Aluminum oxide for photocatalytic organic transformations

The use of sunlight to drive organic reactions constitutes a green and sustainable strategy for organic synthesis. In such reactions, the photoredox requirements of organic reactions are typically fulfilled through using wide band gap semiconductors or redesigning the photocatalyst. Herein, we demonstrated the use of a reverse strategy; instead of redesigning the dye as per convention, the reactant is chemisorbed on the Brønsted base sites of Al2O3 to form a surface complex, causing an upshift in its HOMO to a level accessible for electron abstraction by the dye. This enables the highly selective oxidation of benzylic alcohols to aldehydes by a large variety of dyes, even though negligible reaction occurred in the absence of Al2O3 or with other metal oxides. The charge-transfer surface complex formed between the dye and Al2O3 is also essential in facilitating the transport of electrons from BnOH to O2. Next, we further extended the use of Al2O3 complexation in conjunction with photocatalysis to drive the selective aerobic oxidation of phenylboronic acids. It was discovered that all metal oxides with Brønsted basicity can also enable high yields of phenols. The proximity and strength of the Brønsted base sites appear to be crucial towards the reaction; there seems to be a positive relationship between the yields of alcohol and the quantity of strong Brønsted base sites, rather than with the quantity of weak or all Brønsted base sites. Lastly, we explored the idea of rendering Al2O3 photocatalytically active through carbon-modification, which would eliminate the need for a dye. The resultant carbon doped Al2O3 enables the selective oxidation of benzylic amines to form imines under visible light irradiation. We believe that our aforementioned discoveries may subvert our understanding of the role of Al2O3 in photocatalytic reactions. It may also bring forth a new methodology of utilizing surface complexation mechanisms between the reactants and earth-abundant materials to effectively achieve a wider range of photoredox reactions.Doctor of Philosophy (MSE

Surface complexation for photocatalytic organic transformations

Photocatalysis constitutes an important research interest due to its capability for achieving important chemical reactions in an environmentally green and sustainable manner. The use of heterogeneous photocatalysts adds additional advantages such as ease of separation from reaction mixtures, reusability, as well as photo, thermal and chemical stability. In this account, we showed how the surface complexation of different key players on TiO2 can be used control the reaction pathway to enable difficult organic transformations, as demonstrated by the selective aerobic oxidation of sulfides to sulfoxides. First, we designed a photocatalytic-surface complexation system comprising three fundamental components; visible-light-absorbing dye, TiO2 and TEMPO as the redox mediator. Next, the said system was elegantly simplified into a visible-light-harvesting surface complex generated in-situ between TiO2 and tertiary amines, which enabled O2 to be selectively activated only in the presence of the target sulfide substrate. This was then expanded into the new concept of synergistic photocatalysis, which is based on the interplay of reactants (sulfides and benzylamines) via the aforementioned visible-light-harvesting surface complex to enable two seemingly irrelevant reactions in one photocatalytic system. Lastly, we briefly discussed how surface complexation on heterogeneous catalysts such as metal oxides can be further utilized for photocatalytic organic transformations.MOE (Min. of Education, S’pore)Published versio

Bio‐inspired plasmonic photocatalysts

The conversion of solar energy to sustainable energy sources is a significant field of study for the relief of the world's energy problems, and among the various strategies developed, semiconductor photocatalysts have received significant attention as a promising candidate due to their attractive efficiency, mild reaction conditions, and low cost. The enhancement of such photocatalysts with plasmonic materials, by virtue of the Schottky junction and localized surface plasma resonance phenomenon, could facilitate the rapid progress in enhancement of photocatalytic efficiency under visible light irradiation. To further improve photocatalytic efficiency, scientists look to nature for inspiration, culminating in the evolution of complex hierarchical structures in order to fully utilize the potential of solar energy. In the past decade, there has been significant progress in the development of bio‐inspired plasmonic photocatalysts, such as antireflective surfaces, 3D photonic structures, and branched structures. This review describes the state‐of‐the‐art bio‐inspired light manipulation approaches, as well as future challenges in solar energy harvesting by plasmonic photocatalysts.Accepted versionThe authors thank the financial support from Singapore Ministry of Education (MOE2014- T3-1-004 and MOE2014-T2-2-140)

Thermal-responsive polymers for enhancing safety of electrochemical storage devices

Thermal runway constitutes the most pressing safety issue in lithium-ion batteries and supercapacitors of large-scale and high-power density due to risks of fire or explosion. However, traditional strategies for averting thermal runaway do not enable the charging-discharging rate to change according to temperature or the original performance to resume when the device is cooled to room temperature. To efficiently control thermal runaway, thermal-responsive polymers provide a feasible and reversible strategy due to their ability to sense and subsequently act according to a predetermined sequence when triggered by heat. Herein, recent research progress on the use of thermal-responsive polymers to enhance the thermal safety of electrochemical storage devices is reviewed. First, a brief discussion is provided on the methods of preventing thermal runaway in electrochemical storage devices. Subsequently, a short review is provided on the different types of thermal-responsive polymers that can efficiently avoid thermal runaway, such as phase change polymers, polymers with sol-gel transitions, and polymers with positive temperature coefficients. The results represent the important development of thermal-responsive polymers toward the prevention of thermal runaway in next-generation smart electrochemical storage devices.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Synergistic photocatalytic aerobic oxidation of sulfides and amines on TiO2 under visible-light irradiation

Selective photocatalytic aerobic oxidation, which can be conducted under ambient conditions, is of great importance towards achieving sustainable chemistry. However, its practical applications are undermined by several challenges, such as low selectivity, sluggish reaction rates, and the requirement of UV light irradiation. Herein, we report a new concept of synergistic photocatalytic oxidation, for which two seemingly irrelevant reactions can be achieved in one photocatalytic system through the synergistic interplay of reactants and catalyst. As proof of concept, two challenging reactions, the aerobic oxidation of sulfide and the aerobic oxidative formylation of amine with methanol, were employed to demonstrate such synergistic photocatalytic aerobic oxidation under visible-light irradiation. This work could pave the way for highly selective photoredox catalysis via rational design based on mechanistic insight.Published versio

Elastic substrates for stretchable devices

Stretchable devices, with the capability of retaining their functionalities under stretching, are drawing much attention as a promising solution to address the mechanical mismatch between traditional stiff electronics and soft curvilinear biological systems. Intensive efforts have been made toward the advancement of stretchable devices, such as the development of novel mechanically durable materials, deformable conductors and circuits, novel processing methods, and elastic matrixes for stretchable substrates and system integration. Among these, the elastic substrate constitutes the component that bears the applied strain and thus endows the device with stretchability, rendering its properties crucial to the overall performance of stretchable devices. This article provides a summary of the elastic materials commonly employed as stretchable substrates, as well as reveals fundamental insights into the properties requirements in the selection of stretchable substrates. Important challenges and strategies in the development of elastic matrices for stretchable devices are also discussed.Published versio

Nature-Inspired Structural Materials for Flexible Electronic Devices

Exciting advancements have been made in the field of flexible electronic devices in the last two decades and will certainly lead to a revolution in peoples’ lives in the future. However, because of the poor sustainability of the active materials in complex stress environments, new requirements have been adopted for the construction of flexible devices. Thus, hierarchical architectures in natural materials, which have developed various environment-adapted structures and materials through natural selection, can serve as guides to solve the limitations of materials and engineering techniques. This review covers the smart designs of structural materials inspired by natural materials and their utility in the construction of flexible devices. First, we summarize structural materials that accommodate mechanical deformations, which is the fundamental requirement for flexible devices to work properly in complex environments. Second, we discuss the functionalities of flexible devices induced by nature-inspired structural materials, including mechanical sensing, energy harvesting, physically interacting, and so on. Finally, we provide a perspective on newly developed structural materials and their potential applications in future flexible devices, as well as frontier strategies for biomimetic functions. These analyses and summaries are valuable for a systematic understanding of structural materials in electronic devices and will serve as inspirations for smart designs in flexible electronics.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Tertiary amine mediated aerobic oxidation of sulfides into sulfoxides by visible-light photoredox catalysis on TiO2

The selective oxidation of sulfides into sulfoxides receives much attention due to industrial and biological applications. However, the realization of this reaction with molecular oxygen at room temperature, which is of importance towards green and sustainable chemistry, remains challenging. Herein, we develop a strategy to achieve the aerobic oxidation of sulfides into sulfoxides by exploring the synergy between a tertiary amine and titanium dioxide via visible-light photoredox catalysis. Specifically, titanium dioxide can interact with triethylamine (TEA) to form a visible-light harvesting surface complex, preluding the ensuing selective redox reaction. Moreover, TEA, whose stability was demonstrated by a turnover number of 32, plays a critical role as a redox mediator by shuttling electrons during the oxidation of sulfide. This work suggests that the addition of a redox mediator is highly functional in establishing visible-light-induced reactions via heterogeneous photoredox catalysis.Published versio

Resistive switching memory devices based on proteins

Resistive switching memory constitutes a prospective candidate for next-generation data storage devices. Meanwhile, naturally occurring biomaterials are promising building blocks for a new generation of environmentally friendly, biocompatible, and biodegradable electronic devices. Recent progress in using proteins to construct resistive switching memory devices is highlighted. The protein materials selection, device engineering, and mechanism of such protein-based resistive switching memory are discussed in detail. Finally, the critical challenges associated with protein-based resistive switching memory devices are presented, as well as insights into the future development of resistive switching memory based on natural biomaterials