Understanding the anatase-rutile phase junction in charge separation and transfer in a TiO2 electrode for photoelectrochemical water splitting

Deep insight into regulation mechanism of band distribution in phase junction CdS for enhanced photocatalytic H2 production

An in-depth understanding of structure-activity relationship between the phase constitution and solar-to-hydrogen (STH) conversion efficiency is conducive to guiding the optimization route of targeted photocatalyst candidates, further establishing advanced photocatalytic systems. Herein, based on the concept of phase engineering, we encompassed the crystalline phase of CdS and achieved precise regulation of phase proportion as well as phase boundary width in the phase junction for the first time. The above cooperative effect not only modifies energy band distribution for sufficient redox potentials, but also guarantees the reverse migration orientation of photogenerated carriers in phase junction, thereby endowing photocarriers with a prolonged lifetime. Compared to pure cubic or hexagonal phase (72.6 or 101.1 μmol h-1 g-1), this CdS system with optimized phase junction demonstrates an improved photocatalytic hydrogen evolution activity of 1.04 mmol h-1 g-1 and favorable stability without cocatalyst assistance, which mainly stems from an efficient protons reduction process interacting with long-lived photogenerated electrons. This research explores the mechanism behind phase regulation and its relationship with junction capability, providing a powerful strategy to manipulate crystal phase distribution and paving a feasible avenue for other phase-dependent photocatalysts towards rational design of heterostructures based on different phases in solar energy conversion field.

Modulating crystal facets of photoanodes for photoelectrocatalytic scalable degradation of fluorinated pharmaceuticals in wastewater

Fluorinated pharmaceuticals pollution has become an ever-increasing environmental concern due to its negative impacts. Photoelectrocatalytic (PEC) degradation system is a desirable approach to tackle the pollution problems. However, photogenerated charge separation and interfacial mass transfer are the main bottlenecks for improving the PEC degradation performance. Herein, we report a TiO2 photoanode with tuned (101)/(110) facets in situ grown on a Ti mesh substrate for PEC degradation of fluorinated pharmaceuticals. The exposure of (101) facets facilitates efficient photogenerated charge separation and the desorption of generated •OH radical. Besides, the three-dimensional (3D) architecture of photoanode promotes macroscopic mass transfer. This system performed complete defluorination of 5-fluorouracil and more than 75 % total organic carbon (TOC) removal efficiency. The apparent reaction rate constant of high (101) facet-exposed TiO2 grown on Ti mesh is up to 6.96 h-1, 6‒fold faster than that of photoanode with low (101) facet-exposed TiO2 grown on Ti foil. It is demonstrated that a large-sized PEC system of 1200 cm2 can degrade 100 L of synthetic fluorinated pharmaceutical wastewater with more than 80 % elimination efficiency. This work showcases the facet and substrate modulated strategy of fabricating high-performed photoanode for PEC wastewater purification.

Role of Interfacial Morphology in Cu2O/TiO2 and Band Bending: Insights from Density Functional Theory

Photocatalysis, a promising solution to environmental challenges, relies on the generation and utilization of photogenerated charge carriers within photocatalysts. However, the recombination of these carriers often limits efficiency. Heterostructures, especially Cu2O/TiO2, have emerged as effective solutions to enhance charge separation. This study systematically explores the effect of interfacial morphologies on the band bending within Cu2O/TiO2 anatase heterostructures by employing density functional theory. Through this study, eight distinct interfaces are identified and analyzed, revealing a consistent staggered-type band alignment. Despite variations in band edge positions, systematic charge transfer from Cu2O to TiO2 is observed across all interfaces. The proposed band bending configurations would suggest enhanced charge separation and photocatalytic activity under ultraviolet illumination due to a Z-scheme configuration. This theoretical investigation provides valuable insights into the interplay between interfacial morphology, band bending, and charge transfer for advancing the understanding of fundamental electronic mechanisms in heterostructures.

Solar light driven atomic and electronic transformations in a plasmonic Ni@NiO/NiCO3 photocatalyst revealed by ambient pressure X-ray photoelectron spectroscopy

This work employs ambient pressure X-ray photoelectron spectroscopy (APXPS) to delve into the atomic and electronic transformations of a core-shell Ni@NiO/NiCO3 photocatalyst - a model system for visible light active plasmonic photocatalysts used in water splitting for hydrogen production. This catalyst exhibits reversible structural and electronic changes in response to water vapor and solar simulator light. In this study, APXPS spectra were obtained under a 1 millibar water vapor pressure, employing a solar simulator with an AM 1.5 filter to measure spectral data under visible light illumination. The in situ APXPS spectra indicate that the metallic Ni core absorbs the light, exciting plasmons, and creates hot electrons that are subsequently utilized through hot electron injection in the hydrogen evolution reaction (HER) by NiCO3. Additionally, the data show that NiO undergoes reversible oxidation to NiOOH in the presence of water vapor and light. The present work also investigates the contribution of carbonate and its involvement in the photocatalytic reaction mechanism, shedding light on this seldom-explored aspect of photocatalysis. The APXPS results highlight the photochemical reduction of carbonates into -COOH, contributing to the deactivation of the photocatalyst. This work demonstrates the APXPS efficacy in examining photochemical reactions, charge transfer dynamics and intermediates in potential photocatalysts under near realistic conditions.

Spatial interfacial heterojunctions of TiO2 for photocatalytic degradation of toluene: Effects of interface amorphous region and oxygen vacancy

The catalytic activity of TiO2 is contingent upon its crystal structure and the optoelectronic properties associated with defects. In this study, a one-step method was used to synthesize TiO2 with a spatial interface of rutile/anatase phases, and a simple thermal annealing process was applied to optimize the amorphous regions and oxygen vacancies at the interface between the rutile and anatase phases of TiO2. High-resolution transmission electron microscopy (HRTEM) elucidates the evolution process of the amorphous domain at the interface, skillfully introducing oxygen vacancies at the heterojunction interface by modulating the amorphous domain. The obtained photocatalyst (TiO2-350 °C) after annealing exhibits an optimal interface structure, with its photocatalytic activity and stability in degrading toluene far superior to P25. Photocurrent and photoluminescence (PL) measurements affirm that the existence of interfacial oxygen vacancies heightens the efficiency of electron transfer at the interface, while surface oxygen vacancies significantly enhance the stability and mineralization rate of toluene degradation. The improved photocatalytic properties were attributed to the combined effects of surface/interface oxygen vacancies and spatial interface heterojunctions. The one-step synthesis method developed in this work provides a novel perspective on combining spatially interfaced anatase/rutile phases with surface/interfacial oxygen vacancies.

Key Strategies for Enhancing H2 Production in Transition Metal Oxide Based Photocatalysts

Transition metal oxides (TMOs) were one of the first photocatalysts used to produce hydrogen from water using solar energy. Despite the emergence of many other genres of photocatalysts over the years, TMO photocatalysts remain dominant due to their easy synthesis and unique physicochemical properties. Various strategies have been developed to enhance the photocatalytic activity of TMOs, but the solar-to-hydrogen (STH) conversion efficiency of TMO photocatalysts is still very low (<2 %), which is far below the targeted STH of 10 % for commercial viability. This article provides a comprehensive analysis of several widely used strategies, including oxygen defects control, doping, establishing interfacial junctions, and phase-facet-morphology engineering, that have been adopted to improve TMO photocatalysts. By critically evaluating these strategies and providing a roadmap for future research directions, this article serves as a valuable resource for researchers, students, and professionals seeking to develop efficient energy materials for green energy solutions.

Water effect on the band edges of anatase TiO2 surfaces: A theoretical study on charge migration across surface heterojunctions and facet-dependent photoactivity

The charge migration mechanism across the surface heterojunction constructed on an anatase TiO2 nanocrystal is still under debate. To solve this longstanding question, we present a systematic study of the band edges (vs. standard hydrogen electrode, SHE) of aqueous TiO2 interfaces with anatase (101), (100) and (001) surfaces, using a combination of density functional theory-based molecular dynamics (DFTMD) and efficient computational SHE (cSHE) methods. Our calculations show that the conduction band minimum (CBM) of the (101) surface is lower than that of (001) and (100) surfaces, which is thermodynamically favorable for electrons migrating to the (101) surface through the surface heterojunction, while the hole preferentially accumulates on the (100) surface due to its highest valence band minimum (VBM). In addition, we qualitatively explore the facet-dependent photocatalytic activity of anatase TiO2. Due to the possession of both the beneficial atomic structure (with 100% undercoordinated Ti5c atoms at the surface) and electronic structure (more strongly oxidizing holes in the VBM and efficient electron-hole spatial separation separation), the (001) surface exhibits the most efficient photocatalytic performance for water oxidation. Furthermore, it is confirmed that the use of simplified theoretical models neglecting the detailed atomic structures of water at the aqueous interface is inadequate to predict the band alignment of semiconductors relative to water redox potentials, so that it may result in substantial errors in evaluating the photocatalytic performance of materials to be used for water splitting.

Nanomaterials for photo-electrochemical water splitting: a review

Over the last few decades, the global rise in energy demand has prompted researchers to investigate the energy requirements from alternative green fuels apart from the conventional fossil fuels, due to the surge in CO2 emission levels. In this context, the global demand for hydrogen is anticipated to extend by 4-5% in the next 5 years. Different production technologies like gasification of coal, partial oxidation of hydrocarbons, and reforming of natural gas are used to obtain high yields of hydrogen. In present time, 96% of hydrogen is produced by the conventional methods, and the remaining 4% is produced by the electrolysis of water. Photo-electrochemical (PEC) water splitting is a promising and progressive solar-to-hydrogen pathway with high conversion efficiency at low operating temperatures with substrate electrodes such as fluorine-doped tin oxide (FTO), incorporated with photocatalytic nanomaterials. Several semiconducting nanomaterials such as carbon nanotubes, TiO2, ZnO, graphene, alpha-Fe2O3, WO3, metal nitrides, metal phosphides, cadmium-based quantum dots, and rods have been reported for PEC water splitting. The design of photocatalytic electrodes plays a crucial role for efficient PEC water splitting process. By modifying the composition and morphology of photocatalytic nanomaterials, the overall solar-to-hydrogen (STH) energy conversion efficiency can be improved by optimizing their opto-electronic properties. The present article highlights the recent advancements in cleaner and effective photocatalysts for producing high yields of hydrogen via PEC water splitting.

Carbon Nitride-Based Heterojunction Photoelectrodes with Modulable Charge-Transfer Pathways toward Selective Biosensing

Photoelectrochemical (PEC) sensing enables the rapid, accurate, and highly sensitive detection of biologically important chemicals. However, achieving high selectivity without external biological elements remains a challenge because the PEC reactions inherently have poor selectivity. Herein, we report a strategy to address this problem by regulating the charge-transfer pathways using polymeric carbon nitride (pCN)-based heterojunction photoelectrodes. Interestingly, because of redox reactions at different semiconductor/electrolyte interfaces with specific charge-transfer pathways, each analyte demonstrated a unique combination of photocurrent-change polarity. Based on this principle, a pCN-based PEC sensor for the highly selective sensing of ascorbic acid in serum against typical interferences, such as dopamine, glutathione, epinephrine, and citric acid was successfully developed. This study sheds light on a general PEC sensing strategy with high selectivity without biorecognition units by engineering charge-transfer pathways in heterojunctions on photoelectrodes.

Photocatalytic Performance of Sol-Gel Prepared TiO2 Thin Films Annealed at Various Temperatures

Titanium dioxide (TiO2) in the form of thin films has attracted enormous attention for photocatalysis. It combines the fundamental properties of TiO2 as a large bandgap semiconductor with the advantage of thin films, making it competitive with TiO2 powders for recycling and maintenance in photocatalytic applications. There are many aspects affecting the photocatalytic performance of thin film structures, such as the nanocrystalline size, surface morphology, and phase composition. However, the quantification of each influencing aspect needs to be better studied and correlated. Here, we prepared a series of TiO2 thin films using a sol-gel process and spin-coated on p-type, (100)-oriented silicon substrates with a native oxide layer. The as-deposited TiO2 thin films were then annealed at different temperatures from 400 °C to 800 °C for 3 h in an ambient atmosphere. This sample synthesis provided systemic parameter variation regarding the aspects mentioned above. To characterize thin films, several techniques were used. Spectroscopic ellipsometry (SE) was employed for the investigation of the film thickness and the optical properties. The results revealed that an increasing annealing temperature reduced the film thickness with an increase in the refractive index. Atomic force microscopy (AFM) was utilized to examine the surface morphology, revealing an increased surface roughness and grain sizes. X-ray diffractometry (XRD) and UV-Raman spectroscopy were used to study the phase composition and crystallite size. The annealing process initially led to the formation of pure anatase, followed by a transformation from anatase to rutile as the annealing temperature increased. An overall enhancement in crystallinity was also observed. The photocatalytic properties of the thin films were tested using the photocatalytic decomposition of acetone gas in a home-built solid (photocatalyst)-gas (reactant) reactor. The composition of the gas mixture in the reaction chamber was monitored using in situ Fourier transform infrared spectroscopy. Finally, all of the structural and spectroscopic characteristics of the TiO2 thin films were quantified and correlated with their photocatalytic properties using a correlation matrix. This provided a good overview of which film properties affect the photocatalytic efficiency the most.

3D Flower-like TiO2 Nanostructures: Anatase-To-Rutile Phase Transformation and Photoelectrochemical Application

Flower-like TiO2 nanostructures were obtained by chemical oxidation of Ti foil using H2O2 combined with subsequent annealing. This paper offers an analysis of the phase transformation of 3D flower-like titanium dioxide nanostructures. The role of the annealing atmosphere, sample thickness, grain shape, and nanoflower size are discussed. The nanostructures were examined using SEM, XRD, and Raman spectroscopy. Due to the nature of these two processes, the morphology of these nanomaterials is complex, and is obtained through a reaction involving Ti foil and H2O2 at 80°C. A distinction is made between the layer composed of small grains at the substrate/oxide interface, elongated crystal-like structures, and outer spongle-like film. The annealing parameters, such as atmosphere (air or argon) and temperature (450 or 600 °C), affect phase composition. The photoelectrochemical performance of the anode based on flower-like TiO2 has been shown. The thickness and phase composition of the anodes are factors that strongly affect the photocurrent. The multiphase heterojunctions proposed for 3D flower-like TiO2 photoanodes in photoelectrochemical (PEC) cells suppose that the conduction band of anatase should be above rutile. The highest photoelectrochemical performance was obtained for a photoanode composed of 20–40% anatase and an associated thickness of 0.75–1.5 µm.

In Situ Growth Intercalation Structure MXene@Anatase/Rutile TiO2 Ternary Heterojunction with Excellent Phosphoprotein Detection in Sweat

Abnormal protein phosphorylation may relate to diseases such as Alzheimer's, schizophrenia, and Parkinson's. Therefore, the real-time detection of phosphoproteins in sweat was of great significance for the early knowledge, detection, and treatment of neurological diseases. In this work, anatase/rutile TiO₂ was in situ grown on the MXene surface to constructing the intercalation structure MXene@anatase/rutile TiO₂ ternary heterostructure as a sensing platform for detecting phosphoprotein in sweat. Here, the intercalation structure of MXene acted as electron and diffusion channels for phosphoproteins. The in situ grown anatase/rutile TiO₂ with n-n-type heterostructure provided specific adsorption sites for the phosphoproteins. The determination of phosphoprotein covered concentrations in sweat, with linear range from 0.01 to 1 mg/mL, along with a low LOD of 1.52 μM. It is worth noting that, since the macromolecular phosphoprotein was adsorbed on the surface of the material, the electrochemical signal gradually decreased with the increase of phosphoprotein concentration. In addition, the active sites in the MXene@anatase/rutile TiO₂ ternary heterojunction and synergistic effect of the heterojunction were verified by first-principle calculations to further realize the response to phosphoproteins. Additionally, the effective diffusion capacity and mobility of phosphoprotein molecules in the ternary heterojunction structure were studied by molecular dynamics simulation. Furthermore, the constructed sensing platform showed high selectivity, repeatability, reproducibility, and stability, and this newly developed sensor can detect for phosphoprotein in actual sweat samples. This satisfactory sensing strategy could be promoted to realize the noninvasive and continuous detection of sweat.

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

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

Crystal facet and phase engineering for advanced water splitting

This review covers the principles and recent advances in facet and phase engineering of catalysts for photocatalytic, photoelectrochemical, and electrochemical water splitting. It suggests the basis of catalyst design for advanced water splitting.

Interaction of KRSR Peptide with Titanium Dioxide Anatase (100) Surface: A Molecular Dynamics Simulation Study

Due to its tensile strength and excellent biocompatibility, titanium (Ti) is commonly used as an implant material in medicine and dentistry. The success of dental implants depends on the formation of a contact between the oxidized surface of Ti implant and the surrounding bone tissue. The adsorption of proteins and peptides to the implant surface allows the bone-forming osteoblast cells to adhere to such modified surfaces. Recently, it has been observed that tetrapeptide KRSR (Lys-Arg-Ser-Arg) functionalization could promote osteoblast adhesion to implant surfaces. This may facilitate the establishment of an efficient bone-to implant contact and improve implant stability during the healing process. GROMACS, a molecular dynamics software package was used to perform a 200 ns simulation of adsorption of the KRSR peptide to the TiO₂ (anatase) surface in an aqueous environment. The molecule conformations were mapped with Replica Exchange Molecular Dynamics (REMD) simulations to assess the possible peptide conformations on the anatase surface, and the umbrella sampling method was used to calculate the binding energy of the most common conformation. The simulations have shown that the KRSR peptide migrates and attaches to the surface in a stable position. The dominant amino acid residue interacting with the TiO₂ surface was the N-terminal charged lysine (K) residue. REMD indicated that there is a distinct conformation that is taken by the KRSR peptide. In this conformation the surface interacts only with the lysine residue while the ser (S) and arg (R) residues interact with water molecules farther from the surface. The binding free energy of the most common conformation of KRSR peptide to the anatase (100) surface was ΔG = −8.817 kcal/mol. Our result suggests that the N-terminal lysine residue plays an important role in the adhesion of KRSR to the TiO₂ surface and may influence the osseointegration of dental implants.

Constructing a Z-Scheme Heterojunction Photocatalyst of GaPO4/α-MoC/Ga2O3 without Mingling Type-II Heterojunction for CO2 Reduction to CO

Constructing Z-scheme heterojunction photocatalysts is a prevalent strategy to prolong the lifetime of photoinduced charge carriers without reducing their redox potentials. Nevertheless, these photocatalysts were usually mingled with type-II heterojunction, leading to a decrease in the redox potentials of photoinduced charge carriers. Herein, based on the absolute electronegativity of semiconductors, a Z-scheme heterojunction photocatalyst of GaPO₄/α-MoC/Ga₂O₃ was designed and successfully constructed, in which the formation of type-II heterojunction was prevented between GaPO₄ and Ga₂O₃. In the GaPO₄/α-MoC/Ga₂O₃ photocatalyst, the conduction band (CB) and valance band (VB) potentials and the Fermi level of Ga₂O₃ are higher than those of GaPO₄, respectively. Under irradiation, photoinduced electrons on the CB of GaPO₄ migrate to the electron mediator α-MoC and subsequently recombine with the photoinduced holes of Ga₂O₃, thereby retaining the photoinduced charge carriers with higher redox potentials. As a result, GaPO₄/α-MoC/Ga₂O₃ exhibits a 4-fold enhancement of activity for CO₂ photoreduction, compared to Ga₂O₃. Photocatalytic mechanism studies indicate that superoxide radicals might be an important intermediate for CO₂ reduction to CO. The present work supplies a paradigm to construct a Z-scheme heterostructure without mingling type-II heterojunction via energy band engineering.

Photocatalytic Selective Oxidation of Ammonia in a Semi-Batch Reactor: Unravelling the Effect of Reaction Conditions and Metal Co-Catalysts

Photocatalysis has been used for the oxidation of ammonia/ammonium in water. A semibatch photoreactor was developed for this purpose, and nanostructured TiO2-based materials, either commercial P25 or prepared by flame spray pyrolysis (FSP), were used as catalysts. In the present work, we investigated the effect of (i) metal co-catalysts, (ii) pH, and (iii) ammonia concentration on the efficiency of oxidation and on the selectivity to the undesired overoxidation byproduct, i.e., nitrites and nitrates. Several metals were added to both titania samples, and the physicochemical properties of every sample were studied by XRD, BET, and UV-Vis spectroscopy. The pH, which was investigated in the range of 2.5–11.5, was the most important parameter. The optimum pH values, resulted as 11.5 and 4.8 for P25 and FSP respectively, matching the best compromise between an acceptable conversion and a limited selectivity toward nitrite and nitrate formation. For both titania samples (P25 and FSP), ammonia conversion vs. nitrite and nitrate formation were highly dependent on the pH. At pH ≥ 9, the initial rate of photooxidation was high, with selective formation of overoxidized byproducts, whereas, at a more acidic pH, the conversion was lower, but the selectivity toward nitrogen formation was higher. P25 samples added with noble metal co-catalysts (0.1 mol% Ag, Au, Pd, Pt) at pH = 11.5 remarkably increased the selectivity to nitrite and nitrate, while, in the case of FSP samples (pH = 4.8), the co-catalysts increased the selectivity toward N2 with respect to the unpromoted catalyst and also the conversion in the case of Au and Pt. Reactivity was discussed, leading to the proposing of a mechanism that correlates the activity with either surface adsorption (depending of the surface charge of the catalyst and on pH) or the homogeneous reactivity of oxidizing species.

K, Na and Cl co-doped TiO2 nanorod arrays on carbon cloth for efficient photocatalytic degradation of formaldehyde under UV/visible LED irradiation

A photocatalytic reactor of TiO₂ nanorod arrays on carbon cloth for gaseous formaldehyde degradation.

Flash microwave-assisted solvothermal (FMS) synthesis of photoactive anatase sub-microspheres with hierarchical porosity

The synthesis of nanostructured sub-microspheres of TiO2 anatase with hierarchical nano- and mesoporosity was successfully achieved by using an innovative approach that applies the principles of acidic digestion to microwave (MW) solvothermal synthesis. This process, termed flash microwave-assisted solvothermal (FMS) synthesis, facilitates the formation of spherical particles without surfactants or templating agents, exploiting the rapid reaction kinetics engendered by MW heating. Unlike many other MW-assisted solvothermal methods, the application of constant MW power leads to a rapid increase of the autogenous pressure, inducing burst-nucleation of small primary crystallites and subsequent rapid agglomeration into secondary particles, with reaction times reduced to minute-timescales. The use of non-aqueous polar solvents such as ethanol is key to the production of regular spheres with a narrow size distribution, composed of nanocrystallites. Morphology, porosity, specific surface area, phase composition, crystallite size and optical properties of the particles can be controlled via a judicious selection of physical and chemical synthesis parameters, especially precursor choice and acid concentration. The complex structure of the particles leads to surface areas of up to ca. 500 m2 g-1 with intergranular mesoporosity. The as-synthesised FMS particles show increased adsorption under dark conditions and selective de-ethylation of rhodamine B under visible light compared to a commercial photocatalyst (Degussa P25). The photodegradation mechanism hinges on the capacity of the spheres to accept electrons from the photoexcited state of molecules at the particle surface, with the large sphere surface area maximising adsorption capacity and improving the efficiency of the photocatalytic processes. The singular characteristics and properties of the particles could pave the way for further applications in water purification and optoelectronic devices.

Fe/Ni Bimetallic Organic Framework Deposited on TiO2 Nanotube Array for Enhancing Higher and Stable Photoelectrochemical Activity of Oxygen Evaluation Reaction

Photoelectrochemical (PEC) water splitting is a promising strategy to improve the efficiency of oxygen evolution reactions (OERs). However, the efficient adsorption of visible light as well as long-term stability of light-harvesting electrocatalysis is the crucial issue in PEC cells. Metal–organic framework (MOF)-derived bimetallic electrocatalysis with its superior performance has wide application prospects in OER and PEC applications. Herein, we have fabricated a nickel and iron bimetallic organic framework (FeNi-MOF) deposited on top of anodized TiO₂ nanotube arrays (TNTA) for PEC and OER applications. The FeNi-MOF/TNTA was incorporated through the electrochemical deposition of Ni²⁺ and Fe³⁺ onto the surface of TNTA and then connected with organic ligands by the hydrothermal transformation. Therefore, FeNi-MOF/TNTA demonstrates abundant photoelectrocatalytic active sites that can enhance the photocurrent up to 1.91 mA/cm² under 100 mW/cm² and a negligible loss in activity after 180 min of photoreaction. The FeNi-MOF-doped photoanode shows predominant photoelectrochemical performance due to the boosted excellent light-harvesting ability, rapid photoresponse, and stimulated interfacial energy of charge separation under the UV-visible light irradiation conditions. The results of this study give deep insight into MOF-derived bimetallic nanomaterial synthesis for photoelectrochemical OER and provide guidance on future electrocatalysis design.

Synergistic Design of Anatase–Rutile TiO2 Nanostructured Heterophase Junctions toward Efficient Photoelectrochemical Water Oxidation

Synergistically designing porous nanostructures and appropriate band alignment for TiO2 heterophase junctions is key to efficient charge transfer, which is crucial in enhancing photoelectrochemical (PEC) water splitting for hydrogen production. Here, we investigate the efficiency of PEC water oxidation in anatase–rutile TiO2 nanostructured heterophase junctions that present the type-II band alignment. We specifically prove the importance of a phase alignment in heterophase junction for effective charge separation. The TiO2 heterophase junctions were prepared by transferring TiO2 nanotube (TNT) arrays onto FTO substrate with the help of a TiO2 nanoparticle (TNP) glue layer. The PEC characterization reveals that the rutile (R)-TNT/anatase (A)-TNP heterophase junction has a higher photocurrent density than those of A-TNT/R-TNP junction and anatase or rutile single phase, corresponding to twofold enhanced efficiency. This type-II band alignment of R-TNT/A-TNP for water oxidation, in which photogenerated electrons (holes) will flow from rutile (anatase) to anatase (rutile), enables to facilitate efficient electron-hole separation as well as lower the effective bandgap of heterophase junctions. This work provides insight into the functional role of heterophase junction for boosting the PEC performances of TiO2 nanostructures.

Lattice distortion induced internal electric field in TiO2 photoelectrode for efficient charge separation and transfer

Providing sufficient driving force for charge separation and transfer (CST) is a critical issue in photoelectrochemical (PEC) energy conversion. Normally, the driving force is derived mainly from band bending at the photoelectrode/electrolyte interface but negligible in the bulk. To boost the bulky driving force, we report a rational strategy to create effective electric field via controllable lattice distortion in the bulk of a semiconductor film. This concept is verified by the lithiation of a classic TiO₂ (Li-TiO₂) photoelectrode, which leads to significant distortion of the TiO₆ unit cells in the bulk with well-aligned dipole moment. A remarkable internal built-in electric field of ~2.1 × 10² V m^-1 throughout the Li-TiO₂ film is created to provide strong driving force for bulky CST. The photoelectrode demonstrates an over 750% improvement of photocurrent density and 100 mV negative shift of onset potential upon the lithiation compared to that of pristine TiO₂ film.

Aspect Ratio-Dependent Charge Carrier Dynamics in Matchstick-like Ag2S-ZnS Nanorods for Solar Hydrogen Generation

Matchstick-like Ag₂S-ZnS nanorods (NRs) with a tunable aspect ratio (AR) were synthesized using one-pot thermal decomposition. The ultraviolet photoelectron spectra and time-resolved photoluminescence spectra of the Ag₂S-ZnS NRs were collected to study their electronic band structures and charge carrier dynamics. The energy difference (ΔE) at the interface between the ZnS stem and Ag₂S tip was altered as the AR of Ag₂S-ZnS NRs increased from 11.9 to 18.4, resulting in an enlarged driving force for the delocalized electrons along the conduction band of ZnS being injected into that of Ag₂S. The interfacial electron transfer rate constant (k_et) from ZnS to Ag₂S could be enhanced by ∼2 orders of magnitude from 5.27 × 10⁶ to 3.24 × 10⁸ s^-1, leading to a significant improvement in the efficiency of solar hydrogen generation. This investigation provides new physical insights into the manipulation of charge carrier dynamics by means of AR adjustment in semiconductor nanoheterostructures for photoelectric conversions.

Structural-Phase Catalytic Redox Reactions in Energy and Environmental Applications

The structure-property engineering of phase-based materials for redox-reactive energy conversion and environmental decontamination nanosystems, which are crucial for achieving feasible and sustainable energy and environment treatment technology, is discussed. An exhaustive overview of redox reaction processes, including electrocatalysis, photocatalysis, and photoelectrocatalysis, is given. Through examples of applications of these redox reactions, how structural phase engineering (SPE) strategies can influence the catalytic activity, selectivity, and stability is constructively reviewed and discussed. As observed, to date, much progress has been made in SPE to improve catalytic redox reactions. However, a number of highly intriguing, unresolved issues remain to be discussed, including solar photon-to-exciton conversion efficiency, exciton dissociation into active reductive/oxidative electrons/holes, dual- and multiphase junctions, selective adsorption/desorption, performance stability, sustainability, etc. To conclude, key challenges and prospects with SPE-assisted redox reaction systems are highlighted, where further development for the advanced engineering of phase-based materials will accelerate the sustainable (active, reliable, and scalable) production of valuable chemicals and energy, as well as facilitate environmental treatment.

Scheelite-related MBi1-xV1-xMoxO4 (MII- Ca, Sr) solid solution-based photoanodes for enhanced photoelectrochemical water oxidation

The photoelectrochemical properties of scheelite-related MBi_1-xV_1-xMo_xO₄ (M^II = Ca, Sr, x = 0.1 to 0.9) solid solutions deposited on conductive glass (coated with SnO₂, F-doped) have been investigated as photoanodes in photoelectrochemical (PEC) water splitting. The variation of the final annealing temperature during the preparation of the conduction electrodes as well as the value of substitution x have been shown to affect the PEC performance. The micropowders of MBi_1-xV_1-xMo_xO₄ (M^II = Ca, Sr, x = 0.1 to 0.9) samples were first fabricated vi a solid-state method; they were characterised by SEM microscopy and powder and single crystal X-ray diffraction, and the band gap values were estimated using diffusive reflectance data. The value of substitution x = 0.1 in the cases of samples containing calcium and strontium affords the highest PEC performance reported for the whole range of substitution. These results demonstrate a promising approach for the beneficial utilization of BiVO₄-substituted scheelite-related solid solutions in photo-electrochemical cells towards efficient and inexpensive photoanodes.

Continuous flow hydrothermal synthesis of phase pure rutile TiO2 nanoparticles with a rod-like morphology

Titania nanocrystals are used in numerous applications but specific polymorphs (anatase, rutile, brookite) are typically required in specific applications making synthesis control over the crystal phase essential. Supercritical continuous flow reactors constitute fast, scalable alternatives to conventional autoclave hydrothermal synthesis. They provide outstanding control over nanoparticle characteristics such as size, crystallinity, and morphology but previous studies have always resulted in anatase products. Here we report, for the first time, a continuous hydrothermal flow method for obtaining phase pure rutile nanoparticles thereby significantly broadening the crystal design space for large scale titania applications. Through variation of the reactor temperature, the dimensions of the rod-like rutile crystallites are tunable in a range of 35 to 60 nm in length and 10 to 35 nm in width (maximum aspect ratio of ∼3.5) leading to a tunable band gap (3.2-3.5 eV) and high specific surface areas exceeding 200 m² g^-1.

Ultraviolet-induced Ostwald ripening strategy towards a mesoporous Ga2O3/GaOOH heterojunction composite with a controllable structure for enhanced photocatalytic hydrogen evolution

The design of efficient semiconductor oxide materials with heterojunction nanostructures for photocatalysis holds great promise in the fields of clean energy conversion and storage.

Controllable fabrication of TiO2 anatase/rutile phase junctions by a designer solvent for promoted photocatalytic performance

Highly active coin tree-like TiO₂ anatase–rutile phase junctions were constructed by tailored DESs and the two-phase ratios can be easily tuned.

Unravelling a long-lived ligand-to-metal cluster charge transfer state in Ce–TCPP metal organic frameworks

We report the ultrafast charge separation dynamics in porphyrin-based Ce–TCPP MOFs using optical and X-ray transient absorption (XTA) spectroscopy.

Construction of a triple sequential junction for efficient separation of photogenerated charges in photocatalysis

A triple sequential junction providing a continuous charge separation and transfer channel was successfully fabricated by rational combining the anatase/rutile TiO₂ heterophase and rutile/rutile TiO₂ homophase junctions.

Morphology- and Crystalline Composition-Governed Activity of Titania-Based Photocatalysts: Overview and Perspective

Titania photocatalysts have been intensively examined for both mechanism study and possible commercial applications for more than 30 years. Although various reports have already been published on titania, including comprehensive review papers, the morphology-governed activity, especially for novel nanostructures, has not been reviewed recently. Therefore, this paper presents novel, attractive, and prospective titania photocatalysts, including zero-, one-, two-, and three-dimensional titania structures. The 1D, 2D, and 3D titania structures have been mainly designed for possible applications, e.g., (i) continuous use without the necessity of particulate titania separation, (ii) efficient light harvesting (e.g., inverse opals), (iii) enhanced activity (fast charge carriers’ separation, e.g., 1D nanoplates and 2D nanotubes). It should be pointed out that these structures might be also useful for mechanism investigation, e.g., (i) 3D titania aerogels with gold either incorporated inside the 3D network or supported in the porosity, and (ii) titania mesocrystals with gold deposited either on basal or lateral surfaces, for the clarification of plasmonic photocatalysis. Moreover, 0D nanostructures of special composition and morphology, e.g., magnetic(core)–titania(shell), mixed-phase titania (anatase/rutile/brookite), and faceted titania NPs have been presented, due to their exceptional properties, including easy separation in the magnetic field, high activity, and mechanism clarification, respectively. Although anatase has been usually thought as the most active phase of titania, the co-existence of other crystalline phases accelerates the photocatalytic activity significantly, and thus mixed-phase titania (e.g., famous P25) exhibits high photocatalytic activity for both oxidation and reduction reactions. It is believed that this review might be useful for the architecture design of novel nanomaterials for broad and diverse applications, including environmental purification, energy conversion, synthesis and preparation of “intelligent” surfaces with self-cleaning, antifogging, and antiseptic properties.

Water Oxidation Catalysts for Artificial Photosynthesis

Water oxidation is the primary reaction of both natural and artificial photosynthesis. Developing active and robust water oxidation catalysts (WOCs) is the key to constructing efficient artificial photosynthesis systems, but it is still facing enormous challenges in both fundamental and applied aspects. Here, the recent developments in molecular catalysts and heterogeneous nanoparticle catalysts are reviewed with special emphasis on biomimetic catalysts and the integration of WOCs into artificial photosystems. The highly efficient artificial photosynthesis depends largely on active WOCs integrated into light harvesting materials via rational interface engineering based on in-depth understanding of charge dynamics and the reaction mechanism.

Mixed Titanium Oxide Strategy for Enhanced Photocatalytic Hydrogen Evolution

Titanium dioxide is a promising photocatalyst material for water splitting, but is limited by its low utilization of solar energy and rapid recombination of electron-hole pairs. Herein, a mixed titanium oxide strategy, utilizing TiO₂/Ti₂O₃ heterostructures consisting of in situ grown TiO₂ nanotubes with mixed anatase and rutile phases on bulk Ti₂O₃ materials, is demonstrated for efficient and recyclable hydrogen evolution from photocatalytic water splitting. Taking advantage of the formed heterostructures and the created porous structures, the photogenerated electrons from the conduction band of anatase TiO₂ can be first delivered to rutile TiO₂ and then transferred to Ti₂O₃. Meanwhile, the presence of Ti₂O₃ in TiO₂/Ti₂O₃ heterostructures can substantially promote the charge mobility and suppress the recombination of photogenerated electron-hole pairs. Hence, with a tuned band gap structure that enables rapid electron-hole separation, increased charge carrier density, and enhanced light absorption, the TiO₂/Ti₂O₃ heterostructures provide an enhanced photocatalytic hydrogen evolution rate as high as 1440 μmol g^-1 h^-1 under full-sunlight irradiation and without any other cocatalyst. This mixed titanium oxide strategy may open up new avenues for designing and constructing highly efficient TiO₂-based photocatalytic materials for various applications.

New Insights into the Electron-Collection Efficiency Improvement of CdS-Sensitized TiO2 Nanorod Photoelectrodes by Interfacial Seed-Layer Mediation

Titanium dioxide (TiO₂) nanorods (NRs) are widely used as photoanodes in photoelectrochemical (PEC) solar fuel production because of their remarkable photoactivity and stability. In addition, TiO₂ NR electrode materials can be decorated with active CdS quantum dots (QDs) to expand the sunlight photon capture. The overall photoelectric conversion efficiency for TiO₂ NR or QD-sensitized TiO₂ NR electrode materials in PEC is typically dominated by their interfacial electron transfer (ET) properties. To understand the key factors affecting the ET, the anatase TiO₂ seed layer was added into the interface between the rutile TiO₂ NRs and fluorine-doped tin oxide (FTO) substrate. This seed layer enhanced the photocatalytic performance of both the TiO₂ NR and CdS QD-sensitized TiO₂ NR photoanodes in PEC. Time-resolved photoluminescence spectroscopy and PEC analyses, including Mott-Schottky, electrochemical impedance spectroscopy, and photovoltage ( V_ph) measurements, were used to study the charge-carrier dynamics at the interfaces between the FTO, TiO₂, and CdS QD. Analysis of the results showed that band alignment at the anatase/rutile junction between the TiO₂ and FTO promoted electron-collection efficiency ( e_EC) at the FTO/TiO₂ interface and ET rate constant ( k_ET) at the TiO₂/CdS QD interface. Furthermore, 34% enhancement of the efficiency in hydrogen (H₂) generation demonstrated the potential of the TiO₂ seed-layer-mediated TiO₂/CdS QD NR photoanode in the application of PEC solar fuel production. The current work represents new insights into the mechanism of ET in TiO₂ and TiO₂/CdS QD NR, which is very useful for the development of photoelectrode materials in solar energy conversions.