Superionic phase transition in individual silver selenide nanowires

Tracking Cation Exchange in Individual Nanowires via Transistor Characterization

Cation exchange is a versatile method for modifying the material composition and properties of nanostructures. However, control of the degree of exchange and material properties is difficult at the single-particle level. Successive cation exchange from CdSe to Ag2Se has been utilized here on the same individual nanowires to monitor the change of electronic properties in field-effect transistor devices. The transistors were fabricated by direct synthesis of CdSe nanowires on prepatterned substrates followed by optical lithography. The devices were then subjected to cation exchange by submerging them in an exchange solution containing silver nitrate. By removal of the devices from solution and probing the electrical transport properties at different times, the change in electronic properties of individual nanowires could be monitored throughout the entire exchange reaction from CdSe to Ag2Se. Transistor characterization revealed that the electrical conductivity can be tuned by up to 8 orders of magnitude and the charge-carrier mobility by 7 orders of magnitude. While analysis of the material composition by energy dispersive X-ray spectroscopy confirmed successful cation exchange from CdSe to Ag2Se, X-ray fluorescence spectroscopy proved that cation exchange also took place below the contacts. The method presented here demonstrates an efficient way to tune the material composition and access the resulting properties nondestructively at the single-particle level. This approach can be readily applied to many other material systems and can be used to study the electrical properties of nanostructures as a function of material composition or to optimize nanostructure-based devices after fabrication.

Pre-phase transition of a Cu2-xS template enables polymorph selective synthesis of MS (M = Zn, Cd, Mn) nanocrystals via cation exchange reactions

Utilization of copper-deficient Cu2-xS nanocrystals (NCs) with diverse crystal phases and stoichiometries as cation exchange (CE) templates is a potential route to overcome the current limitations in the polymorph selective synthesis of desired nanomaterials. Among the Cu2-xS NCs, covellite CuS is emerging as an attractive CE template to produce complicated and metastable metal sulfide NCs. The presence of a reducing agent is essential to induce a phase transition of CuS into other Cu2-xS phases prior to the CE reactions. Nevertheless, the effect of the reducing agent on the phase transition of CuS, especially into the hexagonal close packing (hcp) phase and the cubic close packing (ccp) phase, has been scarcely exploited, but it is highly important for the polymorphic production of metal sulfides with the wurtzite phase and zinc blende phase. Herein, we report a reducing agent dependent pre-phase transition of CuS nanodisks (NDs) into hcp and ccp Cu2-xS NCs. 1-Dodecanethiol molecules and oleylamine molecules selectively reduced CuS NDs into hcp djurleite Cu1.94S NDs and ccp digenite Cu1.8S NCs. Afterward, the hcp Cu1.94S NDs and ccp Cu1.8S NCs were exchanged by Zn2+/Cd2+/Mn2+, and the wurtzite phase and the zinc blende phase of ZnS, CdS, and MnS NCs were produced. Without the pre-phase transition, direct CE reactions of CuS NDs are incapable of synthesizing the above wurtzite and zinc blende metal sulfide NCs. Therefore, our findings suggest the importance of the pre-phase transition of the CE template in polymorphic syntheses, holding great promise in the fabrication of other polymorphic nanomaterials with novel physical and chemical properties.

Halide-driven polymorph selectivity in the synthesis of MnX (X = S, Se) nanoparticles

Devising synthetic strategies to control crystal structure is of great importance as materials properties are governed by structure. MnS is a great model system as it has three known stable polymorphs. Herein, we show the selective synthesis of colloidal wurtzite- and rock-salt-type MnS under identical reactions conditions changing only the manganese halide precursor. Mixtures of Mn halides or halide surrogate (e.g., NH₄Cl) also enabled polymorph control. Powder X-ray diffraction aliquot studies of the reactions revealed the crystal structure at the onset of nucleation and that of the final product is the same, unlike the Ostwald ripening transformation observed in other systems. The halide-driven selectivity was also observed in the synthesis of manganese selenide nanoparticles. In this system, variation of the Mn halide precursor allowed access to the wurtzite- and rock salt-type polymorphs of MnSe, as well as the pyrite-MnSe₂ phase. Based on this work, the mixing of metal salts might be a simple and effective strategy towards polymorph control and access materials with new crystal structures.

Superionic conductor Ag2Se modulated CoSe2 nanosheets prepared via monometallic cation release for efficient pH-universal water electrolysis into hydrogen

Superionic conductors regulated transition metal chalcogenides are the newly emerged electrocatalyst in water electrolysis into clean hydrogen and oxygen. However, there is still much room for the development of structural design, electronic modulation and heterogeneous interface construction to improve the overall water splitting performance in pH-universal solutions, especially in alkaline and neutral mediums. Herein, using β-cyclodextrin (β-CD) and citric acid (CA) organics with abundant hydroxyl (-OH) and carboxyl (-COOH), a special Ag₂Se nanoparticles-decorated CoSe₂ flower-like nanosheets loaded on porous and conductive nickel foam substrate (Ag₂Se-CoSe₂/NF) was successfully constructed by a new method of monometallic cation release of coordinated cobalt. The Ag₂Se phase exerts the nature characteristics of superionic conductors to modulate the morphological and electronic structures of CoSe₂ as well as improve its conductivity. The generated rich active interfaces and abundant Se vacancy defects facilitate numerous active sites exposure to accelerate the hydrogen ion transport and charge transfer. Compared to the single-phase Ag₂Se/NF-8 and CoSe₂/NF, the prepared Ag₂Se-CoSe₂/NF-8 with a two-phase synergistic effect achieves an outstanding pH-universal electrocatalytic hydrogen production performance by water electrolysis, as evidenced by a lower overpotential (60 mV, 212 mV and 85 mV vs RHE at 10 mA cm^-2 for pH = 0.36, 7.00 and 13.70, respectively). Only a voltage of 1.55 V at 10 mA cm^-2 is required to implement the overall water splitting in an alkaline electrolyzer. This work provides significant guidance for the future designation and practical development of transition metal chalcogenides with superionic conductors applied in the electrocatalytic field.