Boosted Electrochemical Activity with SnO₂ Nanostructures Anchored on α-Fe₂O₃ for Improved Charge Transfer and Current Density

Abstract

This study presents a straightforward and cost-effective method to enhance the photoelectrochemical (PEC) water-splitting performance of α-Fe₂O₃ (F), SnO₂ (S), and α-Fe₂O₃ decorated with SnO₂ quantum dots (FS) photoanodes in a NaOH electrolyte. The FS electrode demonstrated a notable improvement in PEC efficiency within the electrolyte. In particular, the generated charges of the FS anode in the NaOH electrolyte reached approximately 12.01 mA cm⁻² under illumination, indicating that the developed heterostructures effectively enhanced kinetics, leading to improved separation of induced carrier pairs. This active carrier-pair separation mechanism contributed considerably to the increased PEC activity in the 0.1 M NaOH electrolyte. The reduction in the bandgap of FS increased its absorption capability in visible light, which further enhanced the current density. Furthermore, the reduction in electrolyte resistance (9.71 Ω), internal resistance (20.19 Ω), charge transfer resistance (3.21 kΩ), Tafel slope (45.5 mV dec-1), limiting current density (−2.09 mA cm⁻²), and exchange current density (−3.68 mA cm⁻²) under illumination at the interface enhanced the charge density of FS. Further, a strong interaction among photoanode nanostructures significantly enhances PEC activity by improving efficient charge separation and transport, reducing recombination rates, and enabling quicker movement of charge carriers to the electrode/electrolyte interface. Thus, this study provides an effective approach to increasing the PEC activity of heterostructures.

1. Introduction

The development of functional materials for high-performance electrochemical applications is a critical area of research, particularly in the fields of energy storage and conversion. Among the various materials being explored, metal oxides have demonstrated major potential because of their inherent electrochemical properties [1]. Moreover, metal oxide nanostructures, such as BiVO₄ [2], WO₃ [3], and α-Fe₂O₃ [4], which are stable in aqueous electrolytes and can harvest light in the solar region, have been extensively examined as potential photoelectrodes in electrochemical hydrogen generation. Among these, α-Fe₂O₃ can efficiently absorb energy in visible light, owing to its bandgap of ~2.0 eV, and has a theoretical solar-to-hydrogen conversion efficiency of 17.0%. Despite these promising characteristics, the actual efficiency of α-Fe₂O₃ has yet to achieve a theoretically predicted efficiency [5]. Recently, α-Fe₂O₃ nanostructures were used for enhancing water splitting, which achieved a current density of 0.33 mA cm⁻² at 1.23 V_RHE [6]. Makimizu et al. [7] studied the low-temperature annealing effect on the electrochemical properties of α-Fe₂O₃, resulting in a current density of 0.5 mAcm⁻². Moreover, Geethana et al. [8] studied the rapid annealing effect on the electrochemical activity of the α-Fe₂O₃ photoanode and observed enhanced current density. Fe₂O₃ nanorods were utilized for electrochemical water oxidation in a 1.0 M NaOH electrolyte and achieved the lowest current density of 1.42 mA cm⁻² [9]. Furthermore, water oxidation using Zr-doped α-Fe₂O₃ in a 1 M KOH electrolyte solution achieved a photocurrent density of 1.69 mAcm⁻² [10]. This shortfall is primarily due to its poor electrical conductivity (10⁻⁶ Ω⁻¹ cm⁻¹), short hole diffusion length (~2–4 nm), shorter excited lifetime (<10 ps), and poor reaction kinetics, leading to high electron-hole recombination rates, and inefficient charge separation. To overcome these constraints, widespread efforts have been focused on the development of innovative nanostructures that incorporate hematite. These nanostructures are either decorated or doped with several elements, decoration of nanostructures, and/or composited with other oxides. Among these techniques, the development of a heterostructure can generate an electric field at the junction between the nanostructures, improving charge migration and separation while reducing the recombination rate [11]. In addition, the heterostructure facilitates the combination of structures that can enhance the absorption of visible light, generating more charge pairs and improving electrochemical activity. Pourbakhsh et al. [12] developed the bi-layer structure of α-Fe₂O₃ structures on TiO₂, which achieved a low current density of 0.3 mA cm⁻². The α-Fe₂O₃/CdS heterostructure was synthesized using a modified hydrothermal method to achieve a current density of 0.6 mAcm⁻² [13]. Baldovi et al. developed an α-Fe₂O₃-TiO₂ heterostructure, evaluated its light-harvesting capacity resulting from the coupling of nanostructures, and attained improved light absorption capacity owing to the reduced bandgap of nanostructures [14]. Cai et al. [15] studied the effect of heterostructure formation on photocarrier separation, achieving a current density of 20 µAcm⁻² at 1.23 V_RHE. Zhang et al. synthesized ZnO/α-Fe₂O₃ heterojunction photoelectrode for improving catalyst activity and attained a current generation of 1.82 mAcm⁻² [16]. Furthermore, the Fe₂O₃/BiVO₄ composite photoanode synthesized for electrochemical activity obtained a current density of 1.6 mA cm⁻² [17]. Additionally, Zhang et al. [18] reported that the interface effect of the Z-scheme α-Fe₂O₃/g-C₃N₄ photoanode [18] improved charge transfer.

SnO₂ possesses outstanding electrical conductivity, chemical stability, and catalytic properties, making it an ideal nanostructure for enhancing the electrochemical activity of α-Fe₂O₃. The integration of SnO₂ nanostructures into α-Fe₂O₃ nanoparticles is expected to create a composite material with superior electrochemical characteristics [19]. The SnO₂ nanostructures can deliver a conductive network that facilitates effective electron migration while also providing large active sites for enhancing electrochemical activity. This decoration can effectively improve the charge transfer kinetics and enhance the complete electrochemical performance. Additionally, the catalytic stability imparted by the SnO₂ reduces volume expansion and aggregation issues associated with α-Fe₂O₃, leading to enhanced cycling stability and durability. Furthermore, combining α-Fe₂O₃ and SnO₂ are two such materials that, when combined, can synergistically enhance their individual characteristics. For instance, Rahman et al. [20] developed an α-Fe₂O₃–SnO₂ nanocomposites via the hydrothermal method for electrochemical applications, achieving a very low current density of 0.32 mAcm⁻². In addition, the CoOOH/α-Fe₂O₃/SnO₂ photoanode achieved a current density of 2.05 mA cm⁻² owing to reduced surface states and enhanced absorption capability [21]. Moreover, α-Fe₂O₃/TiO₂ 3D hierarchical nanostructures synthesized for electrochemical activity attained a current density of 0.2 mAcm⁻² [22]. However, no studies have been conducted on the effect of applied potential on the photocurrent generation performance of heterostructures. This study focuses on α-Fe₂O₃ nanoparticles decorated with SnO₂ nanostructures, aiming to improve charge transfer and generation kinetics and achieve high stability and electrochemical activity.

Herein, pristine α-Fe₂O₃ and SnO₂ and the α-Fe₂O₃–SnO₂ heterostructure were synthesized, and the effect of the heterostructure formation on the electrochemical properties of α-Fe₂O₃ nanoparticles was studied. The α-Fe₂O₃–SnO₂ showed enhanced photon harvesting capability and a record-high photo-induced current generation in the pristine samples. This enhancement may be due to increased charge migration and separation and reduced charge-transfer resistance at the electrode surface/electrolyte interface.

2. Preparation Techniques

Initially, 0.42 g of FeCl₂⋅H₂O were poured into 50 mL of H₂O and stirred for 70 min. After the complete dissolution of FeCl₂⋅H₂O, the pH of the solution was adjusted to 10 using an NH₄OH solution with continuous stirring. The final solution was transferred to a Teflon-lined hydrothermal reactor and annealed for 4 h at 180 °C. After cooling to room temperature, the product was centrifuged and cleaned with water and ethanol several times before being oven-dried at 80 °C for 24 h. The resultant sample was annealed for another 2 h at 500 °C.

Additionally, 2.25 g of SnCl₄⋅5H₂O was poured into 105 mL of water and stirred for 80 min, and then 5 mL of N₂H₄ was poured into the SnCl₄⋅5H₂O with continuous stirring at 30 °C. The hydrazine acts as a reducing agent in the preparation of SnO₂ nanostructures, facilitating the reduction of tin precursors to SnO₂ while also controlling the growth and morphology of the nanostructures. Further, its strong reducing properties help in achieving the desired nanostructure properties and purity. The solution was maintained at 112 °C for 20 h, then cooled and separated by centrifugation. The solution was cleaned several times using water and ethanol and oven-dried at 85 °C for 24 h.

Equal weights of each synthesized α-Fe₂O₃ and SnO₂ nanostructure were separately poured into 50 mL of H₂O and dispersed for 2 h using a probe sonicator. The SnO₂ solution was then added dropwise to the α-Fe₂O₃ solution and sonicated for 3 h. The final product was extracted via centrifugation, cleaned with water and ethanol several times, and oven-dried at 90 °C for 26 h.

To fabricate the SnO₂-decorated α-Fe₂O₃ photoanode, each synthesized sample (3 mg) was dispersed in ethylene glycol (5 mL) using probe sonication for 30 min. This dispersed solution was drop-casted onto a clean 1 × 1 cm² ITO glass substrate placed on a hot plate at 130 °C, transferred into an oven, and dried at 130 °C for 24 h.

The crystalline phases of the synthesized samples were characterized using X-ray diffraction (XRD; PANalytical X’pert PRO, Eindhoven,, The Netherlands). Scanning electron microscopy (SEM; Hitachi S-4800, Tokyo, Japan) was used to examine the morphologies. The chemical states and valence bands of the samples were examined using X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific MultiLab 2000, Seoul, South Korea). The optical properties of the samples were investigated using Fourier-transform infrared spectroscopy (FTIR; Perkin Elmer Spectrum 100, city, stat Shelton, Connecticut, e, United States), Raman spectroscopy (XploRA Plus, Osaka, Japan), and UV–Vis (ultraviolet–visible) spectroscopy (Neogen NEO-D3117, Seoul, South Korea).

Photoelectrochemical (PEC) water-splitting properties were evaluated using a three-electrode setup containing the synthesized samples, Ag/AgCl, and Pt as working, reference, and counter electrodes, respectively, with an SP-200 potentiostat (Bio-Logic, Seyssinet-Pariset, Seyssinet-Pariset, France). All the parameters were recorded against the reference electrode in a 0.1 M NaOH electrolyte under both ON and OFF conditions. Electrochemical impedance analysis was performed at a potential of 10 mV over the frequency range of 0.5–100 MHz in 0.1 M NaOH.

3. Results and Discussion

The synthesized samples were analyzed using XRD to gather insights into their structural properties. Figure 1 shows the structural analysis of pristine α-Fe₂O₃ (F), pristine SnO₂ (S), and α-Fe₂O₃-SnO₂ (FS). The structural analysis of F sample showed a characteristic peaks at 24.21° (012), 33.21° (104), 35.65° (110), 40.88° (113), 49.51° (024), 54.11° (116), 57.62° (122), 62.48° (214), 64.07° (300), 71.95° (1010), and 75.51° (220), corresponding to the iron oxide rhombohedral crystal phase has been observed, as per JCPDS file no. 33–0664 [23]. The reflected patterns of the SnO₂ sample showed a peak at 26.07° (110), 33.64° (101), 51.71° (211), and 64.65° (301) corresponding to the tetragonal phase according to the JCPDS file no. 77–0450 [24]. The diffraction pattern of α-Fe₂O₃/SnO₂ indicated composite structures with the characteristic peaks of both α-Fe₂O₃ and SnO₂ samples. Feng et al. [25] observed XRD patterns of SnO₂/Fe₂O₃ hybrid nanofibers.

The morphologies of the synthesized α-Fe₂O₃, SnO₂, and α-Fe₂O₃-SnO₂ samples are shown in Figure 2. The pristine α-Fe₂O₃ sample exhibited an irregular square-shaped structure with smooth surfaces (Figure 2a). Figure 2b shows the agglomeration of SnO₂ nanoparticles. The heterostructure morphology of the synthesized α-Fe₂O₃-SnO₂ sample is shown in Figure 2c. Evidently, SnO₂ nanoparticles were uniformly decorated on the surface of the α-Fe₂O₃ surface, resulting in a rougher surface than pristine α-Fe₂O₃. This indicates that the SnO₂ nanostructures were anchored to the surfaces on the α-Fe₂O₃ nanostructures surface without disturbing the original morphology of the iron oxide. Liu et al. [26] developed a similar type of α–Fe₂O₃@SnO₂ hybrid structure, which exhibited better catalytic activity because of its unique heterostructure and synergic effect between α-Fe₂O₃ and SnO₂ samples.

Figure 3 shows the optical properties of the synthesized α-Fe₂O₃, SnO₂, and α-Fe₂O₃-SnO₂ samples using UV–Vis spectroscopy. As shown in Figure 3a, the absorption edge of pristine α-Fe₂O₃ nanostructures was at 645 nm. For pure SnO₂, an absorption edge was observed at 315 nm (Figure 3b). In contrast, the α-Fe₂O₃-SnO₂ heterostructure displayed sturdier absorption not only in the UV region but also in the visible region [27]. Compared with α-Fe₂O₃, the absorption edge of α-Fe₂O₃-SnO₂ was red-shifted and observed at 713 nm. This red shift in the heterostructure was due to the variance in the optical bandgaps of α-Fe₂O₃ and SnO₂, causing the rearrangement of the bands in the α-Fe₂O₃-SnO₂ [28]. Moreover, the grain boundaries at the junction between the structures created several transitional energy levels that lowered the conduction band of α-Fe₂O₃, facilitating free electron transport from the valence band to the conduction band of the heterostructure [10,29]. This enhanced the electrochemical activity of the α-Fe₂O₃-SnO₂. Figure 3d–f show the plots of (αhν)² vs. hν for the optical bandgaps of α-Fe₂O₃, SnO₂, and α-Fe₂O₃-SnO₂ derived from the UV–Vis absorption spectra. The results showed that the optical bandgaps of α-Fe₂O₃, SnO₂, and α-Fe₂O₃-SnO₂ were at 1.92, 3.93, and 1.73 eV, respectively. The reduced optical bandgap of α-Fe₂O₃-SnO₂ was due to the red-shifting of its absorption onset compared to that of α-Fe₂O₃. Based on the UV–Vis spectral analysis, the SnO2 deposited on the α-Fe₂O₃ heterostructure is expected to exhibit substantial electrochemical activity for water splitting in the visible region, similar to the performance observed in PEC methods [14,15].

Figure 4a shows the FTIR spectra of the pristine samples and heterostructure. Pristine α-Fe₂O₃ exhibited absorption bands at 3407, 1634, 1380, 878, 550, and 470 cm⁻¹ were noticed. The band at 3407 cm⁻¹ was attributed due to the O-H stretching vibration in O-H groups. The characteristic bands at 1634 and 1380 cm⁻¹ were due to Fe–OH vibrations. The stretching vibrations of the Fe–O–Fe bond were observed at 878 cm⁻¹ for the α-Fe₂O₃ sample. The bands at 550 and 470 cm⁻¹ were attributed due to the Fe–O stretching and Fe–O bending vibrations of α-Fe₂O₃. The SnO₂ nanostructures exhibited absorption bands at 3472, 2919, 2880, 1440, 1380, 1119, 670, and 560 cm⁻¹. The band at 3472 cm⁻¹ was attributed to the O-H stretching vibrations and water molecules engrossed from the atmosphere by the SnO₂ sample on its surface. The bands at 2919 and 2880 cm⁻¹ were attributed to the existence of C-H groups. The bands at 1440, 1380, and 1119 cm⁻¹ were assigned to the vibration of Sn-OH vibrations. The bands at 670 and 560 cm⁻¹ are ascribed to stretching and vibrations of O-Sn-O and Sn-O, respectively. Further, the heterostructure showed mixed bands of α-Fe₂O₃ and SnO₂ nanostructures, and no additional peaks were observed, indicating the superior quality of the heterostructure.

Figure 4b shows the Raman spectra of pristine α-Fe₂O₃, SnO_2, and the α-Fe₂O₃-SnO₂ heterostructure. The strong characteristic peaks of α-Fe₂O₃ at 223 and 290 cm⁻¹ were assigned to the A1g symmetry, and the weaker peak at 407 cm⁻¹ corresponded to the Eg symmetry, indicating the formation of α-Fe₂O₃ phase. No additional peaks corresponding to other forms of iron oxide and iron oxyhydroxides were observed, confirming the purity and phase of the α-Fe₂O₃ sample. SnO₂ exhibited no strong Raman bands. However, the heterostructure sample showed α-Fe₂O₃ bands along with a SnO₂ disorder activation Raman band at 584 cm⁻¹. The peak position and band intensity of 584 cm⁻¹ is based on the nanoparticle size [11].

The XPS spectra of α-Fe₂O₃, SnO₂, and α-Fe₂O₃-SnO₂ are shown in Figure 5. The XPS survey spectra confirmed the core elements of the synthesized samples and demonstrated the superior quality of the nanostructure (Figure 5a). The electronic states of the nanostructures were investigated by recording the core-level peaks, as shown in Figure 5b–h. The core-level Fe2p XPS peaks for the α-Fe₂O₃ and α-Fe₂O₃–SnO₂ are shown in Figure 5b,c. Peaks were observed at binding energies of 710 and 723 eV, corresponding to Fe2p_3/2 and Fe2p_1/2, respectively. In addition, the Fe2p core peaks were deconvoluted into four major peaks at 708.8 (2p_3/2, Fe²⁺), 710.1 (2p_3/2, Fe³⁺), 722.9 (2p_1/2, Fe²⁺), and 725.2 (2p_3/2, Fe³⁺) eV, as well as Fe satellite peaks at 712.2 (Fe²⁺), 717.7 (Fe³⁺), 728.4 (Fe²⁺), and 732.2 (Fe³⁺) eV. Consequently, the presence of both Fe³⁺ (α-Fe₂O₃) and Fe²⁺ (FeO) states was revealed in the α-Fe₂O₃ and α-Fe₂O₃–SnO₂. The core XPS analyses of SnO₂ and α-Fe₂O₃–SnO₂ are presented in Figure 3d,e, demonstrating a +4 oxidation state of Sn-based on two major peaks at 486.2 and 494.6 eV, corresponding to Sn3d_5/2 and Sn3d_3/2, respectively. The O1s XPS core spectra of all the synthesized samples showed a peak at a binding energy of 528.8 eV, which may be attributed to the lattice oxygen (O_b) and the adsorbed oxygen (O_a) peak at 530.6 eV, indicating the purity of the samples (Figure 3f–h).

The valence band (VB) spectra of the synthesized samples were recorded to estimate the VB positions, as shown in Figure 6a–c. The VB energies of α-Fe₂O₃, SnO_2, and SnO₂ decorated α-Fe₂O₃ nanostructures were obtained at 0.34, 2.43, and 0.42 eV. The bandgaps obtained from the UV–vis spectra and the VB energies were utilized to determine the electronic structures and band positions of the samples, as given in Figure 6d. The heterostructure of the α-Fe₂O₃–SnO₂ sample displayed narrower bandgaps, high VB state potential, and lower CB (conduction band) state potential than the α-Fe₂O₃ sample, enabling effective light absorption capability and charge excitation as well as adequate energy for enhanced PEC reaction. Consequently, these synthesized catalysts are expected to perform better as photoanodes under light illumination for H₂ production.

Electrochemical analysis is crucial for elucidating the charge kinetics at the bulk/liquid interface and the separation and migration of charge pairs at the anode top surface/liquid interface within the fabricated photoanode [15]. Figure 7a shows the electrochemical impedance analysis results of α-Fe₂O₃, SnO₂, and α-Fe₂O₃-SnO₂ anodes under both ON and OFF conditions in 0.1 M NaOH. Three characteristic regions were observed in the Nyquist spectra: resistance associated with contact and liquid (R_s) at lower applied frequencies, resistance related to charge transfer at the anode surface/electrolyte junction (R_i) at mid-applied frequencies, and resistance attributed to ion diffusion at higher applied frequencies (R_c). The impedance plots of F, S, and FS showed half-circles under ON and OFF conditions, with aberrations in the dimensions of the half-circles and inclined lines demonstrating discrete conductive properties among the various anodes. The lowest half-circle radius was observed for FS in the ON state, indicating enhanced carrier transfer activity of pristine anodes. In addition, the impedance analysis was modeled using equivalent circuits to estimate the kinetic behavior of the photoanodes (Figure 7b). These parameters include the liquid resistance resulting from the formation of an anode/electrolyte junction (R_s), resistance ascribed to developed grains and grain boundaries in the anode and material defects (R_i), resistance associated with charge movement (R_c), space charge capacitance (C_i), and Helmholtz capacitance (Cc). The physical properties of the F, S, and FS anodes are listed in

Table 1. Impedance fitted parameters for α-Fe₂O₃, SnO_2, and α-Fe₂O₃-SnO₂ electrodes.

Anode	State	Rs (Ω)	Ri (Ω)	Rc (Ω)	Ci (μF)	Cc (μF)
α-Fe2O3 (F)	Dark	10.53	20.33	8024.28	0.35	10.01
	Light	9.94	20.23	4800.02	0.37	9.99
SnO2 (S)	Dark	11.89	28.15	1227.61	1.53	7.98
	Light	11.86	27.88	1144.30	1.54	7.93
α-Fe2O3-SnO2 (FS)	Dark	9.80	20.21	3741.74	0.33	13.03
	Light	9.71	20.19	3212.38	0.31	13.01

. Remarkably, the resistance values under light conditions were lower than those in the OFF state, indicating that the carriers enabled electron and hole migration within the anodes and their interfaces. In particular, the FS anode exhibited a lower R_s value in the ON state, suggesting that the FS sample developed enhanced effective charge movement along the anode surface/liquid junction, resulting in higher electrochemical activity. The lowest R_s value for the FS sample connected to the pristine anodes may be attributed to the development of a heterostructure, signifying a reduction in the contact resistance of pristine samples. In the ON state, the smallest R_i value (20.19 Ω) was achieved for the FS sample connected to the pure F and S anodes. This suggests that the heterostructure considerably decreased the recombination rate by varying the band structure, which is a major reason for the improved catalytic activity. The R_c value for FS (3212.38 Ω) was smaller than that for F (4800.02 Ω) and S (1144.30 Ω) in the ON state, emphasizing the enhanced carrier transfer toward the interface in the FS anode, particularly with the heterostructure photoanode. In addition, the noteworthy decrease in R_c is attributed to two major reasons. First, the electric field produced at the α-Fe₂O₃-SnO₂ junction, owing to the development of a heterostructure, considerably promoted charge migration and reduced the recombination of charge pairs, thus reducing charge transfer resistance. Second, the surface of the heterostructure photoanode provided a larger interaction area with a large number of active sites for oxidation at the anode/liquid junction, which considerably increased the interfacial charge migration, leading to a decreased Rc value. This enrichment is advantageous for attaining higher electrochemical activity under light conditions. The C_i and C_c values for the FS electrode were higher than those for the F and S electrodes in the ON and OFF states. In the ON state, the highest C_i and C_c values of 0.31 µF and 13.01 µF, respectively, were achieved for the FS anode. This was due to the development of a heterostructure that prevented the accumulation of charges at the surface/liquid junction. The lower R_i value and improved capacitance of the α-Fe₂O₃-SnO₂ anode facilitated easier migration of generated charges to the anode surface, where water oxidation occurs. Thus, the FS photoanode catalyst produced numerous carrier pairs in the electrolyte under illumination. Furthermore, Bode and phase analyses (Figure 7c,d), respectively) were performed for the F, S, and FS anodes. The impedance spectra of the FS moved towards lower applied frequencies than those of the pristine F and S anodes, owing to the rapid formation and enhanced migration of charge pairs, lowering the recombination rate. Phase analysis of FS showed a peak with a smaller intensity frequency, signifying that the sample may have a prolonged charge lifetime associated with the F and S anodes. This indicates that the constant spreading of the nanostructures increased charge separation and prolonged the carrier lifetime.

Figure 8a shows the Tafel plots of the F, S, and FS electrodes. The plots showed an increase in the anode voltage when switching from an ON to an OFF state. This shift indicated increased charge-pair production, thus improving the catalytic activity. The values derived from the Tafel analysis are listed in

Table 2. Tafel fitted parameters for α-Fe₂O₃, SnO₂ and α-Fe₂O₃-SnO₂ electrodes.

Anode	State	Tafel Slope (mVdec−1)	JLt (mAcm−2)	Jee (mAcm−2)
α-Fe2O3 (F)	Dark	69.2	−0.68	−3.95
	Light	60.9	−0.67	−3.86
SnO2 (S)	Dark	63.6	−1.21	−4.59
	Light	54.3	−1.18	−4.33
α-Fe2O3-SnO2 (FS)	Dark	50.8	−2.60	−3.75
	Light	45.5	−2.09	−3.68

. Clearly, the Tafel slopes, limiting charge density (J_Lt), and exchange charge density (J_ee) in the ON condition were smaller than those in the OFF condition for all anodes. The decreased Tafel slopes suggest that the anodes required a low applied potential to trigger the carrier pairs. Remarkably, the FS anode showed the lowest Tafel slope of 45.5 mV dec⁻¹ under light, indicating its effectiveness in producing induced carriers. This finding emphasized the rapid kinetics of the FS anode. The FS anode achieved J_Lt and J_ee values of −2.09 and −3.68 mA cm⁻², respectively, under light (Table 2), which were higher than those of the F and S anodes, indicating that the FS anode exhibited a higher charge migration rate. Thus, the FS electrode exhibited improved electrochemical activity under illumination.

Voltammetry analysis was performed on the ON/OFF states for synthesized α-Fe₂O₃, SnO_2, and α-Fe₂O₃-SnO₂ samples in ON and OFF states to assess their current generation. Figure 8b shows the outcomes of the voltammetry scan acquired for α-Fe₂O₃, SnO_2, and α-Fe₂O₃-SnO₂ anodes in the OFF state. At 1.0 V vs. Ag/AgCl, the α-Fe₂O₃-SnO₂ samples exhibited a dark current density of 0.65 mAcm⁻², whereas no noticeable values of 0.49 and 0.48 mAcm⁻² were obtained for α-Fe₂O₃ and SnO₂ electrodes at the identical reference voltage. This increase in dark current density for the α- α-Fe₂O₃-SnO₂ anode can be attributed to enhanced charge migration within the heterostructure and reduced recombination of charge pairs, owing to the extensive surface area of the α-Fe₂O₃-SnO₂ anode, which offered additional energetic sites for water oxidation [14]. The heterojunction structure enhanced charge migration within the bulk of the electrode, enabling additional holes to reach the surface, where water oxidation occurs. The presence of more holes at the anode/electrolyte interface and the reduced number of recombination pairs resulting from the additional interfacial surface area led to more efficient hole removal for water oxidation, thereby improving the dark current.

The induced current density is a critical parameter that directly reflects the electrochemical performance of the anodes. Figure 8c shows the current response of the α-Fe₂O₃, SnO_2, and α-Fe₂O₃-SnO₂ anodes in the ON state. The α-Fe₂O₃-SnO₂ photoanode exhibited a current density of 12.01 mAcm⁻² at 1.0 V vs. Ag/AgCl, which is ~21-fold higher than the α-Fe₂O₃ sample, which only achieved 0.58 mAcm⁻². This substantial increase was attributed to the development of a heterostructure that generated an inherent electric field at the intersection. This electric field enables the migration of energized charge pairs, considerably decreasing the charge pair recombination rate and thus improving the current density. Further, a strong interaction among photoanode nanostructures significantly enhances PEC activity by improving several key factors. These interactions facilitate efficient charge separation and transport, reducing recombination rates and enabling quicker movement of charge carriers to the electrode/electrolyte interface [14]. Additionally, they increase the surface area of the photoanodes, providing more active sites for PEC reactions while also improving light absorption through enhanced scattering and multiple reflections within the nanostructured network. Also, the catalytic properties of the photoanodes are often enhanced due to synergistic effects arising from the interactions among the nanostructures. This synergy not only boosts PEC efficiency but also contributes to better stability, maintaining the structural integrity of the nanostructures under operational conditions [15]. Finally, strong interactions allow for controlled defect engineering, tuning the electronic properties of the materials to further improve PEC performance. Collectively, these improvements lead to a significant enhancement in the overall PEC activity of the photoanode nanostructures. Additionally, the decrease in the bandgap of α-Fe₂O₃-SnO₂ increased light absorption in the solar spectrum, enhancing the production of charge carriers production and current response. Furthermore, the obtained results are compared with those from published articles, as shown in

Table 3. Comparative data of α-Fe₂O₃ and SnO₂-based photoanodes.

Structure	Current Density (mA cm−2)	Reference
α-Fe2O3/TiO2	0.3	[12]
α-Fe2O3/CdS	0.6	[13]
ZnO/α-Fe2O3	1.82	[16]
Fe2O3/BiVO4	1.6	[17]
α-Fe2O3/SnO2	0.32	[20]
CoOOH/α-Fe2O3/SnO2	2.05	[21]
α-Fe2O3/TiO2	0.2	[22]
α-Fe2O3/SnO2	12.01	Present study

.

Chronoamperometric analysis was performed to evaluate the behavior of the F, S, and FS anodes at various voltages, as represented in Figure 9a–c. All the anodes showed distinct switching characteristics when subjected to voltages above 0.6 V. Remarkably, during ON/OFF cycles 0.9 V, all the anodes demonstrated current densities significantly higher than those at other given voltages. The FS anode exhibited the highest photocurrent density at 0.6 V. This enhanced performance is attributed to the hybrid structure of the FS anode, which results in decreased resistance and improved capacitance [14]. The induced current densities of the anodes at various potentials decreased in the following order: 0.4 V < 0.6 V < 0.8 < 0.9 V.

4. Conclusions

Herein, α-Fe₂O₃, SnO₂, and α-Fe₂O₃-SnO₂ electrodes were successfully fabricated, and the effect of the heterostructure formation on the electrochemical parameters was studied. XRD and Raman spectroscopy analysis confirmed the quality of the developed α-Fe₂O₃-SnO₂ heterostructures. The α-Fe₂O₃-SnO₂ heterostructure achieved a photocurrent density of 12.01 mA cm⁻² at 1.0 V vs. Ag/AgCl, which is 21-fold higher than that of pristine α-Fe₂O₃. The heterostructure generated an inherent electric field that enabled the migration of energized carriers, thus significantly decreasing the recombination rate and enhancing water splitting. The decreased bandgap of the α-Fe₂O₃-SnO₂ heterostructure improved light absorption in the solar spectrum, which further enhanced the induced current density. The strong interlinked morphology of α-Fe₂O₃-SnO₂ enhanced the charge separation of carrier pairs and improved water splitting. Additionally, improved charge density and reduced carrier transfer resistance at the anode/liquid interface of α-Fe₂O₃-SnO₂ were supplementary signs accompanying the enhancement in current density for the samples. Hence, the development of an α-Fe₂O₃-SnO₂ heterostructure with a strong interconnecting surface is a feasible way to improve the photoresponse of α-Fe₂O₃ anodes for water splitting.