Water Splitting on Rutile TiO2 -Based Photocatalysts

Stable Photoelectrochemical Reactions at Solid/Solid Interfaces toward Solar Energy Conversion and Storage

Electrochemistry has extended from reactions at solid/liquid interfaces to those at solid/solid interfaces. However, photoelectrochemistry at solid/solid interfaces has been hardly reported. In this study, we achieve a stable photoelectrochemical reaction at the semiconductor-electrode/solid-electrolyte interface in a Nb-doped anatase-TiO2 (a-TiO2:Nb)/Li3PO4 (LPO)/Li all-solid-state cell. The oxidative currents of a-TiO2:Nb/LPO/Li increase upon light irradiation when a-TiO2:Nb is located at a potential that is more positive than its flat-band potential. This is because the photoexcited electrons migrate to the current collector due to the bending of the conduction band minimum toward the negative potential. The photoelectrochemical reaction at the semiconductor/solid-electrolyte interface is driven by the same principle as those at semiconductor/liquid-electrolyte interfaces. Moreover, oxidation under light irradiation exhibits reversibility with reduction in the dark. Thus, we extend photoelectrochemistry to all-solid-state systems composed of solid/solid interfaces. This extension would enable us to investigate photoelectrochemical phenomena uncleared at solid/liquid interfaces because of low stability and durability.

Efficient full solar spectrum-driven photocatalytic hydrogen production on low bandgap TiO2/conjugated polymer nanostructures

The development of photocatalysts that can utilize the entire solar spectrum is crucial to achieving efficient solar energy conversion. The utility of the benchmark photocatalyst, TiO2, is limited only to the UV region due to its large bandgap. Extending the light harvesting properties across the entire spectrum is paramount to enhancing solar photocatalytic performance. In this work, we developed low bandgap TiO2/conjugated polymer nanostructures which exhibit full spectrum activity for efficient H2 production. The highly mesoporous structure of the nanostructures together with the photosensitizing properties of the conjugated polymer enabled efficient solar light activity. The mesoporous TiO2 nanostructures calcined at 550 °C exhibited a defect-free anatase crystalline phase with traces of brookite and high surface area, resulting in the best performance in hydrogen production (5.34 mmol g-1 h-1) under sunlight simulation. This value is higher not only in comparison to other TiO2-based catalysts but also to other semiconductor materials reported in the literature. Thus, this work provides an effective strategy for the construction of full spectrum active nanostructured catalysts for enhanced solar photocatalytic hydrogen production.

Boosting Photocatalytic Performance of ZnO Nanowires via Building Heterojunction with g-C3N4

The development of a stable and highly active photocatalyst has garnered significant attention in the field of wastewater treatment. In this study, a novel technique involving a facile stirring method was devised to fabricate an array of g-C₃N₄/ZnO nanowire (ZnO NW) composites. Through the introduction of g-C₃N₄ to augment the generation of electron-hole pairs upon exposure to light, the catalytic efficacy of these composites was found to surpass that of the pristine ZnO NWs when subjected to simulated sunlight. The photocatalytic performance of a 20 mg·L^-1 methylene blue solution was found to be highest when the doping rate was 25 wt%, resulting in a degradation rate of 99.1% after 60 min. The remarkable enhancement in catalytic efficiency can be ascribed to the emergence of a captivating hetero-junction at the interface of g-C₃N₄ and ZnO NWs, characterized by a harmoniously aligned band structure. This alluring arrangement effectively curtailed charge carrier recombination, amplified light absorption, and augmented the distinct surface area, culminating in a notable boost to the photocatalytic prowess. These findings suggest that the strategic engineering of g-C₃N₄/ZnO NW heterostructures holds tremendous promise as a pioneering avenue for enhancing the efficacy of wastewater treatment methodologies.

Ni(NH3)62+ more efficient than Ni(H2O)62+ and Ni(OH)2 for catalyzing water and phenol oxidation on illuminated Bi2MoO6 with visible light

Nickel (hydr)oxide (NiOH) is known to be good co-catalyst for the photoelectrochemical oxidation of water, and for the photocatalytic oxidation of organics on different semiconductors. Herein we report a greatly improved activity of Bi₂MoO₆ (BMO) by nickel hexammine perchlorate (NiNH). Under visible light, phenol oxidation on BMO was slow. After NiNH, NiOH, and Ni²⁺ loading, a maximum rate of phenol oxidation increased by factors of approximately 16, 8.8, and 4.7, respectively. With a BMO electrode, all catalysts inhibited O₂ reduction, enhanced water (photo-)oxidation, and facilitated the charge transfer at solid-liquid interface, respectively, the degree of which was always NiNH > NiOH > Ni²⁺. Solid emission spectra indicated that all catalysts improved the charge separation of BMO, the degree of which also varied as NiNH > NiOH > Ni²⁺. Furthermore, after a phenol-free aqueous suspension of NiNH/BMO was irradiated, there was a considerable Ni(III) species, but a negligible NH₂ radical. Accordingly, a plausible mechanism is proposed, involving the hole oxidation of Ni(II) into Ni(IV), which is reactive to phenol oxidation, and hence promotes O₂ reduction. Because NH₃ is a stronger ligand than H₂O, the Ni(II) oxidation is easier for Ni(NH₃)⁶⁺ than for Ni(H₂O)⁶⁺. This work shows a simple route how to improve BMO photocatalysis through a co-catalyst.

Inhibition of melanoma using a nanoceria-based prolonged oxygen-generating phototherapy hydrogel

Tumor hypoxic environment is an inevitable obstacle for photodynamic therapy (PDT) of melanoma. Herein, a multifunctional oxygen-generating hydrogel loaded with hyaluronic acid-chlorin e6 modified nanoceria and calcium peroxide (Gel-HCeC-CaO2) was developed for the phototherapy of melanoma. The thermo-sensitive hydrogel could act as a sustained drug delivery system to accumulate photosensitizers (chlorin e6, Ce6) around the tumor, followed by cellular uptake mediated by nanocarrier and hyaluronic acid (HA) targeting. The moderate sustained oxygen generation in the hydrogel was produced by the reaction of calcium peroxide (CaO2) with infiltrated H2O in the presence of catalase mimetic nanoceria. The developed Gel-HCeC-CaO2 could efficiently alleviate the hypoxia microenvironment of tumors as indicated by the expression of hypoxia-inducible factor -1α (HIF-1α), meeting the “once injection, repeat irradiation” strategy and enhanced PDT efficacy. The prolonged oxygen-generating phototherapy hydrogel system provided a new strategy for tumor hypoxia alleviation and PDT.

Heterophase Polymorph of TiO2 (Anatase, Rutile, Brookite, TiO2 (B)) for Efficient Photocatalyst: Fabrication and Activity

TiO₂ exists naturally in three crystalline forms: Anatase, rutile, brookite, and TiO₂ (B). These polymorphs exhibit different properties and consequently different photocatalytic performances. This paper aims to clarify the differences between titanium dioxide polymorphs, and the differences in homophase, biphase, and triphase properties in various photocatalytic applications. However, homophase TiO₂ has various disadvantages such as high recombination rates and low adsorption capacity. Meanwhile, TiO₂ heterophase can effectively stimulate electron transfer from one phase to another causing superior photocatalytic performance. Various studies have reported the biphase of polymorph TiO₂ such as anatase/rutile, anatase/brookite, rutile/brookite, and anatase/TiO₂ (B). In addition, this paper also presents the triphase of the TiO₂ polymorph. This review is mainly focused on information regarding the heterophase of the TiO₂ polymorph, fabrication of heterophase synthesis, and its application as a photocatalyst.

Effect of the Deposition of Vanadium-Oxide on the Photocatalytic Activity of TiO2 Nanotubes and Its Photodiode Performance Interfaced with CH3NH3PbI3 Single Crystal

In this study, we report the influence of vanadium oxide (VO), as a photosensitive component, on the photoactivity of TiO2 nanotubes (TNTs). A series of TNTs of varying tube diameter were synthesized by the anodization of titanium foils at different voltages, while vanadium oxide was deposited on TNTs by wet chemical deposition. An improvement in the optical properties of nanotubes was observed after the deposition of vanadium oxide. An improvement in the optical properties (redshift in UV-Vis spectra) of TNTs and TNT/VO was noted. The photocatalytic activity was improved with increasing tube diameter, while it was weakened after the deposition of VO. Furthermore, photoactivity was investigated in photodiodes based on TNTs or TNT/VO and single crystals of CH3NH3PbI3. The photoelectric measurement revealed that different TNT diameters did not influence the I-V characteristic of the photodiodes, while the deposition of VO improved the photocurrent for smaller TNTs.

Construction of TiO2(B)/Anatase Heterophase Junctions via a Water-Induced Phase Transformation Strategy for Enhanced Photocatalytic Hydrogen Production

The development of facile and green solution-phase routes toward the fabrication of TiO₂-based heterophase junctions with a delicate control of phase and structure is a challenging task. Herein, we report a simple and convenient method to controllably fabricate TiO₂(B)/anatase heterophase junctions, which was successfully realized by utilizing the ideal great solvent of water to treat the presynthesized TiO₂(B) nanosheet precursor at a low temperature of 80 °C. On the basis of phase structure transformation and morphology evolution data, the formation of these TiO₂(B)/anatase heterophase junctions was reasonably explained by a novel water-induced TiO₂(B) → anatase phase transformation mechanism. Benefiting from the desirable structural and photoelectronic advantages of more exposed active sites, enhanced light absorbance, and promoted separation of photogenerated electron-hole pairs, the thus-transformed TiO₂(B)/anatase heterophase junctions exhibit fascinating photocatalytic performance in water splitting. Specifically, with the help of Pt as a cocatalyst and methanol as a sacrificial agent, the H₂ production rate of optimized TiO₂(B)/anatase heterophase junction reaches 6.92 mmol·g^-1·h^-1, which is almost 7.1 and 2.1 times higher than those of the pristine TiO₂(B) nanosheets and the final anatase nanocrystals. More interestingly, the TiO₂(B)/anatase heterophase junction also delivers prominent activity toward pure water splitting to simultaneously produce H₂ and H₂O₂, with evolution rates of up to 1.10 and 0.55 mmol·g^-1·h^-1, respectively. Our work may advance the facile green solvent-mediated synthesis of metal oxide-based heterophase junctions for applications in energy- and environmental-related areas.

Synthesis, Structural and Optical Properties of ZrBi2Se6 Nanoflowers: A Next-Generation Semiconductor Alloy Material for Optoelectronic Applications

ZrBi₂Se₆ nanoflower-like morphology was successfully prepared using a solvothermal method, followed by a quenching process for photoelectrochemical water splitting applications. The formation of ZrBi₂Se₆ was confirmed by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The estimated value of work function and band gap were found to be 5.5 and 2.26 eV measured using diffuse reflection spectroscopy and ultraviolet photoelectron spectroscopy, suggesting the potential candidate for water splitting. The highest current density of 9.7 μA/cm² has been observed for the ZrBi₂Se₆ photoanode for the applied potential of 0.5 V vs SCE. The flat-band potential value was -0.46 V, and the 1.85 nm width of the depletion region is estimated from the Mott-Schottky (MS) analysis. It also reveals that the charge carrier density for the ZrBi₂Se₆ nanoflowers is 4.8 × 10¹⁵ cm^-3. The negative slope of the MS plot indicates that ZrBi₂Se₆ is a p-type semiconductor. It was observed that ZrBi₂Se₆ nanoflowers had a high charge transfer resistance of ∼730 kΩ and equivalent capacitance of ∼40 nF calculated using electrochemical impedance spectroscopy (EIS) measurements. Using chronoamperometry, the estimated rise time and decay time were 50 ms and 0.25 s, respectively, which reveals the fast photocurrent response and excellent PEC performance of the ZrBi₂Se₆ photoanode. Furthermore, an attempt has been made to explain the PEC activity of ZrBi₂Se₆ nanoflowers using an energy band diagram. Thus, the initial results on ZrBi₂Se₆ nanoflowers appear promising for the PEC activity toward water splitting.

Synthesis and applications of TiO2/ZnO hybrid nanostructures by ZnO deposition on TiO2 nanotubes using electrochemical processes

In recent years, TiO2/ZnO hybrid nanostructures have been attracting the interest of the scientific community due to their excellent photoelectrochemical properties. The main advantage of TiO2/ZnO hybrid nanostructures over other photocatalysts based on semiconductor materials lies in their ability to form heterojunctions in which the valence and conduction bands of both semiconductors are intercalated. This factor produces a decrease in the band gap and the recombination rate and an increase in the light absorption range. The aim of this review is to perform a revision of the main methods to synthesise TiO2/ZnO hybrid nanostructures by ZnO deposition on TiO2 nanotubes using electrochemical processes. Electrochemical synthesis methods provide an easy, fast, and highly efficient route to carry out the synthesis of nanostructures such as nanowires, nanorods, nanotubes, etc. They allow us to control the stoichiometry, thickness and structure mainly by controlling the voltage, time, temperature, composition of the electrolyte, and concentration of monomers. In addition, a study of the most promising applications for TiO2/ZnO hybrid nanostructures has been carried out. In this review, the applications of dye-sensitised solar cell, photoelectrocatalytic degradation of organic compounds, photoelectrochemical water splitting, gas sensors, and lithium-ion batteries have been highlighted.

Light-induced Li extraction from LiMn2O4/TiO2 in a water-in-salt electrolyte for photo-rechargeable batteries

Photocharging of high-potential spinel LiMn₂O₄ is demonstrated by using a water-in-salt electrolyte and TiO₂ nanoparticles. In a developed half-cell system with an electron acceptor, Li extraction from LiMn₂O₄ proceeds under the illumination of UV-visible light at an estimated rate of ∼23 mA g^-1. This work paves the way for high-potential cathode materials in photo-rechargeable batteries.

Recent Advances in TiO2-based Photoanodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting has attracted a great attention in the past several decades which holds great promise to address global energy and environmental issues by converting solar energy into hydrogen. However, its low solar-to-hydrogen (STH) conversion efficiency remains a bottleneck for practical application. Developing efficient photoelectrocatalysts with high stability and high STH conversion efficiency is one of the key challenges. As a typical n-type semiconductor, titanium dioxide (TiO 2 ) exhibits high PEC water splitting performance, especially high chemical and photo stability. But, TiO 2 has also disadvantages such as wide band gap and fast electron-hole recombination rate, which seriously hinder its PEC performance. This review focuses on recent development in TiO 2 -based photoanodes as well as some key fundamentals. The corresponding mechanisms and key factors for high STH, and controllable synthesis and modification strategies are highlighted in this review. We conclude finally with an outlook providing a critical perspective on future trends on TiO 2 -based photoanodes for PEC water splitting.

Sodium doping in brookite TiO2 enhances its photocatalytic activity

We report in this work that sodium doping of brookite TiO₂ effectively enhances its photocatalytic activity, which becomes three times higher than that of the quasi-spherical brookite TiO₂. The results demonstrated that the sodium-doped brookite Na _x Ti_{1- x} O₂ can be stable up to 500 °C. At 600°C, the sodium in the brookite precipitates in the form of Na₂CO₃, and above 700 °C, the brookite Na _x Ti_{1- x} O₂ transforms into Na₂Ti₆O₁₃ by a twinning process with the orientation relationship of [1-2-3]_Matrix//[1-23]_Twins and (-2-10)_Matrix//(1-1-1)_Twins. The differences in the ionic radius and the electronegativity between Na and Ti destroy the local atomic arrangement of the brookite structure and produce microstructures such as the core-shell structure, local lattice distortion, interstitial atoms, and atomic vacancies, which are critical to its excellent photocatalytic activity.

Synthesis, modification and application of titanium dioxide nanoparticles: a review

Titanium dioxide (TiO₂) has been heavily investigated owing to its low cost, benign nature and strong photocatalytic ability. Thus, TiO₂ has broad applications including photocatalysts, Li-ion batteries, solar cells, medical research and so on. However, the performance of TiO₂ is not satisfactory due to many factors such as the broad band gap (3.01 to 3.2 eV) and fast recombination of electron-hole pairs (10^-12 to 10^-11 s). Plenty of work has been undertaken to improve the properties, such as structural and dopant modifications, which broaden the applications of TiO₂. This review mainly discusses the aspects of TiO₂-modified nanoparticles including synthetic methods, modifications and applications.

Construction of unique heterojunction photoanodes through in situ quasi-epitaxial growth of FeVO4 on Fe2O3 nanorod arrays for enhanced photoelectrochemical performance

By enhancing the separation of photogenerated electron–hole pairs, reducing carrier recombination and the impedance of the interface electrolyte, the FeVO4–Fe2O3 photoanode exhibits obviously enhanced photoelectrochemical performance.

Recent Advances in the Catalytic Applications of Lanthanide-Oxo Clusters

Lanthanide-oxo/hydroxo clusters (LOCs) in this mini-review refer to polynuclear complexes featuring a polyhedral metal-oxo/hydroxo cluster core of lanthanide ions exclusively or with coexisting 3d metal ions. We summarize herein the recent works using this unique family of cluster complexes for catalysis; this aspect of research stands in stark contrast to their extensively studied synthetic and structural chemistry as well as the much-researched magnetic properties. Following a brief introduction of the synthetic strategies for these clusters, pertinent results from available literature reports are surveyed and discussed according to the types of catalyzed reactions. Particular attention was paid to the selection of a cluster catalyst for a specific type of reactions as well as the corresponding reaction mechanism. To the end, the advantages and challenges in utilizing LOCs as multifunctional catalysts are summarized, and possible future research directions are proposed.

Calcined ZnTi-Layered Double Hydroxide Intercalated with H3 PW12 O40 with Efficiently Photocatalytic and Adsorption Performances

Wastewater treatment is of great significance to environmental remediation. The exploration of efficient and stable methods for wastewater treatment is still a challenging issue. Herein, a heterojunction material with photocatalysis and adsorption properties has been designed to remove the complex pollutants from wastewater. The heterojunction material (ZnO/TiO₂ -PW₁₂ , PW₁₂ =[PW₁₂ O₄₀ ]^3- ) was synthesized by calcining the ZnTi-layered double hydroxide (ZnTi-LDH) intercalated with the Keggin-type polyoxometalate H₃ PW₁₂ O₄₀ . In the construction of ZnO/TiO₂ -PW₁₂ it was found that the polyanionic PW₁₂ remained unchanged in the process of forming the proposed heterojunction. The photochemical properties verify that heterojunction synergistic with PW₁₂ facilitated the separation of photoproduced electron-hole pairs and thus suppressed the recombination. Therefore, ZnO/TiO₂ -PW₁₂ exhibits excellent photocatalytic property, and the efficiency of Cr(VI) photoreduction reached more than 90 % in the first 3 min. Furthermore, the electrostatic force between the PW₁₂ and cationic dyes makes ZnO/TiO₂ -PW₁₂ having an outstanding adsorption performance for cationic dyes, such as rhodamine B, crystal violet and methyl blue. Such heterojunction material combined with polyoxometalate puts forward new insights for the design of functional materials for water treatment with low cost and high efficiency.

The Chemical Nature of Ti4O10-: Vibrational Predissociation Spectroscopy Combined with Global Structure Optimization

The gas-phase infrared spectrum of Ti₄O₁₀^- is studied in the spectral range from 400 cm^-1 to 1250 cm^-1 using cryogenic ion trap vibrational spectroscopy, in combination with density functional theory (DFT). The infrared photodissociation (IRPD) spectrum of D₂-tagged Ti₄O₁₀^- provides evidence for a structure of lower symmetry that contains a superoxo group (1121 cm^-1) and two terminal Ti=O moieties. DFT combined with a genetic algorithm for global structure optimization predicts two isomers which feature a superoxo group: the C_s symmetric global minimum-energy structure and a similar isomer (C₁) that is slightly higher in energy. Coupled cluster calculations confirm the relative stability. Comparison of the harmonic DFT spectra (different functionals) with the IRPD spectrum suggests that both of these isomers contribute. Earlier assignments to the adamantane-like C_3v isomer with three terminal Ti-O^• - groups in a quartet state are not confirmed. They were based on the infrared multiple photon photodissociation (IRMPD) spectrum of bare Ti₄O₁₀^- and local DFT structure optimizations.

Efficient solar photocatalytic hydrogen production using direct Z-scheme heterojunctions

We report the preparation of a series of heterojunctions made of Ta₃N₅ and TiO₂ nanoparticles that show good properties for photocatalytic hydrogen production. The composite photocatalyst with a light-response range up to 620 nm shows a hydrogen evolution rate of 250 μmol h^-1. The apparent quantum efficiency at 330 nm can be as high as 46%. Particularly, normalized spectral studies indicate that the heterojunction is more active upon full-spectrum (without using optical filters) irradiation, and its activity is even superior  to the total activity exhibited upon UV-light irradiation (λ ≤ 420 nm) and visible-infrared light irradiation (λ ≥ 420 nm). Moreover, in situ photodeposition of platinum nanoparticles on the surface of the photocatalyst as well as the band alignment analysis demonstrate the Z-scheme mechanism associated with the photocatalytic process. Specifically, photogenerated electrons from TiO₂ will rapidly combine with the photogenerated holes from Ta₃N₅ through interfacial charge transfer, leaving the more active electrons and holes in Ta₃N₅ and TiO₂, respectively, to facilitate redox reactions. Basically, TiO₂ is only UV-light active, while Ta₃N₅ can be activated under visible-light irradiation. In this case, a synergy effect, upon simultaneous UV-light excitation and visible-light excitation, can be achieved by full-spectrum irradiation, leading to a much higher photocatalytic activity. This work thus provides a favorable and upward direction for the establishment of heterojunctions for high-efficiency hydrogen production and solar energy applications.

Mixed Anion Control of the Partial Oxidation of Methane to Methanol on the β-PtO2 Surface

Although the C-H bond of methane is very strong, it can be easily dissociated on the (110) surface of β-PtO2. This is because a very stable Pt-C bond is formed between the coordinatively unsaturated Pt atom and CH3 on the surface. Owing to the stable nature of the Pt-C bond, CH3 is strongly bound to the surface. When it comes to methanol synthesis from methane, the Pt-C bond has to be cleaved to form a C-O bond during the reaction process. However, this is unlikely to occur on the β-PtO2 surface: The activation energy of the process is calculated to be so large as 47.9 kcal/mol. If the surface can be modified in such a way that the ability for the C-H bond activation is maintained but the Pt-C bond is weakened, a catalyst combining the functions of C-H bond cleavage and C-O bond formation can be created. For this purpose, analyzing the orbital interactions on the surface is found to be very useful, resulting in a prediction that the Pt-C bond can be weakened by replacing the O atom trans to the C atom with a N atom. This would be a sort of process to make β-PtO2 a mixed anion compound. Density functional theory simulations of catalytic reactions on the β-PtO2 surface show that the activation energy of the rate-limiting step of methanol synthesis can be reduced to 27.7 kcal/mol by doping the surface with N.

Fabrication of Highly Textured 2D SnSe Layers with Tunable Electronic Properties for Hydrogen Evolution

Hydrogen is regarded to be one of the most promising renewable and clean energy sources. Finding a highly efficient and cost-effective catalyst to generate hydrogen via water splitting has become a research hotspot. Two-dimensional materials with exotic structural and electronic properties have been considered as economical alternatives. In this work, 2D SnSe films with high quality of crystallinity were grown on a mica substrate via molecular beam epitaxy. The electronic property of the prepared SnSe thin films can be easily and accurately tuned in situ by three orders of magnitude through the controllable compensation of Sn atoms. The prepared film normally exhibited p-type conduction due to the deficiency of Sn in the film during its growth. First-principle calculations explained that Sn vacancies can introduce additional reactive sites for the hydrogen evolution reaction (HER) and enhance the HER performance by accelerating electron migration and promoting continuous hydrogen generation, which was mirrored by the reduced Gibbs free energy by a factor of 2.3 as compared with the pure SnSe film. The results pave the way for synthesized 2D SnSe thin films in the applications of hydrogen production.

Rutile TiO2 single crystals delivering enhanced photocatalytic oxygen evolution performance

Owing to their scientific and technological importance, the development of highly efficient photocatalytic water oxidation systems with rapid photogenerated charge separation and high surface catalytic activity is highly desirable for the storage and conversion of solar energy. A promising candidate is rutile phase titanium dioxide (TiO2), which has been widely studied over half a century. Specifically, oriented single-crystalline TiO2 surfaces with high oxidative reactivity would be most desirable, but achieving these structures has been limited by the availability of synthetic techniques. In this study, a facile and green synthetic approach was developed for the first time to synthesize rutile TiO2 single crystals with regulable reductive and oxidative facets. Glycolic acid (GA) and sodium fluoride (NaF) are used as the crucial and effective phase and facet controlling agents, respectively. The selective adsorption of F- ions on the facets of rutile TiO2 crystals not only plays a key role in driving the nucleation and preferential growth of the crystals with desired facets but also significantly affects their photocatalytic gas evolution reactivity. With heat treatment, the highly stable F--free rutile TiO2 single crystals with a high percentage of oxidative facets exhibit a superior photocatalytic gas evolution rate (≈116 μmol h-1 per 0.005 g catalyst), 8.5 times higher than that of previous F--containing samples.

Deposition of zinc cobaltite nanoparticles onto bismuth vanadate for enhanced photoelectrochemical water splitting

During the past few decades, photoelectrochemical (PEC) water splitting has attracted significant attention because of the reduced production cost of hydrogen obtained by utilizing solar energy. Significant efforts have been invested by the scientific community to produce stable ternary metal oxide semiconductors, which can enhance the stability and increase the overall production of oxygen. Herein, we present the ternary metal oxide deposition of ZnCo₂O₄ as a route to obtain a novel photocatalyst layer on BiVO₄ to form BiVO₄/ZnCo₂O₄ a novel composite photoanode for PEC water splitting. The structural, topographical, and optical analyses were performed using field emission scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy, and UV-Vis spectroscopy to confirm the structure of the ZnCo₂O₄ grafted over BiVO₄. A remarkable 4.4-fold enhancement of the photocurrent was observed for the BiVO₄/ZnCo₂O₄ composite compared with bare BiVO₄ under visible illumination. The optimum loading of ZnCo₂O₄ over BiVO₄ yields unprecedented stable photocurrent density with an apparent cathodic shift of 0.46 V under 1.5 AM simulated light illumination. This is also evidenced by the flat-band potential change through Mott-Schottky analysis, which reveals the formation of p-ZnCo₂O₄ on n-BiVO₄. The improvement in the PEC performance of the composite with respect to bare BiVO₄ is ascribed to the formation of thin passivating layer of p-ZnCo₂O₄ on n-BiVO₄ which improves the kinetics of interfacial charge transfer. Based on our study, we have gained an in-depth understanding of the BiVO₄/ZnCo₂O₄ composite as high potential in efficient PEC water splitting devices.

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.

Interfacial Charge Transport in 1D TiO2 Based Photoelectrodes for Photoelectrochemical Water Splitting

1D nanostructured photoelectrodes are promising for application as photoelectrochemical (PEC) devices for solar energy conversion into hydrogen (H₂ ) owing to the optical, structural, and electronic advantages. Titanium dioxide (TiO₂ ) is the most investigated candidate as a photoelectrode due to its good photostability, low production cost, and eco-friendliness. The obstacle for TiO₂ 's practical application is the inherent wide bandgap (UV-lights response), poor conductivity, and limited hole diffusion length. Here, a comprehensive review of the current research efforts toward the development of 1D TiO₂ based photoelectrodes for heterogeneous PEC water splitting is provided along with a discussion of nanoarchitectures and energy band engineering influences on interfacial charge transfer and separation of 1D TiO₂ composited with different dimensional photoactive materials. The key focus of this review is to understand the charge transfer processes at interfaces and the relationship between photogenerated charge separation and photoelectrochemical performance. It is anticipated that this review will afford enriched information on the rational designs of nanoarchitectures, doping, and heterojunction interfaces for 1D TiO₂ based photoelectrodes to achieve highly efficient solar energy conversion.

Crystalline phase regulation of anatase-rutile TiO2 for the enhancement of photocatalytic activity

Biphasic TiO₂ with adjustable crystalline phases was prepared by the hydrothermal-calcination method assisted by nitric acid (HNO₃) and hydrogen peroxide (H₂O₂), using potassium titanate oxalate (K₂TiO(C₂O₄)₂) as the titanium source. The influences of H₂O₂ volume on anatase and rutile contents and photocatalytic activity of biphasic TiO₂ were investigated and the photocatalytic mechanism was explored. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and specific surface area (BET) were employed to characterize crystal structure, physical morphology, absorbable light, chemical composition, specific surface area and pore size distribution. The photocatalytic degradation efficiency towards a methylene blue (MB) solution under xenon light was tested, and the photocatalytic stability of the sample was investigated by photocatalytic cycle experiments. The prepared biphasic TiO₂ was nanorod-shaped and had a large specific surface area. The results showed the anatase TiO₂ content increased and the photocatalytic efficiency was enhanced as the H₂O₂ volume solution increased. Among the catalysts, the biphasic TiO₂ prepared with 30 mL of H₂O₂ had the best photocatalytic effect and could entirely degrade the MB solution after 30 minutes under irradiation. After three repeated degradations, the photocatalytic degradation rate was still estimated to be as high as 95%. It is expected that the work will provide new insights into fabricating heterophase junctions of TiO₂ for environmental remediation.

Biphasic TiO₂ with adjustable crystalline phases was prepared by the hydrothermal-calcination method assisted by nitric acid (HNO₃) and hydrogen peroxide (H₂O₂), using potassium titanate oxalate (K₂TiO(C₂O₄)₂) as the titanium source.

Origin of the enhanced photocatalytic activity of (Ni, Se, and B) mono- and co-doped anatase TiO2 materials under visible light: a hybrid DFT study

The characteristic properties of TiO₂ (anatase) make doping necessary to enhance its photocatalytic activity. Herein, a density functional theory (DFT) study using the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional was performed to precisely investigate the effect of mono- and co-doping (Ni, Se and B) on the structural, electronic and optical properties of anatase TiO₂. Notably, the origin of the enhanced photocatalytic activity of the modified systems was determined. The response to visible light was enhanced for all the mono- and co-doped materials except for B_int, and the highest absorption coefficient was observed for Se⁴⁺ mono-doping and Se/B_int+sub and Ni/B_sub co-doping. The decrease in bandgap is associated with a red shift in the absorption edges with the smallest bandgap calculated for Ni/B_sub (2.49 eV). Additionally, the Ni, Se⁴⁺ and Se²⁻ mono-doped systems and Ni/Se⁴⁺ co-doped systems are proposed as promising photocatalysts for water splitting applications and further experimental validation. Moreover, the Ni/B_int+sub and Se/B_int+sub co-doped materials can also be valuable photocatalysts for other energy applications due to their enhanced visible light activity and the prolonged lifetime of their produced charge carriers.

Hybrid DFT calculations demonstrate that Ni, Se⁴⁺ and Se²⁻ mono-doped and Ni/Se⁴⁺ co-doped TiO₂ are potential photocatalysts for water splitting and hydrogen production.

Fabrication of a TiO2/Fe2O3 Core/Shell Nanostructure by Pulse Laser Deposition toward Stable and Visible Light Photoelectrochemical Water Splitting

Here, we report the fabrication of TiO₂/Fe₂O₃ core/shell heterojunction nanorod arrays by a pulsed laser deposition (PLD) process and their further use as photoelectrodes toward high-performance visible light photoelectrochemical (PEC) water splitting. The morphology, phase, and carrier conduction mechanism of plain TiO₂ and TiO₂/Fe₂O₃ core/shell nanostructure were systematically investigated. PEC measurements show that the TiO₂/Fe₂O₃ core/shell nanostructure enhances photocurrent density by nearly 2 times than the plain ones, increases visible light absorption from 400 to 550 nm, raises the on/off separation rate, and delivers high stability with only a 3% decrease of current density for tests of even more than 14 days. This work provides a method to design an efficient nanostructure by combination of a facile hydrothermal process and high-quality PLD process to fabricate a clean surface and excellent crystallinity for charge separation, transfer, and collection toward enhanced PEC properties.

Toward Standardized Photocatalytic Oxygen Evolution Rates Using RuO2@TiO2 as a Benchmark

Quantitative comparison of photocatalytic performances across different photocatalysis setups is technically challenging. Here, we combine the concepts of relative and optimal photonic efficiencies to normalize activities with an internal benchmark material, RuO₂ photodeposited on a P25-TiO₂ photocatalyst, which was optimized for reproducibility of the oxygen evolution reaction (OER). Additionally, a general set of good practices was identified to ensure reliable quantification of photocatalytic OER, including photoreactor design, photocatalyst dispersion, and control of parasitic reactions caused by the sacrificial electron acceptor. Moreover, a method combining optical modeling and measurements was proposed to quantify the benchmark absorbed and scattered light (7.6% and 81.2%, respectively, of λ = 300-500 nm incident photons), rather than just incident light (≈AM 1.5G), to estimate its internal quantum efficiency (16%). We advocate the adoption of the instrumental and theoretical framework provided here to facilitate material standardization and comparison in the field of artificial photosynthesis.

Role of defects on TiO2/SiO2 composites for boosting photocatalytic water splitting

Defect engineering of semiconductor photocatalysts is considered as an evolving strategy to adjust their physiochemical properties and boost photoreactivity of the materials. Here, hydrogenation and UV light pre-treatment of TiO₂/SiO₂ composite with the ratio of 9 : 1 (9TiO₂/1SiO₂) were conducted to generate Ti³⁺ and non-bridging oxygen holes center (NBOHC) defects, respectively. The 9TiO₂/1SiO₂ composite exhibited much higher photocatalytic water splitting than neat TiO₂ and SiO₂ as a consequence of the electronic structure effects induced by the defect sites. Electron paramagnetic resonance (EPR) indicated that hydrogenated and UV light pre-treated of 9TiO₂/1SiO₂ boosted a higher density of Ti³⁺ and NBOHC defect which could serve to suppress photogenerated electron-hole pair recombination and act as shallow donors to trap photoexcited electron. Overall, both defect sites in 9TiO₂/1SiO₂ delivered advantageous characteristic relative to neat TiO₂ and SiO₂ with the finding clearly illustrating the value of defect engineering in enhancing photocatalytic performance.

Porous Hybrid Materials Based on Mesotetrakis(Hydroxyphenyl) Porphyrins and TiO2 for Efficient Visible-Light-Driven Hydrogen Production

A series of highly robust nano-micro hybrid materials based on meso-tetra(4-hydroxyphenyl) porphyrins (M = H, Pd, Zn) and titanium dioxide (denoted as THPP-TiO2, THPP-Pd-TiO2, and THPP-Zn-TiO2) have been prepared by a facile sol-gel method for the first time. When Pt nanoparticles are incorporated in these hybrids, Pt/THPP-Pd-TiO2 achieves good H2 production activity (2025.4 μmol g−1 h−1 and 12.03 μmol m−2 h−1), higher than that of Pt/THPP-Zn-TiO2 (1239.8 μmol g−1 h−1 and 7.46 μmol m−2 h−1) and Pt/THPP-TiO2 (576.8 μmol g−1 h−1 and 4.02 μmol m−2 h−1), owing to the different central metal ions in porphyrins. The best activity of Pt/THPP-Pd-TiO2 would be attributed to the two-center catalysis from coordination Pd metal ions and Pt nanoparticles, while the higher activity of Pt/THPP-Zn-TiO2 than Pt/THPP-TiO2 could be ascribed to the more effective light harvesting and electron transfer between THPP-Zn and TiO2. In addition, the hybridized Pt/THPP-Pd-TiO2 catalyst exhibits unattenuated hydrogen production stability even after recycling the experiment 10 times (cumulative turnover number of 5111 after 50 h), far superior to that of the surface-sensitized Pt/THPP-Pd/TiO2 catalyst with analogous components, due to the more stable Ti-O bonds between four phenols in porphyrins and TiO2 for the hybrid system. The present study provides a promising approach for constructing stable organic–inorganic hybrid systems with unique hierarchical structures for efficient light absorption and electron transfer.

Novel Architecture Titanium Carbide (Ti3C2Tx) MXene Cocatalysts toward Photocatalytic Hydrogen Production: A Mini-Review

Low dimensional transition metal carbide and nitride (MXenes) have been emerging as frontier materials for energy storage and conversion. Ti3C2Tx was the first MXenes that discovered and soon become the most widely investigated among the MXenes family. Interestingly, Ti3C2Tx exhibits ultrahigh catalytic activity towards the hydrogen evolution reaction. In addition, Ti3C2Tx is electronically conductive, and its optical bandgap is tunable in the visible region, making it become one of the most promising candidates for the photocatalytic hydrogen evolution reaction (HER). In this review, we provide comprehensive strategies for the utilization of Ti3C2Tx as a catalyst for improving solar-driven HER, including surface functional groups engineering, structural modification, and cocatalyst coupling. In addition, the reaming obstacle for using these materials in a practical system is evaluated. Finally, the direction for the future development of these materials featuring high photocatalytic activity toward HER is discussed.

Photocatalysts for H2 Generation from Starburst Triphenylamine/Carbazole Donor-Based Metal-Free Dyes and Porous Anatase TiO2 Cube

A series of novel triphenylamine/carbazole-based D-D-π-π-A dyes DH1-4 and a mesoporous anatase cubic "microcage" TiO₂ material (denoted as MC-TiO₂ ) were synthesized and combined to obtain dye-sensitized photocatalysts (denoted as DHn/Pt/MC-TiO₂ , n=1-4). These catalysts showed better performances in visible-light-driven H₂ evolution from water than DHn/Pt/P25-TiO₂ catalysts based on commercial P25-TiO₂ bulk semiconductor under similar conditions. Compared with P25-TiO₂ particles, the porous MC-TiO₂ had a large Brunauer-Emmett-Teller surface area, porosity, and exposed {0 0 1} crystal plane, which greatly contributed to the photocatalytic activity. The optimized DH2/Pt/MC-TiO₂ photocatalyst exhibited an attractive H₂ production rate (16.28 mmol g^-1  h^-1 based on catalyst mass), and the optimized DH4/Pt/MC-TiO₂ photocatalyst showed good stability [turnover number (TON) of 16 699 in 105 h based on dye number], which represents one of the best performances among all reported visible-light-driven heterogeneous catalytic systems. Compared with the other dyes in this series, the high H₂ production rate of DH2 on Pt/MC-TiO₂ can be attributed to its size-matching effect and thus high dye loading amount, whereas the high TON and durability of DH4/Pt/MC-TiO₂ are probably related to the rapid regeneration kinetics of DH4.

X-ray absorption linear dichroism at the Ti K-edge of rutile (001) TiO2 single crystal

X-ray absorption linear dichroism of rutile TiO2 at the Ti K-edge provides information about the electronic states involved in the pre-edge transitions. Here, linear dichroism with high energy resolution is analyzed in combination with ab initio finite difference method calculations and spherical tensor analysis. It provides an assignment of the three pre-edge peaks beyond the octahedral crystal field splitting approximation and estimates the spatial extension of the corresponding final states. It is then discussed for the first time the X-ray absorption (XAS) of pentacoordinated titanium atoms due to oxygen vacancies and it is found that, similarly to anatase TiO2, rutile is expected to exhibit a transition on the low-energy side of peak A3. Its apparent absence in the experiment is related to the degree of p-d orbital mixing which is small in rutile due to its centrosymmetric point group. A recent XAS linear dichroism study on anatase TiO2 single crystals has shown that peak A2 has an intrinsic origin and is due to a quadrupolar transition to the 3d energy levels. In rutile, due to its centrosymmetric point group, the corresponding peak A2 has a small dipole moment explaining the weak transition. The results are confronted with recent picosecond X-ray absorption spectroscopy on rutile TiO2 nanoparticles.

Effect of aspect ratios of rutile TiO2 nanorods on overall photocatalytic water splitting performance

The spatial separation of reduction and oxidation reaction sites on the different facets of a semiconductor is an ideal and promising route for overall photocatalytic water splitting due to efficient charge carrier separation. Rutile TiO2 has separate oxidation and reduction crystal facets and can be used to achieve direct splitting of pure water under ultraviolet (UV) light irradiation. In order to improve the rate of water oxidation reaction, the ratio of different crystal facets of rutile should be regulated controllably. However, the preparation of rutile TiO2 architecture has been limited by the availability of synthetic techniques. In this study, rutile TiO2 nanorods with various aspect ratios were accurately prepared in the presence of Cl- anions and H+ cations, which were found to play a crucial role in forming the morphology of rutile TiO2 nanorods. In addition, the mechanism involving the growth of rutile TiO2 nanorods with different aspect ratios is proposed. Rutile TiO2 nanorods with a high proportion of oxidative (111) facets provided higher overall water splitting reactivity.

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.

Photocatalytic properties of hybrid materials based on a multicharged polymer matrix with encored TiO2 and noble metal (Pt, Pd or Au) nanoparticles

In this study, we report a synthesis of new nanocomposites, wherein TiO₂ is introduced into multicharged polymeric matrix and covered with noble metals (Pt, Pd or Au) for the photocatalytic application.

Photoelectrochemical water-splitting over a surface modified p-type Cr2O3 photocathode

H₂ generation via solar photoelectrochemical water-splitting by Cr₂O₃ was successfully realized by surface modification with TiO₂ and the following Pt deposition.

TiO2 Nanotubes for Solar Water Splitting: Vacuum Annealing and Zr Doping Enhance Water Oxidation Kinetics

Herein, we report the cooperative effect of Zr doping and vacuum annealing on the carrier dynamics and interfacial kinetics of anodized TiO₂ nanotubes for light-driven water oxidation. After evaluation of different Zr loads and different annealing conditions, it was found that both Zr doping and vacuum annealing lead to a significantly enhanced light harvesting efficiency and photoelectrochemical performance. The substitution of Zr⁴⁺ by Ti⁴⁺ species leads to a higher density of surface defects such as oxygen vacancies, facilitating electron trapping on Zr⁴⁺, which reduced the charge recombination and hence boosted the charge transfer kinetics. More importantly, vacuum annealing promoted the presence of surface defects. Furthermore, the mechanistic study through impedance spectroscopy revealed that both charge transfer and surface conductivity are significantly enhanced due the presence of an oxygen-deficient TiO₂ surface. These results represent an important step forward in the optimization of nanostructured TiO₂-based photoelectrodes, with high potential in photocatalytic applications, including solar fuel production.

Graphitic Carbon Nitride Materials for Photocatalytic Hydrogen Production via Water Splitting: A Short Review

The generation of photocatalytic hydrogen via water splitting under light irradiation is attracting much attention as an alternative to solve such problems as global warming and to increase interest in clean energy. However, due to the low efficiency and selectivity of photocatalytic hydrogen production under solar energy, a major challenge persists to improve the performance of photocatalytic hydrogen production through water splitting. In recent years, graphitic carbon nitride (g-C3N4), a non-metal photocatalyst, has emerged as an attractive material for photocatalytic hydrogen production. However, the fast recombination of photoexcited electron–hole pairs limits the rate of hydrogen evolution and various methods such as modification, heterojunctions with semiconductors, and metal and non-metal doping have been applied to solve this problem. In this review, we cover the rational design of g-C3N4-based photocatalysts achieved using methods such as modification, metal and non-metal doping, and heterojunctions, and we summarize recent achievements in their application as hydrogen production photocatalysts. In addition, future research and prospects of hydrogen-producing photocatalysts are also reviewed.