Bi2O3 nanoparticles: phytogenic synthesis, effect of calcination on physico-chemical characteristics and photocatalytic activity

ABSTRACT The present work reported the bioinspired synthesis of Bi2O3 nanoparticles (NPs) using Ziziphus mauritiana leaves aqueous extract at four annealing temperatures (300, 400, 500, and 600C°) and examined annealing temperatures effect of on various physicochemical characteristics and photocatalytic activities of NPs. The XRD and Raman results reveal the existence of α and β polymorphs of Bi2O3 in all samples but with increase of calcination temperature the extent of α-phase, particle size, and crystallinity were increased while band gap value decreased. The BET results confirmed the mesoporous structure of all samples. Further, decolorization experiments conducted with crystal violet (CV) dye showed an observable effect of annealing temperature on the adsorption and photocatalytic efficacy of the prepared samples, and the utmost (99.88 %) removal of CV dye was computed with the Bi2O3 nanoparticles calcined at 300oC (B-300). The main active species involved in the photo-mineralization of dye was h+ followed by •OH..


Introduction
Bismuth oxide (Bi 2 O 3 ) is a semiconducting metal oxide that is increasingly being explored for different potential applications.It has been reported for diverse applications in numerous fields, for instance, fuel cells [1], photovoltaics [2], gas sensors [3], energy materials [4], photocatalysis [5], biomedical field [6], and antibacterial applications [7].Bi 2 O 3 is found to be exist mainly in six polymorphic forms (α-, β-, γ-, δ-, ε-, and ω-phase) [8].Among them, α-and β-forms of Bi 2 O 3 are stable phases at low temperatures, whereas the other phases are metastable at high temperatures [9].Some distinguishable characteristics, which include absorption from visible light, the high value of the refractive index, and high photoconductivity, render Bi 2 O 3 as a potential candidate for light-driven catalytic applications [10].
The fabrication of Bi 2 O 3 has been accomplished employing various physical and chemical approaches like hydrothermal synthesis [11], sol-gel [12], template synthesis [13], facile chemical method [14], co-precipitation [15], thermal decomposition [16], etc., but such methods are encountered with some problems, for instance, physical methods are laborious and time-consuming, while chemicals are associated with chemical methods that may not be environmentally benign in nature [17].In this scenario, biogenic synthesis has emerged as a good alternative to nanosynthesis because of its cost-effective, efficient, and eco-friendly aspects [18].The use of plant bio-materials for the preparation of nanoparticles possesses additional advantages due to the widespread distribution, abundance, and biodiversity of plant species.Basically, the plant extract prepared from the plant body contains a lot of phytochemicals such as terpenoids, flavonoids, alkaloids, carbohydrates, vitamin C, tannins, saponins, phenolic compounds, steroids, and proteins, which function as reducing or oxidising as well as stabilising agent in nanosynthesis [19].
Few plant species like Anisomeles malabarica [20], Jatropha multifida L [21], Mentha pulegium [22], Ficus benghalensis [23], Cassia fistula [24] and Millettia pinnata [25] have been explored and reported for the bionanosynthesis of Bi 2 O 3 .Ziziphus mauritiana, a thorny shrub or small tree of the dry tropical and subtropical climates that is often known as Indian jujube and is a member of the Rhamnaceae family, is typically grazed by livestock.Many studies reported in the literature suggest that the Ziziphus mauritiana plant itself bestows some medicinal characteristics like antimicrobial, antidiabetic, and antidiarrheal properties [26].Reported phytochemical studies reveal the presence of a lot of phytochemicals, including flavonoids, alkaloids, terpenoids, tannins, saponins, and phenolic compounds in the extract of leaves of the plant [27].The phytochemicals present play a key role in nanomaterial synthesis.So far, various metallic such as Ag [28], Au [29], Cu [30] and metal oxides like CuO [31], MgO [32], and ZnO [33] nanoparticles have been reported using Ziziphus mauritiana plant extract.Here, in a novel attempt, Ziziphus mauritiana plant leaf extract has been employed for the biosynthesis of Bi 2 O 3 nanoparticles.
The calcination temperature plays a significant role in designing the crystallinity, structural, and surface-related characteristics of the obtained product as heat given to some substances may lead to some kinds of changes, which may further result in the generation of different phases having different properties [34].
The influence of annealing temperature on different properties of bismuth oxide nanomaterials has previously been studied by some researchers.Xiaohong et al. (2007) reported the synthesis of bismuth oxide films at varying annealing temperatures.The study demonstrated the highest photocatalytic performance of the bismuth oxide sample calcined at 550°C for Rhodamine B dye degradation [35].Mallahi et al. (2014) presented the fabrication of bismuth oxide NPs employing the sol-gel method at different calcination temperatures, 200°C, 500°C, and 800°C and studied changes in the shape, size, and surface characteristics of prepared nanoparticles [12].Recently, Astuti et al. (2021) have presented variations in physicochemical characteristics, and photocatalytic activity of sol-gel synthesised Bi 2 O 3 nanoparticles because of calcinations at 500°C, 600°C, and 700°C.The results demonstrated the formation of a mixture of crystal structures of monoclinic (α-Bi 2 O 3 ) and body centred cubic (bcc) (γ-Bi 2 O 3 ) phases.The photocatalytic performance of the nanoparticles was evaluated over methyl orange dye solution [36].
In contrast to previous studies, the present study, for the first time, reports a Ziziphus mauritiana plant leaves-based phytogenic approach for the synthesis of bismuth oxide, and further, it aims at determining the impact of annealing temperature on different characteristics such as structure, size, morphology, band-gap value, and adsorption-photodegradation efficiency of biogenic Bi 2 O 3 NPs for the removal of crystal violet dye.

Materials
In this work, analytical-grade reagents (AR) were employed.Crystal violet, bismuth nitrate(III) pentahydrate [Bi(NO 3 ) 3 .5H 2 O], benzoquinone (BQ), disodium salt of ethylenediamine tetraacetate (EDTA-2Na), isopropyl alcohol (IPA), and other chemicals were acquired from Sigma-Aldrich Co., India, and used without further purification in the current investigation.Water that had undergone two distillations (DD) was used to make solutions and for various experimental purposes.Ziziphus mauritiana plant leaves were gathered from the campus of the University of Rajasthan in Jaipur, India.

Preparation of plant leaf extract
To remove dust, grime, and other attachments, the leaves of Ziziphus mauritiana Lam. were first cleansed with tap water and then with distilled water.The cleaned plant leaves were then air-dried at room temperature in a shaded area.Twenty grams of dried leaves in powdered form were combined with 300 mL of DD water to create the aqueous extract.For 30 min, the mixture was heated in a heating mantle at about 70°C before being allowed to cool at ambient temperature.Afterwards, Whatman no.42 filter paper was used to filter the prepared combination.To get the suspended particles out of the filtrate, it was centrifuged for 5 min at a speed of about 3500 rpm.The biosynthesis of Bi 2 O 3 nanoparticles was then carried out using the generated supernatant.

Biogenic synthesis of Bi 2 O 3
The Bi 2 O 3 nanoparticles were fabricated following a similar procedure with necessary modifications reported in our earlier work [24].Briefly, the prepared aqueous leaf extract was taken, heated at 60°C, and then mixed with 4.0 g of Bi (NO 3 ) 3 .5H 2 O (fine powder).The as-obtained mixture was subjected to stirring and heating (at 65°C) for 45 min, which resulted in a yellow-brown coloured suspension.It was first allowed to settle down and then separated out by filtration.The brown-yellow paste so obtained was dried in an electric oven at 90°C.Then, it was divided into four parts and calcined in a muffle furnace, separately for 3 h at four different temperatures (300°C, 400°C, 500°C, and 600°C).The resulting powder samples were labelled as B-300, B-400, B-500, and B-600, respectively, and stored for further characterisation and experimentation.

Characterisation of Nanosamples
A slew of techniques were employed for detailed analysis of prepared nanoparticles.The existence of Bi 2 O 3 in the synthesised nano-samples was confirmed by employing a powder X-ray diffractometer (D/teX Ultra 250) with Kα filter 1D for Cu (λ = 0.154 nm), which operates at 40 Kv and 50 mA in the 2θ range of 10° to 70° (with step width 0.01 and scan speed 2.00° per min).Microscopic analysis was carried out using Nava Nano 450-FEI modelled Field Emission Scanning Electron Microscope.The elemental composition of the prepared samples was determined through Energy Dispersive X-ray (EDX) measurements.Optical properties (band gap determination) were analysed using the UV-Vis spectrophotometer (Shimadzu UV-2600, Japan).Bruker-ALPHA FTIR instrument was employed to get information regarding the possible functional groups present in the obtained materials using the KBr pellet method in the 4000 to 500 cm −1 wave number range.Raman spectra were obtained in the spectral range of 100-2500 cm −1 .BET characterisations of the prepared samples were carried out by using NOVA Quantachrome TouchWin v1.22 BET analyser.

Adsorption-photocatalytic degradation experiments
To investigate the effect of calcination temperature on the adsorption capacity and photocatalytic activity of the green synthesised nanoproducts, experiments were carried out taking crystal violet (CV) as a model dye pollutant.The removal ratios for CV dye, first by adsorption in the dark and subsequently by photodegradation under visible light irradiation, were evaluated.First of all, optimal values of various parameters such as initial dye concentration, adsorbent-catalyst amount, and contact time were decided for the effective removal of dye.In a typical procedure, 20 mg of the Bi 2 O 3 sample was mixed with 60 mL of CV dye solution (30 mg/L), and the mixture was then agitated in the dark until the adsorption-desorption equilibrium was established.The above reaction mixture was subsequently subjected to visible light exposure using a sodium lamp (250-W, Lelesil innovative system).During the dye removal experiment, a certain amount of the reaction mixture was taken out at prearranged time intervals, and after the catalyst was removed from it by centrifugation, the remaining dye concentration was measured using a double-beam UV-visible spectrophotometer (UV-2600, Shimadzu, Japan) at a wavelength of 573 nm.The percentage of the dye eliminated was calculated using the calculation below: Where C t and A t are the dye concentration and absorbance at time interval t, respectively, C o is the initial concentration of CV dye, and A o is the absorbance corresponding to it at zero time.
The sizes of the grown nanocrystallites were calculated using Debye Scherrer's equation as follows.
where D is the crystallite size, k = 0.9, λ = 1.5406Å, β = peak width at half maxima, and θ is the diffraction angle.The high-intensity XRD peaks were used for calculating the average size of the Bi 2 O 3 nanoparticles, and the values so obtained are presented in Table 1.The maximum average nanoparticle size was observed for the B-500 sample and which was 66.51 nm, followed by B-400 (62.95 nm), while it was 49.40 nm and 44.66 nm for B-600 and B-300 samples, respectively.
Additionally, the micro-strain and dislocation density for the prepared samples were also calculated using the equations as follows [38]: Where D is the average crystallite size calculated from the Debye-Scherrer equation, the values of microstrain and dislocation density calculated for the prepared samples are also presented in Table 1.It is evident from the results that the microstrain in the samples was observed to decrease with increasing the value of calcination temperature.
Dislocation density predicts the degree of crystallinity of the samples, and the decrease in its value suggests an increase in crystalline nature [39].B-500 reveals a minimum value of dislocation density predicting the highest degree of crystallinity in the sample, while B-400 and B-600 samples show intermediate values of dislocation density, suggesting the generation of moderate crystallinity.However, B-300 has maximum dislocation density indicating comparatively lower crystalline order for the sample.
Figure 2 depicts the FTIR spectra of all four samples of the Bi 2 O 3 .The peak in all samples at around 629 cm −1 can be attributed to the metal-oxygen (Bi-O-Bi) bending vibrational modes in Bi 2 O 3 [40].At the same time, the peak near 975 cm −1 arising in all samples may be accredited to the stretching vibrations of Bi-O bonds [41].A peak at around 1694 cm −1 is observed due to H 2 O content [42], while a trace of NO 3 is evident from a peak of nearly 1524 cm −1 .The peaks observed around 1404, 2316, and 2885 cm −1 might be arising because of the C-C bond, C-O bond, and C-H bond stretching vibrations, respectively [43].The peaks in 3600-3800 cm −1 regions generally originate due to O-H stretching vibrations.
Furthermore, the force constant k for the Bi-O bond was also determined using the equation given below [44]: Where c denotes the velocity of light and μ is the reduced mass of bismuth and oxygen.The reduced mass (μ) can be obtained by the equation below:  Where M Bi and M O are the atomic mass of bismuth and oxygen, respectively.Furthermore, the average bond length (r) can be calculated by the following relation [45]: Table 2 illustrates the variation in the values of force constant and bond length values for the Bi-O bond in all the prepared samples of bismuth oxide.It is seen that the force constant decreases, while the bond length value increases with an increase in calcination temperature.With increasing temperature, the thermal vibrations weaken the bond strength, thus increasing the bond length and reducing the force constant.The increase in bond length is also evident from the shifting of the wavenumber to lower values from 979 cm −1 to 967 cm −1 (Figure 2).As the bond length increases and the force constant decreases, the bond weakens, and a low amount of energy is required to produce vibration in the metal-oxygen bond that results in shifting of the peak to lower wavenumber [46,47].In addition, the increase in Bi-O bond length is also evident from the increase in the average particle size of the prepared samples with rising temperatures.Thus, the results corroborate the XRD and FESEM findings.
In order to further verify the structural properties of the grown samples, Raman spectra were measured in the 100-2500 cm −1 wavenumber range.Figure 3 illustrates the Raman patterns of all four nanomaterial samples.For the samples calcined at low temperatures (300°C and 400°C), major peaks observed at around 381, 470, and 602 cm −1 can be correlated to the β-phase of Bi 2 O 3 .Contrarily, the Raman spectra of the B-600 sample observe relatively sharp peaks at around 145, 155, 189, 215, 281, 317, 417, and 451 cm −1 that can largely be attributed to the monoclinic phase [37].It is to be noted in the figure that with increasing calcination temperature, the intensity of characteristic β-peaks decreases, while that of α-phase peaks increases.Thus, the Raman data also substantiate the XRD findings of the dominance of α-content in the composition at higher temperatures.

Morphological analysis
The effect of thermal treatment on the morphological properties of the grown Bi 2 O 3 nanocrystallites was  evaluated by the FESEM study.It is obvious from the FESEM images (Figures 4 to 7) that in low-temperature samples, grains are non-homogeneous, whereas with rising calcination temperature, the homogeneity increases.The abnormal growth in particle size may be attributed to the existence of materials in high chemical equilibrium at low temperatures [48].Also, at low temperatures, high magnetic interactions are found because of the small size of crystallites, resulting in high agglomeration [49].However, high annealing temperature does not favour agglomeration, leading to less agglomerated states at higher temperatures.The EDX patterns (Fig S1; supplementary material) of the prepared nanoparticles confirm the occurrence of bismuth and oxygen with an elemental atomic ratio of almost 2:3, which indicate the high purity of the samples.

Optical studies
In order to determine the optical characteristics of as-synthesised Bi 2 O 3 samples, their UV -visible absorption spectra were recorded.photocatalyst can be calculated using Tauc's relation as follows [50,51]: Where α = optical absorption coefficient, k = constant of proportionality depending on the transition probability, E g = optical band gap energy (In electron volts-eV), hv = energy of the photon, and n = 4 and 1 for indirect and direct band gap semiconductors, respectively.The band gap of the prepared Bi 2 O 3 samples was determined using the plot of (ahv) 2 against hv (Figure 9).The obtained band gap values for the Bi 2 O 3 samples are 3.42, 2.84, 2.80, and 2.82 eV for the B-300, B-400, B-500, and B-600, respectively.It is to be noted from the results that the band gap value initially observed to be decreased when the calcination temperature was increased from 300°C to 500°C; however, furthermore boosting in the  temperature at 600°C the band gap was determined to be increased (2.82 eV).

BET analysis
The pore structures of the prepared Bi 2 O 3 samples were analysed by N 2 sorption isotherms, and the pore size distribution was computed by the BJM method.The N 2 sorption isotherms as exhibited in Figure 10, demonstrated hysteresis loops, and type-IV patterns illustrating porous structure and the sharp increase in N 2 adsorption volumes in the P/Po ranges of 0.7 to 0.99, reflecting good homogeneity of samples and mesoporous nature.The average pore size, pore volume, and surface area values for all samples are demonstrated in Table 3.The specific surface areas were estimated by BET analysis and obtained of 9.856, 10.651, 11.662, and 11.504 m 2 g −1 for B-300, B-400, B-500, and B-600, respectively.The multipoint BET adsorption curves and t-plots are shown in Figures 2 and 3, respectively (Supplementary material).The t-plot analysis studies revealed that approximately 100% volume of pores is contributed from mesopores in all prepared samples.

Adsorption-photodegradation performance
The removal performance of CV dyes in the absence of light and when exposed to visible light, respectively, was used to assess the adsorption capacity and photocatalytic activity of the greenly synthesised bismuth oxide samples.The adsorption of CV on the Bi 2 O 3 sample was confirmed prior to the photocatalytic degradation.Figure 11 shows the UV-Vis spectra of CV dye solution that have been decolourised by bismuth oxide B-300, B-400, B-500, and B-600 over 110 min under dark (30 min) and light (80 min) conditions.It is evident from the figure that CV dye shows a maximum absorption peak at 573 nm.With increasing adsorption time, the intensity of the aforesaid peak decreases gradually.Nonetheless, the shape of the absorption peak remained unchanged, and the maximum absorption was still at 573 nm.It indicates the stability of the CV dye during the adsorption process.After 30 min of adsorption time, the intensity of the absorption spectra tended to remain unchanged, suggesting the establishment of the adsorption-desorption equilibrium.After that, the reaction mixture was subjected to visible light exposure for the photodegradation of the remaining amount of the dye.The intensity of the UV-vis absorption spectra again reflected a decreasing trend, and ultimately, the spectral line became almost flat, which was evident in the photodegradation of the remaining CV dye over Bi 2 O 3 nanoparticles in the presence of visible light.The influence of annealing temperature on the adsorption-photodegradation efficiency of the Bi 2 O 3 nanocatalyst was evaluated and represented in Figure 12.The experimental results suggest that the highest adsorption of CV dye (68.62%) was observed for the B-300 sample, followed by B-400 (64.35%).B-500 adsorbed about 53.46% of CV dye, while about 57.70% of CV dye was adsorbed by the B-600 sample in the dark.The high amount of adsorption is indicative of the involvement of electrostatic attraction in the adsorption process [52].The difference in morphology, particularly particle size, is what causes the difference in adsorption activity for each sample.Due to having the narrowest particle size range, the B-300 exhibits the maximum adsorption activity, followed by the B-400 and B-500.The proliferated adsorption efficiency for the B-600 sample can be due to its smaller crystallite volume.Therefore, the smaller the particle size, the bigger the surface area and, consequently, the better the adsorption effectiveness.
The nanomaterials were subsequently tested as a catalyst for photocatalytic degradation of CV.The experimental results demonstrate that all samples exhibit good photocatalytic activity against CV.However, B-300 displays better photocatalytic activity (99.88%) compared to the other three products.99.88%, 97.20%, 92.25%, and 93.59% over all removal capabilities for CV dye were exhibited by B-300, B-500, B-500, and B-600, respectively.This is because bismuth oxide sample B-300 contains a higher content of its β-phase, which exhibits a higher catalytic activity, which can be attributed to a number of factors, including a smaller band gap, a longer lifespan for recombination, crystalline size, and surface potential [37].With increasing calcination temperature, the content of β-phase of Bi 2 O 3 decreases, which results in lower photocatalytic activity of Bi 2 O 3 samples at higher temperatures.In addition, at high temperatures, relatively perfect crystal development takes place, reducing the lattice defects, which leads to the weakening of separation and transmission capabilities of photogenerated electrons and holes [53].Thus, reduction in separation and mobility of photo carriers may lead to lower photocatalytic efficiency at higher temperatures.
Similar results of the dye degradation by bismuth oxide NPs were observed by Astuti et al. (2021) [36] and Hou et al. (2013) [54] in which they clarified that the occurrence of two crystalline forms enhances the dye degradation ability of Bi 2 O 3 .Moreover, the increase in particle size at higher calcination temperature declines the adsorption activity of Bi 2 O 3 nanomaterial [36].As reported by Peng et al. (2004) [55], Weidong et al. (2007) [56], Wen et al. (2009) [57], and Huang et al. (2011) [58] that the β-Bi 2 O 3 polymorph exhibits higher photo-mineralisation potential for dyes in comparison to the α-Bi 2 O 3 .It has been described in XRD results that on increasing of calcination temperature the intensity of XRD peaks  corresponding to β-Bi 2 O 3 decreased, while intensity of peaks related to α-Bi 2 O 3 has shown enhancement, additionally new XRD peaks related to the α-Bi 2 O 3 form emerged in XRD patterns at higher calcination temperature which have confirmed that the β-Bi 2 O 3 polymorphs gradually transform into α-Bi 2 O 3 form at higher temperature, and consequently, the photodegradation performance for CV dyes was enhanced.

Optimisation of experimental variables
In order to optimise the initial dye concentration, catalyst amount, and pH, dye removal were carried out, changing the reaction variables over a range.Figure 13(a) depicts the effect of the initial CV dye amount on its removal percentage, taking other parameters constant.The increase in degradation percentage with increasing dye concentration at the initial level can be attributed to the availability of more dye molecules for adsorption as well as photodegradation.However, further increase in dye amount after a certain concentration leads to a lower removal outcome, which may be due to the deep colour intensity of the dye solution at a higher concentration resulting in less penetration of light photons to the catalyst surface [59,60] in addition to more adsorption of dye molecules, reducing the number of available active sites [61].
Similarly, the effect of the catalyst dose on the percentage removal of CV dye was checked, varying the catalyst dosage from 10 to 40 mg, as illustrated in Figure 13(b).Here also, a higher amount of the catalyst increases the turbidity in the reaction mixture, which leads to a lower reach of the light photon to the catalyst surface.However, at the initial stage,  the increase in catalyst dose amount reasons the rise in removal efficiency because of the greater availability of active sites and surface area for adsorption and photodegradation [61,62].Figure 13(c) illustrates the impact of solution pH on the dye removal performance.pH significantly influences the charge on the catalyst surface and protonation-deprotonation of dye molecules.The results suggest the highest removal efficiency at neutral pH, while it shows decreasing trend at both acidic and alkaline pH.At low pH, the catalyst surface becomes positively charged due to the adsorption of H + ions.At the same time, with lowering pH, the CV dye molecules having three nitrogen centres get protonated, generating a positive charge over it.At very low pH, all three nitrogen of the dye molecule are found carrying positive charges.The positively charged dye molecules are electrostatically repelled by the catalyst having a positive surface in acidic pH values, resulting in low dye removal through adsorption as well as photodegradation [63].Contrarily, in alkaline pH, nucleophilic OH − ions attack the electrophilic carbon centre of the dye molecule, converting it into colourless triphenylmethanol form (neutral).At a high alkaline medium, the hydroxyl ions start deprotonation of the colourless methanolic form of the dye producing its alkoxide form (negatively charged).The negative surface of the catalyst (because of the adsorption of the OH − ions) repels the negatively charged form of dye, causing the reduction in dye removal performance at higher pH [64].

Kinetics of photodegradation
Chemical kinetics studies can be used to calculate the decolourisation rate, which can be used to estimate the photocatalytic efficiency of synthesised bismuth oxide samples against CV dye.The photo-decolourisation activity of dyes generally follows the kinetics of pseudo-first-order reactions [25,65] as stated by the formula: k is the pseudo-first-order reaction rate constant (min −1 ), C 0 and C t are the CV concentration at the initial level and time t, respectively.
The semi-logarithm curve (Figure 14(a)) illustrating degradation of CV dye over different Bi 2 O 3 nanosamples.According to the experimental data, the pseudo-first-order integral equation fits the data better (Figure 14(b)).It can be inferred that the pseudo-first-order kinetic equation can well explain the kinetics of CV photodegradation over the Bi 2 O 3 nanophotocatalyst under visible light illumination.The CV photodegradation rates of bismuth oxides B-300, B-400, B-500, and B-600 were calculated as; 0.0689, 0.0329, 0.0226, and 0.0222 min −1 , respectively, and the regression coefficient values were found to be 0.934, 0.956, 0.942, and 0.967, respectively.On the basis of the values of the degradation rate constant, it can be suggested that bismuth oxide calcined at 300°C has the highest photocatalytic degradation rate against CV in aqueous medium under visible light.

Analysis of reactive species
The investigation of the role of reactive species, the scavenger tests for CV dye degradation over Bi 2 O 3 nanoparticles calcined at 300°C (B-300) were performed using BQ, EDTA-2Na, and IPA as •O 2 − , h + , and •OH trappers [66,67], and results are displaced in Figure 15.When BQ as a •O 2 − trapper was mixed to dye solution, no significant decline in degradation ability was reported; however, on addition of IPA, an apparent hang-up could be reported, reflecting that •OH   demonstrated sufficient part in photo-mineralisation process.On the other hand, the mixing of EDTA-2Na induced the utmost decline in the degradation efficiency which meant that holes were the chief active species.Finally, it can be summarised that in the degradation process of CV dye by green fabricated Bi 2 O 3 (B-300) the participation of active species followed the order of •O 2 − < •OH < h + .

CV dye photodegradation mechanism
The edge potential positions of the valence band (VB) and conduction band (CB) of the photocatalyst decide the redox aptitude of any photocatalyst.On visible light exposure, the excitation of e − from VB of the catalyst to its CB is taken place, leaving behind h + on the VB.The photogenerated e − present on CB can interact with the O 2 molecules present on the photocatalyst surface and reduce them into superoxide radicals, while the h + stayed on VB, can interact with hydroxide and water molecules and oxidise them into hydroxyl radicals.The produced active free radicals take part in the photodegradation of organic pollutants and convert them into non-hazardous products.The edge potentials of the CB and VB can be evaluated by using E CB = X-E e -0.5E g and E VB = E CB +E g .Where the absolute electronegativity of the Bi 2 O 3 semiconductor is X = 5.95 [68], the free electron energy (E e ) on the hydrogen scale is 4.  [70].On the basis of the radical scavenger experiments, and VB and CB edge potential analysis, a plausible mechanism of the photodegradation of CV dye in the presence of prepared Bi 2 O 3 photocatalysts can be proposed as given by the following equations.
The schematic representation of the mechanism of photodegradation of CV dye is depicted in Figure 16.

Catalyst stability and reusability
The reusability and photostability of the catalysts were evaluated for five sequential runs separately.After every adsorption-photodegradation cycle, the each catalyst was recovered by filtration, washed thoroughly with water and alcohol to remove adsorbed dye molecules and other attachments, and then dried inan oven and used in the next experimental cycle.It is evident from the results of the reusability experiments (Figure 17) that a slight reduction in the activity of each catalyst was observed with increasing reuse times, and even after five runs of reuse, each catalyst was found to exhibit significant dye removal activity.The decrease in the efficiency of the catalysts can be ascribed to the adsorbed fraction of the dye molecules on the reactive sites of the catalyst surfaces [71].In addition, the conglomeration of the catalyst particles during the separation process may be another cause for the reduction of the activity of the catalysts [72].

Conclusion
In the present study, the synthesis of Bi 2 O 3 nanoparticles was achieved using the aqueous extract obtained from dried leaves of Ziziphus mauritiana plant species and calcined at four different temperatures (300°C, 400°C, 500°C, and 600°C).
The effect of annealing temperature on the structural, optical, morphological properties, and adsorption-photocatalytic performance of phytogenic bismuth oxide nanoparticles was analysed.The XRD and Raman data confirmed the existence of two distinctive crystallographic phases, i.e. α-phase (monoclinic) and β-phase (tetragonal) of Bi 2 O 3 in all prepared samples.The results demonstrate that the β-phase content in the composition decreased with increasing calcination temperature, while the dominance of α-phase of Bi 2 O 3 was observed at higher temperatures.The morphological study through SEM images demonstrated the presence of a flake-like nanostructure with a size range of 25 to 70 nm.The value of the band gap was observed to decrease with rising calcination temperature indicating a reduction in particle size at the higher temperature.The highest CV dye removal percentage through adsorption-photocatalytic activity was observed for the B-300 sample.Further, the kinetics of the photocatalytic process of Bi 2 O 3 samples calcined at different temperatures over CV was also studied.On the basis of the results, it can be inferred that annealing temperature notably influences the size, morphology, adsorption, and photocatalytic activity of bismuth oxide nanomaterials, and 300°C is the optimum condition temperature for the synthesis of Bi 2 O 3 nanoparticles to be used in adsorption and photocatalytic applications.

Figure 1 .
Figure 1.XRD patterns obtained for Bi 2 O 3 nano-samples calcined at different temperatures.

Figure 13 .
Figure 13.Effect of (a) initial CV concentration (b) catalyst dose amount, and (c) pH on photocatalytic degradation of CV dye.

Figure 14 .Figure 15 .
Figure 14.(A) A/Ao plot and (b) kinetics of photodegradation of CV over Bi 2 O 3 nanoparticles calcined at various temperatures.

Table 2 .
Presenting the calculated values of force constant (k) and bond length (r) of Bi-O bond for Bi 2 O 3 samples.
photo-produced electrons jumped at the CB in all four Bi 2 O 3 photocatalysts unable to reduce O 2 into •O 2 − .However, electrons could interact with O 2 and H + to generate H 2 O 2 and produced H 2 O 2 would be transformed into •OH radicals 3 photocatalysts could oxidise water or hydroxide to generate •OH radicals that limits the recombination of electronholes and convert the toxic CV dye molecules into non-toxic products.Moreover, h + itself could also photo-mineralise CV dye.At the same time, the CB potential values for all four Bi 2 O 3 photocatalysts are less negative than the redox potential of O 2 /•O 2 − (−0.33 eV vs NHE), which indicates that the