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Temperature-controlled growth and optical properties of ZnO nanorods with quadrangular and hexagonal cross sections
Materials Chemistry and Physics 115 (2009) 799–803
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Temperature-controlled growth and optical properties of ZnO nanorods with quadrangular and hexagonal cross sections Jiafu Zhong, Ke Cheng, Binbin Hu, Hechun Gong, Shaomin Zhou, Zuliang Du ∗ Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 May 2008 Received in revised form 20 January 2009 Accepted 19 February 2009 Keywords: Chemical vapor deposition ZnO nanostructures X-ray photoelectron spectroscopy Chemisorbed oxygen
a b s t r a c t High-density ZnO nanorods with quadrangular and hexagonal cross sections were synthesized by chemical vapor deposition combining with a sol–gel method. It was found that the different morphologies could be obtained by changing the growth temperature. The growth mechanism was presented. The photoluminescence (PL) spectra demonstrated that both the quadrangular and hexagonal cross sections ZnO nanorods had a weak ultraviolet (UV) emission but a very strong and wide green emission. Interestingly, the quadrangular ZnO nanorods showed a stronger green emission than the hexagonal ones. Our X-ray photoelectron spectroscopy (XPS) results indicated that the quadrangular ZnO nanorods had a higher oxygen adsorption ability, and which should be the primary reason for its stronger green emission. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, much effort has been devoted to fabricating quasi one-dimensional (Q1D) nanoscale semiconductor materials due to their potential applications in nanodevices and complicated nanocircuits [1–3]. Being a wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), ZnO is recognized as a promising material for short-wavelength optoelectronic applications due to its superior optical and electronic properties [3–7]. Various ZnO nanostructures, including nanobelts [1], nanorods [2], nanowires [4], nanotips [5] nanonails [6], and nanohelices [7] have been developed based on the vapor–solid (VS) and vapor–liquid–solid (VLS) growth process [8]. As we know, the physical properties are expected to have a close correlation with their size and shape in the field of nanomaterials [9–11]. So it is technically important to fabricate nanomaterials with controllable morphologies and structures for deep understanding the influences of these factors on their physical properties. Up to now, many nanostructures with different cross sections have been reported. For example, a rectangular-like cross section ZnO nanobelts [1] and triangular-like cross section ZnO nanorods [12] have been reported by Wang’s group, and the similar structured ZnO has also been reported by other groups using different growth methods [13–15]. However, few reports could be found using the versatile chemical vapor-phase deposition method to synthesize well-crystallized ZnO nanorods with quadrangular (square
∗ Corresponding author. Tel.: +86 378 3881358; fax: +86 378 3881358. E-mail addresses: [email protected], [email protected] (Z. Du). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.02.028
or rhombic) cross section until now. Here, we prepare the different cross section ZnO nanostructures by changing the growth temperature. Specifically, the hexagonal and quadrangular ZnO nanorods are obtained at 850 and 950 ◦ C respectively. The photoluminescence spectra exhibit a different green emission for the hexagonal and quadrangular structured ZnO, indicating a close correlation between the physical properties and the different cross sections they owned. To understand how the cross section impact on their PL properties and to explore their potential applications, the hexagonal and quadrangular structured ZnO nanorods have been characterized using XPS. More adsorbed oxygen has been found on the surface of quadrangular ZnO nanorods due to the special crystal planes it owned, and which should be the primary reason for its stronger green emission. And we think that the novel quadrangular ZnO nanorods not only provided valuable modes in understanding crystal growth mechanisms in nanometer scale, but also exhibited high potential for fabricating novel nanoelectronic and optical devices with enhanced performance. 2. Experimental procedure The procedure for the synthesis of ZnO nanorods was as follows: firstly, 2.97 g Zn(NO3 )2 ·6H2 O was dissolved in 100 ml deionized water (10 M cm). After this, 5.763 g citric acid and 6.6 ml ethylene glycol were added in the solution with stirring at 80 ◦ C (±5 ◦ C) for 1 h to form a sol. Secondly, the sol was polymerized into a gel at 150 ◦ C (±5 ◦ C) for 5 h and subsequently pre-pyrolyzed at 400 ◦ C (±10 ◦ C) for 1 h to become an amorphous composite precursor [16]. The resulting amorphous precursor was ground and loaded in a ceramic boat, and the ceramic boat was put into a one-end-sealed quartz tube. A silicon wafer coated with Au in a thickness of 100 nm was placed about 5 cm downstream from the ceramic boat used as a substrate to collect the products. The Au film on silicon wafer was coated using an Eiko IB-3 ion coater for 20 min operated at a vacuum of 15 Pa
800
J. Zhong et al. / Materials Chemistry and Physics 115 (2009) 799–803
3. Results and discussion
Fig. 1. XRD patterns of ZnO nanorods with quadrangular and hexagonal cross sections.
and current of 3 mA. At last, the furnace was heated up to 950 ◦ C and kept at that temperature for 60 min with N2 as a carrier gas at a flow rate of 200 sccm. Then the furnace was naturally cooled down to room temperature. Q1D ZnO nanorods with quadrangular (square and rhombic) cross sections were obtained on the substrate. For hexagonal structured ZnO nanorods, the synthesis procedure was the same as the quadrangular ones except the growth temperature lower to 850 ◦ C. The crystallinity of the products was characterized by X-ray diffraction (XRD) with X’pert MRD-Philips diffractometer equipment with Cu K␣ radiation. The morphologies were imaged using JSM-5600 scanning electron microscopy (SEM). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) (JEOL-2010, at 200 kV, Japan) were also performed to characterize the ZnO nanorods. XPS (Axis Ultra, Shimadzu-Kratos, England) and the photoluminescence (PL, SPEX F212, SPEX Co. America) spectra were performed to obtain the surface information and optical properties of our synthesized ZnO nanorods.
XRD measurements [Fig. 1(a) and (b)] show that the assynthesized quadrangular and hexagonal ZnO nanorods are consistent with the standard of bulk ZnO (JCPDS Card File, No. 790207), which could be indexed to the wurtzite structure crystalline ZnO. The lattice parameter c calculated from the peak (0 0 2) is about 0.521 nm. The SEM images of quadrangular (square and rhombic) ZnO nanorods synthesized at 950 ◦ C are shown in Fig. 2. From the SEM images we can see that the substrate was coated with large quantities of ZnO nanorods, which have a square or rhombic cross section [inset Fig. 2(a)]. The square cross section can be observed clearly as shown in Fig. 2(b). These quadrangular nanorods have a nonuniform width of about several hundred nanometers and length up to several microns. Typical TEM image of the quadrangular ZnO nanorods is shown in Fig. 2(c). Based on HRTEM image and SAED pattern shown in Fig. 2(d), we can see that the quadrangular ZnO nanorods prefer to grow along [0 0 1] direction. And we can observe a singlecrystalline wurtzite structure and a 0.52 nm distance between the neighboring (0 0 1) planes. The high-density aligned hexagonal ZnO nanorods have also been synthesized at 850 ◦ C successfully. As shown in Fig. 3(a) and (b), the ZnO nanorods have a mean diameter of ∼100 nm and have a length about tens of microns. The regular prismatic hexagonal ZnO nanorods can be observed, as shown in the inset of Fig. 3(a). Furthermore, the hexagonal ZnO nanorods were well aligned vertically with a uniformity in their length and density. The TEM image shown in Fig. 3(c) represents a general morphology of the hexagonal ZnO nanorods. And the HRTEM image shown in Fig. 3(d) indicates that the hexagonal ZnO nanorod has a perfect wurtzite structure. The inset in Fig. 3(c) shows the corresponding SAED pattern, which further confirms our synthesized ZnO nanorod is single-crystalline and grows along caxis.
Fig. 2. SEM images of the quadrangular ZnO nanorods with (a) low and (b) high magnification, and the insets show top-end morphology. (c) A typical TEM image. (d) A HRTEM image with the corresponding SAED patterns.
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Fig. 3. SEM images of the hexagonal ZnO nanorods (a) top view and (b) side view. (c) A typical TEM image of hexagonal ZnO nanorods and an inset for counterpart SAED patterns. (d) A corresponding HRTEM image.
The growth mechanism is presented as follows: the gel is pre-pyrolyzed at 400 ◦ C which makes it become an amorphous composite precursor. And this composite precursor provides the carbon and zinc source for the next reaction process. When the horizontal tube furnace is heated up to 850 ◦ C, the following reactions may occur [17]: ZnO(s) + C(s) = Zn(g) + CO(g)
(1)
ZnO(s) + CO(g) = Zn(g) + CO2 (g)
(2)
ZnO(s) + CO(g) → ZnOx (x < 1) + CO2 (g)
(3)
Subsequently, parts of Zn vapor carried by the N2 are transferred to the substrate region (about 500 ◦ C). Then, the reactions take place as follows: 2Zn(l) + O2 (g) = 2ZnO(s)
(4)
Briefly, the source amorphous Zn–C–O composite precursor is reduced into Zn or its sub-oxide (ZnOx , x < 1) by carbon, which has a lower melting point at about 420 ◦ C. Parts of Zn or ZnOx in vapor will be transferred to the substrate and condensed into liquid nanodroplets. When the liquid droplets become supersaturated, Zn particles precipitate and react with oxygen forming ZnO nuclei on the substrate which serve as the seeds for the ZnO nanostructure growth. Due to ZnO prefer to grow along [0 0 1] direction, this will lead to a rod-like structure finally. Usually, there are two mechanisms for the growth of 1D ZnO nanostructure materials: vapor–solid mechanism (without metal catalyst) and vapor–liquid–solid mechanism (with metal catalyst) [8]. In our case, the deposited Au film on the silicon substrate is used as catalyst, so our growth process is most likely the well-known VLS mechanism. However, it is notable here that the ZnO nanorods with different cross sections are obtained at almost the same conditions except the different growth temperatures. It has been reported that the morphologies of products can be tuned by changing the gas-phase supersaturation during the growth process [12,18]. In our case,
when the Zn or ZnOx vapor is advected by the carrier gas downstream to the substrate, a local supersaturation will be established. As we know, the supersaturation is strongly related to the growth temperature. So the different cross sections of ZnO nanorods we obtained may be due to the different growth temperature [12,19]. On the other hand, the growth rate in different crystal planes is also related to the zinc and oxygen vapor concentration ratio [19]. And the crystal plane with a faster growth velocity was easier to disappear, whereas the plane with a slower growth velocity is prone to be remained [18]. In a word, the difference of gas-phase supersaturation in different growth temperature and the different growth rates in different crystal planes is the primary reason for the formation of the ZnO nanorods with different cross sections. The optical properties of as-synthesized two kinds of ZnO nanorods were investigated here. Generally, there are always two emission bands for ZnO which center at UV range and visible range
Fig. 4. PL spectra of ZnO nanorods with quadrangular and hexagonal cross sections.
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Fig. 5. High-resolution XPS spectra in O 1s region (a and b) and Zn 2p region (c and d).
respectively. The UV emission peak is the intrinsical near band gap emission which can be attributed to the recombination of free excitons [20]. However, the origin of the visible emission is controversial and several mechanisms have been proposed. Commonly, the origin of visible emission of ZnO can be ascribed to impurities (mainly including adsorbed oxygen, hydroxide, moisture, etc) and various intrinsic defects such as the singly ionized oxygen vacancies and interstitial zinc atoms [21–23]. In our case, two separate emission bands are also observed for both the quadrangular and the hexagonal structure ZnO as shown in Fig. 4. They both exhibit a broad and strong green emission peak centered at 516 nm. Whereas, the narrow weaker UV emission peaks center at 385 nm for hexagonal nanorods and 390 nm for quadrangular nanorods respectively. We think that the 5 nm red shift cannot be attributed to the quantum confinement effect, because the size of the ZnO nanorods is far larger than the Bohr radius of ZnO (2.34 nm) [8]. This red shift in the UV emissions may be related to the formation of different shallow levels inside the band gap due to the presence of defects [24]. In order to investigate the properties of defects in the two kinds of ZnO nanostructures, XPS measurement was carried out. The deconvoluted XPS spectra of O 1s of hexagonal structure and quadrangular structure ZnO nanorods are shown in Fig. 5(a) and (b) respectively [25]. We can see that the photoelectron spectra of O 1s could be fitted as two peaks centered at 530.3 and 532.0 eV. The low binding energy component centered at 530.3 eV could be attributed
to O2− oxidation state bound with Zn in the crystal lattice. While the high binding energy component centered at 532.0 eV is corresponding to the presence of loosely bound chemisorbed oxygen [25,26]. From our XPS result, it is easy to see that the relative intensity ratio (peak at 532.0 eV/peak at 530.3 eV) of quadrangular ZnO is higher than that of hexagonal ZnO, indicating more chemisorbed oxygen existed on the surface of quadrangular ZnO. Generally, the larger the surface, the more oxygen will be adsorbed. But in our case, the surface area of the hexagonal ZnO is obviously higher than the quadrangular ZnO due to their smaller nanorod diameter as shown in Figs. 2 and 3. So the different oxygen adsorption ability should not be ascribed to the surface area difference. It has been reported that the oxygen adsorption ability has a correlation with the crystal planes [27,28]. Then we think that the different oxygen adsorption ability of quadrangular ZnO and hexagonal ZnO is due to the different planes they owned. That is, there is more adsorbed oxygen on the surface of quadrangular ZnO nanorods due to the special crystal planes it owned. It is just the more adsorbed oxygen resulted the stronger green emission observed in photoluminescence. At the same time, the binding energy of Zn 2p for two structures is also somewhat different. The Zn 2p3/2 and Zn 2p1/2 peaks of hexagonal ZnO nanorods locate at 1021.5 and 1044.7 eV (shown in Fig. 5c) and those values of the quadrangular ZnO nanorods shift to 1021.8 and 1045.0 eV respectively. There is a 0.3 eV difference between them. As the XPS measurements of two samples are simultaneous, the
J. Zhong et al. / Materials Chemistry and Physics 115 (2009) 799–803
carbon contaminated condition is almost the same. The 0.3 eV difference is significant and indicate that Zn is in the formal different Zn2+ valence state within an oxygen-deficient environment [29]. 4. Conclusion In summary, high-density quadrangular and aligned hexagonal ZnO nanorods were synthesized on Au-coated silicon substrate by sol–gel method combining with chemical vapor deposition. Comprehensive structural investigations including XRD, SAED, and HRTEM showed that both the quadrangular and hexagonal ZnO nanorods are pure wurtzite structure. More adsorbed oxygen was found on the surface of quadrangular ZnO nanorods through our XPS results, which is the primary reason for its stronger green emission. Acknowledgement This work was supported by the Natural Science Foundation of China (Nos. 90306010 and 10874040), State Key Basic Research “973” Plan of China (No. 2007CB616911), and the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No. 708062) References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] J.Y. Park, D.E. Song, S.S. Kim, Nanotechnology 19 (2008) 105503.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Temperature-controlled growth and optical properties of ZnO nanorods with quadrangular and hexagonal cross sections Jiafu Zhong, Ke Cheng, Binbin Hu, Hechun Gong, Shaomin Zhou, Zuliang Du ∗ Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 May 2008 Received in revised form 20 January 2009 Accepted 19 February 2009 Keywords: Chemical vapor deposition ZnO nanostructures X-ray photoelectron spectroscopy Chemisorbed oxygen
a b s t r a c t High-density ZnO nanorods with quadrangular and hexagonal cross sections were synthesized by chemical vapor deposition combining with a sol–gel method. It was found that the different morphologies could be obtained by changing the growth temperature. The growth mechanism was presented. The photoluminescence (PL) spectra demonstrated that both the quadrangular and hexagonal cross sections ZnO nanorods had a weak ultraviolet (UV) emission but a very strong and wide green emission. Interestingly, the quadrangular ZnO nanorods showed a stronger green emission than the hexagonal ones. Our X-ray photoelectron spectroscopy (XPS) results indicated that the quadrangular ZnO nanorods had a higher oxygen adsorption ability, and which should be the primary reason for its stronger green emission. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, much effort has been devoted to fabricating quasi one-dimensional (Q1D) nanoscale semiconductor materials due to their potential applications in nanodevices and complicated nanocircuits [1–3]. Being a wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), ZnO is recognized as a promising material for short-wavelength optoelectronic applications due to its superior optical and electronic properties [3–7]. Various ZnO nanostructures, including nanobelts [1], nanorods [2], nanowires [4], nanotips [5] nanonails [6], and nanohelices [7] have been developed based on the vapor–solid (VS) and vapor–liquid–solid (VLS) growth process [8]. As we know, the physical properties are expected to have a close correlation with their size and shape in the field of nanomaterials [9–11]. So it is technically important to fabricate nanomaterials with controllable morphologies and structures for deep understanding the influences of these factors on their physical properties. Up to now, many nanostructures with different cross sections have been reported. For example, a rectangular-like cross section ZnO nanobelts [1] and triangular-like cross section ZnO nanorods [12] have been reported by Wang’s group, and the similar structured ZnO has also been reported by other groups using different growth methods [13–15]. However, few reports could be found using the versatile chemical vapor-phase deposition method to synthesize well-crystallized ZnO nanorods with quadrangular (square
∗ Corresponding author. Tel.: +86 378 3881358; fax: +86 378 3881358. E-mail addresses: [email protected], [email protected] (Z. Du). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.02.028
or rhombic) cross section until now. Here, we prepare the different cross section ZnO nanostructures by changing the growth temperature. Specifically, the hexagonal and quadrangular ZnO nanorods are obtained at 850 and 950 ◦ C respectively. The photoluminescence spectra exhibit a different green emission for the hexagonal and quadrangular structured ZnO, indicating a close correlation between the physical properties and the different cross sections they owned. To understand how the cross section impact on their PL properties and to explore their potential applications, the hexagonal and quadrangular structured ZnO nanorods have been characterized using XPS. More adsorbed oxygen has been found on the surface of quadrangular ZnO nanorods due to the special crystal planes it owned, and which should be the primary reason for its stronger green emission. And we think that the novel quadrangular ZnO nanorods not only provided valuable modes in understanding crystal growth mechanisms in nanometer scale, but also exhibited high potential for fabricating novel nanoelectronic and optical devices with enhanced performance. 2. Experimental procedure The procedure for the synthesis of ZnO nanorods was as follows: firstly, 2.97 g Zn(NO3 )2 ·6H2 O was dissolved in 100 ml deionized water (10 M cm). After this, 5.763 g citric acid and 6.6 ml ethylene glycol were added in the solution with stirring at 80 ◦ C (±5 ◦ C) for 1 h to form a sol. Secondly, the sol was polymerized into a gel at 150 ◦ C (±5 ◦ C) for 5 h and subsequently pre-pyrolyzed at 400 ◦ C (±10 ◦ C) for 1 h to become an amorphous composite precursor [16]. The resulting amorphous precursor was ground and loaded in a ceramic boat, and the ceramic boat was put into a one-end-sealed quartz tube. A silicon wafer coated with Au in a thickness of 100 nm was placed about 5 cm downstream from the ceramic boat used as a substrate to collect the products. The Au film on silicon wafer was coated using an Eiko IB-3 ion coater for 20 min operated at a vacuum of 15 Pa
800
J. Zhong et al. / Materials Chemistry and Physics 115 (2009) 799–803
3. Results and discussion
Fig. 1. XRD patterns of ZnO nanorods with quadrangular and hexagonal cross sections.
and current of 3 mA. At last, the furnace was heated up to 950 ◦ C and kept at that temperature for 60 min with N2 as a carrier gas at a flow rate of 200 sccm. Then the furnace was naturally cooled down to room temperature. Q1D ZnO nanorods with quadrangular (square and rhombic) cross sections were obtained on the substrate. For hexagonal structured ZnO nanorods, the synthesis procedure was the same as the quadrangular ones except the growth temperature lower to 850 ◦ C. The crystallinity of the products was characterized by X-ray diffraction (XRD) with X’pert MRD-Philips diffractometer equipment with Cu K␣ radiation. The morphologies were imaged using JSM-5600 scanning electron microscopy (SEM). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) (JEOL-2010, at 200 kV, Japan) were also performed to characterize the ZnO nanorods. XPS (Axis Ultra, Shimadzu-Kratos, England) and the photoluminescence (PL, SPEX F212, SPEX Co. America) spectra were performed to obtain the surface information and optical properties of our synthesized ZnO nanorods.
XRD measurements [Fig. 1(a) and (b)] show that the assynthesized quadrangular and hexagonal ZnO nanorods are consistent with the standard of bulk ZnO (JCPDS Card File, No. 790207), which could be indexed to the wurtzite structure crystalline ZnO. The lattice parameter c calculated from the peak (0 0 2) is about 0.521 nm. The SEM images of quadrangular (square and rhombic) ZnO nanorods synthesized at 950 ◦ C are shown in Fig. 2. From the SEM images we can see that the substrate was coated with large quantities of ZnO nanorods, which have a square or rhombic cross section [inset Fig. 2(a)]. The square cross section can be observed clearly as shown in Fig. 2(b). These quadrangular nanorods have a nonuniform width of about several hundred nanometers and length up to several microns. Typical TEM image of the quadrangular ZnO nanorods is shown in Fig. 2(c). Based on HRTEM image and SAED pattern shown in Fig. 2(d), we can see that the quadrangular ZnO nanorods prefer to grow along [0 0 1] direction. And we can observe a singlecrystalline wurtzite structure and a 0.52 nm distance between the neighboring (0 0 1) planes. The high-density aligned hexagonal ZnO nanorods have also been synthesized at 850 ◦ C successfully. As shown in Fig. 3(a) and (b), the ZnO nanorods have a mean diameter of ∼100 nm and have a length about tens of microns. The regular prismatic hexagonal ZnO nanorods can be observed, as shown in the inset of Fig. 3(a). Furthermore, the hexagonal ZnO nanorods were well aligned vertically with a uniformity in their length and density. The TEM image shown in Fig. 3(c) represents a general morphology of the hexagonal ZnO nanorods. And the HRTEM image shown in Fig. 3(d) indicates that the hexagonal ZnO nanorod has a perfect wurtzite structure. The inset in Fig. 3(c) shows the corresponding SAED pattern, which further confirms our synthesized ZnO nanorod is single-crystalline and grows along caxis.
Fig. 2. SEM images of the quadrangular ZnO nanorods with (a) low and (b) high magnification, and the insets show top-end morphology. (c) A typical TEM image. (d) A HRTEM image with the corresponding SAED patterns.
J. Zhong et al. / Materials Chemistry and Physics 115 (2009) 799–803
801
Fig. 3. SEM images of the hexagonal ZnO nanorods (a) top view and (b) side view. (c) A typical TEM image of hexagonal ZnO nanorods and an inset for counterpart SAED patterns. (d) A corresponding HRTEM image.
The growth mechanism is presented as follows: the gel is pre-pyrolyzed at 400 ◦ C which makes it become an amorphous composite precursor. And this composite precursor provides the carbon and zinc source for the next reaction process. When the horizontal tube furnace is heated up to 850 ◦ C, the following reactions may occur [17]: ZnO(s) + C(s) = Zn(g) + CO(g)
(1)
ZnO(s) + CO(g) = Zn(g) + CO2 (g)
(2)
ZnO(s) + CO(g) → ZnOx (x < 1) + CO2 (g)
(3)
Subsequently, parts of Zn vapor carried by the N2 are transferred to the substrate region (about 500 ◦ C). Then, the reactions take place as follows: 2Zn(l) + O2 (g) = 2ZnO(s)
(4)
Briefly, the source amorphous Zn–C–O composite precursor is reduced into Zn or its sub-oxide (ZnOx , x < 1) by carbon, which has a lower melting point at about 420 ◦ C. Parts of Zn or ZnOx in vapor will be transferred to the substrate and condensed into liquid nanodroplets. When the liquid droplets become supersaturated, Zn particles precipitate and react with oxygen forming ZnO nuclei on the substrate which serve as the seeds for the ZnO nanostructure growth. Due to ZnO prefer to grow along [0 0 1] direction, this will lead to a rod-like structure finally. Usually, there are two mechanisms for the growth of 1D ZnO nanostructure materials: vapor–solid mechanism (without metal catalyst) and vapor–liquid–solid mechanism (with metal catalyst) [8]. In our case, the deposited Au film on the silicon substrate is used as catalyst, so our growth process is most likely the well-known VLS mechanism. However, it is notable here that the ZnO nanorods with different cross sections are obtained at almost the same conditions except the different growth temperatures. It has been reported that the morphologies of products can be tuned by changing the gas-phase supersaturation during the growth process [12,18]. In our case,
when the Zn or ZnOx vapor is advected by the carrier gas downstream to the substrate, a local supersaturation will be established. As we know, the supersaturation is strongly related to the growth temperature. So the different cross sections of ZnO nanorods we obtained may be due to the different growth temperature [12,19]. On the other hand, the growth rate in different crystal planes is also related to the zinc and oxygen vapor concentration ratio [19]. And the crystal plane with a faster growth velocity was easier to disappear, whereas the plane with a slower growth velocity is prone to be remained [18]. In a word, the difference of gas-phase supersaturation in different growth temperature and the different growth rates in different crystal planes is the primary reason for the formation of the ZnO nanorods with different cross sections. The optical properties of as-synthesized two kinds of ZnO nanorods were investigated here. Generally, there are always two emission bands for ZnO which center at UV range and visible range
Fig. 4. PL spectra of ZnO nanorods with quadrangular and hexagonal cross sections.
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Fig. 5. High-resolution XPS spectra in O 1s region (a and b) and Zn 2p region (c and d).
respectively. The UV emission peak is the intrinsical near band gap emission which can be attributed to the recombination of free excitons [20]. However, the origin of the visible emission is controversial and several mechanisms have been proposed. Commonly, the origin of visible emission of ZnO can be ascribed to impurities (mainly including adsorbed oxygen, hydroxide, moisture, etc) and various intrinsic defects such as the singly ionized oxygen vacancies and interstitial zinc atoms [21–23]. In our case, two separate emission bands are also observed for both the quadrangular and the hexagonal structure ZnO as shown in Fig. 4. They both exhibit a broad and strong green emission peak centered at 516 nm. Whereas, the narrow weaker UV emission peaks center at 385 nm for hexagonal nanorods and 390 nm for quadrangular nanorods respectively. We think that the 5 nm red shift cannot be attributed to the quantum confinement effect, because the size of the ZnO nanorods is far larger than the Bohr radius of ZnO (2.34 nm) [8]. This red shift in the UV emissions may be related to the formation of different shallow levels inside the band gap due to the presence of defects [24]. In order to investigate the properties of defects in the two kinds of ZnO nanostructures, XPS measurement was carried out. The deconvoluted XPS spectra of O 1s of hexagonal structure and quadrangular structure ZnO nanorods are shown in Fig. 5(a) and (b) respectively [25]. We can see that the photoelectron spectra of O 1s could be fitted as two peaks centered at 530.3 and 532.0 eV. The low binding energy component centered at 530.3 eV could be attributed
to O2− oxidation state bound with Zn in the crystal lattice. While the high binding energy component centered at 532.0 eV is corresponding to the presence of loosely bound chemisorbed oxygen [25,26]. From our XPS result, it is easy to see that the relative intensity ratio (peak at 532.0 eV/peak at 530.3 eV) of quadrangular ZnO is higher than that of hexagonal ZnO, indicating more chemisorbed oxygen existed on the surface of quadrangular ZnO. Generally, the larger the surface, the more oxygen will be adsorbed. But in our case, the surface area of the hexagonal ZnO is obviously higher than the quadrangular ZnO due to their smaller nanorod diameter as shown in Figs. 2 and 3. So the different oxygen adsorption ability should not be ascribed to the surface area difference. It has been reported that the oxygen adsorption ability has a correlation with the crystal planes [27,28]. Then we think that the different oxygen adsorption ability of quadrangular ZnO and hexagonal ZnO is due to the different planes they owned. That is, there is more adsorbed oxygen on the surface of quadrangular ZnO nanorods due to the special crystal planes it owned. It is just the more adsorbed oxygen resulted the stronger green emission observed in photoluminescence. At the same time, the binding energy of Zn 2p for two structures is also somewhat different. The Zn 2p3/2 and Zn 2p1/2 peaks of hexagonal ZnO nanorods locate at 1021.5 and 1044.7 eV (shown in Fig. 5c) and those values of the quadrangular ZnO nanorods shift to 1021.8 and 1045.0 eV respectively. There is a 0.3 eV difference between them. As the XPS measurements of two samples are simultaneous, the
J. Zhong et al. / Materials Chemistry and Physics 115 (2009) 799–803
carbon contaminated condition is almost the same. The 0.3 eV difference is significant and indicate that Zn is in the formal different Zn2+ valence state within an oxygen-deficient environment [29]. 4. Conclusion In summary, high-density quadrangular and aligned hexagonal ZnO nanorods were synthesized on Au-coated silicon substrate by sol–gel method combining with chemical vapor deposition. Comprehensive structural investigations including XRD, SAED, and HRTEM showed that both the quadrangular and hexagonal ZnO nanorods are pure wurtzite structure. More adsorbed oxygen was found on the surface of quadrangular ZnO nanorods through our XPS results, which is the primary reason for its stronger green emission. Acknowledgement This work was supported by the Natural Science Foundation of China (Nos. 90306010 and 10874040), State Key Basic Research “973” Plan of China (No. 2007CB616911), and the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No. 708062) References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] J.Y. Park, D.E. Song, S.S. Kim, Nanotechnology 19 (2008) 105503.
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