Preparation And Properties Of Polyoxometalates-layered Double Hydroxides Ultrathin Films

Layered double hydroxides(LDHs), also known as hydrotalcite,having a structure, which consists of positively charged sheet and negatively charged anions interlayer. Many applications of LDHs in industrial production are attributed to their distinctive structure and properties. Although many researches on LDHs as photo catalyst for the degradation of aqueous organic pollutants have been reported recently, poor visible light photoactivities are usually involved, owing to their low response abilities to the visible light. Polyoxometalates(POMs) are a well-known and significant class of inorganic compounds and currently receiving much attention because of their chemical,structural and electronic versatility. Many important applications of POMs such as catalysis, medicine, biology and materials science have been reported recently. POMs-intercalated layered double hydroxides(LDHs-POMs) are a new type of functional composite materials. The physicochemical properties of the interlayer POMs as well as the positively charged sheet may be improved to a large extent. The convential synthesis methods of LDHs are ion exchange, calcination recovery, one step synthesis, and sol-gel method et al. However, in view of the large volume of POMs such as typical Keggin-type and Dawson-type POMs, it is difficult to synthesis LDHs-POMs with popular methods. In order to solve the problem, we used newly–reported delamination-reassembling method: firstly, LDH monolayer nanosheet is exfoliated from the original LDHs presurors, and then POMs layers were alternately assembled to form LDHs-POMs hybrids by utilizing a layer by layer(LBL) technique.In this paper, three kinds of novel ordered ultra thin films(UTFs), namely, Zn-Cr-POMs-UTFs ? Zn-Al-POMs-UTF ? Zn-Ti-POMs-UTFs were synthesized by using Zn-Cr-NO3- LDH?Zn-Al-NO3- LDH?Zn-Ti-NO3- LDH as precursors, respectively. The UTFs were prepared via the hybrid assembly of exfoliated Zn-Cr-LDH? Zn-Al-LDH? Zn-Ti-LDH monolayer and typical Dawson-type POMs ?-P2W18O626-(P2W18). The UTFs were characterized by X-ray diffraction(XRD), Fourier transform infrared spectra(FT-IR), inductively coupled plasma atomic emission spectrometry(ICP-AES), UV diffuse reflectance spectra(UV/DRS), scanning electron microscopy(SEM). From the results, retention of the structure integrity of the Dawson structures in the hybrid UTFs compositions was confirmed. The thickness of the UTFs were at nano range, and the morphology is uniform. The visible light photocatalytic performances of the UTFs were tested in the degradation of aqueous azo dye Congo red(CR). All the synthesized UTFs showed higher photocatalytic activity than that of the pure P2W18 and the sheet. Among all of the UTFs, Zn-Ti-P2W18-UTFs showed the highest photocatalytic activity, mainly attributed to the improved response ability of P2W18 to the visible light,induced from the strong interaction between the exfoliated Zn-Ti LDH nanosheet and the P2W18. Therfore, a synergetic effect is deduced in the degradation reaction via Zn-Ti-P2W18-UTFs as photocatalyst. In addition, the influence of the p H value of solution, concentration of dyes, the catalyst quality, sedimentary cycle numbers, and the types of the base plate to the photocatalytic activities were investigated.