Optoelectronic insights of lead-free layered halide perovskites.

Two-dimensional organic-inorganic halide perovskites have emerged as promising candidates for a multitude of optoelectronic technologies, owing to their versatile structure and electronic properties. The optical and electronic properties are harmoniously integrated with both the inorganic metal halide octahedral slab, and the organic spacer layer. The inorganic octahedral layers can also assemble into periodically stacked nanoplatelets, which are interconnected by the organic ammonium cation, resulting in the formation of a superlattice or superstructure. In this perspective, we explore the structural, electronic, and optical properties of lead-free hybrid halides, and the layered halide perovskite single crystals and nanostructures, expanding our understanding of the diverse applications enabled by these versatile structures. The optical properties of the layered halide perovskite single crystals and superlattices are a function of the organic spacer layer thickness, the metal center with either divalent or a combination of monovalent and trivalent cations, and the halide composition. The distinct absorption and emission features are guided by the structural deformation, electron-phonon coupling, and the polaronic effect. Among the diverse optoelectronic possibilities, we have focused on the photodetection capability of layered halide perovskite single crystals, and elucidated the descriptors such as excitonic band gap, effective mass, carrier mobility, Rashba splitting, and the spin texture that decides the direct component of the optical transitions.

Nanoplatelet Superlattices by Tin-Induced Transformation of FAPbI3 Nanocrystals.

The transition from 3D to 2D lead halide perovskites is traditionally led by the lattice incorporation of bulky organic cations. However, the transformation into a coveted 2D superlattice-like structure by cationic substitution at the Pb2+ site of 3D perovskite is unfamiliar. It is demonstrated that the gradual increment of [Sn2+ ] alters the FASnx Pb1- x I3 nanocrystals into the Ruddlesden-Popper-like nanoplatelets (NPLs), with surface-absorbed oleic acid (OA) and oleylamine (OAm) spacer ligand at 80 °C (FA+ : formamidinium cation). These NPLs are stacked either by a perfect alignment to form the superlattice or by offsetting the NPL edges because of their lateral displacements. The phase transition occurs from the Sn/Pb ratio ≥0.011, with 0.64 wt% of Sn2+ species. At and above Sn/Pb = 0.022, the NPL superlattice stacks start to grow along [00l] with a repeating length of 4.37(3) nm, comprising the organic bilayer and the inorganic block having two octahedral layers (n = 2). Besides, a photoluminescence quantum yield of 98.4% is obtained with Sn/Pb = 0.011 (n ≥ 4), after surface passivation by trioctylphosphine (TOP).

Synthesis of mesoporous Fe/Al/La trimetallic oxide for photodegradation of various water-soluble dyes: Kinetic, mechanistic, and pH studies.

Phase pure, trigonal, mesoporous Fe/Al/La trimetallic nano-oxide (abbreviated as FAL) was synthesized using energy efficient chemical route with bandgap 1.97 eV and SBET = 50.02 m2/g and an average pore size of 8.95 nm for photodegradation of azo (di and tri) and thiazine class of dyes successfully. The valence band and conduction band potentials were calculated using the Mott-Schottky plot. The highest photodegradation efficiency was 93.85 ± 2% for reactive black 5 (RB5) at pH 7 under solar irradiation. The phase formation of FAL was confirmed by PXRD, TEM, and HRTEM analyses. The other characterizations include FESEM, Raman, EPR, UV, HPLC, LC-MS, etc. The presence of the metal centers and their corresponding oxidation states were confirmed by the SAEDS, elemental mapping, and XPS analyses respectively. FAL was also able to photodegrade direct blue 71 (DB71) and methylene blue (MB) under the same condition at different pH efficiently (pH 2-11). The photodegradation obeyed the pseudo-1st-order kinetics and was reusable up to 5 successive cycles. This study may be an efficient tool to meet UNs' SDG:6.

Cs3Bi2I9 nanodiscs with phase and Bi(III) state stability under reductive potential or illumination for H2 generation from diluted aqueous HI.

The increasingly popular, lead-free perovskite, Cs3Bi2I9 has a vulnerable Bi3+ state under reductive potentials, due to the high standard reduction potential of Bi3+/Biδ+ (0 < δ < 3). Contrary to this fundamental understanding, herein, ligand-coated Cs3Bi2I9 nanodiscs (NDs) demonstrate outstanding electrochemical stability with up to -1 V versus a saturated calomel electrode in aqueous 0.63 M (5% v/v) and 6.34 M (50% v/v) hydroiodic acid (HI), with a minor BiI3 fraction due to the unavoidable partial aqueous disintegration of the perovskite phase after 8 and 16 h, respectively. A dynamic equilibrium of saturated 0.005 M NDs maintains the common ion effect of I-, and remarkably stabilizes ∼93% Bi3+ in 0.63 M HI under a strong reductive potential. In comparison, the hexagonal phase of bulk Cs3Bi2I9 disintegrates considerably in the semi-aqueous media. Lowering the concentration of synthetic HI from the commonly used ∼50% v/v by elevating the pH from -0.8 to 0.2 helps in reducing the cost per unit of H2 production. Our Cs3Bi2I9 NDs with a hexagonal lattice have 4-6 (002) planes stacked along the c-axis. With 0.005 M photostable NDs, 22.5 µmol h-1 H2 is photochemically obtained within 8 h in a 6.34 M HI solution. Electrocatalytic H2 evolution occurs with a turnover frequency of 11.7 H2 per s at -533 mV and outstanding operational stability for more than 20 h.

In Situ Cation Intercalation in the Interlayer of Tungsten Sulfide with Overlaying Layered Double Hydroxide in a 2D Heterostructure for Facile Electrochemical Redox Activity.

The role of electrochemical interfaces in energy conversion and storage is unprecedented and more so the interlayers of two-dimensional (2D) heterostructures, where the physicochemical nature of these interlayers can be adjusted by cation intercalation. We demonstrate in situ intercalation of Ni2+ and Co2+ with similar ionic radii of ∼0.07 nm in the interlayer of 1T-WS2 while electrodepositing NiCo layered double hydroxide (NiCo-LDH) to create a 2D heterostructure. The extent of intercalation varies with the electrodeposition time. Electrodeposition for 90 s results in 22.4-nm-thick heterostructures, and charge transfer ensues from NiCo-LDH to 1T-WS2, which stabilizes the higher oxidation states of Ni and Co. Density functional theory calculations validate the intercalation principle where the intercalated Ni and Co d electrons contribute to the density of states at the Fermi level of 1T-WS2. Water electrolysis is taken as a representative redox process. The 90 s electrodeposited heterostructure needs the relatively lowest overpotentials of 134 ± 14 and 343 ± 4 mV for hydrogen and oxygen evolution reactions, respectively, to achieve a current density of ±10 mA/cm2 along with exceptional durability for 60 h in 1 M potassium hydroxide. The electrochemical parameters are found to correlate with enhanced mass diffusion through the cation and Cl--intercalated interlayer spacing of 1T-WS2 and the number of active sites. While 1T-WS2 is mostly celebrated as a HER catalyst in an acidic medium, with the help of intercalation chemistry, this work explores an unfound territory of this transition-metal dichalcogenide to catalyze both half-reactions of water electrolysis.