Electron Transfer into Electron-Accumulated Nanocrystals: Mimicking

Jul 31, 2018 - We measured electron-transfer (ET) from photoexcited molecular light-harvesters into doped QDs using transient absorption spectroscopy...
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Electron Transfer into Electron-Accumulated Nanocrystals: Mimicking Intermediate Events in Multielectron Photocatalysis II Junhui Wang, Tao Ding, and Kaifeng Wu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05942 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Journal of the American Chemical Society

Electron Transfer into Electron-Accumulated Nanocrystals: Mimicking Intermediate Events in Multielectron Photocatalysis II Junhui Wang, Tao Ding, and Kaifeng Wu* State Key Laboratory of Molecular Reaction Dynamics and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China, 116023

Supporting Information Placeholder and essentially zero electron-injection-yield.6 Thus, for molecular catalysts, this recombination step can be extremely detrimental; for nanocatalysts, however, it remains unknown. Last but not the least, the presence of extra charges on small nanocrystals not only statistically increases the number of recombination channels for charge-separated-states7 but also might open an Auger-assisted CR pathway similar to a recentlyintroduced Auger-assisted CT model.8 These can significantly accelerate CR and impose a big challenge for charge scavenging. Taking together, these effects could make the overall charge-separation-yield of a multielectron reaction much lower than that of a single-electron reaction.

ABSTRACT: The overall efficiency of multielectron photocatalytic reactions is often much lower than the chargeseparation-yield reported for the first charge-transfer (CT) event. Our recent study has partially linked this gap to CT from charge-accumulated light-harvesters. Another possible intermediate event lowering the efficiency is CT into chargeaccumulated nanocatalysts. To study this event, we built a “toy” system using nanocrystal quantum dots (QDs) doped with extra electrons to mimick charge-accumulated nanocatalysts. We measured electron-transfer (ET) from photoexcited molecular light-harvesters into doped QDs using transient absorption spectroscopy. The measurements reveal that the pre-existing electron slows down ET from 37.8±2.2 ps in the neutral sample to 93.4±8.6 ps in the singly-doped sample, accelerates charge-recombination (CR) from 7.02±0.84 ns to 3.69±0.25 ns, and lowers the electron-injection-yield by ~14%. This study uncovers yet another possible intermediate event lowering the efficiency of multielectron photocatalysis.

Many solar-to-fuel conversion processes involve multielectron reactions,1 which require sequential absorption of multiple solar photons coupled with multiple charge-transfer (CT) steps (Scheme 1a). As the turnover rates of charges on catalysts are often slower than the photon absorption rates of lightharvesters (LHs),2 while the first charge is injected into neutral catalysts, the ensuing ones are likely injected into chargeaccumulated catalysts. For molecular catalysts, this effect is primarily reflected as the change in the redox potential (~0.5−1.0 V for small molecules).2a, 3,4 For nanocatalysts, the accumulated charges generate charging energy acting as an additional energy penalty for CT, which was expected to significantly inhibit the second CT step.5

Scheme 1. a) A scheme showing how charged (e.g., negative) catalysts are generated in multielectron photocatalysis. Cat.: catalyst; SD: sacrificial donor. b) To mimick the above event, we dope a nanocrystal QD with an extra electron and then study electron-transfer from light-harvesters (LHs) into this negatively-charged model catalyst. ET: electron-transfer; CR: charge-recombination. Our recent study has partially linked the gap between high charge-separation-yields reported for the first CT step9 and much lower overall photocatalytic efficiencies9b, 9d to CT from charge-accumulated light-harvesters.10 Here we focus on CT into charge-accumulated nanocatalysts as yet another type of possible intermediate event lowering the efficiency of multielectron photocatalysis. In order to build an appropriate model system, we again use QDs doped with extra electrons,11 however, not as donors but rather as electron acceptors, and attach molecular LHs to these n-doped QDs (Scheme 1b). This is radically different from many previous studies of CT from QDs12 and is nontrivial as quantum confinement effect renders the band edges of QDs energetically inaccessible by most light-harvesting materials. This simple model system captures

Besides, the accumulated charges could immediately recombine with the photogenerated charge of-the-opposite-sign in the LH (Scheme 1a), competing with forward CT and lowering charge-injection-yield. For example, a previous study using [Ru(bpy)3]2+ linked to methyl-viologen (MV2+) showed that excitation of [Ru(bpy)3]2+ led to ET from [Ru(bpy)3]2+ to MV2+ in 4 ps; however, if MV2+ was pre-reduced to MV+ using electrolysis, excitation of [Ru(bpy)3]2+ led to rapid recombination (800 fs) between MV+ and the hole in [Ru(bpy)3]2+ 1

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the essence of the photoinduced CT and CR processes in the above event. We found that the pre-existing electron on QDs slowed down ET rate into QDs by ~2.5-fold, enhanced the CR rate of charged-separated-states by ~1.9-fold, and lowered the electron-injection-yield by ~14%. Note that we use QDs to mimick nanocatalysts instead of using real catalysts such as metal nanoclusters/nanoparticles because QDs exhibit distinct spectral features for the doped electrons11, which allows for a neat quantification of the effects of pre-existing electrons on ensuing ET steps. Although QDs have discrete electron levels, which are different from many metal nanoparticles having continuous bands, their charging energy should be comparable when their sizes are similar.

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Photoinduced electron-injection from AZs into QDs was studied using pump-probe transient absorption (TA) spectroscopy; see SI for details. Fig. 1b shows the TA spectra probed at indicated time delays following the excitation by a 600 nm pump pulse (500 µJ/cm2) that selectively excites the AZs. The spectra are dominated by a negative feature at ~450 nm that can be assigned to the exciton bleach (XB) of CdS QDs, indicating electron-transfer (ET) from photoexcited AZs into QDs.12, 15 Note that hole- and energy-transfer channels are energetically forbidden. In addition to QD features, an enlarged view of the TA spectra in Fig. 1b shows weak features from photoexcited AZs; we label the features centered ~530 nm and ~640 nm as AZ-1 and AZ-2, respectively. Another set of TA experiment performed using a 550 nm pump generates similar results (Fig. S2).

We built our model system using CdS QDs and alizarin (AZ) molecules for the following reasons: i) CdS QDs can be controllably doped with band-edge electrons11a; ii) photoexcited AZ molecules can inject electrons into QDs13; iii) the spectral features of CdS QDs and AZs are well-separated from each other. The CdS QDs we used have a first excitonic peak at ~450 nm (Fig. 1a). QD-AZ complexes were prepared by a mixing and sonication procedure; see supporting information (SI) for details. The absorption spectrum of the complexes in hexane exhibits an additional broad absorption band in the 500-600 nm range that can be assigned to surface-adsorbed AZs (Fig. 1a and inset). It is very different from that of pure AZs in toluene (Fig. S1), which has been attributed to chelation between AZs and metal cations that strongly modifies the electronic structure of AZs.13-14

The XB feature grows progressively within ~300 ps (Fig. 1c), which is a measure of interfacial ET from photoexcited AZs into QDs. Meanwhile, ET leads to the decay of the positive signal and the formation of a negative one in the AZ-1 region and an opposite change in the AZ-2 region (Fig. 1d). Based on these observations, we assign AZ-1 to overlapped excitedstate-absorption and ground-state-bleach signals and AZ-2 to overlapped stimulated-emission and AZ cation-absorption signals. After reaching its maximum, the XB feature starts to decay on the ns timescale. Because the band edge electrons in CdS QDs are long-lived (Fig. S3), the decay in Fig. 1c is a measure of charge-recombination (CR) between QD electrons and AZ cations. We simultaneously fit the XB kinetics in Fig. 1c and the AZ-1 and AZ-2 kinetics in Fig. 1d using the same set of ET and CR time constants, according to a model described in the SI. An important parameter in the fit is the excited-state-lifetime of surface-adsorbed AZs, which should be different from that of free AZs in toluene (~78 ps; Fig. S1). To this end, we measured AZs adsorbed to small-size CdS QDs which prohibit ET from AZs to QDs (Fig. S4), resulting in an excited-state-lifetime of 309±10 ps. Based on the fits, the averaged ET and CR time constants are 67.3±0.26 ps and 2.37±0.26 ns, respectively, and the electron-injection-yield from photoexcited AZs into QDs is calculated to be 83.2±7.8% (see SI). On the basis of understanding the spectral and dynamic features of QD-AZ complexes, we examine the effect of preexisting electrons in QDs on interfacial ET from AZs to QDs. we used a photochemical doping procedure to prepare n-doped CdS QDs with an extra electron in the conduction band (see SI).11a According to the absorption spectrum in Fig. 2a, the doping caused a bleach of the QD 1S exciton by ~8.3% (corresponding to 16.6% of QDs doped with single electrons by assuming a bimodal distribution10) as compared to neutral complexes which are obtained by exposing the same solution to the air.11a The doping level is controlled to be low to avoid heavily-charged species that would affect our spectral analysis.

Figure 1. a) Absorption spectrum of CdS QDs (blue) and CdS QD-alizarin (AZ) complexes (red) dispersed in hexane. Inset shows the absorption due to surface-adsorbed AZs. b) TA spectra of QD-AZ complexes, showing a CdS exciton bleach (XB) feature at ~450 nm and two AZ features (enlarged view in the inset) at ~530 nm (AZ-1) and ~640 nm (AZ-2). c) XB kinetics (blue squares) and its multi-exponential fit (black solid line); gray lines show the de-convoluted XB formation and decay kinetics. d) AZ-1 (green circles) and AZ-2 (red triangles) kinetics and their multi-exponential fits (black solid lines).

The ET and CR events following the excitation of AZs in ndoped QD-AZ complexes are illustrated in Fig. 2b. The photoexcited complexes (AZ*-QD-) undergo either ET from AZ* to QD- to form charge-separated-states (AZ+-QD2-), with a rate of kET, or CR to form metastable charged species (AZ--QD), with a rate of kCRa, or intrinsic recombination to the ground-state 2

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Journal of the American Chemical Society complexes (AZ-QD-), with a rate of kAZ. Both AZ+-QD2- and AZ--QD eventually return to AZ-QD-, with rates of kCR and kETa, respectively. In the following, we map out these rate constants and compare them with neutral samples using TA measurements.

doped (neutral) CdS-AZ complexes, the averaged ET time is 93.4±8.6 ps (37.8±4.7 ps), the CR time is 3.69±0.25 ns (7.02±0.25 ns), and the electron-injection-yield is calculated to be 75.4±6.5% (89.6±8.2%). Note that the time constants in the neutral sample are different from those in the pristine sample in Fig. 1, possibly because the QD energy levels are shifted due to surface modifications introduced by the doping procedure16. Nonetheless, the differences between the n-doped and neutral samples can only be ascribed to the doped electron. The differences are summarized as: i) the pre-existing electrons on QDs slow down the rate of ET into QDs by ~2.5-fold; ii) the CR rate of the charged-separated-states is accelerated by ~1.9-fold; iii) the competition between interfacial ET and CR lowers the electron injection yield by ~14%. It remains unclear whether the ~1.9-fold faster CR in the doped sample compared to the neutral one is due to the increase of recombination channels or the opening of Augerassisted CR process. A QD-size dependent study would help clarify this issue.8a The electron-injection-yield in the doped sample is lowered by only ~14% compared to the neutral one, which is in stark contrast to the zero-injection-yield induced by the pre-injected electron on MV+ in the [Ru(bpy)3]2+-MV+ system mentioned above.6 This is likely because the preexisting electrons on nanocatalysts are more delocalized in space and hence couple more weakly to the holes on lightharvesters, as compared to molecular catalysts. This comparison suggests that, at least for the sake of suppressing this CR process, nanocatalysts are likely better than molecular catalysts.

Figure 2. a) Absorption spectra of n-doped (light red) and neutral (dark red) QD-AZ complexes with the same concentration. Inset is the scheme for the doping process. b) A scheme showing the charge separation and recombination processes following the excitation of AZs in the n-doped QD-AZ complexes. c) TA spectra of n-doped QD-AZ complexes. d) XB kinetics in n-doped (blue squares) and neutral (gray squares) complexes and their multi-exponential fits (black solid lines). Inset shows the same kinetic traces but normalized at their maxima.

To summarize, in order to study the effect of accumulated charges on ensuing CT steps in multielectron photocatalytic reactions, we built a model system where the QDs as electron acceptors were doped with an extra electron and then measured ET from photoexcited alizarins into these n-doped QDs. The measurements have uncovered the role of the pre-existing electron in slowing down ET, accelerating CR, and lowering electron-injection-yield, providing important insights into the efficiency-limiting events potentially involved in solar-to-fuel conversion.

Fig. 2c shows the TA spectra of n-doped QD-AZ complexes probed at indicated time delays following the excitation by a 600 nm pump pulse (280 µJ/cm2); the TA spectra for neutral complexes measured under exactly the same conditions are plotted in Fig. S5. Comparing the neutral and n-doped QD-AZ complexes, we find that the QD XB forms with a faster rate in the latter than in the former (Fig. 2d). This is slightly counterintuitive as we expect the pre-existing electron on QDs should slow down electron-injection into them. However, according to the scheme in Fig. 2b, there is a competition between this electron-injection and a charge-recombination process, which leads to a faster apparent formation rate of the QD XB in the doped samples; see the kinetic model in the SI for details. According to this model, the competition should also lower the electron-injection-yield in the doped sample, which is indeed observed (Fig. 2d). In addition, the QD XB in the doped sample decays faster than that in the neutral sample, indicating faster charge-separated-states recombination in the former (AZ+-QD2-).

ASSOCIATED CONTENT Supporting Information. Figures S1-S5, sample preparations, TA experiment set-ups, kinetics fitting models, Table S1.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (21773239).

We fit the kinetics in Fig. 2d using the kinetic model in the SI that accounts for all the processes shown in Fig. 2b. Note that, because the nominally doped sample is a mixture of singlydoped and undoped QD-AZ complexes, these fits use a linear combination of kinetics expressions derived for singly-doped and undoped complexes. According to these fits, in singly-

REFERENCES 1. (a) Lewis, N. S.; Nocera, D. G., Proc. Natl. Acad. Sci. 2006, 103, 15729-15735; (b) Blankenship, R. E.; Tiede, D. M.; Barber, J.; Brudvig, G. W.; Fleming, G.; Ghirardi, M.; Gunner, M. R.; Junge, W.; Kramer, D. M.; Melis, A.; Moore, T. A.; Moser, C. C.; Nocera, D. G.; 3

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Nozik, A. J.; Ort, D. R.; Parson, W. W.; Prince, R. C.; Sayre, R. T., Science 2011, 332, 805-809; (c) Mallouk, T. E., J. Phys. Chem. Lett. 2010, 1, 2738-2739; (d) Bard, A. J.; Fox, M. A., Acc. Chem. Res. 1995, 28, 141145; (e) Chen, X.; Li, C.; Gratzel, M.; Kostecki, R.; Mao, S. S., Chem. Soc. Rev. 2012, 41, 7909-7937; (f) Sakimoto, K. K.; Wong, A. B.; Yang, P., Science 2016, 351, 74-77; (g) Brown, K. A.; Harris, D. F.; Wilker, M. B.; Rasmussen, A.; Khadka, N.; Hamby, H.; Keable, S.; Dukovic, G.; Peters, J. W.; Seefeldt, L. C.; King, P. W., Science 2016, 352, 448-450. 2. (a) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B., Acc. Chem. Res. 2009, 42, 1995-2004; (b) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J.-M., Science 2012, 338, 90-94. 3. Gimbert-Suriñach, C.; Albero, J.; Stoll, T.; Fortage, J.; Collomb, M.-N.; Deronzier, A.; Palomares, E.; Llobet, A., J. Am. Chem. Soc. 2014, 136, 7655-7661. 4. Hammarström, L., Acc. Chem. Res. 2015, 48, 840-850. 5. (a) Amirav, L.; Alivisatos, A. P., J. Am. Chem. Soc. 2013, 135, 13049-13053; (b) Amirav, L.; Alivisatos, A. P., J. Phys. Chem. Lett. 2010, 1, 1051-1054. 6. Reiner, L.; Tilmann, H.; Olof, J.; Leif, H., Chemistry – A European Journal 2002, 8, 102-110. 7. (a) Klimov, V. I., Annual Review of Condensed Matter Physics 2014, 5, 285-316; (b) Huang, J.; Huang, Z. Q.; Yang, Y.; Zhu, H. M.; Lian, T. Q., J. Am. Chem. Soc. 2010, 132, 4858-4864. 8. (a) Zhu, H.; Yang, Y.; Hyeon-Deuk, K.; Califano, M.; Song, N.; Wang, Y.; Zhang, W.; Prezhdo, O. V.; Lian, T., Nano Lett. 2014, 14, 1263-1269; (b) Olshansky, J. H.; Ding, T. X.; Lee, Y. V.; Leone, S. R.; Alivisatos, A. P., J. Am. Chem. Soc. 2015, 137, 15567-15575; (c) HyeonDeuk, K.; Kim, J.; Prezhdo, O. V., J. Phys. Chem. Lett. 2015, 6, 244-249.

Page 4 of 4

9. (a) Wu, K.; Zhu, H.; Lian, T., Acc. Chem. Res. 2015, 48, 851859; (b) Wu, K.; Lian, T., Chem. Soc. Rev. 2016, 45, 3781-3810; (c) Zhu, H.; Lian, T., Energy & Environ. Sci. 2012, 5, 9406-9418; (d) Lian, S.; Kodaimati, M. S.; Dolzhnikov, D. S.; Calzada, R.; Weiss, E. A., J. Am. Chem. Soc. 2017, 139, 8931-8938; (e) Zhukovskyi, M.; Tongying, P.; Yashan, H.; Wang, Y.; Kuno, M., ACS Catalysis 2015, 5, 6615-6623. 10. Wang, J.; Ding, T.; Wu, K., J. Am. Chem. Soc. 2018, 140, 7791-7794. 11. (a) Rinehart, J. D.; Schimpf, A. M.; Weaver, A. L.; Cohn, A. W.; Gamelin, D. R., J. Am. Chem. Soc. 2013, 135, 18782-18785; (b) Shim, M.; Guyot-Sionnest, P., Nature 2000, 407, 981-983; (c) Koh, W.-k.; Koposov, A. Y.; Stewart, J. T.; Pal, B. N.; Robel, I.; Pietryga, J. M.; Klimov, V. I., Scientific Reports 2013, 3, 2004. 12. Zhu, H.; Yang, Y.; Wu, K.; Lian, T., Annu. Rev. Phys. Chem. 2016, 67, 259-281. 13. Dworak, L.; Roth, S.; Wachtveitl, J., J. Phys. Chem. C 2017, 121, 2613-2619. 14. (a) Dworak, L.; Matylitsky, V. V.; Wachtveitl, J., ChemPhysChem 2009, 10, 384-391; (b) Duncan, W. R.; Prezhdo, O. V., J. Phys. Chem. B 2005, 109, 365-373. 15. Klimov, V. I., J. Phys. Chem. B 2000, 104, 6112-6123. 16. (a) Tsui, E. Y.; Hartstein, K. H.; Gamelin, D. R., J. Am. Chem. Soc. 2016, 138, 11105-11108; (b) Carroll, G. M.; Tsui, E. Y.; Brozek, C. K.; Gamelin, D. R., Chem. Mat. 2016, 28, 7912-7918.

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