Ind. Eng. Chem. Res. 2000, 39, 2635-2643
2635
Roles of Metal Ions in the Formation of Hematite Particles from Forced Hydrolysis Reaction Kazuhiko Kandori,* Akemi Yasukawa, and Tatsuo Ishikawa School of Chemistry, Osaka University of Education, Asahigaoka 4-698-1, Kashiwara-shi, Osaka 582-8582, Japan
The roles of metal (Me) ions (Cu(II) and Cr(III)) in the morphologies and structures of hematite particles, produced from a forced hydrolysis reaction of FeCl3-HCl at various concentrations of metal ions ranging from 0 to 0.8 in the Me/(Fe + Me) atomic ratio (XMe), were investigated by separation of the reaction into two steps: precipitation of β-FeOOH and phase transformation from β-FeOOH to hematite. The metal ions inhibited the particle growth of β-FeOOH at a lower concentration region, while they promoted the particle growth at a higher concentration region with incorporation of these ions into the crystal lattice. The β-FeOOH particles produced with metal ions exhibited high specific surface areas and microporosities. It was suggested that the phase transformation from β-FeOOH to hematite is strongly affected by the metal ions incorporated into β-FeOOH particles but not by those adsorbed on the particle surface. Introduction It is well-known that the monodispersed particles controlled in size, shape, and internal structure are utilized for advanced particulate materials such as ceramics, pigments, sensors, cosmetics, electric devices, and so forth. Because of this wide application, many investigators have investigated the formation process and structural character of uniform colloidal particles employing hematite (R-Fe2O3) with various shapes and sizes1-4, which were produced from a forced hydrolysis reaction of FeCl3-HCl solution.5 We revealed previously that the cubic (edge length ≈1.00 µm) and large spherical (diameter ≈0.82 µm) particles are polycrystalline and constructed by aggregation of fine ferric oxide hydroxide primary particles.6,7 The authors also reported the effect of various kinds of organic compounds such as amines, dimethylformamide (DMF), and dioxane (DX) on the precipitation of hematite particles and disclosed the structural character of the characteristic hematite particles produced.8-10 Recently, the authors also found that inorganic metal ions contained in the FeCl3-HCl solutions strongly affect the forced hydrolysis reaction of Fe(III) in acidic solution on the buildup of polynuclears from mononuclear iron species and vary the morphology of hematite particles (called “one-step method” hereafter).11 The morphologies and sizes of the hematite particles were varied with an increase in the concentration of divalent ions (Cu(II), Ni(II), and Co(II)) up to 0.8 in a Me/(Fe + Me) atomic ratio (XMe), though no remarkable change was observed upon doping of Cr(III) at a narrow concentration region of XCr e 0.04, and further no pure hematite particle was precipitated at XCr g 0.05. The hematite particles formed at XCu e 0.4 and XCr e 0.04 were polycrystalline with an enlarged c edge length in a unit cell, while the particles produced at XCu g 0.6 were shown to be single crystal. The change in the phase transformation rate from β-FeOOH to hematite by the addition of metal ions * To whom correspondence should be addressed. E-mail address:
[email protected]. Fax: +81-729-783394.
was suggested to be closely related to the crystal lattice distortion and amount of the lattice OH- ions of the formed particles with various morphologies and structures. A part of the dopants is principally incorporated into the particle surface phase. However, the roles of metal ions and the formation mechanism of characteristic hematite particles were unclear. If they become clear, we will be able to control the morphology and inner structure of hematite particles. The objective of the present study is to reveal the role of metal ions in the formation of hematite by separation of the forced hydrolysis reaction into two steps: precipitation of β-FeOOH (step 1) and phase transformation from β-FeOOH to hematite (step 2). The results of this “twostep method” employed in the present study will contribute to elucidating the formation mechanism of characteristic hematite particles with various morphologies. Experimental Section Materials. Reagent-grade ferric chloride hexahydrate, copper chloride dihydrate, chromium chloride hexahydrate, and hydrochloric acid from Wako Pure Chemicals Ltd. were used as received. Ferric chloride solution (2 mol dm-3) was prepared by distilled deionized water as a stock solution to prevent hydrolysis and was properly diluted before use. Copper and chromium chloride solutions were prepared before use. Particle Preparation. Preparation of hematite particles was done separately by steps 1 and 2 in the twostep method as follows. The scheme of particle preparation is shown in Scheme 1 and compared with the onestep method carried out in our previous paper.11 a. Preparation Procedure of Precursor β-FeOOH Particles (Step 1). To separate the forced hydrolysis reaction into two steps, β-FeOOH particles, denoted as β-FeOOH(XMe), were previously prepared by a forced hydrolysis of FeCl3-HCl aqueous solutions containing various amounts of metal ions under the same conditions employed in the one-step method except for the aging period being reduced.11 A Pyrex glass vial containing a 30-cm3 reacting solution was tightly closed
10.1021/ie990417h CCC: $19.00 © 2000 American Chemical Society Published on Web 06/29/2000
2636
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000
Scheme 1. Particle Preparation Procedures
with a Teflon-lined screw cap and heated in a forced conventional air oven at 100 °C for 6 h. The concentrations of Cu(II) and Cr(III), XCu and XCr, were varied from 0 to 0.8 and from 0 to 0.04, respectively. The concentrations of total metal ions and HCl were fixed at 3.12 × 10-2 and 3.20 × 10-3 mol dm-3, respectively. These conditions provided a single-hematite crystal phase with spherical shape after the solution was aged for another 7 days in the one-step method.11 The pH of the solution after aging was ≈1. The precipitated β-FeOOH particles were filtered through a 0.2-µm Millipore filter, rinsed with deionized-distilled water, and dried in an evacuated dryer at room temperature for 16 h. b. Preparation Procedure of Hematite Particles (Step 2). The preparation of hematite particles was performed by two methods, A and B, utilizing different kinds of β-FeOOH(XMe) particles previously prepared as a precursor. Method A used β-FeOOH(XMe) particles previously produced without metal ions, denoted as β-FeOOH(XMe)0), while method B utilized β-FeOOH(XMe) ones produced with metal ions at corresponding XMe. In these methods, 83.2 mg of dried β-FeOOH(XMe) powders previously prepared, corresponding to the β-FeOOH concentration 3.12 × 10-2 mol dm-3 in the one-step method calculated with the assumption that the hydrolysis of FeCl3 is 100%, was redispersed by ultrasonic agitation in 30 cm3 of 9.70 × 10-2 mol dm-3 HCl solution, dissolving appropriate amounts of Cu(II) or Cr(III). The concentrations of Cu(II) and Cr(III) in the HCl solution, XCu and XCr, were varied from 0 to 0.8 and from 0 to 0.04, respectively, the same as those
for producing β-FeOOH(XMe). It should be noted that the XMe values of the precursor β-FeOOH(XMe) particles used in method B were fitted with those of the HCl solution. In other words, method B using β-FeOOH(XMe) produced with metal ions is a much more satisfactory procedure for reproducing the one-step method employed in our previous paper than method A. The solutions dispersing β-FeOOH(XMe) powders in methods A and B were aged for 7 days at 100 °C in Teflon-lined screw-capped Pyrex glass vials as well as employed in the one-step method.11 In both methods A and B, the same purification method as that for the production of β-FeOOH(XMe) was used for the precipitates obtained after the solution was aged with β-FeOOH(XMe). Characterization of the Particles. Transmission electron microscopy (TEM), X-ray powder diffraction (XRD), in situ Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), adsorption experiments of N2, and CHN elemental analysis were carried out for the characterization of the synthesized β-FeOOH and hematite particles. Fe(III), Cu(II), and Cr(III) contents were determined by inductively coupled plasma spectroscopy (ICP) after the particles were dissolved in diluted HCl solution. Infrared spectra of the particles embedded in KBr pellets (1 mg/500 mg of KBr) were also measured in dry air using an FTIR spectrometer. The details of the characterization of the particles are described in the preceding reports.6-8 Measurement of Metal Ion Adsorption. The amounts of adsorbed Cu(II) and Cr(III) were measured at the same conditions as those in method A. The
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2637
Figure 2. Changes in the mean particle volume (O, 0) and crystallite size from the (110) plane (b, 9) of β-FeOOH(XMe) particles produced with Cu(II) (O, b) and Cr(III) (0, 9) ions.
Figure 1. TEM micrographs of β-FeOOH particles produced at various concentrations of Cu(II) [a-1-a-5] and Cr(III) [b-1-b-5] ions.
β-FeOOH(XMe)0) powders (83.2 mg) were dispersed in Cu(II) and Cr(III) solutions with various XMe containing 3.20 × 10-3 mol dm-3 HCl. The solid and solution were separated by ultrafiltration after the suspensions were shaken at 30 °C for 7 days or 100 °C for 3 h. The equilibrium concentrations of Cu(II) and Cr(III) in the filtrate were determined by the ICP method. The amount of adsorbed metal ions per unit of surface area, the adsorption density (ΓMe; ions nm-2), was obtained by dividing the amount of adsorption by the specific surface area of β-FeOOH(XMe)0). Results and Discussion Effects of Metal Ions on the Formation of β-FeOOH Particles. a. Shape and Crystallinity. To investigate the effects of metal ions on the formation of β-FeOOH particles by step 1 at the same conditions as those in the one-step method, XMe was changed widely from 0 to 0.8 and from 0 to 0.04 for Cu(II) and Cr(III), respectively. Figure 1 displays typical TEM micrographs of precipitated particles after the solution was aged for
6 h at various XMe values. Clearly, the size of rodlike β-FeOOH(XMe) is varied with changes in the concentration of metal ions. The particle size is decreased at low metal ion concentrations of 2.0 × 10-5 e XCu e 2.0 × 10-3 and 2.0 × 10-5 e XCr e 2.0 × 10-4 while it increases above these concentrations. To compare the change in particle size with the addition of metal ions, the mean volumes of the particles were calculated by using the mean width and length of each particle produced with various amounts of Cu(II) and Cr(III) and are displayed in Figure 2 as a function of XMe (open symbols). The crystallite sizes of the β-FeOOH(XMe) particles (D110) estimated by the Scherrer equation from the half-height width of the XRD peaks of the (110) plane are also displayed in Figure 2 (closed symbols). Obviously, small and less crystallized β-FeOOH(XMe) particles are produced at low concentrations of Cu(II) and Cr(III), though D110 of the products with the Cr(III) system monotonically decreases with XCr. These results imply that the metal ions inhibit the particle growth of β-FeOOH at a lower concentration, while they promote the particle growth of β-FeOOH at a higher concentration. Because Al(III) in the aging solution enhances the hydrolysis of Fe(III) in the solution,12 a similar effect can be anticipated for that of Cu(II) and Cr(III). Therefore, the small and less crystallized β-FeOOH particles are produced at the lower concentrations of Cu(II) and Cr(III) by their fast hydrolysis reaction. However, the detailed reasoning of the inhibition and promotion effects of metal ions on the formation of β-FeOOH is unclear at the moment. b. Chemical Composition. To discuss the effects of metal ions on the formation of β-FeOOH particles, we assayed the metal contents in the particles (XMe(P)) by the ICP method. Table 1 summarizes the XMe(P) of the samples produced at various XMe values. Unfortunately, the XMe(P) of the particles produced at XCu e 2.0 × 10-4 could not be measured because the content of Cu(II) was below the detectable limit of the ICP spectrometer. The XCu(P) and XCr(P) values are ranging from 1.0 × 10-4 to 1.4 × 10-2, suggesting that 0.01-1.4 mol % of both metal ions were incorporated into the formed β-FeOOH(XMe) particles. This amount of Cu(II) incorporated into β-FeOOH(XMe) particles is less than the literature value of 6 mol %.13 This difference may be due to the different pH conditions in the particle production; in the literature β-FeOOH particles were produced by aging of the FeCl3 solution with urea at 95-99 °C and pH 7.5-8.0. The surface atomic ratios (XMe(S)) determined by XPS using the peaks of Fe(2p3/2), Cr(2p3/2), and Cu(2p3/2)
2638
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000
Table 1. Amount of Cu(II) and Cr(III) in β-FeOOH Particles Me/(Fe + Me) Me Cu
Cr
a
Me/(Fe + Me)
in solution
in particle
in surface
XMe
XMe(P)a
XMe(S)b
0
0
2 × 10-5 2 × 10-4 2 × 10-3 2 × 10-2 0.2 0.4 0.6 0.8
1.0 × 10-4 9.0 × 10-3 7.0 × 10-3 9.7 × 10-3 5.2 × 10-3 9.2 × 10-3
2 × 10-5 2 × 10-4 2 × 10-3 2 × 10-2 4 × 10-2
1.2 × 10-4 8.9 × 10-3 1.4 × 10-2
Data from ICP method
XMe(S)/XMe(P)
0
b
0.13 0.15 0.15
0
1083 17 11
Figure 3. Changes in the specific surface area of β-FeOOH(XMe) particles produced with Cu(II) (O) and Cr(III) (b) ions.
Data from XPS method.
respectively at 711.0, 577.4, and 935.9 eV are also shown in Table 1. Because the reliable peak of Cu(2p3/2) could not be measured because of the low content of Cu(II) in the particle surface phase, only the XCr(S) values are presented in Table 1. The XCr(S) exhibits 0.13-0.15, though the XMe(S) values of the materials produced at XCr e 2.0 × 10-4 could not be measured because of the low amount of Cr(III) in the surface phase, the same as that of the system doping Cu(II). In a comparison of the ratios of XCr(S)/XCr(P), the XCr(S) is 11-1083 times XCr(P), indicating that Cr(III) ions are concentrated in the particle surface phase. Considering the documented escaping depth of electrons in XPS of Fe(III), Cu(II), and Cr(III) ions are 0.78, 0.53, and 0.94 nm, respectively,14 the thickness of these surface phases can be regarded as ≈1 nm. This high surface metal ion content proves the adsorption and incorporation of metal ions by surface complexation and precipitation reactions, as will be discussed later. c. Surface Area and Porosity. To gain information on the crystallinity and porosity of β-FeOOH(XMe) particles, the N2 adsorption measurement was done after the materials were pretreated at 100 °C in vacuo. All the obtained adsorption isotherms belonged in type II of the IUPAC classification system.15 The specific surface areas (SN2) of the particles from the adsorption isotherms of N2 by fitting to the BET equation are plotted as a function of XMe in Figure 3. The SN2 values of β-FeOOH(XMe) particles produced with both metal ions are increased by the addition of a small amount of metal ions, though they slightly decrease with an increase in XMe. This behavior agrees with the change in particle size, as is seen in Figure 2. However, SN2 values abruptly increased at the higher concentration regions of XCu g 0.6 and XCr g 2.0 × 10-2. This increase of SN2 can be explained by the formation of micropores in the particles. In Figure 4 are shown the characteristic t-plots calculated from the adsorption isotherms of N2 for the particles produced at various XMe values and outgassed at 100 °C. The t-plots of the particles produced without metal ions exhibit a straight line up to t ) 1 nm (open circle), implying nonporous character of β-FeOOH(XMe)0) particles for N2 molecules.16 However, all the other particles produced with metal ions are made of straight lines passing through the origin at a low-pressure branch up to t ) 0.3-0.5 nm, but the plots curved downward above the point, especially for the
Figure 4. t-Plot curves of β-FeOOH(XMe) particles produced with various concentrations of (A) Cu(II) and (B) Cr(III) ions and outgassed at 100 °C. (A) XCu: (O) 0, (0) 2.0 × 10-5, (]) 2.0 × 10-4, (4) 2.0 × 10-3, (3) 2.0 × 10-2, (+) 0.2, (×) 0.4, (9) 0.6, and (2) 0.8. (B) XCr: (O) 0, (0) 2.0 × 10-5, (]) 2.0 × 10-4, (4) 2.0 × 10-3, (3) 2.0 × 10-2, and ([) 0.04.
particles produced at XCu g 0.6 and XCr ) 0.04 (closed symbols), suggesting that these particles possess micropores.16 These micropores could be formed by elimination of adsorbed metal ions, proving strongly the aggregation mechanism on the formation of β-FeOOH particles, proposed by Murphy et al.,17 in which ferric oxide hydroxide particles are formed by aggregation of primary hydrous Fe(III) oxide from FeCl3, Fe(NO3)3, and Fe(ClO4)3 solutions. Here, it should be emphasized that β-FeOOH(XMe) particles produced at XCu g 0.6 and XCr ) 0.04 are highly porous, despite their large size. d. Adsorption of Metal Ions onto β-FeOOH Particles. Figure 5 represents the change of adsorption density (ΓMe) of Cu(II) and Cr(III) on β-FeOOH(XMe)0) with XMe after the suspension was shaken at 30 and 100 °C. The ΓMe values for the system adsorbed at 30 °C are less than 0.21 ions nm-2 over the whole XMe region (see inset of Figure 5), corresponding to the
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2639
dothermic process of reactions (1)-(3).25 Similar endothermic data have been reported for Cu(II) and Cr(III) adsorption on silica gels26 and Zn(II), Cu(II), Pb(II), and Cd(II) on R-FeOOH.27,28 However, the extremely large ΓMe values, especially the ΓCu at XCu g 0.4, which is more than two times ΓOH, cannot be explained only by the surface complexation mechanism. Because the suspensions were shaken at 100 °C, metal ions are presumed to be hydrolyzed in the solution to enhance adsorption by the hydrolysis mechanism. Indeed, Cr(III), which is more readily hydrolyzed in solution than Cu(II),29 gave larger ΓMe’s than Cu(II). It has been reported that the cation that is more readily hydrolyzed has higher affinity to the oxide surface.23,30 The large ΓMe values in a high XMe region can be further explained by the surface precipitation mechanism as expressed by eq 4.19,20,31 Figure 5. Adsorption density of Cu(II) (O, b) and Cr(III) (0, 9) ions on β-FeOOH(XMe)0) particles at various XMe values: (O, 0) 30 °C and (b, 9) 100 °C.
results reported by Kanungo.18 He reported the small ΓMe values,
