Toward efficient dye degradation and the bactericidal behavior of Mo-doped La₂O₃ nanostructures

Abstract

In this study, different concentrations (0, 0.02, 0.04, and 0.06 wt%) of Mo doped onto La₂O₃ nanostructures were synthesized using a one-pot co-precipitation process. The aim was to study the ability of Mo-doped La₂O₃ samples to degrade toxic methylene blue dye in different pH media. The bactericidal potential of synthesized samples was also investigated. The structural properties of prepared samples were examined by XRD. The observed XRD spectrum of La₂O₃ showed a cubic and hexagonal structure, while no change was recorded in Mo-doped La₂O₃ samples. Doping with Mo improved the crystallinity of the samples. UV-Vis spectrophotometry and density functional theory calculations were used to assess the optical characteristics of Mo–La₂O₃. The band gap energy was reduced while the absorption spectra showed prominent peaks due to Mo doping. The HR-TEM results revealed the rod-like morphology of La₂O₃. The rod-like network appeared to become dense upon doping. A significant degradation of MB was confirmed with Mo; furthermore, the bactericidal activities against S. aureus and E. coli were measured as 5.05 mm and 5.45 mm inhibition zones, respectively, after doping with a high concentration (6%) of Mo.

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1. Introduction

Environmental contamination spurred on by rapid industrialization and urbanization impacts human lives directly. Numerous industries, including leather, textiles, paper, rubber, plastic, cosmetics, and food, produce organic toxic dyes as waste products on a regular basis.^1–3 Industrial wastewater contains heavy metals and other inorganic (trace elements, mineral acids, sulfates, inorganic salts, metal complexes with organic compounds) and organic (phenols, dyes, polyaromatic hydrocarbons, pesticides, etc.) pollutants.^4,5 These contaminants endanger aquatic fauna and flora, as well as human health. Dyes, referred to as coloring agents, are organic substances that bind chemically to the fabric surface to produce color. A dye is highly discernible even in small concentrations (less than 1 ppm) and carries toxicologically dangerous effects. It is the most common pollutant in textile industrial wastewater. Methylene blue (C₁₆H₁₈N₃SC) is a cationic dye possessing a heterocyclic aromatic chemical composition that is extensively used for coloring cotton, wool, and silk. Approximately 40% of synthetic dyes, such as methylene blue, are poisonous, mutagenic, and carcinogenic substances that persist in industrial wastes and have the potential to become the source of several serious health and environmental health issues.^6–8 To reduce environmental waste, factories are regulated to transform their dye impurities into acceptable outcome before disposal.⁹ Moreover, waste recovery processes should utilize water in a sustainable and cost-effective manner.¹⁰ Catalysis, electrolysis, photocatalysis, ion exchange, carbon filtering, adsorption, chemical precipitation, nanofiltration, reverse osmosis, and microbial control are a few of the most often used industrial water removal techniques.^3,11–13 Among these, catalysis is a popular method because it is ecologically beneficial, cost-beneficial, and energy-efficient.¹⁴ Mastitis is a significant economic burden on the dairy industry. It is accompanied by chemical, microbiological, and physical alterations in milk, and also pathological abnormalities in mammary gland structures.¹⁵ Mastitis is caused by infectious agents, including bacteria, viruses, and fungi. The most common bacteria pathogens are Coliform and Staphylococcus aureus.^16,17 Multiple drug-resistant Gram-positive and Gram-negative bacterial species have emerged in recent years, posing a substantial risk to public health.^18,19

The discovery of nanomaterials has revolutionized dye-contaminated wastewater treatment technologies. Small nanostructures in the size range between 1 and 100 nm possess remarkable surface-to-volume ratios compared to the bulk chemical composition, instigating significant improvements in chemical (biological, catalytic activity, etc.) and physical properties.^20,21 A considerably large number of metal oxides (MOs) are employed to degrade several organic impurities found in industrial wastewater. Nano-sized metallic oxides have a large number of industrial and technological applications, including environmental, optical, and mechanical gadgets.^22–24 MOs such as ZnO, TiO₂, La₂O₃, CeO₂, and so forth are utilized for dye degradation.^25–29 Among the alluring and potentially beneficial rare-earth MOs is La₂O₃ which has an energy gap of 4.3–5.8 eV.^30,31 Due to the rare electronic configuration [4f electrons] in lanthanides, their applications have been reported in diverse fields.^32,33 La₂O₃ has properties that set it apart from the other oxides in the series, due to the fact that La³⁺ is the only trivalent cation among the rare-earth elements without 4f electrons and possesses a simple xenon-like electronic structure.³⁴ Rare-earth elements have anticoagulant, antiemetic, bactericidal, antitumor, and anticancer effects. They have sparked a lot of attention in medicine and pharmacology. Rare-earth metals are well known for their antioxidant and antibacterial properties in animals.^35,36 Lanthanum-containing inorganic compounds are less expensive than the rest of the rare-earth host materials (Y₂O₃, Gd₂O₃, etc.).³⁷ La₂O₃ has been used in numerous applications, such as H₂ storage materials, superconductors, optoelectronic gadgets, laser crystals, LEDs, biosensors, excessive refraction optical fibers, and catalysts.^30–33,38 Additionally, as a basic element, lanthanum's oxides and hydroxides can react with CO₂ in the air to generate a less crystalline carbonate.³⁹ For the synthesis of La₂O₃ nanostructures, several approaches have been devised, including hydrothermal,⁴⁰ sol–gel,⁴¹ thermal deposition,⁴² co-precipitation,^34,43 and sonochemical,³² as well as other physiochemical techniques. Among these, co-precipitation is the most convenient and economic method, as it does not require any specific equipment.⁴⁴ The application field and catalytic activity of synthesized La₂O₃ nanostructures have been shown to be highly influenced by the preparation procedure.^30,45

Because of its thermal stability and distinctive electronic structure, Mo is a popular co-catalyst and helpful dopant among related elements.⁴⁶ It increases absorption in both the ultraviolet (UV) and visible light, and decreases the optical band gap. The Mo dopant may also increase the charge-carrier concentration. Studies have shown that Mo atoms can increase the metal oxide semiconductor stability and band structure.^47–49 In this work, undoped La₂O₃ and Mo (0.02, 0.04, and 0.06 wt%)-doped La₂O₃ nanostructures were effectively synthesized via a one-pot co-precipitation approach. The synthesized material was employed to remove the toxic dye MB. Its antibacterial potential against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacteria was also evaluated. Theoretical calculations were undertaken to reveal the change in the band gap (E_g) energy and optical properties with an increase in the Mo concentration.

2. Experimental section

2.1 Materials

Lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O, >99%), sodium molybdate dihydrate (Na₂MoO₄·2H₂O, 99%), and sodium hydroxide (NaOH, 98%) were procured from Sigma Aldrich (Germany).

2.2 Preparation of pure and Mo-doped La₂O₃ nanostructures

To prepare La₂O₃ nanostructures, 0.5 M of La(NO₃)₃·6H₂O was prepared under constant stirring at 80 °C for 30 min. The desired amount of NaOH was added dropwise into the stirred solution to maintain pH ∼ 12, under stirring at 100 °C for 3 h. The precipitates were isolated by centrifuge at 7500 rpm. The precipitates were dried for 12 h at 150 °C and milled into a fine powder. The above-mentioned method was adopted for all the doped samples, whereby Mo-doped La₂O₃ was synthesized with various concentrations of Mo (0.02, 0.04, and 0.06 wt%). The synthesis route used is depicted schematically in Fig. 1.

Fig. 1 Schematic diagram of the synthesis of the Mo–La₂O₃ nanostructures.

2.3 Catalytic activity (CA)

The degradation of methylene blue MB (oxidizing agent) was used as a means to assess the catalytic activity of the dopant-free and doped La₂O₃ nanostructures in the presence of sodium borohydride NaBH₄ (reducing agent), with the prepared nanostructures acting as a catalyst in this study. To ensure the purity of the experiment, all the reagents, including MB and NaBH₄, were used immediately after preparation. First, 3 mL aqueous MB solution was combined with freshly prepared (400 μL) 0.1 M NaBH₄ solution followed by adding 400 μL Mo-doped nanostructures (0, 2, 4, and 6 wt%) to the solution. It is worth remembering that a catalyst reduces the reaction activation energy (E_a), thus improving its stability and rate. Adsorption occurs when a catalyst is incorporated into MB in the presence of a reducing agent. The decolorization and variation in dye absorption intensity over time can be used to assess the reaction rate (Fig. 2). The size of the particles influences the catalytic activity of the dye because smaller particles have a higher surface-to-volume ratio, resulting in enhanced catalytic activity. In analytical chemistry, the most widely used redox indicator to regulate catalytic activity during a dye degradation test is MB. Furthermore, when oxidized, MB appears blue, and when reduced, it appears neutral, as shown in Fig. 2.⁵⁰

Fig. 2 Schematic diagram showing the experimental approach to evaluate the catalytic activity.

2.4 Photocatalytic activity (PCA)

The present study involved MB degradation in the presence of Mo-doped La₂O₃ nanostructures under sunlight. UV-Vis spectrophotometry was engaged to record the photocatalytic activity of all the concerned samples at room temperature. A stock solution of MB (g mL⁻¹) was prepared and 10 mg of synthesized samples was mixed with 30 mL stock solution. Before illumination, these samples were kept in the dark for 30 min to achieve a strong adsorption–desorption equilibrium between the MB and the photocatalyst. The solution was then transferred to a photoreactor after vigorous stirring. A Hg lamp (400 W, 20 cm) was used as the visible-light source. Afterward, 3 mL solution was separated for UV-Vis analysis at various time intervals. The maximum absorption (λ_max) for MB was attained at 665 nm for all the samples. The following equation was used to calculate the percentage (%) degradation:where C₀ and C_t are the initial and final concentrations of MB, respectively.

2.4.1 Reaction mechanism and kinetics. A possible PCA mechanism is depicted in Fig. 3. Photocatalytic degradation entails numerous processes, including adsorption–desorption, electron–hole pair formation, electron pair recombination, and chemical reaction.^51,52 The PCA begins with photoexcitation, which is initiated by photons processing energy equal to or more than the material band gap energy (E_g). These photons stimulate valence/lower band (VB) electrons, causing them to move to the conduction/higher band (CB). Electrons leave holes in the VB, resulting in the formation of e–h⁺ pairs, as stated in the equation below.

La₂O₃ + hν → e⁻(CB) + h⁺(VB) (2)

Fig. 3 Schematic illustration of the photocatalysis mechanism of the Mo-doped La₂O₃ nanostructures.

The photon-induced electrons react with oxygen to form the superoxide radical O₂˙⁻.

e⁻ + O₂ → O₂˙⁻ (3)

Later, when these superoxide radicals O₂˙⁻ react with water, oxidizing agents, such as hydroperoxy (HO₂˙) and hydroxyl radicals (OH˙), are produced, which are crucial for the breakdown of organic contaminants.

O₂˙⁻ + H₂O → HO₂˙ + OH (4)

Water on the photocatalyst surface may also be trapped in these irradiation-induced holes, producing hydroxyl radicals (OH˙).

h_VB⁺ + OH⁻ → ˙OH_ad (5)

h_VB⁺ + H₂O → ˙OH + H⁺ (6)

Finally, the oxidation of organic molecules produces CO₂ and H₂O.

˙OH + organic molecules + O₂ → products (CO₂ and H₂O) (7)

Furthermore, even a minor recombination of electron–hole pairs, which may be lost eventually, will affect the photocatalytic activity of the nanocatalyst.⁵³

e_CB⁻ + h_VB⁺ → La₂O₃ (8)

2.5 Segregation and characterization of S. aureus and E. coli

Mastitis-positive ewe milk specimens from Punjab animal hospitals and farms were cultured using 5% blood agar (SBA). Overnight incubation at 37 °C proceeded and the resulting bacteria were purified by streaking in triplicate on MacConkey (MA) and mannitol salt agar (MSA) at pH ∼ 7. Coagulation, catalase, and Gram staining were used to distinguish the extracted commodities biochemically and morphologically.

2.6 Antimicrobial activity

To evaluate the bactericidal capabilities of Mo-doped La₂O₃ nanostructures, Gram-positive (S. aureus) and Gram-negative (E. coli) microbes were recovered effectively from ovine mastitic fluid. The bactericidal content was assessed in vitro by swabbing S. aureus and E. coli strains on MSA and MA, correspondingly. Bacterial remedies containing 0.5 Mc-Farland standard were swabbed onto agar plates, while wells 6 mm wide were formed using a sterile cork borer. In comparison to ciprofloxacin (5 μg/50 μL) used as a standard drug and DIW (50 μL) used as a negative control, the wells were filled with distinct concentrations of the doped nanomaterial, e.g., 500 and 1000 μg/50 μL. The sensitivity of all the generated samples was determined using a Vernier caliper after 24 h incubation (37 °C) of the agar dishes. To assess the antibacterial effect, one-way analysis of variance was employed.

2.7 Characterization

A PANalytical XPert PRO X-ray diffraction (XRD) system employing Cu Kα radiation (λ ∼ 0.0154 nm) was utilized to determine the crystal structure and phase information of the nanostructures in the 2θ range from 20° to 70°. A PerkinElmer spectroscope was used to detect the presence of functional groups from the FTIR spectra. A UV-Vis spectrophotometer (Genesys 10S) was employed to examine the optical properties in the range from 200 to 700 nm. The samples' morphologies and microstructures were recorded with a JSM-6460LV FE-SEM system coupled with an EDX spectrometer. PL spectra were obtained on a JASCO FP-8300 system for the pure and doped La₂O₃. The HR-TEM equipment JEOL JEM 2100F was used to measure the inter-planner d-spacing of the prepared nanostructures.

2.8 Computational details

First-principles computation based on the framework of density functional theory (DFT) within the Perdew, Burke, Ernzerhof (PBE) for the exchange–correlation functional⁵⁴ were carried out using the Quantum Atomistix ToolKit (QuantumATK) package.⁵⁵ Norm-conserving PseudoDojo pseudopotentials were used to describe the ions and core electrons with a medium basis set. A mesh cut-off energy of 10⁵ Ha was employed.⁵⁶ For the relaxation of the structure, the 4 × 4 × 1 Monkhorst–Pack's scheme were applied to sample the Brillouin zone. High accuracy in the electronic structure calculations was derived from using a 7 × 7 × 1 k-point grid. The investigated system was fully optimized until the maximum force was lower than 0.05 eV Å⁻¹ on each atom site using the limited-memory Broyden–Fletcher–Goldfarb–Shanno scheme. For the self-consistent calculations, we utilized a criterion of a total energy convergence of 10⁻⁶ Ha difference. The electronic and optical properties were computed employing the more accurate Heyd–Scuseria–Ernzerhof hybrid functional (HSE06).^57–59

3. Results and discussion

XRD was employed on the control and Mo-doped La₂O₃ in the 2θ range of 20–70° to examine the crystal structure, phase purity, and size of the crystallites (Fig. 4(a)). The diffracted peaks observed at 2θ = 39.48° (012), 48.49° (111), 55.39° (112) were attributed to hexagonal (h-La₂O₃), (JCPDS no: 73-2141), and 27.02° (222) to the cubic phase (c-La₂O₃), (JCPDS no: 65-3185) of La₂O₃, while the peak broadening was related to the formation of smaller particles in the nanosize range.⁶⁰ The additional peak at 64.02°, denoted by ♠, was ascribed to La(OH)₃ (JCPDS no: 36-1481), which arose as La₂O₃ is hygroscopic in nature and upon exposure to a moist environment it quickly changes to La(OH)₃, as reported by Fleming et al.^61–63 No undesired peak appeared in the synthesized samples, suggesting the high purity of the nanostructures. The average size of the crystallites was found to be in the range of 13.1–10.6 nm, as estimated using Scherrer's formula. The shifting of the peaks to higher 2θ reflected that the crystallite size decreased with increasing the dopant amount, due to the smaller ionic radii of the dopant Mo⁴⁺/Mo⁶⁺ (0.68 Å/0.62 Å).^60,64 This smaller radii will alter the local symmetry through the doping of heteroatoms.^65,66 The chemical composition and identification of the functional group of Mo-doped La₂O₃ were measured using FTIR analysis, as represented in Fig. 4(b). The spectra were observed in the frequency range of 500–4000 cm⁻¹. The peaks at 502, 644, 845, and 1077 cm⁻¹ represented La–O bond stretching vibration.^33,34,67,68 The intense peak at 1339 cm⁻¹ and small peak at 3610 cm⁻¹ were ascribed to the bending and stretching of O–H vibrations, respectively, caused by the samples being processed in a moist environment.^69–71 SAED analysis revealed discrete rings, corresponding to various planes, i.e., (222), (012), (111), and (112), of both the pure and doped samples, Fig. 4(c–e). Evidence of the well-crystallized material was related to these findings, which matched the XRD data. To determine the d-spacing, highly magnified images (at 5 nm scale) were deployed to reveal the lattice fringes for the identification of the crystallographic planes. The estimated d-spacing for La₂O₃ was ∼0.185 nm, which satisfies the theoretical d-spacing of the (111) crystallographic plane of La₂O₃, Fig. 4(f). From Fig. 4(g and h), upon doping, the d-spacing of Mo–La₂O₃ was slightly decreased to ∼0.182–0.180 nm. These calculated values matched the XRD results (Fig. 4(a)).

Fig. 4 (a) XRD patterns, (b) FTIR spectra of the synthesized samples, (c–e) SAED patterns, and (f–h) d-spacing of pristine and the 4% and 6% doped nanostructures.

UV-Vis spectroscopy was utilized to investigate the optical properties of the prepared samples in terms of the maximum absorption of ultra-visible light, the band shift, and calculation of the band gap energy. Undoped and Mo-doped La₂O₃ showed absorption in the range of 280–310 nm (Fig. 5(a)). The maximum absorption peak found at 288 nm was presumably due to charge-transfer (O²⁻ → La³⁺) absorption, attributed to electronic transition π–π*.⁷² The band gap energy was evaluated using Tauc's equation, and the indirect E_g was determined to be 4.3 eV, which is consistent with previously published data.⁶⁹ More energy levels were created in the E_g of La₂O₃ upon the addition of Mo. With the increasing concentration of Mo dopant, only a small modification in the E_g was observed, e.g., ∼4.2 eV, at a higher concentration of Mo (Fig. 5(b)).^46,64 The band gap energy of the doped samples decreased as the crystallite size decreased, which might have been caused by reduced orientation realignment and poor crystallinity of the resulting materials.⁷³

Fig. 5 (a) Absorption spectra, (b) band gap energy plot, (c) PL spectra of La₂O₃ and Mo-doped La₂O₃ nanostructures.

The PL spectra of the samples were investigated to determine the changes in the charge-transfer efficiency and recombination rate. The PL spectra of La₂O₃ and Mo–La₂O₃ shown in Fig. 5(c) were measured from 400 to 600 nm with a 350 nm excitation wavelength. There was a single emission peak in the spectra at 414 nm, attributed to deep levels/trap state emission in the E_g of La₂O₃. This emission transition was attributed to oxygen vacancies, singly ionized, in La₂O₃ and it occurs when a photogenerated h⁺ combines with an e⁻ occupying an oxygen vacancy.^74–76 Due to the absence of electrons in the 4f shell of La³⁺, it is unable to emit light from the inner atomic 4f shell when crystalline La₂O₃ is formed.^69,76,77 Near band edge (NBE) emission/transitions between MO-generated defects and the O 2p band induce violet emission between 400 and 430 nm.^78,79 Doping with Mo can suppress the recombination rate of excitons, which causes a drop in the emission peak intensity. The graph demonstrates that for a concentration of 2% Mo, the emission intensity was optimal. After this, the peak intensity decreased with the higher concentration of Mo. The higher concentrations reduce the inter-nuclear separation below a critical distance, and the excitation energy tends to move to energy-killing sites.^80–83

The EDS spectra of the undoped sample, shown in Fig. 6(a), confirmed the existence of La and O, as well as C, which most likely originated from the carbon tape used to mount the samples. The spectra of the doped sample, Fig. 6(b–d), are nearly identical to the undoped one except for the peak of Mo ascribed to the dopant.⁸⁴ The Na peak could be attributed to the use of NaOH to control the pH during the nanostructures synthesis.⁸⁵

Fig. 6 (a–d) SEM-EDS analysis of La₂O₃ and Mo-doped La₂O₃.

The morphology and microstructure of pure and Mo (4% and 6%)-doped La₂O₃ were further characterized by TEM. Fig. 7(a) reveals the rod-like morphology of the undoped La₂O₃ prepared by the co-precipitation method, most likely formed because of the self-assembly of molecules due to the elevated pH and removal of water.^86,87 The pristine sample showed a uniform rod-like network, Fig. 7(a). In Fig. 7(b), it appears that Mo played a role in the process of nucleation, while few undissolved Mo particles can be seen, along with the agglomeration of the rods. The rods network seemed to overlap when the concentration of Mo increased, Fig. 7(c). Agglomeration occurs at the nanoscale due to the strong attractive forces between particles.⁸⁸ Brownian agglomeration occurred here,⁸⁹ where particles stuck together loosely with an overlap of nanorods, resulting in the formation of a network, which facilitates charge transportation, leading to increased catalytic activity.

Fig. 7 HR-TEM images of (a–c) pristine and Mo (4% and 6%)-doped La₂O₃.

MB was utilized to test the catalytic application of nanocatalyst in the presence of a reducing agent (NaBH₄). By measuring the decrease in absorbance with a UV-Vis spectrophotometer, the reactions were monitored. The pure and doped nanostructures illustrated a maximum degradation of 91.15–98.63% in neutral medium (pH 7), 90.86–96.43% in acidic medium (pH 4), and 91.69–99.07% in basic medium (pH 12), as shown in Fig. 8(a–c), respectively. In the presence of a doped nanocatalyst, reduction was enhanced. The maximum catalytic activity was achieved with Mo (6%) in all media. The crystallinity, shape, and surface area of the nanomaterials all influence the catalytic activity.^90,91 During catalysis, the synthesized material serves as electron relays, reducing MB in the presence of NaBH₄, allowing electrons to flow from BH₄⁻ ions (donor) to MB (acceptor), resulting in dye reduction. Nanostructures with abundant active sites increase BH₄⁻ ion and dye molecule adsorption, which encourages them to react with each other.^92,93 In general, a catalyst with a wide surface area has a high catalytic efficiency since it provides more active sites.^94,95

Fig. 8 Catalysis of La₂O₃, Mo–La₂O₃ (2%, 4%, and 6%) in (a) neutral, (b) acidic, and (c) basic media.

The photocatalysis of MB aqueous solution was investigated on undoped and doped La₂O₃ nanostructures, Fig. 9(a–c). Highly competitive degradation of 96.5% was noted for the basic medium (pH 12), within 30 min, whereas, 25.9% degradation was achieved in neutral medium (pH 7) in 60 min, and for the acidic medium (pH 4), the degradation was 21.8% in 15 min for 6% Mo-doped La₂O₃, which showed the lowest intensity of PL in Fig. 5(c), indicating a lower recombination rate. The photocatalytic response is governed by the pH of the solution, which might affect dye adsorption on the photocatalyst surface. MB is a cationic dye and its degradation was modest at low pH. However, with the rise in pH, the maximum degradation was observed. Positively charged catalyst surfaces tend to oppose the adsorption of cationic adsorbates species in an acidic medium. In basic dye solutions, surface charges tend to become negative due to the enhanced electrostatic interaction between the positively charged dye and negatively charged catalyst. As a result, dye adsorption increases, which is consistent with earlier findings.^96–98 The sample with 2% Mo doping had the lowest degradation of MB and showed the highest PL intensity. A higher recombination rate may be the cause of the lower degradation efficiency of this sample. The particle size, morphology, and surface area of nanocatalysts have a significant impact on the degradation performance. Particles with a large surface area offer more active sites for atoms, resulting in an increase in the number of redox reactions and MB reduction. Photogenerated es⁻ and hs⁺ interact with O₂ and H₂O molecules in the solution to form highly reactive O₂⁻, OH⁻, and H⁺ radical species, respectively. Excess H⁺ and es⁻ interact with dye molecules, leading to MB reduction.^99,100 It is evident that degradation increases with an increasing concentration of the dopant.^74,79

Fig. 9 Photocatalysis of La₂O₃, Mo–La₂O₃ (2%, 4%, and 6%) in (a) neutral, (b) acidic, and (c) basic media.

Catalyst reusability is crucial as it allows for the treatment of industrial wastewater and effluent for the highest number of cycles possible. The reusability of the control and Mo (6%) nanostructures was examined by extracting the used catalyst material. This was washed, dried, and used again for degradation in up to 4 cycles, Fig. 10(a and b). On each cycle, the efficiency of the reused catalyst showed only a slight change. It is worth noting that after four cycles, the degradation efficiency was only lowered by a small proportion (2%), demonstrating that the prepared nanostructures were highly stable. Furthermore, the stability of these two samples was tested by storing the degraded solution in the dark for 72 h to see if the dye degradation was stable. Dye degradation was monitored spectrophotometrically every 24 h, as illustrated in Fig. 10(c). The percentage degradation was calculated using eqn (1) and is given as the extracted results.

Fig. 10 Reusability of (a) La₂O₃ and (b) Mo (6%), and (c) stability comparison of the pure sample and Mo (6%).

Table 1 shows the bactericidal efficacy of Mo-doped La₂O₃ for S. aureus and E. coli, as determined by the well diffusion technique. At minimal and maximal doses, substantial (p < 0.05) inhibition areas for E. coli were found (0.95–4.65 mm) and (1.40–5.45 mm), respectively, and (0–5.05 mm) for S. aureus at high dose. Except for 6% Mo–La₂O₃, none of the concentrations had any antibacterial effect against S. aureus at low dose. The inhibition zones for S. aureus and E. coli were compared using a negative control DI water (0 mm) and positive control ciprofloxacin (4.35 mm) and (5.35 mm), respectively. In fact, the antibacterial activity of the nanostructures can be attributed to a variety of phenomena, including electrostatic contact or interactions with OH⁻ and H₂O present at the surface, both of which result in the production of reactive oxygen species (ROS), see the schematic illustration in Fig. 11. These interactions are similar to those seen in photocatalysis. Indeed, the most effective nanostructure, with 6% Mo dopant, for the degradation of MB also exhibited the best bactericidal action. In conclusion, the shape and coarseness of various nanostructures have a significant impact on the antibacterial performance.¹⁰¹ MB degradation through a nanocatalyst is also influenced by the same factors.

Table 1 Antimicrobial activity of pure and Mo-doped La₂O₃ nanostructures

Sample	Inhibition zone^a (mm)		Inhibition zone^b (mm)
	500 μg/50 μL	1000 μg/50 μL	500 μg/50 μL	1000 μg/50 μL
a Inhibition area (mm) of Mo-doped La2O3S. aureus. b Inhibition zone determination of Mo-doped La2O3 for E. coli.
La2O3	0	0	0.95	1.40
2% Mo–La2O3	0	3.35	1.05	2.15
4% Mo–La2O3	0	4.15	1.65	2.55
6% Mo–La2O3	3.15	5.05	4.65	5.45
Ciprofloxacin	4.35	4.35	5.35	5.35
DIW	0	0	0	0

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Fig. 11 Schematic illustration of the antibacterial mechanism of the prepared Mo-doped La₂O₃ nanostructures.

3.1 DFT calculations

We started by performing a structural analysis of defect-free La₂O₃ prior to investigating the defects. The La₂O₃ crystallized in the hexagonal structure (space group P3̄m1), as shown in Fig. 12. The optimized lattice parameters, a = 3.914 Å, c = 6.096 Å, showed a fair agreement with the experimentally reported values (a = 3.939 Å, c = 6.136 Å)¹⁰² and previous theoretical studies.^103,104 To investigate Mo doping into La₂O₃ at the La site, we built a 2 × 2 × 2 supercell (containing 40 atoms) and generated dopants concentration of 6.25% by replacing one La atom with one Mo atom, as illustrated in Fig. 12. The substitution of Mo impurity into the La₂O₃ lattice at the La site showed that the a- and c-lattice parameters (a = 3.902 Å, c = 6.063 Å) were little changed compared to those of the pristine sample. This difference occurs from the hexagonal lattice distortion after the Ag doping. The relaxed La–O average bond length of the pure sample was 2.645 Å, which was rather close to the experimental measurements¹⁰² and theoretical values.^103,104 For Mo doping, the process increased the La–O (2.706 Å) bond distance because of the distortion of the crystalline structure.

Fig. 12 Crystal structure (based on a 2 × 2 × 2 supercell) of Mo-doped La₂O₃. La is shown in green, Mo in purple, and O in red.

Regarding the electronic properties of La₂O₃, the band structures and the density of states (DOS) were analyzed, as shown in Fig. 13. The band structure suggested a semiconducting nature with an indirect band gap (from Gamma to K points in the first Brillouin) of the magnitude of 4.99 eV using the HSE06 functional, which is consistent with our experimental band gap value of 4.3 eV and previous experimental data (5.34, 4.35 eV).^69,102 Comparison with the literature indicates that our HSE06 computed band gap value for La₂O₃ is in excellent agreement with that of 5.1 and 4.95 eV formerly reported using the HSE06 and G₀W₀ methods,^103,104 respectively. The valence band edge states were found to be dominated by O atoms (2p orbitals), while the unoccupied conduction band minimum states were mainly contributed by the La atom (5d orbitals). We therefore performed the electronic calculations on one doped Mo atom in the La₂O₃ by investigating the band structure and DOS, as shown in Fig. 13. As for the substitutional Mo to the La-doped La₂O₃, the band structure showed that flat bands were formed around the Fermi level, which can be beneficial, as they may act as trapping centers. We present the calculated density of electronic states for Mo-doped La₂O₃ in Fig. 13. It was found that Mo doping induced the trap gap state around the Fermi level. From the LDOS plots, the latter state was mainly constructed by Mo 3d orbitals, with a small contribution of O 2p orbitals, indicating the occurrence of the O 2p–Mo 3d hybridization. A more interesting point to notice here is the change in the band gap of the La₂O₃ supercell. However, with Mo doping, the band gap decreased to the value of 4.71 eV, which can be attributed to the variation in the bond length of La–O and the emergence of a new Mo–O bond with Mo introduction. Thus, this result is well aligned with the experimental data. Moreover, with a single La atom substitution with Mo, an in-gap state around the Fermi level was created driving a significant shift down to the VB and CB edges. To explore the absorption characteristics in the pristine and Mo-doped La₂O₃ systems, we calculated the absorption coefficient spectra versus the incident light wavelength using the HSE06 functional, as shown in Fig. 14. The absorption coefficients analysis showed that the pristine La₂O₃ could only absorb in the ultraviolet-light range without any absorption response in the visible-light region. The Mo doping induced a slight red-shift of the absorption edge because of the impurity in-the-gap state.

Fig. 13 Calculated band structures and total and partial DOS of (a) pristine and (b) Mo-doped La₂O₃.

Fig. 14 Calculated absorption coefficient spectra of doping free and Mo-doped La₂O₃.

To prove the favorable adsorption of molecules on the considered materials, the adsorption energy was computed by selecting NaBH₄ as molecules to be absorbed on the surface of pristine and Mo-doped La₂O₃ (001) according to ref. 105 and 106:

E_ads = E_{molecule+La2O3(001)} − E_La2O3(001) − E_molecule (9)

where E_{molecule+La2O3(001)}, E_La2O3(001), and E_molecule denote the total energies of the NaBH₄–La₂O₃(001) system, pure surface, and the isolated molecule, respectively. The La₂O₃ (001) and Mo-doped La₂O₃(001) surfaces were simulated using the adopted slab geometry with a (2 × 2) surface unit cell, separated by a vacuum distance of 15 Å,¹⁰⁷ as shown in Fig. 15, and a corresponding 4 × 4 × 1 k-point mesh was employed for all these configurations. The findings revealed that the adsorption energies were −4.95 and −3.25 eV in the La₂O₃(001) and Mo-doped La₂O₃(001) surfaces adsorbed to NaBH₄.

Fig. 15 Side view of the adsorption of NaBH₄ molecules on the (a) La₂O₃(001) and (b) Mo-doped La₂O₃(001) surfaces. La, O, Mo, B, H, and Na atoms are represented by green, red, purple, blue, gray and yellow balls, respectively.

4. Conclusion

A cost-effective, co-precipitation technique was used to successfully synthesize Mo-doped La₂O₃ nanostructures. The XRD spectra confirmed the presence of hexagonal Mo along with the slight occurrence of a cubic phase in the prepared samples. The crystallite size was found to diminish from 13.1 to 10.6 nm upon doping with Mo. Combined density functional theory and experimental study supported a decrease in the E_g with an increasing amount of Mo dopant. The calculations suggest that Mo doping induces a slight red-shift in the absorption edge due to the impurity in-the-gap state. TEM revealed the rod-like morphology of La₂O₃ synthesized using the co-precipitation approach. The EDS spectra demonstrated the elemental composition and successful Mo doping. When compared to undoped samples, Mo-doped La₂O₃ nanostructures demonstrated better catalytic potential of up to 99%. Furthermore, the synthesized nanostructures exhibited substantial bactericidal efficacy against S. aureus and E. coli bacteria.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are immensely thankful to HEC Pakistan (SRGP# 21-1669) and the Core Research Facilities, KFUPM (Dhahran, Saudi Arabia) for their assistance. S. Goumri-Said thanks the Alfaisal University Research Office in Saudi Arabia for financing this research project C20431.

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Footnote

† M. Ikram and N. Abid are equal contributors.

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