Researchers　commonly　enhance　the　sensitivity　of　gas　sensing　nanomaterials　by　increasing　the　effective　active　area.　It　would　be　more　straightforward　to　instead　optimize　the　grain　boundaries.　In　this　work,　Zn4CO3(OH)(6).H2O　was　synthesized　as　a　precursor　to　ZnO　via　a　facile　hydrothermal　process　and　annealing,　respectively.　ZnO　nanonets　of　hierarchical　crystalline　nodes　with　abundant　grain　boundaries　was　prepared.　The　nanonets　exhibited　an　ultra-sensitive　response　value　of　398,　and　a　fast　response　time　(4　s　of　exposure　to　100　ppm　ethanol),　compared　with　traditional　nanosheets.　The　sensing　enhancement　as　per　the　grain　boundary　was　modeled　and　experiments　confirmed　the　model.　Microstructural　and　microscopic　analyses　indicated　that　the　sensitivity　of　the　nanonets　mainly　is　attributable　to　the　abundant　grain　boundaries　with　lots　of　hierarchical　crystalline　nodes.　The　high-sensitive　response　and　fast　response　time　toward　ethanol　by　the　ZnO　nanonets　is　considered　to　derive　from　the　enhancement　effect　of　grain　boundaries　in　hierarchical　crystalline　nodes,　which　creates　the　additional　depletion　layers　from　abundant　oxygen　vacancies　led　to　a　substantial　change　in　the　surface　potential,　referring　to　the　difference　between　in-air　versus　in-ethanol.　The　proposed　high-performance　sensor　is　inexpensive　to　fabricate,　straightforward　to　synthesize,　and　advantageous　for　practical　use.　(C)　2021　Elsevier　B.V.　All　rights　reserved.