Tailoring β-Bi₂O₃ Nanoparticles via Mg Doping for Superior Photocatalytic Activity and Hydrogen Evolution

Abstract

Bismuth oxide (β-Bi₂O₃) is a promising visible-light-driven photocatalyst due to its narrow direct bandgap, but its practical application is hindered by rapid electron–hole recombination and limited surface active sites. This study demonstrates a sol-gel synthesis approach to tailor β-Bi₂O₃ nanoparticles through magnesium (Mg) doping, achieving remarkable enhancements in the photocatalytic degradation of organic pollutants and hydrogen evolution. The structural analysis through XRD, SEM, and EDX confirmed Mg-doping concentrations of 0.025 to 0.1 M led to crystallite size reduction from 79 nm to 13 nm, while the UV–Vis bandgap measurement showed it decreased from 3.8 eV to 3.08–3.3 eV. The photodegradation efficiency increased through Mg doping at a 0.1 M concentration, with the highest rate constant value of 0.0217 min⁻¹. The doping process led to VB potential reduction between 3.37 V (pristine) and 2.78–2.91 V across the doped samples when referenced to SCE. The photocatalytic performance of Mg_0.075Bi_1.925O₃ improved with its 3.2 V VB potential because the photoelectric band arrangement enhanced both light absorption and charge separation. The combination of modifications through Mg doping yielded an enhanced photocatalytic performance, which proves that magnesium doping is a pivotal approach to modifying β-Bi₂O₃ suitable for environmentally and energy-related applications.

1. Introduction

Extensive research into advanced wastewater treatment approaches has grown due to two parallel factors: rising global water cleanliness needs, together with environmental pollution concerns [1,2]. Semiconductor-based photocatalysis represents a leading wastewater degradation technology because it achieves efficient organic pollutant elimination through sustainable solar-powered solutions [3,4,5]. Bismuth oxide (Bi₂O₃) stands as a popular semiconductor material that scientists have widely examined because of its appropriate bandgap energy, durable photostability, and strong visible light sensitivity [6]. The practical usage of pure Bi₂O₃ faces two main challenges since it recycles electron–hole pairs at high speed and allows minimal sunlight absorption [7,8]. Different researchers have examined the photocatalytic performance of Bi₂O₃ by implementing three main strategies that included doping with other elements, along with oxygen vacancy development and nanostructure design [9,10,11].

The process of incorporating metal ions functions as a successful technique for altering both the electronic characteristics and optical properties of semiconductor materials [12,13]. The alkaline earth metal magnesium represents a viable dopant option since it introduces both defect states and enables specific crystal phase stabilization [14,15]. The introduction of Mg into the material structure creates oxygen vacancies that act as primary facilitators for increased charge-carrier separation and improved photocatalytic performance [16,17]. When Mg is incorporated into the Bi₂O₃ lattice structure, it affects the bandgap properties, which enables more efficient visible light absorption, ideal for solar-powered applications [7].

β-Bi₂O₃ demonstrates excellent potential for photocatalysis because of its narrow bandgap (~2.3–2.8 eV) and strong absorption of visible light [18,19]. The β-phase has a short-lived existence since it transforms to alternate phases when heat exposure increases, thus restricting its potential practical applications [20,21]. Bismuth oxide (Bi₂O₃) functions as an effective photocatalyst under visible light irradiation, owing to its direct bandgap of 2.8 eV [7]. Research indicates that its photocatalytic efficiency can be significantly enhanced through metal ion doping [22,23]. For instance, Hameed et al. [24] demonstrated improved degradation of methyl orange, methylene blue, and phenol using Zn-modified Bi₂O₃. Similarly, Anandan et al. [25] explored the photocatalytic behavior of Au–Bi₂O₃ nanorods. Additionally, Xie et al. [26] reported that Bi₂O₃ doped with transition metals such as vanadate (VV), lead (PbII), silver (AgI), and cobalt (CoII) exhibited superior photocatalytic activity compared to undoped Bi₂O₃ under visible light irradiation [7]. Mg doping stabilizes the β-phase structure of Bi₂O₃ by making it suitable for photocatalytic utilization. Mg doping leads to oxygen-vacancy formation, which creates new locations for pollutant interaction as well as enhancing general photocatalytic capabilities [27,28].

Methylene blue (MB) serves as a widespread organic dye used in textile and printing operations, which substantially pollutes water [29,30]. The durability of methylene blue in water bodies leads to significant environmental dangers, together with health risks for humans [31]. The removal methods for methylene blue utilizing adsorption and biological treatment effectively fail to provide suitable results while creating additional pollutants [32]. Photocatalysis serves as a sustainable treatment method because it transforms organic dyes into harmless substances such as CO₂ and H₂O through light-powered reactions [29,33]. The development of efficient, cost-effective sunlight-responsive photocatalysts exists as an essential obstacle.

This work describes how new Mg-doped β-Bi₂O₃ nanoparticles with oxygen vacancies were synthesized through a simple sol-gel combustion approach. Systematic investigation of the Mg incorporation in β-Bi₂O₃ lattices used different analytical tools, including X-ray diffraction and scanning electron microscopy. Herein, we studied the bandgap modifications together with the optical properties of nanoparticles through UV–Vis absorption spectroscopy. An evaluation of Mg-doped β-Bi₂O₃ nanoparticles took place through the use of MB solutions under sunlight illumination for photocatalytic testing. The laboratory results show that β-Bi₂O₃’s photocatalytic behavior improves because Mg-doping brings together three key effects, which boost charge-carrier separation and oxygen vacancy formation and extend the light absorption range. This study presents novel insights into using alkaline earth metals for improving photocatalytic material performance while demonstrating Mg-doped β-Bi₂O₃ nanoparticles as an effective, sustainable photocatalyst for environmental cleanup. Furthermore, this work aims to create advanced enhanced photocatalytic materials which combine Bi₂O₃ methods with Mg-doping benefits to address real applications in solar energy use and wastewater treatment.

2. Results and Discussion

2.1. Structural Study of Bi₂O₃ and Mg-Doped Bi₂O₃

The XRD patterns of Bi₂O₃ doped with magnesium exist at various molar ratios as shown in Figure 1. The prepared samples demonstrate high crystallinity because of their distinct reflectivity peak pattern. The number of diffraction peaks decreased as the magnesium content in the solution grew. The XRD patterns from both undoped and doped samples matched the tetragonal β-phase Bi₂O₃ structure, which belongs to the space group P-421c (JCPDS Card No. 27-0050), while displaying lattice parameters of a = 7.7425 Å and c = 5.6313 Å. Analysis through X-ray diffraction confirmed complete phase purity for the synthesized samples, since the Mg ion peaks as well as other impurity peaks were absent. The tetragonal Bi₂O₃ crystal structure showed its main characteristic peaks at angular positions which matched with the reflections of the diffraction planes (−1 1 1), (0 2 0), (−1 0 2), (0 0 2), (−1 1 2), (−1 2 1), (−2 0 2), (−2 1 2), (−1 1 3), (0 4 1), (−1 0 4), (−2 4 1), and (−2 2 4). The results of this study match those from past studies that discuss how Bi₂O₃ allotropes form differently based on synthesis temperature ranges [7,34]. The different radii of the ions between bismuth and magnesium indicate that magnesium inserts itself partly into places where bismuth ions exist in the Bi₂O₃ crystal framework. XRD analysis demonstrates that very little Mg substitution of Bi occurs, although the crystal compositions show minimal differences.

The average crystallite size and microstrain ε were calculated using the W.H. equation (Equation (1)) based on the most intense XRD peak [35].

$$\beta \cos \theta ={\displaystyle \frac{k\lambda }{D}}+4\epsilon sin\theta$$

(1)

Here, k = 0.9 represents the numerical shape factor, a constant value; D denotes the crystallite size; λ is the wavelength of the incident radiation; β is the full width at half maximum (FWHM) in radians; and θ is the Bragg angle measured in radians.

After knowing the crystallite size, the dislocation density δ of the material was estimated as given in Equation (2).

$$\mathsf{\delta }={\displaystyle \frac{1}{D^2}}$$

(2)

Table 1. The calculated crystalline size, microstrain, dislocation density, and energy gap via change in concentrations of Mg-doped Bi₂O₃ nanoparticles.

Samples	2θ, °	D-Crystalline Size (nm)	Microstrain (ε)	Dislocation Density × 10−4 (nm−2)	Bandgap Energy (eV)
Bi2O3	27.943 29.076 31.393 32.772 32.772 45.852 47.015 53.599 55.408 57.453 74.290	79.7	74.23	1.8145	3.8
Mg0.025Bi1.975O3	27.943 29.076 31.393 32.772 32.772 45.852 47.015 53.599 55.408 57.453 74.290	18.1	36.741	7.407	3.08
Mg0.05Bi1.95O3	27.943 29.076 31.393 32.772 32.772 45.852 47.015 53.599 55.408 57.453 74.290	43.9	35.307	8.0217	3.13
Mg0.075Bi1.925O3	27.943 29.076 31.393 32.772 32.772 45.852 47.015 53.599 55.408 57.453 74.290	27.2	45.096	4.91714	3.3
Mg0.1Bi1.9O3	27.943 29.076 31.393 32.772 32.772 45.852 47.015 53.599 55.408 57.453 74.290	12.3	29.542	0.00115	3.14

indicates the materials crystallite size, the lattice strain, and the dislocation densities. The introduction of magnesium and different crystallite sizes explained the variable lattice constants and dislocation densities. Table 1 shows that the crystallite size and microstrain reduced with the increasing incorporation of Mg in Bi₂O₃ until 5% of Mg doping; however, the increase up to 7.5% Mg doping led to an increase in the size and strain, as demonstrated in Figure 2. Moreover, the increase up to 10% of Mg doping allowed a decrease in size to around 30 nm may be due to lattice distortion and strain relaxation caused by the smaller ionic radius of Mg²⁺; the inhibition of crystal growth due to the introduction of defects and boundaries; the formation of oxygen vacancies and defects that promote smaller crystallite formation; the stabilization of the β-phase leading to a more uniform and less strained crystal structure; and the enhanced nucleation rate, favoring the formation of smaller crystallites. These combined effects result in a reduction in both crystallite size and microstrain as the Mg-doping concentration increases.

Figure 3a–c exhibits the FESEM images of how Mg-doped Bi₂O₃ particles become smaller in size while their morphology changes with increasing Mg material concentration. The transformation of crystal growth and nucleation properties and the development of lattice defects follow the effects of Mg doping in Bi₂O₃. The higher Mg concentrations enable small particle dimensions as well as superior surface properties which benefit photocatalyst systems that require large surface areas and many defects. The FSEM-EDX mapping technique of the Bi_1.9Mg_0.1O₃ material demonstrates the distribution of Bi, Mg, and O with smaller particles showing rough surface characteristics due to the presence of Mg atoms, as shown in Figure 4a–d. The uniform distribution of Mg between the Bi₂O₃ lattice crystals verifies the successful incorporation of Mg while no impurity phases indicate the synthesized material remains pure. The observations line up with the anticipated Mg doping outcomes that cause Bi_1.9Mg_0.1O₃ to become a material suitable for photocatalyst applications through its smaller crystallites, increased defects, and better surface properties.

2.2. UV–Visible Spectroscopy

Catalyst material responses toward UV or visible light illumination and their light absorption properties, which activate catalyst particles, need thorough examination. The maximum absorbance measurement of Mg-doped Bi₂O₃ nanoparticles used UV–Vis spectroscopy through exposure to wavelengths from 200 to 800 nm. Figure 5a shows that pure Bi₂O₃ absorbs visible light with a maximum absorption edge at 300 nm, but Mg-doped Bi₂O₃ nanoparticles absorb light at a longer wavelength of 425 nm. When Mg is used to modify Bi₂O₃, the material shows two distinct peaks located at 290 and 350 nm. Although pure Bi₂O₃ shows the same level of absorption as Mg-doped Bi₂O₃, the dopant material demonstrates stronger absorption across the visible light spectrum, suggesting its potential for use in photocatalytic systems. The improved light absorption capability of this material enhances visible light usage for the generation of additional charge carriers from photon absorption. Mg incorporation and the production of oxygen vacancies in the bandgap enhance Mg-doped Bi₂O₃ nanoparticles by broadening their wavelength absorption capabilities and raising their absorption intensity. Oxygen vacancies in Mg-doped Bi₂O₃ introduce shallow-trap levels inside the bandgap that trap excited electrons according to Singh [36].

Determining the optical bandgap energy (Eg) is essential to understanding which portion of the solar spectrum can be absorbed by a semiconductor. The optical bandgap (Eg) was calculated by analyzing the fundamental absorption associated with the excitation of electrons from the valence band to the conduction band. The bandgap (Eg) can be derived from a plot of (αhν)² versus photon energy (hν) using the well-known Tauc relation [37]:

$${\left(\alpha h\nu \right)}^2=A \left(h\nu -E_g\right)$$

(3)

where α is the absorption coefficient, A is a constant, Eg is the energy gap, and ν is the light frequency, respectively.

The energy gaps were determined by drawing (αhν)² versus hν plots, followed by extrapolation of the linear parts to the x-axis. The series of bandgaps extending from pristine Bi₂O₃ to magnesium-doped Bi₂O₃ (0.025 to 0.1 M) provided various values of 3.8 eV, 3.08 eV, 3.13 eV, 3.3 eV, and 3.14 eV. The results showed a decrease in the energy gap size of magnesium-doped samples when compared to pure Bi₂O₃.

During these experiments, the bandgap values generated corresponded to the findings documented by Wang et al. [38]. Umebayashi et al. [39] demonstrated that dopant introduction creates occupied electronic states within the valence band or bandgap, along with a dependency on alkaline earth metal oxidation states and the composition of their cationic sublattices. This research added Mg²⁺ ions into the material structure, which generated impurity states in the forbidden zone that modified both donor energy levels above the valence band and acceptor energy levels below the conduction band. Bi₂O₃ experienced a decrease in its bandgap to 3.08 eV because of this occurrence. The photocatalytic properties of Bi₂O₃ become stronger after magnesium doping because the resulting bandgap reduction improves its visible light sensitivity.

2.3. Photocatalytic Activity

The spectrum demonstrates that MB exhibits a powerful initial absorption at 665 nm. Undoped Bi₂O₃ exhibits a slow peak intensity reduction over time because of its restricted photoactivity. Powdered Bi₂O₃ exhibits a slow reaction speed because its wide bandgap prevents it from efficiently absorbing visible light and creating electron–hole pairs. Pure Bi₂O₃ demonstrates weak photocatalytic activity since MB decomposition reaches a small extent after 120 min of exposure to radiation, as shown in Figure 6a. The MB absorption peak starting at 665 nm shows similar strength to that of the undoped material. The photocatalytic activity increases in the sample because the peak reduction happens faster than in pure Bi₂O₃. The performance enhancement comes from Mg introducing defects that shrink the bandgap and enable better visible light absorption and charge-carrier separation. Low Mg doping leads to a major reduction in MB over 120 min, thus demonstrating its positive effects. The UV–Vis absorption spectra confirm that dye-degradation efficiency increases with rising Mg concentration to reach its highest point, except for the 0.75 Mg doping, as shown in Figure 6c. The photocatalytic improvements come from narrow bandgap structures and improved visible light absorption ability while producing efficient charge-carrier separation. Tests have established the value of Mg-doped Bi₂O₃ as a highly potent photocatalytic agent under visible light conditions for environmental cleansing applications, as demonstrated in Figure 6d.

The efficacy of both undoped and Mg-doped Bi₂O₃ photocatalysts increases proportionally with the amount of Mg-doping element; the best performance belongs to the sample with the highest Mg content. The time-dependent MB concentration changes provided an assessment of the efficiency, as shown in Figure 7. When Mg is incorporated into Bi₂O₃, it forms new electronic states that enable additional interband sites. The photocatalyst surface maintains effective charge-carrier separation because electrons become trapped inside sites that create new electronic states along the bandgap. The presence of oxygen vacancies in Mg-doped Bi₂O₃ impedes electron (e⁻) and hole (h⁺) recombination because they enhance the separation of photogenerated charge carriers. The photocatalytic efficiency of 0.5% Mg-doped Bi₂O₃ proves to be lower than other samples which indicates excessive electron–hole recombination takes place. Greater amounts of Mg in the doping process enable enhanced photoproduction of electrons and holes which boosts photocatalytic reaction efficiency.

2.4. Kinetics Study for the Degradation of MB

The photodegradation of dyes follows pseudo-first-order kinetics and is expressed in the following equation:

$$Ln\left({\displaystyle \frac{C_o}{C_t}}\right)=K_1·t$$

(4)

C_o and C_t stand for the initial and remaining concentrations of MB at the particular illumination time before determining K₁, which represents the first-order rate constant. Research data on dye degradation underwent linear least squares analysis to obtain the primary parameters k and R². The experimental data fitted precisely against the model through the presented method, which produced accurate parameter values from the analysis in Figure 8. The model shows excellent effectiveness for describing how the dye reacts to light exposure through its strong correlation with experimental data. Figure 8 shows that Mg/Bi₂O₃ NPs displayed maximum photocatalytic activity by producing the highest K value compared to single Bi₂O₃ and other doping samples. The rate constants k exist as listed values in

Table 2. Parameters related to pseudo-first-order kinetic models for the photodegradation of methylene blue using Bi_2−xMg_xO₃ with varying dopant concentrations (x = 0, 0.025, 0.05, 0.075, and 0.1) as the photocatalyst.

Samples	Removal Efficiency, %	k	R2
Bi2O3	73	0.0105	0.96
Mg0.025Bi1.975O3	91	0.02	0.979
Mg0.05Bi1.95O3	82	0.0146	0.995
Mg0.075Bi1.925O3	92	0.0182	0.98
Mg0.1Bi1.9O3	93	0.0217	0.979

. The rate constants determined through pseudo-first-order analysis corresponded with experimental photodegradation output at x = 0, 0.025, 0.05, 0.075, and 0.1, resulting in values of 0.0105, 0.02, 0.0146, 0.0182, and 0.0217, respectively. The photocatalytic efficiency reaches its peak when Mg doping achieves its optimal concentration level according to the increasing rate constant data. The fitting accuracy of experimental data by the pseudo-first-order model appears through its R² coefficient of determination. The pseudo-first-order model prediction strength indicated by R² was higher in (ln(C_o/C_t) vs. t) than the corresponding value from the pseudo-second-order model (1/Ct vs. t). The data variances explainable by the model reflect its strong representative power for degradation behavior through the high R² value.

The research demonstrates that Bi₂O₃ nanoparticles with added Mg perform outstandingly when used to break down methylene blue and other organic pollutants through photocatalytic processes. The material demonstrates exceptional properties because it possesses a narrow bandgap as well as improved charge-carrier separation and structural robustness, which allows its use in prolonged wastewater applications. Congruent research indicates that Mg doping establishes oxygen vacancies for charge preservation, which form small energy levels between the valence band and conduction band states. Photogenerated electrons follow a new excitation path by moving from the VB to oxygen vacancy states while leaving behind holes in the VB. Additionally, the position of the CB in Mg-Bi₂O₃ facilitates the formation of superoxide radicals (O²⁻) from molecular oxygen in the solution. The photoexcited holes from the catalyst function as highly oxidizing agents, which form hydroxyl radicals (OH) alongside oxygen near the surface. Reactive radicals originating from the catalyst break organic methylene blue molecules in their chemical bonds to produce fragmentary products.

The evaluation of different catalyst efficiencies for separating charge carriers used photocurrent measurements. The chronoamperometric test of photoelectrochemical experiments yielded results that are shown in Figure 9a. The serial on-off changes operated for five seconds revealed a direct and stable current transformation in each switch. The sample Mg_0.075Bi_1.925O₃ showed a remarkably lower photocurrent than each of its composition counterparts. Another parameter measured from the photoelectrochemical experiments was photocurrent density represented by 7, 9, 13, 40, and 12 μA/cm² for Bi₂O₃, Mg_0.025Bi_1.975O₃, Mg_0.05Bi_1.95O₃, and Mg_0.075Bi_1.925O₃, respectively. Photocurrent density analysis showed that Mg_0.025Bi_1.975O₃ achieved the most extensive photocurrent value of 40 μA/cm², surpassing every other sample tested.

The findings confirm that heterostructure formation, the Bi₂O₃ electron sink nature, and Mg incorporation enhance charge-carrier separation efficiency as well as electron-transfer rates. Mott–Schottky measurements confirmed the band structure of Bi₂O₃ along with its Mg-doped variants Mg_0.025Bi_1.975O₃, Mg_0.05Bi_1.95O₃, Mg_0.075Bi_1.925O₃, and Mg_0.1Bi_1.9O₃, thus validating the previous results. The researchers determined freely mobile charge-carrier concentrations that were equivalent to normal-condition donor densities, after considering Mg-doping effects on phase transitions. Measurement procedures took place under dark conditions at a 500 Hz measurement frequency. Figure 9b demonstrates that pure Bi₂O₃ functions as an n-type semiconductor, given its positive slope in the Mott–Schottky analysis. EIS spectra of Bi₂O₃ and fabricated Mg_0.025Bi_1.975O₃, Mg_0.05Bi_1.95O₃, Mg_0.075Bi_1.925O₃, and Mg_0.1Bi_1.9O₃ samples are presented in Figure 9c. Posterior to Bi₂O₃, the radius of the EIS arc diminishes sequentially through the following materials: Mg_0.075Bi_1.925O₃, then Mg_0.05Bi_1.95O₃, then Mg_0.1Bi_1.9O₃, culminating in Mg_0.025Bi_1.975O₃. The semicircle diameter in the Nyquist plot acts as an inverse indicator for material charge transport efficiency. The smaller proportion of the EIS arc demonstrates useful characteristics, including better photogenerated charge separation and accelerated interfacial charge routes, and slower electron–hole (e⁻/h⁺) pair recombination. The optimal electrical conductivity emerges from the Mg_0.075Bi_1.925O₃ compound as revealed in Figure 9c through its minimal charge-transfer resistance values. The optimized Mg doping creates conditions that lead to the reduced carrier recombination of photogenerated carriers. Among the tested samples, Mg_0.075Bi_1.925O₃ stands out as the most effective photocatalyst because it shows superior performance in morphological analysis, together with structural and optoelectronic analysis. The authors selected Mg_0.075Bi_1.925O₃ for detailed examination because this composition demonstrated the best performance.

The efficiency of photocatalysts used for hydrogen production depends heavily on the value of their hydrogen evolution reaction potential (HER). A photocatalytic system for H₂ generation shows better prospects when the overpotential remains low. A linear sweep voltammetry analysis illustrates that Mg_0.075Bi_1.925O₃ possesses the most favorable hydrogen evolution overpotential of −1.34 V vs. SCE among Bi₂O₃, Mg_0.025Bi_1.975O₃, Mg_0.05Bi_1.95O₃, Mg_0.075Bi_1.925O₃, and Mg_0.1Bi_1.9O₃. Mg_0.075Bi_1.925O₃ demonstrates the most suitable conditions for photocatalytic H₂ production because of its optimal HER overpotential value of −1.34 V vs. SCE when compared to virgin Bi₂O₃ with a greater overpotential of −1.87 V vs. SCE. The obtained bandgap structures for all samples stemmed from Tauc’s plot analysis and are displayed as conduction band (CB) values in Figure 10. The valence band (VB) potential readings for the samples Bi₂O₃, Mg_0.025Bi_1.975O₃, Mg_0.05Bi_1.95O₃, Mg_0.075Bi_1.925O₃, and Mg_0.1Bi_1.9O₃ were 3.37 V, 2.8 V, 2.78 V, 3.2 V, and 2.91 V (vs. SCE), respectively, with CB values. The optimized band arrangement of Mg_0.075Bi_1.925O₃ helps to enhance the efficiency of charge separation and light trapping which leads to a better performance in breaking down organic pollutants through photocatalysis.

3. Materials and Methods

3.1. Synthesis of Mg-Doped Bismuth Oxide NPs

Bi₂O₃ nanoparticles were prepared by the solution combustion method using urea as fuel. A cylindrical crucible took the stoichiometric quantities of a mixture containing Bi(NO₃)₃·5H₂O (99.999% Loba Co., Mumbai, India) and fuel urea [CH₄N₂O] (99.999% Loba Co. India). The mixture in the crucible was stirred well to obtain homogeneity for 10 min at 400 rpm, and a clear, transparent solution was formed. This crucible was placed in a muffle furnace and heated to 600 °C for two hours. The resulting product was cooled to room temperature overnight and collected. A yellow powder was obtained, crushed using a pestle and mortar, and characterized. To prepare the samples for doping with different concentrations of Mg, the same procedure was followed for undoped Bi₂O₃ as for doping with varying ratios of molar Mg(NO₃)₃·9H₂O (99.999% Loba Co., India). The molar ratios of Mg/Bi were 0%, 0.25%, 0.5%, 0.75%, and 1%, respectively. Finally, a pale yellow powder with an increase in Mg²⁺ content was easily ground into a powder.

3.2. Characterization

The crystalline structure and morphology of a series of Bi_2-xMg_xO₃ (x = 0, 0.025, 0.05, 0.075, and 0.1) samples were considered by employing XRD, SEM, EDX, and X-ray diffraction (XRD) analysis was performed using a LANScientific instrument (Suzhou, China) with a wavelength (λ = 0.15418 nm) with diffraction peaks recorded between 10° and 80° (2θ), and a step of 0.06 degree/s was exploited to investigate the crystal structure. The morphology, particle size, and elemental concentration were investigated using a scanning electron microscope (SEM, a JEOL JSM 5910, Tokyo, Japan) with an accelerating voltage of 20 kV. Energy-dispersive X-ray (EDX) spectroscopy (JSM-5910, Tokyo, Japan) was used to study the manufactured material’s compositional features. The bandgap energy was measured with a JASCO 670 UV–visible spectrophotometer (Tsukuba-shi, Japan). The acquired materials were examined for their photocatalytic efficacy using organic dye pollutants, specifically methylene blue (MB). The electrochemistry measurements were carried out by Corretest Cs 305 (Wuhan, China). To perform the photocatalysis experiment, 20 milligrams of photocatalyst material was combined with a 500-watt xenon lamp.

3.3. Photocatalytic Performance of Bi_2−xMg_xO₃

The photocatalytic assessment of synthesized Bi₂O₃ particles occurred through the investigation of their ability to break down methylene blue (MB) in water. The UV–Vis spectrophotometer (JASCO 670) determined that the MB absorbance peak existed within the visible spectrum range from 400 to 800 nm while analyzing the absorbance spectrum. The reaction monitoring required the usage of 665 nm as the wavelength where the maximum absorbance was detected. The experiments used visible light illumination, which included a Pyrex-glass-made immersion well photochemical reactor as the light source. A cylindrical reactor contained the light source and had a water-cooling jacket for experimental temperature management. Each test used 0.02 g of photocatalyst, either from Bi₂O₃ or Mg-doped Bi₂O₃, with 50 mL of MB solution at a 10 ppm concentration, which was stirred at 600 rpm. The solution underwent 30 min sonication under dark conditions before irradiation for the achievement of adsorption–desorption equilibrium. The chemical degradation of the dye solution was measured through absorbance detection at 665 nm. The photocatalysis process indicated its dye-removal efficiency through the implementation of this specific calculation [40,41]:

$$Removal \%={\displaystyle \frac{\left(C_o-C_t\right)}{C_o}}\times 100$$

(5)

After resolving the complex process of photocatalytic degradation, the initial (C₀) and final (C_t) concentrations of the pollutant are accurately measured. The catalyst is separated via centrifugation and then re-dispersed in the dye solution for further recycling evaluations.

4. Conclusions

The sol-gel synthesized Mg-doped β-Bi₂O₃ nanoparticles brought about significant photocatalytic enhancements because of their optimized structural properties along with their adjusted electronic features. The addition of Mg doping at concentrations between 0.025 and 0.1 M resulted in crystallite sizes decreasing from 79 nm (pristine Bi₂O₃) down to 13 nm, together with a decrease in bandgap from 3.8 eV to 3.08–3.3 eV. The modified bandgap and optimized valence band potential levels between 2.78 and 2.91 V vs. SCE improved the separation of charges and absorption of sunlight. The sample with Mg_0.075Bi_1.925O₃ composition displayed the optimal band alignment structure that enabled excellent light-trapping abilities and superior charge-transfer capabilities which resulted in a high organic pollutant photodegradation rate constant of 0.0217 min⁻¹ and superior hydrogen evolution performance. Research indicates that Mg doping represents an essential method to design β-Bi₂O₃ materials for the continued development of sustainable environmental cleanup systems and renewable energy solutions. The researchers should study scalability and long-term stability for practical implementation in future investigations.