Controlled synthesis of CuS-decorated CuO pillars over Cu mesh with improved wettability, photothermal and photocatalytic properties
Studies have shown that semiconductor compound photocatalysts have a wide range of promising applications in the treatment of environmental pollution. Meanwhile, in nature, the leaves of green plants are almost always lamellar, and photosynthesis in leaves is the most common solar energy conversion system, which involves the process of light absorption and conversion. In this paper, CuS/CuO nanorod arrays with plant-like structures were prepared on Cu mesh by in situ oxidation and successive ion layer adsorption and reaction (SILAR) method. The effects of oxidation temperature, annealing time and SILAR cycle on the structure and properties of CuS/CuO/Cu Mesh were investigated, and the mechanism of photothermal effect and photocatalysis was further analyzed. The experimental results show that the layered nanostructure and porous morphology of CuS/CuO/Cu mesh can reduce photon spillage and have better photothermal, wettability and photocatalytic properties. CuS/CuO/Cu mesh degraded MB and MO up to 92.7% and 86% in 180 min, while it still has good stability after 8 cycles. Compared with powder and microbial catalyst, CuS/CuO/Cu mesh has the advantages of brief preparation time, good stability, reusable, and no secondary pollution, which can bring greater economic benefits and better sewage treatment effect.
Chen Wang and Weiran Xu have contributed equally to this work.
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Introduction
The rapid development of modern industry not only brings benefits to human society, but also inevitably produces a lot of environmental pollution problems [[1]–[3]]. According to reports, more than 10,000 people die every day from water-borne diseases, and more than 200,000 tons of synthetic dyes are discharged into wastewater by the textile industry every year [[4]]. At present, the main methods of wastewater treatment are biodegradation [[5]], physical adsorption [[6]], chemical reaction [[7]] and photocatalytic degradation [[8]–[10]]. Among these methods, photocatalytic degradation has the advantages of high efficiency, low cost, reusable and environmentally friendly [[11]]. Therefore, the photocatalytic degradation method has received extensive attention, and the rational development of efficient photocatalysts is of great significance in the field of sewage treatment.
As a good semiconductor photocatalyst, CuO is a p-type semiconductor which has a narrow band gap [[13]–[15]]. The narrow band gap makes it have a wider range of visible light absorption, even extended to the infrared region [[16]]. At the same time, CuO has the characteristics of stable performance, non-toxic and harmless, simple process, low cost and environmental friendliness, and can be used as heterogeneous catalysts [[18]], sensors [[19]], super-capacitors [[20]], solar cells [[21]] and photocatalysts [[22]–[24]], etc. Therefore, CuO is often a priority material for both basic research and practical applications. At present, many methods have been developed to prepare CuO nanostructures, including hydrothermal method [[25]], electrodeposition method [[26]], sol–gel method [[27]] and thermal oxidation method [[28]]. Despite the attractive capabilities of CuO catalysts prepared by these methods, most of the CuO prepared are powder, bulk and thin film and undergo complex separation operations such as centrifugation, ultrasound and drying. The cumbersome recycling process not only leads to high recycling costs, but also leads to secondary pollution and hinders its practical application in water purification. According to the research, the in situ oxidation method can construct three-dimensional CuO nanoarrays on the copper substrate, which can effectively solve the complex problem of recycling [[29]–[32]]. On the one hand, the nanoarray ensures a good connection channel between the oxide and the copper substrate, which is conducive to electron transfer; on the other hand, the nanoarray has a large specific surface area and pores, which are conducive to the adsorption of substances and the provision of reaction sites. However, the photocatalytic materials of 3D nanoarrays can effectively reduce light dissipation and have good separation and transfer of photogenerated carriers, but the role of solar thermal radiation in the photocatalytic process is often ignored.
CuS has become a new class of photothermal agents due to its simple synthesis, good biocompatibility, low cost, high photostability and excellent photothermal conversion efficiency [[33]–[35]]. For example, GuO et al. [[36]] synthesized AuxCu-cus core–shell structure in TiO2 nanocavity, and utilized optical drive and thermal effect to oxidize glycerol, even obtaining glycerol conversion efficiency of 72% without external heating. Tian et al. [[37]] synthesized the centimeter-scale Au-CuS material with biomimetic structure, which not only has excellent infrared photothermal conversion performance (30.56%), but also has effect on solar photothermal conversion in low-temperature applications, realizing more intensive broadband solar absorption and lower reflectivity. Sun et al. [[38]] compounded CuS nanoparticles into polyacrylamide hydrogel can effectively convert strong absorption light into local heat and the solar photothermal conversion efficiency can reach 92% under one sun illumination (1000 W/m2), which is characterized by high photothermal conversion efficiency. A large number of studies have shown that CuS as an additive can not only improve the light absorption range of photocatalysts, but also fully absorb the thermal radiation of sunlight. This phenomenon is hardly affected by the particle morphology, shape and surrounding medium, resulting in better stability of the photothermal conversion efficiency [[39]]. Therefore, CuS loaded on CuO nanoarrays is expected to improve the absorption of sunlight in a wide wavelength range and promote photothermal conversion efficiency, thereby enhancing the ability of photocatalysts to degrade organic dyes, and has certain potential applications in the field of environmental management.
At present, studies have shown that CuO and CuS can form heterojunction photocatalysts, which can inhibit the electron–hole recombination process, thus improving the life of photocarriers and promoting the catalytic reaction. For example, Cao et al. [[41]] synthesized pine dendritic ternary CuO/CuS/ZnO on a copper foam substrate. High specific surface area and unique structure enable efficient charge transport and efficient capture of visible light. Kao et al. [[42]] synthesized CuO/CuS core–shell nanowires on copper foil through thermal oxidation and two-step annealing, and the degradation rate increased from 67 to 89%. Zhang et al. [[43]] prepared CuO/Al2O3 microspheres by hydrothermal method. After the deposition of Pt and CuS, the hydrogen production efficiency of oxalic acid increased by 1.44 times. Liu et al. [[44]] constructed a biosensor using CuO/Ag2S/CuS NHs as electrodes, which expanded the available wavelength range and improved the photoelectric response of the sensor by 1.4 times. The heterostructure of the semiconductor can expand the light absorption range and inhibit the recombination of electrons and holes [[45]]. At the same time, CuS can conduct in situ heating of catalyst under light due to its unique thermal effect, which can significantly accelerate the generation and migration of photogenerated carriers, thus promoting the photocatalytic reaction process. Therefore, the synergistic effect of heterojunction structure and thermal effect has a good application prospect.
In this work, we constructed plant-like CuO/CuS nanorod array on the copper mesh by in situ oxidation and successive ion layer adsorption and reaction (SILAR) method, inspired by the leaf structure in plant photosynthesis and maximizing the synergistic absorption effect of CuO and CuS. A series of conditions such as oxidation temperature, annealing time and SILAR cycle were studied, and the optimal reaction conditions were determined according to the influence factors. We analyzed the thermal effect and photocatalysis of CuS/CuO/Cu Mesh in detail, and explored the mechanism of co-photocatalysis. The results show that CuS/CuO/Cu Mesh exhibits good photothermal effect and high photocatalytic performance under light conditions. Better surface wettability enables it to better contact with water pollution and the photocatalyst still shows good stability after 8 cycles of stability experiments. More importantly, the composite catalyst of CuS/CuO with copper net as the skeleton realizes relatively easy recovery process and reduces the problems of difficult recovery and secondary pollution.
Experimental
Chemicals and materials
The Cu mesh with a purity of 99.99% was purchased from Qinghe YuQian Matal Products Co., Ltd, (China). Sodium hydroxide (NaOH), ammonium peroxodisulfate [(NH4)2S2O8], copper nitrate trihydrate [Cu(NO3)2 · 3H2O] and sodium sulfide (Na2S · 9H2O) were of analytical grade and used without further purification or processing. The water in the experiments is ultra-pure water.
Synthesis of CuO nanorod arrays on Cu mesh
Before experiments, commercial Cu mesh was cut into 2.0 × 2.0 cm, which was cleaned in acetone, ethanol, deionized water ultrasonic for 30 min, respectively, followed by drying after using 0.1 M HCl dilute solution to soak for 30 s. The purpose is to remove the surface oxide layer, flushing to deionized water to neutral. In a typical procedure, 15.0 mL of 3 M NaOH and 15.0 mL of 0.15 M (NH4)2S2O8 solutions were mixed under stirring until the solution become transparent. Then, the cleaned Cu mesh was immersed in the etching solution for 25 min at temperature. After rinsed by deionized water and dried in air, the sample was annealed in a muffle furnace at 200 °C for 2 h, the heating rate was 2 °C/min and then cooled gradually to room temperature. Thus, the CuO nanorod arrays were obtained on the Cu mesh, and the optimum conditions were determined by changing reaction temperature and annealing time.
Synthesis of CuS nanosheets on the CuO nanorod arrays
CuS nanosheets were directly grown on CuO nanoarrays by a successive ionic layer adsorption and reaction (SILAR) method. The CuO nanorod arrays were dipped in 20.0 mL of 5 mM Cu(NO3)2 solution for 60 s to adsorbed Cu2+ ions. Subsequently, the treated CuO nanorods were immersed in 20.0 mL of 5 mM Na2S solution for 60 s to adsorbed S2− ions, which was defined as one SILAR cycle. Following each immersion, the CuO nanorod arrays were rinsed with ethanol to remove loosely adsorbed ions for 30 s. The CuS/CuO nanostructure was gained after 3 cycles of continuous ion adsorption, then washed with ethanol and dried under vacuum at 60 °C. Thus, the CuS nanosheets were attached to CuO nanorods, and the optimum conditions are determined by changing the SILAR cycle.
Characterization
The microscopic morphologies of samples were tested by field emission scanning electron microscopy (FESEM, SU8010, Hitachi), and the types of elements were analyzed by energy-dispersive spectrometer (EDS, Apollo xp, Ametek). The physical phase structure and surface components were identified by X-ray diffraction (XRD, d8 advance, Bruker) and X-ray photoelectron spectrum (XPS, K-ALPHA, Thermo Scientific). The light absorption capacity was measured by ultraviolet–visible spectrophotometer (UV–Vis, UV-2600, SHIMADZU), photoluminescence spectroscopy (F-4700, HITACHI) and Fourier transform infrared (FTIR). The surface temperature of the samples was analyzed by the thermal infrared camera (FLUKE Ti32 Fluke Co., Ltd.). The wettability of sample surface was measured by the contact angle measuring instrument (CA, DSA100, KRUSS). It should be noted that during contact angle measurements, droplet volumes of 2 μL are used and each sample is tested at three points.
Photocatalytic measurement
In the photocatalytic experiment, methylene blue (MB) and methylene orange (MO) were used to simulate dye organic matter in water to evaluate the photocatalytic activity of samples, respectively. In this paper, the concentration of MB and MO in this paper was 24 mg/L, a 350 W xenon lamp was as the light source and a UV–Vis spectrophotometer was used to measure the absorbance of the dye organic matter. Prior to the photocatalytic experiment, the samples and 30 ml dye organic matter solution were placed together in a dark environment for 30 min to ensure the adsorption–desorption balance on the photocatalyst surface. Then, the above solution was exposed to the Xenon lamp. Following, the absorbance of the dye organic matter solution was recorded every 30 min using a UV–Vis spectrophotometer and calculated according to the Beer–Lambert Law as the following equation:
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where C0 was the initial absorbance of dye organic matter at adsorption equilibrium and Ct was the absorbance of dye organic matter measured at different times by a UV–Vis spectrophotometer.
Results and discussion
Preparation of CuS/CuO nanoarray structure
Inspired by the leaf structure of plant photosynthesis, plant-structured CuS/CuO nanorod arrays were grown on copper mesh, as shown in Fig. 1. The evolution of CuS/CuO nanorods by in situ oxidation and the successive ionic layer adsorption and reaction (SILAR) method can be divided into three steps, as illustrated by the following proces s [[46]].
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Graph: Figure 1 Schematic diagram of the synthesis and growth process of CuS/CuO/Cu mesh nanostructures. a The growth process of CuO nanorods. b The synthesis process of CuS nanosheets
Copper mesh was chosen as the bottom due to its unique flexibility, thermal conductivity and high electrical conductivity. In the first step, the surface of the copper net is oxidized by S2O82−. Subsequently, Cu is oxidized to Cu2+ and dissolved into the solution. Then, Cu2+ and OH- form blue Cu(OH)2 nanorods in an alkaline environment [[30]]. In the second step, the Cu(OH)2 nanorods are calcined and treated at high temperature to dehydrate and transform into brown-black CuO nanorods, and the nanorods are slightly bent. In the third step, the high coverage density CuS nanosheets are uniformly attached to CuO nanorods, resulting in a plant-like structure of CuS/CuO nanorods and a highly porous surface morphology, which is important for the construction of the wetting surface and the light trapping process.
Morphological characteristics
Morphological control is very important in the process of catalyst preparation. The microstructure and morphological characteristics of CuO nanorods and CuS nanosheets were investigated by scanning electron microscopy (SEM). In order to explore the optimal conditions for CuS/CuO/Cu mesh with plant-like structures, we experimentally explored the optimal oxidation temperature (reaction temperature: 25 °C), the optimal annealing time (reaction time: 2 h) and the optimal SILAR cycle (reaction cycle: 6 cycles).
Figure 2a–d shows the effect of different oxidation temperatures of CuO. The driving force for ion diffusion is the change in chemical potential, from a place of low chemical potential to a place of high chemical potential [[47]]. When the water bath temperature was 25 °C, most of the Cu2+ was transported to the tip and a small portion grew radially to form a one-dimensional CuO nanorod structure with an average length and average diameter of about 5 um and 200 nm, respectively. Radial growth rate and longitudinal growth rate are temperature dependent [[48]]. When the temperature is higher, the radial growth rate of nanorods is significantly higher than the longitudinal growth rate, resulting in very few nanorods growing. At 80 °C, the nanorods were completely transformed into nanosheets and finally into CuO nanoflowers. Therefore, the optimal oxidation temperature was 25 °C.
Graph: Figure 2 SEM images of the samples. a-d SEM images of CuO with different oxidation temperature, a 25 °C, b 40 °C, c 60 °C, d 80 °C; e–h SEM images of CuO with different annealing time, e 0 h, f 2 h, g 4 h, h 6 h; i-l SEM images of CuS/CuO with different SILAR cycles, i 0 cycle, j 3 cycles, k 6 cycles, l 9 cycles
Figure 2e–h shows the effect of different annealing time of CuO. As shown in Fig. 2e, the surface of the unannealed copper mesh is uniformly covered by smooth-surfaced Cu(OH)2 nanorods. After annealing for 2 h, Cu(OH)2 was dehydrated and converted to CuO, and the surface of the nanorods was rough and curved, as shown in Fig. 2f. At 4 h of annealing, the nanorods showed obvious cracks. With the annealing time to 6 h, in order to relieve the internal stress, the CuO nanorods split in all directions from the tip. After severe dehydration, the nanorods become thinner, bend severely and cross over each other, which is not conducive to the attachment of CuS nanosheets. Therefore, the optimal annealing time was 2 h.
Figure 2i–l shows the effect of different SILAR cycles of CuS/CuO. As shown in Fig. 2i, gaps and holes appeared on the CuO nanorods before attachment, which could provide more attachment sites. When cycle was 3, CuS nanosheets were successfully synthesized on the surface of CuO nanorods by successive adsorption of copper and sulfur ions. However, due to insufficient reaction time, the size and distribution of the CuS nanosheet structures were not uniform. When the cycle was 6 as in Fig. 2k, the CuS nanosheets were able to adhere uniformly to the CuO nanorods and the thickness is roughly about 20 nm. There was a small gap between the sheets, which could provide more adsorption sites and a large illumination area. On the contrary, a large number of nanosheets agglomerate into spheres when the number of cycles is higher. Therefore, the optimal SILAR was 6 cycles.
Optical characteristics
To further investigate the optical properties of the sample, the analysis was performed by UV–Vis, PL and FTIR, as shown in Fig. 3. Figure 3a–c shows the effects of different water bath temperatures, annealing times and SILAR cycles, respectively. This indicates that the unmodified CuO nanoarrays have good light absorption in the light range and the water bath temperature of 25 °C and annealing time of 2 h have better absorption intensity. After CuO nanorods were attached to CuS nanorods, the light absorption performance of CuS/CuO nanorods was further improved, but the multiple adsorption had little change. The bandgap evaluation and light absorption at 650 nm for each sample are described in Tables (1, 2 and 3).
Graph: Figure 3 UV–Vis spectra under different conditions: a oxidation temperature, b annealing times, c SILAR cycles of CuS. CuS/CuO/Cu Mesh and CuO/Cu Mesh optical properties: d Tauc plots, e PL, f FTIR
Table 1 The band gap of CuO with different oxidation temperature
Conditions | 25 °C | 40 °C | 60 °C | 80 °C |
|---|
Absorbance (α.μ.) | 0.956 | 0.799 | 0.895 | 1.004 |
Band gap/eV | 2.254 | 2.257 | 2.264 | 2.259 |
Table 2 The band gap of CuO with different annealing time
Conditions | 0 h | 2 h | 4 h | 6 h |
|---|
Absorbance (α.μ.) | 0.628 | 0.958 | 0.883 | 0.917 |
Band gap/eV | 2.434 | 2.258 | 2.273 | 2.290 |
Table 3 The band gap of CuS/CuO with different SILAR cycles
Conditions | 0 cycle | 3 cycles | 6 cycles | 9 cycles |
|---|
Absorbance (α.μ.) | 0.712 | 1.071 | 1.070 | 1.069 |
Band gap/eV | 2.259 | 2.201 | 2.196 | 2.203 |
In order to explore the optical properties of the samples before and after modification, the CuO/Cu mesh in Fig. 2i and CuS/CuO/Cu mesh in Fig. 2k were characterized, respectively, as shown in Fig. 3d–f. The estimated band gaps of CuS/CuO/Cu mesh and CuO/Cu mesh are 2.196 eV and 2.259 eV, respectively. While we measured the photoluminescence intensity of different catalysts using a fluorescence spectrophotometer, as shown in Fig. 3e, the fluorescence intensity of CuS/CuO is diminished relative to CuO, which indicates that less photogenerated electrons and holes are compounded, and thus more photogenerated carriers are involved in the photocatalytic reaction. Meanwhile, Fig. 3f reflects the degree of infrared light absorption of the sample, and the modified CuS/CuO nanostructure has better infrared absorption ability. For the plant-like structure of CuS/CuO/Cu mesh, the characteristic peaks located at 3436 cm−1 and 1577 cm−1 can be attributed to the O–H stretching region, which is related to the O–H vibration caused by the water molecules adsorbed on the surface of the complexes [[33]]. The characteristic peak located at 2916 cm−1 is attributed to symmetric and asymmetric stretching and bending vibrations of C-H [[49]]. At the same time, hydroxyl groups were detected on the surface of CuO and CuS/CuO in FTIR, indicating that they are easily wetted by water and have good hydrophilicity, which can be used in the field of wastewater treatment.
EDS characteristics
CuS/CuO/Cu Mesh and CuO/Cu Mesh were reconstructed according to the optimal conditions (oxidation temperature of 25 °C, annealing time of 2 h, and the cycle SILAR of 6.). Meanwhile, wettability, thermal effect and catalytic performance of the modified samples were discussed.
As shown in Fig. 4, the corresponding energy-dispersive X-ray (EDX) elemental mapping indicates the microscopic morphology of the CuS/CuO/Cu mesh and CuO/Cu mesh surfaces as well as the distribution of Cu, O and S elements. From Fig. 4a, it can be seen that the CuO nanorods with rough surface are staggered on top of the copper mesh, and the pores formed by the staggering can allow the liquid to travel freely in the nanoarray, which can better adsorb the pollutants and thus achieve degradation. In contrast, as shown in Fig. 4d, it is obvious that there are nanosheets uniformly attached to the surface of CuO nanorods, and the average thickness of the nanosheets is roughly about 20 nm. The substrate is a copper mesh with the highest and uniform distribution of copper elements on the sample surface, which is not negligible. Moreover, the presence of elemental S in Fig. 4e, which indicates the successful attachment of CuS nanosheets to CuO nanorods, is consistent with the XPS results. In addition, the presence of CuS nanosheets facilitates the refraction of photons inside the nanoarrays and delays the spillover and annihilation of photons.
Graph: Figure 4 a SEM image of CuO/Cu mesh, b-c EDS mapping of CuO/Cu mesh, d SEM image of CuS/CuO/Cu mesh, e–g EDS mapping of CuS/CuO/Cu mesh, h elemental content diagram of CuO/Cu mesh, i elemental content diagram of CuS/CuO/Cu mesh
Meanwhile, the plant-like structure of CuS/CuO has a layered nanostructure and a multi-vacancy surface morphology, which is important for the wetting surface construction and light trapping process, thus enhancing the light absorption of the catalyst and showing better catalytic performance.
XRD and XPS characteristics
The crystal structure was verified X-ray diffraction (XRD), as shown in Fig. 5a. The untreated Cu mesh with diffraction peaks of 43.3°, 50.43° and 74.13° correspond to the (111), (200) and (220) faces of the cubic crystal system of Cu, respectively (JCPDS no. 04–0836). The Cu mesh appears new diffraction peaks of 23.84°, 34.06°, 35.89° and 38.08° after etching, corresponding to the (021), (002), (111) and (022) faces of the orthorhombic crystal system of Cu(OH)2 (JCPDS no. 35–0505). Cu(OH)2 was transformed to monoclinic CuO after annealing with diffraction peaks of 35.69° and 38.88° corresponding to the (002) and (111) facets, respectively. After SILAR, the diffraction peak of 48° corresponds to the (110) face of CuS in the hexagonal crystal system (JCPDS no. 06–0464).
Graph: Figure 5 a XRD patterns of different samples; XPS spectra of b typical survey spectra of CuS/CuO/Cu mesh, c Cu 2p, d S 2p and e O 1 s
In addition, the chemical bonding states of CuS/CuO/Cu mesh were further characterized by XPS analysis, and the results are shown in Fig. 5(b-e). Cu and S elements can be clearly observed in the XPS results, indicating a successful CuS attachment. In the Cu 2P spectra, the two peaks at 932.8 eV and 952.7 eV correspond to Cu0. The two peaks at 934.2 eV and 953.9 eV and the two oscillating satellite peaks at 943.3 eV and 962.8 eV correspond to Cu2+ on the surface [[50]]. For the S 2p spectra, the peaks at 161.3 eV and 162.5 eV correspond to S 2p3/2 and S 2p1/2, respectively, further demonstrating the presence of CuS [[51]]. The broad and weak peak at 168.0 eV is attributed to the sulfate product, which arises from the oxidation of CuS [[52]]. In Fig. 5e, the peaks at 530.3 eV and 532.4 eV originate from the O element of CuO, while the peak at 531.3 eV corresponds to the O element of Cu(OH)2 [[53]]. This proves the successful preparation of CuS/CuO/Cu mesh in terms of the chemical valence of the complexes.
Photothermal characteristics
Effective light absorption and efficient solar thermal conversion are two important criteria for the design of solar thermal collector materials, which are widely accepted [[31]]. In addition, the catalyst converts the absorbed light energy into thermal energy, providing some of the energy needed for the catalytic reaction, which facilitates the chemical reaction process.
To investigate the thermal effect of the sample, we irradiate the sample surface with simulated sunlight at room temperature body condition for 130 s, and the temperature change images and curves are shown in Fig. 6a, b. It is obvious from the temperature change curve that the temperature of the sample increases rapidly and reaches equilibrium at the same light irradiation density (optical power density: 100 mW/cm2). Figure 6d shows that the surface of Cu mesh is relatively smooth and can reflect most of the photons. In contrast, both Cu(OH)2 and CuO nanoarrays reduce photon spillover and have a better thermal absorption effect with a maximum surface temperature of about 40 °C. The surface of CuO nanorods is a little rougher and has small holes and gaps after dehydration, and its photothermal effect is a little better. However, the CuS/CuO nanorod arrays have a plant-like structure with better light absorption and thermal effectivity, and their surface temperature can be increased from room temperature to about 52 °C with about doubling of the temperature rise trend, which is closely related to their unique plant-like nanostructures.
Graph: Figure 6 a Thermal images of different samples under same illumination intensity (100 mW/cm2) for 130 s, b Time-dependent temperature evolution curves of different samples, c Schematic illustration of the multiple reflection effect of exothermal process on CuS nanosheets, d SEM surface image and section image of different samples: (1–2) Cu mesh, (3–4) Cu(OH)2/Cu mesh, (5–6) CuO/Cu mesh, (7–8) CuS/CuO/Cu mesh
As mentioned above, the homogeneous and layered nanometer coaxial nanorod structure of plant-like CuS/CuO is expected to exhibit excellent light absorption and thermal effects. As shown in Fig. 6c, when light shines on the CuS/CuO nanorod array, photons can be reflected and refracted many times between adjacent nanorods and between adjacent layers of nanoblades, and finally absorbed. Thus, the multiple interactions between plant-like structures and photons can greatly enhance the utilization of solar radiation and can provide more adsorption sites, which is important for the photodegradation of pollutants.
Wettability characteristics
The wettability of the catalyst surface not only affects the mass transport of the reactants, but also plays an important role in the photocatalytic reaction process [[13]]. We tested the surface wettability of Cu mesh, Cu(OH)2/Cu mesh in Fig. 2e, CuO/Cu mesh in Fig. 2i and CuS/CuO/Cu mesh in Fig. 2k, respectively. The result is shown in Fig. 7.
Graph: Figure 7 The change in the contact angle of the samples at different periods
Recently, through experimental operation and theoretical research from Jiang's group, a CA of 65° was proved to define nonwetting and wetting [[54]]. Hydrophobicity is seen at 65° < CA < 150°, hydrophilicity is seen at 10° < CA < 65°. In addition, wettability is influenced by surface roughness and chemical properties [[13]]. From the results, it can be seen that the surface of the untreated Cu mesh is smooth and the contact angle value of 129° indicates that the copper mesh is hydrophobic, which is a property of the metal monomers. After etching by alkaline solution, the contact angle of Cu(OH)2/Cu mesh is 56°, which indicates a good affinity for water and a hydrophilic material. Meanwhile, Cu(OH)2 was converted to CuO by annealing and the hydrophilicity of the sample was further enhanced, which was attributed to the effect of the presence of many gaps and pores after the dehydration of the nanorods, and the CuO nanoarrays showed surface roughness and curvature. However, the modified CuS/CuO coaxial nanorods exhibited superior hydrophilicity with a contact angle value of 25°.
This unique layered coaxial structure is similar to that of plants, where the nano-blades can better disperse the force of water droplets with the solid surface, thus allowing the sample to exhibit better hydrophilicity. Therefore, the better hydrophilicity of CuS/CuO/Cu mesh enables it to fully contact with the pollutants in the water body, thus achieving better wastewater purification results.
Photocatalytic characteristics
The photocatalytic activities of CuS/CuO/Cu mesh and CuO/Cu mesh were evaluated by MB and MO dye degradation experiments. As shown in Fig. 8c, f, the light absorption intensity of the organic dyes did not change significantly under dark conditions in both MO and MB solutions with or without catalyst, indicating that the self-degradation of the organic dyes and the adsorption of catalyst can be neglected. On the contrary, light conditions are an important factor affecting the degradation of organic dyes by the catalyst [[55]].
Graph: Figure 8 UV–Vis spectra curve of MB: a CuO/Cu mesh light, b CuS/CuO/Cu mesh light, c CuS/CuO/Cu mesh dark. UV–Vis spectra curve of MO: d CuO/Cu mesh light, e CuS/CuO/Cu mesh light, f CuS/CuO/Cu mesh dark. g-h the degradation efficiency curves of MB and MO, i the catalysis repeatability tests of CuS/CuO/Cu mesh
According to the Beer-Lambert law, there is a positive correlation between concentration and absorption intensity [[56]]. In the dye solution with photocatalyst, the intensity of characteristic absorption peak of organic dyes decreases gradually with time, and the degradation rate shows a nonlinear trend, until a certain time slows down and stabilizes. In Fig. 8g, h, CuO/Cu mesh degradation rates of MB and MO in 180 min were only 88% and 70.5%, respectively. However, CuS/CuO/Cu mesh degradation rates of MB and MO in 180 min were 92.7% and 86%, respectively. After 8 cycles of MB degradation, the CuS/CuO/Cu mesh still maintained high photodegradation efficiency and had good stability. This is attributed to the uniform layered coaxial nanostructure of CuS/CuO/Cu mesh and the effect of heterojunction, which is of great significance to the surface wettability and optical capture process of catalyst, thus further showing better catalytic performance.
For CuS/CuO/Cu mesh, in terms of selection, it degrades organic matter by generating a large number of strong oxidizing groups through light to purify wastewater. In terms of cost, its preparation time and low cost allow large-scale preparation. In terms of degradation effect, unlike powder and biomaterials, the reticulated catalyst is easy to recycle and reuse without causing secondary water pollution. Therefore, CuS/CuO/Cu mesh has good economic benefits, recycling stability and degradation effect, etc.
Photocatalytic mechanism
As shown in Fig. 9, we proposed a reasonable photocatalytic process and electron–hole pair separation mechanism to explain the principle of CuS/CuO composite nanostructure photocatalytic degradation of organic compounds. The reaction process is as follows:
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Graph: Figure 9 Diagram of the photocatalytic mechanism of CuS/CuO/Cu mesh
The photocatalytic reaction process can be divided into three steps: the adsorption process, the photophysical process and the photochemical process. First, the CuS/CuO/Cu mesh and dye solutions were placed in a dark environment for a period of time to ensure the adsorption–desorption equilibrium on the photocatalytic surface. Then, under the irradiation of the light source, the electrons (e−) in the valence band (VB) of CuS and CuO are excited and leap to the conduction band (CB), thus leaving photogenerated holes (h +) in the valence band (VB), and finally the photogenerated carriers move to the surface. Since the CB potential of CuS is more negative than that of CuO, the excitation electrons (e−) on the CB of CuO can be transferred to the CB of CuS and the holes (h+) generated on the VB of CuS are transferred to the VB of CuO driven by the potential, thus successfully separating the photogenerated carriers purpose [[57]]. In addition, the energy band binding of CuS/CuO/Cu mesh can generate a bending electric field, which promotes the jumping of photogenerated carriers and suppresses the interfacial complexation, resulting in better photocatalytic activity relative to other catalysts. Finally, the photogenerated electrons (e−) on the CuS conduction band (CB) react with oxygen molecules (O2) to form superoxide radical anions (·O2−), and the photogenerated holes (h +) on the CuO valence band (VB) react with water molecules (H2O) and hydroxyl ions (OH−) to form hydroxyl radical groups (·OH). Under the action of ·OH and ·O2−, dye is oxidatively degraded to H2O and CO2, which can effectively degrade the organic pollutants in the wastewater. In addition, the CuS/CuO/Cu mesh has significant wastewater treatment potential due to its shorter preparation time, better stability and ease of recycling.
Conclusion
In conclusion, inspired by the leaf structure of plant photosynthesis, we simply grew layered coaxial CuS/CuO nanorods on Cu mesh by in situ oxidation and successive ion layer adsorption and reaction (SILAR) method as a novel easy-to-recycle reticulated catalyst.
The results show that under simulated solar irradiation (100 mW/cm2), CuS/CuO/Cu mesh can reduce photon spillover and annihilation, and the surface temperature can rise rapidly from room temperature to about 52 °C. Better photothermal effect can promote the generation and transition of photogenerated carriers, which is beneficial to the photocatalytic reaction. In addition, the plant-like structure of the nanorod array has better hydrophilicity, and the leaf structure and a large number of pores enable it to better contact with the organic matter in the wastewater, and the degradation rate of 180 min MB and MO can reach 92.7% and 86%. Because of its unique and strong plant-like structure and hydrophilicity, it can maximize the synergistic effect of CuS nanosheets and CuO nanorods to achieve high thermal effect and light absorption. It is worth noting that due to its unique and flexible properties, CuS/CuO/Cu Mesh can be easily recovered after catalysis, maintaining good cyclic stability and photocatalytic performance even after 8 cycles. Compared with other degradable materials such as powders and microorganisms, CuS/CuO/Cu mesh has the advantages of short preparation time, good stability, reusability and no secondary pollution, which can bring greater economic benefits and better wastewater treatment results.
Acknowledgements
The authors gratefully acknowledge the Open Research Foundation of Engineering Research Center of Nano-Geomaterials of Ministry of Education (No. NGM2020KF021), Hubei Key Laboratory of Forensic Science (Hubei University of Police) (2018KFKT05) and Youth Program of National Natural Science Foundation of China (41807201) for the financial support.
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By Chen Wang; Weiran Xu; Chao Xu; Qi Zhang; Zhicheng Zhang; Xueqi Wang; Zhao Fan and Xudong Xiong
Reported by Author; Author; Author; Author; Author; Author; Author; Author
