Low Temperature Synthesis of Large‐Size Anatase TiO2 Nanosheets with Enhanced Photocatalytic Activities

TiO₂ nanosheets have continuously been intriguing due to their high surface activities as photocatalyst but still challenging to synthesis large‐scale 2D nanostructures. A special microstructure evolution of TiO₂, ripening in aqueous solution at low temperature (≈4 °C), is found for the first time, i.e., from the initial aperiodic atom‐networks gradually into low crystallized continuous spongy structure with small crystal facets and ultimately forming large‐size anatase nanosheets with exposed (101) and (200) facets. Based on this finding, the synthesized anatase TiO₂ nanosheets possess monodispersed large‐scale 2D nanostructure so as to exhibit appreciable quantum size effects and remarkable enhanced optical absorption capacity. Using photocatalytic reduction of Cr (VI) to Cr (III) as the probe reaction to evaluate photocatalytic activities of the TiO₂ nanosheets, the reductivity of Cr (VI) achieves 99.8% in 15 min under irradiation of 200–800 nm light. At the same time, an in situ Cr (III)‐doping occurs spontaneously and triggers pronounced visible light driven photocatalysis, reducing 99% of Cr (VI) in 100 min under irradiation of 400–800 nm light.     

A Photoresponsive Rutile TiO2 Heterojunction with Enhanced Electron–Hole Separation for High‐Performance Hydrogen Evolution

Rutile titanium dioxide (TiO₂) is a promising photocatalyst due to its high thermodynamic stability and few intragrain defects. However, it has not yet achieved photocatalytic activity comparable to that of anatase TiO₂ owing to its higher recombination rate of electron–hole pairs. To effectively separate the electron–hole pairs in rutile TiO₂, a facet heterojunction (FH) structure to prolong the lifetime of the photogenerated electrons is proposed. Ultrathin TiO₂ nanosheets with different facets are coated in situ onto TiO₂ nanorod (NR) substrates, where FHs are built among the nanosheets as well as between the nanosheets and NR substrates. The as‐prepared rutile TiO₂, with an FH structure (FH‐TiO₂), serves as an effective photocatalyst for water splitting. More than 45 and 18 times higher photogenerated current density and H₂ production rate, respectively, are obtained compared to those of pure rutile TiO₂ NRs. Moreover, FH‐TiO₂ delivers a 0.566 mmol g^?1 h^?1 H₂ production rate even in pure water. This study offers important insights into the rational design of rutile TiO₂ structures for highly efficient photocatalytic reactions.     

Growth and <Emphasis Type="Italic">in situ</Emphasis> transformation of TiO<Subscript>2</Subscript> and HTiOF<Subscript>3</Subscript> crystals on chitosan-polyvinyl alcohol co-polymer substrates under vapor phase hydrothermal conditions

A chitosan-polyvinyl alcohol (CS/PVA) co-polymer substrate possessing a large number of amino and hydroxyl groups is used as a substrate to induce the direct growth and in situ sequential transformation of titanate crystals under HF vapor phase hydrothermal conditions. The process involves four distinct formation/transformation stages. HTiOF₃ crystals with well-defined hexagonal shapes are formed during stage I, and are subsequently transformed into {001} faceted anatase TiO₂ crystal nanosheets during stage II. Interestingly, the formed anatase TiO₂ crystals are further transformed into cross-shaped and hollow squareshaped HTiOF₃ crystals during stages III and IV, respectively. Although TiO₂ crystal phases and facet transformations under hydrothermal conditions have been previously reported, in situ crystal transformations between different titanate compounds have not been widely reported. Such crystal formation/transformations are likely due to the presence of large numbers of amino groups in the CS/PVA substrate. When celluloses possessing only hydroxyl groups are used as a substrate, the direct formation of {001} faceted TiO₂ nanocrystal sheets is observed (rather than any sequential crystal transformations). This substrate organic functional group-induced crystal formation/transformation approach could be applicable to other material systems.

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TiO2‐Decorated Graphite Nanoplatelet Nanocomposites for High‐Temperature Sensor Applications

Temperature and/or composition mapping inside high temperature energy conversion and storage devices are challenging, yet of critical importance to improve the material design for optimum performance. Here, the great potential of TiO₂ nanoparticle (NP)‐decorated graphite nanoplatelet (GNP) nanocomposites as high temperature thermal senors or gas sensors is reported. Effects of the GNP substrate on phonon confinement in Raman spectrum, grain growth, and phase stability of anatase TiO₂ NPs at high temperatures are systematically studied. Thermally sensitive Raman signatures, indicating the ultrafast grain growth of TiO₂ NPs in response to short thermal shock treatments (0.1–25 s) at high temperatures, are exploited for high temperature thermal sensing applications. A very high accuracy of nearly 98% in temperature measurements is demonstrated for a given short‐time thermal exposure. Thermal stability of anatase TiO₂ NPs against transformation into the rutile phase in TiO₂‐GNP nancomposites is substantially increased by controlling the surface area of the substrate, which would significantly improve the performance of TiO₂‐based high temperature gas sensors.     

One‐Dimensional Densely Aligned Perovskite‐Decorated Semiconductor Heterojunctions with Enhanced Photocatalytic Activity

By using one‐dimensional rutile TiO₂ nanorod arrays as the structure‐directing scaffold as well as the TiO₂ source to two consecutive hydrothermal reactions, densely aligned SrTiO₃‐modified rutile TiO₂ heterojunction photocatalysts are crafted for the first time. The first hydrothermal processing yielded nanostructured rutile TiO₂ with the hollow openings on the top of nanorods (i.e., partially etched rutile TiO₂ nanorod arrays; denoted PE‐TNRAs). The subsequent second hydrothermal treatment in the presence of Sr²⁺ transforms the surface of partially etched rutile TiO₂ nanorods into SrTiO₃ nanoparticles via the concurrent dissolution of TiO₂ and precipitation of SrTiO₃ while retaining the cylindrical shape (i.e., forming SrTiO₃‐decorated rutile TiO₂ composite nanorods; denoted STO‐TNRAs). The structural and composition characterizations substantiate the success in achieving STO‐TNRA nanostructures. In comparison to PE‐TNRAs, STO‐TNRA photocatalysts exhibit higher photocurrents and larger photocatalytic degradation rates of methylene blue (3.21 times over PE‐TNRAs) under UV light illumination as a direct consequence of improved charge carrier mobility and enhanced electron/hole separation. Such 1D perovskite‐decorated semiconductor nanoarrays are very attractive for optoelectronic applications in photovoltaics, photocatalytic hydrogen production, among other areas.     

Ultrathin Anatase TiO2 Nanosheets for High‐Performance Photocatalytic Hydrogen Production

An ethanol solvothermal route has been developed to prepare ultrathin anatase TiO₂ nanosheets with dominant {001} facets (≈97%), a thickness of ≈2.5 nm, and a side length of ≈200 nm. The introduction of ethanol solvent significantly enhances the content of surface chemisorbed F^? on the TiO₂ nanosheet, which has a higher stability and further lowers the surface energy of {001} facets, giving rise to the large percentage of active {001} facets. Adopting well‐defined morphology, such nanosheets loaded with 1 wt% Pt exhibit an H₂ evolution rate as high as 17.86 mmol h^?1 g^?1, and the corresponding apparent quantum efficiency has been determined to be 34.2%.     

Hydrothermal synthesis and gas sensing properties of different titanate nanostructures

Anatase TiO₂ nanoparticles and other three different morphologies of titanate nanostructures such as nanotubes, nanosheets and nanowires were successfully prepared by hydrothermal method. The structures and morphologies of the final products were characterized with field-emission scanning electron microscopy (FE-SEM). Phase analysis was carried out using X-ray diffraction (XRD). A novel formation mechanism from anatase TiO₂ nanoparticles to titanate nanowires is proposed based on FE-SEM. The gas sensing properties to ethanol were also investigated. The results indicate that nanotubes, nanosheets, nanowires show much less resistance and larger response than nanoparticles.     

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2‐Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS₂) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS₂ is introduced into the 2H‐MoS₂ nanosheets by two‐step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write‐once read‐many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H‐MoS₂‐polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H‐MoS₂‐PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS₂ nanosheets plays an important role on the resistive switching behaviors of the MoS₂‐based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.     

Effect of nitrate ion on formation of TiO2 nanoplate structure in hydrothermal solution

Nanoplate structure of TiO₂ was synthesized by surfactant-assisted hydrothermal method. The existence of nitrate ion with a high concentration prohibited the rolling of the nanosheets and played the key role on the formation of nanoplates of titanate. After calcinations, the titanate was transformed into TiO₂ nanoplates (anatase) of micrometer-size with 20-30 nm in thickness.     

One‐Step Synthesis of Single‐Layer MnO2 Nanosheets with Multi‐Role Sodium Dodecyl Sulfate for High‐Performance Pseudocapacitors

A template‐free, one‐step and one‐phase synthesis of single‐layer MnO₂ nanosheets has been developed via a redox reaction between KMnO₄ and sodium dodecyl sulfate (SDS). The successful formation of single‐layer MnO₂ nanosheets has been confirmed by the characteristic absorption around 374 nm and the typical thickness of ~0.95 nm. The slow redox reaction controlled by the gradual hydrolysis of SDS is found to be the key factor for the successful formation of single‐layer nanosheets. SDS not only serves as the precursor of dodecanol to reduce KMnO₄, but also aids the formation of single‐layer MnO₂ nanosheets as a structure‐inducing agent. The resultant single‐layer MnO₂ nanosheets possess superior specific capacitance, which can be attributed to the extended surface and high porosity of MnO₂ nanosheets on the electrode. The MnO₂ nanosheets also show excellent durability, retaining 91% of the starting capacitance after 10 000 charge/discharge cycles. Moreover, the symmetric pseudocapacitor based on the synthesized single‐layer MnO₂ nanosheets exhibits a high specific capacitance, indicating great potential for real energy storage. Therefore, it has been demonstrated for the first time that a single readily available reagent, SDS, can play multiple roles in reducing KMnO₄ to conveniently yield single‐layer MnO₂ nanosheets as a high‐performance pseudocapacitive material.     

Effect of hydrothermal growth temperature on structural and optical properties of TiO2 nanoparticles

TiO₂ nanoparticles have been prepared by hydrothermal method at different temperatures. The X-ray diffraction results showed that anatase TiO₂ nanoparticles with grain size in the range of 7–27 nm has been obtained. HRTEM images show the formation of TiO₂ nanoparticles with grain size ranging from 7 to 26 nm. The Raman spectra exhibited peaks corresponding to the anatase phase of TiO₂. Optical absorption studies reveal that the absorption edge shifts towards longer wavelength (red shift) with increasing hydrothermal temperature.     

Metallic Graphene‐Like VSe2 Ultrathin Nanosheets: Superior Potassium‐Ion Storage and Their Working Mechanism

Potassium‐ion batteries (KIBs) are receiving increasing interest in grid‐scale energy storage owing to the earth abundant and low cost of potassium resources. However, their development still stays at the infancy stage due to the lack of suitable electrode materials with reversible depotassiation/potassiation behavior, resulting in poor rate performance, low capacity, and cycling stability. Herein, the first example of synthesizing single‐crystalline metallic graphene‐like VSe₂ nanosheets for greatly boosting the performance of KIBs in term of capacity, rate capability, and cycling stability is reported. Benefiting from the unique 2D nanostructure, high electron/K⁺‐ion conductivity, and outstanding pseudocapacitance effects, ultrathin VSe₂ nanosheets show a very high reversible capacity of 366 mAh g^?1 at 100 mA g^?1, a high rate capability of 169 mAh g^?1 at 2000 mA g^?1, and a very low decay of 0.025% per cycle over 500 cycles, which are the best in all the reported anode materials in KIBs. The first‐principles calculations reveal that VSe₂ nanosheets have large adsorption energy and low diffusion barriers for the intercalation of K⁺‐ion. Ex situ X‐ray diffraction analysis indicates that VSe₂ nanosheets undertake a reversible phase evolution by initially proceeding with the K⁺‐ion insertion within VSe₂ layers, followed by the conversion reaction mechanism.     

TiO2 nanorod films grown on Si wafers by a nanodot-assisted hydrothermal growth

We report the controlled hydrothermal growth of rutile TiO₂ nanorods on Si wafers by using an anatase TiO₂ nanodot film as an assisted growth layer. The anatase nanodot film was prepared on the wafer by phase-separation-induced self-assembly and subsequent heat-treatment at 500 °C. The nanodots on the wafer were then subjected to hydrothermal treatment to induce the growth of rutile TiO₂ nanorod films. The size and dispersion density of the resulting TiO₂ nanorods could be varied by adjusting the Ti ion concentration in the growth solution. The TiO₂ nanorods were of the rutile phase and grew in the [001] direction. The growth mechanism reveals that the growth of the rutile nanorods was wholly dependent on the existence of rutile TiO₂ seeds, which could be formed by the dissolution-reprecipitation of the anatase nanodots during hydrothermal treatment or under the high-temperature conditions of the subsequent heat-treatment of the as-prepared nanodots. In controlling the rutile nanorod growth, the anatase nanodots show more efficiency than a dense anatase film. Preliminary evaluations of the rutile nanorod films have demonstrated that the wettability changed from highly hydrophobic to superhydrophilic and that the photocatalytic activity was enhanced with increasing nanorod dispersion density.     

Rutile Nanorod/Anatase Nanowire Junction Array as Both Sensor and Power Supplier for High‐Performance,Self‐Powered,Wireless UV Photodetector

Self‐powered UV photodetectors based on TiO₂ nanotree arrays have captured much attention in recent years because of their many advantages. In this work, rutile/anatase TiO₂ (R/A‐TiO₂) heterostructured nanotree arrays are fabricated by assembling anatase nanowires as branches on rutile nanorods. External quantum efficiencies as high as 90% are reached at 325 nm. These high quantum efficiencies are related to the higher amount of light harvesting due to the larger surface area, the better separation ability of the photogenerated carriers by the rutile/anatase heterostructure, and the faster electron transport, related to the 1D nanostructure and lattice connection at the interface of the two kinds of TiO₂. Furthermore, a self‐powered wireless UV photodetector is shown with excellent wireless detection performance. Such devices will enable significant advances for next‐generation photodetection and photosensing applications.     

Solar photocatalytic degradation of rhodamine B by heat-treated nanometer anatase TiO<Subscript>2</Subscript> powder

The partial phase transformation of nanometer TiO₂ powder from anatase to rutile was realized by heat-treatment, and then a novel photocatalyst which could utilize solarlight was obtained. The heat-treated nanometer TiO₂ powders at different transition stage were characterized by XRD, TEM and UV-vis spectra. In addition, the photocatalytic activity of heat-treated nanometer TiO₂ powder was tested out through the degradation of Rhodamine B dye in aqueous solution under solarlight irradiation. The results reveal that the nanometer anatase TiO₂ powder heat-treated at 500°C for 80 min exhibites the highest photocatalytic activity. That is, Rhodamine B dye can effectively degraded under solarlight irradiation in the presence of heat-treated nanometer TiO₂ powder.     

Heterostructured TiO2 Nanorod@Nanobowl Arrays for Efficient Photoelectrochemical Water Splitting

Heterostructured TiO₂ nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO₂ nanorods grown on the inner surface of arrayed anatase TiO₂ nanobowls are designed and fabricated as a new type of photoanodes for photoelectrochemical (PEC) water splitting. The unique heterostructures with a hierarchical architecture are readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two‐dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO₂ NR@NB arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated electron transfer. Accordingly, the PEC measurements of the TiO₂ NR@NB arrays on the fluoride‐doped tin oxide (FTO) substrate show significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the unmodified TiO₂ NR@NB array photoelectrode yields a photocurrent density of 1.24 mA cm^–2 at 1.23 V with respect to the reversible hydrogen electrode, which is almost two times higher than that of the TiO₂ nanorods grown directly on the FTO substrate.     

Glucose‐Induced Synthesis of 1T‐MoS2/C Hybrid for High‐Rate Lithium‐Ion Batteries

1T phase MoS₂ possesses higher conductivity than the 2H phase, which is a key parameter of electrochemical performance for lithium ion batteries (LIBs). Herein, a 1T‐MoS₂/C hybrid is successfully synthesized through facile hydrothermal method with a proper glucose additive. The synthesized hybrid material is composed of smaller and fewer‐layer 1T‐MoS₂ nanosheets covered by thin carbon layers with an enlarged interlayer spacing of 0.94 nm. When it is used as an anode material for LIBs, the enlarged interlayer spacing facilitates rapid intercalating and deintercalating of lithium ions and accommodates volume change during cycling. The high intrinsic conductivity of 1T‐MoS₂ also contributes to a faster transfer of lithium ions and electrons. Moreover, much smaller and fewer‐layer nanosheets can shorten the diffusion path of lithium ions and accelerate reaction kinetics, leading to an improved electrochemical performance. It delivers a high initial capacity of 920.6 mAh g^?1 at 1 A g^?1 and the capacity can maintain 870 mAh g^?1 even after 300 cycles, showing a superior cycling stability. The electrode presents a high rate performance as well with a reversible capacity of 600 mAh g^?1 at 10 A g^?1. These results show that the 1T‐MoS₂/C hybrid shows potential for use in high‐performance lithium‐ion batteries.     

Glycine assisted synthesis of flower-like TiO2 hierarchical spheres and its application in photocatalysis

Flower-like anatase TiO₂ hierarchical spheres assembled by nanosheets were synthesized by glycine assistant via a simple hydrothermal approach and after-annealing process. These flower-like spheres are about 2 μm in diameter with sheet thickness about 20 nm. Results showed reaction time, temperature, solution pH and glycine dosage all played an important role in control of shape and size of the as-synthesized TiO₂ nanocrystals. The photocatalytic activity of this nano-TiO₂ was evaluated by the photocatalytic oxidation decomposition of methyl orange under sunlight illumination in the presence of hydrogen peroxide (H₂O₂). The photocatalytic activity of the obtained TiO₂ was higher than that of commercial TiO₂.     

Ordered Single‐Crystalline Anatase TiO2 Nanorod Clusters Planted on Graphene for Fast Charge Transfer in Photoelectrochemical Solar Cells

Achieving efficient charge transport is a great challenge in nanostructured TiO₂‐electrode‐based photoelectrochemical cells. Inspired by excellent directional charge transport and the well‐known electroconductibility of 1D anatase TiO₂ nanostructured materials and graphene, respectively, planting ordered, single‐crystalline anatase TiO₂ nanorod clusters on graphene sheets (rGO/ATRCs) via a facial one‐pot solvothermal method is reported. The hierarchical rGO/ATRCs nanostructure can serve as an efficient light‐harvesting electrode for dye‐sensitized solar cells. In addition, the obtained high‐crystallinity anatase TiO₂ nanorods in rGO/ATRCs possess a lower density of trap states, thus facilitating diffusion‐driven charge transport and suppressing electron recombination. Moreover, the novel architecture significantly enhances the trap‐free charge diffusion coefficient, which contributes to superior electron mobility properties. By virtue of more efficient charge transport and higher energy conversion efficiency, the rGO/ATRCs developed in this work show significant advantages over conventional rGO–TiO₂ nanoparticle counterparts in photoelectrochemical cells.     

Tuning Oxygen Vacancies in Ultrathin TiO2 Nanosheets to Boost Photocatalytic Nitrogen Fixation up to 700 nm

Dinitrogen reduction to ammonia using transition metal catalysts is central to both the chemical industry and the Earth's nitrogen cycle. In the Haber–Bosch process, a metallic iron catalyst and high temperatures (400 °C) and pressures (200 atm) are necessary to activate and cleave N?N bonds, motivating the search for alternative catalysts that can transform N₂ to NH₃ under far milder reaction conditions. Here, the successful hydrothermal synthesis of ultrathin TiO₂ nanosheets with an abundance of oxygen vacancies and intrinsic compressive strain, achieved through a facile copper‐doping strategy, is reported. These defect‐rich ultrathin anatase nanosheets exhibit remarkable and stable performance for photocatalytic reduction of N₂ to NH₃ in water, exhibiting photoactivity up to 700 nm. The oxygen vacancies and strain effect allow strong chemisorption and activation of molecular N₂ and water, resulting in unusually high rates of NH₃ evolution under visible‐light irradiation. Therefore, this study offers a promising and sustainable route for the fixation of atmospheric N₂ using solar energy.