Strontium-Doped Tin Oxide Nanofibers for Enhanced Visible Light Photocatalysis

Abstract

This study investigates the photocatalytic degradation of methylene blue (MB) using strontium-doped SnO₂ nanofibers synthesized via electrospinning. The 1% Sr-doped SnO₂ nanofibers exhibited remarkable photocatalytic activity, achieving 84.74% MB degradation under visible light irradiation, substantially outperforming both undoped SnO₂ nanofibers (61%) and the same catalyst under UV light (69%) under identical experimental conditions. Comprehensive electrochemical investigations revealed that Sr doping fundamentally transformed interfacial charge transfer kinetics, with 1% Sr-doped nanofibers exhibiting a remarkable three-fold decrease in charge transfer resistance (404 Ω compared to 1350 Ω for undoped samples), a dramatic enhancement in charge carrier density (5.17 × 10²² versus 9.24 × 10¹⁹ for undoped samples), and an approximately eight-fold increase in diffusion coefficient (8.78 × 10⁻¹⁰ versus 1.13 × 10⁻¹⁰ cm²s⁻¹). These electrochemical improvements were corroborated by comprehensive structural characterization, which demonstrated that strategic Sr incorporation induced beneficial oxygen vacancies, reduced crystallite size, increased microstrain, and enhanced dislocation density, collectively contributing to superior surface reactivity and accelerated photocatalytic mechanisms. This work establishes a quantitative correlation between electrochemical characteristics and photocatalytic activity in Sr-doped SnO₂ nanofibers, revealing the fundamental mechanisms that transform the SnO₂ nanostructure from UV-dependent to efficient visible light-driven catalysts for organic pollutant degradation.

1. Introduction

Water pollution represents one of the most pressing environmental challenges of our time, with industrial effluents containing organic pollutants, which pose significant threats to aquatic ecosystems and human health. Among various treatment approaches, photocatalytic degradation has emerged as a promising advanced oxidation process, offering complete mineralization of pollutants into harmless end products like carbon dioxide and water [1]. This process harnesses the power of semiconductor materials that, when activated by light, generate reactive species capable of degrading organic contaminants without producing secondary waste streams.

Semiconductor-based photocatalysts function through a fundamental mechanism wherein light absorption promotes electrons from the valence band to the conduction band, creating electron-hole pairs. These charge carriers migrate to the catalyst surface where they initiate redox reactions with adsorbed water and oxygen molecules, generating highly reactive hydroxyl (•OH) and superoxide (•O²⁻) radicals [2]. These radicals subsequently attack and decompose organic pollutants through successive oxidation reactions. The efficiency of this process depends critically on three factors: light absorption capacity, charge carrier separation and transport, and surface catalytic activity.

Among various semiconductor photocatalysts, tin oxide (SnO₂) has attracted considerable attention due to its exceptional chemical stability, non-toxicity, earth abundance, and relatively low cost [3,4]. With a wide bandgap of approximately 3.6 eV, high electron mobility (100–200 cm²V⁻¹s⁻¹), and excellent resistance to photocorrosion, SnO₂ shares many advantageous properties with the widely studied titanium dioxide (TiO₂) [5]. However, despite these favorable characteristics, the practical application of SnO₂ in photocatalysis faces two significant challenges. First, its wide bandgap restricts light absorption primarily to the ultraviolet region, which constitutes only about 5% of the solar spectrum, severely limiting its efficiency under natural sunlight [4,6]. Second, the rapid recombination of photogenerated charge carriers diminishes quantum efficiency and overall photocatalytic performance [7].

Various strategies have been explored to enhance the photocatalytic performance of SnO₂. These include forming heterojunctions with other semiconductors, such as ZnO [8] and TiO₂ [9,10], which promote charge separation through interfacial electron transfer. While heterojunction formation has demonstrated improved photocatalytic activity, these composite systems often involve complex synthesis procedures and may introduce new interfaces that can act as recombination centers. Alternatively, doping with various elements offers a more direct approach to modify the electronic structure of SnO₂, enhancing both light absorption and charge carrier dynamics.

Doping introduces impurity levels within the bandgap or creates oxygen vacancies, effectively narrowing the bandgap and extending light absorption into the visible region [11,12]. Additionally, dopants can act as trap sites for either electrons or holes, facilitating charge separation and reducing recombination rates. Previous studies have investigated SnO₂ doping with various elements, including magnesium [13], niobium [14], potassium [15], and several transition metals, reporting enhanced photocatalytic activity. Among potential dopants, strontium (Sr) emerges as a particularly promising candidate due to its unique properties [7]. Strontium is a promising dopant for SnO₂ due to its high stability and larger ionic radius (Sr²⁺ = 1.12 Å) than Sn⁴⁺ (0.71 Å) except potassium (K⁺ = 1.33 Å). But K⁺ has a charge of +1, which may create a charge imbalance in the SnO₂ lattice, whereas Sr²⁺ can more effectively compensate for the charge imbalance by substituting a divalent ion (Sr²⁺) for a tetravalent Sn⁴⁺ in the SnO₂ lattice [7]. Because of the larger ionic radius of Sr²⁺ and the compensating charge imbalance in Sn⁴⁺, Sr is a promising candidate when compared to other doping materials for SnO₂.

Strontium, with its larger ionic radius compared to tin, introduces significant lattice distortion when incorporated into the SnO₂ crystal structure, creating oxygen vacancies to maintain charge neutrality [7]. These oxygen vacancies serve dual functions: They create mid-gap energy levels, effectively reducing the bandgap and enhancing visible light absorption, while also acting as trapping sites that facilitate charge separation. Additionally, the divalent nature of Sr²⁺ (compared to tetravalent Sn⁴⁺) creates a charge imbalance that enhances electronic conductivity, potentially improving charge transport properties. Despite these potential advantages, the application of Sr-doped SnO₂ nanostructures in photocatalysis remains relatively unexplored.

Beyond compositional modifications, the morphological engineering of photocatalysts represents another effective strategy to enhance performance. Nanostructured materials, with their high surface-to-volume ratios, provide abundant active sites for photocatalytic reactions while reducing bulk recombination through shortened charge diffusion distances. Among various nanostructures, one-dimensional (1D) nanofibers offer distinct advantages for photocatalytic applications [16]. Unlike nanoparticles, which tend to aggregate in aqueous media, nanofibers maintain their dispersibility while providing interconnected pathways for electron transport. Their high aspect ratio and nanoscale diameter result in significantly enhanced surface area and reduced average crystallite size compared to conventional nanoparticles [16].

The electrospinning technique represents a versatile and scalable approach for fabricating nanofibers with controlled composition and morphology. This method involves applying a high voltage to a polymer solution, creating a charged jet that undergoes elongation and solvent evaporation, resulting in continuous nanofibers [17]. Subsequent calcination removes the polymer matrix, leaving behind ceramic nanofibers with well-defined structural properties. Despite the advantages of nanofibers for photocatalysis, previous research on SnO₂-based systems has primarily focused on nanoparticle morphologies or composite nanofibers (e.g., TiO₂/SnO₂, ZnO/SnO₂), with limited exploration of pure SnO₂ nanofibers for photocatalytic applications.

The present study addresses these research gaps by investigating Sr-doped SnO₂ nanofibers for enhanced visible light photocatalysis. Through a systematic approach, we explore how Sr doping transforms the structural, optical, and electrochemical properties of SnO₂ nanofibers and correlate these properties with photocatalytic performance. The study introduces several novel elements: the preservation of pristine nanofibrous morphology through optimized electrospinning, enhanced visible light activity through strategic Sr doping, and comprehensive electrochemical characterization establishing direct links between charge carrier dynamics and photocatalytic efficiency.

Methylene blue (MB), a common industrial dye, serves as a model pollutant to evaluate the photocatalytic performance of our synthesized materials [18]. As a cationic thiazine dye widely used in textiles, paper, and pharmaceuticals, MB represents a class of recalcitrant organic pollutants that persist in industrial effluents [18,19]. Its well-defined optical properties, with a characteristic absorption maximum at 664 nm, facilitate quantitative analysis of degradation efficiency, making it an ideal probe molecule for assessing photocatalytic activity.

Through a comprehensive investigation of Sr-doped SnO₂ nanofibers with varying dopant concentrations (0, 1, 3, and 5%), this study demonstrates that 1% Sr-doped SnO₂ nanofibers achieve 84.74% MB degradation under visible light illumination within 180 min, significantly outperforming both undoped SnO₂ nanofibers (61%) and the same catalyst under UV light conditions (69%). This enhanced performance is attributed to the synergistic effects of modified electronic structure, improved charge transfer properties, and optimized nanofibrous morphology, as revealed through detailed electrochemical analyses. The findings provide valuable insights into the design principles for high-performance visible light photocatalysts, with potential applications extending beyond dye degradation to various environmental remediation processes.

2. Experimental Methods

2.1. Materials and Reagents

Tin (II) chloride dehydrate (SnCl₂·2H₂O), tin (IV) oxide (SnO₂) (purity > 99.99%, trace metal basis, average particle size of $\le$100 nm), polyvinylpyrrolidone (PVP), ethanol, N, N-dimethylformamide (DMF), strontium nitrate (Sr (NO₃)₂), methylene blue (MB) (0.05 wt.% in H₂O) were purchased from Sigma-Aldrich (Merk, Melbourne, Australia). All chemicals were used without further purification.

2.2. Fabrication of Strontium-Doped SnO₂ Nanofibers

SnO₂ nanofibers were prepared by electrospinning technology. Briefly, 0.56 gm of SnCl₂·2H₂O was added to 5.5 mL of DMF and 5.5 mL of ethanol. Then, 0.76 gm of PVP was mixed with the solution. For Sr doping, Sr (NO₃)₂ was added to the as-prepared SnO₂ precursor solution at concentrations of 0.1 g (1 wt.%), 0.3 g (3 wt.%), and 0.5 g (5 wt.%) relative to the total solution weight. These concentrations are referred to throughout the manuscript as 1% Sr-doped, 3% Sr-doped, and 5% Sr-doped SnO₂, respectively.

After that, this final solution was stirred at 60 °C for 2 h and then stirred overnight at room temperature. The electrospinning solution was placed in a plastic syringe with a conductive needle (18 G) at a feed rate of 0.5 mL/h. The applied voltage was 17 kV and the distance between the needle tip and the collector wrapped in aluminum foil paper was set to 18 cm. The as-spun nanofiber (NF) was calcined in an oven at 600 °C for 2 h and SnO₂ nanofibers were finally achieved after the removal of polymer and solvents.

2.3. Material Characterization

The samples were characterized by various characterization tools. The morphology of NFs was examined by scanning electron microscopy on a ThermoFisher Phenom™ XL G2 Desktop SEM. ImageJ software (v1.54g) was used to measure the average diameter of the fiber. The crystal structure and phase identification were investigated by X-ray diffraction (XRD) analysis using Cu Kα radiation with the wavelength of 1.54178 Å, in the range of 2θ = 20 − 90° on a Malvern Panalytical powder XRD machine.

2.4. Electrochemical Measurements

Electrochemical measurements were conducted using a Gamry Potentiostat IFC1010-27215 (Gamry Instruments, Warminster, PA, USA) in a three-electrode configuration with 1 M KOH, an alkaline medium as the electrolyte. Alkaline electrolytes, such as KOH, are well-established in supercapacitor research for their ability to enhance ion adsorption/desorption at the electrode surface, leading to improved charge storage. This is particularly relevant for pseudocapacitive materials like SnO₂, where charge storage is not solely based on double-layer formation [20]. The intercalation and deintercalation of alkaline electrolyte ions can significantly influence the pseudocapacitive behavior of SnO₂ nanofibers, potentially improving their performance. Among various alkaline electrolytes, KOH is widely favored in pseudo-capacitance applications of SnO₂ [21,22] due to its high ionic conductivity of K⁺ [23], which accelerates electrochemical kinetics with higher stability and conductivity in an electrochemical environment, dissociating K⁺ and OH⁻ completely in the aqueous solution.

An Ag/AgCl electrode served as the reference, glassy carbon as the working electrode, and platinum as the counter electrode. The working electrode had an active area of 0.071 cm². The binder solution was prepared as 5 wt.% of PVDF in 5 mL of DMF solution [24] and was stirred overnight at room temperature. The active material on the working electrode was prepared by dispersing 0.1 mg of SnO₂ nanofibers in 5 µL of PVDF solution. A paste was then prepared, and a thin layer was deposited on the working electrode’s active area. The electrode was subsequently dried in an oven at 50 °C for 15 min.

The electrochemical study includes cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), a Mott–Schottky (M-S) plot, linear sweep voltammetry (LSV), and chronoamperometry (CA). In our study, we will use CV to (1) determine the accumulated charge storage area and current density to discuss charge transfer kinetics and(2) observe redox potentials to understand the charge transfer process, We will also calculate specific capacitance following Equation (1) [25].

$$C+\int ivdv/2\mu m∆V$$

(1)

where C is capacitance, i is current, v is the potential of the CV curve, m is the mass of active material (0.01 mg), $∆V$ is the potential window (0.8 V), $\mu$ is the scan rate (50 mV/s), and $\int ivdv$ is the integral of current (i) with respect to voltage (v). This represents the area under the CV curve.

Electrochemical Impedance Spectroscopy yields Nyquist plots of real and imaginary impedance, enabling the derivation of an equivalent circuit model. Analysis of this model will allow us to determine key electrochemical parameters: (1) charge transfer resistance to evaluate charge transfer kinetics, (2) double layer capacitance indicative of the charge transfer surface area, (3) series resistance representing the ohmic resistance, (4) diffusion-induced Warburg impedance, and (5) recombination resistance to understand ion diffusion at the electrode–electrolyte interface.

A Mott–Schottky plot, derived from capacitance measurements as a function of applied potential, is employed to determine a semiconductor’s doping concentration and flat band potential. The slope of this plot is directly related to doping concentration, while the x-intercept reveals the flat band potential [26]. This technique, which involves measuring the impedance of an electrochemical interface under varying DC voltage and a fixed AC signal, utilizes the Mott–Schottky equation to analyze the space charge region at semiconductor–electrolyte interfaces. The equation is

C⁻² + (2/εε₀)(1/N_A + 1/N_D)(V − V_fb − kT/q)

(2)

where C is the capacitance of the space charge region; ε is the dielectric permittivity of the semiconductor; ε₀ is the permittivity of free space; N_A is the acceptor doping concentration; N_D is the donor doping concentration; V is the applied potential; V_fb is the flat band potential; k is the Boltzmann constant; T is the temperature; and q is the elementary charge.

From chronoamperometry, we can calculate the diffusion coefficient by fitting the current transient curve with the Cotrell equation. From the diffusion coefficient, we can investigate the movement speed (faster or slower) of charge carriers to identify more efficient charge transport materials. The Cottrell plot is calculated using Equation (3) [27].

$$i+nFAc\surd D{\left(\pi t\right)}^{{\displaystyle \frac{-1}{2}}}$$

(3)

Here, i represents the current, n is the total number of electrons (4) transferred in the reaction, F is Faraday’s constant (96,485 C/mol), c is the initial concentration (6.64 $\times$ 10⁻⁵) of electroactive material, D is the diffusion coefficient, A is the active area of the electrode (0.071 cm²), and t is time.

Linear Sweep Voltammetry is a potentiodynamic technique where the working electrode’s potential is linearly swept over time at a constant scan rate, while the resulting current is measured. This method is employed to technically investigate electrochemical behavior by determining redox potential, studying reaction mechanisms and kinetics, enabling quantitative analysis, assessing material properties like stability and electrocatalytic activity, and facilitating initial screening and characterization of electrochemical systems through the analysis of the resulting current-potential voltammograms. The Tafel plot is derived from LSV, where the potential values are referenced to the reversible hydrogen electrode (RHE) through an RHE calibration. Equation (4) [28] is used to convert potential from Ag/AgCl to RHE.

E_RHE + E_Ag/AgCl + 0.059pH + E⁰Ag_/AgCl

(4)

(E⁰Ag/AgCl = +0.197 V)

The overpotential (η) was calculated [29] at different current densities using Equation (5) as

η = E_RHE − 1.23

(5)

2.5. Photocatalytic Experiment

A 10 mg/L MB solution was prepared using deionized water. For photocatalytic degradation, 6 mg of SnO₂ nanofibers (NFs) were dispersed in 30 mL of the MB solution (0.2 g/L) under continuous stirring in a 100 mL glass beaker. After the addition of varying percentages of Sr-doped NFs to the MB solution, the beakers were wrapped in aluminum foil and stirred for 60 min in the dark medium to allow for adsorption–desorption equilibrium. Subsequently, the solutions were exposed to UV light (Analytik Jena, Jena, Germany 254/302/365 nm, 8 Watt) irradiation in a photoreactor with a dimension of 38.5 $\times$ 19 $\times$ 19 cm (W $\times$ H $\times$ L). Two different light sources, (1) a UV lamp with ${\lambda }_{max} +$ 365 nm and (2) a solar simulator lamp with 100 mW/cm² (1 sun) were used in the experiment. The height between the lamp and the glass beaker was maintained at 10 cm. At predetermined time intervals for 180 min, aliquots of the solution were withdrawn using a pipette and analyzed using UV-Vis spectrophotometry (GENESYS, ThermoFisher, Waltham, MA, USA) to monitor the degradation of the MB absorption peak. The samples were then returned to the beaker for continued irradiation. Finally, degradation efficiency was recorded using the initial concentration and final concentration at different time intervals from the corresponding absorption peaks.

3. Results and Analysis

3.1. Morphological Evaluation of SnO₂ NFs

The SEM micrographs in Figure 1 reveal the morphological evolution of SnO₂ nanofibers as a function of Sr doping concentration. Figure 1a reveals an agglomerated particulate structure characteristic of the SnO₂ nanoparticles (NP), which showcases distinctive morphology compared to the nanofiber (NF). The undoped SnO₂ nanofibers (Figure 1b) exhibit a dense network of interconnected fibers with relatively uniform morphology. Upon introducing 1% Sr (Figure 1c), the fibers maintain their continuous structure while showing a slight increase in diameter, which is quantitatively confirmed in

Table 1. The average diameter of Sr-doped SnO₂ nanofibers.

%Sr-Doped SnO2 NF	Average Diameter (nm)
Pristine	$259.04 \pm$25.57
1	$288.69 \pm$$17.24$
3	$209.49 \pm 15.29$
5	$148.72 \pm$5.98

. The 1% Sr-doped sample presents well-defined, smooth fibers with consistent morphology throughout the network. As the Sr content increases to 3% (Figure 1d), the fiber network remains dense but with visibly thinner fibers compared to the 1% sample. A further increase in Sr doping to 5% (Figure 1e) results in a more pronounced reduction in fiber diameter along with the emergence of structural voids and discontinuities within the network.

Table 1 quantifies the diameter variations observed in the SEM images. The undoped SnO₂ nanofibers display an average diameter of 259.04 ± 25.57 nm. With the incorporation of 1% Sr, the mean diameter increases to 288.69 ± 17.24 nm, representing an 11.4% expansion. This diameter increase can be attributed to the substitution of Sn⁴⁺ (0.71 Å) by the larger Sr²⁺ ions (1.12 Å), which induce lattice distortion. Notably, the standard deviation in diameter measurements decreases from ±25.57 nm to ±17.24 nm with 1% Sr doping, suggesting enhanced uniformity in fiber formation. As Sr doping increases to 3% and 5%, the average fiber diameter progressively decreases to 209.49 ± 15.29 nm and 148.72 ± 5.98 nm, respectively. This represents a 19.1% reduction at 3% and a substantial 42.6% reduction at 5% Sr doping compared to the undoped sample. The standard deviation at 5% Sr doping (±5.98 nm) is significantly lower than for other samples, indicating highly uniform but much thinner fibers. This non-monotonic trend in fiber diameter with increasing Sr content suggests complex structural reconfiguration mechanisms at higher doping concentrations. Smaller fiber diameters lead to a larger specific surface area [30]. Our SnO₂ nanofibers showed a nonlinear relationship with Sr doping in fiber diameter. At 1% doping, the fiber diameter slightly increased to 288 nm from the pristine 259 nm, suggesting a plausible minor reduction in specific surface area. However, with 3% and 5% Sr doping, the fiber diameter decreased, indicating a corresponding increase in specific surface area.

The observed morphological changes with Sr doping concentration are likely driven by multiple competing factors. At low doping levels (1 wt.%), the incorporation of Sr²⁺ ions primarily causes lattice expansion [31], resulting in larger fiber diameters. However, at higher doping concentrations (3 wt.% and 5 wt.%), additional mechanisms such as increased lattice strain, enhanced densification during calcination [32], and the possible formation of secondary phases becomes dominant, leading to fiber shrinkage. The structural discontinuities observed in the 5 wt.% Sr-doped sample suggest that excessive Sr incorporation may compromise the structural integrity of the nanofiber network, potentially affecting the material’s functional properties in catalytic applications.

The EDS (energy-dispersive X-ray spectroscopy) for 1% Sr-doped SnO₂ NF derived from SEM showcased successful incorporation of Sr²⁺ in Figure 2.

3.2. Structural Properties

The XRD patterns presented in Figure 3 provide critical insights into the crystalline structure of the synthesized Sr-doped SnO₂ nanofibers and reveal how Sr incorporation influences the structural properties of these materials. The diffraction pattern of the manufactured SnO₂ nanoparticles (Figure 3a) serves as a reference standard, displaying well-defined, sharp diffraction peaks at 2θ values of approximately 26.6° (110), 33.9° (101), 37.9° (200), 51.8° (211), 54.8° (220), 57.8° (310), 61.9° (112), 64.7° (301), 71.3° (202), and 78.7° (321). These peaks correspond to the tetragonal rutile structure of SnO₂ (JCPDS card file no. 41-1445) [33,34], confirming the phase purity of the reference material. The pristine SnO₂ nanofibers (Figure 3b) exhibit diffraction peaks at identical 2θ positions to the manufactured SnO₂, verifying the successful synthesis of pure SnO₂ nanofibers. However, the diffraction peaks of the pristine nanofibers appear broader and less intense compared to the manufactured SnO₂, indicating smaller crystallite sizes in the electrospun nanofibers. This observation is consistent with the nanofibrous morphology observed in the SEM images (Figure 1a), where the electrospinning process and subsequent calcination result in nanostructured materials with reduced crystallite dimensions.

Upon Sr doping, significant changes in the XRD patterns become evident. The 1% Sr-doped sample (Figure 3c) maintains the primary diffraction peaks of rutile SnO₂ but also exhibits additional peaks at approximately 21.58° (004), 23.53° (202), 30.39° (204), and 46.35° (332), suggesting the formation of a secondary phase [35], displayed in Figure 3f. These new peaks likely correspond to strontium–tin oxide compounds Sr₃Sn_5, with a tetragonal crystal structure matched from Highscore software from Malvern Panalytical (v 5.2), indicating successful incorporation of Sr into the SnO₂ matrix through the formation of Sr-Sn compositions. The diffraction peaks in the 1% Sr-doped sample appear broader than those of pristine SnO₂, indicating further reduction in crystallite size [36] with Sr incorporation.

As Sr doping increases to 3% (Figure 3d) and 5% (Figure 3e), there is a progressive reduction in peak intensity and an increase in peak broadening. This trend suggests a continuous decrease in crystallite size [36] with increasing Sr concentration, which complements the observed reduction in nanofiber diameter from 288.69 nm at 1% Sr to 148.72 nm at 5% Sr, as quantified in Table 1. The 5% Sr-doped sample (Figure 3e) shows significantly suppressed diffraction peaks for most crystallographic planes, with only a few dominant peaks remaining visible. This substantial peak suppression indicates severe lattice distortion [37] and possibly increased amorphization at high Sr doping levels.

The structural changes observed in the XRD patterns correlate well with the morphological evolution seen in the SEM images (Figure 1). The initial increase in fiber diameter with 1% Sr doping, followed by diameter reduction at higher doping levels, parallels the crystallographic changes observed in XRD. The introduction of Sr²⁺ ions, which are significantly larger than Sn⁴⁺ ions, creates lattice distortions and oxygen vacancies that fundamentally alter both the crystalline structure and fiber morphology. These structural modifications are expected to significantly influence the electronic properties and photocatalytic performance of the Sr-doped SnO₂ nanofibers, particularly through the creation of oxygen vacancies that can enhance visible light absorption and charge carrier separation efficiency. The quantitative crystallographic parameters presented in

Table 2. Crystallographic data derived from XRD of manufactured SnO₂, undoped, and doped SnO₂ NFs.

Sample Name	Average Crystallite Size (nm)	Dislocation Density (m−2)	Micro Strain (%)
Manufactured SnO2	47.23	1.9$\times {10}^{15}$	0.23
0%-Sr doped SnO2 NF	12.71	1.37$\times {10}^{16}$	0.77
1%-Sr doped SnO2 NF	11.71	1.78$\times {10}^{16}$	1.02
3%-Sr doped SnO2 NF	28.08	3.36$\times {10}^{15}$	0.41
5%-Sr doped SnO2 NF	7.14	3.53$\times {10}^{16}$	1.42

provide deeper insights into the structural modifications induced by Sr doping and corroborate the qualitative observations from the XRD patterns in Figure 3. Table 2 presents the crystal properties of undoped and doped SnO₂ NFs. Crystallite size was derived from the Debye–Scherrer equation from Equation (6) [38].

$$\mathrm{Crystallite} \mathrm{size}, D = {\displaystyle \frac{k\lambda }{\beta cos\theta }}$$

(6)

where k is the shape factor, typically around 0.9; $\lambda$ is X-ray wavelength, the wavelength of Cu kα is 0.154 nm; $\beta$ is the peak full width of half maximum intensity (FWHM) in radians, and $\theta$ is Bragg’s diffraction angle.

Dislocation density, $\delta$, is calculated using Equation (7) [39].

$$\mathrm{Dislocation} \mathrm{density}, \delta +{\displaystyle \frac{1}{D^2}}$$

(7)

where D is the average crystallite size. The microstrain is extracted from the full width at half maximum β of (2θ − ω) diffraction peaks from Equation (8) [40].

$$\mathrm{Microstrain}, \epsilon +{\displaystyle \frac{\beta }{4\mathit{tan}\theta }}$$

(8)

A comparison between manufactured SnO₂ nanoparticles and electrospun nanofibers reveals striking differences in their crystalline properties, with the electrospinning process significantly altering the microstructural characteristics. The manufactured SnO₂ nanoparticles exhibit a relatively large average crystallite size of 47.23 nm, corresponding to the sharp, well-defined diffraction peaks observed in Figure 3a. In contrast, the pristine SnO₂ nanofibers show a dramatically reduced crystallite size of 12.71 nm, consistent with the peak broadening observed in Figure 3b. This substantial reduction (approximately 73%) in crystallite size can be attributed to the rapid solidification during electrospinning and controlled crystallization during thermal treatment [41], which inhibits extensive crystal growth.

The influence of Sr doping on crystallite size follows a non-monotonic trend. With 1% Sr doping, the crystallite size slightly decreases to 11.71 nm, suggesting that low concentrations of Sr²⁺ ions impede crystal growth by creating lattice distortions [42]. Surprisingly, at 3% Sr doping, the crystallite size increases substantially to 28.08 nm, which deviates from the general trend observed in the XRD patterns. This anomalous increase might result from Sr-induced recrystallization processes or the formation of larger Sr-rich domains within the SnO₂ matrix [43]. However, at 5% Sr doping, the crystallite size decreases dramatically to 7.14 nm, representing a 44% reduction compared to the pristine nanofibers. This significant decrease aligns with the severe peak broadening and suppression observed in Figure 3e and correlates with the substantial reduction in nanofiber diameter (148.72 nm) noted in Table 1.

The dislocation density, which quantifies crystal lattice imperfections, shows an inverse relationship with crystallite size across most samples [44]. The commercial SnO₂ exhibits the lowest dislocation density (1.9 × 10¹⁵ m⁻²), consistent with its larger crystallites and higher crystallinity. The pristine SnO₂ nanofibers show a higher dislocation density (1.37 × 10¹⁶ m⁻²), which further increases to 1.78 × 10¹⁶ m⁻² with 1% Sr doping. At 3% Sr doping, the dislocation density decreases to 3.36 × 10¹⁵ m⁻², mirroring the unexpected increase in crystallite size. The 5% Sr-doped sample exhibits the highest dislocation density (3.53 × 10¹⁶ m⁻²), indicating severe lattice distortion and defect formation at high Sr concentrations [45].

Microstrain values, which reflect lattice distortion, follow a similar pattern [46]. The commercial SnO₂ shows minimal strain (0.23%), while pristine nanofibers exhibit higher strain (0.77%) due to their nanostructured nature. Sr doping progressively increases lattice strain, with values of 1.02% at 1% Sr doping and 1.42% at 5% Sr doping. The 3% Sr-doped sample again deviates from this trend with a lower strain value (0.41%), consistent with its larger crystallite size and lower dislocation density.

From

Table 3. Lattice parameters of undoped and doped SnO₂ derived from XRD peaks (110) and (101).

Sample Name	a = b (Å)	c (Å)	Cell Volume (Å3)
Commercial SnO2	4.72	3.16	70.4
Pristine SnO2 NF	4.70	3.21	71.1
1% Sr-doped SnO2 NF	4.82	3.21	74.76
3% Sr-doped SnO2 NF	4.75	3.19	72.06
5% Sr-doped SnO2 NF	4.72	3.01	67.06

, showing the calculated lattice parameters from XRD at (110) and (101) peaks, it is observed that the length of the edges (a, b, and c) of the unit cell is slightly changed, which indicates subtle lattice distortion upon Sr doping. Also, the ionic radius of Sr²⁺ (1.12 Å) is larger than Sn⁴⁺ (0.71 Å), which is reflected in the total cell volume, highest with 1% Sr doping (74.76 ų) compared to pristine SnO₂ nanofiber (71.1 ų) [47], as Sr²⁺ ions substitute Sn⁴⁺ ions, indicating Sr incorporation. This substitution leads to local strain and a tendency to increase the interatomic distances, resulting in a larger unit cell volume. Conversely, for 5% of Sr doping, the volume (67.06 Å) of the unit cell is significantly contracted from the pristine one. It can be assumed that, when maintaining charge neutrality when Sr²⁺ substitutes Sn⁴⁺, oxygen vacancies are likely created in the SnO₂ lattice. These oxygen vacancies can lead to a contraction of the lattice by reducing the electrostatic repulsion between the remaining oxide ions and allowing the Sn/Sr cations to move closer [48]. Additionally, from Table 2, it is evident that the lattice micro-strain of 5% Sr doping (1.42%) is significantly higher than pristine SnO₂ NF (0.77%), with the smallest average crystallite size (7.14 nm), which could contribute to the non-monotonic volume change [40,44]. The interplay between these factors—the initial lattice expansion due to Sr substitution and the subsequent lattice contraction due to increased strain and reduced crystallite size—likely results in the non-monotonic trend observed in the unit cell volume.

These crystallographic parameters complement the morphological observations from SEM analysis (Figure 1). The initial increase in fiber diameter with 1% Sr doping, followed by a decrease at higher doping levels, can now be understood in terms of competing microstructural processes. The introduction of Sr²⁺ ions creates oxygen vacancies and lattice distortions that influence both crystal growth and fiber formation during electrospinning and calcination. The anomalous behavior of the 3% Sr-doped sample suggests a critical concentration threshold where the microstructural evolution mechanism changes, potentially involving phase segregation or recrystallization processes.

3.3. Electrochemical Studies

3.3.1. Cyclic Voltammetry (CV)

The electrochemical characteristics of the Sr-doped SnO₂ nanofibers were investigated using cyclic voltammetry. Figure 4 presents the CV curves of pristine and Sr-doped SnO₂ nanofibers recorded at a scan rate of 50 mV/s, while

Table 4. Total charge and specific capacitance calculated from CV.

% Sr Doping of SnO2 NFs	Total Charge, Q (C)	Specific Capacitance (F/g)
Commercial SnO2 NP Pristine SnO2 NF	1.80 $\times 10$−5 4.35 $\times 10$−5	22.5 54.4
1% Sr-doped	8.45 $\times 10$−5	106
3% Sr-doped	6.80 $\times 10$−5	85
5% Sr-doped	3.76 $\times 10$−5	47.1

summarizes the quantitative electrochemical parameters derived from these measurements.

The CV profiles exhibit quasi-rectangular shapes with distinctive redox humps, characteristic of both electric double-layer capacitance and pseudocapacitive behavior [30,49]. This dual charge storage mechanism arises from the combined effects of ion adsorption at the electrode–electrolyte interface and surface redox reactions involving oxygen vacancies in the SnO₂ lattice [50]. The enclosed area within the CV loops, which represents the total charge storage capacity, varies significantly with Sr doping concentration, indicating a profound influence of Sr incorporation on the electrochemical performance. Pseudo-capacitance is a Faradaic energy storage mechanism arising from rapid, reversible redox reactions occurring at or near the electrode surface. This process involves electrosorption/electrodesorption coupled with charge transfer, but critically, it proceeds without any bulk phase transformation during charge/discharge cycles [51,52]. The pseudo-capacitance (C_ϕ) is defined by the change in charge (Q) with respect to the potential (V), specifically dQ/dV. Unlike battery processes involving bulk chemical changes, pseudo-capacitance in our system with SnO₂ nanofibers and a KOH electrolyte is attributed to fast surface redox reactions, potentially affecting the intercalation of alkali metal cations (specifically K⁺ from the KOH electrolyte) into the SnO₂ structure. This leads to a charge storage behavior where the electrode potential exhibits a linear dependence on the accumulated charge and is proportional to the surface coverage of electroactive ions, forming a reversible functionalized molecular layer. The possible reaction is given below [51]:

SnO₂ + K⁺ + e⁻ ↔ SnOOK

(9)

The potential of the electrode has a linear dependence on the charge [52]. From Figure S1, the linear relation between the scan rate of CV and peak current is explicitly observed for 1% Sr-doped SnO₂ NF. The relationship between peak current (i_p) and scan rate (v) can be described by the following power law equation [51].

i_p + av^b

(10)

log (i_p) + log (a) + blog (v)

where a is the constant and b is the slope value. The b value is close to 1 (0.7), indicating a faster surface-controlled redox reaction than a diffusion-induced reaction from the bulk of the material with dominant capacitive behavior.

The current response of commercial SnO₂ nanoparticles (NP) is almost identical to 5% Sr-doped SnO₂ NF. From Table 4, the commercial SnO₂ NP exhibits the lowest specific capacitance (22.5 F/g) and total accumulated charge (1.8 × 10⁻⁵ C) compared to SnO₂ NFs (doped or undoped), which explicitly supports the superiority of the nanofibrous morphology of SnO₂. The pristine SnO₂ nanofibers display a relatively modest current response with a total charge of 4.35 × 10⁻⁵ C and a specific capacitance of 54.4 F/g. The specific capacitance was calculated using Equation (1) described in Section 2.4. Upon 1% Sr doping, there is a remarkable enhancement in electrochemical activity [53,54], manifested by the substantially larger CV loop area. This is quantitatively confirmed in Table 4, which shows that the 1% Sr-doped sample exhibits a total charge of 8.45 × 10⁻⁵ C, representing a 94% increase compared to the pristine sample. Correspondingly, the specific capacitance nearly doubles to 106 F/g, indicating significantly improved charge storage capability. As Sr doping increases to 3%, the CV curve area moderately decreases, with the total charge reducing to 6.80 × 10⁻⁵ C and specific capacitance to 85 F/g. This trend continues with 5% Sr doping, where the electrochemical performance deteriorates further, with total charge (3.76 × 10⁻⁵ C) and specific capacitance (47.1 F/g) values falling below those of the pristine SnO₂ nanofibers. This non-monotonic dependence of electrochemical properties on Sr content parallels the trends observed in the morphological and crystallographic analyses.

The superior electrochemical performance of the 1% Sr-doped sample can be correlated with its unique structural and morphological characteristics previously identified. The slight increase in fiber diameter (288.69 nm) compared to pristine SnO₂ (259.04 nm), as shown in Table 1, combined with reduced crystallite size (11.71 nm vs. 12.71 nm) and increased dislocation density (1.78 × 10¹⁶ vs. 1.37 × 10¹⁶ m⁻²) from Table 2, creates an optimal balance of surface area and defect concentration. The increased microstrain (1.02% vs. 0.77%) and the formation of secondary phases observed in the XRD pattern (Figure 3c) suggest the presence of abundant oxygen vacancies at 1% Sr doping, which serve as active sites for charge transfer and storage.

Conversely, the deterioration in electrochemical performance at higher Sr doping levels (3% and 5%) can be attributed to the compromised structural integrity observed in both the SEM images (Figure 1d) and crystallographic data. The excessive lattice distortion and defect concentration at 5% Sr doping, evidenced by the highest microstrain (1.42%) and dislocation density (3.53 × 10¹⁶ m⁻²), likely impedes charge transport within the nanofiber network. Additionally, the significant reduction in fiber diameter (148.72 nm) and the emergence of structural discontinuities may reduce the effective electrode-electrolyte contact area and disrupt electron conduction pathways. These electrochemical findings establish a clear structure–property relationship, demonstrating that moderate Sr doping (1%) optimizes the balance between creating beneficial defects that enhance charge storage capacity and maintaining the structural integrity necessary for efficient charge transport.

3.3.2. Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy measurements were conducted to further elucidate the charge transfer dynamics and interfacial properties of the Sr-doped SnO₂ nanofibers. Figure 5 presents the Nyquist plots for the different samples, while

Table 5. Equivalent circuit parameters fitted with EIS plot for SnO₂ NFs with Sr doping.

Equivalent Circuit Parameters	Commercial SnO2 NP	Pristine SnO2 NF	1 wt.% Sr-Doped	3 wt.% Sr-Doped	5 wt.% Sr-Doped
Rs (Ohm-cm2)	3.8	3.73	2.87	1.73	3.58
CPEdl (F)	7.06$\times {10}^{-5}$	1.42$\times {10}^{-4}$	3.8$\times {10}^{-4}$	1.4$\times {10}^{-4}$	6.97$\times {10}^{-5}$
$\eta$dl	0.94	0.97	0.84	0.89	0.93
Rct (Ohm-cm2)	859	1350	404	1000	450
ZW (Ohm-cm2)	3$\times {10}^4$	1.06 $\times {10}^4$	3.84$\times {10}^3$	1.13$\times {10}^4$	5.71$\times {10}^3$
RGC (Ohm-cm2)	200	200	100	100	200
CPEGC (F)	7.5$\times {10}^{-4}$	7.33$\times {10}^{-4}$	3.5$\times {10}^{-4}$	6.4$\times {10}^{-4}$	2.5$\times {10}^{-3}$
$\eta$GC	0.72	0.68	0.8	0.86	0.45

provides the quantitative parameters derived from fitting these plots to an equivalent Randle circuit model [55].

The Nyquist plots exhibit linear regions in the low-frequency domain, characteristic of diffusion-controlled processes, and semicircular arcs in the high-frequency region, representing charge transfer resistance at the electrode–electrolyte interface. A notable trend is immediately apparent: the 1% Sr-doped sample displays the smallest semicircle diameter, indicating significantly enhanced charge transfer kinetics [56] compared to other samples. The fitted equivalent circuit parameters in Table 5 provide quantitative confirmation of these observations. Commercial SnO₂ NP exhibits the charge transfer resistance of 859 Ohm-cm², lower than pristine SnO₂ NF but higher than 1% (404 Ohm-cm²) and 3% (450 Ohm-cm²) Sr-doped SnO₂ NF. But the double-layer capacitance of SnO₂ NP (7.06$\times 10$⁵ F) is the lowest among all of the conditions, supporting the specific capacitance derived from V (Figure 4). The diffusion-induced impedance, the Warburg impedance (3$\times {10}^4$ ohm-cm²) is also higher than in other nanofibrous morphologies. These values indicate the limitations of nanoparticles compared to nanofibers of SnO₂ in light energy harvesting. The charge transfer resistance (R_ct) of the pristine SnO₂ nanofibers is remarkably high at 1350 Ω-cm², indicating substantial impedance to electron transfer. Upon 1% Sr doping, this resistance drastically decreases to 404 Ω-cm², representing a 70% reduction. This significant improvement in charge transfer kinetics aligns with the enhanced current response observed in the CV measurements (Figure 4) and the increased total charge and specific capacitance values (Table 4).

The Warburg impedance (Z_W), which characterizes ion diffusion resistance in the electrode material, follows a similar trend. The 1% Sr-doped sample exhibits the lowest Z_W value (3.84 × 10³ Ω-cm²), approximately 64% lower than the pristine SnO₂ (1.06 × 10⁴ Ω-cm²). This indicates a greatly enhanced ion diffusion capability, which is critical for efficient electrochemical processes, including photocatalysis. The double-layer capacitance (CPE_dl) increases substantially from 1.42 × 10⁻⁴ F for pristine SnO₂ to 3.8 × 10⁻⁴ F for the 1% Sr-doped sample, suggesting a higher electrochemically active surface area. The solution resistance (R_s) also shows improvement with moderate Sr doping, decreasing from 3.73 Ω-cm² for pristine SnO₂ to 2.87 Ω-cm² and 1.73 Ω-cm² for 1% and 3% Sr-doped samples, respectively.

At higher Sr doping levels, the electrochemical parameters show varying trends. The 3% Sr-doped sample maintains improved performance compared to pristine SnO₂ but does not match the optimal properties of the 1% Sr-doped sample. The 5% Sr-doped nanofibers exhibit complex behavior: while R_ct (450 Ω-cm²) and Z_W (5.71 × 10³ Ω-cm²) remain lower than the pristine sample, other parameters such as CPE_dl (6.97 × 10⁻⁵ F) deteriorate significantly, and R_s increases to 3.58 Ω-cm².

These EIS results provide deeper insight into the electrochemical behavior observed in the CV analysis (Figure 5 and Table 5). The substantially reduced charge transfer resistance and Warburg impedance of the 1% Sr-doped sample explain its superior charge storage capability [57]. The increased CPE_dl value further suggests enhanced surface reactivity [58], which is essential for photocatalytic applications. The trends in electrochemical parameters correlate remarkably well with the previously observed morphological and structural properties. The optimal performance of the 1% Sr-doped sample can be attributed to its unique combination of increased fiber diameter (288.69 nm, Table 1), moderately reduced crystallite size (11.71 nm, Table 2), and increased defect concentration (dislocation density 1.78 × 10¹⁶ m⁻², Table 2). The increased microstrain (1.02%) and the formation of secondary phases observed in XRD (Figure 3c) likely create abundant oxygen vacancies that serve as active sites for charge transfer and storage, explaining the dramatically reduced R_ct and Z_W values.

The deterioration in electrochemical performance at 5% Sr doping, despite having the smallest crystallite size (7.14 nm) and highest defect concentration, can be attributed to excessive lattice distortion [59] and structural discontinuities observed in SEM (Figure 1e) and XRD (Figure 3e). These structural deficiencies likely create barriers to electron transport within the nanofiber network, offsetting the potential benefits of increased surface area and defect sites. The analysis reveals that Sr doping (1%) creates an optimal balance between beneficial defect creation and structural integrity preservation, significantly enhancing charge transfer kinetics and ion diffusion.

3.3.3. Mott–Schottky (MS) Plot

To gain deeper insights into the semiconductor properties and electronic band structure of the Sr-doped SnO₂ nanofibers, Mott–Schottky (MS) analysis was performed. The Mott–Schottky (MS) analysis is a valuable technique for investigating the electronic characteristics of semiconductors. It enables the direct determination of flat band potential and charge carrier density through capacitance-voltage measurements [26]. Figure 6 presents the MS plots for different Sr doping concentrations, while

Table 6. Parameters obtained from the slope of MS graphs for Sr-doped SnO₂ nanofibers.

% of Sr Doping	Flat Band Potential, Vfb	Slope	$\mathbf{Charge} \mathbf{Carrier} \mathbf{Density}, {\mathit{N}}_{\mathit{D}} (\mathbf{c}\mathbf{m}$−3)
Commercial SnO2 NP	−0.48 V	$6.5 \times {10}^8$	1.2$\times {10}^{22}$
Pristine SnO2 NF	−0.45 V	8.4$\times {10}^{10}$	$9.24 \times {10}^{19}$
1% Sr doping	−0.53 V	$1.5 \times {10}^8$	$5.17 \times {10}^{22}$
3% Sr doping	−0.68 V	2.4$\times {10}^{10}$	$3.23 \times {10}^{20}$
5% Sr doping	−0.62 V	3.09$\times {10}^{11}$	$2.51 \times {10}^{19}$

summarizes the key parameters extracted from these measurements. From the slope of the MS plot, we can determine the charge carrier density. The parameters are extracted following Equation (11) [60].

$$N_{D }+{\displaystyle \frac{2}{{\epsilon }_0{\epsilon }_{r}e\times slope}}$$

(11)

where N_D represents charge carrier density, ${\epsilon }_0$ is the dielectric permittivity of air, and ${\epsilon }_r$ corresponds to relative permittivity (for SnO₂, 18 is considered). The MS plots display positive slopes for all samples, confirming the n-type semiconductor nature of the SnO₂ nanofibers, where electrons serve as the majority charge carriers. The flat band potential (V_fb), which approximates the position of the conduction band edge, shows a systematic shift toward more negative values with increasing Sr doping concentration, moving from −0.45 V for pristine SnO₂ to −0.53 V, −0.68 V, and −0.62 V for 1%, 3%, and 5% Sr-doped samples, respectively. This negative shift indicates a favorable energetic band alignment [61] for enhanced reduction reactions, which is beneficial for photocatalytic applications.

The most remarkable finding from the MS analysis is the extraordinary enhancement in charge carrier density (N_D) observed with Sr doping. Commercial SnO₂ NP exhibits a flat band potential of − 0.48 V, more negative than pristine SnO₂ NF (−0.45 V). The charge carrier density of SnO₂ NP ($1.20 \times {10}^{22} {\mathrm{c}\mathrm{m}}^{-3})$ (Table 6) is 130-fold larger than pristine SnO₂ NF ($9.24 \times {10}^{19} \mathrm{c}\mathrm{m}$⁻³). This discrepancy can be explained by the density of surface states (Figure 6) and some secondary phases from XRD peaks of SnO₂ NP (Figure 3). The density of surface states for SnO₂ NP is higher than pristine SnO₂ NF. Also, from XRD peaks, some secondary phases are formed, which might create any defect state or trap centers trapping charge carriers, which increases the charge carrier density.

The pristine SnO₂ nanofibers exhibit a carrier density of 9.24 × 10¹⁹ cm⁻³, which increases dramatically by approximately three orders of magnitude to 5.17 × 10²² cm⁻³ for the 1% Sr-doped sample. This represents an unprecedented 560-fold increase in charge carrier concentration, which explains the superior electrochemical performance [62] observed in CV (Figure 4, Table 4) and EIS (Figure 5, Table 5) measurements. The slope of the linear region in the MS plots, which is inversely proportional to carrier density, decreases substantially from 8.4 × 10¹⁰ for pristine SnO₂ to 1.5 × 10⁸ for 1% Sr-doped SnO₂, confirming the dramatic increase in carrier concentration. At higher Sr concentrations, the carrier density decreases progressively to 3.23 × 10²⁰ cm⁻³ for 3% Sr doping and further to 2.51 × 10¹⁹ cm⁻³ for 5% Sr doping, approaching the value of the pristine sample. This non-monotonic trend in carrier density correlates with the observed changes in charge transfer resistance (R_ct) in the EIS analysis (Table 5), where R_ct increases for 3% Sr doping before decreasing again at 5% Sr doping.

The electronic properties revealed by MS analysis provide a fundamental explanation for the superior electrochemical and photocatalytic performance of the 1% Sr-doped SnO₂ nanofibers. The exceptionally high carrier density, combined with the negative shift in flat band potential, creates optimal conditions for efficient charge separation and transfer during photocatalytic reactions. The increased carrier concentration can be attributed to the oxygen vacancies [7,37,63] generated by the substitution of Sn⁴⁺ with Sr²⁺ ions, which act as electron donors in the SnO₂ lattice. The density of surface states [64,65], indicated by the circled regions in the negative potential domain of Figure 6, also shows variations with Sr doping concentration, with the 1% Sr-doped sample exhibiting distinct surface states that may serve as catalytic sites. The decrease in carrier density at higher Sr concentrations (3% and 5%) may result from several competing mechanisms. Excessive Sr doping could lead to the formation of charge-compensating defects that trap carriers [66,67], clusters of Sr ions that create potential barriers for charge transport or increased structural disorder that reduces carrier mobility. These effects would counteract the beneficial impact of oxygen vacancies on carrier concentration.

3.3.4. Linear Sweep Voltammetry (LSV)

Linear sweep voltammetry and Tafel analysis were conducted to further evaluate the electrocatalytic activity and reaction kinetics of the Sr-doped SnO₂ nanofibers. The LSV curves in Figure 7a show a clear influence of Sr doping on the current response of the SnO₂ nanofibers. The 1% Sr-doped sample exhibits the highest current density across the entire potential range, followed by the 3% Sr-doped sample. Both the pristine and 5% Sr-doped samples display significantly lower current densities, with the 5% Sr-doped sample showing the poorest performance. This trend directly correlates with the electrochemical properties observed in CV (Figure 4) and EIS (Figure 5) measurements, reinforcing the superior electron transfer capabilities of the 1% Sr-doped nanofibers [68].

The Tafel plots in Figure 7b provide critical insights into the reaction kinetics and catalytic efficiency of the samples. The Tafel slope [69], which represents the voltage increase required to enhance the current density by one order of magnitude, is a key indicator of electrocatalytic activity, with lower values indicating more efficient catalysis. The Tafel plot is derived from LSV, where the potential values are referenced to the reversible hydrogen electrode (RHE) through an RHE calibration. Equation (4) [28] is used to convert potential from Ag/AgCl to RHE, and the overpotential (η) was calculated [29] at different current densities using Equation (5) as described in Section 2.4.

The overpotential values of 0, 1, 3, and 5% of Sr-doped SnO₂ NFs are −2.63, −2.43, −2.46, and −3.03 V at the current density of 10 mA/cm². 1% Linear regions of the Tafel plots were analyzed using the Tafel equation: η + b log (j) + a, where η represents overpotential, j denotes current density, and b is the Tafel slope. The extracted slope values are presented in

Table 7. Tafel slope derived from Tafel plot of Sr-doped SnO₂ NFs and commercial SnO₂.

Sample	Tafel Slope (V/dec)
Commercial SnO2 NP	2.02
Pristine SnO2 NF	0.84
1% Sr-doped	0.47
3% Sr-doped	0.62
5% Sr-doped	0.49

. The ascending order of the Tafel slope is 1% < 5% < 3% < 0%-Sr doped SnO₂ NF.

The current-voltage characteristics in Figure 7a demonstrate that the 1% Sr-doped SnO₂ nanofibers exhibit the highest current response at any given voltage, followed by 3% Sr-doped samples. This suggests enhanced electrical conductivity and charge transfer properties in the low-concentration Sr-doped samples. The 5% Sr-doped nanofibers, however, show the lowest current response, indicating that excessive Sr doping may introduce defects or phase segregation that impede charge transport.

The Tafel plots in Figure 7b and the derived Tafel slopes in Table 7 provide crucial insights into the reaction kinetics. The commercial SnO₂ NP exhibits the highest Tafel slope (2.02 V/dec), suggesting sluggish electron transfer kinetics. In contrast, all nanofiber samples demonstrate significantly lower Tafel slopes, with the 1% Sr-doped sample showing the lowest value (0.47 V/dec), followed by 5% Sr-doped (0.49 V/dec), 3% Sr-doped (0.62 V/dec), and pristine SnO₂ NF (0.84 V/dec).

The dramatic reduction in the Tafel slope from commercial NPs to nanofibers, particularly with Sr doping, indicates that the nanofiber morphology and Sr incorporation facilitate faster electron transfer kinetics. A lower Tafel slope corresponds to a higher exchange current density and thus more favorable reaction kinetics [70,71].

As Sr doping increases to 3%, the Tafel slopes increase slightly to 0.62 V/dec and for 5%, reduce to 0.49 V/dec, respectively. These values remain lower than the pristine sample but are higher than the optimal 1% Sr-doped nanofibers. This trend suggests that excessive Sr doping, while still beneficial compared to undoped SnO₂, does not maintain the optimal catalytic efficiency achieved at 1% doping concentration.

The electrocatalytic performance observed in the LSV and Tafel analyses can be directly linked to the electronic and structural properties previously characterized. The exceptionally high charge carrier density of the 1% Sr-doped sample (5.17 × 10²² cm⁻³, Table 6) provides abundant mobile electrons for catalytic reactions, contributing to its superior current response and lower Tafel slope. The reduced charge transfer resistance (404 Ω-cm², Table 5) further facilitates efficient electron transfer at the electrode–electrolyte interface. The slightly higher Tafel slopes for 3% and 5% Sr-doped samples correlate with their lower carrier densities (3.23 × 10²⁰ and 2.51 × 10¹⁹ cm⁻³, respectively) and higher charge transfer resistances compared to the 1% Sr-doped sample. The anomalous behavior of the 3% Sr-doped sample, particularly its increased crystallite size (28.08 nm, Table 2) and charge transfer resistance (1000 Ω-cm², Table 5), aligns with its intermediate performance in the LSV and Tafel analyses. The poor current response of the 5% Sr-doped sample, despite having a relatively low Tafel slope, suggests that while the intrinsic catalytic activity per active site may be reasonable, the overall performance is limited by other factors such as reduced electrochemically active surface area (lower CPE_dl values, Table 5) and structural discontinuities observed in SEM (Figure 1d).

3.3.5. Chronoamperometry (CA)

To further investigate the charge transport dynamics and diffusion properties of the Sr-doped SnO₂ nanofibers, chronoamperometry (CA) measurements were conducted. Figure 8a displays the current response over time following a potential step, while Figure 8b presents the corresponding Cottrell plots.

Table 8. Diffusion coefficient calculated from Cottrell equation for undoped and doped SnO₂ nanofibers.

% of Sr Doping	Slope i/(t)0.5	D, Diffusion Coefficient (cm2s−1)
Commercial SnO2 NP	6.17$\times {10}^{-6}$	3.62$\times {10}^{-11}$
Pristine SnO2 NF	1.09$\times {10}^{-5}$	1.13$\times {10}^{-10}$
1% Sr-doped	3.04$\times {10}^{-5}$	8.78$\times {10}^{-10}$
3% Sr-doped	1.39$\times {10}^{-5}$	1.84$\times {10}^{-10}$
5% Sr-doped	1.04$\times {10}^{-5}$	1.03$\times {10}^{-10}$

summarizes the diffusion coefficients calculated using the Cottrell equation.

The CA curves in Figure 8a reveal significant differences in the initial current surge and subsequent decay behavior among the samples. The 1% Sr-doped nanofibers exhibit a remarkably higher initial current density (approximately 3.2 mA/cm²) compared to other samples, indicating superior charge storage capacity and electrode kinetics. This initial current is approximately three times higher than that of the pristine SnO₂ nanofibers (approximately 1.1 mA/cm²) and 45 times higher than commercial SnO₂ NP (approximately 0.07 mA/cm²); it also significantly higher than both 3% and 5% Sr-doped samples, which show initial currents of approximately 1.2 mA/cm² and 0.6 mA/cm², respectively. The enhanced initial current response of the 1% Sr-doped sample aligns with its superior capacitive properties observed in CV measurements (Figure 3, Table 3).

The Cottrell plots in Figure 8b, which display current versus t⁻¹/², provide quantitative insights into the diffusion behavior [72] of the electroactive species within the nanofiber electrodes. The linear relationship between current and t⁻¹/² confirms that the electrochemical processes are primarily diffusion-controlled. The slope of the Cottrell plot is directly proportional to the diffusion coefficient, with steeper slopes indicating more rapid diffusion of electroactive species. The Cottrell plot is calculated using Equation (3) [27] discussed in Section 2.4.

The diffusion coefficient is a critical parameter for photocatalytic applications, as it directly influences the ability of photogenerated charge carriers to reach reactive sites on the catalyst surface before recombination [73,74]. A higher diffusion coefficient in the context of photocatalytic activity means the reactants (e.g., pollutants) can move quickly through the solution to the catalyst surface. The reaction rate will be higher because of the faster diffusion rate and improved mass transfer. Due to the reduced diffusion length between the catalyst and bulk pollutant, the limited mass transfer can be largely reduced, which may accelerate photocatalytic degradation [75].

As shown in Table 8, the 1% Sr-doped sample exhibits the steepest slope (3.04 × 10⁻⁵), approximately 2.8 times higher than that of pristine SnO₂ (1.09 × 10⁻⁵). This translates to a significantly higher diffusion coefficient (D) of 8.78 × 10⁻¹⁰ cm²s⁻¹ for the 1% Sr-doped sample compared to 1.13 × 10⁻¹⁰ cm²s⁻¹ for pristine SnO₂, representing an almost 8-fold enhancement and 24-fold higher than commercial SnO₂ NP (3.62 × 10⁻¹¹ cm²s⁻¹). This substantial improvement in diffusion properties is a critical factor contributing to the superior electrochemical and photocatalytic performance of the 1% Sr-doped nanofibers. Interestingly, the 3% Sr-doped sample shows a moderate enhancement in diffusion coefficient (1.84 × 10⁻¹⁰ cm²s⁻¹), approximately 63% higher than the pristine sample but significantly lower than the 1% Sr-doped sample. The 5% Sr-doped sample exhibits a diffusion coefficient (1.03 × 10⁻¹⁰ cm²s⁻¹) slightly lower than the pristine sample, indicating compromised diffusion properties at high Sr concentrations.

The diffusion behavior observed in the CA measurements correlates well with the electrochemical impedance results (Figure 5, Table 5), particularly the Warburg impedance (Z_W) values, which characterize diffusion resistance. The 1% Sr-doped sample showed the lowest Z_W (3.84 × 10³ Ω-cm²), consistent with its highest diffusion coefficient. Similarly, the higher Z_W values for other samples align with their lower diffusion coefficients. The significantly enhanced diffusion properties of the 1% Sr-doped nanofibers suggest that photogenerated electrons and holes can more efficiently migrate to the surface to participate in redox reactions, explaining the superior photocatalytic performance observed for this sample.

3.4. Optical Properties

The UV-visible absorption technique was employed to analyze the optical characteristics of undoped and 1% Sr-doped SnO₂ nanofibers (NFs) within the 300–800 nm range (Figure 9). The absorption spectra, influenced by factors such as oxygen vacancies, morphology, band gap, and impurity centers, exhibited a cut-off wavelength between 320 and 360 nm, indicative of electron photo-excitation from the valence to the conduction band [7]. Notably, the absorption peak displayed a subtle red shift (shift towards higher wavelengths) with increased Sr doping, resulting in a reduction in the estimated energy band gap, as determined from the Tauc plot [76]. Using the Tauc plot, the band gap values were calculated to be 3.77 eV for pristine and 3.28 eV for 1% Sr-doped SnO₂ NFs. This band gap reduction is attributed to a shift in the conduction band minimum to lower energies [77] and the introduction of additional energy states within the SnO₂ band gap due to oxygen vacancies formed to maintain charge neutrality upon Sr²⁺ incorporation into the SnO₂ lattice.

3.5. Photocatalytic Degradation

3.5.1. UV Light as a Light Source

Following the comprehensive characterization of morphological, structural, and electrochemical properties, the functional performance of the Sr-doped SnO₂ nanofibers was evaluated through photocatalytic degradation of methylene blue (MB) dye under UV light irradiation. Figure 10 presents the time-dependent absorption spectra of MB in the presence of different doping levels of SnO₂ NF photocatalysts, while

Table 9. UV light-driven photocatalytic degradation efficiency of MB with SnO₂ NF photocatalyst.

Sample	% Degradation
Commercial SnO2 NP	16
0% Sr-doped SnO2 NF	61
1% Sr-doped SnO2 NF	69
3% Sr-doped SnO2 NF	58
5% Sr-doped SnO2 NF	63

summarizes the degradation efficiencies achieved after 180 min of UV irradiation.

The absorption spectra in Figure 10 display the characteristic peaks of MB with maximum absorption at approximately 664 nm. The progressive decrease in absorption intensity with increasing irradiation time indicates the gradual degradation of the dye molecules. The rate and extent of this degradation vary significantly among the different catalysts, revealing their relative photocatalytic activities. As shown in Table 9, the commercial SnO₂ nanoparticles exhibit poor photocatalytic performance, achieving only 16% MB degradation after 180 min of UV irradiation. In contrast, the pristine SnO₂ nanofibers demonstrate substantially higher activity with 61% degradation efficiency. This nearly four-fold enhancement highlights the inherent advantages of nanofiber morphology, which provides higher surface area, enhanced light harvesting, and improved charge transport properties compared to conventional nanoparticles. The 1% Sr-doped SnO₂ nanofibers exhibit the highest photocatalytic activity among all samples, achieving 69% MB degradation after 180 min. This represents a 13% improvement over pristine SnO₂ nanofibers and reinforces the significant beneficial impact of moderate Sr doping on catalytic performance. The enhanced photocatalytic activity of the 1% Sr-doped sample aligns perfectly with its superior electrochemical properties observed in previous analyses, particularly its exceptionally high-charge carrier density (5.17 × 10²² cm⁻³, Table 6), low-charge transfer resistance (404 Ω-cm², Table 5), and enhanced diffusion coefficient (8.78 × 10⁻¹⁰ cm²s⁻¹, Table 8).

Interestingly, the photocatalytic activity exhibits a non-monotonic relationship with Sr doping concentration. The 3% Sr-doped nanofibers show reduced performance (58% degradation) compared to both 1% Sr-doped and pristine samples, while the 5% Sr-doped nanofibers demonstrate intermediate activity (63% degradation). This complex trend mirrors the variations observed in the electrochemical properties, particularly the charge transfer resistance (Table 5) and carrier density (Table 6), which also showed non-monotonic dependence on Sr content. The degradation profiles visible in Figure 10 reveal additional insights into the photocatalytic mechanism. All samples show more rapid degradation during the initial 60 min, followed by slower decay in the later stages. The 1% Sr-doped sample (Figure 10c) exhibits particularly effective degradation in the early phase, consistent with its enhanced charge transfer kinetics and lower Tafel slope (0.47 V/dec, Table 7).

Figure 11 provides a comprehensive view of the photocatalytic degradation kinetics of methylene blue (MB) using various SnO₂-based photocatalysts. Relative concentration profiles (C_t/C₀) in Figure 11a and the corresponding degradation efficiency curves in Figure 11b offer valuable insights into both the rate and extent of the photocatalytic process over the 180 min irradiation period. As illustrated in Figure 11a, the commercial SnO₂ nanoparticles demonstrate minimal photocatalytic activity, with the C_t/C₀ value decreasing only slightly from 1.0 to approximately 0.84 after 180 min, corresponding to a mere 16% degradation efficiency. This poor performance can be attributed to the large crystallite size (47.23 nm, from Table 2) and limited surface area of commercial nanoparticles, resulting in fewer active sites for catalytic reactions and inefficient charge separation. In contrast, all the electrospun SnO₂ nanofibers exhibit significantly enhanced photocatalytic performance. The pristine SnO₂ nanofibers show a steady decrease in relative concentration to approximately 0.39 after 180 min, achieving 61% degradation efficiency. This substantial improvement over commercial nanoparticles highlights the inherent advantages of the nanofiber morphology, including higher surface-to-volume ratio, enhanced light harvesting, and improved charge transport along the one-dimensional fiber structure.

The 1% Sr-doped SnO₂ nanofibers display the most remarkable photocatalytic activity, with the relative concentration decreasing more rapidly and reaching the lowest final value of approximately 0.31 (Figure 11a), corresponding to 69% degradation efficiency after 180 min. Notably, the degradation curve for the 1% Sr-doped sample shows a distinct kinetic profile compared to other samples, with a rapid initial decrease in the first 45 min, followed by a steady decline throughout the experimental duration. This suggests that Sr doping not only enhances the overall catalytic efficiency but also accelerates the initial reaction rate. The degradation kinetics of the 3% and 5% Sr-doped samples exhibit intermediate performance between the pristine and 1% Sr-doped nanofibers. The 3% Sr-doped sample achieves approximately 58% degradation, while the 5% Sr-doped sample reaches approximately 63% degradation after 180 min. Interestingly, both samples show similar degradation profiles to the 1% Sr-doped sample during the first 60 min but diverge in the later stages, indicating that while all Sr-doped samples benefit from enhanced initial reaction rates, only the 1% Sr-doped sample maintains superior performance throughout the entire process.

The degradation efficiency curves in Figure 11b reveal distinct phases in the photocatalytic process. A minimal degradation phase is observed during the first 30 min, likely representing the surface adsorption equilibrium period [1,18]. This is followed by a rapid degradation phase between 30 and 60 min, where significant differences between the catalysts become apparent. The 1% Sr-doped sample shows the steepest increase in degradation efficiency during this phase, reaching approximately 35% at 60 min compared to approximately 25% for other nanofiber samples. In the final phase (60–180 min), all samples show continued degradation but at varying rates, with the 1% Sr-doped sample maintaining the highest degradation rate.

From 40 to 120 min, a plateau is observed where very subtle degradation of MB occurs. But from 120 min to 180 min, a sharp MB degradation is evident. The plateau observed within 40 to 120 min can be described by the Langmuir adsorption isotherm model [78,79]. This model can be mathematically expressed as

$$q+{\displaystyle \frac{QC}{C+{k_{eq}}^{-1}}}$$

(12)

where q is the amount of adsorbate (MB dye) adsorbed per unit of area, Q is the amount of adsorbate adsorbed at surface saturation for a given concentration (C), and k_eq is the Langmuir equilibrium constant.

At the beginning of the experiment, the SnO₂ surface has many vacant adsorption sites. Methylene blue molecules from the solution rapidly adsorb onto these sites, as predicted by the Langmuir isotherm model. The photocatalytic degradation proceeds at a reasonable rate because the dye molecules are in close contact with the photocatalyst surface, where the reactive species (electrons and holes) are generated. As the reaction proceeds and more dye molecules are adsorbed and potentially degraded, the concentration of methylene blue in the solution decreases [80]. According to the Langmuir isotherm [78], as the concentration (C) decreases, the fraction of surface coverage also decreases as q decreases, but not linearly. Initially, a decrease in C leads to a significant decrease in surface coverage. By 40 min, the SnO₂ surface may be approaching saturation with methylene blue molecules covering the active sites of the catalytic surface [81,82]. Even though the photocatalyst is still active under light irradiation, there are fewer available adsorption sites for new dye molecules to bind and be degraded. The rate of degradation becomes limited by the availability of adsorbed dye molecules on the surface. The subsequent sharp degradation from 120 to 180 min might be due to the desorption or further degradation of MB, freeing up active sites for fresh dye molecules to adsorb and undergo photocatalysis.

A pseudo-first-order kinetic model is fitted following the equation of the simplified Langmuir–Hinshelwood model [7].

ln (C₀/C_t) + kt

(13)

where C_t and C₀ are the concentration of MB after and before the illumination time, respectively, and k is the rate constant. For each experiment, the plots of ln (C₀/C_t) vs. irradiation time (t) for MB are presented in Figure 12, respectively, and their calculated rate constant values are mentioned in

Table 10. Reaction rate and R² value derived from the pseudo-first-order model for undoped and doped SnO₂ NFs.

Sample	Reaction Rate, k (min−1), from 40 to 120 min	R2
Pristine SnO2 NF	6.32 × 10−4	0.83
1% Sr-doped	0.00125	0.87
3% Sr-doped	8.76 × 10−4	0.98
5% Sr-doped	5.65 × 10−4	0.99

. From Table 10, it is clear that the 1% Sr-doped SnO₂ has the highest degradation rate (0.00125 min⁻¹) constant with correlation constant, an R² value of 0.87, compared to the undoped (6.32 × 10⁻⁴ min⁻¹, R² = 0.83) and 3% (8.76 × 10⁻⁴ min⁻¹, R² = 0.98) and 5% Sr-doped SnO₂ (5.65 × 10⁻⁴ min⁻¹, R² = 0.99). These findings suggest that the degradation process best fits a pseudo-first-order kinetics model. Even in a plateau, 1% Sr-doping exhibits a better reaction rate than other conditions. The high correlation constant, R², shown in Table 10, showing MB dye degradation approaching unity, indicates that the catalytic degradation process is dominant on the surface [81] of the SnO₂ photocatalyst.

3.5.2. Visible Light as a Light Source

While the UV light photocatalytic experiments demonstrated the significant benefits of the nanofiber morphology and Sr doping, the performance under visible light (solar simulator) provides even more compelling evidence for the transformative impact of strategic Sr doping on SnO₂ photocatalysts. Figure 13 presents the time-dependent absorption spectra of methylene blue (MB) degradation under simulated solar irradiation, while

Table 11. Visible light-driven photocatalytic degradation efficiency of MB with SnO₂ NF.

Sample Name	Commercial SnO2	0% Sr-Doped SnO2 NF	1% Sr-Doped SnO2 NF
% degradation	70%	61%	84.74%

summarizes the degradation efficiencies achieved after 180 min.

The most striking observation from Table 11 is the remarkable enhancement in photocatalytic activity for the 1% Sr-doped SnO₂ nanofibers under visible light, achieving an impressive 84.74% MB degradation after 180 min. This represents a dramatic improvement compared to its performance under UV light (69%, Table 10), suggesting that Sr doping fundamentally alters the light absorption properties of SnO₂, extending its photocatalytic activity into the visible region of the spectrum. This finding is particularly significant considering that pristine SnO₂ NF is known to have a wide bandgap (>3 eV) that limits its activation to UV light [83]. Interestingly, the commercial SnO₂ nanoparticles also show enhanced performance under visible light (70% degradation) compared to UV light (16% degradation), though the improvement mechanism likely differs from that of the Sr-doped nanofibers. The pristine SnO₂ nanofibers maintain similar performance under both UV and visible light (61% degradation), suggesting minimal bandgap modification in the absence of dopants.

The absorption spectra in Figure 13 reveal distinct degradation kinetics for each photocatalyst. The commercial SnO₂ (Figure 13a) shows a significant decrease in absorption intensity between 0 and 60 min, with slower degradation thereafter. The pristine SnO₂ nanofibers (Figure 13b) exhibit more uniform degradation throughout the 180 min. Most notably, the 1% Sr-doped sample (Figure 13c) demonstrates exceptionally rapid degradation within the first 15 min, followed by continued progressive degradation throughout the experiment.

The observed visible light photocatalytic activity of commercial SnO₂ NP surpasses what is typically expected, despite its high bulk purity. This enhanced performance can be attributed to multiple factors. Surface hydroxyl groups and inherent oxygen vacancies may create defect levels within the band gap, enabling some lower-energy photon absorption [84]. The key takeaway from the XRD analysis (Figure 2) reveals the presence of secondary crystalline phases (112), (024) alongside primary peaks of SnO₂. These additional phases, potentially possessing narrower band gaps and the presence of secondary crystalline phases in the commercial SnO₂, as indicated by XRD, likely form heterojunctions with SnO₂. These interfaces can create a more favorable energetic landscape for charge separation and transfer upon visible light excitation of the secondary phases, ultimately leading to the observed enhanced photocatalytic performance under visible light [85]. Electrochemical analysis from the M-S plot (Figure 6) also revealed that the density of surface states of commercial SnO₂ is slightly higher than the pristine nanofibrous morphology of SnO₂, indicating trapping sites or mid-gap states [86] potentially introducing energy levels within the energy band gap, and narrowing the energy band gap.

3.5.3. Photocatalytic Mechanism

SnO₂ acts as an efficient photosensitizer because of its ability to absorb UV light (λ < 380 nm) due to its wider energy bandgap (3.2–3.6 eV) [87,88]. Previously, many research groups established SnO₂ as a photosensitizer for successful photocatalytic degradation [7,63,87,89,90]. The established effectiveness of tin dioxide (SnO₂) as a photosensitizer for photocatalytic degradation stems from its combination of high optical transparency, infrared reflectivity, and robust chemical and thermal stability [91]. As a prominent photocatalyst, SnO₂ shares a rutile-type crystal structure with TiO₂ [88]. In its unit cell, each Sn (IV) ion is octahedrally coordinated by six oxygen ions, while each oxygen is bonded to three Sn (IV) ions, forming a (6, 3) network [88,92]. This octahedral arrangement is considered crucial for high photocatalytic activity [93] as it enhances the mobility of photogenerated electron-hole pairs, increasing their likelihood of participating in surface reactions. Furthermore, the inherent defects within the SnO₂ structure contribute to its electrical conductivity by narrowing the energy band gap [94], thus improving its semiconducting properties. The introduction of additional oxygen anions [95] facilitates the oxidation of Sn²⁺ to Sn⁴⁺, maintaining charge neutrality within the material. Figure 14 illustrates a schematic diagram of the oxidation process taking place on the SnO₂ crystal surface. Figure 15 explicitly describes the reaction steps of Sr-doped SnO₂ NF as a photocatalyst on MB dye degradation.

The enhanced photocatalytic performance of Sr-doped SnO₂ can be attributed to several factors. Firstly, Sr incorporation can effectively suppress charge carrier recombination by trapping electrons from the SnO₂ conduction band. Secondly, the lattice mismatch between Sr²⁺ (1.12 Å) and Sn⁴⁺ (0.71 Å) ions promotes the formation of oxygen vacancies, which act as charge separation centers. Finally, the reduced crystallite size of Sr-doped SnO₂ results in a larger surface area, facilitating enhanced adsorption of organic pollutants and consequently improving photocatalytic activity [96].

Upon UV light irradiation, photogenerated electron-hole pairs are generated within the SnO₂ lattice. In undoped SnO₂, rapid electron-hole pair recombination limits their participation in photocatalytic reactions. Sr doping effectively mitigates this recombination by trapping electrons, enabling their efficient separation. Furthermore, Sr incorporation induces the formation of oxygen vacancies, which act as charge separation centers. Concurrently, photogenerated holes in the valence band of SnO₂ oxidize water molecules to form hydroxyl radicals, while trapped electrons react with oxygen to generate superoxide radicals, which subsequently undergo multi-electron reduction pathways to form additional hydroxyl radicals [90,97]. Oxygen vacancies are easily formed in SnO₂ compared to Sn vacancies due to their lower formation energy [98]. These oxygen vacancies can exhibit a dual role: trapping charge carriers in the valence band or transferring trapped electrons to the conduction band under UV irradiation [63]. The generated hydroxyl or superoxide radicals effectively oxidize organic pollutants, ultimately converting them into non-toxic substances such as CO₂, O_2, or H₂O [99]. The step-by-step reactions associated with this degradation mechanism of MB dye with Sr-doped SnO₂ are provided below in Figure 15.

4. Conclusions

This comprehensive study demonstrates that strategic strontium doping of SnO₂ nanofibers via electrospinning technology creates highly effective photocatalysts for environmental remediation applications. The investigation reveals that precise control of dopant concentration is crucial for optimizing photocatalytic performance, with 1% Sr-doped SnO₂ nanofibers exhibiting exceptional degradation efficiency for methylene blue under both UV (69%) and visible light (84.74%) conditions. The superior photocatalytic activity of the 1% Sr-doped sample stems from synergistic effects across multiple scales. At the morphological level, Sr doping at 1% concentration maintains an ideal nanofiber network with optimal fiber diameter (288.69 nm), providing efficient one-dimensional electron transport pathways while preserving structural integrity. Crystallographically, moderate Sr doping reduces crystallite size (11.71 nm) and increases microstrain (1.02%) and dislocation density (1.78 × 10¹⁶ m⁻²), creating abundant oxygen vacancies that serve as electron traps to enhance charge separation. From UV absorbance spectra, the energy bandgap of 1% Sr-doped SnO₂ is reduced to 3.28 eV from 3.77 eV of the undoped one, reflecting successful incorporation of Sr.

The most remarkable impact of Sr doping manifests in the electronic properties, where 1% Sr incorporation produces an extraordinary three-order-of-magnitude increase in charge carrier density (5.17 × 10²² cm⁻³) compared to pristine SnO₂ nanofibers (9.24 × 10¹⁹ cm⁻³). This dramatic enhancement in carrier concentration, coupled with significantly reduced charge transfer resistance (404 Ω vs. 1350 Ω for undoped) and an approximately eight-fold increase in the diffusion coefficient (8.78 × 10⁻¹⁰ vs. 1.13 × 10⁻¹⁰ cm²s⁻¹), creates an ideal electronic environment for efficient photocatalytic reactions. The most significant finding emerges from the visible light experiments, where 1% Sr-doped SnO₂ nanofibers achieve remarkable 84.74% methylene blue degradation, representing a 39% improvement over pristine SnO₂ nanofibers. This substantial enhancement under visible light demonstrates that strategic Sr doping effectively transforms SnO₂ from a UV-dependent to a visible light-driven photocatalyst, greatly expanding its practical utility for solar-powered environmental applications.

Beyond photocatalysis, the significantly improved electrochemical properties of the Sr-doped SnO₂ nanofibers suggest their potential application in other fields such as sensing, energy storage, and photoelectrochemical water splitting. The substantial capacitance values (106 F/g for 1% Sr-doped vs. 54.4 F/g for pristine) indicate promising capabilities for supercapacitor applications. The established structure–property–function relationships provide a framework for further optimization of metal-doped semiconductor nanofibers for sustainable environmental and energy applications.