Templated Synthesis and Chemical Behavior of Nickel Nanoparticles within High Aspect Ratio Silica Capsules

One-dimensional transition metal nanostructures are of interest in many magnetic and catalytic applications. Using a combination of wet chemical synthesis, optical (infrared), and structural characterization methods (powder X-ray diffraction, scanning and transmission electron microscopy), we have investigated four paths to access 1D nickel nanostructures: (1) direct chemical reduction of a self-assembled nickel-hydrazine coordination complex, (2) thermal decomposition of the silica encapsulated nickel-hydrazine complex, (3) treatment of the silica encapsulated nickel-hydrazine complex with sodium borohydride followed by thermal annealing, and (4) electroless nickel plating using silica encapsulated nickel seed particles. We find that only route 1, which does not require a silica template, results in the formation of nickel nanorods, albeit some particle aggregation is observed. Routes 2 and 3 result in the formation of isotropic nickel structures under a reducing atmosphere. Route 4 results in heterogeneous nucleation and growth of existing particles only when partial etching of the silica capsule occurs. Detailed examination of the encapsulated nickel particles allows studying the effect of silica surface silanols on the oxidation of encapsulated nickel particles, the presence of nanoparticle-silica support interactions, the sintering mechanism of nickel and nickel oxide particles, and the fate of boride impurities. Nickel/silica nanostructures are strongly magnetic at room temperature

Super-Resolution Mapping of Photogenerated Electron and Hole Separation in Single Metal−Semiconductor Nanocatalysts

Metal-semiconductor heterostructures are promising visible light photocatalysts for many chemical reactions. Here, we use high-resolution superlocalization imaging to reveal the nature and photocatalytic properties of the surface reactive sites on single Au-CdS hybrid nanocatalysts. We experimentally reveal two distinct, incident energy-dependent charge separation mechanisms that result in completely opposite photogenerated reactive sites (e- and h+) and divergent energy flows on the hybrid nanocatalysts. We find that plasmon-induced hot electrons in Au are injected into the conduction band of the CdS semiconductor nanorod. The specifically designed Au-tipped CdS heterostructures with a unique geometry (two Au nanoparticles at both ends of each CdS nanorod) provide more convincing high-resolution single-turnover mapping results and clearly prove the two charge separation mechanisms. Engineering the direction of energy flow at the nanoscale can provide an efficient way to overcome important challenges in photocatalysis, such as controlling catalytic activity and selectivity. These results bear enormous potential impact on the development of better visible light photocatalysts for solar-to-chemical energy conversion

Photochemical versus Thermal Synthesis of Cobalt Oxyhydroxide Nanocrystals

Photochemical methods facilitate the generation, isolation, and study of metastable nanomaterials having unusual size, composition, and morphology. These harder-to-isolate and highly reactive phases, inaccessible using conventional high-temperature pyrolysis, are likely to possess enhanced and unprecedented chemical, electromagnetic, and catalytic properties. We report a fast, low-temperature and scalable photochemical route to synthesize very small (~3 nm) monodisperse cobalt oxyhydroxide (Co(O)OH) nanocrystals. This method uses readily and commercially available pentaamminechlorocobalt(III) chloride, [Co(NH3) 5Cl]Cl2, under acidic or neutral pH and proceeds under either near-UV (350 nm) or Vis (575 nm) illumination. Control experiments showed that the reaction proceeds at competent rates only in the presence of light, does not involve a free radical mechanism, is insensitive to O 2, and proceeds in two steps: (1) Aquation of [Co(NH3) 5Cl] 2+ to yield [Co(NH3) 5(H2O)] 3+, followed by (2) slow photoinduced release of NH3 from the aqua complex. This reaction is slow enough for Co(O)OH to form but fast enough so that nanocrystals are small (ca. 3 nm). The alternative dark thermal reaction proceeds much more slowly and produces much larger (~250 nm) polydisperse Co(O)OH aggregates. UV-Vis absorption measurements and ab initio calculations yield a Co(O)OH band gap of 1.7 eV. Fast thermal annealing of Co(O)OH nanocrystals leads to Co3O4 nanocrystals with overall retention of nanoparticle size and morphology. Thermogravimetric analysis shows that oxyhydroxide to mixed-oxide phase transition occurs at significantly lower temperatures (up to T = 64 degrees C) for small nanocrystals compared with the bulk

Templated Synthesis and Chemical Behavior of Nickel Nanoparticles within High Aspect Ratio Silica Capsules

One-dimensional transition metal nanostructures are of interest in many magnetic and catalytic applications. Using a combination of wet chemical synthesis, optical (infrared), and structural characterization methods (powder X-ray diffraction, scanning and transmission electron microscopy), we have investigated four paths to access 1D nickel nanostructures: (1) direct chemical reduction of a self-assembled nickel-hydrazine coordination complex, (2) thermal decomposition of the silica encapsulated nickel-hydrazine complex, (3) treatment of the silica encapsulated nickel-hydrazine complex with sodium borohydride followed by thermal annealing, and (4) electroless nickel plating using silica encapsulated nickel seed particles. We find that only route 1, which does not require a silica template, results in the formation of nickel nanorods, albeit some particle aggregation is observed. Routes 2 and 3 result in the formation of isotropic nickel structures under a reducing atmosphere. Route 4 results in heterogeneous nucleation and growth of existing particles only when partial etching of the silica capsule occurs. Detailed examination of the encapsulated nickel particles allows studying the effect of silica surface silanols on the oxidation of encapsulated nickel particles, the presence of nanoparticle-silica support interactions, the sintering mechanism of nickel and nickel oxide particles, and the fate of boride impurities. Nickel/silica nanostructures are strongly magnetic at room temperature.Reprinted (adapted) with permission from Journal of Physical Chemistry Letters 4 (2013): 3918, doi: 10.1021/jp409878a. Copyright 2013 American Chemical Society.</p

Catalytic activity of MnOx/WO3 nanoparticles: synthesis, structure characterization and oxidative degradation of methylene blue

MnOx/WO3 nanoparticles were synthesized by an impregnation method and the physicochemical properties of compounds were characterized by atomic absorption spectroscopy (AAS), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalytic degradation of an organic dye, methylene blue, in the presence of nano-MnOx supported on WO3 as a catalyst and hydrogen peroxide, H2O2, as the oxidant has been studied at room temperature in water. The effects of oxidant amount, catalyst composition and an OH-radical scavenging agent on the degree of decomposition of MB dye were also studied.This is a manuscript of an article published as Amini, Mojtaba, Behzad Pourbadiei, T. Purnima A. Ruberu, and L. Keith Woo. "Catalytic activity of MnO x/WO 3 nanoparticles: synthesis, structure characterization and oxidative degradation of methylene blue." New journal of chemistry 38, no. 3 (2014): 1250-1255. DOI: 10.1039/C3NJ01563G. Copyright 2014 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique. Posted with permission

Photoelectrochemical Generation of Hydrogen from Water Using a CdSe Quantum Dot-Sensitized Photocathode

The present study reports photelectrochemical H₂ evolution using a water-solubilized S₃-cap-CdSe quantum dot-sensitized NiO as the photocathode and either [Co­(bdt)₂]⁻ (bdt =1,2-benzenedithiolate) or Ni­(DHLA)_<i>x</i> (DHLA= the anion of dihydrolipoic acid) complex as the H₂-forming catalyst. The NiO-S₃-cap-CdSe/[Co­(bdt)₂]⁻ system produces H₂ with a turnover frequency of 3000 per CdSe mol·h. Faradaic efficiency for this system is essentially quantitative. Both systems are stable for more than 16 h

Super-Resolution Mapping of Photogenerated Electron and Hole Separation in Single Metal−Semiconductor Nanocatalysts

Metal-semiconductor heterostructures are promising visible light photocatalysts for many chemical reactions. Here, we use high-resolution superlocalization imaging to reveal the nature and photocatalytic properties of the surface reactive sites on single Au-CdS hybrid nanocatalysts. We experimentally reveal two distinct, incident energy-dependent charge separation mechanisms that result in completely opposite photogenerated reactive sites (e- and h+) and divergent energy flows on the hybrid nanocatalysts. We find that plasmon-induced hot electrons in Au are injected into the conduction band of the CdS semiconductor nanorod. The specifically designed Au-tipped CdS heterostructures with a unique geometry (two Au nanoparticles at both ends of each CdS nanorod) provide more convincing high-resolution single-turnover mapping results and clearly prove the two charge separation mechanisms. Engineering the direction of energy flow at the nanoscale can provide an efficient way to overcome important challenges in photocatalysis, such as controlling catalytic activity and selectivity. These results bear enormous potential impact on the development of better visible light photocatalysts for solar-to-chemical energy conversion.Reprinted (adapted) with permission from Journal of the American Chemical Society 135 (2014): 1398, doi: 10.1021/ja409011y. Copyright 2014 American Chemical Society.</p