Preparation of Core–Shell‐Structured Nanoparticles (with a Noble‐Metal or Metal Oxide Core and a Chromia Shell) and Their Application in Water Splitting by Means of Visible Light

Abstract

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.

Development of Novel Photocatalyst and Cocatalyst Materials for Water Splitting under Visible Light

“…The Cr 2 O 3 shell thickness was also dependent on the pH of the solution in which the photoreduction of K 2 CrO 4 was conducted. 76 As a result, the water-splitting activity of samples prepared at different pHs varied. When the deposition pH ranged from 3.0 to 7.5, there was almost no change in activity, with stoichiometric evolution of H 2 and O 2 under visible light.…”

Solution-phase Synthesis of Zn-doped GaN Photocatalysts: Morphology, Composition, and Catalytic Activity towards Methylene Blue Degradation and 4-nitroaniline Conversion

“…In order to make GaN absorb in the visible region, studies have been conducted to modify the band structure through solid solutions of GaN and metal oxides, especially ZnO. [6,7,8] Metal nitrides and oxynitrides have been developed and studied as photocatalysts owing to their lower band gap energy. Wang and co-workers have studied GaN nanowires synthesized by molecular beam epitaxy and reported its photocatalytic activity towards water splitting.…”

“…Notably, these oxynitrides are capable of utilizing visible light and act as stoichiometric water splitting photocatalysts when loaded with suitable co-catalysts. [7,18,19,20,21] Based on DFT calculations, the bottom of the conduction band of (Zn x Ga 1-x )(O x N 1-x ) is mainly formed from 4s and 4p orbitals of Ga while N 2p and Zn 3d contribute to form the top of the valence band. This orbital arrangement is responsible for p-d repulsion, which creates a valence band maximum, resulting in a decrease in Bandgap.…”

“…[18] In addition to synthesis of photocatalysts, Domen and others have studied the effect of different compositions and structure of co-catalysts. Among the wide range of co-catalysts studied, core-shell type Ni@NiO, [7,22] Rh@Cr 2 O 3 [7] and RuO 2 [19,23] have shown significant increase in activity towards overall water splitting. It is reported that the physicochemical state of the core, in core-shell type, and the dispersion plays a significant role in enhancing water splitting.…”

“…For instance, (Zn x Ga 1-x )(O x N 1-x ) loaded with Rh@Cr 2 O 3 core-shell co-catalyst has produced 6 and 12 times more H 2 than the same core-shell structure in which the core was replaced with Pt and Pd, respectively. [7] Environmental pollution remediation is becoming a key issue due to increasing organic pollutants in air and water systems and consequent stringent regulations. Development of new methods to decompose organic pollutants is an imperative task.…”