Subscriber access provided by Kaohsiung Medical University
Communication
Charge Transfer from n-Doped Nanocrystals: Mimicking Interme-diate Events in Multi-Electron Photocatalysis Junhui Wang, Tao Ding, and Kaifeng Wu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04263 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society
Charge Transfer from n-Doped Nanocrystals: Mimicking Intermediate Events in Multi-Electron Photocatalysis 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 ABSTRACT: In multi-electron photocatalytic reactions, an absorbed photon triggers charge transfer from the lightharvester to the attached catalyst, leaving behind a charge of the opposite sign in the light-harvester. If this charge is not scavenged before the absorption of the following photons, photoexcitation generates not neutral but charged excitons from which the extraction of charges should become more difficult. This is potentially an efficiency-limiting intermediate event in multi-electron photocatalysis. To study the charge dynamics in this event, we doped CdS nanocrystal quantum dots (QDs) with an extra electron and measured hole transfer from n-doped QDs to attached acceptors. We find that the Auger decay of charged excitons lowers the charge separation yield to 68.6% from 98.4% for neutral excitons. In addition, the hole transfer rate in the presence of two electrons (1290 ps) is slower than that in the presence one electron (776 ps), and the recombination rate of charge separated states is about twice faster in the former case. This model study provides important insights into possible efficiency-limiting intermediate events involved in photocatalysis.
extraction of one charge from multi-excitons immediately generates charged excitons and the following events are also CT from charged excitons. Thus, charge extraction from charged excitons likely represents an important intermediate event in multi-electron photocatalysis. Previous mechanistic studies on many photocatalytic systems using ultrafast spectroscopy have been focused on the first CT event, which generally revealed very high-efficiency (near-unity) charge extraction from neutral excitons.4a, 6 Therefore, these exists a longstanding gap between this high charge extraction efficiency and relatively low overall photocatalytic efficiency (a few to tens percents) in many reported systems.4a One of the contributors to this gap might be the rarely-studied intermediate events, charge extraction from charged excitons.
Solar-to-fuel conversion has been envisioned and pursued as a key enabling technology for clean energy resources in the future.1 Many of these conversion processes2 are multielectron reactions in nature, which inevitably involve multiple sequential charge transfer (CT) events.3 These events are illustrated in Scheme 1a using catalyst-decorated semiconductor nanoparticles4 as an example. The semiconductor absorbs one solar photon and creates an electron-hole pair (an exciton), triggering interfacial CT from the nanoparticle to co-catalyst. This generates a charge-separated-state with one charge on the catalyst and the other on the nanoparticle. If the absorption cross section of the nanoparticle is very large or the reaction is performed under concentrated light illumination such that the remaining charge is not scavenged before the absorption of the the next photon, a charged exciton is generated in the nanoparticle, and hence the second CT is not from a neutral exciton but rather from a charged exciton. This process repeats and leads to a nanoparticle accumulated with more and more charges. Alternatively, if multi-excitons are simultaneously generated via the absorption of one high energy photon, as has been demonstrated for semiconductor quantum dots (QDs),5
Scheme 1. a) Generation of charged excitons in multi-electron photocatalysis. Hole instead of electron transfer is used as the example; see the main text for details. b) To mimick the above event, we dope a QD with an extra electron and study hole transfer from this n-doped QD. In very small semiconductor nanoparticles, the decay of a charged exciton is dominated by nonradiative Auger recombination whereby the recombination energy of an exciton is dissipated by exciting the extra carrier, often on the picoseconds timescale.7 This becomes very difficult for interfacial CT to compete with. Yet another difficulty associated with charge extraction from charged excitons is the effect of coulombic attraction by multiple charges on the rate of interfacial CT. In addition, the recombination rate of charge-separated-states should scale with the number of charges in the nanoparticle.8 These fundamental issues need to be investigated such as to obtain a complete picture for the CT events involved in multielectron photocatalysis.
1
ACS Paragon Plus Environment
Journal of the American Chemical Society In this work, we present a study of CT from charged nanocrystal QDs which represents a model system for the aforementioned intermediate photocatalytic events (Scheme 1b). Specifically, photoexcitation of an n-doped QD (i.e., QD doped with an electron) creates a so-called negative trion (X-) in the QD; hole transfer from this negative trion captures the essence of Scheme 1a. We choose QDs as their spectral and dynamical features of both Auger recombination5, 9 and interfacial CT10 have been well-established. More importantly, the development in post-synthetic treatments of colloidal QDs now allows for “digitized” electron doping into the band edge of QDs.7b, 11 We utilized this doping protocol to create n-doped CdS QDs and studied hole transfer to attached phenothiazine (PTZ) using transient absorption (TA) spectroscopy. We note that for realistic photocatalytic systems positively charged (p-doped) nanoparticles are more likely to generated than n-doped ones, as hole transfer is often much slower than electron transfer.12 However, p-doped II-VI QDs were found to be unstable.13 The fundamental physical insights obtained from this study should be translatable to the problem of electron transfer from pdoped QDs.
To prepare n-doped CdS QDs with an extra electron in the conduction band, we adopted a photochemical doping method in which lithium triethylborohydride (LiEt3BH) was added to QD solution under illumination.11a As shown in Fig. 2a, this procedure caused a progressive bleach of 1S exciton absorption, with the bleach amplitude controlled by the amount of added LiEt3BH, indicative of electron injection into the 1Se level partially blocking the optical transition.11a Exposure of the n-doped sample to the air immediately recovers the absorption spectrum due to electron release (Fig. 2a); therefore, all the measurements for n-doped QDs were performed under strictly air-free conditions. To exclude any possible changes to sample introduced by the photodoping procedure, in the following experiments, n-doped samples are compared to their corresponding neutralized samples (termed “neutral”) instead of the pristine sample.17
In our experiment, we chose PTZ as hole acceptors because of its suitable oxidation potential and its characteristic absorption at ~520 nm in the oxidized form that would facilitate spectroscopic analysis14. The QDs we used exhibit a lowest energy exciton peak at ~450 nm (Fig. 1a, top), with an average diameter of 4.8±0.7 nm (see TEM images in Fig. S1); see Supporting Information (SI). Fig. 1a shows the pump-probe TA spectra of QDs probed at indicated time delays following the excitation by a 400 nm pulse. The pump energy density was 28 µJ/cm2 , corresponding to an average exciton number of 0.03 per dot ( ~ 0.03); see Figs. S2 and S3 for details. The TA spectra were dominated by a long-lived exciton bleach (XB) feature that shows only ~25% decay within 7 ns (Fig. 1a). Selective removal of electrons from photoexcited QDs using benzoquinone (BQ) molecules15 shows that within 3 ns the XB feature of CdS-BQ complexes almost decays to zero, indicating that XB in CdS QDs is dominated by state-filling effect of band-edge electrons;10a, 16 see also Fig. S4.
b
CdS QD
16
0
Figure 2. a) Absorption spectra of pristine and n-doped CdS QDs with various doping levels (blue solid lines) and neutralized QDs obtained by exposing the n-doped QDs to the air (blue circles). b) TA kinetics probed at the XB feature of ndoped-1 (light blue) and neutral (blue) CdS QDs. Inset shows that the negative trion (X-) recombination kinetics (blue squares) and its single-exponential fit (black solid line). The XB kinetics in n-doped QDs was measured using TA spectroscopy (see Fig. S5 for TA spectra). As shown in Fig. 2b, after normalizing at the slowly-decaying tail, the XB kinetics of n-doped-1 QDs exhibits faster decay compared to neutral QDs. By performing a subtraction between the two kinetic traces, which removes the contribution from undoped QDs in the doped ensemble, the recombination dynamics of negative trions (X-) can be isolated (Fig. 2b inset), which has a time constant of 1.3±0.1 ns, dominated by Auger recombination. Measurements on n-doped-2 and 3 QDs gave similar results (Fig. S6) except that higher doping levels tend to generate a faster decay component that is likely due to some heavily charged species in the ensemble. We notice that the X- lifetime is ~6-fold longer than the biexciton lifetime (~220 ps; Fig. S3b). It is worth mentioning that, in addition to the trion decay component, the XB kinetics of both neutral and n-doped samples show an additional fast decay on the 10s of ps timescale (~5% amplitude) as compared to that of pristine CdS QDs (Fig. 1b). This is likely due to the electron trapping sites introduced by the photodoping procedure, similar to previous reports.11a
XB
Abs 8 0 -8 -16 -24
XB 450
0 ps 1-3 ps 0.3-0.8 ns 1-3 ns 4-7 ns n-doped
500 550 Wavelength (nm)
600
∆ Abs (mOD)
a Abs or ∆ Abs (mOD)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
-8 CdS CdS-BQ
-16
-24 1
10
100 Delay (ps)
Page 2 of 4
1000
Figure 1. a) Absorption spectrum of CdS QDs (gray line, top) and their TA spectra (blue lines, bottom). The difference between absorption spectra of n-doped and neutral QDs was also shown (scaled) for comparison (gray circles). b) TA kinetics probed at the XB of CdS QDs (blue line) and CdS-BQ complexes (green line).
We then study the effect of the doped electron on interfacial hole transfer (HT) from QDs to attached PTZ.14a, 14c, 14d As illustrated in Fig. 3a, the photoexcited, n-doped CdS-PTZ
2
ACS Paragon Plus Environment
Page 3 of 4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society complexes (QD-*-PTZ) can decay either via HT to form charge-separated-states with two electrons in QD and one hole on PTZ (QD2--PTZ+), with a rate of kHT, or via Auger recombination to form ground state n-doped CdS-PTZ complexes (QD--PTZ), with a rate of kAR. QD2--PTZ+ eventually recombines to form QD--PTZ with a rate of kCR. To map out the dynamics of these processes, we measured the TA spectroscopy of n-doped CdS-PTZ complexes; see SI for sample details and Fig. S7 for their absorption spectra. The average number of PTZ molecules attached per QD is estimated to be ~5.8 (SI).
cesses in neutral and n-doped CdS-PTZ complexes; see details there. Using these models, we simultaneously fit the kinetics of XB and PTZ+ features for both neutral and n-doped samples (SI and Table S1; Fig. S9). Based on the fits, in n-doped (neutral) CdS-PTZ complexes, the averaged HT time is 1290 ps (776 ps), the charge recombination time is 8.3 ns (18 ns), and the charge separation yield is 68.6% (98.4%). This comparison reveals several important effects of the pre-existing charge on the ensuing CT events. First, the HT rate in the presence of two electrons is slower (by ~1.6-fold) than that in the presence of one electron due to a stronger coulombic attraction in the former case. Secondly, the charge recombination rate of a hole with two electrons is considerably faster (by ~2.2-fold) than that of a hole with one electron due to doubling of the number of recombination channels in the former case. Last but not least, the competition between Auger recombination and interfacial HT leads to a considerably lower (by ~1.4-fold) charge separation yield in the doped sample than in the neutral sample. We note that in our current system, because X- lifetime is 1.3 ns and hole transfer time is 776 ps, the effect of X- on lowering the hole extraction yield is not impressively large. However, these time constants vary from system to system, if X- lifetime is similar but hole transfer time is ~4 ns, as reported in a previous system for example,14d the hole extraction yield might be lowered by ~4-fold. In conclusion, we have measured HT from n-doped QDs to investigate the influence of charges accumulated in the lightharvester on the ensuing CT events. By doping CdS QDs with one electron and measuring interfacial HT from doped QDs to attached PTZ acceptors, we revealed three aspects of effects of the doped electron on HT process. These results provide important insights into the possible intermediate events involved in multi-electron photocatalysis.
Figure 3. a) A scheme showing the competition between trion Auger recombination (kAR) and interfacial hole transfer (kHT) in photoexcited, n-doped CdS-PTZ complexes. b) TA spectra of n-doped CdS-PTZ complexes. Inset shows the formation of PTZ+ signal. c) TA kinetics probed at the XB of neutral (red) and n-doped (light red) CdS-PTZ complexes normalized at their long-lived tail. Inset shows the kinetics (light red circles) obtained by performing a subtraction between XB kinetics in the main panel. The black solid line is a fit. d) TA kinetics probed at the PTZ+ signal of neutral (red) and n-doped (light red) CdS-PTZ complexes. The black solid lines are fits to these kinetics. Inset shows the comparison between these kinetics after normalizing them at their maxima.
ASSOCIATED CONTENT Supporting Information. Figures S1-S9, Table S1, sample preparations, TA experiment set-ups, estimation of the number of attached PTZ molecules per QD, kinetics fitting models. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
Fig. 3b and Fig. S7 show the TA spectra of n-doped and neutral CdS-PTZ complexes, respectively. Both sets of spectra show the XB feature of CdS QDs at ~450 nm and another absorptive feature at ~520 nm that can be assigned to the oxidized PTZ (PTZ+).14 Comparing the kinetics of these features in neutral and n-doped CdS-PTZ complexes, we find that XB (Fig. 3c) decays faster in the latter than in the former, consistent with the existence of X- Auger recombination in the latter. The PTZ+ feature, obtained by subtracting the contribution of the overlapping photoinduced absorption background (see Fig. S8 for details), in the n-doped sample shows a faster formation rate, a lower maximum signal amplitude, and a faster decay rate than that in the neutral sample (Fig. 3d). This is also consistent with a competition between interfacial HT and X- Auger recombination. In the SI, we derived kinetic models describing all the charge separation and recombination pro-
*
[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).
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.; 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,
3
ACS Paragon Plus Environment
Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1, 2738-2739; (d) Bard, A. J.; Fox, M. A., Acc. Chem. Res. 1995, 28, 141145. 2. (a) Chen, X.; Li, C.; Gratzel, M.; Kostecki, R.; Mao, S. S., Chem. Soc. Rev. 2012, 41, 7909-7937; (b) Liu, C.; Tang, J.; Chen, H. M.; Liu, B.; Yang, P., Nano Lett. 2013, 13, 2989-2992; (c) Sakimoto, K. K.; Wong, A. B.; Yang, P., Science 2016, 351, 74-77. 3. Hammarström, L., Acc. Chem. Res. 2015, 48, 840-850. 4. (a) Wu, K.; Lian, T., Chem. Soc. Rev. 2016, 45, 3781-3810; (b) Subramanian, V.; Wolf, E.; Kamat, P. V., J. Phys. Chem. B 2001, 105, 11439-11446. 5. Klimov, V. I., Annu. Rev. Phys. Chem. 2007, 58, 635-673. 6. (a) Wu, K.; Zhu, H.; Lian, T., Acc. Chem. Res. 2015, 48, 851859; (b) Zhu, H.; Lian, T., Energy & Environ. Sci. 2012, 5, 9406-9418; (c) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X., J. Phys. Chem. B 2003, 107, 6668-6697; (d) Kodaimati, M. S.; McClelland, K. P.; He, C.; Lian, S.; Jiang, Y.; Zhang, Z.; Weiss, E. A., Inorg. Chem. 2018, 57, 36593670. 7. (a) Jha, P. P.; Guyot-Sionnest, P., ACS Nano 2009, 3, 10111015; (b) Cohn, A. W.; Rinehart, J. D.; Schimpf, A. M.; Weaver, A. L.; Gamelin, D. R., Nano Lett. 2014, 14, 353-358; (c) Klimov, V. I.; Mikhailovsky, A. A.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G., Science 2000, 287, 1011-1013; (d) Robel, I.; Gresback, R.; Kortshagen, U.; Schaller, R. D.; Klimov, V. I., Phys. Rev. Lett. 2009, 102, 177404. 8. Huang, J.; Huang, Z. Q.; Yang, Y.; Zhu, H. M.; Lian, T. Q., J. Am. Chem. Soc. 2010, 132, 4858-4864. 9. Kambhampati, P., J. Phys. Chem. Lett. 2012, 3, 1182-1190. 10. (a) Zhu, H.; Yang, Y.; Wu, K.; Lian, T., Annu. Rev. Phys. Chem. 2016, 67, 259-281; (b) Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V.,
Page 4 of 4
Proc. Natl. Acad. Sci. 2011, 108, 29-34; (c) Harris, R. D.; Bettis Homan, S.; Kodaimati, M.; He, C.; Nepomnyashchii, A. B.; Swenson, N. K.; Lian, S.; Calzada, R.; Weiss, E. A., Chem. Rev. 2016, 116, 12865-12919. 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. Wu, K.; Chen, Z.; Lv, H.; Zhu, H.; Hill, C. L.; Lian, T., J. Am. Chem. Soc. 2014, 136, 7708-7716. 13. Qin, W.; Guyot-Sionnest, P., ACS Nano 2012, 6, 9125-9132. 14. (a) Huang, J. E.; Huang, Z. Q.; Jin, S. Y.; Lian, T. Q., J. Phys. Chem. C 2008, 112, 19734-19738; (b) Christensen, J. A.; Phelan, B. T.; Chaudhuri, S.; Acharya, A.; Batista, V. S.; Wasielewski, M. R., J. Am. Chem. Soc. 2018, 140, 5290-5299; (c) Lian, S.; Weinberg, D. J.; Harris, R. D.; Kodaimati, M. S.; Weiss, E. A., ACS Nano 2016, 10, 6372-6382; (d) Wu, K.; Du, Y.; Tang, H.; Chen, Z.; Lian, T., J. Am. Chem. Soc. 2015, 137, 10224-10230. 15. (a) Wu, K.; Zhu, H.; Liu, Z.; Rodríguez-Córdoba, W.; Lian, T., J. Am. Chem. Soc. 2012, 134, 10337-10340; (b) McArthur, E. A.; MorrisCohen, A. J.; Knowles, K. E.; Weiss, E. A., J. Phys. Chem. B 2010, 114, 14514-14520; (c) Burda, C.; Green, T. C.; Link, S.; El-Sayed, M. A., J. Phys. Chem. B 1999, 103, 1783-1788. 16. Klimov, V. I., J. Phys. Chem. B 2000, 104, 6112-6123. 17. (a) Wu, K.; Park, Y.-S.; Lim, J.; Klimov, V. I., Nat Nano 2017, 12, 1140-1147; (b) Wu, K.; Lim, J.; Klimov, V. I., ACS Nano 2017, 11, 8437-8447.
Insert Table of Contents artwork here
4
ACS Paragon Plus Environment
