Al–Pd Nanodisk Heterodimers as Antenna–Reactor Photocatalysts

Sep 22, 2016 - Photocatalysis uses light energy to drive chemical reactions. Conventional industrial catalysts are made of transition metal nanopartic...
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Al-Pd Nanodisk Heterodimers as Antenna-Reactor Photocatalysts Chao Zhang, Hangqi Zhao, Linan Zhou, Andrea Schlather, Liangliang Dong, Michael McClain, Dayne F. Swearer, Peter Nordlander, and Naomi J. Halas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03582 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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Al-Pd Nanodisk Heterodimers as Antenna-Reactor Photocatalysts Authors: Chao Zhang 1, 4, Hangqi Zhao 1, 4, Linan Zhou 2, 4, Andrea E. Schlather 2, 4, Liangliang Dong 2, 4

, Michael J. McClain 2, 4, Dayne F. Swearer

2, 4

, Peter Nordlander

1, 3, 4

*, and Naomi J.

Halas 1, 2, 3, 4* 1

Department of Electrical and Computer Engineering, 2Department of Chemistry,

3

Department of Physics and Astronomy, and 4Laboratory for Nanophotonics, Rice

University, 6100 Main Street, Houston, Texas 77005, United States.

Key words: Photocatalysis, plasmonics, aluminum, palladium, heterodimer, hydrogen dissociation

Abstract: Photocatalysis uses light energy to drive chemical reactions. Conventional industrial catalysts are made of transition metal nanoparticles that interact only weakly with light, while metals such as Au, Ag, and Al that support surface plasmons interact strongly with light but are poor catalysts. By combining plasmonic and catalytic metal nanoparticles, the plasmonic ‘antenna’ can couple light into the catalytic ‘reactor’. This interaction induces an optical polarization in the reactor nanoparticle, forcing a plasmonic response. When this “forced plasmon” decays it can generate hot carriers, converting the catalyst into a

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photocatalyst. Here we show that precisely oriented, strongly coupled Al-Pd nanodisk heterodimers fabricated using nanoscale lithography can function as directional antennareactor photocatalyst complexes. The light-induced hydrogen dissociation rate on these structures is strongly dependent upon the polarization angle of the incident light with respect to the orientation of the antenna-reactor pair. Their high degree of structural precision allows us to microscopically quantify the photocatalytic activity per heterostructure, providing precise photocatalytic quantum efficiencies. This is the first example of precisely designed heterometallic nanostructure complexes for plasmonenabled photocatalysis, and paves the way for high-efficiency plasmonic photocatalysts by modular design.

Photocatalysis, a process that harvests energy from light to promote chemical reactions, is an attractive and sustainable alternative to current chemical industrial processes which require intensive energy input.1–7 An ideal photocatalyst should interact strongly both with molecules and with light to effectively lower the chemical reaction barrier of interest. Conventional industrial catalysts made of transition metal nanoparticles, such as Pd, Pt, and Ru, possess a favorable electronic structure that allows a variety of molecules to adsorb and react on their surfaces. The use of transition metal nanoparticles alone as photocatalysts for several chemical reactions showed promising quantum efficiencies and favorable selectivities compared with thermal processes,8,9 but further improvement of their performance is hindered by their weak interaction with light.10–12 Conversely, plasmonic metals such as Au, Ag, and Al, are far poorer catalysts, yet support surface plasmon resonances with optical cross sections far larger than their physical cross sections, and large 2 ACS Paragon Plus Environment

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electromagnetic field enhancements.13–15 However, one can combine plasmonic and catalytic metal nanoparticles in close proximity to each other, forming a strongly coupled “antenna-reactor” complex. In that case, when illuminated, the plasmonic antenna induces an optical polarization in the reactor particle through its optical near field, essentially driving a plasmon in the nonplasmonic nanoparticle. When this “forced plasmon” decays it can do so by generating hot carriers, as in plasmonic nanoparticles, making it a photocatalyst in addition to its innate catalytic properties.16 In this Letter, we demonstrate the use of lithographically defined, substrate-supported Al-Pd nanodisk heterodimers as antenna-reactor photocatalysts (Figure 1a). Compared with chemically synthesized antenna-reactor complexes, these heterodimers are patterned with precise control over the size, interparticle distance, and orientation of the coupled nanodisk pairs. Using the hydrogen dissociation reaction as a photocatalytic probe, the reaction rate of these nanostructures shows a clear dependence on gap dimensions, illumination wavelength, and orientation of the nanostructures with respect to the polarization angle of incident light. Moreover, since the number of heterodimers participating in the photoreaction can be precisely controlled, an accurate quantification of reaction rates and quantum efficiencies at the individual reactor level can be performed. This antenna-reactor design demonstrates a simple, modular, and easily adaptable platform for controlled and quantitative highefficiency photocatalysis.

Al-Pd heterodimers, each consisting of an Al (antenna) and a Pd (reactor) nanodisk separated by a few-nanometer gap, were patterned using hole-mask colloidal lithography. This is a highly parallel process that allows for fabrication of highly uniform, precisely 3 ACS Paragon Plus Environment

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defined nanoparticle complexes over large areas (see Supporting Information for a detailed description of the fabrication process)17. The heterodimers were formed by sequentially evaporating Al and Pd through the hole mask at two different angles (Figure 1b). By varying the evaporation angle for each nanodisk, the gap between the Al and the Pd disks can be precisely controlled. Two sets of Al-Pd heterodimers were prepared with equivalent Al and Pd nanodisk diameters: one with small gaps (SG, Al diameter = 75 ± 7 nm, Pd diameter = 50 ± 7 nm, gap = 3.0 ± 7.3 nm, SEM: Figure 1c, S2a, gap distribution: Figure S2c) and one with slightly larger gaps (LG, Al diameter = 73 ± 8 nm, Pd diameter = 45 ± 8 nm, gap = 8.6 ± 8.6 nm, SEM: Figure 1d, S2b, gap distribution: Figure S2d). The height of the disks is 35 nm (cross section images in Figure S4). The actual sizes of the SG and LG Al-Pd heterodimer samples are shown in Figure 1e. For incident polarization transverse to the interparticle axis of the heterodimer structures, surface plasmon peaks were observed at nominally 390 nm in the extinction spectrum of both samples. For longitudinal polarization, the near field of the Al disk dipolar plasmon induces the dipolar plasmon of the Pd disk, and the extinction peaks were broadened and redshifted to 460 nm for the small-gap heterodimers, and to 400 nm for the slightly larger gap heterodimers, due to the distance dependent coupling between the Al and the Pd disks (Figure S3).

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Figure 1. Antenna-reactor effect on plasmonic-catalytic bimetallic heterodimers. a, Schematic of the antenna-reactor mechanism. The plasmonic antenna (gray disk) focuses the light onto the catalytic reactor (red disk), inducing a “forced plasmon” in the reactor particle, which decays by generating hot electrons which promote chemical reactions on its surface. b, Fabrication process of Al-Pd heterodimers using hole-mask colloidal lithography. The heterodimer arrays were fabricated by sequentially evaporating Al and Pd through the Au hole mask at two different angles. c, d, Representative scanning electron microscope (SEM) images of Al-Pd heterodimers with 3 nm gaps (SG, c) and 9 nm gaps (LG, d), respectively. The Al disks are on the left and Pd disks on the right. Scale bar: 200 nm. e, Image showing the actual sizes of SG (left) and LG (right) Al-Pd heterodimer samples next to a dime.

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The photocatalytic hydrogen dissociation reaction was performed at room temperature and atmospheric pressure using a photocatalysis reaction system described earlier18 (see Supporting Information for photocatalysis measurement methods). Hydrogen (H2) and deuterium (D2) were introduced as reactants, so that the photocatalytic desorption of hydrogen and deuterium atoms from the photocatalyst surface could be observed by monitoring the formation of HD. On Pd surfaces, dissociative adsorption of H2 and D2 is nearly barrierless;19–21 the rate limiting step of this reaction being the associative desorption of HD. Hot electron generated by plasmon decay in Pd disk can populate the antibonding orbit of Pd-H(D) bond, which breaks the Pd-H(D) bond and leads to the desorption of HD.16 The generation of HD was monitored in real time using an inline mass spectrometer. Under illumination, the Al-Pd heterodimers show HD generation rates well above the thermal reaction background. In a control experiment, Al-Al heterodimers showed no hydrogen dissociation under light illumination (Figure S5). This result clearly shows that the catalytically active component in the Al-Pd heterodimer is the Pd disk. We note that Al nanocrystals can also work as a photocatalyst for hydrogen dissociation.22 The fact that AlAl dimers here showed no detectable photocatalytic activity does not contradict the observation in Ref. 22, as the number of Al disks in this control experiment is much fewer and the light intensity is much weaker compared with the earlier work. We also monitored the temperature increase of the sample during photocatalysis measurement and calculated the maximum temperature increase due to laser heating. Both results indicated that the temperature increase during the photocatalysis measurement is less than 2 K (see Supporting Information for temperature increase calculation), which

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leads to negligible thermal contribution according to our thermal background calibration under dark condition (Fig. S7).

Wavelength-dependent HD generation for SG and LG samples, for both longitudinal and transverse incident polarization, is shown in Figure 2. The HD generation rates exhibit wavelength dependencies that also clearly indicate that the Al disk serves as an antenna. For the SG heterodimers under longitudinally polarized incident light (along the interparticle axis) (Figure 2a), HD generation was observed over a broad wavelength region centered at 430 nm. The HD generation rate is ~7 times higher than what was observed for a sample of Pd monomer nanodisks of 44 ± 6 nm in diameter, fabricated as a control (Figure S8). Under illumination with transverse polarization, the HD rate is substantially decreased. The wavelength-dependent HD production on Pd monomers performed with linearly polarized light showed only a slow, nearly monotonically decreasing activity from short to long wavelengths (black squares in Figure 2a and 2b). This agrees with the absorption spectrum of Pd monomer, which consists of a weak dipolar plasmon centered around 350 nm and a broad interband transition background across the entire visible range.10 In the wavelength region away from the dimer plasmon resonance, the HD production rate on SG heterodimers is similar to Pd monomers, with no prominent peak. For LG heterodimers (Figure 2b), a maximum in photocatalytic HD generation is also observed in the wavelength range of the surface plasmon of the complex, for both longitudinal and transverse incident polarization. However, compared to the SG heterodimers, the overall polarization dependence of HD generation was reduced, and the peak wavelength for HD generation was blueshifted to 410 nm, where both changes are 7 ACS Paragon Plus Environment

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most likely due to the weaker interparticle coupling for this nanostructure. The contrast between the wavelength dependences of the Al-Pd heterodimers and the Pd monomers clearly indicate that the reaction rate on the Pd reactor is substantially enhanced by its coupling to the Al antenna.

Figure 2. Wavelength dependence. a, b, Wavelength dependent HD production rate on SG (a) and LG Al-Pd heterodimers (b) under longitudinal (solid points) and transverse (hollow points) excitation. The laser power density was kept constant at 160 W/cm2 for all wavelengths and polarizations. HD rates on Pd monomer array are shown in black squares. c, d, Absorption of Pd disk in SG Al-Pd heterodimer (c) and LG Al-Pd heterodimer (d) calculated using finite-difference time-domain (FDTD) simulations for longitudinal (solid lines) and transverse (dashed lines) excitations. Absorption spectrum of a single Pd disk is shown (black curves).

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The antenna-reactor interaction that induces the “forced plasmon” in the reactor particle follows the optical absorption in the reactor particle of the complex. To better understand this antenna-reactor modulation mechanism, we performed finite-difference time-domain (FDTD) simulations to calculate the optical absorption in the Pd disk for the SG and LG heterodimers and the isolated Pd monomers. For longitudinal polarization, as shown in Figure 2c and 2d, the absorption of the Pd disk in the heterodimer (solid curves) is enhanced by the near field of the Al disk compared with the Pd monomer (black curves). This is in agreement with the enhancement of the HD generation rate observed experimentally for longitudinal polarization. For transverse polarization, however, the absorption of the Pd disk in the heterodimer (dashed curves) is weaker than in the Pd monomer. This is consistent with the HD generation rate measurements for the LG heterodimer, where excitation by transverse polarization resulted in a lower HD generation rate than in the Pd monomers. For SG heterodimers, the HD rate was higher than in the Pd monomers, which is most likely due to coupling between nearby heterodimers through the near field or scattering. The effect of incident light polarization on the Al-Pd heterodimer photocatalytic activity was further examined by resonant heterodimer excitation (430 nm for SG and 410 nm for LG) by varying the polarization angle from 0o to 180o. For both samples, the highest HD rate was obtained when the heterodimers were excited longitudinally, along the interparticle axis. As the polarization is rotated toward the transverse direction, the HD rate is gradually decreased (Figure 3a and 3b). The normalized absorption in the Pd disk of the SG and LG heterodimers for this polarization range is shown as the black curves in Figure 9 ACS Paragon Plus Environment

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3a and 3b, and follows the same polarization dependence. The contrast between the local absorption for longitudinal and transverse excitation is further illustrated in Figure 3c and 3d, where the local, spatially dependent absorption (obtained from FDTD calculations) of the SG (Figure 3c) and LG (Figure 3d) heterodimers is plotted on log scale. For longitudinal excitation (upper panels), the coupling of the Al disk to the Pd disk substantially enhances its absorption, with a maximum Pd absorption nearest to the gap between the two nanodisks. For transverse excitation (lower panels), the local optical absorption in the Pd nanodisk is substantially weaker, with the minimum Pd absorption directly adjacent to the Al-Pd gap.

Figure 3. Polarization dependence. a, b, Normalized HD rates on SG Al-Pd heterodimers (a) and LG Al-Pd heterodimers (b) excited at different polarizations. 0 degree corresponds to longitudinal incident polarization. Normalized absorption of Pd disk in the heterodimer is shown as black curves. c, d, Calculated optical absorption

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maps on log scale for SG heterodimer (c) and LG heterodimer (d) with longitudinal (upper) and transverse (lower) excitation. The Al disks are on the left and the Pd disks on the right.

An important advantage of lithographically defined, substrate-supported heterodimers is that the number of reactors under illumination can be accurately determined, based on the area of the substrate and the surface density of the heterodimers. This provides a more detailed level of quantification than would be obtainable with chemically synthesized antenna-reactor photocatalysts16. The dependence of the HD generation rate on the excitation power was measured at the respective resonance wavelengths of the heterodimers, for both polarizations (Figure 4a, 4b). From these measurements, accurate reaction rates in terms of moles of HD per gram of Pd catalyst per second could be determined. For the antenna-reactor samples studied, reaction rates as high as ~10 moles of HD per gram of Pd catalyst per second (mol/g/s) were obtained. These extremely high rates reflect both the high activity of Pd towards hydrogen activation, and more importantly, the extremely small amount of Pd participating in the photoreaction (a few nanograms, see Supporting Information for calculations). The intensity dependence of the reaction rate follows a power law with an exponent ~3, which indicates a multi-electron-induced HD desorption process23–25. The internal quantum efficiency (IQE), defined as the ratio between the number of generated HD molecules and the number of photons absorbed in the Pd disks, can be calculated directly from the absorption cross section of the Pd disk in the heterodimer and the surface density of the heterodimers. As shown in Figure 4c and 4d, the IQE increases with laser intensity and can be as high as ~10 % at high power densities. 11 ACS Paragon Plus Environment

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In addition, since we know the number of Pd disks that are being illuminated, we can also calculate the microscopic reaction rate of each Pd reactor. The number of HD molecules desorbed from one Pd reactor by one laser pulse, as a function of incident laser power, is shown in Figure 4e and 4f for SG and LG heterodimers, respectively. The maximum turnover of ~50 HD molecules per laser pulse per antenna-reactor, for an incident laser power density of 200 W/cm2, provides unprecedented quantitative benchmarks for the design and optimization of modular antenna-reactor photocatalysts for this, and other, chemical reactions.

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Figure 4. Power dependence of reaction rate and internal quantum efficiency (IQE). a, b, HD rate normalized to the total mass of Pd under light on SG heterodimers (a) and LG heterodimers (b). c, d, IQE on SG heterodimers (c) and LG heterodimers (d). e, f, Number of HD molecules desorbed from one Pd disk triggered by one laser pulse on SG heterodimers (e) and LG heterodimers (f). The SG and LG heterodimers were excited with 430 nm and 410 nm light respectively, at longitudinal (solid points) and transverse (hollow points) polarization. All axes are plotted on log scale.

This antenna-reactor heterostructure is modular and can be easily adapted and optimized to photocatalyze other chemical reactions. Plasmonic antennas can be designed to provide enhanced optical absorption over a broad wavelength range overlapping interband transitions in transition metal catalysts. Such interband transitions will result in both hot electrons and holes and can thus in principle be used to enhance and control more general redox reactions16,26,27. The antennas can in principle be tuned to optimize charge transfer between the reactor particle and an adsorbate reactant species of interest28. Reactor particles can be fabricated from a wide variety of catalytic materials, not only pure metals but also alloys with catalytic properties towards reactions of interest29, and also semiconductors, where slow recombination times further enhance reaction rates30–32. The deposition-based approach demonstrated here also provides a highly straightforward, ligand-free strategy for the combinatorial fabrication of antenna-reactor photocatalyst complexes that could be difficult or impossible to synthesize chemically.

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In conclusion, we have demonstrated the use of lithographically defined antenna-reactor heterodimers for plasmon-induced photocatalysis. With this simple structure, we show that the plasmonic antenna can focus light onto the catalytic reactor and through this interaction induce a “forced plasmon” that efficiently generates hot carriers thus transforming the catalytic nanoparticle to a photocatalytic nanoparticle. We show that the photo-induced reactivity in these oriented structures has a wavelength and polarization dependence that closely follows the excitation of the heterodimer plasmon resonance, and is highly sensitive to interparticle distance within the structure. The well-defined structure enables a quantitative analysis of the photocatalytic process, including the efficiency and throughput of the individual reactor-catalyst complexes. The capability of tuning the antenna and reactor materials and dimensions independently could ultimately facilitate a large number of photo-induced chemical reactions using optimized antenna and reactor combinations.

Associated Content Supporting Information Fabrication process and characterization of Al-Pd heterodimers, photocatalysis measurements, control experiments, reaction rate and quantum efficiency calculations, and FDTD simulation can be found in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information: *Corresponding authors: [email protected], [email protected]

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Notes: The authors declare no competing financial interest.

Acknowledgments: This research was supported by the Air Force Office of Science and Research under grant FA9550-15-1-0022 and by the Robert A. Welch Foundation under grants C-1220 (N.J.H.) and C-1222 (P.N.). This project received support from the Defense Threat Reduction Agency-Joint Science and Technology Office for Chemical and Biological Defense Basic Research (Grant no. HDTRA 1-16-1-0042). D.F.S. acknowledges support from the National Science Foundation through a Graduate Research Fellowship (0940902).

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