Karthik Shankar

Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications.

Single-crystal one-dimensional (1D) semiconductor architectures are important in materials-based applications requiring a large surface area, morphological control, and superior charge transport. Titania has widespread utility in applications including photocatalysis, photochromism, photovoltaics, and gas sensors. While considerable efforts have focused on the preparation of 1D TiO2, no methods have been available to grow crystalline nanowire arrays directly onto transparent conducting oxide (TCO) substrates, greatly limiting the performance of TiO2 photoelectrochemical devices. Herein, we present a straightforward low temperature method to prepare single crystal rutile TiO2 nanowire arrays up to 5 microm long on TCO glass via a non-polar solvent/hydrophilic substrate interfacial reaction under mild hydrothermal conditions. The as-prepared densely packed nanowires grow vertically oriented from the TCO glass substrate along the (110) crystal plane with a preferred (001) orientation. In a dye sensitized solar cell, N719 dye, using TiO2 nanowire arrays 2-3 microm long we achieve an AM 1.5 photoconversion efficiency of 5.02%.

Highly-ordered TiO2 nanotube arrays up to 220 µm in length: use in water photoelectrolysis and dye-sensitized solar cells

The fabrication of highly-ordered TiO2 nanotube arrays up to 134 µm in length by anodization of Ti foil has recently been reported (Paulose et al 2006 J. Phys. Chem. B 110 16179). This work reports an extension of the fabrication technique to achieve TiO2 nanotube arrays up to 220 µm in length, with a length-to-outer diameter aspect ratio of ≈1400, as well as their initial application in dye-sensitized solar cells and hydrogen production by water photoelectrolysis. The highly-ordered TiO2 nanotube arrays are fabricated by potentiostatic anodization of Ti foil in fluoride ion containing baths in combination with non-aqueous organic polar electrolytes including N-methylformamide, dimethyl sulfoxide, formamide, or ethylene glycol. Depending upon the anodization voltage, the inner pore diameters of the resulting nanotube arrays range from 20 to 150 nm. As confirmed by glancing angle x-ray diffraction and HRTEM studies, the as-prepared nanotubes are amorphous but crystallize with annealing at elevated temperatures.

High Carrier Density and Capacitance in TiO2 Nanotube Arrays Induced by Electrochemical Doping

The paper describes the electronic charging and conducting properties of vertically oriented TiO 2 nanotube arrays formed by anodization of Ti foil samples. The resulting films, composed of vertically oriented nanotubes approximately 10 mum long, wall thickness 22 nm, and pore diameter 56 nm, are analyzed using impedance spectroscopy and cyclic voltammetry. Depending on the electrochemical conditions two rather different electronic behaviors are observed. Nanotube array samples in basic medium show behavior analogous to that of nanoparticulate TiO 2 films used in dye-sensitized solar cells: a chemical capacitance and electronic conductivity that increase exponentially with bias potential indicating a displacement of the Fermi level. Nanotube array samples in acidic medium, or samples in a basic medium submitted to a strong negative bias, exhibit a large increase in capacitance and conductivity indicating Fermi level pinning. The contrasting behaviors are ascribed to proton intercalation of the TiO 2. Our results suggest a route for controlling the electronic properties of the ordered metal-oxide nanostructures for their use in applications including supercapacitors, dye-sensitized solar cells, and gas sensing.

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%.

Highly Efficient Solar Cells using TiO2 Nanotube Arrays Sensitized with a Donor-Antenna Dye

Donor antenna dyes provide an exciting route to improving the efficiency of dye sensitized solar cells owing to their high molar extinction coefficients and the effective spatial separation of charges in the charge-separated state, which decelerates the recombination of photogenerated charges. Vertically oriented TiO(2) nanotube arrays provide an optimal material architecture for photoelectrochemical devices because of their large internal surface area, lower recombination losses, and vectorial charge transport along the nanotube axis. In this study, the results obtained by sensitizing TiO(2) nanotube arrays with the donor antenna dye Ru-TPA-NCS are presented. Solar cells fabricated using an antenna dye-sensitized array of 14.4 microm long TiO(2) nanotubes on Ti foil subjected to AM 1.5 one sun illumination in the backside geometry exhibited an overall conversion efficiency of 6.1%. An efficiency of 4.1% was obtained in the frontside illumination geometry using a 1 microm long array of transparent TiO(2) nanotubes subjected to a TiCl(4) treatment and then sensitized with the Ru-TPA-NCS dye. Open circuit voltage decay measurements give insight into the recombination behavior in antenna-dye sensitized nanotube photoelectrodes, demonstrating outstanding properties likely due to a reduction in the influence of the surface traps and reduced electron transfer from TiO(2) to ions in solution.

P-type Cu--Ti--O nanotube arrays and their use in self-biased heterojunction photoelectrochemical diodes for hydrogen generation.

Copper and titanium remain relatively plentiful in the earths crust; hence, their use for large-scale solar energy conversion technologies is of significant interest. We describe fabrication of vertically oriented p-type Cu-Ti-O nanotube array films by anodization of copper rich (60% to 74%) Ti metal films cosputtered onto fluorine doped tin oxide (FTO) coated glass. Cu-Ti-O nanotube array films 1 mum thick exhibit external quantum efficiencies up to 11%, with a spectral photoresponse indicating that the complete visible spectrum, 380 to 885 nm, contributes significantly to the photocurrent generation. Water-splitting photoelectrochemical pn-junction diodes are fabricated using p-type Cu-Ti-O nanotube array films in combination with n-type TiO 2 nanotube array films. With the glass substrates oriented back-to-back, light is incident upon the UV absorbing n-TiO 2 side, with the visible light passing to the p-Cu-Ti-O side. In a manner analogous to photosynthesis, photocatalytic reactions are powered only by the incident light to generate fuel with oxygen evolved from the n-TiO 2 side of the diode and hydrogen from the p-Cu-Ti-O side. To date, we find under global AM 1.5 illumination that such photocorrosion-stable diodes generate a photocurrent of approximately 0.25 mA/cm (2), at a photoconversion efficiency of 0.30%.

High efficiency double heterojunction polymer photovoltaic cells using highly ordered TiO2 nanotube arrays

Vertically oriented TiO2 nanotube arrays formed by anodization offer a highly ordered material architecture for efficient charge generation and collection in photoelectrochemical devices. A blend of regioregular poly(3-hexylthiophene) and a methanofullerene (phenyl C71-butyric acid methyl ester) was infiltrated into transparent TiO2 nanotube films. The heterojunction poly(3-hexylthiophene) (P3HT)-([6,6]-phenyl-C71-butyric acid methyl ester) and P3HT-TiO2 interfaces both result in charge separation. The resulting solid state solar cells show a short-circuit current density of 12.4mA∕cm2, 641mV open circuit potential, and a 0.51 fill factor, yielding power conversion efficiencies of 4.1% under AM 1.5 sun.

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.

Tantalum‐Doped Titanium Dioxide Nanowire Arrays for Dye‐Sensitized Solar Cells with High Open‐Circuit Voltage

Liquid-junction dye-sensitized solar cells (DSSCs) based on nanocrystalline titania (TiO2) electrodes constitute a potentially low-cost alternative to traditional inorganic siliconbased photovoltaics and have been studied extensively over the past two decades. Liquid-junction DSSCs now show high short-circuit photocurrent densities (Jsc) and good fill factors (FF) owing to improvements made in the photosensitizer and the titania electrodes. Despite these improvements, a remaining issue of critical importance is the relatively low open-circuit photovoltage (Voc) obtained. The Voc of a liquid-junction DSSC is determined by the energy difference between the quasi-Fermi level (QFL) of the semiconductor and the potential of the redox couple in the electrolyte. For n-type TiO2, the injection of electrons from photoexcited dye molecules raises the QFL towards the conduction band (CB). 5] Thus, the maximum achievable Voc would correspond to the case of a degenerate semiconductor and would therefore equal the energy difference between the TiO2 CB edge and the widely used tri-iodide redox level; this theoretical maximum achievable value of Voc for n-TiO2based DSSCs is 0.95 V. Nevertheless, Voc values of 0.7–0.8 V are typically obtained in reported DSSCs, with the deviation from the theoretical maximum commonly explained by interfacial recombination at the TiO2–dye or TiO2–electrolyte interfaces. Substantial efforts have been made to improve the photovoltage obtained by retarding the recombination losses. For example, a thin overcoat of different insulting metal oxides, such as Nb2O5 and Al2O3, have frequently been used to modify the TiO2 electrode by making a core/shell structure. Several kinds of organic molecular additives, such as deoxycholic acid, 4-guanidinobutyric acid, and 4-tertbutylpyridine (TBP) have also been used in the redox electrolyte. However, these approaches have been found to yield only approximately 50 mV improvement in the open-circuit photovoltage. The intentional incorporation of atomic impurities into semiconducting materials is a common approach for tailoring properties such as band gap or electric conductivity for specific applications. Doping is routinely performed with bulk semiconductors and has recently been extended to nanoscale materials as well. Among nanostructured materials, semiconducting nanowires are widely studied because of their special electrical and optical properties and because it is possible to use them as components in functional devices such as solar cells. Unlike bulk materials, one-dimensional nanowires are usually prepared under non-equilibrium conditions, and it has proved challenging to dope them homogeneously; high-temperature vapor-phase approaches are commonly employed in their synthesis, which are limited in regards to homogeneous doping and alloying because of the high growth temperatures. In contrast, low-temperature hydrothermal synthesis approaches possess an inherent advantage over vapor-phase routes for doping purposes. There have recently been reports on the synthesis of aligned rutile TiO2 nanowire arrays on transparent conducting oxide (TCO) substrates by hydrothermal synthesis; however, no reports of anisotropic transition-metal-doped TiO2 nanowires grown on TCO substrates in the solution phase exist. Herein, we present the synthesis of titania nanowire arrays homogeneously doped with tantalum and prepared under hydrothermal conditions. The synthetic process presented herein should readily be extendable to allow doping of nanowires with different transition metals (e.g., Fe, W, Cr); however, this report is limited to the Ta-doped system. Further, we have translated this advance in materials synthesis into enhanced device performance by demonstrating dye-sensitized solar cells with a very high open-circuit photovoltage of 0.87 V, strikingly close to the theoretical maximum. Figure 1a, b shows field-emission scanning electron microscopy (FESEM) top-surface images of a typical assynthesized nanowire array sample at both low and high magnification. A highly uniform and densely packed array of nanowires is obtained, with an average wire width of approximately 20 nm. Figure 1 c is a cross-sectional view of the same film with a thickness of approximately 3.6 mm, indicating that the nanowires grow almost perpendicularly from the substrate. This finding is confirmed by the X-ray diffraction (XRD) pattern, which shows a remarkably enhanced (002) peak (Figure 1d). XRD patterns indicate the absence of peaks corresponding to the Ta2O5 phase. Owing to the low doping concentration detected by energydispersive X-ray spectroscopy (EDX; 0.83 at%) and to the comparable ionic radii of tantalum (0.064 nm) and titanium ions (0.061 nm), no peak shift was detected after tantalum doping. Figure 1e is a high-resolution TEM (HRTEM) image of the as-prepared nanowire sample, showing the nanowires to be highly crystalline (rutile). The nanowires grow in the (001) direction with the [110] crystal plane parallel to the [*] Dr. X. J. Feng, Dr. K. Shankar, M. Paulose, Prof. C. A. Grimes Materials Research Institute, The Pennsylvania State University University Park, PA 16802 (USA) E-mail: cgrimes@engr.psu.edu

Photoelectrochemical and water photoelectrolysis properties of ordered TiO2 nanotubes fabricated by Ti anodization in fluoride-free HCl electrolytes

Described is the synthesis of TiO2 nanotube array films by anodization of Ti foil in HCl electrolytes containing different H2O2 concentrations. Highly ordered nanotube arrays up to 860 nm in length, 15 nm inner pore diameter, and 10 nm wall thickness were obtained for one hour anodizations using a 0.5 M HCl aqueous electrolyte containing 0.1–0.5 M H2O2 concentrations for anodization potentials between 10–23 V. The use of ethylene glycol as the electrolyte medium significantly alters the anodization kinetics and resulting film morphologies; nanotube bundles several microns in length achieved for anodization potentials between 8 V and 18 V in only a few minutes. The nanotube arrays obtained from the ethylene glycol electrolytes show relatively higher photocurrents, ≈0.8 mA cm−2 under AM 1.5. Under 100 mW cm−2 AM 1.5 illumination a 500 °C annealed 1 cm2 nanotube array sample, obtained by anodization of a Ti foil sample in ethylene glycol + 0.5 M HCl + 0.4 M H2O2 electrolyte, demonstrates a hydrogen evolution rate of approximately 391 μL h−1 by water photoelectrolysis, time-power normalized evolution rate of 3.9 mL W−1 h−1, with water splitting confirmed by the 2 : 1 ratio of evolved hydrogen to oxygen.