Name: synthesis and characterization of fe-doped aluminosilicate nanotubes with enhanced electron conductive properties
Uploaded: 2016-11-15T15:00:00
Description: Watch this Scientific Journal Video about Synthesis and Characterization of Fe-doped Aluminosilicate Nanotubes with Enhanced Electron Conductive Properties at JoVE.com

The authors acknowledge Prof. Claudio Gerbaldi and Nerino Penazzi (Politecnico di Torino) for lending the dry room.

1. Synthesis of 3 g of IMO NTs
<ol>
	<li>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).</li>
	<li>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...

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) phases46,47.
Another crucial point is the fast hyd...

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 synthesized2,3, in addition to being present in nature (mainly in volcanic soils). Imogolite (IMO) is a hydrated aluminosilicate with the formula (OH)3Al2O3SiOH4,5, 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 nm3,5,7. 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 &#176;C. Synthesis at 25 &#176;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 pores10,11 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 water10,11, 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 &#176;C6,14-16 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)3Al2O3Si1-xGexOH19. 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 water28-32.
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 &#34;defective sites&#34;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 (Eg = 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 NTs33,36,38-40.
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)3Al1.975Fe0.025O3SiOH 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 &#950; 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.

<table border="0" cellpadding="0" cellspacing="0" width="628">
	<tbody>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Perchloric Acid (70%) puriss. p.a., ACS reagent, 70% (T)</td>
			<td style="width:95px;">Sigma Aldrich (Fluka)</td>
			<td style="width:120px;">77230</td>
			<td style="width:197px;">Toxic. Use facesheild and respirator filter.</td>
		</tr>
		<tr height="80">
			<td height="80" style="height:80px;width:216px;">Aluminum-tri-sec-butoxide 97%</td>
			<td style="width:95px;">Sigma Aldrich</td>
			<td>201073</td>
			<td style="width:197px;">Skin and eye irritation. Use&#160; eyesheild&#160; and faceshield and respirator filter</td>
		</tr>
		<tr height="80">
			<td height="80" style="height:80px;width:216px;">Tetraethyl orthosilicate&#160;&#160;&#160; (reagent grade 98%)</td>
			<td style="width:95px;">Sigma Aldrich</td>
			<td>131903</td>
			<td style="width:197px;">Toxic, Skin and eye irritation. Use&#160; eye and face shields and respirator filter</td>
		</tr>
		<tr height="60">
			<td height="60" style="height:60px;width:216px;">Iron(III) chloride hexahydrate ACS reagent, 97%</td>
			<td style="width:95px;">Sigma Aldrich</td>
			<td>236489</td>
			<td style="width:197px;">Toxic and corrosive.&#160; Use&#160; eye and face shields and gloves.</td>
		</tr>
		<tr height="60">
			<td height="60" style="height:60px;width:216px;">Orange II Sodium salt for microscopy (Hist.), indicator (pH 11.0-13.0)&#160;</td>
			<td style="width:95px;">Sigma Aldrich&#160;&#160;&#160; (Fluka)</td>
			<td>75370</td>
			<td style="width:197px;">Skin and eye irritation. Use&#160; gloves and dust mask.</td>
		</tr>
	</tbody>
</table>

synthesis and characterization of fe-doped aluminosilicate nanotubes with enhanced electron conductive properties

The goal of the protocol is to synthesize Fe-doped aluminosilicate nanotubes of the imogolite type with the formula (OH)3Al2-xFexO3SiOH. 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&#8226;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 &#176;C, high resolution transmission electron microscopy (HRTEM), diffuse reflectance (DR) UV-Vis spectroscopy, and &#950;-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.

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...

Watch this Scientific Journal Video about Synthesis and Characterization of Fe-doped Aluminosilicate Nanotubes with Enhanced Electron Conductive Properties at JoVE.com

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

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&#8226;6H2O to the mixture containing the Si and Al precursors or by post-synthesis ionic exchange of preformed aluminosilicate nanotubes.

The goal of the protocol is to synthesize Fe-doped aluminosilicate nanotubes of the imogolite type with the formula (OH)3Al2-xFexO3SiOH. Doping with Fe ...

The overall goal of this procedure is to synthesize iron-doped aluminosilicate nanotubes of the imogolite type with defined textural and surface properties and a reduced band gap with respect to proper imogolite. This method allows preparation of iron-doped nanotubes with the maximum iron content of 1.4 weight percent, by either direct synthesis or post-synthesis loading. The main advantages of post-synthesis doping is the simplicity of procedure.As doping occurs by ionic exchange of aluminum in the outer surface of preformed nanotubes. An important consequence of iron-doping is that the imogolite band gap is lowered from 4.9 to 2.8 to 2.4 electron volts, which is particularly relevant for photocatalytic and semi-conductor applications. Surface iron three sides form ducts with anionic azo-dyes, which can be used to investigate electrostatic and ligand interactions at nanotube surfaces.This demonstration of this method is critical as the formation of nanotubes strictly depends on using the proper synthesis conditions. To begin nanotube synthesis, in a dry environment, place 187.7 milliliters of double distilled water in a two liter beaker. Add 1.3 milliliters of 70%perchloric acid to obtain an 80 millimolar solution, and place the mixture on a stir plate.Using a graduated pipette, place eight milliliters of aluminum tri-sec-butoxide in a small beaker. Add 3.8 milliliters of tetraethyl orthosilicate with another graduated pipette, and stir gently until the mixture is clear and colorless. Slight excess of TEOS, with respect to the silicon to aluminum stoichiometry ratio, must be used to prevent the preferential formation of other phases.Add the ATSB TEOS mixture, drop wise to the perchloric acid solution while stirring. Continue stirring for 18 hours to obtain a transparent mixture. Add 1.3 liters of double distilled water to the stirring mixture.Continue stirring for 20 minutes. Place the diluted mixture in a thick-walled PTFE autoclave reactor and heat at 100 degrees Celsius for four days to polymerize the mixture. Dilution of the reaction mixture is critical because condensation of orthosilicic acid hinders the formation of nanotube.The optimum temperature should be between 95 and 100 degree. Decreasing the temperature decrease the formation of nanotubes, whereas increasing the temperature forms impurities. After heating, collect the nanotubes in a 0.02 micron filter.Wash the nanotubes with double distilled water and observe the filtered solid. Dry the nanotubes in an oven at 50 degrees Celsius for one day to obtain imogolite nanotubes as a white powder. To begin post-synthesis loading, disperse 0.25 grams of IMO nanotubes in 15 milliliters of double distilled water.Add 0.025 grams of iron three chloride hexahydrate and stir the mixture for 18 hours. Then add three milliliters of water and 1.5 milliliters of ammonium hydroxide to the reddish-brown mixture to precipitate iron oxo-hydroxide species. Filter the mixture, wash the solids with double distilled water, and dry at 120 degrees Celsius for 48 hours to isolate the iron-L-IMO nanotubes.To begin synthesis of iron-doped nanotubes, in a dry environment, dissolve 0.1 grams of iron three chloride hexahydrate in 190 milliliters of an 80 millimolar perchloric acid solution in double distilled water by stirring. Prepare a mixture of eight milliliters of ATSB and 3.8 milliliters of TEOS using graduated pipettes. And then add the mixture drop wise to the iron solution while stirring.Check that the mixture pH is four and then stir the mixture for 18 hours to obtain a reddish-brown mixture. Add 1.3 liters of double distilled water to the mixture and stir for one hour. Then pour the mixture into a PTFE autoclave reactor.Close the reactor and heat the mixture in an oven at 100 degrees Celsius for four days. Collect the solids, wash with double distilled water, and dry the solids at 50 degrees Celsius overnight to isolate iron 0.70 IMO nanotubes. In a volumetric flask, prepare 200 milliliters of 0.67 millimolar acid orange seven solution in double distilled water.The final solution pH should be 6.8. Fill a cuvette and acquire a transmission UV-Vis spectrum of the dye solution as a starting reference. Determine the intensity of the 484 nanometer band corresponding to the hydrazone tautomer of acid orange seven.For each sample, place 50 milliliters of dye solution in 50 milligrams of isolated nanotubes in a dark bottle and begin stirring the mixture. After five minutes, prepare a tube and centrifuge five milliliters of the mixture at 835 times G for three minutes. Decant the supernatant and determine the intensity of the 484 nanometer band.Leave the sample stirring for three days, centrifuging and analyzing portions of the sample at set times. Monitor the 484 nanometer band intensity to determine the amount of dye adsorbed into the nanotubes over time. Diffuse reflectants UV-Vis spectroscopy indicates isomorphic substitution of iron three plus for octahedral aluminum three plus in all of the iron-doped NTs.Post-synthesis loading favors formation of iron oxo-hydroxide clusters, though the clusters are also seen in NTs synthesized with higher iron content. The surface charge behavior of the iron-doped NTs is similar to undoped IMO NTs, however, the ability to remove an anionic dye from pH 6.8 solution varies. The plateauing behavior in the iron 1.4 and iron 0.7 NTs is attributed to the formation of iron dye adducts.The differences in surface area and volume indicate that post-synthesis loading primarily affects the NT outer surface. The oxo-hydroxide clusters are thus presumed to occupy a large proportion of the iron-L NT surface. Limiting the surface area available to the bulky dye.The band gap decreases in all of the iron-doped NTs. Iron oxo-hydroxide clusters are thought to reduce the band gap of IMO, which is in agreement with the iron-L NTs having the smallest band gap. But the d to d transition from the clusters may make band gap determination more difficult.While attempting this procedure it is important to use a moisture free environment when working with ATSB. By using a dry environment, it is possible to measure the silicon and aluminum precursor with graduated pipette while avoiding ATSB hydrolysis. Following this procedure, the associated behavior of other compounds can be investigated to answer additional questions about their activity of iron-doped nanotubes, such as the photocatalytic activity towards organic pollutants in water.Development of the post-synthesis loading technique open the possibility of exploring substitution of similar cations for aluminum at the imogolite outer surface by simple ionic exchange. After watching this video you should get an understanding how to synthesize and evaluate iron-doped nanotubes with reactive iron three coordinated sites on a surface.

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