Atomic Layer Deposition of the Metal Pyrites FeS2 , CoS2 , and NiS2

2D FeSx Nanosheets by Atomic Layer Deposition: Electrocatalytic Properties for the Hydrogen Evolution Reaction

2-dimensional FeS_x nanosheets of different sizes are synthesized by applying different numbers of atomic layer deposition (ALD) cycles on TiO₂ nanotube layers and graphite sheets as supporting materials and used as an electrocatalyst for the hydrogen evolution reaction (HER). The electrochemical results confirm electrocatalytic activity in alkaline media with outstanding long-term stability (>65 h) and enhanced catalytic activity, reflected by a notable drop in the initial HER overpotential value (up to 26 %). By using a range of characterization techniques, the origin of the enhanced catalytic activity was found to be caused by the synergistic interplay between in situ morphological and compositional changes in the 2D FeS_x nanosheets during HER. Under the application of a cathodic potential in alkaline media, the as-synthesized 2D FeS_x nanosheets transformed into iron oxyhydroxide-iron oxysulfide core-shell nanoparticles, which exhibited a higher active catalytic surface and newly created Fe-based HER catalytic sites.

Atomic Layer Deposition of Cobalt Phosphide for Efficient Water Splitting

Transition-metal phosphides (TMP) prepared by atomic layer deposition (ALD) are reported for the first time. Ultrathin Co-P films were deposited by using PH3 plasma as the phosphorus source and an extra H2 plasma step to remove excess P in the growing films. The optimized ALD process proceeded by self-limited layer-by-layer growth, and the deposited Co-P films were highly pure and smooth. The Co-P films deposited via ALD exhibited better electrochemical and photoelectrochemical hydrogen evolution reaction (HER) activities than similar Co-P films prepared by the traditional post-phosphorization method. Moreover, the deposition of ultrathin Co-P films on periodic trenches was demonstrated, which highlights the broad and promising potential application of this ALD process for a conformal coating of TMP films on complex three-dimensional (3D) architectures.

Electrochemically Driven Cation Exchange Enables the Rational Design of Active CO2 Reduction Electrocatalysts

Metal oxides or sulfides are considered to be one of the most promising CO₂ reduction reaction (CO₂ RR) precatalysts, owing to their electrochemical conversion in situ into highly active electrocatalytic species. However, further improvement of the performance requires new tools to gain fine control over the composition of the active species and its structural features [e.g., grain boundaries (GBs) and undercoordinated sites (USs)], directly from a predesigned template material. Herein, we describe a novel electrochemically driven cation exchange (ED-CE) method that enables the conversion of a predesigned CoS₂ template into a CO₂ RR catalyst, Cu₂ S. By means of ED-CE, the final Cu₂ S catalyst inherits the original 3 D morphology of CoS₂ , and preserves its high density of GBs. Additionally, the catalyst's phase structure, composition, and density of USs were precisely tuned, thus enabling rational design of active CO₂ RR sites. The obtained Cu₂ S catalyst achieved a CO₂ -to-formate Faradaic efficiency of over 87 % and a record high activity (among reported Cu-based catalysts). Hence, this study opens the way for utilization of ED-CE reactions to design advanced electrocatalysts.

Hydrogenation of Functionalized Nitroarenes Catalyzed by Single-Phase Pyrite FeS2 Nanoparticles on N,S-Codoped Porous Carbon

Catalytic hydrogenation of nitroarenes is an industrially very important and environmentally friendly process for the production of anilines; however, highly chemoselective reduction of nitroarenes decorated with one or more reducible groups in a nitroarene molecule remains a challenge. Herein, a novel hybrid non-noble iron-based nanocatalyst (named as FeS₂ /NSC) was developed, which was prepared from biomass as C and N source together with inexpensive Fe(NO₃ )₃ as Fe source through high-temperature pyrolysis in a straightforward and cost-effective procedure. Comprehensive characterization revealed that single-phase pyrite FeS₂ nanoparticles with precisely defined composition and uniform size were homogeneously dispersed on N,S-codoped porous carbon with large specific surface area, hierarchical porous channels, and high pore volume. The resultant catalyst FeS₂ /NSC demonstrated good catalytic activity for hydrogenation of functionalized nitroarenes with good tolerance of various functional groups in water as a sustainable and green solvent. Compared with bulk pyrite FeS₂ and other non-noble metal-based heterogeneous catalysts reported in the literature, a remarkably enhanced activity was observed under mild reaction conditions. More importantly, FeS₂ /NSC displayed exclusive chemoselectivity for the reduction of nitro groups for nitroarenes bearing varying readily reducible groups.

Synthesis of Thin-Film Metal Pyrites by an Atomic Layer Deposition Approach

Late 3d transition metal disulfides (MS₂ , M=Fe, Co, Ni, Cu, Zn) can crystallize in an interesting cubic-pyrite structure, in which all the metal cations are in a low-spin electronic configuration with progressive increase of the e_g electrons for M=Fe-Zn. These metal pyrite compounds exhibit very diverse and intriguing electrical and magnetic properties, which have stimulated considerable attention for various applications, especially in cutting-edge energy conversion and storage technologies. The synthesis of the metal pyrites is certainly very important, because highly controllable, reproducible, and reliable synthesis methods are virtually essential for both fundamental materials research and practical engineering. In this Concept, a new approach of (plasma-assisted) atomic layer deposition (ALD) to synthesize the thin-film metal pyrites (FeS₂ , CoS₂ , NiS₂ ) is introduced. The ALD synthesis approach allows for atomic-precision control over film composition and thickness, excellent film uniformity and conformality, and superior process reproducibility, and therefore it is of high promise for uniformly conformal metal pyrite thin-film coatings on complex 3D structures in general. Details and implications of this ALD approach are discussed in this Concept, mainly from a conceptual perspective, and it is envisioned that, with this new ALD synthesis approach, a significant amount of new studies will be enabled on both the fundamentals, and novel applications of the metal pyrite materials.