ELSEVIER Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet materials letters Controllable synthesis of ZnO ar
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ELSEVIER
Contents lists available at SciVerse ScienceDirect
Materials Letters
journal homepage: www.elsevier.com/locate/matlet
materials letters
Controllable synthesis of ZnO architectures by a surfactant-free hydrothermal process
Guixiang Du **, Lidong Zhang °, Yan Feng *, Yanyan Xu °, YuXiu Sun °, Bin Ding 2, Qian Wang 2
* College of Chemistry, Tianjin Key Laboratory of Structure and Performance for functional Molecule, Tianjin Normal University, Tianjin 300387, China
> School of Energy and Chemical Technology, Tianjin Bohai Vocational Technical College,Tianjin 300402, China
Article history:
Received 26 October 2011
Accepted 3 January 2012
Available online 10 January 2012
Keywords:
Various ZnO architectures like novel flowerlike structures radially assembled by rods, nails or towers and novel radialized bundled tubular structures with diverse diameter in entire length were controllably synthe- sized with different amine precursors by a simple surfactant-free hydrothermal process It suggests that di- verse amine sources, which possibly have different hydroxyl ion releasing rate contributing to different reaction rates, probably play an importance role in controlling the assembly of different ZnO architectures, ZnO besides temperature and time X-ray powder diffraction (XRD) results prove the ZnO belonging to wurtzite
Semiconductors
Microstructure
Surfactant-free
Hydrothermal
structure and room temperature photoluminescence (PL) demonstrates a high quality of the products
© 2012 Elsevier B.V All rights reserved
1 Introduction
Zinc oxide (ZnO), an important wide-band gap semiconductor
material has been viewed as one of the promising nanomaterials
due to the applications in gas sensors, photocatalysts and solar cells
[1-3] Since the properties of the material depend closely on the
microstructure, much effort has been focused on controlling
sizes, morphology and microstructure of ZnO, and diverse ZnO
structures can be achieved by vapor-phase or solution processes
[4-10] Complex procedures and equipment are involved in
vapor-phase processes, and therefore, more effective, simpler
and low cost solution ways are feasible to controllably synthesize
the ZnO architectures in a large-scale by changing reaction time,
surfactants and substrates [9,10] However, it has been rarely
reported that different precursors have been employed to control
the morphologies and structures We think that diverse amine
precursors, which possibly possess different hydroxyl ion releas-
ing rate, contributing to different reaction rates of zinc ion and
hydroxyl ion, maybe control the assembly of different ZnO
architectures
In this letter, we have successfully achieved various ZnO architectures
including novel radialized flowers assembled by rods, towers and nails
with one sharp tip, and novel tubular ZnO bundles composed of many
single tubes with different diameters from bottom to top by four amine
precursors by a surfactant-free hydrothermal process It suggests that dif-
ferent amine precursors play an important role in controlling of ZnO
structures possibly by tuning the reaction rate
* Corresponding author Tel.: + 86 22 23766515; fax: +86 22 23766515
E-mail address: guixiangdu@yahoo.cn (G Du)
0167-577X/$ — see front matter © 2012 Elsevier B.V All rights reserved
doi:10.1016/j.matlet.2012.01.013
2 Experimental
In a typical synthesis, 0.0075 mol of glycol was added into 15 ml of aqueous solution of Zn(NO3)2-6H20 (0.05 M) and ammonia (0.05 M), the mixture was stirred for minutes and was transferred into 25 mL
Teflon-lined stainless steel autoclaves, sealed and maintained at certain temperature and time After the reaction, the samples was filtered out, washed several times with distilled water and alcohol, and then dried
at 60 °C under air atmosphere Hexamethylenetetramine (HMTA), ethanolamine (EA) and ethylenediamine (ED) were also used in the parallel experiments
XRD analyses were conducted on a Bruker D8A X-ray diffractometer with a Cu Ka radiation (X= 0.15418 nm) Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopic (FE-TEM) images were performed on a FEI Nova Nano SEM 230 micro- scope and on a FEI Tecnai G? F20 microscopic, respectively The room temperature PL spectrum was recorded with a HORIBA JY FL-3 spectro- photometer excited by a He-Cd laser with a wavelength of 325 nm
3 Results and discussion
XRD results of all the samples have nearly same peaks A represen- tative XRD spectrum of the ZnO samples is shown in Fig 1 All diffraction peaks can be indexed within the experimental error as a wurtzite struc- ture of ZnO (a = 0.324982 nm and c=0.520661 nm) No peaks associat-
ed with other crystalline forms are detected
We first performed the experiments from four amine sources in the presence of glycol at 90 °C for 20 h Fig 2a and the insert present well-defined hexagonal ZnO rods with smooth ends obtained from ammonia When HMTA was used, the hexagonal ZnO bundles radially assembled by tower-like structures (Fig 2b) were gained, and it is
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S
L S
> =
a a
20(degree)
Fig 1 A representative XRD pattern of the ZnO architectures obtained from HMTA
clear that there are some steps on the side surface of a single tower
Fig 2b’ shows a typical TEM image of a ZnO tower with diameter of
300 nm, and the corresponding HRTEM image (upper right inset of
Fig 2b) and SEAD pattern (lower right inset of Fig 2b’) of the tower
were recorded The (0002) lattice plane of hexagonal ZnO, with a lattice
spacing of about 0.52 nm, can be clearly identified in the lattice-
resolved HRTEM image It indicates that the ZnO tower is single crystal-
line in nature and preferentially grows along the [0001] direction The
wurtzite structure of tower was further confirmed by the SAED pattern
Flowerlike ZnO bundles radially assembled by well-defined hexagonal
rods with even diameters were obtained from EA (Fig 2c) But when
ED was replaced, novel flowerlike architectures (Fig 2d), radially
assembled by ZnO nails with a single sharp tip, quite different to
those in previous reports [9,10] and those of Fig 2b and c formed
It suggests that diverse amine sources, possibly having different
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Nova NanoSEM
hydroxyl ion releasing rate, contributing to different reaction rates, probably cause the formation of different ZnO architectures Certainly, hereinto, glycol maybe plays a certain role in controlling the morphologies in the process, and the correlative study is underway
When reactions were performed at 100 °C for 10h by the four amine sources, various novel bundled tubes and flowers (Fig 3) were produced Novel hexagonal tubular ZnO with obvious different diameter through entire length, presenting one closed sharp tip and growing into bundles from the open ends in various directions, were obtained from ammonia (Fig 3a) The typical diameters of the tubes at the open end and tips range between 500-600 nm and between 100-150 nm, respectively, and the typical lengths are in the range of 10-12 um To our best of knowledge, large-scale fabrication of ZnO tu- bular radial bunches with changed diameter in each tube has been rarely reported in a surfactant-free hydrothermal process When HMTA was employed, the hexagonal ZnO tubular bunches radially assembled by closely packed nanotubes with open ends exposed were formed (Fig 3b), which is similar to the report by Yu et al [11] Fig 3c reveals
the flowerlike ZnO with unsmooth surface obtained from EA, which is
different from traditionally rod-based flowerlike ZnO The typical flower shown in the right of Fig 3c indicates that it is composed of several sharp-tip petals, in which each petal consists of some short, non- smooth rods, similar to the underdeveloping ones The ends of these rods attach to each other and form the tip-like petal (the insert of the right flower of Fig 3c) In addition, instead of the petal-based flowers, some unusual rod-based ZnO without obvious petals (typically displayed in the left of Fig 3c) extending radially from center to
form flowerlike structures are clearly seen, in which the end
faces of these rods seem to be “dissolved” and the hexagonal contour
is rough and blurry (the insert corresponding to the left flower of Fig 3c) Itis easy to imagine that the rod-based ZnO flowers without ob- vious petals could be gradually “dissolved”, and they are maybe the forerunners of the petal-based flowers Fig 3d illustrates the rods or
Fig 2 ZnO architectures obtained from ammonia (a), HMTA (b), EA (c), ED (d) at 90 °C for 20 h The insets of Fig 2b are the SAED pattern and the HRTEM image of a nanotower, respectively.
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Fig 3 ZnO architectures obtained from ammonia (a), HMTA (b), EA (c), ED (d) at 100 °C for 10h
tower structures obtained from ED, in which many of them were
composed of discrete subunits, orientated and connected along
c-axis All the above results suggest that it is an effective way to
controllably synthesize ZnO by changing reactant precursors with
different ion releasing rate, besides changing the temperature
and time
The optical properties of ZnO flowerlike bundles from HMTA were
observed by PL (Fig 4) Generally, the UV emission at about 391 nm is
band-edge emission resulting from the recombination of free excitons
[11], while the green—yellow emission can be attributed to the recombi-
nation of photo-generated hole with electrons in singly occupied oxygen
vacancies The green-yellow emission in our sample can be neglectable
compared with the intensive sharp UV emission Therefore, the results of
PL indicate that our growth method can produce a low concentration of
oxygen vacancies and high optical quality of single-crystal ZnO [8,12]
3
a
= |
=
an
c +
oO
—_
£7
a
350 400 450 500 550 600 ~~ 650
wavelength(nm)
Fig 4 Room temperature PL spectrum of the ZnO architectures obtained from HMTA
4 Conclusions
In summary, various ZnO architectures, especially novel radialized flowerlike structures assembled by nails with one sharp tip and novel tubular ZnO bundles composed of many single tubes with different diameters from bottom to top were controllably synthesized with dif- ferent amine precursors by a simple surfactant-free hydrothermal process It suggests that different amine precursors play an important role in controlling the formation of various ZnO architectures possibly
by tuning the reaction rate, besides temperature and time It not only gives an effective way to controllably synthesize various ZnO archi- tectures but also provides valuable information for the controlled synthesis of other functional nanomaterials
Acknowledgements This work was supported by National Natural Scientific Foundation of China (Nos 51102180 and 21001081), Tianjin Science and Technology Fund Project for High Education (Nos 20110311 and 20100504) and the talent fund projects in Tianjin Normal University (No 5RLO78)
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