Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor-metal hybrid nanoparticles (HNPs) focusing on metal-chalcogenides-metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor-metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.

Hole Scavenging and Electron-Hole Pair Photoproduction Rate: Two Mandatory Key Factors to Control Single-Tip Au-CdSe/CdS Nanoheterodimers

Metal/semiconductor hetero-nanostructures are now considered as benchmark functional nanomaterials for many light-driven applications. Using laser-driven photodeposition to control growth of gold nanodots (NDs) onto CdSe/CdS dot-in-rods (DRs), we show that the addition of a dedicated hole scavenger (MeOH) is the cornerstone to significantly reduce to less than 3.5% the multiple-site nucleation and 2.5% the rate of gold-free DRs. This means, from a synthetic point of view, that rates up to 90% of single-tip DRs can be reproducibly achieved. Moreover, by systematically varying this hole scavenger concentration and the Au/DRs ratio on the one hand, and the irradiation intensity and the time exposure on the other hand, we explain how gold deposition switches from multisite to single-tipped and how the growth and final size of the single photodeposited ND can be controlled. A model also establishes that the results obtained based on these different varying conditions can be merged onto a single "master behavior" that summarizes and predicts the single-tip gold ND growth onto the CdSe/CdS DRs. We eventually use data from the literature on growth of platinum NDs onto CdS nanorods by laser-deposition to extend our investigation to another metal of major interest and strengthen our modeling of single metallic ND growth onto II-VI semiconducting nanoparticles. This demonstrated strategy can raise a common methodology in the synthesis of single-tip semiconductor-metal hybrid nanoheterodimers (NHDs), leading to advanced nanoparticles architectures for applications in areas as different as photocatalysis, hydrogen production, photovoltaics, and light detection.

Efficient Hot Electron Transfer from Small Au Nanoparticles

Many important chemical transformations enabled by plasmonic hot carrier photocatalysis have been reported, although their efficiencies are often too low for practical applications. We examine how the efficiency of plasmon-induced hot electron transfer depends on the Au particle size in Au-tipped CdS nanorods. We show that with decreasing Au size, the plasmon width increases due to enhanced surface damping contributions. The excitation of Au nanoparticles leads to an instrument response time-limited ultrafast hot electron transfer process to CdS (≪140 fs). The quantum efficiency of this process increases from ∼1% to ∼18% as the particle size decreases from 5.5 ± 1.1 to 1.6 ± 0.5 nm due to both enhanced hot electron generation and transfer efficiencies in small Au particles. Our finding suggests that decreasing plasmonic particle size is an effective approach for improving plasmon-induced hot carrier transfer efficiency and provides important insight for the rational improvement of plasmonic hot carrier-based devices.

Photoinduced ultrafast charge separation in colloidal 2-dimensional CdSe/CdS-Au hybrid nanoplatelets and corresponding application in photocatalysis

Multicomponent hybrid nanocrystals (HNC) consisting of a semiconductor and metallic domains are an important class of nanostructured materials demonstrating useful applications and interesting basic knowledge. In this scenario, Au nanoparticle (NP) islands of ∼2 nm have been grown on unique two dimensional (2D) CdSe/CdS core@shell hexagonal nanoheteroplatelets of 20 nm diameter to form unprecedented 2D CdSe/CdS-Au HNCs and detailed optical characterization has been carried out to determine the dimensionality based electron transfer dynamics on the ultrafast scale. Steady state optical absorption studies show that upon growing Au NPs onto the 2D nanoplates, a new band appears in the red region of the spectra (500-800 nm), which suggests a strong interaction between the exciton of the core-shell and the plasmon of the metal NPs. Fluorescence studies showed the quenching of emission of the semiconductor domains upon the growth of the metallic domains. Detailed optical and TRPL studies suggested efficient charge transfer from the 2D CdSe/CdS to the Au domains, irrespective of excitation wavelength. Femtosecond transient absorption studies suggest that the electron transfer from the 2D hybrid nanocrystals to the metal domain is on an ultrafast time scale (∼800 fs). No evidence is observed for charge transfer from the 2 nm Au domains to the semiconductor seeds. The broad absorption in the visible region of the hybrid nanocrystals and the ultrafast charge transfer facilitates very efficient photo-catalytic reactions under direct sun light, as a case study.

Chloride Ion Mediated Synthesis of Metal/Semiconductor Hybrid Nanocrystals

A synthetic route to prepare metal-semiconductor hybrid nanoparticles is presented, along with the possibility to tune the ratio of primary to secondary nucleation and the morphology of the semiconductor material grown on the metal nanoparticle seeds. Gold and cobalt-platinum nanoparticles are employed as metal seeds, on which CdS or CdSe is grown. Using transmission electron microscopy, absorption spectroscopy (UV-vis), and powder X-ray diffraction as characterization techniques, a significant influence of chloride ions on the type of nucleation (that is, secondary or primary nucleation) as well as on the shape of the resulting heterostructures is observed. Partially replacing the commonly used cadmium precursor CdO by varying amounts of CdCl2 opens access to rod-like, multiarmed, flower-like, and bullet-like structures. The results suggest that neither pure CdO nor pure CdCl2 as precursors but only a mixture of both make these structures obtainable. In this article, the influence of the chloride ion concentration during semiconductor growth on metal seeds is investigated in depth. The morphology of the resulting heterostructures is characterized carefully, and a growth mechanism is suggested. Furthermore, it is shown that this synthetic approach can be transferred to seeds of various metals such as platinum, gold, and cobalt platinum.

Probing Bioluminescence Resonance Energy Transfer in Quantum Rod-Luciferase Nanoconjugates

We describe the necessary design criteria to create highly efficient energy transfer conjugates containing luciferase enzymes derived from Photinus pyralis (Ppy) and semiconductor quantum rods (QRs) with rod-in-rod (r/r) microstructure. By fine-tuning the synthetic conditions, CdSe/CdS r/r-QRs were prepared with two different emission colors and three different aspect ratios (l/w) each. These were hybridized with blue, green, and red emitting Ppy, leading to a number of new BRET nanoconjugates. Measurements of the emission BRET ratio (BR) indicate that the resulting energy transfer is highly dependent on QR energy accepting properties, which include absorption, quantum yield, and optical anisotropy, as well as its morphological and topological properties, such as aspect ratio and defect concentration. The highest BR was found using r/r-QRs with lower l/w that were conjugated with red Ppy, which may be activating one of the anisotropic CdSe core energy levels. The role QR surface defects play on Ppy binding, and energy transfer was studied by growth of gold nanoparticles at the defects, which indicated that each QR set has different sites. The Ppy binding at those sites is suggested by the observed BRET red-shift as a function of Ppy-to-QR loading (L), where the lowest L results in highest efficiency and furthest shift.

Complete assembly of Cu2ZnSnS4 (CZTS) nanorods at substrate interfaces using a combination of self and directed organisation

Herein, we report the two stage assembly of semiconductor nanorods at substrate interfaces using both self and field directed assembly methods.

Phosphine-free, low-temperature synthesis of tetrapod-shaped CdS and its hybrid with Au nanoparticles

Tetrapod-shaped CdS colloidal nanocrystals are synthesized using a facile, phosphine-free synthesis approach at low temperature. The arm length and diameter of CdS tetrapods can be easily tuned by using different source of sulphureous precursors, i.e., sulfur powder, thioacetamide, and sodium diethyldithiocarbamate. Moreover, the growth of Au nanoparticles onto CdS to form metal-semiconductor hybrid nanocrystals is also demonstrated. The tetrapod-shaped CdS nanocrystals exhibit strong arm-diameter-dependent absorption and photoluminescence characteristics. Importantly, the as-obtained CdS tetrapods exhibit promising photocatalytic activity for the water-splitting reaction in photoelectrochemical cells.

Photocatalysis from Fluorescence-Quenched CdSe/Au Nanoheterostructures: A Size-Dependent Study

We report a systematic study of the electron-transfer process from CdSe quantum dots (QDs) to the Au tips as a function of the QD diameter and also the size of the Au-tip. For Au-tips smaller than ∼3 nm, that is, when they are still not metallic, a reduction in PL behavior is observed as the excited electrons are transferred from the QD-conduction band to Au, with quenching being higher for larger tips and smaller QDs. A combination of steady state and time-resolved studies establish the mechanism of charge transfer that is further confirmed by dye-degradation studies, showing the possibility of ambient day light photocatalysis.

Colloidal polymers via dipolar assembly of magnetic nanoparticle monomers

In this Spotlight on Applications, we describe our recent progress in the preparation of hierarchical one-dimensional (1-D) materials constructed from polymer-coated ferromagnetic cobalt nanoparticles. We begin with a general discussion of nanoparticles capable of 1-D self-organization to form 1-D assemblies, which we term colloidal polymers. The need for efficient, highly directional interactions prompted our investigation with polymer-coated ferromagnetic nanoparticles, which spontaneously form linear assemblies through coupling of north and south magnetic poles present in these single-domain ferromagnetic nanoparticles. These highly directional N-S interactions and the resulting formation of 1-D assemblies can be understood in the context of traditional polymer-forming reactions. The dipolar assembly of these ferromagnetic nanoparticles into chains and binary assemblies while dispersed in organic media has been investigated as a key foundation to form novel magnetic materials and heterostructured nanocomposites. These studies enabled the fabrication of magnetic nanoactuating systems resembling "artificial cilia and flagella". We then discuss our recent efforts to prepare cobalt oxide nanowires using various nanoparticle conversion reactions through a process termed colloidal polymerization. A series of novel functional "colloidal monomers" based on dipolar cobalt nanoparticles were also prepared, incorporating noble metal or semiconductor nanoinclusions to form heterostructured cobalt oxide nanocomposites.

Engineering nanointerfaces for nanocatalysis

We focus on recent advances in the delicate design of well-defined nanointerfaces to promote nanocatalysis towards renewable energy.

Origin of chains of Au-PbS nano-dumbbells in space

Self-assembled, one-dimensional (1D) nanomaterials are amenable building blocks for bottom-up nanofabrication processes. A current shortcoming in the self-assembly of 1D nanomaterials in solution phase is the need for specific linkers or templates under very precise conditions to achieve a handful of systems. Here we report on the origin of a novel self-assembly of 1D dumbbells consisting of Au tipped PbS nanorods into stable chains in solution without any linkers or templates. A realistic multi-particle model suggests that the mesophase comprises 1D dumbbells arrayed in chains formed by anisotropic van der Waals type interactions. We demonstrate an alternative recognition mechanism for directing the assembly of the 1D dumbbells, based on effective interaction between the neighboring dumbbells consisting of Au tips with complementary crystallographic facets that guides the entire assembly in space.

Directing the deposition of ferromagnetic cobalt onto Pt-tipped CdSe@CdS nanorods: synthetic and mechanistic insights

A methodology providing access to dumbbell-tipped, metal-semiconductor and metal oxide-semiconductor heterostructured nanorods has been developed. The synthesis and characterization of CdSe@CdS nanorods incorporating ferromagnetic cobalt nanoinclusions at both nanorod termini (i.e., dumbbell morphology) are presented. The key step in the synthesis of these heterostructured nanorods was the decoration of CdSe@CdS nanorods with platinum nanoparticle tips, which promoted the deposition of metallic CoNPs onto Pt-tipped CdSe@CdS nanorods. Cobalt nanoparticle tips were then selectively oxidized to afford CdSe@CdS nanorods with cobalt oxide domains at both termini. In the case of longer cobalt-tipped nanorods, heterostructured nanorods were observed to self-organize into complex dipolar assemblies, which formed as a consequence of magnetic associations of terminal CoNP tips. Colloidal polymerization of these cobalt-tipped nanorods afforded fused nanorod assemblies from the oxidation of cobalt nanoparticle tips at the ends of nanorods via the nanoscale Kirkendall effect. Wurtzite CdS nanorods survived both the deposition of metallic CoNP tips and conversion into cobalt oxide phases, as confirmed by both XRD and HRTEM analysis. A series of CdSe@CdS nanorods of four different lengths ranging from 40 to 174 nm and comparable diameters (6-7 nm) were prepared and modified with both cobalt and cobalt oxide tips. The total synthesis of these heterostructured nanorods required five steps from commercially available reagents. Key synthetic considerations are discussed, with particular emphasis on reporting isolated yields of all intermediates and products from scale up of intermediate precursors.

Size- and shape-dependent growth of fluorescent ZnS nanorods and nanowires using Ag nanocrystals as seeds

High-quality, monodisperse, and size-controlled Ag-ZnS nanorods or nanowires have been synthesized successfully using Ag nanocrystals as seeds. Such one-dimensional colloidal Ag-ZnS nanorods or nanowires having a purposefully controlled diameter in the range of 5-9 nm and a length of 18-600 nm were obtained by altering the reaction conditions, such as concentration, reaction time, reaction temperature, and diameter of Ag nanocrystals. The conjunction interface of Ag-ZnS nanorods or nanowires consists of the (200) plane of Ag nanocrystal and (101) plane of ZnS rod or wire, the <101> directions of ZnS nanorods grow preferentially. Based on the photoluminescence and lifetime of Ag-ZnS nanorods, it was found that Ag nanocrystals enhanced the radiative rate eventually, the fluorescence intensity of Ag-ZnS nanorods can be tuned by changing the size of the Ag seeds. The Ag-ZnS nanorods or nanowires showed greatly improved optical properties as compared to ZnS nanocrystals, the maximum emission was around 402 nm and the photoluminescence quantum yield was up to 30% when 5 nm Ag nanocrystals were used as seeds.

A facile method for gold decoration of Te@CdTe nanorods in aqueous solution

Colloidal synthesis of metal-semiconductor hybrid nanostructures is mainly achieved in organic solution. In some applications of hybrid nanoparticles relevant in aqueous media, phase transfer of hydrophobic metal-semiconductor hybrid nanostructures is essential. In this work, we present a simple method for direct synthesis of water-soluble gold (Au) decorated Te@CdTe hybrid nanorods (NRs) at room temperature by using aqueous Te@CdTe NRs as templates, which were preformed by using CdTe nanocrystals (NCs) as precursor in the presence of hydrazine hydrate (N(2)H(4)). Our results showed that NRs were decorated with Au islands both on tips and along the surface of the NRs. The size and density of Au islands can be controlled by varying the amount of Au precursor (mixture of HAuCl(4) and thioglycolic acid (TGA)) and TGA/HAuCl(4) ratio. A possible growth mechanism for the Au decoration of Te@CdTe NRs is concluded as three steps: (1) the formation of AuTe(1.7) via the substitution reaction of Cd(2+) by Au(3+), (2) adsorption of Au-TGA complex onto the preformed AuTe(1.7) anchors and following reduction by CdTe and N(2)H(4), leading to the formation of small Au NCs, (3) Au NCs grow to bigger ones, followed by reduction of more Au precursor by N(2)H(4).

Direct determination of polarity, faceting, and core location in colloidal core/shell wurtzite semiconductor nanocrystals

The ability to determine the atomic arrangement and termination of various facets of surfactant-coated nanocrystals is of great importance for understanding their growth mechanism and their surface properties and represents a critical piece of information that can be coupled to other experimental techniques and to calculations. This is especially appealing in the study of nanocrystals that can be grown in strongly anisotropic shapes, for which the relative growth rates of various facets can be influenced under varying reaction conditions. Here we show that in two representative cases of rod-shaped nanocrystals in the wurtzite phase (CdSe(core)/CdS(shell) and ZnSe(core)/ZnS(shell) nanorods) the terminations of the polar facets can be resolved unambiguously by combining advanced electron microscopy techniques, such as aberration-corrected HRTEM with exit wave reconstruction or aberration-corrected HAADF-STEM. The [0001] and [000-1] polar directions of these rods, which grow preferentially along their c-axis, are revealed clearly, with one side consisting of the Cd (or Zn)-terminated (0001) facet and the other side with a pronounced faceting due to Cd (or Zn)-terminated {10-1-1} facets. The lateral faceting of the rods is instead dominated by three nonpolar {10-10} facets. The core buried in the nanostructure can be localized in both the exit wave phase and HAADF-STEM images.

Electrophoretic deposition of poly(3-decylthiophene) onto gold-mounted cadmium selenide nanorods

Molecular mechanisms of electrophoretic deposition (EPD) of poly(3-decylthiophene) (P3DT) molecules onto vertically aligned cadmium selenide arrays have been studied using large-scale, nonequilibrium molecular dynamics (MD), in the absence and presence of static external electric fields. The field application and larger polymer charges accelerated EPD. Placement of multiple polymers at the same lateral displacement from the surface reduced average deposition times due to "crowding", giving monolayer coverage. These findings were used to develop and validate Brownian dynamics simulations of multilayer polymer EPD in scaled-up systems with larger inter-rod spacings, presenting a generalized picture in qualitative agreement with random sequential adsorption.