Hai-Fa Zhai

Preparation, characterization and photocatalytic properties of ZnTiO3 powders

A novel photocatalyst ZnTiO(3) powder was prepared by a modified alcoholysis method, using ethylene glycol as reagent/solvent and acetylacetone as stabilizer. A series of analytical techniques were used to characterize the crystallinity, composition, bandgap, morphology, specific surface area and grain size of ZnTiO(3) powders. The relationship between the physicochemical property and the photocatalytic activity was deeply investigated, too. It is found that the photocatalytic activity is dependent on the phase of catalysts. The product of ZnTiO(3) with pure hexagonal-phase calcined at 800 degrees C for 3h exhibits the maximum photocatalytic performance in the photochemical degradation of the azo dye methyl violet under solar light irradiation. The processing parameters such as the concentration of catalysts and the pH value also play an important role in tuning the photocatalytic activity. The optimal concentration and pH value of the pure hexagonal-phase ZnTiO(3) is around 4g/L and 8 in a 10mg/L dye-aqueous solution, respectively.

The Antibacterial Activity of Ta-doped ZnO Nanoparticles

A novel photocatalyst of Ta-doped ZnO nanoparticles was prepared by a modified Pechini-type method. The antimicrobial study of Ta-doped ZnO nanoparticles on several bacteria of Gram-positive Bacillus subtilis (B. subtilis) and Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa) were performed using a standard microbial method. The Ta-doping concentration effect on the minimum inhibitory concentration (MIC) of various bacteria under dark ambient has been evaluated. The photocatalytical inactivation of Ta-doped ZnO nanoparticles under visible light irradiation was examined. The MIC results indicate that the incorporation of Ta5+ ions into ZnO significantly improve the bacteriostasis effect of ZnO nanoparticles on E. coli, S. aureus, and B. subtilis in the absence of light. Compared to MIC results without light irradiation, Ta-doped ZnO and pure ZnO nanoparticles show much stronger bactericidal efficacy on P. aeruginosa, E. coli, and S. aureus under visible light illumination. The possible antimicrobial mechanisms in Ta-doped ZnO systems under visible light and dark conditions were also proposed. Ta-doped ZnO nanoparticles exhibit more effective bactericidal efficacy than pure ZnO in dark ambient, which can be attributed to the synergistic effect of enhanced surface bioactivity and increased electrostatic force due to the incorporation of Ta5+ ions into ZnO. Based on the antibacterial tests, 5xa0% Ta-doped ZnO is a more effective antimicrobial agent than pure ZnO.

Bipolar resistive switching based on SrTiO3/YBa2Cu3O7 epi-layers

Epitaxial SrTiO3/YBa2Cu3O7 bilayers are deposited on (0u20090u20091) SrTiO3 substrates. The resistive switching characteristics of Pt/SrTiO3/YBa2Cu3O7 sandwiches are investigated for nonvolatile memory applications. The Pt/SrTiO3/YBa2Cu3O7 structure exhibits liable bipolar resistance switching with an On/Off current ratio of about 30 and long data retention. Poole–Frenkel conduction is found to dominate the high-resistance state, while Schottky emission is suggested for the low-resistance one. A switching between the bulk and the interface-controlled conduction by voltage pulses with different polarities, which is governed by the migration and redistribution of oxygen vacancies around the SrTiO3/YBa2Cu3O7 interface, is proposed to explain the resistive switching in the heterostructures.

The Polymerization Effect on Synthesis and Visible-Light Photocatalytic Properties of Low-Temperature β-BiNbO4 Using Nb-Citrate Precursor

Low-temperature β-BiNbO4 powders (denoted as Low-β) were prepared by citrate and Pechini methods using homemade water-soluble niobium precursors. The addition of ethylene glycol and the resultant polymerization effect on the synthesis and visible-light photocatalytic performance of β-BiNbO4 powders were fully investigated. The polymerization effect is beneficial to lower the phase formation temperature and obtain smaller particle catalysts. Both methods can synthesize catalysts with excellent performance of visible-light degradation of methyl violet (MV). The Low-β BiNbO4 powder prepared by citrate method shows better degradation rate of about 1xa0h to decompose 80xa0% of MV and also displays good photocatalytic stability. The photodegradation of MV under the visible-light irradiation followed the pseudo-first-order kinetics according to the Langmuir-Hinshelwood model, and the obtained first-order rate constant and half-time are 2.85u2009×u200910−2xa0min−1 and 24.3xa0min, respectively. The better photocatalytic performance of BiNbO4 powders prepared by citrate method can be attributed to its smaller band gap and better crystallinity.

Efficient Visible-Light Photocatalytic Properties in Low-Temperature Bi-Nb-O System Photocatalysts

Low-temperature Bi-Nb-O system photocatalysts were prepared by a citrate method using homemade water-soluble niobium precursors. The structures, morphologies, and optical properties of Bi-Nb-O system photocatalysts with different compositions were investigated deeply. All the Bi-Nb-O powders exhibit appreciably much higher photocatalytic efficiency of photo-degradation of methyl violet (MV), especially for Bi-Nb-O photocatalysts sintered at 750xa0°C (BNO750), only 1.5xa0h to completely decompose MV, and the obtained first-order rate constant (k) is 1.94/h. A larger degradation rate of Bi-Nb-O photocatalysts sintered at 550xa0°C (BNO550) can be attributed to the synergistic effect between β-BiNbO4 and Bi5Nb3O15. Bi5Nb3O15 with small particle size on β-BiNbO4 surface can effectively short the diffuse length of electron. BNO750 exhibits the best photocatalytic properties under visible-light irradiation, which can be attributed to its better crystallinity and the synergistic effect between β-BiNbO4 and α-BiNbO4. The small amount of α-BiNbO4 loading on surface of β-BiNbO4 can effectively improve the electron and hole segregation and migration. Holes are the main active species of Bi-Nb-O system photocatalysts in aqueous solution under visible-light irradiation.

The Effects of Li/Nb Ratio on the Preparation and Photocatalytic Performance of Li-Nb-O Compounds

The effects of Li/Nb ratio on the preparation of Li-Nb-O compounds by a hydrothermal method were studied deeply. Li/Nb ratio has a great impact on the formation of LiNbO3; the ratio smaller than 3:1 is beneficial to the formation of LiNbO3, while larger than 3:1, forms no LiNbO3 at all and the morphology and chemical bond of Nb2O5 raw material are totally modified by Li ions. The reason can be attributed to the large content of LiOH, which is beneficial to form Li3NbO4 not LiNbO3, and also, even if LiNbO3 particle locally forms, it is easily dissolved in LiOH solution with strong alkalinity. Pure LiNb3O8 powders are obtained with two absolutely opposite Li/Nb ratios: 8:1 and 1:3; the former shows a unique porous and hollow structure, quite different from the particle aggregation (the latter shows). Compared with Li/Nbxa0=xa01:3, the 4.2 times higher photocatalytic performance of LiNb3O8 (Li/Nbxa0=xa08:1) are observed and it can be attributed to the unique porous and hollow structure, which provides a high density of active sites for the degradation of MB. Compared to LiNbO3, the improved photocatalytic performance of LiNb3O8 can be attributed to its layered structure type with the reduced symmetry enhancing the separation of electrons and holes.

Hydrothermal-Assisted Sintering Strategy Towards Porous- and Hollow-Structured LiNb3O8 Anode Material

Porous- and hollow-structured LiNb3O8 anode material was prepared by a hydrothermal-assisted sintering strategy for the first time. The phase evolution was studied, and the formation mechanism of the porous and hollow structure was proposed. The formation of the unique structure can be attributed to the local existence of liquid phase because of the volatilization of Li element. As the anode material, the initial discharge capacity is 285.1xa0mAhg−1 at 0.1 C, the largest discharge capacity reported so far for LiNb3O8. Even after 50xa0cycles, the reversible capacity can still maintain 77.6xa0mAhg−1 at 0.1 C, about 2.5 times of that of LiNb3O8 samples prepared by traditional solid-state methods. The significant improvement of Li storage capacity can be attributed to the special porous and hollow structure, which provides a high density of active sites and short parallel channels for fast intercalation of Li+ ions through the surface.

Preparation and Photocatalytic Performance of Hollow Structure LiNb3O8 Photocatalysts

Hollow structure LiNb3O8 photocatalysts were prepared by a hydrothermal method assisting sintering process. The particles’ aggregation to form hollow structures with obvious cavities can be attributed to the Li element volatilization during calcination process. All the LiNb3O8 powders show high photocatalytic efficiency of degradation of methylene blue (MB), especially for the sample calcined at 700xa0°C (LNO700), with only 3xa0h to completely decompose MB. The photo-degradation of MB follows the pseudo-first-order kinetics, and the obtained first-order rate is 0.97/h. The larger degradation rate of LNO700 can be attributed to its hollow structure which provides a larger specific surface area and more active sites to degrade the MB molecules. The cycling test of photo-degradation and adsorption of MB over LNO700 powder indicates that the hollow structure of the LiNb3O8 photocatalyst is stable and the LiNb3O8 photocatalyst is an efficient photocatalyst with good reusability, confirmed by the XRD and X-ray photoelectron spectroscopy tests before and after photo-degradation of MB.

In situ polarization and dielectric property measurements of Pb(Zr0.52Ti0.48)O3 ferroelectric nanocrystals

Pb(Zr0.52Ti0.48)O3/polycarbonate (PZT/PC) composite films with different concentration of PZT ferroelectric nanocrystals are prepared. The polarization and dielectric relaxation behavior of PZT ferroelectric nanocrystals are characterized using in situ transmittance and X-ray diffraction (XRD) measurements for the first time. It’s found that 10% PZT/PC composite film has the largest orientation change and negligible dielectric relaxation after poling (the φ value of 13.8% is almost constant with time even for 168 h). Based on the XRD results, we consider that the preferential orientation of PZT nanocrystals to align in PC matrix after poling is [001] direction.