1 Introduction

Iwaki et al. in 1983 [1] and later Pleskov et al. in 1987 [2] introduced diamond as an electrode material. Since then boron-doped diamond has been recognized as one of the best electrode materials. It shows numerous unique physical and chemical properties [3–5], such as (i) high chemical stability in harsh environments and/or at high voltage/current densities; (ii) weak or no surface bio-fouling; (iii) biocompatibility; (iv) low and stable capacitive currents in aqueous and non-aqueous solutions; and (v) a wide electrochemical potential window. Moreover, the diamond surface can be terminated with hydrogen, hydroxyl, and oxygen, which allow tuning the electronic properties of the solid/electrolyte interface with respect to energy alignments of interacting levels. Furthermore, diamond is ultra-hard (50–150 GaP) and various diamond nanostructures can be formed [6–9].

Many successful reports have been thus shown about the use of planar macroscopic diamond electrodes for electrochemistry, bio electrochemistry, electroanalysis, electrocatalysis, and environmental related applications [3–6]. However due to the non-uniform doping in diamond, boundary effects, and the varied ratios of graphite to diamond, in most studies only averaged electrochemical signals are detected over the full electrode. To solve this problem, the investigation of diamond electrochemistry at the nanoscale, in other words, electrochemistry of diamond nanostructures and nanoparticles, is highly needed. In this way the difference of electric and electrochemical properties from their bulk electrodes might be detected. The effect of the size, shape, and composition of diamond nanostructures and nanoparticles on their electrochemical properties can be clarified [10–12]. These studies will play significant role in their applications for electrochemical energy storage and conversion, electrocatalysis, electrochemical sensor development, and related fields.

We therefore summarize in this chapter recent progress and achievements about diamond electrochemistry using diamond nanostructures (e.g., nanotextures, nanowires) [13–56], diamond nanoparticles (un-doped and doped nanoparticles) [57–128], and diamond nanoelectrode arrays [129–132]. The applications of these nanostructures and nanoparticles for electroanalytical (e.g., electrochemical, biochemical sensing) and electrochemical applications (e.g., energy storage with capacitors and batteries, electrocatalysis, etc.) [133–141] are shown. Diamond nanoelectrode array is introduced and highlighted as a promising tool to investigate diamond electrochemistry at the nanoscale. After comparing the results published in literature, we close this chapter with a conclusion about the future and tendency of electrochemistry using diamond nanostructures and nanoparticles.

2 Diamond Nanostructures

2.1 Fabrication Approaches

Diamond nanostructures, including nanotextures (also called nanograss, nanowinkles, nanocone, etc.) with dimensions of typically a few nanometers, nanowires (also called nanoneedles, nanoforests, etc.) with lengths of a few micrometers, and networks (also called porous films, etc.) with pore sizes from few nanometers to micrometers have been fabricated using top-down, bottom-up, or template-free approaches. In the top-down approach, diamond is etched away with reactive ions in a plasma using an etching (hard) mask. Figure 1a shows schematically such an approach where nickel nanoparticles are applied as the etching mask. The morphology (e.g., size, length, density, etc.) of the resulting nanostructures are determined mainly by the etching mask (e.g., nature, size, shape, etc.) and etching conditions (e.g., temperature, gas, pressure, time, etc.). Various mask materials have been thus applied, including Al, SiO2, Au, Ni, Mo, polymers, oxides, nitrides, and diamond nanoparticles, etc. [6–9]. For example, the etching mask of 5–10 nm diamond nanoparticles was applied to form diamond nanotextures [13]. With an etching time of 10 s the textures (1–5 nm in diameters, 11 nm in distance) were produced. One typical AFM tapping-mode image of such nanotextures is shown in Fig. 1b. If nickel nanoparticles were applied as the etching mask, the formed diamond nanowires [14] have a height of 1200 (±200) nm, a width of 35 (±5) nm, and a density of ~1010 cm−2, as shown in the SEM image in Fig. 1c. In a bottom-up approach, diamond nanostructures are produced by the overgrowth of other nanostructures. Up to the morphology of the templates, various diamond nanostructures have been formed [6–9]. The templates from silicon nanowires, carbon nanotubes (CNTs), SiO2 spheres, etc. have been employed. Figure 1d shows one SEM image of diamond foam fabricated with the overgrowth of SiO2 sphere. Figure 1e gives the SEM image of a diamond network [48], synthesized with a template-free approach, namely through selective and wet-chemical removal of silicon carbide from a diamond/SiC composite film with a mixture of HF and HNO3.

Fig. 1
figure 1

a Schematic illustration of forming diamond nanowires using a top-down approach and Ni nanoparticles as the etching mask; b AFM tapping-mode image of diamond nanotextures; SEM images of c diamond nanowires, d diamond foam and e diamond networks [137]

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2.2 Electrochemical Properties

Voltammetry and impedance were applied to characterize the interfacial properties (e.g., the real electrode areas, their electrochemical activities) of diamond nanotextures and nanowires [13–15]. The analysis of Mott-Schottky plots [13] in the absence of redox probes, the analysis of electrochemical impedance spectra using an electric equivalent circuit [15] and the cyclic voltammograms in the presence of redox probes were conducted to determine the surface areas of diamond nanostructures as well as the electrode processes of redox probes on diamond nanostructures based electrodes [13–15].

For example, Fig. 2 shows the Mott-Schottky plots of a smooth diamond electrode and diamond nanotextures based electrode (for an etching time of 10 s and using diamond nanoparticles as the etching mask) in 0.1 M pH 7.4 phosphate buffer. The measurements were done at a fixed frequency of 1.0 kHz. The built-in potential of diamond nanotextures, calculated by use of Mott-Schottky equation, was 1.6 V. Its surface area was 2.1 times higher than that of a smooth diamond [13].

Fig. 2
figure 2

Mott-Schottky plots of a a smooth diamond and (b) diamond nanotextures for an etching time of 10 s in 0.1 M pH 7.4 phosphate buffer at a fixed frequency of 1.0 kHz [13]

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Figure 3a shows the cyclic voltammograms of a smooth diamond electrode and diamond nanowires based electrode (using a top-down approach and nickel nanoparticles as the etching mask) in 0.1 M KCl. The surface area of diamond nanowires based electrode, calculated from the capacitive current, is 10-times larger than that of a smooth diamond electrode [14]. On such an electrode the electrode process of Fe(CN) 3−/4−6 was quasi-reversible and diffusion-controlled, as shown in Fig. 3b. The calculated electrode active area of diamond nanowires based electrode is however only 1.5 times larger than that of a smooth diamond. Similar result, namely the electrode area calculated from the capacitive current is much larger than that from the Faradaic current of redox probes, was obtained on diamond networks [48]. This is because the faradaic current of ferri-/ferro- cyanide is dominated by their diffusion lengths of analytes, which are typically in the range of tens of micrometers. Since the distance in between nanowires or diamond pores is much smaller than the value of diffusion lengths of analytes, diamond nanowires or diamond networks based electrode actually behaves as a macro-sized electrode [14].

Fig. 3
figure 3

Cyclic voltammograms of (solid lines) a smooth diamond electrode and (dashed lines) diamond nanowires based electrode in a 0.1 M KCl at a scan rate of 0.050 V s−1 and in b 1.0 mM Fe(CN) 3−/4−6 \(+\) 0.1 M KCl at a scan rate of 0.1 V s−1 [14]

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2.3 Electrochemical Applications

Due to improved reactive sites, promoted electrocatalytic activities, and accelerated electron transfer rates, diamond nanotextures/nanowires are promising electrodes to improve the efficiency, sensitivity, selectivity and reproducibility of biomedical and chemical sensors [16–39]. Due to their wide electrochemical potential windows, enhanced surface areas, as well as high chemical stability, diamond nanostructures have been used for energy storage and related applications [42–56].

2.3.1 Electroanalytical Applications

Diamond nanostructures have been widely applied for electroanalytical applications [16–55], such as for electrochemical sensing of dopamine in the presence of ascorbic acid and uric acid [16, 17], for non-enzymatic monitoring of glucose [18–20], for the detection and immobilization of biomolecules (e.g., tryptophan [21, 22], tyrosine [22], catechol [23], and cytochrome c [24–26]), for enhanced electron transfer of shewanella loihica PV-4 [27], and for the construction of toxicity sensor by use of shewanella loihica PV-4 planktonic cells as the recognition element in bioelectrochemical systems [28].

For example, N-doped diamond nanowires showed excellent electrocatalytic activity towards the oxidation of ascorbic acid, dopamine, and uric acid. The electrocatalytic activity results from the increased sp2 graphitic phase and the nanowire-like structure [17]. Diamond nanowires allowed non-enzymatic oxidation of glucose [18–20]. On diamond nanowires (e.g., with the length of about 3 µm and the diameter from 10 to 50 nm), a detection limit of 60 µM glucose was reported [19]. A fast and stable glucose oxidation process (less than 20 s) was reported on diamond nanowires as well [18]. At 0.7 V (vs. SCE), the stable steady-state (only 8 % reduction after 150 repetitive cycles) oxidation current was linearly enhanced with the concentration of glucose from 0 to 7 mM. The sensitivity of these detections was 8.1 µA mM−1 cm−2 and the detection limit was 0.2 ± 0.01 µM [18]. On diamond nanowires (e.g., with 20 nm in diameter, 200 nm in length and 50 nm in distance), electrocatalytic detection of catechol was realized in a working concentration range of 5 to 100 µM with a sensitivity of 719.71 µA M−1 cm−2 and a detection limit of 1.3 µM [23]. Differential pulse voltammetric detection of tryptophan on diamond nanowires was reported with a detection limit of 5 × 10−7 M [21]. Simultaneous detection of tryptophan and tyrosine was conducted successfully as well by differential pulse voltammetry on diamond nanowires when the amount ratio of tryptophan to tyrosine was less than 0.5 [22].

Direct electrochemistry of cytochrome c was realized on the OH-terminated diamond nanotextrues [24–26]. As shown in Fig. 4a, diamond nanotextures act as molecular traps, leading to a more efficient electron transfer process. The surface coverage of cytochrome c on diamond nanotextures was evaluated to be 4.2 × 1012 cm−2 and its electron transfer rate constant was (1.43 ± 0.05) s−1, higher than some of reported values [24–26]. The enhancement of electron transfer rate results from the electrostatic and hydrophobic interaction of cytochrome c with the OH-terminated diamond. Electrocatalytic reactions towards oxygen reduction as well as AFM tapping and scratching experiments in buffer [29–31] further confirmed that the electrostatic interaction controls coarse orientation of cytochrome c while hydrophobic interaction assists in the formation of the electron transfer complex, as schematically shown in Fig. 4b. Diamond nanograss showed an enhanced electron transfer from outer membrane c-type cytochromes of shewanella loihica PV-4 to the electrode [27]. Using such a shewanella loihica PV-4 planktonic cell as the recognition element in bioelectrochemical systems (BES), a toxicity sensor based on the electrochemical and impedance detection of tobramycin was proposed [28].

Fig. 4
figure 4

Schematic demonstration of a trapping of cytochrome c using diamond nanotextures for an efficient electron transfer process and b the interaction of cytochrome c with the OH-terminated diamond surface [26, 30]

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Decorated diamond nanotextures/nanowires with nanoparticles (e.g., from nickel [19, 32, 33] and platinum [34]), with nitrophenyl [35–37], and with carboxylic acid-terminated poly(pyrrole) [38] were applied to construct electrochemical [19], DNA biosensors [35–37, 39], and immunosensors [32, 33, 38].

For example, tip coating of diamond nanowires with nickel nanoparticles improved the detection limit for glucose detection to 10 µM [19]. Non-enzymatic oxidation of glucose and electrocatalytic oxidation of methanol have been reported on diamond nanotextures decorated with platinum nanoparticles [34]. Diamond nanotextures were functionalized via electrochemical grafting at −0.05 V (vs. Ag/Ag+) for 4 s with 1.0 mM diazonium salts. The density of phenyl molecules at diamond nanotextures was about 2 × 1013 cm−2 [36]. Constant-current mode scanning tunneling microscope (STM) conducted before and after grafting with nitrophenyl on diamond textures indicated the preferential bonding of nitrophenyl to the tips of wires [13]. Such tip-functionalized diamond nanotextures were applied to immobilize DNA with aid of SSMCC chemistry, as shown in Fig. 5a [35, 36]. Electrochemical detection of DNA hybridization on such an interface was realized by use of negatively charged redox indicators [35, 36]. Figure 5b shows the differential pulse voltammograms of Fe(CN) 3−/4−6 on diamond nanotextures based electrode before and after functionalized with single-strand DNA (ss DNA) and double-strand DNA (ds DNA). The difference of peak currents of redox indicators on these interfaces was applied to detect DNA hybridization. A detection limit of about 2 pM was realized on 0.03 cm2 sensor area over 30 hybridization/denaturation cycles. The discrimination of single-base mismatched complementary DNA was achieved [35–37, 39].

Fig. 5
figure 5

a Schematic plots of the tip-functionalization of diamond nanotextures for DNA immobilization and b differential pulse voltammetric detection of DNA hybridization on diamond nanotexures using negatively charged redox mediators [35, 36]

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Furthermore, several diamond nanowires based immunosensors have been reported [33, 34, 38]. For instance, biotinylated anti-IgG was specifically linked to nickel particles. The charger transfer resistance, detected from electrochemical impedance spectroscopy, was linear with IgG concentration in the range of 0.3–400 ng mL−1. The detection limit of IgG was found to be 0.3 ng mL−1 [33].

2.3.2 Electrochemical Capacitors

Diamond nanowires [42–50, 133, 134, 139] and porous diamond films [51–53] have been utilized extensively for supercapacitors construction, although the capacitance of a smooth diamond electrode itself is not so huge, in comparison with other non-carbon electrodes. Moreover, up to the electrolyte applied, boron-doping level, and the surface morphology as well as terminations, the capacitance of a diamond electrode varies greatly. For a nanocrystalline diamond electrode with a boron-doping level of 5 × 1020 cm−3, its double-layer capacitance is about 3.6–7, 14–20, 11–15 µF cm−2 in aqueous, organic, and ionic liquid solution, respectively. However due to enhanced surface areas as well as the wide working potential windows [42, 43] (ca. 2.5 V in aqueous electrolytes and 7.3 V in organic electrolytes [44–46]), diamond nanostructures are very promising for supercapacitors construction. Taking diamond networks as an example, a diamond network with a porosity from 15 to 68 % led to hundreds of times enhancement of the surface areas than that of flat diamonds (e.g., 490-fold for a 3 µm thick diamond network).

Electric double layer capacitors (EDLCs) based on diamond honeycomb nanostructures showed a capacitance of 3910 µF cm−2 and 666 µF cm−2 in aqueous and organic solution, respectively [44–46]. Diamond foam based EDLCs attained specific capacitances of 598 and 436 µF cm−2 in aqueous and organic solutions, respectively. A high power density of 807 W cm−3 was achieved, which touched the best power performance of electrolytic capacitors [47]. In 0.1 M H2SO4, the double layer capacitance of a diamond network was calculated to be 13.7 F g−1 or 17.3 F cm−3 at a scan rate of 0.1 V s−1 [48]. Figure 6a compares the capacitive behaviour of a smooth diamond electrode with two diamond networks in 0.1 M H2SO4 [48]. Silicon nanowires coated with a thin diamond film (240 nm in thickness) was employed for EDLCs application. The capacitance of such an EDLC was 105 µF cm−2 in a mixture of propylene carbonate with 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide, one room temperature ionic liquid. A high energy density of 84 µJ cm−2 and power density of 0.94 mW cm−2 and good stability (retention stability of 93.3 % after 10,000 cycles at a scan rate of 5 V s−1) were achieved [49]. Figure 6b presents a very recent retention stability test result for a diamond EDLC in 1.0 M Na2SO4 [43]. Only 5 % reduction of the capacitance is seen. This is due to high chemical stability of diamond electrodes in any kind of media as well as at different working potentials. On TiO2 nanotubes coated with boron-doped diamond (200–500 nm in thickness), the specific capacitance is dependent on the boron concentration. In 0.1 M NaNO3, the specific capacitance of 2.10, 4.79, and 7.46 mF cm−2 was obtained at a scan rate of 0.01 V s−1 for a [B]/[C] ratio of 2000, 5000 and 10,000, respectively. The substantial improvement of electrochemical performance and the excellent rate capability was explained with the synergistic effect of TiO2 treatment in CH4:H2 plasma and the high electrical conductivity of boron-doped diamond layers [50]. Recently, “diamond paper” showed in aqueous electrolyte a capacitance of 0.688 mF cm–2 per layer, or 0.645 F g–1. The specific power of these diamond based supercapacitors reached 1 × 105 W kg–1 [134].

Fig. 6
figure 6

Cyclic voltammograms of a flat diamond film (I), diamond network-1 (II) and diamond network-2 (III) in 0.1 M H2SO4 at a scan rate of 0.1 V s−1; b Capacitance retention of a diamond EDLC as a function of charge/discharge cycles, tested in 1.0 M Na2SO4 at a scan rate of 0.1 V s−1 [43, 48]

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Other porous diamond films/membranes [51–53] have been employed for supercapacitor applications. For example, a two-step thermal treatment method was developed for the fabrication of porous conductive boron-doped diamond films [51]. The sizes of the pores were from several tens to several hundred nanometer. Such a porous membrane exhibited a double-layer capacitance of ca. 140 μF cm−2 in an aqueous electrolyte, estimated from cyclic voltammetry and galvanostatic measurements [52]. Porous boron-doped diamond films overgrew on CNTs, showed c.a. 450 times greater electroactive areas and double-layer capacitance values than those for the equivalent flat boron-doped diamond electrodes [51]. Another porous diamond, based on the overgrowth of a highly porous polypyrrole scaffold with a thin boron-doped diamond film, exhibited remarkable electrochemical properties, including a large double layer capacitance up to 3 mF cm−2 in aqueous LiClO4 and a low electrochemical impedance [53].

Diamond nanowires were coated as well with nickel hydroxide [133] to construct diamond pseudocapacitors. Although much higher capacitances of diamond pseudocapacitors than those of diamond EDLCs were obtained, the big shortcoming of diamond pseudocapacitors is the poor retention stability of their capacitances. This results from low stability of pseudo or redox-active species on diamond electrode surface (e.g., these metal oxides are easily stripped from diamond during the charging/discharging processes) [43, 133].

2.3.3 Other Applications

Diamond nanostructures have been proposed as well for other electrochemical applications [40, 54–56, 135, 138]. For example, porous diamond membrane was applied as a filter to separate differently charged 5-carboxyfluorescein and substance P under different biases [55]. Diamond nanowires was also applied to enhance the intensity of electrogenerated chemiluminescence (ECL) of a ruthenium tris(2,2′)bipyridyl/tripropylamine system [40]. Owing to their superior properties (e.g., large surface areas, improved electrocatalytic activities, and accelerated electron transfer rates), the enhancement of ECL intensity was attributed mainly to the highly facile oxidation of tripropylamine on diamond nanowires. This study revealed an effective method for the ultrasensitive detection of ruthenium tris(2,2′)bipyridyl. It will help to increase the efficiency of immunoassays and DNA analysis based on ruthenium tris(2,2′)bipyridyl electrogenerated chemiluminescence. Diamond nanowires have been utilized as well for electrocatalytic applications [135, 138]. For example, diamond nanowires coated with Pt have been applied for electrocatalytic hydrogen evolution [135]. Very recently, electrochemical CO2 reduction has been achieved on diamond coated silicon nanowires, where doped diamond acts as a metal-free electrocatalyst [138]. Recently, diamond foams composed of hollow spheres of polycrystalline boron-doped diamond have been chemically modified with two donor–acceptor type molecular dyes and further utilized as electrode materials for p-type dye-sensitized solar cells in an aqueous electrolyte solution containing methyl viologen as a redox mediator [141].

3 Diamond Nanoparticles

3.1 Synthesis Methods

3.1.1 Un-doped Diamond Nanoparticles

Un-doped diamond nanoparticles with size down to 20 nm have been produced via ball-milling of micro-sized high-pressure-high-temperature (HPHT) diamond films. In this way, diamond nanoparticles less than 10 nm were seldomly obtained. With the aid of bead-assisted sonic disintegration, the production of diamond nanoparticles 70–80 nm in diameter was realized by milling polycrystalline chemical-vapor-deposited (CVD) diamond films. These nanoparticles have faceted shapes with sharp edges, which correspond to fractured crystallographic planes. Using such a method, few spherical nanoparticles were found. Another more widely applied approach to produce spherical diamond nanoparticles these days is to use dynamic processes from molecules of explosives and different graphite precursors [5, 57], including the direct transformation of graphite by an external shock wave, the detonation of graphite mixed with explosive, and the detonation of high energy explosive. Diamond nanoparticles synthesized from these detonation methods have a core of sp3 diamond with a size of 4–5 nm and a shell of a mixture of sp2 and sp3 carbon as well as oxygen based functional groups (e.g., carboxylic acids, esters, lactones, etc.) [5, 57].

3.1.2 Doped Diamond Nanoparticles

Doped diamond particles, mainly boron-doped diamond particles, have been obtained mainly via the overgrowth of insulating diamond particles with boron-doped diamond, solid-state diffusion, and milling [119, 120]. For example, boron-doped diamond powder was synthesized under HPHT using B-doped graphite intercalation compositions as carbon sources [121]. Recently, Kruger et al. [123] applied a multistep milling process followed by purification and surface oxidation to produce 10–60 nm boron-doped diamond nanoparticles. The starting material used for such a milling process was heavily boron-doped diamond films.

3.2 Electrochemical Properties

To investigate electrochemistry of diamond nanoparticles, diamond nanoparticles are always treated and/or cleaned. The applied processes include acid boiling, oxidation by Osswald method, plasma treatment, thermal annealing, etc. For example, surface treatment of diamond nanoparticles by heating in air and in a hydrogen flow results in oxygenated (O-) and hydrogenated (H-) nanoparticles, respectively. The acid boiling removes effectively the responses associated with sp2 carbon impurities. On these diamond nanoparticles based film electrodes, a potential independent capacitive signal has been obtained and checked by recording their cyclic voltammograms in solutions without any redox species [67].

Diamond nanoparticles based film electrodes have been fabricated with various techniques [129], such as drop coating from their ethanol suspensions, smearing a mineral oil paste of diamond nanoparticles, grinding diamond powders into the tip of a Pt wire sealed in a small pipette, electrophoretically deposition, co-deposition, non-contact microprinting, and layer-by-layer self-assembly by a high pressure/high-temperature methodology, etc.

3.2.1 Un-doped Diamond Nanoparticles

Diamond nanoparticles, one form of un-doped diamond, showed different electrochemical properties from un-doped bulk diamond. They are surprisingly electrochemically active. Their electrochemical activities depend greatly on the nature/type (e.g., detonation, HPHT particles), surface terminals, as well as the pH value of the solutions.

3.2.1.1 Detonation Diamond Nanoparticles

Novoselova et al. [58] studied for the first time the redox activity of diamond powders based film electrode in aqueous electrolytes. Redox couples of [Fe(CN)6]3−/4− and Ce3+/4+ were applied as the probes. The voltammetric response of Fe(CN) 3−/4−6 was however superimposed on a linearly sloping background and with small peak currents. The reduction currents were two times larger than the oxidation currents. In the case of the Ce3+/4+ redox couple, additional current responses were noted, besides the expected current peaks. Zang et al. [59] used cavity electrodes to investigate redox activities of diamond nanoparticles. Stable background currents in KCl electrolyte over a wide potential range (−1.2 to 2.0 V vs. Ag/AgCl) and a quasi-reversible reversible electrode reaction for the Fe(CN) 3−/4−6 couple with the electrode reaction rate constant of 2.87 × 10−3 cm s−1 were obtained. The recorded AC impedance spectra were consistent with those obtained on a porous electrode [59].

Holt and her colleague [60–65] made extensive investigation on the electrochemistry of detonation diamond nanoparticles. Differential pulse voltammetry of electrode-immobilised layers of diamond nanoparticles in the absence of solution redox species revealed oxidation and reduction peaks, resulting from direct electron transfer reactions of diamond nanoparticles themselves [60]. Moreover, the presence of detonation diamond nanoparticles on the (diamond) electrode modified the cyclic voltammetric response of Fe(CN) 3−/4−6 and IrCl 3−/2−6 when the scan rate was slow and the concentration of redox couples was low. For example, enhancements of oxidation currents were noted at potentials where the oxidation of the surface of diamond nanoparticles started. The enhancements of reduction currents were likewise observed where diamond nanoparticle was reducible [60]. Attenuated total reflectance infrared spectroscopy was then used to monitor spectral features of the surface of diamond nanoparticles [61]. Aqueous IrCl6 2− was added in these studies. They found that electron transfer between the surface of diamond nanoparticle and the solution redox species results in the oxidation of 8.5 % of surface alcohol groups, with concomitant formation of unsaturated ketone or quinone-like moieties [61]. Scanning electrochemical microscopy (SECM) [62] was applied as well to investigate the redox behavior of detonation diamond nanoparticles based film electrode (Fig. 7). Different collection modes and various redox mediators were used to estimate quantitatively heterogeneous rate constants and overpotentials of redox probes. On such an electrode, extremely sluggish kinetics were found for all redox couples, but the reduction of Fe(CN) 3−6 was found to be especially slow when compared to the oxidation of Fe(CN) 4−6 . Overall, the rate constants were about 10 times faster at diamond nanoparticles based film electrode than that on the boron-doped diamond film [62]. Supported with in-situ infrared (IR) experimental results, they stated that electron transfer at the diamond nanoparticle surface takes place at similar sites as on the boron-doped diamond film. But the reaction sites are present at higher relative concentrations due to the higher surface to bulk atom ratio of the nanoparticles. The modification of an electrode with an immobilised layer of diamond nanoparticles was found later to enhance significantly the redox currents for reversible oxidation of ferrocene methanol. Current enhancement is dependence of the diameter of diamond nanoparticles, with enhancement increasing in the order 1000 nm < 250 nm < 100 nm < 10 nm < 5 nm [64].

Fig. 7
figure 7

Schematic illustration of SECM investigation of redox activities of diamond nanoparticles based film electrode with different redox probes and collection modes [62]

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To explain such the ‘molecule-like’ redox behaviour of diamond nanoparticles, Holt et al. proposed a so-called feedback mechanism [60–64]. Figure 8 shows schematically such a mechanism. A self-conducted oxidation and reduction process via surface states at specific potentials triggers the redox activities of diamond nanoparticles. The magnitude of current enhancement depends on the standard potential of the redox couple relative to those of the surface states of diamond nanoparticles. Provided that at the nanoscale surface properties of diamond nanoparticles dominate over those of the bulk, electron transfer occurs between these essentially insulating particles and a redox species in solution or an underlying electrode. The occurrence of reversible reduction of diamond nanoparticles via electron injection into available surface states at well-defined reduction potentials is thus speculated. In this process, diamond nanoparticles act as a source and a sink of electrons for the promotion of solution redox reactions [67]. They then concluded that the electrochemical activity of un-doped diamond nanoparticles is attributed to unsaturated bonding at the diamond nanoparticle surface [60]. Electron transfer occurs between electroactive species generated at the underlying electrode during voltammetry and the immobilized diamond nanoparticles in the interfacial region, leading to regeneration of the starting species and hence enhancement in currents.

Fig. 8
figure 8

Schematic plot of a catalytic feedback mechanism: catalytic reactions of ferrocene methanol on diamond nanoparticles based film electrode [60]

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Moreover, electrochemical activity of detonation diamond nanoparticles (5 nm in diameter) based film electrode was tunable via surface functionalizations and graphitization degree of diamond nanoparticles. For example, both redox reactions of Fe(CN) 3−/4−6 and Fe3+/2+ were quasi-reversible on the pristine diamond nanoparticles based film electrodes. The O-terminated diamond nanoparticles exhibited the greatest electrochemical activity for the redox couples Ru(NH3) 3+/2+6 and Fe(CN) 4−/3−6 while the H-terminated diamond nanoparticles the least. After fluorination of diamond nanoparticles, the electrode reactions of Fe(CN) 3−/4−6 became slower, while the amino modification accelerated the electron transfer process of Fe(CN) 3−/4−6 anions but slowed the redox reaction of Fe3+/2+ cations [69]. Annealing of diamond nanoparticles in vacuum led to the variation of electrochemical activities of diamond nanoparticles. The electron transfer rate of the Fe(CN)6 3−/4− redox couple in aqueous solutions decreased with an increase of the annealing temperature. Re-annealing in air restored the original electrochemical activity. This is because vacuum annealing below 850 ℃ removed parts of the oxygen-containing surface functionalities from the surface of diamond nanoparticles and produced more sp2 carbon atoms in the shell. When the annealing temperature was at 900–1100 ℃, more serious graphitization produced a continuous fullerenic shell wrapped around a diamond core, which had a high conductivity and electrochemical activity [68].

Furthermore, electrochemical activity of detonation diamond nanoparticles based film electrode was found to be affected by pH values of the solution, namely the by solution proton concentration [60, 62]. In the solutions with pH value of 4 and 5, well-defined peaks in the potential range of −0.1 to 0.5 V (vs. Ag/AgCl) were seen from differential pulse voltammograms. As the solution pH increased, they became much smaller in magnitude and far less resolved [60, 62]. This is because the electrochemical response of diamond nanoparticles based film electrode resulted from the oxidation and reduction of surface states of diamond nanoparticles. The potentials of these surface states are however pH-dependent; moreover they are able to interact with solution redox species [60, 62].

3.2.1.2 HPHT Diamond Nanoparticles

Fermin et al. [66, 67] combined zeta potential measurements in the solutions and electrochemical studies in thin-layer assemblies of diamond nanoparticles to investigate electrochemical properties of undoped HPHT diamond particles. The estimated point-of-zero zeta potential was 6.6. The zeta potentials of these nanoparticles depended on pH. They found that in a single electron transfer process 1 × 104 redox centres per particle were involved. Electrochemical signals were rather sensitive to the extent of sp2 hybridisation at the surface of diamond powders [66]. Electrochemical field-effect transistors were employed to investigate the charge transport properties of O- and H-terminated diamond particles in the presence and absence of metal nanostructures [67]. The assembly of H-terminated diamond particle was characterized by a charging process at a potential above 0.1 V (vs. Ag/AgCl). The responses were found to be associated with hole-injection into the valence band edge, which is shifted to approximately −4.75 eV (vs. vacuum) upon hydrogenation. The position of the valence band edge as well as hole number density at the H-terminated diamond particle surface varied as a function of the applied potential [70]. Through the discussion in terms of the electrochemical formation of charge carriers in the diamond particles, percolation theory, and charge screening at the double layer, Fermin et al. suggested that charge transport on un-doped diamond particles is not only determined by the intrinsic surface conductivity of individual diamond particles, but also by particle-to-particle charge transfer [71]. The latter contribution effectively controls the assembly conductivity in the presence of an electrolyte solution as the difference between hydrogenated and oxygenated particles vanishes. The conductivity in the presence of metal nanoparticles is mainly determined by the metal volume fraction, while diamond surface termination and the presence of electrolyte solutions exert only minor effects [71].

3.2.2 Doped Diamond Nanoparticles

To check electrochemical activities of boron-doped diamond particles, redox probes of Fe(CN) 3−∕4−6  and Ru(NH3) 3+∕2+6 and a polytetrafluoroethylene binder were used. These particles were produced via microwave plasma-assisted CVD growth of a thin boron-doped layer on insulating diamond powders (8–12 μm in diameter). At scan rates between 10 and 500 mV s−1, the peak difference of the anodic wave from the cathodic wave was high (in the range 0.14–0.35 V vs. Ag/AgCl) for both redox probes, suggesting significant ohmic resistance within the powder/binder electrode [119]. Later the same technique was applied to overgrow small-sized diamond powders (100 and 500 nm in diameter). Both powders had increased conductivities. Well-defined electrochemical responses were obtained on these powders based film electrode for the redox reactions of Fe(CN) 3−∕4−6 , Ir(Cl) 2−∕3−6 , and Fe2+∕3+, in comparable to typical responses shown on the high-quality, boron-doped nanocrystalline diamond thin-film [120]. Later the electrochemical characteristics of boron-doped diamond powders based film electrodes were investigated by measuring the cyclic voltammetric curves and AC impedance spectra [121]. The powders were synthesized under HPHT using B-doped graphite intercalation compositions as carbon sources. For the Fe(CN) 3−∕4−6  redox couple, the electrode reaction process is reversible or quasi-reversible at the scan rates of 0.01–1.0 V s−1. At the low scan rates the linear relation between peak current and square root of scan rate indicates that the electrode process is a diffusion-controlled mass-transport process. The electrochemical behavior is similar to a planar electrode. With an increase of the scan rate the electrode process is controlled by the mass transport plus kinetic process. AC impedance spectra exhibited the porous structure characteristic of boron-doped diamond powders based film electrode [121]. The boron-doped diamond nanoparticles made by solid-state diffusion method showed a lower capacitance but a higher conductivity than undoped diamond nanoparticles [122]. In the potential range of −0.3 to 1.8 V (vs. SCE), a featureless voltammetric response was obtained. Recently high quality boron-doped diamond nanoparticles with a size of 10–60 nm and a boron concentration of approximately 2.3 × 1021 cm−3 have been produced by Kruger et al. [123]. However the electrochemistry of those nanoparticles has not been reported yet.

3.3 Electrochemical Applications

3.3.1 Un-doped Diamond Nanoparticles

3.3.1.1 Electroanalytical Applications

Due to the features of giant specific surface areas and large numbers of surface defects as well as the cluster structure, detonation diamond nanoparticles have increased electrical conductivities. As a novel type of electrode materials, they have been employed frequently for electrochemical and biochemical sensing applications [72–86].

Many electrochemical sensors based on diamond nanoparticles have been reported, including the sensors for the detection of azathioprine [72], epinephrine and uric acid in the presence of ascorbic acid [73], nitrite [74], tryptophan, and 5-hydroxytryptophan [75]. The matrix of diamond and silver nanoparticles were applied for electrochemical monitoring of thioridazine [76] and hydrogen peroxide [77]. For example, on an electrode based on a chitosan matrix and the mixture of nanographite and diamond nanoparticles, electrocatalytic detection of azathioprine was realized in a concentration range from 0.2 to 100 µM with a detection limit of 65 nM [72]. The same electrode was applied successfully to detect epinephrine (0.01–10 μM) and uric acid (0.01–60 μM) in the presence of ascorbic acid. The detection limit was 3 nM for both epinephrine and uric acid [73]. Voltammetric monitoring of 30 nM tryptophan and 6 nM 5-hydroxytryptophan was shown to be possible on diamond nanoparticles based film electrode [75]. By decorating diamond nanoparticles with silver nanoparticles, voltammetric determination of thioridazine was achieved in the concentration range of 0.08–100 µM with a detection limit of 0.01 µM [76]. Synergistic effect of two kinds of nanoparticles was proposed to demonstrate the satisfactory electrochemical activity [76]. Such a matrix was applied to fabricate a non-enzymatic hydrogen peroxide sensor [77]. On such a sensor, hydrogen peroxide was detected in the range of 0.1–34.0 µM with a detection limit of 0.01 μM and a sensitivity of 1.59 × 106 µA M−1 [77]. TiO2 nanoparticles coated diamond nanoparticles exhibited higher electrochemical activity than the pristine diamond nanoparticles, especially higher catalytic ability towards the oxidation of nitrite anions. A detection limit of 0.55 µM and a linear range of 0.05–1.0 mM for the detection of nitrite ions were achieved [78].

Diamond nanoparticles based electrochemical biosensors have been reported. Glucose oxide, cytochrome c, hemoglobin, horseradish peroxidase, alcohol dehydrogenase, and lactate oxidase have been immobilized on diamond nanoparticles for the detection of glucose [79–83], and alcohols [84], lactate [85], respectively. For example, alcohol dehydrogenase (ADH) has been adsorbed on oxidized diamond nanoparticles. The adsorption of the non-covalently immobilized ADH, estimated with Langmuir isotherms, was dependent on pH values of the solutions. A higher packing density was achieved at the isoelectric point of ADH. Its relative activity was retained up to 70 % under optimum pH conditions. An ethanol bioelectrochemical cell and an alcohol biosensor were then proposed [84]. On the diamond nanoparticles and polyaniline based electrode, direct electrochemistry of cytochrome c was achieved, leading to electrocatalytic detection of nitrite ions in a concentration range from 0.5 μM to 3 mM with a detection limit of 0.16 μM [81]. Diamond nanoparticles and porous poly(aniline)–poly(2-acrylamido 2-methyl propane sulfonic acid) network based sponges were prepared to entrap horseradish peroxidase [83]. On such a matrix, electrocatalytic reduction of hydrogen peroxide was realized in a concentration range of 1–45 mM with a rapid response time of 5 s, a high sensitivity of 129.6 μA M−1 and a low detection limit of 59 μM [83]. An electrochemical biosensor based on diamond nanoparticles was proposed as well for lactate determination. The workable concentration range was from 50 µM to 0.7 mM, the sensitivity was 4.0 µA mM−1, and the detection limit was 15 µM [85]. Antibody immobilization was reported on diamond nanoparticles seeded inter-digitated electrodes (IDEs). Such an impedance biosensor improved the overall detection sensitivity, namely the resistance to charge transfer. The sensor performance was better than those based on gold or ITO electrodes. When sensing bacteria from 106 cfu mL−1 E. coli O157:H7, the resistance to charge transfer at the IDEs decreased by 38.8 %, which is nearly 1.5 times better than that reported previously using redox probes. Further in the case of 108 cfu mL−1 E. coli O157:H7, the charge transfer resistance changed by 46 % [86].

3.3.1.2 Electrocatalysts

Different kinds of diamond nanoparticles (e.g., surface graphitized diamond nanoparticles with a diamond core covered by a graphitic carbon shell, bucky diamond nanoparticles with a nanoscale diamond core surrounded by a fullerene shell, and graphene coated diamond nanoparticles) have been used as the support to load catalysts (e.g., Pt [87–93], Ni [94, 95], Ti [96], Pd [97], Pt/Ni [98], Pt/Ru [99–102], Sn/Pb [103], Pt/Eu [91, 99], and metal oxides [104–106]). The mostly investigated electrocatalytic reactions include electrocatalytic oxidation of methanol/formic acid/CO as well as oxygen reduction reaction. To load these catalysts on diamond nanoparticles, numerous approaches have been developed, including electrodeposition [88, 92], chemical reduction [93], and microwave-assisted polyol synthesis [91, 99]. For example, electrodeposition of Pt nanoparticles on un-doped diamond nanoparticles (5–100 nm in diameter) were conducted in 1.1 mM chloroplatinic acid solution. The electrodeposited Pt nanoparticles were well-dispersed on the facet surfaces of diamond nanoparticles [88, 107]. Pt and Ru nanoparticles were chemically deposited on un-doped and boron-doped diamond nanoparticles through the use of NaBH4 as reducing agent and sodium dodecyl benzene sulfonate as a surfactant [93]. Microwave-assisted reduction method [91, 98] has been used for the preparation of catalysts of Pt [91], Pt/Ni [98], and Pt/Ru [102] on the surface of diamond nanoparticles. A microwave heating polyol method was used to prepare the Pt/Ru electrocatalyst on the surface of un-doped diamond nanoparticles. It was found that Ru was partly dissolved in the face-centered cubic Pt lattice. The Pt/Ru nanoparticles were small and uniform with the size of 2–4 nm, and highly dispersed on the surface of diamond nanoparticles [102]. A simple ultrasonic treatment in the presence of diamond powders prior to electrodeposition improved spatial distribution and a higher Pt dispersion over the electrode [99–102]. A two-step method was reported to modify diamond nanoparticles with Pt and TiO2 nanoparticles, namely first by a microwave hydrolysis step, and then electrodeposition of Pt nanoparticles [104–106].

For the first time Fermin et al. [97] utilized Pd and HPHT diamond nanoparticles (500 nm in diameter) based electrocatalysts for the electrochemical stripping of CO and oxidation of formic acid in the acid solutions. Later the same group studied the electrocatalytic reactivity of Pt nanoparticles coated HPHT diamond particles towards the oxidation of adsorbed CO, methanol, and formic acid with differential electrochemical mass spectrometry. Diamond nanoparticles with different surface terminations were used for these electrocatalytic oxidation reactions, leading to different oxidation mechanisms [89]. Towards the ability of the electrocatalytic oxidation of methanol, diamond nanoparticles with smaller diameters (e.g., 5 nm) exhibited better electrocatalytic activity than bigger ones (e.g., 100 nm) after the surface of diamond nanoparticles were coated with electrodeposited Pt nanoparticles [88, 107]. The application of these Pt modified diamond electrodes in the electrochemical oxidation of hydrogen peroxide was demonstrated [92]. The ink-paste method was used to prepare the membrane electrode assembled with Pt and Pt/Ru modified un-doped and boron-doped diamond nanoparticle catalytic systems [100]. Their performances were examined in a direct methanol fuel cell system [100]. The Pt/Ru catalyst exhibited higher activity and stability for methanol electrooxidation reaction than individual Pt catalyst [102]. The investigation of electrocatalytic reduction of oxygen on metal catalysts coated diamond nanoparticles has been conducted further using cyclic voltammetry, chronoamperometry and linear sweep voltammetry [98]. The Pt/Ni catalysts exhibited better electrocatalytic activities than the Pt catalysts either for methanol oxidation reaction or for oxygen reduction reaction [98]. In the acid medium the Pt/TiO2 catalyst system possessed higher electrocatalytic activity for methanol oxidation reaction compared with the individual Pt catalyst [96]. TiN coated diamond nanoparticles showed higher catalytic activity and better stability in methanol oxidation and oxygen reduction reactions compared with the individual catalysts on carbon and on diamond nanoparticles [106].

3.3.1.3 Energy Storage

Diamond nanoparticles have been employed as the electrode material for energy storage, such as for electrochemical capacitors [108–113, 136], lithium batteries [114, 115], and dye-sensitive solar cells [116]. For supercapacitors, diamond nanoparticles are always thermally annealed at the temperatures above 1000 ℃. Due to the generation of carbon onions, the energy is possible to be stored under a high current density and a high capacitance.

For example, Gogotsi et al. investigated and compared in organic and aqueous electrolytes the performance of EDLCs based on carbon onions, diamond nanoparticles, carbon black and multi-walled carbon nanotubes [108]. Different methods were applied, including galvanostatic cycling, electrochemical impedance spectroscopy and cyclic voltammetry. To construct pseudocapacitors, the surface of diamond nanoparticles and carbon onions was coated with a layer of phosphomolybdate [111] or polyanilline [112, 113] (produced via electropolymerization in a cavity electrode or a chemical oxidation approach [113]). Carbon onions and phosphomolybdate based pseudocapacitance exhibited a 20 % increase in the capacitance (up to 600 mF cm−2 at 5 V s−1) [111]. Due to the porous network structure [113], the pseudocapacitance based on diamond nanoparticles and polyaniline (with the weight ratios of 3–28 %) increased to 640 F g−1 in 1.0 M H2SO4. This capacitance was 3–4 times higher than that of the activated carbons and more than 15 times higher than that of diamond nanoparticles and carbon onions. Moreover the charge-discharge characteristics were stable for 10,000 cycles [113].

Some experiments to use diamond nanoparticles for lithium batteries were reported as well [114, 115]. For example, a volumetric capacity of less than 23 mA h cm−3 has been shown, although it was much lower than 450–700 mA h cm−3 offered by state-of-the-art high-density cathodes used in commercial Li-ion batteries [115]. The composite system of polyaniline and diamond nanoparticles prepared via electrochemical polymerization techniques was applied toward the iodine/iodide redox couple for the construction of dye-sensitised solar cells [116].

3.3.1.4 Other Applications

Diamond nanoparticles have been applied as corrosion inhibition [117], and as the metal-free catalysts for oxidant- and steam-free dehydrogenation [118, 140]. For example, the nanocomposite of polyaniline and diamond nanoparticles showed an ohmic junction and wide potential values, independent of redox characteristics of both polyaniline and diamond nanoparticles. This is due to its chain conformation and electronic properties (achieved by the interaction of the free electron pairs of the nitrogen atoms in the polyaniline with a charged molecule on the surface of diamond nanoparticles), leading to excellent corrosion inhibitor characteristics [117].

3.3.2 Doped Diamond Particles

Boron-doped diamond powders based film electrodes were shown to be dimensionally stable at 1.4 V (vs. Ag∕AgCl) for 1 h in 0.5 M H2SO4 at 80 ℃. They are thus corrosion-resistant during anodic polarization. In contrast, glassy carbon powders polarized under identical conditions underwent significant microstructural degradation and corrosion [124]. Electrocatalytic oxidation of methanol was tested on the composite based on boron-doped diamond particles (500 nm and 5 μm in diameter) and metallic oxides [125]. By coating boron-doped diamond nanoparticles with Ni(OH)2, a non-enzymatic glucose sensor was constructed. The detection limit for glucose detection was 1.2 μM [126]. Recently, boron-doped diamond nanoparticles have been electrostatically self-assembled on carbon nanotubes. Such a 3D network showed a low electron transfer resistance but a large effective surface area, resulted in an improved electrochemical performance in glucose detection [128]. The construction of electrochemical capacitors using RuO2 coated diamond powders has been reported as well [127].

4 Diamond Nanoelectrode Arrays

Small-dimensional electrodes (e.g., nanoelectrodes) offer various benefits over planar macroscopic electrodes [10, 142, 143], such as reduced Ohmic resistance, enhanced mass transport, decreased charging currents, decreased deleterious effects of solution resistance, and high possibility for fast voltammetric measurements. However, single nanoelectrode only generates a small current that is relatively difficult to detect with conventional electrochemical setups. This has been circumvented by fabricating nanoelectrode arrays or ensembles that operate in parallel. They amplify the signal of individual nanoelectrodes but do not lose their beneficial characteristics. If diamond nanoelectrode arrays or ensembles are applied for sensor applications, their performances with respect to the sensitivity, detection limit, life time, and reproducibility, will be highly improved [10, 130, 142, 143]. This is because boron-doped diamond is one of the most appropriate and optimized material for the fabrication of these arrays and ensembles [6, 137]. Moreover, since a macroscopic diamond electrode shows a higher degree of inhomogeneity with respect to boron-doping level and termination effects due to its macroscopic dimensions, one would thus expect a homogenized behavior on a diamond nanoelectrode array (NEA) or nanoelectrode ensemble (NEE) [131, 132]. This is due to the small grains of diamond films as well as a more effective termination of these small electrochemical active areas.

4.1 Production Procedures

NEAs and NEEs were fabricated using E-beam lithography and nanosphere lithography, respectively [131]. The following is the fabrication steps of NEAs [131]. On a 200 nm thin boron-doped nanocrystalline diamond film, a 200 nm thick SiO2 is deposited. This oxide layer is structured using E-beam lithography with subsequent nickel deposition and SF6 etching of SiO2. In the next step, metal contacts are deposited using photolithography to allow electrical contact for electrochemical characterization. In the crucial step, a 140 nm thin insulating nanocrystalline diamond film is grown on the part of the boron-doped nanocrystalline diamond layer that is exposed to the CVD plasma and not protected by SiO2 islands. With the removal of SiO2 in hydrofluoric acid, the arrays of recessed boron-doped nanocrystalline diamond nanoelectrodes surrounded by insulating diamond (NEAs) are obtained. In the NEAs we fabricated the nanoelectrodes are distributed in a hexagonal order, having a well-defined radius of 250 nm and a distance of 10 μm next to other nanoelectrodes and an electrode density of 11 × 105 cm−2. One SEM image of such a NEA is shown in Fig. 9a.

Fig. 9
figure 9

a SEM image of a diamond NEA; Schematic plots of volumetric behavior of methyl viologen on b H- and c O-terminated diamond NEAs, respectively [131, 132]

Full size image

Nanosphere lithography was developed to fabricate NEEs [131]. Initially, a photolithography step is used to deposit metal contacts on diamond. Thereafter, the sample is immersed in a solution of SiO2 spheres. The next step involves the growth of insulating diamond around the above mentioned spheres. Insulating diamond selectively grows on the area exposed to the plasma. After the removal of SiO2 spheres in hydrofluoric acid, nanoelectrodes having a concave shape are fabricated. These nanoelectrodes thus have the same size as the diameter of SiO2 spheres. Since the concentration of the SiO2 solution is directly correlated to the density of spheres on the diamond surface as well as to the average distance of neighboring spheres, the density of those nanoelectrodes are controllable. The size of the nanoelectrodes can be well-defined by selecting market-available SO2 spheres as required. For example, to obtain sigmoidal voltammograms, we chose a SiO2 concentration of 9.55 × 108 cm−3, corresponding to a surface density of 9.7 × 105 cm−2 and an average distance of neighboring spheres of ~10 μm. The size of the nanoelectrodes in a NEE is about 175 nm. The density of nanoelectrodes is about 8.5 × 105 cm−2 [131].

4.2 Electrochemical Properties

Cyclic voltammetry of Fe(CN) 3−/4−6 , Ru(NH3) 2+/3+6 and IrCl 2−/3−6 were conducted on the NEA and NEE in 0.1 M KCl solution. The scan rates were varied from few mV s−1 up to 10 V s−1. At small scan rates (e.g., 20 mV s−1 for the NEA and 1 mV s−1 for the NEE), the voltammograms have mixed shapes, indicating partially overlapping diffusion hemispheres. Increasing the scan rates leads to typical steady-state sigmoidal voltammograms on both electrodes. The change is more distinct on the NEA than on the NEE [131]. Impedance was performed on the NEA and NEE in 0.1 M KCl at open circuit potentials. The redox couple of Fe(CN) 3−/4−6 (1.0 mM) was added. Their impedance spectra have similar characteristics, exhibiting a large semicircle in the high-frequency regime and at low frequencies a transition to linear diffusion with unity slope (particularly observable for the NEA) [131]. A semicircle at high frequency regime is due to a three-dimensional hemispherical diffusion on the diamond NEA and NEE [144–146]. The transition at low frequencies represents the regime of overlapping diffusion hemispheres. These behaviors are similar with those obtained from voltammetry [131].

Moreover, the voltammetric response of Ru(NH3) 2+/3+6 and IrCl 2−/3−6 on a diamond NEA show the dependence of surface termination on the charge of the analytes [131]. Please note that on planar macroscopic diamond electrodes both analytes show no dependence of electron transfer rate constants on the surface termination of diamond electrodes. On the H-terminated diamond NEAs, the voltammogram of the anion IrCl 2−/3−6 shows a fast electron transfer while at the O-terminated surface, the steady-state current as well as the slope of the transition from reduction to oxidation decreases, indicative of a slower electron transfer. This tendency is similar for another negatively charged redox couple of Fe(CN) 3−/4−6 . However, the opposite effect is observed for the positively charged redox molecules Ru(NH3) 2+/3+6 . That is, on an O-terminated diamond NEA, the electron transfer rate for Ru(NH3) 2+/3+6 is faster than that on a H-terminated surface. It is known that H-terminated diamond surface has a positive surface dipole layer (“positive” refers to the interface of diamond to the liquid) and the O-terminated surface results in a negative surface dipole layer. Such behaviors are therefore probably due to either an electrostatic or a site blocking effect [131].

4.3 Electrochemical Applications

Diamond NEAs have been applied for electrochemical sensing [130] and the investigation of surface-sensitive adsorption phenomena [132]. The adsorption of neutral methyl viologen (MV0) was used as a model system. Diffusion-controlled processes manifest themselves as sigmoidal-shaped voltammograms on O-terminated diamond NEAs, whereas adsorption-controlled processes result in peaks in the voltammogram for H-terminated diamond NEAs. The change in the shapes of these voltammograms is due to the drastic changes that occur in the diffusion profiles during the transition. It alters from hemispherical diffusion on the O-terminated surface to thin-layer electrochemistry upon the adsorption on the H-terminated surface. In this way the de-convolution of diffusion-controlled current from adsorption-controlled current was conducted. By analysing anodic stripping process at high scan rates, the deposition of amorphous MV0 was approved on H-terminated diamond NEAs. These results are schematically shown in Fig. 9b, c for H- and O-terminated diamond NEAs, respectively [132]. The types and the concentration of the buffer solutions were changed to alter the interaction of MV0 with H-terminated diamond NEAs [132]. Increasing urea concentrations leads to the same impact on the adsorption of MV0 as guanidine, which weakens hydrophobic interaction. This effect of ions on the interaction of MV0 and the hydrophobic diamond surface is correlated with the Hofmeister series [132]. Subsequently, the adsorption of MV0 on H-terminated diamond NEAs is controlled by hydrophobic interaction [132]. Therefore diamond NEA is ideal for the study of adsorption phenomena at the liquid-solid interface in voltammetry [132].

5 Summary and Outlook

Electrochemistry using diamond nanostructures, nanoparticles, nanoelectrodes, in other words, diamond nanoelectrochemistry, has been paid much attention in the fields of electrochemical sensors, energy, and electrocatalysts during the past years. In such electrochemical systems, nanostructured diamond (e.g., textures, wires, pores, nanoelectrodes, etc.) and diamond nanoparticles were used as the working electrode, instead of macro-sized diamond bulk electrode. From fundamental aspects of diamond nanoelectrochemistry, future activities should focus on the effect of surface termination of diamond at the nanoscale, diamond-to-graphite ratios, surface defects, and morphology (e.g., size, shape, etc.) effects of diamond nanostructures on their electrochemical properties in the absence and presence of redox probes. Coating these diamond nanostructures and nanoparticles with stable and electroactive modifiers is important for their applications for sensors, energy storage and conversion, and catalytic reactions. Doping diamond nanostructures and nanoparticles with dopants such as N and other atoms will widen their applications such as for electrocalaytic reactions (e.g., oxygen reduction/evolution reaction, CO2 reduction reaction, etc.). Combination of diamond nanostructures with diamond nanoparticles, or the formation of their hybrid nanocomposites, for example, diamond nanowires based diamond nanoelectrodes, diamond nanoparticles coated diamond nanoelectrodes, will take full advantages of diamond as well as nanoelectrochemistry, leading to more novel concepts and applications [147, 148]. For example, on such hybrid nanocomposites, the investigation of diamond electrochemistry and electroanalysis at the nanoscale (e.g., capacitive current at a single nanowire, single molecule detection, etc.), which is hard to be realized on other electrode materials, will be feasible.

In conclusion, progress and achievements on electrochemistry using diamond nanostructures, nanoparticles and nanoelectrodes are summarized. Through the input from material scientists, chemists, physics, and engineers, more and nicer results in the fields of electrochemical properties and applications of diamond nanostructures and nanoparticles will be obtained in coming years. By showing and comparing the results published in literature, we believe this chapter will help the readers to know more how electrochemistry of diamond nanostructures and nanoparticles started as well as where and how it goes in future.