  Control of water ice formation on surfaces is of key technological and
economic importance, but the fundamental understanding of ice nucleation and
growth mechanisms and the design of surfaces for controlling water freezing
behaviour remain incomplete. Graphene is a two-dimensional (2D) material that
has been extensively studied for its peculiar wetting properties with liquid
water incl. a heavily debated wetting transparency. Furthermore, graphene is
the parent structure of soot particles that are heavily implicated as nuclei in
atmospheric ice formation and consequently graphene is often used as a model
surface for computational ice nucleation studies. Despite this, to date
experimental reports on ice formation on scalable graphene films remain
missing. Towards filling this gap, we here report on the water freezing
behaviour on scalably grown chemical vapour deposited (CVD) graphene films on
application-relevant polycrystalline copper (Cu). We find that as-grown CVD
graphene on Cu can be (as we term it) freezing transparent i.e. the graphene
presence does not change the freezing temperature curves of liquid water to
solid ice on Cu in our measurements. Such freezing transparency has to date not
been considered. We also show that chemical functionalization of the graphene
films can result in controllable changes to the freezing behaviour to
lower/higher temperatures and that also the observed freezing transparency can
be lifted via functionalization. Our work thereby introduces the concept of
freezing transparency of graphene on a metal support and also introduces
scalable CVD graphene/Cu as an ultimately thin platform towards control of ice
nucleation behaviour on a technologically highly relevant metal.
