Synergistic effect of N-decorated and Mn(2+) doped ZnO nanofibers with enhanced photocatalytic activity

Abstract

Here we report a high efficiency photocatalyst, i.e., Mn(2+)-doped and N-decorated ZnO nanofibers (NFs) enriched with vacancy defects, fabricated via electrospinning and a subsequent controlled annealing process. This nanocatalyst exhibits excellent visible-light photocatalytic activity and an apparent quantum efficiency up to 12.77%, which is 50 times higher than that of pure ZnO. It also demonstrates good stability and durability in repeated photocatalytic degradation experiments. A comprehensive structural analysis shows that high density of oxygen vacancies and nitrogen are introduced into the nanofibers surface. Hence, the significant enhanced visible photocatalytic properties for Mn-ZnO NFs are due to the synergetic effects of both Mn(2+) doping and N decorated. Further investigations exhibit that the Mn(2+)-doping facilitates the formation of N-decorated and surface defects when annealing in N2 atmosphere. N doping induce the huge band gap decrease and thus significantly enhance the absorption of ZnO nanofibers in the range of visible-light. Overall, this paper provides a new approach to fabricate visible-light nanocatalysts using both doping and annealing under anoxic ambient.

Figure 1

SEM images of (a) as-spun 15% Mn²⁺-doped ZnO nanofibers, (b) after annealing at 550 °C for 1 h in N₂, (c) enlarged view of the annealed nanofibers. (d) TEM image of 15% Mn²⁺-doped ZnO nanofibers. The inset shows the SAED pattern. (e) HRTEM image taken from the nanofiber indicated by the ellipse in (d). (f) EDS spectra from the single nanofiber.

Figure 2

(a) XRD patterns of 5%, 10% and 15% Mn²⁺-doped ZnO nanofibers. (b) The step-scanning XRD of all the nanofibers.

Figure 3

(a) UV−vis diffuse reflectance spectra (DRS) of undoped and Mn²⁺-doped ZnO fibers. The inset shows the corresponding plots of (αhν)2 versus photon energy (hν). (b) UV-vis absorption spectra of RhB at different time in the presence of 15% Mn²⁺-doped ZnO nanofibers under visible light. The inset illustrates the SEM image of the specimen reclaimed after photocatalytic measurement. (c) Photodegradation of RhB by ZnO nanofibers doped with different Mn²⁺ concentration. (d) Cycling tests of photodegradation of the specimen. (e) Degradation rate constants and apparent quantum efficiencies of ZnO nanofibers with different doping concentrations.

Figure 4

(a) XPS spectra of undoped and Mn²⁺-doped ZnO NFs. (b–e) Mn 2p, N 1s, Zn 2p, and O 1s scan, respectively, of the four samples. The black solid lines are the experimental data, whereas the red dotted lines and thin lines are the fitting results.

Figure 5

(a) 2 × 2 × 3 ZnO supercell with a wurtzite structure and view of the Mn²⁺-doped ZnO (1 0 0) surface. (b) Calculated DOS of ZnO and 4.2% Mn²⁺-doped ZnO in the form of a bulk crystal. (c,d) Electron densities of states of bulk ZnO and 4.2% Mn²⁺-doped ZnO. The energy of the valence band maximum of the bulk phase is taken to be zero.

Figure 6

(a) Visible light photocatalysis process in NFs. (b,c) Schematic of trapping and photocatalytic mechanism in a single NF under dark and visible light.