Tuning the structural properties of cadmium-aluminum layered double hydroxide for enhanced photocatalytic dye degradation.

The distinctive layered structure, chemical stability and tunability of layered double hydroxides (LDHs) have led to extensive investigations in various areas of photocatalysis, including photocatalytic water splitting, carbon dioxide photoreduction, and degradation of organic pollutants. Here, a series of visible light active cadmium-aluminum layered double hydroxides (CdAl LDHs) with various Cd2+ : Al3+ ratios is synthesized via the reaction-diffusion framework (RDF) leading thereby to a hierarchal spherical structure of the LDH. The aim of this study is to develop an optimal CdAl LDH photocatalyst that is activated by solar light irradiation and tested for methylene blue (MB) degradation. The structural and physicochemical properties of the synthesized materials are determined by several imaging and spectroscopic techniques. The photocatalytic study reveals a strong dependence of the photocatalytic activity of the CdAl LDH on the cationic ratio with an optimal performance at a ratio Cd2+ : Al3+ equal to 3 : 1. A mechanism is proposed whereby the activity is ascribed to the formation of intermediate reactive oxidative species (ROS) during the photodegradation reactions and scrutinised by invoking different ROS quenchers and corroborated by density functional theory (DFT) calculations.

Band Propagation, Scaling Laws, and Phase Transition in a Precipitate System III: Effect of the Anions of Precursors.

In this third part of this work, we study the effect of the nature of the anion (nitrate, chloride, bromide, and sulfate) of the nickel salt precursor on the emerging Liesegang pattern in the nickel hydroxide/ammonia system in agar gel. We show that multiple Liesegang patterns, such as direct, irregular, revert, and pulse, can thereby result from such an effect. The microscopic analysis of the solids in the bands reveals that the different anions, present in the gel and previously thought of as spectator ions, interact chemically and variably with the crystalline solid in the leading band, thus affecting drastically the resulting pattern exhibited by the trailing bands. Moreover, we demonstrate that, in these four systems, the hydrotalcite-like α-Ni(OH)2 in the leading band redissolves to form the brucite-like polymorph ß-Ni(OH)2 in the trailing bands via an Ostwald ripening mechanism. The macroscopic features of the various Liesegang patterns in the four anion systems as portrayed by the spacing and width laws are determined for various initial concentrations.

Crystal Growth of ZIF-8, ZIF-67, and Their Mixed-Metal Derivatives.

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.

Reaction-diffusion framework: the mechanism of the polymorphic transition of α- to ß-cobalt hydroxide.

A new and simple method is proposed to explore the mechanism of the intercalation/deintercalation of a variety of anions throughout the formation of α-Co(OH)2 crystals and their polymorphic conversion to ß-Co(OH)2. This method is based on the reaction-diffusion of hydroxide ions in a gel matrix containing the cobalt salt. The spatiotemporal evolution of each polymorph and their interaction is revealed by tracking the location of the two sharp interfaces between the two polymorphs (conversion zone) and between the gel and α-Co(OH)2 (formation zone) and by measuring the weight composition of each zone. We thereby find that the dynamics of the transformation reaction are correctly described by the two-dimensional Avrami-Erofe'ev equation at different temperatures. The data suggest that the structural redistribution of the atoms inside the α-Co(OH)2 particles plays the fundamental role in establishing the overall rate of the reaction. On the other hand, we notice that other factors such as the nature of the intercalated anions and the concentration of the polymer matrix alter considerably the final rate of the transition reaction through increasing the stability of the α phase.

Band propagation, scaling laws and phase transition in a precipitate system. I: Experimental study.

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 ß-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.