Hierarchically Porous Cu-, Co-, and Mn-Doped Platelet-Like ZnO Nanostructures and Their Photocatalytic Performance for Indoor Air Quality Control

Abstract

Several parameters, including specific surface area, morphology, crystal size, and dopant concentration, play a significant role in improving the photocatalytic performance of ZnO. However, it is still unclear which of these parameters play a significant role in enhancing the photocatalytic activity. Herein, undoped and Mn-, Co-, and Cu-doped platelet-like zinc oxide (ZnO) nanostructures were synthesized via a facile microwave synthetic route, and their ultraviolet (UV) and visible-light-induced photocatalytic activities, by monitoring the gaseous acetaldehyde (CH₃CHO) degradation, were systematically investigated. Both the pure and doped ZnO nanostructures were found to be UV-active, as the CH₃CHO oxidation photocatalysts with the Cu-doped ZnO one being the most UV-efficient photocatalyst. However, upon visible light exposure, all ZnO-nanostructured samples displayed no photocatalytic activity except the Co-doped ZnO, which showed a measurable photocatalytic activity. The latter suggests that Co-doped ZnO nanostructures are potent candidates for several indoor photocatalytic applications. Various complementary techniques were utilized to improve the understanding of the influence of Mn-/Co-/Cu-doping on the photocatalytic performance of the ZnO nanostructures. Results showed that the synergetic effects of variation in morphology, surface defects, that is, V_O, high specific surface areas, and porosity played a significant role in modulating the photocatalytic activity of ZnO nanostructures.

Figure 1

XRD patterns of undoped and Mn-, Co-, and Cu-doped ZnO nanostructures.

Figure 2

SEM images of undoped ZnO and Mn- Co-, and Cu-doped ZnO nanostructures.

Figure 3

N₂ adsorption–desorption isotherms of undoped and Mn-, Co-, and Cu-doped ZnO nanostructures.

Figure 4

[F(R_∞)hν]² vs (hν) curves of undoped and Mn-, Co-, and Cu-doped ZnO nanostructures. The inset shows the reflectance spectra of both undoped and doped ZnO nanostructures.

Figure 5

Comparison of the PL spectra of undoped and Mn-, Co-, Cu-doped ZnO nanostructures using 350 nm excitation wavelengths light.

Figure 6

High-resolution XPS spectra for O 1s Gaussian fitted peaks of (a) undoped, (b) Co-doped ZnO, (c) Mn-doped ZnO, and (d) Cu-doped ZnO nanostructures.

Figure 7

High-resolution X-ray photoelectron spectra of (a) Cu 2p, (b) Mn 2p, and (c) Co 2p.

Figure 8

UV-induced photocatalytic reactivity of undoped ZnO. (a) Adsorption–desorption competitive processes under dark conditions (black circles), UV light-induced (purple circles) temporal loss of CH₃CHO. (b) Typical variation of CO₂ (centered at ∼2350 cm^–1) and CO (centered at ∼2144 cm^–1) levels under dark conditions (black line), Vis irradiation (green line), and UV exposure (purple line) of pure ZnO sample in CH₃CHO balanced with synthetic air at 296 K and 700 Torr.

Figure 9

UV-induced photocatalytic reactivity of Mn–ZnO sample toward CH₃CHO. (a) Upper panel: Dark (black circles) and UV (purple circles)-induced CH₃CHO temporal loss. (b) Lower panel: CO₂ levels under dark conditions (black circles) and UV irradiation (purple circles).

Figure 10

UV-induced photocatalytic reactivity of Cu–ZnO sample toward CH₃CHO. Upper panel: (a) Dark (black circles) and UV (purple circles)-induced CH₃CHO temporal loss. Lower panel: (b) CO formation under sample UV irradiation (purple circles).

Figure 11

UV-induced photocatalytic reactivity of Co–ZnO sample toward CH₃CHO. (a) Dark (black circles) and Vis (green circles) and UV (purple circles)-induced CH₃CHO temporal loss. (b) Vis-induced CO product yield measurement (green circles) employing Beer–Lambert law for determining CH₃CHO and CO concentrations. Linear least-squares analysis resulted in an yield of 0.22 ± 0.05, with the quoted uncertainty to be the 2σ of the fit.

Figure 12

Schematic representation of the synthetic procedure employed for the preparation of ZnO nanostructures.