CHAPTER VIII PHOTOCATALYTIC ACTIVITY OF Ag, CuO, NiO AND ...

CHAPTER VIII

PHOTOCATALYTIC ACTIVITY OF Ag, CuO,

NiO AND ZnO NANOPARTICLES

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Figure. 8

by photon

8.1 Schem

n acting o

matic diag

on the sem

gram of th

miconduct

he photoc

tor [4].

catalytic pprocess in

nitiated




107

8.1 Introduction

Semiconductors (such as TiO2, NiO, ZnO, CuO, Fe2O3, CdS, and

ZnS) can act as sensitizers for light-induced redox-processes due to

the electronic structure of the metal atoms in chemical combination,

which is characterized by a filled valence band, and an empty

conduction band [1]. Nanopowders, controlled to nanocrystalline size

(less than 100 nm) can show atom �like behaviors which result from

higher surface energy. It is due to their large surface area as well as

the wider band gap between the valence and conduction band when

they are divided to near the atomic scale.

Upon irradiation, valence band electrons are promoted to the

conduction band leaving a hole behind. These electron�hole pairs can

either recombine or can interact separately with other molecules. The

holes may react either with electron donors in the solution, or with

hydroxide ions to produce powerful oxidizing species like hydroxyl

(oxidation potential 2.8 V) or superoxide radicals [2]. In other word,

semiconductor materials are materials whose valence band and

conduction band are separated by an energy gap or band-gap. When a

semiconductor molecule absorbs photons with energy equal or greater

than its band-gap, electrons in the valence band can be excited and

jump up into the conduction band, and thus charge carriers are

generated. In order to have a photocatalyzed reaction, the e �h+

recombination, subsequent to the initial charge separation, must be

prevented as much as possible [3].




108

Among all these semiconductors, the most widely used

semiconductor catalysts in photoinduced processes are titanium

dioxide (TiO2), zinc oxide, copper oxide and nickel oxide. Though these

semiconductors have the disadvantage of not being activated by

visible light, but by ultraviolet (UV) light, it is advantageous over the

others in that it is chemically and biologically inert, photocatalytically

stable, relatively easy to produce and to use, able to efficiently catalyze

reactions, cheap and without risks to the environment or humans .

Figure. 8.1 shows the schematic diagram of the photocatalytic process

initiated by photon acting on the semiconductor.

Higher crystallinity is a positive factor for improving the

photocatalytic activity because the higher the crystalline quality is, the

smaller the number of defects is. The defects act as trapping and

recombination centers for photogenerated electrons and holes,

resulting in a decrease in the photocatalytic activity. On the other

hand, the particle size determines the migration distance for

photogenerated electrons or holes to the reaction sites at the particle

surface, and bigger size results in the increase of the recombination

probability, which is a negative factor for the photocatalytic activity.

The resulting photocatalytic activity is dominated by the balance

between these two factors.

For estimating quantitatively photocatalytic activities of the

samples, the pseudo-first order reaction is employed to calculate the

rate constant (k) of Methyl Orange (MO) degradation. In this UV light,




109

since the ratio of absorbance At of MO at time t to its value Ao

measured at t=0 must be equal to the concentration ratio Ct/Co of

MO, the kinetic equation for the reduction can be written as [5]

tappt Ck

dtdC

or tkAA

CC

apptt

00

lnln

where Ct is the concentration of MO at time t and kapp is the apparent

rate constant, which can be obtained from the decrease of the peak

intensity at 464 nm with time.

8.2 Photocatalytic activity of Ag Nanoparticles

Figure 8.2 shows the UV-visible spectra showing the

degradation curves. Figure 8.3 shows the plot of ln(At/Ao) against

reaction time for AG, ACA, TNP, ACA+TNP and AG+TNP in the

catalytic reduction of MO. A good linear correlation, ln(At/A0) versus

time, is obtained for all the systems studied. The samples ACA and AG

didn�t show any photocatalytic activity. The apparent rate constant of

this catalytic reaction for TNP, ACA+TNP, AG+TNP, was found to be

0.006/min, 0.014/min and 0.017/min respectively, as measured from

the plot of ln(At/Ao) versus time. It can be seen that Ag NP+TNP

samples show higher photocatalytic activities than that of the TNP

sample under UV irradiation and this is an effective way to enhance

the photocatalytic activity. And comparing the combination of

ACA+TNP and AG+TNP, AG+TNP shows the excellent photocatalytic




Figure 8

a) TNP b)

8.2 UV v

AG+TNP

isible spe

and c) AC

ectra sho

CA+TNP.

owing the

e degradaation of M

MO by




Figure 8

ACA+TNP

.3 Plot of

P and AG+

f ln(At/Ao

+TNP in th

o) against

he catalyt

reaction

tic reducti

time for

ion of MO

AG, ACA

O.

A, TNP,




110

activity than ACA which can be explained on the basis of the size

effect.

The following reasons may attribute to the observed

photocatalytic activity of Ag nanoparticles+TNP. The metal

nanoparticles increases the rate of photocatalytic activity of

semiconductor [6]. The Fermi levels of the Ag nanoparticles and TiO2

align in such a way that the electrons from TiO2 flow to Ag

nanoparticles when Ag nanoparticles are added to TiO2 [6]. When the

light incident on TiO2, the electron and hole pair will be produced. The

electron will move to Ag nanoparticles which traps the electron. This

will suppress the electron-hole combination. The hole is diffuse to the

surface of TiO2. It will induce the oxidation process in MO which was

adsorbed on the surfaces of TiO2. The decrease in the electron density

in TiO2 tends to an increase in the hydroxyl group acidity [7]. This in

turn enhances the photocatalytic activity of the TiO2.

As the size of the metal nanoparticles added with semiconductor

decreases, the photocatalytic efficiency of the semiconductor increases [8].

This may be due to the ability of the small metal nanoparticles to trap

photo electrons produced in the semiconductor. If the size of the metal

nanoparticles added to the semiconductor increases, it acts as

recombination sites due to their ability to capture both photo

electrons and holes. The observed high apparent rate constant for

GAg+TNP was due the smaller size of the nanoparticle. The observed

enhanced photocatalytic activity of TiO2 by silver nanoparticle was




111

also confirmed by the observed rough atomic rough structure due to

the silver twinning crystalline structure. Since the atomic

arrangements in nanoparticles determine the efficiency of the catalyst.

The addition of small size Ag nanoparticles on TiO2 is attributing to

acceleration of O2- formation and decreases the recombination ability

and increases the generation of H+ or OH. When the small size Ag

nanoparticles combine with electron it is transferred to Ag-. This Ag-

interacts with Ti4+ to form Ag and Ti3+. In addition, Ag- interacts with

O2 form O2- and Ag. The observed high amount of oxygen anion

radicals and the reactive center of the semiconductor surface are

responsible for the enhanced photocatalytic activity of TiO2 [9].

The photosensitization degradation reaction can be summarized

in the following five steps:

adsdye +h ( >420nm) *adsdye (excitation reaction)

*adsdye + TiO2 adsdye + TiO2 (ecb -) (electroninjection reaction)

adsdye + TiO2 (ecb -) dye ads (charge recombination reaction)

TiO2 (ecb -) +O2 2O (electron scavenging reaction)

adsdye + ( 2O or OH ) degradation

Firstly, the excited dye molecules are generated upon absorption

of visible photos and subsequently injecting electrons into TiO2

conduction band. After addition of Ag nanoparticles, it disperses




112

randomly on the TiO2 surface, the charge transfer between TiO2 and

MO is promoted. Two main reactions can take place due to suitable

energetic positions of MO and Ag nanoparticles:

adsdye + Ag adsdye + Ag (ecb -)

Ag (ecb -) TiO2 (ecb -)

These can be described as (i) the electron injection from the

excited state of MO to the conduction band of Ag and (ii) the

relaxation process of the hot electron from the conduction band of Ag

to TiO2. Thus, an electron injection into the conduction band of TiO2

can be enhanced by the synergistic capture of electron reaction due to

the presence of Ag nanoparticles with TiO2 nanoparticles. The injected

electron reacts with the surface adsorbed O2 to yield active oxygen

radicals (e.g., 2O , OH , OOH ). Subsequently, the dyes are degraded or

mineralized by these oxygen radicals. Therefore, the resultant MO

degradation rate is enhanced.

The enhanced UV light photocatalytic activity of AgNPs +TNP

may be attributed to the following reasons also. Because the Fermi

level of Ag is lower than that of titanium, the photo induced electrons

can transfer to the Ag nanoparticles. Then, the transferred electrons

are trapped by Ag nanoparticles and separated effectively. The strong

interaction between Ag nanoparticles and TNP can effectively inhibit

the recombination of excited electrons and holes and then enhance

the photocatalytic performance. Accordingly, more photoinduced




113

electrons and holes can be produced, resulting in more oxidizing

peroxy or superoxy species to participate photocatalytic degradation

reaction [6]. Therefore the enhanced photocatalytic activity is observed

for Ag NPs+TNP compared to TNP.

8.3 Photocatalytic activity of nano CuO

CuO nanoparticles have been widely used as powerful

heterogeneous catalysts because of its high activity and selectivity in

oxidation/reduction reactions. The main purpose of nanoparticle

catalysis lies with activity promotion and selectivity enhancement with

respect to bulk materials. In order to demonstrate the photocatalytic

activity of the prepared CuO nanoflowers CuO nanorods the

degradation of MO under UV irradiation was investigated. It was

observed that the absorbance value of MO decreases with the increase

in the time of irradiation. A good linear correlation, ln(At/Ao) versus

time, is obtained. The apparent rate constants of this catalytic

reaction for floral nano CuO and rod like nano CuO were 0.018/min

and 0.017/min respectivley. Figure 8.4 shows the UV visible

degradation spectra and Figure 8.5 shows the plot for degradation of

MO.

The relevant reactions at the CuO surface causing the

degradation of dyes can be expressed as follows:

CuO +h (UV) CuO (eCB +hVB +)

CuO (hVB +) + H2O CuO +H++OH�




Fig 8.4 U

and b) CC

UV visible

CA.

e spectra

showing the degrradation o

of MO by a) CG




Figure 8

b) CCA in

8.5 Plot

n the cata

of ln(At/

alytic redu

/Ao) again

uction of M

nst reacti

MO.

ion time

for a) CCG and




114

CuO (hVB +) + OH CuO +OH�

CuO (eCB ) + O2 CuO +O2 �

O2 � + H+ HO2�

Dye + OH� degradation products

Dye + hVB+ oxidation products

Dye + eCB reduction products

8.4 Photocatalytic activity of NiO nanoparticles

The recombination of the electron and the hole must be

prevented as much as possible if a photocatalyzed reaction must be

favored. The ultimate goal of the process is to have a reaction between

the activated electrons with an oxidant to produce a reduced

productivity, and also a reaction between the generated holes with a

reluctance to produce an oxidized product. The photogenerated

electrons could reduce the dye or react with electron acceptors such

as O2 adsorbed on the semiconductor surface or dissolved in water,

reducing it to superoxide radical anion O2 � [10]. The photogenerated

holes can oxidize the organic molecule to form R+, or react with OH or

H2O oxidizing them into OH� radicals. Together with other highly

oxidant species (peroxide radicals) they are assumed to be responsible

for the NiO photodecomposition of organic substrates as dyes. The

resulting �OH radical, being a very strong oxidizing agent (standard

redox potential +2.8 V) can oxidize most azo dyes to the mineral end-




Figure 8

a) NG and

8.6 UV v

d b) NCA.

isible spe

ectra sho

owing thee degrada

ation of MMO by




Figure 8.

in the cat

.7 Plot of

talytic red

ln(At/Ao)

duction of

against r

f MO.

reaction tiime for a)

NG and bb) NCA




115

products. According to this, the relevant reactions at the NiO surface

causing the degradation of dyes can be expressed as follows:

NiO +h (UV) NiO (eCB +hVB +)

NiO (hVB +) + H2O NiO +H++OH�

NiO (hVB +) + OH NiO +OH�

NiO (eCB ) + O2 NiO +O2 �

O2 � + H+ HO2�

Dye + OH� degradation products

Dye + hVB+ oxidation products

Dye + eCB reduction products

where h is photon energy required to excite the semiconductor

electron from the valence band (VB) region to the conduction band

(CB) region.

Figure 8.6 shows the UV- visible spectra for MO degradation. Figure

8.7 shows the degradation curves of MO over different samples. A good

linear correlation, ln(At/A0) versus time, is obtained for all the systems

studied. The apparent rate constant of this catalytic reaction for NG

and NCA were found to be 0.021/min and 0.049/min respectively, as

measured from the plot of ln(At/A0) versus time. On comparing NCA

shows the excellent photocatalytic activity than NG which can be

explained on the basis of fine crystalline nature.




116

8.5 Photocatalytic activity of nano ZnO

Under UV light the reactive species such as super oxide radical

and hydroxyl radical, generated by the ZnO powders can oxidize the

dye molecules to the degraded products then the ratio of At/Ao

decreased with the irradiation time. The UV- visible spectra for

degradation is shown in Figure 8.8. The efficiency of photo

degradation of MO was indicated by the decrease of the ratio between

the remaining dye in the solution after irradiation at time t (At) and

the initial absorbance (Ao) as shown in Figure 8.9. The photocatalytic

discoloration of dye is believed to take place according to the following

mechanism:

ZnO + h ZnO (hvb+ +ecb-)

hvb+ + H2O �OH+ H+

ecb- + O2 �O2-

�O2- + H2O H2O2 2�OH

�OH + dye dyeox intermediates CO2 + H2O

The photocatalytic activities are then suppressed if the photo-induced

electrons recombine with the holes as follows:

hvb+ +ecb- heat

This process is possibly inhibited by oxygen valencies. A good

linear correlation, ln(At/A0) versus time, is obtained for all the systems

studied. The apparent rate constant of this catalytic reaction for ZG




Figure 8

a) ZG and

8.8 UV v

d b) ZCA.

isible speectra shoowing thee degrada

ation of MMO by




Figure 8.

in the cat

.9 Plot of

talytic red

ln(At/Ao)

duction of

against r

f MO.

reaction tiime for a)

) ZG and b) ZCA




117

and ZCA were found to be 0.010/min and 0.011/min respectively, as

measured from the plot of ln(At/A0) versus time. On comparing ZCA

shows the higher photocatalytic activity than ZG which can be

explained on the basis of fine crystalline nature.

8.6 Comparison of photocatalytic activity

There are two linked parts of the photocatalysis mechanism

(photo and catalysis). The first part (the photo part) concerns a

phenomenon linked to the light material interaction which includes

photos absorption, charge carrier creation and dynamics, and also

surface trapping. The second part (catalysis part) concerns a

phenomenon linked to surface radical formation and surface

reactivity, that is the interaction between H2O, O2, organic pollutant

and the oxide surface. For the photo part the most effective structural

parameter on photocatalysis is the crystalline quality, which can be

expressed in terms of crystallite size. For the catalysis part the specific

surface area is the most effective structural parameter. Together with

these parameters, atmospheric pollution, OH groups and particle size

distribution also contribute to the photocatalytic process [5]. In the

same way as Boujday et al. discussed, in the present study also, the

size and crystalline nature of the nanoparticles plays a vital role in

determining the photocatalytic activity.

On the other hand, the shape of the crystals affects the

absorbencies and direct band gaps and then impacts on the

photocatalytic performance due to the different activity of the




118

dominant facet. The specific surface dominated by the structural and

crystal size is an important factor affecting the reaction activity. High

absorbency in the visible region results in the visible-light induced

photocatalytic activity As a result, the different morphologies of metal

oxide crystals lead to the different specific surface areas, direct band

gaps and UV-visible absorbencies which cause the different

photocatalytic activities. So the shape is also an important factor

which determines the photocatalytic activity. Table 8.1 gives the

comparative study of the Ag, CuO, NiO, and ZnO nanoparticles. From

the table, it is seen that, the crystal shape, size, uniformity, crystalline

nature is holding the complete responsibility of determining the

photocatalytic activity. The ocatahedral shape with high crystalline

quality is producing the maximum photocatalytic activity. The flower

and rod like nanoparticles with high crystalline quality is also showing

very good photocatalytic activity. In the case of flakes shape, the

activity is low compared to other shapes which can be explained

under the fact of crystalline nature.

8.7 Conclusion

The photocatalytic activity of Ag, CuO, NiO and ZnO were

investigated. The silver nanoparticles were acting as a very good co-

catalyst when they are added to TNP nanoparticles. The octahedral

shape is exhibiting excellant photocatalytic activity than other

samples. Flower like and rod like nanoparticles also showing high

photocatalytic activity. The nanoflakes also showing good activity but




119

compared to other samples it is showing the lowest activity. The

lowest photocatalytic activity can be explained under the fact of the

size and crystalline nature. By comparing the photocatalytic activity, it

is known that, it completely depends on size, shape, uniformity,

crystalline nature and quality.

References

[1] M.R. Hoffmann, S.T. Martin,W. Choi, Chem. Rev., 95(1995) 69.

[2] W.Z. Tang, H. An, Chemosphere, 31(1995)4171.

[3] T.V. Gerven, G. Mul, J. Moulijn, A. Stankiewicz, Chem. Eng.

Proc., 46(2007)781.

[4] U.I. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C, 9(2008)1.

[5] S. Boujday, F Wunsch, P Portes , J.F Bocquet, C.C. Justin,

Sol. Energ. Mat. Sol. C., 83(2004)421.

[6] A.L. Linsebigler, G.Q. Lu, J.T. Yates Jr., Chem. Rev., 95 (1995)

735

[7] N. J-Renault, P. Pichat, A. Foissy, R. Mercier, J. Phys. Chem.,

90 (1986) 2733.

[8] H. Kobayashi, F. Mizuno, Y. Nakato, H.J. Tsubomura, Phys.

Chem., 95 (1991)819.

[9] F. Meng, Z.Sun, Mater. Chem. Phys., 118 (2009) 349.

[10] X. Lin, F. Rong, D. Fu, C. Yuan, Powder Tech., 219(2012)173.




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CHAPTER VIII PHOTOCATALYTIC ACTIVITY OF Ag, CuO, NiO AND ...

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