Xinjian Feng

Bio: Xinjian Feng is an academic researcher from Chinese Academy of Sciences. The author has contributed to research in topics: Nanowire & Materials science. The author has an hindex of 26, co-authored 38 publications receiving 8872 citations. Previous affiliations of Xinjian Feng include Soochow University (Suzhou) & Northern Illinois University.

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

Abstract: Photocorrosion stable WO3 nanowire arrays are synthesized by a solvothermal technique on fluorine-doped tin oxide coated glass. WO3 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/cm2 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.

Super-Hydrophobic PDMS Surface with Ultra-Low Adhesive Force†

Abstract: Summary: Rough polydimethylsiloxane (PDMS) surface containing micro-, submicro- and nano-composite structures was fabricated using a facile one-step laser etching method. Such surface shows a super-hydrophobic character with contact angle higher than 160° and sliding angle lower than 5°, i.e. self-cleaning effect like lotus leaf. The wettabilities of the rough PDMS surfaces can be tunable by simply controlling the size of etched microstructures. The adhesive force between etched PDMS surface and water droplet is evaluated, and the structure effect is deduced by comparing it with those own a single nano- or micro-scale structures. This super-hydrophobic PDMS surface can be widely applied to many areas such as liquid transportation without loss, and micro-pump (creating pushing-force) needless micro-fluidic devices. Etched PDMS surface containing micro-, submicro-, and nano-composite structures shows a self-cleaning effect with water CA as high as 162° and SA lower than 5°.

Rapid charge transport in dye-sensitized solar cells made from vertically aligned single-crystal rutile TiO(2) nanowires.

Abstract: 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.

Abstract: 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

Solar Water Splitting Cells

Abstract: Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

TiO2 photocatalysis: Design and applications

Abstract: TiO 2 photocatalysis is widely used in a variety of applications and products in the environmental and energy fields, including self-cleaning surfaces, air and water purification systems, sterilization, hydrogen evolution, and photoelectrochemical conversion. The development of new materials, however, is strongly required to provide enhanced performances with respect to the photocatalytic properties and to find new uses for TiO 2 photocatalysis. In this review, recent developments in the area of TiO 2 photocatalysis research, in terms of new materials from a structural design perspective, have been summarized. The dimensionality associated with the structure of a TiO 2 material can affect its properties and functions, including its photocatalytic performance, and also more specifically its surface area, adsorption, reflectance, adhesion, and carrier transportation properties. We provide a brief introduction to the current situation in TiO 2 photocatalysis, and describe structurally controlled TiO 2 photocatalysts which can be classified into zero-, one-, two-, and three-dimensional structures. Furthermore, novel applications of TiO 2 surfaces for the fabrication of wettability patterns and for printing are discussed.

Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting

Abstract: We report the first demonstration of hydrogen treatment as a simple and effective strategy to fundamentally improve the performance of TiO2 nanowires for photoelectrochemical (PEC) water splitting. Hydrogen-treated rutile TiO2 (H:TiO2) nanowires were prepared by annealing the pristine TiO2 nanowires in hydrogen atmosphere at various temperatures in a range of 200–550 °C. In comparison to pristine TiO2 nanowires, H:TiO2 samples show substantially enhanced photocurrent in the entire potential window. More importantly, H:TiO2 samples have exceptionally low photocurrent saturation potentials of −0.6 V vs Ag/AgCl (0.4 V vs RHE), indicating very efficient charge separation and transportation. The optimized H:TiO2 nanowire sample yields a photocurrent density of ∼1.97 mA/cm2 at −0.6 V vs Ag/AgCl, in 1 M NaOH solution under the illumination of simulated solar light (100 mW/cm2 from 150 W xenon lamp coupled with an AM 1.5G filter). This photocurrent density corresponds to a solar-to-hydrogen (STH) efficiency of ∼1...

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

Abstract: Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.