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1.
Sci Rep ; 9(1): 15579, 2019 Oct 30.
Artigo em Inglês | MEDLINE | ID: mdl-31666532

RESUMO

The Brønsted acidity of graphene oxide (GO) materials has shown promising activity in organic synthesis. However, roles and functionality of Lewis acid sites remain elusive. Herein, we reported a carbocatalytic approach utilizing both Brønsted and Lewis acid sites in GOs as heterogeneous promoters in a series of multicomponent synthesis of triazoloquinazolinone compounds. The GOs possessing the highest degree of oxidation, also having the highest amounts of Lewis acid sites, enable optimal yields (up to 95%) under mild and non-toxic reaction conditions (85 °C in EtOH). The results of FT-IR spectroscopy, temperature-programed decomposition mass spectrometry, and X-ray photoelectron spectroscopy identified that the apparent Lewis acidity via basal plane epoxide ring opening, on top of the saturated Brønsted acidic carboxylic groups, is responsible for the enhanced carbocatalytic activities involving Knoevenagel condensation pathway. Recycled GO can be effectively regenerated to reach 97% activity of fresh GO, supporting the recognition of GO as pseudocatalyst in organic synthesis.

2.
Small ; 15(50): e1903363, 2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-31608571

RESUMO

Thin film electrocatalysts allow strong binding and intimate electrical contact with electrodes, rapid mass transfer during reaction, and are generally more durable than powder electrocatalysts, which is particularly beneficial for gas evolution reactions. In this work, using cobalt manganese oxyhydroxide, an oxygen evolution reaction (OER) electrocatalyst that can be grown directly on various electrodes as a model system, it is demonstrated that breaking a continuous film into discontinuous patches can significantly enhance the overall OER performance without sacrificing long-term stability even under elevated electrocatalytic stress. Discontinuous films with higher edge-to-area ratios exhibits reduced overpotentials, increased turnover frequency, and more pronounced current increase after electrochemical conditioning. Operando Raman spectroscopy studies during electrocatalysis reveal that the film edges require lower potential barrier for activation. Introducing discontinuity into thin film electrocatalysis can thus lead to the realization of high performance yet highly robust systems for harsh gas evolution reactions.

3.
Nanoscale Res Lett ; 14(1): 271, 2019 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-31399837

RESUMO

Photoluminescent zero-dimensional (0D) quantum dots (QDs) derived from transition metal dichalcogenides, particularly molybdenum disulfide, are presently in the spotlight for their advantageous characteristics for optoelectronics, imaging, and sensors. Nevertheless, up to now, little work has been done to synthesize and explore photoluminescent 0D WS2 QDs, especially by a bottom-up strategy without using usual toxic organic solvents. In this work, we report a facile bottom-up strategy to synthesize high-quality water-soluble tungsten disulfide (WS2) QDs through hydrothermal reaction by using sodium tungstate dihydrate and L-cysteine as W and S sources. Besides, hybrid carbon quantum dots/WS2 QDs were further prepared based on this method. Physicochemical and structural analysis of QD hybrid indicated that the graphitic carbon quantum dots with diameters about 5 nm were held onto WS2 QDs via electrostatic attraction forces. The resultant QDs show good water solubility and stable photoluminescence (PL). The excitation-dependent PL can be attributed to the polydispersity of the synthesized QDs. We found that the PL was stable under continuous irradiation of UV light but can be quenched in the presence of hydrogen peroxide (H2O2). The obtained WS2-based QDs were thus adopted as an electrodeless luminescent probe for H2O2 and for enzymatic sensing of glucose. The hybrid QDs were shown to have a more sensitive LOD in the case of glucose sensing. The Raman study implied that H2O2 causes the partial oxidation of QDs, which may lead to oxidation-induced quenching. Overall, the presented strategy provides a general guideline for facile and low-cost synthesis of other water-soluble layered material QDs and relevant hybrids in large quantity. These WS2-based high-quality water-soluble QDs should be promising for a wide range of applications in optoelectronics, environmental monitoring, medical imaging, and photocatalysis.

4.
Environ Sci Pollut Res Int ; 26(20): 20701-20711, 2019 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-31102232

RESUMO

Chlorpheniramine is a pharmaceutical pollutant and a precursor of carcinogenic nitrosamines during disinfection/oxidation. In our previous study, graphene oxide coated with magnetite (GO-Fe3O4) was capable of removing chlorpheniramine in deionized water by adsorption. This study investigated the removal of chlorpheniramine and its nitrosamine formation potentials (FPs) by adsorption onto magnetic GO-Fe3O4, with respect to the influence by using real municipal wastewaters as the background. In the results, the adsorption performances of chlorpheniramine in wastewaters decreased in the order: GO-Fe3O4 suspension > GO-Fe3O4 particles > activated carbon. Chlorpheniramine adsorptions on GO-Fe3O4 particles and activated carbon were reduced by using real wastewaters as the background, whereas chlorpheniramine adsorption on GO-Fe3O4 suspension was enhanced due to the effects of surface charge on GO-Fe3O4 and ionic strength variation in water. The fittings of adsorption isotherms indicated that the wastewater background reduced the surface heterogeneity of GO-Fe3O4 suspension and improved the adsorption performance. Appreciable removal efficiencies of NDMA and other nitrosamine FPs were observed when GO-Fe3O4 particles were added in real wastewaters. However, when chlorpheniramine was present in wastewaters, chlorpheniramine adsorption and degradation reaction simultaneously occurred on the surface of GO-Fe3O4, increasing NDMA and other nitrosamine FPs in wastewaters after GO-Fe3O4 addition for chlorpheniramine adsorption. The assumption was further demonstrated by observing the NDMA-FP increase during chlorpheniramine adsorption on GO-Fe3O4 in deionized water. GO-Fe3O4 is a potential adsorbent for chlorpheniramine removal. Nevertheless, the low treatment efficiencies at high doses limit its application for nitrosamine FP adsorptions in real wastewaters.


Assuntos
Clorfeniramina/isolamento & purificação , Nitrosaminas/química , Nitrosaminas/isolamento & purificação , Poluentes Químicos da Água/isolamento & purificação , Purificação da Água/métodos , Adsorção , Clorfeniramina/química , Dimetilnitrosamina/química , Dimetilnitrosamina/isolamento & purificação , Desinfecção , Óxido Ferroso-Férrico/química , Grafite/química
5.
Nat Commun ; 10(1): 422, 2019 01 24.
Artigo em Inglês | MEDLINE | ID: mdl-30679461

RESUMO

Graphene oxide (GO) sheets have been used to construct various bulk forms of GO and graphene-based materials through solution-based processing techniques. Here, we report a highly cohesive dough state of GO with tens of weight percent loading in water without binder-like additives. The dough state can be diluted to obtain gels or dispersions, and dried to yield hard solids. It can be kneaded without leaving stains, readily reshaped, connected, and further processed to make bulk GO and graphene materials of arbitrary form factors and tunable microstructures. The doughs can be transformed to dense glassy solids of GO or graphene without long-range stacking order of the sheets, which exhibit isotropic and much enhanced mechanical properties due to hindered sliding between the sheets. GO dough is also found to be a good support material for electrocatalysts as it helps to form compliant interface to access the active particles.

6.
Chemistry ; 24(37): 9305-9315, 2018 Jul 02.
Artigo em Inglês | MEDLINE | ID: mdl-29726635

RESUMO

Nanoscale transition-metal dichalcogenide materials showed promising potential for visible-light responsive photocatalysis. Here, we report our investigations on the synthesis of heterodimensional nanostructures of two-dimensional (2D) ultrathin MoS2 nanosheets interspersed with ZnO nanoparticles by using a facile two-step method consisting of sonication-aided exfoliation technique followed by a wet chemical process. The photocatalytic activity of the nanocomposites was examined by studying the degradation of different organic dye pollutants and tetracycline, a common antibiotic, under visible-light irradiation. It is found that within 30 min more than 90 % of the model organic dye was photodegraded by the optimized quasi-0D/2D hybrid nanomaterial. The reaction rate of pollutant degradation is about five and eight times higher than those of the pristine MoS2 naonosheets and P25 photocatalysts, respectively. The outstanding photocatalytic activity of the heterodimensional hybrids can be attributed to a few beneficial features from the synergetic effects. Most importantly, the intimate junction between ZnO and MoS2 facilitates the separation of photogenerated carriers, leading to the enhancement of photocatalytic efficiency. A tentative photocatalytic degradation mechanism was proposed and tested. Overall, the present work provides valuable insights for the exploration of cost-effective nanoscale heterodimensional hybrids constructed from atomically thin layered materials.


Assuntos
Antibacterianos/química , Dissulfetos/química , Molibdênio/química , Nanocompostos/química , Poluentes Químicos da Água/química , Óxido de Zinco/química , Catálise , Corantes/química , Luz , Tamanho da Partícula , Processos Fotoquímicos , Fotólise , Propriedades de Superfície , Tetraciclina/química
7.
Chem Commun (Camb) ; 54(57): 7900-7903, 2018 Jul 12.
Artigo em Inglês | MEDLINE | ID: mdl-29845163

RESUMO

We report a two-step hybridization of N-doped graphene and Ag-decorated Fe2O3 hematite to realize a balanced oxygen adsorption/desorption equilibrium and a laser-coupled ORR (LORR). The stable plateau currents with n values of 3.9 in a wide potential range (0.2-0.7 V) and 7.5% peroxide inhibition of the LORR are found to be directly associated with the Ag/Fe2O3 heterojunction, where interactions of semiconductor band gap excitation and plasmonic resonance-induced hot electrons are proposed to occur.

8.
Opt Lett ; 42(22): 4768-4771, 2017 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-29140365

RESUMO

Graphene oxide (GO) ultrathin film can be wafer-scale deposited by spin coating, can be patterned by laser interference lithography and oxygen plasma etching, can be thinned atomically (0.26 nm/min) and oxidized by ozone treatment, and is a relatively transparent and low-refractive-index material compared to pristine graphene. All those unique properties prompt us to realize a low-loss (∼5 dB/cm), high-extinction-ratio (19 dB), and narrowband (0.425 nm) GO/silicon hybrid waveguide Bragg reflector by transferring 7-nm-thick GO gratings (n=1.58) atop a silicon strip waveguide. Unlike a sidewall-corrugated strip waveguide Bragg reflector that generally exhibits distorted corrugation profiles and is sensitive to fabrication errors, the as-realized GO-grating-covered strip waveguide Bragg reflector exhibits a stable reflecting wavelength and controllable reflection bandwidth that can be well predicted by numerical simulations.

9.
Sci Rep ; 7(1): 3908, 2017 06 20.
Artigo em Inglês | MEDLINE | ID: mdl-28634339

RESUMO

Demand for rapid and massive-scale exfoliation of bulky graphite remains high in graphene commercialization and property manipulation. We report a procedure utilizing "preformed acidic oxidizing medium (PAOM)" as a modified version of the Hummers' method for fast and reliable synthesis of graphene oxide. Pre-mixing of KMnO4 and concentrated H2SO4 prior to the addition of graphite flakes enables the formation of effectively and efficiently oxidized graphene oxide (EEGO) featured by its high yields and suspension homogeneity. PAOM expedites diffusion of the Mn-oxidants into the graphite galleries, resulting in the rapid graphite oxidation, capable of oxidizing bulky graphite flakes (~0.8 mm in diameter) that can not be realized by the Hummers' method. In the scale-up tests, ten-time amount of graphite can be completely exfoliated by PAOM without need of extended reaction time. The remarkable suspension homogeneity of EEGO can be exploited to deposit ultra-flat coating for wafer-scale nanopatterning. We successfully fabricated GO optical gratings with well-defined periodicity (300 nm) and uniform thickness (variation <7 nm). The combination of the facile and potent PAOM approach with the wafer-scale patterning technique may realize the goal for massive throughput graphene nanoelectronics.

10.
Sci Rep ; 6: 37174, 2016 11 17.
Artigo em Inglês | MEDLINE | ID: mdl-27853187

RESUMO

A general solvent-dependent protocol directly influencing the oxygen reduction reaction (ORR) in metal oxide/graphene nanohybrids has been demonstrated. We conducted the two-step synthesis of cobalt oxide/N-doped graphene nanohybrids (CNG) with solvents of water, ethanol, and dimethylformamide (DMF), representing tree typical categories of aqueous, polar organic, and organic N-containing solvents commonly adopted for graphene nanocomposites preparation. The superior ORR performance of the DMF-hybrids can be attributed to the high nitrogen-doping, aggregation-free hybridization, and unique graphene porous structures. As DMF is the more effective N-source, the spectroscopic results support a catalytic nitrogenation potentially mediated by cobalt-DMF coordination complexes. The wide-distribution of porosity (covering micro-, meso-, to macro-pore) and micron-void assembly of graphene may further enhance the diffusion kinetics for ORR. As the results, CNG by DMF-synthesis exhibits the high ORR activities close to Pt/C (i.e. only 8 mV difference of half-wave potential with electron transfer number of 3.96) with the better durability in the alkaline condition. Additional graphene hybrids comprised of iron and manganese oxides also show the superior ORR activities by DMF-synthesis, confirming the general solvent-dependent protocol to achieve enhanced ORR activities.

11.
Chemistry ; 22(42): 14950-14961, 2016 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-27576479

RESUMO

The synthesis of noble metal/semiconductor hybrid nanostructures for enhanced catalytic or superior optical properties has attracted a lot of attention in recent years. In this study, a facile and all-solution-processed synthetic route was employed to demonstrate an Au/ZnO platform with plasmonic-enhanced UV/Vis catalytic properties while retaining strengthened luminescent properties. The visible-light response of photocatalysis is supported by localized surface plasmon resonance (LSPR) excitations while the enhanced performance under UV is aided by charge separation and strong absorption. The enhancement in optical properties is mainly due to local field enhancement effect and coupling between exciton and LSPR. Luminescent characteristics are investigated and discussed in detail. Recyclability tests showed that the Au/ZnO substrate is reusable by cleaning and has a long shelf life. Our result suggests that plasmonic enhancement of photocatalytic performance is not necessarily a trade-off for enhanced near-band-edge emission in Au/ZnO. This approach may give rise to a new class of versatile platforms for use in novel multifunctional and integrated devices.

12.
Chemistry ; 22(36): 12777-84, 2016 Aug 26.
Artigo em Inglês | MEDLINE | ID: mdl-27483050

RESUMO

We exploit the utilization of two-dimensional (2D) molybdenum oxide nanoflakes as a co-catalyst for ZnO nanorods (NRs) to enhance their photocatalytic performance. The 2D nanoflakes of orthorhombic α-MoO3 were synthesized through a sonication-aided exfoliation technique. The 2D MoO3 nanoflakes can be further converted to substoichiometric quasi-metallic MoO3-x by using UV irradiation. Subsequently, 1D-2D MoO3 /ZnO NR and MoO3-x /ZnO NR composite photocatalysts have been successfully synthesized. The photocatalytic performances of the novel nanosystems in the decomposition of methylene blue are studied by using UV- and visible-illumination setup. The incorporated 2D nanoflakes show a positive influence on the photocatalytic activity of the ZnO. The obtained rate constant values follow the order of pristine ZnO NR

13.
Chem Commun (Camb) ; 52(59): 9291-4, 2016 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-27366795

RESUMO

Interaction between adsorbed t-butyl peroxybenzoate and photoexcited graphene rendered trapped phenyl and t-butoxy radicals. Post-irradiation thermal desorption showed benzene, t-butanol, and isobutylene oxide as the end products. The required hydrogen atoms were obtained via the radical disproportionation. Graphene enabled radical species to be captured and their on-surface chemistry to be revealed.

14.
Inorg Chem ; 54(21): 10163-71, 2015 Nov 02.
Artigo em Inglês | MEDLINE | ID: mdl-26451851

RESUMO

Due to the excellent catalytic performance of manganese oxide (K-OMS-2) in a wide range of applications, incorporation of various dopants has been commonly applied for K-OMS-2 to acquire additional functionality or activities. However, the understanding of its substitution mechanism with respect to the catalytic performance of doped K-OMS-2 materials remains unclear. Here we present the structural distortion (from tetragonal to monoclinic cell) and morphological evolution in K-OMS-2 materials by doping hexavalent molybdenum. With a Mo-to-Mn ratio of 1:20 (R-1:20) in the preparation, the resultant monoclinic K-OMS-2 shows a small equidimensional particle size (∼15 nm), a high surface area of 213 m(2) g(-1), and greatly improved catalytic activity toward CO oxidation with lower onset temperatures (40 °C) than that of pristine K-OMS-2 (above 130 °C). HR-TEM analyses reveal direct evidence of structural distortion on the cross-section of 2 × 2 tunnels with the absence of 4-fold rotation symmetry expected for a tetragonal cell, which are indexed using a monoclinic cell. Our results suggest that substitution of Mo(6+) for Mn(3+) (rather than Mn(4+)) coupled with the vacancy generation results in a distorted structure and unique morphology. The weakened Mn-O bonds and Mn vacancies associated with the structural distortion may be mainly responsible for the enhanced catalytic activity of monoclinic K-OMS-2 instead of dopant species.

15.
Nanoscale ; 6(21): 12805-13, 2014 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-25226177

RESUMO

Exceptional photocatalytic enhancement of graphene-semiconductor composites has been widely reported, but our understanding of the role that graphene plays in this enhancement remains limited, which arises from the difficulty of precisely controlling graphene hybridization. Here we present a general platform of a graphene-semiconductor hybrid panel (GHP) system wherein a precise number of layers of graphene are hybridized with photoactive semiconductors (e.g. TiO2, ZnO) to study systematically how graphene affects the photocatalysis. The results show that the graphene enhancement of the photocatalysis depends on the number of graphene layers, with the maximum performance observed at 3 layers. Photodeposited indicators of gold particles further reveal that graphene thickness governs the density of photocatalytic sites and charge transfer efficiency at the graphene-semiconductor interfaces. We suggest that quantized energy levels caused by different numbers of stacked graphene sheets along the vector normal to the graphene basal plane affect the charge transfer routes and lead to the graphene thickness-controlled photocatalysis. GHP substrates deposited with gold particles are promising, uniform substrates for surface enhanced Raman scattering (SERS) applications with the enhancement factor as high as ∼10(8) on 3-layer graphene.

16.
Nanoscale ; 6(1): 334-41, 2014 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-24196690

RESUMO

High-performance hydrogen peroxide sensors provide valuable signals of biological interactions, disorders, and developing of diseases. Low-cost metal oxides are promising alternatives but suffer from low conductivity and sensing activity. Multi-component metal oxides are excellent candidates to accomplish these challenges, but the composition inhomogeneity is difficult to manage with conventional material preparation. We demonstrated redox preparation strategies to successfully synthesize highly homogeneous, noble metal-free H2O2 sensors of spinel nanostructured cobalt manganese oxides with enhanced conductivity, multiple mixed-valence features, and efficient H2O2 sensing activities. The designed redox reactions accompanied with material nucleation/formation are the key factors for compositional homogeneity. High conductivity (1.5 × 10(-2) S cm(-1)) and H2O2 sensing activity (12 times higher than commercial Co3O4) were achieved due to the homogeneous multiple mixed-valence systems of Co(ii)/(iii) and Mn(iii)/(iv). A wide linear detection range (from 0.1 to 25 mM) with a detection limit of 15 µM was observed. Manganese species assist the formation of large surface area nanostructures, enhancing the H2O2 reduction activities, and inhibit the sensing interference. The material controls of hierarchical nanostructures, elemental compositions, porosity, and electrochemical performances are highly associated with the reaction temperatures. The temperature-dependent properties and nanostructure formation mechanisms based on a reaction rate competition are proposed.


Assuntos
Técnicas Eletroquímicas , Peróxido de Hidrogênio/análise , Compostos de Manganês/química , Nanoestruturas/química , Óxidos/química , Catálise , Cobalto/química , Eletrodos , Oxirredução , Porosidade , Propriedades de Superfície , Temperatura
17.
Chem Commun (Camb) ; 49(29): 3025-7, 2013 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-23462684

RESUMO

A general redox procedure was successfully developed for the controlled synthesis of substituted cobalt oxides with hierarchical flower-like nanostructures comprising unique Y-shaped interconnections. The substitution and nanostructures synergistically enhance the material's electrochemical activities for highly efficient sensing of H2O2.

18.
Nanotechnology ; 23(23): 235603, 2012 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-22595887

RESUMO

Site-specific stamping has the potential of becoming a low-cost, high-throughput method for depositing specific-shaped graphene micro-patterns over large areas on a wide variety of substrates. The use of an approach involving flexible stamps presented here represents an important advance towards reaching that potential. This approach entails lithographic creation (dry etching) of high-quality micro-pillar arrays of highly oriented pyrolytic graphite (HOPG) over large areas. This is followed by embedding the micro-pillar arrays in polydimethylsiloxane (PDMS), and detaching them from the HOPG base. This results in flexible stamps containing embedded HOPG micro-pillar arrays with freshly cleaved stamping surfaces. The flexible HOPG/PDMS stamps are then brought into contact with substrate surfaces to site-specifically stamp graphene or few-layer graphene (FLG) arrays over large areas. The freshly cleaved nature of the micro-pillar surfaces in the flexible stamps, the low elastic modulus of the flexible stamps and the elimination of sidewall deposits on the micro-pillars allow for more uniform stamping, relative to the use of stiff HOPG stamps from earlier studies. This approach has the potential to expand the substrate choice for graphene or FLG stamping to include curved and/or flexible substrates that could have an impact on the burgeoning field of flexible/stretchable electronics.


Assuntos
Cristalização/métodos , Grafite/química , Membranas Artificiais , Impressão Molecular/métodos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Módulo de Elasticidade , Substâncias Macromoleculares/química , Teste de Materiais , Conformação Molecular , Tamanho da Partícula , Propriedades de Superfície
19.
Chem Commun (Camb) ; 46(32): 5945-7, 2010 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-20596572

RESUMO

A facile single-step method was developed for synthesizing todorokite-type manganese oxide octahedral molecular seieves (OMS-1) and 2 x 4 tunnel structured manganese oxide (OMS-5) materials. Selection of starting materials and initial pH conditions in the syntheses are crucial.

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