Defective TiO2 Research Articles

Recent progress on photocatalytic evolution of hydrogen gas over TiO2-x-based emerging nanostructures

In the recent decade, there has seen a significant advancement in TiO2-based photocatalytic production of hydrogen, as a powerful alternative to fossil fuels and in coping with the corresponding environmental pollution. Among the explored strategies for promoting the performance of pure TiO2, induction of structural defect in this semiconductor has been proven as robust approach for effective H2 evolution. This review brings together the recent information on emerging approaches to induce Ti3+ species and oxygen vacancies (OVs) in the structure of TiO2 and the related nanostructures/hybrids. Moreover, the employed characterization techniques and evidences of OVs and Ti3+ species presented in various TiO2-based structures were thoroughly discussed and depicted. Furthermore, the key determinants for enhanced H2 production over defective TiO2 were underlined for TiO2-x with homo/heterophase structures and/or doped with other materials, and in the presence/absence of cocatalyst. Besides, the plausible charge carriers transfer route in TiO2-x-based photocatalysts was indicated for H2 production under light illumination. The provided information in this review will light up the way toward economically viable and practical approaches for fabricating defective TiO2 nanomaterials, detecting defects, and producing H2 at higher rates and yields.

MXene-intercalation induced ordered brick-mortar structures of allomorph junctions for enhanced flexibility in TiO2 nanofibers and photocatalytic efficiency

Continuous TiO2 nanofibers (NFs) with the merits of high photochemical activity, adjustable active sites, and easy to recycle have triggered a huge interest in the catalysis fields. Nevertheless, they are easy to break due to the intrinsic brittleness of ceramic TiO2. Here, we report a Mxene-intercalation toughening strategy to fabricate flexible TiO2 NF films with a sol–gel electrospinning method followed by a low-temperature calcination. The in-situ doping of MXene (Ti3C2Tx) nanosheets in TiO2 NFs reduces the grain sizes and introduces 2D defective MXene-derived TiO2 planes, the former creates sinuous grain boundaries that can reduce crack propagation, while the latter generates elastic brick–mortar structures of allomorph junctions that can absorb more external work. The dual functions remarkably enhance the breaking strength of TiO2 NFs from 0.06 to 0.16 MPa. Fascinatingly, this strategy reduces the bandgap of TiO2 and more efficiently conduct photogenerated carriers. The evolution rate of photocatalytic CO2 reduction into CH4 reaches 31.6 μmol/g/h without using sacrificing agents and photosensitizers. The reported facile strategy simultaneously enhances the flexibility of TiO2 nanofibers and photocatalytic efficiency.

Defective TiO2- x for High-Performance Electrocatalytic NO Reduction toward Ambient NH3 Production.

Synthesis of green ammonia (NH3 ) via electrolysis of nitric oxide (NO) is extraordinarily sustainable, but multielectron/proton-involved hydrogenation steps as well as low concentrations of NO can lead to poor activities and selectivities of electrocatalysts. Herein, it is reported that oxygen-defective TiO2 nanoarray supported on Ti plate (TiO2- x /TP) behaves as an efficient catalyst for NO reduction to NH3 . In 0.2m phosphate-buffered electrolyte, such TiO2- x /TP shows competitive electrocatalytic NH3 synthesis activity with a maximum NH3 yield of 1233.2µgh-1 cm-2 and Faradaic efficiency of 92.5%. Density functional theory calculations further thermodynamically faster NO deoxygenation and protonation processes on TiO2- x (101) compared to perfect TiO2 (101). And the low energy barrier of 0.7eV on TiO2- x (101) for the potential-determining step further highlights the greatly improved intrinsic activity. In addition, a Zn-NO battery is fabricated with TiO2- x /TP and Zn plate to obtain an NH3 yield of 241.7µgh-1 cm-2 while providing a peak power density of 0.84mWcm-2 .

Improving photocatalytic performance of defective titania for carbon dioxide photoreduction by Cu cocatalyst with SCN- ion modification

Titanium dioxide (TiO2) with defects is a promising semiconductor photocatalyst for photoreduction CO2, due to its unique electronic properties. However, defective TiO2 is difficult to achieve a high yield of CO2 photoreduction, especially the high-value-added C2+ hydrocarbons, due to the slow transfer of multielectron/proton and sluggish C-C coupling kinetics. Here, we combine the photoinduced deposition of copper (Cu) nanoparticle on defective titania (TiO2-SBO) with thiocyanate anion (SCN-, KSCN) surface modification via impregnation route to prepare a series of high performance photocatalysts (SCN-Cu/TiO2-SBO). It was found that the production rate and selectivity of C2H4 of CO2 over sample SCN-Cu/TiO2-SBO-3 as a representative are 4.7 μmol·g−1·h−1 and 40%, while the CO and CH4 yields rise by 2 times and 5 times as compared with those over bare TiO2-SBO. This is because the Cu cocatalyst can serve as the active site for C-C coupling (via intermediate *CO-COH) and the SCN-induced surface dipole effect can enhance the carrier’s dynamic behavior and accelerate the multielectron transfer, while the SCN- ions with nucleophilicity are conducive to CO2 adsorption as well as CO hydrogenation and the multi-proton coupling process of C2+ products. Furthermore, first principle calculations illustrate that the Cu species can effectively reduce the reaction energy of the key intermediate *CO-COH of ethylene, and SCN- ions benefit to the adsorption of CO2 molecules, and thereby being favor for the generation of ethylene.

Physicochemical properties of Fe-doped TiO2 and the application in Dye-sensitized solar cells

Fe-doped TiO2 was here used as a semiconductor material for Dye-sensitized solar cells (DSSCs) to improve the solar energy conversion to electricity as compared to precursor TiO2. The physicochemical properties of Fe-doped TiO2 were clarified by different techniques such as X-ray diffractions, Transmission Electron Microscope, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV–vis Diffuse Reflectance Spectra, and Photoluminescence Spectra. The material resulted in a defective TiO2 with oxygen vacancies and Ti3+ along with Fe ions into the structure. The formation of superficial defects contributed to the prevention of electron-hole recombination, thereby enhancing the lifetime of the photoelectrons. The presence of defects was measured by the deconvolution of Ti 2p and O 1s, while the low recombination rate was calculated with the PL spectrum of the semiconductor. The higher lifetime of the e−/h+ pairs lead to a result with the efficiency of Fe–TiO2-based DSSC increased to 30% in comparison with undoped-TiO2-based DSSC.

Oxygen Vacancy Diffusion in Rutile TiO2: Insight from Deep Neural Network Potential Simulations

Defects play a crucial role in the surface reactivity and electronic engineering of titanium dioxide (TiO2). In this work, we have used an active learning method to train deep neural network potentials from the ab initio data of a defective TiO2 surface. Validations show a good consistency between the deep potentials (DPs) and density functional theory (DFT) results. Therefore, the DPs were further applied on the extended surface and executed for nanoseconds. The results show that the oxygen vacancy at various sites are very stable under 330 K. However, some unstable defect sites will convert to the most favorable ones after tens or hundreds of picoseconds, while the temperature was elevated to 500 K. The DP predicated barriers of oxygen vacancy diffusion were similar to those of DFT. These results show that machine-learning trained DPs could accelerate the molecular dynamics with a DFT-level accuracy and promote people's understanding of the microscopic mechanism of fundamental reactions.

Defective TiO2-Supported Dual-Schottky Heterostructure Boosts Fast Reaction Kinetics for High Performance Lithium-Ion Storage

TiO2, as a potential anode for lithium-ion batteries, suffers from low theoretical capacity and inferior reaction kinetics. Herein, we innovatively designed a unique dual-Schottky regulated ternary defective-TiO2-supported Co-nanodots-anchored N-doped-carbon-coated (defective TiO2@Co@NC) electrode, which was expected to possess advanced electrochemical reaction kinetics and increased Li-ion capacity. Molecular mechanics calculations indicated that the isolated cobalt nanodot can be stably formed in defective TiO2@Co@NC. And such a Co nanodot can not only firmly bridge the defective TiO2 matrix and Co-embedded N-doped carbon coating forming a dual-Schottky heterostructure, which boosts the fast reaction kinetics, but also effectively suppress the irreversibility of Li-ion intercalation. Correspondingly, as-designed binder-free defective TiO2@Co@NC anode was successfully synthesized via carbonization of the 3D hierarchical defective TiO2@Co-MOF precursor. In particular, the defective TiO2 nanotube array was for the first time used as nonreactive template to prepare Co-MOF composites. And TEM characterizations revealed that plentiful isolated cobalt nanodots were factually formed in derivative defective TiO2@Co@NC. As expected, defective TiO2@Co@NC electrodes exhibit remarkably high areal capacity (1191.2 μAh cm–2/490.9 mAh g–1 at 100 μA cm–2/41 mA g–1, 2.8 times of the pristine anatase TiO2 anodes), excellent cyclic stability (a capacity fading rate of 0.026% per cycle, at 500 μA cm–2/206.0 mA g–1 for 600 cycles), and superb rate properties (405.0 μAh cm–2/166.9 mAh g–1 at 1000 μA cm–2/412.0 mA g–1). Furthermore, the realization of in situ growth of uniform Co-MOF coatings with controllable thickness in defective TiO2@Co-MOF precursors facilitates the preparation of defective TiO2@Co@NC derivatives with tailored specific capacities, which paves the way for future research on MOF composites and Li-ion batteries.

A Type-I Heterojunction by Anchoring Ultrafine Cu2O on Defective TiO2 Framework for Efficient Photocatalytic H2 Production

The metal–organic framework (MOF) contains abundant metal ions and organic ligands, and is an excellent self-sacrificial template or precursor for further assembly of functional composites. Herein, MOF-derived defective TiO2 anchored by ultrafine Cu2O nanoparticles has been successfully prepared. In situ formation of type-I heterojunctions between defective TiO2 and ultrafine Cu2O nanoparticles and their application in photocatalytic H2 evolution were demonstrated. Series of composites with varying amounts of Cu2O from 1% to 10% were synthesized. 5 wt % Cu2O loading leads to a H2 generation activity of 4.81 mmol g –1 h –1 under UV–visible light, which is about 3.2 times larger than that of MOF-derived defective TiO2 (1.49 mmol g –1 h –1). It is found that the existence of Ti3+ and Ov defects in the prepared Cu2O/D-TiO2 (defective TiO2) introduces a new local energy level in the forbidden band, which is responsible for effective charge separation. The uniform distribution of Cu2O nanoparticles in defective TiO2 enables close surface contact, enhanced visible light absorption, and electron-transfer efficiency through the formation of heterojunction. This work provides valuable guidelines for exploiting high-performance MOF-derived photocatalysts for the photocatalytic H2 evolution.

A Review of the Single-Step Flame Synthesis of Defective and Heterostructured TiO2 Nanoparticles for Photocatalytic Applications

Titanium dioxide (TiO2) is an excellent UV-photocatalytic material that is widely used in various applications, including clean energy production, environmental remediation, and chemical production. However, the use of TiO2 is limited in the field of visible light photocatalysis due to its large bandgap and fast recombination rate between electron and hole pairs, which generally results in a low photocatalytic reaction. Defect/bandgap engineering by doping and the introduction of heterojunctions has been successfully employed to improve the photocatalytic activities of TiO2 over a wide wavelength. To apply the unconventional structured TiO2 with high photocatalytic performance to industries, the development of efficient methods for large-scale production is of high importance. Flame synthesis is a very promising method for the rapid production of nanoparticles. In this article, we summarize the latest reports on the synthesis of defective and heterostructured TiO2 using the single-step method of flame synthesis. Fundamental understandings of reactor configurations, synthesis conditions, precursor preparation and their physicochemical properties are intensively discussed.

Selective photoelectrochemical oxidation of glucose to glucaric acid by single atom Pt decorated defective TiO2

Photoelectrochemical reaction is emerging as a powerful approach for biomass conversion. However, it has been rarely explored for glucose conversion into value-added chemicals. Here we develop a photoelectrochemical approach for selective oxidation of glucose to high value-added glucaric acid by using single-atom Pt anchored on defective TiO2 nanorod arrays as photoanode. The defective structure induced by the oxygen vacancies can modulate the charge carrier dynamics and band structure, simultaneously. With optimized oxygen vacancies, the defective TiO2 photoanode shows greatly improved charge separation and significantly enhanced selectivity and yield of C6 products. By decorating single-atom Pt on the defective TiO2 photoanode, selective oxidation of glucose to glucaric acid can be achieved. In this work, defective TiO2 with single-atom Pt achieves a photocurrent density of 1.91 mA cm−2 for glucose oxidation at 0.6 V versus reversible hydrogen electrode, leading to an 84.3 % yield of glucaric acid under simulated sunlight irradiation.

Defective TiO2 nanomaterial for photocatalytic degradation of carbamazepine drug

The remarkable optical properties of defective TiO2 make it an exceptional photocatalyst showing high photocatalytic activity under ultraviolet–visible (UV–Vis) and near-infrared (NIR) irradiations. Herein, a facile technique is provided to synthesize colored TiO2 by degassing TiO2 obtained by a microwave-assisted hydrothermal method. The optical bandgap of TiO2 was shifted from 3.2 eV to 3.06 eV with the formation of new energy levels located below the conduction band at about 1.96 eV. The defective TiO2 was investigated for the photocatalytic degradation of carbamazepine, one of the persistent drugs in water systems and an emerging environmental concern. The highest photocatalytic degradation rate (94.6%) was achieved within 240 min under sunlight irradiation, while 66.3% and 80.7% of 20 ppm CBZ were degraded within 360 min under simulated solar light and visible light. Moreover, the total organic carbon (TOC) analysis demonstrates that about 88.84% was mineralized within 240 min under sunlight irradiation. This modification strategy of TiO2 is found to be one of the best available pathways to achieve a high photocatalytic performance of TiO2 under sunlight irradiation for the degradation of non-removed drugs such as carbamazepine in wastewater treatment plants.

Mechanism of photocatalytic reduction of CO2 to CH4 on F-doped defective anatase TiO2(101) surface: A density functional theory study

Photocatalytic reduction of CO2 into CH4 has attracted comprehensive concerns as an effective solution to meet the energy crisis and global warming. F-doped defective anatase TiO2 was reported to effectively improve the photocatalytic efficiency and the selectivity of CH4 generation. However, the mechanism behind it remains unclear. Herein, we systematically studied the photocatalytic pathway of CO2 reduction to CH4 on F-doped defective anatase TiO2(101) by density functional calculations. First, we found that F doping leads to the enhanced reduction potential of photoexcited electrons. Furthermore, we identified a thermodynamically feasible reaction path: CO2→CO/HCOO→CHO→CH2O→CH3O→CH3OH→CH3→CH4. The whole reaction is exothermic, elucidating an extremely strong reduction activity of the TiO2(101) surface. Our work provides some new insights into the CO2 reduction mechanisms of F-doped TiO2, which will help to design TiO2-related photocatalysts with high catalytic performance.

Enhanced localized electron density from PdCu nanoparticle loading on a defective TiO2 support for selective nitrate electroreduction to ammonia

The localized electrons of OVs on a TiO2−x support are regulated by PdCu nanoparticles, leading to accelerated hydrogenation and inhibited N2 formation; hence PdCu NPs/TiO2−x exhibits superior NH3 selectivity and yield.

Fast carrier diffusion via synergistic effects between lithium-ions and polarons in rutile TiO2.

Carrier mobility in titanium dioxide (TiO2) systems is a key factor for their application as energy materials, especially in solar cells and lithium-ion batteries. Studies on the diffusion of Li-ions and polarons in rutile TiO2 systems have attracted extensive attention. However, how their interaction affects the diffusion of Li-ions and electron polarons is largely unclear and related studies are relatively lacking. By using first-principles calculations, we systematically investigate the interaction between the intercalated Li-ions and electron polarons in rutile TiO2 materials. Our analysis shows that the diffusion barrier of the electron polarons decreases around the Li-ion. The interaction between the Li-ions and polarons would benefit their synergistic diffusion both in the pristine and defective rutile TiO2 systems. Our study reveals the synergistic effects between the ions and polarons, which is important for understanding the carrier properties in TiO2 systems and in further improving the performance of energy materials.

Correction: Core–shell defective TiO2 nanoparticles by femtosecond laser irradiation with enhanced photocatalytic performance

Correction for ‘Core–shell defective TiO2 nanoparticles by femtosecond laser irradiation with enhanced photocatalytic performance’ by Bersu Bastug Azer et al., Mater. Adv., 2023, https://doi.org/10.1039/d3ma00019b.

Core–shell defective TiO2 nanoparticles by femtosecond laser irradiation with enhanced photocatalytic performance

Herein, the authors present a new defect formation mechanism related to the laser wavelength. Short pulse width lasers only modify the surface layer, resulting in core-shell defective TiO2. Unlike long pulse lasers, the bulk of particles remains unchanged.

Anatase TiO2-x and zwitterionic porphyrin polymer-based nanocomposite for enhanced cancer photodynamic therapy.

Photodynamic therapy has been used as a treatment option for cancer; however, the existing TiO2 photosensitizer does not have the ability to specifically target cancer cells. This lack of selectivity reduces its effectiveness in overcoming cancer resistance. To improve photodynamic therapy outcomes, an innovative solution is proposed. In this study, we report on the compounding of a zwitterionic covalent organic polymer (COP) with a TiO2 photosensitizer for the first time. The aim is to overcome cancer cellular resistance. A one-pot synthetic strategy, which includes the construction of a porphyrin-based COP has been employed. This strategy has also been applied to the rapid preparation of anatase defective TiO2 (TiO2-x). To improve the hydrophilic and antifouling properties of the polymer, zwitterion L-cysteine has been conjugated with a porphyrin-based COP using a thiol-ene "click chemistry" reaction. The novel zwitterionic porphyrin-based COP has the ability to trigger biodegradation under the acid microenvironment due to the presence of acid-sensitive β-thioether esters. When combined with TiO2-x, the resultant nanocomposite produces an enhanced photodynamic therapy effect for drug-resistant cancer cells under NIR laser irradiation. This is due to the strong mutual sensitization of zwitterionic porphyrin-based COP and TiO2-x. Importantly, the nanocomposite delivery system exhibits excellent cytocompatibility in the dark and has the potential to improve the accuracy of cancer diagnosis through fluorescence imaging. The results of this study demonstrate the potential application of this alternative nanocomposite delivery system for remote-controllable photodynamic therapy of tumors.

Metalloporphyrin modified defective TiO2 porous cages with the enhanced photocatalytic activity for coupling of hydrogen generation and tetracycline removal.

Integration of molecular transition-metal complexes and semiconductors is an appealing method to develop high-performance hybrid photocatalysts based on improvement of their solar energy harvesting ability and photogenerated charge carrier separation efficiency. Herein, Cu-TCPP modified TiO2 porous cages with oxygen vacancy defects, derived from NH2-MIL-125(Ti) nanocrystals, are successfully prepared to form PC-TiO2-d/Cu-TCPP hybrids via a surface assembly process. The PC-TiO2-d/Cu-TCPP hybrid shows an enhanced photodegradation efficiency (73.7%, 95.4%) towards tetracycline in the air under visible light or the simulated sunlight irradiation compared to PC-TiO2-d (33.7%, 81.1%) within 100 min. Moreover, the photocatalytic system is applicable to coupling both processes of solar fuel production and pollutant degradation. The PC-TiO2-d/Cu-TCPP hybrid exhibits a high hydrogen evolution rate of ∼2 mmol g-1 h-1 in the aqueous solution of tetracycline in an inert atmosphere upon irradiation by the simulated sunlight. In contrast, an inferior photocatalytic performance of hydrogen evolution is observed in pure water without the addition of tetracycline. Finally, the high sustainability of PC-TiO2-d/Cu-TCPP is mainly attributed to the strong interaction between the molecular photosensitizer and the semiconductor photocatalyst by oxygen vacancies and Cu(ii) ions.

Pd supported on defective TiO2 polymorphic mixtures: Effect of metal-support interactions upon glycerol selective oxidation

Palladium catalysts supported on defective mixes of anatase, TiO2 (II) and rutile crystalline phases, previously obtained by high-energy ball milling, were synthesized and tested for glycerol selective oxidation. A deep characterization of these unusual materials was carried out to elucidate catalytic and physicochemical features. Electron density transfer from support to metal or vice versa, depending on the polymorphs present, could not only alter palladium particle sizes and its surface oxidation state but also reducibility and oxygen mobility of catalysts. Furthermore, acid-base properties achieved also influenced catalytic activity under mild conditions of liquid-phase glycerol oxidation. A conversion of 94% and a selectivity to glyceric and lactic acids of 48% and 22% respectively were obtained for the Pd catalyst supported on mechanochemically activated anatase. The presence of several polymorphs in a metal oxide support could therefore benefit or handicap catalytic cycle for a particular reaction. Metal-support interactions play a key role in heterogenous catalysts and thus the rational design of supports comes on the scene.

In situ evolved defective TiO2 as robust support for CoB-catalyzed hydrolysis of NaBH4

Sodium borohydride (NaBH4) has been proposed to be a realizable hydrogen supplier for proton-exchange membrane fuel cell. Cobalt boride (CoB) is one of the most promising non-precious metal-based catalysts toward NaBH4 hydrolysis. In this work, we develop a facile method to synthesize defects-rich TiO2 supported CoB nanoparticles (CoB/TiO2-x). In this CoB/TiO2-x, an electronic metal-support interaction is induced by transferring electrons from the defective TiO2 to CoB to form electron-rich CoB particles, facilitating the hydrolysis of NaBH4. The as-synthesized CoB/TiO2-x catalyst shows a good catalytic performance with hydrogen generation rate of 3070 mL min−1 gCo−1 and activation energy of 57.03 kJ mol−1.