Letter pubs.acs.org/JPCL
Realizing Visible Photoactivity of Metal Nanoparticles: Excited-State Behavior and Electron-Transfer Properties of Silver (Ag8) Clusters Wei-Ta Chen,† Yung-Jung Hsu,† and Prashant V. Kamat* Radiation Laboratory, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: Silver nanoclusters complexed with dihydrolipoic acid (DHLA) exhibit molecular-like excited-state properties with well-defined absorption and emission features. The 1.8 nm diameter Ag nanoparticles capped with Ag8 clusters exhibit fluorescence maximum at 660 nm with a quantum yield of 0.07%. Although the excited state is relatively short-lived (τ 130 ps), it exhibits significant photochemical reactivity. By introducing MV2+ as a probe, we have succeeded in elucidating the interfacial electron transfer dynamics of Ag nanoclusters. The formation of MV+• as the electron-transfer product with a rate constant of 2.74 × 1010 s−1 confirms the ability of these metal clusters to participate in the photocatalytic reduction process. Basic understanding of excited-state processes in fluorescent metal clusters paves the way toward the development of biological probes, sensors, and catalysts in energy conversion devices. SECTION: Plasmonics, Optical Materials, and Hard Matter using laser flash photolysis.41,42 Surface complexation of these few atom clusters with different ligands dictates the overall size and shape of clusters.11,43,44 The ability to synthesize highly emissive metal clusters has rejuvenated interest in understanding excited-state dynamics of noble-metal clusters.4,45,46 We have now synthesized fluorescent silver clusters stabilized by dihydrolipoic acid in aqueous solutions. The femtosecond transient absorption measurements that elucidate the excitedstate behavior and its participation in the interfacial electron transfer are discussed. The results discussed here provide the basis to bridge seemingly different properties of metal nanoparticles Absorption and Emission Properties of Silver Clusters. Dihydrolipoic acid (DHLA)-capped silver nanoclusters exhibit unusually strong emission in the visible with maximum around 660 nm.16 DHLA, with its dithiol and carboxy functional groups, is considered to be a suitable capping agent to control the size of Ag clusters. The strong interaction between thiol and Ag enables well-defined cluster size control.47,48 On the other hand, the carboxylate functional group of DHLA renders suspendability to these clusters in polar solvents. The synthetic steps involved in the Ag cluster synthesis are illustrated in Scheme 1. Figure 1 shows the absorption and fluorescence spectra of DHLA-stabilized Ag clusters with characteristic spectral features in the visible region. Blank synthetic controls prepared in the absence of silver source do not exhibit any absorption and emission in the visible region. Ag cluster suspension
M
onolayer-protected noble-metal nanoclusters with size control have unraveled many new interesting optical and electronic properties.1−11 The capping layer, usually consisting of thiol derivatives, enables tuning of metal nanoparticle size and their surface plasmon resonance properties.12,13 The localized surface plasmon resonance (LSPR) behavior, which is a dominant property of metal nanoparticles, disappears as the size of the metal nanoparticle attains the Fermi wavelength of the electron (2 nm or less). The energy levels become discrete, and such discrete structures have been observed for different metal clusters.8,14−16 Thus, these quantized metal clusters that exhibit molecular-like properties with discrete electronic transitions represent the intermediate state between metal atoms and nanoparticles. The noble-metal clusters offer new fascinating opportunities in the development of fluorescent probes,17−20 two-photon absorbers,21,22 and catalysts.23−28 Gold clusters with a magic number of core atoms have been synthesized.4,9,29−31 Dickson and coworkers have synthesized fluorescent Ag2−Ag8 clusters using second- and fourth-generation OH terminated poly(amidoamine) dendrimers as stabilizers.25 Similarly, mercaptosuccinic acid has been used as a capping agent to produce blueand red-emitting Ag7 and Ag8 clusters.20 Whereas metal nanoparticles have been coupled to semiconductors to exploit surface plasmon effect or promote photoinduced charge separation, very few studies report photoinduced energy and electron-transfer processes.32,33 The chemistry of silver clusters is well-known in silver halide photography.34,35 Early events of Ag+ reduction to Ag0, followed by the formation of Ag2+ and Ag42+, have been welldocumented.36−40 More recently, Scaiano and coworkers have photochemically synthesized fluorescent particle-supported small silver clusters and probed the formation of Ag2 clusters © 2012 American Chemical Society
Received: July 12, 2012 Accepted: August 15, 2012 Published: August 15, 2012 2493
dx.doi.org/10.1021/jz300940c | J. Phys. Chem. Lett. 2012, 3, 2493−2499
The Journal of Physical Chemistry Letters
Letter
clusters. On the basis of the absorption and emission spectral characteristics, we confirm the dominance of Ag8 clusters in our metal particle suspension prepared using DHLA as a stabilizing ligand. In future discussion, we will refer to them as Ag8 or silver clusters. The transmission electron micrograph (TEM) of silver clusters synthesized with DHLA as capping agent is shown in Figure 2. The TEM image shows the presence of small
Scheme 1. Synthetic Steps That Involve Surface Complexation with DHLA Followed by the Formation of Ag Core/Ag8 Shell Nanoparticles
Figure 2. TEM image of Ag core/Ag8 clusters. Insert shows the HRTEM showing the crystalline Ag core. Figure 1. (a) Absorption and (b) emission spectra of silver clusters in aqueous suspension. The excitation wavelength was at 450 nm for recording emission spectra. Inset shows the cluster solution before and during UV excitation.
nanoparticles of an average diameter of ∼1.8 nm with minimal aggregation. The high-resolution TEM image exhibits a lattice spacing of 0.23 nm corresponding to the Ag (111) plane, thus confirming the crystalline nature of Ag nanocore with an fcc structure. Usually ∼2 nm diameter metal nanoparticles are nonfluorescent. So, how can we account for the fluorescence emission observed for the 1.8 nm diameter crystalline silver nanoparticles? We can reconcile these observations if we consider a core−shell geometry with the Ag8 clusters existing as a capping layer on the top of crystalline metal nanocore. This assignment of Ag crystalline core and Ag8 cluster shell structure is in agreement with the fluorescent Ag and Au nanoparticles that have been proposed in previous studies.7 Excited-State Interaction with Methyl Viologen. Because the excited Ag8 clusters show molecular-like optical activity, we wanted to probe their ability to participate in photoinduced charge-transfer process. We employed methyl viologen (MV2+) as a probe to elucidate the interfacial electron-transfer process. Figure 3 shows the Ag8 cluster emission spectrum recorded at different concentrations of MV2+. The Ag8 cluster emission intensity decreases with increasing the concentration of MV2+. More than 90% fluorescence intensity is quenched when the MV2+ in the solution is increased to 0.09 mM. The ability to quench efficiently the Ag8 cluster emission is an indication that the excited-state deactivation is occurring via an electron transfer pathway. Previous studies have discussed the excitedstate quenching of small metal clusters on the basis of energytransfer and electron-transfer processes.32 Because MV2+ can be reduced to form MV+• radical via an one-electron reduction process, we are able to follow the reduction process by monitoring its absorption characteristics with maxima at 398 (ε = 41 100 M−1 cm−1) and 605 nm (ε =
displays three absorption peaks at 325, 425, and 500 nm. These absorption features are distinctively different than surface plasmon absorption seen in the region of 380−420 nm region for silver nanoparticles of >5 nm diameter. Smaller size metal nanoparticles with diameter 420 nm). The difference absorption spectra (recorded after and before irradiation) show the characteristic absorption corresponding to MV+• radical. No such changes were noticeable if we excluded Ag8 cluster from the sample. These results confirm that the charge transfer from Ag8 cluster to MV2+ is the dominant mechanism of the fluorescence quenching seen in Figure 3 (reactions 1 and 2). (Ag)8 + hν → (Ag)8 *
(1)
(Ag)8 * + MV2 + → (Ag)8+ + MV +·
(2)
Probing Electron Transfer Using Femtosecond Transient Absorption Spectroscopy. The excited state of metal clusters is usually short and requires detection with subnanosecond resolution. For example, femtosecond pump−probe and upconversion spectroscopy techniques have been successfully employed to probe the energy and electron-transfer processes in Au25 clusters.10,21,32 We employed time-resolved transient absorption spectroscopy to establish the excited-state behavior of the Ag8 cluster and its participation in the interfacial chargetransfer process. The transient absorption spectra recorded at different delay times following 387 nm laser pulse excitation of Ag8 cluster suspension (in the absence of MV2+) are shown in
Figure 4. (A) Absorption spectra of Ag8 cluster and 0.09 mM MV2+ in deaerated water (a) before and (c) after irradiation with visible light for 3 min. The absorption spectra (b) and (d) were recorded with 0.09 mM MV2+ in deaerated water before and after irradiation with visible light (>420 nm). Inset shows the difference absorption spectrum recorded after and before visible irradiation ((spectrum c) − (spectrum a)). (B) Absorption growth at 605 nm corresponding to MV+. radical formation during λ > 420 nm light irradiation: (a) Ag8-MV2+ and (b) neat MV2+ in deaerated aqueous solution. 2495
dx.doi.org/10.1021/jz300940c | J. Phys. Chem. Lett. 2012, 3, 2493−2499
The Journal of Physical Chemistry Letters
Letter
Figure 5. Time-resolved difference absorption spectra of (A) Ag8 cluster and (B) Ag8 cluster and 0.09 mM MV2+ in deaerated aqueous solution. (C,D) Transient absorption-time profiles recorded at 465 and 605 nm, respectively. Excitation wavelength = 387 nm, power intensity = 8.16 mW/ cm2.
Small photocurrents seen under visible-light irradiation were attributed to injection of hot electrons from excited gold nanoparticles into semiconductor nanoparticles.60−62 In support of this argument, a few spectroscopic studies have evoked the plasmon-induced charge injection from excited gold nanoparticles into TiO2 nanostructures.63 Recently, gold nanocluster with
