Influence of Anions and Cations on the Formation of Iron Oxide Nanoparticles in Aqueous Media

Recently, various types of functionalized metal oxide nanoparticles have been used for many applications because of their unique chemical and physical properties. To synthesize metal oxide nanoparticles, liquid-phase synthesis techniques have been developed. The production process of metal oxide nanoparticles in aqueous media is extremely complex because the formation, crystal structure, crystallinity, chemical composition, and morphology of the particles are considerably dependent on the preparation conditions (e.g., anion and cation concentrations and species, additives, solution pH, and reaction temperature and time). Accordingly, clarifying these effects is fundamental to accurately understand the formation mechanism of metal oxide nanoparticles to further develop new functionalized nanoparticles. In this review, the influence of anions (Cl – , SO 42– , and NO 3– ) and cations (Ni 2+ , Cu 2+ , and Cr 3+ ) on the formation and structure of iron oxide nanoparticles in aqueous media is described.


Introduction
In industries, various types of functionalized metal oxide nanoparticles, such as iron oxide, silica gel, alumina, zinc oxide, and titania, have been developed and used for many applications (e.g., catalysis, adsorbent, biomedicine, magnetic data storage, agriculture, fuel cell, pigment, and sensor material) because of their unique chemical and physical properties (Iler R.K., 1979;Cornell R.M. and Schwertmann U., 1996;2000;Fujishima A. et al., 1999;Rodríguez J.A. and Fernández-García M., 2006;Chavali M.S. and Nikolova M.P., 2019;Korotcenkov G. et al., 2019). To further improve the particle properties, controlling the particle shape and size, monodispersity, and crystallinity is necessary. However, in general, the synthesis of high-quality nanoparticles, such as monodisperse single-crystal nanoparticles, is considerable difficult (Matijević E., 1985;Sugimoto T., 1987;2001;Pelizzetti E., 2011). Metal oxide nanoparticles can be produced using solid, liquid, and gasphase synthesis techniques. Among these, the liquid-phase synthesis affords several advantages (Gutch A. et al., 2004;Feng J. et al., 2015;Karatutlu A. et al., 2018): (1) low-cost particle production can be performed; (2) surface active nanoparticles can be obtained; (3) particles from a homoge-neous solution can be prepared; (4) preparation conditions (e.g., reaction temperature and time, anion and cation concentrations, additives, solution pH, and raw materials) can be easily controlled; (5) particle shape and size can be regulated by the abovementioned preparation conditions and method. Based on these benefits, various liquid-phase synthesis techniques for the production of metal oxide nanoparticles have been developed, and approximately 45 % of functionalized nanoparticles are synthesized through these methods in the industry (Charitidis C.A. et al., 2014). The liquid-phase synthesis can be classified into two approaches: "Top-down approach" and "Bottom-up approach." In the "Top-down approach," nanoparticles are synthesized by etching the macro-sized or micro-sized bulk materials in aqueous media (e.g., wet milling, lithography, laser ablation, and chemical etching). In the "Bottom-up approach," nanoparticles are prepared by assembling small blocks, such as atoms and molecules, in aqueous media (e.g., sol-gel, precipitation, pyrolysis, and chemical reduction methods).
Reactions (1)-(4) continuously proceed to yield multinuclear Fe 3+ complex ions. These ions further grow to produce an embryo through ion collisions in which the reaction continuously breaks down and reforms during the reversible reaction stage. When the embryo exceeds a certain critical size, it becomes a nucleus, which is considerably more stable. The generated nucleus is further grown to form fine primary particles, which transform into crystals via two possible routes: "aggregation mechanism" and "ion diffusion growth mechanism". In the former route, aggregates are generated by the agglomeration of primary particles. During aging, the primary particles in the aggregates solidify into crystalline particles. In the latter mechanism, solute ions are diffused and deposited on the primary particles to form crystalline particles. The growth process of α-FeOOH particles is depicted in Fig. 2. In this case, the primary particles composed of fine crystals are regularly combined by -O-and -OH-bonds to crystallize as α-FeOOH. Accordingly, the formation process of metal oxide nanoparticles is considerably complex and is not fully understood. The formation, crystal structure, crystallinity, chemical composition, and particle morphology are extremely dependent on the preparation conditions, such as anion and cation concentrations and species, additive, solution pH, and reaction temperature and time. Among these, anion and cation were observed to markedly affect the formation and structure of iron oxides. The phase stability of γ-FeOOH was lowered by Cland SO 4 2- (Sudakar S. et al., 2003). Moreover, the increase of [SO 4 2-]/[Cl -] ratio in the aqueous Fe 2 (SO 4 ) 3 -FeCl 3 solution accelerated the α-FeOOH formation (Oh S.J. et al., 2002). Further, the addition of SO 4 2and SO 3 2significantly impeded the formation and crystallization of β-FeOOH Kamimura T. et al., 2005;Tanaka H. et al., 2016a;b). The crystallization and growth of α-FeOOH and β-FeOOH particles were inhibited by adding Cu 2+ , Ni 2+ , Cr 3+ , and Ti 4+ (Inouye K. et al., 1968;1972;Ishikawa T. et al., 2000;2001;2002;2009;Nakayama T. et al., 2005;Tanaka H. et al., 2013a;b). Accordingly, elucidating these effects  is fundamental to accurately understand the formation mechanism of metal oxide nanoparticles.
In this review, the influence of anions and cations on the formation and structure of iron oxide nanoparticles is described based on our research. The iron oxide nanoparticles were synthesized in the presence of anions (Cl -, SO 4 2-, and NO 3 -) and cations (Ni 2+ , Cu 2+ , and Cr 3+ ) (Tanaka H. et al., 2013a;b;2016a;. The results can aid in developing new functionalized nanomaterials using the liquid-phase synthesis technique.

Influence of anions on formation of iron oxide particles
The influence of anions on the formation of iron oxide nanoparticles was examined by synthesizing the particles from Fe 3+ solutions containing Cl -, NO 3 -, and SO 4 2-. A mixture of aqueous FeCl 3 , Fe 2 (SO 4 ) 3 , and Fe(NO 3 ) 3 solutions were aged at 50 °C for 48 h. Prior to aging, the solution pH was approximately 2. The generated precipitates were filtered out, thoroughly washed with deionized-distilled water, and air-dried at 50 °C.
The amount of products formed is shown in Fig. 3. This amount is increased by incrementing the quantity of FeCl 3 added to the FeCl 3 -Fe(NO 3 ) 3 system. In the Fe(NO 3 ) 3 -Fe 2 (SO 4 ) 3 , Fe 2 (SO 4 ) 3 -FeCl 3 , and FeCl 3 -Fe(NO 3 ) 3 -Fe 2 (SO 4 ) 3 systems, increasing the quantity of Fe 2 (SO 4 ) 3 through addition reduces the amount of formed products. The amount of these products eventually become zero when higher amounts of SO 4 2are added. These results indicate that the formation of iron oxides is inhibited by the addition of SO 4 2-. The major formation species of iron oxides is shown in Fig. 4. Iron-oxyhydroxysulfate (Fe 8 O 8 (OH) 6 (SO 4 )·nH 2 O: Schwertmannite) is formed by adding SO 4 2-, however, the addition of a large amount of SO 4 2prevents the generation of iron oxides. In contrast, β-FeOOH is mainly yielded by the FeCl 3 -Fe(NO 3 ) 3 system, and α-FeOOH is only generated in the Fe(NO 3 ) 3 system, indicating that the FeCl 3 -Fe(NO 3 ) 3 system favors the formation of β-FeOOH rather than α-FeOOH.
Transmission electron microscopy (TEM) images of the formed particles are shown in Fig. 5. The spherical α-FeOOH particles with an average diameter of 358 nm are generated in the Fe(NO 3 ) 3 system. In the FeCl 3 -Fe(NO 3 ) 3 system, the spherical α-FeOOH particles virtually diminish, and rod-shaped β-FeOOH particles are observed. Further, as the amount of added Clincreases, the mean length and width of β-FeOOH particles increase from 677 nm to 1.48 μm and from 130 to 238 nm, respectively. Hence, the incorporation of NO 3 slightly suppresses the growth of β-FeOOH particles. In the Fe(NO 3 ) 3 -Fe 2 (SO 4 ) 3 system, needle-like Schwertmannite particles with a mean length and width of approximately 260 and 15 nm, respectively, are mainly formed. By adding NO 3 -, the Schwertmannite particles disappear, and irregular-shaped α-FeOOH particles with an average size of 68 nm are generated. In the Fe 2 (SO 4 ) 3 -FeCl 3 system, increasing the amount of added SO 4 2decreases the particle size of β-FeOOH and forms Schwertmannite particles (mean length = 136 nm, mean width = 49 nm). In the FeCl 3 -Fe(NO 3 ) 3 -Fe 2 (SO 4 ) 3 system, due to the Schwertmannite, only needle-like particles with a length and width of approximately 310 and 40 nm are recognizable. These facts reveal that the presence of SO 4 2in the aforementioned system remarkably precludes the formation of α-FeOOH and β-FeOOH particles and produces needle-like Schwertmannite particles. Note that a higher SO 4 2content in the solution inhibits the formation of particles. Thus, the effect of SO 4 2on the formation of iron oxide particles is greater than that of Cland NO 3 -. These results indicate that anions, such as SO 4 2-, Cl -, and NO 3 -, affect the formation, crystallization and particle growth of iron oxides, and the effect is in the order SO 4 2->> Cl -> NO 3 -. The affinity of anions to Fe 3+ can be evaluated through the stability constant (log K) of Fe 3+ complexes with anions. The stability constant of Fe 3+ complexes with SO 4 2is 4.1 and considerably larger than that with NO 3 -(1.0) and Cl -(0.6), indicating the strong   Consequently, when a small amount of SO 4 2is added, the major product becomes Schwertmannite containing SO 4 2-. Upon increasing the amount of added SO 4 2-, the SO 4 2content in the [Fe(SO 4 ) m (H 2 O) 6-m ] 3+ ions increases and the number of active >Fe-OH groups is reduced, resulting in the suppression of olation reaction (3). Accordingly, no precipitates are generated at higher SO 4 2contents in the solution. In contrast, β-FeOOH is mainly produced in the FeCl 3 -Fe(NO 3 ) 3 system. The stability constants of Fe 3+ complexes with Cland NO 3 are considerably lower than that with SO 4 2-. Hence, the addition of Cland NO 3 prevents the progress of reactions (1)-(4), leading to the formation of the amorphous precursor of α-FeOOH and β-FeOOH particles during aging. It has been established that the isoelectric point (iep) of iron oxides and oxyhydroxides, such as α-FeOOH, β-FeOOH, γ-FeOOH, Fe 3 O 4 , and α-Fe 2 O 3 , is a neutral pH, and aforementioned materials have a positive surface charge in the acidic solution (Cornell R.M. and Schwertmann U., 1996;2000). Moreover, the ionization energy of NO 3 is larger than that of Cl - (Lang P.F. and Smith B.C., 2003). Therefore, the added Clpreferentially adsorbs on the surface of the amorphous precursor to form Cl --containing β-FeOOH particles. However, the decrease in the amount of added Clreduces the size of β-FeOOH particles, indicating the inhibition of their growth as a result of the adsorption of NO 3 on the precursors. To clarify the influence of anion and solution pH on the formation of iron oxide nanoparticles, the particles were synthesized from the FeCl 3 -Fe 2 (SO 4 ) 3 -Fe(NO 3 ) 3 system at different OH -/3Fe 3+ molar ratios. The OH -/3Fe 3+ ratio ranges 0-1.5 through the addition of NaOH solutions. The pH titration curves of aqueous FeCl 3 , Fe 2 (SO 4 ) 3 , and Fe(NO 3 ) 3 solutions are shown in Fig. 6. All the curves are practically identical, indicating that the solution pH is independent of the anions. At OH -/3Fe 3+ = 0-1.0, the solution pH becomes practically constant (i.e., 2-3) because of the reaction of Fe 3+ with the added OHto precipitate Fe(OH) 3 . At OH -/3Fe 3+ > 1.0, the pH level of the solution suddenly increases because all the Fe 3+ transforms into Fe(OH) 3 . The major products synthesized at OH -/3Fe 3+ = 0-1.5 are shown in Fig. 7. At OH -/3Fe 3+ = 0.5, the hydrated iron oxide Fe 2 O 3 •nH 2 O particles are mainly produced in the FeCl 3 -Fe(NO 3 ) 3 system. In contrast, the Fe 2 (SO 4 ) 3 -FeCl 3 , Fe(NO 3 ) 3 -Fe 2 (SO 4 ) 3 , and FeCl 3 -Fe 2 (SO 4 ) 3 -Fe(NO 3 ) 3 systems enhance the α-FeOOH formation. This is caused by the transformation of Schwertmannite during aging because this material is among the precursors of α-FeOOH (Cornell R.M. and Schwertmann U., 1996;2000;Tanaka Fig. 5 TEM images of the particles formed in FeCl 3 -Fe(NO 3 ) 3 -Fe 2 (SO 4 ) 3 system. Adapted with permission from Ref. (Tanaka H. et al., 2013a). Copyright: (2013) Elsevier B.V. H. et al., 2013a;2016a;Muguruma et al., 2015). The major products at OH -/3Fe 3+ = 1.0 and 1.5 (pH levels are approximately 7 and 14, respectively) are ferrihydrites (Fe 5 HO 8 ·4H 2 O) and α-FeOOH, respectively. The foregoing means that raising the solution pH before aging lowers the influence of anions on the structure and composition of iron oxide nanoparticles.

Influence of cations on formation of iron oxide nanoparticles
The influence of cations on the formation of iron oxide nanoparticles was examined by synthesizing the α-FeOOH particles with Ni 2+ , Cu 2+ , and Cr 3+ . Metal salts, such as NiSO 4 , CuSO 4 , or Cr(NO 3 ) 3 , and FeSO 4 were dissolved in O 2 -free deionized-distilled water, of which the metal/Fe atomic ratio of the solution was 0-0.1. The aqueous NaOH solution was added to the solution until the OH -/2Fe 2+ ratio reached 0.05. The generated suspension was aged in bubbling air at 50 °C for 24 h. The resulting precipitate was filtered out, washed with deionized distilled water, and air-dried at 50 °C.
X-ray diffraction (XRD) patterns of the products synthesized with Ni 2+ , Cu 2+ and Cr 3+ are shown in Fig. 8. As shown in Fig. 8(A), with the addition of Ni 2+ , the intensity and position of α-FeOOH peaks remain virtually unchanged. As shown in Fig. 8(B), with Cu 2+ addition, the α-FeOOH peaks are weakened and broadened by increasing the Cu/Fe ratio. A similar tendency is observed with the addition of Cr 3+ as shown in Fig. 8(C). Besides, the α-FeOOH peaks vanish at Cr/Fe ≥ 0.04 and broad peaks ascribed to Schwertmannite are developed. Furthermore,  sharp peaks due to Natrojarosite NaFe 3 (SO 4 ) 2 (OH) 6 appear at Cr/Fe ≥ 0.08. Hence, the addition of Cu 2+ and Cr 3+ inhibits the crystallization of α-FeOOH and a considerable amount of added Cr 3+ impedes the transformation of Schwertmannite into α-FeOOH. The crystallite size of α-FeOOH, which is calculated using the Scherrer equation, is plotted against the metal/Fe ratio in Fig. 9. The crystallite size is 24.1 nm when metal/Fe = 0 and it decreases as the Cu/Fe ratio increases up to 0.02, indicating that the crystallization of α-FeOOH is inhibited. The Cr 3+ addition also reduces the crystallite size and its effect is slightly higher than that of Cu 2+ addition. Nonetheless, no considerable influence is observed by Ni 2+ addition. These results indicate that the addition of Cu 2+ and Cr 3+ inhibits the crystallization of α-FeOOH and the effect is in the order Cr 3+ > Cu 2+ >> Ni 2+ .
In Fig. 10 are plotted the metal/Fe atomic ratio of the particles estimated from the Ni, Cu, Cr, and Fe contents against the metal/Fe ratio of the solution. It is clearly observed that the Cr/Fe ratio of the particles is considerably larger than that of the solution. However, the Cu/Fe and Ni/  Fig. 9 Crystallite size of α-FeOOH vs. atomic ratio metal/Fe. Adapted with permission from Ref. (Tanaka H. et al., 2018). Copyright: (2018) Elsevier B.V.
Fe ratios of the particles are lower than those of the starting solutions. These indicate that the added Cr 3+ is more easily incorporated in the particles than Fe 3+ , in contrast, the uptake of Ni 2+ and Cu 2+ is difficult.
TEM images of the particles generated in the presence of Ni 2+ , Cu 2+ , and Cr 3+ are shown in Fig. 11. When metal/ Fe = 0, rod-shaped α-FeOOH particles with a length and width of approximately 270 and 40 nm, respectively, are recognized. The morphology of α-FeOOH particles is unchanged by Ni 2+ addition, indicating that the added Ni 2+ has no considerable influence on the growth of α-FeOOH particles. At Cu/Fe = 0-0.01, no significant change in the morphology of α-FeOOH particles is observed. At Cu/ Fe ≥ 0.02, the rod-shaped α-FeOOH particles transform into irregular α-FeOOH particles with a size of approximately 65 nm. Similar to Cu 2+ addition, with Cr 3+ addition, increasing the Cr/Fe ratio up to 0.01 substantially reduces the size of α-FeOOH particles to approximately 50 nm and alters the rod-shaped particles into irregular ones. These results indicate that the growth of α-FeOOH particles is considerably suppressed by Cu 2+ and Cr 3+ addition. At Cr/ Fe ≥ 0.04, irregular-shaped Schwertmannite particles with a size of approximately 25 nm are observed. Furthermore, hexagonal Natrojarosite particles with a size of approximately 0.8 μm appear when Cr/Fe ≥ 0.06.
The formation and crystallization of iron oxide particles considerably depend on the solution pH (Misawa T. et al., 1974;Blesa M.A. and Matijević E., 1989;Cornell R.M. and Schwertmann U., 1996;2000;Tamura H., 2008). The solution pH before aging is plotted as a function of metal/Fe atomic ratio in Fig. 12. The solution pH before aging is 7.0 at metal/Fe = 0 and is essentially not altered by Ni 2+ addition. Further, the solution pH before aging is reduced by adding Cu 2+ and Cr 3+ up to metal/Fe = 0.02 and remains practically constant at approximately 4.5 and 4.0, respectively. This is due to the formation of Cu(OH) 2 Fig. 10 Plots of atomic ratio metal/Fe in particles against atomic ratio metal/Fe in solution. Adapted with permission from Ref. (Tanaka H. et al., 2018). Copyright: (2018) Elsevier B.V. Fig. 11 TEM images of the particles generated with Ni 2+ , Cu 2+ and Cr 3+ . Adapted with permission from Ref. (Tanaka H. et al., 2018). Copyright: (2018) Elsevier B.V. and Cr(OH) 3 through the addition of NaOH solution because the reactions of Cu 2+ and Cr 3+ with OHprogress at pH ≥ 4.2 and ≥ 4.0, respectively (Baes Jr C.F. and Mesmer R.E., 1986). Hence, lowering the solution pH before aging by adding Cu 2+ and Cr 3+ suppresses the crystallization and particle growth of α-FeOOH. To verify this, α-FeOOH particles were prepared at pH = 4.0 and 7.0 without metal ion addition. The XRD patterns and TEM images of the obtained particles are depicted in Fig. 13. It is evident that the reduction in solution pH before aging significantly weakens and broadens the α-FeOOH peaks. Moreover, the crystallite size of α-FeOOH obtained at pH = 4.0 is 7 nm less than that at pH = 7.0 (24 nm). Further, the rodshaped particles become irregular with a size of ca. 30 nm.
These results indicate that reducing the solution pH before aging by adding Cu 2+ and Cr 3+ considerably prevents the crystallization and particle growth of α-FeOOH. The crystal structure of α-FeOOH consists of double chains of edge-shared Fe 3+ octahedron, as shown in Fig. 2 (Cornell R.M. and Schwertmann U., 1996;2000). The ionic radii of octahedral Ni 2+ (0.069 nm) and Cu 2+ (0.072 nm) are larger than that of octahedral Fe 3+ (0.064 nm), hence, the substitution of Ni 2+ and Cu 2+ with Fe 3+ in the α-FeOOH crystal is difficult (Shannon R.D., 1976). The solution pH before aging is markedly decreased by addition of Cu 2+ and Cr 3+ , consequently impeding the growth of α-FeOOH crystals and particles, nonetheless, Ni 2+ addition exhibits no distinct influence. In contrast, the added Cr 3+ is easily incorporated into the α-FeOOH crystal by substitution with Fe 3+ to form α-Cr x Fe 1-x OOH solid solution because the ionic radius of Cr 3+ in the octahedron (0.063 nm) is the same as that of Fe 3+ (0.064 nm) (Shannon R.D., 1976). Further, increasing the amount of added Cr 3+ induces the formation of Cr 3+ -doped Schwertmannite particles. As mentioned above, Schwertmannite is one of the precursors of α-FeOOH. Therefore, the incorporation of Cr 3+ into Schwertmannite stabilizes its crystal structure, thus inhibiting its transformation into α-FeOOH. Consequently, the formation, crystallization and particle growth of α-FeOOH are markedly impeded by Cr 3+ , and the effect is in the order Cr 3+ > Cu 2+ >> Ni 2+ .

Conclusions
Recently, various functionalized metal oxide nanoparticles with unique chemical and physical properties have been developed and applied in the fields of chemical Fig. 12 Plots the solution pH before aging as a function of atomic ratio metal/Fe. Adapted with permission from Ref. (Tanaka H. et al., 2018). Copyright: (2018) Elsevier B.V.

Fig. 13
XRD patterns and TEM images of α-FeOOH particles prepared at pH = 4.0 and 7.0. Adapted with permission from Ref. (Tanaka H. et al., 2018). Copyright: (2018) Elsevier B.V. engineering, medicine, food, cosmetics, environment, etc. The particle properties are extremely dependent on the crystal structure, crystallinity, chemical composition, and particle morphology. Therefore, nanoparticles with high crystallinity, homogeneous chemical composition, uniform shape and size, and narrow particle size distribution possess excellent chemical and physical properties. However, the synthesis of such particles is extremely difficult. Hence, many researchers have investigated the synthesis of high-quality nanoparticles using the liquid-phase synthesis technique. The formation of nanoparticles is considerably affected by a number of factors, such as the anion and cation concentrations and species, additive, solution pH, and reaction temperature and time. To derive high-quality nanoparticles, gaining insight on the fundamental principles of the formation mechanism and process of nanoparticles is necessary. Accordingly, a systematic study of the particle formation is absolutely essential for the development of new functionalized nanoparticles.