ZnO:CuO Composites Obtained by Rapid Joule Heating for Photocatalysis

Abstract

Semiconductor oxides belonging to various families are ideal candidates for application in photocatalytic processes. One of the challenges facing photocatalytic processes today is improving their efficiency under sunlight irradiation. In this study, the growth and characterization of semiconductor oxide nanostructures and composites based on the ZnO and CuO families are proposed. The selected growth method is the resistive heating of Zn and Cu wires to produce the corresponding oxides, combined with galvanic corrosion of Zn. An exhaustive characterization of the materials obtained has been carried out using techniques based on scanning electron microscopy and optical spectroscopies. The method we have followed and the conditions used in this study present promising results, not only from a degradation efficiency point of view but also because it is a cheap, easy, and fast growth method. These characteristics are essential in order to scale the process beyond the laboratory.

Keywords: Joule heating; copper oxide; galvanic corrosion; photocatalysis; zinc oxide.

Figure 1

(a) A schematic illustration of the photocatalysis process in a TMO. (b) A schematic comparison between the bandgaps of some relevant TMOs and some common redox pairs.

Figure 2

Experimental diagram of resistive heating system.

Figure 3

SWZN samples. (a) A SEM image of the ZnO nanowires obtained on the surface of the Zn wire. (b) The EDS spectrum recorded on the surface. (c) The BSE image and EDS map of the distribution of Zn (red) and oxygen (cyan) through the cross-section of the Zn wire. (d) The EDS signal profile along a line crossing the oxide layer. (e) CL (up) and PL (down) spectra. (f) The resonant Raman spectrum (λexc = 325 nm) obtained on the surface covered with nanowires.

Figure 4

SWZN samples. (a) A SEM image of the CuO nanowires and microcrystals obtained on the surface of the Cu wire. (b) The Raman spectrum (λexc = 633 nm) obtained on the surface covered with nanowires. (c) The EDS spectrum recorded on the surface. In the inset, EDS map of the distribution of Cu (red) and oxygen (cyan) through the cross-section of the Cu wire is presented.

Figure 5

IWZN samples. (a) A SEM image of the surface of the Zn wire immersed in CuO powder after Joule heating. (b) EDS measurements on the IWZN samples. The inset shows the compositional map, where the Zn signal is presented in red and the Cu signal in blue. (c) Raman spectra (λexc = 325 nm) of the surface of the IWZN wires.

Figure 6

IWCU samples. (a) A SEM image of the surface of the Cu wire immersed in ZnO powder after Joule heating. (b) EDS measurements on the IWCU samples. The inset shows the compositional map, where the Zn signal is presented in red and the Cu signal in blue. (c) The Raman spectrum (λexc = 633 nm) of the surface of the IWCU wires.

Figure 7

(a) A SEM image of the braided nanowires: Cu wire (right) and Zn wire (left). (b) X-ray maps (center) and spectra (Cu wire (right) and Zn wire (left)).

Figure 8

Photocatalytic performance of different sets of samples.

Figure 9

(a) Rhodamine UV-Vis absorption spectra variation over time with BWs as photocatalysts and UV+Vis illumination. (b) C/C₀ variation over time for BW samples. (c) Pseudo-first-order kinetics for Rhodamine dye in the presence of BWs as photocatalysts. Black squares and red circles indicated the two linear behaviors observed.

Figure 10

SEM images of the surface of the BW (Cu wire up and Zn wire down) after (a) 3 h, (b) 5 days, and (c) 1 month in the solution. (d) SEM images of the surface of the BW (Cu wire up and Zn wire down) once the sample has been left in air.

Figure 11

SEM images of the surface of the BW after (a) 3 h and (b) 1 month in the solution. EDS maps for the BW after (c) 3 h, (d) 5 days, and (e) 1 month in the solution. In the EDS maps, the Zn signal is presented in red and the Cu signal in blue.

Figure 12

Raman spectra (λexc = 325 nm) recorded on the surface of the BW samples after (a) 3 h and (b,c) 1 month in the solution. (d) Raman spectra (λexc = 325 nm) when the sample is left in air for several days after galvanic corrosion.