18 Cluster - American Chemical Society

Article pubs.acs.org/JPCC

A Quantum Alloy: The Ligand-Protected Au25−xAgx(SR)18 Cluster

Douglas R. Kauffman,*,†,‡ Dominic Alfonso,† Christopher Matranga,† Huifeng Qian,§ and Rongchao Jin†,§ †

National Energy Technology Laboratory, United States Department of Energy, Pittsburgh, Pennsylvania 15236, United States URS, Post Office Box 618, South Park, Pennsylvania 15129, United States § Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ‡

S Supporting Information *

ABSTRACT: Recent synthetic advances have produced very small (sub-2 nm), ligand-protected mixed-metal clusters. Realization of such clusters allows the investigation of fundamental questions: (1) Will heteroatoms occupy specific sites within the cluster? (2) How will the inclusion of heteroatoms affect the electronic structure and chemical properties of the cluster? (3) How will these very small mixed-metal systems differ from larger, more traditional alloy materials? In this report we provide experimental and computational characterization of the ligand-protected mixed-metal Au25−xAgx(SC2H4Ph)18 cluster (abbreviated as Au25−xAgx, where x = 0−5 Ag atoms) compared with the unsubstituted Au25(SC2H4Ph)18 cluster (abbreviated as Au25). Density functional theory analysis has predicted that Ag heteroatoms will preferentially occupy sites on the surface of the cluster core. X-ray photoelectron spectroscopy revealed Au−Ag state mixing and charge redistribution within the Au25−xAgx cluster. Optical spectroscopy and nonaqueous electrochemistry indicate that Ag heteroatoms increased the cluster lowest unoccupied molecular orbital (LUMO) energy, introduced new features in the Au25−xAgx absorbance spectrum, and rendered some optical transitions forbidden. In situ spectroelectrochemical experiments revealed charge-dependent Au25−xAgx optical properties and oxidative photoluminescence quenching. Finally, O2 adsorption studies have shown Au25−xAgx clusters can participate in photomediated charge-transfer events. These results illustrate that traditional alloy concepts like metal-centered state mixing and internal charge redistribution also occur in very small mixed-metal clusters. However, resolution of specific heteroatom locations and their impact on the cluster’s quantized electronic structure will require a combination of computational modeling, optical spectroscopy, and nonaqueous electrochemistry. can affect the electronic structure12,27−30 and chemical properties11,17 of mixed-metal clusters, but it remains unclear how these properties evolve with respect to the cluster stoichiometry. These newly realized mixed-metal clusters allow the exploration of fundamental questions: (1) Where do the heteroatoms reside within the cluster? (2) How will heteroatom inclusion affect the discrete energy levels and chemical properties of small metal clusters? (3) How will very small mixed-metal systems differ from larger, more traditional alloy materials? While seemingly straightforward, these questions are important for understanding and designing molecular-scale mixed-metal nanostructures. The atomically precise, ligand-protected Au25(SC2H4Ph)18 cluster (abbreviated as Au25) is an attractive platform to study very small mixed-metal systems because it can be synthesized with molecular purity,31 its crystal structure has been solved,5,32,33 and it readily accepts heteroatoms.11−15,17−19,21,27,34 Moreover, the Au25 electronic structure is highly quantized,5 and relatively small perturbations can induce characteristic optical and electrochemical changes.35−40

1. INTRODUCTION Alloys often demonstrate unique properties compared to singlecomponent systems.1 Interactions between constituent atoms can modify the alloy’s electronic structure and surface composition, often leading to enhanced chemical or optical properties.2−4 It is interesting to consider how such effects may evolve in very small (sub-2 nm) metal clusters that only contain a few dozen atoms. Quantum effects become significant in this size regime, and electron confinement within the small cluster can lead to molecule-like energy level quantization.5 The substitution of just one heteroatom will significantly alter the stoichiometry of such small systems and may lead to unique electronic structure perturbations. This provides an exciting opportunity to investigate whether traditional alloy concepts like d-band mixing,6,7 charge rearrangement,8,9 and surface segregation1,2,10 can adequately describe very small mixed-metal systems. In particular, heteroatom substitution will likely occur at specific, energetically favorable locations within the cluster, and electronic effects may require the consideration of discrete energy levels and characteristic optical transitions. Along these lines, small mixed-metal clusters have been synthesized by substituting known numbers of Pt,11,12 Pd,13−18 Ag,19−26 or Cu27 heteroatoms into atomically precise Aun clusters. Initial reports suggest that heteroatom substitution © XXXX American Chemical Society

Received: February 5, 2013 Revised: March 18, 2013

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dx.doi.org/10.1021/jp4013224 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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formation of nanoclusters. The reaction mixture was then stirred for an additional ∼8 h. 2.2. Cluster Purification. The Au25−xAgx reaction products were first transferred to a single-neck 100 mL round-bottom flask and THF was removed by rotary evaporation at room temperature. Remaining in the flask were a water phase and a phase containing nanoclusters with an oily consistency. The product was washed with methanol to remove excess phenylethanethiol, the aqueous phase was removed, and the precipitates were collected by centrifugation at ∼5000 rpm. This process was repeated five times to remove the excess phenylethanethiol. Cluster products in the precipitate were then extracted with acetonitrile and the supernatant was collected by centrifugation at ∼5000 rpm. The acetonitrile solution was dried with rotary evaporation at room temperature. The cluster products were redissolved in minimal dichloromethane (1−3 mL) and finally dried in a small vial under a nitrogen gas stream. 2.3. Cluster Characterization. The cluster sample was analyzed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). This was conducted on a PerSeptiveBiosystems Voyager DE super-STR time-of-flight (TOF) mass spectrometer with trans-2-[3-(4-tert-butylphenyl)2-methyl-2-propenylidene]malononitrile (DCTB) as the matrix.20 Dimethylformamide (DMF) was dried over 3 Å molecular sieves and used as a solvent for spectroscopic and electrochemical measurements. N2 and O2 gases were ultrahigh purity grade. For spectroscopic measurements, Au25 and Au25−xAgx were dissolved in DMF, immediately placed into a sealable septum-capped quartz cuvette (Starna), and then purged with N2 for 1 h prior to measurement. Optical absorbance spectra were collected with Perkin-Elmer Lambda 1050 and Agilent 8453 spectrophotometers. Photoluminescence (PL) spectra were collected on a Horiba Jobin-Yvon Fluorolog-3 spectrofluorometer with a liquid N2 cooled InGaAs detector. Nonaqueous electrochemistry was performed with a Biologic SP-150 potentiostat. Square wave voltammetry (SWV) curves were collected at a scan rate of 10 mV·s−1. Au25 and Au25−xAgx were dissolved in DMF with a supporting electrolyte of 0.1 M tetrabutylammonium perchlorate (TBAP). Solutions were thoroughly purged with N2 prior to electrochemical measurement and kept under a blanket of flowing N2 during experiments. Pt wires were used as the working and counter electrodes, and a nonaqueous Ag/Ag+ reference electrode (0.01 M AgNO3 + 0.1 M TBAP in acetonitrile) completed the cell. Electrochemical potentials were calibrated into the standard hydrogen electrode (SHE) scale after each experiment by adding ferrocene into the cluster solution; the ferrocene Fc/Fc+ redox couple has a literature value of +0.7112 V versus SHE in DMF + 0.1 M TBAP.44 In situ spectroelectrochemistry was performed by use of a commercially available quartz cell (BASi; model EF-1350) with a Pt mesh working electrode, a Pt wire counter electrode, and the above-noted Ag/Ag+ reference electrode. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI 5600ci spectrometer employing Al Kα X-rays. Samples were loaded directly onto conductive carbon tape, and binding energies were calibrated to the C 1s peak of adventitious carbon at 284.6 eV. The purity of the bulk Au and Ag foils was >99%. 2.4. Computational Details. Cluster geometries were constructed by use of the previously density functional theory (DFT)-optimized Au25(SH)181− structure as a starting point.45

This sensitivity is advantageous, and it can be used to monitor the effects of heteroatom substitution in ways that are not possible with traditional alloy materials. Herein we describe the experimental and computational characterization of ligandprotected mixed-metal Au25−xAgx(SC2H4Ph)18 clusters (abbreviated as Au25−xAgx, where x = 0−5 Ag atoms) in comparison to the unsubstituted Au25 cluster. Exhaustive structural screening was conducted within the density functional theory (DFT) framework to identify energetically favorable locations for heteroatom substitution. DFT is a valuable complement to experimental efforts because it allows predictive determination of difficult-to-measure structural properties at the atomic level. Mass spectrometry identified Au22Ag3 as the most abundant species in the Au25−xAgx sample, and DFT was used to analyze all 176 symmetry-inequivalent Au22Ag3 configurations within the defined cluster geometry. This type of brute force approach has been computationally plausible in the last several years in view of the advances in parallel algorithms for DFT coupled with rapid improvements in computer hardware.41 Our calculations predict that Ag heteroatoms will preferentially occupy equivalent sites on the surface of the cluster’s core, rather than the peripheral (−S−Au−S−Au−S−)6 ligand shell or the central atomic position. Experimentally, X-ray photoelectron spectroscopy (XPS) revealed some aspects of the mixed-metal cluster were analogous to traditional alloys, and Au−Ag state mixing and charge redistribution were noted within the Au25−xAgx cluster. On the other hand, techniques like optical spectroscopy and nonaqueous electrochemistry resolved heteroatom-related changes in the cluster’s quantized electronic structure. In particular, Ag heteroatoms increased the cluster lowest unoccupied molecular orbital (LUMO) energy, whereas the energy of the highest occupied molecular orbital (HOMO) was unaffected. Ag substitution also split some optically active levels and rendered other transitions forbidden. In situ spectroelectrochemistry benchmarked charge-dependent optical properties of Au25−xAgx, and O2 adsorption experiments identified photomediated cluster oxidation. Notably, Au25−xAgx showed oxidative photoluminescence (PL) quenching, whereas the unsubstituted Au25 cluster typically shows increased PL intensity after oxidation.39,40,42 Our results illustrate that sub-2 nm mixed-metal clusters, that is, “quantum alloys”, do share some similarities with larger, more traditional alloy materials. However, an accurate description of such systems requires consideration of discrete, molecule-like energy levels and energetically favorable heteroatom locations.

2. METHODS 2.1. Cluster Synthesis. Au25−xAgx was synthesized through an alteration of the reported Au25 synthesis.43 All synthetic steps were performed at room temperature and under air atmosphere. Gold salt (HAuCl4·3H2O, 0.080 g) and silver salt (CH3COOAg, 0.007 g) were mixed in a 5:1 ratio and dissolved with tetraoctylammonium bromide (TOAB; 0.155 g) in 15 mL of tetrahydrofuran (THF) in a 100 mL trineck round-bottom flask. The solution was stirred for 15 min and became red. Phenylethanethiol (0.168 mL) was added to the flask and the mixture was stirred for 15 min. Over this time period the solution color slowly changed to pale yellow. NaBH4 (0.093 g, dissolved in 5 mL of cold water) was added to the flask and the solution color immediately turned black, indicating the B



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Figure 1. (a) Experimentally determined Au25 crystal structure showing the “core” and “shell” components of the cluster;5,32 the organic portion of the ligands and the tetraoctylammonium (TOA+) counterion have been omitted from this presentation. (b) DFT-optimized Au22Ag3 structure. (c) DFT analysis of 176 symmetry-inequivalent Au22Ag3 configurations. We have defined “center” as the central atomic position within the cluster, “core” as the 12 atomic positions on the surface of the icosahedral core, and “shell” as the 12 Au atoms within the −S−Au−S−Au−S− semiring structures in the ligand shell. The data in Figure 1c are summarized in Table S1 (Supporting Information).

→ 0 was employed. As part of our analysis, the electronic partial density of states (PDOS) was determined for the examination of electronic properties. It corresponds to the splitting of the total density of states N(E) = ∑ jδ(E − εj) into contributions from various orbitals centered on each atom, with the sum spanning all Kohn−Sham eigenvalues εj. In VASP,41,46 this quantity is calculated as PDOSilm(E) = ∑ j,occδ(E − εj)|⟨ilm| ϕj⟩|2, where the squared term contains projection of the Kohn− Sham eigenfunctions ϕj on a set of orthogonalized atomic wave functions. In this expression, i and lm correspond to atomic site and angular quantum numbers, respectively. The δ function is approximated as a Gaussian function with a chosen smearing width. In this work, a width of 0.05 eV was adopted.

Three Au atoms in various sites of the cluster were replaced by Ag, giving rise to structures with a 22:3 Au:Ag ratio. This Au:Ag ratio was chosen to reflect the most abundant species in the experimental sample. The energetics of all 176 symmetryinequivalent Au22Ag3 configurations within the defined cluster geometry were examined with DFT as implemented in the Vienna ab initio simulation package (VASP) code.41,46 This implementation includes total energy and atomic force calculations. The generalized gradient approximation (GGA) with the Perdew−Burke−Enzerhoff (PBE) functional was used to calculate the exchange−correlation energy.47 The interactions of the valence electrons with the core electrons and the nuclei were described by the projector-augmented wave (PAW) all-electron potentials within the frozen-core approximation.48 For Ag and Au, we used the standard PAW potentials acting on 11 (Ag, 4d10 and 5s1 ; Au, 5d10 and 6s1) outer core/valence electrons. Plane-wave basis sets with a cutoff energy of 600 eV were used to expand the DFT-PAW pseudowave functions. In order to ensure the decoupling of periodic images, the cluster was placed in a cubic box with dimensions of 24 Å/side and a uniform compensating background charge was assumed. A Γ-point sampling of the Brillouin zone was utilized in calculations of the ground state. During the geometry optimization, the coordinates of the atoms were allowed to relax by use of a quasi-Newton variable metric algorithm until the total force on the atoms was < 0.03 eV/Å. A Gaussian smearing of σ = 0.2 eV was used, and the corrected energy for σ

3. RESULTS AND DISCUSSION 3.1. Computational Modeling of Au25−xAgx. The formation of mixed-metal Au25−xMx clusters has been achieved through direct synthetic routes11,19,27,49 or the solution-phase displacement of Au atoms.21,34 We directly synthesized Agsubstituted clusters through a modified Au25 synthetic procedure,43 using a 5:1 ratio of Au and Ag precursor salts. Previous reports have indicated that a maximum of 11 Ag heteroatoms can be substituted into Au25 to form Au14Ag11.19 In the present case, MALDI-MS identified Au22Ag3 as the peak (i.e., most abundant) species in a distribution of Au25−xAgx clusters (x = 0−5 Ag atoms; Figure S1, Supporting Information). Cluster fragmentation can occur during C



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MALDI-MS analysis,50,51 and lower-mass signals indicated Au21−xAgx fragment formation through the loss of ligandcontaining Au4(SC2H4Ph)4 species from the cluster shell.52 XPS analysis identified an average cluster composition of Au22.1Ag2.9 (Figure S2, Supporting Information). These results reflect the MALDI-MS data and indicate a sample-wide average of ∼3 Ag atoms/cluster, that is, Au22Ag3. Our computational efforts were focused on the Au22Ag3 cluster. We specifically chose to model the Au22Ag3 cluster because it represents the peak species identified by MALDI-MS (Figure S1, Supporting Information). Our Au22Ag3 model was based on the reported Au25 crystal structure shown in Figure 1a.5,32 The Au25 cluster contains an icosahedral Au13 core protected by a peripheral shell of six −S−Au−S−Au−S− semiring structures; the organic −C2H4Ph portion of the ligands and the TOA+ counterion have been omitted from this presentation. Au and Ag share equivalent atomic radii and valence electron counts,53 and Ag substitution is not expected to significantly change the overall structure or negative groundstate charge of Au25.19,27,29,30,54 As such, we systematically replaced three Au atoms throughout a previously DFToptimized Au25(SH)18− cluster45 with Ag atoms to form Au22Ag3. The geometry of the Au22Ag3 cluster was then optimized by DFT to obtain the minimum energy structure. We analyzed the energetics of all 176 symmetry-inequivalent Au22Ag3 configurations within the defined cluster geometry. The results of this exhaustive DFT treatment are presented in Figure 1b,c and in Table S1 in Supporting Information. The different Au22Ag3 configurations were categorized into unique families based on the location of the Ag heteroatoms, for example, core−core−core, shell−core−core, shell−core−center, etc. We have defined “center” as the central atomic position within the cluster, “core” as the 12 atomic positions on the surface of the icosahedral core, and “shell” as the 12 Au atoms within the −S−Au−S−Au−S− semiring structures in the ligand shell. The different families were energetically unique from one another at a confidence level greater than 99.9%, and 0.81 ± 0.03 eV separated the most stable (core−core−core) and least stable (shell−shell−center) Au22Ag3 configurations. These results indicate the synthesized Au25−xAgx clusters likely contain Ag atoms on the surface of the Au-centered icosahedral core, whereas Ag occupation of the central atomic position or ligand shell is unfavorable. The narrow energy distribution within the core−core−core family implies the specific location of Ag atoms on the core surface may vary from cluster to cluster,30 and the Au22Ag3 structure presented in Figure 1b is a representative member of the core−core−core family. Previous reports have also suggested preferential incorporation of Ag heteroatoms into the surface of the cluster core.19,29,30,55 Our thorough analysis of different Au22Ag3 structures provides firm computational support for such reports. On the other hand, some studies have predicted that Cu, Pd, and Pt heteroatoms will preferentially occupy the cluster’s central atomic position.11,12,14,27−29 The underlying reason for different heteroatom locations and capacities is still an unresolved topic; however, differences in atomic radii and valence electron counts are suspected to influence heteroatom substitution and the stability of the resulting mixed-metal cluster.19,27,29 3.2. X-ray Photoelectron Spectroscopy. XPS is a useful technique for analyzing alloy materials because it can interrogate both core-level and valence band (VB) electrons. Samples were loaded onto conductive carbon tape for analysis,

and binding energies were calibrated to the C 1s peak at 284.6 eV; all peaks were found to be in good agreement with literature values (Table S2, Supporting Information).56 Figure 2a presents the Au 4f spectra of bulk Au and the nanocluster samples. The Au 4f region of Au25 was shifted to slightly higher binding energies compared to Au25−xAgx and bulk Au. This phenomenon has been observed for other ligand-protected Au clusters, and increased Au 4f binding energies are attributed to the thiolate ligands withdrawing electronic density from Au atoms.57,58 On the other hand, Au25−xAgx displayed Au 4f binding energies that were slightly negative of bulk Au. Figure 2b compares the Ag 3d spectra of Au25−xAgx and bulk Ag. The Ag 3d spectral region of Au25−xAgx was shifted to lower binding energies compared with bulk Ag. A negative shift in Ag 3d binding energies is consistent with Ag oxidation.56,59,60 This shift may seem counterintuitive because oxidation typically increases the binding energy of metals. However, Ag does not follow this trend, and oxidation-based changes to the lattice potential, work function, and/or extra-atomic relaxation energies are thought to negatively shift the Ag 3d peaks.59 Negatively shifted Ag 3d binding energies have also been observed in bulk Au−Ag alloys, and Ag is thought to donate a small fraction of electron density to Au.6,7 Negishi et al.19 have reported similar Au 4f and Ag 3d binding energies for other Au25−xAgx clusters. They interpreted their results to indicate electron donation from the less electronegative Ag heteroatoms (XAg = 1.93) to the more electronegative Au atoms (XAu = 2.4); electronegativity values were taken from ref 53. We also suspect that Au−Ag charge redistribution occurred within the Au25−xAgx cluster, as evidenced by the cluster’s negatively shifted Ag 3d spectrum. Further evidence for charge redistribution within the Au25−xAgx cluster comes from the Au 4f binding energies noted in Figure 2a. We hypothesize that Au−Ag charge redistribution balanced the electron-withdrawing effects of the thiolate ligands and produced Au 4f binding energies that were comparable to bulk Au. We also used XPS to investigate the VB spectral region (Figure 2c); we note the term “valence band” is somewhat of a misnomer because the clusters possess discrete energy levels rather than bulk-like energy bands. Nevertheless, this spectral region is particularly interesting for very small mixed-metal clusters because it probes electronic states near the Fermi level. The lack of a Fermi edge at ∼0 eV in the cluster spectra represents a reduction in metallic (conducting) electronic states.58 Other small Au clusters have been found to lack Fermi edge spectral features,58,61 and a so-called metal-to-insulator transition (energy level quantization) is expected for clusters that possess less than ∼150 metal atoms. 61−63 This quantization leads to the development of molecular orbitals, and the onset of the VB spectrum represents the cluster HOMO.64 Both clusters showed VB onsets at a binding energy of 0.42 eV; the VB onset was determined by linearly extrapolating the leading edge of the spectrum to the baseline (Figure S3, Supporting Information).64 This observation suggests the substitution of Ag heteroatoms did not affect the energy of the Au25−xAgx HOMO. The cluster VB spectra were dominated by the Au 5d doublet centered at ∼5 eV.7 The cluster Au 5d5/2 peaks were shifted to higher binding energy and their apparent Au 5d peak separations were narrowed compared to bulk Au (Figure 2c; Table S2 in Supporting Information). Similar Au 5d narrowing has been observed in other small clusters58,61−63 as well as traditional alloys,6,7 and the phenomenon points to reduced D



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Figure 2. X-ray photoelectron spectroscopy. (a) Au 4f region of Au25, Au25−xAgx, and bulk Au. (b) Ag 3d spectral region of Au25−xAgx and bulk Ag; the raw Au25−xAgx data are represented by points, and the solid curve is the fitted spectral envelope. (c) Valence band (VB) spectral region of Au25, Au25−xAgx, bulk Au, and bulk Ag; the raw cluster data are represented by points, and the solid curves are the fitted spectral envelopes. Samples were loaded onto conductive carbon tape, and all binding energies were calibrated against the C 1s peak at 284.6 eV.

broad PL that extends below Eg. This low-energy PL originates from the relaxation of photoexcited electrons into emissive mid-gap states.37,42,70 Figure 3b compares the optical absorbance and PL spectra of Au25 and Au25−xAgx dissolved in N2-purged DMF; the labeled absorption peaks correspond to the transitions defined in Figure 3a. PL spectra were collected with excitation wavelengths (λex) corresponding to the b absorbance peak of each cluster. Both experimental11,13,19,20,27 and computational studies27,28,30,54 have noted heteroatom-induced modification of the cluster Eg. We found the Au25−xAgx absorbance onset (Eg; Figure S4, Supporting Information), a, a′, and b absorbance features, and PL maximum were all shifted to higher energies by 0.08−0.16 eV. The spectral data are summarized in Table 1. Au25−xAgx also possessed an apparently new absorbance feature at 2.60 eV that we have labeled b′. The energy separation between the a/a′ and b/b′ absorbance features were quite similar (0.27 vs 0.26 eV), and we hypothesize the b′ transition stems from the HOMO − 1 and − 2 levels splitting into two optically active states. This hypothesis is analogous to the HOMO splitting that separates the a′ and a transitions.38,47−49 The Au25−xAgx absorbance spectrum also lacked a characteristic c transition. In the Au25 cluster this transition originates from a deeper HOMO − 5 energy level (Figure 3a), and mixing between Au and Ag electronic states may have rendered this transition forbidden in the Au25−xAgx cluster. Computational studies have predicted that heteroatom substitution may split optically active levels and alter characteristic Au25 absorbance features.28,30 Our observation that Ag

coupling between neighboring Au atoms and a disruption in long-range periodicity.6,7 This interpretation is reasonable because the clusters contain a maximum of 25 Au atoms, whereas the bulk Au sample contains an extended network of Au−Au neighbors. The Au 5d doublet of Au25−xAgx appeared “filled in”, and the apparent Au 5d peak separation was equivalent to isolated Au atoms (1.5 eV).7 This spectral change likely stemmed from a combination of two phenomena: mixing between Au 5d and Ag 4d states, and a further reduction in Au−Au coupling caused by Ag heteroatom insertion.6,7 Taken as a whole, the XPS data show that some aspects of mixedmetal clusters are analogous to traditional alloys, and the cluster stoichiometry, Au−Ag state mixing, charge rearrangement, and reduced Au−Au coupling can be inferred from the core-level and VB spectra. However, additional techniques are required to investigate heteroatom-induced perturbations to the quantized electronic structure of very small mixed-metal clusters. 3.3. Optical Spectroscopy. The quantized energy levels of very small, ligand-protected clusters can lead to molecule-like optical transitions.5,37 A simplified Au25 energy level diagram is presented in Figure 3a.5 Au25 has a theoretically predicted HOMO−LUMO energy gap (Eg) of 1.37 eV that corresponds to the a′ and a transitions. The Au25 Eg was experimentally determined to be 1.36 eV from the onset of the absorbance spectrum (Figure S4, Supporting Information),37,65 but splitting of the degenerate HOMO levels separated and blueshifted the a and a′ transitions.66−69 Absorbance feature b is a mixture of HOMO − 2 → LUMO and HOMO → LUMO + 1 and + 2 transitions, and absorbance feature c represents a HOMO − 5 → LUMO transition. Au25 also demonstrates E



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Figure 3. (a) Simplified Au25 energy level diagram created from the data in ref 5. (b) Optical absorbance and PL spectra of Au25 and Au25−xAgx in N2-purged DMF. (c) Partial density of states (PDOS) plots for the Au25 and Au22Ag3 cluster models. The energy scale of the PDOS plots has been set relative to the particular cluster’s Fermi level (EFermi = 0); Figure S5 (Supporting Information) presents the PDOS plots over a larger energy range. (d) Experimental square-wave voltammograms (SWVs) of Au25 and Au25−xAgx in N2 purged DMF + 0.1 M TBAP. Curves were collected at a scan rate of 10 mV·s−1, and potentials were calibrated into the SHE scale by use of ferrocene.44

heteroatoms introduced a new b′ transition and eliminated the characteristic c transition provides experimental support for such computational predictions. The spectral data in Figure 3b and Table 1 indicate that Ag heteroatom substitution increased the cluster Eg. Figure 3c plots the partial density of states (PDOS) for the model Au25 and Au22Ag3 clusters. The energy scale of the PDOS plots have been set relative to the particular cluster’s Fermi level (EFermi = 0); Figure S5 (Supporting Information) presents the PDOS plots over a larger energy range. The location of the Au25 and Au22Ag3 HOMO levels were equivalent at −0.3 eV versus the Fermi level. This computational prediction qualitatively agrees with the XPS observation of equivalent HOMO binding energies (Figure 2c and Figure S3 in Supporting Information). On the other hand, the Au22Ag3 LUMO energy was found to increase by 0.1 eV compared to Au25. The predicted LUMO increase is in excellent agreement with the experimentally observed 0.09 eV increase in Eg and 0.08−0.16 eV blue shift in Au25−xAgx spectral features (Table 1). Heteroatom-induced alteration of LUMO energy has also been theoretically predicted by Häkkinen and co-workers.28 Their computational model studied Au25 and Au24Pd1 clusters, and similar to our results, they found equivalent HOMO energies for the two clusters. However, in contrast to the

present case, their calculations indicated that Pd substitution would decrease the energy of the Au24Pd1 LUMO. These results illustrate the unique ways that particular heteroatoms can perturb the electronic structure of small metal clusters. 3.4. Electrochemistry. Nonaqueous electrochemistry is a useful technique because it can resolve the HOMO and LUMO energy levels of small metal clusters.37,71 Figure 3d presents the SWVs of Au25 and Au25−xAgx in N2-purged DMF with a supporting electrolyte of 0.1 M TBAP. Curves were collected Table 1. Summary of Optical Spectroscopy Data for Au25 and Au25−xAgxa Eg, eV a′, eV a, eV b′, eV b, eV c, eV PL max., eV

Au25

Au25−xAgx

Δ

1.36 1.56 1.82

1.45 1.71 1.98 2.60 2.86

0.09 0.15 0.16

1.24

0.11

2.78 3.09 1.13

0.08

a

Eg stands for the HOMO−LUMO energy gap as determined from the onset of the absorbance spectrum (Figure S4, Supporting Information). F



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with a scan rate of 10 mV·s−1, and potentials were calibrated into the SHE scale by use of ferrocene.44 Please note the electrochemical potential scale in Figure 3d (volts vs SHE) is not the same as the arbitrarily set energy scale in Figure 3c (electronvolts vs cluster Fermi level; EFermi = 0 eV). Au25 and Au25−xAgx displayed several characteristic redox peaks that correspond to charge injection into the HOMO and LUMO. A charging energy associated with the electrochemical addition or removal of electrons splits the HOMO into two peaks and increases the apparent HOMO−LUMO peak separation. The electrochemical charging energy was estimated to be 0.2 V from the HOMO redox peak separation.37,71,72 ́ Murray and co-workers37 and Garcia-Raya et al.71 have shown the Eg of small metal clusters can be estimated by subtracting the charging energy from the (0/−1) and (−1/−2) redox peak separation. This analysis provided Eg values of 1.40 eV for Au25 and 1.45 eV for Au25−xAgx. The electrochemical determination of a larger Eg for Au25−xAgx is consistent with both the spectroscopic and computational results in Figure 3b,c. The open circuit potentials (equilibrium potentials) of both clusters were negative of their respective (0/−1) redox peaks, indicating both clusters possessed an inherent −1 charge.37,73 This observation supports the hypothesis that Ag heteroatom substitution should not change the negative ground-state charge of Au25.19,27,29,30,54 The electrochemical data are summarized in Table S3 in Supporting Information. Particular interest was paid to positions of the cluster HOMO and LUMO redox peaks. The HOMO (0/−1) redox peaks of both clusters were located at +0.34 V versus SHE. The electrochemical determination of equivalent HOMO potentials is consistent with the XPS (Figure 2c) and computational (Figure 3c) results, and it provides firm experimental evidence that Ag insertion did not perturb the energy of the cluster HOMO. On the other hand, the Au25−xAgx LUMO (−1/−2) redox peak was shifted to more cathodic potentials by 50 mV. This cathodic shift translates into a 50 meV increase in the Au25−xAgx LUMO energy, and it agrees with the experimentally observed spectral blue shifts (Figure 3b) and the computationally predicted LUMO increase (Figure 3c). The electrochemical data allow us to unambiguously attribute the spectral blue shift to a heteroatom-induced LUMO increase, whereas the HOMO energy was unaffected. The Au25−xAgx SWV also contained apparent electrochemical features superimposed upon the background at roughly +0.16, −0.85, and −1.0 V. Murray and co-workers13 and Negeshi et al.14 noted similarly anomalous electrochemical features in Pd-substituted Au25 clusters. The origin of these features remains unclear; however, their position with respect to the HOMO (0/−1) and LUMO (−1/−2) redox peak suggests they may represent optically inactive states within the Au25−xAgx HOMO−LUMO gap. In comparison, Negishi et al.27 successfully incorporated Cu atoms into the Au25 structure. They also found equivalent HOMO potentials for the Au25 and Au24Cu1 clusters, but incorporation of Cu heteroatoms anodically shifted the cluster’s LUMO. This anodic shift decreased the Eg and red-shifted the Au24Cu1 spectral features. Qian et al.11 found a similarly redshifted HOMO−LUMO transition for Au24Pt1 clusters; however, this spectral change resulted from an apparent increase in HOMO energy. These observations further illustrate the unique influence that particular heteroatoms can have on the electronic structure of small metal clusters, and they underscore the difference between sub-2 nm mixed-metal clusters and traditional alloy materials. Specifically, a combina-

tion of computational modeling, optical spectroscopy, and nonaqueous electrochemistry can resolve the specific location of heteroatoms and their impact on discrete energy levels within very small mixed-metal clusters. 3.5. In Situ Spectroelectrochemistry and Photomediated Charge Transfer. Charge transfer and charge rearrangement can modify the spectral features of atomically precise clusters. Accordingly, the in situ application of electrochemical potentials is a convenient way to study the charge-state dependent optical properties of Au25−xAgx.37,42 For example, the application of an electrochemical potential that is more positive than the (−1/0) redox peak will oxidize the cluster into the neutral Au25−xAgx0 form. Similarly, the application of an electrochemical potential that is more positive than the (+1/0) redox peak should produce cationic Au25−xAgx1+. Figure 4a contains the absorbance and PL spectra of Au22Ag3 during in situ application of electrochemical potentials; please

Figure 4. (a) In situ spectroelectrochemical modification of Au25−xAgx absorbance and PL in N2-purged DMF with a supporting electrolyte of 0.1 M TBAP. (b) Absorbance and PL spectra of an initially N2-purged Au25−xAgx solution (black curves) that was then saturated with O2 for 1 h in ambient room light (red curves). Light-free O2 exposures produced substantially smaller spectral changes (Figure S8, Supporting Information), indicating a primarily photomediated process.

see Figure 3d for the position of the applied potentials with respect to the Au25−xAgx redox peaks. The initial spectra (black curves) were collected in the absence of electrochemical potential, that is, in an open circuit configuration. The open circuit potential (equilibrium potential) of the Au25−xAgx cluster was negative of the (0/−1) redox peak, indicating it was initially in the −1 charge state.37,73 The red curves in G



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tional results predict Ag atoms will preferentially occupy sites on the surface of the cluster core, but occupation of the central atomic position or sites within the ligand shell was found to be energetically unfavorable. Our experimental data show that mixing between Au and Ag states perturbs discrete energy levels within the cluster’s electronic structure. Interestingly, the Ag heteroatoms increase the LUMO energy while leaving the HOMO energy unaffected. These perturbations shift characteristic optical absorbance and PL features to higher energy. Heteroatom-induced splitting of discrete energy levels also introduces new features in the Au25−xAgx absorbance spectrum. In situ spectroelectrochemistry revealed unique charge-dependent optical properties and oxidative PL quenching. Finally, O2 adsorption experiments confirmed Au25−xAgx could participate in photomediated charge-transfer events. These results highlight the unique ways in which heteroatom substitution can affect the electronic structure and photophysical behavior of very small clusters and that descriptions of mixed-metal clusters require consideration of their quantized energy levels.

Figure 4a were collected during the application of +0.42 V. Depopulation of the Au25−xAgx HOMO bleached the a′ transition, slightly bleached and blue-shifted the a transition, condensed the b′ and b transitions into a single peak at 2.78 eV, and decreased the PL intensity by 64%. Further oxidation of the Au25−xAgx cluster at +0.72 V significantly bleached the a′ and a transitions, red-shifted the b′/b absorbance peak, and increased the absorbance at 2.4 eV (blue curves). Equivalent PL intensities were observed during the application of +0.42 and +0.72 V. The ability to oxidatively “turn off” the Au25−xAgx PL is particularly striking because the parent Au25 cluster typically shows increased PL after oxidation.39,42 The origins of the Au25 PL increase have yet to be resolved, but oxidation is thought to further polarize Au−S bonds39,40 and slightly change the Au25 geometry.35 Either one of the above-noted phenomena could increase Au25 PL intensity through a reduction in nonradiative relaxation pathways. The presence of Ag heteroatoms may limit further bond polarization and/or geometric changes, and the oxidative Au25−xAgx PL quenching likely stems from a depletion of photoexcitable electrons from the cluster HOMO. Finally, we investigated the chemical interaction between Au25−xAgx and O2. Our previous work with the Au25−O2 couple revealed that photoexcitation was required to initiate charge transfer.42 The O2 electron-accepting level is located at −0.66 V versus SHE in aprotic, nonaqueous solvents like DMF.42,74 This potential resides within the Au25 HOMO−LUMO gap, and O2 cannot spontaneously remove electrons from the ground-state Au25 HOMO. However, once an electron was photoexcited above the cluster LUMO, it could relax into the O2 acceptor level and the cluster became oxidized. Notably, cluster oxidation did not occur in the absence of photoexcitation. The electrochemical potentials of the Au25−xAgx HOMO and LUMO suggest it should also participate in photomediated charge transfer with O2. Figure 4b presents the absorbance and PL spectra of an initially N2-purged Au25−xAgx solution that was subsequently saturated with O2 in the presence of ambient room light. Au25−xAgx showed oxidation-like spectral changes only after solutions were bubbled with O2 exposure in the presence of ambient room light. Specifically, O2 exposure conducted in ambient room light bleached the a′ transition, blue-shifted the a transition, condensed the b′ and b transitions into a single peak, and decreased the PL intensity by 84% ± 1%. These changes were reproducible from sample to sample (Figure S6, Supporting Information). Purging O2 from solution did not reverse the absorbance spectrum changes; however, it did restore the PL intensity by approximately 15% (Figure S7, Supporting Information). These observations indicate that Au25−xAgx remained oxidized after O2 was purged from solution and that some portion of the PL quenching was reversible. Substantially smaller spectral changes were observed after lightfree O2 exposure (Figure S8, Supporting Information), confirming that the phenomenon was photomediated. The O2 exposure experiments further strengthen our hypothesis that photomediated charge transfer is a general chemical process available to clusters with appropriate HOMO−LUMO energy levels.42 Finally, the observed oxidative PL quenching highlights the way in which heteroatom substitution can influence the photophysical properties of small mixed-metal clusters.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data and analysis, eight figures, and three tables as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: Douglas.Kauff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. J. Baltrus (NETL) for access to XPS instrumentation. R.J. acknowledgments financial support by the Air Force Office of Scientific Research under AFOSR Award FA9550-11-1-9999 (FA9550-11-1-0147) and the Camille Dreyfus Teacher-Scholar Awards Program. As part of the National Energy Technology Laboratory’s Regional University Alliance (NETL-RUA), a collaborative initiative of the NETL, this technical effort was performed under RES Contract DE-FE0004000. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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4. CONCLUSIONS We have provided experimental and computational characterization of the mixed-metal Au25−xAgx cluster. Our computa-

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