pH-Induced Surface Modification of Atomically Precise Silver

Article pubs.acs.org/IC

pH-Induced Surface Modification of Atomically Precise Silver Nanoclusters: An Approach for Tunable Optical and Electronic Properties Lina G. AbdulHalim,† Zahra Hooshmand,‡ Manas R. Parida,† Shawkat M. Aly,† Duy Le,‡ Xin Zhang,§ Talat S Rahman,‡ Matthew Pelton,∥,⊥ Yaroslav Losovyj,# Peter A. Dowben,§ Osman M. Bakr,† Omar F. Mohammed,*,† and Khabiboulakh Katsiev*,† †

King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division, Solar and Photovoltaics Engineering Research Center (SPERC), Thuwal 23955-6900, Saudi Arabia ‡ Department of Physics, University of Central Florida, Orlando, Florida 32816, United States § Department of Physics and Astronomy, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States ∥ Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, United States ⊥ Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States # Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *

ABSTRACT: Noble metal nanoclusters (NCs) play a pivotal role in bridging the gap between molecules and quantum dots. Fundamental understanding of the evolution of the structural, optical, and electronic properties of these materials in various environments is of paramount importance for many applications. Using state-of-the-art spectroscopy, we provide the first decisive experimental evidence that the structural, electronic, and optical properties of Ag44(MNBA)30 NCs can now be tailored by controlling the chemical environment. Infrared and photoelectron spectroscopies clearly indicate that there is a dimerization between two adjacent ligands capping the NCs that takes place upon lowering the pH from 13 to 7.

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INTRODUCTION Surface engineering is a unique approach for generating stable, multifunctional, and highly ordered nanostructured materials. Ligand modification lies at the forefront of postsynthetic surface engineering tools for rendering nanoscale materials soluble in a desired solvent and ultimately suitable for particular applications.1−5 For instance, to improve transport properties of films of quantum dots, capping ligands are exchanged with shorter and conducting ones.6−9 Another approach employs interfacing nanoscale materials with functional systems such as proteins,10 porphyrins11 or cyclodextrins12 via supramolecular chemistry. On the other hand, atomically precise noble metal nanoclusters (NCs), as a new class of nanoparticles that consist of a few tens of atoms (of order 2 nm in size) capped by organic ligands, have also been engineered.13−15 These compounds possess unique physicochemical properties, intriguing geometrical structures, and quantized charging behaviors.16−19 However, microscopic interactions governing the detailed structure, dynamics, and orderliness of this class of materials need to be explored in order to deploy them in functional devices.20,21 © XXXX American Chemical Society

Similar to the case of semiconductors, quantum size effects, so-called quantum confinement,22,23 have been proposed as a major influence on the optical and electronic properties of metal NCs.24 The quantum size effect view of the electronic structure of NCs is oversimplified because it disregards the impact of nanocluster surface and ligands engineering. The latter is a very effective approach for tailoring the optical and electronic properties of NCs.25−31 At such small length scale (less than 2 nm), nearly 30−70% of the atoms of a nanocluster reside on the surface. Thus, the chemistry of organic ligands, the surface structure of the NCs, and their combined effect on the electronic properties are significant and of great importance for the applications based on these clusters. Theoretical work and various spectroscopic and microscopic studies on selfassembled monolayers-covered planar Au and Ag single crystal surfaces indicate that the alignment of the molecular states relative to the metal Fermi level and the work function can be tuned by changing the docking chemistry.32−35 The choices of Received: August 26, 2016

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DOI: 10.1021/acs.inorgchem.6b02067 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

photoemission spectroscopy resolution was limited by an instrumental line width of approximately 400 meV. For XPS experiments we used a PHI Versa Probe II instrument equipped with a monochromatic Al K (alpha) source. The instrument base pressure was ca. 8 × 10−10 Torr. The X-ray power of 65 W at 15 kV was used for all experiments with 260 μm beam size at the X-ray incidence and take off angles of 45°.The instrument work function was calibrated to give a binding energy (BE) of 84.0 eV for the Au 4f7/2 line for metallic gold, and the spectrometer dispersion was adjusted to give a BE of 284.8 eV for the C 1s line of adventitious (aliphatic) carbon. The PHI double charge compensation system was used on all samples. The electron charge neutralizer settings were adjusted for each sample to give a BE of 284.8 eV for the C 1s line. The ultimate Versa Probe II instrumental resolution was determined to be 0.35 eV using the Fermi edge of the valence band for metallic silver. The resolution of the charge compensation system was 13).44 At such high pH levels, all carboxylate groups of the ligands are deprotonated, and the NCs are readily soluble in aqueous solutions. The red curve in Figure 1 shows the UV−vis absorption spectrum of deprotonated Ag44(MNBA)30 NCs.

For structural details see the Supporting Information.

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EXPERIMENTAL SECTION

Chemicals and Reagents. All chemicals, including 5,5′-dithiobis(2-nitrobenzoic acid) (DTNBA, 99%), silver nitrate (AgNO3, 99%), sodium hydroxide (NaOH), and sodium borohydride (NaBH4, 99.99% metals basis), were purchased from Sigma-Aldrich and used without further purification. Synthesis of Ag44(MNBA)30 NPs. To 5 mL of water, 53 mg of DTNBA was added, followed by a fresh solution of NaBH4 (50 mg, 5 mL of water). After around 15 min, an aqueous solution of AgNO3 (43 mg, 5 mL of water) was added to the DTNBA/NaBH4 solution and allowed to stir for around 1 h, during which the NPs were formed. The NPs were purified by repeated dispersion in 50% methanol in 1 M NaOH solution followed by repeated centrifugation at 9000 rpm for 10 min. Varying pH. The pH of the purified NCs was maintained at pH 13 as recorded with the pH meter. To protonate the NCs, a sample of Ag44(MNBA)30 was first concentrated by centrifugation at 5000 rpm for 30 min using Millipore Amicron centrifuge membrane filters. Then Milli-Q water at pH 5.8 was added to the filtrate, and the sample was centrifuged again. This process was repeated several times until the pH of the final solution was maintained at pH 7. Characterization. The UV−vis absorption spectra of silver NCs in solution were recorded using a Cary 5000 UV−vis−NIR spectrophotometer (Varian Inc.). The steady-state IR spectra of NCs were measured with a Cary 600 series FTIR spectrometer (Agilent technologies). The IPES spectra were obtained by using incident electrons with varying kinetic energy while detecting the emitted photons at a fixed energy (9.7 eV) using a Geiger−Müller detector. The inverse

Figure 1. UV−vis absorption spectra of Ag44(MNBA)30 NCs in aqueous solutions at different pH. The inset in the upper panel shows a structure of the capping ligands.

To protonate the ligands capping the nanocluster, a concentrated solution of NCs was diluted with Milli-Q water at pH 5.8 and was immediately filtered with Millipore Amicron centrifuge membrane filters centrifuged at 5000 rpm for 30 min. This process was repeated at least 5 times, where each time the filtered NCs were diluted with Milli-Q water, to ensure the protonation of the majority of the ligands. The pH of the final solution was 7, and the corresponding UV−vis spectrum is displayed in Figure 1 (black curve). Comparing the two spectra at pH 13 and 7, we observe substantial changes in the optical absorption: at lower pH, the peak at 392 nm is broadened whereby no clear transitions appear at 483 or 553 nm. Additionally, the peak at 652 nm is blue-shifted by 27 nm, and most importantly, the peak at 857 vanishes completely, turning B


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Inorganic Chemistry the solution from bold ruby red to faint yellow. Interestingly, the optical absorption and the color of the NCs are recovered (Figure S1) when the pH is raised again by adding aliquots of 1 M sodium hydroxide solution. It is worth pointing out that the mass spectra of the Ag44 NC at pH 13 have already been reported.44 As for the case of Ag44 at pH 7, we could not find conditions where the cluster ionized intact in either the ESI or MALDI. The band assignment in the UV−vis spectra of Ag44 NCs was previously reported in detail by Zheng and co-workers.40 Optical transitions present in the absorption spectrum were qualitatively reproduced by the means of time-dependent density functional theory, and the energy diagram of the molecular orbitals responsible for the transitions along with the contribution of each atomic orbital, the ligand, and metal bands were shown. On the other hand, a change in pH from 13 to 2 results in a single plasmonic peak at 450 nm due to degradation of the Ag44 NC. Consequently, the differences in the UV−vis spectrum between pH 13 and pH 7 can be explained by changes in the ligand (from fully deprotonated to fully protonated) of the intact Ag44 NC. Additionally, it is reported that when the ligands used to stabilize the NCs were hydrophobic, the NCs exhibited some photoluminescence. However, when water-soluble ligands were used, such as MNBA, no photoluminescence was detected.45 The aggregation may also quench photoluminescence, but TEM images were recorded for NCs which showed no aggregation, and the clusters are still monodisperse in nature, as shown in Figure S3. As is well established, the changes in the absorption spectra of the molecular−metal nanocluster complex could be related to changes in the underlying geometry of the metal cluster,46,47 initiated by changing ligand interactions, but changes in the ligand electronic structure cannot be ignored. In this case, we suspect that a structural change of the organic ligand is involved. We suggest that protonation may cause dimerization of the adjacent carboxyl groups. Simulations of such model systems were performed using density functional theory (DFT) employing the generalized gradient approximation (GGA) exchange-correlation functional parametrized by the Perdew− Burke−Ernzerhof48 (PBE) functional with the DFT-D3 correction49 for accounting for van de Waals interactions (See the Supporting Information for more details). On this basis, in the unsolvated case, the dimerization of the carboxylate moieties is the result of direct dimerazation between adjacent ligands, as depicted in Scheme 1. A similar phenomenon was also predicted theoretically by Gell et al. on Ag44 NCs using 4mercaptobenzoic acid as stabilizing ligand.42 They simulated the absorption spectra of the nanocluster at different pH values, and concluded that dimer interactions on the surface of the NCs were responsible for the observed changes in the optical spectra. Obviously when the clusters are deprotonated, the carboxylates cannot dimerize as in Scheme 1, and a different molecular configuration must be adopted. In the case of a solvated ligand−silver nanocluster, model DFT cluster calculations suggest that dimerization through hydrogen bonding via a water molecule is possible, as seen in Scheme 2, and indeed is energetically very favorable. Deprotonation of the carboxylate moiety, as would occur at high pH, will change the nature of the hydrogen bonding mediated by water, but also likely leads to different interactions of the sulfur with the silver cluster, as indicated in Scheme 2. As discussed in the following paragraphs, dimerization alters the

Scheme 2. Schematic of Two Dimerization Schemes for Partially Solvated Ag44(MNBA)30 (Corresponding to pH 7), Where the MNBA Ligands Are Protonated (a), and at pH 13, Where the MNBA Ligands Are Deprotonated (b), i.e. Corresponding to pH 13a

The configurations are stable with adsorption energies of −2.04 eV (a) and −1.83 eV (b), respectively. For structural details see the Supporting Information.

a

electronic structure of NCs, which in turn affects electronic transitions. Vibrational spectroscopy may indicate benzoic acid dimer formation,50,51 we used FTIR spectroscopy, with the particular focus on the CO stretching vibration, to probe any local interaction of the organic ligands including dimerization. The steady-state IR spectra in Figure 2 show that the CO stretch vibration for pure ligands is located at 1695 cm−1. For Ag44(MNBA)30 nanocluster powder, prepared from aqueous solution, this value is downshifted to 1583 cm−1, pointing to the formation of dimers upon protonation of the NCs; on the other hand, for those prepared from deuterated aqueous solution, the

Figure 2. FTIR spectra of (a) pure capping ligands in powder form (solid curve) and in MeOH (dotted curve), (b) solid NCs prepared from protonated deuterated water, and (c) solid NCs prepared from water. C


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Inorganic Chemistry CO band is upshifted by approximately 13 cm−1 compared to the one prepared from normal aqueous solution. This provides clear experimental evidence for the involvement of the hydrogen bonding interactions,52 possibly mediated by a water molecule, as suggested by our DFT calculations. The dimerization of Scheme 2 is also consistent with the changes of the C 1s core level peak structure, as measured by Xray photoelectron spectroscopy (XPS). When deprotonation occurs, the dimerization takes place between two adjacent ligands and can lead to significantly different sulfur interactions. Changing sulfur interactions, as occurs with protonation/ deprotonation, will lead to vastly different screening by the metallic Ag44 nanocluster core. Such a changing interaction will effectively decrease the electron kinetic energy, upon deprotonation, thus adding spectral weight to an “unscreened” C 1s core level feature at 288.3 eV (Figure S2).53,54 The electronic structure of a solvated ligand stabilized silver cluster will differ, however, from the supported cluster.55 The modification of the COOH group also results in significant changes to the electronic structure, as is expected. The complete electronic structure was studied by a combination of photoelectron spectroscopy (PES) and inverse photoelectron spectroscopy (IPES). The valence and conduction band (CB) structures are shown in Figure 3. The

ground state HOMO−LUMO gap because the optical excitation leaves a hole that, through Columbic interaction, reduces the excitation energy below the ground state gap.59 The small feature at the conduction band minimum, seen in Figure 3, is very similar to the surface state feature associated with clean silver surfaces, and is certainly consistent with the unoccupied frontier’s weighted state related to the coinage metals (Cu, Ag, Au).60 When Na in the COONa group is replaced by H, significant changes in the electronic structure are observed. The valence band is now comprised of two spectral features located at −2.5 eV and −5.6 eV, with the valence band maximum at −5.7 eV and a significantly sharper band edge. The features in the conduction band become less pronounced, shifted to higher energies (2.3 eV and 3.5 eV), and in some cases, they cannot be identified. There is a tendency for the band gap to widen by about 0.7 eV, and the band gap value is estimated to be about 2.0 eV. This is consistent with the optical measurements, suggesting that this is not simply a ligand effect, affecting only the surface of the NCs. The energy shifts here, in inverse photoemission, are about 0.5 eV and around 100−200 meV in optical spectroscopy, so that the energy shifts for the optical transition are less than the unoccupied molecular orbital shifts suggested by inverse photoemission. The greater the shift in inverse photoemission, the more surface sensitive the technique when compared to optical absorption, provide compelling evidence that the surface or ligand envelope has a significant contribution to the changes. The inverse photoemission feature at around 1 eV, with a stronger density of states just above the chemical potential in the spectra of the Ag nanocluster’s COONa sample, closely resembles the expected unoccupied 5s band of silver and suggests that this is a more “metallic” nanocluster at the higher pH than seen with COOH formation at the lower pH. This is consistent with a change in the sulfur to silver bonding configuration. Transient absorption measurements provide additional evidence for the modified electronic structure of the clusters, as Na+ is substituted with H. Our previous transient absorption measurements, on clusters with 4-fluorothiophenol, 2-naphthalenethiol, or 4-mercaptobenzoic acid, showed that excitation of the clusters with a laser pulse results in ultrafast charge separation followed by slow charge recombination.61 In these more recent samples, with solvated COONa moieties, the transient absorption spectra measured using ultrafast systems Helios (experimental details are described elsewhere)62 are very similar to those prior results,61 as shown in Figure 4. The transient absorption spectra show ground-state bleach features at 400 nm (pH 7 and pH 13) and 480 nm (pH 13) corresponding to peaks in the steady-state absorption spectra, and additional excited-state absorption bands, which appear at 450, 532, and 612 nm shown in Figure 4a−b. At pH 13, there is a ground bleach at 533 nm, but it is superimposed on the induced-absorption feature that extends from 500−625 nm. The data comparison at pH 7 and pH 13 makes this clear. The induced absorption is due to transitions between the excited state produced by the pump and higher-lying levels in the cluster. The isosbestic point between this peak and the ground state bleaches shows that the features all originate from the same states. The disappearance of the 450 nm band at pH 7 is understood in line with the changes brought to the ground state absorption getting broader at this pH value, which is manifested also in the ground-state bleach formed at this pH value. Thus, the ESA at 450 nm at pH 7 diminished because the

Figure 3. UPS/IPES spectra of Ag44(MNBA)30 nanocluster thin films deposited from solutions of different pH. All binding energies are denoted as E − EF, making occupied state binding energies negative.

valence band of the deprotonated Ag44(MNBA)30 nanocluster is comprised of three spectral features located at −2.2 eV, −4 eV, and −5.6 eV, with the valence band maximum at −5.5 eV. The conduction band consists of four spectral features centered around −1 eV, −2.6 eV, −4.5 eV, and −5.5 eV. The electronic band gap is estimated to be 1.35 eV, while the valence band maximum (VBM) positioned at −0.45 eV and the conduction band minimum (CBM) at 0.9 eV above the Fermi level, as plotted in Figure 3, and thus is in a good agreement with the optical measurements. A slightly larger band gap value with the combination of UPS/IPES is expected and is due to the fundamental differences between optical absorption and electronic spectroscopy. UPS and IPES are final state spectroscopies that, in the absence of full screening, lead to an apparent increase in the measured gap to values greater than the ground state HOMO− LUMO gap (depending on the final state screening),56 even in molecular systems.57−59 On the other hand, optical absorption measurements may result in a smaller value than the actual D


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Inorganic Chemistry

structure inferred from the linear absorption, PES, and IPES spectra. As stated above, substituting Na+ with H increases the energy level for separation in the cluster, increasing the difference in free energy between the initial and final states for charge separation, and leading to slower carrier dynamics, especially charge recombination. Both time constants vary exponentially with pH; this would be consistent with charge separation in the normal Marcus regime, if the free-energy differences for charge separation depend linearly on pH. Finally, the pH dependence of charge separation and recombination times may also be partially due to changes in the overlap between the initial and final electronic wave functions as the conformational state of the ligands is changed.

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CONCLUSION In summary, we report, for the first time, a new approach to tune both the optical and electronic properties of Ag44(MNBA)30 NCs, by simply controlling the pH of the ligand shell. The changes in the optical and electronic properties have been confirmed by a combination of steadystate and femtosecond time-resolved experiments. Interestingly, our results clearly demonstrate that a dimerization between two adjacent ligands capping the NCs takes place upon changing the pH from 13 to 7. We believe that the novel insights reported in this work provide a fundamental understanding of the key variables involved in silver NCs engineering, thus paving the way toward their exploitation for precise control of the electronic structure and photophysics of NCs in general.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 4. Femtosecond transient absorption spectra for Ag44 NCs at pH 7 (a) and at pH 13 (b). Femtosecond kinetic traces of Ag44 at excited-state absorption and at ground-state bleach wavelengths (c), using photoexcitation at 350 nm. All the solid red lines are fits of the kinetic traces.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02067.

TA signal is dominated by ground-state bleach. The strong adsorption at 400−410 nm is in surprisingly good agreement with the density of states at the top of the valence band seen in photoemission and the bottom of the conduction band seen in inverse photoemission, suggesting strong absorption would occur at about 3 eV. As can be seen, the ground-state bleach signals are partially recovered within 4 ns time delay while the excited-state absorption bands decay up to 90% due to the charge recombination. The decay at both the ground-state bleach signal at 405 nm and the excited-state absorption bands at 600 nm are described by two time constants as shown in Figure 4c: an ultrafast decay time around 1 ps, corresponding to charge separation, and a slow decay time of approximately 2 ns, corresponding to charge recombination. Both of these time constants are comparable to those previously observed for clusters in water.61 For the protonated sample, in contrast, Figure 4a shows that the transient spectra are dominated by a single bleach feature, at a wavelength corresponding to the single peak within the UV spectral region. It is observed that, within 5.44 ns time delay, the ground-state bleach signal is partially recovered and excitedstate absorption bands decay up to only 70%, corresponding to charge recombination, which is slower compared to the deprotonated cluster. The change in the carrier dynamics at different pH values is consistent with the change in electronic

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Absorption spectra, and XPS and DFT calculations (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by King Abdullah University of Science and Technology (KAUST), and part of this work was supported by Saudi Arabia Basic Industries Corporation (SABIC) grant RGC/3/2470-01.The work at U Nebraska was partly supported by the U. S. Department of Energy through grant #DE-FG02-07ER15842. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility under Contract No. DE-AC02-06CH11357. DFT calculations (ZH, DL, and TSR) were performed at the UCF Advanced Research Computing Center and partially supported by NSF grant CHE-1310327. We thank Sampyo Hong for fruitful discussions. E


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