Shock-consolidated TiO2 bulk with pure anatase phases fabricated by explosive compaction using underwater shockwave

Titanium dioxide (TiO₂) bulk with pure anatase phases was fabricated by an explosive compaction technique using an underwater shockwave. Dynamic shock pressure of 6 GPa was used to consolidate anatase TiO₂ powders. Its microstructural, crystalline structural and photocatalytic characteristics were observed and measured by various techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and photocatalytic activity measurement system. It was confirmed that the relative density of anatase TiO₂ compact is about 96% (3.73 g/cm³) of the theoretical density (3.89 g/cm³) and a strong surface bonding between particles is formed by a shock energy. In X-ray diffraction analysis, high purity anatase phases, broadened peaks due to lattice defects and decreased crystallite size were found. For the photocatalytic activities, the anatase TiO₂ compact was quite excellent compared to the commercial sintered TiO₂ bulk.     

Nanocrystalline TiO2 on single walled carbon nanotube arrays: Towards the assembly of organized C/TiO2 nanosystems

Arrays of single walled carbon nanotube bundles organized following different architectures have been coated by a homogeneous deposit of nanocrystalline titania. The nanotubes were grown treating nanosized C powders with atomic H in a purpose-designed chemical vapor deposition (CVD) reactor, the subsequent TiO₂ deposition was performed at 400 °C using the metal-organic CVD (MOCVD) technique and titanium tetraisopropoxide Ti(OⁱPr)₄ as a precursor. X-ray diffraction and Raman spectroscopy evidence the anatase structure of the TiO₂ coatings, formed by grains with an average size of about 55 nm. The structural and compositional characteristics of the TiO₂ deposits are not sensitive to the organization of the nanotube arrays, which maintain their pristine architectures. The adopted synthetic procedure opens a new route for the immobilization of anatase-type TiO₂ nanocrystallites onto geometrically varied structures and for the integration of composite nanotube/TiO₂ systems in effective devices.     

Mesoporous polyaniline or polypyrrole/anatase TiO2 nanocomposite as anode materials for lithium-ion batteries

A functional composite as anode materials for lithium-ion batteries, which contains highly dispersed TiO₂ nanocrystals in polyaniline matrix and well-defined mesopores, is fabricated by employing a novel one-step approach. The as-prepared mesoporous polyaniline/anatase TiO₂ nanocomposite has a high specific surface area of 224 m² g⁻¹ and a predominant pore size of 3.6 nm. The electrochemical performance of the as-prepared composite as anode material is investigated by cyclic voltammograms and galvanostatic method. The results demonstrate that the polyaniline/anatase nanocomposite provides larger initial discharge capacity of 233 mAh g⁻¹ and good cycle stability at the high current density of 2000 mA g⁻¹. After 70th cycles, the discharge capacity is maintained at 140 mAh g⁻¹. The excellent electrochemical performance of the polyaniline/TiO₂ nanocomposite is mainly attributed to its special structure. Furthermore, it is accessible to extend the novel strategy to other polymer/TiO₂ composites, and the mesoporous polypyrrole/anatase TiO₂ is also successfully fabricated.     

Tailoring high-surface-area nanocrystalline TiO2 polymorphs for high-power Li ion battery electrodes

The crystallization and morphology of brookite and anatase titania (TiO₂) were controlled using the urea-mediated hydrolysis/precipitation route in the presence of the Ti³⁺ ions. Without the strong complexing agents and the non-hydrothermal conditions, simple alterations to the urea concentration led to the synthesis from brookite nanorods to anatase nanoflowers at a low temperature below 100 °C, whereas the BET specific surface area evolved from 102 to 268 m² g⁻¹, respectively. A possible formation mechanism was also proposed for these TiO₂ nanostructures. The excellent reversible capacity and rate capability were achieved for the anatase nanoflowers because of the small crystallite size and significantly large surface area.     

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.     

Direct synthesis of TiO2 nanoparticles by using the solid-state precursor TiH2 powder in a thermal plasma reactor

We attempt the direct synthesis of TiO₂ by using the solid state precursor TiH₂ powder with oxygen in a thermal plasma reactor. Nanocrystalline titanium dioxide powder has been synthesized by using thermal plasma synthesis in a non-transferred arc thermal plasma reactor. The thermal plasma-synthesized powder product consists of nano-sized particles of anatase and rutile phases of titanium dioxide. Particle compositions were observed on collecting powder from different positions of the reactor and varying the amount of flow rate of reactive gases (O₂). The characteristics of the powder such as particle size, size distribution and phases were analyzed using various techniques such as XRD, SEM, TEM, XPS, EDS and particle size analyzer. UV–visible reflection spectroscopy of the plasma-synthesized TiO₂ powders showed the absorbance in the visible region leading to effective photocatalytic activity, which is clearly confirmed from the XPS analysis. XPS analysis reveals the presence of –OH bonds on the surface of nanoparticles, which is the significant evidence of better quality of powders in comparison to other methods. Also, we have investigated the phase transformation phenomenon of anatase to rutile. At 1000 °C, complete transformation of the anatase to rutile occurs. Powders prepared in this procedure are white in colour and their diameter varies from 10 nm to 150 nm. Average particle size distributes in the range of 20–50 nm. The unique property about the plasma-synthesized powders is high resistance to heat treatment, with enhanced photocatalytic activity.     

Super/subcritical fluid extractions for preparation of the crystalline titania

Various extraction techniques which use fluids in super/subcritical state (CO₂, H₂O, CH₃OH) were used for the preparation of crystalline TiO₂ from surfactant-mediated titania organogels (the amorphous reaction intermediate products of surfactant-mediated sol-gel synthesis). Crystallinity, purity and texture of TiO₂ powders (titania aerogels) were studied in detail for different treatment modes of titania organogels. Suitable combinations of extraction solvents and extraction temperature and pressure were considered for synthesis of TiO₂ powder crystalline phase (anatase) with high specific surface area and purity (carbon content < 0.1 wt.%). Optimum results were achieved by combined extraction procedures—subcritical water extraction and supercritical fluid extraction or pressurised liquid extraction. Moreover, the possible tuning of the TiO₂ powder pore-size distribution by regulation of temperature at constant pressure for the combined subcritical water extraction and pressurised liquid extraction was found.     

Porous titania microspheres with uniform wall thickness and high photoactivity

Highly porous titania particles were prepared by depositing thin films of titania, using alternating reactions of TiCl₄ and hydrogen peroxide, on poly(styrene-divinylbenzene) (PS-DVB) template particles via atomic layer deposition (ALD) at 77 °C. The composition of the titania films was verified by XPS analysis and the titania films were directly observed by TEM. TGA/DSC was used to study the thermal decomposition of the polymer template. Porous titania particles with uniform wall thicknesses were successfully obtained after the template PS-DVB was removed by oxidation in air at 400 °C for 24 h. Verification of the resulting porous structure of the titania particles was done by cross-sectional SEM and nitrogen adsorption–desorption analysis. Porous titania particles were treated at different temperatures. XRD analysis was used to determine the microstructure and phase transformation of titania at elevated temperatures. The photocatalytic activity of these porous titania particles was studied by methylene blue decomposition under UV light at room temperature and was found to be comparable to that of commercial anatase titania nanoparticles (~20 nm). Depositing Na₂SO₄ on TiO₂ retarded the TiO₂ phase transformation from anatase to rutile during calcination and, thus, greatly increased the photoactivity of the porous titania particles.     

Nano-twinned structure and photocatalytic properties under visible light for undoped nano-titania synthesised by hydrothermal reaction in water-ethanol mixture

Nano-TiO₂ crystals showing visible light driven photocatalytic activity were synthesized by hydrothermal reaction in an ethanol-water mixture. The experiments were conducted to optimise the synthesis conditions for nano titania, in the range of temperature from 200 to 400 °C. X-ray diffraction depicted that the products obtained were anatase at 250 °C and above. For the products obtained at 250 °C, detailed analysis was conducted since it depicted high crystallinity with smallest particle sizes. Shape of the crystal was rounded rectangular with the size of 4 ± 1 nm to 7 ± 1 nm. The high-resolution transmission electron microscopy (HRTEM) revealed the existence of novel nano-twin structure in anatase grains and surface defects around the nanocrystals. Photocatalytic property was investigated for these undoped titania samples under UV and visible light. The nano twin structure, surface defects, and nano-meter size of the synthesized titania are believed to play a crucial role for the high catalytic activity.     

Effects of annealing temperature on the photocatalytic activity of N-doped TiO2 thin films

The effects of annealing temperature on the photocatalytic activity of nitrogen-doped (N-doped) titanium oxide (TiO₂) thin films deposited on soda-lime-silica slide glass by radio frequency (RF) magnetron sputtering have been studied. Glancing incident X-ray diffraction (GIAXRD), Raman spectrum, scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectra were utilized to characterize the N-doped TiO₂ thin films with and without annealing treatment. GIAXRD and Raman results show as-deposited N-doped TiO₂ thin films to be nearly amorphous and that the rutile and anatase phases coexisted when the N-doped TiO₂ thin films were annealed at 623 and 823 K for 1 h, respectively. SEM microstructure shows uniformly close packed and nearly round particles with a size of about 10 nm which are on the slide glass surface for TiO₂ thin films annealed at 623 K for 1 h. AFM image shows the lowest surface roughness for the N-doped TiO₂ thin films annealed at 623 K for 1 h. The N-doped TiO₂ thin films annealed at 623 K for 1 h exhibit the best photocatalytic activity, with a rate constant (k_a) of about 0.0034 h⁻¹.     

Removal of the herbicide Bentazon from contaminated water in the presence of synthesized nanocrystalline TiO2 powders under irradiation of UV-C light

A solution-based processing method has been used to synthesize nanocrystalline TiO₂ powders by controlling the hydrolysis of TiCl₄ in an aqueous solution in both anatase and rutile phases. The primary particle sizes of the powders were in the range of 5-15 nm. To determine the crystal phase composition and size of the prepared photocatalysts, X-ray diffraction (XRD) measurements were used. We also studied the photocatalytic removal of the herbicide, Bentazon, from contaminated water in the presence of synthesized nanocrystalline TiO₂ powders under UV light illumination (30 W). The removal efficiency of Bentazon was 16% when the photolysis was carried out in the absence of TiO₂ and it was negligible in the absence of UV light. We have studied the influence of the basic operational parameters such as the different kinds of TiO₂, amount of TiO₂, irradiation time and initial concentration of Bentazon on the photocatalytic removal efficiency of Bentazon. Our results indicated that 99% removal of the herbicide from the solution containing 15 ppm of Bentazon after selecting desired operational parameters could be achieved in a relatively short time, about 90 min. A kinetic model was successfully established for the prediction of removal of Bentazon by the UV/TiO₂ system with any concentration of the herbicide. In this work, we also compared the photocatalytic activity between the commercial TiO₂ and synthesized nanocrystalline TiO₂ powders. The photocatalytic activities of different photocatalysts were tested using the herbicide solution.     

Microwave-hydrothermal synthesis of extremely high specific surface area anatase for decomposing NOx

Apparently C-doped and undoped or pure nanoparticles of anatase were synthesized using a microwave hydrothermal process in the temperature range of 140–180 °C for 1 h from several Ti precursors, such as Ti ethoxide, Ti isopropoxide and Ti oxysulfate. Nanoparticles of anatase samples were characterized by powder X-ray diffraction, transmission electron microscopy (TEM) and photocatalytic activity measurements. Results showed that nanoparticles in the size range of 4–17 nm of anatase were obtained in all cases with surface areas in the range of 151–267 m²/g. The photocatalytic activity of the prepared titanias was measured using methylene blue (MB) and NO_x molecules. Because MB has very strong adsorption on the samples, photocatalytic degradation under either solar light or black light irradiation was found to be very limited. However, the DeNO_x abilities of carbon-doped titanias were higher than those of Degussa P25 commercial titania sample and undoped or pure titanias especially under irradiation by long wavelength or visible light (>500 nm).     

A novel approach for preparation of highly crystalline anatase TiO2 nanopowder from the agglomerates

A new approach for synthesis of dry and well-shaped anatase titania nanocrystals by different methods of synthesis, like sol-gel and precipitation, is reported. The significance of this new method is the dehydration of titania agglomerates by simple azeotropic distillation to afford uniform and reproducible anatase nanocrystals with particle size < 20 nm and average surface area as high as 106.44 m² g^− 1.     

Synthesis of nanocrystalline ytterbium-doped yttria by citrate-gel combustion method and fabrication of ceramic materials

Nanosized ytterbium doped yttria powders were prepared by citrate-gel combustion techniques. As-synthesized precursor and calcined powders were characterized for their crystalline structure, particle size and morphologies. Nanocrystalline Yb³⁺:Y₂O₃ powders with pure cubic yttria crystal structure were obtained by calcination of as-prepared precursors at 1100 °C for 3 h. Powders obtained were well dispersed with an average particle size of 60 nm. By using the obtained powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were produced by vacuum sintering at 1800 °C for 12 h. The emission spectrum of the sintered ceramics under the excitation wavelength of 905 nm illustrates that there are three fluorescence peaks locating at 976 nm, 1030 nm and 1075 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of ytterbium ion.     

TiO2 nanotubes manufactured by anodization of Ti thin films for on-chip Li-ion 2D microbatteries

The use of self-organized TiO₂ nanotube arrays electrochemically grown onto Si is investigated for the fabrication of an alternative electrode dedicated to on-chip Li-ion 2D microbatteries. Discharge/charge curves and cycling performance are studied in lithium-anode electrochemical test cells for both amorphous and crystalline titania nanotubes. At 5 μA cm⁻² amorphous TiO₂ nanotube layers onto Si deliver a maximum areal capacity of 89 μAh cm⁻² in the first reversible discharge and 56 μAh cm⁻² over 50 cycles. We demonstrate that these nanostructured thin film electrodes showing such electrochemical performances are compatible with IC technology.     

Greener photocatalysts: Hydroxyapatite derived from waste mussel shells for the photocatalytic degradation of a model azo dye wastewater

This paper demonstrates for the first time the feasibility of utilising waste mussel shells for the synthesis of hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ (denoted as HAP) to be used as a greener, renewable photocatalyst for recalcitrant wastewater remediation. HAP was synthesised from Perna canaliculus (green-lipped mussel) shells using a novel pyrolysis–wet slurry precipitation process. The physicochemical properties of the HAP were characterised using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The HAP produced was of comparable quality to commercial (Sulzer Metco) HAP. The synthesised HAP had good photocatalytic activity, whereby methylene blue (a model textile wastewater compound) and its azo dye breakdown products were degraded with an initial rate of 2.5 × 10⁻⁸ mol L⁻¹ min⁻¹. The overall azo dye degradation was nearly 54% within 6 h and 62% within 24 h in an oxygen saturated feed in a batch reactor using a HAP concentration of 2.0 g/L, methylene blue concentration of 5 mg L⁻¹, UV irradiation wavelength of 254 nm and a stirring speed of 300 rpm. The kinetics were well described by three first order reactions in series, reflecting the reaction pathway from methylene blue to azo dye intermediates, then to smaller more highly oxidised intermediates and finally degradation of the recalcitrants. The final two steps of the reaction had significantly slower rates than the initial step (rates constants of 6.2 × 10⁻³ min⁻¹, 1.2 × 10⁻³ min⁻¹ and approximately (due to limited data points) 1.6 × 10⁻⁴ min⁻¹ for the first, second and third step respectively), which tie in with this mechanism, however it could also indicate that the reaction is either product inhibited and/or affected by catalyst deactivation. FTIR analysis of the post-reaction HAP revealed surface PO₄³⁻ group loss. Since there is good photocatalytic activity with oxygen in limited and excess supply during the photoreaction, this indicates the possibility of lattice oxygen participation in the photocatalytic reaction, which needs to be characterised more fully. However, overall, these results indicate that the HAP derived from the mussel shells is a promising greener, renewable photocatalyst for the photocatalytic degradation of wastewater components.     

Study on photocatalytic deposition of bismuth onto nanocrystalline TiO2 by quartz crystal microbalance and electrochemical method

In situ techniques of quartz crystal microbalance (QCM), differential pulse voltammetry (DPV) and amperometric measurement were employed to investigate the adsorption Bi(III) ions and the photocatalytic deposition Bi process at the surface of nanocrystalline TiO₂. It was obtained that the adsorption of Bi(III) ions onto nanocrystalline TiO₂ accords with the pseudo-second-order reaction and the reaction rate constant k was about 13.3 g mol⁻¹ min⁻¹. In addition, the photocatalytic deposition of Bi onto the surface of TiO₂ was further investigated. It was found that photocatalytic deposition rate at the surface of TiO₂ was enhanced by increasing pH value or initial concentration of Bi(III) ions. The influence of organic hole-scavegeners on the photocatalytic deposition of Bi was also investigated, and it was obtained that formic acid may be the best for the photocatalytic reduction of Bi. The mass ratio between the Bi(III) and Bi metal deposition was calculated as 7.48:1. Therefore, it can be concluded that QCM, DPV and amperometric measurement may be effective and reliable for the investigation of the photocatalytic deposition of Bi onto the surface of nanocrystalline TiO₂.     

A comparative study of the effect of Pd-doping on the structural,optical, and photocatalytic properties of sol–gel derived anatase TiO2 nanoparticles

Pd-doped anatase TiO₂ nanoparticles were synthesized by a modified sol–gel deposition technique. The synthetic strategy is applicable to other transition and post-transition metals to obtain phase-pure anatase titania nanoparticles. This is important in the sense that anatase titania forms the most hydroxyl radicals (compared to other polymorphs like rutile, brookite, etc.) for better photocatalytic performance. XRD and Raman data confirm the phase-pure anatase formation. Doping of Pd²⁺ into Ti⁴⁺ sites (for substitutional doping) or interstitial sites (for interstitial doping) creates strain within the nanoparticles and is reflected in the XRD peak broadening and Raman peak shifts. This is because of the ionic radii difference between Ti⁴⁺(∼68 pm) and Pd²⁺(∼86 pm). XPS data confirm the formation of high surface titanol groups at the nanoparticle surface and a large number of loosely bound Ti³⁺–O bonds, both of which considerably enhance the photocatalytic activity of the doped nanoparticles. A comparative study with other metal doping (Ga) shows that TiO₂: Pd nanoparticles have more Ti³⁺–O bonds, which enhance the charge transfer rate and hence improve the photocatalytic activity compared to other transition and post-transition metal-doped titania nanostructures.     

Study on the effect of EDTA on the photocatalytic reduction of mercury onto nanocrystalline titania using quartz crystal microbalance and differential pulse voltammetry

In-situ technique quartz crystal microbalance (QCM), differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were employed to investigate the effect of disodium ethylenediamine tetraacetate (EDTA) on photocatalytic reduction of mercury onto nanocrystalline titania (TiO₂). Effects of EDTA on adsorption of Hg(II) and its photocatalytic reduction process at the surface of TiO₂ in different pH solutions had been studied in detail. From the in-situ response to the adsorption of Hg(II) onto TiO₂, the reaction rate and saturation adsorption amount were estimated about 4.71 × 10⁻⁶ g mol⁻¹ min⁻¹ and 46.36 (mg Hg(II)/g TiO₂) via the model of pseudo-second-order kinetics respectively. The photocatalytic reduction of Hg at the surface of TiO₂ was influenced by pH and the mole ratio of Hg(II) to EDTA. When the ratio of Hg(II) to EDTA 1:1, it was most favorable for the photocatalytic reduction of mercury. In addition, the effects of HCOOH and EDTA on the reduction of Hg(II) was comparatively investigated and the mechanism on the photocatlytic reduction of mercury was illustrated. Therefore, it could be concluded that QCM, DPV and CV were effective methods for the investigation of photocatalytic reduction of complex heavy metal ions onto the surface of nanocrystalline TiO₂.     

Li–Y doped and codoped TiO2 thin films: Enhancement of photocatalytic activity under visible light irradiation

Sol−gel synthesis based on the self-assembling template method has been applied to synthesize Li–Y doped and co-doped TiO₂ not only to improve simultaneously the structural and electronic properties of TiO₂ nanomaterials but also to achieve Li–Y doping of titania with high photocatalytic reactivity. The characterization of the samples was performed by GXRD, GSDR, FT-IR, and Raman spectroscopy. According to the GXRD patterns, all the observed reflections can be indexed using the anatase form of TiO₂, Which is confirmed by ground state diffuse reflectance and micro-Raman spectra. The Li–Y doped titania materials immobilized as nanostructured thin films on glass substrates exhibit high photocatalytic efficiency for the degradation of toluidine and benzoic acid under visible light irradiation. The development of these visible light-activated nanocatalysts has the potential of providing environmentally benign routes for water treatment.