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Proceedings of the National Academyof Sciences, India Section A: PhysicalSciences ISSN 0369-8203Volume 83Number 1 Proc. Natl. Acad. Sci., India, Sect. A Phys.Sci. (2013) 83:7-14DOI 10.1007/s40010-012-0063-3

Effect of Silver Impregnation on Efficiencyof ZnO for Photocatalytic Degradation ofEosin Y

S. Siva Kumar, V. Ranga Rao &G. Nageswara Rao

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RESEARCH ARTICLE

Effect of Silver Impregnation on Efficiency of ZnOfor Photocatalytic Degradation of Eosin Y

S. Siva Kumar • V. Ranga Rao • G. Nageswara Rao

Received: 10 July 2012 / Revised: 17 October 2012 / Accepted: 7 November 2012 / Published online: 12 February 2013

� The National Academy of Sciences, India 2013

Abstract The photocatalytic degradation of Eosin Y, a

xanthenes dye has been investigated under visible light

irradiation in the presence of aqueous suspension of ZnO

and silver impregnated ZnO at various conditions. ZnO

was impregnated with Ag of varying concentrations such as

0.5, 1.0, 1.5 at.% and the samples were calcined at various

temperatures for 2 h. The catalyst was characterized by

XRD, SEM and UV–Visible diffuse reflectance spectro-

scopic studies. The photocatalytic activity of ZnO was

found to be enhanced by silver impregnation and 1 at.% of

silver impregnated ZnO has high photocatalytic activity.

Calcinations of samples at higher temperature decreased

the photocatalytic activity. Effect of various parameters

such as catalyst loading, initial concentration of dye and

pH of the solution on dye degradation was also studied.

The kinetics of dye degradation showed that the data were

well fitted to pseudo-first order kinetic model.

Keywords Photocatalysis � Silver impregnated ZnO �Eosin Y � Dye degradation � Textile effluents

Introduction

Dyes are used in many industries like textile, paper, plastic,

leather, ceramic, cosmetics, ink and food processing. The

waste water from these industries contains large quantities of

residual dye effluents, which are toxic and non-biodegrad-

able [1]. These dyes create severe environmental pollution

problems by releasing toxic and potential carcinogenic

substances into the aqueous phase. Decolourisation of dye

effluents has therefore received increasing attention. Various

physical and chemical processes such as precipitation,

adsorption, flocculation, reverse osmosis and ultra filtration

can be used for colour removal from textile effluents [2–5].

However, these methods merely transfer dyes from the liquid

to the solid phase causing secondary pollution, which

requires further treatment [6, 7].

During the last two decades, advanced oxidation pro-

cesses (AOPs) have been proposed as an alternative way

for treating undesirable organic pollutants, including dye-

stuffs [8]. These methods were based on generation of

highly reactive species like hydroxyl radicals that oxidizes

a broad range of pollutants. Among AOPs, heterogeneous

photocatalysis includes a combination of semiconductors

and light that seems to be an attractive method for the

degradation of various organic pollutants [9–12]. The

reason for increased interest towards photocatalytic deg-

radation is that the process may use atmospheric oxygen as

the oxidant and can be carried out under ambient condi-

tions that lead to total mineralization of organics to CO2,

water and mineral acids.

Titanium dioxide (TiO2) is considered as one of the best

photocatalyst and has the ability to detoxify water from a

number of organic pollutants [13, 14]. However, the

shortcoming of TiO2 is that it absorbs only a small portion

of solar spectrum in the UV region. Hence interest has been

S. Siva Kumar

Department of Chemistry, Anil Neerukonda Institute of

Technology and Sciences, Sangivalasa,

Visakhapatnam 531162, India

V. Ranga Rao

Department of Chemistry, Government College (Autonomous),

Rajahmundry 533105, India

G. Nageswara Rao (&)

School of Chemistry, Andhra University,

Visakhapatnam 530003, India

e-mail: gollapallinr@yahoo.com

123

Proc. Natl. Acad. Sci., India, Sect. A Phys. Sci. (January–March 2013) 83(1):7–14

DOI 10.1007/s40010-012-0063-3

Author's personal copy

drawn towards the search for suitable alternatives to TiO2.

Many attempts have been made to study photocatalytic

activity of different semiconductors such as SnO2, ZrO2,

CdS and ZnO [15–17]. Lizama et al. [16] reported that ZnO

is a more efficient catalyst than TiO2 for degradation of

reactive blue 19. The advantage of ZnO is its absorption

over a larger fraction of solar spectrum than TiO2 [18]. For

this reason ZnO seems to be the most suitable catalyst for

photo degradation in the presence of sun light.

Eosin Y is the widely used counter stain in routine

staining of histological tissue sections (erythrocytes, col-

lagen, epithelial cells, etc.) [19, 20]. Biological stains such

as Eosin Y, Auramine O are used in biomedical research

laboratories and also for diagnostic purposes. These stains

are known to be toxic or mutagenic for human and animals

[21, 22]. High concentration of dye in stain solutions

results in the formation of waste waters of high toxicity. So

in the present study, the photocatalytic degradation of

Eosin Y in aqueous solutions under visible light irradiation

by using ZnO and Silver impregnated ZnO was investi-

gated. Effect of pH, initial dye concentration and photo-

catalyst doses on dye degradation was also reported.

Experimental

Materials

Zinc oxide and Eosin Y were obtained from Merck India.

All the solutions were prepared using double distilled

water. All the chemicals used in the study were of ana-

lytical reagent grade.

Preparation of Silver-Impregnated ZnO Photocatalyst

Silver impregnated ZnO containing 1 at.% of silver was

prepared by the following procedure. Silver nitrate solution

containing 105 mg in 100 ml was added to 5 g of ZnO and

the mixture was stirred for 4 h. The suspension was

allowed to stand for 24 h undisturbed and water was then

evaporated by heating at 100 �C for 16 h. The dried solid

was grounded to fine powder in an agate mortar and cal-

cined at 200, 400 and 600 �C for 2 h in muffle furnace.

Silver impregnated ZnO of 0.5 and 1.5 at.% were prepared

by taking 52.5 and 157.5 mg of silver nitrate and following

the same procedure.

Characterization

XRD and SEM

The catalysts were characterized for their structure and

morphology by XRD (X-ray diffraction) and SEM (scanning

electron microscopy). The XRD patterns of the powdered

samples were recorded by Bruker 8 D advanced X-ray dif-

fractometer and SEM images of the samples were taken by

using Philips XL 30 ESEM Scanning electron microscope.

UV–Vis Diffuse Reflectance Spectroscopy

UV–Vis spectroscopy was used to characterize the optical

absorption properties of ZnO and silver impregnated ZnO

(Ag–ZnO). The UV–Vis absorption spectra of the samples

were recorded in the wavelength range 200–800 nm using

Shimadzu UV 3600 UV–Vis–NIR spectrometer in diffuse

reflectance mode (DRS) using BaSO4 as reference. Spectra

were recorded at room temperature and the data were

transformed through Kubelka–Munk function.

20 40 60 80

0

500

1000In

ten

sity

/a.u

2 Theta/Degree

Undoped ZnO

0.5% Ag-ZnO

1.0% Ag-ZnO

1.5% Ag-ZnO

a

045303

0

600

12006000C

4000C

2000C

Inte

nsi

ty/a

.u

2 Theta/Degree

1000C

Ag

Ag

Ag

b

Fig. 1 Powdered XRD patterns of ZnO and various at.% of Ag–ZnO

samples. a Dried at 100 �C and b 1 at.% Ag–ZnO calcined at various

temperatures

8 S. Siva Kumar et al.

123

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Photocatalytic Studies

The experimental setup for the photocatalytic degrada-

tion consisted of a 250 ml beaker with water circulation

outside. The beaker was placed on a magnetic stirrer;

above which high pressure mercury vapour lamp

(125 W, Philips) emitting visible light was placed.

100 ml of the dye solution was taken in the beaker and

ZnO or Ag–ZnO at a dose of 1 g l-1 was added to it.

The distance of the light source from the upper level of

dye solution in the beaker was 18 cm. The solution was

stirred in the dark for 5 min to establish the adsorption

equilibrium. The zero time reading was taken and the

solution was then irradiated. Aliquots were taken at

regular time intervals and centrifuged to analyze the

percent degradation of the dye.

Analysis

The percent degradation of Eosin Y was determined

spectrophotometrically by measuring absorbance of the

dye solution at kmax 518 nm. The decolourisation effi-

ciency (%) was calculated as:

Efficiency %ð Þ ¼ C0 � C

C0

� 100

where C0 is the initial concentration of dye and C is the

concentration of dye after irradiation. Similar experiments

Fig. 2 SEM images of a pure ZnO and b 1 at.% Ag impregnated ZnO

350 375 400 425

0.0

0.3

0.6

b

Ab

sorb

ance

/a.u

Wavelength /nm

a

Fig. 3 UV–Vis absorption spectra of ZnO and Ag impregnated ZnO;

a pure ZnO, b 1 at.% Ag–ZnO

3.0 3.2 3.4 3.6

0

20

40

60

b

[F(R

)hv]

2

Photon energy /eV

a

Fig. 4 Kubelka–Munck function versus photon energy plots for ZnO

and Ag impregnated ZnO; a pure ZnO and b 1 at.% Ag–ZnO

Effect of Silver Impregnation on Efficiency of ZnO 9

123

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were carried out by varying concentration of dye solution,

catalyst loading and pH of the solution.

Results and Discussion

Catalyst Characterization

XRD and SEM Analysis

The XRD patterns obtained for powdered ZnO and Ag–

ZnO of various concentrations of silver loading 0.5, 1.0 and

1.5 at.% dried at 100 �C are shown in Fig. 1a indicating

well indexed phases of ZnO wurtzite structure (JCPDS No.

36–1451). The intensity of XRD peaks increases with

increase in addition of silver up to 1 at.% indicating better

crystallinity of the samples and further addition of silver

again decreased the intensity. Impregnation of ZnO with

Ag? ions doesn’t show any change in crystal structure may

be due to low percent of silver loading and surface depo-

sition. Calcination of samples at 200, 400 and 600 �C

introduced a small extra peak in XRD pattern (Fig. 1b)

characteristic of metallic silver [23, 24]. The intensity of

the peak is very low due to lower percent of silver incor-

porated into ZnO crystal lattice.

Based on the photocatalytic degradation studies, further

characterization was done for 1 % Ag–ZnO, which exhibits

better photocatalytic activity.

The SEM images of ZnO and 1 % Ag–ZnO heated at

100 �C are given in Fig. 2, which shows that ZnO particles

are in different shapes and sizes and, silver doping does not

change the morphology of ZnO.

Diffuse Reflectance Analysis

Figure 3 shows UV–Vis absorption spectra of ZnO as well

as ZnO impregnated with 1.0 at.% of silver. Both the

samples have a strong absorbance below 400 nm. The

addition of silver caused a slight shift in absorption to

higher wavelength side (red shift). The band gap energies

of ZnO and 1 % Ag–ZnO samples were calculated by

extrapolating the linear portion of the plots of modified

Kubelka–Munk function, [F(R)hm]2 versus the photon

energy (hm) shown in Fig. 4 [25]. The band gap energies of

pure ZnO and 1 % silver impregnated ZnO samples were

found to be 3.28 and 3.27 eV respectively. The band gap

0 30 60 90 1200

25

50

75

100

% D

egra

dat

ion

Time/min

Fig. 5 Effect of silver doping on degradation of Eosin Y, (Eosin

Y) = 7.22 9 10-5 M, (Catalyst) = 1 g l-1; (open square) pure ZnO,

(open circle) 0.5 % Ag–ZnO, (up-pointing triangle) 1.0 % Ag–ZnO,

(down-pointing triangle) 1.5 % Ag–ZnO

0 30 60 90 1200

25

50

75

100

% D

egra

dat

ion

Time /min

a

0

25

50

75

100

% D

egra

dat

ion

Time /min

b

0 30 60 90 120

Fig. 6 Effect of catalyst loading on degradation of Eosin Y, (Eosin Y) = 7.22 9 10-5 M; a pure ZnO and b 1 at.% Ag impregnated ZnO; (opensquare) 0.25 g l-1, (open circle) 0.50 g l-1, (star) 1.00 g l-1, (open diamond) 1.50 g l-1, (open triangle) 2.00 g l-1

10 S. Siva Kumar et al.

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energy is decreased slightly for 1 % Ag–ZnO compared to

pure ZnO, which supports the red shift in absorption

spectra.

Adsorption and Direct Photolysis

Aqueous solution of Eosin Y (7.22 9 10-5 M) mixed with

ZnO (1 g l-1) was stirred in dark to observe whether there

was any colour removal due to adsorption. There was no

observable adsorption on the surface of the photocatalyst

on magnetic stirring in dark for 1 h. Direct photolysis of

dye solution was also studied by irradiation in the absence

of the photocatalyst with magnetic stirring for 1 h. It

showed no observable degradation.

Photocatalytic Degradation

The aqueous solutions of Eosin Y (7.22 9 10-5 M) were

found to degrade up to 68 and 88 % on visible light irra-

diation for 1 h in the presence of ZnO (1 g l-1) and Ag–

ZnO (1 g l-1), respectively. The degradation efficiency of

Ag–ZnO was found to be higher than ZnO as shown in

Fig. 5.

The effect of calcination temperature of Ag–ZnO on

degradation of Eosin Y was also studied. The percent

degradation decreased with increase in calcination tem-

perature. Samples heated at 100 and 200 �C have high

activity than samples calcined at 400 and 600 �C for 2 h.

Higher temperature calcination has a negative effect on

photocatalytic activity because the presence of silver pro-

motes the densification and grain growth of ZnO at higher

temperatures by forming a silver island in the ZnO matrix.

This causes reduction in active surface sites of the photo-

catalyst for the adsorption of dye and light [26]. The

degradation efficiency also varied with concentration of

silver ion impregnation. With increase in silver loading

from 0.5 to 1.5 at.%, the degradation efficiency was found

to be maximum for 1 at.% of silver impregnation on ZnO.

This is the optimal concentration of Ag impregnation on

ZnO by this method.

The photocatalytic degradation of a dye in solution is

initiated by the photoexcitation of the semiconductor, fol-

lowed by the formation of electron-hole pair on the surface

of catalyst. The high oxidative potential of the hole in the

catalyst permits the direct oxidation of the dye to reactive

intermediates. Another reactive intermediate which is

responsible for the degradation is hydroxyl radical formed

by the decomposition of water or by reaction of hole with

OH-. The hydroxyl radical is an extremely strong non-

selective oxidant which leads to partial or complete min-

eralization of several organic chemicals [27]. The role of

reductive pathway in heterogeneous photocatalysis is very

little as compared to oxidation [28]. The enhancing effect

of Ag doping may be explained by its ability to trap

electrons and this process reduces the recombination of

charges and favours the oxidation of the substrate [29].

Higher concentrations of silver doping could be unfa-

vourable to photocatalytic efficiency. It is assumed that

silver doping below optimum value acts as electron-hole

separation center [30], where as above optimum value it

can also act as charge carrier recombination center [31].

Effect of Catalyst Concentration

In order to determine the effect of catalyst loading, the

experiments were performed by varying catalyst concentra-

tion from 0.25 to 2.0 g l-1 for dye solution of 7.22 9 10-5 M

at natural pH of dye solution 5.97. The percent degradation of

0 30 60 90 120 150 180

0 30 60 90 120 150 180

0

25

50

75

100

% D

egra

dat

ion

Time/min

a

0

25

50

75

100

% D

egra

dat

ion

Time/min

b

Fig. 7 Effect of dye concentration on degradation of Eosin Y,

(Catalyst) = 1 g l-1. a Pure ZnO and b 1 at.% Ag–ZnO; (star)

7.22 9 10-6 M, (open diamond) 1.44 9 10-5 M, (down-pointingtriangle) 2.89 9 10-5 M, (up-pointing triangle) 7.22 9 10-5 M,

(open circle) 1.44 9 10-4 M, (open square) 2.89 9 10-4 M

Effect of Silver Impregnation on Efficiency of ZnO 11

123

Author's personal copy

Eosin Y for various doses of catalyst has been shown in Fig. 6.

The percent degradation increases with increase in catalyst

dose and higher concentrations of catalyst beyond 1.5 g l-1

does not have much effect on degradation. Similar trend was

observed in case of Ag–ZnO too. This can be explained on the

basis of increased total active surface area with the catalyst

dose and availability of more active sites on the surface. At the

same time due to increased turbidity of the suspension with

high dose of photocatalyst, penetration of light and photoac-

tivated volume of suspension decrease.

Effect of Dye Concentration

The effect of initial dye concentration on the photodegra-

dation of Eosin Y was studied at different concentrations in

the range of 7.22 9 10-6–2.89 9 10-4 M (Fig. 7). It was

observed that the percent degradation gradually decreased

with increase in initial dye concentration. As concentration

of dye solution increases, the photons get intercepted by

dye molecules before they reach the catalyst surface,

decreasing the absorption of photons by the catalyst.

Hence, the generation of relative amounts of OH� and O2�-

on the surface of the catalyst decreases. Thus the dye

degradation efficiency decreases as the dye concentration

increases [32].

Effect of pH

Industrial effluents containing dyes are discharged at dif-

ferent pH values. Hence, the effect of pH on the decolo-

urisation efficiency was studied at various pH values

ranging from 2 to 12 for constant dye concentration and

catalyst loading (Fig. 8). It was observed that the decolo-

urisation efficiency was increased slightly with increasing

pH in the acidic range but was greatly increased in the

basic range above pH 9.0. Similar behavior was reported in

the literature [16–18]. The pH of the solution is an

important parameter in the photocatalytic reactions taking

place on the surface of the particles as it dictates the charge

on the particle surface. The zero point charge for ZnO is

9.0 and ZnO surface is positively charged below pH 9.0

and above this pH, surface is negatively charged by

adsorbed OH- ions. The presence of large quantities of

OH- ions on the particle surface as well as in the reaction

medium favours the formation of OH�, which is responsible

for enhancement in photocatalytic activity [17].

Kinetic Analysis

The kinetics of degradation of Eosin Y for an initial con-

centration as the reaction proceeds have been described

well by the Langmuir–Hinshelwood kinetic model [33].

r ¼ dC

dt¼ kKC

1þ KC

As KC is very small compared to unity, neglecting KC

in the denominator and integrating with respect to time t,

simplifies the above equation to pseudo first order kinetic

equation

lnC0

C

� �¼ kKt ¼ kapp t

where r is rate of dye degradation, C0 is initial concen-

tration of dye, C is the concentration of dye at time t, k is

reaction rate constant and K is absorption coefficient of the

dye onto the photocatalyst particle.

The kinetic plots for the degradation of Eosin Y under

optimal catalyst dose of ZnO and Ag–ZnO are shown in

0 20 40 60 80 100 1200

25

50

75

100

% D

egra

dat

ion

Time/min

a

0

25

50

75

100

% D

egra

dat

ion

Time/min

b

0 20 40 60 80 100 120

Fig. 8 Effect of initial pH of dye solution on degradation of Eosin Y, (Eosin Y) = 7.22 9 10-5 M, (Catalyst) = 1 g l-1, a pure ZnO and

b 1 at.% Ag–ZnO; (open square) 2.21, (open circle) 4.17, (up-pointing triangle) 6.05, (down-pointing triangle) 9.07, (star) 11.17

12 S. Siva Kumar et al.

123

Author's personal copy

Fig. 9. The rate constants were calculated to be 0.018 and

0.0296 min-1 for degradation of Eosin Y on ZnO and Ag–

ZnO, respectively.

Conclusions

The impregnation of silver ions enhanced the photocata-

lytic activity of ZnO. The optimum concentration of silver

loading in this method was found to be 1 at.%. Calcination

of the samples at higher temperature has a negative impact

on photocatalytic activity. The characterization of samples

by XRD and SEM analysis indicated that silver addition

did not change the crystal structure and morphology of

ZnO, but DRS studies indicate a small decrease in band

gap of 1 % Ag–ZnO. This may be one of the reasons for

higher activity of 1 % Ag–ZnO samples over others in

visible region. The percent degradation of Eosin Y

increased with increase in catalyst dose up to an optimum

loading. Further increase in catalyst dose showed a little

effect. The photocatalytic degradation of Eosin Y followed

pseudo-first order kinetics.

Acknowledgments The authors thank the UGC Networking pro-

gram, University of Hyderabad, India for providing instrumental

facility to carry out XRD and SEM analyses.

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0 15 30 45 600.0

0.5

1.0

1.5

2.0ln

(C0/C

)

Time/min

R2= 0.9912

R2= 0.9823a

b

Fig. 9 Kinetic plots for degradation of Eosin Y on a ZnO and

b 1 at.% Ag–ZnO (Eosin Y) = 7.22 9 10-5 M, (Catalyst) = 1 g l-1

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