•Multiphasic TiO2/rGO composite was synthesized via one-step hydrothermal method.•The reaction parameters appreciably affect the phase and morphology of TiO2.•Photo-absorption ability of the nanocomposite was studied.•Morphology evolution of the Ag NCs on each nanocomposite was elaborated in detail.•Ag surface grown on triphasic TiO2/rGO holds the potential for SERS applications.TiO2/reduced graphene oxide (rGO) nanocomposites were prepared via a facile one-step hydrothermal method using TiCl3 as the TiO2 precursor. Cetyltrimethyl ammonium bromide (CTAB) was introduced as a stabilizer for GO in solution. The effects of GO content, Ti3+ concentration and urea additive on phase constituent and morphology of the TiO2 crystallites in the nanocomposites were systematically investigated. UV–vis absorption ability of the as-made composites was further tested and discussed. Ag nanocrystals (NCs) were photocatalytically grown on the surfaces of biphasic (anatase + brookite) and triphasic (anatase + brookite + rutile) TiO2/rGO nanocomposites to evaluate their surface-enhanced Raman scattering (SERS) performances. Morphology evolution of the Ag NCs in response to different photocatalytic ability of the TiO2/rGO nanocomposite was also investigated in detail. The nanocomposite with triphasic TiO2 of proper phase constituents was confirmed to favor the growth of Ag particles of two distinctly different sizes and to produce SERS substrates of substantially better performance.Multiphasic TiO2/rGO nanocomposites were hydrothermally synthesized and were used as the photocatalysts to deposit Ag NCs for SERS applications.Download high-res image (61KB)Download full-size image

In2O3 particles with three distinctive morphologies of 1D rods, 2D disks and 3D cubes were converted from their respective precursors synthesized by a facile urea-based homogeneous precipitation method. Two kinds of precursor phases, including In(OH)3 and InOHSO4(H2O)2, were obtained. In the case of high urea concentration, mesocrystalline rod-like In(OH)3 particles were produced by oriented primary particle aggregation induced by the coordination of urea on {012} of the primary particles. In contrast, cube-like In(OH)3 was obtained by Ostwald ripening in low-concentration urea solution. The addition of K2SO4 facilitates the formation of an In–sulfate complex, and InOHSO4(H2O)2 precursor disks about 2 μm in diameter were formed. It is suggested that the adsorption of urea on the active growth sites of the precursor leads to the formation of round disks instead of hexagonal plates. Upon blue light excitation, the three types of In2O3 particles obtained from their respective precursors exhibited a morphology-dependent photoluminescence behavior (disks > cubes > rods), but did not differ greatly in the positions of the PLE and PL bands of the luminescence. The emission (in the range of 500–540 nm) from In2O3 is associated with oxygen vacancies and is highly dependent on the annealing atmosphere.

Nanosized anatase TiO2 particles anchored on nanocarbon substrates have great potential for practical applications in high-performance lithium ion batteries and efficient photocatalysts. The synthesis of this material usually utilizes calcination to crystallize amorphous titania, which normally causes the formation of aggregates and some side effects. In this work, we demonstrated that sub-20 nm anatase particles uniformly anchored on graphene oxide and reduced graphene oxide nanosheets in aqueous solution at a temperature of 90 °C and atmospheric pressure, without further calcination. The photocatalytic oxidation activity and electrochemical properties of graphene oxide/anatase TiO2 (GO/A) and reduced graphene oxide/anatase TiO2 (RGO/A) were comparatively investigated. We found that GO/A showed higher photocatalytic oxidation activity than RGO/A under UV light irradiation. Graphene oxide accepted electrons and suffered reduction, which finally decreased GO/A’s photocatalytic oxidation activity to an extent similar to RGO/A. We also found that, as anode material for Li-ion battery, the specific capacity of RGO/A was nearly three times that of GO/A at the same current rate. This study will inspire better design of metal oxide/nanocarbon nanocomposites for high performance lithium ion battery and photocatalysis applications.

In this study, a NiCrAlY + AlNiY composite coating was prepared by arc ion plating technique and subsequent annealing treatment. Cyclic hot corrosion tests of the composite coating and a reference NiCrAlY coating coated with mixed salts of Na2SO4 + K2SO4 and Na2SO4 + NaCl were carried out at 700 °C. The results indicated that the composite coating performed better against the corrosion due to the gradient element distribution in Al-enriched outer layer and Cr-enriched inner layer. The corrosion mechanisms for the two coatings were also discussed.

•A facile synthetic technique of growing SERS active Ag substrates onto Cu micro-grid has been systematically studied.•Changing processing parameters has yielded Ag crystals of various morphologies and SERS performances.•PVP additive was observed to suppress Ag dendrite crystallization for nearly monodispersed Ag polyhedrons/nanoplates.•PVP modified SERS substrate exhibits excellent EF and RSD values in the repeated detection of 10 μM R6G analyte.Galvanic growth of Ag nano/micro-structures on Cu micro-grid was systematically studied for surface-enhanced Raman scattering (SERS) applications. Detailed characterizations via FE-SEM and HR-TEM showed that processing parameters, (reaction time, Ag+ concentration, and PVP addition) all substantially affect thermodynamics/kinetics of the replacement reaction to yield substrates of significantly different microstructures/homogeneities and thus varied SERS performances (sensitivity, enhancement factor, and reproducibility) of the Ag substrates in the detection of R6G analyte. PVP as an additive was shown to notably alter nucleation/growth behaviors of the Ag crystals and promote the deposition of dense and uniform Ag films of nearly monodisperse polyhedrons/nanoplates through suppressing dendrites crystallization. Under optimized synthesis (50 mM of Ag+, 30 s of reaction, and 700 wt.% of PVP), Ag substrates exhibiting a high Raman signal enhancement factor of ~ 1.1 × 106 and a low relative standard deviation of ~ 0.13 in the repeated detection of 10 μM R6G were obtained. The facile deposition and excellent performance reported in this work may allow the Ag microstructures to find wider SERS applications. Moreover, growth mechanisms of the different Ag nano/micro-structures were discussed based on extensive FE-SEM and HR-TEM analysis.

A novel and facile approach based on an aqueous reaction of ammonium fluotitanate with an alkaline substance was developed in this work to prepare foamed single-crystals (about 50 nm) of anatase TiO2 exhibiting excellent photocatalytic performances. It is shown that multicrystalline and rice-shaped anatase nanoparticles (around 100 nm in length, 30 nm in width), which have a foamed microstructure and F-dopants, were initially precipitated as a precursor, which can then be readily transformed by calcination at 500 °C into foamed anatase single crystals (about 50 nm in size) with significant amounts of entrapped nanopores (4–10 nm). A fluorine-assisted dehydration/assembly mechanism was believed to be responsible for the formation of such a peculiarly nano-structured precursor, and recrystallization may account for its transformation into the foamed single crystals. Both the precursor nanoparticles and the resultant single crystals were found to have narrowed bandgaps. Though the precursor has already shown good photocatalytic performance in the decomposition of methyl orange, the calcination derived single crystals exhibited even higher activity due to their better crystallinity, smaller size, single crystalline nature and hollow structure. The alkaline substance used for precipitation was found to influence the microstructure and consequently photoactivity of the anatase nanoparticles. It is also shown that, among all the obtained anatase nanomaterials, the foamed single-crystalline nanocrystals, precipitated with sodium hydroxide and having the highest pore density, exhibited photocatalytic activities far superior to Degussa P25, especially under simulated sunlight irradiation. The results obtained herein may shed light on the photoactivity improvement of anatase TiO2 and investigations of other foamed functional nanostructures.

A homogeneous co-precipitation method was developed to synthesize well dispersed yttrium aluminum garnet (YAG, Y3Al5O12) powders. This method ensures the co-precipitation of Y3+ and Al3+, solving the problem of differential precipitation of Y3+ and Al3+ in the conventional homogeneous precipitation method. Furthermore, the pH value can be controlled to be a plateau (around 5.2) during the titration process due to the continuous decomposition of the heated urea which can compensate for the pH reduction when adding the mother salts, which is beneficial to the narrow size distribution of the precipitates. Pure YAG nanopowders can be obtained at 900 °C without the formation of impurity phases such as YAM (Y4Al2O9) or YAP (YAlO3). The resultant YAG powders are well dispersed and have excellent sinterability that can be sintered into transparent ceramics at 1700 °C by vacuum sintering.

•Gd2Zr2O7/ZrO2(3Y) composites were prepared using vacuum sintering.•The phase composition and microstructure are studied.•Gd2Zr2O7/ZrO2(3Y) materials show superior mechanical properties.•The solid solution strengthening and stress-induced phase transformation toughening mechanism are proposed.•Two kinds of mechanisms explain the improvement of mechanical properties.Gd2Zr2O7/ZrO2(3Y) composite ceramics were prepared by vacuum sintering using Gd2Zr2O7 and ZrO2(3Y) nanoparticles. The ceramics were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), the three-point-bending technique and single-edge-notched-beam tests. The effect of various proportions of ZrO2(3Y) on the phase composition, microstructure, bending strength and fracture toughness of the final Gd2Zr2O7/ZrO2(3Y) composite ceramics was also analyzed. The change from m-ZrO2 to t-ZrO2 phase contents, before and after fracture, was measured using XRD quantitative phase analysis. The results confirm that, with the increasing content of ZrO2(3Y), a phase transition from solid solution to saturated precipitation occurs and the bending strength and fracture toughness of the samples increase gradually. When the content of ZrO2(3Y) reached 95 vol.%, the Gd2Zr2O7/ZrO2(3Y) composite ceramics had a bending strength of 547 MPa and a fracture toughness of 5.5 MPa m1/2, indicating that stress-induced phase transformation toughening was an efficient way to increase the mechanical properties of the Gd2Zr2O7 ceramics.

Gadolinium zirconate (Gd2Zr2O7) nanocrystals were prepared via two different combustion methods: citric acid combustion (CAC) and stearic acid combustion (SAC). The effects of the different preparation methods on the phase composition, microtopography, and sintering densification of the resulting Gd2Zr2O7 nanopowders were investigated by thermal-gravimetric and differential thermal analysis (TG-DTA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. The results indicated that both methods could produce Gd2Zr2O7 nanopowders with an excellent defective fluorite structure. The reaction time was reduced by the SAC method, compared with the CAC method. The nanopowders synthesized by the two methods were different in grain size distribution. The resulting nanoparticle diameter was about 50 nm for CAC and 10 nm for SAC. After vacuum sintering, the sintered bodies also had a different relative density of about 93% and 98%, respectively. Thus the preparation of Gd2Zr2O7 nanopowders by SAC was the first choice to achieve the desired sintering densification.SEM images of the Gd2Zr2O7 ceramic prepared by the SAC method, sintered at 1550 °C for 2 h in vacuum

Ag nanocrystals (NCs) were photocatalytically grown on the surfaces of brookite and rutile nanocrystals, respectively, and their surface-enhanced Raman scattering (SERS) performance was evaluated. The resultant Ag NCs exhibit different morphologies owing to the different photocatalytic capabilities of the two types of TiO2 under otherwise identical synthetic conditions. The effects of AgNO3 concentration, UV irradiation time, and UV light power on the morphology evolution and growth kinetics of the Ag NCs were systematically investigated. Moreover, PVP was found to serve as both a reductant and a capping agent in the photocatalytic reaction systems, and its presence allows morphological control of the Ag NCs. A proper amount of PVP was confirmed to favor Ag nanoplates of larger sizes and to produce SERS substrates of substantially better performance.Keywords: photocatalysis; poly(vinylpyrrolidone) (PVP); silver nanocrystals; surface-enhanced Raman scattering; titania;

•Ag–SnO2 composite powders were synthesized by the sol–gel auto-combustion method.•Citric acid facilitates sol formation and reduces particle agglomeration.•SnO2 nanoparticles disperse homogenously in the Ag matrix.•Citric acid improves microstructure and properties of the Ag-SnO2 composite.A sol–gel auto-combustion method was developed to synthesize the Ag–SnO2 composite powders, which were then used as the starting material to prepare Ag–SnO2 electrical contact materials by hot pressing. It was found that citric acid strongly influences the thermal behaviors, phase evolution, morphology and composition distribution of the Ag–SnO2 composite powders. During the synthesis of Ag–SnO2 composite powders, citric acid can facilitate the formation of sol solution, lower the energy required for the decomposition of dry gel, and reduce the agglomeration of particles. Besides, a comparatively high molar ratio of citric acid to metal ions prevents effectively the occurrence of composition segregation in the Ag–SnO2 electrical contact materials. Therefore, the prepared Ag–SnO2 electrical contact materials have good performances in density, hardness and electric conductivity.

•Stearate melting method can synthesize well dispersed fine YAG nanopowders.•The method ensures precise Y:Al ratio and cations mixing at atomic level.•Pure and dispersed YAG powders can be obtained at low temperature of 750 °C.•The YAG powders can be sintered into transparent ceramics at 1700 °C.Ultrafine YAG powders were synthesized by a novel stearate melting method, in which yttrium stearate and aluminum tristearate, having similar physical and chemical properties, were co-melted and then calcined to produce fine YAG nanopowders. This method has the advantages of precise control of Y:Al ratio, homogeneous mixing of cations at atomic level, fine particle size, and good particle dispersion. The formation mechanism of the precursor and the YAG nanopowder was studied by means of XRD, FT-IR, TG–DTA, BET and FE-SEM. Pure YAG nanopowder can be obtained by calcining the co-melted precursor at a relatively low temperature (750 °C), much lower than those of the traditional solid-state reaction method and various wet chemical synthesis methods. The resultant YAG powders are well dispersed and have excellent sinterability. For the YAG powder calcined at 1000 °C, the green compact has the maximum shrinkage rate at about 1450–1550 °C and a total shrinkage of ∼16.70% during constant heating rate sintering. The compact can be sintered to 99.4% of the theoretical density at 1600 °C. The prepared YAG powder can be sintered into transparent ceramics at 1700 °C for 5 h by vacuum sintering.Graphical abstract

•Highly textured Ca9Co12O28 ceramics have been prepared by hot pressing method.•TE properties of the samples show bidimensional anisotropic character.•Lattice thermal conductivity is the main thermal transference in the samples.•At 600 °C the samples have a ZT = 0.18 in the direction perpendicular to the hot pressing direction.Ca9Co12O28 thermoelectric ceramics were fabricated by both sintering and hot-pressing methods. The hot pressed ceramic has a layered structure, and exhibits {0 l 0} preferred orientation and a high orientation degree of 0.84 calculated from the XRD patterns. Thermoelectric properties of the sintered and hot pressed samples were measured from 100 °C to 600 °C. For the hot pressed sample, the electrical conductivity in the direction perpendicular to the pressing direction is much higher than that in the direction parallel to the pressing direction. Thermal conductivity of the hot pressed sample is also anisotropic, and the lattice thermal conductivity is the main thermal transference. In the direction perpendicular to the pressing direction, the hot pressed sample shows a high ZT value of 0.18 at 600 °C.

Homogeneous precipitation method for synthesizing (Gd0.99,Pr0.01)2O2S sub-microphosphor was developed, using the commercially available Gd2O3, Pr6O11, H2SO4 and (NH2)2CO (urea) as the starting materials. It was found that the as-synthesized precursor is mainly composed of (Gd0.99,Pr0.01)2(OH)2(CO3)(SO4)·nH2O. Pure quasi-spherical shaped (Gd0.99,Pr0.01)2O2S particles can be synthesized by calcining the precursor at a temperature higher than 700 °C for 1 h in flowing hydrogen. The (Gd0.99,Pr0.01)2O2S particles have a narrow size distribution with a mean grain size of about 300–400 nm. Photoluminescence spectra of (Gd0.99,Pr0.01)2O2S under 303 nm UV excitation show a green emission at 515 nm as the most prominent peak, which corresponds to the 3P0 → 3H4 transition of Pr3+ ions. Decay study reveals that the 3P0 → 3H4 transition of Pr3+ ions in Gd2O2S host lattice has a single exponential decay behavior.Research highlights► A homogeneous precipitation for synthesizing (Gd0.99,Pr0.01)2O2S sub-microphosphor. ► Pure quasi-spherical (Gd0.99,Pr0.01)2O2S particles with narrow size distribution. ► No toxic carbon disulfide or hydrogen sulfide in the synthesis process. ► High luminescence intensity and suitable decay time.

(Gd1−x,Eux)2O2SO4 sub-microphosphors were synthesized by homogeneous precipitation method from commercially available Gd2O3, Eu2O3, H2SO4 and (NH2)2CO (urea) starting materials. Fourier transform infrared spectra show that the precursors with different molar ratios of (NH2)2CO to Gd2(SO4)3 (the m value) are mostly composed of gadolinium hydroxyl, carbonate and sulfate groups with some crystal water. X-ray diffraction indicated that the precursor (m = 5) can be transformed into pure Gd2O2SO4 phase after heat treated at 900 °C for 2 h in air. Field emission scanning electron microscope micrographs illustrate that the Gd2O2SO4 phosphor particles (m = 5) are quasi-spherical in shape and well dispersed, with a mean particle size of about 300–500 nm. Photoluminescence spectroscopy reveals that the strongest emission peak for (Gd1−x,Eux)2O2SO4 sub-microphosphors is located at 618 nm under 270 nm light excitation, which corresponds to the 5D0 → 7F2 transition of Eu3+ ions. The quenching concentration of Eu3+ ions is 5 mol% and the concentration quenching mechanism is due to the electric dipole–dipole interaction. Decay study reveals that the 5D0 → 7F2 transition of Eu3+ ions fits with a mono exponential function.

A novel co-precipitation method for synthesizing (Gd1−x, Prx)2O2S nano-phosphors was developed, using the commercially available Gd2O3, Pr6O11, H2SO4 and NaOH as the starting materials. This method has the advantage of controllable phosphor particle size and shape, simplicity in processing, low cost, and not involving toxic carbon disulfide or hydrogen sulfide. It was found that the as-synthesized precursor is mainly composed of (Gd1−x, Prx)2(OH)4SO4·nH2O. By calcining the (Gd1−x, Prx)2(OH)4SO4·nH2O precursor at a temperature higher than 700 °C for 1 h in flowing hydrogen, pure quasi-spherical shaped (Gd1−x, Prx)2O2S particles can be synthesized. The (Gd1−x, Prx)2O2S particles have a narrow size distribution with a mean grain size of about 20–30 nm. UV–vis spectra indicates that the absorption edge of the samples has a blue shift with increasing calcination temperature, while PL spectra of (Gd1−x, Prx)2O2S under 301 nm excitation show a green emission at 511 nm as the most prominent peak, which corresponds to the 3P0 → 3H4 transition of Pr3+ ions. The optimal x value is 0.01 for the highest luminescent emission intensity.

High photocatalytic efficiency, easy recovery, and no biological toxicity are three key properties related to the practical application of anatase photocatalyst in water cleaning, but seem to be incompatible. Nanoparticles-constructed hierarchical anatase microspheres with high crystallinity and good dispersion prepared in this study via one-step solution processing at 90 °C under atmospheric pressure by using ammonium fluotitanate as the titanium source and urea as the precipitant can reconcile these three requirements. The hierarchical microspheres were found to grow via an aggregative mechanism, and contact recrystallization occurred at high additions of the FeCl3 electrolyte into the reaction system. Simultaneous incorporation of fluorine and iron into the TiO2 matrix was confirmed by combined analysis of X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and UV−vis absorption spectroscopy. Surface structure and morphology changes of the microspheres induced by high-temperature annealing were clearly observed by field-emission scanning electron microscopy, especially for the phase-transformed particles. The original nanoparticles-constructed rough surfaces partially became smooth, resulting in a sharp drop in photocatalytic efficiency. Interestingly, iron loading has detrimental effects on the visible-light photocatalytic activity of both the as-prepared and the postannealed anatase microspheres but greatly enhances the photocatalytic activity of the as-prepared anatase microspheres under UV irradiation. No matter under UV or visible-light irradiation, the fluorine-loaded anatase microspheres and especially the postannealed ones show excellent photocatalytic performance. The underlying mechanism of fluorine and iron loading on the photocatalytic efficacy of the anatase microspheres was discussed in detail. Beyond photocatalytic applications, this kind of material is of great importance to the assembling of photoactive photonic crystal that can control light motion.

A novel and facile approach based on an aqueous reaction of ammonium fluotitanate with an alkaline substance was developed in this work to prepare foamed single-crystals (about 50 nm) of anatase TiO2 exhibiting excellent photocatalytic performances. It is shown that multicrystalline and rice-shaped anatase nanoparticles (around 100 nm in length, 30 nm in width), which have a foamed microstructure and F-dopants, were initially precipitated as a precursor, which can then be readily transformed by calcination at 500 °C into foamed anatase single crystals (about 50 nm in size) with significant amounts of entrapped nanopores (4–10 nm). A fluorine-assisted dehydration/assembly mechanism was believed to be responsible for the formation of such a peculiarly nano-structured precursor, and recrystallization may account for its transformation into the foamed single crystals. Both the precursor nanoparticles and the resultant single crystals were found to have narrowed bandgaps. Though the precursor has already shown good photocatalytic performance in the decomposition of methyl orange, the calcination derived single crystals exhibited even higher activity due to their better crystallinity, smaller size, single crystalline nature and hollow structure. The alkaline substance used for precipitation was found to influence the microstructure and consequently photoactivity of the anatase nanoparticles. It is also shown that, among all the obtained anatase nanomaterials, the foamed single-crystalline nanocrystals, precipitated with sodium hydroxide and having the highest pore density, exhibited photocatalytic activities far superior to Degussa P25, especially under simulated sunlight irradiation. The results obtained herein may shed light on the photoactivity improvement of anatase TiO2 and investigations of other foamed functional nanostructures.