We chose g-C3N4 as the host materia

We chose g-C3N4 as the host material for the synthesis of
the electrocatalyst mainly for the following reasons: (1) g-C3N4
is composed of inexpensive, earth-abundant elements, (2) gC3N4 can easily be prepared from commercially available and
inexpensive starting materials, (3) g-C3N4 is chemically quite
stable due to the strong covalent bonds involved in it [30–32],
and (4) g-C3N4 contains nanosize cavities composed of three
heptazine units (Fig. 1E), which can function as macrocyclic ligands for inclusion of various catalytically active metallic ions
or nanoparticles [33–35]. However, these appealing structural
features of g-C3N4 need to be structurally/compositionally “upgraded” with electrocatalytically active groups to fully take
advantage of them and utilize the material for electrocatalysis of
HER.
To this end, we have carried out the synthesis of Cu-C3N4 catalysts. The synthetic method we employed to make the catalysts
involved a one-step self-assembly procedure (see Section 2 for
details of the experimental procedures). This method was adopted
from previously reported synthesis of Fe3+(Zn2+)-doped g-C3N4
hybrid materials by Wang et al. [33]. It is worth emphasizing here
though that despite the similarities in structures between the CuC3N4 reported here and the Fe3+(Zn2+)-C3N4 reported in Ref. [33], as
well as the similarities of the synthetic methods employed in both
cases, it was only the Cu-doped g-C3N4 that we made showed electrocatalytic activity toward H2 evolution reaction. In other words,
our attempted tests of electrocatalysis of HER using the Fe3+(Zn2+)-
C3N4 materials that we made as reported by Wang et al. [33] as
well as many other types of metal ion containing C3N4 were all
unsuccessful.
In a typical synthesis, copper(II) salt (e.g., CuCl2) was used as
the source of copper, and dicyandiamide was used as an organic
monomer for making the g-C3N4. When the mixture of copper(II)
salt and dicyandiamide was thermally treated at elevated temperature (500 ◦C) under N2 protection, the dicyandiamide became
g-C3N4 while, at the same time, the copper ions were directly
self-assembled in situ within the dicyandiamide-derived g-C3N4,
forming the desired Cu-C3N4 materials. The relative amount of
copper in the Cu-C3N4 material, which can be determined by thermogravimetric (TG) analysis (see below), was varied (or controlled)
by changing the molar ratio of copper(II) salt and dicyandiamide. In
this work, two Cu-C3N4 samples with different amount of copper,
denoted hereafter as 0.12Cu-C3N4 and 0.31Cu-C3N4, because the
molar ratios of Cu/g-C3N4 in them were found to be 0.12:1.00 and
0.31:1.00, respectively, (see below) were synthesized. Pure g-C3N4
was also synthesized to serve as a reference material to compare
the structures, properties, and electrocatalytic activities with those
of Cu-C3N4 materials. The BET surface areas of g-C3N4 and Cu-C3N4
materials are found to be nearly similar (e.g., 10 cm2/g and 6 cm2/g
for g-C3N4 and 0.12Cu-C3N4, respectively).