Inherently-Forced Tensile Strain in Nanodiamond-Derived Onion-like Carbon: Consequences in Defect-Induced Electrochemical Activation

Abstract

We analyzed the nanodiamond-derived onion-like carbon (OLC) as function of synthesis temperature (1000~1400 °C), by high-resolution electron microscopy, electron energy loss spectroscopy, visible-Raman spectroscopy, ultraviolet photoemission spectroscopy, impedance spectroscopy, cyclic voltammetry and differential pulse voltammetry. The temperature dependences of the obtained properties (averaged particle size, tensile strain, defect density, density of states, electron transfer kinetics, and electrochemical oxidation current) unanimously coincided: they initially increased and saturated at 1200 °C. It was attributed to the inherent tensile strains arising from (1) the volume expansion associated with the layer-wise diamond-to-graphite transformation of the core, which caused forced dilation of the outer shells during their thermal synthesis; (2) the extreme curvature of the shells. The former origin was dominant over the latter at the outermost shell, of which the relevant evolution in defect density, DOS and electron transfer kinetics determined the electrochemical performances. In detection of dopamine (DA), uric acid (UA) and ascorbic acid (AA) using the OLC as electrode, their oxidation peak currents were enhanced by factors of 15~60 with annealing temperature. Their limit of detection and the linear range of detection, in the post-treatment-free condition, were as excellent as those of the nano-carbon electrodes post-treated by Pt-decoration, N-doping, plasma, or polymer.

Figure 1

HR-TEM images of OLCs synthesized by annealing the detonation nanodiamond at temperatures of (a) 1000 °C, (b) 1100 °C, (c) 1200 °C and (d) 1400 °C.

Figure 2. The average particle size vs annealing temperature profile of the OLCs.

10~20 HR-TEM pictures were taken for each OLC sample, from which 50–55 clearly isolated particles (See Fig. S4) were selected for the particle size measurement for each OLC sample. See Fig. S5 where the particle size distributions of each OLC samples were given.

Figure 3

The peak current vs annealing temperature profiles of the OLC electrodes corresponding to the (a) CV profiles shown in Fig. S8a–c and (b) DPV profiles shown in the Fig. S9.

Figure 4. Strain of OLC nanoparticles estimated from the shift in 2D peak frequency of Raman spectra, relative to that of the flat multilayer graphene (MLG); the effect of the number of layers on the 2D band peak frequency, for the flat MLG, was included in the calculation.

Figure 5

(a) EELS profiles of OLC samples (black: OLC-1000, red: OLC-1100, blue: OLC-1200, dark cyan: OLC-1400) and (b) The annealing-temperature dependence of sp² and sp³ contents corresponding to EELS profiles (a).

Figure 6. The annealing-temperature dependences of the I_D/I_G ratio, DOS, and Rct⁻¹ corresponding to Figs S14, S15 and S18.

Figure 7

DPV responses from OLC-1200 electrode for 0.1 M PBS containing (a) 1–1200 μM AA, (b) 0.1–700 μM DA, (c) 0.5–600 μM UA.