Dopant-Controlled Selenization in Pd Nanocrystals: The Triggered Kirkendall Effect

Doping foreign impurities in host nanomaterials can induce new materials properties. In addition, doping can also influence the crystallization process and change the shape and/or phase of the host material. While dopant-induced changes in the properties of materials have been well studied, the concept of doping and its chemistry in the design of different nanostructures has rarely been investigated. In order to further understand the doping chemistry, this study investigated the dopant-controlled enhancement of the rate of the chemical reaction during the transformation from one doped material to another and the consequent effect on the shape evolution of the nanostructures. These are performed during the selenization of metal Pd(0), using Ag dopant. While the controlled process produced cuboidal Pd₁₇Se₁₅ from the quasi-spherical nanocrystals of Pd(0), on doping, the shape of Pd₁₇Se₁₅ transformed into hollow cubes. The rate was also enhanced by more than 30 times for the doped case in comparison to undoped Pd(0). Importantly, while for the undoped nanocrystals, the selenization approached in one direction, where for the doped particles, it occurred all around the nanocrystals and triggered the Kirkendall effect. Detailed investigations were conducted to elucidate the influence of the dopant on both the rate and directional approach of selenization in Pd(0), initiation of the fast diffusion of Pd, change in shape, and formation of the hollow structures. To our understanding, the role of dopants in controlling chemical processes is of fundamental importance, and this will undoubtedly broaden the scope of research on the chemistry of doping and crystal growth in solution

Monodisperse SnS Nanocrystals: In Just 5 Seconds

As per the classical growth mechanism, tuning the reaction parameters in the growth stage remains pivotal to control the shape, size, dispersity, and size distribution of the colloidal nanocrystals, but what would be the case when the growth is very fast and the nanomaterials are formed instantaneously? Certainly, it needs a different chemical protocol. We investigate here one of such cases: the formation of different shapes of SnS nanostructures. With proper programming of chemical reaction, highly monodisperse α-SnS nanocubes and nanotetrahedrons are obtained within 5 s of the reaction. Furthermore, tuning the density of nucleation, the size of the nanostructures is tuned in a wide window. These two shapes of SnS are also explored for the study of photocatalytic dye degradation, and the facet-dependent rate for this photocatalytic activity has been compared

The Redox Chemistry at the Interface for Retrieving and Brightening the Emission of Doped Semiconductor Nanocrystals

Photo-oxidation of semiconductor quantum dots is the prime concern during their processability, as it often induces nonradiative states and quenches the band edge excitonic emission. Nevertheless, similar effects have been observed for light emitting doped semiconductor nanocrystals, and the dopant emissions are also quenched due to the surface oxidation. This is more pronounced for selenide-based host semiconductors. To overcome this, we study the interface chemistry of Cu-doped and Mn-doped ZnSe nanocrystals and report here the retrieving and brightening of the emission from completely quenched months old doped nanocrystals. This has been obtained by treating the doped nanocrystals with appropriate organic thiol ligands which remove the surface oxidative states as well as resist further oxidation of the nanocrystals. Here, we investigate details of the redox chemistry at the interface and study related photophysics in retrieving the dopant emission

Diffusion-Induced Shape Evolution in Multinary Semiconductor Nanostructures

The classical mechanism of crystal growth for architecting different nanomaterials in solution, although widely studied, is mainly restricted to binary semiconductor systems. However, this method is not applicable to multinary nanomaterials, which have multivalent cations possessing different reactivity under identical reaction conditions. Hence, the shape architectures of these nanostructures, which require a more sophisticated approach, remain relatively unexplored compared to those of binary semiconductors. Owing to the importance of the multinary materials, which are emerging as excellent green materials for both light harvesting and light emission, we investigated the diffusion-rate-controlled formation of ternary AgGaSe₂ nanostructures and studied their heterostructures with noble metals. Controlling the changes in the rate of diffusion of the Ag ions resulted in the formation of tadpole-shaped AgGaSe₂ ternary nanostructures. In situ study by collecting a sequential collection of samples has been carried out, and the conversion of amorphous Ga-selenide to crystalline AgGaSe₂ has been monitored. In addition, heterostructures of tadpole AgGaSe₂ with noble metals, Au and Pt, were designed, and their photocatalytic behaviors were studied

Efficient Superionic Conductor Catalyst for Solid in Solution–Solid–Solid Growth of Heteronanowires

How efficient could a superionic conductor catalyst be? Beyond the traditionally used molecular precursors when the solution dispersed solid nanomaterials of variable size, shape and phase are introduced under certain reaction condition; the catalyst is found to digest all these structures in minutes irrespective of their phase and morphology, resulting unique heteronanowires. This has been inspected here by employing different ZnSe nanostructures as precursor for Ag₂Se nanocrystal catalyst in its superionic conductor phase to obtain the Ag₂Se-ZnSe heteronanowires. This dissolution and formation process of these nanostructures is correlated with the change in the reaction temperature profile, the phase of the catalyst, the shape/phase and surface ligands of the source nanostructures, and the possible mechanism of the unique heteronanowires growth has been investigated

Modulated Binary–Ternary Dual Semiconductor Heterostructures

A generic modular synthetic strategy for the fabrication of a series of binary-ternary group II-VI and group I-III-VI coupled semiconductor nano-heterostructures is reported. Using Ag2Se nanocrystals first as a catalyst and then as sacrificial seeds, four dual semiconductor heterostructures were designed with similar shapes: CdSe-AgInSe2, CdSe-AgGaSe2, ZnSe-AgInSe2, and ZnSe-AgGaSe2. Among these, dispersive type-II heterostructures are further explored for photocatalytic hydrogen evolution from water and these are observed to be superior catalysts than the binary or ternary semi-conductors. Details of the chemistry of this modular synthesis have been studied and the photophysical processes involved in catalysis are investigated

Thermal-Undoping-Induced 2D Sheet Exfoliations in 1D Nanomaterial

Exfoliations leading to monolayer sheets are mostly reported in 2D materials such as graphene, WS₂, MoS₂, etc. However, theoretically it is established that exfoliations can also be possible for 1D materials like Sb₂S₃, though this has not been experimentally reported yet. Furthermore, most of the reported exfoliations are carried out with physical processes, and only in few cases complicated chemical pathways are also established. Keeping a view on the importance of both materials and methods, herein the exfoliation of 1D Sb₂S₃ nanostructures was reported via a unique thermal undoping approach where annealing expelled Sn atoms from the crystal lattice of 1D Sn-doped Sb₂S₃ nanostructures, leading to 2D sheets via very intriguing 1D–2D coupled structures. Sb₂S₃ is a 1D material but associated with 2D van der Waals forces, and in our dopant removal approach, exfoliation was exclusively carried out in directions perpendicular to the major axis of doped nanostructures. Apart from experimental supports, DFT calculation was also carried out keeping Sn in substitutional and interstitial positions to support our claim. These results suggest that designing a proper chemical process could successfully exfoliate the 1D materials, and the same might be extended to other materials of the same family

Au-SnS Hetero Nanostructures: Size of Au Matters

In nanoscale, with size variation, Au shows different optical behaviors. For the small size clusters (sub-5 nm), it behaves more like semiconductors having sp and d band electronic energy levels splitting and also do not show the characteristic plasmon. However, for larger size particles (>5 nm), it shows the plasmonic absorption. Considering these two structures of Au⁰, we report here their coupling with a low bandgap semiconductor SnS and study the difference in their formation chemistry and materials’ properties. Following a common synthetic approach in which a smaller size SnS cube and tetrahedron shapes result in Au cluster decorated Au-SnS heterostructures, larger size SnS cubes form coupled Au-SnS nanostructures. Contrastingly, the nonplasmonic Au⁰ cluster-SnS hinders the photocatalytic activity, whereas the plasmonic coupled Au-SnS enhances the catalytic activity toward reduction of organic dye methylene blue. However, both types of heterostructures show enhanced photocurrent as well as photoresponse activities. Details of the chemistry of formation, epitaxy at the junction, and change in the materials’ properties are studied and reported here in this article

Coincident Site Epitaxy at the Junction of Au–Cu₂ZnSnS₄ Heteronanostructures

Considering the chemistry of the formation and physics at interfaces, we report on the heterostructure of a promising new energy material, Au–Cu₂ZnSnS₄ (Au-CZTS), and investigate the impact of coupling on Au on improving both the photostability and the photoresponse behavior. We focus primarily on the fundamental issues involved in bringing together two dissimilar materials having different chemical and physical properties in a single building block where one is a multinary semiconductor nanomaterial and the other is a plasmonic noble metal. The formation of heteroepitaxy at the junction of Au and CZTS was investigated for two different phases of CZTS. Considering epitaxy formation along the {111} planes of Au, it was observed that the wurtzite and tetragonal phases of CZTS exhibit coincident site epitaxy with different periodic intervals. A detailed study of this epitaxy formation with Au in both phases of CZTS has been carried out and reported. Because Au-CZTS is a promising new material, we have further investigated its photocurrent and photoresponse behavior and compared them with the properties and behavior of pure CZTS. We believe that these findings will help the energy-materials community, providing guidelines for investigating new functional materials and their applications