In this transfer process, the ZnO QDs reduce the energy barrier between formaldehyde gas and graphene as shown in Fig. 7(d). For the ZnO materials, the reported energy level of the valence band (VB) and conduction band (CB) is 2.89 V and 0.31 V vs. NHE (normal hydrogen electrode), respectively. As shown in Fig. 7(d), the CB energy level of ZnO is located between the LUMO level of formaldehyde and the Fermi level of graphene, which facilitates the electron transfer from formaldehyde to graphene.