Charge-Transfer Dynamics in Nanorod Photocatalysts with Bimetallic

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Charge-Transfer Dynamics in Nanorod Photocatalysts with Bimetallic Metal Tips Maria Wächtler, Philip Kalisman, and Lilac Amirav J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09265 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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The Journal of Physical Chemistry

Charge-Transfer Dynamics in Nanorod Photocatalysts with Bimetallic Metal Tips Maria Wächtlera*, Philip Kalismanb#, Lilac Amiravb a

Leibniz Institute of Photonic Technology Jena, Albert-Einstein-Straße 9, 07745 Jena,

Germany b

Schulich Faculty of Chemistry, Technion−Israel Institute of Technology, Haifa 32000, Israel

AUTHOR INFORMATION *Corresponding Author E-Mail: [email protected] Phone +49 3641 206 167 #

Current Address

QD Vision, 29 Hartwell Avenue, Lexington, MA 02421

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ABSTRACT CdSe@CdS dot-in-rod nanostructures tipped with AuPt bimetallic nanoparticles as cocatalyst show increased photon-to-hydrogen conversion efficiency compared to their analogues with pure Au or Pt tips. The underlying charge-separation and recombination processes are investigated by time-resolved transient absorption spectroscopy, to unravel whether the observed enhancement of photocatalytic activity is due to charge-separation/recombination properties of the system, or to higher reactivity for proton reduction at the surface of the metal nanoparticle. We find that in the catalytically active Pt and AuPt functionalized structures charge separation occurs with similar time constants (Pt 3.5, 35 and 49 ps; AuPt 2.6, 31, and 66 ps) and the charge-separated state shows a lifetime of ~20 µs in both cases. Hence, these processes should not be regarded as source of the increased catalytic efficiency in the AuPt functionalized nanorods. The results indicate that the proton reduction at the metal nanoparticle surface itself determines the overall efficiency.


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INTRODUCTION Solar energy is going to play a decisive role in satisfying future energy demands, providing a clean and sustainable source of energy, however, the conversion of solar energy into affordable electricity, or high-energy chemicals (fuels) presents a challenge. In particular, solar-driven photocatalytic water splitting into hydrogen and oxygen can provide a source of clean and renewable fuel.1-5 In this respect semiconductor heteronanostructures, e.g., CdSe@CdS seeded rods6-8 functionalized with catalytically active metal nanoparticles (Figure 1), have shown high potential.9-15 In these structures the semiconductor acts as sensitizer. Upon excitation by light an electron hole pair is generated, which subsequently can dissociate. The hole, driven by the band gap offset between the two semiconductors, is localized in the seed.6, 16-18 The electron can be transferred to the metal nanoparticle, where it is available for the reduction of protons.11, 19-22 The metal co-catalyst promotes separation of photogenerated charge carriers and offers lower activation potentials for hydrogen evolution.11,

23

Besides

optimization in the dimensions of the nanorods (e.g. size of the seed, length of the nanorod, diameter of the rod),11, 21, 24-27 the impact of variations in the nature of the catalytic reaction center with respect to variations in size, composition, and morphology, are of utmost importance to achieve optimal efficiencies for solar to hydrogen conversion.28-34 A recent comparative study of the catalytic efficiency of CdSe@CdS seeded nanorods with metal nanoparticle tips of differing composition (Au, Pt and Au core decorated with Pt islands, illustrated in Figure 1) revealed a fourfold increase of the activity of the system containing bimetallic tips, relative to the pure Pt or Au tipped systems.29 Utilization of a bimetallic, rather than a pure metal catalyst, may enable decoupling of the different interfaces within the heterostructure photocatalyst system. This might enable optimization of the semiconductor–metal interface for improved charge-transfer and charge-accumulation processes, alongside optimization of the metal–liquid junction for enhanced activity of the 3 ACS Paragon Plus Environment


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proton-reduction reaction at the metal nanoparticle surface.23,

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28-29, 35-37

The answer to the

question whether the observed enhancement is due to charge separation/recombination properties of the system or due to higher reactivity of the catalytic reaction at the surface of the metal nanoparticle remained open. In this study we address this question by investigating the charge-separation and recombination processes in these metal-tipped seeded rods by applying time-resolved transient absorption (TA) spectroscopy.

Figure 1. Structures and absorption spectra of the investigated semiconductor-metal heterostructures dispersed in toluene: bare rod (black), Au tipped rod (yellow), Pt tipped rod (red), AuPt tipped rod (orange). EXPERIMENTAL METHODS The synthesis and the detailed structural analysis of the nanorods investigated in this study is reported by Amirav and colleagues.29 The fs time-resolved transient absorption measurements were performed in a cuvette with 1 mm path length under inert argon atmosphere. The setup used has been described elsewhere.38-39 In short, the samples were excited at 390 nm by 100 fs pulses with an energy density of 80 µJ/cm2. A white light continuum generated in a sapphire crystal was used to probe the sample between 450 and 700 nm. The relative temporal delay between the pump and probe pulses was varied over a maximum range of 2 ns. The mutual polarization of the 4 ACS Paragon Plus Environment

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pump and probe pulses was set to magic angle. To avoid contributions from the coherent artifact the pulse overlap region (±200 fs) around time zero was excluded in the fitting procedure. The setup for the ns time-resolved transient absorption (TA) measurements was described in detail previously.40 The samples were excited by 5 ns pulses at 520 nm with a repetition rate of 10 Hz in 1 cm cell under oxygen free atmosphere. The probe light is delivered by a pulsed 75 W Xe arc lamp and detected on a PMT after passing a monochromator. For a more detailed description of the experimental set-ups and the fitting procedure the reader is referred to the SI.

RESULTS AND DISCUSSION The synthesis and a detailed structural analysis of the nanostructures investigated in this study (Figure 1) was reported recently.29 We focused on the set that was found to be the most active for hydrogen production, using seeded CdSe@CdS rods that have an average length of 30-40 nm and a diameter of 4.5 nm. The diameter of the CdSe core is 2.5 nm and hence these structures possess quasi type-II band alignment.18, 27, 41-43 The conduction band edge of the core and the shell possess similar energy, while the valence band shows a band edge offset between the CdSe core and the CdS rod. Such band alignment supports intrinsically separation of charge carriers as the holes tend to localize in the seed, while the electrons are delocalized between core and shell. The confinement of the hole supports directional charge transfer to acceptors and prolongs the lifetime of such charge separation.11,

19-20, 44-45

The

seeded rods are functionalized by different metal nanoparticles grown selectively at one end, possessing the following composition: Au, Pt and mixed AuPt. The morphology of the mixed metal tip was thoroughly analyzed and can be described as Au core decorated with Pt islands. The diameter of the metal tips is between 2 and 4 nm. 5 ACS Paragon Plus Environment


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Absorption spectra of the investigated heterostructures dispersed in toluene are depicted in Figure 1. The absorption spectrum of the bare rods shows the characteristic absorption features of the lowest exciton transition between the valence band (VB) edge and the lowest conduction band (CB) level localized in the CdSe core (570 nm). The features at 460 and 410 nm correspond to the CdS rod based 1Σ (1σe-1σh) and 1Π (1πe-1πh) exciton transitions, respectively, between discrete CB and VB levels that result from quantum confinement in the radial direction.22,

24, 43, 46-48

Further, minor contributions of an excitonic CdS transition

stemming from the CdS shell surrounding the CdSe seed, which is slightly redshifted compared to the CdS rod 1Σ transition, is hidden in the red shoulder of the CdS rod band edge transition.21,

24, 49

The spectra of the metal tip functionalized rods can be regarded as a

superposition of these transitions of the seeded rod and broad absorption features of the metal nanoparticles spanning from the NIR to the UV region consisting of interband d-sp transitions, and for Au nanoparticles, additionally the surface plasmon resonance (SPR) band at ~ 520 nm.22,

50-53

The time-resolved spectroscopic measurements were performed upon

excitation at 390 nm, exciting CdS rod localized 1Π exciton transitions with minor contribution of above band edge exciton transitions of the CdSe seed due to the small volume of the CdSe seed and d-sp interband transitions of the metal nanoparticles. The weight of the metal contributions depends on the metal tip and increases from Pt to Au. To address the charge-separation processes the timescale up to 2 ns is investigated with 100 fs time-resolution by TA spectroscopy. The transient spectra of all four samples (Figure 2A and Figures S1 and S2 in the SI) show the characteristic bleach features of the CdSe and the CdS exciton transitions observable at 570 and 458 nm in agreement with the static absorption spectra. The bleach feature of the exciton transition from the CdS shell around the seed region is, as expected, weak due to its low volume and hidden underneath and not distinguishable from the strong bleach of the exciton transition localized in the CdS rod. Previous studies have shown that the bleach features are due to filling of the conduction band 1σe electron 6 ACS Paragon Plus Environment

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levels. Hence, these bleach features are indicators for the presence of electrons in the conduction band, and enable characterization of exciton recombination, as well as electron transfer processes to suited acceptors (e.g. metal nanoparticles or molecular acceptors).11, 22, 50 Though under the chosen excitation conditions (λex= 390 nm) in all samples additional direct excitation of d-sp interband transitions of the metal nanoparticles is possible, only for the Au tipped rods contributions of the metal tip can be identified in the transient spectra (SI, Figure S1 and S2). Around ~ 520 nm the characteristic negative bleach feature of the SPR band, with additional positive absorption contributions at longer and shorter wavelengths around the maximum of the SPR band, are observable in the transient spectra. This signature is caused by a broadening of the SPR band induced by the generation of a non-thermal electron distribution.52-53 Similar signatures for Pt and AuPt nanoparticles are not detectable in the available probing range.22, 36, 54-55 A comparison of the temporal evolution of these bleach features, for the bare rods and the functionalized samples (Figure 2 and SI), reveals, that on the sub-ps timescale a general buildup of the signal is observed for both the bare rod and the functionalized samples. This buildup results from relaxation of hot excitons produced by excitation at 390 nm to the band edge, populating the 1σ level.22, 24 On the longer timescale a decay of the bleach features occurs. In the bare rod sample, only exciton recombination processes within the semiconductor structure can be responsible for the decay of the bleach signal. In the Pt and the AuPt functionalized samples a faster decay of the bleach features is observed, which indicates the presence of additional processes decreasing the filling of the conduction band edge levels. These processes are assigned to electron transfer from the conduction band to the metal nanoparticle, leading to charge separation in these structures.



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Figure 2.

(A) Plot of the transient absorption data of the AuPt tipped sample; (B-C)

Normalized kinetic traces at chosen probe wavelengths for the Pt (red), AuPt (orange), and Au (yellow) tipped samples compared to the bare rod (black) at (B) 458 nm, in the region of the exciton bleach in the CdS rod and shell region, (C) at 580 nm, probing the CdSe seed localized exciton, fits are represented by dashed lines; (D) normalized kinetic traces of the Au tipped sample (yellow) compared to the bare rods (black) at a probe wavelength of 505 nm, showing strong contributions of the bleach of the SPR band superimposed on the nanorod intrinsic dynamics. Further indication for charge-separation processes that occur within the first 2 ns after excitation is delivered by a comparison of the transient spectra at late delay times (1.8 ns) (Figure 3). In the spectra of the Pt and the AuPt tipped samples an additional positive feature forms at 475 nm, in the red flank of the bleach feature of the CdS rod localized exciton 8 ACS Paragon Plus Environment

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transition. This feature is reported in many studies to be a characteristic indicator for the formation of a charge-separated state with various acceptors, both molecular species and metal nanoparticles.11, 19, 22, 24, 50 This signal is caused by a stark effect induced shift of the exciton transition, resulting from the electrical field which is generated by charge separation in the nanostructure.

Figure 3. (A) Transient spectra at 1800 ps delay time recorded in the fs TA measurements for the bare rod (black), the Pt (red), AuPt (orange), and Au (yellow) tipped samples, and average spectra over delay times between 2 and 20 μs resulting from the ns TA measurements (inset), (B) ns time-resolved TA decay kinetics averaged over spectral regions with dominating positive and negative signal contributions on the μs scale for the bare rod (black), the Pt (red) and AuPt (orange) and Au (yellow) tipped sample. 9 ACS Paragon Plus Environment


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The temporal evolution of the spectral signatures in the transient spectra of the Au tipped sample differs from the Pt and AuPt tipped samples (Figure 2). After the fast build-up of the overall signal we observe a fast decay within the first 10 ps of the Au SPR bleach feature at probe wavelengths 505 nm in the maximum of the SPR bleach feature. At a probe wavelength of 580 nm the Au nanoparticles add a positive fast decaying contribution to the overall signal,52 causing the rise of the negative bleach signal intensity of the CdSe seed to appear slower compared to the other samples. After that, only the characteristic features of the CdSe@CdS nanostructure exciton bleach remain (see transient spectra in the SI), and their decay follows closely the behavior in the bare rod without metal tip. In the spectrum recorded at long delay times (1.8 ns), no indication of the formation of a charge-separated state can be detected (Figure 3). The absence of the characteristic positive feature that indicates the presence of long-lived charge separated states leads to the conclusion that in our Au tipped sample charge separation occurs to only a very low extent or does not occur at all. In order to extract the timescale of charge separation, kinetic traces at probe wavelengths representing the development of certain excitons were chosen for evaluation and fitted with a multi-exponential function: the CdS rod and shell exciton are probed at 465 nm, the exciton localized in the CdSe seed is probed at 580 nm (for details see SI). For all samples the formation process of the signal corresponds to a time constant in the range ≤ 200 fs, which is limited by the time-resolution of our experimental setup. In the non-functionalized sample the decay of the exciton bleach localized in the CdS portion can be described by applying three exponents, 14 ps, 192 ps and 3000 ps and a constant offset covering for recovery processes much slower than our observation window. The slow decay of the CdSe seed bleach feature is fit biexponentially (284 and 4000 ps). Additionally, a second slower formation component with a time constant of 22 ps is needed to describe the build-up of the CdSe seed bleach feature probed at 580 nm (see SI Figure S3). This corresponds to the fastest decay process of 10 ACS Paragon Plus Environment

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the CdS exciton bleach. It describes the relaxation of excitons generated in the CdS portion of the nanorod to form excitons that are localized in the CdSe seed driven by the localization of the generated hole in the CdSe seed.19, 21, 24 The timescale of ~ 20 ps observed here is slower than recently reported for similar systems (0.4-10 ps).11, 21, 24, 56 The localization process can be described by a diffusion mechanism and the localization time is strongly dependent on the rod length, which can account for the observed discrepancy.13,

25, 56

Additionally, the

localization time is sensitive to the type and quality of the junction, which also could play a role in this respect.19, 24, 56-57 The slower decay processes of the exciton bleach features are related to electron-hole recombination processes within the semiconductor nanostructure. To describe the deviation of the decay dynamics of the bleach features in the Pt and AuPt tipped nanorods from the bleach recovery dynamics of the bare nanorods, additional processes that account for possible transfer of electrons from the conduction band of the semiconductor to the metal domain should be taken into account (for details see SI). Three rates for electron transfer to the metal domain can be found with time constants of 3.5, 35 and 48 ps for the Pt and nearly similar values of 2.6, 31, and 65 ps for the AuPt tipped rods. The two fast processes contribute to bleach recovery at the probe wavelength 458 nm and, hence, describe electron transfer originating from the CdS rod and shell excitonic state, while the slowest process accounts for the accelerated bleach recovery at 580 nm and can be related to electron transfer to the metal tip from the CdSe seed (Figure 4). The variation of the time-constants observed for charge separation is due to the differing spatial localization of the distinct excitonic states and the increasing distance to the electron accepting metal tip, which decreases the overlap between the electron wave function and the metal acceptor states states.21,

24

Our determined values are in good agreement with literature reports for related

structures.11, 20-22



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Figure 4. Illustration of localization of the three distinct exciton states in CdSe@CdS seeded nanorods and the charge-separation and charge recombination processes in the Pt and AuPt tipped samples. As indicated by the quantitative analysis of the development of the spectral shape, and by comparing the kinetic traces in Figure 2, the recovery dynamics in the Au tipped sample can be described by a simple superposition of semiconductor nanostructure intrinsic electron-hole recombination dynamics, with Au nanoparticle intrinsic electron relaxation dynamics. The description of temporal evolution of the signal at chosen probe wavelengths requires only the consideration of a single additional process, with a time constant of 2.2 ps. This process shows the strongest contributions to the temporal evolution of the signal around the maximum of the SPR band, and does not contribute to bleach decay and hence is attributed to Au intrinsic heat dissipation. The value of 2.2 ps is in very good agreement with electron-phonon coupling times reported for colloidal Au nanoparticles.52-53, 58 The absence of any indication for charge separation in our sample is in contrast to literature reports for similar systems.31, 50, 59-60

This discrepancy could be attributed to differences in the synthesis61 and resulting

semiconductor-metal interface,62-64 as well as due to utilization of different capping ligand and solvent.50, 60 Further, there are reports of charge separation in Au tipped nanorods occuring on 12 ACS Paragon Plus Environment

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a sub 100 fs time scale, which we cannot address due to the limited time resolution of our setup.59 If such ultrafast charge separation would occur in combination with fast recombination, the charge separated state might not be detectable. Last but not least, we investigated the charge-recombination processes in Pt and AuPt functionalized rods by performing transient absorption measurements on a timescale up to 200 μs. In Figure 3, we compare the signal for the bare sample and the Pt, AuPt and Au tipped samples. We observed in the transient spectra of the Pt and AuPt tipped rods the characteristic negative bleach feature, with a maximum at 460 nm, and an additional positive feature at 475 nm, which serves (as discussed above) as an indicator for the charge-separated state. This positive feature is missing in the non-functionalized and Au tipped samples in agreement with the missing formation of a charge separated state in the latter. Due to very low signal intensity, these measurements merely allow us to estimate the lifetime of the charge-separated state. The minor signal remaining in the bare sample and Au tipped sample could be assigned to residual long-lived deep trap states.22, 24, 65 The signal of the charge-separated states shows a slower decay, which is nearly identical for both the Pt and the AuPt tipped sample. The average charge-recombination time can be estimated to be ca. 20 μs independent of the nature of the metal tip. This is considerably longer than the values found recently for non-seeded metal tipped CdS nanorods (1.2 μs)22,

50

and confirms the improved charge-separation

properties of CdSe@CdS nanorods compared to CdS nanorods, which helps preventing backward recombination.11, 19-20, 44-45

CONCLUSIONS For the samples with Pt and AuPt tips, which are the catalytically active species, we observe charge separation that occurs on three timescales (Figure 4). We suggest that these timescales originate from different localization of the excitonic state within the semiconductor 13 ACS Paragon Plus Environment


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nanostructure from which the electron is transferred to the metal domain. The difference in the time constants for the transfer process observed for Pt and AuPt tipped rods were only marginal. This also accounts for the charge-recombination time constants, which are basically identical in both types of samples. On the other hand, in agreement with the sample’s low activity, we observe limited or absent charge separation (below our detection limit) for the Au tipped sample. This indicates that the decoration of the Au nanoparticles with Pt islands functions as activating factor for charge separation. Two possible explanations might account for this interesting finding: (1) The metal nanoparticles are grown in a co-photodeposition procedure, with both Pt and Au precursor present in the solution.29 The resulting structure of Au core decorated with islands of Pt results from the substantial differences in deposition kinetics of the different metals. However, limited Pt nucleation at the CdS surface could not be ruled out. The presence of Pt at the interface, even to very low degree, could affect the energy barrier for charge transfer to the metal tip. (2) The presence of Pt on the surface affects the electronic character of the metal nanoparticle, e.g. the density of states close to the Fermi level. It has been reported that even low amounts of Pt in bimetallic AuPt nanoparticles may increase the density of states at the Fermi level significantly compared to pure Au nanoparticles,53, 55, 66-67 rendering the electronic structure of the AuPt particle more similar to the pure Pt nanoparticles. This could account for the observed similar electron transfer properties from the conduction band to the Fermi level of the metal domain in the Pt and AuPt samples. Nevertheless, the rates of the charge-separation and recombination processes in the Pt and AuPt samples are very similar and cannot account for the significant improvement in catalytic activity of the bimetallic tip. Hence, the reduction reaction at the nanoparticle surface should be regarded as the efficiency determining process. The morphology of the bimetallic tip (Au core decorated with Pt islands on the surface) introduces numerous exposed interfaces 14 ACS Paragon Plus Environment

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between the two phases. Such lateral heterogeneity is reported to have strong effect on binding energy of adsorbates such as oxygen, CO and hydrogen.35-37, 68-69 DFT calculations indicate that the strongest binding sites are formed by Pt atoms that are adjacent to Au.69 Further, the catalytic reaction might be supported by an interplay between the good electron storage capabilities of Au, which may support accumulation of electrons, creating a reservoir,70 and the catalytically active Pt, which mediates electron transfer to the surface and the surrounding medium, e.g. adsorbed H+.12,

23, 37

These results extend the fundamental

understanding of charge-transfer dynamics at the semiconductor-metal interface on the nanoscale, and sheds light on the interesting features that are introduced by utilization of bimetallic nanoparticles. We believe that ultimately this work will support rational design of efficient photocatalytic systems for artificial water splitting and solar-to-fuel conversion. ACKNOWLEDGMENT We acknowledge the support by the COST Action CM1202 PERSPECT-H2O. LA gratefully acknowledges the support of the I-CORE Program of the Planning and Budgeting Committee, and The Israel Science Foundation (Grant No. 152/11), as well as the German-Israeli Foundation (GIF) for Scientific Research and Development (Grant 2307-2319.5/2011). This research was partially carried out in the framework of Russell Berrie Nanotechnology Institute (RBNI) and the Nancy and Stephen Grand Technion Energy Program (GTEP). We thank Xiaoting Zhang for her assistance during sample preparation and data acquisition as a part of her master project. ASSOCIATED CONTENT Supporting Information Available: The supporting information includes detailed description of the experimental setup and conditions, transient spectra, and details of the fitting procedure. This information is available free of charge via the Internet at http://pubs.acs.org. 15 ACS Paragon Plus Environment


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TOC Graphics 85x46mm (300 x 300 DPI)

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Figure 1. Structures and absorption spectra of the investigated semiconductor-metal heterostructures dispersed in toluene: bare rod (black), Au tipped rod (yellow), Pt tipped rod (red), AuPt tipped rod (orange). Figure 1 82x67mm (300 x 300 DPI)



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Figure 2. (A) Plot of the transient absorption data of the AuPt tipped sample; (B-C) Normalized kinetic traces at chosen probe wavelengths for the Pt (red), AuPt (orange), and Au (yellow) tipped samples compared to the bare rod (black) at (B) 458 nm, in the region of the exciton bleach in the CdS rod and shell region, (C) at 580 nm, probing the CdSe seed localized exciton, fits are represented by dashed lines; (D) normalized kinetic traces of the Au tipped sample (yellow) compared to the bare rods (black) at a probe wavelength of 505 nm, showing strong contributions of the bleach of the SPR band superimposed on the nanorod intrinsic dynamics. Figure 2 177x141mm (300 x 300 DPI)


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Figure 3. (A) Transient spectra at 1800 ps delay time recorded in the fs TA measurements for the bare rod (black), the Pt (red), AuPt (orange), and Au (yellow) tipped samples, and average spectra over delay times between 2 and 20 µs resulting from the ns TA measurements (inset), (B) ns time-resolved TA decay kinetics averaged over spectral regions with dominating positive and negative signal contributions on the µs scale for the bare rod (black), the Pt (red) and AuPt (orange) and Au (yellow) tipped sample. Figure 3 82x128mm (300 x 300 DPI)



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Figure 4. Illustration of localization of the three distinct exciton states in CdSe@CdS seeded nanorods and the charge-separation and charge recombination processes in the Pt and AuPt tipped samples. Figure 4 76x70mm (300 x 300 DPI)


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Charge-Transfer Dynamics in Nanorod Photocatalysts with Bimetallic

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