Advanced photocatalysts: Pinning single atom co-catalysts on titania nanotubes

Single atom (SA) catalysis, over the last 10 years, has become a forefront in heterogeneous catalysis, electrocatalysis, and most recently also in photocatalysis. Most crucial when engineering a SA catalyst/support system is the creation of defined anchoring points on the support surface to stabilize reactive SA sites. Here, a so far unexplored but evidently very effective approach to trap and stabilize SAs on a broadly used photocatalyst platform is introduced. In self-organized anodic TiO2 nanotubes, a high degree of stress is incorporated in the amorphous oxide during nanotube growth. During crystallization (by thermal annealing), this leads to a high density of Ti3+-O-v, surface defects that are hardly present in other common titania nanostructures (as nanoparticles). These defects are highly effective for SA iridium trapping. Thus a SA-Ir photocatalyst with a higher photocatalytic activity than for any classic co-catalyst arrangement on the semiconductive substrate is obtained. Hence, a tool for SA trapping on titania-based back-contacted platforms is provided for wide application in electrochemistry and photoelectrochemistry. Moreover, it is shown that stably trapped SAs provide virtually all photocatalytic reactivity, with turnover frequencies in the order of 4 x 10(6) h(-1) in spite of representing only a small fraction of the initially loaded SAs.Web of Science3130art. no. 210284

One-dimensional suboxide TiO2 nanotubes for electrodics applications

Finanziert aus dem Open-Access-Publikationsfonds der Universität Siegen für ZeitschriftenartikelThis mini-review summarizes the recent research on the applications of anodic one-dimensional suboxide titania nanostructures with enhanced electrodics properties by introducing atomic-scale defects into the titania systems. Due to the unique semiconductive properties of TiO2 nanostructures, so far, the main focus of research has been on photocatalytic applications of this semiconductor. The limited conductivity of pristine anodized titania restrains the application of one-dimensional (1D) nanostructured TiO2 as an electrode. However, recently published works present that suboxide TiO2 nanotubes, due to the enhanced conductivity, are promising electrodes for electrochemical applications. In this mini-review, we highlight the unique advantages of defective TiO2-x nanotubes as an electrode and address the recent electrodics applications of 1D anodic TiO2 nanotubes

TiO2 nanotube arrays decorated with Ir nanoparticles for enhanced hydrogen evolution electrocatalysis

Designing cost-effective hydrogen evolution reaction (HER) electrocatalysts containing highly active, but expensive platinum group metals (PGMs) is key to the commercialization of polymer electrolyte membrane water electrolysis systems for green hydrogen production. Our recent investigations have shown that efficient and durable HER composite cathodes can be prepared by spontaneous deposition of PGM nanoparticles on self-aligned titania nanotube (TNT) arrays formed by anodization [1]. In this synthesis route, anatase TNTs are first cathodically protonated (H-TNT), and then used as the reducing agent for PGM ions at room temperature. Herein, we employ the galvanic displacement strategy to decorate H-TNT arrays with ultrafine Ir nanoparticles [2]. We demonstrate that transforming the top surface morphology of supporting TNT arrays from ordered open-top tubes to bundled nanowires (“nanograss”) is beneficial for exposing more Ir active centers during the HER operation. Consequently, applying very low concentrations of Ir(III) ions in the galvanic displacement step is sufficient to produce exceptionally active nanograss-modified Ir@TNT composites. An optimum Ir@TNT, possessing a low Ir loading of 5.7 μgIr cm–2, requires an overpotential of only –63 mV to reach a current density of –100 mA cm–2 and shows a stable long-term performance in a 1 M HClO4 solution. Computational simulations suggest that the hydrogen-rich TiO2 support not only strongly interacts with anchored Ir particles and weakens their H binding strength to a moderate level, but also actively provides hydrogen for rejuvenation of the Ir active sites at the Ir|H-TiO2 interface, thereby significantly enhancing HER catalysis. [1] U.Č. Lačnjevac, R. Vasilić, T. Tokarski, G. Cios, P. Żabiński, N. Elezović and N. V. Krstajić, Nano Energy 47 (2018) 527. [2] U. Lačnjevac, R. Vasilić, A. Dobrota, S. Đurđić, O. Tomanec, R. Zbořil, S. Mohajernia, N.T. Nguyen, N. Skorodumova, D. Manojlović, N. Elezović, I. Pašti, P. Schmuki, Journal of Materials Chemistry A 8 (2020) 22773

As a single atom Pd outperforms Pt as the most active co-catalyst for photocatalytic H-2 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 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 TiO2 surface