Enhanced charge separation through modulation of defect-state in wide band-gap semiconductor for potential photocatalysis application: Ultrafast spectroscopy and computational studies

Structural defects of wide band gap semiconductors play important role in their functionality. Defect mediated recombination of photoinduced electron-hole pair in the semiconductors for their photo catalytic activities, is detrimental. In the case of ZnO nanostructures, radiative recombination upon band gap photo-excitation (337 eV) originated from different surface defects (mainly Oxygen vacancies at 2.50 eV (V+) and 2.25 eV (V++) with respect to valance and conduction bands respectively) of crystal lattice, acquires immense interest for both fundamental scientific point of view and for the betterment of their manifold applications. The present work indicates that for transition metal semiconducting oxides, use of anionic attachment like Cl- as surface defect healer proves to be more useful for photocatalytic application than bulk doping using cationic dopant like Mn. ZnO NPs of different sizes (5 nm and 30 nm) are synthesized via precipitation method and allowed to interact with chloride ions in aqueous solution. A variety of electron microscopy and picosecond resolved spectroscopic techniques have been employed to study the role of chloride ions for the enhanced photoinduced charge separation in the aqueous environments. Our first principles density-functional calculations for ZnO nanoclusters with surface oxygen vacancy indicate introduction of trap states within the band gap of the nanoclusters. These states effectively confine the photoinduced electrons and thus essentially reduce the photocatalytic yield with respect to pristine ZnO. However, upon Cl- attachment to the defect states, the energy of the trap states were found to be healed, recovering the efficacy of reactive oxygen species (ROS) generation in the aqueous solution. We have also impregnated Mn2+ ions to the ZnO lattice using precipitation method in solution phase. The DFT calculation on the Mn2+ ion impregnated ZnO lattice reveals more defect states compare to that of pristine lattice. The rate of electron recombination is found to be much faster through non-radiative pathway (ground state recovery), leading to a decrease in photocatalytic activity in the case of Mn-doped ZnO. Attachment of Cl- to the Mn-doped ZnO partially recovers the ROS generation, which is consistent with healing of deep trap states. The present work is anticipated to provide a new insight into the surface defect modulation of ZnO in potential photocatalysis application. (C) 2016 Elsevier B.V. All rights reserved.