Critical Coupling of Visible Light Extends Hot-Electron Lifetimes for H2O2 Synthesis

Devices driven by above-equilibrium “hot” electrons are appealing for photocatalytic technologies, such as in situ H2O2 synthesis, but currently suffer from low (<1%) overall quantum efficiencies. Gold nanostructures excited by visible light generate hot electrons that can inject into a neighboring semiconductor to drive electrochemical reactions. Here, we designed and studied a metal–insulator–metal (MIM) structure of Au nanoparticles on a ZnO/TiO2/Al film stack, deposited through room-temperature, lithography-free methods. Light absorption, electron injection efficiency, and photocatalytic yield in this device are superior in comparison to the same stack without Al. Our device absorbs >60% of light at the Au localized surface plasmon resonance (LSPR) peak near 530 nm—a 5-fold enhancement in Au absorption due to critical coupling to an Al film. Furthermore, we show through ultrafast pump–probe spectroscopy that the Al-coupled samples exhibit a nearly 5-fold improvement in hot-electron injection efficiency as compared to a non-Al device, with the hot-electron lifetimes extending to >2 ps in devices photoexcited with fluence of 0.1 mJ cm−2. The use of an Al film also enhances the photocatalytic yield of H2O2 more than 3-fold in a visible-light-driven reactor. Altogether, we show that the critical coupling of Al films to Au nanoparticles is a low-cost, lithography-free method for improving visible-light capture, extending hot-carrier lifetimes, and ultimately increasing the rate of in situ H2O2 generation.

We immersed samples with ZnO films in 7.5 ml of the pre-irradiated gold solution in a 50 ml beaker. We shook the beaker at 80 rpm under a pulsed UV-A illumination of 30 W m -2 , with a pulse pattern of 5 sec on + 5 sec off repeated for a full 60 seconds. Samples were then removed from the solution, rinsed with ethanol, and dried with N2. The sample surface exhibited a consistent purple color, with the absorbance profile confirmed via UV-Vis spectroscopy in an integrating sphere.

Material Characterization -SEM, XRD, and ICP-OES:
We analyzed the morphology of samples with a FEI Quanta 3D scanning electron microscope (SEM), employing both secondary electron (SE) and backscattered electron (BSE) detectors. We used a SE detector to image the morphology of the ZnO film and size of Au nanoparticles, while the BSE detector enhanced the atomic number contrast, permitting better differentiation between the Au nanostructures and underlying ZnO film.
The crystal structure of our ZnO films was identified through out-of-plane XRD using a PANalytical X-ray diffractometer operating at 45 kV and 40 mA. The θ-2θ radial scan was performed over the range 30-45º with a step size of 0.01º and dwell time of 120 s, using Cu Kα (λ=1.54 Å) as the radiation source.
We quantified the Au and ZnO mass composition of our samples with a PerkinElmer Optima 8000 Optical Emission Spectrometer (OES). Samples for OES measurement were digested in 10 ml Aqua Regia and the resulting solution was diluted with 40 ml of de-ionized water. Metal standards (Au, Zn) were purchased from BDH (VWR Analytical) and diluted to the measurable 4 range with a similar HCl/HNO3 acid matrix as the experimental solutions. The linear fit of each standard was > 0.9999 across a concentration range spanning 3 orders of magnitude.

Finite Element Method Simulations:
We used the Maxwell equation solver JCMsuite v3.18 (JCMwave GmbH) to calculate the absorption in the various layers of the MIM structure as a function of wavelength and ZnO thickness. JCMsuite uses hp-refined finite element method to solve for the full electromagnetic field. We used a polynomial degree of four for all electromagnetic simulations. Typical meshes for the geometries are shown in Fig. 2a. We modeled the Au nanoparticles as isolated structures with a perfectly matched layer (PML) on all boundaries of the mesh; PML thickness was tuned by JCMsuite's internal algorithm. We excited all geometries with normal incidence plane waves and TM and TE polarizations. Due to the symmetry of the geometry the polarization did not affect the results.
Ultrafast Pump-Probe Spectroscopy, cont.: The time-resolved spectroscopic data was first processed using Surface Xplorer software to correct for any artifacts introduced by temporal dispersion of the probe (chirp). Time zero was identified for four different wavelengths, then a third order polynomial was fit to the data points to correct for time zero over the full spectrum. This procedure was performed for both transmission and reflection data sets. 5 A FLIR Blackfly photodiode camera was used to capture the pump and probe profile and a Gaussian function was used to extract the FWHM of the pump and probe beams to measure fluence using an effective area of the overlap of the pump and the probe: The FWHM at the sample position were 157 µm for 600 nm pump, 164 µm for 540 nm pump, 150 µm for 535 nm pump, 192 µm for 500 nm pump, 179 µm for 450 nm pump, 225 µm for 330 nm pump, and ~90 µm for the probe.

Ultraviolet Photoelectron Spectroscopy (UPS):
We performed ultraviolet photoelectron spectroscopy (UPS) measurements at the 5-meter toroidal grating monochromator (5m-TGM) beamline at the Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University, described in detail elsewhere. 1 The beamline is equipped with an ultrahigh vacuum (UHV) chamber endstation, which carries an Omicron EA125 hemispherical electron energy analyzer with a five channel detector. The endstation was kept under UHV with a pressure of 10 −10 mbar. The samples were held on tantalum holder by placing two tantalum stripes over the films. All samples were gently sputtered (1 keV Ne + for 2 minutes at 1x10 -5 Torr) to remove the contaminants from the surface. All spectra were collected in normal emission geometry and normalized to a point on the background formed by inelastic secondary electrons. UPS spectra were collected with a constant pass energy of 10 eV and a photon energy of 65 eV. The binding energies in UPS spectra are referenced with respect to the Fermi level of a gold foil in electrical contact with the sample holder.

S3: XRD and Scherrer Equations to estimate ZnO grain size
The Scherrer equation can be used to estimate the grain size of crystalline materials by: Where τ is mean size of ordered (crystalline) domains, K is dimensionless shape factor (assumed to be 0.9 here), λ is the X-ray wavelength (Cu kα1 = 1.5416 Å), B is the full-width half max (FWHM) in radians, and θ is the Bragg angle. With XRD, we examined the (100), (002), and (101) orientation peaks of our wurtzite ZnO and performed a Gaussian fit to calculate B, and by extension τ. The same test was performed on two samples, the data for which is represented in Figure S8. The ZnO grain size, averaged across both samples and all orientations, is estimated to be 12.1 ± 2.9 nm. Note the sharp peak at 2θ ≈ 38.5° represents the (111) orientation of the Al substrate upon which the ZnO was spun.  As observed in the Figure S11, utilizing a back-scatter detector with our SEM imaging provided better clarity between the Au nanoparticles and ZnO film. Using ImageJ software (freely available online), we adjusted the image contrast and binarized the image to select the Au domains ( Figure S10). Further filtering of particles based on overall size (> 2 nm diameter and < 80 nm) and circularity (> 0.6) were used to eliminate artificial Au clusters from the particle size analysis. Results for Au nanoparticle size were binned in 5 nm size increments for three images of varying magnification. For each image, the number of particles analyzed was > 500.

S5: ICP-OES measurements of Au loading
To determine the mass loading of Au, we started by estimating and measuring the total

S6: Additional Pump-Probe Data
The two-temperature model, adapted from Sun et al. 2

is based on Equation S2
: Where H(t) is the Heaviside function, ℎ ′ is the decay time of the nonthermal population, ℎ is  Quantitative data for the Au e-ph coupling times (fit by TTM) and ZnO rise time is provided in Table S1.  was used for the UV pump.

Sample dT/T |NT τth (ps) dT/T |Th τe-ph (ps) dT/T |L
Where EG is the bandgap of ZnO, is the initial binding energy (i.e. VBM), is the initial binding energy to the Zn core level (without Au), is the binding energy of the core level with Au over layer, and is the difference of initial binding energy to core level binding.
From the measured values of the parameters through UV-Vis absorbance and UPS, we determine that our effective Schottky barrier height is ~ 0.3 eV.