Letter pubs.acs.org/NanoLett
Photocatalytic Hydrogen Generation by CdSe/CdS Nanoparticles Fen Qiu,† Zhiji Han,† Jeffrey J. Peterson,† Michael Y. Odoi,† Kelly L. Sowers,† and Todd D. Krauss*,†,‡ †
Departments of Chemistry and ‡The Institute of Optics, University of Rochester, Rochester, New York 14627, United States S Supporting Information *
ABSTRACT: The photocatalytic hydrogen (H2) production activity of various CdSe semiconductor nanoparticles was compared including CdSe and CdSe/CdS quantum dots (QDs), CdSe quantum rods (QRs), and CdSe/CdS dot-in-rods (DIRs). With equivalent photons absorbed, the H2 generation activity orders as CdSe QDs ≫ CdSe QRs > CdSe/CdS QDs > CdSe/ CdS DIRs, which is surprisingly the opposite of the electron−hole separation efficiency. Calculations of photoexcited surface charge densities are positively correlated with the H2 production rate and suggest the size of the nanoparticle plays a critical role in determining the relative efficiency of H2 production. KEYWORDS: Nanoparticles, photocatalysis, hydrogen generation, water splitting he abundance of solar flux striking the earth makes it an attractive sustainable energy source, independent of fossil fuels.1,2 Solar H2 production by photocatalytic water splitting in particular has attracted great interest as a potential method to store solar energy in chemical bonds.3,4 Because of the complexity and difficulty of solving the complete water splitting problem, often model systems are studied that focus on either the oxidative or the reductive half-reactions. A molecular-based system for the reductive half-reaction, photoreduction of protons to produce H2, typically consists of a photosensitizer (PS), a catalyst, and sources of protons and electrons. The relevant photoreduction processes, depicted in Figure 1, involve (i) absorption of light by the PS and subsequent internal charge separation, (ii) intermolecular charge transfer (i.e., reduction of the catalyst by the PS and reduction of the PS by direct holedonation from a sacrificial electron donor), and (iii) catalytic formation of H2 by the reduced catalyst. The efficiency of photoreduction is directly dependent on the relative rates of these three processes, and researchers have sought to optimize each of these steps.5−9 We recently reported a photocatalytic H2 generation system with exceptional activity and robustness using dihydrolipoic acid (DHLA)-capped CdSe nanocrystals (NCs) as the PS, a NiDHLA complex as the catalyst, and ascorbic acid as the sacrificial donor.10 The motivation for exploring colloidal semiconductor NCs as photosensitizers is that they have several potential advantages over small molecule PSs, including broad absorption spectra, size-tunable band gaps,11,12 the ability
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© 2016 American Chemical Society
to store and deliver two electrons,13 and enhanced photostability.10,14 Advances in the wet chemical synthesis of complex nanoparticles offer exciting possibilities to direct electron flow and thus optimize H2 generation by tailoring the sizes, shapes, and compositions of nanoparticle-based PSs.15−17 For example, one-dimensional quantum rods (QRs) with more facile internal charge separation than zero-dimensional quantum dots (QDs) could more easily facilitate charge transfer to a catalyst.14,18 Type II core/shell nanoparticles are expected to confine one carrier in the core and the other carrier in the shell, thereby promoting efficient charge separation as well.19,20 For example, Zhu et al. showed efficient H2 generation using a CdSe/CdS dot-in-rod (DIR) PS, a Pt nanoparticle catalyst, a methyl viologen dication (MV2+) as electron relay and a 3mercaptopropionic acid (MPA) sacrificial donor.5 In this system, MPA-capped water-soluble CdSe/CdS DIRs were the most active for H2 generation, compared to other nanoparticles, perhaps due to the enhanced charge separation in the CdSe/ CdS DIRs. In another recent example, 100% photon-tohydrogen production efficiency was reported for a proton reduction reaction for Pt-tipped CdSe/CdS DIRs with excitation of the CdS.21 By employing more highly engineered nanoparticles that are designed to efficiently separate photoexcited electrons and holes, we hypothesized further improveReceived: March 12, 2016 Revised: July 28, 2016 Published: August 1, 2016 5347
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Figure 1. (a) Schematic illustrating photoreduction processes for H2 generation using nanoparticles as the PS, a Ni-DHLA complex as the catalyst, and ascorbic acid (AA) as the sacrificial donor. (HT, hole transfer; ET, electron transfer). Energy levels for a typical core/shell CdSe/CdS QD,27 AA, and Ni-DHLA catalyst10 are given versus the normal hydrogen electrode (NHE). (b) Normalized absorbance spectra of DHLA-capped nanoparticles in water. Spectra are offset for clarity.
approximately −1.2 V (vs NHE).28 Estimates of the conduction band level in the other nanoparticles are predicted to occur between −1.2 and −1.10 V and electron transfer to the NiDHLA catalyst (−0.9 V) is thermodynamically favorable for all the nanoparticles. A cartoon depicting the relevant energy levels for H2 generation with CdSe/CdS core/shell QDs is shown in Figure 1a. Photocatalytic H2 generation measurements were performed in a custom-built 16-sample apparatus using optimized conditions similar to our previous study.10 Each 40 mL sample vessel contained 5.0 mL of solution and a sensor cap that monitored head space pressure in real-time. Rather than explore all possible experimental conditions for H2 production, here we focused on conditions that allowed us to observe clear differences in catalytic activity due to differences in charge separation efficiency between the nanoparticles: 4 μM of Ni(NO3)2, approximately 4 μM of DHLA-capped nanoparticles, and 0.5 M AA in EtOH/H2O at pH = 4.5. As was shown in Han et al., the Ni(NO3)2 and DHLA form a soluble, molecular catalyst with no trace of Ni metal formed during the photocatalytic reaction, and the AA serves as an optimized sacrificial donor.10 With regards to the nanoparticle concentrations, samples were prepared such that they all had the same peak absorbance. This condition corresponds to a 20% variation in the absorbed photon flux at the excitation wavelength. The absolute nanoparticle concentration between the core and core−shell QDs and the DIRs varied within 20%; the QRs were approximately a factor of 2 less concentrated than the QDs.29,30 Photoexcitation was provided by a 70 mW, 520 nm light-emitting diode source. The relative uncertainty in the amount of H2 produced is 7% based on an average over multiple-run experiments. The H2 evolution over a 46 h period for the four nanoparticles is shown in Figure 2. The nanoparticles are all robust and stable even after 46 h, matching the excellent stability and robustness to the QD system studied previously.10 In the QR, core/shell QD, and DIR samples the amount of photogenerated H2 increases linearly with time. In the QD sample the H2 evolution is linear for the first ∼10 h (inset in Figure 2) and then decreases due to the depletion of the AA.10 The total amount of H2 (produced during the first 10 h), turnover numbers (TONs, with respect to Ni-DHLA catalyst), turnover frequencies (TOFs, with respect to Ni-DHLA catalyst), and internal quantum efficiencies (Φ) are summarized in Table 1. There is a large spread in relative H2 generation
ments to the H2 production activity in the QD/Ni-DHLA photocatalytic system may be possible. Here, we report studies of light-driven H2 generation using four different types of nanoparticles as PSs in the nanoparticle/ Ni-DHLA photocatalytic system: spherical CdSe QDs, spherical core/shell CdSe/CdS QDs, one-dimensional CdSe QRs, and one-dimensional CdSe/CdS DIRs. Although all systems were quite robust with activity over 2 days, nanoparticles with greater internal charge separation efficiency (i.e., core/shell QDs and DIRs) surprisingly exhibited worse H2 generation activity. Fluorescence quenching studies yield electron and hole transfer kinetics that are consistent with the expected relative activity for H2 generation. Simple calculations suggest that electron and hole densities over the nanoparticle surface are important for the electron and hole transfer rates, respectively, and thus determine the relative efficiency of H2 generation in these nanoparticles. The CdSe QDs, CdSe QRs, CdSe/CdS core/shell QDs, and CdSe/CdS DIRs used in this study were synthesized by variations of literature methods22−25 and synthetic details are given in the Supporting Information (SI). CdSe/CdS DIRs were synthesized via a seeded-growth method starting from a CdSe seed nanoparticle with the first exciton absorption peak at 490 nm (2.3 nm in diameter).26 Transmission electron microscopy (TEM) confirmed the sizes and the shapes of each nanoparticle (Figure S1 in the SI). The majority of the studies were performed on CdSe QDs that were 2.9 ± 0.4 nm in diameter, CdSe QRs that were 3.0 ± 0.7 in diameter and 7.9 ± 2.7 nm in length, CdSe/CdS core/shell QDs that were 4.0 ± 0.4 nm in diameter, and CdSe/CdS DIRs that were 3.8 ± 0.8 nm in diameter and 11.5 ± 1.8 nm in length. Histograms showing the distribution of nanoparticle diameters and lengths in the samples studied are shown in Figure S2 in the SI. All nanoparticles were synthesized such that they had the same first exciton absorption peak at ∼545 ± 5 nm (Figure 1b). The absorption spectra for all samples show narrow and wellresolved absorption features, suggesting monodisperse and high quality samples. As-synthesized nanoparticles are soluble in organic solvents, therefore, the nanoparticles were subsequently rendered water-soluble by performing a ligand exchange with DHLA (Figure S3 in SI).27 After ligand exchange, the bare QDs and QRs have no measurable photoluminescence quantum yield (PL QY), while the core−shell and DIR nanoparticles had PL QY values in the range of 3−5%. For 2.9 nm diameter CdSe QDs, the conduction band energy level is known to occur at 5348
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core,19,20 leading to better internal charge separation compared to pure CdSe nanoparticles. One-dimensional nanoparticles are also expected to exhibit superior internal charge separation efficiency compared to zero-dimensional structures.18,21 Yet, the ordering of H2 generation activity between nanoparticles appears anticorrelated with the expected charge separation efficiency. The apparent inverse relationship between H2 production activity and internal charge separation was also seen in measurements of CdSe/CdS DIRs of different lengths and CdSe/CdS QDs of different shell thicknesses. DIRs were synthesized with identical core CdSe QDs (diameter = 3.9 ± 1.2 nm) but with different rod lengths (varied from 8.0 to 12.6 nm, Figure S4 in SI). Comparisons of H2 activity show that longer DIRs exhibit worse activity for H2 production than shorter DIRs (Figure S5 in SI). In core/shell CdSe/CdS QDs in which the shell thickness varied between 1.5 and 3.5 monolayers, the thicker shells resulted in the poorer H2 generation activity (Figure S6 in SI), although H2 generation activity for CdSe/CdS QDs is also likely impacted by a decrease in reduction potential as the CdS shell is made thicker. In previous studies of CdSe nanoparticle systems,5,31 it was shown that the initial electron transfer to the catalyst was extremely fast, and therefore the H2 production activity was determined by the relative rates of hole-donation and back electron transfer. One possible explanation for the ordering of catalytic activity is that the intermolecular electron or hole transfer rates are very different for different nanoparticles. To test this hypothesis, fluorescence quenching studies of the various nanoparticles in the presence of Ni-DHLA catalyst or AA were performed. Such studies are not possible on all samples (specifically CdSe QDs) because the fluorescence intensity is completely lost following ligand exchange and transfer to aqueous solvents, even without any quenching agent present. However, the CdSe/CdS heterostuctures do maintain sufficient fluorescence intensity after ligand exchange to permit quenching studies (Figure S7 in SI). Photochemically driven electron-transfer steps were studied separately for nanoparticles in the presence of both Ni2+ and AA in EtOH/H2O mixtures at pH 7. Experimental details are given in the SI. The nanoparticles’ fluorescence was quenched in the presence of both Ni2+ and AA, indicating charge transfer from the nanoparticle excited state, as expected. The concentration dependence of the quenching yields for both the DIRs and core/shell QDs and example PL spectra from Ni2+ quenching of the CdSe/CdS DIRs and are shown in
Figure 2. Photocatalytic hydrogen production for a 46 h period from a system containing various nanoparticle PSs (4.0 μM), Ni(NO3)2 (4.0 μM), and AA (0.5 M) in 5.0 mL EtOH/H2O at pH 4.5 upon irradiation with 520 nm excitation. Inset: the first 10 h period of photocatalytic hydrogen production.
Table 1. Hydrogen Production Activities photolysis (10 h)
H2 (μmol)
TON ( × 103)
TOF (h−1)
Φinternal (%)
CdSe QDs CdSe QRs CdSe/CdS QDs CdSe/CdS DIRs
691 149 67.7 15.9
34.6 7.45 3.39 0.795
3460 745 339 79.5
59 13 5.8 1.4
activity (4−40 times) among the various nanoparticles, much larger than the spread in absolute nanoparticle concentration (∼2 times). On a per photon absorbed basis (Φinternal), the ordering of H2 generation activity was CdSe QDs > CdSe QRs > CdSe/CdS core/shell QDs > and CdSe/CdS DIRs. In order to ensure accurate comparisons, nanoparticle concentrations were purposely kept low enough such that the total H2 produced was limited by the equal number of photons absorbed in each case. Thus, the external quantum efficiencies (2 × H2 production rate/incident photon arrival rate) are about 2/3 of the internal values reported in Table 1. This ordering of H2 production activity in the current system is surprising based on expectations from differences in internal charge separation efficiency between the nanoparticles. For quasi-type II CdSe/CdS heterostructures, the electron wave function is more delocalized across the whole nanoparticle, while the hole wave function is largely confined to the CdSe
Figure 3. (a) Quenching yield as a function of quencher concentration. (b) The emission quenching of DHLA-CdSe/CdS DIRs (4 μM) by the Ni2+ catalyst at various concentrations in 1:1 EtOH/H2O. (c) Stern−Volmer plot of the data shown in (b) with the equation of the best fit line (red line). Error bar (4%) was estimated from the standard deviation of repeated measurements. 5349
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photocatalytic systems in which H2 production activity is strongly correlated with internal charge separation efficiency (oxidative quenching ∼0.5 ps−1 and reductive quenching ∼0.5 ns−1).5 Because the relative magnitudes of the charge transfer rates are similar for oxidative or reductive quenching, the PL quenching data does not suggest one process or the other is strongly rate limiting. We speculated that there were differences in electron and hole surface charge densities between nanostructures, which could be calculated and may provide further insight into the specific rate-limiting step. Calculations were completed using the effective mass approximation34 with a spherical or cylindrical finite potential well that assumes a quasitype-II band alignment between CdSe and CdS,35 with hole wave functions confined to the core of the nanoparticle and electron wave functions penetrating to the outer surface. Further details are given in the SI. Calculated electron and hole surface charge densities are both correlated with the relative H2 production activity for the various nanoparticles (Table 2). Although the precise values of the charge densities are dependent on parameters for which a wide range of values have been reported (e.g., the CdSe/CdS band offset, finite surface potential), the relative ordering of the calculated surface charge density is only weakly dependent on these effects. In all cases, the nanoparticle surface charge densities order as CdSe QDs ≫ CdSe QRs > CdSe/CdS core/ shell QDs > CdSe/CdS DIRs. One can rationalize this ordering based on the differences in surface to volume ratio of the various nanostructures. For quantum confined nanoparticles, increasing the amount of confinement, and thus the magnitude of the wave function at the surface, is equivalent to increasing the surface to volume ratio. A second contributing factor to the relative ordering of proton reduction efficiency is the relative surface area of the nanoparticles. For equal concentrations of nanoparticles and catalysts, the one-dimensional structures have a larger surface area per catalyst molecule than the zerodimensional structures because one-dimensional structures have larger surface to volume ratios. This surface area difference would also contribute to a decrease in catalytic activity for the one-dimensional structures compared to the zero-dimensional structures with the relative area difference being at most (for the QDs to the longest DIRs) a factor of 6.4. While we would not expect a monotonic scaling of proton reduction efficiency with available surface area per catalyst molecule among nanoparticles of the same shape,36 we would expect surface area effects remain important given the different nanoparticle morphologies studied. In general, the relative magnitudes of electron surface charge density more closely follow the relative magnitudes of H2 production activity than the hole surface charge density (as shown in Table 2). This calculational result may suggest photocatalytic H2 production is efficiency is determined by electron transfer to the catalyst. Indeed, recent ultrafast optical spectroscopic studies of electron transfer from CdSe nanoparticles to the Ni-DHLA catalyst support this conclusion.31 For example, it was shown that while electron transfer to the Ni-DHLA catalyst was extremely fast, within tens of picoseconds, the transfer rate slowed for larger QDs and for core− shell QDs.31 Thus, ultrafast spectroscopy also suggests that the relative magnitude of electron surface charge density determines the rate of electron transfer and this is in agreement with the relative ordering of the H2 production activity. However, given the uncertainty in the absolute magnitude of
Figure 3a,b, respectively. The concentration-dependence of quenching was fit following a Stern−Volmer (SV) model (Figure 3c)32 I0 = 1 + kqτ[Q] I
where I0 and I are the fluorescence intensities without and with the presence of quencher, kq is the quenching rate constant, τ is the intrinsic excited state lifetime of the nanoparticle determined by time-correlated single photon counting (Figure S8 in SI), and [Q] is the quencher concentration. Fluorescence quenching and SV fitting for CdSe/CdS DIRs in the presence of AA and CdSe/CdS core/shell QDs in the presence of Ni2+ and AA are shown in the SI (Figure S9). The quenching constants as determined from the fits with Ni2+ and AA for both CdSe/CdS DIRs and CdSe/CdS core/shell QDs are summarized in Table 2. Table 2. Measured Quenching Rate Constants and Calculated Electron and Hole Surface Charge Densities kq/1012 s‑1 M‑1 (with Ni2+)
k′q /1012 s‑1 M‑1 (with AA)
ρe/nm‑2 (rel ρe)a
71 ± 4
9±1
0.32 (1.0) 0.052 (0.16) 0.049 (0.15)
0.089 (1.0) 0.013 (0.15) 0.001 (0.01)
13 ± 1
1.3 ± 0.1
0.002 (0.006)
7 × 10−5 (7 × 10−4)
CdSe QDs CdSe QRs CdSe/ CdS core/ shell QDs CdSe/ CdS DIRs
ρh/nm‑2 (rel ρh)a
relative H2 production 1.0 0.18
0.13
0.025
a
The charge density value relative to that of CdSe QDs is given in parentheses.
The relative magnitude of quenching constants between core/shell QDs and DIRs for both oxidative quenching (core/ shell QDs 5× greater than DIRs) and reductive quenching (core/shell QDs 7× greater than DIRs) are positively correlated with their relative H2 production activity (core/ shell QDs 5× greater than DIRs). This data is consistent with the hypothesis that differences in H2 production activity between nanoparticles originate from differences in intermolecular charge transfer rates. The absolute rate of charge transfer can be estimated from the product of the quenching constant and quencher concentration. Even though kq for oxidative quenching is ten times greater than k′q for reductive quenching (Table 2), when performing photocatalytic proton reduction the reductive quenching of nanoparticles by AA likely competes with their oxidation by the Ni-DHLA catalyst because of the higher concentration of AA (0.5 M) relative to Ni2+ catalyst (4 μM). We note that the AA quenching behavior reached a maximum at ∼15 ± 5 μM (Figure 3c and Figure S9 in SI), indicating that the AA quenching behavior saturates at ∼4:1 mol ratio of AA/nanoparticle. Considering these factors, the approximate oxidative and reductive quenching rates for core/shell QDs are 3.6 and 7.6 ns−1, respectively. These values are similar to systems with a molecular catalyst and QD PSs, in which oxidative and reductive quenching both occur with nanoseconds lifetimes.33 However, they are significantly slower than those reported for 5350
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Nano Letters calculated charge densities, a higher level of theory, for example, one that considers the change in reduction potential with QD radius, is required to make more robust conclusions. The calculations do suggest that core/shell structures are worse in H2 production than core-only structures because they have larger radii than the core-only structures and consequently lower values of their electron and hole surface charge density. An interesting question concerns possible reasons for the unexpected ordering we observe versus other studies of photocatalytic H2 production5 and/or electron transfer to acceptors,5,37 whereby the most efficient nanoparticles for proton reduction were the DIRs and the least efficient the core QDs. One important difference to note is the excitation wavelength used in the current work here versus the other studies. In our case, by using green wavelengths for excitation the CdSe core is directly excited, whereas in other studies excitation was in the blue, thus creating an exciton in the core CdSe QD and the CdS shell. In the latter case, absorption into the nanorod produces a bound exciton with a strong probability of remaining in the CdS shell.37 Thus, the electron and hole are both surface accessible and will have strong electronic interactions with an electron shuttle (i.e., MV2+) and a sacrificial electron donor, respectively. By contrast, for excitation of the CdSe, the hole remains in the CdSe core, while the electron can diffuse into the CdS nanorod.38,39 Thus, in our case the hole always must pass through a significant tunneling barrier in order to escape into the sacrificial donor. This tunneling barrier slows down hole transfer significantly and thus for the full catalytic system leads to worse performance. A second important difference concerns the relative amount of catalyst versus nanoparticle. In previous work,5 the mole ratio of Pt catalyst to nanoparticle was 4000:1, while in our case it was close to 1:1. Thus, as the nanoparticle is elongated, the surface area per photoexcited electron per catalyst increases, which leads again to worse performance. It would be interesting for future studies to saturate the surface of the nanoparticles with catalyst and see if the ordering would be inverted. In summary, a comparative study of a photocatalytic process for H2 generation employing a Ni-DHLA catalyst, ascorbic acid sacrificial donor, and various CdSe/CdS nanoparticle PSs is reported. We find that bare CdSe QDs are the best PSs, which is surprising because this nanoparticle has poorer internal charge separation in the excited state compared to QRs or DIRs. This results suggests that highly engineered heterostructures are not necessarily better at proton reduction than the simplest nanostructures when using molecular catalysts. While we found for this system that amine sacrificial donors that operate at very basic pH values did not give good photocatalytic performance, it would be worthwhile in the future to investigate other possible sacrificial donor species (e.g., alcohols21 and sodium sulfite). Ongoing work will also study the proton reduction efficiency with catalyst concentration for the different nanoparticles, as well as address specific electron and hole charge transfer pathways on ultrafast time scales that will help direct further possible optimizations of the catalytic properties of the complete system.
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Experimental section and additonal figures, tables, and references (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, under Grant DESC0002106. The authors thank Haiming Zhu and Cunming Liu for helpful discussions about the calculation for electron and hole surface charge density.
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