# ZnO Electrodeposition on Boron-Doped Diamond: Effects of Zinc Precursor Concentration

Pierrick Gautier, Anne Vallée, Arnaud Etcheberry, Nathalie Simon

► To cite this version:

Pierrick Gautier, Anne Vallée, Arnaud Etcheberry, Nathalie Simon. ZnO Electrodeposition on Boron-Doped Diamond: Effects of Zinc Precursor Concentration. ECS Transactions, 2015, 66 (6), pp.141 - 149. 10.1149/06606.0141ecst. hal-04963533

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# ZnO Electrodeposition on Boron-Doped Diamond: Effects of zinc precursor concentration

P. Gautier, A. Vallée, A. Etcheberry, N. Simon*

Lavoisier Institute (UMR 8081), Université de Versailles 45 avenue des Etats Unis
78035 Versailles cedex, France

Corresponding author*: nathalie.simon@uvsq.fr

The cathodic electrodeposition of zinc oxide has been widely studied on various substrates in the last decades. The boron doped diamond (BDD) is a non conventional wide band gap semiconductor, which offers new possibilities as a substrate for this electrochemical deposition. Thus, in the present work, we study the deposition process performed with zinc chloride and dissolved oxygen as a precursor. The effect of zinc precursor concentration in a range of 0.2 to 5 mM is tested at 60°C. The current transient curves coupled with X-Ray Diffraction and Scanning Electron Microscopy analysis, reveal the effect of this parameter on the morphology of obtained ZnO/BDD deposits. Interestingly, the formation of ZnO pyramids was highlighted for the first time.

## Introduction

ZnO is a wide band gap (3.32 eV) semiconductor with n-type conductivity, which is used in many devices for different applications, including gas sensors, transparent electrodes, and acousto- and electro-optical devices. Recently, the combination of ZnO with boron-doped-diamond (BDD) was used in order to make heterojunctions (1), surface acoustic-wave (SAW) devices (2) and biosensors electrodes (3). Usually, ZnO was deposited on BDD by chemical vapor deposition (CVD) (1) (3) (4), magnetron sputtering (5), atomic layer deposition (ALD) (6) or by co-precipitation (2). The electrodeposition represents an interesting new way for the deposition of ZnO on BDD. This chemical route presents many advantages such as low cost and possibility to obtain a large range of structures from the dense layer up to well-defined nanostructures.

In 2006, the electrochemical deposition of ZnO on BDD (7) was tested for the first time. Based on nitrate reduction as a precursor of hydroxide; the formation of a dense ZnO layer constituted of micrometric grains, was observed. Recently, following the process developed by Peulon and Lincot (8), our research group has shown that the cathodic deposition of nanostructures of ZnO was promising on BDD films (9). This process is based on the reduction of dissolved oxygen as precursor of hydroxide (steps [1] and/or [2]). Then, the zinc hydroxide precipitation occurs [3] and finally, its dehydration [4].

$$O_2 + 2 H_2O + 4 e^- \rightarrow 4 OH^- \quad [1]$$

$$O_2 + 2 H_2O + 2 e^- \rightarrow H_2O_2 + 2 OH^- \quad [2]$$

$$Zn^{2+} + 2 OH^- \rightarrow Zn(OH)_2 \quad [3]$$

$$Zn(OH)_2 \rightarrow ZnO + H_2O \quad [4]$$
Our previous work (9), was devoted to the study of the bath temperature influence, while the ZnCl₂ concentration was set at 5 mM. In the present work, the zinc precursor concentration was varied from 0.2 to 5 mM while the temperature was set at 60°C.

## Experimental

Boron doped polycrystalline diamond films (1.5-2 µm) deposited on polycrystalline silicon substrates in a hot-filament-assisted chemical vapor deposition reactor supplied with diborane and methane in hydrogen are provided by Neocoat. As grown diamond films were tested and one doping level was studied: N<sub>A</sub> ≈ 6x10<sup>20</sup> B.cm<sup>-3</sup> checked by SIMS measurements.

The electrodeposition was performed in a three-electrode electrochemical cell with the substrate (BDD) as the cathode, a Zn wire as the counter electrode and a Mercury Sulfate Electrode (MSE) placed in a separate compartment maintained at room temperature as reference. The electrolyte was an aqueous solution of 0.1 M KCl (Alpha Aesar, purity > 99%) and ZnCl₂ (Carlo erba, purity > 97%) at different concentrations: 5 mM, 1 mM and 0,2 mM. Molecular oxygen was supplied to the solution by O₂ bubbling 45 minutes prior to starting the experiment. A moderate bubbling of O₂ was maintained in the cell during the deposition process. ZnO deposition was performed at constant applied potential (-1,4 V/MSE) during 1 hour with a Parstat-2263.

Morphologies studies have been performed with a field emission scanning electron microscope Jeol 7001-F at 15 kV accelerating voltage. X-ray diffraction was used to examine the crystal structure of electrodeposited ZnO. X-ray diffraction data were collected at 293 K on a Siemens D-5000 diffractometer using CuKα radiation (l = 1,5418 Å; step size 0.02° (2θ); angular range 20-65° (2θ))

## Results and Discussion

The influence of zinc precursor (ZnCl₂) concentration is studied on ZnO electrodeposition on BDD. Various concentrations of ZnCl₂ have been tested: 0.2 mM, 1 mM and 5 mM at 60°C. In this system, chloride concentration is maintained by KCl; the influence of chloride concentration is thus negligible. The concentration of Zn²⁺ is therefore the only parameter varying. Deposits have been done at a constant applied potential of -1,4 V/MSE during one hour as chosen in previous work (9). The current transients (C-t) registered during the processes are presented on figure 1.
<figure><img src="image_2.png" /><figcaption>Figure 1. Current transients corresponding to oxygen reduction at 60°C on BDD without ZnCl₂ (dashed line) and with [ZnCl₂] = 5 mM, 1 mM and 0.2 mM (full lines)</figcaption></figure>

As a reference, the *C*-t curve corresponding to oxygen reduction on naked BDD is presented (dashed lines fig. 1). For this experiment, a platinum wire was used as counter electrode to avoid Zn²⁺ contribution in the solution during the process. As observed on the curve, the cathodic current increases strongly during the first 1000 s and then tends to stabilize after one hour of treatment at 2.10⁻³ A/cm².

Addition of zinc chloride in solution modifies the system, and the electrodeposition of ZnO may occur at BDD surface during the process. The corresponding *C*-t curves (full lines) have a similar shape compared to that recorded without zinc precursor. However, the measured cathodic current is lower than that corresponding to oxygen reduction on bare BDD, whatever the ZnCl₂ concentration is (table I). Figure 1 shows that the cathodic current is decreased as the concentration in precursor is increased. This result indicates that oxygen reduction on the BDD surface is linked to the presence and the concentration of zinc precursor in the solution. This suggests that by varying Zn²⁺ concentration, the oxygen reduction, in fact, occurs on different ZnO/BDD structures.

A similar *C*-t curve (same shape and current plateau) was reported on SnO₂ (10) for comparable deposition conditions ([ZnCl₂] = 5 mM and T= 70°C), which confirms the opening to use BDD as a substrate for the electrodeposition of zinc oxide.

<table><caption>TABLE I. Current density plateau measured after 1 hour of treatment for various [ZnCl₂].</caption><thead><tr><th>[ZnCl<sub>2</sub>]</th><th>0 mM</th><th>0.2 mM</th><th>1 mM</th><th>5 mM</th></tr></thead><tbody><tr><th>Current density after 1 hour of deposition (A/cm<sup>2</sup>)</th><td>2 10<sup>-3</sup></td><td>1.8 10<sup>-3</sup></td><td>1.1 10<sup>-3</sup></td><td>5 10<sup>-4</sup></td></tr></tbody></table>

To study the different deposits, XRD analyses have been performed (fig 2.). Additionally to the substrate diffraction peaks labeled with an asterisk, the diffractograms highlight peaks characteristic of wurtzite crystallized zinc oxide for each deposits. The
crystallinity of ZnO deposit is improved when the zinc precursor concentration increases, as shown with diffraction peaks which get thinner.

<figure><img src="image_3.png" /><figcaption>Figure 2. XRD of naked diamond and ZnO films deposited on diamond with different precursor concentration (0.2 mM, 1 mM and 5 mM).</figcaption></figure>

From the XRD analysis, the texturation coefficient of the three ZnO deposits has been calculated with Eq. 5 (11). This coefficient is a comparison between peaks intensity of electrodeposited ZnO and a reference compound (JCPDS 36-1451). If the ZnO growing is isotropic, the coefficient value is 1. A preferential growth along an axis, leads to a value superior to 1.

$$TC_{hkl} = \frac{I_{hkl} / I_{0hkl}}{\frac{1}{n} \sum_{l}^{hkl} I_{0hkl}} \quad [5]$$

I<sub>hkl</sub> : Peak intensity for a given polar plane

I<sub>0,hkl</sub> : Peak intensity of reference compound (JCPDS 36-1451) for a given polar plane

N : Number of considering polar planes for the study

<table><caption>TABLE II. Texture coefficient as a function of zinc precursor concentration</caption><thead><tr><th>Diffraction peak</th><th>[ZnCl<sub>2</sub>] = 0,2 mM</th><th>[ZnCl<sub>2</sub>] = 1mM</th><th>[ZnCl<sub>2</sub>] = 5 mM</th></tr></thead><tbody><tr><td>002</td><td>1.5</td><td>2.5</td><td>2.2</td></tr><tr><td>101</td><td>1.3</td><td>0.5</td><td>0.4</td></tr><tr><td>100</td><td>0.8</td><td>0.6</td><td>0.6</td></tr></tbody></table>

Values of texture coefficient were calculated for three different diffraction peaks: 101, 002 and 100 and are summarized in table II. Our results show that the ZnO deposits obtained with the different concentrations of zinc precursor have a texture coefficient superior to 1 along 002 axes, indicating a preferential orientation of c-axis. The highest
texture coefficient's value is obtained for the ZnO prepared with a concentration of 1 mM, meaning that this deposit is the most oriented along c-axis. The preferential orientation is in agreement with results described by Elias et al (12) on other substrates in similar deposition conditions.

SEM analyses have been recorded to study the morphology of the three zinc oxyde's deposits (Fig. 3). Very different ZnO/BDD structures are evidenced.

<figure><img src="image_4.png" /><figcaption>Figure 3. SEM images of naked BDD (a), ZnO deposited with [ZnCl₂] = 0.2 mM (b), 1 mM (c) and 5 mM (d). Sectional views are added as insets.</figcaption></figure>

With a concentration of 0.2 mM of ZnCl₂ (fig. 3.b), nanorods of ZnO are observed with a section of 400 nm and a height of 1.5 µm. In these conditions, the deposit is not fully covering the BDD surface and the substrate is still observable. Similar morphologies with however, different section and height, have already been reported on many substrates as conducting glass (13), on polycrystalline SnO₂ (8) and single crystal GaN (14).

With a concentration of 1 mM, pyramids are obtained with a base of 1 µm and a height of 6-7 µm. This morphology corresponds to a new morphology of electrodeposited ZnO, never described in literature on other substrates. It should be noted that in similar experimental conditions, El Belghiti et al. (15) obtained nanorods on SnO₂ substrate.

Finally, with a concentration of 5 mM, a dense layer of ZnO is observed, totally covering the substrate, as already described on BDD (9), on polycrystalline SnO₂ (8), on single crystal GaN (16) and on single crystal Au (17). ZnO grains with a section of 1-1.5 µm and a height of 1.7 µm compose this dense layer.

As revealed by the top and sectional SEM views, the ZnO morphologies of the three deposits reveal the formation of rods and/or pyramids meaning a preferential growing
along c-axis which is consistent with DRX analysis. Figure 3-c reveals that the nanostructure obtained with [ZnCl₂] = 1 mM presents the highest length over diameter ratio (table III), thus confirming the highest coefficient texture deduced from XRD analysis.

The morphologies of deposits are also consistent with the current transients registered during the processes (fig. 1). Indeed, with [ZnCl₂] = 0,2 mM, the ZnO deposit is not fully covering and the substrate is still visible (fig 3-b). Therefore oxygen reduction probably continues to take mainly place on BDD surface, leading to a current transient very close to that obtained on naked BDD. When the zinc precursor concentration increases to 1 mM and 5 mM, electrodeposited ZnO is more and more covering (fig3-c and 3-d), implying oxygen reduction on ZnO deposit in formation rather than on BDD. As evidencing on figure 1, oxygen reduction seems in these conditions less effective on ZnO deposit on formation than on naked BDD.

From SEM analyses (fig.3) and cathodic current transients (fig.1), the estimation of the faradic yield of ZnO electrodeposition have been made by comparing the thickness of obtained deposit (SEM) and cathodic charge involved during electrodeposition (transients). Based on the involved cathodic charge during electrodeposition, the theoretical thickness is calculated thanks to equation [6] with a faradic yield of 100%.

$$ Thickness_{Theo} = \frac{q.M}{n.e.N.d.S} \quad [6] $$

q : Involved cathodic charge (C)

M : Molar mass of ZnO (81.4 g.mol⁻¹)

n : implied electrons for the formation of ZnO (2 electrons)

e : electric charge of an electron (1.6 10⁻¹⁹ C)

N : Avogadro constant (6.02 10²³ mol⁻¹)

d : ZnO density (5.6 g.cm⁻³)

S : Electrode surface (0.1 cm²)

For each deposit, an average thickness of ZnO (experimental thickness) is estimated and compared to the theoretical thickness [7].

$$ R = \frac{Thickness_{exp}}{Thickness_{theo}} \quad [7] $$

Table III shows the coulometric charge corresponding to the deposition, experimental thickness (SEM), theoretical thickness and calculated faradic yields for each deposit.

As shown on table III, when the zinc precursor concentration decreases, the cathodic charge increases while the average thickness estimated by SEM decreases. Consequently, faradic yield of ZnO electrodeposition declines as the Zn²⁺ concentration is reduced: for a concentration of 5 mM, the faradic yield is close to 100%, for 1 mM it is close to 79% and for 0.2 mM, it is very low (2%). These results suggest that as the zinc precursor concentration decreases, locally the Zn²⁺ amount is not sufficient to fully precipitate the
hydroxides produced during steps [1] and [2], in other words, the production of $OH^-$ becomes faster than the supply by diffusion of $Zn^{2+}$. This was already reported by El Belghiti et al. on $SnO_2$ substrate (15).

<table><caption>**TABLE III.** Experimental and theoretical thicknesses; Faradic yields of ZnO electrodeposition; Height/diameter ratio of ZnO structures.</caption><thead><tr><th></th><th>[ZnCl<sub>2</sub>] = 0,2 mM</th><th>[ZnCl<sub>2</sub>] = 1mM</th><th>[ZnCl<sub>2</sub>] = 5 mM</th></tr></thead><tbody><tr><th>Charge (mC)</th><td>800</td><td>450</td><td>200</td></tr><tr><th>Experimental thickness (μm)</th><td>0.13</td><td>3.0</td><td>1.7</td></tr><tr><th>Theoritical Thickness (μm)</th><td>6.7</td><td>3.8</td><td>1.7</td></tr><tr><th>Faradic Yield</th><td>2 %</td><td>79 %</td><td>100 %</td></tr><tr><th>Height/diameter ratio</th><td>3-4</td><td>6-7</td><td>1-1,5</td></tr></tbody></table>

Interestingly, on BDD surface at 60°C, a concentration of 1 mM of $ZnCl_2$ seems to be a good balance to obtain a dense ZnO layer preferentially oriented along c-axis, with a good faradic yield.

From these results, growing mechanisms for the electrodeposited zinc oxide on BDD, as a function of zinc precursor concentration, have been proposed and illustrated on figure 4.

With $[ZnCl_2] = 5$ mM, the faradic yield is close to 100%, meaning that all the produced $OH^-$ is involved in ZnO precipitation. In these conditions, the local $Zn^{2+}$ concentration is enough to permit a lateral and vertical growing of ZnO crystals as observed by SEM.

With $[ZnCl_2] = 1$ mM, the faradic yield decreases up to 79%, suggesting that the local concentration of $Zn^{2+}$ is not enough to fully precipitate the produced hydroxides. The production of $OH^-$ becomes a little bit faster than the supply by diffusion of $Zn^{2+}$. In these conditions, an important vertical growing along c-axis is favored associated to a weak lateral development of ZnO crystals leading to the growth of pyramids.

Finally, with the lowest zinc precursor concentration (0.2 mM), the poor faradic yield (2%) suggests that the amount of $OH^-$ species is locally much larger than that of $Zn^{2+}$. The production of $OH^-$ becomes much faster than the supply by diffusion of $Zn^{2+}$. In these conditions, the preferential ZnO precipitation on the most accessible plane i.e. at the top of crystals (002 polar plane) occurs, and the lateral development is limited, explaining the growth of nanorods as already reported on other substrates (18).
<figure><img src="image_5.png" /><figcaption>Figure 4. Schematic illustration of ZnO deposition mechanism as a function of zinc precursor concentration.</figcaption></figure>

Conclusion

This work studies the influence of ZnCl₂ concentration on the cathodic electrodeposition of ZnO on Boron Doped Diamond surfaces. For experiments made at T = 60°C and an applied potential of -1.4V/MSE, a high concentration of Zn²⁺ cations (5 mM) leads to the growth of a dense layer of ZnO, totally covering the BDD substrate. The decrease of the Zn²⁺ concentration to 1mM induces an important preferential growth along c-axis and the formation with a good faradic yield of a layer made of pyramids is observed. Finally, a very low zinc precursor concentration (0.2 mM) causes a notable decrease of the faradic yield and leads to the formation of ZnO nanorods.

This work evidences that, as on other substrates, the zinc precursor concentration is a key factor of the morphology of ZnO electrodeposited on BDD. Very different and interesting structures are grown on the diamond substrate. Our results are thus very encouraging for many applications as for example the preparation *via* this liquid route of new BDD/ZnO heterostructures.

Finally, It should be noted that the pyramids obtained on BDD with [ZnCl₂] = 1 mM are a new ZnO morphology, never reported for electrodeposition on another substrate up to now. This result proves the importance of the substrate for the ZnO electrodeposition and is again very encouraging, as BDD is a new possibility for this process which has not really been explored
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