•PdAg/SiO2-NH2 catalyst for catalytic nitrate reduction with HCOOH as reducing agent.•The closed reaction system enhanced catalytic nitrate reduction activity.•Surface -NH2 modification enhanced nitrate reduction efficiency.•PdAg/SiO2-NH2 demonstrated better nitrate reduction efficiency than PdCu/SiO2-NH2.•Different optimal Ag contents for catalytic HCOOH decomposition and nitrate reduction.Nitrate pollution in water is becoming a severe problem all over the world, and the catalytic reduction of nitrate by reducing agents had been considered as one of the most promising methods because it could convert nitrate to harmless nitrogen gas with a high efficiency. In this work, the PdAg/SiO2-NH2 catalyst was developed by firstly loading Pd onto NH2 surface-modified SiO2 catalyst support, followed by a controlled surface reaction to load Ag and create PdAg alloy nanoparticles on SiO2-NH2 catalyst support. For the first time, the PdAg/SiO2-NH2 catalyst was found to be able to effectively reduce nitrate with HCOOH as the reducing agent precursor. Its enhanced nitrate reduction performance could be attributed to the combination effects from the surface NH2 modification and the better electron transfer from Ag to Pd than from other metals due to their larger difference of work function, both of which were beneficial for the catalytic HCOOH decomposition to provide the in situ sources of the reducing agent of H2 and buffer of CO2 for the catalytic nitrate reduction process. To optimize the nitrate reduction performance of the PdAg/SiO2-NH2 catalyst, Ag content was carefully modulated and the Pd:Ag molar ratio of 1:0.5 was found to have the most efficient nitrate reduction capability.Download high-res image (145KB)Download full-size image

Superparamagnetic nanocatalysts could minimize both the external and internal mass transport limitations and neutralize OH– produced in the reaction more effectively to enhance the catalytic nitrite reduction efficiency with the depressed product selectivity to undesirable ammonium, while possess an easy magnetic separation capability. However, commonly used qusi-monodispersed superparamagnetic Fe3O4 nanosphere is not suitable as catalyst support for nitrite reduction because it could reduce the catalytic reaction efficiency and the product selectivity to N2, and the iron leakage could bring secondary contamination to the treated water. In this study, protective shells of SiO2, polymethylacrylic acid, and carbon were introduced to synthesize Fe3O4@SiO2/Pd, Fe3O4@PMAA/Pd, and Fe3O4@C/Pd catalysts for catalytic nitrite reduction. It was found that SiO2 shell could provide the complete protection to Fe3O4 nanosphere core among these shells. Because of its good dispersion, dense structure, and complete protection to Fe3O4, the Fe3O4@SiO2/Pd catalyst demonstrated the highest catalytic nitrite reduction activity without the detection of NH4+ produced. Due to this unique structure, the activity of Fe3O4@SiO2/Pd catalysts for nitrite reduction was found to be independent of the Pd nanoparticle size or shape, and their product selectivity was independent of the Pd nanoparticle size, shape, and content. Furthermore, their superparamagnetic nature and high saturation magnetization allowed their easy magnetic separation from treated water, and they also demonstrated a good stability during the subsequent recycling experiment.Keywords: catalytic nitrite reduction; Fe3O4@SiO2/Pd; product selectivity; protective layer; superparamagnetic core

Plasmonic photocatalysis could provide a promising solution to the two fundamental problems of current TiO2-based visible-light photocatalysis on low photocatalytic efficiency and low usage of solar illumination. But till now, most plasmonic photocatalysts have relied on noble metal nanostructures of Au or Ag due to their easy synthesis and efficient absorption of visible light. In this study, a TiN/TiO2 nanocomposite photocatalyst was synthesized by the in situ growth of TiO2 nanoparticles on TiN nanoparticles with a fluorine-free, vapor-phase hydrothermal process. In this composite photocatalyst system, the desirable visible light absorption could be attributed to the LSPR effect of a nanostructured TiN phase. Thus, a plasmonic photocatalyst without noble-metal components was developed, and its good visible light photocatalytic activity was demonstrated by both the photodegradation of organic pollutants of RhB and 4-NP and the disinfection of microorganisms of E. coli. From the energy alignment analysis, hot electrons were expected to be completely injected from TiN to TiO2 once they were excited above the Fermi energy level of TiN because no barrier existed, resulting in better electron injection efficiency than previous reported noble-metal-based plasmonic photocatalysts.

The creation of photocatalysts with controlled facets has become an important approach to enhance their activity. However, how the formation of heterojunctions on exposed facets could affect their photocatalytic performance ranking had not yet been investigated. In this study, Cu2O@TiO2 core–shell structures were created, and Cu2O/TiO2 p–n heterojunctions were formed on various exposed facets of Cu2O cubes, Cu2O cuboctahedra, and Cu2O octahedra, respectively. These Cu2O@TiO2 polyhedra demonstrated an enhanced photocatalytic degradation effect on Methylene Blue (MB) and 4-nitrophenol (4-NP) under visible light illumination, because of the enhanced charge carrier separation by the formation of Cu2O@TiO2 p–n heterojunctions. It was further found that their photocatalytic performance was also facet-dependent as pure Cu2O polyhedra, while the photocatalytic performance ranking of these Cu2O@TiO2 polyhedra was different with that of their corresponding Cu2O polyhedron cores. By the combination of optical property measurement and XPS analysis, the energy band alignments of these Cu2O@TiO2 polyhedra were determined, which demonstrated that Cu2O@TiO2 octahedra had the highest band offset for the separation of charge carriers. Thus, the charge-carrier-separation-driven force in Cu2O@TiO2 polyhedra was different from their corresponding Cu2O polyhedron cores, which resulted in their different surface photovoltage spectrum (SPS) responses and different photocatalytic performance rankings.Keywords: Cu2O@TiO2 polyhedra; energy band alignment; exposed facets; p-n heterojunctions; visible light photocatalytic activity

One-dimensional nanomaterials may organize into macrostructures to have hierarchically porous structures, which could not only be easily adopted into various water treatment apparatus to solve the separation issue of nanomaterials from water but also take full advantage of their nanosize effect for enhanced water treatment performance. In this work, a novel template-based process was developed to create Mn3O4/CeO2 hybrid nanotubes, in which a redox reaction happened between the OMS-2 nanowire template and Ce(NO3)3 to create hybrid nanotubes without the template removal process. Both the Ce/Mn ratio and the precipitation agent were found to be critical in the formation of Mn3O4/CeO2 hybrid nanotubes. Because of their relatively large specific surface area, porous structure, high pore volume, and proper surface properties, these Mn3O4/CeO2 hybrid nanotubes demonstrated good As(III) removal performances in water. These Mn3O4/CeO2 hybrid nanotubes could form paper-like, free-standing membranes spontaneously by a self-assembly process without high temperature treatment, which kept the preferable properties of Mn3O4/CeO2 hybrid nanotubes while avoiding the potential nanomaterial dispersion problem. Thus, they could be readily utilized in commonly used flow-through reactors for water treatment purposes. This approach could be further applied to other material systems to create various hybrid nanotubes for a broad range of technical applications.Keywords: As(III) removal; free-standing membrane; Mn3O4/CeO2 hybrid nanotubes; redox precipitation reaction; toxicity reduction

The development of highly efficient As(III) adsorbents is critical to largely simplify the arsenic treatment process and lower its cost. For the first time, SnO2 nanospheres were demonstrated to possess a highly efficient As(III) adsorption capability from water in a near neutral pH environment as predicted by the material criterion we recently developed for the selection of highly efficient arsenic adsorbents. These SnO2 nanospheres were synthesized by a simple and cost-effective hydrolysis process with the assistance of ethyl acetate under ambient conditions, which had a good dispersity, a narrow size distribution, a relatively large specific surface area, and a porous structure. A fast As(III) adsorption was observed in the kinetics study on these SnO2 nanospheres, and their Langmuir adsorption capacity was determined to be ∼112.7 mg g−1 at pH ∼7. The As(III) adsorption mechanism on SnO2 nanospheres was examined by both macroscopic and microscopic techniques, which demonstrated that it followed the inner-sphere complex model. These SnO2 nanospheres demonstrated effective As(III) adsorption even with exceptionally high concentrations of co-existing ions, and a good regeneration capability by washing with NaOH solution.

Hydrous manganite (MnOOH) nanorods were synthesized by a simple precipitation process in ethanol at room temperature, which eliminated high temperature calcination or a hydrothermal process in the creation of most manganese oxide-based adsorbents and resulted in low energy consumption and subsequently low production cost. These MnOOH nanorods had a high specific surface area at ∼165.9 m2 g−1 and their total pore volume was ∼0.561 cm3 g−1, which was beneficial to their arsenic removal performance. These MnOOH nanorods demonstrated a superior As(III) removal performance from an aqueous environment. At near neutral conditions (pH ∼ 7), their arsenic adsorption capacity was over 431.2 mg g−1, which was among the highest reported values in the literature. The superior As(III) removal performance of these MnOOH nanorods relied on the adsorption and subsequent oxidation of As(III) to less mobilized/toxic As(V), and its fixation on their surface to form inner-sphere arsenic surface complexes.

Palladium-modified nitrogen-doped titanium oxide (TiON/PdO) thin film was synthesized by the ion-beam-assisted deposition technique, which enabled a heavy nitrogen doping and the subsequent light absorption extension to ∼700 nm for a better usage of the solar spectrum. Based on TiON/PdO thin film and a phase contrast microscope, a micro-reaction chamber was developed, which allowed the simultaneous optical excitation of the photocatalytic thin film and the phase contrast image observation of cells in it. The real time, in situ observation of the photocatalytic destruction of Saccharomyces cerevisiae (S. cerevisiae), an essential eukaryotic unicellular model of living cells, was conducted with this new observation technique, which demonstrated clearly that the photocatalytic destruction effect was much stronger than the photodamage effect caused by the visible light irradiation alone in the disinfection process.

•Palladium-modified nitrogen-doped titanium oxidephotocatalyst (TiON/PdO)•Effective visible-light photocatalytic disinfection of yeast cells by TiON/PdO•Real time, in situ observation technique was developed for photocatalytic disinfection.•The fluorescence/phase contrast microscope with a photocatalytic micro-reactor•Yeast cell disinfection happened before the cell structure collapsed.An in situ microscopy technique was developed to observe in real time the photocatalytic inactivation process of Saccharomyces cerevisiae (S. cerevisiae) cells by palladium-modified nitrogen-doped titanium oxide (TiON/PdO) under visible light illumination. The technique was based on building a photocatalytic micro-reactor on the sample stage of a fluorescence/phase contrast microscopy capable of simultaneously providing the optical excitation to activate the photocatalyst in the micro-reactor and the illumination to acquire phase contrast images of the cells undergoing the photocatalytic inactivation process. Using TiON/PdO as an example, the technique revealed for the first time the vacuolar activities inside S. cerevisiae cells subjected to a visible light photocatalytic inactivation. The vacuoles responded to the photocatalytic attack by the first expansion of the vacuolar volume and then contraction, before the vacuole disappeared and the cell structure collapsed. Consistent with the aggregate behavior observed from the cell culture experiments, the transition in the vacuolar volume provided clear evidence that photocatalytic disinfection of S. cerevisiae cells started with an initiation period in which cells struggled to offset the photocatalytic damage and moved rapidly after the photocatalytic damage overwhelmed the defense mechanisms of the cells against oxidative attack.

A novel Cu2O/TiO2 composite photocatalyst structure of Cu2O nanospheres decorated with TiO2 nanoislands were synthesized by a facile hydrolyzation reaction followed by a solvent-thermal process. In this Cu2O/TiO2 composite photocatalyst, Cu2O served as the main visible light absorber, while TiO2 nanoislands formed heterojunctions of good contact with Cu2O, beneficial to the photoexcited electron transfer between them. Their band structure match and inner electrostatic field from the p–n heterojunction both favored the transfer of photoexcited electrons from Cu2O to TiO2, which effectively separated the electron–hole pairs. Photogenerated holes on Cu2O could react with water or organic pollutants/microorganisms in water to avoid accumulation on Cu2O because of the partial TiO2 nanoislands coverage, which enhanced their stability during the photocatalysis process. Their superior photocatalytic performance under visible light illumination was demonstrated in both the degradation of methyl orange and the disinfection of Escherichia coli bacteria. An interesting post-illumination catalytic memory was also observed for this composite photocatalyst as demonstrated in the disinfection of Escherichia coli bacteria in the dark after the visible light was shut off, which could be attributed to the transfer of photoexcited electrons from Cu2O to TiO2 and their trapping on TiO2 under visible light illumination, and their release in the dark after the visible light was shut off.Keywords: catalytic memory; Cu2O/TiO2 composite photocatalyst; enhanced performance/stability; p−n heterojunction; visible-light-activated photocatalysis;

Well-defined WO3·H2O hollow spheres composed of nanoflakes were successfully synthesized by a template-free solvothermal process with i-PrOH–H2O mixture solvent. In this process, the tungsten precursor was firstly hydrolyzed to form solid spheres composed of both WO3·2H2O and WO3·H2O phases. Then, these solid spheres underwent an Ostwald ripening process and the WO3·2H2O phase was dehydrated to WO3·H2O at the same time to form hollow spheres with a pure WO3·H2O phase. With appropriate calcination temperature, hollow spheres with WO3 (major) and WO3·H2O (minor) mixture phases were created. These hollow spheres with WO3 and WO3·H2O mixture phases demonstrated a largely enhanced photocatalytic activity for RhB degradation under visible light irradiation compared with either pure WO3·H2O hollow spheres or pure WO3 hollow spheres which could be attributed to the matched band structure between the WO3 and WO3·H2O phases. Thus, an effective charge carrier separation can occur between the WO3 and WO3·H2O phases of these hollow spheres under visible light irradiation, contributing to the observed largely enhanced photocatalytic performance.

A novel superparamagnetic Ag@silver-based salt photocatalyst, MFNs@SiO2@Ag4SiW12O40/Ag, was created, which demonstrated highly efficient photocatalytic performance under visible light illumination in both the degradation of methylene blue (MB) and the disinfection of Escherichia coli (E. coli) bacteria. In this composite photocatalyst, well-dispersed, superparamagnetic magnesium ferrite nanoparticles (MFNs) were used as the core because of their easy magnetic separation capability. A passive SiO2 mid-layer was used to separate MFNs and Ag4SiW12O40 and form a strong bond with silver ions for their loading after –SH surface modification. The Ag4SiW12O40 layer was subsequently formed by the reaction with silicotungstic acid to avoid the commonly adopted calcination procedure after deposition/precipitation, and silver nanoparticles were formed on the surface of Ag4SiW12O40 layer after UV irradiation to further enhance their photocatalytic performance and stability under visible light illumination. The surface modification on the passive SiO2 mid-layer and the bridging procedure for material loading developed in our approach could be readily applied to other material systems for the creation of novel composite materials with various functions.

Arsenic is a highly toxic element and its contamination in water bodies is a worldwide problem. Arsenic adsorption with metal oxides/hydroxides-based adsorbents is an effective approach to remove arsenic species from water for the health of both human beings and the environment. However, no material criterion had been proposed for the selection of potential candidates. Equally puzzling is the fact that no clear explanation was available on the poor arsenic adsorption performance of some commonly used adsorbents, such as active carbon or silica. Furthermore, in-depth examination was also not available for the dramatically different competing adsorption effects of various anions on the arsenic adsorption. Through the arsenic adsorption mechanism study on these highly efficient arsenic adsorbents, we found that ionic potential could be used as a general material criterion for the selection of highly efficient arsenic adsorbents and such a criterion could help us to understand the above questions on arsenic adsorbents. This material criterion could be further applied to the selection of highly efficient adsorbents based on ligand exchange between their surface hydroxyl groups and adsorbates in general, which may be used for the prediction of novel adsorbents for the removal of various contaminations in water.

A novel quasi-monodisperse, superparamagnetic Pd/Fe3O4 catalyst was synthesized for effective catalytic bromate reduction. The catalyst was prepared by dispersing nanoparticles of Pd (weight percent up to 1%) on the surface of superparamagnetic Fe3O4 microspheres with 300–500 nm in diameter and 10–20 nm in grain size. Complete reduction of bromate by this Pd/Fe3O4 catalyst was demonstrated within a short period (<2 h) over a range of pH values, in the presence of a variety of co-existing ions, and after multiple cycles. In addition, the superparamagnetic nature of the catalyst enhanced its good dispersion in water during water treatment when there was no external magnetic field, and its high saturation magnetization allowed an easy magnetic separation from water when an external magnetic field was applied after the water treatment. Thus, it could be easily recycled and reused, further enhancing its application potential.

Ultrafine superparamagnetic magnesium ferrite nanocrystallites were successfully synthesized by doping Mg2+ into α-Fe2O3 in a solvent thermal process. The dopant altered the microstructure of the α-Fe2O3 which resulted in an enhanced arsenic adsorption performance and an easy magnetic separation capability. At a concentration of 10%, Mg-doping largely increased the surface area to ∼438 m2 g−1, enhanced the dispersion and contact with arsenic species in water due to its superparamagnetic behavior, and largely improved the arsenic adsorption performance in both lab-prepared and natural water samples at near neutral pH when compared with ultrafine α-Fe2O3 nanoparticles. At an Mg concentration of 10%, the amounts of As(III) and As(V) adsorbed reached 9.3 mg g−1 and 10 mg g−1, respectively, at a low equilibrium arsenic concentration of less than 10 μg L−1. The nanocrystallites could be easily separated from water when an external magnetic field was present after water treatment due to their high saturation magnetization, which solved the potential nanomaterial dispersion problem and facilitated arsenic desorption and reuse of the material.

SiO2 coated Fe3O4 submicrometer spherical particles (a conducting core/insulating shell configuration) are fabricated using a hydrothermal method and are loaded at 10 and 20 vol % into a bisphenol E cyanate ester matrix for synthesis of multifunctional composites. The dielectric constant of the resulting composites is found to be enhanced over a wide frequency and temperature range while the low dielectric loss tangent of the neat cyanate ester polymer is largely preserved up to 160 °C due to the insulating SiO2 coating on individual conductive Fe3O4 submicrometer spheres. These composites also demonstrate high dielectric breakdown strengths at room temperature. Dynamic mechanical analysis indicates that the storage modulus of the composite with a 20 vol % filler loading is twice as high as that of neat resin, but the glass transition temperature considerably decreases with increasing filler content. Magnetic measurements reveal a large saturation magnetization and negligibly low coercivity and remanent magnetization in these composites.Keywords: dielectric properties; mechanical stiffening; polymer−matrix composites; SiO2 coated Fe3O4 submicrometer spheres; superparamagnetic behavior;

Fusarium graminearum is the pathogen for Fusarium head blight (FHB) on wheat, which could significantly reduce grain quality/yield and produce a variety of mycotoxins posing a potential safety concern to human foods. As an environmentally friendly alternative to the commonly used chemical fugicides, a highly effective photocatalytic disinfection of F. graminearum macroconidia under visible light illumination was demonstrated on a visible-light-activated palladium-modified nitrogen-doped titanium oxide (TiON/PdO) nanoparticle photocatalyst. Because of the opposite surface charges of the TiON/PdO nanoparticles and the F. graminearum macroconidium, the nanoparticles were strongly adsorbed onto the macroconidium surface, which is beneficial to the photocatalytic disinfection of these macroconidia. The photocatalytic disinfection mechanism of TiON/PdO nanoparticles on these macroconidia could be attributed to their cell wall/membrane damage caused by the attack from reactive oxygen species (ROSs) as demonstrated by the fluorescence/phase contrast microscopy observations, while a breakage of their cell structure was not necessary for their loss of viability.Keywords: fluorescence staining; Fusarium graminearum macroconidia; photocatalytic disinfection; TiON/PdO nanoparticles; visible light;

A facile approach was developed to create well-dispersed, ultrasmall superparamagnetic magnesium ferrite nanocrystallites with controlled hydrophilicity/hydrophobicity and high saturation magnetization by a room temperature precipitation reaction followed by a solvent thermal process at low temperature. The in situ NaCl “cage” which self-formed during the room temperature precipitation reaction confined the formation, crystallization, and growth of the magnesium ferrite nanocrystallites, resulting in an ultrasmall average particle size of ∼3.7 nm, while a high saturation magnetization comparable with that of the bulk MgFe2O4 material was achieved with a low Mg2+ content of ∼10%. The surfaces of these superparamagnetic magnesium ferrite nanocrystallites could be modified with oleic acid/citric acid ester coatings to have controlled hydrophobicity/hydrophilicity to ensure their good dispersion in either nonaqueous or aqueous environments. This approach could be readily applied to other ferrite material systems and promises a wide range of technical applications.

Hydrous zirconium oxide (ZrO2·xH2O) were synthesized by a low-cost hydrothermal process followed with heat treatment. ZrO2·xH2O nanoparticles ranged from 6 nm to 10 nm and formed highly porous aggregates, resulting in a large surface area of 161.8 m2 g–1. The batch tests on the laboratory water samples demonstrated a very high degree of As(III) and As(V) removal by ZrO2·xH2O nanoparticles. The adsorption mechanism study demonstrated that both arsenic species form inner-sphere surface complexes on the surface of ZrO2·xH2O nanoparticles. Higher arsenic removal effect of these ZrO2·xH2O nanoparticles were demonstrated, compared with commercially available Al2O3 and TiO2 nanoparticles. Ionic strength and competing ion effects on the arsenic adsorption of these ZrO2·xH2O nanoparticles were also studied. Testing with natural lake water confirmed the effectiveness of ZrO2·xH2O nanoparticles in removing arsenic species from natural water, and the immobilization of ZrO2•xH2O nanoparticles on glass fiber cloth minimized the dispersion of nanoparticles into the treated body of water. The high adsorption capacity of ZrO2·xH2O nanoparticles is shown to result from the strong inner-sphere surface complexing promoted by the high surface area, large pore volume, and surface hydroxyl groups of zirconium oxide nanoparticles.

The development of highly efficient As(III) adsorbents is critical to largely simplify the arsenic treatment process and lower its cost. For the first time, SnO2 nanospheres were demonstrated to possess a highly efficient As(III) adsorption capability from water in a near neutral pH environment as predicted by the material criterion we recently developed for the selection of highly efficient arsenic adsorbents. These SnO2 nanospheres were synthesized by a simple and cost-effective hydrolysis process with the assistance of ethyl acetate under ambient conditions, which had a good dispersity, a narrow size distribution, a relatively large specific surface area, and a porous structure. A fast As(III) adsorption was observed in the kinetics study on these SnO2 nanospheres, and their Langmuir adsorption capacity was determined to be ∼112.7 mg g−1 at pH ∼7. The As(III) adsorption mechanism on SnO2 nanospheres was examined by both macroscopic and microscopic techniques, which demonstrated that it followed the inner-sphere complex model. These SnO2 nanospheres demonstrated effective As(III) adsorption even with exceptionally high concentrations of co-existing ions, and a good regeneration capability by washing with NaOH solution.

A novel quasi-monodisperse, superparamagnetic Pd/Fe3O4 catalyst was synthesized for effective catalytic bromate reduction. The catalyst was prepared by dispersing nanoparticles of Pd (weight percent up to 1%) on the surface of superparamagnetic Fe3O4 microspheres with 300–500 nm in diameter and 10–20 nm in grain size. Complete reduction of bromate by this Pd/Fe3O4 catalyst was demonstrated within a short period (<2 h) over a range of pH values, in the presence of a variety of co-existing ions, and after multiple cycles. In addition, the superparamagnetic nature of the catalyst enhanced its good dispersion in water during water treatment when there was no external magnetic field, and its high saturation magnetization allowed an easy magnetic separation from water when an external magnetic field was applied after the water treatment. Thus, it could be easily recycled and reused, further enhancing its application potential.

Ultrafine superparamagnetic magnesium ferrite nanocrystallites were successfully synthesized by doping Mg2+ into α-Fe2O3 in a solvent thermal process. The dopant altered the microstructure of the α-Fe2O3 which resulted in an enhanced arsenic adsorption performance and an easy magnetic separation capability. At a concentration of 10%, Mg-doping largely increased the surface area to ∼438 m2 g−1, enhanced the dispersion and contact with arsenic species in water due to its superparamagnetic behavior, and largely improved the arsenic adsorption performance in both lab-prepared and natural water samples at near neutral pH when compared with ultrafine α-Fe2O3 nanoparticles. At an Mg concentration of 10%, the amounts of As(III) and As(V) adsorbed reached 9.3 mg g−1 and 10 mg g−1, respectively, at a low equilibrium arsenic concentration of less than 10 μg L−1. The nanocrystallites could be easily separated from water when an external magnetic field was present after water treatment due to their high saturation magnetization, which solved the potential nanomaterial dispersion problem and facilitated arsenic desorption and reuse of the material.