Bio-fabrication of selenium nanoparticles/micro-rods using cabbage leaves extract for photocatalytic dye degradation under natural sunlight irradiation

ABSTRACT This work reports the synthesis of semiconductor selenium nanoparticles/micro-rods (Se-NP/MRs) using the bio-fabrication approach and studies its photocatalytic activity under natural sunlight irradiation. Cabbage leaf extract was used as a reducing agent to fabricate Se-NP/MRs. X-ray diffraction patterns shown the crystalline structure with a crystallite size of 28.17 nm, conferring to planes of hexagonal structure. A characteristic sharp resonance peak in Raman spectra at 235 cm−1 endorsed to the Se – Se bond of crystalline Se-NP/MRs. The prominent peak in the FTIR spectra at 621 cm−1 supported the formation of Se-NP/MRs. Scanning electron micrograph revealed a nearly spherical shape with irregular layers in the 35‒95 nm size range. In contrast, the dimension of the micro-rods was observed to be ~ 10 µm in length and 2.5 µm in diameter. The bandgap energy of Se-NP/MRs was analysed to be 2.14 eV using the Tauc plot. The exciting photocatalytic performance of the bio-fabricated Se-NP/MRs revealed that the methylene blue degradation was ~ 86% in 40 min and ~ 97% in 140 min. Kinetic analysis shows that the pseudo-second-order correlation coefficient is well fitted compared to the pseudo-first-order. Therefore, this work opens the door to further exploration of nano-materials/composites with mixed nanoparticles/micro-rods morphology for sustainable applications in the removal of various textile and industrial contaminants.


Introduction
Nanoparticles with exciting morphology are an emerging interest in the research community and their applications in various fields have been demanding in science and technology [1].Researchers have focused enormously on pioneering research in nanomaterial investigation and exploration to discover various applicability [2,3].Nanostructured materials have been widely explored in recent decades due to their unique physical, chemical characteristics, and enormous applications [4].Most importantly, there are many ways to tune the properties of nanostructured materials based on applications.The key parameter that governs the morphology of the nanoparticles is the synthesis method [5].Despite the fact that there are so many physical and chemical methods available to fabricate the concerned metallic or non-metallic nanoparticles, these methods require higher thermal environments, harmful chemicals, and acidic pH that are very harmful to the environment [6][7][8][9][10][11][12].Therefore, efforts and research on the eco-friendly synthesis of nanoparticles have been greatly intensified.
In recent times, the use of plant extracts to fabricate different nanoparticles has become one of the promising techniques [13][14][15][16][17][18].More importantly, the usage of hazardous reducing agents can be avoided and can be a cost-effective approach to develop a wide range of nanoparticles [19].Phyto-constituents such as polyphenol, alcohol, flavonoid, terpenoid, etc. are the backbone, and play a crucial role in synthesising nanoparticles [20,21].The bioactive components and secondary metabolites present in cabbage leaf extracts such as flavonoid, sterol, polysaccharide, vitamin, phenolic compound, organic acid, enzyme, and protein act as natural stabilisers and reducers to facilitate the formation of selenium nanoparticles/micro-rods [22].Selenium nanoparticles (Se-NPs) are gaining much more interest among the various nanoparticles available due to their excellent antimicrobial, anticancer activity, and low toxicity [23].Se-NPs synthesised by physical and chemical methods degrade/disintegrate toxic and hazardous chemicals into their intermediate products on their surface, which is very terrible to separate, limiting its applications in many fields [24].Therefore, an alternative sustainable green synthesis technique is adapted to prepare semiconductor Se-NPs.Green synthesised selenium nanoparticles can be easily tuned in shape and size for various uses and a variation in properties can be obtained for different applications, such as antioxidants, photocatalysts, or dye degradations [23,25,26].
In this modern industrialised era, various synthetic dyes such as anthraquinone, methylene blue, triphenylmethane, azo dyes including organic pollutants and antibiotics discharged by many industries including textiles, paper, pharmaceutical, cosmetics, and leather, persisted in environmental degradation and created severe health problems in addition to crucial environmental threats [15,[27][28][29][30]. Effluent treatment to remove organic contaminants is an essential feature of environmental technology.Dyes are organic pollutants and one of the major components of effluent discharged by various industries.Photocatalytic treatment of effluents to remove organic pollutants provided an excellent tool.There is an urgent call for the degradation of dyes causing severe water contamination due to their discharge into the water bodies [31].The aquatic niche is highly contaminated with methylene blue causing health risks for both humans and animals.Therefore, the dye industries are trying to eliminate the contaminants from their effluents [32,33].For this, there are numerous methods in practice, but all these methods are expensive and not sufficiently effective against all types of effluents.
In recent years, a great deal of investigation has been carried out to develop a convenient method to remove these toxic materials from water bodies [34].Photocatalysis is an advanced technical tool in which light radiation is used to generate active species that play an important role in wastewater treatment [35].And, when the photocatalyst is synthesised using the sustainable green method to eradicate the harmful dyes, it helps the environment for safety and sustainability [36].Therefore, the investigation of the photocatalytic nature using biosynthesised nanoparticles is relevant to today's world.The photosensitive catalytic material, such as the selenium nanoparticles/microrods, generates active charge carriers for the activity of photocatalysis.The mechanism involved behind the photocatalytic activity of selenium nanoparticles/micro-rods is the photo-generated holes in valence band (VB) h vb + by reacting with H 2 O to produce OH − and H + species.Moreover, e − in conduction band (CB) produces electron resonance plasma on the surface of selenium nanoparticles/micro-rods and they react with O 2 to create O 2 − .The OH − radicals and O 2 − radical anions produced to allow the oxidation process on the surface of the selenium nanoparticles/micro-rods catalyst to degrade methylene blue [37].Though there are some reports for Se-nanoparticles via chemical synthesis using catalysis.We believe there are limited studies on the preparation of Se-NPs through plant extract and its application in the field of photocatalysis of dyes [31,38,39].Also, traditional methods used for the manufacture or synthesis of photocatalytic materials face shortcomings such as limited reproducibility, higher costs, and longer processing times.In our report, we have achieved simplistic techniques for manufacturing nanostructure and microstructure photocatalytic material with an economical and ecofriendly approach with the exciting performance of the photocatalytic activity.In this research work, we have studied the formation of a crystalline mixture of selenium nanoparticles/micro rods (Se-NP/MRs) in a single container with an ecological approach.
The prominent role of cabbage leaf extract has stimulated the formation of mixed morphology (nanoparticles and micro rods).Bio-fabricated Se-NP/MRs has been studied for their exciting photocatalytic performance in the degradation of a model dye pollutant i.e. methylene blue (MB) under natural irradiation of sunlight.The degradation behaviour was also studied in pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models.This work represents a simplistic view of the ecological synthesis of Se-NP/MRs with mixed morphology (nanoparticles and micro-rods) for sustainable application in the photocatalytic degradation of various textile/industrial dye pollutants.

Materials
The fresh cabbage was obtained from the local market of Prem Nagar, Dehradun, Uttarakhand, India (30°19′59″N 77°57′39″E).Sodium selenite (Na 2 O 3 Se, 98%), methylene blue (C 16 H 18 ClN 3 S.xH 2 O), potassium bromide (KBr), and ethanol used in current research work were purchased from Sigma-Aldrich, India.All the chemicals used in these experiments were analytical grade and used without any further purification.Milli-Q water was used for the preparation of plant extract and experimental use.

Isolation of cabbage leaves extract
The isolated fresh cabbage leaf was thoroughly washed with running water and then with Millipore water to remove dust and other impurities.The aqueous extract of cabbage leaves was prepared by the slight modification of the reported procedure [40].The washed cabbage leaf was cut into small pieces and crushed in the mortar and pestle.
The raw material was boiled with Millipore water at 60°C for approximately 20 min.After 20 minutes of continuous boiling, the crude material was filtered by Whatman filter paper no. 1.And then, the filtrate was centrifuged at 10,000 rpm for approximately 5 minutes to remove the biomaterials.Then, the isolated extract was kept in the refrigerator for further studies.

Synthesis of selenium nanoparticles/micro-rods (Se-NP/MRs)
A conical flask containing isolated leaf extract (25 mL) was placed on a magnetic stirrer and the sodium selenite (10 mM) was added dropwise to the sodium selenite solution with continuous stirring.The addition of the isolated extract was continued until the colour of the solution transformed from light yellow to brick red.This colour change specifies the formation of selenium nanoparticles [41].The resulting solutions were kept in the dark for approximately 24 h.Subsequently, at 24 h, the resulting solution was centrifuged to isolate the synthesised nanoparticles.The schematic representation has been shown for the bio-fabrication of selenium nanoparticles/micro-rods in Figure 1.

Characterisation of bio-fabricated Se-NP/MRs
The X-ray diffraction pattern (XRD) of the fabricated nanoparticles/micro-rods was analysed using an X-ray diffractometer (X'Pert PRO, PAN-analytical) with Cu Kα radiation (λ = 1.5406Å) in the range of 10-70° with a step size of 0.015°.The chemical nature of the fabricated nanoparticles/micro-rods was analysed using the LASER Raman spectroscopy (RENISHAW in Via Raman confocal microscope system) at an excitation wavelength of 785 nm.The FTIR spectroscopy (Make: Thermo Nicolet, Model: 6700) was used to perform the analysis of chemical functional groups in the bio-synthesised nanoparticles/ micro-rods using the KBr pellet technique in the range of 4000-400 cm −1 .The SEM images of the fabricated nanoparticles/micro-rods were captured by using a scanning electron microscope (Make: Hitachi, Model: S-3400 N).The size distribution of the fabricated nanoparticles/micro-rods was analysed by dynamic light scattering (Make: Malvern Panalytical, Model: Zetasizer Ver.7.13; Serial Number: MAL1043157) particle size analyser.UV-visible absorbance and reflectance spectra of the fabricated nanoparticles/micro-rods were recorded using a UV-VIS-NIR spectrophotometer (Make: Varian, Model: 5000) with the spectrum in the range of 200-800 nm.The analysis of methylene blue degradation/ decolourisation was also performed using absorbance spectra of the UV-VIS-NIR spectrophotometer in the range 200-800 nm.

Photocatalytic degradation of methylene blue using bio-fabricated Se-NP/ MRs
The photocatalytic activity of bio-fabricated Se-NP/MRs using cabbage leaf extract was tested against the most popular dye, i.e. methylene blue under natural sunlight irradiation.The aqueous solution of MB was prepared at a concentration of 10 mg/L.The bio-fabricated photocatalyst Se-NP/MRs (100 mg/L) was added to the 100 ml of MB solution (10 ppm) and kept in agitation in the environmental conditions (temperature 32°C/23°C) under natural sunlight (average solar irradiance ~ 6.84 kWh/m 2 ; 12°00ʹ57" N 79°51ʹ31" E) until the solution loses colour (Figure 1).A similar concentration MB solution (without Se-NP/MRs photocatalyst) was also kept under natural sunlight to notice any colour variation.The observation of the photocatalytic activity was measured using a centrifuged aliquot (2 ml) at a regular interval of 20 min in a quartz cuvette in the UV-VIS-NIR spectrophotometer in the scanning range of 500-800 nm.The percentage of photocatalytic degradation of MB and rate constant was determined by the following Eqn.( 1) and ( 2), respectively: Where C 0 is the initial concentration and C t is the concentration at time t.
Where t is the reaction time and k is the rate constant.

X-ray diffraction analysis of Se-NP/MRs
The XRD study is an essential non-destructive technique to elaborate on the chemical compositions and phase structure of synthesised nanoparticles.

Raman spectra of Se-NP/MRs
The vibratory characteristics of the fabricated nanoparticles/micro-rods were investigated by laser Raman spectroscopy.A characteristic sharp resonance peak at 235 cm −1 was observed in the Raman spectra of the fabricated Se-NP/MRs, which was clearly attributed to the crystalline nature of the fabricated Se-NP/MRs, given in Figure 3.This shows the Se -Se bond for the Se-NP/MRs synthesised by cabbage extract.A small hump is also observed at 457 cm −1 which is ascribed to the Se-NP/MRs [43,44].Raman spectra have demonstrated the efficient reduction of sodium selenite by the cabbage extract competently to form selenium nanoparticles/micro-rod crystallites.It has been in good agreement with the XRD spectra of the Se-NP/MRs.

FTIR analysis of Se-NP/MRs
Figure 4, demonstrates the result of FT-IR analysis for the synthesised Se-NP/MRs by using the cabbage extract.In order to approve the role of the phytoconstituents of cabbage leaf in the bio-fabrication of selenium nanoparticles/micro-rods, an FTIR analysis of the biofabricated nanoparticles/micro-rods was performed.FTIR analysis of the fabricated nanoparticles/micro-rods showed the attendance of various functional groups on the surface of the Se-NP/MRs (Figure 4).The prominent peak that appears at 621 cm −1 describes the formation of Se-NP/MRs.The previous report also reveals selenium nanoparticles' formation [44].The broad peak that appears at 3429 cm −1 corresponds to the -NH and -OH groups, the band that appears at 2933 cm −1 has corresponded to the -CH aliphatic groups.The peak appears at 1634 cm −1 and 1594 cm −1 attribute to the C═O groups.The peak appears around 1386 cm −1 and 1146 cm −1 was attributed to the secondary amine and the symmetrical bending of the CH 3 groups, respectively.The aforementioned functional groups support the remnant of the bioactive components and secondary metabolites present in the extracts of cabbage leaves which facilitated the biofabrication of selenium nanoparticles/micro-rods.Thus, the Se-NP/MRs crystallinity was confirmed by the XRD, supported by the Raman and FTIR analysis.

SEM analysis of Se-NP/MRs
The morphological structure and size of the Se-NP/MRs was investigated by scanning electron micrograph analysis.The low and high magnifications of the SEM micrographs of the prepared Se-NP/MRs were represented in Figure 5(Figure 5a, b,and Figure 5c).The image captured by the scanning electron microscope revealed that the average particle size of the bio-fabricated Se-NP/MRs in the range of 35-95 nm with an almost spherical shape with irregular layers, which was the most reported form of selenium nanoparticles.In addition, it has been observed that the size of Se-NP/MRs is larger than its crystallite size due to the ensemble of some crystallites.Interestingly, the micro-rods or bars-shaped morphological structure of the selenium was also seen extensively.The phytoconstituents in cabbage leaves extract have played a notable role in reducing sodium selenite (Na 2 SeO 3 ) in a controlled manner to grow selenium nanoparticles, and also, stabilise the developed selenium nanoparticles.Moreover, the mechanisms behind the development of selenium micro-rods could be due to the phenomenon of ageing in the air of the selenium nanoparticles.The ageing could be due to the elimination of the cabbage leaves extract during the cleaning process that has worked as a stabiliser of the fabricated selenium nanoparticles.The results obtained have been supported by previous reports, in which it has been reported that the growth of rods from small selenium nanoparticles is due to the ageing of the particles in the air [45].The dimension of the observed selenium micro-rods was ~ 10 µm in length and ~ 2.5 µm in diameter.Fascinatingly, the mixed morphology of Se-NP /MRs, nanospheres were grown onto the micro-rods like budding on the stem that has been obtained in a single synthesis.The uniform distribution of the Se-NP/MRs confirmed the proper nucleation of nanoparticles by reduction from cabbage extract.This may be due to the sufficient amount of reducing agents (functional groups) in cabbage extracts.The previous report showed Se-NPs exhibited higher agglomerated nano-spherical particles [38].

Dynamic light scattering spectroscopy
The DLS pattern of the prepared nanoparticles/micro-rods elucidated that the optimised Se-NP/MRs range with the average particle size distribution of 204 nm and exhibited a narrow peak indicating that the synthesised nanoparticles are more in the mono-dispersed form with a lesser poly-disparity index as shown in Figure S1 (a and b).The polydispersity index was observed to be 0.096 which specifies the polydispersity nature and narrow size distribution of nanoparticles.The contrary of the size with SEM analysis is due to the presence of micro-rods or agglomeration of some Se-NP/MRs grains, which has been revealed by the mean intensity percentage of 2.  S1 (a and b).

UV-visible spectroscopy
The UV visible absorbance and reflectance spectra of the synthesised nanoparticles/ micro-rods have been recorded in the range of 200-800 nm as represented in Figure S2 (a and b).The UV visible absorption spectrum of the fabricated Se-NP/MRs was used to evaluate the optical properties.An absorption band appeared at 219, 271, 355, 422, and 636 nm in the absorbance spectra indicating preliminary confirmation of the formation of selenium nanoparticles/micro-rods using cabbage leaves extract as a reducing agent.A similar investigation was described by other investigators [46,47].The evaluation of the band gap energy (E g ) by Tauc's relation according to the following equation (3): Where Ɛ is the absorption coefficient, hν is the energy of the incident photons, and n = 2 is the power for the direct band gap, which characterises the electronic transition during the absorption process in the k-space.The extrapolation on the x-intercept of Tauc's plot had shown the band gap energy of Se-NP/MRs to be 2.14 eV (Figure 6).The band gap of green synthesised Se-NP/MRs is higher than commercial Se (1.8 eV).This detected blue shift may be due to the morphology of Se-NP/MRs and the influence on the nanomaterial's band gap or due to the capping ligands and proteins occur on the surfaces [45,48].Such optical properties of UV-Vis studies revealed that the Se-NP/MRs can be a potential candidate for the photocatalytic degradation of dye.

Methylene blue dye degradation
The photocatalytic performance of the bio-fabricated Se-NP/MRs was investigated to determine the MB degradation efficiency under the irradiation of natural sunlight and the performance is shown in Figures 7 and Figure 8.The photocatalytic degradation of MB was visually evaluated by observing the gradual changes of the solution from a bright colour to a colourless transparent solution (Figure 1).The distinctive UV-Vis spectroscopic absorption peaks of methylene blue were noticed to be at 665 nm with a shoulder band at 615 nm.Between these two peaks, the high-intensity absorbance peak at 665 nm was  considered to evaluate the gradual photocatalytic degradation activities.The degradation of the methylene blue using Se-NP/MRs was confirmed by a gradual reduction in the UV-Vis absorption intensity in continuous exposure to sunlight for 0-140 min, as displayed in Figure 7.
The prolonged exposure to natural sunlight irradiation has demonstrated evidence that the absorbance of methylene blue at maximum absorption wavelength (665 nm) was progressively declined due to the disintegration of methylene blue chromophoric structure.After 140 min of exposure under natural sunlight irradiation, the degradation rate of methylene blue reached up to ~ 97%, indicating that maximum methylene blue in the solution has been degraded.Figure S3 (a) shown the degradation rate (C/C 0 ) of methylene blue in an aqueous solution for bio-fabricated Se-NP/MRs, and to compare, direct photolysis of the dye solution was also implemented without adding any photocatalysts.The degradation rate [-ln(C/C 0 )] of MB with and without catalyst has also been plotted in Figure S3 (b), indicating an insignificant degradation pattern for the blank solution (without catalyst).The degradation rate of MB with Se-NP/MRs photocatalysts has been represented as the -ln(C/C 0 ) as the function of time using the Eqn.( 2).An excellent photocatalytic activity has been observed under natural solar irradiation, which could be due to the superior crystallinity and mixed morphology (nanoparticles and micro-rods) of the bio-fabricated Se-NP/MRs [49][50][51][52][53]. Table S2 shows a comparison of the current study with earlier reported investigations for the photocatalytic degradation of MB [12,[54][55][56][57][58].Tripathi et al., (2020) has reported selenium nanoparticles synthesised from Ficus benghalensis that were applied for photocatalysis with methyl bromide and it was found that only 57.63% degradation [38].However, the present work confirmed that the Se-NP/MRs obtained from cabbage leaves extract is efficient in photocatalytic performance.In view of that, the mixed morphology (nanoparticles and micro-rods) of biofabricated Se-NP/MRs may have provided a large active catalytic area for the degradation of MB.Together with the nanoparticles, the micro-rods would have contributed to the photocatalysis reaction.Henceforth, the synthesis of the Se-NP/MRs assortment is prudent for the photocatalysis of methylene blue.Therefore, Se-NP/MRs exhibited excellent photocatalytic activity for dye degradation under natural sunlight irradiation.

Degradation kinetics of Se-NP/MRs
MB degradation was successfully achieved in the presence of bio-fabricated Se-NP/MRs photocatalysts with a significant reduction in MB concentration under natural sunlight irradiation as observed with UV-visible analysis.However, the blank MB solution (without catalyst) was also exposed to sunlight simultaneously to observe any change in a bright colour or MB degradation.From observational and kinetic studies, it was established that the use of bio-fabricated Se-NP/MRs photocatalysts instigated the degradation of MB, while no significant changes/degradation have been observed in the MB solution without the use of a catalyst (blank MB solution) under exposure to solar irradiation.The degradation kinetics of MB is shown in Figure 9.The time-dependent absorbance spectra have indicated the maximum photocatalytic reaction during the time interval of 30-40 min with the applied bio-fabricated Se-NP/MRs photocatalyst.After 40 minutes of reaction time, the degradation rate became consecutively slower, which could be due to the hindrance/impediment of the active catalytic sites or the decrease of the interaction with the catalysts due to the presence of degradation products, since the maximum catalytic activity was perceived before the exposure time of 40 min.The continuous shrinkage in absorbance intensity designated the conversion of MB to leuco-MB through the catalytic behaviour of bio-fabricated Se-NP/MRs.
The behaviour of methylene blue degradation was mostly explained using two types of kinetics models i.e. pseudo-first-order (PFO) and pseudo-second-order (PSO) (Figure 9).These kinetics models considered that the rate of degradation for methylene blue on the surface of bio-fabricated Se-NP/MRs photocatalysts is proportional to the number of unoccupied sites.The PFO kinetics model has been governed by the physical process, and the PSO kinetics model is governed by chemical processes, including valence forces sharing or exchanging electrons between the Se-NP/MRs photocatalysts and methylene blue.The mathematical models of PFO and PSO are shown in Eqn. ( 4) and ( 5) [59,60].
Where q t and q e are the amounts of methylene blue degraded at time t and at equilibrium, respectively (mg/g).k 1 (min −1 ) and k 2 (g mg −1 min −1 ) are the degradation rate constants of PFS and PSO, respectively.
The linear plots of PFO and PSO for the degradation of MB are presented in Figure 9(a) and (Figure 9b), respectively, and the relevant kinetic parameters values are listed in Table 1.The results indicate that the correlation coefficient (R 2 ) of PSO is well fitted in comparison to the PFO kinetic correlation coefficient, which assumes that the photocatalytic degradation of methylene blue on bio-fabricated Se-NP/MRs surfaces is a chemical process, such as valence forces sharing or exchanging electrons between the methylene blue and Se-NP/MRs photocatalysts.

Possible mechanism of photocatalytic degradation
The possible photocatalytic mechanism for MB degradation using bio-fabricated Se-NP /MRs has been represented in Figure 10.Methylene blue (methylthioninium chloride; C 16 H 18 ClN 3 S) is a synthetic basic dye of a heterocyclic aromatic chemical nature with a conjugative aromatic structure of 3 hexagonal rings (N and S in para positions).It is an organic chloride salt that has 3,7-bis (dimethylamino)phenothiazin-5-ium as a counter ion.The characteristic absorption spectrum of MB comprises the absorption band at high  (superoxide anionic radicals) generated [37,38].
The mechanism depicted in Figure 10 has signified that the superoxide radical and the free hydroxyl radical play an important role in the degradation of the MB.Due to the oxidative properties of selenium, the reduction mechanism turns the blue colour of MB into colourless.Rapid electron transfer on Se-NP/MRs experiences to reduce MB, making it an effective photocatalyst.In photocatalytic degradation activities, the functions of generation of reactive oxygen species are very essential, which have been investigated by several researchers using specific chemical quenchers.The generation of reactive oxygen species offers fundamental information about their respective roles in the photocatalytic degradation of MB.The dominant species contributing the greatest effect on MB degradation have been reported to be • OH radicals followed by holes species using benzoquinone as O 2 •− radical species quenchers, triethanolamine as holes (h + ) quenchers, and isopropanol as • OH species quenchers [61].It has been reported that the main contributor to the high photocatalytic degradation of MB was the • OH species, which signifies the species largely responsible for the attack of the dye molecules.Another, the photo-excitation of a photocatalyst reduces the dissolved oxygen in the solution, forming O 2 •− radical species, while the h + species generated oxidises the H 2 O molecules into • OH species.To summarise the proposed mechanism, light sensitisation of bio-fabricated Se-NP/MR stimulates the excitation of ground state electrons in the VB to the CB of Se-NP/MRs.Therefore, it generates an electron-hole pair that migrates further to the surface for consecutive use in the photodegradation reaction [61,62].Furthermore, the scavenger test represents the mechanisms for the generation of active species to describe that the position of VB and CB plays a notable role in redox capacity.After absorption of light, the electrons get excited to CB and leave VB with h + .And then, h + captures the H 2 O and OH − to produce the OH • radical, while the O 2 traps the electron and generates the O 2 • radical [63].

Conclusion
Cabbage leaf extract contains various active phytoconstituents such as alkaloids, flavonoid terpenoids, alcoholic compounds, phenolic compounds, etc.These phytochemicals play an imperative role in the bio-fabrication of Se-NP/MRs.In the bio-fabrication of Se-NP/MRs, phytochemicals act as reducers and stabilisers during the formation of selenium nanoparticles.The XRD and Raman spectroscopic studies have evidence that the prepared selenium nanoparticles/micro-rods were highly crystalline in nature.The DLS and SEM study established that the fabricated nanoparticles were uniformly distributed with spherical to microrods or bar-shaped structures.Furthermore, photochemical investigations of the fabricated selenium nanoparticles/micro-rods conclude that the synthesised selenium nanoparticles/ micro-rods efficiently degrade the methylene blue dye under the irradiation of sunlight.Therefore, the results of our investigations are very useful in the treatment of effluents for the dye in the textile industry.In summary, the present study established a cost-effective and eco-friendly method for the fabrication of photocatalytic selenium nanoparticles/micro-rods.

Figure 1 .
Figure 1.Schematic representation for the bio-fabrication of Se-NP/MRs.

Figure 5 .
Figure 5. Scanning electron micrograph of bio-fabricated Se-NP/MRs (a, b, and c) with nano-spheres and micro-rods.

Figure 7 .
Figure 7. UV-Vis absorbance spectra of MB degradation in the presence of bio-fabricated Se-NP/MRs.

Figure 8 .
Figure 8. Percentage of MB degradation in the presence of bio-fabricated Se-NP/MRs.

Figure 9 .
Figure 9. Pseudo-first-order and pseudo-second-order kinetics models for the degradation of methylene blue.

Figure 10 .
Figure 10.Schematic view of the photocatalytic mechanism of bio-fabricated Se-NP/MRs.

Table 1 .
Kinetic parameters of the linear plots of PFO and PSO for the degradation of methylene blue.* of the benzene ring), at low energy (around 660-670 nm) (it moves according to the pH of the solution), and corresponding to n -π* transitions (n is the free doublet on the nitrogen atom of the C═N bond and the free doublet of the S atom at the S═C bond).A shoulder peak absorbance around 610-620 nm corresponds to a 0-1 vibratory transition (ground state level 0 to excited state level 1).Selenium is an element that belongs to the oxygen family of the periodic table and acts as a mild oxidising agent.The photocatalytic degradation of MB denotes the decolourisation effectiveness was increased progressively with the longer exposure time to natural solar irradiation (up to 140 min).However, compared to the MB blank solution (without Se-NP/MRs), the Se-NP /MRs assisted photocatalytic degradation efficiency to be ~ 97% in 140 min(Figure).Generally, MB is bleached and degraded to generate intermediates due to free radicals.The mechanism of the photocatalytic degradation of the methylene blue dye indicates the bio-fabricated photocatalysts Se-NP/MRs photo-generates holes in VB(valence band) h vb + to react with H 2 O and produce OH − and H + species.Another, an ERP (electron resonance plasma) has been created by e − in CB (conduction band) on the surface of Se-NP/MRs photocatalysts to react with O 2 to produce O 2 − .Consequently, the oxidation process has been enabled on the surface of Se-NP/MRs photocatalysts due to the OH − (hydroxyl radicals) and O 2 −