Tammy P. Chou

Aggregation of ZnO Nanocrystallites for High Conversion Efficiency in Dye‐Sensitized Solar Cells

As a relatively new class of photovoltaic devices with a photoelectrochemical system consisting of a dye-sensitized semiconductor film and an electrolyte, dye-sensitized solar cells (DSSCs) have been regarded as a promising alternative to conventional solid-state semiconductor solar cells. They are relatively cost-effective, are easy to manufacture, and can be readily shaped with flexible substrates to satisfy the demands of various applications. Avery important feature of DSSCs is the photoelectrode, which includes mesoporous wide-bandgap oxide semiconductor films with an enormous internal surface area, typically a thousand times larger than that of bulk films. To date, the highest solar-to-electric conversion efficiency of over 11% has been achieved with films that consist of 20-nm TiO2 nanocrystallites sensitized by ruthenium-based dyes. However, further improving the energy conversion efficiency of DSSCs remains a challenge. Competition between the generation and recombination of photoexcited carriers in DSSCs is a main bottleneck for developing higher conversion efficiency. One possible solution is to use one-dimensional nanostructures that are able to provide a direct pathway for the rapid collection of photogenerated electrons and, therefore, reduce the degree of charge recombination. However, such one-dimensional nanostructures seem to have insufficient internal surface area, which limits their energy conversion efficiency at a relatively low level, for example, 1.5% for ZnO nanowires and 4.7% for TiO2 nanotubes. [9] Another way to increase efficiency is to increase the light-harvesting capability of the photoelectrode film by utilizing optical enhancement effects, which can be achieved by means of light scattering by introducing scatterers into the photoelectrode film. Usami, Ferber and Luther, and Rothenberger et al. have demonstrated theoretically that optical absorption by TiO2 nanocrystalline films can be promoted by additionally admixing large TiO2 particles in an optimal volume ratio. This idea was verified experimentally when TiO2 nanocrystalline films were combined with large SiO2, Al2O3, or TiO2 particles. [14–17] By coupling a photonic crystal layer to conventional TiO2 nanocrystalline films as the light scatterer, Nishimura et al. and Halaoui et al. also succeeded in enhancing the light-harvesting capability of solar-cell photoelectrodes. However, the drawback is that the introduction of larger particles into nanocrystalline films will unavoidably lower the internal surface area of the photoelectrode film and, therefore, counteract the enhancement effect of light scattering on the optical absorption, whereas the incorporation of a layer of TiO2 photonic crystal may lead to an undesirable increase in the electron diffusion length and, consequently, increase the recombination rate of photogenerated carriers. Herein we report hierarchically structured ZnO films as the photoelectrodes in DSSCs for the enhancement of energy conversion efficiency. The films are comprised of polydisperse ZnO aggregates consisting of nanosized crystallites. The aggregates are submicrometer-sized and, thus, can function as efficient light scatterers, while the nanocrystallites provide the films with the necessary mesoporous structure and large internal surface area. An overall energy conversion efficiency up to 5.4% has been achieved from the film including polydisperse ZnO aggregates, much higher than 1.5–2.4% for ZnO nanocrystalline films, 0.5–1.5% for ZnO nanowire films, 23] and 2.7–3.5% for uniform ZnO aggregate films. Polydisperse ZnO aggregates were synthesized by the hydrolysis of zinc salt in polyol medium at 160 8C, similar to the method reported by Jezequel et al. Rapid heating at a rate of 10 8Cmin 1 was intentionally used to obtain polydisperse aggregates, that is, with a relatively wide size distribution. The resulting colloidal dispersion was drop-cast onto a fluorine-doped tin oxide (FTO) coated glass substrate to form a film of approximately 9 mm in thickness, and the film was subsequently annealed at 350 8C for 1 h in air to remove residual solvents and any organic compounds as well as to improve the contact between the film and the substrate and the connection between the nanocrystallites and between the aggregates. Figure 1 shows the scanning electron microscopy (SEM) images of ZnO film with polydisperse aggregates and a schematic illustration showing the structure of an aggregate. Figure 1a indicates that the film is well stacked with submicrometer-sized ZnO aggregates. Figure 1b presents the highly disordered structure of the film assembled by polydisperse ZnO aggregates with diameters ranging from several tens to several hundreds of nanometers. Figure 1c is a magnified SEM image of an individual ZnO aggregate, revealing that the ZnO aggregate is nearly spherical in [*] Dr. Q. F. Zhang, Dr. T. P. Chou, B. Russo, Prof. G. Z. Cao Department of Materials Science and Engineering University of Washington Seattle, WA 98195 (USA) Fax: (+1)206-543-3100 E-mail: gzcao@u.washington.edu Homepage: http://depts.washington.edu/solgel/

Template‐Based Growth of Various Oxide Nanorods by Sol–Gel Electrophoresis

The ability to form oxide nanorods is of great interest in a number of areas. In this paper, we report the template-based growth of nanorods of several oxide ceramics, formed by means of a combination of sol–gel processing and electrophoretic deposition. Both single metal oxides (TiO2, SiO2) and complex oxides (BaTiO3, Sr2Nb2O7, and Pb(Zr0.52Ti0.48)O3) have been grown by this method. Uniformly sized nanorods of about 125–200 nm in diameter and 10 μm in length were grown over large areas with near unidirectional alignment. Desired stoichiometric chemical composition and crystal structure of the oxide nanorods was readily achieved by an appropriate procedure of sol preparation, with a heat treatment (700 °C for 15 min) for crystallization and densification.

Organic–inorganic hybrid coatings for corrosion protection

Abstract The corrosion resistance of sol–gel derived, organic–inorganic, silica-based hybrid coatings with various amounts of organic content was studied. Hybrid sols were prepared by copolymerizing tetraethylorthosilicate (TEOS) and 3-methacryloxypropyltrimethoxysilane (MPS) with a two-step acid-catalyst process. Hybrid coatings were dip-coated on 304 stainless steel substrates and annealed at 300 °C for 30 min. Such prepared hybrid coatings were found to be relatively dense, uniform and defect free. The adhesion and flexibility of the coatings were characterized. The influences of the amount of organic component incorporated into the coatings and the aging of sols on corrosion protection were studied. Electrochemical analyses showed that the relatively dense hybrid coatings provided excellent corrosion protection by forming a physical barrier, which effectively separated the anode from the cathode. Some preliminary biocompatibility tests were also conducted on the hybrid coatings.

Electrophoretic Growth of Lead Zirconate Titanate Nanorods

rise to mesoporous nanoparticles with radial channels. The uniform fragment length within the interior of the nanoparticles can be explained by the radial nature of the deformation field around each defect, or site-specific breakage of the 300 nm long native TMV rods. Indeed, chemical degradation of TMV does produce a stable fragment about 50 nm in length, which is similar in size to those encapsulated in the silica shell of the nanoparticles. However, we observed only a random distribution of particle lengths in control samples containing broken TMV tubes. Further work is in progress to confirm the model. In conclusion, nematic liquid crystals of TMV can be used to prepare silica mesostructures and nanoparticles with parallel or radial arrays of linear channels, respectively. The mesostructures are produced as micrometer-size inverse replicas of the nematic phase, and have a periodicity of approximately 20 nm, which is larger than that generally attainable by current methods. In contrast, the nanoparticles are less than 150 nm in size and consist of a dense silica core surrounded by an unusual radial array of mineralized TMV fragments, 50 nm in length. The channeled nanoparticles are produced at lower reactant concentrations and appear to originate from topological defects associated with the deformation and fracturing of silica±TMV clusters as the liquid crystalline state is re-established in the reaction mixture. The general stability of TMV liquid crystals suggests that it should be possible to use a similar approach to prepare a wide range of inorganic oxides, semiconductors and metal-based mesophases and nanoparticles with mesostructured interiors.

Doping effect in layer structured SrBi2Nb2O9 ferroelectrics

This article reports a systematic study of doping effects on the crystal structure, microstructure, dielectric, and electrical properties of layer-structured strontium bismuth niobate, SrBi2Nb2O9 (SBN), ferroelectrics. Substitution in both the A site (Sr2+ by Ca2+ and Ba2+) and B site (Nb5+ by V5+) up to 30 at % were studied. It was found that crystal lattice constant, dielectric, and electrical properties of SBN ferroelectrics varied appreciably with the type and amount of dopants. The relationships among the ionic radii, structural constraint imposed by [Bi2O2]2+ interlayers, and properties were discussed.

Sol-gel-derived hybrid coatings for corrosion protection

The corrosion resistance of sol-gel-derived, organic-inorganic, silica-based hybrid coatings was studied. Hybrid sols were prepared by copolymerizing tetraethylorthosilicate (TEOS) and 3-methacryloxypropyltrimethoxysilane (MPS) with a two-step acid-catalyst process. Hybrid coatings were dip-coated on 304 and 316 stainless steel substrates and annealed at 300°C for 30 minutes. The adhesion, flexibility, and biocompatibility of the coatings were examined. Hybrid coatings were found to be relatively dense, uniform and defect free. Electrochemical analyses showed that the coatings provided excellent corrosion protection by forming a physical barrier, which effectively separated the anode from the cathode. In addition, further experimental results revealed that the corrosion patterns are strongly dependent on the nature of the stainless steel substrates. Some possible mechanisms for corrosion breakdown associated with each type of substrate are also introduced.

Oxygen-vacancy-related dielectric relaxation in SrBi2Ta1.8V0.2O9 ferroelectrics

The strontium bismuth tantalate vanadate, SrBi2Ta1.8V0.2O9, (SBTV) layered perovskite ferroelectrics were made by solid state powder sintering. It was found that the SBTV ferroelectrics had the same crystal structure as that of strontium bismuth tantalate, SrBi2Ta2O9 (SBT), but an increased paraferroelectric transition temperature at ∼360 °C as compared to 305 °C for SBT. In addition, SBTV ferroelectrics showed a frequency dispersion at low frequencies and broadened dielectric peaks at the paraferroelectric transition temperature that shifted to a higher temperature with a reduced frequency. However, after a postsintering annealing at 850 °C in air for 60 h, SBTV ferroelectrics showed reduced dielectric constants and tangent loss, particularly at high temperatures. In addition, no frequency dependence of paraferroelectric transition was found in the annealed SBTV ferroelectrics. Furthermore, there was a significant reduction in dc conductivity with annealing. The prior results implied that the dielectric ...

Patterned Microstructure of Sol–Gel Derived Complex Oxides Using Soft Lithography

While the semiconductor industry continues to push the limits of small-scale devices, the cost of the equipment needed to achieve such small geometries has also kept similar pace. The average research lab does not have the funds to acquire, nor maintain the photolithographic and etching equipment needed to create such patterned structures. This unavailability of resources effectively prevents many groups from being able to perform research in cutting edge fields, such as microelectromechanical system (MEMS) technology, which require the ability to pattern a wide range of materials on a very small scale. Recently however, much attention has been given to a collection of non-photolithographic patterning techniques collectively known as soft lithography, which have the potential of becoming versatile and low cost methods for creating micrometer and sub-micrometer sized structures. To date, several devices have been fabricated using soft lithographic techniques, such as polymeric field effect transistors (FETs), electro-optic devices, Schottky diodes, and silicon metal oxide FETs (MOSFETs), to name a few. In addition to cost benefits, soft lithography has many advantages over traditional photolithography. Photolithography is very sensitive to surface topography. If the substrate to be patterned is not extremely flat, then acceptable results can not be obtained. In contrast, since an elastomeric mold is used in soft lithography, good conformity over curved surfaces is possible, and thus non-planar substrates can be patterned with ease. Material selection is another limitation of photolithography. Only photosensitive materials (such as photo-resists) can be directly patterned, and any other material that needs to be patterned must be susceptible to some type of etching technique. These restrictions seriously limit the arsenal of materials that can be used. On the other hand, with soft lithography, any material that can be derived from liquid precursor can be patterned, provided that the solvent used does not swell the elastomeric mold. Another benefit of this method is that it is an inherently mild process. As a result, many chemically and physically sensitive materials such as dyes and biomolecules can be patterned using this technique, showing again the versatility of this process. As a result of the vast potential of these techniques, many reports describing the use of soft lithography to create patterned structures can be found in the open literature. However, most of these experiments were focused on relatively simple, single component oxide systems or polymeric materials. No work on the direct patterning of complex oxide materials has been reported in the literature. However, these materials possess many important physical properties such as ferroelectricity, piezoelectricty, pyroelectricity, and high Tc superconductivity, which make complex oxides very useful for industrial and modern technological applications. For example, piezoelectric materials play a critical role in MEMS. As such, it would be very beneficial to develop a convenient and low cost method of patterning these materials. This report describes our preliminary work on patterning complex oxide ceramics using soft lithography in conjunction with sol±gel processing. Pb(ZrTi)O3 (PZT) and strontium niobate (Sr2Nb2O7), both of which are piezoelectric ceramics, were chosen as model systems to form patterned structures on silicon substrates using soft lithography. Specifically, the microstructures were patterned by micro-molding in capillaries, or MIMIC molding. In the preparation of the Sr2Nb2O7 sol, the inorganic precursors used were strontium nitrate Sr(NO3)2 (99 %) and niobium pentachloride (NbCl5) (99.8 %). The procedure implemented is outlined in the flowchart, Figure 1, which differs from other reported methods. Using ethylene glycol as a cross-linking agent and ethanol as a solvent, a transparent, stable sol was obtained. Although only 0.018 mol of water was added, hydrated citric acid was used, which provided the extra water needed to achieve the desired molar ratio of Sr/

Adhesion of Sol-Gel-Derived Organic-Inorganic Hybrid Coatings on Polyester

Inorganic coatings, including metal-oxide coatings, provide polymer surfaces with excellent abrasion and wear resistance, and protection against environmental degradation. However, one drawback associated with the incorporation of such ceramic coatings to polymeric materials is the adhesion characteristic at the ceramic-polymer interface. In this paper, two strategies for adhesion enhancement of ceramic coatings on polymer substrates were proposed: (1) formation of chemical bonds through surface condensation reactions, and (2) development of interlocked ceramic and polymeric networks through diffusion of alkoxide precursors. The current research has focused on the adhesion of sol-gel-derived organic-inorganic hybrid coatings on polyester by forming chemical bonds between the polymer substrate and the hybrid coatings, as well as developing interlocked polymeric and inorganic networks at the interface. Contact angle, wettability tests, and chemicalanalysis were done to verify the effectiveness of the adhesion of organic-inorganic hybrid coatings on polyester substrates. In addition, dry and wet thermal cycling tests were done to analyze the adhesion behavior of the hybrid coatings on polyester, followed by microscopy examination. It was found that although both approaches resulted in excellent adhesion of hybrid coatings on polyester, adhesion with interlocked ceramic and polymeric networks was far better than that with chemical bonds in the presence of water at elevated temperatures.

Organic-inorganic sol-gel coating for corrosion protection of stainless steel

One of the most effective corrosion control techniques is the electrical isolation of the anode from the cathode [1, 2]. The chromium oxide (Cr2O3) passivation layer formed on the surface of stainless steel in oxidizing environments is one example. This is the main reason for the durability and corrosion resistance behavior of this particular metal [2, 3]. A more generic approach to enhance corrosion resistance is to apply protective films or coatings. Through the modification of chemical composition of the coatings, such protective coatings can also permit the introduction of other desired chemical and physical properties, such as mechanical strength and hydrophobicity. Various organic coatings have been studied for corrosion protection [4–6]. Specifically, various oxide coatings by sol-gel processing have been studied extensively for corrosion protection of stainless steel [9–13]. In spite of all the advantages of sol-gel processing, sol-gel oxide coatings suffer from several drawbacks. In general, sol-gel coatings are highly porous with low mechanical integrity; annealing or sintering at high temperatures (>800 ◦C) is required to achieve a dense microstructure [14–17]. Consequently, sintering at high temperatures might introduce cracks and/or delamination of sol-gel coatings due to a large mismatch of thermal expansion coefficients and possible chemical reactions at the interface. Sintering at high temperatures also limits application of sol-gel coatings on temperature sensitive substrates and devices. One viable approach to dense, sol-gel-derived coatings without post-deposition annealing at elevated temperatures is to synthesize organic-inorganic hybrid coatings. When appropriate chemical composition and processing conditions are applied, relatively dense organic-inorganic hybrid coatings can be developed for applications, including wear resistance [18, 19] and corrosion protection [20–22]. Messaddeq et al. [21] studied corrosion resistance of organic-inorganic hybrid coatings on stainless steel. The coatings were made by dispersing various amounts of polymethylmethacrylate (PMMA) into zirconia (ZrO2) sol and fired at 200 ◦C for 30 min. PMMA-ZrO2 coatings demonstrated promising corrosion resistance and increased the lifetime of the stainless steel by a factor 30 [21]. However, phase segregation, incomplete coverage, and delamination were observed when the coatings consisted of a high content of organic components. In this paper, we studied the corrosion resistance of sol-gel-derived, organic-inorganic hybrid single-layer coatings on two types of stainless steel. Sol-gel-derived coatings were made from tetraethylorthosilicate (TEOS) and 3-methacryloxypropyltrimethoxysilane (MPS) using a two-step acid catalysis process, and were annealed at 300 ◦C for 30 min. It was demonstrated that sol-gel derived hybrid coatings could significantly enhance the corrosion protection of both 304 and 316 stainless steel substrates. Furthermore, the corrosion resistance behavior of the hybrid coatings on both types of stainless steel was compared and possible mechanisms were discussed. The silica-based organic-inorganic hybrid sol was prepared with an acid-catalyzed, two-step hydrolysiscondensation process. The hybrid sol was prepared by admixing a silica precursor, tetraethylorthosilicate (TEOS, Si(OC2H5)4), and an organic component, 3-methacryloxypropyltrimethoxysilane (MPS, H2CC (CH3)CO2(CH2)3Si(OCH3)3), to control the flexibility and density of the sol-gel network. Silica (SiO2) sol containing 10 mol% MPS with a TEOS : MPS ratio of 90 : 10 was used for analysis. An initial stock solution was made by adding amounts of TEOS and MPS in a mixture of ethanol (C2H5OH), deionized water (DI H2O), and 1N hydrochloric acid (HCl), resulting in a TEOS : MPS : C2H5 : DI-H2O : HCl nominal molar ratio of 0.90 : 0.10 : 3.8 : 5 : 4.8 × 10−3. The mixture was vigorously stirred at a rate of 500 RPM for 90 min at a temperature of 60 ◦C, and further processing of the sol required an additional 3.6 mL 1N HCl and 1.2 mL DI H2O to 30 mL of the stock solution. The sol was stirred again at a rate of 500 RPM for 60 min at a temperature of 60 ◦C. Ethanol was added to dilute the sol in order to obtain a volume ratio of 2 : 1 ethanol to solution. The substrates (10 mm × 40 mm in dimension) used for the analysis of the sol-gel coatings were 304 and 316 stainless steel that had been electropolished. The exposure of the substrates to nitric acid (HNO3) decreased the iron content and increased the chromium content