Tunable sulphur doping on CuFe2O4 nanostructures for the selective elimination of organic dyes from water

In this work, sulphur doped copper ferrites (S-CuFe2O4) photocatalysts were successfully synthesized for the first time using the facile hydrothermal method. The as-synthesized photocatalysts were characterized through XRD, Raman, TGA, FT-IR, UV–Vis-DRS, SEM, EDX and PL techniques. The results revealed that doping with sulphur has been found to be a suitable alternative that causes strain in the lattices as anions replace the oxygen from the CuFe2O4 nanostructures. Due to sulphur dopants, photocatalysts are able to efficiently trap and transfer the photoinduced charges, which readily suppress charge recombination. A UV–Vis spectrophotometer was used to monitor the degradation of selective toxic organic dyes (RhB, CR, MO, and CV) in aqueous media. The dye degradation results provide evidence for the surprisingly superior performance of S-CuFe2O4 over pristine CuFe2O4. On the basis of its efficiencies, this work can be assigned as an excellent candidate for photocatalysis science.

state reaction, hydrothermal method and coprecipitation 32 .Hydrothermal method is preferred due to versatility, uniformity, purity and ease of synthesis 33 .
However, due to the rapid recombination of photo-generated charge carriers, it is necessary to improve the effectiveness of CuFe 2 O 4 34 .To improve photocatalytic efficiencies, a variety of techniques, such as construction of nanocomposite 35 , p-n junctions 36 and metals and non-metals doping 37 have been used.Selective doping can recast the band structure by generating quasi-stable energies 38 .Recently, sulphur dopants on g-C 3 N 4 , graphene sheets 39,40 and ZnO have been reported to improve photocatalytic applications 41 .Moreover, because of the significant difference in size and electronegativity between S and O, S dopants modifies the optical, electrical and photocatalytic properties of semiconductor oxide 42 .
To modify the conventional ferrites and acquire the advantage of S-dopants, the hydrothermal method was adopted to synthesize S-doped copper ferrite (S-CuFe 2 O 4 ) photocatalysts.The as-prepared CuFe 2 O 4 and S-CuFe 2 O 4 catalysts were used for degradation of the selective dyes like rhodamine B (RhB), congo red (CR), crystal violet (CV) and methyl orange (MO) as a probe reaction.The results demonstrate that S-CuFe 2 O 4 photocatalysts demonstrate higher photocatalytic efficiencies as compared to pristine CuFe 2 O 4.

Experimental
Catalyst preparation.Copper ferrites were synthesized by hydrothermal method as follows: Solutions of 0.1 M copper nitrate and 0.2 M iron nitrate magnetic stirred for 30 min.After that, 0.01 M glucose solution was transfer to above solution and left it for stirring (22 h, 50 °C), the preparations of solutions were explained in (supporting information) section.The obtained mixture solution was then sonicated for 30 min by adjusting the sweep frequency (≈ 37 kHz, 300W).The final solution was kept in autoclave (Sanfa, DHG-9030) for 6 h at 140 °C, which is then cooled down and filtered by using high grade filter paper (WHA 1001325, Grade-1).The obtained precipitates were washed thoroughly using deionized water and then with absolute ethanol.To prepare sulphur doped copper ferrites (S-CuFe 2 O 4 ), 500 mg of CuFe 2 O 4 powder was transferred to autoclave reactor and appropriate amount of 1% thiourea solution (5 mg, 0.01 M) was added into this mixture and mixture was again kept in autoclave for 6 h at 140 °C.The synthesis scheme was illustrated in Fig. 1.

Characterization.
To predict the morphology and crystal structures of S-CuFe 2 O 4, powder X-ray diffraction (XRD) results were obtained at Philips-X Pertpro, with Cu kα radiation, λ = 1.5406Å.For the surface morphology of as-prepared catalysts, SEM analyses were conducted on a Field Emission Scanning Electron microscope (FEI-Nova Nano SEM-450).Energy dispersive X-ray analysis (EDX, Horiba 7593 H) afforded the chemical composition of the prepared catalysts.The ultraviolet-visible-diffuse reflectance (UV-Vis-DRS) results were obtained at AvaSpec-2048 TEC spectrometer.The Photoluminescence (PL) results were recorded using Fluorescence spectrometer (LS-45, Perkin Elmer), where PL intensities were measured at 300-650 nm range.Dye degradation experiments were performed using UV-Vis spectrophotometer (PerkinElmer/λ-365.TGA was performed using TG209 F3Tarsus ® , NETZSCH Germany.Fourier Transform Infra-Red (FT-IR) analyses were performed on Bruker Tensor-27.Raman analysis was done by using JASCO NRS-5000/7000 dispersive Raman spectrometers.were investigated in the presence of visible-light radiations within Pyrex reactor.The photoreactor was cylindrical, having a 500 mL volume capacity with an 8 cm internal and 10 cm external diameter.In order to prevent light emission into the surrounding environment, the outer wall of the reactor was covered by aluminum foil.
The cool water was allowed to circulate around the walls of the reactor in order to keep it at room temperature.A halogen lamp having λ ˃ 400 nm was used as a radiation source.The 10 mg amount of photocatalyst was fixed for each photoreaction at pH 7 for 15 ppm dye solution (vol = 100 mL).Prior to photoreaction, the solutions were stored in the dark (absence of light) to ensure the adsorption equilibrium.For the photoreaction, a halogen lamp was fixed at a distance of 30 cm from the reactor.The photocatalytic degradation efficiency was examined at different time intervals using a spectrophotometer (PerkinElmer/λ-365).

Results and discussion
The synthesis procedure of our S-CuFe 2 O 4 photocatalysts is shown in Fig. 1 (see details in the "Experimental" section).To remove the impurities and enhance the crystallinity, the as-synthesized photocatalysts were calcined at 350 °C for 3 h.

XRD.
The phase purity and crystalline nature of as-prepared product was determined by XRD.The XRD pattern of copper ferrite nanostructures is shown in Fig. 2a.The XRD patterns exactly match the standard JCPDS file No. 77-0010 43 .The diffraction peaks of as-synthesized catalysts are located at where λ is 1.54060 Å, β is FWHM and θ is diffraction angle (Cu-Kα).The Williamson-Hall method was also used to calculate the crystallite size of both pure CuFe 2 O 4 and S-CuFe 2 O 4 Eq.( 2).
where d is crystalline size, K is constant = 0.94, ε is constant = 1 45 .The crystallite size (D) was calculated using the Scherer formula and Williamson-Hall method are tabulated in Table 1, whereas, the Williamson-Hall plots are depicted in Fig. S1.

Raman spectroscopy.
Raman spectroscopy is an efficient technique for direct probe of structural transformation, cation redistribution, bonding in metal oxides and lattice distortion 30 . Figure 2b represents Raman spectra of CuFe 2 O 4 and S-CuFe 2 O 4 .Group theory analysis yields four Raman active modes denoted by A 1g , E g and two T 2g modes in range of 200-800 cm −131 .The A 1g mode observed at ∼ 703 cm −1 , E g mode at ∼ 273 cm −1 , the two T 2g modes ∼ 482 cm −1 and ∼ 552 cm −1 .The obtained Raman frequency relies on the Fe(Cu)-O bond length, that varies with the major factors, like phase transformation, lattice distortion and cationic redistribution 32 .It has been noted that sulphur dopants in copper ferrite shift the E g mode slightly to a smaller wave number (273 to 270 cm −1 ).Lattice distortion was observed, between two T 2g modes ascribed majorly due to the sulphur dopants as compared to oxygen contents present in catalysts 33 .

FT-IR.
The Fourier transforms infrared spectrum of CuFe 2 O 4 and S-CuFe 2 O 4 nanoparticles are shown in Fig. 2c.In the present work, major FTIR peak observed at 530 cm −1 is due to M-O vibration of tetrahedron where M represents copper or iron.Similarly, peak observed at 436 cm −1 was referred to the stretching of octahedron planes of spinel copper oxides.The broad band located at around 3500 cm −1 are related to bending vibration of O-H, which correspond to the hydroxyl groups or absorbed water molecules over the catalyst's surfaces.
It is obvious that upon doping sulphur content, the metal sulphur vibrations lies at 500 cm −1 , exhibit low intensity than that of the Cu-O/Fe-O vibration peaks.FTIR patterns located approximately at 1020-1030 cm −1 is due to the C-N stretching, which are likely to be caused by thiourea 46 .These stretching vibrations are also observed by other researchers working on S dopants 47 .These nitrogen/carbon (N-C) vibrations are absent in the FT-IR spectra of pristine CuFe 2 O 4 .

Thermo gravimetric analysis (TGA).
Thermo gravimetric analysis of as-synthesized S-CuFe 2 O 4 (without calcination) was performed to investigate physical behavioral changes (Fig. 2d).These evaluations were performed under N 2 atmosphere; within the temperature range 35-800 °C where increment in heating rate was maintained 5 °C/min.It has been observed that 1.6% weight loss was observed up to 165 °C, this loss can be attributed to the volatilization of absorbed water and solvent molecules 48 .Moreover, further 2.4% weight loss up to 220 °C was observed, endorsed to decomposition of metal hydroxide 48,49 .Afterwards, by increasing the temperature up to 800 °C no weight loss was observed, this exhibited excellent thermal stability of S-CuFe 2 O 4 nanoparticles.
SEM with EDX analysis.PL analysis.The photoluminescence (PL) of as-synthesized photocatalysts provides the fundamental information to determine the photo-induced electron-hole (e -/h + ) pairs to trap and transfer processes.Figure 5c represents the comparative results for CuFe 2 O 4 and S-CuFe 2 O 4 which exhibit the broad peaks at 421 nm, 468 nm and 553 nm 55 .The decrease in the intensity in case of S-CuFe 2 O 4 indicates the sulphur incorporation 56 .Moreover, this decrease intensity evidenced a greater transfer of electrons to active centers.PL peak at 468 nm corresponding to blue emission is due to imperative defects associated with the interface traps existing at the grain boundaries.Similarly, the peak at 553 nm corresponds to defect sites of oxygen vacancies.In S-CuFe 2 O 4 emission is slightly shifted to 549 nm, attributed to the appearance of electronic levels of intrinsic characteristics 57 .It specifies that back recombination of electrons and holes are remarkably lower in case of S-CuFe 2 O 4 catalysts.Due to doping of sulphur, several mid-gap states are formed between valance bands as well as in conduction bands within the structures of prepared catalysts.These mid-gap energy states result in electron/hole pair separation successively because sulphur serves as a charge-separating center 58 .In PL spectroscopy, a broader peak reveals better electron-hole separation, while high intensity peaks exhibit fast electron-hole recombination 59 .It is obvious from the PL spectra that S-CuFe 2 O 4 display highly enhanced photocatalytic performance due to an efficient charge carrier separation 60 .
( (302 mg/g) dye in the absence of visible light (dark).This adsorptive removal is accomplished without change in their λ-max.The amount of adsorbed dye at equilibrium q e (mg/g) was calculated by using Eq. ( 4): where m is catalyst weight, V (L) is volume of solution and Co and Ce are concentrations at initial and equilibrium state of RhB solution 61 .Then sample was irradiated with visible light to examine the photocatalytic efficiency of the S-CuFe 2 O 4 .After 100 min, the concentration of degraded dye reached to ~ 99%.Comparison of degradation efficiency of pristine CuFe 2 O 4 with S-CuFe 2 O 4 shows 34% difference, see Fig. 7a.On the basis of PL and UV-VIS spectrophotometric studies, it could be concluded that presence of sulphur has enhanced the photocatalytic efficiency of copper ferrites by decreasing the electron hole recombination 47 .Rhodamine B has one carboxylic acid (-COOH) and two amino groups (-NH 2 ).Ethyl molecules are electron promoters and donate electron to xanthene rings, as a result shifting the λ max to higher wavelength 62 .The higher λ max is due to electron donating effect of ethylene molecules.During the photoreaction, ethylene molecules are removed  (de-ethylation), due to which the absorbance shifts towards lower wavelength called blue shift.It is observed that λ max in the graph shifted from 550 to 498 nm.It has been obvious that, blue shift may arise due to energy difference between HOMO and LUMO when rhodamine B is grafted with S-CuFe 2 O 4 catalysts.HOMO being filled; its electrons get attracted to the polar S-CuFe 2 O 4 by weak dipole-dipole interactions.This attraction lowers the energy of HOMO while empty LUMO energy states of catalysts remain unaffected.This transition shifts the λ max to the lower wavelength 63 .Besides this, the appearance of blue shift is due to the step by step N-de-ethylation 64 .This de-ethylation gives rise to N,N,N-triethylrhodamine, N,N-diethyl rhodamine and N-ethyl rhodamine.
The final photo-degraded product is assumed to be a completely de-ethylated compound (rhodamine) that has characteristic absorbance maximum at 498 nm 65 .Figure 6b represents absorption spectra of Congo red (CR) at different time intervals.Photoreaction of CR dye solution by S-CuFe 2 O 4 photocatalysts starts after it attains equilibrium between adsorption and desorption in the dark (no light).This adsorption of dye in dark is due to the attraction of two oxygen atoms of sulfonate groups (CR dye) with surfaces of catalysts 65 .This removal is accomplished by parallel decrease in intensity of both peaks without change in their λ max, that is 340 nm for naphthalene rings and 500 nm for π-π* azo bonds 66 .After that, it was irradiated with visible light in order to initiate photocatalytic degradation.The absorption spectrum of CR shows two distinct peaks; peak at 500 nm is due to π-π* azo bonds and second peak at 340 nm is due to naphthalene rings.Actually, π-π* azo bonds are degraded easily as compared to naphthalene ring.After 100 min, the dye is almost completely removed due to destruction of its chromophoric structure in the vicinity of azo-linkages 67 .
Figure 6c represents absorption spectra of methyl orange (MO).The UV-vis spectra of methyl orange have a distinct broad band in the visible region, with maximum absorption at 460 nm due to the chromophore structure of the dye 68 .It is clear from results that after 70 min, the dye has degraded as a result of photoreaction with S-CuFe 2 O 4 photocatalysts.This is due to the destruction of homo-and hetero polyaromatic rings present in methyl orange 69 .It has been investigated that during photoreaction, the -N=N-double bond breaks and simple phenyl rings are formed with amine groups as degraded products 70 .
Crystal violet (CV) degradation is exhibited in Fig. 6d which represents a sharp peak at about 590 nm.Previously reported studies explain that the degradation of crystal violet dye can take place via different mechanisms such as N-demethylation, chromophore cleavage and breaking in ring structure 71 .The degradation mechanism and pathway depends on the type of catalysts, degradation method and oxidizing agents used 72 .After 50 min, the violet color of dye solution had completely faded away.The comparative efficiencies of reported ferrites as-well as-prepared S-CuFe 2 O 4 are illustrated in the Table 2. because sulphur atoms introduce new active sites that enhance the interactions with the adsorbed dye molecules.Moreover, sulphur dopants modify the electronic structure of the photocatalysts, leading to changes in the bandgap and energy levels.This improves the separation of electron-hole pairs and reduces recombination, leading to more efficient use of the generated charge carriers for dye degradation.Another advantage associated with the use of S-CuFe 2 O 4 photocatalysts is its good magnetic properties that allows it to be easily separated from reaction mixture by using external magnet.This separation technique is much easier, faster and efficient as compared to other conventional techniques such as filtration and centrifugation 86 .This also eliminates the risk of secondary pollution from catalysts as well as its full utilization for long times.The recyclability tests were performed for each cycle after careful washing and drying.Figure 7b shows recyclability results of S-CuFe 2 O 4 for MO degradation.After being used three times for the degradation of MO, a slight difference in catalyst's efficiency was observed due to loss of catalyst during recovery process.).These superoxide radicals ( ⋅ O 2 -) react with dyes and convert them to degraded byproducts 87 .At the same time, holes available in valance band levels of catalysts oxidize the dyes over the surfaces of catalysts.Presence of sulphur in the catalysts not only captures electrons but also prevents the recombination of charges (back reaction) and also can produce superoxide radicals ( ⋅ O 2 -) over active sites of catalyst by transporting electrons to oxygen 47 .Moreover, holes oxidize the H 2 O molecules into ˖ OH radicals, these hydroxyl radicals further enhance the dye adsorption at the surfaces of catalysts 88,89 .Due to higher dye adsorption at catalysts surfaces, rate of dye degradation is readily increased (Scheme 1).

Conclusion
In this work, CuFe 2 O 4 and S-CuFe 2 O 4 photocatalysts were successfully synthesized by employing hydrothermal methods.For structural morphologies XRD, Raman and FT-IR techniques were subjected to confirm crystallinity and sulphur doping.The results depicts that the doping of non-metallic anionic sulphur causes strain in the lattice and enter the lattice as anions by replacing oxygen.SEM results confirm that sulphur doping reduces the agglomeration and increases the actives cites for photoreactions.EDX confirms the presence of sulphur whereas  S-CuFe2O4+ hʋ S-CuFe2O4 (e -+ h + ) Dye + (h + ) degraded product e -CB + O2
www.nature.com/scientificreports/pristine CuFe 2 O 4 and S-CuFe 2 O 4 photocatalysts respectively.Results have confirmed the purity and existence of sulphur dopants in CuFe 2 O 4 photocatalysts.UV-VIS-DRS.The optical properties of pristine CuFe 2 O 4 and S-CuFe 2 O 4 were investigated by their UV-Vis-DRS study.The UV-Vis-DRS has advantage over UV-Vis absorption spectroscopy specifically to obtain the optical properties of powdered sample because it has less scattering effects than the absorption of liquids52 .Figure5aillustrates an absorption edge that indicates the Urbach resemblance absorption tail within visible region53,54 .CuFe 2 O 4 shows maximum absorption at ~ 688 nm, corresponding to its optical band gap i.e.Eg = 1.80 eV.Sulphur doping slightly red-shifted the visible absorption to around ~ 696 nm (Eg = 1.78 eV), showing excellent visible light response which results in higher photocatalytic activities, the band gap energy plot showed in Fig.5b.The optimal band gap of pure and doped CuFe 2 O 4 was determined by Eq. (3).
44ese XRD patterns indicate that as-synthesized catalysts emphasize cubic spinel structures, where Fe +3 ions occupy octahedral while Cu +2 ions occupy tetrahedral sites.It has been noted that due to sulphur doping major peaks shifts to the lower 2 theta whereas intensity of diffraction peaks has been observed to be decreased as well.The XRD results depict that the replacement of larger sulphur anions was accomplished with the smaller oxygen ions in the CuFe 2 O 4 structure.No other crystalline phases of Fe 2 O 3 , FeS or CuS were detected that confirm the purity of the catalysts, some extra peaks beyond 65° were observed due to some CuO contents44.The sharpness of peaks indicates that the designed catalysts of this work are highly crystalline in nature.The crystalline sizes of pure CuFe 2 O 4 and S-CuFe 2 O 4 have been calculated by using Debye-Scherrer's Formula in Eq. (1).

Table 1 .
51e morphologies of pristine CuFe 2 O 4 and S-CuFe 2 O 4 nanostructures were investigated by SEM (equipped with ETD/TLD detector).The SEM images of pristine CuFe 2 O 4 are demonstrated in Fig.3a,b.The results show that the particles are agglomerated and have irregular flacks-like morphologies.The particles have a tendency to agglomerate in clusters due to the attractive forces (i.e., magnetic dipole-dipole)50.Whereas, the SEM results of as-prepared S-CuFe 2 O 4 catalysts are shown in Fig.4a-dat different magnifications (i.e., 5 µm, 2 µm, 1 µm and 500 nm).It is clear from SEM results that by doping with sulphur, the flakes-like morphology of catalysts became more regular and agglomeration was reduced, which indicate the porosity of catalysts51.Thus, more active sites are available for photocatalytic reactions.These results depict that S-doping enhances the surface area and assist to increase the catalytic activity of CuFe 2 O 4 .EDX analyses provide valuable information on the composition and distribution of elements.The EDX results of as-synthesized pristine CuFe 2 O 4 and S-CuFe 2 O 4 photocatalysts are illustrated in Figs.3c and 4e, respectively.The EDX results reveal the relative amounts of copper, iron, oxygen and sulphur in the material that are tabulated in TableS1and S2 for Comparison of crystallite sizes and band gap energies of as-synthesized photocatalysts.

Table 2 .
Comparison of photocatalysts used for the degradation of dyes.
Photocatalytic reactions involved in dye degradation.