Na Du

Synthesis, characterization, and visible-light photocatalytic activity of BiOI hierarchical flower-like microspheres

BiOI hierarchical flower-like microspheres were hydrothermally prepared, using tetrabutylammonium iodide as an iodine source and template. Many BiOI hierarchical structures have been synthesized using KI, NaI, HI or ionic liquids as the iodine source, but the use of alkyl ammonium iodide as the iodine source was not reported in the literature. The so-obtained BiOI samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen sorption measurements, and ultraviolet-visible diffuse reflectance spectroscopy. The effects of hydrothermal temperature and time on the BiOI crystal structure and morphology were investigated. The BiOI microspheres were composed of BiOI nanosheets. The photocatalytic performance of the BiOI samples was determined from the degradation of Rhodamine B, under visible light irradiation. BiOI microspheres prepared at 160 °C over 30 h exhibited excellent photodegradation efficiency, which was more than five and seven times higher than those of BiOI nanoplates and N-doped TiO2, respectively. The high photocatalytic performance was attributed to the high specific surface area and low nanosheet thickness. A morphologic factor was proposed to represent the ratio of specific surface area to nanosheet thickness, and correlated to the photodegradation efficiency of the BiOI samples. The photocatalytic efficiency increased with increasing morphologic factor. The BiOI photocatalyst exhibited excellent stability and reusability, and has potential in environment remediation.

Sorption of Cr(VI) on Mg–Al–Fe layered double hydroxides synthesized by a mechanochemical method

Mg–Al–Fe layered double hydroxides (LDHs) with a constant Mg2+/(Fe3+ + Al3+) molar ratio but varying Fe3+/(Al3+ + Fe3+) molar ratios (RFe) from 0–1 were synthesized by a mechanochemical method. Sorption of Cr(VI) on the LDHs in aqueous solutions was investigated by a batch technique. The sorption process obeyed pseudo-second-order kinetics, and the sorption equilibrium could be well described with the Freundlich isotherm. The saturated sorption amount increased with increasing RFe, indicating that the replacement of Al by Fe in LDHs is favorable for Cr(VI) sorption. The mechanisms of Cr(VI) sorption included the intercalation of Cr(VI) oxyanions into the LDH gallery, and the surface complexation between dichromate anions and hydroxyl groups of the LDHs. The driving forces of Cr(VI) sorption on LDHs included physical binding (or electrostatic attraction) and chemical binding. The physically sorbed amount of Cr(VI) on the LDHs was almost independent of RFe, while the chemically sorbed amount obviously increased with increasing RFe. That is, the increase of RFe causes the chemical activity of surface sorption sites of the LDHs to increase, resulting in their sorption capacity for Cr(VI) increasing. In addition, the sorption capacity of the LDHs synthesized by the mechanochemical method is comparable with those of LDHs synthesized by coprecipitation and hydrothermal methods. Thus, Mg–Fe LDHs are promising sorbents for treating Cr(VI)-containing wastewater, and the mechanochemical method can be used to synthesize LDH sorbents.

A sorbent concentration-dependent Freundlich isotherm

During study of the adsorption isotherms at solid–liquid interfaces, a sorbent concentration effect (Cs-effect) phenomenon was observed. In order to describe the Cs-effect, we proposed a new adsorption model, i.e., surface component activity (SCA) model. It supposes that the interaction between the sorbent particles exists in the real adsorption system, which induces the deviation of a real adsorption system from an ideal one. It is the deviation that induces the emerging of the Cs-effect. Based on the SCA model, the activity coefficient of adsorption sites should be a function of the sorbent concentration (Cs), and a Cs-dependent Freundlich equation (Freundlich-SCA equation) was derived. It was confirmed that the Freundlich-SCA equation can describe the Cs-effect observed in adsorption experiments. Its parameters (nS and KS) can be simulated with experimental adsorption data and are independent of Cs. Thus, these parameters obtained at given Cs values can be used to predict the adsorption behavior of the adsorbate at any Cs value.

Preparation and photovoltaic properties of CdS quantum dot-sensitized solar cell based on zinc tin mixed metal oxides

The present study reports a new type of quantum dot sensitized solar cells (QDSSCs) using the zinc tin mixed metal oxides (MMO) as the anode materials, which were obtained from the layered double hydroxide (LDH) precursor. The successive ionic layer adsorption and reaction (SILAR) method is applied to deposit CdS quantum dots. The effects of sensitizing cycles on the performance of CdS QDSSC are studied. Scanning electron microscopy (SEM), Transmission electron microscope (TEM) and X-ray diffraction (XRD) are used to identify the surface profile and crystal structure of the mixed metal oxides anode. The photovoltaic performance of the QDSSC is studied by the electrochemical method. The new CdS QDSSC exhibits power conversion efficiency (PCE) up to 0.48% when the anode was sensitized for eight cycles.

Rough Glass Surface-Mediated Transition of Micelle-to-Vesicle in Sodium Dodecylbenzenesulfonate Solutions

In this paper, we report a micelle-to-vesicle transition in aqueous solution of the anionic single-tailed surfactant (STS) sodium dodecylbenzenesulfonate (SDBS), with the mediation of a rough glass surface (RGS) in the absence of cosurfactants or additives. This transition produced a mixed solution of vesicles and micelles. Interestingly, the obtained SDBS vesicles in the solution displayed good stability during a long-term storage (at least 6 months at room temperature), exposure to high temperature (80 °C for 2 h), and freeze-thawing (-20 or -196 °C for 2 h to approximately 25 °C) after the RGS was removed. Our results confirmed that SDBS could adsorb on the RGS to form bilayers, in which the molecular packing parameter of SDBS was in the range of 1/2-1. The bilayer adsorption and the roughness of the solid surface played an important role in the vesicle formation. In addition, we propose a possible mechanism for the RGS-mediated transition of micelle-to-vesicle in SDBS solutions: SDBS micelles and molecules adsorb on the RGS to form curved bilayers; the curved bilayers detach from the RGS, and then close to form vesicles.

Rough glass surface-mediated formation of vesicles from lauryl sulfobetaine micellar solutions.

We report novel vesicles composed of the zwitterionic surfactant lauryl sulfobetaine (LSB), which is a simple single-tailed surfactant (STS). The novel vesicles spontaneously formed from LSB micellar solutions with the mediation of a rough glass surface (RGS) in the absence of any cosurfactants or additives. Importantly, the obtained STS vesicles displayed good stability upon long-term storage, exposure to high temperature, and freeze-thawing after the RGS was removed. The pH of the LSB solution (4.0-9.0) and the presence of NaCl (1.0 × 10(-5) and 1.0 × 10(-4) mol/L) in the LSB solution had no obvious influence on the formation and stability of the vesicles. The adsorption configuration of LSB on the RGS was investigated via water contact angle measurements and atomic force microscope observations. The results showed that LSB adsorption bilayers could form on the RGS, and the bilayer adsorption of LSB on the RGS and the roughness of the solid surface played a key role in the vesicle formation. A possible mechanism for the RGS-mediated formation of LSB vesicles is proposed: LSB micelles and molecules adsorb on the RGS to form curved bilayers, and the curved bilayers are then detached from the RGS and close to form vesicles. To the best of our knowledge, this is the first report of LSB alone forming vesicles. This finding extends our understanding of the nature of vesicle systems.

Thickness-determined photocatalytic performance of bismuth tungstate nanosheets

Bismuth tungstate (Bi2WO6) nanosheets with dominant exposed (010) facets and various thicknesses (H) and lateral sizes were hydrothermally synthesized via pH adjustment of precursor suspensions. As the pH increases from <1 to 8, the resultant nanosheets exhibit improved crystallinity and photoabsorption, decreased specific surface area, increased H, and decreased photoactivity in the degradation of rhodamine B (RhB), methylene blue (MB), and Eosin Y (EY) under visible light irradiation. The photoactivity of the Bi2WO6 sample obtained at pH < 1 is about 6, 100, and 25 times of that at pH 8 for RhB, MB, and EY degradation, respectively. The photoactivity enhancement is ascribed to reduction of the H. The photocatalytic efficiencies are inversely proportional to the reduction of H2 when the nanosheets can be penetrated by incident light. This work reveals the structure–performance relationship of Bi2WO6 nanosheets and provides significant guidance for preparation of high efficient two-dimensional photocatalysts.

Spontaneous vesicle formation and vesicle-to-micelle transition of sodium 2-ketooctanate in water

Single-tailed short-chain alkyl keto-acids/salts, a class of fatty acid/salt derivatives, such as sodium 2-ketooctanate (KOCOONa), are a kind of weakly acid/salt type amphiphiles and plausible prebiotic molecules, and the current understanding of their aggregation behavior in aqueous solutions is still limited. Herein, the aggregation behavior of KOCOONa in aqueous solution was studied by changing its concentration (C), using equilibrium surface tension, conductivity, and fluorescence measurements. The aggregates formed were characterized using freeze-fracture and cryogenic transmission electron microscopy, dynamic light scattering, atomic force microscopy, and confocal laser scanning microscopy. A concentration-driven stepwise aggregation was identified in the KOCOONa solution. Vesicles can spontaneously form from the single-component aqueous solution, with a critical vesicle concentration (CVC) of ∼15mM, which is obviously lower than that of octanoic acid/salt (120-200mM). With increasing C, a vesicle-to-micelle transition can occur, showing a critical micelle concentration (CMC) of ∼80mM. In addition, the membrane permeability of the KOCOONa vesicles was examined using small-size Calcein and large-size FITC-BSA as fluorescence probes, showing a size-selective permeability, similar to short-chain (C8-C11) fatty acid vesicles. For the first time, the aggregation behavior of single-tailed keto-acid salt surfactant is reported.

Formation of simple single-tailed vesicles mediated by lipophilic solid surfaces

Adsorption and aggregation of surfactants at solid-liquid interfaces were fairly well understood, but there was limited knowledge regarding the effect of the presence of a solid surface on aggregate structures in bulk solution. Except for the fatty acid system, most simple single-tailed surfactants (STSs) are well known to form micelles but not vesicles in aqueous solution. Herein, we report a novel phenomenon: with the mediation of lipophilic solid surfaces (LSSs), the zwitterionic STS lauryl sulfobetaine (LSB) formed vesicles from its micellar solution without any additives, producing a mixed solution of vesicles and micelles. More interestingly, the STS vesicles coexisted stably with micelles in the solution and were thermally insensitive even after the removal of LSSs. The quantity of LSB vesicles decreases with the addition of ethanol. The pH effects (4.0-9.0) did not have an obvious influence on the formation and stability of the LSB vesicles. Similar results were obtained from the other STSs, suggesting that the LSS-mediated micelle-to-vesicle transition may be a general phenomenon. We proposed a possible mechanism that adsorption, the matrix effect, and interdigitated bilayer structures were probably crucial for the formation and stability of STS vesicles. We expect this work to provide important insights into the effect of the solid/liquid interface on the self-assembly chemistry of surfactants in bulk solution.

Iron-Doped Bismuth Tungstate with an Excellent Photocatalytic Performance

The metal‐doping of Bi‐based oxide photocatalysts to enhance the photocatalytic performance has been a research hotspot in recent years, but the research on the mechanism of doping‐induced photoactivity improvement is still incomplete. To illustrate the mechanism completely and clearly, Fe‐doped Bi2WO6, as an example, was synthesized by a facile hydrothermal route. Fe‐doped Bi2WO6 exhibits a much higher photoactivity than pure Bi2WO6 in the degradation of Rhodamine B (RhB) and salicylic acid (SA). By replacing W6+, the Fe3+ doping narrows the energy band gap and generates many oxygen vacancies, which can increase the adsorption of both RhB and SA, and the introduced defect and Fe‐doping energy levels in the band gap of Bi2WO6 increase the separation efficiency of photoinduced charge carriers significantly. These lead to the considerable photoactivity enhancement of Fe‐doped Bi2WO6. The photoactivity of Fe‐doped Bi2WO6 is stable and superior to that of most reported modified Bi2WO6 photocatalysts. This work illustrates in detail the mechanism for the photoactivity improvement of Fe‐doped Bi2WO6, which benefits the research of other metal‐doped photocatalysts.