Wanneng Ye

Photo-to-current response of Bi2Fe4O9 nanocrystals synthesized through a chemical co-precipitation process

Pure-phase Bi2Fe4O9 nanocrystals of mullite-type structure were synthesized through a facile chemical co-precipitation process. The nanocrystals consist of nanorods with transverse size of 100–200 nm and nanoparticles of 80–150 nm in dimension. The nanorods are preferentially grown along [001] and show flat side facets (110) and (10). The nanocrystals exhibit two narrow bandgaps of 2.05 eV and 1.53 eV. A steady photocurrent of 37 μA cm−2 was observed in the Bi2Fe4O9 nanocrystals under visible-light irradiation. The photo-to-current response of Bi2Fe4O9 has not been reported before. Such a high photo-to-current conversion efficiency is ascribed to the synergistic effect of energy band structure and the high reactivity of exposed (110) and (10) facets.

Structural Regulation of PdCu2 Nanoparticles and Their Electrocatalytic Performance for Ethanol Oxidation

Two types of PdCu2 nanoparticles were prepared through one-pot synthesis and a two-step reducing process, named as PdCu2-1 and PdCu2-2, respectively. The morphology and structure of as-prepared samples were investigated by transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma-optical emission spectrometry. Results showed that more Pd atoms were buried in the inside of PdCu2-1, whereas more available Pd sites were distributed on the surface of PdCu2-2. The electrochemical measurements indicated that both PdCu2-1 and PdCu2-2 nanoparticles showed a higher electrocatalytic activity than that for pure Pd nanoparticles. In particular, PdCu2-2 predictably exhibited a better stability and durability as well as a lower onset potential and a higher catalytic current density than that of PdCu2-1 toward ethanol oxidation in alkaline media. On the basis of these studies, the formation mechanisms of both the PdCu2 catalysts and the relationship between their structure and properties were discussed in this paper.

An RAPET approach to in situ synthesis of carbon modified Li4Ti5O12 anode nanocrystals with improved conductivity

Carbon modified lithium titanate (Li4Ti5O12) anode nanocrystals for Li-ion batteries were synthesized by directly treating the titanium alkoxide and lithium acetate ethanol solution via the Reaction under Autogenic Pressure at Elevated Temperature (abbreviated to RAPET). The mixture of the liquid precursors decomposed during the RAPET process and then reacted in situ and transformed into carbon-modified Li4Ti5O12 anode nanocrystals. The organic moieties in the titanium alkoxide and the lithium salt provided both the oxygen and carbon for the synthesis. The resulting products were characterized by X-ray diffraction (XRD), elemental analysis, scanning electronic microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), nitrogen adsorption–desorption measurements, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge testing. The influences of the titanium alkoxide precursors, i.e. the length of the alkoxy group, on the properties of the final products and the presence of the in situ resulting carbon on the electrochemical performance have been investigated.

Types and configurations of domain walls in ferroelectric Bi4Ti3O12 single crystals

In the ferroelectric Bi4Ti3O12, the major component Ps(a) of spontaneous polarization Ps lies along the a axis; the component Ps(c) along the c axis is small. The two switchable components are expected to make up various types of domain walls (DWs). According to group-theory analysis, 11 permissible types of DWs are predicted to exist theoretically, and they include five types of ferroelectric DWs, one type of anti-phase boundary (APB) and five types of APB-combined ferroelectric DWs. The five types of ferroelectric DWs are Ps(a)-90° DWs, Ps(a)-180° DWs, Ps(c)-180° DWs, Ps(a)-90°/Ps(c)-180° DWs and Ps-180° DWs. In Bi4Ti3O12 single crystals, just the five types of ferroelectric DWs were observed using transmission electron microscopy, with no trace of APBs or APB-combined ferroelectric DWs seen. The Ps(a)-90° domains are lamellar and do not have even thickness. Both the Ps(a)-90° DWs and Ps(a)-90°/Ps(c)-180° DWs lie mainly on the (110) plane, but often fold to the (001) plane. The Ps(a)-180° domains are predominantly flaky. Both the Ps(a)-180° DWs and Ps-180° DWs lie mainly on the (001) plane. The Ps(c)-180° DWs observed are irregularly curved.

Retention loss in the ferroelectric (SrBi2Ta2O9)-insulator (HfO2)-silicon structure studied by piezoresponse force microscopy

Metal-ferroelectric-insulator-silicon (MFIS) structures with SrBi2Ta2O9 as ferroelectric thin film and HfO2 as insulating buffer layer were fabricated by pulsed-laser deposition. The interfaces and memory window of the MFIS structure were investigated. Piezoresponse force microscopy was used to observe the change of domain images in order to investigate the retention characteristics, which demonstrated that the MFIS structure experiences retention loss via a random-walk–type process, identified by a stretched exponential-decay model. The corresponding mechanism was discussed based on the time-dependent depolarization field.

Electrospun γ-Fe 2 O 3 nanofibers as bioelectrochemical sensors for simultaneous determination of small biomolecules

Nanofibers of α-Fe2O3 and γ-Fe2O3 have been obtained after the controlled calcination of precursor nanofibers synthesized by electrospinning. α-Fe2O3 nanofibers showed an irregular toruloid structure due to the decomposition of poly (4-vinyl) pyridine in air while γ-Fe2O3 nanoparticles decorated nanofibers were observed after the calcination under N2 atmosphere. Electrochemical measurements showed that different electrochemical behaviors were observed on the glassy carbon electrodes modified by α-Fe2O3 and γ-Fe2O3 nanofibers. The electrode modified by γ-Fe2O3 nanofibers exhibited high electrocatalytic activities toward oxidation of dopamine, uric acid and ascorbic acid while α-Fe2O3 nanofibers cannot. Furthermore, the γ-Fe2O3 modified electrode can realize the selective detection of biomolecules in ternary electrolyte solutions. The synthesis of nanofibers of α-Fe2O3 and γ-Fe2O3 and their electrochemical sensing properties relationship have been discussed and analyzed based on the experimental results.

In situ observation of the motion of ferroelectric domain walls in Bi4Ti3O12 single crystals

Five types of ferroelectric domain walls (DWs) are present in Bi4Ti3O12 single crystals (Ye et al., 2015). Here their motion was investigated in situ using transmission electron microscopy and optical microscopy. The motion of P(a)-90° DWs, P(a)-180° DWs and P(c)-180° DWs was observed through electron beam poling in a transmission electron microscope. The growth of new Ps(a)-180° nanodomains was frequently seen and they tended to nucleate at preexisting Ps(a)-90° DWs. Irregularly curved P(c)-180° DWs exhibit the highest mobility, while migration over a short range occurs occasionally for faceted Ps(a)-90° DWs. In addition, the motion of Ps(a)-90° DWs and the growth/annihilation of new needle-like Ps(a)-90° domains in a 20 µm-thick crystal were observed under an external electric field on an optical microscope. Most of the new needle-like Ps(a)-90° domains nucleate at preexisting Ps(a)-90° DWs and the former are much smaller than the latter. This is very similar to the situation for Ps(a)-180° domain switching induced by electron beam poling in a transmission electron microscope. Our observations suggest the energy hierarchy for different domains of Ps(c)-180° ≤ Ps(a)-180° ≤ Ps(a)-90° ≤ new needle-like Ps(a)-90° in ferroelectric Bi4Ti3O12.