Pil J. Yoo

Green Synthesis of Biphasic TiO2–Reduced Graphene Oxide Nanocomposites with Highly Enhanced Photocatalytic Activity

A series of TiO(2)-reduced graphene oxide (RGO) nanocomposites were prepared by simple one-step hydrothermal reactions using the titania precursor, TiCl(4) and graphene oxide (GO) without reducing agents. Hydrolysis of TiCl(4) and mild reduction of GO were simultaneously carried out under hydrothermal conditions. While conventional approaches mostly utilize multistep chemical methods wherein strong reducing agents, such as hydrazine, hydroquinone, and sodium borohydride are employed, our method provides the notable advantages of a single step reaction without employing toxic solvents or reducing agents, thereby providing a novel green synthetic route to produce the nanocomposites of RGO and TiO(2). The as-synthesized nanocomposites were characterized by several crystallographic, microscopic, and spectroscopic characterization methods, which enabled confrimation of the robustness of the suggested reaction scheme. Notably, X-ray diffraction and transmission electron micrograph proved that TiO(2) contained both anatase and rutile phases. In addition, the photocatalytic activities of the synthesized composites were measured for the degradation of rhodamine B dye. The catalyst also can degrade a colorless dye such as benzoic acid under visible light. The synthesized nanocomposites of biphasic TiO(2) with RGO showed enhanced catalytic activity compared to conventional TiO(2) photocatalyst, P25. The photocatalytic activity is strongly affected by the concentration of RGO in the nanocomposites, with the best photocatalytic activity observed for the composite of 2.0 wt % RGO. Since the synthesized biphasic TiO(2)-RGO nanocomposites have been shown to effectively reduce the electron-hole recombination rate, it is anticipated that they will be utilized as anode materials in lithium ion batteries.

Fabrication of Graphene Thin Films Based on Layer-by-Layer Self-Assembly of Functionalized Graphene Nanosheets

In this study, we present a facile means of fabricating graphene thin films via layer-by-layer (LbL) assembly of charged graphene nanosheets (GS) based on electrostatic interactions. To this end, graphite oxide (GO) obtained from graphite powder using Hummers method is chemically reduced to carboxylic acid-functionalized GS and amine-functionalized GS to perform an alternate LbL deposition between oppositely charged GSs. Specifically, for successful preparation of positively charged GS, GOs are treated with an intermediate acyl-chlorination reaction by thionyl chloride and a subsequent amidation reaction in pyridine, whereby a stable GO dispersibility can be maintained within the polar reaction solvent. As a result, without the aid of additional hybridization with charged nanomaterials or polyelectrolytes, the oppositely charged graphene nanosheets can be electrostatically assembled to form graphene thin films in an aqueous environment, while obtaining controllability over film thickness and transparency. Finally, the electrical property of the assembled graphene thin films can be enhanced through a thermal treatment process. Notably, the introduction of chloride functions during the acyl-chlorination reaction provides the p-doping effect for the assembled graphene thin films, yielding a sheet resistance of 1.4 kΩ/sq with a light transmittance of 80% after thermal treatment. Since the proposed method allows for large-scale production as well as elaborate manipulation of the physical properties of the graphene thin films, it can be potentially utilized in various applications, such as transparent electrodes, flexible displays and highly sensitive biosensors.

Single-step solvothermal synthesis of mesoporous Ag–TiO2–reduced graphene oxide ternary composites with enhanced photocatalytic activity

With growing interest in the photocatalytic performance of TiO2-graphene composite systems, the ternary phase of TiO2, graphene, and Ag is expected to exhibit improved photocatalytic characteristics because of the improved recombination rate of photogenerated charge carriers and potential contribution of the generation of localized surface plasmon resonance at Ag sites on a surface of the TiO2-graphene binary matrix. In this work, Ag-TiO2-reduced graphene oxide ternary nanocomposites were successfully synthesized by a simple solvothermal process. In a single-step synthetic procedure, the reduction of AgNO3 and graphene oxide and the hydrolysis of titanium tetraisopropoxide were spontaneously performed in a mixed solvent system of ethylene glycol, N,N-dimethylformamide and a stoichiometric amount of water without resorting to the use of typical reducing agents. The nanocomposites were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, along with different microscopic and spectroscopic techniques, enabling us to confirm the successful reduction of AgNO3 and graphite oxide to metallic Ag and reduced graphene oxide, respectively. Due to the highly facilitated electron transport of well distributed Ag nanoparticles, the synthesized ternary nanocomposite showed enhanced photocatalytic activity for degradation of rhodamine B dye under visible light irradiation.

Stamped microbattery electrodes based on self-assembled M13 viruses.

The fabrication and spatial positioning of electrodes are becoming central issues in battery technology because of emerging needs for small scale power sources, including those embedded in flexible substrates and textiles. More generally, novel electrode positioning methods could enable the use of nanostructured electrodes and multidimensional architectures in new battery designs having improved electrochemical performance. Here, we demonstrate the synergistic use of biological and nonbiological assembly methods for fabricating and positioning small battery components that may enable high performance microbatteries with complex architectures. A self-assembled layer of virus-templated cobalt oxide nanowires serving as the active anode material in the battery anode was formed on top of microscale islands of polyelectrolyte multilayers serving as the battery electrolyte, and this assembly was stamped onto platinum microband current collectors. The resulting electrode arrays exhibit full electrochemical functionality. This versatile approach for fabricating and positioning electrodes may provide greater flexibility for implementing advanced battery designs such as those with interdigitated microelectrodes or 3D architectures.

Rutile TiO2-based perovskite solar cells

A perovskite solar cell based on rutile TiO2 film was prepared and its photovoltaic performance was compared to an anatase TiO2-based perovskite solar cell. Rutile TiO2 nanoparticles with aspect ratio of 0.2 (20 nm wide and 100 nm long) were prepared by hydrolysis of TiCl4 at ambient temperature. Anatase TiO2 nanoparticles with diameter of about 50 nm were hydrothermally synthesized. The annealed rutile film showed porosity of 60.6%, while lower porosity of 49.1% was detected in the anatase TiO2 film. CH3NH3PbI3 perovskite was deposited on TiO2 film using either a one-step spin coating or two-step dipping method. 2,2′,7,7′-Tetrakis(N,N-p-dimethoxy-phenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) was used as a hole transporting material. One-step deposition led to average power conversion efficiency (PCE) of 8.19% from the rutile-perovskite solar cells and 7.23% from the anatase-perovskite solar cells, while two-step deposition resulted in higher average PCE of 13.75% for the former device and 13.99% for the latter one. Regardless of the deposition methodologies, the rutile–perovskite solar cell showed generally higher Jsc and lower Voc. Slower electron transport and longer electron lifetime were observed for the rutile-based perovskite solar cell than for the anatase-based one. Although the same perovskite material was used for both rutile and anatase TiO2, the difference in electronic behavior indicates that photo-excited electrons are in part injected to TiO2 and the extent of electron injection can be influenced by the crystal phase of TiO2. Despite longer electron lifetime, the slightly lower voltage of the rutile-based device might be due to the fact that the amount of injected electrons was relatively larger for rutile than anatase, leading to a lower Fermi energy level at equilibrium between TiO2 and perovskite. Using a 260 nm-thick rutile TiO2 film, the highest PCE of 14.46% was achieved by depositing CH3NH3PbI3 using a two-step method, in which photocurrent density of 20.02 mA cm−2, open-circuit voltage of 1.022 V and fill factor of 0.71 were demonstrated.

Controlling Surface Mobility in Interdiffusing Polyelectrolyte Multilayers

The phenomenon of interdiffusion of polyelectrolytes during electrostatic layer-by-layer assembly has been extensively investigated in the past few years owing to the intriguing scientific questions that it poses and the technological impact of interdiffusion on the promising area of electrostatic assembly processes. In particular, interdiffusion can greatly affect the final morphology and structure of the desired thin films, including the efficacy and function of thin film devices created using these techniques. Although there have been several studies on the mechanism of film growth, little is known about the origin and controlling factors of interdiffusion phenomena. Here, we demonstrate a simple but robust method of observing the process of polyelectrolyte interdiffusion by adsorbing charged viruses onto the surface of polyelectrolyte multilayers. The surface mobility of the underlying polycation enables the close-packing of viruses adsorbed electrostatically to the film so as to achieve a highly packed structure. The ordering of viruses can be controlled by the manipulation of the deposition pH of the underlying polyelectrolyte multilayers, which ultimately controls the thickness of each layer, effective ionic cross-link density of the film, and the surface charge density of the top surface. Characterization of the films assembled at different pH values were carried out to confirm that increased quantities of the mobile polycation LPEI incorporated at higher pH adsorption conditions are responsible for the ordered assembly of viruses. The surface mobility of viruses atop the underlying polyelectrolyte multilayers was determined using fluorescence recovery after photobleaching technique, which leads to estimate of the diffusion coefficient on the order of 0.1 microm(2)/sec for FITC-labeled viruses assembled on polyelectrolyte multilayers.

Dramatically Enhanced Mechanosensitivity and Signal-to-Noise Ratio of Nanoscale Crack-Based Sensors: Effect of Crack Depth

The sensitivity of a nanoscale crack-based sensor is enhanced markedly by modulating the crack depth. The crack-depth-propagated sensor exhibits ≈16 000 gauge factor at 2% strain and a superior signal-to-noise ratio of ≈35, which facilitates detection of target signals for voice-pattern recognition.

Solvent-Assisted Patterning of Polyelectrolyte Multilayers and Selective Deposition of Virus Assemblies

We introduce a simple method to pattern electrostatic assemblies of viruses onto a polyelectrolyte multilayer. The increased mobility of weak polycation chains in the multilayer above a given thickness ensures the surface mobility of viruses required for spontaneous ordering of densely packed viruses atop polymeric patterns. To pattern the polyelectrolyte multilayer film, we employ a nonconventional patterning method known as solvent-assisted capillary molding for the first time on multilayer films, and demonstrate micrometer-scaled dense patterns of viruses, where the accessible feature size can be correlated by the length scale of virus and swelling property of underlying patterned polyelectrolyte multilayer. We further examine the ability to modify the top surfaces of these assemblies with biological ligands, which extends the applicability of patterned viruses to biological detection purposes. We expect that the present method described here can be generally applied to the patterning of other polyelectrolyte multilayers and combined with the ordered assembly of anisotropic nanomaterials such as polymeric nanotubes or inorganic nanowires for a broad range of applications.

Complex Pattern Formation by Adhesion-Controlled Anisotropic Wrinkling

We report complex pattern formation and shape control in the confinement-induced wrinkling that occurs when a poly(dimethylsiloxane) (PDMS) mold is placed on a bilayer of metal and polymer and then heated. Various complex structures that are different from the mold pattern form through the self-organization of wrinkles. These complex structures could be inverted in shape by manipulating the work of adhesion at the interface between the mold and the metal surface. Convex wrinkles result when the work of adhesion is relatively large. However, inverted concave wrinkles emerge when it is relatively small. The ratio of the mold period to the intrinsic wrinkling wavelength is another factor that determines the shape. The ability to tailor the shape of a surface is expected to have a broad range of applications in electro-optics and microfluidics.

Solvent-driven dewetting and rim instability

An experimental method suitable for reproducible results has been used to investigate dewetting behavior of thin films of solvent-laden polymer. This solvent-driven dewetting enables one to change spreading coefficient by an order of magnitude that is not readily realizable in thermal dewetting and to study polar interactions that have not been fully exploited experimentally. While the film instability is similar to that found in thermal dewetting, the rim instability is quite different. Two different types of the rim instability have been found. With a polar solvent, the rim instability changes from one type to another with increasing film thickness whereas the unstable rim becomes stable for an apolar solvent.