TiO2 nanofibers doped with rare earth elements and their photocatalytic activity

In the present study rare earth doped (Ln³⁺–TiO₂, Ln = La, Ce and Nd) TiO₂ nanofibers were prepared by the sol–gel electrospinning method and characterized by XRD, SEM, EDX, TEM, and UV-DRS. The photocatalytic activity of the samples was evaluated by Rhodamine 6G (R6G) dye degradation under UV light irradiation. XRD analysis showed that all the synthesized pure and doped titania nanofibers contain pure anatase phase at 500 °C but at 700 °C it shows both anatase and rutile phase. XRD result also shows that Ln³⁺-doped titania probably inhibits the phase transformation. The diameter of nanofibers for all samples ranges from 200 to 700 nm. It was also observed that the presence of rare-earth oxides in the host TiO₂ could decrease the band gap and accelerate the separation of photogenerated electron–hole pairs, which eventually led to higher photocatalytic activity. To sum up, our study demonstrates that Ln³⁺-doped TiO₂ samples exhibit higher photocatalytic activity than pure TiO₂ whereas Nd³⁺-doped TiO₂ catalyst showed the highest photocatalytic activity among the rare earth doped samples.     

Decomposition of nonionic surfactant on a nitrogen-doped photocatalyst under visible-light irradiation

A visible-light-active N-containing TiO₂ photocatalysts were prepared from crude amorphous titanium dioxide by heating amorphous TiO₂ in gaseous NH₃ atmosphere. The calcination temperatures ranged from 200 to 1000 °C, respectively. UV–vis/DR spectra indicated that the N-doped catalysts prepared at temperatures <400 °C absorbed only UV light (E_g = 3.3 eV), whereas samples prepared at temperatures ≥400 °C absorbed both, UV (E_g = 3.10–3.31 eV) and vis (E_g = 2.54–2.66 eV) light. The chemical structure of the modified photocatalysts was investigated using FT-IR/DRS spectroscopy. All the spectra exhibited bands indicating nitrogen presence in the catalysts structure. The photocatalytic activity of the investigated catalysts was determined on a basis of a decomposition rate of nonionic surfactant (polyoxyethylenenonylphenol ether, Rokafenol N9). The most photoactive catalysts were those calcinated at 300, 500 and 600 °C. For the catalysts heated at temperatures of 500 and 600 °C Rokafenol N9 removal was equal to 61 and 60%, whereas TOC removal amounted to 40 and 35%, respectively. In case of the catalyst calcinated at 300 °C surfactant was degraded by 54% and TOC was removed by 35%. The phase composition of the most active photocatalysts was as follows: (a) catalyst calcinated at 300 °C—49.1% of amorphous TiO₂, 47.4% of anatase and 3.5% of rutile; (b) catalyst calcinated at 500 °C—7.1% of amorphous TiO₂, 89.4% of anatase and 3.5% of rutile; (c) catalyst calcinated at 600 °C—94.2% of anatase and 5.8% of rutile.     

Microstructure and photocatalytic activity of electrospun carbon nanofibers decorated by TiO2 nanoparticles from hydrothermal reaction/blended spinning

Recently, carbon nanofibers@TiO₂ (CNFs@TiO₂) composites as photocatalysts for dye degradation have attracted intense attention. However, only few contributions had been made to investigate systematically the differences between the various preparation approaches and the influence of thermal treatment on the photocatalytic activity. In this work, the electrospun CNFs@TiO₂ composites which were prepared by hydrothermal reaction and blended spinning, respectively, have been fabricated via stabilization in air at 280 °C and then carbonization in N₂ at heat treatment temperature between 500 and 1100 °C. The composites which were prepared by hydrothermal reaction and blended spinning showed the outstanding photocatalytic activity at 900 °C and 1100 °C, respectively. And the photocatalytic activity of composites prepared by hydrothermal reaction was higher than that prepared by blended spinning, but reversibility of the composites showed a reverse trend. These results indicated that the effect of heat treatment temperature on the photocatalytic activity depended on the synergistic effect among the adsorptive property of CNFs, TiO₂ loading amount and anatase phase content in composites. Hence, combining the merits of hydrothermal reaction and blended spinning, a novel method for preparing CNFs@TiO₂ composites with high TiO₂ loading amount and strong interfacial interaction could be envisioned.     

Hydrothermal synthesis of titanate nanostructures with high UV absorption characteristics

The titanate nanostructures with high UV absorption characteristics could be fabricated by hydrothermal method within a temperature range of 90–150 °C. TEM, XRD, BET analyses, and UV–vis spectroscopy were employed to elucidate the synthesized titanate nanostructure characteristics which were microstructure, phase transformation, specific surface area, and band gap energy, respectively. With an increase in the hydrothermal treating temperature from 90 to 120 °C, the specific surface area of titanate nanostructures was increased from 83 to 258 m²/g, while the band gap energy of titanate nanostructures was increased from 3.44 to 3.84 eV and then slightly decreased to 3.81 eV at 150 °C. The fabricated titanate nanostructures could exhibit higher UV adsorption capability but lower photocatalytic activity when compared with that of commercial TiO₂ powders.     

Synthesis and photocatalytic activity of ring-like anatase TiO2 with {001} facts exposed

Developing photocatalysts with specific morphology and good photocatalytic activities promises good opportunities to discover the geometry-dependent properties. In the present work, ring-like anatase TiO₂ with dominant {001} facets exposed were successfully synthesized via a one-pot solvothermal process of tetrabutyl titanate and hydrofluoric acid solution at 180 °C for 8 h. We found that hydrofluoric acid plays an important role in the formation of ring-like TiO₂. The morphology and microstructure were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller N₂ gas adsorption–desorption isotherms. The photocatalytic activity was evaluated by photocatalytic oxidation degradation of methylene blue aqueous solution under UV light. Results showed that ring-like TiO₂ with {001} facets exposed exhibited an excellent photocatalytic activities due to its unique structure: Nanosheets with hole.     

Supercritical hydrothermal synthesis of titanium dioxide nanostructures with controlled phase and morphology

A novel template- and organic-free synthesis of TiO₂ nanostructures with controlled phase and morphology was realized through batch supercritical hydrothermal treatment (400 °C) of titanate nanotubes (TNTs) with H₂O₂ in NaOH aqueous solution. Well-defined 3D titanate hierarchical spheres (THSs), 2D multilayered titanate nanosheets (TNSs), and 1D monodisperse anatase nanorods (ANRs) exposing (0 1 0) facets were prepared in 15 min by slightly varying the NaOH solution pH. Specifically, the obtained Na/H-THSs (without/with HCl neutralization) exhibited highly porous structures with large specific surface area (109 m² g⁻¹ and 196 m² g⁻¹, respectively). Temperature-dependent phase and morphology evolutions of products under subcritical condition (200 and 300 °C) were investigated. The formation of the TiO₂ nanostructures from TNTs was proposed mainly following a dissolution–nucleation-growth mechanism, suggesting that both supercritical temperature and NaOH solution pH were determinant factors governing the nucleation and growth process and thus the phase and morphology.     

Ordered mesoporous carbon–TiO2 materials for improved electrochemical performance of lithium ion battery

A series of ordered mesoporous carbon–TiO₂ (OMCT) materials with various weight percentages of TiO₂ (50–75 wt%) were synthesized by evaporation-induced self-assembly and in-situ crystallization at various calcination temperatures (600–1200 °C) to evaluate the Li-ion storage performance. The OMCT has ordered 2D hexagonal mesoporous structures and the TiO₂ nanocrystals with different phases are embedded into the frameworks of carbonaceous matrix. The reversible capacity of OMCT is highly dependent on the phase and content of TiO₂, and the anatase TiO₂ is a superior crystalline phase to rutile and TiN for Li-ion insertion. The OMCT₆₅ which contains 35 wt% carbon and 65 wt% TiO₂ shows a high capacity of 500 mAh g^?1 at 0.1C after 80 cycles. In addition, OMCT₆₅ exhibits a good cyclability and rate capability. The reversible capacity remains at 98 mAh g^?1 at a high rate of 5C, and then recoveries to 520 mAh g^?1 at 0.1C after 105 cycles. The excellent reversible capacity and rate capability of OMCT₆₅ are attributed to the embedment of well-dispersed anatase TiO₂ nanocrystals into the specific porous structure of OMCT, which can not only facilitate the fast Li-ion charge transport but can also strengthen the carbon–TiO₂ co-constructing channels for lithiated reactions.     

Enhanced photocatalytic activity of TiO2-ZnO thin films deposited by dc reactive magnetron sputtering

The effect of deposition conditions on the photocatalytic activity of TiO₂-ZnO thin films was studied. By using a (Ti)₉₀-(Zn)₁₀ alloy target, the samples were deposited at room temperature on glass substrates by dc reactive magnetron sputtering and post-annealed in air at 500 °C. The dependence of the physical properties of the films on the O₂/Ar gas ratio and the deposition working pressure was investigated. XRD patterns showed mainly the formation of the anatase phase of TiO₂. Optical absorption measurements exhibited a blue shift of the band-gap energy with increasing working pressure. XPS spectra indicated the presence of the Ti⁴⁺ and Zn²⁺ oxidation states, which correspond to TiO₂ and ZnO, respectively. The chemical state of Ti was further analyzed by means of the modified Auger parameter, α’, which gave a value of ca. 873 eV. The photocatalytic property of the films was assessed by the degradation of a methylene blue aqueous solution. The maximum photocatalytic performance was observed for the samples deposited at 3.0 mTorr and O₂/Ar gas ratio of 10/90. These results are explained in terms of the structural, optical, and morphological properties of the films.     

Visible-light-induced photocatalytic activity of TiO2−xNy prepared by solvothermal process in urea–alcohol system

Nitrogen-doped anatase, rutile and brookite titania photocatalyst TiO_2−xN_y which can be excited by visible light were prepared by mixing aqueous TiCl₃ solutions with urea ((NH₂)₂CO) and various type of alcohols followed by solvothermal treatment at 190 °C. The phase composition, crystallinity, microstructure and specific surface area of titania powders greatly changed depending on the pH and type of solvents. Violet, yellowish and grayish TiO_2−xN_y with excellent visible light absorption and photocatalytic activity were prepared. The TiO_2−xN_y powders prepared in urea–methanol solution showed excellent photocatalytic ability for the oxidative destruction of nitrogen monoxide under irradiation of visible light λ > 510 nm.     

Photocatalytic activity of titanium dioxide coatings: Influence of the firing temperature of the chemical gel

Heterogeneous photocatalysis can be exploited for the decomposition of micro-organisms which have developed on the surfaces of building materials. In this work, the efficiency of titanium dioxide coatings on fired clay products is examined. The sol–gel method is used to synthesize a fine TiO₂ powder with a specific surface area of 180 m² g^?1. Thermal treatment of the chemical gel at 340 °C leads to crystallisation in the anatase phase and with further temperature increase, crystallite growth. For thermal treatments in the range 580–800 °C, there is a progressive transition from anatase to rutile. However, despite a decrease in specific surface area of the powder attributed to aggregation/agglomeration, the coherent domain size deduced from X-ray diffraction measurements remains almost constant at 23 nm. Once the transition is completed, increase of thermal treatment temperature above 800 °C leads to further crystallite growth in the rutile phase. The thermally treated titania powders were then sprayed onto fired clay substrates and the photocatalytic activity was assessed by the aptitude of the coating to degrade methylene blue when exposed to ultraviolet light. These tests revealed that the crystallite size is the important controlling factor for photocatalytic activity rather than the powder specific surface area or the anatase/rutile polymorph ratio.     

Synergy effect in the photocatalytic degradation of methylene blue on a suspended mixture of TiO2 and N-containing carbons

N-containing carbon materials were obtained from waste plum stones submitted to pyrolysis under Ar flow at 700 °C or to activation under steam at 800 °C and enriched with nitrogen by heating in a NH₃/air mixture at 270 °C or in NO at 300 °C. In situ mixtures of TiO₂ and carbons were prepared by the slurry method and methylene blue photodegradation was chosen as a model reaction to verify the influence of N-containing carbons on the photocatalytic activity of TiO₂ under artificial visible light irradiation. From the kinetics of methylene blue degradation an important synergy effect between both solids was detected with a remarkable increase up to a factor of 5.3 higher in the photocatalytic activity on TiO₂–C than that on TiO₂ alone. A mechanism for the photoassisting role of N-containing carbons upon the photoactivity of TiO₂ under visible light is discussed.     

Enhanced photocatalytic activity of electrospun TiO2/ZnO nanofibers with optimal anatase/rutile ratio

The photocatalytic characteristics of the TiO₂/ZnO nanofibers synthesized by electrospinning followed by calcinating at different temperatures to alter the anatase-to-rutile ratio are investigated. The results demonstrate that the photocatalytic activity of TiO₂/ZnO nanofibers is enhanced by optimizing the anatase/rutile ratio among the trade-off effects of the band-gap energy, the electron/hole recombination rate, and the surface area. When calcined at 650 °C, the TiO₂/ZnO nanofibers with optimal anatase/rutile ratio (48:52) balancing these trade-off effects have the highest photocatalytic efficiency both in the degradation of RhB in liquid and conversion of NO gas.     

Control of refractive index by annealing to achieve high figure of merit for TiO2/Ag/TiO2 multilayer films

We modified the refractive index (n) of TiO₂ by annealing at various temperatures to obtain a high figure of merit (FOM) for TiO₂/Ag/TiO₂ (45 nm/17 nm/45 nm) multilayer films deposited on glass substrates. Unlike the as-deposited and 300 °C-annealed TiO₂ films, the 600 °C-annealed sample was crystallized in the anatase phase. The as-deposited TiO₂/Ag/as-deposited TiO₂ multilayer film exhibited a transmittance of 94.6% at 550 nm, whereas that of the as-deposited TiO₂/Ag/600 °C-annealed TiO₂ (lower) multilayer film was 96.6%. At 550 nm, n increased from 2.293 to 2.336 with increasing temperature. The carrier concentration, mobility, and sheet resistance varied with increasing annealing temperature. The samples exhibited smooth surfaces with a root-mean-square roughness of 0.37–1.09 nm. The 600 °C-annealed multilayer yielded the highest Haacke's FOM of 193.9×10⁻³ Ω⁻¹.     

Effect of microstructural evolution on wettability and tribological behavior of TiO2 nanotubular arrays coated on Ti–6Al–4V

Self-organized TiO₂ nanotubular arrays were fabricated by electrochemical anodization of Ti–6Al–4V plates in an NH₄F/H₃PO₄ electrolyte. The effect of microstructural evolutions on the wettability and tribological behavior of the TiO₂ nanotubes was investigated. Based on the XRD profiles of the fabricated material, the characteristic TiO₂ peaks were not recognized after anodization; however, highly crystalline TiO₂ (anatase and rutile) was formed due to crystallization during annealing at 500 °C for 1.5 h. The nanotube arrays were converted entirely to rutile at 700 °C. From a microstructure point of view, a highly ordered nanotube structure was achieved when the specimen was annealed at 500 °C, with a length of 0.72 μm and a pore diameter of 72 nm. Further increasing the annealing temperature to 700 °C resulted in the complete collapse of the tubular structure. The results indicate that the improved wettability of the anodized specimens was due to the combination of the effects of both the surface oxide layer and the increased surface roughness achieved after anodization. Moreover, the wear resistance and wettability of the sample annealed 500 °C were improved due to the high hardness (435 HV) and low coefficient of friction (0.133–168) of the highly crystalline structure of the TiO₂ nanotubes.     

Evaluating the photocatalytic activity of Pt/TNT film catalyst

TiO₂ nanotubes (TNT) were prepared by a hydrothermal method from the commercially available TiO₂-P25. Five types of TNT were produced at different temperatures (120 °C, 130 °C, and 150 °C) and by using different reaction times (12 h, 24 h, and 30 h). The photocatalytic reactor that was used is a film catalytic reactor, in which the height of the catalyst is 1.0 mm. The BET and FESEM analysis results showed that TNT130-24 (130 °C, 24 h) and TNT150-12 (150 °C, 12 h) possessed well-formed tubular structures with a high specific surface area (282.9–316.7 m² g⁻¹) and large pore volumes (0.62–0.70 cm³ g⁻¹). However, TNT120-30 (120 °C, 30 h) presented the best photocatalytic activity upon CO removal due to the synergistic effect of TiO₂ nanotubes and TiO₂ particles. After the TNT catalysts were modified with Pt particles, the removal efficiency was in the order of Pt/TNT120-30>Pt/TNT130-24>Pt/P25. Pt/TNT120-30 showed 99% removal efficiency in a continuous photoreactor with a high space velocity of 1.79×10⁴ h⁻¹. The results of the TEM and DRS analyses confirmed that the Pt particles enhanced the photocatalytic reaction, which was attributed to the well-dispersed nature of the 1 nm nanoscaled Pt particles on the surfaces of the TNT catalysts, and narrowed the band gap from 3.22 eV to 3.01 eV.     

Functionalizing slag wool fibers with photocatalytic activity by anatase TiO2 and CTAB modification

Anatase TiO₂ coatings prepared by solvothermal process in a neutral ethanol solution of isopropyl titanate at 160 °C have been grown on slag wool fibers (SWF) which were modified by hexadecyltrimethylammonium bromide (CTAB) in advance. X-ray diffraction patterns confirmed the coatings are of a nanocrystalline anatase structure, and scanning electron microscopy observations and energy-dispersive X-ray spectrum revealed a continuous coverage of TiO₂ formed on the fiber surfaces. The photocatalytic activity of the samples was tested by the photocatalytic degradation of methylene blue (MB) solution. The results show that CTAB modified slag wool fibers (CMSWF) are not a suitable adsorbent for MB due to their weaker negative surface charges. Anatase TiO₂ coated CMSWF display higher photocatalyst activity than anatase TiO₂ coated SWF without CTAB modification, and Anatase TiO₂ coated CMSWF are relatively stable under UV-light irradiation.     

SnS2/TiO2 nanocomposites with enhanced visible light-driven photoreduction of aqueous Cr(VI)

SnS₂/TiO₂ nanocomposites have been synthesized via microwave assisted hydrothermal treatment of tetrabutyl titanate in the presence of SnS₂ nanoplates in the solvent of ethanol at 160 °C for 1 h. The physical and chemical properties of SnS₂/TiO₂ were studied by XRD, FESEM, EDS, TEM, XPS and UV–vis diffuse reflectance spectra (DRS). The photocatalytic activity of SnS₂/TiO₂ nanocomposites were evaluated by photoreduction of aqueous Cr(VI) under visible light (λ>420 nm) irradiation. The experimental results showed that the SnS₂/TiO₂ nanocomposites exhibited excellent reduction efficiency of Cr(VI) (~87%) than that of pure TiO₂ and SnS₂. The SnS₂/TiO₂ nanocomposites were expected to be a promising candidate as effective photocatalysts in the treatment of Cr(VI) wastewater.     

Synergic effect of cation doping and phase composition on the photocatalytic performance of TiO2 under visible light

The synergic effect of cation doping and phase composition for the further improvement of the photocatalytic activity of TiO₂ under visible light is reported for the first time. Fe^3 + and Sn^4 + co-doped TiO₂ with optimized phase composition were synthesized through a simple soft-chemical solution method. The visible-light-driven photocatalytic activity of Fe^3 + and Sn^4 + co-doped TiO₂ was 5 times of that of Evonik P25 TiO₂ using degradation of methylene blue as model reaction. The synthesized photocatalysts were characterized by powder X-ray diffraction, UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, ¹¹⁹Sn Mössbauer spectroscopy, and X-ray absorption fine structure spectroscopy. It is indicated that Sn^4 + doping can facilitate the phase transition from anatase to rutile. The different ratios of anatase and rutile can be achieved by tuning the amount of Sn^4 + doped into the lattice. Furthermore, the doping of Sn^4 + into TiO₂ lattice can stabilize the phase composition when Fe^3 + is co-doped. In the Fe^3 + and Sn^4 + co-doped TiO₂, Sn^4 + is mainly used to tune and stabilize the phase composition of TiO₂ and Fe^3 + acts as a doping cation to narrow the band gap of TiO₂. Both band gap and phase composition of TiO₂ can be tuned effectively by the simultaneous introduction of Fe^3 + and Sn^4 +. The synergic effect of optimized phase composition (anatase/rutile = 25/75) and narrowed band gap should be the two main reasons for the promoted photocatalytic activity of TiO₂ under visible light.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

Synthesis and photocatalysis of TiO2 hollow spheres by a facile template-implantation route

Process variables such as reaction temperature (55 to 90 °C), calcination temperature (450 to 750 °C), and concentration of TiCl₄ precursor (26 to 105 mM) have been examined in order to tailor the surface area, crystallite size, and the anatase/rutile ratio of the polycrystalline TiO₂ microcapsules prepared by a template-implantation route in heptane solvent. The hollow capsules are all non-aggregating with nanoporous shell structure. Among the process variables examined, the Brunauer–Emmett–Teller (BET) surface area and the anatase/rutile ratio are found critically dependent on the reaction temperature, in which a reduced reaction temperature (from 90 to 55 °C) leads to a higher BET value (from 8.4 to 36.4 m^?2 g^?1), a predominant anatase phase (weight fraction of the anatase phase increases from 0.20 to 0.84), and an improved photodegradation of aqueous methylene blue (MB) dye under UV exposure (the degradation rate increases from 0.5×10^?2 to 5.5×10^?2 min^?1).