Mechanism of Efficient Viologen Radical Generation by Ultrafast Electron Transfer from CdS Quantum Dots

Abstract

Artificial photosynthetic systems consisted of nanocrystal light absorbers, molecular redox mediators, and catalysts are one of the most promising and flexible approaches for solar fuel generation because their constituents can be independently tuned. In this work, we investigate the photoreduction of three viologen derivatives, one of the most widely investigated molecular redox mediators, of different redox potentials, 7,8-dihydro-6H-dipyrido­[1,2-a:2′,1′-c]­[1,4]­diazepinediium (PDQ2+), methyl viologen (MV2+), and benzyl viologen (BV2+), using CdS quantum dots (QDs) as the light absorber and mercaptopropionic acid as a sacrificial electron donor in aqueous (pH = 7) solution. Under continuous 405 nm light-emitting diode illumination, the steady-state radical generation quantum yield (QY) follows the order of PDQ•+ (15.99%) > MV•+ (12.61%) > BV•+ (6.56%). Transient absorption spectroscopy studies show that while the rates of initial electron transfer (ET) from the excited QD conduction band to the mediators, following the order of BV2+ > MV2+ > PDQ2+, decrease for mediators with more negative redox potentials (and lower ET driving force), the initial transient charge separation QYs are unity in all samples because these ET rates are much faster than the intrinsic exciton decay within the QD. The steady-state QYs are much smaller than unity because of charge recombination (CR), whose rates, following the order of BV2+ > MV2+ > PDQ2+, decrease for mediators with more negative redox potentials (and higher ET driving force in the Marcus’ inverted regime). In these systems, there exists a long-lived component in the radial decay kinetics, whose amplitudes determine the steady-state radical generation QYs. We speculate that the desorption of the radical from the QD surface is essential for the suppression of CR and is responsible for the steady-state generation of radicals. This work provides new insight for rational design and improvement of efficient QD/redox mediator-based photoreduction systems.

CsPbBr3–CdS heterostructure: stabilizing perovskite nanocrystals for photocatalysis

“…If indeed the CsPbBr 3 –CdS heterostructure is responsible for photocatalytic reduction, we should be able to observe the buildup of stable viologen radical under continuous photoirradiation. 62,63 …”

“…If indeed the CsPbBr 3 -CdS heterostructure is responsible for photocatalytic reduction, we should be able to observe the buildup of stable viologen radical under continuous photoirradiation. 62,63 Deaerated CsPbBr 3 -CdS nanocrystal suspensions in toluene : ethanol (85 : 15 v/v%) mixed solvent containing 100 mM of EV 2+ were subjected to steady-state visible light illumination (>400 nm; 200 mW cm À2 ). Absorbance spectra were recorded periodically during the steady state photolysis experiment.…”

Rational design of artificial redox-mediating systems toward upgrading photobioelectrocatalysis

“…Consequently, the photo-induced high energy electron flux is partially dissipated in extraneous metabolic activity reducing the capacity to generate electrical or chemical energy. Collectively, these complications are known as poor electrical-'contact' or 'wiring' between photosynthetic entities and electrode surfaces, which necessitate additional electron transfer routes to efficiently pass on the reducing energy [36][37][38][39]. The future scale-up of photobioelectrocatalysis relies heavily on improving this electric contact at the biohybrid interface.…”

Hot electron transfer in Zn–Ag–In–Te nanocrystal–methyl viologen complexes enhanced with higher-energy photon excitation

“…Charge separation dynamics with the use of various electron acceptors has been reported in many papers. [5][6][7][8][9][10][11][12][13][14][15][16][17] For example Banin et al reported that hybrid nanostructures of Au-tipped CdS nanorods with a larger Au size exhibited a higher probability for charge separation of multiple excitons formed in the CdS nanocrystals, improving the photocatalytic activity for H 2 generation. 15 The rate of photoinduced electron transfer from CdSe NCs to methyl viologen (MV 2+ ) was shown to be dependent on the particle morphology: plate-like CdSe NCs exhibited a slower rate than that of spherical or rod-shaped CdSe NCs due to the weak electronic coupling along the short and long axes in the lateral dimension.…”