Core-shell-structured nanoparticles, consisting of a noble metal or metal oxide core and a chromia (Cr(2)O(3)) shell, were studied as promoters for photocatalytic water splitting under visible light. Core nanoparticles were loaded by impregnation, adsorption or photodeposition onto a solid solution of gallium nitride and zinc oxide (abbreviated GaN:ZnO), which is a particulate semiconductor photocatalyst with a band gap of approximately 2.7 eV, and a Cr(2)O(3) shell was formed by photodeposition using a K(2)CrO(4) precursor. Photodeposition of Cr(2)O(3) on GaN:ZnO modified with a noble metal (Rh, Pd and Pt) or metal oxide (NiO(x), RuO(2) and Rh(2)O(3)) co-catalyst resulted in enhanced photocatalytic activity for overall water splitting under visible light (lambda>400 nm). This enhancement in activity was primarily due to the suppression of undesirable reverse reactions (H(2)-O(2) recombination and/or O(2) photoreduction) and/or protection of the core component from chemical corrosion, depending on the core type. Among the core materials examined, Rh species exhibited relatively high performance for this application. The activity for visible-light water splitting on GaN:ZnO modified with an Rh/Cr(2)O(3) core-shell configuration was dependent on both the dispersion of Rh nanoparticles and the valence state. In addition, the morphology of the Cr(2)O(3) photodeposits was significantly affected by the valence state of Rh and the pH at which the photoreduction of K(2)CrO(4) was conducted. When a sufficient amount of K(2)CrO(4) was used as the precursor and the solution pH ranged from 3 to 7.5, Cr(2)O(3) was successfully formed with a constant shell thickness (approximately 2 nm) on metallic Rh nanoparticles, which resulted in an effective promoter for overall water splitting.
Harmonische Zusammenarbeit: Nanopartikel aus Mn3O4 und aus Kern/Schale‐strukturiertem Rh/Cr2O3 als Cokatalysatoren auf der Oberfläche eines GaN‐ZnO‐Mischkristalls als Katalysator beschleunigen die O2‐ bzw. H2‐Entwicklung unter sichtbarem Licht (λ>420 nm) und verbessern so die Wasserspaltungsaktivität gegenüber analogen Systemen, die nur mit Mn3O4 oder Rh/Cr2O3 modifiziert wurden.
A gradual size control of poly(N-vinyl-2-pyrrolidone)-protected
Rh nanoparticles (PVP-Rh NPs) was successfully achieved in the range
of 1.7–7.7 nm by tuning the pH value and the reaction temperature
of the ethylene glycol (EG) solution; smaller NPs were formed at higher
pH and at higher temperature. This trend can be interpreted by the
change in the nucleation rate caused by tuning the pH value and/or
the temperature of the solution. When the size tuned PVP-Rh NPs were
applied for use as cocatalysts in a photocatalyst (solid solution
of GaN and ZnO (Ga1–x
Zn
x
)(N1–x
O
x
)) for overall water splitting under visible light,
it was demonstrated that smaller Rh cores gave higher activity than
the larger ones.
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