Mixed-phase TiO2 Research Articles

Facile Formation of Anatase/Rutile TiO2 Nanocomposites with Enhanced Photocatalytic Activity.

Anatase/rutile mixed-phase TiO2 nanoparticles were synthesized through a simple sol-gel route with further calcination using inexpensive titanium tetrachloride as a titanium source, which effectively reduces the production cost. The structural and optical properties of the prepared materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis adsorption. The specific surface area was also analyzed by Brunauer–Emmett–Teller (BET) method. The anatase/rutile mixed-phase TiO2 nanocomposites containing of rod-like, cuboid, and some irregularly shaped anatase nanoparticles (exposed {101} facets) with sizes ranging from tens to more than 100 nanometers, and rod-like rutile nanoparticles (exposed {110} facets) with sizes ranging from tens to more than 100 nanometers. The photocatalytic activities of the obtained anatase/rutile mixed-phase TiO2 nanoparticles were investigated and compared by evaluating the degradation of hazardous dye methylene blue (MB) under ultraviolet light illumination. Compared to the commercial Degussa P25-TiO2, the mixed-phase TiO2 nanocomposites show better photocatalytic activity, which can be attributed to the optimal anatase to rutile ratio and the specific exposed crystal surface on the surface. The anatase/rutile TiO2 nanocomposites obtained at pH 1.0 (pH1.0-TiO2) show the best photocatalytic activity, which can be attributed to the optimal heterojunction structure, the smaller average particle size, and the presence of a specific exposed crystal surface. The enhanced photocatalytic activity makes the prepared anatase/rutile TiO2 photocatalysts a potential candidate in the removal of the organic dyes from colored wastewater.

UV enhanced ammonia gas sensing properties of PANI/TiO2 core-shell nanofibers

TiO2/PANI core-shell nanofiber thin film sensors are fabricated by in-situ chemical polymerization on electrospun TiO2 nanofibers, and evaluated for NH3 gas sensing application with UV irradiation at room temperature (25 °C). Fully rutile and 30% anatase-70% rutile mixed phase TiO2 nanofibers are used as n-type core material. PANI as a p-type shell has porous morphology and FE-SEM results reveal that the average thickness of PANI shell is about 20 nm, which is in the order of Debye length resulting a higher sensitivity. Experimental result reveals that the sensor with combined phase core has a higher photo-assisted response. Moreover, the effect of various UV light intensities on TiO2/PANI core-shell nanofiber show maximum response at intermediate UV light intensity. Besides, UV light exposure led to a considerable reduction in recovery and response times to approximately one-fifth of their value. Ammonia sensing property is also evaluated at different humidity levels upon exposure to UV light and shows that higher humidity content leads to increase in sensor response. High sensitivity and low detection limit as low as 50 ppb under high humidity condition and excellent selectivity for various VOCs make the proposed sensor to be a good candidate for breath analysis applications.

CO2 adsorption and conversion of epoxides catalyzed by inexpensive and active mesoporous structured mixed-phase (anatase/brookite) TiO2

An efficient adsorption of CO2 and the conversion of epoxides into valuable products by inexpensive and active catalysts are of great interests and are really daunting challenge. Herein, we report the synthesis of propylene carbonate (PC) with excellent yield (100%) from the cycloaddition of CO2 and propylene oxide (PO) (fixation or adsorption of CO2) catalysed by NiO-modified-TiO2 (designated as NiO/TiO2) nanoparticles (NPs) with tetrabutylammonium iodide (nBu4NI) used as co-catalyst under solvent free conditions. For this purpose, mesoporous structured mixed-phase (anatase/brookite) TiO2 and NiO/TiO2 NPs were synthesized by an eco-friendly and simple sol-gel method. Synthesized NiO/TiO2 NPs catalyst was thoroughly characterized by various analytical techniques such as Fourier-Transform Infrared (FTIR) Spectroscopy, Energy Dispersive X-Ray (EDX) analysis, Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), X-Ray Diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) method, Scanning Electron Microscopy (SEM), and High-Resolution Transmission Electron Microscopy (HRTEM). To broaden the scope of cycloaddition of CO2, several other epoxides as substrates were also examined using NiO/TiO2 NPs catalyst and the cyclic carbonates products (86.9–100%) were accomplished. Moreover, the enhanced adsorption studies of CO2 with solid mixed oxides catalysts are also remarked. Furthermore, NiO/TiO2 NPs catalyst was simply recovered, reused for four successive runs without significant loss of its original activity, and regenerated for further use.

Improved charge collection and photo conversion of bacteriorhodopsin sensitized solar cells coupled with reduced graphene oxide decorated one-dimensional TiO2 nanorod hybrid photoanodes

Nanohybrid photoanodes made of mixed phase TiO2 nanorods (NRs) decorated with reduced graphene oxide (rGO) is fabricated by one step hydrothermal technique. Addition of rGO influences the phase formation of the TiO2 and optimum mixed phase formation of rutile (16%) and anatase (84%) TiO2 polymorphs is achieved at 1.0 wt% rGO decoration. Using rGO decorated TiO2 hybrid photoanodes, bacteriorhodopsin sensitized solar cells is fabricated with liquid and quasi-state electrolyte. Decoration of 1.0 wt% rGO upon TiO2 NRs boosted the photocurrent (Jsc = 2.2 mA/cm2) generation and photoconversion (η = 1.3%) efficiency which is 94% enhanced than pristine TiO2 NRs photoanodes. Experimental observation showcase higher rGO decoration (>1.0 wt% rGO) suppress the charge transport due to the formation of self-induced trapping sites within the transfer network. Minimum charge transfer resistance and maximum charge collection efficiency of ∼98% is achieved at 1.0 wt% rGO decorated TiO2 nanorods photoanodes. The ideal decoration of rGO (1.0 wt%) in photoanodes significantly improves the bio-sensitizer loading, charge transfer, electron lifetime and reduces the recombination of the BSSCs. Thus nanohybrid (rGO-TiO2 NR) photoanode assembled BSSC exhibit higher photoconversion efficiency than earlier BSSCs fabricated with nanoparticle (P25 mixed-phase TiO2) and nanorod (rutile and anatase pristine TiO2) based photoanodes.

Immobilized Anatase/Rutile Mixed Phase Titanium Dioxide on Glass Beads Prepared via Dip Coating Technique.

In this work, anatase/rutile mixed phase titanium dioxide (TiO₂) particles prepared via sonochemical combined with calcination process was immobilized on borosilicate glass beads via sol-gel dip coating technique utilizing titanium tetraisopropoxide (TTIP). The optical properties of mixed phase TiO₂ was evaluated by diffuse reflectance UV-Vis spectrophotometer to confirm phase transformation of anatase to rutile phase at different calcination temperatures. Phase identification and structural properties of TiO₂ particles were analyzed using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Surface morphology of mixed phase TiO₂ coated on glass beads with different coating precursor solution were monitored by scanning electron microscope. The chemical composition of coated beads was characterized by energy dispersive spectroscopy to affirm the presence of TiO₂ on glass bead substrate.

Preparation, Characterization and Visible Light Photocatalytic Activities of Samarium-Doped Mixed Crystalline Phase TiO2 Nanoparticles

Mixed crystalline phase TiO2 nanoparticles with different content of Sm ion were prepared by a facile sol-gel method. The samples were characterized by XRD, XPS, UV-Vis-DRS, and PL. The influence of Sm-doping on structure and visible light photocatalytic activity of mixed phase TiO2 was evaluated by photocatalytic degradation of MB under the irradiation of fluorescent lamp. The results indicate that samarium appears two kinds of Sm3+ and Sm2+ valence state on the surface of TiO2. Sm doping can restrain the phase transition from anatase to rutile, and prevent the grain growth. Appropriate amount of Sm doping and the proportions of rutile and anatase mixed phase can effectively inhibit the recombination of photo-generated electron and hole pairs, expand the wavelength range of absorption spectrum. Moreover, the photocatalytic activity of Sm-TiO2 in MB degradation is evidently enhanced. The MB degradation rate of Sm-TiO2 samples with n(Sm): n(Ti)=0.006 is the best under the irradiation of fluorescent light when the sintered temperature T=500°C, it is 97% within 6 h, which is significantly higher than 56.4% of pure TiO2 under the same experimental conditions.

Influence of Ytterbium Doping on the Visible Light Photocatalytic Activity of Mixed Phase TiO<sub>2</sub> Nanoparticles

The TiO2 nanoparticles with different Yb doping amount were prepared by a facile sol-gel method. The samples were characterized by XRD, TEM, XPS, UV-Vis, and PL, etc. The influence of Yb-doping on structure and visible light photocatalytic activity of mixed phase TiO2 was evaluated by photocatalytic degradation of methylene blue (MB) under the irradiation of fluorescent lamp. The results show that ytterbium appeared two kinds of Yb3+ and Yb2+ valence state on the surface of TiO2. Ytterbium doping can restrain the phase transfer from anatase to rutile, prevent the grain growth, and increase the surface hydroxyl content. Appropriate amount of Yb doping and the proportions of rutile and anatase mixed phase can effectively inhibit the recombination of photogenerated electron and hole, expand the wavelength range of absorption spectrum, and improve the photocatalytic performance of TiO2. Meanwhile, the photocatalytic activity of the Yb-TiO2 samples was the best under the irradiation of fluorescent light when the sintered temperature T=500°C and n(Yb):n(Ti)=0.009, it was 95.2% within 6 h and significantly higher than 56.4% of pure TiO2 under the same experimental conditions.

Introduction of Ti3+ ions into heterostructured TiO2 nanotree arrays for enhanced photoelectrochemical performance

The mixed-phase heterostructured TiO2 nanotree arrays were synthesized by a secondary hydrothermal route, and Ti3+ ions were introduced into the arrays for enhanced photoelectrochemical performance. The results show that the arrays consist of heterostructures with anatase and rutile phases. XPS results indicate that during the hydrogenation process, part of Ti4+ ions transform to Ti3+ ions. Although the band gap shifts by only 0.1 eV, introduced defects and Ti3+ ions can greatly increase the efficiency of electron-hole separation. Photocurrent of the heterostructured TiO2 nanotree arrays with introduced Ti3+ ions increases by 41 times, and the photocurrent density enhances from 0.0055 to 0.2272 mA/cm2.

Facile synthesis, structural, optical and photocatalytic properties of anatase/ rutile mixed phase TiO2 ball-like sub-micron structures

We report facile synthesis of ball-like sub-micron structures of TiO2 with strong photocatalytic activity. FESEM and TEM studies showed ball-like sub-micron TiO2 structures. XRD results showed that the synthesized TiO2 sub-micron structures annealed at 300 °C exist in pure anatase phase, while annealing at 600 °C led to the creation of mixed phase TiO2 structures. The photocatalytic studies showed that anatase/ rutile mixed phase TiO2 ball-like sub-micron structures exhibit superior photocatalytic activities for the decomposition of Rhodamine B (RhB) and methyl orange (MO) dyes. We attribute the observed improved photodegradation activities of anatase/ rutile mixed phase TiO2 sub-micron structures to the synergistic effects of large surface area of ball-like TiO2 structures facilitating enhanced adsorption of dye molecules and the mixed phase composition resulting in improved charge separation efficiency.

Interfacial structure and properties of TiO2 phase junction studied by DFT calculations

Mixed-phase (i.e. phase junction) TiO2 has been widely used in commercial photocatalysis applications, because of its excellent photocatalytic performance. However, the interfacial composition, micro-structure, and properties of mixed-phase TiO2 are not fully understood so far. Consequently, the phase transition process TiO2 from anatase to rutile is still not entirely clear; and there is no consensus on the mechanism of mixed-phase enhancing photocatalytic performance of TiO2. In this article, 16 interface models for TiO2 phase junctions were constructed by the low-index surfaces of TiO2 with anatase and rutile phases. The interfacial energy, microstructure, electronic structure and properties have been systematically calculated and analyzed. Based on the interfacial formation energy and the configuration of [TiO6] octahedron at A/R interface, the phase transition of TiO2 is most likely to occur at the interface between anatase (110) plane and rutile (011) plane, owing to the favorable interface formation energy and the smallest deformation of [TiO6] octahedrons. However, because the migration direction of photo-generated carriers is opposite to the direction of the built-in electric field at the A/R interface in A(110)/R(011) phase junction, the hetero-direction migration of photo-generated electron-hole pairs will be prevented. Thereby, the existence of A(110)/R(011) phase junction is unfavorable to improve the photocatalytic performance. On the other hand, when the calcination temperature rises, the favorable phase junctions (such as A(110)/R(001) phase junction) gradually occupies the growth advantage, which can improve the photocatalytic performance of TiO2. This is a possible explanation for the excellent photocatalytic properties of Degussa P25 with mixed-phase.

Carbon-Coated and Interfacial-Functionalized Mixed-Phase Mox Ti1-x O2-δ Nanotubes as Highly Active and Durable PtRu Catalyst Support for Methanol Electrooxidation.

A synchronous carbon-coating and interfacial-functionalizing approach is proposed for the fabrication of Mo-doped Mox Ti1-x O2-δ nanotubes (C@IF-MTNTs) under mild hydrothermal reaction with subsequent annealing as advanced catalyst supports for PtRu nanoparticles (NPs) towards methanol electrooxidation. The carbonation of glucose and Mo-doping takes place simultaneously at the interface of pristine anatase TiO2 nanotubes (TNTs), generating a unique concentric multilayered one-dimensional (1D) structure with crystalline an anatase/rutile mixed-phase TiO2 core and Mo-functionalized interface and subsequently a carbon shell. The obtained PtRu/C@IF-MTNTs catalyst exhibits an over 2 times higher mass activity with comparable durability than that of the unmodified PtRu/C@TNTs catalyst and over 1.7 times higher mass activity with over 20 % higher stability than that of PtRu/C catalyst. Such superior catalytic performance towards methanol electrooxidation is ascribed to the Mo-functionalized interface, concentric multilayered 1D architecture, and anatase/rutile mixed-phase core, which facilitates the charge transport through 1D structural support and electronic interaction between C@IF-MTNTs and ultrafine PtRu NPs. This work reveals the critical application of a 1D interfacial functionalized architecture for advanced energy storage and conversion.

Photodegradation of Microcystin-LR Using Visible Light-Activated C/N-co-Modified Mesoporous TiO2 Photocatalyst

Microcystin-LR (MC-LR), a potent hepatotoxin produced by the cyanobacteria, is of increasing concern worldwide because of severe and persistent impacts on humans and animals by inhalation and consumption of contaminated waters and food. In this work, MC-LR was removed completely from aqueous solution using visible-light-active C/N-co-modified mesoporous anatase/brookite TiO2 photocatalyst. The co-modified TiO2 nanoparticles were synthesized by a one-pot hydrothermal process, and then calcined at different temperatures (300, 400, and 500 °C). All the obtained TiO2 powders were analyzed by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscope (TEM), specific surface area (SSA) measurements, ultraviolet-visible diffuse reflectance spectra (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, and photoluminescence (PL) analysis. It was found that all samples contained mixed-phase TiO2 (anatase and brookite), and the content of brookite decreased with an increase in calcination temperature, as well as the specific surface area and the content of non-metal elements. The effects of initial pH value, the TiO2 content, and MC-LR concentration on the photocatalytic activity were also studied. It was found that the photocatalytic activity of the obtained TiO2 photocatalysts declined with increasing temperature. The complete degradation (100%) of MC-LR (10 mg L−1) was observed within 3 h, using as-synthesized co-modified TiO2 (0.4 g L−1) at pH 4 under visible light. Based on the obtained results, the mechanism of MC-LR degradation has been proposed.

Synergistic effect of ZnO QDs and Sn4+ ions to control anatase-rutile phase of three-dimensional ordered hollow sphere TiO2 with enhanced photodegradation and hydrogen evolution

A novel three-dimensional ordered hollow spherical Sn4+ doped TiO2 material with ZnO quantum dot sensitization (ZnO QDs@Sn-TiO2) was synthesized by a simple colloidal template method. The characterization results indicate that the crystal phase of the prepared composite could be controlled by modifying the loading of ZnO QDs under the synergistic action of ZnO QDs and Sn4+ while maintaining the favorable three-dimensional ordered hollow sphere structure. Based on diffuse reflectance and fluorescence spectroscopy, ZnO QDs provide multiple migration pathways for photogenerated carriers, effectively inhibiting the recombination of photogenerated electron–hole pairs. And the photocatalytic experiments show that the photocatalyst ZnO QDs@Sn-TiO2 has significantly improved photocatalytic performance than pure TiO2. Simultaneously, the optimized results show that 2.0 ZnO QDs@Sn-TiO2 has the best degradation rate and photocatalytic activity. In summary, this work provides a new approach for the synthesis of multi-morphological mixed phase TiO2 photocatalysts for photocatalytic applications.

Highly efficient visible-light-driven photocatalytic degradation of VOCs by CO2-assisted synthesized mesoporous carbon confined mixed-phase TiO2 nanocomposites derived from MOFs

Improving the visible light response and efficient separation of electron-hole pairs play vital roles in commonly used TiO2 photocatalyst for VOCs degradation. Herein, N-doped mesoporous carbon encapsulated anatase-rutile phase junction TiO2 (TiO2@C-N(x)) was successfully synthesized via the pyrolysis of a representative amine functionalized Ti-based MOF, NH2-MIL-125, under the atmosphere of Ar and subsequent CO2 treatment. Our synthesis stragety was based on the rational regulation of the formation of TiO2 phase junction and the decomposition of amorphous carbon onto the TiO2@C-N (without subsequent CO2 process) using CO2 as both anatase-rutile phase transformation promoter and mild oxidant. Compared with TiO2@C-N, TiO2@C-N(x) nanocomposites with subsequent CO2 process exhibit significantly improved photocatalytic activity as well as mineralization efficiencies. For example, the mineralization efficiency reached 51.9% at 62.4% of styrene degradation within 240 min of visible-light irradiation by using the optimal TiO2@C-N(30) nanocomposites as compared with only 19.7% mineralization efficiency at 31.0% of styrene degradation under the same conditions of TiO2@C-N. Furthermore, the primary radicals involved in degradation of VOCs was identified by electron paramagnetic resonance spectroscopy, and the possible degradation intermediates were also monitored by means of proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). Finally, the radicals involved degradation reaction mechanism was also tentatively proposed.

Enhancing the performance of hole-conductor free carbon-based perovskite solar cells through rutile-phase passivation of anatase TiO2 scaffold

Surface passivation of the electron transport layer in perovskite solar cells has been proven to be one crucial strategy to improve the power conversion efficiency. In this study, we introduce rutile phase titania shell to passivate anatase TiO2 scaffold, forming anatase@rutile mixed-phase TiO2 scaffold in hole-conductor-free carbon based perovskite solar cells. Anatase@rutile mixed phase TiO2 films exhibit higher conductivity, faster charge extraction, lower trap density and suppressed charge recombination at the interface of TiO2/perovskite films in the modified devices as well, achieving a champion power conversion efficiency of 15.21% for carbon based perovskite solar cells. More importantly, benefiting from the stable and better ultraviolet light filtration properties of rutile phase TiO2, our devices exhibit excellent stability maintaining 92% of their initial efficiency after 200 days storage in ambient air, highlighting their merits as a potential competitor for future industrial photovoltaics.

Can mixed anatase and rutile photocatalyst TiO2 be synthesized under high pressure in water?

We have reported for the first time the synthesis of mixed anatase and rutile TiO2 under 27 MPa in water at the temperature as low as 375 °C. XRD patterns show that the content of rutile in the mixed phase TiO2 is increased by 47.7% compared with the raw TiO2. The average particle size of the mixed phase TiO2 is increased obviously compared with the raw TiO2. Compared with that of the raw TiO2, the band gap of the mixed phase TiO2 is reduced to 2.94 eV while the absorbance is enhanced. Although the particle size increases, the mixed phase TiO2 still exhibits better photocatalytic performance.

Radiative and Non-Radiative Recombination Pathways in Mixed-Phase TiO2 Nanotubes for PEC Water-Splitting

Anatase and rutile mixed-phase TiO2 with an ideal ratio has been proven to significantly enhance photoelectrochemical (PEC) activity in water-splitting applications due to suppressing the electron–hole recombination. However, the mechanism of this improvement has not been satisfactory described yet. The PEC water oxidation (oxygen evolution) at the interface of TiO2 photoanode and electrolyte solution is determined by the fraction of the photogenerated holes that reach the solution and it is defined as the hole transfer efficiency. The surface and bulk recombination processes in semiconductor photoanodes majorly influence the hole transfer efficiency. In this work, we study the hole transfer process involved in mixed-phase TiO2 nanotube arrays/solution junction using intensity-modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS); then, we correlate the obtained hole transfer rate constants to (photo)electrochemical impedance spectroscopy (PEIS) measurements. The results suggest that the enhanced performance of the TiO2 mixed-phase is due to the improved hole transfer rate across the TiO2/liquid interface as well as to the decrease in the surface trap recombination of the holes.

In-situ synthesis of mixed phase electrospun TiO2 nanofibers: a novel visible light photocatalyst

TiO2 nanofibers were synthesized by electrospinning method using mixed titanum isopropoxide and polyvinylpyrrolidone precursors in ethanol followed by calcination at high temperature. The obtained TiO2 e-spun nanofibers were characterized by FESEM, TEM, XRD, XPS and BET surface area analytical techniques. XRD result shows that the nanofibers contain both anatase and rutile mixed phases. FESEM and TEM study indicates the development of precise fine tiny nanoparticles with diameter in the range of 10–20 nm, which are oriented along one dimensional direction to form the fibrous structure with diameter in the range of 300–500 nm. The mixed phase TiO2 nanofibers were used as photocatalysts for degradation of Rhodamine-B and Methyl orange under solar light irradiation and also under visible light irradiation. More than 99% degradation was achieved within 90 min of irradiation time for both the dyes. Furthermore, it is observed that the TiO2 photocatalyst is efficiently degrading (98%) the tetracycline hydrochloride solution within 70 min of contact time. The excellent visible light photocatalytic activity may be attributed to the combine factors of hetero-conjunction at anatase–rutile interface and immediate charge transfer due to 1D structure, which inhibit the electron–hole recombination. Rutile-Anatase dual phase TiO2 nanofibers exhibit excellent visible light photocatalytic activity due to the combine factors of hetero-conjunction at anatase-rutile interface and immediate charge transfer due to 1D structure, which inhibits the electron-hole recombination.

Cu and Zr surface sites in photocatalytic activity of TiO2 nanoparticles: The effect of Zr distribution

The present work is focused on the role of ZrO2 modification in the performance of CuO modified TiO2. Zirconia loading leads to formation of more resistant photocatalytic layers compared to samples modified with only copper containing species. Surface modification of mixed phase TiO2 with CuO/ZrO2 improves the degradation of Reactive blue 19 dye under simulated solar irradiation. An in-depth investigation of the catalysts showed that in case of CuO/ZrO2 modification, the covering of the TiO2 surface with zirconium containing species prevents morphological and harmful energetic changes induced by copper species formed on the rutile TiO2 phase at a higher copper loading.

Enhanced photocatalytic characteristics by Ag-sensitized TiO2 photocatalysts with mixed phases

Abstract The Ag-sensitized TiO2 photocatalysts with various contents of Ag were fabricated by a facile sol-gel method. The Ag nanoparticles with an average diameter of only a few nanometers distributed uniformly over the TiO2 surface. Their presence resulted in suppressed recombination and more effective utilization of the visible-light radiation through surface plasmon resonance. However, excess Ag nanoparticles led to higher recombination rates, reduced surface area, and thereby lower photocatalytic activity. Comparative study on the photocatalytic performance of the photocatalysts without and with various Ag loadings was systematically carried out. The moderate reduction of Ag nanoparticles by an appropriate surface treatment condition to increase the area for dye adsorption was favorable to photodegradation. By double calcinations, mixed phases of anatase and rutile were obtained, which exhibited better performance than single phase. The synergistic effect of Ag nanoparticles, mixed phase TiO2 and high surface area has qualified the Ag-sensitized TiO2 nanocomposites as highly promising photocatalysts.