Aqueous Photogeneration of H2 with CdSe Nanocrystals and Nickel

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Aqueous Photogeneration of H with CdSe Nanocrystals and Nickel Catalysts: Electron Transfer Dynamics Cunming Liu, Fen Qiu, Jeffrey J. Peterson, and Todd D. Krauss J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp510935w • Publication Date (Web): 19 Dec 2014 Downloaded from http://pubs.acs.org on December 31, 2014

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The Journal of Physical Chemistry

Aqueous Photogeneration of H2 with CdSe Nanocrystals and Nickel Catalysts: Electron Transfer Dynamics Cunming Liu,§ Fen Qiu,† Jeffrey J. Peterson† and Todd D. Krauss§,†,#,∗ §

Rochester Advanced Materials Program, †Department of Chemistry and #the Institute of

Optics, University of Rochester, Rochester, New York, 14627

∗

To whom correspondence should be addressed, [email protected].

1 ACS Paragon Plus Environment


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Abstract CdSe quantum dots (QDs) and simple aqueous Ni2+ salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen in water. Using ultrafast transient absorption (TA) spectroscopy, the electron transfer (ET) processes from the QDs to the Ni catalysts have been characterized. CdSe QDs transfer photoexcited electrons to a Ni-dihydrolipoic acid (Ni-DHLA) catalyst complex extremely fast and with high efficiency: the amplitude-weighted average ET lifetime is 69 ± 2 ps, and ~90% of ultrafast TA signal is assigned to ET processes. The impacts of Auger recombination, QD size and shelling on ET are also reported. These results help clarify the reasons for the exceptional photocatalytic H2 activity of the CdSe QD/Ni-DHLA system and suggest direction for further improvements of the system.

Keywords: quantum dots, transient absorption, electron transfer, Auger recombination


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Introduction Artificial conversion of sunlight to chemical fuels has attracted attention for several decades as a potential approach to store and transport energy.1 One of the most attractive strategies from the perspective of clean, renewable sources of energy is lightdriven proton reduction to molecular hydrogen, in which a strong light-absorbing molecule (the photosensitizer) rapidly transfers a photoexcited electron to a highly efficient catalyst for reducing protons.2,3 Generally speaking, the most efficient and active homogeneous catalytic systems for proton reduction have photosensitizers and catalysts that contain extremely rare metals, such as ruthenium and/or platinum.2-5 As such, these systems are not considered viable for large scale energy production. Recently, an active area of research has focused on discovering photosensitizers and catalysts that are inexpensive and contain elements highly abundant in the Earth’s crust.6-8 However, the combination of organic or organometallic photosensitizers with molecular catalysts containing abundant transition metals (such as cobalt or nickel) typically leads to middling performance with respect to proton reduction activity and longevity.6-8 Often, the poor stability of the system can be traced to decomposition of the photosensitizer in minutes to hours while under external illumination.9 Colloidal semiconductor nanocrystals, also called quantum dots (QDs), can in theory provide several advantages over traditional molecular photosensitizers in artificial



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photosynthesis systems for H2 generation. Compared to organic molecules, QDs possess superior photostability,10,11 have much larger absorption cross sections (10-16-10-15 cm2)12,13 and broad absorption spectra which can be tuned by controlling the precise size of the QDs.12-14, QDs also possess long (up to ~30 ns) excited state lifetimes,15 which should lead to more facile reduction of a catalyst, and have tunable surface chemistry that allows for a wide variety of chemical attachment schemes for linking to a catalyst.16,17 Importantly, QDs have the ability to transfer multiple electrons from an excited state to a proton reduction catalyst.18-20 Indeed, photochemical proton reduction systems involving QDs such as CdSe/ZnS,20 CdSe/CdS dot-in-rod,21 and CdTe22-24 with a variety of abundant, transition metal catalysts, including Co,20,24 Mo21 and Fe

22,23

, have been

reported. However, generally speaking the efficiency and longevity of proton reduction in these QD systems does not significantly exceed that which can be achieved using more traditional organic chromophores and catalysts.20-24 We have recently discovered a novel system for light-driven H2 production employing water-soluble, dihydrolipoic acid (DHLA)-capped CdSe QDs as the photosensitizer and a Ni-DHLA complex as the catalyst. The system has exceptional longevity of H2 production (≥ 360 hours) and generates over 600,000 turn-over numbers with respect to catalyst.25 The overall photon to H2 quantum efficiency of this system is ~36%, which is noticeably higher than typical photocatalytic systems employing either organic or inorganic photosenitizers.6-9,20-24,26-28 Although the performance of the QD/NiDHLA system is quite extraordinary with respect to photochemical proton reduction to H2, almost nothing is known about why the system works as well as it does. For example,


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ultrafast photoexcited electron transfer (ET) rates from the QD to the Ni-DHLA catalyst, one of the most basic steps in the photocatalytic process, are unknown. Here, we report measurements of the ultrafast ET dynamics between CdSe QDs and Ni-DHLA catalysts using ultrafast TA spectroscopy. Photoexcited electrons can transfer very fast (sub-100 ps) from the CdSe QDs to the Ni-DHLA catalysts, with a ~90% efficiency. The impacts of Auger recombination, QD size and shelling on ET are also characterized. These results help provide insights into the design and optimization of artificial photocatalytic systems for efficient H2 production.

Experimental CdSe QDs and CdSe/CdS QDs were synthesized by use of previously reported procedures.29,30 The as-synthesized QDs are capped by trioctylphosphine oxide (TOPO) lignads and are very hydrophobic, therefore, a ligand exchange with dihydrolipoic acid (DHLA) was used to render the QDs water soluble (see supporting information, SI).25,31 DHLA-capped QDs were dispersed in a MeOH/H2O mixture (volume ratio 1 : 1), and the size and concentration of the QDs were determined using reported empirical formulas.12 The Ni-DHLA catalyst was synthesized by a modified procedure as proposed by Brown et al..32 In order to form the QD/Ni-DHLA complex system, DHLA-capped QDs and NiDHLA solutions were mixed and then diluted with MeOH/H2O to the desired QD concentration. The Ni-DHLA:QD molar ratios were generally 20:1, although certain experiments used alternate values as described in the text. The prepared QD/Ni-DHLA samples were stored in the dark prior to measurements. Absorption spectra of samples were characterized by a Perkin-Elmer Lambda 950 UV/vis/NIR spectrophotometer.



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Further details regarding sample preparation and characterization procedures can be found in the SI. Ultrafast TA measurements were performed in a typical pump-probe setup as previously described.33 Femtosecond laser pulses at 800 nm were first generated by a Ti: sapphire oscillator (Spectra Physics, Tsunami) and then amplified by a 1.0 kHz regenerative amplifier (Spectra Physics, Spitfire) to a fluence of ~0.90 mJ/pluse. The 800-nm pulses from regenerative amplifier were introduced into an optical parametric amplifier (Spectra Physics, OPA 800C), the output of which produced ~180 fs pulses tunable from 1100 to 2300 nm. One or two BaB2O4 (BBO) crystals were used to double or quadruple, respectively, the frequency of signal or idler beam from OPA to generate the wavelengths for the probe and reference beams. The 400-nm pump beam was generated by doubling residual 800 nm pulses from OPA with a BBO crystal. The reference beam was used to correct the probe signal for pulse-to-pulse fluctuations. A motorized delay stage was used to control the delay time between pump and probe pulses. Samples were placed in a quartz cuvette with a 2 mm path length and vigorously shaken to avoid laser-induced artifacts in the TA spectra. The optical density of QDs in all samples was constant at ~0.30 at the pump wavelength of 400 nm. The average number of excitons absorbed per QD was determined by characterizing the pump fluencedependence of the intensity change of the TA signal from the first exciton transition of QDs at the delay time of 2 ps.34 = 1.0 was determined from the point where the TA intensity change deviates from a linear dependence on pump fluence (an average single exciton regime) to a nonlinear dependence (an average multiexciton regime).34 For


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example, a pump fluence of 1.6 × 1015 photons/cm2 corresponds to =1.0 for the CdSe QDs with the first exciton state 1Se-1S3/2 (1S) at 580 nm (see SI Figure S1).

Results and Discussion

Figure 1. (a) Absorption spectra of the Ni-DHLA catalyst and CdSe540 QDs with (red, [QD]:[Ni-DHLA] = 1:20) and without (black) the presence of Ni-DHLA catalyst. (b) Potentials of 1S electron and hole states of CdSe540 QDs and the reduction potential of the Ni-DHLA. Potential values given vs. normal hydrogen electrode (NHE).

The absorption spectra of CdSe QDs without and with the Ni-DHLA catalyst exhibit the same primary spectral features (Figure 1a). The first and second absorption peaks are located at 540 and 450 nm and are identified as the 1Se-1S3/2 (1S) and 1Pe-1P3/2 (1P) transitions, respectively.35 The QD sample is here referred to as CdSe540 in terms of the 1S exciton peak position. When the Ni-DHLA catalyst is present, there is a slight 7 ACS Paragon Plus Environment


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increase in the QD absorption background, which is associated with direct absorption of the Ni-DHLA catalyst. The reduction and oxidation potentials of the CdSe540 QDs are estimated to be at -1.20 V and +1.10 V (vs. NHE), respectively.36 For the Ni-DHLA catalyst, the reduction potential occurs at -0.90 V (vs. NHE),25 thus ET from the CdSe540 QD is thermodynamically favorable (Figure 1b).

Figure 2. TA spectra of CdSe540 QDs (a) without and (b) with Ni-DHLA catalyst ([QD]:[Ni-DHLA] = 1:20) under = 0.54. TA transients probed at 540 nm with (red) and without (green) the presence of the Ni-DHLA catalyst under (c) = 0.54 and (d) = 1.92, respectively. The blue line in (c) is the TA signal of pure Ni-DHLA probed at 540 nm. Solid black lines in (c) and (d) are exponential fits according to Eq. 1.

The TA spectra of CdSe540 QDs without and with Ni-DHLA catalyst present are shown in Figure 2a and b, respectively. These spectra are consistent with published reports of TA spectra from bare CdSe QDs.34,37-39 In particular, two photobleaching 8 ACS Paragon Plus Environment

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features at 540 nm and 450 nm are observed, associated with state filling of the 1S and 1P exciton transitions.34,37-39 Two photoabsorption bands at 470 nm and 580 nm, associated with a carrier-induced Stark shift of the 1P transition and a biexciton transition related to the 1S state, are also observed.34,37 Of note, the 1S exciton photobleaching signal of CdSe540 QDs without Ni-DHLA catalyst present recovers only 40% of its maximum value in 1 ns (Figure 2a), suggesting that the majority of the photoexcited QDs have a long-lived (> 1 ns) excited state. In contrast, the TA spectra of CdSe 540 QDs with NiDHLA catalyst present indicate that nearly 95% of the 1S exciton photobleaching recovers within 1 ns (Figure 2b). Because the bleaching signal of the 1S exciton transition is mainly dominated by electron filling of 1S electron state,34 the faster recovery of the 1S exciton photobleaching in presence of Ni-DHLA catalyst strongly suggests ET from QDs to the catalyst. The ET process from the CdSe540 QDs to the Ni-DHLA catalyst is further quantified by comparing the TA kinetics at the 1S exciton transition of CdSe540 QDs with and without the Ni-DHLA catalyst present (Figure 2c). Without Ni-DHLA catalyst present, the QD TA signals exhibit a bi-exponential decay, with time constants of 37 ± 2 ps (14% of the signal magnitude) and 3.1 ± 0.1 ns (86% of the signal magnitude) (Table 1). The ps and ns components are associated with electron relaxation to shallow and deep trapping defects, respectively.38,39 Upon adding Ni-DHLA catalyst, an additional fast decay component clearly arises in the TA kinetics (Figure 2c). Control TA experiments with pure Ni-DHLA catalyst (i.e., no QDs present) at the same concentration and monitored at 540 nm do not exhibit any fast component and are nearly 60 times weaker than the TA kinetic signal from the CdSe540 QDs, clearly indicating that the fast TA



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decay in the QD/Ni-DHLA system is not associated with the pure catalyst. Assuming that the intrinsic lifetimes of electron relaxation are unaffected by the addition of Ni-DHLA catalyst, the ET lifetimes can be estimated by fitting the 1S TA kinetics data of the QD/Ni-DHLA with a simple multiple exponential model as follows:20,40 ∆T (t ) = ∑ i Ai exp(−t / τ i ) + ∑ j Bj exp(−t / τ jET ) where the first term

∑

i

(1)

Ai exp(−t / τ i ) is responsible for the intrinsic electron relaxation

in the QDs without catalyst present and the second term

∑ B exp(−t / τ j

j

jET

) accounts for

the ET processes from the QDs to Ni-DHLA catalyst. In these expressions, Ai and Bj are time independent coefficients whose magnitudes represent the weights of different decay components τi and τj. In the fitting of the TA transients with Ni-DHLA present, all preexponential factors (e.g., Ai and Bi) are varied, but the lifetimes of the intrinsic electron relaxation processes are fixed to the same values measured when the Ni-DHLA is absent.

Table 1. Lifetime components (τi , τj ) and the relative contribution of intrinsic electron relaxation and ET processes to the TA signal.

Sample

τ1, ps (A1, %)

τ2, ns (A2, %)

CdSe540

37± 2 (14)

3.1± 0.1 (86)

τ1ET, ps (B1, %)

τ2ET, ps (B2, %)

< τ ET>, ps (B1+B2, %)

CdSe540/ 37± 2 (2) 3.1± 0.1 (10) 15±1 (59) 180±6 (29) 69±2 (88) Ni-DHLA *Amplitude-weighted average lifetime = ∑ /(∑ ) and the relative ET fraction = ∑ /(∑ + ∑ ).

Parameters describing the excited state dynamics of CdSe QDs with and without Ni-DHLA catalyst (monitored at CdSe540 1S exciton transition, [QD]:[Ni-DHLA] =


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1:20, under = 0.54) are summarized in Table 1. A single exponential function could not model the fast decay of the CdSe QD excited state dynamics upon addition of the NiDHLA catalyst; two exponential components were required. From the exponential prefactors (B1 and B2) it is clear that the majority of the electrons transfer extremely fast, within ~15 ps, while the remainder transfers in a slow process with a decay constant of ~180 ps (Table 1). The amplitude-weighted average lifetime for ET to the catalyst is 69 ± 2 ps. The TA data also suggests the ET process is highly efficient: nearly 90% of the 1S TA signal is assigned to ET processes. The relatively large percentage of electrons that undergo ET to the Ni-DHLA catalyst is certainly one important factor that can help to explain the unusually high hydrogen production efficiency of QD/Ni-DHLA system.25 TA studies as a function of the relative Ni-DHLA concentration all exhibit biexponential dynamics although the ET rate and the relative fraction of TA signal due to ET are concentration-dependent. As the Ni-DHLA concentration with respect to QDs decreases, both the ET rate and the relative fraction of signal due to ET also decrease (See SI Figure S2 and Table S1). In particular, a decrease in molar concentration of NiDHLA catalyst from 20-fold to 5-fold excess with respect to QDs results in changes of the fast ET decay constant from 15 ± 1 ps to 28 ± 3 ps, and of the relative fraction of transferred electrons decreases from 88% to 36% (See SI Table S1). Such concentration dependent ET lifetimes are expected for a molecular like system with freely diffusing donors and acceptors.41 The deviation of ET rates from simple, single exponential kinetics is reasonably expected given the complexity of the process, and has been similarly observed in other systems.20,42-44

Nonetheless, control experiments were performed to ensure that the



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change in excited state dynamics upon the addition of Ni-DHLA were the result of ET to the Ni-DHLA catalyst. Previously, we showed that even in the presence of DHLA, ethylenediaminetetraacetic acid (EDTA) is able to outcompete the DHLA for coordinating the Ni2+ and thus sequester in the Ni2+ in a form that is catalytically inactive.25 Successful sequestration of Ni2+ from the Ni-DHLA complex is supported by absorption spectra (Figure 3a); upon addition of EDTA the long absorption tail to the red (between 600 to 650 nm) due to Ni-DHLA disappears and the absorption spectrum resembles to that of free QDs. As the amount of EDTA added increases, progressively slower decays are also seen in the TA dynamics of CdSe540 QDs (Figure 3b), suggesting that the Ni2+ in Ni-DHLA is chelated by the EDTA. Fitting the TA data with Eq. 1 allows for a quantification of the ET lifetimes with different molar concentrations of EDTA added (See SI Table S2). Adding EDTA was seen to affect both the fast and slow ET components, resulting in change of amplitude-weighted average ET lifetime from 69 ± 2 ps to 157 ± 7 ps. due to the reduced availability in solution of catalytic Ni-DHLA. The slowing down of the ET process is accompanied with a decrease in relative fraction of electrons transferred from 88% to 27%. These control experiments demonstrate that ET from the QDs to the Ni-DHLA catalyst is responsible for the dramatic change in excited state dynamics as measured with TA spectroscopy. Given that the accurate structure for the Ni-DHLA catalyst and the nature of its interaction with the QD are unknown,25 fully understanding the origins of the two decay components is extremely difficult at present. The observation of bi-exponential kinetics in the ET between the CdSe QD and the Ni-DHLA catalyst implies that at least two physical processes are responsible for the effect. One possible explanation is that the fast


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decay component corresponds to reduction of Ni2+ to Ni+ while the slow decay corresponds to adding a second electron to catalyst that has been reduced previously.45 Two decay components could also arise from ET to slightly different Ni-DHLA catalysts, for example one that is closely associated with the QD surface, and one that is freely diffusing.46

Figure 3. (a) Absorption spectra of CdSe540 QDs (green) and of the CdSe540/Ni-DHLA with different molar concentrations of EDTA. Molar ratios of CdSe540:Ni-DHLA: EDTA are listed in the figure. (b) 1S TA kinetics of the samples shown in (a). All TA measurements are performed under = 0.54. Solid black lines are multi-exponential fits obtained using Eq. 1.

One important property of QDs with respect to solar H2 production is their ability to support an excited state with multiple electrons without significant change in the QD



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absorption properties. Transferring two electrons simultaneously to a catalyst (as opposed to two consecutive light absorption/electron transfer events) could also significantly enhance catalytic efficiencies. For QDs multiple electrons could be produced through absorption of multiple photons, or through absorption of a single high energy photon, a processes known as multiple exciton generation (MEG)47 or carrier multiplication (CM).48,49 Efficient MEG/CM processes in QDs are relevant to the production of solar fuels for similar reasons to their relevance for QD-based photovoltaics: the efficiency of the light absorbing process could be significantly enhanced if multiple electrons were produced per photon. On the other hand, it is well known that multiple excitons in QDs quickly annihilate each other through efficient Auger recombination (AR) processes, 49-51 thus, potentially mitigating any benefit from MEG/CM. To evaluate the relevance of MEG/CM processes for photocatalysis, we studied the competition between ET and Auger recombination in the CdSe QDs using TA spectroscopy with varying pump fluence. Transients of CdSe540 QDs with and without the Ni-DHLA catalyst under high pump fluence ( = 1.92) are shown in Figure 2d. For CdSe QDs without Ni-DHLA catalyst, increasing the pump fluence causes a noticeable change in the TA signal: a fast component that is attributed to AR between two or more excitons in the QD appears in the first 100 ps.49-51 The decay constant for biexciton AR in CdSe540 QDs was determined to be 93 ± 3 ps (described in the SI, Figure S3), consistent with other reports for CdSe QDs.49-51 In contrast, for CdSe QDs with Ni-DHLA catalyst present, the change in TA signal upon increasing pump fluence is noticeably smaller, as a result of a fast electron extraction by the Ni-DHLA catalyst on a 50-ps time scale (Figure 2d).


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To quantitatively analyze the TA kinetics three independent, competitive relaxation pathways for excited electrons are characterized: intrinsic electron trapping to the surface (trap), AR, and ET to the Ni-DHLA catalyst. TA data at high pump fluence was fit using Eq. 1 and, in order to compare the relative fractions of the three relaxation pathways, the AR decay constant was fixed to 93 ± 3 ps and the lifetimes of electron trapping relaxation and ET were fixed to those in Table 1, but all amplitudes were floated. The relative contributions to the TA signal for each process are summarized in Table 2. Interestingly, under high fluence, sufficient to create on average almost 2 excitons per QD, the relative fraction of TA signal due to ET remains well over 80%, and electron depopulation from AR is minimal (6%). This result suggests that biexciton AR will negligibly affect ET transfer efficiency and supports the possibility for transfer of multiple electrons to the catalyst. Together these findings indicate that MEG/CM strategies may be useful to improve photochemical H2 production.

Table 2 Relative fractions of intrinsic electron trapping (trap), Auger recombination (AR) and ET processes in CdSe540 QDs without and with Ni-DHLA catalyst present under = 0.54 and = 1.92. CdSe540 (%)

CdSe540/Ni-DHLA (%)

Trap

AR

Trap AR ETfast ETslow ETtotal

0.54

100

NA

12

NA

59

29

88

1.92

66

34

11

6

60

23

83

*Relative ET fraction = ∑ /(∑ + ∑ ) . trap%=(A1+A2)%, ETfast%=B1%, ETslow%=B2%, ETtotal%=(B1+B2)% in Table 1 and AR% is for an additional AR component companying with the trapping and ET components under = 1.92.



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Previously, we have found that larger sized QDs had a lower photocatalytic activity for hydrogen production compared to smaller sized QDs, which may be attributed to a reduced overpotential.25 To study the effect of QD size on ET rates, CdSe QDs were synthesized with a first exciton absorption feature (1S) at 580 nm, referred to as CdSe580. The simplest reasoning suggests that for the larger QDs, a reduced overpotential should lead to slower electron transfer kinetics to the Ni-DHLA catalyst.52 Figure 4a shows TA transients from CdSe580 QDs with and without Ni-DHLA catalyst present under = 0.65. As expected, the 1S photobleaching signal for the larger CdSe580 QDs recovers about 58% in 1 ns with Ni-DHLA catalyst present, whereas the CdSe540 QDs exhibit near complete recovery (~95%) in the same time period. The lifetimes and relative fraction of ET were determined by fitting the TA traces in Figure 4a as previously described for CdSe540 QDs (see SI Table S3). The CdSe580 QDs have slower ET compared to the smaller CdSe540 QDs, for both the short component (15 ± 1 ps vs. 29 ± 4 ps) and the longer component (180 ± 6 ps vs. 276 ± 8 ps). The relative fraction of the TA signal due to ET is also smaller for CdSe580 QDs compared to CdSe540 QDs (23% vs. 88%). The slower and less efficient ET of CdSe580 QDs compared to CdSe540 QDs is consistent with the previously observed decrease in photocatalytic H2 production with increasing QD size.25 Growing a CdS shell around the CdSe core has also been investigated as a method to improve H2 production activity.18,53,54 CdSe/CdS core-shell QDs have a quasi-type II core-shell structure, in which the electron wavefunction is more delocalized over the whole QD while the hole is confined to the CdSe core.53 Thus, CdSe/CdS core-shell QDs are expected to lead to a better electron-hole separation and thereby facilitate ET to the


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catalyst. Surprisingly, we found that CdSe/CdS core-shell QDs do not perform nearly as well as CdSe QDs in photochemical H2 production, and thus, internal charge separation must not be a major factor in driving the catalytic performance of the system.54 In order to better understand this effect, we studied the ET kinetics between CdSe/CdS QDs and Ni-DHLA catalyst. The particular CdSe/CdS QD was prepared by overcoating a CdSe core with a 1S absorption peak at ~490 nm with approximately 1 monolayer of CdS.30 After CdS shelling, the core-shell QD exhibits a 1S exciton peak at 510 nm due to delocalization of the electron from CdSe core to CdS shell and it is referred to as CdSe/CdS510. (See absorption spectra in SI Figure S4.)

Figure 4. (a) TA transients of CdSe580 QDs with (red) and without (blue) the presence of Ni-DHLA, probed at the 1S exciton transition under = 0.65. (b) TA transients of CdSe/CdS510 QDs with (red) and without (blue) the presence of Ni-DHLA catalyst, 17 ACS Paragon Plus Environment


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probed at the 1S exciton transition under = 0.63. The molar ratio of QDs to catalysts is 1:20 in both TA measurements. Solid lines are multi-exponential fits using Eq. 1.

TA kinetics of CdSe/CdS510 core/shell QDs without and with Ni-DHLA catalyst are presented in Figure 4b. Different from the CdSe QDs, the presence of the CdS surface capping layer greatly reduces the accessibility of surface traps for the electron, resulting in a TA decay that can be well fit by a single exponential decay with a lifetime of 3.2 ± 0.1 ns (See SI Table S4). As expected, adding Ni-DHLA catalyst to the CdSe/CdS510 QD solution leads to a faster recovery of the 1S exciton photobleaching signal as electrons are rapidly transferred to the Ni-DHLA catalyst. The ET process was modeled as with other QDs (Eq. 1) and is fit by a bi-exponential decay with a fast time constant 24 ± 1 ps and a slow time constant 241 ± 12 ps. The relative fraction of TA signal attributed to ET ~66% (See SI Table S4). Like CdSe580 QDs, the slower and less efficient ET of CdSe/CdS510 QDs compared to CdSe540 QDs is consistent with its previously reported poorer photochemical hydrogen production compared to the CdSe540 QDs.54 In the case of CdSe580 QDs, the slower ET was speculated to result from the lower thermodynamic driving force in larger QDs compared to smaller QDs. In the case of the CdSe/CdS510 QDs, the particles have the nearly same driving forces as CdSe540 QDs (-0.34 eV vs. 0.30 eV),36 yet the amplitude-weighted average ET rate is approximately 2 times slower, which means that predictions of relative ET rates based on driving force alone are insufficient and other effects must be included to explain the present observations. To further clarify the impacts of QD size, driving force and shelling effects on the ET between the CdSe QDs and Ni-DHLA catalyst, the ET rates for CdSe540, CdSe/CdS510 and CdSe580 QDs have been modeled using Marcus theory.52,55 In this 18 ACS Paragon Plus Environment

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model, the ET between the QD and Ni-DHLA catalyst is simplified to a system involving only two energy states: the QD donor state and the Ni-DHLA acceptor state. The lifetime can be calculated according to the equation:52,55

1

τ ET where

∆GET

2

H  (λ + ∆GET ) 2  2π = exp  −  h 4πλ k BT 4λ k BT   is

the

(2)

driving

force

( ∆GET = −e[ E (CdSe + /CdSe*) − E (Ni-DHLA - /Ni-DHLA)] ), |H |2 is the electronic coupling between the 1S electron state of the QD and the lowest unoccupied molecular orbital of the Ni-DHLA catalyst, λ is the reorganization energy, T is the temperature, and all other variables are fundamental constants. The electronic coupling |H |2 is proportional 2

to the amplitude of the electron wavefunction at the particle surface ( Φ1Se (r0 ) ).55 In 2

2 order to estimate |H |2 , the radial distribution functions ( ∝ r Φ1Se (r ) ) for the 1S

electron for CdSe540, CdSe/CdS510 core-shell and CdSe580 QDs were initially calculated using an effective mass approximation (EMA) method, with the electron confined in a spherical potential well of a finite depth.55-59

Details of the EMA

calculations are included in SI and the calculated radial distribution functions for CdSe540, CdSe/CdS510 core-shell and CdSe580 QDs are shown in Figure 5a. ∆GET between the QD and Ni-DHLA catalyst are taken from the literature in the cases of CdSe540 and CdSe580 QDs.36 In the case of CdSe/CdS510 QDs, ∆GET was estimated from a comparison of the EMA-calculated energy levels for CdSe490 QDs prior to CdS shelling and the CdSe/CdS510 core-shell QDs.

The specific ∆GET values used are

CdSe540: -0.30 eV, CdSe/CdS510 -0.34 eV and CdSe580: -0.20 eV. A reorganization



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energy λ = 0.24 eV was found to best describe the data (See SI Figure S7), which is within the 0.2-0.5 eV range reported in comparable systems 55,60,61

2

(a) Normalized radial distribution functions ( ∝ r 2 Φ1Se (r) ) of the 1S electron wavefunctions calculated using the EMA method (lines A, B and C correspond to r0,CdSe540 = 1.43 nm, r0,CdSe/CdS510 = 1.48 nm and r0,CdSe580 = 1.90 nm, respectively) and (b) comparison of the experimental and theoretical ET lifetimes predicted by Eq. 2 for various QD sizes (blue line: CdSe/CdS core-shell QD, red line: CdSe QD). The driving force ∆GET decreases (becomes less negative) as the QD radius r0 increases (See SI Figure S8).


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As shown in Figure 5b, Eq. 2 accurately models the size dependence of the ET rate in the CdSe and CdSe/CdS QDs. The decrease in ET rate as the CdSe QD size increases is driven largely by the smaller surface amplitude of electron wavefunction in CdSe580 compared to CdSe540 QDs. In the case of CdSe/CdS510 QDs, the core/shell structure produces a driving force that is smaller than would be expected for a pure CdSe QD with a 1S exciton transition at 510 nm, due to the bigger size of the core-shell QD. In fact, the driving force ∆GET of a CdSe/CdS510 QD is within ~10% of the value for a CdSe540 QD. Furthermore, the size of the CdSe/CdS510 QD and CdSe540 QD are within ~3%, and the simplest expectation is that the two samples should exhibit similar ET lifetimes. However, the EMA calculations show that amplitude value of the electron wavefunction of CdSe/CdS510 QDs is ~40% smaller at the particle surface than that of CdSe540 QDs. This effect originates from the heavier effective mass of the electron in the CdS shell versus the CdSe core (0.21m0 vs. 0.14 m0),51,62 which rapidly attenuates the electron wavefunction in the CdS shell, and accounts for the ~2 times longer ET lifetime in CdSe/CdS510 QDs compared to CdSe540 QDs. Note that this effect would be general to all CdSe/CdS nanostructures. In fact, our modeling predicts that ET rates will always be lower in CdSe/CdS core/shell structures compared to core-only structures with the same driving forces (Figure 5b). Interestingly, the model also predicts that CdSe540 QD is near the optimum size for maximized ET rates. This conclusion depends on the precise value of the re-organization energy, and further increases in the ET rate may be possible with smaller sized particles (See SI Figure S7). While the Marcus theory model predicts that small-sized, CdSe QDs will produce the maximum ET rate, which is consistent with the observation that the smaller CdSe QDs are the optimum particles for aqueous



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photocatalytic generation of H2 with Ni-DHLA catalysts,.25 we should be cautious about applying the model too aggressively. Our analysis would certainly benefit from larger number of available data points, as well as having more accurate values for the reorganizational energy and better models for the electron wave function at the QD surface.

Conclusion We have investigated the ET processes between CdSe QDs and a Ni-DHLA catalyst, a system with unprecedented photocatalytic H2 production activity, with ultrafast TA spectroscopy. ET from the QDs to the Ni-DHLA catalyst is fast and efficient: the amplitude-weighted average ET lifetime is 69 ± 2 ps with a majority component of 15 ± 1 ps and nearly 90% of the TA signal attributable to ET.

The presence of multiple

electron-hole pairs in CdSe QDs has negligible impact on the efficiency of ET, suggesting MEG/CM may be a viable strategy to improve photocatalytic H2 production. Measured ET rates in QDs of different sizes and with different core/shell structures show that increasing the QD size and shelling the QD both decrease the ET rate. Coupled with simple theoretical models, these results suggest further improvements in ET may be achieved by working with small CdSe QDs. Future work will seek to explore such optimized structures and to better understand the electron transfer kinetics of the QD/NiDHLA system.

Acknowledgment


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This work was supported by the National Science Foundation (Grant Award Number CHE-1307254). The authors gratefully acknowledge Bradford Loesch for his assistance with data acquisition and Sanela Lampa-Pastirk for fruitful discussions of the manuscript.

Supporting Information Supporting Information Available: sample preparation, dependence of TA signals on pump fluence for CdSe580 QDs, influence of Ni-DHLA concentration on ET, TA transient fitting for Auger component extraction, rates and fractions of ET and AR for bare CdSe580 QDs and CdSe/CdS510 core-shell QDs, EMA calculation of radial charge densities of electron for bare and core-shell QDs, size and driving force-dependent predicted ET lifetimes for bare QD under different λ, change of driving force with QD radius. This material is available free of charge via the Internet http://pubs.acs.org.



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Aqueous Photogeneration of H2 with CdSe Nanocrystals and Nickel

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