The reason may be due to the low quality of diamond phase when a high nitrogen
concertation (70% N2 in reacting gas) is involed during the deposition
process. Moreover, the potential window range in our work
(−1.2 V–1.9 V) is more positive compared to that in Shalini et al.’s
work (−1.7 V–1.3 V) [18], which shows the possibility of the current
electrode in the determination of Ag (+0.2 V), Hg (+0.4 V) and Au
(+0.8 V) ions in the ASV process. The hybrid diamond/graphite film
with 9% methane shows a narrower potential window range (from
−0.8 V to 1.7 V) and a higher background current (∼60 A/cm2),
which is attributed to the decrease of diamond phase in the film.
Moreover, the high intensity of the cathodic peak presented at
around
−0.5 V also reflects a high amount of non-diamond phases
[35].
In order to give a further insight into the effect of surface
structures of hybrid diamond/graphite films deposited at different
methane levels on the electrochemical window, SEM images and
Raman spectroscopies of the films grown at methane concentrations
of 7% and 9% are shown in Fig. 4. In Fig. 4(a), the 7% hybrid
film shows a morphology of the transition state, which is composed
of both granular and nanostructured diamond. The diamond
nanostructure in this sample is not so obvious and only a small
amount of short diamond nanostructure exists in some areas of the
hybrid film. From its Raman spectroscopy (Fig. 4(c)) we can also
find that the characteristic diamond peak locating at 1333 cm−1 is
much sharper than that for 8% sample (Fig. 2(c)), implying a relative
higher amount of diamond phase in the film. Theoretically, the
amount of non-diamond phase increases with the methane concentration
in feed gas. The SEM image (Fig. 4(b)) of the sample
deposited with 9% methane shows more and longer diamond nanostructure
in the hybrid film. According to the Raman spectroscopy
in Fig. 4(d), its graphite amount is even larger than that in 8%
sample, which results in a narrower potential window and higher
background current of the film. The above results strongly indicate
that less sp2 content in the film leads to slow kinetics while more
sp2 content in the film narrows the potential window and enhances
the background current. Therefore, there should be an optimized
sp2 carbon level which allows the hybrid film to have fast electrochemical
kinetics as well as wider potential window. Based on the
above results, the hybrid diamond/graphite film grown under 8%
methane concentration fulfills the above requirements. It is therefore
more suitable as an electrode for ASV trace heavy metal ions
detection. In the following, more detailed electrochemical characterization
of this electrode will be carried out.