Effects of pretreatment of substrates on the preparation of large scale ZnO nanotube arrays
Department of Physical Chemistry, University of Science and Technology Beijing
作者简介：GUO Min E-mail: guomin@metall.ustb.edu.cn;
收稿日期：17 January 2009
基金：supported by the National Basic Research Program of China (No. 2007CB613608);the National Natural Science Foundation of China (Nos. 50872011 and 50672006);the New Century Excellent Talents Support Program of the Ministry of Education of China (No. 08-0723);
Effects of pretreatment of substrates on the preparation of large scale ZnO nanotube arrays
Abstract：
Well-aligned hexagonal ZnO nanotubes (NTs) arrays were synthesized on pretreated indium tin oxide (ITO) substrates by a simple hydrothermal method. The morphology and structure of the products were characterized by scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). A new method of substrate pretreatment was introduced to prepare ZnO coated films. The size of ZnO seeds and the formation rate of ZnO NTs were investigated. Further, the mechanism of the preparation of ZnO NTs was discussed. The photoluminescence (PL) spectrum measurement shows fairly internal defects existing in ZnO nanotubes.
Keyword：
nanotube arrays; zinc oxide; substrate; pretreatment; hydrothermal method; annealing time;
Received： 17 January 2009
1. Introduction
Recently,zinc oxide(Zn O)has been attracting more and more attention due to its possession of unique properties such as near-UV emission,optical transparency,electric conductivity,and piezoelectricity[1-3].
Earlier progress has been achieved in the study o one-dimensional Zn O structures[4-6],while nowadays con siderable efforts have focused on the synthesis of Zn O nanotubes[7-11].There have been many methods used on this subject,including the hydrothermal method[12-13]electrodeposition[14-15],and vapor phase growth.Elias e al.[16]obtained arrays of single-crystal Zn O nanotubes with tailored dimensions,while the three-step process in cluding electrochemical and chemical approaches made the preparation complicated and added undesirable variables Xing et al.[17]demonstrated NTs in large-area substrates by vapor phase growth.However,the nature of high tem perature in this method limited its application.Compared to these techniques,a simple hydrothermal method is more ap plicable because it is cheaper,template-free,and versatile Vayssieres et al.[18]reported well-distributed Zn O micro tube arrays by this approach in 2001.
Nevertheless,the size was beyond nano scale and no de tails of microtube arrays were reported.Almost all reports about the hydrothermal method failed to face the fact tha the appearance of NTs is usually unrepeatable and the preparation process is almost out of control.In this manuscript,repeatable and well-controllable Zn O NTs arrays were obtained on premodified ITO substrates by a simple hydrothermal method.
2. Experimenta
2.1. Materials
All chemicals were of analytical reagent grade and used without further purification,and all the aqueous solutions were prepared using de-ionized water.
2.2. Pretreatment of ITO substrate
The ITO substrate premodified by Zn O nanoparticles was fabricated by a three-step procedure.Firstly,zinc nitrate(0.5mol/L)and methenamine(0.5 mol/L)aqueous solutions were respectively dripped onto ITO glass for a natural mixture of 10 min.Then the substrate was spun for 30 s at a rate of 3000 r/min.Finally,the pretreated substrate was heated in a muffle oven at 300°C.The annealing time was in the range of 5 min to 4 h.The procedure was repeated three times.The obtained ITO substrate was cooled to room temperature for further preparation.
2.3. Preparation of Zn O NTs
15 m L zinc nitrate(0.2 mol/L)and 15 m L methenamine(0.2 mol/L)aqueous solutions were separately added into a Teflon-lined autoclave with a capacity of 50 m L.The premodified substrate was immersed into the solution with a gradient angle.Subsequently,the autoclave was sealed and heated at 90°C for days,and then it was cooled to room temperature.The ITO glass was then rinsed with ethanol and dried in air for further characterization.
2.4. Characterization of Zn O NTs arrays
The Zn O NTs grown on the substrate were characterized and analyzed by scanning electron microscopy(SEM,SU-PRA55)andX-raydiffractionanalysis(XRD,M21XXRHF).The PL spectrum was measured by a spectrophotometer(Hitachi F4500 FL)using an excitation wavelength of 320 nm.
3. Results and discussion
It was observed in experiments that Zn O crystal seeds first grew into Zn O nanorods(NRs)arrays,and then formed a hollow structure with the absence of polar{0001}faces.The pretreatment of substrates was crucial to control the formation rate of NTs.The obtained nano structures were typically 200-350 nm in diameter and up to about 4?m long.
Fig.1 shows the SEM images of Zn O crystal seeds and well-aligned Zn O NTs obtained by the hydrothermal method for 96 h.It can be observed that all of the seeds appear as spheres and are well dispersed.After pretreatment with the substrates,the Zn O seeds obtained after annealing are more mature and controllable;this is consistent with Fig.1.The statistics reveal that the Zn O crystal seeds grow larger with increasing annealing time.It is obvious that longer annealing time results in larger Zn O crystal seeds because small seeds tend to conglomerate and grow into more mature ones.Also,the size and maturation of Zn O crystal seeds on substrates had significant effects on the NTs formed in the following process.In the following couple of days,Zn O NRs were fully prepared due to the fact that Zn O presents a tendency to minimize total surface energy by growing along the[0001]direction.It is well known that the integrality,distribution,and formation rate of NTs depend on the maturation and size of NRs.However,the essential problem of NRs quality is rooted in the preparation of Zn O seeds,which can be solved by our precursory approach.The size and formation rate statistics of Zn O NRs and NTs arrays are listed in Table 1.
Fig.1.SEM images of Zn O seeds(a,c,e)and NTs(b,d,f)grown after 96 h with the substrates annealed for 5 min,5 min,1 h(a,b);5 min,5 min,2 h(c,d)and 5 min,5 min,4 h(e,f).
Accordingly,the whole process can be clearly explained by the following reactions:
It is obvious that the production of NH3 helps create the proper pressure.The numbers of Zn O crystal seeds on the substrate and Zn2+ions in the solution are critical in the determination of the morphology and distribution of Zn O NTs.
Table 1 shows that larger Zn O crystal seeds easily grow into Zn O NRs and NTs with larger diameters of polar{0001}faces within the first 72 h.This phenomenon can be explained by previous studies on Zn O crystal seeds.The conglomeration during the annealing process resulted in less but bigger seeds;larger ones grew into NRs much faster than smaller ones.In the later dissolution period,larger NRs retained their dominance over small ones and turned into larger NTs.Therefore,Zn O NTs formed in the aging process were larger if the annealing time was longer.
It is worth noting from Table 1 that the average size of Zn O NTs was even smaller at 96 h than at 72 h in group 3#while groups 1#and 2#retained the same pattern,i.e.,NTs were larger with a longer aging time.The pattern in the first two groups was the result of the rapid dissolution of small Zn O NRs and NTs in the original solution due to asymmetric Zn O-film-coated substrates.It is well known that all the products were simultaneously dissolving at the same time;however,small ones tended to dissolve much faster because of higher surface energy.As a result,the extra Zn2+ions in the solution combined with OH-and then formed Zn O powder at the bottom of the autoclave.Therefore,the average sizes of Zn O NTs of the two groups were larger at 96 h than at 72 h for statistical reasons.Since the third group at96 h had an obvious decrease in size,it was explained that some of the Zn O NTs were totally broken because the NTs in group 3#were larger and more mature for the reason of having favorable crystal seeds in the first place,so they reached the equilibrium of growth and dissolution much faster.The larger NRs began dissolving into NTs earlier than the rest.After the inner walls were formed,the nonpolar faces were left to dissolve in advance.It is worth mentioning that some of the Zn O crystal seeds were not monocrystalline due to conglomeration,indicating that Zn O NTs tend to break into several parts during dissolution.As a result,the remaining Zn O NTs were more integrated than the smaller ones.
Table 1.Original crystal seeds and the following NT formation under three pretreatment conditions 下载原图
Note:1#,2#,3#—Zn O prepared under the annealing time of 5 min,5 min,1 h;5 min,5 min,2 h;and 5 min,5 min,4 h,respectively.
Table 1.Original crystal seeds and the following NT formation under three pretreatment conditions
It is clearly observed that group 2#had better results than the rest of the groups,considering the NTs’size distribution(Fig.2)and formation rate.The diameter of Zn O NTs is around 350 nm as shown in Fig.2.With moderate Zn O crystal seeds and the maturation degree of NRs during preparation,the NTs of group 2#had the best maintenance condition with a NT formation rate of 80%,as demonstrated in Table.1.
According to the X-ray diffraction pattern,the Zn O NTs obtained were highly pure(Fig.3)without noticeable impurities detected.The diffraction peaks are indexed to hexagonal wurtzite structures of Zn O.Additionally,the(002)plane is relatively weak compared with the Zn O NRs pattern because the(00n)planes are absent in the hollow structure.
Fig.2.Size distribution of Zn O NTs after 96 h in group 2#under the annealing time of 5 min,5 min,2 h.
Fig.3.XRD patterns of highly distributed Zn O NTs(a)and Zn O NRs(b)on an ITO substrate premodified by Zn O nanoparticles.
To present the extraordinary optical properties,a room temperature photoluminescence spectrum was conducted.Fig.4 illustrates the emitting band centered at around 390nm which shows the optical property of Zn O nanostructures.Tong et al.[19]provided the PL spectrum of Zn O NRs by a different preparation procedure with a visible emission ranging from 440 nm to 600 nm due to abundant surface defects associated with oxygen vacancies.Compared with his work,NRs obtained with special pretreatment show negligible defects and impurities at the surface of Zn O NTs.This might indicate that different preparation techniques have great influence on the crystallization degree of NTs,and the pretreatment of substrates clearly has the advantage of a better crystallization with much fewer surface defects.The blue band at 466.8 nm in Fig.4,which can be observed clearly,may possibly be due to the internal defects which can also be verified from the SEM images.
4. Conclusions
In this paper,well-aligned Zn O NTs were prepared by a hydrothermal method on pretreated ITO substrates.The formed Zn O crystal seeds on the substrates were mature and well distributed.SEM images revealed that nanotubes obtained under the annealing time of 5 min,5 min,2 h present the best tube formation rate of 80%.The effects of crystal seeds and the unique coating method were demonstrated briefly.The formation mechanism was discussed using three different conditions.XRD analysis shows a Zn O hexagonal structure with a weak diffraction peak of(002)plane,which reflects the tubular structure.The PL spectrum measurement shows internal defects existing in Zn O NTs.
Fig.4.SEM image(a)and PL spectrum(b)of Zn O NTs grown at 90°C for 96 h.
参考文献
[6] Ohta H. and Hosono H., Transparent oxide optoelectronics, Mater. Today, 2004, 7 (6): 42
