Synthesis of CaCO3 nanocomposite from natural carbonate source and its effect on the inclusion of Eu3+ ions for photocatalytic activity

Abstract Nano CaCO3 (nC), various polymer mediated nano CaCO3 and Eu3+ anchored nano CaCO3/PEG were synthesized from natural carbonate source (Dolomite) (CaMg(CO3)2) using a novel, low cost, non-toxic and effective route biomimetic synthesis. The mineralogy, structural, thermal stability, and morphology analysis of the products were assessed through FTIR, XRD, TG-DTA, FE-SEM with EDX mapping, and HR-TEM with SAED. Results show that the prepared samples were in the form of calcite with rhombohedral (25-36 nm) structure. The prepared products exhibit good thermal stability up to 844° C. The products show a spherical and porous like structure. The elemental compositions of the products were confirmed through EDX analysis. All the plane values (observed from SAED) were matched with XRD results. An increase of Eu3+ in CaCO3 shows that the absorption and emission peaks are blue shifted and the band gap values are red shifted. The measured photoluminescence quantum yield values of 0.02 Eu3+:CaCO3/PEG (nCE1) and 0.08 Eu3+: CaCO3/PEG (nCE4) are 21% and 19%, respectively. Finally, the products (nC, nCE1, and nCE4) were subjected to photodegradation application and results shows that nCE1 has good photocatalytic activity against methylene blue (87%) for 90 min.


Introduction
Globally, in recent decades, due to the vast developments in industries including chemical, mining, and medical industries, and socio-economic development, serious environmental pollution is unavoidable one with various dyes and pigments.Specifically, water pollution is a major threat to the people.The color and quality of the water vary with the level of pollutants.The presence of dyes and pigments in the water can prevent the penetration of sunlight into the bottom of the river, lake, or other water bodies, causing a reduction in dissolved oxygen content and a threat to aquatic plants, animals, and finally humans.Thus, several attempts have been made nowadays to identify and remove the same from the water.Many literatures have been reported on the photocatalytic process in the aqueous medium (Mostafa et al. 2019;Lu et al. 2019;Sun et al. 2015).Based on the above applications and needs, many numbers of researches are required to analyze the materials using current technology.However, it is important to identify the best photocatalyst material with increased efficiency and low cost.To achieve the above, the present study is concentrated on natural materials like carbonates.
Calcium carbonate (CaCO 3 ) is one of the wellknown and extremely important minerals in the Earth's crust and finds the applications in fundamental research and industry.Because of its beneficial properties (structural, optical, and surface), synthesized CaCO 3 can be used in a variety of applications include filler materials, paints, plastics, paper, textile, rubber, sealants, cosmetics, toothpaste, foodstuffs, photocatalyst, biosensor, medicine, pharmaceutical industry, and drug delivery systems (Yamanaka et al. 2012;Price et al. 2011;Kim et al. 2009;Xu et al. 2007).
In the present study, natural dolomitic (CaMg(CO 3 ) 2 )) rock was selected as a source to extract the CaCO 3 nanomaterials.It is a common, major constituent of sedimentary formation in association with calcite and rock-type carbonate mineral that contains Ca, Mg, and trace elements (Sr, Cu, Fe, Na, and Si) (Somarathna et al. 2016).It is more abundant (2%) than calcite in the Earth's crust, geographically vast and ranging in thickness from hundreds to thousands of feet in the Earth's crust.The majority of natural carbonate source-rich rocks were initially calcium carbonate muds that underwent post depositional magnesium alteration to become natural carbonate sources in the Earth's crust.The availability and quality of natural carbonate sources were investigated by some researchers (Soliman et al. 2004;Anderle 1997).Rich deposits are available in many states of India, such as Rajasthan, Madhyapradesh, Tamilnadu, Andra Pradesh, Assam, Mizoram, Meghalaya, Orissa, Bihar, Himachal Pradesh, Uttar Pradesh, Sri Lanka, Indonesia, and China (Ramasamy and Rajkumar 2005).Researchers have taken CaO, MgO, Ca, and CaCO 3 from the above deposits (Arifin et al. 2019;Mantilaka et al. 2013) to use in different ways in both small and large scale industries.
Precipitation, sol-gel, atomized microemulsion (Chatterjee and Mishra 2013), hydrothermal (Karimi and Ranjbar 2016), mechanochemical (Tsuzuki et al. 2000), co-precipitation (Nassar et al. 2015), and microwave solid-state synthesis (Cheng et al. 2015) have all been used to prepare nano CaCO 3 .Some limitations of these methods are high by-products, usage of hot chemicals, high cost, and complex procedures (Barhoum et al. 2015;Nassar et al. 2015).Thus, a straightforward, environmentally friendly, very costeffective and simple biomimetic method has been adopted in the present study to extract nano CaCO 3 .This method is suitable for the preparation of the high surface area, better thermal stability, and highly effective noble nano-CaCO 3 without any unnecessary impurities.The present study is mainly focused on the extraction of pure and well crystalline nano CaCO 3 and Eu 3þ doped nano CaCO 3 from naturally abundantly available impure dolomite rock (CaMg(CO 3 ) 2 ) through biomimetic method.Based on the literature survey and to the best of our knowledge, it is the first study to synthesize above products.Ramasamy et al. (2018) and Yao et al. (2010) were extensively executed biomimetic synthesis and precipitated nano calcium carbonate from various natural sources.
Due to their versatility, dependability, and stability, inorganic materials have recently attracted a lot of attention for research and development, particularly lanthanide ion-doped phosphors.Due to their incompletely occupied 4f and unoccupied 5d orbits and unique spectrum characteristics, rare earth elements have numerous great optical, electrical, and magnetic properties.As we know, trivalent Europium (Eu 3þ ) has been extensively studied as an activator ion because of its distinct 4f-4f transition.It could be a good dopant in various hosts such as oxides, alumino-silicates, oxy-sulfides, silicates, and fluorides.There has been growing interest in the preparation and behavior of the CaCO 3 : Eu 3þ by various methods, including the carbonation method, double decomposition method, solid state synthesis, and microwave-co precipitation method.
The scope of this present work is to extract the CaCO 3 nanoparticles from natural carbonate (dolomite) source.The effect of Eu 3þ doping in extracted nano CaCO 3 from a natural carbonate source was explored using the biomimetic method.The efficiency of the products was analyzed for the photodegradation of methylene blue dye.

Materials
The following chemicals were used in the present study: Anhydrous Sodium Carbonate, Sucrose, EuCl 3 , PEG 6000 (Poly Ethylene Glycol), PVA (Poly Vinyl Alcohol), CTAB (Cetyltrimethylammomium Bromide) and Hydrochloric acid.These chemicals were purchased from Sigma-Aldrich.Acid-cleaned glassware was used.For all sample preparation and dilution, ultra-pure millipore water was used.

Sample collection: Natural carbonate source
Ten naturally well-grown crystallized natural carbonate sources (Dolomite rock) were recently collected from Arisipalayam (Latitude-1165' 92'' N; Longitude-7814' 00'' E), Salem District, Tamilnadu, India with 5 Kg (approximately) of weight.Collected samples were put through a variety of pretreatments using diluted HCl to separate organic debris and other contaminants.
All the samples were ground to micron-sized particles and subjected to FTIR analysis to assess the mineralogy of all the samples.There are not many variations in the peak positions and intensities of the peaks of all the samples.Hence, only one sample is taken for further analysis.
Nano CaCO 3 preparation (with and without the polymer) The nano CaCO 3 (nC) was extracted from the Natural carbonate source according to the following procedure.In the first step, the Natural carbonate source was crushed and blended.A crushed natural carbonate source sample was ground using agate mortar to get a fine powder.The powder was sieved using a stainless laboratory test sieve (90 mm).The sieved sample was calcined at 900 C for two hours in a muffle furnace to get CaO.MgO. 10 g of CaO.MgO powder was dissolved in 100 ml of 1.15 M sucrose solution and the solution was vigorously stirred for one hour at room temperature.Under suction, the solution was filtered to get a soluble calcium sucrose solution.The calcium sucrose (CS) solution was collected in a 500 mL three-necked rounded bottom flask.The reaction mixture contained 80 ml of 1 M sodium carbonate solution, and the polymer was added drop wise to obtain nano calcium carbonate according to the following reactions. (1) The obtained CS solution was shifted into a 500 ml three-necked glass flask.10 ml of PEG, 80 ml of 1 M sodium carbonate, and a various concentration (0.02, 0.04, 0.06 and 0.08 Mole) of EuCl 3 solution were added drop wise to get a Eu 3þ doped CaCO 3 nanomaterial.The solution is continuously stirred and maintained at 80 C for two hours.The possible mechanism for synthesizing doped nano calcium carbonate is given above.

Characterization techniques
The mineralogical and functional group characterizations were carried out by FT-IR (SHIMADZU-8400) having a resolution of ± 4 cm À1 in transmittance mode within region 4000-400 cm À1 through KBr pellet method.The purity and crystalline properties were examined by X-ray diffractometer (X'pert PRO) with CuK a radiation (k ¼ 1.5406 Å), a current of 30 mA and 40 kV as an operating voltage.Thermo gravimetric (TGA) and differential thermal analysis (DTA) of the samples were performed by Simultaneous Thermal Analyzer (NETZSCH -STA 449F3 -JUPITER) at a heating rate of 10 C/min in the nitrogen gas atmosphere.The morphology of the sample was analyzed using Field Emission Scanning Electron Microscopy (CARL ZEISS-SIGMA-300) [Accelerating voltage range: 0.3 kV to 30 kV, resolution: 15 nm (1 kV) in high vacuum mode (HV)].Mapping with energydispersive X-ray analysis (EDX) measurement was done using Brucker (129 eV).To know the internal morphology and particle size, Selected Area Electron Diffraction (SAED) patterns were captured using High Resolution Transmission Electron Microscopy (HR-TEM) [maker: FEI, model: TECNAI G2-20TWIN].The operating voltage was 80-200 kV and the resolution was 2.4Ao (Adoptive Optics).The diffuse reflectance spectra were recorded using a UV-Vis-DRS (SHIMADZ U 3600 plus) spectrophotometer.The photoluminescence property was studied using a Horiba Jobinyvon Fluoromax-4 spectrometer [The operating wavelength was 200-700 nm; a Xenon arc lamp of 150 W was used as a source].The quantum yield measurements were taken by a fluorescence spectrometer [model-F7100].The photocatalytic activity was recorded by a UV-Vis spectrophotometer [SHIMADZU-UV 1800] in absorbance mode (accuracy 0.3 nm, 20-W halogen lamp).

Photocatalytic analysis
The photocatalytic performances of nC, nCE1, and nCE 4 for degradation of Methylene Blue (MB) dye were analyzed through cylindrical water jacket mini glass reactor with xenon lamp (Model: HML-MP 88, Power supply: 220/230V, Wavelength: 365 nm and 50 Hz,) as a source of UV light illumination.The reactor (photochemical reactor) was used to maintain constant temperature (25 C).At first, a photocatalytic suspension was prepared by taking 20 mg of the prepared catalyst powder in 250 ml of aqueous solution.The photocatalysis of 20 ml of an aqueous MB dye (0.0001 M) solution without catalyst was tested for 15 min under UV light irradiation.After 15 min, the suspension was exposed to UV light.To evaluate the change in concentration of MB dye, 20 ml of suspension was collected at a regular interval of 15 min.Subsequently, the collected solution was loaded into a UV-Vis spectrophotometer to estimate the concentration of MB dye.

Results and discussion
FTIR analysis of natural carbonate source, synthesized CaCO 3 , CaCO 3 /PEG and PEG/CaCO 3 : Eu 31 nanoparticles Figure 1(a) shows the FTIR spectrum of natural carbonate sources.The present results are in good accord with the absorption frequencies reported by Adler and Kerr (1962), Ghosh (1978), andRussell (1987).The absorption peaks observed in the spectrum exhibit the presence of a natural carbonate (Dolomite) source (major) and Orthoclase feldspar (minor).According to Herzberg (1945), the frequency assignments of the minerals of carbonates are a symmetric stretching t 1 (1080-1099 cm À1 ), an out-of-plane bending t 2 (833-909 cm À1 ), an asymmetric stretching t 3 (1420-1450 cm À1 ) and a planer bending t 4 (666-769 cm À1 ).But the symmetric oscillation (t 1) is infrared inactive in dolomite and calcite.The presence of a natural carbonate source (CaMg(CO 3 ) 2 ) is assessed by the observed absorption bands at 728 (t 4 ), 880 (t 2 ), 1439 (t 3 ), 1820 (t 1 þ t 4 ), 2527 (t 1 þ t 3 )), 2624, 2892, and 3019 cm À1 (Ramasamy and Rajkumar 2005;Russell 1987).The observed peak around 1439 cm À1 in this case confirms the presence of a predominant and aggregate natural carbonate source (Adler and Kerr 1963a).The peak at 1439 cm À1 , which lies between 1400 and 1450 cm À1 , also dictates the formation of a natural carbonate source at low pressure (William B. White 1971).According to Ramasamy et al. (2018), the appearance of other frequencies such as 463 (Si-O-Si bending vibration) and 1042 cm À1 (Si (Al)-O stretching vibration) indicates the presence of orthoclase feldspar.In the view of Kuppayee et al. (2011), the presence of the absorption peak at 3441 cm À1 represents the presence of O-H.
Figure 1(b) depicts the FTIR spectrum of synthesized nano CaCO 3 through the biomimetic method.Based on the view of Adler and Kerr (1963b), the observed peaks at 710, 875, 1423, 1796, and 2512 cm À1 show the presence of calcite (CaCO 3 ).Figure 1(c) reveals the FTIR spectrum of PEG medicated nano CaCO 3 (represented as nCP).From the figure, the presences of all major and minor peaks are matched with the Figure 1(b).Additionally the peak observed at 1062 cm À1 (Figure 1c) indicates the presence of PEG on the surface of nano CaCO 3 (Dobre et al. 2021).The peaks at 3440 and 3425 cm À1 exhibit the presence of OH molecules on the product surface (Kuppayee et al. 2011).Other than calcite, there is no additional peak found.It is noteworthy to mention that the products are in well-purified form of CaCO 3 in the phase of calcite D 3h state (Ramasamy et al. 2018).
Figure 2(a-d) depicts the FTIR spectra of various concentrations of (0.02 M, 0.04 M, 0.06 and 0.08 M) Eu 3þ doped PEG/CaCO 3 nanoparticles (represented as nCE 1, nCE 2, nCE 3 and nCE 4 ).The observed major and minor peaks are very well matched with the calcite peaks (Figure 1b).The observation of a peak in the range of 1062-1104 cm À1 depicts the presence of PEG in the product (Leon et al. 2017).The peaks present in the range of 451-456 cm À1 show the inclusion of Eu 3þ in the synthesized products (Bispo et al. 2017).From the figure, the width of the peaks t 3 and t 4 of Eu-containing samples are narrower than the synthesized CaCO 3 (without doping).It is due to the impact of Eu 3þ ions in the CaCO 3 lattice (Gao et al. 2016).The characteristics peak in the range of 3440-3429 cm À1 shows the OH molecules in the products (Kuppayee et al. 2011).
Figure 3(c) shows the XRD pattern of CaCO 3 /PEG composite.The observed major and minor planes in all three cases are matched with planes of pure CaCO 3 (Figure 3b).Here also, no other planes are observed.While adding the PEG the major plane at 29.34 is slightly shifted to the lower angle side.This shows that PEG could be  embedded in the product.From the FTIR and XRD analysis, as no impurity peaks are observed, the achieved product has high purity.It is noteworthy to mention that impurities available in natural carbonate source rock could be filtered or removed by the synthesis.The crystallite sizes of synthesized nC and nCP are calculated using Scherrer's formula (Scherrer 1918).
Where D -is the crystallite size (nm), k -is the wavelength of the Cuk a (1.5406), k-constant (0.94), b -is the FWHM (full-width half maximum), and h À is the diffracted angle.The calculated average crystallite sizes (Table 1) of synthesized nC and nCP are 25 and 29 nm, respectively.Average crystallite sizes of synthesized products are also calculated through W-H plot (method and plots (Figure S1) are given in Supplementary Material) and calculated values are presented in Table 1.This result shows the role of polymers in controlling the agglomeration of the products or reducing the nucleation of growth products as it is represented in elsewhere.In this instance, PEG with more functional groups covers a huge surface area of the nanoparticles with more steric hindrance, which has a detrimental influence on the crystal particles ability to develop (Ramasamy et al. 2018).
The XRD patterns of various concentrations of Eu 3þ anchored CaCO 3 are shown in Figure 4(ad).The results reveal that the observed diffraction patterns are exactly matched with the diffraction planes of calcite (CaCO 3 ) (JCPDS card NO: 86-2343) as observed earlier.No other secondary phases are detected, which indicates that the Eu 3þ ions have been successfully anchored into the CaCO 3 lattice (Zhou et al. 2014;Candelario-Flores et al. 2021).The intensities of all diffraction peaks are decreases as the concentration of Eu 3þ anchored increases (Figure S2).This dictates that the dopant Eu 3þ (ionic radius: 0.095 nm) could be incorporated into the inner lattice of Ca 2þ ions (ionic radius: 0.099 nm) (Sharma et al. 2012).
The average crystallite sizes of nCE 1 , nCE 2 , nCE 3 and nCE 4 are 30, 29, 27 and 26 nm respectively.This shows that the size of nanoparticles decreased with the increase of Eu 3þ (Table 1) concentrations.The reduction of crystallite size is mainly due to the distortion of the host CaCO 3 lattice by the foreign impurity Eu 3þ that reduces the nucleation and succeeding growth rate of CaCO 3 nanoparticles (Gao et al. 2016).The nCE 1 and nCE 4 were chosen for further analysis based on crystallite size.
In addition to the crystallite domain size, microstrain-induced lattice distortion also contributes to the XRD peak broadening (Danilchenko et al. 2002).The crystalline nature of nanoparticles is changed due to the influence of microstructure and dislocation density.So, the following equations (Dutta et al. 2013) can be used to figure out things like microstrain (e) and dislocation density (d).
D is the particle size (nm), b-FWHM, and h-Bragg's angle.The above parameters are calculated (Table 1).From the table, it is seen that all the parameters are increased with a decrease in crystallite size.According to Theivasanthi and Alagar (2012), the existence of dislocations greatly influences many properties of the TG-DTA analysis of a natural carbonate source, nC , nCE 1, and nCE 4 nanoparticles The TG and DTA plot of natural carbonate source (CaMg(CO 3 ) 2 ) is shown in Figure S3.Two successive weight losses are observed in the sample.In the first step, a very small weight loss (2.3%) occurs at 359 C due to dehydration of chemically and physically adsorbed H 2 O or destruction of organic compounds (Zhao et al. 2015).The second (major) weight loss (44.11%) is observed at 877 C. It is due to removal of CO 2 and thermal decomposition of CaMg(CO 3 ) 2 to CaO.MgO (Mantilaka et al. 2014).
Figure 5(a-c) shows the TG-DTA curve of nC , nCE 1, and nCE 4 nanoparticles.From the figure, the first weight losses (3.72%), (3.41%), and (2.35%) were observed at 213 C, 277 C , and 231 C in nC, nCE 1 , and nCE 4 respectively.It is due to the dehydration of adsorbed water or organic compounds (Zhao et al. 2015).The presence of physically bound water can be possible in the nano CaCO 3 since it has a high surface area.It enables water molecules to easily adsorb on the surface.The small weight losses (1.06%) and (1.75%) are observed at 345 C and 338 C in Figure 5(b and c).This is owing to the combustion of PEG on the surface of products (Balaganapathi et al. 2017).The major weight losses were recorded in the endothermic range at 844 C, 819 C and 813 C with an estimated mass loss of 42.2%, 40.11% and 40.25% for synthesized nC , nCE 1 and nCE 4 respectively.This may be correlated to the decomposition/phase transformation of CaCO 3 to CaO (Ramasamy et al. 2018).
The thermal stability of nC , nCE 1, and nCE 4 are 844 C, 819 C and 813 C. It is observed that the increase in Eu 3þ concentration decreases the thermal stability.This demonstrates that the Eu 3þ has been doped successfully in the CaCO 3 host lattice (Cheng et al. 2014).Hussein et al. (2020), Akhtar and Yousafzai (2020) and Ghadami Jadval Ghadam and Mohammad (2013) have prepared CaCO 3 and reported the decomposition temperature of CaCO 3 in the range of 500 C to 755 C. Mostafa et al. (2019)   temperature of the product is 680 C to 780 C with a weight loss of 44%.As well, Abeywardena et al. (2020) reported that the decomposition temperature of nano CaCO 3 extracted from natural carbonate source with surfactant and SDS assisted by the same biomimetic method is in the range of 600-800 C respectively.They had achieved comparable weight loss (42%).In the present work, nC, nCE 1 , and nCE 4 composites have higher thermal stability with low weight loss when compared to the above pioneer works. .This shows that the presences of elements in the synthesized products are Ca, C, and O only.This shows that the products are in a pure form of nano-CaCO 3 .This was confirmed already through FTIR analysis (Figure 1b). Figure 7(a-b) depicts the morphology of nCE 1 .The addition of PEG and doping of 0.02 Eu 3þ in nano CaCO 3 shows aggregated spherical and rhombohedral-like structures.PEG acts as an encapsulating agent to control the process of agglomeration and maintain that distinct structure by the interaction of Ca ions with ethylene oxygen groups (-CH 2 -CH 2 -O-) of PEG (Qiu et al. 2008).Also, the strong electrostatic interaction between lone pair electrons on the oxygen atoms of PEG and Ca 2þ ions helps the formation of PEG/CaCO 3 and to control the agglomeration.PEG can combine with Ca 2þ ions and easily adsorb on the surfaces of CaCO 3 , and thus the activities of the CaCO 3 are greatly suppressed.In addition, the length of PEG molecular chain, viscosity of the solution, and the shape of PEG adsorption layer are all important factors for the formation of different morphologies of CaCO 3 (Xu et al. 2011).nCE 4 has a distinct rhombohedral morphology with a small crystallite size (Figure 8a and b).When a high concentration of Eu 3þ is doped in nC, the nucleation and growth of CaCO 3 nanocomposites could be suppressed considerably (Gao et al. 2016).The morphology observed in the present study is preferable for photocatalytic and other industry-related applications.It is important to mention that rough, porous, spherical-like particles are highly needed for effective adsorption of pollutants (Qomariyah et al. 2020).

FE-SEM with EDX mapping
Figures 7 and 8(c and d) shows the chemical composition of nCE 1 and nCE 4 , which exhibit the presence of Ca, C, O, and Eu.No other additional elements are observed.It is confirmed that the products are Eu 3þ doped CaCO 3 nanoparticles.

HR-TEM analysis
Figure 9(a-c) shows the HR-TEM micrographs of the nC.All the particles are hexagonal and rhombohedral with a few spherical-like nanoparticles.The HR-TEM fringes show well-defined inter-planer d spacing of 0.214 corresponding to the (1 0 4) plane, which confirms the prepared product is in rhombohedral CaCO 3 .From the SAED pattern (Figure 9(d)), all the plane of (hkl)

UV-Vis DRS analysis
The UV-Vis DRS spectra (absorption) of the natural carbonate source, synthesized nC, nCE 1 , and nCE 4 are shown in Figure 12(a-d).Figure 12(a) shows four low-intensity peaks for a natural carbonate source (CaMg(CO 3 ) 2 ) at 256, 318, 335, and 348 nm. Figure 12b shows that the peaks of UV light absorption for nano CaCO 3 (nC) are at 259 and 320 nm.
However, the absorption curves of nCE 1 and nCE 4 (Figure 12(c and d)) exhibit three absorption peaks at 266, 303, 320 nm and 260, 302, and 319 nm, respectively.This may be due to the inducement of new energy levels by the Eu 3þ species in CaCO 3 .Regarding the energy level, new energy levels and vacancies of oxygen are generated by the doping of metal, which induces the bathochromic shift in the band transition and the absorption of visible light through a transfer of charge between a dopant and a valance or conduction band in the crystal field (Yadav et al. 2014).Based on the above, the absorption peaks of nCE 1 and nCE 4 are red-shifted when compared with pure nC (259 nm).The absorption peak shift toward the high-order wavelength is found in the order: nCE 1 > nCE 4 > pure nC.The charge transfer between the f electrons of Eu 3þ ions and the valance or conduction band of CaCO 3 may be responsible for the red shift in the absorption maxima or the formation of impurity levels below the conduction band of CaCO 3 after rare earth ion (Eu 3þ ) doping (Zhang et al. 2019;Stengl et al. 2009).The findings suggest that the doped materials have improved light absorption properties.Since more photons can be used to make photogenerated carriers, absorbing more light is usually good for the photocatalytic activity.Gupta et al. (2015) reported the absorption values for bulk commercial synthetic calcite at 220, 252, 341 and 460 nm.In the same area, they also generated the values theoretically for calcite, which are very similar to one another.The energy transfer of fluorescent calcite was studied by Sidike et al. (2006).They reported 208, 243, 295, 325, and 620 nm as UV excitation values.They reported that trace and rare earth elements are responsible for these values.According to Schulman et al. (1947), the presence of contaminants in the sample frequently causes the optical excitation in the fundamental absorption band to exhibit a variety of fluorescence bands.Based on their theories, the observation of trace elements or a defect in the crystal of the sample may be the cause of the increase in heterostructured peaks and the shifting of the absorption peaks.Ramasamy et al. (2018) reported the absorption values of synthesized nano CaCO 3 and PMMAmediated CaCO 3 in the range of (321 and 257 nm) and (313 and 265 nm) respectively.In the present study, the absorption peaks of synthesized nC, nCE 1 and nCE 4 show that there are no other impurities in the prepared products.This result is consistent with the XRD results.
The band gap energy is determined using the Tauc plot method (Ramasamy et al. 2018).Figure 13(a-c) shows the direct and indirect band gap energy curves of synthesized nC , nCE 1 , and nCE 4 .The direct band gap energy of synthesized nC , nCE 1, and nCE 4 is 5.12, 4.02, and 4.46 eV, respectively.nC, nCE 1 , and nCE 4 have indirect band gap energies of 4.90, 3.84, and 4.21 eV, respectively.Ramasamy et al. (2018) reported that the direct and indirect band gap energies of the synthesized nano CaCO 3 and PMMA template CaCO 3 nanocomposites were 3.15 eV, 3.18 eV, 3.36 eV, and 2.91 eV.Ghadami Jadval Ghadam et al. (2013) have reported the indirect band gap energy of calcium carbonate (bulk) to be 5.60 eV, 5.40 eV, and 5.36 eV for 30 wt %, 36 wt %, and 42 wt% surfactants at room temperature.
The band gap energy is slightly decreased after the doping of Eu 3þ .This shows that the introduction of Eu 3þ into CaCO 3 can decrease the band gap energy of the samples, which may be due to incorporate rare earth ions entering into the CaCO 3 lattice.The degree of photocatalytic activity will be greatly influenced by the band gap of the oxide semiconductor.It is noteworthy to mention that the band gap energy of nCE 4 (Figure 13c) was higher when compared with nCE 1 .It was due to the impact of increasing more Eu 3þ ions in the lattice host which results decreasing crystallite size of the products (as reported in XRD section).When the gap between the two bands is smaller, light is absorbed better, which helps move electrons from the valence band to the conduction band (Killivalavan et al. 2020).

PLE and PL analysis
The destiny of photogenerated electron-hole pair recombination on irradiated semiconducting materials is commonly studied using PLE and PL analysis.The relationship between PL intensity and electronhole pair recombination rate is well established.The PLE and PL spectra of raw natural carbonate source, synthesized nC, nCE 1 , and nCE 4 samples are shown in Figure 14(a-d).Figure 14(a) shows that the excitation values of natural carbonate source (CaMg(CO 3 ) 2 ) are 379 nm and 401 nm and 448 nm for synthesized nC, respectively, while nCE 1 excitation is 380 nm and nCE 4 excitation is 378 nm.The corresponding emission wavelengths are 415 and 439 nm for (CaMg(CO 3 ) 2 ), 448 and 453 nm for nC, and 450 nm for nCE 1 and 450 nm for nCE 4 .In Figure 14(a), the observed two excitation and emission peaks in CaMg(CO 3 ) 2 may be due to the presence of impurities in the material (Ramasamy et al. 2018).
However, in the other three cases (Figure 14(b-d)), only one excitation and emission are observed.It demonstrates the purity of the product (all the impurities are removed).From the figure, the emission values of nCE 1 and nCE 4 are red-shifted compared with nC.The findings clearly show that the electron-hole pair recombination could be reduced by rare earth metal doping, resulting in better separation of photogenerated charge carriers and ultimately improving the photocatalytic activity of the materials (Manikandan et al. 2017).CaCO 3 nanoparticles that have been doped with Eu 3þ exhibit considerable dampening in the PL emission signal when compared to pure CaCO 3 particles, indicating that Eu 3þ doping may improve separation effectiveness and consequently, photocatalytic activity.

Quantum yield
The fluorescence quantum yield is determined by the ratio of the number of photons absorbed to the number of photons released.In the present study, relative fluorescence quantum yield is measured by taking aluminum oxide as the  reference.The measured quantum yield of synthesized nC , nCE 1, and nCE 4 are 17%, 21%, and 19%, respectively (Table 2) under 380 nm excitation.The increase in the quantum yield dictates the reason for the larger crystallite size of the product.On the other hand, a larger crystallite size is favored for better luminescent properties due to the reduced non-radioactive process that acts as a luminescent quencher from the surface of the material (Dai et al. 2017).

Photocatalytic measurements
The photocatalytic performances of nC, nCE 1 , and nCE 4 were studied for degradation of Methylene Blue (MB) dye under UV light irradiation at ambient condition.The adsorption-desorption equilibrium under dark conditions for the adsorption of MB dye on the surface of the nC, nCE 1 and nCE 4 was observed for 45 min (Figure S4).It is noteworthy to mention that nCE 1 exhibits better adsorption when compared with nC and nCE 4 .Thus, more MB molecules can be adsorbed by nCE 1 which was the favorable for improving its visible light activities.Figure 15(a-c) shows the results of photocatalytic analysis of synthesized nC , nCE 1, and nCE 4 .It is evident that as irradiation time increases, the strength of the major MB absorption peak (k max ¼ 655 nm) gradually diminishes.As a result of the constant k max , the chromophoric structure of the dye broke down.The absorption maxima gradually lose their strength under visible light exposure.The percentage of degradation for the 90 min period of the samples (Figure 15(a-c)) is calculated using the following formula (9) (Jarvin et al. 2021).The percentages of degradation of nC , nCE 1 and nCE 4 are 72%, 87% and 82%, respectively.According to Arfaoui et al. (2018), through reduction, the photogenerated electron in the conduction band interacts with molecular oxygen to produce anionic superoxide radicals (O 2 ).To produce the greatest amount of reactive OH o radicals by oxidation, the holes in the valence band attract electrons from the H 2 O molecule or OH ions.The dye (MB) molecule adsorbed on the surface of the catalyst is attacked by these superoxide radicals (O 2 -) and OH o radicals, which causes it to fragment into  CO 2 and H 2 O (Arfaoui et al. 2018).Figure 16 shows a possible way that dye breaks down when CaCO 3 is made (with or without a dopant).
where C 0 is the initial concentration of dyes and C t is the concentration of dyes at various irradiation times.Figure 17(a) exhibits the comparison of the photocatalytic activities of synthesized nC, nCE 1 and nCE 4 .Among these, the nCE 1 sample has high degradation efficiency.It may be due to the red-shift of the absorption edge after the doping of Eu 3þ , showing that new energy levels are formed in the band gap of CaCO 3 .The Eu 3þ dopant may accept the electrons in the conduction band of nC to form Eu 2þ , and the as-formed Eu 2þ may transport an electron to dissolved O 2 to produce superoxide radical anions, which inhibit the recombination of photo induced electrons and holes (Mahmoodi et al. 2018).This means that  Eu 3þ on the surface of nC may act as an electron scavenger.Because of the E (h þ ) strong positive potential, h þ can directly oxidize MB dye (major) and H 2 O to produce superoxide and OH (minor).Also, the rate constant values of dye degradation are calculated by the pseudo-first-order kinetics equation (Mahmoodi et al. 2018 andCharles Prabakar et al. 2019).
where K is the first-order rate constant.determined.It can be seen that the nCE 1 sample has achieved the maximum value.The photocatalytic performance of nCE 1 is greatly improved because k values that are bigger lead to a faster rate of photocatalytic reactions.

Comparison of photocatalytic efficiency with other reported photocatalyst
The photcatalytic efficiency of present nC, nCE 1 and nCE 4 are compared with other reported literature and comparative results are presented in Table 3. From the  3).Finally, efficiencies of present products are higher than four photocatalysts such as ZnO-CeO 2 , TiO 2 @CNTs/AgNPs, Fe 3 O 4 /CaCO 3 /PEG and E 1 //CaCO 3 /Titania nanocomposites (Table 3).

Mechanism
Based on the information described above and documented in the literature, a photocatalytic mechanism for the degradation of methylene blue over Eu 3þ doped CaCO 3 nanoparticles is proposed and shown in Figure 16.A UV light source was used to assess the degradation of MB since the band gap energy of Eu 3þ doped CaCO 3 nanoparticles is in the UV range.An electron hole pair (exciton) is produced when a semiconductor is exposed to photons with energies that are equal to or greater than the semiconductor band gap.A hole is made in the valence band and the electron is accelerated to the conductive band.With or without a recombination hole, an  electron can diffuse toward the surface after charges have been separated.As was already established, rapid excitons recombination can significantly reduce photocatalytic efficiency.With Eu 3þ doping, the electron is prevented from recombining with the hole to form super oxide radicals by being confined to the energetically advantageous Eu 3þ ion (Zong et al. 2014;Xiaohong et al. 2007;Yang et al. 2002).Ionized oxygen vacancies can accept an electron, which can then combine with adsorbate O 2 to produce super oxide radicals.A greater number of O 2 radicals created when Eu 3þ ions are added to CaCO 3 nanoparticles results in enhanced photocatalytic activity (Tang et al. 2012;Weber et al. 2012).

Conclusion
A biomimetic route was used to synthesized nano CaCO 3 (nC), various polymer mediated CaCO 3 , and various concentrations of Eu 3þ anchored CaCO 3 from a natural carbonate source (CaMg(CO 3 ) 2 ).The functional groups of the prepared samples were analyzed through FTIR.The confirmation of the rhombohedral structure and crystallite size of the samples was obtained by XRD analysis.The addition of polymers on the CaCO 3 surface reduces the nucleation growth of the particle.The plane intensities of the Eu 3þ anchored CaCO 3 /PEG samples decreased with an increase in the concentration of Eu 3þ ions.It is due to the successful incorporation of Eu 3þ ions into the Ca 2þ lattice.The TG-DTA analysis exhibits high thermal stability, compared with the pioneer workers.The aggregated spherical and rhombohedral structures were observed through FESEM and HR-TEM analysis.The elemental composition and atomic ratio of the samples were confirmed by EDX analysis.Due to the impact of Eu 3þ in CaCO 3 , the absorption and emission values of Eu 3þ anchored CaCO 3 /PEG samples were red-shifted compared with nC.
With increasing Eu 3þ concentration, the direct and indirect band gap values increase slightly.Relative quantum yields were also calculated as 17%, 21%, and 19%.The photocatalytic analysis of the nCE 1 sample exhibits good degradation efficiency (87%) against MB dye for 90 min, compared with nC and nCE 4 .These results show that the nCE 1 material that was made is a good candidate for use in photocatalysis.

Figure 3 .
Figure 3. XRD spectrum of (a) Raw Natural carbonate source rock (b) nC and (c) nCP.

Figure 6 (
Figure 6(a and b) shows the FESEM images of synthesized CaCO 3 nanoparticles with different magnifications.The images show the agglomerated rhombohedral crystal-like morphology.Ramasamy et al. (2018) and Somarathna et al.

Figure 6 .
Figure 6.(ab) FE-SEM images of nC with different magnifications, (c) mapping of nC and (d) EDX spectrum of nC.
(ac) shows the HR-TEM micrographs of nCE 1 and nCE 4 respectively.It shows the spherical and rhombohedral like structures having well-defined inter-planer d spacing of 0.212 and 0.216, corresponding to the (1 0 4) plane.Figures 10 and 11(d) show the SAED pattern of nCE 1 and nCE 4 .The presence of all plane (hkl) values is very well matched with the XRD patterns.

Figure 9 .
Figure 9. (ac) HR-TEM images of nC with different magnifications and (d) SAED pattern of nC.
Figure17(b)  shows the linear relationship between ln(C 0 /C) and reaction time, which indicates that the photocatalytic degradation process follows a pseudo first-order form.From the slope of the graph (Figure17b), rate constant values are calculated.The rate constant value of synthesized nC is 2.767 Â 10 À3 min À1 , nCE 1 is 3.421 Â 10 À3 min À1 and nCE 4 is 3.301 Â 10 À3 min À1 .Based on the value of K, the rate of photocatalytic activity is

Figure 16 .
Figure 16.Possible mechanism of dye degradation using synthesized nano CaCO 3 with and without dopant.

Figure 17 .
Figure 17.(a) Degradation curve of MB in the presence of synthesized nC , nCE 1 and nCE 4, and (b) pseudo first order kinetics of MB in the presence of synthesized nC , nCE 1 and nCE 4 . Photocatalyst

Table 2 .
Quantum yield data for nC , nCE 1 and nCE 4 .

Table 3 .
Comparison of photocatalytic efficiency of nC, nCE