A facile method to produce zeolitic imidazolate frameworks (ZIF-8, ZIF-67, and solid-solution ZIFs (mixed Co and Zn)) is reported. ZIF crystals are produced via a reaction-diffusion framework (RDF) by diffusing an outer solution at a relatively high concentration of the 2-methyl imidazole linker (HmIm) into an agar gel matrix containing the metal ions (zinc(II) and/or cobalt(II)) at room temperature. Accordingly, a propagating supersaturation wave, initiated at the interface between the outer solution and the gel matrix, leads to a precipitation front with a gradient of crystal sizes ranging between 100 nm and 55 μm along the reaction tube. While the precipitation fronts of ZIF-8 and ZIF-67 travel the same distance for the same initial conditions, ZIF-8 crystals therein are consistently smaller than the ZIF-67 crystals due to the disparity of their rate of nucleation and growth. The effects of the temperature, the concentration of the reagents, and the thickness of the gel matrix on the growth of the ZIF crystals are investigated. We also show that by using RDF we can envisage the formation mechanism of the ZIF crystals, which consists of the aggregation of ZIF nanospheres to form the ZIF-8 dodecahedrons. Moreover, using RDF, the formation of a solid-solution ZIF via the incorporation of Co(II) and Zn(II) cations within the same framework is achieved in a controlled manner. Finally, we demonstrate that doping ZIF-8 by Co(II) enhances the photodegradation of methylene blue dye under visible light irradiation in the absence of hydrogen peroxide.

In the first part of this work, we present an experimental study of the precipitation/redissolution reaction-diffusion system of initially separated components in two distinct organic gels: agar and gelatin. The system is prepared by diffusing a concentrated ammonia solution into a gel matrix that contains nickel sulfate. In agar, the system exhibits a pulse propagation due to the concomitant precipitation reaction between Ni(II) and hydroxide ions and redissolution due to ammonia. At a later stage of propagation, a transition to Liesegang banding is shown to take place. The dynamics of the distance traveled by the precipitation pulse, its width, and mass are shown to exhibit power laws. Moreover, the mass of the bands is shown to oscillate in time, indicating the emergence of a complex mass enrichment mechanism of the formed Liesegang bands. At the microscopic level, we show evidence that the system undergoes a continuous polymorphic transition concomitant with a morphological change whereby the solid in the pulse, which consists of nanospheres of α-nickel hydroxide transforms to form the bands, which consists of larger platelets of β-nickel hydroxide. This clearly indicates the existence of a dynamic Ostwald ripening mechanism that underlies the dynamics on both scales. On the other hand, in gelatin, although we can still obtain similar power laws as in the case of agar, no transition to bands was observed. It is shown that in this case, the propagating pulse is made of nanoparticles of α-nickel hydroxide with an average diameter ~50 nm.

Here, the formation of periodic precipitation (Liesegang bands) from hybrid (organic–inorganic) components is reported, namely ZIF‐8, ZIF‐67, and their mixed metal derivatives. The spacing and width laws that characterize the Liesegang system are determined and the resulting pattern is exploited to crystal engineer various sizes and doping of the ZIF material. Several key parameters that govern the crystallization process of ZIF‐8 are investigated in each band and the growth of the particles in each band is found to follow an Ostwald ripening mechanism.

In this paper, we continue the simulation of periodic precipitation systems with redissolution using the model of Müller and Polezhaev (MP). In the first paper of this sequence (J. Phys. Chem. A 2001, 105, 8053), we proposed a reaction scheme involving precipitation and redissolution to which the MP model was adapted. Maps of Liesegang bands and diffusion profiles were calculated at different concentrations of the inner and outer electrolytes. In this paper, we extend the study to the investigation of the effect of an applied constant electric field on the front propagation. Two reaction schemes, representative of the Co(OH) 2 and Cr(OH) 3 precipitation/redissolution systems, are studied. In the former scheme, the band stratum propagates faster, and the spacing between the bands increases with increasing field strength, exactly reproducing the experimental behavior. In the latter, treated under conditions yielding a single propagating band, two opposing trends are observed: at high field, the propagation is slower at higher field strength, whereas at sufficiently low field, the propagation is enhanced with increasing field strength. Wave stopping occurs at the location reached at the same time in a field-free experiment. This results in the field-on curve crossing the field-free curve at the stopping time, just resembling the experimental observation. Other interesting patterning properties are reported and discussed.

This study investigates and compares arsenic, As(V), removal from aqueous media using the water-stable zinc metal−organic frameworks (Zn-MOF-74) prepared via room-temperature precipitation (RT-Zn-MOF-74) and a solvothermal procedure (HT-Zn-MOF-74). The Zn-MOF-74 crystals possess average particle sizes of 66 nm and 144 μm for RT-Zn-MOF-74 and HT-Zn-MOF-74, respectively. Moreover, nanosized RT-Zn-MOF-74 exhibited a superior performance to HT-Zn-MOF-74. While the Brunauer− Emmett−Teller surface area of RT-Zn-MOF-74 was smaller than that of HT-Zn-MOF-74, higher adsorption uptake took place on the room-temperature-synthesized ones because of their small particle size and better dispersion. Adsorption isotherm studies showed that the Langmuir isotherm was effective for the adsorption of As(V) onto RT-Zn-MOF-74 and HT-Zn-MOF-74 with maximum adsorption uptake (q max ) values of 99.0 and 48.7 mg g −1 , respectively. These values exceed most reported maximum adsorption capacities at neutral pH. The thermodynamics of adsorption revealed a spontaneous endothermic process that is due to the substitution of adsorbed water molecules by arsenate in the pores of the MOF crystal. This was further investigated using plane-wave density functional theory calculations. This study constitutes direct evidence for the importance of tuning the size of the MOF crystals to enhance their properties.

We study the kinetics and mechanism of intercalation and de-intercalation of small anions during the formation of crystalline a-Co(OH) 2 and its transformation to b-Co(OH) 2 within a reaction-diffusion framework. We therein use fluorescence spectroscopy with Rhodamine 6G (Rh6G) as a probe as well as other spectroscopic and imaging techniques. The method is based on the reaction and diffusion of hydroxide ions into a gel matrix containing the Co(II) ions, the conjugate anions to be intercalated and Rh6G. The advantage of this simple method is that it allows us to separate throughout space the various stages during the formation of a-Co(OH) 2 and its transformation to b-Co(OH) 2 , thus enabling fluorescence measurements of the those stages by simply focusing on different areas of the tube. It also permits us to extract with ease the solids for characterization and image analysis. The macroscopic evolution of the system, which consists of a leading blue front designating the formation of a-Co(OH) 2 followed by a sharp blue/pink interface designating the transformation to the pink b-Co(OH) 2 , exhibits different dynamics depending on the anion present in the gel. At a certain stage, the blue/pink interface stops its propagation and only the blue front continues. This represents clear evidence of the dependence of the kinetics of intercalation and de-intercalation on the nature of the anion. The coexisting polymorphs were collected and characterized using XRD, FTIR, Raman and UV-Vis. The fluorescence images of the a-Co(OH) 2 reveal clearly the presence of Rh6G between its layers, whereas images from the b polymorph indicate the opposite. Moreover, the fluorescence of Rh6G is monitored during the formation of a-Co(OH) 2 and its conversion to b-Co(OH) 2 . During the formation, the fluorescence intensity and lifetime are significantly increased whereas the opposite happens during the transformation to the b phase. We are able to calculate the activation energies associated with the intercalation and de-intercalation of anions and show using SEM that the polymorphic transformation is accompanied by an Ostwald ripening mechanism whereby the smaller crystals of a-Co(OH) 2 dissolve to reappear as larger crystals of b-Co(OH) 2 . We find that the activation energies of de-intercalation are systematically smaller than those of intercalation. Our results confirm many aspects of the results of Du et al. on the conversion of a chloride-intercalated a-Co(OH) 2 to b-Co(OH) 2 .

There has been a considerable interest in recent years in developing polymer gel matrices for many important applications such as 2DE for quantization and separation of a variety of proteins and drug delivery system to control the release of active agents. However, a well-defined knowledge of the ultrastructures of the gels has been elusive. In this study, we report the characterization of two different polymers used in 2DE: Gelatin, a naturally occurring polymer derived from collagen (protein) and agar, a polymer of polysaccharide (sugar) origin. Low-temperature SEM is used to examine the internal structure of these gels in their frozen natural hydrated states. Results of this study show that both polymers have an array of hollow cells that resembles honeycomb structures. While agar pores are almost circular, the corresponding Gaussian curve is very broad exhibiting a range of radii from nearly 370 to 700 nm. Gelatin pores are smaller and more homogeneous reflecting a narrower distribution from nearly 320 to 650 nm. Overall, these ultrastructural findings could be used to correlate with functions of the polymers.

We report the cosynthesis of highly stable laminated single crystal alpha- and beta-Co(OH) 2 using the reaction and diffusion of a hydroxide solution into a gel containing Co(II). The obtained alpha-Co(OH) 2, which is known to be thermodynamically unstable and transforms in a short period of time to the beta form, has been stabilized in the gel medium for weeks. The system also exhibits Liesegang banding where complicated spatial dynamics during the formation of the two polymorphs are shown to take place.