Size-selected TiO₂ nanocluster catalysts for efficient photoelectrochemical water splitting

Low density phases of TiO2 by cluster self-assembly

The interest in titanium dioxide (TiO2 ) phases is growing due to the number of applications in cosmetics, food industry and photocatalysis, an increase that is driven by its exceptional properties when engineered at the nanoscale like in the form of nanoparticles. Our goal is to discover unknown low-density phases of TiO2 , with potential for applications in various fields. We then use well-known TiO2 clusters as fundamental building blocks to be self-assembled into unique structures to study their distinct characteristics. Density functional calculations are employed to relax the structures and identify the most stable TiO2 structures within an energy range of 0.1 eV per atom from the rutile and anatase phases, which are confirmed, validating our methodology. Going beyond conventional phases, we found two-dimensional TiO2 structures, previously explored in separate studies, and showing typical structures of transition metal dichalcogenide layers, that forge a bridge between different TiO2 structures. It is noteworthy that our investigation uncovered an entirely novel class of TiO2 structures featuring hexagonal cages like beehive channels, opening novel phases with huge potential. These discovered low-density phases are interesting, particularly the hexagonal cage structures with remarkable large gaps, because they introduce other dimensions for uncharted applications in the ever-growing TiO2 landscape.

Unlocking More Potentials in Two-Dimensional Space: Disorder Engineering in Two-Dimensional Amorphous Carbon

The theory of the nature of glass has been described as the deepest but unsolved problem in solid state theory. The fundamental understanding of the structural characteristics of glassy materials and disorder-property correspondence remains incomplete due to difficulties in fully characterizing disordered structures in three-dimensional materials. Recently, two-dimensional amorphous materials were treated as an atomic-level playground to uncover previously unknown structure-property relationships in vitreous materials. Here, we summarize recent research on one prototypical material, two-dimensional amorphous carbon, including atomic structural characterizations, controllable synthesis, exotic properties, and application potentials. Fundamental discrepancies only induced by the amorphous nature, when compared with crystalline materials, will be highlighted. Finally, we discuss the restricted definition of two-dimensional amorphous carbon, existing challenges, and future research directions.

Boosting sluggish photocatalytic hydrogen evolution through piezo-stimulated polarization: a critical review

To address the growing energy demand, remarkable progress has been made in transferring the fossil fuel-based economy to hydrogen-based environmentally friendly photocatalytic technology. However, the sluggish production rate due to the quick charge recombination and slow diffusion process needs careful engineering to achieve the benchmark photocatalytic efficiency. Piezoelectric photocatalysis has emerged as a promising field in recent years due to its improved catalytic performance facilitated by a built-in electric field that promotes the effective separation of excitons when subjected to mechanical stimuli. This review discusses the recent progress in piezo-photocatalytic hydrogen evolution while elaborating on the mechanistic pathway, effect of piezo-polarization and various strategies adopted to improve piezo-photocatalytic activity. Moreover, our review systematically emphasizes the fundamentals of piezoelectricity and piezo-phototronics along with the operational mechanism for designing efficient piezoelectric photocatalysts. Finally, the summary and outlooks provide insight into the existing challenges and outline the future prospects and roadmap for the development of next-generation piezo-photocatalysts towards hydrogen evolution.

Induced Complementary Resistive Switching in Forming-Free TiOx/TiO2/TiOx Memristors

The undesirable sneak current path is one of the key challenges in high-density memory integration for the emerging cross-bar memristor arrays. This work demonstrates a new heterojunction design of oxide multilayer stacking with different oxygen vacancy contents to manipulate the oxidation state. We show that the bipolar resistive switching (BRS) behavior of the Pt/TiO_x/Pt cross-bar structure can be changed to complementary resistive switching (CRS) by introducing a thin TiO₂ layer in the middle of the TiO_x layer to obtain a Pt/TiO_x/TiO₂/TiO_x/Pt device architecture with a double-junction active matrix. In contrast to the BRS in a single-layer TiO_x matrix, the device with a double-junction matrix remains in a high-resistance state in the voltage range below the SET voltage, which makes it an efficient structure to overcome the sneak path constraints of undesired half-selected cells that lead to incorrect output reading. This architecture is capable of eliminating these half-selected cells between the nearby cross-bar cells in a smaller programming voltage range. A simplified model for the switching mechanism can be used to account for the observed high-quality switching performance with excellent endurance and current retention properties.

Photosensitization Dynamics of Stable Copper Nanoclusters inside the Aqueous Core of Reverse Micelles with Different Pool Sizes

The perennial problem of instability of fluorescent copper nanoclusters (Cu NCs), stemming principally from aerial oxidation, has prevented their vivid usage in energy harvesting compared to the other metal NCs. However, replacement of the much expensive metal NCs with the cheaper Cu NCs is desirable if the functions are met with. Although thiolate protection of Cu NCs could bring some stability to them, appreciably decentlystable Cu NCs were produced inside the aqueous core of reverse micelles (RMs). However, this recent development has not been further explored on the photosensitization of the Cu NCs inside the RMs and their controlled modulation as energy antenna. Here we have synthesized stable Cu NCs inside the aqueous core of RMs with three different pool sizes and established photoinduced electron transfer (PET) to an electron acceptor. Considering the bulk quencher concentration, it appears that the extent of PET increases with decrease in the size of the aqueous core of RMs. However, calculating the effective concentration of the electron acceptor inside the RMs and considering the polarity of the microheterogeneous systems, it becomes clear that the extent of PET actually decreases with decrease in the size of the aqueous pool (w₀, i.e., [H₂O]/[AOT]) = 5-20) in the RMs. This proof of concept and the results are promising toward applications in PET-driven phenomena such as solar cells or batteries.

Defect-Rich Dopant-Free ZrO2 Nanoclusters and Their Size-Dependent Ferromagnetism

As an intermediate form of matter between a single atom or molecule and the bulk, nanoclusters (NCs) provide novel properties because of their high surface area-to-volume ratios and distinct physical and electronic structures. These ultrasmall NCs offer a new approach to advance charge-spin manipulation for novel devices, including spintronics and magnetic tunneling junctions. Here, we deposit monosized ZrO₂ NCs over a large area by using gas-phase aggregation followed by in situ size selection by a quadrupole mass filter. These size-specific NCs exhibit sub-oxide photoemission features at binding energies that are dependent on the cluster size (from 3 to 9 nm), which are attributed to different oxygen vacancy defect states. These dopant-free ZrO₂ NCs also show strongly size-dependent ferromagnetism, which provides distinct advantages in solubility and homogeneity of magnetism when compared to traditional dilute magnetic semiconductors. A defect-band hybridization-induced magnetic polaron model is proposed to explain the origin of this size-dependent ferromagnetism. This work demonstrates a new protocol of magnetization manipulation by size control and promises potential applications based on these defect-rich size-selected NCs.

Hollowsphere Nanoheterojunction of g-C3N4@TiO2 with High Visible Light Photocatalytic Property

In this work, g-C₃N₄@TiO₂ nanostructures with hollow sphere morphology, small grain size, high crystalline quality, and high surface area are successfully synthesized by the annealing method using melamine and hollowsphere precursor, which could be a universal method to synthesis hollow sphere nanoheterojunction. Excellent photocatalytic property was observed from the as-prepared g-C₃N₄@TiO₂ nanostructure with 466.43 μmol·g^-1·h^-1 hydrogen generation rate under visible light irradiation (>420 nm), which was 5.5 times as much as the control couple, nanoparticle nanoheterojunction g-C₃N₄@TiO₂. No apparent deactivation was found during the follow-up cycle performance test. The special morphology and the heterojunction construction contribute to both visible light absorption and photogenerated electron-hole pair separation efficiency and finally to the photocatalytic property. The content of g-C₃N₄ was proved to be an important parameter for the promotion of the photocatalytic property. Overlarge content may lead to lower photogenerated electron-hole pair separation efficiency.

Magnetron-sputtered copper nanoparticles: lost in gas aggregation and found by in situ X-ray scattering

Magnetron discharge in a cold buffer gas represents a liquid-free approach to the synthesis of metal nanoparticles (NPs) with tailored structure, chemical composition and size. Despite a large number of metal NPs that were successfully produced by this method, the knowledge of the mechanisms of their nucleation and growth in the discharge is still limited, mainly because of the lack of in situ experimental data. In this work, we present the results of in situ Small Angle X-ray Scattering measurements performed in the vicinity of a Cu magnetron target with Ar used as a buffer gas. Condensation of atomic metal vapours is found to occur mainly at several mm distance from the target plane. The NPs are found to be captured preferentially within a region circumscribed by the magnetron plasma ring. In this capture zone, the NPs grow to the size of 90 nm whereas smaller ones sized 10-20 nm may escape and constitute a NP beam. Time-resolved measurements of the discharge indicate that the electrostatic force acting on the charged NPs may be largely responsible for their capturing nearby the magnetron.

Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics

Ab initio molecular dynamics simulations are reported for water-embedded TiO₂ surfaces to determine the diffusive and reactive behavior at full hydration. A three-domain model is developed for six surfaces [rutile (110), (100), and (001), and anatase (101), (100), and (001)] which describes waters as "hard" (irreversibly bound to the surface), "soft" (with reduced mobility but orientation freedom near the surface), or "bulk." The model explains previous experimental data and provides a detailed picture of water diffusion near TiO₂ surfaces. Water reactivity is analyzed with a graph-theoretic approach that reveals a number of reaction pathways on TiO₂ which occur at full hydration, in addition to direct water splitting. Hydronium (H₃O⁺) is identified to be a key intermediate state, which facilitates water dissociation by proton hopping between intact and dissociated waters near the surfaces. These discoveries significantly improve the understanding of nanoscale water dynamics and reactivity at TiO₂ interfaces under ambient conditions.

Isomerism in Au-Ag Alloy Nanoclusters: Structure Determination and Enantioseparation of [Au9Ag12(SR)4(dppm)6X6]3

Revealing structural isomerism in a nanocluster remains significant but challenging. Herein, we have obtained a pair of structural isomers, [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-C and [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-Ac [dppm = bis(diphenyphosphino)methane; HSR = 1-adamantanethiol/ tert-butylmercaptan; X = Br/Cl; C stands for one of the structural isomers being chiral; Ac stands for another being achiral], that show different structures as well as different chiralities. These structures are determined by single-crystal X-ray diffraction and further confirmed by high-resolution electrospray ionization mass spectrometry. On the basis of the isomeric structures, the most important finding is the different arrangements of the Au₅Ag₈@Au₄ metal core, leading to changes in the overall shape of the cluster, which is responsible for structural isomerism. Meanwhile, the two enantiomers of [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-C are separated by high-performance liquid chromatography. Our work will contribute to a deeper understanding of the structural isomerism in noble-metal nanoclusters and enrich the chiral nanocluster.

Photoelectrocatalytic effect of unbalanced RF magnetron sputtered TiO2 thin film on ITO-coated patterned SiO2 nanocone arrays

Highly increased photocurrent response of unbalanced RF magnetron sputtered TiO₂ thin film on ITO-coated patterned SiO₂ nanocone arrays.

High-Performance Single-Active-Layer Memristor Based on an Ultrananocrystalline Oxygen-Deficient TiOx Film

The theoretical and practical realization of memristive devices has been hailed as the next step for nonvolatile memories, low-power remote sensing, and adaptive intelligent prototypes for neuromorphic and biological systems. However, the active materials of currently available memristors need to undergo an often destructive high-bias electroforming process in order to activate resistive switching. This limits their device performance in switching speed, endurance/retention, and power consumption upon high-density integration, due to excessive Joule heating. By employing a nanocrystalline oxygen-deficient TiO_x switching matrix to localize the electric field at discrete locations, it is possible to resolve the Joule heating problem by reducing the need for electroforming at high bias. With a Pt/TiO_x/Pt stacking architecture, our device follows an electric field driven, vacancy-modulated interface-type switching that is sensitive to the junction size. By scaling down the junction size, the SET voltage and output current can be reduced, and a SET voltage as low as +0.59 V can be obtained for a 5 × 5 μm² junction size. Along with its potentially fast switching (over 10⁵ cycles with a 100 μs voltage pulse) and high retention (over 10⁵ s) performance, memristors based on these disordered oxygen-deficient TiO_x films promise viable building blocks for next-generation nonvolatile memories and other logic circuit systems.

Efficient photoelectrochemical water splitting on ultrasmall defect-rich TaOx nanoclusters enhanced by size-selected Pt nanocluster promoters

Formation of nanoclusters has attracted a lot of attention in recent years because of their distinct properties from isolated atoms and bulk solids. Here, we focus on the catalytic properties of supported transition metal oxide nanoclusters, such as TaO₂, with a well-defined size distribution below 10 nm. We show that their catalytic performance can be greatly enhanced by introducing a reaction promoter such as Pt. Different combinations of precisely size-selected, defect-rich TaO_x and Pt nanoclusters are produced by a gas-phase aggregation technique in a special DC magnetron sputtering system. Argon flow rate and aggregation length are carefully optimized to control the sizes of these ultrasmall TaO_x and Pt nanoclusters by using a quadrupole mass filter, and TEM studies reveal the different crystalline nature of TaO_x (amorphous) and Pt (crystalline) nanoclusters. We have further demonstrated the size-dependent photoanode activity of (TaO_x, Pt) nanocluster systems in a photoelectrochemical water splitting reaction, where the Pt nanocluster promoters are found to provide a significant enhancement in the photocurrent density, approximately tripled that was observed from just TaO_x nanocluster catalysts alone. The photocurrent density and photoconversion efficiency tend to reduce when Pt nanoclusters become overpopulated due to blocking of the photosensitive TaO_x surface. Reducing the Pt nanocluster size resolves this problem by incorporating a greater number of smaller nanocluster promoters without blocking TaO_x, which leads to further enhancement in the photocurrent density. The enhanced photocatalytic activity is attributed to synergetic effects introduced by the Pt nanoclusters that act as temporary charge storage sites to facilitate effective separation of a large number of electron-hole pairs, generated from a large number of active sites on the defect-rich amorphous TaO_x nanoclusters upon illumination.

Modeling the formation of TiO2 ultra-small nanoparticles

The structures of TiO₂ ultra-small nanoparticles (USNPs) at the atomistic level have been predicted because of their potential importance in catalytic, environmental, biological and energy applications. Low energy (TiO₂)_n clusters and USNPs (n up to 80 at the B3LYP/DZVP2 level, and up to 384 at the PM6 level) were found using a novel bottom-up global optimization approach that is based on all-atom real-space calculations. These structures include USNPs that belong to 1-D, 2-D and 3-D USNP series where all the members share the same fragment types and local translational symmetries. Most of the metastable 2-D and 3-D USNPs contain tubular building blocks similar to the 1-D USNPs. The 3-D USNPs that resemble the bulk anatase are predicted to be energetically favorable structures for 64 ≤ n ≤ 384. A fragment-based model was developed to relate the energy with geometry for the 1-D, 2-D and 3-D USNPs. Surface energy densities were predicted for surface fragments at the different positions of the USNPs using this new model. Based on the predicted surface energy densities and the partial density of states, the most catalytically active sites for the anatase-like 3-D USNPs were predicted to be the kink sites on Face-x surfaces consisting of an octahedral-Ti, the step (edge) sites between the Face-x and Face-y surfaces consisting of a square pyramidal-Ti (on Face-x), and the step sites consisting of a trigonal bipyramidal Ti on the Face-y surfaces.

Embedding Au Quantum Dots in Rimous Cadmium Sulfide Nanospheres for Enhanced Photocatalytic Hydrogen Evolution

Rational design and development of new-generation photocatalysts with high hydrogen evolution activity is recognized as an effective strategy to settle energy crisis. To this regard, hybrid photocatalysts of Au quantum dots embedded in rimous cadmium sulfide nanospheres are synthesized by using a simple hydrothermal process followed by photoreduction. The rimous cadmium sulfide nanospheres with rough surface and irregular fissures greatly strengthen their adhesion and interaction with Au quantum dots, which effectively facilitates the separation, restrains the recombination, and accelerates the consumption of photoinduced electron-hole pairs. Impressively, the highest photocatalytic activity for hydrogen generation (601.2 μmol h^-1 g^-1 ) and organic pollutant degradation (100% degradation in 80 min) is obtained by adjusting the Au mass loading to achieve uniform distribution. This work paves new way to the exploitation of highly efficient metal/semiconductor hybrid photocatalysts for clean energy generation and environment restoration.

Solar-rechargeable battery based on photoelectrochemical water oxidation: Solar water battery

As an alternative to the photoelectrochemical water splitting for use in the fuel cells used to generate electrical power, this study set out to develop a solar energy rechargeable battery system based on photoelectrochemical water oxidation. We refer to this design as a "solar water battery". The solar water battery integrates a photoelectrochemical cell and battery into a single device. It uses a water oxidation reaction to simultaneously convert and store solar energy. With the solar water battery, light striking the photoelectrode causes the water to be photo-oxidized, thus charging the battery. During the discharge process, the solar water battery reduces oxygen to water with a high coulombic efficiency (>90%) and a high average output voltage (0.6 V). Because the reduction potential of oxygen is more positive [E(0) (O2/H2O) = 1.23 V vs. NHE] than common catholytes (e.g., iodide, sulfur), a high discharge voltage is produced. The solar water battery also exhibits a superior storage ability, maintaining 99% of its specific discharge capacitance after 10 h of storage, without any evidence of self-discharge. The optimization of the cell design and configuration, taking the presence of oxygen in the cell into account, was critical to achieving an efficient photocharge/discharge.

Reactive wetting properties of TiO2 nanoparticles predicted by ab initio molecular dynamics simulations

Small-sized wet TiO2 nanoparticles have been investigated by ab initio molecular dynamics simulations. Chemical and physical adsorption of water on the TiO2-water interface was studied as a function of water content, ranging from dry nanoparticles to wet nanoparticles with monolayer coverage of water. The surface reactivity was shown to be a concave function of water content and driven by surface defects. The local coordination number at the defect was identified as the key factor to decide whether water adsorption proceeds through dissociation or physisorption on the surface. A consistent picture of TiO2 nanoparticle wetting at the microscopic level emerges, which corroborates existing experimental data and gives further insight into the molecular mechanisms behind nanoparticle wetting. These calculations will facilitate the engineering of metal oxide nanoparticles with a controlled catalytic water activity.

Two New Armtype Polyoxometalates Grafted on Titanium Dioxide Films: Towards Enhanced Photoelectrochemical Performance

Two new carboxyethyltin-functionalized polyoxometalates (POMs) were successfully obtained and confirmed with physicochemical and spectroscopic methods including X-ray crystallography. The lowest unoccupied molecular orbitals of both compounds are higher in energy than that of TiO2 , and the optical band gaps of these compounds are smaller than that of TiO2 . Grafting them onto a TiO2 film created two kinds of novel photoanode materials that showed significantly enhanced photovoltaic and photocurrent responses, as well as improved photoelectrooxidation activities for methanol relative to that shown by a single TiO2 film. Further, P2 W15 -Co-SnR produced the largest photocurrent by exploring the photoelectric activities of a series of carboxyethyltin POM derivatives. This work provides new insight into the photoelectrochemical functionalization of POM-based organic-inorganic hybrids.

Titanium oxide nanoparticle dispersions in a liquid monomer and solid polymer resins prepared by sputtering

A transparent nanocomposite resin with titanium oxide nanoparticles was prepared using a sputtering technique.

Comprehensive Study of an Earth-Abundant Bifunctional 3D Electrode for Efficient Water Electrolysis in Alkaline Medium

We report efficient electrolysis of both water-splitting half reactions in the same medium by a bifunctional 3D electrode comprising Co3O4 nanospheres nucleated on the surface of nitrogen-doped carbon nanotubes (NCNTs) that in turn are grown on conductive carbon paper (CP). The resulting electrode exhibits high stability and large electrochemical activity for both oxygen and hydrogen evolution reactions (OER and HER). We obtain a current density of 10 mA/cm(2) in 0.1 M KOH solution at overpotentials of only 0.47 and 0.38 V for OER and HER, respectively. Additionally, the experimental observations are understood and supported by analyzing the Co3O4:NCNT and NCNT:CP interfaces by ab initio calculations. Both the experimental and the theoretical studies indicate that firm and well-established interfaces along the electrode play a crucial role on the stability and electrochemical activity for both OER and HER.

Facile Synthesis of Defective TiO2-x Nanocrystals with High Surface Area and Tailoring Bandgap for Visible-light Photocatalysis

A facile hydrothermal approach has been developed to prepare defective TiO_2−x nanocrystals using Ti(III)-salt as a precursor and L-ascorbic acid as reductant and structure direction agent. The prepared TiO_2−x nanocrystals are composed of a highly crystallized TiO₂ core and a disordered TiO_2−x outer layer, possessing high surface area, controlled oxygen vacancy concentration and tunable bandgap via simply adjusting the amount of added L-ascorbic acid. The defective TiO_2−x shows high photocatalytic efficiency in methylene blue and phenol degradation as well as in hydrogen evolution under visible light, underlining the significance of the present strategy for structural and bandgap manipulation in TiO₂-based photocatalysis.

Modular construction of size-selected multiple-core Pt–TiO2 nanoclusters for electro-catalysis

Modular construction of platinum–titanium dioxide clusters, which exhibit multiple Pt cores with a preferred size of 30 ± 6 atoms.