Tailoring Energy Transfer from Hot Electrons to Adsorbate Vibrations

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Tailoring Energy Transfer from Hot Electrons to Adsorbate Vibrations for Plasmon-Enhanced Catalysis Priyank V. Kumar, and David J. Norris ACS Catal., Just Accepted Manuscript • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017

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Tailoring Energy Transfer from Hot Electrons to Adsorbate Vibrations for Plasmon-Enhanced Catalysis Priyank V. Kumar and David J. Norris* Optical Materials Engineering Laboratory, ETH Zurich, 8092 Zurich, Switzerland

ABSTRACT. Chemical reactions can be enhanced on surfaces of bimetallic nanoparticles composed of a core plasmonic metal and a catalytically active shell when illuminated with light. However, the atomic-level details of the steps that govern such photochemical reactions are not yet understood. One critical process is the non-adiabatic energy transfer from hot electrons that transiently populate the unoccupied electronic orbitals of the adsorbate to the vibrational modes of the adsorbed reactants. This occurs via electron-vibration coupling and could potentially be tailored by changing the composition of the shell. Here, we apply an ab initio method based on density functional theory to investigate this coupling at various sp- and d-band metal-adsorbate interfaces. Our calculations demonstrate the importance of d-bands in enhancing and tuning this energy transfer at the interface. Further, they highlight specific choices of metals that could be utilized as shells for efficient photochemical reactions. From these calculations, we extract a simple descriptor (dependent on the coupling matrix element and equilibrium bond length) that can account for the coupling strength at a metal-adsorbate interface, thus representing a valuable tool for rational shell design for different reactions. We show the utility of this descriptor for photocatalysis with calculations for a specific photochemical reaction. The introduction of this descriptor should also impact other processes such as light-triggerred drug release that exploit hot electrons, and surface-enhanced Raman spectroscopy, where electron-vibration coupling plays a key role.

KEYWORDS. Plasmonic catalysis, hot electrons, electron-vibration coupling, core-shell, DFT

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1. INTRODUCTION Plasmon-enhanced catalysis strives to control chemical reactions using light.1,2 The interaction of photons with a metallic nanoparticle can excite localized surface plasmons that decay nonradiatively into energetic charge carriers (referred to as hot electrons and hot holes). These carriers can then transfer their energy to the vibrational modes of adsorbed molecules, which can trigger chemical transformations.3 As a stand-alone technology or in conjunction with conventional heterogeneous catalysis, plasmon-induced photochemistry offers numerous potential advantages. This includes enhancing reaction rates, facilitating new reaction outcomes, engineering product selectivity, and exploiting sunlight as a renewable energy source, among many others.1,3,4 Several groups have already demonstrated photochemical reactions directly on typical plasmonic nanoparticles. Some of these examples include H2 dissociation on Al,5 O2 dissociation on Ag for ethylene epoxidation,6 CO desorption on Pt in the context of syngas purification,7 and organic molecule degradation on Ag particles.8 However, despite these advantages, the photocatalytic efficiencies measured in current experiments have remained low with poor control over product selectivity, rendering them impractical for industrial use.9 This is largely due to the fact that photochemical reactions are carried out directly on plasmonic metals such as Cu, Ag, Au, and Al, which may not represent ideal catalytic surfaces for the reaction under consideration.3 To overcome these limitations, the concept of bimetallic nanoparticle catalysts has been postulated recently (see Refs. 2 and 3). The idea is to have a core-shell type structure, where the core is composed of a plasmonic metal that enhances the production rates of hot electrons and holes from plasmons, while the shell is composed of a different metal that is catalytically active (Figure 1a). While this idea has been shown to be experimentally powerful,10-13 a detailed atomic-level understanding of the different energy flow processes that are involved in such bimetallic catalyst architectures is lacking. For this, theoretical calculations are invaluable since each process can be


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investigated separately, which is challenging to do in experiments. So far, a number of calculations have focused on studying hot-carrier generation and decay processes in plasmonic metals.14 In particular, jellium models and ab initio electronic structure methods have been useful for computing hot-carrier distributions15-17 and the strength of electron-phonon (e-ph) coupling18,19 in various bulk plasmonic metals, thus providing essential guidelines for catalyst design. A potential advantage of the bimetallic design, which remains largely unexplored, is that the shell could be utilized to enhance the coupling of hot carriers (that transiently populate the unoccupied electronic orbitals of the adsorbate) to the vibrational modes of the adsorbate, depending on the choice of the metal. Theoretical work in the past have described this coupling between hot electrons and the different vibrational modes of the adsorbate.20-32 This electron-vibration (e-vb) coupling is often discussed using two approaches: the electronic friction theory33 and the Newns-Anderson model.32,34 It is well known from these approaches that the non-adiabatic e-vb coupling provides the driving force for bond-breaking in light-induced chemical reactions.20 Although these studies have highlighted the importance of energy transfer via e-vb coupling, they have been limited to specific choices of metal-adsorbate interfaces. As such, a comprehensive comparison of the non-adiabatic e-vb coupling strengths at various sp- and d-band metal-adsorbate interfaces is missing. Performing such a study would not only help understand the microscopic origin of this coupling on the surfaces of various types of metals, but also potentially allow the design of shells with tailored energy-transfer rates from hot carriers to different adsorbate bonds. This would be possible if we could identify general material parameters that describe the coupling strengths of hot carriers to the vibrational modes of the adsorbate at the surfaces of various metals. Thus, the aim of this work is three-fold. First, we employ an ab initio method based on density functional theory (DFT) to compute the interaction of hot carriers with different vibrational modes of a metal-adsorbate complex.35 This approach enables us to go beyond the free-electron/jellium models and explicitly account for the effect of d-bands, which, as we show, plays an important role


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in e-vb coupling at a metal surface. Second, based on calculations for a range of metallic substrates, we wish to extract simple material parameters (descriptors) that primarily dictate e-vb coupling at the metal-adsorbate interface and could help guide catalyst-shell design. Third, we seek to understand the consequences of such descriptors for utilizing hot electrons in applications such as photocatalysis and light-triggerred drug release,36 and other applications such as surface-enhanced Raman spectroscopy (SERS), where e-vb coupling plays an important role.37,38

2. METHODOLOGY For this work, we choose CO and NO molecules as model adsorbates, and various sp and 4d/5d transition metals as model substrates (that could be utilized as shells). Our selection of metals allows us to explore the impact of sp- and d-bands as well as a diverse set of surface parameters known to govern chemisorption. It is likely that the same parameters will also influence e-vb coupling at the metal-adsorbate interface.39,40 More specifically, these parameters include the coupling matrix element between the adsorbate states and the metal d-states (or the sp-states), the so-called d-band center, and the filling of the bonding/anti-bonding states determined by the Fermi level of the metaladsorbate system. We focus on hot electrons in this work since it is known that the photochemical reaction for CO and NO adsorbed on metal surfaces proceeds via hot-electron transfer.7,32 We perform calculations on a unit cell that consists of two layers of metal atoms (4 atoms in each layer) in order to simulate a metal surface (see Figure 1b). We consider one CO molecule adsorbed directly on top of a metal atom. We choose the (0001) surface for the hexagonal close-packed (HCP) structures of Ru, Tc, Zr, Os, Re, and Hf, the (110) surface for the body-centered cubic (BCC) structures of Nb and Ta, and the (111) surface for the face-centered cubic (FCC) structures of Al, Ag, Pd, Rh, Au, Pt, and Ir. These choices represent experimentally stable surfaces and also reproduce the trends in adsorption energies accurately, consistent with previous calculations,41,42 and hence represent meaningful configurations to probe the relationship between chemisorption and e-vb


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coupling (see Figure S1 in the Supporting Information for adsorption energies). Similar unit-cell configurations are also adopted for the NO molecule. All calculations were performed using the ABINIT code.43,44 The ground-state electronic structure was computed within the generalized gradient approximation (GGA) of DFT using the revised Perdew-Burke-Ernzerhof (RPBE) exchange-correlation functional,45 which is known to treat chemisorption on the d-band metals successfully. However, care must be taken when using semilocal functionals to determine stable binding sites, as they can fail drastically for CO adsorption on transition metals, often stabilizing the wrong site (atop vs. bridge/hollow). We employed normconserving pseudopotentials46 and a kinetic-energy cutoff of 30 Ha for the plane-wave basis set. Lattice-dynamical properties were computed using density functional perturbation theory.47,48,49 The structures were relaxed to less than 2×10!! Ha/Bohr, and a 4×4×1 k-point grid was used. The coupling strength (and hence the energy transfer) between the electrons and a particular molecular vibration (or a phonon mode of the metallic lattice) can be quantified using a parameter called linewidth. Therefore, the phonon and vibrational linewidths serve as important quantities to compare energy transfer rates associated with different modes. We computed the phonon and vibrational linewidths using Fermi’s “golden rule” as follows:50,51 𝛾𝐪! = 2𝜋𝜔𝐪!

𝐤!"

𝐪!

𝑔𝐤!𝐪!,𝐤!

!

𝛿 𝜖𝐤! − 𝜖! 𝛿 𝜖𝐤!𝐪! − 𝜖! ,

(1)

where, the matrix elements are given by: 𝐪!

𝑔𝐤!𝐪!,𝐤! = 𝜓𝐤!𝐪! 𝛿 𝐪! 𝑉 𝜓𝐤! ,

(2)

with 𝜖! representing the Fermi level, 𝜓𝐤! being the electronic wavefunction for band n, wavevector k, and eigenvalue 𝜖𝐤! . 𝛿 𝐪! 𝑉 is the derivative of the self-consistent potential associated with a lattice vibration of wavevector q, branch index ν, and frequency 𝜔𝐪! . The total linewidth associated with a specific mode ν is evaluated as 𝛾! =

𝐪 𝑤𝐪 𝛾𝐪! ,

where 𝑤𝐪 is the Brillouin zone weight associated


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with the wavevector q. The matrix elements were computed on 8×8×1 q-point and 16×16×1 kpoint grids (see Figure S2 in the Supporting Information for convergence tests).

3. RESULTS AND DISCUSSION In Figure 2a, we show the phonon and the vibrational modes of our metal-CO system by plotting the phonon density of states (DOS), taking Pt as an example substrate. The phonon modes arise from the metal atoms, which have frequencies in the range 0-180 cm-1. The vibrational modes arise due to the presence of the adsorbate. In the particular case of CO adsorbed on the metal, four such vibrational modes exist: 1 and 2 are rocking modes that appear at ~400 cm-1; 3 is the center-of-mass mode at ~500 cm-1; and 4 is the internal stretching mode at ~2050 cm-1.52 In our work, we wish to understand how the hot electrons generated in the metal-adsorbate system couple to each of these four vibrational modes of the adsorbate, and how this coupling varies across different metals. First, we quantify the interaction between hot electrons and mode 3 (the center-of-mass mode). This vibration primarily involves metal-CO bond stretching and is thus expected to be strongly affected by the type of metal. Additionally, such a mode is expected to play a key role in regulating energy transfer in the case of larger adsorbates, as evidenced from chemical Raman enhancement of larger organic molecules (see Refs. 38 and 53). The upper panel of Figure 2b shows the computed vibrational linewidths for this mode at different energies above the Fermi level exemplifying the case of CO adsorbed on Pt. This was achieved by artificially setting the value of the Fermi energy above its actual ground-state value, in steps of 0.2 eV.54 As expected, the computed linewidths show a trend similar to the electronic DOS of the CO molecule (bottom panel of Figure 2b), with the peak occurring at the energy of the lowest unoccupied molecular orbital (LUMO) of the adsorbate. Similar plots were obtained for the other metals and for the other three vibrational modes associated with the adsorbate (see Figure S3 in the Supporting Information).


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In Figure 3a, we plot the peak linewidth values obtained (indicated by the green arrow in Figure 2b) for mode 3 of CO adsorbed on different metal substrates. Recall that the greater the linewidth value, the greater the e-vb coupling and the associated energy transfer. We omit the cases of CO on Ag and Au, since CO is repelled from these noble-metal surfaces, as van der Waals (vdW) interactions are not included in our study.55 The plot reveals several interesting features: (i) The peak linewidth values increase from Zr (Hf) through Ru (Os) for the 4d (5d) series and then decrease. In other words, the plot reveals a “volcano” shape. (ii) The linewidths are also higher for the 5d metals compared to the 4d metals. A similar analysis on mode 4 (the internal stretching mode) leads to the plot shown in Figure 3b. In contrast to mode 3, the linewidths for mode 4 monotonically increase as we go from the left to right along both series. Once again, these values are higher on the 5d metals than on the 4d metals. The volcano shape observed in mode 3 is also present for modes 1 and 2 (see Figure S4 in the Supporting Information). Additionally, we find that the linewidths for these modes are significantly smaller than for modes 3 and 4. Hence, we expect them to play a minor role in controlling the energy transfer at the interface. The above trends are also observed for an NO molecule, providing evidence that these concepts are transferable to different adsorbates (see Figures S5 and S6 in the Supporting Information). We have also performed calculations on a larger unit cell containing four layers of metal atoms and compared the results obtained with the two-layer case. These calculations show that although the peak linewidth values are quantitatively modified, the qualitative trends discussed here remain unaffected (see Figures S7 and S8 in the Supporting Information). Our calculations on the Mg (0001), Al (111) and Ca (111) surfaces (sp-band metals) yielded peak linewidth values of 0.12, 0.24, and 0.30 meV, respectively, for mode 3. For mode 4, we obtained values of 13.03, 11.98, and 9.08 meV, respectively. These results suggest that the linewidths on the d-band metals are generally much higher than that on sp-band metals, indicating the importance of d-bands for enhanced e-vb coupling and energy transfer. These results further demonstrate that the


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energy transfer from hot electrons to the different vibrational modes of the adsorbate can be tuned by altering the shell composition, indicating a vast, yet untapped opportunity for catalyst design. They also highlight the limitations of jellium models and the importance of ab initio computations in describing e-ph and e-vb couplings,14 to the extent that the former are unable to capture the vital effects induced by the d-bands at the metal-adsorbate interface. To explain the trends observed in our calculations and to extract the factors that primarily control the e-vb coupling at the metal-adsorbate interface, we utilized the deformation potential approach formalized by Bardeen and Shockley to compute the matrix elements:56,57,58 𝐪!

𝑔𝐤!𝐪!,𝐤!

!

= ℏ

2𝑀𝜔𝐪! . Δ!! 𝐪 . 𝐼 ! (𝐪) ,

(3)

where Δ! 𝐪 is the deformation potential for the vibrational mode 𝜐, 𝐼 𝐪 is the overlap integral over the unit cell between the initial and final electronic states, 𝑀 is the mass of the unit cell, and ℏ is the reduced Planck constant. In our case, the strong modification of the linewidths by different metal substrates can chiefly be explained by evaluating the deformation potential for the relevant mode. This is done by computing the shift of the adsorbate’s LUMO level relative to the Fermi level 𝜖! , induced by a particular vibration, i.e. Δ! 𝐪 = 𝜕(𝜖! − 𝜖!"#$ ) 𝜕𝐪! .38 For a CO molecule, the LUMO lies at the degenerate 2𝜋 levels. Its interaction with the metal dstates leads to different degrees of hybridization and, hence, broadening of the LUMO in the energy range 3.0-3.4 eV above the Fermi level (see bottom panel in Figure 2b).41 This makes it difficult to extract a particular energy, 𝜖!"#$ , for computing the deformation potential.37 To overcome this difficulty, we chose to evaluate 𝜖!"#$ as the 2𝜋-band center, given by the center of gravity of the 2𝜋 states above the Fermi level. In Figure 3c,d, we plot the deformation-potential values for the two key modes involved in energy transfer, modes 3 and 4, calculated on different metal substrates using a displacement of ± 0.1 Å along a particular vibrational mode.38 The deformation-potential values show trends that correlate well with the linewidths obtained through ab initio computations in Figure 3a,b. This suggests that


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the electronic factors governing the deformation potential primarily dictate the differences in linewidths observed in our computations. In the deformation-potential approach, it is assumed that the atomic displacements can be described by long-wavelength acoustic waves, and these can be related to the elastic strain of the crystal.57,59 The primary effect of such long-wavelength vibrational modes (displacements along a vibrational mode) is known to be the modification of the interatomic coupling matrix elements between the relevant atoms.59 For instance, if we consider mode 3, the coupling matrix element, 𝑉!" , between the adsorbate states (a) and the d-states (d) of the metal would be the most relevant. Hence, we expect the deformation potential per unit displacement to primarily scale as 𝜕𝑉!" 𝜕𝑑! , where 𝑑! is the bond length between the adsorbate and the metal atom. In our case, since the 2𝜋-states of CO interact with the d-states of the metal atom, the relevant matrix element for mode 3 is 𝑉!! , which can be evaluated using the muffin-tin orbital theory.41,60 Namely, 𝑉!! = 𝜂!"#

! !

ℏ! !!

! !

!!!

,

(4)

where 𝜂!"# is a universal dimensionless constant, 𝑚 is the electron mass, 𝑟! is the characteristic length related to the spatial extent of the d orbitals of the metal atom, which can be obtained from Ref. 61, and 𝑑! is the metal-adsorbate bond length, which can be obtained from our DFT calculations. Based on these equations, one expects the square of the deformation potential per unit displacement along mode 3 (and, hence, the linewidth) to scale as 𝑉!! 𝑑!

!

or simply 𝑟!! 𝑑!!

(referred to as 𝑓!" for mode 3), if we neglect the constants. In Figure 4a, we evaluate 𝑓!" and compare the values to the corresponding deformation potential and linewidth values obtained in Figure 3a,c. The factor 𝑓!" increases as we go from left to right along the series and then decreases, reproducing the volcano shape observed for the peak linewidths. Additionally, it predicts higher values for the case of 5d metals, in line with our linewidth calculations. However, the factor predicts peak linewidths at the surfaces of Rh (and Ir) instead of


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Ru (and Os), which we attribute to the simplicity of the analytical model as compared to our ab initio result. For mode 4, the relevant coupling matrix element to consider would be 𝑉!" , representing the coupling between the C and O sp-states. This is given by:61 ℏ!

𝑉!" = 𝜂!" !!! , !"

(5)

where 𝜂!" is a universal dimensionless constant, and 𝑑!" is the C-O bond length. Based on this ! expression, we expect the deformation-potential and linewidth values to scale as 1 𝑑!" (referred to

as 𝑓!" for mode 4). Figure 4b shows that this factor increases from left to right along the series, in contrast to 𝑓!" . Again we obtain a good correlation with the peak linewidths in Figure 3b. We note that the descriptor 𝑓!" shows a volcano shape, even though one might naively expect it to decrease monotonically from left to right across a series just as 𝑟! does (dotted lines in Figure 4c). However, the volcano shape arises because the equilibrium metal-adsorbate bond length 𝑑! also decreases from left to right (solid lines in Figure 4c), which enters into 𝑓!" in its denominator. This change in metal-adsorbate bond length is attributed to the dominant “repulsion term” (sometimes referred to as Pauli repulsion) due to the energy cost associated with the orbital reorganization during the formation of a chemical bond. While this effect plays an important role by dictating the metal-adsorbate bond length, we do not discuss this here as detailed explanations of this effect can be found in Refs. 40 and 60. In contrast, the C-O bond length 𝑑!" decreases from left to right along a series (Figure 4d), and hence the descriptor 𝑓!" increases accordingly (Figure 4b). Together, our analyses suggest that descriptors such as 𝑓!" and 𝑓!" could represent excellent starting points to gauge and potentially tailor the energy transfer from hot electrons to the different vibrational modes of the adsorbate. Figure 5 summarizes this critical relationship between the linewidths and the descriptors we obtained for modes 3 and 4, and such plots could potentially be utilized in designing photocatalysts. We note that the linewidth values for mode-4 are highly


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sensitive to the changes in the 𝑓!" descriptor value (or in other words, changes in 𝑑!" ) compared to those of mode-3. We add that the discovery of similar descriptors, nearly two decades ago, for the adsorption energy of adsorbates on metals has impacted the field of heterogeneous catalysis profoundly.40,62 Now that we have extracted a simple descriptor to understand and potentially control e-vb coupling at the metal-adsorbate interface, we discuss some of the consequences for photocatalysis. It is well known that the adsorption energy of the adsorbate on the metal surface is another key factor that controls the overall photochemical reaction rate3,4,32,52 and, like e-vb coupling, depends critically on the parameters of the metal substrate.40 For instance, Kale et al. showed photochemical conversion of CO to CO2 on Pt nanoparticles by activating the metal-adsorbate bonds.7 Since the desorption of CO is the rate-limiting step here, one can apply the following expression to obtain an estimate of the desorption probability, derived using the modified friction model proposed by Brandbyge and co-workers:33 𝑃!"# = −𝐸!"#

! !(!) 𝑑𝑡 ! (!) exp ! !"#

𝐸!"#

𝑇!"# (𝑡) ,

(6)

where 𝐸!"# is the adsorption energy of the CO molecule on the metal surface, 𝑡 is time, 𝜂 is the electronic friction term related to e-vb coupling, and 𝑇!"# is the adsorbate temperature. This equation clearly demonstrates the importance of understanding and controlling both the e-vb coupling and the adsorption energy to improve photochemical reaction rates. Thus, it would be beneficial to (i) distinguish the electronic factors governing e-vb coupling and the adsorption energy, and (ii) investigate if these two properties could be controlled independently. In what follows, we clarify the relevant factors and demonstrate that it is possible to exercise control over the e-vb coupling and the adsorption energy independently, taking the aforementioned CO oxidation reaction on Pt as an example reaction. The main electronic factor governing the adsorption energy of an adsorbate on a d-band metal is the so-called “d-band center”, which is defined as the center of gravity of the d-band density of


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states with respect to the Fermi level.40 The key idea is that the magnitude of the adsorption energy increases (decreases) as the d-band center decreases (increases). On the other hand, for the mode-3 vibration for example, we know from our results that the main factor controlling the e-vb coupling strength is 𝑓!" = 𝑟!! 𝑑!! . Since 𝑟! is fixed for a given metal surface, these descriptors collectively suggest that by using methods that solely vary the d-band center but not the resulting metaladsorbate bond length 𝑑! , one could in principle tune the adsorption energy without altering the e-vb coupling associated with mode-3. To test this, we strained the Pt slab in-plane by -6, -3, +3 and +6% to alter the d-band center.63 Figure 6a shows a plot comparing the adsorption energy and the linewidths of the mode-3 and mode4 vibrations for the different values of strain considered. Interestingly, the mode-3 linewidths remain mostly unchanged across these calculations, while the adsorption energy changes by almost 1 eV (see Figure 6b). This is because straining the slab alters the value of the d-band center and consequently, the adsorption energy as discussed before.63 However, since 𝑟! is fixed and the value of 𝑑! across these calculations is negligibly affected by strain (less than 1% relative to the unstrained case), the corresponding 𝑓!" values are largely unaffected. This explains the constant linewidth values computed for mode-3. On the other hand, we observed that the linewidth values for mode-4 vary between 78.33 meV and 44.10 meV, as 𝑑!" varies between 1.141 Å and 1.145 Å. This is consistent with our prediction before, and can be explained by the higher sensitivity of the mode-4 linewidth values to the changes in the C-O bond length, 𝑑!" . These calculations clearly demonstrate that there are significant opportunities to control energy transfer via e-vb coupling and the adsorption energy independently, and should be explored further. Importantly, these additional calculations demonstrate that 𝑓!" and 𝑓!" are indeed suitable descriptors to describe e-vb coupling and could guide catalyst surface design in photochemical reactions.


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We once again remark that our results are important as most experiments thus far employ metallic nanoparticles made from plasmonic metals, without optimizing for energy transfer from hot electrons to the adsorbate vibrations.3 Therefore, in the context of core-shell bimetallic nanoparticles, our work represents a first step toward rational design of shells. The ultimate choice of the surface metal would depend on additional factors such as (i) the electron transfer rates (not studied here), and (ii) the chemical reaction involved and the associated rate-limiting step, which would decide the vibrational mode that needs to be activated. Finally, we also note the broader implications of our results in diverse applications. For example, resonant illumination of Au nanoparticle-DNA complex results in light-triggered release of DNA, taking advantage of hot electrons generated during the process. In another example, surfaceenhanced Raman spectroscopy (SERS), chemical enhancements up to 2-3 orders of magnitude are observed where deformation potentials associated with vibrational modes involving the adsorbatemetal bond play an important role.38,53,37 As such, one could use the principles developed here to further optimize and enhance therapeutic cargo release and Raman intensity of the relevant vibrational modes, respectively, for the two applications discussed above.

4. CONCLUSIONS In summary, we employed an ab initio approach to probe e-vb coupling of a metal-adsorbate complex and extracted microscopic information that would be challenging to obtain from experiment. Our key conclusions are: (i) one can tune the energy transfer (coupling) from hot electrons to different vibrational modes of the adsorbate depending on the d-band metal substrate, (ii) the coupling matrix element and the equilibrium bond length are expected to be key parameters controlling this e-vb coupling strength at the interface, (iii) considerable potential exists to control the adsorption energy and the e-vb coupling strength independently, presenting additional opportunities to affect photochemical reaction outcomes favorably. We also expect the picture


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developed here to be applicable to other adsorbates, although more calculations are necessary to confirm this claim. In general, we believe that the ideas presented here can help guide the design of core-shell bimetallic nanoparticles for plasmon-enhanced catalysis and have broad impact in different applications of hot electrons.

ASSOCIATED CONTENT Supporting Information Adsorption energies of CO and NO on different metals; Convergence tests for linewidth values; Linewidth values for all the vibrational modes associated with CO; Linewidth values for all the vibrational modes associated with NO; Linewidth calculations on a four-layer slab model.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS P.V.K. thanks the Marie Curie ETH Zurich Postdoctoral Fellowship for financial support. Computations were performed at the ETH High-Performance Computing Clusters, namely Euler and Brutus. The authors acknowledge C. Faber, E. De Leo, F. Rabouw, A. Riedinger, F. Ott, B. le Feber, and P. Nordlander for helpful discussions. REFERENCES 1. Brongersma, M. L.; Halas, N. J.; Nordlander, P. Nat. Nanotechnol. 2015, 10, 25-34. 2. Linic, S.; Christopher, P; Ingram, D. B. Nat. Mater. 2011, 10, 911-921. 3. Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M. Nat. Mater. 2015, 14, 567-576.


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4. Kale, M. J.; Avanesian, T.; Christopher, P. ACS Catal. 2014, 4, 116-128. 5. Zhou, L.; Zhang, C.; McClain, M.; Manjavacas, A.; Krauter, C. M.; Tian, S.; Berg, F.; Everitt, H. O.; Carter, E. A.; Nordlander, P.; Halas, N. J. Nano Lett. 2016, 16, 1478-1484. 6. Christopher, P.; Xin, H; Linic, S. Nat. Chem. 2011, 3, 467-472. 7. Kale, M. J.; Avanesian, T.; Xin, H.; Yan, J.; Christopher, P. Nano Lett. 2014, 14, 5405-5412. 8. Boerigter, C.; Campana, R.; Morabito, M; Linic, S. Nat. Commun. 2016, 7, 10545. 9. Nordlander, P. In Quantum Plasmonics Workshop, London, UK, Aug 24-26, 2016. 10. Aslam, U.; Chavez, S.; Linic, S. Nat. Nanotechnol. 2017, 12, 1000-1005. 11. Robatjazi, H.; Zhao, H; Swearer, D. F.; Hogan, N. J.; Zhou, L.; Alabastri, A.; McClain, M. J.; Nordlander, P.; Halas, N. J. Nat. Commun. 2017, 8, 27. 12. Swearer, D. F.; Zhao, H; Zhao, L; Zhang, C.; Robatjazi, H.; Martinez, J. M. P.; Crauter, C. M.; Yazdi, S.; McClain, M. J.; Ringe, E.; Carter, E. A.; Nordlander, P.; Halas, N. J. Proc. Natl. Acad. Sci. 2016, 113, 8916-8920. 13. Li, K.; Hogan, N. J.; Kale, M. J.; Halas, N. J.; Nordlander, P.; Christopher, P. Nano Lett. 2017, 17, 3710-3717. 14. Narang, P.; Sundararaman, R; Atwater, H. A. Nanophotonics 2016, 5, 96-111. 15. Manjavacas, A.; Liu, J. G.; Kulkarni, V.; Nordlander, P. ACS Nano 2014, 8, 7630-7638. 16. Sundararaman, R.; Narang, P.; Jermyn, A. S.; Goddard III, W. A; Atwater, H. A. Nat. Commun. 2014, 5, 5788. 17. Bernardi, M.; Mustafa, J.; Neaton, J. B.; Louie, S. G. Nat. Commun. 2015, 6, 7044. 18. Brown, A. M.; Sundararaman, R.; Narang, P.; Goddard, W. A.; Atwater, H. A. ACS Nano 2016, 10, 957-966. 19. Brown, A. M.; Sundararaman, R.; Narang, P.; Goddard, W. A.; Atwater, H. A. Phys. Rev. B 2016, 94, 075120. 20. Frischkorn, C.; Wolf, M. Chem. Rev. 2006, 106, 4207-4233.


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21. Saalfrank, P. Chem. Rev. 2006, 106, 4116-4159. 22. Cavanagh, R. R.; King, D. S.; Stephenson, J. C; Heinz, T. F. J. Phys. Chem. 1993, 97, 786-798. 23. Bonn, M.; Funk, S.; Hess, C.; Denzler, D. N.; Stampfl, C.; Scheffler, M.; Wolf, M.; Ertl, G. Science 1999, 285, 1042-1045. 24. Wodtke, A. M.; Matsiev, D.; Auerbach, D. J. Prog. Surf. Sci. 2008, 83, 167-214. 25. Arnolds, H.; Bonn, M. Surf. Sci. Rep. 2010, 65, 45-66. 26. Klüner, T. Prog. Surf. Sci. 2010, 85, 279-345. 27. Petek, H. J. Chem. Phys. 2012, 137, 091704. 28. Gadzuk, J. W. Phys. Rev. B 1991, 44, 13466-13477. 29. Aizawa, H.; Tsuneyuki, S. Surf. Sci. 1997, 377, 610-614. 30. Yan, L.; Ding, Z.; Song, P.; Wang, F.; Meng, S. Appl. Phys. Lett. 2015, 107, 083102. 31. Yan, L.; Wang, F.; Meng, S. ACS Nano 2016, 10, 5452-5458. 32. Avanesian, T.; Christopher, P. J. Phys. Chem. C 2014, 118, 28017-28031. 33. Brandbyge, M.; Hedegård, P.; Heinz, T. F.; Misewich, J. A.; Newns, D. M. Phys. Rev. B 1995, 52, 6042-6056. 34. Newns, D. M. Phys. Rev. 1969, 178, 1123-1135. 35. We neglect the linewidths arising from electron-electron (e-e) interactions in our study. The process we focus on, i.e. the coupling of electrons to the vibrational modes of the adsorbate, occurs after a fraction of hot electrons have already been transferred to the adsorbate. The fraction of electrons that gets transferred indeed depends on the e-e interaction and is another important factor that dictates the overall efficiency of a photochemical reaction. Further, we also recognize that e-e interactions can alter the residence times of the hot electrons in the adsorbate. Nevertheless, the question we aim to answer here is: Given a fraction of transferred electrons, what is the coupling strength of the electrons to the vibrational modes of the adsorbate on different metals? Hence, for the purpose of this work, the e-e interaction has been neglected.


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36. Goodman, A. M.; Hogan, N. J.; Gottheim, S.; Li, C.; Clare, S. E.; Halas, N. J. ACS Nano 2017, 11, 171-179. 37. Morton, S. M.; Jensen, L. J. Am. Chem. Soc. 2009, 131, 4090-4098. 38. Zayak, A. T.; Hu, Y. S.; Choo, H.; Bokor, J.; Cabrini, S.; Schuck, P. J.; Neaton, J. B. Phys. Rev. Lett. 2011, 106, 083003. 39. Hammer, B.; Nørskov, J. K. Surf. Sci. 1995, 343, 211-220. 40. Hammer, B.; Nørskov, J. K. Adv. Catal. 2000, 45, 71-129. 41. Hammer, B.; Morikawa, Y.; Nørskov, J. K. Phys. Rev. Lett. 1996, 76, 2141-2144. 42. Gajdoš, M.; Eichler, A.; Hafner, J. J. Phys. Condens. Matter 2004, 16, 1141-1164. 43. Gonze, X.; Amadon, B.; Anglade, P.-M.; Beuken, J.-M.; Bottin, F.; Boulanger, P.; Bruneval, F.; Caliste, D.; Caracas, R.; Côté, M.; Deutsch, T.; Genovese, L.; Ghosez, Ph.; Giantomassi, M.; Goedecker, S.; Hamann, D. R.; Hermet, P.; Jollet, F.; Jomard, G.; Leroux, S.; Mancini, M.; Mazevet, S.; Oliveira, M. J. T.; Onida, G.; Pouillon, Y.; Rangel, T.; Rignanese, G.-M.; Sangalli, D.; Shaltaf, R.; Torrent, M.; Verstraete, M. J.; Zerah, G.; Zwanziger, J. W. Comput. Phys. Commun. 2009, 180, 2582-2615. 44. Gonze, X.; Beuken, J.-M.; Caracas, R.; Detraux, F.; Fuchs, M.; Rignanese, G.-M.; Sindic, L.; Verstraete, M.; Zerah, G.; Jollet, F.; Torrent, M.; Roy, A.; Mikami, M.; Ghosez, Ph.; Raty, J.-Y.; Allan, D. C. Comput. Mater. Sci. 2002, 25, 478-492. 45. Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. Rev. B 1999, 59, 7413-7421. 46. Fuchs, M.; Scheffler, M. Comput. Phys. Commun. 1999, 119, 67-98. 47. Gonze, X. Phys. Rev. B 1997, 55, 10337-10354. 48. Gonze, X.; Lee, C. Phys. Rev. B 1997, 55, 10355-10368. 49. Baroni, S.; de Gironcoli, S.; Dal Corso, A.; Giannozzi, P. Rev. Mod. Phys. 2001, 73, 515-562. 50. Savrasov, S. Y.; Savrasov, D. Y. Phys. Rev. B 1996, 54, 16487-16501.


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51. Allen, P. B.; Mitrović, B. Theory of Superconducting TC, In Solid State Physics; Seitz, F., Turnbull, D., Ehrenreich, H., Eds.; Academic Press, 1983; Vol. 37, p 1-92. 52. Olsen, T.; Gavnholt, J.; Schiøtz, J. Phys. Rev. B 2009, 79, 035403. 53. Hilty, F. W.; Kuhlman, A. K.; Pauly, F.; Zayak, A. T. J. Phys. Chem. C 2015, 119, 2311323118. 54. Anaddb help - Abinit. http://www.abinit.org/doc/helpfiles/for-v7.6/users/anaddb_help.html. (Accessed: December 7, 2016). 55. Including the vdW interactions can make CO adsorption on noble metals energetically favorable. For example, we estimated that CO adsorption on a two-layer Ag (111) surface is more favorable by 0.21 eV when vdW interaction is included using the DFT+D2 method at the PBE level of theory. 56. Giustino, F. Rev. Mod. Phys. 2016, 89, 015003. 57. Ziman, J. M. Electrons and Phonons: The Theory of Transport Phenomena in Solids; Clarendon Press: Oxford, 1960. 58. Bernardi, M.; Vigil-Fowler, D.; Ong, C. S.; Neaton, J. B.; Louie, S. G. Proc. Natl. Acad. Sci. 2015, 112, 5291-5296. 59. Harrison, W. A. Elementary Electronic Structure; World Scientific: New York, 1999. 60. Xin, H.; Linic, S. J. Chem. Phys. 2010, 132, 221101. 61. Harrison, W. A. Electronic Structure and the Properties of Solids: The Physics of the Chemical Bond; W. H. Freeman and Company: San Francisco, 1980. 62. Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Nat. Chem. 2009, 1, 37-46. 63. Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Phys. Rev. Lett. 1998, 81, 2819-2822.


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ACS Catalysis

FIGURES

a Chemically active metal (shell) Plasmonic metal (core)

Ab initio study of electron-pho b

Priyank V

CO

Optical Materials Eng Unit cell

Introduction

Computational Metho

shell metal

on-induced transformationsZr of Nb chemically atTc Ru Rh adsorbates on metallic nanoparticles (NPs) Hf Ta Re Os Ir ning significant attention since they provide Ca orm to utilize sunlight as a renewable source gy [1].

[3] Pt Au 5d metals • Norm-conserving pseudopotentials Mg Al sp metals Gradient Approximation • Generalized • Relaxation: 30 Ha cut-off, 4◊4◊1 k-p • electron-phonon calculation: Figure 1. (a) An illustration of the bimetallic core-shell nanoparticle design for efficient8◊8◊1 q photochemical reactions (not to scale). (b) Structural model showing k-pt a 0.25-monolayer (ML) 16◊16◊1 grid Pd

Ag 4d metals • ABINIT code

coverage of the adsorbate (CO)bond(breaking(( on a two-layer metal slab. The surface unit cell is highlighted and events(the shell are shown. We compute the phonon linewidths, the different metal surfaces constituting

electron-phonon coupling (with adsorbate) surface(( plasmons( decay e.h( excita3ons(

electron transport

at t energy level corresponding to the antibitals of the adsorbate, given by the Fe rule” [4]: “q‹ = 2ﬁÊq‹

ÿ

kjj Õ

q‹ 1: Illustration of energy propagation mechanisms in a Õ,kj where, gk+qj urface plasmon-mediated chemical reaction. ACS Paragon Plus Environment

q‹ 2 Õ,kj | ”(‘kj ≠ ‘F )”( |gk+qj

= Èk + qj Õ|” q‹ V ef f |kjÍ 19

Figure(2:(Phonon(modes,(LWD,(DOS( ACS Catalysis

Page 20 of 31

Conclusion

1 2 3 4 5 6 7 Mode 4 Pt slab Mode 1 & 2 8 9 Mode 3 10 11 12 13 (×2) 14 15 ad 16 17 0 100 200 300 400 500 600 2000 2050 2100 18 19 Phonon frequency (cm-1) 20 21 22 10 23 24 25 26 27 5 28 ad 29 30 31 32 0 33 34 35 36 mar 37 Aslam, Calvin Boerigter, and Matthew 38 39 4(6):567–576, 06 2015. 40 41 skov, 42 Frank Abild-Pedersen, Felix Studt, and 43 aard. 0 1 2 3 4 5 44 of 45 the National Academy of Sciences, Energy (eV) 3,46 2011. 47 l. 48 49 Figure 2. (a) The phonon and vibrational density of states shown for CO chemisorbed on the Pt hys. 50 Comm., 180:2582–2615, 2009. (111) surface. This plot indicates the different phonon and vibrational modes of the metal-adsorbate 51 v and Savrasov. complex. The eigenvectors for the four vibrational modes (named 1-4) relevant to the adsorbate are 52 D. Y. 53 illustrated.Dec (b, upper , 54:16487–16501, 1996.panel) The mode-3 vibrational linewidth values calculated at different energies 54 above the Fermi level (placed at 0 eV) for CO chemisorbed on Pt. The green arrow indicates the 55 56 peak linewidth value. (b, lower panel) The partial electronic density of states of the CO molecule. 57 58 59 60

DOS (arb. units)

d an ab initio approach to study a on coupling at the metal-adsorbate inxtracted microscopic information that enging to obtain from an experimental ur key conclusions are:

matrix element V between the ates and the metal d-states, is be a key parameter controlling the non coupling strength at the b Linewidth (meV)

rgy and the electron-phonon coupling be independently controlled through center and V , respectively.

DOS (arb. units)

References

Acknowledgements

he Marie Curie ETH Zurich Postdoctoral felncial support. Computations were performed ACSand Paragon Plus Environment h-Performance Computing Clusters Euler uthors acknowledge C. Faber, E. De Leo, F.

20

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Figure(3:((mode(3(and(4)(LWD,(DeformaAon(potenAal( 25

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ACS Catalysis

3 5 2

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21

ACS Catalysis

Figure(4:((mode(3(and(4)(Descriptor(vs.(metal( c 4.5

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Page 22 of 31

1.15

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22

1.9 3

1 2 3 4 5 6 metal 7 metal 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ACS Catalysis

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1.15 30

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Figure 5. Plot summarizing the key relationship between linewidths and the descriptors, 𝑓!" and 𝑓!" , for modes 3 and 4. The lines are drawn as a guide to the eye.


23

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-2

1 5 -2.2 2 -2 -1.5 -1 -1.8 -1.7 3 4 5 -0.8 6 7 Mode-3 80 8 -1 Mode-4 9 10 -1.2 11 60 12 -1.4 13 14 40 15 -1.6 16 17 -1.8 18 20 19 -2 20 21 22 -2.2 0 -1.8 -1.7 -1.6 -1.5 -2 -1.4 -1.3 -1.5 23 -1 24 Adsorption energy (eV) 25 26 27 -0.8 28 29 -1 30 31 -1.2 32 -3% 33 34 -1.4 35 36 -6% -1.6 37 unstrained 38 -1.8 39 40 41 -2 42 43 -2.2 3 44 -1.5 -1 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 45 d-band center (eV) 46 47 48 49 Figure 6. (a) Peak linewidth values for the mode-3 and mode-4 vibrations, and the corresponding 50 51 adsorption energies of a CO molecule on a Pt slab with different amounts of strain. The data points, 52 from left to right, are for strain values of -6, -3, 0, 3 and 6%. (b) The adsorption energies as a 53 function of the corresponding d-band centers of the surface Pt atom. In (a) and (b), the lines are 54 55 drawn as a guide to the eye. 56 57 58 59 60

Linewidth (meV)

a

Adsorption energy (eV)

b


24

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ACS Catalysis

TOC(ﬁgure( TOC FIGURE energy transfer via e-vb coupling e- e

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1 2 Chemically active metal 3 (shell) 4 5 Plasmonic metal 6 (core) 7 8 9 10 11 12 13 14 15 CO 16 17 18 19 20 Unit 21 cell 22 23 24 25 26 shell metal 27 28 29 Zr Nb Tc Ru Rh Pd Ag 4d metals 30 • 31 Hf Ta Re Os Ir Pt Au 5d metals 32 • ACS Paragon Plus Environment 33 Ca Mg Al sp metals 34 • 35

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ACS Catalysis15

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