Photocatalytic Carbon Dioxide Conversion by Structurally and Materially Modified Titanium Dioxide Nanostructures.

TiO2 has aroused considerable attentions as a promising photocatalytic material for decades due to its superior material properties in several fields such as energy and environment. However, the main dilemmas are its wide bandgap (3-3.2 eV), that restricts the light absorption in limited light wavelength region, and the comparatively high charge carrier recombination rate of TiO2, is a hurdle for efficient photocatalytic CO2 conversion. To tackle these problems, lots of researches have been implemented relating to structural and material modification to improve their material, optical, and electrical properties for more efficient photocatalytic CO2 conversion. Recent studies illustrate that crystal facet engineering could broaden the performance of the photocatalysts. As same as for nanostructures which have advantages such as improved light absorption, high surface area, directional charge transport, and efficient charge separation. Moreover, strategies such as doping, junction formation, and hydrogenation have resulted in a promoted photocatalytic performance. Such strategies can markedly change the electronic structure that lies behind the enhancement of the solar spectrum harnessing. In this review, we summarize the works that have been carried out for the enhancement of photocatalytic CO2 conversion by material and structural modification of TiO2 and TiO2-based photocatalytic system. Moreover, we discuss several strategies for synthesis and design of TiO2 photocatalysts for efficient CO2 conversion by nanostructure, structure design of photocatalysts, and material modification.

Morphology dependent electrical conduction and breakdown in single TiO2 nanotubes.

Understanding the electrical conduction properties of a single nanostructure is essential for gaining insight into the fundamental charge transport through 1D materials and also for exploring the collective behavior of an array of such nanostructures. TiO2 nanostructures, such as electrochemically grown nanotubes, have been widely studied in recent times for several applications. The electrolyte plays a vital role in deciding the morphology, which, in turn, governs the charge transport behavior. Here we present a comparative study of the charge transport through a single TiO2 nanotube grown by electrochemical anodization using ethylene glycol and dimethyl sulphoxide electrolytes. The individual nanotubes are assembled into nanodevices using photolithography without relying on complex and sophisticated process like electron beam lithography or focused ion beam deposition. The electric field dependent charge transport properties show Schottky emission at a lower field regime and Poole-Frenkel emission in the higher region. The temperature-dependent electrical conduction (110 K-410 K) is mediated by two thermal activation processes, attributed to shallow impurities in the low-temperature range (T < 230 K) and to the donors at deep intermediate levels at higher temperatures (T > 230 K). The activation energies for EG based nanotubes are found to be higher than those for DMSO nanotubes owing to the double wall morphology of the formed tubes. Also, the study of the electrical breakdown phenomena of these nanotubes reveals three distinct categories of collapse. 'Model A' type breakdown is characterized by a stepwise rise of the current up to the breakdown point and a fall to zero following a non-uniform step by step decrease, which is driven by crack formation near the electrode interface and its propagation. 'Model B' shows a transient rise and fall in current, leading to breakdown due to electromigration, whereas 'Model C' type breakdown observed in a bundle of nanotubes shows a mixed trend of 'Model A' and 'Model B'. The data and analysis provide insight into the current limit through an individual nanotube or bundle of nanotubes and will be useful for designing prototype nanodevices from titania nanostructures.

Localized thermal spike driven morphology and electronic structure transformation in swift heavy ion irradiated TiO2 nanorods.

Irradiation of materials by high energy (∼MeV) ions causes intense electronic excitations through inelastic transfer of energy that significantly modifies physicochemical properties. We report the effect of 100 MeV Ag ion irradiation and resultant localized (∼few nm) thermal spike on vertically oriented TiO2 nanorods (∼100 nm width) towards tailoring their structural and electronic properties. Rapid quenching of the thermal spike induced molten state within ∼0.5 picosecond results in a distortion in the crystalline structure that increases with increasing fluences (ions per cm2). Microstructural investigations reveal ion track formation along with a corrugated surface of the nanorods. The thermal spike simulation validates the experimental observation of the ion track dimension (∼10 nm diameter) and melting of the nanorods. The optical absorption study shows direct bandgap values of 3.11 eV (pristine) and 3.23 eV (5 × 1012 ions per cm2) and an indirect bandgap value of 3.10 eV for the highest fluence (5 × 1013 ions per cm2). First principles electronic structure calculations corroborate the direct-to-indirect transition that is attributed to the structural distortion at the highest fluence. This work presents a unique technique to selectively tune the properties of nanorods for versatile applications.

Template-free fabrication of BiFeO3 nanorod arrays: multiferroic and photo-electrochemical performances.

BiFeO3 (BFO) has been widely investigated in many forms and morphologies because of its combined multiferroic and photovoltaic properties. However, direct growth of vertically aligned BFO nanorods on an underlying substrate has remained a challenge. In this work, we report template free growth of BiFeO3 nanorod arrays on fluorine doped tin oxide coated glass substrate. This has been achieved by a two-step process, in which FeOOH nanorods are grown by chemical bath deposition and converted into BFO using bismuth (Bi) coating by physical vapour deposition (PVD). Both DC sputtering and thermal evaporation are attempted under PVD and the results suggest that Bi deposited by DC sputtering leads to well-defined BFO nanorods, which show superior performance in both multiferroic and photoelectrochemical studies. Piezoelectric force microscopy data shows the signature butterfly loop that confirms piezoelectric behaviour with a d 33 value of 8 pmV-1 in the BFO nanorods grown by DC sputtering. Further, the M-H hysteresis curve for the same samples reveals a remanent magnetization (Mr) value of 0.54 emu cc-1 and antiferromagnetic nature at room temperature. Finally, a stable photocurrent density of 0.05 mA cm-2 is achieved at 0.8 V vs Ag/AgCl under 1 Sun illumination. This work opens up new avenues for BFO in applications involving 1D nanostructures.

High photoelectrochemical performance of reduced graphene oxide wrapped, CdS functionalized, TiO2 multi-leg nanotubes.

Absorption of visible light and separation of photogenerated charges are two primary pathways to improve the photocurrent performance of semiconductor photoelectrodes. Here, we present a unique design of tricomponent photocatalyst comprising of TiO2 multileg nanotubes (MLNTs), reduced graphene oxide (rGO) and CdS nanoparticles. The tricomponent photocatalyst shows a significant red-shift in the optical absorption (∼2.2 eV) compared to that of bare TiO2 MLNTs (∼3.2 eV). The availability of both inner and outer surfaces areas of MLNTs, the visible light absorption of CdS, and charge separating behavior of reduced graphene oxide layers contribute coherently to yield a photocurrent density of ∼11 mA cm-2 @ 1 V vs. Ag/AgCl (100 mW cm-2, AM 1.5 G). Such a high PEC performance from TiO2/rGO/CdS photoelectrode system has been analyzed using diffused reflectance (DRS) and electrochemical impedance (EIS) spectroscopy techniques. The efficient generation of charge carriers under light irradiation and easy separation because of favourable band alignment, are attributed to the high photoelectrochemical current density in these tricomponent photocatalyst systems.

An improved strategy for transferring and adhering thin nanoporous alumina membranes onto conducting transparent electrodes for template assisted electrodeposition of high aspect ratio semiconductor nanowires with increased optical absorption.

This article presents a new method for transferring and enhancing the adhesion of thin nanoporous alumina (NPA) membranes onto non-atomically flat substrates like fluorine-doped tin oxide (FTO) coated glass. The study reports use of glycerol as an additive to reduce the brittleness of the polystyrene filler that was used to fill the pores of the NPA membrane. Additionally, a new reflux-based method is reported here for the complete removal of the polystryrene filler from the porous channels of alumina. The adhesion between an NPA membrane and an underlying electrode was enhanced by electrodepositing a thin (∼40 nm) intermediate layer of the conducting polymer polyaniline (PANI). The PANI layer acts as an efficient electrostatic adhesive between the NPA and the conducting glass electrode and ensures ultra-strong adhesion of the NPA membrane, which can survive the harsh conditions of CdTe nanowire electrodeposition (60 °C temperature and an acidic electrolyte) without delamination for 30 min. The resulting nanowires clearly templated the structure of NPA and displayed free-standing nanowires over a large area with a diameter of around 60 nm, a length of approximately 2.8 µm (aspect ratio ∼47) and an areal density of 5.9 × 1012 nanowires cm-2. Total optical absorption measurement on the free-standing CdTe nanowires exhibited a 45% enhancement over a wavelength range of 350-1400 nm as compared to a CdTe planar thin film of same thickness.

Photo-electrochemical properties of graphene wrapped hierarchically branched nanostructures obtained through hydrothermally transformed TiO2 nanotubes.

Hierarchically structured nanomaterials play an important role in both light absorption and separation of photo-generated charges. In the present study, hierarchically branched TiO2 nanostructures (HB-MLNTs) are obtained through hydrothermal transformation of electrochemically anodized TiO2 multi-leg nanotubes (MLNT) arrays. Photo-anodes based on HB-MLNTs demonstrated 5 fold increase in applied bias to photo-conversion efficiency (%ABPE) over that of TiO2 MLNTs without branches. Further, such nanostructures are wrapped with reduced graphene oxide (rGO) films to enhance the charge separation, which resulted in ∼6.5 times enhancement in %ABPE over that of bare MLNTs. We estimated charge transport (η tr) and charge transfer (η ct) efficiencies by analyzing the photo-current data. The ultra-fine nano branches grown on the MLNTs are effective in increasing light absorption through multiple scattering and improving charge transport/transfer efficiencies by enlarging semiconductor/electrolyte interface area. The charge transfer resistance, interfacial capacitance and electron decay time have been estimated through electrochemical impedance measurements which correlate with the results obtained from photocurrent measurements.

Graphene Oxide Modified TiO2 Micro Whiskers and Their Photo Electrochemical Performance.

Harnessing the solar energy and producing clean fuel hydrogen through efficient photo-electrochemical water splitting has remained one of the most challenging endeavors in materials science. The core problem is to develop a suitable photo-catalyst material that absorbs a significant part of the solar spectrum and produces electron-hole pairs that can be easily separated without recombination. In the recent times, the composite of Titanium dioxide with graphene have been investigated to explore the advantages of both class of materials. Here we report on the photo-electrochemical properties of reduced graphene oxide functionalised TiO2 whiskers. The TiO2 whiskers are obtained from potassium titanium oxide (KTi8O16) synthesized through hydrothermal technique followed by ion exchange method and heat treatment. Graphene oxide was deposited on the as prepared TiO2 whiskers using hydrothermal method. As formed samples were characterized by Raman spectroscopy to confirm the presence of reduced graphene oxide (RGO) attached to TiO2 whiskers. Comparative photo electrochemical studies were carried out for TiO2 and reduced graphene oxide modified TiO2 whiskers. Among these, RGO modified TiO2 whiskers show significantly higher photo current density possibly due to enhancement in charge separation ability and longer electron life times.

Theory, instrumentation and applications of magnetoelastic resonance sensors: a review.

Thick-film magnetoelastic sensors vibrate mechanically in response to a time varying magnetic excitation field. The mechanical vibrations of the magnetostrictive magnetoelastic material launch, in turn, a magnetic field by which the sensor can be monitored. Magnetic field telemetry enables contact-less, remote-query operation that has enabled many practical uses of the sensor platform. This paper builds upon a review paper we published in Sensors in 2002 (Grimes, C.A.; et al. Sensors 2002, 2, 294-313), presenting a comprehensive review on the theory, operating principles, instrumentation and key applications of magnetoelastic sensing technology.

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.

Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons.

The past several decades have seen a significant rise in atmospheric carbon dioxide levels resulting from the combustion of hydrocarbon fuels. A solar energy based technology to recycle carbon dioxide into readily transportable hydrocarbon fuel (i.e., a solar fuel) would help reduce atmospheric CO2 levels and partly fulfill energy demands within the present hydrocarbon based fuel infrastructure. We review the present status of carbon dioxide conversion techniques, with particular attention to a recently developed photocatalytic process to convert carbon dioxide and water vapor into hydrocarbon fuels using sunlight.

Eliminating unwanted nanobubbles from hydrophobic solid/liquid interfaces: a case study using magnetoelastic sensors.

Air bubbles are known to form at the liquid/solid interface of hydrophobic materials upon immersion in a liquid (Holmberg, M.; Kdühle, A.; Garnaes, J.; Mørch, K. A.; Boisen, A. Langmuir 2003, 19, 10510-10513). In the case of gravimetric sensors, air bubbles that randomly form at the liquid-solid interface result in poor sensor-to-sensor reproducibility. Herein a superhydrophilic ZnO nanorod film is applied to the originally hydrophobic surface of a resonance-based magnetoelastic sensor. The superhydrophilic coating results in the liquid completely spreading across the surface, removing unwanted air bubbles from the liquid/sensor interface. The resonance amplitude of uncoated (bare) and ZnO-modified sensors are measured in air and then when immersed in saline solution, ethylene glycol, or bovine blood. In comparison to the bare, hydrophobic sensors, we find that the standard deviation of the resonance amplitudes of the liquid-immersed ZnO-nanorod-modified sensors decreases substantially, ranging from a 27% decrease for bovine blood to a 67% decrease for saline. The strategy of using a superhydrophilic coating can be applied to other systems having similar interfacial problems.

The effect of TiO2 nanotubes in the enhancement of blood clotting for the control of hemorrhage.

The main biological purpose of blood coagulation is formation of an obstacle to prevent blood loss of hydraulic strength sufficient to withstand the blood pressure. The ability to rapidly stem hemorrhage in trauma patients significantly impacts their chances of survival, and hence is a subject of ongoing interest in the medical community. Herein, we report on the effect of biocompatible TiO2 nanotubes on the clotting kinetics of whole blood. TiO2 nanotubes 10 microm long were prepared by anodization of titanium in an electrolyte comprised of dimethyl sulfoxide and HF, then dispersed by sonication. Compared to pure blood, blood containing dispersed TiO2 nanotubes and blood in contact with gauze pads surface-decorated with nanotubes demonstrated significantly stronger clot formation at reduced clotting times. Similar experiments using nanocrystalline TiO2 nanoparticles showed comparatively weaker clot strengths and increased clotting times. The TiO2 nanotubes appear to act as a scaffold, facilitating fibrin formation. Our results suggest that application of a TiO2 nanotube functionalized bandage could be used to help stem or stop hemorrhage.