Mechanism of Efficient Viologen Radical Generation by Ultrafast

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Mechanism of Efficient Viologen Radical Generation by Ultrafast Electron Transfer from CdS Quantum Dots FengJiao Zhao, Qiuyang Li, Ke-Li Han, and Tianquan Lian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06551 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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The Journal of Physical Chemistry

Mechanism of Efficient Viologen Radical Generation by Ultrafast Electron Transfer from CdS Quantum Dots Fengjiao Zhao†,‡,§, Qiuyang Li†, Keli Han‡ and Tianquan Lian†*

†

Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States

‡

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical

Physics, Chinese Academy of Sciences, Dalian 116023, China §

University of the Chinese Academy of Sciences, Beijing 100049, China.

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 of 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 sacrificial electron donor in aqueous (pH=7) solution. Under continuous 405 nm LED 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 (CB) to the mediators, following the order of BV2+ > MV2+ > PDQ2+, decreases 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 !" "

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than unity because of charge recombination, whose rates, following the order of BV2+ > MV2+ > PDQ2+, decreases for mediators with more negative redox potentials (and higher ET driving force in the Marcus’ inverted regime). In these systems, there exists a long live 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 charge recombination 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.

Key words: redox mediator; viologens; photoreduction; quantum dots, electron transfer, charge recombination.

Introduction Converting renewable and abundant solar energy into clean chemical fuels through artificial photosynthesis has attracted intensive attentions in recent years.1-4 Quantum confined semiconductor nanocrystals have shown great potentials as light-harvesting materials in many photosynthetic systems because their optical properties and redox potentials can be tuned by their sizes through the quantum confinement effect, and charge recombination times can be controlled by their morphologies.4-14 One of the most promising approaches is to use redox mediators to deliver electrons or holes from the light harvesting component to catalysts to drive the overall photo-driven catalytic transformation. Viologens are frequently used as mediators in many light-driven hydrogen generation and CO2 reduction systems, because their redox potentials conveniently fall between the CB edge of the semiconductor materials and the reduction potential of these important fuel forming reactions.10-13,

15-16

Moreover, viologen

radicals are relatively stable in anaerobic conditions and have distinct spectral signatures that enable unambiguous in situ spectroscopic studies.14-18 Photoreduction of methyl viologens, one of the most common viologen derivatives, #" "


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by quantum confined nanocrystals has been extensively studied.15, 17-19 This process was shown to depend sensitively on the morphology and composition of the nanocrystals and a near unity photoreduction QY has been reported using CdSe@CdS dot-in-rod nanorods. However, it has also been reported that the redox potential of methyl viologens is not sufficiently negative to drive H2 reduction reaction effectively at pH 7 or higher and the use of viologen derivatives with more negative potentials can significantly improve the overall QY.15-16 In such artificial photosynthetic systems, the overall reaction is complex and its QY depends on both the QYs of initial radical generation and the subsequent catalytic conversion driven by the radical, both of which can be influenced by the redox potentials of the mediators. Therefore, a detailed understanding of both processes is essential for the rational design of such mediatorbased photocatalytic systems. In this paper, we report a study of the photoreduction of viologen derivatives of different

redox

potentials,

7,

8-Dihydro-6H-Dipyrido[1,2-a:2',1'-c][1,4]

diazepinediium dibromide (PDQ2+, -550mV vs. NHE), methyl viologen (MV2+, -448 mv) and benzyl viologen (BV2+, -370 mv),20 using CdS QDs as a photo-sensitizor and 3-mercaptopropionic acid (MPA) as a sacrificial donor (Figure 1A &B). We show that the steady-state radical generation QY of these radical generation systems increases from BV2+, MV2+, to PDQ2+. To understand how the photoreduction QY depends on their redox potential, we carried out a detailed transient absorption (TA) spectroscopic study of the charge separation and recombination processes. We discuss how the rates and the efficiencies of these processes depend on the redox potentials of the mediator and how these dependences affect the overall steady-state radical generation efficiencies.

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Figure 2. Radical generation in CdS QD/viologen systems. (a) UV-vis absorbance spectra of CdS QDs with PDQ2+ after different illumination time (0-60s in 10s intervals). Inset: UV-Vis difference absorbance spectra (spectrum at time t – spectrum at t=0) showing the contribution of PDQ+•. (b) Amount of generated PDQ+•, MV+• and BV+• radicals as a function of illumination time in pH 7 buffer. (c) Viologen radical generation quantum yields within the first 5s. Experimental conditions: excitation by 405nm LED light at 2.3 mW; sample optical density of 0.3 at 405nm and in pH 7 aqueous solution with phosphate buffer; and 5mM mediator concentration.

Ultrafast ET from CdS QDs to viologens At

the

excitation

wavelength

used

for

steady-state

quantum

yield

measurements(405 nm), the viologen molecules have negligible absorption, and their photoreduction occurs by ET from the excited CdS QDs to viologen LUMO. As shown in Figure 1(B), driving forces for charge separation (CS), i.e. ET from the QD CB edge (at -3.40 eV vs vacuum) to PDQ2+, MV2+ and BV2+ are ~0.55 eV, ~0.65 eV and ~0.73 eV, respectively.20, 28-34 Details of energy level calculation are shown in the Supporting Information (SI2). The photogenerated radical can undergo charge recombination (CR) with the holes in the QD (VB or trapped below the band edge) or oxidized MPA molecules.35-39 Thus, effective suppression of the CR process is essential for achieving steady-state radical generation. To understand the observed trend of steady-state radical generation yield, we carried out transient absorption (TA) spectroscopy measurements to directly follow the charge separation and recombination processes. We first compare the TA spectra of CdS QDs with and without viologens in pH=7 buffer with 400 nm excitation (Figure 3a and 3b). The TA spectra of free CdS QDs (Figure 3a) show a 1S exciton bleach (XB), the negative peak at ~451nm. According to our previous studies, XB results from CB edge electron state-filling, probing the electron population on 1S CB level.40-43 Hole statefilling contributes negligibly to XB due to the strong degeneracy and mixing between dense hole levels in VB.15, 40, 42-43 The TA spectra of CdS QD-PDQ2+ complexes (Figure 3b) shows that in the presence of PDQ (5 mM), the 1S XB recovers faster than that of '" "


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To quantify ET rates from CdS QDs to different viologens, we fit the viologen concentration dependent XB kinetics to a model that is described in the Supporting Information (SI6). This model assumes a Poisson distribution of the number of acceptors on QDs (see Eq.S4),44 whose average (m) is related to the solution concentration by the Langmuir adsorption isotherm(SI7).32 It is also assumed that the ET rate increases linearly with the number of acceptors (n) on the QD. The best fits (solid lines in Figure 3d) give the intrinsic ET rates, kint, the ET rate per adsorbed viologen, of 0.21f0.01 ps-1, 0.13f0.01 ps-1, and 0.03f0.01 ps-1 for BV2+, MV2+ and PDQ2+, respectively (Figure 4a). This reveals that ET from the QD to BV2+ molecules is the fastest, followed by MV2+, and then PDQ2+. The transient ET yields are calculated by considering the competition between ET (kET) and exciton recombination (kx) (Eq. S9) as 97.40f0.05%, 95.87f0.06%, and 84.27f0.16% for BV2+, MV2+ and PDQ2+, respectively (see SI8). As shown in Figure 1, the reduction potential becomes less negative from BV2+, MV2+, and PDQ2+, which corresponds to a decreasing driving force for ET from the QD CB edge. Our result suggests that the ET rate from QDs to these viologens increases at larger driving force, consistent with previously observed trends from similar QDs to methyl viologen and other adsorbates.32, 45 The best fit also reveals the average number of adsorbed viologens on the QD surface, m, as a function of viologen concentration (Figure 4b). With increasing total concentration, the number of viologens adsorbed on QD surfaces first grows at low concentration (,1">5=/."

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Mechanism of Efficient Viologen Radical Generation by Ultrafast

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