Hierarchical superstructure of alkylamine-coated ZnS nanoparticle assemblies

'Beneficial impurities' in colloidal synthesis of surfactant coated inorganic nanoparticles

Colloidal synthesis of nanoparticles (NP) has advanced tremendously over the past 25 years, with an increasing number of research papers introducing nanomaterials with a variety of compositions, shapes, sizes, and phases. Although much progress has been achieved, commonly used synthetic procedures often fail to reproduce results, and the fine details of the syntheses are often disregarded. Reproducibility issues in synthesis can be ascribed to the effects of impurities, trace amounts of chemical moieties which significantly affect the reaction products. Impurities in NP synthesis are rarely reported or regularly studied, despite their impact, deleterious, or beneficial. This topical review discusses several case studies of colloidal NP synthesis where the sources and the chemistry of impurities are highlighted, and their role is examined.

Beneficial Impurities and Phase Control in Colloidal Synthesis of Tin Monoselenide

The effect of impurities in colloidal synthesis of SnSe in oleylamine surfactant was investigated. Specifically, it was found that impurities such as water, hydrochloric acid, and carbon dioxide stabilize the recently discovered cubic phase of tin monoselenide, π-SnSe. We describe the reaction that releases HCl to the reaction medium through reaction of SnCl₂ with moisture and its subsequent reaction with oleylamine, transforming it from neutral to charged surfactant. A similar path occurs in the case of CO₂, which reacts with oleylamine to give charged oleylammonium-oleylcarbamate molecular pairs. By exposing the reaction to controlled concentrations of "beneficial contaminants", hitherto a major source of irreproducibility in this synthesis, we demonstrate phase and shape control from π-SnSe cubes to α-SnSe rods.

Stability of cubic tin sulphide nanocrystals: role of ammonium chloride surfactant headgroups

New semiconducting metastable cubic phases have been recently discovered in the tin monosulfide and monoselenide systems. Surface energy calculations and experimental studies indicate that this cubic π-phase is stabilized by specific ligand adsorption on the surface. In this work, it is shown experimentally that the synthesis carried out using mixtures of oleylamine and oleylammonium chloride (OACl) surfactants results in the cubic phase, transforming the growth from orthorhombic to cubic nanoparticles with increasing OACl concentration up to a limiting point. Complementary ab initio calculations find that adsorbed ligands lower the surface energies for both the cubic phase and the orthorhombic phase, relative to the pristine surfaces. The decrease in the surface energy increases with ligand coverage. Stronger binding energies to the cubic phase suggest a higher coverage, and therefore preferential stabilization of this phase. Upon further increasing the coverage, the surface energy becomes negative, effectively destabilizing the particles in agreement with experimental observations. Bonding analysis shows that Cl bonds to Sn and replaces missing Sn-S bonds at the surface of the cubic structure. In the competing orthorhombic layered phase, Cl also bonds to a Sn atom but at the expense of one of the Sn-S bonds of this Sn atom. This observation can explain the trends of the surface energies. This combined experimental and computational analysis sheds light on the stabilization processes of these nano-materials and indicates the path to improve synthetic routes.

Titanium Dioxide (TiO2) Mesocrystals: Synthesis, Growth Mechanisms and Photocatalytic Properties

Hierarchical TiO2 superstructures with desired architectures and intriguing physico-chemical properties are considered to be one of the most promising candidates for solving the serious issues related to global energy exhaustion as well as environmental deterioration via the well-known photocatalytic process. In particular, TiO2 mesocrystals, which are built from TiO2 nanocrystal building blocks in the same crystallographical orientation, have attracted intensive research interest in the area of photocatalysis owing to their distinctive structural properties such as high crystallinity, high specific surface area, and single-crystal-like nature. The deeper understanding of TiO2 mesocrystals-based photocatalysis is beneficial for developing new types of photocatalytic materials with multiple functionalities. In this paper, a comprehensive review of the recent advances toward fabricating and modifying TiO2 mesocrystals is provided, with special focus on the underlying mesocrystallization mechanism and controlling rules. The potential applications of as-synthesized TiO2 mesocrystals in photocatalysis are then discussed to shed light on the structure–performance relationships, thus guiding the development of highly efficient TiO2 mesocrystal-based photocatalysts for certain applications. Finally, the prospects of future research on TiO2 mesocrystals in photocatalysis are briefly highlighted.

Efficient artificial mineralization route to decontaminate Arsenic(III) polluted water - the Tooeleite Way

Increasing exposure to arsenic (As) contaminated ground water is a great threat to humanity. Suitable technology for As immobilization and removal from water, especially for As(III) than As(V), is not available yet. However, it is known that As(III) is more toxic than As(V) and most groundwater aquifers, particularly the Gangetic basin in India, is alarmingly contaminated with it. In search of a viable solution here, we took a cue from the natural mineralization of Tooeleite, a mineral containing Fe(III) and As(III)ions, grown under acidic condition, in presence of SO₄²⁻ ions. Complying to this natural process, we could grow and separate Tooeleite-like templates from Fe(III) and As(III) containing water at overall circumneutral pH and in absence of SO₄²⁻ ions by using highly polar Zn-only ends of wurtzite ZnS nanorods as insoluble nano-acidic-surfaces. The central idea here is to exploit these insoluble nano-acidic-surfaces (called as INAS in the manuscript) as nucleation centres for Tooeleite growth while keeping the overall pH of the aqueous media neutral. Therefore, we propose a novel method of artificial mineralization of As(III) by mimicking a natural process at nanoscale.

Surface plasmon resonance in surfactant coated copper sulfide nanoparticles: Role of the structure of the capping agent

HYPOTHESIS

The optical properties of as-synthesized CuS nanoparticles are affected by shape, size and morphology and exhibit increased optical absorbance in the infrared range due to localized surface plasmon resonance (LSPR), which is also affected by these parameters. An additional parameter which affects the LSPR-related absorbance is crystallinity of the surfactant coating.

EXPERIMENTS

CuS nanoparticles with varying morphologies were synthesized using a single source, single surfactant/solvent route. Thereafter, the particles were heat treated at temperatures varying from 130 °C to 230 °C with and without protective environment. Prior to and following the treatments, the particles were characterized using various techniques. Additionally, temperature resolved structural study and thermal analysis of the surfactant coating were performed.

FINDINGS

We confirm that the previously reported effects of particle dimensions and chemical composition on LSPR apply for the synthesized particles. Moreover, we report an additional, previously unreported effect, connecting the crystal structure of the nanoparticle surfactant coating to LSPR. This in turn allows control over LSPR peak position by varying the degree of crystallinity of the capping surfactant layer. Thermal study of the surfactant coating showed gradual structural transition and high dependence of phase transformation on atmospheric environment during treatment.

New nanocrystalline materials: a previously unknown simple cubic phase in the SnS binary system

We report a new phase in the binary SnS system, obtained as highly symmetric nanotetrahedra. Due to the nanoscale size and minute amounts of these particles in the synthesis yield, the structure was exclusively solved using electron diffraction methods. The atomic model of the new phase (a = 11.7 Å, P2(1)3) was deduced and found to be associated with the rocksalt-type structure. Kramers-Kronig analysis predicted different optical and electronic properties for the new phase, as compared to α-SnS.

Assembly of metals and nanoparticles into novel nanocomposite superstructures

Controlled assembly of nanoscale objects into superstructures is of tremendous interests. Many approaches have been developed to fabricate organic-nanoparticle superstructures. However, effective fabrication of inorganic-nanoparticle superstructures (such as nanoparticles linked by metals) remains a difficult challenge. Here we show a novel, general method to assemble metals and nanoparticles rationally into nanocomposite superstructures. Novel metal-nanoparticle superstructures are achieved by self-assembly of liquid metals and nanoparticles in immiscible liquids driven by reduction of free energy. Superstructures with various architectures, such as metal-core/nanoparticle-shell, nanocomposite-core/nanoparticle-shell, network of metal-linked core/shell nanostructures, and network of metal-linked nanoparticles, were successfully fabricated by simply tuning the volume ratio between nanoparticles and liquid metals. Our approach provides a simple, general way for fabrication of numerous metal-nanoparticle superstructures and enables a rational design of these novel superstructures with desired architectures for exciting applications.

Stacking of Short DNA Induces the Gyroid Cubic-to-Inverted Hexagonal Phase Transition in Lipid-DNA Complexes

Lyotropic phases of amphiphiles are a prototypical example of self-assemblies. Their structure is generally determined by amphiphile shape and their phase transitions are primarily governed by composition. In this paper, we demonstrate a new paradigm for membrane shape control where the electrostatic coupling of charged membranes to short DNA (sDNA), with tunable temperature-dependent end-to-end stacking interactions, enables switching between the inverted gyroid cubic structure (QIIG) and the inverted hexagonal phase (HIIC). We investigated the structural shape transitions induced in the QIIG phase upon complexation with a series of sDNAs (5, 11, 24, and 48 bp) with three types of end structure ("sticky" adenine (A)-thymine (T) (dAdT) overhangs, no overhang (blunt), and "nonsticky" dTdT overhangs) using synchrotron small-angle X-ray scattering. Very short 5 bp sDNA with dAdT overhangs and blunt ends induce coexistence of the QIIG and the HIIC phase, with the fraction of QIIG increasing with temperature. Phase coexistence for blunt 5 bp sDNA is observed from 27 °C to about 65 °C, where the HIIC phase disappears and the temperature dependence of the lattice spacing of the QIIG phase indicates that the sDNA duplexes melt into single strands. The only other sDNA for which melting is observed is 5 bp sDNA with dTdT overhangs, which forms the QIIG phase throughout the studied range of temperature (27 °C to 85.2 °C). The longer 11 bp sDNA forms coexisting QIIG and HIIC phases (with the fraction of QIIG again increasing with temperature) only for "nonsticky" dTdT overhangs, while dAdT overhangs and blunt ends exclusively template the HIIC phase. For 24 and 48 bp sDNAs the HIIC phase replaces the QIIG phase at all investigated temperatures, independent of sDNA end structure. Our work demonstrates how the combined effects of sDNA length and end structure (which determine the temperature-dependent stacking length) tune the phase behavior of the complexes. These findings are consistent with the hypothesis that sDNAs and sDNA stacks with lengths comparable to or larger than the cubic unit cell length disfavor the highly curved channels present in the QIIG phase, thus driving the QIIG-to-HIIC phase transition. As the temperature is increased, the breaking of stacks due to thermal fluctuations restores increasing percentages of the QIIG phase.

Nanometer size effects in nucleation, growth and characterization of templated CdS nanocrystal assemblies

We report on the oriented nucleation of CdS nanocrystals on well-defined polydiacetylene Langmuir film templates. Nucleation on the red phase of polydiacetylene resulted in ordered linear arrays of CdS nanocrystals that are aligned with respect to the template. High resolution transmission electron microscopy showed crystalline particles of ~5 to 8 nm size. Selected area electron diffraction micrographs showed spot patterns which are attributed to the well-defined orientations of both polymorphs: the cubic zinc blende and the hexagonal wurtzite polymorphs of CdS. We present a unique growth mechanism where oriented nucleation of CdS on the polydiacetylene template initially takes place in the zinc blende phase. Beyond a certain size threshold, growth proceeds in the more stable wurtzite phase. This transformation keeps the stacking direction of the close packed planes, while altering only their stacking sequence. Notably, size-confinement effects were observed in electron diffraction patterns from the wurtzite phase. These effects originated from off-axis planes that do not fulfill the Bragg conditions, yet their elongated Bragg rods intersect with the Ewald sphere, giving rise to unexpected reflections.

Directed coassembly of oriented PbS nanoparticles and monocrystalline sheets of alkylamine surfactant

We demonstrate control over the orientation of PbS nanoparticles by way of directed assembly, which in turn affects the crystal structure of alkylamine surfactants such as octadecylamine (ODA, C(18)H(37)NH(2)) and hexadecylamine (HDA, C(16)H(33)NH(2)). This directed assembly method results in the arrangement of PbS nanoparticles with a well-defined epitaxial orientation on lamellar alkylamine sheets, which undertake a new crystal structure to accommodate these relations. Understanding these surfactant-nanoparticle inter-relations is very instrumental in understanding surfactant-assisted nanoparticle synthesis and assembly.

Origin of the contact angle hysteresis of water on chemisorbed and physisorbed self-assembled monolayers

Self-assembled monolayers (SAMs) are known to form on a variety of substrates either via chemisorption (i.e., through chemical interactions such as a covalent bond) or physisorption (i.e., through physical interactions such as van der Waals forces or "ionic" bonds). We have studied the behavior and effects of water on the structures and surface energies of both chemisorbed octadecanethiol and physisorbed octadecylamine SAMs on GaAs using a number of complementary techniques including "dynamic" contact angle measurements (with important time and rate-dependent effects), AFM, and electron microscopy. We conclude that both molecular overturning and submolecular structural changes occur over different time scales when such SAMs are exposed to water. These results provide new insights into the time-dependent interactions between surfaces and colloids functionalized with SAMs when synthesized in or exposed to high humidity or bulk water or wetted by water. The study has implications for a wide array of phenomena and applications such as adhesion, friction/lubrication and wear (tribology), surfactant-solid surface interactions, the organization of surfactant-coated nanoparticles, etc.