Exploiting Plasmonic and Plexcitonic Nanomaterials in Industrial Catalysis

能源密集型的化学反应，如二氧化碳还原和蒸汽重整，能否在接近室温的条件下使用光作为能源？能否在未来5-10年内通过高效的阳光驱动的水分解实现氢经济？量子准粒子如激子、等离激子和声子在非均相催化下是如何相互作用的？在一系列催化反应中，我们能否减少人类文明对铂、钯、金、银等贵金属的过度依赖？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。等离子体是金属中自由电子气体的集体和相干振荡。在金属纳米粒子中，金属-介电界面上电子集体激发的空间限制导致了强局域表面等离子体共振（LSPR）。光催化应用需要可见光和近红外波长的LSPR共振。Ag、Au、Cu、Al、TiN、ZrN和Cu2S等材料的纳米粒子在可见光和近红外波段均存在LSPR峰。表面等离子体在飞秒内通过朗道阻尼（LD）或化学界面阻尼（CID）衰减，形成能量远高于波尔兹曼分布的热电子-空穴对。因此，热载体作为化学转化试剂尤其具有吸引力，可以驱动化学反应，如水裂解生成H2、二氧化碳还原成增值产品、偶氮染料合成、氨合成、碳氢化合物重整、有机化合物光氧化等。然而，热电子在~ 100fs时间尺度上经历电子-电子散射和与声子在~ 1ps时间尺度上的碰撞，导致非常快的弛豫到纯粹的热载流子分布。该方案所要解决的关键技术挑战是如何将热电子从金属中转移出来，并在它们多余的能量在各种耗散过程中损失之前使它们驱动化学反应。等离子体金属纳米粒子（纳米棱镜、纳米立方、纳米壳等）与无机半导体和偶联有机染料的异质结为等离子体热载子的高效分离和开发提供了最有前途的途径。这些异质结的潜在物理过程尚未完全了解。我们也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50亿美元的半导体光催化产业和300亿美元的全球催化剂产业（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).