The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201908505.Single-atom (SA) catalysis is a novel frontline in the catalysis field due to the often drastically enhanced specific activity and selectivity of many catalytic reactions. Here, an atomic-scale defect engineering approach to form and control traps for platinum SA sites as co-catalyst for photocatalytic H 2 generation is described. Thin sputtered TiO 2 layers are used as a model photocatalyst, and compared to the more frequently used (001) anatase sheets. To form stable SA platinum, the TiO 2 layers are reduced in Ar/H 2 under different conditions (leading to different but defined Ti 3+ -O v surface defects), followed by immersion in a dilute hexachloroplatinic acid solution. HAADF-STEM results show that only on the thin-film substrate can the density of SA sites be successfully controlled by the degree of reduction by annealing. An optimized SA-Pt decoration can enhance the normalized photocatalytic activity of a TiO 2 sputtered sample by 150 times in comparison to a conventional platinum-nanoparticle-decorated TiO 2 surface. HAADF-STEM, XPS, and EPR investigation jointly confirm the atomic nature of the decorated Pt on TiO 2 . Importantly, the density of the relevant surface exposed defect centers-thus the density of Pt-SA sites, which play the key role in photocatalytic activity-can be precisely optimized.Single-atom (SA) or single-site catalysis (SACs) has over the past years become an increasingly fascinating topic in the catalysis field. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] SACs have allowed new approaches in heterogeneous catalysis, [12,13] minimized the use of precious metals, [14]
Surface exposed Ti3+ and lattice embedded Ti3+ in an optimum ratio is the determining factor for optimized photocatalytic H2 evolution.
Summary Here, we evaluate three different noble metal co-catalysts (Pd, Pt, and Au) that are present as single atoms (SAs) on the classic benchmark photocatalyst, TiO 2 . To trap the single atoms on the surface, we introduced controlled surface vacancies (Ti 3+ -O v ) on anatase TiO 2 nanosheets by a thermal reduction treatment. After anchoring identical loadings of single atoms of Pd, Pt, and Au, we measure the photocatalytic H 2 generation rate and compare it to the classic nanoparticle co-catalysts on the nanosheets. While nanoparticles yield the well-established the hydrogen evolution reaction activity sequence (Pt > Pd > Au), for the single atom form, Pd radically outperforms Pt and Au. Based on density functional theory (DFT), we ascribe this unusual photocatalytic co-catalyst sequence to the nature of the charge localization on the noble metal SAs embedded in the TiO 2 surface.
Separation and transfer of photogenerated charge carriers are key elements in designing photocatalysts. TiO2 in numerous geometries has been for many years the most studied photocatalyst. To overcome kinetic limitations and achieve swift charge transfer, TiO2 has been widely investigated with cocatalysts that are commonly randomly placed nanoparticles on a TiO2 surface. The poor control over cocatalyst placement in powder technology approaches can drastically hamper the photocatalytic efficiencies. Here in contrast it is shown that the site‐selective placement of suitable charge‐separation and charge‐transfer cocatalysts on a defined TiO2 nanotube morphology can provide an enhancement of the photocatalytic reactivity. A TiO2–WO3–Au electron‐transfer cascade photocatalyst is designed with nanoscale precision for H2 production on TiO2 nanotube arrays. Key aspects in the construction are the placement of the WO3/Au element at the nanotube top by site‐selective deposition and self‐ordered thermal dewetting of Au. In the ideal configuration, WO3 acts as a buffer layer for TiO2 conduction band electrons, allowing for their efficient transfer to the Au nanoparticles and then to a suitable environment for H2 generation, while TiO2 holes due to intrinsic upward band bending in the nanotube walls and short diffusion length undergo a facilitated transfer to the electrolyte where oxidation of hole‐scavenger molecules takes place. These photocatalytic structures can achieve H2 generation rates significantly higher than any individual cocatalyst–TiO2 combination, including a classic noble metal–TiO2 configuration.
In the present work we report on the key factors dictating the photoelectrochemical (PEC) performance of suboxide titania (TiO ) nanotubes. TiO nanotubes were produced by a systematic variation of reduction heat treatments of TiO in Ar/H . The properties of the TiO tubes were investigated by electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), solid-state conductivity, reflectivity measurements, photocurrent spectroscopy, and photoelectrochemical hydrogen evolution. In line with earlier literature, these suboxide tubes show a drastically improved photoelectrochemical water-splitting performance compared to non-reduced anatase TiO tubes. In this work we show that the key improvement in water-splitting performance is due to the strongly improved conductivity of TiO semimetalic tubes, reaching 13.5 KΩ per tube compared to 70 MΩ (for non-reduced anatase), and is not due to the enhanced visible-light absorbance.
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