Anatase-Type TiO2 and ZrO2-Doped TiO2 Directly Formed from Titanium(III) Sulfate Solution by Thermal Hydrolysis: Effect of the Presence of Ammonium Peroxodisulfate on their Formation and Properties

Anatase-type TiO₂ powder containing sulfur with absorption in the visible region was directly formed as particles with crystallite in the range 15–88 nm by thermal hydrolysis of titanium(III) sulfate (Ti₂(SO₄)₃) solution at 100°–240°C. Because of the presence of ammonium peroxodisulfate ((NH₄)₂S₂O₈), the yield of anatase-type TiO₂ from Ti₂(SO₄)₃ solution was accelerated, and anatase with fine crystallite was formed. Anatase-type TiO₂ doped with ZrO₂ up to 9.8 mol% was directly precipitated as nanometer-sized particles from the acidic precursor solutions of Ti₂(SO₄)₃ and zirconium sulfate in the presence and the absence of (NH₄)₂S₂O₈ by simultaneous hydrolysis under hydrothermal conditions at 200°C. By doping ZrO₂ into TiO₂ and with increasing ZrO₂ content, the crystallite size of anatase was decreased, and the anatase-to-rutile phase transformation was retarded as much as 200°C. The anatase-type structure of ZrO₂-doped TiO₂ was maintained after heating at 1000°C for 1 h. The favorable effect of doping ZrO₂ to anatase-type TiO₂ on the photocatalytic activity was observed.     

Scandium-Doped Anatase (TiO2) Nanoparticles Directly Formed by Hydrothermal Crystallization

Anatase-type TiO₂ solid solutions doped with 0–10 mol% scandium were formed by hydrothermal crystallization under weak basic conditions above 180°C for 5 h from amorphous co-precipitates that were obtained from the aqueous precursor solutions of TiOSO₄ and Sc(NO₃)₃ using aqueous ammonia. The anatase particles were spindle-like and consisted of nanosized-crystallites (23–25 nm). The lattice parameter c ₀ of anatase and the length and width of the spindle-like anatase gradually increased when the scandium content was increased. The diffuse reflectance spectra of the as-prepared TiO₂ doped with scandium showed that the onset of absorption slightly shifted to longer wavelengths with increasing scandium content. The band gap of anatase was slightly increased by making solid solutions with scandium oxide.     

Direct Formation and Photocatalytic Performance of Anatase (TiO2)/Silica (SiO2) Composite Nanoparticles

Anatase (TiO₂)/silica (SiO₂: 23.9–27.7 mol%) composite nanoparticles were directly synthesized from (i) the reaction of titanyl sulfate (TiOSO₄) and sodium metasilicate (Na₂SiO₃) under mild hydrothermal conditions, (ii) the acidic precursor solutions of TiOSO₄ and tetraethylorthosilicate (TEOS) by thermal hydrolysis, and (iii) the metal alkoxides, i.e., tetraisopropoxide (TTIP) and TEOS, by the sol–gel method. Their photocatalytic activities were evaluated by measurements of the relative concentration of methylene blue after UV irradiation. The as-prepared TiO₂/SiO₂ composite nanoparticles showed far more improved photocatalytic activity than the pure anatase-type TiO₂. The composite nanoparticles formed from (i) TiOSO₄ and Na₂SiO₃ as well as those from (ii) TiOSO₄ and TEOS showed fairly good photocatalytic activity, and it was better than that of those synthesized from (iii) the metal alkoxides, which was suggested to be due to the difference in crystallinity of the anatase.     

Fabrication of Bifunctional Titania/Silica-Coated Magnetic Spheres and their Photocatalytic Activities

A magnetic photocatalyst is prepared by coating a magnetic core with a dual layer of inert silica (SiO₂) and photoactive titania (TiO₂). The complete photoactive TiO₂ shell is directly formed on the SiO₂-coated magnetic spheres. Subsequent hydrothermal treatment improves the degree of crystallinity of TiO₂. TiO₂/SiO₂-coated magnetic spheres show good photocatalytic activities in the degradation of methylene blue in the aqueous solution. The photocatalytic TiO₂ surface shell is electrically insulated by the intermediate SiO₂ layer from the magnetic core to maintain good photocatalytic activity of the TiO₂ shell. The magnetic photocatalyst can be easily recovered by applying external magnetic field.     

Preparation of Titanium Dioxide Nanocrystallite with High Photocatalytic Activities

Anatase nanocrystalline titanium dioxide (TiO₂) with high photocatalytic activities was prepared by hydrothermal crystallization with the addition of acetic acid. The presence of acetic acid could arrest the growth of the nanocrystallites during the hydrothermal processes and be propitious to form nanocrystallites with round edges and corners. This morphology of (TiO₂) had more active sites in the photocatalytic process and showed high photocatalytic activity in the degradation of the methylene blue in the aqueous solution.     

Preparation and Properties of Nanocrystalline α-Fe2O3-Sensitized TiO2 Nanosheets as a Visible Light Photocatalyst

We present a novel approach for the fabrication of α-Fe₂O₃-sensitized TiO₂ nanosheets. The TiO₂ nanosheets with partly curly surfaces and large surface areas can rapidly absorb the methylene blue (MB). The absorption and degradation of MB are investigated. Results show that the photocatalytic activities of α-Fe₂O₃ make the MB degradation efficient under visible light irradiation. By coupling two semiconductor systems, electrons can be injected from an excited small band gap semiconductor into TiO₂ under visible light. Such a modification is expected to lead to new or enhanced properties.     

Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline cerium(IV) oxide (CeO₂) powders were prepared by heating solutions of cerium(IV) salts in the presence of urea under hydrothermal conditions at 120° to 180°C. The effects of the concentration of urea and hydrothermal treatment temperature on the morphology and crystallite size of the synthesized particles were investigated. The synthesized particles were angular, ultrafine CeO₂, with a cubic fluorite structure. Their crystallite size decreased from 20 to 10 nm with increasing urea concentration from 2 times to 8 times that of the Ce⁴⁺ ion. The size only slightly changed by calcining at temperatures below 600°C.     

Direct Formation of Iron(III)-Doped Titanium Oxide (Anatase) by Thermal Hydrolysis and Its Structural Property

Anatase-type TiO₂ (titania) doped with iron up to 19.8 mol% was directly formed as nanometer-sized particles from acidic precursor solutions of TiOSO₄ and Fe(NO₃)₃ by simultaneous hydrolysis, under mild hydrothermal conditions at 180°3C. Iron content in the anatase-type TiO₂ was much less than that of the starting composition of the precursor solutions because of slower hydrolysis rate of Fe(NO₃)₃ than that of TiOSO₄ at 180°3C. The XRD data, TEM selected-area diffraction patterns, and Mössbauer effect measurement showed that iron(III) formed a solid solution in the anatase-type TiO₂ precipitates and that there was no iron oxide precipitated as secondary phase without making a solid solution with TiO₂ present in the precipitates. Doping of Fe₂O₃ into TiO₂ shifted the phase transformation from anatase-type to rutile-type structure to a low temperature. On the phase transformation from anatase to rutile, iron oxide was precipitated as Fe₂TiO₅ (pseudobrookite) phase. When the iron content was increased in the anatase phase, onset of optical absorption shifted to longer wavelengths, and absorption in the UV-light region and in the visible-light region over 400–600 nm clearly appeared in the diffuse reflectance spectra of the as-prepared Fe(III)-doped TiO₂.     

Photocatalytic Activity of Monodispersed Spherical TiO2 Particles with Different Crystallization Routes

Monodispersed spherical TiO₂ particles were prepared by hydrothermal crystallization and/or calcination of spherical amorphous particles, synthesized by thermal hydrolysis of TiCl₄. The crystallized spherical particles were secondary agglomerates of primary nanocrystallites. Different crystallization routes and conditions provided the spherical TiO₂ particles with wide particle characteristics, such as the fraction of crystallization, the size and shape of the primary nanocrystallites, and the specific surface area. The photocatalytic activity showed complex dependence on the crystallization routes and conditions. The complex dependence behavior could be explained by combining the effects of the fraction of crystallization, the specific surface area, and the adsorption ability for hydroxyl ions. Especially, in the present study, the hydrothermally crystallized TiO₂ particles with large primary nanocrystallites showed the highest photocatalytic activity. The high photocatalytic activity mainly resulted from the high surface adsorption ability for hydroxyl ions, which was closely related to the well-developed (flat and faceted) morphology of primary nanocrystallite.     

Environmentally friendly coloured materials: cellulose/titanium dioxide/inorganic pigment composite spherical microbeads prepared by viscose phase-separation method

In order to develop environmentally friendly coloured materials, cellulose composite spherical microbeads hybridised with titanium dioxide (TiO₂) particles and inorganic pigment were prepared by a phase-separation method using viscose and an aqueous solution containing sodium polyacrylate. Findings regarding the relationships between cellulose xanthate and the electronic characteristics of TiO₂ particles used in the cellulose/inorganic material composite sphering process are also reported. These findings suggest that the location of TiO₂ particles in cellulose microbeads is related to electrical repulsion between the xanthate (CSS⁻) group and TiO₂. The use of TiO₂ powder as colour pigment is limited, as its colour is white. The cellulose composite spherical microbeads covered with TiO₂ and Fe₂O₃ particles were developed by addition of iron oxide (Fe₂O₃). Their surfaces were viewed by laser microscope and using SEM images. These composite microbeads retained the photocatalytic property of TiO₂. Cellulose/TiO₂/Fe₂O₃ composite spherical microbeads with both colour function and photocatalytic properties were successfully prepared.     

Coprecipitation and Hydrothermal Synthesis of Ultrafine 5.5 mol% CeO2-2 mol% YO1.5ZrO2 Powders

Ultrafine 5.5 mol% CeO₂—2 mol% YO_1.5ZrO₂ powders with controllable crystallite size were synthesized by two kinds of coprecipitation methods and subsequent crystallization treatment. The amorphous gel produced by ammonia coprecipitation and hydrothermal treatment at 200°C for 3.5 h results in an ultrafine powder with a surface area of 206 m²/g and a crystallite size of 4.8 nm. The powder produced by urea hydrolysis and calcination exhibits a purely tetragonal phase. In addition, the powders crystallized by hydrothermal treatment exhibit high packing density and can be sintered at lower temperature (,1400°C) with nearly 100% tetragonal phase achieved.     

Preparation of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films Via a Polyvinylpyrrolidone-Assisted Aqueous Sol–Gel Process and Dielectric Properties

Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) thin films were prepared by spin coating using aqueous solutions of metal salts containing polyvinylpyrrolidone, where niobium oxide layers and lead—magnesium–titanium oxide layers were laminated on Pt(111)/TiO _x /SiO₂/Si(100) substrates and fired at 750° or 800°C. 250 ± 20 nm thick 0.7PMN–0.3PT thin films of a single-phase perovskite could be prepared, and the film fired at 750°C had dielectric constants and dielectric loss of 1900 ± 350 and 0.13 ± 0.03, respectively, exhibiting polarization-electric field hysteresis with a remanent polarization of 5.1 μC/cm² and a coercive field of 21 kV/cm.     

Direct Formation of Zirconia-Doped Titania with Stable Anatase-Type Structure by Thermal Hydrolysis

Nanoparticles of anatase-type titania (TiO₂) doped with zirconia (ZrO₂) were directly synthesized from acidic precursor solutions of TiOSO₄ and Zr(SO₄)₂ by simultaneous hydrolysis, under mild hydrothermal conditions, at 200° and 240°C. Doping ZrO₂ into TiO₂ suppressed the crystal growth of anatase and shifted the phase transformation from anatase-type to rutile-type structure to a high temperature. The presence of an anatase-type structure with high crystallinity and high phase stability, even after annealing at 1000°C for 1 h, was fully achieved by both the doping of ZrO₂ into TiO₂ through direct precipitation and the simultaneous hydrolysis of the sulfate solutions.     

Photocatalytic Activity of Ni 8 wt%-Doped TiO2 Photocatalyst Synthesized by Mechanical Alloying Under Visible Light

Ni 8 wt%-doped titanium dioxide (TiO₂) was synthesized by mechanical alloying. The photocatalytic activity of Ni 8 wt%-doped TiO₂ powder was evaluated by measuring the visible light absorption ability by ultraviolet visible diffuse reflectance spectroscopy (UV/Vis-DRS) and photoluminescence (PL) spectroscopy. Ni 8 wt%-doped TiO₂ powders had only a rutile phase and spherical particles with an average grain size of less than 10 nm. The UV/Vis-DRS analysis showed that the UV absorption for the Ni 8 wt%-doped TiO₂ powder moved to a longer wavelength and the photoreactivity was rapidly enhanced. And PL results revealed that the new absorption was believed to be induced by localization of the trapping level near the valance band or conduction band. Moreover, Ni 8 wt%-doped TiO₂ had a high reaction activity for decomposition of 4-chlorophenol in aqueous solution under UV and visible light. To obtain the electronic structure of Ni-doped TiO₂, we have performed ab initio pseudopotential plane wave methods based on the density functional theory. The band gap of Ni-doped TiO₂ narrowed more than pure TiO₂. These results agree with the experimentally observed phenomenon.     

Synthesis and Properties of CdSe-Sensitized Rutile TiO2 Nanocrystals as a Visible Light-Responsive Photocatalyst

CdSe-modified rutile TiO₂ nanocrystals with mercaptoacetic acid as a stabilizer were prepared by the hydrothermal method. The photoactivities of the new photocatalyst under visible light illumination were demonstrated by the degradation of methylene blue (MB). The effects of CdSe content on the absorption spectra and photoactivities were discussed. After sensitization, TiO₂ significantly responded to visible light illumination and the visible light photoactivities also increased.     

(Sulfur, Nitrogen)-Codoped Rutile-Titanium Dioxide as a Visible-Light-Activated Photocatalyst

Many elements have been used to dope titanium dioxide (TiO₂) so that it absorbs light in the visible region of the solar spectrum. We have synthesized, for the first time, (S,N)-codoped rutile-TiO₂ photocatalyst using a hydrothermal method and subsequent nitridation under NH₃ flow. The (S,N)-codoping causes the absorption edge of TiO₂ to be shifted to the longer-wavelength region and two optical absorption bandgaps in the visible-light region. (S,N)-codoping has a better photocatalytic activity than S-doping and N-doping for degradation of methylene blue solution under visible light.     

Enzyme-Assisted Synthesis of Titania under Ambient Conditions

A low-temperature procedure for the synthesis of TiO₂ was developed using a water-soluble titanium complex and enzymes. Titanium–lactate and titanium–glycolate complexes were used as precursors of TiO₂. Digestive enzymes were chosen as biocatalysts for the synthesis of TiO₂ in aqueous solutions. TiO₂ powders were deposited from reaction solutions having optimal pH values for the enzymes to exhibit their catalytic activities for their original substrates. TiO₂ has been synthesized by an enzymatic reaction in an aqueous solution under ambient conditions. The XRD measurements indicated that TiO₂ powders thus obtained usually have an amorphous phase. However, after calcination at 550°C, the anatase phase was confirmed. Interestingly, the minor phase of TiO₂, brookite, was found in the sample obtained from the titanium–glycolate complex catalyzed by lipase at pH 10 after calcination at 550°C.     

The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.     

Preparation, Characterization, and Photocatalytic Activity of N, S-Codoped TiO2 Nanoparticles

The doping of titanium dioxide (TiO₂) with various metal or nonmetal elements has been considered as an effective strategy to extend the photoactive wavelength region to visible light. In this paper (nitrogen [N] and sulfur [S])-codoped anatase TiO₂ nanoparticles were prepared via a sol–gel route, followed by a heat treatment at elevated temperatures. The as-prepared samples were extensively characterized by X-ray diffraction, UV–Vis absorption spectroscopy, and X-ray photoelectron spectroscopy. The N, S-codoped TiO₂ nanoparticles showed a strong visible light absorption and exhibited an enhanced photocatalytic activity for the degradation of methylene blue as compared with the pure, N- or S-doped TiO₂ under either UV light or solar light irradiation.     

Crystalline Transition from H2Ti3O7 Nanotubes to Anatase Nanocrystallines Under Low-Temperature Hydrothermal Conditions

In this paper, we first reported the crystalline transition from H₂Ti₃O₇ nanotubes to anatase TiO₂ nanocrystallines under low-temperature hydrothermal conditions (∼140°C). A newly proposed mechanism for the crystalline transition from H₂Ti₃O₇ nanotubes to anatase TiO₂ nanocrystallines under low-temperature conditions is discussed in detail, which is supported by X-ray diffraction, high-resolution transmission electronic microscope, selected-area electron diffraction, and crystal structure models.