Iron Phosphate Prepared by Coupling Precipitation and Aging


Jan 8, 2013 - Iron Phosphate Prepared by Coupling Precipitation and Aging: Morphology, Crystal Structure, and Cr(III) Adsorption. Tong B. Zhang, Yang ...
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Iron Phosphate Prepared by Coupling Precipitation and Aging: Morphology, Crystal Structure, and Cr(III) Adsorption Tong B. Zhang, Yang C. Lu,* and Guang S. Luo State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 10084, China ABSTRACT: In this work, pure iron phosphate products with various morphologies and crystal structures were obtained by coupling fast precipitation in a microreactor and the aging process. Morphology control from amorphous nanoparticle to monoclinic microsheet or microsphere can be simply realized by only changing the addition of H3PO4 in the reaction system. Microscopic observations on morphology evolvement revealed the formation of different morphology, or crystal structure could be illustrated by the synergetic effect of precursor produced in the mixing unit and postgrowth from solution in aging generally. Furthermore, Cr(III) adsorption determinations confirmed monoclinic-phase FePO4 had a better adsorption capacity than amorphous FePO4, and microsheet FePO4 has potential as a Cr(III) adsorbent with rationality in crystal structure, surface area, and macroscopic size.

1. INTRODUCTION The synthesis of inorganic materials with controllable morphology and structure is gaining expansive research interest due to some fascinating structure- and shape-dependent properties with respect to applications in catalysis, electronics, adsorption, and biotechnology.1−6 Abundant works, mostly concentrating on noble metals and semiconductor nanocrystals such as CuO2 and Pd nanocubes,7−10 have been reported on morphology and structure manipulation and their relationship between morphologies and properties. As one of the most promising iron-based materials with low cost, environmental friendliness, and biocompatibility, iron phosphate with various crystal structures has shown several successful applications in the fields of catalysis, wastewater purification, ferroelectrics, lithium batteries, and steel and glass industries.11−15 For examples, the orthorhombic heterosite FePO4, obtained from delithiated LiFePO4,16,17 is one of the most promising cathode materials for lithium batteries,18 whose electrochemical activity is higher than the monolithic FePO4 characterized recently.19 α-Quartz FePO4 is the most stable state with low electrochemical activity compared with other crystal structures. When used as a catalyst for the gas-phase selective oxidation of benzene to phenol with N2O, only quartzphase crystalline iron phosphate exhibits catalytic activity due to the tetrahedral-coordinated Fe as the active center. 20 Amorphous FePO4, showing various properties and application performances related with the amorphicity, has uncertain morphology, which is strongly affected by the synthesis conditions, such as the aqueous environment and temperature.21 Triclinic FePO4, with a space group of P1, has been reported to be anode material for lithium batteries.22As iron phosphate is used as an adsorbent for the removal of heavy © 2013 American Chemical Society

metal ions, the crystal structure and morphology are also reported as crucial properties.15 Several synthetic routes have been reported for synthesizing iron phosphate according to specific needs for different applications, such as nanoparticle FePO4 fabricated by using CTAB for lithium batteries, nanotube FePO4 synthesized by the solvothermal method for catalysis, naonosphere FePO4 synthesied by the microwave method for biosensoring, and so forth.23−30 However, few of them are concerned about systematic manipulation of the morphology and structure. Especially, more detailed and intuitive evidence is still scarce for better understanding the kinetic process and mechanism of crystal and morphology growth. To date, controllable preparation of iron phosphate with varieties of morphology and structure is still a challenge in a general platform. It is a hinderance to recognize the relationship between morphology/ crystal structure and application performance in a specific aspect and to develop high-performance iron phosphate materials as well. In our previous work,31 nanoparticle iron phosphate with well dispersion and high purity is obtained through coupling fast precipitation in a membrane microcontactor and thermal treatment for aging. Thanks to the distinguished ability of controlling micromixing of the microcontactor, the pH value could be controlled accurately to ensure a nanoparticle precursor with well dispersion and depletion of iron and phosphorus resources in solution. Thus, when being thermally treated, only conversion of the impurity to iron phosphate in the precursor precipitate occurs, and the growth process is Received: October 2, 2012 Revised: December 23, 2012 Published: January 8, 2013 1099

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Figure 1. Experimental setup used during the experiments. 2.2. Experimental Procedure. First, Fe(NO3)3·9H2O and (NH4)3PO4 were dissolved in deionized water, separately. H3PO4 was used to adjust the pH value of the reaction system. It could reduce the concentration of hydroxyl and restrain the hydrolysis of iron ions, which would suppress the formation of Fe(OH)3. The dispersed feed (the aqueous solution containing (NH4)3PO4) was pressed through the membrane into the microchannel to mix with the continuous feed (the aqueous solution containing Fe(NO3)3) coming from the continuous feed inlet. The two solutions were mixed in the microchannel to generate precipitate as the precursor immediately, and then, the precursor was aged in an oil bath with temperature of 110 °C and stirring speed of 1200 rpm under atmospheric pressure for 1 h for thermal treatment. Following this step, the powders were filtered from the slurry, washed with distilled water at room temperature three times or more, and dried at 105 °C in air overnight to obtain the final product. 2.3. Analysis and Characterization. The morphologies of products were observed using scanning electron microscopy (SEM; JSM-7401F, 3 kV), transmission electron microscopy (TEM; JEM2010, 120 kV), and the high-resolution scanning electron microscopy (HR-SEM; HITACHI S-5500, 5 kV). Elemental composition of the samples (referring to iron and phosphorus) was determined by inductively coupled plasma−optical emission spectroscopy (ICP-OES; IRIS Intrepid II XSP). The samples were dissolved by hydrochloric acid solution. The power of the plasma was 1150 W. Peristaltic pump rotation was 100 r/min. The precipitates as prepared were characterized by powder X-ray diffraction analysis (XRD; D8-Advance) using Cu Kα radiation (40 kV and 40 mA) at a scanning rate of 10°/min. The nanoparticle product was calcined (heated from 30 to 800 °C at a heating rate of 10 K/min in air atmosphere). Brunauer−Emmett−Teller (BET) surface areas, pore size distribution, and pore volume of the as prepared samples were measured at 77 K using a surface area analyzer (Quantachrome Autosorb-1-C chemisorptions−physisorption analyzer). The pore size distribution was calculated from the desorption branches using the Barrett− Joyner−Halenda (BJH) method. The concentration of Cr(III) ion in aqueous solution was determined by a polarized Zeeman atomic absorption spectrophotometer (HITACHI Z-5000) with a slit width of 1.3 nm, a wavelength of 359 nm, and a lamp current of 9 mA. The flame type was an air− C2H2 gas mixture with a 2.8 L/min total flow.

negligible. It is natural to think that, when controlling the pH value of the micromixing region, the amount and composite of precipitates will change, as the same of solution environment for the follow-up thermal treatment. Different morphologies and crystal structures may be fabricated theoretically. Thus, this coupling technique could act as a platform for systematic manipulation of morphology and structure of FePO4, while basic principles of the technique deserved to be deeply investigated. In this work, we applied the coupling technique to prepare iron phosphate under various synthesis conditions. The morphologies of the obtained FePO4 changed from nanoparticle to microsheet and microsphere, and the crystal structure changed from amorphous to monoclinic, correspondingly. The mechanism for evolution of different morphologies was proposed on the basis of direct observation during the experiments. Taking Cr(III) ion adsorption as an example, the morphology crystal structure dependent performance of FePO4 was revealed, providing a foundation for FePO4 to be developed as functional materials.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Apparatus. All of the reagents used in these experiments including ferric nitrate nonahydrate (Fe(NO3)3·9H2O), ammonium phosphate ((NH4)3PO4·3H2O), and phosphoric acid (H3PO4) were of analytical reagent grade. Deionized water was used throughout the process. All these chemicals were supplied by Beijing Chemicals Company (Beijing, China) and used directly without further purification. Figure 1 shows the experimental setup used during the experiments, which coupling a mixing unit of a membrane dispersion microcontactor and an aging unit for thermal treatment in an oil bath. The membrane dispersion microcontactor device is mainly consisted of two stainless steel sample plates (50 mm × 50 mm × 18 mm) using laser etching techniques and a stainless steel microfiltration membrane. The stainless steel microfiltration membrane (shown in inset) with a pore size of 5 μm is used in the microcontactor as the dispersion medium. The active area of the microporous membrane is 12.56 mm2, and the geometric size of the microchannel is 15 mm × 0.5 mm × 0.5 mm (length × width × height). 1100

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3.1.3. Crystal Structure. XRD analysis was used to determine the crystal structure and phase purity of the obtained products with different morphologies. The corresponding XRD patterns of products are shown in Figure 3. The nanoparticle product as prepared is suggested to be amorphous, since no intensive peaks can be detected. But, after calcination, its diffraction peaks appeared and showed a perfect match with the standard data of JCPDF file no. 29-0715,31 reflecting the high chemical purity. Different from an amorphous nanoparticle, both the microsheet product and the microsphere product possess a monoclinic system with the space group P21/n indexed from the standard data of PDF no. 72-0471. The structure is built up from PO4 tetrahedral and FeO6 octahedra sharing corners to form a three-dimensional framework. The corresponding crystal structure is shown in Figure 4. Each Fe octahedron shares corners with four P tetrahedrals, and the remaining two corners are occupied by two water molecules. Each PO4 tetrahedron, in its turn, shares four corners with FeO6 octahedra.32 3.1.4. Morphology in Detail. During the formation of different morphologies of iron phosphate, we found that the morphologies of the products strongly depended on the reaction conditions, especially, the amount of H3PO4, which directly determined the morphology of the final products. However, for a complete view of the formation process of different morphologies, more detailed information needed to be presented. Thus, TEM and HR-SEM were used for a complete view of different morphologies. A TEM image of the nanoparticle product is illustrated in Figure 5a. The inset is a typical selected area electron diffraction (SAED) pattern, which confirms the nanoparticle is amorphous. The corresponding size distribution is presented in Figure 5b. The average size of the primary particles is about 50 nm. The size of the particles seems to be uniform. This narrow size distribution mainly depends on the good micromixing performance of the microcontactor. For FePO4 with microsheet morphology, aggregates would be disassociated from each other and become single microsheets after ultrasonic treatment. Thus, single microsheets would exist under the scope of our observation. A dark field TEM image of one single microsheet is presented in Figure 6, which clearly exhibits that the size of the microsheet is around 1 μm. The microsheet with a particle-like edge seems to be formed by nanoparticles arrayed in an ordered manner. As expected, the inset, which is the high-magnification SEM image of the corresponding microsheet presented in Figure 6, confirms that the microsheet is composed of irregular polyhydral nanoplates. These nanoplates aggregate together without fusion and form cracks on the microsheet. The highresolution TEM (HR-TEM) image shown in Figure 7a indicates clear crystalline lattice fringes. The fringe spacings of 0.278 and 0.683 nm correspond to {013} and {011} planes of monoclinic iron phosphate dihydrate. The SAED pattern (Figure 7b) further confirms the single-crystal structure. The high-magnification SEM image on the surface of the microsphere in Figure 8a further demonstrates that the microspheres are composed of nanorods. The average length of the nanorod is about 300 nm measured from randomly selected nanorods that appear completely in the image. A representative TEM image of a few nanorods is presented in Figure 8b. The lattice fringes are continuous, and they have the same orientation, demonstrating that the nanorod is single crystalline, which is further confirmed by the SAED pattern in

3. RESULTS AND DISCUSSION 3.1. Characterization of Products. An aqueous solution of mixed Fe(NO3)3 and H3PO4 was used for the continuous phase feed. The molar concentration of Fe(NO3)3 was 0.1 mol/ L, while the concentration of H3PO4 changed from 0.5 to 5 times that of Fe(NO3)3. The mixed aqueous solution of (NH4)3PO4 and H3PO4 was used for the dispersed phase feed. The molar ratio of (NH4)3PO4 and H3PO4 varied from 1:0 to 1:3, and the concentration of (NH4)3PO4 was set at 0.1 mol/L. Two feeds were mixed in the microcontactor with equal volume flow rates of 50 mL/min. Then, the precursor was aged by thermal treatment for 1 h as previously described. Some typical compositions of continuous and dispersed phases were shown in Table 1. Table 1. Some Typical Compositions of Continuous and Dispersed Phases conc (mol/L)

continuous phase

dispersed phase

entry

Fe(NO3)3/(NH4)3PO4

Fe(NO3)3/H3PO4

(NH4)3PO4/H3PO4

1 2 3

0.1

1:1 1:5 1:5

1:0 1:0.5 1:3

3.1.1. Morphology. The morphologies of as-prepared products obtained under different reaction conditions were studied by SEM. The morphology changed from nanoparticle to microsheet and microsphere with different amounts of H3PO4 being added to the reaction system. The product obtained at a relatively high pH condition, namely, entry 1 shown in Table 1, is mainly consisted of aggregation of small nanoparticles, the dispersion of which is good, as shown in Figure 2a. Further insight into the morphology of these aggregated particles is shown in Figure 2a′. It can be found that these nanoparticles form a spongelike morphology. Figure 2b,b′ shows different magnification images of the product obtained with gradually increasing the amount of H3PO4. Detailed composition of continuous and disperse phases is illustrated as entry 2 in Table 1. The lowmagnification SEM image in Figure 2b shows that the asprepared product is mainly composed of microsheets, which are relatively uniform in size. The relatively high-magnification SEM image in Figure 2b′ illustrates that some of the microsheets aggregate together to form a flowerlike morphology. With the addition of more H3PO4 to the reaction system, that is, the amount of H3PO4 in the dispersed phase increasing to 3 times that of (NH4)3PO4 shown as entry 3 in Table 1, iron phosphate with microsphere morphology is produced, as shown in Figure 2c. The size of them differs with each other, ranging from 3 to 6 μm. The relatively high-magnification SEM image, illustrated in Figure 2c′, clearly exhibits that the microsphere is composed of nanorods with different orientations. 3.1.2. Composition. In order to determine the chemical composition of different product morphologies quantitatively, we used ICP-OES to determine the chemical composition of products with various morphologies. The results are shown in Table 2. It indicates that every product has contents of P and Fe and their mole ratios are very close to the stoichiometry of FePO4, indicating all these products with different morphologies are of pure FePO4. 1101

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Figure 2. SEM images of different product morphologies produced under conditions of (a, a′) entry 1, (b, b′) entry 2, and (c, c′) entry 3 in Table 1, respectively.

Table 2. P/Fe Molar Ratios of Products with Different Morphologies morphology

P (wt %)

Fe (wt %)

molar ratio of P/Fe

nanoparticle microsheet microsphere

16.16 16.32 16.06

28.58 28.82 28.52

1.021 1.023 1.017

the inset of Figure 8c. The distance between two adjacent lattice fringes is 0.341 nm, close to {120} plane spacing of monoclinic iron phosphate dihydrate. The crystalline growth orientation is along the ⟨001⟩ zone axis. 3.2. Mechanism Exploring. 3.2.1. Effect of Phosphoric Acid. As mentioned before, the amount of H3PO4 plays the most important role for determining the final morphology of the product. As far as we can see, for different conditions, H3PO4 mainly plays two roles. One is determining the amount of initially produced precipitate, and the other is determining the solution environment for postgrowth during the thermal treatment process, including pH value, ratio of ion species, ionic strength, and so forth.

Figure 3. XRD spectra of FePO4·2H2O with (a) nanoparticle morphology, (b) microsphere morphology, and (c) microsheet morphology.

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Figure 6. Dark field TEM image of one single microsheet. Inset is HRSEM image of corresponding microsheet.

3.2.2. Morphology and Crystal Structure Evolvement during the Aging Process. In order to understand the formation mechanism of different morphologies, experiments on morphology evolvement were conducted with controlling different thermal treatment times for microsheet and microsphere FePO4. A similar method could also be found in the work of other researchers.33 Discussions on the formation mechanism of amorphous nanoparticle FePO4 can be found in our previous work.31 Morphologies of microsheet FePO4 with controlled thermal treatment times being 10, 20, 30, 32, 36, 40 min are illustrated in Figure 10. Figure 11 shows the corresponding XRD spectrum. As seen, when the precursor was thermally treated for 10 min, the product was mainly composed of amorphous nanoparticles with well dispersion. As the thermal treatment time goes to 30 min, the morphology of the product seemed to be staying as nanoparticles, except for a little change in size and further aggregation due to their high surface energy. Although the morphology did not change much, the composition of the product kept changing based on the fact that, after thermal treatment, the product became pure FePO4 while the precursor contained hydrogen phosphate impurities. However, in the following 2 min, the morphology of the product changed substantially. Microsheet morphology with weak diffraction peaks (curve c in Figure 11) began to appear. When thermal treatment time increased to 36 min, microsheet morphology became dominant in the product. The formation of the microsheet kept going until all of the nanoparticles disappeared. Corresponding sharp diffraction peaks (curve e in Figure 11)

Figure 4. Schematic representation of the monoclinic structure of FePO4·2H2O. Blue corresponds to FeO6 octahedra, and green corresponds to PO4 tetrahedra.

In order to clarify the function of H3PO4 that we proposed, a series of experiments were conducted with gradually increase of the amount of H3PO4 addition to the reaction system. During the experiments, we found that the amount of precipitate decreased with increased H3PO4 concentration, until no precipitate was produced in the mixing unit. However, the composition of the initial precipitate produced under different conditions did not change much according to the energydispersive spectrometry (EDS) results presented in Table 3. A typical EDS spectrum of initially produced precipitate is also illustrated in Figure 9. P/Fe molar ratios are all around 1.4, indicating some iron hydrogen phosphate coprecipitated with iron phosphate at the initial stage of precipitation. Consequently, it will directly lead to a different solution environment for the follow-up growth process. These precursors and postgrowth from solution environment are responsible for determining the morphology of the final product collectively. In summary, in the process reported here, iron phosphate with different morphologies from nanoparticle to microsheet and microsphere has been obtained through controlling the total amount of H3PO4 added to the reaction system. The detailed conditions for preparing some typical morphologies are listed in Table 4.

Figure 5. (a) TEM image of nanoparticle product and (b) corresponding particle size distribution. The inset is the SAED pattern of spot A. 1103

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Figure 7. (a) HR-TEM image of microsheet product and (b) the SAED pattern corresponding to spot B shown in the inset.

Figure 8. Details of the microsphere: (a) HR-SEM image of the surface; (b) TEM image of the nanorods; and (c) HR-TEM image corresponding to spot D. The inset is the SAED pattern.

Table 3. Composition of Initially Produced Precipitate under Different Conditions sample 1:1, 1:1, 1:3, 1:3, 1:3, 1:5, 1:5,

1:0.5 1:1 1:0 1:1 1:3 1:0 1:3

a

P (atom %)

Fe (atom %)

molar ratio of P/Fe

17.6 17.6 18.0 17.8 16.9 17.5 18.2

13.2 12.2 12.2 12.9 11.7 12.1 13.6

1.3 1.4 1.5 1.4 1.4 1.4 1.3

a

1:1, 1:0.5 means the ratio of Fe(NO3)3 and H3PO4 in the continuous phase is 1:1 and (NH4)3PO4 and H3PO4 in the disperse phase is 1:0.5. The others indicate similar meanings. Figure 9. Typical EDS spectrum of initially produced precipitate under the condition of 1:1, 1:1 in Table 3.

demonstrated a fine crystal was obtained. Surprisingly, the formation of microsheet FePO4 with monoclinic crystal 1104

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Table 4. Conditions for Preparing Some Typical Morphologies of Iron Phosphate Fe(NO3)3 (mol/L)

(NH4)3PO4 (mol/L)

total H3PO4 (mol/L)

crystal structure

morphology

0.1

0.1

0.5

amorphous monoclinic monoclinic

nanoparticle microsheet microsphere

structure from amorphous nanoparticles could be achieved within just 10 min. The previous 30 min seemed to be pivotal for the growth of the crystal and the morphology. Besides converting the impurity in the precursors to products, the conversion from Fe(H2PO4)3 to FePO4 and H3PO4 in solution may carry out gradually. As the solubility product of FePO4 in solution surpasses a threshold value, postgrowth of the crystal will take place and change the morphology or crystal structure evidently. Morphology evolvement of microsphere FePO4 is illustrated in Figure 12. When mixing the continuous phase and the dispersed phase in the microcontactor under this condition, no precipitate was produced. When the solution was thermally treated for 10 min, a small amount of particles with an average diameter of 1 μm was produced. The situation was kept to 30

Figure 11. XRD spectra of microsheet product being thermal treated for (a) 10, (b) 30, (c) 32, (d) 36, and (e) 40 min.

min. These small amount particles were produced mainly owing to uneven composition in solution. However, in the time

Figure 10. Morphology evolvement of microsheet FePO4·2H2O under thermostatic condition. 1105

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Figure 12. Morphology evolvement of microsphere FePO4·2H2O under thermostatic condition.

that followed, a sharp increase of the precipitate amount was observed at 50 min, leading to the formation of microsphere morphology with a monoclinic crystal structure. Analogous to what happened in the formation of the microsheet, conversion from hydrophosphate to phosphate carried out continuously until a critical point. The microsphere morphology also formed in rapid speed within 10 min, which we observed also benefited from the previous 30 min of preparation time. The corresponding XRD spectra of microsphere products with different thermal treatment time are presented in Figure 13. The product was amorphous during the formation period for microspheres. When the formation process finished, a monoclinic crystal structure was obtained with the same pace. 3.2.3. Proposed Mechanism. Following the undoubted evidence shown above, we suggest the mechanism for the

formation of different morphologies is the synergistic effect of the initial precipitate process and the postgrowth process. For that reason, the acidity of the solution, determined by the amount of H3PO4 added to the reaction system, plays a critical role in the process of FePO4 morphology and crystal growth because it will not only determine the amount of initial produced precipitate but also influence the rates of the postgrowth process through changing the solution environment. Under conditions of producing FePO4 of microsheet morphology, the proper H3PO4 amount makes the initial precipitate process and the postgrowth process play an equivalent effect. When thermally treated, conversion of impurity to FePO 4 in the precursor occurred. More importantly, FePO4 with a monoclinic crystal structure produced through conversion of Fe(H2PO4)3 in solution would grow on initially produced amorphous precursors. The difference of different crystal faces would motivate the oriented attachment process. For microsheet morphology, the jagged borders, shown in Figure 13, obviously suggested the occurrence of two-dimensionally oriented attachment growth between the edge surfaces of adjacent nanoplates. Recent molecular dynamics simulations showed that the edge−edge attachment had a much higher probability than the face−face attachment for nanoplates due to the lower energy barrier at the small sideface.34 Rotation of adjacent nanoblocks was believed to be occurring to find a perfect matching orientation. The oriented attachment was initiated at multiple sites,35 like a modular assembly, which would accelerate the process of forming microsheet morphology. Thus, microsheet morphology with a monoclinic crystal structure of FePO4 could be achieved at a rapid speed. With an increasing amount of H3PO4 added to the reaction system, no precipitate was produced through the microreactor due to the low acidity of the mixed aqueous solution. Apparently, the postgrowth process was dominant in

Figure 13. XRD spectra of the product being thermally treated for (a) 10, (b) 30, (c) 40, and (d) 50 min. 1106

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Figure 14. Sketch map of FePO4·2H2O morphology evolution.

determining the final morphology. Similar to the process that happened in the formation of the microsheet, conversion of Fe(H2PO4)3 in solution to FePO4 nucleis occurred continuously during thermal treatment. When the solubility product of FePO4 in solution surpassed a threshold value, there would be a sudden sharp increase of the precipitate amount. Thus, the fast forming of nanorods of FePO4 with a monoclinic crystal structure was understandable. The anisotropic growth of nanorods of FePO4, probably accompanied by the Ostwald ripening process, was mainly arisen from the specific adsorption of phosphate anions on the surface planes parallel to the [001] zone of the FePO4 nucleis, according to relevant research.36−38 As a result, the crystals were in favor of crystallizing in an ordered manner, producing the final nanorod morphology. In conclusion, the amount of H3PO4 added to the reaction system determines the winner between the initial precipitate process and the postgrowth process. When H3PO4 concentration is low, no iron and phosphorus are left in solution. The postgrowth process is negligible. Only conversion of Fe2(HPO4)3 to FePO4 in the precursor occurs during thermal treatment leading to uniform nanoparticle morphology. As H3PO4 concentration increases, the amount of iron and phosphorus left in solution gradually increases while the initial precipitate decreases. The changing of the initial precipitate amount and the solution environment is believed to be responsible for the forming of microsheet morphology of FePO4 produced by the oriented attachment process, because under such conditions, the initial precipitate process and the postgrowth process are well matched. When the H3PO4 concentration is high enough, the postgrowth process is dominant for deciding the final morphology. Due to the specific adsorption of phosphate ions, anisotropic growth of

nanorods is formed. The corresponding sketch map of FePO4 morphology evolution caused by H3PO4 is shown in Figure 14. 3.3. Adsorption Experiment. FePO4 has long been reported as a green adsorbent for removal of heavy metal ions in aqueous solutions. For different metal ions, the sorption mechanism differs. For example, ion exchange between protons from the surface of FePO4 is found to be responsible for Zn(II) sorption,39 while it is specific adsorption for Cr(III) and can be reused through using 0.1% H2O2 and 0.05 mol/L NH3 aqueous mixture as a stripping reagent.15 However, to date, no literature was concerned about the influence of morphology and crystal structure of FePO4 on the specific adsorption property. In this work, by taking Cr(III) as the target adsorption ions, the influence of morphology and crystal structure of FePO4 on adsorption property was studied. The adsorption behavior of Cr(III) on iron phosphate was investigated by using batch adsorption mode experiments. Cr(III) was provided from the dissolving of CrCl3·6H2O. In general, 20 mL of aqueous solution containing Cr(III) ions was mixed with a different amount of adsorbent of different morphologies. The pH value was adjusted to 5.9 using 0.1 mol/ L sodium hydroxide for maintaining favorable retention of Cr(III).15 The initial concentration of Cr(III) was 13.573 ppm. The quantity of nanoparticle, microsheet, and microsphere product was 0.0410, 0.0259, and 0.0363 g, respectively. The mixture was shaken in a 30 °C, 150 rpm oscillating water bath for 24 h. After centrifugation at 4000 rpm for 10 min, the supernatant was analyzed by atomic absorption spectroscopy (AAS) for quantifying the residual chromium in the solution after adsorption. The maximum adsorption capacities of Cr(III) for different morphologies of nanoparticle, microsheet, and microsphere are 5.30, 14.17, and 9.90 mg/g, respectively. The result is not 1107

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pure iron phosphate with different morphologies and crystal structures. Basic principles of the coupling technique were thoroughly investigated. By using Fe(NO3)3 and (NH4)3PO4 as iron and phosphorus resources, respectively, morphology control from amorphous nanoparticle to monoclinic microsheet or microsphere can be simply realized by only changing the addition of H3PO4 in the reaction system. Microscopic observations on morphology evolvement revealed the formation of different morphology or crystal structure could be illustrated by the synergetic effect of precursor produced in the mixing unit and postgrowth from solution in aging generally. The construction of monoclinic crystal structure relying on temperature is not in the mixing unit at room temperature but in the aging unit at high temperature. Furthermore, Cr(III) adsorption determinations confirmed monoclinic-phase FePO4 had better adsorption capacity than amorphous FePO4, and microsheet FePO4 has potential as a Cr(III) adsorbent with rationality in crystal structure, surface area, and macroscopic size. All in all, coupling fast and uniform precipitation and aging provides a platform for preparing various functional FePO4 materials, of which the mechanism can be further verified and popularized for establishing new preparation methods and developing new materials in the future.

proportional to surface area and pore volume, shown in Table 5. Despite the biggest surface area and pore volume that a Table 5. Characteristics of Various FePO4 Products and Their Adsorption Performance for Cr(III) morphology crystal structure

nanoparticle amorphous

microsheet monoclinic

microsphere monoclinic

S/(m2 g−1) pore volume (cc/g) adsorption capacity (mg/g)

58.6 10.0 × 10−1 5.30

40.0 1.3 × 10−1 14.17

33.9 1.3 × 10−1 9.90

product with nanoparticle morphology possesses, its maximum adsorption capacity of Cr(III) surprisingly is the minimum. All of the different morphologies have a similar pore size distribution with mesoporous size around 3 nm shown in Figure 15. According to the adsorption mechanism proposed



AUTHOR INFORMATION

Corresponding Author

*Phone: +86 10 62773017; fax: +86 10 62770304; e-mail: [email protected] Author Contributions

The manuscript was written through contributions of all authors Notes

The authors declare no competing financial interest.



Figure 15. Pore size distribution of different morphologies. From top to bottom represents nanoparticle, microsheet, and microsphere, respectively. The inset is the isothermal adsorption curve.

ACKNOWLEDGMENTS The authors gratefully thank for Professor Whittingham of State University of New York at Binghamton for providing the crystal structure database of monoclinic iron phosphate dihydrate. The support of the National Natural Science Foundation of China (20876084, 21036002, and 21176136) and National Basic Research Program of China (2007CB714302) on this work is highly acknowledged.

by Zhang et al.,15 the adsorption mechanism is Cr(III) (1s22s22p63s23p63d3) with two empty 3d orbits competing with Fe for affluent electrons of O. It is confirmed by the facts that there is no obvious effect of ionic strength on Cr(III) retention and the interaction between Fe and O becomes weaker and redshift after Cr(III) loading revealed by UV−vis spectra. Thus, it is an easy job to deduce that the crystal structure is the most important factor responsible for the difference in the adsorption capacity of Cr(III). As we mentioned before, the microsheet and microsphere morphology products possess a monoclinic crystal structure, in which the Fe octahedron shares corners with four P tetrahedral and the remaining two corners are occupied by two water molecules. In short, more O is located around Fe. When adsorbing Cr(III), more electrons can be supplied. As a result, the maximum adsorption capacity of Cr(III) can be increased sharply. For the same crystal structure, the maximum adsorption capacity increases with the increase in surface area and pore volume. Thus, microsheet FePO4 has a better adsorption capacity than microsphere FePO4 due to a bigger surface area and pore volume.



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4. CONCLUSIONS In this paper, through coupling fast precipitation in a membrane dispersion microcontactor and aging, we obtained 1108

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