TiO2 Crystals Research Articles

Influence of calcination temperatures on the anatase and rutile mixed phase composition and photocatalytic activity of the carbon doped mesoporous TiO2

Utilizing dicyandiamide as carbon source, the carbon doped mesoporous TiO2 (C–TiO2) with high photocatalytic activity was synthesized by sol-gel method, and the influence of calcination temperatures on the phase composition and photocatalytic properties of C–TiO2 photocatalyst were investigated. The XRD analysis shows carbon dopant in TiO2 can change the phase structure of the TiO2 and retard the TiO2 phase transformation from anatase to rutile. The N2 adsorption-desorption isotherms further inform the presence of mesoporous structure and XPS analysis clearly demonstrates that carbon atoms are successfully incorporated into the TiO2 crystals, which could cause a long-tail absorption in the visible-light region. Methylene blue was used to evaluate the photocatalytic activity of C–TiO2 under a xenon lamp irradiation. Compared with TiO2(550), the photocatalytic activity of as-prepared C–TiO2 is greatly improved, and C–TiO2 photocatalyst synthesized at 550 °C exhibits the most excellent photocatalytic performance due to its appropriate TiO2 phase composition of 74.7 wt% anatase and 25.3 wt% rutile, a larger specific surface area and effectively separating of e− and h+.

Effect of non-ionic surfactant in the solvothermal synthesis of anatase TiO2 nanoplates with a high percentage of exposed {001} facets and its role in the photocatalytic degradation of methylene blue dye

The synthesis of anatase TiO2 nanoparticles with controlled morphology and increased {001} facets exposed without the presence of fluorine-derived substances is a challenge. Herein, we report a highly effective approach to fabricate anatase TiO2 nanoplates with exposed {001} facets and their exploitation as robust photocatalytic materials for dye remediation. These materials were synthesized under controlled hydrolysis and condensation reactions, using titanium (IV) n-butoxide in an ethanolic solution, with acetic and sulfuric acids, by a solvothermal method at 190 °C with or without the presence of the non-ionic surfactant Triton® X-100 and then characterized. During TiO2 crystal synthesis, the effect of a non-ionic surfactant on the TiO2 particle growth was investigated. Our results demonstrate that the proposed method can synthesize pure and crystalline anatase TiO2 square nanoplates that form nanostructured spheres with high surface area, uniformly sized mesopores, and exposed {001} facets. The presence of non-ionic surfactant increased the exposed {001} facets percentage of the formed nanoplates from 69 to 80%, decreased the crystallite thickness, but unaffected its crystalline phase and band gap energy. The kinetic constants (Ka e Kb) for the synthesized TiO2 anatase nanoplates are considerably higher than the commercial TiO2 anatase constant (Kc). The synthesized photocatalysts show higher efficiency in the photocatalytic removal of methylene blue (MB) than commercial TiO2 (for t = 120 min).

Preparation of different crystal types TiO2 materials and its photodegradation performance in Congo Red wastewater

ABSTRACT TiO2 gel was synthesized by the sol–gel method, and TiO2 photodegradation material (TiO2-PDM) was obtained after drying, grinding, and heat treatment, which was used for photodegradation of Congo Red wastewater. The Rietveld refinement method confirmed that the TiO2 material was anatase crystal when the calcination temperature was lower than 600°C. When the calcination temperature was between 600 and 750°C, the TiO2 material was anatase-rutile mixed crystal, and the proportion of rutile increased gradually. The content of the rutile crystal reached 98.3% at 800°C. Compared with TiO2-PDM at 800°C, TiO2-PDM at 500°C has a smaller crystal size and better dispersibility. The best photodegradation performance was identified as the 77.1% when the drying temperature was 80°C, the calcination temperature was 500°C, and the calcination time was 1.5 h.

Fabrication of highly luminescent TiO2:Eu3+ thin film with low annealing temperature requirement by co-doping with Sn4+ ions

Rare earth (RE)-doped TiO2 thin films possess important applications in modern optoelectronic devices. However, the high annealing temperature requirement remains a critical restriction in device fabrications. In this work, TiO2 thin films doped with trivalent europium (Eu3+) ions were fabricated by a convenient sol-gel approach and subsequent annealing treatment. Interestingly, it is found that the optimal post-annealing temperature of TiO2:Eu3+ thin film is dramatically reduced from 800 to 300 °C by incorporating 10 at% Sn4+ into the sol precursor. The Sn-incorporated film annealed at 300 °C shows a 4-fold enhancement in photoluminescence (PL) intensity related to Eu3+ ions to the optimized Sn-free film. Systematic analysis reveals that this enhancement is attributed to Sn element promoting the crystallization of both anatase TiO2 and rutile SnO2 clusters at 300 °C, which provides sufficient SnO2/TiO2 crystal boundary facilitating Eu3+ to occupy and sensitized by SnO2 crystals. These results provide valuable insights into fabricating highly luminescent RE-doped TiO2 thin films with low annealing temperature requirement, which is essential for designing optoelectronic devices based on RE materials more freely in the future.

Adsorption and Oxidation of NO2 on Anatase TiO2: Concerted Nitrate Interaction and Photon-Stimulated Reaction

The catalytic and photon-induced oxidation of NO2 on anatase TiO2 has been studied and compared with the surface nitrate species obtained after adsorption of HNO3. Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface–NO3– species, where NO2 bonds to the lattice oxygen, and freely adsorbed NO3– ions. These products can be obtained either by dissociative adsorption of HNO3 or by catalytic/photocatalytic oxidation of NO2, which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface–NO3– species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N2O5) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO2 adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO2 crystal. In particular, a stable surface–NO3– and NO3– ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO2 and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO2 particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N2O during continued UV light illumination.

Facile fabrication of Z-scheme TiO2/ZnO@MCM-41 heterojunctions nanostructures for photodegradation and bioactivity performance

Designing Z-scheme heterojunction materials as a highly efficient photocatalyst is essential in environmental remediation. Herein, a new type of TiO2/ZnO@MCM-41 (TZM-41) heterojunction photocatalyst was designed for degrading organic dyes from wastewater and antibacterial activity on Staphylococcus (S. aureus). First, mesoporous MCM-41 silica material was fabricated with the sol–gel method and then calcinated under 550 °C to form porous material. Subsequently, ZnO and TiO2 crystals were grown onto the MCM-41 surface to obtain a TZM-41 heterojunction photocatalyst. TZM-41 nanocomposite with visible light excitations revealed a maximum degradation of methylene blue (MB) and methyl orange (MO) under visible light irradiation. The reusability tests, reaction mechanism, reaction kinetics as well as TOC and COD tests were investigated on the photocatalytic performance. The findings obtained from the reaction mechanism, quenching radical test, and electron spin resonance (ESR) technique indicated that hydroxyl radicals (•OH) and superoxide anion (•O2−) play an essential role in organic dyes degradation. Furthermore, the antibacterial properties of ZnO and TZM-41 nanocomposite were evaluated on S. aureus. The current study provides new horizons to develop Z-scheme heterojunction materials for improvement in water treatment and remediation.

Preparation and immobilisation of Brookite-type Ti3+-TiO2 for photocatalytic ammonia synthesis from N2 and H2O

In this work, a Ti3+-doped brookite-type TiO2 crystal (Ti3+–TiO2) was prepared using a NaBH4 thermal reduction method. A composite photocatalyst Ti3+–TiO2/C was obtained on a glass plate by heat treatment to remove organic substances. The structure of the as-prepared photocatalyst was characterised, and the immobilisation conditions were optimised. Furthermore, photocatalytic ammonia synthesis from N2/H2O was investigated. The results show that the introduction of the carbon carrier not only can enhance the ability of Ti3+–TiO2/C to transport photogenerated electrons but also can prevent the recombination of photogenerated electrons and holes through the transfer of electrons by the carbon substrate. Meanwhile, the oxygen-containing groups linked to the carbon carrier can improve the N2 adsorption and activation ability of the photocatalyst, which improved the photocatalytic ammonia synthesis activity of Ti3+–TiO2/C. The ammonia yield on the Ti3+–TiO2/C composite photocatalyst reached 0.313 µmol/(L·cm2) after 4 h of photocatalytic ammonia synthesis reaction from N2/H2O, which is 2.51 times higher than that on pure Ti3+–TiO2 under the same conditions. In addition, in this work, we used immobilised Ti3+–TiO2/C as a photocatalyst, which simplifies the method of recycling the catalyst and provides an idea for the industrialisation of the photocatalytic reaction.

Novel visible light active ternary TiO2-ZnO-WO3 composites: Facile preparation strategy and performance evaluation using the response surface design

This work reports the design and optimization of the preparation conditions of high performance visible light active TiO2-ZnO-WO3 ternary composite (TC) photocatalyst using minimum trial experiments based on response surface methodology (RSM). Calcination temperature, the weight percent (%) of ZnO and WO3 (with respect to TiO2) and nitrogen oxides (NOx) removal efficiency were chosen as the key variables toward optimizing the preparation conditions using RSM. Minitab Factorial design predicts trial experiments that are to be performed. The response coefficient difference between experimental and RSM and model verification approaches confirm that the TiO2-ZnO-WO3 TC containing 3% of ZnO and 3% of WO3 and calcined at 600 °C (designated as MinT(Z3%,W3%)) is superior in photocatalytic NOx removal performances. The microstructural and optical properties as well the surface composition of the composites prepared at the Maximum, Medium and Minimum conditions as predicted by RSM were evaluated by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) and the reasons for the excellent performance of MinT(Z3%,W3%) are corroborated with XRD, XPS, and DRS results. The generation of intermediate energy levels, formation of non-stochiometric solid solution like Wx Ti1−x O2 via substitution of W ionic states in the crystal lattice of TiO2 crystal and merging and reforming of particles contribute to visible light activation and high photocatalytic performance for MinT(Z3%,W3%). The possible photocatalytic mechanism informs that holes and bound hydroxyl radicals play predominant roles in the reaction pathways.

Methods to Tune the Optical Response of Porous Photonic Crystal Structures

Photonic crystals are periodic dielectric structures which selectively tune the wavelengths of light propagating through the material1 2. The highly ordered, repeating structural lattice induces a photonic bandgap or stopband which inhibits or partially attenuates certain frequencies of light, similar to the electronic bandgap with forbidden energies present in semiconductor materials3. These forbidden frequencies are blocked in transmission and reflected from the material surface. The inherent sensitivity of this photonic response to repeating lattice size dimensions and the magnitude of the refractive index contrast between the constituent materials allows for tailored optical behaviour by adjusting the photonic crystal structural parameters or environment4 5.A range of interesting applications using both the photonic bandgap and material porosity have emerged, predicated on the ability to accurately forecast the wavelength position of the photonic response. Colorimetric sensors6 7, photocatalysts8 9 and solar cells10 are prime examples of these types of applications; the porosity of the photonic crystal facilitates greater material infiltration and reactions, while the photonic bandgap acts to enhance the optical component of the process. Critically, the use of these structures is tied to our ability to predict and interpret the signature optical response.Here, we examine several techniques which can be used modify the photonic bandgap/stopband for photonic crystal structures. For TiO2 and SnO2 inverse opal photonic crystals, we explore how solvent infiltration into the highly porous network red-shifts the observed photonic response. Using solvents with different refractive indices, we apply the shifted photonic stopband data to determine the fill fraction of solid material comprising the photonic crystal network. We also examine functionalization of artificial opal and inverse opal photonic crystals with metal films. We detail the emergence of a consistent photonic stopband blue-shift with increasing metal content and propose a reduction in the effective refractive index of the entire photonic crystal introduced by the specific properties of the metal film. Importantly, the effects investigated here are broadly applicable to a range of realistic operating conditions across many disciplines where an understanding of the photonic stopband is paramount to the application. References Yablonovitch, E., Inhibited Spontaneous Emission in Solid-State Physics and Electronics. Physical Review Letters 1987, 58 (20), 2059-2062.John, S., Strong localization of photons in certain disordered dielectric superlattices. Physical Review Letters 1987, 58 (23), 2486-2489.Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S., Photonic crystals: putting a new twist on light. Nature 1997, 386 (6621), 143-149.Blanford, C. F.; Schroden, R. C.; Al-Daous, M.; Stein, A., Tuning Solvent-Dependent Color Changes of Three-Dimensionally Ordered Macroporous (3DOM) Materials Through Compositional and Geometric Modifications. Advanced Materials 2001, 13 (1), 26-29.Aguirre, C. I.; Reguera, E.; Stein, A., Tunable Colors in Opals and Inverse Opal Photonic Crystals. Advanced Functional Materials 2010, 20 (16), 2565-2578.Zhang, Y.; Qiu, J.; Hu, R.; Li, P.; Gao, L.; Heng, L.; Tang, B. Z.; Jiang, L., A visual and organic vapor sensitive photonic crystal sensor consisting of polymer-infiltrated SiO2 inverse opal. Physical Chemistry Chemical Physics 2015, 17 (15), 9651-9658.Li, H.; Chang, L.; Wang, J.; Yang, L.; Song, Y., A colorful oil-sensitive carbon inverse opal. Journal of Materials Chemistry 2008, 18 (42), 5098-5103.Chen, J. I. L.; von Freymann, G.; Choi, S. Y.; Kitaev, V.; Ozin, G. A., Amplified Photochemistry with Slow Photons. Advanced Materials 2006, 18 (14), 1915-1919.Collins, G.; Lonergan, A.; McNulty, D.; Glynn, C.; Buckley, D.; Hu, C.; O'Dwyer, C., Semiconducting Metal Oxide Photonic Crystal Plasmonic Photocatalysts. Advanced Materials Interfaces 2020, 7 (8), 1901805.Liu, L.; Karuturi, S. K.; Su, L. T.; Tok, A. I. Y., TiO2 inverse-opal electrode fabricated by atomic layer deposition for dye-sensitized solar cell applications. Energy & Environmental Science 2011, 4 (1), 209-215. Figure 1 SEM images and optical transmission spectra for (a) TiO2 and (b) SnO2 inverse opals. In each case the wavelength position of the photonic stopband is red-shifted significantly when a solvent infiltrates the porous photonic crystal network. SEM images and optical transmission spectra for (c) artificial polystyrene opals coated with a gold film and (d) TiO2 inverse opals coated with a copper film. Metal film incorporation into the photonic crystal network acts to consistent blue-shift the observed photonic stopband. Figure 1

(Invited) Solar CO2 Conversion into Liquid Fuels By Photoelectrochemical Approaches

Photoelectrochemical (PEC) system for the reduction of CO2 into liquid fuels of formaldehyde/acetaldehyde, methanol and ethanol with trace amount of hydrogen gas bubbling have been described in the aspect of thermodynamics and kinetics of the CO2 reduction reaction to have efficient method by lowering activation energy of CO2 reduction on the electron transfer reaction and to do reduction potential tuning of CO2 reduction reaction for the selective reduction products. Ca/Fe doped TiO2 photoanode oxidizes water and generates the large amount of O2, electrons and protons. On the other side, rGO(reduced graphene oxide)/PVP(poly(4-vinyl)pyridine)/Nafion multi-layers have been coated and fabricated on the surface of Cu foil cathode to reduce CO2 into formaldehyde and acetaldehyde. When solar light was irradiated on the surface of photoanode, electrons get excited to the conduction band of Ca/Fe doped TiO2 and transported to the cathode via external wire with low external bias potential. rGO on Cu foil has been used for the dual functions of reduction potential tuning and multi-electron shuttling process of CO2 reduction reaction. The multi-electron shuttling function was illuminated by larger number and longer life-time of excited electrons and generating electron cloud, which have been confirmed by time-resolved photoluminescence (TR-PL) decay curves and 2D time-resolved photoluminescence (TR-PL) mapping images of Cu/rGO electrode. This allows the sequential multi-electron transport process from Cu/rGOcathode to CO2, which was studied with time-resolved chronoamperometry. N-heterocyclic poly(4-vinyl)pyridine (PVP) helps to capture and do chemical activation of reactant CO2 molecule by complexation as [PVP-CO2*] complex via charge separation and lowering transition state energy level of the electron transfer for the formation of anion radical of carbon dioxide complex with PVP as [e- + PVP-CO2*]≠ complex via electron delocalization, which is basically possible due to high basicity of lone pair electrons of nitrogen atoms of N-heterocyclic PVP compounds. These functions result in the lowered activation energy for the CO2 reduction reaction to have faster kinetics. The decreased amount of activation energy was determined with Tafel plots by measuring the slopes of it and semicircles of the electrochemical impedance spectroscopy (EIS) Nyquist plots at different temperatures. Nafion film coated on the surface of cathode assists the faster transport of H+ ions to CO2 reduction reaction sites for the simultaneous one-pot reaction of proton coupled electron transfer reactions. In the photocataytic system with semi-conducting polymers, CO2 was conversed into liquid solar fuels like formaldehyde and alcohols with solar to fuel efficiency over 2.5% to produce liquid fuel products over mM concentration for 4 hour reaction. Chang Woo Kim, So Jin Yeob, Hui-Ming Cheng, Young Soo Kang*, Selectively Exposed Crystal Facet-Engineered TiO2 Thin Film Photoanode for the Higher Performance of Photoelectrochemical Water Splitting Reaction, Energy and Environmental Sciences. 2015, 8, 3646.Amol U.Pawar, Chang WooKim, Myung Jong Kang, Young Soo Kang*, Crystal facet engineering of ZnO photoanode for the higher water splitting efficiency with proton transferable nafion film, Nano Energy 2015, 20, 156.Chang Woo Kim, Young Seok Son, Myung Jong Kang, Do Yoon Kim, Young Soo Kang*, (040)-Crystal Facet Engineering of BiVO4 Plate Photoanode for Solar Fuel Production, Energy Mater., 2016, 6, 1501754.Jin You Zheng, Amol Uttam Pawar, Chang Woo Kim, Yong Joo Kim, Young Soo Kang*, Highly enhancing photoelectrochemical performance of facilely-fabricated Bi-induced (002)-oriented WO3 film with intermittent short-time negative polarization, Applied Catalysis B 2018, 233, 88.Chang Woo Kim, Myung Jong Kang, Sohyun Ji, and Young Soo Kang*, Artificial Photosynthesis for Formaldehyde Production with 85% of Faradaic Efficiency by Tuning the Reduction Potential, ACS Catalysis 2018, 82, 968-974.Myung Jong Kang, Chang Woo Kim, Amol U. Pawar, Hyun Gil Cha, Sohyun Ji, Wen-Bin Cai, and Young Soo Kang*, Selective Alcohol on Dark Cathode by Photoelectrochemical CO2 Valorization and Their in-situ Characterization, minor revision, ACS Energy Lett. 2019, 4, 1549.Hong Ryeol Park, Amol Uttam Pawar, Umapada Pal, Tierui Zhang, Young Soo Kang, Enhanced solar photoreduction of CO2 to liquid fuel over rGO grafted NiO-CeO2 heterostructure nanocomposite, Nano Energy, 2021, 79, 105483.Amol U. Pawar, Umapada Pal, Jin You Zheng, Chang Woo Kim and Young Soo Kang*, Dynamics controlled CO2 reduction on Cu/rGO/PVP/Nafion multi-layered cathode for selective production of Acetaldehyde and Formaldehyde, Applied Catalysis B: Environmental, 2022, 303, 120921.

(Invited) Ceramic Nano-Heterostructures By Materials Design: Platforms for Sensing Applications – Opportunities and Challengess

This talk summarizes R&D efforts in the author’s laboratory on the fabrication of oxide nano-heterostructures, exploiting intrinsic material properties, that are highly scalable and do not require use of lithography. One such process creates crystallographically oriented nanofiber arrays of single crystal TiO2 in H2/N2 environment. H2/N2 heat treatment was also used to grow nanofibers on polycrystalline SnO2, showing directional growth on grains with crystal facets. We have also developed a process to create nanofibers of TiO2 on Ti metal/alloys via oxidation under a limited supply of oxygen. In another process, SnO2 nanowires grown from commercial FTO slides using the vapor-liquid-solid (VLS) method were placed in a microwave-assisted hydrothermal chamber where TiO2 nanorods nucleated radially from the SnO2 nanowire cores. We developed yet another interesting nano-structure (nanoislands and/or nanobars) during thermal annealing of an oxide (GDC) on top of another oxide (YSZ) substrate that self-assembles along the softest elastic direction of the substrate. What is common about these structures is that they are fabricated without the use of lithographic techniques and involves simple processes such as gas-phase reactions and stress-driven processes. These nano-heterostructures can be used as platforms for chemical sensing, catalysis, photocatalysis, photovoltaics and biomedical applications. Sensing application presents opportunities and challenges that are presented including an Open access Database Of Resistive type gas Sensors (ODORS) that has been developed and can be used to select suitable sensing materials.

(Invited) In Situ Time-Resolved Probe of Charge Carrier Dynamics at Planar Semiconductor Photoelectrode/Liquid Interface

Photoelectrochemistry is a promising approach for converting solar energy to storable chemical energy in chemical fuels. The efficiency of photoelectrochemcial devices depends critically on efficient transfer of charge carrier across the electrode liquid interface and the efficiency and selectivity of the catalytic reaction on electrode surface. Although transient absorption spectroscopy is a powerful tool for probing interfacial dynamics in high surface area electrodes, it is often not applicable on planar electrodes because the observed signal is often dominated by carrier dynamics in the bulk. In this talk, we discuss our recent effort in developing in situ time-resolved linear and nonlinear spectroscopic tools that can be used to probe carrier dynamics and reactions at planar electrode/liquid interfaces, focusing on transient reflection spectroscopy and electric field induced second harmonic generation (EFISH) spectroscopy. Compared to transient absorption, transient reflection spectroscopy is more sensitive to surface carrier density that can be more directly correlated to catalysis. We showed that transient reflectance change can be used to follow charge carrier dynamics and their amplitude can be directly correlated to efficiency of initial interfacial charge separation on p-GaP single crystals protected by TiO2 ALD layer. Our finding suggests that IPCE is determined by the product of the efficiency of initial ultrafast (< hundreds of ps) charge separation across the GaP/TiO2 interface and the efficiency of water reduction reaction on the slower time scale. Key loss pathways that limit these efficiencies are discussed. In a second approach, we showed the EFISH signal in a n-doped single crystal TiO2 can be used to probe the accumulation of photogenerated carriers at surface states and these trapped carriers are responsible for the water oxidation reaction.

Room-Temperature 181Ta(TiO2): An e-γ TDPAC Study

In this work, we report on the hyperfine parameters of the foreign 181Ta probe in the rutile structure of the single crystal TiO2 using the e−γ and γ−γ time differential perturbed angular correlation (TDPAC) technique. We implanted 181Hf ions into a sample of single crystal rutile TiO2 in the Bonn Isotope Separator. The implanted sample was then thermally annealed at a temperature of 873 K for 315 min in a vacuum. The 181Hf radioisotopes decayed by β− emission, followed by a cascade to the ground of γ rays or conversion electrons into a stable state 181Ta. The 181Ta probe substitutes the Ti lattice site with a unique nuclear quadrupole interaction, allowing for the precise measurement of the largest electric field gradient (Vzz) and asymmetry parameter (η). The hyperfine parameters obtained from the e−γ TDPAC spectroscopy agree with those of the γ−γ TDPAC spectroscopy at room temperature, apart from a calibration factor, both from our experiments and the literature. This suggests that the electronic recombination following the internal conversion of the L shell electron takes less time (ps) than the intermediate lifetime of the metastable 181Ta state (ns).

Verneuil growth of TiO2 (rutile) crystals of large size and low dislocation density at low gas flow rate

Large (30 mm in diameter) TiO2 (rutile) single crystals with low dislocation density were grown by the Verneuil method using a single crystal furnace designed and improved by the authors. The structure of the burner was optimized by numerical simulation analysis so that the crystal could be grown at a low gas flow rate. The investigation of the growth process parameters (i.e., growth rate, outer flow rate of O2, inner flow rate of O2, and the increment of H2 flow rate) shows that the inner flow rate of O2 and the increment of H2 flow rate have the strongest influence on the crystal growth process. On this basis, other growth parameters (growth rate, outer flow rate of O2) were also optimized. Compared with the conventional Verneuil method, the crystal can be grown at a considerably low gas flow rate (40–50% lower) with the method in this work, which reduces the melt turbulence impacted by gas flow, enabling a steady and clear solid-liquid interface and improving the crystal quality. The optimum growth conditions are for the growth rate of 6 mm/h, O2 outer flow rate of 3.5 L/min, O2 inner flow rate of 5.5 L/min, and increment of H2 flow rate of 0.1 L/4 min. The etch pit density of the rutile crystals is 3.29 × 104 cm−2, an order of magnitude lower than that of the crystals grown by the conventional Verneuil method. The optical properties of the crystal are comparable to those grown by the floating zone method. Especially, it is easier to obtain a larger crystal size with lower production costs. Our results provide a possible route for industrializing the Verneuil production of large, high-quality and low-cost rutile single crystals.

Colossal dielectric behavior of (Nb, Ga) co-doped TiO2 single crystal

(Nb0.5Ga0.5)xTi1−xO2 (x = 0.05%,0.2%,0.5%,1%,5%) single crystals were prepared by Verneuil method. Even if the doping amount is 0.05%, the dielectric permittivity (ε′) is obviously higher than that of pure rutile TiO2 (260), reaching more than 104. As the doping amount x increase, the ε′ increases, and reaches the maximum value when x = 5%, but the dielectric loss increases correspondingly. The best dielectric performance (ε′=1.0 ×105, tanδ=0.034 under 1 kHz) is obtained when the doping amount is 0.5%. Even at 106 Hz, the dielectric loss is only 0.019, much lower than the reported ceramic samples. This may be due to the fact crystalline materials can effectively reduce defects such as grain boundary, second phase, porosity and segregation. The dielectric spectra, impedance analysis, XPS and DC bias results show that the colossal permittivity (CP) properties could be attributed to EPDD polarization. With the increase of doping concentration (>1%), the hopping polarization occurs, resulting in a slight increase in dielectric loss. These results indicate that TiO2 crystal materials with lower dielectric loss are good candidates for dielectric materials. Taking TiO2 crystal as the research object, we can have a clearer understanding of the origin of TiO2 dielectric properties and the influence of doping ion content, which is of great significance to the development of TiO2-based dielectric materials.

Titanium-Pillared Clay: Preparation Optimization, Characterization, and Artificial Neural Network Modeling.

Titanium-pillared clay (Ti-PILC), as one of the most suitable types of porous adsorbents/(photo)catalysts, was prepared from a local type of Iranian clay and titanium isopropoxide. The production process was optimized by changing three operating parameters, including the clay suspension concentration (in the range of 0.5–10% w/v), the H+/Ti ratio (2–8 mol/mol), and the calcination temperature (300–700 °C). The largest specific surface area for the Ti-PILC was about 164 m2/g under the clay suspension of 0.5% w/v, H+/Ti = 6, with a surface area 273% larger than that of the raw clay. The surface areas obtained from more concentrated clay suspensions were, however, comparable (159 m2/g for 3% w/v clay and H+/Ti = 4). An increase in the calcination temperature has a negative effect on the porous texture of Ti-PILC, but based on modeling with artificial neural networks, its contribution was only 7%. Clay suspension and H+/Ti ratio play a role of 56 and 37% of the specific surface area. The presence of rutile phase, and in some cases anatase phase of TiO2 crystals was detected. FTIR and SEM investigations of Ti-PILCs produced under different operating parameters were analyzed.

Branched core-shell a-TiO2@N-TiO2 nanospheres with gradient-doped N for highly efficient photocatalytic applications

A branched core-shell nanosphere composed of an anatase TiO2 (a-TiO2) core and a TiO2 nanobranch shell with gradient-doped N (a-TiO2@N-TiO2) is synthesized by a simple in situ doping method, in which mixed crystal anatase-rutile TiO2 (ar-TiO2) nanosphere is first prepared by oxidizing Ti using H2O2, and then is etched by NH3·H2O to form (NH4)2TiO3 nanobranches, which is converted into a-TiO2@N-TiO2 following an ambient annealing process. The diameter of a-TiO2 core is ∼500 nm, and the thickness of N-TiO2 branched shell is ∼100 nm with gradually increased N concentration from the bottom to the edge. Ultra-thin amorphous coating layers on the branches are also observed. The morphology of the composites could be further tuned by the amount of NH3·H2O, and its effect on the photocatalytic performance is also investigated. The optimized a-TiO2@N-TiO2 shows an outstanding hydrogen evolution rate of 308.1 µmol g−1 h−1 under air mass (AM) 1.5 illumination, and also exhibits highly active in photocatalytic degradation of various refractory organic pollutants, including organic dyes, phenols, antibiotics, and personal care products, with removal ratios higher than 96% after 2 h operation. This can be due to the gradient-doped N-TiO2 nanobranches, which not only provide bending band structure and defect level derived from the N impurities and O vacancies, resulting the formation of n-n+ heterojunctions to improve the charge separation, but also enhance the charge transfer at the liquid-solid interface due to the numerous nanobranches and amorphous coating layers.

N and S co-doping of TiO2@C derived from in situ oxidation of Ti3C2 MXene for efficient persulfate activation and sulfamethoxazole degradation under visible light

Visible-light-driven photocatalysts with synergistic persulfate (PS) activation and photocatalysis possess enormous pollution-control potential. Herein, nitrogen and sulphur co-doped TiO2@C hybrid (N/S-TiO2@C) featuring the complementary visible-light photocatalysis and synergetic PS activation was synthesized by in situ oxidation of Ti3C2 MXene and thereafter co-doping with N and S. Both anatase and rutile phases of TiO2 were attained in N/S-TiO2@C. The decrease of TiO2 bandgap (about 1.81 eV) and the defect of carbon layer caused by the N and S co-doping in both TiO2 crystal and carbon layer enabled the synergistic enhancement of PS activation and photocatalysis. Complete degradation and 93.4% mineralization of sulfamethoxazole (SMZ, 10 mg/L) were achieved by dosing PS (2 mM) and N/S-TiO2@C (0.4 g/L) under visible light within 30 min. The degradation rate by N/S-TiO2@C through PS activation under visible light was 27.9-, 11.1-, and 2.30-fold higher than that by PS, N-TiO2@C, and S-TiO2@C alone, respectively. Reactive species SO4−,1O2, OH, and O2−, especially the former two, were determined dominating the SMZ degradation, and the degradation pathways were illustrated through intermediates identification. Moreover, N/S-TiO2@C kept excellent performance in activating PS to degrade SMZ in real water samples. These results indicate that N/S-TiO2@C is a stable and promising PS activator under visible light and has a good application potential in organic wastewater treatment.

N–TiO2 crystal seeds incorporated in amorphous matrix for enhanced solar hydrogen generation: Experimental & first-principles analysis

We present here a combined study on the photoelectrochemical activity of highly active Nitrogen doped titanium dioxide thin-film using experiments and First principle density (DFT) based calculation. Hybridization of N 2p with O 2p and localized valence band upshifting leads to the reduction in band-gap of N–TiO2. To validate theoretical findings, the role of nitrogen in TiO2 is revisited with a focus on partial crystallinity. The best-case photoelectrode, nanostructured partially crystalline nitrogen-doped titanium dioxide (PCNDTO) offered photocurrent density of 24.3 mA/cm2 at 1 V versus saturated calomel electrode (SCE). The absence of well-defined peaks and long-range order in XRD pattern and Raman spectrum respectively suggests partially crystallinity. High-resolution transmission electron microscopy (HR-TEM) images confirm the presence of TiO2 crystals in the amorphous matrix. High photoelectrochemical response can be attributed to the abundance of hydroxyl groups, high electrochemical active surface area, reduced charge transfer resistance, and reduced charge carrier recombination rate.

Graphene-assisted titanium dioxide Z-mechanism photoelectrode as enzymatic glucose biosensor

Highly exposed surfaces of anatase TiO2 crystals are of interest due to their excellent photogenerated electron–hole pair separation effect and high photocatalytic activity. In this work, a Z-mechanism biosensor with reduced graphene oxide (rGO) and anatase TiO2 nanocrystals combined with PDA (TiO2/rGO/PDA) was successfully synthesized. rGO was combined with the co-exposed (001) and (101) facets of TiO2 nanocrystals to construct a Z-mechanism in which rGO acts as an electron transport medium and provides a channel for the transfer of electrons, resulting in TiO2 nanocrystals with high photoelectron–hole pair separation efficiency and strong redox ability. The coated PDA polymer not only could absorb visible light but also has good stability and biocompatibility, which helps to increase the adsorption of target enzyme molecules. The TiO2/rGO/PDA/GOx biosensor with the Z-mechanism displays the sensitivity of 13.82 μA mM−1 cm−2 in 0.1M PBS solution (pH = 7.4) with a linear range of 0–3 mM and the LOD of 0.034 µM. In general, the application of the Z-mechanism in biosensors would provide a new orientation for the design of biosensors.