Titanium porous scaffolds from precursor powders: rheological optimization of TiH2 slurries

Titanium macroporous scaffolds were prepared from TiH₂ suspensions by reactive sintering, using inverse replication of a sacrificed polymer sponge. This work describes investigation on the rheological behaviour of TiH₂ suspensions with a maximum solid content of 45 vol.% in water. Stabilization of the suspensions was investigated by sedimentation tests and rheology measurements and optimized by using the proper concentration of dispersant and gelling agent.     

Photocatalytic reduction of selenium ions using different TiO2 photocatalysts

Comparative performance studies between different photocatalysts for reduction reactions are scarce. Here, Millennium PC500 and PC50 photoreactivities were investigated for selenite (Se(IV)) and selenate (Se(VI)) reduction to their elemental form (Se(0)) and compared with that of Degussa P25 TiO₂. Millennium PC500, with the highest surface area demonstrated the fastest photoreduction of Se(IV) and Se(VI), compared to P25 and Millennium PC50. Millennium PC50 and P25, which have comparable surface areas, showed similar photoreactivities. UV-Vis reflectance measurements and XRD characterisation revealed that the Se(0) deposits underwent phase transformation from amorphous to stable crystalline Se(0) during the drying and ageing process. Overall, the Millennium PCs appear to be promising photocatalysts for the photoreduction process.     

Continuous supercritical hydrothermal synthesis of controlled size and highly crystalline anatase TiO2 nanoparticles

The production of size-controlled and highly crystalline anatase titanium dioxide (TiO₂) nanoparticles was carried out under supercritical hydrothermal conditions (400 °C and 30 MPa) in a continuous flow apparatus with a residence time of 1.7 s. An industrially useful titanium sulfate (Ti(SO₄)₂) solution was used as the starting solution. KOH was used to change TiO₂ solubility and pH and thereby control the particle size. The apparatus comprised two micromixers operating at high temperature. The first mixer was configured to prepare a supercritical aqueous KOH solution from supercritical water (SC-H₂O) and KOH. The second mixer combined this KOH solution with aqueous Ti(SO₄)₂. In situ pH control and homogeneous nucleation were achieved in the second mixer. This two-step high-temperature micromixing process produced reasonably small and homogeneous particles. The particles were characterized by transmission electron microscopy (TEM) on the basis of morphology, average size, and size distribution, together with the coefficient of variation (CV). Powder X-ray diffraction (XRD) was used to determine the crystal structure and crystalline size. The weight loss of material was found through thermogravimetric (TG) measurement. The crystal structure of the product was assigned to the anatase single phase. The average particle size could be adjusted in the range 13–30 nm while maintaining a CV of 0.5 by changing the KOH concentration. At low pH, the powder XRD results for crystallite size were in good agreement with the average particle size measured by TEM, confirming that the products were single crystals of TiO₂ nanoparticles. When the reactor temperature was increased from 400 to 500 °C, the weight loss decreased from 4.5 to 2.5%, keeping the average particle size and high crystallinity of the TiO₂ particles unchanged.     

Gas-phase photo-oxidation of toluene using nanometer-size TiO2 catalysts

A modified sol–gel process was used to prepare nanostructured TiO₂ catalysts of controlled particle size (i.e. 6, 11, 16 and 20 nm). The influence of the TiO₂ particle size in the gas phase photocatalytic oxidation of toluene was investigated under both dry and humid conditions. The main products of reaction were carbon dioxide and water, although small amounts of benzaldehyde were also detected. The smaller particle size (i.e. 6 nm) lead to higher conversion and complete mineralization of toluene into CO₂ and H₂O. Both electronic and structural effects (i.e. size and ensemble effects) are responsible for the excellent performance of 6 nm TiO₂ catalyst for toluene photo-degradation. The structural differences between 6 nm TiO₂ and larger catalysts were analyzed using EPR spectroscopy.     

Photocatalytic activity of Cu2O/TiO2, Bi2O3/TiO2 and ZnMn2O4/TiO2 heterojunctions

Cu₂O/TiO₂, Bi₂O₃/TiO₂ and ZnMn₂O₄/TiO₂ heterojunctions were studied for potential applications in water decontamination technology and their capacity to induce an oxidation process under VIS light. UV–vis spectroscopy analysis showed that the junctions-based Cu₂O, Bi₂O₃ and ZnMn₂O₄ are able to absorb a large part of visible light (respectively, up to 650, 460 and 1000 nm). This fact was confirmed in the case of Cu₂O/TiO₂ and Bi₂O₃/TiO₂ by photocatalytic experiments performed under visible light. A part of the charge recombination that can take place when both semiconductors are excited was observed when a photocatalytic experiment was performed under UV–vis illumination. Orange II, 4-hydroxybenzoic and benzamide were used as pollutants in the experiment. Photoactivity of the junctions was found to be strongly dependent on the substrate. The different phenomena that were observed in each case are discussed.     

Atomic layer deposition of TiO2 films on particles in a fluidized bed reactor

Atomic layer deposition (ALD) of controlled-thickness TiO₂ films was carried out on particle substrates in a fluidized bed reactor for the first time. Films were deposited on 550 nm SiO₂ spheres and 65 nm ZnO nanoparticles for enhanced optical properties. Nanoparticles were fluidized with the assistance of a magnetically-coupled stirring unit. The metalorganic precursor titanium tetraisopropoxide was used here followed by either H₂O or H₂O₂ to deposit TiO₂ at various substrate temperatures. Growth rates of 0.01 nm/cycle and 0.04 nm/cycle were achieved when using H₂O and H₂O₂ as the oxidizer, respectively. These conformal TiO₂ films were verified using HRTEM, ICP-AES, XPS and UV absorbance measurements. The specific surface area changed appropriately after the particle size increased by the deposition of films with a given density, which showed that primary particles were not agglomerated together due to the coating process. In situ mass spectrometry was used to monitor reaction progress throughout each ALD reaction cycle. Bulk quantities of powder were successfully functionalized by TiO₂ nanofilms without wasting excess precursor.     

Preparation and characterizations of a new PS/TiO2 hybrid membranes by sol-gel process

New organic-inorganic hybrids based on PS/TiO₂ hybrid membranes were prepared by sol-gel and phase inversion process. The membranes were characterized in terms of morphology, structure, hydrophlicity, UF performance and thermal stability. The results showed that macrovoids were nearly suppressed with formation of a sponge like membrane structure. The TiO₂ particles were uniformly dispersed in membrane. The nanodispersed inorganic network formed after sol-gel process and the strong interaction between inorganic network and polymeric chains led to the improvement of porosity and thermal stability. In particular hydrophilicity and permeability increased drastically with the increasing of TiO₂ content in the range of 0-9.3 wt%, without changing retention properties of membrane. However, high-TiO₂ concentration induces nanoparticles aggregate, resulting in the decline of hydrophilicity and permeability. Thus, PS/TiO₂ hybrid membranes with proper TiO₂ content are desirable to meet some specific requirements in industrial separation.     

Modified titanium electrodes: Application to Ti/TiO2/PbO2 dimensionally stable anodes

Titanium is a valve metal able to withstand corrosion, due to the presence of a passivating layer of titanium oxide on its surface. But, due to that more or less insulating layer, titanium cannot be used directly as an anodic material. However, modification of the surface of a Ti/TiO₂ substrate may lead to the formation of new structures: Ti/TiO₂/M or Ti/TiO₂/OX, in which M is a metal such as platinum and OX a conducting oxide exhibiting electrocatalytic properties. These structures have interesting electrochemical properties and may be used as efficient electrode materials.In this paper, after a review of the electrochemical behaviour of these structures, we give new results concerning the selective electrodeposition of lead dioxide on Ti/TiO₂ substrates and we propose an interpretation of the results taking into account the dielectric properties of the underlying TiO₂. It is shown that there is a dramatic decrease of the resistance of the electrode when a PbO₂ layer is electrodeposited onto a Ti/TiO₂ structure. That effect allows the preparation of electrodes (low-cost DSAs) that may be used as anodes in spite of the presence of the underlying TiO₂ layer, that layer being useful to avoid corrosion of the titanium substrate. At last, the effect of stabilization of the underlying TiO₂ layer is discussed.     

Preparation of polyaniline-modified TiO2 nanoparticles and their photocatalytic activity under visible light illumination

Titanium dioxide nanoparticles were modified by polyaniline (PANI) using ‘in situ’ chemical oxidative polymerization method in hydrochloric acid solutions. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy spectrum (XPS) and UV–vis spectra were carried out to characterize the composites with different PANI contents. The photocatalytic degradation of phenol was chosen as a model reaction to evaluate the photocatalytic activities of the modified catalysts. Results show that TiO₂ nanoparticles are deposited by PANI to mitigate TiO₂ particles agglomeration. The modification does not alter the crystalline structure of the TiO₂ nanoparticles according to the X-ray diffraction patterns. UV–vis spectra reveal that PANI-modified TiO₂ composites show stronger absorption than neat TiO₂ under the whole range of visible light. The resulting PANI-modified TiO₂ composites exhibit significantly higher photocatalytic activity than that of neat TiO₂ on degradation of phenol aqueous solution under visible light irradiation (λ ≥ 400 nm). An optimum of the synergetic effect is found for an initial molar ratio of aniline to TiO₂ equal to 1/100.     

Multifunctional TiO2 coatings for Cultural Heritage

Environmental pollution arising from industrial implants and urban factors is constantly increasing, causing aesthetical and durability concerns to urban structures exposed to the atmosphere.     

Photocatalytic degradation of imidazolinone fungicide in TiO2-coated optical fiber reactor

The photocatalytic degradation of the fungicide fenamidone is studied in a TiO₂-coated optical fiber photoreactor. Fenamidone is slowly transformed with a kinetic order of 1 and a degradation rate of 0.02 h⁻¹. Intermediate products were isolated and identified by means of solid phase extraction (SPE) coupled to liquid chromatography-mass spectrometry techniques (LC-MS). A proposed degradation pathway of fenamidone is presented, involving mainly hydroxylation and oxidation reactions. Carboxylic acids and sulfate ions resulting from the same reaction in a powder reactor were also identified.     

Photocatalytic degradation of Chromotrope 2R using nanocrystalline TiO2/activated-carbon composite catalysts

Composite catalysts made of nanocrystalline TiO₂ and carbon were prepared by a modified sol–gel method over activated carbon (AC). The composite catalysts were characterized by N₂ adsorption–desorption isotherm, TG, diffuse reflectance UV–vis spectroscopy, XRD and SEM. The photocatalytic activity was tested on the degradation of Chromotrope 2R (C2R) in aqueous medium under UV radiation. The composite catalysts exhibited higher activities than commercial Degussa P25 alone and the photocatalytic process was more efficient than the pure photolytic degradation. A modified Langmuir–Hinshelwood approach was used to study the kinetics and to determine the adsorption equilibrium constant and the reaction rate constant. Two different mechanisms are proposed and discussed in order to explain the observed synergy.     

Photocatalytic degradation of methylene blue with TiO2 nanoparticles prepared by a thermal decomposition process

The generation of TiO₂ nanoparticles by the thermal decomposition of titanium tetraisopropoxide (TTIP) was carried out experimentally using a tubular electric furnace at various synthesis temperatures (700-1300 °C) and TTIP heating temperatures (80-110 °C). The photocatalytic activity of the resulting TiO₂ nanoparticles was examined by measuring the rate of methylene blue decomposition. The TiO₂ nanoparticles were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) measurements and transmission electron microscopy (TEM). The crystallite size and crystallinity increased with increasing synthesis temperature and TTIP heating temperature. A TTIP heating temperature and synthesis temperature of 95 °C and 900 °C, respectively, were found to be the optimal synthesis conditions. The primary particle diameter obtained under optimum synthesis conditions was considerably smaller than the commercial photocatalyst (Degussa, P25). The specific surface areas were more than 134.4 m² g^− 1. Under the optimal conditions, the photocatalytic activity for methylene blue was higher than that of the commercial photocatalyst.     

Preparation of TiO2 thin films on glass beads by a rotating plasma reactor

We experimentally coated the TiO₂ thin films on the glass beads by a rotating cylindrical plasma chemical vapor deposition (PCVD) process. The precursors for the thin films were generated by the plasma reactions, and they deposited on the glass beads to become the grains on the films. The TiO₂ thin films grow more quickly on the glass beads by increasing the reactor pressure, or the rotation speed of the reactor. As the applied power increases, the thickness of the thin films on the glass beads decreases. As the thickness of the TiO₂ thin films increases, the uniformity of the TiO₂ thin films decreases due to the deposition of larger grains or due to the increase of crack size. The rotating cylindrical PCVD process can be a good method to prepare the particles coated with metal or organic-doped thin films for highly functionalized materials.     

二氧化钛分级结构材料的研究进展

二氧化钛(TiO2)具有优越的光电性能和物理特性而备受关注。通过制备良好形貌的TiO2,来优化其相关性能是一条有效改善性能的方式。综述了不同的TiO2分级结构的制备及其对相关性能的优化作用机理,并对今后制备不同分级结构的TiO2材料做出展望。     

Photocatalytic treatment of water-soluble pesticides by photo-Fenton and TiO2 using solar energy

The technical feasibility and performance of photocatalytic degradation of four water-soluble pesticides (diuron, imidacloprid, formetanate and methomyl) have been studied at pilot scale in two well-defined systems of special interest because natural-solar UV light can be used: heterogeneous photocatalysis with titanium dioxide and homogeneous photocatalysis by photo-Fenton. The pilot plant is made up of compound parabolic collectors (CPCs) specially designed for solar photocatalytic applications. Experimental conditions allowed disappearance of pesticide and degree of mineralisation achieved in the two photocatalytic systems to be compared. In order to assure that the photocatalytic results are consistent, hydrolysis and photolysis tests have been performed with the four pesticides. The initial concentration tested with imidacloprid, formetanate and methomyl was 50 and 30 mg/l with diuron, and the catalyst concentrations were 200 mg/l and 0.05 mM with TiO₂ and iron, respectively. Total disappearance of the parent compounds and 90% mineralisation have been attained with all pesticides tested, methomyl being the most difficult to be degraded with both treatments. First-order rate constants, initial rate, time necessary for mineralising 90% of the initial TOC and hydrogen peroxide consumption were calculated in all cases, enabling comparison both of treatments and of the selected pesticide reactivity.     

Characteristics of in-situ synthesized Hydroxyapatite on TiO2 ceramic via plasma electrolytic oxidation

The biocompatibility properties of Ti scaffolds can be improved significantly by hydroxyapatite (HA) composite coating. We successfully coated the surface of the Ti substrates by in-situ formation of HA nanocrystals on TiO₂ sublayer under calcium acetate and trisodium phosphate electrolytes through the plasma electrolytic oxidation (PEO) process. The effects of the process parameters and passivation on the characteristics of the coated substrates were studied using X–ray diffraction, Fourier-transform infrared spectroscopy, Field emission scanning electron microscopy, and Energy–dispersive X–ray spectroscopy. The systematically controlled experimental studies indicated that using the higher calcium/phosphorous ratio in the electrolyte enhances the micro arcs power and consequently, thickens the synthesized HA layer. The HA nanocrystals were tailored on the walls and edges of the discharge channels due to the locally concentrated heating zones, which consequently resulted in noticeable amounts of Ca and P dopants in porous TiO₂.     

Nano-particle synthesis of titanium oxides from ilmenite in a thermal plasma reactor

Nano-particle of titanium oxides was synthesised from the precursor ilmenite powder in an argon plasma jet at atmospheric pressure. The decomposition of ilmenite concentrates was investigated in a non-transferred arc thermal plasma reactor. Argon and Argon–Nitrogen were taken as plasma gases in the plasma torch. The effect of input power, plasma gases and particle size was carried out on the quality and yield of the product. Nano-powders were characterized using various techniques, viz., XRD, SEM, and TEM. Thermal equilibrium diagram was plotted to predict the thermodynamic behaviour of the system under various operating conditions. The present investigation shows significant segregation of titanium and iron-rich products in thermal plasma processing. Average particles diameters are from 10 nm to 200 nm.     

Gravity consolidation-sedimentation behaviors of concentrated TiO2 suspension

Consolidation-sedimentation behaviors of consolidated sediment under action of gravity were investigated using highly concentrated suspension of titanium dioxide particles under conditions of various pHs, initial heights, and initial concentrations. The average consolidation ratio of the consolidated sediment was analyzed on the basis of the simplified analytical solution obtained using the modified Terzaghi's model under the moving Lagrangian coordinate system. Although the solution pH strongly affects consolidation-sedimentation behaviors, the modified average consolidation coefficient was little influenced by the solution pH. Also, the consolidation coefficient increased in almost direct proportion to the total volume of solids per unit cross-sectional area, which changes the driving force of consolidation-sedimentation due to the overlying weight of the solid particles. The variations with time of the height of the consolidated sediment were adequately described using the analytical solution describing the average consolidation ratio with the aid of the relation that the equilibrium height was represented by a power function of the total volume of solids for a specified pH.     

Continuous photodegradation of naphthalene in water catalyzed by TiO2 supported on glass Raschig rings

Polycyclic aromatic hydrocarbons (PAH's) are a class of persistent organic pollutants of special concern since they are carcinogenic and mutagenic. In this paper, the design of a continuously stirred tank reactor is reported for the photodegradation of the simplest and most water-soluble PAH, naphthalene, in water using TiO₂ (in the crystalline form of anatase), supported on glass Raschig rings as catalyst, with oxygen as electron acceptor. A first order kinetic rate constant has been calculated for this photodegradation. The irradiated solution after the reaction has been analysed and only traces of 1-naphthol, 1,4-naphthalenedione and phthalates have been found as intermediate products of the photodegradation.