Fabrication of graphene films on TiO2 nanotube arrays for photocatalytic application

Graphene film was formed on the surface of titanium dioxide nanotube (TiO₂ NT) arrays through in situ electrochemical reduction of a graphene oxide dispersion by cyclic voltammetry. The residual oxygen-containing groups and other structural defects such as sp³-hybridized carbons in the electrodeposited graphene were further removed by photo-assisted reduction of the underlying TiO₂ NTs, thus achieving the maximum restoration of π-conjugation in the graphene planes. Spectroscopic, electrochemical, and photoelectrochemical techniques were used to characterize the graphene films, and the use of the resulting graphene–TiO₂ NT material in photocatalysis was investigated. The results showed that the graphene–TiO₂ NT material exhibited a greatly improved photocatalytic activity compared with unmodified TiO₂ NTs.     

Synthesis,HRTEM and electron diffraction studies of B/N-doped C and BN nanotubes

Large-scale synthesis of multi-walled BN and single-walled B/N-doped C nanotubes (NT) from C nanotube templates was carried out. The NTs were produced through heating of C templates with B₂O₃ in a flowing N₂ atmosphere at 1503–1773 K. The NTs were analyzed by means of a JEM-3000F high-resolution field emission transmission electron microscope operated at 300 kV and equipped with a parallel detection electron energy loss spectrometer. Particular attention was given to the effects of C template morphology, synthesis temperature and metal oxide promoters on the yield and chemical composition of NTs. Ropes consisting of tens of multi-walled BN NTs were synthesized at 1773 K using MoO₃ and PbO promoters. Ropes of single-walled B/N-doped C NTs were produced at high yields by synthesis at 1553 K. Packing of NTs in the ropes and nanotube helicities were studied by electron diffraction. B/N-doped C and BN NT morphologies, formation mechanism and atomic structures are discussed in this paper.     

Influence of seed layer treatment on low temperature grown ZnO nanotubes: Performances in dye sensitized solar cells

Non-aligned and highly densely aligned ZnO nanotube (NTs), synthesized by low temperature solution method were applied as photoanode materials for the fabrication of efficient dye-sensitized solar cells (DSSCs). The crystalline and the morphological analysis revealed that the grown aligned ZnO NTs possessed a typical hexagonal crystal structure of outer and inner diameter ∼250 nm and ∼100 nm, respectively. ZnO seeding on FTO substrates is an essential step to achieve the aligned ZnO NTs. A DSSC fabricated with aligned ZnO NTs photoanode achieved high solar-to-electricity conversion efficiency of ∼2.2% with short circuit current (J_SC) of 5.5 mA/cm², open circuit voltage (V_OC) of 0.65 V and fill factor (FF) of 0.61. Significantly, the aligned ZnO NTs photoanode showed three times improved solar-to-electricity conversion efficiency than DSSC fabricated with non-aligned ZnO NTs. The enhanced performances were credited to the aligned morphology of ZnO NTs which executed the high charge collection and the transfer of electrons at the interfaces of ZnO NTs and electrolyte layer.     

Quantum chemical simulations reveal acetylene-based growth mechanisms in the chemical vapor deposition synthesis of carbon nanotubes

Nonequilibrium quantum chemical molecular dynamics (QM/MDs) simulation of early stages in the nucleation process of carbon nanotubes from acetylene feedstock on an Fe₃₈ cluster was performed based on the density-functional tight-binding (DFTB) potential. Representative chemical reactions were studied by complimentary static DFTB and density functional theory (DFT) calculations. Oligomerization and cross-linking reactions between carbon chains were found as the main reaction pathways similar to that suggested in previous experimental work. The calculations highlight the inhibiting effect of hydrogen for the condensation of carbon ring networks, and a propensity for hydrogen disproportionation, thus enriching the hydrogen content in already hydrogen-rich species and abstracting hydrogen content in already hydrogen-deficient clusters. The ethynyl radical C₂H was found as a reactive, yet continually regenerated species, facilitating hydrogen transfer reactions across the hydrocarbon clusters. The nonequilibrium QM/MD simulations show the prevalence of a pentagon-first nucleation mechanism where hydrogen may take the role of one “arm” of an sp² carbon Y-junction. The results challenge the importance of the metal carbide formation for SWCNT cap nucleation in the VLS model and suggest possible alternative routes following hydrogen-abstraction acetylene addition (HACA)-like mechanisms commonly discussed in combustion synthesis.     

Site-specific Surface Chemistry on Nanotubes

A simple ultra-high vacuum sample temperature ramping technique can be used to characterize the confinement effects and different adsorption sites on nanotube (NT) bundles deposited on a support such as silica. Results for inorganic NTs such as WS₂ and TiO₂ NTs are compared with data for carbon nanotubes and inorganic WS₂ fullerene-like nanoparticles.     

Increasing of Photocatalytic Performance of TiO2 Nanotubes by Doping AgS and CdS

Highly ordered titanium nanotubes (TiO₂ NTs) photocatalyst was prepared by the anodic oxidation method, and AgS, CdS, and AgS/CdS nanoparticles were doped on the surface of TiO₂ NTs by the successive ion adsorption and reaction (SILAR) method. The photocatalysts were characterized by SEM, EDS, XRD, and potentiostat system. The SEM and EDS analyses respectively show that the average outer diameter of prepared photocatalysts is in the range of 50–120?nm, and the presence of Ti, O, Ag, and Cd is successfully proved. The photocatalytic properties of TiO₂ NTs and doped TiO₂ NTs were studied by measuring the degradation of Methylene Blue (MB) solution. The experimental results show that AgS/CdS/TiO₂ photocatalyst exhibited most efficient photocatalytic activity with 340?µA/cm² photocurrent value. AgS/CdS/TiO₂ NTs photocatalyst shows up to 22.20% higher than TiO₂ NTs, 16.42% higher than CdS/TiO₂ NTs, and 4.3% higher than AgS/TiO₂ NTs.     

Fabrication and characterization of copper doped TiO2 nanotube arrays by in situ electrochemical method as efficient visible-light photocatalyst

Highly ordered copper doped TiO₂ nanotube arrays (CuTiO₂NTs) thin-film were prepared in an aqueous solution containing NH₄F and different concentrations of copper nitrate via the electrochemical oxidation of titanium substrates. The resulting nanotubes were characterized by FE-SEM, XRD, XPS and EDX. The CuTiO₂NTs showed a tube diameter of 40–90 nm and wall thickness of 20–30 nm. Diffuse reflectance spectra showed a shift toward longer wavelengths relative to pure TiO₂ nanotubes (TiO₂NTs). The visible light photo-catalytic activity of the CuTiO₂NTs electrodes was evaluated by the removal of methylene blue (MB) dye and the production of hydrogen. The results showed that CuTiO₂NTs samples exhibited better photo-catalytic activity than the TiO₂NTs. This work demonstrated a feasible and simple anodization method to fabricate an effective, reproducible, and inexpensive visible-light-driven photo-catalyst for hydrogen evolution and environmental applications.     

Diversified Structural Basis of a Conserved Molecular Mechanism for pH‐Dependent Dimerization in Spider Silk N‐Terminal Domains

Conversion of spider silk proteins from soluble dope to insoluble fibers involves pH‐dependent dimerization of the N‐terminal domain (NT). This conversion is tightly regulated to prevent premature precipitation and enable rapid silk formation at the end of the duct. Three glutamic acid residues that mediate this process in the NT from Euprosthenops australis major ampullate spidroin 1 are well conserved among spidroins. However, NTs of minor ampullate spidroins from several species, including Araneus ventricosus (^AvMiSp NT), lack one of the glutamic acids. Here we investigate the pH‐dependent structural changes of ^AvMiSp NT, revealing that it uses the same mechanism but involves a non‐conserved glutamic acid residue instead. Homology modeling of the structures of other MiSp NTs suggests that these harbor different compensatory residues. This indicates that, despite sequence variations, the molecular mechanism underlying pH‐dependent dimerization of NT is conserved among different silk types.     

Modification of Mg<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript>(OH)<Subscript>4</Subscript> nanotubes by magnetite nanoparticles

The magnetite nanoparticles and nanocomposite “Nanotube of hydrosilicate Mg—magnetite nanoparticles—Mg-ChR-NT/Fe₃O₄-NP” were obtained by coprecipitation. The composition of the synthesized samples has been established by X-ray diffraction. Using transmission electron microscopy, the presence of magnetite nanoparticles has been detected both inside the NTs and at the external surface of the NT walls. The specific surface of the NTs, nanoparticles, and composite is determined.     

The Investigations of Construction and in vitro Release of Curcumin Lyotropic Liquid Crystals: Phase Diagrams,Small Angle X-ray Scattering,Rheological Technology,and Release Kinetics

In this study, hexagonal and cubic lyotropic liquid crystals (LLC) were constructed in Brij 97-Tween 40 (MS₈₂, MS₆₄ and MS₄₆)/OLA/H₂O systems to encapsulate curcumin. MS₈₂, MS₆₄, and MS₄₆ indicated that the mass ratio of Brij 97/Tween 40 was 8/2, 6/4, and 4/6. The microstructure of curcumin LLC was studied using small angle X-ray scattering (SAXS). Phase diagrams showed that the increase of MS reduced the phase transition temperature (T_C). Particularly, the T_C of sample C₁Cur [Brij 97-Tween 40 (MS₄₆)/OLA/H₂O = 50.0/2.8/47.2] and C₂Cur [Brij 97-Tween 40 (MS₄₆)/OLA/H₂O = 50.0/25.0/25.0] was 37.6 and 35.4 °C, respectively, close to the temperature of the human body. Thus, the shear rheology and SAXS were used to study the structural changes of samples C₁Cur and C₂Cur with temperature. The moduli values of samples C₁Cur and C₂Cur decreased with the increase of temperature, showing various structural strengths. in vitro release experiment was used to study the drug release kinetics. The release of curcumin from LLC conformed to the concentration diffusion model. Due to a similar a_S, the release of curcumin from samples A₁Cur, B₁Cur, and C₁Cur (Brij 97-Tween 40/OLA/H₂O = 50.0/2.8/47.2 and the MS is MS₈₂, MS₆₄, and MS₄₆) showed a similar release behavior under different MS. The release behavior of curcumin was related to the structure of samples C₁Cur and C₂Cur at different temperatures. Curcumin exhibited the fastest release rate when the samples behaved as the micellar phase.     

The effects of substitution on the electrochemical reduction of naphthacenetetrone in N,N-Dimethylformamide

The polarographic reduction of 5,6,11,12-naphthacenetetrone (NT) and several of its chloromethyl, and nitro derivatives were investigated. The electrochemical behavior of NT was compared with 6, 11-dihydroxy-5, 12-naphthacencequinone (DHNQ) previously studied. Like DHNQ, NT exhibits two reduction waves in N,N-dimethylformamide. However, two additional post adsorption waves which were not present in DHNQ reduction were also observed. The first step reduction of NT, representing the formation of the radical anion NT^?, has a potential (0.01 V vs sce) much more anodic than that of its DHNQ counterpart ( ?0.75 V vs sce). The substituent effects of NTs were noted. A linear Hammett relationship similar to that of the DHNQ series was also found for the NTs. In both cases, the averaged values of σ_meta and σ_para were used as the substituent constants     

High-efficiency photoelectrochemical performance of PbS nanoparticles sensitized TiO2 nanotube arrays

TiO₂ nanotube arrays sensitized by PbS nanoparticles (TiO₂ NTs/PbS) with enhanced visible-light activity were synthesized by a two-step approach including an electrochemical anodization technique followed by an in situ photodeposition approach. The structural investigations indicated that PbS nanoparticles grew uniformly on the walls of the TiO₂ NTs. The TiO₂ NTs/PbS exhibited more excellent photoelectrochemical properties than that of the TiO₂ NTs under visible-light irradiation. The enhanced photoelectrochemical activity of the TiO₂ NTs/PbS could be attributed to the improvement of visible-light absorption and charge separation derived from the coupling effect of the PbS nanoparticles and TiO₂ NTs.     

Highly sensitive acetone-sensing properties of Pt-decorated CuFe2O4 nanotubes prepared by electrospinning

Pristine and Pt-decorated copper ferrite nanotubes (Pt-CuFe₂O₄ NTs, 0.1%, 0.5%, and 1.0%, mole percent) were prepared by a simple electrospinning method. The samples were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. Their gas-sensing properties were evaluated by a commercial CGS-4TPs system. Microscopic images showed that all samples consisted of well-defined nanotubes with diameter of 70?100 nm. Gas-sensing measurements revealed that the Pt-CuFe₂O₄ NTs had an improved acetone-sensing properties compared with pristine CuFe₂O₄ NTs. In particular, 0.5% Pt-CuFe₂O₄ NT-based sensor showed a high response (16.5 at 100 ppm), good selectivity, and long-term stability for acetone at 300 °C. In addition, more Pt dopants would have a greater effect on promoting the sensing properties of the CuFe₂O₄ NTs at high acetone concentrations. A gas-sensing enhancement mechanism of Pt-CuFe₂O₄ NT-based sensors was proposed, according to the catalytic oxidation process of acetone molecules, which could be due to the kinetic competition between Pt dopants and CuFe₂O₄ NTs.     

Fabrication of plasmonic Au/TiO2 nanotube arrays with enhanced photoelectrocatalytic activities

Uniformly dispersed Au nanoparticles (NPs) deposited on the surface of highly ordered TiO₂ nanotube arrays (Au/TiO₂ NTs) were synthesized through a two-step process including anodization method and microwave-assisted chemical reduction route. The investigation indicated that Au NPs grew uniformly on the walls of TiO₂ NTs. Au/TiO₂ NTs exhibited excellent visible light absorption due to the LSPR effect of Au NPs. Au/TiO₂ NTs exhibited much higher photocurrent density and the photoconversion efficiency of Au decorated TiO₂ NTs was about 2.05 times greater than that of bare TiO₂ NTs. Besides, the PL intensity of Au/TiO₂ NTs was much lower than that of TiO₂ NTs, revealing a decrease in charge carrier recombination. The prepared Au/TiO₂ NTs exhibited superior photoelectrocatalytic activity and stability in the degradation of MB under simulated solar light irradiation. The synergy effect between nanotubular structures of TiO₂ and uniformly dispersed Au nanoparticles, as well as the small bias potential and strong interaction between Au and TiO₂, facilitated the Au plasmon-induced charge separation and transfer, which lead to highly efficient and stable photoelectrocatalytic activity.     

Extreme-length carbon nanofilaments with single-walled nanotube cores grown by pyrolysis of methane or acetylene

We describe a process of catalytic hydrocarbon pyrolysis which yields a new class of carbon nanotubes/nanofilaments (NT/NFs). It is characterized by high temperature (1000-1050 °C), and higher concentrations of gaseous species normally active in coke formation. Usually coke growth quickly encapsulates a catalyst particle in such environments, which in turn blocks further NT/NF growth. In our case the NT/NF lengths can reach 3-5 mm at a virtually constant filament diameter of 50-130 nm. HRTEM studies revealed a novel filament structure consisting of a SWNT core 2-4.5 nm in diameter covered by an outer shell of disordered carbon with a soot-like structure. The growing tip of a NT/NF is conical without any visible evidence of a catalyst particle at its end. The growth rate of the observed NT/NFs reaches 5-10 μm/s, far exceeding that of common catalytically grown NTs and filaments. Variation of the catalyst metal (iron, nickel, cobalt) and in the feedstock gas (acetylene, methane) did not result in any significant changes in the NT/NFs structure. These features all point to a non-catalytic growth mechanism of NT/NFs. The process described here can be a basis for development of a technology for production of very long carbon NTs with the aspect ratio as high as several tens of thousands.     

Preparation of CdS/TiO2 nanotube arrays and the enhanced photocatalytic property

In this paper, a series of CdS/TiO₂ NTs have been synthesized by SILAR method. The as-prepared CdS/TiO₂ NTs have been analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectrometer (EDS), and ultraviolet–visible (UV–vis). And their photocatalytic activities have been investigated on the degradation of methylene blue under simulated solar light irradiation. XRD results indicate that TiO₂ NTs were anatase phase, CdS nanoparticles were hexagonal phase. FESEM results indicate that low deposition concentration can keep the nanotubular structures. UV–vis results indicate that CdS can be used to improve the absorbing capability of TiO₂ NTs for visible light, and the content of CdS affects the band gap. Photocatalytic results indicate that CdS nanoparticles are conducive to improve the photocatalytic efficiency of TiO₂ NTs, and the highest degradation rate can reach 93.8%. And the photocatalytic mechanism of CdS/TiO₂ NTs to methylene blue is also described.     

Open-ended TiO2 nanotubes formed by two-step anodization and their application in dye-sensitized solar cells

We demonstrate a simple method to fabricate open-ended TiO(2) nanotube (NT) based dye-sensitized solar cells (DSSCs), where the NTs are attached to either TiO(2) nanorods (NRs) grown on fluorine-doped tin oxide (FTO) or FTO directly by nanoparticles (NPs). A completely hole-through TiO(2) NT layer is fabricated via a two-step anodization with heat treatment immediately after the first anodization. DSSCs with the open-ended NTs show better photovoltaic performance than those with close-ended NTs, due to the enhanced charge transport in the open-ended structure. Under optimum conditions, DSSCs fabricated with the open-ended NT layer exhibit a short circuit current density (J(sc)) of 19.10 mA cm(-2), an open circuit voltage (V(oc)) of 0.68 V, a fill factor (FF) of 0.49, and a power conversion efficiency (eff) of 6.3%.     

The density of water in carbon nanotubes

In this paper, the density of water confined in carbon nanotubes of different sizes and chirality is calculated. Molecular dynamics is used to simulate the spontaneous filling of the nanotube with water molecules coming from an external bath. Three H₂O filling modes are found and a correlation, which relates the density with the nanotube diameter, is proposed.     

Bioinspired multilevel interconnected networks with porous multiwalled nanotubes built by heterogeneous nanocrystallites

Inspired by fiber-interwoven eggshell membrane (ESM) with associated specific permeability and transport performance, interconnected nanotube networks are constructed to empolder relevant distinctive properties and functional advantages. In this work, a typical ESM-templated procedure combined with stepwise impregnation and gradient calcination is developed to accomplish the fabrication of multilevel frameworks from TiO₂ nanocrystallites to porous multiwalled nanotubes (NTs) to final interconnected networks. During the impregnation process, in situ mineralization occurs on ESM fiber substrate to generate Ti-impregnant coating with variant composition and layered structure. Followed by the calcination, anatase TiO₂ porous NTs come into being as ESM templates decompose at 500°C, then heterogeneous nanocrystallites are achieved at higher temperature. Thus, the final ESM-morphic nanocomposites present interconnected networks woven by porous multiwalled NTs. Benefiting from interconnected multichannels of multilevel networks and controllable heterogeneous nanocrystallites, the ESM-inspired TiO₂ would behave higher light-excitation and transport efficiency, which can achieve more valuable applications such as photocatalytic degradation to some organic pollutants.     

Fabrication and microwave absorption properties of BaTiO3 nanotube/polyaniline hybrid nanomaterials

Hybrid nanomaterials consisting of BaTiO₃ nanotubes (BTO NTs) and polyaniline (PANI), hereafter denoted as BTO NT/PANI, were successfully prepared by a one‐step in situ oxidative polymerization process of aniline monomers in the presence of BaTiO₃ nanotubes, with ammonium persulfate as oxidant and hydrochloric acid as dopant. The structure and morphology of the nanocomposites were characterized by field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X‐ray powder diffraction, thermogravimetric analysis, and X‐ray photoelectron spectroscopy. The conductivity property of the hybrid nanomaterials was also investigated. Compared with pure polyaniline, BTO NT/PANI hybrid nanomaterials exhibited enhanced reflection loss properties, which can even be further improved with appropriate electromagnetic impedance matching. More importantly, microwave‐absorbing properties of the nanocomposites can be simply modulated simply by controlling the BaTiO₃ NTs content of the absorber for the required frequency bands. Therefore, these composites may be used as lightweight andhighly effective microwave absorbers. POLYM. COMPOS., 2013 © 2013 Society of Plastics Engineers