Interconnected TiO2 Research Articles

(Invited) Template Assisted Synthesis of Porous Metal Oxide and Metal Nanostructures by ALD

Atomic layer deposition (ALD) has been intensively used to coat porous templates and duplicate the morphology after template removal. Here, we present a systematic investigation on the synthesis of nanoporous Pt and metal oxides via ALD on a sacrificial carbonaceous template: multiwalled carbon nanotubes (MWCNTs).1-3 Pt, Al2O3, TiO2, ZnO and VOx were deposited on MWCNTs, and the samples were subsequently annealed in air to remove the MWCNTs. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) showed surprising morphological transitions at the nanoscale, while still maintaining the overall forest-type structure at the micro scale. Hollow Al2O3 nanotubes, chains of TiO2 nanoparticles and 3D networks of interconnected V2O5 and ZnO nanoparticles and Pt nanowires were observed after the removal of the MWCNTs templates (see Fig.1). The following photocatalytic test also revealed that the interconnected TiO2 nanoparticle chains showed high catalytic activity towards the degradation of acetaldehyde under UV light. The investigation in the morphological transition in the case of TiO2 nanoparticle chains showed that the crystallization temperature plays an important role in the formation of the final morphology. In the case of 3D network of Pt nanowires, it showed that a continuous coating of metal on the MWCNTs is not indispensable for achieving a self-supported porous structure after template removal. 1S. Deng et al., Nanoscale, 2014, 6, 6939. 2S. Deng et al., J. Mater. Chem. A., 2015, 3, 2642. 3S. Deng et al., RSC Advances, 2014, 4, 11648. Figure 1

Highly Crystallized C-Doped Mesoporous Anatase TiO2 with Visible Light Photocatalytic Activity

Highly crystallized C-doped mesoporous anatase TiO2 is prepared using a multi-walled carbon nanotube (MWCNT) mat as both a “rigid” pore template and a carbon doping source. SEM and TEM characterization shows that the MWCNT template imposed a pore structure in reverse of that of the MWCNT mat. The pore walls are formed by chain-like interconnected TiO2 nanocrystals with an average diameter about 10 nm, and pores are derived from spaces occupied by MWCNTs before removal. XRD characterization shows that TiO2 is crystallized with a pure anatase phase. XPS characterization reveals that the relative carbon content in the TiO2 is related to the duration of TiO2/MWCNT composite annealing before removal of MWCNT template. Three samples prepared contain 2.3%, 2.8% and 3.9% carbon; show a ~30 nm red shift and a plateau of adsorption from 450–800 nm in UV–Vis spectra in comparison to that of P25; and display visible light photocatalytic activity for decomposition of methyl orange (MO) in relationship with the carbon content and crystallinity of the anatase TiO2.

Enhanced bond strength and bioactivity of interconnected 3D TiO2 nanoporous layer on titanium implants

Titanium and its alloys have been extensively used as implant materials due to their excellent mechanical properties, adequate corrosion resistance and good biocompatibility. In this study, an interconnected 3D TiO2 structure on Ti implants is fabricated via anodization method in a fluoride-containing electrolyte with water content larger than 50%. The pore size ranges from 30 to 200 nm with the current density increased from 50 to 100 mA·cm2. Compared with other kinds of porous TiO2 layers, which were fabricated by the same method with different electrolyte composition, the prepared 3D TiO2 structure shows the highest hardness and apparent Young's modulus, which are over 4 times higher than that of the TiO2 nanotube arrays. Moreover, the 3D TiO2 possess the highest adhesion strength as the critical load for the delamination occurred is 0.7 N, while for the nanotube-structured layer and the transition structure, the load are 0.2 and 0.4 N, respectively. Additionally, the 3D TiO2 structure also shows better corrosion resistance than the others. Titanium with 3D oxide layer has also shown good performance in osteogenic cells culturing, since in the cell viability assay, the cells adhered on the 3D oxide layer spread extensively and showed a spindle shape while on the TiO2 nanotube arrays or titanium with transition structure the cells were unfavorable to spread. This work can provide guidelines for improving structural and environmental conditions responsible for modification of Ti surfaces for biomedical applications.

C-doped mesoporous anatase TiO2 comprising 10 nm crystallites

We report a C-doped mesoporous anatase TiO2 with high surface area synthesized using multi-walled carbon nanotube (MWCNT) mat as a “rigid” template and carbon doping source. The characterization by SEM, HRTEM, X-ray diffraction and nitrogen adsorption revealed that TiO2 samples have a porous structure which are figuratively a inverse copy of MWCNT network and pore walls are formed by interconnected TiO2 nanoparticles with average diameter of ∼10nm. We found that annealing temperatures from 400 to 1000°C before MWCNT template removal had very limited effect on particle size (∼10nm), surface area (112–129m2/g) and total pore volume (0.74–0.85m2/g) of the samples through a significantly delayed phase transition from anatase to rutile started at 800°C, resulting in only ∼9.1% conversion at 1000°C. The pore size distribution is in mesopore range from 6 to 60nm peaked at ∼24nm. XPS analysis showed a relatively strong C1s peak at 288.4eV, indicating C doping at Ti sites, which is responsible for red shift of adsorption edge of UV–vis spectra and photocatalytic activity in visible-light region.

Single-step synthesis process of interconnected spiderweb-like TiO2 films as photoanode for self-powered ultraviolet-detector

Anatase TiO2 films with tunable nanoarchitectures were fabricated directly on conductive substrates using one-step electrostatic spray deposition (ESD). The morphology of TiO2 films gradually evolved from one-dimensional (1D) rice-like to three-dimensional (3D) interconnected spiderweb-like architecture by only varying the flow rate of precursor solution with no need for templates or binders. The synergistic integration of the unique morphology and higher crystallite size imparted 3D-interconnected nanospiderwebs with remarkably enhanced photoconversion efficiency (6.52%) compared with commercial TiO2 and other 1D nanoarchitectures. The self-powered ultraviolet (UV)-photodetector based on nanospiderwebs showed high photosensitivity (405300%), fast response and excellent photosensitivity linearity in a wide light intensity range. This self-powered device is a promising candidate for high-sensitivity and high-accuracy UV photodetectors.

Enhanced performance of dye-sensitized solar cells with fluorine doped nanocrystalline TiO2 as photoanode

In order to enhance the performance of dye-sensitized solar cells (DSSCs), the fluorine doped TiO2 nanoparticles were employed as photonode. Using TiF4 as starting material, fluorine atom is introduced into nanocrystalline TiO2 by a simple method. The TiO2 nanoparticles with uniform F doping were prepared by hydrothermal method, and were further exploited as the application in DSSCs via doctor-blading technique successfully. In addition, the highest conversion efficiency of 6.30% was achieved under one sun illumination (AM 1.5G, 100mW/cm2), which is about 13% higher than that of the devices using commercial P25 TiO2 nanoparticles, the probably reason could mainly ascribed to the improvement in fill factor and open-circuit voltage. Moreover, the mechanism study demonstrated that the improved cell performance could attributed to the higher amount of dye adsorption on TiO2, the enhanced interfacial charge transfer rate between interconnected TiO2, higher injection rate from dye to TiO2 nanoparticles which was induced by the formation of Ti3+ and F− ions in the semiconductor, and the reduced recombination at the interfacial face between TiO2 and electrolyte.

A Biomineralization Strategy for "Net"-Like Interconnected TiO2 Nanoparticles Conformably Covering Reduced Graphene Oxide with Reversible Interfacial Lithium Storage.

A green and simple biomineralization‐inspired method to create “net”‐like interconnected TiO2 nanoparticles conformably covering reduced graphene oxide (RGO) with high loading density is reported. This method uses polyamine as both the biomineralization agent and linker to manipulate the nucleation, growth, and crystallization of TiO2 nanoparticles on the surface of graphene oxide. The obtained TiO2/RGO composites demonstrate sub‐10‐nm TiO2 nanoparticles with (001) facets, ultrathin thickness (10–12 nm), and a high surface area of 172 m2 g−1. When used as anode material for lithium ion batteries, the material displayed excellent rate capability and long cycle life; a capacity of 155 mAh g−1 is obtained after 50 cycles at the rate of 5C (1C = 168 mA g−1) and a specific capacity of 115 mAh g−1 is retained after 2000 cycles at the rate of 25C, which is much higher than that of mechanically mixed TiO2/graphene composites. Detailed discharge curve analysis reveals that the high rate and cycle performance are partly a result of the reversible interfacial lithium storage of materials, which might be attributed to the pores in the TiO2 nets on the RGO and may provide a sufficient number of interfaces for accepting both electrons and lithium ions.

Scalable synthesis of three-dimensional interconnected mesoporous TiO2 nanotubes with ultra-large surface area

Mesoporous nanotubes represent a unique class of nanostructures with numerous applications including biomaterials, lithium ion battery anodes, and photocatalysts. Herein we describe for the first time a general, scalable, and reproducible approach for the production of 3D interconnected mesoporous TiO2 nanotubes with ultrahigh specific surface area and large pore volume by using a nanofibrous template of bacterial cellulose (BC). The obtained samples were characterized by Brunauer–Emmett–Teller (BET) surface area measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results demonstrated that TiO2 nanotubes sustained the 3D interconnected structure of pristine BC and exhibited an average diameter of 36nm and a mesoporous wall with a mean mesopore size of 3.2nm. The particular structure endows TiO2 nanotubes with ultra-large surface area (1629m2g−1) and high photocatalytic activity.

From Quasi-2D TiO2(B) and Graphene Nanosheets to 3D Hierarchical Nanostructured Electrodes for Li-Ion Batteries

Two-dimensional (2D) layered materials are at the emerging frontiers of research and development. It is expected that quasi-2D layered materials will also find applications for energy storages. It has been known that nanostructured electrodes hold the promise for development of high-performance rechargeable batteries with ultrafast charging and discharging rates that are in great needs for portable electronic devices and electrical vehicles applications. A 3D nanostructured electrodes potentially can address the interwoven issues related with energy density, power density, and cycling stability of electrochemical energy storages, especially lithium ion batteries. In this regard, the 2D nanosheets might prove being superior over nanoparticals and nanowires. Quasi-2D TiO2(B) nanosheet has the desirable intrinsic properties as well as the merits of 2D materials as anode material to design 3D nanostructured electrodes for high-performance Li-ion battery. In our study, perpendicularly oriented graphene network was first grown by plasma-enhanced chemical vapor deposition that encircles the struts of a nickel foam as template,1 on which quasi-2D TiO2(B) nanosheets, perpendicular to the underlying graphene sheets, were then self-assembled through hydrothermal synthesis. The micro-scale morphology of these quasi-2D TiO2(B) nanosheets grown on perpendicularly oriented graphene network is presented by the scanning electron microscopy images in the inset of figure 1(b). Such a rationally-designed hierarchical 3D nanostructured electrode based on quasi-2D materials combines the merits such as a short diffusion length and a large surface area that facilitates the ion intercalation, an ordered porous geometry that assists ion migration and offers space for electrode material expansion, and an interconnected network with enhanced structural stability and minimized electron transport resistance. Using such a 3D ordered macrostructure as an electrode that is based on oriented 2D nanosheets of TiO2(B) and graphene grown inside a nickel foam, coin cells were assembled using lithium metal as a second electrode. Extensive electrochemical studies were conducted to test the potential of such an quasi-2D nanosheets based nanostructured electrode for Li-ion batteries. An extraordinary performance was found in terms of capacity, charge/discharge rates and cycling stability. Such an electrode exhibits a large capacity of 320 mAh/g at 0.3C, 250 mAh/g at 2C, and 100mAh/g at the extremely fast 103C. After 2000 charge/discharge cycling at a current density of 2A/g (12C), it maintains 89% of its initial capacity, and when returning to a lower current density of 0.5 A/g (2C) after the 2000-cycles cycling stress at 2A/g, a negligible capacity lose was found. Figure 1(a) shows the discharge capacities at several different rates, and figure 1(b) exhibits the cycling stability when stressed at 12C charge/discharge rate. These results indicate that such a hierarchical nanostructure based on interconnected quasi-2D TiO2(B) nanosheets could be developed as a very stable and high-rate anode for high-performance Li-ion batteries. The detail synthesis, material structure and properties, and electrochemical studies will be reported. Figure 1

Self-assembled TiO2 agglomerates hybridized with reduced-graphene oxide: A high-performance hybrid photocatalyst for solar energy conversion

Spherical TiO2 agglomerates incorporated with reduced-graphene oxide (rGO) sheets were fabricated using aerosol assisted self-assembly. Compared to that of conventional graphene–TiO2 composites, the new hybrid geometry of the self-assembled agglomeration of TiO2 and rGO enabled a porous structure, a more efficient charge separation through interconnected TiO2 nanoparticles and rGO, and improved the contact between TiO2 and rGO to maximize the role of rGO as an electron reservoir. The as-prepared rGO–TiO2 composites were characterized with a diverse range of analytical techniques, and their photocatalytic activity was tested in terms of H2 production and gaseous CH3CHO degradation. The incorporation of the rGO sheets into the TiO2 agglomerates promoted the photocatalytic H2 production and CH3CHO oxidation; our results showed that the performance of our systems is directly proportional to the content of rGO (when added up to 10 wt%). This finding confirmed that rGO, acting as an electron collector and mediator, can facilitate charge-pair separation; only a small light-shielding effect by rGO was observed. Furthermore, open-circuit potential decay measurements revealed that the presence of rGO in the agglomerated TiO2 can significantly suppress charge recombination; this further confirmed the role of rGO as an effective electron conduit. The new geometry of the rGO–TiO2 composite proposed in this work shows several advantages compared to various types of graphene–TiO2 composites previously reported, such as the stronger electronic coupling between rGO sheets and TiO2, minimized light-shielding effect by rGO (even when a relatively large amount of rGO is used), and facile scale-up for mass production. Therefore, our rGO–TiO2 composite can be considered as a promising hybrid photocatalyst for solar energy conversion.

Templated synthesis of TiO2 nanotube macrostructures and their photocatalytic properties

Controlled synthesis of hierarchically assembled titanium dioxide (TiO2) nanostructures is important for practical applications in environmental purification and solar energy conversion. We present here the fabrication of interconnected TiO2 nanotubes as a macroscopic bulk material by using a porous carbon nanotube (CNT) sponge as a template. The basic idea is to uniformly coat an amorphous titania layer onto the CNT surface by the infiltration of a TiO2 precursor into the sponge followed by a subsequent hydrolysis process. After calcination, the CNTs are completely removed and the titania is simultaneously crystallized, which results in a porous macrostructure composed of interconnected anatase TiO2 nanotubes. The TiO2 nanotube macrostructures show comparable photocatalytic activities to commercial products (AEROXIDE TiO2 P25) for the degradation of rhodamine B (RhB). Moreover, the TiO2 nanotube macrostructures can be settled and separated from water within 12 h after photocatalysis, whereas P25 remains suspended in solution after weeks. Thus the TiO2 nanotube macrostructures offer the advantage of easy catalyst separation and recycle and can be a promising candidate for wastewater treatment.

Interconnected TiO2 nanowires consisting of nanosized crystallites for high conversion efficiency in dye-sensitized solar cells with gel electrolyte

In this paper, we present a method to prepare interconnected TiO2 nanowires consisting of nanosized crystallites (INCNCs). A high light-to-electricity conversion yield of 8.20% was achieved by applying the structure of the thin INCNCs electrode film in DSSC with gel electrolyte, much higher than 6.84% of TiO2 nanoparticulate film with same thickness (6.5 μm).

Constructing hierarchical submicrotubes from interconnected TiO2 nanocrystals for high reversible capacity and long-life lithium-ion batteries

Here, we report a facile hydrothermal approach for synthesizing anatase TiO2 hierarchical mesoporous submicrotubes (ATHMSs) with the aid of long-chain polymer as soft template. The TiO2 nanocrystals, with sizes of 6–8 nm, are well interconnected with each other to build tubular architectures with diameters of 0.3–1.5 μm and lengths of 10–25 μm. Such highly porous structures give rise to very large specific surface area of 201.9 m2 g−1 and 136.8 m2 g−1 for the as-prepared and annealed samples, respectively. By using structurally stable ATHMSs as anode materials for lithium-ion batteries, they exhibited high reversible capacity, long cycling life and excellent cycling stability. Even after 1000 cycles, such ATHMS electrodes retained a reversible discharge capacity as high as 150 mAh g−1 at the current density of 1700 mA g−1, maintaining 92% of the initial discharge capacity (163 mAh g−1).

Multi-Functionality of Macroporous TiO2 Spheres in Dye-Sensitized and Hybrid Heterojunction Solar Cells

Micron-sized macroporous TiO2 spheres (MAC-TiO2) were synthesized using a colloidal templating process inside emulsions, which were then coated on a nanocrystalline TiO2 light absorption film to prepare a bilayered photoanode for liquid-based dye-sensitized solar cells (DSSC) and hybrid heterojunction solid-state solar cells. MAC-TiO2 layers can enhance light scattering as well as absorption, because their pore size and periodicity are comparable to light wavelength for unique multiple scattering and a porous surface can load dye more. Moreover, due to the bicontinuous nature of macropores and TiO2 walls, electrolyte could be transported much faster in between the TiO2 spheres rather than within the small TiO2 nonporous architectures. Electron transport was also facilitated along the interconnected TiO2 walls. In DSSCs with these MAC-TiO2 scattering layers, efficiency was higher than conventional DSSCs incorporating a commercial scattering layer. The unique geometry of MAC-TiO2 results in strong improvements in light scattering and infiltration of hole-transporting materials, thereby the MAC-TiO2-based solid-state device showed comparatively higher efficiency than the device with conventional nanocrystalline TiO2.

Mesoporous Au–TiO2 Nanoparticle Assemblies as Efficient Catalysts for the Chemoselective Reduction of Nitro Compounds

ABSTRACTHere, we propose novel mesoporous Au-loaded TiO2 nanoparticle assemblies (Au-MTA) as highly effective catalysts for the reduction of nitroaromatic compounds into the corresponding aryl amine products. The obtained materials possess a continuous network of interconnected gold and anatase TiO2 (ca. 9 nm in size) nanoparticles with controllable gold particle size (i.e. ranging from ∼3.2 to ∼9.4 nm) and exhibit large and accessible pore surface area (ca. 100–160 m2/g), as evidenced by SAXS, XRD, TEM and N2 physisorption measurements. Interestingly, the Au-MTA mesophases have exhibited remarkable activity and selectivity for the reduction of nitro into amine groups using NaBH4 as reducing agent. Indeed, the Au loading and particle size have a key effect on hydrogenation reactions, affecting significantly the yield and product composition.

Interconnected carbon-decorated Tio2 nanocrystals with enhanced lithium storage performance

A novel direct preparation method was developed to prepare interconnected carbon-decorated TiO2 nanocrystals. The as-prepared composite exhibited a morphology of interconnected 10nm TiO2 nanocrystals with uniform carbon coverage. As anode in lithium ion batteries, high reversible capacity of 184mAhg−1 with 87.4% initial coulomb efficiency, outstanding rate performance and stable cycle life were observed, which was attributed to ultrafine particle size, uniform carbon coating and high preparation temperature.

Hollow mesoporous frameworks without the annealing process for high-performance lithium–ion batteries: A case for anatase TiO2

Hierarchically porous hollow TiO2 microspheres composed of interconnect nanocrystals have been synthesized by a one-pot hydrothermal method without further thermal annealing. The as-prepared materials are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and electrochemical measurements. FESEM shows that the resultant hierarchical mesoporous TiO2 hollow microspheres (HTMs) comprised mesoporous shells with interconnected anatase nanocrystals. TEM reveals that the interconnected structure of hollow microspheres almost remained unchanged with increasing hydrothermal reaction time. The electrochemical measurements results showed that the initial Li insertion/extraction capacity of the electrodes obtained at 180°C for 15h is 158mAhg−1 and 151mAhg−1 at 5C, respectively. Moreover, in the 100th cycle, the reversible capacity still remains about 131mAhg−1, indicating excellent cycling stability and good high-rate performance. The electrochemical impedance spectroscopy measurements indicated that the surface reaction kinetics of TiO2 nanocrystals were improved significantly, which may be attributed to fast charge transfer in the interconnected TiO2 nanocrystals structure. With the advantages of low cost and ease of processing, these HTMs are not only promising LIBs anode materials but also provide a conceptual route of fabricating oxides electrodes in high-performance LIBs in the future.

High Performance N-Doped Mesoporous Carbon Decorated TiO2 Nanofibers as Anode Materials for Lithium-Ion Batteries

N-doped mesoporous carbon decorated TiO2 nanofibers were synthesized for the first time by a facile electrospinning process combined with subsequent heat treatment and investigated as an anode material for lithium-ion battery applications. The N-doped mesoporous carbon decorated TiO2 nanofibers had continuous one-dimensional (1-D) geometry with a smooth surface and an average thickness of ∼250 nm. The nanofibers comprised both interconnected polycrystalline TiO2 and numerous mesopores in N-doped carbon. After 100 cycles at current density of 33 mA/g, the N-doped mesoporous carbon decorated TiO2 nanofibers still exhibited a high capacity of 264 mAh/g. This superior electrochemical performance is attributed to small TiO2 crystallites, N-doped mesoporous carbon, favorable 1-D nanostructures, and smooth accommodation of the strain occurring during the charge–discharge process.

Inverted Organic Photovoltaic Cells Using Three-Dimensionally Interconnected TiO<SUB>2</SUB> Nanotube Arrays

Here we describe a simple sol-gel method to fabricate inverted organic photovoltaics (OPV) using interconnected TiO2 nanotubes (inter-TiO2 NT) as an efficient electron transport layer. Three-dimensionally inter-TiO2 NT arrays were prepared by spin-coating a TiO2 precursor solution on the ZnO nanorod (NR) template grown via the liquid phase deposition method. Upon etching of ZnO NRs, inter-TiO2 NT arrays were generated, as confirmed by X-ray diffraction (XRD), energy-filtering transmission electron microscopy (EF-TEM) and field-emission scanning electron microscopy (FE-SEM). A blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) deeply infiltrated into the pores of inter-TiO2 NT, as revealed by FE-SEM and atomic force microscopy (AFM) images. The power conversion efficiency (PCE) of inter-TiO2 NT-based inverted OPV reached 3.0% at an air mass of 1.5 (100 mW/cm2), which is a 25% performance improvement compared to flat TiO2 films derived from the sol-gel process or commercial paste. The efficiency improvement arises from facilitated charge separation and collection due to the increased TiO2 interface arera and efficient transport pathway.

Mesoporous Au–TiO2 nanoparticle assemblies as efficient catalysts for the chemoselective reduction of nitro compounds

In this article, we demonstrate novel mesoporous Au-loaded TiO2 nanoparticle assemblies (Au–MTA) as high-effective catalysts for the selective transformation of nitroaromatics into the corresponding aryl amine products. These materials feature a three-dimensional open porous structure consisting of interconnected uniform gold and TiO2 nanoparticles, large internal surface area (ca. 104–120 m2 g−1), and narrow mesopores (ca. 7.3–7.6 nm). Au–MTA displays outstanding performance for the selective reduction of nitro into amine groups using sodium borohydride as a reducing agent under ambient conditions. We show that both chemoselectivity and hydrogenation activity of the Au–MTA catalysts are highly related to the Au loading and particle size. As a result the 2% Au–MTA associated with 5 nm sized Au particles was found to be a prominent catalyst for hydrogenation reactions, providing exceptionally high selectivity (>96%) and conversion yield (>92%) to the corresponding amines. The implication of these results is that mesoporous ensembles of gold and TiO2 nanoparticles should be considered for the synthesis of fine amine chemicals.