TECHNICAL FIELD

[0001] The present invention generally relates to solution chemistry, and more particularly relates to rational design of visible -light- sensitive bismuth-based material with exceptional photocatalytic degradation properties.

BACKGROUND OF THE DISCLOSURE

[0002] Photocatalysis has been extensively recognized as a process with potential applications in many fields, such as water splitting, water treatment and purification, hydrogen evolution, photoelectrochemical conversion, treatment of volatile organic compounds and antimicrobial applications. For instance, titanium dioxide (TiO ₂ ) is known as the most studied semiconductor with first published reports as early as 1956. Since TiO ₂ can absorb only ultraviolet-light (UV -light) from the solar spectrum, the demand in visible-light-sensitive photocatalysts has skyrocketed and has led to development of other materials. Among such other materials, bismuth-based (Bi- based) semiconductors rapidly attracted attention due to stability of Bi ³⁺ and its atomic structure. In Bi ³⁺ -based materials the hybridized oxygen 2p and bismuth 6s ² orbitals can cause an upshift of the valence band (VB) towards the conduction band (CB) making the band gap smaller and sensitive to visible light.

[0003] Among metal oxides and simple binary compounds, Bi ₂ O ₃ is the most promising material due to its narrow band gap (2.0-2.8 eV) and unique availability of polymorphs. In general, Bi ₂ O ₃ exists in four different phases: monoclinic a, tetragonal b, body-centered cubic g, and face-centered cubic d, where a and d are stable phases and b and g are high-temperature metastable phases. Successful formation of metastable phases depends on temperature and the applied synthesis method. As b -phase has the lowest band gap and the best photocatalytic properties among other Bi ₂ O ₃ polymorphs, it is especially desirable to synthesize a/b-Bi ₂ O ₃ composite with p-n heterojunctions that can potentially lead to enhanced electron-hole mobility. However, such synthesize of photocatalytic material has been found challenging.

[0004] Thus, there is a need for methods design and synthesize of visible-light- sensitive bismuth-based material with exceptional photocatalytic degradation properties. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.

SUMMARY

[0005] According to at least one aspect of the present embodiments, a method for rational design of visible-light-sensitive photocatalysts for photocatalytic degradation of organic compounds is provided. The method includes matching valence band (VB) and conduction band (CB) of a photocatalyst with redox potentials of the organic compounds and matching VB and CB of the photocatalyst with reactive oxygen species (ROS). The method further includes matching redox potentials of the organic compounds with the ROS.

[0006] According to another aspect of the present embodiments, a first, second and third material for visible-light- sensitive photocatalyitc degradation of organic compounds are provided. The first material includes palladium-loaded a/b-Bi ₂ O ₃ nanorods. The second material includes a palladium-loaded a/b-Bi ₂ O ₃ p-n heterojunction photocatalyst. And the third material includes Ag-BiVO ₄ . BRIEF DESCRIPTION OF THE DRAWINGS [0007] The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to illustrate various embodiments and to explain various principles and advantages in accordance with present embodiments.

[0008] FIG. 1 depicts a table summarizing dye degradation data for selected doped Bi ₂ O ₃ catalysts.

[0009] FIG. 2 depicts X-ray diffraction (XRD) profiles for pristine Bi ₂ O ₃ and Pd- loaded Bi ₂ O ₃ with 1, 2 and 5 wt% Pd loading in accordance with present embodiments. [0010] FIG. 3, comprising FIGs. 3A and 3B, depicts Fourier transform infrared ( FTIR) absorbance spectra of pristine Bi ₂ O ₃ and Pd-loaded Bi ₂ O ₃ (1, 2 and 5 wt% Pd) in accordance with the present embodiments, wherein FIG. 3A depicts the absorbance spectrum in the range of 400-1200 cm ^-1 and FIG. 3B depicts a magnified spectrum range of 850-890 cm ^-1 .

[0011] FIG. 4, comprising FIGs. 4A to 4E, depicts X-ray photoelectron spectroscopy (XPS) spectra of 2 wt% Pd-loaded Bi ₂ O ₃ in accordance with the present embodiments, wherein FIG. 4 A depicts a survey spectrum, FIG. 4B depicts Gaussian fitting of an O Is core level spectrum, FIG. 4C depicts a Bi 4f spectrum, FIG. 4D depicts a Na 1s spectrum and FIG. 4E depicts Gaussian fitting of a Pd 3d spectrum.

[0012] FIG. 5 depicts six scanning electron microscopy (SEM) representative images of pristine a/b-Bi ₂ O ₃ , 2 wt% Pd-loaded Bi ₂ O ₃ and 5 wt% Pd-loaded Bi ₂ O ₃ in accordance with the present embodiments. [0013] FIG. 6 depicts a graph of UV-Vis absorbance spectroscopy of a/b-Bi ₂ O ₃ and Pd-loaded B1 ₂ O ₃ in accordance with the present embodiments.

[0014] FIG. 7, comprising FIGs. 7A to 7F, depicts UV-Vis spectroscopy graphs of normalized photocatalytic degradation profiles and kinetic plots with degradation rate of BG, MB and AR by pristine B1 ₂ O ₃ and Pd- loaded B1 ₂ O ₃ (1, 2 and 5 wt% Pd), wherein FIGs. 7A and 7B depict the kinetics of BG degradation, FIGs. 7C and 7D depict the kinetics of MB degradation, and FIGs. 7E and 7F depict the kinetics of AR degradation. [0015] FIG. 8 depicts a table of total degradation percentage values after twenty minutes of visible light irradiation and first-order degradation constants for MB, BG and AR dyes for pristine and Pd-loaded B1 ₂ O ₃ at various Pd loadings in accordance with the present embodiments.

[0016] FIG. 9, comprising FIGs. 9A to 9D, depicts plots of photocatalytic degradation with 2 wt% Pd- loaded B1 ₂ O ₃ in accordance with the present embodiments, wherein FIG. 9A depicts the photocatalytic degradation of MB in a MB-AR dyes mixture of 1:1, 2:1, 1:2 volume ratios, FIG. 9B depicts the photocatalytic degradation of AR in the MB-AR dyes mixture of 1:1, 2:1, 1:2 volume ratios, FIG. 9C depicts the photocatalytic degradation of BG in a BG-AR dyes mixture of 1:1, 2:1, 1:2 volume ratios, and FIG. 9D depicts the photocatalytic degradation of AR in the BG-AR dyes mixture of 1:1, 2:1, 1:2 volume ratios.

[0017] FIG. 10, comprising FIGs. 10A to 10C, depicts normalized photocatalytic degradation curves with 2 wt% Pd-loaded Bi ₂ O ₃ at different pH values in accordance with the present embodiments, wherein FIG. 10A depicts normalized photocatalytic degradation curves at different pH values for MB, FIG. 10B depicts normalized photocatalytic degradation curves at different pH values for BG, and FIG. 10C depicts normalized photocatalytic degradation curves at different pH values for AR. [0018] FIG. 11, comprising FIGs 11A to 11C, depicts graphs of normalized photocatalytic degradation curves with 2 wt% Pd-loaded Bi ₂ O ₃ in the presence of carrier scavengers (BQ, EDTA, IP) in accordance with the present embodiments, wherein FIG. 11A depicts normalized photocatalytic degradation curves with Pd- loaded Bi ₂ O ₃ in the presence of carrier scavengers for MB, FIG. 1 IB depicts normalized photocatalytic degradation curves with Pd-loaded Bi ₂ O ₃ in the presence of carrier scavengers for AR, and FIG. 11C depicts normalized photocatalytic degradation curves with Pd-loaded Bi ₂ O ₃ in the presence of carrier scavengers for BG.

[0019] FIG. 12 depicts a graph of XRD profiles for 2 wt% Pd-loaded Bi ₂ O ₃ before and after interaction with MB, AR and BG in accordance with the present embodiments. [0020] FIG. 13 depicts an illustration of relative positions of VB/CB of a- Bi2O3 , b- Bi203, PdO and Pd NPs and HOMO/LUMO of MB, AR and BG dyes derived from the investigations herein in accordance with the present embodiments.

[0021] FIG. 14 depicts an illustration of relative positions of VB/CB of BiVO ₄ , Ag ₂ O and Ag NPs and HOMO/LUMO of cationic BG, RhB, MB and anionic AR and MO dyes derived from the investigations hereinafter in accordance with present embodiments.

[0022] FIG. 15 depicts X-ray diffraction (XRD) profiles for pristine BiVO ₄ and Ag- B1VO ₄ with 2, 5 and 7 wt% Ag loading in accordance with present embodiments. [0023] FIG. 16, comprising FIGs. 16 A to 16D, depicts X-ray photoelectron spectroscopy (XPS) spectra of 7 wt% Ag-BiVO ₄ in accordance with the present embodiments, wherein FIG. 16A is XPS spectra for C Is, FIG. 16B is XPS spectra for Bi 4f, FIG. 16C is XPS spectra for O 1s and V 2p, and FIG. 16D is XPS spectra for Ag

3d. [0024] FIG. 17 depicts a graph of UV-Vis absorbance spectroscopy of pristine monoclinic BiVO ₄ and Ag- BiVO ₄ composites in accordance with the present embodiments.

[0025] FIG. 18, comprising FIGs. 18A to 18D, depicts SEM representative images in accordance with the present embodiments, wherein FIGs. 18A and 18B depict pristine monoclinic BiVO ₄ and FIGs. 18C and 18D depict 7 wt% Ag-BiVO ₄ .

[0026] FIG. 19, comprising FIGs. 19A to 19D, depicts graphs of normalized photocatalytic degradation by pristine BiVO ₄ and Ag- BiVO ₄ (2, 5 and 7 wt% Ag) in accordance with the present embodiments, wherein FIG. 19A depicts for photocatalytic degradation by pristine BiVO ₄ and Ag-BiVO ₄ (2, 5 and 7 wt% Ag) for AR dye and FIG. 19C depicts for photocatalytic degradation by pristine BiVO ₄ and Ag-BiVO ₄ (2, 5 and 7 wt% Ag) for MO dye; FIG. 19B depicts a first-order kinetic plot for AR dye and FIG. 19D depicts a first-order kinetic plot for MO dye.

[0027] FIG. 20 depicts a table of total degradation (%) and k constant values (min- ¹ ) for AR and MO dyes, respectively, in accordance with the present embodiments. [0028] FIG. 21, comprising FIGs. 21 to 2 IF, depicts graphs of normalized photocatalytic degradation by pristine BiVO ₄ and Ag-BiVO ₄ (2, 5 and 7 wt% Ag) for MB dye (FIG. 21 A). BG dye (FIG. 21C) and RhB dye (FIG. 21E) and first-order kinetic plots for MB dye (FIG. 21B), BG dye (FIG. 21D) and RhB dye (FIG. 18F-21F) in accordance with the present embodiments.

[0029] FIG. 22 depicts a table of total degradation (%) and k constant values (min- ¹ ) for MB, BG and RhB dyes, respectively.

[0030] And FIG. 23, comprising FIGs. 23A to 23E, depict graphs of normalized photocatalytic degradation curves with 7 wt% Ag-BiVO ₄ in the presence of scavengers (BQ, EDTA, IP) for dyes in accordance with the present embodiments, wherein FIG. 23A depicts normalized photocatalytic degradation curves in the presence of scavengers for MB dye, FIG. 23B depicts normalized photocatalytic degradation curves in the presence of scavengers for BG dye, FIG. 23C depicts normalized photocatalytic degradation curves in the presence of scavengers for RhB dye, FIG. 23D depicts normalized photocatalytic degradation curves in the presence of scavengers for AR dye, and FIG. 23E depicts normalized photocatalytic degradation curves in the presence of scavengers for MO dye.

[0031] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale.

DETAILED DESCRIPTION

[0032] The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. It is the intent of present embodiments to present a simple sono-chemical method to fabricate Pd-loaded a/b-Bi ₂ O ₃ nanorods with exceptional methylene blue (MB), brilliant green (BG) and acid red 1 (AR) dye degradation properties. The unique nanorod morphology was accomplished in accordance with present embodiments by treating bismuth complexes with sodium hydroxide at room temperature. Palladium (Pd) was introduced by an ultraviolet (UV) photodeposition method from a palladium (II) chloride source. The successful substitution of Bi ³⁺ by Pd ²⁺ in bulk Pd-loaded a/b-Bi ₂ O ₃ was confirmed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The surface presence of α-, b-Bi ₂ O ₃ , PdO and metallic Pd ⁰ was confirmed by X-ray photoelectron spectroscopy (XPS). [0033] The rational design and fabrication approach in accordance with the present embodiments is based on (a) matching valence band (VB) and conduction band (CB) of an as-prepared catalyst with redox potentials of dyes, (b) matching VB and CB with reactive oxygen species (ROS), and (c) matching redox potentials of dyes with ROS. By using the approach of rational design in accordance with the present embodiments, the resultant material is able to degrade cationic MB (90.8%), BG (83.3%) and anionic AR (83.2%) dyes within twenty minutes under light emitting diode (LED) irradiation of 100 mW/cm ² , as well as to degrade mixtures of cationic/anionic MB-AR and BG- AR dyes. Dye degradation parameters, such as initial concentration of dye, amount of catalyst, and solution pH, can be further optimized in accordance with the present embodiments. A mechanism of dye degradation based on ROS scavenging experiments and mapping out of semiconductor band structures with redox potentials of dyes and ROS in accordance with the present embodiments is also presented. Pd-loaded a/b- Bi ₂ O ₃ catalysts in accordance with the present embodiments has potential applications in wastewater treatment, water splitting and healthcare industries.

[0034] While many studies of Bi-based materials are focused on Bi ₂ O ₃ , BiVO ₄ , BiNbO ₄ , Bi2WO ₆ , BiPO ₄ , BiFeO ₃ , Bi ₂ O ₃ is the most promising material due to narrow band gap (2.0-2.8 eV) and unique availability of polymorphs. Bi ₂ O ₃ exists in a monoclinic a stable phase, a tetragonal b high-temperature metastable phase, a body- centered cubic g high-temperature metastable phase, and a face-centered cubic d stable phase. Successful formation of the metastable phases depends on temperature and the applied synthesis method. A solvothermal a/b - Bi ₂ O ₃ synthesis method for 17a- ethynylestradiol has been demonstrated. In addition, conventionally prepared a/b- Bi ₂ O ₃ nanowires that improved degradation of rhodamine B and methyl orange, preparation of a /b- Bi ₂ O ₃ nanowires for photo-degradation of orange G, and degradation of rhodamine B by a/b- Bi ₂ O ₃ nanostructures under natural sunlight have been presented in technical literature. In addition, the nature of enhanced photocatalytic performance of oxide was also investigated by computational tools which, for example, have shown from first-principle calculations that the enhanced photocatalytic performance of a/b- Bi ₂ O ₃ originates from downshift of b- Bi ₂ O ₃ energy bands, resulting in the form the staggered gaps.

[0035] Another approach to tune up the B1 ₂ O ₃ properties is doping by noble metals. While noble metal doping of a/b- Bi ₂ O ₃ has not been discussed, noble metals, such as rhodium (Rh), palladium (Pd), silver (Ag), gold (Au) and platinum (Pt) were introduced as dopants into single-phase Bi ₂ O ₃ structure. FIG. 1 depicts a table which summarizes some quantifiable parameters, relevant to dye degradation, such as type of dye, degradation time, light spectrum, power of the lamp, catalyst, and dye amounts for selected doped-Bi ₂ O ₃ oxides. Instead of acid red 1 (AR), methyl orange (MO) was chosen as both dyes represent family of anionic azoic dyes and little published material was found on degradation of AR by doped-BCCF.

[0036] hi accordance with the present embodiments, the above-mentioned methods are combined and a new Pd-loaded a/b - Bi ₂ O ₃ p-n heterojunction photocatalyst is prepared by a sono-chemical method. Further, a rational design approach in accordance with the present embodiments advantageously enables prediction of dye degradation properties and minimization of the development cost of photocatalyst. Palladium- loaded a/b - Bi ₂ O ₃ nanorods are designed by (a) matching VB and CB of as-prepared catalyst with redox potentials of dyes, (b) matching VB and CB with reactive oxygen species (ROS), and (c) matching redox potentials of dyes with ROS. The obtained materials were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and X-Ray photoelectron spectroscopy (XPS). In addition, their photocatalytic properties were examined by degradation of cationic and anionic dyes (methylene blue (MB), acid red 1 (AR) and brilliant green (BG)) monitored by UV-Vis spectroscopy. The catalyst developed in accordance with the present embodiments demonstrated remarkable dye removal properties (within 20 min degradation of dyes). Moreover, the material exhibited an ability to degrade a mixture of cationic-anionic dyes (MB-AR and BG-AR). For the purpose of simplicity hereafter in this application, Bi ₂ O ₃ is used instead of a/b-Bi ₂ O ₃ .

[0037] Bismuth nitrate pentahydrate (Bi(NO ₃ ) ₃ -5H ₂ O, 98%, Sigma- Aldrich) was use as a precursor to provide a bismuth source. Palladium chloride (PdCl ₂ ), also from Sigma- Aldrich, was used as a source of palladium. Sodium hydroxide (NaOH) and acetic acid (CH ₃ COOH) were used for pH adjustment and obtained from Alfa Aesar and Scharlau, respectively. Ethylenediaminetetraacetic acid (C ₁₀ H ₁₆ N ₂ O ₈ , 98%, Sigma- Aldrich), isopropanol (C ₃ H ₈ O, 99.5%, Sigma-Aldrich) and benzoquinone (C ₆ H ₄ O ₂ , 98%, Sigma-Aldrich) were used as scavengers for holes, •OH radicals and electrons, respectively. Ethanol (CH ₃ CH ₂ OH, 96%, Sigma-Aldrich) and deionized water (DI, 50 KW) were used throughout experiments as solvents. Methylene Blue (dye content ³ 82%), Brilliant Green (dye content ³ 95%) and Acid Red 1 (dye content 60%) were obtained from Sigma-Aldrich.

[0038] The nanosized Bi ₂ O ₃ synthesis was accomplished by a sono-chemical method involving the following four chemical reactions: Initially, 5.0 g of bismuth nitrate pentahydrate is added into 100 mL of DI water and stirred for 60 min to disperse it (reaction (1)). Once a white solution was obtained, sodium hydroxide (NaOH) was added stepwise to adjust the pH level to 10 (reaction (2)). The mixture was vigorously stirred for 120 min and exposed to sonication (60 min) to achieve an orange precipitate. The solution was stirred overnight and collected by centrifuge. The obtained powder was annealed at 300 °C (at a 2°C/minute heating rate) for 120 min and naturally cooled down to obtain a/b-Bi ₂ O ₃ nanosized rods (reactions (3) and (4)). The orange powder was collected and stored under room temperature.

[0039] The role of NaOH during synthesis is crucial as it appears to play a role for the material’s morphology. Bismuth complexes reacted with NaOH to form Bi(OH) ₃ nuclei, then grew into nanocrystals governed by the O s tw aid-ripening process. Eventually, crystal growth nanocrystals were bound to the enhanced number of hydroxyl groups from the NaOH source to form nanorods.

[0040] The Pd-loaded a/b-Bi ₂ O ₃ nanorods were synthesized by a photo-deposition method. In this method, 0.1 g Bi ₂ O ₃ was dispersed in one ml ethanol and a corresponding amount of palladium chloride was added to obtain to 1, 2 and 5 wt% Pd, respectively. The obtained solution was stirred and exposed to UV light (14 mW/cm ² intensity) for 180 min to reduce Pd ²⁺ ions into metal Pd ⁰ . After UV light exposure, ethanol was evaporated (100 °C) and the as-prepared powder was used for further dye degradation experiments.

[0041] X-ray diffraction (XRD) measurements were conducted on a Bruker D8 Advance X-ray diffractometer with Cu Ka radiation of 0.154 nm wavelength to evaluate powder’s composition and phase. The scanning angle was adjusted from 20 °C to 60 °C. UV-Vis reflectance spectra were recorded using a PG Instruments TIIO+ UV- Vis spectrophotometer. Surface structure and morphology were examined by a scanning electron microscopy (SEM) JEOL JSM-7600F. The elemental composition of elements was evaluated by energy-dispersive X-ray spectroscopy (EDX). In order to identify the chemical composition _* Fourier-transform infrared spectroscopy (FTIR) spectra were obtained from 300 cm ^-1 to 3800 cm ^-1 by a Vertex 70 (Bruker). In order to investigate phase structure and chemical composition of synthesized materials, X-ray photoelectron spectroscopy (XPS) was recorded by a Thermo Fischer Scientific Theta Probe system. The powder calcination was performed in a box furnace (Anhui Haibei 1100 model). Photoluminescence (PF) tests (254 nm excitation wavelength) were conducted by a QE Pro spectrofluorometer (Ocean Optics, USA). A Prizmatix Ultra High Power FED lamp (48 W) was used as a light source during dye degradation experiments. The light intensity was recorded by an Optical Power Meter PM100D from Thorlabs.

[0042] The photocatalytic evaluation was carried out for MB, AR and BG dyes in an aqueous solution under visible light irradiation of 100 mW/cm ² . During a typical experiment, 15 mg of catalyst were dispersed in 15 mL of water (1 g L ^-1 concentration) and sonicated for 1 hour. After that, the solution was mixed with dye with a concentration of 1 g L ^-1 and kept in the dark for thirty minutes for dye molecules to adsorb on the catalyst’s surface. At the next step, the solution was transferred into 1.5 mL centrifuge tubes and the UV lamp was switched on. Every 5 min 1 mL of the solution was collected, centrifuged (5 min) and transferred to the cuvette for the UV- Vis absorption measurements.

[0043] In order to investigate phase structure and chemical composition of synthesized materials, the a /b-Bi ₂ O ₃ and Pd-loaded Bi ₂ O ₃ were analyzed by X-ray diffraction (XRD) as shown in FIG. 2. FIG. 2 depicts XRD profiles 100 for pristine BbCT and Pd-loaded Bi ₂ O ₃ with 1, 2 and 5 wt% Pd loading. A right panel 150 displays the peak associated with PdO (002) and the a (120) peak as the Pd loading increases. [0044] The main 2q peaks of monoclinic a-Bi ₂ O ₃ are21.97°, 24.59°, 25.82°, 26.11°, 28.03°, 33.4°, 46.35°, 52.22°, 53.25°, and 54.49° are consistent with (020), (102), (002), (111), (012), (200), (041), (-321), (124), and (241) lattice planes, respectively. The main 2q peaks of tetragonal b-Bi ₂ O ₃ are 30.25°, 31.83°, 54.81°, 55.65°, and 55.91° which are assigned to (211), (002), (203), (421), and (402), respectively. These results are consistent with previously reports. However, the major (120) peak ascribed to a- Bi ₂ O ₃ has a noticeable shift towards a lower angle as compared to such reports. The angle decreases from 27.39° to 26.96° and can be explained by the presence of Na from NaOH source during synthesis. As a result, Na ¹⁺ with a larger ionic radius (113 pm) tends to substitute for Bi ³⁺ which has a smaller ionic radius (100pm), the crystal lattice expands, and the (120) peak shifts to the lower angle.

[0045] With increased Pd loading, the small diffraction peak at 33.6° is observed, which corresponds to the (002) lattice plane and indicates deposition of PdO on theBi ₂ O ₃ surface. Furthermore, with increased Pd content, the slight angle shift of the (120) peak towards a higher angle occurs (from 26.96° to 27.18°, 27,18° to 27.23° and 27.23° to 27.31° for 1, 2 and 5 wt% Pd loading, respectively) indicating the successful introduction of Pd ²⁺ (ionic radius 86 pm) instead of Bi ³⁺ /Na ¹⁺ into the crystal lattice, which is also supported by the FTIR analysis as shown in FIG. 2 and discussed below. [0046] Referring to FIGs. 3 A and 3B, Fourier transform infrared ( FTIR) absorbance spectra 200, 250 of pristine Bi ₂ O ₃ and Pd-loaded Bi ₂ O ₃ (1, 2 and 5 wt% Pd) in accordance with the present embodiments are depicted. The FTIR absorbance spectra 200 (FIG. 3A) is a FTIR absorbance spectra of pristine and Pd-loaded Bi ₂ O ₃ were recorded in the range of 400-1200 cm ^-1 . Separate peaks at 864 and 879 cm ^-1 are shown in the FTIR absorbance spectra 250 (FIG. 3B). The peak at 515 cm ^-1 is very pronounced and assigned to the stretching vibration of Bi-O. The peak at 864 cm ^-1 is visible for Bi ₂ O ₃ and NaOH but disappears and shifts towards higher wavenumber to 879 cm ^-1 for Pd-loaded Bi ₂ O ₃ . Here, the peak shift can be attributed to a decrease in bond length: while a Na-0 bond has 0.278-0.283 nm length (864 cm ^-1 ), the Pd-0 has 0.2 nm bond length (879 cm ^-1 ). The peak at 1060 cm ^-1 exhibits a weak signal for Bi ₂ O ₃ but increases with Pd content. It has been attributed to Bi-0 vibrations due to interaction between Bi-0 bonds and its surrounding environment. The bond lengths for Bi-0 and Pd-0 are comparable (~0.2-0.22 nm vs. 0.2 nm), so the increased peak intensity can be ascribed to the increased number of Pd-0 bonds. In summary, the FTIR absorbance spectra 200, 250 suggest the successful substitution of Bi ³⁺ /Na ¹⁺ by Pd ²⁺ in bulk of Pd-loaded Bi ₂ O ₃ . [0047] X-ray photoelectron spectroscopy (XPS) analysis was canned out to investigate composition and chemical states of pristine and 2 wt% Pd-loaded Bi ₂ O ₃ . Referring to FIGs. 4A to 4E, X-ray photoelectron spectroscopy (XPS) spectra of 2 wt% Pd-loaded Bi ₂ O ₃ in accordance with the present embodiments are depicted. FIG. 4A depicts a survey spectrum 300, FIG. 4B depicts Gaussian fitting of an O Is core level spectrum 320, FIG. 4C depicts a Bi 4f spectrum 340, FIG. 4D depicts a Na Is spectrum 360, and FIG. 4E depicts Gaussian fitting of a Pd 3d spectrum 380. The survey spectra 300 confirms the presence of Bi 4f, O Is, Na Is and Pd 3d in 2 wt% Pd-loaded Bi ₂ O ₃ . Gaussian fitting of the O ls core level spectrum 320 displays three peaks centered at 529.8 eV, 531.6 eV and 533.0 eV. The largest peak at 531.6 eV is generally attributed to Bi-0 bonding. The second largest peak at 529.8 eV can be ascribed to Pd-0 bonding. The peak at 533.0 eV is characteristic of adsorbed H ₂ O. The peak at the highest energy (536.0 eV) is a typical Na Auger overlap with O ls. The Bi 4f spectrum 340 (FIG. 3G 4C) comprises two peaks at 164.4 eV and 159.1 eV. These peaks can be assigned to Bi 4f _5/2 and Bi 4f _7/2 of Bi ³⁺ (Bi ₂ O ₃ ), respectively. The Na Is peak 360 (FIG. -3D-4D) is centered at 1071.5 eV and generally attributed to Na ⁰ state, indicating UV light reduction during synthesis of Na ¹⁺ to Na ⁰ . Gaussian fitting of Pd 3d spectrum 380 (FIG. TE-4E) has two symmetrical peaks at 340.7 eV and 335.5 eV, corresponding to Pd 3d _3/2 and Pd 3d _3/2 state. The peak values are in a good agreement for metallic Pd ⁰ . The lower intensity peaks at 342.0 eV and 337.3 eV correspond to PdO oxide. Thus, UV light ejected photoelectrons (during synthesis) are able to reduce the major part of (a) Pd ²⁺ to metallic Pd ⁰ and (b) Na ¹⁺ from NaOH source to Na ⁰ .

[0048] In summary, XRD and FTIR techniques confirm the substitution of Bi ³⁺ /Na ¹⁺ by Pd ²⁺ in bulk of Pd-loaded Bi ₂ O ₃ . XPS analysis suggests the presence of metallic Pd ⁰ , PdO and Na ⁰ on the surface of Bi ₂ O ₃ .

[0049] Surface morphology of the synthesized samples was examined by Scanning Electron Microscopy (SEM). FIG. 5 depicts six scanning electron microscopy (SEM) representative images 400, 410, 430, 440, 460, 470 in accordance with the present embodiments. SEM images 400, 410 depict pristine a/b- Bi ₂ O ₃ . SEM images 430, 440 depict 2 wt% Pd-loaded Bi ₂ O ₃ . And SEM images 460, 470 depict 5 wt% Pd-loaded Bi ₂ O ₃ . It can be observed that the samples possess rod-like structure and have a length of 400-500 nm and diameter of 100 nm. Pd particles exhibit sheet- and sphere-like morphology and are evenly distributed across the Bi ₂ O ₃ surface. The Pd particle size varies from a few nanometers up to 20 nm.

[0050] Optical band gap and absorbance properties are important parameters controlling photocatalytic degradation kinetics. Referring to FIG. 6, a graph 500 depicts UV-Vis absorbance spectroscopy of a /b- Bi ₂ O ₃ and Pd-loaded Bi ₂ O ₃ in accordance with the present embodiments. An inset 550 of the graph 500 shows band gap values based on extrapolation of the linear segment of (ahv) ² versus hv for a/b- Bi ₂ O ₃ and Pd-loaded Bi ₂ O ₃ in accordance with the present embodiments. The absorbance of pristine Bi ₂ O ₃ and Pd-loaded Bi ₂ O ₃ as the function of wavelength is shown in the graph 500. The corresponding band gap based on Kubelka-Munk equation is illustrated in the inset 550. It can be seen that for all samples the absorption edge is significantly shifted towards the visible light and located in the 585-626 nm range. The absorption threshold shifts from a yellow colour for Bi ₂ O ₃ to orange for 1 wt% and 5 wt% Pd loading and to red for 2 wt% Pd loading.

[0051] The largest band gap of 2.12 eV is attained to Bi ₂ O ₃ and with increased amount of dopant the band gap value drops to 1.98 eV for 2 wt% Pd-loaded Bi ₂ O ₃ . There are many factors contributing to the change in band gap energy. First, dopant in bulk and on the surface of catalyst can act as shallow level impurities that create energy levels in the band gap near the conduction band edge and shallow acceptor defects near the valence band edge, respectively. Higher amount of dopant leads to an increase in density of states, the continuum of states is formed, and band gap shrinks. Second, as Pd is a noble metal, the red shift can be explained by the surface plasma resonance (SPR) effect. Other factors include lattice strain or compression, surface effects, and amount of impurities.

[0052] Dyes serve as prototyping pollutants, mimicking real waste water treatment situations. Approximately 70% of all the dyes used in the textile industry can be ascribed to azoic dyes (with azo bond linkage - N = N -). Inside azoic groups, dyes can be classified as cationic and anionic dyes. Thus, cationic azo dyes (MB and BG) and anionic azo dye (AR) were selected to be treated with Pd-loaded Bi ₂ O ₃ catalyst in aqueous solution under LED light irradiation. The degradation kinetics was monitored by UV-Vis spectroscopy. [0053] Referring to FIGs. 7 A to 7F, UV-Vis spectroscopy graphs 600, 610, 620, 630, 640, 650 depict normalized photocatalytie degradation profiles and kinetic plots with degradation rate of BG, MB and AR by pristine Bi ₂ O ₃ and Pd-loaded Bi ₂ O ₃ (1, 2 and 5 wt% Pd), wherein the graphs 600, 610 depict the kinetics of BG degradation, the graphs 620, 630 depict the kinetics of MB degradation, and the graphs 640, 650 depict the kinetics of AR degradation. Insets 615, 635, 655 display the UV-Vis absorption spectra of 2 wt% Pd-loaded Bi ₂ O ₃ for BG, MB and AR, respectively. Error bars in the graphs 600, 610, 620, 630, 640, 650 are based on the standard deviation for n = 3. [0054] Kinetics of BG degradation are shown in the graphs 600, 610, where C ₀ is the concentration of BG prior to LED light exposure and C is the concentration after the test. The main absorption band is centered at 619 nm and exhibits gradual decrease within twenty minutes. The lowest degradation rate is attained to pristine Bi ₂ O ₃ (58.3 ± 3.0 %) and with higher amounts of dopant the reaction rate gradually increases. The highest degradation rate is attained to 5 wt% of Pd (83.3 ± 3.1 %), followed by 2 wt% Pd (79.5 ± 3.2 %) and by 1 wt% Pd (71.3 ± 3.6 %).

[0055] Interestingly, the percentage of Pd loading does not dramatically affect the degradation rate of BG. The major difference is observed between Bi ₂ O ₃ and 1 wt% Pd loading at 13.0 % while with higher Pd loading the degradation rate slows down to 8.2% and to 3.8% (difference between 2 wt% and 1 wt% of Pd and between 5 wt% and 2 wt% of Pd, respectively). The optimized amount of dopant acts as a trap for charge carriers and prevents electron-hole recombination. However, over the concentration threshold, the active sites of catalyst are blocked, photon absorbance rates are reduced and quantum yield decreases or slows down.

[0056] Another important factor to consider is a BG degradation pathway. In general, any dye-semiconductor interaction follows this path: (a) transfer of dye molecules to the surface of a catalyst; (b) adsorption of molecules on the catalyst’s surface; (c) photoeatalytic reaction; (d) desorption of the intermediates from the surface; and (e) transfer of intermediates to the bulk liquid. As step (c) (photoeatalytic reaction) is the focus, possible degradation mechanisms for each dye is considered. It is known that BG degrades in five steps including (i) A'-dcmcthylation reactions (removal of -CH ₃ from N=N); (ii) the adduct reaction and the cleavage of the central carbon with the bond between C ₁ -C ₄ ; (iii) removal of benzene; (iv) open ring reaction and (v) mineralization process. The breakdown leads to the formation of small organic molecules and inorganic ions. Alternatively, BG can undergo degradation due to cleavage of the whole chromophore structure (color-bearing groups). It is also known that A-demethylation reaction is accompanied with hypsochromie shift of the main peak at 619 nm towards blue region. No significant shift of the major peak was observed, which is evidence of BG chromophore destruction and, as a result, degradation.

[0057] The best MB degradation kinetics in the graphs 620, 630 was ascribed to 2 wt% Pd loading (90.8 ± 3.2 %) followed by 5 wt% Pd (81.2 ± 3.0 %), 1 wt% Pd (57.7 ± 3.3 %) and pristine Bi ₂ O ₃ (51.2 ± 2.4 %), respectively. Unlike BG, MB degradation was observed with a strong blue shift from 661 nm to 629 nm in a period of twenty minutes, respectively. The largest shift of 19 nm was recorded between zero and five minutes indicating iV-demethylation and generation of intermediates. It is possible to hypothesize that a major attack of reactive oxygen species on methyl group occurs within five minutes after dye-semiconductor exposure to LED light. It has been reported that hydroxyl radicals ( ^. OH) and holes are the major species to attack methyl groups (weak electron-donor substituents) during degradation of MB.

[0058] The best decomposition rate of AR is attained to 2 wt% and 5 wt% Pd with 83.2 ± 3.3 % and 76.3 ± 3.4 %, respectively, as shown in the graphs 640, 650. The addition of 1 wt% Pd to Bi ₂ O ₃ does not significantly affect degradation rate with low- to-moderate efficiency (41.1 ± 2.9 % and 38.4 ± 2.9 % for 1 wt% Pd- Bi ₂ O ₃ and pristine, respectively). Anionic AR dye belongs to the azo group of dyes with the main peak centered at around 525 nm. This is the characteristic absorption of a conjugated electronic structure of azo group and hydrazonc form as a predominant species, where the hydroxyl group appears as a carbonyl group and its hydrogen is linked to another group. Fast AR degradation can be ascribed to the destruction of the reactive conjugated electronic structure -C-N=N-C- connecting the two aromatic groups where the main attack is facilitated by •OH radicals. Eventually, the attack leads to formation of toxic compounds (intermediates and carboxylic acids) and NH ₄ ⁺ , NO ₃ - and SO ₄ ^2- ions during AR degradation.

[0059] Overall, the photocatalytic properties of Pd-loaded Bi ₂ O ₃ nanorods towards degradation of MB, BG, and AR dyes over performs existing materials summarized in the table of FIG. 1 hereinbefore. Degradation rate and corresponding first-order degradation constants for all three dyes are summarized in the table of FIG. 8. Average and standard deviation values are based on n = 3 measurements and the k constant value reported is the highest constant obtained.

[0060] The dye degradation tests were carried out in the presence of 2 wt% Pd- loaded Bi ₂ O ₃ and the mixture of cationic-anionic dyes (MB-AR and BG-AR) under visible light. Referring to FIGs. 9A to 9D, plots 700, 720, 740, 760 of photocatalytic degradation with 2 wt% Pd- loaded Bi ₂ O ₃ in accordance with the present embodiments are depicted. The plot 700 depicts the photocatalytic degradation of MB in a MB-AR dyes mixture of 1:1, 2:1, 1:2 volume ratios and the plot 720 depicts the photocatalytic degradation of AR in the MB-AR dyes mixture of 1:1, 2:1, 1:2 volume ratios. The plot 740 depicts the photocatalytic degradation of BG in a BG-AR dyes mixture of 1:1, 2:1, 1:2 volume ratios and the plot 760 depicts the photocatalytic degradation of AR in the BG-AR dyes mixture of 1 : 1 , 2: 1 , 1:2 volume ratios. Error bars are based on the standard deviation from n = 3 measurements.

[0061] It can be observed in the plot 720 that for the MB-AR dye mixture the degradation rate of AR is weakly affected by the variation in the volume ratio (77.4 ± 3.1 % - 82.8 ± 3.4 % after thirty minutes of irradiation). On the other hand, the best MB decomposition rate was influenced by the presence of AR and attained 77.3 ± 3.3 %, followed by 64.2 ± 2.5 % and 44.7 ± 2.7 % for 1:2, 1:1 and 2:1 (MB-AR), respectively. The results of the BG-AR mixture of dyes are shown in the plots 740, 760. Unlike the MB-AR mixture, the degradation of both BG and AR dyes is heavily dependent upon volume ratio. Superior results for BG were ascribed to 1:2 and 1:1 (BG-AR) volume ratio with 89.9 ± 2.9 % and 87.2 ± 2.9 % rate, respectively. The highest AR degradation rate was found to be 98.6 ± 3.7 % (BG-AR 2:1), followed by 74.3 ± 3.6 % (BG-AR 1:1) and 63.4 ± 2.9 % (BG-AR 1:2) mixture of dyes, respectively. [0062] In real wastewater environments, it is crucially important to deal with different pH levels. As a result, the effect of different pH on photocatalysts ’ dye degradation capabilities as a function of reaction time is an important factor. FIGs. 10A to 10C depict normalized photocatalytic degradation curves 800, 830, 860 with 2 wt% Pd-loaded Bi ₂ O ₃ at different pH values in accordance with the present embodiments. The curves 800 depict normalized photocatalytic degradation curves at different pH values for MB. The curves 830 depict normalized photocatalytic degradation curves at different pH values for BG. And the curves 860 depict normalized photocatalytic degradation curves at different pH values for AR. Error bars are based on the standard deviation for n = 3 measurements. [0063] Degradation of MB pollutant by 2 wt% Pd- Bi ₂ O ₃ at different pH values is shown in the plots 800. It can be seen that pH levels from 7 to 12 are favorable for degradation of MB and the values of 90.8 ± 3.2 %, 83.5 ± 3.6 % and 69.5 ± 2.7 % stand for pH = 7, 10 and 12, respectively. Such dependency is not easy to explain by using only the well-established approach that in alkaline solutions semiconductors’ surfaces are negatively charged due to presence of OH-. It is a complex interaction that includes (i) the nature of the dye (cationic, anionic); (ii) the pH environment - acidic, alkaline or neutral; (iii) the type of catalyst (n-type or p-type) and, therefore, the predominant charge carriers (electrons or holes); (iv) the presence of nanojunctions; (v) the band- gap of the dyes and the participating catalysts; (vi) the redox values for valence band (VB), conduction band (CB), highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO) for the catalysts and the dyes, respectively; (vii) the formation of reactive oxygen species and their redox values; and (viii) other factors, such as quantum confinement effect, if nanoparticles (NPs) (e.g., Pd) are presented. [0064] MB is a cationic dye and the higher presence of OH- (higher pH) causes faster dye degradation due to electrostatic force. Additionally, as can be seen from scavengers experiment and diagram described hereinafter (see FIGs. 9A to 9C and FIG. 11), MB degradation is driven mainly by electrons which can migrate from CB of n-type b- Bi ₂ O ₃ to LUMO of MB. Consequently, the synergetic effect of electrons and OH induces the formation of other reactive species (like, •OH and O2 ). The lowest MB degradation rate values were observed to pH = 4 and pH = 2 (48.6 ± 1.9 % and 20.3 ± 0.9 %, respectively), which contribute to efficient dye degradation.

[0065] The necessary condition for degradation of any dye is an adsorption of dye molecules on the photocatalysts’ surface. If the surface is only positively charged (due to H ⁺ in acidic environment) such interaction cannot occur due to repulsion forces which result in extremely low degradation efficiency. One of the confirmation of this hypothesis is the absence of blue l _max peak shift towards lower wavelength (it is fixed on 661 nm which is not shown in the plots 800) as blue shift is known as a sign of intermediates generation during interaction of dye molecules and catalyst.

[0066] The alkaline environment is also preferred by cationic BG pollutant and results are shown in the plots 830. The best degradation rate is attained for pH = 12, pH = 10 and pH = 7 at rates of 93.5 ± 3.4 %, 86.3 ± 3.1 % and 79.4 ± 3.3 %, respectively. Unlike MB, the degradation rate of BG is more dependent on the higher presence of OH- in the solution, which manifests the major role of •OH in the degradation process. [0067] The degradation kinetics of anionic AR is the fastest in neutral and acidic environments as shown in the plots 860. It exhibits the rate of 83.2 ± 3.3 %, 79.3 ± 3.5 % and 78.7 ± 3.0 % for pH = 7, pH = 4 and pH = 2, respectively. As the degradation rate was not significantly changed, it is hypothesized that H ⁺ plays a minor role in the acidic environment. The main degradation mechanism is based on holes from VB (p- type ot-BUOs). H ⁺ ions from the acidic environment are repulsive to holes and, therefore, do not participate in further reactions to generate reactive oxygen species •OH and H ⁺ . As a result, AR degradation rate does not change with higher acidity. The negligible degradation of AR was observed for alkaline pH = 10 and pH = 12 (25.4 ± 2.5 % and 20.5 ± 2.1 %, respectively) and ascribed to the large amount of OH- ions. It can be concluded that cationic MB and BG dyes favour neutral and alkaline environments for faster degradation, while anionic AR dye “prefers” neutral and acid conditions, which is in line with previous works.

[0068] The mechanism of dye degradation was investigated by adding p- benzoquinone (BQ), EDTA and isopropanol (IP) as scavengers to capture electrons, holes and •OH radicals, respectively. Dye degradation kinetics in the presence of 2 wt% Pd-loaded Bi ₂ O ₃ and without scavengers are shown for comparison purposes in FIGs. 11 A, 11B and TIC. FIGs. IIA to 11C depictgraphs 900, 930, 960 of normalized photocatalytic degradation curves with 2 wt% Pd-loaded Bi ₂ O ₃ in the presence of carrier scavengers (BQ, EDTA, IP) in accordance with the present embodiments. The graph 900 depicts normalized photocatalytic degradation curves with Pd-loaded Bi ₂ O ₃ in the presence of carrier scavengers for MB, the graph 930 depicts normalized photocatalytic degradation curves with Pd-loaded Bi ₂ O ₃ in the presence of carrier scavengers for AR, and the graph 960 depicts normalized photocatalytic degradation curves with Pd-loaded Bi ₂ O ₃ in the presence of carrier scavengers for BG. Error bars are based on the standard deviation for n=3 measurements.

[0069] Decomposition of MB shown in the graph 900 is primarily governed by electrons (21.0 ± 1.9 %) and •OH radicals (23.5 ± 1.9 %) with lesser contribution of holes (39.2 ± 2.0%). For anionic AR shown in the graph 930, the reaction almost completely stops when holes are taken out of the system (10.5 ± 1.5 % degradation rate). Holes contribute about 80% of the total degradation rate. The exclusion of electrons and •OH radicals does not significantly influence photocatalytic reaction with

71.1 ± 2.9 % and 78.0 ± 3.7 % for BQ and IP, respectively.

[0070] Degradation of BG is shown in the graph 960. It decomposes similarly to MB with major participation of electrons and •OH radicals (26.2 ± 2.3 % and 29.4 ±

2.1 %, respectively), followed by holes (40.8 ± 2.1 %). It can be concluded, that for cationic MB and BG pollutants, electrons and •OH radicals play the major role in catalytic reaction, while AR dyes degrade due to contribution of holes.

[0071] For post-dye interaction analysis with MB, BG and AR, the 2 wt% Pd-loaded B 126)3 composite was centrifuged and removed from dye-water suspension, washed with ethanol and dried under 120°C for 10 hours. Compared to the original material, the used powder exhibited a brighter colour. FIG. 12 depicts a graph 1000 of XRD profiles for 2 wt% Pd-loaded Bi ₂ O ₃ before and after interaction with MB, AR and BG in accordance with the present embodiments. The XRD analysis, shown in the graph 1000, revealed the formation of bismuth subcarbonate ( CBi ₂ O ₅ ). Four major very pronounced peaks were found which could be matched to CBi ₂ O ₅ . These peaks are (011), (013), (110) and (020) corresponding to 23.87°, 30.19°, 32.69° and 46.93°, respectively.

[0072] Post-dye interaction morphology of 2 wt% Pd-loaded Bi ₂ O ₃ catalyst was further investigated by SEM, Post-dye morphology of the catalyst did not change significantly whereby nanorods were aggregated into larger structures of a few micrometres. Separate nanorods were 50-100 nm in diameter and 400-500 nm in length which is comparable to the original composites in the SEM images of FIG. 5.

[0073] Further analysis of structural composition of catalyst before and after interaction with dyes was carried out by EDX. Due to limitations of the EDX technique, it is difficult to make reliable conclusions, however, the general trend indicates a decreased presence of Na and Pd after photocatalytic reactions. No residual species from dyes were found. The formation of CBi ₂ O ₅ is of interest due to its antibacterial applications against Helicobacter Pylori and potential involvement in development of new nanodrugs.

[0074] Referring to FIG. 13, an illustration 1100 depicts relative positions of VB/CB of a-Bi203, b-Bi203, PdO and Pd NPs and HOMO/LUMO of MB, AR and BG dyes derived from the investigations herein in accordance with the present embodiments. The values of VB/CB vs. NHE of a- Bi ₂ O ₃ , b- Bi ₂ O ₃ and Pd NPs (no exact value but relative position to VB/CB of b- Bi ₂ O ₃ ) are available in the literature. The band gap of PdO is known and electronegativity value was calculated from the Mulliken theory. Consequently, corresponding E (VB) and E (CB) of PdO were found. HOMO/LUMO values of MB, AR and BG are also available in the literature. Finally, the reduction potentials of O ₂ , H ₂ O ₂ , O ₂ - and •OH were found from these sources.

[0075] In reality, the kinetics of dye degradation is affected by a plethora of factors. Herein, a few of them are considered, like (a) ROS generation; (b) HOMO/LUMO of dyes with respect to the position of VB and CB of catalyst; (c) Self-sensitization mechanism and the formation of Schottky barrier. All the redox potentials of ROS, VB and CB values of catalysts and HOMO/LUMO numbers as well as the diagram are shown in the illustration 1100.

[0076] ROS generation is heavily affected by the relative positions of VB and CB and their match to reduction potentials of oxygen species. The proposed sequence of chemical reactions is based on the value of reduction potential (from most negative to most positive vs. Normal Hydrogen Electrode (NHE). As all the dyes are stable under ambient light, their charge carriers are excluded from further ROS analysis.

[0077] The ionization reaction of O ₂ has a reduction potential of -0.33 eV. As this value is far from VB and CB values of all participating catalysts, the reaction is excluded from further analysis. [0078] The production of hydrogen peroxide (H ₂ O ₂ ) reaction has a reduction potential of 0.32 eV. Holes (h ⁺ ) from p-type a- Bi ₂ O ₃ and PdO can both participate in the reaction and yield the large amount of H ₂ O ₂ . The reaction has 2.3 eV reduction potential. As electrons (e ) from CB of n-type b- Bi ₂ O ₃ (0.23 eV) and Pd NPs (no exact value but the relative position of VB/CB is close to VB/CB of b-Bi ₂ O ₃ ) can participate in the reaction, the amount of •OH is produced. However, due to travel distance between CB (0.23 eV) and redox value of •OH (2.3 eV), the generated amount of •OH is relatively small.

The reaction has reduction potential of 0.94 eV which makes it accessible for h ⁺ migration from p-type a-Bi ₂ O ₃ and PdO.

[0079] This reaction has 0.32 eV reduction potential and e- from CB of n-type b- Bi ₂ O ₃ and Pd NPs can interact with H ₂ O ₂ . As CB of both catalysts are located close to the redox value, a large amount of H ₂ O ₂ is produced.

[0080] The reaction has 2.3 eV reduction potential. It results in interaction of h ⁺ from VB a-Bi ₂ O ₃ and PdO. As both VB values of PdO and a-Bi ₂ O ₃ are located close to the redox value of •OH, a large number of •OH is produced.

[0081] To sum up, both electrons and holes participate in ROS generation but in different aspects. Holes from reactions (6), (8), and (10) produce a large number of hydrogen peroxide (H ₂ O ₂ ), superoxide anions (O ₂ -) and hydroxyl radicals (•OH), respectively. Electrons from reactions (7) and (9) induce the production of •OH and H ₂ O ₂ , respectively. However, electrons have to travel a long distance to produce •OH (reaction (7)) and the final yield is relatively small. The generated amount of H ₂ O ₂ (reaction (9)) is large due to participation of electrons from both n-type b-Bi ₂ O ₃ and Pd NPs. If electrons are removed from the system it should inevitably decrease the amount of H ₂ O ₂ and •OH. [0082] The dye degradation mechanism can also be approached by matching HOMO/LUMO values of dyes and VB/CB of catalysts. From FIGs. 9A, 9B and 9C, it was shown that degradation of MB and BG dyes is primarily governed by electrons (graphs 900, 960), while degradation of anionic AR is governed by holes (graph 930). MB and BG cationic dyes have LUMO redox potentials of 0.065 eV and 0.04 eV, respectively, which is close to the CB value of n-type b-B 120.3 (0.23 eV). As a result, electrons can freely flow from the CB of the catalyst and participate in degradation of MB and BG. Another factor is n-type Pd NPs, which has a CB value of 0.54 eV and can potentially contribute to degradation of cationic dyes (it is known that during the degradation process the pH level of catalytic system changes can affect LUMO level of MB and it shifts to match the CB level of Pd NPs). Anionic AR has a HOMO value of 2.14 eV, which makes it very accessible for holes from VB of p-type a-Bi ₂ O ₃ (2,88 eV) and PdO (2.79 eV)

[0083] Another contributing factor for AR degradation is a possibility of a self- sensitization mechanism. It involves a few steps. First, dye molecules are adsorbed on the photocatalyst surface, visible light is absorbed from a LED source and electrons from LUMO of AR migrate to the CB of the semiconductor. Secondly, due to lost electrons, dye molecules become a positively charged radical. This unstable dye molecule can be easily attacked by ROS (for example, O ₂ - ). Simultaneously, electrons can react with oxygen from reaction (5) to form O ₂ -. Eventually, H ₂ O ₂ are generated through reaction (9) which is converted to •OH.

[0084] Also, a factor to consider is the formation of a Schottky barrier between b- Bi ₂ O ₃ and Pd/PdO nanostructures. As the CB of b-Bi ₂ O ₃ is more negative than the CB of Pd and PdO nanostructures, electrons from the CB of b-Bi ₂ O ₃ can quickly migrate towards the CB of Pd and PdO nanostructures through the Schottky barrier at the metal- photocatalyst interface. Consequently, those electrons can be recombined with holes from p-type PdO, inhibit electron-hole recombination in b-Bi ₂ O ₃ and contribute to the formation of ROS.

[0085] Finally, it can be concluded, that degradation of cationic MB and BG dyes rely on a matching strategy of CB of n-type b-Bi ₂ O ₃ and Pd NPs with dyes’ LUMO values. Rapid degradation of anionic AR also follows this matching approach (VB of p-type a-Bi ₂ O ₃ and PdO and dye’s HOMO value) with an additional contributing factor of self-sensitization mechanism.

[0086] The rational design strategy described hereinabove predicts the efficiency of dye degradation process based on (a) the mapping of a catalysts’ energy bands with reactive oxygen species (ROS) redox potentials, (b) the mapping of ROS redox potentials with molecular orbitals of dyes, and (c) the mapping of molecular orbitals with energy bands of dyes and catalysts, respectively. Based upon the proposed chemical reactions and positions of ROS redox values vs. valence band (VB)/conduction band (CB) of catalysts in the system, the relative yield of O ₂ , H ₂ O ₂ , and •OH during photocatalytie process can be estimated. Finally, based upon the estimated relative yield of ROS and HOMO/LUMO positions of pollutants, the role of ROS in the photo-oxidation process is predicted. HOMO and LUMO stand for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively, and define energy levels in molecules, such as organic dyes.

[0087] Referring to FIG. 14, an illustration 1200 depicts relative positions of VB/CB of BiVO ₄ , Ag ₂ O and Ag NPs and HOMO/LUMO of cationic BG, RhB, MB and anionic AR and MO dyes derived from the investigations hereinafter in accordance with present embodiments. Ag-BiVO ₄ , relevant redox potentials are shown in the illustration 1200, where CB of BiVO ₄ is 0.00 eV and VB is 2.25 eV vs. NHE (from absorbance measurements in FIG. 17 hereinbelow). CB/VB of Ag nanoparticles (NPs) are 0.69/2.8 eV and Ag ₂ O are 0.34/2.34 eV vs. NFIE, respectively. The redox values of ROS are - 0.33, 0.94, 0.32, and 2.33 V vs. NHE for O ₂ , O ₂ -, H ₂ O ₂ , and •OH, respectively. HOMO/LUMO values of cationic methylene blue (MB) and rhodamine B (RhB) are 0.065/1.77 eY and 0.54/2.78 eV, respectively. HOMO/LUMO values of anionic acid red 1 (AR) and methyl orange (MO) are -0.2/2.14 eV and -0.036/1.644 eV vs. NHE, respectively. Molecular orbital values of brilliant green (BG) were estimated from conventional reports. BiVO ₄ and Ag NPs are n-type materials, while Ag ₂ O is a p-type material with photoelectrons and photo-holes as predominant charge carriers, respectively. Consequently, electrons are responsible for photocatalytic degradation of cationic dyes (MB, RhB and BG), while photo-holes drive degradation of anionic pollutants (AR and MO).

[0088] The rational design approach in accordance with the present embodiments is mapped in the illustration 1200 to discern between photocatalytic and photo-oxidative degradation pathways. Through a series of radicals, electrons, and holes scavenging experiments and matching strategy of dyes’ and catalysts’ energy bands with ROS redox potentials, it was found that MB, MO and AR dyes degrade due to interaction with photoelectrons and photo-holes, while RhB and BG dyes degrade due to combination of photo-oxidation by ROS and photo-holes. For such a task, Ag-BiVO ₄ was synthesized by sol-gel and UV photo -reduction methods and a comprehensive dye degradation study was carried out under visible light of cationic MB, RhB and BG and anionic AR and MO.

[0089] Bismuth nitrate pentahydrate (Bi(NO ₃ ) ₃ ·5H ₂ O, 98%, Sigma- Aldrich) and ammonium metavanadate (H ₄ NO ₃ V, 99%, Sigma- Aldrich) were utilized as precursors for bismuth and vanadium sources, respectively. Silver nitrate (AgNO ₃ ) was used as a source of silver from Sigma- Aldrich. Acetic acid (CH ₃ COOH, 99%, Alfa Aesar), nitric acid (HNO ₃ , 70%, Sigma-Aldrich), ethanol (CH ₃ CH ₂ OH, 96%, Sigma-Aldrich) and deionized water (DI, 50 KW) were used as solvents throughout all experiments. Methylene Blue (dye content ³ 82%), Brilliant Green (dye content ³ 95%), Acid Red 1 (dye content 60%), Rhodamine B (dye content ³ 95%) and Methyl Orange (dye content 85%) from Sigma-Aldrich were used.

[0090] Monoclinic BiVO ₄ was prepared by following the recipe hereinabove. Ag- BiVO ₄ with 2 wt%, 5 wt% and 7 wt% loading of Ag were synthesized by a photo- deposition method. During preparation, 0.1 g of pristine BiVO ₄ was dispersed in 1 mL of ethanol and the corresponding amount of AgNO ₃ was added to obtain 2-7 wt% Ag. The solution was stirred for 180 minutes, the intensity of UV light was set for 14 mW/cm ² , the distance between the vial and the light source was kept at two centimeters. At the last stage of synthesis, ethanol was evaporated and Ag- BiVO ₄ composites were used for further characterization and dye degradation experiments.

[0091] As before, X-ray diffraction (XRD) experiments were carried out on a Bruker D8 Advance X-ray diffractometer with Cu Ka radiation of 0.154 nm wavelength to evaluate powder’s composition and phase. The scanning angle was set from 25 °C to 60 °C. UV-Vis absorbance spectra were recorded and monitored by using a PG Instruments TIIO+ UV-Vis spectrophotometer. Surface morphology of samples was examined by scanning electron microscopy (SEM) JEOL JSM-7600F. Fourier- transform infrared spectroscopy (FTIR) spectra was carried out from 400 cm ^-1 to 1200 cm ^-1 by a Vertex 70 (Bruker). Chemical composition and phase structure were recorded by X-ray photoelectron spectroscopy (XPS) with a Thermo Fischer Scientific Theta Probe system. Annealing of the sample was performed in a box furnace (Anhui Haibei 1100 model). Photoluminescence (PL) tests with 254 and 365 nm excitation wavelength, was carried out by a QE Pro spectrofluorometer (Ocean Optics, USA). A Prizmatix Ultra High Power LED lamp (48 W) was used as a light source during dye degradation experiments. And the light intensity was monitored by an Optical Power Meter PM100D from Thorlabs and the zeta potential was measured by NanoBrook Omni device in PALS mode.

[0092] The photocatalytic tests were conducted for MB, BG, RhB, AR and MO dyes in an aqueous solution under visible light irradiation of 100 mW/cm ² . During a typical experiment 15 mg of catalyst were dispersed in 15 mL of water (1 g L ^-1 concentration) and sonicated for 1 hour. Then, the solution was mixed with dye with the concentration 1 g L ^-1 and kept in the dark for thirty minutes to achieve adsorption-desorption equilibrium. The same concentration was used for all dyes. Next, the suspension was transferred into 1.5 mL centrifuge tubes and the LED lamp was switched on. Lor each measurement, 1 mL of the solution was collected, centrifuged for 5 min, and transferred to the cuvette for UV-Vis absorption experiments.

[0093] Referring to FIG. 15, X-ray diffraction (XRD) profiles for pristine BiVO ₄ and Ag-BiVO ₄ with 2, 5 and 7 wt% Ag loading in accordance with present embodiments are depicted in a graph 1300. A panel 1350 displays (121) and (040) peaks associated with a shift to lower angles. It can be seen that material possess monoclinic scheelite structure (JCPDS Card no. 14-0688). No peaks from other phases were detected, indicating high purity of the synthesized composite. The major (121) peak exhibits shift to the lower angle from 28.698° for pristine BiVO ₄ to 28.685°, 28.659° and 28.631° for 2, 5, and 7 wt% Ag, respectively. Lacet (040) also demonstrates lower angle shift from 30.461° for pristine BiVO ₄ to 30.434°, 30.408° and 30.381° for 2, 5, and 7 wt% Ag, respectively. Lower angle shift can be attributed to either (a) cationic or (b) anionic doping as discussed hereinabove. Cationic doping is consistent with the fact that Bi ⁺³ ions (100 pm) can be substituted by slightly larger Ag ⁺ ions (110 pm). Vanadium (V ⁺⁵ ) ions are less likely to be replaced due to smaller size (49.5 pm). Due to the low Ag presence, no corresponding XRD peaks were detected.

[0094] The Fourier-transform infrared spectroscopy (FTIR) at 400-1200 cm ^-1 range was applied to characterize chemical bonds of BiVO ₄ and Ag-BiVO ₄ catalysts. No Ag- related bands in bulk were observed indicating the absence of chemical bonding between Ag and BiVO ₄ . The intense band at 474 cm ^-1 is ascribed to symmetric bending of VO ₄ ^3- . The weak and intense absorption bands at 554 cm ^-1 and 606 cm ^-1 are the signs of bending vibration of Bi-O bonds, respectively. Bands at 661 cm ^-1 and 811 cm ^-1 are associated with asymmetric and symmetric stretching of VO ₄ ^3- , respectively. In summary, the FTIR spectra confirm the presence of monoclinic BiVO ₄ [0095] Referring to FIGs. 16A to 16D, graphs 1400, 1420, 1440, 1460 show high- resolution X-ray photoelectron spectroscopy (XPS) spectra to evaluate chemical states of C, Bi, V, O and Ag elements of 7 wt% Ag-BiVO ₄ . The XPS spectra of C Is is displayed in the graph 1400 and suggests the presence of C=0 and C-OH bonds (290.1 eV and 287.4 eV, respectively). The Bi 4f peaks in the XPS spectra of the graph 1420 at 158.9 eV and 164.2 eV represent metallic Bi ⁰ , while peaks at 164.1 eV and 169.4 eV are ascribed to Bi ⁺³ chemical state. The O Is and V 2p peaks are overlapped and shown in the XPS spectra of the graph 1440. The V 2p peaks at lowest binding energies of 516.5 eV and 524.1 are ascribed to metallic V ⁰ , while doublets at 521.8 eV and 529.4 eV attributed to V ⁺⁵ state. The largest 01 s peak at 529.7 ascribed to O ^2- anions (Bi-0 bonds). The O Is peaks at 530.8 eV and 534.6 eV can be assigned to V-0 bond and surface adsorbed oxygen species, respectively. The XPS spectra in the graph 1460 confirms the presence of metallic Ag (368.3 eV and 374.3 eV) and Ag ₂ O (371.3 eV and 377.4 eV). With larger Ag loading the amount of Ag ⁰ gradually increases from 2.55 at. % to 6.56 at. %, while Ag ⁺ increases slower (2.19-2.91 at. %). The total presence of Ag in both chemical states increases from 4.74 atomic % for 2 wt% Ag to 6.37 atomic % and 9.47 atomic % for 5 wt% Ag and 7 wt% Ag, respectively.

[0096] The optical properties of samples were investigated by measuring absorbance as a function of the wavelength as shown in a graph 1500 of FIG. 17. The graph 1500 depicts UV-Vis absorbance spectroscopy of pristine monoclinic BiVO ₄ and Ag-BiVO ₄ composites in accordance with the present embodiments. The corresponding band gap was calculated from a plot depicting (ahv) ² versus (hv) and shown an inset graph 1550 in FIG 17. Absorption edges were found to be 551.11 nm, 556.05 nm, 570.73 nm and 573.48 nm for BiVO ₄ , 2 wt% Ag, 5 wt% Ag and 7 wt% Ag, respectively. It can be seen that the absorbance spectra of Ag-BiVO ₄ are red- shifted in comparison with pristine monoclinic BiVO ₄ , which can be explained by surface plasmon resonance (SPR) effects. As a result, incorporation of Ag reduces the band gap: for example, for 2 wt% Ag loading the band gap slightly drops in comparison with pristine BiVO ₄ , 2.23 eV vs. 2.25 eV, respectively. Higher Ag loading leads to enhanced absorption capacity of visible light with the band gaps of 2.17 and 2.16 eV for 5 wt% and 7 wt% Ag, respectively.

[0097] The room temperature photo luminescence (PL) technique (l _e = 254 nm) was employed to investigate the electron-hole recombination rate in as-prepared samples, where the less intense PL signal manifests enhanced charge carrier separation efficiency. It was found that the most intense UV -light- induced peak is located at 537 nm and attributed to pristine BiVO ₄ The peak corresponds to electron -hole recombination where holes formed in the O 2p band and electrons are in the V 3d band. The intensity of the PL peak drastically drops when Ag introduced. The weakest PL signal was recorded from 2 wt% Ag, indicating major inhibition of electron-hole recombination. Furthermore, the red-shift from 537 nm for BiVO ₄ to 516-520 nm for Ag-BiVO ₄ composites demonstrates the interaction among metallic Ag, Ag ₂ O andBiVO ₄ . Therefore, incorporation of Ag leads to suppressed recombination rate of charge carriers. Referring to FIGs. 18A to 18D, representative SEM images 1600, 1620, 1640, 1660 depict the as -synthesized pristine BiVO ₄ (SEM images 1600, 1620) and 7 wt% Ag-BiVO ₄ (SEM images 1640, 1660). BiVO ₄ particles have micro sphere morphology with 1-2 mm diameter. When Ag is added there is no significant morphological change observed. Based on the SEM results 1600, 1620, 1640, 1660, it can be concluded that the UV photo-reduction synthesis method does not affect the morphology of the Ag-BiVO ₄ catalyst.

[0098] The photocatalytic activity of as -prepared composites was evaluated for 60 minutes and 120 minutes under LED light by degradation of anionic AR and MO dyes, respectively. Degradation kinetics was routinely monitored by UV-Vis spectroscopy of the main characteristic bands at 517 nm for AR and at 465 nm for MO, with results shown in FIGs. 19A to 19D. FIGs. 19A and 19C depict graphs 1700, 1740 of normalized photocatalytic degradation by pristine BiVO ₄ and Ag-BiVO ₄ (2, 5 and 7 wt% Ag for AR dye (graph 1700) and MO dye (graph 1740) showing the change in degradation rate, where C ₀ is the concentration of pollutant after adsorption-desorption equilibrium and C is the concentration at the certain irradiation time. FIGs. 19B and 19D depict first-order kinetic plots 1720, 1760 for AR dye (plot 1720) and MO dye (plot 1760) which show the Langmuir- Hinshelwood model by plotting the first-order reaction curves 1720, 1760. Blank experiments (photolysis) were carried out to confirm the major role of the Ag-BiVO ₄ catalyst in the degradation process. The error bars in the graphs 1700, 1740 and the plots 1720, 1760 are based on the standard deviation for n = 3 measurements.

[0099] Visible-light degradation of AR by pristine BiVO ₄ is negligible (8.1 ±0.7 %) and comparable to photolysis results. Once Ag is introduced on the BiVO ₄ surface, degradation rate increases to 39.3 ± 1.9 % and 78.8 ± 2.5 % for 2 wt% Ag and 5 wt% Ag, respectively, as seen from the graph 1700, and reaches a maximum value of 79.0 ± 2.5 % for 7 wt% Ag. Interestingly, no significant change was observed in the removal rate between 5 wt% Ag and 7 wt% Ag, which can be attributed to minor p-type Ag ₂ O content difference on the BiVO ₄ surface. The increase in AR removal rate from 8.1 ± 0.7 % for pristine BiVO ₄ to 39-3 ± 1.9 % for 2 wt% Ag can also be explained by major contribution of 2.19 atomic % of p-type Ag ₂ O . On the other hand, the drastic difference in AR degradation between 2 wt% Ag and 7 wt% Ag cannot be ascribed to the effect of Ag ₂ O only (0.72 atomic % Ag ₂ O increase between 2 wt% Ag and 7 wt% Ag). Here, rapid AR degradation was likely induced by the formation of •OH radicals from interaction of photo-generated charge carriers, electrons from metallic Ag and holes from Ag ₂ O, respectively, with water molecules.

[00100] The graph 1720 and the plot 1760 show degradation of anionic MO dye in the presence of photocatalyst under visible light. After 120 minutes, a notable intensity decrease was observed at 465 nm, indicating continuous process of intermediates formation. First, one of the benzene rings was replaced with OH- group due to attack of •OH radicals. Second, the breakage of N-C bond in dimethylamino group causes the replacement of methyl group with protons. Finally, the breakage of the main azo double bond -N=N- (color-bearing group) due to ROS attack leads to formation of inorganic sub-products. Evidently, the highest degradation rate was achieved with 7 wt% Ag (64.9 ± 2.0 %), while with 5 wt% Ag and 2 wt% Ag the removal rate dropped to 41.5 ± 1.6 % and 34.2 ± 1.3 %, respectively. Similar to anionic AR, the difference in degradation rate between pristine BiVO ₄ and 2 wt% Ag-BiVO ₄ can be attributed to p- type Ag ₂ O, while the difference in removal rate between 2 wt% Ag and 7 wt% Ag- BiVO ₄ cannot be explained by Ag ₂ O contribution only (0.72 atomic % Ag ₂ O increase between 2 wt% and 7 wt%). The latter change in the MO removal rate is possibly attributable to the detrimental effect of ROS, like •OH, H ₂ O ₂ and O2- radicals.

[00101] The degradation kinetics of AR and MO dyes was routinely analysed by the Langmuir-Hinshelwood model. The first-order model was applied: where C ₀ /C is the ratio of concentration of dye solution at adsorption-desorption equilibrium, and k is the first order constant (min ). Based on a first-order model, k values were collected and analysed, where the largest k implies better dye degradation properties. Both total degradation, % and k constant values for anionic dyes are summarized in the table of FIG. 20, wherein the average and standard deviation values are based on n = 3 measurements, the k constant value reported is the highest rate constant obtained, and the dye degradation value is below 5 %.

[00102] Photocatalytic degradation of cationic MB, BG and RhB was monitored under LED irradiation and recorded by using UV-Vis spectroscopy. Decolorization of MB and BG was evaluated for twenty minutes with measurements taken every five minutes, while the removal rate of RhB was studied for 120 minutes with measurements taken every thirty minutes. The results are shown in FIGs. 21A to 21F. FIGs. 21A to 21F depict graphs 1800, 1820, 1840 of normalized photocatalytic degradation by pristine B1VO ₄ and Ag-BiVO ₄ (2, 5 and 7 wt% Ag) for MB dye (graph 1800), BG dye (graph 1820) and RhB dye (graph 1840) and first-order kinetic plots 1810, 1830, 1850 for MB dye (plot 1810), BG dye (plot 1830) and RhB dye (plot 1850) in accordance with the present embodiments. The error bars in the graphs 1800, 1820, 1840 and the plots 1810, 1830, 1850 are based on the standard deviation for n = 3 measurements [00103] MB degradation is shown in the graph 1800 and the plot 1810. Under visible light, decomposition of MB is largely independent from the Ag presence with the best results ascribed to 2 wt% Ag and 7 wt% Ag (91.7 ± 3,1 % and 90.9 ± 2.4, respectively). The results can be explained by direct match of CB from n-type BiVO ₄ with LUMO level of MB , where p-type Ag ₂ O and metallic Ag do not participate in the photoeatalytic reaction. MB exhibits two main absorption peaks at 668 nm and 615 nm. The major peak at 668 nm is characteristics of conjugation system between the two dimethylamine substituted aromatic rings through the nitrogen and sulfur, while the minor peak at 615 nm is attributed to dye dimmer. Both peaks retard gradually with slightly faster kinetics ascribed to the 615 nm peak.

[00104] Degradation of BG is shown in the graph 1820 and the plot 1830. Cationic BG degrades mainly due to contribution of n-type BiVO ₄ (51.1 ± 2.0 %). With Ag presence, the degradation rate increases to 65.2 ± 2.7 %, 73.0 ± 2.3 % and 84.0 ± 2.7 % for 2, 5, and 7 wt% Ag, respectively, which can be explained by the self- sensitization mechanism and photo-oxidation by ROS. No hypsochromic shift of main absorption band at 619 nm was observed, indicating BG chromophore destruction.

[00105] The degradation of RhB was investigated for 120 minutes and shown in the graph 1840 and the plot 1850. Pristine BiVO ₄ is not sufficient to induce breakage of the dye given a decomposition rate of 12.1 ± 0.7. A higher presence of Ag leads to enhanced degradation rate with 85.5 ± 2.8 %, 86.9 ± 3.0 % and 90.5 ± 3.3 %, for 2, 5 and 7 wt% Ag, respectively. The main absorption peak is located at 535 nm wavelength indicating the n p ^* transition of C=N and C=O groups. No hypsochromic peak shift was observed, suggesting a degradation pathway via (i) chromophore cleavage, (ii) hydroxylation, (iii) aromatic ring opening and, finally (iv) mineralization· The major role in RhB degradation is attributed to n-type metallic Ag NPs on the surface and photo-oxidation by ROS. Although, the CB level of Ag is slightly more positive than LUMO level of RhB, breakage of dye molecules can be accomplished by the self- sensitization mechanism. Both total degradation, % and k constant values for cationic dyes MB, BG and RhB are summarized in the table of FIG. 22 wherein the average and standard deviation values are based on n=3 measurements and the k constant value reported is the highest rate constant observed.

[00106] Species involved in the process of dye degradation were further identified by adding p-benzoquinone (BQ), EDTA and isopropanol (IP) to the analyte solutions as scavengers to capture electrons, holes and •OH radicals, respectively. The results are shown in FIGs. 23A to 23E. Figs. 23A to 23E depict graphs 1900, 1920, 1940, 1960, 1980 of normalized photocatalytic degradation curves with 7 wt% Ag-BiVO ₄ in the presence of scavengers (BQ, EDTA, IP) for MB dye (graph 1900), BG dye (graph 1920), RhB dye (graph 1940), AR dye (graph 1960), and MO dye (graph 1980). The error bars are based on the standard deviation for n=3 measurements.

[00107] Anionic AR and MO dyes degrade due to contribution of photo-holes, as shown by degradation rates of 12.8 ± 1.5 % and 14.6 ± 1.8 %, respectively. Cationic MB decomposes at a rate of 21.2 ± 3.0 % due to photoelectrons with very minor role of photo-holes and •OH radicals. Decomposition of cationic BG and RhB is governed by photoelectrons and •OH radicals, with decomposition rates of 22.1 ± 1.8 % and 27.6 ± 2.3% for BG and 24.7 ± 2.3 % and 27.5 ± 2.4 % for RhB, respectively.

[00108] The efficiency of dye degradation depends upon many parameters including (i) band gap of the catalysts, (ii) light intensity and generated number of charge carriers, (iii) efficiency of charge carriers (electron-hole pairs) and corresponding recombination rate, (iv) amount and type of generated ROS, (v) morphology, (vi) structure of dye molecule, and (vii) self-sensitization mechanism. In general, photo -degradation of dyes occurs due to photocatalytic attack of electron-holes after transfer and adsorption of dye molecules on the catalyst’s surface. Consequently, ROS (O ₂ -, H ₂ O ₂ , and •OH) are produced from the reaction of charge carriers with water molecules, known as photo- oxidation, and also participate in oxidation of pollutants.

[00109] Degradation of cationic MB is driven by photoelectrons from n-type BiVO ₄ . As the CB level of BiVO ₄ is less positive than LUMO level of MB (0.00 eV vs. 0.065 eV), electrons can freely flow causing rapid degradation within twenty minutes. Ag NPs as another n-type material have minor influence on degradation of MB due to a mismatch of their CB and LUMO levels which is in line with our experimental results seen in the table of FIG. 22. Decomposition of cationic BG and RhB follows similar principles with direct electrons transition from CB of BiVO ₄ or Ag NPs to LUMO level of BG and RhB. Unlike MB, BG and RhB pollutants exhibit more efficient degradation in the presence of Ag, which can be explained by (a) n-type Ag NPs and further self- sensitization mechanism and (b) the influence of ROS (from the graphs 1920, 1940 both dyes degrade due to influence of photoelectrons and •OH radicals). Degradation of anionic AR and MO exclusively depends upon surface Ag, which is attributed to p- type Ag ₂ O. From the illustration 1200 (FIG. 14), it can be observed that the VB level of Ag ₂ O is more positive than the HOMO of both AR and MO, manifesting the major role of photo-holes in the degradation process, which is confirmed by the scavenging tests, the results of which are shown in FIGs. 23A to 23E.

[00110] From the above explanation, the degradation mechanism of cationic BG and RhB is still unclear and the role of ROS needs more explanation. It can be approached by photo-oxidation mechanism and the formation of ROS from n-type Ag NPs. Here, a sequence of chemical reactions, from most negative to most positive vs. NHE, excluding photolysis as all the dyes are stable under LED light. The VB/CB values of catalysts, HOMO/LUMO of dyes and redox values of ROS are shown in the illustration 1200.

The reduction potential of this reaction is -0.33 eV, which is far from the CB/VB values of catalysts as well as the HOMO/LUMO values of dyes. Thus, this reaction is excluded from further discussion.

The reduction potential of the reaction is 0.32 eV (production of H ₂ O ₂ ). Therefore, holes from VB of p-type Ag ₂ O (2.34 eV) can participate in the reaction.

[00111] The reaction of •OH production from H ₂ O ₂ has the reduction potential of 2.3 eV. Electrons from CB of both Ag NPs and BiVO ₄ can participate in the reaction. However, the CB level of Ag NPs is more positive than of BiVO ₄ . As a result, electrons from the CB of Ag NPs have to travel a shorter distance, yielding a larger amount of •OH.

The reduction potential of production of 0 ₂ - by oxidation of H ₂ O ₂ is 0.94 eV. Therefore, holes from VB of p-type Ag ₂ O can participate in the reaction.

The reduction potential of H ₂ O ₂ production is 0.32 eV. However, only electrons from the CB of BiVO ₄ can participate in this reaction as the CB of Ag NPs is more positive, making their electrons inaccessible for the above reaction. [00112] From the proposed sequence of chemical reactions, it becomes clear that electrons from n-type Ag NPs can yield a large number of •OH radicals (reaction (14). Additionally, •OH radicals participate in a reaction with holes from the VB of Ag ₂ O to produce H ₂ O ₂ (reaction (13)). Based on a matching of ROS redox values with HOMO/LUMO of BG and RhB, it can be concluded, that RhB and BG are more likely to be degraded due to attack of •OH (within the HOMO/LUMO range of RhB) and H ₂ O ₂ radicals (within the HOMO/LUMO range of BG), respectively. The proposed mechanism finds support in the degradation of MO and AR by photoholes on the surface, stabilized by a negative potential in the double layer induced by the anionic dye molecules. Further, BG and RhB are degraded by photo-oxidation by radical oxygen species, such as •OH an H ₂ O ₂ , whose precursors are HO- and O ₂ -, as displayed by the negative Zeta potential value. Thus, the degradation of BG and RhB has a partially photo-oxidation nature, while MB, AR and MO pollutants undergo pure photocatalytic degradation, which was confirmed by scavenging experiments and the energy mapping approach.

[00113] As seen herein, by employing a rational design approach of matching redox potentials of dye, catalysts, and reactive oxygen species (ROS), Ag-BiVO ₄ photo- catalysts of 2, 5 and 7 wt% Ag were successfully prepared via sol-gel and UV photo- reduction methods. The as-prepared composites over-perform existing counterparts by exhibiting excellent photocatalytic properties towards degradation of cationic methylene blue (MB, 20 min, 90.9 ± 2.4 %), brilliant green (BG, 20 min, 84.0 ± 2.7 %) and rhodamine B (RhB, 120 min, 90.5 ± 3.3 %) as well as anionic acid red 1 (AR, 60 min, 79.0 ± 2.5 %) and methyl orange (MO, 120 min, 64.9 ± 2.0 %) dyes. Furthermore, based on the novel approach of rational design disclosed herein, the degradation of MB dye was ascribed to photoelectrons from n-type BiVO ₄ , while the degradation of MO and AR was attributed to photo-holes from p-type Ag ₂ O. Meanwhile, BG and RhB dyes degrade due to photoelectrons and photo-oxidation by ROS, which was confirmed by carrier scavenger experiments. Results indicate that Ag-BiVO ₄ oxide can be efficiently used for wastewater treatment, solar energy, water splitting, and medicine. [00114] Thus, it can be seen that the present embodiments provide methods and systems for rational design of visible-light-sensitive bismuth-based material with exceptional photocatalytic degradation properties. Novel Pd-loaded a/b-Bi ₂ O ₃ nanorods (1, 2 and 5 wt% Pd) were synthesized by using sono-chemical method and their photocatalytic properties were investigated by (a) degradation of cationic MB, BG and anionic AR and (b) mixed dyes (MB-AR) and (BG-AR) under visible light irradiations. Moreover, for the first time a/b-Bi ₂ O ₃ catalyst was successfully coupled with noble metal (Pd) to achieve superior dye degradation properties. The results indicate that the best dye degradation properties were attained to 2 wt% and 5 wt% Pd- loaded Bi ₂ O ₃ catalyst. The extraordinary photocatalytic performance was achieved by matching VB and CB of semiconductor with the respective electronic levels of dyes and ROS. Comprehensive optimization of the catalyst, such as amount of semiconductor, amount of dye, pH level, ROS scavenging tests, PL tests, was performed and dye degradation mechanism was proposed. Furthermore, characterization of the catalyst after interaction with dye was carried out. XRD revealed the formation of a new material (CBi ₂ O ₅ ) which has never been reported in dye degradation of Bi-based composite. Due to the remarkable photocatalytic properties, Pd-loaded a/b-Bi ₂ O ₃ can have wide applications in wastewater treatment, water splitting and antibacterial fields.

[00115] In addition, methods and systems for rational design of a monoclinic visible- light- sensitive Ag-BiVO ₄ catalyst by using the UV photo-reduction method is disclosed and its photocatalyst properties are tested on several pollutants. By employing a combination of (a) a strategy where photo-catalysts and dye molecule energy bands match with ROS reduction potentials and (b) charge carrier trapping experiments, the major degradation pathway for each pollutant tested was predicted and identified. Thus, cationic MB degrades due to mobile photoelectrons from n-type BiVO ₄ , while anionic AR and MO break down due to photo-holes from p-type Ag ₂ O. Cationic RhB and BG undergo the combination of photocatalytic degradation from both n-type BiVO ₄ and Ag NPs and photo-oxidation by •OH and H ₂ O ₂ radicals, respectively. Furthermore, the synthesized composites were able to degrade MB and RhB dyes in twenty minutes and 120 minutes, respectively, which over-performs previous materials, and other pollutants, such as BG (in 20 min), AR (in 60 min) and MO (in 120 min), which have not been previously tested with Ag- BiVO ₄ . The present embodiments suggest that Ag- BiVO ₄ materials are an efficient agent for wastewater treatment, with other potential applications in the fields of solar energy, water splitting and medicine.

[00116] While exemplary embodiments have been presented in the foregoing detailed description of the present embodiments, it should be appreciated that a vast number of variations exist. It should further be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, operation, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing exemplary embodiments of the invention, it being understood that various changes may be made in the function and arrangement of steps and method of operation described in the exemplary embodiments without departing from the scope of the invention as set forth in the appended claims.