Synthesis and Characterization of Fe-doped Aluminosilicate Nanotubes with Enhanced Electron Conductive Properties

Summary

Here, we present a protocol to synthesize and characterize Fe-doped aluminosilicate nanotubes. The materials are obtained by either sol-gel synthesis upon addition of FeCl₃•6H₂O to the mixture containing the Si and Al precursors or by post-synthesis ionic exchange of preformed aluminosilicate nanotubes.

Abstract

The goal of the protocol is to synthesize Fe-doped aluminosilicate nanotubes of the imogolite type with the formula (OH)₃Al_2-xFe_xO₃SiOH. Doping with Fe aims at lowering the band gap of imogolite, an insulator with the chemical formula (OH)₃Al₂O₃SiOH, and at modifying its adsorption properties towards azo-dyes, an important class of organic pollutants of both wastewater and groundwater.

Fe-doped nanotubes are obtained in two ways: by direct synthesis, where FeCl₃ is added to an aqueous mixture of the Si and Al precursors, and by post-synthesis loading, where preformed nanotubes are put in contact with a FeCl₃•6H₂O aqueous solution. In both synthesis methods, isomorphic substitution of Al³⁺ by Fe³⁺ occurs, preserving the nanotube structure. Isomorphic substitution is indeed limited to a mass fraction of ~1.0% Fe, since at a higher Fe content (i.e., a mass fraction of 1.4% Fe), Fe₂O₃ clusters form, especially when the loading procedure is adopted. The physicochemical properties of the materials are studied by means of X-ray powder diffraction (XRD), N₂ sorption isotherms at -196 °C, high resolution transmission electron microscopy (HRTEM), diffuse reflectance (DR) UV-Vis spectroscopy, and ζ-potential measurements. The most relevant result is the possibility to replace Al³⁺ ions (located on the outer surface of the nanotubes) by post-synthesis loading on preformed imogolite without perturbing the delicate hydrolysis equilibria occurring during nanotube formation. During the loading procedure, an anionic exchange occurs, where Al³⁺ ions on the outer surface of the nanotubes are replaced by Fe³⁺ ions. In Fe-doped aluminosilicate nanotubes, isomorphic substitution of Al³⁺ by Fe³⁺ is found to affect the band gap of doped imogolite. Nonetheless, Fe³⁺ sites on the outer surface of nanotubes are able to coordinate organic moieties, like the azo-dye Acid Orange 7, through a ligand-displacement mechanism occurring in an aqueous solution.

Introduction

The term nanotube (NT) is universally associated with carbon nanotubes¹, one of the most-studied chemical objects today. Less known is the fact that aluminosilicate NTs can also be synthesized^2,3, in addition to being present in nature (mainly in volcanic soils). Imogolite (IMO) is a hydrated aluminosilicate with the formula (OH)₃Al₂O₃SiOH^4,5, occurring as single-walled NT with Al(OH)Al and Al-O-Al groups on the outer surface and non-interacting silanols (SiOH) on the inner one⁶. Concerning geometry, the length varies from a few nm to several hundred nm^3,5,7. The inner diameter is constant at 1.0 nm⁵, whereas the outer diameter is ~2.0 nm in natural IMO, increasing to 2.5-2.7 nm in samples synthetized at 100 °C. Synthesis at 25 °C yields NTs with outer diameters close to that of natural IMO instead⁸. Recently, it has been shown that NTs with different external diameters may also be obtained by changing the acid used during the synthesis⁹. In the dry powder, IMO NTs assemble in bundles with nearly hexagonal packing (Figure 1). Such an array of NTs gives rise to three kinds of pores^10,11 and related surfaces¹². Besides proper intra-tube A pores (1.0 nm in diameter), smaller B pores (0.3-0.4 nm wide) occur among three aligned NTs within a bundle, and, finally, larger C pores occur as slit-mesopores among bundles (Figure 1). Both chemical composition and pore dimension affect the adsorption properties of the material. The surfaces of A pores are very hydrophilic, as they are lined with SiOH, and are able to interact with vapors and gases like H₂O, NH₃, and CO¹². Because they are small, B pores are hardly accessible, even to small molecules like water^10,11, whereas C pores may interact with larger molecules like phenol⁶ and 1,3,5-triethylbenzene¹². Amara et al. have recently shown that hexagonalization of NTs organized in closely-packed bundles occurs with (imogolite analogue) aluminogermate NTs¹³. This phenomenon, though not observed so far with aluminosilicate NTs, could affect the accessibility of B pores as well.

Interest in IMO-related chemistry has increased recently, partly due to the possibility of changing the composition of both the inner and the outer surface of NTs. The presence of a plethora of hydroxyls renders IMO extremely sensitive to thermal degradation, since dehydroxylation occurs above 300 °C^6,14-16 with consequent NT collapse.

The inner surface may be modified by several methods, including the substitution of Si atoms with Ge atoms¹⁷, which causes the formation of either single- or double-walled¹⁸ NTs with the formula (OH)₃Al₂O₃Si_1-xGe_xOH¹⁹. Post-synthesis grafting of organic functionalities leads to the formation of NTs with the formula (OH)₃Al₂O₃SiO-R, where R is the organic radical²⁰. Through one-pot synthesis in the presence of a Si precursor containing one organic radical directly linked to the Si atom, formation hybrid NTs form, with the formula (OH)₃Al₂O₃Si-R (R = -CH₃, -(CH₂)₃-NH₂)^21,22.

Modification of the outer surface is of the utmost interest for the fabrication of imogolite/polymer composites²³ and involves either electrostatic interactions or covalent bonding. The former method is based on the charge matching between the outer surfaces of the NTs and a proper counter-ion (e.g., octadecylphosphonate)^24,25; the latter method implies a reaction between pre-formed IMO NTs and an organosilane (e.g., 3-aminopropylsilane)²⁶.

In water, electrostatic interactions between IMO and ions are possible due to the following equilibria²⁷

Al(OH)Al + H⁺ = Al(OH₂)⁺Al (1)

SiOH = SiO^– + H⁺ (2)

leading to charged surfaces that have been tested in anion/cation retention from polluted water^28-32.

The present work concerns yet another modification of the outer surface (i.e., the isomorphic substitution of (octahedral) Al³⁺ with Fe³⁺, hereafter referred to as Al³⁺/Fe³⁺ IS). This phenomenon is indeed common in minerals, whereas less is known about Al³⁺/Fe³⁺ IS in IMO NTs.

Concerning doping, the first issue is the total amount of iron that can be hosted by the NTs without causing severe structural strains. A pioneering experimental work on Fe-doped IMO showed that NTs do not form at Fe mass fractions higher than 1.4%³³. Successive theoretical calculations showed that Fe could either isomorphically substitute for Al or create "defective sites"³⁴. Such defects (i.e., iron oxo-hydroxide clusters) were supposed to reduce the band gap of IMO (an electrical insulator)^34,35 from 4.7 eV to 2.0-1.4 eV³⁴. Accordingly, we have recently shown that the presence of Fe³⁺ imparts the solid with new chemical and solid-state properties, lowering the band gap of IMO (E_g = 4.9 eV) to 2.4-2.8 eV³⁶.

A recent report on Fe-doped aluminum-germanate NTs, isostructural with IMO, showed that actual Al³⁺/Fe³⁺ IS is limited to a mass fraction of 1.0% Fe, since the formation of iron oxo-hydroxide particles unavoidably occurs at a higher Fe content due to the natural tendency of Fe to form aggregates³⁷. Similar results were obtained with Fe-doped IMO NTs^33,36,38-40.

From a scientific point of view, the determination of the state of Fe and of its possible reactivity and adsorption properties in Fe-doped IMO is an important issue that requires several characterization techniques.

In this work, we report the synthesis and characterization of Fe-doped IMO. Two samples were synthesized with a mass fraction of 1.4% Fe by either direct synthesis (Fe-x-IMO) or post-synthesis loading (Fe-L-IMO); a third sample with a lower iron content (corresponding to a mass fraction of 0.70%) was obtained through direct synthesis in order to avoid cluster formation and to obtain a material in which mostly Al³⁺/Fe³⁺ IS occurred. In this case, the formation of NTs with the chemical formula (OH)₃Al_1.975Fe_0.025O₃SiOH is expected. Morphological and textural properties of the three Fe-doped IMO are compared to those of proper IMO. In addition, surface properties related to Fe(OH)Al groups are studied in water by measuring the ζ potential and the interaction with the (bulky) anion of the azo-dye Acid Orange 7 (NaAO7), a model molecule of azo-dyes, which are an important class of pollutants of both wastewater and groundwater⁴¹_. AO7^– structure and molecular dimensions are reported in Figure 2a, along with the UV-Vis spectrum (Figure 2b) of a 0.67 mM water solution (natural pH = 6.8). Due to its molecular dimensions⁴², the AO7^– species should mainly interact with the outer surface of NTs, limiting parasitic interactions possibly deriving from diffusion within IMO inner pores, so it can be used as a probe molecule of the outer surface.

Protocol

1. Synthesis of 3 g of IMO NTs In a dry room, prepare an 80 mM HClO4 solution by slowly adding 1.3 ml of perchloric acid with a mass fraction of 70% to 187.7 ml of double-distilled water at room temperature (r.t.). Use a 2,000-ml beaker that will be useful for successive dilutions (step 1.6). In a smaller beaker in the dry room, mix 8 ml of aluminum-tri-sec-butoxide (97%) (ATSB; the source of aluminum)43,44 and 3.8 ml of tetraethyl orthosilicate (98%) (TEOS; the source…

Representative Results

Concerning the synthesis of IMO and Fe-doped IMO NTs, the most relevant issues are i) the formation of NTs, especially during Fe-doping by direct synthesis; ii) the actual environment of Fe species in the final materials; and iii) the effect of Fe on the physicochemical properties of the material, especially its band gap and its adsorption properties. The presence of Fe at the outer surface of NTs is indeed expected to modify the interactions between the NTs and the adsorbate species, esp…

Discussion

In order to be successful, the reported protocol has to be carefully followed, since formation of NTs strictly depends on the synthesis conditions. The following steps are critical: in steps 1.2 and 2.3, a slight excess of TEOS has to be used with respect to the Si/Al stoichiometry ratio (i.e., TEOS:ATBS = 1.1:2). The excess of TEOS prevents the preferential formation of gibbsite (Al(OH)₃) and/or boehmite (AIOOH) phases^46,47.

Another crucial point is the fast hyd…

Materials

Perchloric Acid (70%) puriss. p.a., ACS reagent, 70% (T)	Sigma Aldrich (Fluka)	77230	Toxic. Use facesheild and respirator filter.
Aluminum-tri-sec-butoxide 97%	Sigma Aldrich	201073	Skin and eye irritation. Use  eyesheild  and faceshield and respirator filter
Tetraethyl orthosilicate    (reagent grade 98%)	Sigma Aldrich	131903	Toxic, Skin and eye irritation. Use  eye and face shields and respirator filter
Iron(III) chloride hexahydrate ACS reagent, 97%	Sigma Aldrich	236489	Toxic and corrosive.  Use  eye and face shields and gloves.
Orange II Sodium salt for microscopy (Hist.), indicator (pH 11.0-13.0)	Sigma Aldrich    (Fluka)	75370	Skin and eye irritation. Use  gloves and dust mask.