Nanocrystalline TiO2 Photocatalytic Membranes with a Hierarchical Mesoporous Multilayer Structure: Synthesis,Characterization, and Multifunction

A novel sol–gel dip‐coating process to fabricate nanocrystalline TiO₂ photocatalytic membranes with a robust hierarchical mesoporous multilayer and improved performance has been studied. Various titania sols containing poly(oxyethylenesorbitan monooleate) (Tween 80) surfactant as a pore‐directing agent to tailor‐design the porous structure of TiO₂ materials at different molar ratios of Tween 80/isopropyl alcohol/acetic acid/titanium tetraisopropoxide = R:45:6:1 have been synthesized. The sols are dip‐coated on top of a homemade porous alumina substrate to fabricate TiO₂/Al₂O₃ composite membranes, dried, and calcined, and this procedure is repeated with varying sols in succession. The resulting asymmetric mesoporous TiO₂ membrane with a thickness of 0.9 μm exhibits a hierarchical change in pore diameter from 2–6, through 3–8, to 5–11 nm from the top to the bottom layer. Moreover, the corresponding porosity is incremented from 46.2, through 56.7, to 69.3 %. Compared to a repeated‐coating process using a single sol, the hierarchical multilayer process improves water permeability significantly without sacrificing the organic retention and photocatalytic activity of the TiO₂ membranes. The prepared TiO₂ photocatalytic membrane has great potential in developing highly efficient water treatment and reuse systems, for example, decomposition of organic pollutants, inactivation of pathogenic microorganisms, physical separation of contaminants, and self‐antifouling action because of its multifunctional capability.     

A Unique Disintegration–Reassembly Route to Mesoporous Titania Nanocrystalline Hollow Spheres with Enhanced Photocatalytic Activity

A novel disintegration–reassembly route is reported for the synthesis of mesoporous TiO₂ nanocrystalline hollow spheres with controlled crystallinity and enhanced photocatalytic activity. In this unique synthesis strategy, it is demonstrated that sol–gel‐derived mesoporous TiO₂ colloidal spheres can be disintegrated into discrete small nanoparticles that are uniformly embedded in the polymer (polystyrene, PS) matrix by surface‐induced photocatalytic polymerization. Subsequent reassembly of these TiO₂ nanoparticles can be induced by an annealing process after further coating of a resorcinol–formaldehyde (RF) resin, which forms self‐supported hollow spheres of TiO₂ at the PS/RF interface. The abundant phenolic groups on the RF resin serve as anchoring sites for the TiO₂ nanoparticles, thus enable the reassembly of the TiO₂ nanoparticles and prevent their sintering during the thermal crystallization process. This unique disintegration–reassembly process leads to the formation of self‐supported TiO₂ hollow spheres with relatively large surface area, high crystallinity, and superior photocatalytic activity in dye degradation under UV light irradiation.     

Inorganic Macroporous Films from Preformed Nanoparticles and Membrane Templates: Synthesis and Investigation of Photocatalytic and Photoelectrochemical Properties

Colloidal dispersions of titania, zirconia, tin oxide, indium oxide, and ceria have been successfully used to impregnate membrane templates and form the respective metal oxide (MO) porous films. The use of alumina and iron oxide sols in the same procedure, however, resulted in compact structures. By mixing different nanoparticle solutions before impregnation, final inorganic films containing two metal oxides, of variable metal oxide ratios, were obtained. The porous inorganic materials were analyzed in terms of surface area, pore size, film thickness, and crystallinity. The mechanism of nanoparticle infiltration and particle adsorption to the template walls is proposed based on the stability of the inorganic film and a study of the influence of either the sol concentration or washing times on the amount of inorganic substance incorporated in the hybrid material. The photocatalytic decomposition of an organic pollutant, 2‐chlorophenol, was demonstrated for the porous titania material along with the structures containing mixtures of titania with zirconia, indium oxide, and tin oxide. A ratio of 9:1 TiO₂/MO gave the highest photocatalytic activity, which was higher than the activity of Degussa P25 for the TiO₂/In₂O₃ and TiO₂/SnO₂ systems under the same conditions. The titania films have also been attached to substrates—glass or indium tin oxide (ITO) surfaces—and the photoelectrochemical properties of the porous film attained. A comparison with a spin‐coated titania film (prepared from the same colloidal dispersion) showed that the structured porous inorganic film has two times the photoelectrochemical efficiency as the spin‐coated film.     

Mesoporous Anatase Titania Hollow Nanostructures though Silica‐Protected Calcination

The crystallization of nanometer‐scale materials during high‐temperature calcination can be controlled by a thin layer of surface coating. Here, a novel silica‐protected calcination process for preparing mesoporous hollow TiO₂ nanostructures with a high surface area and a controllable crystallinity is presented. This method involves the preparation of uniform silica colloidal templates, sequential deposition of TiO₂ and then SiO₂ layers through sol–gel processes, calcination to transform amorphous TiO₂ to crystalline anatase, and finally etching of the inner and outer silica to produce mesoporous anatase TiO₂ shells. The silica‐protected calcination step allows crystallization of the amorphous TiO₂ layer into anatase nanocrystals, while simultaneously limiting the growth of anatase grains to within several nanometers, eventually producing mesoporous anatase shells with a high surface area (～311 m² g^?1) and good water dispersibility upon chemical etching of the silica. When used as photocatalysts for the degradation of Rhodamine B under UV irradiation, the as‐synthesized mesoporous anatase shells show significantly enhanced photocatalytic activity with greater enhancement for samples calcined at higher temperatures thanks to their improved crystallinity.     

Well‐Ordered Large‐Pore Mesoporous Anatase TiO2 with Remarkably High Thermal Stability and Improved Crystallinity: Preparation,Characterization, and Photocatalytic Performance

Thermally‐stable, ordered mesoporous anatase TiO₂ with large pore size and high crystallinity has been successfully synthesized through an evaporation‐induced self‐assembly technique, combined with encircling ethylenediamine (EN) protectors to maintain the liquid crystal mesophase structure of TiO₂ primary particles, followed by calcination at higher temperature. The structures of the prepared mesoporous TiO₂ are characterized in detail by small‐angle and wide‐angle X‐ray diffraction, Raman spectra, N₂ adsorption/desorption isotherms, and transmission electron microscopy. Experimental results indicate that the well‐ordered mesoporous structure could be maintained up to 700 °C (M700) and also possesses large pore size (10 nm), high specific BET surface area (122 m² g^?1), and high total pore volumes (0.20 cm³ g^?1), which is attributed to encircling EN protectors for maintaining the mesoporous framework against collapsing, inhibiting undesirable grain growth and phase transformation during the calcination process. A possible formation mechanism for the highly stable large‐pore mesoporous anatase TiO₂ is also proposed here, which could be further confirmed by TG/FT‐IR in site analysis and X‐ray photoelectron spectroscopy. The obtained mesoporous TiO₂ of M700 exhibit better photocatalytic activity than that of Degussa P25 TiO₂ for degradation of highly toxic 2,4‐dichlorophenol under UV irradiation. This enhancement is attributed to the well‐ordered large‐pore mesoporous structure, which facilitates mass transport, the large surface area offering more active sites, and high crystallinity that favors the separation of photogenerated electron‐hole pairs, confirmed by surface photovoltage spectra.     

Transparent Mesoporous Nanocomposite Films for Self‐Cleaning Applications

A versatile approach is studied for the elaboration of TiO₂ based photocatalytic coatings for self‐cleaning applications on transparent substrates. The basic principle of the synthesis relies on the use of preformed TiO₂ colloidal particles that are further dispersed within a transparent silica binder with a mesoporous structure. Film porosity in the nanometer range is controlled by achieving the sol–gel silica condensation around self‐organized micellar assemblies of a templating copolymer surfactant. The latter also acts as a stabilizer for the TiO₂ particles, thus preserving their high dispersion within the film so that excellent optical properties are maintained even for high TiO₂ loading (up to 50 %). Studies of photodegradation kinetics show that such mesoporous films are at least 15 times more active than films synthesized with a usual microporous silica binder. Moreover, the measured quantum‐yield efficiency (1.1 %) is found to be among the highest reported up to now. Improved photoactivity of the films is discussed as resulting from the closer proximity between the organic molecules and the surface of the TiO₂ crystallites as well as the improved diffusion rate of water and oxygen through the interconnected pore network.     

High‐Performance Multifunctional TiO2 Nanowire Ultrafiltration Membrane with a Hierarchical Layer Structure for Water Treatment

A novel, multifunctional TiO₂ nanowire ultrafiltration (UF) membrane with a layered hierarchical structure is made via alkaline hydrothermal synthesis, followed by a filtration and hot‐press process. The TiO₂ UF membrane has high surface porosity (21.3%) and pore size values around 20 nm. The membrane possesses multifunctional capabilities under UV irradiation, such as anti‐fouling, anti‐bacterial, concurrent separation, and photocatalytic oxidation. The unique properties of the membrane indicate its potential in applications for environmental purification.     

Conformal Nano‐Sized Inorganic Coatings on Mesoporous TiO2 Films for Low‐Temperature Dye‐Sensitized Solar Cell Fabrication

Here, a new method based on sol–gel electrophoretic deposition to produce uniform high‐quality inorganic conformal coatings on mesoporous nano‐particulate films is presented. This novel sol preparation method allows for very fine control of the coating properties, thus inducing new adjustable functionalities to these electrodes. It is shown that the deposition of an amorphous TiO₂ and/or MgO shell onto photoanodes used in dye‐sensitized solar cells (DSSCs) improves their light‐to‐electric‐power conversion efficiency without the need for sintering. It is proposed that the amorphous TiO₂ coating improves the electronic inter‐particle connection and passivates the surface states. The insulating MgO coating further reduces the electron transfer from the conduction band into the electrolyte while the electron injection from the excited dye state remains unperturbed for thin coatings. Using a low‐temperature method for DSSC production on plastic substrates, a maximum efficiency of 6.2% applying pressure together with an optimized TiO₂ coating is achieved. For systems that cannot be pressed a conversion efficiency of 5.1% is achieved using a double shell TiO₂/MgO coating.     

Pore‐Filling of Spiro‐OMeTAD in Solid‐State Dye Sensitized Solar Cells: Quantification,Mechanism, and Consequences for Device Performance

In this paper, the pore filling of spiro‐OMeTAD (2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)9,9′‐spirobifluorene) in mesoporous TiO₂ films is quantified for the first time using XPS depth profiling and UV–Vis absorption spectroscopy. It is shown that spiro‐OMeTAD can penetrate the entire depth of the film, and its concentration is constant throughout the film. We determine that in a 2.5‐µm‐thick film, the volume of the pores is 60–65% filled. The pores become less filled when thicker films are used. Such filling fraction is much higher than the solution concentration because the excess solution on top of the film can act as a reservoir during the spin coating process. Lastly, we demonstrate that by using a lower spin coating speed and higher spiro‐OMeTAD solution concentration, we can increase the filling fraction and consequently the efficiency of the device.     

Sono‐ and Photochemical Routes for the Formation of Highly Dispersed Gold Nanoclusters in Mesoporous Titania Films

A sono‐ and photochemical approach has been developed to incorporate highly dispersed gold nanoclusters into mesoporous TiO₂ films. The first step involves the sonication of a TiO₂ film immersed in a gold chloride solution. This effectively removes the air trapped in the porous film matrix and drives the gold chloride into the pore channels, leading to a homogeneous adsorption of ionic Au in the TiO₂ mesoporous matrix. The second step takes advantage of the photocatalytic property of TiO₂ to reduce the adsorbed Au ions to Au⁰. As the gold nanoclusters thus produced are stabilized by the TiO₂ mesonetwork, no organic capping molecules are required. Highly dispersed Au/TiO₂ nanoheterojunction arrays can be obtained using this interesting approach.     

Large‐scale Synthesis of Urchin‐like Mesoporous TiO2 Hollow Spheres by Targeted Etching and Their Photoelectrochemical Properties

A versatile targeted etching strategy is developed for the large‐scale synthesis of urchin‐like mesoporous TiO₂ hollow spheres (UMTHS) with tunable particle size. Its key feature is the use of a low‐temperature hydrothermal reaction of surface‐fluorinated, amorphous, hydrous TiO₂ solid spheres (AHTSS) under the protection of a polyvinylpyrrolidone (PVP) coating. With the confinement of PVP and water penetration, the highly porous AHTSS are selectively etched and hollowed by fluoride without destroying their spherical morphology. Meanwhile TiO₂ hydrates are gradually crystallized and their growth is preferentially along anatase (101) planes, reconstructing an urchin‐like shell consisting of numerous radially arranged single‐crystal anatase nanothorns. Complex hollow structures, such as core–shell and yolk–shell structures, can also be easily synthesized via additional protection of the interior by pre‐filling AHTSS with polyethylene glycol (PEG). The hollowing transformation is elucidated by the synergetic effect of etching, PVP coating, low hydrothermal reaction temperature, and the unique microstructure of AHTSS. The synthesized UMTHS with a large surface area of up to 128.6 m² g^‐1 show excellent light‐harvesting properties and present superior performances in photocatalytic removal of gaseous nitric oxide (NO) and photoelectrochemical solar energy conversion as photoanodes for dye‐sensitized mesoscopic solar cells.     

Syntheses,Li Insertion,and Photoactivity of Mesoporous Crystalline TiO2

Ordered mesoporous rutile and anatase TiO₂ samples are prepared using mesoporous silica SBA‐15 as template and freshly synthesized titanium nitrate and titanium chloride solutions as precursors. The rutile material formed from the nitrate solution is monocrystalline and contains minimal amounts of Si with a Si:Ti ratio of 0.031(4), whereas the anatase material formed from the chloride solution comprises nanocrystals and contains a higher content of Si with a Si:Ti ratio of 0.18(3). It is found that control of temperature and selection of Ti‐containing precursor play important roles in determining the crystal phase and crystallinity. A possible formation mechanism of porous crystalline TiO₂ is suggested. Characterization of these porous materials is performed by XRD, HRTEM, and nitrogen adsorption/desorption. SBA‐15‐templated mesoporous rutile TiO₂ exhibits a higher Li ion insertion capability than KIT‐6‐templated TiO₂ due to its larger surface area. Likewise mesoporous anatase TiO₂:SiO₂ composite has a better photoactivity than bulk TiO₂ or TiO₂‐loaded SBA‐15 for bleaching methylene blue.     

Sol–gel fabrication and enhanced optical properties,photocatalysis, and surface wettability of nanostructured titanium dioxide films

TiO₂ photocatalytic film, annealed at temperatures of 500 °C and 700 °C, was prepared on SiO₂ pre-coated glass via sol–gel technique for photocatalytic purposes and effects of catalyst-type on its properties were investigated by an X-ray diffractometer (XRD), Scanning Electron Microscope, UV–vis spectrophotometer, and contact angle measurements. The XRD results showed that present phases depend upon catalyst used in the solution and phase transformation behaves in a temperature-dependent manner. For the layers derived from sols containing acidic catalysts, the anatase structure dominated and exhibited much better photocatalytic activity. The results indicated that the sample derived from sol comprises H₂SO₄ as catalyst, and exhibits anatase grains with the lowest size. This could be the reason for its better photocatalytic activity. Finally, samples derived from sol containing acidic catalysts showed superhydrophilicity and superior cleaning ability.     

Influence of the Relative Humidity on the Performance of Polymer/TiO2 Photovoltaic Cells

Hydrolysis of titanium(IV ) isopropoxide (TTIP) is a well‐known method for the fabrication of TiO₂. Normally it is made via a sol–gel reaction in the presence of water. In this paper we report on the preparation of flat TiO₂ films for conjugated polymer/TiO₂ photovoltaic cells, from a TTIP/isopropanol solution. It is shown that the morphological structure of the TiO₂ film is strongly dependent on the relative humidity during spin‐coating of the TTIP/isopropanol solution. In bilayer devices consisting of TiO₂/poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV), a low relative humidity (< 25 %, room temperature) is needed in order to form smooth, transparent TiO₂ films. Increasing the relative humidity results in porous TiO₂ films with a high surface roughness, which leads to shunted devices. Apart from bilayer devices, bulk‐heterojunction (BHJ) hybrid TiO₂:MDMO‐PPV photovoltaic cells have been made, by spin‐coating a mixture of TTIP and MDMO‐PPV in toluene. Again a strong relation was found between the relative humidity during spin‐coating and the current–voltage characteristics of the devices. However, in contrast to the bilayer devices, the best BHJ devices were made at higher relative humidity. The observed performance dependence on relative humidity is discussed in relation to the TiO₂ morphology.     

Hierarchical Double‐Shell Nanostructures of TiO2 Nanosheets on SnO2 Hollow Spheres for High‐Efficiency,Solid‐State,Dye‐Sensitized Solar Cells

A high‐energy conversion efficiency of 8.2% at 100 mW cm^?2 is reported, one of the highest values for N719‐based, solid‐state, dye‐sensitized solar cells (ssDSSCs). The solar cells are based on hierarchical double‐shell nanostructures consisting of inner SnO₂ hollow spheres (SHS) surrounded by outer TiO₂ nanosheets (TNSs). Deposition of the TNS on the SHS outer surface is performed via solvothermal reactions in order to generate a double‐shell SHS@TNS nanostructure that provides a large surface area and suppresses recombination of photogenerated electrons. An organized mesoporous (OM)‐TiO₂ film with high porosity, large pores, and good interconnectivity is also prepared via a sol‐gel process using a poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer template. This film is utilized as a matrix to disperse the double‐shell nanostructures. Such nanostructures provide good pore‐filling for solid polymer electrolytes, faster electron transfer, and enhanced light scattering, as confirmed by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency (IPCE), and intensity‐modulated photocurrent spectroscopy (IMPS)/intensity‐modulated photovoltage spectroscopy (IMVS).     

Alizarin Complexone on Nanocrystalline TiO2: A Heterogeneous Approach to Anion Sensing

A novel, heterogeneous approach to “naked‐eye” colorimetric and spectrophotometric anion sensing is demonstrated, employing the molecular receptor alizarin complexone adsorbed onto a nanocrystalline, mesoporous TiO₂ film. pH buffer action intrinsic to the TiO₂ film allows dip sensing in aqueous solutions. This heterogeneous sensing system exhibits a high selectivity to F^– and CN^– anions, high sensitivity, a rapid response time, and excellent reusability.     

Polyphenol‐Sensitized Atomic Layer Deposition for Membrane Interface Hydrophilization

Improvements in energy–water systems will necessitate fabrication of high‐performance separation membranes. To this end, interface engineering is a powerful tool for tailoring properties, and atomic layer deposition (ALD) has recently emerged as a promising and versatile approach. However, most non‐polar polymeric membranes are not amenable to ALD processing due to the absence of nucleation sites. Here, a sensitization strategy for ALD‐coating is presented, illustrated by membrane interface hydrophilization. Facile dip‐coating with polyphenols effectively sensitizes hydrophobic polymer membranes to TiO₂ ALD coating. Tannic acid‐sensitized ALD‐coated membranes exhibit outstanding underwater crude oil repulsion and rigorous mechanical stability through bending and rinsing tests. As a result, these membranes demonstrate outstanding crude oil‐in‐water separation and reusability compared to untreated membranes or those treated with ALD without polyphenol pretreatment. A possible polyphenol‐sensitized ALD mechanism is proposed involving initial island nucleation followed by film intergrowth. This polyphenol sensitization strategy enriches the functionalization toolbox in material science, interface engineering, and environmental science.     

Nanocrystalline Metal Oxides from the Injection of Metal Oxide Sols in Coordinating Solutions: Synthesis,Characterization, Thermal Stabilization,Device Processing,and Gas‐Sensing Properties

Metal oxide (SnO₂, TiO₂, In₂O₃, ZnO) sols are prepared by various sol–gel processes in such a way as to hinder the condensation reactions. The obtained sols are injected at 160 °C into a solution of tetradecene and dodecylamine, and kept under heating for different periods of time. Depending on the starting sol, variously crystallized oxide nanoparticles are obtained, whose phase compositions and chemical structure have been studied by X‐ray diffraction (XRD) and Fourier transform IR spectroscopy. The elimination of the organic residuals has been carried out by thermal treatment, and the thermal evolution of the nanoparticles has been studied by thermal analyses and Raman spectroscopy. High‐resolution transmission electron microscopy studies coupled with XRD measurements show that the thermal treatment does not markedly affect the particle size, which remains in the nanometer‐sized regime (from 3.5 to 8.5 nm, depending on the system), except in the case of ZnO. The thermally purified and stabilized powders, drop‐coated onto alumina substrates with pre‐deposited electrical contacts, have been tested as gas‐sensing devices, displaying outstanding sensing properties even at room temperature.     

Battery Performance and Photocatalytic Activity of Mesoporous Anatase TiO2 Nanospheres/Graphene Composites by Template‐Free Self‐Assembly

Nanosized mesoporous anatase TiO₂ particles have important applications in high‐performance lithium ion batteries and efficient photocatalysis. In contrast to the conventional synthesis routes where various soft or hard templates are usually employed, the direct growth of uniform mesoporous anatase TiO₂ nanospheres on graphene sheets by a template‐free self‐assembly process is presented. Compared to the conventional mesoporous anatase particles consisting of polycrystalline TiO₂, the microstructure of obtained mesoporous anatase nanospheres on graphene sheets is single‐crystal‐like. The growth mechanism, lithium ion battery performance, and photocatalytic activity of the resultant mesoporous anatase TiO₂ nanospheres/graphene composites are thoroughly investigated. In comparison to the reference TiO₂, the composite shows substantial improvement in lithium specific capacity from 1 C to 50 C, and photocatalytic removing organic pollutant and hydrogen evolution. More strikingly, the specific capacity of the composite at the rate of 50 C is as high as 97 mA h g^?1, 6 times higher than that of the reference TiO₂.     

Evaporation‐Induced Coating and Self‐Assembly of Ordered Mesoporous Carbon‐Silica Composite Monoliths with Macroporous Architecture on Polyurethane Foams

A facile approach of solvent‐evaporation‐induced coating and self‐assembly is demonstrated for the mass preparation of ordered mesoporous carbon‐silica composite monoliths by using a polyether polyol‐based polyurethane (PU) foam as a sacrificial scaffold. The preparation is carried out using resol as a carbon precursor, tetraethyl orthosilicate (TEOS) as a silica source and Pluronic F127 triblock copolymer as a template. The PU foam with its macrostructure provides a large, 3D, interconnecting interface for evaporation‐induced coating of the phenolic resin‐silica block‐copolymer composites and self‐assembly of the mesostructure, and endows the composite monoliths with a diversity of macroporous architectures. Small‐angle X‐ray scattering, X‐ray diffraction and transmission electron microscopy results indicate that the obtained composite monoliths have an ordered mesostructure with 2D hexagonal symmetry (p6m) and good thermal stability. By simply changing the mass ratio of the resol to TEOS over a wide range (10–90%), a series of ordered, mesoporous composite foams with different compositions can be obtained. The composite monoliths with hierarchical macro/mesopores exhibit large pore volumes (0.3–0.8 cm³ g^?1), uniform pore sizes (4.2–9.0 nm), and surface areas (230–610 m² g^?1). A formation process for the hierarchical porous composite monoliths on the struts of the PU foam through the evaporation‐induced coating and self‐assembly method is described in detail. This simple strategy performed on commercial PU foam is a good candidate for mass production of interface‐assembly materials.