# ZnO Electrodeposition on Boron-Doped Diamond: Effects of Diamond Surface Terminations

Nathalie Simon, Anne Vallée, Anne-Marie Gonçalves, Pierrick Gautier, Arnaud Etcheberry

► To cite this version:

Nathalie Simon, Anne Vallée, Anne-Marie Gonçalves, Pierrick Gautier, Arnaud Etcheberry. ZnO Electrodeposition on Boron-Doped Diamond: Effects of Diamond Surface Terminations. ECS Transactions, 2017, 77 (4), pp.109 - 118. 10.1149/07704.0109ecst. hal-04963539

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# ZnO Electrodeposition on Boron-Doped Diamond: Effects of diamond surface terminations.

N. Simon*, A. Vallée, A.M. Gonçalves, P. Gautier and A. Etcheberry

Lavoisier Institute (UMR 8081), Université de Versailles

*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. In the present work, we focus on the influence of the BDD surface chemistry either H- or O-terminated obtained by different oxidation treatments (electroless or anodic) on the ZnO deposition process. Surface terminations are followed by contact angle measurements and XPS analyses. The morphologies and structures of ZnO deposits are characterized by XRD and SEM. Besides playing on deposit morphology, this work shows that the surface terminations of BDD (-CH, -CHx, C-OH, -COOH, -COC or -C=O) have also a strong effect on the ZnO deposit adhesion.

## Introduction

Diamond and zinc oxide (ZnO) are two highly important, wide band gap, semiconductors with a great number of attractive properties. Recently the combination of n-type ZnO and p-type diamond has attracted much attention for different applications as the fabrication of heterojunctions (1), surface acoustic-wave (SAW) devices (2) or biosensors electrodes (3). In these works, ZnO was mainly deposited on diamond surfaces by high temperature processes such as chemical vapor deposition (CVD) (4), magnetron sputtering (5) or atomic layer deposition (ALD) (6).

Electrochemical deposition (ECD) is emerging as an important method to prepare semiconductor thin films such as ZnO. This chemical route presents many advantages such as low cost, low temperature, soft processing of materials and possibility to obtain a large range of structures from dense layers up to well-defined nanostructures by means of appropriate growth parameters.

Since the last decades, the electrodeposition of ZnO has been successfully performed on many conductive substrates (7) leading to the growth of various structures but it was studied only one time on diamond from a Zn(NO₃)₂ solution (8).

Whatever the process, the first step in the ECD of ZnO consists in the production of hydroxides near the electrode surface to raise the local pH. Three precursors can be used to produce the OH⁻ ions: Hydrogen peroxide, nitrate salts or dissolved oxygen. Unlike zinc nitrate, using oxygen as a precursor allows the modulation of [OH⁻]/[Zn²⁺] ratio in the bath.

Recently our group has investigated the ECD of ZnO on Boron Doped Diamond (BDD) electrodes using the method based on the reduction of dissolved O₂, developed by Peulon and Lincot (9) The reduction of dissolved oxygen leads to the formation of hydroxide
(step [1] or [1]'), by raising the local pH the precipitation of zinc hydroxide (step [2]) occurs in presence of Zn²⁺ ions, which is finally transformed into zinc oxide by dehydration (step [3]).

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

$$O_2 + 2 H_2O + 2 e^- \rightarrow H_2O_2 + 2 OH^- \quad [1]'$$

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

$$Zn(OH)_2 \rightarrow ZnO + H_2O \quad [3]$$

We studied the influence of bath temperature and zinc chloride concentrations and evidenced that the ECD of zinc oxide was promising on as-grown H terminated BDD electrodes (10, 11).

In the present work, we focus on the influence of the BDD surface chemistry either H- or O-terminated obtained by different oxidation treatments (electroless (12, 13)) or anodic (14, 15)) on the ZnO deposition process. Surface terminations are followed XPS analyses. The morphologies and structures of ZnO deposits are characterized by XRD and SEM. Beside the deposit morphology, this work also focuses on the possible effect of the surface terminations of BDD (-CH, -CHx, C-OH, -COOH, -COC or -C=O) on the ZnO deposit adhesion.

## Experimental

### BDD Substrate preparation and characterization:

BDD 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. The doping level checked by SIMS measurements was 6x10²⁰ B.cm⁻³. Due to the deposition process, the surface of as deposited BDD layers being partly hydrogenated, samples are termed H-BDD.

Oxidations of H-BDD substrates were performed either at Open Circuit Potential (OCP) conditions in presence of an oxidizing agent at room temperature or in galvanostatic conditions.

For OCP oxidations, H-BDD surfaces were immersed in desaerated solutions containing either 0.06 mol.l⁻¹ of Ce⁴⁺ (as sulfate hydrate) or MnO₄⁻ 0.1 mol.l⁻¹ (as potassium) in 0.5 M H₂SO₄ for 5 days. The resulting oxidized samples are respectively termed O<sub>Ce</sub>-BDD and O<sub>MnO4</sub>-BDD.

Anodizations of H-BDD surfaces were performed in H₂SO₄ 0.5 M following the procedure described in reference (15). To produce different kind of surface terminations, H-BDD samples were oxidized either with low (0.3 mA.cm⁻² for 10 min) or high current densities (100 mA.cm⁻² for 10 sec). The resulting samples are respectively termed O<sub>LC</sub>-BDD and O<sub>HC</sub>-BDD.

X-ray photoelectron spectroscopy (XPS) analysis was used to examine the BDD substrate surface terminations. XPS measurements were performed on a VG220iXL system with a base pressure of 5x10⁻¹⁰ Torr and using the AlKa (1486.5 eV) X-ray monochromatized radiation with a pass energy of 20 eV or 8 eV (resolution 0.2 eV). Energy levels were calibrated with Au single crystal. The spectra were processed using VG Advantage datasystem.

### ZnO films deposition:
ZnO electrodeposition was performed in a classical 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 deposition solution contained 5 mM ZnCl₂ (Carlo erba, purity > 97%) and 0.1 M KCl (Alpha Aesar, purity > 99%). The solution was saturated with molecular oxygen and a moderate O₂ bubbling was maintained in the cell during the deposition process. Five different BDD surfaces either Hydrogen or Oxygen terminated were used as substrates. For each deposition, the applied potential was -1.4 V/MSE, the bath temperature was 60°C and the duration was 1 hour.

The films morphology was observed 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

### Substrate preparation and characterization

Four different oxidizing treatments were used to convert hydrogen terminations to oxygen ones on the BDD surfaces. Oxidations of H-BDD surfaces were performed both at Open Circuit Potential (OCP) conditions, with either Ce⁴⁺ or MnO₄⁻ species as oxidizing agent at room temperature and in galvanostatic conditions with two different anodic currents.

To get qualitative and quantitative information about the resulting oxygen functionalities, XPS was performed on BDD interfaces before and after oxidizing treatments. Figures 1 and 2 give the high resolution C1s XPS spectrum of diamond electrode surfaces before and after oxidations and table 1 reports the evolution of each component before and after the different oxidizing treatments.

The total Oxygen amount (table 1) was obtained by dividing the relevant peak integrated area of well-defined O1s core level spectra by the appropriate bulk sensitivity factor following the method described in reference (16). Due to air exposure, the as-grown H-BDD samples present about 7% of oxygen. After OCP treatments, the oxygen quantity increases from 7 to 12.5 % with MnO₄⁻ and to 9.5 % with Ce⁴⁺. After both anodic processes, the oxygen quantity increases from 7 to 8.5 % whatever current density value.

<table><caption>**TABLE I.** Oxygen amount and proportions of each C1s contribution obtained by peak fitting for BDD before (H-BDD) and after chemical (O<sub>Ce</sub>-BDD and O<sub>MnO4</sub>-BDD) and electrochemical oxidations (O<sub>LC</sub>-BDD and O<sub>HC</sub>-BDD)</caption><thead><tr><th rowspan="2">Samples</th><th colspan="6">C1s atomic %</th><th rowspan="2">O1s atomic %</th></tr><tr><th>C<sub>0</sub></th><th>C<sub>1</sub></th><th>C<sub>2</sub></th><th>C<sub>3</sub></th><th>C<sub>4</sub></th><th>C<sub>5</sub></th></tr></thead><tbody><tr><td>H-BDD</td><td>1.5</td><td>73.5</td><td>8</td><td>7.5</td><td>1</td><td>1</td><td>7</td></tr><tr><td>O<sub>Ce</sub>-BDD</td><td>2</td><td>68</td><td>12.5</td><td>6</td><td>1</td><td>1</td><td>9.5</td></tr><tr><td>O<sub>MnO4</sub>-BDD</td><td>1.5</td><td>60.5</td><td>11</td><td>12.5</td><td>1</td><td>1</td><td>12.5</td></tr><tr><td>O<sub>LC</sub>-BDD</td><td>2</td><td>64</td><td>18.5</td><td>4.5</td><td>1.5</td><td>1</td><td>8.5</td></tr><tr><td>O<sub>HC</sub>-BDD</td><td>2.5</td><td>48</td><td>32.5</td><td>6.5</td><td>1</td><td>1</td><td>8.5</td></tr></tbody></table>
To get information about the kind of chemical functionalities present at BDD before and after oxidation, the chemical shifts of the C1s core level were investigated. The C1s peak (Figures 1 and 2) exhibits two main components at 284 eV (noted C1) and at + 0.8 eV (noted C2) respectively attributed to the bulk diamond component and to polyhydride carbon species, CHx (17). Due to air exposure H-BDD surface (Fig. 1) exhibits additional components shifted towards higher energies, corresponding to oxidized carbon atoms. The peak at +2eV with respect to the bulk (noted C3) is attributed to singly oxidized carbons, in other words to hydroxyl or ether groups. (17). The peak at -0.8eV noted C₀ corresponds to sp² carbon.

<figure><img src="image_2.png" /><figcaption>Figure 1. High resolution C1s XPS spectra of as-grown (H-BDD) diamond electrode surface.</figcaption></figure>

The “C-O” terminations present on H-BDD sample are mainly singly oxidized carbon atoms either hydroxyl or ether groups (component C3).

After the chemical oxidation with ceric specie the C4 and C5 contributions are constant and the C3 component decreases to 6% while the total amount of oxygen increases. This apparent contradiction suggests the growth of hydroxide terminations at the detriment of ether's groups initially present on H-BDD. After the chemical oxidation with permanganate ion, the C3 component increases to 12.5% similarly with the total amount of oxygen, which suggests the preferential growth of C-O-C groups. The contact angle with water decreases from 100° on H-BDD to 47° after oxidation with Ce⁴⁺ and 57° after oxidation with MnO₄⁻. While the total amount of Oxygen is higher on O MnO₄⁻-BDD surface, the wettability is lesser than that recorded on O Ce⁻-BDD, which confirm the growth of more polar C-OH groups on O Ce⁻-BDD surface and less polar C-O-C groups on O MnO₄⁻-BDD, as already reported by Charrier et al. (12-13).

After both anodic oxidations, C4 (small increase) and C5 contributions are nearly constant, the C3 component decreases while the total amount of oxygen increases a little, suggesting the preferential growth of hydroxide groups. The main evolution observed after both anodic processes concerns the C2 component. Indeed, the “CHx” contribution evolutes from 8% on H-BDD to 18.5 and 32.5% after respectively low current density or high current density treatments.
<figure><img src="image_3.png" /><figcaption>Figure 2. High resolution C1s XPS spectra of diamond electrode surfaces after chemical (O<sub>Ce</sub>-BDD and O<sub>MnO4</sub>-BDD) and electrochemical oxidations (O<sub>LC</sub>-BDD and O<sub>HC</sub>-BDD)</figcaption></figure>

Deposition: Current – time (C-t) curves :

Figures 3 depict the chronoamperograms (C-t) corresponding to the deposition of ZnO at -1.4 V/MSE, during 1 hour on the five different BDD substrates. The cathodic current, *j*<sub>c</sub>, corresponds to the ORR (eq. (1) and/or (1)'). As already reported by many authors, current transients are very informative on the ECD mechanism (18, 19, 20-22). C-t curves recorded on H-BDD and O<sub>Ce</sub>-BDD substrates are very similar; the |*j*<sub>c</sub>| values increase during the process and reach a limiting plateau value of -0.8 mA.cm<sup>-2</sup> after about 1000 s. The global coulometric charge corresponding to these ECD is about 250 mC. Differently, chronoamperograms registered during the ZnO deposition on O<sub>MnO4</sub>-BDD, O<sub>LC</sub>-BDD and O<sub>HC</sub>-BDD present a peak current of about 1 mA.cm<sup>-2</sup> characteristic of a 3D island growth mechanism (19,22,23). The peak is much more marked for the substrate prepared with the galvanostatic process and high current density. The absence of such peak in the current-time transients recorded on as-grown and chemically oxidised with Ce<sup>4+</sup> BDD electrodes could be explained by a lesser density of nucleation sites, as already reported by Pauporté *et al*. for other ECD conditions (19).
<figure><img src="image_4.png" /></figure>

Figure 3. Current-time curves recorded during electrodeposition of ZnO on BDD surfaces, in 5 mM ZnCl₂ / 0.1 M KCl solution saturated in dissolved O₂. Applied potential = -1.4 V/MSE; Temperatures of bath = 60 °C.

Left: Black curve: H-BDD, Grey curve: BDD oxidised with Ce⁴⁺ and light grey curve: BDD oxidized with MnO₄⁻.

Right: Black curve: H-BDD, Grey curve: BDD anodised with high current density and light grey curve: BDD anodised with low current density.

SEM Analysis:

The SEM images of the deposits prepared on the four different BDD substrates are given on figure 4.

For the one hour ECD, very similar morphologies are observed on H-BDD and O<sub>Ce</sub>-BDD substrates. The SEM top and cross section views of these samples (figure 4 : left), show a ZnO layer constituted of hexagonal micrometric rods with a smooth surface, a diameter around 1µm and a height of 1.7 µm. The density of these layers is about 140 grains per 100 µm².

The ZnO deposits on OMnO₄-BDD, O<sub>LC</sub>-BDD and O<sub>HC</sub>-BDD are a little bit different. For these three oxidized BDD substrates, a ZnO layer constituted of columnar grains are still observed, but the grains density is higher (respectively 200, 200 and 250 grains per 100 µm²) and the thickness of the deposits is also higher about 1.9 µm.

A correlation appears between *C*-t curves shape and ZnO morphologies. The two substrates which do not show peak current (e.g. H-BDD and O<sub>Ce</sub>-BDD) are also those where less dense deposits are observed. This confirms the assumption that a lesser density of nucleation sites are present both on as-grown and on the oxidized with Ce⁴⁺ diamond surfaces. To confirm this assumption, very short deposition processes have been made on the five BDD surfaces to observe the first stages of deposition. Figure 4 (Right part) shows the SEM images registered for ECD corresponding to a global charge of 50 mC.cm⁻². These SEM pictures confirm the assumption; the density of nucleation sites is much higher for the O-BDD surfaces anodized or treated with MMnO₄ compared to the as-grown and O<sub>Ce</sub>-BDD surfaces.
<figure><img src="image_5.png" /><figcaption>**Figure 4:** SEM images of ZnO structures obtained after electrodeposition at 60°C on the various BDD surfaces (H-BDD and O-BDD). Left: structures corresponding to a 1h00</figcaption></figure>
ECD (The insets give cross sectional view of deposits); Right: deposits obtained with an anodic charge of 50 mC.cm⁻².

## Adhesion Test

To estimate the adhesion of ZnO layers on the various BDD substrates, current-voltage experiments have been made, by cycling the ZnO/BDD electrode in a KCl solution 0.1M at 20°C from 0V/SME to 1.5 V/SME. The results are presented on Figure 5. The anodic polarisation of ZnO/BDD electrodes could induce the decomposition of ZnO deposits from the following equation:

$$ZnO + 2 h^{+}_{BV} \rightarrow Zn^{2+} + 1/2 O_{2}$$

The anodic potential corresponding to the decomposition reaction and the current peak intensity can be used as a first evaluation of the adhesion of ZnO on BDD substrate. The more anodic and intense are the values, the more adherent should be the ZnO deposit on BDD substrate.

<figure><img src="image_6.png" /><figcaption>Figure 5. Current-voltage curves recorded in KCl 0.1M solution at T = 20°C, on the various BDD/ZnO interfaces. V = 50 mV.s⁻¹.</figcaption></figure>

Left: Black curve: ZnO on H-BDD; grey curve ZnO on BDD oxidised with KMnO₄; light grey ZnO on BDD oxidised with Ce⁴⁺.

Right: Black curve: ZnO on H-BDD; grey curve ZnO on BDD anodised with high current density; light grey ZnO on BDD anodised with low current density

On H-BDD, the peak corresponding to ZnO oxidation appears at + 1.35 V/SME with an anodic current of about 2 mA.cm⁻². For surfaces treated with Ce⁴⁺ and MnO₄⁻ the anodic peak is respectively at the same potential position (+ 1.35 V/SME) or shifted to less positive values (+ 1.2 V/SME), while the current density are much less important (0.2 mA.cm⁻²) or in the same order of magnitude (1.2 mA.cm⁻²). These results suggest that the deposit is more adherent on the BDD surface treated with Ce⁴⁺, and fewer adherent on the BDD surface treated with KMnO₄ compared to the as-grown diamond substrate.
On the BDD surface anodically treated with the lowest current, the peak corresponding to ZnO decomposition is still at + 1.35 V/MSE, while the current density is higher which suggest a less adherent ZnO deposit compared to H-BDD substrate.

Very differently, when the substrate has been oxidised with a high current, the zinc oxide deposit seems to be much more adherent than that deposited on H-BDD. As observed on figure 5b, no real decomposition peak is visible while the polarisation of the ZnO/BDD electrode is put from 0V/SME to 1.5 V/SME.

## Discussion

The increase of ether's groups concentration at the diamond surface, observed after the chemical oxidation in presence of KMnO₄, is related to an increase of nucleation sites. The hypothesis is that such C-O-C terminations could reduce the adsorption of Cl⁻ ions during the ECD process as already described (19), leading to the growth of a denser ZnO layer. The presence of ether's group seems also induce a decrease of the adhesion of the deposit. At the opposite, the formation of hydroxyls groups after the chemical treatment with the ceric specie, would not limit the adsorption of Cl⁻ ions leading to a substrate with a number of nucleation sites very similar to that on as-grown diamond. In these conditions, a very similar ZnO deposit will be obtained in terms of morphology, while it adhesion seems to be favored by the presence of C-OH species at the BDD surface.

The density of nucleation sites and the adhesion of ZnO deposit seem thus related to the kind of single “C-O” bonds which are present at the BDD surface. An important point is that in these conditions, the adhesion of deposits and the number of nucleation sites, vary in opposite ways.

After both anodic processes, the number of nucleation sites increases in the same time with the adherence. In these conditions, more than an effect of “C-O” type, the huge evolution of the proportion of CHx seems to explain the adhesion and morphology of the zinc oxide deposits.

## Conclusion

This work studies the influence of the BDD surface chemistry on the ZnO electrochemical deposition.

BDD electrodes with different surface's chemistry were tested as substrate for the ECD of zinc oxide. As-grown hydrogen terminated diamond electrode was compared to samples oxidized either at Open Circuit Potential (OCP) conditions in presence of an oxidizing agent at room temperature or in galvanostatic conditions. The five BDD electrodes have been characterized by XPS analysis before ECD process at 60°C, during 1h at -1,4 V/MSE in KCL 0.1M/ZnCL₂ 5mM solutions.

On diamond surfaces oxidized at OCP conditions, the density of nucleation sites and the adhesion of ZnO deposit seem related to the kind of single “C-O” bonds which are present at the BDD surface. Ether groups lead to a denser and less adherent ZnO layer while hydroxyl groups seem to favor the growth of a less dense and more adherent deposit.

On diamond surfaces anodized, the key point appears to be the presence of CHx groups which lead in both cases to denser and more adherent zinc oxide deposits.

Besides playing on deposit morphology, this work evidences that the surface terminations of BDD (-CH, C-OH, -COC or -CHx) have a strong effect on the ZnO deposit adhesion.
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