Exploiting Plasmonic and Plexcitonic Nanomaterials in Industrial Catalysis

二氧化碳减排和水蒸气重整等能源密集型化学反应能否在接近室温的条件下，以光为能源进行？氢经济能否通过高效的阳光驱动的水分裂在未来5-10年内实现？在多相催化的背景下，激子、等离子体和声子等量子准粒子是如何相互作用的？我们能否减少文明对贵金属如铂、钯、金和银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检查和回答的科学问题和技术可能性的种类。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圆周率共轭染料分子的异质结。等离子体是金属中自由电子气的集体和相干振荡。在金属纳米粒子中，金属-介质界面电子集体激发的空间限制导致了强烈的局域表面等离子体共振(LSPR)。LSPR在可见光和近红外波长的共振是光催化应用所需要的。少数几种材料的纳米粒子，如Ag、Au、Cu、Al、TiN、ZrN和Cu2S，已被证明在可见光和近红外有LSPR峰。表面等离子体通过朗道衰减(LD)或化学界面衰减(CID)在飞秒内形成热电子-空穴对，其能量远高于玻尔兹曼分布的能量。因此，热载流子作为化学反应的转换剂特别有吸引力，以驱动化学反应，如水裂解生成氢气，二氧化碳还原为增值产品，偶氮染料的合成，氨合成，烃类重整，有机化合物的光氧化等。然而，热载流子经历了电子-电子散射，并在~1ps时间尺度上与声子发生碰撞，导致非常快地弛豫到纯热载流子分布。这一提议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，并在它们的过剩能量被各种耗散过程损失之前，让它们驱动化学反应。等离子体金属纳米粒子(纳米棱柱、纳米立方体、纳米壳层等)与无机半导体和pi共轭有机染料的异质结为等离子体热载流子的高效分离和开发提供了最有前途的途径。这些异质结处的潜在物理过程还没有完全被理解。我们还试图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价值50亿美元的半导体光催化行业和价值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).