we investigate nickel–iron-based electrocatalytic coatings on stainless-steel substrates for commercial alkaline water electrolysers. Stainless steel electrodes for water electrolysis have received attention lately, showing that they can be a low-cost substrate for water electrolysis. Coating stainless steel with low-cost electrocatalysts can prove beneficial to lower overpotential for the oxygen evolution reaction (OER), thereby reducing the overall energy consumption of water electrolysis at an affordable cost. We show that NiFe-deposited substrates have an overpotential of 514 mV at 10 mA cm−2 current. The substrates also exhibited excellent stability in strong alkaline condition for 60 h under continuous 1.2 V working potential vs SCE. The results in full-cell electrolysers demonstrate that the electrolyser with the NiFe-coated anode could generate nearly six times as much current density compared with the bare stainless-steel substrate.

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Emerging single-atom iron catalysts for advanced catalytic systems

Due to the elusive structure-function relationship, traditional nanocatalysts always yield limited catalytic activity and selectivity, making them practically difficult to replace natural enzymes in wide industrial and biomedical applications. Accordingly, single-atom catalysts (SACs), defined as catalysts containing atomically dispersed active sites on a support material, strikingly show the highest atomic utilization and drastically boosted catalytic performances to functionally mimic or even outperform natural enzymes. The molecular characteristics of SACs (e.g., unique metal-support interactions and precisely located metal sites), especially single-atom iron catalysts (Fe-SACs) that have a similar catalytic structure to the catalytically active center of metalloprotease, enable the accurate identification of active centers in catalytic reactions, which afford ample opportunity for unraveling the structure-function relationship of Fe-SACs. In this review, we present an overview of the recent advances of support materials for anchoring an atomic dispersion of Fe. Subsequently, we highlight the structural designability of support materials as two sides of the same coin. Moreover, the applications described herein illustrate the utility of Fe-SACs in a broad scope of industrially and biologically important reactions. Finally, we present an outlook of the major challenges and opportunities remaining for the successful combination of single Fe atoms and catalysts.

Challenges of modeling nanostructured materials for photocatalytic water splitting

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano-photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

Recent advances and perspectives for solar-driven water splitting using particulate photocatalysts

The conversion and storage of solar energy to chemical energy via artificial photosynthesis holds significant potential for optimizing the energy situation and mitigating the global warming effect. Photocatalytic water splitting utilizing particulate semiconductors offers great potential for the production of renewable hydrogen, while this cross-road among biology, chemistry, and physics features a topic with fascinating interdisciplinary challenges. Progress in photocatalytic water splitting has been achieved in recent years, ranging from fundamental scientific research to pioneering scalable practical applications. In this review, we focus mainly on the recent advancements in terms of the development of new light-absorption materials, insights and strategies for photogenerated charge separation, and studies towards surface catalytic reactions and mechanisms. In particular, we emphasize several efficient charge separation strategies such as surface-phase junction, spatial charge separation between facets, and polarity-induced charge separation, and also discuss their unique properties including ferroelectric and photo-Dember effects on spatial charge separation. By integrating time- and space-resolved characterization techniques, critical issues in photocatalytic water splitting including photoinduced charge generation, separation and transfer, and catalytic reactions are analyzed and reviewed. In addition, photocatalysts with state-of-art efficiencies in the laboratory stage and pioneering scalable solar water splitting systems for hydrogen production using particulate photocatalysts are presented. Finally, some perspectives and outlooks on the future development of photocatalytic water splitting using particulate photocatalysts are proposed.

Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective

Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

Electron and Hole Mobilities in Bulk Hematite from Spin-Constrained Density Functional Theory

Transition metal oxide materials have attracted much attention for photoelectrochemical water splitting, but problems remain, e.g. the sluggish transport of excess charge carriers in these materials, which is not well understood. Here we use periodic, spin-constrained and gap-optimized hybrid density functional theory to uncover the nature and transport mechanism of holes and excess electrons in a widely used water splitting material, bulk-hematite (α-Fe₂O₃). We find that upon ionization the hole relaxes from a delocalized band state to a polaron localized on a single iron atom with localization induced by tetragonal distortion of the six surrounding iron-oxygen bonds. This distortion is responsible for sluggish hopping transport in the Fe-bilayer, characterized by an activation energy of 70 meV and a hole mobility of 0.031 cm²/(V s). By contrast, the excess electron induces a smaller distortion of the iron-oxygen bonds resulting in delocalization over two neighboring Fe units. We find that 2-site delocalization is advantageous for charge transport due to the larger spatial displacements per transfer step. As a result, the electron mobility is predicted to be a factor of 3 higher than the hole mobility, 0.098 cm²/(V s), in qualitative agreement with experimental observations. This work provides new fundamental insight into charge carrier transport in hematite with implications for its photocatalytic activity.

Research Progress of Oxygen Evolution Reaction Catalysts for Electrochemical Water Splitting

The development of a low-cost and high-efficiency oxygen evolution reaction (OER) catalyst is essential to meet the future industrial demand for hydrogen production by electrochemical water splitting. Given the limited reserves of noble metals and many competitive applications in environmental protection, new energy, and chemical industries, many studies have focused on exploring new and efficient non-noble metal catalytic systems, improving the understanding of the OER mechanism of non-noble metal surfaces, and designing electrocatalysts with higher activity than traditional noble metals. This Review summarizes the research progress of anode OER catalysts for hydrogen production by electrochemical water splitting in recent years, for noble metal and non-noble metal catalysts, where non-noble metal catalysts are highlighted. The categories are as follows: (1) Transition metal-based compounds, including transition metal-based oxides, transition metal-based layered hydroxides, and transition metal-based sulfides, phosphides, selenides, borides, carbides, and nitrides. Transition metal-based oxides can also be divided into perovskite, spinel, amorphous, rock-salt-type, and lithium oxides according to their different structures. (2) Carbonaceous materials and their composite materials with transition metals. (3) Transition metal-based metal-organic frameworks and their derivatives. Finally, the challenges and future development of the OER process of water splitting are discussed.

Anticatalytic Strategies to Suppress Water Electrolysis in Aqueous Batteries

Aqueous electrolytes are the leading candidate to meet the surging demand for safe and low-cost storage batteries. Aqueous electrolytes facilitate more sustainable battery technologies due to the attributes of being nonflammable, environmentally benign, and cost effective. Yet, water's narrow electrochemical stability window remains the primary bottleneck for the development of high-energy aqueous batteries with long cycle life and infallible safety. Water's electrolysis leads to either hydrogen evolution reaction (HER) or oxygen evolution reaction (OER), which causes a series of dire consequences, including poor Coulombic efficiency, short device longevity, and safety issues. These are often showstoppers of a new aqueous battery technology besides the low energy density. Prolific progress has been made in the understanding of HER and OER from both catalysis and battery fields. Unfortunately, a systematic review on these advances from a battery chemistry standpoint is lacking. This review provides in-depth discussions on the mechanisms of water electrolysis on electrodes, where we summarize the critical influencing factors applicable for a broad spectrum of aqueous battery systems. Recent progress and existing challenges on suppressing water electrolysis are discussed, and our perspectives on the future development of this field are provided.

Photoelectrochemical Behavior and Computational Insights for Pristine and Doped NdFeO3 Thin-Film Photocathodes

Among the different strategies that are being developed to solve the current energy challenge, harvesting energy directly from sunlight through a tandem photoelectrochemical cell (water splitting) is most attractive. Its implementation requires the development of stable and efficient photocathodes, NdFeO3 being a suitable candidate among ternary oxides. In this study, transparent NdFeO3 thin-film photocathodes have been successfully prepared by a citric acid-based sol-gel procedure, followed by thermal treatment in air at 640 °C. These electrodes show photocurrents for both the hydrogen evolution and oxygen reduction reactions. Doping with Mg2+ and Zn2+ has been observed to significantly enhance the photoelectrocatalytic performance of NdFeO3 toward oxygen reduction. Magnesium is slightly more efficient as a dopant than Zn, leading to a multiplication of the photocurrent by a factor of 4-5 for a doping level of 5 at % (with respect to iron atoms). This same trend is observed for hydrogen evolution. The beneficial effect of doping is primarily attributed to an increase in the density and a change in the nature of the majority charge carriers. DFT calculations help to rationalize the behavior of NdFeO3 by pointing to the importance of nanostructuring and doping. All in all, NdFeO3 has the potential to be used as a photocathode in photoelectrochemical applications, although efforts should be directed to limit surface recombination.

Tuning the Electronic Structures of Multimetal Oxide Nanoplates to Realize Favorable Adsorption Energies of Oxygenated Intermediates

Highly active oxygen evolution reaction (OER) electrocatalysts are important to effectively transform renewable electricity to fuel and chemicals. In this work, we construct a series of multimetal oxide nanoplate OER electrocatalysts through successive cation exchange followed by electrochemical oxidation, whose electronic structure and diversified metal active sites can be engineered via the mutual synergy among multiple metal species. Among the examined multimetal oxide nanoplates, CoCeNiFeZnCuO_x nanoplates exhibit the optimal adsorption energy of OER intermediates. Together with the high electrochemical active surface area, the CoCeNiFeZnCuO_x nanoplates manage to deliver a small overpotential of 211 mV at an OER current density of 10 mA cm^-2 (η₁₀) with a Tafel slope as low as 21 mV dec^-1 in 1 M KOH solution, superior to commercial IrO₂ (339 mV at η₁₀, Tafel slope of 55 mV dec^-1), which can be stably operated at 10 mA cm^-2 (at an overpotential of 211 mV) and 100 mA cm^-2 (at an overpotential of 307 mV) for 100 h.

A boron-decorated melon-based carbon nitride as a metal-free photocatalyst for N2 fixation: a DFT study

On the basis of the electron "acceptance-donation" concept, a boron decorated melon-based carbon nitride (CN) is studied as a metal-free photocatalyst to efficiently reduce N2 to NH3 under visible light irradiation. The results revealed that a boron-interstitial (Bint)-decorated melon-based CN has an outstanding N2 reduction capacity through the enzymatic mechanism with a rather low overpotential (0.32 V). The excellent efficiency and selectivity of Bint-decorated melon-based CN in N2 reduction reaction (NRR) are attributed to the concentrated spin polarization on the B atom, the significant enhancement of visible and infrared light absorption, and the effective inhibition of the competitive hydrogen evolution reaction (HER). Importantly, B-doped melon-based CN has been successfully synthesized in the experiments, so obtaining Bint-decorated melon is promising, while proton transfer from the -NH2 group in CN to the B atom surely will affect the functionality of the catalyst through deactivation of the N2 adsorption site. Our study provides a novel single atom metal-free photocatalyst with high efficiency for NRR, which is conducive to the sustainable synthesis of ammonia.

Predicted CsSi compound: a promising material for photovoltaic applications

Exploration of photovoltaic materials has received enormous interest for a wide range of both fundamental and applied research. Therefore, in this work, we identify a CsSi compound with a Zintl phase as a promising candidate for photovoltaic material by using a global structure prediction method. Electronic structure calculations indicate that this phase possesses a quasi-direct band gap of 1.45 eV, suggesting that its optical properties could be superior to those of diamond-Si for capturing sunlight from the visible to the ultraviolet range. In addition, a novel silicon allotrope is obtained by removing Cs atoms from this CsSi compound. The superconducting critical temperature (T_c) of this phase was estimated to be of 9 K in terms of a substantial density of states at the Fermi level. Our findings represent a new promising CsSi material for photovoltaic applications, as well as a potential precursor of a superconducting silicon allotrope.

Importance of catalyst–photoabsorber interface design configuration on the performance of Mo-doped BiVO4 water splitting photoanodes

Photoelectrochemical water splitting is mostly impeded by the slow kinetics of the oxygen evolution reaction. The construction of photoanodes that appreciably enhance the efficiency of this process is of vital technological importance towards solar fuel synthesis. In this work, Mo-modified BiVO4 (Mo:BiVO4), a promising water splitting photoanode, was modified with various oxygen evolution catalysts in two distinct configurations, with the catalysts either deposited on the surface of Mo:BiVO4 or embedded inside a Mo:BiVO4 film. The investigated catalysts included monometallic, bimetallic, and trimetallic oxides with spinel and layered structures, and nickel boride (NixB). In order to follow the influence of the incorporated catalysts and their respective properties, as well as the photoanode architecture on photoelectrochemical water oxidation, the fabricated photoanodes were characterised for their optical, morphological, and structural properties, photoelectrocatalytic activity with respect to evolved oxygen, and recombination rates of the photogenerated charge carriers. The architecture of the catalyst-modified Mo:BiVO4 photoanode was found to play a more decisive role than the nature of the catalyst on the performance of the photoanode in photoelectrocatalytic water oxidation. Differences in the photoelectrocatalytic activity of the various catalyst-modified Mo:BiVO4 photoanodes are attributed to the electronic structure of the materials revealed through differences in the Fermi energy levels. This work thus expands on the current knowledge towards the design of future practical photoanodes for photoelectrocatalytic water oxidation.

Polaronic structure of excess electrons and holes for a series of bulk iron oxides

With the use of a gap-optimized hybrid functional and large supercells, it is found that while the electron hole polaron generally localises onto a single iron site, the electron polaron localises across two iron sites of the same spin layer.

Ni-Fe-Cr-Oxides: An Efficient Catalyst Activated by Visible Light for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a thermodynamically and kinetically demanding process, therefore it requires the use of catalysts enabling to meet technologically relevant conditions. Here, we realized efficient OER catalysts fabricated using relatively cheap precursors consisting of earth-abundant metal oxides, i.e. nickel, iron, and chromium, and a simple one-step preparation method. It is shown that the catalyst is activated by anodic polarization under irradiation with visible light, which allows decreasing the overpotential necessary for the OER.

Strategies of Anode Materials Design towards Improved Photoelectrochemical Water Splitting Efficiency

This review presents the latest processes for designing anode materials to improve the efficiency of water photolysis. Based on different contributions towards the solar-to-hydrogen efficiency, we mainly review the strategies to enhance the light absorption, facilitate the charge separation, and enhance the surface charge injection. Although great achievements have been obtained, the challenges faced in the development of anode materials for solar energy to make water splitting remain significant. In this review, the major challenges to improve the conversion efficiency of photoelectrochemical water splitting reactions are presented. We hope that this review helps researchers in or coming to the field to better appreciate the state-of-the-art, and to make a better choice when they embark on new research in photocatalytic water splitting.

Water Splitting Reaction at Polar Lithium Niobate Surfaces

Water splitting is a highly promising, environmentally friendly approach for hydrogen production. It is often discussed in the context of carbon dioxide free combustion and storage of electrical energy after conversion to chemical energy. Since the oxidation and reduction reactions are related to significant overpotentials, the search for suitable catalysts is of particular importance. Ferroelectric materials, for example, lithium niobate, attracted considerable interest in this respect. Indeed, the presence of surfaces with different polarizations and chemistries leads to spatial separation of reduction and oxidation reactions, which are expected to be boosted by the electrons and holes available at the positive and negative surfaces, respectively. Employing the density functional theory and a simplified thermodynamic approach, we estimate the overpotentials related to the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) on both polar LiNbO3 (0001) surfaces. Our calculations performed for ideal surfaces in vacuum predict the lowest overpotential for the hydrogen evolution reaction (0.4 V) and for the oxygen evolution reaction (1.2 V) at the positive and at the negative surfaces, respectively, which are lower than (or comparable with) commonly employed catalysts. However, calculations performed to model the aqueous solution in which the reactions occur reveal that the presence of water substantially increases the required overpotential for the HER, even inverting the favorable polarization direction for oxidation and reduction reactions. In aqueous solution, we predict an overpotential of 1.2 V for the HER at the negative surface and 1.1 V for the OER at the positive surface.

Integrated Design and Control of Various Hydrogen Production Flowsheet Configurations via Membrane Based Methane Steam Reforming

This work focuses on the development and implementation of an integrated process design and control framework for a membrane-based hydrogen production system based on low temperature methane steam reforming. Several alternative flowsheet configurations consisted of either integrated membrane reactor modules or successive reactor and membrane separation modules are designed and assessed by considering economic and controller dynamic performance criteria simultaneously. The design problem is expressed as a non-linear dynamic optimization problem incorporating a nonlinear dynamic model for the process system and a linear model predictive controller aiming to maintain the process targets despite the effect of disturbances. The large dimensionality of the disturbance space is effectively addressed by focusing on disturbances along the direction that causes the maximum process variability revealed by the analysis of local sensitivity information for the process system. Design results from a multi-objective optimization study, where only the annualized equipment and operational costs are minimized, are used as reference case in order to evaluate the proposed design framework. Optimization results demonstrate the controller’s ability to track the imposed setpoint changes and alleviate the effects of multiple simultaneous disturbances. Also, significant economic improvements are observed by the implementation of the integrated design and control framework compared to the traditional design methodology, where process and controller design are performed sequentially.

The importance of charge redistribution during electrochemical reactions: a density functional theory study of silver orthophosphate (Ag3PO4)

The charge redistribution during oxygen evolution reaction relates to the electrochemical activity as shown for Ag₃PO₄ structures.

Effects of Metal Combinations on the Electrocatalytic Properties of Transition-Metal-Based Layered Double Hydroxides for Water Oxidation: A Perspective with Insights

Transition-metal-based layered double hydroxides (TM LDHs) have emerged as highly efficient water oxidation catalysts. They are promising and have the potential to replace the rare and expensive precious metal-based ones such as RuO2 and IrO2, which have been well established. In this perspective, we will summarize the current development of TM LDHs as oxygen evolution reaction (OER) catalysts toward electrochemical water splitting. Particular emphasis will be placed on the roles of the transition-metal cations and the effects of their combination on their catalytic performance for the OER. It is hoped that this perspective will provide fundamental guidelines for future researches in this booming area.

Plasmonic Surface Lattice Resonances: A Review of Properties and Applications

When metal nanoparticles are arranged in an ordered array, they may scatter light to produce diffracted waves. If one of the diffracted waves then propagates in the plane of the array, it may couple the localized plasmon resonances associated with individual nanoparticles together, leading to an exciting phenomenon, the drastic narrowing of plasmon resonances, down to 1–2 nm in spectral width. This presents a dramatic improvement compared to a typical single particle resonance line width of >80 nm. The very high quality factors of these diffractively coupled plasmon resonances, often referred to as plasmonic surface lattice resonances, and related effects have made this topic a very active and exciting field for fundamental research, and increasingly, these resonances have been investigated for their potential in the development of practical devices for communications, optoelectronics, photovoltaics, data storage, biosensing, and other applications. In the present review article, we describe the basic physical principles and properties of plasmonic surface lattice resonances: the width and quality of the resonances, singularities of the light phase, electric field enhancement, etc. We pay special attention to the conditions of their excitation in different experimental architectures by considering the following: in-plane and out-of-plane polarizations of the incident light, symmetric and asymmetric optical (refractive index) environments, the presence of substrate conductivity, and the presence of an active or magnetic medium. Finally, we review recent progress in applications of plasmonic surface lattice resonances in various fields.

Au Nanoparticle Sub-Monolayers Sandwiched between Sol-Gel Oxide Thin Films

Sub-monolayers of monodisperse Au colloids with different surface coverage have been embedded in between two different metal oxide thin films, combining sol-gel depositions and proper substrates functionalization processes. The synthetized films were TiO₂, ZnO, and NiO. X-ray diffraction shows the crystallinity of all the oxides and verifies the nominal surface coverage of Au colloids. The surface plasmon resonance (SPR) of the metal nanoparticles is affected by both bottom and top oxides: in fact, the SPR peak of Au that is sandwiched between two different oxides is centered between the SPR frequencies of Au sub-monolayers covered with only one oxide, suggesting that Au colloids effectively lay in between the two oxide layers. The desired organization of Au nanoparticles and the morphological structure of the prepared multi-layered structures has been confirmed by Rutherford backscattering spectrometry (RBS), Secondary Ion Mass Spectrometry (SIMS), and Scanning Electron Microscopy (SEM) analyses that show a high quality sandwich structure. The multi-layered structures have been also tested as optical gas sensors.

Water splitting catalyzed by titanium dioxide decorated with plasmonic nanoparticles

The development of active, cheap, efficient and visible-light-driven water splitting catalysts is currently the center of intense research efforts. Amongst the most promising avenues, the design of titania and plasmonic nanoparticle hybrids is particularly appealing. Titania has been known for long to be an active photocatalyst, able to perform water splitting under light irradiation. However, this activity is limited to the ultraviolet spectrum and suffers from too rapid charge carrier recombination. The addition of plasmonic nanostructures enables to push absorption properties to the visible region and prevent unwanted charge recombination. In this review, we explain the principles behind the activity of such nanohybrids towards visible light water splitting and detail the recent research developments relying on plasmonic metals, namely Au, Ag and Cu.

Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for Solar Fuel Photocatalysis

We use plasmonic Au-TiO₂ aerogels as a platform in which to marry synthetically thickened particle-particle junctions in TiO₂ aerogel networks to Au∥TiO₂ interfaces and then investigate their cooperative influence on photocatalytic hydrogen (H₂) generation under both broadband (i.e., UV + visible light) and visible-only excitation. In doing so, we elucidate the dual functions that incorporated Au can play as a water reduction cocatalyst and as a plasmonic sensitizer. We also photodeposit non-plasmonic Pt cocatalyst nanoparticles into our composite aerogels in order to leverage the catalytic water-reducing abilities of Pt. This Au-TiO₂/Pt arrangement in three dimensions effectively utilizes conduction-band electrons injected into the TiO₂ aerogel network upon exciting the Au SPR at the Au∥TiO₂ interface. The extensive nanostructured high surface-area oxide network in the aerogel provides a matrix that spatially separates yet electrochemically connects plasmonic nanoparticle sensitizers and metal nanoparticle catalysts, further enhancing solar-fuels photochemistry. We compare the photocatalytic rates of H₂ generation with and without Pt cocatalysts added to Au-TiO₂ aerogels and demonstrate electrochemical linkage of the SPR-generated carriers at the Au∥TiO₂ interfaces to downfield Pt nanoparticle cocatalysts. Finally, we investigate visible light-stimulated generation of conduction band electrons in Au-TiO₂ and TiO₂ aerogels using ultrafast visible pump/IR probe spectroscopy. Substantially more electrons are produced at Au-TiO₂ aerogels due to the incorporated SPR-active Au nanoparticle, whereas the smaller population of electrons generated at Au-free TiO₂ aerogels likely originate at shallow traps in the high surface-area mesoporous aerogel.

Phase-Selective Syntheses of Cobalt Telluride Nanofleeces for Efficient Oxygen Evolution Catalysts

Cobalt-based nanomaterials have been intensively explored as promising noble-metal-free oxygen evolution reaction (OER) electrocatalysts. Herein, we report phase-selective syntheses of novel hierarchical CoTe₂ and CoTe nanofleeces for efficient OER catalysts. The CoTe₂ nanofleeces exhibited excellent electrocatalytic activity and stablity for OER in alkaline media. The CoTe₂ catalyst exhibited superior OER activity compared to the CoTe catalyst, which is comparable to the state-of-the-art RuO₂ catalyst. Density functional theory calculations showed that the binding strength and lateral interaction of the reaction intermediates on CoTe₂ and CoTe are essential for determining the overpotential required under different conditions. This study provides valuable insights for the rational design of noble-metal-free OER catalysts with high performance and low cost by use of Co-based chalcogenides.