2 research outputs found
On-Surface Carbon Nitride Growth from Polymerization of 2,5,8-Triazido-s-heptazine
Carbon nitrides have recently come into focus for photo- and thermal
catalysis, both as support materials for metal nanoparticles as well as
photocatalysts themselves. While many approaches for the synthesis of
three-dimensional carbon nitride materials are available, only top-down
approaches by exfoliation of powders lead to thin film flakes of this
inherently two-dimensional material. Here, we describe an in situ on-surface
synthesis of monolayer 2D carbon nitride films, as a first step towards precise
combination with other 2D materials. Starting with a single monomer precursor,
we show that 2,5,8-triazido-s-heptazine (TAH) can be evaporated intact,
deposited on a single crystalline Au(111) or graphite support, and activated
via azide decomposition and subsequent coupling to form a covalent
polyheptazine network. We demonstrate that the activation can occur in three
pathways, via electrons (X-ray illumination), photons (UV illumination) and
thermally. Our work paves the way to coat materials with extended carbon
nitride networks which are, as we show, stable under ambient conditions
Does Cluster Encapsulation Inhibit Sintering? Stabilization of Size-Selected Pt Clusters on Fe3O4(001) by SMSI
The metastability of supported metal nanoparticles limits their application in heterogeneous catalysis at elevated temperatures due to their tendency to sinter. One strategy to overcome these thermodynamic limits on reducible oxide supports is encapsulation via strong metal-support interaction (SMSI). While annealing-induced encapsulation is a well-explored phenomenon for extended nanoparticles, it is as yet unknown whether the same mechanisms hold for subnanometer clusters, where concomitant sintering and alloying might play a significant role. In this article, we explore the encapsulation and stability of size-selected Pt5, Pt10, and Pt19 clusters deposited on Fe3O4(001). In a multimodal approach using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM), we demonstrate that SMSI indeed leads to the formation of a defective, FeO-like conglomerate encapsulating the clusters. By stepwise annealing up to 1023 K, we observe the succession of encapsulation, cluster coalescence, and Ostwald ripening, resulting in square-shaped crystalline Pt particles, independent of the initial cluster size. The respective sintering onset temperatures scale with the cluster footprint and thus size. Remarkably, while small encapsulated clusters can still diffuse as a whole, atom detachment and thus Ostwald ripening are successfully suppressed up to 823 K, i.e., 200 K above the Hüttig temperature that indicates the thermodynamic stability limit