Mechanistic insight into deep holes from interband transitions in Palladium nanoparticle photocatalysts

Abstract

Summary Utilizing hot electrons generated from localized surface plasmon resonance is of widespread interest in the photocatalysis of metallic nanoparticles. However, hot holes, especially generated from interband transitions, have not been fully explored for photocatalysis yet. In this study, a photocatalyzed Suzuki-Miyaura reaction using mesoporous Pd nanoparticle photocatalyst served as a model to study the role of hot holes. Quantum yields of the photocatalysts increase under shorter wavelength excitations and correlate to “deeper” energy of the holes from the Fermi level. This work suggests that deeper holes in the d -band catalyze the oxidative addition of aryl halide R-X onto Pd 0 at the nanoparticles' surface to form R-Pd II -X complex, thus accelerating the rate-determining step of the catalytic cycle. The hot electrons do not play a decisive role. In the future, catalytic mechanisms induced by deep holes should deserve as much attention as the well-known hot electron transfer mechanism.

Low-Temperature Ethylbenzene Conversion on Rutile TiO2(100) via Photocatalysis: The Strong Photon Energy Dependence

“…Such a wavelength-dependent LT C 6 H 5 CHCH 2 formation cannot be explained well by the simplified TiO 2 photocatalysis model. With reference to several previous works on other photocatalytic systems, “deep holes” originated from higher energy photoexcitation are considered to be have a stronger oxidizing ability, , consistent with the higher yield of low-temperature C 6 H 5 CHCH 2 under 257 nm irradiation in this work. However, the use of excess energy of deep holes can follow two different pathways .…”

Photocatalysis of Metallic Nanoparticles: Interband vs Intraband Induced Mechanisms

“…The crystal temperature rises as soon as this scattering starts, and heat transfer to the surrounding environment happens with a time constant of several hundred picoseconds. , These time constants were mainly collected from ultrafast spectroscopies in which each nanocrystal absorbed multiple photons within an ultrashort laser pulse (about 100 fs) in order to achieve a significant transient signal for detection. This condition is quite different from continuous-wave irradiation where each nanocrystal roughly absorbs the two consecutive photons within an estimated interval of about a hundred nanoseconds . This estimation was based on experimental conditions where a high power LED (∼500 mW) was used, and the colloidal solution of the photocatalysts has a high optical absorption (OD = 0.42).…”

“…As mentioned in Section , the LSPR-IB spectral overlap can be tricky to assign the correct energy levels of the hot holes and their exact contribution to the photocatalytic mechanisms. To reduce this overlap and focus on the photocatalytic properties of the d-band hot holes, our group utilized mesoporous palladium nanocrystals whose LSPR is shifted to the near-infrared region, but their IBs are in the visible . The deeper holes generated from shorter-wavelength excitation with stronger oxidation power catalyze better the oxidation addition of aryl halide onto the palladium surface (the rate-determining step of the Suzuki–Miyaura reaction, Figure c), thus offering a higher product yield .…”

“…While the hot-electron mediated reactions have been well demonstrated, the hot-hole mediated ones have recently emerged. ,, Hot holes generated from LSPR can catalyze metal etching, organic transformations, − polymerization, and oxygen evolution reactions . The hot holes generated from IBs have much lower potentials below the Fermi level and lower energy than those generated from LSPR.…”

d-Band Holes React at the Tips of Gold Nanorods

“…d-band holes are promising for driving chemical reactions. − When plasmonic nanoparticles are illuminated, the conduction electrons in the nanoparticle oscillate with the electromagnetic field, leading to the excitation of plasmonic modes. , Nonradiative plasmon decay leads to the excitation of hot electrons above the Fermi level of the metal, and hot holes below the Fermi level with energies exceeding the lattice temperature. , These excited-state carriers can drive chemical reactions and have attracted attention for photocatalytic applications. − Many plasmonic materials, including gold, exhibit interband transitions between the d-band and sp-band. , Because of the high density of states in the d-band, plasmon decay above the interband threshold leads to rapid interband damping and the generation of deep-lying holes in the d-band. − The interband threshold has been reported as low as 1.7 eV for gold, indicating that d-band holes can contribute to photochemical reactions across most of the visible spectrum . Excitation of direct interband transitions above the plasmon energy in gold also generates hot d-band holes. − While plasmon-induced chemistry has long focused on exploiting hot electrons and holes in the sp-band, , it has recently become clear that d-band holes play an important role in plasmon-induced chemistry by enabling new reaction pathways. − …”