Doping level influence on chemical surface of diamond electrodes

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Abstract

The modification of surface bond termination promoted by the doping level on diamond electrodes is analyzed. The films were prepared by hot filament chemical vapor deposition technique using the standard mixture of H2/CH4 with an extra H2 flux passing through a bubbler containing different concentrations of B2O3 dissolved in methanol. Diamond morphology and quality were characterized by scanning electron microscopy and Raman scattering spectroscopy techniques while the changes in film surfaces were analyzed by contact angle, cyclic voltammetry and synchrotron X-ray photoelectron spectroscopy (XPS). The boron-doped diamond (BDD) films hydrophobicity, reversibility, and work potential window characteristics were related to their physical properties and chemical surface, as a function of the doping level. From the Mott–Schottky plots (MSP) and XPS analyzes, for the lightly (1018 cm−3) and highly (1020 cm−3) BDD films, the relationship between the BDD electrochemical responses and their surface bond terminations is discussed.

Highlights

► Two boron doped diamond films were grown with different doping levels. ► Different electrochemical response between the samples was observed. ► There was a direct relationship between the measurements of XPS, electrochemical and contact angle. ► The doping process results in different chemical terminations. ► The doping process affects the band diagram result in NEA or PEA.

Introduction

The electrochemical behavior of boron doped diamond (BDD) electrodes strongly depend on its surface properties such as the grain size, the crystallographic orientation, and the sp2 content at the film grain boundary [1]. The stabilization of diamond surface may occur as H-terminated or O-terminated and presents singular physical and chemical effects [2]. Among these effects are the changes in wettability behavior of these films, which is hydrophobic for H-terminated or hydrophilic for an O-terminated diamond. Also, the diamond presents a difference in electron affinity (EA) that are mainly caused by surface dipole difference between “C–H” and “C–O” bonds with a negative electron affinity (NEA) for H-terminated and positive electron affinity (PEA) for O-terminated films [3], [4], [5]. These differences have important consequences for the kind of surface chemistry that can be successfully performed on diamond electrodes.

The importance of the hydrogen role on the diamond surface has been studied by some authors [2], [6], [7]. A highly unusual property of undoped, hydrogen-terminated diamond is the appearance of p-type surface conductivity when exposed to air [2]. This conductivity arises from positive charge carries (holes) and is confined to a narrow near surface region. Several mechanisms for p-type conductivity have been proposed, including the surface transfer doping mechanism involving the H2/H+ redox couple, as originally proposed, that is in agreement with the experimental observations of the surface conductivity in the diamond [7]. Chakrapani et al. have reported the study of the charge transfer equilibrium between hydrogen-terminated, macroscopic diamonds and diamond powders with aqueous solutions of controlled pH and oxygen concentration [2]. From these results, an adsorbate from the atmosphere on the hydrogen-terminated diamond surface is required to induce the surface accumulation layer, resulting in a positive space charge layer in diamond with 1013 cm−2 holes.

For BDD electrodes, the influence of hydrogen and oxygen terminations on the reversibility of the ferro/ferricyanide and other redox couples was studied by Granger and Swain [8]. They argued that the Fe(CN)64−/3− redox reaction takes place through a specific surface interaction on the hydrogen-terminated surface that is blocked for oxygen-terminated diamond. According to the electrode application, it is important to quantify the presence of each group on the diamond. Wang et al. [9] have discussed a novel method to differentiate among hydroxyl, ether, and carbonyl groups on BDD electrode surfaces of oxidized diamond. This method is based on simple esterification between trifluoroacetic acid and the surface hydroxyl groups of oxidized diamond. The formation of hydroxyl groups is for the most part desired over ether and carbonyl groups, as well known chemical routes can be used to link functional groups to –OH units [9]. However, from XPS analyzes, they showed that the oxygen functions on the surface of as-deposited BDD film is not of the C–OH type, but ether bonds instead. They concluded that the advantage of this approach is that it helps to solve important experimental problems not only in electrochemistry but in surface of BDD.

Concerning the discussion above, the cause of different electrochemical behaviors corresponding to different surface-terminated diamond films is far from being clarified. So, complementary studies are justified to correlate the electrode response and its surface termination evolution taking into account how the doping levels may influence the diamond surface. This study is presented for BDD electrodes produced at two different doping levels, called A and B samples, with acceptor concentrations around 1018 and 1020 atoms  cm−3. These values were estimated from Mott–Schottky plots (MSP) using frequencies of 1, 10 and 50 kHz. The physical properties and chemical surface of BDD electrodes were correlated with their hydrophobicity, reversibility, and work potential window for the two doping levels studied.

Section snippets

Experimental

BDD films were grown by hot filament-assisted chemical vapor deposition (HFCVD) technique at 1050 K from 1.0vol% CH4 in the H2/CH4 mixture at a total pressure of 6.5×103 Pa. The films were deposited on silicon substrate after seeding pre-treatment during 16 h [10]. Boron was obtained from an additional H2 flow forced to pass through a bubbler containing B2O3 dissolved in methanol. For all our experiments, the H2 and the B2O3/CH3OH/H2 flows were controlled in order to obtain the desired B/C ratios.

Surface and physical analyzes

The morphological characterization by SEM analyzes of the BDD films are presented in Fig. 1(a and b), which demonstrated faceted grains with symmetrical and smooth faces with uniform texture. The average grain size variation, also evaluated from SEM images, was not significant and remained between 5.0 and 3.0 μm for both samples. Conversely, the highly doped film (sample B) showed a significant increase in the smallest grain population compared to that for film grown with lower boron addition.

Conclusion

The results clearly indicate that the reversibility of the electrochemical response for Fe(CN)63−/4− redox couple on lightly and highly BDD is significantly changed by the boron addition. From the XPS analyzes the oxygen amount increased as a function of the doping level and presented a close agreement with the contact angle measurements. The doping level influenced the sp2 amount at the film grain boundary as well as the defect level in the diamond lattice leading to different surface

Acknowledgments

We are very grateful to the Brazilian Agencies CNPq and FAPESP for the financial support. Special thanks to Laboratório Nacional de Luz Sincrotron by the XPS analyzes.

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