Anatase Crystals Research Articles

Producing photoactive, transparent and hydrophobic SiO2-crystalline TiO2 nanocomposites at ambient conditions with application as self-cleaning coatings

Nowadays, the enhancement of atmospheric pollution is dramatically increasing the presence of soiling on buildings in every city of the world. Thus, the development of photocatalysts as self-cleaning coatings is a promising challenge. The first object of this work was to develop a simple synthesis route for obtaining SiO2-crystalline TiO2 nanocomposites at ambient temperature. Thus, it meets the requirements to produce photoactive coatings on buildings and, additionally, it can be used in other applications requiring low temperature. The second objective was to gain insights into the structure of these materials and to establish the relationship between their structure and their performance as photocatalysts.The synthesis process involves mixing titanium and silicon alkoxides in the presence of oxalic acid. An organic silica oligomer is also added to reduce surface energy and consequently, to give hydrophobic properties to the material. These products were applied as a coating on marble specimens in order to investigate their effectiveness. The results obtained highlight that oxalic acid acts a chelating agent of the titanium precursor, giving rise to a homogeneous TiO2–SiO2 material. In addition and most importantly, oxalic acid produces photoactive anatase crystals at ambient temperature. From our investigation on the structure of these materials, we conclude that the formation of independent domains of TiO2 inside of SiO2 matrix, and the anatase crystal size are key factors for improving the photoactivity of the coatings. We also conclude that the oxalic acid concentration plays a crucial role in the formation of this optimum structure.

Nanocrystalline TiO2 and Halloysite clay mineral composite films prepared by sol-gel method: Synergistic effect and the case of silver modification to the photocatalytic degradation of Basic Blue- 41 azo dye in water.

<div> <p>Tubular halloysite clay mineral and nanocrystalline TiO<sub>2 </sub>were incorporated in the preparation of nanocomposite films on glass substrates via sol-gel method at 450ºC. The synthesis involves a simple chemical method employing nonionic surfactant molecule as pore directing agent along with the acetic acid-based sol-gel route without addition of water molecules. Drying and thermal treatment of composite films ensure elimination of organic material and lead to the formation of TiO<sub>2</sub> nanoparticles homogeneously distributed on the surface of the halloysite. Nanocomposite films without cracks of active anatase crystal phase and small crystallite size on halloysite nanotubes are characterized by microscopy techniques and porosimetry methods in order to examine their structural properties. The composite halloysite-TiO<sub>2 </sub>films with variable quantities of halloysite were examined as photocatalysts to the discoloration of Basic Blue 41 azo dye in water. These nanocomposite films proved to be very promising photocatalysts and highly effective to dye’s discoloration in spite of small amount of halloysite/TiO<sub>2</sub> catalyst immobilized onto glass substrates. It also has been shown that the efficiency of the halloysite/TiO<sub>2</sub> films could be further improved when silver particles were deposited on their surface after successful adsorption from an aqueous solution of a silver salt and UV reduction of the adsorbed ions.</p> </div> <p> </p>

Effect of the RE (RE = Eu, Er) doping on the structural and textural properties of mesoporous TiO2 thin films obtained by evaporation induced self-assembly method

Polymeric sol–gel route has been used for the preparation of TiO2 and RE2O3–TiO2 (RE=Eu, Er) mesoporous thin films by evaporation induced self-assembly method using Si (100) as a substrate. The influence of the relative humidity (RH) on the preparation of the film has been studied being necessary to work under 40% RH in order to obtain homogeneous and transparent thin films. The films were annealed at different temperatures until 900°C/1h and the anatase crystallization and its crystal size evolution were followed by low angle X-ray diffraction. Neither the anatase–rutile transition nor the formation of other compounds was observed in the studied temperature range. Ellipsoporosimetry studies demonstrated that the thickness of the thin films did not change after calcination at 500°C, the porosity was constant until 700°C, the pore size increased and the specific surface area decreased with temperature. Moreover, the effect of the doping with Er3+ and Eu3+ was studied and a clear inhibition of the crystal growth and the sintering process was detected (by transmission electron and atomic force microscopy) when the doped films are compared with the undoped ones. Finally, Eu3+ and Er3+ f–f transitions were detected by PL measurements.

N-doped TiO2 visible light photocatalyst films via a sol–gel route using TMEDA as the nitrogen source

Abstract N-doped TiO 2 thin films were successfully prepared using N,N,N′,N′-tetramethylethane-1,2-diamine (TMEDA) via a single step sol–gel synthesis. Characterisation by X-ray diffraction and Raman spectroscopy confirmed that the anatase crystal type of TiO 2 was present in all samples. UV/Vis spectroscopy was used to determine the absorbance and band-gap of the materials. Stearic acid and resazurin dye Photo-degradation measurements were used to show the enhanced photocatalytic properties of the materials. Both of these methods are well known for the assessment of photocatalytic films for self cleaning applications with stearic acid being the preferred model for bacteria. The N-doped films showed superhydrophilicity under filtered white light conditions, whereas the undoped films did not show any photo-induced superhydrophilicity. The N-doped TiO 2 films show interstitial nitrogen content rather than substitutional and a substantial red-shift of the band-gap. They were shown to be superior visible light photocatalysts when compared to similar non-doped films. The nature of the chelating ligand, TMEDA, has a profound effect on the oxidation state of the Ti centres, with low concentrations of TMEDA promoting the formation of Ti 3+ centres giving a superior photocatalyst. This method is to the best of our knowledge the easiest, reliable and most reproducible one step route to achieve N-doped TiO 2 films for photocatalytic applications. These N-doped TiO 2 films could have potential applications; as self-cleaning surfaces in healthcare, water sterilisation, and solar energy harvesting.

Cerium-modified TiO2 nanocrystalline films for visible light photocatalytic activity

Abstract Cerium modified thin nanocrystalline TiO 2 photocatalytic films for visible light utilization, are prepared via sol–gel route at 500 °C on glass substrates. The synthesis involves a simple procedure employing nonionic surfactant molecule as pore directing agent along with the acetic acid-catalyzed sol–gel route without direct addition of water molecules while cerium was present at different concentrations. Drying and thermal treatment of the films led to the formation of TiO 2 nanoparticles with homogeneous particle size distribution. The presence of cerium induced a decrease of the TiO 2 nanoparticle size. Nanocomposite films were formed without cracks and consisted of anatase crystal phase and small crystallite size ranging from 7 to 10 nm. All materials were characterized by microscopy techniques, UV–vis and porosimetry methods in order to examine their structural properties. The Ce-modified TiO 2 films with various cerium contents were evaluated as visible light photocatalysts for the photo-oxidation of potential organic pollutants in water.

Synthesis of high surface area TiO2 nanoparticles by mild acid treatment with HCl or HI for photocatalytic propene oxidation

Nanostructured TiO2 photocatalysts with small crystalline sizes have been synthesized by sol–gel using the amphiphilic triblock copolymer Pluronic P123 as template. A new synthesis route, based on the treatment of TiO2 xerogels with acid–ethanol mixtures in two different steps, synthesis and extraction–crystallization, has been investigated, analyzing two acids, hydrochloric and hydriodic acid. As reference, samples have also been prepared by extraction–crystallization in ethanol, being these TiO2 materials amorphous and presenting higher porosities. The prepared materials present different degrees of crystallinity depending on the experimental conditions used. In general, these materials exhibit high surface areas, with an important contribution of microporosity and mesoporosity, and with very small size anatase crystals, ranging from 5 to 7nm.The activity of the obtained photocatalysts has been assessed in the oxidation of propene in gas phase at low concentration (100ppmv) using a UVA lamp with a wavelength of 365nm. Under the conditions studied, these photocatalysts show different activities in the oxidation of propene which do not depend on their surface areas, but on their crystallinity and band gap energies. Sample prepared with HCl both during synthesis and in extraction–crystallization steps is the most active one, with superior performance than Evonik P25.

Optimisation of HiPIMS photocatalytic titania coatings for low temperature deposition

Titanium dioxide in its anatase form is widely used in photocatalytic applications due to its high photocatalytic activity, stability and low cost. Titania coatings directly deposited by conventional magnetron sputtering tend to have an amorphous microstructure. For the anatase structure to develop, substrate heating or post-deposition thermal treatment is usually required, with the anatase crystal phase generally forming at temperatures in excess of 400°C. This precludes the choice of thermally sensitive substrate materials for the photoactive coating.Depending on the nature of the driving voltage waveform, high power impulse magnetron sputtering (HiPIMS) has been shown to deliver a relatively low thermal flux to the substrate, whilst still allowing the direct deposition of crystalline titania coatings. Consequently, this technique offers the potential to deposit photocatalytically active titania coatings directly onto polymeric substrates and, therefore, opens up a range of new applications. In the present work a range of titanium dioxide thin films were deposited by HiPIMS onto glass substrates in order to study the influence of various process parameters, such as pressure, pulse frequency and pulse duration on coating structure and photocatalytic properties. The photocatalytic properties of the coatings were assessed by their ability to degrade the organic dye methylene blue under UV and fluorescent light irradiation. The degradation rate of methylene blue was calculated by measuring its absorption peak height at 665nm in continuous mode under UV/fluorescent light source. The hydrophilic properties of the coatings were also investigated by measuring the contact angle of water droplets on the coating surfaces. Experimentally, the optimum conditions to maximise the photocatalytic performance of the coatings were found. The influence of various deposition parameters on the photocatalytic properties and crystal structure of the coatings is discussed.Optimised coatings then were deposited onto polymeric substrates, such as polyethylene terephthalate (PET) and polycarbonate, to assess the suitability of using this method for high-energy, low-temperature deposition of photoactive titania coatings and the relevance of the optimised condition was tested for other types of substrates other than glass. It was found that titania coatings deposited by HiPIMS directly onto polymeric substrates showed relatively high levels of activity in their as-deposited state.The ability to deposit crystalline titania with photocatalytic functionality at temperatures low enough to enable the use of polymer substrates is a significant advancement in the field. It could potentially allow the production of high volumes of photocatalytic material on substrates, such as polymer web, which is not possible with current deposition techniques.

Photoinduced properties of nanocrystalline TiO2-anatase coating on Ti-based bone implants

The paper reports on the photoinduced properties of hydrothermally treated (HT) titanium used for bone implants. The anatase coatings composed of 30–100nm anatase crystals exhibited high photocatalytic activity and good photo-induced wettability, reaching a superhydrophilic state, despite the larger crystal dimensions than the previously reported optimal ones. These properties are due to a suitable combination of surface texture, roughness, thickness, crystal morphology and particle size, which allowed the two independent photo-induced phenomena to occur simultaneously. The results on caffeine degradation by photocatalysis and the prolonged effect (up to two weeks) of photo-induced wettability in dark suggested a possible applicability of the HT anatase coatings as bacteria-repelling surfaces for body implants, in favor of a better osseointegration in vivo.

Sol–gel derived Ta-containing TiO2 films on surface roughened NiTi alloy

The surface of NiTi alloy was roughened by NaOH–HCl treatment, and the Ta-containing TiO2 films were coated on the pretreated NiTi alloy by the sol–gel method. Thermal analyses indicate that the evaporation temperature of the organics decreases with the addition of tantalum ethoxide in the TiO2 sol, but the crystallization temperature of anatase increases. The NaOH–HCl pretreatment improves the film integrity, but cracks still form in the films at high Ta contents (≥20 %, molar ratio) owing to the increasing film thickness. X-ray diffraction (XRD) confirms that the addition of Ta suppresses the crystallization of anatase. X-ray photoelectron spectroscopy (XPS) reveals that Ta exists as Ta2O5 in the film. With the increase of Ta content, the hydrophilic conversion of the films under UV illumination is impeded, but their corrosion resistance in 0.9 % NaCl solution increases, tested by the potentiodynamic polarization. The coating samples have acceptable hemolysis ratios for biomaterials (<5). The introduction of Ta improves the anti-aggregating function of the TiO2 film in the platelet adhesion test.

Synthesis of iron-doped TiO2 for degradation of reactive Orange16

In this study the optimum conditions for preparing the iron-doped TiO2 nanoparticles were investigated. Samples were synthesized by sol–gel impregnation method. Three effective parameters were optimized using Taguchi method, consisted of: (i) atomic ratios of Fe to Ti; (ii) sintering temperature; (iii) sintering time. The characterization of samples was determined using X-ray diffraction, BET- specific surface area, UV- Vis reflectance spectra (DRS) and scanning electron microscope (SEM). The XRD patterns of the samples indicated the existence of anatase crystal phase in structure. UV- Vis reflectance spectra showed an enhancement in light absorbance in the visible region (wavelength > 400 nm) for iron-doped samples. The photocatalytic activity of samples was investigated by the degradation of RO 16 (RO 16) dye under UV irradiation. The results illustrated that the photocatalytic activity of iron-doped TiO2 was more than pure TiO2, because of the smaller crystal size, grater BET surface area and higher light absorption ability.

WO3/TiO2heterostructures tailored by the oriented attachment mechanism: insights from their photocatalytic properties

Efficient WO3/TiO2 photocatalysts were obtained through the Oriented attachment mechanism via hydrothermal annealing, which entailed using previously prepared WO3 particles and commercial TiO2 nanoparticles as “building blocks”. The photoreactivity of this material towards organic contaminant degradation was enhanced compared to commercial TiO2 because the coupling of the WO3/TiO2 structures reduced the recombination of the photogenerated charges in anatase TiO2. The pristine WO3 nanoparticles were prepared by hydrothermally assisted degradation of a W-peroxocomplex and all of the prepared materials were characterized by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, electron microscopy, Near-Infrared spectroscopy, nitrogen physical adsorption (BET method), photoluminescence spectroscopy (PL) and diffuse reflectance spectra (DRS). The optimal compositional ratio for the WO3/TiO2 heterostructures was 30–40 wt% of WO3. We also provided the interface formation in the HRTEM images by observing the presence of the lattice spacing of the (101) and (120) planes of TiO2 anatase and WO3 orthorhombic crystals, respectively, which allows the charge migration during the photocatalyzed degradation of the heterostructures, thereby improving their photoactivity.

Coexistence of an anatase/TiO2(B) heterojunction and an exposed (001) facet in TiO2 nanoribbon photocatalysts synthesized via a fluorine-free route and topotactic transformation

In this work, we report a novel approach to fabricate hierarchical TiO2 microspheres (HTMS) assembled by ultrathin nanoribbons where an anatase/TiO2(B) heterojunction and high energy facet coexist. The as-adopted approach involves (1) nonaqueous solvothermal treatment of a mixture of tetrabutyl titanate and acetic acid and (2) topotactical transformation into HTMS via thermal annealing. By this approach, the TiO2(B) phase usually synthesized from an alkaline treatment route could be initially formed. Subsequently, phase transition from TiO2(B) to anatase TiO2 occurs upon thermal treatment. It is demonstrated that such phase transition is accompanied by crystallographic orientation along the c-axis of anatase and TiO2(B) crystals, resulting in not only a coherent interface between two phases but also oriented attachment of anatase mesocrystals along the [001] direction, and finally high-energy (001) facet exposure. Interestingly, this work provides an alternative fluorine-free route for the synthesis of TiO2 crystals with high-energy (001) facet exposure. The structural analysis reveals that lattice-match induced topotactic transformation from TiO2(B) to anatase is the sole reason for the (001) facet exposure of anatase TiO2. The photocatalytic test for acetaldehyde decomposition shows that HTMS with anatase/TiO2(B) heterojunction and high-energy (001) facet exhibits superior photocatalytic efficiency compared with the relevant commercial product P25, which can be ascribed to the synergistic effect of large surface area, anatase/TiO2(B) heterojunction as well as high-energy facet exposure.

Influence of CeO2 addition on the physicochemical and catalytic properties of Pd/TiO2 catalysts in CO oxidation

The influence of CeO2 addition on the formation of the microstructure, electronic state, and catalytic properties of Pd/TiO2 supported catalysts in CO oxidation were investigated. It was shown that, when Pd is supported on titanium dioxide modified with cerium dioxide, annealing at 500°C results in the formation of Pd/(CeO2-TiO2) catalysts with a nanocrystalline structure composed of incoherently intergrown fine anatase crystals and interblock boundaries in which palladium and cerium are stabilized. The higher catalytic activity of Pd/(CeO2-TiO2) catalysts compared to Pd/TiO2 catalysts is explained by the smaller size of Pd particles and the higher proportion of palladium in the Pdδ+ state.

Photocatalytic Activity of Hierarchically Structured TiO2Films Synthesized by Chemical Vapor Deposition

Hierarchically structured TiO2photocatalyst films were synthesized using low-pressure metal-organic chemical vapor deposition (LPMOCVD) method to examine their photocatalytic activity. The thickness of the TiO2films increased proportionally with increasing deposition time. The TiO2film synthesized at 773 K showed a hierarchical structure composed of vertically grown laminar (112)-oriented anatase crystals. With increasing deposition time, the grain became larger and the morphology became sharper. In the initial CVD stage, small particular crystals were formed, above which sequential growth of layers of columnars with increasing size took place, forming hierarchical structure. The hierarchically structured TiO2film exhibited much higher photocatalytic activity than unhierarchically structured TiO2film. The photocatalytic activity increased with increasing film thickness.

Improvement of Orange II Photobleaching by Moderate Ga3+ Doping of Titania and Detrimental Effect of Structural Disorder on Ga Overloading

Highly photoactive Ga3+‐doped anatase modification of titania was prepared by homogeneous hydrolysis of aqueous solutions mixture of titanium oxo‐sulphate TiOSO4 and gallium(III) nitrate with urea. Incorporation of Ga3+ into the anatase lattice has a clear positive effect on the photocatalytic activity under UV and Vis light irradiation up to a certain extent of Ga. Ga3+ doping decreased the size of the crystallites, increased surface area, and affected texture of the samples. Higher amount of gallium leads to the formation of a nondiffractive phase, probably photocatalytically inactive. The titania sample with 2.18 wt.% Ge3+ had the highest activity during the photocatalysed degradation in the UV and visible light regions; the total bleaching of dye Orange II was achieved within 29 minutes. Ga concentration larger than 5% (up to 15%) significantly inhibited the growth of the anatase crystal domains which formed the nondiffractive phase content and led to remarkable worsening of the photobleaching efficiency.

Control of Exposed Facet and Morphology of Anatase Crystals through TiOxFy Precursor Synthesis and Impact of the Facet on Crystal Phase Transition

Anatase crystals with 30% and 80% of {001} facet exposures are prepared by decomposition of two fluorine-contained precursors (TiOxFy). The two precursors were obtained via solvothermal process by controlling the F– and Ti4+ concentrations and had a flower-like and hexagonal pencil-like structure, respectively. The two anatase products by calcination of the corresponding precursors at 700 °C have unique morphologies: sheet-like single crystals with square pores and oriented assembly of truncated-octahedron anatase crystals, respectively. Both of them are inherited from the morphologies of the precursor particles. We reveal for the first time that the phase transition temperature of the crystal with larger {001} facet percentage is lower, and the in situ SEM experiments indicate that numerous defect sites first form on {001} facet during the phase transformation of anatase to rutile, while the {101} facet on the same crystal remains smooth. The formation of defect sites is attributed to the preferential (r...

A novel 3D structure composed of strings of hierarchical TiO2 spheres formed on TiO2 nanobelts with high photocatalytic properties

A novel hierarchical titanium dioxide (TiO2) composite nanostructure with strings of anatase TiO2 hierarchical micro-spheres and rutile nanobelts framework (TiO2 HSN) is successfully synthesized via a one-step hydrothermal method. Particularly, the strings of hierarchical spheres are assembled by very thin TiO2 nanosheets, which are composed of highly crystallized anatase nanocrystals. Meanwhile, the HSN has a large surface area of 191m2/g, which is about 3 times larger than Degussa P25. More importantly, the photocatalytic activity of HSN and P25 were evaluated by the photocatalytic oxidation decomposition of methyl orange (MO) under UV light illumination, and the TiO2 HSN shows enhanced photocatalytic activity compared with Degussa P25, as result of its continuous hierarchical structures, special conductive channel and large specific surface area. With these features, the hierarchical TiO2 may have more potential applications in the fields of dye-sensitized solar cells and lithium ion batteries.

Synthesis of photocatalytic TiO2 nano-coatings by supersonic cluster beam deposition

In this paper we report on the photocatalytic behavior in gas phase propane oxidation of well-defined TiO2 nanoparticle (NP) coatings prepared via Supersonic Cluster Beam Deposition (SCBD) on Si-wafers and quartz substrates. The temperature dependent crystal phase of the coatings was analyzed by Raman spectroscopy, and the morphology by High Resolution-Scanning Electron Microscopy.SCBD deposition in the presence of oxygen enables the in situ synthesis of TiO2 layers of amorphous NP at room temperature. Adapting the deposition temperature to 500°C or 650°C leads to Anatase crystals of variable size ranges, and layers showing significant porosity. At 800°C mainly Rutile is formed. Post annealing by wafer heating of the amorphous NP prepared at room temperature results in comparable temperature dependent phases and morphologies.Photocatalytic activity in propane oxidation was dependent on the morphology of the samples: the activity decreases as a function of increasing particle size. The presence of water vapor in the propane feed generally increased the activity of the wafer-heated samples, suggesting OH groups are not profoundly present on SCBD synthesized layers. In addition, a remarkable effect of the substrate (Si or Quartz) was observed: strong interaction between Si and TiO2 is largely detrimental for photocatalytic activity.The consequences of these findings for the application of SCBD to synthesize samples for fundamental (spectroscopic) study of photocatalysis are discussed.

Solvothermal synthesis and photocatalytic performance of Mg2+-doped anatase nanocrystals with exposed {0 0 1} facets

The photocatalytic activity of TiO2 and magnesium doped TiO2 nanocrystals in dopant range of 2–6.2at%, was studied. The doped and undoped nanocrystals with exposed {001} crystal facets were synthesized by a solvothermal method. Several studies have shown that the (001) surface of the TiO2 anatase crystal is more reactive than the thermodynamically stable (101) surface. The crystal structure as well as the shape of the TiO2 and Mg2+/TiO2 anatase nanoparticles were determined using two different techniques, such as X-ray powder diffraction (XRD) and transmission electron microscopy (TEM), which both lead to the agreeable conclusion that the nanocrystals are in the form of plates. Chemical analysis of the photocatalyst was carried out with X-ray photoelectron spectroscopy (XPS) and showed the presence of magnesium ions in the TiO2 nanoplates. UV–vis diffuse reflectance spectroscopy (DRS) showed that there is an adsorption shift for doped TiO2 to visible light region.The photocalaytic efficiency of the synthesized catalysts was investigated by the photocatalytic oxidation of the gaseous nitric oxide (NO) and decomposition of the gaseous acetaldehyde (CH3CHO) under UV irradiation. It was demonstrated that the low Mg2+ doped catalysts exhibited higher photocatalytic activity than the pure TiO2. The optimal concentration of dopant that is beneficial for the photocatalytic activity was studied.

Room-temperature ferromagnetism in Ni2+ doped TiO2 nanocrystals synthesized from nanotubular precursors

Hydrothermal synthetic route for synthesis of Ni2+ doped TiO2 nanocrystals showing room temperature ferromagnetism, using dispersion of titania nanotubes in the presence of Ni2+ ions at different pHs as precursors, is reported. Morphologies of tubular precursors and Ni2+ doped TiO2 nanocrystals were characterized by transmission electron microscopy. Titania nanotubes have outer diameter of about 10nm while the length varies widely even reaching few hundred nanometers. Ni2+ doped TiO2 nanoparticles synthesized at pH 3 have polygonal shape and dimension of about 20nm. Transmission electron microscopy analysis revealed presence of mixed shapes (polygonal and rod like) in the sample of Ni2+ doped TiO2 nanoparticles synthesized at pH 5. X-ray diffraction analysis of resultant powder and selected area electron diffraction pattern confirmed the anatase crystal phase of Ni2+ doped TiO2 nanoparticles independently of dopant concentrations (0.09, 0.25, 0.86, 1.48 and 1.80at%), followed by slight changes of their microstructural parameters. Reflection spectra of Ni2+ doped TiO2 nanoparticles revealed their altered optical properties in comparison to undoped TiO2 nanoparticles. Room temperature ferromagnetic ordering with saturation magnetic moment in the range of 10−3–5×10−2μB per Ni atom was observed for all measured films made of Ni2+ doped TiO2 nanoparticles.