Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid-Base Parameters

Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

Influence of Anatase-Rutile Ratio on Band Edge Position and Defect States of TiO2 Homojunction Catalyst

Removal of toxic air and water dissociation in the environment has become a major challenging issue throughout the world. Mixed phase rutile-anatase titanium dioxide catalysts are very effective in photocatalysis and have been studied extensively. However, the mechanism causing this effect and band alignment of the two phases are not fully understood. Pointing to the issue, we have designed one-dimensional mixed-phase TiO₂ and introduced defects near the valence band. Experimental results showed that band alignment between two phases, up-shift of the band edge, and optimum anatase percentage play a key role in the enhancement of the photocatalytic activity. We predicted shifts in band edge originating from surface electric dipole layer induced by defects.

Low Temperature Deposition of TiO2 Thin Films through Atmospheric Pressure Plasma Jet Processing

Titanium dioxide (TiO2) has been widely used as a catalyst material in different applications such as photocatalysis, solar cells, supercapacitor, and hydrogen production, due to its better chemical stability, high redox potential, wide band gap, and eco-friendly nature. In this work TiO2 thin films have been deposited onto both glass and silicon substrates by the atmospheric pressure plasma jet (APPJ) technique. The structure and morphological properties of TiO2 thin films are studied using different characterization techniques like X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and field emission scanning electron microscopy. XRD study reveals the bronze-phase of TiO2. The XPS study shows the presence of Ti, O, C, and N elements. The FE-SEM study shows the substrate surface is well covered with a nearly round shaped grain of different size. The optical study shows that all the deposited TiO2 thin films exhibit strong absorption in the ultraviolet region. The oleic acid photocatalytic decomposition study demonstrates that the water contact angle decreased from 80.22 to 27.20° under ultraviolet illumination using a TiO2 photocatalyst.

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 of uniform ordered mesoporous TiO2 microspheres with controllable phase junctions for efficient solar water splitting

As a benchmark photocatalyst, commercial P25-TiO2 has been widely used for various photocatalytic applications. However, the low surface area and poorly porous structure greatly limit its performance. Herein, uniform ordered mesoporous TiO2 microspheres (denoted as Meso-TiO2-X; X represents the rutile percentage in the resultant microspheres) with controllable anatase/rutile phase junctions and radially oriented mesochannels are synthesized by a coordination-mediated self-assembly approach. The anatase/rutile ratio in the resultant microspheres can be facilely adjusted as desired (rutile percentage: 0-100) by changing the concentration of hydrochloric acid. As a typical one, the as-prepared Meso-TiO2-25 microspheres have a similar anatase/rutile ratio to commercial P25. But the surface area (78.6 m2 g-1) and pore volume (0.39 cm3 g-1) of the resultant microspheres are larger than those of commercial P25. When used as the photocatalyst for H2 generation, the Meso-TiO2-25 delivers high solar-driven H2 evolution rates under air mass 1.5 global (AM 1.5 G) and visible-light (λ > 400 nm), respectively, which are significantly larger than those of commercial P25. This coordination-mediated self-assembly method paves a new way toward the design and synthesis of high performance mesoporous photocatalysts.

Pulsed photoinitiated fabrication of inkjet printed titanium dioxide/reduced graphene oxide nanocomposite thin films

This work reports a new technique for scalable and low-temperature processing of nanostructured TiO₂ thin films, allowing for practical manufacturing of TiO₂-based devices such as perovskite solar cells at low-temperature or on flexible substrates. Dual layers of dense and mesoporous TiO₂/graphitic oxide nanocomposite films are synthesized simultaneously using inkjet printing and pulsed photonic irradiation. Investigation of process parameters including precursor concentration (10-20 wt%) and exposure fluence (4.5-8.5 J cm^-2) reveals control over crystalline quality, graphitic oxide phase, film thickness, dendrite density, and optical properties. Raman spectroscopy shows the E _g peak, characteristic of anatase phase titania, increases in intensity with higher photonic irradiation fluence, suggesting increased crystallinity through higher fluence processing. Film thickness and dendrite density is shown to increase with precursor concentration in the printed ink. The dense base layer thickness was controlled between 20 and 80 nm. The refractive index of the films is determined by ellipsometry to be 1.92 ± 0.08 at 650 nm. Films exhibit an energy weighted optical transparency of 91.1%, in comparison to 91.3% of a thermally processed film, when in situ carbon materials were removed. Transmission and diffuse reflectance are used to determine optical band gaps of the films ranging from 2.98 to 3.38 eV in accordance with the photonic irradiation fluence and suggests tunability of TiO₂ phase composition. The sheet resistance of the synthesized films is measured to be 14.54 ± 1.11 Ω/□ and 28.90 ± 2.24 Ω/□ for films as-processed and after carbon removal, respectively, which is comparable to high temperature processed TiO₂ thin films. The studied electrical and optical properties of the light processed films show comparable results to traditionally processed TiO₂ while offering the distinct advantages of scalable manufacturing, low-temperature processing, simultaneous bilayer fabrication, and in situ formation of removable carbon nanocomposites.

Core/Shell Microstructure Induced Synergistic Effect for Efficient Water-Droplet Formation and Cloud-Seeding Application

Cloud-seeding materials as a promising water-augmentation technology have drawn more attention recently. We designed and synthesized a type of core/shell NaCl/TiO₂ (CSNT) particle with controlled particle size, which successfully adsorbed more water vapor (∼295 times at low relative humidity, 20% RH) than that of pure NaCl, deliquesced at a lower environmental RH of 62-66% than the hygroscopic point (h_g.p., 75% RH) of NaCl, and formed larger water droplets ∼6-10 times its original measured size area, whereas the pure NaCl still remained as a crystal at the same conditions. The enhanced performance was attributed to the synergistic effect of the hydrophilic TiO₂ shell and hygroscopic NaCl core microstructure, which attracted a large amount of water vapor and turned it into a liquid faster. Moreover, the critical particle size of the CSNT particles (0.4-10 μm) as cloud-seeding materials was predicted via the classical Kelvin equation based on their surface hydrophilicity. Finally, the benefits of CSNT particles for cloud-seeding applications were determined visually through in situ observation under an environmental scanning electron microscope on the microscale and cloud chamber experiments on the macroscale, respectively. These excellent and consistent performances positively confirmed that CSNT particles could be promising cloud-seeding materials.

Bioinspired Hierarchical Nanofibrous Silver-Nanoparticle/Anatase-Rutile-Titania Composite as an Anode Material for Lithium-Ion Batteries

A new bioinspired hierarchical nanofibrous silver-nanoparticle/anatase-rutile-titania (Ag-NP/A-R-titania) composite was fabricated by employing a natural cellulose substance (e.g., commercial laboratory cellulose filter paper) as the structural scaffold template, which was composed of anatase-phase titania (A-titania) nanotubes with rutile-phase titania (R-titania) nanoneedles grown on the surfaces and further silver nanoparticles (AgNPs) immobilized thereon. As it was employed as an anode material for lithium-ion batteries (LIBs), high reversible capacity, enhanced rate performance, and excellent cycling stability were achieved as compared with those of the corresponding cellulose-substance-derived nanotubular A-titania, R-titania, heterogeneous anatase/rutile titania (A-R-titania) composite, and commercial P25 powder. This benefited from its unique porous cross-linked three-dimensional structure inherited from the initial cellulose substance scaffold, which enhances the sufficient electrode/electrolyte contact, relieves the severe volume change upon cycling, and improves the amount of lithium-ion storage; moreover, the high loading content of the silver component in the composite improves the electrical conductivity of the electrode. The structural integrity of the composite was maintained upon long-term charge/discharge cycling, indicating its significant stability.

Self-induced synthesis of phase-junction TiO2 with a tailored rutile to anatase ratio below phase transition temperature

The surface phase junction of nanocrystalline TiO₂ plays an essential role in governing its photocatalytic activity. Thus, facile and simple methods for preparing phase-junction TiO₂ photocatalysts are highly desired. In this work, we show that phase-junction TiO₂ is directly synthesized from Ti foil by using a simple calcination method with hydrothermal solution as the precursor below the phase transition temperature. Moreover, the ratio of rutile to anatase in the TiO₂ samples could be readily tuned by changing the ratio of weight of Ti foil to HCl, which is used as the hydrothermal precursor, as confirmed by the X-ray diffraction analysis. In the photocatalytic reaction by the TiO₂ nanocomposite, a synergistic effect between the two phases within a certain range of the ratio is clearly observed. The results suggest that an appropriate ratio of anatase to rutile in the TiO₂ nanocomposite can create more efficient solid-solid interfaces upon calcination, thereby facilitating interparticle charge transfer in the photocatalysis.

Solution processable formation of a few nanometer thick-disordered overlayer on the surface of open-ended TiO2 nanotubes

Herein, we designed vertically aligned TiO₂ nanotube arrays, in which a very thin disordered overlayer approximately a few nm thick was formed via a room-temperature solution process.

S-doped mesoporous nanocomposite of HTiNbO5 nanosheets and TiO2 nanoparticles with enhanced visible light photocatalytic activity

S-doped mesoporous TiO₂/HTiNbO₅ nanocomposite showed dramatically enhanced visible-light photocatalytic activity and stability owing to the combined effects of nano-heterojunction, S doping and morphology engineering.

Fabrication of anatase/rutile hierarchical nanospheres with enhanced n/p type gas sensing performance at room temperature

Porous Pt decorated anatase/rutile sensing nanospheres with high crystallinity and large surface area synthesized through psHT treatment present enhanced sensitivity and selectivity to VOCs vapor at room temperature.

In-Situ-Reduced Synthesis of Ti³⁺ Self-Doped TiO₂/g-C₃N₄ Heterojunctions with High Photocatalytic Performance under LED Light Irradiation

A simple one-step calcination route was used to prepare Ti(3+) self-doped TiO2/g-C3N4 heterojunctions by mixture of H2Ti3O7 and melamine. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) spectroscopy, and UV-Vis diffuse reflectance spectroscopy (UV-vis DRS) technologies were used to characterize the structure, crystallinity, morphology, and chemical state of the as-prepared samples. The absorption of the prepared Ti(3+) self-doped TiO2/g-C3N4 heterojunctions shifted to a longer wavelength region in comparison with pristine TiO2 and g-C3N4. The photocatalytic activities of the heterojunctions were studied by degrading methylene blue under a 30 W visible-light-emitting diode irradiation source. The visible-light photocatalytic activities enhanced by the prepared Ti(3+) self-doped TiO2/g-C3N4 heterojunctions were observed and proved to be better than that of pure TiO2 and g-C3N4. The photocatalysis mechanism was investigated and discussed. The intensive separation efficiency of photogenerated electron-hole in the prepared heterojunction was confirmed by photoluminescence (PL) spectra. The removal rate constant reached 0.038 min(-1) for the 22.3 wt % Ti(3+) self-doped TiO2/g-C3N4 heterojunction, which was 26.76 and 7.6 times higher than that of pure TiO2 and g-C3N4, respectively. The established heterojunction between the interfaces of TiO2 nanoparticles and g-C3N4 nanosheets as well as introduced Ti(3+) led to the rapid electron transfer rate and improved photoinduced electron-hole pair's separation efficiency, resulting in the improved photocatalytic performance of the Ti(3+) self-doped TiO2/g-C3N4 heterojunctions.

Carbon nanotubes enhanced Seebeck coefficient and power factor of rutile TiO2

Ti–C substitution occurs when carbon nanotubes were thermally dispersed in rutile TiO₂ and the electrical conductivity as well as Seebeck coefficient were simultaneously promoted at a low filling fraction of tubes.

Polymorphic phase transition among the titania crystal structures using a solution-based approach: from precursor chemistry to nucleation process

Nanocrystalline titania are a robust candidate for various functional applications owing to its non-toxicity, cheap availability, ease of preparation and exceptional photochemical as well as thermal stability. The uniqueness in each lattice structure of titania leads to multifaceted physico-chemical and opto-electronic properties, which yield different functionalities and thus influence their performances in various green energy applications. The high temperature treatment for crystallizing titania triggers inevitable particle growth and the destruction of delicate nanostructural features. Thus, the preparation of crystalline titania with tunable phase/particle size/morphology at low to moderate temperatures using a solution-based approach has paved the way for further exciting areas of research. In this focused review, titania synthesis from hydrothermal/solvothermal method, conventional sol-gel method and sol-gel-assisted method via ultrasonication, photoillumination and ILs, thermolysis and microemulsion routes are discussed. These wet chemical methods have broader visibility, since multiple reaction parameters, such as precursor chemistry, surfactants, chelating agents, solvents, mineralizer, pH of the solution, aging time, reaction temperature/time, inorganic electrolytes, can be easily manipulated to tune the final physical structure. This review sheds light on the stabilization/phase transformation pathways of titania polymorphs like anatase, rutile, brookite and TiO2(B) under a variety of reaction conditions. The driving force for crystallization arising from complex species in solution coupled with pH of the solution and ion species facilitating the orientation of octahedral resulting in a crystalline phase are reviewed in detail. In addition to titanium halide/alkoxide, the nucleation of titania from other precursors like peroxo and layered titanates are also discussed. The non-aqueous route and ball milling-induced titania transformation is briefly outlined; moreover, the lacunae in understanding the concepts and future prospects in this exciting field are suggested.

Photocatalytic Finishing of Cotton Fabrics Based on Nano-Titanium Dioxide and Polyurethane Binder

In the textile industry, binders are used in fabric finishing processes to promote adhesion between a fiber surface and the particles of desired properties. In this research, a commercial polyurethane binder, Evo®Fin PUS, was used to attach photocatalytic TiO2nanoparticles (Degussa®, P-25) onto a cotton surface, in order to impart self-cleaning properties and to improve wash fastness. The cotton fabrics were finished with the aqueous dispersion of TiO2and the binder, consisting of 2, 4, and 6 % w/v for each component. Finishing was done via a pad-dry-cure process. Then a direct dye (C.I. Direct Blue 199), which was used as a model stain, was dropped onto each sample prior to 24-h illumination with simulated solar light. The self-cleaning properties were triggered by light and evaluated in terms of the reduction in color strength values (K/S) of the stain after exposure. The self-cleaning performance was preserved when the binder was added to the TiO2coating. However, washing reduced the self-cleaning performance of all samples because of the detachment of some TiO2particles, as observed by scanning electron microscopy. Overall, improved wash fastness was observed with the help of binder. Samples were further analyzed for mechanical properties, crease recovery, and drapeability.

Fabrication of Ti3+ self-doped TiO2(A) nanoparticle/TiO2(R) nanorod heterojunctions with enhanced visible-light-driven photocatalytic properties

Ti³⁺ self-doped TiO₂(A)/TiO₂(R) heterojunctions were synthesized and the samples exhibit high visible light photocatalytic activity.

Synthesis of the double-shell anatase-rutile TiO2 hollow spheres with enhanced photocatalytic activity

A novel double-shell TiO2 hollow sphere with an inner anatase shell and an outer rutile shell was synthesized by a simple sol-gel method and silica protected calcination process. The structure and formation mechanism was proposed based on characterization using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The double-shell spheres have a uniform diameter of 360 nm and a typical yolk-shell structure. Moreover, the double-shell TiO2 hollow spheres possess a large specific surface area (169 m(2) g(-1)). Due to the high surface area, multiple light reflection and beneficial electron conduction between the inner anatase and outer rutile shell of this special structure, the as-prepared double-shell TiO2 catalysts show remarkably enhanced photoactivity compared to the commercial P25 catalyst. In particular, rhodamine B molecules can be completely decomposed in the presence of the double-shell spheres after 60 minutes of irradiation with UV light. In addition, the high activity is retained after five cycles, indicating the stability and reusability of the double-shell catalyst.

Type-II ZnO nanorod-SnO2 nanoparticle heterostructures: characterization of structural, optical and photocatalytic properties

In this work we report, for the first time, on the preparation of ZnO nanorod-SnO2 nanoparticle (ZnO NR-SnO2 NP) heterostructures by a simple two-step thermal evaporation approach. Systematical characterization of the product reveals that the rutile SnO2 NPs, with a diameter of about 20 nm, are uniformly and tightly decorated on the entire ZnO NRs. Photoluminescence (PL) investigation on the ZnO NR-SnO2 NP heterostructures shows that they exhibit a significantly decreased UV emission compared with the bare ZnO NRs, revealing an efficient charge separation arising from the type-II band alignment. Enlightened by this merit, photocatalytic behavior of the synthesized heterostructures is studied, which shows a remarkably enhanced photodegradation performance of rhodamine B (RhB) in contrast to the pure ZnO NRs. We also carry out the stability test of the ZnO NR-SnO2 NP heterostructures and the result indicates an extremely high adhesion nature between the ZnO NR and the coated SnO2 NPs. This advantage endowed with the thermal evaporation approach can lead to an efficient spatial charge separation between the ZnO NR and the SnO2 NPs and thus effectively minimize the charge recombination along three-dimensional heterointerfaces, which makes such ZnO NR-SnO2 NP architectures highly promising for a wide range of photovoltaic and photocatalytic applications.