Anatase TiO2 Nanosheets Research Articles

Synthesis and Characterization of Anatase TiO2 Microspheres Self-Assembled by Ultrathin Nanosheets.

In this paper, we report a novel and simple method for synthesizing the microspheres self-assembled from ultrathin anatase TiO2 nanosheets with a high percentage of (001) facets via the hydrolysis process of the single-reagent (potassium fluorotitanate). We then used optical microscopy, scanning electron microscopy, and high-resolution confocal laser Raman spectroscopy to characterize the microspheres generated under different conditions. The study found that the size of the anatase TiO2 microspheres synthesized was 0.5–3 μm. As the synthesis time increased, the corroded surface of the microspheres gradually increased, resulting in the gradual disappearance of the edges and corners of the anatase nanosheets. The exposure percentage of the (001) facets of ultrathin anatase nanosheets synthesized for 2 h at 180–200 °C are close to 100%. The microsphere whose surface is completely covered by these anatase nanosheets also has nearly 100% exposed (001) facets. This new anatase nanosheet-based self-assembled microsphere will have great application potential in pollution prevention, environmental protection, and energy fields.

Photoelectrochemical performance of facet-controlled TiO2 nanosheets grown hydrothermally on FTO.

Single crystal anatase TiO2 nanosheets (TiO2-NSs) are grown hydrothermally on fluorine-doped tin oxide (FTO). By systematically changing the hydrothermal conditions such as reaction time, initial concentration of Ti precursor, F precursor, and HCl as an additive, a wide variety of TiO2-NSs, with different morphologies and faceting have been synthesized. For the different morphologies and different facet ratios (anatase S001/S001+101), the photoelectrochemical response is characterized and compared. We find that for photoanodes in neutral electrolytes, the magnitude of the photocurrent depends strongly on the growth parameters, that is, peak IPCEs can vary from 11.7% to 61%. For a wide range of parameters, the key parameter deciding on the photocurrent is the effective electrochemically active area of the electrode. Only for very high facet ratios >91%, the photoresponse can be strongly influenced by faceting – for samples with a S001/S001+101 of 91%, IPCE value of ≈84% is obtained. This work defines not only optimized synthesis conditions for a most effective growth of these single crystalline electrode, but also represents fundamental data for further applications of such electrodes.

Au-Pd nanoparticles immobilized on TiO2 nanosheet as an active and durable catalyst for solvent-free selective oxidation of benzyl alcohol

TiO2nanocrystals with controlled facets have been extensively investigated due to their excellent photocatalytic performance in sustainable and green energy field. However, the applications in thermal catalysis without applying UV irradiation are comparably less and the identification of their intrinsic roles, especially the different catalytic behaviors of each crystal facet, remains not fully recognized. In this study, bimetallic AuPd nanoparticles supported on anatase TiO2 nanosheets exposing {001} facets or TiO2 nanospindles exposing {101} as a catalyst were prepared by sol-immobilization method and used for solvent-free benzyl alcohol oxidation. The experimental results indicated that the exposed facet of the support has a significant effect on the catalytic performance. AuPd/TiO2-001 catalyst exhibited a higher benzyl alcohol conversion than that of the AuPd/TiO2-101. Meanwhile, all the prepared AuPd/TiO2 catalysts were characterized by XRD, ICP-AES, XPS, BET, TEM, and HRTEM. The results revealed that the higher number of oxygen vacancies in TiO2-sheets with the exposed {001} facets of higher surface energy could be responsible for the observed enhancement in the catalytic performance of benzyl alcohol oxidation. The present study displays that it is plausible to enhance the catalytic performance for the benzyl alcohol oxidation by tailoring the exposed facet of the TiO2 as a catalyst support.

Nitrogen doping-mediated oxygen vacancies enhancing co-catalyst-free solar photocatalytic H2 production activity in anatase TiO2 nanosheet assembly

Introducing oxygen vacancies (Vo) into TiO2 photocatalyst has been considered an effective strategy for improving co-catalyst-free solar photocatalytic activity. However, the methods used to synthesize it require high pressure/temperature and/or hazardous/costly reagents. Here we propose Vo introduction, concomitant with N-doping in TiO2, as an alternative strategy for achieving efficient co-catalyst-free solar photocatalytic activity under less extreme conditions. After calcination at 450 °C of mesoporous spherical assemblies of a layered titanate nanosheet containing N,N-dimethylformamide as its synthesis solvent in the structure, we successfully synthesized mesoporous spherical assemblies of nanosheets composed of anatase TiO2 nanoparticles with Vo mediated by N doping. This material exhibits good co-catalyst-free solar photocatalytic activity for hydrogen evolution via water splitting under irradiation with simulated solar light, which is considerably higher than that of typical co-catalyst-free defective TiO2 materials. We discuss the possible role of the introduced Vo in facilitating charge separation and raising photocatalytic efficiency.

Engineering self-doped surface defects of anatase TiO2 nanosheets for enhanced photocatalytic efficiency

Anatase TiO2 nanosheets with controlled locations of defects in the bulk or on the surface were successfully synthesized by finely adjusting the temperatures and amounts of added HF during hydrothermal synthesis. The TiO2 nanosheets with dominant surface defects exhibit much higher activity in photocatalytic water reduction to hydrogen than those with dominant bulk defects, unambiguously demonstrating the promotion effect of surface defects on the TiO2 photocatalysis. The surface defects can be act as the site both for trapping the photogenerated charges on the surface and for adsorbing and activating reactants, subsequently facilitating the photogenerated charges-participated surface reactions. These results offer a novel strategy to enhance the photocatalytic efficiency of TiO2 photocatalysts via engineering self-doped surface defects.

Controllable Synthesis of Anatase TiO2 Nanosheets Grown on Amorphous TiO2/C Frameworks for Ultrafast Pseudocapacitive Sodium Storage.

Pseudocapacitance has been confirmed to significantly improve the rate capability and cycling durability of electrode materials. However, rational design and controllable synthesis of intercalation pseudocapacitive materials for sodium-ion batteries (SIBs) still remain greatly challenging. Herein, a core-shell TiO2-based anode composed of S-, Co-, and N-doped amorphous TiO2/C framework cores and ultrathin anatase TiO2 nanosheet shells (SCN-TC@UT) was synthesized using Ti-based metal-organic frameworks (Ti-MOFs) as self-sacrificing templates coupled with a solvothermal sulfidation process. Thanks to heteroatom doping, integration of carbon species, and 2D nanosheet coating, the kinetic properties of SCN-TC@UT have been significantly improved. As a consequence, the anode achieves ultrahigh capacitive contributions up to 90.9 and 96.3% of the total capacity at scan rates of 5 and 10 mV s-1 and delivers unprecedented capacities of 211, 201, and 100 mA h g-1 at 1, 5, and 30 C (1 C=335 mA g-1) for over 800, 2000, and 18,000 cycles, respectively. Even at an ultrahigh rate of 50 C, the anode can still deliver a capacity of 108 mA h g-1. This work demonstrates the most efficient TiO2-based anode ever reported for SIBs and holds great potential in directing the development of amorphous materials for intercalation pseudocapacitance.

Construction of anatase@rutile core@shell TiO2 nanosheets with controllable shell layer thicknesses for enhanced ethanol sensing

Anatase@rutile core@shell TiO2 (A@R-TiO2) nanosheets with varied thicknesses of rutile shell layers have been constructed via two-step hydrothermal processes. Rutile shell layers with controllable thicknesses (1.9–6.8 nm) are generated on the surfaces of anatase TiO2 (A-TiO2) nanosheets by regulating HCl concentrations and hydrothermal reaction times. This new type of A@R-TiO2 nanosheets exhibits an evident enhancement in ethanol sensing response and a reduction in optimum operation temperature in comparison with A-TiO2 nanosheets. A rutile shell layer thickness-dependent ethanol sensing response is revealed. The A@R-TiO2 nanosheets with a shell layer of approximately 5.0 nm show the highest response of 43.9–500 ppm ethanol at 270 °C, which is 4.35 times higher than that of A-TiO2 nanosheets. The formation of TiO2 polymorphism junctions leads to the electron transferring from inner anatase cores to outer rutile shells and the enrichment of active O− species on the surfaces of rutile shells, contributing greatly to the improved ethanol sensing properties of A@R-TiO2 nanosheets. The unique core@shell structure with enhanced surface area is also responsible for the further resistance modulation and increased ethanol sensing performances of A@R-TiO2 nanosheets.

Synthesis and characterization of TiO2-based nanostructures via fluorine-free solvothermal method for enhancing visible light photocatalytic activity: Experimental and theoretical approach

Surface-fluorinated anatase TiO2 nanosheets have been widely prepared using hydrofluoric acid (HF) as morphology controlling agent with several titanium alkoxides. However, hazardous and other scientific problems associated with HF are major obstacles towards its global commercialization. Therefore, this study introduces a facile, green and fluorine-free solvothermal synthetic approach to prepare anatase TiO2 nanosheets (TNSs), involving commercial TiO2 nanoparticles as titanium source and N, N dimethylformamide (DMF) as morphology controlling agent under alkaline environment. A Box-Behnken-Design 3-level experimental design was applied to optimize the major key synthesis parameters for tuneable TNSs performance by manipulating the reaction temperature (120−180 °C), reaction time (3−48 hr) and dose of DMF (5−40 mL). The proposed model was verified through the analysis of variance and performing experiments at predicated optimal conditions (temperature =165.35 °C, time = 25.50 h and DMF dose =22.50 mL). The quadratic model was found to be best-fitted with the maximum correlation coefficients (R2 = 0.989 and adjusted R2 = 0.975) between predicted and actual results. The crystal growth mechanism of TNSs was also proposed. The formation of tetragonal TNSs (10 nm) containing the highest surface area with least bandgap upto 2.62 eV, confirmed through XRD, TEM, BET, DRS, FTIR and XPS measurements. The excellent photocatalytic performance was exhibited by as-developed TNSs for phenol degradation upto 95.33 % under visible-light. The photoactivity of TNSs was found to be strongly associated with synthesis parameters including hydrothermal temperature, reaction time and DMF dose. The phenol degradation data was well explained via pseudo-first-order kinetics and Langmuir isotherm models. Thermodynamic parameters were suggested the nature of phenol degradation process. This enhanced degradation activity of TNSs was attributed to the well-defined morphology, larger surface area, improved energy harvesting aptitude and abated recombination rate of photogenerated electron hole-pairs resulting an enhancement in the photogenerated charge carrier.

In Situ Grown Monolithic Au/TiO2 Catalysts on Flexible Ti Mesh for Efficient Low‐Temperature CO Oxidation

AbstractThe development of advanced catalyst integration strategy is of great importance and promising for long‐term and stable gas catalysis in environmental processing. However, conventional catalyst fabrication techniques including spray method and spin coating cannot overcome the key problems of catalyst peeling off from substrate and related severe performance degradation. In this work, an in situ integration strategy of monolithic Au/TiO2 catalysts on Ti mesh for efficient and stable low‐temperature CO oxidation is reported. Plasma electrolytic oxidation technology can create a porous TiO2 seeding layer with strong substrate adherence. Subsequent hydrothermal reaction, ion exchange, and calcination processes can induce the in situ nucleation of sodium titanate nanosheets and their follow‐up conversion to TiO2 nanosheets with huge surface area. The oxygen defects generated in anatase TiO2 nanosheet supports can not only provide sufficient anchoring sites for the deposition and attachment of gold nanoparticles (Au NPs) during deposition–precipitation process, but also absorb and activate oxygen species. Meanwhile, the strong interactions between Au NPs and TiO2 support make the catalysts more chemically active. As a result, the Au/TiO2 monolithic catalysts can eliminate CO completely at low temperature and exhibit considerably high stability, suggesting their potential in practical applications.

Photocatalytic degradation and adsorption of phenol by solvent-controlled TiO2 nanosheets assisted with H2O2 and FeCl3: Kinetic, isotherm and thermodynamic analysis

Abstract Hydrofluoric acid (HF) has been widely employed to regulate the surface characteristics of TiO2 containing sheet-like morphology with (001) facets for various environmental applications. However, carcinogenic effects associated with HF are the main stumbling blocks on its way towards global commercialization. In the same line of action, an eco-friendly approach for the synthesis of anatase TiO2 nanosheets (TNSs) via employing hydrothermal process and N, N dimethylformamide (DMF) as a novel morphology-controlling agent has been reported. The as-produced TNSs were characterized by XRD, HRTEM, Raman, FTIR, XPS and DRS characterizations. The photoactivity of as-produced TNSs was studied for photocatalytic degradation of phenol under visible light irradiation in the presence of green oxidants such as hydrogen peroxide (H2O2) and ferric chloride (FeCl3) to produce free hydroxyl radicals for speedily reduction of recombination of photogenerated electrons hole-pairs. The results revealed that as-produced TNSs could activate by green oxidants with yielding up to 94.19 and 97.12% phenol degradation in the presence of H2O2 and FeCl3, respectively. Moreover, phenol adsorption data was well explained via pseudo first order and pseudo second order kinetics while Langmuir and Freundlich isotherms were more suitable to explain the adsorption of phenol onto TNSs, providing maximum adsorption capacity up to 23.596 mg/g. Various thermodynamic parameters were evaluated, suggesting the favourable, spontaneous and endothermic adsorption process. The values of activation energy (18.505 kJ/mol) confirmed the physical adsorption of phenol onto TNSs. The excellent aptitude of anatase TNSs to produce hydroxyl radicals and super-oxides radicals with promptly lessening of recombination of photogenerated electron hole-pairs makes them motivated applicant for wastewater treatment.

Ultrafast Response and High Selectivity toward Acetone Vapor Using Hierarchical Structured TiO2 Nanosheets.

For the development of high-performance gas sensors, ultrafast response and high selectivity are critical requirements for many practical applications. An alternative strategy is to employ hierarchical nanostructured materials in gas sensors. In this work, we report newly synthesized TiO2 hexagonal nanosheets with a hierarchical porous structure, which demonstrate an ultrafast gas response and high selectivity toward acetone vapor for the first time. A simple one-step annealing process to prepare hierarchical TiO2 nanosheets derived from layered TiSe2 nanosheet templates is reported. The hierarchical structure interlaced with anatase TiO2 nanosheets showed an open porous characteristic. The average pore size was about 20 nm examined using a high-resolution TEM. The gas sensing properties toward acetone vapor of the novel hierarchical structured TiO2 nanosheets were characterized in detail including optimal operation temperature, sensitivity, selectivity, response/recovery time, and long-term stability. The gas sensing response and recovery times were 0.75 s and 0.5 s, respectively. We attribute these superior response properties to its unique hierarchical pore structure with a high specific surface area. The results show great potential for acetone vapor detection, particularly in dynamic ultrafast monitoring by using the synthesized hierarchical structured TiO2 nanosheets.

Engineering surface defects on two-dimensional ultrathin mesoporous anatase TiO2 nanosheets for efficient charge separation and exceptional solar-driven photocatalytic hydrogen evolution

Two-dimensional ultrathin mesoporous anatase TiO2 nanosheets with engineered surface defects are fabricated via evaporation-induced self-assembly, solvothermal and hydrogenation, which exhibit excellent photocatalytic hydrogen evolution.

TiO2 nanosheet/NiO nanorod hierarchical nanostructures: p–n heterojunctions towards efficient photocatalysis

TiO2 nanosheet/NiO nanorod heterojunction hybrids have been developed through a hydrothermal route, where NiO nanorods (size: 5 nm in diameter and 20–40 nm in length) are deposited at the {0 0 1} facet of anatase TiO2 nanosheets. The photocatalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 adsorption-desorption analysis, UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy and time-resolved fluorescence. The TiO2/NiO photocatalysts exhibited good photocatalytic activities towards the degradation of methyl blue (MB) and phenol, and hydrogen generation efficiency under visible light irradiation. The maximum rate constant can be reached 0.0279 min−1 and 0.0135 min−1 respectively, which are about 12 and 10 times higher than that of TiO2 nanosheets. And the hydrogen generation efficiency is 10 times higher than physical mixing of TiO2 and NiO. Photocatalytic degradation efficiency remains more than 90% after 6 times cycle dye degradation, and the H2 production efficiency is almost the same after four cycles, suggesting good stability and reusability. The enhanced photocatalytic activities are associated with the rational design of TiO2/NiO hierarchical heterojunctions which ensues high photogenerated charge separation efficiency. With the improved photocatalytic performance, the TiO2/NiO heterojunction hybrids are expected to be potential photocatalysts in environmental and energy related areas.

Hierarchical TiO2 microsphere assembled from nanosheets with high photocatalytic activity and stability

TiO2 hierarchical microsphere assembled from anatase TiO2 nanosheets has been successfully prepared and exhibited excellent catalytic activities of H2 production and benzyl alcohol selective oxidized to benzaldehyde. These enhanced photoactivity can be mainly ascribed to the charge separation promoted by the surface F and nanosheet units, enhancement of substrate molecular transport profited from the hierarchical structure.

TiO2/Graphitic Carbon Nitride Nanosheets for the Photocatalytic Degradation of Rhodamine B under Simulated Sunlight

Surface-fluorinated ultrathin anatase TiO2 nanosheets with exposed (001) crystal planes were synthesized via a relatively low temperature solvothermal route. These two-dimensional (2D) TiO2 nanoshe...

Direct Visualization of Independent Ta Centers Supported on Two-Dimensional TiO2 Nanosheets.

Highly dispersed, supported oxides are ubiquitous solid catalysts but can be challenging to characterize with atomic precision. Here, it is shown that crystalline anatase TiO2 nanosheets (∼5 nm thick) are ideal supports for imaging highly dispersed active sites. Ta cations were deposited by several routes, and high-resolution high angle annular dark-field scanning transmission electron microscopy was used to determine the location of Ta with respect to the TiO2 lattice and quantify Ta-Ta distances. In the best case, it is shown that >80% of Ta atoms are isolated from one another, whereas other techniques are blind to this critical catalytic property or give only qualitative estimates. TiO2 nanosheets may prove to be a useful platform for other types of catalysis studies.

One-pot synthesis of C-dots modified TiO2 nanosheets/UiO-66-NH2 with improved photocatalytic activity under visible light

A simple one-pot method was employed to synthesize C-dots modified TiO2 nanosheets (NS)/UiO-66-NH2 for enhancing photocatalytic activity. Anatase TiO2 NS and UiO-66-NH2 were closely combined and C-dots were well modified on their surfaces. The photocatalytic activities of catalysts were assessed through photodegrading ketoprofen (10 mg/L) under visible light. C-dots/TiO2 NS/UiO-66-NH2 degraded 90% of ketoprofen in 80 min, and owned the higher kinetic rate than TiO2 NS (24 times) or TiO2 NS/UiO-66-NH2 (2.1 times). The results indicated that C-dots/TiO2 NS/UiO-66-NH2 exhibited the most superb visible-light photocatalytic activity. Besides, C-dots/TiO2 NS/UiO-66-NH2 showed good stability and reusability in four runs of degrading ketoprofen. The improved photocatalytic performance was caused by the heterostructure of TiO2 NS and UiO-66-NH2 and photoelectrical performance of C-dots, effectively absorbing visible light and promoting photoelectrons transfer.

Surface Defect-Controlled Growth and High Photocatalytic H2 Production Efficiency of Anatase TiO2 Nanosheets.

Facet engineering of anatase TiO2 by controlling the {001} exposure ratio has been the focus of numerous investigations to optimize photocatalytic activity. In particular, an introduction of fluoride ions during the crystal growth has been demonstrated to be very effective and decisive in realizing the facet exposure of the crystals. However, a key role of fluoride ions in stabilizing {001} exposure and improving subsequent photocatalytic activity of anatase TiO2 remains unclear up to date. Herein, a controlled thickness of anatase TiO2 nanosheets has been realized by introducing different amounts of ethanol into a HF acid-assisted hydrothermal reaction. The thinnest nanosheets with a thickness of ∼2.9 nm were evaluated to have the highest H2 production rate of 41.04 mmol·h-1·g-1 under ultraviolet light irradiation, and the corresponding quantum efficiency was determined to be 41.6% (λ = 365 nm). Moreover, it is proved for the first time that fluoride ions are bonded with Ti vacancies on {001} facets, and such defects are crucial for stabilizing the ultrathin nanosheets and improving their electron-hole separation, therefore leading to a highly efficient photocatalytic activity. The findings offer an opportunity to engineer facets and functionality of anatase TiO2 by controlling surface defects.

Significant improvement in photocatalytic activity by forming homojunction between anatase TiO2 nanosheets and anatase TiO2 nanoparticles

Abstract Novel homojunction TiO2 samples of anatase TiO2 nanoparticles with dominant {101} facets (TP) supported on anatase TiO2 nanosheets with dominant {001} facets (TS) in different ratio of TP/TS (denoted as R%TP/TS) were prepared by a vapor-induced hydrothermal method and followed by photothermocatalytic treatment. The TiO2 samples were characterized with TEM, XRD, Raman, XPS, BET, PL spectra, UV–vis absorption spectra, photocurrent measurement, and fluorescence emission decay. It is found that the formation of the TP/TS homojunction significantly enhances the photocatalytic activity for acetone photodegradation. The TiO2 samples of 2.5%TP/TS, 5%TP/TS, and 10%TP/TS exhibit much higher photocatalytic activity than TS and TP. Especially, the specific CO2 production rate for acetone degradation of the optimized 5%TP/TS is 4.0, 5.4 and 3.3 times higher than that of TS, TP, and P25, respectively. The results of photocurrent, PL, and fluorescence emission decay demonstrate that the significant enhancement in photocatalytic activity is attributed to the efficient improvement in photogenerated electron-hole separation efficiency by forming homojunction. This study provides a new approach by building homojunction to significantly enhancing the separation efficiency of the photogenerated electron-hole and photocatalytic activity.

Excellent catalysis of TiO2 nanosheets with high-surface-energy {001} facets on the hydrogen storage properties of MgH2.

Transition metal compounds are one of the highly efficient types of catalyst for improving the reaction kinetics of hydrogen storage materials. Among all the transition metals, titanium and its compounds show great catalytic effects on magnesium hydride. In this study, for the first time, TiO2 nanosheets with exposed {001} facets were synthesized and doped into MgH2. The TiO2 nanosheet (NS)-doped MgH2 showed superior kinetic performance and the lowest desorption temperature. The onset temperature of MgH2 + 5 wt% TiO2 NS for the release of hydrogen was 180.5 °C and the corresponding peak temperature was 220.4 °C, which are much lower than those of pure MgH2 and also distinctly lower than those of MgH2 + 5 wt% TiO2 nanoparticles (NP). For the isothermal dehydrogenation analysis, the MgH2 + 5 wt% TiO2 NS could release 6.0 wt% hydrogen within 3.2 min at 260 °C and desorb 5.8 wt% hydrogen within 6 min at 240 °C. It is worth noting that the MgH2 + 5 wt% TiO2 NS can even release 1.2 wt% hydrogen at a temperature as low as 180 °C within 300 min. The hydrogenation kinetics of MgH2 + 5 wt% TiO2 NS is also greatly improved, which could absorb hydrogen within only a few seconds at mild temperature. It can take up 3.3 wt% hydrogen at 50 °C and 5.4 wt% at 100 °C within 10 s. It is demonstrated that the tremendous enhancement in reaction kinetics of MgH2 can be ascribed to the nanometer size and highly active {001} facets of anatase TiO2. The higher average surface energy can significantly reduce the hydrogen desorption activation energy of MgH2 to 67.6 kJ mol-1, thus easily improving the hydrogen desorption properties.