Studies On The Preparation And Structure Of Zinc Sulfide/Cadmium Sulfide Nanoparticles Intercalated Layered Double Hydroxides

Based on the conception of intercalation chemistry, nanosized ZnS/CdS have been synthesized in the interlayer galleries of Mg-Al layered double hydroxides (LDHs) by a process involving ion-exchange of an Mg₂Al-NO₃ LDH precursor with different metal(Zn, Cd)-acid (citrate, EDTA, NTA)complexes followed by reaction between the intercalated complex anions and H₂S. The supramolecular structure, the growth of the semiconductor particles in the interlayer as well as the relationship between the host (LDHs) and guest (organic acid complex) in each reaction have been systematically investigated. The main results are as follows:1. Intercalation of different metal-acid complexes in MgAI-LDHs were successfully carded out by the classical anion exchange (at atmosphere) and anionic exchange under hydrothermal conditions in antoclave, respectively. The materials have been characterized by elemental analysis, powder X-ray diffraction (XRD), transmission electron microscopy, FT-IR spectroscopy, MAS ¹³C NMR spectroscopy, UV-visible diffuse reflectance spectroscopy and hign resolution transmission electron microscopy (HRTEM) and supramolecular structural models have been proposed. (1) The nitrate anions in the LDH-NO₃ precursor is completely displaced during the classical exchange reactions with [Zn(Cit)]^2-(H₄Cit=citrate). The interlayer anions of [Zn(Cit)]^2- are oriented in monolayer arrangements with its long axis perpendicular to the layers and the pendant carboxylate group of adjacent anions hydrogen bonded to opposite layers. In the monolayer arrangement, altemate Zinc atoms are shifted along the c axis above and below the midpoint of the interlayer galleries. (2) Intercalation of [Zn(NTA)]^- in [MgAl] LDHs was successfully carded out by the anionic exchange under hydrothermal conditions in antoclave with appropriate parameters, such as heating duration(1 hour), temperature(120℃), excess of anion to intercalate (n～1.5), which can avoid the decomposition of the [Zn(NTA)]^- complex in the exchange process. The guest anions are accommodared in a monolayer arrangements with its long axis tilted to the layers and Zinc atoms are located in the midpoint of the interlayer galleries. As a result of the guest anions with bulky volume and one negative charge, it is not possible to balance the layer charge by intercalation of [Zn(NTA)]^- alone. So the [Zn(NTA)]^- and NO₃^- coexist in the interlayers. (3) If the amount of [Zn(EDTA)]^2- complex is low (n＜4), the materials suffer from poor crystallinity and the coexistence of more than one phase in the classical anion exchange. In an attempt to overcome this problem we chose to employ a hydrothermal synthesis using only a two-fold excess of [Zn(EDTA)]^2- complex. The orientation of the guest anion in the interlayer galleries is markedly dependent on layer charge density. The guest anions are positioned in a condensed bilayer of guest species with the minimal dimension of each anion perpendicular to the layers with the Mg:A1～2 and alternate Zinc atoms are shifted along the c axis above and below the midpoint of the interlayer galleries. When the Mg:A1 ratio is varied to 3.0, the guest anions are orientated a monolayer assembly with the maximal dimension of each anion tilted with respect to the hydroxide layers and Zinc atoms are located in the midpoint of the interlayer galleries. In the case of Mg₂Al-Zn(EDTA) LDH, a metastable phase with a similar monolayer structure to that found in Mg₃Al-Zn(EDTA) LDH was detected by XRD in the range 140-300℃. The reversible nature of the structural transformation indicates that a reorientation of the guest [Zn(EDTA)]^2- complex in the interlayer galleries occurs on heating. (4) [Cd(EDTA)]^2- complex was intercalated into LDHs with low charge density by a hydrothermal synthesis, which is impossible to be completed by classical anion exchange process. The arrangement of the guest anions is similar to that of the [Zn(EDTA)]^2- intercalated LDHs with the Mg:A1～3.2. The relatively mobility of guest anions was strengthened in the solution and the layered structure of LDHs was destroyed due to the further aggregation of the small ZnS nanoparticls in the inerlayer. The larger particles of cubic ZnS (5 nm) could be prepared by liquid-liquid reactions between the [Zn(Cit)]^2- intercalated LDH and Na₂S in solution. However, the smaller cubic ZnS (sphalerite) nanoparticles are synthesized by gas-solid reaction in the confined interlayer gallery space of LDHs.(1) The cubic ZnS (sphalerite) was successfully formed in the interlamellar domain rather than on the external surfaces and was co-intercalated with citrate dianions through the gas-solid phase reaction between the [Zn(Cit)]^2- intercalated LDHs and H₂S gas. H₂S diffuses into the interlayer and firstly reacts with the H₂O in the interlayer. Due to the confinement effect of the layers, the small ZnS nanoparticles may only grow in the two dimensional gallery spaces and the semiconductor particles have a disk or plate-like morphology (2 nm). The ZnS nanoparticles have been liberated from the interlayer galleries by treatment with carbonate anions and, free of the constraint imposed by the layers, grow into larger crystallites with an average particle diameter of about 6.0 nm on the external surfaces of the LDH. (2) In the case of the [Zn(EDTA)]^2- or [Zn(NTA)]^- intercalated LDHs, few amount of ZnS nanoparticles is formed in room temperature as result of the high stability of the complexes. However, the samples were heated from room temperature up to 120℃, the reactivity of the complex with the H₂S gas was enhanced and more ZnS nanoparticles with a diameter of about 3.0 nm was formed in the interlayer. (3) The [Cd(EDTA)]^2- intercalated LDHs could be easy to completely react with the H₂S gas under 120℃and the CdS nanoparticles with a plate-like morphology (3 nm) was formed in the interlayer. As the layer charge density increasing, the amount of the [Cd(EDTA)]^2- complex in the interlayer was decreased which lead to a decrease in the particle size of CdS nanoparticles. This method provides a simple approach for the synthesis of semiconductor particles in layered double hydroxide materials.