Transfer Channel of Photoinduced Holes on a TiO2 Surface As Revealed by Solid-State Nuclear Magnetic Resonance and Electron Spin Resonance Spectroscopy

Crystal Water in Minerals Modulates Oxygen Activation for Hydrogen Peroxide Photosynthesis

Sunlight-responsive minerals contribute significantly to biogeochemical cycles by activating oxygen (O2) to generate reactive oxygen species (ROS). However, the role of crystal water, incorporated into minerals through hydration during rock cycles, in O2 activation remains largely unexplored. Here, we construct tungstite models containing oxygen vacancies to elucidate the modulation of mineral-based ROS dynamics by the synergy between oxygen vacancy and crystal water. Crystal water promotes the protonation process of superoxide anion radicals to produce hydrogen peroxide (H2O2) and alleviates its decomposition. This mineral-based H2O2 photosynthesis system efficiently eliminates organic pollutants in a sequential light-dark reaction. Furthermore, this synergy effect can extend to other metal oxide minerals such as TiO2, SnO2, CuO, ZnO, and Bi2O3. Our results illuminate an overlooked pathway for modulating the protonation process by immobilized water in hydrous minerals, playing a crucial role in ROS storage and migration and pollutant dynamics in a natural environment throughout the day/night cycle.

Positive and Negative Impacts of Interfacial Hydrogen Bonds on Photocatalytic Hydrogen Evolution

Understanding the behavior of water molecules at solid-liquid interfaces is crucial for various applications such as photocatalytic water splitting, a key technology for sustainable fuel production and chemical transformations. Despite extensive studies conducted in the past, the impact of the microscopic structure of interfacial water molecules on photocatalytic reactivity has not been directly examined. In this study, using real-time mass spectrometry and Fourier-transform infrared spectroscopy, we demonstrated the crucial role of hydrogen bond (H-bond) networks on the photocatalytic hydrogen evolution in thickness-controlled water adsorption layers on various TiO2 photocatalysts. Under controlled water vapor environments with relative humidity (RH) below 70%, we observed a monotonic increase in the H2 formation rate with increasing RH, indicating that reactive water molecules were present not only in the first adsorbed layer but also in several overlying layers. In contrast, at RH > 70%, when more than three water layers covered the catalyst surface, the H2 formation rate turned to decrease dramatically because of the structural rearrangement and hardening of the interfacial H-bond network induced during further water adsorption. This unique many-body effect of interfacial water was consistently observed for various TiO2 particles with different crystalline structures, including brookite, anatase, and a mixture of anatase and rutile. Our results demonstrated that depositing several water layers in a water vapor environment with RH ∼ 70% is optimal for photocatalytic hydrogen evolution rather than liquid-phase reaction conditions in aqueous solutions. This study provides molecular-level insights into designing interfacial water conditions to enhance photocatalytic performance.

Physical interpretation of entropy, Boltzmann constant, and temperature

Through the previously reported the quantum-identity, the light-model, and the T(temperature) · S(entropy) energy, the implied meaning of temperature and entropy, respectively, which it was difficult to intuitively recognize, was clearly defined. In order to minimize possible errors at this time, the interrelationship of the SI base unit, which is the smallest unit, and the T(temperature) · S(entropy) unit integration was used. In the process of converting to Planck units, each unit (criterion) for entropy and temperature was calculated, and their physical and chemical meanings were compared and reinterpreted. Thus, the unit of entropy is related to the Boltzmann constant, and the temperature is the oscillation of pure mass units. Therefore, the intuitive recognition of physical and chemical factors based on the unit of meter(m)-time(s) is considered sufficient as an initiator to move closer to new science beyond the current limited application.

Unraveling the atomic structure and dissociation of interfacial water on anatase TiO2 (101) under ambient conditions with solid-state NMR spectroscopy

Anatase TiO2 is a widely used component in photo- and electro-catalysts for water splitting, and the (101) facet of anatase TiO2 is the most commonly exposed surface. A detailed understanding of the behavior of H2O on this surface could provide fundamental insights into the catalytic mechanism. This, however, is challenging due to the complexity of the interfacial environments, the high mobility of interfacial H2O, and the interference from outer-layer H2O. Herein, we investigate the H2O/TiO2 interface using advanced solid-state NMR techniques. The atomic-level structures of surface O sites, OH groups, and adsorbed H2O have been revealed and the detailed interactions among them are identified on the (101) facet of anatase TiO2. By following the quantitative evolution of surface O and OH sites along with H2O loading, it is found that more than 40% of the adsorbed water spontaneously dissociated under ambient conditions on the TiO2 surface at a loading of 0.3 mmol H2O/g, due to the delicate interplay between water-surface and water-water interactions. Our study highlights the importance of understanding the atomic-level structures of H2O on the surface of TiO2 in catalytic reactions. Such knowledge can promote the design of more efficient catalytic systems for renewable energy production involving activation of water molecules.

Accumulation of Long-Lived Photogenerated Holes at Indium Single-Atom Catalysts via Two Coordinate Nitrogen Vacancy Defect Engineering for Enhanced Photocatalytic Oxidation

Visible-light-driven photocatalytic oxidation by photogenerated holes has immense potential for environmental remediation applications. While the electron-mediated photoreduction reactions are often at the spotlight, active holes possess a remarkable oxidation capacity that can degrade recalcitrant organic pollutants, resulting in nontoxic byproducts. However, the random charge transfer and rapid recombination of electron-hole pairs hinder the accumulation of long-lived holes at the reaction center. Herein, a novel method employing defect-engineered indium (In) single-atom photocatalysts with nitrogen vacancy (Nv) defects, dispersed in carbon nitride foam (In-Nv-CNF), is reported to overcome these challenges and make further advances in photocatalysis. This Nv defect-engineered strategy produces a remarkable extension in the lifetime and an increase in the concentration of photogenerated holes in In-Nv-CNF. Consequently, the optimized In-Nv-CNF demonstrates a remarkable 50-fold increase in photo-oxidative degradation rate compared to pristine CN, effectively breaking down two widely used antibiotics (tetracycline and ciprofloxacin) under visible light. The contaminated water treated by In-Nv-CNF is completely nontoxic based on the growth of Escherichia coli. Structural-performance correlations between defect engineering and long-lived hole accumulation in In-Nv-CNF are established and validated through experimental and theoretical agreement. This work has the potential to elevate the efficiency of overall photocatalytic reactions from a hole-centric standpoint.

Thermal-Stabilized Protonated TiO2 for Heat-Accelerated Photoelectrochemical Water Splitting

Enhancing the charge separation efficiency is a big challenge that limits the energy conversion efficiency of photoelectrochemical (PEC) water splitting. Surface states generated by protonation of TiO2 are the efficient charge separation passageways to massively accept or transfer the photogenerated electrons. However, a challenge is to avoid the deprotonation of a protonated TiO2 photoelectrode at the operation temperature. Here, we found that the terminal hydroxyl group (OHT) as surface states on the TiO2 surface generated via electrochemical protonation of TiO2 at 90 °C [90-TiO2-x-(OH)x] is thermally stable. As a result, the thermally enhanced photocurrent of the 90-TiO2-x-(OH)x electrode reached 1.05 mA cm-2 under 80 °C, and the stability was maintained up to 10 h with a slight photocurrent decrease of 3%. The thermally stable surface states as charge separation paths provide an effective method to couple the heat field with the PEC process via thermal-stimulating hopping of polarons.

Confined Heterojunction in Hollow-Structured TiO2 and Its Directed Effect in Photodriven Seawater Splitting

The high salinity of seawater often strongly affects the activity and stability of photocatalysts utilized for photodriven seawater splitting. The current investigation is focused on the photocatalyst H-TiO2/Cu2O, comprised of hydroxyl-enriched hollow mesoporous TiO2 microspheres containing incorporated Cu2O nanoparticles. The design of H-TiO2/Cu2O is based on the hypothesis that the respective hollow and mesoporous structure and hydrophilic surfaces of TiO2 microspheres would stabilize Cu2O nanoparticles in seawater and provide efficient and selective proton adsorption. H-TiO2/Cu2O shows hydrogen production performances of 45.7 mmol/(g·h) in simulated seawater and 17.9 mmol/(g·h) in natural seawater, respectively. An apparent quantum yield (AQY) in hydrogen production of 18.8% in water (and 14.9% in natural seawater) was obtained at 365 nm. Moreover, H-TiO2/Cu2O displays high stability and can maintain more than 90% hydrogen evolution activity in natural seawater for 30 h. A direct mass- and energy- transfer mechanism is proposed to clarify the superior performance of H-TiO2/Cu2O in seawater splitting.

Photoreforming for Lignin Upgrading: A Critical Review

Photoreforming of lignocellulosic biomass to simultaneously produce gas fuels and value-added chemicals has gradually emerged as a promising strategy to alleviate the fossil fuels crisis. Compared to cellulose and hemicellulose, the exploitation and utilization of lignin via photoreforming are still at the early and more exciting stages. This Review systematically summarizes the latest progress on the photoreforming of lignin-derived model components and "real" lignin, aiming to provide insights for lignin photocatalytic valorization from fundamental to industrial applications. Considering the complexity of lignin physicochemical properties, related analytic methods are also introduced to characterize lignin photocatalytic conversion and product distribution. We finally put forward the challenges and perspective of lignin photoreforming, hoping to provide some guidance to valorize biomass into value-added chemicals and fuels via a mild photoreforming process in the future.

Red anatase TiO2 microspheres with exposed major {001} facets and boron-stabilized hydrogen-occupied oxygen vacancies for visible-light-responsive water oxidation

In pursuit of efficient solar energy to chemical energy conversion through band engineering of wide-bandgap photocatalysts such as TiO₂, a compromise occurs between a narrow bandgap and high-redox-capacity photo-induced charge carriers, which impairs the potential advantages associated with the widened absorption range. The key to this compromise is an integrative modifier that can simultaneously modulate both the bandgap and band edge positions. Herein, we theoretically and experimentally demonstrate that oxygen vacancies occupied by boron-stabilized hydrogen pairs (OV_BH) serve as an integrative band modifier. Compared to hydrogen-occupied oxygen vacancies (OV_H), which require the aggregation of nanosized anatase TiO₂ particles, oxygen vacancies coupled with boron (OV_BH) can be easily introduced into large and highly crystalline TiO₂ particles, as shown by density functional theory (DFT) calculations. The coupling with interstitial boron facilitates the introduction of paired hydrogen atoms. The red-colored {001} faceted anatase TiO₂ microspheres with OV_BH benefit from the narrowed bandgap of 1.84 eV and the down-shifted band position. These microspheres not only absorb long-wavelength visible light up to 674 nm but also enhance visible-light-driven photocatalytic oxygen evolution.

Insights into Photothermally Enhanced Photocatalytic U(VI) Extraction by a Step-Scheme Heterojunction

In this work, a CdS/BiVO4 step-scheme (S-scheme) heterojunction with self-photothermally enhanced photocatalytic effect was synthesized and applied for efficient U(VI) photoextraction. Characterizations such as transient absorption spectroscopy and Tafel test together confirmed the formation of S-scheme heterojunctions, which allows CdS/BiVO4 to avoid photocorrosion while retaining the strong reducing capacity of CdS and the oxidizing capacity of BiVO4. Experimental results such as radical quenching experiments and electron spin resonance show that U(VI) is rapidly oxidized by photoholes/•OH to insoluble UO2(OH)2 after being reduced to U(IV) by photoelectrons/•O2 -, which precisely avoids the depletion of electron sacrificial agents. The rapid recombination of electron-hole pairs triggered by the S-scheme heterojunction is found to release large amounts of heat and accelerate the photocatalysis. This work offers a new enhanced strategy for photocatalytic uranium extraction and presents a direction for the design and development of new photocatalysts.

A spatial homojunction of titanium vacancies decorated with oxygen vacancies in TiO2 and its directed charge transfer

The n-p homojunction design in semiconductors could enable directed charge transfer, which is promising but rarely reported. Herein, TiO₂ with a spatial n-p homojunction has been designed by decorating TiO₂ nanosheets with Ti vacancies around nanostructured TiO₂ with O vacancies. 2D ¹H TQ-SQ MAS NMR, EPR and XPS show the junction of titanium vacancies and oxygen vacancies at the interface. This spatial homojunction contributes to a significant enhancement in photoelectrochemical and photocatalytic performance, especially photocatalytic seawater splitting. Density functional theory calculations of the charge density reveal the directional n-p charge transfer path at the interface, which is proposed at the atomic-/nanoscale to clarify the generation of rational junctions. The spatial n-p homojunction provides a facile strategy for the design of high-performance semiconductors.

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.

Insights into Photothermally Enhanced Photocatalytic U(VI) Extraction by a Step-Scheme Heterojunction

In this work, a CdS/BiVO 4 step-scheme (S-scheme) heterojunction with self-photothermally enhanced photocatalytic effect was synthesized and applied for efficient U(VI) photoextraction. Characterizations such as transient absorption spectroscopy and Tafel test together confirmed the formation of S-scheme heterojunctions, which allows CdS/BiVO 4 to avoid photocorrosion while retaining the strong reducing capacity of CdS and the oxidizing capacity of BiVO 4 . Experimental results such as radical quenching experiments and electron spin resonance show that U(VI) is rapidly oxidized by photoholes/ • OH to insoluble UO 2 (OH) 2 after being reduced to U(IV) by photoelectrons/ • O 2 - , which precisely avoids the depletion of electron sacrificial agents. The rapid recombination of electron-hole pairs triggered by the S-scheme heterojunction is found to release large amounts of heat and accelerate the photocatalysis. This work offers a new enhanced strategy for photocatalytic uranium extraction and presents a direction for the design and development of new photocatalysts.

Insights into thermally assisted photocatalytic overall water splitting over ZnTi-LDH in a gas–solid reaction system

H2O2 and bridge hydroxyl groups form because of water splitting. This process occurs intensely with the addition of heat, resulting in generation of more intermediates. Meanwhile, the separation of electrons and holes is accelerated by the heat.

The bifunctional Lewis acid site improved reactive oxygen species production: a detailed study of surface acid site modulation of TiO2 using ethanol and Br−

Modulation of surface acid sites (SAS) can effectively enhance the efficiency of reactive oxygen species (ROS) production.

Surface-Induced Desolvation of Hydronium Ion Enables Anatase TiO2 as an Efficient Anode for Proton Batteries

Hydrogen ion is an attractive charge carrier for energy storage due to its smallest radius. However, hydrogen ions usually exist in the form of hydronium ion (H₃O⁺) because of its high dehydration energy; the choice of electrode materials is thus greatly limited to open frameworks and layered structures with large ionic channels. Here, the desolvation of H₃O⁺ is achieved by using anatase TiO₂ as anodes, enabling the H⁺ intercalation with a strain-free characteristic. Density functional theory calculations show that the desolvation effects are dependent on the facets of anatase TiO₂. Anatase TiO₂ (001) surface, a highly reactive surface, impels the desolvation of H₃O⁺ into H⁺. When coupled with a MnO₂ cathode, the proton battery delivers a high specific energy of 143.2 Wh/kg at an ultrahigh specific power of 47.9 kW/kg. The modulation of the interactions between ions and electrodes opens new perspectives for battery optimizations.

Titanium Vacancies in TiO2 Nanofibers Enable Highly Efficient Photo-Driven Seawater Splitting

Photo-driven seawater splitting is considered as one of the most promising techniques for sustainable hydrogen production. However, the high salinity of seawater would deactivate catalysts and consumes the photogenerated carriers. Metal vacancies in metal oxide semiconductors are critical to directed electron transfer and high salinity resistance, thus desirable but remains a challenge. We demonstrate a facile controllable calcination approach to synthesize TiO 2 nanofibers with rich Ti-vacancies with excellent photo/electro performances and long-time stability in photo-driven seawater splitting, including photocatalysis and photoelectrocatalysis. Experimental measurements and theoretical calculations reveal the formation of titanium vacancies, as well as its unidirectional electron trap and superior H + adsorption ability for efficient charge transfer and corrosion resistance of seawater. Therefore, the characteristics and mechanism have been proposed at an atomic-/nanoscale to clarify the generation of titanium vacancies and the corresponding interfacial electron transfer.

Efficient and selective photocatalytic CH4 conversion to CH3OH with O2 by controlling overoxidation on TiO2

The conversion of photocatalytic methane into methanol in high yield with selectivity remains a huge challenge due to unavoidable overoxidation. Here, the photocatalytic oxidation of CH₄ into CH₃OH by O₂ is carried out on Ag-decorated facet-dominated TiO₂. The {001}-dominated TiO₂ shows a durable CH₃OH yield of 4.8 mmol g⁻¹ h⁻¹ and a selectivity of approximately 80%, which represent much higher values than those reported in recent studies and are better than those obtained for {101}-dominated TiO₂. Operando Fourier transform infrared spectroscopy, electron spin resonance, and nuclear magnetic resonance techniques are used to comprehensively clarify the underlying mechanism. The straightforward generation of oxygen vacancies on {001} by photoinduced holes plays a key role in avoiding the formation of •CH₃ and •OH, which are the main factors leading to overoxidation and are generally formed on the {101} facet. The generation of oxygen vacancies on {001} results in distinct intermediates and reaction pathways (oxygen vacancy → Ti–O₂^• → Ti–OO–Ti and Ti–(OO) → Ti–O^• pairs), thus achieving high selectivity and yield for CH₄ photooxidation into CH₃OH.

The photocatalytic conversion of CH₄ into CH₃OH with high activity and selectivity must avoid product overoxidation. Here, authors minimize overoxidation by using a (001)-dominated TiO₂ nanosheet to circumvent CH₄ overoxidation intermediates plus reaction pathways that occur on (101) facets.

Mechanistic Studies on Photocatalytic Overall Water Splitting over Ga2O3-Based Photocatalysts by Operando MS-FTIR Spectroscopy

Photocatalytic water splitting on semiconductor photocatalysts is one of the most important physichemical processes, but its surface reaction mechanisms are not fully understood. Based upon the ATR-FTIR investigations combining with the mass spectroscopy (MS) analysis, a direct hydroxyl radical formation mechanism that is different from those observed for other semiconductor photocatalysts is proposed. This study provides the insight into overall water splitting mechanism on Ga₂O₃-based photocatalysts at a molecular level, and it helps one to further understand the photocatalysis on semiconductor photocatalysts.

Hydroxyl-enriched highly crystalline TiO2 suspensible photocatalyst: facile synthesis and superior H2-generation activity

A facile ethanol-controlled strategy was reported to simultaneously realize the excellent suspendability and high-crystallinity of a hydroxyl-enriched TiO₂ nanocrystal for efficient photocatalytic H₂ generation.

Insights into Designing Photocatalysts for Gaseous Ammonia Oxidation under Visible Light

Excessive emission of ammonia (NH₃) gives rise to a number of negative effects on the environment and human health. Photocatalysis is an efficient method to eliminate gaseous NH₃; however, photocatalytic oxidation (PCO) of NH₃ in the visible light region has not been achieved to date. Herein, we test a set of typical visible-light-sensitive photocatalysts (N-TiO₂, g-C₃N₄, and Ag₃PO₄) for NH₃ oxidation and reveal for the first time that the semiconductor Ag₃PO₄ can harness visible light to realize ambient NH₃ oxidation. Combining the activity testing results with the photochemical properties of samples, we confirm that photoexcited holes are responsible for triggering the initial key step of NH₃ oxidation (NH₃ to ^•NH₂), and therefore, the redox potential of photoexcited holes plays the decisive role in the reaction. We propose that an active visible light photocatalyst for NH₃ oxidation requires both a suitable band gap for visible light response and a low valence band edge associated with a high oxidation potential for activating NH₃ to ^•NH₂. Our findings provide new insights into the PCO of pollutants under visible light and will benefit future design of more efficient visible-light-sensitive photocatalysts.

Visualizing the Interfacial Charge Transfer between Photoactive Microcystis aeruginosa and Hydrogenated TiO2

Exploring photoactive biotic-abiotic conjugations is of great importance for a variety of applications, but it remains difficult to probe the interfacial transfer of photoinduced charge carriers. In this work, Kelvin probe force microscopy, together with fluorescence imaging technique, were used to visually observe the spatial distribution and interfacial behavior of photocarriers in Microcystis aeruginosa/TiO₂ hybrids. Experimental investigations suggested that photosynthetic microalgae cells were prone to trap photoholes from TiO₂ photocatalysts. Oxygen vacancy defects in semiconductor exhibited significant impact on the charge migration, as the surface photovoltage of hydrogenated TiO₂/microalgae hybrid was much higher than the pristine system. Profiting from the bioenhanced charge separation, biotic-abiotic architecture presented remarkably increased activity for photocatalytic inactivation of microalgae microorganisms. This work not only highlights the visual techniques for understanding the charge transfer around biotic-abiotic interface, but also provides a bioenhanced conjugation for the photocatalytic elimination of microorganisms in water treatment applications.

Enhanced Photocatalytic CO2 Reduction over TiO2 Using Metalloporphyrin as the Cocatalyst

The photocatalytic reduction of carbon dioxide (CO2) into CO and hydrocarbon fuels has been considered as an ideal green technology for solar-to-chemical energy conversion. The separation/transport of photoinduced charge carriers and adsorption/activation of CO2 molecules play crucial roles in photocatalytic activity. Herein, tetrakis (4-carboxyphenyl) porphyrin (H2TCPP) was incorporated with different metal atoms in the center of a conjugate macrocycle, forming the metalloporphyrins TCPP-M (M = Co, Ni, Cu). The as-obtained metalloporphyrin was loaded as a cocatalyst on commercial titania (P25) to form TCPP-M@P25 (M = Co, Ni, Cu) for enhanced CO2 photoreduction. Among all of the TCPP-M@P25 (M = Co, Ni, Cu), TCPP-Cu@P25 exhibited the highest evolution rates of CO (13.6 μmol⋅g−1⋅h−1) and CH4 (1.0 μmol⋅g−1⋅h−1), which were 35.8 times and 97.0 times those of bare P25, respectively. The enhanced photocatalytic activity could be attributed to the improved photogenerated electron-hole separation efficiency, as well as the increased adsorption/activation sites provided by the metal centers in TCPP-M (M = Co, Ni, Cu). Our study indicates that metalloporphyrin could be used as a high-efficiency cocatalyst to enhance CO2 photoreduction activity.

Controlled hydrogenation into defective interlayer bismuth oxychloride via vacancy engineering

Hydrogenation is an effective approach to improve the performance of photocatalysts within defect engineering methods. The mechanism of hydrogenation and synergetic effects between hydrogen atoms and local electronic structures, however, remain unclear due to the limits of available photocatalytic systems and technical barriers to observation and measurement. Here, we utilize oxygen vacancies as residential sites to host hydrogen atoms in a layered bismuth oxychloride material containing defects. It is confirmed theoretically and experimentally that the hydrogen atoms interact with the vacancies and surrounding atoms, which promotes the separati30on and transfer processes of photo-generated carriers via the resulting band structure. The efficiency of catalytic activity and selectivity of defective bismuth oxychloride regarding nitric oxide oxidation has been improved. This work clearly reveals the role of hydrogen atoms in defective crystalline materials and provides a promising way to design catalytic materials with controllable defect engineering.

Interfacial co-existence of oxygen and titanium vacancies in nanostructured TiO2 for enhancement of carrier transport

The interfacial co-existence of oxygen and metal vacancies in metal oxide semiconductors and their highly efficient carrier transport have rarely been reported. This work reports on the co-existence of oxygen and titanium vacancies at the interface between TiO₂ and rGO via a simple two-step calcination treatment. Experimental measurements show that the oxygen and titanium vacancies are formed under 550 °C/Ar and 350 °C/air calcination conditions, respectively. These oxygen and titanium vacancies significantly enhance the transport of interfacial carriers, and thus greatly improve the photocurrent performances, the apparent quantum yield, and photocatalysis such as photocatalytic H₂ production from water-splitting, photocatalytic CO₂ reduction and photo-electrochemical anticorrosion of metals. A new "interfacial co-existence of oxygen and titanium vacancies" phenomenon, and its characteristics and mechanism are proposed at the atomic-/nanoscale to clarify the generation of oxygen and titanium vacancies as well as the interfacial carrier transport.

Interfacial Chemical-Bond-Modulated Charge Transfer of Heterostructures for Improving Photocatalytic Performance

Interface engineering of heterostructured photocatalysts plays a very important role in the transfer and separation process of interfacial charge carriers, but how to regulate the transfer and separation of photogenerated charge carriers still is a huge challenge at the nanometric interface of heterostructures (HCs). Herein, we demonstrate that interfacial chemical bonds can effectively modulate photogenerated charge transfer in nanoclay-based HCs constructed by natural Kaolinite (Kaol) nanosheets and P25-TiO₂. Experimental results and density functional theory (DFT) calculations confirm that stable Al-O-Ti bonds form at the interfaces by interactions of the Al-OH groups of Kaol and (101) surfaces of anatase TiO₂. The Al-O-Ti bond strengthens the energy band bending of the space charge region near the interfacial bond and thus provides a fast transfer channel for interfacial photogenerated charge, resulting in the boosted charge transfer and separation ability of Kaol/P25 HCs. The findings reported here provide a deeper insight into modulating interfacial charge transfer by chemical bonds and shed new light on interface engineering of efficient heterostructured photocatalysts for environmental applications.

Preparation and Photocatalytic Property of Ag Modified Titanium Dioxide Exposed High Energy Crystal Plane (001)

TiO2 exposed high energy crystal plane (001) was prepared by the sol-gel process using butyl titanate as the titanium source and hydrofluoric acid as the surface control agent. Ag-TiO2 was prepared by depositing Ag on the crystal plane of TiO2 (101) with a metal halide lamp. The surface morphology, interplanar spacing, crystal phase composition, ultraviolet absorption band, element composition, and valence state of the samples were characterized by using field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD), ultraviolet-visible absorption spectrum (UV-Vis-Abs), and X-ray photoelectron spectroscopy (XPS), respectively. The formation mechanism of high energy crystal plane (001) was discussed, and the photocatalytic activities were evaluated by following degradation of methyl orange. The results show that TiO2 exposed the (001) crystal plane with a ratio of 41.8%, and Ag can be uniformly deposited on the crystal plane of TiO2 (101) by means of metal halide lamp deposition. Under the same conditions, the degradation rate of methyl orange by deposited Ag-TiO2 reaches as much as 93.63% after 60 min using the metal halide lamp (300 W) as an illuminant, 81.89% by non-deposited samples and 75.20% by nano-TiO2, causing a certain blue shift in the light absorption band edge of TiO2. Ag-TiO2 has the best photocatalytic performance at a pH value of 2.

Surface Defect-Controlled Growth and High Photocatalytic H2 Production Efficiency of Anatase TiO2 Nanosheets

Facet engineering of anatase TiO₂ by controlling the {001} exposure ratio has been the focus of numerous investigations to optimize photocatalytic activity. In particular, an introduction of fluoride ions during the crystal growth has been demonstrated to be very effective and decisive in realizing the facet exposure of the crystals. However, a key role of fluoride ions in stabilizing {001} exposure and improving subsequent photocatalytic activity of anatase TiO₂ remains unclear up to date. Herein, a controlled thickness of anatase TiO₂ nanosheets has been realized by introducing different amounts of ethanol into a HF acid-assisted hydrothermal reaction. The thinnest nanosheets with a thickness of ∼2.9 nm were evaluated to have the highest H₂ production rate of 41.04 mmol·h^-1·g^-1 under ultraviolet light irradiation, and the corresponding quantum efficiency was determined to be 41.6% (λ = 365 nm). Moreover, it is proved for the first time that fluoride ions are bonded with Ti vacancies on {001} facets, and such defects are crucial for stabilizing the ultrathin nanosheets and improving their electron-hole separation, therefore leading to a highly efficient photocatalytic activity. The findings offer an opportunity to engineer facets and functionality of anatase TiO₂ by controlling surface defects.

Multiple Doped Carbon Nitrides with Accelerated Interfacial Charge/Mass Transportation for Boosting Photocatalytic Hydrogen Evolution

The interaction of water molecule with catalysts is crucial to photocatalysis, but the surface property manipulation still remains a great challenge. In this study, we report an in situ multiple heteroelement (sodium, oxygen, and iodide) doping strategy based on a molten salt-assisted route to prepare a green-colored carbon nitride (GCN). The as-prepared GCN yields 25.5 times higher H₂ evolution rate than that of bulk polymeric carbon nitride under visible light. Experimental characterization data demonstrate that the GCN delivers upshift of the conduction band and increased specific surface area and hydrophilicity. As confirmed by time-resolved PL spectra, DMPO spin-trapping EPR analysis, and so on, the excellent activity is dominantly ascribed to the greatly enhanced hydrophilicity and, subsequently, efficient interfacial charge transfer and hydrogen liberation. Moreover, through surface charge modification, the GCN also shows an increased degradation activity of rhodamine B. This work highlights the importance of surface modulation through multiple earth-abundant element incorporation for designing of advanced and practical photocatalysts.

Surface Bridge Hydroxyl-Mediated Promotion of Reactive Oxygen Species in Different Particle Size TiO2 Suspensions

Reactive oxygen species (ROS) play an essential role in TiO₂ photocatalysis. They arise from the transfer of light-initiated carriers to the TiO₂ surface and react with oxygen or water, in which the TiO₂ surface is crucial. However, how the TiO₂ surface affects ROS production is unclear. Herein, dynamic generation of ROS in suspensions of TiO₂ of different particle sizes was investigated under ultraviolet-light irradiation. It is surprising to find that more ROS were produced more quickly for 100-140 nm TiO₂ than for 20-60 nm TiO₂. Further research suggested that ROS production was intrinsically correlated with the surface bridging hydroxyls per unit area. More bridging hydroxyls induced lower IEP and more negative charges on the TiO₂ surface, which favored the transfer of photogenerated carriers, resulting in the promotion of ROS and photocatalytic activity. This provided insight into designing high-efficiency photocatalysts to solve the problem of small particle sizes causing loss and blockage in wastewater treatment.

Hydrogen-Location-Sensitive Modulation of the Redox Reactivity for Oxygen-Deficient TiO2

Hydrogenated black TiO₂ is receiving ever-increasing attention, primarily due to its ability to capture low-energy photons in the solar spectrum and its highly efficient redox reactivity for solar-driven water splitting. However, in-depth physical insight into the redox reactivity is still missing. In this work, we conducted a density functional theory study with Hubbard U correction (DFT+U) based on the model obtained from spectroscopic and aberration-corrected scanning transmission electron microscopy (AC-STEM) characterizations to reveal the synergy among H heteroatoms located at different surface sites where the six-coordinated Ti (Ti_6C) atom is converted from an inert trapping site to a site for the interchange of photoexcited electrons. This in-depth understanding may be applicable to the rational design of highly efficient solar-light-harvesting catalysts.

Effects of the Structure of TiO2 Nanotube Arrays on Its Catalytic Activity for Microbial Fuel Cell

To enhance the microbial fuel cell (MFC) for wastewater treatment and chemical oxygen demand degradation, TiO2 nanotubes arrays (TNA) are successfully synthesized on Ti foil substrate by the anodization process in HF and NH4F solution, respectively (hereafter, denoted as TNA-HF and TNA-NF). The differences between the two kinds of TNA are revealed based on their morphologies and spectroscopic characterizations. It should be highlighted that 3D TNA-NF with an appropriate dimension can make a positive contribution to the high photocatalytic activity. In comparison with the TNA-HF, the 3D TNA-NF sample exhibits a significant enhancement in current generation as the MFC anode. In particular, the TNA-NF performs nearly 1.23 times higher than the TNA-HF, and near twofold higher than the carbon felt. It is found that the two kinds of TiO2-based anodes have different conductivities and corrosion potentials, which are responsible for the difference in their current generation performances. Based on the experimental results, excellent stability, reliability, and low cost, TNA-NF can be considered a promising and scalable MFC bioanode material.

Synthesis of hydrophobic and hydrophilic TiO2 nanofluids for transformable surface wettability and photoactive coating

TiO₂ nanofluids, possessing a highly dispersed TiO₂ core and an organic shell, have been used for the fabrication of coatings with transformable wettability.

Water adsorption, dissociation and oxidation on SrTiO3 and ferroelectric surfaces revealed by ambient pressure X-ray photoelectron spectroscopy

Unveiling surface adsorbates under atmospheric conditions and in surface water redox reactions on TiO₂ terminated surfaces and ferroelectric oxides, as studied by AP-XPS.

Enhancement of mitochondrial ROS accumulation and radiotherapeutic efficacy using a Gd-doped titania nanosensitizer

Radiotherapy is an extensively used treatment modality in the clinic and can kill malignant cells by generating cytotoxic reactive oxygen species (ROS). Unfortunately, excessive dosages of radiation are typically required because only a small proportion of the radiative energy is adsorbed by the soft tissues of a tumor, which results in the nonselective killing of normal cells and severe systemic side effects. An efficient nanosensitizer that makes cancer cells more sensitive to radiotherapy under a relatively low radiation dose would be highly desirable.

Methods: In this study, we developed a Gd-doped titania nanosensitizer that targets mitochondria to achieve efficient radiotherapy. Upon X-ray irradiation, the nanosensitizer triggers a “domino effect” of ROS accumulation in mitochondria. This overabundance of ROS leads to mitochondrial permeability transition and ultimately irreversible cell apoptosis. Confocal laser imaging, western blotting and flow cytometry analysis were used to explore the biological process of intrinsic apoptosis induced by the nanosensitizer. Clonogenic survival assay, cell migration and invasion experiments were employed to evaluate the radiosensitizing effect of the nanosensitizer in vitro. Finally, to evaluate the therapeutic outcome of the nanosensitizer in vivo, MCF-7 tumor model was used.

Results: Confocal laser images and western blotting data demonstrated that the nanosensitizer in conjunction with X-ray irradiation could induce cell apoptosis in ROS-mediated apoptotic signal pathways. A clonogenic survival assay revealed that cells treated with the prepared nanosensitizer exhibited a lower number of viable cell colonies than that of the nontargeted group under X-ray irradiation. Notably, with only a single dose of radiotherapy, the mitochondria-targeted nanosensitizer elicited the complete ablation of tumors in a mouse model.

Conclusion: The designed nanosensitizer in combination with X-ray radiation exposure could be used for radiotherapy against cancer in living cells and in vivo. Moreover, the nanosensitizer with mitochondria targeting played a pivotal role in triggering a “domino effect” of ROS and cell apoptosis. The current strategy could provide new opportunities in designing efficient radiosensitizers for future cancer therapy.

Synthesis of Particulate Hierarchical Tandem Heterojunctions toward Optimized Photocatalytic Hydrogen Production

Photocatalytic hydrogen production using semiconductors is identified as one of the most promising routes for sustainable energy; however, it is challenging to harvest the full solar spectrum in a particulate photocatalyst for high activity. Herein, a hierarchical hollow black TiO₂ /MoS₂ /CdS tandem heterojunction photocatalyst, which allows broad-spectrum absorption, thus delivering enhanced hydrogen evolution performance is designed and synthesized. The MoS₂ nanosheets not only function as a cost-effective cocatalyst but also act as a bridge to connect two light-harvesting semiconductors into a tandem heterojunction where the CdS nanoparticles and black TiO₂ spheres absorb UV and visible light on both sides efficiently, coupling with the MoS₂ cocatalyst into a particulate photocatalyst system. Consequently, the photocatalytic hydrogen rate of the black TiO₂ /MoS₂ /CdS tandem heterojunction is as high as 179 µmol h^-1 per 20 mg photocatalyst under visible-light irradiation, which is almost 3 times higher than that of black TiO₂ /MoS₂ heterojunctions (57.2 µmol h^-1 ). Most importantly, the stability of CdS nanoparticles in the black TiO₂ /MoS₂ /CdS tandem heterojunction is greatly improved compared to MoS₂ /CdS because of the formation of tandem heterojunctions and the strong UV-absorbing effect of black TiO₂ . Such a tandem architectural design provides new ways for synthesizing particulate photocatalysts with high efficiencies.

Crystalline TiO2 protective layer with graded oxygen defects for efficient and stable silicon-based photocathode

The trade-offs between photoelectrode efficiency and stability significantly hinder the practical application of silicon-based photoelectrochemical devices. Here, we report a facile approach to decouple the trade-offs of silicon-based photocathodes by employing crystalline TiO₂ with graded oxygen defects as protection layer. The crystalline protection layer provides high-density structure and enhances stability, and at the same time oxygen defects allow the carrier transport with low resistance as required for high efficiency. The silicon-based photocathode with black TiO₂ shows a limiting current density of ~35.3 mA cm^-2 and durability of over 100 h at 10 mA cm^-2 in 1.0 M NaOH electrolyte, while none of photoelectrochemical behavior is observed in crystalline TiO₂ protection layer. These findings have significant suggestions for further development of silicon-based, III-V compounds and other photoelectrodes and offer the possibility for achieving highly efficient and durable photoelectrochemical devices.

Crystalline TiO2 protective layer with graded oxygen defects for efficient and stable silicon-based photocathode

The trade-offs between photoelectrode efficiency and stability significantly hinder the practical application of silicon-based photoelectrochemical devices. Here, we report a facile approach to decouple the trade-offs of silicon-based photocathodes by employing crystalline TiO₂ with graded oxygen defects as protection layer. The crystalline protection layer provides high-density structure and enhances stability, and at the same time oxygen defects allow the carrier transport with low resistance as required for high efficiency. The silicon-based photocathode with black TiO₂ shows a limiting current density of ~35.3 mA cm⁻² and durability of over 100 h at 10 mA cm⁻² in 1.0 M NaOH electrolyte, while none of photoelectrochemical behavior is observed in crystalline TiO₂ protection layer. These findings have significant suggestions for further development of silicon-based, III–V compounds and other photoelectrodes and offer the possibility for achieving highly efficient and durable photoelectrochemical devices.

While silicon-based materials can convert sunlight directly to fuel and electricity, balancing their stability and efficiency constrains usage. Here, authors protect silicon photocathodes with crystalline titanium dioxide layers with graded oxygen defects to improve both durability and efficiency.

Defect-Enhanced Charge Separation and Transfer within Protection Layer/Semiconductor Structure of Photoanodes

Silicon (Si) requires a protection layer to maintain stable and long-time photoanodic reaction. However, poor charge separation and transfer are key constraint factors in protection layer/Si photoanodes that reduce their water-splitting efficiency. Here, a simultaneous enhancement of charge separation and transfer in Nb-doped NiO_x /Ni/black-Si photoanodes induced by plasma treatment is reported. The optimized photoanodes yield the highest charge-separation efficiency (η_sep ) of ≈81% at 1.23 V versus reversible hydrogen electrode, corresponding to the photocurrent density of ≈29.1 mA cm^-2 . On the basis of detailed characterizations, the concentration and species of oxygen defects in the NiO_x -based layer are adjusted by synergistic effect of Nb doping and plasma treatment, which are the dominating factors for forming suitable band structure and providing a favorable hole-migration channel. This work elucidates the important role of oxygen defects on charge separation and transfer in the protection layer/Si-based photoelectrochemical systems and is encouraging for application of this synergistic strategy to other candidate photoanodes.

A Study on Doped Heterojunctions in TiO2 Nanotubes: An Efficient Photocatalyst for Solar Water Splitting

The two important factors that affect sunlight assisted water splitting ability of TiO₂ are its charge recombination and large band gap. We report the first demonstration of nitrogen doped triphase (anatase-rutile-brookite) TiO₂ nanotubes as sun light active photocatalyst for water splitting with high quantum efficiency. Nitrogen doped triphase TiO₂ nanotubes, corresponding to different nitrogen concentrations, are synthesized electrochemically. Increase in nitrogen concentration in triphase TiO₂ nanotubes is found to induce brookite to anatase phase transformation. The variation in density of intra-band states (Ti³⁺ and N 2p states) with increase in nitrogen doping are found to be critical in tuning the photocatalytic activity of TiO₂ nanotubes. The presence of bulk heterojunctions in single nanotube of different nitrogen doped TiO₂ samples is confirmed from HRTEM analysis. The most active nitrogen doped triphase TiO₂ nanotubes are found to be 12 times efficient compared to pristine triphase TiO₂, for solar hydrogen generation. The band alignment and charge transfer pathways in nitrogen doped TiO₂ with triphase heterojunctions are delineated. Bulk heterojunctions among the three phases present in the nanotubes with intra-band defect states is shown to enhance the photocatalytic activity tremendously. Our study also confirms the theory that three phase system is efficient in photocatalysis compared to two phase system.