Synthesis of ultrathin Bi2Se3 nanosheets/graphene nanocomposite with defects/vacancies-dependent transient photocurrent performance

The electronic structure of topological insulator nanosheets largely depends on their layer thickness; thus fine-tuning the layer thickness is a reasonable way to manipulate their specific optoelectronic properties. However, it is elusive how defects/vacancies on the surface of topological insulators can affect their optoelectronic properties. Herein, we report that ultrathin bismuth selenide (Bi2Se3) nanosheets with two, seven and nineteen layers can grow on reduced graphene oxide in a solvothermal process. The positron annihilation spectroscopy showed that two-layer Bi2Se3 ultrathin nanosheets possess hextuple vacancy associates (V-BiSeSeSeSeSe), seven-layer Bi2Se3 nanosheets contain octuple vacancy associates (V-BiSeSeSeSeSeSe) and isolated vavancy V-Bi whereas nineteen-layer Bi2Se3 nanosheets have predominant duple vacancy associates V-B(iSe). First-principle computations indicate that the adsorption of I-2 on duple, hextuple and octuple vacancy associates are energetically preferable, and the order of density of states (DOS) around the Fermi level is V-BiSeSeSeSeSeSe > V-BiSeSeSeSeSe > V-BiSe, implying that V-BiSeSeSeSeSeSe shows the highest migration capacity of electron. Using these nanosheets as counter electrodes, seven-layer Bi2Se3 ultrathin nanosheets exhibited excellent electrocatalytic activity in comparison with two-layer and 19-layer nanosheets, indicating that defects/vacancies on the surface of topological insulators could cause obvious change of optoelectronic properties.