Research Projects

Per- and polyfluoroalkyl substances (PFAS) are persistent, man-made pollutants that have contaminated water resources worldwide, including India. Recently, high PFAS levels were reported in Chennai’s Adyar River by Prof. Indumati's group at IIT Madras. The presence of C–F bonds (∼130 kcal/mol), the strongest in organic chemistry, makes PFAS extraordinarily stable and resistant to degradation. PFAS gets accumulated in living organisms and is linked to serious health risks, including cancer. As such, there is a need to degrade these harmful chemicals. Although conventional treatments, such as filtration, adsorption, and oxidation, can remove PFAS to some extent, they cannot break the C–F bonds. Moreover, no method currently achieves complete mineralization of a wide range of PFAS (there are about 14,000 varieties). To circumvent this challenge, we recently proposed the use of hot carriers (HCs) generated from the plasmonic metal nanoclusters (NCs) to degrade PFAS. In plasmonic NCs, incident light can excite localized surface plasmons (collective electron oscillations), which subsequently decay to produce energetic hot electrons and holes. These HCs can transfer to adsorbed molecules, driving otherwise inaccessible chemical reactions. In one of the preliminary studies (Nat. Catal. 3, 564–573, 2020), the dissociation of a C–F bond (hydrodefluorination of fluoromethane) via plasmonic excitation was demonstrated. However, the direct activation of a PFAS molecule was first demonstrated by us, where we showed that upon plasmonic excitation, Ag nanoclusters can generate hot electrons that transfer directly to PFOA, inducing its near-complete degradation within tens of femtoseconds. Motivated by this breakthrough, we propose a comprehensive computational study to unravel how plasmonic NCs can be engineered to completely degrade a wide variety of PFAS. Using first-principles real-time time-dependent density functional theory (RT-TDDFT) and Ehrenfest molecular dynamics (EMD), we will explore various key parameters that affect the generation of hot carriers and their transfer to PFAS for their efficient degradation. These include (i) NC composition (Ag, Au, Cu, Ir, Rh, Pt, and their doped variants), (ii) size (between 1 to 2 nm), (iii) geometry (icosahedral, cuboctaheral, etc., and arrays of NCs), (iv) surface chemistry (ligand environment), (v) PFAS chemistry (short vs long chain, and various functional groups), and (vi) excitation conditions (light frequency, intensity, and polarization). For each NC–PFAS complex, we will simulate plasmon excitation and decay, quantify the direct hot carrier transfer to PFAS, and follow the ensuing dynamics to understand the degradation pathways. We will particularly focus on the direct hot-electron and hot-hole transfer (DHET and DHHT) channels as functions of all parameters, and identify the combinations that can yield maximum defluorination. The role of multiple NCs or plasmonic “hot spots” will be studied, where field enhancement is expected to boost carrier generation. Finally, we will investigate plasmon decay mechanisms (Landau vs chemical interface damping) and identify the relations between dipole dephasing times and hot-carrier yields. We strongly believe that the successful completion of this project will have a high impact in the fields of both physical and environmental chemistry. Specifically, unlocking a photophysical route to break the “C–F bond” could revolutionize PFAS cleanup and inspire plasmonic solutions for other persistent pollutants. In addition to this, we will have a wealth of information on both the plasmon decay across multiple NCs and efficient PFAS degradation strategies. Particularly, we will provide general guidelines for NC design (e.g., optimal sizes, materials, and light conditions) that experimentalists can use to create plasmonic PFAS remediators, quantitative metrics of carrier transfer efficiency, and detailed degradation mechanisms for multiple PFAS classes.