Novel Nanostructured Titania and Titania Nanocomposites for Photovoltaics and Photocatalysis

With the consumption of energy continually increasing around the world and the main source of this energy, fossil fuels, slowly being depleted, the need for alternate sources of energy is becoming more and more pertinent. Demand for solar energy has experienced exponential increase over the last decade. Nanostructured TiO2 has attracted significant attention due to its nontoxicity, low cost and wide applications in photovoltaics and photocatalysis. This research is focused on novel synthesis and surface modification of TiO2 nanotube arrays for applications in advanced dye-sensitized solar cells (DSSCs) and efficient photocatalysis. The first part of this work entails fast synthesis of bamboo-type TiO2 nanotube arrays with large surface area via anodization of Ti substrates for applications as photo-anodes in high-efficiency DSSCs. In addition, titania nanotubes are modified with other nanomaterials for further increased efficiency of DSSCs. For example, uniformly-sized Ag nanoparticles are deposited onto TiO2 nanotube array via pulse electrodeposition for plasmon effect, leading to enhanced light absorption in DSSCs. Also, reduced graphene oxide nanosheets are deposited onto a TiO2 nanotube array using electrophoretic deposition, for increased electronic conductivity and improved electron transport in DSSCs. Additionally, ultra-thin two dimensional TiO2 nanosheets are synthesized via exfoliation of layered protonated titanate into separate layers using bulky organic ions, for application as photo-anodes with enhanced light scattering and dye loading in high-efficiency DSSCs. The second part of the work concentrates on synthesis of Ag-modified bamboo-type TiO2 nanotube arrays for efficient photocatalysis. Such novel titania-based nanocomposite structure provides large surface area for organic pollutant absorption and subsequent degradation; the ordered structure of nanotube array also offers direct pathway for fast electron transport. Moreover, Ag nanoparticles deposited onto TiO2 nanotubes function as reservoirs for photogenerated electrons to improve charge separation and facilitate catalytic reactions

Enhancing high-rate and elevated-temperature performances of nano-sized and micron-sized LiMn2O4 in lithium-ion batteries with ultrathin surface coatings

LiMn2O4 suffers from severe capacity degradation when used as a cathode material in rechargeable lithium-ion batteries, especially when cycled at high rates and elevated temperatures. To enhance its high-rate electrochemical performance at elevated temperature (55 degrees C), we use atomic layer deposition (ALD) to deposit ultrathin and highly conformal Al2O3 coatings (as thin as 0.72 nm) onto micron-sized and nano-sized LiMn2O4 with precise thickness-control at atomic scale. All ALD-modified electrodes exhibit significantly improved capacities and cycling stability compared to bare electrodes. In particular, the effect of ALD coating to improve electrochemical performance of LiMn2O4 is more distinct for nano-sized LiMn2O4 than for micron-sized LiMn2O4, and more distinct for electrochemical cycling at higher charge/discharge rates. For example, nano-LiMn2O4 electrode coated with 6 Al2O3 ALD layers delivers higher initial capacity (124.7 mA h/g) and final capacity (106.7 mA h/g) after 100 cycles than bare electrode with an initial capacity of 112.3 mA h/g and a final capacity of only 95.5 mA h/g, when cycled at a very high rate of 5 C at 55 degrees C. In addition, nano-LiMn2O4 electrodes show much better rate performance than micron-LiMn2O4 electrodes at 5 C. The enhanced electrochemical performance of ALD-modified LiMn2O4 is ascribed to high-quality ALD oxide coatings that are highly conformal, dense, complete, and thus protect active material from severe dissolution, and to a formed robust glass layer on the surface of LiMn2O4 that suppress its crystallographic transformation during electrochemical cycling. Surface modifications of LiMn2O4 are also carried out by either ALD coating directly onto the entire LiMn2O4/carbon/PVDF composite electrode or coating only on LiMn2O4 particles in electrode. The former results in more significantly improved electrochemical performance of cathode, possibly because ALD coating onto entire electrode provides better mechanical integrity and preserves contact between LiMn2O4 particles and carbon/poly-vinylidenefluoride network

Improving energy relay dyes for dye-sensitized solar cells by use of a group of uniform materials based on organic salts (GUMBOS)

© 2016 The Royal Society of Chemistry. In this study, GUMBOS (a group of uniform materials based on organic salts) derived from rhodamine B chloride, 1,1′-diethyl-2,2′-carbocyanine iodide, 3,3′-diethylthiacarbocyanine iodide, and meso-tetra(4-carboxyphenyl)porphine were synthesized and characterized for application as energy relay dyes (ERDs) in dye-sensitized solar cells (DSSCs). A facile ion exchange reaction was employed for synthesis of GUMBOS. These GUMBOS exhibited improved characteristics in comparison to their respective parent dyes, including increased solubility, thermal stability, molar extinction coefficient, and fluorescence quantum yield. In addition, excellent spectral overlap integral and Förster resonance energy transfer efficiency were observed between various GUMBOS based-ERDs (donors) and the photosensitizing dye, N719 [di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(ii)] (acceptor). DSSC devices were fabricated and solar efficiency was evaluated in the absence and presence of ERDs using N719 as the photosensitizing dye. DSSCs in the presence of GUMBOS-based ERDs exhibited increased solar efficiencies in comparison to DSSCs in the absence of ERDs. Moreover, increases in solar efficiencies were found to be dependent on the counterions used in GUMBOS synthesis

Gate-Tunable Electron Injection Based Organic Light-Emitting Diodes for Low-Cost and Low-Voltage Active Matrix Displays

Low-cost and low-voltage active matrix displays were fabricated by simply patterning gate electrode arrays on a polymer electrolyte (PE)-coated polymer light-emitting diode (PLED). Structurally, a PE capacitor seamlessly stacked on a PLED by sharing a common Al:LiF composite electrode (PEC|PLED). This monolithic integrated organic optoelectronic device was characterized and interpreted as the tunable work function (surface potential) because of the perturbation of accumulated ions on Al:LiF composite electrode by PEC charging and discharging. The modulation of electron injection by the PEC resulted in increases in the electroluminescent brightness, from <100 cd m^–2 to >8000 cd m^–2, and the external quantum efficiency from <0.025% to 2.4%

Electrolyte-Gated Red, Green, and Blue Organic Light-Emitting Diodes

We report vertical electrolyte-gated red, green, and blue phosphorescent small-molecule organic light-emitting diodes (OLED), in which light emission was modified by tuning the electron injection via electrochemical doping of the electron injection layer 4,4-bis­(<i>N</i>-carbazolyl)-1,1-biphenyl (CBP) under the assistance of a polymer electrolyte. These devices comprise an electrolyte capacitor on the top of a conventional OLED, with the interfacial contact between the electrolyte and electron injection layer CBP of OLEDs achieved through a porous cathode. These phosphorescent OLEDs exhibit the tunable luminance between 0.1 and 10 000 cd m^–2, controlled by an applied bias at the gate electrode. This simple device architecture with gate-modulated luminance provides an innovative way for full-color OLED displays