Xinggui Zhou

In Situ Formation of Cobalt Oxide Nanocubanes as Efficient Oxygen Evolution Catalysts

Oxygen evolution from water poses a significant challenge in solar fuel production because it requires an efficient catalyst to bridge the one-electron photon capture process with the four-electron oxygen evolution reaction (OER). Here, a new strategy was developed to synthesize nonsupported ultrasmall cobalt oxide nanocubanes through an in situ phase transformation mechanism using a layered Co(OH)(OCH3) precursor. Under sonication, the precursor was exfoliated and transformed into cobalt oxide nanocubanes in the presence of NaHCO3-Na2SiF6 buffer solution. The resulting cobalt catalyst with an average particle size less than 2 nm exhibited a turnover frequency of 0.023 per second per cobalt in photocatalytic water oxidation. X-ray absorption results suggested a unique nanocubane structure, where 13 cobalt atoms fully coordinated with oxygen in an octahedral arrangement to form 8 Co4O4 cubanes, which may be responsible for the exceptionally high OER activity.

Mechanistic insight into size-dependent activity and durability in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane

We report a size-dependent activity in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane. Kinetic study and model calculations revealed that Pt(111) facet is the dominating catalytically active surface. There is an optimized Pt particle size of ca. 1.8 nm. Meanwhile, the catalyst durability was found to be highly sensitive to the Pt particle size. The smaller Pt particles appear to have lower durability, which could be related to more significant adsorption of B-containing species on Pt surfaces as well as easier changes in Pt particle size and shape. The insights reported here may pave the way for the rational design of highly active and durable Pt catalysts for hydrogen generation.

DFT study of propane dehydrogenation on Pt catalyst: effects of step sites

Self-consistent periodic slab calculations based on gradient-corrected density functional theory (DFT-GGA) have been conducted to examine the reaction network of propane dehydrogenation over close-packed Pt(111) and stepped Pt(211) surfaces. Selective C-H or C-C bond cleaving is investigated to gain a better understanding of the catalyst site requirements for propane dehydrogenation. The energy barriers for the dehydrogenation of propane to form propylene are calculated to be in the region of 0.65-0.75 eV and 0.25-0.35 eV on flat and stepped surfaces, respectively. Likewise, the activation of the side reactions such as the deep dehydrogenation and cracking of C(3) derivatives depends strongly on the step density, arising from the much lower energy barriers on Pt(211). Taking the activation energy difference between propylene dehydrogenation and propylene desorption as the descriptor, we find that while step sites play a crucial role in the activation of propane dehydrogenation, the selectivity towards propylene is substantially lowered in the presence of the coordinatively unsaturated surface Pt atoms. As the sole C(3) derivative which prefers the cleavage of the C-C bond to the C-H bond breaking, propyne is suggested to be the starting point for the C-C bond breaking which eventually gives rise to the formation of ethane, methane and coke. These findings provide a rational interpretation of the recent experimental observations that smaller Pt particles containing more step sites are much more active but less selective than larger particles in propane dehydrogenation.

One-Pot Noninjection Synthesis of Cu-Doped ZnxCd1-xS Nanocrystals with Emission Color Tunable over Entire Visible Spectrum

Unlike Mn doped quantum dots (d-dots), the emission color of Cu dopant in Cu d-dots is dependent on the nature, size, and composition of host nanocrystals (NCs). The tunable Cu dopant emission has been achieved via tuning the particle size of host NCs in previous reports. In this paper, for the first time we doped Cu impurity in Zn(x)Cd(1-x)S alloyed NCs and tuned the dopant emission in the whole visible spectrum via variation of the stoichiometric ratio of Zn/Cd precursors in the host Zn(x)Cd(1-x)S alloyed NCs. A facile noninjection and low cost approach for the synthesis of Cu:Zn(x)Cd(1-x)S d-dots was reported. The optical properties and structure of the obtained Cu:Zn(x)Cd(1-x)S d-dots have been characterized by UV-vis spectroscopy, photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The influences of various experimental variables, including Zn/Cd ratio, reaction temperature, and Cu dopant concentration, on the optical properties of Cu dopant emission have been systematically investigated. The as-prepared Cu:Zn(x)Cd(1-x)S d-dots did show PL emission but with quite low quantum yield (QY) (typically below 6%). With the deposition of ZnS shell around the Cu:Zn(x)Cd(1-x)S core NCs, the PL QY increased substantially with a maximum value of 65%. More importantly, the high PL QY can be preserved when the initial oil-soluble d-dots were transferred into aqueous media via ligand replacement by mercaptoundeconic acid. In addition, these d-dots have thermal stability up to 250 °C.

Fabrication of silicon-based multilevel nanostructures via scanning probe oxidation and anisotropic wet etching

A rational approach is described for fabricating multilevel silicon-based nanostructures via scanning probe oxidation (SPO) and anisotropic wet etching. Using silicon oxide nanopatterns on Si(100) and Si(110) surfaces created by SPO as masks, two-dimensional (2D) nanostructures with high aspect ratio and a variety of patterns can be formed by anisotropic wet etching with KOH. By employing a mixture of KOH solutions and isopropyl alcohol (IPA) as an alternative to KOH alone, control of the morphology of the etched silicon surfaces, crucial for further fabrication, was greatly improved. The SPO and etching processes can be continually repeated on the 2D nanostructures, permitting the formation of various multilevel silicon-based nanostructures, including a T-gate structure useful for electronic circuitry. In addition, these multilevel silicon structures can be used as nanoimprint moulds for their rapid replication.

Carbon mediated catalysis：A review on oxidative dehydrogenation

Abstract Carbon mediated catalysis has gained an increasing attention in both areas of nanocatalysis and nanomaterials. The progress in carbon nanomaterials provides many new opportunities to manipulate the types and properties of active sites of catalysts through manipulating structures, functionalities and properties of carbon surfaces. The present review focuses on progresses in carbon mediated oxidative dehydrogenation reactions of ethylbenzene, propane, and butane. The state-of-the-art of the developments of carbon mediated catalysis is discussed in terms of fundamental studies on adsorption of oxygen and hydrocarbons, reaction mechanism as well as effects of carbon nanomaterial structures and surface functional groups on the catalytic performance. We highlight the importance and challenges in tuning of the electron density of carbon and oxygen on carbon surfaces for improving selectivity in oxidative dehydrogenation reactions.

Single-Crystal Bi2S3 Nanosheets Growing via Attachment–Recrystallization of Nanorods

High-quality Bi(2)S(3) discrete single-crystal nanosheets with orthorhombic structure have been synthesized through the thermal decomposition of a single-source precursor, Bi(S(2)CNEt(2))(3), in amine media. The morphology evolution reveals that the Bi(2)S(3) nanosheets are developed through the assembly of nanorods, and an attachment-recrystallization growth mechanism is proposed for the formation of nanosheets with the use of nanorods as building blocks. High-resolution transmission electron microscopy studies reveal that the nanosheets have the largest exposed surface of (100) facets. The effects of experimental variables, such as the reaction temperature, time, precursor concentration, and media, on the morphology of the obtained nanocrystals have been systematically investigated in which the amine has served as the solvent, surfactant, and electron donor.

Recyclable hollow Pd–Fe nanospheric catalyst for Sonogashira-, Heck-, and Ullmann-type coupling reactions of aryl halide in aqueous media

Hollow Pd-Fe nanospheres were fabricated through a vesicle-assisted chemical reduction method. With the characterization of X-ray diffraction, selected area electron diffraction, X-ray photoelectron spectroscopy, scanning electron micrography, transmission electron micrography, and N(2) physisorption experiment, the resulting Pd-Fe material was identified to be hollow spherical with mesoporous shell. During aqueous Sonogashira-, Heck-, and Ullmann-type coupling reactions of aryl halide, the as-prepared hollow Pd-Fe nanospheres exhibited much higher activity than the dense counterpart nanoparticles. The enhanced reactivity was attributed to both the hollow chamber structure and the promotional effect of Fe-dopants, which provided more Pd active sites for the reactants. Moreover, this hollow material displayed other advantages such as low-cost, recyclability and easy experimental handling.

Facile Synthesis of Monodisperse CdS Nanocrystals via Microreaction

CdS-based nanocrystals (NCs) have attracted extensive interest due to their potential application as key luminescent materials for blue and white LEDs. In this research, the continuous synthesis of monodisperse CdS NCs was demonstrated utilizing a capillary microreactor. The enhanced heat and mass transfer in the microreactor was useful to reduce the reaction temperature and residence time to synthesize monodisperse CdS NCs. The superior stability of the microreactor and its continuous operation allowed the investigation of synthesis parameters with high efficiency. Reaction temperature was found to be a key parameter for balancing the reactivity of CdS precursors, while residence time was shown to be an important factor that governs the size and size distribution of the CdS NCs. Furthermore, variation of OA concentration was demonstrated to be a facile tuning mechanism for controlling the size of the CdS NCs. The variation of the volume percentage of OA from 10.5 to 51.2% and the variation of the residence time from 17 to 136 s facilitated the synthesis of monodisperse CdS NCs in the size range of 3.0–5.4 nm, and the NCs produced photoluminescent emissions in the range of 391–463 nm.

Modified carbon nanotubes by KMnO4 supported iron Fischer–Tropsch catalyst for the direct conversion of syngas to lower olefins

Manganese and potassium promoters coated carbon nanotubes (i.e., MnK-CNTs) were synthesized by a redox reaction between CNTs and KMnO4, in which the CNTs act as reducing agent and as substrate for the heterogeneous nucleation of K-doped manganese oxide. The as-synthesized MnK-CNTs were employed to support Fe catalyst (i.e., Fe/MnK-CNTs, the loadings of 7.9 wt% Fe, 15.7 wt% Mn and 1.9 wt% K) for the direct conversion of syngas to lower olefins. It is revealed that Fe/MnK-CNTs catalyst is more active and stable than FeMnK/CNTs catalyst prepared by co-impregnation method using CNTs as a support. Furthermore, under similar CO conversion, the Fe/MnK-CNTs catalyst exhibits higher selectivity of hydrocarbons especially lower olefins. This could be related to the small-sized and uniform nanoparticles, the well-distributed promoters, the weak metal–support interaction and the greater defects on support, which are the consequences of the unique structural transformation of MnK-CNTs as a function of temperature and atmosphere.