Reduced Titanium Dioxide Research Articles

Visible light driven (VLD) reduced TiO2-x nanocatalysts designed by inorganic and organic reducing agent-mediated solvothermal methods for electrocatalytic and photocatalytic applications.

This work presents a comparative study on the structural, optical and electrochemical characteristics of visible light driven (VLD) reduced titanium dioxide (TiO2-x ) nanocatalysts synthesized via inorganic and organic synthetic routes. X-ray diffraction (XRD) patterns, Raman spectra and X-ray absorption fine structure (XAFS) analyses reflected anatase phase titania. Whereas, the quantitative EXAFS fit and XANES analysis revealed structural distortion due to the presence of oxygen and titanium vacancies with low valent Ti states in anatase lattices of certain nanocatalysts, which subsequently leads to better electrochemical and photocatalytic activities. Moreover, owing to the large surface area and mesoporous structures, the Mg-TiO2-x nanocatalysts exhibited enhanced water adsorption and ultimately increased overall water splitting with an OER overpotential equal to 420 mV vs. RHE at a current density of 10 mA cm-2 (Tafel slope = 62 mV dec-1), extended visible light absorbance, decreased photoluminescence (PL) intensity and increased carrier lifetime in comparison with commercial titania.

Sensitive analysis of heat transfer enhancement in ternary Casson nanofluid flow between a conical surface and disk

This study explores the ternary nanofluid flow within the canonical gap between a cone and a disk with particle deposition and magnetic field effects. Reduced titanium dioxide, magnetite, and graphene oxide are used as nanoparticles in the base fluid ethylene glycol. The governing equations of the problem are in the form of partial differential equations, which are converted to nonlinear ordinary differential equations by using appropriate similarity transformations, and they are solved numerically by using Runge–Kutta–Fehlberg fourth fifth-order (RKF 45) technique. The main agenda of this work is to discuss the impacts of parameters on three cases. The effects of essential aspects on fluid flow, heat and mass transfer rates were studied and analyzed using a graphical representation. Additionally, the response surface methodology and sensitivity analysis are carried out to enhance the importance of the heat transfer rate. The results reveal that the flow field increases significantly with increased Reynolds numbers for both cone and disk rotations. It is observed that the sensitivity analysis of the Nusselt number toward the Eckert number is more for all the radiation parameter values and the Eckert number’s middle level.

Heterostructure of Reduced Titanium Dioxide Nanorods and Small Molecules Leads to Photo‐Induced Negative Differential Resistance Behaviors

AbstractNegative differential resistance (NDR) is of considerable interest due to the uncommon characteristic of having a region where the current decreases as the applied voltage bias increases. However, it is difficult to reproduce NDR devices and evaluate their reliability and stability because of the unstable NDR behaviors. In this paper, a highly stable photo‐induced hybrid photonic (HP)‐NDR device is presented for practical applications, which has heterostructure between TiO2‐x nanorods formed by oblique angle deposition and dinaphtho[2,3‐b:2′,3′‐f]‐thieno[3,2‐b]thiophene. The mechanism of NDR behaviors is investigated through morphological analysis, energy band structure, and electrical characteristics. In addition, diverse bias stress and endurance measurements are performed to demonstrate the reliability of the HP‐NDR device. As a result, the NDR behavior lasts up to 1,000 s in each bias stress and 38,850 cycles in the repeated current–voltage (I–V) characteristics Thus, the study has successfully achieved robust HP‐NDR devices with potential for practical applications.

Organic dye-loaded reduced titanium dioxide as a broadband saturable absorber for ultrafast fiber lasers.

As a rising star among metal oxide nanomaterials, titanium dioxide (TiO2) has been widely investigated and employed in optical applications because of its excellent optical properties. In this work, we demonstrate the efficient and broadband nonlinear photonic properties of methylene blue (MB)-loaded reduced TiO2 (TiO2-x-MB) and explore the performance of a TiO2-x-MB-microfiber photonic device in broadband ultrafast photonics. Within an erbium-doped fiber laser (EDFL) system, utilizing the TiO2-x-MB-microfiber photonic device as a saturable absorber (SA), steady mode-locked pulses together with chaotic pulses were successfully achieved at the wavelength of 1.55 μm. Furthermore, by incorporating the TiO2-x-MB SA into a thulium-doped fiber laser (TDFL) system, an ultrashort single pulse and multiple pulses were obtained at 2.0 μm. These results indicate that TiO2-x-MB is an excellent nanomaterial for use in mode-locked lasers, being an alternative candidate for ultrafast fiber lasers via exploiting the chemical and physical properties of oxide nanomaterials.

The Role of Lattice Defects on the Optical Properties of TiO2 Nanotube Arrays for Synergistic Water Splitting.

In this study, we report a facile one-step chemical method to synthesize reduced titanium dioxide (TiO2) nanotube arrays (NTAs) with point defects. Treatment with NaBH4 introduces oxygen vacancies (OVs) in the TiO2 lattice. Chemical analysis and optical studies indicate that the OV density can be significantly increased by changing reduction time treatment, leading to higher optical transmission of the TiO2 NTAs and retarded carrier recombination in the photoelectrochemical process. A cathodoluminescence (CL) study of reduced TiO2 (TiO2-x) NTAs revealed that OVs contribute significantly to the emission bands in the visible range. It was found that the TiO2 NTAs reduced for a longer duration exhibited a higher concentration of OVs. A typical CL spectrum of TiO2 was deconvoluted to four Gaussian components, assigned to F, F+, and Ti3+ centers. X-ray photoelectron spectroscopy measurements were used to support the change in the surface chemical bonding and electronic valence band position in TiO2. Electron paramagnetic resonance spectra confirmed the presence of OVs in the TiO2-x sample. The prepared TiO2-x NTAs show an enhanced photocurrent for water splitting due to pronounced light absorption in the visible region, enhanced electrical conductivity, and improved charge transportation.

Photocatalytic potential of a titanium dioxide–supported platinum catalyst against VOCs with complicated composition under varying humidity conditions

It is more important to assess the performance of photocatalysts against volatile organic compounds (VOCs) as a mixture rather than as single-component systems to understand the factors and processes governing photocatalytic (PC) destruction of VOCs under real-world operation condition. It is often critical because VOCs in ambient air generally exist in the form of complicated mixtures. In this context, the PC destruction of 14 VOCs as a mixture was studied using a reduced titanium dioxide (TiO2)-supported platinum (1 wt%) catalyst under varying humidity conditions (room temperature (RT) and ultraviolet irradiation (λmax: 254 nm)). The removal efficiencies for aromatic hydrocarbons (e.g., benzene, toluene, m-xylene, and styrene), when examined in the relative humidity (RH) range of 0 to 100%, were in four distinctive ranges of 8.93% and 13.3%, 29.1% and 39.4%, 61.9% and 75.6%, and 92.9% and 96.6%, respectively. In contrast, the removal of ketones increased with RH between 0% and 20%, such as 30.7%–73.8% for methyl ethyl ketone and 68.7%–95.9% for methyl isobutyl ketone. Aldehydes were removed efficiently in the dark through a synergy between thermal oxidation and adsorption at RT. Overall, RH had positive and/or negative effects on photocatalysis depending on the VOC type. The overall results of this study are expected to offer a theoretical framework and practical insights into the PC processes occurring toward complex VOC systems.

Thermodynamic analysis of titanium dioxide-based isothermal methanothermal and carbothermal redox cycles for solar fuel production

Due to the less efficiency-penalty and avoidance of thermal stress resulting from temperature swings, isothermal thermochemical redox cycle has emerged as a promising way to produce solar fuels. However, the isothermal thermochemical cycles based on the currently reported metal oxides have not shown a competitive theoretical solar-to-fuel efficiency. Herein, titanium dioxide-based isothermal methanothermal redox cycle and carbothermal redox cycle for efficient solar fuel production have been proposed and thoroughly evaluated thermodynamically. The calculations suggest that the reactant ratio of 0.5 is beneficial for oxygen release and fuel selectivity in the reduction step. Excessive reducing agents could cause the generation of the by-product titanium carbide, resulting in the loss of redox capacity. Attributing to the considerable thermodynamic driving force for water splitting, the reduced titanium dioxide could be almost fully reoxidized even at high temperatures ⩾1600 K without excess water. As expected, the efficiency evaluation indicates that the studied isothermal redox cycles are more thermodynamically favorable than the non-isothermal ones. Furthermore, according to the efficiency optimization strategy, the methanothermal and carbothermal redox cycles should be performed at the reactant ratios of 0.4 (reduction step) and 0.5 (water-splitting step) under the isothermal operating at 1500 K and 1600 K, respectively, to achieve the highest solar-to-fuel efficiencies of 40.0% and 35.7% without heat recuperation. The results indicate that the titanium dioxide-based isothermal methanothermal and carbothermal redox cycles both have the potential of becoming competitive routes for solar fuel production.

Simple Ethanol Refluxing Method for Production of Blue-Colored Titanium Dioxide with Oxygen Vacancies and Visible Light-Driven Photocatalytic Properties

We show that a simple ethanol (EtOH) refluxing treatment at mild temperature (120 °C) allows producing blue-colored and reduced titanium dioxide (TiO2–x) exhibiting improved visible-light (VIS) photocatalytic properties. The treatment causes an increase in the density of Ti(III) species and the appearance of two optical absorption features: a broad absorption band—responsible for the blue coloration—extending from the green region (∼2.3 eV) up to the near-infrared and a subgap absorption tail close to the band gap energy. The experimental results combined with a computation of the density of states via hybrid Hartree–Fock density functional support the hypothesis that the EtOH reflux treatment leads to formation of surface and subsurface oxygen (O) vacancies. We also show that the excitation-resolved photoluminescence technique allows a high-contrast detection of a subgap optical excitation band peaked at about 430 nm (∼2.9 eV), associated with anatase photoluminescence, whose intensity increases after the EtOH reflux treatment. This result gives a very direct support to the debated hypothesis identifying O vacancy states as the energy levels involved in the radiative transition of anatase TiO2. Improved photocatalytic degradation by the processed TiO2 under VIS illumination is demonstrated, and the possible mechanism involved in the formation of surface O vacancies is discussed. The method outlines a very simple, low-cost, and fast procedure to target the formation of O vacancies in the TiO2 surface region.

Influence of carbon or nitrogen dopants on the electronic structure, optical properties and photocatalytic activity of partially reduced titanium dioxide

Influence of carbon or nitrogen dopants on the electronic structure, optical properties and photocatalytic activity of partially reduced titanium dioxide

UV and Visible Light-Driven Production of Hydroxyl Radicals by Reduced Forms of N, F, and P Codoped Titanium Dioxide.

The photocatalytic activities of reduced titanium dioxide (TiO2) materials have been investigated by measuring their ability to produce hydroxyl radicals under UV and visible light irradiation. Degussa P25 TiO2 was doped with nitrogen (N), fluorine (F), and/or phosphorus (P) and then subjected to surface modification employing a thermo-physicochemical process in the presence of reducing agent sodium borohydride (NaBH4). The reduced TiO2 materials were characterized by a number of X-ray, spectroscopic and imaging methods. Surface doping of TiO2 was employed to modulate the band gap energies into the visible wavelength region for better overlap with the solar spectrum. Hydroxyl radical generation, central to TiO2 photocatalytic water purification applications, was quantitated using coumarin as a trap under UV and visible light irradiation of the reduced TiO2 materials. At 350 nm irradiation, the yield of hydroxyl radicals generated by the reduced forms of TiO2 was nearly 90% of hydroxyl radicals generated by the Degussa P25 TiO2. Hydroxyl radical generation by these reduced forms of TiO2 was also observed under visible light irradiation (419 and 450 nm). These results demonstrated that simple surface modification of doped TiO2 can lead to visible light activity, which is important for more economical solar-driven applications of TiO2 photocatalysis.

Defective black Ti3+ self-doped TiO2 and reduced graphene oxide composite nanoparticles for boosting visible-light driven photocatalytic and photoelectrochemical activity

We present a facile and environmentally friendly way to successfully build partially reduced titanium dioxide nanoparticles integrated with reduced graphene oxide composite nanosheets (RGO/TiO2−x) through an environmental solvothermal method at relatively low temperature. The Ti3+ self-doping of TiO2−x is conducive to enhance its visible light absorption, and the existence of RGO as the electron receiver makes the electrons on TiO2−x transport better to its surface, effectively promoting the separation of electrons and holes. Experiments on photocatalytic degradation of methylene blue dye and phenol colorless organic pollutant by visible light demonstrate that the RGO/TiO2−x assemblies exhibit the highest photocatalytic activity, and that methylene blue solution concentration of used in the experiment is 40 mg L−1, which was more higher than the concentration (10 mg L−1) used in many of reported articles. Compared to the pure TiO2 and TiO2−x, the photodegradation efficiency of methylene blue and phenol can be increased to 100% within 120 min and 150 min under visible light. Moreover, the mineralization rates of MB, such as the total organic carbon (TOC) degradation rate, can reach to 75.7%, 86.6% and 89.4% for m-RGO/TiO2−x, respectively. And the reaction efficiency of RGO/TiO2−x is basically not much changed after 4th cycles, suggesting its excellent photocatalytic stability.

Dielectric study of the interplay between charge carriers and electron energy losses in reduced titanium dioxide

The transport mechanism in titanium dioxide through polarons is an open issue. High-resolution electron energy-loss spectroscopy (HREELS) is in principle of great relevance in such context, provided the fingerprints on the loss spectrum of the charge carriers involved in the material are disclosed. This paper aims at evidencing those fingerprints. Through a suitable parametrization of the dielectric function, a theoretical analysis of EELS excitations in defective ${\mathrm{TiO}}_{2}$ rutile is developed in the framework of the semiclassical dielectric theory. The focus is put on the interplay between phonons, interband transitions, and defect-related excitations, namely, plasmon and band-gap states. Transport properties are demonstrated to be more efficiently grasped through the screening they induce on phonons than through the existence of a defined surface plasmon peak. While the corresponding imaginary part of the dielectric function only yields a slight broadening and temperature dependence of the quasielastic peak due to the large static dielectric function and electron effective mass, a sizable upward shift in energy and a decrease in intensity of phonons due to the real part are predicted. Band-gap states also screen phonons but with downward shift in energy loss. Due to its large oscillator strength, the high-energy-lying surface phonon at 95 meV is a very sensitive reporter of the combined effects of transport behavior and band-gap states. Finally, it is highlighted that extracting quantitative information out of EELS experiments requires an accurate modeling of the depth profile.

In situ glow discharge plasma electrolytic synthesis of reduced TiO2 for enhanced visible light photocatalysis

Reduced titanium dioxide (TiO2−x) due to its extraordinary visible light absorption has been widely investigated in photodegradation and water splitting nowadays. However, conventional routes to synthesize reduced TiO2 usually demand multiple preparation steps, harsh controlled conditions or expensive facilities. Here we developed a single-step in situ approach to prepare the gray TiO2−x nanoparticles (sub-10 nm) effectively by the glow discharge plasma electrolysis (GDPE) under atmospheric pressure. The co-existence of self-doped oxygen vacancies and Ti3+ in the generated TiO2−x nanoparticles is demonstrated by electron paramagnetic resonance (EPR). The tunable ratio of bulk/surface defect can be realized by controlling the glow discharge power directly. It should be noticed that Ti3+ in the synthesized TiO2−x are quite stable in ambient air. The UV–vis spectra of gray TiO2−x show an enhanced visible light absorption, which leads to high visible-light photocatalytic activity. Moreover, the as-prepared TiO2−x after 6 months storage still shows excellent stability during photocatalytic reactions. Owing to its simplicity and effectivity, this preparation method with GDPE should provide a large-scale production for TiO2−x with high photoactivity.

Gentle way to build reduced titanium dioxide nanodots integrated with graphite-like carbon spheres: From DFT calculation to experimental measurement

Extending the ultraviolet response of anatase TiO2 into visible light range and effectively decreasing the recombination rate of photo-generated carriers are vital to promote the practical applications in the fields of hydrogen generation and water pollutant treatment of TiO2-based photocatalysts. Here we present a mild way to build composite of partial reduced TiO2 nanodots coated graphite-like carbon spheres (g-CS@TiO2-X) via a green solvothermal approach with relative low temperature. The chemical structure and electronic property of 3D g-CS@TiO2-X composites have been investigated by DFT. Our DFT calculation results indicates that some mid-levels are formed between the CB and VB of TiO2-X, which is beneficial to its visible absorption ability. The calculation results also show that bridge of TiOC formed between g-CS and TiO2-X may let the composite possesses superior charge separation property. The experimental results demonstrate that the obtained 3D g-CS@TiO2-X can serve as an effective visible-light-driven photocatalyst for hydrogen production and organic pollutants degradation. The good visible light absorption introduced by Ti3+ and 3D g-CS, the well-known electron transfer capability of g-CS, the matched electron structures of TiO2-X and g-CS, and perfect interface between TiO2-X and g-CS (TiOC bridge), efficiently boost visible light photocatalytic performance via one-level electron transfer process.

Universal tight binding model for chemical reactions in solution and at surfaces. III. Stoichiometric and reduced surfaces of titania and the adsorption of water.

We demonstrate a model for stoichiometric and reduced titanium dioxide intended for use in molecular dynamics and other atomistic simulations and based in the polarizable ion tight binding theory. This extends the model introduced in two previous papers from molecular and liquid applications into the solid state, thus completing the task of providing a comprehensive and unified scheme for studying chemical reactions, particularly aimed at problems in catalysis and electrochemistry. As before, experimental results are given priority over theoretical ones in selecting targets for model fitting, for which we used crystal parameters and band gaps of titania bulk polymorphs, rutile and anatase. The model is applied to six low index titania surfaces, with and without oxygen vacancies and adsorbed water molecules, both in dissociated and non-dissociated states. Finally, we present the results of molecular dynamics simulation of an anatase cluster with a number of adsorbed water molecules and discuss the role of edge and corner atoms of the cluster.

Sulfur cathodes with hydrogen reduced titanium dioxide inverse opal structure.

Sulfur is a cathode material for lithium-ion batteries with a high specific capacity of 1675 mAh/g. The rapid capacity fading, however, presents a significant challenge for the practical application of sulfur cathodes. Two major approaches that have been developed to improve the sulfur cathode performance include (a) fabricating nanostructured conductive matrix to physically encapsulate sulfur and (b) engineering chemical modification to enhance binding with polysulfides and, thus, to reduce their dissolution. Here, we report a three-dimensional (3D) electrode structure to achieve both sulfur physical encapsulation and polysulfides binding simultaneously. The electrode is based on hydrogen reduced TiO2 with an inverse opal structure that is highly conductive and robust toward electrochemical cycling. The relatively enclosed 3D structure provides an ideal architecture for sulfur and polysulfides confinement. The openings at the top surface allow sulfur infusion into the inverse opal structure. In addition, chemical tuning of the TiO2 composition through hydrogen reduction was shown to enhance the specific capacity and cyclability of the cathode. With such TiO2 encapsulated sulfur structure, the sulfur cathode could deliver a high specific capacity of ∼1100 mAh/g in the beginning, with a reversible capacity of ∼890 mAh/g after 200 cycles of charge/discharge at a C/5 rate. The Coulombic efficiency was also maintained at around 99.5% during cycling. The results showed that inverse opal structure of hydrogen reduced TiO2 represents an effective strategy in improving lithium sulfur batteries performance.

Influence of niobium doping on crystal structure and thermoelectric property of reduced titanium dioxide ceramics

Titanium oxide ceramics doped with niobium is synthesized in reduced atmosphere at 1200 ℃ by conventional solid-state reaction technique. From their crystal structures determined by the powder X-ray diffraction(XRD), the samples have multiple-phase with low Nb concentration, but they have single tetragonal rutile phase when Nb content is larger than 0.02. The electrical conductivities, the Seebeck coefficients and the thermal conductivities of the samples with single phase are measured at a temperature range between room temperature and 900 K. The electrical conductivity and the Seebeck coefficient show non-metallic behaviors. According to the fitting, it is found that the samples show thermal-activation mechanism at low temperatures and small-polaron hopping conduction mechanism at high temperatures. Moreover, the analyses of XRD, electrical conductivity and Seebeck coefficient show that the concentration of oxygen vacancy decreases with Nb content increasing. Thermal conductivity decreases with temperature increasing, dominating by lattice thermal conductivity. In the measurement region, the figure of merit (ZT) reaches a highest value of 0.19 at 873 K in the Ti0.98Nb0.02O2- sample.

A model for the electrochemical reduction of metal oxides in molten salt electrolytes

The reduction of numerous metal oxides is being investigated by electro-deoxidation in molten salts due to the low-oxygen content advantage these processes offer. One of these processes is the Fray–Farthing–Chen (FFC) process that involves the direct reduction of titanium dioxide to titanium. A model for the multi-stage reduction of titanium dioxide to titanium is reported herein. The modeling approach adopted is based on the porous electrode theory used for studying lithium-ion batteries. [J.S. Newman, C.W. Tobias, J. Electrochem. Soc. 109 (12) (1962) 1183; K.E. Thomas, J.S. Newman, T.M. Darling, in: B. Scrosati, W. van Schalkwijk (Eds.), Advances in Lithium-Ion Batteries, Kluwer Academic Publishers, New York, 2002; J.S. Newman, K.E. Thomas, Electrochemical Systems, 3rd ed., Wiley-Interscience, 2004; V. Srinivasan, J.S. Newman, J. Electrochem. Soc. 151 (10) (2004) A1517; V. Srinivasan, J.S. Newman, J. Electrochem. Soc. 151 (10) (2004) A1530; J. Christensen, V. Srinivasan, J.S. Newman, J. Electrochem. Soc. 153 (3) (2006) A560; J.S. Newman, K.E. Thomas, H. Hafezi, D.R. Wheeler, J. Power Sources 119 (SI) (2003) 838; C.R. Pals, J.S. Newman, J. Electrochem. Soc. 142 (10) (1995) A3274]. The reduction of an individual sintered pellet of TiO 2 as it undergoes electro-deoxidation in a molten salt bath of CaCl 2 is modeled. This model has been applied to study the effect of physical variables such as the porosity, radius of pellet, radius of grains (particle size) in the pellet and the effect of starting with a partially reduced oxide. The effect of calcium titanate formation on the reduction is also incorporated and it is shown that if the reduction is started with a pellet of partially reduced titanium dioxide, one can avoid titanate formation. Though the FFC process is not completely successful in complete reduction due to titanate formation and needs to be developed further it has succeeded in the reduction of metals that are not as complex as the titanium oxygen system. This model could also be used for studying the reduction of other metal and mixed oxides such as SiO 2, NiO, Cr 2O 3, Nb 2O 5 and ZrO 2 that are simpler to reduce than TiO 2.

Interesting magnetic behavior from reduced titanium dioxide nanobelts

The magnetic behavior of titanium dioxide nanobelts has been investigated with and without Co doping. Room temperature ferromagnetism was observed when the Co-doped anatase TiO2 nanobelts were prepared via vacuum annealing of 2.5at.% Co-doped titanate nanobelts, while annealing them in air resulted in paramagnetic ordering. Interestingly, by vacuum annealing the undoped titanate nanobelts under the same conditions, superparamagnetic ordering was observed in the resulting anatase TiO2 nanobelts. The electron paramagnetic resonance of this latter sample shows a strong symmetrical signal at g=2.003 suggesting some sort of exchange interactions among the localized electrons’ spin moments from single electron trapped in oxygen vacancies.

Room-temperature ferromagnetic properties in Mn-doped rutile [formula omitted] thin films

Room-temperature ferromagnetic properties of Mn-doped reduced titanium dioxide ( Ti 1 - x Mn x O 2 - δ ) thin films with rutile structure synthesized by a sol–gel method have been detected for a limited range of Mn composition ( x ) . The Ti 1 - x Mn x O 2 - δ films were found to be p-type semiconducting with hole concentration near 10 19 cm - 3 . The observed ferromagnetism in the Ti 1 - x Mn x O 2 - δ films is not related to the hole concentration but related to x. The room-temperature ferromagnetism is attributable to magnetic polaron formed by trapped electron in oxygen vacancy and magnetic ions around it. Thus, the existence of oxygen vacancies is necessary for the room-temperature ferromagnetism.