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Journal of Physics and Chemistry of Solids 69 (2008) 1495–1497

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Scratch test for immobilization of photocatalytic ZnO nanopowderssynthesized by solution combustion method

Sung Parka,�, Hye-Jung Parka, Kang Yooa, Hong Sick Kima, Jae Chun Leea,Yun-Joong Chunga, Ju-Hyeon Leeb, Myong Kyun Parkc

aDepartment of Materials Science and Engineering, Myongji University, Yongin, Kyunggi 449-728, Republic of KoreabDepartment of Electronic Materials Engineering, Sunmoon University, Asan, Choongnam 336-708, Republic of Korea

cDepartment of Mechanical Engineering, Myongji University, Yongin, Kyunggido 449-728, Republic of Korea

Received 25 June 2007; accepted 31 October 2007

Abstract

Four kinds of organic binders and three kinds of inorganic binders were prepared. All of the binding materials are commercially

available except zinc phosphate. The zinc phosphate was synthesized by the reaction of ZnO powders and phosphoric acid. All of these

binders were used to immobilize the solution combustion method (SCM) ZnO powders on the glass substrates. Scratch test result showed

that the zinc phosphate has the best overall adhesion properties such as high cohesive force with the ZnO powders and high adhesive

force with the glass substrate. Furthermore, it showed the highest photocatalytic efficiency among organic and inorganic binders. Its

photocatalytic efficiency reached to about 80% of the powder-type photocatalytic reaction. In general, organic binders have good

adhesion properties but they could be decomposed by photocatalytic reaction. Inorganic binders, however, have poor adhesion

properties but they are not decomposed by photocatalytic reaction. Contrary to general aspect, the zinc phosphate inorganic binder

synthesized in this work showed good adhesion properties, high photocatalytic efficiency and good mechanical stability. This surprising

result might be due to the chemical reaction between ZnO powders and the zinc phosphate binder.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; A. Oxides; B. Chemical synthesis; C. X-ray diffraction; D. Mechanical properties

1. Introduction

Photocatalysis has attracted much attention because ofits application potential for water and air purification.Especially, for water treatment, the photocatalyst powderis usually suspended in water, the photoreactor being aslurry reactor. This type of reactor has obvious advantagesfrom the standpoint of the reactor itself. When thephotocatalyst powder is perfectly mixed, there is nosegregation of phases, and when the photocatalyst powderis small enough, their entire external surface can beirradiated during the reaction time. However, for applica-tions, it cannot be allowed that treated water can stillcontain solid photocatalyst particles, even though theparticles are harmless. Therefore the remained solid

e front matter r 2007 Elsevier Ltd. All rights reserved.

cs.2007.10.133

ng author. Tel.: +8231 330 6463; fax : +82 31 338 6911.

ss: spark@mju.ac.kr (S. Park).

photocatalyst particles should be removed by a liquid–solidseparator, the installation and operation of which increasethe cost. This is why the immobilization of photocatalystparticles is very important [1–3].There are two approaches to immobilize the photo-

catalysts: one is to immobilize them by a binding method,and the other one is to immobilize them by directformation of photocatalyst films. Since the direct forma-tion method usually results in poorer crystalline quality ofphotocatalyst materials compared to the binding method,the binding method is more desirable to keep the crystallineproperties of photocatalysts. The key issue for the bindingmethod is proper binding materials. Even though organic-type binders have superior adhesion properties, they couldbe decomposed by photocatalytic reaction. Inorganic-typebinders are not decomposed, but they have poor adhesionproperties. Systematic studies for the binding materialshave not been reported as long as authors know. In this

ARTICLE IN PRESSS. Park et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1495–14971496

paper, the binding materials were studied systematicallyand the results will be reported.

Table 1

Results of scratch test for various binders, the unit of CF (force for

cohesive-type failure) and AF (force for adhesive-type failure) is kg

(vertical force or normal force)

Organic binder Inorganic binder

Binder CF AF Binder CF AF

Epoxy 0.16 0.39 Sodiumsilicate 0.15 1.80

INO 0.25 3.60 Potassiumsilicate 0.21 0.35

PMMA 0.55 7.20 Zinc phosphate 0.65 7.5

PVA 0.20 8.50

60

80

100

120

ncentr

ation (

ppm

)

(c)

(d)

2. Experimental procedure

Zinc nitrate powder (Zn(NO3)2 6H2O, High puritychemicals) and glycine powder (H2NCH2COOH, Yakuripure chemicals) were used for oxidizer and fuel, respec-tively. The oxidizer and fuel were then dissolved in distilledwater. The solution was heated on hot plate with stirring.When all of the distilled water was evaporated, the nitrategroup (NO3

�) reacted with the fuel and generated intenseheat (about 1500–1800 1C). This intense heat resulted invery tiny local explosions. This explosion energy was usedto synthesize ZnO nanopowders. Authors call this uniquesynthesis method—solution combustion method (SCM).

The synthesized SCM ZnO nanopowders were immobi-lized by various binders such as organic binders andinorganic binders. The organic binders tried wereepoxy (Samsung Chem.), INO (Samsung Chem.), PMMA(Degussa), PVA (Junsei Chem.) and PVB(Aldrich). Theinorganic binders tried were sodiumsilicate (Shinheungslicate), potassiumsilicate (Shinheung silicate) and zincphosphate. The zinc phosphate was synthesized by thereaction of ZnO powders and phosphoric acid. Thesebinders were coated on glass substrates and then thesynthesized ZnO nanopowders were spray-coated on thesubstrates.

-10 0 10 20 30 40 50 60 70

0

20

40

Ag C

o

Irradiation Time (min.)

(a)

(b)

Fig. 2. Photocatalytic removal of Ag ions from wastewater with SCM

ZnO powder bound with (a) no binder, (b) zinc phosphate, (c) PMMA and

(d) potassiumsilicate.

3. Results and discussion

Fig. 1 shows the cross sectional SEM photograph of theimmobilized ZnO samples. The composition of each layerwas confirmed by EDX. The synthesized ZnO nanopow-ders were immobilized by PMMA. It seems that most ofthe PMMA was spread out to the ZnO nanopowders andacted as a binder. That is why the remained PMMA layerwas very thin.

Fig. 1. Cross-section of immobilized ZnO pow

Scratch test was performed to evaluate the adhesionproperty of binders for the immobilization of SCM ZnOpowders. Although the scratch test does not provide adirect measure of adhesion, it does measure a critical loadwhich is representative of the coating adhesion. The resultsof the scratch test were summarized in Table 1. There are

der (SEM photograph and EDX profile).

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73

27

54

65

22

48

0

20

40

60

80

100

INO PMMA PVA

XR

D I

nten

sity

(%

)

100

5055

92

48 52

0

20

40

60

80

100

Zinc phosphate Sodiumsilicate Potasiumsilicate

XR

D I

nten

sity

(%

)

Fig. 3. Results of ultrasonic test: XRD intensity from SCM ZnO powders bound with (a) organic binders and (b) inorganic binders before (white column)

and after (gray column) 10min of ultrasonic vibration.

S. Park et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1495–1497 1497

two kinds of failure types; one is cohesive type and theother one is adhesive type. The cohesive type of failureoccurs between the ZnO particles and the binder, indicat-ing the displacement of the ZnO particles from the binder.The evidence of cohesive-type failure was detected by thefirst change in slope from the friction coefficient vs. normalforce plot. On the other hand, the adhesive type of failureoccurs between the substrate and the binder. The onset ofthis failure was detected by acoustic emission. Crackingphenomena in materials are associated with the generationof high-frequency vibrations in both the binder andsubstrate, and each failure mode has a characteristicacoustic emission signal. The critical loads for adhesivefailure (AF) in Table 1 indicate the failure of the binders.Average adhesion force of organic binders was higher thanthat of inorganic binders. PMMA showed highest cohesiveforce in organic binders. Zinc phosphate presented highestcohesive force in inorganic binders. In general inorganicbinders have poor cohesive and adhesive forces. However,zinc phosphate showed abnormally high cohesive andadhesive forces. This is probably due to the reactionbetween the SCM ZnO powder and zinc phosphate.

Fig. 2 represents Ag ion concentration change as afunction of UV irradiation time. PMMA and zincphosphate were selected as best organic and inorganicbinders, respectively, to investigate photocatalytic effi-ciency. Potassiumsilicate binder and SCM ZnO powderwithout binder were also tried for reference. The zincphosphate sample showed highest photocatalytic efficiency.Its Ag ion removal rate reached to about 80% of powder-type photocatalyst. This is a surprising result. Generally, inview of mechanical properties, organic binders have beenknown as better binders although they could be decom-posed by photocatalytic reaction. However, zinc phosphatesynthesized in this work showed the best binding property.Furthermore, it is not decomposed by photocatalyticreaction because it is an inorganic material.

To investigate mechanical stability of binders, ultrasonicvibration was applied to the bound SCM ZnO powders inwater for 10min.Fig. 3 shows the XRD intensity change ofbound SCM ZnO powders after 10min of ultrasonic

vibration. The zinc phosphate sample showed highestmechanical stability. Only 8% of bound SCM ZnOpowders were removed after 10min.

4. Conclusions

The zinc phosphate was prepared by the reaction of ZnOpowders and phosphoric acid. It was used for an inorganicbinder to immobilize the SCM ZnO powders on the glasssubstrate. The scratch test result showed that it has the bestoverall adhesion properties such as high cohesive force withthe ZnO powders and high adhesive force with the glasssubstrate. Furthermore, it showed the highest photocatalyticefficiency among organic and inorganic binders. Its photo-catalytic efficiency reached to about 80% of the powder-type photocatalytic reaction. In general, organic bindershave good adhesion properties but they could be decom-posed by photocatalytic reaction. On the other hand,inorganic binders have poor adhesion properties but theyare not decomposed by photocatalytic reaction. However,the zinc phosphate inorganic binder synthesized in this workshowed good adhesion properties, high photocatalyticefficiency and good mechanical stability. This surprisingresult might be due to the chemical reaction between ZnOpowders and the zinc phosphate binder.

Acknowledgment

This work has been supported financially by KoreaEnergy Management Corporation (KEMCO).

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