Hot Hole Photoelectrochemistry on Au@SiO2@Au Nanoparticles

Letter pubs.acs.org/JPCL

Hot Hole Photoelectrochemistry on Au@SiO2@Au Nanoparticles Andrea E. Schlather,†,‡ Alejandro Manjavacas,§ Adam Lauchner,‡,∥ Valeria S. Marangoni,‡,⊥ Christopher J. DeSantis,‡,∥ Peter Nordlander,‡,∥,# and Naomi J. Halas*,†,‡,∥,# †

Department of Chemistry, Rice University, Houston, Texas 77005, United States Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States § Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131, United States ∥ Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States ⊥ Nanomedicine and Nanotoxicology Group, Physics Institute of Sao Carlos, University of Sao Paulo, San Carlos, BR-13560970, Brazil # Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States ‡

S Supporting Information *

ABSTRACT: There is currently a worldwide need to develop efficient photocatalytic materials that can reduce the highenergy cost of common industrial chemical processes. One possible solution focuses on metallic nanoparticles (NPs) that can act as efficient absorbers of light due to their surface plasmon resonance. Recent work indicates that small NPs, when photoexcited, may allow for efficient electron or hole transfer necessary for photocatalysis. Here we investigate the mechanisms behind hot hole carrier dynamics by studying the photodriven oxidation of citrate ions on Au@SiO2@Au core− shell NPs. We find that charge transfer to adsorbed molecules is most efficient at higher photon energies but still present with lower plasmon energy. On the basis of these experimental results, we develop a simple theoretical model for the probability of hot carrier−adsorbate interactions across the NP surface. These results provide a foundation for understanding charge transfer in plasmonic photocatalytic materials, which could allow for further design and optimization of photocatalytic processes.

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This effect may be enhanced when the holes are generated at the d bands of noble metals due to their large electronic density of states.10 Plasmon-induced hot carrier generation is the result of plasmon decay. The initial carrier distribution, that is, the transient energetic state prior to thermal relaxation by electron−electron collisions, depends on the size and shape of the nanostructure.17 While hot carrier lifetimes of metal crystals and films are extremely short, there is mounting evidence that hot carrier lifetimes may increase with decreasing metal NP size18,19 and that hot carriers produced through plasmon decay may have longer lifetimes at electrochemical interfaces20,21 that could be exploited to drive chemical reactions. Nanomatryoshkas (NMs) are of particular interest from this perspective because they may possess plasmon resonances at wavelengths similar to larger spherical solid NPs and nanoshells, while also maintaining the high absorption-toscattering ratio characteristic of small NPs.22 Additionally, the

he demand for environmentally friendly chemistry has led to intense study of catalytic reactions on photoexcited metals, where sunlight could ultimately drive important reactions. Metallic nanoparticles (NPs) are gaining rapid interest as efficient and recyclable platforms for light-driven chemistry.1−5 Additionally, NPs offer tunable light absorption and scattering properties throughout the visible and infrared through manipulation of their geometrical features, such as size and shape. Plasmonic photocatalysis occurs through separation of hot electrons and holes that transfer to adsorbate molecules, reducing the barrier to bond dissociation.6−13 The dynamics of these charge transfer processes is an active field of study due to its importance in optimizing the photocatalytic and photovoltaic properties of metal NP systems.14,15 Previously, we probed the mechanism of hot carrier-induced chemistry such as room-temperature hydrogen dissociation on Au and Al NPs.2,16 Similar results were found for photocatalytic oxidation of ethylene by elemental oxygen on plasmonic Ag NPs.4 In these cases, hot electron transfer to the antibonding orbitals of adsorbate molecules had been shown to effectively lower the activation barrier of a reaction that would otherwise be energetically unfavorable. Additionally, hot holes can behave as strong oxidants, driving the transfer of electrons from the HOMO levels of adsorbate molecules to the metal substrate. © 2017 American Chemical Society

Received: March 7, 2017 Accepted: April 20, 2017 Published: April 20, 2017 2060

DOI: 10.1021/acs.jpclett.7b00563 J. Phys. Chem. Lett. 2017, 8, 2060−2067

Letter

The Journal of Physical Chemistry Letters

observations, we observe a large increase in photovoltage with increasing photon energy,27 corresponding to the onset of d band transitions in Au; this is likely due to greater energetic overlap between the citrate ion HOMO level and d band holes generated far below the Fermi level in Au. Importantly, we observe that the photocharging rate of AuNMs resulting from hot hole photo-oxidation of surface citrate ions depends on the properties of the plasmon resonance. We show that the measured photo-oxidation rate is proportional to the energy overlap between the local optical absorption spectrum at the AuNM surface and the energy of the HOMO level of the adsorbed citrate. Electrodes were fabricated by a two-step process of dropcasting onto polyvinylpyrrolidone (PVP)-coated ITO substrates and drying in an ethanol environment (Suppporting Information Figure S3). The native ITO surface is hydrophobic and has a high liquid contact angle, resulting in NP aggregation during evaporation (Figure 2a). Treatment with O2 plasma hydroxylates the surfaces, drastically lowering the contact angle,32 but this produces very low particle density (Figure 2b). Proper modification of the ITO surface to optimize the AuNM distribution was achieved by minimizing exposure to hydroxylating agents and treating the substrate overnight with PVP in EtOH (Figure 2c). By slightly lowering the contact angle, surface wetting was controlled and resulted in a reproducible drop-casted NP spot. Evaporation in an ethanol environment promoted even distribution of particles through suppression of the surface tension forces of water during droplet drying.33 A SEM image of a representative AuNM electrode with a particle density of 1.1 × 109 particles/cm2 (Figure 1a) shows minimal particle aggregation and an average particle separation greater than 100 nm, sufficient to prevent noticeable plasmonic coupling.34 AuNMs with average dimensions of [r1,r2,r3] = [25.5 ± 3.0, 38.4 ± 1.3, 53.0 ± 3.3] nm (see Figure 1b), as measured directly from TEM images using 50−100 particles per average, indicates good monodispersity in size. Extinction spectrum measurements of AuNMs, both in solution and dried on the substrate, reveal two extinction maxima due to hybridization between the Au core and Au shell plasmons (Figure 1c).22,35 This feature only occurs once the Au shell is grown on the silica-coated Au NP seeds (Supporting Information Figure S1). The high-energy and low-energy extinction peaks in the AuNM spectrum are attributed to antibonding (ABD) and bonding (BD) dipolar plasmon modes, located at 550 and 780 nm, respectively.22,31,36 Theoretical extinction spectra were calculated using Mie theory,37 with the dielectric functions for gold and silica taken from Johnson and Christy38 and Palik,39 respectively. To account for the ∼2.25 nm size distribution of our particle ensemble, we averaged 27 separate theoretical extinction spectra for variations of the nominal AuNM dimensions. This procedure results in a theoretical extinction spectrum in good agreement with the ensemble extinction measurement in water (Supporting Information Figure S2). To investigate the hot carrier generation and extraction, we inserted the AuNM electrode in a photoelectrochemical cell containing a solution of 0.1 M potassium nitrate electrolyte and 0.5 mM sodium citrate, which acts as a sacrificial electron donor in the photocharging process.25 Sodium citrate was chosen because it is a widely used reagent in colloidal synthesis of noble metals such as Au and Ag, first acting as a weak reducing agent to form NPs in solution from metal ions and then as an

production of hot electrons can be spectrally adjusted in such hybridized NP systems by tuning the plasmon resonance.9,23 The remarkable tunability of NMs permits rational synthesis of a NP with features that are particularly desirable for this investigation. First, multiple plasmon resonances in the visible regime allow for a more general characterization of plasmonic resonances while utilizing only a single NP system. Second, they possess significant absorption very near to the surface of the NP, facilitating charge transfer across the interface. We note that carrier relaxation rates were shown to increase with the strength of electronic coupling between surface states and adsorbate molecules, suggesting that electron scattering at the metal−molecule interface can be tuned with surface ligand binding strength.24 In this Letter, we investigate the optical efficiencies of photogenerated hot holes in a multilayered Au@SiO2@Au plasmonic nanomatryoshka (AuNM), using the sacrificial photo-oxidation of citrate ions on a AuNM-decorated electrode in an open-circuit (OC) electrochemical cell (see Figure 1a−c).

Figure 1. Gold−silica−gold nanomatryoshka (AuNM) electrode characterization. (a) SEM micrograph of the AuNMs on an ITO electrode and (inset) single AuNM particle. (Scale bar: 500 nm; inset: 50 nm) (b) Schematic of AuNM depicting radii definitions used within this Letter. (c) Calculated averaged extinction spectrum from Mie theory (black line) and experimental extinction spectra of the AuNMs before deposition in aqueous solution (blue dash) and after deposition on ITO-coated glass in electrolyte/citrate solution (red dots).

Citrate oxidation on plasmonic nanocrystals through a photoKolbe reaction has been well-characterized in both Ag and Au NPs.25−27 It has been shown that electron transfer from surface citrate ions to photoexcited hot holes can occur in parallel with hot carrier relaxation in metal NPs.28 We used a sample of AuNMs, which exhibits two distinct plasmon resonances, each with roughly the same scattering and absorption ratios, to provide a novel quantification standard for this investigation. AuNMs permit a broad range of tunability with large absorption efficiencies29,30 and strong field enhancements arising from hybridization between the inner Au core and outer Au shell resonances.22,31 Consistent with other recent 2061


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Figure 2. Demonstrating the effect of ITO surface functionalization on the liquid contact angle and NM deposition. (a) An unhydroxylated metal oxide film will have a liquid contact angle of around 90° and is considered hydrophobic. A drop of 1010 NM/mL solution evaporated in an EtOH environment will result in a dense monolayer of aggregated NMs. (b) A metal oxide film that is hydrophilic will exhibit a much smaller contact angle. The 1010 NM/mL drop will wet the entire ITO surface and lead to a sparse coverage of AuNMs. (c) Controlled lowering of the contact angle can be accomplished through surface functionalization by a polar, aprotic polymer, such as PVP. Scale bars: 5 mm (black) and 500 nm (white).

ionic surfactant. As a surfactant, both steric and electrostatic repulsive forces of citrate adsorbates on NP surfaces provide excellent charge stability in aqueous solutions40 due to the formation of a negative double layer at the NP−solution interface. The double layer remains stable until a sufficient cathodic photovoltage (i.e., negative electrode polarization) is reached (VOC = −100 mV in the case of a Ag NP electrode25), resulting in surface charge repulsion of the negatively charged citrate anions inside of the double layer.41 OC measurements were obtained after allowing the electrodes to equilibrate in the citrate−electrolyte solution overnight before laser illumination; a typical equilibrium rest potential VOC for the NM electrode in citrate−electrolyte solution ranged between VOC = 90 and 150 mV vs Ag/AgCl. AuNM plasmons decay nonradiatively into hot electron− hole pairs. Holes with an energy greater than the oxidation potential of citrate may be injected to adsorbed citrate ions at the NM surface, yielding 1,3-acetonedicarboxylate via a photoKolbe reaction.25−27,42 The photogenerated hot electrons accumulate as excess charge at the interfacial double layer and increase the Fermi level of the AuNM electrode, resulting in a decrease of the OC potential.43,44 In our experiment, laser irradiation at the AuNM peak extinction wavelength (550 nm) for 40 min resulted in a decrease in the OC photovoltage of −38 mV relative to VOC (Figure 3b). At this point, the electrode potential approached a constant value as the photooxidation of citrate ions was in equilibrium with the electron discharging through surface interactions. The photovoltage action spectrum (Figure 4a), plotted as the equilibrium photovoltage versus illumination wavelength, illustrates the efficiency of hole collection by the citrate at the AuNM as a function of photon energy. Consistent with other recent work for Au and Ag NPs, the photo-oxidation appears to coordinate only weakly with the measured extinction spectrum.26,27 The total far-field absorption calculated by Mie theory (Figure 4a) is roughly equal for both BD and ABD modes of the AuNM, indicating that the lower photovoltage measured near the BD is not strictly a function of absorption. Unlike previous studies, however, the presence of two distinct peaks in our photovoltage measurements shows a clear correlation with the plasmon energy resonances of the NP. To explain the observed photovoltage action spectrum, we developed a model for the hot carrier−adsorbate overlap

Figure 3. (a) Mechanism of photocharging in Au NPs. The schematic shows laser illumination of the NP electrode in an electrochemical cell at OC conditions, that is, net zero current flow. Nonradiative plasmon decay creates a hot electron−hole pair. Hot holes participate in photooxidation of surface citrate ions, while hot electrons collect on the NP surface, resulting in measurable photocharging. (b) OC potential vs time for a AuNM electrode before, during, and after 550 nm laser irradiation. The laser power density was 18 mW/cm2.

describing the probability of a photogenerated carrier to participate in the photocatalytic reaction by injection from the AuNM to the adsorbate. This model considers the spatial and energetic overlap between the local photon absorption, hot carrier energy distribution, and adsorbate HOMO energy distribution. First, the local absorption spectrum at 1 nm below the spherical NM surface is calculated using the boundary element method (BEM).45,46 A depth of 1 nm was chosen for the calculation as it is on the order of magnitude of the 2062


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Figure 4. Photovoltage action spectrum, calculated hot carrier−adsorbate overlap, and particle absorption maps. (a) The measured photovoltage (red squares) is plotted as a function of the excitation wavelength. The calculated far-field absorption of the AuNM ensemble is shown in black, and the calculated hot carrier−adsorbate overlap is shown in blue. (b) Schematic showing the theoretical model for the energy overlap between a photogenerated hot hole and the citrate HOMO level. εF is the Fermi level of the NM, EH is the energy of the photogenerated hot hole, and xℏω is the maximum hot carrier energy with respect to the Fermi level. Local absorption maps calculated at (c) 450, (d) 516, (e) 640, and (f) 768 nm, corresponding to the absorption peaks on the calculated hot carrier−adsorbate overlap spectrum, show the absorption cross section of a AuNM due to near-field enhancements and the imaginary component of the dielectric constant. Absorption maps were calculated assuming bottom illumination of the NP.

expected hot carrier mean free path in this system;17,47 thus, absorption deeper than 1 nm has a decreasing probability of yielding carriers of sufficient energy for injection into a citrate molecule. The local absorption was calculated for 27 different AuNM radii and uniformly averaged to reflect the experimental size distribution, in the same manner as our reported theoretical extinction spectra that was calculated using Mie theory. Second, the efficiency of the hot holes to induce citrate oxidation was calculated as the overlap between the hot hole energy distribution and the HOMO level of the citrate.48 We modeled the hole energy distribution as a triangular function that is equal to 1 at the Fermi level and becomes 0 at a distance xℏω from the Fermi level, as shown in Figure 4b, with ℏω being the light energy and x being a fit parameter. This simple approximation for the initial carrier energy distribution captures the key features of the expected hot carrier (electrons and holes) distribution in a finite system as derived from recent theoretical models,23 which are higher near the Fermi level and decrease to zero away from the Fermi level. The fit parameter will impact the relative degree of the plasmonic effect compared to the d band transitions, but the features of the expected local absorption do not change drastically (see Supporting Information Figure S5-2); similar adjustments could be expected from other models for the carrier distribution.49,50 We modeled the HOMO level of the citrate as a Gaussian function,48 centered at EH = 1 eV below the Fermi level. This value was inferred from measurement of the citrate oxidation potential (Supporting Information, section S4) with respect to the OC potential (see Supporting Information, sections S4 and

S6 for methodology). Finally, the oxidation efficiency was multiplied by local absorption to obtain the theoretical hot carrier−adsorbate overlap. The hot carrier−adsorbate overlap spectrum reveals that the strong BD resonance observed in both the extinction spectrum (∼750 nm) and corresponding absorption maps (Figure 4f; see Supporting Information section S5 for methods) is significantly diminished due to a lower density of hot holes near the ∼1 eV citrate HOMO level. The ABD resonance lies at an energy only ∼1.4× greater than that of the BD and with roughly equal absorption probability but produces a 3-fold improvement in the hot carrier−adsorbate interaction. At wavelengths below 525 nm, interband transitions from filled d band states to partially filled sp bands damp the plasmon resonance and dominate the absorption at the NM surface.51 This mechanism is corroborated by the uniform absorption (i.e., absence of a polar charge distribution) across the outer shell in the absorption map (Figure 4c,d). The strong absorption of the d band is under-represented in the extinction spectrum due to the strong scattering intensity at wavelengths near the plasmon resonances and the strong absorption in the center gold NP (Figure 4d); this further contributes to the observed difference between the extinction and the photovoltage. Hot holes generated from d band transitions have the added benefit of being located 2.3 eV below the Fermi level,51 while plasmonic hot holes have a higher probability of being generated closer to the Fermi level.9 Our model for the hot carrier distribution results in an injection probability that is in good agreement with the experimental results. 2063


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Figure 5. Energy diagrams depicting possible hot hole- mediated electron transfer mechanisms by citrate oxidation. (a) A 2.36 eV excitation results in the formation of a hot hole in the Au d bands. Electron transfer from the citrate ion HOMO proceeds exergonically. (b) A 1.65 eV photon results in surface plasmon decay, which may produce a hot electron, but the resulting cold hole remains near the Fermi level misaligned with the HOMO level. (c) A warmer hole generated via plasmon decay from a 1.65 eV excitation may overlap better with the citrate HOMO level for more efficient hole transfer.

dependent on the electrochemical properties of the analyte; for example, if the oxidation energy of the analyte was shifted closer to the Fermi level of the metal, we would see an increased response in that spectral region. Second, as seen in other systems,16 the d band can be very efficient for holeinjection reactions because the large density of electrons compensates for the lesser degree of field enhancement in the d band regime for a plasmonic particle. Finally, plasmon resonances with near-surface absorption at high energies will produce highly energetic carriers that should be capable of driving reactions more efficiently than a larger number of charge carriers generated at lower energies or further away from the surface. Plasmon-induced hot hole transfer into adsorbed molecules depends on the relative energies of the hole, the HOMO level of the molecule, and the Fermi level of the metal. Here, we study hole transfer-induced photo-oxidation of citrate ions adsorbed on plasmonic AuNMs on an electrode in an electrochemical solution. The photocharging rates, as measured through the OC voltage, are shown to be strongly wavelength dependent with local maxima at the two plasmon resonances of the NP. We develop a simple and intuitive model relating the plasmonic light absorption to the photo-oxidation rate. This model, which is based on the Fermi energy of the metal, the plasmon energy of the NP, and the energy of the HOMO level, provides excellent agreement with the experimental results and can easily be applied to other NP−adsorbate systems. Because of the remarkable light-focusing properties of plasmonic nanostructures, hot-carrier-driven photochemical reactions are a promising means of converting light energy to chemical energy. The high surface-to-volume ratio and double-layer capacitances of NPs in solution make them ideal candidates for electron transfer-driven chemistry.

Our study presents a hybrid experimental−theoretical framework for probing NP−molecule photoreactions, which we use to explain several photoreaction regimes within the AuNM−citrate system (Figure 5). Electron transfer from the HOMO level of the surface citrate ions to hot holes located in the d band of the AuNM gold shell occurs due to the exergonic relationship of the citrate electrons and hot holes (Figure 5a). Even as the Fermi level of the AuNM electrode increases due to the buildup of photovoltage, the transition remains energetically favorable because direct excitation of the electron-rich d band produces a significant number of highly energetic holes concentrated near the edge of the d band. These results are not consistent with holes created by plasmon decay, which can be located throughout the sp band, depending on the incident photon energy. A plasmon-induced hot electron leaves a cold hole near the Fermi level with insufficient energy for transferring into the citrate HOMO (Figure 5b). Warmer holes created deeper below the Fermi level have larger overlap with the citrate HOMO and can participate in HOMO electron transfer (Figure 5c). While the generation of maximally energetic hot electrons (Figure 5b) is favored for experimental systems that use high-energy hot electrons, it does not produce holes with sufficient oxidizing power. Warm holes (Figure 5c) produced within the gold sp band are a good match to the citrate HOMO level, resulting in the improved photo-oxidation rate observed near 1.65 eV; yet, this is limited, in comparison to the 2.36 eV excitation, due to the lower generation probability reflected in our triangular energy distribution model. We can use this framework to provide insights into the design of efficient charge transfer reaction involving metal NPs. First, we highlight the importance of the NP system being tailored to that of the analyte to achieve the highest photochemical efficiency. Charge transfer properties from a NP to analyte will change with the analyte and are thus equally 2064


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reduction of Au3+ from a plating solution onto the Au-seeded NPs, where the seeds act as nucleation sites for Au growth. The plating solution was made 9−16 h before fabrication by combining 200 mL of ultrapure water, 50 mg of anhydrous potassium carbonate, and 3 mL of a 1% w/v HAuCl4 solution into a 500 mL amber bottle and shaking vigorously. In a 4.5 mL methacrylate cuvette, 1.5 mL of plating solution was combined with 10−20 μL of the NM or NS seed particle solution. Next, 7.5 μL of 37% formaldehyde solution was placed in the cuvette cap, and once closed, the cuvette was shaken vigorously for 1 min, followed by 3 min of rest to allow for the shell growth to complete. A visible color change of the solution from red to purple was witnessed due to formation of the outer shell. The extinction spectrum of the AuNMs was measured using a UV− vis−NIR spectrophotometer (Varian Cary 5000, Agilent Technologies). This process was repeated with varying seed particle solution volumes until reaching the desired plasmon resonance extinction maxima. The completed NM solutions were diluted to 5 mL and centrifuged in 50 mL tubes for 20 min at 700 rcf to remove small colloids that formed in the shell reduction. The washed AuNMs were redispersed in 250 μL of ultrapure water to a final concentration of 3.4 × 1010 particles/ mL. AuNM imaging and sizing were done using a JEOL 1230 high contrast transmission electron microscope (TEM). Fabrication of Nanomatryoshka Electrodes. ITO-coated glass slides (Delta Technologies, 15−25 Ω/sq) were cut to a size of 1 cm × 3 cm and cleaned by stepwise sonication in acetone and isopropyl alcohol for 5 min each. The slides were dried with a stream of N2 and submerged in a 2% w/v ethanolic solution of PVP for 24 h. This was followed by thorough rinsing with ethanol and drying with N2. Then, 50 μL of a solution of AuNMs (3.4 × 1010 particles/mL, as described above) was drop-casted onto the slides, placed in an airtight canister with a beaker containing 1 mL of ethanol, and dried overnight. After drying, the NM electrodes were cleaned in a 40 W oxygen plasma for 2 min to remove organic contaminants from the NM surface and excess polymer from the electrode. The area of the NM spot on the 1 × 3 cm2 ITO surface was ∼0.70−0.75 cm2. The electrodes were imaged using an FEI Quanta 650 scanning electron microscope (SEM). The extinction spectra of the AuNM electrodes in electrolyte solution (see below) were recorded using a liquid light guide attached to a white light source and an Ocean Optics fiber-coupled spectrometer. Photoelectrochemical Measurements. Photoelectrochemical and dark electrochemical measurements used a three-electrode cell configuration. The AuNM electrode was attached to a silver wire using copper tape and functioned as the working electrode, a Pt mesh soldered to a silver wire (100 mesh, 99.9% trace metals basis, Sigma-Aldrich) served as the counter electrode, and a Ag/AgCl electrode in 3.5 M potassium chloride (KCl) isolated by a fritted glass tube (Pine Instruments) served as the reference electrode. The cell was a 4 cm × 1 cm fused quartz cuvette (Starna Cells), and the electrolyte/analyte solution was 0.1 M potassium nitrate (KNO3) with 5 mM sodium citrate in ultrapure water. All measurements were conducted at room temperature (21 °C) and without stirring. Electrochemical experiments were conducted with and without prior purging with Ar gas, without any noticeable difference in results. OC, voltammetric, and chronoamperometric experiments were carried out using a PARSTAT 4000 potentiostat (Princeton Applied Instruments). Wavelength-selective laser excitation was accomplished with a Fianium supercontinuum laser source equipped with a

METHODS Reagents. A 50 nm Au NanoXact gold colloid (7.0 × 1010 particles/mL, citrate-capped) was purchased from Nanocomposix. Ethanol (200 proof) was purchased from Koptec. 3-Aminopropyl-triethoxysilane (APTES) and tetraethoxysilane (TEOS) were purchased from Gelest. Potassium carbonate (anhydrous) was purchased from Fischer Scientific. Ammonium hydroxide solution (28% in H2O), formaldehyde (37% in H2O), gold(III) chloride hydrate (99.999% trace metals basis), PVP (avg. mol. wt. 40 000), trisodium citrate dihydrate (≥99.0%), and potassium nitrate (≥99.0%) were purchased from Sigma-Aldrich. All chemicals were used without further purification. Synthesis of Nanomatryoshka Seeds. The synthesis of multilayered AuNMs was completed over a period of 4−5 consecutive days. First, Au colloid was coated with APTESdoped silica using a modified Stöber process. A 250 mL Erlenmeyer flask with a ground glass joint and a large stir bar was cleaned by soaking in aqua regia (3:1 HNO3/HCl) (Caution! Strong oxidizer) and subsequently rinsed with ultrapure Millipore water at least 20 times. Finally, the flask and stir bar were rinsed with ethanol and dried in an oven at 100 °C. The flask was removed and allowed to cool to room temperature, after which 21 mL of 50 nm Au colloid was added under medium stirring, followed by slow pouring of 180 mL of 200 proof ethanol and swift injection of 1.8 mL of ammonium hydroxide. Next, 36 μL of f resh 10% solutions of APTES and TEOS, both in ethanol, were added. The flask was sealed, and the solution was stirred at room temperature for 65 min, followed by stirring at 4 °C for 24 h. The solution was then transferred to a dialysis membrane (Spectra/POR6, MWCO = 10 000), which was prepared by soaking for 1 h in ultrapure water. This was followed by several rinses with ultrapure water to remove surfactant chemicals and a final ethanol rinse to remove water from the membrane. The NP solution was dialyzed in 1 gallon of ethanol for 16 h under medium stirring to remove free silanes. Next, the NP solution was transferred to an airtight jar (cleaned using the same protocol as the flask) and sat undisturbed for 3 h at 4 °C to let aggregated NPs and free silica settle to the bottom. Using a pipet, 15 mL aliquots of silica-coated NPs were removed from the top of the solution and centrifuged in 50 mL polypropylene tubes for 30 min at 1500 rcf. All but ∼300 μL of supernatant was then pipetted from the tubes, and the pellet was redispersed in the remaining liquid by sonication and vortexing. The volumes of the tubes were combined and added under sonication to 40 mL of Duff colloid (aged for 1−2 weeks; protocol found elsewhere52) preceded by 600 μL of 1 M NaCl. The solution was then vortexed and sonicated for an additional 20 min. The NP cores + Duff colloid solution was left in a dark drawer for 1.5 days so that the APTES-doped silica was simultaneously etched and functionalized by the 1−2 nm Duff colloid to serve as seeds for the final Au shell. The seeded NPs were sonicated for 10 min and then separated into 10 mL aliquots and centrifuged for 25 min at 900 rcf. The supernatant was removed and immediately centrifuged again. Each pellet was redispersed in 1 mL of ultrapure water, sonicated for 5 min, and transferred to a 2 mL centrifuge tube. After a final centrifugation for 20 min at 800 rcf, the pellets were redispersed in ultrapure water to a final volume of 500 μL. Fabrication of Au Nanomatryoshkas f rom Seeded Precursors. Growth of the final Au shell for AuNMs occurs through 2065


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grating; the bandwidth of each illumination band was set to 25 nm. The beam was expanded 4× using spherical plano-convex lenses and had a measured area of 0.636 cm2. The output laser power was measured using a low-power thermal power sensor (Thor Laboratories). The photon flux for each illumination band was normalized to 1 × 1016 photons/s using a neutral density gradient filter wheel.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00563. Optical characterization of nanomatroyshka synthesis, Mie theory modeling of nanomatroyshkas (single particle vs ensemble), AuNM deposition on ITO, electrochemical characterization of redox reactions on a AuNM electrode, hot carrier−adsorbate overlap calculation details, and dependence of the AuNM electrode open-circuit photovoltage and closed-circuit photocurrent on laser power (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alejandro Manjavacas: 0000-0002-2379-1242 Peter Nordlander: 0000-0002-1633-2937 Naomi J. Halas: 0000-0002-8461-8494 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Air Force Office of Scientific Research Multidisciplinary Research Program of the University Research Initiative (AFOSR MURI FA9550-15-1-0022) and the Welch Foundation under Grants C-1220 (N.J.H.) and C1222 (P.N.). A.E.S. acknowledges financial support from the Welch foundation through the Welch Pre-Doctoral fellowship. A.M. acknowledges financial support from the Department of Physics and Astronomy and the College of Arts and Sciences of the University of New Mexico and the UNM Center for Advanced Research Computing for computational resources used in this work. A.E.S. wishes to thank Bob Zheng, Amanda Goodman, Linan Zhou, Chao Zhang, Nate Hogan, and Nicholas King for helpful discussions.

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REFERENCES

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on April 25, 2017. Figure S2 of the Supporting Information was updated. The revised paper was reposted on April 26, 2017.

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Hot Hole Photoelectrochemistry on Au@SiO2@Au Nanoparticles

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