Paul G. O'Brien

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

The Rational Design of a Single-Component Photocatalyst for Gas-Phase CO2 Reduction Using Both UV and Visible Light

The solar‐to‐chemical energy conversion of greenhouse gas CO2 into carbon‐based fuels is a very important research challenge, with implications for both climate change and energy security. Herein, the key attributes of hydroxides and oxygen vacancies are experimentally identified in non‐stoichiometric indium oxide nanoparticles, In2O3‐x(OH)y, that function in concert to reduce CO2 to CO under simulated solar irradiation.

Photomethanation of Gaseous CO2 over Ru/Silicon Nanowire Catalysts with Visible and Near‐Infrared Photons

Gaseous CO2 is transformed photochemically and thermochemically in the presence of H2 to CH4 at millimole per hour per gram of catalyst conversion rates, using visible and near‐infrared photons. The catalyst used to drive this reaction comprises black silicon nanowire supported ruthenium. These results represent a step towards engineering broadband solar fuels tandem photothermal reactors that enable a three‐step process involving i) CO2 capture, ii) gaseous water splitting into H2, and iii) reduction of gaseous CO2 by H2.

Selectively Transparent and Conducting Photonic Crystals

[*] Prof. N. P. Kherani, Dr. A. Chutinan The Edward S. Rogers Sr. Department of Electrical and Computer Engineering University of Toronto 10 King’s College Road, Room GB254B Toronto, ON M5S 3G4 (Canada) E-mail: kherani@ecf.utoronto.ca Prof. G. A. Ozin, D. P. Puzzo, L. D. Bonifacio Materials Chemistry Research Group, Department of Chemistry University of Toronto 80 St. George Street, Toronto, ON M5S 3H6 (Canada) E-mail: gozin@chem.utoronto.ca P. G. O’Brien Department of Materials Science and Engineering University of Toronto 184 College Street Room 140, Toronto, ON M5S 3E4 (Canada)

Tailoring the Electrical Properties of Inverse Silicon Opals ‐ A Step Towards Optically Amplified Silicon Solar Cells

electrical conductivity of i-Si-o, and how these material properties correlate with the optical properties of i-Si-o. To reach our objectives, we fabricated i-Si-o by infiltrating the voids of a silica-opal template with Si using chemical vapor deposition (CVD), and subsequently etching the silica spheres with hydrofluoric acid (HF). [19] Accurate measurements of the dc electrical dark conductivities (sd) require electrodes and electrical contacts on the i-Si-o films. Therefore, instead of using glass substrates, which are most commonly employed, we prepared all samples on sapphire substrates due to their high stability against HF etching. Note that the i-Si-o fabricated on glass substrates usually gets detached and becomes free-standing after HF etching, while high- quality intact i-Si-o films can be achieved on sapphire substrates.

See-Through Dye-Sensitized Solar Cells: Photonic Reflectors for Tandem and Building Integrated Photovoltaics

See-through dye-sensitized solar cells with 1D photonic crystal Bragg reflector photoanodes show an increase in peak external quantum efficiency of 47% while still maintaining high fill factors, resulting in an almost 40% increase in power conversion efficiency. These photoanodes are ideally suited for tandem and building integrated photovoltaics.

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H2 Treatment

Thermal treatment of ultrathin films of hematite (α-Fe2 O3 ) under an atmosphere of 5 % H2 in Ar is presented as a means of activating α-Fe2 O3 towards the photoelectrochemical splitting of water. Spin-coated films annealed in air exhibited no photoactivity, whereas films treated in hydrogen exhibited a photocurrent response. X-ray photoelectron spectroscopy and UV/Vis absorption spectroscopy results showed that the H2 -treated films contain oxygen vacancies, which suggests improved charge transport. However, Tafel slopes, scan-rate dependent measurements, and kinetic analyses performed by using H2 O2 as a hole scavenger suggested that surface modification may also contribute to their induced photoactivity. Electrochemical impedance spectroscopy results revealed the buildup of a surface trap capacitance at the point of photocurrent onset for the hydrogen-treated films under illumination. A decrease in charge trapping resistance was also observed, which suggests improved transport of charges away from the surface.

Flash nano-welding: investigation and control of the photothermal response of ultrathin bismuth sulfide nanowire films.

Ultrathin Bi₂S₃ nanowires undergo a pronounced photothermal response to irradiation from a commercial camera flash. Controlled nano-welding was shown by using single walled carbon nanotube mats as an electrically and thermally conductive substrate. The resulting welded nanowire film is denser and has significantly lower resistance than unflashed bilayer films.

Enhancing photovoltaics with broadband high-transparency glass using porosity-tuned multilayer silica nanoparticle anti-reflective coatings

The performance of optoelectronic devices using glass envelopes can be improved substantially by the application of an effective anti-reflective coating. In this paper, we investigate the preparation of low index films through modulation of the porosity of silica nanoparticle films. Porosity variation is accomplished by introducing polystyrene porogen within colloidal silica nanoparticle films, which are deposited in a controlled manner, followed by pyrolysis of the porogen. Multilayer stacks of nanoparticle films with varying degrees of porosity were fabricated by sequentially spin coating and sintering various silica–polystyrene mixtures. The average transmittance (400–1000 nm) of Corning glass was improved from 91.0% to 95.2% using a three layer stack on one glass–air interface, and to 99.0% using three layer stacks on both interfaces – the highest reported values for facile synthesized multilayer structures. Utilization of the single and dual interface high transparency glass placed on a crystalline silicon solar cell leads to increased photocurrent densities by 4.0% and 6.0%, respectively, relative to uncoated glass.

From Bare Metal Powders to Colloidally Stable TCO Dispersions and Transparent Nanoporous Conducting Metal Oxide Thin Films

06 Transparent conductive oxides (TCOs) are a technologically important class of materials. [ 1 ] Herein we describe a facile, universal, and green ‘one-pot’ approach to produce stable dispersions comprising TCO nanoparticles (NPs) such as SnO 2 , In 2 O 3 , ATO ( ≡ SnO 2 :Sb), ITO ( ≡ In 2 O 3 :Sn), and ZTO ( ≡ SnO 2 :Zn). The synthesis begins by etching the bare metal powder precursors (Sn, In, Sb, and Zn) with HCl and is completed by adding aqueous hydrogen peroxide below room temperature. No complex work-up or time-consuming purifi cation is required, only simple fi ltration. Moreover, this approach avoids organic surfactants, capping ligands and/ or organic solvents, metal halides like SnCl 4 or SbCl 3 , coordination compounds and sol–gel precursors like Sn(O t Bu) 4 , which are commonly used in all reported syntheses of TCOs NPs and which often contaminate and therefore complicate their subsequent utilization and purifi cation. [ 2 , 3 ] It is noteworthy that TCO nanoparticles have been synthesized in different reaction media including ionic liquids, [ 4 ] polyols, [ 5 ] and water. [ 6 ] The herein reported TCO NPs possess diameters of 3–6 nm, are colloidally stable, can be produced on a multigram scale and are well-suited for spin-coating nanoporous, transparent, and conductive TCO thin fi lms [ 7 ] (see Scheme 1 ) with potential utility in lithium ion batteries, solar and photoelectrochemical cells, electrochromics and sensors, fl at-panel displays, transparent thin-fi lm transistors, optoelectronic devices, and photonic crystal architectures. [ 8–12 ] Additionally,