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Electrodeposition from ZnO nano-rods to nano-sheets with only zinc nitrate electrolyte and its photoluminescence
Applied Surface Science 257 (2011) 10317–10321
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electrodeposition from ZnO nano-rods to nano-sheets with only zinc nitrate electrolyte and its photoluminescence Bai Xue, Yi Liang, Liu Donglai, Nie Eryong, Sun Congli, Feng Huanhuan, Xu jingjing, Jin Yong, Jiao Zhifeng, Sun Xiaosong ∗ College of Materials Science and Engineering, Sichuan University, Wangjiang Road No. 29 Chengdu 610064, Sichuan, PR China
a r t i c l e
i n f o
Article history: Received 11 January 2011 Received in revised form 21 April 2011 Accepted 10 May 2011 Available online 13 June 2011 Keywords: ZnO Electrochemical deposition Morphology evolution Growth mechanism Photoluminescence
a b s t r a c t This very paper focuses on the synthesis of ZnO nano-structures by means of electro-chemical-deposition process. The crystalline structure and morphologies of the prepared ZnO were characterized with X-ray diffraction and scanning electronic microscopy, respectively. It is found that in case of low Zn(NO3 )2 ·6H2 O electrolyte concentration the fast growth mode in the c-axis direction leaded to the formation of 1D nanostructure of ZnO. On the other hand, at high concentration, this fast growth mode was restricted because the absorbed NO3 − on (0 0 0 1) plane would bond with Zn2+ ions, which, therefore, resulted in the formation of 2D nanostructure of ZnO. Room temperature photoluminescence performances of different ZnO structures were also investigated. A blue shift of 15 nm for ZnO nano-sheets has been found as the shapes of ZnO evolved from nano-rods to nano-sheets. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO), a wide band-gap (3.37 eV) and large excitation binding energy (60 meV) material, is a promising II–VI compound semiconductor. Due to its unique optical and electrical properties, a great deal of researches have been conducted, which focuses on the potential applications such as sensors [1,2], photo-catalysts [3], dye-sensitized solar cells (DSSCs) [4], light-emitting diodes and laser diodes in the UV–vis range [5–7], etc. Hence, so far, many kinds of ZnO nano-structures, particularly, one-dimensional (1D) nano-wires or nano-rods, and two-dimensional (2D) nano-sheets or nano-plates have been synthesized by the technique processes including vapor–liquid–solid (VLS) [8], chemical vapor deposition (CVD) [9], electron beam evaporation (EBE) [10], spray pyrolysis (SP) [11], hydrothermal method [12], and electrochemical deposition (ECD) [13–19]. In comparison with the VLS, CVD, EBE, and SP methods that require high temperature, rigorous condition and as well as complex operation, from the view point of technology, ECD process presents much more cheaper experimental setups, a rapid and cost-effective method for preparation of large-area ZnO nano-structures with high orientation degree, in terms of its lowtemperature processing, arbitrary substrate shapes, and precise control of film thickness [20].
Recently, it has been found that the evolution from 1D to 2D ZnO nano-structures can be manipulated by introducing different capping agents in the electrolyte [21–23] during the ECD process. In fact, as we know, ZnO is one kind of polar crystal, whose polar (0 0 0 1) plane with higher surface energy leads to the fast crystal growth rate along c-axis, 0 0 0 1, and, thus, preferentially results in the formation of 1D ZnO nanostructures [24]. On the other hand, the presence of capping agents can evolve the morphology of ZnO nanostructures from nano-rod to nano-sheet. For example, appropriate Cl− capping agents tend to preferentially adsorb onto the (0 0 0 1) planes, the positively charged planes, therefore, to hinder the crystal growth along the c-axis of ZnO, resulting in the formation of platelet-like crystals [25]. However, to our best knowledge, there is little investigation on the evolution of ZnO nanostructures from nano-rods to nano-sheets without additional electrolyte. In this paper, we report the attempt of growing two different ZnO nanostructures, nano-rods and nano-sheets by ECD process. It will be found out that ZnO nano-sheets can be evolved from ZnO nano-rods on TCO glass substrates by using ECD process in the sole Zn(NO3 )2 ·6H2 O electrolyte without any anion capping agents used. An electrolyte concentration-induced growth mechanism has been discussed, hereby. In addition, the room temperature photoluminescence (PL) performance of ZnO nanostructures are investigated.
2. Experimental ∗ Corresponding author. Tel.: +86 028 85471569; fax: +86 028 85416050. E-mail address: [email protected] (S. Xiaosong). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.132
The ECD process was performed with an electrochemical analytical instrument (LK9805) that involves a three-electrode
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Fig. 1. Plan-view and cross-sectional (insets) SEM images of the ZnO nanostructures obtained at different concentration of Zn(NO3 )2 ·6H2 O: 0.005 M (a), 0.01 M (b), 0.05 M (c), and 0.1 M (d), respectively.
electrochemical cell. A platinum foil and a standard calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. The working electrode was the TCO glass substrates, which were sequentially cleaned in an ultrasonic bath of acetone, ethanol and then deionized water for 10 min in each step. Deionized water was used as the solvent for the Zn(NO3 )2 aqueous solution. The ZnO nano structures were then electrodeposited onto the TCO glass substrates from the prepared aqueous solution of Zn(NO3 )2 ·6H2 O for 30 min at 80 ◦ C in a water bath for further characterizations. The characterizations on the crystalline structure and morphologies of the ZnO prepared were performed by X-ray diffraction ˚ and scanning (XRD, DX1000, Cu K␣ radiation, K␣ = 1.54178 A) electron microscopy (SEM, JEOL JSM-5900LV), respectively. The photoluminescence spectrum of the ZnO prepared was measured at room temperature with 325 nm excitation by a fluorescence spectrophotometer (F-7000, Hitachi) with Xe lamp as the excitation source. 3. Results and discussion The reactions that corresponds to the ECD process of growing ZnO from Zn(NO3 )2 aqueous solution is supposed to be listed as bellow: Zn(NO3 )2 → Zn2+ + 2NO3 − −
−
NO3 + H2 O + 2e → NO2 + 2OH
(1) −
(2)
−
(3)
Zn(OH)2 → ZnO + H2 O
(4)
2+
Zn
+ 2OH → Zn(OH)2
and NO3 − ions in the deionized water (Eq. (1)). Then, the cathode reduction of nitrate ions liberates NO2 − ions and OH− ions in the vicinity of the First, the zinc nitrate salt dissolves and turns into Zn2+
cathode (Eq. (2)). Later, Zn2+ ions and OH− ions combine to form zinc hydroxide (Eq. (3)), in terms of hydroxylation reaction. Finally, zinc hydroxide spontaneously dehydrates into ZnO and H2 O (Eq. (4)) if the solution temperature is higher than 34 ◦ C [26].
3.1. The effect of electrolyte concentration on the morphology of ZnO nanostructures Fig. 1 shows the plan-view and cross-sectional (insets) SEM images of ZnO nanostructures prepared at different concentrations of Zn(NO3 )2 ·6H2 O from 0.005 M to 0.1 M while keeping the applied potential (−1 V vs. SCE), temperature (80 ◦ C), and deposition time (1800s) constant. It can be seen that randomly oriented nano-rods with 100–200 nm in diameter were grown on the TCO glass substrate with 0.005 M Zn(NO3 )2 ·6H2 O (Fig. 1a). At the 0.01 M Zn(NO3 )2 ·6H2 O, the diameter of nano-rods turned to be 150–250 nm and were mostly aligned on the TCO glass substrate (Fig. 1b). An even higher Zn(NO3 )2 ·6H2 O electrolyte concentration of 0.05 M resulted in vertically aligned ZnO nano-rods with well-defined hexagonal flat tops. Meanwhile, it is worthy noting that these rods were 400 nm in mean diameter and some of them appeared to merge with one another (Fig. 1c). Further increasing the Zn(NO3 )2 ·6H2 O electrolyte concentration to 0.1 M resulted in the growth of ZnO nano-sheets of 50–100 nm in thickness on the TCO glass substrate (Fig. 1d). The cross-sectional SEM images (insets) in Fig. 1d shows that these sheets were nearly vertical to the substrate, which is similar to the earlier report [27]. The XRD patterns of ZnO nanostructures prepared at various Zn(NO3 )2 ·6H2 O electrolyte concentration from 0.005 M and 0.1 M are illustrated in Fig. 2. Apart from the reflections corresponding to the TCO glass substrate, marked with *, the diffraction peaks can be indexed to the wurtzite hexagonal ZnO (JCPDS 36-1451). Three weak diffraction peaks corresponding to ZnO (1 0 0), (2 0 0)
B. Xue et al. / Applied Surface Science 257 (2011) 10317–10321
Fig. 2. XRD pattern of the ZnO nanostructures obtained at different concentration of Zn(NO3 )2 ·6H2 O: 0.005 M (a), 0.01 M (b), 0.05 M (c), 0.1 M (d), respectively. The diffraction peaks marked with * corresponds to the TCO-glass substrates.
and (1 0 1) planes, respectively, can be indexed in the XRD spectrum in Fig. 2a. This is in good agreement with the SEM image of randomly oriented ZnO nano-rods shown in Fig. 1a. With increasing Zn(NO3 )2 ·6H2 O electrolyte concentration from 0.005 to 0.05 M, the intensity of the (0 0 2) peak increased significantly, which indicates that the ZnO nano-rods were grown along the c-axis and perpendicular to the substrate, which is also in good agreement with the SEM observations, as shown in Fig. 1b. However, in Fig. 2d, the relative intensity of the (0 0 2) peak is found to be slightly decreased for the ZnO nano-sheets with respect to Fig. 2c, suggesting the evolution of the morphology from nano-rods to nano-sheets, as shown in Fig. 1d.
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ZnO has a wurtzite structure and the stacking sequences of Zn2+ and O2− make it one kind of polar crystal. The oppositely charged ions produce positively Zn2+ -terminated (0 0 0 1) and neg¯ polar surfaces, respectively, which atively O2− -terminated (0 0 0 1) produce a net induced dipole moment along its c-axis. Hence, it can be inferred that the electrostatic force will attract negative ions in the solution onto the positive polar face of (0 0 0 1) that allows the anisotropic growth of the crystal along the c-axis. When the Zn(NO3 )2 ·6H2 O electrolyte concentration is low, the OH− ions adsorb on the (0 0 0 1) face and combine with Zn2+ ions, then immediately convert Zn(OH)2 into ZnO. This fast growth along the c-axis leads the formation of 1D nanostructure of ZnO, as shown in Fig. 1a. At the higher Zn(NO3 )2 ·6H2 O electrolyte concentration, the hydroxylation occurs at a much faster rate on the electrode surface before complete dehydration to ZnO. It has been reported that this will lead to OH− ligand terminals occurring on the (0 0 0 1) surface, which will prevent the new OH− ions from incorporating effectively into the as-formed ZnO nano-crystallites along the c-axis direction [28]. Thus, the crystal growth along the c-axis is partially suppressed and hence the other direction growths are partially enhanced, leading to form 2D nanostructure of ZnO, as shown in Fig. 1d.
3.2. The effect of nitrate ions concentration on the morphology of ZnO nano rods Previous studies [29] have found that a higher Zn2+ concentration leads to an increase of rod diameter of ZnO. To study the effect of nitrate ions concentration ([NO3 − ]) on the electrodeposition of ZnO nanostructure, we performed a ECD procedure with 0.005 M Zn(NO3 )2 ·6H2 O solution and spontaneously controlling NO3 − concentrations by adding KNO3 to make the nitrate ions concentration ([NO3 − ]) ranging from 0.005 M to 0.1 M while keeping other electrochemical parameters constant ([Zn2+ ] = 0.005 M,
Fig. 3. SEM images of the ZnO nanostructures obtained at different concentration of nitrate ions [NO3 − ]: 0.005 M (a), 0.01 M (b), 0.05 M (c), 0.1 M (d), at [Zn2+ ] = 0.005 M.
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Fig. 4. SEM images of the ZnO nanostructures electrodeposited under different potentials: −0.8 V (a), −1.0 V (b), −1.2 V (c), and the current density–time curves for electrodeposition at different potentials (d).
V = −1 V vs. SCE, T = 80 ◦ C, Time = 1800s). Fig. 3 shows the approximate diameters of the nano-rods are 150 nm at 0.005 M (Fig. 3a), 250 nm at 0.01 M (Fig. 3b), 300 nm at 0.05 M (Fig. 3c), and 350 nm at 0.1 M (Fig. 3d), respectively. It can be deduced, therefore, that the increase of nitrate ions concentration can provide more OH− ions that will lead an increase of the diameter of the nano-rods, as shown in Eq. (2). This validates the surplus OH− ions preferably adsorb on the (0 0 0 1) planes of ZnO, which prevents the new OH− ions from incorporating effectively into the as-formed ZnO nano-crystallites along the c-axis direction. 3.3. The effect of electrodeposition potential on the morphology of ZnO nano-rods
325 nm in the wavelength range from 350 nm to 800 nm, respectively. Both spectra present an ultraviolet (UV) peak, say 398 nm for Fig. 5a and 384 nm for Fig. 5b, which corresponds to the near band edge emission (NBE) procedure, but with different relative intensity and exact peak position. The similar results of UV emission at 384 nm [30] and 398 nm [21,31] have been reported for ZnO bundles and films. In general, ZnO can emit three luminescence bands in the UV, green, and as well as yellow regions. The UV emission is due to the direct recombination of photo-generated charge carriers (exciton emission), whose blue shift might happen because of the quantum confine effect or the native defect and free carrier concentrations in different samples [29]. There-
The reduction of zinc nitrate also plays an important role in the ECD process of ZnO growth in our experiments as shown in Eq. (2). To investigate the effect of the potential on the morphology of ZnO nano-rods, we prepared the ZnO nano-rods by controlling the applied potential from −0.8 to −1.2 vs. SCE, while keeping the other electrochemical parameters constant ([Zn(NO3 )2 ]) = 0.01 M, T = 80 ◦ C, Time = 1800s). Fig. 4a–c indicates that the diameter of the nano-rods increase as the deposition potential becomes more negative with respect to SCE. The current density–time curves for electrodeposition at different potentials are illustrated in Fig. 4d, which clearly presents that the current density significantly increased as the deposition potential change from −0.8 to −1.2 V, indicating that much more OH− ions were produced as shown in Eq. (2). And this would leads to the increase of diameter of ZnO nano-rods as we have discussed before. 3.4. Photoluminescence properties Fig. 5a and b illustrates the room temperature PL spectra of prepared ZnO nano-rods and nano-sheets under an excitation of
Fig. 5. Room temperature PL spectra of ZnO nano-rods (a) and that of nano-sheets (b).
B. Xue et al. / Applied Surface Science 257 (2011) 10317–10321
fore, the detailed explanation on the blue shift from 398 nm to 384 nm as the shapes evolve from nano-rods to nano-sheets needs additional PL investigation at low temperature. [32] A blue emission peak centered at 466 nm is also observed for both spectra. It is reported that the visible emission generally nominated as deep level emission (DLE) is probably related to the variation of the intrinsic defects in ZnO, such as zinc vacancy (VZn ), oxygen vacancy (VO ), interstitial zinc (Zni ), interstitial oxygen (Oi ), and antisite oxygen (OZn ) [33]. 4. Conclusions In conclusion, the morphology evolution from nano-rods to nano-sheets was achieved by increasing Zn(NO3 )2 ·6H2 O electrolyte concentration without additional capping agents. When Zn(NO3 )2 ·6H2 O electrolyte concentration is low, the OH− ions, adsorbing onto the (0 0 0 1) face, combine with Zn2+ ions and then immediately convert Zn(OH)2 into ZnO. This fast growth mode along the c-axis leads to the formation of 1D nanostructure of ZnO. With increasing Zn(NO3 )2 ·6H2 O electrolyte concentration, the hydroxylation occurs at a much faster rate on the electrode surface before complete dehydration to ZnO. The surplus OH− ions preferably adsorb onto the (0 0 0 1) planes of ZnO, which prevents the new OH− ions from incorporating effectively into the as-formed ZnO nano-crystallites along the c-axis direction. Therefore, the crystal growth along the c-axis is partially suppressed and the other direction growths are, therefore, enhanced, which leads to form 2D nanostructure of ZnO. It is found that the diameter of ZnO nano-rod was also controlled by the Zn2+ concentration. Room temperature PL spectra indicate that the PL of ZnO nanosheets had blue-shifted of 15 nm with respect to that of ZnO nano-rods. The present work presents a simple and cost-effective method to deposit ZnO nanostructure thin film on TCO glass substrate. Acknowledgement The SEM images are taken at the analytical and testing center by Hui Wang, Sichuan University.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electrodeposition from ZnO nano-rods to nano-sheets with only zinc nitrate electrolyte and its photoluminescence Bai Xue, Yi Liang, Liu Donglai, Nie Eryong, Sun Congli, Feng Huanhuan, Xu jingjing, Jin Yong, Jiao Zhifeng, Sun Xiaosong ∗ College of Materials Science and Engineering, Sichuan University, Wangjiang Road No. 29 Chengdu 610064, Sichuan, PR China
a r t i c l e
i n f o
Article history: Received 11 January 2011 Received in revised form 21 April 2011 Accepted 10 May 2011 Available online 13 June 2011 Keywords: ZnO Electrochemical deposition Morphology evolution Growth mechanism Photoluminescence
a b s t r a c t This very paper focuses on the synthesis of ZnO nano-structures by means of electro-chemical-deposition process. The crystalline structure and morphologies of the prepared ZnO were characterized with X-ray diffraction and scanning electronic microscopy, respectively. It is found that in case of low Zn(NO3 )2 ·6H2 O electrolyte concentration the fast growth mode in the c-axis direction leaded to the formation of 1D nanostructure of ZnO. On the other hand, at high concentration, this fast growth mode was restricted because the absorbed NO3 − on (0 0 0 1) plane would bond with Zn2+ ions, which, therefore, resulted in the formation of 2D nanostructure of ZnO. Room temperature photoluminescence performances of different ZnO structures were also investigated. A blue shift of 15 nm for ZnO nano-sheets has been found as the shapes of ZnO evolved from nano-rods to nano-sheets. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO), a wide band-gap (3.37 eV) and large excitation binding energy (60 meV) material, is a promising II–VI compound semiconductor. Due to its unique optical and electrical properties, a great deal of researches have been conducted, which focuses on the potential applications such as sensors [1,2], photo-catalysts [3], dye-sensitized solar cells (DSSCs) [4], light-emitting diodes and laser diodes in the UV–vis range [5–7], etc. Hence, so far, many kinds of ZnO nano-structures, particularly, one-dimensional (1D) nano-wires or nano-rods, and two-dimensional (2D) nano-sheets or nano-plates have been synthesized by the technique processes including vapor–liquid–solid (VLS) [8], chemical vapor deposition (CVD) [9], electron beam evaporation (EBE) [10], spray pyrolysis (SP) [11], hydrothermal method [12], and electrochemical deposition (ECD) [13–19]. In comparison with the VLS, CVD, EBE, and SP methods that require high temperature, rigorous condition and as well as complex operation, from the view point of technology, ECD process presents much more cheaper experimental setups, a rapid and cost-effective method for preparation of large-area ZnO nano-structures with high orientation degree, in terms of its lowtemperature processing, arbitrary substrate shapes, and precise control of film thickness [20].
Recently, it has been found that the evolution from 1D to 2D ZnO nano-structures can be manipulated by introducing different capping agents in the electrolyte [21–23] during the ECD process. In fact, as we know, ZnO is one kind of polar crystal, whose polar (0 0 0 1) plane with higher surface energy leads to the fast crystal growth rate along c-axis, 0 0 0 1, and, thus, preferentially results in the formation of 1D ZnO nanostructures [24]. On the other hand, the presence of capping agents can evolve the morphology of ZnO nanostructures from nano-rod to nano-sheet. For example, appropriate Cl− capping agents tend to preferentially adsorb onto the (0 0 0 1) planes, the positively charged planes, therefore, to hinder the crystal growth along the c-axis of ZnO, resulting in the formation of platelet-like crystals [25]. However, to our best knowledge, there is little investigation on the evolution of ZnO nanostructures from nano-rods to nano-sheets without additional electrolyte. In this paper, we report the attempt of growing two different ZnO nanostructures, nano-rods and nano-sheets by ECD process. It will be found out that ZnO nano-sheets can be evolved from ZnO nano-rods on TCO glass substrates by using ECD process in the sole Zn(NO3 )2 ·6H2 O electrolyte without any anion capping agents used. An electrolyte concentration-induced growth mechanism has been discussed, hereby. In addition, the room temperature photoluminescence (PL) performance of ZnO nanostructures are investigated.
2. Experimental ∗ Corresponding author. Tel.: +86 028 85471569; fax: +86 028 85416050. E-mail address: [email protected] (S. Xiaosong). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.05.132
The ECD process was performed with an electrochemical analytical instrument (LK9805) that involves a three-electrode
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B. Xue et al. / Applied Surface Science 257 (2011) 10317–10321
Fig. 1. Plan-view and cross-sectional (insets) SEM images of the ZnO nanostructures obtained at different concentration of Zn(NO3 )2 ·6H2 O: 0.005 M (a), 0.01 M (b), 0.05 M (c), and 0.1 M (d), respectively.
electrochemical cell. A platinum foil and a standard calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. The working electrode was the TCO glass substrates, which were sequentially cleaned in an ultrasonic bath of acetone, ethanol and then deionized water for 10 min in each step. Deionized water was used as the solvent for the Zn(NO3 )2 aqueous solution. The ZnO nano structures were then electrodeposited onto the TCO glass substrates from the prepared aqueous solution of Zn(NO3 )2 ·6H2 O for 30 min at 80 ◦ C in a water bath for further characterizations. The characterizations on the crystalline structure and morphologies of the ZnO prepared were performed by X-ray diffraction ˚ and scanning (XRD, DX1000, Cu K␣ radiation, K␣ = 1.54178 A) electron microscopy (SEM, JEOL JSM-5900LV), respectively. The photoluminescence spectrum of the ZnO prepared was measured at room temperature with 325 nm excitation by a fluorescence spectrophotometer (F-7000, Hitachi) with Xe lamp as the excitation source. 3. Results and discussion The reactions that corresponds to the ECD process of growing ZnO from Zn(NO3 )2 aqueous solution is supposed to be listed as bellow: Zn(NO3 )2 → Zn2+ + 2NO3 − −
−
NO3 + H2 O + 2e → NO2 + 2OH
(1) −
(2)
−
(3)
Zn(OH)2 → ZnO + H2 O
(4)
2+
Zn
+ 2OH → Zn(OH)2
and NO3 − ions in the deionized water (Eq. (1)). Then, the cathode reduction of nitrate ions liberates NO2 − ions and OH− ions in the vicinity of the First, the zinc nitrate salt dissolves and turns into Zn2+
cathode (Eq. (2)). Later, Zn2+ ions and OH− ions combine to form zinc hydroxide (Eq. (3)), in terms of hydroxylation reaction. Finally, zinc hydroxide spontaneously dehydrates into ZnO and H2 O (Eq. (4)) if the solution temperature is higher than 34 ◦ C [26].
3.1. The effect of electrolyte concentration on the morphology of ZnO nanostructures Fig. 1 shows the plan-view and cross-sectional (insets) SEM images of ZnO nanostructures prepared at different concentrations of Zn(NO3 )2 ·6H2 O from 0.005 M to 0.1 M while keeping the applied potential (−1 V vs. SCE), temperature (80 ◦ C), and deposition time (1800s) constant. It can be seen that randomly oriented nano-rods with 100–200 nm in diameter were grown on the TCO glass substrate with 0.005 M Zn(NO3 )2 ·6H2 O (Fig. 1a). At the 0.01 M Zn(NO3 )2 ·6H2 O, the diameter of nano-rods turned to be 150–250 nm and were mostly aligned on the TCO glass substrate (Fig. 1b). An even higher Zn(NO3 )2 ·6H2 O electrolyte concentration of 0.05 M resulted in vertically aligned ZnO nano-rods with well-defined hexagonal flat tops. Meanwhile, it is worthy noting that these rods were 400 nm in mean diameter and some of them appeared to merge with one another (Fig. 1c). Further increasing the Zn(NO3 )2 ·6H2 O electrolyte concentration to 0.1 M resulted in the growth of ZnO nano-sheets of 50–100 nm in thickness on the TCO glass substrate (Fig. 1d). The cross-sectional SEM images (insets) in Fig. 1d shows that these sheets were nearly vertical to the substrate, which is similar to the earlier report [27]. The XRD patterns of ZnO nanostructures prepared at various Zn(NO3 )2 ·6H2 O electrolyte concentration from 0.005 M and 0.1 M are illustrated in Fig. 2. Apart from the reflections corresponding to the TCO glass substrate, marked with *, the diffraction peaks can be indexed to the wurtzite hexagonal ZnO (JCPDS 36-1451). Three weak diffraction peaks corresponding to ZnO (1 0 0), (2 0 0)
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Fig. 2. XRD pattern of the ZnO nanostructures obtained at different concentration of Zn(NO3 )2 ·6H2 O: 0.005 M (a), 0.01 M (b), 0.05 M (c), 0.1 M (d), respectively. The diffraction peaks marked with * corresponds to the TCO-glass substrates.
and (1 0 1) planes, respectively, can be indexed in the XRD spectrum in Fig. 2a. This is in good agreement with the SEM image of randomly oriented ZnO nano-rods shown in Fig. 1a. With increasing Zn(NO3 )2 ·6H2 O electrolyte concentration from 0.005 to 0.05 M, the intensity of the (0 0 2) peak increased significantly, which indicates that the ZnO nano-rods were grown along the c-axis and perpendicular to the substrate, which is also in good agreement with the SEM observations, as shown in Fig. 1b. However, in Fig. 2d, the relative intensity of the (0 0 2) peak is found to be slightly decreased for the ZnO nano-sheets with respect to Fig. 2c, suggesting the evolution of the morphology from nano-rods to nano-sheets, as shown in Fig. 1d.
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ZnO has a wurtzite structure and the stacking sequences of Zn2+ and O2− make it one kind of polar crystal. The oppositely charged ions produce positively Zn2+ -terminated (0 0 0 1) and neg¯ polar surfaces, respectively, which atively O2− -terminated (0 0 0 1) produce a net induced dipole moment along its c-axis. Hence, it can be inferred that the electrostatic force will attract negative ions in the solution onto the positive polar face of (0 0 0 1) that allows the anisotropic growth of the crystal along the c-axis. When the Zn(NO3 )2 ·6H2 O electrolyte concentration is low, the OH− ions adsorb on the (0 0 0 1) face and combine with Zn2+ ions, then immediately convert Zn(OH)2 into ZnO. This fast growth along the c-axis leads the formation of 1D nanostructure of ZnO, as shown in Fig. 1a. At the higher Zn(NO3 )2 ·6H2 O electrolyte concentration, the hydroxylation occurs at a much faster rate on the electrode surface before complete dehydration to ZnO. It has been reported that this will lead to OH− ligand terminals occurring on the (0 0 0 1) surface, which will prevent the new OH− ions from incorporating effectively into the as-formed ZnO nano-crystallites along the c-axis direction [28]. Thus, the crystal growth along the c-axis is partially suppressed and hence the other direction growths are partially enhanced, leading to form 2D nanostructure of ZnO, as shown in Fig. 1d.
3.2. The effect of nitrate ions concentration on the morphology of ZnO nano rods Previous studies [29] have found that a higher Zn2+ concentration leads to an increase of rod diameter of ZnO. To study the effect of nitrate ions concentration ([NO3 − ]) on the electrodeposition of ZnO nanostructure, we performed a ECD procedure with 0.005 M Zn(NO3 )2 ·6H2 O solution and spontaneously controlling NO3 − concentrations by adding KNO3 to make the nitrate ions concentration ([NO3 − ]) ranging from 0.005 M to 0.1 M while keeping other electrochemical parameters constant ([Zn2+ ] = 0.005 M,
Fig. 3. SEM images of the ZnO nanostructures obtained at different concentration of nitrate ions [NO3 − ]: 0.005 M (a), 0.01 M (b), 0.05 M (c), 0.1 M (d), at [Zn2+ ] = 0.005 M.
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Fig. 4. SEM images of the ZnO nanostructures electrodeposited under different potentials: −0.8 V (a), −1.0 V (b), −1.2 V (c), and the current density–time curves for electrodeposition at different potentials (d).
V = −1 V vs. SCE, T = 80 ◦ C, Time = 1800s). Fig. 3 shows the approximate diameters of the nano-rods are 150 nm at 0.005 M (Fig. 3a), 250 nm at 0.01 M (Fig. 3b), 300 nm at 0.05 M (Fig. 3c), and 350 nm at 0.1 M (Fig. 3d), respectively. It can be deduced, therefore, that the increase of nitrate ions concentration can provide more OH− ions that will lead an increase of the diameter of the nano-rods, as shown in Eq. (2). This validates the surplus OH− ions preferably adsorb on the (0 0 0 1) planes of ZnO, which prevents the new OH− ions from incorporating effectively into the as-formed ZnO nano-crystallites along the c-axis direction. 3.3. The effect of electrodeposition potential on the morphology of ZnO nano-rods
325 nm in the wavelength range from 350 nm to 800 nm, respectively. Both spectra present an ultraviolet (UV) peak, say 398 nm for Fig. 5a and 384 nm for Fig. 5b, which corresponds to the near band edge emission (NBE) procedure, but with different relative intensity and exact peak position. The similar results of UV emission at 384 nm [30] and 398 nm [21,31] have been reported for ZnO bundles and films. In general, ZnO can emit three luminescence bands in the UV, green, and as well as yellow regions. The UV emission is due to the direct recombination of photo-generated charge carriers (exciton emission), whose blue shift might happen because of the quantum confine effect or the native defect and free carrier concentrations in different samples [29]. There-
The reduction of zinc nitrate also plays an important role in the ECD process of ZnO growth in our experiments as shown in Eq. (2). To investigate the effect of the potential on the morphology of ZnO nano-rods, we prepared the ZnO nano-rods by controlling the applied potential from −0.8 to −1.2 vs. SCE, while keeping the other electrochemical parameters constant ([Zn(NO3 )2 ]) = 0.01 M, T = 80 ◦ C, Time = 1800s). Fig. 4a–c indicates that the diameter of the nano-rods increase as the deposition potential becomes more negative with respect to SCE. The current density–time curves for electrodeposition at different potentials are illustrated in Fig. 4d, which clearly presents that the current density significantly increased as the deposition potential change from −0.8 to −1.2 V, indicating that much more OH− ions were produced as shown in Eq. (2). And this would leads to the increase of diameter of ZnO nano-rods as we have discussed before. 3.4. Photoluminescence properties Fig. 5a and b illustrates the room temperature PL spectra of prepared ZnO nano-rods and nano-sheets under an excitation of
Fig. 5. Room temperature PL spectra of ZnO nano-rods (a) and that of nano-sheets (b).
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fore, the detailed explanation on the blue shift from 398 nm to 384 nm as the shapes evolve from nano-rods to nano-sheets needs additional PL investigation at low temperature. [32] A blue emission peak centered at 466 nm is also observed for both spectra. It is reported that the visible emission generally nominated as deep level emission (DLE) is probably related to the variation of the intrinsic defects in ZnO, such as zinc vacancy (VZn ), oxygen vacancy (VO ), interstitial zinc (Zni ), interstitial oxygen (Oi ), and antisite oxygen (OZn ) [33]. 4. Conclusions In conclusion, the morphology evolution from nano-rods to nano-sheets was achieved by increasing Zn(NO3 )2 ·6H2 O electrolyte concentration without additional capping agents. When Zn(NO3 )2 ·6H2 O electrolyte concentration is low, the OH− ions, adsorbing onto the (0 0 0 1) face, combine with Zn2+ ions and then immediately convert Zn(OH)2 into ZnO. This fast growth mode along the c-axis leads to the formation of 1D nanostructure of ZnO. With increasing Zn(NO3 )2 ·6H2 O electrolyte concentration, the hydroxylation occurs at a much faster rate on the electrode surface before complete dehydration to ZnO. The surplus OH− ions preferably adsorb onto the (0 0 0 1) planes of ZnO, which prevents the new OH− ions from incorporating effectively into the as-formed ZnO nano-crystallites along the c-axis direction. Therefore, the crystal growth along the c-axis is partially suppressed and the other direction growths are, therefore, enhanced, which leads to form 2D nanostructure of ZnO. It is found that the diameter of ZnO nano-rod was also controlled by the Zn2+ concentration. Room temperature PL spectra indicate that the PL of ZnO nanosheets had blue-shifted of 15 nm with respect to that of ZnO nano-rods. The present work presents a simple and cost-effective method to deposit ZnO nanostructure thin film on TCO glass substrate. Acknowledgement The SEM images are taken at the analytical and testing center by Hui Wang, Sichuan University.
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