Wenjing Lou

Gold-ionic liquid nanofluids with preferably tribological properties and thermal conductivity

Gold/1-butyl-3-methylimidazolium hexafluorophosphate (Au/[Bmim][PF6]) nanofluids containing different stabilizing agents were fabricated by a facile one-step chemical reduction method, of which the nanofluids stabilized by cetyltrimethylammonium bromide (CTABr) exhibited ultrahighly thermodynamic stability. The transmission electron microscopy, UV-visible absorption, Fourier transform infrared, and X-ray photoelectron characterizations were conducted to reveal the stable mechanism. Then, the tribological properties of these ionic liquid (IL)-based gold nanofluids were first investigated in more detail. In comparison with pure [Bmim][PF6] and the nanofluids possessing poor stability, the nanofluids with high stability exhibited much better friction-reduction and anti-wear properties. For instance, the friction coefficient and wear volume lubricated by the nanofluid with rather low volumetric concentration (1.02 × 10-3%) stabilized by CTABr under 800 N are 13.8 and 45.4% lower than that of pure [Bmim][PF6], confirming that soft Au nanoparticles (Au NPs) also can be excellent additives for high performance lubricants especially under high loads. Moreover, the thermal conductivity (TC) of the stable nanofluids with three volumetric fraction (2.55 × 10-4, 5.1 × 10-4, and 1.02 × 10-3%) was also measured by a transient hot wire method as a function of temperature (33 to 81°C). The results indicate that the TC of the nanofluid (1.02 × 10-3%) is 13.1% higher than that of [Bmim][PF6] at 81°C but no obvious variation at 33°C. The conspicuously temperature-dependent and greatly enhanced TC of Au/[Bmim][PF6] nanofluids stabilized by CTABr could be attributed to micro-convection caused by the Brownian motion of Au NPs. Our results should open new avenues to utilize Au NPs and ILs in tribology and the high-temperature heat transfer field.

Relationship between dispersion state and reinforcement effect of graphene oxide in microcrystalline cellulose–graphene oxide composite films

In this study, highly ordered, flexible, homogeneous and reinforced microcrystalline cellulose (MCC)–graphene oxide (GO) composite films regenerated from MCC/1-butyl-3-methylimidazolium chloride ([Bmim]Cl) solutions were prepared, and their nanostructures, thermal stability and mechanical properties were investigated by Fourier-transform infrared spectra, X-ray diffraction spectra, scanning electron microscopy images, thermal gravimetric analyses and tensile strength measurements. Moreover, the effect of the dispersion state of GO in MCC/[Bmim]Cl solutions with varying GO contents was studied by rheological tests. The mechanical properties of composite films could be remarkably improved over those of pure MCC film and there is a close relationship between the dispersion state and reinforcement effect of GO. Specifically, in comparison with pure MCC film, the composite film containing 0.5 wt% of GO exhibits a 64.7% enhancement in tensile strength and an 85.1% enhancement in strain-to-failure whereas the mechanical properties of the composite films are inferior to that of pure MCC film when the GO content is higher than 1 wt%.

Ionic liquid-based stable nanofluids containing gold nanoparticles.

A one-phase and/or two-phase method were used to prepare the stable ionic liquid-based nanofluids containing same volume fraction but different sizes or surface states of gold nanoparticles (Au NPs) and their thermal conductivities were investigated in more detail. Five significant experiment parameters, i.e. temperature, dispersion condition, particle size and surface state, and viscosity of base liquid, were evaluated to supply experimental explanations for heat transport mechanisms. The conspicuously temperature-dependent and greatly enhanced thermal conductivity under high temperatures verify that Brownian motion should be one key effect factor in the heat transport processes of ionic liquid-based gold nanofluids. While the positive influences of proper aggregation and the optimized particle size on their thermal conductivity enhancements under some specific conditions demonstrate that clustering may be another critical effect factor in heat transport processes. Moreover, the remarkable difference of the thermal conductivity enhancements of the nanofluids containing Au NPs with different surface states could be attributed to the surface state which has a strong correlation with not only Brownian motion but also clustering. Whilst the close relationship between their thermal conductivity enhancements and the viscosity of base liquid further indicate Brownian motion must occupy the leading position among various influencing factors. Finally, a promisingly synergistic effect of Brownian motion and clustering based on experimental clues and theoretical analyses was first proposed, justifying different mechanisms are sure related. The results may shed lights on comprehensive understanding of heat transport mechanisms in nanofluids.

Ionothermal synthesis of bismuth sulfide nanostructures and their electrochemical hydrogen storage behavior

Bismuth sulfide (Bi2S3) nanoflowers and nanorods were synthesized by thermal decomposition of single source precursor, bismuth di-n-octyl-dithiophosphate (Bi[S2P(OC8H17)2]3), in an ionic liquid (IL) solvent. Both of these products belong to the orthorhombic phase by the analysis of the X-ray powder diffraction spectra. The morphology evolution of Bi2S3 from nanoflowers to nanorods was investigated based on transmission electron microscopy observations. The electrochemical hydrogen storage behavior of these Bi2S3 nanostructures was studied in detail. It was found that the morphology and structure played an important role on the hydrogen storage capacity of such nanomaterials. The Bi2S3 nanoflower structures have a discharging capacity of 100 mAh g−1 at room temperature, which makes it a potential candidate for applications in hydrogen storage, high-energy batteries, and catalytic fields.

Stable Blue‐ and Green‐Emitting Zinc Oxide from Ionic Liquid Crystal Precursors

Photoluminescent nanoparticles, because of their unique properties and potential applications in biomedical fluorophores, LEDs, and photovoltaic devices, have attracted intensive investigation in the past years. Efficient green and red emissions of cadmium sulfide (selenide, tellurium) have been obtained by controlling the particle size, whereas there are few reports about the preparation of stable blue-emitting nanocrystals (NCs). Wide band gap ZnO (3.4 eV), nontoxic and more stable towards air, is a good candidate for a blue emitter. However, ZnO nanoparticles tend to aggregate and grow spontaneously because of their high surface energies. Hence, their fluorescence, red-shifts, broadens, and weakens gradually at room temperature. For this reason, the surface modification of ZnO nanomaterials is thought as an effective way for stable luminescence. The polymer-stabilized ZnO nanoparticles have been fabricated for remarkable blue fluorescence. It is noted, however, that in this pioneering work, only blue fluorescent emission has been obtained. Herein, we developed a new luminescent system to achieve stable blue and green fluorescent emissions at room temperature. Recently, ionic liquid precursors (ILPs) or ionic liquid crystal precursors (ILCPs) have been investigated intensively, for use as the precursor of the inorganic material, the solvent of the reaction, and the template of the final inorganic particle, simultaneously. The repelled charges of ILPs or ILCPs form a protective shell and prevent these nanoparticles from undergoing aggregation or Ostwald ripening. Furthermore, ILPs or ILCPs can be designed by using different cations and anions, which is facilitated to achieve new physicochemical properties and morphologies of nanocrystals. Chen and co-workers have described a new approach for the preparation of an ionic liquidstabilized nano-ZnO with high and stable luminescence. They used ammonium ionic liquid as precursor and obtained blue to yellow fluorescent emissions by varying the ratios of reaction reagents. In this communication, a new kind of ionic liquid crystal precursor containing Zn cation was prepared from more common imidazolium ionic liquid. Blue and green fluorescent nano-ZnO were obtained by using different alkyl precursors. In a typical synthesis, as shown in Scheme 1, the precursors (abbr. Zn-ILC-C1 and Zn-ILC-C16) were obtained from the reaction of Zn(OH)2 and 1-acetic acid-3-alkyl imidazolium chloride

Reduced graphene oxides by microwave-assisted ionothermal treatment

Given their potential applications in electronic and optoelectronic devices and circuits, reduced graphene oxides (RGOs) have attracted considerable interest. However, more facile and environmentally friendly reduction methods, whether thermal or chemical reduction methods, still need to be further exploited. In this paper, a facile and environmentally friendly method was developed to reduce the graphene oxides (GOs) homogeneously exfoliated in ionic liquids by microwave-assisted ionothermal treatment under relative low temperature (200 °C) and atmospheric pressure. UV-visible absorption spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray diffraction pattern, atomic force microscopy, X-ray photoelectron spectroscopy, elemental analysis, thermogravimetric analysis and electrical conductivity measurement were used to confirm the formation of RGOs with a high reduction degree. The large enhancement of C/O atomic ratio (from 1.32 to 7.65) and at least four orders of electrical conductivity rise of RGOs compared with GOs revealed the high deoxygenation and graphitization efficiency of this method. In addition, the conductive RGOs could be successfully exfoliated to a single layer in some organic solvents, which is paramount for their scalable applications. In consideration of the involatile and recyclable nature of ionic liquids, this novel method can be considered as an economical and green thermal reduction route.

Spin-polarized charge trapping cell based on a topological insulator quantum dot

We demonstrate theoretically that a topological insulator quantum dot can be formed via double topological insulator constrictions (TICs). The TICs are created by appropriate split-gate electrode patterns on the top of a HgTe/CdTe quantum well (QW) with inverted band structures. In sharp contrast to conventional semiconductor quantum dots, the presence or absence of topological insulator edge states in the proposed quantum Hall bar system leads to distinct propagating behaviors. This topological insulator quantum dot can be used as a charge and/or spin carrier trap memory element with near perfect program/erase efficiency by properly adjusting the voltages applied to the split-gates. For completeness, we also demonstrate that a small perturbation of the Rashba spin orbit interaction (RSOI) or a magnetic field in the quantum dot does not destroy the topological edge states and has negligible impact on the on-(edge)-state transport behaviors of the quantum Hall bar.

Mechanical synthesis of chemically bonded phosphorus–graphene hybrid as high-temperature lubricating oil additive

Red phosphorus (P) was covalently attached to graphene nanosheets (Gr) using high-energy ball-milling under a nitrogen atmosphere. Benefiting from the formation of phosphate and P–O–C bonds on graphene surfaces, the resulting phosphorus–graphene (P–Gr) hybrids exhibited excellent dispersion stability in polyalkylene glycol (PAG) base oil compared with graphene. Moreover, tribological measurement indicated that addition of 1.0 wt% P–Gr into PAG resulted in significant reduction in friction coefficient (up to about 12%) and wear volume (up to about 98%) for steel/steel contact at 100 °C, which was likely due to the formation of a boundary lubrication film on the sliding surfaces during the friction and wear processes. XPS analysis demonstrated that the tribofilm is composed of FeO, Fe3O4, FeOOH, FePO4, and the compounds containing C–O–C and P–O bonds.

A gel–sol transition phenomenon of oxidation multi-walled carbon nanotubes–glycerol nanofluids induced by polyvinyl alcohol

Oxidation multi-walled carbon nanotubes (O-MWCNTs)–glycerol nanofluids were prepared, either with or without any polyvinyl alcohol (PVA), and their thermal conductivity (TC), stability, fluidity, and rheological properties were investigated. The results demonstrated that the TC enhancements of the nanofluids were almost linear with the nanotube concentration but showed little dependence on temperature. The largest TC enhancement was up to 21.0% when the volume concentration was only 1.4 vol%. The nanofluids exhibited high stability for more than 2 months and a very interesting gel–sol transition phenomenon induced by a low amount of PVA (about 0.06 wt%), which can be observed by an inverted and side tube method and supported by the rheological data. As the fluidity loss of carbon nanotube-based nanofluids has long been one of the major obstacles for their potential applications in energy transfer technologies, the novel gel–sol transition phenomenon introducing the fluidity recovery has great significance in both academic research and practical application fields of the nanofluids. Furthermore, a reasonably stable and disperse mechanism was also proposed to explain this phenomenon.

Preparation of WS2 nanocomposites via mussel-inspired chemistry and their enhanced dispersion stability and tribological performance in polyalkylene glycol

Abstract A novel biomimetic surface modification method utilizing mussel-inspired chemistry was used to prepare tungsten disulfide (WS2) nanocomposites, which enhanced the dispersion stability and tribological performance of WS2 in polyalkylene glycol (PAG). Herein, WS2-polydopamine-methoxypolyethylene glycol amine (WS2-PDA-MPGA) was first synthesized via mussel-inspired chemistry and used as a lubricant additive in PAG. After modification, the dispersion stability of WS2 nanosheets in PAG was obviously improved. Moreover, the tribological performance of WS2-PDA-MPGA in PAG at high temperature was evaluated by the oscillating reciprocating tribometer. Compared to pure PAG, the lubricant composition containing WS2-PDA-MPGA exhibited excellent performance in friction reduction and anti-wear properties at high temperature. The optimal tribological performance could be obtained when the percentage of additives was 0.9 wt%. The tribological results indicate that WS2-PDA-MPGA, with its good dispersion stability, has better friction reduction and anti-wear properties than does WS2 in PAG base oil. The chemical composition analysis of the wear surface indicated that a stable protective film had been formed by physical adsorption and tribo-chemical reactions. Therefore, the surface modification strategy is an effective way to improve the dispersion stability of WS2 in PAG, which can be expanded application of WS2 in the tribological field. Graphical Abstract