Photocatalytic and Cathode Active Abilities of Ni-Substituted α-FeOOH Nanoparticles

1. Introduction

Metal oxide nanoparticles (MO NPs) exhibit a large surface area and a pronounced quantum size effect due to superparamagnetism and the quantum tunneling phenomenon, distinguishing them from bulk MO materials. Among the wide array of MO NPs, iron oxide NPs have emerged as a novel functional material class that has gained considerable attention across various domains. These include biomedical applications, wherein they demonstrate antimicrobial and anticancer activities [1], as well as catalysts [2], photocatalysts [3,4], and agents for environmental purification [5].

In particular, goethite (α-FeOOH) NPs represent the most readily available form of iron oxyhydroxide in nature. Their crystal structure comprises interconnected FeO₃(OH)₃ octahedra with shared edge and vertex linkages. The versatile applications of α-FeOOH NPs encompass traditional roles as pigments, catalysts, photocatalysts [6], adsorbents, gas sensors, photoelectrodes [7], and battery electrodes [8,9]. Notably, the utilization of α-FeOOH NPs for wastewater purification and as secondary battery electrodes holds immense significance in realizing future sustainable development goals. This is primarily due to their ability, non-toxic nature, stability, electrical conductivity, and photocatalytic activity under visible light exposure [10].

As for UV- and visible-light activated photocatalytic ability of α-FeOOH NPs, a first-order rate constant (k) of 7.4×10^-4 min^-1 was recorded for 15 mg of hematite (α-Fe₂O₃) NPs formed by the combustion method, in a reaction with 50 mL of an aqueous solution containing 10 mg of Rhodamine B (RhB) under UV light irradiation [3]. In this reaction, 95% of the RhB decomposed after 180 min [3]. In addition, the photocatalytic reaction between RhB–H₂O₂ aqueous solutions and 10 mg of powdered, 4 mol% Cu-doped goethites under exposition to visible light resulted in 60% degradation of RhB in 1 h, which could be attributed to a combined effect of nanoneedle morphology and of Cu doping [11].

In relation to the developments of visible-light activated photocatalysts and secondary battery electrodes of iron oxide NPs, we confirmed the methylene blue (MB) degradation effect of a mixture of Fe NPs and γ-Fe₂O₃ NPs [12]. In this study, a 10-day MB_aq degradation test using a 1:3 molar ratio of Fe NPs to γ-Fe₂O₃ NPs mixed powder showed a significant decrease in MB concentration from 20.0 to 0.85 μM with a maximum k value of 2.84 day^-1, indicating that iron oxide NPs effectively degrade organic compounds [13]. This result proves that the nano size of Fe and γ-Fe₂O₃ showed larger reactivity because of their high surface area. Another result of methylene blue decomposition was reported on Sn-incorporated FeOOH NPs showing a smaller band gap energy and higher decomposition performance compared to pure FeOOH. It is assumed that the band gap energy of α-FeOOH NPs decreased due to the introduction of Sn^IV [14].

On the other hand, as for the application of α-FeOOH as an electrode for a secondary battery, an excellent cycling performance with a reversible specific capacity of 870 mAh g⁻¹ under the current rate of 100 mA g⁻¹ of LIB was reported. The anode contained α-FeOOH corner-truncated prisms (CTPs) with a length of about 1 μm and a width of about 200 nm obtained by hydrothermal treatment [9].

An intriguing and notable characteristics of α-FeOOH NPs is their inherent capability to incorporate diverse metal cations into their crystal structure. This opens the field to the possibility of modifying various crystal properties, including morphology, size, crystallinity, and color, by replacing Fe³⁺ ions within the structure. In particular, the doping of α-FeOOH NPs with a range of metal cations, such as Al³⁺, In³⁺, Cu²⁺, Cr²⁺, Mn²⁺, and Zn²⁺, has been extensively investigated to enhance further the aforementioned physical properties [11,12]. These studies have explored the potential for improving and refining the characteristics of α-FeOOH NPs by introducing and incorporating these selected metal cations into their structure.

In addition, Cu²⁺-for-Fe³⁺ substitution in goethite caused a gradual elongation and narrowing of nanorods with the formation of nanoneedles which led to a slight decrease in direct and indirect optical band gaps compared with pure phases [11].

The introduction of nickel (Ni) as a dopant in α-FeOOH NPs is anticipated to impact their crystalline structure and oxidation state. Ni can substitute for Fe ions within the crystal lattice of α-FeOOH NPs [12]. This incorporation of Ni is likely to result in slight modifications of the crystalline structure, which predominantly consists of interconnected FeO₆ octahedra with shared edges and vertices [11,12]. Upon doping with Ni, the dopant ions can occupy available sites within the lattice [11,12]. The extent of structural changes will depend on many factors such as the concentration of the dopant and its compatibility with the lattice structure of α-FeOOH [15].

The oxidation state of Ni within the doped α-FeOOH NPs will primarily depend on the specific synthesis conditions and the prevailing redox environment [15]. Typically, Ni is commonly found in a +2-valence state. However, it is important to note that the local chemical environment and interactions with neighboring atoms within the lattice structure can influence the oxidation state of the dopant ions. Therefore, Ni may exhibit alternative oxidation states in certain conditions, such as +3.

Herein, we report the relationship between the structure, photocatalytic ability, and active cathode property of a secondary battery of Ni-substituted α-FeOOH NPs prepared by hydrothermal synthesis. For this purpose, structural characterization was performed by X-ray diffractometry (XRD), X-ray absorption spectroscopy (XANES/ EXAFS), and ⁵⁷Fe-Mössbaur spectroscopy. Moreover, for the properties of photocatalytic ability, electrochemical performance, and active cathode evaluation, Brunauer-Emmett-Teller (BET) surface area analysis, diffuse reflectance ultraviolet-visible (UV-Vis) spectroscopy, Impedance spectroscopy (IS), and active cathode performance of LIBs and SIBs were carried out.

2. Results and Discussion

2.1. XRD Patterns of α-Ni_xFe_1-xOOH NPs

Figure 1 shows the XRD patterns of Ni0, Ni5, Ni10, Ni15, Ni20, Ni30, Ni40, and Ni50 samples. The XRD peaks of goethite (ICDD card No.00-029-0713) with space group (Pbnm) were observed in all XRD patterns with increasing Ni content. Moreover, a new peak appeared at 2θ = 11.56° corresponding to the (0 0 3) plane of α-Ni(OH)₂ crystalline phase (ICDD card No.01-076-6904) with space group (R3m: H), and the intensity of this peak increased with increasing x content from 0.30 to 0.50 as shown in the yellow rectangle in Figure 1. The percentage of the α-Ni(OH)₂ crystalline phase increased from 7.63% to 9.35% while ‘x’ increased from 0.30 to 0.50 as shown in Table 1. Additionally, the elevation of background intensity observed in the XRD patterns depicted in Figure 1, as the Ni content increased from 0.30 to 0.50, suggests the appearance of an amorphous Ni hydroxide phase. The diffraction lines of α-FeOOH were slightly shifted towards lower 2θ values with increasing Ni content, implying a minor expansion in the unit cell due to Ni/Fe substitution. The unit cell parameters of goethite in Ni0, Ni5, Ni10, Ni15, Ni20, Ni30, Ni40, and Ni50 samples obtained by Rietveld refinement of XRD patterns are listed in Table 1. The slight variation in ionic radius between Fe³⁺ (0.645 Å) and Ni²⁺ (0.69 Å) [16] ions caused a small expansion of the unit cell of α-FeOOH. In line with previous studies [17,18], it was observed that the expansion along the b-axis direction was most remarkable. The non-uniform expansion is likely to be attributed to different distortions in the octahedral sites occupied by Ni²⁺ and Fe³⁺ ions, stemming from their disparate electron configurations (3d⁸ and 3d⁵, respectively). The incorporation of Ni into α-FeOOH can be attributed partially to its incongruent release from the initial Fe-Ni hydroxide (Ni-ferrihydrite) [19]. The calculated average size of α-Ni_xFe_1-xOOH NPs by the Scherrer equation from the FWHM of the (110) diffraction line is summarized in Table 1.

2.2. Thermogravimetric and Differential Thermal Analysis (TG-DTA) of α-Ni_xFe_1-xOOH

Thermogravimetric (TG) and differential thermal analysis (DTA) curves for Ni_xFe_1-xOOH NPs with ‘x’ 0, 0.10, 0.20, 0.30, 0.40, and 0.50 are shown in Figure 2a and Figure 2b, respectively.The weight loss at temperatures up to approximately 140 °C corresponds to the release of H₂O molecules adsorbed on the surface of Ni_xFe_1-xOOH NPs crystals or trapped within the interstitial spaces between them [19,20]. This low-temperature weight loss increased in the Ni_xFe_1-xOOH NPs samples with increased Ni content, which can be explained by more adsorbed water in the NiFeOOH NPs samples with a large surface area [20]. Also, different molar fractions of incorporated Ni²⁺ ions and different crystal facets in Ni-doped Ni_xFe_1-xOOH NPs crystals can also affect the amount of adsorbed water in Ni_xFe_1-xOOH NPs samples. The weight loss at temperatures between about 150 and 290 °C can be attributed to the dehydroxylation of surface hydroxyl groups [18,19,20]. This weight loss also increases in the Ni_xFe_1-xOOH NPs with a higher Ni content due to the larger surface area. The total weight loss percentage for Ni_xFe_1-xOOH NPs in the temperature range from RT to 1000 °C increased with Ni content increased, from 14.24% in Ni0 to 26.63% in Ni50, as shown in Figure 2 (a).

Figure 2 (b) shows the DTA curves of the Ni_xFe_1-xOOH NPs with ‘x’ 0, 0.10, 0.20, 0.30, 0.40, and 0.50. DTA curve of Ni_xFe_1-xOOH NPs showed a single endothermic peak in the temperature range from 230- 300°C due to the phase transformation from α-FeOOH to α-Fe₂O₃ [18,19]. As the amount of Ni increased from 0 to 0.2 mol.%, the endothermic peak became smaller and somewhat shifted to higher temperatures from 251°C in Ni0 to 262°C in Ni20, while in Ni30 shifted in the opposite direction and observed at 241 °C. The DTA curves of Ni40 and Ni50 showed new endothermic reactions at lower temperatures caused by the lower degree of crystallinity or the increased amorphous phase [19]. These results agree with XRD data (see Figure 1 and Table 1).

2.3. FTIR Spectra of α-Ni_xFe_1-xOOH NPs

FTIR spectra of Ni_xFe_1-xOOH NPs with ‘x’ 0, 0.10, 0.20, 0.30, 0.40, and 0.50, and Fe₂O₃ and NiO as standard reference materials, carried out in the wavenumber range of 400–4000 cm⁻¹, are shown in Figure 3. The transmission band showed approximately 3454 cm^-1 ascribed to -OH [21] in Ni0 and shifted to a lower wavenumber, and the peak intensity increased with increasing Ni content. The small transmission peak observed at 3130 cm^-1 corresponds to the O-H stretching vibration of the hydroxyl group [18,22]. The band center of the hydroxyl group gradually shifts to a higher wave number, and the peak intensity decreases with increasing Ni content, as shown in Figure 3. The intense two transmission peaks were observed at 797 cm^-1 and 895 cm^-1 related to Fe-O-H bending vibrations out of plane and in plane [17,18,19,22], respectively. The intensity of both transmission peaks gradually decreased with Ni content. These two bands slightly shifted towards higher wave numbers up to 0.30 mol% Ni content, and with high Ni content (0.40 and 0.50), the center of these bands changed to lower wavenumbers. It is known that bending bands' positions depend on crystallinity degree and the transition metal substituted with Fe [17,22]. In this study, Ni substituted with Fe caused a shift toward lower wavenumbers with increasing Ni content from Ni0 to Ni30, while in Ni40 and Ni50, an increase in particle size caused a change to a higher wavenumber. These results agree with XRD and DTA (see Figure 1 and Figure 2). The transmission band was observed at 645 cm^-1 in pure-FeOOH due to Fe-O symmetric stretching [22], and the band center gradually shifted to a lower wavenumber with increasing Ni content from 0 to 0.5. A new transmission peak appeared in higher Ni concentration samples (Ni40 and Ni50) at 472 and 476 cm^-1, respectively. It was noted that the recent peak compared with that shown in the FTIR spectrum of NiO, can be attributed to Ni-O [23]. The small transmission bands located at 1639 cm^-1 in Fe₂O₃ are due to the stretching vibration of -OH from absorbed water [21,23,24]. Also, this peak showed in Ni_xFe_1-xOOH NPs at 1639 cm^-1 in Ni0 and slightly shifted to 1643 cm^-1, and the peak intensity gradually increased with increasing Ni content.

2.4. XAFS Spectra of α-Ni_xFe_1-xOOH NPs

The XAFS and EXAFS measurements are powerful tools for investigating the local atomic and electronic properties of materials, providing a comprehensive understanding of their structure and reactivity [25,26].

Figure 4 shows the XANES spectra and Fourier transforms of Fe-/Ni- K-edges of α-Ni_xFe_1-xOOH NPs with ‘x’ 0, 0.05, 0.10, 0.15, 0.20, 0.30, 0.40, and 0.50 and Fe foil, Fe₃O₄, and α-Fe₂O₃ as standard materials in the Fe K-edge and with ‘x’ from 0.10, 0.15, 0.20,0.30, 0.40, and 0.50 and Ni-foil and NiO as references materials at Ni K-edge.

Figure 4 (a) displays the normalized Fe K-edge XAFS spectra of α-Ni_xFe_1-xOOH NPs. The Fe K absorption edge was observed at normalized intensity= 0.5 for Fe-foil, Fe₃O₄, and Fe₂O₃ being 7117.26±0.02, 7120.67±0.02, and 7121.79±0.02 eV, respectively. The oxidation state in Fe-foil, Fe₃O₄, and α-Fe₂O₃ is 0, 2.66, and 3, respectively. The absorption edge of α-Ni_xFe_1-xOOH NPs was observed at 7122.65±0.01, 7122.49±0.01, 7122.27±0.01,7122.15±0.01, 7122.23±0.01, 7121.91±0.01, 7122.08±0.01, and 7122.28±0.01 with ‘x’ 0, 0.05, 0.10, 0.15, 0.20, 0.30, 0.40, and 0.50, respectively as shown in insert figure in Figure 4 (a). Increasing oxidation state of Fe results in a shift of the absorption edge to higher energy [27]; according to the above, values of the absorption edge of α-Ni_xFe_1-xOOH NPs are higher than the values of Fe₂O₃ (Fe³⁺), this implies that none of the α-Ni_xFe_1-xOOH NPs samples contained Fe²⁺, and they contained only Fe³⁺. The pre-edge peak was found at 7113 eV for α-Ni_xFe_1-xOOH NPs, at 7114 eV for both Fe-foil and Fe₂O₃, being designated as the absorption edge. The pre-edge features in the XANES spectrum reveal electronic states and symmetry of unoccupied orbitals in the absorbing atom [27,28]. Analyzing the intensity, energy position, and shape of pre-edge peaks helps understand the electronic structure, oxidation state, and local environment of Fe atoms in the material [29]. The intensity of the pre-edge peak is lower when the octahedral site has high symmetry. α-FeOOH with the octahedral structure of FeO₆ shows an increase in the pre-edge peak intensity and therefore a decrease in symmetry, with increasing Ni content.

Figure 4 (c) displays the normalized Ni K-edge XANES spectra of Ni_xFe_1-xOOH NPs with ‘x’ 0.10, 0.15, 0.20, 0.30, 0.40, and 0.50 and Ni-foil and NiO as reference materials. The pre-edge peak of Ni foils, NiO, and Ni_xFe_1-xOOH NPs was shown at 8321.40, 8319.68, and 8319.48 eV, respectively [30]. The height of the pre-edge peak intensity of NiO and Ni_xFe_1-xOOH NPs has approximately the same value, and the change in the pre-edge is minimal. Therefore, we cannot find any significant difference in the oxidation state of Ni and the symmetry of the structure [31].

In contrast, the Ni K normalized absorption peak of Ni_xFe_1-xOOH NPs and reference materials are shown in the inserted figure in Figure 4 (c). The absorption edge of Ni-foil, Ni_xFe_1-xOOH NPs, and NiO is at 8327.40, (8329.21-8329.48), and 8328.76 eV, respectively. According to the absorption edge's higher value compared to NiO, the oxidation state of Ni in all Ni_xFe_1-xOOH NPs samples is higher than +2. Both Ni15 and Ni20 are samples containing less Ni³⁺, while Ni10 has the highest level of Ni³⁺.

Figure 4 (b) shows the Fourier transform of Fe K-edge EXAFS (FT-EXAFS) for α-Ni_xFe_1-xOOH NPs. The FT-EXAFS of α-Ni_xFe_1-xOOH NPs, in the first coordination, was observed at 1.57 Å, and the peak intensity decreased with increasing ‘x’ from 0.10 to 0.50, and this peak could be attributed to Fe-O [29,30]. Moreover, the second peak of Fe-foil, Fe₃O₄, and Fe₂O₃ was shown at 2.21 Å, 2.91 Å, and 2.68 Å, respectively. The second peak of Ni_xFe_1-xOOH NPs was also shown between the Fe₂O₃ and Fe₃O₄. Also, this peak was shifted to a higher Fe-Fe distance, and the peak intensity decreased by reducing the Fe content from 1 to 0.50. All observed peaks in this range (2.21 - 2.91) Å are attributed to Fe-Fe (or Fe-Ni) [32,33].

Figure 4 (d) shows the Ni-K FT-EXAFS of Ni_xFe_1-xOOH NPs, and it was clear that two peaks at 1.63 Å and 2.77 Å, can be attributed to Ni-O and Ni- Me (Me = Ni or Fe), respectively [33,34,35]. The first peak was moved to a higher distance between Ni and O 1.75 Å, but the second peak shifted to a lower Ni- Me distance of 2.62 Å for Ni_xFe_1-xOOH NPs [35]. The intensity of the first peak decreased, and the peak position was moved to a higher radius with increasing Ni content. These results obtained from Ni K and Fe K FT-EXAFS and XANES of the Ni_xFe_1-xOOH NPs agree with XRD results.

2.5. ⁵⁷Fe- Mössbauer Spectra of α-Ni_xFe_1-xOOH NPs

Figure 5 shows ⁵⁷Fe-Mössbauer spectra of α-Ni_xFe_1-xOOH NPs ‘x’ from 0.00 to 0.50 measured at RT in the velocity range from 12 mm s^-1 to -12 mm s^-1 and from 3 mm s^-1 to -3 mm s^-1. The Mössbauer parameters in these two velocity ranges are listed in Table 2 and Table 3. Figure 5 (a) shows the ⁵⁷Fe- Mössbauer spectra of Ni0, Ni10, Ni20, Ni30, Ni40, and Ni50 measured at RT in the wide velocity range from 12 mm s^-1 to -12 mm s^-1. The Mössbauer spectra exhibits a sextet with Mössbauer parameters characteristic of α-Ni_xFe_1-xOOH NPs ‘x’ from 0.00 to 0.30 as the predominant component, while the sextet disappeared after increasing the Ni content to 0.30. The α-FeOOH NPs and α-Ni_xFe_1-xOOH NPs spectra were analyzed by incorporating the hyperfine magnetic distribution (B_hf); their average values are presented in Table 2. The slight changes in isomer shift, except for the Ni0 sample, indicate that the site is distorted goethite, as shown in Table 2. Moreover, the presence of a quadrupole doublet, which increases as the Ni content increases, can be ascribed to the presence of Fe³⁺ ions in a low-crystalline or amorphous phase. This phase could be attributed to compounds such as Ni-Fe hydroxide, low crystalline Ni-ferrite, or Ni-goethite. The higher proportion of the low-crystalline or amorphous phase aligns with the increased background observed in the XRD patterns, as depicted in Figure 1. Also, the line width value increased from 0.196 to 0.53 mm s^-1, this may be confirmed that the result obtained from XRD, wherein the amorphous phases increased with increasing Ni content. In addition, the particle size of the goethite drops to a nano-scale, and the superparamagnetic component increases as a function of increasing Ni content [11,14,15]. The presence of the doublet suggests nanosized goethite particles in Ni40 and Ni50. To analyze the doublet component of FTMS spectra of Ni20, Ni30, Ni40, and Ni50 measured at RT in the wide velocity range for more detail, a narrower velocity range from 3 mm s^-1 to -3 mm s^-1 was applied and the spectra are presented in Figure 5 (b). One doublet showed in Ni20, Ni30, Ni40, and Ni50 spectra in the wide velocity range from 12 mm s^-1 to -12 mm s^-1 was analyzed as a doublet component. The two doublets were considered to be derived from the superparamagnetic and amorphous (or ferrihydrite) components [15].

On the other hand, the FTMS spectra of Ni0, Ni5, Ni10, and Ni20 were measured in the range of velocity from 12 mm s^-1 to -12 mm s^-1 at 86 K, as shown in Figure 6 (a). Goethite was present in all spectra, and a doublet derived from amorphous (or ferrihydrite) are also present in Ni10 and Ni20. Additionally, the FTMS spectra of Ni20 and Ni50 were measured in a narrower range, as shown in Figure 6 (b).

A sextet of FeOOH and a doublet of amorphous (ferrihydrite) were present in Ni20, and Ni50 had two doublets derived from ferrihydrite and a superparamagnetic phase. These results are in good agreement with XRD and XAFS measurements, as shown in Figure 1 and Figure 4. From Table S2, the quadrupole splitting of the doublet derived from amorphous or ferrihydrite increased from 0.70 mm s^-1 for Ni20 to 0.89 mm s^-1 for Ni50. Therefore, it is considered that amorphous aging is progressing, and ferrihydrite is being formed [17,19]. This is consistent with the fact that Ni50 has a specific surface area lower than Ni20 as shown by BET analysis [35]. In addition, Sample Ni20 contains goethite and an amorphous phase, while sample Ni50 also contains crystalline α-Ni(OH)₂, which is a possible reason for the lower surface area. The FTMS spectra of Ni10 and Ni20 were measured at low temperatures from 20K to 300K, as shown in Figure S1, and the Mössbauer parameters of Ni10 and Ni20 are summarized in Table S3 and Table S4.

As shown in Table 2 the Mössbauer parameters measured at room temperature indicated in a sextet and a doublet related to goethite and a superparamagnetic phase. Also, a doublet shown at RT is divided into two doublets related to superparamagnetic and amorphous phases. Additionally, with decreasing temperature from 300 K to 20 K, the doublets disappeared in both samples Ni10 and Ni20.

3.6. BET Analysis of α-Ni_xFe_1-xOOH NPs

The experiment involved varying the relative pressure (P/P₀), where P represents the adsorption equilibrium pressure, and P₀ represents the saturation vapor pressure) within the range of 0 to 1. The number of gas molecules adsorbed was quantified and graphed against the relative pressure, resulting in an isotherm. Figure 7 displays the isotherms of α-Ni_xFe_1-xOOH NPs. There are many factors that influence the shape of the curve of an isotherm, such as the existence and dimensions of pores and adsorption energy. In 1985, the International Union of Pure and Applied Chemistry (IUPAC) published a classification system comprising six types of adsorption/desorption isotherms. Over the past three decades, two additional types have been introduced. The isotherm shape observed for Ni0 and Ni10 is classified as type II according to this classification. Type II indicates the absence of pores or the potential presence of macropores (pores with sizes exceeding 50 nm) [36]. The IV and V types exhibit a unique phenomenon known as hysteresis, wherein the adsorption and desorption processes do not align as typically [37].

Hysteresis, closely associated with capillary condensation, primarily occurs in the mesopore range [38]. The hysteresis pattern observed can offer insights into the shape and structure of the pore. However, due to the diverse nature of hysteresis patterns, establishing a direct relationship between them and pore shape and structure is challenging.

On the other hand, the adsorption isotherms for Ni20, Ni30, Ni40, and Ni50 fall into the H4-type hysteresis classification. Type H4 is indicative of the presence of slit-type pores. This type of hysteresis pattern can sometimes be observed when micropores (pores with sizes of 2 nm or less) are present, as seen in type I isotherms [39]. Based on the isotherm shape depicted in Figure 7, it can be inferred that Ni0 to Ni10 possesses macropores, while Ni20 to Ni50 exhibits micropores. The presence of micropores can also be deduced from the rapid increase in adsorption at low relative pressures and the characteristics of the hysteresis loop. The BET method can extract information regarding the adsorption process, specifically from monolayer adsorption to multilayer adsorption. The sample's surface area can be determined by accurately calculating the degree of monolayer adsorption.

This can be achieved by multiplying the quantity of single-molecule adsorption by the cross-sectional area occupied by a single gas molecule as follows [37].

$$\frac{P}{V\left(P_0-P\right)}=\frac{1}{V_{m}C}+\left(\frac{C-1}{V_{m}C}\right)\left(\frac{P}{P_0}\right)$$

(1)

P, P₀, V_m, and C are parameters related to adsorption equilibrium pressure, saturation vapor pressure line, monolayer adsorption volume (e.g., adsorption volume when the gas molecules form a monolayer on a solid surface), and adsorption heat, respectively. The established relationship between P/P₀ values in the range of 0.05 to 0.35 reveals significant insights.

The surface area, determined by applying the BET method, is presented in Table 4. Based on these calculations, the surface area of Ni0 was found to be 45.1 m²g^-1. With an increase in the Ni content from Ni0 to Ni20, the surface area progressively rises, reaching the highest value of 174.0 m²g^-1 for Ni20. However, as the Ni content is further increased, the surface area decreases, reaching 96.3 m²g^-1 for Ni40. Based on this result, it can be expected that the electrochemical properties of Ni20 will be the best due to the largest surface area compared to all prepared samples.

3.7. Bandgap Energy Derived from DRS of α-Ni_xFe_1-xOOH NPs

The band gap energy (E_g) of α-Ni_xFe_1-xOOH NPs was determined using diffuse reflection UV-Vis spectroscopy. In this method, a powder sample is irradiated by incident light, and the incident light undergoes multiple transmissions, absorptions, and reflections within the powder. Consequently, absorption patterns are obtained from the reflections in different directions. The spectra obtained were analyzed using the Kubelka-Munk K-M function [40]. E_g, which represents the energy difference between the valence band's top and the conduction band's bottom level [41], is a crucial parameter in assessing photocatalytic activity. E_g can be determined by constructing a Tauc plot [42,43]:

$${\left(h\nu \alpha \right)}^{1/n}=A\left(h\nu -E_g\right)$$

(2),

where h is Planck's constant, ν is the frequency, α is the absorption coefficient, A is the proportionality constant, E_g is the band gap energy, and n is a parameter depending on the type of transition in the material. For direct allowed transitions, it is taken as n = 1/2; for indirect allowed transitions n = 2 [44]. Figure 8 displays the plot of (hνα) ^² versus (hν) obtained from diffuse reflectance spectroscopy (DRS) measurements of Ni_xFe_1-xOOH NPs, enabling estimation of the E_g values. The E_g value for pure goethite (Ni0) was determined to be 2.71±0.01 eV. Table 4 shows that the E_g values for all Ni-doped samples were smaller than that of Ni0. Notably, the smallest E_g value of 2.06±0.01 eV was observed for the Ni30 sample, determined as the x-axis intercept of the fitted straight lines depicted in Figure 8.

This comparison highlights the greater effectiveness of Ni addition in reducing the band gap. Photogenerated carriers are produced when a photocatalyst absorbs photons with energy larger than its E_g. Consequently, in this study, the Ni_xFe_1-xOOH NPs exhibit high excitability when exposed to a metal-halide lamp emitting light with wavelengths ranging from 250 to 750 nm. This excitation greatly enhances the photocatalytic performance [45]. In conclusion, the above observations validate that the Ni_xFe_1-xOOH NPs photocatalysts investigated in this study possess an appropriate band gap and effectively utilize light.

3.8. Photo-Fenton Catalytic Ability of α-Ni_xFe_1-xOOH NPs

The reaction rate constant k of methylene blue decomposition is calculated using the following equation (3) [14]:

$$\mathrm{Ln}\left(c_t/c_0\right)=-kt$$

(3)

c₀ is the concentration before the reaction of MB, and ct means the concentration of MB at time t.

Figure 9 illustrates the relationship between ln(ct/c₀) and t for α-Ni_xFe_1-xOOH NPs. The values of k, representing the rate constant for MB decomposition, were determined and are presented in Table 4. For Ni0, the value of k is 6.4±0.1×10^-3, while the maximum value of k, 15.8±0.6×10^-3 min^-1, is observed for Ni10. As Ni is added to goethite, the k values show an increasing trend from Ni5 to Ni10. However, for Ni15 and subsequent samples, the k values are smaller than that for Ni0.

In the case of Ni15 and later samples, a decrease in goethite content is observed, accompanied by the generation of a significant amount of low-crystalline substances. This change in composition is believed to be the underlying cause of the lower k values observed. Notably, Ni10 exhibits the highest k value among the samples. This can be attributed to the findings from DRS and FeMS data, which suggest a reduction in the band gap and non-oxidation processes occurring in the Ni10 sample.

3.9. Electrical Properties - Impedance Spectra and DC Conductivity

Experimental data obtained through Solid-State Impedance Spectroscopy (SS-IS) are presented in a complex impedance plane, known as the Nyquist diagram, see Figure 10 (a-c). Analysis of these plots involves employing electrical equivalent circuit (EEC) modeling, which utilizes a complex nonlinear least-square (CNLLSQ) fitting procedure. It is apparent that the impedance spectra observed in all examined samples exhibit a distinct semicircle, representing the bulk electrical process within the investigated α-Ni_xFe_1-xOOH NPs. This behavior can be effectively described by an equivalent R-CPE circuit. The parameters for each circuit element (R, A, and α) were directly derived from the measured impedance data using the CNLLSQ method.

In the present study, we took a step further and examined the conductivity spectra of all the samples. The conductivity spectra of samples with Ni10 and Ni20 at different temperatures are shown in Figure 10 (a-b), respectively. Although the conductivity isotherms have a similar shape, overall spectral features can be observed as follows. Firstly, there exists a frequency-independent conductivity plateau at low frequencies. This particular feature is associated with the long-range transportation of charge carriers and represents the overall resistance observed in the impedance spectra or DC conductivity. In addition, frequency-dependent conductivity, commonly referred to as conductivity dispersion, manifests itself with increasing frequency in the form of a power-law. This behavior arises from localized movements of charge carriers occurring over short distances. We used values of the fitting parameter R obtained from modelling along with sample geometry to determine the total DC conductivity, as shown in Table 5. The DC conductivity obtained is in good correlation with observed DC plateaus in conductivity spectra; see Figure 11 (a-b).

The DC conductivity in our samples demonstrates a temperature-dependent behavior that follows an Arrhenius relationship, indicating semiconducting characteristics, see Figure 11 (c). Consequently, the activation energy for the DC conductivity, E_DC, was determined for individual samples from the slope of log(σ_DCT) vs. 1000/T using the equation:

$${\sigma }_{DC}T={\sigma }_0^{*}\exp \left(\frac{-E_{DC}}{k_{B}T}\right)$$

(4)

where σ_DC is the DC conductivity, σ₀*is the pre-exponent, k_B is the Boltzmann constant, and T is the temperature (K). The activation energy, E_DC, and DC conductivity, σ_DC, at 90 °C for all investigated α-Ni_xFe_1-xOOH NPs samples are presented in Table 5.

Continuing our analysis, we turn the focus to the comparison of the conductivity spectra at 110 °C, shown in Figure 12 (a). Firstly, as previously mentioned, the shape of the conductivity spectra does not change with the composition. This consistency indicates that the mechanism of electrical transport remains unaffected. However, it is evident that the modification of α-Ni_xFe_1-xOOH NPs and the increase in Ni content has a negative effect, resulting in a decrease in DC conductivity, see Figure 12 (b) and Table 5. As the Ni/(Fe+Ni) ratio increases, the DC conductivity exhibits a nearly linear decline, from 5.52×10^-10 (Ω cm)^-1 for Ni10 to 5.30×10^-12(Ω cm)^-1 for the Ni50 sample. Conversely, the activation energy for DC conductivity, E_DC, follows the opposite trend, with values increasing in the 73.0–82.5 kJmol^-1 range.

The goethite structure can be described as parallel double chains of edge-sharing octahedra. These chains consist of Fe^III bonded to three oxide ions and three hydroxides extending along the [001] direction and are linked to neighboring double chains by corner sharing. Guskos et al. [46] conducted an electrical study on goethite and proposed that charge transport occurs through thermally activated three-dimensional hopping of electrons via oxygen vacancies, based on DC electrical measurements conducted over a wide temperature range.

However, it is important to note that our understanding of the electronic structure of iron oxides and oxyhydroxides, including goethite, remains incomplete. In our study for Ni_xFe_1-xOOH NPs, the obtained values for activation energy (0.75–0.85 eV), see Table 5, are almost 3 times lower in comparison to goethites studied by Guskos et al [46]. Our findings align more closely with the activation energy values reported for magnetite and hematite. Similar values are observed in the literature for various materials with disordered or partially disordered structures and dominant electron transport [47,48,49,50].

Furthermore, Vitaly et al. [51], showed that RT charge transport in goethite is primarily governed by thermally activated hopping of small polarons, with the associated mobility being higher compared to other iron oxyhydroxide (FeOOH) polymorphs. Small polarons are formed when electrons self-trap onto an iron center, resulting in conduction through phonon-mediated hops between centers [52]. Different inequivalent paths for electron hopping characterized by different Fe–Fe bond distances and species bridging two neighboring Fe atoms are identified [51]. The pathway involving migration along the double chain ([001] direction) through shared octahedral edges, with electron transport mediated by O and OH species, is characterized by Fe atoms with parallel spins and with the shortest Fe²⁺-Fe³⁺ distance of approximately 3 Å. Su et al. [53], studied the electrical conduction mechanism of goethite under pressures up to 17.1 GPa using Impedance spectroscopy. The results indicate a pressure-induced conduction mechanism transition around 5 GPa from mixed protonic-electronic conduction to pure electronic conduction which is associated with the pressure-induced magnetic state transition.

In our study, we used IS method in an inert atmosphere (LN2), so the protonic contribution is expected to be inhibited and does not contribute to total conductivity. Thus, the obtained trend in our study could lead to the conclusion that as the Ni content is increasing, the structure and bonding are affected in the studied Ni_xFe_1-xOOH NPs samples, which leads to the increase of polaron hopping activation energies, thus decreasing the ground state carrier mobility compared to pure goethite. Also, at the same time, with Ni doping and a decrease in Fe content, the charge carrier concentration decreases. Additionally, the presence of mixed metal centers (Fe, Ni) does not appear to have a favorable effect on polaron transport. Overall, the effects above collectively contribute to a decrease in the DC conductivity as the Ni content increases in the studied Ni_xFe_1-xOOH nanoparticles.

3.10. The Electrochemical Properties of α-Ni_xFe_1-xOOH NPs/Li- and Na-Ion Batteries

Table 6 shows the discharge capacity and capacity retention of Ni0, Ni10, Ni20, and Ni30 samples as a cathode in LIB under a low current rate at 5 mA g^-1; the initial discharge capacity of Ni0 was the highest, reaching 1069 mAhg^-1. The discharge capacity decreased with the increase in Ni content to 250 mAhg^-1 for Ni30. Moreover, the capacity retention of all samples suddenly dropped after 10 cycles. The capacity retention of Ni0, Ni20, and Ni30 was less than 1 %, while Ni10 was 7.4 %.

On the other hand, after a tenfold increase of the current rate to 50 mA g^-1, the initial discharge capacity of Ni0, Ni10, and Ni20 was 333, 350, and 494 mAhg^-1, respectively, and the discharge capacity increased with the Ni content.

However, capacity retention decreased when the Ni content was increased. We can conclude that after increasing the current rate from 5 to 50 mA g^-1, the initial capacity of all samples decreased, while the capacity retention enhanced, as shown in Table 6 and Figure 13 (a).

In contrast, the discharge capacities of Ni0, Ni10, Ni20, Ni30, and Ni50 were 110, 116, 223, 189, and 202 mAhg^-1, respectively, which was proportional to the size of the surface area as shown in Table 4 [54,55].

Additionally, the capacity retention after 30 cycles of all samples after adding Ni is lower than that for Ni0, as shown in Table 7 and Figure 13 (b). According to the aforementioned results concerning α-Ni_xFe_1-xOOH NPs as cathode material in both LIB and SIB, with a specific focus on capacity retention after 30 cycles, one can deduce that the performance of Ni_xFe_1-xOOH NPs as active cathode materials is more comparable in SIB compared to LIB.

3. Materials and Methods

3.1. Preparation of α-Ni_xFe_1-xOOH NPs

For preparing the α-Ni_xFe_1-xOOH NPs, iron (III) chloride hexahydrate (Wako-095-00875) and nickel (II) chloride (Wako-141-01062) were dissolved in 30 mL of pure water under magnetic stirring. The concentration and molar ratio of nickel are presented in Table 8. Once the mixture solution was thoroughly homogenized, 3 mol. L^-1 sodium hydroxide (Wako-194-02135) aqueous solution was added until the pH reached 13.

The mixture solution underwent ultrasonication for 20 min after vigorous stirring. Furthermore, the mixture solution was stirred for an additional 20 min. The obtained mixture was then transferred into a Teflon-lined stainless-steel autoclave and maintained at 80°C for a hydrothermal reaction period of 24 h. After naturally cooling to RT, the solid products were washed with pure water and ethanol to remove the neutral electrolyte. The solid products were dried at 60°C for 12 h and allowed to cool naturally. These samples were respectively abbreviated as Ni0, Ni5, Ni10, Ni15, Ni20, Ni30, Ni40, and Ni50.

3.2. Structural Characterization

X-ray diffraction (XRD) patterns were measured by using a RINT-TTR III (Rigaku) X-ray diffractometer with Cu-Kα: λ=1.54 Å. The measurements were conducted over a 2 θ range of 10° to 80°, with a data interval and scanning rate of 0.02° and 5° min^-1, respectively. The X-ray generator was operated at 50 kV and 300 mA. The obtained XRD patterns were analyzed using Smartlab Studio II Powder XRD software with the database of ICDD.

Thermogravimetry differential thermal analysis (TG-DTA) was performed by (Thermo plus TG8120, Rigaku), under the heating rate of 10 K min^-1 and the temperature range (RT- 1000°C). The weight of the α-Ni_xFe_1-xOOH NPs and α-Al₂O₃ reference was fixed to be 10 mg.

Fourier Transform Infrared Spectroscopy (FTIR) transmission spectra of α-Ni_xFe_1-xOOH NPs were recorded by (FTIR PerkinElmer; spectral resolution: 1 cm^-1, USA) spectrometer in the range 400-4000 cm^-1. The α-Ni_xFe_1-xOOH NPs samples as a powder were mixed with KBr in a pellet of 10 mm diameter under a pressure of 90 kg/mm² (KBr pellets technique), with the ratio of α-Ni_xFe_1-xOOH NPs samples weight to KBr at ∼1%.

⁵⁷Fe Mössbauer spectra were measured at RT and at 86 K by a constant acceleration method with a ⁵⁷Co(Rh) source, and α-Fe was used as a standard reference material. The obtained spectra were analyzed using the Mosswinn 4.0 software. RT- X-ray absorption spectra of Fe-K and Ni-K (XANES/EXAFS) were measured in transmission mode by a beamline BL-12C at High Energy Accelerator Research Organization (KEK-PF, 1-1 Oh-ho, Tsukuba, Ibaraki, Japan). The X-ray beam emitted from the synchrotron was monochromatized using a Si(111) double crystal and further attenuated to suppress higher harmonics by employing a Ni mirror. The intensity of the X-ray beam was measured by setting an ionization box before and after transmission. In the front chamber, a mixture of N₂+He gases (N₂: 30%, He: 70 %) was filled, while in the rear chamber, a mixture of Ar+N₂ gases (Ar: 30 %, N₂: 70 %) was utilized for X-ray intensity measurements. The required amount of sample needed for an excellent spectrum was calculated by SAMPLEM4M software. The sample and boron nitride were mixed and homogenized in a mortar for 20 min, followed by pressing to make pellets with a diameter of 1.0 cm. X-ray absorption spectra were analyzed by Athena software. The optical bandgap was recorded with a Shimadzu UV-3600 spectrometer with an integrating sphere attachment (ISR-3100, Shimadzu). Diffuse reflectance UV-Vis spectroscopy, combined with the Kubelka-Munk equation and Tauc plots, were utilized to estimate the optical band gap. The specific surface area (SSA) was measured by BERSORP MINI X (MICRO TRAC BEL) with N₂ gas as the adsorbate, and the SSA was calculated by BET (Brunauer-Emmett-Teller) method. The preprocessing was performed at 60 °C for 24 h using the BELSORP VAC II (MICRO TRAC BEL). The obtained results were analyzed by BELMaster7(MICROTRAC BEL).

3.3. Photocatalytic Activity

To evaluate the photocatalytic performance of the α-Ni_xFe_1-xOOH NPs, degradation of methylene blue (MB) in an aqueous solution was measured, combining the produced sample (10 mg), MB (Wako 7220-79-3) (20 µM, 10 mL) and H₂O₂ (Wako 081-04215) (9.75M, 40 µL). UV-Vis spectra of MB aqueous solution were measured by GENESYS^TM 10S UV-Vis spectrophotometer in a wavelength region from 200 to 800 nm every 10 minutes with photoirradiation and stirring. The concentration of MB aqueous solution after each interval was measured using the absorbance at the wavelength of 665 nm. The visible-light Fiber-Lite MH-100 metal-halide lamp emitted in a wavelength range from 250 to 750 nm, and the output power was 200 W.

3.4. Solid-State Impedance Spectroscopy (SS-IS) of α-Ni_xFe_1-xOOH NPs

Electrical properties were studied by Impedance spectroscopy. Powder samples were pressed into cylindrical pellets 10 mm in diameter and thickness of about 0.5 mm under a uniform load of 2 tons using a hydraulic press. Gold electrodes 6 mm in diameter were sputtered onto both sides of the sample pellets using Sputter coater SC7620, Quorum Technologies for the electrical contact. Complex impedance was measured using an impedance analyzer (Novocontrol Alpha-AN Dielectric Spectrometer, Novocontrol Technologies GmbH & Co. KG, Germany) over a wide frequency range from 0.01 Hz to 1 MHz at temperatures between 30 °C and 170 °C (step 20 °C). The temperature was controlled to an accuracy of ±0.2 K.

Experimental data were analyzed by electrical equivalent circuit (EEC) modeling employing the complex nonlinear least-square (CNLLSQ) fitting procedure. Depending on the EEC used and obtained fitting parameters, various process(es) can be identified and separated. Typically, a single impedance semicircle can be represented by an EEC that combines a resistor and a capacitor connected in parallel. Ideally, this semicircle should pass through the origin of a complex plot and yield a low-frequency intercept on the real axis, corresponding to the resistance, R, of the observed process. However, in cases where the experimentally observed semicircle appears depressed, an alternative component known as the constant-phase element (CPE) is used instead of the standard capacitor in the equivalent circuits. The CPE is an empirical impedance function of the type: Z*CPE = 1/A(iω)α where A and α are the constants. For the samples in this study, the complex impedance spectrum is described by an equivalent R-CPE circuit. The parameters for each circuit element (R, A, and α) were determined directly from measured impedance data using the CNLLSQ method. The DC conductivity values are calculated based on the modeling process, considering both the parameters obtained and the sample geometry.

2.5. Preparation of SIB Containing α-NixFe1-xOOH NPs Cathode

To prepare the cathode, first, 250 mg of α-Ni_xFe_1-xOOH NPs, and 95 mg of acetylene black (AB, Strem chemicals 06-0025) were mixed by a ball mill (Pulverisette7, Fritch) at 800 rpm for 15min. Then, 5 mg of polytetrafluoroethylene (PTFE, Wako 165-13412) was added to the 95 mg ball-milled mixture powder and mixed in the mortar until the powder became semi-solid and pressed in a pellet with a diameter of 1 cm and weight of 30 mg. For the sodium-ion battery, metallic Na (Kishida 750-70852) (90 mg) and an electrolyte of 1 M NaClO₄ / propylene carbonate solution (Tomiyama LIPASTE-P/S1) were used, while metallic Li (Wako 127-06001)(30 mg) was used with LiPF₆/Ethylene carbonate and Dimethyl carbonate 1: 1 v/v% ratio (Kishida LBG-00022) for lithium-ion battery. The 2032-type coin battery was assembled in a glove box under an oxygen concentration less than 0.99 ppm. TOSCAT-3100SK (TOYO-SYSTEM) measured the charge-discharge capacity performance at 30 cycles under the voltage range of 0.8-4.0 V, a current rate of 5 and 50 mA g^-1, and a lower limit current of 0.1 mA. This process started with the discharge process in discharge-CC and charge-CC/CV.5.

5. Conclusions

In this paper, we studied the relationship between local structure, photo-Fenton catalytic ability, and electrochemical properties in LIBs and SIBs of Ni-doped goethite with the composition of α-Ni_xFe_1-xOOH NPs (x= 0, 0.10, 0.20, 0.30, 0.40, and 0.50, abbreviated as Ni100*x). XRD patterns of α-Ni_xFe_1-xOOH showed the α-FeOOH crystalline phases in all prepared samples. Moreover, a new crystalline phase is related to Ni(OH)₂, for Ni contents from 0.30 to 0.50. The weight loss at temperatures up to approximately 140 °C corresponds to the release of H₂O molecules adsorbed on the surface of Ni_xFe_1-xOOH NPs crystals or trapped within the interstitial spaces between them. This low-temperature weight loss increased in the Ni_xFe_1-xOOH NPs samples with increased Ni content, which can be explained by more adsorbed water in the NiFeOOH NPs samples with a large surface area. The weight loss at temperatures between about 150 and 290 °C can be attributed to the dehydroxylation of surface hydroxyl groups. This weight loss also increases in the Ni_xFe_1-xOOH NPs with a higher Ni content due to the larger surface area. Moreover, the transmission peak observed at 3130 cm^-1 corresponds to the O-H stretching vibration of the hydroxyl group. The strong two transmission peaks were observed at 797 cm^-1 and 895 cm^-1 related to Fe-O-H bending vibrations out of the plane and in the plane, respectively. A new transmission peak appeared in higher Ni concentration samples (Ni40 and Ni50) at 472 and 476 cm^-1, respectively. Compared with that shown in the FTIR spectrum of NiO can be attributed to Ni-O. The absorption edge energy of α-Ni_xFe_1-xOOH NPs is higher than the values of Fe₂O₃ (Fe³⁺); this implies that none of the α-Ni_xFe_1-xOOH NPs samples contained Fe²⁺ and they contained only Fe³⁺. Moreover, the Ni-K absorption edge energy of Ni_xFe_1-xOOH NPs is higher than that expected for pure Ni²⁺. Samples Ni15 and Ni20 contain less Ni³⁺, while Ni10 has the highest Ni³⁺ content. The ⁵⁷Fe Mössbauer spectrum of Ni0, measured at RT, displayed a sextet corresponding to goethite, with an isomer shift (δ) of 0.36 mm s^-1 and a hyperfine magnetic distribution (B_hf) of 32.4 T. Additionally, a doublet with δ and quadrupole splitting (Δ) values of -0.26 mm s^-1 (for both) was observed due to a superparamagnetic component. The absorption area of the superparamagnetic component increased from 4.8% to 82.9%, with an increase in Ni content up to an 'x' value of 0.2. The surface area of Ni100*x changed from 45.1 to 73.9, 174, 145, and 178 m² g⁻¹ with ‘x’ 0, 0.10, 0.20, 0.30, 0.40, and 0.50, respectively. In addition, the optical band of Ni_xFe_1-xOOH NPs decreased from 2.71 eV to 2.15 with increasing Ni content from 0 to 0.50. The largest first-order rate constant (k) of 14.6×10^-3 min^-1 was measured for Ni10. The DC conductivity decreased from 5.52×10^-10 to 5.30×10^-12 (Ω.cm)^-1 with ‘x’ increasing from 0.10 to 0.50. The highest initial capacity was recorded at 494 mAh g^-1 for Ni30 at the current rate of 50 mA g^-1 as a cathode material in LIB. Meanwhile, in SIB, the largest initial capacity was found as 233 mAh g^-1 at a current rate of 5 mA g^-1. In conclusion, α-Ni_xFe_1-xOOH NPs can be effectively utilized as visible-light-activated catalysts and active cathode materials in secondary batteries.