Conducting Interface in Oxide Homojunction: Understanding of Superior Properties in Black TiO2

Analysis of the Inhomogeneous Growth of Sputtered Black TiO2 Thin Films

Black titanium dioxide (B-TiO2) is a highly active photoelectrochemical material compared to pure titanium dioxide due to its increased light absorption properties. Recently, we presented the deposition of thin-film B-TiO2 using an asymmetric bipolar reactive magnetron sputter process. The resulting samples exhibit excellent photoelectrochemical properties, which can be fine-tuned by varying the process parameters. In this article, results of morphological, electrical, and photoelectrochemical measurements are discussed to better understand the surprisingly high electrochemical activity of the films. In order to study the influence of the dynamic process on film formation, we use static sputtering with a fixed substrate covering the entire chamber area in front of the two targets. This allows the material composition of the sputtered film to be analyzed depending on its relative position to the targets. The results lead to the conclusion that the asymmetric bipolar sputtering mainly produces two phases, a transparent, nonconductive crystalline phase and a black, conductive amorphous phase. As a consequence, the dynamically sputtered samples are multilayers of these two materials. We discuss that the significantly better electrical and photoelectrochemical properties emerge from the inhomogeneous nature of the laminates, like also found in core-shell nanoparticles of B-TiO2.

Thermal vacuum de-oxygen fabrication of new catalytic pigments: SiO2@TiO2-x amorphous photonic crystals for formaldehyde removal

Eliminating benzene and formaldehyde pollution is indispensable after the popularity of colorful home decoration in current society. The possibility and advantages of vividly colorful amorphous photonic crystals (APCs) as catalytic pigments were established. Biomimetic synthesis of APCs is an effective approach to obtaining angle-independent structural colors. Herein, we introduce oxygen vacancies through thermal vacuum de-oxygenation to synthesize SiO₂@TiO_2-x APCs for angle-independent structural colors and enhanced photocatalytic performance in one step. Core-shell nanospheres with controllable particle size were synthesized using a mixed-solvent method as the structural unit of APCs to prepare seven structural colors: red, orange, yellow, green, cyan, blue, and purple. The photocatalytic activity of in situ fabricated SiO₂@TiO_2-x APCs was conspicuously enhanced by thermal vacuum deoxidation. An amorphous layer formed on the TiO₂ nanocrystals provides TiO_2-x with excellent spectral response to visible light, transient photocurrent, and surface photovoltage up to 38.44 μA cm^-2 and 28.8 mV, respectively. Black TiO_2-x absorbs incoherent scattering, causing APCs to generate vividly angle-independent structural colors. The existence of oxygen vacancies in TiO_2-x promotes electron activation and a synergistic effect with the photonic local effect of APCs in improving the degradation of formaldehyde by catalytic pigments, effectively protecting the beautiful living environment of human beings.

Surface-redox sodium-ion storage in anatase titanium oxide

Sodium-ion storage technologies are promising candidates for large-scale grid systems due to the abundance and low cost of sodium. However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO₂(A)). During the initial sodiation process, a thin surface layer (~3 to 5 nm) of crystalline TiO₂(A) becomes amorphous but still undergoes Ti⁴⁺/Ti³⁺ redox reactions. A model explaining the role of the amorphous layer and the dependence of the specific capacity on the size of TiO₂(A) nanoparticles is proposed. Amorphous nanoparticles of ~10 nm seem to be optimum in terms of achieving high specific capacity, on the order of 200 mAh g^-1, at high charge/discharge rates. Kinetic studies of TiO₂(A) nanoparticles indicate that sodium-ion storage is due to a surface-redox mechanism that is not dependent on nanoparticle size in contrast to the lithiation of TiO₂(A) which is a diffusion-limited intercalation process. The surface-redox properties of TiO₂(A) result in excellent rate capability, cycling stability and low overpotentials. Moreover, tailoring the surface-redox mechanism enables thick electrodes of TiO₂(A) to retain high rate properties, and represents a promising direction for high-power sodium-ion storage.

Single Atom Catalysts for Selective Methane Oxidation to Oxygenates

Direct conversion of methane (CH₄) to C_1-2 liquid oxygenates is a captivating approach to lock carbons in transportable value-added chemicals, while reducing global warming. Existing approaches utilizing the transformation of CH₄ to liquid fuel via tandemized steam methane reforming and the Fischer-Tropsch synthesis are energy and capital intensive. Chemocatalytic partial oxidation of methane remains challenging due to the negligible electron affinity, poor C-H bond polarizability, and high activation energy barrier. Transition-metal and stoichiometric catalysts utilizing harsh oxidants and reaction conditions perform poorly with randomized product distribution. Paradoxically, the catalysts which are active enough to break C-H also promote overoxidation, resulting in CO₂ generation and reduced carbon balance. Developing catalysts which can break C-H bonds of methane to selectively make useful chemicals at mild conditions is vital to commercialization. Single atom catalysts (SACs) with specifically coordinated metal centers on active support have displayed intrigued reactivity and selectivity for methane oxidation. SACs can significantly reduce the activation energy due to induced electrostatic polarization of the C-H bond to facilitate the accelerated reaction rate at the low reaction temperature. The distinct metal-support interaction can stabilize the intermediate and prevent the overoxidation of the reaction products. The present review accounts for recent progress in the field of SACs for the selective oxidation of CH₄ to C_1-2 oxygenates. The chemical nature of catalytic sites, effects of metal-support interaction, and stabilization of intermediate species on catalysts to minimize overoxidation are thoroughly discussed with a forward-looking perspective to improve the catalytic performance.

Vacancy-rich graphene supported electrocatalysts synthesized by radio-frequency plasma for oxygen evolution reaction

Cobalt compounds supported on reduced graphene oxides using radio frequency plasma method. The plasma creates vacancy defects on the cobalt compound.

Hydrogenated Amorphous TiO2-x and Its High Visible Light Photoactivity

Hydrogenated crystalline TiO₂ with oxygen vacancy (O_V) defect has been broadly investigated in recent years. Different from crystalline TiO₂, hydrogenated amorphous TiO_2−x for advanced photocatalytic applications is scarcely reported. In this work, we prepared hydrogenated amorphous TiO_2−x (HA-TiO_2−x) using a unique liquid plasma hydrogenation strategy, and demonstrated its highly visible-light photoactivity. Density functional theory combined with comprehensive analyses was to gain fundamental understanding of the correlation among the O_V concentration, electronic band structure, photon capturing, reactive oxygen species (ROS) generation, and photocatalytic activity. One important finding was that the narrower the bandgap HA-TiO_2−x possessed, the higher photocatalytic efficiency it exhibited. Given the narrow bandgap and extraordinary visible-light absorption, HA-TiO_2−x showed excellent visible-light photodegradation in rhodamine B (98.7%), methylene blue (99.85%), and theophylline (99.87) within two hours, as well as long-term stability. The total organic carbon (TOC) removal rates of rhodamine B, methylene blue, and theophylline were measured to 55%, 61.8%, and 50.7%, respectively, which indicated that HA-TiO_2−x exhibited high wastewater purification performance. This study provided a direct and effective hydrogenation method to produce reduced amorphous TiO_2−x which has great potential in practical environmental remediation.

Defective Black TiO2: Effects of Annealing Atmospheres and Urea Addition on the Properties and Photocatalytic Activities

A series of black TiO₂ with and without the addition of urea were successfully prepared using a simple one-step synthetic method by calcination under different atmospheres (vacuum, He, or N₂). The physicochemical, optical, and light-induced charge transfer properties of the as-prepared samples were characterized by various techniques. It was found that a vacuum atmosphere was more beneficial for the formation of oxygen vacancies (OVs) than the inert gases (He and N₂) and the addition of urea-inhibited OVs formation. The samples annealed in the vacuum condition exhibited better visible-light adsorption abilities, narrower bandgaps, higher photo-induced charge separation efficiency, and lower recombination rates. Hydroxyl radicals (·OH) were the dominant oxidative species in the samples annealed under a vacuum. Finally, the samples annealed under vacuum conditions displayed higher photocatalytic activity for methylene blue (MB) degradation than the samples annealed under He or N₂. Based on the above, this study provides new insights into the effects of annealing atmospheres and urea addition on the properties of black TiO₂.

Recent Progress on Intercalation-Based Anode Materials for Low-Cost Sodium-Ion Batteries

Intercalation-based anode materials can be considered as the most promising anode candidates for large-scale sodium-ion batteries (SIBs), owing to their long-term cycling stability and environmental friendliness, as well as their natural abundance. Nevertheless, their low energy density, low initial coulombic efficiency, and poor cycling lifespan, as well as sluggish sodium diffusion dynamics are still the main issues for the application of intercalation-based anode materials in SIBs in terms of meeting the benchmark requirements for commercialization. Over the past few years, tremendous efforts have been devoted to improving the performance of SIBs. In this Review, recent progress in the development of intercalation-based anode materials, including TiO₂ , Li₄ Ti₅ O₁₂ , Na₂ Ti₃ O₇ , and NaTi₂ (PO₄ )₃ , is summarized in terms of their sodium storage performance, critical issues, sodiation/desodiation behavior, and effective strategies to enhance their electrochemical performance. Additionally, challenges and perspectives are provided to further understand these intercalation-based anode materials.

Black TiOx Films with Photothermal-Assisted Photocatalytic Activity Prepared by Reactive Sputtering

Titanium oxide is widely applied as a photocatalyst. However, its low efficiency and narrow light absorption range are two main disadvantages that severely impede its practical application. In this work, black TiOx films with different chemical compositions were fabricated by tuning target voltage and controlling O2 flow during reactive DC magnetron sputtering. The optimized TiOx films with mixed phases (TiO, Ti2O3, Ti3O5, and TiO2) exhibited fantastic photothermal and photocatalytic activity by combining high light-absorptive Ti2O3 and Ti3O5 phases with the photocatalytic TiO2 phase. The sample prepared with oxygen flow at 5.6 ± 0.2 sccm and target voltage near 400 V exhibited excellent optical absorbance of 89.29% under visible light, which could improve surface temperature to 114 °C under sunlight. This film could degrade Rhodamine-B up to 74% after 150 min of UV irradiation. In a word, this work provides a guideline for fabricating black TiOx films with photothermal-assisted photocatalytic activity by reactive DC magnetron sputtering, which could avoid the usage of hydrogen and is convenient for quantity preparation.

Distance Effect of Ni-Pt Dual Sites for Active Hydrogen Transfer in Tandem Reaction

Unveiling the distance effect between different sites in multifunctional catalysts remains a major challenge. Herein, we investigate the distance effect by constructing a dual-site distance-controlled tandem catalyst with a five-layered TiO₂/Pt/TiO₂/Ni/TiO₂ tubular nanostructure by template-assisted atomic layer deposition. In this catalyst, the Ni and Pt sites are separated by a porous TiO₂ interlayer, and the distance between them can be precisely controlled on the subnanometer scale by altering the thickness of the interlayer, while the inner and outer porous TiO₂ layers are designed for structural stability. The catalyst exhibits superior performance for the tandem hydrazine hydrate decomposition to hydrogen and subsequent nitrobenzene hydrogenation when the Ni and Pt site distance is on the subnanometer level. The performance increases with the decrease of the distance and is better than the catalyst without the TiO₂ interlayer. Isotopic and kinetic experiments reveal that the distance effect controls the transfer of active hydrogen, which is the rate-determining step of the tandem reaction in a water solvent. Reduced Ti species with oxygen vacancies on the TiO₂ interlayer provide the active sites for hydrogen transfer with -Ti-OH surface intermediates via the continuous chemisorption/desorption of water. A smaller distance induces the generation of more active sites for hydrogen transfer and thus higher efficiency in the synergy of Ni and Pt sites. Our work provides new insight for the distance effect of different active sites and the mechanism of intermediate transfer in tandem reactions.

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The distance effect is an interesting and important topic in catalysis

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The distance of Ni-Pt dual sites is precisely controlled in subnanometer scale on a TiO₂/Pt/TiO₂/Ni/TiO₂ five-layer catalyst by ALD

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The distance controls the water-assisted hydrogen transfer, determining the overall efficiency of the tandem reaction

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A close distance in subnanometer induces more active sites for hydrogen transfer and efficient synergy of Ni and Pt sites

Pt nanoparticle-decorated two-dimensional oxygen-deficient TiO2 nanosheets as an efficient and stable electrocatalyst for the hydrogen evolution reaction

Developing novel hydrogen evolution reaction (HER) catalysts with high activity, high stability and low cost is of great importance for the ever-broader applications of hydrogen energy. Among the conventionally used platinum-based heterogeneous catalysts, the high consumption and low utilization efficiency of precious platinum are the most crucial issues. Herein we present a facile approach to prepare an effective HER catalyst with platinum nanoparticles dispersed on oxygen-deficient TiO2-x nanosheets (NSs). The fabricated Pt-TiO2-x NS electrocatalyst shows an overpotential of 35 mV at 10 mA cm-2 for the HER in 0.5 M H2SO4, which is highly comparable to that of commercial Pt/C (34 mV). More attractively, the Pt-TiO2-x NS electrocatalyst largely enhanced the mass activity (MA) of Pt and electrochemical stability compared to commercial Pt/C. The excellent HER performance of Pt-TiO2-x NSs is attributed to the synergetic effect between highly dispersed Pt species and TiO2-x NSs with oxygen vacancies, which enhances both electrocatalytic activity and durability over a wide pH range. This strategy can provide insights into constructing highly efficient catalysts and their support for different energy-related applications.

Grey Rutile TiO2 with Long-Term Photocatalytic Activity Synthesized Via Two-Step Calcination

Colored titanium oxides are usually unstable in the atmosphere. Herein, a gray rutile titanium dioxide is synthesized by two-step calcination successively in a high-temperature reduction atmosphere and in a lower-temperature air atmosphere. The as-synthesized gray rutile TiO₂ exhibits higher photocatalytic activity than that of white rutile TiO₂ and shows high chemical stability. This is attributed to interior oxygen vacancies, which can improve the separation and transmission efficiency of the photogenerated carriers. Most notably, a formed surface passivation layer will protect the interior oxygen vacancies and provide long-term photocatalytic activity.

Atomic-Scale Mapping of Impurities in Partially Reduced Hollow TiO2 Nanowires

The incorporation of impurities during the chemical synthesis of nanomaterials is usually uncontrolled and rarely reported because of the formidable challenge in measuring trace amounts of often light elements with sub-nanometer spatial resolution. And yet, these foreign elements (introduced by doping, for example) influence functional properties. We demonstrate how the hydrothermal growth and a partial reduction reaction on hollow TiO2 nanowires leads to the introduction of parts per millions of boron, sodium, and nitrogen. This doping explains the presence of oxygen vacancies and reduced Ti states at the surface, which enhance the functional properties of TiO2 . Our results were obtained on model metal oxide nanomaterials and they shed light on a general process that leads to the uncontrolled incorporation of trace impurities in TiO2 , thereby, having a strong effect on applications in energy-harvesting.

Recover the activity of sintered supported catalysts by nitrogen-doped carbon atomization

The sintering of supported metal nanoparticles is a major route to the deactivation of industrial heterogeneous catalysts, which largely increase the cost and decrease the productivity. Here, we discover that supported palladium/gold/platinum nanoparticles distributed at the interface of oxide supports and nitrogen-doped carbon shells would undergo an unexpected nitrogen-doped carbon atomization process against the sintering at high temperatures, during which the nanoparticles can be transformed into more active atomic species. The in situ transmission electron microscopy images reveal the abundant nitrogen defects in carbon shells provide atomic diffusion sites for the mobile atomistic palladium species detached from the palladium nanoparticles. More important, the catalytic activity of sintered and deactivated palladium catalyst can be recovered by this unique N-doped carbon atomization process. Our findings open up a window to preparation of sintering-resistant single atoms catalysts and regeneration of deactivated industrial catalysts.

Highly distorted mesoporous S/C/Ti3+ doped black TiO2 for simultaneous visible light degradation of multiple dyes

S–B-TiO₂ exhibited 90 and 96% visible light simultaneous degradation of rose bengal and methylene blue dyes in 80 min, respectively.

Surface Reconstruction of La0.8Sr0.2Co0.8Fe0.2O3-δ for Superimposed OER Performance

Perovskites have become important OER electrocatalysts. Herein, as-prepared La_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ (LSCF-0) is chosen as a sample to exhibit the superimposed effect of surface reconstruction accompanied by reduction of Co³⁺ to Co²⁺ on the further improvement of its activity and stability. As-synthesized LSCF-0 perovskite is chemically treated by simply immersing in an aqueous solution of NaBH₄ for 1.0 h at room temperature. The optimized LSCF (LSCF-2) owns an amorphous layer consisting of nanosized particles of ∼20 nm (vs smooth bulk crystalline surface for untreated LSCF), which exhibits superior OER performance to LSCF-0. LSCF-2 has an overpotential of 248 mV (10 mA cm^-2) and a Tafel slope of 51 mV dec^-1 (vs 355 mV and 76 mV dec^-1 for LSCF-0 and 381 mV and 91 mV dec^-1 for LCO) and an excellent cycle stability for 20 h running. This work supplies a new strategy to enhance OER performance through surface reconstruction of as-prepared perovskites.

Influence Mechanism of Cu Layer Thickness on Photoelectric Properties of IWO/Cu/IWO Films

Transparent conductive IWO/Cu/IWO (W-doped In₂O₃) films were deposited on quartz substrates by magnetron sputtering of IWO and Cu in the Ar atmosphere. The X-ray diffraction (XRD) patterns identified the cubic iron-manganese ore crystal structure of the IWO layers. The influence of the thickness of the intermediate ultra-thin Cu layers on the optical and electrical properties of the multilayer films was analyzed. As the Cu layer thickness increases from 4 to 10 nm, the multilayer resistivity gradually decreases to 4.5 × 10^-4 Ω·cm, and the optical transmittance in the mid-infrared range increases first and then decreases with a maximum of 72%, which serves as an excellent candidate for the mid-infrared transparent electrode.

Doping strain induced bi-Ti3+ pairs for efficient N2 activation and electrocatalytic fixation

The electrochemical N₂ fixation to produce ammonia is attractive but significantly challenging with low yield and poor selectivity. Herein, we first used density function theory calculations to reveal adjacent bi-Ti³⁺ pairs formed on anatase TiO₂ as the most active electrocatalytic centers for efficient N₂ lying-down chemisorption and activation. Then, by doping of anatase TiO₂ with Zr⁴⁺ that has similar d-electron configuration and oxide structure but relatively larger ionic size, the adjacent bi-Ti³⁺ sites were induced and enriched via a strained effect, which in turn enhanced the formation of oxygen vacancies. The Zr⁴⁺-doped anatase TiO₂ exhibited excellent electrocatalytic N₂ fixation performances, with an ammonia production rate (8.90 µg·h⁻¹·cm⁻²) and a Faradaic efficiency of 17.3% at −0.45 V versus reversible hydrogen electrode under ambient aqueous conditions. Moreover, our work suggests a viewpoint to understand and apply the same-valance dopants in heterogeneous catalysis, which is generally useful but still poorly understood.

Electrocatalytic N₂ conversion to ammonia offers a low-carbon alternative to industrial ammonia production, although the rational design of active, efficient catalysts remains limited. Here, authors show Ti³⁺-pair sites in anatase TiO₂ to be active for aqueous N₂ electroreduction to ammonia.

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.

Large Scale Synthesis of Nanopyramidal-Like VO₂ Films by an Oxygen-Assisted Etching Growth Method with Significantly Enhanced Field Emission Properties

The present investigation reported on a novel oxygen-assisted etching growth method that can directly transform wafer-scale plain VO₂ thin films into pyramidal-like VO₂ nanostructures with highly improved field-emission properties. The oxygen applied during annealing played a key role in the formation of the special pyramidal-like structures by introducing thin oxygen-rich transition layers on the top surfaces of the VO₂ crystals. An etching related growth and transformation mechanism for the synthesis of nanopyramidal films was proposed. Structural characterizations confirmed the formation of a composite VO₂ structure of monoclinic M1 (P21/c) and Mott insulating M2 (C2/m) phases for the films at room temperature. Moreover, by varying the oxygen concentration, the nanocrystal morphology of the VO₂ films could be tuned, ranging over pyramidal, dot, and/or twin structures. These nanopyramidal VO₂ films showed potential benefits for application such as temperature-regulated field emission devices. For one typical sample deposited on a 3-inch silicon substrate, its emission current (measured at 6 V/μm) increased by about 1000 times after the oxygen-etching treatment, and the field enhancement factor β reached as high as 3810 and 1620 for the M and R states, respectively. The simple method reported in the present study may provide a protocol for building a variety of large interesting surfaces for VO₂-based device applications.

SnS2 Nanosheets/H-TiO2 Nanotube Arrays as a Type II Heterojunctioned Photoanode for Photoelectrochemical Water Splitting

Improving the separation efficiency of photogenerated electron-hole pairs and the conductivity of electrons to photoanode substrates are critical to achieve high-performance photoelectrochemical (PEC) water splitting. Here, a SnS₂ /H-TiO₂ /Ti heterojunction photoanode was fabricated with SnS₂ nanosheets vertically grown on hydrogen-treated TiO₂ (H-TiO₂ ) nanotube arrays on a Ti substrate. It showed a significantly enhanced photocurrent of 4.0 mA cm^-2 at 1.4 V (vs. reversible hydrogen electrode) under AM 1.5 G illumination, 70 times higher than that of SnS₂ /TiO₂ /Ti. Kelvin probe force microscopy measurements indicated that photogenerated electrons could be easily transported through the SnS₂ /H-TiO₂ interface but not through the SnS₂ /TiO₂ interface. Through hydrogen treatment, defects were created in H-TiO₂ nanotubes to convert type I junctions to type II with SnS₂ nanosheets. As a result, a high efficiency of electron-hole separation at the SnS₂ /H-TiO₂ interface and a high electron conductivity in H-TiO₂ nanotubes were achieved and improved PEC performance. These findings show an effective route towards high-performance photoelectrodes for water splitting.

Recent Advances in the Use of Black TiO2 for Production of Hydrogen and Other Solar Fuels

Black TiO₂ has emerged as one of the most promising photocatalysts recently discovered. The reason behind its catalytic activity is considered to be due to the presence of defects and Ti³⁺ species at the surface of black TiO₂ nanostructures, which are crucial for its diverse applications. Moreover, disordered/crystalline surface layers and bulk regions have been identified and appear to influence the intrinsic properties of the material. Here, we present the latest studies on the use of black TiO₂ for metal free hydrogen production, as well as for CO₂ photoreduction and N₂ photofixation. After highlighting the structure/property relations, we conclude with some critical questions and suggest further topics of research in order to better understand the underlying mechanisms of light absorption in black TiO₂ , especially towards solar fuels production.

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Here, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

Atomic Substitution Enabled Synthesis of Vacancy-Rich Two-Dimensional Black TiO2- x Nanoflakes for High-Performance Rechargeable Magnesium Batteries

Rechargeable magnesium (Mg) batteries assembled with dendrite-free, safe, and earth-abundant metal Mg anodes potentially have the advantages of high theoretical specific capacity and energy density. Nevertheless, owing to the large polarity of divalent Mg²⁺ ions, the insertion of Mg²⁺ into electrode materials suffers from sluggish kinetics, which seriously limit the performance of Mg batteries. Herein, we demonstrate an atomic substitution strategy for the controlled preparation of ultrathin black TiO_{2- x} (B-TiO_{2- x}) nanoflakes with rich oxygen vacancies (OVs) and porosity by utilizing ultrathin 2D TiS₂ nanoflakes as precursors. We find out that the presence of OVs in B-TiO_{2- x} electrode material can greatly improve the electrochemical performances of rechargeable Mg batteries. Both experimental results and density functional theory simulations confirm that the introduction of OVs can remarkably enhance the electrical conductivity and increase the number of active sites for Mg²⁺ ion storage. The vacancy-rich B-TiO_{2- x} nanoflakes exhibit high reversible capacity and good capacity retention after long-term cycling at large current densities. It is hoped that this work can provide valuable insights and inspirations on the defect engineering of electrode materials for rechargeable magnesium batteries.

Preparation of a New Type of Black TiO2 under a Vacuum Atmosphere for Sunlight Photocatalysis

Black TiO₂ as a solar-driven photocatalyst has attracted enormous attention from scientists and engineers in water and wastewater treatment field. Most of the methods used for the preparation of black TiO₂ are thermal treatment under a hydrogen atmosphere. Nevertheless, it is well known that working with hydrogen is not safe and needs special maintenance. Here, for the first time, we prepared black TiO₂ by sintering P25 pellets at different temperatures (500-800 °C) under a vacuum atmosphere that showed the same performance with the prepared black TiO₂ under a hydrogen atmosphere. The samples were characterized by X-ray diffraction, Raman spectra field emission scanning electron microscopy, transmission electron microscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, and ultraviolet-visible deep resistivity sounding techniques. The differences between the formation of oxygen vacancy density and color turning in sintered powder and pellet were also studied. The results showed that the color of the P25 powder became darker after sintering but not completely turning to black, whereas the P25 pellets completely turned black after sintering. The resultant black TiO₂ was used for the photocatalytic degradation of the acetaminophen (ACE) in aqueous solution under AM 1.5G solar light illumination; it was found that the P25 pellet sintered in 500 °C had the highest photocatalytic performance for ACE degradation under AM 1.5G solar light illumination. The photocatalytic activity of prepared black TiO₂ under vacuum and hydrogen atmospheres was also compared together; the results showed that photocatalytic activities of both samples were so close together. The existence of the oxygen vacancy after 6 months and long and short-term stability (by application for photocatalytic degradation of ACE in an aqueous solution) of the black TiO₂ pellets was also studied; the results showed that the TiO₂ pellets in aqueous phase had acceptable stability.

Defect-engineered TiO2 Hollow Spiny Nanocubes for Phenol Degradation under Visible Light Irradiation

Herein, we mainly report a strategy for the facile synthesis of defect-engineered F-doped well-defined TiO2 hollow spiny nanocubes, constructed from NH4TiOF3 as precursor. The topological transformation of NH4TiOF3 mesocrystal is accompanied with fluorine anion releasing, which can be used as doping source to synthesize F-doped TiO2. Our result shows that the introduction of oxygen vacancies (Vo's) and F dopant can be further achieved by a moderate photoreduction process. The as prepared sample is beneficial to improve photocatalystic degradation and Photoelectrochemical (PEC) efficiency under visible light irradiation. And this improvement in photocatalytic and photoelectrocatalytic performance can be ascribed to the significant enhancement of visible light absorption and separation of excited charges resulted from the presence of oxygen vacancies, F- ions and hollow structure of TiO2.

Tuning defects in oxides at room temperature by lithium reduction

Defects can greatly influence the properties of oxide materials; however, facile defect engineering of oxides at room temperature remains challenging. The generation of defects in oxides is difficult to control by conventional chemical reduction methods that usually require high temperatures and are time consuming. Here, we develop a facile room-temperature lithium reduction strategy to implant defects into a series of oxide nanoparticles including titanium dioxide (TiO₂), zinc oxide (ZnO), tin dioxide (SnO₂), and cerium dioxide (CeO₂). Our lithium reduction strategy shows advantages including all-room-temperature processing, controllability, time efficiency, versatility and scalability. As a potential application, the photocatalytic hydrogen evolution performance of defective TiO₂ is examined. The hydrogen evolution rate increases up to 41.8 mmol g⁻¹ h⁻¹ under one solar light irradiation, which is ~3 times higher than that of the pristine nanoparticles. The strategy of tuning defect oxides used in this work may be beneficial for many other related applications.

Defective oxides are attractive for energy conversion and storage applications, but it remains challenging to implant defects in oxides under mild conditions. Here, the authors develop a versatile lithium reduction strategy to engineer the defects of oxides at room temperature leading to enhanced photocatalytic properties.

Hierarchical Nanotubular Anatase/Rutile/TiO2(B) Heterophase Junction with Oxygen Vacancies for Enhanced Photocatalytic H2 Production

Oxygen vacancies have been demonstrated to enhance the interfacial charge separation in TiO₂-based photocatalysts. In this report, we explored a facile route to synthesize hierarchical nanotubular anatase/rutile/TiO₂(B) nanostructures with high surface area and defective electronic structure. The formation of oxygen vacancies in the heterophase junction was analyzed by UV-vis absorption spectra, electron spin resonance, and X-ray photoelectron spectroscopy. The enhanced interfacial charge separation and transportation ensured the excellent photoactivity of oxygen-deficient junctions for the photocatalytic production of hydrogen. As a result, the defective anatase/rutile/TiO₂(B) junction showed a high hydrogen evolution rate of 2.79 mmol/h, which was 19 times higher than blank TiO₂ nanotubes. The results demonstrate that defect modulation is a powerful tool to enhance the catalytic performances of TiO₂-based photocatalysts.

Enhanced Photoelectrochemical Behavior of H-TiO2 Nanorods Hydrogenated by Controlled and Local Rapid Thermal Annealing

Recently, colored H-doped TiO₂ (H-TiO₂) has demonstrated enhanced photoelectrochemical (PEC) performance due to its unique crystalline core-disordered shell nanostructures and consequent enhanced conduction behaviors between the core-shell homo-interfaces. Although various hydrogenation approaches to obtain H-TiO₂ have been developed, such as high temperature hydrogen furnace tube annealing, high pressure hydrogen annealing, hydrogen-plasma assisted reaction, aluminum reduction and electrochemical reduction etc., there is still a lack of a hydrogenation approach in a controlled manner where all processing parameters (temperature, time and hydrogen flux) were precisely controlled in order to improve the PEC performance of H-TiO₂ and understand the physical insight of enhanced PEC performance. Here, we report for the first time a controlled and local rapid thermal annealing (RTA) approach to prepare hydrogenated core-shell H-TiO₂ nanorods grown on F:SnO₂ (FTO) substrate in order to address the degradation issue of FTO in the typical TiO₂ nanorods/FTO system observed in the conventional non-RTA treated approaches. Without the FTO degradation in the RTA approach, we systematically studied the intrinsic relationship between the annealing temperature, structural, optical, and photoelectrochemical properties in order to understand the role of the disordered shell on the improved photoelectrochemical behavior of H-TiO₂ nanorods. Our investigation shows that the improvement of PEC performance could be attributed to (i) band gap narrowing from 3.0 to 2.9 eV; (ii) improved optical absorption in the visible range induced by the three-dimensional (3D) morphology and rough surface of the disordered shell; (iii) increased proper donor density; (iv) enhanced electron-hole separation and injection efficiency due to the formation of disordered shell after hydrogenation. The RTA approach developed here can be used as a suitable hydrogenation process for TiO₂ nanorods/FTO system for important applications such as photocatalysis, hydrogen generation from water splitting and solar energy conversion.

Laser induced photocatalytic activity enhancement of TiO2 thin films

In this paper, the photocatalytic activity enhancement of TiO₂ thin films was realized by laser irradiation. The H₂ yield of the as-irradiated film is 79 μmol/(h*m²), which is 33% more than that of the as-deposited TiO₂ film. Spectrophotometer, X-ray diffraction and Raman system were employed to characterize the samples. The results showed that both the scanning rate and line spacing of the laser modification have effects on photocatalytic activity. It suggests that a phase junction is formed between the amorphous and rutile phases. The increment of H₂ generation could be attributed to the alignment of Fermi levels in the phase junction.

Tuning the Electronic Conductivity in Hydrothermally Grown Rutile TiO₂ Nanowires: Effect of Heat Treatment in Different Environments

Hydrothermally grown rutile TiO₂ nanowires are intrinsically full of lattice defects, especially oxygen vacancies. These vacancies have a significant influence on the structural and electronic properties of the nanowires. In this study, we report a post-growth heat treatment in different environments that allows control of the distribution of these defects inside the nanowire, and thus gives direct access to tuning of the properties of rutile TiO₂ nanowires. A detailed transmission electron microscopy study is used to analyze the structural changes inside the nanowires which are correlated to the measured optical and electrical properties. The highly defective as-grown nanowire arrays have a white appearance and show typical semiconducting properties with n-type conductivity, which is related to the high density of oxygen vacancies. Heat treatment in air atmosphere leads to a vacancy condensation and results in nanowires which possess insulating properties, whereas heat treatment in N₂ atmosphere leads to nanowire arrays that appear black and show almost metal-like conductivity. We link this high conductivity to a TiO_2-x shell which forms during the annealing process due to the slightly reducing N₂ environment.

Surface Localization of Defects in Black TiO2: Enhancing Photoactivity or Reactivity

In the past several years, surface-disordered TiO₂, which is referred to as black TiO₂ and can absorb both visible and near-infrared solar light, has triggered an explosion of interest for many important applications. Despite the excellent optical and electrical features of black TiO₂ for various photoelectrochemical (PEC) and photochemical reactions, the current understanding of the photocatalytic mechanism is unsatisfactory and incomplete. On the basis of previous studies, we present new insight into the surface localization of defects and perspectives on the liquid/solid interface. The future prospects for understanding black TiO₂ from this perspective suggest that defect engineering at the liquid/solid interface is a potential method of guiding nanomaterial design.