Abundant salts and complex constituents pose challenges to the enrichment of trace-level uranyl from field water samples. On the basis of the chemical behavior of the uranyl cation in several physical–chemical conditions (in the presence of MgCl2 etc.), we developed a facile and green strategy to improve the uranyl extraction from field water samples, different from the traditional way with organic impregnation. In our systems about nano-Mg(OH)2 treating salt lake brine, when the adsorbent is pretreated with a trace amount of UO22+ ion, the ability to extract uranyl from brine can increase by 2–5 times. The molecular dynamics simulation and time-resolved laser-induced fluorescence spectroscopy analysis show that, in the preadsorption stage, UO22+ ions can steadily bind to the Mg(OH)2 (001) surface because of the interaction with surface hydroxyl groups. When pretreated Mg(OH)2 was exposed to brine, the coordination interaction between the preloaded UO22+ ion and the [UO2(CO3)x(2x–2)–] in solution engenders good selectivity and affinity for uranyl over competitive ions in brine. Besides, the coordination number of UO22+ by carbonate oxygen is large, which further facilitates subsequent adsorption. Such strategy does not introduce other impurities and can apply to metal-oxides-type adsorbents, e.g., TiO2. This study should shed light on further improvement of efficient uranyl extraction from field samples and give insights into the mechanism understanding of uranyl adsorption in real systems.Keywords: Adsorption; Low concentration; Mg(OH)2; Molecular dynamics simulation; Nanoadsorbents; Selectivity; Uranyl

Because of its remarkably high adsorption affinity to uranyl ions, Mg(OH)2 can effectively extract trace-level uranyl and has been exploited for the treatment of field water samples. In this work, we used molecular dynamics simulation to systematically study the dynamics, energetics and structure aspects of uranyl adsorption on the Mg(OH)2 (001) surface. The approach of the uranyl cation causes the redistribution of surface OH groups and the emergence of a negatively charged surface region, which accommodates the adsorption of uranyl. The adsorption stability of uranyl is largely attributed to the coordination interaction with surface OH groups, and the calculated adsorption free energy is in quantitative agreement with experimental results. On the other hand, the adsorbed uranyl affects the orientation of surrounding OH groups, which may hinder the additional uranyl adsorption to the adjacent region and limit the adsorption capacity. The estimation of monolayer surface coverage is also well consistent with the experiments. Taken together, our results reveal the mechanisms of both adsorption affinity and capacity of Mg(OH)2. As suggested by this work, comprehensive studies about uranyl adsorption can provide insight into the adsorption properties and should be helpful for the further development of uranyl adsorbents.

A novel visible-light-driven photocatalyst CuS/ZnS with nanoflower architectures has been synthesized by a simple hydrothermal method and a successive cation exchange treatment. The visible light photocatalytic hydrogen production activity was estimated from a mixed Na2S and Na2SO3 aqueous solution. The experimental results reveal that the photocatalytic performance of ZnS nanomaterials can be enhanced dramatically with the deposition of a small percentage of CuS. When loading a 1.97 mol% CuS content, the as-prepared CuS/ZnS sample reaches an optimal hydrogen production rate of 5152 μmol h−1 g−1 under visible light and an apparent quantum efficiency of 26.2% at 420 nm (without the assistance of a Pt co-catalyst). The high photocatalytic performances are attributed to the low energy level provided by the deposited CuS on the ZnS surface, which can be activated under visible light. Furthermore, the interpolar electric field (IPEF) existing in ZnS nano-architectures can also promote the efficient separation of the photogenerated charge carriers and thus enhance the hydrogen production activity.

The selective and efficient removal of pollutants is essential for wastewater treatment and resource recycling. In this study, an ultra-thin molecularly imprinted polymers (MIPs) membrane with a thickness of 1 nm was synthesized by a facile atom transfer radical polymerization (ATRP) method using layered double hydroxides (LDH) as the template substrate. The as-prepared MIPs membrane was characterized by XRD, TEM, AFM, FTIR and BET measurements. This material was then applied for the selective preconcentration of Rhodamine B (RhB) in water. Benefiting from the surface-imprinting technique, a high adsorption capacity of 100.1 mg g−1 was obtained with an enrichment multiple of 27.3 times. It was interesting that the MIPs adsorption was temperature-sensitive, which was employed as a simple and environmentally friendly desorption strategy by changing the operating temperature. The applicability of the material was demonstrated by recovering RhB from real water samples. These membrane-structured MIPs are highly selective, reproductive, and can be produced on a large scale, and thus are promising adsorbents for wastewater treatment and organic resource recycling.

In this article, a novel composite (Mg(OH)2 supported nanoscale zerovalent iron (denoted as nZVI@Mg(OH)2) was prepared and characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy method. The morphology analysis revealed that Mg(OH)2 appeared as self-supported flower-like spheres, and nano Fe0 particles were uniformly immobilized on the surface of their “flower petals”, thus aggregation of Fe0 particles was minimized. Then the Pb(II) removal performance was tested by batch experiments. The composite presented exceptional removal capacity (1986.6 mg/g) compared with Mg(OH)2 and nanoscale zerovalent iron due to the synergistic effect. Mechanisms were also explored by a comparative study of the phase, morphology, and surface valence state of composite before and after reaction, indicating that at least three paths are involved in the synergistic removal process: (1) Pb(II) adsorption by Mg(OH)2 (companied with ion exchange reaction); (2) Pb(II) reduction to Pb0 by nanoscale zerovalent iron; and (3) Pb(II) precipitation as Pb(OH)2. The hydroxies provided by Mg(OH)2 can dramatically promote the role of nanoscale zerovalent iron as reducer, thus greatly enhancing the whole Pb(II) sequestration process. The excellent performance shown in our research potentially provides an alternative technique for Pb(II) pollution treatment.Keywords: magnesium hydroxide; nanoscale zerovalent iron (nZVI); nZVI@Mg(OH)2 composite; synergistic sequestration;

Abrupt crystallization from ∼2–5 nm (amorphous) to ∼12–15 nm (crystalline) was observed in hydrothermal coarsening of Ag2S. The desorption behavior of capping ligands could be associated with the aggregation and fusion of amorphous particles into crystals.

•[Ru(bpy)3]2+-mediated photoelectrochemical sensor was developed for BPA detection.•Molecularly imprinted polypyrrole was modified on a SnO2 electrode as the recognition element.•The measurement was realized using a visible light source.•This sensor was highly sensitive and selective.•Practicality of the sensor was demonstrated by the analysis of BPA spiked water samples.A ruthenium-mediated photoelectrochemical sensor was developed for the detection of BPA, using molecularly imprinted polymers (MIPs) as the recognition element, a tin oxide (SnO2) nanoparticle-modified ITO as the electrode, and a blue 473-nm LED as the excitation light source. Photoelectrochemical oxidation of BPA on SnO2 electrode was achieved by [Ru(bpy)3]2+ under the irradiation of light. It was found that BPA was oxidized by Ru3+ species produced in the photoelectrochemical reaction, resulting in the regeneration of Ru2+ and the concomitant photocurrent enhancement. MIPs film was prepared by electropolymerization of pyrrole on SnO2 electrode using BPA as the template. Surface morphology and properties of the as-prepared electrode were characterized by SEM, electrochemical impedance spectroscopy, and photocurrent measurement. In the presence of BPA, an enhanced photocurrent was observed, which was dependent on the amount of BPA captured on the electrode. A detection limit of 1.2 nM was obtained under the optimized conditions, with a linear range of 2–500 nM. Selectivity of the sensor was demonstrated by measuring five BPA analogs. To verify its practicality, this sensor was applied to analyze BPA spiked tap water and river water. With advantages of high sensitivity and selectivity, low-cost instrument, and facile sensor preparation procedure, this sensor is potentially suitable for the rapid monitoring of BPA in real environmental samples. Moreover, the configuration of this sensor is universal and can be extended to organic molecules that can be photoelectrochemically oxidized by Ru3+.

Taking SnO2 quantum dots with random orientation as a precursor, NaOH induces self-assembly of SnO2 dots to form the nanowires, side-by-side attachment of which generates hierarchically ordered structures. The multistep oriented attachment mechanism can help to describe the oriented assembly of big nanocrystals.

Despite its significant potential as an adsorbent, graphene oxide (GO) is still not widely used mainly because of the difficulties in redispersing after drying. To break the strong forces between GO layers and improve adsorption ability, a titanium phosphate (TiP) modified GO composite was fabricated in this work. Hierarchically structured GO@TiP composites (GTiP) were synthesized via a mild in situ chemical precipitation method. The products were thoroughly characterized by various methods such as SEM, TEM, XRD, XPS and BET. This new material was then applied as an adsorbent for Eu3+ in water. GTiP exhibited excellent adsorption characteristics for europium ions. It took shorter than 1.5 min to reach half-equilibrium, while its adsorption capacity can be as high as 64.33 mg g−1. GTiP also shows considerable adsorption ability both in a wide range of pH and salinity. The excellent adsorption capability of GTiP may have been derived from both materials in which GO increased the specific surface area and adsorption capacity, while TiP enhanced hydrophilicity and mechanical properties of GTiP.

The mechanism of the interaction between nano-Mg(OH)2 adsorbent and uranyl in water was studied. At trace levels, the uranyl is adsorbed as a monolayer on nano-Mg(OH)2, and occupied a small proportion of the adsorption sites. As the uranyl concentration crosses over a threshold, continuous increase of adsorption capacity takes place. It indicates that, by taking the pre-adsorbed uranyl as the nucleation centers, the additional uranyl crystallizes and forms U-rich nanocrystals well-scattered on the surface of nano-Mg(OH)2. A strategy of inducing fast crystal growth of nano-Mg(OH)2 to micrometer-sized Na2Mg(CO3)2 enables the desorption and enrichment of uranyl. The recycling and reuse of nano-Mg(OH)2 can be achieved simultaneously. The finding in this work provides fundamental understanding of the efficient usage of nano-Mg(OH)2 in practical applications.Keywords: adsorption; crystallization; enrichment; mechanism; nano-Mg(OH)2; uranyl;

In the past two years, three-dimensional graphene (3DG) was introduced to the environmental treatment area as a promising new material. Despite much progress in its synthesis and applications, 3DG is still limited in terms of green large-scale synthesis and practical environmental applications. In this work, a 3DG synthetic method was developed at 95 °C in an EDTA-induced self-assembly process. Because little EDTA was found to be consumed during synthesis, which might be due to its great stability and poor reducibility, 3DG with complete structure can be successively obtained by reusing the EDTA solution more than 10 times. Furthermore, 3DG was found to possess a superior adsorption capacity of 119 mg g–1 (pH 6.0) for paraquat, a highly toxic herbicide with positive charges and a conjugated system of π bonds in its molecular structure. The adsorption capacity was much higher than those in classic paraquat adsorbents, such as clay and activated carbon. To address the problem of 3DG damage by stirring, a pyramid-shaped nylon teabag was adopted to protect the soft hydrogel during the repeated adsorption–desorption processes. After five cycles, the 3DG teabag still maintained 88% of the initial adsorption capacity. This facile method may be easily applied in other environmental treatment conditions.Keywords: EDTA; green synthesis; paraquat; self-assembly; teabag recycling; three-dimensional graphene

The PL lifetime optimization of CdTe QDs capped with TGA has yet to be understood from a perspective of growth kinetics. In this work, the growth kinetics and PL properties of CdTe QDs growing in aqueous solutions of two TGA concentrations, 0 mM and 57 mM, were systematically investigated using UV, TEM, and PL methods. CdTe QDs in 0 mM TGA solution were found to follow the mixed OA (Oriented Attachment)–OR (Ostwald Ripening) growth kinetics. The PL peaks experienced a red-shift with almost unchanged intensity and the PL lifetimes increased gradually. In 57 mM TGA solution, the QDs followed the OA dominated growth mechanism. The PL peak broadened greatly with a red-shift and its intensity decreased significantly. The PL lifetime increased much higher than that in 0 mM TGA solution. Based on the different growth kinetic models of the two systems, we suggest that in the low (0 mM) TGA solution, the increased surface defects induced by TGA desorption and the existence of partial internal defects caused by OA growth were the main reasons for the gradual increase of PL lifetime, while in high (57 mM) TGA solution, the increase of PL lifetime was ascribed to the abundant internal defects produced by OA collision. Finally, kinetic data showed the effect of the TGA concentration on crystal growth and PL lifetime of CdTe QDs. The results might provide guidance for understanding the mechanism behind the phenomena of ligand-related PL properties.

So far, the photoluminescence (PL) properties optimization of mercaptopropionic acid (MPA) capped CdTe has yet to be understood from a perspective of growth kinetics. In this work, the growth kinetics, size distribution and PL properties of CdTe quantum dots (QDs) grown in MPA aqueous solutions of two extreme concentrations, i.e. 0 mM and 50 mM, were systematically investigated. The studies revealed that CdTe QDs coarsen in low and high concentrations of MPA followed different growth kinetics, i.e. a mixed OA (Oriented Attachment)–OR (Ostwald Ripening) growth and an OA dominated growth, respectively. Moreover, the OR growth in a low MPA concentration solution was found to be disadvantageous for narrowing the size distribution but favourable for enhancing fluorescence, while the OA growth in a high MPA concentration solution is beneficial for focusing and controlling the particle size. Further activation energy calculations indicate that the extra MPA that forbids the OR growth and retards the OA growth is favourable for slowing the particle growth, thus achieving CdTe QDs with a controlled size and narrow distribution. Furthermore, the kinetics study reveals how the introduction of concentrated surface ligands affects the PL properties (FWHM and intensity) of the material.

Crystal growth mechanism, kinetics, and microstructure development play fundamental roles in tailoring materials with controllable sizes and morphologies. Oriented attachment (OA) involves the spontaneous self-organization of adjacent nanocrystals, resulting in crystal growth by addition of solid particles that share a common crystallographic orientation. It is common for crystal growth to occur by more than one mechanism simultaneously, such as OA and coarsening. This complexity leads to the difficulty in studying the OA growth mechanism. Here, we briefly review progress in kinetic models involving OA and the impact of this mechanism on materials science. We concentrate mostly on recent findings that relate different OA behaviors to surface chemistry and growth conditions, aiming to elucidate this crystal growth mechanism. To explore OA-limited growth, the influence of the Ostwald ripening (OR) mechanism needs to be known and quantified. The introduction of capping ligands has been reported to play multiple roles, including i) promoting or inhibiting aggregation and thus influencing OA growth, or ii) hindering OR from occurring and thus facilitating OA. A detailed survey of nanocrystal growth kinetics under the effect of surface adsorption is presented and summarized.

One difficult issue that environmental scientists are facing is how to convert soluble U(VI) into insoluble U(IV) and recycle it. In the present study, a method, which was widely reported in the literature, was used to collect soluble U(VI) using general biomass (including bacteria and yeast extract), and then a strategy was developed to transform the amorphous uranium-containing precipitates (Uranium–Phosphorus Amorphous Compound, UPAC) into large-sized insoluble UO2 nanoparticles. The results show that the biomass could precipitate more than 90% of the U(VI) (0.42 mmol L−1) within 10 min. The maximum precipitation capacity of the biomass (dry weight) ranged from 120 to 187 mg U g−1. The UPAC can be further converted into soluble uranyl phosphate compounds (HUO2PO4) at room temperature for 90 days or under the hydrothermal condition at 150 °C for 48 h. However, once the hydrothermal temperature was raised to 240 °C, insoluble UO2 nanoparticles of around 10 nm could be obtained within 48 h. This work provides a new possibility for the cost-effective preparation of nuclear fuel (UO2) with inexpensive raw materials. The mechanism correlating to the transformation of the UPAC into inorganic UO2 is also discussed here.

As a chemical and structural simple hydrophilic material, Mg(OH)2 exhibits great potential in water environment remediation. In this work, we use molecular dynamics (MD) simulation to investigate the distribution of water molecules on the Mg(OH)2 (001) surface as well as the dynamic behaviors of interfacial water. Even though Mg(OH)2 substrate can considerably affect the density profile of water molecules as well as the water dipole orientations, no specific adsorption sites for water can be observed on the surface of Mg(OH)2. Meanwhile, the interaction of water molecules with Mg(OH)2 substrate does not disturb the hydrogen bonds between interfacial water molecules. More interestingly, the substrate has modest effect on the dynamic behaviors of interfacial water, e.g., the residence time, hydrogen bond lifetime, and self-diffusion coefficient, which is in sharp contrast to many other hydrophilic materials.

•A novel photoelectrochemical DNAzyme sensor was developed for Pb2+ detection.•Flower-like ZnO nanostructure was employed as the electrode material.•The detection limit of the sensor was 0.1 nM.•The sensor allows selective and sensitive detection of Pb2+ in human serum and water samples.Lead contamination is now widespread, and exposure to lead may cause adverse effects on human beings. In this study, a photoelectrochemical sensor based on flower-like ZnO nanostructures was developed for Pb2+ detection, using a Pb2+-dependent DNAzyme as the recognition unit and a double-strand DNA intercalator, Ru(bpy)2(dppz)2+ (bpy=2,2'-bipyridine, dppz=dipyrido[3,2-a:2',3'-c] phenazine) as the photoelectrochemical signal reporter. The ZnO nanoflower was fabricated on an indium tin oxide (ITO) electrode by the convenient hydrothermal decomposition method. The morphology and photoelectrochemical property of the ZnO nanoflowers were characterized by SEM, XRD and photocurrent measurements. DNAzyme-substrate duplex was assembled on an ITO/ZnO electrode through electrostatic adsorption. In the presence of Pb2+, RNA-cleavage activity of the DNAzyme was activated and its substrate strand was cleaved, resulting in the release of Ru(bpy)2(dppz)2+ from the DNA film and the concomitant photocurrent decrease. The detection principle was verified by fluorescence measurements. Under the optimized conditions, a linear relationship between photocurrent and Pb2+ concentration was obtained over the range of 0.5–20 nM, with a detection limit of 0.1 nM. Interference from other common metal ions was found negligible. Applicability of the sensor was demonstrated by analyzing lead level in human serum and Pb2+ spiked water samples. This facile and economical sensor system showed high sensitivity and selectivity, thus can be potentially applied for on-site monitoring of lead contaminant.

Here we reported the antibacterial effect and related mechanism of three nano-Mg(OH)2 slurries using Escherichia coli as model bacteria. X-ray diffraction (XRD), scanning electron microscopy (SEM) and laser particle size analysis revealed that the as-synthesized Mg(OH)2_MgCl2, Mg(OH)2_MgSO4 and Mg(OH)2_MgO are all composed by nanoflakes with different sizes, and their aggregates in water are 5.5, 4.5, and 1.2 μm, respectively. Bactericidal tests showed that the antibacterial efficiency is conversely correlated with the size of Mg(OH)2 aggregates. Transmission electron microscopy (TEM) observation have not provided evidence of cellular internalization, however, the antibacterial effect is positive correlation to the loss of integrity of cell walls. SEM and zeta potential analysis revealed that the adhering ability of Mg(OH)2 on the bacterial surface is Mg(OH)2_MgCl2 > Mg(OH)2_MgSO4 > Mg(OH)2_MgO, indicating the toxicity of Mg(OH)2 may be caused by the electrostatic interaction-induced external adsorption. Confocal laser scanning microscopy (CLSM) further revealed that the adhering of Mg(OH)2 on the bacterial surface could increase the permeability of cell membranes. Taken together, the antibacterial mechanism of nano-Mg(OH)2 could be as follows: nano-Mg(OH)2 adsorbed on the bacterial surface by charge attraction first, and then destroyed the integrity of cell walls, which resulting in the final death of bacteria.Keywords: antibacterial activity; Escherichia coli; Mg(OH)2 nanoparticles; microscopic; permeability; surface charges;

It is still a big challenge to treat large amount of water with low concentration of pollutant. In this study, a hierarchical (micro/nano) structured Mg(OH)2 adsorbent was introduced by the in situ hydration of porous MgO in the dye solution. The adsorbent showed high selective adsorption capacity (Q0 ≈ 155 mg/g for acid alizarine blue) and fast adsorption rate for the removal of anionic dyes down to the mg/L levels. Moreover, the adsorbed dye was successfully desorbed by carbonation, resulting in a ∼4000 fold enrichment of the dye solution. It was demonstrated that by establishing a reversible switch between the Mg(OH)2 micro/nanorod and the bulk MgCO3·3H2O, a continuous preconcentration of low-concentration dye wastewater could be achieved.Keywords: anionic dye; carbonation; dye wastewater; Mg(OH)2 micro/nanorods; preconcentraion; selective adsorption;

Treatment of wastewater containing low-concentration yet highly-expensive rare earth elements (REEs) is one of the vital issues in the REEs separation and refining industry. In this work, the interaction and related mechanism between self-supported flowerlike nano-Mg(OH)2 and low-concentration REEs wastewater were investigated. More than 99% REEs were successfully taken up by nano-Mg(OH)2. Further analysis revealed that the REEs could be collected on the surface of Mg(OH)2 as metal hydroxide nanoparticles (<5 nm). An ion-exchange model was proposed as a critical factor for both guaranteeing the reaction speed and maintaining the self-supported structure of the materials. In addition, a method was developed to further separate the immobilized REEs and the residual magnesium hydroxide by varying the solution pH. In a pilot-scale experiment, the REEs from practical wastewater were immobilized effectively at a high flow rate. We anticipate this work can provide a good example for the recycling of valuable REEs in practical industrial applications.Keywords: ion-exchange; Mg(OH)2 nanoparticles; rare earth elements; recycling; self-supported; water treatment;

The desorption of CrVI from CrVI-adsorbed layered double hydroxide (CrVI-LDH) and the recycling of LDH adsorbent are the bottlenecks that limit the practical application of LDH in treating CrVI-containing industrial wastewater. Given the strong affinity of LDH for CO2, we studied desorption and enrichment of CrVI from CrVI-LDH as well as recycling of LDH in the presence of high-pressure CO2. Results showed that CrVI solution with concentration of 500 mg/L could be enriched more than 20 times in each adsorption–desorption cycle. The regenerated LDH maintained the layer structure and the sheets as revealed by XRD and TEM patterns. FT-IR data showed CO2 formed HCO3– at high pressure. The transformation from CO2 to HCO3– followed by the anion-exchange with CrO42– was the critical factor for CrVI desorption and LDH regeneration. A pilot-scale experiment was carried out with 20 L CrVI-containing electroplating wastewater. The concentration of the desorbed CrVI solution could reach up to 10000 mg/L, which could be used in electroplating after appropriate adjustment. The main advantages of this method are high concentration of CrVI, direct reuse of enriched CrVI, and efficient regeneration of LDH adsorbent. This method showed promises in recycling CrVI and regenerating LDH in treating industrial wastewater.Keywords: chromium(VI); CO2; concentration; electroplating wastewater; layered double hydroxides (LDHs); recycle;

The mechanism by which the maximum band-edge emission of quantum dots (QDs) occurs remains unclear. In this work, systematic kinetic studies revealed that the growths of thioglycolic acid (TGA)-capped CdS QDs at three concentrations (7.0, 2.3, and 0.8 mM) undergo a two-stage process: an initial oriented attachment (OA) dominant stage and a subsequent Ostwald Ripening (OR) stage. At the transition point from the OA dominant to the OR stage, the band-edge PL peaked at 450–470 nm and reached its “maximum”, with the narrowest peak width about 28 nm. Investigation on particle size distribution (PSD) showed size “focusing” in the OA dominant stage. Nevertheless, the ideally narrowest PSD occurs far earlier than the maximum band-edge emission. Furthermore, its PL emission was found broadened by competitive defect-related emissions, reasonably assigned to lattice defects generated by the initial OA growth. Annealing of defects at the later OA stage can be responsible for enhanced PL band-edge emission to a certain maximum value, which will decrease in the subsequent OR growth due to the broadening PSD. The role of surface-capping was also discussed and proved to be the key factor to achieve the maximum band-edge emission of QDs by tuning the crystal growth kinetics.

As a critical issue in “green” synthesis, the mechanism by which the growth temperature influences the photoluminescence (PL) properties of quantum dots (QDs) remains unclear. In this work, temperature sensitive growth kinetics and PL evolution of thioglycolic acid (TGA) capped CdS QDs were systematically investigated. This revealed that TGA capped CdS QDs coarsened below and above 90 °C experienced completely different growth kinetics, i.e. a two-stage growth from oriented attachment to Ostwald ripening (OA–OR) and a hybrid growth of the two mechanisms (OA + OR), respectively. With the evolution of the OA–OR growth, the corresponding PL shows characteristics of a gradual weakness of the defect-related emissions centered at ∼650 nm and a steadily enhanced band-edge emission peaking at ∼450–470 nm. For the OA + OR hybrid growth, the defect-related emission band steadily coexists with the band-edge emission after an initial decrease. In-depth kinetics and high-resolution transmission electron microscopy (HRTEM) investigation indicate that a low temperature favors a long-term OA self-integration stage, by which internal defects could be relaxed or even eliminated. For a high temperature, this tendency could be greatly inhibited by the concurrent OR growth just around these defects. The FTIR analysis further revealed that the adsorption–desorption equilibrium of capping ligands was temperature-sensitive, resulting in different growth kinetics and PL characteristics of CdS QDs.

This work aims at the investigation of nano-Mg(OH)2 as a promising adsorbent for uranium recovery from water. Systematic analysis including the uranium adsorption isotherm, the kinetics and the thermodynamics of adsorption of low concentrations of uranyl tricarbonate (0.1–20 mg L−1) by nano-Mg(OH)2 was carried out. The results showed a spontaneous and exothermic uranium adsorption process by Mg(OH)2, which could be well described with pseudo second order kinetics. Surface site calculation and zeta potential measurement further demonstrated that UO2(CO3)34− was a monolayer adsorbed onto nano-Mg(OH)2 by electrostatic forces. Accordingly, the adsorption behavior met the conditions of the Langmuir isotherm. Moreover, in most of the reported literature, nano-Mg(OH)2 had a higher UO2(CO3)34− adsorption affinity b, which implied a higher adsorption amount at equilibrium in a dilute adsorbate system. The significance of the adsorption affinity b for choosing and designing adsorbents with respect to low concentration of resources/pollutants treatment has also been assessed.

A novel environmentally friendly nano-adsorbent is developed by doping Cu+ cations into the lattice of ZnS microspheres. The adsorbent shows selective adsorbability for cationic dyes in low concentrations in wastewater. The adsorbed dye could be successfully eluted with alcohol, resulting in a 1000 fold enrichment of the dye solution.

With the existence of CO2 and H2O, an unusual “jump of size” phenomenon from nano-Mg(OH)2 into bulk MgCO3·3H2O was investigated. It was revealed that the fast growth mode undergoes an in situ conversion of aggregated nano-particles to bulk crystal.

Investigations about how to recycle the deactivated nanomaterials are well-needed. This work was designed to explore the recycling strategy of Mg(OH)2 nanoadsorbent during treating low concentration of CrVI solution as an example. It was demonstrated that a reversible route between Cr-adsorbed nano-Mg(OH)2 and Cr-desorbed bulk-MgCO3·3H2O can be established by using CO2 as a phase transformation medium. In each adsorption−desorption cycle, CrVI solution with initial concentration of 10 mg·L−1 could be enriched over 40 times. An aggregation-induced rapid phase transformation mechanism from nano-Mg(OH)2 to bulk-MgCO3·3H2O was discovered, which was one of the critical factors to ensure the disposing efficiency of this environmental-friendly CrVI disposal system. A pilot-scale experiment was conducted with this strategy to deal with 50 L CrVI-containing simulated industrial wastewater. The enrichment of CrVI and the recycle of nano-Mg(OH)2 can be successfully achieved simultaneously.

Ideal Al-doped ZnO (AZO) thin films should have high carrier mobility and carrier concentration, as well as high thermal and chemical stability. To achieve these properties, ZnO should be heavily doped with Al and perfectly crystallized. Through analyzing the possible valence state of the elements and local lattice structures of AZO films during the gas-phase deposition process, we find that the current gas-phase deposition method may encounter an intrinsic obstacle that heavy doping of Al, high thermal stability, and high mobility (perfect crystallinity) cannot be achieved simultaneously. However, based on the understanding that an AZO thin film prepared in oxidizing atmosphere is actually accompanied with a high concentration of zinc vacancy, we propose a strategy to obtain an AZO film with ideal characteristics. Under an oxidizing atmosphere, a heavily doped AZO film with a high concentration of zinc vacancy is prepared using a gas-phase deposition method. Then a zinc vapor annealing treatment is employed to improve the crystallinity and conductivity of the film by filling the zinc vacancies with zinc atoms. The prepared AZO films possess the highest mobility (36.8 cm2 V−1 s−1) ever reported. Moreover, the films also show remarkable stability in carrier concentration, mobility, and resistivity under damp heat treatment (85 °C) over months.

Crystal growth kinetics of thioglycolic acid (TGA)-capped CdTe nanoparticles in H2O was studied at different particle concentrations, with the aim to understand the effects of concentration and surface state of primary particles on the growth mechanism of nanocrystals. The growth process of CdTe nanoparticles was monitored by in situ UV–vis absorbance spectra, which reveal the coexisting growth mechanisms of oriented attachment (OA) and Ostwald ripening (OR). Increasing the primary nanoparticle concentration facilitated the OA-based growth at the initial stage, while the OR growth which was mainly occurring at the latter stage was restrained. On the basis of the fitting activation energy and experimental zeta potential, the effect of surface charge was further discussed and proved to be the critical factor that affected the OA process by directing the particle–particle interaction.

The multifunctional thin films (BW12/Ag NPs)n (BW12 = BW12O40, NPs = nanoparticles) were prepared by layer-by-layer self-assembly method. The (BW12/PEI–Ag+)n (PEI = polyethylenimine) composite films were achieved through alternately depositing anionic BW12 and cationic PEI–Ag+ complex. The deposition process of (BW12/PEI–Ag+)10 multilayer is linear layer-by-layer self-assembly. Under UV irradiation, Ag ions in (BW12/PEI–Ag+)n multilayer films were reduced photochemically into Ag NPs and (BW12/Ag NPs)10 films were obtained. Through UV–vis measurements, the presence of surface plasma absorption peak at 445 nm demonstrated the formation of silver NPs. The electrochemical and antibacterial activities of (BW12/Ag NPs)n films were investigated. The electrochemical results indicate that the glassy carbon electrode modified with (BW12/Ag NP)n film exhibits the electroreduction toward O2. Moreover, the (BW12/Ag NP)10 multilayer films exhibit long-lasting antibacterial properties toward Escherichia coli (E. coli).

Water-soluble mercaptoacetic acid-coated 3.1 nm CdS quantum dots (QDs) with two concentrations were selected for studying the correlation between the photoluminescence and the crystal growth mechanism. By achieving the classic Ostwald ripening mechanism and oriented attachment (OA) growth mechanism, we have shown that the evolution of the emission spectra were obviously different. The change in both the surface and internal defects during OA crystal growth were responsible for the specific variation of the photoluminescence of CdS QDs. Strategies for obtaining QDs with different luminescent properties are suggested.

The crystal growth mechanism, kinetics, and microstructure development play a fundamental role in tailoring the materials with controllable sizes and morphologies. The classical crystal growth kinetics—Ostwald ripening (OR) theory is usually used to explain the diffusion-controlled crystal growth process, in which larger particles grow at the expense of smaller particles. In nanoscale systems, another significant mechanism named “oriented attachment (OA)” was found, where nanoparticles with common crystallographic orientations directly combine together to form larger ones. Comparing with the classical atom/molecular-mediated crystallization pathway, the OA mechanism shows its specific characteristics and roles in the process of nanocrystal growth. In recent years, the OA mechanism has been widely reported in preparing low-dimension nanostructural materials and reveals remarkable effects on directing and mediating the self-assembly of nanocrystals. Currently, the interests are more focused on the investigation of its role rather than the comprehensive insight of the mechanism and kinetics. The inner complicacy of crystal growth and the occurrence of coexisting mechanisms lead to the difficulty and lack of understanding this growth process by the OA mechanism. In this context, we review the progress of the OA mechanism and its impact on materials science, and especially highlight the OA-based growth kinetics aiming to achieve a further understanding of this crystal growth route. To explore the OA-limited growth, the influence of the OR mechanism needs to be eliminated. The introduction of strong surface adsorption was reported as the effective solution to hinder OR from occurring and facilitate the exclusive OA growth stage. A detailed survey of the nanocrystal growth kinetics under the effect of surface adsorption was presented and summarized. Moreover, the development of OA kinetic models was systematically generalized, in which the “molecular-like” kinetic models were built to take the OA nanocrystal growth behavior as the collision and reaction between molecules. The development of OA growth kinetics can provide a sufficient understanding of crystal growth, and the awareness of underlying factors in the growth will offer promising guidance on how to control the size distribution and shape development of nanostructural materials.

An “infinite recycling” method for enhancing the durable applications of a ZnS nano-photocatalyst is shown. Based on the finding of thermodynamic stable nanophase of ZnS, we designed a strategy in which the deactivated ZnS nano-photocatalyst could be recovered into its original state. This ZnS photocatalyst can be used repeatedly without being released into environment as nano-waste. The strategy uses material highly efficiently and is environmentally friendly.

Bioremediation of Cr(VI) through reduction relies on the notion that the produced Cr(III) may be precipitated or efficiently immobilized. However, recent reports suggest that soluble organo-Cr(III) complexes are present in various chromate-reducing bacterial systems. This work was designed to explore the factors that affect the immobilization of Cr(III) in the Ochrobactrum anthropi system. X-ray absorption fine structure analysis on the cell debris clearly verified that coordination of Cr(III) occurs on the surfaces via the chelating coordination with carboxyl- and amido-functional groups. However, competitive coordination experiments of Cr(III) revealed that the small molecules such as amino acids and their derivatives or multicarboxyl compounds hold stronger coordination ability with Cr(III) than with cell debris. We speculate that it is the preferential coordination of Cr(III) to the soluble organic molecules in the bacterial culture medium that inhibits effective immobilization of Cr(III) on the cells. On the basis of this understanding, a strategy with two-step control of the medium was proposed, and this achieved successful immobilization of Cr(VI) as Cr(III) by O. anthropi and Planococcus citreus in 5−50 L pilot-scale experiments.

In this article, we report a novel molten hydroxide strategy to prepare bulk-sized complexes of three-dimensional (3D) nanostructures at a large scale. Taking cadmium hydroxide as an example, we revealed that the bulk-sized architectures were composed by cross-linked nanoplates, which could be fabricated by a one-step hydrothermal route in the presence of molten composite-hydroxide (NaOH + KOH) at 200 °C. Powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analysis show that the growth of the well-defined nanostructures actually undergoes two stages. Initially, with the mixture of cadmium chloride and a large excess of composite-hydroxide, a prestructure of aggregating nanoparticles is formed. Afterward, the prestructure recrystallizes into nanoplates of 150 nm thickness and with the (001) plane as exposed surface. Meanwhile, these nanoplates are interweaved and organized into a globe-like structure and 3D aggregates. We speculate that the molten hydroxide plays a critical role not only in providing a strong surface effect to allow recrystallization and anisotropic growth but also in realizing the self-assembly of nanoparticles in inorganic solution. The molten composite-hydroxide solution strategy was further extended into a calcium hydroxide system and the same assembling nanoplate architectures were successfully achieved.

The environmental behavior, biological and ecological effects of nanomaterial have attracted much attention, and nanomaterial is prevalently used in the process of industrial manufacturing. However, the investigation on how to resolve the environmental problems of nanowastes is scarcely reported. We have proposed a novel method in which the toxicity of the waste is reduced during the fast growth of nanoparticals. It is suggested that this method could be used in the treatment of the Cr(VI)-containing nanowastes from the chlorate plants. In this work, the efficiencies of the treatment at both room and high temperatures were further studied. The craft route is advanced for the treatment at the high temperature. The phase, morphology, size, and thermal behavior of the detoxified solid were analyzed by XRD, SEM, and DTA. The solid was also tried as the raw of glaze. The disposal provided here will be a good example for the recycling of the hazardous nanowastes.

By comparing the photocatalytic bactericidal effect on different crystal faces of bulk ZnO crystal, we found that an electron degradation mechanism dominates the photocatalytic processes of ZnO material.

In this work, crystal growth kinetics of surfactant-free nanocrystalline SnO2 in distilled water at 175–250 °C were investigated. The growth rate followed the type of asymptotic curve in hundreds of hours, which could be fitted by the multistep oriented attachment (OA) kinetic model. High-resolution transmission electron microscope (HRTEM) data also indicated crystal growth occurring via the multistep OA mechanism. During the growth of SnO2, the concentration of Sn ions in the aqueous solution was examined. It reveals that the unsaturated situation of SnO2 results in crystal growth via the pure OA mechanism. A growth model of self-integration of conjugated nanocrystals was discussed for understanding the OA behavior.

As-synthesized 3.5-nm sphalerite CdS was hydrothermally coarsened in a series of Na2S solutions at 150 °C, aiming to understand the effect of the Na2S additive on the growth and phase-transition mechanisms. X-ray diffraction (XRD) data indicate that, with increasing Na2S concentration, both the growth rate and the ratio of wurtzite CdS increase. Moreover, high-resolution transmission electron microscopy (HRTEM) data reveal that, in contrast to the regular surface nucleation mode in aqueous solution, the new wurtzite phase of CdS in Na2S solution appears in the interior regions of crystal, and the interfacial nucleation mode greatly promotes the generation of the wurtzite-type structure. The oriented-attachment (OA) mechanism during growth was found to contribute to the unique mode of phase transformation. We propose that the strong interfacial adsorption effect of Na2S promotes the growth of CdS via the OA mechanism and the OA-mediated phase transformation.

A basic understanding related to the immobilization of chromium by bacteria is essential for chromate pollutant remediation in the environment. In this work, we studied the Cr(VI) uptake mechanism of living Ochrobactrum anthropi and the influence of a bacterial culture medium on the Cr-immobilization process. It was found that the Cr-immobilization ratio of bacteria in Tris-HCl buffer is higher than in LB medium. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) analysis revealed that the chromium accumulated on bacteria were mostly in Cr(III) states. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations showed that noticeable Cr(III) precipitates were accumulated on bacterial surfaces. AFM roughness analysis revealed that the surface roughness of bacteria increased greatly when the bacteria−Cr(VI) interaction was in Tris-HCl buffer rather than in LB solution. Transmission electron microscopy (TEM) thin section analysis coupled with energy-dispersive X-ray spectroscopy showed that Cr(III) is also distributed in bacterial inner portions. A chromium-immobilization mechanism considering the participation of both bacterial inner portions and bacterial surfaces of living Ochrobactrum anthropi was proposed, whereas the bacterial surface was the dominant part of the immobilization of Cr(III). This work also proved that the control of Cr immobilization by living Ochrobactrum anthropi could be achieved via adjusting the bacterial culture medium.

Mg–Al–Cl layered double hydroxide (Cl-LDH) was prepared to simultaneously remove Cu(II) and Cr(VI) from aqueous solution. The coexisting Cu(II) (20 mg/L) and Cr(VI) (40 mg/L) were completely removed within 30 min by Cl-LDH in a dosage of 2.0 g/L; the removal rate of Cu(II) was accelerated in the presence of Cr(VI). Moreover, compared with the adsorption of single Cu(II) or Cr(VI), the adsorption capacities of Cl-LDH for Cu(II) and Cr(VI) can be improved by 81.05% and 49.56%, respectively, in the case of coexisting Cu(II) (200 mg/L) and Cr(VI) (400 mg/L). The affecting factors (such as solution initial pH, adsorbent dosage, and contact time) have been systematically investigated. Besides, the changes of pH values and the concentrations of Mg2+ and Al3+ in relevant solutions were monitored. To get the underlying mechanism, the Cl-LDH samples before and after adsorption were thoroughly characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. On the basis of these analyses, a possible mechanism was proposed. The coadsorption process involves anion exchange of Cr(VI) with Cl− in Cl-LDH interlayer, isomorphic substitution of Mg2+ with Cu2+, formation of Cu2Cl(OH)3 precipitation, and the adsorption of Cr(VI) by Cu2Cl(OH)3. This work provides a new insight into simultaneous removal of heavy metal cations and anions from wastewater by Cl-LDH.Cl− intercalated Mg–Al layered double hydroxide was demonstrated to be an efficient absorbent for simultaneous removal of Cu(II) and Cr(VI) in different concentrations through: (i) anion exchange of Cr(VI) with Cl− in the interlayer of LDH, (ii) formation of Cu2Cl(OH)3 precipitation, (iii) adsorption of Cr(VI) by Cu2Cl(OH)3, and (iv) isomorphic substitution of Mg2+ with Cu2+.Download high-res image (248KB)Download full-size image

By comparing the photocatalytic bactericidal effect on different crystal faces of bulk ZnO crystal, we found that an electron degradation mechanism dominates the photocatalytic processes of ZnO material.

Despite its significant potential as an adsorbent, graphene oxide (GO) is still not widely used mainly because of the difficulties in redispersing after drying. To break the strong forces between GO layers and improve adsorption ability, a titanium phosphate (TiP) modified GO composite was fabricated in this work. Hierarchically structured GO@TiP composites (GTiP) were synthesized via a mild in situ chemical precipitation method. The products were thoroughly characterized by various methods such as SEM, TEM, XRD, XPS and BET. This new material was then applied as an adsorbent for Eu3+ in water. GTiP exhibited excellent adsorption characteristics for europium ions. It took shorter than 1.5 min to reach half-equilibrium, while its adsorption capacity can be as high as 64.33 mg g−1. GTiP also shows considerable adsorption ability both in a wide range of pH and salinity. The excellent adsorption capability of GTiP may have been derived from both materials in which GO increased the specific surface area and adsorption capacity, while TiP enhanced hydrophilicity and mechanical properties of GTiP.

In this work, crystal growth kinetics of surfactant-free nanocrystalline SnO2 in distilled water at 175–250 °C were investigated. The growth rate followed the type of asymptotic curve in hundreds of hours, which could be fitted by the multistep oriented attachment (OA) kinetic model. High-resolution transmission electron microscope (HRTEM) data also indicated crystal growth occurring via the multistep OA mechanism. During the growth of SnO2, the concentration of Sn ions in the aqueous solution was examined. It reveals that the unsaturated situation of SnO2 results in crystal growth via the pure OA mechanism. A growth model of self-integration of conjugated nanocrystals was discussed for understanding the OA behavior.

The PL lifetime optimization of CdTe QDs capped with TGA has yet to be understood from a perspective of growth kinetics. In this work, the growth kinetics and PL properties of CdTe QDs growing in aqueous solutions of two TGA concentrations, 0 mM and 57 mM, were systematically investigated using UV, TEM, and PL methods. CdTe QDs in 0 mM TGA solution were found to follow the mixed OA (Oriented Attachment)–OR (Ostwald Ripening) growth kinetics. The PL peaks experienced a red-shift with almost unchanged intensity and the PL lifetimes increased gradually. In 57 mM TGA solution, the QDs followed the OA dominated growth mechanism. The PL peak broadened greatly with a red-shift and its intensity decreased significantly. The PL lifetime increased much higher than that in 0 mM TGA solution. Based on the different growth kinetic models of the two systems, we suggest that in the low (0 mM) TGA solution, the increased surface defects induced by TGA desorption and the existence of partial internal defects caused by OA growth were the main reasons for the gradual increase of PL lifetime, while in high (57 mM) TGA solution, the increase of PL lifetime was ascribed to the abundant internal defects produced by OA collision. Finally, kinetic data showed the effect of the TGA concentration on crystal growth and PL lifetime of CdTe QDs. The results might provide guidance for understanding the mechanism behind the phenomena of ligand-related PL properties.

A novel visible-light-driven photocatalyst CuS/ZnS with nanoflower architectures has been synthesized by a simple hydrothermal method and a successive cation exchange treatment. The visible light photocatalytic hydrogen production activity was estimated from a mixed Na2S and Na2SO3 aqueous solution. The experimental results reveal that the photocatalytic performance of ZnS nanomaterials can be enhanced dramatically with the deposition of a small percentage of CuS. When loading a 1.97 mol% CuS content, the as-prepared CuS/ZnS sample reaches an optimal hydrogen production rate of 5152 μmol h−1 g−1 under visible light and an apparent quantum efficiency of 26.2% at 420 nm (without the assistance of a Pt co-catalyst). The high photocatalytic performances are attributed to the low energy level provided by the deposited CuS on the ZnS surface, which can be activated under visible light. Furthermore, the interpolar electric field (IPEF) existing in ZnS nano-architectures can also promote the efficient separation of the photogenerated charge carriers and thus enhance the hydrogen production activity.

Abrupt crystallization from ∼2–5 nm (amorphous) to ∼12–15 nm (crystalline) was observed in hydrothermal coarsening of Ag2S. The desorption behavior of capping ligands could be associated with the aggregation and fusion of amorphous particles into crystals.

Taking SnO2 quantum dots with random orientation as a precursor, NaOH induces self-assembly of SnO2 dots to form the nanowires, side-by-side attachment of which generates hierarchically ordered structures. The multistep oriented attachment mechanism can help to describe the oriented assembly of big nanocrystals.

The selective and efficient removal of pollutants is essential for wastewater treatment and resource recycling. In this study, an ultra-thin molecularly imprinted polymers (MIPs) membrane with a thickness of 1 nm was synthesized by a facile atom transfer radical polymerization (ATRP) method using layered double hydroxides (LDH) as the template substrate. The as-prepared MIPs membrane was characterized by XRD, TEM, AFM, FTIR and BET measurements. This material was then applied for the selective preconcentration of Rhodamine B (RhB) in water. Benefiting from the surface-imprinting technique, a high adsorption capacity of 100.1 mg g−1 was obtained with an enrichment multiple of 27.3 times. It was interesting that the MIPs adsorption was temperature-sensitive, which was employed as a simple and environmentally friendly desorption strategy by changing the operating temperature. The applicability of the material was demonstrated by recovering RhB from real water samples. These membrane-structured MIPs are highly selective, reproductive, and can be produced on a large scale, and thus are promising adsorbents for wastewater treatment and organic resource recycling.