Structure,photocatalytic and antibacterial activity study of Meso porous Ni and S co-doped TiO2 nano material under visible light irradiation

K.V.Divya Lakshmi,T.Siva Rao,J.Swathi Padmaja,I.Manga Raju,M.Ravi Kumar

Department of Inorganic and Analytical Chemistry,Andhra University,Visakhapatnam 530003,India

ABSTRACT Undoped and Ni-S co-doped mesoporous TiO2 nano materials were synthesized by using sol-gel method.The characteristic features of as prepared catalyst samples were investigated using various advanced spectroscopic and analytical techniques.The characterization results of the samples revealed that all the samples exhibited anatase phase (XRD),decreasing band gap (2.68 eV) (UV-Vis-DRS),small particle size(9.2 nm)(TEM),high surface area(142.156 m2·g-1)(BET),particles with spherical shape and smooth morphology (SEM);there is a frequency shift observed for co-doped sample (FT-IR) and the elemental composition electronic states and position of the doped elements (Ni and S) in the TiO2 lattice analyzed by XPS and EDX.These results supported the photocatalytic degradation of Bismarck Brown Red (BBR)achieved with in 110 min and also exhibited the antibacterial activity on Staphylococcus aureus (MTCC-3160),Pseudomonas fluorescence (MTCC-1688) under visible light irradiation.

Keywords:Sol-gel Ni-S co-doped TiO2 Photocatalysis under visible light Degradation of Bismarck Brown Red Antibacterial activity

1.Introduction

Over the last few decades,semiconductors were highly used in the field of photocatalysis for degradation of organic dye pollutants[1].Among the various semiconductors being studied,TiO2is considerable one of the most suitable semiconductors,[2-4]due to its biological and chemical inertness,long term stability against photo corrosion and high photocatalytic efficiency under UV-light irradiation.However,TiO2having large band gap(3.2 eV)[5-7]and high recombination rate of photo generated charge carriers limits the use of TiO2as a photocatalyst under visible light.To improve the quantum efficiency of TiO2in visible region there is a need to decrease a band gap and electron hole (e-/h+) recombination by doping TiO2with metal and non-metal elements[8-10].The metal doped TiO2was obtained by dissemination of metal nanoparticles in to TiO2matrix.An appended benefit of transition metal doping like Mn,Cr,Ni,and Cu [11-14]is to improve the trapping of electron to inhibit the e-/h+recombination during irradiation.The main drawback of metal doping is thermal instability of doped TiO2and requirement of more expensive ion implantation facilities[15].The doping of various non-metal elements such as S,N,C and P[16-19]have been used to extension of the photocatalytic activity of TiO2under visible light and minimize the e-/h+recombination centers.According to previous literature survey co-doped TiO2have been improved better photocatalytic when compared to single doped TiO2[15].The simultaneous doping of metal and non-metal in to TiO2lattice exerts the reduction of band gap and minimize the e-/h+by separation of e-/h+pairs.To coin these properties in a single catalyst we aimed to synthesize Ni and S co-doped TiO2by using sol-gel method.In this investigation,Ni2+has been preferred due to its low valence state when it is incorporated into TiO2lattice greatly suppressing the recombination of e-/h+at the surface of the catalyst and providing a number of e-/h+pairs per particles which significantly enhance the photocatalytic activity of the catalyst [20,21].Whereas introduction of Sulfur into TiO2lattice causes the narrowing the band gap of TiO2.[16,22-24].The main advantage of sol-gel method is that the using of catalytic material has an excellent control over the product properties and is accessible in all key processing steps of gel,aging,drying heat treatment[25].Based on the above said facts of Ni and S,and to achieve both the properties in a single catalyst.Hence,the present investigation is aimed to synthesize Ni and S co-doped TiO2nanomaterial by using sol-gel method.

Furthermore,the photocatalytic efficiency of the synthesized catalyst was determined by degradation of Bismarck Brown Red(BBR) dye.It is an azo dye,mostly used as dyeing agent in leather and Textile industry [26,27].The releasing of these dyes into the water bodies as washouts causes intensive hazard to the ecosystem.The same photocatalytic methodology has been proposed to apply for testing antibacterial activity of the catalyst on pathogenic bacteria such as Gram positive Staphylococcus aureus(MTCC-3160)and Gram negative(Pseudomonas fluorescence,MTCC-1688)[28,29].

2.Experimental

2.1.Materials

All the chemicals used in the synthesis process were reagent grade and the solutions were prepared using double distilled water without further purification.N-butyl tetra ortho titanate(Ti(OBu)4),manganese nitrate [Ni(NO3)2]·6H2O and thiourea(CH4N2S) are obtained from E-Merck (Germany) were used as a precursors of titanium,nickel,sulfur for preparing undoped TiO2and co-doped TiO2catalysts respectively.Bismarck Brown Red dye was used as a model dye pollutant obtained from High media,India.Ethanol and nitric acid are obtained from E-Merck (India)used as a solvent in the reaction procedure.

2.2.Preparation of Ni &S co-doped Titania nano materials

Nickel and sulfur co-doped nano titania was synthesized by solgel method.40 ml of ethanol along with the 20 ml of n-Butyl ortho titanate taken in a Beaker-1 and acidified with 3.2 ml of Nitric acid and continued stirring for 30 min,this solution is considered as solution-1.40 ml of ethanol,the calculated (required weight percentage) amount of dopants and 7.2 ml of water taken in a beaker-2 and continued stirring for 30 min.Later this solution is considered as solution-2.Then the solution-2 was added to solution-1 drop wise slowly and stirred until turbidity obtained.Keep the turbidity solution aside for aging 48 h at room temperature to obtain a gel.The gel was dried at 70°C in an oven and ground for fine powder.This powder was calcined at 450°C for about 5 h in a muffle furnace.The same procedure was adopted for the preparation of undoped TiO2without addition of dopants.Various weight percentages of dopants(0.25 wt%-1.0 wt%)contain TiO2prepared samples were assigned to NiST-1-NiST-5 are presented in Table 1.

2.3.Description of the instruments used for characterization of the catalyst

The crystalline structure of photocatalyst was determined by powder X-ray diffraction (XRD) spectra taken (model ultima IV Rigaku) using anode Cu Kα (λ=1.5406 nm) radiation with a nickel filter.The applied current and voltage were 40 mA and 40 kV respectively.The average crystallite sizes of anatase was determined according to the Scherer equation using (FWHM) data of the selected peak.X-ray photo electron spectroscopy (XPS) was recorded with a PH1 quantum ESCA microprobe system using the Al Kα line of a 250 W X-ray tube as a radiation source with the energy of 1453.6 eV,16 mA×12.5 kV and a working pressure lower than 133.3×10-8Pa.The fitting of XPS curves was analyzed with multipack 6.0 A software.The surface area and porosity measurements were carried out with a micrometrics Gemini VII surface area analyzer.FT-IR spectra of the samples were recorded on a FT-IR spectrometer(Nicolet Avatar 360).The diffuse reflectance spectra (DRS) were recorded with a Shimadzu 3600 UV-Visible-NIR Spectrophotometer equipped with an integrating sphere diffuse reflectance accessory,using BaSO4as reference scatter.Powder samples were loaded into a quartz cell and spectra were recorded in the range of 200-900 nm.The nitrogen adsorption/desorption isotherms were recorded 2-3 times to obtain reproducible results and reported by BJH surface/volume mesopore analysis.The micro pore volume was calculated using the Frenkel-Halsey-Hill isotherm equation.Each sample was degassed at 300°C for 2 h.The size and shape of the nano particles were recorded with TEM measurements using JEOL/JEM 2100,operated at 200 kV.The morphology and elemental composition of the catalyst were characterized using scanning electron microscope (SEM) (ZEISS-SUPRA 55 VP)equipped with an energy dispersive X-ray (EDX) spectrophotometer and operated at 20 kV.Photoluminescence spectral analysis was done using Horiba Jobin Fluoro Max-4 instrument with a PMT voltage of 150 V and slit set both at 2.5 nm.The extent of BBR degradation was monitored using UV-Vis spectrophotometer(Shimadzu 1601).

Table 1 Name assigned to different weight percentages of TiO2 co-doped catalysts

2.4.Photocatalytic activity of the catalyst-degradation of Bismarck Brown Red dye (BBR)

The photocatalytic activity of the synthesized catalyst,Ni and S co-doped nanotitania was determined by degradation of BBR dye under visible light irradiation in the photocatalytic reactor [30].A high pressure 400 W (35000 lm) mercury vapor lamp with UV filter (Oriel,51472) was used as a visible light irradiation source.The degradation procedure was performed by taking 100 ml of BBR dye solution of required concentration(1-10 mg·L-1)containing sufficient amount of the catalyst in a 150 ml Pyrex glass vessel under continuous stirring placed about 20 cm away from the light source.The running water was circulated around the sample container to filter IR radiation to maintain constant solution temperature in a pyrex glass reaction vessel.The solution was stirred in dark for 30 min to attain adsorption-desorption equilibrium between surface of the catalyst and BBR dye.After visible light illumination 5 ml of aliquots of sample were collected from the reaction mixture using Millipore syringe (0.45 μm) at different intervals of time to observe the change in BBR dye concentration by measuring its absorbance at 459 nm using UV-visible (Milton Roy Spectronic 1201)Spectrophotometer.A pH meter(Elico Digital pH meter model 111E,EI)was used for adjusting and investigation of pH variation during the degradation process.The pH of the dye solutions was adjusted prior to irradiation by addition of 0.1 N NaOH/0.1 N HCl to get required pH [31].The percent of degradation of BBR dye was calculated from the following equation.

where,A0is initial absorbance of dye solution before expose with visible light and Atis absorbance of dye solution at time t.

The optimum reaction conditions are attained by varying the reaction parameters,such as dopant concentration,effect of pH,catalyst dosage and initial dye concentration and results are discussed in Sections 4.1-4.5.

2.5.Antibacterial activity study of photocatalyst

Antibacterial activity study of NiST-2 was carried out by Agarwell diffusion method [32]against bacterial strains namely Gram positive bacteria Staphylococcus aureus (MTCC-3160) and Gram negative bacteria Pseudomonas fluorescence (MTCC-1688).Nutrient Agar (High media-India) dissolved in water was distributed in 100 ml conical flask and sterilized in an autoclave at 121°C 15 lbp for 15 min.After autoclaved the media,poured into sterilized petri plates and swabbed using L-shaped glass rod with 100 μl of 24 h mature broth culture of bacterial strain.The wells were made by sterile cork-borer.Wells are created in the petri plates and different concentrations of TiO2co-doped (NiST-2)nanoparticles are injected(200 μg·ml-1,300 μg·ml-1,400 μg·ml-1)into bore wells.The TiO2nanoparticles were dispersed in sterile water and it was used as a negative control and simultaneously the standard Antibiotic Chloramphenicol(100 μg·ml-1)as positive control were tested against the bacterial pathogen,then the plates were incubated 24 h at 37°C.The minimum zone of inhibition of every well measured in millimeter.

3.Results and Discussions

3.1.XRD

Fig.1.The XRD pattern of the synthesized undoped and co-doped TiO2 with different wt% of Ni 2p and S 2p.

The X-ray diffraction patterns of Ni and S co-doped and undoped TiO2samples were given in Fig.1.From the figure all diffraction peaks were showed the anatase phase at 2θ=25.3o.The diffraction peaks at 2θ values 25.3o,38.3o,48.2o,54.7oare corresponding to planes of (100) (004),(200) and (211) respectively.No extra peak observed at 2θ=27.8o,indicated that there is no formation of rutile phase.Because,all the samples including undoped TiO2were calcined at 450°C for 5 h.Hence,at this temperature there is no phase transformation from anatase to rutile.In general,rutile phase starts formation at higher temperature.There is no diffraction lines for nickel and sulfur oxides or other compounds[33]observed.This is maybe indicated that Ni and S incorporated in to TiO2lattice by substituting Ti4+ions.The Ni2+cannot occupies the interstitial sites because the radius of Ni is 0.072 nm and Ti4+is 0.068 nm,hence Ni2+can easily replace some Ti4+ions[34,35]and also Sulfur as S6+ion which can replaces the lattice Ti4+into TiO2matrix [36,37].The average crystallite size of undoped and codoped samples were calculated using Scherrer equation and they are found to be 13.5 nm and 5.5-7.2 nm.The average crystallite sizes of undoped and co-doped samples TiO2samples were given in Table 2.From Table 2 it is observed that the crystallite size decreases due to the fact that increasing the metal ion (Ni2+) content into TiO2lattice crystal strain increases while the grain growth decreases.Hence,the particle size decreases [25,37].

3.2.UV-Vis-DRS

The UV-Vis-diffuse reflectance spectra of undoped and codoped TiO2(Ni and S) samples were recorded λ vs Abs as shown in Fig.2(a).The co-doping of Ni and S in to TiO2lattice extended the strong absorption of TiO2in visible light towards 400-800 nm.This may be due to the fact that Ni2+and S6+deflected the formation of extra energy level near the CB and VB edge and instigated a prodigious narrowing of band gap [36].The band gap of the samples was calculated from reflectance spectra[F(R)]using Kubelka-Munk formalism and Tauc plot method [25]as given in Fig.2(b)it shows a Lenoir region just above optical absorption edge for n=1/2 the band gap is direct allowed transition.The exponent depends upon the type of transition and it may have values such as 1/2,2,3/2 and 3 corresponding to the allowed direct,allowed indirect,forbidden direct and forbidden indirect transition respectively[38,39].The co-doped sample band gap ranging from 2.62 eV to 2.74 eV and undoped TiO2is 3.2 eV which is comparable with the literature value [40].The band gap energies of all the synthesized sample results are given in Table 2.The results from the table shown that all the co-doped samples had reduced band gap when compared to single doped TiO2[35](Table 3).Hence,it is active in visible region.

3.3.BET

Fig.2.(a)The DRS spectra of undoped TiO2 and co-doped TiO2 with different wt%of Ni&S.(b)Kubelka-Munk function(F(R)hv)1/2 vs photon energy(hv)for determining band gap energy values.

Table 2 The results of crystallite size (XRD),Band gap (UV-Vis-DRS) &BET surface area

Table 3 The comparative crystallite size and band gap values of Ni and S single doped and Ni and S co-doped TiO2

The surface area,pore volume and pore size of all the prepared samples including undoped TiO2were determined by N2adsorption desorption measurements by using BET surface area analyzer and the data was presented in Table 2.The isotherms are given in Fig.3(a) and pore size is given in Fig.3(b).The results from the table clearly authenticated that the pore volume and pore size of the prepared samples are close to be standard values.Hence,the catalysts follow the Type-IV isotherm.It is considered to be mesoporous [42].This can be concluded that the catalysts have better textural values i.e.high surface area,pore volume and pore size which enhance the better photocatalytic activity of the catalyst.The surface area increases may be due to the incorporation of Ni and S into TiO2lattice which decreases the particle size.Before proceeds for characterization of the catalyst with XPS,SEM-EDX,TEM and FT-IR,conducted trial degradation with all the catalysts at required reaction parameters was conducted for the degradation of Bismarck Brown Red (BBR) with all the catalysts.Based on degradation results,NiST-2 shows the best catalytic activity.Hence,we have carried out the characterization only for NiST-2 catalyst with the following instrumental techniques.

3.4.XPS

The composition and its electronic states of elements in the catalyst samples (NiST-2) were analyzed by using X-ray photo electronic spectroscopy.Fig.4(a) represented the total survey spectrum of the elemental composition of NiST-2 catalyst such as Ti,O,Ni and S.This spectrum indicated the presence of doped elements in the catalyst.The oxidation states of doped elements and their position in the host TiO2lattice were determined by interpreting the binding energies of magnifying spectra of the elements from Fig.4(b-e).Fig.4(b) shows the double peaks at Ti 2p1/2 and Ti 2p3/2 corresponding with binding energies at 464.11 eV and 458.003 eV.The splitting energy between these two peaks is 6.10 eV indicating the presence of Ti4+ion.The binding energies of Ti 2p1/2 and Ti 2p3/2 of Ti4+in undoped TiO2corresponding to 458.9 and 464.6 eV [43].The peak splitting energy difference is 5.7 eV this is less than that of co-doped TiO2.This small shift is may be attributed to presence of incorporation of Ni and S into TiO2lattice.The Fig.4(c) represents the binding energies of S 2p corresponding to doublet peaks at 167.78 eV and 169.80 eV [44].The peak splitting energy difference is 1.92 eV.This energy difference is correlated with the elemental sulfur peak binding energy(165.0 eV-164.0 eV)[42].The co-doped sample sulfur has a higher binding energy that is 0.9 eV.This indicated that sulfur has entered as a cation (S6+) replacing the Ti4+ion.In addition,the existing Ti-O bond energy is larger (672.4 kJ·mol-1) than Ti-S bond(418 kJ·mol-1).Therefore,sulfur substitution for oxygen in TiO2is energetically not favorable.S6+cation which is substituted for lattice Ti is more favorable than O2-by S2-.So it leads to the formation of Ti-O-S bond in TiO2matrix [36].From Fig.2(d),the binding energies of Ni in co-doped sample were observed at 868.39 eV and 853.67 eV corresponding to Ni 2p1/2 and Ni 2p 3/2 [45,46].The peak splitting energy is 14.7 eV.It is confirmed that Ni is stabilized in a divalent state,when the energy difference(14.7 eV) is less than that of metallic Nickel (852.7-870.0=17.3 eV) [43].This can be interpreted that a shift in the Ni 2p peak and a shift of Ti 2p peaks corresponds to the rearrangement of Ti4+ions,due to the substitution doping of Ni2+in place of Ti4+ion.Fig.4(e)shows the XPS spectra of oxygen binding energies at 527.88 eV and 529.72 eV corresponding to O1S.These binding energies are related to two chemical states of oxygen,one is lattice oxygen (Ti-O-Ti) and another one is for chemiadsorbed oxygen on the surface of the catalyst [46].These energies are less when compared with undoped TiO2(Ol-530.2 and Oh-531.5),this decreased energy may be attributed that oxygen may be bonding with nickel (Ni-O).These results concluded that Ni and S doped in TiO2lattice by replacing Ti4+ions.

3.5.SEM-EDX

The surface morphology and elemental composition of synthesized co-doped TiO2(NiST-2)nano particles have been investigated by SEM and EDX.The morphologies of undoped and co-doped(NiST-2) TiO2are confided here in Fig.5 (a) and (b).The figures indicated that the samples exhibited small particles with agglomerated and spherical shape with smooth morphology.The EDX analysis indicated the presence of constituent elements at their respective weight percentages Fig.5 (c).Based on the XRD,XPS and EDX results Ni and S are doped in to TiO2lattice by substituting Ti4+ions.

Fig.3.(a) The N2 adsorption-desorption isotherms and (b) pore diameter distribution curves of NiST-2 co-doped TiO2 and undoped TiO2.

Fig.4.a) XPS survey spectrum of co-doped TiO2 and high resolution spectrum of Ti 2p (b) S 2p (c) Ni 2p (d) O1s (e) respectively.

3.6.TEM

The particle size distribution was obtained by measuring the particle diameter from representative TEM images of undoped and co-doped (NiST-2) TiO2samples are shown in Fig.6a) and(b).From the images it is noticed that the particle size of NiST-2 is smaller in size compared to undoped TiO2.The diffraction rings are observed for co-doped (NiST-2) TiO2catalyst from SAED pattern which clearly reveals that no structural change of anatase TiO2was found;the planes are (101),(004),(200) and (211).Fig.6(c) shows that the average size of the prepared nanoparticle is 9.5 nm,which was calculated from Gaussian fitting of the size histogram [47,48].These results confirmed that the co-doping of Ni and S decreases the particle size.

3.7.FT-IR

Fig.5.The SEM images of a) undoped TiO2 b) NiST-2 c) EDX spectrum of NiST-2.

Fig.6.The TEM images of a) undoped TiO2,b) NiST-2 and SAED pattern of co-doped TiO2,c) histogram and Gaussian fitting of NiST-2.

The FT-IR spectra for undoped and co-doped (NiST-2) TiO2were given in Fig.7.The peaks appeared around at 3012 cm-1-3464 cm-1,1620 cm-1-1635 cm-1[45]corresponding to stretching vibrations of OH which belong to Ti-OH on the surface and bending vibrations of adsorbed H-OH molecule.The strong absorption band around 569 cm-1is due to stretching vibrations of Ti-O-Ti and Ti-O band in undoped TiO2which is in good agreement with previous studies[49].From Fig.7,it is seen that after co-doping of Ni and S in to TiO2lattice the stretching vibrations of skeletal Ti-O-Ti shifted to 460 cm-1-560 cm-1indicating the formation of Ni-O,and also 1054 cm-1-1064 cm-1represented the deformation of Ti-O-S,which confirms the Ni2+and S6+doped into TiO2lattice[50,51].Further this can be explained by comparing Ni and S co-doped TiO2stretching frequency values of with Ni and S single doped TiO2[41,52].These values are presented in Table 4,obtained from literature.

Table 4 The comparative FT-IR stretching frequencies of results of Ni and S single doped and co-doped TiO2 sample

4.Assessment of Photocatalytic and Antibacterial Activity of the Catalyst (NiST-2) by Degradation of Bismarck Brown Red (BBR)Dye and Pathogens

4.1.Study of photocatalytic activity of NiST-2

To establish optimum reaction parameters for efficient photocatalytic degradation of BBR dye using NiST-2 catalyst,experiments were carried out under visible light irradiation.The rate of degradation of dye with NiST-2 catalyst was determined by measuring its absorbance(at λ max-459 nm)during the reaction at different intervals of time.Before proceeding for the above process,the following reactions are conducted to understand the interdependency of the light,catalyst and dye.In the first instance,a fixed amount of dye solution (10 mg·L-1) was taken in a 150 ml beaker and stirred for the solution 15 min in the dark and exposed to visible light and measured the absorbance of dye in both the cases(dark and presence of light).No significant change in the absorbance of the dye was observed.This indicated that only the visible light cannot degrade the dye.In the second instance both catalyst and dye was taken in a beaker and stirred for 60 min in the dark and then exposed to visible light.In the dark,a small decrease in the absorbance of the dye was observed,due to the adsorption of the dye on the surface of the catalyst.But in the presence of visible light a drastic change in the absorbance of dye was noticed by the activation of the catalyst.This indicated that to get the degradation of the dye,light and catalyst are the interdependent factors.To achieve a complete degradation of dye not only these two parameters but also other parameters are needed such as effect of pH of the solution,catalyst dosage,initial dye concentration.

4.2.Effect of dopant concentration

The effect of the catalyst containing various dopant (Ni &S)concentrations was studied by degradation of BBR dye at other parameters kept constant such as pH=2,catalyst dosage 100 mg·L-1and initial dye concentration 10 mg·L-1.The results are given in Fig.8.From the figure it is observed that among all the co-doped catalysts NiST-2 (0.75 wt% Ni-0.25 wt%S) showed high rate of degradation.This is may be attributed that by increasing the metal dopant concentration,the state of surface of the catalyst becomes+ve charge which facilitates the adsorption of water and favors the formation of Titanol (Ti-OH) at the surface of the catalyst which is confirmed by FT-IR O-H stretching frequencies.This Titanol formation enables the OH.formation on absorption of visible light.This absorption of visible light may be due to the decrease in band gap energy (2.62 eV) by doping of S.

4.3.Effect of pH

Fig.8.Effect of dopant concentration on photocatalytic of co-doped titania for the rate of degradation of BBR dye.Here,catalyst dosage 50 mg·L-1,pH 2 and BBR=10 mg·L-1.

Fig.9.The effect of pH on the rate of degradation of BBR dye by Ni 2p &S 2p codoped TiO2.Here,catalyst dosage is 100 mg·L-1 and BBR=10 mg·L-1.

The solution pH is an important parameter in the photocatalytic reaction,it changes the surface charge properties of the photocatalyst.This can be achieved by the change in the pattern of addition of the contents in the reaction vessel i.e.required amount of catalyst dosage was taken into 150 ml beaker and adjusted the required pH of the solution with 0.1 mol·L-1HCl/0.1 mol·L-1NaOH whichever is needed and stirred for 10 min.This pattern of addition changes the surface of the catalyst +ve charge or -ve charge which is made available for the dye molecules to adsorb.The photo degradation efficiency of the catalyst was evaluated by the degradation of BBR dye at various pH values (2-8) of the reaction solution.The results were shown in Fig.(9).From the figure it is observed that at pH 2 the rate of degradation is very high compared to other pH values.The rate of degradation decreases from 3 to 6 maybe due to decreases the H+ion concentration it diminishes the+ve charge on surface of the catalyst[57].Hence,adsorption of dye molecules decreases slowly.Hence rate of degradation BBR decreases.

Further,the increase in pH (pH=8) rate of BBR degradation decreases due to the adsorption of dye molecules on the surface of the catalyst which is negligible.This is maybe attributed that at basic pH surface of the catalyst become-ve,Hence the adsorption of -ve charge dye molecules may not be possible,the rate of degradation is almost insignificant [56].

4.4.Effect of catalyst dosage

Catalyst dosage has been widely studied in degradation process.It is carried out by varying the catalyst amount from 50 mg·L-1to 250 mg·L-1in 10 mg·L-1of BBR solution.According to the Fig.10 it was observed that with increasing catalyst dosage up to 100 mg·L-1,the rate of degradation increases,which later on decreases.As the catalyst dosage increases,the number of photons adsorbed by the catalyst particles increases(or)the number of catalyst particles activated by the adsorbed quantum of radiation increases.Further increase in the catalyst dose the solution turbidity increases.This restricted the penetration of light to activate the catalyst particles,leading to a decrease in the rate of BBR degradation [57,58].

4.5.Effect of initial dye concentration

Fig.10.Effect of catalyst dosage on the degradation of BBR by NiST-2 co-doped TiO2 here pH=2 and BBR=10 mg·L-1.

The effect of initial dye concentration of BBR is a great influence on the rate of degradation and this was studied by varying the dye concentrations from 5 mg·L-1to 30 mg·L-1of BBR at a fixed dosage of 100 mg·L-1co-doped (NiST-2) TiO2nanopowder at a solution maintained at pH 2.These results(Fig.11)illustrated that degradation increases with increasing the concentration up to 10 mg·L-1;further increasing the dye concentration the rate of degradation decreases due to the limited number of active catalyst particles that are available even though dye molecules increase.In other words the rate of degradation of dye decreases [59]explained in terms of blanket effect,i.e.once a layer of dye molecules are adsorbed on the catalyst surface,another layer adsorbed on it may not be possible.Hence,there is a delay in the adsorption of the second layer,until the first layer gets completely degraded;this is called the blanket effect.This can lead to slowing down of the rate of degradation with the increase in the dye concentration.The Comparative study for photocatalytic efficiency of Ni and S single doped and co-doped TiO2values are given in Table 5.

4.6.Evaluation of antibacterial activity of NiST-2 catalyst on

Staphylococcus aureus (MTCC-3160),Pseudomonas fluorescence(MTCC-1688)

Fig.11.Effect of initial concentration of the dye on the rate of degradation of BBR dye.Here,pH=2 and catalyst dosage—100 mg·L-1.

Table 5 Comparative table for photocatalytic efficiency of Ni and S single doped and co-doped TiO2

The antibacterial activity of undoped and co-doped (NiST-2)TiO2nanoparticles was checked against different bacterial strains carried out by Agar-well diffusion method.The bacterial strains like Staphylococcus aureus (MTCC-3160),Pseudomonas fluorescence(MTCC-1688) at different concentrations of NiST-2 nanoparticles were taken in different wells in a petri dish with a concentration ranging from 200 μg·ml-1,300 μg·ml-1,400 μg·ml-1and in another well a standard chloramphenicol was taken at concentration of 100 μg·ml-1.The petri plates are showed in Fig.12(a),(b)and (c).For each bacterial growth,zones of inhibition diameter were determined for 3 replicates of each catalyst dosage and standard (chloramphenicol) at 200 μg·ml-1,300 μg·ml-1,400 μg·ml-1and 100 μg·ml-1respectively.For each dose and standard (chloramphenicol),the mean and their standard deviation values were calculated and presented in Table 6.From the table,the values represent the zone of inhibition of bacterial growth for Staphylococcus aureus (MTCC-3160) and was found to be (21.5±0.15) at 400 μg·ml-1which is better than other catalyst dosages but less than that of standard value (chloramphenicol (25.96±0.05)) .Whereas in the case of Pseudomonas fluorescence (MTCC-1688)(19.9±0.17)same dose shows better zone of inhibition of bacterial growth when compared to standard value of (chloramphenicol(16.71±0.2)) .Further we have calculated the T-test value for all concentrations at three replicates for each dose using SPSS software and the calculated values are tabulated in Tables 7 and 8.The values from the table indicated that the significant P values of each concentration is ＜0.05.This is concluded that*400 μg·ml-1is the best concentration against these two pathogens.Therefore the co-doped TiO2nanoparticles exhibiting better antibacterial activity when compared to undoped TiO2.This inhibition of bacterial growth may be due to the electron hole which forms in valence band of TiO2by irradiation of catalyst with visible light.These e-/h+(+ve) act as strong oxidizing agents that could degrade the protein coat of the bacteria and lead to the inhibition of the growth of the organism.(See Fig.13.)

4.7.Detection of e-/h+, and .OH

During the photocatalytic and anti-bacterial activity studies the formation of active species in the reaction of the key entities to drawn a reaction pathway.Hence,identification of these species attempted by using a scavenging reagents [60]such as,Ethylene di amine tetra acetate (EDTA) for VB/h+,1,4 Benzo quinine forand coumarin for .OH.

4.8.Detection of e-/h+,

The main reactive species (e-/h+) detected can form reactive oxygen species.The main reactive species (e-/h+) were detectedby di-sodium salt of EDTA(scavenging reagent),100 mg·L-1of catalyst solution was taken along with 10 mg·L-1of dye solution in a 150 ml pyrex glass beaker and exposed to visible light for up to 20 min.Then,5.0 ml of aliquot of solution was withdrawn into a cuvette using a mille pore syringe to measure the decreasing absorbance of the BBR dye.After collection of the 1st aliquot at 20 min,1.0 ml of 1 mmol·L-1di-sodium salt of EDTA solution has been immediately added to reaction mixture.Then the 2nd aliquot was collected at 20 min and the absorbance was measured.This was continued up to 110 min at the rate of 5 ml for 20 min.The results are presented in Fig.14.From this figure it is observed that the rate of degradation of BBR increased up to 20 min after the addition of EDTA (scavenger);the reaction rate slows down and becomes constant after 40 min.This may be attributed that EDTA can inhibit the activity of e-/h+which is an important active site for the degradation of dyes.Simultaneously theand .OH are important active species.The role ofis explained in the mechanism.The production ofis tested by using 1,4 Benzo quinone(BQ).When it was added to the reaction mixture,the photocatalytic activity of BBR was decreased as shown in Fig.14.Which implies that the O2.is an important reaction intermediate for the proceedings of the further reaction for production of .OH.

Table 6 Antibacterial activity of undoped and co-doped TiO2 nano particles (NiST-2) against Staphylococcus aureus (MTCC-3160) and Pseudomonas fluorescence (MTCC-1688)

Table 7 Inhibition zone of Staphylococcus aureus (MTCC-3160) with NiST-2 nano catalyst

Table 8 Inhibition zone of Pseudomonas fluorescence (MTCC-1688) with NiST-2 nano catalyst

In photocatalytic reactions,hydroxyl radical is considered as highly reactive species in the reactions and responsible for oxidative decomposition of pollutants.The OH radicals having short life and high reactivity but the direct detection of hydroxyl radicals are difficult.Hence,we have used the photoluminescence technique(PL)to investigate the production of hydroxyl radicals;the spectra are given in Fig.15.Coumarin was used as a probe molecule,which on reaction with hydroxyl radicals forms 7-hydroxy coumarin.The catalyst was dispersed in 10 mg·L-1of coumarin solution in acidic condition and exposed to visible light.At every 30 min the reaction solution was filtered and photo luminescent spectra of the gener-ated 7-hydroxy coumarin show maximum absorption at 450 nm.In this spectrum the intensity was observed with increasing irradiation time.The results further confirmed that synthesized sample,NiST-2 showed increases rate of formation of.OH.This is due to the fact that the catalyst particles can generate of .OH during the illumination.

Fig.12.Zone of inhibition of a) Undoped TiO2 b) Staphylococcus aureus (MTCC-3160).C) Pseudomonas fluorescence (MTCC-1688).

Fig.13.Antibacterial activity of NiST-2 against Staphylococcus aureus and Pseudomonas fluorescence,error bars indicated the Mean of three replicates±Standard deviation.

Fig.14.The effect of h+ scavenger by the degradation of BBR dye,catalyst-NiST—2,pH—2,catalyst dosage 100 mg·L-1.

Fig.15.Photoluminescence spectra of catalyst NiST-2,Catalyst dosage 100 mg·L-1

4.9.Mechanism for photocatalysis and antibacterial activity

Based on the scavenging test,the reactive species plays the key role in the photocatalytic degradation process.OH radical is a very important reactive species in the degradation of organic pollutants.When co-doped TiO2(NiST-2) is irradiated with visible light,the electrons get excited to conduction band leaving a hole creating in the valence band Eqs.(1) and (2).The holes react with surface hydroxyl groups/with adsorbed water molecules on surface of TiO2to generate hydroxyl radicals and hydrogen ions Eqs.(3)and (4).The electrons are transferred to adsorbed oxygen producing superoxide anions,these superoxide radicals react with adsorbed water molecules producing peroxide radicals and hydroxyl ions Eqs.(5) and (6).The peroxide radicals combined with H+result in the formation of hydroxy radicals and hydroxy ions.Hydrogen peroxide is formed as an intermediate product Eqs.(7)and (8).

Fig.16.Recyclability test for NiST-2 catalyst.

These holes oxidized with hydroxyl radicals to facilitate the formation of.OH.This OH radical reacts with adsorbed dye molecules(BBR) and performs degradation.Similarly the strong oxidants HO./h+react with the outer part of peptidoglycon of bacteria and lead to the destruction of bacterial growth.

4.10.Recycling process for evaluating the photo stability of as prepared catalyst (NiST-2)

The stability of the catalyst is a very important parameter to access its practicability.Photocatalysis should be maintaining high photocatalytic activity during long-term use for their practical applications.Moreover,the catalyst should be easily separated from the reaction mixture simply through centrifugation.

To evaluate the stability of the catalyst recyclability experiments were conducted under visible light irradiation.After 110 min of photocatalytic reaction with Ni and S co-doped TiO2,the photocatalyst was separated from the solution and then washed with distilled water.The concentration of BBR and photocatalyst was maintained as constant.Fig.16 represented that after 4 cycles,the catalytic activity of the catalyst towards the pollutant slightly decreased but it was still high.It revealed that the good repeatability of the photocatalytic activity may be attributed to the coverage of the photocatalyst surface by BBR dye molecules.The photocatalyst surface is very difficult to clean thoroughly,so it influences the surface properties and photocatalytic efficiency of Ni and S co-doped TiO2.

5.Conclusions

· The co-doping of Ni and S into TiO2did not change the crystal lattice of TiO2and exhibited the anatase phase at 400°C calcination temperature.

· The doping of Ni and S into TiO2lattice by substitution of Ti4+ions.This can be confirmed by XRD,XPS and FT-IR results.

· Due to Ni and S co-doping there is significant decreasing band gap for co-doped sample (2.62 eV-2.74 eV) when compared to undoped TiO2(3.2 eV).

· The particle size(9.5 nm)and high surface area(142.15 m2·g-1)of the catalyst samples were determined by HR-TEM and N2adsorption-desorption analysis by BET.These results highly correlated with the degradation results of BBR dye.

· Among all the catalysts (NiST-1 to NiST-5),NiST-2 shows highest rate of degradation compared to undoped TiO2.

· NiST-2 having highest inhibition growth rate of bacteria (Pseudomonas fluorescence (MTCC-1688-(19.9±0.17)) at 400 μg·ml-1of catalyst dose under visible light.Which is better than that of standard value (Chloromphenicol-(16.7±0.28)).

Acknowledgements

K.V.Divya Lakshmi is thankful to the University Grants Commission (UGC) for providing BSR fellowship.