Comparison of the evolution and growth processes of films of M/Al-layered double hydroxides with M=Ni or Zn

Abstract

Films of layered double hydroxides (M/Al-LDHs) with M=Ni, Zn have been fabricated on a porous anodic alumina/aluminum (PAO/Al) substrate via an in situ crystallization technique. The Ni/Al-LDH film has an orientation in which the ab-faces of the platelets are all perpendicular to the substrate whilst the LDH crystallites in the Zn/Al-LDH film are randomly orientated. Furthermore, the interlayer galleries of Ni/Al-LDH contain ${\mathrm{CO}}_3^{2-}$ anions whereas NO₃⁻ anions were found in the interlayer galleries of Zn/Al-LDH. These differences between the Ni/Al-LDH and Zn/Al-LDH films are, at first sight, surprising because the films were fabricated under identical experimental conditions. Two different mechanisms, homogeneous nucleation and the heterogeneous nucleation, have been proposed in order to account for the different growth processes of the Ni/Al-LDH and Zn/Al-LDH films, respectively. The two distinct growth mechanisms can satisfactorily account for the different crystallite orientations and types of interlayer ion in the M/Al-LDH (M=Ni, Zn) films.

Introduction

There has been considerable recent interest in developing ways for microstructure-controlled fabrication of inorganic films from aqueous solutions at low temperatures in view of their potential applications as sensors, electrodes or high density magnetic storage devices (Niesen and De Guire, 2001). Numerous studies (Chu et al., 2008) have shown that the nucleation and growth processes involved in forming an inorganic film have a significant effect on the orientation of the resulting films (Damman et al., 2002; Adachi et al., 2004). The degree of preferred orientation is greatly dependent on how well the crystallite nucleation is confined to the substrate surface. For example, a large number of experimental and theoretical studies have described the oriented growth of zeolite crystals on various substrates (den Exter et al., 1997; Tavolaro and Drioli, 1999; Hedlund, 2000; Wang and Yan, 2001; Lai et al., 2002; Zhang et al., 2005a, Zhang et al., 2005b). Two main mechanisms—heterogeneous nucleation (Lovallo et al., 1998) and homogeneous nucleation (Li et al., 2004)—have been proposed in the literature to account for the formation of inorganic films.

Layered double hydroxides (LDHs), also known as hydrotalcite-like materials or anionic clays, are one of the most useful classes of inorganic layered compounds and have received considerable attention in the fields of catalysis, separation, and environmental remediation (Leroux and Taviot-Guého, 2005; Evans and Duan, 2006; Li and Duan, 2006; Williams and O’Hare, 2006). Their structure consists of positively charged brucite-like layers of the form [M^II_1−xM^III_x(OH)₂]^x+ with trivalent cations partially substituting for divalent cations. The excess positive charges because of the trivalent cations in the hydroxide layers are compensated by intercalation of anions, usually accompanied by water molecules, in the interlayer galleries. LDHs are thus represented (Rives, 2001; Auerbach et al., 2004; Evans and Slade, 2006; He et al., 2006) by a general formula [MII_1−xM^III_x(OH)₂]^x+(Aⁿ⁻)_x/n·yH₂O, where M^II is a divalent cation such as Mg²⁺, Ni²⁺, Zn²⁺, Cu²⁺, or Mn²⁺; M^III is a trivalent cation such as Al³⁺, Fe³⁺, or Cr³⁺; and Aⁿ⁻ is an interlayer anion such as ${\mathrm{CO}}_3^{2-}$, ${\mathrm{SO}}_4^{2-}$, NO₃⁻, Cl⁻, or OH⁻. As a result, a large class of isostructural materials, which can be considered complementary to aluminosilicate clays, with widely varied physicochemical properties can be obtained by changing the identity of the metal cations, the molar ratios of M²⁺/M³⁺ as well as the interlayer anions. In addition, their high versatility, easily tailored properties and low cost make it possible to produce materials tailored to fulfill specific requirements.

As a result of their anisotropic structure, layered materials such as LDHs tend to form platelet-like crystallites in which the a and b dimensions (parallel to the layers) are much larger than the c dimension (perpendicular to the layers). In films of such crystallites the anisotropic platelets may be arranged in an ordered manner (all platelets parallel to one another) or in a random fashion. Whilst the majority of layered materials can be fabricated in the form of films with control over orientation (Thompson, 1994; Kryszewski, 2000), formation of oriented films of LDHs is more difficult and until recently they have mostly been studied using powder samples (Rives, 2001). Oriented LDH films have great potential for use in a variety of areas including as chemical sensors (Shan et al., 2003, Shan et al., 2004), clay-modified electrodes (He et al., 2001a, He et al., 2001b), corrosion-resistant coatings (Leggat et al., 2002; Buchheit and Guan, 2004), monolithic catalysts, optoelectronic devices, and biological micro-reactors (Li and Duan, 2006) and therefore the development of protocols for the microstructure-controlled oriented immobilization of LDHs for use in devices is an important topic in LDH research (Wang et al., 2007). There are two main approaches for the fabrication of oriented LDH films: one is the colloidal assembly technique (Lee et al., 2003, Lee et al., 2004, Lee et al., 2007; Liu et al., 2006; Wang et al., 2007), and the other is the in situ crystallization technique (Lei et al., 2005; Chen et al., 2006; Gao et al., 2006; Lü et al., 2008; Zhang et al., 2008). The former directly uses colloidal nanocrystals or delaminated nanosheets of LDHs as building blocks which are immobilized on a substrate via various interactions. For example, Lee et al., 2003, Lee et al., 2004, Lee et al., 2007 have obtained both monolayer and multilayer LDH films with a preferred orientation with the c-axis perpendicular to (ab-face parallel) the surface of Si substrates by use of LDH nanocrystals as building blocks. Our group also obtained large continuous transparent and oriented self-supporting LDH films by a simple solvent evaporation method from dilute suspension of monodisperse colloidal nanoparticles (Wang et al., 2007). The delaminated nanosheets can also be used as building blocks for the construction of various functional oriented organic-LDH thin films. For example, Liu et al. (2006) used delaminated CoAl-LDH nanosheets to prepare multilayer nanocomposite films through layer-by-layer self-assembly techniques. All the above oriented LDH films have their ab-face parallel to the substrate owing to the intrinsic propensity of the crystallites to align in an orientation that leads to maximum face-to-face contact between the crystallite and the substrate. The in situ crystallization technique involves direct growth of LDH crystallites on a substrate and LDH films with their crystallite ab-face perpendicular to the substrate surface can be prepared by this method. Recently, we (Chen et al., 2006) have succeeded in preparing Ni/Al-LDH films on a PAO/Al substrate via this technique. We subsequently employed this method to fabricate other types of Al-containing LDH films such as Zn/Al-LDHs (Zhang et al., 2008). Most interestingly, however, we found that the Ni/Al-LDH and Zn/Al-LDH films had both different crystallite orientations and interlayer anions, despite both being fabricated by the same in situ crystallization technique under identical experimental conditions.

Since the performance of LDH films in many cases is dependent on the alignment of the individual crystallites in the films, it is desirable to be able to control the crystallite orientation during synthesis process. An understanding of the mechanism of the growth process of a film should help to provide a means of controlling the orientation of the crystallites. In this paper, in order to understand the oriented growth of LDH crystallites, two types of M/Al-LDH (M=Ni or Zn) films have been fabricated on PAO/Al substrates by the in situ crystallization technique under identical conditions, and detailed investigation of the evolution of the LDH films has been carried out in an attempt to explain the different orientations and interlayer anions found for the LDH films.