Ti 3d Research Articles

Rational engineering of morphology modulated Ti-ZnO thin films coupled monolayer Ti3C2 MXene for efficient visible light PEC water splitting activity

• Morphology modulated Ti 3 C 2 /Ti-ZnO composite constructed. • Well-dispersed grain-like morphology of Ti-ZnO anchored on monolayer Ti 3 C 2 MXene. • Ti 3 C 2 /Ti-ZnO composite show improved visible light PEC activity. • Ti 3 C 2 /Ti-ZnO composite promote the long-term PEC activity for 21 min. • The charge transport mechanism under visible light PEC activity is evaluated. Novel morphology modulated monolayer 2D Ti 3 C 2 MXene coupled Ti-doped ZnO (Ti 3 C 2 /Ti-ZnO) photoelectrode developed for exploring interfacial charge transfer and visible-light photoelectrochemical (PEC) water splitting. Here, we altered the flake-like ZnO into grain-like morphology by selective Ti (2 at.%) doping into ZnO under simultaneous radio frequency (RF) and direct-current (DC) magnetron sputtering. This study describes the versatility of the Ti 3 C 2 /Ti-ZnO (monolayer/grain-like) interface. Transmission electron microscopy findings suggest that Ti 3 C 2 MXene developed in a monolayer. Moreover, scanning electron microscopy proved a consistent grain-like Ti-ZnO thin film interface with monolayer Ti 3 C 2 MXene. We also find Hall effect measurements to uncover the interfacial impact on charge carrier generation. The fabricated monolayer/grain-like interface modulated the band gap towards visible region. With all the above benefits, Ti 3 C 2 /Ti-ZnO achieved stable visible water splitting for 21 min. Overall, these findings provide the potentiality of Ti 3 C 2 /Ti-ZnO photoelectrode to extend the PEC activity towards the visible region.

The electronic, structural, ferroelectric and optical properties of strontium and zirconium co-doped BaTiO[formula omitted]: First-principles calculations

Based on the first-principles calculations, we investigated the electronic, structural, ferroelectric and optical properties of (Ba0.875Sr0.125)(Zr0.125Ti0.875)O3(BSZT). The optimized structure and electron charge density analysis show that the tetragonal structure of BaTiO3 is distorted after doping with Sr and Zr, and the displacement of Zr atoms in the same layer is observed to be smaller than that of Ti atoms. The double potential well curves of pure BaTiO3 and BSZT fit well with the Landau–Ginzburg model. Meanwhile, the spontaneous polarization of BSZT is smaller than that of pure BaTiO3. Furthermore, the analysis of the density of states demonstrates that the ferroelectric properties of the BSZT material are derived from the strong hybridization between Ti 3d, Zr 4d and O 2p orbitals. Finally, we calculated the optical properties such as the real ϵ1(ω) and imaginary ϵ2(ω) part of the complex dielectric function, absorption coefficient α(ω), energy loss function L(ω), reflectivity R(ω) and refractive index n(ω) of BSZT and pure BaTiO3. The results show that the optical properties of BSZT are blue-shifted and exhibit good optical response in the UV region with the appearance of new peaks. The increase of band gap and the change of optical properties suggest that BSZT can be effectively used in ceramic capacitors, resistors with high dielectric insulation and optoelectronic devices.

Near-field radiative heat transfer modulated by nontrivial topological surface states

Three-dimensional topological insulators (3D TIs), such as Bi2Se3 and Bi2Te3, possessing insulating bulk states and nontrivial topological surface states protected by time-reversal symmetry, have attracted considerable attention. In this paper, we investigate the near-field radiative heat transfer in 3D TIs, which can be strongly modulated by the non-trivial surface states. Full 3D TI properties including both the bulk and surface states are considered in our theoretical study. We show that the hybrid polaritons in single-material TI films, resulting from the coupling of Dirac plasmons on the top and bottom surfaces with the multiple phonon polariton modes supported by the bulk states including hyperbolic phonon polaritons (HPP), surface phonon polaritons (SPhP) and epsilon-near-pole (ENP) modes, can strongly mediate and enhance the near-field thermal radiation. The dispersions of hybrid polaritons intensely depend on the chemical potential of the surface and the film thickness, and by tuning both separately, we can control the coupling and competition between the surface and bulk modes, and thus modulate the radiative spectrum and heat flux. In addition, the nonlocal optical response of the surface states has a remarkable impact on the mode hybridization. This work demonstrates the potential of 3D TIs in manipulating near-field thermal radiation, which may provide a useful platform for energy conversion and thermal management.

Prediction of phonon-mediated superconductivity in new Ti-based M_2AX phases

A high-throughput computational method is used to predict 39 new superconductors in the Ti-based M_2AX phases, and the best candidates are then studied in more detail using density functional theory electron–phonon coupling calculations. The detailed calculations agree with the simple predictions, and Ti_2AlX (X: B, C and N) materials are predicted to have higher values of T_c than any currently known hexagonal M_2AX phases. The electronic states at the Fermi level are dominated by the Ti 3d states. The choice of X (X: B, C and N) has a significant impact on the electronic density of states but not on the phonon characteristics. The electron–phonon coupling parameter for Ti_2AlX (X: B, C and N) was determined to be 0.685, 0.743 and 0.775 with a predicted T_c of 7.8 K, 10.8 K and 13.0 K, respectively.

Enhanced charge storage performance of MXene based all-solid-state supercapacitor with vertical graphene arrays as the current collector

The dwindling depletion of fossil fuels and ever-growing energy demand are the two sole persuasive reasons of the search for renewable clean green energy sources and the storing of these energy sources are called for immediate attention for the fabrication of supercapacitor like efficient energy storage devices. Variety of electrode materials have been investigated over the past years to surpass the shortcomings (low energy density, poor stability, high internal resistance etc.) of the two main category of supercapacitors i.e., electric double layer (EDL) capacitor and pseudocapacitor. To tie the advantages of both of these types of supercapacitive materials, herein we have fabricated a two-electrode symmetric device using Ti3C2Tx MXene and passivated vertical graphene nanosheets (VGN) as a current collector material. The easily synthesized material was investigated through several characterization techniques like X-ray diffraction, field emission scanning electron microscopy, high resolution transmission electron microscopy, contact angle measurements, surface area analysis and finally the electrochemical measurements. The device displayed a high areal capacitance of 199 mF/cm2 at 0.08 mA/cm2 along with an excellent stability and columbic efficiency of 90 % and nearly 100 % over 6000 repetitive charge-discharge cycles. The energy density and corresponding power density were also found to be remarkable and as high as 13.57 μWh/cm2 and 31.07 μW/cm2 respectively. To corroborate our experimental observations qualitatively, we have carried out the extensive Density Functional Theory (DFT) simulations and presented the structural and electronic properties of pristine VGN and MXene sheets and MXene@VGN. The interaction between VGN and MXene is due to charge transfer from Ti 3d orbital of MXene to C 2p orbital of VGN. Enhanced quantum capacitance and lower diffusion energy barrier for the electrolyte's ions in MXene@VGN justify the superior charge storage performance in the modified structure compared to pristine VGN and MXene as observed in the experiment. The computed quantum capacitance follows the trend MXene@VGN > MXene > VGN, matching nicely with the trend of specific capacitance obtained in the experiment.

Modulating Tit2gOrbital Occupancy in a Cu/TiO2Composite for Selective Photocatalytic CO2Reduction to CO

The electronic structure of composites plays a critical role in photocatalytic conversion, whereas it is challenging to modulate the orbital for an efficient catalyst. Herein, we regulated the t2g orbital occupancy state of Ti to realize efficient CO2 conversion by adjusting the amount of photo-deposited Cu in the Cu/TiO2 composite. For the optimal sample, considerable electrons transfer from the Cu d orbital to the Ti t2g orbital, as proven by X-ray absorption spectroscopy. The Raman spectra results also corroborate the electron enrichment on the Ti t2g orbital. Further theoretical calculations suggested that the orbital energy of the Ti 3d orbital in TiO2 is declined, contributing to accepting Cu 3d electrons. As a result, the Cu/TiO2 composite exhibited an extremely high selectivity of 95.9 % for CO, and the productivity was 15.27 μmol g-1 h-1 , which is almost 6 times that of the original TiO2 . Our work provides a strategy for designing efficient photocatalysis as a function of orbital regulation.

Effective lifting of the topological protection of quantum spin Hall edge states by edge coupling

The scientific interest in two-dimensional topological insulators (2D TIs) is currently shifting from a more fundamental perspective to the exploration and design of novel functionalities. Key concepts for the use of 2D TIs in spintronics are based on the topological protection and spin-momentum locking of their helical edge states. In this study we present experimental evidence that topological protection can be (partially) lifted by pairwise coupling of 2D TI edges in close proximity. Using direct wave function mapping via scanning tunneling microscopy/spectroscopy (STM/STS) we compare isolated and coupled topological edges in the 2D TI bismuthene. The latter situation is realized by natural lattice line defects and reveals distinct quasi-particle interference (QPI) patterns, identified as electronic Fabry-Pérot resonator modes. In contrast, free edges show no sign of any single-particle backscattering. These results pave the way for novel device concepts based on active control of topological protection through inter-edge hybridization for, e.g., electronic Fabry-Pérot interferometry.

Effects of Nb concentration on Nb-doped anatase TiO2: DFT + U calculations

The crystal structure, formation energy, electronic structure, electrical properties and optical properties of anatase TiO2 with various Nb concentrations were studied by first-principles calculations based on density functional theory (DFT) and the Hubbard U correction. Firstly, the crystal structures of TiO2 with various Nb concentrations were optimized successfully. The higher concentrations of Nb facilitate the synthesis of Nb-doped TiO2 systems in an O-rich environment. Furthermore, the band structures and density of states prove that the electronic structure of Nb-doped TiO2 is mainly transformed by the hybridization of Ti 3d, Nb 4d and O 2p states. Next, broadening of the valence band and occupied states at the bottom of the conduction band enhance the conductivity of Nb-doped TiO2, and the conductivity reaches the maximum value at the high Nb-doping concentration of 6.25 at.%. Last but not least, the calculated optical properties show that the absorption edge of Nb-doped TiO2 produces an blue shift. TiO2 with Nb-doping concentration below 14.58 at.% exhibits the high transmittance in the visible region.

A facile strategy toward hierarchically porous composite scaffold for osteosarcoma ablation and massive bone defect repair

The surgical treatment of osteosarcoma remains a challenge because of massive bone defects and tumor recurrence. Herein, a facile strategy inspired by cooking is developed to fabricate hierarchically porous composite scaffold for bone tumor ablation and massive bone defect repair. Silk fibroin (SF) as a biological foaming agent is foamed by mechanical stirring to form physical hydrogel through strong interaction with 2D Ti 3 C 2 MXene nanosheets . Bacterial cellulose (BC) is used to stabilize air bubbles to keep porous structure . Hierarchically porous SF/BC/MXene (FSCM) scaffold with ∼200 μm macropore and nanofibrillar wall is acquired by lyophilization. Such FSCM composite scaffold presents prominent osteosarcoma ablation performance after near-infrared (NIR) light irradiation, and features excellent bone defect repair ability. This work represents a newly strategy to fabricate hierarchically porous bifunctional scaffold for enhancing bone tumor ablation and facilitating massive bone defect repair in the clinical treatment of osteosarcoma. • Protein foaming is a valid strategy for fabricating multifunctional bone tissue engineering composite scaffold. • MXene endowed FSCM scaffold with excellent osteosarcoma ablation performance. • Hierarchically porous bio-mass scaffold presented prominent bone defect repair capability. • Multifunctional bio-mass composite scaffold showed great prospect for the clinical treatment of osteosarcoma.

Constructing S-scheme 2D/0D g-C3N4/TiO2 NPs/MPs heterojunction with 2D-Ti3AlC2 MAX cocatalyst for photocatalytic CO2 reduction to CO/CH4 in fixed-bed and monolith photoreactors

• 2D MAX Ti 3 AlC 2 anchored g-C 3 N 4 /TiO 2 synthesized for photocatalytic CO 2 to fuels. • 2D MAX mediated TiO 2 /g-C 3 N 4 heterojunction found efficient for CO 2 to CH 4 reduction. • Photoactivity of g-C 3 N 4 /MAX/TiO 2 found 97.70 folds higher than TiO 2 under visible light. • MAX promoted CO 2 methanation in TiO 2 /g-C 3 N 4 composite with higher selectivity. • Performance of fixed-bed and monolith photoreactor further confirms higher productivity. Exfoliated 2D MAX Ti 3 AlC 2 conductive cocatalyst anchored with g-C 3 N 4 /TiO 2 to construct 2D/0D/2D heterojunction has been explored for enhanced CO 2 photoreduction in a fixed-bed and monolith photoreactor. The TiO 2 particle sizes (NPs and MPs) were systematically investigated to determine effective metal-support interaction with faster charge carrier separation among the composite materials. When TiO 2 NPs were anchored with 2D Ti 3 AlC 2 MAX structure, 10.44 folds higher CH 4 production was observed compared to anchoring TiO 2 MPs. Maximum CH 4 yield rate of 2103.5 µmol g −1 h −1 achieved at selectivity 96.59% using ternary g-C 3 N 4 /TiO 2 /Ti 3 AlC 2 2D/0D/2D composite which is 2.73 and 7.45 folds higher than using binary g-C 3 N 4 /Ti 3 AlC 2 MAX and TiO 2 NPs/Ti 3 AlC 2 samples, respectively. A step-scheme (S-scheme) photocatalytic mechanism operates in this composite, suppressed the recombination of useful electron and holes and provides higher reduction potential for efficient CO 2 conversion to CO and CH 4 . More importantly, when light intensity was increased by 5 folds, CH 4 production rate was increased by 3.59 folds under visible light. The performance of composite catalyst was further investigated in a fixed-bed and monolith photoreactor and found monolithic support increased CO production by 2.64 folds, whereas, 53.99 times lower CH 4 production was noticed. The lower photocatalytic activity in a monolith photoreactor was due to lower visible light penetration into the microchannels. Thus, 2D MAX Ti 3 AlC 2 composite catalyst can be constructed for selective photocatalytic CO 2 methanation under visible light in a fixed-bed photoreactor.

Self-assembly construction of hollow Ti3C2Tx Submicro-Tubes towards efficient alkali metal ion storage

Hollow Ti 3 C 2 T x submicro-tubes are smartly designed by a single-step wet electro-spin strategy and exhibit remarkable reversible capacities and cycling stability at high rates towards alkali metal ion batteries. • An electrospinning involved self-assembly methodology is first designed to construct hollow Ti 3 C 2 T x submicro-tubes. • Ti 3 C 2 T x submicro-tubes with an ultra-thin wall are assembled with monolayered nanosheets. • Intrinsic formation mechanism of the submicro-tubes is rationally put forward. • The optimized submicro-tubes are endowed with abundant active sites and convenient ion diffusion/transport pathways. • The Ti 3 C 2 T x submicro-tubes show excellent alkali metal ion storage behaviors with superb electrochemical utilization. MXenes have shown enormous potential in various electrochemical energy-related fields due to their excellent electronic and mechanical properties. However, the serious stacking of two-dimensional (2D) Ti 3 C 2 T x nanosheets (NSs) always results in modest reversible capacities and reaction kinetics when serving as an anode material for rechargeable batteries. To well address the issues, an electrospinning involved self-assembly methodology is first designed here to construct hollow Ti 3 C 2 T x submicro-tubes (Ti 3 C 2 T x -SMT) assembled with monolayered NSs. Moreover, the underlying formation mechanism of the Ti 3 C 2 T x -SMT is rationally put forward with comprehensive physicochemical analysis. The optimized Ti 3 C 2 T x -SMT with an ultra-thin wall of just several Ti 3 C 2 T x layers in thickness is endowed with abundant active sites and convenient ion diffusion/transport pathways, enhancing the electrochemical utilization, which guarantees its excellent alkali metal ion (Li + , Na + and K + ) storage behaviors in terms of both high-rate reversible capacities and long-duration cycle stability. More significantly, our contribution here provides a guiding design of 2D ultrathin materials as universal electrodes for advanced alkali metal ion batteries and beyond.

Influence of Nd concentrations on the structural, electronic and optical properties of anatase TiO2: A first-principles approach

Titanium dioxide (TiO2) and TiO2 doped with impurities offer good potential for many applications such as photoanode in dye-sensitized solar cells (DSSCs) and photocatalysts used in hydrogen production and degradation of organic pollutants. First-principles calculations on the structural, electronic and optical properties of pure anatase TiO2 with different Nd concentrations are performed using 2 × 1 × 1 supercell with a stoichiometry of Ti1-xNdxO2 (x = 0, 0.0625, 0.125 and 0.1875). The doping effect to the properties of TiO2 can be observed from the changes in the structural parameters, modification of the band structure, presence of impurity energy levels (IELs) in density of states (DOS) and difference in optical properties results. At a higher Nd concentration doping, the band gap of Nd-doped TiO2 decreases as a result of the overlapping of states among Nd 4f, Ti 3d and O 2p, which can enhance the light absorption activity in the visible light region. These findings indicate that Nd-doped TiO2 can increase the performance of photocatalyst and photovoltaic due to its capability to harvest visible light.

Understanding of the bond covalency nature with ionic electronegativity in perovskite cuprates La2CuMO6-x (M = Ti, Mn, Ru)

A bond valence sums (BVS) modified ionic electronegativity (EN) scale was suggested for better accommodating the double perovskite La2CuMO6-x (M ​= ​Ti, Mn, Ru) with mixed bonding nature. Both the M ​− ​O bond covalency fraction (f) and the cooperative B-site sublattice distortion are highlighted to connect structure and physical properties. Particularly, the Ti-based compound hosts the largest M/O ionic EN difference and hence yields the smallest f value while the Ru-based analogue exhibits the opposite result. As the consequence, robust spin disordering accompany with poor dc conductivity is convinced in the Ti-based sample owing to its featuring B-site ionic arrangement and profound suppression of 3d(Ti)-2p(O) orbital hybridization. For the Ru-based sample, strong ferromagnetic spin frustration and low charge carrier hopping potential might be essentially ascribed to its large f value through enhancing orbital degeneracy between 4d(eg)-2p σ and 4d(t2g)-2p π bonding states. The Mn-based sample with the moderate f value exhibits the similar magnetic and electric behaviors as those of Ru-based sample. Both the d(M)-p(O) orbital hybridization and the M ​− ​O bond covalency could be tuned by rationally selecting the M cation with proper ionic EN value. This work provides a deep understanding of the bond covalency nature in perovskite cuprates and might help to develop novel electronic functional materials.

Edge spin transport in the disordered two-dimensional topological insulator WTe2

The spin conductance of two-dimensional topological insulators (2D TIs) is not expected to be quantized in the presence of perturbations that break the spin-rotational symmetry. However, the deviation from the pristine-limit quantization has yet to be studied in detail. In this paper, we define the spin current operator for the helical edge modes of a 2D TI and introduce a four-terminal setup to measure spin conductances. Using the developed formalism, we consider the effects of disorder terms that break spin-rotational symmetry or give rise to edge-to-edge coupling. We identify a key role played by spin torque in an out-of-equilibrium edge. We then utilize a tight-binding model of topological monolayer ${\mathrm{WTe}}_{2}$ and scattering matrix formalism to numerically study spin transport in a four-terminal 2D TI device. In particular, we calculate the spin conductances and characteristic spin decay length in the presence of magnetic disorder. In addition, we study the effects of interedge scattering in a quantum point contact geometry. We find that the spin Hall conductance is surprisingly robust to spin symmetry-breaking perturbations, as long as time-reversal symmetry is preserved and interedge scattering is weak.

Quasi zero-dimensional MoS2 quantum dots decorated 2D Ti3C2Tx MXene as advanced electrocatalysts for hydrogen evolution reaction

Developing advanced noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) is still a great challenge. Herein, a novel HER catalyst with quasi zero-dimensional (0D) MoS 2 quantum dots (QDs) supported on two-dimensional (2D) Ti 3 C 2 T x MXene nanosheets is facilely synthesized. The MoS 2 QDs/Ti 3 C 2 T x nanohybrid retains the unique layer structure, and the MoS 2 QDs are in situ formed and distributed uniformly. The obtained MoS 2 QDs/Ti 3 C 2 T x catalyst exhibits superior electrocatalytic activity due to its excellent conductivity, abundant of active sites exposed and a high percentage of 1T metallic phase (∼76%) of MoS 2 QDs. Remarkably, an early HER overpotential of 220 mV at 10 mA cm −2 and a small Tafel slope of 72 mV dec −1 of MoS 2 QDs/Ti 3 C 2 T x are achieved in 0.5 M H 2 SO 4 solution. In addition, the exchange current density of MoS 2 QDs/Ti 3 C 2 T x is ∼5 times larger compared with pure MoS 2 , thus demonstrating an accelerated charge transfer during the electrocatalytic process. • A facile strategywas developed to synthesize MoS 2 QDs on Ti 3 C 2 T x MXene. • The catalyst contains highly conductive 1T phase MoS 2 and rich accessibleactive sites. • MoS 2 QDs/Ti 3 C 2 T x shows outstanding electrocatalytic performance towards HER.

Superimposed Effect of La Doping and Structural Engineering to Achieve Oxygen-Deficient TiNb2O7 for Ultrafast Li-Ion Storage.

TiNb2O7 (TNO) is a competitive candidate of a fast-charging anode due to its high specific capacity. However, the insulator nature seriously hinders its rate performance. Herein, the La3+-doped mesoporous TiNb2O7 materials (La-M-TNO) were first synthesized via a facile one-step solvothermal method with the assistance of polyvinyl pyrrolidone (PVP). The synergic effect of La3+ doping and the mesoporous structure enables a dual improvement on the electronic conductivity and ionic diffusion coefficient, which delivers an impressive specific capacity of 213 mAh g-1 at 30 C. The capacity retention (@30C/@1C) increases from 33 to 53 and 74% for TNO, M-TNO, and La-M-TNO (0.03), respectively, demonstrating a step-by-step improvement of rate performance by making porous structures and intrinsic conductivity enhancement. DFT calculations verify that the enhancement in electronic conductivity due to La3+ doping and oxygen vacancy, which induce localized energy levels via slight hybridization of O 2p, Ti 3d, and Nb 4d orbits. Meanwhile, the GITT result indicates that PVP-induced self-assembly of TNO accelerates the lithium ion diffusion rate by shortening the Li+ diffusion path. This work verifies the effectiveness of the porous structure and highlights the significance of electronic conductivity to rate performance, especially at >30C. It provides a general approach to low-conductivity electrode materials for fast Li-ion storage.

Electric field induced Mott transition and bipolar resistive switching in La2Ti2O7-x thin film

The oxygen ion migration-induced resistive switching is a complex process with multiple mechanisms. This study investigated resistive switching properties and mechanism for strongly correlated materials lanthanum titanium oxide La2Ti2O7-x (LTO) using spectromicroscopy and density functional theory simulation. Bipolar resistive switching without an electroforming process was first reported in LTO film. The mechanism can be explained by oxygen ion migration induced band-filling controlled Mott transition in LTO. Oxygen ion migration-induced reduction and oxidation of LTO can increase and decrease Ti 3d orbital filling state, which in turn increases and reduces Mott-Hubbard gap and LTO resistance, respectively. In addition, the RRAM device, TiN/LTO/Pt, shows stable endurance, retention, and small variability at room temperature and 85 ℃, with a stable on/off-state current density, which is independent of the electrode size. This work not only demonstrates that LTO and other strongly correlated materials are potential candidates to prepare resistive random access memory based on oxygen ion migration induced Mott-transition mechanism but also indicates that intrinsic resistance change in switching layers between top and bottom electrodes also contribute to overall resistive switching.

Synchrotron based x-ray absorption spectroscopy investigation and temperature dependent ferroelectric properties of Ni doped BaTiO3 nanostructures

In this report, the local structure geometry of BaTi1-xNixO3 (0.0 ≤ x ≤ 0.06) ceramics has been studied through synchrotron based x-ray-absorption spectroscopy, measured at the Ti K, Ba L3 and Ni K-edges at room temperature. The x-ray absorption near edge spectroscopy (XANES) at Ti K-edge confirms the TiO6 octahedron geometry of Ti atom due hybridization between Ti(3d)-O(2p) orbitals and signifies the non-centrosymmetric nature of BaTiO3 samples. However, Ti–O off-center displacement (non-centrosymmetric) is disturbed under the effect of Ni doping as a result of oxygen vacancies formation. More interestingly, XANES studies at Ni K-edge ensure successful substitution of Ni in BaTiO3 as Ni2+ ions. Extended x-ray absorption fine structure (EXAFS) data at Ni K-edge have been fitted to estimate pertinent local structural parameters of the Ni–O, Ni–Ba, Ni–Ti and Ni–Ni shells (viz. bond lengths and disorder parameters) and it also reveal that the structural disorders around the Ti sites in the doped BaTiO3 expand with Ni doping. The mixed-valence states of titanium ion, i.e., Ti4+ and Ti3+ in the doped samples were established with the x-ray photoelectron spectroscopy (XPS). Moreover, XPS divulges the creation of oxygen vacancies due to Ni doping in BaTiO3 matrix. The complementary information about the lattice vibration is analyzed through Raman studies that approve the softening of the transverse optical (TO) mode present at 515 cm−1. The temperature dependent ferroelectric studies affirm that the ferroelectricity vanishes in the doped samples due to the decrement in the off-center displacement between the Ti4+ and O2− ions in the TiO6 octahedral geometry. Differential thermal analysis (DTA) exhibits a ferroelectric tetragonal to para-electric cubic phase transition in the pristine as well as Ni doped BaTiO3 samples.

Dopamine Adsorption on Rutile TiO2(110): Geometry, Thermodynamics, and Core-Level Shifts from First Principles.

The modification of the rutile TiO2(110) surface with dopamine represents the best example of the functionalization of TiO2-based nanoparticles with catecholamines, which is of great interest for sunlight harvesting and drug delivery. However, there is little information on the dopamine–TiO2(110) adsorption complex in terms of thermodynamic properties and structural parameters such as bond coordination and orientation of the terminal ethyl–amino group. Here, we report a density functional theory (DFT) investigation of dopamine adsorption on the TiO2(110) surface using the optB86b-vdW functional with a Hubbard-type correction to the Ti 3d orbitals, where Ueff = 3 eV. Guided by available X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) data, our simulations identify enolate species with bidentate coordination at a submonolayer coverage, which are bonded to two neighboring 5-fold-coordinated Ti atoms at the TiO2(110) surface through both deprotonated oxygen atoms of the dopamine, i.e., in a bridging fashion. The process is highly exothermic, involving an adsorption energy of −2.90 eV. Calculated structural parameters suggest that the molecule sits approximately upright on the surface with the amino group interacting with the π-like orbitals of the aromatic ring, leading to a gauche-like configuration. The resulting NH···π hydrogen bond in this configuration can be broken by overcoming an energy barrier of 0.22 eV; in this way, the amino group rotation leads to an anti-like conformation, making this terminal group able to bind to other biomolecules. This mechanism is endothermic by 0.07 eV. Comparison of existing spectroscopic data with DFT modeling shows that our computational setup can reproduce most experimentally determined parameters such as tilt angles from NEXAFS and chemical shifts in XPS, which allows us to identify the preferred mode of adsorption of dopamine on the TiO2(110) surface.

Photo- and Electrocatalytic Reduction of CO2 over Metal-Organic Frameworks and Their Derived Oxides: A Correlation of the Reaction Mechanism with the Electronic Structure.

A Ce/Ti-based bimetallic 2-aminoterephthalate metal-organic framework (MOF) was synthesized and evaluated for photocatalytic reduction of CO2 in comparison with an isoreticular pristine monometallic Ce-terephthalate MOF. Owing to highly selective CO2 adsorption capability, optimized band gaps, higher flux of photogenerated electron-hole pairs, and a lower rate of recombination, this material exhibited better photocatalytic reduction of CO2 and lower hydrogen evolution compared to Ce-terephthalate. Thorough probing of the surface and electronic structure inferred that the reducibility of Ce4+ to Ce3+ was due to the introduction of an amine functional group into the linker, and low-lying Ti(3d) orbitals in Ce/Ti-2-aminoterephthalate facilitated the photoreduction reaction. Both the MOFs were calcined to their respective oxides of Ce1-xTixO2 and CeO2, and the electrocatalytic reduction of CO2 was performed over the oxidic materials. In contrast to the photocatalytic reaction mechanism, the lattice substitution of Ti in the CeO2 fluorite cubic structure showed a better hydrogen evolution reaction and consequently, poorer electroreduction of CO2 compared to pristine CeO2. Density functional theory calculations of the competitive hydrogen evolution reaction on the MOF and the oxide surfaces corroborated the experimental findings.