Photoconductive Interlocked Molecules and Macromolecules

Organic compounds and materials with photoconductive properties have been studied for many years because of their importance in many technological applications such as dye-sensitized solar cells, photodiodes, photoresistors, electronics, biomolecular sensing, etc. For multiple purposes, such molecules require intense protection from various factors which can decrease their durability and cause fatigue. Interlocked molecules and macromolecules involving photoconductive organic components and various types of macrocycles, such as cyclodextrins, cyclophanes, or macrocyclic ethers, are promising candidates for new photoconductivity-related applications. In this chapter, a review in this emerging research area in materials science and technology is provided. Focus is placed on photoconductive (poly)rotaxanes and (poly)catenanes. Various types of such materials and compounds are reviewed, and recent examples are provided. The relation between their structure and photoconductive behavior is discussed

Electrochemical generation-collection sensors for bioanalysis

Electrochemical generation-collection devices have potential for application in a wide range of analytical devices, both as free standing sensor systems and as components in analytical systems such as high performance liquid chromatography (HPLC) or flow injection analysis (FIA). The underlying principle is that an electrochemical reaction on one element is used to generate some chemical species which is then transported by diffusion or diffusion and convection to a second electrode where the electrochemical reaction is reversed. The fraction collected is a function of device geometry, operating conditions, any reaction of the generated species in bulk solution and mass transport. This thesis describes the construction of generation-collection sensors comprising coplanar inlaid microelectrodes of different size and shape: ring-disc and disc-disc in sizes ranging from 10 m to 100 m. Fabrication techniques including sputter-coating and embedding of wires were developed and are described. The sensors were characterised using scanning electron microscopy and their electrochemical characterisation was achieved using reversible outer sphere redox couples including ferrocene derivatives, ruthenium hexaammine (III/II) and ferro/ferricyanide. Experimental data are given for the effects of geometry, size and generator current density, particularly where this would affect the uniformity of the flux. Tests were also undertaken in specially built flow cells to assess their viability for application in FIA and LC detection. Where possible, performance in both the steady state and transient mode were compared with numerical and analytical models. Three specific bioanalytical applications were investigated and are described: (i) detection of peptides and amino acids using electrochemically generated bromine/ hypobromite (ii) electrochemical biuret detection and (iii) determination of titratable acidity and alkalinity (buffer capacity). Results for (i) showed more complex behaviour with the Pt-Pt dual microelectrode than that has been reported for macroelectrodes under similar system. Voltammetric results are reported. Electrochemical biuret systems showed less improvement in sensitivity with the generation-collection mode to small peptide molecules. For the buffer capacity sensor, both disc-disc and ring-disc geometries were investigated in a range of sizes 25 to 75 m for the disc and approximately 1 m for the ring. A pH sensitive surface was prepared by the deposition of hydrated iridium oxide on gold. Local pH changes were effect by electrolysis of the buffer solution. Experimental works emphasised the importance of controlling the current density to ensure that buffer capacity was not exceeded. An analytical model was of great use in optimising the device. Results are reported for various buffer systems comparing experiments with steady-state theory.Open Acces

Pyridine assisted CO₂ reduction to methanol at high pressure

Significant research efforts have been directed towards exploring electrocatalysts for the selective reduction of CO₂ to fuels such as methanol. Bocarsly et al (Princeton University) have recently reported the use of aromatic amines (e.g. pyridine (C₅H₅N)) as electrocatalysts in aqueous electrolytes for the reduction of CO₂ at low overpotentials (50-150 mV). Importantly, the CO₂-pyridine reduction process was claimed to selectively produce methanol with Faradaic efficiencies of ~100% on p-GaP electrode and 22-30% on Pt and Pd electrodes. Moreover, the initially proposed mechanism based on a radical intermediate interaction with CO₂ as a key step toward the production of methanol was subsequently disproved. In this project, methanol formation by the CO₂-pyridine (C₅H₅N) system was assessed by conducting electrolysis under various conditions at platinum electrodes. High pressure CO₂ was used with the aim of increasing the methanol yield. In the course of the present study, the bulk electrolysis confirmed the methanol production at 1 bar and at 55bar of CO₂ in the presence of pyridine. However, the methanol yield was found to be persistently limited to sub-ppm level (<1ppm) under all conditions investigated. The observed methanol yield limitation could not be overcome by the electrode reactivation techniques used. Moreover, the methanol formation seemed unaffected by the current density or the biasing mode. This was an indication of the independence of methanol production from the charge transfer on the electrode. In agreement with these observations, analysis of the voltammetric data supported by simulation revealed that the CO₂-pyridine reduction system is mainly pyridinium assisted molecular hydrogen production under all conditions investigated. In particular, protonated pyridine (C₅H₅N) ‘pyridinium’ was confirmed to behave as a weak acid on platinum. It was found that CO₂ is merely a proton source of pyridine reprotonation via the hydration reaction followed by carbonic acid dissociation. The reprotonation reaction coupled to the electrode reaction ultimately leads to the dihydrogen production. No direct contribution of CO₂ in the reduction process was observed. The production of methanol seems to occur chemically rather than directly driven by the charge transfer on the electrode. The role of pyridine (C₅H₅N) appears to be restricted to assisting in the generation of the hydrogen necessary for the alcohol production

Low Cost, Carbon-Based Micro- and Nano-Structured Electrodes for High Performance Supercapacitors

Advances in the development of sustainable, low-cost, and reliable energy storage technologies have become a high priority as the demand for high power, and high energy storage devices has risen with emerging technologies in electronics, transportation, and renewable energy systems. Supercapacitors, due to their relatively high energy density and power density, provide an attractive alternative to bridge the gap between conventional batteries and capacitors. Materials ranging from high surface area, inert carbons to Faradaic metal oxides and conducting polymers have been used to achieve a range of performance properties in supercapacitors. However, the development of new technologies faces many challenges, such as sustainability, charge efficiency, capacity, cycle stability and scalable manufacturing processes. In this work, to overcome some of these challenges, we developed straightforward, low-cost approaches for the design of micro- and nano-structured electrodes with enhanced electrochemical performance. Two main pathways were taken (1) manipulation of the electrode composition through the incorporation of lignin, as a redox polymer, into the active electrode material, for enhanced energy density, and (2) modification of the electrode structure through changes in the synthesis process of the electrode materials to improve the electrochemical performance. For the first approach, lignin polymers were incorporated into a conducting polymer during electrochemical polymerization, providing increased Faradaic charge storage from the phenolic lignin groups. Polypyrrole (PPy) electrodes were prepared with alkali lignin (AL) and sulfonated lignin (SLS), and the electrochemical performance was compared with pure PPy films. We demonstrated an increase in capacitance of 30% in PPy/AL compared to PPy/SLS and 56% to PPy. Subsequently, AL and SLS were combined with porous carbon, which is electrochemically inert and non-reactive with lignin to improve the electrode stability and study the electrochemical performance of lignin without possible chemical/physical interactions with PPy. We found that intermediate pore sizes (\u3e40 nm) led to optimal redox activity as lignin cannot get inside small pores, and large pores do not adsorb significant amounts of polymer. In the second approach, lignin was used as a precursor to make high surface area carbon fibers, in which the structure of conventional fibers (polyacrylonitrile) was manipulated to produce porous materials. Decreasing the fiber diameter (115 to 8.5 µm) led to an increase in capacitance from ~2 F/g to ~70 F/g and a chemical activation process resulted in capacitances of ~192 F/g. Under the same scope, high surface area resorcinol–formaldehyde carbon aerogels reinforced with a backbone material allowed the fabrication of free-standing electrodes, eliminating the need for a binder and current collector during supercapacitor assembly. Finally, we developed a template-free synthesis method for creating microstructured electrodes to improve ion transport within thick conducting polymer films (~16 µm) while maintaining high energy storage capacity. Electrodes comprising these materials validate low cost, high energy density and innovative ways to manipulate the chemical composition and physical structure of Faradaic and non-Faradaic materials

New visible light absorber for solar fuels : Ga(Sbx)N1-x alloys.

Solar energy conversion to fuels via photoelectrochemical water splitting is one of the most important technological directions toward meeting the global energy and environmental challenge. However, till date, there are no suitable semiconductor materials available that can absorb visible light, possess right kind of band edge energetics and are stable in aqueous environments. In this work, a new III-V alloy material Ga(Sbx)N1-x with dilute antimony concentration is proposed and developed for photoelectrochemical water splitting. Experimental studies were conducted first to synthesize the proposed alloy materials to understand structure-property relationships and compare them to those obtained using first principles computations. Finally, efforts were made to improve the quality of materials synthesized within the context of improving their photoactivity with water splitting reaction. In general, III-V nitrides have garnered immense interest as suitable materials for solar hydrogen generation due to their tunable band gaps with composition, high carrier mobilities and high absorption coefficients. Computations using first principles density functional theory (DFT+U) revealed that a small amount of Sb incorporation is sufficient to achieve a significant band gap reduction in GaN from 3.4eV to 2eV. Theoretical computations predicted that Ga(Sb)xN1-x alloys with 2 eV band gap straddle the electrochemical redox potentials. The synthesis of dilute GaSbxN1-x alloys is conducted using a custom-built metalorganic chemical vapor deposition reactor. Extensive characterization of the resulting films suggests that there is a large band gap bowing even with small amounts (few percent ~ 2-3%) of antimony incorporation into GaN. In addition, photoelectrochemical characterization confirmed the band edges straddling redox potentials. All the experimental data regarding band gap bowing, lattice expansion and band edge straddling matched very well with the theoretical predictions. Moreover, the alloys with Sb incorporation \u3e7% exhibited indirect band gap transition as predicted by DFT + U calculations. The polycrystalline Ga(Sbx)N1-x thin films were shown to be capable of unassisted water splitting but with low efficiencies. So, two different approaches are investigated to improve the quality of resulting films: thick films with high texture and single crystal quality, Ga(Sbx)N1-x nanowires. The use of a pre-treatment step at 900° C, 40:1 ratio of antimony to gallium precursors and temperatures above 750 °C allowed for good quality crystal growth while allowing for incorporation of antimony. Photoactivity as high as 1 mA/cm2 was obtained. In addition, VLS approach has been demonstrated to obtain high crystalline quality films using copper as catalyst. Vapor-liquid-solid growth experiments using copper particles allowed for tip led growth of GaSbxN1-x nanowires at temperatures beyond 600° C. The antimony composition in the resulting nanowires increased with growth temperature up to 5 at% while improving the quality. Also the photoactivity obtained from nanowires has been increased by two orders of magnitude when compared with polycrystalline films. In summary, a new class of III-V nitride alloys using dilute antimonides is demonstrated to have suitable properties for solar fuels applications but can find other applications

축전용량 향상을 위한 퀴논 기반 새로운 전극 첨가제 개발

학위논문(석사) -- 서울대학교대학원 : 융합과학기술대학원 융합과학부(나노융합전공), 2022.2. 이강원.Redox active organic compounds have proven to be functional materials for devices such as switches, molecular wires, sensors and, importantly, electronic/optoelectronic devices. These organic molecules are functionalized with various redox active substances that can undergo redox reactions. Among these compounds, quinone-based materials have received considerable attention as an electrode material for an energy storage device because they are cost-effective, environmentally friendly and exhibit high reversibility. In this study, quinone-based derivatives are synthesized and then blended physically with activated carbon. Three quinone based derivatives (HBU 680, HBU 888, and HBU 889) are synthesized and used as electrode additives for supercapacitors. Organic additives including activated carbon and their complexes are primarily characterized by a PhQ-PhQH2 redox transition involving a two-electron two-proton process. The composite electrodes show specific capacitance of 176 F/g, 262 F/g, and 145 F/g for HBU 680, HBU 888 and HBU 889 respectively at current density 5 mA/cm2. Cycle performance can also be enhanced by the adoption of PhQH2- based organic additives for their pore-filling morphology to increase the packing density of composite electrodes. The composite electrodes also show 100% capacity retention over 10,000 cycles. These results show that the new quinone-based derivates can enhance pseudocapacitive behavior and warrant their use as electrode addiitves for supercapacitors. The quinone derivatives were obtained by a simple one or two-pot synthesis process using less harmful organic solvents. A coating slurry using activated carbon as an active material, quinone derivatives as a redox additive, polyvinylidene fluoride as a polymer binder, and N-methyl-2-pyrrolidinone as a dispersion solvent was perfectly mixed to provide an excellent composite electrode as shown by the SEM data. The cyclic voltammetry performed in a three-electrode cell using a composite electrode as the working electrode of 1 M H2SO4 showed a high potential window in the -0.2 ~ 0.8 V range and good capacitance retention compared to Ag/AgCl. The experimental results showed that the capacitive performance of the composite electrode is due to the synergistic effect between activated carbon and organic additives. This effective method can be used to obtain good performing composite electrodes for supercapacitor applications.산화환원 활성 유기 화합물은 스위치, 분자 와이어, 센서 및 중요하게는 전자/광전자 장치의 기능성 재료로 입증되었다. 이러한 유기 분자는 산화 환원 반응을 겪을 수 있는 다양한 산화 환원 활성 물질로 기능 화된다. 이들 화합물 중 퀴논계 물질은 합리적인 비용과 친환경적이며 높은 가역성을 나타내어 에너지 저장 장치용 전극 물질로 상당한 주목을 받고 있다. 퀴논과 그 유도체는 전기화학 분야의 유망한 유기 물질이다. 본 연구에서는 퀴논계 유도체를 합성한 후 활성탄과 물리적으로 혼합하였다. 3개의 퀴논계 유도체(HBU 680, HBU 888 및 HBU 889)가 합성되어 슈퍼커패시터용 전극 첨가제로 사용된다. 활성탄 및 그 착물을 포함한 유기 첨가제는 주로 2개의 전자, 2개 양성자 과정을 포함하는 PhQ-PhQH2 산화환원 전이를 특징으로 한다. 복합 전극은 전류 밀도 5 mA/cm2에서 HBU 680, HBU 888 및 HBU 889에 대해 각각 176 F/g, 262 F/g 및 145 F/g의 비정전용량을 나타낸다. 사이클 성능 또한 복합 전극의 패킹 밀도를 증가시키기 위해 기공 충전 형태에 PhQH2 기반 유기 첨가제를 채택함으로써 향상될 수 있다. 복합 전극은 10000회 주기 동안 100% 용량 유지를 보였고 이러한 결과는 새로운 퀴논 기반 유도체가 의사용량성 거동을 향상시킴으로써 슈퍼커패시터용 전극 첨가제로서의 사용을 보증할 수 있음을 보여준다. 퀴논 유도체는 덜 유해한 유기 용매를 사용하여 간단한 1 또는 2단계 합성 공정으로 얻었다. 활성탄을 활물질로, 퀴논 유도체를 산화환원 첨가제로, polyvinylidene fluoride을 고분자 바인더로, N-methyl-2-pyrrolidinone을 분산 용매로 사용하는 코팅 슬러리를 완벽하게 혼합하여 우수한 복합 전극을 제조하였고 SEM을 통하여 확인하였다. 1M H2SO4 용액에 복합전극을 작업전극으로 사용하여 Ag/AgCl 기준 -0.2 ~ 0.8V의 넓은 전위창에서 cyclic voltammety를 확인한 결과 우수한 용량 유지율을 나타냈다. 이러한 결과는 복합 전극의 정전 용량 성능이 활성탄과 유기 첨가제 사이의 시너지 효과에 기인한다는 것을 보여주었다. 이 효과적인 방법을 통해 슈퍼커패시터로서 우수한 성능의 복합 전극을 얻을 수 있음을 확인했다.Chapter 1. Introduction 10 1.1. Historical overview of supercapacitors 10 1.2. Design and fundamental concepts of supercapacitors 12 1.3. Energy storage mechanism in supercapacitors 14 1.3.1. Electric-double-layer capacitances (EDLCs) 14 1.3.2. Pseudocapacitors 17 1.4. Requirements for electrode material 20 1.5. Materials for supercapacitors' electrodes 21 1.5.1. Carbon-based materials 21 1.5.2. Pseudocapacitive electrode materials 24 Chapter 2. Experimental Section 26 2.1. Synthesis of quinone based derivatives(HBU additives) 26 2.1.1. Synthesis of HBU 889 27 2.1.2. Synthesis of HBU 888 29 2.1.3. Synthesis of HBU 680 32 2.2. Redox mechanisms of the HBU additives 35 2.3. Composite electrode preparation 38 2.4. Electrochemical characterization 40 Chapter 3. Results and Discussion 41 3.1. Characterization of quinone based composite electrode 41 3.2. Cyclic voltammetry analysis 46 3.3. Galvanostatic charge-discharge 49 3.4. Specific capacitance 52 Chapter 4. Conclusion 54 References 55 Abstract in Korean 66석

Parallel manipulation of individual magnetic microbeads for lab-on-a-chip applications

Many scientists and engineers are turning to lab-on-a-chip systems for cheaper and high throughput analysis of chemical reactions and biomolecular interactions. In this work, we developed several lab-on-a-chip modules based on novel manipulations of individual microbeads inside microchannels. The first manipulation method employs arrays of soft ferromagnetic patterns fabricated inside a microfluidic channel and subjected to an external rotating magnetic field. We demonstrated that the system can be used to assemble individual beads (1-3µm) from a flow of suspended beads into a regular array on the chip, hence improving the integrated electrochemical detection of biomolecules bound to the bead surface. In addition, the microbeads can follow the external magnet rotating at very high speeds and simultaneously orbit around individual soft magnets on the chip. We employed this manipulation mode for efficient sample mixing in continuous microflow. Furthermore, we discovered a simple but effective way of transporting the microbeads on-chip in the rotating field. Selective transport of microbeads with different size was also realized, providing a platform for effective sample separation on a chip. The second manipulation method integrates magnetic and dielectrophoretic manipulations of the same microbeads. The device combines tapered conducting wires and fingered electrodes to generate desirable magnetic and electric fields, respectively. By externally programming the magnetic attraction and dielectrophoretic repulsion forces, out-of-plane oscillation of the microbeads across the channel height was realized. Furthermore, we demonstrated the tweezing of microbeads in liquid with high spatial resolutions by fine-tuning the net force from magnetic attraction and dielectrophoretic repulsion of the beads. The high-resolution control of the out-of-plane motion of the microbeads has led to the invention of massively parallel biomolecular tweezers.Ph.D.Committee Chair: Hesketh, Peter; Committee Member: Allen, Mark; Committee Member: Degertekin, Levent; Committee Member: Lu, Hang; Committee Member: Yoda, Minam

Office of Research -- Annual Report 2005-2006

Table of Contents Physical Sciences & Engineering Exploring the final frontier – extreme light . . . . . . . . . . . 2 Nano discovery is golden . . . . . . . . . . . . . . . . . . . . . . . . 4 New center fuels energy research . . . . . . . . . . . .. . . . . . 6 Imparting the human touch . . . . . . . . . . . . . . . . . . . . . . . 8 Tackling transportation challenges . . . . . . . . . . . . . . . . .10 Concrete results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Dunes divulge mega-drought clues . . . . . . . . . . .. . . . . .12 New drought tools aid tough decisions . . . . . . . . . . . . . .13 Life Sciences Unraveling immune system intricacies . . . . . . . . . . . . . .14 Solving evolutionary mysteries . . . . . . . . . . . . . . . . . . . .15 Plant transformation lab is biotech pipeline . . . . .. . . . . .16 Organic farming research expanding . . . . . . . . . . . . . . .17 Training international HIV/AIDS researchers . . . . . . . . .18 Expanding partnerships with Zambia, China . . . .. . . . . .19 Humanities & The Arts ‘Poet of democracy’ goes digital . . . . . . . . . . . . . . . . . . .20 GIS atlas reveals railroad’s instrumental role . . . . . . . . .20 Center enhances humanities research . . . . . . . . . . . . . .21 The arts in action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 Social Sciences Smoking’s impact on babies’ behavior . . . . . . . . . . . . .24 Horses a powerful prevention tool . . . . . . . . . . . . . . . . .25 Genes and political temperament . . . . . . . . . . . . . . . . .26 Technology Development Moving discoveries to the marketplace . . . . . . . . . . . .27 Education & Outreach Turning loose Tekbots as teaching tools . . . . . . . . . . .28 Undergrads experience research firsthand . . . . . . . . .30 Goldwater recipients . . . . . . . . . . . . . . . . . . . . . . . . . .31 Fossils going online for easy access . . . . . . . . . . . . .32 Training Native American teachers . . . . . . . . . . . . . .33 Extending Our Reach Celebrating excellence, collaboration . . . . . . . . . .. . .34 Financials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Iron-oxide catalyzed silicon photoanode for water splitting

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 130-139).This thesis presents an integrated study of high efficiency photoanodes for water splitting using silicon and iron-oxide. The fundamental limitations of silicon to water splitting applications were overcome by an ultrathin iron-oxide film stack and a pH-adjusted electrochemical environment. It was experimentally demonstrated that this functional photoanode has very strong photoactivity exceeding the performance of previously reported systems. A complete theoretical explanation is provided with experimental substantiations. Two major obstacles of the application of silicon to water splitting are its high valence band edge and the lack of catalytic functionality. The solution for band edge mismatch is to facilitate the unique pH response of silicon with respect to electrolyte pH. As opposed to the low pH, a high pH electrolyte allows the silicon valence band edge to be located below the oxygen evolution potential, providing physical platform for the water oxidation reaction. In this platform, the introduction of a thin iron-oxide layer was proved to effectively catalyze the anode reaction which is otherwise impossible. The very thin film enables the separation of the two key functions of the photoelectrode: photocarrier generation and catalysis. Since the iron-oxide film has a very low absorption of incident light, phorocarriers are generated in the silicon layer, while the surface still maintains catalytic functionality. By this functional separation, it was possible to overcome the loss incurred by poor electronic and photovoltaic properties of iron-oxide. The thin semiconducting film also allows a space charge region to span beyond the catalyst layer to silicon, inducing a large built-in potential to lower the required overpotential. Additional improvement was made by adopting silicon microfabrication techniques to maximize light harvesting and minimize potential losses in silicon layer. By vertical wires and ohmic contact formation, the onset potentials were decreased and the current-potential slopes were steepened, resulting in current density as high as 17 mA/cm² at zero overpotential. These results were obtained with two of the most abundant materials, and as such shows the prospect of an efficient solar-driven water splitting pathway. In addition, the comprehensive theoretical explanations will contribute to searching for further system improvements.by Kimin Jun.Ph.D