Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure-Activity Relationship

Cobalt Hydroxide Modification of TiO2 Nanosheets for Visible-Light-Responsive Photocatalysts

To make full use of sunlight for water splitting reactions for hydrogen production, a visible-light-driven photocatalyst was developed by modifying TiO2 nanosheets with Co(OH)2. By adding an aqueous Co(NO3)2·6H2O solution to a TiO2 nanosheet suspension, the TiO2 nanosheets aggregated and Co(OH)2 was formed. In the ultraviolet-visible (UV-vis) diffuse reflectance spectrum of the photocatalyst, new absorption bands attributable to Co(OH)2 and the interfacial charge transfer between Co(OH)2 and the TiO2 nanosheets appeared at around 600 and 400 nm, respectively. The photocatalytic activity of Co(OH)2/TiO2 nanosheets was evaluated in terms of the O2 evolution reaction in an aqueous AgNO3 solution, finding that the reaction proceeds under visible light. Furthermore, the investigation of the wavelength dependence of the photocatalytic activity revealed that the photocatalytic reaction on Co(OH)2/TiO2 nanosheets proceeds via Co(OH)2 photocatalysis and interfacial charge transfer between Co(OH)2 and the TiO2 nanosheets under visible light irradiation.

Vacancy-Driven High-Performance Metabolic Assay for Diagnosis and Therapeutic Evaluation of Depression

Depression is one of the most common mental illnesses and is a well-known risk factor for suicide, characterized by low overall efficacy (<50%) and high relapse rate (40%). A rapid and objective approach for screening and prognosis of depression is highly desirable but still awaits further development. Herein, a high-performance metabolite-based assay to aid the diagnosis and therapeutic evaluation of depression by developing a vacancy-engineered cobalt oxide (Vo-Co3O4) assisted laser desorption/ionization mass spectrometer platform is presented. The easy-prepared nanoparticles with optimal vacancy achieve a considerable signal enhancement, characterized by favorable charge transfer and increased photothermal conversion. The optimized Vo-Co3O4 allows for a direct and robust record of plasma metabolic fingerprints (PMFs). Through machine learning of PMFs, high-performance depression diagnosis is achieved, with the areas under the curve (AUC) of 0.941-0.980 and an accuracy of over 92%. Furthermore, a simplified diagnostic panel for depression is established, with a desirable AUC value of 0.933. Finally, proline levels are quantified in a follow-up cohort of depressive patients, highlighting the potential of metabolite quantification in the therapeutic evaluation of depression. This work promotes the progression of advanced matrixes and brings insights into the management of depression.

Acetate Ions Facilitated Immobilization of Highly Dispersed Transition Metal Oxide Nanoclusters in Mesoporous Silica

The immobilization of tiny active species within inert mesoporous silica imparts a range of functions, enhancing their applicability. A significant obstacle is the spontaneous migration and aggregation of these species within the mesopores, which threaten their uniform distribution. To address this, we propose a postmodification method that involves grafting transition metal oxide nanoclusters into silica mesopores via interfacial condensation, catalyzed by acetate ions. Specifically, CuO nanoclusters, in the form of oligomeric [O1-x-Cu2-(OH) 2x]n2+, have a strong interaction with the silica framework. This interaction inhibits their growth and prevents mesopore blockage. Theoretical calculation results reveal that the acetate ion promotes proton transfer among various hydroxy species, lowering the free energy and thereby facilitating the formation of Cu-O-Si bonds. This technique has also been successfully applied to the encapsulation of four other types of transition metal oxide nanoclusters. Our encapsulation strategy effectively addresses the challenge of dispersing transition metal oxides in mesoporous silica, offering a straightforward and widely applicable method for enhancing the functionality of mesoporous materials.

Nanoclay-Modulated Interfacial Chemical Bond and Internal Electric Field at the Co3 O4 /TiO2 p-n Junction for Efficient Charge Separation

To achieve a high separation efficiency of photogenerated carriers in semiconductors, constructing high-quality heterogeneous interfaces as charge flow highways is critical and challenging. This study successfully demonstrates an interfacial chemical bond and internal electric field (IEF) simultaneously modulated 0D/0D/1D-Co₃ O₄ /TiO₂ /sepiolite composite catalyst by exploiting sepiolite surface-interfacial interactions to adjust the Co²⁺ /Co³⁺ ratio at the Co₃ O₄ /TiO₂ heterointerface. In situ irradiation X-ray photoelectron spectroscopy and density functional theory (DFT) calculations reveal that the interfacial Co²⁺ OTi bond (compared to the Co³⁺ OTi bond) plays a major role as an atomic-level charge transport channel at the p-n junction. Co²⁺ /Co³⁺ ratio increase also enhances the IEF intensity. Therefore, the enhanced IEF cooperates with the interfacial Co²⁺ OTi bond to enhance the photoelectron separation and migration efficiency. A coupled photocatalysis-peroxymonosulfate activation system is used to evaluate the catalytic activity of Co₃ O₄ /TiO₂ /sepiolite. Furthermore, this work demonstrates how efficiently separated photoelectrons facilitate the synergy between photocatalysis and peroxymonosulfate activation to achieve deep pollutant degradation and reduce its ecotoxicity. This study presents a new strategy for constructing high-quality heterogeneous interfaces by consciously modulating interfacial chemical bonds and IEF, and the strategy is expected to extend to this class of spinel-structured semiconductors.

Trends and progress in application of cobalt-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water splitting

There has been a growing interest in water oxidation in recent two decades. Along with that, remarkable discovery of formation of a mysterious catalyst layer upon application of an anodic potential of 1.13 V vs. standard hydrogen electrode (SHE) to an inert indium tin oxide electrode immersed in phosphate buffer containing Co^(II) ions by Nocera et.al, has greatly attracted researchers interest. These researches have oriented in two directions; one focuses on obtaining better understanding of the reported mysterious catalyst layer, further modification, and improved performance, and the second approach is about designing coordination complexes of cobalt and investigating their properties toward the application in water splitting. Although there have been critical debates on true catalysts that are responsible for water oxidation in homogeneous systems of coordination complexes of cobalt, and the case is not totally closed, in this short review, our focus will be mainly on recent major progress and developments in the design and the application of cobalt oxide-based materials in catalytic, electrocatalytic, photocatalytic, and photoelectrocatalytic water oxidation reaction, which have been reported since pioneering report of Nocera in 2008 (Kanan Matthew and Nocera Daniel in Science 321:1072-1075, 2008).

Constructing porous carbon nitride nanosheets for efficient visible-light-responsive photocatalytic hydrogen evolution

The photocatalytic performance of polymeric carbon nitride (CN) is mainly restricted by the poor mass charge separation efficiency and poor light absorption due to its polymeric nature. The conventional strategies to address these problems involved constructing a nanosheets structure would result in a blue shifted light absorption and increased exciton binding energy. Here, with combination of ammonia etching and selectively hydrogen-bond breaking, holey carbon nitride nanosheets (hCNNS) were constructed, thus widening the light absorption range, and spontaneously shortening the migration distance of electrons and holes in the lateral and vertical directions, respectively. Further analysis also found out the reserved atomic structure order endowed hCNNS with the relatively high redox potential. When irradiated with visible light (λ > 420 nm) and loaded with 3 wt% Pt as the cocatalyst, the hydrogen evolution rate of hCNNS was about 40 times higher than the bulk CN, and the apparent quantum yield (AQY) of hCNNS is 1.47% at 435 ± 15 nm. We expect this research can provide a new sight for achieving highly efficient solar utilization of CN-based photocatalysts.

Hollow Cobalt Oxide/Carbon Hybrids Aid Metabolic Encoding for Active Systemic Lupus Erythematosus during Pregnancy

A noninvasive, easy operation, and accurate diagnostic protocol is highly demanded to assess systemic lupus erythematosus (SLE) activity during pregnancy, promising real-time activity monitoring during the whole gestational period to reduce adverse pregnancy outcomes. Here, machine learning of serum metabolic fingerprints (SMFs) is developed to assess the SLE activity for pregnant women. The SMFs are directly extracted through a hollow-cobalt oxide/carbon (Co₃ O₄ /C)-composite-assisted laser desorption/ionization mass spectrometer (LDI MS) platform. The Co₃ O₄ /C composite owns enhanced light absorption, size-selective trapping, and better charge-hole separation, enabling improved ionization efficiency and selectivity for LDI MS detection toward small molecules. Metabolic fingerprints are collected from ≈0.1 µL serum within 1 s without enrichment and encoded by the optimized elastic net algorithm. The averaged area under the curve (AUC) value in the differentiation of active SLE from inactive SLE and healthy controls reaches 0.985 and 0.990, respectively. Further, a simplified panel based on four identified metabolites is built to distinguish SLE flares in pregnant women with the highest AUC value of 0.875 for the blind test. This work sets an accurate and practical protocol for SLE activity assessment during pregnancy, promoting precision diagnosis of disease status transitions in clinics.

Visible Light Trapping against Charge Recombination in FeOx-TiO2 Photonic Crystal Photocatalysts

Tailoring metal oxide photocatalysts in the form of heterostructured photonic crystals has spurred particular interest as an advanced route to simultaneously improve harnessing of solar light and charge separation relying on the combined effect of light trapping by macroporous periodic structures and compositional materials' modifications. In this work, surface deposition of FeOx nanoclusters on TiO2 photonic crystals is investigated to explore the interplay of slow-photon amplification, visible light absorption, and charge separation in FeOx-TiO2 photocatalytic films. Photonic bandgap engineered TiO2 inverse opals deposited by the convective evaporation-induced co-assembly method were surface modified by successive chemisorption-calcination cycles using Fe(III) acetylacetonate, which allowed the controlled variation of FeOx loading on the photonic films. Low amounts of FeOx nanoclusters on the TiO2 inverse opals resulted in diameter-selective improvements of photocatalytic performance on salicylic acid degradation and photocurrent density under visible light, surpassing similarly modified P25 films. The observed enhancement was related to the combination of optimal light trapping and charge separation induced by the FeOx-TiO2 interfacial coupling. However, an increase of the FeOx loading resulted in severe performance deterioration, particularly prominent under UV-Vis light, attributed to persistent surface recombination via diverse defect d-states.

Isolated Cobalt Centers on W18O49 Nanowires Perform as a Reaction Switch for Efficient CO2 Photoreduction

Isolated cobalt atoms have been successfully decorated onto the surface of W₁₈O₄₉ ultrathin nanowires. The Co-atom-decorated W₁₈O₄₉ nanowires (W₁₈O₄₉@Co) greatly accelerate the charge carrier separation and electron transport in the catalytic system. Moreover, the surface decoration with Co atoms modifies the energy configuration of the W₁₈O₄₉@Co hybrid and thus boosts the redox capability of photoexcited electrons for CO₂ reduction. The decorated Co atoms work as the real active sites and, perhaps more importantly, perform as a reaction switch to enable the reaction to proceed. The optimized catalyst delivers considerable activity for photocatalytic CO₂ reduction, yielding an impressive CO generation rate of 21.18 mmol g^-1 h^-1.

Heterostructured CoOx-TiO2 Mesoporous/Photonic Crystal Bilayer Films for Enhanced Visible-Light Harvesting and Photocatalysis

Heterostructured bilayer films, consisting of co-assembled TiO₂ photonic crystals as the bottom layer and a highly performing mesoporous P25 titania as the top layer decorated with CoO_x nanoclusters, are demonstrated as highly efficient visible-light photocatalysts. Broadband visible-light activation of the bilayer films was implemented by the surface modification of both titania layers with nanoscale clusters of Co oxides relying on the chemisorption of Co acetylacetonate complexes on TiO₂, followed by post-calcination. Tuning the slow photon regions of the inverse opal supporting layer to the visible-light absorption of surface CoO_x oxides resulted in significant amplification of salicylic-acid photodegradation under visible and ultraviolet (UV)–visible light (Vis), outperforming benchmark P25 films of higher titania loading. This enhancement was related to the spatially separated contributions of slow photon propagation in the inverse opal support layer assisted by Bragg reflection toward the CoO_x-modified mesoporous P25 top layer. This effect indicates that photonic crystals may be highly effective as both photocatalytically active and backscattering layers in multilayer photocatalytic films.

Water Oxidation through Interfacial Electron Transfer by Visible Light Using Cobalt-Modified Rutile Titania Thin-Film Photoanode

TiO₂ is a good photoanode material for water oxidation to form O₂; however, UV light (λ < 400 nm) is necessary for this system to operate. In this work, cobalt species were introduced onto a rutile TiO₂ thin film grown on a fluorine-doped tin oxide (FTO) substrate for visible-light activation of TiO₂ and to construct water oxidation sites. TiO₂ thin films were prepared on the FTO surface by the thermohydrolysis of TiCl₄, followed by annealing at 723 K in air; the loading of the cobalt species was achieved simply by immersing TiO₂/FTO into an aqueous Co(NO₃)₂ solution at room temperature, followed by heating at 423 K in air. Physicochemical analyses revealed that the cobalt species deposited on the TiO₂ film was α-Co₃(OH)₄(NO₃)₂ and that the cobalt-modified TiO₂ thin-film electrode had a visible-light absorption band that extended to 700 nm due to interfacial electron transitions from the cobalt species to the conduction band of TiO₂. Upon anodic polarization in the presence of visible light, the cobalt-modified TiO₂ thin-film electrode generated an anodic photocurrent with an onset potential of +0.1 V vs RHE, which was consistent with that of pristine rutile TiO₂. Product analysis during the controlled potential photoelectrolysis in the presence of an applied bias smaller than 1.23 V under visible light showed that water oxidation to O₂ occurred on the cobalt-modified TiO₂/FTO. This study demonstrates that a visible-light-driven photoelectrochemical cell for water oxidation can be constructed through the use of earth-abundant metals without the need for a complicated preparation procedure.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

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.

Targeted modulation of g-C3N4 photocatalytic performance for pharmaceutical pollutants in water using ZnFe-LDH derived mixed metal oxides: Structure-activity and mechanism

Pharmaceuticals have been frequently detected in various water bodies, posing potential threat to human health and ecological environment. In this work, ZnFe-LDH derived mixed metal oxides (ZnO/ZnFe₂O₄, ZnFeMMO) were innovatively adopted to modulate the g-C₃N₄ photocatalytic performance for the enhanced degradation of ibuprofen (IBF) and sulfadiazine (SDZ) as targeted pollutants. Characterization analyses indicated that the g-C₃N₄/ZnFeMMO composites were in the feature of rationally-designed microarchitecture, increased specific surface area, improved light absorbance and efficient charge separation, thereby resulting in promoted photocatalytic activities. Furthermore, the ratio of g-C₃N₄ to ZnFeMMO in the composites was found to exert significant effects on the resulted microstructures and properties. The results showed that the composite with low g-C₃N₄ content of 1.0 wt% or high g-C₃N₄ content of 90 wt% exhibited the optimum catalytic activity for the degradation of IBF or SDZ, respectively. Such distinct structure-activities can be attributed to the different dominated reactive species in two cases: h⁺ for IBF degradation but OH for SDZ degradation. A Z-scheme mechanism was proposed for the charge separation, together with ZnFe₂O₄ as a light sensitizer. Degradation pathways for IBF and SDZ were established by ESI-QToF-MS technology. This work provided a new perspective to develop rationally-architectured g-C₃N₄ based photocatalysts for the decontamination of water polluted by pharmaceuticals.

Co3O4 Nanopetals on Si as Photoanodes for the Oxidation of Organics

Cobalt oxide nanopetals were grown on silicon electrodes by heat-treating metallic cobalt films deposited by DC magnetron sputtering. We show that cobalt oxide, with this peculiar nanostructure, is active towards the photo-electrochemical oxidation of water as well as of organic molecules, and that its electrochemical properties are directly linked to the structure of its surface. The formation of Co3O4 nanopetals, induced by oxidizing annealing at 300 °C, considerably improves the performance of the material with respect to simple cobalt oxide films. Photocurrent measurements and electrochemical impedance are used to explain the behavior of the different structures and to highlight their potential application in water remediation technologies.

Water Splitting on Rutile TiO2 -Based Photocatalysts

Water splitting using a semiconductor photocatalyst with sunlight has long been viewed as a potential means of large-scale H₂ production from renewable resources. Different from anatase TiO₂ , rutile enables preferential water oxidation, which is useful for the construction of a Z-scheme water-splitting system. The combination of rutile TiO₂ with a suitable H₂ -evolution photocatalyst such as a Pt-loaded BaZrO₃ -BaTaO₂ N solid solution enables solar-driven water splitting into H₂ and O₂ . While rutile TiO₂ is a wide-gap semiconductor with a bandgap of 3.0 eV, co-doping of rutile TiO₂ with certain metal ions and/or nitrogen produces visible-light-driven photocatalysts, which are also useful as a component for water oxidation in visible-light-driven Z-scheme water splitting. The key to achieving highly efficient water oxidation is to maintain a charge balance of dopants in the rutile, because single doping typically produces trap states that capture photogenerated electrons and/or holes. Here we provide a concise summary of rutile TiO₂ -based photocatalysts for water-splitting systems.

Efficient difunctional photocatalyst prepared in situ from Prussian blue analogues for catalytic water oxidation and visible-light absorption

Co(OH)₂-modified CuO (Co(OH)₂/CuO) nanoparticles serve as both visible-light-driven water oxidation catalysts and visible-light-absorption centers.

Simultaneous Ni Doping at Atom Scale in Ceria and Assembling into Well-Defined Lotuslike Structure for Enhanced Catalytic Performance

Oxide materials with redox capability have attracted worldwide attentions in many applications. Introducing defects into crystal lattice is an effective method to modify and optimize redox capability of oxides as well as their catalytic performance. However, the relationship between intrinsic characteristics of defects and properties of oxides has been rarely reported. Herein, we report a facile strategy to introduce defects by doping a small amount of Ni atoms (∼1.8 at. %) into ceria lattice at atomic level through the effect of microstructure of crystal on the redox property of ceria. Amazingly, a small amount of single Ni atom-doped ceria has formed a homogeneous solid solution with uniform lotuslike morphology. It performs an outstanding catalytic performance of a reduced T₅₀ of CO oxidation at 230 °C, which is 135 °C lower than that of pure CeO₂ (365 °C). This is largely attributed to defects such as lattice distortion, crystal defects and elastic strain induced by Ni dopants. The DFT calculation has revealed that the electron density distribution of oxygen ions near Ni dopant, the reduced formation energy of oxygen vacancy originated from local chemical effect caused by local distortion after Ni doping. These differences have a great effect on increasing the concentration of oxygen vacancies and enhancing the migration of lattice oxygen from bulk to a surface which is closely related to optimized redox properties. As a result, oxygen storage capacity and the associated catalytic reactivity has been largely increased. We have clearly demonstrated the change of crystal lattice and the charge distribution effectively modify its chemical and physical properties at the atomic scale.

Effects of the SrTiO3 support on visible-light water oxidation with Co3O4 nanoparticles

SrTiO₃ particle as a support material had a strong influence on the formation of Co₃O₄ nanoparticles, resulting in different photocatalytic activities for visible-light water oxidation (λ > 480 nm).