Synthesis of ZnO/Si Hierarchical Nanowire Arrays for Photocatalyst Application

Abstract

ZnO/Si nanowire arrays with hierarchical architecture were synthesized by solution method with ZnO seed layer grown by atomic layer deposition and magnetron sputtering, respectively. The photocatalytic activity of the as-grown tree-like arrays was evaluated by the degradation of methylene blue under ultraviolet light at ambient temperature. The comparison of morphology, crystal structure, optical properties, and photocatalysis efficiency of the two samples in different seeding processes was conducted. It was found that the ZnO/Si nanowire arrays presented a larger surface area with better crystalline and more uniform ZnO branches on the whole sidewall of Si backbones for the seed layer by atomic layer deposition, which gained a strong light absorption as high as 98% in the ultraviolet and visible range. The samples were proven to have a potential use in photocatalyst, but suffered from photodissolution and memory effect. The mechanism of the photocatalysis was analyzed, and the stability and recycling ability were also evaluated and enhanced.

Keywords: Hierarchical architecture; Nanowire arrays; Photocatalytic activity; Semiconductor.

Fig. 1

Top view (top row) and cross-sectional view (bottom row) SEM images of ZnO/Si nanowire arrays of sample ALD (a, c) and sample MS (b, d)

Fig. 2

EDX spectra of Si nanowire arrays with seed layer at different positions of sample ALD ( a–c, g) and sample MS (d–f, h)

Fig. 3

Concentration variation of elements of Si and Zn (a) and Zn/Si ratio (b) of seeded sample ALD and sample MS at different positions in Fig. 2

Fig. 4

XRD patterns of sample ALD and sample MS. The ALD curve was shifted upward in contrast to MS curve for comparison

Fig. 5

PL spectra of sample ALD and sample MS. The curves were both median smoothed (7 points) and Savitsky–Golay smoothed (3rd order, 7 points) automatically in the same measuring conditions

Fig. 6

Diffuse reflection spectra of sample ALD and sample MS

Fig. 7

Absorption spectra of degraded MB aqueous solutions as catalyzed by sample ALD (a) and sample MS (b) under irradiation of UV light, in which 0, 1st, 2nd, and 3rd hours represent spectrum of MB solution before photodegradation, after photodegradation in the 1st, 2nd, and 3rd hours, respectively

Fig. 8

Absorption spectra of degraded MB aqueous solutions as catalyzed by silicon nanowire arrays under irradiation of UV light, in which 0, 1st, 2nd, and 3rd hours represent spectrum of MB solution before photodegradation, after photodegradation in the 1st, 2nd, and 3rd hours, respectively

Fig. 9

Schematic energy band diagram and proposed photocatalytic mechanisms of the ZnO/Si nanowire arrays under UV light irradiation

Fig. 10

Cross-sectional view SEM images of sample ALD (a, b) and sample MS (c, d) before and after photocatalytic performance

Fig. 11

SEM images of sample ALD with and without the coating layer. a Pristine trees. b Trees coated by TiO₂. c Trees coated by Ag. d Trees coated by Ag/TiO₂. e Trees coated by TiO₂/Ag. f Amplified image of trees coated by Ag

Fig. 12

Diffuse reflection spectra of sample ALD with the coating layer. The spectra of trees in other coating layers were shifted upward in contrast to the spectrum of trees coated by TiO₂ for comparison

Fig. 13

Absorption spectra of degraded MB aqueous solutions as catalyzed by sample ALD in Ag/TiO₂ coating layer under irradiation of UV light, in which 0, 1st, 2nd, and 3rd hours represent spectrum of MB solution before photodegradation, after photodegradation in the 1st, 2nd, and 3rd hours, respectively. The inset shows the enlarged image of the absorption peak