Preparation　of　holey,　single-crystal,　2D　nanomaterials　containing　in-plane　nanosized　pores　is　very　appealing　for　the　environment　and　energy-related　applications.　Herein,　an　in　situ　topological　transformation　is　showcased　of　2D　layered　double　hydroxides　(LDHs)　allows　scalable　synthesis　of　holey,　single-crystal　2D　transition　metal　oxides　(TMOs)　nanomesh　of　ultrathin　thickness.　As-synthesized　2D　Co/NiO-2　nanomesh　delivers　superior　photocatalytic　CO2-syngas　conversion　efficiency　(i.e.,　V-CO　of　32460　mu　mol　h(-1)　g(-1)　CO　and　VH2${V_{{{\rm{H}}_2}}}$　of　17840　mu　mol　h(-1)　g(-1)　H-2),　with　V-CO　about　7.08　and　2.53　times　that　of　NiO　and　2D　Co/NiO-1　nanomesh　containing　larger　pore　size,　respectively.　As　revealed　in　high-angle　annular　dark-field　scanning　transmission　electron　microscopy　(HAADF-STEM),　the　high　performance　of　Co/NiO-2　nanomesh　primarily　originates　from　the　edge　sites　of　nanopores,　which　carry　more　defect　structures　(e.g.,　atomic　steps　or　vacancies)　than　basal　plane　for　CO2　adsorption,　and　from　its　single-crystal　structure　adept　at　charge　transport.　Theoretical　calculation　shows　the　topological　transformation　from　2D　hydroxide　to　holey　2D　oxide　can　be　achieved,　probably　since　the　trace　Co　dopant　induces　a　lattice　distortion　and　thus　a　sharp　decrease　of　the　dehydration　energy　of　hydroxide　precursor.　The　findings　can　advance　the　design　of　intriguing　holey　2D　materials　with　well-defined　geometric　and　electronic　properties.