Spontaneous Electroless Galvanic Cell Deposition of 3D Hierarchical and Interlaced S-M-S Heterostructures

Geometry-tailored freestanding epitaxial Pd, AuPd, and Au nanoplates driven by surface interactions

Freestanding epitaxial metal nanoplates can be utilized as advanced three-dimensional platforms for various novel applications. Here we report the vapor-phase epitaxial growth of freestanding Pd, AuPd, and Au nanoplates on an a-cut sapphire substrate as well as the comprehensive study of their growth mechanisms and geometry tailoring. All as-grown Pd, AuPd, and Au nanoplates possess twin-free single crystallinity as well as are aligned three-dimensionally on the substrate with the same orientation. Interestingly, depending on their composition, they have the following three distinct geometries: trapezoid (Pd), hexagon (AuPd), or rhombus (Au). By analyzing the correlation of the geometry and orientation of the as-synthesized nanostructures, we reveal that all the nanoplates grow from square pyramidal seed crystals. The interfacial lattice mismatch between the bottom plane of the square pyramidal seeds and a-cut sapphire substrate increases in the following order: Pd < AuPd < Au. Consequently, the length of the interface between the bottom of the nanoplate and the substrate decreases in the following order: Pd > AuPd > Au; this leads to the resulting geometries of the synthesized nanoplates. Such a fundamental understanding of the growth mechanism would aid the growth of epitaxial metal nanostructures with the desired geometry, which is very attractive for building macroscale functional nanoarchitectures.

Epitaxially aligned submillimeter-scale silver nanoplates grown by simple vapor transport

Epitaxially aligned large silver (Ag) nanoplate arrays with ultraclean surfaces are very attractive for novel plasmonic applications. Although solution-phase methods have been extensively employed to synthesize Ag nanoplates, these cannot be used to grow epitaxial large Ag nanoplates on substrates. Here we report a vapor-phase synthetic strategy to epitaxially grow submillimeter-scale Ag nanoplates on a variety of substrates. By simply transporting Ag vapor to the substrates at an optimal temperature (820 °C), we synthesize ∼100 μm-sized Ag nanoplates with atomically clean surfaces, which are three-dimensionally aligned on the substrates. We demonstrate that both the type of supported seed and their interfacial lattice matching with the substrates determine the epitaxial growth habit of the nanoplates, directing their crystallinity, shape, and orientation. (i) On r-cut sapphire substrates, twinned pentagonal nanoplates grow vertically from twinned triangular seeds through a seed → nanoplate process. (ii) On m-cut sapphire substrates, twinned trapezoidal Ag nanoplates grow slantingly from twinned decahedral seeds through a seed → NW → nanoplate process. (iii) Interestingly, twin-free single-crystalline trapezoidal Ag nanoplates grow from twin-free square pyramidal seeds on STO (001) substrates through a seed → NW → nanoplate process. The epitaxially aligned Ag nanoplate arrays could serve as a new platform for two-dimensional (2D) guiding of surface plasmons as well as for hierarchical 3D plasmonic nanoarchitecturing.

Toward Efficient Preconcentrating Photocatalysis: 3D g-C3N4 Monolith with Isotype Heterojunctions Assembled from Hybrid 1D and 2D Nanoblocks

The macroscopic integration of the microscopic catalyst is one of the most promising strategies for photocatalytic technology in facing practical applications. However, in addition to the unsatisfactory photoactivated exciton separation, a new problem restricting the catalytic efficiency is the unmatched kinetics between the reactant diffusion and the photochemical reaction. Here, we report an isotype heterojunctional three-dimensional g-C₃N₄ monolith which is assembled from the hybrid building blocks of the nanowires and nanosheets. Benefiting from its hierarchically porous network and abundant heterojunctions, this catalytic system exhibits inherently promoted efficiency in light absorption and exciton separation, thus leading to a desirably improved photocatalytic performance. Furthermore, thanks to the structural and functional advantages of the constructed g-C₃N₄ monolith, a novel strategy of preconcentrating photocatalysis featuring the successive filtration, adsorption, and photocatalysis has been further developed, which could technically coordinate the kinetic differences and result in over-ten-time enhancement on the efficiency compared with the traditional photocatalytic system. Beyond providing new insights into the structural design and innovative application of the monolithic photocatalyst, this work may further open up novel technological revolutions in sewage treatment, air purification, microbial control, etc.

Effects of Au nanoparticles and ZnO morphology on the photocatalytic performance of Au doped ZnO/TiO2 films

Au doped ZnO nanocomposite films on TiO₂ seeding layer (AuZ/T) were fabricated by hydrothermal processing and their photocatalytic performance was investigated. It could be found that the AuZ/T with micrometer(μm)-sized, lying ZnO bulks revealed optimal photocatalytic performance toward methyl orange under simulated sunlight, whose apparent degradation rate constant K _app of 1.31 was about 20% higher compared to that of ZnO/TiO₂ and 3 times higher compared to that of ZnO. The Au nanoparticles, TiO₂ seeding layer and hydrothermal processing time imposed vital influence on the morphology of ZnO nanostructures, which played key roles in the formation of ZnO/TiO₂ heterojunction and charge transfer (CT) inside it, as demonstrated by kinetics of transient photoluminescence (PL) decaying. The incorporation of Au nanoparticles not only induced the variations of ZnO crystallinity and reduction of ZnO band gap (E _g), but also generated the Schottky heterojunction of metal-semiconductor, which would be beneficial to the CT inside nanocomposite films and separation of photo-generated electron-hole pairs, as verified by the remarkable PL suppression. The mechanism responsible for photocatalysis enhancement, which was resulted from the hybrid effects of Au nanoparticles and the ZnO morphology was discussed in details.

Correlations among morphology, composition, and photoelectrochemical water splitting properties of InGaN nanorods grown by molecular beam epitaxy

The mechanism underlying the effect of growth condition on the morphology evolution of InGaN nanorods (NRs) has been systematically investigated. The increased Ga flux enhances both the axial and the radial growth at the growth stage. However, the changed Ga flux influences not only the growth but also the nucleation of InGaN NRs. At the nucleation stage, the increased Ga flux shortens the delay time for NR formation, and prolongs the growth stage for a fixed total growth time. Those two aspects result in the increase of NR diameter and height with the supplied Ga flux. In addition, the continuous nucleation is ended much earlier due to the accelerated saturation of substrate area with the increased Ga flux, resulting in a decreased final NR density. In addition to the morphology evolution with the Ga flux, the composition characteristic of InGaN NRs has been also studied. The In distribution of InGaN NRs depends critically on the NR diameter along the NR growth direction, and the NRs show a morphology-dependent In incorporation. Interestingly, the InGaN NRs discussed here show a radial Stark effect induced by the pinned Fermi level. The radial Stark effect shifts the absorption edge of the InGaN NRs toward longer wavelengths, makes the InGaN NRs attractive for photoelectrochemical water splitting applications. The photoelectrochemical measurements present a significant increase in the photocurrent with the increased total surface area of the InGaN NRs, which is due to the enhanced light absorption effects and the enlarged interfacial area of the semiconductor/electrolyte.

Inverse Stellation of CuAu-ZnO Multimetallic-Semiconductor Nanostartube for Plasmon-Enhanced Photocatalysis

One-dimensional (1D) metallic nanocrystals constitute an important class of plasmonic materials for localization of light into subwavelength dimensions. Coupled with their intrinsic conductive properties and extended optical paths for light absorption, metallic nanowires are prevalent in light-harnessing applications. However, the transverse surface plasmon resonance (SPR) mode of traditional multiply twinned nanowires often suffers from weaker electric field enhancement due to its low degree of morphological curvature in comparison to other complex anisotropic nanocrystals. Herein, simultaneous anisotropic stellation and excavation of multiply twinned nanowires are demonstrated through a site-selective galvanic reaction for a pronounced manipulation of light-matter interaction. The introduction of longitudinal extrusions and cavitation along the nanowires leads to a significant enhancement in plasmon field with reduced quenching of localized surface plasmon resonance (LSPR). The as-synthesized multimetallic nanostartubes serve as a panchromatic plasmonic framework for incorporation of photocatalytic materials for plasmon-assisted solar fuel production.

Self-Assembled Heterostructures: Selective Growth of Metallic Nanoparticles on V2 -VI3 Nanoplates

Precise control of the selective growth of heterostructures with specific composition and functionalities is an emerging and extremely challenging topic. Here, the first investigation of the difference in binding energy between a series of metal-semiconductor heterostructures based on layered V₂ -VI₃ nanostructures is investigated by means of density functional theory. All lateral configurations show lower formation energy compared with that of the vertical ones, implying the selective growth of metal nanoparticles. The simulation results are supported by the successful fabrication of self-assembled Ag/Cu-nanoparticle-decorated p-type Sb₂ Te₃ and n-type Bi₂ Te₃ nanoplates at their lateral sites through a solution reaction. The detailed nucleation-growth kinetics are well studied with controllable reaction times and precursor concentrations. Accompanied by the preserved topological structure integrity and electron transfer on the semiconductor host, exceptional properties such as dramatically increased electrical conductivity are observed thanks to the pre-energy-filtering effect before carrier injection. A zigzag thermoelectric generator is built using Cu/Ag-decorated Sb₂ Te₃ and Bi₂ Te₃ as p-n legs to utilize the temperature gradient in the vertical direction. Synthetic approaches using similar chalcogenide nanoplates as building blocks, as well as careful control of the dopant metallic nanoparticles or semiconductors, are believed to be broadly applicable to other heterostructures with novel applications.