Letter pubs.acs.org/NanoLett
Enhanced Photochemistry of Ethyl Chloride on Ag Nanoparticles Gil Toker, Alexander Bespaly, Liat Zilberberg, and Micha Asscher* Institute of Chemistry, The Hebrew University of Jerusalem, Edmund J. Safra CampusGivat Ram, Jerusalem 91904, Israel S Supporting Information *
ABSTRACT: Enhanced photodecomposition of ethyl chloride (EC) adsorbed on SiO2/Si (100) supported silver nanoparticles (Ag NPs) under ultrahigh vacuum (UHV) conditions has been studied in order to assess the potential contribution of plasmonic effects. The cross section for photodecomposition of EC and overall photoyield were found to increase with increasing photon energy regardless of the plasmon resonant wavelength and with Ag coverage without any noticeable particle size effect. The influence of EC−Ag NPs separation distance on the rate of EC decomposition was studied in order to examine potential local electric field influence on the photodissociation process. Long (∼5 nm) photoactivity decay distance has been observed which excludes local surface plasmon dominance in the photodecomposition event. These findings suggest that the alignment of excited electron energy and adsorbate affinity levels is central for efficient photochemical reactions, whereas short-range electric field enhancement by plasmon excitation on top and at the immediate vicinity of silver nanoparticles does not have any measurable effect. KEYWORDS: Surface photochemistry, metal nanoparticles, surface plasmon resonance, photocatalysis
A
chemistry on extended metal surfaces via the Menzel− Gomer−Redhead (MGR) model.14−16 Spherical colloid Ag NPs can transform into larger particles with different morphologies (e.g., prisms) upon visible light illumination.9 This phenomenon was explained by plasmoninduced photooxidation of the citrate ligand.10 A recent study based on a photoelectrochemical setup has demonstrated that the photooxidation rate of citrate attached to different forms of silver NPs correlate only with the excitation photon energy regardless of their plasmon resonance frequency.11 Excitation at higher photon energy is thought to generate holes which better energetically overlap the ligand’s HOMO electronic state leading to oxidation. Ever since the discovery of surface enhanced raman spectroscopy (SERS)17 more than three decades ago and the theoretical prediction in those days that local field enhancement due to plasmon excitation may result in accelerated chemical transformation,18 the quest to find, characterize, and understand plasmonic photochemistry has expanded to cover many diverse systems. To date, plasmon-induced photochemical experiments are usually performed in flow reactors6−8 or in solution.9−11 Sample preparation and characterization of such reactions under ultra-high vacuum (UHV) conditions eliminate the uncertainty regarding the effect of ligands, solvents, or other interacting chemicals. Under a clean and well-defined UHV environment, with in situ surface inspection before and following irradiation, detection of yields of reactivity
ttempts to enhance utilization of solar energy for chemical transformation have been at the focus of research for decades.1 This is motivated by the need for alternative sources of energy as well as ecological reasons. Among the various ways to enhance photoinduced chemical activity, coinage (Ag and Au) metal nanoparticles (NPs)-induced photochemistry has been studied2−11 due to the unique optical properties of these materials.12 Significantly enhanced local electric fields at particle edges and increased absorbance and scattering upon irradiation at the resonant surface plasmon frequency has been postulated early on and formulated by the Mie theory.13 The plasmon frequencies of Ag NPs and Au NPs can be tuned across the visible range, depending on their size and dielectric surroundings, making them attractive candidates for enhanced solar energy transformation into desired chemical activity. Plasmon-induced photochemistry can be divided into indirect and direct pathways. A plasmon-induced photochemical reaction is considered indirect if the plasmon excitation enhances the photochemical rate of an adjacent photocatalyst (see review in ref 3). In direct plasmon-induced photoreactions, discussed below, the metal nanoparticle is the photocatalyst.4−11 Reactions such as NO desorption from Ag NPs,5 ethylene epoxidation on Ag nanocubes,6 and hydrogen dissociation on TiO27- and SiO28-supported Au NPs show enhanced photoactivity when plasmon excitation is at peak resonant absorbance. The reaction mechanism suggested in all cases was transient negative ion−hot electron (excited by the plasmon energy) attachment to the adsorbate’s affinity level as the precursor for desorption or dissociation. Such mechanisms were introduced five decades ago for adsorbates photo© 2015 American Chemical Society
Received: September 26, 2014 Revised: December 20, 2014 Published: January 2, 2015 936
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Figure 1. (A) HR-SEM images of as prepared Ag BLAG sample (left) and a similar sample that was used in photochemical experiments following 20 Ne+ sputter−anneal cycles (right) (see text). (B) Particle size histograms exhibit a bimodal size distribution for each sample. Note the different scales in the y axis of the two histograms. (C) Surface plasmon absorbance spectra of Ag NPs deposited on sapphire samples with (red) and without (black) a 3 ML titania capping layer.28 The surface plasmon resonance wavelength of the titania covered particles is significantly red-shifted due to the change in the dielectric environment of the silver particles. Particles size was calculated using an image processing software (WSXM). The resulting particles’ perimeters were divided by pi. For spherical particles the result is the diameter and for the larger particles that tend to be non spherical, the result reflects a mean size parameter.
and selectivity are possible.5,19 Moreover, it has enabled us to study the effect of molecule−Ag NPs separation distance at subnanometer resolution for the first time, to the best of our knowledge. Alkyl halide surface photochemistry has been studied extensively.20−23 The thermal and photochemical interaction of EC with extended Ag (111) single crystal surface was reported by Zhou and White.20 One ML EC adsorption decreased the silver work function by 1 eV, from 4.7 to 3.7 eV. No thermal decomposition was observed upon sample heating. Photoinduced C−Cl bond cleavage was reported with a threshold at 350−380 nm (3.5−3.3 eV). The cross section for photofragmentation was found to increase with photon energy up to a value of 2.6 × 10−20 cm2 at 254 nm irradiation (4.9 eV), the highest photon energy used in that study. Butane (C4H10) and AgCl were the only reaction products found in postirradiation temperature programmed desorption (TPD) analysis, although XPS data suggested some ethyl radical (C2H5) photodesorption during irradiation. Substrate-mediated photochemistry through a transient hot electron attachment to EC was suggested20 and later on supported by ab initio calculations.21 Methyl chloride (MC) photodissociation was used by Gilton et al.22 for the evaluation of electron scattering properties of different spacer layers: MC was adsorbed on top of a spacer layer of varying thickness on Ni (111) and subsequently the sample was irradiated by 248 nm photons. Using water as the spacer material, electron scattering reduced the photofragment signal significantly within 2−3 monolayers (ML) after an initial increase by an order of magnitude at the first monolayer due to reduced quenching. In contrast, when Xe was used as a spacer layer, an initial decrease was observed (probably because of work function change) after which the photofragment signal
remained constant up to 65 ML Xe. Similar electron scattering results were presented by Jo et al.23 employing an electron analyzer instead of using probe molecules. In this Letter, we show that ethyl chloride (EC) photodissociation on a wide range of Ag NPs sizes (2 to 150 nm) does not exhibit resonant behavior: the reaction cross section and photoyield increase with increasing photon energy with no apparent enhancement for irradiation at wavelengths close to the surface plasmon frequency. A maximum in photoresponse observed at exactly 1 ML Xe as a spacer layer is reported and discussed. The studies described here were performed under UHV conditions with SiO2/Si(100) as the substrate for Ag NPs deposition and EC adsorption. Several discrete light sources at 532−248 nm were employed to study the photochemical response of the EC molecules on top of the Ag NPs and at precise distance away (0−15 nm) from the metal particles using Xe as a spacer layer. More details, including sample preparation and characterization are given in the Supporting Information section. All photochemistry studies reported here were performed on Ag nanoparticles grown via the buffer layer assisted growth (BLAG24−27) method on top of the SiO2/Si(100) substrates (see Figure 1) except for the continuous thin Ag films prepared by direct deposition (see details below). The surface plasmon resonance of the clean silver particles shown in Figure 1C (black curve, maximum at 420 nm) was measured on top of a sapphire substrate. We compared this spectrum to a sample where the silver particles were grown the same way on a quartz (SiO2) substrate. The surface plasmon resonance was practically identical (at 422 nm, not shown). TPD experiments of the parent EC molecule (mass 64) from clean SiO2/Si (100) reveal a single desorption peak at 85 K, 937
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shown similar TPD profile to that of allyl radical, suggesting that both arise from butane fragmentation within the QMS ionizer29 (Figure S2 in the Supporting Information section). The desorption temperature maxima shown in Figure 2B decrease with increasing density of the photofragments, consistent with second order kinetics (recombinative desorption). Assuming a pre-exponential factor of 1013 Hz the binding energy of butane (detected by its allyl radical fragment at mass 41) was estimated as 7.3 ± 0.5 kcal/mol using TPD line shape analysis. The initial formation rate of allyl radical (mass 41) was used to derive the cross section for EC photodissociation (Figure S3 in the Supporting Information section) due to poisoning and saturation at higher photon doses (see discussion on the effect of poisoning in Figures S4−S5 in the Supporting Information section). The photodissociation cross sections found (7 ± 3 × 10−21, 2 ± 1 × 10−20, and 6 ± 3 × 10−20 cm2 for 355, 266, and 248 nm irradiation, respectively) are larger by a factor of 2−4 than those reported by Zhou et al.20 for EC photodissociation on a single crystal Ag (111) surface (changing from 2 × 10−21 cm2 at 365 nm excitation to 2.6 × 10−20 cm2 for 254 nm). A possible explanation for this enhancement is the higher work function of single crystal Ag (111) compared to that of our polycrystalline Ag-4.72 and 4.26 eV, respectively.30 The lower work function allows hot electrons better energetic overlap with the adsorbed EC affinity level, see below. Photoinduced desorption of NO dimers from Ag NPs, studied by Mulugeta and co-workers, was reported to increase by a factor of 3 compared with single crystal Ag (111) for nonresonant excitation, attributed to the confined nature of electron excitation in the small particles (up to 10 nm in diameter).5 This mechanism is not expected to play a significant role in our samples, where the average particle size was larger by an order of magnitude. TPD spectra at mass 41 and 30 following irradiation of the standard Ag NPs/SiO2/Si(100) sample at different wavelengths are shown in Figure 3. Photons at 532 nm do not induce any photodissociation of the EC molecules (red curve), whereas higher energy photons lead to an increase in the photoyield of both photoproducts. For allyl radical (mass 41) at photon energies above 3.5 eV, a low temperature desorption peak at 87 K emerges similar to the one assigned α by Zhou et al.20 Their work attributed the low temperature peak (which appears at excitation wavelengths λ < 285 nm) to butane formed during irradiation, whereas the high temperature peak centered at 110 K, to recombinative desorption of butane formed during TPD (see Figure 2). The ratio between the integrated signals of C3H5 and C2H6 decreases from 10 to 1.7 when the photon energy increases from 3.5 to 5 eV. This observation suggests that the higher energy photons may lead to C−H bond cleavage necessary for hydrogenation of ethyl radical to ethane, competing with surface butane formation. Interestingly, on Ag (111), butane was the only carbon-containing product for all wavelengths tested (250−400 nm).20 We also found that the photoproduct selectivity was sensitive to and affected by the substrate reactivation (sputter−annealing) procedure (see Figure S6 in the Supporting Information section). The resonant wavelength for plasmon excitation of large Ag spheres (120 ± 30 nm diameter) was calculated to be 420 ± 10 nm based on the quasistatic approximation.12 Spectroscopic measurements of large Ag NPs deposited on a sapphire substrate (Figure 1C) are in good agreement with the theoretical predictions and show small size dependence. In
whereas similar experiments from Ag BLAG particles-covered substrate exhibit an additional (small) peak at 100 K. We assign this extra peak to EC molecules residing on and desorbing from silver particles or their interface with the substrate (Supporting Information Figure S1). Using these TPD results and the coverage independent sticking coefficient found, we estimate that an exposure of 1 L EC (corrected for ion gauge sensitivity factor) leads to 1 ML EC coverage on the substrate. In order to study the photodissociation of EC, postirradiation TPD experiments were performed. Figure 2A compares TPD
Figure 2. (A) TPD at the indicated masses following exposure of 0.4 ML EC on top of Ag NPs (50 nm average size)/SiO2/Si (100) sample after (red) 355 nm irradiation (4 mJ/cm2 per pulse at 10 Hz, 2.1 × 1019 photons/cm2) and without irradiation (black). (B) Postirradiation TPD at mass 41 following exposure of 0.4 ML EC on Ag NPs, same as in (A), at the indicated photon dose at 355 nm.
spectra of 0.4 ML EC from a standard Ag NPs sample (Figure 1) following UV irradiation (red) to the desorption of EC without irradiation (black). The laser irradiation consisted of 3000 pulses at 4 mJ/cm2 per pulse, overall 2.1 × 1019 photons/ cm2 using the third harmonic (355 nm) of a Nd-YAG laser (Spectra Physics INDI 20, pulse width 6−7 ns, lasing rate 10 Hz). Postirradiation TPD spectra indicate that at this wavelength (355 nm), the parent molecules’ population is depleted (note the order of magnitude larger signal at mass 64, parent molecule, compared with masses 30 and 41, fragments). A simultaneous increase in the yield of photoproducts ethane (mass 30, C2H6) and allyl radical (mass 41, C3H5) takes place. Ethane exhibits a sharp desorption peak at 72 K, whereas allyl radical desorbs at 100−140 K with no signature of the low temperature peak. Butane molecule and propyl radical have 938
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resonant plasmon wavelength at laser pulse duration of 6−7 ns does not affect the photodissociation of EC. Xe layers were utilized as a spacer between the Ag NPs and the EC molecules in order to assess the effect of separation distance on the efficiency of EC photochemistry. The standard sample (Figure 1, particles size ≈ 50 ± 10 nm) was sputtered and annealed to 300 K prior to each EC/Xe adsorption and irradiation to avoid accumulation of photochemical fragments (e.g., Cl) in previous excitation cycles (see Supporting Information). Figure 4A presents the normalized integrated postirradiation TPD signal of C3H5 (allyl radical, mass 41) versus the Xe spacer layer thickness in monolayers following irradiation at the three photochemically active wavelengths: 355, 266, and 248 nm. The results of the same experiments for C2H6 (ethane, mass 30) are shown in Figure 4B. The normalized photoyields at mass 41 monotonically decrease (Figure 4A) with increasing molecule−nanoparticle separation distance in a similar way for all wavelengths studied here (except for the α desorption peak at 248 nm irradiation that will be discussed below). For thick Xe layers (50 ML), the signal is smaller by an order of magnitude compared to that obtained when the molecule is adsorbed on the bare Ag NPs surface. The decay distance of the photoyield is 5 ± 2 nm. Ethane photoyield initially increases with increasing spacer layer thickness for 266 and 248 nm irradiation but not when excited by 355 nm photons (Figure 4B). Similar behavior was observed in the case of the allyl radical in the α desorption peak (near 87 K, Figure 3A) for 248 nm excitation, saturating at 1 ML. Thicker spacer layers have led to a monotonic decrease of the photoactivity. The results shown in Figure 4 need to be addressed in two levels: The first is why the EC−Ag separation distance dependence shows monotonic behavior in some cases (355 nm excitation and β allyl radical formation for all wavelengths) and not in others. The second issue is how to explain this nonmonotonous behavior as a function of the separation distance and the excitation wavelength. An important aspect to address the first issue is the difference between two populations of butane (detected by their allyl radical fragment at mass 41), β and α, formed as a result of the 248 nm irradiation. As mentioned above, the α population was attributed to ethyl radical recombination during irradiation, whereas the higher temperature β population was suggested to arise from recombinative desorption of ethyl fragments adsorbed on the surface in postirradiation TPD experiments.20 Such recombinative desorption of the β population was also seen in our studies (Figure 2). The ethyl groups are formed from the dissociative electron attachment (DEA) of the parent EC molecule. The similar non monotonous trend seen for ethane and α butane as a function of separation distance indicate that both species are produced on the cold substrate (45 K) during or directly following the laser excitation, unlike β butane. Excitation at 355 nm is not energetic enough or effective to form butane (or ethane) during the laser excitation, resulting in monotonous decay for both photoproducts. For this wavelength, no α butane was detected and ethane photoyield was low (see Figure 3). The behavior displayed by the fragments on the cold substrate at spacer layer thickness of less than 1 ML resembles that reported for methyl chloride photodecomposition with water as a spacer layer:22 an increase in photoreactivity that saturates at 1 ML. This phenomenon was attributed to the diminishing quenching rate due to the physical separation
Figure 3. TPD of allyl radical (mass 41, A) and ethane (mass 30, B) following irradiation by 2.2 × 1020 cm−2 photons at 532 nm (red), 2.1 × 1019 cm−2 photons at 355 nm (green), 1.6 × 1019 cm−2 photons at 266 nm (blue), and 1.3 × 1019 cm−2 photons at 248 nm (cyan). Reference spectra without irradiation (black) are also displayed. The heating rate was 1 K/sec. The peak at 80−120 K at mass 30 (B) arises from parent molecule fragmentation within the QMS. All the measurements were preformed on Ag NPs (50 nm average size)/ SiO2/Si (100).
order to shift the plasmon resonance toward the excitation wavelength available to us (532 nm), a hybrid sample was prepared in which the standard Ag NPs were the intermediate layer between two layers of TiOx (x ≤ 2) using reactive layer assisted deposition method (RLAD,28 see Supporting Information section for more details). A similar hybrid sample grown on a transparent sapphire sample revealed the spectral red shift to a resonant plasmon wavelength of 540 nm (Figure 1C). We assume that TiOx/Ag NPs/TiOx/SiO2/Si (100) and TiOx/Ag NPs/TiOx/sapphire respond optically the same with respect to the surface plasmon resonance position near 530−540 nm. Photochemical experiments performed on the hybrid sample (TiOx/Ag NPs/TiOx/SiO2/Si (100)) have shown no EC decomposition upon 532 nm irradiation. Moreover, we have performed two-color, coherent excitation experiments in which the above sample was simultaneously irradiated by 532 and 355 nm pulses, originating from the same Nd:YAG laser. These two pulses overlapped on the surface both laterally and temporally. No increased photodecomposition was found under these conditions. The TiOx protecting layer on top of the AgNPs (about 1.4 nm thick) actually leads to a reduced photochemical activity at 355 nm compared to the standard bare Ag NPs by a factor of 2, yet it enabled us to test the effect of two colors excitation scheme. These findings indicate that excitation at the 939
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increase at 1 ML Xe is two (α butane) and three (ethane) fold as a result of irradiation at 248 nm, whereas 266 nm photons lead to a more modest increase of photoreactivity of only 40% in the ethane population. This difference can be attributed to work function difference: as schematically shown in Figure 4C, 5.0 eV electrons (generated by 248 nm photons) excited from near the Fermi level (Ef) of the silver particles, are slightly too energetic to optimally overlap the LUMO anti bonding state (affinity level) of the adsorbed EC molecules (in the absence of a Xe spacer). When a monolayer Xe is used as a spacer, the work function of the Xe−Ag system is about 0.4−0.5 eV higher than the EC−Ag case.20,31 Now, the energetic electrons better overlap the EC molecules’ LUMO level. The lower energy photoelectrons excited by 4.66 eV above the Fermi energy (using 266 nm photons) are less effective than the 5.0 eV electrons in driving this process: Although these lower energy electrons overlap the EC-LUMO state at the same energy as the 5.0 eV electrons, they do so in the EC−Ag system (no Xe spacer case). In this case, some quenching by the silver particles competes and decreases the rate of ethane formation, as indeed experimentally demonstrated in Figure 4B. To conclude, a combination of diminishing quenching rate and work function tuning can explain the enhanced reactivity for spacer layers up to 1 ML Xe and its wavelength dependence. For both photoproducts (ethane and butane), the monotonous decay at Xe layers higher than 1 ML arises from gradual electron scattering within the Xe matrix, leading to a decreased flux of electrons that may collide with EC molecules adsorbed on top. Low energy (1 eV) electron scattering in solid Xe on single crystal surfaces has a mean free path (MFP) of 10 and 25 nm for disordered and ordered Xe matrix32 (adsorbed at 17 and 45 K, respectively). It is possible that Xe adsorption on Ag NPs sample will lead to a larger degree of disorder than that obtained in low temperature adsorption on a flat single crystal substrate. Such disorder leads to an electron mean free path of 5 nm, consistent with the observed decay distance in photochemical activity in our system. This can also explain why photoyield decay was not observed in the case of methyl chloride in similar Xe spacer experiments on Ni (111)22 because of the longer MFP in ordered Xe layers. On the other hand, electron MFP is known to be rather sensitive to electron energy, in contrast to our results that show similar decay distance for different excitation energy. The residual reactivity at large separation distances (20 nm) can originate from direct dissociation, especially for the high energy photons. In order to assess potential Ag nanoparticles size effect on the photoyield, postirradiation (1000 pulses, 4 mJ/cm2 per pulse at 355 nm) TPD of C3H5 (allyl radical, mass 41) are shown in Figure 5 for four different samples: native oxide SiO2/ Si(100), 2 Ag BLAG cycles (particles size ≈ 15 ± 3 nm), 20 Ag BLAG cycles (particles size ≈50 ± 10 nm), and direct deposition of an extended Ag film. On top of the reference native silicon oxide substrate, EC photodecomposition at 355 nm irradiation does not occur, indicating its total inactive behavior, as expected from an insulating, wide band-gap material. Excitation of the sample having 15 nm average diameter AgNPs, results in some (minor) photochemical activity, with fragments desorbing at 175 K. The sample with larger Ag NPs (particles size ≈ 50 ± 10 nm) exhibits higher activity by a factor of 4 and a further factor of 4 increase has been detected when the amount of evaporated silver has grown by another order of magnitude, through the formation of a homogeneous Ag thin film (see SEM image in
Figure 4. (A) Integrated QMS signal for mass 41 (allyl radical, C3H5) versus the Xe spacer layer thickness in monolayers (ML) and nanometers. Results were normalized relative to the area under the TPD peak obtained without a spacer layer. (B) Similar results for mass 30 (ethane, C2H6). (C) Energy level diagram for EC adsorbed on polycrystalline Ag. Electron energy (and distribution) following excitation by 248 and 266 nm photons (5.0 and 4.66 eV, respectively) with respect to the relevant Fermi energy, EF, are schematically depicted.
between the excited species and the metal substrate. The same explanation applies in our system: reduced quenching increases the excited ethyl lifetime, leading to selective enhancement of the photoproducts formed during excitation (i.e., ethane and α butane). The physical separation inhibits also direct ethyl adsorption on the Ag NPs, enabling further reactivity even after de-excitation. However, reduced quenching should affect both 266 and 248 nm irradiation equally, but Figure 4 reveals that the relative 940
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photoelectron scattering within the Xe matrix. The initial enhancement of both butane and ethane formation with increasing separation distance (up to 1 ML Xe) for both 266 and 248 nm excitation originates from a combination of work function tuning of band alignment (excited electron energy vs the EC molecule affinity levels or antibonding LUMO states) and diminishing quenching of excited molecules by the silver particles. These results imply that plasmonic enhancement of photochemical reactions is not a universal phenomenon: energetic overlap between the plasmon-induced carriers and the adsorbate’s unoccupied energy levels is required for plasmon enhancement of such reactions.
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ASSOCIATED CONTENT
S Supporting Information *
Details on the experimental scheme, sample preparation, and characterization procedure, TPD spectra of EC as a function of exposure, postirradiation TPD spectra of several photoproducts (butane and allyl and propyl radical), allyl radical coverage versus photon density (for cross section calculation), Auger electron spectra before and following photochemistry, photochemical yield from de- and reactivated (sputtered) samples displaying enhanced photoproduct selectivity. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 5. Postirradiation TPD spectra (0.4 ML EC, 7 × 1018 photons/ cm2 at 355 nm, heating rate 1 K/sec) of mass 41 (C3H5-allyl radical) from native oxide SiO2/Si(100) (black) as a reference, 2 BLAG cycles of 0.3 nm Ag on 10 ML Xe (red, particles size ∼ 15 ± 3 nm), identical Ag−Xe quantities but 20 stacked Ag BLAG cycles (green, particles size ∼50 ± 10 nm) and Ag direct deposition (blue) forming an almost uniform film of silver by the evaporation of 60 nm Ag. Inset: integrated mass 41 signal versus Ag coverage as determined by ex situ HR-SEM. SEM images of directly deposited Ag thin film (left) and 20 Ag BLAG cycles (right) are shown to demonstrate the silver particles shape.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partially supported by the US−Israel Binational Science Foundation and by the Israel Science Foundation. Partial support by the INNI-FTA program for all oxide PV is acknowledged.
Figure 5). The inset depicts the Ag coverage dependence of the integrated area under the photodissociation fragment desorption peak at mass 41. The overall product formation rate was observed to correlate and increase with increasing silver coverage, emphasizing the lack of sensitivity of the photoactivity of the EC molecules to the size of the Ag NPs. In summary, the photofragmentation of ethyl chloride (EC) adsorbed on Ag nanoparticles at a wide range of sizes has been studied using a nanosecond pulsed UV laser excitation procedure. Postirradiation TPD experiments and Auger electron spectroscopy indicate that the C−Cl bond dissociates under UV irradiation to form surface-bound Cl atoms, ethane, and butane. The product yield and photochemical cross section were found to increase with increasing photon energy for both photoproducts. Using a two-color excitation scheme where one wavelength overlaps the modified surface plasmon resonance of the silver particles (532 nm) and the other at 355 nm has not resulted in any measurable enhancement of photoreactivity. The photoyield increased with the Ag NPs coverage, up to a uniform silver film. These observations support our conclusion that no plasmon enhanced photochemistry could be identified in the present EC−Ag NPs system. The effect of molecule−Ag NPs separation distance on the photochemistry has been studied using Xe as a spacer layer. The overall decrease in reactivity with increasing EC−Ag separation distance, found for both photoproducts (ethane and butane) and all three UV excitation wavelengths used in this study, has been attributed to
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