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Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays A. Prunaa,b,n, D. Pullinic, D. Busquetsb b
a University of Bucharest, 405 Atomistilor Str., 077125 Bucharest-Magurele, Romania Institute of Materials Technology, University Politecnica of Valencia, Camino de Vera s/n, 46022 Valencia, Spain c Fiat Research Centre, 50 Torino Str., 10043 Orbassano, Italy
Received 23 April 2015; received in revised form 5 July 2015; accepted 14 July 2015
Abstract The fabrication of whole Al-doped ZnO nanostructured electrodes is reported by a two-step technology. Magnetron sputtered Al-doped ZnO (AZO) films were investigated as seeding substrates for the electrodeposition of Al-doped ZnO nanorod arrays. The nucleation, density, morphological and structural properties of Al-doped ZnO NRAs were analyzed as a function of precursor concentration and substrate and compared to undoped ZnO NRAs. High density and good crystal quality are reported for the Al-doped ZnO NRAs grown on AZO films from low precursor concentration. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Grain growth; B. Electron microscopy; B. X-ray methods; D. ZnO
1. Introduction A wide range of applications in fields including solar cells, sensors or transducers have made use of transparent conductive oxides (TCOs) thanks to their remarkable properties [1–3]. Indium tin oxide (ITO) is one of the most employed TCO materials [1]. Nevertheless, it presents various drawbacks such as toxicity or low chemical stability beside the fact that indium is a scarce element on Earth. ZnO has received extensive research lately as another TCO material for optoelectronic devices thanks to its versatility [1]. However, the control over the its physical, electronic and optical properties is very important for these applications while the presence of dopants is known to enhance them and derive sometimes in rather unique properties that are useful for the fabrication of devices [4]. The dopant atoms of higher valence elements are mostly used to improve the electrical conductivity [1]. Amongst them, Al gained much attention due to its low cost, high abundance, n Corresponding author at: University of Bucharest, 405 Atomistilor Str., 077125 Bucharest-Magurele, Romania. E-mail address: [email protected] (A. Pruna).
non-toxicity and most of all, the ease in its incorporation in ZnO structure [5]. Thus, Al-doped ZnO (AZO), an n-type semiconductor with a bandgap around 3.4 eV [6], could be proposed as a cheaper, less toxic alternative to ITO thanks to its high transmittance and low resistivity. Lately, nanostructuring of undoped/doped ZnO has also attracted increased interest for various fields of applications due to the ease in fabrication of nanocrystals with different shapes and dimensions. Generally, the physical properties of ZnO nanorod arrays (ZnO NRAs) are strongly affected by their morphology in terms of shape, surface density, size, and vertical alignment besides doping [7]. A proper design of ZnO nanostructures could result in improved performance in sensors, transducers or solar cells [8]. For example, nanostructured arrays replaced the traditional nanoparticle film in order to improve solar cell efficiency by optimizing the electron transport, while photocatalytic activity could be optimized by tuning the face orientation of ZnO crystals [9]. Other various properties including electron-phonon coupling have been observed to be size-dependant [10,11]. Up to now, un-doped/doped ZnO nanostructures have been obtained by many techniques including chemical and physical
http://dx.doi.org/10.1016/j.ceramint.2015.07.087 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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vapor deposition that require sophisticated expensive equipments and employ vacuum and/or high temperature. The electrochemical deposition technique has become an increasingly active research area in recent years due to its low cost, mild conditions and accurate process control [1]. It is well known that electrodeposition can be applied as nanostructuring technology to a wide range of materials including semiconductor oxides (such as ZnO nanowires) [12,13], graphene materials [14,15], metals (such as Cu nanowires) [16] and conducting polymers (such as poly(o-phenylene diamine)) [17]. The study on the electrochemical growth of doped ZnO NRAs with varying size and morphology results to be a demanding factor in order to design and optimize devices in addition to novel material properties. In order to explain the properties of nanostructured doped ZnO, one should take into account factors including the size effect, grain crystallographic orientation and the influence of the growth substrate. Thus, in order to minimize the negative influence of a possible lattice mismatch between the substrate and the nanostructures to be grown, a seed layer that could offer the nucleation centers for the subsequent electrochemical growth and promote and improve the vertical alignment of nanostructures growth by nucleation effect could be used, in the consideration that the same crystal type shows the best degree of structural matching [18]. A well known industrially-applicable method to grow seed layers based on ZnO or AZO could be offered by magnetron sputter deposition thanks to the advantage of producing homogeneous films on large areas at moderate temperature [5,19]. While there are many studies on ZnO/AZO nanostructured electrodes involving seed layers, these treated the effects of sputtered AZO seed layers on the electrochemical growth of ZnO arrays [19] or the effects of sputtered ZnO buffer layer on the sputter deposition of AZO films [20], the effects of spincoated ZnO seed layer on the hydrothermal synthesis of Aldoped ZnO nanorod arrays [21,22] or the effects of spin-coated AZO on the template-electrodeposition of ZnO [23]. To the best of our knowledge, while there are many papers studying doped-ZnO NRAs by an electrochemical deposition method, there are no works a whole Al-doped ZnO electrodeposited electrode. Moreover, although ITO replacement is highly desirable, it is still employed in devices. Thus, the fabrication of whole Al-doped ZnO nanostructured electrodes becomes of extreme importance because of their potential for various applications, while tuning the properties of such electrodes by electrodeposition is still a challenging topic. Anthony et al. demonstrated, for example, that optical band gap of the ZnO nanowire arrays could be tuned by electrodeposition on ZnO seed layer [24]. Here, we exploited the advantage of the crystal lattice matching of the electrodeposited Al-doped ZnO nanostructures with a room-temperature AZO sputtered substrate which makes the need of a seed layer null and eliminates the use of ITO as substrate. The growth, morphology and structure of Al-doped ZnO NRAs were investigated in a self-seeding aqueous solution route as a function of deposition parameters and reported to the growth of ZnO arrays and on ITO
substrates. This unique low-temperature processing method could be of great importance for the application of Al-doped ZnO nanostructures. It is expected that such structure may enhance device efficiency by significantly improving interfacial quality, beside of having the benefits of a one-step growth method by removal of any substrate pre-treatment, absence of catalyst or surfactants. 2. Experimental 2.1. Materials All chemical were of reagent grade and used without further purification. Double distilled water was used to prepare all the aqueous solutions. The glass substrates (Schott AF-45 borosilicate) were cleaned by standard procedure prior to use. An AZO target (Lesker, 2% Al2O3, 99.99% purity) was used for the magnetron sputter deposition. 2.2. Fabrication of AZO substrates AZO films were deposited according to our previous work [5]. Briefly, radio-frequency (rf) magnetron sputter deposition was employed to provide the AZO film on the surface of glass substrates. The deposition was performed by applying a sputtering power of 100 W to the system kept at a total pressure of 30 mTorr and room temperature. The AZO target was placed 9 cm away from substrate. Post-annealing treatment of AZO films was performed for 1 h at 400 1C in Ar atmosphere. The thickness of the AZO films was determined from step-height measurements using an atomic force microscope (AFM). 2.3. Deposition of Al-doped ZnO nanostructures A conventional three-electrode electrochemical cell provided with a saturated calomel reference electrode, a Pt disk counter electrode and the AZO-coated glass as working electrode was employed. The electrolyte was kept at 70 1C during the deposition. Al-doped ZnO nanostructures were electrochemically deposited at 1 V from Zn(NO3)2 aqueous solution. The Zn2 þ precursor concentration varied from 0.1 to 0.001 M and the Al3 þ doping ratio was set at a Al3 þ /Zn2 þ molar ratio of 1:100. ZnO counterparts were electrodeposited in the same working conditions but in the absence of AlCl3 as doping precursor. 2.4. Characterization methods The electrochemical analysis was performed with a 500 AMEL potentiostat. The surface morphology of the substrates, ZnO and Al-doped ZnO nanostructures, respectively was observed with a FEI Quanta 200 3D scanning electron microscope by using an acceleration voltage of 30 kV. AZO film topography was studied by atomic force microscopy (AFM) using a Veeco 3100 microscope in tapping mode. The crystal structure of the materials was investigated with an X-ray diffraction Seifert 3003 PTS diffractometer with Cu-Kα and λ ¼ 0.15419 nm and operating at 40 kV and 40 mA.
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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3. Results and discussion 3.1. Characterization of the AZO substrate The fabrication of AZO films was optimized in terms of morphological, optical and structural properties as a function of sputtering parameters as we previously showed [5]. Briefly, in order to obtain crystalline AZO film with low roughness and good optical and electrical properties, the sputtering power and plasma pressure were set at 100 W and 30 mTorr, respectively. In addition, the 100 W rf sputtering power has been shown to lead to high crystalline electrodeposited ZnO based nanostructures with the (002) ZnO peak emerged as the dominant peak [25]. Considering the marked influence of the seed layer on the morphology of the electrodeposited NRAs as demonstrated by previous works where thicker seed layer have been shown to induce the formation of homogeneous structures [26], the thickness of the seed layer was set at 100 nm. Fig. 1 presents the morphology of AZO film with reference to the one of ITO-coated glass substrate. As one can see, the ITO in Fig. 1a shows a nonhomogeneous morphology with closely packed nanocrystallites of variable size and growth orientation. On the other hand, the representative AFM image of AZO film showed highly homogenous morphology and grain size, as depicted in Fig. 1b. The grains collectively form a slightly openpacked structure. The root mean square roughness RMS for AZO film was found below 3 nm while the RMS for the plain glass was 0.20 nm. Although it was suggested that growth of ZnO is defect site-driven and increased surface roughness increase the rate of reduction of nitrate ions [27], a homogeneous ordered array of nanostructures is less likely to be obtained on such substrates. To this end, the AFM image in Fig. 1 clearly indicates that the homogeneous crystallite size of the AZO films sputtered under the given conditions may offer facile nucleation sites for the growth of homogeneous AZO NRAs and no seeding steps are further required since the interfacial energy can be highly reduced. Previous results on the doping with Al atoms have shown that it has low effect on the structure and crystal orientation, but it markedly affects the electrical and optical properties e.g.,
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the electrical resistivity was found to firstly increase and then decrease with increasing Al content due to the increase of carrier concentration and the decrease of mobility, while the transmission in the visible region was found to increase but with lower yield at higher Al doping levels [21]. The increase in the transmission was explained by a larger energy bandgap resulted from the shift of the Fermi level into the conduction band induced by the free carriers provided by the incorporation of Al atoms [28], while the decrease in transmission at higher doping level was attributed to an increase in the ionized impurity scattering centers [29]. As one can see, the crystal structure and optical property of the sputtered AZO seed films presented in Fig. 2 are in agreement with previous findings. The XRD spectra of AZO film in Fig. 2a revealed crystalline hexagonal structure and a preferred orientation with the c-axis perpendicular to the substrate. The (002) preferred orientation of the obtained seed layer is highly desired for the growth of well defined vertical structures to the substrate, as previously shown [30]. In addition, no significant peaks for metal Al or Al2O3 were observed, thus revealing that the doping of Al atoms did not result in significant changes in the structure and crystal orientation of the wurtzite-type ZnO film. On the other hand, excellent optical properties of sputtered AZO film were recorded with very high transmittance and a bandgap of 3.37 eV as shown in Fig. 2b [5].
3.2. Electrodeposition of Al-doped ZnO NRAs on AZO substrate The nucleation and growth mechanism of ZnO in aqueous solutions have been previously investigated by numerous groups [12,13,31]. Here, we focus on zinc nitrate approach as the precursor of both the hydroxide ions and the zinc ions. The electrodeposition mechanism of ZnO through nitrate precursors is based on the formation of hydroxide ions from the reduction of nitrates at the substrate. The progressing formation of hydroxide ions leads to a local pH increase that induces the formation of intermediate species (zinc hydroxides). After dehydration of Zn(OH)2, the final product (ZnO) forms
Fig. 1. (a) SEM images of ITO and (b) AFM image of AZO film. Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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Fig. 2. (a) XRD and (b) transmittance spectra of sputtered AZO film.
Zn(NO3)2-Zn2 þ þ 2NO3
(1)
NO3 þ 2e þ H2O-2OH þ 2NO2
(2)
Zn2 þ þ 2OH -Zn(OH)2
(3)
Zn(OH)2-ZnO þ H2O
(4)
In order to characterize the self-seeding properties of the AZO substrates for the electrodeposition of doped ZnO NRAs, the deposition progress was monitored both on ITO and AZO, as a function of precursor concentration and doping. The chronoamperometric curves in Fig. 3 evidence the differences between the deposition of pure ZnO and Al-doped ZnO NRAs at 1 V on both substrates. As expected, the deposition current for ZnO NRAs on ITO depicted in Fig. 3a decreased with ZnO precursor concentration from 100 mM to 1 mM. This is due to the fact that zinc ions diffusion is not limited while their concentration is high, thus the reaction (3) is accelerated and the electrodeposition rate of ZnO is higher. In the case of lower ZnO precursor concentration, the electrodeposition process is restricted and induces a decrease of current density. Upon the addition of Al3 þ into the electrolyte, a synergistic effect occurred in all cases, that is, the current employed for deposition of Al-doped ZnO increased with respect to the current of un-doped ZnO at all stages of the deposition process. Since the images in Fig. 3 depict for exemplification only the growth up to 600 s, it should be stated that the trend of the current for ZnO deposition from low precursor concentration is higher than for corresponding doped ZnO only in this first stage and it further stabilizes at the same value as for the doped one (after 30 min). This behavior is due to the low concentration of the precursor and the diffusion limited process. It should be noted as well, that the concentration of Zn2 þ ions in the bath is controlled to be 100 times larger than that of Al3 þ , thus the deposition of ZnO dominates the process and Al3 þ ions serve as dopants. Since a higher nitrate concentration is discarded by using chloride salt for doping, it cannot be used to explain the increase in current which is rather attributed to a catalytic effect of Al3 þ ions for the process of reduction of nitrate ions and confirms the successful doping of ZnO.
On the other hand, a general decrease in the current value for the deposition of the counterparts on AZO substrates is observed in Fig. 3b. This is attributed to the higher resistivity of AZO film with respect to ITO and confirms the importance of substrates properties for the nucleation process. As observed in Fig. 3b, the decrease in the ZnO precursor concentration and the doping procedure results in a similar trend of the deposition current on AZO. This result confirms the successful doping of the ZnO NRAs on AZO substrates, as well. The trend of current transients in Fig. 3a shows that the current density remains constant after an initial stage. This observation is valid for all cases (pure and doped) ZnO on both substrates and it indicates the two steps of the electrodeposition process–the first one related to the reduction reaction (1) when the growth rate is lower and the cathodic current markedly increases with the increase in the hydroxide ions near/on the ITO surface resulting in the formation of many small ZnO crystals on the ITO ones. After the initial step, the growth proceeds faster according to the reactions (3) and (4). The formation of the diffusion layer on the ITO surface for the ZnO growth is barely affected by doping procedure at high concentration although the duration of nuclei formation is increased. At low precursor concentration, the slopes of the current transient suggest a change in the diffusion layer formation upon doping and the duration of nuclei formation appears increased with respect to higher precursor concentration. As it is known that low concentrations are needed for the growth of NRAs, Fig. 3b presents for exemplification the growth of pure and doped ZnO on AZO substrates from 5 and 1 mM precursor concentration, respectively. One can observe that diffusion is clearly affected by the nature of substrate suggesting that nucleation is governed by the buffer crystallites during its first step. However, the coalescence of neighboring nucleation sites during a second step, cannot be excluded and further study is necessary in order to understand the nucleation and growth mechanisms. In order to confirm the influence of the buffer crystallites of AZO on the growth of NRAs, the Scharifker and Hills [32] model is further used to evidence the nucleation process during
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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Fig. 3. Current transients for electrodeposition of ZnO and Al-doped ZnO on ITO substrate (a) and AZO substrate (b) as a function of Zn(NO3)2 concentration. (Al3 þ :Zn2 þ molar ratio 1:100).
Fig. 4. Experimental current transients plotted in reduced current–time coordinates for the growth of ZnO from 1 mM Zn(NO3)2 on (a) ITO and (b) AZO and (c) from 5 mM Zn(NO3)2 on AZO substrate. (progressive and instantaneous nucleation models are presented in gray shaded lines).
the initial few seconds using chronoamperometric techniques. Progressive nucleation corresponds to slow growth of nuclei on a small number of active sites, all activated at the same time while the instantaneous nucleation corresponds to fast growth of nuclei on many active sites, all activated during the course of electro-reduction [33,34]. Hence, the method of obtaining information about kinetics of the nucleation process extensively used for a wide variety of systems [35,36] is to analyze the transients in a reduced form in terms of the maximum cathodic current (im) and the time when this is observed (tm). The fitting of experimental curves with theoretical ones for electrodeposition of ZnO on both ITO as well as AZO substrate is shown in Fig. 4. For exemplification, the image in Fig. 4a and b depicts the transients for the growth of ZnO from 1 mM Zn(NO3)2. One can see that the data in Fig. 4 neither follow the progressive nor the instantaneous growth mechanism and most probably it follows an intermediate nucleation mechanism. Anyhow, it can be observed that the nucleation on ITO is closer to the instantaneous model while on AZO substrate is closer to the progressive one. This can be explained on the fact that the preferential growth in an electrochemical process is determined by the energetically favorable way. Thus, for a
substrate with higher resistivity, the electron transfer in the electrodeposition process takes place on the seed layer where the crystals prefer the growth along the direction with low resistivity as per to reduce the difficulty of charge transfer. On the contrary, on more conductive substrate such as ITO, the crystals would nucleate more easily occupying all the available nucleation sites. However, the complexity of the nucleation and growth of ZnO on AZO substrate (see Fig. 4c) indicates a marked influence of both precursor concentration and buffer crystallites of the substrate [37]. 3.3. Crystal structure of Al-doped ZnO NRAs The structural changes and identification of phases in Al-doped ZnO NRAs were studied with the help of the XRD technique. We firstly performed XRD measurements to investigate the effect of the ZnO precursor concentration on the crystal structure of the electrodeposited nanostructures. The XRD patterns observed for the intrinsic ZnO NRAs electrodeposited on AZO substrates consisted of Bragg reflections of hexagonal ZnO with the preferential growth oriented along the c-axis perpendicular to the substrate, as depicted in Fig. 5a. The peak of electrodeposited intrinsic ZnO is observed at 34.51, irrespective of the precursor
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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A. Pruna et al. / Ceramics International ] (]]]]) ]]]–]]] Table 1 Average grain size calculated from XRD measurements as a function of precursor concentration and doping. Material
ZnO Al-doped ZnOa a
Zn(NO3)2 concentration 1 mM
5 mM
85 nm 76 nm
60 nm 52 nm
Al3 þ :Zn2 þ molar ratio 1:100.
Fig. 6. Top-view SEM image of ZnO NRAs grown on ITO substrate.
Fig. 5. XRD spectra of NRAs electrodeposited on AZO substrates: (a) ZnO as function of Zn(NO3)2 concentration, (b) Al-doped ZnO NRAs as function of Zn(NO3)2 concentration (Al3 þ :Zn2 þ molar ratio 1:100) and (c) (002) peak intensity evolution.
concentration, while the peak of AZO substrate was 34.41 (see Figs. 5a and 2a). To identify the effect of Al doping on the peak shift of the Bragg reflection, XRD measurements were performed also on
Al-doped ZnO NRAs electrodeposited from the same electrolytes, but in the presence of doping precursor. The spectra in Fig. 5b feature the characteristic peak pattern of hexagonal ZnO (wurtzite type) with c-axis preferential orientation also in the case of Al-doped ZnO NRAs. Considering that the AZO seed layer used as substrate showed (002) preferential orientation and the atoms of electrodeposited Al-doped ZnO crystals are connected by covalent bond, the in-plane electrical conductivity was much higher than along other direction, which made it easier for the crystals to grow along (002) direction during electrodeposition. The appearance of doped ZnO peak at 34.46–34.491 indicates a change in the average lattice constant by the down-shift of the peak with respect to the intrinsic ZnO NRAs which can be attributed to the replacement of a Zn atom by the Al atom introduced into the ZnO NRAs during electrodeposition. The contraction of the ZnO lattice was expected upon introduction of Al3 þ into ZnO due to the smaller radius of Al3 þ than for zinc and it is in agreement with previous results observed for doping of ZnO with other elements [38]. Further, in order to assess the effect of the dopant concentration in the electrolytic bath, different ZnO precursor concentrations (Al3 þ /Zn2 þ molar ratio of 1:100) were studied. It was observed that the (002) peak shift of Bragg reflection for the Al-doped ZnO NRAs presented in Fig. 5b increased with increasing Zn(NO3)2 concentration employed for the electrodeposition. Moreover, the (002) peak intensity depicted in Fig. 5c is observed to decrease with higher dopant concentration, suggesting a degradation of the
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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Fig. 7. Top-view SEM images of ZnO NRAs grown on AZO substrate from: (a) 5 mM Zn(NO3)2 and (b) 1 mM Zn(NO3)2.
Fig. 8. Tilt SEM images of ZnO (top row) and Al-doped ZnO (low row) NRAs grown on AZO substrate from: (a, c) 5 mM Zn(NO3)2 and (b, d) 1 mM Zn(NO3)2.
crystal structure. Although the growth along c-axis remained preferential, the crystal degradation of doped ZnO deposited from higher content of Al3 þ is supported also by the presence of very weak diffraction peaks of (100), (101) and (102) ZnO planes and the presence of (006) peak of Al2O3 peak at 41.71. The weakening of (002) preferential orientation of doped ZnO with increasing dopant concentration might be attributed to higher growth rate of ZnO (higher current density). Thus, less-aligned Al-doped ZnO formed as the precursor concentration increased
above 5 mM. Considering the increase in the (002) peak shift, the decrease of the (002) peak intensity and the appearance of Al2O3 peak in the spectrum, a further reducing of the ZnO lattice size with the Al3 þ content in the electrolyte is suggested and it can be attributed to the introduction of a higher dopant content in the lattice. These results come into good agreement with the electrochemical results and confirm the doping and the increased Al content in the electrodeposited doped ZnO NRAs with increasing dopant precursor concentration.
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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Table 2 Average tip diameter and length of the ZnO rods electrodeposited on AZO substrate calculated from SEM images, as a function of precursor concentration and doping. Property
Diameter, nm Length, nm Density, 10 7 cm 2 a
Material
ZnO Al-doped ZnOa ZnO Al-doped ZnOa ZnO Al-doped ZnOa
Zn(NO3)2 concentration 1 mM
5 mM
95 76 530 360 4.8 6.8
68 60 620 510 4.4 5.6
Al3 þ :Zn2 þ molar ratio 1:100.
The grain size was further estimated from the width at halfmaximum of (002) X-ray diffraction peak—see Table 1. The average values were in all cases greater than the value corresponding to AZO substrate (48 nm). It should be noted that the crystallite size can also be interpreted as the mean size of a distribution of single domain grains of different sizes. Upon the addition of dopant in the electrolytic bath, a decrease was observed in the grain size with respect to corresponding un-doped ZnO, indicating the formation of increased number of nuclei and a slower growth on these. These results confirm the electrochemical ones in regards to the catalytic effect of Al3 þ doping on the electroreduction of nitrate ions at the surface of the electrode.
to another and along the length for the NRs electrodeposited from low precursor concentration, while a bimodal distribution of the NRs diameter was revealed for the counterparts electrodeposited from relatively high precursor concentration. The unimodal distribution of NR diameter obtained from low precursor concentration is probably due to the spatial hindrance caused by the formation of more nuclei, that can explain the increase in the perpendicular orientation of the NRs and confirms the XRD results. An analysis of SEM images in Figs. 7 and 8 in regards to the average values of nanorod tip diameter, length and density of the rods is presented in Table 2. The results indicate the formation of shorter nanorods with the decrease in precursor concentration due to an increased nuclei density, while the doping with Al3 þ appeared to fasten the growth on an increased number of nucleation sites thus, resulting in thinner rods. 4. Conclusions Vertically aligned, crystalline Al-doped ZnO NRs with wurtzite hexagonal phase were successfully electrodeposited on AZO films by a self-seeding aqueous solution approach. The results indicated that nanocrystalline AZO substrate allows an increase of the nanorods density in comparison to ITO one. A progressive tendency and a more complex process were observed for the nucleation of ZnO and Al-doped Zno nanorods, respectively at the surface of AZO films. The lattice size and length of doped ZnO nanorods decreased with the Al3 þ content while the decrease in the Zn2 þ precursor resulted in an increased number of nanorods.
3.4. Morphology analysis of electrodeposited AZO NRAs Acknowledgments SEM analysis was performed in order to analyze the influence of the precursor concentration, doping and the substrate on the morphology of the electrodeposits. First, the surface morphology of the ZnO NRAs deposited on ITO substrate was investigated. As one can see from the representative image depicted in Fig. 6, the bare ITO substrate was covered in a non-uniform distribution by randomlyoriented ZnO NRAs of about 180 nm in diameter. This could be due to the relatively higher precursor concentration and the increased reduction rate of nitrate ions at the defect sites on the ITO surface, as shown in Fig. 1. On the other hand, it is known that films with low sheet resistance facilitate electrochemically induced growth reaction, favoring the nucleation of ZnO at the electrode interface. Conversely, the diameter of the nanorods can be controlled by the AZO grains, since the nucleation events generate nanocrystals aligned to the wurtzite c-axis of AZO grains and thus, a homogeneous coverage is facilitated. Simultaneously, the small lattice mismatch and large structural similarity of AZO substrates offer plentiful low energy sites for ZnO crystallites, as confirmed by the increased density of ZnO NRs exhibited in Fig. 7. The evolution of ZnO morphology with precursor concentration (see Fig. 7) indicates a higher density of nanorods with the decrease in precursor concentration. Moreover, a careful analysis of the tilted views of doped and un-doped ZnO NRAs (see Fig. 8) shows a homogeneous morphology from one rod
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Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays A. Prunaa,b,n, D. Pullinic, D. Busquetsb b
a University of Bucharest, 405 Atomistilor Str., 077125 Bucharest-Magurele, Romania Institute of Materials Technology, University Politecnica of Valencia, Camino de Vera s/n, 46022 Valencia, Spain c Fiat Research Centre, 50 Torino Str., 10043 Orbassano, Italy
Received 23 April 2015; received in revised form 5 July 2015; accepted 14 July 2015
Abstract The fabrication of whole Al-doped ZnO nanostructured electrodes is reported by a two-step technology. Magnetron sputtered Al-doped ZnO (AZO) films were investigated as seeding substrates for the electrodeposition of Al-doped ZnO nanorod arrays. The nucleation, density, morphological and structural properties of Al-doped ZnO NRAs were analyzed as a function of precursor concentration and substrate and compared to undoped ZnO NRAs. High density and good crystal quality are reported for the Al-doped ZnO NRAs grown on AZO films from low precursor concentration. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Grain growth; B. Electron microscopy; B. X-ray methods; D. ZnO
1. Introduction A wide range of applications in fields including solar cells, sensors or transducers have made use of transparent conductive oxides (TCOs) thanks to their remarkable properties [1–3]. Indium tin oxide (ITO) is one of the most employed TCO materials [1]. Nevertheless, it presents various drawbacks such as toxicity or low chemical stability beside the fact that indium is a scarce element on Earth. ZnO has received extensive research lately as another TCO material for optoelectronic devices thanks to its versatility [1]. However, the control over the its physical, electronic and optical properties is very important for these applications while the presence of dopants is known to enhance them and derive sometimes in rather unique properties that are useful for the fabrication of devices [4]. The dopant atoms of higher valence elements are mostly used to improve the electrical conductivity [1]. Amongst them, Al gained much attention due to its low cost, high abundance, n Corresponding author at: University of Bucharest, 405 Atomistilor Str., 077125 Bucharest-Magurele, Romania. E-mail address: [email protected] (A. Pruna).
non-toxicity and most of all, the ease in its incorporation in ZnO structure [5]. Thus, Al-doped ZnO (AZO), an n-type semiconductor with a bandgap around 3.4 eV [6], could be proposed as a cheaper, less toxic alternative to ITO thanks to its high transmittance and low resistivity. Lately, nanostructuring of undoped/doped ZnO has also attracted increased interest for various fields of applications due to the ease in fabrication of nanocrystals with different shapes and dimensions. Generally, the physical properties of ZnO nanorod arrays (ZnO NRAs) are strongly affected by their morphology in terms of shape, surface density, size, and vertical alignment besides doping [7]. A proper design of ZnO nanostructures could result in improved performance in sensors, transducers or solar cells [8]. For example, nanostructured arrays replaced the traditional nanoparticle film in order to improve solar cell efficiency by optimizing the electron transport, while photocatalytic activity could be optimized by tuning the face orientation of ZnO crystals [9]. Other various properties including electron-phonon coupling have been observed to be size-dependant [10,11]. Up to now, un-doped/doped ZnO nanostructures have been obtained by many techniques including chemical and physical
http://dx.doi.org/10.1016/j.ceramint.2015.07.087 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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vapor deposition that require sophisticated expensive equipments and employ vacuum and/or high temperature. The electrochemical deposition technique has become an increasingly active research area in recent years due to its low cost, mild conditions and accurate process control [1]. It is well known that electrodeposition can be applied as nanostructuring technology to a wide range of materials including semiconductor oxides (such as ZnO nanowires) [12,13], graphene materials [14,15], metals (such as Cu nanowires) [16] and conducting polymers (such as poly(o-phenylene diamine)) [17]. The study on the electrochemical growth of doped ZnO NRAs with varying size and morphology results to be a demanding factor in order to design and optimize devices in addition to novel material properties. In order to explain the properties of nanostructured doped ZnO, one should take into account factors including the size effect, grain crystallographic orientation and the influence of the growth substrate. Thus, in order to minimize the negative influence of a possible lattice mismatch between the substrate and the nanostructures to be grown, a seed layer that could offer the nucleation centers for the subsequent electrochemical growth and promote and improve the vertical alignment of nanostructures growth by nucleation effect could be used, in the consideration that the same crystal type shows the best degree of structural matching [18]. A well known industrially-applicable method to grow seed layers based on ZnO or AZO could be offered by magnetron sputter deposition thanks to the advantage of producing homogeneous films on large areas at moderate temperature [5,19]. While there are many studies on ZnO/AZO nanostructured electrodes involving seed layers, these treated the effects of sputtered AZO seed layers on the electrochemical growth of ZnO arrays [19] or the effects of sputtered ZnO buffer layer on the sputter deposition of AZO films [20], the effects of spincoated ZnO seed layer on the hydrothermal synthesis of Aldoped ZnO nanorod arrays [21,22] or the effects of spin-coated AZO on the template-electrodeposition of ZnO [23]. To the best of our knowledge, while there are many papers studying doped-ZnO NRAs by an electrochemical deposition method, there are no works a whole Al-doped ZnO electrodeposited electrode. Moreover, although ITO replacement is highly desirable, it is still employed in devices. Thus, the fabrication of whole Al-doped ZnO nanostructured electrodes becomes of extreme importance because of their potential for various applications, while tuning the properties of such electrodes by electrodeposition is still a challenging topic. Anthony et al. demonstrated, for example, that optical band gap of the ZnO nanowire arrays could be tuned by electrodeposition on ZnO seed layer [24]. Here, we exploited the advantage of the crystal lattice matching of the electrodeposited Al-doped ZnO nanostructures with a room-temperature AZO sputtered substrate which makes the need of a seed layer null and eliminates the use of ITO as substrate. The growth, morphology and structure of Al-doped ZnO NRAs were investigated in a self-seeding aqueous solution route as a function of deposition parameters and reported to the growth of ZnO arrays and on ITO
substrates. This unique low-temperature processing method could be of great importance for the application of Al-doped ZnO nanostructures. It is expected that such structure may enhance device efficiency by significantly improving interfacial quality, beside of having the benefits of a one-step growth method by removal of any substrate pre-treatment, absence of catalyst or surfactants. 2. Experimental 2.1. Materials All chemical were of reagent grade and used without further purification. Double distilled water was used to prepare all the aqueous solutions. The glass substrates (Schott AF-45 borosilicate) were cleaned by standard procedure prior to use. An AZO target (Lesker, 2% Al2O3, 99.99% purity) was used for the magnetron sputter deposition. 2.2. Fabrication of AZO substrates AZO films were deposited according to our previous work [5]. Briefly, radio-frequency (rf) magnetron sputter deposition was employed to provide the AZO film on the surface of glass substrates. The deposition was performed by applying a sputtering power of 100 W to the system kept at a total pressure of 30 mTorr and room temperature. The AZO target was placed 9 cm away from substrate. Post-annealing treatment of AZO films was performed for 1 h at 400 1C in Ar atmosphere. The thickness of the AZO films was determined from step-height measurements using an atomic force microscope (AFM). 2.3. Deposition of Al-doped ZnO nanostructures A conventional three-electrode electrochemical cell provided with a saturated calomel reference electrode, a Pt disk counter electrode and the AZO-coated glass as working electrode was employed. The electrolyte was kept at 70 1C during the deposition. Al-doped ZnO nanostructures were electrochemically deposited at 1 V from Zn(NO3)2 aqueous solution. The Zn2 þ precursor concentration varied from 0.1 to 0.001 M and the Al3 þ doping ratio was set at a Al3 þ /Zn2 þ molar ratio of 1:100. ZnO counterparts were electrodeposited in the same working conditions but in the absence of AlCl3 as doping precursor. 2.4. Characterization methods The electrochemical analysis was performed with a 500 AMEL potentiostat. The surface morphology of the substrates, ZnO and Al-doped ZnO nanostructures, respectively was observed with a FEI Quanta 200 3D scanning electron microscope by using an acceleration voltage of 30 kV. AZO film topography was studied by atomic force microscopy (AFM) using a Veeco 3100 microscope in tapping mode. The crystal structure of the materials was investigated with an X-ray diffraction Seifert 3003 PTS diffractometer with Cu-Kα and λ ¼ 0.15419 nm and operating at 40 kV and 40 mA.
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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3. Results and discussion 3.1. Characterization of the AZO substrate The fabrication of AZO films was optimized in terms of morphological, optical and structural properties as a function of sputtering parameters as we previously showed [5]. Briefly, in order to obtain crystalline AZO film with low roughness and good optical and electrical properties, the sputtering power and plasma pressure were set at 100 W and 30 mTorr, respectively. In addition, the 100 W rf sputtering power has been shown to lead to high crystalline electrodeposited ZnO based nanostructures with the (002) ZnO peak emerged as the dominant peak [25]. Considering the marked influence of the seed layer on the morphology of the electrodeposited NRAs as demonstrated by previous works where thicker seed layer have been shown to induce the formation of homogeneous structures [26], the thickness of the seed layer was set at 100 nm. Fig. 1 presents the morphology of AZO film with reference to the one of ITO-coated glass substrate. As one can see, the ITO in Fig. 1a shows a nonhomogeneous morphology with closely packed nanocrystallites of variable size and growth orientation. On the other hand, the representative AFM image of AZO film showed highly homogenous morphology and grain size, as depicted in Fig. 1b. The grains collectively form a slightly openpacked structure. The root mean square roughness RMS for AZO film was found below 3 nm while the RMS for the plain glass was 0.20 nm. Although it was suggested that growth of ZnO is defect site-driven and increased surface roughness increase the rate of reduction of nitrate ions [27], a homogeneous ordered array of nanostructures is less likely to be obtained on such substrates. To this end, the AFM image in Fig. 1 clearly indicates that the homogeneous crystallite size of the AZO films sputtered under the given conditions may offer facile nucleation sites for the growth of homogeneous AZO NRAs and no seeding steps are further required since the interfacial energy can be highly reduced. Previous results on the doping with Al atoms have shown that it has low effect on the structure and crystal orientation, but it markedly affects the electrical and optical properties e.g.,
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the electrical resistivity was found to firstly increase and then decrease with increasing Al content due to the increase of carrier concentration and the decrease of mobility, while the transmission in the visible region was found to increase but with lower yield at higher Al doping levels [21]. The increase in the transmission was explained by a larger energy bandgap resulted from the shift of the Fermi level into the conduction band induced by the free carriers provided by the incorporation of Al atoms [28], while the decrease in transmission at higher doping level was attributed to an increase in the ionized impurity scattering centers [29]. As one can see, the crystal structure and optical property of the sputtered AZO seed films presented in Fig. 2 are in agreement with previous findings. The XRD spectra of AZO film in Fig. 2a revealed crystalline hexagonal structure and a preferred orientation with the c-axis perpendicular to the substrate. The (002) preferred orientation of the obtained seed layer is highly desired for the growth of well defined vertical structures to the substrate, as previously shown [30]. In addition, no significant peaks for metal Al or Al2O3 were observed, thus revealing that the doping of Al atoms did not result in significant changes in the structure and crystal orientation of the wurtzite-type ZnO film. On the other hand, excellent optical properties of sputtered AZO film were recorded with very high transmittance and a bandgap of 3.37 eV as shown in Fig. 2b [5].
3.2. Electrodeposition of Al-doped ZnO NRAs on AZO substrate The nucleation and growth mechanism of ZnO in aqueous solutions have been previously investigated by numerous groups [12,13,31]. Here, we focus on zinc nitrate approach as the precursor of both the hydroxide ions and the zinc ions. The electrodeposition mechanism of ZnO through nitrate precursors is based on the formation of hydroxide ions from the reduction of nitrates at the substrate. The progressing formation of hydroxide ions leads to a local pH increase that induces the formation of intermediate species (zinc hydroxides). After dehydration of Zn(OH)2, the final product (ZnO) forms
Fig. 1. (a) SEM images of ITO and (b) AFM image of AZO film. Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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Fig. 2. (a) XRD and (b) transmittance spectra of sputtered AZO film.
Zn(NO3)2-Zn2 þ þ 2NO3
(1)
NO3 þ 2e þ H2O-2OH þ 2NO2
(2)
Zn2 þ þ 2OH -Zn(OH)2
(3)
Zn(OH)2-ZnO þ H2O
(4)
In order to characterize the self-seeding properties of the AZO substrates for the electrodeposition of doped ZnO NRAs, the deposition progress was monitored both on ITO and AZO, as a function of precursor concentration and doping. The chronoamperometric curves in Fig. 3 evidence the differences between the deposition of pure ZnO and Al-doped ZnO NRAs at 1 V on both substrates. As expected, the deposition current for ZnO NRAs on ITO depicted in Fig. 3a decreased with ZnO precursor concentration from 100 mM to 1 mM. This is due to the fact that zinc ions diffusion is not limited while their concentration is high, thus the reaction (3) is accelerated and the electrodeposition rate of ZnO is higher. In the case of lower ZnO precursor concentration, the electrodeposition process is restricted and induces a decrease of current density. Upon the addition of Al3 þ into the electrolyte, a synergistic effect occurred in all cases, that is, the current employed for deposition of Al-doped ZnO increased with respect to the current of un-doped ZnO at all stages of the deposition process. Since the images in Fig. 3 depict for exemplification only the growth up to 600 s, it should be stated that the trend of the current for ZnO deposition from low precursor concentration is higher than for corresponding doped ZnO only in this first stage and it further stabilizes at the same value as for the doped one (after 30 min). This behavior is due to the low concentration of the precursor and the diffusion limited process. It should be noted as well, that the concentration of Zn2 þ ions in the bath is controlled to be 100 times larger than that of Al3 þ , thus the deposition of ZnO dominates the process and Al3 þ ions serve as dopants. Since a higher nitrate concentration is discarded by using chloride salt for doping, it cannot be used to explain the increase in current which is rather attributed to a catalytic effect of Al3 þ ions for the process of reduction of nitrate ions and confirms the successful doping of ZnO.
On the other hand, a general decrease in the current value for the deposition of the counterparts on AZO substrates is observed in Fig. 3b. This is attributed to the higher resistivity of AZO film with respect to ITO and confirms the importance of substrates properties for the nucleation process. As observed in Fig. 3b, the decrease in the ZnO precursor concentration and the doping procedure results in a similar trend of the deposition current on AZO. This result confirms the successful doping of the ZnO NRAs on AZO substrates, as well. The trend of current transients in Fig. 3a shows that the current density remains constant after an initial stage. This observation is valid for all cases (pure and doped) ZnO on both substrates and it indicates the two steps of the electrodeposition process–the first one related to the reduction reaction (1) when the growth rate is lower and the cathodic current markedly increases with the increase in the hydroxide ions near/on the ITO surface resulting in the formation of many small ZnO crystals on the ITO ones. After the initial step, the growth proceeds faster according to the reactions (3) and (4). The formation of the diffusion layer on the ITO surface for the ZnO growth is barely affected by doping procedure at high concentration although the duration of nuclei formation is increased. At low precursor concentration, the slopes of the current transient suggest a change in the diffusion layer formation upon doping and the duration of nuclei formation appears increased with respect to higher precursor concentration. As it is known that low concentrations are needed for the growth of NRAs, Fig. 3b presents for exemplification the growth of pure and doped ZnO on AZO substrates from 5 and 1 mM precursor concentration, respectively. One can observe that diffusion is clearly affected by the nature of substrate suggesting that nucleation is governed by the buffer crystallites during its first step. However, the coalescence of neighboring nucleation sites during a second step, cannot be excluded and further study is necessary in order to understand the nucleation and growth mechanisms. In order to confirm the influence of the buffer crystallites of AZO on the growth of NRAs, the Scharifker and Hills [32] model is further used to evidence the nucleation process during
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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Fig. 3. Current transients for electrodeposition of ZnO and Al-doped ZnO on ITO substrate (a) and AZO substrate (b) as a function of Zn(NO3)2 concentration. (Al3 þ :Zn2 þ molar ratio 1:100).
Fig. 4. Experimental current transients plotted in reduced current–time coordinates for the growth of ZnO from 1 mM Zn(NO3)2 on (a) ITO and (b) AZO and (c) from 5 mM Zn(NO3)2 on AZO substrate. (progressive and instantaneous nucleation models are presented in gray shaded lines).
the initial few seconds using chronoamperometric techniques. Progressive nucleation corresponds to slow growth of nuclei on a small number of active sites, all activated at the same time while the instantaneous nucleation corresponds to fast growth of nuclei on many active sites, all activated during the course of electro-reduction [33,34]. Hence, the method of obtaining information about kinetics of the nucleation process extensively used for a wide variety of systems [35,36] is to analyze the transients in a reduced form in terms of the maximum cathodic current (im) and the time when this is observed (tm). The fitting of experimental curves with theoretical ones for electrodeposition of ZnO on both ITO as well as AZO substrate is shown in Fig. 4. For exemplification, the image in Fig. 4a and b depicts the transients for the growth of ZnO from 1 mM Zn(NO3)2. One can see that the data in Fig. 4 neither follow the progressive nor the instantaneous growth mechanism and most probably it follows an intermediate nucleation mechanism. Anyhow, it can be observed that the nucleation on ITO is closer to the instantaneous model while on AZO substrate is closer to the progressive one. This can be explained on the fact that the preferential growth in an electrochemical process is determined by the energetically favorable way. Thus, for a
substrate with higher resistivity, the electron transfer in the electrodeposition process takes place on the seed layer where the crystals prefer the growth along the direction with low resistivity as per to reduce the difficulty of charge transfer. On the contrary, on more conductive substrate such as ITO, the crystals would nucleate more easily occupying all the available nucleation sites. However, the complexity of the nucleation and growth of ZnO on AZO substrate (see Fig. 4c) indicates a marked influence of both precursor concentration and buffer crystallites of the substrate [37]. 3.3. Crystal structure of Al-doped ZnO NRAs The structural changes and identification of phases in Al-doped ZnO NRAs were studied with the help of the XRD technique. We firstly performed XRD measurements to investigate the effect of the ZnO precursor concentration on the crystal structure of the electrodeposited nanostructures. The XRD patterns observed for the intrinsic ZnO NRAs electrodeposited on AZO substrates consisted of Bragg reflections of hexagonal ZnO with the preferential growth oriented along the c-axis perpendicular to the substrate, as depicted in Fig. 5a. The peak of electrodeposited intrinsic ZnO is observed at 34.51, irrespective of the precursor
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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A. Pruna et al. / Ceramics International ] (]]]]) ]]]–]]] Table 1 Average grain size calculated from XRD measurements as a function of precursor concentration and doping. Material
ZnO Al-doped ZnOa a
Zn(NO3)2 concentration 1 mM
5 mM
85 nm 76 nm
60 nm 52 nm
Al3 þ :Zn2 þ molar ratio 1:100.
Fig. 6. Top-view SEM image of ZnO NRAs grown on ITO substrate.
Fig. 5. XRD spectra of NRAs electrodeposited on AZO substrates: (a) ZnO as function of Zn(NO3)2 concentration, (b) Al-doped ZnO NRAs as function of Zn(NO3)2 concentration (Al3 þ :Zn2 þ molar ratio 1:100) and (c) (002) peak intensity evolution.
concentration, while the peak of AZO substrate was 34.41 (see Figs. 5a and 2a). To identify the effect of Al doping on the peak shift of the Bragg reflection, XRD measurements were performed also on
Al-doped ZnO NRAs electrodeposited from the same electrolytes, but in the presence of doping precursor. The spectra in Fig. 5b feature the characteristic peak pattern of hexagonal ZnO (wurtzite type) with c-axis preferential orientation also in the case of Al-doped ZnO NRAs. Considering that the AZO seed layer used as substrate showed (002) preferential orientation and the atoms of electrodeposited Al-doped ZnO crystals are connected by covalent bond, the in-plane electrical conductivity was much higher than along other direction, which made it easier for the crystals to grow along (002) direction during electrodeposition. The appearance of doped ZnO peak at 34.46–34.491 indicates a change in the average lattice constant by the down-shift of the peak with respect to the intrinsic ZnO NRAs which can be attributed to the replacement of a Zn atom by the Al atom introduced into the ZnO NRAs during electrodeposition. The contraction of the ZnO lattice was expected upon introduction of Al3 þ into ZnO due to the smaller radius of Al3 þ than for zinc and it is in agreement with previous results observed for doping of ZnO with other elements [38]. Further, in order to assess the effect of the dopant concentration in the electrolytic bath, different ZnO precursor concentrations (Al3 þ /Zn2 þ molar ratio of 1:100) were studied. It was observed that the (002) peak shift of Bragg reflection for the Al-doped ZnO NRAs presented in Fig. 5b increased with increasing Zn(NO3)2 concentration employed for the electrodeposition. Moreover, the (002) peak intensity depicted in Fig. 5c is observed to decrease with higher dopant concentration, suggesting a degradation of the
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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Fig. 7. Top-view SEM images of ZnO NRAs grown on AZO substrate from: (a) 5 mM Zn(NO3)2 and (b) 1 mM Zn(NO3)2.
Fig. 8. Tilt SEM images of ZnO (top row) and Al-doped ZnO (low row) NRAs grown on AZO substrate from: (a, c) 5 mM Zn(NO3)2 and (b, d) 1 mM Zn(NO3)2.
crystal structure. Although the growth along c-axis remained preferential, the crystal degradation of doped ZnO deposited from higher content of Al3 þ is supported also by the presence of very weak diffraction peaks of (100), (101) and (102) ZnO planes and the presence of (006) peak of Al2O3 peak at 41.71. The weakening of (002) preferential orientation of doped ZnO with increasing dopant concentration might be attributed to higher growth rate of ZnO (higher current density). Thus, less-aligned Al-doped ZnO formed as the precursor concentration increased
above 5 mM. Considering the increase in the (002) peak shift, the decrease of the (002) peak intensity and the appearance of Al2O3 peak in the spectrum, a further reducing of the ZnO lattice size with the Al3 þ content in the electrolyte is suggested and it can be attributed to the introduction of a higher dopant content in the lattice. These results come into good agreement with the electrochemical results and confirm the doping and the increased Al content in the electrodeposited doped ZnO NRAs with increasing dopant precursor concentration.
Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087
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Table 2 Average tip diameter and length of the ZnO rods electrodeposited on AZO substrate calculated from SEM images, as a function of precursor concentration and doping. Property
Diameter, nm Length, nm Density, 10 7 cm 2 a
Material
ZnO Al-doped ZnOa ZnO Al-doped ZnOa ZnO Al-doped ZnOa
Zn(NO3)2 concentration 1 mM
5 mM
95 76 530 360 4.8 6.8
68 60 620 510 4.4 5.6
Al3 þ :Zn2 þ molar ratio 1:100.
The grain size was further estimated from the width at halfmaximum of (002) X-ray diffraction peak—see Table 1. The average values were in all cases greater than the value corresponding to AZO substrate (48 nm). It should be noted that the crystallite size can also be interpreted as the mean size of a distribution of single domain grains of different sizes. Upon the addition of dopant in the electrolytic bath, a decrease was observed in the grain size with respect to corresponding un-doped ZnO, indicating the formation of increased number of nuclei and a slower growth on these. These results confirm the electrochemical ones in regards to the catalytic effect of Al3 þ doping on the electroreduction of nitrate ions at the surface of the electrode.
to another and along the length for the NRs electrodeposited from low precursor concentration, while a bimodal distribution of the NRs diameter was revealed for the counterparts electrodeposited from relatively high precursor concentration. The unimodal distribution of NR diameter obtained from low precursor concentration is probably due to the spatial hindrance caused by the formation of more nuclei, that can explain the increase in the perpendicular orientation of the NRs and confirms the XRD results. An analysis of SEM images in Figs. 7 and 8 in regards to the average values of nanorod tip diameter, length and density of the rods is presented in Table 2. The results indicate the formation of shorter nanorods with the decrease in precursor concentration due to an increased nuclei density, while the doping with Al3 þ appeared to fasten the growth on an increased number of nucleation sites thus, resulting in thinner rods. 4. Conclusions Vertically aligned, crystalline Al-doped ZnO NRs with wurtzite hexagonal phase were successfully electrodeposited on AZO films by a self-seeding aqueous solution approach. The results indicated that nanocrystalline AZO substrate allows an increase of the nanorods density in comparison to ITO one. A progressive tendency and a more complex process were observed for the nucleation of ZnO and Al-doped Zno nanorods, respectively at the surface of AZO films. The lattice size and length of doped ZnO nanorods decreased with the Al3 þ content while the decrease in the Zn2 þ precursor resulted in an increased number of nanorods.
3.4. Morphology analysis of electrodeposited AZO NRAs Acknowledgments SEM analysis was performed in order to analyze the influence of the precursor concentration, doping and the substrate on the morphology of the electrodeposits. First, the surface morphology of the ZnO NRAs deposited on ITO substrate was investigated. As one can see from the representative image depicted in Fig. 6, the bare ITO substrate was covered in a non-uniform distribution by randomlyoriented ZnO NRAs of about 180 nm in diameter. This could be due to the relatively higher precursor concentration and the increased reduction rate of nitrate ions at the defect sites on the ITO surface, as shown in Fig. 1. On the other hand, it is known that films with low sheet resistance facilitate electrochemically induced growth reaction, favoring the nucleation of ZnO at the electrode interface. Conversely, the diameter of the nanorods can be controlled by the AZO grains, since the nucleation events generate nanocrystals aligned to the wurtzite c-axis of AZO grains and thus, a homogeneous coverage is facilitated. Simultaneously, the small lattice mismatch and large structural similarity of AZO substrates offer plentiful low energy sites for ZnO crystallites, as confirmed by the increased density of ZnO NRs exhibited in Fig. 7. The evolution of ZnO morphology with precursor concentration (see Fig. 7) indicates a higher density of nanorods with the decrease in precursor concentration. Moreover, a careful analysis of the tilted views of doped and un-doped ZnO NRAs (see Fig. 8) shows a homogeneous morphology from one rod
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Please cite this article as: A. Pruna, et al., Effect of AZO film as seeding substrate on the electrodeposition and properties of Al-doped ZnO nanorod arrays, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.087