X-ray diffraction

X-ray diffractograms of Fh-6 and Fh-SW-NH₄SCN-2, and Fh-SW samples presented six characteristic peaks of 6-line ferrihydrite and two characteristic peaks of 2-line ferrihydrite, respectively (Fig. 1) (Cornell and Schwertmann, Reference Cornell and Schwertmann2003). Artificial seawater 4.0 Gy only had an effect on the crystallinity of ferrihydrite, since instead of 6-line ferrihydrite, 2-line ferrihydrite was obtained. However, Samulewski et al., observed that in the synthesis of magnetite, besides magnetite, goethite (26.9%) and gypsum (19.4%) were also obtained (Samulewski et al., Reference Samulewski, Gonçalves, Urbano, da Costa, Ivashita, Paesano and Zaia2020). The Fh-SW-NH₄SCN-2 sample showed lower crystallinity when compared to the Fh-6 sample, probably due to the high quantity of salts present in seawater 4.0 Gy that interferes with the formation of the ferrihydrite structure. It should be pointed out that ammonium thiocyanate was in the burette along with potassium hydroxide and this solution was dripped into the beaker for 1 h. Thus, for most of the time of the synthesis, the ammonium thiocyanate was at a much lower concentration than the ferrous nitrate or the salts of the artificial seawater 4.0 Gy. Therefore, it probably did not have an effect on the synthesis products, because of its low concentration. Since artificial seawater 4.0 Gy contains a high quantity of Mg²⁺, there is a possibility that this cation is being incorporated into the ferrihydrite structure, causing crystallinity loss (Giovanoli and Schwertmann, Reference Giovanoli and Schwertmann1992). For the Fh-SW-NH₄SCN-2 sample, the SCN⁻ anion was added to the burette along with the OH⁻ anion. Since the SCN⁻ anion is dripped into the beaker, its concentration is always much lower than the SO₄²⁻ anion concentration. Thus, the SO₄²⁻ anion from artificial seawater interacts with the Fe³⁺ cation forming complexes and the SCN⁻anion does not interact with the Fe³⁺ cation (Broadhurst and Preez, Reference Broadhurst and du Preez1993).

Fig. 1. X-ray diffractograms of the samples (a) Fh-6-synthesis of ferrihydrite-6-lines; Fh-SW-4.0-synthesis of ferrihydrite in artificial seawater 4.0 and Fh-SW-NH₄SCN-2-synthesis of ferrihydrite in artificial seawater 4.0 being that NH₄SCN plus KOH together in the burette and (b) Fh-SW-4.0-NH₄SCN-1-synthesis of ferrihydrite in artificial seawater 4.0 being that NH₄SCN plus Fe(NO₃)₃9H₂O together in the beaker; Fh-DW-NH₄SCN-1-synthesis of ferrihydrite in distilled water being that NH₄SCN plus Fe(NO₃)₃9H₂O together in the beaker and Fh-DW-NH₄SCN-2-synthesis of ferrihydrite in distilled water being that NH₄SCN plus KOH together in the burette. Mt-magnetite, Hm-hematite, Gt-goethite.

X-ray diffractograms of Fh-DW-NH₄SCN-2 and Fh-SW-NH₄SCN-1 samples presented characteristic peaks of goethite, hematite and magnetite (Fig. 1). For the Fh-DW-NH₄SCN-1 sample, the X-ray diffractogram presented characteristic peaks of hematite and magnetite (Fig. 1). However, the FT-IR spectra of Fh-DW-NH₄SCN-2, Fh-SW-NH₄SCN-1 and Fh-DW-NH₄SCN-1 samples showed bands at 890 cm⁻¹ (O-H bending in plane) and 794 cm⁻¹ (O-H bending out of the plane) which are characteristic of goethite (Table 1) (Cornell and Schwertmann, Reference Cornell and Schwertmann2003). In addition, Samulewski et al. (Reference Samulewski, Gonçalves, Urbano, da Costa, Ivashita, Paesano and Zaia2020) also observed that when magnetite was synthesized in the presence of SCN⁻anion, goethite was also obtained. For Fh-DW-NH₄SCN-1 and Fh-DW-NH₄SCN-2 samples, the (hematite, (018)) and (magnetite, (311)) peaks should be the highest intensity, but probably due to orientation, they are not (Fig. 1). The Fh-6 sample showed weak bands at 890 cm⁻¹ and 801 cm⁻¹ (Table 1). These bands could be due to the formation of a small amount of goethite, which was not detected by X-ray diffractometry. For the Fh-SW and Fh-SW-NH₄SCN-2 samples these bands were not observed (Table 1).

Table 1. Assignments of frequencies (cm⁻¹) in FTIR spectra of the samples

Sh-shoulder, w-weak, vw-very weak; Fh-6-synthesis of ferrihydrite-6-lines; Fh-DW-NH₄SCN-1-synthesis of ferrihydrite in distilled water being that NH₄SCN plus Fe(NO₃)₃9H₂O together in the beaker; Fh-DW-NH₄SCN-2-synthesis of ferrihydrite in distilled water being that NH₄SCN plus KOH together in the burette; Fh-SW-4.0-synthesis of ferrihydrite in artificial seawater 4.0; Fh-SW-4.0-NH₄SCN-1-synthesis of ferrihydrite in artificial seawater 4.0 being that NH₄SCN plus Fe(NO₃)₃9H₂O together in the beaker and Fh-SW-NH₄SCN-2-synthesis of ferrihydrite in artificial seawater 4.0 being that NH₄SCN plus KOH together in the burette.

^a Cornell and Schwertmann, Reference Cornell and Schwertmann2003

^b Mazzetti and Thistlethwaite, Reference Mazzetti and Thistlethwaite2002

^c Ristić et al., Reference Ristić, de Grave, Musić, Popović and Orehovec2007

^d Nakamoto, Reference Nakamoto1978

^e Colthup et al., Reference Colthup, Daly and Wiberley1990.

When thiocyanate was added together with Fe³⁺ (Fh-SW-NH₄SCN-1 and Fh-DW-NH₄SCN-1), at the beginning of the synthesis the concentration of ammonium thiocyanate was high, so the iron oxides showed good crystallinity (Fig. 1). For the sample Fh-DW-NH₄SCN-2, the SCN⁻ anion was added to the burette along with the OH⁻ anion, however, unlike the Fh-SW-NH4SCN-2 sample, goethite, magnetite and goethite were obtained (Fig. 1). Probably, in this case, the SCN⁻ anion is not competing with seawater salts, mainly SO₄²⁻ anion. It should be noted that ferrihydrite has low stability, so ferrihydrite is an intermediate for the synthesis of other iron oxides (Cornell and Schwertmann, Reference Cornell and Schwertmann2003).

Pearson's theory

For the synthesis of these materials, Pearson's theory could provide information to the interaction between SCN⁻ and the Fe³⁺ ion on the formation of the M-SCN or M-NCS bond. The SCN⁻ ion can bind metals through the sulphur atom or nitrogen atom. When the metal is a hard acid, SCN⁻ binds through the nitrogen atom and when the metal is a soft acid SCN⁻ binds through the sulphur atom. Since the Fe³⁺ ion is considered a hard acid, the metal coordinates with the SCN⁻ ion through nitrogen to form isothiocyanate complexes (Pearson, Reference Pearson1963). It is well known that goethite is formed when sulphur compounds interact with the Fe³⁺ cation (Cornell and Schneider, Reference Cornell and Schneider1989; Cornell et al., Reference Cornell, Schneider and Giovanoli1989; Cornell and Schwertmann, Reference Cornell and Schwertmann2003; Vu and Moreau, Reference Vu and Moreau2015; Carneiro et al., Reference Carneiro, Ivashita, de Souza Junior, de Souza, Paesano, da Costa, di Mauro, de Santana, Zaia and Zaia2013; Samulewski et al., Reference Samulewski, Gonçalves, Urbano, da Costa, Ivashita, Paesano and Zaia2020). Therefore, this may be an indication that Fe³⁺ interacted with the SCN⁻ ion also by sulphur. In addition, the formation of magnetite could be an indication that the SCN⁻ ion oxidized forming thiocyanogen-(SCN)₂ or trithiocyanate ion-(SCN)⁻₃ and the reduction of Fe³⁺ to Fe²⁺ (see reactions below) (Broadhurst and Preez, Reference Broadhurst and du Preez1993)

$${\rm 2SC}{\rm N}^ \hbox{-} {\rm} + {\rm 2F}{\rm e}^{{\rm 3\ + \ }}\Leftrightarrow \lpar {{\rm SCN}} \rpar _ 2{\rm} + {\rm 2F}{\rm e}^{{\rm 2\ + \ }}$$

$${\rm 3SC}{\rm N}^ \hbox{-} {\rm} + {\rm 2F}{\rm e}^{{\rm 3\ + \ }}\Leftrightarrow \lpar {{\rm SCN}} \rpar ^ \hbox{-} _ 3 {\rm} + {\rm 2F}{\rm e}^{{\rm 2\ + \ }}$$

Infrared spectroscopy

The main purpose of using infrared spectroscopy was not to confirm the results of X-ray diffractometry but to verify details in the samples that diffractometry would not show. Using infrared spectroscopy, it was possible to verify that some samples have several substances adsorbed on them.

The Fh-DW-NH₄SCN-1, Fh-DW-NH₄SCN-2 and Fh-SW-NH₄SCN-1 samples showed a band at 2050 cm⁻¹ due to C≡N stretching of SCN⁻ (Table 1, online Supplementary Figure S1) (Nakamoto, Reference Nakamoto1978; Colthup et al., Reference Colthup, Daly and Wiberley1990). It should be noted that SCN⁻ was preconcentrated in all samples where goethite was synthesized (Table 1, Fig. 1). This could be an indication of a mechanism for the preconcentration of SCN⁻. In addition, for the goethite formation, sulphur of SCN⁻ interacts with Fe³⁺ (Cornell and Schneider, Reference Cornell and Schneider1989; Cornell et al., Reference Cornell, Schneider and Giovanoli1989; Cornell and Schwertmann, Reference Cornell and Schwertmann2003; Vu and Moreau, 2005; Carneiro et al., Reference Carneiro, Ivashita, de Souza Junior, de Souza, Paesano, da Costa, di Mauro, de Santana, Zaia and Zaia2013; Samulewski et al., Reference Samulewski, Gonçalves, Urbano, da Costa, Ivashita, Paesano and Zaia2020). The Fh-SW-NH₄SCN-2 sample showed a band at 880 cm⁻¹, which could be an indication of the interaction of SCN⁻ with Fe³⁺ by nitrogen. However, the band at 2050 cm⁻¹ due to C≡N stretching of SCN⁻ was not observed for this sample (Table 1). The Fh-SW-NH₄SCN-2 sample showed three bands at 2976 cm⁻¹, 2931 cm⁻¹ and 2897 cm⁻¹ due to N–H stretching of NH₄⁺(Table 1, online Supplementary Figure S1) (Nakamoto, Reference Nakamoto1978; Colthup et al., Reference Colthup, Daly and Wiberley1990).

The adsorption of SCN⁻ anion onto ferrihydrite occurs in a very acidic pH (Vu and Moreau, Reference Vu and Moreau2015) or in a neutral pH in the presence of artificial seawater 4.0 Gy, which contains high Mg²⁺, Ca²⁺ and SO₄²⁻ concentrations (Zaia et al., Reference Zaia, de Carvalho, Samulewski, de Carvalho Pereira and Zaia2020). It should be noted that the pH of the oceans 4.0 billion years ago was in the range from 6.3 to 7.2 (Halevy and Bachan, Reference Halevy and Bachan2017; Krissansen-Totton et al., Reference Krissansen-Totton, Arney and Catling2018). Thus, the sorption/precipitation of SCN⁻ and NH₄⁺ onto the samples is very important for prebiotic chemistry, as this is a way to preconcentrate these molecules and they could be used for further reactions in molecular evolution.

Fh-SW and Fh-SW-NH₄SCN-2 samples showed bands at 1096 cm⁻¹ and 1096/1050 cm⁻¹, respectively (Table 1). These bands are due to S-O stretching from the SO₄²⁻ of the artificial seawater (Nakamoto, Reference Nakamoto1978; Colthup et al., Reference Colthup, Daly and Wiberley1990). These samples (Fh-SW, Fh-SW-NH₄SCN-2) are ferrihydrites (Fig. 1). It should be noted that when SO₄²⁻ interacts with one or more oxygen atoms, the change in symmetry alters the number of vibrational modes. Thus, more bands in the 1097 cm⁻¹ region should be observed (Peak et al., Reference Peak, Ford and Sparks1999; Fukushi et al., Reference Fukushi, Aoyama, Yank, KItadai and Nakashima2013; Jonhston and Chrysochoou, Reference Johnston and Chrysochoou2016). Therefore, the interaction of SO₄²⁻ with the Fh-SW sample is characteristic of an outer-sphere complex and with the Fh-SW-NH₄SCN-2 sample an inner-sphere complex (Peak et al., Reference Peak, Ford and Sparks1999; Fukushi et al., Reference Fukushi, Aoyama, Yank, KItadai and Nakashima2013; Jonhston and Chrysochoou, Reference Johnston and Chrysochoou2016).

For all samples, bands were observed at 1472 cm⁻¹ and 1345 cm⁻¹ (Table 1, online Supplementary Figure S1). These bands are due to CO₃²⁻ adsorbed onto the samples (Mazzetti and Thistlethwaite, Reference Mazzetti and Thistlethwaite2002; Ristić et al., Reference Ristić, de Grave, Musić, Popović and Orehovec2007). The CO₃²⁻ was from CO_2(g) in the air which was absorbed by the synthesis solutions.

Surface analysis

The pore size, volume and surface area are important parameters in material science as well as in prebiotic chemistry, as the material with a high surface area will adsorb more molecules than a low surface area. In addition, pore size is important in prebiotic chemistry because molecules can enter the pores and mimic the walls of a cell. In the pores, the molecules will be protected against UV radiation or hydrolysis and the molecules will be separated from the environment, where reactions could occur (Brasier et al., Reference Brasier, Matthewman, McMahon and Wacey2011; Impey et al., Reference Impey, Lunine and Funes2012). Among the synthesized samples, the Fh-SW sample presented the highest surface area, pore volume and pore size (Table 2). It should be noted that this sample is a 2-line ferrihydrite and showed the lowest crystallinity (Fig. 1). The surface area, pore volume and pore size for the ferrihydrite samples (Fh-6, Fh-SW, Fh-SW-NH₄SCN-2) are close to the values obtained by other studies (Schultz et al., Reference Schultz, Benjamin and Ferguson1987; Stanjek and Weidler, Reference Stanjek and Weidler1992; Cornell and Schwertmann, Reference Cornell and Schwertmann2003; Pereira et al., Reference Pereira, Anizelli, di Mauro, Valezi, da Costa, Zaia and Zaia2019). As the Fh-DW-NH₄SCN-1 and Fh-DW-NH₄SCN-2 samples presented much lower crystallinity than the Fh-SW-NH₄SCN-1 sample (Fig. 1), they showed much higher surface areas than the Fh-SW-NH₄SCN-1 sample (Table 2). It should be noted that these samples are a mixture of magnetite, hematite and goethite (Fig. 1). Fh-SW and Fh-DW-NH₄SCN-2 samples could be classified as mesoporous materials (2 nm −50 nm) and the other samples as microporous materials (< 2 nm) (Table 2) (Cornell and Schwertmann, Reference Cornell and Schwertmann2003).

Table 2. Results of adsorption/desorption of N₂ at 77 K in ferrihydrite synthesis (surface analysis BET) and pH_pzc

*BET Brunauer-Emmett-Teller; **BJH Barrett-Joyner-Halenda; ^#pH_pzc = pH at the Point of Zero Charge Fh-6-synthesis of ferrihydrite-6-lines; Fh-DW-NH₄SCN-1-synthesis of ferrihydrite in distilled water being that NH₄SCN plus Fe(NO₃)₃9H₂O together in the beaker; Fh-DW-NH₄SCN-2-synthesis of ferrihydrite in distilled water being that NH₄SCN plus KOH together in the burette; Fh-SW-4.0-synthesis of ferrihydrite in artificial seawater 4.0; Fh-SW-4.0-NH₄SCN-1-synthesis of ferrihydrite in artificial seawater 4.0 being that NH₄SCN plus Fe(NO₃)₃9H₂O together in the beaker and Fh-SW-NH₄SCN-2-synthesis of ferrihydrite in artificial seawater 4.0 being that NH₄SCN plus KOH together in the burette.

The pH at the point of zero charge

When the pH is above the pH_PZC, the material is negatively charged and when it is below this value, the material is positively charged (Parks and de Bruyn, Reference Parks and de Bruyn1962). For prebiotic chemistry, the pH_PZC is an important parameter, since, in general, positively charged molecules will preferably adsorb onto negatively charged minerals; the reverse is also true (Lambert, Reference Lambert2008).

The Fh-6 sample contains only ferrihydrite and its pH_PCZ was 8.24, which is in accordance with the data obtained in the literature for ferrihydrite, which vary from 7.0 to 8.8 (Kosmulski, Reference Kosmulski2018). In the case of the Fh-SW sample, which also contains only ferrihydrite (Fig. 1), the pH_PCZ was 7.25 (Table 2). This decrease in pH_PCZ is probably related to the adsorption of SO₄²⁻ onto the sample (Table 1, 2). Fh-SW-NH₄SCN-2 presented a pH_PCZ similar to the Fh-6 sample (Table 2). It should be noted that this sample adsorbed SO₄²⁻ and NH₄⁺ (Table 1).

Fh-DW-NH₄SCN-1, Fh-DW-NH₄SCN-, and Fh-SW-NH₄SCN-1 samples contain a mixture of goethite, hematite and magnetite (Table 1, Fig. 1), and their pH_PCZ values were 6.35, 6.43 and 7.11, respectively (online Supplementary Figure S3). These values are in the range of pH_PCZ for these samples (Cornell and Schwertmann, Reference Cornell and Schwertmann2003; Kosmulski, Reference Kosmulski2018). All these samples adsorbed SCN⁻.

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is an excellent tool for probing the electronic structures of Fe-O bonds in Fe_1−yO (wüstite), α-Fe₂O₃ (hematite), γ-Fe₂O₃ (maghemite), Fe₃O₄ (magnetite) and FeO(OH) (goethite) compounds (Fujii et al., Reference Fujii, de Groot, Sawatzky, Voogt, Hibma and Okada1999; Yamashita et al., Reference Yamashita and Hayes2008). However, even in the absence of dissolved oxygen, Fe⁰ can react with water to form soluble Fe²⁺ and not only iron oxides (Kruger and Calvertm, Reference Kruger and Calvertm1967), but also amorphous black ferric oxyhydroxide (Ding et al., Reference Ding, de Jong, Roosendaal and Vredenberg2000) and ferrihydrite (Fh) (Chernyshova et al., Reference Chernyshova, Hochella and Madden2007), which is an abundant mineral in the terrestrial crust with an ordered form Fe₁₀O₁₄(OH)₂ + nH₂O where n is close to 1.

Fh is a low-crystalline nanoparticulate and highly defective material containing both tetrahedrally and octahedrally coordinated ferric ions (Chernyshova et al., Reference Chernyshova, Ponnurangam and Somasundaran2010; Michel et al., Reference Michel, Barron, Torrent, Morales, Serna, Boily, Liu, Ambrosini, Cismasu and Brown2010). Metastable iron oxyhydroxides and particularly maghemite with inverse spinel structure denoted as (Fe³⁺)^tet₈ [Fe³⁺Fe²⁺]^oct₈O₃₂, where 16 Fe³⁺ and 8 Fe²⁺ occupy tetrahedral (tet) and octahedral (oct) interstitial sites of the oxygen anions, forming a closed-packed FCC lattice and can form Fh by dehydration.

Next, comparisons of the XPS measurements of samples with pure Fh and their mixture with iron oxides are performed. The nomenclature of samples includes the first letter of their constituent compounds as identified by XRD measurements; (i.e.), the addition of Gt for goethite, Hm for hematite and Mt for magnetite. Although there is evidence that the binding-energy values of the Fe 2p_3/2 and O 1s peak positions can be dependent on the calibration experimentally obtained from C 1s peak position (Yamashita and Hayes, Reference Yamashita and Hayes2008), we observed systematic changes in O 1s peak position with site occupancy and oxidation state of Fe in the samples containing a mixture of iron oxides and oxyhydroxides.

The XPS O 1s spectra from the samples containing Fh with mixtures of compounds are shown in Fig. 2a, whereas the XPS peaks of O 1s for the pure Fh samples prepared with current seawater (w0.0) and proxy seawater from 4 billion years ago (w4.0) are shown in Fig. 2b.

Fig. 2. The XPS spectra of O 1s photopeak from the (a) ferrihydrate mixtured with iron (hidr)oxides compounds and (b) pure ferrihydrate samples.

The contributions Fe-O, H-O and adsorbed water are indicated by dashed lines and arrows in the photopeak region of the O 1s core-level. The peak position of O 1s has been investigated by many works and values between 529 and 532 eV have been reported (Ding et al., Reference Ding, de Jong, Roosendaal and Vredenberg2000; Weiss et al., Reference Weiss and Ranke2002; Yamashita and P. Hayes, Reference Yamashita and Hayes2008). It is worth noting that the binding energy of the O 1s peak is almost independent of the oxide phase at 530 eV in the presence of both Fe³⁺ and Fe²⁺ species (Schedel-Niedrig et al., Reference Schedel-Niedrig, Weiss and Schlogl1995; Fujii et al., Reference Fujii, de Groot, Sawatzky, Voogt, Hibma and Okada1999; Weiss and Ranke, Reference Weiss and Ranke2002). However, the covalency reduces the number of filled states with O 2p character, so that the strength of the O 1s signal is related to the degree of covalency (Schedel-Niedrig et al., Reference Schedel-Niedrig, Weiss and Schlogl1995; Weiss and Ranke, Reference Weiss and Ranke2002). All XPS O 1s spectra shown in Fig. 2a exhibit broad lines at about 529.5 eV, whereas a broader structure with a peak at about 531 eV is observed for pure Fh samples, as shown in Fig. 2b. Furthermore, strong absorption of water is observed in samples containing Fh mixed with magnetite, hematite and goethite. Particularly, Fh mixed with goethite exhibits a pronounced peak at the position of 533 eV corresponding to water absorption. The water absorption occurs in Fe oxides and a goethite-like phase is reported on the surface of water exposed magnetite samples (Kendelewicz et al., Reference Kendelewicz, Liu, Doyle, Brown, Nelson and Chambers2000). In hematite, O-H, oxygen-containing carbon species and H₂O surface spectral features are found around 531, 532 and 533 eV, respectively (Yamamoto et al., Reference Yamamoto, Kendelewicz, Newberg, Ketteler, Starr, Mysak, Andersson, Ogasawara, Bluhm, Salmeron, Brown and Nilsson2010). The chemisorption of water on the iron oxide surfaces occurs initially on defect sites. No significant changes are observed between samples denoted as Fh w0.0 and Fh w4.0, which exhibit much less adsorbed water after the thermal desorption procedure.

The XPS peaks of Fe 2p_3/2 and Fe 2p_1/2 for the samples containing Fh mixed with iron compounds are shown in Fig. 3a. The peaks Fe 2p_3/2 are narrower and stronger than Fe 2p_1/2 and the area of the Fe 2p_3/2 peak is greater than that of Fe 2p_1/2 because Fe 2p_3/2 has degeneracy of four states while Fe 2p_1/2 has only two from the spin-orbit splitting. The peak position of Fe 2p_3/2 is found at 709 eV for FhGHM and FhGH samples, whereas it appears to be dominated by a component positioned at 711.5 eV for the FhG sample. In addition, these values have been reported by many works (Ding et al., Reference Ding, de Jong, Roosendaal and Vredenberg2000; Yamashita and Hayes, Reference Yamashita and Hayes2008). Whereas the Fe²⁺ species has a spectral component positioned at 709 eV, the Fe³⁺ species in tetrahedral and octahedral sites are positioned between 710.7 and 711.0 eV (Fujii et al., Reference Fujii, de Groot, Sawatzky, Voogt, Hibma and Okada1999; Weiss and Ranke, Reference Weiss and Ranke2002; Yamashita and Hayes, Reference Yamashita and Hayes2008). For Fe³⁺, the Fe 2p_3/2 peak also has an associated shake-up satellite peak which are located approximately 8 eV higher than the main Fe 2p_3/2 peak (Fujii et al., Reference Fujii, de Groot, Sawatzky, Voogt, Hibma and Okada1999; Weiss and Ranke, Reference Weiss and Ranke2002; Yamashita and Hayes, Reference Yamashita and Hayes2008).

Fig. 3. The XPS spectra of Fe 2p core level from the (a) ferrihydrate mixtured with iron-oxide-based compounds and (b) pure ferrihydrate samples.

A particular problem in the case of iron oxides is that Fe₂O₃ can reduce to Fe₃O₄ not only by Ar⁺ sputtering but also X-ray exposure in the XPS analysis (Paparazzo, Reference Paparazzo1986; Lad and Henrich, Reference Lad and Henrich1988; Yamashita and Hayes, Reference Yamashita and Hayes2008). Magnetite contains both Fe³⁺ and Fe²⁺ species, which also cause a broader Fe 2p signal than that of the hematite (Weiss and Ranke, Reference Weiss and Ranke2002), whereas goethite is an iron (oxy)hydroxide containing ferric iron. Clearly, the contribution from (Fe²⁺, Fe³⁺) species are predominant in Fh and FhG samples, conforming to Figs. 3a and 3b. The shake-up satellite line characteristic for the Fe³⁺ species at 719 eV binding energy is also visible in XPS spectra of pure Fh samples. The Fe 2p_3/2 signal in the FhG spectrum is asymmetrically broadened, when compared to the FhGHM and FhGH spectra, which can be attributed to excitations of the Fe²⁺ species in the Fh compound. The three Fe-oxides in the FhGHM sample led to a sharper Fe 2p_3/2 profile, indicating a predominance of Fe²⁺ species. Direct comparisons between XPS Fe 2p spectra from Fh w0.0 and Fh w4.0 samples again revealed no significant differences. It is worth noticing that the satellite structures in XPS spectra Fe 2p became unresolved which is likely evidence of the reduction in the Fe₂O₃-Fe₃O₄ system. This was not only due to the formation of Fe²⁺ ions, but also to nonhomogeneous changes in the hybridization parameters between octahedral and tetrahedral Fe³⁺ ions (Fujii et al., Reference Fujii, de Groot, Sawatzky, Voogt, Hibma and Okada1999).

The XPS spectra of Fe 3p for the samples are shown in Figure4a and 4b. Although the Fe 3p core-level consists of both Fe 3p_3/2 and Fe 3p_1/2, the separation energy of the XPS peaks proportional to the spin–orbit coupling is smaller than the instrumental resolution. Despite the appearance of the Fe 3p peak as a single peak, it is possible to obtain physical parameters from peak position, full width at half maximum (FWHM) and asymmetry of the profile, mainly in Fig. 4a. Peak positions and FWHM of the XPS Fe 3p peak are distinct for samples with Fe only present as Fe³⁺ and Fe²⁺ species (Yamashita and Hayes, Reference Yamashita and Hayes2008). The vertical dashed lines in Fig. 4 indicate the Fe 3p peak positions for Fe²⁺ and Fe³⁺ ions at 54 and 56 eV, respectively.

Fig. 4. The XPS spectra of Fe 3p core level from the (a) ferrihydrate mixtured with iron-oxide-based compounds and (b) pure ferrihydrate samples.

In Fig. 4a it can be observed that the FWHM and asymmetry of Fe 3p profile for Fe²⁺ are larger than Fe³⁺ for FhG and FhGH samples compared to the FhGHM sample.

Although the Fe 3p peak enables the quantitative analysis of Fe³⁺ and Fe²⁺ using relative areas of each constituent peak assigned to Fe²⁺ and Fe³⁺ without the interference of satellite peaks intervening in Fe 2p peaks, the electronic similarities seen in Fe oxides/oxyhydroxides makes this task complex. Essentially, a certain ubiquity for components arises in the oxide/oxyhydroxide compounds involving essentially close-packed arrangements of oxygen ions, with the Fe ions occupying two kinds of interstitial positions. Excitations of the type 3p⁶3dⁿ towards 3p⁵3dⁿ⁺¹ dominate at the Fe 3p threshold and compete with the direct photoemission process 3p⁶3dⁿ decaying 3p⁶3d^n‒1 + e^‒. Nevertheless, the degeneracy of the five Fe 3d states depend on their different orbital overlaps with the O ligands and ligand-field splitting is inversely shifted in octahedral and tetrahedral sites. Thus, the number of unoccupied 3d states available for mixing with O 2p states (hybridization) is related to either the different local surroundings of the Fe cations and the covalent bonding between Fe 4sp states and O 2p states (Weiss and Ranke, Reference Weiss and Ranke2002).

Among the reasons why Fe 3p peaks were not used in the present study for the quantitative analysis of the Fe²⁺/Fe³⁺ ratios in the samples are difficulties in determining linear or Shirley background subtractions, asymmetry factor values and unique Gaussian–Lorentzian ratios in the fitting procedures. High-resolution XPS measurements are required to obtain reliable proportions of iron present in different oxidation states.