shows the XRD patterns of CdS photocatalysts. It was
noted that both hexagonal phase and cubic phase were
detected in every photocatalyst [27]. However, the ratio of the
two phases was influenced by NaOH. For these CdS, more
NaOH added, more hexagonal phase obtained and higher
crystallinity achieved, except for that prepared in 0.1 mol
NaOH condition which presented an opposite result. It was
considered that higher concentration of NaOH could produce
more Cd(OH)2 nucleus, and was favorable for the rapid formation
of CdS nucleus, as well as the growth of crystals.
However, excessive NaOH which could produce a very high
basicity, would dissolve CdS crystals, in other words, would
restrict their growth, resulting a poor crystallinity. It has been
reported that, CdS photocatalysts with hexagonal crystal
structures always exhibit much higher photocatalytic activities
than those with cubic structures [27]. Combining the results
offered by Fig. 1, the photocatalytic activities of samples
with hexagonal crystal structure (nNaOH ¼ 0.06 mol and
nNaOH ¼ 0.08 mol) presented higher photoactivities than those
with cubic structures (nNaOH ¼ 0 mol and nNaOH ¼ 0.02 mol),
which further confirmed the above conclusion.