Nanoparticles with uniform characteristics have been made also by flame-assisted spray pyrolysis (FASP) of inexpensive inorganic precursors and solvents, (e.g., ethanol). In FASP, cheap fuel gases (acetylene or methane) are used to promote solution droplet evaporation and gas-to-particle conversion preventing formation of inhomogeneous product by droplet-to-powder conversion., Acetylene was chosen as the main fuel gas for its high adiabatic flame temperature. Here FASP is explored for synthesis of iron phosphate nanoparticles with potential applications in food fortification and as cathode materials in lithium batteries.
Here, FASP and FSP are explored for synthesis of homogeneous FePO4 nanoparticles from inexpensive precursors and fuels (C2H2 or CH4) at increased production rates. The focus is on iron phosphate having high solubility in dilute hydrochloric acid (pH 1) after 30 min, a measure that is used to reliably predict the relative bioavailability of iron compounds.
To overcome this, costly metal organic precursors (i.e., alkoxides, acetylacetonates, or carboxylates) are typically used. Furthermore, by selecting appropriate solvents, more homogeneous nanoparticles are made. For example, in Bi2O3 synthesis from bismuth nitrate, using acetic acid as solvent resulted in volatile bismuth acetates and eventually homogeneous nanoparticles, whereas the use of ethanol resulted in inhomogeneous products. Another way to reduce product inhomogeneity is the conversion of cheap inorganic metal precursors to metal organic complexes by chemical methods. The presence of a carboxylic acid in the flame spray pyrolysis (FSP) precursor solution resulted in homogeneous Al2O3, Fe2O3, and Co3O4 nanoparticles by formation of a more volatile carboxylate from the metal nitrate. However, such solvents are quite costly compared to pure ethanol or water.
Low-cost synthesis of iron phosphate nanostructured particles is attractive for large scale fortification of basic foods (rice, bread, etc.) as well as for Li-battery materials. This is achieved here by flame-assisted and flame spray pyrolysis (FASP and FSP) of inexpensive precursors (iron nitrate, phosphate), solvents (ethanol), and support gases (acetylene and methane). The iron phosphate powders produced here were mostly amorphous and exhibited excellent solubility in dilute acid, an indicator of relative iron bioavailability. The amorphous and crystalline fractions of such powders were determined by X-ray diffraction (XRD) and their cumulative size distribution by X-ray disk centrifuge. Fine and coarse size fractions were obtained also by sedimentation and characterized by microscopy and XRD. The coarse size fraction contained maghemite Fe2O3 while the fine was amorphous iron phosphate. Furthermore, the effect of increased production rate (up to 11 g/h) on product morphology and solubility was explored. Using increased methane flow rates through the ignition/pilot flame of the FSP-burner and inexpensive powder precursors resulted in also homogeneous iron phosphate nanoparticles essentially converting the FSP to a FASP process. The powders produced by FSP at increased methane flow had excellent solubility in dilute acid as well. Such use of methane or even natural gas might be economically attractive for large scale flame-synthesis of nanoparticles.
The standard precursor (0.2 M iron nitrate with 0.2 M tributyl phosphate in denaturized ethanol) solution is similar to Hilty et al. without, however, addition of costly 2-ethylhexanoic acid (2-EHA). This formulation cuts the raw materials (precursor and solvent) cost by one-third for the iron and phosphate concentrations (0.2 M) used here. During standard FSP synthesis of iron phosphate using this precursor solution, inhomogeneous product is obtained as with Fe2O3. Addition of 2-EHA in this solution (1:1, ethanol/2-EHA) prevents product inhomogeneities, and Raman measurements confirmed the product was FePO4.
shows XRD patterns of powders made by FASP of 2 mL/min standard precursor solution at different C2H2 flow rates with constant spray parameters. Common to all XRD patterns, a broad hump appears at 15–40°, indicating amorphous iron phosphate. At 1 and 1.5 L/min C2H2, the patterns show peaks corresponding to Fe2O3 with crystallite sizes of 26 and 30 nm, respectively. Maghemite is typically formed in flames;- however, it is difficult to distinguish it from magnetite, as their major XRD peaks overlap. FSP using Fe(III)-naphthenate or even nitrate in reducing flame conditions (equivalence ratios, Φ, near or above 1) resulted in magnetite and wustite. To make magnetite particles at such Φ, the flames were enclosed to control the combustion atmosphere., At the present highly oxidizing conditions (Φ ≤ 0.8), maghemite formation is expected and its structure well fitted the XRD patterns (goodness of fit 2H2 thus indicate inhomogeneous product powders that consist of amorphous iron phosphate and crystalline maghemite. At higher C2H2 flow rates, however, the peaks of iron oxide gradually disappear resulting in purely amorphous iron phosphate. This is consistent to FASP synthesis of homogeneous bismuth oxide at similar conditions.
Although required by plants in small amounts, Fe is involved in many important compounds and physiological processes in plants. Iron is involved in the manufacturing process of chlorophyll, and it is required for certain enzyme functions. Fe’s involvement in chlorophyll synthesis is the reason for the chlorosis (yellowing) associated with Fe deficiency. Iron is found in the iron-containing (heme) proteins in plants, examples of which are the cytochromes. Cytochromes are found in the electron transfer systems in chloroplasts and mitochondria. Iron is also associated with certain non-heme proteins such as ferredoxin.
(bottom) shows the XRD pattern from a FSP-made powder at 6 mL/min precursor solution flow rate and 1.25 L/min CH4. The maghemite peaks (dXRD = 43 nm) are clearly visible (especially the strongest peak at 35.6°), indicating an inhomogeneous product. Compared to the pattern with the same CH4 flow rate but lower precursor flow rate (2 mL/min), the peaks intensities are decreased. The maghemite crystallite contents are reduced from 8 to 3 wt % with the increased precursor solution feed rate from 2 to 6 mL/min. This reduction is explained with the increased enthalpy density content in the spray flame from the increased flow rate of the combustible precursor solution as with FSP synthesis of bismuth oxide and cerium oxide. The increased powder production rate promoted the formation of more homogeneous powders by FSP as hotter flames were created by their increased enthalpy content. This resulted in longer high temperature residence time of the spray droplets and thus facilitated precursor evaporation from the droplets and promotion of gas-to-particle conversion. This was seen with FASP also in and where increased precursor solution feed rates led to larger iron phosphate particles but free from maghemite. Note that by increasing the solution flow rate (e.g., by doubling it to 12 mL/min) does not remove the crystalline residue or prevent the formation of large particles by droplet-to-particle conversion. At this precursor flow rate, the SSA was reduced to 90 m2/g. As the precursor droplet concentration increases with increased precursor solution feed rate, there is not enough energy in that solution to fully evaporate these droplets without providing additional energy, e.g., by increasing the flow of the supporting CH4 gas.