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Density-controlled growth and passivation of ZnO nanorod arrays by electrodeposition
Thin Solid Films 638 (2017) 426–432
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Density-controlled growth and passivation of ZnO nanorod arrays by electrodeposition Lida Guo, Yang Tang ⁎, Fu-Kuo Chiang, Linge Ma, Jie Chen Beijing Engineering Research Center of Nanostructured Thin Film Solar Cell, National Institute of Clean-and-Low-Carbon Energy, Future Science & Technology City, Changping District, Beijing, 102211, People's Republic of China
a r t i c l e
i n f o
Article history: Received 15 June 2016 Received in revised form 4 August 2017 Accepted 9 August 2017 Available online 09 August 2017 Keywords: Nanostructures Electrochemical techniques Crystal growth Photoluminescence spectroscopy
a b s t r a c t Here we established a simple electrochemical method of synthesizing ZnO nanomaterials with high quality and the ability to control the nanostructures' density For ZnO nanorod growth, a thin layer of Al doped ZnO (AZO) was deposited on glass substrates and ZnO nanorod arrays were grown on these substrates from aqueous solution containing Zn(NO3)2 and NH4NO3. It was found that the density, morphology, and quality of the nanorod arrays can be controlled by changing the applied potentials or the concentration of NH4NO3 in the solution as growing and etching were carried out simultaneously. Moreover, the nanorod density decrease resulted in an improvement of the quality of the as-grown nanorods. Scanning electron microscope (SEM), transmission electron microscopy (TEM), photoluminescence and X-ray diffraction (XRD) have been used to characterize the samples. The density of ZnO nanorods can be controlled by varying the concentrations of the precursor solutions or applied potentials. Passivated low-density ZnO nanorod arrays with high quality were fabricated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanomaterials have captured the attention of many researchers in recent years due to their good prospects in various technological areas [1,2]. ZnO is one of the most important semiconductor materials with a direct wide band gap (3.37 eV) and a large excitation binding energy (60 meV). It is nontoxic, cheap, stable, and environmentally friendly [3]. Since the ZnO nanorods exhibit fewer defects than its thin-film structure, it is, therefore, a promising material for photoelectronic devices, such as light-emitting diodes [4–7] and solar cells [8]. Among the various methods used for the fabrication of ZnO nanorods, the electrochemical deposition method is of particular interest since this technique enables fast fabrication and relatively low cost [3]. It also achieves easy control of both the morphology and the thickness of the as-grown ZnO. Also, high production efficiency can be achieved at low temperature with this technique, which requires inexpensive equipment. According to past research report, the density of the nanoarrays and the distance between the nanorods affected the photoelectric response and carrier transport characteristic of photoelectric devices based on semiconductor nanostructures [9]. Furthermore, the tailoring of nanostructures' density is necessary in some applications.
⁎ Corresponding author. E-mail address: [email protected] (Y. Tang).
http://dx.doi.org/10.1016/j.tsf.2017.08.015 0040-6090/© 2017 Elsevier B.V. All rights reserved.
For instance, in dye-sensitized solar cells, the ability of the dye to interpenetrate the semiconducting material is related to the nanorods density [10]. Also, for applications in copper indium gallium selenide (CIGS) solar cells, the nanostructures with a low density used as substrate is desired because the absorber material needs to be intercalated between the nanorods. Despite various ZnO nanostructures that have been electrochemically deposited, low-density ZnO nanorod arrays are difficult to prepare and this limits their applications in nanodevices. The technical problem is to fabricate ZnO nanorod arrays with controllable density and distance between the nanorods. In this paper, we report the preparation and characterization of ZnO nanorod arrays electrochemically deposited from an aqueous solution of zinc nitrate (Zn(NO3)2) and ammonium nitrate (NH4NO3) at a low temperature down to 75 °C. Specifically, we investigated the effects of varying the concentration of NH4NO3 in the precursor solution and the applied potential on the morphology, structure and optical properties of the obtained nanopillars.
2. Experimental details Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99.998%, Alfa) and ammonium nitrate (NH4NO3, 99%, J&K Scientific) were dissolved in water that had a resistivity of 18.2 MΩ·cm (prepared by a Millipore Milli-Q Advantage A10 system). The AZO-P002 glass substrates with thickness of 2 mm were purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd. The substrates are covered by AZO (Al ~ 2%) with thickness of approximate 800 nm and a sheet resistance below 10 Ω/□.
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The AZO substrate was firstly cleaned by acetone, ethanol and ultrapure water in a sonic bath. Then the ZnO nanorod arrays were electrodeposited on these substrates in an electrolyte aqueous solution of
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Zn(NO3)2 and NH4NO3. The solution was prepared at room temperature and then placed in a 75 °C water bath heated by a hot plate. A platinum sheet, a platinum wire and the AZO substrate served as the counter electrode, the reference electrode and the working electrode, respectively. During the growth process, a constant electric potential as referred to the reference electrode was applied to the substrate. The duration of the depositions was 900 s. Different concentrations of NH4NO3 (0– 205 mM) with an applied potential of − 1.32 V were used, while Zn(NO3)2 was kept at 5 mM in the source solution. The effect of the applied potential in the range of −1.10 V to −1.38 V was also tested for one growth solution. At the end of the growth, the AZO substrates covered with ZnO nanorods were taken out of the aqueous solution and rinsed in flowing deionized water immediately so as to remove any residual salt from the surface. Potentiostat by Ivium was used for the electrochemical deposition. pH value of the solution was measured by a Mettler S40K pH meter. The morphology and the crystal structure of the as-grown ZnO nanorods were investigated by a scanning electron microscope (SEM, FEI Nova NanaSEM 450) and transmission electron microscopy (TEM, JEOL ARM200F). The cross-sectional TEM sample for high resolution images was prepared by gluing the Si wafer with the thin film. The sample was cut and mechanically thinned by abrasive papers and dimpler. Then attached the sample on a Mo-ring and carefully milled the selected area by Ar ion beam for obtaining an ultra-thin region with the thickness of about 20–30 nm. In the final step, the surface amorphous layer was removed by low-voltage (0.1 kV) Ar ion beam for making sure the high quality TEM image to be obtained. The optical qualities of excitonic recombination in as-grown ZnO nanorods were analyzed by photoluminescence (PL) experiments under room temperature. The PL was excited by a He-Cd laser (325 nm line). It is generally assumed that a large UV to visible emission ratio suggests excellent quality of the ZnO nanorods, in other words, the defect density is low. 3. Results and discussions The reactions for ZnO nanorod deposition from Zn(NO3)2 and NH4NO3 precursors are as below [11]. An increase in the applied potential leads not only to a high vertical growth rate but also to a high lateral growth rate which has already been reported by Lee et al. [12]. ZnðNO3 Þ2 ⇆Zn2þ þ 2NO3 −
ð1Þ
NO3 − þ H2 O þ 2e− ⇆NO2 − þ 2OH−
ð2Þ
Zn2þ þ 2OH− ⇆ZnðOHÞ2
ð3Þ
ZnðOHÞ2 ⇆ZnO þ H2 O
ð4Þ
As reaction equilibrium moves to the right side, ZnO forms through dehydration of Zn(OH)2 and precipitate onto the substrate, resulting in the formation of ZnO nanostructures. Dehydration and precipitation reactions in the solutions (Eq. (3)) will continuously provide matter for the growth of ZnO nanorods. However, if the pH values of the
Table 1 pH value of solution as a function of NH4NO3 concentration.
Fig. 1. SEM top and cross-sectional images of as-grown ZnO nanorod arrays prepared with increasing concentrations of NH4NO3: (a, f) 0 mM, (b, g) 60 mM, (c, h) 100 mM, (d, i) 150 mM, (e, j) 205 mM. The concentration of Zn(NO3)2 was 5 mM and the applied potential was −1.32 V. Scale bar: 500 nm.
Sample
Zn(NO3)2 concentration (mM)
NH4NO3 concentration (mM)
Solution pH
1 2 3 4 5
5 5 5 5 5
0 60 100 150 205
5.533 4.560 4.545 4.500 4.496
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Table 2 Length, distance, diameter, density and PL ratio of ZnO nanostructures as a function of NH4NO3 Concentration. The concentration of Zn(NO3)2 was 5 mM and the applied potential was −1.32 V. Sample
Concentration of NH4NO3 (mM)
Thickness of AZO (nm)
Length (nm)
Distance (nm)
Diameter (nm)
Density (/cm2)
PL ratio
1 2 3 4 5
0 60 100 150 205
874 ± 10 859 ± 19 530 ± 40 536 ± 52 441 ± 50
80 ± 25 600 ± 46 318 ± 35 241 ± 33 208 ± 91
0±5 18 ± 17 38 ± 20 69 ± 38 97 ± 50
91 ± 37 191 ± 41 88 ± 24 81 ± 22 89 ± 30
9.2 × 109 2.0 × 109 6.0 × 109 5.6 × 109 3.1 × 109
6 2 19 20 22
solution changed, the above reversible reactions may be altered and shift in the opposite direction [14]. When the reactions occur in acidic solutions, H2O will slowly decompose to generate OH– (Eq. (2)). However, the hydrolytic processes (Eq. (4)) can still occur as Zn(OH)2 is easily hydrolysable [14]. In an acidic solution, ZnO is unstable, so the dissolving ratio and process can be different [13]. 3.1. Effect of NH4NO3 Fig. 1 shows SEM images of ZnO nanorods grown on AZO substrates. The concentrations of NH4NO3 were in the range from 0 mM to 205 mM. Image 1a–1e were taken perpendicular to the surface to record the distance between the nanopillars and the vertical alignment of the ZnO nanorods. With some tilting, images 1f–1j gave morphological dimensions of the nanorod arrays, namely the average diameter of the asgrown nanorods, as well as the thickness of AZO film as a function of the NH4NO3 concentration. The distances are from 0 to 97 nm, the diameters of the nanorods range from 60 nm to 191 nm, and the densities are in the range from 2 × 109 to 9.2 × 109/cm2. It is apparent in Table 2 that the distance between the nanorods increases with the concentration of NH4NO3. The approximate distances are 0 nm, 18 nm, 38 nm, 69 nm and 97 nm at NH4NO3 concentration of 0 mM, 60 mM, 100 mM, 150 mM and
Fig. 2. TEM images of the as-grown ZnO nanorods at 5 mM Zn(NO3)2 and 205 mM NH4NO3 with an applied potential of −1.32 V. (a) The overall morphology, (b) ZnO segments found aside the nanorods.
205 mM respectively. Normally, the current intensity increases with electrolyte concentration, causing the increase of deposition rate, which is defined as the amount of deposited material per unit time. Therefore, the ZnO nanorods increases in both diameter and length as NH4NO3 concentration increased from 0 to 60 mM (Table 2). However, the pH of the solution drops with increases of the concentration of NH4NO3 (as shown in Table 1), resulting in the corrosion of the ZnO nanorods. Although the mechanism is debatable [15–18], we noted that, by varying the pH of the reaction mixture, the rate of nucleation and growth events could be changed [15]. The reversible reactions are altered and shift in the opposite direction. Moreover, the etching and growing of ZnO/AZO are carried out simultaneously. Furthermore, as mentioned, the acidic solution can result in corrosion of ZnO, because ZnO is unstable under this condition. On the whole, NH4NO3 plays a role of accelerating the growth of ZnO nanorods, as long as the concentration of NH4NO3 is below 100 mM. As the NH4NO3 concentration is increased to over 100 mM, the pH continues to drop, and corrosion overwhelms precipitation, which leads to decreases of nanoarray
Fig. 3. High-resolution TEM images of the as-grown ZnO nanorods prepared with an applied potential of −1.32 V. (a) ZnO nanorods prepared with 5 mM Zn(NO3)2 and 205 mM NH4NO3, (b) ZnO nanorods prepared with 5 mM Zn(NO3)2.
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Fig. 4. Macro-viewing of one of the non-uniform ZnO nanorods at 5 mM Zn(NO3)2 and 205 mM NH4NO3 with an applied potential of −1.32 V. (a), (b) TEM morphology of the ZnO nanorod; (c), (d) HAADF images show the interest area that may lead to the observed ZnO segments.
density, nanorod diameter, thickness of AZO, on the other hand an increase in distance between nanorods (Table 2). The real-space images of the as-grown ZnO nanorods acquired by TEM may give a glance on the described etching-and-growth mechanism above. Fig. 2 exhibits the morphologies of the resultant as-grown ZnO nanorods on AZO after chemical reaction in 5 mM Zn(NO3)2 and 205 mM NH4NO3 solution with an applied potential of −1.32 V. In Fig. 2a, the interface between AZO and ZnO (cross-sectional view) characterizes the equilibrium boundary of etching and growth processes simultaneously, in which the thickness of the residual AZO layer is about 600 nm and ZnO achieves 100–400 nm in thickness (white dotline). It is consistent with the SEM image in Fig. 1j. Selected area electron
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diffraction (SAED) in the insert of Fig. 2a shows ZnO with the hexagonal structure with space group P63cm (No. 186), which is identical to that of AZO. It indicates the identical crystal structure is one of the essential ingredients of ZnO grown onto AZO which determines the optical properties. Except the as-grown ZnO nanorods, some crystalline fragments (white arrow in Fig. 2b) were found as well. They were not introduced by the TEM specimen preparation since they were all composed by Zn and O inspected by Electron Dispersive Spectroscopy (EDS) equipped in TEM. Most of them are well-crystallized and joint the as-grown nanorods with an extremely thin amorphous layer or apart from the mixing layer composed by polycrystalline ZnO and amorphous matters (Fig. 2b). Particularly, most of the surface of the nanostructures is coated by a ZnO layer (Fig. 3a) with the thickness of about 10 nm forming a special core-shell structure. They were coherent to the as-grown ZnO nanorods and well-deposited on the surface, but easy-damaged by electron bombardment into poly-crystalline structure. The etching rate of AZO was not uniform due to different crystal structures on the surface. Therefore, a rough and uneven surface was formed, and the raised part worked as a seed allowing ZnO to grown on. These images of microstructure imply that the simultaneous etching and growing processes not only effectively corroded the AZO substrate to give advantage to ZnO growth but also eroded the as-grown nanorods leading to partial collapse. Eventually, an external layer homogeneously covered the surfaces of nanorods as residuals. On the contrary, high-resolution TEM images (Fig. 3b) shows that no core-shell structure was found on the surface (dotted line) of the ZnO nanorod prepared at 5 mM Zn(NO3)2. Although it is not clear how the layer was formed, this difference on structure affects the optical quality of the nanorods to a great extent. High Angle Annual Dark Field (HAADF) images of the sample prepared under 5 mM Zn(NO3)2 and 205 mM NH4NO3 acquired by Scanning Transmission Electron Microscopy (STEM) could give a clear picture of what mentioned above. In Fig. 4, the real-space TEM images (Fig. 4a and b) show a ZnO nanorod re-growth on the AZO starting by a distinguishable interface (white arrow). Similar to most of the other as-grown nanorods, its morphology inclines to be non-uniform pillar which is consistent with SEM images indicating it was not artificially caused during the TEM specimen preparation. The corresponding HAADF images are shown in Fig. 4c and d, in which the image contrast indicates the structural variation inside the nanorod. Here the most
Fig. 5. Photoluminescence emission spectra of ZnO nanorods prepared with 5 mM Zn(NO3)2 and increasing concentrations of NH4NO3. The applied potential was −1.32 V. (a) normalized PL intensity, (b) non-normalized PL intensity.
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Fig. 6. XRD scans for the ZnO nanostructure synthesized without and with 205 mM NH4NO3 on AZO substrate. The Zn(NO3)2 concentration was 5 mM and the applied potential was −1.32 V.
interesting thing is on the interface in which relatively low diffracted signals were detected, which could be denoted as a structural “weak area”. Therefore nanorods would be corroded easily and form the ZnO segments as observed. Generally, the relatively low signals in HAADF image are due to lower in thickness, lower atomic number of composite elements, and relatively poor crystallinity. The photoluminescence spectra are important because they can be important criteria for producing ZnO nanopillars with low defect densities. The large UV-to-visible ratio (PL ratio) is evidence of the high quality of the ZnO nanorod arrays [12]. The PL spectra of the as-grown ZnO nanostructures on AZO substrates were studied at room temperature. Fig. 5a shows the PL spectra of the ZnO nanopillars electrodeposited on AZO substrates obtained at concentration of NH4NO3 between 0 mM and 205 mM. The UV emission at ~ 375 nm corresponds to the nearband-edge emission of ZnO which is due to the recombination of exitons. For the samples deposited at lower NH4NO3 concentrations, a broad visible-light luminescence (450–700 nm) centered at 550– 630 nm (yellow and orange color) was observed, accompanied by a red shoulder at 680 nm. The origin of the visible emission is highly controversial [18]. The yellow emission is assumed from oxygen interstitial defects [19,20] or Zn(OH)2 on the surface [21,22]. The orange emission is attributed to defects associated with excess oxygen [23]. Although the size of the nanopillars increases with NH4NO3 concentration whenever the concentration is below 100 mM, the defect density was found to increase as a result of the fast growth and coalescence of the nanopillars [12], as well as increasing surface defects. As the NH4NO3 concentration increases to 100 mM, defect density drops leading to
lower defect emission (450–700 nm). We believe the possible reason is that the surface defects have been reduced due to lower density and less coalescence, also the above-mentioned core-shell structure possibly plays an important role in surface passivation. Under low concentration of NH4NO3, the high lateral growth rate results in coalescent between adjacent nanopillars, and there exist many defects at the joint. As the NH4NO3 concentration becomes higher, highly separated nanopillars are formed with few defects on the surface, and it is speculated that most surface defects were passivated by that thin coherent layer of ZnO in the core shell structure. Therefore, the less spatially coalescent nanopillars deposited at high NH4NO3 concentrations have fewer defects. Moreover, the UV-to-visible ratio of the ZnO nanopillars, as shown in Table 2, first drops and then increases with increasing NH4NO3 concentrations, reaching a maximum at a concentration of 205 mM. The falloff at concentration of 60 mM is possibly due to the increase in the defect density resulting from the increased coalescence of the nanopillars. The increase of UV-to-visible ratio is due to decreased surface defects. In addition, the AZO film became thinner as more NH4NO3 was added to the solution. The non-normalized PL spectrums in Fig. 5b show the PL intensity at 375 nm fell while the concentration was 100 mM or higher. This implies that more ZnO/AZO was etched away with increasing NH4NO3. When NH4NO3 concentrations higher than 205 mM were used, it was clear that part of the AZO layer was eroded away. The X-ray diffraction (XRD) spectra corresponding to the sample prepared with and without NH4NO3 are shown in Fig. 6, and the crystallinity of ZnO nanostructures was confirmed by X-ray diffraction. A notable intensity at 34.4° is obtained for the (002) diffraction peak, which means that the ZnO preferred the orientation of (002). The intensity of (002) peak decreases with increase of NH4NO3 concentration, because ZnO has been etched with solution containing 205 mM NH4NO3. Conventional XRD generally produces a weak signal from the nanorods and an intense signal from the substrate [24], because the ZnO nanorods are short. To obtain more signals from the nanorods and the material surface [25,26], glancing incidence XRD (GIXRD) was used in this work. The GIXRD scans of ZnO nanostructures synthesized without and with NH4NO3 are shown in Fig. 7a and b. The dominant peaks are identified at 34.5° and 63.0°, which can be indexed as the (002) and (103) reflections of ZnO [27]. The intensity of (103) peak is higher than that of (002) peak, which demonstrates that those surfaces in ZnO nanostructures preferred the orientation of (103). To further confirm the effect of NH4NO3, GIXRD patterns of ZnO nanostructure synthesized without and with NH4NO3 at glancing-incident angle 0.5° are shown in Fig. 7c in a log scale. It is clear that both (002) and (103) peaks of the sample synthesized without NH4NO3 are lower, due to relatively low density and etched AZO substrate.
Fig. 7. GIXRD scan of ZnO nanostructure prepared with 5 mM Zn(NO3)2 and an applied potential was −1.32 V. (a) GIXRD scan of ZnO nanostructure synthesized without NH4NO3 at different glancing-incident angle, (b) GIXRD scan of ZnO nanostructure synthesized with 205 mM NH4NO3 at different glancing-incident angle, (c) GIXRD scan of ZnO nanostructure synthesized with and without NH4NO3 at glancing-incident angle 0.5° in a log scale.
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Table 3 Length, distance, diameter, density, PL ratio as a function of applied potential. The concentration of Zn(NO3)2 and NH4NO3 was kept at 5 mM and 195 mM respectively. Potential (V)
Length (nm)
Distance (nm)
Diameter (nm)
Density (/cm2)
PL ratio
−1.38 −1.32 −1.26 −1.10
345 ± 53 398 ± 55 402 ± 50 364 ± 44
35 ± 23 55 ± 31 225 ± 105 280 ± 128
69 ± 24 80 ± 26 68 ± 26 84 ± 32
3.0 × 109 2.9 × 109 1.3 × 109 6.4 × 108
15 15 22 14
increase in the absolute value of the applied potential leads to a high vertical growth rate and a high lateral growth rate [12]. On the contrary, high concentration of NH4NO3 leads to notable corrosion on ZnO nanopillars. Therefore as the applied potential becomes less negative, the growth rate decreases, and the corrosion effect becomes dominant leading to lower density of ZnO nanorod arrays and higher distance between nanorods. The distance was significantly increased when the applied potential was varied from −1.32 V to −1.26 V, because dominant factor was altered from growth rate to etching rate. At − 1.26 V, the growth rate was low enough that the etching began to dominate the deposition process. When the growth rate was further reduced at −1.1 V, the dominant role was still etching, so the distance slightly increased when the applied potential was varied from −1.26 V to −1.1 V. Fig. 9a gives the normalized PL spectra of the ZnO nanopillars electrodeposited on AZO substrates obtained at cathode potential between −1.38 V and −1.1 V. The defect density is found to decrease as the applied potential becomes less negative, because highly separated nanopillars have few surface defects. Furthermore, the UV-to-visible ratio of the ZnO nanopillars, as shown in Table 3, first increases and then drops with increasing cathode potential, reaching a maximum at a potential of −1.26 V. This can be seen from Fig. 9b the defect emission at − 1.26 V was the lowest. The increase of PL ratio results from decreased surface defects. However, as the applied potential became less negative, the growth rate was much smaller than the erosion rate. As a result, most ZnO and AZO were eroded away, so the falloff at −1.1 V is due to the smaller amount of the rest ZnO. When potential less negative than −1.1 V was used, on some part of the substrate ZnO layer was eroded away, leaving only uncovered glass substrate. 4. Conclusions
Fig. 8. SEM top and cross-sectional images of as-grown ZnO nanorod arrays prepared with decreasing cathodic voltage: (a, e) −1.38 V, (b, f) −1.32 V, (c, g) −1.26 V, (d, h) –1.1 V. The concentration of Zn(NO3)2 and NH4NO3 was kept at 5 mM and 195 mM respectively. Scale bar: 500 nm.
3.2. Effect of applied potential In order to achieve lower density, we fabricated ZnO nanorod arrays under different applied potentials. Fig. 8 shows SEM images of ZnO nanorods grown on AZO substrates under different potentials while the NH4NO3 concentration remained 195 mM and Zn(NO3)2 concentration was kept at 5 mM. The temperature was 75 °C, and deposition time was 900 s. Nanorod arrays with different density grew vertically on the substrate. The images show that the distance between the nanopillars increases and the density of the ZnO nanorod arrays decreases as the deposition potential becomes less negative. The approximate distances and densities are 35 nm and 3 × 109/cm2 at − 1.38 V, 55 nm and 2.9 × 109/cm2 at −1.32 V, 225 nm and 1.3 × 109/cm2 at −1.26 V, 280 nm and 6.4 × 108/cm2 at −1.1 V respectively (Table 3). As mentioned, an
In summary, we reported a low-temperature NH4NO3-assisted electrochemical method for fabrication of density-controlled high-quality as-grown ZnO nanorod arrays from an aqueous solution of Zn(NO3)2 and NH4NO3. We conclude that the density-controlled ZnO nanorods can be grown by varying the concentrations of the precursor solutions or applied potentials. Both the vertical and lateral growth rates of ZnO nanorods are remarkably enhanced using NH4NO3concentrations below 100 mM, due to fast growth and coalescence of the nanopillars. As long as the concentration is higher than 100 mM, the nanorods are corroded during growth process which results in passivated low-density ZnO nanorod arrays with considerable improvement in the PL UV to visible emission ratio. Besides, the nanoarray density will be further reduced without a significant effect on the optical qualities whenever the applied potential becomes less negative. Finally, XRD profiles confirm that ZnO nanostructures possess (002) and (103) preferred orientations. Overall, our works provide a simple method of synthesizing high-quality and low density nanorod arrays, which have promising advantages for photoelectric applications, including light-emitting diodes, nanostructured solar cells, and so on. Acknowledgment This work was supported by the National Natural Science Foundation of China [grant number 61404007] and the Beijing Talents Fund [grant number 2015000021223ZK38]
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Fig. 9. (a) Normalized photoluminescence emission spectra of ZnO nanorods with increasing cathodic voltage, (b) Enlarged visible emission spectra of (a). The concentration of Zn(NO3)2 and NH4NO3 was 5 mM and 195 mM respectively.
References [1] J. Hu, T.W. Odom, C.M. Lieber, Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes, Acc. Chem. Res. 32 (1999) 435–445. [2] M. Willander, O. Nur, Q.X. Zhao, L.L. Yang, M. Lorenz, B.Q. Cao, J.Z. Pérez, C. Czekalla, G. Zimmermann, M. Grundmann, A. Bakin, A. Behrends, M. Al-Suleiman, A. El-Shaer, A. Che Mofor, B. Postels, A. Waag, N. Boukos, A. Travlos, H.S. Kwack, J. Guinard, D. Le Si Dang, Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers, Nanotechnology 20 (2009) 332001. [3] J. Chen, Y. Tang, D. Greiner, L. Ae, R. Baier, J. Lehmann, S. Sadewasser, M.C. LuxSteiner, Fast growth of high work function and high-quality ZnO nanorods from an aqueous solution, J. Phys. Chem. C 115 (2011) 5239–5243. [4] Y. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, B.J. Kim, Y.S. Park, C.J. Youn, Next generation of oxide photonic devices: ZnO-based ultraviolet light emitting diodes, Appl. Phys. Lett. 88 (2006) 241108. [5] R. Könenkamp, R.C. Word, M. Godinez, Ultraviolet electroluminescence from ZnO/ polymer heterojunction light-emitting diodes, Nano Lett. 5 (2005) 2005–2008. [6] L.K. Van Vugt, S. Ruhle, D. Vanmaekelbergh, Phase-correlated nondirectional laser emission from the end facets of a ZnO nanowire, Nano Lett. 6 (2006) 2707–2711. [7] L. Aé, J. Chen, M.C. Lux-Steiner, Hybrid flexible vertical nanoscale diodes prepared at low temperature in large area, Nanotechnology 19 (2008) 475201. [8] L. Aé, D. Kieven, J. Chen, R. Klenk, T. Rissom, Y. Tang, M. Ch Lux-Steiner, ZnO nanorod arrays as an antireflective coating for Cu(In,Ga)Se2 thin film solar cells, Prog. Photovolt. Res. Appl. 18 (2010) 209–213. [9] S.H. Jo, J.Y. Lao, Z.F. Ren, R.A. Farrer, T. Baldacchini, J.T. Fourkas, Field-emission studies on thin films of zinc oxide nanowires, Appl. Phys. Lett. 83 (2003) 4821–4823. [10] A.M. Lockett, P.J. Thomas, P. O'Brien, Influence of seeding layers on the morphology, density, and critical dimensions of ZnO nanostructures grown by chemical bath deposition, J. Phys. Chem. C 116 (2012) 8089–8094. [11] M. Izaki, T. Omi, Electrolyte optimization for cathodic growth of zinc oxide films, J. Electrochem. Soc. 143 (1996) L53–L55. [12] S.J. Lee, S.K. Park, C.R. Park, J.Y. Lee, J. Park, Y.R. Do, Spatially separated ZnO nanopillar arrays on Pt/Si substrates prepared by electrochemical deposition, J. Phys. Chem. C 111 (2007) 11793–11801. [13] X. Yan, Z. Li, R. Chen, W. Gao, Template growth of ZnO nanorods and microrods with controllable densities, Cryst. Growth Des. 8 (2008) 2406–2410.
[14] T.P. Niesen, M.R. De Guire, Review: deposition of ceramic thin films at low temperatures from aqueous solutions, Solid State Ionics 151 (2002) 61–68. [15] W.J. Li, E.W. Shi, W.Z. Zhong, Z.W. Yin, Growth mechanism and growth habit of oxide crystals, J. Cryst. Growth 203 (1999) 186–196. [16] A. Wander, N.M. Harrison, Ab-initio study of ZnO(1120), Surf. Sci. 468 (2000) L851–L855. [17] J.D.H. Donnay, D. Harker, A new law of crystal morphology extending the law of Bravais, Am. Mineral. 22 (1937) 446–467. [18] J.B. Baxter, F. Wu, E.S. Aydil, Growth mechanism and characterization of zinc oxide hexagonal columns, Appl. Phys. Lett. 83 (2003) 3797–3799. [19] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doǧan, V. Avrutin, S.J. Cho, H. Morko, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 041301. [20] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Low-temperature wafer-scale production of ZnO nanowire arrays, Angew. Chem. 42 (2003) 3031–3034. [21] H. Zhou, H. Alves, D.M. Hofmann, W. Kriegseis, B.K. Meyer, G. Kaczmarczyk, A. Hoffmann, Behind the weak excitonic emission of ZnO quantum dots: ZnO/ Zn(OH)2 core-shell structure, Appl. Phys. Lett. 80 (2002) 210–212. [22] N.S. Norberg, D.R. Gamelin, Influence of surface modification on the luminescence of colloidal ZnO nanocrystals, J. Phys. Chem. B 109 (2005) 20810–20816. [23] A. Teke, Ü. Özgür, S. Doǧan, X. Gu, H. Morkoç, B. Nemeth, J. Nause, H.O. Everitt, Excitonic fine structure and recombination dynamics in single-crystalline ZnO, Phys. Rev. B 70 (2004) 195207. [24] A. Lee, G. Kim, S.J. Yoo, I.S. Cho, H. Seo, B. Ahn, H.K. Yu, Growth of ZnO thin film on graphene transferred Si (100) substrate, Thin Solid Films 619 (2016) 68–72. [25] D. Pradhan, K.T. Leung, Vertical growth of two-dimensional zinc oxide nanostructures on ITO-coated glass: effects of deposition temperature and deposition time, J. Phys. Chem. C 112 (2008) 1357–1364. [26] D. Pradhan, K.T. Leung, Controlled growth of two-dimensional and one-dimensional ZnO nanostructures on indium tin oxide coated glass by direct electrodeposition, Langmuir 24 (2008) 9707–9716. [27] O. Lupan, L. Chow, G. Chai, B. Roldan, A. Naitabdi, A. Schulte, H. Heinrich, Nanofabrication and characterization of ZnO nanorod arrays and branched microrods by aqueous solution route and rapid thermal processing, Mater. Sci. Eng. B 145 (2007) 57–66.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Density-controlled growth and passivation of ZnO nanorod arrays by electrodeposition Lida Guo, Yang Tang ⁎, Fu-Kuo Chiang, Linge Ma, Jie Chen Beijing Engineering Research Center of Nanostructured Thin Film Solar Cell, National Institute of Clean-and-Low-Carbon Energy, Future Science & Technology City, Changping District, Beijing, 102211, People's Republic of China
a r t i c l e
i n f o
Article history: Received 15 June 2016 Received in revised form 4 August 2017 Accepted 9 August 2017 Available online 09 August 2017 Keywords: Nanostructures Electrochemical techniques Crystal growth Photoluminescence spectroscopy
a b s t r a c t Here we established a simple electrochemical method of synthesizing ZnO nanomaterials with high quality and the ability to control the nanostructures' density For ZnO nanorod growth, a thin layer of Al doped ZnO (AZO) was deposited on glass substrates and ZnO nanorod arrays were grown on these substrates from aqueous solution containing Zn(NO3)2 and NH4NO3. It was found that the density, morphology, and quality of the nanorod arrays can be controlled by changing the applied potentials or the concentration of NH4NO3 in the solution as growing and etching were carried out simultaneously. Moreover, the nanorod density decrease resulted in an improvement of the quality of the as-grown nanorods. Scanning electron microscope (SEM), transmission electron microscopy (TEM), photoluminescence and X-ray diffraction (XRD) have been used to characterize the samples. The density of ZnO nanorods can be controlled by varying the concentrations of the precursor solutions or applied potentials. Passivated low-density ZnO nanorod arrays with high quality were fabricated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor nanomaterials have captured the attention of many researchers in recent years due to their good prospects in various technological areas [1,2]. ZnO is one of the most important semiconductor materials with a direct wide band gap (3.37 eV) and a large excitation binding energy (60 meV). It is nontoxic, cheap, stable, and environmentally friendly [3]. Since the ZnO nanorods exhibit fewer defects than its thin-film structure, it is, therefore, a promising material for photoelectronic devices, such as light-emitting diodes [4–7] and solar cells [8]. Among the various methods used for the fabrication of ZnO nanorods, the electrochemical deposition method is of particular interest since this technique enables fast fabrication and relatively low cost [3]. It also achieves easy control of both the morphology and the thickness of the as-grown ZnO. Also, high production efficiency can be achieved at low temperature with this technique, which requires inexpensive equipment. According to past research report, the density of the nanoarrays and the distance between the nanorods affected the photoelectric response and carrier transport characteristic of photoelectric devices based on semiconductor nanostructures [9]. Furthermore, the tailoring of nanostructures' density is necessary in some applications.
⁎ Corresponding author. E-mail address: [email protected] (Y. Tang).
http://dx.doi.org/10.1016/j.tsf.2017.08.015 0040-6090/© 2017 Elsevier B.V. All rights reserved.
For instance, in dye-sensitized solar cells, the ability of the dye to interpenetrate the semiconducting material is related to the nanorods density [10]. Also, for applications in copper indium gallium selenide (CIGS) solar cells, the nanostructures with a low density used as substrate is desired because the absorber material needs to be intercalated between the nanorods. Despite various ZnO nanostructures that have been electrochemically deposited, low-density ZnO nanorod arrays are difficult to prepare and this limits their applications in nanodevices. The technical problem is to fabricate ZnO nanorod arrays with controllable density and distance between the nanorods. In this paper, we report the preparation and characterization of ZnO nanorod arrays electrochemically deposited from an aqueous solution of zinc nitrate (Zn(NO3)2) and ammonium nitrate (NH4NO3) at a low temperature down to 75 °C. Specifically, we investigated the effects of varying the concentration of NH4NO3 in the precursor solution and the applied potential on the morphology, structure and optical properties of the obtained nanopillars.
2. Experimental details Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99.998%, Alfa) and ammonium nitrate (NH4NO3, 99%, J&K Scientific) were dissolved in water that had a resistivity of 18.2 MΩ·cm (prepared by a Millipore Milli-Q Advantage A10 system). The AZO-P002 glass substrates with thickness of 2 mm were purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd. The substrates are covered by AZO (Al ~ 2%) with thickness of approximate 800 nm and a sheet resistance below 10 Ω/□.
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The AZO substrate was firstly cleaned by acetone, ethanol and ultrapure water in a sonic bath. Then the ZnO nanorod arrays were electrodeposited on these substrates in an electrolyte aqueous solution of
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Zn(NO3)2 and NH4NO3. The solution was prepared at room temperature and then placed in a 75 °C water bath heated by a hot plate. A platinum sheet, a platinum wire and the AZO substrate served as the counter electrode, the reference electrode and the working electrode, respectively. During the growth process, a constant electric potential as referred to the reference electrode was applied to the substrate. The duration of the depositions was 900 s. Different concentrations of NH4NO3 (0– 205 mM) with an applied potential of − 1.32 V were used, while Zn(NO3)2 was kept at 5 mM in the source solution. The effect of the applied potential in the range of −1.10 V to −1.38 V was also tested for one growth solution. At the end of the growth, the AZO substrates covered with ZnO nanorods were taken out of the aqueous solution and rinsed in flowing deionized water immediately so as to remove any residual salt from the surface. Potentiostat by Ivium was used for the electrochemical deposition. pH value of the solution was measured by a Mettler S40K pH meter. The morphology and the crystal structure of the as-grown ZnO nanorods were investigated by a scanning electron microscope (SEM, FEI Nova NanaSEM 450) and transmission electron microscopy (TEM, JEOL ARM200F). The cross-sectional TEM sample for high resolution images was prepared by gluing the Si wafer with the thin film. The sample was cut and mechanically thinned by abrasive papers and dimpler. Then attached the sample on a Mo-ring and carefully milled the selected area by Ar ion beam for obtaining an ultra-thin region with the thickness of about 20–30 nm. In the final step, the surface amorphous layer was removed by low-voltage (0.1 kV) Ar ion beam for making sure the high quality TEM image to be obtained. The optical qualities of excitonic recombination in as-grown ZnO nanorods were analyzed by photoluminescence (PL) experiments under room temperature. The PL was excited by a He-Cd laser (325 nm line). It is generally assumed that a large UV to visible emission ratio suggests excellent quality of the ZnO nanorods, in other words, the defect density is low. 3. Results and discussions The reactions for ZnO nanorod deposition from Zn(NO3)2 and NH4NO3 precursors are as below [11]. An increase in the applied potential leads not only to a high vertical growth rate but also to a high lateral growth rate which has already been reported by Lee et al. [12]. ZnðNO3 Þ2 ⇆Zn2þ þ 2NO3 −
ð1Þ
NO3 − þ H2 O þ 2e− ⇆NO2 − þ 2OH−
ð2Þ
Zn2þ þ 2OH− ⇆ZnðOHÞ2
ð3Þ
ZnðOHÞ2 ⇆ZnO þ H2 O
ð4Þ
As reaction equilibrium moves to the right side, ZnO forms through dehydration of Zn(OH)2 and precipitate onto the substrate, resulting in the formation of ZnO nanostructures. Dehydration and precipitation reactions in the solutions (Eq. (3)) will continuously provide matter for the growth of ZnO nanorods. However, if the pH values of the
Table 1 pH value of solution as a function of NH4NO3 concentration.
Fig. 1. SEM top and cross-sectional images of as-grown ZnO nanorod arrays prepared with increasing concentrations of NH4NO3: (a, f) 0 mM, (b, g) 60 mM, (c, h) 100 mM, (d, i) 150 mM, (e, j) 205 mM. The concentration of Zn(NO3)2 was 5 mM and the applied potential was −1.32 V. Scale bar: 500 nm.
Sample
Zn(NO3)2 concentration (mM)
NH4NO3 concentration (mM)
Solution pH
1 2 3 4 5
5 5 5 5 5
0 60 100 150 205
5.533 4.560 4.545 4.500 4.496
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Table 2 Length, distance, diameter, density and PL ratio of ZnO nanostructures as a function of NH4NO3 Concentration. The concentration of Zn(NO3)2 was 5 mM and the applied potential was −1.32 V. Sample
Concentration of NH4NO3 (mM)
Thickness of AZO (nm)
Length (nm)
Distance (nm)
Diameter (nm)
Density (/cm2)
PL ratio
1 2 3 4 5
0 60 100 150 205
874 ± 10 859 ± 19 530 ± 40 536 ± 52 441 ± 50
80 ± 25 600 ± 46 318 ± 35 241 ± 33 208 ± 91
0±5 18 ± 17 38 ± 20 69 ± 38 97 ± 50
91 ± 37 191 ± 41 88 ± 24 81 ± 22 89 ± 30
9.2 × 109 2.0 × 109 6.0 × 109 5.6 × 109 3.1 × 109
6 2 19 20 22
solution changed, the above reversible reactions may be altered and shift in the opposite direction [14]. When the reactions occur in acidic solutions, H2O will slowly decompose to generate OH– (Eq. (2)). However, the hydrolytic processes (Eq. (4)) can still occur as Zn(OH)2 is easily hydrolysable [14]. In an acidic solution, ZnO is unstable, so the dissolving ratio and process can be different [13]. 3.1. Effect of NH4NO3 Fig. 1 shows SEM images of ZnO nanorods grown on AZO substrates. The concentrations of NH4NO3 were in the range from 0 mM to 205 mM. Image 1a–1e were taken perpendicular to the surface to record the distance between the nanopillars and the vertical alignment of the ZnO nanorods. With some tilting, images 1f–1j gave morphological dimensions of the nanorod arrays, namely the average diameter of the asgrown nanorods, as well as the thickness of AZO film as a function of the NH4NO3 concentration. The distances are from 0 to 97 nm, the diameters of the nanorods range from 60 nm to 191 nm, and the densities are in the range from 2 × 109 to 9.2 × 109/cm2. It is apparent in Table 2 that the distance between the nanorods increases with the concentration of NH4NO3. The approximate distances are 0 nm, 18 nm, 38 nm, 69 nm and 97 nm at NH4NO3 concentration of 0 mM, 60 mM, 100 mM, 150 mM and
Fig. 2. TEM images of the as-grown ZnO nanorods at 5 mM Zn(NO3)2 and 205 mM NH4NO3 with an applied potential of −1.32 V. (a) The overall morphology, (b) ZnO segments found aside the nanorods.
205 mM respectively. Normally, the current intensity increases with electrolyte concentration, causing the increase of deposition rate, which is defined as the amount of deposited material per unit time. Therefore, the ZnO nanorods increases in both diameter and length as NH4NO3 concentration increased from 0 to 60 mM (Table 2). However, the pH of the solution drops with increases of the concentration of NH4NO3 (as shown in Table 1), resulting in the corrosion of the ZnO nanorods. Although the mechanism is debatable [15–18], we noted that, by varying the pH of the reaction mixture, the rate of nucleation and growth events could be changed [15]. The reversible reactions are altered and shift in the opposite direction. Moreover, the etching and growing of ZnO/AZO are carried out simultaneously. Furthermore, as mentioned, the acidic solution can result in corrosion of ZnO, because ZnO is unstable under this condition. On the whole, NH4NO3 plays a role of accelerating the growth of ZnO nanorods, as long as the concentration of NH4NO3 is below 100 mM. As the NH4NO3 concentration is increased to over 100 mM, the pH continues to drop, and corrosion overwhelms precipitation, which leads to decreases of nanoarray
Fig. 3. High-resolution TEM images of the as-grown ZnO nanorods prepared with an applied potential of −1.32 V. (a) ZnO nanorods prepared with 5 mM Zn(NO3)2 and 205 mM NH4NO3, (b) ZnO nanorods prepared with 5 mM Zn(NO3)2.
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Fig. 4. Macro-viewing of one of the non-uniform ZnO nanorods at 5 mM Zn(NO3)2 and 205 mM NH4NO3 with an applied potential of −1.32 V. (a), (b) TEM morphology of the ZnO nanorod; (c), (d) HAADF images show the interest area that may lead to the observed ZnO segments.
density, nanorod diameter, thickness of AZO, on the other hand an increase in distance between nanorods (Table 2). The real-space images of the as-grown ZnO nanorods acquired by TEM may give a glance on the described etching-and-growth mechanism above. Fig. 2 exhibits the morphologies of the resultant as-grown ZnO nanorods on AZO after chemical reaction in 5 mM Zn(NO3)2 and 205 mM NH4NO3 solution with an applied potential of −1.32 V. In Fig. 2a, the interface between AZO and ZnO (cross-sectional view) characterizes the equilibrium boundary of etching and growth processes simultaneously, in which the thickness of the residual AZO layer is about 600 nm and ZnO achieves 100–400 nm in thickness (white dotline). It is consistent with the SEM image in Fig. 1j. Selected area electron
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diffraction (SAED) in the insert of Fig. 2a shows ZnO with the hexagonal structure with space group P63cm (No. 186), which is identical to that of AZO. It indicates the identical crystal structure is one of the essential ingredients of ZnO grown onto AZO which determines the optical properties. Except the as-grown ZnO nanorods, some crystalline fragments (white arrow in Fig. 2b) were found as well. They were not introduced by the TEM specimen preparation since they were all composed by Zn and O inspected by Electron Dispersive Spectroscopy (EDS) equipped in TEM. Most of them are well-crystallized and joint the as-grown nanorods with an extremely thin amorphous layer or apart from the mixing layer composed by polycrystalline ZnO and amorphous matters (Fig. 2b). Particularly, most of the surface of the nanostructures is coated by a ZnO layer (Fig. 3a) with the thickness of about 10 nm forming a special core-shell structure. They were coherent to the as-grown ZnO nanorods and well-deposited on the surface, but easy-damaged by electron bombardment into poly-crystalline structure. The etching rate of AZO was not uniform due to different crystal structures on the surface. Therefore, a rough and uneven surface was formed, and the raised part worked as a seed allowing ZnO to grown on. These images of microstructure imply that the simultaneous etching and growing processes not only effectively corroded the AZO substrate to give advantage to ZnO growth but also eroded the as-grown nanorods leading to partial collapse. Eventually, an external layer homogeneously covered the surfaces of nanorods as residuals. On the contrary, high-resolution TEM images (Fig. 3b) shows that no core-shell structure was found on the surface (dotted line) of the ZnO nanorod prepared at 5 mM Zn(NO3)2. Although it is not clear how the layer was formed, this difference on structure affects the optical quality of the nanorods to a great extent. High Angle Annual Dark Field (HAADF) images of the sample prepared under 5 mM Zn(NO3)2 and 205 mM NH4NO3 acquired by Scanning Transmission Electron Microscopy (STEM) could give a clear picture of what mentioned above. In Fig. 4, the real-space TEM images (Fig. 4a and b) show a ZnO nanorod re-growth on the AZO starting by a distinguishable interface (white arrow). Similar to most of the other as-grown nanorods, its morphology inclines to be non-uniform pillar which is consistent with SEM images indicating it was not artificially caused during the TEM specimen preparation. The corresponding HAADF images are shown in Fig. 4c and d, in which the image contrast indicates the structural variation inside the nanorod. Here the most
Fig. 5. Photoluminescence emission spectra of ZnO nanorods prepared with 5 mM Zn(NO3)2 and increasing concentrations of NH4NO3. The applied potential was −1.32 V. (a) normalized PL intensity, (b) non-normalized PL intensity.
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Fig. 6. XRD scans for the ZnO nanostructure synthesized without and with 205 mM NH4NO3 on AZO substrate. The Zn(NO3)2 concentration was 5 mM and the applied potential was −1.32 V.
interesting thing is on the interface in which relatively low diffracted signals were detected, which could be denoted as a structural “weak area”. Therefore nanorods would be corroded easily and form the ZnO segments as observed. Generally, the relatively low signals in HAADF image are due to lower in thickness, lower atomic number of composite elements, and relatively poor crystallinity. The photoluminescence spectra are important because they can be important criteria for producing ZnO nanopillars with low defect densities. The large UV-to-visible ratio (PL ratio) is evidence of the high quality of the ZnO nanorod arrays [12]. The PL spectra of the as-grown ZnO nanostructures on AZO substrates were studied at room temperature. Fig. 5a shows the PL spectra of the ZnO nanopillars electrodeposited on AZO substrates obtained at concentration of NH4NO3 between 0 mM and 205 mM. The UV emission at ~ 375 nm corresponds to the nearband-edge emission of ZnO which is due to the recombination of exitons. For the samples deposited at lower NH4NO3 concentrations, a broad visible-light luminescence (450–700 nm) centered at 550– 630 nm (yellow and orange color) was observed, accompanied by a red shoulder at 680 nm. The origin of the visible emission is highly controversial [18]. The yellow emission is assumed from oxygen interstitial defects [19,20] or Zn(OH)2 on the surface [21,22]. The orange emission is attributed to defects associated with excess oxygen [23]. Although the size of the nanopillars increases with NH4NO3 concentration whenever the concentration is below 100 mM, the defect density was found to increase as a result of the fast growth and coalescence of the nanopillars [12], as well as increasing surface defects. As the NH4NO3 concentration increases to 100 mM, defect density drops leading to
lower defect emission (450–700 nm). We believe the possible reason is that the surface defects have been reduced due to lower density and less coalescence, also the above-mentioned core-shell structure possibly plays an important role in surface passivation. Under low concentration of NH4NO3, the high lateral growth rate results in coalescent between adjacent nanopillars, and there exist many defects at the joint. As the NH4NO3 concentration becomes higher, highly separated nanopillars are formed with few defects on the surface, and it is speculated that most surface defects were passivated by that thin coherent layer of ZnO in the core shell structure. Therefore, the less spatially coalescent nanopillars deposited at high NH4NO3 concentrations have fewer defects. Moreover, the UV-to-visible ratio of the ZnO nanopillars, as shown in Table 2, first drops and then increases with increasing NH4NO3 concentrations, reaching a maximum at a concentration of 205 mM. The falloff at concentration of 60 mM is possibly due to the increase in the defect density resulting from the increased coalescence of the nanopillars. The increase of UV-to-visible ratio is due to decreased surface defects. In addition, the AZO film became thinner as more NH4NO3 was added to the solution. The non-normalized PL spectrums in Fig. 5b show the PL intensity at 375 nm fell while the concentration was 100 mM or higher. This implies that more ZnO/AZO was etched away with increasing NH4NO3. When NH4NO3 concentrations higher than 205 mM were used, it was clear that part of the AZO layer was eroded away. The X-ray diffraction (XRD) spectra corresponding to the sample prepared with and without NH4NO3 are shown in Fig. 6, and the crystallinity of ZnO nanostructures was confirmed by X-ray diffraction. A notable intensity at 34.4° is obtained for the (002) diffraction peak, which means that the ZnO preferred the orientation of (002). The intensity of (002) peak decreases with increase of NH4NO3 concentration, because ZnO has been etched with solution containing 205 mM NH4NO3. Conventional XRD generally produces a weak signal from the nanorods and an intense signal from the substrate [24], because the ZnO nanorods are short. To obtain more signals from the nanorods and the material surface [25,26], glancing incidence XRD (GIXRD) was used in this work. The GIXRD scans of ZnO nanostructures synthesized without and with NH4NO3 are shown in Fig. 7a and b. The dominant peaks are identified at 34.5° and 63.0°, which can be indexed as the (002) and (103) reflections of ZnO [27]. The intensity of (103) peak is higher than that of (002) peak, which demonstrates that those surfaces in ZnO nanostructures preferred the orientation of (103). To further confirm the effect of NH4NO3, GIXRD patterns of ZnO nanostructure synthesized without and with NH4NO3 at glancing-incident angle 0.5° are shown in Fig. 7c in a log scale. It is clear that both (002) and (103) peaks of the sample synthesized without NH4NO3 are lower, due to relatively low density and etched AZO substrate.
Fig. 7. GIXRD scan of ZnO nanostructure prepared with 5 mM Zn(NO3)2 and an applied potential was −1.32 V. (a) GIXRD scan of ZnO nanostructure synthesized without NH4NO3 at different glancing-incident angle, (b) GIXRD scan of ZnO nanostructure synthesized with 205 mM NH4NO3 at different glancing-incident angle, (c) GIXRD scan of ZnO nanostructure synthesized with and without NH4NO3 at glancing-incident angle 0.5° in a log scale.
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Table 3 Length, distance, diameter, density, PL ratio as a function of applied potential. The concentration of Zn(NO3)2 and NH4NO3 was kept at 5 mM and 195 mM respectively. Potential (V)
Length (nm)
Distance (nm)
Diameter (nm)
Density (/cm2)
PL ratio
−1.38 −1.32 −1.26 −1.10
345 ± 53 398 ± 55 402 ± 50 364 ± 44
35 ± 23 55 ± 31 225 ± 105 280 ± 128
69 ± 24 80 ± 26 68 ± 26 84 ± 32
3.0 × 109 2.9 × 109 1.3 × 109 6.4 × 108
15 15 22 14
increase in the absolute value of the applied potential leads to a high vertical growth rate and a high lateral growth rate [12]. On the contrary, high concentration of NH4NO3 leads to notable corrosion on ZnO nanopillars. Therefore as the applied potential becomes less negative, the growth rate decreases, and the corrosion effect becomes dominant leading to lower density of ZnO nanorod arrays and higher distance between nanorods. The distance was significantly increased when the applied potential was varied from −1.32 V to −1.26 V, because dominant factor was altered from growth rate to etching rate. At − 1.26 V, the growth rate was low enough that the etching began to dominate the deposition process. When the growth rate was further reduced at −1.1 V, the dominant role was still etching, so the distance slightly increased when the applied potential was varied from −1.26 V to −1.1 V. Fig. 9a gives the normalized PL spectra of the ZnO nanopillars electrodeposited on AZO substrates obtained at cathode potential between −1.38 V and −1.1 V. The defect density is found to decrease as the applied potential becomes less negative, because highly separated nanopillars have few surface defects. Furthermore, the UV-to-visible ratio of the ZnO nanopillars, as shown in Table 3, first increases and then drops with increasing cathode potential, reaching a maximum at a potential of −1.26 V. This can be seen from Fig. 9b the defect emission at − 1.26 V was the lowest. The increase of PL ratio results from decreased surface defects. However, as the applied potential became less negative, the growth rate was much smaller than the erosion rate. As a result, most ZnO and AZO were eroded away, so the falloff at −1.1 V is due to the smaller amount of the rest ZnO. When potential less negative than −1.1 V was used, on some part of the substrate ZnO layer was eroded away, leaving only uncovered glass substrate. 4. Conclusions
Fig. 8. SEM top and cross-sectional images of as-grown ZnO nanorod arrays prepared with decreasing cathodic voltage: (a, e) −1.38 V, (b, f) −1.32 V, (c, g) −1.26 V, (d, h) –1.1 V. The concentration of Zn(NO3)2 and NH4NO3 was kept at 5 mM and 195 mM respectively. Scale bar: 500 nm.
3.2. Effect of applied potential In order to achieve lower density, we fabricated ZnO nanorod arrays under different applied potentials. Fig. 8 shows SEM images of ZnO nanorods grown on AZO substrates under different potentials while the NH4NO3 concentration remained 195 mM and Zn(NO3)2 concentration was kept at 5 mM. The temperature was 75 °C, and deposition time was 900 s. Nanorod arrays with different density grew vertically on the substrate. The images show that the distance between the nanopillars increases and the density of the ZnO nanorod arrays decreases as the deposition potential becomes less negative. The approximate distances and densities are 35 nm and 3 × 109/cm2 at − 1.38 V, 55 nm and 2.9 × 109/cm2 at −1.32 V, 225 nm and 1.3 × 109/cm2 at −1.26 V, 280 nm and 6.4 × 108/cm2 at −1.1 V respectively (Table 3). As mentioned, an
In summary, we reported a low-temperature NH4NO3-assisted electrochemical method for fabrication of density-controlled high-quality as-grown ZnO nanorod arrays from an aqueous solution of Zn(NO3)2 and NH4NO3. We conclude that the density-controlled ZnO nanorods can be grown by varying the concentrations of the precursor solutions or applied potentials. Both the vertical and lateral growth rates of ZnO nanorods are remarkably enhanced using NH4NO3concentrations below 100 mM, due to fast growth and coalescence of the nanopillars. As long as the concentration is higher than 100 mM, the nanorods are corroded during growth process which results in passivated low-density ZnO nanorod arrays with considerable improvement in the PL UV to visible emission ratio. Besides, the nanoarray density will be further reduced without a significant effect on the optical qualities whenever the applied potential becomes less negative. Finally, XRD profiles confirm that ZnO nanostructures possess (002) and (103) preferred orientations. Overall, our works provide a simple method of synthesizing high-quality and low density nanorod arrays, which have promising advantages for photoelectric applications, including light-emitting diodes, nanostructured solar cells, and so on. Acknowledgment This work was supported by the National Natural Science Foundation of China [grant number 61404007] and the Beijing Talents Fund [grant number 2015000021223ZK38]
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Fig. 9. (a) Normalized photoluminescence emission spectra of ZnO nanorods with increasing cathodic voltage, (b) Enlarged visible emission spectra of (a). The concentration of Zn(NO3)2 and NH4NO3 was 5 mM and 195 mM respectively.
References [1] J. Hu, T.W. Odom, C.M. Lieber, Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes, Acc. Chem. Res. 32 (1999) 435–445. [2] M. Willander, O. Nur, Q.X. Zhao, L.L. Yang, M. Lorenz, B.Q. Cao, J.Z. Pérez, C. Czekalla, G. Zimmermann, M. Grundmann, A. Bakin, A. Behrends, M. Al-Suleiman, A. El-Shaer, A. Che Mofor, B. Postels, A. Waag, N. Boukos, A. Travlos, H.S. Kwack, J. Guinard, D. Le Si Dang, Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers, Nanotechnology 20 (2009) 332001. [3] J. Chen, Y. Tang, D. Greiner, L. Ae, R. Baier, J. Lehmann, S. Sadewasser, M.C. LuxSteiner, Fast growth of high work function and high-quality ZnO nanorods from an aqueous solution, J. Phys. Chem. C 115 (2011) 5239–5243. [4] Y. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, B.J. Kim, Y.S. Park, C.J. Youn, Next generation of oxide photonic devices: ZnO-based ultraviolet light emitting diodes, Appl. Phys. Lett. 88 (2006) 241108. [5] R. Könenkamp, R.C. Word, M. Godinez, Ultraviolet electroluminescence from ZnO/ polymer heterojunction light-emitting diodes, Nano Lett. 5 (2005) 2005–2008. [6] L.K. Van Vugt, S. Ruhle, D. Vanmaekelbergh, Phase-correlated nondirectional laser emission from the end facets of a ZnO nanowire, Nano Lett. 6 (2006) 2707–2711. [7] L. Aé, J. Chen, M.C. Lux-Steiner, Hybrid flexible vertical nanoscale diodes prepared at low temperature in large area, Nanotechnology 19 (2008) 475201. [8] L. Aé, D. Kieven, J. Chen, R. Klenk, T. Rissom, Y. Tang, M. Ch Lux-Steiner, ZnO nanorod arrays as an antireflective coating for Cu(In,Ga)Se2 thin film solar cells, Prog. Photovolt. Res. Appl. 18 (2010) 209–213. [9] S.H. Jo, J.Y. Lao, Z.F. Ren, R.A. Farrer, T. Baldacchini, J.T. Fourkas, Field-emission studies on thin films of zinc oxide nanowires, Appl. Phys. Lett. 83 (2003) 4821–4823. [10] A.M. Lockett, P.J. Thomas, P. O'Brien, Influence of seeding layers on the morphology, density, and critical dimensions of ZnO nanostructures grown by chemical bath deposition, J. Phys. Chem. C 116 (2012) 8089–8094. [11] M. Izaki, T. Omi, Electrolyte optimization for cathodic growth of zinc oxide films, J. Electrochem. Soc. 143 (1996) L53–L55. [12] S.J. Lee, S.K. Park, C.R. Park, J.Y. Lee, J. Park, Y.R. Do, Spatially separated ZnO nanopillar arrays on Pt/Si substrates prepared by electrochemical deposition, J. Phys. Chem. C 111 (2007) 11793–11801. [13] X. Yan, Z. Li, R. Chen, W. Gao, Template growth of ZnO nanorods and microrods with controllable densities, Cryst. Growth Des. 8 (2008) 2406–2410.
[14] T.P. Niesen, M.R. De Guire, Review: deposition of ceramic thin films at low temperatures from aqueous solutions, Solid State Ionics 151 (2002) 61–68. [15] W.J. Li, E.W. Shi, W.Z. Zhong, Z.W. Yin, Growth mechanism and growth habit of oxide crystals, J. Cryst. Growth 203 (1999) 186–196. [16] A. Wander, N.M. Harrison, Ab-initio study of ZnO(1120), Surf. Sci. 468 (2000) L851–L855. [17] J.D.H. Donnay, D. Harker, A new law of crystal morphology extending the law of Bravais, Am. Mineral. 22 (1937) 446–467. [18] J.B. Baxter, F. Wu, E.S. Aydil, Growth mechanism and characterization of zinc oxide hexagonal columns, Appl. Phys. Lett. 83 (2003) 3797–3799. [19] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doǧan, V. Avrutin, S.J. Cho, H. Morko, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 041301. [20] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Low-temperature wafer-scale production of ZnO nanowire arrays, Angew. Chem. 42 (2003) 3031–3034. [21] H. Zhou, H. Alves, D.M. Hofmann, W. Kriegseis, B.K. Meyer, G. Kaczmarczyk, A. Hoffmann, Behind the weak excitonic emission of ZnO quantum dots: ZnO/ Zn(OH)2 core-shell structure, Appl. Phys. Lett. 80 (2002) 210–212. [22] N.S. Norberg, D.R. Gamelin, Influence of surface modification on the luminescence of colloidal ZnO nanocrystals, J. Phys. Chem. B 109 (2005) 20810–20816. [23] A. Teke, Ü. Özgür, S. Doǧan, X. Gu, H. Morkoç, B. Nemeth, J. Nause, H.O. Everitt, Excitonic fine structure and recombination dynamics in single-crystalline ZnO, Phys. Rev. B 70 (2004) 195207. [24] A. Lee, G. Kim, S.J. Yoo, I.S. Cho, H. Seo, B. Ahn, H.K. Yu, Growth of ZnO thin film on graphene transferred Si (100) substrate, Thin Solid Films 619 (2016) 68–72. [25] D. Pradhan, K.T. Leung, Vertical growth of two-dimensional zinc oxide nanostructures on ITO-coated glass: effects of deposition temperature and deposition time, J. Phys. Chem. C 112 (2008) 1357–1364. [26] D. Pradhan, K.T. Leung, Controlled growth of two-dimensional and one-dimensional ZnO nanostructures on indium tin oxide coated glass by direct electrodeposition, Langmuir 24 (2008) 9707–9716. [27] O. Lupan, L. Chow, G. Chai, B. Roldan, A. Naitabdi, A. Schulte, H. Heinrich, Nanofabrication and characterization of ZnO nanorod arrays and branched microrods by aqueous solution route and rapid thermal processing, Mater. Sci. Eng. B 145 (2007) 57–66.