Exploiting Plasmonic and Plexcitonic Nanomaterials in Industrial Catalysis

能源密集型化学反应，如CO2还原和蒸汽重整，可以在接近室温的条件下使用光作为能源进行吗？氢经济能否在未来5-10年内通过高效的阳光驱动水分解实现？激子、等离子体激元和声子等量子准粒子在多相催化中是如何相互作用的？我们能否减少文明对铂、钯、金和银等贵金属在一系列催化反应中的过度依赖？这些都是本提案中描述的研究计划试图研究和回答的科学问题和技术可能性。这项研究的核心是等离子体纳米粒子，以及它们与半导体和π共轭染料分子的异质结。 等离子体激元是金属中自由电子气的集体和相干振荡。 在金属纳米颗粒中，电子的集体激发在金属-电介质界面处的空间限制导致强的局部表面等离子体共振（LSPR）。 在可见光和近红外波长下的LSPR共振对于光催化应用是期望的。一些材料如Ag、Au、Cu、Al、TiN、ZrN和Cu 2S的纳米颗粒已经显示出在可见光和近红外中具有LSPR峰。表面等离子体通过朗道阻尼（LD）或化学界面阻尼（CID）在飞秒内衰减，形成热电子-空穴对，其能量远高于玻尔兹曼分布的预期。 因此，热载体作为化学转化的试剂特别有吸引力，以驱动化学反应，例如水裂解以产生H2、将CO2还原成增值产物、偶氮染料的合成、氨合成、烃重整、有机化合物的光氧化等。热电子在~ 100 fs时间尺度上经历电子-电子散射，在~ 1 ps时间尺度上经历与声子的碰撞，导致非常快地弛豫到纯热载流子分布。该提案寻求解决的关键技术挑战是如何将热电子从金属中穿梭出来，并在其多余的能量损失到各种耗散过程之前让它们驱动化学反应。 等离子体金属纳米粒子（纳米棱镜、纳米立方体、纳米壳等）与无机半导体和π共轭有机染料的异质结为等离子体热载流子的有效分离和开发提供了最有前途的途径。这些异质结的基本物理过程尚未完全了解。我们还寻求推进我们对等离子体金属-半导体异质结的基本科学理解。该提案中的研究有可能影响50亿美元的半导体封装行业和300亿美元的全球催化剂行业（2018年）。

Could energy intensive chemical reactions such as CO2 reduction and steam reforming be performed close to room temperature using light as the energy source? Could the hydrogen economy be realized within the next 5-10 years through efficient sunlight-driven water-splitting ? How do quantum quasiparticles such as excitons, plasmons and phonons interact in the context of heterogeneous catalysis? Could we reduce civilization's extraordinary reliance on precious metals such as platinum, palladium, gold and silver for a range of catalytic reactions? These are the sorts of scientific questions and technological possibilities that the research program described in this proposal seeks to examine and answer. At the heart of this research are plasmonic nanoparticles, and their heterojunctions with semiconductors and pi-conjugated dye molecules. Plasmons are collective and coherent oscillations of the free electron gas in metals. In metal nanoparticles, the spatial confinement of collective excitations of electrons at the metal-dielectric interface result in strong localized surface plasmon resonances (LSPR). LSPR resonances at visible & near-infrared wavelengths are desired for photocatalytic applications. Nanoparticles of a handful of materials such as Ag, Au, Cu, Al, TiN, ZrN and Cu2S have been shown to have LSPR peaks in the visible and near-infrared. Surface plasmons decay in femtoseconds through either Landau damping (LD) or chemical interface damping (CID) to form hot electron-hole pairs with energies much higher than what would be expected from a Boltzmann distribution. Therefore, hot carriers are particularly attractive as agents of chemical transformation in order to drive chemical reactions such as water-splitting to generate H2, reduction of CO2 into value-added products, synthesis of azo dyes, ammonia synthesis, hydrocarbon reforming, photooxidation of organic compounds, etc. However, hot electrons experience electron-electron scattering over ~ 100 fs timescales and collisions with phonons over ~ 1 ps timescales resulting in very fast relaxation to a purely thermal carrier distribution. The key technological challenge that this proposal seeks to address is to how to shuttle the hot electrons away from the metal and get them to drive a chemical reaction before their excess energies are lost to a variety of dissipation processes. Heterojunctions of plasmonic metal nanoparticles (nanoprisms, nanocubes, nanoshells, etc) with inorganic semiconductors and pi-conjugated organic dyes, offer the most promising routes for efficient separation and exploitation of plasmonic hot carriers. The underlying physical processes at these heterojunctions are not completely understood. We also seek to advance our fundamental scientific understanding of plasmonic metal-semiconductor heterojunctions. The research in this proposal has the potential to impact the $5 billion semiconductor photocatalysis industry and the $30 billion global catalyst industry (2018).