Electrodeposition of Zn(OH)2, ZnO thin films and nanosheet-like Zn seed layers and influence of their morphology on the growth of ZnO nanorods

Electrochimica Acta 98 (2013) 255–262

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Electrodeposition of Zn(OH)2 , ZnO thin films and nanosheet-like Zn seed layers and influence of their morphology on the growth of ZnO nanorods a ˛ Kamila Zarebska , Maciej Kwiatkowski a , Marianna Gniadek b , Magdalena Skompska a,∗ a b

Laboratory of Electrochemistry, Faculty of Chemistry, University of Warsaw, Pasteura 1, 02 093 Warsaw, Poland Laboratory of Theory and Applications of Electrodes, Faculty of Chemistry, University of Warsaw, Pasteura 1, 02 093 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 3 November 2012 Received in revised form 8 March 2013 Accepted 9 March 2013 Available online 17 March 2013 Keywords: Electrodeposition Seed layer Zn(OH)2 thin film Zn nanosheets ZnO nanorods

a b s t r a c t In this work we have studied the influence of experimental conditions (temperature, presence of oxygen in the bath solution and deposition time) on morphology, optical properties and composition of the seed layers electrochemically deposited on indium tin oxide (ITO) electrodes in aqueous solutions of Zn(NO3 )2 and Zn(CH3 COO)2 . The scanning electron microscope images of the samples prepared at different deposition charge densities were correlated with the transmittance spectra and SEM pictures of ZnO nanorods grown on the seeded substrates to determine the optimum conditions of preparation of highly oriented ZnO nanorods arrays for solar cells applications. We have also presented a method of deposition of Zn nanosheets, uniformly distributed and vertically oriented to the substrate and discussed the influence of oxygen on the form of Zn nanostructures. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction ZnO nanowires deposited on the conducting substrate are promising components in a wide range of devices for various practical applications such as field emission transistors [1–6] dye sensitized- or extremely thin absorber (ETA) solar cells [7–12], chemical sensors and biosensors [13]. Such geometry of the deposit offers a high length to diameter ratio and therefore, its total surface area might be even 1000 times larger than the geometric surface area of the substrate. This is highly advantageous for the solar cell application, because the ZnO nanorods deposited on the transparent conducting substrate may be covered with a large amount of light harvesting material, which is important in achieving high performance of the device [14,15]. Aligned nanorods may be easily obtained by means of electrochemical or hydrothermal methods by two-steps procedure. The first step consists in preparation of a seed layer on the substrate [16–19], whereas the second one is the growth of nanostructures. The seeding stage may be omitted but it is determining for the quality of the obtained nanorods (their uniformity and ordering) [20–22]. The ZnO underlayer provides an optimum degree of structural fit, plays a role of nucleation centers for the growth of ZnO

∗ Corresponding author. Tel.: +48 22 822 02 11; fax: +48 22 822 59 96. E-mail address: [email protected] (M. Skompska). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.03.051

nanostructures and also acts as buffer layer between the conducting substrate and the electrolyte. In general, the seeding may be realized by chemical methods (for example spray pyrolysis [23] or spin coating of zinc acetate solution followed by annealing at 350 ◦ C in atmosphere of O2 [16]) or by electrochemical deposition in the solutions of Zn(NO3 )2 [1,18,21,24], ZnCl2 saturated with O2 [19,25–28] or Zn(CH3 COO)2 [29,30]. The key reaction consists in precipitation of Zn2+ ions with hydroxyl anions, followed by dehydration of the deposit according to the scheme: Zn2+ + 2OH− → Zn(OH)2 → ZnO + H2 O

(1)

Two different approaches have been reported for transformation of Zn(OH)2 into ZnO: annealing of the seed layer prepared at room temperature [21,23] or one step deposition carried out at elevated temperature [19,20,22]. According to the studies reported by Goux et al. [26] performed in ZnCl2 saturated with oxygen in a broad range of temperatures (25–90 ◦ C), deposition at low temperatures (below 34 ◦ C) leads to passivation of the surface with non conducting film which undergoes to progressive dehydration at higher temperature to well-crystallised ZnO seeds. This stage seems to be crucial for the growth of ZnO nanorods on the seeded substrate and is important from the point of view of further applications of the nanostructured electrodes. For example, a seed layer for the use in photovoltaic hybrid cells should cover the whole substrate with a thin and uniform film. A full coverage is necessary to prevent the growth of irregular nanostructures as well as to avoid the electrical

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shortcuts between cathode and anode after deposition of the hole transporting layer. Additionally, the ZnO underlayer should be transparent to visible light to achieve a high efficiency of back-side illuminated device. In contrast, the ZnO nanostructurs for catalytic or sensor applications may be branched to provide a large surface area and easy access of the reagents. Therefore, it is important to control well the seeding process to obtain the ZnO nanostructures of required morphology. In this work we present the results of the studies on electrochemical seeding of ITO electrode in the solutions of Zn(NO3 )2 and Zn(CH3 COO)2 . The goal of the studies was to correlate the deposition conditions with morphology of the resultant layer and with arrangement of ZnO nanostructures grown on the seeded substrates. The electrochemical data were interpreted in correlation with SEM images, transmittance spectra and XRD results of the prepared samples and discussed in terms of mechanism of the formation of the seed layer. To our best knowledge this is the first report on electrosynthesis of seed layer in the form of Zn nanosheets vertically oriented to the substrate and application of such layer for the growth of ZnO nanorods.

2. Experimental Electrochemical deposition of the seed layers was performed in a three-electrode, single compartment electrochemical cell with ITO working electrode, a Pt counter electrode and aqueous Ag/AgCl/Cl− (3 M) reference electrode. Prior to the deposition, the ITO surface was ultrasonically cleaned and degreased with acetone and then immersed for 10 s in 3 M NaOH, rinsed with distilled H2 O, etched for 10 s in concentrated H2 SO4 and finally rinsed again thoroughly with distilled water. According to the literature [21], etching of ITO in acid peels off an “old” layer and exposes a “fresh” surface which is more active due to higher surface energy. Moreover, the etching increases the density of hydroxyl groups on the ITO surface and in consequence density of the nucleation centers for nucleation-growth process. Therefore, seeding on the etched surface could be easier than on the non-activated surface. Electrodeposition of the seed layers was carried out in two different aqueous solutions: 0.05 M Zn(NO3 )2 and 0.1 M Zn(CH3 COO)2 by cyclic voltammetry and potentiostatic method using AUTOLAB PGSTAT 30 (Ecochemie, The Netherlands). A hydrothermal growth of ZnO nanorods on the seeded substrate was performed at 80 ◦ C in the solution of 0.05 M Zn(NO3 )2 + NH3 . Amount of NH3 (28 wt % NH3 in water) was adjusted by disappearance of white precipitate of Zn(OH)2 which is formed at low amount of ammonia added but disappears at pH >10 due to formation of Zn(NH3 )4 2+ complex. The UV–vis transmittance spectra were measured by means of UV-vis spectrometer Lambda 12, (Perkin Elmer) in the wavelength range from 320 nm to 1100 nm. The crystalline structure of seed layers deposited on ITO was studied by means of the broad angle X-ray diffractometer (Bruker D8 Discover, Germany) using CuK˛ radiation source. Morphology of the samples was examined by field emission scanning electron microscopy (FESEM) (Zeiss Merlin, Germany)

3. Results and discussion 3.1. Electrodeposition of a seed layer in the solution of Zn(NO3 )2 Electrodeposition of the seed layer in the solution of 0.05 M Zn(NO3 )2 was carried out on ITO electrode at constant potential

of −1.2 V. According to the literature, the key step of the process is reduction of NO3 − ions according to the reaction: NO3 − + H2 O + 2e → NO2 − + 2OH−

(2)

Zn2+

ion on the electrode [31]. The catalyzed by adsorption of hydroxyl ions formed in this process at the electrode precipitate Zn2+ to Zn(OH)2 . An additional source of OH− is reduction of molecular oxygen present in the solution: O2 + 2H2 O + 4e → 4OH−

(3)

A general shape of the current transients presented in Fig. 1, obtained at different temperatures is similar to that reported by Lincot et al. [25,26] for deposition of ZnO from the solution of ZnCl2 saturated with molecular oxygen, on the rotating disc electrode. A monotonic decrease of the reduction current density in the experiments performed at 25 ◦ C and 40 ◦ C, typical of passivation of the electrode, is due to formation of poorly conducting film of Zn(OH)2 which is deposited on the substrate in the form of compact and uniform film of fibrillar morphology (see SEM image A in Fig. 1). The increase of deposition temperature above 40 ◦ C leads to the change of the shape of j–t curve from descending to sigmoidal one. The sigmoidal curve with three distinct regions: deep minimum followed by the increase of the current and formation of a plateau is characteristic of the kinetically controlled process of nucleation and growth [32]. This process consists in transformation (after some induction time) of the insulating hydroxide layer initially formed on the electrode into crystalline and conducting ZnO [26,33] A gradual transformation of zinc hydroxide into oxide is well illustrated by the SEM images of the samples prepared at 60 ◦ C at different deposition times (images B–E). The process seems to occur by curling the fibrils into the grains which then play the role of the growing centers. It is worth noting that before full overlapping of the growing centers, some parts of the substrate become naked (image D). This was confirmed by detailed inspection of the images obtained at higher resolution. This may have important consequences for the further growth of ZnO nanostructures, which will be discussed later in the text. Crystallinity of the seed layers prepared at two different temperatures, 25 ◦ C and 80 ◦ C was examined by X-ray diffraction. Since the samples were deposited on ITO, several characteristic signals of the substrate (at 2: 30.2 ◦ , 35.2 ◦ and 51.3 ◦ [34], denoted by asterisks) appear in all spectra presented in Fig. 2. Another peaks, observed at 34.5 ◦ , 31.6◦ and 36.5◦ are ascribed respectively to diffractions from (0 0 2), (1 0 0) and (1 0 1) planes of wurtzite ZnO [35]. In both spectra there is also an additional small signal at 45.3◦ (denoted by a hash), which is not characteristic of ZnO. We suppose that this peak may be ascribed to zinc hydroxide nitrate (Zn5 (OH)8 (NO3 )2 • 2H2 O) [36], which is often an intermediate compound during the preparation of ZnO from nitrate solutions. However, the most interesting observation is that the same signals are observed in the spectra of both samples and no signals from Zn(OH)2 appear in the spectrum of the film deposited at the room temperature. It is also important to know that intensity of the signals for the sample prepared at 25 ◦ C were very weak and therefore, the spectrum was collected for several hours. This means that already the fibrillar film of amorphous Zn(OH)2 deposited at 25 ◦ C contains some zones of polycrystalline ZnO, playing the role of nucleation centers in further transformation of zinc hydroxide into ZnO. However, the crystals in this sample are oriented randomly, which is evidenced by much lower relative intensity of the peaks corresponding to the planes (0 0 2) and (1 0 1) (I(0 0 2) /I(1 0 1) = 2) in comparison to that for the sample deposited at 80 ◦ C (I(0 0 2) /I(1 0 1) = 12.7). The reason of formation of ZnO even at low temperature is not clear. However, the formation of ZnO may be facilitated by

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Fig. 1. A comparison of chronoamperometric curves obtained for polarization of ITO at the potential of −1.2 V in the solution of 0.05 M Zn(NO3 )2 at different temperatures: 25 ◦ C, 40 ◦ C, 50 ◦ C, 60 ◦ C, 80 ◦ C and SEM images of seeded substrates obtained at different stages of deposition carried out at 60 ◦ C (images B-E). The arrows indicate the end of deposition process.

1

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electrostatic interactions of Zn2+ or complex ions with the negatively charged electrode [37]. According to the equilibrium constants of hydroxy complexes of zinc at the temperature of 25 ◦ C, in the solution of pH between 7 and 9, zinc ions exist mainly in the form of Zn2+ and Zn(OH)+ complex [38]. As it has been mentioned above, application of the ZnO nanorod arrays in DSSC solar cells requires a full coverage of the transparent conducting substrate with the seed layer to avoid the shortcuts between the collecting electrodes. This may occur if the light harvesting material is deposited not only on ZnO nanorods, but also directly on the uncovered ITO substrate. Therefore, a series of experiments was performed at 25 ◦ C and 80 ◦ C to determine the value of the deposition charge which should be spent for full coverage of the substrate with the seed layer. The SEM picture of the sample seeded at the charge density of 30 mC cm−2 at the room temperature (25 ◦ C) have shown that even at such low value the

40 2 / degree

*

1

50

Fig. 2. X-ray diffraction patterns of ZnO seed layers deposited in the solution of 0.05 M Zn(NO3 )2 at the temperature 25 ◦ C (1) and 80 ◦ C (2). The peaks labeled with asterisks correspond to ITO.

1

60

40

20

400

500

600

700

800

900

/nm Fig. 3. (a) Transmittance spectra of the samples deposited at 80 ◦ C and at the charge densities: 30 mC cm−2 (curve 1), 60 mC cm−2 (curve 2) and 90 mC cm−2 (curve 3) and 200 mC cm−2 (curve 4). (b) Transmittance spectra of the sample deposited at room temperature and at the charge density of 30 mC cm−2 (curve 1) and of the same sample after annealing at 350 ◦ C for 1 h (curve 2).

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3.1.1. Hydrothermal growth of ZnO nanorods on the seed layers electrodeposited in the solution of Zn(NO3 )2 In order to determine how the morphology of the seed layer influence on the growth of ZnO nanowires, a hydrothermal deposition of ZnO was performed on the ITO plates seeded potentiostatically in the solutions of Zn(NO3 )2 at the charge density of 30 mC cm−2 , at two temperatures: 25 ◦ C and 80 ◦ C. The

seeded substrates before hydrothermal deposition were annealed in air for 1 h at 350 ◦ C. The hydrothermal growth was carried out for 2.5 h at the temperature of 80 ◦ C in the solution of 0.05 M Zn(NO3 )2 + NH3 (see details in Section 2). As visible in Fig. 4, the substrate seeded at the room temperature is covered with uniform and well ordered nanorods with an average diameter of about 30 nm. In contrast, in the sample seeded at 80 ◦ C one can notice some naked areas without nanorods. This means that the ZnO nanorods have grown only on the nucleation centers generated by seeding. Moreover, since the rods located at the border of naked areas (denoted by circle in Fig. 4b) had more space to grow, their average diameter is greater than that of the rest of nanorods in the sample and some of them are tilted with respect to the substrate. 3.2. Deposition of a seed layer in the solution of Zn(CH3 COO)2 Zinc acetate is a precursor more often used for chemical seeding [22] or chemical bath deposition [41] of the ZnO nanostructures than for electrochemical seeding of the substrate.

0

a 1.5

-1

1.0

/ mC cm

2 1

-2

-3

-2

whole electrode was covered with a uniform fibrillar film. In contrast, the deposition performed at the same charge density at 80 ◦ C led to the formation of white spots randomly distributed on the surface. The increase of deposition charge density to 90 mC cm−2 improved the coverage but some parts of the substrate remained still naked. The full coverage was achieved above 110 mC cm−2 . On the other hand, as it is visible in Fig. 3a, the increase of the deposition charge leads to decrease of transmittance of the sample which may be disadvantageous for their further application in the solar cells with a back-illuminated configuration. Therefore, it seems more relevant to perform the seeding at the room temperature at the charge density of 30 mC cm−2 with post-annealing at 350 ◦ C (for about 1 h) to convert amorphous Zn(OH)2 into ZnO. The SEM pictures have indicated that high temperature does not decrease quality of the film, i.e. no cracks across the film or uncovered parts of ITO substrate were observed after annealing. On the other hand, the annealing improved significantly transmittance of the sample by the shift of a transmission edge to 370 nm (see curves 1 and 2 in Fig. 3b) and increase of the transmittance in the wavelength range from 400 to 700 nm. The transmittance oscillations visible in Fig. 3a (line 3), called as Fabry-Perot fringes [39], are connected with interferences between light beam transmitted and partially reflected at interface of two phases. These features appear when a transparent thin film of material of high reflection index is deposited on a lower refractive index substrate [40].

j/ mA cm

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0.4 0.0

0

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0.2 t/s

40

50

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Fig. 4. SEM images of the ZnO nanorods obtained by hydrothermal synthesis on the ITO seeded in Zn(NO3 )2 at 25 ◦ C (a) and at 80 ◦ C (b).

Fig. 5. (a) Cyclic voltammograms on ITO electrodes in the solution of 0.1 M Zn(CH3 COO)2 in the presence of oxygen (curve 1) and in oxygen-free solution (curve 2), at the temperature 25 ◦ C at the scan rate of 40 mV s−1 . Insets: plots of the charge density vs. polarization potential obtained by integration of the CVs. (b) A comparison of chronoamperometric curves obtained during polarization of ITO at the potential of −1.2 V in the solution of 0.1 M Zn(CH3 COO)2 in the presence of oxygen (curve 1) and in deoxygenated solution (curve 2) at 25 ◦ C. Inset: an initial part of the current–time profile at the time scale from 0 to 0.4 s.

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*

a

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55

60

65

2 / degree

*

*

b

Zn(CH3 COO)2 + 2H2 O → Zn2+ + 2CH3 COOH + 2OH− OH−

(4)

An additional source of ions needed for precipitation of Zn(OH)2 may be electrochemical reduction of molecular oxygen present in the solution, according to the scheme (3). In order to study the influence of oxygen on the seeding process, the electrodeposition was performed both in the solutions containing oxygen and after deaeration. The voltammogram obtained in oxygen-free solution of 0.1 M Zn(CH3 COO)2 , presented in Fig. 5a (curve 2) is typical of the process of electrodeposition/dissolution of Zn. The charge balance between reduction and oxidation charges (see curve 2 in the inset) indicates that no concurrent irreversible reaction occurs on the electrode. In contrast, the reoxidation peak observed in the cyclic voltammogram recorded in the presence of oxygen (curve 1) is much lower than that in deaerated solution and the charge involved in reoxidation is only 55% of the reduction charge (see curve 1 in the inset). There are two reasons of this unbalance: reduction of oxygen dissolved in the solution (Reaction (3)) and passivation of deposited Zn. The former process occurs at the potentials more negative than −0.7 V as it is evidenced by the increase of the current density in the potential range from −0.7 V to −1.1 V (curve 1) with respect to the voltammogram recorded in the oxygen-free solution. According to the literature, the critical value of pH at which the solution becomes supersaturated with respect to the Zn(OH)2 at 25 ◦ C is 6.68 [42]. Since the pH of the solution used for electrodeposition was 6.2, the reduction of oxygen at the electrode probably results in the local increase of pH above this threshold value leading to precipitation of Zn(OH)2 .

*

*

*

*

Zn (101)

Zn (100)

ZnO (002)

1

Fig. 6. SEM images of the seed layers obtained on ITO electrodes by potentiostatic deposition at −1.2 V and temperature of 25 ◦ C in deaerated solution of 0.1 M Zn(CH3 COO)2 (a) and in the presence of oxygen (b). The deposition charge 140 mC cm−2 .

In aqueous solution Zn(CH3 COO)2 undergoes to hydrolysis according to the scheme:

Zn (002)

Intensity / a.u.

2

25

30

35

40 45 50 2 / degree

55

60

65

Fig. 7. The XRD patterns of the seed layers deposited in the solution of 0.1 M Zn(CH3 COO)2 at constant potential of −1.2 V (a) and the sample after annealing at 350 ◦ C (1 h) in the presence of oxygen (b). The peaks labeled with asterisks correspond to ITO.

Fig. 5b presents the chronoamperometric curves obtained at the polarization potential of −1.2 V in the presence of oxygen (curve 1) and in deaerated solution (curve 2). Although the current density is lower in the presence of oxygen, the overall shape of the two current transients is similar and typical of the nucleation and growth process. Namely, the current after a short minimum, located at about 0.03 s, rises to the plateau and then increases very slowly but continuously in the longer time scale, as for kinetically controlled process. The influence of oxygen dissolved in the deposition bath on morphology of the samples obtained after the passage of the charge density of 140 mC cm−2 is illustrated in Fig. 6. The surface of ITO seeded in de-oxygenated solution was covered with a mosaic of very thin, two-dimensional sheet-like (lamellar) structures oriented vertically to the substrate (Fig. 6a). In contrast, two quite different types of structures were formed on ITO seeded in the oxygen-containing solution: two-dimensional lamellas randomly located on the substrate and large (micrometer-size) three dimensional crystals (Fig. 6b). The formation of 2D sheet-like structures has been also reported for electrochemical deposition of Zn/Zn5 (OH)8 Cl2 • H2 O [43] and for Zn(OH)2 synthesized chemically in liquid–liquid biphasic (organic–aqueous) system containing hydrophobic long-chain

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Fig. 8. SEM images illustrating different stages of the formation of Zn crystals during potentiostatic seeding of ITO at −1.2 V in the solution of 0.1 M Zn(CH3 COO)2 in the presence of oxygen, at the temperature of 25 ◦ C.

carboxylate ions (as heptanoate, decanoate) in organic (xylene) phase [44]. However, in XRD spectrum of the sample prepared in this work in the deoxygenated solution of Zn(CH3 COO)2 one can observe only the signals corresponding to reflections from (0 0 2), (1 0 0), (1 0 1) and (1 0 2) planes of hexagonal Zn, respectively at 2 : 36.4◦ , 39.0◦ , 43.4◦ , 54.5◦ (JCPDS 03-065-5558) and several peaks ascribed to the ITO substrate (labeled with asterisks) (Fig. 7a). Annealing of the sample for 1 h in the presence of oxygen at 350 ◦ C leads to oxidation of Zn to ZnO, confirmed by appearance in the XRD spectrum of additional signal at 2 = 34.5◦ corresponding to (0 0 2) lattice plane of ZnO wurzite (Fig. 7b). At the same time the intensities of the peaks corresponding to the planes of hexagonal Zn decreased, which may be illustrated by the change of intensity ratios of the selected peaks of Zn and ITO substrate before and after annealing. This comparison indicated that the ratio of the most intensive peaks IZn(0 0 2) /IITO(2 2 2) , where IZn(0 0 2) and IITO(2 2 2) are diffraction intensities respectively of (0 0 2) plane of Zn and (2 2 2) plane of cubic In2 O3 , decreased in effect of annealing from 1.2 to 0.4. It also important to note that the peak corresponding to (0 0 2) lattice plane of ZnO, appeared as well in the sample seeded in the zinc acetate solution the presence of oxygen but its intensity was very low. The Zn nanosheet structures presented in Fig. 6a are unique because of their excellent vertical arrangement to the substrate. Although the formation of plate-like Zn nanostructures by electrochemical method or by thermal metal–vapor deposition have been already reported in the literature [45,46], the zinc plates obtained by means of these methods were much thicker (above 100 nm), randomly oriented with respect to the surface and had tendency to grow in multi-layered patterns. The Zn nanosheets have been also synthesized by thermochemical reduction of ZnS in the atmosphere of N2 in the presence of CO and H2 [47] or by the hot filament metal-oxide vapor deposition (HFMOVD) technique [48] but their morphology was different from that presented in Fig. 6a.

The questions which arise concern the mechanism of formation of the 3D crystals deposited together with Zn sheet-like structures in the oxygen-containing solution and the reason non-uniform coverage of the electrode with 2-D nanosheets, different from that obtained in deoxygenated solution. It seems that the latter effect may be ascribed to adsorption and reduction of oxygen on the electrode disturbing the uniform arrangement of the lamellar Zn structures. The answer to the first question may be found by analysis of SEM images of the samples at different stages of the nanocrystals growth, presented in Fig. 8. The samples were prepared at different deposition charge densities, from 70 to 280 mC cm−2 . As visible, the first step of the process consists in the formation of pentagonal or hexagonal templates probably by folding the adjacent nanosheets (see image (a)) which are subsequently building up with the neighboring lamellar structures (image b). Then, these templates are filled in with other nanosheets (image c) which leads to the formation of the crystals of a plate structure (image d). 3.2.1. Hydrothermal growth of ZnO nanorods on the seed layers deposited in the solution of 0.1 M Zn(CH3 COO)2 Hydrothermal synthesis of ZnO was carried out on two ITO plates seeded potentiostatically in the solution of Zn(CH3 COO)2 at the potential −1.2 V and the charge density of 140 mC cm−2 . One of the seed layers was prepared in the presence of oxygen, whereas the second one in deaerated solution. Then, the substrates were annealed in air for 1 h at 350 ◦ C and placed in the autoclave containing solution of 0.05 M Zn(NO3 )2 + NH3 (prepared according to the procedure described in the Section 2) for 2.5 h. The temperature of synthesis was 80 ◦ C. The differences in the morphology of the resultant samples presented in Fig. 9, may be directly correlated with the images of the seed layers presented in Fig. 6. A dense and uniform layer of nanorods (Fig. 9a) were formed on the substrate covered with the uniform film of sheet-like structures deposited in deoxygenated

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solution (Fig. 6a). In contrast, the forms of nanorods obtained on the substrate seeded in the presence oxygen are more diversified, from regular nanorods to the structures jointed at the bottom and branched at the top (Fig. 9b), randomly distributed in the layer. This evidently results from coexistence of 2D sheet-like and 3D block structures in the layer seeded in the presence of oxygen. To understand better the way of nanorod growth on the substrate seeded in the solution of Zn(CH3 COO)2 in the presence of oxygen, the hydrothermal synthesis was terminated after a short time (15 and 30 min) to examine the nanorods at early stage of the growth. As visible in Fig. 10, the growth of nanorods took place mainly on the walls of the zinc blocks, perpendicular to their surface, leading to non-uniform arrangement of the rods in the final ZnO layer. Since the blocks are of different size, this also results in non-uniform thickness of the film. However, such hedgehog-like ZnO nanostructures may be useful for catalytic or sensor applications where large area of ZnO surface and easy access of the reagents to the nanorods are advantageous.

4. Conclusions

Fig. 9. SEM images of the ZnO nanorods obtained by hydrothermal synthesis on ITO seeded in the solution of Zn(CH3 COO)2 at room temperature in the presence of oxygen (a) and in oxygen-free solution (b).

The seed layers of different morphology and composition were electrochemically deposited on ITO electrodes in aqueous solutions of Zn(NO3 )2 and Zn(CH3 COO)2 . The potentiostatic deposition at the potential of −1.2 V in Zn(NO3 )2 solution at low temperature (25–40 ◦ C) leads to the formation of a resistive fibrillar layer of Zn(OH)2 which at higher temperatures transforms into polycrystalline ZnO. The XRD investigations indicated, than amorphous zinc hydroxide layer obtained at the room temperature contains the zones of ZnO (of wurzite structure) which act as the nucleation centers for transformation of Zn(OH)2 to ZnO. Zinc hydroxide obtained at room temperature even at low deposition charge (30 mC cm−2 ) covers the substrate with very uniform layer, which after annealing is an excellent seed layer for hydrothermal or electrochemical growth of well aligned ZnO nanorods arrays. Seeding of the substrate in Zn(NO3 )2 solution at elevated temperatures leads to formation of the polycrystalline crystalline ZnO but full coverage of the substrate is achieved at deposition charge above 110 mC cm−2 . When the substrate seeded at lower charge density is used for synthesis of ZnO nanorods, the naked areas remain uncovered with the nanorods. The higher electroseeding charge improves the coverage but on the other hand leads to decrease of the transmittance of the film. The potentiostatic seeding of ITO in deaerated solution of Zn(CH3 COO)2 leads to the formation of 2D Zn nanosheets, perpendicularly oriented with respect to the substrate and densely distributed on the whole substrate. The Zn nanostructures may be transformed into ZnO by annealing at 350 ◦ C in the presence of oxygen. The seeding in the solution of Zn(CH3 COO)2 containing oxygen results in non uniform coverage of the substrate with the sheet-like Zn nanostructures and formation of 3D block-like Zn crystals. The ZnO nanorods grown by hydrothermal method on the sheetlike Zn layer are vertically aligned with respect to the substrate, whereas these grown on the Zn blocks create the hedgehog-like ZnO structures.

Acknowledgement Fig. 10. SEM images of ZnO nanorods after 15 min (a) and 30 min (b) of hydrothermal growth on Zn crystals seeded in the solution of Zn(CH3 COO)2 at room temperature in the presence of oxygen.

The authors thank Polish Ministry of Science and Higher Education for financial support through research project No. N N204 117039.

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