Fabrication of Piezoelectric ZnO Nanowires on Laser Textured Copper Substrate to Enhance Catalytic Properties

Abstract

In this work, 3D periodic “grid-type” CuO/Cu₂O layers were fabricated on a copper sheet using laser processing techniques, and the laser processing parameters were optimized for favorable ZnO nanowire growth. It was found that ZnO nanowires could be successfully prepared to form a CuO-Cu₂O-ZnO heterojunction structure without an extra catalyst or seed layer coating, which could be attributed to the copper oxide active sites induced via laser texturing. ZnO nanowires on laser textured “grid-type” copper substrates demonstrated an effective piezocatalytic performance with different morphologies and the generation of abundant reactive oxygen species in the CuO-Cu₂O-ZnO catalytic system, providing a fundamental mechanism for the degradation of organic dye in water. This simple and low-cost method could provide a useful guide for the large-scale efficient and versatile synthesis of immobilized piezoelectric catalysts.

1. Introduction

Over the past few decades, as the adverse effects of rapid global industrialization have intensified, water pollution has become a serious global problem that poses a major challenge to the sustainable development of human civilization. Therefore, research into the removal of contaminants from natural water systems is urgent and necessary. Among various water treatment solutions, utilizing energy from nature to purify water sources is likely to be the most promising technology because it is a green, energy-saving, and facile method [1,2,3]. Currently, piezoelectric catalysts such as BaTiO₃ [4], MoS₂ [5,6], TiO₂ [7], and ZnO [8] have been widely studied and reported. Among them, ZnO nanowires have shown excellent performance in harvesting mechanical energy in the natural environment (such as wind energy, tidal energy, energy from acoustic waves, and atmospheric pressure) [9,10,11,12], and can be applied in the fabrication of piezoelectric nanogenerators [13,14,15], high-performance visible/ultraviolet photoelectric detectors [16,17], the decomposition of water to produce sustainable hydrogen fuel [18,19], and wastewater treatment in industry, etc. When ZnO nanowires are deformed under the action of an external force, a piezoelectric field can be generated on the surface, which induces electrons and holes in the nanowires to migrate in opposite directions [20], and the separated charges react with oxygen and OH⁻ in the water and produce reactive oxygen species (ROS) that continuously degrade pollutants in the water [21]. However, the high recombination rate of separated electron–hole pairs during piezoelectric catalytic degradation consumes external input mechanical energy and reduces piezoelectric catalytic efficiency. Many efforts have been made to prevent the recombination of electron–hole pairs, such as the development of various novel ZnO nanostructures [22,23,24], the construction of efficient ZnO nanowire heterojunctions [25,26,27], the introduction of external electric fields, and the coupling of piezoelectric with photoelectric effects of ZnO nanowires [28,29]. Among them, the construction of heterojunctions is considered to be one of the most effective methods to improve the efficiency of electron and hole separation within ZnO nanowires, such as ZnO-CuO [30], ZnO-CeO₂ [31], ZnO-Au@Pt [32], ZnO-GO [33], ZnO-Ag [34], etc. These as-prepared hybrid catalysts are mainly divided into non-immobilized and immobilized catalysts, which have inherent limitations in their use. For example, non-immobilized catalysts are prone to the introduction of secondary contamination in operation and cannot be used on a large scale. Most immobilized catalysts require a seed layer to be implemented before the reaction, and their high cost and complex fabrication processes likewise limit their practical application at the industrial level [35]. Therefore, it is of great significance to develop a seed-layer-free, immobilized ZnO catalyst fabrication process.

Recently, some works have reported the seedless hydrothermal synthesis of ZnO-immobilized catalysts. However, most of them required expensive single-crystal substrates (e.g., GaN [36]), deposited Ti/Au film [37], or the introduction of an external electrical field to enhance the ZnO nucleation [38]. Therefore, the preparation of periodic and patterned arrays of CuO-Cu₂O-ZnO heterojunction on 3D micro-/nano-structured copper substrates remains a great challenge. In recent years, laser processing technology has attracted a lot of attention in the preparation of water treatment materials due to its unique advantages such as high energy density and short processing time [39,40,41], including processing various superhydrophobic surfaces with self-cleaning functions and preparing graphene materials with anti-fouling and anti-bacterial functions [42,43]. Our group also reported the fabrication of copper mesh with an oil–water separation function using laser processing technology for oil spill treatment in the environment, along with other uses [44,45]. The unique advantages of laser processing technology have opened up a new path for the development of efficient water treatment materials.

In this work, a laser-processing-technique-assisted hydrothermal method was proposed to fabricate a CuO-Cu₂O-ZnO heterojunction array with efficient piezoelectric catalytic capacity for water purification, where a periodic three-dimensional CuO-Cu₂O-Cu substrate was fabricated via laser ablation, followed by the hydrothermal-synthesis-induced growth of ZnO nanowires on a 3D CuO-Cu₂O-Cu substrate for the formation of a CuO-Cu₂O-ZnO heterojunction structure. The CuO-Cu₂O-Cu substrates generated via laser processing simplified the hydrothermal growth process of ZnO nanowires and helped to promote the separation of electron and hole pairs inside ZnO nanowires, improving their piezoelectric catalytic water purification capability. The piezocatalytic activity of the CuO-Cu₂O-ZnO heterojunction array was evaluated via MB decomposition under ultrasonic vibration in the dark. The results showed that the CuO-Cu₂O-ZnO heterojunction array possessed up to 34% degradation capacity within 30 min. Furthermore, the hydroxyl radicals and superoxide anions generated during piezoelectric catalysis were detected via the terephthalic acid photoluminescence (TA-PL) method and the nitroblue tetrazolium (NBT) transformation method, respectively. The results showed that a large amount of reactive oxygen species were generated in the CuO-Cu₂O-ZnO catalytic system, which provided a basic mechanism for the degradation of organic dyes in water. The combination of the unique advantages of laser processing technology and hydrothermal methods could provide a highly efficient and cost-effective method for developing new and versatile immobilization catalysts.

2. Materials and Methods

2.1. Materials

All of the chemicals were of analytical grade and used without further purification. Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 1 mol/L), copper sheet, hexamethylenetetramine (HMTA; C₆H₁₂N_4, 30 wt%), ammonia water, terephthalic acid (TA), and nitroblue tetrazolium (NBT) were obtained from Aladdin Chemistry Co. Ltd. (Shanghai, China).

2.2. Preparation of Periodic 3D CuO-Cu₂O-ZnO Heterojunction Array

The copper sheet, with a thickness of ≈300 µm, was cut into pieces of around 1 cm × 1 cm and cleaned with deionized water, followed by hotplate drying. A nanosecond pulsed laser ablation system was applied to create a patterned “grid-type” substrate microstructure in ambient air, as shown in Figure 1a. A laser with a wavelength of 1064 nm, pulse repetition rate (PRR) of 25 kHz, spot size of ≈30 μm, scanning speed of 500 mm s⁻¹, and scan line spacing of ≈0.05 mm was used, whereas the number of scanning repetitions was set at 1, 5, and 10, respectively. To optimize laser processing parameters for the growth of ZnO nanowires, Zn(NO₃)₂ and HAMT reactant concentrations at 50 mM in 3 M ammonia aqueous solution were chosen to prepare the ZnO nanowire growth solution, and then the solution was transferred to a 25 mL reaction kettle and stirred with a glass rod for one minute to mix it well. Then, we placed the processing surface of the copper sheet downward against the inner wall of the reaction kettle. The reaction temperature was set to 150 °C and the growth time was two hours. After the reaction, the copper sheets were removed, rinsed several times with deionized water, and placed on a drying table at 100 °C to dry. Figure 1b,c shows a schematic diagram of hydrothermally grown ZnO nanorods on the “grid-type” substrate. The laser processing parameters for the “grid-type” substrate were set at 10 rounds of laser scanning and spot line spacing of 50 μm. ZnO nanowires were obtained under hydrothermal conditions in 3 M ammonia aqueous solution with both Zn(NO₃)₂ and HAMT reactant concentrations of 1 mM, 25 mM, 50 mM, 100 mM, 200 mM, 400 mM, 600 mM, and 800 mM (molar concentrations and volume ratios, Zn(NO3)2:HAMT = 1:1), respectively, which are labeled as S1, S2, S3, S4, S5, S6, S7, and S8, in order.

2.3. Mechanical Stability of ZnO Nanowires on Laser Ablated Substrate

The adhesion of the ZnO nanowires was tested using an ultrasonic cleaning experiment. First, the grid-type substrate was put in a beaker. A total of 20 mL of deionized water was added, and it was treated in an ultrasonic cleaner with 120 W power for 5 min; then, it was placed on a drying table at 100 °C to dry it.

2.4. Piezocatalytic Performance Evaluation

As shown in Figure 1d, in order to test the catalytic activity of the CuO-Cu₂O-ZnO catalysts, 0.3 mg of methylene blue powder was dissolved in 500 mL of deionized water, and then 30 mL of methylene blue solution was taken out at a time to conduct the catalytic degradation experiment, ensuring the same initial concentration of MB at the beginning of each experiment for each experimental group. At room temperature, each group of samples was placed into beakers containing MB solution and kept in the dark for 5 min to reach dissociation and adsorption equilibrium. The beaker was then placed in an ultrasonic cleaner with 120W and a frequency of 40 kHZ for degradation experiments. After taking out 3–4 mL of the solution every five minutes and performing centrifuge treatment, the change in MB concentration was measured using a UV-Vis spectrophotometer. To prevent the effect of heat generated from ultrasonic vibrations, the water in the ultrasonic cleaner was changed every ten minutes until the experiment was completed. The catalytic capacity was defined as C/C₀ × 100%; the corresponding degradation rate was defined as (1 − C/C₀) × 100%; C₀ was the concentration of MB at T = 0 min when the adsorption equilibrium was reached; and C was the concentration of MB after 30 min of catalysis [46].

2.5. Active Species Trapping

Radical trapping experiments were carried out to identify the primary reactive oxygen species (ROS) generated in each of the eight CuO-Cu2O-ZnO heterojunction array systems. To identify the ∙OH radicals, the reaction of terephthalic acid (TA) with ·OH to produce the highly fluorescent 2-hydroxy terephthalic acid (TAOH) was investigated. At fixed time intervals of the piezocatalytic treatment, an aliquot of the solution was withdrawn and transferred in a cuvette, and the PL spectrum was recorded using a fluorescence spectrometer operating at an excitation wavelength of 315 nm and an emission wavelength of 429 nm. To detect the radical ∙O2- formed under piezocatalytic conditions, a reaction was carried out of NBT with ∙O2- to form the insoluble purple formazan, spectrophotometrically monitoring the changes in the concentration of NBT. For this purpose, an NBT solution with a concentration of 0.03 mM was prepared and the UV-Vis spectra of the solution were recorded at different times of reaction; we observed a continuous decrease in the NBT absorption maximum centered at 259 nm as the reaction proceeded and the reagents were consumed.

Firstly, a 5 mM terephthalic acid solution was prepared in a NaOH solution (10 mM) to analyze the radical of ∙OH. Then, 20 pieces of experimental samples were added to 45 mL of the above solution and exposed to ultrasonic irradiation. The fluorescence spectra were recorded using a fluorescence spectrometer operating at an excitation wavelength of 315 nm and an emission wavelength of 429 nm. NBT reacted with ∙O₂⁻ to form the insoluble purple formazan, thus leading to a decrease in absorbance at 259 nm. The NBT solution was prepared with a concentration of 0.03 mM. The production of ∙O₂⁻ could be tested by monitoring the concentration change in NBT using a UV-Vis spectrophotometer (PerkinElmer, USA).

2.6. Characterization

The surface morphology, composition, and chemical valence of the samples were studied via scanning electron microscopy (SEM, Germany, GeminiSEM 500), X-ray powder diffraction (Japan, Shimadzu XRD-7000), and X-ray photoelectron spectroscopy (XPS, USA, Thermo Scientific, ESCALAB Xi+). The efficiency of MB degradation using CuO-Cu₂O-ZnO catalysts and the detection of superoxide anion production during piezoelectric catalysis were examined via UV-Vis spectroscopy (USA, LAMBDA 1050+). The hydroxyl radical production was detected via transient fluorescence spectroscopy (UK, Edinburgh FLS1000, FLS1000).

3. Results and Discussion

3.1. Growth of ZnO Nanowires on Laser Textured Copper Substrate

Figure 2a,b shows the “grid-type” substrate fabricated on a copper sheet using laser processing techniques. The regular 3D “dimple” pits were left where the laser spot scanned [47], “tapered bumps” were formed in the middle of every four 3D “dimple” pits, and a large number of nanoparticles and inhomogeneous corrugated transition layers were deposited next to them. The regular 3D “dimple” pits and 3D “tapered bumps” were vertically cross-distributed with a diameter of about 40 μm on the bottom. These experimental results can be explained in terms of a dynamic ablation process using a laser. At high laser fluences, the ablated copper atoms are anisotropically ejected into the air with a plasma plume formation, demonstrating the high kinetic energy of the species. Then, the plasma strongly interacts with the air molecules, leading to plasma nucleation and oxidation of the copper atoms, forming nanoparticles and aggregate. Finally, these particles are deposited on the copper surface under the operation of gravity and air resistance [48].

From Figure 2c, it can be seen that large amounts of CuO and Cu2O were produced on the surface of the 3D “grid-type” substrate, which are key to growing ZnO without a catalyst or seed layer coating. The physical reason for such enhanced nucleation and regular growth in the presence of CuO-Cu₂O heterojunction may be attributed to a galvanic effect in the substrate [49]. Although the system is quite complex, we have represented the main chemical reactions in the following equations [50]:

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{N}}_4+\mathrm{Z}{\mathrm{n}}^{2+}\to [\mathrm{Z}\mathrm{n}\left({\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{N}}_4\right){]}^{2+}$$

(1)

$$\mathrm{N}{\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}\to \mathrm{N}{\mathrm{H}}_4^{+}+\mathrm{O}{\mathrm{H}}^{-}$$

(2)

$$\mathrm{Z}{\mathrm{n}}^{2+}+4\mathrm{N}{\mathrm{H}}_3\to \mathrm{Z}\mathrm{n}(\mathrm{N}{\mathrm{H}}_3{)}_4^{2+}$$

(3)

$$\mathrm{Z}{\mathrm{n}}^{2+}+4\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{Z}\mathrm{n}(\mathrm{O}\mathrm{H}{)}_4^{2-}$$

(4)

$$\mathrm{Z}\mathrm{n}(\mathrm{N}{\mathrm{H}}_3{)}_4^{2+}+2\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{Z}\mathrm{n}\mathrm{O}+4\mathrm{N}{\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}$$

(5)

$$\mathrm{Z}\mathrm{n}(\mathrm{O}\mathrm{H}{)}_4^{2-}\to \mathrm{Z}\mathrm{n}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+2\mathrm{O}{\mathrm{H}}^{-}$$

(6)

$$[\mathrm{Z}\mathrm{n}\left({\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{N}}_4\right){]}^{2+}+2\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{Z}\mathrm{n}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{N}}_4$$

(7)

Under the action of the built-in electric field of the CuO-Cu₂O heterojunction, the positively charged intermediates Zn(NH₃)₄²⁺, [Zn(C₆H₁₂N₄)]²⁺, and OH⁻ formed in the hydrothermal reaction migrate towards the substrate containing CuO and Cu₂O and then react to form ZnO crystals. These enable the laser-processed copper substrate to grow ZnO nanorods without any catalyst or seed layer coating, which is consistent with recent investigations into the Al-based galvanic-cell effect for synthesizing ZnO nanorods on conducting substrates [51].

Figure 2d–i shows the SEM images of the ZnO nanowires, grown via hydrothermal synthesis, on “grid-type” substrates prepared using different numbers of laser scans. The patterned ZnO nanowire arrays mainly consisted of nanorods with pinpoint tops, which were densely and uniformly distributed on the “grid-type” substrate. The nanorod lengths were in the range of 1~5 μm. Furthermore, as the number of laser scans increased, more and more ZnO nanowires were grown on the substrate, which were attributed to the greater number of induced copper oxide active sites as the number of laser scans increased. The XRD patterns of the 3D “grid-type” substrates prepared with different numbers of laser scans are shown in Figure 2c. As the number of laser scans increased, the peak intensity of copper decreased, relative to that of CuO and Cu₂O, indicating that the copper substrate was more severely ablated by the laser, resulting in a larger surface area of the copper substrate being oxidized, producing more CuO-Cu₂O particles and inhomogeneous corrugated transition layers, which is beneficial for the growth of ZnO nanowires.

The long-term performance of immobilized hybrid catalysts is governed by the stability and adhesion of ZnO nanowires to the immobilizing substrate. Ultrasonic cleaning experiments for ZnO nanowires are shown in Figure 2j–l. Figure 2j shows the SEM images before ultrasonic stripping, and Figure 2k,l shows the SEM images after ultrasonic cleaning in deionized water for 5 min. The figure shows that most of the ZnO nanowires remain on the copper sheet, which further indicates the strong adhesion of the ZnO nanowires grown on the copper sheet [52]. As shown in Figure 2a,b, a large number of nanoparticles, regular pits [53], and corrugated transition layers were produced on the copper sheet, increasing the roughness of the substrate surface, which may be one of the important reasons for the strong adhesion of ZnO nanowires on the 3D CuO-Cu₂O substrate [54,55].

Figure 3a shows the XRD pattern of the CuO-Cu₂O-ZnO heterojunction constructed on a copper sheet. The sharp diffraction peaks indicate that the grown ZnO nanowires exhibit a good crystalline quality, and all of the diffraction peaks correspond to the wurtzite structured ZnO (JCPDS card no. 36-1451) [56]. Other peaks are attributed to the Cu (JCPDS card no. 04-0836), Cu₂O (JCPDS card no.99-0041), and cubic CuO (JCPDS card no. 71-0251) [57]. This demonstrates the successful preparation of CuO-Cu₂O-ZnO heterojunction catalysts on a “grid-type” substrate. We further analyzed the elemental valence state of the substrate copper surface after laser processing via XPS. Figure 3b shows the XPS scan spectrum of a “grid-type” copper sheet after the hydrothermal growth of ZnO nanowires, and from which the main binding energy peaks of carbon (C 1s), oxygen (O 1s), zinc (Zn 2p), and copper (Cu 2p) can be observed. As shown in Figure 3c, the peak at binding energy of 935.2 eV corresponds to CuO [58], while the peak at binding energy of 933.5 eV corresponds to Cu₂O [59]. This is a good indication of the generation of CuO and Cu₂O on the copper surface after laser processing. The XPS spectrum of Zn 2p is shown in Figure 3d and the two strong peaks at binding energies of 1023.06 eV and 1046.20 eV correspond to Zn 2p^3/2 and Zn 2p^1/2, respectively, which indicates the presence of Zn in the form of ZnO. Figure 3e shows the XPS spectra of C 1S, and the three strong peaks at binding energies 284.8 eV, 286.5 eV, and 288.7 eV can be assigned to the C-C bond, C-O-C bond, and HO-C=O bond, respectively. Figure 3f shows the spectra of O 1S, and the three strong peaks at binding energies 530.1 eV, 531.5 eV, and 532.5 eV can be seen to originate from the Cu-O bond, Zn-OH bond, and -OH bond. It has been documented that these oxygen-containing functional groups facilitate the anchoring of ZnO nanorods [33].

In this section, 3D periodic “grid-type” CuO-Cu₂O-ZnO heterojunctions were successfully fabricated via the laser-processing-technique-assisted hydrothermal method, and the laser processing parameters were optimized for favorable ZnO nanowire growth. The CuO-Cu₂O-ZnO heterojunction structure was further verified by XRD and XPS characterizations. Furthermore, it was found that laser-induced copper oxide active sites are essential for the preparation of CuO-Cu₂O-ZnO heterojunction structures without the need for catalysts and seed layer coatings.

3.2. Modulating the Morphology of ZnO Nanowires

Figure 4 shows the SEM images of ZnO nanowire morphology obtained under hydrothermal conditions with reactant concentrations of 1 mM, 25 mM, 50 mM, 100 mM, 200 mM, 400 mM, 600 mM, and 800 mM, respectively. From Figure 4a,e, it can be seen that at an initial concentration of 1 mM, the grown ZnO nanowires are extremely short, with a length of about 0.33 um and a diameter of about 0.13 µm, due to the low concentration of Zn²⁺. When the concentration of reactants exceeds 50 mM, as shown in Figure 4c,g, the length of the ZnO nanowires is 3.55 µm and the diameter is 0.34 µm, and the growing density of ZnO can be seen to have increased tremendously by this time. As shown in Figure 4i,m, when the concentration increases to 200 mM, the length of the ZnO nanowires continues to increase to approximately 4.6 µm and the growth density of the ZnO nanowires is almost maximized, but the nanowires are not uniform, and the coarse and fine ones are mixed, with diameters varying from approximately 0.36 µm to 0.7 µm. As shown in Figure 4j,n, when the growth concentration reaches 400 mM, the morphology of the ZnO nanowires starts to change from a cylindrical rod to a hexagonal prism with a length of 4.7 µm and a diameter of 1.2 µm. There is a small bump on top of the hexagonal prism. As shown in Figure 4k,o, when the concentration exceeds 600 mM, the small bumps on the hexagonal prism disappear, and the morphology of the ZnO nanowires becomes completely hexagonal prismatic and does not change with the increase in concentration.

As shown in Figure 4, it is clear that the orderliness and growth position of ZnO nanorods grown on “grid-type” copper substrates were retained at constant in the eight sets of experiments, but the growing concentration of ZnO caused a dramatic change in the morphology of ZnO nanowires [60]. This effect of reactant concentration on morphology can be understood in terms of the critical concentration difference between the (0001) and (00$\stackrel{-}{1}$0) facets of the ZnO nanowires, which is related to the surface energy difference between the crystalline facets. In ZnO nanowires, (0001) polar planes have the highest surface energy (2.0 J/m²) [56] compared to other crystalline planes (e.g., (00$\stackrel{-}{1}$0) planes, 1.16 J/m²) [60], and crystalline planes with larger surface energies have lower nucleation-free energy barriers (i.e., lower critical nucleation concentrations). Furthermore, crystal growth occurs when the concentration of reactants exceeds the critical concentration of each crystalline surface. In the early stages of the hydrothermal reaction, the reactant solution is supersaturated for both the (0001) and (00$\stackrel{-}{1}$0) planes, so that ZnO crystals cannot only grow axially but also laterally; however, even in the same concentration of the reaction solution, the ZnO nuclei of the (0001) compared to the (00$\stackrel{-}{1}$0) planes also face greater supersaturation, which leads to faster axial growth of ZnO crystals than lateral growth, resulting in the formation of ZnO nanowires with a length greater than the diameter. Furthermore, as the concentration of reactants increases, the supersaturation between the different crystalline planes is lifted relatively more slowly, leading to a continuous increase in the length and diameter of the ZnO nanowires and finally to the formation of ZnO nanowires with different aspect ratios [56,60]. The trends of length, diameter, and aspect ratio of ZnO nanowires grown at growth concentrations from 1 mM to 800 mM are shown in Figure 4q–s. As the growth concentration was increased, the length of the ZnO nanowire increased from 0.33 µm to 5.4 µm and the diameter increased from 0.14 µm to 1.6 µm, with the ZnO nanowire length increasing much faster than the diameter; this growth process of ZnO nanowires can be divided into two stages. In the first growth stage, as the concentration of the reactant increased from 1 mM to 50 mM, the length of the ZnO nanowires increased from 0.33 µm to 3.55 µm; the most significant change in the length of the ZnO nanowires was observed during this process. In contrast, the diameter of the ZnO nanowires increased more slowly as the concentration increased from 1 mM to 50 mM, and an increase in diameter of the ZnO nanowires was seen from 0.14 µm to 0.34 µm, with the aspect ratio of the ZnO nanowires at 1 mM being 2.35 and increasing with increasing reactant concentration, reaching a maximum aspect ratio of 10.44 at 50 mM. In the second growth stage, as the concentration of reactants increased from 100 mM to 800 mM, the growth rate of the length of the ZnO nanorods began to slow down, increasing from approximately 3.6 µm to 5.4 µm in length. Conversely, the growth rate of the diameter of the ZnO nanorods increased from 0.5 µm to 1.6 µm when the growth concentration was increased from 100 mM to 800 mM. Particularly, when the concentration of reactants increased from 200 mM to over 400 mM, the morphology of ZnO changed from nanowires to hexagonal prisms, and the diameter of the ZnO nanomaterials increased from 0.7 µm to 1.2 µm, so that at these concentrations the most significant change was observed in the diameter of ZnO. Furthermore, the aspect ratio of the ZnO nanorods was 7.22 at a reactant concentration of 100 mM and decreased to 2.62 when the reactant concentration was increased to 800 mM. This demonstrates that different concentrations of reactants cause different growth rates of ZnO nanowires along the radial and axial directions, which ultimately leads to the formation of ZnO nanowires with tunable aspect ratios.

3.3. Piezocatalytic Performance Evaluation

The piezocatalytic activity of the as-prepared samples was analyzed via MB decomposition with ultrasonication in the dark with an initial concentration of MB at 0.6 mg/L [33]. Figure 5a,b shows the piezoelectric catalytic degradation performance of the mixed ZnO nanowire arrays from sample 1 to sample 8. As expected, within a certain range, increasing the growth concentration of ZnO nanowires is beneficial for improving the piezoelectric catalytic degradation of immobilized hybrid ZnO nanowires. Sample 1 was grown at a concentration of 1 mM and had the lowest piezoelectric catalytic degradation performance (8%), while sample 8 was grown at a concentration of 800 mM and had the highest degradation performance (34%). As shown in Figure 4, as the growth concentration was increased, the length of the ZnO nanowire increased from 0.33 µm to 5.4 µm and the diameter increased from 0.14 µm to 1.6 µm; the increase in length of the ZnO nanowires was much faster than the increase in diameter. The higher piezocatalytic degradation performance of the mixed ZnO nanowire arrays with higher growth concentrations can be attributed to the larger active surface area under external forces from the longer length of the ZnO nanowires; there were more reaction sites for pollutant decomposition. Furthermore, the more significant the increase in the length of the ZnO nanowires, the more likely the ZnO nanowires are to bend and deform under the same external pressure, determining a more efficient internal migration of electrons and holes, thus leading to better piezoelectric catalytic degradation performance of the ZnO nanowires [18,52].

To further evaluate the piezoelectric degradation performance of CuO-Cu2O-ZnO piezoelectric catalysts, we recorded the UV-Vis absorption spectra of sample 8 during catalytic degradation. The illustration in the top left corner of Figure 5b shows the UV–Vis absorption spectra (500–730 nm) of the MB solution, acquired at different ultrasonic stirring times. It can be observed that the intensity of the maximum absorption peak, located at 664 nm, progressively decreases with the prolongation of the ultrasonic stirring time. Furthermore, in the box of Figure 5b, the MB solutions at increasing ultrasonic stirring times are shown in the top left corner, highlighting how the initial blue color of the MB solution gradually fades, thus fully demonstrating the piezoelectric catalytic capacity of ZnO nanowires grown on copper substrates.

To elucidate the mechanism of piezocatalysis, sample S8 was examined for reactive oxygen species (ROS) produced during piezoelectric catalytic degradation. Hydroxyl radicals and superoxide anions generated during piezoelectric catalysis were detected via the terephthalic acid photoluminescence (TA-PL) method and NBT transformation method, respectively. NBT can react with ·O2– to form insoluble purple formazan in aqueous solution; therefore, the generated superoxide ions can be determined spectrophotometrically using NBT as a probe by monitoring the concentration changes in NBT because of the catalytic reaction. Figure 5c shows the NBT spectra recorded at different times during the piezoelectric catalysis process, from 0 min to 30 mins, highlighting the decreasing intensity of the absorbance signal of NBT at 259 nm in the solution. This indicates that the ∙O2- produced in the piezoelectric catalysis process is involved in the degradation of MB [61,62]. The rapid and selective reaction of terephthalic acid (TA) with ·OH produces highly fluorescent 2-hydroxy terephthalic acid (TAOH), which fluoresces at 429 nm. The PL intensities of TAOH (see Figure 5d), significantly enhanced with the prolonged catalytic reaction time, demonstrate that ·OH radicals are continuously generated in the piezocatalytic process [63,64]. The findings confirm that CuO-Cu2O-ZnO can efficiently produce ∙O2- and ∙OH for the degradation of pollutants.

The initial occurrence required for the degradation process is the generation of conduction band electrons (e⁻) and valence band holes (h⁺) (electron–hole pairs) in semiconductors [49]. The mechanism of ∙O₂⁻ and ∙OH-activated oxide generation by the CuO-Cu₂O-ZnO hybrid catalyst for the degradation of organic pollutants is shown in Figure 5e. It can be concluded that the efficient piezoelectric catalytic performance of ZnO nanowires on copper sheets can be attributed to the coupling effect of the efficient charge transfer process of the hybrid CuO-Cu₂O-ZnO heterojunction and the piezoelectric field generated by the ZnO nanowires under the action of an ultrasound. Under intense ultrasonic irradiation, the water produces extremely active bubbles, which subsequently collapse and generate localized high water pressure, causing the ZnO nanowires to bend and allowing the ZnO nanowires to generate built-in potential to induce the separation of electrons and holes. As shown in Figure 3c, CuO and Cu₂O are generated after laser processing, and CuO and Cu₂O are p-type semiconductors, while ZnO is an n-type semiconductor. As shown in Figure 5e, when p-type and n-type semiconductors are in contact, the electrons moved from the CB (conduction band) of ZnO to the VB (valence band) of CuO and Cu₂O via electrostatic attraction, which results in the electrons staying in the CB of CuO and Cu₂O and the hole remaining in the VB of ZnO, respectively. This process could reduce the recombination of electron and hole pairs on the surface of ZnO [65]. It has been shown that the CB potential of CuO is −0.92 eV versus the NHE (normal hydrogen electrode) potential, and the CB potential of Cu₂O is −1.2 eV versus the NHE [66], which is more negative than the standard redox potential of O₂/∙O₂⁻ (−0.33 eV versus NHE); so, the electrons in the CB of CuO and Cu₂O can reduce the oxygen dissolved in water to produce ∙O₂⁻. Meanwhile, the VB potential of ZnO (+3.01 eV versus NHE) is more positive than the standard redox potential of ∙OH/H₂O (+2.38 eV versus NHE), and so the holes in the VB of ZnO can oxidize H₂O to produce ∙OH. Under the coupling effect of the efficient charge transfer process of the hybrid ZnO heterojunction and the piezoelectric field generated via the bending of ZnO nanorods, a large number of reactive oxygen species (ROS) are generated in the hybrid catalytic degradation system, in which hydroxyl radicals can oxidize methylene blue in water to produce non-toxic CO₂ and H₂O. Figure 5e [67] shows the reaction equations associated with the process.

4. Conclusions

In this work, immobilized CuO-Cu₂O-ZnO hybrid catalysts were fabricated on a 3D CuO-Cu₂O-Cu substrate via the laser-processing-technology-assisted hydrothermal method without any catalyst or seed layer coating before the hydrothermal reaction. A periodic three-dimensional CuO-Cu₂O-Cu substrate was firstly fabricated via laser ablation, followed by the hydrothermal-synthesis-induced growth of ZnO nanowires on a 3D CuO-Cu₂O-Cu substrate for the formation of a CuO-Cu₂O-ZnO heterojunction structure. The laser processing parameters were optimized for favorable ZnO growth. The surface morphology and chemical composition of CuO-Cu₂O-ZnO hybrid catalysts were characterized using SEM, XRD, and XPS techniques, indicating the successful preparation of CuO-Cu₂O-ZnO heterojunction catalysts on a grid-type substrate. Furthermore, it was found that the formation of Cu₂O-CuO on the copper sheet via laser processing was crucial for the preparation of the CuO-Cu₂O-ZnO heterojunction catalyst. The process of change in the morphology of ZnO nanowires and their physical mechanisms were investigated in detail using reactant concentrations ranging from 1 mM to 800 mM. ZnO nanorods on “grid-type” copper substrates demonstrated effective piezoelectric catalytic abilities with different morphologies, and it was found that longer ZnO nanowires have a stronger piezoelectric catalytic degradation performance. This can be attributed to the fact that the longer the length of the ZnO nanowires, the greater the active surface area under external forces, providing more reaction sites for pollutant decomposition. Furthermore, the easier it is to bend and deform the ZnO nanowire under the same external pressure, the more efficient the internal migration of electrons and holes is, which leads to better piezoelectric catalytic degradation performance of ZnO nanowires. Abundant reactive oxygen species were detected in the hybrid piezoelectric ZnO catalysis system, which indicates that the catalytic piezoelectric pathway degrades organic compounds in water. This simple and low-cost method could provide a useful guide for the large-scale efficient and versatile synthesis of immobilized piezoelectric catalysts for the water redemption industry.