Rapid Growth of Zinc Oxide Nanotube-Nanowire Hybrid Architectures and Their Use in Breast Cancer-Related Volatile Organics Detection.

A simple direct method for the rapid fabrication of zinc oxide nanotube-nanowire hybrid structure in an environmentally friendly way is described here. Zinc foils were anodized in an aqueous solution of washing soda and baking soda at room temperature in order to obtain the hybrid architecture. At the beginning of the process nanowires were formed on the substrate. The wider nanowires transformed into nanotubes in about a minute and grew in length with time. The morphological integrity was maintained upon heat treatment at temperatures up to the melting point of the substrate (∼400 °C) except that the nanotube wall became porous. The chemiresistor devices fabricated using the heat-treated structure exhibited high response to low-concentration volatile organic compounds that are considered markers for breast cancer. The response was not significantly affected by high humidity or presence of hydrogen, methane, or carbon dioxide. The devices are expected to find use as breath sensors for noninvasive early detection of breast cancer.

Catalyst-Free Plasma Enhanced Growth of Graphene from Sustainable Sources.

Details of a fast and sustainable bottom-up process to grow large area high quality graphene films without the aid of any catalyst are reported in this paper. We used Melaleuca alternifolia, a volatile natural extract from tea tree plant as the precursor. The as-fabricated graphene films yielded a stable contact angle of 135°, indicating their potential application in very high hydrophobic coatings. The electronic devices formed by sandwiching pentacene between graphene and aluminum films demonstrated memristive behavior, and hence, these graphene films could find use in nonvolatile memory devices also.

Stable and Efficient Perovskite Solar Cells Based on Titania Nanotube Arrays.

Highly ordered 1D TiO2 nanotube arrays are fabricated and applied as nanocontainers and electron transporting material in CH3 NH3 PbI3 perovskite solar cells. The optimized device shows a power conversion efficiency of 14.8%, and improved stability under an illumination of 100 mW cm(-2). This is the best result based on 1D TiO2 nanostructures so far.

Femtosecond to millisecond studies of electron transfer processes in a donor-(π-spacer)-acceptor series of organic dyes for solar cells interacting with titania nanoparticles and ordered nanotube array films.

Time-resolved emission and absorption spectroscopy are used to study the photoinduced dynamics of forward and back electron transfer processes taking place between a recently synthesized series of donor-(π-spacer)-acceptor organic dyes and semiconductor films. Results are obtained for vertically oriented titania nanotube arrays (inner diameters 36 nm and 70 nm), standard titania nanoparticles (25 nm diameter) and, as a reference, alumina nanoparticle (13 nm diameter) films. The studied dyes contain a triphenylamine group as an electron donor, cyanoacrylic acid part as an electron acceptor, and differ by the substituents in a spacer group that causes a shift of its absorption spectra. Despite a red-shift of the dye absorption band resulting in an improved response to the solar spectrum, smaller electron injection rates and smaller extinction coefficients result in reduced dye sensitized solar cell (DSSC) conversion efficiencies. For the most efficient dye, TPC1, electron injection from the hot locally excited state to titania on a time scale of about 100 fs is suggested, while from the relaxed charge transfer state it proceeds in a non-exponential way with time constants from 1 ps to 50 ps. Our results imply that the latter process involves the trap states below the conduction band edge (or the sub-bandgap tail of the acceptor states), localized close to the dye radical cation, and is accompanied by fast electron recombination to the parent dye's ground state. This process should limit the efficiency of DSSCs made using these types of organic dyes. The residual, slower recombination can be described by a stretched exponential decay with a characteristic time of 0.5 µs and a dispersion parameter of 0.33. Both the electron injection and back electron transfer dynamics are similar in titania nanoparticles and nanotubes. Variations between the two film types are only found in the time resolved emission transients, which are explained in terms of the difference in local electric fields affecting the position of the emission bands.

Single-Step Synthesis of Nitrogen-Doped Graphene Oxide from Aniline at Ambient Conditions.

Single-step, single-precursor synthesis of nitrogen-doped graphene oxide (N-GO) was demonstrated in this work. By choosing aniline as the sole source of carbon and nitrogen, N-GO films were fabricated using microwave plasma at a power as low as 80 W in atmospheric conditions. The aniline vapor dissociated under plasma formed islands of N-GO nanosheets on the substrates or walls of the quartz deposition chamber. The interplanar spacing in the pristine N-GO films was observed to be lower than that of GO films, which indicated a lower concentration of oxygen and other species present in the space between the N-GO layers. The as-fabricated N-GO demonstrated superior antiscaling and algicidal properties that are deemed imperative for water purification applications.

High-efficiency Förster resonance energy transfer in solid-state dye sensitized solar cells.

Solid-state dye-sensitized solar cells (SS-DSCs) offer the potential to make low cost solar power a reality, however their photoconversion efficiency must first be increased. The dyes used are commonly narrow band with high absorption coefficients, while conventional photovoltaic operation requires proper band edge alignment significantly limiting the dyes and charge transporting materials that can be used in combination. We demonstrate a significant enhancement in the light harvesting and photocurrent generation of SS-DSCs due to Förster resonance energy transfer (FRET). TiO(2) nanotube array films are sensitized with red/near IR absorbing SQ-1 acceptor dye, subsequently intercalated with Spiro-OMeTAD blended with a visible light absorbing DCM-pyran donor dye. The calculated Förster radius is 6.1 nm. The donor molecules contribute a FRET-based maximum IPCE of 25% with a corresponding excitation transfer efficiency of approximately 67.5%.

Ta3N5 nanotube arrays for visible light water photoelectrolysis.

Tantalum nitride (Ta3N5) has a band gap of approximately 2.07 eV, suitable for collecting more than 45% of the incident solar spectrum energy. We describe a simple method for scale fabrication of highly oriented Ta3N5 nanotube array films, by anodization of tantalum foil to achieve vertically oriented tantalum oxide nanotube arrays followed by a 700 degrees C ammonia anneal for sample crystallization and nitridation. The thin walled amorphous nanotube array structure enables transformation from tantalum oxide to Ta3N5 to occur at relatively low temperatures, while high-temperature annealing related structural aggregation that commonly occurs in particle films is avoided. In 1 M KOH solution, under AM 1.5 illumination with 0.5 V dc bias typical sample (nanotube length approximately 240 nm, wall thickness approximately 7 nm) visible light incident photon conversion efficiencies (IPCE) as high as 5.3% were obtained. The enhanced visible light activity in combination with the ordered one-dimensional nanoarchitecture makes Ta3N5 nanotube arrays films a promising candidate for visible light water photoelectrolysis.

Molecular design of near-IR harvesting unsymmetrical squaraine dyes.

The functionalized unsymmetrical benzothiazole squaraine organic sensitizers 5-carboxy-2-({3-[(3-hexylbenzothiazol-2(3H)-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene}methyl)-1-hexyl-3,3-dimethyl-3H-indolium (hereafter named as SK-11) and 5-carboxy-2-({3-[(3-hexyl-5-methoxybenzothiazol-2(3H)-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene}methyl)-1-hexyl-3,3-dimethyl-3H-indolium (coded as SK-12) are designed and developed to observe an intense and wider absorption band in the red/NIR wavelength region. DFT/TDDFT calculations have been performed on the two unsymmetrical squaraine sensitizers to gain insight into their electronic and optical properties. The utility of these dyes in solid state dye sensitized solar cells (SS-DSSCs) is demonstrated.

Synthesis and applications of electrochemically self-assembled titania nanotube arrays.

Highly ordered vertically oriented TiO(2) nanotube arrays fabricated by electrochemical anodization offer a large surface area architecture with precisely controllable nanoscale features. These nanotubes have shown remarkable properties in a variety of applications including, for example, their use as hydrogen sensors, in the photoelectrochemical generation of hydrogen, dye-sensitized and solid-state heterojunction solar cells, photocatalytic reduction of carbon dioxide into hydrocarbons, and as a novel drug delivery platform. Herein we consider the development of the various nanotube array synthesis techniques, different applications of the TiO(2) nanotube arrays, unresolved issues, and possible future research directions.

Visible to near-infrared light harvesting in TiO2 nanotube array-P3HT based heterojunction solar cells.

The development of high-efficiency solid-state excitonic photovoltaic solar cells compatible with solution processing techniques is a research area of intense interest, with the poor optical harvesting in the red and near-IR (NIR) portion of the solar spectrum a significant limitation to device performance. Herein we present a solid-state solar cell design, consisting of TiO(2) nanotube arrays vertically oriented from the FTO-coated glass substrate, sensitized with unsymmetrical squaraine dye (SQ-1) that absorbs in the red and NIR portion of solar spectrum, and which are uniformly infiltrated with p-type regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) that absorbs higher energy photons. Our solid-state solar cells exhibit broad, near-UV to NIR, spectral response with external quantum yields of up to 65%. Under UV filtered AM 1.5G of 90 mW/cm(2) intensity we achieve typical device photoconversion efficiencies of 3.2%, with champion device efficiencies of 3.8%.

Fabrication of Nano-Onion-Structured Graphene Films from Citrus sinensis Extract and Their Wetting and Sensing Characteristics.

Graphene and its derivatives have acquired substantial research attention in recent years because of their wide range of potential applications. Implementing sustainable technologies for fabricating these functional nanomaterials is becoming increasingly apparent, and therefore, a wide spectrum of naturally derived precursors has been identified and reformed through various established techniques for the purpose. Nevertheless, most of these methods could only be considered partially sustainable because of their complexity as well as high energy, time, and resource requirements. Here, we report the fabrication of carbon nano-onion-interspersed vertically oriented multilayer graphene nanosheets through a single-step, environmentally benign radio frequency plasma-enhanced chemical vapor deposition process from a low-cost carbon feedstock, the oil from the peel of Citrus sinensis orange fruits. C. sinensis essential oil is a volatile aroma liquid principally composed of nonsynthetic hydrocarbon limonene. Transmission electron microscopy studies on the structure unveiled the presence of hollow quasi-spherical carbon nano-onion-like structures incorporated within graphene layers. The as-fabricated nano-onion-incorporated graphene films exhibited a highly hydrophobic nature showing a water contact angle of up to 1290. The surface energies of these films were in the range of 41 to 35 mJ·m-2. Moreover, a chemiresistive sensor directly fabricated using C. sinensis-derived onion-structured graphene showed a p-type semiconductor nature and a promising response to acetone at room temperature. With its unique morphology, surface properties, and electrical characteristics, this material is expected to be useful for a wide range of applications.

PEGylated (NH4)xWO3 nanorod mediated rapid photonecrosis of breast cancer cells.

Photothermal therapy has emerged as a potential minimally invasive technique to destroy malignant cells with high selectivity. It utilizes low band gap nanoscale materials as photothermal agents dispersed at the affected area to increase the temperature locally by absorbing radiation in the near infrared (NIR) region and destroys the cells. In an effort to develop a photothermal agent with high efficacy for photothermal therapy, we found that (NH4)xWO3 nanorods of length 0.5-1.0 µm and diameter ∼100 nm could destroy breast cancer cells rapidly when irradiated with a wavelength in the therapeutic window. The material was prepared using a solvothermal route followed by PEGylation for improving the biocompatibility. X-ray diffraction and transmission electron microscopy studies revealed the hexagonal crystal lattice of the material. The uniform wrapping of polyethylene glycol (PEG) around the nanorods was confirmed using energy dispersive spectroscopy elemental mapping. An 808 nm laser was used to investigate the photothermal responses of the material on SUM-159 and MCF-7 breast cancer cells in vitro. The PEGylated-(NH4)xWO3 nanorods exhibited rapid temperature elevation from 20 °C to 60 °C within 3 min upon irradiation. A significant growth inhibition of SUM-159 and MCF-7 breast cancer cells with photonecrosis was observed. PEGylated (NH4)xWO3 nanorods could potentially be used in cancer therapy due to their strong photonecrotic properties at specific NIR wavelengths that suffer from minimal attenuation while passing through biological tissues.

Observation of a low temperature n-p transition in individual titania nanotubes.

Manipulating the transport properties of titania nanotubes (NTs) is paramount in guaranteeing the material's successful implementation in various solid state applications. Here we present the unique semiconducting properties of individual titania NTs as revealed from thermoelectric and structural studies performed on the same individual NTs. The NTs were in the anatase phase fabricated by anodic oxidation and doped with intrinsic defects created by reducing the lattice thermally. Despite their polycrystalline nature and nanoscale walls, the doped NTs were found to be 4-5 orders of magnitude more electrically conducting than TiO2 nanowires and thin films, with values approaching the bulk single crystal conductivity. The reason for the high conductivity was found to be the high carrier concentration on the order of 1022 cm-3, which counteracted the low mobility values ∼0.006 cm2 V-1 s-1. Furthermore, this high level of carrier concentration transitioned the NTs to a degenerate state, which is the first such example in thermally doped titania NTs. More importantly, our study showed the creation of acceptor states along with donor states in individual nanotubes upon lattice reduction. These acceptor levels were found to be active at low temperatures when donor states were not ionized, shifting the Fermi level (Ef) from the conduction band to the valence band.

Exfoliation of a non-van der Waals material from iron ore hematite.

With the advent of graphene, the most studied of all two-dimensional materials, many inorganic analogues have been synthesized and are being exploited for novel applications. Several approaches have been used to obtain large-grain, high-quality materials. Naturally occurring ores, for example, are the best precursors for obtaining highly ordered and large-grain atomic layers by exfoliation. Here, we demonstrate a new two-dimensional material 'hematene' obtained from natural iron ore hematite (α-Fe2O3), which is isolated by means of liquid exfoliation. The two-dimensional morphology of hematene is confirmed by transmission electron microscopy. Magnetic measurements together with density functional theory calculations confirm the ferromagnetic order in hematene while its parent form exhibits antiferromagnetic order. When loaded on titania nanotube arrays, hematene exhibits enhanced visible light photocatalytic activity. Our study indicates that photogenerated electrons can be transferred from hematene to titania despite a band alignment unfavourable for charge transfer.

Anodically Grown Titania Nanotube Induced Cytotoxicity has Genotoxic Origins.

Nanoarchitectures of titania (TiO2) have been widely investigated for a number of medical applications including implants and drug delivery. Although titania is extensively used in the food, drug and cosmetic industries, biocompatibility of nanoscale titania is still under careful scrutiny due to the conflicting reports on its interaction with cellular matter. For an accurate insight, we performed in vitro studies on the response of human dermal fibroblast cells toward pristine titania nanotubes fabricated by anodic oxidation. The nanotubes at low concentrations were seen to induce toxicity to the cells, whereas at higher concentrations the cell vitality remained on par with controls. Further investigations revealed an increase in the G0 phase cell population depicting that majority of cells were in the resting rather than active phase. Though the mitochondrial set-up did not exhibit any signs of stress, significantly enhanced reactive oxygen species production in the nuclear compartment was noted. The TiO2 nanotubes were believed to have gained access to the nuclear machinery and caused increased stress leading to genotoxicity. This interesting property of the nanotubes could be utilized to kill cancer cells, especially if the nanotubes are functionalized for a specific target, thus eliminating the need for any chemotherapeutic agents.

Anodic growth of highly ordered TiO2 nanotube arrays to 134 microm in length.

Described is the fabrication of self-aligned highly ordered TiO(2) nanotube arrays by potentiostatic anodization of Ti foil having lengths up to 134 mum, representing well over an order of magnitude increase in length thus far reported. We have achieved the very long nanotube arrays in fluoride ion containing baths in combination with a variety of nonaqueous organic polar electrolytes including dimethyl sulfoxide, formamide, ethylene glycol, and N-methylformamide. Depending on the anodization voltage, pore diameters of the resulting nanotube arrays range from 20 to 150 nm. Our longest nanotube arrays yield a roughness factor of 4750 and length-to-width (outer diameter) aspect ratio of approximately 835. The as-prepared nanotubes are amorphous but crystallize with annealing at elevated temperatures. In initial measurements, 45 mum long nanotube-array samples, 550 degrees C annealed, under UV illumination show a remarkable water photoelectrolysis photoconversion efficiency of 16.25%.

Fabrication of highly ordered TiO2 nanotube arrays using an organic electrolyte.

Anodization of titanium in a fluorinated dimethyl sulfoxide (DMSO) and ethanol mixture electrolyte is investigated. The prepared anodic film has a highly ordered nanotube-array surface architecture. Using a 20 V anodization potential (vs Pt) nanotube arrays having an inner diameter of 60 nm and 40 nm wall thickness are formed. The overall length of the nanotube arrays is controlled by the duration of the anodization, with nanotubes appearing only after approximately 48 h; a 72 h anodization results in a nanotube array approximately 2.3 mum in length. The photoelectrochemical response of the nanotube-array photoelectrodes is studied using a 1 M KOH solution under both UV and visible (AM 1.5) illumination. Enhanced photocurrent density is observed for samples obtained in the organic electrolyte, with an UV photoconversion efficiency of 10.7%.

Numerical simulation of light propagation through highly-ordered titania nanotube arrays: dimension optimization for improved photoabsorption.

Propagation of electromagnetic waves in the ultraviolet-visible range (300 to 600 nm) through a unique highly-ordered titania nanotube array structure is studied using the computational technique of Finite Difference Time Domain (FDTD). Through numerical simulation the transmittance, reflectance and absorbance of the nanotube-arrays are obtained as a function of tube length and diameter. The nanotube-arrays are found to completely absorb light having wavelengths less than approximately 330 nm. For wavelengths above 380 nm absorption increases as a function of nanotube length, while above 435 nm absorption increases with decreasing pore size. Computational simulations closely match experimental measurements, indicating the suitability of the computational technique for guiding material optimization.

Water-photolysis properties of micron-length highly-ordered titania nanotube-arrays.

We report the water photoelectrolysis and photoelectrochemical properties of the titania nanotube arrays as a function of nanotube crystallinity, length (up to 6.4 microm), and pore size. Most noteworthy of our results, under 320-400 nm illumination (98 mW/cm2) the titania nanotube-array photoanodes (area 1 cm2), pore size 110 nm, wall thickness 20 nm, and 6 microm length, generate hydrogen by water photoelectrolysis at a rate of 7.6 mL/hr, with a photoconversion efficiency of 12.25%. The energy-time normalized hydrogen evolution rate is 80 mL/hrW, the largest reported hydrogen photoelectrolysis generation rate for any material system by a factor of four. The highly-ordered nanotubular architecture appears to allow for superior charge separation and charge transport, with a calculated quantum efficiency of over 80% for incident photons with energies larger than the titania bandgap.

Thermal-structural relationship of individual titania nanotubes.

The thermal properties of nano-scale materials are largely influenced by their geometry. The zero, one and quasi one dimensional forms of the same material could exhibit unique thermal transport properties depending upon the shape and nano-scale feature size. In order to gain a clear understanding of the contributions from geometrical scattering effects on thermal transport, it is required to study these nano-materials in a single isolated form rather than in clusters or films. In the past decade, titanium dioxide nanotube arrays fabricated by anodic oxidation of titanium emerged as a useful semiconductor architecture for a variety of applications, particularly for solar energy conversion. Nonetheless, the thermal properties of individual nanotubes that are important for their use in high temperature applications have not been clearly understood. Here we report the thermal transport properties of individual titania nanotubes as revealed by our preliminary study using a suspended microdevice that facilitates the thermal conductivity measurements and crystal structure investigation on the same nanotube. The nanotubes were prepared by anodic oxidation of a titanium foil in HF-DMSO electrolyte at 60 V, having outer diameters in the range of 200 to 300 nm and wall thicknesses of ∼30 to 70 nm in either amorphous or polycrystalline anatase phase. The thermal conductivity of single nanotubes was found to be very close to that of the amorphous phase (1.5 W mK(-1) and 0.85 W mK(-1) respectively) and it was only half of the thermal conductivity of the nanotube arrays in the film form. The thermal conductivity of bulk TiO2 is known to be almost six times higher. The observed thermal conductivity suppression in single nanotubes was explained using a transport model developed by considering diffuse phonon-surface scattering and scattering of phonons by ionized impurities of concentrations in the order of 10(18)-10(19) cm(-3).