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 of Si) in the molar ratio Al:Si = 2:1.1. Use graduated pipettes to measure the volumes of reagents.</li>
	<li>Leave the mixture under mild stirring for one min until a clear and uniform mixture (without any suspended solid particles) is obtained.</li>
	<li>Immediately after, with a Pasteur pipette, dropwise add the whole mixture to the aqueous solution of HClO4 under stirring (the final molar ratios are Si:Al:HClO4 = 1.1:2:1). By adding the mixture to the HClO4 aqueous solution, white clusters form and the pH increases to 5.</li>
	<li>Stir the final mixture at r.t. for about 18 hr, until a transparent solution is obtained.</li>
	<li>Under stirring, add 1.3 L of double-distilled water (measured with a graduated cylinder) to dilute the solution to 20 mM with respect to Al. Stir the obtained 20 mM Al solution for about 20 min.</li>
	<li>Pour the mixture into a polytetrafluoroethylene autoclave (with thick walls) and leave it within a stove for 4 d at 100 &#176;C without stirring.</li>
	<li>After 4 d, filter the clear and transparent solution (use a 0.02 micron filter) to collect the NTs and wash with double-distilled water, obtaining a dense, transparent mixture.</li>
	<li>Dry the mixture in a stove at 50-60 &#176;C for 1 d. The final IMO powder has a white color.</li>
</ol>
2. Synthesis of 3 g Fe-x-IMO NTs (with a mass fraction of either 0.70% or 1.4% Fe)
<ol>
	<li>In a dry room, prepare an 80 mM solution of HClO4 by slowly adding 1.3 ml of perchloric acid with a mass fraction of 70% to 187.7 ml of double-distilled water (pH = 1.0). Use a 2,000-ml beaker that will be useful for successive dilutions (step 2.6).</li>
	<li>Dissolve 0.1 g of FeCl3&#8226;6H2O in the HClO4 acid solution to obtain Fe-0.70-IMO NTs.</li>
	<li>Dropwise add 8 ml of ATSB and 3.8 ml of TEOS into the iron-containing solution. Use graduated pipettes to measure reagent volumes. Check that the pH is equal to 4. Leave the mixture under stirring at r.t. for 18 hr.</li>
	<li>After 18 hr, dilute the resulting solution to 20 mM in Al by adding 1.3 L of double-distilled water (measured with a graduated cylinder) and maintain it under stirring for 1 hr. Afterwards, pour it inside a sealed polytetrafluoroethylene autoclave (with thick walls) and leave it within a stove for 4 d at 100 &#176;C.</li>
	<li>Filter the solution, wash the resulting reddish-brown powder with double-distilled water, and dry it overnight at 50 &#176;C in an oven.</li>
	<li>In order to prepare Fe-1.4-IMO NTs, repeat all the steps with 0.2 g of FeCl3&#8226;6H2O.</li>
</ol>
3. Synthesis of Fe-L-IMO NTs
<ol>
	<li>Disperse 0.25 g IMO in 15 ml of double-distilled water.</li>
	<li>Add 0.025 g FeCl3&#8226;6H2O to the mixture (weight calculated considering a slight excess of iron(III) chloride hexahydrate). Leave under stirring for 18 hr; after 18 hr of stirring, the color of the mixture turns from yellow to reddish-brown, indicating the initial formation of iron oxo/hydroxide species.</li>
	<li>Add 3.0 ml of water and 1.5 ml of NH4OH solution (mass fraction of 33%) to precipitate all Fe3+ species as oxo/hydroxide.</li>
	<li>Filter the mixture, wash the resulting powder with double-distilled water, and dry it in a stove at 120 &#176;C for 48 hr.</li>
</ol>
4. Sample Characterization
<ol>
	<li>Before measuring low-angle X-ray diffraction (XRD) patterns of the sample, mill 100 mg of the powder in an agate mortar, deposit it on a sample holder, and press it with care in order to get a uniform and smooth surface. Instrumental parameters of XRD patterns reported here are detailed in Ref. 36.</li>
	<li>To obtain high resolution electron transmission microscopy (HRTEM) micrographs, mill 10 mg of powder in an agate mortar. In order to obtain a well-dispersed sample for HRTEM inspection, put the milled powder in contact with a Cu grid covered with a Lacey carbon film.
	<ol>
		<li>Remove the excess by gently shaking the grid in order to leave only a few grains electrostatically interacting with the sample holder. Avoid dispersing the powder in a solvent, which could modify the NT arrangement. Instrumental parameters of HRTEM measurements reported here are detailed in Ref. 36 and 39.</li>
	</ol>
	</li>
	<li>To determine the BET SSA (Brunauer-Emmett-Teller Specific Surface Area) and porous volume values reported in Table 1, measure N2 adsorption/desorption isotherms at -196 &#176;C. Before measurement, outgas the samples at 250 &#176;C in order to remove water and other atmospheric contaminants10 while still preserving NTs6,14-16. The instrumental details are reported in Ref. 39.</li>
	<li>Outgas the powder in a UV-Vis quartz cell connected to a standard vacuum frame (residual pressure below 10-3 mbar) and take its diffuse reflectance (DR) UV-Vis spectrum. The instrumental parameters of DR-UV-Vis spectra reported here are detailed in Ref. 36. 
	NOTE: Experimental and instrumental details concerning electrophoretic mobility measurements are reported in Ref. 39.</li>
	<li>NaAO7 adsorption experiments
	<ol>
		<li>Prepare 200 ml of 0.67 mM NaAO7 solution by adding double-distilled water to 0.047 g NaAO7 in a volumetric flask. The solution pH should be 6.80.</li>
		<li>Pour 50 ml of the solution inside a dark bottle and add 50 mg of IMO (powder concentration 1 g/L). Keep the solution under stirring during the experiment. Repeat this step with the other powders (powder concentration 1 g/L).</li>
		<li>At regular time intervals (t: 0 sec, 5 min, 10 min, 45 min, 2 hr, 5 hr, 24 hr, and 72 hr), recover 5 ml of the supernatant by centrifugation at 835 x g for 3 min.</li>
		<li>Analyze the supernatant by transmission UV-Vis spectroscopy in a 1-mm path cuvette. In water, AO7- undergoes azo-hydrazone tautomerism, whereas the hydrazone form is stable in the solid phase, as shown by the UV-Vis spectrum in Figure 2b. Determine the amount of AO7- removed from the solution by measuring the decrease of the 484 nm band intensity of the hydrazone form, according to the literature38,39,41.</li>
	</ol>
	</li>
</ol>

The authors have nothing to disclose.

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 hydrolysis of ATBS. To prevent this, a moisture-free environment is needed (e.g., the dry room adopted in this work). In a dry environment, it is possible to measure the volumes of both TEOS and ATBS using graduated pipettes, avoiding ATBS hydrolysis that would lead to an unsuccessful synthesis.
Another crucial point is the dilution in step 1.6. At higher reagent concentrations, condensation of orthosilicic acid would hinder the formation of NTs.
During polymerization, the temperature has to be controlled carefully. The temperature of polymerization during step 1.7 should not exceed 100 &#176;C. To the best of our knowledge, the optimum range of temperature for polymerization in order to obtain a high yield of pure NTs is 95-100 &#176;C. At lower temperatures, the NT formation rate decreases, whereas at higher temperatures, other impurities (e.g., aluminum oxides) form48,49. The use of a thermostat would be the best solution, but measuring the temperature close to the autoclave within the stove may be enough, as done in this work.
The main limitation of the synthesis protocol is that NTs do not form at mass fractions higher than 1.4% Fe, as reported in the literature by authors using a slightly different protocol33. This can be due to some structural strains induced by Fe in this type of structure. The second major limitation is the degree of Al3+/Fe3+ IS that can be reached, corresponding to a maximum mass fraction of 1.0% Fe. It has to be noted, however, that the same was also observed with Fe-doped aluminogermanate NTs37.
On the contrary, the occurrence of IS by post-synthesis loading is an interesting and encouraging result, likely related to the fact that Al(OH)Al groups on the outer surface of NTs are able to undergo ionic exchanges in solution, according to reaction (4):
[Al(OH) Al](s) + Fe(H2O)6(aq)3+ = [Fe(OH) Al](s) + Al(H2O)6(aq)3+ (4)
This result is particularly relevant and shows that the proposed protocol has important outcomes, since it opens the possibility of changing the composition of the IMO outer surface by ionic exchange, thus avoiding more complicated procedures (i.e., direct synthesis). As mentioned before, the synthesis of IMO requires several precautions in order to be successful, and the mere addition of another reagent-in the present case, FeCl3&#8226;6H2O-will perturb the synthesis environment. It is indeed simpler to add the Fe precursor to an aqueous solution of preformed NTs, as done for the Fe-L-IMO sample. The same procedure could be extended to other cations with proper charges and radii, like Cr3+ and Ti3+. In the case of Ti3+, however, there could be some limitations due to the stability of the Ti3+ species and of its precursor.
Another important consequence of the successful doping procedure is the lowering of the IMO band gap. This result is particularly relevant if applications involving semiconductors are concerned, such as photocatalysis. Moreover, the presence of reactive Fe3+ surface species could be exploited in a photo-Fenton reaction for the removal of organic pollutants from water.
The formation of Fe(OH)Al groups as a consequence of Al3+/Fe3+ IS provides Fe3+ sites that are accessible to species able to coordinate iron in water, as observed during the AO7- adsorption experiments reported here. This concept could be extended to the retention of other organic pollutants, making it possible to exploit the outer surface of NTs in adsorption processes involving not only mere electrostatic interaction, but ligand displacement as well.
The worst performance of Fe-L-IMO towards AO7- adsorption is ascribable to the occurrence of a larger fraction of clusters. After the addition of ammonia during loading, cluster formation mostly occurs on the outer surfaces of NTs, as confirmed by the pore volumes, reported in Table 1. There is an increase in the total pore volume by loading, whereas the micropore volume, amenable to type A pores, remains unchanged. IS Fe3+ sites likely acted as crystallization centers for iron oxo-hydroxide clusters. Due to cluster formation, IS Fe3+ sites were no more accessible to AO7- species, finally lowering the adsorption capacity of Fe-L-IMO towards the dye39.

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><tbody><tr><td>Perchloric Acid (70%) puriss. p.a., ACS reagent, 70% (T)</td><td>Sigma Aldrich (Fluka)</td><td>77230</td><td>Toxic. Use face shield and respirator filter.</td></tr><tr><td>Aluminum-tri-&lt;em&gt;sec&lt;/em&gt;-butoxide 97%</td><td>Sigma Aldrich</td><td>201073</td><td>Skin and eye irritation. Use eye and face shields and respirator filter.</td></tr><tr><td>Tetraethyl orthosilicate (reagent grade 98%)</td><td>Sigma Aldrich</td><td>131903</td><td>Toxic. Skin and eye irritation. Use eye and face shields and respirator filter.</td></tr><tr><td>Iron(III) chloride hexahydrate ACS reagent, 97%</td><td>Sigma Aldrich</td><td>236489</td><td>Toxic and corrosive. Use eye and face shields and gloves.</td></tr><tr><td>Orange II Sodium salt for microscopy (Hist.), indicator (pH 11.0-13.0)</td><td>Sigma Aldrich (Fluka)</td><td>75370</td><td>Skin and eye irritation. Use 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, especially in water solutions. The above aspects have to be assessed by multiple characterization techniques. DR-UV-Vis spectroscopy is used to assess the coordination of iron species in doped samples, as well as the presence of isolated Fe3+ sites and/or iron oxo-hydroxide clusters. &#950;-potential measurements allow studies of the surface charge of the samples in an aqueous environment and adsorption of AO7- to assess the behavior of the material towards an azo-dye used to probe the NT external surface.
The successful synthesis of NTs is documented by XRD patterns and HRTEM analyses. All the samples showed the typical XRD pattern ascribable to NTs organized in a hexagonal array (Figure 3a)43. The main peak corresponds to the d100 reflection, from which the cell parameter, which corresponds to the center-to-center distance between two aligned NTs in a hexagonal packing (Figure 1), is calculated as a = 2d100/&#8730;3. The d100 peak is in the same position (2&#920; = 3.88&#176;) with both IMO and Fe-L-IMO, whereas it shifts to slightly higher angles with samples prepared by direct synthesis, leading to a decrease in the corresponding values of both d100 and a. This phenomenon was ascribed to the replacement of (bulkier) ClO4- ions, present in the synthesis batch of IMO16, by (smaller) Cl- ions deriving from the Fe precursor, with a consequent decrease of the interspace between adjacent NTs39. Formation of NTs is confirmed by HRTEM analysis; the reported micrographs concerning the Fe-0.70-IMO sample show a bundle of NTs (Figure 3b) in a hexagonal array (Figure 3c). N2 isotherms (not reported) allowed for the measurement of BET SSA and pore volumes (Table 1). As a whole, the presence of Fe leads to an increase in surface area. The sample obtained by loading has a larger total volume; the difference with respect to micropore volume, mainly related to A pores, indicates that the loading procedure mainly affected the outer surface of NTs.
The state of Fe is studied by means of DR-UV-Vis spectra in Figure 4a. Similar curves are seen for the Fe-containing samples, whereas IMO (a white powder) weakly absorbs in the UV-Vis range. Both Fe-0.70-IMO and Fe-1.4-IMO mainly absorb at 270 nm; a minor absorption at 480 nm is clearly visible with Fe-1.4-IMO, but it is almost negligible with Fe-0.70-IMO. Such results indicate the occurrence of Al3+/Fe3+ IS with both samples, since the band at 270 nm is due to charge-transfer transitions (CT) from O2- to isolated octahedral Fe3+ sites, and of the formation of iron oxo-hydroxide clusters at high Fe contents, with the 480 nm band occurring due to d-d transitions of Fe2O3 clusters36,39. The spectrum of Fe-L-IMO, similar to that of Fe-1.4-IMO, is slightly shifted towards higher wavelengths and is more intense in the d-d transition range. Such results indicate that iron oxo-hydroxide cluster formation is favored by post-synthesis exchange, although Al3+/Fe3+ IS also occurs.
The Tauc&#39;s plot in Figure 4b shows that IMO has a band gap Eg = 4.9 eV, in agreement with the calculated value (4.6 eV)34. Doping with Fe brings about a significant decrease of the band gap. With the Fe-0.70-IMO sample, where mostly IS Fe3+ species occur, Eg is 2.8 eV, indicating that Fe-doping has the effect of lowering the band gap since the sample is approaching semi-conductor behavior. With Fe-L-IMO, an even lower band gap is measured (Eg = 2.4 eV), although the presence of iron oxo-hydroxide clusters hampers a more precise determination of Eg in this sample.
The samples&#39; behavior in water is investigated by means of &#950;-potential measurements and adsorption of AO7- from an aqueous solution at pH = 6.8. Since IR spectra reported elsewhere39 did not reveal relevant differences between IMO and Fe-0.70-IMO (due to the low Fe content), only the samples with mass fractions of 1.4% Fe were considered. The &#950;-potential curve of IMO (Figure 5a) shows that it is positively charged at low pH values, with a point of zero charge (PZC) at pH = 9.8 (i.e., very close to that of alumina)44,45. Fe-doped samples show a very similar behavior: the net difference in both surface charge and PZC among the samples is not relevant. As a whole, the samples are positively charged at low pH and negatively charged at high pH; therefore, all of them should be able to adsorb anions and cations above and below their PZC, respectively.
AO7- adsorption results are reported in Figure 5b as percentage of dye adsorbed as a function of time. The best performance is given by Fe-1.4-IMO, followed by IMO, indicating that the main process is an electrostatic attraction between AO7- anions and the positively-charged outer surfaces of the samples (the solution pH was 6.8).
However, some important differences are observed: Fe-1.4-IMO gives the best performance in terms of AO7- removal, with a steep increase in the dye adsorption in the very first minutes (accompanied by a pH drop, as reported in Ref. 39). The same occurs, to a minor extent, with Fe-0.70-IMO, because reaction (3) takes place:
Fe(OH)2+Al + AO7- = FeAO7- + Al-OH + H+ (3)
which implies the formation of FeAO7- adducts through a ligand displacement phenomenon, with N atoms of the dye coordinating IS Fe3+ sites.
<img alt="Figure 1" src="/files/ftp_upload/54758/54758fig1.jpg" /> 
Figure 1. Representation of two bundles of IMO NTs in hexagonal packing. When occurring in powder form, IMO NTs are packed in bundles having nearly hexagonal symmetry. The cell parameter a evidenced in the figure corresponds to the center-to-center distance between two aligned NTs within a hexagonal bundle. Pores A, B, and C correspond to proper IMO nanopores (~1.0 nm wide), nanopores among three aligned NTs (~0.30-0.40 nm wide), and slit mesopores among bundles, respectively. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54758/54758fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54758/54758fig2.jpg" /> 
Figure 2. The azo-dye Acid Orange 7 in water: azo-hydrazone tautomerism and UV-Vis spectrum. Part (a) reports the chemical formula and molecular dimensions42 of the dye along with its azo- and hydrazone forms, both present in water solution due to tautomerism. Part (b) reports the UV-Vis spectrum of the initial 0.67 mM solution of the dye used for the adsorption experiments39. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54758/54758fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54758/54758fig3.jpg" /> 
Figure 3. Textural characterization of the synthesized powder samples. Part (a) reports low-angle XRD patterns of IMO (curve 1), Fe-L-IMO (curve 2), Fe-0.70-IMO (curve 3), and Fe-1.4-IMO (curve 4). Parts (b) and (c) refer to sample Fe-0.70-IMO; a selected HRTEM micrograph of the powder sample is reported (b), along with a magnification of the frontal view of a bundle with NTs forming a hexagonal array (c). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54758/54758fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/54758/54758fig4.jpg" /> 
Figure 4. DR-UV-Vis spectroscopic characterization of the synthesized powder samples. Part (a) reports the DR-UV-Vis spectra of IMO (curve 1), Fe-0.70-IMO (curve 2), Fe-1.4-IMO (curve 3), and Fe-L-IMO (curve 4). Part (b) reports the corresponding Tauc&#39;s plots, from which the band gap values (Eg, eV) reported in Table 1 were determined. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54758/54758fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/54758/54758fig5.jpg" /> 
Figure 5. Measurements of the samples&#39; surface charges and Acid Orange 7 adsorption experiments. Part (a) reports the &#950;-potential curves of IMO (squares), Fe-1.4-IMO (triangles), and Fe-L-IMO (circles); error bars correspond to 10% of the measured value, according to previous works38,39. Part (b) reports the percentage of AO7- removed versus time for IMO (squares), Fe-0.70-IMO (stars), Fe-1.4-IMO (triangles), and Fe-L-IMO (circles). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54758/54758fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Sample</td>
			<td>Fe, wt%</td>
			<td>BET SSA (m2 g-1)</td>
			<td>Total Volume (cm3 g-1)</td>
			<td>Micropore Volume (cm3 g-1)</td>
			<td>d100 (nm &#177;0.01)</td>
			<td>a (nm)</td>
			<td>Band gap (eV)</td>
			<td>PZC</td>
			<td>AO7- removed&#160;</td>
		</tr>
		<tr>
			<td>IMO</td>
			<td>-</td>
			<td>383</td>
			<td>0.21</td>
			<td>0.13</td>
			<td>2.27</td>
			<td>2.62</td>
			<td>4.9</td>
			<td>9.8</td>
			<td>95%</td>
		</tr>
		<tr>
			<td>Fe-L-IMO</td>
			<td>1.4</td>
			<td>400</td>
			<td>0.27</td>
			<td>0.13</td>
			<td>2.27</td>
			<td>2.62</td>
			<td>2.4</td>
			<td>10.0</td>
			<td>30%</td>
		</tr>
		<tr>
			<td>Fe-0.70-IMO</td>
			<td>0.70</td>
			<td>450</td>
			<td>0.22</td>
			<td>0.15</td>
			<td>2.19</td>
			<td>2.53</td>
			<td>2.8</td>
			<td>-</td>
			<td>37%</td>
		</tr>
		<tr>
			<td>Fe-1.4-IMO</td>
			<td>1.4</td>
			<td>455</td>
			<td>0.22</td>
			<td>0.14</td>
			<td>2.17</td>
			<td>2.51</td>
			<td>2.8</td>
			<td>10.4</td>
			<td>96%</td>
		</tr>
	</tbody>
</table>
Table 1. Relevant properties of the samples, as determined by N2 isotherms at -196 &#176;C, XRD patterns, DR-UV-Vis spectroscopy, &#950;-potential measurements, and adsorption experiments with AO7- water solutions.

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 ...

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Nanyang Technological University

Research

JoVE Journal

Chemistry

9.2K Views.  Politecnico di Torino.  Hier presenteren we een protocol te synthetiseren en karakteriseren Fe gedoteerde aluminosilicaat nanobuizen. De materialen worden verkregen met één sol-gel-synthese na toevoeging van FeCl3 6H • 2 O het mengsel dat het Si en Al precursors of post-synthese ionenuitwisseling voorgevormde aluminosilicaat nanobuisjes. 

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

Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) membranes have been under development since the 1990's and have become a common method to template the synthesis of high aspect ratio nanostructures, mostly by electrochemical growth or pore-wetting. Recently, these membranes have become commercially available in a wide range of pore sizes and densities, leading to an extensive library of functional nanostructures being synthesized from AAO membranes. These include composite nanorods, nanowires and nanotubes made of metals, inorganic materials or polymers 3-10. Nanoporous membranes have been used to synthesize nanoparticle and nanotube arrays that perform well as refractive index sensors, plasmonic biosensors, or surface enhanced Raman spectroscopy (SERS) substrates 11-16, as well as a wide range of other fields such as photo-thermal heating 17, permselective transport 18, 19, catalysis 20, microfluidics 21, and electrochemical sensing 22, 23. Here, we report a novel procedure to prepare gold nanotubes in AAO membranes. Hollow nanostructures have potential application in plasmonic and SERS sensing, and we anticipate these gold nanotubes will allow for high sensitivity and strong plasmon signals, arising from decreased material dampening 15.

Solution-suspendable gold nanotubes with controlled dimensions can be synthesized by electrochemical deposition in porous anodic aluminum oxide (AAO) membranes using a hydrophobic polymer core. Gold nanotubes and nanotube arrays hold promise for applications in plasmonic biosensing, surface-enhanced Raman spectroscopy, photo-thermal heating, ionic and molecular transport, microfluidics, catalysis and electrochemical sensing.

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) ...

Synthesis and Functionalization of Nitrogen-doped Carbon Nanotube Cups with Gold Nanoparticles as Cork Stoppers

Nitrogen-doped carbon nanotubes consist of many cup-shaped graphitic compartments termed as nitrogen-doped carbon nanotube cups (NCNCs). These as-synthesized graphitic nanocups from chemical vapor deposition (CVD) method were stacked in a head-to-tail fashion held only through noncovalent interactions. Individual NCNCs can be isolated out of their stacking structure through a series of chemical and physical separation processes. First, as-synthesized NCNCs were oxidized in a mixture of strong acids to introduce oxygen-containing defects on the graphitic walls. The oxidized NCNCs were then processed using high-intensity probe-tip sonication which effectively separated the stacked NCNCs into individual graphitic nanocups. Owing to their abundant oxygen and nitrogen surface functionalities, the resulted individual NCNCs are highly hydrophilic and can be effectively functionalized with gold nanoparticles (GNPs), which preferentially fit in the opening of the cups as cork stoppers. These graphitic nanocups corked with GNPs may find promising applications as nanoscale containers and drug carriers.

We discussed the synthesis of individual graphitic nanocups using a series of techniques including chemical vapor deposition, acid oxidation and probe-tip sonication. By citrate reduction of HAuCl4, the graphitic nanocups were effectively corked with gold nanoparticles due to the chemically reactive edges of the cups.

Nitrogen-doped carbon  nanotubes consist of many cup-shaped graphitic compartments termed as  nitrogen-doped carbon nanotube cups (NCNCs).  These ...

Engineering

Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

In recent years &pi;-conjugated organic semiconductors have emerged as the active material in a number of diverse applications including large-area, low-cost displays, photovoltaics, printable and flexible electronics and organic spin valves. Organics allow (a) low-cost, low-temperature processing and (b) molecular-level design of electronic, optical and spin transport characteristics. Such features are not readily available for mainstream inorganic semiconductors, which have enabled organics to carve a niche in the silicon-dominated electronics market. The first generation of organic-based devices has focused on thin film geometries, grown by physical vapor deposition or solution processing. However, it has been realized that organic nanostructures can be used to enhance performance of above-mentioned applications and significant effort has been invested in exploring methods for organic nanostructure fabrication.	
A particularly interesting class of organic nanostructures is the one in which vertically oriented organic nanowires, nanorods or nanotubes are organized in a well-regimented, high-density array. Such structures are highly versatile and are ideal morphological architectures for various applications such as chemical sensors, split-dipole nanoantennas, photovoltaic devices with radially heterostructured "core-shell" nanowires, and memory devices with a cross-point geometry. Such architecture is generally realized by a template-directed approach. In the past this method has been used to grow metal and inorganic semiconductor nanowire arrays. More recently &pi;-conjugated polymer nanowires have been grown within nanoporous templates. However, these approaches have had limited success in growing nanowires of technologically important &pi;-conjugated small molecular weight organics, such as tris-8-hydroxyquinoline aluminum (Alq3), rubrene and methanofullerenes, which are commonly used in diverse areas including organic displays, photovoltaics, thin film transistors and spintronics.	
Recently we have been able to address the above-mentioned issue by employing a novel "centrifugation-assisted" approach. This method therefore broadens the spectrum of organic materials that can be patterned in a vertically ordered nanowire array. Due to the technological importance of Alq3, rubrene and methanofullerenes, our method can be used to explore how the nanostructuring of these materials affects the performance of aforementioned organic devices. The purpose of this article is to describe the technical details of the above-mentioned protocol, demonstrate how this process can be extended to grow small-molecular organic nanowires on arbitrary substrates and finally, to discuss the critical steps, limitations, possible modifications, trouble-shooting and future applications.

We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

In  recent years &pi;-conjugated  organic semiconductors have emerged as the active material in a number of  diverse applications including large-area, ...

Preparation and Evaluation of Hybrid Composites of Chemical Fuel and Multi-walled Carbon Nanotubes in the Study of Thermopower Waves

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs along the interface between the fuel and core materials. Simultaneously, dynamic changes in thermal and chemical potentials across the micro/nanostructured materials result in concomitant electrical energy generation induced by charge transfer in the form of a high-output voltage pulse. We demonstrate the entire procedure of a thermopower wave experiment, from synthesis to evaluation. Thermal chemical vapor deposition and the wet impregnation process are respectively employed for the synthesis of a multi-walled carbon nanotube array and a hybrid composite of picric acid/sodium azide/multi-walled carbon nanotubes. The prepared hybrid composites are used to fabricate a thermopower wave generator with connecting electrodes. The combustion of the hybrid composite is initiated by laser heating or Joule-heating, and the corresponding combustion propagation, direct electrical energy generation, and real-time temperature changes are measured using a high-speed microscopy system, an oscilloscope, and an optical pyrometer, respectively. Furthermore, the crucial strategies to be adopted in the synthesis of hybrid composite and initiation of their combustion that enhance the overall thermopower wave energy transfer are proposed.

A protocol for conducting thermopower wave experiments is presented. The synthesis of hybrid composites of a chemical fuel and micro/nanostructured material, manufacturing of a thermopower wave generator, and methods for measuring the corresponding physical phenomena are described.

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs ...

Facile Preparation of Ultrafine Aluminum Hydroxide Particles with or without Mesoporous MCM-41 in Ambient Environments

An aqueous suspension of nanogibbsite was synthesized via the titration of aluminum aqua acid [Al(H2O)6]3+ with L-arginine to pH 4.6. Since the hydrolysis of aqueous aluminum salts is known to produce a wide array of products with a wide range of size distributions, a variety of state-of-the-art instruments (i.e., 27Al/1H NMR, FTIR, ICP-OES, TEM-EDX, XPS, XRD, and BET) were used to characterize the synthesis products and identification of byproducts. The product, which was comprised of nanoparticles (10-30 nm), was isolated using gel permeation chromatography (GPC) column technique. Fourier transform infrared (FTIR) spectroscopy and powder X-ray diffraction (PXRD) identified the purified material as the gibbsite polymorph of aluminum hydroxide. The addition of inorganic salts (e.g., NaCl) induced electrostatic destabilization of the suspension, thereby agglomerating the nanoparticles to yield Al(OH)3 precipitate with large particle sizes. By utilizing the novel synthetic method described here, Al(OH)3 was partially loaded inside the highly ordered mesoporous framework of MCM-41, with average pore dimensions of 2.7 nm, producing an aluminosilicate material with both octahedral and tetrahedral Al (Oh/Td = 1.4). The total Al content, measured using energy-dispersive X-ray spectrometry (EDX), was 11% w/w with a Si/Al molar ratio of 2.9. A comparison of bulk EDX with surface X-ray photoelectron spectroscopy (XPS) elemental analysis provided insight into the distribution of Al within the aluminosilicate material. Furthermore, a higher ratio of Si/Al was observed on the external surface (3.6) as compared to the bulk (2.9). Approximations of O/Al ratios suggest a higher concentration of Al(O)3 and Al(O)4 groups near the core and external surface, respectively. The newly developed synthesis of Al-MCM-41 yields a relatively high Al content while maintaining the integrity of the ordered silica framework and can be used for applications where hydrated or anhydrous Al2O3 nanoparticles are advantageous.

An ultrafine aluminum hydroxide nanoparticle suspension was prepared via the controlled titration of [Al(H2O)]3+ with L-arginine to pH 4.6 with and without cage-effect confinement within mesoporous channels of MCM-41.

An aqueous suspension of nanogibbsite was synthesized via the titration of aluminum aqua acid [Al(H2O)6]3+ with L-arginine to pH 4.6. Since the hydrolysis ...

Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning electron microscope (ESEM) operating with a low pressure water vapor ambient. In this technique, a portion of the electron beam interacts with the water vapor in the vicinity of the CNT sample, dissociating the water molecules into hydroxyl radicals and other species by radiolysis. The remainder of the electron beam interacts with the CNT forest sample, making it susceptible to oxidation from the chemical products of radiolysis. This technique may be used to trim a selected region of an individual CNT, or it may be used to remove hundreds of cubic microns of material by adjusting ESEM parameters. The machining resolution is similar to the imaging resolution of the ESEM itself. The technique produces only small quantities of carbon residue along the boundaries of the cutting zone, with minimal effect on the native structural morphology of the CNT forest.

Low pressure scanning electron microscopy in a water vapor ambient is used to machine nanoscale to microscale features in carbon nanotube forests.

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning ...

Electrospinning of Photocatalytic Electrodes for Dye-sensitized Solar Cells

This work demonstrates a protocol to fabricate a fiber-based photoanode for dye-sensitized solar cells, consisting of a light-scattering layer made of electrospun titanium dioxide nanofibers (TiO2-NFs) on top of a blocking layer made of commercially available titanium dioxide nanoparticles (TiO2-NPs). This is achieved by first electrospinning a solution of titanium (IV) butoxide, polyvinylpyrrolidone (PVP), and glacial acetic acid in ethanol to obtain composite PVP/TiO2 nanofibers. These are then calcined at 500 &#176;C to remove the PVP and to obtain pure anatase-phase titania nanofibers. This material is characterized using scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The photoanode is prepared by first creating a blocking layer through the deposition of a TiO2-NPs/terpineol slurry on a fluorine-doped tin oxide (FTO) glass slide using doctor blading techniques. A subsequent thermal treatment is performed at 500 &#176;C. Then, the light-scattering layer is formed by depositing a TiO2-NFs/terpineol slurry on the same slide, using the same technique, and calcinating again at 500 &#176;C. The performance of the photoanode is tested by fabricating a dye-sensitized solar cell and measuring its efficiency through J-V curves under a range of incident light densities, from 0.25-1 Sun.

The overall goal of this project was to use electrospinning to fabricate a photoanode with improved performance for dye-sensitized solar cells.

This work demonstrates a protocol to fabricate a fiber-based photoanode for dye-sensitized solar cells, consisting of a light-scattering layer made of ...

Growth of Gold Dendritic Nanoforests on Titanium Nitride-coated Silicon Substrates

In this study, a high-power impulse magnetron sputtering system is used to coat a flat and firm titanium nitride (TiN) film on silicon (Si) wafers, and a fluoride-assisted galvanic replacement reaction (FAGRR) is employed for the rapid and easy deposition of gold dendritic nanoforests (Au DNFs) on the TiN/Si substrates. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy patterns of TiN/Si and Au DNFs/TiN/Si samples validate that the synthesis process is accurately controlled. Under the reaction conditions in this study, the thickness of the Au DNFs increases linearly to 5.10 &#177; 0.20 &#181;m within 15 min of the reaction. Therefore, the employed synthesis procedure is a simple and rapid approach for preparing Au DNFs/TiN/Si composites.

This study presents a feasible procedure for synthesizing gold dendritic nanoforests on titanium nitride/silicon substrates. The thickness of gold dendritic nanoforests increases linearly within 15 min of a synthesis reaction.

In this study, a high-power impulse magnetron sputtering system is used to coat a flat and firm titanium nitride (TiN) film on silicon (Si) wafers, and a ...

Iron Nanowire Fabrication by Nano-Porous Anodized Aluminum and its Characterization

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and data storage. We demonstrate a fabrication method for iron (Fe) nanowires&#160;via electrochemical deposition into anodic&#160;alumina oxide (AAO) templates. The templates are fabricated by anodization of aluminum (Al) discs, and the pore length and diameter are controlled by changing the anodizing conditions. Pores with an average diameter of around 120 nm are created using oxalic acid as the electrolyte. Using this method, cylindrical nanowires are synthesized, which are released by dissolving the alumina using a selective chemical etchant.

In this work, we describe a&#160;protocol to fabricate iron nanowires, including the formation of the porous alumina membrane that is used as the template, electrodeposition into templates using electrolyte solution, and the release of the nanowires into the solution.

Magnetic nanowires possess unique properties that have attracted the interest of different fields of research, including basic physics, biomedicine, and ...

Synthesis of Metal Nanoparticles Supported on Carbon Nanotube with Doped Co and N Atoms and its Catalytic Applications in Hydrogen Production

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented herein. The novel strategy is based on a facile one-pot pyrolysis treatment of cobalt (II) acetylacetonate and nitrogen-rich organic precursors under Ar atmosphere at 800 &#176;C, resulting in the formation of Co- and N- co-doped carbon nanotube with earthworm-like morphology. The obtained catalyst was found to have a high density of defect sites, as confirmed by Raman spectroscopy. Here, cobalt (II) nanoparticles were stabilized on the atomically dispersed cobalt- and nitrogen-doped carbon nanotubes. The catalyst was confirmed to be effective in the catalytic hydrolysis of ammonia borane, in which the turnover frequency was 5.87 mol H2&#183;molCo-1&#183;min-1, and the specific hydrogen generation rate was determined to be 2447 mL H2&#183;gCo-1&#183;min-1. A synergistic function between the Co nanoparticle and the doped carbon nanotubes was proposed for the first time in the catalytic hydrolysis of ammonia borane reaction under a mild condition. The resulting hydrogen production with its high energy density and minimal refueling time could be suitable for future development as energy sources for mobile and stationary applications such as road trucks and forklifts in transport and logistics.

Here, we present a protocol to synthesize Co nanoparticles supported on carbon nanotubes with Co- and N- dopants for hydrogen productions.

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented ...

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

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