V. V. Popov, E. F. Levina, A. I. Gorbunov, and V. V. Shcher binin, The Oxidation Mechanism of Iron(II) Compounds. Synthesis of Iron(III) Oxide Hydroxides, ...
ISSN 00360236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 7, pp. 995–1001. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.V. Popov, A.I. Gorbunov, E.F. Levina, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 7, pp. 1063–1069.
SYNTHESIS AND PROPERTIES OF INORGANIC COMPOUNDS
Regularities of the Formation of Iron(III) Nanocrystalline Particles during the Oxidation of Iron(II) Compounds in a Neutral Medium V. V. Popova, A. I. Gorbunovb, and E. F. Levinab a
Moscow Engineering Physics Institute, (State University), Kashirskoe sh. 31, Moscow, 141700 Russia b State Research Institute of Chemistry and Technology of Organoelement Compounds, sh. Entuziastov 38, Moscow, 115409 Russia Received January 20, 2009
Abstract—The regularities of the formation of iron(III) oxide hydroxides as nanocrystalline particles via oxi dation of iron(II) compounds in a nearneutral pH region were studied by potentiometric titration, electron microscopy, chemical analysis, and Xray diffraction. The oxidation process comprises two steps. The first step produces Fe(II)–Fe(III) hydroxo salts having a “green rust” structure in the form of nanocrystalline par ticles shaped as hexagons. The second step produces anisotropic nanocrystalline particles of iron(III) oxide hydroxides via the dissolution–oxidation–precipitation mechanism and via solidphase oxidation. The oxi dation of chlorinecontaining suspensions helps the formation of singlephase nanocrystalline lepidocrocite, while oxidation in the presence of sulfate ions yields nanocrystalline goethite. DOI: 10.1134/S0036023610070028
Iron(III) oxides and oxide hydroxides represent an abundant class of compounds widely used as inorganic pigments, magnetic materials, catalysts, ect. [1]. Cur rently, an important method for manufacturing them is a process comprising the oxidation of iron(II) to form particles having diverse morphologies and crystal structures [1, 2]. As oxidation occurs in aqueous solu tion, the mechanism and kinetics of the process are primarily controlled by the pH value [3]. In a high acidity medium (pH ≤ 1–2), oxidation occurs in a homogeneous system (reagents and products are in solution). Increasing pH to 2 induces precipitation from the solution; that is, the system becomes hetero geneous because of the appearance of solidphase par ticles produced by the oxidation, these particles con sisting of various iron(III) compounds. The mecha nism and kinetics of the process change radically in the range of pH ≥ 6, where precipitated iron(II) hydroxide Fe(OH)2 becomes the major oxidizable iron(II) species [3]. In the range 6.5–7.0 < pH < 8.5– 9.0, the oxidation process, leading to the formation of anisotropic particles of lepidocrocite γFeOOH [4] or goethite αFeOOH [5], involves an intermediate forma tion of iron(II)–iron(III) hydroxo salts with the general II III x+ formula [ Fe (1 · [(x/n)An– · (m/n)H2O]x– − x)Fe x ( OH ) 2] (socalled green rust (GR)) [6, 7]. In the highalkalin ity region (pH ≥ 12), Fe(OH)2 oxidation immediately yields anisotropic αFeOOH particles [8]. At the same time, temperature elevation to 70°С or higher enhances the formation of cubeshaped magnetite Fe3O4 particles [5]. Thus, synthesis parameters (namely, the type and concentration of reagents, reagent ratio, pH, temperature, and oxidation rate)
controlling the kinetics and mechanism of the process considerably influence the phase composition, sizes, and shapes of the resulting particles [1–5]. Chemical analysis in tandem with electron micros copy and Xray diffraction is an efficient tool for studying the formation and growth of nanocrystalline particles and gaining information on the evolution of their morphology and atomic and crystal structure during the synthesis [5]. One goal of this work was to study, in a detailed way, the regularities of the particle formation of oxygenated iron(III) compounds during the oxidation of iron(II) hydroxide suspensions in a nearneutral pH region. The other goal was to eluci date how process parameters influence the physico chemical properties of the resulting nanocrystalline particles. EXPERIMENTAL The initial solutions of iron(II) salts were prepared by dissolving weighed portions of FeСl2 · 4H2O (chemi cally pure grade) or FeSO4 · 7H2O (pure for analysis grade) in distilled water acidified to pH of 2.0 by hydrochloric acid or sulfuric acid depending on the salt anion. The thus prepared salt solutions (their con centration ranges: [FeCl2]in = 65–175 g/L, [FeSO4]in = 60–225 g/L) were filtered through a “Blue Band” paper filter for removing suspended insoluble particu lates. Solutions of bases were prepared by diluting 25.5% aqueous ammonia (high purity grade) with dis tilled water to obtain concentrations of 100–110 g/L, or by dissolving solid NaOH (pure for analysis grade) to obtain solutions with concentrations of 100–750 g/L. The initial iron(II) hydroxide suspensions with molar
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ratios ranging within 0.8 < [OH–]/[FeII] < 2 (accord ingly, pHin was 7.0–9.0) were prepared by adding aqueous ammonia or aqueous NaOH to an FeСl2 or FeSO4 solution under vigorous stirring. In this case, FeII means all ferrous iron species present in the sys tem (specifically, Fe2+ and Fe(OH)+ ionic species as well as Fe(OH)2). The reagent mass was purged with a nitrogen flow (Russian State Standard (GOST) 9293 74, 99.95 vol %) to inhibit uncontrolled oxidation dur ing neutralization and for thermostating to the set temperature (6–40°С). The resulting suspensions were then oxidized by bubbling with an air flow. A spe cialdesign stirrer was used to provide the maximal possible homogenization of the reaction mass. The end of the process was judged from the dramatic shift of pH down to below 5.0–5.5. The process course was monitored by continuously measuring pH in the reaction medium and periodi cally sampling aliquots for chemical, Xray diffrac tion, and electronmicroscopic analyses. Iron(II) concentration [FeII] in the suspension was determined by volumetric titration (chromatometry) [9] after a sampled volume (10 mL) was dissolved in 30 mL of aqueous hydrochloric acid (1 : 1). [Fetot] was deter mined after iron(III) was reduced to iron(II). [FeIII] in the sample was found as the difference ([Fetot] – [FeII]). The filtrate was analyzed in a similar manner. Preliminary experiments showed that the viscosity of the reaction suspension changes considerably dur ing oxidation (by one order of magnitude or greater, depending on the initial [FeII] value), which hampers sampling of equal suspension amounts for analyses. Therefore, along with the absolute values of [FeII], [FeIII], and [Fetot], we embedded a relative quantity, ox namely, percent suspension oxidation αsusp : III [Fe ]susp ox α susp= tot [Fe ]susp
RESULTS AND DISCUSSION × 100%
[Fe tot]susp − [Fe II]susp × 100% , = [Fe tot]susp
(1)
Because of an incomplete FeII precipitation at 0.8 < [OH–]/[FeII] < 2, the results of chemical analysis were used to calculate the actual percent precipitation value (βprec), which was determined in a completely oxidized sample: tot
[ Fe ]s.ph. × 100% tot [ Fe ]susp
[ Fe tot]susp − [ Fe tot]filtr = × 100%, [ Fe tot]susp
In the nearneutral pH region, the oxidation of iron(II) compound occurs with decreasing pH according to the following general scheme: 4Fe(II) + 6H2O + O2 → 4FeOOH↓ + 8H+.
where [FeIII]susp, [Fetot]susp, and [FeII]susp are, respec tively, FeIII, Fetot, and FeII concentrations in the sus pension.
β prec =
where [Fetot]s.ph., [Fetot]susp, and [Fetot]filtr are, respec tively, Fetot concentrations in the solid phase (precipi tate), suspension, and filtrate. Xray diffraction analysis was carried out during oxidation as follows: A suspension sample (10– 15 mL) was centrifuged at 15 000 rpm for 5 min. The resulting precipitate was transferred under an inert atmosphere to the target and coated with a film to keep it from oxidizing during subsequent measurements. Powder Xray diffraction analysis was carried out on a DRON3 diffractometer interfaced with a computer (monochromatized СuKα radiation, quartz mono chromator, pointbypoint mode, 0.02° steps, expo sure time per point: 2 s). The phase composition was determined using the JCPDS–ICDD 1997 File. Crys tallite sizes were estimated as coherent diffraction domains (CDDs) derived from the broadening of the width at halfheight of the diffraction peak using the Debye–Scherrer equation [10]. The size and shape of the resulting particles were monitored using transmission electron microscopy. This analysis was carried out during oxidation as fol lows: A suspension sample (1–1.5 mL) was diluted with distilled water (250 mL), which had been freed from oxygen and cooled to 0–2°С. After stirring, a drop of the solution was applied to a copper net stage coated by an amorphous carbon film, the stage being then immediately transferred to the vacuum chamber of a JEM 2000 EX electron microscope, evacuated, and then recorded in the transmission electron microscopy mode (lightfield and darkfield images) and electedarea electron diffraction mode (the mini mal size of the selector diagram was ~200 nm) at an accelerating voltage of 160 kV.
(2)
(3)
Figure 1 displays the evolution of the properties of an {Fe(OH)2 + FeCl2} suspension during oxidation. The suspension oxidation rate remains unchanged during the entire process and is virtually unaffected by the iron(II) concentration and pH (Fig. 1). The per cent oxidation value asymptotically approaches the precipitation value of the system oxidized; that is, the Fe(OH)2 precipitate is the first to oxidize, while iron(II) oxidation in the solution occurs at a far slower rate. The discovered tendency allowed us to estimate the percent oxidation of the solid phase αox s.ph. in analyzed samples in view of [Fe ]filtr ≈ [Fe ]filtr and [ Fe tot]s.ph. = [ Fe tot]susp · β prec, as follows: II
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70
7
2 60
6 50 40
5
1
30
4
20 10
4
3
0
20
40
60
τ, min
80
100
120
3 2 140
Fig. 1. Evolution of the properties of {Fe(OH)2 + FeCl2} suspensions having various percent precipitation values ((1, 3) 44% and (2, 4) 69%) during oxidation at 20°C: (1, 2) percent oxidation of the suspension and (3, 4) pH of the suspension. Dashed lines show the percent precipitation of the suspensions.
[ Fe ]s.ph. [ Fe ]s.ph. − [Fe ]s.ph. × 100% = tot tot [ Fe ]s.ph. [ Fe ]s.ph. III
α s.ph. = ox
× 100% ≈
tot
[ Fe tot]susp − [ Fe II]susp tot [ Fe ]susp β prec
II
(4)
α susp × 100%, β prec ox
× 100% =
where [FeIII]s.ph., [Fetot]s.ph., and [FeII]s.ph. are, respec tively, FeIII, Fetot, and FeII concentrations in the solid phase. Noteworthy is that, despite an invariable iron(II) oxidation rate in the system, the pH curve has a com plex trend. A characteristic feature of the pH curve is a “indent,” which appears regardless of the anion of the initial iron(II) salt, percent precipitation (which is proportional to the initial suspension pH), the con centration of the oxidized phase, oxidation rate, and temperature (Fig. 1). This pH = f(τ) trend was first described by us in [5] and was later repeatedly sup ported by other researchers [11–14]. Proceeding from the different rates of dissolution in acid of solid phases containing iron(II) and iron(III) compounds as observed by Tamaura et al. [15], we discovered that the appearance of final oxidation products (in the case at hand, iron oxide hydroxides FeOOH) is observed only in the samples obtained after the “indent” appeared in the pH curve. The indent appears on the pH curve at a roughly constant value of the percent oxidation of the solid RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
phase containing iron(II)–iron(III) compounds. Chemical analysis allowed us to determine that, for chloride systems, the percent oxidation of the solid phase at the moment when the “indent” appears is 35– 45%, corresponding to the composition of the oxi + 3+ dized solid phase formulated as Fe 20.55 −0.70 Fe 0.30 −0.45, II III i.e., the ratio Fe : Fe ≈ (2.0–1.2) : 1; in sulfate sus pensions, the respective value is 45–50%, correspond + 3+ ing to the composition Fe 20.50 −0.55 Fe 0.45−0.50 , or the ratio II III Fe : Fe ≈ 1 : 1. These results correspond to chemical analysis data obtained for GR1(Cl–) and GR2(SO 24−) [16]. Electron microscopy and Xray diffraction were used to study the oxidation mechanism of iron(II) compounds in a more detailed way. In the initial sus pension samples taken several minutes after neutral ization, the most substance amount was found to con sist of aggregates with sizes of several hundreds of nanometers comprising smaller particles with sizes of 100–150 nm for the {Fe(OH)2 + FeCl2} system (Fig. 2) and 20–40 nm for the {Fe(OH)2 + FeSO4} system. Along with aggregates of uncertain shape, a small amount of particles with sizes of 200–300 nm shaped as hexagons was observed in the initial samples (Fig. 2). Xray diffraction identified GR1(Cl–) (a rhombo hedral unit cell [6, 17]) and GR2(SO 24− ) (a hexagonal unit cell [6, 18]) in these samples with CDDs of ~10– 12 nm for the chloride systems (Table 1) and sulfate systems (Table 2). The formation of Fe(II)–Fe(III) double hydroxo salts is likely due to the small amount Vol. 55
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(а)
400 nm (b)
(c)
1000 nm (d)
(e)
(f)
Fig. 2. Lightfield images of a {Fe(OH)2 + FeCl2} suspension (percent precipitation: 74%; pHin: 7.80; 20°C) for various percent oxidation values of the solid phase: (a) 10, (b) 19, (c) 36 (immediately after passing the indent), (d) 53, (e) 74, and (f) 99%. The same scale is used in Fig. 2a and Fig. 2b, as well as in Figs. 2c–2f. The electron diffraction pattern for a hexagonal particle is in the inset to Fig. 2c.
of iron(III) compounds (5–10%) contained in the ini tial solutions and, accordingly, in iron(II) suspensions. Noteworthy is the absence of an Fe(OH)2 phase in the initial suspension; we earlier found this phase in the strongly alkaline region [8]. Once the oxidation begins, aggregate and agglom erate increase in size to several micrometers and appear surrounded by hexagonshaped crystals. Con currently, the CDDs of GR increase to 20–50 nm
(depending on the synthesis parameters; Cf. Tables 1, 2). Similar crystallite size data were obtained using darkfield images. Electron diffraction experiments showed that the GR crystal structure belongs to the hexagonal particles (Fig. 2c). The appearance of a indent on the pH curve corresponds to the appearance in samples of anisotropic nanoparticles of αFeOOH (orthorhombic crystal system, space group Pbnm (62), PDF 441415) or γFeOOH (orthorhombic crystal system, space group Bbmm (63), PDF 290713). Elec
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Table 1. Evolution of the composition of a {Fe(OH)2 + FeCl2} suspension (percent precipitation: 74%; pHin: 7.80) during oxi dation at 20°C Sample no.
α s.ph. , %
1
10
GR1 (10)
2
19
GR1 (13)
3
4
ox
Phase composition (CDD, nm) IGR1/IγFeOOH
GR1 (20) + γFeOOH (traces) 36 Immediately after passing the “indent” 53 GR1 (20) + γFeOOH (13)
5
74
GR1 (20) + γFeOOH (15)
6
99
γFeOOH (15)
Appearance and size of structural elements (nm) Aggregates ~350–700; primary particles 100–150; hexagonal particles 200–300 Aggregates 700–1000; agglomerates of up to 3000; primary particles 100–160; hexagonal particles 200–300 Aggregate1300–2000; hexagonal particles 200–300; needleshaped particles: length 200–350, width 15–25
~50
6.3 0.5
Aggregates 1000–2000; anisotropic particles (ag gregates): length 600–1000, width 60–100 Aggregate 350–670; Anisotropic particles (aggre gates): length 1500–2300, width180–250 Anisotropic particles (aggregates): length 2300– 2700, width 200–400
Notes: I is the main Xray diffraction peak intensity for a given compound. Samples in Table 1 are numbered to match the symbolic notations in Fig. 2 as follows: “1” stands for “a,” “2” for “b,” “3” for “c,” “4” for “d,” “5” and “6” for “f.”
Table 2. Evolution of the composition of a {Fe(OH)2 + FeSO4} suspension (percent precipitation: 63%; pHin 8.20) during ox idation at 40°C ox Sample α s.ph. , % Phase composition (CDD, nm) IGR2/IαFeOOH no.
1 2
15 29
GR2 (12) GR2(15)
3
45 “indent”
4
72
GR2 (50) + αFeOOH (22)
5
96
αFeOOH (25)
GR2 (50) + αFeOOH (traces)
~40
tron microscopy shows that the formation of these nanoparticles may occur in two concurrent routes: via the formation of needleshaped particles near the GR surface thanks to the dissolution–oxidation–precipi tation mechanism (Fig. 3a) and via growth of aniso tropic particles from GR aggregates as a result of solid phase oxidation (Fig. 3b). Legrand et al. [19] arrived at similar inferences in studying the oxidation of carbon ate green rust GR1(CO32− ). We also discovered the solidphase character of the oxidation process when a sample known to contain γFeOOH anisotropic parti cles and some residual amount of Fe(II)–Fe(III) compounds was subjected to an electron beam in the vacuum column of the electron microscope. Heat and residual oxygen cause a gradual oxidation of the iron(II)–iron(III) complex, formation of fine RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
0.8
Appearance and size of structural elements (nm) Aggregates ~200–300; primary particle 20–40; Aggregates 2000–3000; primary particle 70–90; hexa gonal particles 700–1000 Aggregates 2000–3000; hexagonal particles 1000– 1300; needleshaped particles: length 300–400, width 30–40 Hexagonal particles 1000–1100; needleshaped parti cles: length 450–500,width 40; anisotropic particles (aggregates): length 500–800, width 50–100 Anisotropic particles (aggregates): length 1300–1700, width 130–270
FeOOH crystallites, and their subsequent oriented incorporation into existing particles. A further oxidation of the suspensions brings about a decrease in the percent amount and size of the initial aggregates and agglomerates, gradual disappearance of hexagonal particles, and an increase in the amount and size of anisotropic particles (Fig. 2). Along with growing in size, the primary anisotropic particles are aggregated so that the resulting iron oxide hydroxides are aggregates of structural elements (anisotropic platelets in the case of αFeOOH and needles in the case of γFeOOH) oriented along the major crystallo graphic axis [001] (Fig. 4). Noteworthy, the resulting αFeOOH crystalline particles contain, along with anisotropic aggregates, a small amount of intergrowths in the form of dendrites (Fig. 4a). However, γFeOOH Vol. 55
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(а)
200 nm (b)
400 nm
Fig. 3. Lightfield images of a {Fe(OH)2 + FeCl2} suspension (percent precipitation: 74%; pHin: 7.80; 20°C) immediately after passing the indent (percent oxidation of the solid phase: 36%).
aggregates obtained under similar conditions are larger and more anisotropic (compared to αFeOOH) and do not contain dendrites (Fig. 4b). Comparing the particle size values determined by electron microscopy and Xray diffraction, we found that all compounds formed during oxidation are polycrystalline (both GR
(а)
200 nm
(b)
400 nm
Fig. 4. Lightfield images of (a) αFeOOH and (b) γ FeOOH particles.
and FeOOH). Our results well match the Domingo et al.’s theoretical and experimental data on the structure of FeOOH particles [20]. We should mention that the oxidation of {Fe(OH)2 + FeCl2} suspensions at temperatures no higher than 20–25°С produces singlephase nanocrystalline γFeOOH. As temperature increases further to 30°С, an αFeOOH impurity appears, and then at 40°С, magnetite Fe3O4 appears. γFeOOH still remains the major phase. At the same time, the oxidation of {Fe(OH)2 + FeSO4} suspensions at temperatures below 20°С induces the formation of a mixture of αFeOOH + γFeOOH nanocrystalline phases. Temperature eleva tion leads to the γFeOOH αFeOOH phase tran sition and the formation of nanocrystalline αFeOOH at 40°С. It seems that the occurrence of chloride ions enhances γFeOOH formation. In summary, our study showed that the oxidation rate of iron(II) compounds in a nearneutral pH is practically unaffected by [FeII] and pH, being invari able over a wide range of the percent oxidation values of the solid phase (5–10 to 85–90%). Two steps of the process have been recognized. The first step produces iron(II)–iron(III) hydroxo salts having the GR struc ture in the form of nanocrystalline particles shaped as hexagons, which is accompanied with a dramatic shift down of pH. The second step (where pH changes weakly) produces anisotropic nanocrystalline particles of iron(III) oxide hydroxides via the dissolution–oxi dation–precipitation mechanism and via solidphase oxidation. GR1(Cl–) oxidation enhances the forma tion of γFeOOH nanocrystalline particles, whereas GR2(SO 24− ) oxidation enhances the formation of an αFeOOH nanocrystalline phase. Oxyhydroxides are synthesized as polycrystalline particles and represent aggregates of anisotropic primary structural elements aligned along the major crystallographic axis.
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