Al-Pd Nanodisk Heterodimers as Antenna-Reactor Photocatalysts

Transcription

1 Al-Pd Nanodisk Heterodimers as Antenna-Reactor Photocatalysts 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, 2 Department of Chemistry, 3 Department of Physics and Astronomy, and 4 Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, United States *Corresponding authors: halas@rice.edu; nordland@rice.edu Supporting Information 1

2 Sample fabrication: The fabrication process of Al-Pd heterodimers was adapted from Ref.1, as illustrated in Fig. S1. Microscope cover glass (Fisherbrand, B) was used as the substrate. Poly(methyl methacrylate) (PMMA, MircoChem, 950 PMMA A4) was spin-coated onto the substrate at 2000 rpm for 1 min followed by baking at 180 o C for 10 min. Ar-O 2 plasma cleaning (Fischione, 660 W) was then performed for 8 sec to reduce the hydrophobicity of the PMMA surface. 0.2 wt.% poly(diallyldimethylammonium chloride) (PDDA) aqueous solution was drop casted onto the substrate to functionalize the PMMA surface with positive charges. After rinsing the substrate in water and drying in N 2 flow, 0.02 wt.% 100 nm polystyrene (PS, Invitrogen) bead aqueous solution was drop casted onto the substrate and let sit for 5 min. The PS beads dispersed on the PMMA surface randomly due to the attractive force between the beads and the surface as well as the repulse force between the beads. The substrate was then dried in swift N 2 flow. 20 nm Au was evaporated onto the substrate to form the hole-mask. Then the PS beads were peeled off the substrate using PDMS stamps. Ar-O 2 plasma etching (Fischione, 660 W) was performed for 12 min to etch through the PMMA layer under the holes to expose the substrate. 35 nm Al and 35 nm Pd were then evaporated sequentially at different angles to form the heterodimers. Vacuum was broken between Al evaporation and Pd evaporation to form a native alumina layer around the Al disks so that the Al metal and the Pd metal are not in direct contact. The gap size was controlled by varying the evaporation angles. The PMMA layer and the hole-mask were finally removed by liftoff in acetone. 2

3 Photocatalysis measurement: After fabrication, the Al-Pd heterodimer substrate was cut into ~5 10 mm 2 small substrates for photocatalysis measurement. Three layers of Al-Pd heterodimers were stacked into a customized reaction chamber (Harrick Scientific) to increase the number of heterodimers under the light. The layers were separated by ~0.2 mm using ZrO 2 powder placed at the edge of the substrate to allow gas flow between the layers. 15 sccm H 2 and 15 sccm D 2 (Matheson) were introduced into the reaction chamber as reactants. The reaction chamber was kept at room temperature and atmospheric pressure during the measurement. A mass spectrometer (Hiden Analytical) was used to analyze the concentration of HD in the outlet in real time. Measurement was performed after HD reached steady level. A tunable Ti:sapphire pulsed laser (Coherent Ultra II, nm, 150 fs, 80 MHz) equipped with a second harmonic generator (APE) was used as light source for photocatalysis measurement. The laser emits linearly polarized light which is suitable for the characterization of the heterodimers. The power of the light was controlled using a set of neutral density filters. The light was focused to a ~ mm 2 spot on the substrate by an off-axis parabolic mirror (Thorlabs, MPD269-F01). The polarization of the light was controlled either by rotating the substrates or using a half-wave plate (Thorlabs, AHWP05M-600). HD production rate under illumination was calculated by subtracting HD level in the Laser OFF state from the Laser ON state to exclude the contribution of steady thermal reaction. For wavelength and polarization dependence experiments, the 3

4 HD rates for different wavelength and polarizations were measured in random order. The values and error bars of HD rate data shown in Fig. 2 and Fig. 3 are calculated based on multiple measurements. Electromagnetic field simulation: Modeling of the heterodimers was performed using the finite difference time domain method (FDTD, Lumerical). The heterodimer was placed on top of an infinite silica substrate and excited with a plane wave at normal incidence. The dimensions of heterodimers were chosen to be identical to the fabricated structures apparent from SEM images. The dielectric function of Al was approximated by the Bruggeman effective medium theory 2 with a volume fraction of 0.85 in the Al/Al 2 O 3 composite, and a 3 nm covering of Al 2 O 3 layer was included to account for the surface oxidation. Tabulated data 3 was used for the optical responses of all other materials. The optical absorption was calculated by integrating the Ohmic loss within the nanostructures. All simulations were checked for convergence to ensure numerical accuracy. Plasmonic heating effect was calculated for the heterodimers under pulse illumination following the model developed in Ref. 4. The maximum temperature increase on the nanoparticle was calculated separately for Pd and Al by T I f V c, max abs / where I and f are the average intensity and pulse repetition rate of laser, σ abs is the absorption cross section of Pd in the heterodimer calculated by FDTD, V, ρ and c are the volume, mass density and heat capacity of Pd, respectively. The peak temperature 4

5 increase due to laser heating for both SG and LG heterodimers is smaller than 2 K for the experimental conditions. Microscopy and spectroscopy characterizations: Large scale scanning electron microscopy (SEM) images of the SG and LG heterodimers are shown in Fig. S2a and S2b, respectively, with Al disks on the left and Pd disks on the right. The gap size distributions for the SG and LG heterodimers were obtained from more than 200 heterodimers for each sample, as shown in Fig. S2c and S2d, respectively. Heterodimers with overlapping Al and Pd disks are represented by negative gap sizes. The density of SG and LG heterodimers was calculated based on 6 large area SEM images for each. The structural parameters of SG and LG heterodimers are listed in Table S1: Table S1: Structural Parameters of SG and LG Heterodimers Heterodimer Al Disk Diameter Pd Disk Diameter Gap Surface Density (nm) (nm) (nm) (μm -2 ) SG 75 ± 7 50 ± ± ± 0.8 LG 73 ± 8 45 ± ± ± 0.7 The extinction spectra of the SG and LG heterodimers at longitudinal and transverse polarizations were measured using a Cary UV-vis-NIR spectrometer (Fig. S3a, S3b). The measured spectra are confirmed by FDTD simulation using average structural parameters in Table S1 (Fig. S3c, S3d). The broadened plasmon peaks in the experimental spectra result from inhomogeneous broadening mostly due to the distribution of gap sizes. 5

6 SEM images of the cross section of two Al-Pd heterodimers are shown in Fig. S4. The scale bars are 100 nm. The stability of the heterodimers during hydrogen dissociation reaction is confirmed by SEM images before and after reactions, as shown in Fig. S5. No observable morphology change of the heterodimers was observed for both samples after week-long hydrogen dissociation measurements. Thermal reaction rate of hydrogen dissociation on Al-Pd heterodimers was measured using the SG heterodimer substrate with light turned off. The temperature of the reaction chamber was increased stepwise up to 370 K and the HD generation rates at each temperature were recorded. The thermal reaction rate is weak compared to photoreaction. Control: To verify the active component in the Al-Pd heterodimer, we prepared Al-Al dimers by evaporating Al instead of Pd in the second round of evaporation (Fig. S8). Real time monitoring of HD rate on SG-Al-Pd heterodimers and Al-Al dimers are shown in Fig. S5. When illuminated, the Al-Pd heterodimers showed activity towards hydrogen dissociation while the Al-Al dimers had no such activity. This result, however, does not contradict previous report on hydrogen dissociation on Al nanocrystals 5 since here the 6

7 number of Al disks is ~100 time fewer and the laser power is ~25 time weaker than in the Al nanocrystal case. Pd monomer array was prepared using similar process (Fig. S9). 60 nm PS beads were used instead of 100 nm PS beads and 35 nm Pd was evaporated normally through the hole-mask. The average diameter of the Pd disk was 44 ± 6 nm. Wavelength dependent HD production on Pd monomers was measured using identical procedure as for Al-Pd heterodimers. Reaction Rate calculation: The absolute HD rate r HD was converted from mass spectrometer HD intensity through internal calibration using steps of HD concentration. The conversion rate is 1 c/s = 0.43 ± pmol/s. The total number of Pd disks under light: N Pd Disk 3 Pd Disk S, where ρ Pd Disk and S are the surface density of heterodimers (Table S1) and the area of light spot ( mm 2 ), respectively. Multiplying by 3 takes into account the three layers. The mass of one Pd disk 4 2 mpd Disk Pd h D, where ρ Pd, h, and D are the density of Pd metal, the height of Pd disk (35 nm), and the diameter of Pd disk (Table S1), respectively. The total mass of Pd under light: mpd NPd Disk m Pd Disk. The reaction rates in Fig. 4a and 4b are normalized to m Pd. 7

8 r Normalized HD r m HD Pd (1) At a given laser power P, the total number of photon absorbed by the Pd disks in unit time: N P P S S h h photon abs Pd Disk OD 2OD abs Pd Disk OD 2OD (2) where hν, σ abs, and OD are the photon energy, absorption cross section of the Pd disk in the heterodimer, and the optical density of the heterodimer layer at the excitation wavelength, respectively. The last factor takes into account of the three layers under illumination. The internal quantum efficiency (IQE) is calculated as the ratio between the HD rate and the photon absorbed by Pd: rhd IQE 100% (3) N photon The number of HD desorbed from one Pd disk excited by one laser pulse: N HD per Pd disk per pulse N r HD Pd Disk f (4) where f is the repetition frequency of the laser. 8

9 Figure S1. Fabrication process of Al-Pd heterodimers using hole-mask colloidal lithography. 9

10 Figure S2. Al-Pd heterodimers. a, b, representative large-scale SEM images of SG (a) and LG (b) Al-Pd heterodimers. Scale bar is 1 μm. c, d, gap size distribution of SG (c) and LG (d) Al-Pd heterodimers. The average gap sizes for SG and LG heterodimers are 3.0 and 8.6 nm, respectively. Negative gap size indicates overlapping Al and Pd disks. 10

11 Figure S3. Extinction spectra of SG and LG Al-Pd heterodimers. a, b, Experimental UVvis-NIR extinction spectra of SG (a) and LG (b) Al-Pd heterodimers at longitudinal (solid lines) and transverse (dashed lines) polarizations. c, d, Calculated extinction spectra of SG (c) and LG (d) Al-Pd heterodimers at longitudinal (solid lines) and transverse (dashed lines) polarizations. 11

12 Figure S4. a, b Side view of Al-Pd heterodimers. The Al disks are on the left and the Pd disks on the right. Scale bar: 100 nm. 12

13 Figure S5. Real-time monitoring of HD production rate on SG Al-Pd heterodimers (red line) and Al-Al dimers (green line). Al-Pd heterodimers are catalytic under light illumination for hydrogen dissociation while Al-Al dimers show no such reactivity. The light was 430 nm, 200 W/cm 2 and at longitudinal polarization. 13

14 Figure S6. Stability of Al-Pd heterodimers before and after reaction. Representative SEM images of SG (left column) and LG (right column) Al-Pd heterodimers before (upper row) and after (bottom row) hydrogen dissociation reaction. No noticeable morphology change of the Al-Pd heterodimers was observed. Scale bar: 400 nm. 14

15 Figure S7. Thermal reaction rate of HD production on Al-Pd heterodimers under dark condition. 15

16 Figure S8. Al-Al dimers. a, Representative SEM image of Al-Al dimers. Scale bar: 400 nm. b, UV-vis-NIR extinction spectra of Al-Al dimer array at longitudinal (solid line) and transverse (dashed line) polarization. 16

17 Figure S9. Pd monomers. a, Representative SEM image of Pd monomers with average diameter = 44 ± 6 nm. Scale bar: 400 nm. b, UV-vis-NIR extinction spectrum of Pd monomer array. 17

18 References: (1) Fredriksson, H.; Alaverdyan, Y.; Dmitriev, A.; Langhammer, C.; Sutherland, D. S.; Zäch, M.; Kasemo, B. Adv. Mater. 2007, 19, (2) Choy, T. C. Effective medium theory : principles and applications, 2nd ed.; Oxford University Press: Oxford, United Kingdom, (3) Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: San Diego,CA, USA, (4) Baffou, G.; Rigneault, H. Phys. Rev. B 2011, 84, (5) Zhou, L.; Zhang, C.; McClain, M. J.; Manjavacas, A.; Krauter, C. M.; Tian, S.; Berg, F.; Everitt, H. O.; Carter, E. A.; Nordlander, P.; Halas, N. J. Nano Lett. 2016, 16,