Given extreme energy crisis, semiconductor technology offers a perfect solution in harnessing solar energy and effectively dealing with the environmental pollution [1]. Titanium dioxide (TiO₂) is widely studied and is most popular among photocatalysts that are being used. It is also known to be non-toxic with high oxidation capacity and chemical stability [2]. Henceforth it is extensively used in creams and cosmetics [3]. A major drawback with TiO₂ is its wide bandgap, which facilitates it to respond only to ultraviolet light, i.e., about 4% of total solar energy [4]. Thus, visible light photocatalysts like bismuth vanadate (BiVO₄) have come to limelight. BiVO₄ is widely studied due to their lower bandgap of 2.4 eV [5]. Photocatalytic property of BiVO₄ is identified to vary with their crystal structure. They exist in tetragonal zircon, monoclinic and tetragonal scheelite structures, among which the most active phase under visible light radiation is known to be monoclinic scheelite structure [6–8]

Staphylococcus aureus (S. aureus) by far a major infectious agent among hospitals and within the community. Methicillin, a β-lactam antibiotic, acts by inhibiting penicillin-binding proteins (PBPs) that area unit concerned within the synthesis of peptidoglycan, a necessary mesh-like chemical compound that surrounds the cell. S. aureus will become proof against methicillin and different β-lactam antibiotics through the expression of a far off PBP, PBP2a, that’s proof against the action of methicillin however which might perform the functions of the host PBPs. Methicillin-resistant S. aureus isolates area unit typically proof against different categories of antibiotics (through completely different mechanisms) creating treatment choices restricted, and this has light-emitting diode to the rummage around for new compounds active against these strains. associate understanding of the mechanism of methicillin resistance has light-emitting diode to the invention of accent factors that influence the amount and nature of methicllin resistance. Several researchers round the world have experimented the chance of activity of nanoparticles such as silver, gold, zinc oxide, TiO₂ and so on, in terminating the growth of MRSA [9–11].

The synthetic routes available for the synthesis of BiVO₄ include majorly sol-gel method, hydrolysis of metal alkoxides, hydrothermal method, etc. [12]. Among these, the hydrothermal route is known to be most effective for processing advanced functional materials as it facilitates control over the size and morphology of the nanoparticles and does not involve in post-synthesis treatments [13]. In the present work, the preparation, characterization and photocatalytic efficiency of Al-doped BiVO₄ materials synthesised by hydrothermal method at 180ºC for 4 h and 8 h duration, keeping pH 7 as constant, without the addition of any structure enhancing agents have been reported. Methicillin-resistant S. aureus (MRA, MTCC - 6908) is gram-positive cocci that is a significant infective agent that causes dental caries, skin and soft tissue infections (SSTIs), respiratory organ infection and so on [10]. It is an associate opportunistic parasite and statistically two thirds of the cocci in domestic water is of S. aureus. Antibacterial activity of aluminium doped BiVO₄ crystals against S. aureus has been completely investigated in this research.

XRD spectra of the synthesised photocatalysts show the formation of well crystalline scheelite monoclinic structures, at both the durations of hydrothermal treatment. The graphs obtained are in good agreement with JCPDS card No. 14-0688 as shown in Fig. 1, which corresponds to I2/a space group of monoclinic bismuth vanadates with a=5.195, b=11.70, c=5.092 and α=γ≠90°, β=90°. Although, notable differences were observed upon Al doping (Al⁺³ from Al₂O₃). An occurrence of interstitial doping of Al on BiVO₄ was evident from XRD spectra, which displays additional Al peaks, along with characteristic monoclinic peaks of BiVO₄. The increase in peak intensity with hydrothermal synthesis duration from 4 h to 8 h was attributed to the formation of well crystalline structures [15, 16].

The values of 2θ, Muller’s indices, full-width half maximum, calculated crystallite size were tabulated in Table 1.

Crystallite size of the particles was calculated using the following formula (Eq. (1)). d-spacing was calculated using Bragg’s law.

Where,

FTIR spectra (Fig. 2) of the synthesised Al:BiVO₄ samples show a band around 1,000-1,100 cm⁻¹, which was attributed to Al-O, 730–1,000 cm⁻¹ was attributed to the stretching mode of VO₄ ⁻³ and its branch at 613 cm⁻¹ attributes to Bi-O [17]. The peak observed at 1,386 cm⁻¹ was assigned to stretching mode of C-H group and the one at 1,760 cm⁻¹ attributes to C=O stretch from SDS which was used as the surfactant.

FE-SEM micrographs (Fig. 3) show distinct morphological changes on Al doping and hydrothermal duration. Micrographs of pure BiVO₄ sample synthesised at 4 h period show irregular cheese-puff like particles (Fig. 3 (a0)). Whereas, upon addition of Al⁺³ at 4 h synthesis flake-like particles were formed (Fig. 3(a1), (b1), (c1)). With the increase in hydrothermal duration to 8 h, Al:BiVO₄ showed perfect rod like monoclinic crystals (Fig. 3(a2), (b2), (c2)) and FE-SEM images show the presence Al (or Al⁺³) on the surface of BiVO₄. Addition of Al to BiVO₄ showed a significant influence on its. Hence, we see the formation of flake like structures at 4 h synthesis. Significant growth of well-formed rod like crystals during the hydrothermal reaction has been observed, as the crystallinity increased with longer hydrothermal reaction time of 8 h.

Structural changes in the products are in agreement with XRD spectra obtained. The increase in peak intensity with upon longer duration of hydrothermal treatment resulted in well crystalline monoclinic structures. Atomic percentages of synthesised materials were determined by EDS spectra as shown in Fig. 3(a0) and 3(d). It shows a comparison between atomic percentages of pure BiVO₄ and changes upon Al doping. Percentage oxygen varied from 74.77% to 74.16%, vanadium 13.48% to 13.81% and bismuth 11.75% to 10.17%. Although 1% Al₂O₃ has been used for doping, because of the presence of 2Al⁺³ in the molecule, EDS spectrum shows 1.86% of Al has been doped.

UV-Vis absorption spectroscopy (Fig. 4) was studied for catalysts that were synthesised at different hydrothermal durations. The obtained materials showed a characteristic absorption edge at around 550 nm, which correlates with n to π^* transition that corresponds to the excitation of an electron from one of the unshared pair to the π^* orbital [18]. It is noted that upon Al doping, absorption intensity was increased and further increased with increase in duration of synthesis. Band gap energies of the synthesised materials was determined from Tauc plot of (αhν)2 vs. photoenergy (hν)

Where, α = absorption coefficient.

Estimated Eg values are 2.35, 2.23, 2.21 eV for BiVO₄, Al:BiVO₄ (4 h) and Al:BiVO₄ (8 h), respectively (Table 1 and Fig. 4). Thus, Al doping decreased the band gap energy of BiVO₄ significantly. The peaks below 400 nm are nothing but the maximum absorption intensity, usually observed when the initial impact of the photons on the material takes place.

Photoxidation inactivation of MRSA using the synthesized semiconductors was studied by the proposed 9 h experiment (Fig. 5). It was evident from the results obtained that all the synthesized samples showed a good activity against the growth of bacteria at all concentrations. It was also observed that due to insufficiency of direct interaction between the particles surface and the bacteria, the results were not spontaneous [19]. Thus, prolonged exposure to solar radiation facilitated the formation of more O-H radicals, which interact with the bacterial cell wall, causing its disruption. As proposed by Pablos et al., an electrostatic repulsion between the negatively charged bacterial surface and the photocatalyst surface would occur unless the intervention of an intermediate counter ion layer of cations takes place [20]. Thus, it was assumed that hydroxyl radical formation was hindered and hole transfer could be favoured under visible light [21].

As prepared photocatalysts showed good antibacterial activity in all concentrations. It is notable that 0.002 g/L of Al doped BiVO₄ particles showed higher antibacterial activity than the pure BiVO₄ and the reference material. Upon increasing the concentration, synthesized materials showed even higher antibacterial effect with highest being 94% by the particles synthesized at 8 h hydrothermal duration. The improved photocatalytic antibacterial performance of Al:BiVO₄ can be ascribed to the critical lessening in band-gap energies from 2.35 eV (unadulterated BiVO₄) to 2.23 eV (Al:BiVO₄ (4 h)) and further diminishment to 2.21 eV (Al:BiVO₄ (8 h)) with an increase in crystallinity of the material [18]. The presence of Al⁺³ (due to the usage Al₂O₃ as dopant) on the surface of the catalyst also improved charge transfer characteristics which resulted in significant increase in photocatalytic efficiency [19, 20].

MIC values of the nanoparticles against MRSA have been determined based in %T, calculated from OD values. MIC₅₀ of both Al:BiVO₄ (4 h) and Al:BiVO₄ (8 h) has been determined as 5 μg/mL. Whereas, MIC₉₀ of Al:BiVO₄ (4 h) is 50 μg/mL and MIC₉₀ of Al:BiVO₄ (8 h) is 10 μg/mL. This shows that there is a tremendous increase in antibacterial property of Al:BiVO₄ with increase in crystallinity. MIC₉₉ of Al:BiVO₄ (8 h) is 50 μg/mL, where 100% growth inhibition occurred.

Monoclinic BiVO₄ and Al/Al⁺³ could be excited to generate electrons and holes upon adequate visible light illumination, as shown in Fig. 7. These electrons further interact with O₂ and produce O₂·⁻, which splits H₂O to form OH⁻ and ·OH. It also reacts with proton (H⁺) to form hydroperoxyl radical (HO₂ ⁻). These reactive oxygen species (ROS) in turn react with dyes, effluents, organic material present in water to release majorly CO₂ and H₂O, thus assisting in water treatment [21].

Oxidative stress shoulders a major role in antibacterial activity causing cytotoxic effects. This marks for a superior property of BiVO₄ in inhibiting the growth of pathogenic bacteria, which can be harnessed in real time applications after few toxicological studies.

Al modified BiVO₄ nanocrystals were synthesised successfully using hydrothermal treatment for the first time to enhanced photocatalytic properties of the material. It was evident that Al doping had a significant influence on the morphology of synthesised products. Upon Al⁺³ doping (valency of Al in Al₂O₃ is Al⁺³) on the surface of BiVO₄ matrix, irregular cheese-puff like morphology of BiVO₄ changed to flake-like structures, which gradually formed rod like monoclinic structures upon increasing hydrothermal duration. This modification projected in a distinct enhancement of photocatalytic antibacterial property of the as-prepared materials. The highest antibacterial rate has been performed by Al:BiVO₄ which was synthesised at 8 h of hydrothermal treatment. The enhanced photocatalytic activity of Al:BiVO₄ can be attributed to the significant decrease in band-gap energy from 2.35 eV (pure BiVO₄) to 2.23 eV (Al:BiVO₄ (4 h)) and further reduction to 2.21 eV (Al:BiVO₄ (8 h)) with an increase in crystallinity of the material. The presence of Al/Al⁺³ on the surface of the catalyst also improved charge transfer characteristics which resulted in significant increase in photocatalytic efficiency. This research shows promising results for real time usage of Al:BiVO₄ for waste water treatment, after conducting few toxicological analyses.