3. Results and discussion
3.1. Microstructure of the hybrid diamond/graphite electrodes
Fig. 1(a) shows a typical SEM image of the diamond film
deposited at a microwave power of 10 kW with a methane concentration
of 8%. Rough surface is clearly observable, which provides
an enhanced surface effect in electrochemical detection. One can
see that the nanostructures with high density are distributed on
the surface. To understand the formation conditions of the nanostructures,
various process parameters were applied, as shown in
Fig. 1(b)–(d). Under these CVD process parameters, the size of the
diamond grains decreases with decreasing of microwave power
and methane concentration. Although the shape and size of the diamond
grains under these parameters are different, these films all
exhibit granular grains in contrast to the nanostructure of diamond
films at higher microwave power and methane level. Therefore,
the one dimensional structure only emerges under high microwave
power and methane concentration, while either lower microwave
power or methane concentration leads to the disappearance of
this diamond nanostructure. From the above results, we believe
that both higher CH4 concentration and microwave power are the
necessary conditions to prepare one dimensional diamond nanostructures
in this work.
To obtain a perspective view in the distribution of phases, TEM
characterization was employed to characterize the microstructure
of the as-deposited films. TEM image shown in Fig. 2(a) exhibits
the morphology of diamond/graphite films with rod-like structures
sporadically distributed in the surface of the films. Selected
area electron diffraction (SAED) pattern in the inset of Fig. 2(a)
consists of diffraction spots and rings. The lattice spacings are
estimated to be 0.36 nm, 0.21 nm, 0.13 nm and 0.11 nm, corresponding
to the (0 0 0 2) plane of graphite phase, (1 1 1), (2 2 0) and
(3 1 1) planes of diamond, respectively. Hence, the TEM investigation
identifies the co-existence of diamond and crystalline graphite
phases. From the high resolution TEM (HRTEM) image (Fig. 2(b)),
it is clear that the rod-like diamond nanostructures corresponding
to the diamond nanostructure is surrounded by a graphite shell.
In the Raman spectroscopy in Fig. 2(c), the characteristic peak of
diamond is around 1333 cm−1 while the peaks locating at about
1400 cm−1 and 1580 cm−1 respectively correspond to D and G band
of sp2 structure, which suggests the co-existence of diamond and
graphite phases. This can also be confirmed by X-ray diffraction
(XRD) pattern as shown in Fig. 2(d), in which graphite (0 0 2) and
diamond (1 1 1) and (2 2 0) peaks are clearly observable. The XRD
result is in good consistence with the HRTEM and SAED results. It is
believed that the conductive graphitic shells around the diamond
nanostructure make the whole material conductive, endowing it
with the ability in the applications of electrical or electrochemical
nanodevices. Currently, the conductive diamond/graphite nanostructure
has already been synthesized using different gas