Light-Induced Agglomeration of Single-Atom Platinum in Photocatalysis.

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.

Facet-Control versus Co-Catalyst-Control in Photocatalytic H2 Evolution from Anatase TiO2 Nanocrystals.

Titanium dioxide (TiO2 ) and, in particular, its anatase polymorph, is widely studied for photocatalytic H2 production. In the present work, we examine the importance of reactive facets of anatase crystallites on the photocatalytic H2 evolution from aqueous methanol solutions. For this, we synthesized anatase TiO2 nanocrystals with a large amount of either {001} facets, that is, nanosheets, or {101} facets, that is, octahedral nanocubes, and examined their photocatalytic H2 evolution and then repeated this procedure with samples where Pt co-catalyst is present on all facets. Octahedral nanocubes with abundant {101} facets produce >4 times more H2 than nanosheets enriched in {001} facets if the reaction is carried out under co-catalyst-free conditions. For samples that carry Pt co-catalyst on both {001} and {101} facets, faceting loses entirely its significance. This demonstrates that the beneficial role of faceting, namely the introduction of {101} facets that act as electron transfer mediator is relevant only for co-catalyst-free TiO2 surfaces.

A drastic improvement in photocatalytic H2 production by TiO2 nanosheets grown directly on Ta2O5 substrates.

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 nanosheets (NSs), while the most frequently reported substrate used for its synthesis is a fluorine-doped tin oxide (FTO). Herein we demonstrate that anatase TiO2 NS layers can be similarly grown on a Ta2O5 substrate. We found that a Ta2O5 back contact provides a remarkable improvement of the photocatalytic activity of the TiO2 NSs in comparison to a FTO back contact. The TiO2 NSs on Ta2O5 exhibit a 170-fold increase in photocatalytic H2 production rate than that obtained by TiO2 NSs on FTO substrate. The proposed mechanism reveals that such a drastic enhancement of optimized TiO2 NS arrays on the Ta2O5 substrate is attributed to the blocking nature of Ta2O5 for photo-generated electrons in the TiO2 NSs.

As a single atom Pd outperforms Pt as the most active co-catalyst for photocatalytic H2 evolution.

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, TiO2. To trap the single atoms on the surface, we introduced controlled surface vacancies (Ti3+-Ov) on anatase TiO2 nanosheets by a thermal reduction treatment. After anchoring identical loadings of single atoms of Pd, Pt, and Au, we measure the photocatalytic H2 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 TiO2 surface.

Photoelectrochemical performance of facet-controlled TiO2 nanosheets grown hydrothermally on FTO.

Single crystal anatase TiO2 nanosheets (TiO2-NSs) are grown hydrothermally on fluorine-doped tin oxide (FTO). By systematically changing the hydrothermal conditions such as reaction time, initial concentration of Ti precursor, F precursor, and HCl as an additive, a wide variety of TiO2-NSs, with different morphologies and faceting have been synthesized. For the different morphologies and different facet ratios (anatase S 001/S 001+101), the photoelectrochemical response is characterized and compared. We find that for photoanodes in neutral electrolytes, the magnitude of the photocurrent depends strongly on the growth parameters, that is, peak IPCEs can vary from 11.7% to 61%. For a wide range of parameters, the key parameter deciding on the photocurrent is the effective electrochemically active area of the electrode. Only for very high facet ratios >91%, the photoresponse can be strongly influenced by faceting - for samples with a S 001/S 001+101 of 91%, IPCE value of ≈84% is obtained. This work defines not only optimized synthesis conditions for a most effective growth of these single crystalline electrode, but also represents fundamental data for further applications of such electrodes.

On the Controlled Loading of Single Platinum Atoms as a Co-Catalyst on TiO2 Anatase for Optimized Photocatalytic H2 Generation.

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 H2 generation is described. Thin sputtered TiO2 layers are used as a model photocatalyst, and compared to the more frequently used (001) anatase sheets. To form stable SA platinum, the TiO2 layers are reduced in Ar/H2 under different conditions (leading to different but defined Ti3+ -Ov 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 TiO2 sputtered sample by 150 times in comparison to a conventional platinum-nanoparticle-decorated TiO2 surface. HAADF-STEM, XPS, and EPR investigation jointly confirm the atomic nature of the decorated Pt on TiO2 . 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.