Ultrathin　two-dimensional　(2D)　nanostructures　have　attracted　increasing　research　interest　for　energy　storage　and　conversion.　However,　tackling　the　key　problem　of　lattice　mismatch　inducing　the　instability　of　ulreathin　nanostructures　during　phase　transformations　is　still　a　critical　challenge.　Herein,　we　describe　a　facile　and　scalable　strategy　for　the　growth　of　ultrathin　nickel　phosphide　(Ni2P)　nanosheets　(NSs)　with　exposed　(001)　facets.　We　show　that　single-layer　functionalized　graphene　with　residual　oxygen-containing　groups　and　a　large　lateral　size　contributes　to　reducing　the　lattice　strain　during　phosphorization.　The　resulting　nanostructure　exhibits　remarkable　hydrogen　evolution　activity　and　good　stability　under　alkaline　conditions.　©　2018　Wiley-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim