Srebri Petrov

Large‐Scale Synthesis of Ultrathin Bi2S3 Necklace Nanowires

One of the most fascinating areas of nanoscience is the study of one-dimensional nanostructures. From our materials chemistry viewpoint the idea of bringing the size of nanowires down to a point where nanoscale colloidal analogues of polymers can be studied is most stimulating. This direction has been the subject of intense study that promises to develop a new class of materials in which the topological properties of polymers can be coupled with quantum size effects and inorganic crystalline materials. Our strategy goes in a singular direction: instead of connecting nanocrystals by ligand chemistry or dipole interactions we aim to synthesize colloidal nanowires thin enough to display polymer-like behavior. Colloidal chemistry has in recent years shown that virtually any composition can be obtained as colloidal nanocrystals of the most diverse shapes. Much work though still needs to be done in the area of pnictide chalcogenides for which very few syntheses are available for obtaining colloidally stable products and none to our knowledge has shown fully demonstrated quantum size effects. Pnictide chalcogenides (Bi2S3 in particular) are in fact known to show extremely wide changes in the bandgap energy and conductivity with changes in stoichiometry. In the present work we report a “low”-temperature, gramscale route to Bi2S3 nanowires of unprecedented thickness (< 2 nm), with a necklace architecture, strong excitonic features never before seen in bismuth chalcogenides, and extremely high extinction coefficients. The wires are colloidally stable for months even after extensive purification. The nanowires were obtained by injecting a solution of sulfur in oleylamine into a saturated solution of bismuth citrate in oleylamine at 130 8C (see Supporting Information). The nanowires start nucleating immediately after injection and are shown in Figure 1a. The wires are below 2 nm in diameter and display remarkable size uniformity as well as a lack of extensive branching (even though branching points can be seldomly observed). The length of the wires could not be measured rigorously due to their melting under e-beam irradiation, but light scattering measurements indicate their length can be in the order of several microns. Strikingly, the diameter of the wires does not change during growth (Supporting Information), as it will be later shown. In Figure 1b we show a Z-contrast TEM image, which indicates the melting the wires undergo upon e-beam irradiation. The inhomogeneity in the droplet spacing is a hint to the nanowire9s texture that will be highlighted later. The HRTEM image obtained from the wires (Figure 1c) shows some slightly elongated nanocrystals. The fact that only certain parts of the wires show lattice fringes at a given time indicates their polycrystalline microstructure. The lattice spacing that is evidenced in Figure 1c corresponds to the (021) planes of the Bi2S3 structure. The reaction is highly scalable due to the high concentration of the reagents and its high yield (> 60%): multigram quantities can be routinely obtained in a laboratory environment. Figure 1d depicts over 17 grams of nanowires produced in the course of a single reaction (ca. 350 mL). Unlike for single-crystalline ultrathin nanowires, the powder X-ray diffraction (XRD) pattern for the Bi2S3 nanowires (Figure 2a) is devoid of relatively sharp features, which seems to indicate a rather isotropic nanocrystalline component. In ultrathin nanowires, the different coherence lengths along the different crystallographic directions, which are due to the high aspect ratio and small size, give rise to sharp peaks on an broad background. In this case, the XRD pattern is more consistent with a spherical nanocrystal system: Rietveld refinement of the XRD pattern was successfully accomplished by using the Bi2S3 unit cell as well as a spherical particle model with a 1.6 nm size, consistent with the microscopy data (Supporting Information). It is important here not to overestimate the accuracy of Rietveld refinement when it comes to nanocrystal shape determination but it is safe to say that we are not dealing with a one-dimensional single crystal. [*] L. Cademartiri, R. Malakooti, Dr. S. Petrov, Prof. G. A. Ozin Lash Miller Chemical Laboratories Department of Chemistry, University of Toronto 80 St. George Street, Toronto, ON, M5S 3H6 (Canada) Fax: (+1)416-971-2011 E-mail: gozin@chem.utoronto.ca Homepage: http://www.chem.toronto.edu/staff/GAO/group.html

Cross-linking Bi2S3 ultrathin nanowires: a platform for nanostructure formation and biomolecule detection.

This paper describes the use of chemical cross-linking of ultrathin inorganic nanowires as a bottom-up strategy for nanostructure fabrication as well as a chemical detection platform. Nanowire microfibers are produced by spinning a nanowire dispersion into a cross-linker solution at room temperature. Nanomembranes with thicknesses down to 50 nm were obtained by injecting the nanowire dispersion at the cross-linker-solution/air interface. Furthermore, the sensitivity of the nanowire to amine cross-linkers allowed development of a novel sensing platform for small molecules, like the neurotransmitter serotonin, with detection limits in the picomolar regime.

Barium Titanate Inverted Opals—Synthesis, Characterization, and Optical Properties

Barium titanate inverted opals with powder and film morphologies were synthesized from barium ethoxide and titanium isopropoxide in the interstitial spaces of a polystyrene opal. This procedure involves infiltration of precursors into the interstices of the polystyrene opal template followed by hydrolytic polycondensation of the precursors to amorphous barium titanate and removal of the polystyrene opal by solvent extraction or calcination. In-situ variable temperature powder X-ray diffraction and micro-Raman spectroscopy allow one to observe the thermally induced transformation of the as-synthesized amorphous barium titanate inverted opal to the nanocrystalline form. In this way, a nanocrystalline barium titanate inverted opal can be engineered as either the cubic or tetragonal polymorph. Control of this process is key to the practical realization of a room-temperature stable ferroelectric barium titanate inverted opal that can be thermally tuned through the ferroelectric–paraelectric transition around the Curie temperature. Optical characterization demonstrated photonic crystal behavior of the inverted barium titanate opals and results were in good agreement with photonic band structure calculations. The synthesis of optical quality ferroelectric barium titanate inverted opals provides an opportunity to electrically and optically engineer the photonic band structure and the possibility of developing tunable three-dimensional photonic crystal devices.

Ultrathin Bi2S3 nanowires: surface and core structure at the cluster-nanocrystal transition.

Herein, we present the structural characterization of the core and surface of colloidally stable ultrathin bismuth sulfide (Bi(2)S(3)) nanowires using X-ray Absorption Spectroscopy (EXAFS and XANES), X-ray Photoelectron Spectroscopy (XPS), and Nuclear Magnetic Resonance (NMR). These three techniques allowed the conclusive structural characterization of the inorganic core as well as the coordination chemistry of the surface ligands of these structures, despite the absence of significant translational periodicity dictated by their ultrathin diameter (1.6 nm) and their polycrystallinity. The atomic structure of the inorganic core is analogous to bulk bismuthinite, but Bi atoms display a remarkably higher coordination number than in the bulk. This can be only explained by a model in which each bismuth atom at the surface (or in close proximity to it) is bound to at least one ligand at any time.

Nanoparticle Films and Photonic Crystal Multilayers from Colloidally Stable, Size-Controllable Zinc and Iron Oxide Nanoparticles

We report a facile sol-gel synthesis of colloidally stable Fe(2)O(3) and ZnO nanoparticles in alcoholic solvents, ROH, where R = methyl, ethyl, n-propyl, isopropyl, and tert-butyl. We show that nanoparticles of ZnO (4-42) nm and Fe(2)O(3) (4-38 nm) monotonically increase in size upon increasing the alkyl chain length and branching of the alcohol solvent. These colloidally stable and size-controllable metal oxide nanoparticles enable the formation of high optical quality films and photonic crystal multilayers whose component layer thickness, refractive index, porosity, and surface area are found to scale with the nature of the alcohol. Utility of these colloidally stable nanoparticles is demonstrated by preparation of one-dimensional porous photonic crystals comprising ncZnO/ncWO(3) and ncFe(2)O(3)/ncWO(3) multilayers whose photonic stop band can be tuned by tailoring nanoparticle size. Myriad applications can be envisaged for these nanoparticle films in, for example, heterogeneous catalysis, photocatalysis, electrocatalysis, chemical sensors, and solar cells.

Polyselenophenes with distinct crystallization properties

The polythiophene derivative poly(3-hexyl)thiophene (P3HT) has become one of the most well studied organic materials due to its interesting and important chemical and physical properties. Two different crystal structures have been observed for P3HT, type-1 and type-2, however a pure type-2 structure has never been obtained. Herein, we investigate the crystal structure of polyselenophene analogs (P3HS), and discover that a pure type-2 phase is formed in low molecular weight P3HS (Mn = 5.9 kg mol−1). Wide-angle X-ray scattering shows that the type-2 phase is readily formed and stable at room temperature, which is very distinct from what is observed in control experiments with P3HT. Absorption spectra of P3HS films with the pure type-2 phase lack the typical shoulder peaks indicating that π–π stacking is relatively poor in the type-2 phase. Scanning transmission electron microscopy (STEM) images, however, show that large nanofibers are formed by type-2 crystallization thereby demonstrating the potential of P3HS to drive unique types of self-assembled structures through crystallization, and should motivate continued efforts on selenophene analogs of P3HT for a variety of studies and uses.

Modular assembly and phase study of two- and three-dimensional porous tin(IV) selenides

The work presented focuses attention on a detailed powder and single crystal XRD structure analysis of materials that emerge successively in the hydrothermal synthesis of tetramethylammonium-templated porous tin(IV) selenides. Four phases have been identified in this system and three of them, orthorhombic and monoclinic polymorphs of two-dimensional porous layered TMA 2 Sn 3 Se 7 (TMA=tetramethylammonium) plus a novel tetragonal three-dimensional open-framework TMA 2 Sn 5 Se 10 O have been structurally characterized by single crystal XRD. The order of appearance and interconversion of these materials is shown to follow Oswalds law of successive reactions, which is further supported by Connolly surface calculations. In addition the thermal stability of the orthorhombic TMA 2 Sn 3 Se 7 phase is explored by in situ variable temperature PXRD under nitrogen and in vacuum, from which it is determined that the structural integrity of the framework is retained up to the point of template removal around 250 and 300 °C, respectively. The accumulated knowledge that has emerged from this study and earlier work allows a modular assembly pathway to be proposed that can rationalize the formation of the observed phases.

Electrochromic Bragg Mirror: ECBM

IO N We describe herein the fi rst example of an electrochromic Bragg mirror (ECBM), combining nanoporous multilayers made of NiO and WO 3 nanoparticles. Because NiO and WO 3 are complementary in their coloration effects (e.g. cathodic coloration for WO 3 and anodic coloration for NiO) [ 1 ] and their corresponding change in refractive index, tunability can be achieved by combining these electrochromic components in a 1D Bragg mirror tandem arrangement. The high nanoporosity of this ECBM allows protons and electrons to be quickly shuttled into and out of the multilayers, altering the mix of intervalence charge transfer optical effects within the layers and Bragg diffraction effects between the layers. A proper choice of electrolyte guarantees cycling of the optical properties with negligible degradation. Electrochromic [ 2 , 3 ] devices change their electronic structure and color via electrically-induced storage of ions and electrons in the material, which can be reversed by applying an opposing electrical bias. In comparison, photonic crystals change their color by alterations in the dimension and/or refractive index of the photonic lattice, which can also be changed electrically and reversibly. A prominent example for reversible color changes emanating from alterations of the geometrical structure are voltage-driven, swellable and shrinkable 3D inverse opals built from cross-linked polyferrocenylsilane. [ 3–5 ] This effect is known as the electrophotonic effect. [ 4 , 5 ] Electrochromic materials present themselves as a good alternative, as the coloration usually comes with a change in the refractive index. In the fi eld of electrochromic devices, tungsten trioxide W(VI)O 3 and nickel oxide Ni(II)O are the inorganic material archetypes, and function according to Equations 1–2 . [ 6–8 ]

Nanocrystal Self‐Assembly with Rod–Rod Block Copolymers

Two distinct morphologies of hexylselenophene-hexylthiophene rod-rod block copolymer films can be prepared depending on the molecular weight of the sample (see picture: left M(n) =12.9, right M(n) =3.9 kg mol(-1)). These polymers can be used to organize spherical CdSe nanocrystals (yellow) into either dispersed or aligned hierarchical structures. Scale bars: 200 nm.

Spatially Confined Redox Chemistry in Periodic Mesoporous Hydridosilica–Nanosilver Grown in Reducing Nanopores

Periodic mesoporous hydridosilica, PMHS, is shown for the first time to function as both a host and a mild reducing agent toward noble metal ions. In this archetypical study, PMHS microspheres react with aqueous Ag(I) solutions to form Ag(0) nanoparticles housed in different pore locations of the mesostructure. The dominant reductive nucleation and growth process involves SiH groups located within the pore walls and yields molecular scale Ag(0) nanoclusters trapped and stabilized in the pore walls of the PMHS microspheres that emit orange-red photoluminescence. Lesser processes initiated with pore surface SiH groups produce some larger spherical and worm-shaped Ag(0) nanoparticles within the pore voids and on the outer surfaces of the PMHS microspheres. The intrinsic reducing power demonstrated in this work for the pore walls of PMHS speaks well for a new genre of chemistry that benefits from the mesoscopic confinement of Si-H groups.