Sol–gel synthesis of mesoporous anatase–brookite and anatase–brookite–rutile TiO2 nanoparticles and their photocatalytic properties

PEGylated doped- and undoped-TiO2 nanoparticles for photodynamic Therapy of cancers

Titanium dioxide has been widely known for its phototoxicity in the environmental context, but little is known for its use in the photodynamic therapy of cancers. Previous studides have shown the hazardous effects of undoped-titanium dioxide nanoparticles (undoped-TiO₂ NPs) in the ecosystem; however, it remains to explore the effect of polyethylene glycol (PEG) conjugation and doping of metal and non-metal on the photodynamic activity of TiO₂. Here we report the synthesis, characterizations, and applications of doped- and undoped-TiO₂ NPs stabilized by PEG in the photodynamic therapy of cancers. Our results demonstrate that in vitro PEG-NPs significantly reduced the survival of human cervical cancer cells (HeLa) upon solar and ultraviolet (UV) radiations. We found that doping of the metal (cobalt) and non-metal (nitrogen) onto TiO₂ nanocrystals enhanced the photoactivation of doped-TiO₂ NPs in the visible/near infrared (Vis/NIR) region, but these nanocrystals were revealed by cytotoxicity assays to be less potent in killing cancer cells compared to PEGylated undoped-TiO₂. The significant photodynamic effect was shown by PEGylated undoped-TiO₂ synthesized through the sol-gel method with 75% killing of HeLa cells at 5.5 μg/mL concentrations in exposure to UV or sunlight radiations. In vitro cytotoxicity was measured by Sulforhodamine B (SRB) and 3-(4, 5-Dimethylthiazol-2-Yl)-2,5-Diphenyltetrazolium Bromide (MTT) assays after irradiations with IR, UV, and sunlight for 15-30 minutes (min). All the synthesized NPs were characterized by XRD, AFM, SEM, EDX and DRS chemical analysis. Taken together, our data demonstrate that water-soluble PEGylated TiO₂ NPs maybe a good candidate for the photodynamic therapy of cervical cancer cells. Our data propose that the use of PEG surfactant can enhance the potency of already available photochemical therpeutic agents.

Photocatalytic Activity of TiO₂ Nanofibers: The Surface Crystalline Phase Matters

The crystal phases and surface states of TiO₂ can intrinsically determine its performance in the applications of photocatalysis. Here, we prepared TiO₂ nanofibers with different crystal phase contents by electrospinning followed via calcination at different temperatures. The TiO₂ nanofibers were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectrometry, transmission electron microscopy (TEM), and photocatalytic performance testing. The results showed that the phases of TiO₂ nanofibers were layered, that surface crystal phase transition rate was faster than that of internal layers contributed the difference in the ratio of anatase and rutile in the outer and inner layer of TiO₂ nanofibers. The TiO₂ nanofibers obtained at 575 °C had the best photocatalytic activity, taking only 25 min to degrade Rhodamine B. At 575 °C, the rutile content of the sample surface was about 80 wt.%, while the internal rutile content was only about 40 wt.%. Subsequently, we prepared two different structures of anatase-rutile core-shell TiO₂ nanofibers. The core-shell structure can be clearly seen by TEM characterization. The photocatalytic activity of two kinds of core-shell TiO₂ nanofibers was tested. The results showed that the photocatalytic activity was close to that of the pure phase TiO₂ nanofibers, which corresponded with the surface phase. This further proves that the photocatalytic activity of the material is mainly affected by its surface structure.

Synthesis and applications of nano-TiO2: a review

TiO₂-based nanomaterials have attracted prodigious attention as a photocatalysts in numerous fields of applications. In this thematic issue, the mechanism behind the photocatalytic activity of nano-TiO₂ as well as the critical properties have been reviewed in details. The synthesis routes and the variables that affect the size and crystallinity of nano-TiO₂ have also been discussed in detail. Moreover, a newly emerged class of color TiO₂, TiO₂ in aerogel form, nanotubes form, doped and undoped form, and other forms of TiO₂ have been discussed in details. Photocatalytic and photovoltaic applications and the type of nano-TiO₂ that is more suitable for these applications have been discussed in this review.

Synthesis of Au-decorated three-phase-mixed TiO2/phosphate modified active carbon nanocomposites as easily-recycled efficient photocatalysts for degrading high-concentration 2,4-DCP

It is of great significance to fabricate easily-recycled TiO₂ photocatalysts with high activity. Herein, dominant-anatase three-phase (anatase/rutile/brookite)-mixed nanosized TiO₂ with high photocatalytic activity for degrading a high-concentration of 2,4-DCP has been synthesized via a hydrothermal process with HCl as a phase-directing agent, and interestingly the apparent photoactivity could be greatly improved by decorating Au nanoparticles and then coupling phosphate-treated active carbon. The amount-optimized nanocomposite displays ∼12-fold enhancement in degradation rate constant (k) compared to anatase TiO₂. Based on the steady-state surface photovoltage spectra, fluorescence spectra related to the produced ·OH amount, temperature-programmed desorption and O₂ electrochemical reduction curves, it is confirmed that the exceptional photoactivity is mainly attributed to the greatly-enhanced charge separation from the phase-mixed composition, and from the decorated Au as electron acceptors and its promotion effects on O₂ activation. Moreover, the use of phosphate-modified AC as a support is also positive for efficient photocatalytic reactions by accepting electrons and concentrating the pollutants, with recyclable features. This work provides a feasible strategy to fabricate TiO₂-based nano-photocatalysts for degrading high-concentration pollutants to remediate the environment.

Au decorated three-phase-mixed nanosized TiO₂ coupled with phosphate-treated AC as recyclable nanocomposite photocatalysts exhibit excellent photoactivity for degrading high-concentration 2, 4-DCP, mainly due to the improved charge separation and specific surface area.

Electrochemical tuning of heterojunctions in TiO2 nanotubes for efficient solar water splitting

The electrochemical assisted biphase ↔ triphase switching in TiO₂ nanotubes and their comprehensive photocatalytic hydrogen generation efficiencies are presented.

Effect of the Preparation Method (Sol-Gel or Hydrothermal) and Conditions on the TiO₂ Properties and Activity for Propene Oxidation

Since the two most commonly used methods for TiO₂ preparation are sol-gel (SG) and hydrothermal (HT) synthesis, this study attempts to compare both methods in order to determine which one is the most suitable to prepare photocatalysts for propene oxidation. In addition, this work studies how the concentration of the HCl used for hydrolysis of the TiO₂ precursor affects the properties of the obtained materials. Also, the effect of avoiding the post-synthesis heat-treatment in a selection of samples is investigated. The photocatalysts are characterized by XRD, N₂ adsorption-desorption isotherms and UV-vis spectroscopy, and the study tries to correlate the properties with the photocatalytic performance of the prepared TiO₂ samples in propene oxidation. TiO₂ materials with high crystallinity, between 67% and 81%, and surface area (up to 134 m²/g) have been obtained both by SG and HT methods. In general, the surface area and pore volume of the TiO₂-HT samples are larger than those of TiO₂-SG ones. The TiO₂-HT catalysts are, in general, more active than TiO₂-SG materials or P25 in the photo-oxidation of propene. The effect of HCl presence during the TiO₂ synthesis and of the post synthesis heat treatment are much more marked in the case of the SG materials.

A Study on Doped Heterojunctions in TiO2 Nanotubes: An Efficient Photocatalyst for Solar Water Splitting

The two important factors that affect sunlight assisted water splitting ability of TiO₂ are its charge recombination and large band gap. We report the first demonstration of nitrogen doped triphase (anatase-rutile-brookite) TiO₂ nanotubes as sun light active photocatalyst for water splitting with high quantum efficiency. Nitrogen doped triphase TiO₂ nanotubes, corresponding to different nitrogen concentrations, are synthesized electrochemically. Increase in nitrogen concentration in triphase TiO₂ nanotubes is found to induce brookite to anatase phase transformation. The variation in density of intra-band states (Ti³⁺ and N 2p states) with increase in nitrogen doping are found to be critical in tuning the photocatalytic activity of TiO₂ nanotubes. The presence of bulk heterojunctions in single nanotube of different nitrogen doped TiO₂ samples is confirmed from HRTEM analysis. The most active nitrogen doped triphase TiO₂ nanotubes are found to be 12 times efficient compared to pristine triphase TiO₂, for solar hydrogen generation. The band alignment and charge transfer pathways in nitrogen doped TiO₂ with triphase heterojunctions are delineated. Bulk heterojunctions among the three phases present in the nanotubes with intra-band defect states is shown to enhance the photocatalytic activity tremendously. Our study also confirms the theory that three phase system is efficient in photocatalysis compared to two phase system.

Influence of acidic pH on the formulation of TiO2 nanocrystalline powders with enhanced photoluminescence property

Titanium dioxide (TiO2) nanoparticles were prepared by the sol-gel method at different pH values (3.2-6.8) with a hydrochloric acid (HCl) solution. Raw samples were calcined at 500 °C for 2 h. The effects of pH on the structural, morphological and optical properties of TiO2 nanoparticles were investigated. At pH 4.4-6.8, only the anatase phase of TiO2 was observed. Under strong acidic condition at pH 3.2 rutile, brookite and anatase co-exist, but rutile is the predominant phase. The strain value increased and the crystallite size decreased as the HCl content increased. The increased crystallite sizes in the range 21-24 nm and enhanced blue emission intensity around 432 nm was obtained for the sample at pH 5.0. Experimental results showed that TiO2 nanoparticles synthesized at pH 5.0 exhibited the best luminescence property with pure anatase phase.

Mutual-stabilization in chemically bonded graphene oxide–TiO2 heterostructures synthesized by a sol–gel approach

The Ti–O–C interactions between GO and TiO₂ are strong enough to induce mutual shaping during synthesis and thermal treatment.

Bicrystalline TiO2 heterojunction for enhanced organic photodegradation: engineering and exploring surface chemistry

Bicrystalline TiO₂ anatase/rutile (TiAR) and anatase/brookite (TiAB) have been studied for photocatalytic degrdation of organics, while H₂O₂ treatment induced opposite mechanisms on the surface.

Investigation of the influence of vanadium, iron and nickel dopants on the morphology, and crystal structure and photocatalytic properties of titanium dioxide based nanopowders

Photoactive V, Fe and Ni doped TiO2 (M-TiO2) nanopowders were synthesized by a modified two-step sol-gel process in the absence of additives. Titanium oxychloride, which is a rarely-used TiO2 precursor was used to yield M-TiO2 photocatalysts with preferential photochemical performance in the presence of natural solar irradiation. The obtained samples were calcined at different calcination temperatures ranging from 450 to 800°C to evaluate the influence of the sintering on the physicochemical properties. The properties of the obtained samples were examined by XRF, XRD, Raman spectroscopy, UV-visible DRS, XPS, nitrogen gas physisorption studies, SEM-EDAX and HRTEM analyses. Structural characterization of the samples revealed the incorporation of these transition metal element into TiO2. It was also depicted that the morphology, crystal structure, optical and photochemical properties of the obtained samples were largely dependent on the calcination temperature and the type of dopant used during the preparation process. The photochemical performance of the samples was investigated in the photodegradation of methylene blue in the presence of natural sunlight. The experimental results indicated that the VT600 sample possessed the highest activity due to its superior properties. This study provides a systematic preparation and selection of the precursor, dopant and calcination temperature that are suitable for the formation of TiO2-based heterogeneous photocatalysts with appealing morphology, crystal structure, optical and photochemical properties for myriad of applications.