Growth of GaN films on circle array patterned Si (111) substrates

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Formation of ZnO rods with varying diameters from

ε

-Zn(OH)

2 Jing Wang, Lan Xiang

n

Department of Chemical Engineering, Tsinghua University, Beijing 10084, China

a r t i c l e i n f o

Available online 16 February 2014 Keywords:

A1. Nanostructures A1. Crystal morphology B1. Nanorods B1. Zinc oxide

B2. Semiconducting II–VI material

a b s t r a c t

The inﬂuence of temperature on the formation of one-dimensional (1D) ZnO from

ε

-Zn(OH)2via solution

route was studied in this paper, using ZnSO4and NaOH as the raw materials. The experimental results

indicated that the increase of temperature from 251C to 80 1C accelerated the conversion of

ε

-Zn(OH)2to

ZnO, leading to the formation of 1D ZnO with comparative big diameters. A two-stage route was then developed to synthesize ZnO nanorods with diameters of 20–100 nm and lengths of 1–3

μ

m by aging Zn (OH)2in 1.0 mol L1NaOH at 251C for 72 h followed by aging the slurry at 80 1C for 2 h. Compared with

the ZnO submicron-rods (diameters: 100–500 nm, lengths: 1–3

μ

m) formed at 801C, the ZnO nanorods formed via the two-stage route exhibited a thinner diameter and a higher photo-degradation activity for Rhodamine B owing to the pre-formation of ZnO nanorods (diameters: 10–50 nm) on

ε

-Zn(OH)2surface

at 251C as well as the existence of more oxygen defects.

1. Introduction

The controllable synthesis of one-dimensional (1D) ZnO nano-materials has been widely studied in recent years owing to their unique electronic/optoelectronic properties and potential applica-tions in nanoscale electronics or photonic devices, catalysts and

ceramics, etc. [1–4]. ZnO nanorods with small diameters were

usually used for the fabrication of gas sensors, ﬁeld emission

devices, solar cells and the photo-catalysts, etc. [4_–7] owing to

their distinctive geometries and size effects. Many methods have been developed to synthesize ZnO nanorods such as chemical

vapor deposition, electrochemical deposition, sol–gel and aqueous

solution method, etc. [8–11], specially, the aqueous solution

method has attracted much attention owing to the moderate conditions and the easy control of ZnO properties.

The former aqueous solution synthesis of ZnO nanorods were

difﬁcult to be scaled-up since they were usually carried out in dilute

solutions. For example, Morin et al. produced ZnO nanowires with

diameters of about 22 nm and lengths up to 10

μ

m in 3

106mol L1 Zn(NO3)2solution[12]. Liu et al.[13]synthesized the

ZnO nanorods with diameters of 10–30 nm and lengths of about 1

μ

m

at room temperature from the solution containing 0.03–0.05 mol L1

Zn(NO3)2and 1.0 mol L1NaOH. The surfactant-assisted routes were

usually employed to adjust the diameters of ZnO nanorods. For

example, Qiu et al. reported that the presence of 0.008–0.01 moL L1

polyethylenimine (PEI) decreased the diameters of 1D ZnO from

250 nm to 100 nm [14]. Li et al. synthesized ZnO nanowires with

diameters of 30–50 nm and lengths of 2

μ

m by hydrothermal

treat-ment of the Zn OHð Þ2

4 solution at 1401C for 24 h in the presence of

polyethylene glycol (PEG400)[15].

Recently, increasing attention has been paid to the solution

growth of 1D ZnO from

ε

-Zn(OH)2 precursor owing to the

moderate conditions, the high efﬁciency and the capability for

scaling-up[16–23]. For example, Mcbride et al.[16]reported the

formation of ZnO micro-rods with an average diameter of 1

μ

m

and lengths up to 9.5

μ

m from

ε

-Zn(OH)2in 0.375 mol L1NaOH

at 1011C; Wang et al.[21]fabricated the micro-ﬂowers composed

of ZnO needles with diameters of 100–300 nm and lengths of

3–5

μ

m from

ε

-Zn(OH)2in 1.60 mol L1NaOH at 801C for 10 h; Li

et al. [17] developed a SDS-assisted method to produce ZnO

submicro-rods with diameters of 450–550 nm and lengths of

15

μ

m in 1.0 mol L1NaOH at 801C. It was noticed that the former

work only reported the formation of ZnO rods with diameters in

micro- or submicron-scale from

ε

-Zn(OH)2, the fabrication of ZnO

nanorods was still a challenge.

Herein, a facile solution method was developed to synthesize

ZnO rods with varying diameters from

ε

-Zn(OH)2precursor. The

inﬂuence of temperature on the formation of ZnO rods was

studied and a two stage route was developed to synthesize ZnO

nanorods with diameters of 20–100 nm and lengths of 1–3

μ

m.

2. Experimental

Commercial reagents with analytical grade were used in the

experiments without further puriﬁcation.

ε

-Zn(OH)2 precursor

Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/jcrysgro

Journal of Crystal Growth

http://dx.doi.org/10.1016/j.jcrysgro.2014.01.070

n_{Corresponding author. Tel:}_{þ86 10 62788984.}

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were prepared by drop-wise addition (3 ml min1) of 2.0 mol L1

ZnSO4 into 4.0 mol L1NaOH under stirring (300 rpm) at 251C,

keeping the molar ratio of OH to Zn2þ as 2:1. The slurry

containing

ε

-Zn(OH)2 precipitate was kept stirring for 10 min,

thenﬁltrated, washed with deionized water and dried at 25 1C for

24 h. 1.50 g of

ε

-Zn(OH)2was dispersed in 40.0 ml of 1.0 mol L1

NaOH, the slurry was kept aging at 25–80 1C for 2–72 h, the ﬁnal

product wasﬁltrated, washed and dried at 60 1C for 12 h.

The morphology and microstructures of the samples were

examined with a _{ﬁeld emission scanning electron microscopy}

(FESEM, JSM 7401F, JEOL, Japan) and a high resolution transmis-sion electron microscopy (HRTEM, JEM-2010, JEOL, Japan). The

structures of the samples were identiﬁed by X-ray powder

diffractometer (XRD, D8 Advance, Bruker, Germany) using CuK

α

(

λ

¼0.154178 nm) radiation. The surface compositions of the

sam-ples were characterized by X-ray photoelectron spectrometer (XPS, PHI-5300, PHI, USA). The concentrations of soluble Zn(II) were analyzed by EDTA titration. The room temperature photo-luminescence spectra were measured on a Hitachi F-7000 lumi-nescence spectrometer using Xe lamp with an excitation wave-length of 325 nm.

The photo-catalytic activities of ZnO rods were evaluated by detecting the degradation of RhB aqueous solution containing

4 mg L1RhB and 200 mg L1ZnO under ultraviolet light

irradia-tion originated from 16 W UV lamp. The suspensions were sampled and centrifuged every 20 min and the RhB concentrations were determined by measuring the absorption at 554 nm using an

UV–vis spectrophotometer (UV-8453, Agilent, USA).

Fig. 1. Morphology and XRD patterns of the samples formed at different temperatures and time. Temperature (1C): (a) 80, (b) 60, (c and d) 25; time (h): (a) 2.0, (b)6.0, (c) 72, and (d) 0;♦: ε-Zn(OH)2,n: ZnO.

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3. Results and discussion

3.1. Inﬂuence of temperature on the formation of ZnO rods

from

ε

-Zn(OH)2

Fig. 1shows the morphology and XRD patterns of the samples formed at different temperatures. ZnO submicron-rods with

dia-meters of 100–500 nm and lengths of 1–3

μ

m formed after aging

the

ε

-Zn(OH)2precursor in 1.0 mol L1NaOH solution at 801C for

2.0 h. The time needed for complete conversion of

ε

-Zn(OH)2to

ZnO was prolonged to 6.0 h at 601C and ZnO submicron-rods with

diameters of 50–200 nm and lengths of 1–3

μ

m were obtained.

Compared with

ε

-Zn(OH)2 precursor composed mainly of the

octahedral particles, the surfaces of the product formed by aging

ε

-Zn(OH)2at 251C for 72 h became cracked and pitted with some

rod-like particles with diameters of 10–50 nm, implying the

possible formation of a minor amount of ZnO rods at room temperature even though no XRD peaks for ZnO phase occurred.

The inﬂuence of temperature on the solution growth of ZnO rods

has been widely investigated previously. Li et al.[24]studied the

chemical bath deposition of ZnO nanorod array at temperatures

ranging from 651C to 95 1C and demonstrated that low

tempera-ture favored the formation of thinner nanorods. However, in their case, ZnO rods were directly precipitated from solution, not grown

from

ε

-Zn(OH)2intermediate. Recently, Nicholas et al.[22]

inves-tigated the temperature-dependent precipitation of

ε

-Zn(OH)2and

ZnO phase, and demonstrated that low temperature favored the

formation of

ε

-Zn(OH)2phase, however, the inﬂuence of

tempera-ture on the growth of 1D ZnO from

ε

-Zn(OH)2 has not been

reported. In the present work, the conversion of

ε

-Zn(OH)2to ZnO

and the anisotropic growth of ZnO rods were connected with the

temperature, and the increase of temperature from 251C to 80 1C

accelerated the formation of 1D ZnO with big diameters. Fig. 2 shows the TEM and HRTEM images of the samples

formed after aging

ε

-Zn(OH)2 in 1.0 mol L1 NaOH at 251C for

72 h. The HRTEM image inFig. 2b (corresponding to the framed

part of the nanorod inFig. 2a) demonstrated that the interplanar

spacing was 0.26 nm which was quite similar to that of (001) planes of ZnO, indicating the preferential growth of the ZnO nanorods along the c-axis.

Fig. 3shows the O1s XPS spectra of the samples formed before

(a) and after (b) aging

ε

-Zn(OH)2in 1.0 mol L1NaOH at 251C for

72 h. In the case of

ε

-Zn(OH)2 raw material, the detected O1s

spectrum for

ε

-Zn(OH)2occurring at 532.6 eV could be resolved to

two peaks located at 533.3 eV and 531.7 eV, corresponding to the

O1s binding energies of O in H2O (532.9–533.3 eV[25]) and Zn–

OH (531.9–531.5 eV [26]), respectively. In the case of the aged

sample, the detected O1s spectrum occurring at 531.5 eV could be

resolved to three peaks located at 532.9 eV (H2O), 531.5 eV

(Zn–OH) and 530.8 eV, respectively. The peak occurred at

530.8 eV should be assigned to the lattice oxygen in ZnO [25].

The above results reconﬁrmed the formation of ZnO on

ε

-Zn(OH)2

surface after aging

ε

-Zn(OH)2 in 1.0 mol L1 NaOH at 251C

for 72 h.

3.2. Formation mechanism of ZnO rods

The dissolution–precipitation and the in-situ crystallization

mechanisms have been suggested by the former researchers to

explain the formation of ZnO from

ε

-Zn(OH)2, but the detailed

Fig. 2. TEM (a) and HRTEM (b) images of the samples formed after agingε-Zn(OH)2in 1.0 mol L1NaOH solution at 251C for 72 h.

536 534 532 530 528

Intensity (a.u.)

Binding Energe (eV) Raw Fit Background H2O Zn-OH 536 534 532 530 528 Intensity (a.u.)

Binding Energe (eV) Raw Fit Background Zn-OH H2O Zn-O

Fig. 3. O1s XPS spectra of the samples formed before (a) and after (b) agingε-Zn (OH)2in 1.0 mol L1NaOH at 251C for 72 h.

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phase transformation behaviors were still unclear [18,20–22]. Fig. 4 shows the variation of soluble Zn(II) concentrations with

reaction time during the aging processes of

ε

-Zn(OH)2 in

1.0 mol L1 NaOH at 25–80 1C. In the cases of 80 1C and 60 1C,

the Zn(II) concentrations increased gradually in the initial time

owing to the quicker dissolution of

ε

-Zn(OH)2than the

precipita-tion of ZnO and decreased in the later time due to the faster

formation of ZnO than the dissolution of

ε

-Zn(OH)2, revealing the

existence of the dissolution–precipitation mechanism in the phase

transformation processes. In previous work, the dissolution_–

precipitation mechanism was often proposed based on the SEM

observations[21,24]and the variation of dissolution–precipitation

rates with temperature has not been considered. Here, the lower

concentrations of soluble Zn(II) at 601C than those at 80 1C

indicated the slower dissolution of

ε

-Zn(OH)2 and the formation

of ZnO at 601C, which favored the formation of ZnO rods with thin

diameters[12]. The concentrations of soluble Zn(II) became much

lower at 251C than those at 60–80 1C, leading to the formation

of merely a minor amount of ZnO nanorods with diameters of

10–50 nm on

ε

-Zn(OH)2 surfaces even after aging

ε

-Zn(OH)2 in

1.0 mol L1NaOH at 251C for 72 h.

Fig. 5shows the morphology of the products formed at different reaction time and temperatures. The occurrence of the small holes

on

ε

-Zn(OH)2surface and the appearance of ZnO nanorods in some

holes implied the possible in-situ crystallization route in the initial formation of ZnO. The formation of the holes should be connected mainly with the volume shrinkage in the dehydration process of

ε

-Zn(OH)2 due to the density difference between

ε

-Zn(OH)2

(3.1 g cm3) and ZnO (5.6 g cm3). The subsequent oriented

growth of the ZnO rods outside

ε

-Zn(OH)2 crystals should be

attributed mainly to the dissolution–precipitation mechanism

[21]. Thus the in-situ crystallization and dissolution–precipitation

mechanisms coexisted in the present conditions, the former one dominated in the initial nucleus and growth stages and the latter one guided the subsequent oriented growth processes. It was also noticed that the prolongation of reaction time led to the slight

increase of the diameters of the ZnO nanorods from 50–400 nm to

100–500 nm at 80 1C and 20–100 nm to 50–200 nm at 60 1C,

indicating that the diameters of the ZnO rods were dependent mainly on the initial diameters of the ZnO formed at the beginning stage.

3.3. Formation of ZnO nanorods via two-stage route

A two-stage route was employed to synthesize ZnO nanorods

with thin diameters.

ε

-Zn(OH)2 precursor was ﬁrstly aged in

1.0 mol L1 NaOH at 251C for 72 h to form primarily a minor

amount of ZnO nanorods with small diameters (10–50 nm) on

ε

-Zn(OH)2surfaces, the slurry was then aged at 801C for 2 h to

promote the quick and complete conversion of

ε

-Zn(OH)2to ZnO

nanorods.Fig. 6shows the SEM image, TEM and inserted HRTEM

images of the ZnO nanorods prepared via the two-stage route. ZnO

nanorods with diameters of 20–100 nm and lengths of 1–3

μ

m

were synthesized via the two-stage route. The occurrence of the lattice fringes with a spacing of 0.26 nm in the HRTEM image in Fig. 6b indicated the c-axis oriented growth of the ZnO nanorods.

0 1 2 3 4 5 6 0.040 0.045 0.050 0.055 0.060 0.065 c Time (h) a b Soluble Zn (mol L -1)

Fig. 4. Variation of soluble Zn(II) with reaction time. Temperature (1C): (a) 80, (b) 60, and (c) 25.

Fig. 5. Morphology of the products formed at different time and temperatures. Temperature (1C): (a–c) 80, (d–f) 60; time (h): (a) 0.25, (b) 0.5, (c) 0.67, (d) 0.5, (e) 1.0, and (f) 3.0.

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Controlling the morphology of ZnO via two-stage route was also

suggested in some former studies, but the roles of the_{ﬁrst stage}

were different. Qiu et al. [27] reported the fabrication of ZnO

nanorod arrays using a preheating hydrothermal process and found that the preheating stage lowered the precursor concentra-tion which favored the formaconcentra-tion of ultralong ZnO nanorods. Xu

et al.[28]developed a two-stage hydrothermal route for synthesis

of long ZnO nanowire arrays by dividing the growth process into two steps at two different temperatures to enhance the growth rate difference between crystals. In the present case, the role of the ﬁrst stage is controlling the diameters of the initially formed ZnO

nanorods on

ε

-Zn(OH)2surface.

Fig. 7 shows the photoluminescence (PL) spectra of the ZnO submicron- and nano-rods formed via the one-stage route (aging

ε

-Zn(OH)2in 1.0 mol L1NaOH at 801C for 2 h ) and the two-stage

route. The broad band emission peaks located at 551 nm should be attributed to the green emission originated from the surface oxygen defects of ZnO and the intensity of the green emission

was proportional to the amount of the oxygen defects[29]. The

stronger intensity in curve b than that in curve a indicated that compared with the ZnO submicron-rods formed via the one-stage route, the ZnO nanorods formed via the two-stage route contained more oxygen defects.

Fig. 8shows the photo-degradation of RhB in the presence of ZnO submicron-rods formed via one stage route and ZnO nanorods formed via two-stage route. After 2.3 h of irradiation, 90% and 53% of RhB were degraded in the presence of ZnO nanorods and ZnO submicron-rods, respectively. The faster degradation of RhB in the presence of ZnO nanorods should be attributed to the existence of

more oxygen defects in the ZnO nanorods, which were usually

considered as the active sites of the ZnO photo-catalyst[30].

4. Conclusions

A facile two-stage route was developed to synthesize ZnO nanorods with thin diameters and high photo-degradation activity

by aging

ε

-Zn(OH)2 precursor in 1.0 mol L1 NaOH at 251C for

72 h followed by aging of the slurry at 801C for 2 h. The initial

formation of minor amount of ZnO nanorods with diameters of

10–50 nm on the

ε

-Zn(OH)2 surface at 251C promoted the

sub-sequent formation of ZnO nanorods with thin diameters at 801C.

Compared with the ZnO submicron-rods formed at 801C, the ZnO

nanorods formed via the two-stage route possessed more surface oxygen defects and enhanced photo-degradation activity.

Acknowledgments

This work was supported by the National Science Foundation of China (no. 51174125, no. 51234003 and no. 51374138) and National Hi-Tech Research and Development Program of China (863 Pro-gram, 2012AA061602).

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