Preparation and Characterization of Silver Doped ZnO Nanostructures

doi:10.4236/ojsta.2012.12004

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Open Journal of Synthesis Theory and Applications, 2012, 1, 18-22

http://dx.doi.org/10.4236/ojsta.2012.12004 Published Online July 2012 (http://www.SciRP.org/journal/ojsta)

Preparation and Characterization of Silver Doped ZnO

Nanostructures

Nguyen Van Nghia*, Tran Nam Trung, Nguyen Ngoc Khoa Truong, Doan Minh Thuy

Department of Physics, Quy Nhon University, Quy Nhon, Vietnam

Email: *[email protected]

Received April 25, 2012; revised June 7, 2012; accepted July 3, 2012

ABSTRACT

ZnO was prepared by hydrothermal method. The result of scanning electron microscopy showed that the materials had

nano rod structures. Ag-doped ZnO was prepared by UV-photoreduction. Crystalline phases and optical absorption of

the prepared Ag-doped ZnO samples were determined by X-ray diffraction, Raman spectrum, UV-visible, and UV-

photoreduction spectrophotometer. X-ray analyses revealed that Ag was doped ZnO crystallizes in hexagonal wurtzite

structure. The incorporation of Ag+ in the site of Zn2+ provoked an increase in the size of nanocrystals as compared to

pure ZnO. The photocatalytic and photoluminescence properties of materials were considered.

Keywords: Nanostructures; Photocatalysis; Hydrothermal; ZnO; Silver Doping

1. Introduction

Nanostructured ZnO materials have received consider-

able interest from scientists due to their remarkable per-

formance in electronics, optics and photonics. ZnO is a

wide band gap (3.37 eV at room temperature) compound

semiconductor that is appropriate for short wavelength

optoelectronics applications. The large exciton binding en-

ergy (60 meV) in ZnO crystal allows efﬁcient excitonic

emission at room temperature. Therefore, ZnO nanos-

tructures have had a wide range of high technology ap-

plications like surface acoustic wave ﬁlters, photonic

crystals, gas sensors, photocatalysis [1-3]. Because of

having a wide bandgap, ZnO can only be activated by

ultraviolet light of wavelength below 385 nm. The ultra-

violet light reaching the earth’s surface is less than 5% of

the solar energy, which is too low to attain significant

photodegradation in commercial application. Some in-

teresting approaches have been adopted to extend the

photoresponse of ZnO toward the visible spectral region,

such as implanting transitional metal ions [4,5].

Metal silver is also a significant visible light photosen-

sitizer, which is stable and nontoxic. Ag is also relatively

cheap; thus Ag modification is of great significance for

industrial practice. The improvement in efficiency of

photocatalytic reactions under visible light is explained

as the result of a vectorial transfer of photogenerated elec-

trons and holes from metal to semiconductor. Moreover,

ZnO modified by Ag can improve the distribution of

surface charges, accept a conduction band generated by

solar light irradiation during photoreaction, prevent the

recombination of the photogenerated electron-hole. Many

researchers reported that ZnO thin film with Ag doping,

which enhances ultraviolet emission and improves elec-

trical and optical properties, was prepared by wet chemi-

cal [6], DC magnetron sputtering [7] and pulsed laser

deposition [8].

In this work, silver nanoparticles were deposited on

the surface of ZnO nanorods (prepared by hydrothermal

method) by using a photochemical reduction under UV

irradiation. We also have compared the photocatalytic

properties of Ag-doped ZnO and ZnO nanorods (ZnO-

NRs).

2. Experimental

2.1. ZnO -NRs Preparation

4,6 g of zinc acetate (Zn(OAc)2 powders were dissolved

in 40 ml a solvent etanol under stirring for 1 h (called M1

solution). The M2 solution was prepared by adding 2,5 g

oxalic acid (H2C2O4) into 40 ml etanol under stirring. M2

was slowly poured in the M1 solution under ultrasonic

wave (35 kHz, 100 W) for 30 minutes. The sonicated

solution was then moved to a teflon vessel and put in a

stainless steel autoclave for carrying out the hydrother-

mal treatment at 140˚C for 20 hours. The stainless steel

was then opened at room temperature and the precipitates

were separated and washed repeatedly by deionized wa-

ter until the pH value of the washing solution became

lower than 7. The final as-prepared product was dried

under vacuum at temperature of 80˚C for 12 hours. Then

*Corresponding author.

N. Van NGHIA ET AL. 19

the product was calcined at temperature of 450˚C for 1

hour in air.

2.2. Ag-Doped ZnO Preparation

0,5 g of ZnO-NRs were dispersed into deionized water

under stirring. The suspension was sonicated for 30 min

by ultrasonic wave. After ultrasonically treated the sus-

pension was further magnetically stirred for 30 min un-

der UV irradiation. Then 0.007 g AgNO3 were added into

the suspension (mAg:mZn = 1%) and followed by UV il-

lumination for 4 h under stirring. The black powder was

centrifuged, rinsed with deionized water repeatedly to

purify the product, and ﬁnally dried at 70˚C under air for

5 h. The product was denoted Zn-Ag1.

Using the same method, we prepared products Zn-Ag2,

Zn-Ag3, Zn-Ag4, Zn-Ag5. The more silver ion was doped,

the darker in colour of the product was.

2.3. Characterization and Measurement

The morphology dimensions and microstructure meas-

urements of the samples were performed using a Hitachi

S4800 scanning electron microscope (SEM). The crystal-

line phases of the obtained samples were characterized

by using a Siemens D-5005 X-ray powder diffractometer

(XRD) with a monochromatized Cu-Kα irradiation (λ =

1.54056 Å). The Raman spectra were recorded on a

Nicolet spectrometer equipped with an optical micro-

scope at room temperature. UV-Vis spectra were meas-

ured by a Scan UV-Vis spectrophotometer (Varian, Cary

500).

The photocatalytic activity was evaluated by measur-

ing the decomposition of the aqueous solution of me-

thylene blue (with a concentration of 10 mg/L) under

sunlight irradiation for 30 min of pure ZnO-NRs and

Ag-doped ZnO. The reactor was equipped with water

circulation in the outer jacket in order to maintain a

constant temperature. Prior to irradiation, the suspensions

were magnetically stirred in dark for 1 h to ensure an

establishment of adsorption/desorption equilibrium. Then

the solution was filtered to remove “particles”. The re-

sulted solution was analyzed by recording variations in

the absorption in UV-visible spectra of methylene orange

using a Shimadzu 1601-PC Ultraviolet-visible spectro-

meter.

3. Results and Discussion

The XRD patterns of ZnO-NRs and Zn-Ag5 are exhibited

in Figure 1.

As can be seen from Figure 1, all the samples were

well crystalline, and of hexagonal wurtzite phase (JCPDS

ﬁle No. 36-1451). However, the Ag-doped samples re-

vealed some additional diffraction peaks marked with

“*” associated with the face-centered-cubic phase of me-

tallic Ag (JCPDS ﬁle No. 04-0783). The appearance of

Ag peaks in the diffraction patterns indicates clearly the

formation of crystalline silver clusters in the nanoparti-

cles. This can be demonstrated by SEM image of Zn-Ag5.

Figure 2(a) shows the image of ZnO nanorods cal-

cined at 450˚C, a large number of open-ended ZnO-NRs

with uniform diameters around 300 nm and lengths about

several micrometer can be clearly seen in the picture. No

obvious damage is found and the materials remain in

good shape. Figure 2(b) shows that Ag-sensitized ZnO

prepared by photoreduction does not cause any change in

the morphology compared with pure ZnO. But some

newly formed small Ag particles with diameter a few

nanometer on the surface of ZnO can be observed clearly.

The EDS spectrum (Figure 3) showed that the Zn-Ag5

material include elements such as Zn, O and Ag. The

quality of Ag content deduced from the EDS spectrum

about 3% indicate that Ag particles are losed in the reac-

tion process.

The room temperature UV-Vis spectrum of undoped

ZnO and Zn-Ag5 are presented in Figure 3. As can be

seen from curve A in Figure 4, ZnO-NRs have a broad

intense absorption below wavelength 400 nm. It is the

characteristic absorption of ZnO corresponding to the

charge transfer process from the valence band to conduc-

tion band in ZnO. In the UV-Vis spectra of the Zn-Ag5

(curve B), it can be seen that there is a absorption peak

Figure 1. XRD patterns of ZnO and Zn-Ag5.

Figure 2. SEM images of ZnO nanorods (a) and Zn-Ag5 (b).

N. Van NGHIA ET AL.

Figure 3. EDS spectrum of Zn-Ag5.

Figure 4. UV-Vis spectrums of ZnO and Zn-Ag5.

around 460 nm in the visible range. This is the character-

istic of surface plasmon absorption corresponding to Ag0

particles [9]. So the modiﬁcation decrease in bandgap

energy of ZnO.

Figure 5 presents the room temperature Raman spec-

trum of ZnO-NRs and Zn-Ag5. It can be seen that the

spectrum of ZnO (curve A) consists of five peaks located

at about 100, 140, 380, 437, and 580 cm–1 which corre-

spond to the fundamental phonon modes of hexagonal

ZnO, respectively. The curve B which is the spectrum of

Zn-Ag5 has four peaks located at about 100, 140, 241,

and 580 cm–1. There appeared a broad Raman peak at

about 241 cm–1 exclusively for the Ag-doped samples.

The intensity of this peak decreased drastically on doping

the silver in the samples. We can recall that the incorpo-

ration of Ag in our ZnO nanoparticles reduces their crys-

tallinity [8].

Figure 6 shows room-temperature PL spectrums of

ZnO and Zn-Ag5 at the excitation wavelength of 325 nm.

We can see that a blue-green emissions at about 520 nm

200 400 600

8000

10000

12000

14000

16000

200 400 600

Intensity (A.u)

B: Zn-Ag5

A: ZnO-NRs

Wavenumber (cm-1)

Figure 5. Raman spectrums of ZnO and Zn-Ag5.

were observed in them. The emission centered at 520 nm

of Zn-Ag5, which can be ﬁtted to two peaks centered at

516 nm and 630 nm, respectively, as shown in the inset

of Figure 5. The peak 516 might be attributed to the in-

trinsic defects (O and Zn vacancies or interstitials and

their complexes) in ZnO. The other hand, the Ag-doped

ZnO, which prepared, exhibits a new and unusual PL

phenomenon at 615 nm; there is the possibility of the

formation of a new surface state. The dopant Ag has a

great effect on the separation and recombination process

of photo-induced charge carriers of ZnO, which can fur-

ther effect on PL performance. It indicates that the pho-

toluminescence mechanism of Ag/ZnO is very complex

and further research is needed.

Figure 7 shows the effect of the Ag content on the

photocatalytic activity of Ag-doped ZnO. The photo-

catalytic degradation ratio of methyl blue (MB) increases

rather rapidly initially with the increase of the Ag content

and reaches a plateau at the Ag content of 4%. It is worth

that the amount of doped silver ion is very important to

photoactivity. But an increase in dopant ion can make

increasable rate recombination of electron-hole pairs,

because silver ions play a central role of recombination,

that can make decreasing the photocatalytic activity of

material. The photocatalytic mechanism of Ag/ZnO is

also complex and it’s studied deeply.

We considered the UV-photoreduction mechanisms

from the view point of photolysis at ZnO catalyst. Oxida-

tion and reduction occur at the same time in Ag ion

photoreduction. In the reduction, the conduction band

electrons generated in the ZnO (e−(CB)) by UV irradition

can reduce adsorbed Ag+ ions, giving rise to Ag atoms

(Ag0). The reduced Ag is deposited on the ZnO surface.

Photochemical reactions induced by ZnO-light are sum-

maized as: r

N. Van NGHIA ET AL.

400 500 600 700800 900400 500 600 700800 900

1000

2000

3000

4000

5000

6000

Zn-Ag5

ZnO

Intensity (A.u)

Wavelength (nm)

500600 700 800 9001000

1000

2000

3000

4000

5000

Fit Curve

Intensity (au)

Wavelength (nm)

Figure 6. Room-temperature PL spectrum of Zn-Ag5. In the inset, the curves “…”show Gaussian curve fitting.

012345

100

The degradation of MB (%)

The ratio Ag-Doped (%)

Figure 7. The degradation of MB on the Ag-doped ZnO.

ZnO + h



→ e–(CB) + h+(VB) (1)

Ag+ + e–(CB) → Ag0 (2)

2H2O + h+(VB) → O2 + 4H+ (3)

4. Conclusion

Nanorod ZnO was successfully synthesized by a simple

hydrothermal method. Ag-doped ZnO composites were

prepared by method of UV-photoreduction with as-syn-

thesized ZnO-NRs. Photocatalytic reactions show that

doping Ag into ZnO hole remarkably improves the pho-

tocatalytic activity of ZnO under simulated solar light.

The PL spectrum result shows that Ag-sensitized ZnO

composite not only has the emission intensities at about

318 nm and 520 nm, but also exhibits a new and unusual

PL phenomenon at 615 nm, indicating that the dopant Ag

has great effects on separation and recombination proc-

esses of photo-induced charge carriers of ZnO.

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