Muhammad Saeed *,Majid Muneer Nida Mumtaz Mohsin Siddique ,Nadia Akram Muhammad Hamayun

1 Department of Chemistry,Government College University Faisalabad,Pakistan

2 Department of Chemistry,Bacha Khan University Charsadda,Pakistan

3 Department of Chemistry,University of Gujrat,Hafiz Hayat Campus Gujrat,Pakistan

Keywords:Co3O4 Ag-Co3O4 Photo-catalysis Photo-degradation Calotropis gigantea Rhodamine B Elay–Rideal mechanism

A B S T R A C T Synthesis,characterization of Co3O4 and Ag-Co3O4 composites and evaluation of their photo-catalytic activities towards photo-degradation of aqueous solution of rhodamine B dye under irradiation of visible light have been described in this paper.Co3O4 was prepared by solid phase mechano chemical process using Co(NO3)2·6H2O and NH4HCO3 as precursor materials.Ag was deposited on Co3O4 from AgNO3 using Calotropis gigantea extract as reducing agent.XRD,SEM and FTIR were used for characterization of prepared composites.Photo-catalytic efficiencies of as-prepared Co3O4 and Ag-Co3O4 were evaluated for aqueous phase photo-degradation of rhodamine B.It was found that deposition of Ag on Co3O4 highly enhanced the photo-catalytic activity of Co3O4.Photo-catalytic degradation followed the Eley–Rideal mechanism.About 100%and 91%photo-degradation of 40 ml dye solution achieved at 313 K in 90 and 120 min over 0.05 g of Ag-Co3O4 as photo-catalyst using 100 and 200 mg·L?1 as initial concentration of dye respectively.

1.Introduction

Studies on oxides of metal have gained much attention for several years as these materials have many technological applications like sensor,coating,catalysis,optical fibers,electrochemistry etc.[1].Cobalt is one of these metals which is more favorable in many applications in comparison to other metals due to its higher abundance and economic feasibility.Cobalt oxide is functional in diverse fields such as Fischer–Tropsch synthesis,gas sensors,pollution control and rechargeable batteries.It exists in different forms of oxides like Co2O3,CoO(OH),CoO,CoO2and Co3O4[2,3].As Co3O4is more stable among other oxides of cobalt,therefore it has been extensively investigated.It is an important material used in magnetic material,battery cathodes,electrochromic films,gas sensors and catalysis.Catalytic applications of Co3O4is one of the most important applications,therefore research on cobalt oxide clusters as heterogeneous catalysts has received considerable attention recently[4–6].To achieve desired demands,Co3O4has to be prepared by asynthetic route which allows the control over the structure,valence state of the cobalt ions and morphology.Therefore,the essential issue is to produce Co3O4with defined morphology,high porosity and a narrow distribution of particle/crystallite sizes.Hence various approaches including hydrothermal treatment[7],polymer combustion[8],thermal decomposition[9]and sol–gel synthesis[10]have been adopted for fabrication of Co3O4.Thermal decomposition of cobalt salts such as cobalt acetate,cobalt sulphate,and cobalt nitrate can also be used for synthesis of cobalt oxide[11–14].How ever,the draw back of this method is the production of mixture of oxides of cobalt due to incomplete decomposition.There is a need for effective and facilesynthetic method for synthesis of Co3O4nanoparticles.We have reported a simple solid phase mechanochemical process at room temperature for synthesis of Co3O4.According to this method,Co3O4can be synthesized on large scale by milling of solid phase NH4HCO3and Co(NO3)2·6H2O in mortar at room temperature[15].Co3O4is an intrinsic p-type semiconductor with direct optical bandgaps at 1.48 and 2.19 eV[16].It can be effectively used as photo-catalyst for photo-degradation of organic pollutants under irradiation of visible light.Irradiation on photo-catalyst results in electron–hole(e/h+)pairs between valence and conduction bands in the photo-catalyst.Photo excited electron reacts with oxygen and produces superoxide anion(O2?)which yields OH radical on protonation.The positive hole,on reaction with water,also produces OH radical.Doping of semiconductors with nanosized silver remarkably reinforces the photo-catalytic efficiencies of semiconductors for degradation of pollutants by preventing the re-combination of electron–hole pair[17–19].A number of methods can be used for deposition of Ag on semiconductor photo catalyst.How ever green synthesis using plants extract as stabilizing and reducing agent has gained much attention.Here in we report the synthesis of Co3O4and Ag-Co3O4.Calotropis gigantea plant extract was used as reducing agent for the deposition of Ag on Co3O4to get Ag-Co3O4nanoparticles.The prepared Co3O4and Ag-Co3O4were employed as photo-catalysts for photo-degradation of rhodamine B.The enhancement of photo-catalytic activities of Co3O4by deposition of Ag using plant extract is economical and environmentally friendly as compared to chemical methods and this is the novelty of present work.

2.Experimental

2.1.Synthesis of Co3O4

Co3O4was synthesized by mechanical mixing of NH4HCO3and Co(NO3)2·6H2O in molar ratio of 5:2 in a mortar.During mixing,a regular change in color of the mixture and smell of ammonia gas was observed and finally it was changed to grey.After mixing and milling for 1 h,the powder obtained was washed and dried at 383 Kfor 12 h in the ambient air.Then the product obtained was calcined at 573 K for 3 h in ambient air.Co3O4was formed according to following reactions[20].

C.gigantea(Fig.1)leaves were dried at room temperature in shade until all moisture was lost(30 days).20 g of leaves was then ground and washed and then refluxed at 373 K for 2 h in 700 ml distilled water.Leaves were separated by filtration and C.gigantea leaf extract was stored for further experiments.The aqueous extract of C.gigantea leaves contain biomolecules such as proteins,enzymes,amino acids,phenolics,alkaloids,glycosides,calotopin,calotoxin,uscharin,gigantin,flavonoids and calactin.These compounds reduce and stabilize the silver ions to silver metal[21,22].

Fig.1.Calotropis gigantea plant.

2.2.Preparation of Ag-Co3O4

In synthesis of Ag-Co3O4samples with 1,2,3 and 4 wt%of Ag,30 ml plant extract was dropped into a 50 ml well-mixed slurry of AgNO3solution and Co3O4particles under constant stirring of reaction mixture at 323 K.The stirring of mixture continued for 2 h to complete the reduction of Ag1+to Ag.Then the mixture was filtered followed by washing and drying of resultant solid at 353 K for 12 h.The resultant Ag-Co3O4was stored for further studies.

2.3.Characterization

The crystalline phases of Co3O4and Ag-Co3O4were identified by X-ray diffraction(XRD)pattern using X-ray Diffractometer(JEOL-JDX-3532 Japan).FTIR analyses were performed with Bruker ALPHA FT-IR Spectrometer.Morphology of Co2O3and Ag-Co3O4was studied with JOEL-JSM-5910 Scanning Electron Microscope(Japan).

2.4.Photo-catalytic experiments

The photo-catalytic efficiencies of as-prepared Co3O4and Ag-Co3O4were evaluated towards pho-degradation of rhodamine B.For a typical experiment,40 ml of dye solution(200 mg·L?1)was stirred at agitation speed of 500 r·min?1.Gaseous oxygen(at the rate of 20 ml·min?1)was passed through the reaction mixture.After 30 min of stirring,a 0.5 ml sample was taken from reaction mixture and was analyzed.Then,0.05 g of Co3O4/Ag-Co3O4was added as catalyst to reaction mixture.The resultant mixture was stirred under visible light irradiation using an incandescent light bulb of 100 W.Sample of 0.5 ml was taken from reaction mixture at regular interval of 15 min and absorbance of each sample was measured spectrophotometrically.Measure absorbance of each sample was converted to concentration by the previously prepared calibration plot of rhodamine B.

Degradation of dye was calculated using

w here D,C0and Ctrepresent degradation,original concentration and concentration of rhodamine B dye at time t respectively.

3.Results and Discussion

3.1.Characterization

XRD of as-prepared Co3O4and Ag-Co3O4(Fig.2)shows that different peaks can be observed which indicate the crystallinity of the prepared particles.Peaks at 2θ =18.59°,31.36°,36.46°,38.22°,44.48°and 59.17°are assigned to(1 1 1),(2 2 0),(3 1 1),(2 2 2),(4 0 0)and(5 1 1)lattices of Co3O4respectively[23,24].The XRD pattern indicates that prepared Co3O4is polycrystalline in nature with cubic spinel structure.The obtained results are well matched with JCPDS data(JCPD:89–1970).Similarly,other researchers like Wadekar et al.[25];Manigandan et al.[26];and Makhlouf et al.[27];have also reported similar results.In the spectrum of Ag-Co3O4,additional peaks at 2θ =38.20°,44.40°and 64.50°which correspond to face centered cubic(fcc)of Ag(JCPDS No.04-0783)can be observed.Debye–Scherrer equation was applied for calculation of crystallite size,which was found as 15 nm for both Co3O4and Ag-Co3O4[28,29].

Fig.2.X-Rays diffracto grams of prepared catalysts.a)Co3O4 b)Ag-Co3O4.

Fig.3 shows the FTIR spectra of as-prepared Co3O4and Ag-Co3O4samples indicating various absorption bands.Peaks at 774,670 and 555 cm?1are attributed to cobalt-oxygen bond.Bhagade et al.[30]have assigned the absorption band at 555 and 658 cm?1to Co--O stretching and O--Co--O bending vibrations respectively.Similarly,Glaspell and co-w orkers[31]have assigned the peaks at 660 and 570 cm?1to spinal of Co2O3.The absorption band at 3405 cm?1shows the presence of hydroxyl--OH group.The bands present between 3200–3500 cm?1and at 1600 cm?1are due to the water on the catalyst surface[15].

Fig.3.Fourier transformed infra-red spectra of prepared catalysts.a)Co3O4 b)Ag-Co3O4.

Fig.4 represents the scanning electron micrographs of as-prepared Co3O4and Ag-Co3O4indicating that well define particles have been synthesized.How ever,the particles are irregular in shape and morphology.The micrographs indicate that particles are in the range of micron size.

3.2.Photo-catalytic properties of Co3O4 and Ag-Co3O4

Photo-catalytic activities of Co3O4and Ag-Co3O4have been investigated by irradiating 40 ml aqueous solution of rhodamine B dye(200 mg·L?1)by visible light at 313 K in the presence of 0.05 g of either Co3O4or Ag-Co3O4particles.It was found that deposition of Ag greatly reinforced the photo-catalytic activity of Co3O4in photo-degradation of rhodamine B(Fig.5).Irradiation of Co3O4results in excitation of electron to conduction band leaving behind a positive hole in valance band.Ag nano particles immobilized on Co3O4prevent the recombination of photo excited electron and positive hole,thus reinforce the photo-catalytic efficiency[32,33].The effect of Ag content on photo-catalytic efficiency of Co3O4was also investigated.It was noted that 38%,44%,57%and 52%photo-degradation of 40 ml solution of dye(200 mg·L?1)was achieved at 313 K in 60 min with 0.05 g of 1%,2%,3%and 4%Ag-Co3O4respectively.Access to the active centers of Co3O4becomes difficult in the presence of high Ag doping,there fore higher Ag load caused a decrease in catalytic activity[14].To recognize the advantages of this work over the frontier study in relevant area,the catalytic efficiency of Ag-Co3O4has been compared with reported data as given in Table 1.Table 1 shows that Ag-Co3O4is superior catalyst than reported catalysts.This comparison decodes that present protocol is preferable in performance than many existing reports.

Fig.4.Scanning electron micrographs of prepared catalysts.a)Co3O4 b)Ag-Co3O4.

Fig.5.Comparison of photo catalytic activities of prepared catalysts towards rhodamine B dye degradation.a)Co3O4 b)Ag-Co3O4.

Table 1Catalytic activities of various catalysts reported in the literature

3.3.Effect of pH

In aqueous phase heterogeneous photo-catalytic reactions,p H significantly affects the position of conductance and valence bands,catalyst aggregate size and charges on the surface of catalyst.The dependence of photo-catalytic efficiency of Ag-Co3O4on pH of reaction mixture was studied in the range of pH 2–12.For this purpose,photodegradation experiment was carried out at 313 K using a 40 ml solution of rhodamine B(200 mg·L?1)and 0.05 g Ag-Co3O4as photo-catalyst for 120 min.For the adjustment of p H,dilute solution of NaOH and HCl were used.The experimental results showed that photo-catalytic activity of Ag-Co3O4increases with pH as given in Fig.6.The enhancement in photo-catalytic activity of Ag-Co3O4with pH might be due to the presence of hydroxyl ions.These ions generate hydroxyl radicals under irradiation.

Fig.6.Dependence of photo-catalytic efficiency of Ag-Co3O4 on pH for photo-degradation of dye.

3.4.Reaction kinetics

The kinetics of present study can be described by the Eley–Rideal(ER)mechanism,according to which dye molecule reacts in fluid phase with oxygen adsorbed at catalyst surface.Irradiation of photo-catalyst results in electron–hole pair formation between valence and conduction bands in the photo-catalyst.Photo excited electron reacts with oxygen and produce super oxide anion(O2?)which yields OH radical on protonation.The positive hole also produces OH radical on reaction with water,take part in degradation of dye molecules[42].The role OH radicals were confirmed by isopropyl alcohol which acts as OH radical scavenger.In the presence of isopropyl alcohol,the photo-degradation of a 40 ml(200 mg·L?1)solution of rhodamine B dye decreased from 91%to 68%at 313 K[43].

The E-R mechanism can be described by following expression(kr=Rate constant,C=Concentration of dye,θO2=Concentration of adsorbed oxygen)[44,45].

Under continuous flow of oxygen,the reaction becomes independent of surface concentration of oxygen,hence Eq.(4)changes to Eq.(5),which changes to Eq.(6)on integration(kAp=Apparent rate constant).

Experimental data of Ag-Co3O4catalyzed photo-degradation of rhodamine B was subjected to kinetics analysis according to Eq.(6)(As given in Fig.7).The apparent rate constants(regression coefficients,R2)were 0.0121(0.97),0.0140(0.99)and 0.0198(0.99)min?1at 303,313 and 323 K respectively.19.9 kJ·mol?1was determined as energy of activation from the slop of Arrhenius plot(Fig.8).The observed lower activation energy favored increased photo-degradation efficiency.

Fig.7.Application of kinetic exp ression(Eq.(6))to Ag-Co3O4 catalyzed photodegradation of rhodamine B data at various temperatures.

Fig.8.Application of Arrhenius equation to apparent rate constants(Arrhenius plot).

Fig.9.Application of kinetic expression(Eq.(6))to Ag-Co3O4 catalyzed photodegradation with different concentrations of rhodamine B.

To explore the variation of rate of reaction with initial concentration of rhodamine Bdye,separate photo-degradation experiments were performed at 323 K over 0.05 g Ag-Co3O4with initial concentration of 100,200,300 and 400 mg·L?1.The data obtained was subjected to kinetics analysis according to Eq.(4)as given in Fig.9.The slop of the curve gives the apparent rate constants.Apparent rate constants(regression coefficient,R2)for Ag-Co3O4catalyzed photo-degradation of rhodamine B were determined as 0.0237(0.98),0.0139(0.99),0.0119(0.98)and 0.0082(0.98)min?1with initial concentration of 100,200,300 and 400 mg·L?1respectively.As dye inhibits the penetration of photon to the solution,the increase in concentration of dye results in the decrease in apparent rate constants.Furthermore,as the catalyst dose and flow of oxygen are kept constant,the number of OH radicals is also constant.With the increase of concentration of rhodamine B dye,the relative number of OH radicals attacking on dye molecules decreases,hence the rate of degradation decreases considerably with the increase in the concentration of the dye[46,47].

4.Conclusions

Co3O4and Ag-Co3O4loaded with 1 wt%,2 wt%,3 wt%,and 4 wt%Ag were synthesized successfully via a mechanochemical and green method,respectively.Deposition of 3%Ag enhanced the photo catalytic efficiency Co3O4from 41%to 91%photo-degradation of a solution of rhodamine B dye(200 mg·L?1)at 313 K.The deposition of Ag on the surface of Co3O4enhanced the photo-catalytic efficiency by decreasing the band gap and inhibiting the recombination of electron–hole pair formed as a result of irradiation of Co3O4.Almost 100%and 91%photo-degradation of 40 ml rhodamine B dye achieved in 90 and 120 min when 0.5 g of Ag-Co3O4was used as photo-catalyst at 313 K using 100 and 200 mg·L?1as initial concentration of dye respectively.In the range of used concentrations,pseudo first order kinetics was most suitable to describe the relationship between rhodamine B concentration and irradiation time in the initial photo-catalytic dyedisposal phase.The apparent rate constant of Co3O4for degradation of rhodamine B at 313 K was0.0046 min?1,which was lower than rate constant of Ag-Co3O4(0.0140 min?1)under identical experimental conditions.Therefore,the as-prepared Ag-Co3O4is a very promising photo-catalyst for practical applications for water purification because of its high activity and stability.

Acknowledgments

The World Academy of Sciences(TWAS)(13-301 RG/MSN/AS_C)is acknowledged for financial support under COMSTECH-TWASG rants Program.Punjab Higher Education Commission(PHEC)Lahore,Pakistan is also acknowledged for providing Travel Grant for presenting this paper at CAMURE-10 and ISMR-9 during July 7–10,2017 in Qingdao,China.