Xinli Guo

Crystallization, phase evolution and ferroelectric properties of sol–gel-synthesized Ba(Ti0.8Zr0.2)O3–x(Ba0.7Ca0.3)TiO3 thin films

A lead-free piezoelectric material with ultra-high properties, Ba(Ti0.8Zr0.2)O3–x(Ba0.7Ca0.3)TiO3(BZT–xBCT) nanocrystals was synthesized via a sol–gel method, and the corresponding thin films were also deposited on Pt/Ti/SiO2/Si substrates by a spin-coating approach. The BZT–xBCT thin film exhibited a high remnant polarization of 22.15 μC cm−2 with a large coercive field of 68.06 kV cm−1. The resultant gel is calcined at various elevated temperatures and studied with FTIR/XRD/Raman/DSC-TGA/AFM/SEM techniques for gel composition, crystallization, phase transition, thermochemistry and the morphology of the film. Although the room temperature crystal structure of the BZT–xBCT nanocrystals appears to be a standard perovskite structure by conventional X-ray diffraction (XRD), Raman spectroscopy demonstrates the presence of non-centrosymmetric regions arising from the off-centering of the titanium (zirconium) atoms. The Raman spectra findings demonstrate the degree by which the tetragonal phase grows with the increase of calcining temperature in BZT–0.5BCT, and the characteristic ferroelectric–ferroelectric phase transition in BZT–xBCT while going through the MPB process. The structural and constituent evolution for the conversion process from gel to ceramic, as well as the formation mechanism of the BZT–0.5BCT crystallite, were also elucidated.

Synergistic effects of silica nanoparticles/polycarboxylate superplasticizer modified graphene oxide on mechanical behavior and hydration process of cement composites

In this paper, the graphene oxide (GO) modified by polycarboxylate superplasticizer (PC) (PC@GO) was decorated by silica nanoparticles (SiO2 NPs) to form a hybrid structure of SiO2 NPs/PC@GO. The as-prepared SiO2 NPs/PC@GO hybrid was incorporated in cement matrix for further reinforcement. The results show that SiO2 NPs are uniformly distributed on the surface of GO nanosheets. The addition of SiO2 NPs/PC@GO hybrid (1.5% SiO2 and 0.02% GO by weight of cement) can increase the compressive strength by factors of 38.31%, 44.47% and 38.89% at 3, 7, 28 days, respectively, which are much higher than those of cement composites reinforced by a single-promoter (either PC@GO or SiO2 NPs). Moreover, our results demonstrate that SiO2 NPs/PC@GO hybrid can accelerate the cement hydration, improve the degree of pozzolanic reaction and refine the microstructure of hydration products. The excellent reinforcement is attributed to the synergistic effects of SiO2 NPs/PC@GO hybrid including better dispersion property, higher degree of pozzolanic reaction and cross-linking structure of [SiO2–GO–CSH]. This work provides a very effective way to reinforce cement composites by SiO2 NPs/PC@GO hybrid.

Carbon Nanowires Spontaneously Formed on Surface of Freshly Cleaved Highly Ordered Pyrolytic Graphite Wafer

Carbon nanowires (CNWs) spontaneously formed on the surface of a freshly cleaved highly ordered pyrolytic graphite (HOPG) wafer have been observed for the first time by atomic force microscopy (AFM), an ultrahigh vacuum scanning tunneling microscopy (UHV-STM), and a scanning electron microscopy (SEM). The observed CNWs grow straight on the surface of one terrace or across the step edges with a diameter range from ~10 to 200 nm and have ametallic conductivity. The growth of such CNWs seems to originate from surface defects such as step edges and protrusions during the fabrication of the HOPG wafer, or by the rolling of graphene layers into cylinders during cleavage. This result has provided a new method of obtaining CNWs.

High-performance Cu nanoparticles/three-dimensional graphene/Ni foam hybrid for catalytic and sensing applications

A novel hybrid of Cu nanoparticles/three-dimensional graphene/Ni foam (Cu NPs/3DGr/NiF) was prepared by chemical vapor deposition, followed by a galvanic displacement reaction in Ni- and Cu-ion-containing salt solution through a one-step reaction. The as-prepared Cu NPs/3DGr/NiF hybrid is uniform, stable, recyclable and exhibits an extraordinarily high catalytic efficiency for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with a reduction rate constant K = 0.056 15 s-1, required time ∼30 s and excellent sensing properties for the non-enzymatic amperometric hydrogen peroxide (H2O2) with a linear range ∼50 μM-9.65 mM, response time ∼3 s, detection limit ∼1 μM. The results indicate that the as-prepared Cu NPs/3DGr/NiF hybrid can be used to replace expensive noble metals in catalysis and sensing applications.

Toward advanced sodium-ion batteries: a wheel-inspired yolk–shell design for large-volume-change anode materials

Yolk–shell structures have found great potential in addressing the huge volume change of alloy-type anodes for lithium-/sodium-ion batteries (LIBs/SIBs). The main challenges associated with yolk–shell structures are the sluggish electron/ion transfer at the yolk–shell interface caused by the point-to-point contact between the yolk and the shell, and the rupture of self-supporting carbon shells during a long-term cycle. Here, inspired by the structure of a wheel, we designed and fabricated a novel yolk–shell structure with a multipoint contact between the yolk and the shell (wheel–shell structure) through a general and scalable approach. The multipoint contact is achieved by bridging SnO2 yolks and graphene shells using carbon nanoribbons, which allows a high-efficiency transfer of electrons and ions inside/outside the wheel–shell structure. Moreover, the interconnected graphene shells function not only as an electrical highway so that all active materials are electrochemically active, but also as a mechanical backbone to maintain the structural integrity. As an anode for SIBs, the wheel–shell structure exhibits extraordinary rate capability (153.3 mA h g−1 at 10.0 A g−1) and robust cycling stability (248.2 mA h g−1 remaining after 1000 cycles at 1.0 A g−1 with a capacity retention of 86.9%). These results demonstrate the most efficient SnO2-based anode ever reported for SIBs. More importantly, the proposed strategy opens up new avenues to boost the electrochemical performance of large-volume-change anode materials for advanced battery systems.

Au/ITO dual-layer-coated optical fiber probe for multifunctional scanning tunneling microscopy

We have fabricated an Au/ITO (indium-tin-oxide) dual-layer optical fiber probe with functions for high-quality scanning tunneling microscope (STM) imaging and near-field excitation and detection of STM-induced electroluminescence (STML). The inner ITO layer is kept approximately 150 nm in thickness to ensure the conductivity of the fiber probe. The outer Au layer is kept approximately 13 nm in thickness to obtain a better combination of high electron injection efficiency and optical transparency. High-quality STM images, and STML corresponding to maximum quantum efficiencies of 7 x 10(-4) and 1 x 10(-4) photons/electron are obtained on Au(111)/mica and p-GaAs(110) surfaces at room temperature (RT), respectively. The outer Au layer on the Au/ITO-coated fiber probe shows an effective role in improving the quality of STM imaging, the tip apex geometry and durability of the fiber probe, the stability and reproducibility of STML and enhancing the intensity of plasmon-mediated STML from the Au(111) surface but it shows no effect in enhancing the intensity of STML from the GaAs(110) surface.