VIETNAM JOURNAL OF CHEMISTRY VOL. 51(2) 146-150 APRIL 2013
TRANSFORMATION OF THE ZnO STRUCTURE FROM MICROPARTICLES TO NANOTUBES UNDER HYDROTHERMAL
CONDITIONS
Vu Thi Thu Ha*, Cao Thi Thuy, Nguyen Thi Ngoc Quynh, Do Thanh Hal, Pham Thi Nam Binh, Au Thi Hang
National Key Laboratory for Petrochemical and Refinery Technologies. Hanoi. Vietnam Received 23 October 2012
Abstract
The ZnO nanottibes were prepared by hydrothermal method in the presence of hydrogen peroxide (H2O2). X-ray diffraction (XRD). transmission electron microscope (TEM) and BET analysis were used to characterize the structure, morphology of as-prepared ZnQ nanotubes. It was found that 100% nansformation was obtained after 90 h and the length of the nanotubes can increase from a several hundred nanometers up to several micrometers by stirring the reaction mixture before aging. In addition, roles of HjOj, aging and stirring process for the formation of nanoUibes were discussed in detail.
1. INTRODUCTION
Zinc oxide is one of the important II-VI semiconductor fimctional materials with a direct band gap of about 3.4 eV at the room temperature and a large exciton binding energy of about 60 meV.
Hence, ZnO has been became a potential material for a widely range of technological applications (gas sensors, varistors, blue lasers, fransparent conductive coatings for flat panels and solar cells ...) [1-18].
ZnO nanotube is one of the important nanostructures of zinc oxide, because not only the existence of tubular structure means a larger surface-to-volume ratio. It was found that ZnO nanotubes not only have large specific surface area but also are the one-dimensional (1 -D) nanostructural semiconductor matenals with their imique stmctural and physical properties [3, 27, 31].
They play important roles in fabricating elecfronic, optoelecfronic, elecfrochemical and electromechanical nanodevices and are used as catalysis-adsorption matenals. ZnO nanotubes are synthesized by some methods as: chemical vapour deposition (CVD) [19]; elecfrochemical deposition [20, 21]; thermal evaporation [22, 23]; combining electrospinning and sputtering method [24], sol-gel [25, 26] and hydrothermal metiiod [27-31].The hydrothermal method has a simple produce. In the hydrothermal method, synthesis of tubes in the presence of templates or the surfactants is a need.
except study of C. Wang et al [31]. However, C.
Wang only studied the effect of aging time to the tubes growth of the ZnO tubes and lengths of these tubes did not exceed 500 nm by static hydrothermal process.
In this study, the ZnO nanotubes were prepared without templates or surfactants by hydrothermal method. The as-synthesized ZnO nanotubes have lengths of several himdred nanometers up to several micrometers. The effect of stirring conditions and aging time on the sizes of ZnO nanotubes was studied. With the same size of ZnO nanotubes, the time of hydrothemal process with stimng is shorter than that of the hydrothemal method without stimng. The several micrometers ZnO tubes were synthesized by confrolling time of stirring process.
The nanostructure of ZnO was characterized by transmission elecfron microscope. X-ray diffraction and N2 adsorption. The mechanism of ZnO nanotube formation also was discussed.
2. EXPERIMENTAL 2.1. Material
Zinc oxide (99.9%, particles size < 5 pm, specific surface are 10.89 mVg, Sigma- Aldrich Co., USA) was used without further freatment. 30% H2O2 solution was purchased from Beijing Chemicals Ltd.
Co., China.
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2.2. Synthesis of ZnO nanotubes with different length
2.2.1. Synthesis of ZnO nanotubes with length of several hundered nanometers [31]
1 ZnO powder was mixed with 10 ml hydrogen peroxide. Then, this mixture was placed in a teflon - lined stainless steel autoclave. Then 40 ml hydrogen peroxide was added. Autoclave was sealed and kept inside an electric oven at 180°C with the different time: 24, 48, 72 and 90 hours. After reaction, the autoclave was cooled to room temperature. The mixture was washed with distilled water, and then with ethanol. The obtained white product was dried at 60°C for 6 h.
2.2.2. Syntiiesis of ZnO nanotubes with length of several fim
1 g ZnO powder and 50 ml hydrogen peroxide were placed into a teflon - lined stainless steel autoclave. The autoclave was sealed, mixture in autoclave was stirred at 80-90°C with different time (3, 5 va 10 h), and then kept inside an elecfric oven at 180°C for 48 hours. After reaction, the autoclave was cooled to room temperature. The mixture was washed with distilled water, and then with ethanol.
The obtained white product was dried at 60''C for 6 h. To study the appropriate conditions for the transformation, we have synthesized some samples under various conditions: aging and sturing time, stirring temperature.
2.3. Characterization of as-synthesized samples The morphology of obtained products was observed by transmission electron microscope (TEH JEOL JEM-IOIO). The crystal shuctures were characterized by X-ray diffractometer (XRD, Brucker, D8 ADVANCE with Cu Ka radiation, X = 1.54178 A). The Brunauer-Emeft-Teller (BET) surface area of the as-prepared samples was evaluated from the N2 adsorption isothermal using a BET sorptometer (Automated BET sorptometer BET 201-A).
3. RESULTS AND DISCUSSION
VuThi Thu Ha, etal.
30-50 nm while their average lengths relative slow increased from 300 to 500 nm when aging time was increased rapidly from 24 h to 90 h. Hence, the tubes growth is preferential along one dimension and relatively slow.
Figure 1: TEM images of ZnO nanotubes with different aging time (a): 24 h, (b): 48 h, (c): 72 h,
(d): 90 h
Fig. 2(a), (b), and (c) show conversion ofthe as- obtained product when aging time is 48 h, 72 h, 90 h, respectively. It can see that the conversion of products with aging time of 48 h and 72 h is much smaller than that of 90 h. For aging time of 90 h sample, it can be considered transfering ZnO from particles to nanotubes Is 100 % and the as-obtained ZnO nanotubes have a uniform size, their lengths of 400-600 nm. Moreover, XRD patterns of these products show they did not contain an impurity and their crystallization are high (the XRD parttem Is not shown).
3.1. Synthesis of ZnO nanotubes with length of several hundred nanometers
Fig. 2: TEM image ofthe ZnO nanotubes with different aging:(a): 48 h, (b): 72 h, (c) 90 h Typical TEM images of the synthesized ZnO Thus, when reation time reached 90 h, the lengths nanotubes are shown in Fig. 1. Fig. 1 shows the of as-synthesised nanotubes is 400-600 nm and diameters of nanotubes in all cases are the same of transformation of ZnO from particles to nanotubles
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is the largest. Furthermore, the growth ofthe tubes is 1-D growth type and relatively slow. These results are also proven in [31]. C. Wang et al. showed that length of the nanotubes increase slowly and reach 500 nm when aging time increase from 0 h to 48 h and the growth of tubes was the 1-D growth type.
However, C. Wang did not attend the conversion degree of.products.
3.2. Synthesis of ZnO nanotubes with length several ^m
TEM images of the samples prepared with stirring time of 3 h. 5 h and 10 h are shown in Fig.
3(a), (b), and (c), respectively.
Transformation ofthe ZnO structure...
3.3. The mechanism of the transforming ZnO particles to the nanotubes
3.3.1. TheroleofHiOi
To investigate the role of H2O2 in the h-ansformation from ZnO particles to Znp nanotubes, two samples were prepared in the presence of H2O2 and without Hz02. Results of the TEM images are shown in Fig. 4. The sample as- prepared without H2O2 show the final products not found ZnO nanotubes (Fig. 4(a)). The as-prepared sample in solution of the H2O2 frace formed a snull amount of ZnO nanotubes (Fig. 4(b)).
Fig. 3: TEM images of samples ZnO with stfrring time:(a): 3h, (b): 5h and (c): lOh The results of TEM images in Fig. 3 show that when stimng time increase, the synthesized samples compose of more micrometer tubes that were much longer in length, however, the diameter of these tubes is nearly constant. The about 400-600 nm nanotubes were obtained with stimng time of 3 h (Fig. 2(a)) and the about 8pm nanotubes when the stirring time reached 10 h (Fig. 2(c)). This behavior can be explained as following: when the stirring time of the mixture was prolonged in the closed reaction system, amount of formed ZnO nuclei is increased, leading to increase amount of formed tubes and increase their length. Hence, stirring factor are increased rapidly growth rate of tubes in a dimensional. In addtion, companng the 3 h stimng time sample above with the 90 h aging time, it shows that both of these cases obtain the ZnO nanotubes with 400-600 nm length, but the reaction time decrease from 90 to 51 h. Thus, the stimng at high temperature are also considerably decreased the reaction time.
(b) ^
Fig. 4: TEM images of samples with different concentration of H2O2 (a) 0%; (b) solution ofthe
H2O2 trace
Thus, the existence of H2O2 plays a crucial role in the formation of the ZnO nanotubes. The transformed process of ZnO from particles to nanotubes carmot occur if not presence of HjOi in the reaction solutions. Therefore, it can be indicate that H2O2 is not only playing role of a substance formed pressure (H2O2 decomposes and releases gaseous oxygen), but also the substances involved in the structure transferred process of ZnO.
3.3.2. Mechanism ofthe ZnO nanotubes formation Through the results of experiments and a "vapor- liquid-solid" growth process [3, 27, 31], it provides some mechanisms of transferring fix)m ZnO particles to nanotubes as follows: At high temperature, the decomposition of the H2O2 is fast and releases gaseous oxygen, leading to the pressure of closed reaction system is quickly increased [31]. The formation process of the ZnO nanotubes occurs via the vapor - liquid - solid mechanism (fig. 7). After absorpted H2O2 on the ZnO solid surfece, these molecules will decompose and release the active 0 atom (O*) and O* interacts with the soUd surface and forms ZnO nanotubes nuclei, then ttiese nuclei are orderly rearranged imder high pressure condition and grow preferentially along one direction, which results the ZnO nanotubes formation [3, 27, 31]. The ZnO nanotubes formation is promoted by increasing
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temperature and time of the stirring before aging, increasing time of aging.
3.3.3. The role of stirring process
The samples were synthesized by stirring the reaction solution in close system at different temperatures, for the same stirring time for 3 h and whhout aging process. The size and morphology of as-prepared products were characterized with transmission electron microscope (TEM) in Fig. 5 and their specific surface area are shown in table 1.
Table 1 shows specific surface area of as- prepared samples at stirring temperatures of 25°C, SO-eOT and 80-90''C. It can see that the specific surface area of these samples is much higher than the inhial ZnO used as raw materials, and specific surface area of the 80-90°C stirring samples is the largest. This resuh also confirmed by their TEM images in Fig. 5. This can be used as a simple method to improve porosity of ZnO original material.
Fig. 5" TEM images of ZnO nanoparticles with reaction temperature:
(a): 25*'C, (b): 50-60°C and (c): 80-90°C Table 1: Specific surface area ofthe as-prepared
samples at different temperatures Stirring
temperature, "C Specific surface
area, m^/g 25 53.32
50-60 52.17
80-90 77.41 TEM images of the samples (Fig. 5) show that as-obtained products composed of nanoparticles, particles have smaller sizes when increasing temperature of stirring. At room temperature, under the effect of H2O2, the micrometer ZnO particles (< 5 micrometers) were divided into 100 nm particles (Fig. 5(a)). Increasing temperature of stirring up to 50-60''C, it can be observed hapening
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the separation of the 100 nm particles (fig. 5(b)).
When the temperature of stirring reached 80-90°C, particle separation occurs more easily and completes the form of quick small ZnO nuclei [27, 31], about 10 nm (fig. 5(c)). Hence, as the synthetic process above occurs without hydrothermal aging, the as- obtained ZnO products are nanoparticles, especially 80-90''C sizes of as-obtained nanoparticles were the smallest. Moreover, according to the mechanism is shown in [27, 31], to form tubes, the first stage is the form of nuclei. In this case, stirring factor can be preferential the form of more ZnO nuclei. Thus, the stirring factor in the closed system at high temperatures plays a role forming the ZnO nanotubes nuclei.
4. CONCLUSIONS
This study has more clearly defined the roles of aging and stirring processes to the form of nanotubes. The transformation of ZnO particles structure to tubes under hydrothermal conditions can be reached 100% and the as-synthesized nanotubes have lengths about several hundreds of nanometer up to several micrometers. In addition, it was given the simple method to improve the porosity of commercial zinc oxide material (specific surface area of ZnO increased from 10 to 77 mVg).
Aknowledgement: We would like to thank for supporting the finance ofthe Viet Nam Science and Technology Ministry to finish this project.
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Corresponding author: Vu Thi Thu Ha
National Key Laboratory for Petrochemical and Refinery Technologies, Hanoi, Vietnam
2 Pham Ngu Lao, Hoan Kiem, Hanoi, Vietnam.