Preparation and efficient visible light-induced photocatalytic activity of m-BiVO4 with different morphologies

The monoclinic scheelite BiVO₄ crystals with peanut-like, oval, twin-quadrangle and twin-four-pointed star morphologies were synthesized via a facile one step hydrothermal method by using sodium citrate as the chelating agent. The X-ray diffraction and scanning electron microscopy were employed to elucidate the structures and mophologies of the as-prepared BiVO₄ samples. The results showed that the formation of m-BiVO₄ with different morphologies relied on the pH value of the precursor solution. The band gaps values (E_g) of all the BiVO₄ samples were around 2.37–2.45 eV according to the UV–vis diffuse reflectance spectrum, which indicated that samples could strongly absorb in the visible light region. The photocatalytic activities of BiVO₄ crystals were evaluated by degradation of MB in aqueous solution under artificial solar-light. The BiVO₄ samples obtained at different pH values showed different photocatalytic activities during the sunlight-driven photodegradation of methylene blue (MB). The sample with peanut-like-shape prepared at pH=1 exhibited the highest activity, and the photocatalytic conversion could reach above 90% after 3 h of irradiation. The result suggested that m-BiVO₄ with peanut-like-shape could be used as an effective photocatalyst in practical application for organic pollutants degradation.     

One-pot synthesis of Ag+ doped BiVO4 microspheres with enhanced photocatalytic activity via a facile hydrothermal method

The Ag⁺/BiVO₄ photocatalyst was fabricated through a facile hydrothermal method by using K₆V₁₀O₂₈·9H₂O as the vanadium source. The impact of Ag⁺ on the product's structure and morphology was studied. It was shown that the amount of Ag⁺ has no effect on the product’s crystal phases but plays an important role on the morphology of the nanoparticles that construct the shell of BiVO₄ microspheres. In addition, the Ag⁺-doped photocatalysts have much higher photocatalytic activities in removing RhB and MB under the UV light illumination than the pure BiVO₄. A possible photocatalytic mechanism was proposed in photoexcitation of the BiVO₄ electrons which subsequently captured by the dopant. The present work may offer a novel route to reach higher photocatalytic activity by doping the Ag⁺ in the semiconductor catalysts.     

Synthesis,photoluminescence and thermoluminescence properties of LiNa3P2O7:Tb3+ green emitting phosphor

The alkaline phosphate based LiNa₃P₂O₇:Tb³⁺ phosphors are prepared by solid state reaction method. X-ray diffraction (XRD) analysis shows that all the powders possess orthorhombic structure. Fourier transform infrared (FTIR) spectroscopy studies suggest that the phosphor belong to the diphosphate family. The morphology of the phosphors is identified by scanning electron microscopy (SEM). Upon 378 nm excitation, the LiNa₃P₂O₇:Tb³⁺ phosphors shown emission bands at 482, 545, 588 and 620 nm corresponding to the transitions ⁵D₄→⁷F₆, ⁵D₄→⁷F₅, ⁵D₄→⁷F₄ and ⁵D₄→⁷F₃, respectively. The optimized concentration of Tb³⁺ in LiNa₃P₂O₇ phosphor is found to be 9 mol%. The concentration quenching mechanism was proved to be the exchange interaction between two nearest Tb³⁺ ions with the critical distance (R_c) of 1.18 nm. The Commission International de l'Eclairage (CIE) coordinates evidence that the phosphors emit in the green light region. Thermoluminescence properties of the prepared phosphors are studied by pre-irradiating the powders with different doses of UV irradiation. The kinetic parameters of TL glow curves are calculated using Chen's peak shape method.     

Kinetic study of isothermal crystallization process of Gd2Ti2O7 precursor's powder prepared through the Pechini synthetic approach

Crystallization process of Gd₂Ti₂O₇ precursor's powder prepared by Pechini-type polymerized complex route has been studied under isothermal experimental conditions in an air atmosphere. It was found that the crystallization proceeds through two-parameter Šesták–Berggren (SB) autocatalytic model, in the operating temperature range of 550 °C≤T≤750 °C. Based on the behavior of SB parameters (M, N), it was found that in the lower operating temperature range, the crystallites with relatively low compactness exist, which probably disclosed low dimensionality of crystal growth from numerous nucleation sites, where the amorphous solid is produced. In the higher operating temperature region (above 750 °C), it was established that a morphological well-defined and high-dimensional particles of the formed pyrochlore phase can be expected. It was found that at T=850 °C, there is a change in the rate-determining reaction step, from autocatalytic into the contracting volume mechanism.     

g-C3N4 modified Bi2O3 composites with enhanced visible-light photocatalytic activity

Novel g-C₃N₄ modified Bi₂O₃ (g-C₃N₄/Bi₂O₃) composites were synthesized by a mixing-calcination method. The samples were characterized by thermogravimetry (TG), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), UV–vis diffuse reflection spectroscopy (DRS), photoluminescence (PL) and photocurrent-time measurement (PT). The photocatalytic activity of the composites was evaluated by degradation of Rhodamine B (RHB) and 4-chlorophenol (4-CP) under visible light irradiation (>400 nm). The results indicated that the g-C₃N₄/Bi₂O₃ composites showed higher photocatalytic activity than that of Bi₂O₃ and g-C₃N₄. The enhanced photocatalytic activity of the g-C₃N₄/Bi₂O₃ composites could be attributed to the suitable band positions between g-C₃N₄ and Bi₂O₃. This leads to a low recombination between the photogenerated electron–hole pairs. The proposed mechanism for the enhanced visible-light photocatalytic activity of g-C₃N₄/Bi₂O₃ composites was proven by PL and PT analysis.     

Fabrication and efficient visible light-induced photocatalytic activity of Bi2MoO6/BiPO4 composite

Novel Bi₂MoO₆/BiPO₄ composites with heterojunction structure were fabricated by a one-step hydrothermal method. The photocatalytic properties of Bi₂MoO₆/BiPO₄ composites were evaluated by photocatalytic degradation of rhodamine B (Rh B) under visible light irradiation (λ>420 nm). The results showed that Bi₂MoO₆/BiPO₄ photocatalysts showed much higher photocatalytic activity for the Rh B degradation than the pure BiPO₄ and Bi₂MoO₆ under visible light. The best photocatalytic performance of Bi₂MoO₆/BiPO₄ with about 98.0% Rh B degradation located at molar ratio of 2:1 under visible light illumination for 30 min. The enhanced photocatalytic activity could be mainly ascribed to the formation of heterojunction interface in Bi₂MoO₆/BiPO₄ composites, which is beneficial to the transfer and separation of photogenerated electron–hole pairs, as well as the strong visible light absorption resulting from the sensitization role of Bi₂MoO₆ to BiPO₄. It was also observed that the photodegradation of Rh B is chiefly attributed to the oxidation action of the generated O₂⁻ radicals and the action of h_vb⁺ through direct hole oxidation process.     

A novel ammonia-assisted method for the direct synthesis of Mn3O4 nanoparticles at room temperature and their catalytic activity during the rapid degradation of azo dyes

In this study, we prepared trimanganese tetroxide nanoparticles from MnCl₂ solution in an ammonia atmosphere using a new surfactant-free method at room temperature. We analyzed and characterized the effects of different processing conditions, such as the concentrations of manganese and the ammonia source, as well as the reaction time, on the structure, purity, and morphology of the products using powder X-ray diffraction (XRD), scanning electron microscopy, and Fourier transformation infrared spectroscopy (FTIR) techniques. The XRD and FTIR analyses confirmed that the prepared products comprised single phase Mn₃O₄. At room temperature, the paramagnetic characteristics were also verified by vibrating sample magnetometry. Furthermore, we tested the catalytic activity of the nanoparticles during the degradation of methyl orange and Congo red, which are organic pollutants. Our experiments demonstrated the rapid color removal and reduction in the chemical oxygen demand (>70% and >50% within 10 min, respectively) using aqueous solutions of azo dyes.     

Preparation, conversion, and comparison of the photocatalytic property of Cd(OH)2, CdO, CdS and CdSe

In this paper, we report the hydrothermal preparation of Cd(OH)₂ nanowires and further conversion to CdO nanobelts, CdS nanowires and CdSe nanoparticles through thermal treatment, solvothermal and mixed-solvothermal routes, respectively. The as-obtained products were characterized by means of powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FEMSEM). Research showed that four cadmium compounds were good photocatalysts for the degradation of organic dyes such as Safranine T and Pyronine B, under irradiation of 365 nm UV light. The order of catalytic activity of different materials was found to be Cd(OH)₂<CdO<CdS<CdSe.     

Aqueous synthesis and enhanced photocatalytic activity of ZnO/Fe2O3 heterostructures

We report a facile synthesis of ZnO/Fe₂O₃ heterostructures based on the hydrolysis of FeCl₃ in the presence of ZnO nanoparticles. The material structure, composition, and its optical properties have been examined by means of transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and diffuse reflectance UV–visible spectroscopy. Results obtained show that 2.9 nm-sized Fe₂O₃ nanoparticles produced assemble with ZnO to form ZnO/Fe₂O₃ heterostructures. We have evaluated the photodegradation performances of ZnO/Fe₂O₃ materials using salicylic acid under UV-light. ZnO/Fe₂O₃ heterostructures exhibited enhanced photocatalytic capabilities than commercial ZnO due to the effective electron/hole separation at the interfaces of ZnO/Fe₂O₃ allowing the enhanced hydroxyl and superoxide radicals production from the heterostructure.     

Preparation and photocatalytic activity of nitrogen-doped TiO2 hollow nanospheres

TiO₂ hollow nanospheres were prepared using silicon oxide as a template. N-doped titanium oxide hollow spheres, TiO_2−xN_x were synthesized by reacting TiO₂ hollow spheres with thiourea at 500 °C. XRD and XPS data showed that oxygen was successfully substituted by nitrogen through the nitrogen-doping reaction, and finally N-doped TiO₂ hollow spheres were formed. The N-doped TiO₂ hollow spheres showed new absorption shoulder in visible light region so that they were expected to exhibit photocatalytic activity in the visible light. The photocatalytic activity of N-doped TiO₂ hollow spheres under visible light was similar to that of normal spherical TiO_2−xN_x in spite of the structural difference.     

Effect of annealing temperature on the microstructures and photocatalytic property of colloidal ZnO nanoparticles

Colloidal ZnO nanoparticles were prepared in ethanol solutions and annealed at different temperatures (150-500 °C) subsequently. The size, morphology and surface characteristics of ZnO nanoparticles were examined by TEM, XRD, UV-vis absorption spectrum and FTIR technique. With the increase of annealing temperature, the mean size of ZnO nanoparticles was increased from 10 to 90 nm, while the bonding structure of acetate groups coordinating with zinc ions evolved from unidentate to bidentate type. The UV-induced degradation results of methyl orange verified that the photocatalytic process of colloidal ZnO nanoparticles without annealing and the sample annealed at 150 °C was unstable for the weakly bonding unidentate type of acetate groups. However, the sample annealed above 150 °C demonstrated their photocatalytic stability in the whole catalytic process for the stable bidentate bonding type of acetate groups. In addition, the change of particle size in the annealing process significantly affected the catalytic activity of photocatalysts. ZnO nanoparticles annealed at 300 °C would be a prospective photocatalysts with a high catalytic activity and stability compared with the other samples.     

Direct sunlight responsive Ag–ZnO heterostructure photocatalyst: Enhanced degradation of rhodamine B

The catalytic activity of Ag–ZnO heterostructure on the photocatalytic degradation of rhodamine B was investigated. It demonstrated that Ag–ZnO heterostructure exhibited an enhanced photocatalytic activity compared to pure ZnO nanoparticles under direct sunlight. The possible factors to the photocatalytic acitivity of the sample were explored, including Ag content, dispersity and calcination temperature. It was shown that the sample dispersed by PVP, with 5% mol ratio Ag content, calcined at 400 °C showed the highest photocatalytic acitivity and this catalyst was reusable.     

Synthesis and enhanced photocatalytic activity of NaNbO3 prepared by hydrothermal and polymerized complex methods

We prepared NaNbO₃ by several methods, namely solid-state reaction (SSR), hydrothermal (HT) and polymerized complex (PC) methods, and investigated the relationships between the photocatalytic activity and the particle size and morphology. The photocatalytic activity was evaluated by H₂ evolution from an aqueous methanol solution and pure water splitting in the presence of the Pt(0.5 wt%)/NaNbO₃ and RuO₂(1.25 wt%)/NaNbO₃, respectively. It is found that the sample prepared by PC with smallest particles exhibits the highest photocatalytic activity in both reactions. Moreover, the HT sample with the cubic and rectangular shape also shows the enhanced photocatalytic activity for H₂ evolution from an aqueous methanol solution in comparison with that of the sample prepared by SSR.     

Catalyst efficiency, photostability and reusability study of ZnO nanoparticles in visible light for dye degradation

Polydispersed ZnO nanoparticles (ZnO1000 and ZnO600) with two different windows of particle size distributions (∼120 and 30 nm) were synthesized using citrate gel route and different annealing treatments (1000 and 600 °C, respectively). Photocatalytic efficiency of these samples was compared with TiO₂ in its commercial form-P25, on two dyes, Methylene blue (MB) and Methyl orange (MO). The X-ray diffraction data showed wrutzite ZnO and anatase and rutile phases of P25. UV-visible absorbance spectra of ZnO1000 showed broad absorption range from UV-to-visible (from 382 to 700 nm), as against sharp absorption peaks in UV range for both ZnO600 and P25. The microstructural morphology as seen through scanning electron micrographs showed ZnO1000 with tetrapod-like structures while the ZnO600 showed almost spherical morphologies. Upon subjecting these catalysts to dye solutions in sunlight it was found that both the dyes were completely decolorised within 20 min by ZnO1000, as against partial decolorisation by ZnO600 and P25 ( 53% and 78% for MO and 77% and 88% for MB samples). The effect of catalyst loading (from 125 mg to 1 g) on decolorisation showed that ZnO1000 had good efficiency for all concentrations which was followed by P25 and then by ZnO600. Small perturbations are attributed to the competition between sunlight scattering-induced, reduced irradiation field and the exposed surface area offered by catalyst, which work as active sites for decolorisation. The reusability of the catalysts when studied on fresh dye samples (4 trials), the decolorisation efficiency decreased merely from 99.2% to 99.12% for ZnO1000 as compared to ZnO600 (53.3% to 19.94%) and P25 (78.3% to 31.42%), indicating the efficient reusability of ZnO1000. The effective half life of the catalysts, in terms of number of reuses, were calculated and found to be ∼3 for both ZnO600 and P25 and was >3000 for ZnO1000, which justifies its extremely high reuse. The byproduct analysis (compared with standards prescribed by World Health Organisation (WHO) and Central Pollution Control Board of India (CPCB)) showed cleavage of the chromophore and of other bonds with opening of benzene rings, indicating degradation of the dyes in concurrence with decolorisation, in the stipulated time. Further, cytotoxicity studies performed on SiHa cell lines showed non-toxicity of the byproducts with ZnO1000 as compared to ZnO600 and P25.     

Hydrothermal synthesis of well-dispersed ultrafine N-doped TiO2 nanoparticles with enhanced photocatalytic activity under visible light

Ultrafine nitrogen-doped TiO₂ nanoparticles with narrow particle size distribution, good dispersion, and high surface area were synthesized in the presence of urea and PEG-4000 via a hydrothermal procedure. TEM observation, N₂ adsorption, XRD, UV-vis spectroscopy, the Raman spectroscopy and XPS analysis were conducted to characterize the synthesized TiO₂ particles. The synthesized TiO₂ particles were a mixture of 49.5% anatase and 50.5% rutile with a size of around 5 nm. The photocatalytic activities were tested in the degradation of an aqueous solution of a reactive Brilliant Blue KN-R under both UV and visible light. The synthesized TiO₂ particles showed much higher photocatalytic activity than a commercial P25 TiO₂ powder under both UV and visible light irradiations. The high performance is associated to N doping, the reduced particle size, good dispersion, high surface area, and a quantum size effect.     

Thermoelectric properties of Ni-doped CuInTe2

Polycrystalline samples of composition Cu_1−xNi_xInTe₂ (for x=0–0.05) were synthesized from elements of 5 N purity using a solid-state reaction. The phase purity of the products was verified by X-ray diffraction. Samples for measurement of the transport properties were prepared using hot-pressing. The samples were then characterized by the measurement of electrical conductivity, the Hall coefficient, the Seebeck coefficient, and the thermal conductivity over a temperature range of 300–675 K. All of the samples demonstrate p-type conductivity. We discuss the influence of Ni substitution on the free carrier concentration and the thermoelectric performance. The investigation of the thermoelectric properties shows an improvement up to 50% of ZT in the temperature range of 300–600 K.     

Solvent-controlled syntheses of Ni12P5 and Ni2P nanocrystals and photocatalytic property comparison

Nickel phosphide nanocrystals with various phases have been successfully synthesized via a simple solvothermal route at 180 °C for 16 h, employing nickel chloride and white phosphorus (WP) as starting reactants in the presence of sodium dodecylbenzene sulfonate (SDBS). X-ray powder diffraction (XRD) research showed that the pure Ni₁₂P₅ phase with a high yield could be obtained in an ethanol solution, and the pure Ni₂P form was prepared in a mixed system with the volume ratio of water/ethanol of 10:10. Namely, the presence of water molecules induced the phase conversion of nickel phosphides. Furthermore, in order to investigate the correlation between properties and phases, as a case, the photocatalytic degradation abilities of two nickel phosphide phases for organic dyes were compared.     

Hydrothermal synthesis of chalcopyrite CuInS2, CuInSe2 and CuInTe2 nanocubes and their characterization

CuInS₂, CuInSe₂ and CuInTe₂ nanocubes of chalcopyrite structure have been successfully synthesized by hydrothermal process using deionized water as solvent at 180 °C for 20 h. The crystallinity, compositional, morphological and optical properties of the synthesized samples were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Raman and photoluminescence (PL) spectra analyses. The Raman spectra of the synthesized CuInS₂, CuInSe₂ and CuInTe₂ samples show the dominant A₁ modes at 293, 172 and 121 cm⁻¹ respectively. The possible chemical reaction and mechanism of nanocubes formation were discussed. The emission wavelength of as synthesized CuInS₂, CuInSe₂ and CuInTe₂ samples were blue shifted at 746 nm (1.66 eV), 863 nm (1.43 eV) and 859 nm (1.44 eV) respectively.     

Effect of electric-field-assisted thermal treatment on the strain and ferroelectric properties of (100)-oriented ferroelectric Pb(Zr0.52Ti0.48)O3 thin films

Ferroelectric Pb(Zr_0.52Ti_0.48)O₃ thin films were deposited on the Pt/Ti/SiO₂/Si substrate by a sol-gel method. As a direct electric field was applied on the films during thermal treatment, strain behavior and ferroelectric properties have been investigated. X-ray diffraction patterns show that great tensile strain exists nearby the interface of the 250 nm thin film while thermal treatment assisted with direct electric field can obviously relax it. The analysis of hysteresis loops indicates that the remnant polarization increases with the thermal treatment time. These results suggest that electric-field-assisted thermal treatment is an effective way to reduce films' tensile strain through the local plastic deformation in Pt layer and enhance the remnant polarization.     

Formation mechanism of H2Ti3O7 nanotubes

Formation mechanism of H2Ti3O7 nanotubes by single-step reaction of crystalline TiO2 and NaOH has been investigated via transmission electron microscopy examinations of series specimens with different reaction times and extensive ab initio calculations. It was found that the growth mechanism includes several steps. Crystalline TiO2 reacts with NaOH, forming a highly disordered phase, which recrystallized into some H2Ti3O7 thin plates. H-deficiency on the top surface leads to an asymmetrical environment for the surface Ti3O2-7 layer. The calculations of the surface tension, elastic strain energy, interlayer coupling energy, and Coulomb force indicated that the asymmetrical environment is the principal driving force of the cleavage of the single sheets of H2Ti3O7 from the plates and the formation of the multiwall spiral nanotubes.