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]

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.

The engineering of the electron transport layer (ETL)/light absorber interface is explored in perovskite solar cells. Single‐crystalline TiO2 nanorod (NR) arrays are used as ETL and methylammonium lead iodide (MAPI) as light absorber. A dual ETL surface modification is investigated, namely by a TiCl4 treatment combined with a subsequent PC61BM monolayer deposition, and the effects on the device photovoltaic performance were evaluated with respect to single modifications. Under optimized conditions, for the combined treatment synergistic effects are observed that lead to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to suppression of hysteresis. The devices show JSC, VOC, and fill factor as high as 23.2 mA cm−2, 1.1 V, and 77%, respectively. These results are ascribed to a more efficient charge transfer across the ETL/perovskite interface, which originates from the passivation of defects and trap states at the ETL surface. To the best of our knowledge, this is the highest cell performance ever reported for TiO2 NR‐based solar cells fabricated with conventional MAPI light absorber. Perspective wise, this ETL surface functionalization approach combined with more recently developed and better performing light absorbers, such as mixed cation/anion hybrid perovskite materials, is expected to provide further performance enhancements.

With recent advances in the field of single‐atoms (SAs) used in photocatalysis, an unprecedented performance of atomically dispersed co‐catalysts has been achieved. However, the stability and agglomeration of SA co‐catalysts on the semiconductor surface may represent a critical issue in potential applications. Here, the photoinduced destabilization of Pt SAs on the benchmark photocatalyst, TiO2, is described. In aqueous solutions within illumination timescales ranging from few minutes to several hours, light‐induced agglomeration of Pt SAs to ensembles (dimers, multimers) and finally nanoparticles takes place. The kinetics critically depends on the presence of sacrificial hole scavengers and the used light intensity. Density‐functional theory calculations attribute the light induced destabilization of the SA Pt species to binding of surface‐coordinated Pt with solution‐hydrogen (adsorbed H atoms), which consequently weakens the Pt SA bonding to the TiO2 surface. Despite the gradual aggregation of Pt SAs into surface clusters and their overall reduction to metallic state, which involves >90% of Pt SAs, the overall photocatalytic H2 evolution remains virtually unaffected.

Trapping sites in single atom (SA) catalysts are critical to the stabilization and reactivity of isolated atoms. Herein, we show that anchoring of Pt SAs on TiO 2 nanosheets is strongly aided by lattice incorporated fluorine species. Tailoring the speciation of fluorine on TiO 2 nanosheets is a key factor for uniform and stable dispersion of the Pt SAs and high efficiency in Pt SA co-catalyzed photocatalytic H 2 production. Fluorinestabilized uniformly dispersed Pt SAs on the (001) surface of TiO 2 can provide a remarkable photocatalytic activity (a H 2 production rate of 45.3 mmol h −1 mg −1 Pt for 65 mW/cm 2 365 nm light). This high (maximized) efficiency can be achieved with a remarkably low loading amount of Pt SAs on TiO 2 nanosheets (0.03 wt %), which is far superior to Pt nanoparticles on a TiO 2 nanosheet with the same or a higher loading amount. F-stabilized Pt SAs on TiO 2 nanosheets also exhibit an excellent stability in long-term photocatalytic reactions.

Titanium dioxide (TiO2) is the most frequently studied semiconducting material for photocatalytic water splitting. One of the favored forms of TiO2 for photocatalytic applications is layers of erected single-crystalline anatase...

Single crystal anatase TiO2 nanosheets (TiO2-NSs) are grown hydrothermally on fluorine-doped tin oxide (FTO).

TiO2 has been the benchmark semiconductor for the production of photocatalytic H2 from aqueous media (with and without sacrificial agent). On TiO2 surfaces, the photocatalytic H2 evolution reaction in aqueous environments is kinetically severely hampered. To overcome this limitation and reach reasonable H2 generation rates, a well-elucidated approach is the use of noble metal co-catalysts. In contrast to costly noble metal approaches, it recently has been reported that titania reduction treatments can lead to a noble-metal-free photocatalytic H2 generation. So-called “grey” titania due to Ti3+ states shows intrinsically activated photocatalytic H2 evolution [1–3]. The present work demonstrates the feasibility to use in-situ photoinduced reduction to create Ti3+ states that act as intrinsic catalyst and activate hydrothermal synthetized anatase nanosheets for H2 generation to mediate the transfer of photo-induced charge carriers to the electrolyte.