Xinjian Feng

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%.

Vertically Aligned WO3 Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis and Photoelectrochemical Properties

Photocorrosion stable WO(3) nanowire arrays are synthesized by a solvothermal technique on fluorine-doped tin oxide coated glass. WO(3) morphologies of hexagonal and monoclinic structure, ranging from nanowire to nanoflake arrays, are tailored by adjusting solution composition with growth along the (001) direction. Photoelectrochemical measurements of illustrative films show incident photon-to-current conversion efficiencies higher than 60% at 400 nm with a photocurrent of 1.43 mA/cm(2) under AM 1.5G illumination. Our solvothermal film growth technique offers an exciting opportunity for growth of one-dimensional metal oxide nanostructures with practical application in photoelectrochemical energy conversion.

Rapid Charge Transport in Dye-Sensitized Solar Cells Made from Vertically Aligned Single-Crystal Rutile TiO2 Nanowires†

A rapid solvothermal approach was used to synthesize aligned 1D single-crystal rutile TiO(2) nanowire (NW) arrays on transparent conducting substrates as electrodes for dye-sensitized solar cells. The NW arrays showed a more than 200 times faster charge transport and a factor four lower defect state density than conventional rutile nanoparticle films.

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

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.

Enhanced Harvesting of Red Photons in Nanowire Solar Cells: Evidence of Resonance Energy Transfer

Modern excitonic solar cells efficiently harvest photons in the 350-650 nm spectral range; however, device efficiencies are typically limited by poor quantum yields for red and near-infrared photons. Using Forster-type resonance energy transfer from zinc phthalocyanine donor molecules to ruthenium polypyridine complex acceptors, we demonstrate a four-fold increase in quantum yields for red photons in dye-sensitized nanowire array solar cells. The dissolved donor and surface anchored acceptor molecules are not tethered to each other, through either a direct chemical bond or a covalent linker layer. The spatial confinement of the electrolyte imposed by the wire-to-wire spacing of the close-packed nanowire array architecture ensures that the distances between a significant fraction of donors and acceptors are within a Förster radius. The critical distance for energy transfer from an isolated donor chromophore to a self-assembled monolayer of acceptors on a plane follows the inverse fourth power instead of the inverse sixth power relation. Consequently, we observe near quantitative energy transfer efficiencies in our devices. Our results represent a new design paradigm in excitonic solar cells and show it is possible to more closely match the spectral response of the device to the AM 1.5 solar spectrum through use of electronic energy transfer.

Synthesis and deposition of ultrafine Pt nanoparticles within high aspect ratio TiO2 nanotube arrays: application to the photocatalytic reduction of carbon dioxide

Using a rapid microwave-assisted solvothermal approach ultrafine Pt nanoparticles are synthesized and deposited in situ within high aspect ratio nanotube arrays. Adjusting the initial concentration of metal ion precursor inside the nanotube support controls the resulting Pt nanoparticle sizes. The Pt-nanoparticle/TiO2 nanotube composite is shown to greatly promote the photocatalytic conversion of carbon dioxide and water vapor into methane, a behavior attributed to the homogeneous distribution of metal co-catalyst nanoparticles over the TiO2 nanotube array surface providing a large number of active reduction sites. The novelty and flexibility of the technique, described herein, could prove useful for the deposition of metal, metal alloy, or metal oxide nanoparticles within a variety of nanotubular or nanoporous material systems with the resulting nanocomposites useful in catalysis, photocatalysis, photovoltaic, and photoelectrochemical applications.

Oriented Assembled TiO2 Hierarchical Nanowire Arrays with Fast Electron Transport Properties

Developing high surface area nanostructured electrodes with rapid charge transport is essential for artificial photosynthesis, solar cells, photocatalysis, and energy storage devices. Substantial research efforts have been recently focused on building one-dimensional (1D) nanoblocks with fast charge transport into three-dimensional (3D) hierarchical architectures. However, except for the enlargement in surface area, there is little experimental evidence of fast electron transport in these 3D nanostructure-based solar cells. In this communication, we report single-crystal-like 3D TiO2 branched nanowire arrays consisting of 1D branch epitaxially grown from the primary trunk. These 3D branched nanoarrays not only demonstrate 71% enlargement in large surface area (compared with 1D nanowire arrays) but also exhibit fast charge transport property (comparable to that in 1D single crystal nanoarrays), leading to 52% improvement in solar conversion efficiency. The orientated 3D assembly strategy reported here can be extended to assemble other metal oxides with one or multiple components and thus represents a critical avenue toward high-performance optoelectronics.

Fabrication of coaxial TiO2/Sb2S3 nanowire hybrids for efficient nanostructured organic–inorganic thin film photovoltaics

We report on low-cost, all solution fabrication of efficient air-stable nanostructured thin film photovoltaics comprised of n-type Sb(2)S(3) chemically deposited onto TiO(2) nanowire array films, forming coaxial Sb(2)S(3)/TiO(2) nanowire hybrids vertically oriented from the SnO(2):F coated glass substrate, which are then intercalated with poly(3-hexylthiophene) (P3HT) for hole transport and enhanced light absorption.

UV-Manipulated wettability between superhydrophobicity and superhydrophilicity on a transparent and conductive SnO2 nanorod film

A smart surface with wettability that can be switched between superhydrophobicity and superhydrophilicity has been realized on a transparent and conductive SnO2 nanorod film by the alternation of UV-irradiation and dark storage.