Introduction

During the last two decades, great advances have been made in nanofabrication and nanotechnology, which led the production of integrated electronics and microprocessors, to a worldwide practice and propelled their integration in a large range of equipments and technologies. While functional systems and corresponding components become smaller due to research and development, the electric sources required for their proper functioning are still based on temporary storages such as batteries and capacitors. This is where nanogenerators come into play [1–4]. In a world where integrated circuits are used daily in our smartphones, personal computers, security systems, or sensors, the dream of self-powered devices directly generating electricity from the environment is finally within our grasp. Piezoelectric nanogenerators are one of the most promising devices as they have the ability of harvesting energy from a wide range of sources. Piezoelectric materials, such as barium titanate (BaTiO\(_{3}\)), lead titanate (PbTiO\(_{3}\)), lead zirconium titanate (PZT; Pb Zr\(_{x}\)Ti\(_{1-x}\)O3), or zinc oxide (ZnO), allow the conversion of mechanical energy, in forms of oscillation, vibration, contact pressure, or bending, into electricity [5].

Zinc oxide (ZnO) is a semiconductor with a wide band gap of 3.3 eV [6]. It usually presents a wurtzite crystalline structure belonging to the \(P6_3mc\) space group, which results in a non-centrosymmetric structure and the appearance of piezoelectricity. To maximize this effect, ZnO must have a preferential growth along the [002] crystallographic plane [7]. ZnO also possesses a relatively high coupling coefficient, making it excellent for a wide range of piezoelectric applications, especially when nanostructured [8]. In particular, by tuning the morphology of ZnO nanostructures, one can enhance the efficiency of future devices [8–10]. ZnO can be deposited in thin films or grown in various nanostructure forms [11, 12], such as nanowires, nanotubes, nanorings, or nanobelts, using a wide range of techniques like electrochemical deposition [13–17], chemical vapor deposition (CVD), magnetron sputtering, pulsed laser deposition (PLD), thermal evaporation [18], atomic layer deposition (ALD) [19], spin-coated sol–gel [20–23], vapor liquid solid (VLS) [1, 24], solochemical process [25], or hydrothermal growth [5, 26–29]. Among these, the electrochemical deposition is one of the most promising bottom-up nanofabrication approaches due to its simple apparatus, low cost execution, and high control over the geometrical dimensions of the fabricated nanostructures.

In this work, we study the potentiostatic electrochemical deposition of ZnO thin films with tunable thicknesses and morphologies. The influence of the electrolyte concentration, temperature, and applied potential during the deposition process is addressed. Depositions were made on top of flexible polyethylene terephthalate (PET) substrates with a conducting indium tin oxide (ITO) layer, for the fabrication of piezoelectric ZnO thin films with a preferential crystallographic structure.

Experimental procedure

ZnO thin films were electrodeposited in conducting substrates (PET with ITO) using an aqueous solution of zinc nitrate hexahydrate (ZNH) as electrolyte. To better understand the electrodeposition process of ZnO thin films, we studied the influence of three different parameters on the deposited films (Table 1): the concentration of ZNH on the electrolyte, 0.1 and 0.01 M; the electrolyte temperature, room temperature (RT; ~19 °C) and 80 °C; and the applied deposition potential, −1.0, −1.1, and −1.5 V vs. Ag/AgCl [30].

Table 1 Parameters varied during the electrochemical deposition process: temperature (RT and 80 °C), concentration of ZNH (0.1 and 0.01 M), and applied potential (−1.0, −1.1, and −1.5 V vs. Ag/AgCl)
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The potentiostatic electrodepositions were performed in a standard electrochemical cell with a three-electrode system [31]: a Pt mesh as counter electrode, a PET/ITO substrate as working electrode, and a reference electrode of Ag/AgCl (in 4 M KCl) (Fig. 1a). For the electrodepositions at 80 °C (Fig. 1b), we added a hot plate that heats a water bath with the previous set-up inside, magnetic stirring to homogenize the electrolyte temperature, and an extra sealing lid on top to prevent the loss of the electrolyte through evaporation. All depositions lasted 1 h and were performed in substrates with 0.5 × 1.4 cm2. Scanning electron microscopy (SEM) images of the samples' surfaces were obtained with high resolution FEI Quanta 400FEG. The chemical composition of the samples was analyzed by energy dispersive X-ray spectroscopy (EDS). The X-ray diffraction (XRD) technique was performed using a Siemens D5000 diffractometer on representative samples and compared to a reference spectrum of the substrate. To estimate the film thickness of the obtained samples, the total charge deposited was analyzed by integrating the deposition current transients.

Figure 1
figure 1

Schematic representation of the electrochemical deposition experimental setup at a room temperature and b 80 °C, illustrating, from left to right, the counter, reference, and working electrodes, respectively

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Results and discussion

Deposition current transients

The monitorization of the current transients [i(t)] during the electrodeposition process allows one to better understand the mechanism of ZnO formation. Figure 2 shows the current transients during the electrodeposition process of selected ZnO thin films on PET substrates with an ITO coating. From the obtained results, we observe an increase of the deposition current with decreasing potential, at room temperature and 80 °C (Fig. 2a). Also, an increase in the deposition current is verified when increasing the solution concentration (Fig. 2b). Finally, comparing representative electrodeposited samples at RT and 80 °C (Fig. 2c), we see that the deposition current also increases with the heated electrolyte. The process is temperature-assisted and thus the heating of the electrolyte increases the deposition rate (seen as an increase in the measured current). Nevertheless, small irregularities occur in the current transients of the heated samples due to the use of magnetic stirring and the existence of convection currents. Note also that the small differences found in the current transients may come from the lower surface conductivity and the different growth mechanisms observed by SEM, which affect the morphology of the ZnO deposits and consequently the deposition area.

Figure 2d shows the complete curve of the deposition process for a solution with 0.01 M of ZNH, at room temperature and under −1.5 V. Two characteristic zones can be defined in the curve, representing two different deposition behaviors. During the first 1250 s of deposition, the current exhibits high values that progressively decrease until a minimum is reached. This is then followed by the second region of the process, where the current remains constant in its minimum value. This effect will be addressed at the end of "Morphology: deposition at room temperature" section when analyzing the distinct surface morphologies deposited during each stage.

Figure 2
figure 2

Deposition current transients recorded during the electrodeposition process of selected ZnO thin films electrodeposited at a 0.1 M (with different temperatures and applied potentials) and b −1.0 V (with different temperatures and concentrations). c Comparison plot of representative electrodeposited samples at 0.1 M and −1.0 V, with varying electrolyte temperatures. d Complete \(i ({\textit{t}})\) current curve for a sample electrodeposited at room temperature with 0.01 M of ZNH under −1.5 V. Note the different time scales

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Deposited charge

When the electrodeposition efficiency is of 100 %, which means that all the measured current is used in the electrodeposition process, one can estimate the deposited charge (Q) by integrating the current transient curve (\(Q = \int i {\rm d}t\)). In this case, we assume that all measured current is used to deposit the Zn\(^{2+}\) ions and neglect the current used in any side chemical reactions. The chemical reactions taking place at the substrate (PET with ITO) during ZnO electrodeposition are as follows [32]:

$$\begin{aligned}&{\mathrm{Zn(NO}_{3})_{2}} \longrightarrow {\mathrm{Zn}^{2+}} + {\mathrm{2NO}_{3}^{-}}\\&{\mathrm{NO}_{3}^{-}} + {\mathrm{H}_{2}}{\mathrm{O}} + 2_{\mathrm{e}}^{-} \longrightarrow {\mathrm{NO}_{2}^{-}} + {\mathrm{2OH}^{-}}\\&{\mathrm{Zn}^{2+}} + {\mathrm{2OH}^{-}} \longrightarrow {\mathrm{Zn(OH)}_{2}}\\&{\mathrm{Zn(OH)}_{2}} \longrightarrow {\mathrm{ZnO}} + {\mathrm{H}}_{2}{\mathrm{O}}. \end{aligned}$$

Using the standard size of the PET/ITO substrates (0.5 × 1.4 cm2), we estimated the deposited charge density for all prepared samples (Fig. 3). We can verify that the charge density increases rather linearly over time, during the first 500 s of deposition, showing a constant deposition rate without major changes in the deposition process (Fig. 3a, b). The charge density slopes are also seen to increase with the ZNH concentration and with decreasing applied potential. However, for the values studied in this work, the variation of the deposition potential (from −1.0 and −1.1 to −1.5 V) was found to have a higher influence on the deposited charge values than varying the concentration of ZNH (from 0.01 to 0.1 M). Figure 3a, b clearly shows that lowering the cathodic potential to more negative values produces a faster increase in the deposited charge than increasing the concentration.

Figure 3c compares the deposited charge density plots of representative electrodeposited samples at room temperature and 80 °C. These show that the deposited charge also increases by heating the electrolyte, achieving its maximum value at 80 °C under the larger negative applied potential of −1.5 V. Also, we can again verify that the applied deposition potential produces larger variations of charge density than all the other parameters. For example, the charge density at the larger negative potentials of −1.5 V, combined with the lowest concentration (0.01 M) and temperature (RT) conditions, is still higher than that at the smallest negative potential of −1.0 V, even with the highest and optimized concentration (0.1 M) and temperature (80 °C) conditions (Fig. 3c).

Finally, we can analyze Fig. 3d, where the complete charge density curve of the sample electrodeposited at −1.5 V, with 0.01 M of ZNH and at RT is shown. As in Fig. 2d, which represents the current transients during the deposition process, this curve can also be divided into two distinct behaviors. The first region, lasting for about 1000 s, shows a linear behavior, similar to other graphs, in which the deposited charge is monotonously increasing. In the second region, from around 1000 s until 3600 s, a linear charge deposition with a much lower slope (almost constant charge density) is observed.

Figure 3
figure 3

Time dependence of the electrodeposition charge density during the growth of representative ZnO thin films, illustrating the effect of increasing ZNH concentration and applied potential at a room temperature and b 80 °C. c Electrodeposition charge density plots comparing representative electrodeposited samples with varying electrolyte temperature, concentration, and applied potential. d Charge density transient curve for the sample electrodeposited at room temperature with 0.01 M of ZNH and under −1.5 V. Note the different time scales

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Film thickness

The total deposited charge calculated by integration of the current transient curves allowed us to estimate the total mass of ZnO deposited, using Faraday’s laws of electrolysis [33, 34]:

$$ m = \frac{QM}{nF} , $$

where m is the deposited mass in grams (g), M is the molar density of the deposited material (81.40 g mol\(^{-1}\) for ZnO), Q is the deposited charge in coloumbs (C), n is the electronegativity of the deposited ions (\(n = 2\) for Zn\(^{2+}\)), and F is Faraday’s constant (F = 96485 C mol\(^{-1}\)). Knowing the volumetric density of ZnO (\(\rho \) = 5.61 g cm\({^{-3}}\)) and the deposition area (A = 0.7 cm\(^{2}\)), one can estimate the thin film’s thickness (h) through:

$$h = \frac{m}{A\cdot\rho }. $$

The results are plotted in Fig. 4, where we can see that the highest thickness was obtained at the largest negative applied potential of −1.5 V and at the highest temperature of 80 °C, as expected from the deposited charge transient curves plotted in Fig. 3. At these high values of applied potential and temperature, the deposition process occurs under charge-transfer control [35], in which the concentration of ZNH in the electrolyte becomes negligible. On the other hand, at room temperature, the highest thickness is again obtained at the largest negative potential of −1.5 V, but using the highest concentration of 0.1 M. At this lower temperature, the applied potential is no longer high enough to limit the deposition growth, and this becomes a diffusion-controlled process, which is limited by the mass transport to the samples surface [35]. Therefore, the thickness of the electrodeposited ZnO films at room temperature and at −1.5 V will still depend on the concentration of ZNH in the electrolyte solution, becoming higher at larger concentrations.

Figure 4
figure 4

Estimated thickness of the ZnO thin films electrodeposited at different temperatures, concentrations, and applied potentials

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Finally, we verify that the estimated thickness tends to increase with the modulus of the applied potential. However, a sudden increase is seen from −1.1 to −1.5 V, which may indicate an unstable deposition at more negative potentials. This effect was also evidenced by the distinct current transients and inhomogeneous morphologies found for the samples deposited at −1.5 V. In the estimated thickness of the electrodeposited thin films, a linear dependency with the applied potential was expected. Nonetheless, the sudden increase at −1.5 V could be associated with the inaccurate consideration of a 100 % deposition efficiency process. This loss in efficiency at more cathodic applied potentials can be ascribed to the hydrogen evolution reaction (HER) that is usually found at lower deposition potentials [36]. In these cases, the current efficiency (\(\sigma \)) is then estimated by [36]

$$ \sigma = \frac{j_{\rm {ZnO}}}{(j_{\rm {ZnO}} + j_{\rm {HER}})}, $$

where j \(_{\rm {ZnO}}\) and j \(_{\rm {HER}}\) are the current density values of the ZnO deposition process and the HER, respectively. To improve the accuracy of the current efficiency measurements, one should resort to more sophisticated methods [37–40]. Nevertheless, even when considering a 90 % drop of current efficiency at higher cathodic potentials [36], the thicker ZnO films would still be obtained for applied potentials of −1.5 V and temperatures of 80 °C (either at 0.01 or 0.1 M), thus making these values the optimum conditions for the growth of thicker ZnO films.

Crystallographic structure

Figure 5 shows the XRD spectra of the substrate prior to the deposition process and of a representative ZnO thin film sample (−1.1 V with a concentration of 0.01 M at 80 °C). We can observe that the obtained spectra are very similar, thus showing only the presence of the crystallographic peaks of the substrate. Such spectrum suggests that the electrodeposition of ZnO on PET/ITO flexible substrates does not produce crystalline structures but rather an amorphous (or nanocrystalline) phase, for the studied parameters in this work. Nevertheless, further analysis of the crystallographic structure of the deposited films should be performed using grazing incidence XRD measurements and/or selected area electron diffraction techniques, to better understand the overall crystallographic structure and/or the possible formation of nanocrystalline phases.

Figure 5
figure 5

XRD spectra of a PET substrate (black line) and a representative ZnO electrodeposited thin film on top of a PET substrate (red line)

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Morphology: deposition at room temperature

Figure 6
figure 6

SEM images of electrodeposited ZnO, at different scales, at room temperature, for 0.1 M of ZNH with applied potentials of (a \(_{\bf 1}\) and a \(_{\bf 2}\)) −1.0 V, (b \(_{\bf 1}\) and b \(_{\bf 2}\)) −1.1 V, and (c \(_{\bf 1}\) and c \(_{\bf 2}\)) −1.5 V

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Figure 6 shows the SEM images of the deposited samples using 0.1 M of ZNH, illustrating the different surface morphologies obtained when varying the applied potential: −1.0 V (a\(_{1}\) and a\(_{2}\)), −1.1 V (b\(_{1}\) and b\(_{2}\)), and −1.5 V (c\(_{1}\) and c\(_{2}\)). All samples revealed traces of Zn and O on the EDS spectra (not shown), with increasing relative counts of Zn and O for lower potential, indicating an increase of the deposition rate at more negative applied potentials. The deposited films’ morphology revealed the nucleation of ZnO nanostructures in the substrate surface for the highest (−1.0 V) potential, without the formation of a continuous film. As shown in Figs. 6a\(_{1}\), b\(_{1}\), c\(_{1}\), the electrodeposition of ZnO tends to form clusters of nanostructures without forming a continuous film. This means that such deposition favors an island-like growth, creating areas with higher ZnO density. In the final stage of the deposition, the ZnO forms almost flat zones at the top of the islands, as seen in Fig. 6c\(_{1}\).

Figure 7
figure 7

Electron microscopy of ZnO-deposited surface, at different scales, with an electrolyte of 0.01 M of ZNH at room temperature, with applied potentials of (a \(_{\mathbf{1 }}\) and a \(_{\mathbf{2 }}\)) −1.0 V, (b \(_{1}\) and b \(_{2}\)) −1.1 V, and (c \(_{\mathbf{1 }}\) and c \(_{\mathbf{2 }}\)) −1.5 V

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When changing the electrolyte concentration to a tenth of the previous amount, the prepared samples revealed a much different morphology, displaying now planar microramifications of very small height, especially when applying a potential of −1.0 or −1.1 V (Fig. 7a, b). Furthermore, we can verify that the deposition at room temperature with 0.01 M and under −1.5 V has two morphologies (Fig. 7c): an initial deposition of microflowers followed by a top flat layer with nanospheres clustered on the surface. Such behavior can be related with the corresponding deposition current transient (Fig. 2d) and the deposited charge curves (Fig. 3d) where, as previously detailed, two distinct behaviors can be noticed. An initial one, during the first thousand seconds with high current values that progressively decrease and linear charge density transients, where ZnO microflowers are formed. Then, a second stage starts where the current remains constant at its minimum value, forming a flat ZnO surface with clustered nanospheres. The initially deposited ZnO microflowers have an effective deposition area much larger than the followed ZnO flat surface with small nanospheres. This can thus explain the large deposition currents measured during the first stage (Fig. 2d), corresponding to the formation of ZnO microflowers, and the subsequent drop in current, due to the final deposition of a flat ZnO surface.

Morphology: deposition at 80 °C

Figure 8
figure 8

Electron microscope results for 0.1 M of ZNH solution at 80 °C, with deposition potentials of (a \(_{\mathbf{1 }}\) and a \(_{\mathbf{2 }}\)) −1.0 V, (b \(_{\mathbf{1 }}\) and b \(_{\mathbf{2 }}\)) −1.1 V, and (c \(_{\mathbf{1 }}\) and c \(_{\mathbf{2 }}\)) −1.5 V

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The obtained results for the electrodeposited samples with 0.1 M electrolyte at 80 °C are shown in Fig. 8 and can be compared with the ones deposited at room temperature (Fig. 6). One immediately notices a clear change in the samples morphology upon decreasing the applied potential to more negative values. At −1.0 V, ZnO nucleates in clustered nanostructures, which are absent at lower deposition potentials (Fig. 8a). At −1.1 V, ZnO forms localized webs that grow on top of each other, as seen in Fig. 8b\(_{1}\), b\(_{2}\). On the other hand, at −1.5 V, the formed nanostructures are much different, resembling coral-like structures of assembled particles (Fig. 8c). Thus the heating of the electrolyte provided a change in the ZnO nanostructures compared to the room temperature samples and to the different potentials applied at 80 °C.

Figure 9
figure 9

SEM images of ZnO-deposited thin films, at different scales, at 80 °C, for 0.01 M of ZNH with deposition potentials of (a \(_{\mathbf{1 }}\) and a \(_{\mathbf{2 }}\)) −1.0 V, (b \(_{\mathbf{1 }}\) and b \(_{\mathbf{2 }}\)) −1.1 V, and (c \(_{\mathbf{1 }}\) and c \(_{\mathbf{2 }}\)) −1.5 V

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Finally, we electrodeposited ZnO using an heated electrolyte at 80 °C with a concentration of 0.01 M. The produced samples (Fig. 9) revealed the nucleation of nanostructures similar to those previously presented in Fig. 8a, although with a lower density, due to the lower concentration of the electrolyte. Differently from the 0.1 M concentration, all the applied potentials show the same type of ZnO nanostructures, with increasing dimensions for more negative potentials.

Conclusions

A thorough study was performed on the morphology of ZnO thin films electrodeposited on PET/ITO substrates upon varying the ZNH concentration, applied potential, and electrolyte temperature. The increase in the temperature of the electrolyte and ZNH concentration, along with the decrease of the applied negative potential, were found to increase the deposition current and thus the total charge deposited. By varying the applied potential one can greatly change the morphology of the electrodeposited films. For 0.01 M of ZNH at 80 °C, the nanostructures increase in size when decreasing the applied potential to more negative values, maintaining the same shape. Also, in the specific case of 0.01 M, −1.5 V at RT, two distinct morphologies were observed by SEM and correlated with the two different regions exhibited by the current deposition transients, indicating the influence of the applied potential during deposition. All three parameters tend to change the films’ morphology, as seen by SEM, allowing the exploration of different ZnO nanostructures for a wide range of purposes. Finally, the obtained results provided a better understanding of the deposition of ZnO nanostructures on flexible conducting substrates using electrochemical processes.