Elsevier

Analytica Chimica Acta

Volume 397, Issues 1–3, 4 October 1999, Pages 145-161
Analytica Chimica Acta

Polycrystalline diamond electrodes: basic properties and applications as amperometric detectors in flow injection analysis and liquid chromatography

https://doi.org/10.1016/S0003-2670(99)00400-6Get rights and content

Abstract

Diamond films, fabricated by chemical vapor deposition, provide electrochemists with an entirely new type of carbon electrode that meets the requirements of activity, conductivity, and stability for a wide range of applications. In this manuscript, the basic electrochemical properties of high quality diamond thin-films (3-6 μm thick) are highlighted. The films are polycrystalline, hydrogen terminated and doped with boron (ca. 1019–1020 cm−3). Some preliminary results using diamond in amperometric detection schemes, coupled with flow injection analysis and liquid chromatography, are presented for azide and nitrite, chlorpromazine, ascorbic acid and catecholamines. The use of diamond for the voltammetric detection of trace metal ions is also illustrated. The detector figures of merit (e.g., dynamic range, sensitivity, detection limit, response variability and response stability) for diamond are compared with freshly polished glassy carbon. Diamond exhibits as good or superior detector performance for each of these analytes. For example, the detection limit (S/N = 3) for chlorpromazine at diamond is 4 nM or 26 pg, and the response variability is 0.3%, while for glassy carbon the detection limit (S/N = 3) is 40 nM or 260 pg, and the response variability is 1%. The properties of diamond electrochemical interfaces are far from being fully understood, but the results reported herein portend the favorable possibilities for applications of diamond in electroanalysis.

Introduction

Carbon electrode materials (e.g., carbon fibers, glassy carbon and graphite) are used in a variety of electrochemical technologies including electroanalysis, energy storage and generation devices (batteries and fuel cells), and electrosynthesis. These materials have a microstructure consisting of condensed layers of six-membered aromatic rings with sp2-hybridized carbon atoms trigonally bonded to one another. The crystallite size, extent of microstructural order and chemical composition can vary from material-to-material (i.e., edge-to-basal plane ratio). This can have important implications on the electron transfer kinetics for a given redox analyte, particularly those analytes that are sensitive to the surface microstructure and chemistry [1], [2], [3], [4]. The electrochemical performance of these materials has been studied extensively over the past three decades. As a result, much is known regarding the structure–reactivity relationship ([1], [2], [3], [4] and refs. therein). One carbon material that has not been extensively investigated so far is diamond.

The electrochemical use of synthetic conductive and semiconductive diamond thin-films has only recently been reported [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. (See [5], [6], [7] for a more complete publication listing). Therefore, the relationship between the physical, chemical and electronic properties of the material, and the observed electrochemical or photoelectrochemical performance is not well understood. Diamond is one of nature’s best insulators, but when doped with boron, the material can possess either semiconducting or semimetallic electronic properties depending on the doping level. The doping level (e.g., boron) and hydrogen content in the film will influence the carrier concentration [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Diamond shows p-type conductivity if the concentration of substitutional boron impurities exceeds the total concentration of donor centers (e.g., nitrogen) [27]. For example, diamond thin-films deposited by hot filament or microwave-assisted chemical vapor deposition (CVD) can be doped as high as 10 000 ppm B/C, resulting in films with resistivities of 0.01 Ω cm or less. Since boron is easily incorporated, heavily doped, low resistivity films can be produced. Such films are sufficiently conductive for electrochemistry. The boron segregation is not uniform and varies with the different growth sectors [28], [29], [30], [31]. It should be noted that both the charge carrier concentration and the carrier mobility will control the conductivity.

Each carbon atom in the bulk of diamond is tetrahedrally bonded to four others using sp3-hybrid orbitals. Microstructurally, the atoms arrange themselves in stacked, six-membered rings in a ‘puckered’ rather than a planar conformation. There are two types of diamond structures: cubic diamond (most common) and hexagonal lonsdaleite (less common). In cubic diamond, the ‘puckered’ six-membered rings adopt a ‘chair’ configuration in both the in-plane and stacking directions, while the rings in lonsdaleite adopt a ‘chair’ configuration in-plane and a ‘boat or eclipsed’ configuration in the stacking direction. [19], [20], [21], [22], [23], [29] In boron-doped polycrystalline films, some of the impurity atoms substitutionally insert for carbon atoms during deposition, while others accumulate in the grain boundary regions. Evidence suggests that the substitutional boron is most responsible for the conductivity. Energetically, the boron atoms (electron acceptors) in low doped films (∼1017 cm−3) form a discrete band located approximately 0.35 eV above the valence band edge. However, the impurity band broadens and the edge shifts toward the valence band with increasing dopant concentration. This is due to the mutual interaction of the boron centers. For instance, a doping level of 1020 cm−3 yields an activation energy of 0.013 eV. [18], [19] In the ideal case at room temperature, a fraction of the valence band electrons are thermally promoted to the boron acceptor level producing free electrons in the dopant band and holes, or vacancies, in the valence band. The electrons and holes move in the presence of an electric field and give rise to electrical conductivity. Additionally, boron-doped diamond thin-films often possess a rough, polycrystalline morphology with grain boundaries, and a small volume fraction of amorphous or nondiamond carbon impurity. As a consequence, the electrochemical and photoelectrochemical properties are probably influenced, in a complex manner, by (i) the dopant type and concentration, (ii) morphological features such as extended and point defects, (iii) the nondiamond impurity content, (iv) the primary crystallographic orientation, (v) the surface termination (H, F or O, etc.) and (vi) the fraction of grain boundary carbon.

Highly doped, hydrogen terminated diamond films are semimetals, and possess several important and unique properties: ( [5], [6], [7] and refs. therein )(i) low and stable voltammetric and amperometric background currents, (ii) wide working potential window in aqueous electrolyte solutions, (iii) reversible to quasi-reversible electron transfer kinetics for several inorganic redox analytes, and enhanced signal-to-background ratios for these analytes due to the low background currents, (iv) morphological and microstructural stability at extreme anodic and cathodic potentials, (v) low adsorption of polar molecules from aqueous solutions like anthraquinone-2,6-disulfonate, and (vi) long-term response stability. These material properties are the impetus for our efforts to study and develop diamond film electrodes for applications as amperometric detectors in flow injection analysis (FIA) and liquid chromatography (LC). It is supposed that some of the shortcomings of conventional sp2 carbon electrodes, namely response variability from material-to-material, reproducible control over the physicochemical properties, the need for frequent pretreatment to activate the surface, surface oxidation and corrosion processes, and response stability can be lessened or aleviated by the use of diamond.

In this manuscript, some of the basic electrochemical properties of high quality diamond films are discussed. The films are polycrystalline, hydrogen terminated and doped with boron (ca. 1019–1020 cm−3). Some of the initial results using diamond in amperometric detection schemes for azide and nitrite, chlorpromazine, ascorbic acid and catecholamines are presented. The use of diamond for the voltammetric detection of trace metal cations is also illustrated. Most of the amperometric data were obtained using flow injection analysis; however, some results for azide and nitrite were also obtained by ion chromatography. The detector figures of merit (e.g., dynamic range, sensitivity, detection limit, response variability and response stability) for diamond are compared with freshly polished glassy carbon. No attempt is made to compare the results presented herein with data from other laboratories reported in the literature. Instead, focus is placed on comparing the performance diamond and freshly polished glassy carbon under identical conditions. It is shown that the detector performance for diamond is as good or superior to glassy carbon. Diamond electrochemical interfaces are far from completely understood, but the results reported herein demonstrate the favorable possibilities for the application of this material in electroanalysis.

Section snippets

Diamond thin-film deposition and characterization

The polycrystalline diamond thin-films were grown using the following deposition protocol. This protocol produces high quality films in our reactor and, most importantly, yields films with fairly reproducible electrochemical properties. The diamond was deposited on conducting p-Si (100) substrates (Virginia Semiconductor, Fredericksburg, VA) using an in-house microwave-assisted CVD reactor. The substrates (0.1 cm thick × 1 cm2 in area) were pretreated by solvent cleaning in toluene, methylene

Morphology and microstructure

Fig. 1(A) and (B) show typical AFM images of a polycrystalline diamond film. This particular film was grown with a 0.33% methane concentration. The large scale image, shown in Fig. 1(A), reveals a well-faceted and continuous film composed of large microcrystallites, grain boundaries, secondary growths (i.e., smaller growth features on top of the large facets and grain boundaries), growth hillocks, and isolated steps. Several of the microcrystallites are triangular in shape with base diameters

Conclusions

High quality, boron-doped diamond films show much promise for applications in electroanalysis. Electrochemical applications require stable, conductive, chemically robust, and economical electrodes. Diamond films, fabricated by chemical vapor deposition, provide electrochemists with an entirely new type of carbon electrode that meets these requirements for a wide range of applications, particularly in the field of electroanalysis. Some of the basic electrochemical properties of high quality,

Acknowledgements

Financial support for this research was generously provided by the National Science Foundation (CHE-9505683) and the donors to the Petroleum Research Fund, administered by the American Chemical Society. The FIA data for ascorbic acid were obtained by Alicia Albritton, and she acknowledges a WAESO Fellowship that supported her work. The authors would like to express their appreciation to Dr. James E. Butler of the Chemistry Division at the Naval Research Laboratory for providing the high quality

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