Introducing Hybrid Defects of Silicon Doping and Oxygen Vacancies into MOF-Derived TiO2–X@Carbon Nanotablets Toward High-Performance Sodium-Ion Storage

Titanium dioxide (TiO₂) is a promising anode material for sodium–ion batteries (SIBs), which suffer from the intrinsic sluggish ion transferability and poor conductivity. To overcome these drawbacks, a facile strategy is developed to synergistically engineer the lattice defects (i.e., heteroatom doping and oxygen vacancy generation) and the fine microstructure (i.e., carbon hybridization and porous structure) of TiO₂-based anode, which efficiently enhances the sodium storage performance. Herein, it is successfully realized that the Si-doping into the MIL-125 metal-organic framework structure, which can be easily converted to SiO₂/TiO_2–x@C nanotablets by annealing under inert atmosphere. After NaOH etching SiO₂/TiO_2–x@C which contains unbonded SiO₂ and chemically bonded Si O Ti, thus the lattice Si-doped TiO_2–x@C (Si-TiO_2–x@C) nanotablets with rich Ti³⁺/oxygen vacancies and abundant inner pores are developed. When examined as an anode for SIB, the Si-TiO_2–x@C exhibits a high sodium storage capacity (285 mAh g⁻¹ at 0.2 A g⁻¹), excellent long-term cycling, and high-rate performances (190 mAh g⁻¹ at 2 A g⁻¹ after 2500 cycles with 95.1% capacity retention). Theoretical calculations indicate that the rich Ti³⁺/oxygen vacancies and Si-doping synergistically contribute to a narrowed bandgap and lower sodiation barrier, which thus lead to fast electron/ion transfer coefficients and the predominant pseudocapacitive sodium storage behavior.     

Improved Na storage and Coulombic efficiency in TiP2O7@C microflowers for sodium ion batteries

Ti-based anode materials in sodium ion batteries have attracted extensive interests due to its abundant resources,low toxicity,easy synthesis and long cycle life.However,low Coulombic efficiency and limited specific capacity affect their applications.Here,cubic-phase TiP₂O₇is examined as anode materials,using in-situ/ex-situ characterization techniques.It is concluded that the redox reactions of Ti4⁺/Ti³⁺and Ti³⁺/Ti⁰consecutively occur during the discharge/charge processes,both of which are highly reversible.These reactions make the specific capacity of TiP₂O₇even higher than the case of TiO2 that only contains a simple anion,0^2-.Interestingly,Ti species participate only one of the redox reactions,due to the remarkable difference in local structures related to the sodiation process.The stable discharge/charge products in TiP₂O₇reduce the side reactions and improve the Coulombic efficiency as compared to T i02.These features make it a promising Ti-based anode for sodium ion batteries.Therefore,TiP₂O₇@C microflowers exhibit excellent electrochemical performances,?109 mAh·g^-1after 10,000 cycles at 2 A·g^-1,or 95.2 mAh·g^-1at 10 A·g^-1.The results demonstrate new opportunities for advanced Ti-based anodes in sodium ion batteries.     

Light Field-Enhanced Single-Site Cu Electrocatalyst for Nitrogen Fixation

Direct electrocatalytic reduction of N₂ to NH₃ under mild conditions is attracting considerable interests but still remains enormous challenges in terms of respect of intrinsic catalytic activity and limited electrocatalytic efficiency. Herein, a photo-enhanced strategy is developed to improve the NRR activity on Cu single atoms catalysts. The atomically dispersed Cu single atoms supported TiO₂ nanosheets (Cu SAs/TiO₂) achieve a Faradaic Efficiency (12.88%) and NH₃ yield rate (6.26 µg h⁻¹ mg_cat⁻¹) at −0.05 V versus RHE under the light irradiation field, in which NH₃ yield rate is fivefold higher than that under pure electrocatalytic nitrogen reduction reaction (NRR) process and is remarkably superior in comparison to most of the similar type electrocatalysts. The existence of external light field improves electron transfer ability between Cu O and Ti O, and thus optimizes the accumulation of surface charges on Cu sites, endowing more electrons involved in nitrogen fixation. This work reveals an atomic-scale mechanistic understanding of field effect-enhanced electrochemical performance of catalysts and it provides predictive guidelines for the rational design of photo-enhanced electrochemical N₂ reduction catalysts.     

Efficient Nitrate Synthesis via Ambient Nitrogen Oxidation with Ru-Doped TiO2/RuO2 Electrocatalysts

A facile pathway of the electrocatalytic nitrogen oxidation reaction (NOR) to nitrate is proposed, and Ru-doped TiO₂/RuO₂ (abbreviated as Ru/TiO₂) as a proof-of-concept catalyst is employed accordingly. Density functional theory (DFT) calculations suggest that Ru^δ+ can function as the main active center for the NOR process. Remarkably doping Ru into the TiO₂ lattice can induce an upshift of the d-band center of the Ru site, resulting in enhanced activity for accelerating electrochemical conversion of inert N₂ to active NO*. Overdoping of Ru ions will lead to the formation of additional RuO₂ on the TiO₂ surface, which provides oxygen evolution reaction (OER) active sites for promoting the redox transformation of the NO* intermediate to nitrate. However, too much RuO₂ in the catalyst is detrimental to both the selectivity of the NOR and the Faradaic efficiency due to the dominant OER process. Experimentally, a considerable nitrate yield rate of 161.9 µmol h⁻¹ g_cat⁻¹ (besides, a total nitrate yield of 47.9 µg during 50 h) and a highest nitrate Faradaic efficiency of 26.1% are achieved by the Ru/TiO₂ catalyst (with the hybrid composition of Ru_xTi_yO₂ and extra RuO₂ by 2.79 wt% Ru addition amount) in 0.1 m Na₂SO₄ electrolyte.     

Green preparation of in-situ oxidized TiO2/Ti3C2 heterostructure for photocatalytic hydrogen production

The preparation of high-efficiency photocatalysts by green and mild methods is very attractive for photocatalysis field. Here, TiO₂/Ti₃C₂ was prepared by in-situ oxidation with a green method, in which only water or H₂O₂ was used. A heterojunction interface formed between the anatase phase TiO₂ and the Ti₃C_2, which was demonstrated by HRTEM and XRD. Fluorescence spectra and i-t transient photocurrent are used to study the separation efficiency of photogenerated electron-hole pairs. The close contact between Ti₃C₂ and TiO₂ speed up the separation of photo-generated electron-hole pairs, promoting a higher hydrogen production performance around 2520.4 μmol·g⁻¹·h⁻¹ on TCT-1.     

Controllable Exfoliation of MOF-Derived Van Der Waals Superstructure into Ultrathin 2D B/N Co-Doped Porous Carbon Nanosheets: A Superior Catalyst for Ambient Ammonia Electrosynthesis

The electrocatalytic nitrogen reduction reaction (NRR) to synthesize NH₃ under ambient conditions is a promising alternative route to the conventional Haber–Bosch process, but it is still a great challenge to develop electrocatalysts’ high Faraday efficiency and ammonia yield. Herein, a facile and efficient exfoliation strategy to synthesize ultrathin 2D boron and nitrogen co-doped porous carbon nanosheets (B/N C NS) via a metal–organic framework (MOF)-derived van der Waals superstructure, is reported. The results of experiments and theoretical calculations show that the doping of boron and nitrogen can modulate the electronic structure of the adjacent carbon atoms; which thus, promotes the competitive adsorption of nitrogen and reduces the energy required for ammonia synthesis. The B/N C NS exhibits excellent catalytic performance and stability in electrocatalytic NRR, with a yield rate of 153.4 µg·h⁻¹·mg⁻¹ cat and a Faraday efficiency of 33.1%, which is better than most of the reported NRR electrocatalysts. The ammonia yield of B/N C NS can maintain 92.7% of the initial NRR activity after 48 h stability test. The authors’ controllable exfoliation strategy using MOF-derived van der Waals superstructure can provide a new insight for the synthesis of other 2D materials.     

Highly Enhanced Pseudocapacitive Performance of Vanadium‐Doped MXenes in Neutral Electrolytes

2D titanium carbide (Ti₃C₂T_x MXene) is recognized as a promising material for pseudocapacitor electrodes in acidic solutions, while the current studies in neutral electrolytes show much poorer performances. By a simple hydrothermal method, vanadium‐doped Ti₃C₂T_x 2D nanosheets are prepared to tune the interaction between MXene and alkali metal adsorbates (Li⁺, Na⁺, and K⁺) in the neutral electrolyte. Maintaining the 2D morphology of MXene, the coexisting V³⁺ and V⁴⁺ are confirmed to form surface V–C and V–O species. At a medium doping level of V:Ti = 0.17:1, the V‐doped MXene exhibits the highest capacitance of 365.9 F g^?1 in 2 m KCl (10 mV s^?1) and excellent stability (5% loss after 5000 cycles), compared to only 115.7 F g^?1 of pristine MXene. Density functional theory calculations reveal the stronger alkali metal ion–O interaction on V‐doped MXene surface than unmodified MXene and a further capacitance boost to 404.9 F g^?1 using Li⁺‐containing neutral electrolyte is reported, which is comparable to the performance under acidic conditions.     

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.     

Thickness-Independent Capacitive Performance of Holey Ti3C2Tx Film Prepared through a Mild Oxidation Strategy

The Ti₃C₂T_x film with metallic conductivity and high pseudo-capacitance holds profound promise in flexible high-rate supercapacitors. However, the restacking of Ti₃C₂T_x sheets hinders ion access to thick film electrodes. Herein, a mild yet green route has been developed to partially oxidize Ti₃C₂T_x to TiO₂/Ti₃C₂T_x by introducing O₂ molecules during refluxing the Ti₃C₂T_x suspension. The subsequent etching away of these TiO₂ nanoparticles by HF leaves behind numerous in-plane nanopores on the Ti₃C₂T_x sheets. Electrochemical impedance spectroscopy shows that longer oxidation time of 40 min yields holey Ti₃C₂T_x (H-Ti₃C₂T_x) with a much shorter relax time constant of 0.85 s at electrode thickness of 25 µm, which is 89 times smaller than that of the pristineTi₃C₂T_x film (75.58 s). Meanwhile, H-Ti₃C₂T_x film with 25 min oxidation exhibits less-dependent capacitive performance in film thickness range of 10–84 µm (1.63–6.41 mg cm⁻²) and maintains around 60% capacitance as the current density increases from 1 to 50 A g⁻¹. The findings clearly demonstrate that in-plane nanopores not only provide more electrochemically active sites, but also offer numerous pathways for rapid ion impregnation across the thick Ti₃C₂T_x film. The method reported herein would pave way for fabricating porous MXene materials toward high-rate flexible supercapacitor applications.     

Visible light response and superior dispersed S-doped TiO2 nanoparticles synthesized via ionic liquid

Visible light response and superior dispersed S⁶⁺-doped TiO₂ nanoparticle catalysts (S-TiO₂) were prepared via ionic liquid of 1-butyl-3-methylimidazolium hexafluorophosphate. The phenol was used for the evaluation of the S-TiO₂ photocatalytic activity. S-TiO₂ was characterized by XPS, UV–vis DRS, FE-SEM, TEM, XRD, TG/DSC, FTIR, and BET. The results showed that S-TiO₂ with appropriate S doping prepared via ionic liquid had smaller particle size, better dispersion, higher activity, and higher surface area (S_BET) than that prepared in water. Cationic S⁶⁺ incorporation into TiO₂ lattice substitutes for Ti⁴⁺ lattice site, generates Ti–O–S bonds in TiO₂, and leads to the formation of donor defect levels in band gap, so that the photocatalytic sensitization of TiO₂ has been extended to visible light region. The optimal content in S doping for the better photocatalytic performance can optimize electrical properties of the intrinsic n-type TiO₂ by adding the adequate amount of donor defect of S_Ti²⁺ for considering the lifetime of the photo-induced charge pairs.     

In Situ Fabrication of Hierarchically Branched TiO2 Nanostructures: Enhanced Performance in Photocatalytic H2 Evolution and Li–Ion Batteries

1D branched TiO₂ nanomaterials play a significant role in efficient photocatalysis and high‐performance lithium ion batteries. In contrast to the typical methods which generally have to employ epitaxial growth, the direct in situ growth of hierarchically branched TiO₂ nanofibers by a combination of the electrospinning technique and the alkali‐hydrothermal process is presented in this work. Such the branched nanofibers exhibit improvement in terms of photocatalytic hydrogen evolution (0.41 mmol g⁻¹ h⁻¹), in comparison to the conventional TiO₂ nanofibers (0.11 mmol g⁻¹ h⁻¹) and P25 (0.082 mmol g⁻¹ h⁻¹). Furthermore, these nanofibers also deliver higher lithium specific capacity at different current densities, and the specific capacity at the rate of 2 C is as high as 201. 0 mAh g⁻¹, roughly two times higher than that of the pristine TiO₂ nanofibers.     

Effects of Ti and TiO2 on Combustion Synthesis of (Ti,V)2AlC/Al2O3 Solid Solution Composites

(Ti,V)₂AlC/Al₂O₃ solid solution composites were prepared by solid state combustion simultaneously incorporating reduction reactions of V₂O₅ and TiO₂/V₂O₅ with aluminum. Two reaction systems composed of Ti–V₂O₅–Al–Al₄C₃ and TiO₂–V₂O₅–Al–Al₄C₃ powder mixtures were studied. Combustion exothermicity was enhanced by increasing V₂O₅ and Al, which not only caused an increase in the combustion temperature and reaction front velocity, but also facilitated evolution of the (Ti,V)₂AlC phase. Between two reaction systems, the Ti-containing samples were more energetic and produced (Ti_1–xV_x)₂AlC/Al₂O₃ composites with x = 0.2–0.8. The degree of element substitution was reduced for the samples adopting TiO₂, which yielded Al₂O₃-added (Ti_1–yV_y)₂AlC with y = 0.4–0.8.     

Facile synthesis of rutile TiO2/carbon nanosheet composite from MAX phase for lithium storage

Titanium oxide (TiO₂), with excellent cycling stability and low volume expansion, is a promising anode material for lithium-ion battery (LIB), which suffers from low electrical conductivity and poor rate capability. Combining nano-sized TiO₂ with conductive materials is proved an efficient method to improve its electrochemical properties. Here, rutile TiO₂/carbon nanosheet was obtained by calcinating MAX (Ti₃AlC₂) and Na₂CO₃ together and water-bathing with HCl. The lamellar carbon atoms in MAX are converted to 2D carbon nanosheets with urchin-like rutile TiO₂ anchored on. The unique architecture can offer plentiful active sites, shorten the ion diffusion distance and improve the conductivity. The composite exhibits a high reversible capacity of 247 mA h g⁻¹, excellent rate performance (38 mA h g⁻¹ at 50 C) and stable cycling performance (0.014% decay per cycle during 2000 cycles) for lithium storage.     

Effects of Nd-substitution on microstructures and dielectric characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

Ca_1−3x/2Nd _x Cu₃Ti₄O₁₂ (x = 0, 0.1, 0.2) ceramics were prepared by a solid state reaction process, and single-phased structures were obtained for all the compositions. The dielectric characteristics of pure and Nd-substituted CaCu₃Ti₄O₁₂ ceramics were investigated together with the microstructures. The mixed-valent structures of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ in the present ceramics were confirmed by X-ray photoelectron analysis. The dielectric relaxation in the low temperature range was examined in detail and the variation of dielectric constant and dielectric loss was attributed to the modification mixed-valent structures.     

Au Sub‐Nanoclusters on TiO2 toward Highly Efficient and Selective Electrocatalyst for N2 Conversion to NH3 at Ambient Conditions

As the N?N bond in N₂ is one of the strongest bonds in chemistry, the fixation of N₂ to ammonia is a kinetically complex and energetically challenging reaction and, up to now, its synthesis is still heavily relying on energy and capital intensive Haber–Bosch process (150–350 atm, 350–550 °C), wherein the input of H₂ and energy are largely derived from fossil fuels and thus result in large amount of CO₂ emission. In this paper, it is demonstrated that by using Au sub‐nanoclusters (≈0.5 nm ) embedded on TiO₂ (Au loading is 1.542 wt%), the electrocatalytic N₂ reduction reaction (NRR) is indeed possible at ambient condition. Unexpectedly, NRR with very high and stable production yield (NH₃: 21.4 µg h^?1 mg^?1_cat., Faradaic efficiency: 8.11%) and good selectivity is achieved at ?0.2 V versus RHE, which is much higher than that of the best results for N₂ fixation under ambient conditions, and even comparable to the yield and activation energy under high temperatures and/or pressures. As isolated precious metal active centers dispersed onto oxide supports provide a well‐defined system, the special structure of atomic Au cluster would promote other important reactions besides NRR for water splitting, fuel cells, and other electrochemical devices.     

MOF-Derived Mn2O3 Nanocage with Oxygen Vacancies as Efficient Cathode Catalysts for Li–O2 Batteries

Designing efficient and cost-effective electrocatalysts is the primary imperative for addressing the pivotal concerns confronting lithium–oxygen batteries (LOBs). The microstructure of the catalyst is one of the key factors that influence the catalytic performance. This study proceeds to the advantage of metal-organic frameworks (MOFs) derivatives by annealing manganese 1,2,3-triazolate (MET-2) at different temperatures to optimize Mn₂O₃ crystals for special microstructures. It is found that at 350 °C annealing temperature, the derived Mn₂O₃ nanocage maintains the structure of MOF, the inherited high porosity and large specific surface area provide more channels for Li⁺ and O₂ diffusion, beside the oxygen vacancies on the surface of Mn₂O₃ nanocages enhance the electrocatalytic activity. With the synergy of unique structure and rich oxygen vacancies, the Mn₂O₃ nanocage exhibits ultrahigh discharge capacity (21 070.6 mAh g⁻¹ at 500 mA g⁻¹) and excellent cycling stability (180 cycles at the limited capacity of 600 mAh g⁻¹ with a current of 500 mA g⁻¹). This study demonstrates that the Mn₂O₃ nanocage structure containing oxygen vacancies can significantly enhance catalytic performance for LOBs, which provide a simple method for structurally designed transition metal oxide electrocatalysts.     

Regulating d-Band Center of Ti2C MXene Via Nb Alloying for Stable and High-Efficient Supercapacitive Performances

Ti₂C MXene with the lowest formula weight is expected to gain superior advantages in gravimetric capacitances over other heavier MXenes. Nevertheless, its poor chemical and electrochemical stability is the most fatal drawback and seriously hinders its practical applications. Herein, an alloy engineering strategy at the transition metal-sites of Ti₂C MXene is proposed. Theoretical calculations reveal that the electronic redistribution of the solid-solution TiNbC MXene improves the electronic conductivity, induces the upward d-band center, tailors the surface functional groups, and increases the electron loss impedance, resulting in its excellent capacitive performance and high chemical stability. The as-prepared flexible TiNbC film delivers specific capacitance up to 381 F g⁻¹ at a scan rate of 2 mV s⁻¹ and excellent electrochemical stability without capacitance loss after 10000 charge/discharging cycles. This work provides a universal approach to develop high-performance and chemically stable MXene electrodes.     

Preparation and magnetic properties of Ti1−xVxPO4

The solid solutions of Ti_1?xV_xPO₄ were prepared under the reductive circumstance, although there seems to be immiscibility near x=0.5. Their magnetic susceptibility was analysed by the temperature-independent and Curie-Weiss contribution. The temperature-independent contribution increased gradually with x until x=0.4 and then decreased to 0 at x=1.0. A bent was observed at x=0.5 in the curve for the Curie constant against x and at x>0.5 the Curie temperature became strongly negative with x.In the region of x≦0.4, the Curie constant corresponded to the electronic configuration of d¹ for a vanadium ion. On the other hand, the Curie constant reflected the d² configuration for a vanadium ion in the range of $\text{x}\text{≧0.6}$. This fact indicates that Ti³⁺/1bV³⁺ homopolar bonds are formed in accordance with the replacement of Ti³⁺ with V³⁺ at x≦0.4. Vanadium ions added further, which substitute for Ti³⁺ in Ti³⁺/1bV³⁺ pairs, do not form the homopolar bond with V³⁺ ions and both V³⁺ ions contribute magnetically as ions with d² configuration.     

Dielectric relaxations in SrTiO3 doped with La2O3 and MnO2 at low temperatures

In addition to the dielectric relaxation around 170 K in the cubic structure of Sr_1–3x/2La _x TiO₃, a similar relaxation was observed at about 70 K in the tetragonal structure with an activation energy in the range 0.13–0.16 eV which increases as x in Sr_1–3x/2La _x TiO₃ varies from 0.60–3.00 at%. This relaxation is explained by Skanavi's model and is discussed in terms of thermal motions of Ti⁴⁺ between potential minima produced by lattice distortions in the tetragonal structure. In order to provide a direct evidence for the suggestion that the dielectric relaxation around 170 K in the cubic is due to thermal motions of Ti⁴⁺, dielectric properties on manganese-doped specimens, Sr_1–3x/2La _x Mn _y Ti_1–y ),O₃ with x=1.40×10^–2 and y=0.1×10^–2, were investigated because Mn⁴⁺ substitutes for Ti⁴⁺. As well as the relaxation due to Ti⁴⁺, this specimen exhibits another peak due to Mn⁴⁺ with an activation energy somewhat smaller than that of Ti⁴⁺. The activation energies and the relative intensity of these relaxation processes are explained by the difference in ionic radii of Mn⁴⁺ and Ti⁴⁺ and the difference in formation energies of a strontium vacancy adjacent to Mn⁴⁺ and that to Ti⁴⁺, which were calculated theoretically using a shell model.     

Ce–Nd codoping effect on the structural and optical properties of TiO2 nanoparticles

We report the changes in the structural and optical property of TiO₂ nanoparticles on codoping Ce–Nd ions. X-ray diffraction clearly demonstrates the structural changes occurring in the codoped TiO₂ nanoparticle. Oxygen defects disturb the TiO bonds in the TiO₆ octahedra and result in the shifting and broadening of the Raman E_g peak. Pure TiO₂ nanoparticles show absorption peak in the UV region. However, codoped TiO₂ nanoparticles exhibit absorption peaks in the visible region corresponding to the f–d and f–f electronic transition of Ce³⁺ and Nd³⁺ in the crystalline environment of TiO₂. The visible emission peaks of pure and codoped TiO₂ nanoparticles are mainly associated with oxygen vacancies. Incorporation of cerium intensifies the visible emission peaks of TiO₂ nanoparticles. On the other hand, codoping of Nd forms some non radiative recombination centres and increases the possibility of emission energy transfer among dopants, defects, thereby quenching the intensity of the visible emission peaks.