We devised a hot-injection synthesis to prepare colloidal double-perovskite Cs 2 NaBiCl 6 nanocrystals (NCs). We also examined the effects of replacing Na + with Ag + cations by preparing and characterizing Cs 2 Na 1– x Ag x BiCl 6 alloy NCs with x ranging from 0 to 1. Whereas Cs 2 NaBiCl 6 NCs were not emissive, Cs 2 Na 1– x Ag x BiCl 6 NCs featured a broad photoluminescence band at ∼690 nm, Stokes-shifted from the respective absorption by ≥1.5 eV. The emission efficiency was maximized for low Ag + amounts, reaching ∼3% for the Cs 2 Na 0.95 Ag 0.05 BiCl 6 composition. Density functional theory calculations coupled with spectroscopic investigations revealed that Cs 2 Na 1– x Ag x BiCl 6 NCs are characterized by a complex photophysics stemming from the interplay of (i) radiative recombination via trapped excitons localized in spatially connected AgCl 6 –BiCl 6 octahedra; (ii) surface traps, located on undercoordinated surface Bi centers, behaving as phonon-assisted nonradiative decay channels; and (iii) a thermal equilibrium between trapping and detrapping processes. These results offer insights into developing double-perovskite NCs with enhanced optoelectronic efficiency.

The combination of two or more metals, forming alloys, core−shells, or other complex heterometallic nanostructures, has substantially spanned the available options to finely tune electronic and structural properties, opening a myriad of opportunities that has yet to be fully explored in different fields. In catalysis, the rational exploitation and design of bimetallic and trimetallic catalysts has just started. Several major aspects such as stability, phase segregation, and alloy− dealloy mechanisms have yet to be deeply understood and correlated with intrinsic factors such as nanoparticle size, composition, and structure and with extrinsic factors, or external agents, such as temperature, reaction gases, and support. Here, by combining model catalysts based on AuCu nanoparticles supported on silica or alumina with in situ characterization techniques under redox pretreatments and CO oxidation reaction, we demonstrate the crucial role of the support with regard to determining the stable active phase of bimetallic supported catalysts. This strategy, associated with theoretical studies, could lead to the rational design of unique active sites.

Metallic two-dimensional transition-metal dichalcogenides (TMDs) of the group 5 metals are emerging as catalysts for an efficient hydrogen evolution reaction (HER). The HER activity of the group 5 TMDs originates from the unsaturated chalcogen edges and the highly active surface basal planes, whereas the HER activity of the widely studied group 6 TMDs originates solely from the chalcogen-or metal-unsaturated edges. However, the batch production of such nanomaterials and their scalable processing into high-performance electrocatalysts is still challenging. Herein, we report the liquid-phase exfoliation of the 2H-TaS 2 crystals by using 2-propanol to produce single/few-layer (1H/2H) flakes, which are afterward deposited as catalytic films. A thermal treatment-aided texturization of the catalytic films is used to increase their porosity, promoting the ion access to the basal planes of the flakes, as well as the number of catalytic edges of the flakes. The hybridization of the H-TaS 2 flakes and H-TaSe 2 flakes tunes the Gibbs free energy of the adsorbed atomic hydrogen onto the H-TaS 2 basal planes to the optimal thermo-neutral value. In 0.5 M H 2 SO 4 , the heterogeneous catalysts exhibit a low overpotential (versus RHE, reversible hydrogen electrode) at the cathodic current of 10 mA cm −2 (η 10 ) of 120 mV and high mass activity of 314 A g −1 at an overpotential of 200 mV. In 1 M KOH, they show a η 10 of 230 mV and a mass activity of 220 A g −1 at an overpotential of 300 mV. Our results provide new insight into the usage of the metallic group 5 TMDs for the HER through scalable material preparation and electrode processing. KEYWORDS: transition-metal dichalcogenides (TMDs), tantalum disulfide (TaS 2 ), tantalum diselenide (TaSe 2 ), hydrogen evolution reaction (HER), heterogeneous catalysts

of using molecular hydrogen (H 2) as carbon-free fuel produced by renewable energy sources. [2] To embrace H 2 as game changer, academic research is struggling to develop advanced electrolysers, while reducing their investment and operational costs. [3] In this context, proton exchange membrane (PEM) water electrolysers overcome several operational drawbacks of commercial alkaline ones, [4] for example, low maximum achievable current density (between 200 and 400 mA cm −2), low operating pressure (<30 bar), inefficient dynamic operation (acceptable part-load operation between 10 and 40% of the nominal load), and gas crossover phenomena (typical gas purity <99.9%). [3,5] Nonetheless, the costs and the scarcity of their most effective catalysts, for example, Pt-group elements for the hydrogen evolution reactions (HER) at the cathode, [6,7] and RuO 2 /IrO 2 for the oxygen evolution reactions at the anode, [8,9] hinder massive commercial products. [10] To face the cost-related barriers of the PEM electrolysers, it is mandatory to search for alternative nonprecious catalysts, [11,12] or at least to reduce the content of precious metals, while still maintaining the electrochemical The nanoengineering of the structure of transition metal dichalcogenides (TMDs) is widely pursued to develop viable catalysts for the hydrogen evolution reaction (HER) alternative to the precious metallic ones. Metallic group-5 TMDs have been demonstrated to be effective catalysts for the HER in acidic media, making affordable real proton exchange membrane water electrolysers. Their key-plus relies on the fact that both their basal planes and edges are catalytically active for the HER. In this work, the 6R phase of TaS 2 is "rediscovered" and engineered. A liquid-phase microwave treatment is used to modify the structural properties of the 6R-TaS 2 nanoflakes produced by liquid-phase exfoliation. The fragmentation of the nanoflakes and their evolution from monocrystalline to partly polycrystalline structures improve the HER-activity, lowering the overpotential at cathodic current of 10 mA cm −2 from 0.377 to 0.119 V. Furthermore, 6R-TaS 2 nanoflakes act as ideal support to firmly trap Pt species, which achieve a mass activity (MA) up 10 000 A g Pt −1 at overpotential of 50 mV (20 000 A g Pt −1 at overpotentials of 72 mV), representing a 20-fold increase of the MA of Pt measured for the Pt/C reference, and approaching the state-of-the-art of the Pt mass activity.

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