Strong near‐field coupling between small catalytic Pt nanoparticles (NPs) and their dielectric supports can be achieved while visible light absorption in the Pt NPs is greatly enhanced via rationally designing the dielectric–Pt core–satellite configuration. Herein, it is demonstrated that the absorption enhancement is mediated by the Mie resonances of the support and the absorption peaks are induced by the magnetic Mie resonances. Utilizing both multipole decomposition analysis and numerical near‐field mapping, resonances including magnetic dipole, magnetic quadrupole, and higher modes are verified. For the generation of strong Mie resonances, a dielectric support with a high refractive index, like TiO2, can be used not only as the core directly, but also as a shell coated beneath or above the Pt NP layer. Additionally, the redshifts between the absorption peaks contributed by the Pt NPs and the scattering peaks contributed by the dielectric core are ascribed to the intrinsical spectral deviation between near‐ and far‐field optical properties. These findings benefit significantly both the fundamental understanding and optimized design of dielectric supported Pt photocatalysts, providing new opportunities for solar energy conversion and visible light photocatalysis using nonplasmonic catalytic transition metals.
The design of antenna–reactor photocatalysts has become a powerful strategy to covert transition metal reactors from traditional thermocatalysts to novel photocatalysts. Plasmonic metals are often used as the optical antenna. Here, we demonstrate that conventional dielectric supports with high refractive index are able to achieve comparable performance as the plasmonic antennas, giving rise to a huge enhancement of the visible light absorption in the small Pt nanoparticles (NPs) of the core–satellite antenna–reactor photocatalysts through resonance energy transfer. The absorption enhancement can be mediated not only by the electric resonances of the plasmonic antenna but also by the magnetic resonances of the dielectric antenna. A large enough dielectric antenna or a small plasmonic antenna is desired for the generation of strong optical resonances. Judged by those Pt NPs with strong visible light absorption enhancement, the potential catalytically active sites are mainly distributed at the back side of the dielectric antenna, while they may be widely distributed over the surface of the plasmonic antenna with probably lower activities. The expanding scope of antenna–reactor photocatalysts offers new opportunities for solar to chemical energy conversion using nonplasmonic catalytic transition metals.
Visible-light-responsive WO3 porous films were synthesized via step-voltage anodization in NH4F/(NH4)2SO4 solution and calcined at various temperatures. The crystalline phase and surface morphology were characterized using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The as-anodized nanoporous films converted to a monoclinic phase with preferential orientation in the (020) planes, and the pore diameters of the films calcined below 450°C were estimated to be in the region of 50-100 nm. The photocatalytic activity was evaluated via photodegradation of methyl orange. The film calcined at 450°C showed the highest photocatalytic activity. Photoelectrochemical measurements showed that the incident photon-to-current conversion efficiency (IPCE) values of the film calcined at 450°C were 87.4% at 340 nm and 22.1% at 400 nm. Under visible light (λ ≥400 nm), the photocurrent density in 0.5 mol •L-1 H2SO4 solution at 1.2 V (vs Ag/AgCl (KCl saturated)) was 5.11 mA•cm-2. Electrochemical impedance spectroscopy (EIS) measurements showed that the film calcined at 450°C exhibited the smallest interface charge transfer resistance and optimal electroconductivity. Perfect crystallinity, high porosity and low resistance can therefore be obtained by controlling the calcination temperature. A large surface area and a porous structure are important factors in affecting photocatalytic activity.
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