Synthesis, characterization and environmental remediation studies of Bi-substituted Li-Co spinel ferrites

Photocatalytic ability of Lithium-Cobalt ferrites was improved by doping with Bi. For the synthesis of different compositions of Bi-doped Li-Co Ferrites, the Micro-Emulsion method was used. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) and Scanning Electron Microscopy (SEM) analyses confirmed its spinel structure and morphology. Bi-substituted Li-Co ferrites exhibit 93.8% and 83.9% degradation of methylene blue and crystal violet, respectively. While using scavenger this value of degradation of crystal violet exceeds up to 98.5%. The rate constants of LiCo0.5Bi0.4Fe1.6O4 for methylene blue (MB) and crystal violet (CV) are 0.0203 and 0.01615 min−1, respectively. The band gap energy value calculated using tauc plot is 2.70 eV and 3.95Ev for Li-Co ferrites and Bi-doped Li-Co ferrites, respectively. Electrochemical impedance spectroscopy (EIS) studies confirmed that the doping of Bi in Li-Co spinel ferrites enhances its conductance. The use of these ferrites for photocatalytic degradation is more beneficial because they can be easily separable afterwards due to their magnetic nature which is confirmed by studying the re-usability of the as-synthesized samples. Highlights Li-Co ferrites and Bi-substituted Li-Co ferrites were synthesized by the micro-emulsion method. XRD and FTIR confirmed their spinel structural features. The prepared catalyst successfully degraded the MB (> 90%) and CV (> 80%). Antibacterial studies of as-synthesized samples were also done for S. aureus (Positive Strain) and P. aeruginosa (Negative Strain) microorganisms. GRAPHICAL ABSTRACT


Introduction
Energy crisis and environmental pollution are the major consequences of the world with emerging industries. Organic dyes, such as methylene blue, rhodamine b and congo red, are the main components of industrial waste water [1]. Scientists are striving to develop certain methods to overcome the increase in the release of organic dyes from different industries, including food and textile industry, directly into water bodies [2]. The best way for the degradation of these pollutants is photocatalysis, as it uses solar energy for the removal of these contaminants [3]. Photocatalysis plays a vital role in areas such as hydrogen production through water splitting, photoreduction and degradation of water contaminants. Besides, the energy crisis can be solved by an economic and environmentally safe photocatalysis technique [4]. Photocatalysis gained great attention for environmental remediation after 1972 when water splitting by UV light was done by using TiO 2 photoanode [5], due to its high efficiency and abundance [6]. There are various factors such as higher value of band gap and lesser absorption energy, which limit the applications of TiO 2 . So, it is the main concern of scientists to explore cheap nanomaterials that will help them to meet their required needs. The assembly for various composites for the photocatalytic degradation of pollutants has also gained much attention during the last few years [7,8]. Different photocatalysts, such as pristine TiO 2 , WO 3 , CdS, Fe 2 O 3 , ZnS or TiO 2 composites, N-Doped ZnO, Ag-based compounds and their composites, can be used for the removal of contaminants owing to their outstanding properties [9][10][11].
Owing to their chemical, electrical and physical properties, nanomaterials have attained great attention for the last few decades [12,13]. The properties of these nanomaterials are greatly dependent on their size and shape [14]. Among various ferromagnetic materials, ferrites are the most important due to the presence of double oxides between iron and other metals [15,16].
The key feature of ferrites includes the separation of sample from the solution due to their distinctive magnetic properties [17]. Additionally, these materials can be used as alternative catalyst supports due to their magnetic properties (high surface area, high dispersion and outstanding stability) [18]. Metal ferrites find their applications in water treatment, gas sensors, microwave and medical devices etc. because of their composition, high resistivity, grain size and magnetic nature etc. [19][20][21]. The electromagnetic properties of ferrites can be altered by changing the operating conditions such as frequency, temperature, pressure and time [22][23][24]. The spinal ferrites, such as MFe 2 O 4 (M = Cu, Co, Mn and Ni), exhibit unique catalytic effectiveness [25]. These spinal ferrites were initially prepared by using the ceramic method with very low yield at micrometric scale. Currently, wet chemical methods, such as co-precipitation, sol-gel and micro-emulsion, are used for the synthesis of ferrites [26]. The spinal ferrites manufactured by the wet chemical method have more distinctive properties than those made by the ceramic method [27,28].There are various methods, such as reverse micelle, sol-gel and citrate gel that can be used for the preparation of metal-substituted Li-Co ferrites [29][30][31]. The microemulsion route has more merits than other methods due to their low cost, simplicity and good control on particle size. At present, various shapes of ferrites have been produced (it may be thin films or in the form of powder). In a variety of electrical instruments, such as voltage-controlled rectifiers, low noise amplifiers and filters, ferrites find its applications as inductive components [32,33]. Typically, Fe-based spinal or Fe d-orbital materials show narrow band gap that causes extension of visible light absorption region [25]. The capability of ferrites can be increased by decreasing the eddy current and enhancing the electrical resistivity. Different types of bismuth and bismuth-substituted compounds are presently used in various fields including photocatalysis [34][35][36].The semiconductors, based on Bi doping or Bi a AO b structure, exhibit better properties due to their suitable band potential and considerable chemical stability [37]. At present, several bismuth-substituted compounds with different band gap energy values are developed that could lessen the recombination rate of e/h and be used for the photocatalytic degradation of various dyes/contaminants [5].
Nanotechnology plays a vital role in controlling the microbial activities. On the basis of microbial growth or potential ability for the inhibition of biofilm formation, various NPs have been investigated. It is concluded that these NPs generate the reactive oxygen species (ROS) by reducing the adhesion of cells caused by electrostatic force neutralization rendered by nanostructured surfaces [38]. The antibacterial activity depends mainly on its properties such as high surface area, reactive sites and unique crystal morphology.
In the current research, Li-Co ferrites and different concentrations of Bi-substituted Li-Co spinal ferrites have been synthesized by the micro-emulsion method.
All these spinal ferrites were tested for their ability to degrade various dyes under solar light and their results were also compared with Bi-substituted Li-Co spinal ferrites. We can assume that Bi-substituted Li-Co spinal ferrites will depict better antibacterial and photocatalytic activities. Cetyltrimethylammonium bromide (CTAB) Sigma Aldrich. All these chemicals of analytical grade were obtained from Sigma Aldrich (Germany) and no further purification was required prior to use.

Micro-emulsion technique
By utilizing the micro-emulsion methodology, Li-Co spinel ferrites doped with different concentrations of bismuth LiCo 0.5 Bi a Fe 2−a O 4 (0.0 ≤ a ≤ 0.4) were synthesized. The chemical compounds utilized in the synthesis of these ferrites are as follows: Bismuth (III) nitrate pentahydrate (Bi(NO 3 ) 3  36M solution of Cetyltrimethylammonium bromide were prepared in beakers separately and stirred continuously. The temperature of all the solutions was then raised up to 60°C. Then the ammonia solution was added until the pH of the solutions became 10 and then all the solutions were mixed and stirred continuously for 6 h. The solutions were kept for 24 h to wait for the precipitates to settle at the bottom of the beakers. Then they were washed using distilled water until the pH became neutral. The precipitates were then dried in an oven at a temperature of 100°C. Afterwards, the dried precipitates were ground into fine powder by pestle and mortar. To avoid the addition of impurities, pestle and mortar was washed by using acetone prior to the grinding of precipitates of each sample. At the end, the calcination of these ferrites was done in a muffle furnace (Vulcan a-550) at 800°C for 7 h (Figure 1).

Characterization techniques
To obtain the XRD patterns of the as-prepared sintered powdered samples, XRD Phillips X'Pert Pro MPD diffractometer was used with Cu Kα (λ = 0.15406 nm). The XRD patterns were studied at room temperature. To understand the nature and structure of the prepared ferrites, the FTIR analysis was done. FTIR spectrometer with built-in Zinc Selenide ATR and Zinc Selenide optics for high humidity was used to carry out the FTIR analysis. To confirm the morphology thermal emission a scanning electron microscope was used. Optical properties of as-synthesized samples were studied by Agilent Cary 60 spectrophotometer having spectral range between 200 and 800 nm. Electrochemical impedance spectroscopy was carried out using Gamry interface 5000 under the frequency 10 2 -10 6 Hz in 50 ml of 0.1 M KCl solution, containing 50 ml of 1 mM ferrocyanide/ferricyanide (1:1) electrolyte.

Photocatalytic degradation
The removal efficiency of methylene blue (MB) and crystal violet (CV) was examined by using LiCo 0.5 (Fe 2 O 4 ) and Bi-doped LiCo 0.5 (Fe 2 O 4 ) as a photocatalysts under visible light irradiation for 110 min. 5 mg of photocatalyst was added into 100 ml of (5 ppm) MB and CV solution. The whole solution was placed in the dark with continuous stirring for 60 min. This was done to attain the adsorption-desorption equilibrium between dye solution and photocatalyst. Then the whole solution was placed under visible light irradiation with constant stirring. 10 ml of this solution was taken after regular time intervals and the removal efficiency of dye was measured by using UV-VIS spectrophotometer. The percentage degradation of both dyes was calculated by using the following equation: where C t is the concentration at time t and C o is the initial concentration of dye solution.

Antibacterial activity
Disc diffusion method was used to carry out the antibacterial activity of as-synthesized magnetic samples. This method was employed against two test microorganisms S. aureus (Positive Strain) and P. aeruginosa (Negative Strain). Different concentrations of as-synthesized magnetic samples were sonicated with distilled water and placed on the surface of the nutrient agar plate by using forcep. The standard antibiotic Ciprofloxacin was taken as a positive control. The inhibition zone was seen clearly on the corners of agar plate after incubation at 37°C for 24 h. This zone of inhibition was measured in mm units.

X-ray diffraction (XRD)
The study of phases of the as-synthesized bismuthdoped Li-Co spinel ferrites LiCo 0.5 Bi a Fe 2−a O 4 (0.0 ≤ a ≤ 0.4) was done by using Kα radiations of Cu (λ = 1.5406 Å), on an X-ray diffractometer. XRD spectrum of LiCo 0.5 Fe 2 O 4 in 2θ range from 20°to 70° [39] shows  511) and (440), which confirms their spinel structure [40]. An additional secondary peak is obtained at 33.1°which is shown by ( * ) in the graph. The assynthesized ferrites have cubic structure containing small concentration of α-Fe 2 O 3 . By increasing the bismuth content the secondary peak vanishes and the crystal has spinel structure with single phase only [41]. The XRD pattern (ICDD No. 88-0671) was used to confirm the structure of the synthesized ferrites. The crystalline size can be calculated by using Sherre's formula where λ is the wavelength of X-rays (0.15406 nm), K is a crystallinity constant (0.9), β is the FWHM and θ is the angle of diffraction.    Figure 3 shows the XRD spectra of the recovered photocatalyst. The characteristic peaks remain unchanged, but there is a slight variation in the intensity of the peaks.

SEM
The SEM analysis was performed for the study of structural and surface morphology of as-prepared nanoparticles. In Figure 4(a), aggregation is due to the

FTIR
FTIR spectra of Bi-doped Li-Co spinel ferrites are shown in the figure. FTIR (Fourier transform infrared) spectroscopy has gained more attention than other methods of infrared spectroscopy. FTIR spectra give spectral analysis of the sample and clearly indicate about the position of bonds and ions in the crystal [42]. FTIR spectra also estimate the cationic distribution and chemical changes on both A and B sites. The absorption bands present at around 508 and 410 cm -1 , as shown in Figure 5, are the main characteristic bands which confirm the formation of spinel ferrites [43]. The absorption bands in given spectra show the stretching vibration of metal and oxygen atom present in the crystal lattice. The spinel structure of synthesized ferrites is also confirmed by the fact that all absorption bands of Bi-doped Li-Co ferrites are present between 400 and 600 cm −1 [39].

Optical properties
UV-VIS spectroscopy is used to measure the optical properties of Bi-doped Li-Co ferrites. Figure 6 shows that Li-Co ferrites have absorption bands both in UV and in visible regions. Its absorption peaks were observed between 350 and 500 nm. The presence of absorption bands in the visible light area is the reason behind the photocatalytic activity of Li-Co ferrites. Absorption capacity in the visible region is enhanced in the case of Bi-doped Li-Co ferrites. It is the main reason behind the exceptional photocatalytic activity of Bi-doped Li-Co ferrites than pure Li-Co ferrites. Tauc plot is used to measure the direct band gap of as-synthesized samples by using the following equation: where hν is the photon energy, α is the absorption coefficient, k is a constant value and n represents the transition type (For direct transition, n = 2) [44]. Extrapolation of the linear curve was done to find out the value of E g . The band gap value calculated for Li-Co ferrites and Bi-doped Li-Co ferrites is 2.70, 3.01, 3.03, 3.72 and 3.95 (Figure 7).

Photocatalysis
where C t is the concentration at time t and C o is the initial concentration of dye solution [45,46]. Table 1 shows the comparison between %degradation of MB and CV by using as-synthesized products as photocatalysts. Figures 8(g,h) and 9 (g,h) show the plots between ln(C o /C t ) and irradiation time (t). According to the first-order kinetic law: [ln(C o /C t ) = kt], from the slope of the straight line drawn using linear regression, the value of degradation rate constant (k) can be determined [47]. All these values show that Bi-doped LiCo 0.5 (Fe 2 O 4 ) exhibit phenomenal photocatalytic performance. The reason behind this increased photocatalytic activity is the increased number of reacting species, greater surface area and large number of active sites (Figures 10 and 11). Table 2 shows the comparison of photocatalytic degradation of the already reported literature and the current work, which confirms the novelty of the current work.

Mechanism of photocatalysis
The very first step involved in the mechanism of photocatalytic degradation of dyes is the adsorption of dye molecules on the surface of NPs of catalyst. When samples are irradiated with solar radiations, the electrons (e − ) in the valence band (VB) of LiCo 0.5 (Fe 2 O 4 ) and Bi-doped LiCo 0.5 (Fe 2 O 4 ) are excited to the conduction band (CB) [52]. This will ultimately create a gap with positive charge in the VB [53]. The charge separation occurs due to the difference in energy bands between LiCo 0.5 (Fe 2 O 4 ) and Bi-doped LiCo 0.5 (Fe 2 O 4 ). It will also hinder the recombination process. The photogenerated electron will react with O 2 present in the solution to give O 2− ions which will form OH hydroxyl radical on reaction with H 2 O [54,55]. In photocatalytic reaction, these photocatalytic reactive species are able to attack and degrade the organic pollutants and convert it into simple compounds such as CO 2 , H 2 O and other by-products. In LiCo 0.5 (Fe 2 O 4 ), the low degradation efficiency is due to the immediate recombination of photogenerated electron and hole pairs which will not allow the electron and hole pairs to move and interact with dye solutions. It is clear from the literature that when solar radiation irradiates the photocatalyst, the photons directly cause the breakdown of bonds of dyes and cause an enormous enhancement in the kinetic energy of free radicals formed [56,57] (Table 3).
The whole mechanism behind this photocatalytic activity can easily be understood by Figure 12

EIS
Electrochemical impedance spectroscopy was performed using the frequency range of 10 2 -10 6 Hz and 50 mL of 0.1M KCl solution containing 50 mL 0f 1 mM ferrocynide/ferricyanide (1:1) electrolyte. Platinum wire and silver/silver chloride Ag(s)/AgCl(s) were utilized as counter and reference electrodes, respectively. For the preparation of working electrode, slurry was made by mixing 5 mg of as-synthesized samples with nafion and it was coated on Indium-Tin oxide (ITO)-coated glass substrates. The prepared working electrode was then dried for 1 h at room temperature. The main purpose  behind this EIS analysis is to analyse the charge transfer properties of as-prepared photocatalysts. It can be seen in Figure 13 that the diameter of Nyquist semicircle of Bi-doped Li-Co ferrites is much smaller than that of undoped Li-Co ferrites. The main reason behind this smaller diameter is the lower charge transfer resistance that causes the effective separation of photogenerated e --h + pair, transport charges between interfaces and lessens the chances of recombination of electron/hole. On the other hand, greater impedance of undoped Li-Co ferrites causes less conductive transfer of photogenerated e − which plays an important role in photocatalytic reaction.

Role of scavenger
Different types of scavenger have been used during the photocatalysis process such as 2-butanol, H 2 O 2 , Ascorbic acid (Vit.C) etc [44]. And their effect on the photocatalytic activity of the synthesized particles is studied. In the present study, Ascorbic acid or Vit.C is used as a scavenger with CV dye and its effect is studied. Ascorbic acid enhances the photocatalytic activity of Bidoped LiCo 0.5 (Fe 2 O 4 ) photocatalyst. Table 1 shows that the photocatalytic degradation of CV is less than that of MB which is 83.9% and 93.8%, respectively, with Bidoped LiCo 0.5 (Fe 2 O 4 ) but with the addition of Vit.C as a scavenger the degradation of CV is enhanced up to 98.5%. Figure 10 shows  with the water molecules and form hydrogen peroxide. The electrons and hole on the surface of the photocatalyst then undergo redox reaction with hydrogen peroxide and form hydroxyl radicals which then degrade the organic dyes to the degradation products like CO 2 and H 2 O. The photocatalytic degradation of methylene blue in the presence of Ascorbic acid is highly reversible. As the light source is removed, the oxidized product changes rapidly to its coloured reduced form at such velocity to make the study of photocatalytic degradation by using spectrophotometer difficult [58] ( Figures  14 and 15).

Reusability
The structure stability and reusability are the important requirements for the development of a photocatalyst.
The recycling experiments of repetitive cycles were performed for the photodegradation of methylene blue to evaluate the durability and stability of as-prepared material [59,60]. The Bi-doped Li-Co ferrites showed similar photocatalytic efficiency after three successive cycles (Figures 16 and 17). It is also noticed that the XRD patterns did not show certain variations after three cycles. It suggests that Bi-doped Li-Co ferrites can be widely utilized for the treatment of industrial waste water.

Antibacterial activity
Antibacterial activity of as-synthesized magnetic samples was carried out by using the disc diffusion method. The results of this method are reported in Table 4.
On the basis of zone of inhibition, Bi-doped LiCo (Fe2O4) showed maximum antibacterial activity against both microorganisms S. aureus (Positive Strain) and P. aeruginosa (Negative Strain) among all the synthesized nanoparticles [61]. This happened due to the doping of Bismuth with LiCo(Fe2O4). The enhanced antibacterial activity of Bismuth nanoparticles and Bismuth-doped compounds is already reported in the literature [62,63]. The whole scenario behind this increase in antibacterial activity is the presence of attraction between positively charged heavy metal ions and negatively charged cell membrane. These ions penetrate in the cell membrane and inactivate the proteins present on the surface of the bacterial cell. This results in reduction in membrane permeability and ultimately causes the demise of microbes (Figures 18  and 19).

Conclusion
Micro-emulsion method was used to synthesize magnetically recyclable Bi-doped Li-Co ferrites. Characterization of these spinel ferrites was done by using XRD, SEM and FTIR for structure morphology and spectral analysis, respectively. The antibacterial activity of these ferrites was studied by using S. aureus (Positive Strain) and P. aeruginosa (Negative Strain) and it can be clearly seen that Bi-doped Li-Co ferrites exhibit more antibacterial activity than pure LI-Co ferrites. The photocatalytic degradation of methylene blue and crystal violet was studied by using as-synthesized samples as photocatalysts. The results show that Bi-doped Li-Co ferrites exhibit exceptional photocatalytic activity than bare Li-Co ferrites which is confirmed by the EIS analysis. These Bi-doped Li-Co ferrites find their potential applications in both environmental and biological fields due to their sustainability, recyclability and cost effectiveness.

Disclosure statement
No potential conflict of interest was reported by the author(s).