Fabrication of Bimetallic Au–Pd–Au Nanobricks as ... - ACS Publications

Article Cite This: Chem. Mater. 2018, 30, 204−213

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Fabrication of Bimetallic Au−Pd−Au Nanobricks as an Archetype of Robust Nanoplasmonic Sensors Ka Chon Ng,†,○ Fan-Cheng Lin,‡,○ Po-Wei Yang,§,○ Yu-Chun Chuang,§ Chung-Kai Chang,§ Ai-Hsuan Yeh,† Chin-Sheng Kuo,† Chen-Rui Kao,† Chia-Chi Liu,⊥ U-Ser Jeng,§ Jer-Shing Huang,*,‡,∥,¶ and Chun-Hong Kuo*,†,□ †

Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan § National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ⊥ Interdisciplinary Program of Sciences, National Tsing Hua University, Hsinchu 30013, Taiwan ∥ Leibniz Institute of Photonic Technology, Albert-Einstein Strasse 9, Jena D-07745, Germany ¶ Research Center for Applied Science, Academia Sinica, Taipei 11529, Taiwan □ Institute of Materials Science and Engineering, National Central University, Jhongli 32001, Taiwan ‡

S Supporting Information *

ABSTRACT: Conventional gas sensors work upon changes in mechanical or conductive properties of sensing materials during a chemical process, which may limit availabilities of size miniaturization and design simplification. However, fabrication of miniaturized sensors with superior sensitivities in real-time and label-free probing of chemical reactions or catalytic processes remains highly challenging, in particular with regard to integration of materials into a desired smaller volume without losing the recyclability of sensing properties. Here, we demonstrate a unique bimetallic nanostructure, the Au−Pd− Au core−shell−frame nanobrick, as a promising archetype for fabrication of miniaturized sensors at nanoscale. Upon analysis of the aqueous synthesis, both ex situ and in situ, the formation of Au frames is consistent with selective deposition and aggregation of NaBH4-reduced Au nanoparticles at the corners and edges of cubic Pd shells, where the {100} surfaces, capped by iodide ions, are growth-limited. By virtue of the thin Pd shell (∼3.5 nm) sandwiched in-between the two Au layers of the core and the frame, the Au−Pd−Au nanobrick yields excellent optical sensitivity in hydrogen gas sensing, leading to a large 13 nm spectral shift of light scattering between Pd and PdHx. The composite nanostructure with a size of ∼60 nm offers an archetype for miniaturized sensors possessing label-free, real-time, and highresolution probing abilities and hence paves the way for fabrication of highly efficient nanosensors via sustainable methods.

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INTRODUCTION Conventional gas sensors exhibit signals based upon changed currents or resistances of sensing materials after exposure to analytes.1−3 The design of sensors used to require extra electric energy input so as to reach high sensitivity. In recent years, utilization of surface plasmon resonances (SPRs) of metals for optical sensing has received great attention because of simple, energy-saving operation.4−6 In the two types of SPRs, which include propagating surface plasmon polaritons (SPPs) and nonpropagating localized SPRs (LSPRs), the property of LSPRs has been exploited for surface-based optical sensing where adsorbate-induced refractive index changes near or on plasmonic nanostructures and are used to monitor binding events in real time.7 LSPRs are known for their intense and spatially nonhomogeneous oscillating electromagnetic fields which concentrate the energy of incoming photons in the small © 2017 American Chemical Society

volumes surrounding metallic nanocrystals. Hence, they create enhancement of the near field in the intermediate vicinity of the nanocrystal surfaces.8 A further elevated enhancement of the near field can occur in-between two neighboring metallic nanocrystals owing to the LSPR coupling.9 The enhancement is distance-dependent and decays exponentially with increasing spacing between the metallic nanocrystals.10 Importantly, the spectral LSPR absorption varies with the metallic nanocrystal’s size, shape, composition, and local dielectric environment.11,12 These unique properties render metallic nanocrystals efficient LSPR generators that can be applied to sensing, photocatalysis, and photovoltaics.13−16 Received: October 5, 2017 Revised: December 3, 2017 Published: December 8, 2017 204

DOI: 10.1021/acs.chemmater.7b04200 Chem. Mater. 2018, 30, 204−213

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Synthesis of Au Nanocubes. In the synthesis, a seed-mediated growth method was utilized. To prepare Au seeds in 2 nm, a solution of 0.02 M NaBH4 was first prepared in an ice bath for later use. Another 9.55 mL of solution containing 1 mmol of CTAC and 2.5 mmol of HAuCl4 was then prepared followed by adding 0.45 mL of 0.02 M NaBH4 with vigorous stirring at room temperature. The solution color quickly turned to brownish red from yellow after the addition of NaBH4, indicating the formation of Au seeds. The seed solution was stirred for 5 more minutes and left still on the bench for 30 min to deplete excess NaBH4. Meanwhile, two growth solutions were prepared in the glass vials labeled A and B. The vials were used to contain 98.45 mL of aqueous solutions dissolving 10 mmol of CTAC, 2.5 × 10−3 mmol of HAuCl4, and 10−3 mmol of NaBr. Afterward, the existing AuCl4− ions in both of the vials were reduced to AuCl2− form with the addition of 0.9 mL of 0.1 M AA. To perform the seedmediated growth of Au nanocubes, a two-step progressive procedure was carried out by transferring 0.65 mL of seed solution to the vial A with shaking for 10 s followed by adding 0.65 mL of solution from the vial A to the vial B after a pale pink solution color appeared. The final solution in the vial B was left undisturbed on the bench for 30 min after mild shaking for 10 s. At last, the products were collected, washed twice, and redispersed in 2 mL of water by centrifuging at 8000 rpm for 10 min (Eppendorf Centrifuge 5804). Synthesis of Pd Nanocubes. Typically, a 20 mL glass vial was taken to contain 9.3 mL of DI water followed by dissolving 1 mmol CTAB powder with sonication. Next, 0.5 mL of 0.01 M H2PdCl4 and 0.2 mL of 0.1 M NaI were added into the CTAB solution. After the color turned dark brown, the glass vial was heated in an oil bath at 90 °C for 5 min. Ascorbic acid (0.23 mL, 0.04 M) was then added to trigger reduction. The reaction was kept under mild stirring and lasted for 30 min. Finally, the Pd nanocubes were collected, washed, and redispersed in 2 mL of water by centrifuging at 11 000 rpm for 20 min (Eppendorf Centrifuge 5804). Synthesis of Au−Pd Core−Shell Nanocubes. The Au−Pd core−shell nanocubes were prepared by conformal overgrowth of Pd on Au nanocubes. Before the synthesis, 0.01 M H2PdCl4 was prepared by dissolving 0.5 mmol of PdCl2 in 50 mL of 0.02 M HCl with vigorous stirring at 60 °C. In the synthesis, a stock solution was prepared by mixing 7.7 mL of 14.5 mM CTAB, 1 mL of concentrated Au nanocube solution, 0.844 mL of 0.01 M H2PdCl4, and 0.01 mL of 0.01 M NaI. Next, 0.428 mL of 0.1 M AA was added with mild shaking and left undisturbed at 40 °C in a water bath for 12 h. The resulting products were collected, washed twice, and redispersed in 2 mL of water by centrifuging at 5500 rpm for 5 min (Thermo Scientific Heraeus Pico 17). Synthesis of Au−Pd−Au Core−Shell-Frame Nanobricks. The formation of core−shell-frame nanobricks was achieved through selective deposition and epitaxial growth of Au nanoparticles on the Au−Pd core−shell nanocubes. The Au nanoparticles were premade by adding 0.8 mL of 0.01 M ice-cold NaBH4 solution to a 10 mL solution comprising 0.1 M CTAB and 2.5 × 10−4 M HAuCl4 under vigorous stirring at room temperature. In the synthesis of Au−Pd−Au nanobricks, 0.05 mL of washed Au−Pd core−shell nanocubes, 0.35 mL of water, and 0.6 mL of Au nanoparticle solutions were mixed with shaking for 5 s. The solution remained undisturbed at ambient temperature for 4 h. Afterward, the resulting nanocrystals were collected, washed twice, and redispersed in 0.05 mL of water by centrifuging at 5000 rpm for 5 min (Thermo Scientific Heraeus Pico 17). Characterization. UV−vis absorption spectra were taken on a HITACHI U-3310 spectrophotometer. The X-ray diffraction experiments were performed at BL-01C2 in Taiwan National Synchrotron Radiation Research Center (NSRRC). The diffraction data were collected using 18 keV X-rays (0.68881 Å in wavelength) and Mar345 image plate detector with Debye−Scherrer geometry. The patterns were converted by GSAS-II program, and the angle calibration was performed according to LaB6 (SRM 660c) standard. The PXRD peak deconvolution was performed using the commercial OriginPro 9.0 program, and the fitting model was constructed based on some reference data and literature results. In Figure 3b, to fit the (111) peak

Among plasmonic metals, Au is generally utilized because of its strong LSPR absorption of visible light, mature synthesis for tuning the absorption wavelength and bandwidth, and chemical stability. However, the chemical inertness of Au nanocrystals results in poor optical sensitivity to nonbinding or flowing small molecules, especially at low concentrations. For this reason, intermediate sensors are required to induce changes in the dielectric environment around the Au LSPR generators. This method is known as indirect nanoplasmonic sensing (INPS) pioneered by Larsson et al. by which they successfully followed catalytic reactions of CO, O2, and H2 in real time.17−19 Active metallic nanocrystals like Pt and Pd are more often used for INPS due to their high surface reactivities. In 2011, Liu et al. found that triangular Au nanoplates with sharp corners (hot spots) act as nanoantennae with a large amplification of hydrogen sensing at the single-particle level when Pd nanospheres were placed nearby plate corners.20 The enhancement factor was negatively affected by increasing the distance between the Au corners and the Pd nanoparticles. This nanoantenna-enhanced sensing is a step toward the observation of single catalytic processes in nanoreactors and biosensing on the single-molecule level. Recently, Syrenova et al. investigated in detail the thermodynamics of hydride formation and analyzed the hysteresis in the metal-to-hydride phase transition with Au-enhanced Pd nanoparticles prepared in different sizes and shapes.21 They concluded that hydride formation enthalpy and entropy in the considered size range (18−63 nm) were nearly independent of particle size or shape. In addition, the results from comparison between ensemble and single-particle measurements clearly revealed that the typical slope on equilibrium plateau observed for Pd nanoparticle ensembles was not only due to irregular particle size but also affected by the specific defect structures of the individual particles such as dislocations to accommodate lattice strain. In this work, we successfully synthesized bimetallic triplelayered Au−Pd−Au nanobricks as the miniaturized yet highly sensitive nanosensors for low-concentration hydrogen gas detection. The triple-layered nanobrick forms in a core−shellframe structure where an ultrathin Pd shell for H2 uptake was sandwiched in-between two Au layers, the core and the frame. The Au frame was formed via selective deposition of tiny Au nanoparticles on the corners and the edges of the Pd shell, facilitated by facet-selective iodide coverage. By virtue of the sandwich structure, the single nanobrick exhibited, to our best knowledge, the largest spectral shift at 2% H2 concentration. Meanwhile, this superior sensitivity was reversible and recyclable, indicating the nanobrick is a stable platform for H2 uptake. The findings reveal that the core−shell-frame nanostructure is a useful archetype for metallic nanosensors and therefore opens a new door for fabrication of miniaturized optical sensors.

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EXPERIMENTAL SECTION

Chemicals. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4, 99.99%, Alfa Aesar), palladium(II) chloride (PdCl2, ≥99%, UniRegion Bio-Tech), hydrochloric acid (HCl, 37%, Merck), sodium bromide (NaBr, 99.5%, J. T. Baker), sodium iodide (NaI, ≥99%, SigmaAldrich), L-ascorbic acid (AA, >99.7%, Sigma-Aldrich), hexadecyltrimethylammonium bromide (CTAB, ≥98.0%, TCI), hexadecyltrimethylammonium chloride (CTAC, ≥95.0%, TCI), and sodium borohydride (NaBH4, ≥98.0%, Sigma-Aldrich) were used as received. Ultrapure deionized water (18.2 MΩ cm−1) was used for all solution preparations. 205

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Chemistry of Materials region of the Au−Pd−Au nanobricks, four contributions were taken into account, including those from Au nanocubes, Au frames, Pd shells, and Au−Pd alloys. The initial peak centers of the Au nanocubes and the Pd shells were determined by referring to those of the pure Au nanocubes and the pure Pd nanocubes, respectively. For the Au−Pd alloys, a broad peak was located at the shoulder of the Au (111) peak. To locate the peak center of the Au frames, a peak at 2θ position slightly lower than that of the Au nanocubes was assigned because of the lattice expansion from smaller particle size. The multicomponent fitting converged well and fit the transmission electron microscopy (TEM) result.22 The small-angle X-ray scattering (SAXS) experiment was conducted on the BL-23A beamline at NSRRC. For measurement, the sample solution was sealed in a quartz capillary and irradiated by an X-ray beam of 12 keV. SAXS data were collected by a Pilatus 1 M area detector located at 5.0 m behind the sample position. SAXS data were corrected for detector sensitivity, buffer solution subtraction, and normalized to the absolute intensity scale (scattering cross sections per unit volume with unit cm−1).23 Scanning electron microscopy (SEM) images were recorded by Zeiss Ultra Plus operated at the accelerating voltage of 10 keV. Bright-field (BF), high-angle annular dark-field (HAADF) TEM images, selected area electron diffraction (SAED) patterns, and energy dispersive X-ray (EDX) analyses in both map and line-scan modes were collected using JEOL JEM-2010F microscope operating at 200 kV. Optical Sensing of Hydrogen Gas. In the experiment, the sensing signals were acquired by measuring the changes in the darkfield scattering spectra of single nanoparticles before and after H2 uptake. The nanoparticle was picked from a batch dropped on the ITO-coated face of a cover glass in our homemade transparent gas chamber. Coating a thin ITO layer made the cover glass conductive and benefited searching for a single nanoparticle in SEM. Successful experimental measurement was accomplished by virtue of the specifically homemade gas chamber for integrating the flow and optical systems (Figure S1). The chamber is composed by four pieces of steel. The central two pieces are stacked and fixed with screws to create the space for gas flows (Figure S1b). Two cover slides (22 × 22 × 0.17 mm) and O-rings are placed in the grooves on the two sides of the central steel pieces and then sandwiched beneath the A and B steel pieces. The cover slide on the A side is the one with nanoparticles for sensing (Figures S1c), but the other on the B side is a blank to seal the chamber and allow the light to transmit (Figure S1d). The gap between the two cover slides is H in distance. The chamber is finally loaded into a matching holder for fitting on the sample stage of the optical microscope (OM), as shown in Figures S1e and S1f. The A side of the chamber (with nanoparticles) is placed facing upward for the top light illumination. The Teflon tubes from the mass flow controllers are connected to the two inlets on the B side on the bottom of the OM sample stage, as shown in Figure S1g. A gas flow system consisting of digital mass flow controllers was utilized to have a precise control on the gas flow rates and therefore the mixing ratios between H2 and N2 gases (Figures S1a). In the setup, two of the mass flow meters (MC-200SCCM-D, Alicat Scientific) were used to control and modulate the flow of H2 and N2 gases into the chamber, and an additional one was used to monitor the total flow out of the chamber. Using a consistent total gas flow rate of 100.0 sccm (H2 + N2) with the total pressure of 14.5 ± 0.1 psi (ca. 1 atm), the mixing ratio of H2/N2 was modulated in the range of 0−2%. Typically, 4% H2 concentration is the critical limit to sensing exploration at ambient conditions, and thus, the H2 sensors have to be made with high sensitivity to the hydrogen concentration before reaching this threshold.24 For this reason, a range of 0−2% H2 concentrations was employed in the experiment. An inverted optical microscope (Axio Imager A1m, Zeiss) was used to record all scattering spectra of nanoparticles. An incident white light from a halogen lamp (HAL 100 illuminator with quartz collector, Zeiss) was focused onto the nanoparticles by a high numerical aperture condenser (Achromatic−aplanatic condenser 1.4 HD Ph DIC, Zeiss) operating under dark-field mode. The scattering light from the single nanoparticle was collected by an air objective (LD Plan-Neofluar 40×/0.6 Corr M27, Zeiss) on the opposite side of the chamber to the incident light. The collected scattered light was then

aligned into the entrance slit of a spectrometer (SR-303i-A with DU401A-BV CCD, Andor) for recording the scattering spectra. The optimal exposure time and entrance slit width of the spectrometer were 10 s and 150 μm, respectively. Before H2 input in every measurement, N2 gas was introduced into the gas chamber to purge the air away over 1 h. For every change of H2 concentration, 30 min were passed before the record of a scattering spectrum to ensure a complete phase change from Pd to PdHx. Meanwhile, the white light was also turned off for 30 min to avoid any unwanted photothermal effects that might influence the hydrogen diffusion into the nanoparticles.6 FDTD Simulation. To explore the resonance modes and the sensing mechanism of the Au−Pd−Au nanobrick, three-dimensional simulations were carried out using the finite-difference time-domain (FDTD) method (FDTD Solutions, v8.16, Lumerical), by which the far-field scattering spectra and the near-field intensity distributions could be obtained. The geometries and dimensions of nanoparticles used in the simulations were adopted according to the results in Figure S2. The complex refractive indexes of Au and Pd metals were obtained from the experimental data measured by Johnson and Christy.25,26 The permittivity of PdHx was estimated by multiplying that of pure Pd by a factor of 0.8, according to the report of Bévenot.27 The surroundings index of refraction was set to 1.0 because the change of the refractive index of the gas mixtures at different H2 concentration was negligible. The mesh size in FDTD simulation was set to 0.2 nm for x, y, and z directions. The total-field scatter-field (TFSF) plane wave source was used to illuminate the nanoparticles from the substrate side to simulate the scattering spectra.

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RESULTS AND DISCUSSION Structural Analyses of Au−Pd−Au Nanobricks. For shaped Au LSPR generators, strong photoelectric fields are typically generated over sharp domains where there are local hot spots. To utilize these hot spots for INPS, we synthesized Au nanocubes as the major LSPR generators and the architectural basis for the sensors. Figure 1a displays the

Figure 1. SEM images of (a and d) Au, (b and e) Au−Pd core−shell nanocubes, and (c and f) Au−Pd−Au core−shell-frame nanobricks.

large-area SEM images of the Au nanocubes self-assembly owing to their highly uniform sizes and shapes. These Au nanocubes are truncated at the corners, as shown in Figure 1d. By statistical counting over 100 nanocubes, the average size of them is 46.8 nm with a low percent size deviation of 5.1% (Figure S2a). Figures 1b and 1e are SEM images of Au−Pd core−shell nanocubes created by reducing H2PdCl4 in the presence of Au nanocubes in the water phase. Their size is 54.6 nm on average with a low percent size deviation of 4.9%, revealing a very homogeneous coating of Pd shells upon the 206

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Figure 2. (a) HAADF-STEM image and (b) the single-particle SAED pattern of a single Au−Pd−Au core−shell-frame nanobrick. (c) STEM-EDS maps of Au and Pd elements and (d) their overlapped image with that of a single nanobrick. (e and f) Line-scan profiles by STEM-EDS analysis across edges and corners.

of Pd coating, iodide ions were introduced to facilitate the formation of the well-defined cubic shape as their adsorption stabilized the {100} crystal faces (promoting growth in the 110 and 111 directions).28−31 The deposition of the premade Au nanoparticles (NPs) with NaBH4 on the preformed Au−Pd cubes was also dominated by a preference for edges and corners due to the iodide-adsorption on {100} crystal faces (Figures 1c and 1f). This was verified by excluding (Figures S4a−f) and enriching the iodide ions (Figures S4g−l) during the coating of Pd shells. As shown in Figures S4a and S4d, the overgrowth of both Au−Pd and Au−Pd−Au nanocrystals take on irregular shapes in the absence of iodide ions. In particular, the latter formed without the frames. By STEM-EDS analysis, it was revealed that the Au−Pd nanocrystals consist of Au cores and Pd shells (Figures S4b and S4c), while the Au−Pd−Au ones consist of Au cores and alloy shells with a high Au content on the outmost surfaces (Figures S4e and S4f). It follows that these nanocrystals formed due to the interdiffusion between the

uniform Au nanocubes. Figure S3a shows a HAADF-STEM image of a single Au−Pd nanocube in which an outer shell is observable. The thickness of the shell is about 2 nm on the edges, while it is 4 nm at the corners. In Figure S3b, the batch of diffraction spots in the SAED pattern of the Au−Pd nanocube is composed by a group of strong and weak spots from the Au core and the Pd shell, respectively (dashed circle). The SAED pattern displays a homogeneous square motif composed by these spots, indicating the epitaxial overgrowth of the shell on the core. According to the results of STEM-EDS, the elemental map reveals that the nanocube is truly composed by an Au core and a Pd shell (Figure S3c). In the line-scan plot (Figure S3d), the boundaries of the Pd profile across the nanocube edges validate that the exposed surface is the pure Pd crystal face (pink arrows). This is validated by the atomresolved TEM image where the lattice spacings close to the nanocube surface are 1.38 and 1.99 Å, corresponding with those of Pd {220} and {200} crystal faces. Notably, in the stage 207

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Figure 3. (a) XRD patterns of Au, Pd, and Au−Pd core−shell nanocubes and Au−Pd−Au core−shell-frame nanobricks acquired by synchrotron radiation with the X-rays energy of 18 keV. (b) Deconvoluted pattern of the (111) peak of the Au−Pd−Au nanobricks. (c) Schematic illustration for the interfacial structures from the core to frame.

deposited Au layers and the Pd shells. Here, the possibility of galvanic replacement is excluded because there should be only a limited amount of unreacted AuCl4− complexes (Au NP solution prepared by reducing HAuCl4 with fourfold excess NaBH4), and the absence of hollow structures supports this hypothesis. Figure S4g shows the concave Au−Pd core−shell nanocubes synthesized with 2 μmol NaI (20 times higher), which were verified by STEM-EDS analysis (Figures S4h and S4i). More importantly, the concave surfaces again prove the capping effect of iodide ions on the Pd 100 crystal faces, reflecting the reliability in our observed results without adding NaI. Coating Au on the concave Au−Pd nanocubes, the nanobricks with distorted or crumbled frames were obtained (Figure S4j). Despite the poor quality of the Au frames, the nanobricks still had well-defined Au−Pd−Au core−shell-frame structures, as shown in Figures S4k and S4l. The irregular formation of Au frames possibly came from the heterogeneous deposition of Au atoms on the top faces and the exposed side faces of the concave Pd edges. Figure 2a is the HAADF-STEM image of the Au−Pd−Au nanobrick assembly showing the contours of the layered structures after Au frame coating. Given the homogeneous SAED pattern composed by square motifs (Figure 2b), the selective deposition of the Au frames on the edges and the corners is also epitaxial, as seen in the Pd coating. Figures 2c and 2d include the elemental distributions of Au and Pd on a single nanobrick, which clearly demonstrate the inner and outer Au crystal domains, and the Pd interlayer. The STEM-EDS edge-to-edge line-scan profile shows the outmost surface is Au (Figure 2e, pink arrows). A similar distribution is observed in the profile taken corner-to-corner (Figure 2f). Figure 3 collects the XRD patterns of Au−Pd−Au nanobricks, Au−Pd, and Au and Pd nanocubes obtained by synchrotron radiation with the

X-ray energy of 18 keV (λ = 0.68881 Å). Au and Pd nanocubes are the references for peak comparison in which the Pd nanocubes were prepared with iodide ions to obtain high uniformity in both the size and the shape (Figure S5). In both patterns of Au−Pd nanocubes and Au−Pd−Au nanobricks, two sets of peaks distinguishably correspond with those of Au and Pd nanocubes. However, there are weak shoulders to the high angle side of the Au peaks observed (arrows in the insets). To explore the possible reasons for this, deconvolution of the 111 peak in the pattern of Au−Pd−Au nanobricks across the 2θ range of 16−18.5° was carried out (Figure 3b). The 111 peak is composed by two peaks of Au (cores and frames), one weak peak of alloy, and the other weak peak of pure Pd crystal domains. It stands to reason that the pure Pd crystal domains are mostly contributed by those within the exposed areas that have not been covered by Au frames. The alloy band should result from the interfaces between Au frames and Pd shells, which was a dominant phenomenon in the case without using iodide ions (Figures S4d−f). Low-temperature interdiffusion between Au and Pd atomic layers is commonly observed in their heterostructures.32 It is mainly due to the higher association energy after formation where Au−Pd bonding stabilizes the interfaces (Au−Pd: 142.7 kJ/mol > Pd−Pd: 136 kJ/mol). Accordingly, for the Au−Pd nanocubes, interfacial diffusion took place during the epitaxial overgrowth of Pd shells, and for the Au−Pd−Au nanobricks, it occurred at the less iodide-covered Pd edges and corners after the deposition of the Au frames, schematically interpreted in Figure 3c. Formation Mechanism of Au−Pd−Au Nanobricks. Iodide ions play a key role in the synthesis of the Au−Pd− Au nanobricks, but the requisite amount is low. When the amount is raised by a factor of 10, the synthesized Au−Pd nanocubes have concave faces rather than frames, which is 208

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Figure 4. (a) Time-dependent UV−vis spectra of the mixed solution of Au−Pd nanocubes and NaBH4-reduced Au nanoparticles and (b) summary plot of the variations in peak position and intensity versus time. SEM images of the captured intermediates at different reaction times of (c) 5, (d) 15, (e) 60, and (f) 120 min. (g) Schematic interpretation for the growth mechanism of the Au frames on the Pd shells.

contrast, the red shift and the intensity decay during the growth of Au frames is a function of aging time and is summarized in Figure 4b. The intermediates at the reaction times of 5, 15, 60, and 120 min were captured to investigate the surface evolution (Figures 4c−f). We observe that a limited quantity of the preformed Au nanoparticles begin to adsorb on the edges and the corners of the Pd shells in the initial 5 min and the coverage increased in the following 15 min. In the period of growth, the adsorbed particle sizes grew, but a portion of Au nanoparticles still attached to the faces, which probably resulted from either a heterogeneous coverages of iodide ions or sample drying for SEM characterization. After 60 min, the frame shapes of Au agglomerates became clear, and few individual Au nanoislands were observed. When extended to 120 min, the formation of Au frames was completed. This suggests surface reconstruction occurred but took place slowly. It is possible the presence of some face-adsorbed Au nanoparticles was lessened because of slow dissolution of Au0 with AuBr4− to form AuBr2− complexes, which were reduced again to deposit on the edge or the corner

further evidence that iodide ions cover and inhibit growth in the ⟨100⟩ directions (Figure S6). The growth mechanism was investigated by examining time-dependent variations of Au LSPR absorption and recording the evolving particle morphologies in the stage of Au frame growth. Figure 4a is the corresponding time-dependent UV−vis spectra of the nanobrick solution aged for 4 h after addition of the NaBH4reduced Au nanoparticles. A trend of red shifting in the peak position from 498 to 524 nm is observed as well as a gradual decrease in the peak intensity. The initial absorption at 498 nm is attributed to the mixed contributions of NaBH4-reduced Au nanoparticles and Au−Pd nanocubes, while the ultimate absorption at 524 nm is simply from the fully formed Au− Pd−Au nanobricks. The spectrum inset shows the LSPR absorption spectra of as-synthesized Au NPs with NaBH4, left undisturbed without mixing with Au−Pd nanocubes at ambient conditions for 12 h. There is no significant difference in the two curves except for some intensity drop, possibly owing to the loss of particles adsorbed onto the inner walls of the vial. In 209

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Figure 5. (a) Energy diagram of the BDP resonance model, which illustrates the spectral shift as a result of the phase change from Pd to PdHx. (b) Experimental dark-field scattering spectra of a single Au−Pd−Au nanobrick recorded at 0% (black symbols) and 2% H2 concentrations (red symbols). The experimental spectra are fitted with a Lorentz resonance model (solid curves) to obtain the resonance wavelengths. (c) Simulated scattering spectra of a Au−Pd−Au nanobrick at 0% H2 concentration (black curve) and a Au-PdHx-Au at 2% H2 concentration (red curve). (d) A series of scattering spectra taken by periodic switches of 0 and 2% H2 concentrations in turns, demonstrating the reversible spectral shifts of the Au− Pd−Au nanobrick due to the successive hydrogen gas uptake and release. The average spectral shift is ∼10 nm.

slow diffusion of Au atoms from the adsorbed clusters to the edges of the Au−Pd core−shell nanocubes, rich with truncated Pd surfaces (as evidenced with the SEM images in Figures 4c− f). Optical Hydrogen Gas Sensing. By virtue of the thin Pd shell (∼3.5 nm) sandwiched between two Au layers of the core and the frame, the Au−Pd−Au nanostructure is an excellent plasmonic resonator where the thin Pd shell serves as a conductive gap. Such a plasmonic resonator is particularly advantageous to the bonding dipolar plasmons (BDP) in the visible frequency range and the charge-transfer plasmons (CTP) in the mid-infrared regime, according to the conductivity of the material in the gap.36−40 For H2 sensing with the Au−Pd−Au nanobricks, the principle is based upon the absorption of H2 gas by the Pd shells, where hydrogen atoms gradually diffuse into the Pd crystal lattices after adsorption to convert Pd into PdHx. The phase change induces the reduction in the density of freely moving conduction electrons and thereby the conductance of the Pd shells.41 As a consequence, the CTP is suppressed, and the BDP exhibits red shifts due to the coupling of two gold layers across the capacitive PdHx gap layer. This change is schematically illustrated in Figure 5a, where the green arrow indicates the phase transition of Pd to PdHx and therefore the red shift of the BDP. Figure 5b is the frequency window in the visible range to observe the change in the BDP of a single Au−Pd−Au nanobrick after H2 absorption. In the beginning, the scattering of a single nanobrick is measured under the inert N2 gas flow (100 sccm) in the chamber, which showed the scattering peak value of 620 nm. After 2% H2 gas was introduced into the

sites. The mechanism is schematically illustrated in Figure 4g. On the basis of these observations, the time-dependent variations in UV−vis spectra and morphologies are connected. First, the intensity drop of the Au LSPR absorption corresponds with the aggregation of Au nanoparticles in the solution and therefore the decrease of the suspended particle concentration. Second, the Au agglomerates cause a red shift of the Au LSPR peak, leading to the negative relation between the peak shift and the intensity drop. Apart from the ex itu examination of the time-dependent intermediates by SEM, in situ SAXS was also performed to trace the growth of the Pd shells and the final Au frames onto the Au and the Au−Pd nanocubes, respectively. As shown in the inset of Figure S7a, the nanocube edge length a = 2Rg, deduced from the radius of gyration Rg of the time-resolved SAXS data (Figure S7a), increases from 46.5 nm (original gold cubes) to 53 nm in the first 10 min, corresponding to a relatively fast growth of Pd-shell thickness of ca. 4 nm.33−35 The result is consistent with the average size growth measured with SEM (Figure S2). In contrast, the SAXS result in Figure S7b reveals a slower growth of the Au frame onto the Au−Pd core−shell nanocubes from a = 53 to 57 nm in nearly 30 min. This is followed slow decay in a to 55 nm in the rest of reaction to 100 min. The observed size, growth, and decay correspond, respectively, to the faster adsorption of Au-clusters onto the Pdshell surface, followed by slow flattening and redistribution of the Au clusters to cover more the Pd edge and corner surface. Such result strongly supports the model proposed in Figure 4g. We note that Au surface energy (0.094 eV/Å2) is slightly smaller than that (0.125 eV/Å2) of Pd, which rationalizes the 210

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Figure 6. (a) Time-dependent X-ray diffraction patterns of Au−Pd−Au nanobricks in the stages of hydrogen (b) uptake and (c) release. The reversible variation in the crystal structure of Pd shells is in good agreement with the result observed in optical sensing. Asterisks indicate the saturation of hydrogen uptake and the beginning of desaturation.

decreases the sensitivity. The superior sensitivity can be attributed to the ultrathin yet highly homogeneous Pd shell between two Au layers, which effectively changes the junction conductance in respond to the concentration of hydrogen in the environment. For recyclable sensing, the effective response by the Pd shells is expected to be a reversible process. To remove hydrogen atoms from PdHx shells, pure N2 gas was purged into the chamber to reverse the conversion of the Pd crystal phase (from PdHx to Pd) and restore the conductance of the Pd shells. Ideally, it leads to the blue shift of the BDP peak back to the original or a close position. In the test of reversibility, another nanobrick was picked and measured to collect its scattering spectra at the H2 concentrations of 0 and 2% switched in turns. For every change of the H2 concentration, we left the sample in the dark without illumination for 30 min before we acquired the scattering spectrum that allowed the phase changes of Pd/PdHx to reach an equilibrium. Figure 5d shows the spectra recorded at the altered H2 concentrations for

chamber, the experimental scattering exhibited a pronounced red shift from 620 (0%, black symbol) to 633 nm (2%, red symbol), giving a red shift of 13 nm. To verify the experimental result, FDTD simulations were also performed to obtain the theoretical scattering spectra of the nanobrick at 0 and 2% H2 concentrations. As a result, the shift between the simulated spectra was 15 nm, in good agreement with that obtained in the experiment (Figure 5c). To the best of our knowledge, it is a very large spectral shift in H2 gas sensing with a single nanoparticle, compared with most of those obtained in previous works.20,41 As shown in Figure 5a, a core−shell-frame nanobrick behaves like a gap nanoantenna whose resonance greatly depends on the gap size and the material therein.36,37,42 Smaller gap size guarantees stronger coupling, higher local field and, therefore, larger sensitivity to the change of the gap material. Moreover, because the gap in our nanobricks is along the extended area under the frame, homogeneous thickness of the Pd layer is critical to suppress variation of coupling strength and minimize the broadening of the resonance peak, which 211

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three cycles. The spectral shifts are 11, −15, 10, −8, and 10 nm in turns, where the negative values denote the blue shifts after pure N2 gas was purged in the chamber for 30 min. Clearly, the phase change between Pd and PdHx is reversible, and the corresponding spectral shifts are profound. This finding is extremely significant as it confirms that the bimetallic Au−Pd− Au core−shell−frame nanobrick is a promising archetype for miniaturized gas sensors and could be applicable to a wide range of materials combinations. To confirm that the reversibility of scattering signals came from the reversible structure change of Pd shells, in situ XRD analysis with synchrotron radiation (18 keV X-rays) was carried out. Here, we examined the time-dependent variations of the diffraction peaks of Pd and the hydride phase. The same gas chamber used for the optical measurement was also applied for this experiment. Noticeably, as shown in Figure S1h, the blank cover slide (without nanoparticles) in the gas chamber was replaced by the Kapton tape to reduce the absorption of X-ray photons by the glass slides. To carry out the diffraction, the chamber was set to stand vertically with the Kapton side facing the incoming synchrotron X-ray, and behind the other side, a 2D detector was set for collecting the diffracted X-ray photons. Figure 6a shows the collection of XRD patterns across the 2θ range of 15.5−18.5° recorded in a cycle of H2 uptake (Figure 6b) and release (Figure 6c). After introducing 2% H2 gas (98% N2), the diffraction pattern was recorded every 5 min until the 50th minute when the mixed H2/N2 gas was switched to 100% N2 gas, followed by continuously recording the XRD patterns. In the stage of H2 uptake (Figure 6b), the 111 peak of Pd at 17.54° weakens, and the shoulder at 16.98° grows with time. This denotes the evolution of the Pd shells to cubic PdHx (ICSD-201089), which corresponds with that observed in the case of Pd nanocubes (Figure S8). The H2 uptake reached saturation after 25 min and remained stable in the cubic PdHx structure under the continuous flow of 2% H2 gas. After closing the H2 gas feed, the peak of PdHx decreased, while that of Pd returned in 10 min, indicating a process of desaturation (Figure 6b). The desaturation went on for 15 more minutes, and ultimately, the Pd shells were recovered, completing the cycle of H2 uptake and release. This validates that the changes in the scattering of Au−Pd−Au nanobricks come from changes in the crystal structure of the Pd shells, which control the dielectric. The nanobricks are thus durable structures (Figure S9) that render promising applications in low H2 sensing.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04200. Setup scheme, setup photos, size-distribution histograms, SEM images, HAADF-STEM images, STEM-EDS maps, and line-scan profiles (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.-S.H.) E-mail: [email protected]. *(C.-H.K.) E-mail: [email protected]. ORCID

U-Ser Jeng: 0000-0002-2247-5061 Chun-Hong Kuo: 0000-0001-6633-8985 Author Contributions ○

K.C.N., F.-C.L., and P.-W.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the technical support from NanoCore, the Core Facilities for Nanoscience and Nanotechnology at Academia Sinica in Taiwan. We specially thank Ms. Mei-Ying Chung, the technician in the Institute of Chemistry at Academia Sinica in Taiwan, for carrying out SEM analyses. C.-H.K. thanks the Ministry of Science and Technology, Taiwan (MOST 104-2113-M-001-007-MY2 and 106-2113-M001-030-MY2), Academia Sinica, Taiwan (Program of Nanotechnology No. 2393) and Executive Yuan, Taiwan (Government Policy Allocation Plan for Key S&T Developments) for support. J.-S.H. thanks the Ministry of Science and Technology, Taiwan (MOST 103-2113-M-007-004-MY3) for support.

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REFERENCES

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CONCLUSIONS In this work, we developed a facile and green synthesis of bimetallic Au−Pd−Au nanobricks in an aqueous phase with low thermal energy input. We described the core−shell-frame structure and synthesis of ultrathin Pd interlayers sandwiched between the Au nanocubes and the Au nanoframes. By virtue of their unique sandwich structure, the nanobricks function as superior optical nanosensors for H2 sensing, exhibiting extremely large shifts in the scattering spectra at a very low H2 concentration (2%). Apart from the superior sensitivity, the profound response was reversible, and the H2 sensing was recyclable on the same nanobrick in the reversibility test for three cycles. These findings demonstrate the core−shell-frame nanobricks can serve as a structural archetype for miniaturized gas detection nanosensors, i.e. Au−Cu2O−Au for H2S sensing. 212

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