BITS Faculty Publications
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Item Single-Site Photocatalytic H2 Evolution from Covalent Organic Frameworks with Molecular Cobaloxime Co-Catalysts(ACS, 2017-10-12) Banerjee, TanmayWe demonstrate photocatalytic hydrogen evolution using COF photosensitizers with molecular proton reduction catalysts for the first time. With azine-linked N2-COF photosensitizer, chloro(pyridine)cobaloxime co-catalyst, and TEOA donor, H2 evolution rate of 782 μmol h–1 g–1 and TON of 54.4 has been obtained in a water/acetonitrile mixture. PXRD, solid-state spectroscopy, EM analysis, and quantum-chemical calculations suggest an outer sphere electron transfer from the COF to the co-catalyst which subsequently follows a monometallic pathway of H2 generation from the CoIII-hydride and/or CoII-hydride species.Item H2 Evolution with Covalent Organic Framework Photocatalysts(ACS, 2018-01-05) Banerjee, TanmayCovalent organic frameworks (COFs) are a new class of crystalline organic polymers that have garnered significant recent attention as highly promising H2 evolution photocatalysts. This Perspective discusses the advances in this field of energy research while highlighting the underlying peremptory factors for the rational design of readily tunable COF photoabsorber–cocatalyst systems for optimal photocatalytic performance.Item Tailor-Made Photoconductive Pyrene-Based Covalent Organic Frameworks for Visible-Light Driven Hydrogen Generation(Wiley, 2018-06-28) Banerjee, TanmayCovalent organic frameworks (COFs) have emerged as a new class of crystalline porous polymers displaying molecular tunability combined with structural definition. Here, a series of three conjugated, photoactive azine-linked COFs based on pyrene building blocks which differ in the number of nitrogen atoms in the peripheral aromatic units is presented. The structure of the COFs is analyzed by combined experimental and computational physisorption as well as quantum-chemical calculations, which suggest a slipped-stacked arrangement of the 2D layers. Photocurrents of up to 6 µA cm−2 with subsecond photoresponse times are measured on thin film samples for the first time. While all COFs are capable of producing hydrogen from water, their efficiency increases significantly with decreasing number of nitrogen atoms. The trending activities are rationalized by photoelectrochemical measurements and quantum-chemical calculations which suggest an increase in the thermodynamic driving force with decreasing nitrogen content to be the origin of the observed differences in hydrogen evolution activities.Item Sustained Solar H2 Evolution from a Thiazolo[5,4-d]thiazole-Bridged Covalent Organic Framework and Nickel-Thiolate Cluster in Water(ACS, 2019-06-20) Banerjee, TanmaySolar hydrogen (H2) evolution from water utilizing covalent organic frameworks (COFs) as heterogeneous photosensitizers has gathered significant momentum by virtue of the COFs’ predictive structural design, long-range ordering, tunable porosity, and excellent light-harvesting ability. However, most photocatalytic systems involve rare and expensive platinum as the co-catalyst for water reduction, which appears to be the bottleneck in the development of economical and environmentally benign solar H2 production systems. Herein, we report a simple, efficient, and low-cost all-in-one photocatalytic H2 evolution system composed of a thiazolo[5,4-d]thiazole-linked COF (TpDTz) as the photoabsorber and an earth-abundant, noble-metal-free nickel-thiolate hexameric cluster co-catalyst assembled in situ in water, together with triethanolamine (TEoA) as the sacrificial electron donor. The high crystallinity, porosity, photochemical stability, and light absorption ability of the TpDTz COF enables excellent long-term H2 production over 70 h with a maximum rate of 941 μmol h–1 g–1, turnover number TONNi > 103, and total projected TONNi > 443 until complete catalyst depletion. The high H2 evolution rate and TON, coupled with long-term photocatalytic operation of this hybrid system in water, surpass those of many previously known organic dyes, carbon nitride, and COF-sensitized photocatalytic H2O reduction systems. Furthermore, we gather unique insights into the reaction mechanism, enabled by a specifically designed continuous-flow system for non-invasive, direct H2 production rate monitoring, providing higher accuracy in quantification compared to the existing batch measurement methods. Overall, the results presented here open the door toward the rational design of robust and efficient earth-abundant COF–molecular co-catalyst hybrid systems for sustainable solar H2 production in water.Item Rational Design of Covalent Cobaloxime–Covalent Organic Framework Hybrids for Enhanced Photocatalytic Hydrogen Evolution(ACS, 2020-06-20) Banerjee, TanmayCovalent organic frameworks (COFs) display a unique combination of chemical tunability, structural diversity, high porosity, nanoscale regularity, and thermal stability. Recent efforts are directed at using such frameworks as tunable scaffolds for chemical reactions. In particular, COFs have emerged as viable platforms for mimicking natural photosynthesis. However, there is an indisputable need for efficient, stable, and economical alternatives for the traditional platinum-based cocatalysts for light-driven hydrogen evolution. Here, we present azide-functionalized chloro(pyridine)cobaloxime hydrogen-evolution cocatalysts immobilized on a hydrazone-based COF-42 backbone that show improved and prolonged photocatalytic activity with respect to equivalent physisorbed systems. Advanced solid-state NMR and quantum-chemical methods allow us to elucidate details of the improved photoreactivity and the structural composition of the involved active site. We found that a genuine interaction between the COF backbone and the cobaloxime facilitates recoordination of the cocatalyst during the photoreaction, thereby improving the reactivity and hindering degradation of the catalyst. The excellent stability and prolonged reactivity make the herein reported cobaloxime-tethered COF materials promising hydrogen evolution catalysts for future solar fuel technologies.