Direct Observation of Photoexcited Hole Localization in CdSe

Apr 22, 2016 - Cooney , R. R.; Sewall , S. L.; Anderson , K. E. H.; Dias , E. A.; Kambhampati , P.Breaking the Phonon Bottleneck for Holes in Semicond...
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Direct Observation of Photoexcited Hole Localization in CdSe Nanorods Ye Yang,† Kaifeng Wu,‡ Andrew Shabaev,§ Alexander L. Efros,§ Tianquan Lian,‡ and Matthew C. Beard*,† †

Chemical and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States The Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States § Center for Computational Material Science, Naval Research Laboratory, Washington, DC 20375, United States ‡

S Supporting Information *

ABSTRACT: Quantum-confined 1D semiconductor nanostructures are being investigated for hydrogen generation photocatalysts. In the photoreaction, after fast electron transfer, holes that remain in the nanostructure play an important role in the total quantum yield of hydrogen production. Unfortunately, knowledge of hole dynamics is limited due to lack of convenient spectroscopic signatures. Here, we directly probe hole localization dynamics within CdSe nanorods (NRs) by combining transient absorption (TA) and time-resolved terahertz (TRTS) spectroscopy. We show that when methylene blue is used as an electron acceptor, the resulting electron transfer occurs with a time constant of 3.5 ± 0.1 ps and leaves behind a delocalized hole. However, the hole quickly localizes in the Coulomb potential well generated by the reduced electron acceptor near the NR surface with time constant of 11.7 ± 0.2 ps. Our theoretical investigation suggests that the hole becomes confined to a ∼±0.8 nm region near the reduced electron acceptor and the activation energy to detrap the hole from the potential well can be as large as 235 meV. is much more efficient.13−15 Compared with well-characterized electron dynamics in NRs, the hole dynamics remain an open question due, in part, to the lack of convenient spectral signatures.18−22 For example, time-resolved photoluminescence (TRPL) measures only hole dynamics in the absence of electron acceptors because in only that case, the PL decay is determined by hole transfer.19,23,24 Transient absorption (TA) spectroscopy is only sensitive to the electron in CdSe systems. In this Letter, we directly investigate the photoinduced hole localization in CdSe NRs in the presence of an electron acceptor by combining time-resolved THz spectroscopy (TRTS) with TA spectroscopy. The CdSe NRs studied here are synthesized by following published reports.25 The diameter and length, determined from TEM analysis (Figure 1A), are 3.5 ± 0.3 and 21.7 ± 2.3 nm, respectively. The absorption spectrum (Figure 1B, red trace) shows a prominent exciton transition band peaked at 564 nm. As demonstrated in previous publications,26,27 ultrasonicating a mixture of methylene blue chloride (MB+Cl−) and a CdSe NR heptane solution results in a NR−MB complex.27,28 Because MB+Cl− is not soluble in heptane, after filtration, all remaining MB+ in the solution is assumed to be absorbed onto the NR surface. Because MB+ molecules are positively charged, their adsorption is accompanied by Cl− to preserve charge neutrality,

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ecently, quantum-confined semiconductors have emerged as a new class of photocatalyst. They possess many unique properties such as large absorption coefficient, tunable band gap energy, and tailorable wave function, making them promising in solar fuel applications such as water splitting and CO2 reduction.1−10 Among them, semiconductor nanorods (NRs) have been intensively studied for photosynthetic hydrogen production.1,3,4,8 Decorating the NR surface with a hydrogen evolution catalyst such as platinum, transition-metal compounds, enzyme, and/or other systems improves their photocatalytic activity. The catalytic reaction starts by the capture of photons by the NRs resulting in electron−hole pair generation within quantum-confined states of the NR. Subsequently, photogenerated electrons undergo charge transfer to a hydrogen generation catalyst within a characteristic time of 0.1 ps−1 ns,7,11−13 and the remaining holes are removed by oxidizing sacrificial electron donors on the time scale of 0.1−100s ns.13−17 Finally, the electrons at the catalyst are removed by reducing protons, and the whole photocatalytic system recovers, ready for the next reaction cycle. In the reaction cycle outlined above, charge separation is the primary step and is directly related to the hydrogen generation quantum yield. Spectral resolving electron and hole features, in time-resolved spectroscopic experiments, can probe such charge-separation dynamics and allow for tailoring specific parameters so as to design efficient systems. Previous studies have shown that hydrogen generation is limited by the rate of hole removal because the electron transfer (ET) to the catalyst © XXXX American Chemical Society

Received: March 28, 2016 Accepted: April 22, 2016

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DOI: 10.1021/acsenergylett.6b00036 ACS Energy Lett. 2016, 1, 76−81

Letter

http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters

complex and that for the NR (red and blue dashed traces in Figure 1B) are the same; thus, MB+ does not absorb the pump light in these experiments. The TRTS apparatus has been described in detail elsewhere.29 Briefly, the sample is photoexcited by a 400 nm pulse and subsequently probed by a single-cycle THz pulse. The optical excitation of the sample results in a change of the transmitted THz electric field, denoted ΔE(t;τp), where t and τp are the THz delay and pump−probe delay, respectively. The frequency-dependent complex photoconductivity, σ̃(w ; t p), can be determined at any pump−probe delay, τp (see SI for details). Due to the lack of knowledge concerning the NR concentration, the photoconductivity measured in this work is the effective photoconductivity of the NR solution, σ̃eff . CdSe NRs. The complex photoconductivities of the NR solution at three different delays are displayed in Figure 2A−C, where the blue circles represent the real part of the complex conductivity, Re[σ (̃ ω; τp)], and the red circles are the imaginary part, Im[σ (̃ ω; τp)]. At the short delay (τp = 0.3 ps), both real and imaginary components are nonzero. The nonzero real conductivity arises from the response of free carriers that are generated by absorption of 400 nm photons, and they act as free particles in the longitudinal direction of NRs, while the negative imaginary component is indicative of the spatial restriction of the carrier motion in the transverse direction of the NRs.30−32 As the delay increases, the real component decays faster than the imaginary part. As shown by panels B and C, the imaginary component begins to dominate the total photoconductivity at 2 ps, while the real component decreases and completely vanishes by 10 ps. The remaining imaginary component decreases linearly with increasing THz

Figure 1. (A) TEM image of a CdSe NR sample. The scale bar represents 100 nm. (B) Absorption spectra of the NR and NR− MB+Cl− complex in the visible-NIR region.

and the result is an overall neutral complex, which we denote as NR−MB. The absorption spectrum of the NR−MB complex (Figure 1B, blue trace) shows an additional peak centered at 670 nm, assigned to the ground-state (GS) absorption band of MB+, and the increased absorption at the NR exciton peak is due to the overlap with the tail of that absorption. The absorption at 400 nm (pump wavelength) for the NR−MB

Figure 2. THz photoconductivity of NRs. Real (blue circles) and imaginary (red triangles) components of photoconductivity determined from THz measurement at pump−probe delays of (A) 0.3, (B) 2, and (C) 10 ps. Real and imaginary components are plotted with blue and red symbols, respectively. The y-axes represent the effective photoconductivity of the solution, which is proportional to the photoconductivity of the NR for a constant concentration. (D) Pump-induced change in the transmitted THz electric field with respect to pump−probe delay time (black circles), compared with the first exciton bleach recovery from TA measurement (red triangles). The sample was excited at 400 nm with a photon fluence of 2.8 × 1014 cm−2 for both THz and TA measurements. The green and blue dashed lines represent a triexponential function in which two exponential terms describe the multiexciton annihilation and one term describes the exciton formation (THz data) or hot carrier relaxation (TA data). We find an average multiexciton recombination time of 58 ± 1 ps, while the exciton formation and hot carrier relaxation time constants are 1.5 and 0.3 ps, respectively. 77

DOI: 10.1021/acsenergylett.6b00036 ACS Energy Lett. 2016, 1, 76−81

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ACS Energy Letters frequency, which is characteristic of excitonic response to the THz electric field. Such spectral differences between free carriers and excitons have been discussed previously in the literature.33−37 Thus, the THz dynamics at early delay times tracks the formation of excitons within the NRs. A similar THz spectral evolution was also observed in polymer semiconductors,36 ZnO nanocrystals (NCs),33 and silicon quantum dots (QDs).38 Carrier dynamics are measured by fixing the THz delay at the maximum value of the THz pulse, tmax, and scanning the pump delay, yielding ΔE(tmax;τp)/E, and can be related to the averaged time-dependent photoconductivity of the sample.39,40 A representative dynamical trace is shown in Figure 2D. As discussed above, the THz modulation mainly arises from exciton polarization in the range of 2−100 ps; therefore, the kinetics reflects the exciton dynamics on this time scale. Also plotted is the exciton bleach kinetics obtained from TA measurement (see the SI for details) under the same excitation condition (red trace, Figure 2D). The maximum amplitudes of the two kinetic traces are normalized to −1 for an easier comparison. The exciton bleach in the TA spectra (Figure S1) results from state filling of the lowest electron level in the CdSe NRs,41,42 thus tracking electron dynamics. The decay of the THz signal is consistent with the TA bleach recovery in the region of 4−100 ps, suggesting that both electrons and excitons undergo the same depopulation mechanism. Therefore, we rule out electron surface trapping due to the lack of decay of the exciton bleach under low excitation intensity (Figure S2). The depopulation of photocarriers can therefore only be caused by exciton−exciton annihilation via an Auger process. At very early delay (5 ns according to the results at longer delays (Figure S3), consistent with the negligible recovery of MB+ GSB by 100 ps. Figure 4A−C shows the frequency-dependent complex photoconductivity of the NR−MB complex. From 2 to 3 ps, the real component does not change, while the imaginary component decays. As discussed before, exciton polarization leads to a linear imaginary component and a zero real component. Thus, the exciton dissociation is mainly responsible for the change in photoconductivity prior to 3 ps, indicating that excitons form on the same time scale as ET. At

Figure 3. TA measurement of the NR−MB complex. (A) TA spectra of the complex at indicated delays. The two bleach peaks are assigned to the exciton bleach of the NR and the ground-state bleach (GSB) of MB+. (B) The kinetics of exciton bleach recovery and the MB+ GSB formation obtained from the TA measurement of the NR−MB complex. The black solid lines are the fitting curves. The sample was excited at 400 nm with a photon fluence of 2.8 × 1014 cm−2.

longer delay (≥3 ps), the real component of the photoconductivity for the NR−MB complex is as significant as the imaginary component (in stark contrast to the NR case), and it increases with increasing THz frequency, indicative of the presence of free charge carriers. Similar photoconductivity spectra have also been observed in many spatially confined systems such as carbon nanotubes,40 silicon nanoparticles and nanowires,29,31 disordered semiconductor polymers,36 and QDs and rods.30,44 Because the TA experiment has already determined that most electrons are extracted by MB+ before 5 ps, the THz measures the photoconductivity of the remaining holes in NRs. Figure 4D shows normalized ΔE(τp;t)/E. The magnitude of the kinetics decays more than 90% by 100 ps, and the time constant is determined to be 11.7 ± 0.2 ps from exponential fitting. The decay of the THz signal can occur either through the decay of photogenerated holes or through the reduction of the hole mobility. Due to the slow recombination rate revealed from the TA measurement (Figure 3B), the depopulation of the hole is negligible by 100 ps, and therefore, the decay of the THz response must be caused by the reduction of the hole mobility. Thus, the kinetics in Figure 4D directly reflects the immobilization of the hole in the NRs (schematically shown in Figure 5A).

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THEORY After electrons undergo charge transfer from the NRs to MB+, the whole system undergoes electronic and/or spatial rearrangement of the polarization in the NR, electron acceptor, and the surrounding medium to compensate for the charge redistribution and reach quasi-equilibrium. To explore the effect of the reduced electron acceptor on the process of hole 78

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Figure 4. Frequency-dependent complex photoconductivity of the NR−MB complex at (A) 2, (B) 3, and (C) 10 ps pump−probe delays. Real and imaginary components are plotted with blue circles and red triangles, respectively. (D) Kinetics of the electric field change of the THz probe. The green line is the exponential fit. The sample was excited at 400 nm with a photon fluence of 2.8 × 1014 cm−2.

have undergone one and two ET events. Multiple point charges located at different positions of the NR will create a wider potential well than that considered here, which may affect the hole localization dynamics. As illustrated in Figure 5A, the negative charge creates an electric potential well in the NR that can localize the hole due to the Coulomb attraction. We follow the method developed previously to predict the electronic structure and exciton binding energy in CdSe NRs45 (greater detail is provided in the SI). As in those calculations, here we assume that the perpendicular motion of the hole remains unperturbed by the interaction with the electron. The Coulomb potential in the NR can then be approximated as V (z ) =

−e 2 κ m(|z| + ρeff )

(1)

where V(z) is the potential well depth, z is the distance from the negative charge in the longitudinal direction, κm is the dielectric constant for the dispersion medium, and ρeff is the effective screening length, which is a function of distance of the negative charge from the NR surface (d). In this potential, the hole states are quantized, and the two lowest energy states are listed in Table S1 as a function of d (distance of the electron from the NR surface). In the following, we approximate d as the radius of the basic aromatic ring in MB+ (d = 0.14 nm) and κm = 2 for nonpolar solvent (hexane); the calculated potential well, energies, and wave functions are plotted in Figure 5B. The model shows that the potential energy of the hole is quantized, and the two lowest energy levels E1 and E2 are determined to be −235 and −156 meV, respectively. The hole occupies the E1 state at room temperature and is localized within ±0.8 nm around the electron in the z direction (Figure 5B). The activation energy to detrap the hole from the potential well is equal to −E1 (235 meV). For a NR with length of ∼21 nm, the Coulomb potential can exert influence on the hole at any position of the rod. The kinetics measured by the THz probe (Figure 4D) depicts the dynamics of the hole being captured in this potential.

Figure 5. (A) Schematic illustration of the hole localization after ET. The shaded area represents the Coulomb potential profile along the longitudinal direction. The red shadow area represents the distribution of the hole wave function density. (B) Coulomb potential well (black line) and hole wave function density distribution in charge-separated NR−MB. The energy of the hole in the Coulomb potential well is quantized, and the wave functions of the two lowest states are displayed.

localization, we simplify this quasi-equilibrium system to a static electric system, in which the reduced molecular acceptor associated with the surrounding environment is idealized as negative point charge outside of the NRs and the hole is delocalized in the NRs. Futhermore, we consider only the case where one electron is localized at the surface of the NR while in the experiment there is a distribution of NR−MB species that 79

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It was estimated in ref 45 that in NRs with a radius of 3 nm, the binding energy of the delocalized 1D exciton (the case without an electron acceptor) is ∼150 meV; for a radius of 1.75 nm, the binding energy should be about 50 meV higher (∼217 meV). We find that when the transferred electron is localized with d < 0.3 nm, the “charge-separated” state has a larger binding energy than the delocalized excitons (in the absence of MB+). Our results suggest that in the initial charge separation step, it is preferable to spatially separate the electron from NRs with d > 0.3 nm to facilitate the removal of holes. We can understand the larger binding energy by considering how the effective masses of the carriers are modified in the two cases. In 1D nanostructures, the electron−hole binding energy is proportional to the reduced electron and hole mass.45 In the case of the charge-separated state, the hole is attracted to the electron, which is localized on the reduced molecule, and the reduced mass is approximately the hole effective mass because the electron is immobilized. In the case of the delocalized exciton, the reduced mass can be approximated as electron effective mass because the hole is much heavier than the electron in CdSe. Therefore, the binding energy in the NR− MB case is higher than that in the NR case. Furthermore, the large effective mass results in a small Bohr radius, and the hole in the NR and the electron localized on the MB are thus tightly bound due to the small Bohr radius of the hole. On the other hand, the average electron−hole distance in the NR exciton (exciton radius) is significantly larger due to the much smaller effective mass of the electron, with a correspondingly larger Bohr radius. It is important to note that contrary to NRs, in spherical NCs, the electron−hole Coulomb interaction decreases when the electron is transferred from the NC to the MB+. When the electron and hole are both inside of a NC, the Coulomb energy is 1.8e2/aNC, where aNC is the NC radius. This energy decreases to e2/aNC if the electron is trapped at the NC surface (the 1s hole distribution is assumed to be spherical). The change in the binding energy contributes to the apparent driving force of the ET process.46 In the ET process, the increase of the binding energy for NRs should result in a driving force larger than that for NCs when the electron energy levels are similar in both nanomaterials. Consequently, a higher ET rate from NRs than that from NCs is expected when the driving force falls in the Marcus normal region. By combining the TRTS and TA techniques, we directly investigate the photoinduced hole localization in CdSe NRs after fast ET. Our results demonstrate that after the ET, the hole is localized near the electron acceptor within a time constant of 11.7 ps. The localized hole and electron are longlived, with a lifetime over 5 ns. In a NR−photocatalyst system, the quantum yield can be reduced by the charge recombination between the electrons in catalysts and the hole in the NRs.47,48 When the hole is trapped around the reduced catalysts, the chance for charge recombination increases. Thus, our results suggest three directions to improve the yield for NR photocatalyst systems: (1) use of an electron donor that can fill the hole faster than the hole localization; (2) employment of a dot-in-rod heterostructure that can localize the hole in a spatially separated location from the reduced catalyst; (3) use of an electron reducing agent that localizes the electron at a distance that is greater than ∼1 nm from the NR surface. Combining TRTS and TA measurements allows for tracking both electron and hole dynamics.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00036. Transient absorption measurement of nanorods, derivation of photoconductivity, and details of theoretical calculations (PDF)

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Work at NREL (Contract Number DE-AC36-08GO28308) and Emory (Grant No. DE-FG02-12ER16347) was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, Office of Science, of the U.S. Department of Energy. Theoretical calculations were supported as part of the Center for Advanced Solar Photophysics, an Energy Frontiers Research Center funded by the Office of Basic Energy Science, Office of Science within the U.S. Department of Energy. A.L.E. acknowledges the financial support of the Office of Naval Research (ONR) through the Naval Research Laboratory Basic Research Program.

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DOI: 10.1021/acsenergylett.6b00036 ACS Energy Lett. 2016, 1, 76−81