The Hot-Electron Photodynamics of Silver Containing Nanosized

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The Hot-Electron Photodynamics of Silver Containing Nanosized Zeolite Films Revealed by Transient Absorption Spectroscopy Matteo Bryckaert, Anastasia Kharchenko, Oleg I Lebedev, Biao Dong, Isabelle De Waele, Guy Buntinx, Olivier Poizat, Svetlana Mintova, and Vincent De Waele J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10727 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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The Journal of Physical Chemistry

The Hot-Electron Photodynamics of SilverContaining Nanosized Zeolite Films Revealed by Transient Absorption Spectroscopy Matteo Bryckaert1, Anastasia Kharchenko1,2, Oleg Lebedev3, Biao Dong2†, Isabelle De Waele1, Guy Buntinx1, Olivier Poizat1, Svetlana Mintova2, Vincent De Waele1* 1

Univ. Lille, CNRS, UMR 8516 - LASIR - Laboratoire de Spectrochimie et Raman, F-59000

Lille, France 2

Laboratoire Catalyse et Spectrochimie, ENSICAEN-Université de Caen-CNRS, 14050 Caen,

France 3

Laboratoire CRISMAT UMR 6508, CNRS-ENSICAEN, 14050 Caen, France

E-mail: [email protected]

1 Environment ACS Paragon Plus


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Abstract Supported metal nanoparticles are attractive materials for hot-electron driven chemistry applications. Here very small Ag nanoparticles (NPs) have been stabilized in LTL nanozeolites that have been assembled in dense and transparent films (AgLTL). The photodynamics of the hot electrons in AgLTL was investigated by femtosecond transient absorption measurements. It is shown that in this material, the absorption of a single photon induces the formation of highly reactive hot-electron distribution characterized by an initial temperature higher than 4000 K. The excess of energy is dissipated by electron-phonon coupling into the metal lattice while the zeolite framework does not perturb significantly the electron dynamics. These results reveal the high potential of silver containing nanosized zeolites for plasmonic chemistry applications under solar illumination conditions.


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Introduction There is a widespread interest for researches in plasmonic chemistry

1-6

that aim to take

advantage of the hot electrons generated by the photoexcitation of the metal nanoparticles in view of solar-to-chemical energy conversion applications. The large-scale exploitation of this novel approach is conditioned by the discovery of efficient materials whose performance relies on the optical and electronic properties of the metal NPs, but also on the metal-support interactions.7 Microporous materials and more specifically zeolites are for long times recognized as appropriate supports for metal NPs and clusters8-11 with potential applications in (photo) catalysis and photonics.12-14 The controlled formation and the elucidation of the structure and the properties of these zeolite-supported nanoparticles and clusters is still highly debated and challenging.15-19 Plasmonic chemistry recently also entered the field of application of zeolites. For example, gold NPs supported on the external surface of zeolite crystals were considered for the plasmonic photocatalytic oxidation of aromatic alcohols.20-21 The benefits of using zeolites as support for metal NPs for plasmonic chemistry are related to the ability (i) to disperse and stabilize very small metal NPs, without the need of a capping agent, so that the surface of the NPs remains accessible to the reactants, and (ii) to concentrate the reactants by adsorption on the zeolite surface, thus favoring their interactions with the supported NPs. In addition, in view of optical applications, synthesis strategies are developed to prepare zeolites as thin transparent layer.22-25 During the last decades, there is also a growing interest in the preparation and application of nanosized zeolites26, i.e., zeolite crystallites with a size in the range 5-100 nm. Recent advances of the preparation methods permitted the preparation of organic template-free nanozeolites that are of great interest for host-guest chemistry applications.27-29 These nanosized zeolites possess characteristics similar to those of their bulky counterpart with, in addition, a much higher external



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surface to bulk volume ratio. By post-synthesis engineering they can be assembled in the form of thin-to-thick films or three-dimensional bodies with very good transport and optical properties3031

. They are also suitable hosts for the stabilization of metal nanoparticles.19,32-34 Although they

are well adapted to design metal-containing optical materials, so far, metal-containing nanosized zeolites have still never been considered as a potential material for hot-electron driven chemistry applications. The plasmonic chemistry is based on an ultrafast transfer of energy between the hot electrons generated by the photoexcitation of a metal NP and adsorbates at the surface of the metal NP1-6 (Scheme 1). This reactive transfer of energy occurs under specific conditions, notably when the excited electrons occupy energy levels above the Fermi level and in resonance with the empty orbitals of the adsorbate. In parallel with this mechanism, the hot electrons dissipate the excess of energy directly into the metal lattice by electron-phonon coupling (Scheme 1). These processes take place in the femto-picosecond timescale and the time-resolved spectroscopy is proved to be a method of choice to characterize the photodynamics of the hot electrons

35-38

and the

modification of the plasmonic response in the presence of coupling with species at the surface of the metal.35,39-41 In most of the applications, the metal NPs are supported and therefore the role of the metal-support interaction may become critical because the hot electrons can interact with the substrate of the metal NPs in the same manner as with the adsorbates, and then transfer part of their energy directly into the substrate (Scheme 1). In extreme cases, the electrons of the metal are photoinjected inside the support, and the plasmonic character is lost.42-44 Zeolites are in principle concerned with this potential issue. Indeed,, the vacant orbitals of the oxygen atoms constituting the zeolite framework are lying in the vicinity of the Fermi level of silver NPs 45 and because of ligand to metal charge transfers occurring between the oxygen atoms of the zeolite


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framework and metal atoms.46 Therefore, these empty orbitals could act as accepting energy levels for the more energetic hot electrons and the delocalization of the hot electrons toward the zeolite framework is potentially competing with the electron attachment process into an adsorbate (Scheme 1). For all of these reasons, the plasmonic response of metal nanoparticles dispersed in the pores or at the surface of zeolitic materials cannot be anticipated without the elucidation of the hot-electron dynamics. Toward this end, we are reporting the first experimental investigation of the hot-electron dynamics of metal-contained zeolite by carrying out UV-vis femtosecond transient absorption spectroscopy.

Scheme 1: Principle of the plasmonic chemistry using Ag-LTL zeolite. a) Silver nanoparticles stabilized inside the LTL channel (18 atoms, D ~ 0.8 nm) and at the surface of the LTL zeolite (108 atoms, V ~ 1 nm3), b) Possible energy flow from the hot electrons generated upon the photoexcitation of a metal NPs in contact with the zeolite framework and adsorbates.



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Ultrafast time-resolved spectroscopy is a method of choice to investigate the electronic properties of metal nanoparticles, It is however worth noting that up until now, pump-probe measurements in zeolites have still remained quite seldom and have been performed essentially either on micron-sized crystalline powders47-50 or on colloidal suspensions consisting of nanosized zeolites.33,51,52 The latter approach allows performing transient absorption measurements in transmission but suffers from the lack of control of the metal surrounding in the solution. As an alternative, we are exploring a novel route that consists of performing the transient absorption measurements on transparent zeolite films. This approach is well-adapted to perform transient absorption measurements with the possibility of controlling by adsorption the micro-environment of the metal nanoparticles in the zeolite host-guest systems. This experimental condition is indispensable to envision a long term exploration of the plasmonic chemistry reactivity in porous materials. More specifically, in the current study, we have investigated the plasmonic response of a novel material consisting in silver nanoparticles (Ag NPs) stabilized in LTL-type nanozeolite films. The LTL framework is constituted by a one-dimensional channel system with large pore opening, which permits the mobility of the oligomeric metal clusters that can aggregate inside the large cage constituting the monodimensional channel or on the external surface (Scheme 1). The LTL type of framework favors the formation of nm-size NPs whereas alternative zeolite frameworks built of sodalites cages (FAU, LTA, EMT, …) usually lead to the stabilization of oligomeric molecular-like clusters15,16,19 that are luminescent and so, in principle, less interesting for hot-electron driven chemistry. This manuscript is organized as follows. The first part will present the preparation and the characterization of the AgLTL films in terms of silver NPs content and dispersion, then regarding the spatial homogeneity and thickness of the films. In the second part, the investigation by


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femtosecond UV-Vis transient absorption of the plasmonic response of AgLTL film is reported, and the transient data are then discussed regarding the potential application of AgLTL for hotelectron driven chemistry.

Experimental part Synthesis and characterization of silver-containing LTL zeolite crystals Nanosized LTL zeolite crystals (20-40 nm) were prepared according to the procedure described in SI and reference 53, and stabilized in water at pH=7.5-8 with a concentration of solid particles of 7.5 wt %. Silver cations were introduced into the void of the zeolite framework by ionic exchange. The ion-exchange was carried out directly in the colloidal suspension without preliminary drying of the sample. The silver nanoparticles (Ag NPs) in the LTL zeolite suspensions (2 wt.%, 7 mL) were prepared under microwave irradiation (120 ° C, 10 min, 1000 W) in the presence of a triethylamine (>99 %, Sigma-Aldrich) as a reducing agent (C2H5N, 2 mL). The reduced suspension was again purified three times and dispersed in water. The details are given in SI. The crystalline structure of the nanosized zeolite crystals before and after ion exchange with silver were determined by recording the powder X-ray diffraction (XRD) patterns using a PANalytical X’Pert Pro diffractometer with Cu Ka monochromatized radiation (1:5418 Å). The samples were scanned in the range 4 to 80 °2θ, with a step size of 0.02 °2θ. The size and location of the Ag NPs were further studied by high angle annular dark field (HAADF) and annular bright field (ABF) scanning transmission electron microscope (STEM). These studies were performed using a JEOL ARM200F cold FEG double aberration corrected electron microscope operated at



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200 kV equipped with a large solid-angle centurio EDX detector and Quantum EELS spectrometer. Preparation and characterization of silver-containing LTL zeolite films CaF2 optical plates (1 mm thick) were used as film supports for all spectroscopic measurements; the plates were pre-cleaned with ethanol and acetone prior to film deposition. The zeolite films were prepared by spin-coating approach using Wafer Spinner SPIN150-NPP. The surface of the plates

was

modified

by

chemical

anchoring

of

seed

crystals

(parent

LTL)

on

Poly(diallyldimethylammonium chloride) adsorbed on the surface of substrate. Before deposition, the Ag-LTL suspension with a concentration of solid particles (2 wt.%) was mixed with PVP10 (5 wt.% in ethanol or water) in the ratio of 1:10., sonicated and filtered. This coating procedure was repeated until the desired film thickness was obtained. High temperature treatment of the films at 450°C for 3 h under argon atmosphere was applied to improve the mechanical properties of the films and to remove the binder. The as-prepared films were further calcined under the same conditions (450°C for 3 h) prior to time-resolved measurements. A more detailed description of the spin-coating procedure is given as Supporting Information. The UV−vis absorption spectra of the zeolite films containing Ag NPs were recorded on a Varian Cary 4000 Spectrophotometer working in transmission mode. The thickness and homogeneity of the films were determined by FTIR imaging using a Bruker Hyperion 3000 microscope coupled to a FTIR spectrometer, after an appropriate calibration of the IR band intensity (See SI).

Femtosecond transient absorption measurements of silver-containing LTL zeolite films


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Owing to the good transparency of the zeolite films, the femtosecond transient absorption measurements were performed in transmission using a conventional transient absorption set-up 54

, Briefly, the pump and probe pulses are generated using an amplified 1kHz Ti:Sa laser system

(Libra, Coherent) that delivers 90 fs (1.1 mJ) laser pulses at 800 nm. The pump excitation at 400 nm is generated by frequency-doubling the 800 nm fundamental in a BBO crystals (1 mm thick). As a probe beam, a white light super-continuum is generated by focusing a part of the fundamental beam (800 nm) in a 1 mm thick CaF2 rotating plate. The polarization of the pump was set parallel to the probe polarization. The pump intensity in the sample was in the range 0,1 – 0,25 mJ.cm-2. The sample was mounted inside a stainless steel optical vacuum cell that has been purposely designed and assembled for the present spectroscopic investigation of nanosized zeolite films. This cell is coupled to a vacuum line and a gas delivering systems. The reported experiments were carried out under high-vacuum conditions (P < 10-4 mbar). The complete set-up is described and illustrated in the supplementary information materials.

Results - Discussion 1 Formation of nm-sized Ag NPs in LTL nanocrystals stabilized as colloidal suspension. LTL nanocrystals (Si:Al = 3.6) were prepared according to the procedure described in ref [53] and then silver was introduced into the porous volume by ionic exchange. The chemical composition of both samples is reported in Table S1. According to the data, 13% of the initial amount of potassium was exchanged by the silver cation Ag+, leading to the final silver content in



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the range of 1 Ag+ per unit cell. So only 1 of the 3.6 exchangeable monovalent cations of LTL have been exchanged. Following the ion-exchange, silver NPs were synthesized in nanosized zeolite LTL by microwave heating in presence of a large excess of trimethylamine in the colloidal suspension. Under these reduction conditions, the formation of metal NPs exhibiting a plasmon resonance band is favored, as it was reported in the case of the preparation of silver NPs in EMT nanocrystals.19 After the microwave treatment, the silver-containing colloidal zeolite suspension, initially white, became green-yellow, which is the first indication that the silver cations were reduced and that silver nanoparticles were formed. This silver containing LTL suspension is stable for more than 24 months at room temperature. Figure 1 shows HAADF-STEM images of nanosized zeolite crystals taken after the reduction of Ag+ to Ag0 in the LTL suspension. The hexagonal pores structure of LTL is observed in Figure 1a, whereas the crystalline fringes associated with the LTL lattice appear clearly in Figure 1c and 1d, thus showing the preservation of the crystal structure during the post-synthesis (ion exchange and chemical reduction) treatment. This point is confirmed by the XRD pattern of the reduced AgLTL (see Figure S1). Homogeneously dispersed Ag NPs appearing as white spots are observed on the images of the LTL nanocrystals (Figure 1b-d). The size distribution of Ag NPs determined from the HAADFSTEM images (Figure 1) is narrow with a maximum at 1.3 nm and with ∼80 % of the NPs having a diameter between 1.1 and 2.65 nm (500 particles measured). The smallest NPs (d < 1.1 nm) can be accommodated inside the large 12-channel cages along the LTL channels (see scheme 1). The enlarged pictures shows small Ag NPs with a size slightly larger than the pore size (Fig 1a) or aligned along the LTL channels suggesting that they fit inside the LTL nanocrystals (Fig 1c,d). However, from the particle size distribution most of the NPs are larger than the 12-channel ellipsoidal cage (1.26 nm × 0.75 nm) constituting the LTL channels and most probably attached


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to the external surface of the nanocrystals. The relative size of the Ag NPs and zeolite pores is illustrated in Scheme 1.



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Figure 1: (top) HAADF STEM image of nanosized LTL zeolite crystals containing silver nanoparticles. (bottom) Histogram of the particles size distribution determined from a set of HAADF-STEM images (pink bars), and plot of the number of silver atoms per nanoparticle, as a function of the NP size (blue trace) calculated assuming an icosahedral NP shape.

2 Preparation and characterization of dense and transparent films AgLTL. After chemical reduction, the silver-containing zeolite nanocrystals were assembled in dense and homogeneous films on 1 mm thick CaF2 plates, hereafter named AgLTL. AgLTL films were


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calcined at 750 K under vacuum to remove the organic compounds. Figure 2 shows a picture of a calcined film together with its absorption spectrum recorded in transmission. The as-prepared films present a good transparency over the UV-Vis spectral range and a weak scattering of the visible light. The spectrum exhibits the expected spectral signature of nm-size silver NPs: (i) the onset of the interband transition around 320 nm, and (ii) a broad plasmon band with a maximum at 420-430 nm. After calcination, the color of the film doesn’t change upon hydration in air as it was reported for silver cluster stabilized in zeolite A.[46] It confirms that the observed optical transition in AgLTL results mostly from the plasmonic response of Ag NPs with a metallic character, and not from a charge transfer between the zeolite framework and the stabilized silver clusters.

Figure 2: UV-vis absorption spectrum of an AgLTL zeolite film deposited on CaF2 support. (Shown in inset)

Nanozeolites assembled as films exhibit also a good transparency in the mid-IR spectral range. We exploited here this particularity to characterize the spatial homogeneity of our zeolite films



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(AgLTL) by using the FTIR spectro-imaging technique. Figure 3 shows FTIR spectra recorded for areas of 100 µm2 that are located at different positions regularly spaced along the diameter of the AgLTL film shown in Figure 2 (inset). The strongest bands of the spectra correspond to the TO4 (T = Al, Si) stretching vibration of the zeolite framework (1100 and 900 cm-1). Their intensity is proportional to the density of the zeolite framework and can be used to quantify the thickness of the film after proper calibration of the IR intensity. The linear correlation between the area of the band and the thickness was determined using a LTL film deposited on a silicon wafer and its corresponding SEM images (see SI for details). So, from the FTIR spectra, we calculated the thickness of the film AgLTL at different positions along the 9 mm line at the center of the sample (Figure 3 inset). The long-range profile is quite homogeneous and, apart localized defaults, the thickness of the film is found to be quasi-constant over several millimeters-long distance. This homogeneity is sufficient to perform transient absorption measurement. The average thickness of the film that is calculated from the IR band intensity is approximately 600 nm. From the value of the film thickness and of the absorbance of the film at 420 nm (Figure 2), and using the extinction coefficient ε(400 nm) = 10 000 Lmole-1cm-1 for the silver atom55, the average concentration of reduced silver atoms (Ag0) in the film is calculated equal to 2×10-1 atoms / nm3 or, C ≈ 0.4 Ag0/u.c. This value is less than the amount of Ag+ introduced by ion-exchange, so the reduction of the silver cations is partial. Probably, at a certain stage, the growth of NPs inside the channel block the diffusion of the quite large trimethylamine molecules. However, because only a relatively low amount of silver is reduced in the zeolite structures, it results a high dispersion of the NPs, as revealed by the HRTEM measurements, and each zeolite nanocrystal supports only a few nm-sized silver NPs and the crystalline structure of the zeolite is preserved, (Figure 1)


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Figure 3: FTIR spectra of a non-calcined AgLTL film recorded for 30 areas of 100 µm2 separated by 300 µm and aligned along a 9 mm line at the center of the sample. The blue-filled area corresponds to the TO4 (T = Si, Al) stretching vibration of the zeolite framework. Inset: thickness at the different positions of the AgLTL film deduced from the area of the filled vibrational bands and from the proportionality coefficient a =30.8 nm/(absorbance unit×cm-1). The red trace corresponds to the average value of the thickness. 4 Ultrafast plasmonic response of AgLTL films

Figure 4: Transient absorption spectra recorded for AgLTL sample upon excitation at 400 nm with a pump intensity of 0.1 mJ.cm-2.

Transient spectra: The transient absorption spectra recorded in AgLTL after excitation at 400 nm (pulse intensity I=0.1mJ.cm-2) for different pump-probe delays are depicted in Figure 4. The



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transient signal is formed within the instrumental response of the set-up (300 fs rise time). The spectrum recorded at 500 fs exhibits an absorption contribution below 340 nm that is associated with the perturbation of the interband transition, and a bipolar-like signal composed by a bleach contribution (395 nm) and an absorption (470 nm) in the red part of the plasmon band that is the typical signature of the formation of hot electrons in silver NPs.56,57.

During the first 5

picoseconds, the transient signal decays without significant modification of its shape. After 5 ps, the interband contribution has vanished and the contribution of the absorption in the red part is relatively more important and slightly shifted to the blue. After 10 ps the bleach contribution is close to zero, whereas a small absorption centered at 460 nm is still observed and shifts to the red while disappearing within hundreds of picoseconds. The first part of the process (t < 10 ps) corresponds to the cooling of the hot electrons into the metal lattice via the electron-phonon coupling. The slower kinetics contribution observed in the red part of the spectrum is assigned to the hot metal lattice that is dissipating its excess of heat into the zeolite framework. The evolution of the transient spectra following the excitation of AgLTL is qualitatively in good agreement with the transient response reported for nm-sized silver nanoparticles.56 Analysis using the TTM model: To get a better insight into the microscopic processes governing this plasmonic response, the temporal evolution of the transient spectra was analyzed. However, because the transient spectra reflect the global changes of the dielectric constant of the sample, the correlation between the measured signal and a microscopic description of the hot-electron dynamics is not straightforward. To tackle away this difficulty, previous investigations35-38,57 have pointed out that for a probe wavelength in the vicinity of the maximum of the bipolar contribution, the real part of the dielectric constant dominates the transmission changes57, which can then be considered proportional to the change of the internal energy of the electrons ∆Ue56,57.


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So, the variation of the internal energy of the electrons (∆Ue = Ce∆Te, where Ce is the heat capacity of the electrons) follows the time-evolution of the transmission changes. In this approximation, the measured transient signal can be related to the temperature change (∆Te) of the hot-electron distribution which follows the Two-Temperature Model (TTM). In the TTM, The kinetics of the heat dissipation between the electrons (temperature Te) and the phonons of the lattice of the metal NPs (temperature Tl) is described by the following equation:

Ce

dT = −G (Te − Tl ) dx

(1)

Cl

dT = G (Te − Tl ) dx

(2)

Equation 1 expresses the cooling of the hot electrons using the electron-phonon coupling constant G = 3.5 x 1016 Wcm-3K-1s-1 for silver. Equation 2 describes the heating of the silver metal lattice whose heat capacity is Cl = 2.2 x 106 Jcm-3K-1. The heat capacity of the electrons depends on the temperature in the range 1000-7000 K according to the formula: Ce = γTe, where γ = 65 Jm-3K-2. Previously, this model was applied successfully to investigate the electron dynamics in silver nanoparticles.35-38,57



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Figure 5: Fit by the TTM model of the kinetics recorded at 380 nm upon excitation of AgLTL at 400 nm with the laser intensity I = 0.1 mJcm-2. The fit in red has been obtained for an initial electronic temperature Te(t=0) = 4145 K. Inset: experimental data and the fit plotted in a semi-log scale.

An attractive aspect of the TTM model is that, for a given coupling constant G, specific to the metal, the amplitude and the temporal evolution of the decay are fully determined by a single parameter that is the initial temperature of the hot electrons (Te(t=0)) induced by the pump excitation. The TTM model was applied to fit the kinetics of AgLTL for the probe wavelength 380 nm (see SI for the detail fitting procedure). In Figure 5, the fit of the decay (λprobe = 380 nm) reconstructed from the data presented in Figure 4 is plotted in a linear and semi-log scale (inset) for comparison. The fit was performed for pump-probe delays longer than 400 fs, in which case the thermalization of the electrons by electron-electron scattering and the subsequent establishment of the Fermi distribution do not contribute anymore to the transient signal.57 The fit reproduces well the experimental data and notably the specific temporal evolution of the decay associated with the TTM model that is characterized by a progressive acceleration of the 18 Environment ACS Paragon Plus

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temperature decay in the beginning of the process, followed by an asymptotic convergence to the final equilibrium temperature (Figure 5). The fitted value for the initial hot-electron temperature is Te(t=0) = 4145 K. Solving the TTM equation also gives the theoretical final equilibrium temperature of the metal at the end of the electron-phonon relaxation. The calculated value is Te(>10ps) = Tl(>10 ps) = 550 K, which represents an ultrafast heating of the metal NPs of 250 K above the ambient temperature in less than 10 ps. The temporal evolution of the temperature of the hot electrons and of the metal lattice of AgLTL modeled by the TTM model is reproduced in Figure 6,

Figure 6: Temporal evolution of the electronic (T(electrons)) and lattice temperature (T(phonons)) following the absorption of a single-photon at 400 nm by AgLTL films. The curves are the solution of the TTM model for Te(t=0) = 4145 K

To test the limit of our approximation, the same fitting procedure was also applied for several probe wavelengths in the spectral range 370-390 nm. (Figure S6). The results are qualitatively similar, the value of Te(t=0) of the TTM varies only within the range 3700-4100 K for the 19 Environment ACS Paragon Plus


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selected wavelengths. The TTM model gives a good reproduction of the kinetics in the considered spectral region and thus, it is concluded that the picosecond dynamics of the hot electrons generated in AgLTL is dominated by the electron-phonon relaxation mechanism.

The value of the initial hot-electron temperature obtained from the above fit (4145 K) is compared with the theoretical value calculated from the energy absorbed by the Ag NPs in the film. If we assume that the totality of the energy of the photons absorbed by a NP is converted into internal energy of the hot electrons, then the initial temperature of the hot-electron distribution Te(t=0) is given by Equation 3 that depends on the energy of the photon (Ep) and on the number of photons (n) adsorbed within the volume (V) of the nanoparticle.[57]

√

T e= T 2l + 2×

nEp (3) Vγ

(3)

Given our pump intensity (Ipump = 0.1 mJcm-2) and using the value σ = 1.5×10-17 cm2 of the absorption cross-section of a silver atom in a NP[55], the numerical application predicts that, in our experiments, less than 1 photon per 300 silver atoms is absorbed. Equation 3 depends on the density of energy absorbed in a NP. So, regarding the initial electronic temperature Te(t=0) obtained by the absorption of one photon, we must distinguish the particles based on their number of silver atoms. The largest ones (above 300 Ag/NPs) absorb 1 photon / 300 atoms constituting the nanoparticle and, in this case, the highest value of the initial electron temperature amounts to 2000 K (Equation 3) that is well below the value deduced from the TTM fit (4145 K). It is concluded that the Ag NPs with a diameter larger than 2,2-2,5 nm (Figure 1) are not


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contributing significantly to the transient signal. Otherwise, a faster decay should be detected. The smallest NPs, typically with a diameter less than 2,2 nm are constituted by less than 300 silver atoms, so the initial electronic Te(t=0) is strongly depending on their size. In Table 1, we have calculated the initial temperature (Te(t=0)) reached by the absorption of a single photon for the different particle diameter. The predicted temperature range for the particles with a diameter in the range 1,1-1,6 nm is in agreement with that predicted by the TTM fit, suggesting that these NPs contribute the most to the transient absorption (TA) signal. The fact that a particular class of NPs contributes mostly to the TA signal can be understood by noticing that the amplitude of the absorption change due to the NPs of a given size must be proportional to (Npart×Natom×Te2) where Npart is the number of NPs, Natom the number of atoms in the NPs, and Te the electronic temperature. So the quadratic dependence with the temperature favors the signal from the smallest NPs. Therefore, for this particular AgLTL sample, characterized by the Ag NPs distribution given in Figure 1, the NPs with a diameter 1,1-1,6 nm which constitute more than 25 % of the NPs and which have a high initial electronic temperature, contribute the most to the transient signal. So, we assign the transient data to the NPs with a diameter roughly in the range 1,1-1,6 nm and conclude that in AgLTL the absorption of a single photon by these NPs induces the generation of highly excited hot electrons with a lifetime of several picoseconds, that decay by electron-phonon coupling without a strong perturbation by the zeolite framework From the values in Table 1, it appears clearly that theoretically most of the NPs in AgLTL would be compatible with the generation of highly excited hot electrons by the absorption of a single photon. However, since we didn’t detect in our measurements any kinetic component with a time constant corresponding to these other electronic temperature, we cannot extend our conclusion to the complete set of supported silver NPs in AgLTL, notably to the smallest ones that are located



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inside the channel of the LTL nanozeolite. So, additional measurements with different NPs size distribution and different excitation conditions are necessary to elucidate the effect of the metalframework interaction on the electron dynamics.

Figure 7: Calculated Fermi distributions showing the energy level occupied by a hot electron with a temperature Te = 300 K, Te = 2000 K and Te = 4000 K.

Consequences on the potential plasmonic activity of AgLTL: We have demonstrated that the small NPs in AgLTL (about 1,1-1,6 nm) permit the formation of highly excited electrons with a relatively long picosecond lifetime. In the rest of this manuscript we will consider the denomination “highly excited hot electrons” as referring to a hot-electron distribution characterized with a temperature Te for which the energy levels located at least 0.5 eV above the Fermi level are statistically populated. In Figure 7, we have reported the calculated Fermi distribution corresponding to a hot electron in silver NPs with a temperature of 300 K, 2000 K, and 4000 K. It is clear on this graph that for the value of Te = 4145 K found by fitting the transient data, the energy levels located up to 1.5 eV above the Fermi level are accessible upon 22 Environment ACS Paragon Plus

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photoexcitation of AgLTL. Let’s discuss briefly the consequences of this highly excited hot electron on the plasmonic chemistry reactivity. It is well accepted from the literature that the mechanism governing the plasmonic chemistry is based on the electron attachment (EA) process into the orbitals of an adsorbate at the surface of the metal NPs1-3,58-60. EA consists in the tunneling through the interfacial barrier between the conduction band and the energy level of the adsorbate and the probability of EA is directly related to the hot-electron temperature and to the lifetime of the hot-electron distribution.42,44,60 For large nanoparticles (d > 10 nm), the athermal electrons instantaneously formed upon the absorption of a single photon disappear by electronelectron scattering in typically 100 fs56 to form a hot-electron distribution of low to moderate temperature (Te ~ 100 K). This implies that only these short-lived athermal electrons can populate energy levels located several electron-volts above the Fermi level of the metal and have a chance to interact with the adsorbate during a very short sub-picoecond time duration. In comparison, in the case of AgLTL, we have experimentally demonstrated that hot electrons with a temperature higher than 2000 K survive for more than 5 ps (Scheme 1), and so the probability of EA is notably increased. For this reason, we believe that AgLTL is a good candidate for plasmonic chemistry applications under solar light illumination. Further studies are in progress to monitor the transfer of energy from the photoexcited NPs to adsorbed probe molecules and to elucidate the impact of the NPs size distribution on the reactivity.



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Table 1: Initial electronic temperature and probability of photoexcitation calculated for the Ag NPs distribution in AgLTL. Diametera) Atoms/NP

Te(t=0)b)

(nm)

(Kelvin)

0.5 -1.1

13-55

>7000c)-5900

1.1-1.6

55-147

5900-3400

1.6-2.2

147-309

3400-2100

2,2-2.75

309-561

2100-1500

2,75-3.3

561-923

1500-1200

3.3-3.8

923-1415

1200-1000

3,8-5

1415-2869 1000-700

a)

We assumed icosahedral-shaped NPs and calculated the diameters corresponding to the magic number (13, 55, 147…) using the silversilver interatomic distance 0.29 nm. b) Te is calculated assuming that the volume of the cluster is equal to 0.69×Vs, where Vs is the volume of the circumscribed sphere of the icosahedra. c) above 7000 K, the relationship Ce = γTe is not valid anymore.

Conclusions To summarize, in this work we have considered for the first time the potential of using silver containing nanosized zeolites for hot-electron driven chemistry applications by performing UVvis transient absorption measurements in order to characterize the ultrafast dynamics of the hot


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electrons that are generated upon photoexcitation of silver NPs stabilized in nanozeolite LTL For these experiments, the nanozeolites have been successfully prepared as thin and spatially homogeneous transparent films. Silver NPs in AgLTL were prepared by chemical reduction under microwave heating in presence of trimethylamine. We have shown that it results from this protocol a relatively narrow distribution of small particle size, with most of NPs with a diameter less than 2.5 nm, and a high spatial dispersion of the Ag NPs inside and on the outer surface of the zeolite nanocrystals. For AgLTL sample, reliable transient spectra have been recorded and from both the shape of the spectra and the analysis of the kinetics by suing the TTM model, the transient signal in AgLTL is assigned to highly excited electrons (Te(t=0) = 4145 K) that are generated upon the absorption of a single photon in Ag NPs which have a diameter in the range 1-1,1.6 nm. These hot electrons are not significantly perturbed by possible interactions with the zeolite framework and dissipate the excess of photoinjected energy mostly by electron-phonon coupling within 10 ps. The possibility of generating a long-lived highly excited hot-electron distribution, under low light illumination conditions, constitutes an objective criteria to consider AgLTL or related metal-containing (nano)zeolites more deeply in view of plasmonic chemistry applications. Beyond the scope of the work, the experimental approach described in this manuscript to perform transient absorption measurements in zeolite films under controlled atmosphere is also promising to be applied to host-guest photochemistry studies in zeolites.



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Supporting Information The supporting information is available free of charge…. -

AgLTL sample preparation, XRD data and chemical composition, The FTIR spectroimaging methods and data used for the determination of the films thickness and homogeneity, Description of the transient absorption set-up designed to measure in the zeolite films and the characterization of the spatial homogeneity by transient absorption, Details of the fit procedure and additional fits.

Acknowledgments: The financial support of this work was provided from TAR-G-ED ANR project. Chevreul institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Nord – Pas de Calais and FEDER are also acknowledged for supporting and funding this work. FTIR data have been measured on the platform for vibrational spectroscopy of the University of Lille. The authors thank J. Dubois for assistance with the transient absorption setup and J. P. Verwaerde for technical support in the development of the optical cell. Author information:

†

present address: Jilin University, College of Electronic Science and Engineering, State Key

Laboratory on Integrated Optoelectronics, 2699 Qianjin Street Changchun changchun, CN 130012 Notes: The authors declare no competing financial interest.


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The Hot-Electron Photodynamics of Silver Containing Nanosized

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