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.
