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Growth and photocatalytic properties of one-dimensional ZnO nanostructures

prepared by thermal evaporation

Hongwei Yan a, Jianbo Hou a, Zhengping Fu a, Beifang Yang a,*, Pinghua Yang a, Kaipeng Liu a,Meiwang Wen a, Youjun Chen a, Shengquan Fu b, Fanqing Li b

a Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR Chinab Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, PR China

1. Introduction

ZnO is a wide band-gap (3.37 eV) semiconductor with a high

exciton binding energy of 60 meV at room temperature, which

exhibits semiconducting and piezoelectric dual properties [1].

One-dimensional ZnO nanostructures have been extensively

studied by many researchers due to their unique properties,

which have novel applications in room-temperature ultraviolet

laser, gas sensors and biomedical sciences [24]. Recently the

photocatalytic performance of ZnO has attracted much attention

which was considered as an alternative to TiO2[57]. Though TiO2has been thought to be the most excellent semiconductor

photocatalyst, ZnO is still worth to be investigated which has

even higher photocatalytic efficiency compared to TiO2 in the

treatment of some organic pollutants[812].

Nanoparticles with higher photocatalytic efficiency than theirbulk phase counterparts were extensively applied in the photo-

catalytic reactions, as which have effective separation of electron

hole pairs andbroadened band-gap from quantum size effects [13].

However, there are some drawbacks for nanoparticles such as the

tendency to aggregateduring aging anddifficulty in separation and

recovery from solutions, which greatly limit their extensive

applications. A good solution is the immobilization of semicon-

ductor photocatalysts on the substrates. When semiconductor

nanoparticles were immobilized on the substrates, the surface-to-

volume area of photocatalysts will be decreased resulting in the

reduction of photocatalytic efficiency. It is a challenge to

synthesize a photocatalyst which not only is immobilized on the

substrate but also has high photocatalytic efficiency. The synthesis

of one-dimensional nanostructure films is prospective to overcome

the above-mentioned drawbacks. Recently various types of ZnO

nanostructures have been investigated in the photodegradation of

organic contaminants, which were synthesized by the various

fabrication techniques[1417]. However, there is still a need to

track the kinetics of photocatalytic process and the photocatalytic

stability of nanostructured ZnO for its practical applications.

Moreover, the growth mechanism of the ZnO nanorods and

nanotubes is also open to question.

In this paper, aligned ZnO nanorods and nanotubes on thesilicon substrates were synthesized by thermal evaporation of high

pure Zn powders without using any other metal catalyst. The

morphology evolution of the nanostructures with prolonged

growth time was studied. ZnO nanoneedle and nanoparticle films

were also grown on even larger size silicon wafers, and their

photocatalytic and recycle performances were studied in detail.

2. Experimental

The films were deposited on the (1 0 0) oriented n-type silicon

wafers in two sizes: 7 mm 20 mm and 20 mm 20 mm. Before

Materials Research Bulletin 44 (2009) 19541958

A R T I C L E I N F O

Article history:

Received 30 September 2008

Received in revised form 5 June 2009

Accepted 25 June 2009

Available online 5 July 2009

Keywords:

A. Nanostructures

A. Semiconductors

B. Vapor deposition

C. Electron microscopy

D. Catalytic properties

A B S T R A C T

Aligned ZnO nanorods and nanotubes were grown on the silicon substrates by thermal evaporation of

high pure zinc powders without any other metal catalyst. The morphology evolution of ZnO

nanostructures with prolonged growth time suggested that the growth of the ZnO nanorods and

nanotubesfollows the vaporliquidsolid mechanism.ZnO nanoneedle and nanoparticle filmswere also

synthesized by the same way, and their photocatalytic performances were tested for the degradation of

organic dye methylene blue.The ZnO nanoneedle films exhibited very high photocatalytic activities.The

decomposition kinetics of the organic pollutant was discussed. Moreover, it is found that the ZnO

nanoneedle films showed very stable photocatalytic activity.

2009 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +86 551 3603194; fax: +86 551 3601592.

E-mail address: bfyang@ustc.edu.cn(B. Yang).

Contents lists available at ScienceDirect

Materials Research Bulletin

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t r e s b u

0025-5408/$ see front matter 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2009.06.014

mailto:bfyang@ustc.edu.cnhttp://www.sciencedirect.com/science/journal/00255408http://dx.doi.org/10.1016/j.materresbull.2009.06.014http://dx.doi.org/10.1016/j.materresbull.2009.06.014http://www.sciencedirect.com/science/journal/00255408mailto:bfyang@ustc.edu.cn

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use, the wafers were ultrasonically cleaned in acetone, ethanol and

distilled water in sequence, and then were boiled for 10 min in the

solutions of NH3H2O:H2O2:H2O (1:1:3) andHCl:H2O2:H2O (3:1:1),

respectively. Then the wafers were etched by HF acid and washed

with distilled water and dipped in ethanol until it was used. The

silicon wafers with polished face downward were laid on a quartz

boat in which 0.4 g high purityzinc powders (99.99%) were loaded.

The vertical distance of the zinc powders and the silicon substrates

was about 2 mm. A horizontal tube furnace was heated to the

reaction temperature and evacuatedusing a mechanical pump and

then purged with pure argon gas. After the quartz boat was loaded

into the furnace, a mixture of 4% O2 in Ar (20 sccm) and pure argon

gas (30 sccm) was introduced. After deposition for a certain time,

the quartz boat was taken out from the tube and a white/gray layer

was formed on the silicon substrate. The X-ray diffraction patterns

were recorded on a Philips Xpert prosuper diffractometer using Cu

Ka irradiation (l= 1.5419 A). The morphologies of the products

were analyzed by field emission scanning electron microscopy

(FESEM) (JSM-6700F).

The photocatalytic degradation of organic dye methylene blue

(MB) (10 mg/L) was executed under irradiation by a 20 W low-

pressure mercury lamp with main wavelength of 254 nm. The

photocatalytic activities of the deposited films were tested as

following methods: (1) dynamic: the ZnO films were suspended in60 mL MB solution with magnetic stirring and a certain amount of

solution was taken out every 30 min, which was then analyzed by

absorption spectra on an ultraviolet-visible recording spectro-

photometer (Shimadzu UV-2401); (2) static: the ZnO films were

dipped in 8 mL MB solution without magnetic stirring under 2 h

irradiation, and then the solution was tested. This photocatalytic

degradation test was repeated 13 times on the same sample for

detecting the photocorrosion of ZnO. And the sample did not

display an apparent surface change after the whole test.

3. Results and discussion

Generally, the vaporliquidsolid (VLS) and vaporsolid (VS)

processes are used to interpret the growth mechanism of one-

dimensional (1D) nanostructures[1821]. Under the existence of

some metal catalysts such as Au, Pt, and Sn, etc., the growth of 1D

ZnO nanostructures usually follows the VLS mechanism. In this

process, a droplet of liquid alloy will form and guide the

anisotropic crystal growth. The existence of nanoparticles capping

at the end of a 1D nanostructure is a characteristic of the VLS

mechanism. When no metal catalysts are used, the VS process is

conventionally used to interpret the growth mechanism of 1D ZnO

nanostructures in our experiment. To prove this VS mechanism,

reaction time-dependent experiments were carried out.

Fig. 1displays the morphologies of the ZnO films grown on the

substrate of 7 mm 20 mm in size at 620 8C at different times:

10 min, 20 min and 30 min. It is clearly seen that a morphologyevolution has occurred with increasing the growth time. For the

sample grown for 10 min, vertically aligned nanorods were formed

with some particles capping on their tips. When growing for

Fig. 1.The low and high magnification SEM images of the ZnO films grown on the silicon substrates at 620 8C at different times: (a and b) 10 min; (c and d) 20 min; (e and f)

30 min.

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20 min, the capping particles on the tips of the nanorods decreased

gradually and a concaved surface was formed. When extending the

growth time to 30 min, a mass of nanotubes were formed mixedwith few nanorods. As seen from the high magnification SEM

images, the walls of the nanotubes are not smooth and capped by

some particles. The diameter of the nanotubes decreases from the

root to thetip. A typical XRDpatternof ZnOnanorodswas shown in

Fig. 2. All the strong peaks can be readily indexed to hexagonal

wurtzite ZnO with cell constants comparable to the reported data

(JCPDS 89-0511). The higher intensity of (0 0 2) peak relative to

other peaks exhibits high c-axis growth orientation of the ZnO

nanorods.

As seen from Fig. 1a the top surface of the nanorods is not

smooth or faceted which is different from the conventional

morphology of the nanorods prepared by this method. It should be

noted that the surface capped by nanoparticles still exists in the

nanotubes. In our experiments no metal catalysts were intention-ally introduced. However, considering that the melting point of

pure metal zinc is 419 8C at atmospheric pressure, it is implied that

the droplet of liquid Zn would emerge on the silicon substrate at

the growth temperature of 620 8C. The liquid Zn is the Zn source of

ZnO as well as being a self-catalyst in the growth process. A liquid

phase Zn/ZnOx(x< 1) would form at the early stage when oxygen

gas was adsorbed on liquid Zn [22]. The highly reactive liquid

droplet was quickly oxygenated and nucleated into nanoparticles,

and then grew orientedly into nanorods. The evaporated Zn vapor

and flowing O2 gas would continuously adsorb on the surface of

liquiddropletand supplythe growthof nanorods.As increasing the

growth time, the morphology of top surface transformed from

nanoparticles to concave surface and then to nanowalls. In the

previous reports, Mo et al. has demonstrated a similar morphology

transform from ZnO nanorods to microhemispheres and nano-

tubes under hydrothermal conditions [23,24]. They proposed a

growthmechanism in which themother rods mayattach thetiny

rods at high surface-energy growing fronts and grow larger. In our

work, as increasing the growth time, the decreasing concentration

of Zn vapor lead to a selective deposition of ZnO nanoparticles on

the high energy surface of ZnO nanorod top, and then the concave

top surface was formed. Moreover, as further increasing the

growth time the partial pressure of Zn vapor decreased as a result

of the consumption of reaction materials, which resulted also in

the formation of a thin wall on the top of nanorods. Thus, the

morphology of top surface of ZnO changed from nanoparticles to

concave surface and then to nanowalls. The morphology evolution

process of ZnO nanostructures with prolonged growth time

suggested that the growth of the ZnO nanorods and nanotubes

follows a self-catalytic vaporliquidsolid mechanism [25].

Although the formation process of the tubes is deduced, the

intrinsic cause of the shape transformation from rod to tube is still

unclear. In addition, in view of the high vapor pressure of Zn metal

(10 Torr at 600 8C), the concave morphology may also originate

from re-evaporation of Zn element, which is rich in ZnOx. The

mechanism described above needs to be examined and improved

by more studies.For the photocatalytic tests of the ZnO films, the larger size

wafers were used. Fig. 3 displays themorphologies of the ZnOfilms

grown on the substrate of 20 mm 20 mm in size at two growth

temperatures for 30 min: 620 8C and 650 8C, respectively. When

the substrates were changed from small size to large size, the

surface morphologies of the deposited films have changed very

much. As shown in Fig. 3a, the film grown at 620 8C is composed of

a mass of poorlyaligned nanoneedles.On theedge of thefilm, there

exist some aggregated nanoparticles. As compared with it, the film

grown at 650 8C (Fig. 3b) is mainly composed of loose nanopar-

ticles on a compact nanoparticle base. Compared to the

7 mm 20 mm wafer, the growth control of ZnO nanostructures

onthe 20 mm 20 mm waferbecomes moredifficult. The possible

reasons are that the morphology of the nanostructures isinfluenced by many experimental parameters, including reaction

temperature, the distance between the source material and the

substrate, vapor dynamics, oxidation rate, etc.[26]. These factors

may result in the big difference of the surface morphologies of ZnO

nanostructures with dissimilar sizes substrates. The nanoneedles

grown on the 20 mm 20 mm wafers looks disorderly that maybe

the result of the above-mentioned factors. Though aligned

nanorods have more anticipation on the photocatalysis than the

disorderly grown nanoneedles, the latter was tested in our

photocatalysis experiments in view of the synthetic facility.

Fig. 4 shows the photocatalytic activities of different photo-

catalysts including ZnO nanoneedles, nanoparticles, TiO2 films and

flowerlike ZnO nano/microstructures. TiO2films were synthesized

Fig. 2. A typical XRD pattern of the ZnO nanorodsgrown on the silicon substrates at

620 8C.

Fig. 3.The SEM images of ZnO nanoneedle (a) and nanoparticle (b) films.

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by a solgel dip-drawing process and annealed in air at 500 8C for

2 h. Flowerlike ZnO nano/microstructures were prepared by a

hydrothermal method and annealed in air at 300 8C for 1 h [27]. Allthe samples were the same in size and tested under the same

conditions. As a comparison, a blank silicon wafer of

20 mm 20 mm in size was used as the reference sample for

testing the influence of the ultraviolet lamp on the degradation of

MB. After 3 h irradiation under ultraviolet light without photo-

catalysts, the degradation ratio of MB is about 15%, which is 96%,

75%, 62% and 56% for the ZnO nanoneedles, nanoparticles, TiO2films and flowerlike ZnO during the same time. The ZnO

nanoneedles display much better photocatalytic activity than

the other samples. Moreover, the samples synthesized by thermal

evaporation show the distinct advantages in the degradation of

organic pollutants. It may be a result of the higher separation

efficiency of electronhole pairs in the ZnO nanostructures with

high crystallinity synthesized by thermal evaporation.Without magnetic stirring the diffusion of MB molecules in

solution is an important rate-controlled process. This is confirmed

by the fact that the color of solution became deep gradually far

from the ZnO films after the photocatalytic reaction in the static

mode. Under magnetic stirring the MB solution was uniformly

dispersed, and the reaction rate was controlled by the surface

reaction process on the ZnO film. The decomposition of organic

compounds by photocatalysts is a solidliquid interface process,

andthe reactions take place at the surface of the photocatalyst. The

LangmuirHinshelwood (LH) model has been shown to success-fully describe the heterogeneous photocatalytic degradations of

organic pollutants in previous works [6,2830]. The reaction rate is

proportional to the fraction of the surface covered by the reactant

in terms of the LHmodel, which could be defined as following [31]:

r dC

dt ku

kKC

1 KC (1)

whereris the reaction rate, Cis the equilibrium concentration of

organic pollutants, t is the time, k is the rate constant, u is the

fraction of the surface covered by reactants, and Kis the adsorption

equilibrium constant. When the concentration of organic com-

pounds is very high (KC 1)or verylow(KC 1), the equation (1)

can be simplified to a zero order reaction (dC/dt=k) or a pseudo-

first reaction (dC/dt=kKC). In our system it was found that the

experiment data presented inFig. 6could not be fitted very well

only by zero order or pseudo-first order reaction. By integration of

Eq.(1) we got the following expression:

KC0 1C

C0

ln

C

C0

kKt (2)

C0 is the initial concentration of organic pollutants. By taking

C0= 10 mg/L and fitting the experimental data in Fig. 5(the solid

lines), we obtained k1= 0.107 mg/(L min) (2.86 107 M/min),

K1= 0.116 L/mg (4.34 104 M1) for nanoparticles, and

k2= 0.140 mg/(L min) (3.74 107 M/min), K2= 0.225 L/mg

(8.41 104 M1) for nanoneedles. It reveals that the ZnO

nanoneedle films have the faster reaction rate and higheradsorption ability than the nanoparticle films. Compared with

the ZnO nanoparticle films in the same size, the ZnO nanoneedle

films have higher specific surface area which could adsorb more

MB molecules. When the ultraviolet light irradiated on the films,

the ZnO nanoneedles could harvest more light and generate more

electronhole pairs resulting in a higher reaction rate. As a result,

the ZnO nanoneedle films exhibited higher photocatalytic activity

than the nanoparticle films under the same initial concentration of

MB. Furthermore, the experimental results of Yi and co-workers

revealed recently,that the increase in aspectratio of TiO2 nanorods

resulted in the effect of reduction of e/h+ recombination[32]. In

our samples, the aspect ratio of the ZnO nanoneedles was

obviously larger than that of the ZnO nanoparticles, thus it should

induce a higher photocatalytic activity for the ZnO nanoneedleFig. 5. Variation with time of the relative concentration of methylene blue.

Fig. 4.The decomposition ratio of methylene blue with time. Fig. 6.The decomposition ratio of methylene blue vs. the repeated test times.

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films than the nanoparticle films. Moreover, the aggregation of

nanoparticles on the substrate decreased their specific surface

area, which would result in the relative lower photocatalytic

efficiency of the nanoparticle films. These results indicate that the

synthesis of one-dimensional ZnO nanostructure films is an

effective way to prepare immobilized photocatalysts with high

photocatalytic activity.

The photocatalytic stability of the ZnO films is an important

concern for the repeated use of the photocatalysts. ZnO will

dissolve in both acidic and highly alkaline conditions, and it will

also dissolve in neutral solution under light illumination[10]. A

disadvantage for ZnO photocatalyst is the photocorrosion induced

by photogenerated holes. This process can be described by the

following reaction equation[10]:

ZnO 2hvb!Zn2O

where hvb+ is the hole in the valence band, and O* is an

intermediate oxygen species with high reaction activity. The

above-mentioned disadvantages do not imply the decrease of the

photocatalytic activity of ZnO according to previous works

[10,33,34]. Without a doubt, the photocorrosion is unfavorable

to the recycle use of the photocatalysts. Interestingly, our ZnO

nanoneedle films are relative stable against photocorrosion, unlike

traditional ZnO powder photocatalysts. As shown in Fig. 6, the ZnOnanoneedle film does not exhibit any great loss in activity even

after 13 times cycles for the degradation of MB in the static mode

condition. Some researchers have recently found that the

photocorrosion of ZnO can be successfully inhibited via hybridiza-

tion with other material, such as monolayer polyaniline [35],

graphite-like carbon[36], perfluorinated ionomer[37], and so on.

However, both photocatalysis and photocorrosion are very

complex processes, and the photocatalytic activity of the ZnO

nanoneedle films may depend on, for example, their aspect ratio of

nanoneedle, size distribution, and/or surface/bulk compositions.

Therefore, the detailed mechanism for the enhanced photocata-

lytic activity and stability of the 1D ZnO nanostructures is still an

open question.

4. Conclusions

In conclusion, aligned ZnO nanorods and nanotubes were

grown on the silicon substrates by a simple thermal evaporation

process. The growth and morphology evolution of the ZnO

nanostructures were interpreted by a self-catalytic vapor

liquidsolid mechanism. ZnO nanoneedle and nanoparticle films

were also synthesized by the same way, and their photocatalytic

performance were tested by decoloring the organic dyes MB,

compared with the TiO2 films and flowerlike ZnO nano/micro-

structures films. It was found that the ZnO nanoneedle films had

much better photocatalytic efficiency than the other samples. The

decomposition kinetics of the organic pollutant MB was well

explained by the LangmuirHinshelwood model. The repeatedphotocatalytic tests confirmed the long-time photocatalytic

stability of the ZnO nanoneedle films, which showed a good

application prospect in the treatment of organic pollutants.

Acknowledgment

This work is supported by National Natural Science Research

Foundation of China.

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