Anatase TiO2 Mesocrystals Research Articles

Cube-like anatase TiO2 mesocrystals as effective electron-transporting materials toward high-performance perovskite solar cells

Electron-transporting materials (ETMs) with higher carrier mobility and a suitable band gap structure play a significant role in determining the photovoltaic performance of perovskite solar cells (PSCs). Herein, cube-like mesoporous single-crystal anatase TiO2 (Meso-TiO2) nanoparticles synthesized by using a facile hydrothermal method were utilized as an efficient ETM for PSCs. The superior semiconducting properties of the Meso-TiO2 based ETM enabled the best power conversion efficiency (PCE) of 20.05% for a PSC. Moreover, the device retained 80% of its initial PCE after being stored in ambient conditions for 20 days under 25 ± 5% relative humidity. In contrast to the commercial TiO2 ETM, the Meso-TiO2 ETM based PSC showed a distinguished interface with better interfacial conditions and improved carrier extraction originating from the cube-like mesoporous single-crystal anatase TiO2 ETM.

Fast growth of precursors for 3D ordered TiO2 mesocrystals: from (NH4)2TiOF4 plates to NH4TiOF3 disks

In this work, ammonium oxofluorotitanate mesocrystals were prepared via an electrochemical route and converted to anatase TiO2 mesocrystals. A unique transition process from (NH4)2TiOF4 rods to crosses and finally to NH4TiOF3 disks was revealed.

TiO2 Mesocrystals Processed at Low Temperature as the Electron-Transport Material in Perovskite Solar Cells.

TiO2 is the most widely used material for preparing the electron-transporting layer (ETL) in perovskite solar cells (PSCs). However, it requires a high-temperature sintering process. Moreover, the intrinsic defects and low electron mobility of TiO2 ETLs cause instability and hysteresis effects in PSCs. In this study, a mesoporous film composed of anatase TiO2 mesocrystals was facilely fabricated by a low-temperature route and then used as an ETL in PSCs for the first time. A satisfactory efficiency of 20.26 % can be achieved through delicate control of the entire device fabrication procedure. The optimal device, with an area of 1 cm2 , achieves an efficiency of 17.07 %. In comparison to the common TiO2 ETLs, those composed of TiO2 mesocrystals show the enhanced electron extraction and suppression of charge accumulation at the perovskite/ETL interface, resulting in improved photovoltaic performance and reduced hysteresis.

Simultaneous photocatalytic degradation of ibuprofen and H2 evolution over Au/sheaf-like TiO2 mesocrystals

Considerable effort has been devoted to the efficient degradation of pharmaceuticals and personal care products (PPCPs), while the chemical energy in these processes has been widely overlooked. In this study, we demonstrated the simultaneous hydrogen production and ibuprofen degradation through heterogeneous photocatalysis. By anchoring Au nanoparticles (NPs) on the (101) surface of sheaf-like TiO2 mesocrystals with [001] orientation, efficient charge separation is achieved, which is essential for the photocatalytic redox reactions. XPS analysis showed that the binding energies of Ti 2p and O 1s indicated no shift after Au addition. Peaks observed at 81.8 and 85.5 eV due to Au 4f7/2 and Au 4f5/2 of metallic gold on the surface of Au/meso-TiO2, confirmed the formation of Au NPs. The as-synthesized anatase TiO2 mesocrystals are composed of small nanocrystals with a size of 8 nm and exhibit the uniform sheaf-like morphology along [001] orientation. As expected, the 1 wt% Au/TiO2 mesocrystals shows the largest photocurrent density, highest H2-evolution rate, and fastest photodegradation rate of ibuprofen under simulated sunlight irradiation among all the studied catalyst. Furthermore, the effect of solution pH, common anions (Cl−, NO3−, and SO42−) and cations (Na+, K+, and Ca2+) on photocatalytic H2 evolution and degradation of ibuprofen were individually investigated and discussed. A mechanism for the simultaneous photocatalytic hydrogen generation and degradation of ibuprofen has also been proposed. This work opens up new opportunities for the development of energy efficient techniques for PPCPs degradation.

Facet-dependent decoration of TiO2 mesocrystals on TiO2 microcrystals for enhanced photoactivity

Bottom-up constructions of hierarchical TiO2 are effective to enhance their photoactivity towards degradations of organic pollutants. Thanks to highly active facets, {001} exposed anatase TiO2 microcrystals attract much attention in photocatalysis; yet their efficiency is limited by the large crystal size. Herein, we report a facile solution approach to deposit anatase TiO2 mesocrystals only on {101} facets of anatase TiO2 microcrystals. The selective surface decoration enhances the photoactivity through replacing the less active {101} facets with more active TiO2 mesocrystals; whilst the highly active {001} facets remain untouched. When utilized to assist photodegradation of phenol in water under UV light illumination, the hierarchical TiO2 exhibited a reaction rate constant doubled that of the pristine {001} exposed TiO2 microcrystals. The present tactic to selectively decorate TiO2 microcrystals may give hints to other applications involving facet-dependent properties.

Synthesis of anatase TiO2 mesocrystals with highly exposed low-index facets for enhanced electrochemical performance

Cube-like anatase TiO2 mesocrystals have been fabricated successfully by using amorphous titanate as a precursor. The size and shape of the synthesized mesocrystals are uniform with an average size of 100 nm. Interestingly, the low index facets of (100), (010) and (001) are exposed completely, forming the special cube-like shape. When used as an anode for lithium-ion batteries, anatase TiO2 mesocrystals exhibit an excellent lithium storage ability. A capacity of 166 mA h g−1 can be remained after 800 cycles at 5C, demonstrating a good long-term cycling performance. Furthermore, an average reversible capacity of 148 mA h g−1 can be also maintained even at a rate as high as 20C. Such an excellent electrochemical performance can be attributed to the special cube-like shape of TiO2 mesocrystals with exposed facets, which facilitate lithium-ion diffusion.

Cucurbit[7]uril encapsulated dye-sensitized enhanced solar photocatalysis using positively charged sheet-like anatase TiO2 mesocrystals

Abstract Recent progress in the field of advanced oxidation process (AOP) has triggered the use of surface engineered visible light active TiO2 as a competent photocatalyst material in solar photocatalysis. Herein, we adopted host-guest inclusion complexation for the site-directed immobilization of a model cationic dye, Rhodamine B (RhB) on to the surface of oxygen-rich positively charged TiO2, resulting in rapid mineralization of the dye pollutant under solar irradiation. Relatively recent macrocyclic host, cucurbit[7]uril (CB7) is used as the host for the formation of inclusion complex with RhB. In this study, solar photocatalysis of dye encapsulated system results in 97% mineralization of dye molecules within 10 min with an excellent kinetic rate of 351 × 10−3 min−1. Here, the formation of CB7-RhB inclusion complex enables direct anchoring of encapsulated dye molecules through carboxylic (-COOH) groups of RhB, which in turn facilitate heterogeneous electron transfer resulting in instantaneous and direct cleavage of the chromophore. The binding of CB7 encapsulated RhB to microwave-assisted sonochemically synthesized TiO2 (MST) surface via monodentate ester-like linkage was established by XPS and FT-IR analysis. Gas chromatography (GC) and high-resolution mass spectrometry (HR-MS) substantiate the complete mineralization of RhB and the intactness of CB7 molecular structure after photodegradation, confirming the reusability of CB7/MST as an efficient and economically viable photocatalytic system. Encapsulation of pollutant dye to CB7 host proves to be an excellent approach to facilitate the photoelectron transfer to the photocatalyst and faster mineralization of dye molecules under solar irradiation. Thus, our approach would be a feasible solution for environment-friendly remediation of dye pollutants.

Photocatalytic Properties Dependent on the Interfacial Defects of Intergrains within TiO2 Mesocrystals.

The regulation of interfacial defects of nanocrystals aligned orderly in a superstructured photocatalyst is an effective approach to improve the photocatalytic performance. However, the synthesis of ordered self-assemblies with abundant interfacial defects and reactive sites is hard to achieve, and applications are limited due to the unclear physicochemical properties, which result from the unique mesocrystalline microstructures. It is reported herein that the photocatalytic properties depend on the interfacial defects of intergrains in anatase TiO2 mesocrystals (TMCs). Research reveals that the photocatalytic activity largely depends on defects, such as lattice distortion and oxygen vacancies, which are located at highly aligned interfaces of intergrains within TMCs. Moreover, the mesocrystalline TiO2 photocatalysts exhibit higher photocatalytic performance in organic degradation and hydrogen evolution, compared with single crystals and polycrystals; this can be ascribed to an appropriate amount of interfacial defects at the intergrains and improved carrier separation efficiency through the highly oriented interfaces. In addition, the photocatalytic performance of the TMCs could be further improved through regulation of defects by undergoing an annealing process under a redox atmosphere. This work can provide an avenue to defect engineering for improved photocatalytic performance.

Highly Efficient Degradation of Tobacco Specific N‐Nitrosamines by TiO2 Mesocrystals with Robust and Tailored Microporous Structure

AbstractIt is essential to control tobacco specific N‐nitrosamines (TSNAs) pollution for the protection of public health, which were identified as the most abundant carcinogens in tobacco and tobacco smoke. Herein, we provide microporous TiO2 mesocrystals as a new material for the remove of TSNAs with high degradation efficiency. The anatase TiO2 mesocrystals with tunable microporous structure were synthesized by a facile method under the acid solution. The TiO2 mesocrystals were firstly used to the remove of TSNAs through photocatalytic degradation. It's exciting that TSNAs of tobacco extraction solution can be almost entirely reduced (up to 97%) by anatase TiO2 mesocrystals after 30 mins through photocatalytic degradation. Moreover, it's found that the microporous size of mesocrystals play a key role on the degradation capability. The TiO2 mesocrystals with microporous size comparable to the size of TSNAs molecule exhibit the best degradation performance. This finding offers a robust microporous material and efficient method for removing the carcinogen like TSNAs as well as other pollutants in environment.

Facet Control of Subunits in Anatase TiO₂ Mesocrystal by Diethanolamine for Enhanced Photocatalytic Activity.

Controllable TiO2 architectures with different facets play a key role in the adjustment of the surface area and charge recombination efficiency in photocatalysts. In this study, we synthesized anatase TiO2 mesocrystals with diethanolamine (DEA, 0-5 mM) as a nontoxic capping agent to control the facet of subunits using hydrothermal method, and determine the effect of DEA on subunits, by-products, and consequent photocatalytic performance of TiO2 mesocrystals. The pure TiO2 was spindle-like shaped mesocrystal having bipyramidal subunits with high portion of {101} facets. As DEA concentration was increased, the percentage of {001} exposed facet of subunits increased, but the number of subunits was gradually reduced. When a large amount of DEA (≥3 mM) was added, TiO2 exhibited single crystals rather than mesocrystal, probably because the long chain of DEA interfered with the oriented attachment of subunits. The photocatalytic activity of the TiO2 improved at 0.5 mM of DEA due to low charge recombination rate by increasing {001} facet. On the other hand, subsequent deterioration with further increase of DEA was attributed to the increased by-products and reduced the number of subunits despite the increase in specific surface area. These results imply that charge separation efficiency, rather than specific surface area, is highly responsible for the photocatalytic activity of TiO2.

Combined electrochemiluminescent and electrochemical immunoassay for interleukin 6 based on the use of TiO2 mesocrystal nanoarchitectures.

A dual-responsive sandwich-type immunosensor is described for the detection of interleukin 6 (IL-6) by combining electrochemiluminescent (ECL) and electrochemical (EC) detection based on the use of two kinds of TiO2 mesocrystal nanoarchitectures. A composite was prepared from TiO2 (anatase) mesocages (AMCs) and a carboxy-terminated ionic liquid (CTIL) and then placed on a glassy carbon electrode (GCE). In the next step, the ECL probe Ru(bpy)3(II) and antibody against IL-6 (Ab1) were immobilized on the GCE. Octahedral anatase TiO2 mesocrystals (OAMs) served as the matrix for immobilizing acid phosphatase (ACP) and secondary antibody (Ab2) labeled with horseradish peroxidase (HRP) to form a bioconjugate of type Ab2-HRP/ACP/OAMs. It was self-assembled on the GCE by immunobinding. 1-Naphthol, which is produced in-situ on the surface of the GCE due to the hydrolysis of added 1-naphthyl phosphate by ACP, is oxidized by HRP in the presence of added H2O2. This results in an electrochemical signal (typically measured at 0.4V vs. Ag/AgCl) that increases linearly in the 10fg·mL-1 to 90ng·mL-1 IL-6 concentration range with a detection limit of 0.32fg·mL-1. Secondly, the oxidation product of 1-naphthol quenches the ECL emission of Ru(bpy)32+. This leads to a decrease in ECL intensity which is linear in the 10 ag·mL-1 to 90ng·mL-1 concentration range, with a detection limit of 3.5 ag·mL-1. The method exhibits satisfying selectivity and good reproducibility which demonstrates its potential in clinical testing and diagnosis. Graphical abstract A dual-responsive sandwich-type immunosensor was fabricated for the detection of interleukin 6 by combining electrochemiluminescence and electrochemical detection based on the use of two kinds of TiO2 mesocrystal nanoarchitectures.

Anatase TiO2 Mesocrystals: Green Synthesis, In Situ Conversion to Porous Single Crystals, and Self-Doping Ti3+ for Enhanced Visible Light Driven Photocatalytic Removal of NO.

Mesocrystals are of great interest for a wide range of applications owing to their unique structural features and properties. The realization of well-defined metal oxide mesocrystals through a facile and green synthetic approach still remains a great challenge. Here, a novel synthesis strategy is reported for the production of spindle-shaped anatase TiO2 mesocrystals with a single-crystal-like structure, which was simply achieved through the one-step hydrolysis reaction of TiCl3 in the green and recyclable media polyethylene glycol (PEG-400) without any additives. Such anatase mesocrystals were constructed from small nanocrystal subunits (≈1.5-4.5 nm in diameter) and formed through oriented aggregation of the nanocrystals pre-formed in the reaction system. Owing to their novel structural characteristics, the as-synthesized anatase mesocrystals could be easily fused in situ into porous single crystals by annealing in air. More significantly, after being annealed in vacuum, Ti3+ sites could be easily induced in the anatase crystal lattice, resulting in the formation of Ti3+ self-doped anatase mesocrystals. The thus-transformed mesocrystals exhibited enhanced visible light activity towards the photocatalytic oxidation of nitric oxide (NO) to NO3- , which could be largely attributed to their intrinsic Ti3+ self-doped nature, as well as high crystallinity and high porosity of the mesocrystalline architecture.

Low Temperature Synthesis of High Energy Facets Exposed Sheet–like Anatase TiO2 Mesocrystals Show Reduced e−/h+ Pair Recombination Rates and Enhanced Photoactivity

AbstractHerein, we report low temperature synthesis of anatase TiO2 mesocrystals with {001} facets exposed sheet‐like morphology via fluorine‐free synthesis by the synergistic effect of microwave and ultrasound treatments. The synthesized TiO2 powder showed higher surface area and enhanced photocatalytic degradation of a model dye under white light irradiation. The sheet‐like anatase TiO2 mesocrystals resulted long‐lived charge carriers and separation that was confirmed from photoluminescence and photocurrent measurements. A five‐fold enhancement in the photocurrent was obtained for the mesocrystals compared to reference sample, which revealed that the orientation of nanocrystallites in sheet‐like morphology play a pivotal role in electron transport. The effect of charge carriers on photocatalytic activity was studied in detail and it followed the order, hydroxyl radicals> holes>electrons. Additionally, the TiO2 sol showed excellent stability. Further, the photoinactivation of Escherichia coli and Candida albicans was evaluated using zone of inhibition test under indoor irradiation and synthesized anatase TiO2 showed a clear zone of inhibition with diameter 37.2 mm and 40.0 mm, respectively whereas commercial TiO2 shows meager inactivation. The results indicate that our approach of combined microwave and sonication can serve as a facile approach for the synthesis of {001} facets exposed anatase sheet‐like TiO2 mesocrystals under halide‐free and low temperature conditions.

Effects of Hydrothermal Temperature on Crystallinity, Microstructure, and Photocatalytic Activity of Anatase TiO2 Mesocrystals

Effects of Hydrothermal Temperature on Crystallinity, Microstructure, and Photocatalytic Activity of Anatase TiO2 Mesocrystals

A Potentiometric Addressable Photoelectrochemical Biosensor for Sensitive Detection of Two Biomarkers.

It is a great challenge to fabricate multiplex and convenient photoelectrochemical biosensors for ultrasensitive determination of biomarkers. Herein, a fascinating potentiometric addressable photoelectrochemical biosensor was reported for double biomarkers' detection by varying the applied bias in the detection process. In this biosensor, the nanocomposite of cube anatase TiO2 mesocrystals and polyamidoamine dendrimers modified a dual disk electrode as an excellent photoelectrochemical sensing matrix. Subsequently, two important biomarkers in serum for prostate cancer, prostate-specific antigen and human interleukin-6, were immobilized onto the different disks of modified electrode via glutaraldehyde bridges. Then another two photosensitizers, graphitic-carbon-nitride-labeled and CS-AgI-labeled different antibodies, were self-assembled onto the electrode surface by a corresponding competitive immune recognition reaction. The change in photocurrent with the target antigen concentration at different critical voltages enables us to selectively and quantitatively determine targets. The results demonstrated that this potentiometric addressable photoelectrochemical biosensing strategy not only has great promise as a new point-of-care diagnostic tool for early detection of prostate cancer but also can be conveniently expanded to multiplex biosensing by simply change biomarkers. More importantly, this work provides an unambiguous operating guideline of multiplex photoelectrochemical immunoassay.

Understanding the growth and photoelectrochemical properties of mesocrystals and single crystals: a case of anatase TiO2

Anatase TiO2 mesocrystals and single crystals with dominant {101} facets were successfully synthesized without any additives using titanate nanowires as precursors under solvothermal and hydrothermal conditions, respectively. It is proposed that the oriented self-assembly process for the formation of TiO2 mesocrystals was controlled by the same thermodynamic principle as that of single crystals in this simple reaction system. Furthermore, the TiO2 mesocrystals were applied in photoelectrochemical (PEC) water splitting and demonstrated much enhanced photocurrent, almost 191% and 274% compared with that of TiO2 single crystals and commercial P25, respectively. Electrochemical impedance measurements under illumination revealed that the photocurrent increase was largely ascribed to the effective charge separation of electron-hole pairs and fast interfacial charge transfer. This could be attributed to the intrinsic characteristics of the mesostructured TiO2 composed of highly oriented nanocrystal subunits offering few grain boundaries, nanoporous nature and a short transport distance.

Enhanced dye-sensitized solar cells performance using anatase TiO2 mesocrystals with the Wulff construction of nearly 100% exposed {101} facets as effective light scattering layer

Anatase TiO2 mesocrystals with a Wulff construction of nearly 100% exposed {101} facets were successfully synthesized by a facile, green solvothermal method. Their morphology, and crystal structure are characterized by powder X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). Accordingly, a possible growth mechanism of anatase TiO2 mesocrystals is elucidated in this work. The as-prepared single anatase TiO2 mesocrystal's mean center diameter is about 500 nm, and the length is about 1 μm. They exhibit high light adsorbance, high reflectance and low transmittance in the visible region due to the unique nearly 100% exposed {101} facets. When utilized as the scattering layer in dye-sensitized solar cells (DSSCs), such mesocrystals effectively enhanced light harvesting and led to an increase of the photocurrent of the DSSCs. As a result, by using an anatase TiO2 mesocrystal film as a scattering overlayer of a compact commercial P25 TiO2 nanoparticle film, the double layered DSSCs show a power conversion efficiency of 7.23%, indicating a great improvement compared to the DSSCs based on a P25 film (5.39%) and anatase TiO2 mesocrystal films, respectively. The synergetic effect of P25 and the mesocrystals as well as the latters unique feature of a Wulff construction of nearly 100% exposed (101) facets are probably responsible for the enhanced photoelectrical performance. In particular, we explore the possibility of the low surface area and exposed {101} facets as an efficient light scattering layer of DSSCs. Our work suggests that anatase TiO2 mesocrystals with the Wulff construction is a promising candidate as a superior scattering material for high-performance DSSCs.

Anatase TiO2 mesocrystals with exposed (0 0 1) surface for enhanced photocatalytic decomposition capability toward gaseous styrene

Anatase TiO2 mesocrystals with exposed (001) surface have been successfully synthesized by a facile one-step solvothermal method using NH4F as the structure regulator in glacial acetic acid environment. The exposed (001) surface of the obtained anatase TiO2 mesocrystal was consisted of numerous nanocrystals with exposed {001} facet. The results indicated that both the added amount of NH4F and solvothermal reaction time played significant roles in the formation of anatase TiO2 mesocrystals with exposed (001) surface. A possible formation mechanism of anatase TiO2 mesocrystal with exposed (001) surface was proposed based on the experimental data. As UV active photocatalysts, the resultant anatase TiO2 mesocrystals were evaluated in detail by photocatalytic decomposition of gaseous styrene. The results demonstrated that the anatase TiO2 mesocrystals fabricated by solvothermal treating a mixture of 0.50g NH4F, 2.50mL Ti(OC4H9)4 and 50mL of glacial acetic acid at 210°C for 24h exhibited the highest photocatalytic activity to the decomposition of styrene. This is due to the synergistic effects of excellent crystallinity, high energy {001} crystal facets, relatively large surface area, enhanced bandgap energy and unique mesoporous structure.

One-step and large-scale synthesis of anatase TiO2 mesocrystals along [001] orientation with enhanced photocatalytic performance

Mesocrystals have attracted considerable attention due to their novel structure and special property, which are different from traditional polycrystals and single crystals. In this work, unique anatase TiO2 mesocrystals along the [001] orientation were successfully synthesized on a large scale via a facile one-pot solvothermal method due to the synergistic effect of tetraethyl silicate (TEOS) stabilizer and sodium acetate trihydrate (NaAc·3H2O) additive. Various characterization methods, including transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 adsorption–desorption isotherms, and X-ray photoelectron spectra (XPS), were applied to investigate the physicochemical properties of the as-prepared samples. The obtained anatase TiO2 mesocrystals show the uniform and monodisperse spindle shape growing along the [001] direction and exhibit obvious mesoporous structure with a BET specific surface area of 85.3 m2 g−1. The as-synthesized anatase TiO2 mesocrystals have remarkable crystalline-phase stability and the pure anatase phase can be retained even when the sample is annealed at 1000 °C for 2 h. The formation mechanism and growth process of anatase TiO2 mesocrystals was investigated with a few controlled experiments. The anatase TiO2 mesocrystals exhibit high photocatalytic activity in the photodegradation of methylene blue (MB) dye and excellent photocatalytic H2-evolution performance from water splitting with a Pt co-catalyst as a result of the high crystallinity, large specific surface area and proper crystal facets.

A facile solid phase reaction to prepare TiO2 mesocrystals with exposed {001} facets and high photocatalytic activity

As one of the most widely investigated and used materials, TiO2 is expected to play a leading role in helping solve environmental and energy issues. In this work, we developed a facile, greener and more controllable approach of preparing anatase TiO2 mesocrystals with largely exposed {001} facets. This approach utilizes the solid–solid topotactic transformation of NH4TiOF3 to anatase TiO2 and only needs a simple sintering process. In order to obtain the exposed {001} facets on the surfaces of final products, the sintering temperature must be between 700 °C and 900 °C. Temperatures below 700 °C lead to unexposed {001} facets. Temperatures above 900 °C lead to a mixed phase of anatase and rutile. The photocatalytic activity of the TiO2 mesocrystals is much higher than that of the corresponding TiO2 polycrystalline materials and comparable with that of commercially available P25. The exposed {001} facets are believed to play a very important role in the photocatalysis process.