Sol-gel based alumina powders with catalytic applications

The sol-gel process provides a new approach to the preparation of oxide materials and offers many advantages for making catalysts. Since homogeneous mixing can be achieved at the molecular scale, the chemical reactivity of the oxide surface can be greatly enhanced; thus powders with high surface area and optimized pore size distribution can be obtained at low temperatures. In the present work NiO/Al₂O₃ sol-gel catalysts were obtained by simultaneous gelation of aluminium isopropoxide and nickel nitrate. A comparative study with pure sol-gel alumina was also realized. By physical-structural studies the changes induced by the introduction of the Ni precursor, before and after aluminium alkoxide hydrolysis were highlighted. The introduction of Ni at the beginning of the reaction favors γ-Al₂O₃ crystallization. When Ni is added at the end of reaction, it delays the alumina crystallization and induces the disorder of the lattice. The obtained Ni doped sol-gel derived alumina has been used as catalyst in the finished form for glycerol reforming to generate H₂ for fuel cell applications. Some evaluation results of Ni-doped alumina combined with TiO₂ in photocatalytic glycerol reforming reaction have been included.     

Preparation of ZnO–Al2O3 Particles in a Premixed Flame

Zinc oxide (ZnO) and alumina (Al₂O₃) particles are synthesized by the combustion of their volatilized acetylacetonate precursors in a premixed air–methane flame reactor. The particles are characterized by XRD, transmission electron microscopy, scanning mobility particle sizing and by measurement of the BET specific surface area. Pure (-)alumina particles appear as dendritic aggregates with average mobile diameter 43–93 nm consisting of partly sintered, crystalline primary particles with diameter 7.1–8.8 nm and specific surface area 184–229 m²/g. Pure zinc oxide yields compact, crystalline particles with diameter 25–40 nm and specific surface area 27–43 m²/g. The crystallite size for both oxides, estimated from the XRD line broadening, is comparable to or slightly smaller than the primary particle diameter. The specific surface area increases and the primary particle size decreases with a decreasing flame temperature and a decreasing precursor vapour pressure. The combustion of precursor mixtures leads to composite particles consisting of zinc aluminate ZnAl₂O₄ intermixed with either ZnO or Al₂O₃ phases. The zinc aluminate particles are dendritic aggregates, resembling the alumina particles, and are evidently synthesized to the full extent allowed by the overall precursor composition. The addition of even small amounts of alumina to ZnO increases the specific surface area of the composites significantly, for example, zinc aluminate particles increases to approximately 150 m²/g. The gas-to-particle conversion is initiated by the fast nucleation of Al₂O₃ or ZnAl₂O₃, succeeded by a more gradual condensation of the excess ZnO with a rate probably controlled by the cooling rate for the flame.     

First-principles study of cobalt silicide nanosheet and nanotubes: Stability and electronic properties

We perform first-principle calculations to study the geometric and electronic structures of cobalt silicide (CoSi₂) nanosheet and nanotubes. The structure of layered CoSi₂ is characterized by a CoSi₂ nanosheet, analogous to the (1 1 1) surface of CoSi₂ crystal. The strain energy involved in rolling up a CoSi₂ nanosheet to CoSi₂ nanotubes is very low. Both the CoSi₂ nanosheet and nanotubes are energetically stable. CoSi₂ nanotubes prefer to form bundles to further release strain energy. All CoSi₂ nanotubes exhibit uniformly metallicity and steady work functions, independent of tube chirality.     

First-principles study of cobalt silicide nanosheet and nanotubes: Stability and electronic properties

We perform first-principle calculations to study the geometric and electronic structures of cobalt silicide (CoSi₂) nanosheet and nanotubes. The structure of layered CoSi₂ is characterized by a CoSi₂ nanosheet, analogous to the (1 1 1) surface of CoSi₂ crystal. The strain energy involved in rolling up a CoSi₂ nanosheet to CoSi₂ nanotubes is very low. Both the CoSi₂ nanosheet and nanotubes are energetically stable. CoSi₂ nanotubes prefer to form bundles to further release strain energy. All CoSi₂ nanotubes exhibit uniformly metallicity and steady work functions, independent of tube chirality.     

Microstructures and electrical properties of La0.8Sr0.2MnO3 films synthesized by sol-gel method

La_0.8Sr_0.2MnO₃ (LSMO) thin films were fabricated on alumina substrates by an improved sol-gel dip-coating process. It was found that multiple dip-coating process could not be performed until the pre-firing temperature reached 600 °C. Different amounts of LSMO powders were added to precursor solution with an aim to avoid cracks in LSMO thin films during calcining caused by the shrinkage mismatch between the film and the substrate. The structure and surface morphology of the films prepared from precursors with and without LSMO powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the addition of 56.4 wt.% LSMO powders into the sol-gel precursor solution significantly modified the microstructure of films. A single LSMO perovskite phase was obtained on alumina substrate after calcining at 800 °C for 4 h by the improved sol-gel method. The sheet resistance of the films prepared with different processing parameters was measured by four-point dc method. Results indicated that the sheet resistance of films decreased with increasing the number of coating applications and the amount of LSMO powders.     

Localized Surface Plasmon Resonance Enhanced Singlet Oxygen Generation and Light Absorption Based on Black Phosphorus@AuNPs Nanosheet for Tumor Photodynamic/Thermal Therapy

Although photodynamic therapy is an efficient therapeutic strategy for cancer treatment, it always suffers from the low singlet oxygen (¹O₂) yields owing to the weak absorption in optical transparent window of biological tissues. Herein, the black phosphorus (BP) nanosheet is integrated with gold nanoparticles (AuNPs) to simultaneously enhance the singlet oxygen generation and hyperthermia by localized surface plasmon resonance (LSPR) in cancer therapy. In the design, BP nanosheet employed as two‐dimension (2D) inorganic photosensitizer is hybridized with AuNPs through polyetherimide (PEI) as bridge to form BP‐PEI/AuNPs hybrid nanosheet. Such hybridation not only significantly increases the ¹O₂ production of BP nanosheet through maximizing the local field enhancement of AuNPs, but also significantly enhances the light absorption of BP nanosheet to promote photothermal effect by LSPR. Accordingly, about 3.9‐fold enhancement of ¹O₂ production and 1.7‐fold increasement of photothermal conversion efficiency are achieved compared with BP‐PEI alone upon single 670 nm laser irradiation. As a proof‐of‐concept model, BP‐PEI/AuNPs hybrid nanosheet with simultaneous dual‐modal phototherapy functions result in effective suppression of tumor growth with minimized side effects both in vitro and in vivo, indicating the great potential of the BP‐PEI/AuNPs hybrid nanosheet as an effective strategy to enhance the cancer therapy efficiency.     

氧化石墨烯猝灭罗丹明6G荧光的机理研究

The fluorescence quenching of Rhodamine 6G (R6G) by graphene oxide (GO) was interrogated by R6G fluorescence measurements using a set of controlled GO samples with varied C/O ratios as the quencher.The carbonyl groups on the GO nanosheet turned to play a dominant role in quenching the R6G fluorescence.The quenching in the static regime can be described by the "sphere of action" model.The significant absorption of the R6G fluorescence by the ground-state complex formed between R6G and GO was identified to be responsible for the static quenching.This work offers helpful insights into the fluorescence quenching mechanisms in the R6G/GO system.     

Fabrication of amine-functionalized magnetite nanoparticles for water treatment processes

The anatase phase of titania (TiO₂) nano-photocatalysts was prepared using a modified sol gel process and thereafter embedded on carbon-covered alumina supports. The carbon-covered alumina (CCA) supports were prepared via the adsorption of toluene 2,4-diisocyanate (TDI) on the surface of the alumina. TDI was used as the carbon source for the first time for the carbon-covered alumina support system. The adsorption of TDI on alumina is irreversible; hence, the resulting organic moiety can undergo pyrolysis at high temperatures resulting in the formation of a carbon coating on the surface of the alumina. The TiO₂ catalysts were impregnated on the CCA supports. X-ray diffraction analysis indicated that the carbon deposited on the alumina was not crystalline and also showed the successful impregnation of TiO₂ on the CCA supports. In the Raman spectra, it could be deduced that the carbon was rather a conjugated olefinic or polycyclic hydrocarbons which can be considered as molecular units of a graphitic plane. The Raman analysis of the catalysed CCAs showed the presence of both the anatase titania and D and G band associated with the carbon of the CCAs. The scanning electron microscope micrographs indicated that the alumina was coated by a carbon layer and the energy dispersive X-ray spectra showed the presence of Al, O and C in the CCA samples, with the addition of Ti for the catalyst impregnated supports. The Brunauer Emmet and Teller surface area analysis showed that the incorporating of carbon on the alumina surface resulted in an increase in surface area, while the impregnation with TiO₂ resulted in a further increase in surface area. However, a decrease in the pore volume and diameter was observed. The photocatalytic activity of the nanocatalysts was studied for the degradation of Rhodamine B dye. The CCA-TiO₂ nanocatalysts were found to be more photocatalytically active under both visible and UV light irradiation compared to the free TIO₂ nanocatalysts.     

Multiple Soaking with Different Solution Concentration in Doped Silica Preform Fabrication Using Modified Chemical Vapor Deposition and Solution Doping

Abstract

Incorporation of alumina (Al₂O₃) into a silica matrix by modified chemical vapor deposition and a solution doping technique is investigated in this study. Multiple soaking cycles were used to increase the aluminum content in the core layer. The effect of alumina retention in silica matrix soot is focused by multiple cycles of soaking with different solution concentrations, while the effect of the adsorption mechanism is fixed by maintaining the soot deposition process (such as temperature [1,800°C], precursor, total gas flow, and soaking time). The deposited soot is examined for porosity characteristics and effective surface area by a gas adsorption technique with Brunauer–Emett–Teller surface area analysis and the surface and cross-section morphology using scanning electron microscopy. Three different concentrations are used in this work (0.3, 0.7, and 1.2 M) with multiple cycles of soaking. Sintering and the collapsing process is controlled for each preform. The result shows that the alumina content is increased substantially as the number of soaking processes is increased, which may be due to the retention effect as only a small amount of adsorption process takes place as indicated by the slight decrease in the surface area of soot. The collapsed preforms are analyzed using a preform analyzer. Energy dispersive x-ray spectrometry is used to check aluminum content and distribution into the core layer.     

Development of monodispersed MnCO<Subscript>3</Subscript>/graphene nanosheet composite as anode for lithium-ion battery by hydrothermal synthesis

A unique monodispersed MnCO₃/graphene nanosheet composite is synthesized by a simple one-step hydrothermal method and used as anode of lithium-ion battery. X-ray diffraction patterns show the typical rhombohedral structure of MnCO₃. A transmission electron micrograph reveals that MnCO₃ is evenly distributed on the graphene nanosheet surface with a uniform diameter of 100 nm. Electrochemical performance results show that the specific discharge capacities of MnCO₃/graphene nanosheet composite remain above 1015.9 mAh g^?1 at a rate of 0.2 C after 85 cycles in the potential window of 0.01–2.0 V and even at a high rate of 1.0 C this parameter remains at 683.5 mAh g^?1 after 100 cycles. Thus, the composite also exhibits favorable rate performance. The excellent reversible capacities are attributed to the highly dispersed and large nanosheet structure of the composite, which may not only facilitate the fast transport of Li⁺ ions between the electrode and electrolyte but also provide enough surfaces to accommodate extra Li⁺ ions that contribute to partial interfacial storage capacities. Additionally, graphene nanosheet can effectively improve electrical conductivity of the composite. Therefore, MnCO₃/graphene nanosheet composite can be a great potential anode material for lithium-ion batteries.     

Template-free synthesis of hierarchical NiO microtubes as high performance anode materials for Li-ion batteries

Hierarchical nanostructured NiO (h-NiO) microtubes were prepared by a simple wet-chemical synthesis without the use of template or surfactant, followed by the calcination of α-Ni(OH)₂ precursor. The structural characterization of the h-NiO microtubes were performed by scanning microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), the results of which indicated that the obtained h-NiO microtubes are covered by the nanosheet grown perpendicularly on the tube surface. The unique hierarchical nanostructure of h-NiO microtubes with high surface area and many voids facilitates the electrochemical reaction as well as the short ion and electron transport pathway. Therefore, as anode electrode of Li-ion batteries, the h-NiO microtubes deliver largely enhanced cycle capacity of 770 mAh·g⁻¹ at a current density of 0.5 C after 200 cycles with high columbic efficiency, compared to the NiO rods. These results suggest that the h-NiO microtubes can be a promising anode material for Li-ion batteries.     

Enhanced β Crystalline Phase in Poly(vinylidene fluoride) via the Incorporation of Graphene Oxide Sheets assisted by Supercritical CO2 Treatment

Graphene oxide (GO) sheets were pre-modified with a typical piezoelectric polymer, poly(vinylidene fluoride) (PVDF), using a simple supercritical carbon dioxide (SC CO₂) method, and then the PVDF-decorated GO was added into a PVDF matrix by solution blending. Transmission electron microscopy (TEM) revealed that the decorating degree of PVDF on the surface of the GO increased significantly with increasing of SC CO₂ pressure and PVDF concentration. The mechanism of the polymer adsorption on the GO sheets through favorable interaction between the GO and PVDF chains was identified via Fourier transform infrared spectroscopy (FTIR). Further, the crystallization behavior of PVDF/GO composites was investigated by differential scanning calorimetry (DSC), FTIR and polarized optical microscopy (POM). Interestingly, the composite with PVDF-decorated GO as the filler showed higher β-phase content compared to the composite with pristine GO as the filler. The study showed that the supercritical fluid-induced epitaxial crystallization process has significant potential for fabricating functional GO-based nanocomposties containing piezoelectric or conducting materials.     

China rose petal as biotemplate to produce two-dimensional ceria nanosheets

China rose petal was used as robust biotemplate for the facile fabrication of novel ceria nanosheet with a thickness of about 7 nm via a continuous infiltration process. The presence of well-resolved peaks ([111], [200], [220], and [311]) for the products revealed the formation of the fluorite-structured CeO₂. The detailed characterization by field-emission scanning electron microscope (FESEM), field-emission transmission electron microscope (FETEM), and atomic force microscopy (AFM) exhibited the biomorphic structure of polycrystalline ceria film with the nanoparticle size of ca. 6.98 nm. Based on the surface chemistry and biochemistry processes, a possible mechanism for the formation of CeO₂ nanosheets is proposed. Furthermore, nitrogen adsorption–desorption measurement and photoluminescence spectrum (PL) were employed to characterize the samples. The ceria nanosheet showed the existence of mesopores (pores 2–4 nm diameter) on its surface and a broad emission ranging from 350 to 500 nm in photoluminescence spectrum. X-ray photoelectron spectroscopy analysis (XPS) confirmed that the mesoporous nanosheets possessed more surface vacancies than the bulk CeO₂; hence these hierarchical CeO₂ layers appear to be potential candidates for catalytic applications.     

Tuned CuCr layered double hydroxide/carbon-based nanocomposites inducing sonophotocatalytic degradation of dimethyl phthalate

This study is the first to explore the possibility of utilizing CuCr LDH decorated on reduced graphene oxide (rGO) and graphene oxide (GO) as sonophotocatalysts for the degradation of dimethyl phthalate (DMP). CuCr LDH and its nanocomposites were successfully fabricated and characterized. Scanning electron microscopy (SEM) along with high-resolution transmission electron microscope (HRTEM) both evidenced the formation of randomly oriented nanosheet structures of CuCr LDH coupled with thin and folded sheets of GO and rGO. The impact of diverse processes on the degradation efficiency of DMP in the presence of the so-prepared catalysts was compared. Benefiting from the low bandgap and high specific surface area, the as-obtained CuCr LDH/rGO represented outstanding catalytic activity (100 %) toward 15 mg L⁻¹ of DMP within 30 min when subjected to light and ultrasonic irradiations simultaneously. Radical quenching experiments and visual spectrophotometry using an O-phenylenediamine revealed the crucial role of hydroxyl radicals compared to holes and superoxide radicals. Overall, outcomes disclosed that CuCr LDH/rGO is a stable and proper sonophotocatalyst for environmental remediation.     

Hydrothermal synthesis of flower-like TiO2 nanocrystals/graphene oxide nanocomposites

A facile hydrothermal method has been developed to be capable of decorating graphene oxide (GO) with flower-like TiO₂ nanocrystals without using any bridging species. The flower-like TiO₂ nanocrystals were uniformly self-assembled on the surface of GO nanosheets. The photocatalytic activity experiment indicated that the prepared TiO₂/GO nanocomposites exhibited a higher photocatalytic activity for the photocatalytic degradation of rhodamine B (RB) aqueous solution under the UV illumination, this methodology made the synthesis of TiO₂/GO nanocomposites possible and may be further extended to prepare more complicated nanocomposites based on GO for technological applications.     

Effect of the size on the aggregation and sedimentation of graphene oxide in seawaters with different salinities

Graphene oxide (GO) with different sizes is inevitably released into the water environment during its production, use, and disposal. Aggregation and sedimentation would occur when GO entered into the water with high ionic strength. However, the environmental behavior and fate of GO in the coastal water are not well known. Therefore, in the present study, the aggregation and sedimentation of GO nanosheets with different sizes in seawater with different salinities were investigated. GO nanosheets with different sizes were prepared by the ultrasonic pulverization. Compared to original GO, the ultrasonically pulverized GO was more stably dispersed in deionized water. In artificial seawater, the aggregation–sedimentation process became more intense with increasing GO concentration and salinity. With the decrease of the GO nanosheet size, the aggregation–sedimentation rate increased, while the critical aggregation and sedimentation salinity decreased. As GO could deposit in wide coastal waters, which might cause potential ecological risks to marine benthic organisms, its environmental behavior, fate, and ecological risks in the coastal water should be further investigated.     

Preparation, electronic structure, and photocatalytic properties of Bi2O2CO3 nanosheet

Bi₂O₂CO₃ nanosheet with a thickness of less than 20 nm was synthesized via hydrothermal and solvothermal process. The properties of the as-prepared nanosheet were characterized by X-ray diffraction, scanning electron microscopy, and diffuse reflectance spectra. The electronic structure was investigated using first-principle calculations. Application of the as-prepared Bi₂O₂CO₃ nanosheet in photocatalysis was also studied.     

Ultrasonication-assisted synthesis of 2D porous MoS2/GO nanocomposite catalysts as high-performance hydrodesulfurization catalysts of vacuum gasoil: Experimental and DFT study

In this study, a novel, simple, high yield, and scalable method is proposed to synthesize highly porous MoS₂/graphene oxide (M−GO) nanocomposites by reacting the GO and co-exfoliation of bulky MoS₂ in the presence of polyvinyl pyrrolidone (PVP) under different condition of ultrasonication. Also, the effect of ultrasonic output power on the particle size distribution of metal cluster on the surface of nanocatalysts is studied. It is found that the use of the ultrasonication method can reduce the particle size and increase the specific surface area of M−GO nanocomposite catalysts which leads to HDS activity is increased. These nanocomposite catalysts are characterized by XRD, Raman spectroscopy, SEM, STEM, HR-TEM, AFM, XPS, ICP, BET surface, TPR and TPD techniques. The effects of physicochemical properties of the M−GO nanocomposites on the hydrodesulfurization (HDS) reactions of vacuum gas oil (VGO) has been also studied. Catalytic activities of MoS₂-GO nanocomposite are investigated by different operating conditions. M9-GO nanocatalyst with high surface area (324 m²/g) and large pore size (110.3 Å), have the best catalytic performance (99.95%) compared with Co-Mo/γAl₂O₃ (97.91%). Density functional theory (DFT) calculations were also used to elucidate the HDS mechanism over the M−GO catalyst. It is found that the GO substrate can stabilize MoS₂ layers through weak van der Waals interactions between carbon atoms of the GO and S atoms of MoS₂. At both Mo- and S-edges, the direct desulfurization (DDS) is found as the main reaction pathway for the hydrodesulfurization of DBT molecules.     

Structure and electronic properties calculation of ultrathin α-Al2O3 films on (0 0 0 1) α-Cr2O3 templates

Ab initio total energy Hartree-Fock calculations of ultrathin films of α-Al₂O₃ on (0 0 0 1) α-Cr₂O₃ templates are presented. The surface relaxation, the in-plane reconstruction and the surface and strain energies of the slabs are studied as a function of alumina film thickness. The surface Al layer is found to relax inwards considerably, with the magnitude of the inwards relaxation depending on the thickness of the ultrathin alumina film in a non-linear manner. The calculations also reveal that ultrathin films of alumina lower the surface energy of (0 0 0 1) α-chromia substrates. This indicates that the (0 0 0 1) α-chromia surface provides favourable conditions for the templated growth of α-alumina. However, increasing the alumina film thickness is found to give rise to a significant increase in strain energy. Finally, the electronic properties at the surface of the (0 0 0 1) α-Al₂O₃/α-Cr₂O₃ slabs are investigated. Here it is found that the alumina coating gives rise to an increase in the covalency of the bonds at the surface of the slabs. In contrast, the influence of an alumina layer on the electrostatic potential at the surface of the chromia slab is relatively minor, which should also be beneficial for the templated growth of α-alumina on (0 0 0 1) α-chromia substrates.     

Study of Au/SnO x –Al2O3 catalysts used in CO oxidation by in situ Mössbauer spectroscopy

The active catalytic components in tin oxide containing alumina-supported gold catalyst were examined by comparing and analysing the in situ Mössbauer spectra of the SnO _x –Al₂O₃ support and the 3 wt.% Au/SnO _x –Al₂O₃ catalyst (1.1 wt.% Sn, Au/Sn = 3:2 atomic ratio). Samples were prepared by using organometallic precursor of ¹¹⁹SnMe₄ (enriched). First tin was grafted to the alumina surface from the organometallic precursor compound. In the next step the grafted complexes were decomposed in flowing oxygen. Gold was deposited onto the SnO _x –Al₂O₃ support in the subsequent step. Analysis of in situ spectra shows that in Au/SnO _x –Al₂O₃ catalyst after activation in hydrogen at 620 K tin may occur in three different oxidation states [Sn (IV), Sn(II) and Sn(0)] simultaneously. The metallic tin is a component of the bimetallic AuSn alloy phase. Data presented provide the first evidence for the formation of alloy-type supported Sn–Au catalyst on alumina. Furthermore, from the spectra recorded at different temperatures, values of the Debye temperatures and recoilless fractions were also determined for the various species. The results show that in catalytic oxidation of carbon monoxide at room temperature the dominant part of Sn(II) and the AuSn alloy is oxidized.