Supplementary Figure S1. Scheme for the fabrication of Au nanohole array pattern and

Transcription

1 Supplementary Figure S1. Scheme for the fabrication of Au nanohole array pattern and the growth of hematite nanorods on the Au nanohole array substrate. (a) Briefly, the 500 nm sized PS monolayer was assembled on the FTO substrate, and then etched for a certain time. Four layers including Ti, Au, Ti and SiO 2 were then deposited. Subsequently, the PS spheres were removed, which led to the formation of Au nanohole array on the FTO substrate. Finally, the hematite nanorods were grown on the nanohole pattern by the hydrothermal method. The SiO 2 layer was employed to protect the Au and Ti not to be etched during the growth of hematite nanorods. (b) and (c), The SEM images with different magnifications on the bottom was obtained from the ordered Au hole array pattern. The Au hole array displays both long-range and short-range order. The short-range disorder, highlighted in the yellow circle, was formed due to defects in the self-assembled PS template. The exposed FTO area inside the holes was 44.2% of whole geometric area of photoanode.

2 Supplementary Figure S2. SEM images of different-length hematite nanorod array. SEM images and the corresponding optical photos of the hematite nanorods on FTO (the bottom row) and on the Au hole array pattern (the upper row) with different lengths: (a) ~150 nm, (b) 500 nm, (c) 1 μm, and (d) 2 μm, respectively. In the case of (a), the Au nanohole array still could be recognized, as marked as the red circles. The nanohole array was fully covered for longer hematite nanorods (b-d). The scale bar is 100 nm for all SEM images except the first panel in (a).

3 Supplementary Figure S3. TEM (inset) and HRTEM images of hematite nanorod. HRTEM image shows clearly the lattice fringe, which agrees well with the (110) plane of hematite (d=2.54 Å). The scale bar is 2 nm.

4 Supplementary Figure S4. XRD patterns obtained from the hematite nanorod array on the bare FTO and on the Au nanohole pattern, respectively. X-ray diffraction (XRD) also indicated the complete conversion from FeOOH to hematite. It is noted that weak diffraction peak for Au was also detected, indicating the presence of the Au nanohole array pattern.

5 Supplementary Figure S5. UV-Visible absorption spectra obtained from an ordered Au nanohole array pattern, a disordered Au nanohole array and a solid Au film of the same thickness. The SPP peak at 475 nm is seen to decrease in the intensity as the disorder increases. As well, the LSPR red-shifts as the disorder increases due to separation and aggregation of the individual nanoholes. The curves are offset for comparison.

6 Supplementary Figure S6. Scheme for the water splitting experiments with the hematite nanorods as the photoanode in photoelectrochemical cell (PEC), and the J-V curves for the hematite nanorods with different lengths (L). To optimize the nanostructure, the hematite nanorods with different lengths (L) on the bare FTO substrate were examined with PEC measurement. The results show that the shortest hematite nanorod sample (150 nm) demonstrated the best performance among all the samples. An increase in the hematite nanorod length would lead to an increase in the light absorption but without any benefit to the photocurrent. This was due to two reasons. First, given the short carrier diffusion length in hematite, the longer nanorod, the larger the charge recombination chance is. Second, the light absorption depth in hematite is ~120 nm. Hence any increase in the length of the nanorod will only lead to a minor increase in the light absorbed. The rest of the PEC results presented in the paper therefore correspond to the 150 nm long hematite nanorods.

7 Supplementary Figure S7. Comparison and separation of SPP and LSPR modes. (a) SPP are travelling charge oscillation waves that form due to coupling of incident light with the periodicity of the nanohole array; (b) LSPR is isolated in each hole and is present regardless of the periodicity; (c) The SPP can lead to an enhanced transmission while the LSPR leads to an increased absorption. When the periodicity of the cell is removed, the SPP and increased transmission no longer exists; (d) The local plasmonic field in the SPP can reradiate the incident light, leading to an increased transmission; (e) The LSPR local field enhancement is centered at the edges of the hole and the interface between the metal and dielectric.

8 Supplementary Figure S8. Transient dynamics with different applied potentials. Transient dynamics of photocurrent obtained from the 150 nm long hematite nanorods on the bare FTO substrate (a) and on the Au nanohole array (b) with a different applied potentials. The presence of current spikes when the light is on/off, which is the typical features for hematite photoelectrode due to its lower oxygen evolution dynamics and high sensitivity to the surface states. Increasing the applied bias would be beneficial to the charge separation and reduce the charge recombination. Photocurrent spikes are typically observed when the illumination is switched both on and off for the hematite nanorods. The anodic current spike results from the accumulated photogenerated holes at the semiconductor-liquid junction due to the slow oxygen evolution reaction kinetics or the oxidization trap states in the bulk or on the surface. 13,14 Higher applied bias weakens the transient spike since a larger proportion of holes have a sufficient potential to oxidize water.

9 Supplementary Figure S9. Photoanode transient dynamics with different irradiation intensities. The as-measured and normalized curves of photocurrent versus time for the hematite nanorod array on the bare FTO substrate (a,b) and on the Au nanohole array pattern (c,d) with an applied bias of 0.23 V vs. Ag AgCl under different light irradiation intensities, respectively. As the irradiation intensity decreases, the photocurrent decreases correspondingly. The normalization was preceded based on the maximum photocurrent value for each curve.

10 Supplementary Figure S10. Technical Details Mode Calculations. (a) Normalized mode intensity of the hematite nanorod in the ordered Au nanohole array. At 450 nm, the mode is mainly centered in the middle of the hematite nanorod with minimal coupling to the surface. (b) As the wavelength increases, the waveguiding efficiency decreases quickly, and the mode is located more at the surface of the nanorod and into the local environment. (c)the modal index changes from the refractive index of the semiconductor to the refractive index of air as the waveguiding efficiency goes from localized to nonexistent. The mode calculation was run in Optimode. The 120 nm diameter fibers were simulated using the full refractive index of hematite. 53 The modal index was solved for at each wavelength using a linearly polarized solver in 2D. The modal index can be thought of as representing how efficiently the incident light is guided (c). A high modal index means all incident light is trapped in a propagating mode, whereas a modal index of one means no waveguiding is present.

11 Supplementary Figure S11. SEM images obtained from the disordered Au hole array pattern with low (a) and high (b) magnifications. It can be seen that the Au hole array pattern prepared by spin-coating has long-range disorder. However, it short-ranged order still exists and cannot be completely removed. Compared to the ordered Au hole array, the disordered one exhibits a reduced transmission at 475 nm, and the LSPR peak becomes broadened and red-shifts to 800 nm, as shown in Supplementary Figure S5. The scale bars are 1 μm and 100 nm for low and high magnifications, respectively.

12 Supplementary Figure S12. Microstructure and optical characterization for hematite on disordered Au hole array. SEM images (a,b) and UV-visible absorption spectra (c,d) obtained from the hematite nanorods array on the bare FTO substrate and on the disordered Au hole array pattern. The scale bar is 1 μm for each SEM. The hematite nanorods mainly grew inside the holes on the FTO surface (red arrows); no hematite nanorods were observed on the SiO 2 -covered region. The disordered Au hole array pattern can still lead to an absorption enhancement in the regions of 350 nm to 500 nm (due to SPP), and 700 nm to 900 nm (due to LSPR), which match well with the SPP and LSPR of the disordered Au hole array pattern. The exposed FTO area inside the holes was 34% of whole geometric area of photoanode.

13 Supplementary Figure S13. PEC performance of the hematite nanorod photoelectrode on the disordered Au hole array pattern. (a) J-V curve, (b) IPCE; The insert is the IPCE in nm. (c) wavelength-dependent enhancement factor spectrum versus the absorption spectra. An enhancement in both the photocurrent and IPCE can be observed for the hematite nanorod array photoelectrode after coupling with the disordered Au hole array. The enhancement factor, however, is much less than that of the ordered Au hole array. The IPCE shows that the local order in the disordered pattern can still lead to an enhanced transmission and an enhancement from the SPP at wavelengths below the band edge of hematite. The LSPR red-shifted to 800 nm, leading to no overlap with the absorbance band edge of hematite. This disallowed PIRET. Thus LSPR did not enhance the IPCE. Error bars are defined by the standard deviation over 3 trials.

14 Supplementary Figure S14. FDTD simulation of the effects of disorder on the Au nanohole array. (a) Single holes have a very weak LSPR (hole size: 350 nm). The dimer-holes display a red-shifted LSPR band at around 800 nm. The trimer-holes have a broadened and red-shifted LSPR band. The random mixture of dimer and trimer holes in a small area shows a broad LSPR band. (The curves are offset for clarity). (b) For a local disordered unit cell containing two single holes, three dimers, and two trimer holes in a small area, a red-shifted LSPR peak is present while the SPP mode is absent due to the lack of periodicity over a large-area. A large-area periodic replica of the local disordered unit cell shows the weak SPP modes and a red-shifted LSPR peak. A large-area long-range ordered hole array exhibits the strong SPP mode and a blue-shifted LSPR peak due to the absence of inter-hole interaction present with dimer and trimer hole pairs. The FDTD simulation results reveal that the LSPR peak depends on the assembly (lay-out) of holes in the local area. The SPP peak depends on the order of hole array and the periodicity over a large area. The intensity of SPP mode increases with increasing the periodicity of the hole array.

15 Supplementary Note 1 Nature of SPP and LSPR in Disordered Verse Ordered Hole Array FDTD simulations were run with a pattern that consisted of single hole, hole dimer, and hole trimer pairs, replicating the disordered hole array (Supplementary Fig. S12). The calculated absorption was averaged over both X and Y polarizations with the wave vector perpendicular to the surface. The FDTD simulations prove the red-shift of the LSPR in the disordered array is due to the coupling between the nanoholes in the dimer and trimer pairs. This is clearly seen by comparing the weak LSPR peak of a single nanohole to the bi-hole and tri-hole simulations in Supplementary Fig.14a. The absorption increases with wavelength from the Drude absorption of the Au. However, the offset in curvature from the LSPR peak can still be distinguished for the bi-hole, tri-hole, and random mixture of holes when compared to the LSPR of the single nanohole. As the distance between the nanoholes in a local area is varied, the LSPR peak shifts in various wavelengths, leading to the broad absorption peak seen in experiment (Supplementary Fig.S5). 55 The FDTD simulations show that the LSPR peak red-shifts to 800 nm when disorder dominates, which is consistent with the previous studies on disordered nanohole arrays. 50,54 To clarify the change in SPP mode with disorder, initially a simulation was run for a single unit cell of two single holes, three dimers, and two trimer holes of different orientations, replicating a very disordered local area (Supplementary Fig.S14b). The effects of disorder on the SPP and LSPR modes can be investigated by systematically changing the amount of times the disordered unit cell is replicated, and the degree with which the nanohole pairs inside the unit cell are misaligned. For example, Supplementary Fig.S14b compares the evolution of the SPP and LSPR modes as the periodicity of the hole array changes from a unit cell of misaligned holes (local disorder), to the same unit cell but duplicated multiple times using periodic boundary conditions (periodic replica of local disorder), and finally to a perfectly aligned set of well-ordered holes in a unit cell duplicated many times over (perfect both long-range order and short-range order). As long-range order increases and the local disorder decreases, the SPP peak increases in the transmission intensity, which is in agreement with our own experiments and

16 other s previous results. 50,54,55 Since SPP is due to the momentum coupling of the Bragg grating, a periodic order of several wavelengths is enough for the mode to start, with further order acting only to increase the peak quality. In highly disordered structures, the SPP modes are expected to be spatially localized, leading to a wide variety of resonant frequencies and broadening the transmission peak. 56,57 On the other hand, the LSPR peak is relatively unaffected by increasing the long-range periodicity. The LSPR peak becomes narrower as the trimer hole groups are aligned along the same axis and repeated multiple times, but the absorbance intensity does not vary like the SPP mode and is always present. 58,59 Again it should be noted the LSPR peak shifts when going from a tri-hole array to a single hole array due to the reduced inter-hole coupling as shown in Supplementary Fig.S14b. In short, The FDTD simulation results reveal that the LSPR peak depends on the assembly (lay-out) of holes in the local area. On the other hand, the SPP peak depends on the order of holes and the periodicity over a large area. The intensity of SPP mode increases with increasing periodicity of the hole array. The IPCE of the disordered Au hole array in Supplementary Fig.S13 accurately reflects the conclusions of the FDTD analysis. Even though the SPP is weak and highly broadened in the disordered structure, an enhancement is still seen in the solar energy conversion efficiency at the SPP transmission wavelength. The results of the disordered hole array prove that, even in cases of extreme disorder, localized modes can still exist, which leads to an increase in the IPCE. As the pattern becomes more ordered like that used in experiments, the SPP mode becomes stronger and the transmission increases, further increasing the strength of the enhancement but not changing the origin of the enhancement. Even with large disorder, the SPP mode is still responsible for the enhancement in the IPCE at the wavelengths below hematite s band edge. However, the red-shift in the LSPR peak to 800 nm no longer allows the spectral overlap necessary for PIRET, removing the LSPR-associated enhancement mechanism in the disordered IPCE curve.

17 Supplementary References 54. Murray, W. A., Astilean, S. & Barnes, W. L. Transition from localized surface plasmon resonance to extended surface plasmon-polariton as metallic nanoparticles merge to form a periodic hole array. Phys. Rev. B. 69, (2004). 55. Eftekhari, F.& Gordon, R. Enhanced second harmonic generation from noncentrosymmetric nanohole arrays in a gold film. J. Selected Topics Quan. Elect. 14, (2008). 56. Stockman, M. I., Faleev, S. V., David J. & Bergman, D. J. Localization versus Delocalization of Surface Plasmons in Nanosystems: Can One State Have Both Characteristics? Phys. Rev. B. 87, (2001). 57. Khurgin, J. B. & Sun, G. Impact of disorder on surface plasmons in two-dimensional arrays of metal nanoparticles. Appl. Phys. Lett. 94, (2009). 58. Yang, Z. L., Li, Q. H., Ren, B. & Tian, Z. Q. Tunable SERS from aluminium nanohole arrays in the ultraviolet region. Chem. Commun. 47, (2011). 59. Chu, Y. & Crozier, K. B. Experimental study of the interaction between localized and propagating surface plasmons. Opt. Lett. 34, (2009).