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.     

Nano Titanium Monoxide Crystals and Unusual Superconductivity at 11 K

Nano TiO₂ is investigated intensely due to extraordinary photoelectric performances in photocatalysis, new‐type solar cells, etc., but only very few synthesis and physical properties have been reported on nanostructured TiO or other low valent titanium‐containing oxides. Here, a core–shell nanoparticle made of TiO core covered with a ≈5 nm shell of amorphous TiO_1+x is newly constructed via a controllable reduction method to synthesize nano TiO core and subsequent soft oxidation to form the shell (TiO_1+x). The physical properties measurements of electrical transport and magnetism indicate these TiO@TiO_1+x nanocrystals are a type‐?? superconductor of a recorded T_c^onset = 11 K in the binary Ti–O system. This unusual superconductivity could be attributed to the interfacial effect due to the nearly linear gradient of O/Ti ratio across the outer amorphous layer. This novel synthetic method and enhanced superconductivity could open up possibilities in interface superconductivity of nanostructured composites with well‐controlled interfaces.     

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.     

Interfacial Lattice‐Strain‐Driven Generation of Oxygen Vacancies in an Aerobic‐Annealed TiO2(B) Electrode

Oxygen vacancies play crucial roles in defining physical and chemical properties of materials to enhance the performances in electronics, solar cells, catalysis, sensors, and energy conversion and storage. Conventional approaches to incorporate oxygen defects mainly rely on reducing the oxygen partial pressure for the removal of product to change the equilibrium position. However, directly affecting reactants to shift the reaction toward generating oxygen vacancies is lacking and to fill this blank in synthetic methodology is very challenging. Here, a strategy is demonstrated to create oxygen vacancies through making the reaction energetically more favorable via applying interfacial strain on reactants by coating, using TiO₂(B) as a model system. Geometrical phase analysis and density functional theory simulations verify that the formation energy of oxygen vacancies is largely decreased under external strain. Benefiting from these, the obtained oxygen‐deficient TiO₂(B) exhibits impressively high level of capacitive charge storage, e.g., ≈53% at 0.5 mV s^?1, far surpassing the ≈31% of the unmodified counterpart. Meanwhile, the modified electrode shows significantly enhanced rate capability delivering a capacity of 112 mAh g^?1 at 20 C (≈6.7 A g^?1), ≈30% higher than air‐annealed TiO₂ and comparable to vacuum‐calcined TiO₂. This work heralds a new paradigm of mechanical manipulation of materials through interfacial control for rational defect engineering.     

In‐Situ Carbon Doping of TiO2 Nanotubes Via Anodization in Graphene Oxide Quantum Dot Containing Electrolyte and Carburization to TiOxCy Nanotubes

Anodic production of self‐organized titania nanotubes (TNTs) in an electrolyte enriched with graphene oxide quantum dots (GOQDs) is reported. The TNT‐GOQD composites grown under these conditions show in‐situ carbon doping, leading to the formation of anatase TiO₂ domains and to the reduction to substoichiometric oxide (TiO_x) and TiC. Surface science and electrochemical techniques are used in synergy to reveal that graphitic carbon is incorporated into TiO₂ upon anodic nanotube growth promoting the formation of oxygen vacancies and thus TiO₂ reduction. Upon annealing in ultrahigh vacuum, titanium oxycarbide (TiO_xC_y) is formed at temperatures ≥400 °C, where the material changes from a semiconductor to a semimetal. At the solid/liquid interface, the apparent electron donor density increases from as‐grown TNTs to as‐grown TNT‐GOQD composites due to the carbon doping, and the conductivity increases further with annealing temperature due to the increasing concentration of coordinatively unsaturated C atoms, crystallinity, and TiO₂ reduction. The materials synthesized and characterized in this study find application in different areas ranging from visible light photocatalysis and photo‐electrochemistry to use as Li‐ion battery anodes and electrocatalyst supports, because it is possible to gradually tune the density of states below the Fermi level, which can be referred to as band‐gap engineering.     

Multichannel Porous TiO2 Hollow Nanofibers with Rich Oxygen Vacancies and High Grain Boundary Density Enabling Superior Sodium Storage Performance

TiO₂ as an anode for sodium‐ion batteries (NIBs) has attracted much recent attention, but poor cyclability and rate performance remain problematic owing to the intrinsic electronic conductivity and the sluggish diffusivity of Na ions in the TiO₂ matrix. Herein, a simple process is demonstrated to improve the sodium storage performance of TiO₂ by fabricating a 1D, multichannel, porous binary‐phase anatase‐TiO₂–rutile‐TiO₂ composite with oxygen‐deficient and high grain‐boundary density (denoted as a‐TiO_2?x /r‐TiO_2?x ) via electrospinning and subsequent vacuum treatment. The introduction of oxygen vacancies in the TiO₂ matrix enables enhanced intrinsic electronic conductivity and fast sodium‐ion diffusion kinetics. The porous structure offers easy access of the liquid electrolyte and a short transport path of Na⁺ through the pores toward the TiO₂ nanoparticle. Furthermore, the high density of grain boundaries between the anatase TiO₂ and rutile TiO₂ offer more interfaces for a novel interfacial storage. The a‐TiO_2?x /r‐TiO_2?x shows excellent long cycling stability (134 mAh g^?1 at 10 C after 4500 cycles) and superior rate performance (93 mAh g^?1 after 4500 cycles at 20 C) for sodium‐ion batteries. This simple and effective process could serve as a model for the modification of other materials applied in energy storage systems and other fields.     

Stability of Halide Perovskite Solar Cell Devices: In Situ Observation of Oxygen Diffusion under Biasing

Using in situ electrical biasing transmission electron microscopy, structural and chemical modification to n–i–p‐type MAPbI₃ solar cells are examined with a TiO₂ electron‐transporting layer caused by bias in the absence of other stimuli known to affect the physical integrity of MAPbI₃ such as moisture, oxygen, light, and thermal stress. Electron energy loss spectroscopy (EELS) measurements reveal that oxygen ions are released from the TiO₂ and migrate into the MAPbI₃ under a forward bias. The injection of oxygen is accompanied by significant structural transformation; a single‐crystalline MAPbI₃ grain becomes amorphous with the appearance of PbI₂. Withdrawal of oxygen back to the TiO₂, and some restoration of the crystallinity of the MAPbI₃, is observed after the storage in dark under no bias. A subsequent application of a reverse bias further removes more oxygen ions from the MAPbI₃. Light current–voltage measurements of perovskite solar cells exhibit poorer performance after elongated forward biasing; recovery of the performance, though not complete, is achieved by subsequently applying a negative bias. The results indicate negative impacts on the device performance caused by the oxygen migration to the MAPbI₃ under a forward bias. This study identifies a new degradation mechanism intrinsic to n–i–p MAPbI₃ devices with TiO₂.     

Ab initio study of magnetism at the TiO<Subscript>2</Subscript>/LaAlO<Subscript>3</Subscript> interface

In this article, we study the possible relation between the electronic and magnetic structures of the TiO₂/LaAlO₃ interface and the unexpected magnetism found in undoped TiO₂ films grown on LaAlO₃. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO₃ has a layered structure along the (001) direction with alternating LaO and AlO₂ planes, with nominal charges of +1 and −1, respectively. As a consequence of that, an oxygen-deficient TiO₂ film with anatase structure will grow preferently on the AlO₂ surface layer. We have therefore performed ab initio calculations for superlattices with TiO₂/AlO₂ interfaces with interfacial oxygen vacancies. Our main results are that vacancies lead to a change in the valence state of neighbor Ti atoms but not necessarily to a magnetic solution and that the appearance of magnetism depends also on structural details, such as second neighbor positions. These results are obtained using both the local spin density approximation (LSDA) and LSDA + U approximations.     

Graphene Wrapped TiO2 Based Catalysts with Enhanced Photocatalytic Activity

Reduced graphene oxide (RGO) wrapped titanium dioxide nanocrystals (TiO₂ NCs@RGO) with oxygen vacancies (Vo) and Ti³⁺ defects have been synthesized by electrostatically wrapping GO around TiO₂ NCs followed by thermal annealing at 400 °C. Transmission electron microscope observations have shown that TiO₂ NCs@RGO has a unique crystalline core/crystalline shell structure, which is different from the original amorphous TiO₂ covered TiO₂ NCs. Raman spectroscopy, X‐ray photoelectron spectroscopy, and electron paramagnetic resonance have demonstrated that Vo‐Ti³⁺ species are more readily formed in TiO₂ NCs@RGO than in TiO₂ NCs. As a result, TiO₂ NCs@RGO exhibits enhanced optical absorption in a wide wavelength range from visible light to near IR and red‐shifted absorption edge. In the photocatalytic degradation reaction of methyl orange, the photodegradation rate constant for TiO₂ NCs@RGO is 2.4 times higher than that of TiO₂ NCs. The enhanced photocatalytic performance can be attributed to the improved charge separation at the interface of TiO₂ NCs and RGO layer and the enhanced optical absorption in visible light region due to the donor levels of the defects such as Vo‐Ti³⁺ species. This work establishes a new method for preparing Vo defect contained TiO₂ catalysts.     

Fabrication and photoluminescence properties of ridged TiO2 nanotube arrays

Highly-ordered ridged TiO₂ nanotube (TNT) arrays have been fabricated by a simple anodic method. Their structures have been systematically investigated using X-ray diffractometer, scanning electron microscope, energy dispersive spectrometer and HRTEM, and the results clearly presented the ridged morphologies with the outer diameters of about 200 nm. We also found that the annealing procedure induced the phase transformation from amorphous phase to anatase phase. The room-temperature photoluminescence (PL) properties were studied, and the much stronger PL emission for the anatase phase was observed compared to the amorphous counterparts. Furthermore, the broad emission for the anatase phase could be decomposed of three peaks locating at 540, 622 and 773 nm, respectively. The oxygen vacancies and the trapping of free excitons by TiO₆ octahedra near defects are the key factors to cause these peaks.     

Facile synthesis of blue anatase TiO<Subscript>2</Subscript> films by solvent evaporation method

With arousing interest in the preparation of defect rich TiO₂ films for several green energy applications, various methods have emerged in recent days. Herein, we report the properties of blue anatase TiO₂ films obtained by combined solvent evaporation and subsequent calcination. The X-ray diffraction and Raman studies indicated the formation of anatase TiO₂ films, while the field emission scanning electron microscopic images confirmed the distribution of spherical particles. X-ray photoelectron spectroscopic data revealed the presence of surplus Ti³⁺ ions and the associated oxygen vacancies. These defect related informations are further evident from the photoluminescence study. The optical study confirmed the extended absorption in the visible region of the blue TiO₂ films. These intrinsic defects play a major role in effective charge carrier separation and in tailoring the band gap absorption properties.     

Enhanced photocatalytic activity of (Zn,N)-codoped TiO2 nanoparticles

(Zn, N)-codoped TiO₂ nanoparticles were prepared by the sol–gel method. X-ray diffraction (XRD) results testified that anatase samples were obtained. Transmission electron microscopy (TEM) patterns revealed that the average grain size of all the samples is about 15 nm and the Zn doping caused obvious particle aggregation. The Brunauer–Emmett–Teller (BET) surface areas of the samples were measured to testify the aggregation. The Zn doping caused slight blue-shift of absorption edge by the ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) measurements. The photocatalytic activity for the degradation of methylene blue (MB) solution was best enhanced in the (Zn, N)-codoped TiO₂ with 1 at.% Zn doping level. Photoluminescence (PL) emission spectra of the samples were also investigated, which revealed that the oxygen vacancy and isolated N 2p states played important roles in the photo-generated charge carrier recombination in the (Zn, N)-codoped TiO₂. It was suggested that the doping induced oxygen vacancies could promote the photocatalytic oxidation processes.     

Synthesis of nano titanium oxide with controlled oxygen content using pulsed discharge in water

Different titanium oxide nanoparticles were formed through pulsed discharge of Ti wires in distilled water and H₂O₂ solution. The recovered samples were characterized by various techniques, such as XRD, SEM and TEM. The results confirm the presence of various titanium oxide nanoparticles including TiO₂ phases (anatase and rutile) and various nonstoichiometric TiO_2−x in recovered samples owing to the oxygen deficient circumstance through pulsed discharge. The titanium oxide nanoparticles exhibit a spherical shape with a size of 10–300 nm. The results show that the energy input adjusted by charging voltage is one major factor to control the phases of titanium oxide and the overall oxygen content of recovered samples. In addition, the H₂O₂ content in distilled water also affects the oxygen content of recovered samples. The sample recovered from 10% H₂O₂ solution is pure TiO₂ consisting of anatase and rutile without nonstoichiometric TiO_2−x. Moreover, the UV–Vis absorption spectra of recovered samples show their intensive visible light absorption and the correlation between the visible light absorption and the experimental conditions (charging voltage and H₂O₂ content).     

An Unusual Strong Visible‐Light Absorption Band in Red Anatase TiO2 Photocatalyst Induced by Atomic Hydrogen‐Occupied Oxygen Vacancies

Increasing visible light absorption of classic wide‐bandgap photocatalysts like TiO₂ has long been pursued in order to promote solar energy conversion. Modulating the composition and/or stoichiometry of these photocatalysts is essential to narrow their bandgap for a strong visible‐light absorption band. However, the bands obtained so far normally suffer from a low absorbance and/or narrow range. Herein, in contrast to the common tail‐like absorption band in hydrogen‐free oxygen‐deficient TiO₂, an unusual strong absorption band spanning the full spectrum of visible light is achieved in anatase TiO₂ by intentionally introducing atomic hydrogen‐mediated oxygen vacancies. Combining experimental characterizations with theoretical calculations reveals the excitation of a new subvalence band associated with atomic hydrogen filled oxygen vacancies as the origin of such band, which subsequently leads to active photo‐electrochemical water oxidation under visible light. These findings could provide a powerful way of tailoring wide‐bandgap semiconductors to fully capture solar light.     

Multifunctional Gadolinium‐Doped Mesoporous TiO2 Nanobeads: Photoluminescence,Enhanced Spin Relaxation,and Reactive Oxygen Species Photogeneration,Beneficial for Cancer Diagnosis and Treatment

Materials with controllable multifunctional abilities for optical imaging (OI) and magnetic resonant imaging (MRI) that also can be used in photodynamic therapy are very interesting for future applications. Mesoporous TiO₂ sub‐micrometer particles are doped with gadolinium to improve photoluminescence functionality and spin relaxation for MRI, with the added benefit of enhanced generation of reactive oxygen species (ROS). The Gd‐doped TiO₂ exhibits red emission at 637 nm that is beneficial for OI and significantly improves MRI relaxation times, with a beneficial decrease in spin–lattice and spin–spin relaxation times. Density functional theory calculations show that Gd³⁺ ions introduce impurity energy levels inside the bandgap of anatase TiO₂, and also create dipoles that are beneficial for charge separation and decreased electron–hole recombination in the doped lattice. The Gd‐doped TiO₂ nanobeads (NBs) show enhanced ability for ROS monitored via ^?OH radical photogeneration, in comparison with undoped TiO₂ nanobeads and TiO₂ P25, for Gd‐doping up to 10%. Cellular internalization and biocompatibility of TiO₂@x Gd NBs are tested in vitro on MG‐63 human osteosarcoma cells, showing full biocompatibility. After photoactivation of the particles, anticancer trace by means of ROS photogeneration is observed just after 3 min irradiation.     

Inverted type bulk-heterojunction organic solar cell using electrodeposited titanium oxide thin films as electron collector electrode

We developed an inverted type bulk-heterojunction organic solar cell with 1 cm² active area using a fluorine-doped tin oxide/electrodeposited amorphous (TiO_x) or anatase (TiO₂) titanium oxide electrode instead of the low work-functional electrode such as Al. The cell with TiO₂ showed the power conversion efficiency (η) of 2.5% by irradiating AM 1.5-100 mW cm^− 2 simulated sunlight. While, the performance of the cell with TiO_x was almost maintained in an ambient atmosphere under continuous light irradiation of 10 h, although slightly small initial η value of 2.1% was observed.     

Structural and electronic impact of SrTiO3 substrate on TiO2 thin films

We demonstrate that the atomic structures, electronic states, and bonding nature of the interface between SrTiO₃ substrate and anatase TiO₂ thin films could be related and technologically manipulated at the atomic level. Applying advanced transmission electron microscopy, the grown anatase TiO₂ thin films are found to make a clean and direct contact to the SrTiO₃ substrates in an epitaxial, coherent, and atomically abrupt way. The atomic-resolution microscopic images reveal that the interface comprises SrO-terminated SrTiO₃ and Ti-terminated TiO₂ with the interfacial Ti of TiO₂ sitting above the hollow site, which is confirmed theoretically to be the most energetically favorable. Quantitatively, the first-principles calculations predict that the oxygen sublattice at the interface undergoes a notable reconstruction, i.e., the interfacial O atoms of TiO₂ are displaced largely toward the SrO plane of the SrTiO₃, flattening the originally zigzag TiO₂ atomic chains. Consequently, the interfacial layers suffer a remarkable modification in the charge accumulation and also a deviation in the density of states from their bulk counterparts, indicating that the substrate can have an impact on the deposited thin films electronically. Using several analytic methods, the SrTiO₃/TiO₂ interface is found to take on a metallic nature, and the interfacial bonding is determined to be of a mixed covalent and ionic character. This combined experimental and theoretical investigation gains insight into the complex atomic and electronic structures of the buried interface, which are fundamental for relating the atomic-scale structures to their properties on a quantum level.     

Facile preparation of TiO2/C nanocomposites as anode materials for lithium-ion batteries

TiO₂/C nanospheres with diameter of 300–400 nm were synthesized by controlled thermal decomposition of titanium glycolate spheres in inert atmosphere. The effect of the calcination temperature and atmosphere on the structure and composition of the product are investigated. The products obtained by calcination of the precursor in nitrogen at 500°C consist of anatase and rutile nanoparticles, and amorphous carbon that is in situ generated from the organic components of glycolate precursor. When used as anode material for lithium-ion batteries, the as-prepared TiO₂/C nanocomposite delivers a capacity of 166 mAh/g after 250 charge/discharge cycles at a current rate of 0.2 C and give a good rate capability. The native carbon not only improves the local conductivity but also prevents the aggregation and growth of TiO₂ nanoparticles during calcination, allowing efficient electronic conductivity and Li ion diffusion.     

Structural and optical properties of nitrogen‐iron co‐doped titanium dioxide films prepared via sol‐gel dip‐coating: Effect of urea and iron nitrate concentration in the sol

In this study, nitrogen‐iron co‐doped titanium dioxide films were prepared via sol‐gel dip‐coating method using urea and iron nitrate as nitrogen and iron source, respectively. Nonmetal doping of TiO₂ have some disadvantages such as massive charge carrier recombination and losing the photo‐catalytic capability. Three different nitrogen‐iron co‐doped titanium dioxide sols with different urea and iron nitrate concentration were prepared. The resulting sols were homogeneous and transparent, and no precipitation was observed in any of them. It was observed that the film prepared with middle urea‐iron nitrate concentration sol got opaque in a short time after the dip‐coating process. All prepared films were characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy, scanning electron microscopy, confocal microscopy and ultraviolet–visible (UV‐Vis) spectroscopy. It was found that the concentration of the urea and iron nitrate in the sol had an effect on the crystal structure, microstructure, surface morphology and optical properties of the resulting films. Samples with middle concentrations had amorphous structure and bigger particle size. It was seen that sample with higher iron amount has lower band‐gap. It is concluded that we can prepare transparent anatase, transparent amorphous and opaque amorphous titanium dioxide films by changing the urea and iron nitrate concentration in the sol.     

Poly(methyl methacrylate)-TiO<Subscript>2</Subscript> nanocomposite obtained by non-hydrolytic sol–gel synthesis

Poly(methyl methacrylate) (PMMA)/titanium dioxide (TiO₂) nanocomposites were prepared by means of in situ generation of TiO₂ through a non-hydrolytic sol–gel process (NHSG), starting from titanium chloride, as titania precursor, benzyl alcohol, as oxygen donor, and commercial PMMA. TiO₂ nanoparticles (average size of 30 nm) were obtained in the anatase and amorphous forms. The in situ generation led to a very homogeneous distribution of particulate fillers within the polymeric matrix avoiding the problems related to distributive and dispersive mixing of conventional compounding methods (top down approach). A slight increase of glass transition temperature was observed for all prepared composites with respect to the pristine PMMA. The NHSG process did not affect the molecular weight of the polymers indicating the absence of any degradation reaction for PMMA. The presence of titania in the anatase phase increases the photodegradation of the PMMA matrix due to UV irradiation.