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ZnO pha tạp Cu được tổng hợp bằng phương pháp solgel kết hợp xử lý nhiệt với hàm lượng Cu pha tạp khác nhau. Vật liệu được ứng dụng làm vật liệu quang xúc tác phân hủy Rhdamine B, các yếu tố ảnh hưởng tới quá trình quang xúc tác, động học và cơ chế quá trình đã được nghiên cứu đầy đủ.
Environmental Science and Pollution Research https://doi.org/10.1007/s11356-021-17106-0 RESEARCH ARTICLE Synthesis, characterization of novel ZnO/CuO nanoparticles, and the applications in photocatalytic performance for rhodamine B dye degradation Thi Thao Truong1 · Truong Tho Pham2,3 · Thi Thuy Trang Truong4 · Tien Duc Pham4 Received: August 2021 / Accepted: 14 October 2021 © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2021 Abstract Photocatalytic deg radation of environmental pollutants is being up to date for the treatment of contaminated water In the present study, ZnO/CuO nanomaterials were successfully fabricated by a simple sol-gel method and investigate the photodegradation of rhodamine B (RhB) The synthesized ZnO/CuO nanoparticles were characterized by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UVVis diffuse reflectance spectroscopy (UV-Vis-DRS), thermal analysis (TGA), surface charge, and Fourier transform infrared spectroscopy (FTIR) The photo-degradation of the dye RhB was followed spectroscopically The overall composition of ZnO/CuO material was found to be wurtzite phase, with particle size of 30 nm, and the Vis light absorption increased with an increase of Cu content The ZnO/CuO nanomaterials were highly active leading to a photo-degradation of 10 ppm RhB reaching 98% within 180 at 0.1 g/L catalyst dosage The change in surface charge after degradation evaluated by ζ potential measurements and the differences in functional vibration group monitored by Fourier transform infrared spectroscopy (FTIR) indicates that the RhB adsorption on the Zn45Cu surface was insignificant And scavenging experiments demonstrate that the RhB degradation by ZnO/CuO nanomaterials involves to some degree hydroxyl radicals Keywords ZnO/CuO · Photocatalyst · Solgel method · Rhodamine B · Degradation mechanism Introduction Responsible Editor: Sami Rtimi * Tien Duc Pham tienduchphn@gmail.com; tienducpham@hus.edu.vn; ducpt@vnu.edu.vn Department of Chemistry, TNU-University of Sciences, Tan Thinh Ward, Thai Nguyen City, Thai Nguyen 250000, Vietnam Laboratory of Magnetism and Magnetic Materials, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Vietnam Faculty of Technology, Van Lang University, Ho Chi Minh City, Vietnam Faculty of Chemistry, University of Science, Vietnam National University, Hanoi, 19 Le Thanh Tong, Hanoi, Hoan Kiem 1000 00, Vietnam Metal oxides are a widely used material for various applications in industrial Among numerous metal oxides, ZnO is known as a multifunctional material due to the potential application in many fields, such as electronics, optoelectronics, sensor, converter, energy generator, and photocatalyst in hydrogen production, event for biomedicine and pro-ecological systems (Kolodziejczak-Radzimska et al 2014) The ZnO is high chemical stability, high thermal and mechanical stability at room temperature, hardness, rigidity, and piezoelectric constant while its hybrid property is low toxicity, biocompatibility, and biodegradability (Ghahramanifard et al 2018) ZnO is also a well-known semiconductor in groups II–VI, whose covalence is on the boundary between ionic and covalent semiconductors with a broad energy band of 3.37 eV (Chou et al 2017) and large exciton binding energy (Shashanka et al 2020) One advantage is that ZnO has quantum yield and easily controlled synthesis processes (Singh and Soni 2020) Furthermore, the structural, morphology, optical, and electrical properties of the nanoscaled ZnO can also be easily modified or improved for many applications (Belkhaoui 13 Vol.:(0123456789) et al 2019) Therefore, an extensive study investigated the ZnO as a potential photocatalytic degradation of various both organic and inorganic pollutants such as ionic dyes, antibiotics, pesticides, peptis, and heavy metal ion (Boon et al 2018; Pirhashemi et al 2018; Raizada et al 2019) A high number of studies using ZnO for photocatalysis is increasing every year to approximately 2400 at 2019 (Frederichi et al 2021) Nevertheless, the photocatalyst of ZnO has several limitations For example, the absorption in the visible (Vis) region is less than 5% (Yu et al 2019) and the high recombination rate of photo-induced charge carriers against the movement of electron and hole to material surface reacts (Kumari et al 2020) To improve this feature, ZnO was combined with many metal oxides to form effective materials, such as Cu2O (Mamba et al 2018; Yu et al 2019), WO3, NiO, CoFe2O4, Au, Pt/Ga, Sr–Au, graphene, Mn, Co, Ce, Nd, Gd (Koe et al 2019; Zhai and Huang 2016), Sn (Venkatesh et al 2020), and Ag (Ramasamy et al 2021) Among these composite materials, a mixed oxide system of ZnO and Cu has attracted many researchers (Huo et al 2019; Jiang et al 2019; Maleki et al 2015; Vaiano et al 2018) That is because CuO is a low-cost metal oxide with a narrow bandgap (1.2–2.1 eV) (Singh and Soni 2020) Nevertheless, CuO shows a low photocatalytic performance (Pirhashemi et al 2018) because of the high recombination rate of charge carriers in the CuO system Therefore, the integration of ZnO and CuO could decrease the rate of recombination of photogenerated carriers (Sahu et al 2020) Moreover, S Rtimi indicated that the signal for the iso-energetic charge transfer among Cu2O and ZnO and the electrostatic interaction between p-type Cu2O and ZnO accelerated the electron migration to the ZnO n-type semiconductor (Mamba et al 2018), enhanced the response to Vis light, and increased the photocatalytic performance under the sunlight Many methods are studied to fabricate the ZnO/CuO materials known as metallurgical process, mechanochemical process, or chemical processes as precipitation, solgel, solvothermal, and hydrothermal methods, using an emulsion or microemulsion, or growing from a gas phase, pyrolysis spray, sonochemical method, or synthesis using microwave Among them, the sol-gel method allows the using of a wide selection of solvents, surfactants, and heat treatment, making it easier to control the particle size and shape For instance, the hollow microsphere with a diameter of approximately μm composed of uniform nanoparticles with a diameter of approximately 20 nm was observed in the work of Chen et al (2020); the quasi-sphere shape with uniform morphologies has been reported (Acedo-Mendoza et al 2020); the hybrid nanocomposite in which CuO nanoparticle is attached to the ZnO–T tetrapod surface (Sharma et al 2020), ZnO nanorod (Patil et al 2019), nanoflower-like structure (Mardikar et al 2020), nanowires (Chou et al 2017), and thin film (Asikuzun et al 2018) was investigated The composite of ZnO and CuO was studied with various ratio of Zn/Cu:Cu content with very 13 Environmental Science and Pollution Research low Zn/Cu ratio in the range 1000/3÷1000/9 (Singh and Soni 2020), with the mass ratios as 99.9/0.1, 98.0/2.0, and 95.0/5.0 (Ruan et al 2020), or different percentages of Cu in the catalyst of 1, 3, 5, and 10% (Harish et al 2017), event component from 100% Zn to 100% Cu (Lavín et al 2019) The previous work indicated that the ZnO/CuO composite showed a better photocatalytic efficiency than the ZnO Nevertheless, to the best of our knowledge, the optimum composition of CuO and the influence of Cu content have not been reported Kavita Sahu et al (Sahu et al 2020) founded that the formation of p-n 2D CuO–ZnO hybrid nanoheterojunctions enhanced the photogenerated charge carrier separation, so that they exhibited excellent photocatalytic decomposition for methylene blue (MB), methylene orange (MO), and nitrophenol (4-NP) under sunlight radiation Ruan et al (2020) indicated that the ZnO/CuO n-n heterojunction photocatalysts, electron on the conduction band (CB) of ZnO, move to the valence band (VB) of CuO by the electrostatic attraction, and form electrons in the CB of CuO and holes in the VB of ZnO, respectively Thus, the recombination of the electrons and hole pairs was reduced on the surface of ZnO, and the photocatalytic activity of ZnO/CuO for Acid Orange under the solar light was improved compared to pure ZnO Rhodamine B (RhB) is a well-known fluorescent cationic dye in organic chemistry and biological studies RhB is usually used as a colorant in many industries such as the plastic, textile, or paint, or illegally used for coloring different confectionery by sweet markets or bakers RhB is soluble in water, and stable with light, temperature, chemicals, or microbes However, RhB is an extremely toxic pollutant for water environment that strongly affects humans and organisms (Yen Doan et al 2020) RhB has no deadly effects on the ecosystem as pesticides (Rani et al 2021) but in the body, RhB can cause oxidative stress, injury, increase in cell apoptosis, and brainstem (Sulistina and Martini 2020) In this study, for the first time, we synthesized the series of ZnO/CuO nanomaterials with different ratios by sol-gel method The characterization of the materials was systematically examined by different physicochemical methods including X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS), thermal analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and surface charge evaluation Their photocatalytic performance for RhB degradation under the solar light was thoroughly investigated The mechanisms for RhB removal using ZnO/CuO were also studied based on the presence of different radicals as well as the changes in charging behavior and surface functional group after RhB degradation Environmental Science and Pollution Research Experimental Materials Oxalic acid (H2C2O4) (purity ≥ 99.5%), cupric nitrate trihydrate (Cu(NO3)2·3H2O) (purity = 99.0–101.0%), zinc acetate dihydrate (Zn(CH3COO)2·2H2O) (purity ≥ 99.0%), and ethanol (C2H5OH) (96%, for HPLC), rhodamine B (≥ 97%, for HPLC), n-butanol (n-C4H9OH) (purity ≥ 99.5%), diammonium oxalate monohydrate ((NH4)2C2O4·H2O) (purity ≥ 99%), and silver nitrate ( AgNO3) (purity ≥ 99.8%) were purchased from Sigma-Aldrich, India, to be used without further purification Deionized water was used throughout the whole study Synthesis of ZnO/CuO nanoparticles ZnO/CuO nanoparticles were prepared by sol-gel method (Kolodziejczak-Radzimska et al 2014; Siddiqui et al 2018) Firstly, zinc acetate dihydrate and cupric nitrate trihydrate were dissolved in ethanol at 60°C for 30 to 60 min, leading to the formation of a clear and homogeneous solution The Zn:Cu molar ratios were 45:1; 30:1; 20:1; 15:1; and 10:1 (labeled as Z nxCu) Secondly, a solution of oxalic acid in ethanol was added dropwise into the initial solution under vigorous stirring at 60°C and maintained for h to obtain the milky white suspension gel form The mole ratio of oxalic acid:total metal ions was 1:1 The gel formed was dried at 80°C for 36 to 48 h to completely dry (xerogel) The xerogel was subjected to thermal analysis to determine the sample annealing temperature After that, the xerogel was finely ground and annealed at 450°C in air for h, heating rate 200 °C/h Characterization After heat treatment, crystal structure and phase composition of obtained products were studied by XRD using a X-ray diffractometer (Max 18XCE, Japan) with Cu Kα radiation (λ = 0.154056 nm) at a scan rate of 0.02° s −1 in the 2θ range from 20 to 80° The morphology and particle size of the materials were evaluated by SEM (Leo 1430VP) and TEM (Jeol JEM 2100F microscope) at an accelerating voltage of 200 kV Elemental analysis of samples was examined using a JSM-7900F SEM attached with EDS Optical absorption, reflectance, and bandgap properties were analyzed using UV-Vis diffuse reflectance spectroscopy (DRS) by a Scinco 4100 instrument The surface charge before and after interaction with RhB was determined by zeta potential using Zetasizer Nano ZS (Malvern, England) The zeta (ζ) potential was calculated from electrophoretic mobility with Smoluchowski’s equation Fourier transform infrared (FTIR) spectroscopy was used to evaluate the change of vibration functional surface groups The FTIR spectra were conducted on JASCO, Japan (FT/IR-4600 type A), using TGS detector with a solution of cm−1 The wavenumber was recorded from 400 to 4000 cm−1 Evaluation of photocatalytic activity The photocatalytic performance of materials was investigated in aqueous RhB solution using natural sunlight (on a sunny day, between 9:00 am and 15:00 pm), at different concentrations of RhB from 10 to 50 ppm with the range of photocatalyst dosage of 0.1–0.5 g/L In the photodegradation, different amounts of catalyst were dispersed in 250-mL RhB solutions Before sunlight irradiation, the suspensions were agitated using magnetic stirrer in the dark for 60 to achieve the contacted RhB molecules with the photocatalyst About mL RhB solution was withdrawn, centrifuged, and filtered, and the solution was collected to determine the RhB concentrations by using a UV-Visible spectrophotometer (UV-1700 Pharma Spec, Shimadzu, Kyoto, Japan) The relative RhB concentration (C/C0) was determined using the relative absorbance (A/A0) at a wavelength of 554 nm (Jiang et al 2019), where A0 and A were the absorbances of RhB solutions at the lighting start time (t0) and at any time t, respectively The photocatalytic efficiency was calculated using Eq (1) (Venkatesh et al 2020): H (%) = A − At Co − Ct 100% = o 100% Co Ao (1) where H is the photocatalytic efficiency, C0 is the initial concentration, and Ct is the concentration of RhB after illumination t For cycle tests, the used photocatalyst was washed several times with ethanol and deionized water and dried at 80°C for 12h after each run For the scavenger tests, di–ammonium oxalate monohydrate (AO) (Sakib et al 2019), tert–butanol (BuOH) (Raja et al 2019), and silver nitrate ( AgNO3) (Osotsi et al 2018), were used as h+, ·OH, and e− reactive species, respectively Results and discussion The TGA/DTA of Z n10Cu and Z nCu0 xerogels have been done and shown in Fig. 1 The TGA-DTA curves of two samples are found to be similar with two weight-loss segments, corresponding to the endothermic process The weight-loss segment between 30 and 200 °C, containing three endothermic processes at 81.21, 125.15, and 162.27°C (ZnCu0 sample) and 83.34, 143.73, and 169.41°C ( Zn10Cu sample), is about 52.275% and 20.77% for Z nCu and Zn10Cu, respectively This effect can be attributed to the removal of water and residual solvent (Mel’nik et al 2006), the decomposition of non-carbonized anion N O3−, and other nitrogen-containing molecules (Xu et al 2009) The second weight-loss segment was located between 366–427°C and 13 Environmental Science and Pollution Research 100 (a) 10 TG-ZnO 100 80 80 -2 -5 60 DTA-Zn10Cu -10 TG (%) 60 40 DTA (µV/mg) DTA-ZnO TG (%) (b) TG-Zn10Cu 40 -15 -20 20 20 -4 200 400 o 600 Temperature ( C) 800 -25 -30 200 400 o 800 600 Temperature ( C) Fig. 1 Schematic of thermal analysis of Z nCu0 (a) and Z n10Cu (b) xerogels 13 Z-(103) Z-(110) Z-(102) C-(111) Z-(100) Z-(002) Z-(101) Z-(200) Z-(112) Z-(201) ZnO-(101) ZnO: Z-(hkl) CuO: C-(hkl) Intensity (a.u) 312–394°C for Z nCu0 and Z n10Cu xerogel, respectively The weight loss in this segment is about 22.49% for Z nCu0 and 39.96% for Zn10Cu, and it may concern to several mechanisms: the volatilization of excess oxalic acid (the boiling point 365.1±25.0 °C), the decomposition of the gel network, or a combustions of organic materials A decrease in the weight is insignificant and thermal effect is observed in the temperature range after those segment, indicating that the calcination temperatures at above 450°C is the crystallization process Therefore, we have decided to treat all samples at 450°C Characterization of ZnO/CuO materials The crystal structures of ZnO/CuO nanoparticles were analyzed by XRD method The diffraction patterns of ZnO/ CuO nanoparticles with different Zn/Cu ratio are shown in Fig. 2 In the ZnO sample, there are a total of nine diffraction peaks in the 2θ ranging from 25 to 70° These peaks locate at 31.76°, 34.61°, 36.29°, 47.56°, 56.98°, 62.84°, 66.20°, 68.00°, and 69.08° and match well with the PDF card (JCPDS No.36-1451) of the Wurtzite structure of ZnO (Shukla and Shukla 2018) The Miller indices of ZnO of the Wurtzite structure are denoted in Fig. 2 There are not any peaks concerning the impurity phase which can be observed within the XRD detection limit In other samples, the change in the Zn/Cu ratio influences the shifting and rising a new peak in the XRD patterns but the Wurtzite structure remains unchanged By increasing the Cu content, the diffraction patterns of ZnO/CuO nanoparticles tend to shift toward a high angle, meaning a shrinkable of the volume of unit cell, as Zn10Cu Zn15Cu Zn20Cu Zn30Cu Zn45Cu ZnO 30 40 50 60 2θ (degree) 70 80 Fig. 2 XRD pattern of synthesized ZnO/CuO materials The inset show the diffraction profile of ZnO-(101) seen in the inset of Fig. 2 This observation is highly contrary to what was observed in the work of Lu et al (2017), where the lattice parameters of ZnO/CuO nanocomposites had not varied with changing the Zn/Cu ratio Ping et al explained this phenomenon by considering a similar ionic radius of Zn2+ (0.074 nm) and Cu1+ (0.074 nm) ions (supported by XPS measurement) It is worth noting that Cu has doped at the Zn site of wurtzite structure of ZnO to form Environmental Science and Pollution Research the tetrahedral coordination of Zn/Cu surrounded by four oxygens In general, a partial substitution of Cu at the Zn site can vary the oxidation state of Cu ions in either C u2+ 1+ (0.071nm) or C u (0.074 nm) depending on the synthesis conditions (Lu et al 2017; Rooydell et al 2017) The different oxidation state of Cu ions is a crucial reason for changing the lattice parameters (the shift of XRD pattern) of ZnO/CuO nanoparticles Therefore, the shift of the XRD patterns observed in Fig. 2 is mainly due to the incorporation of Cu2+ into the ZnO lattice This also suggests that oxygen vacancies not play a crucial role on the optical properties of our samples Our observation is consistent with the previous reports (Rooydell et al 2017) Furthermore, an increase in the Cu content also enhances intensity of the diffraction peak at 38.56° This peak belongs to the main intense peak (111) of the monoclinic structure of CuO (JCPDS card no 45-0937) The appearance of this peak in the ZnO/CuO nanoparticles approves that the Cu doping on the ZnO is not only incorporation inside the ZnO lattice to construct the Z n1−xCuxO compounds but also buildup of CuO lattice for forming the Zn1−xCuxO/CuO nanocomposites Obviously, an increase in Cu doping concentration prefers to form the Z n1−xCuxO/CuO nanocomposites as evidence from an enhanced intensity of the (111) peak of CuO and an unchanged peak position of (101) of ZnO in the Z15Cu and Z10Cu samples The crystallite sizes calculated from Sherrer’s equation indicates that the ZnO crystallite sizes of the Zn0Cu, Zn45Cu, Zn30Cu, Zn20Cu, Zn15Cu, and Zn10Cu system are about 28.0, 25.2, 23.6, 18.0, 27.7, and 29.0 nm, respectively (Zn10Cu) (Zn30Cu) Elemen t OK Cu L Zn L Totals Weight (%) 18.89 06.47 74.64 100.00 Atomic (%) 48.70 04.20 47.10 Element Weight (%) 22.71 2.25 75.04 100.00 Atomic (%) 54.53 1.36 44.10 OK Cu K Zn K Totals The presence of Cu in materials was confirmed by the EDX spectra (Fig. 3) The sharp peaks of Zn, Cu, and O were obtained; no other peaks related to any other element were detected in the spectrum within the detection limit The calculated Zn/Cu ratio from the EDX spectrum of Zn10Cu is about 47.1/4.2 (11.2/1.0), which is quite close to the design value The other samples also show a matching between theoretical and calculated Zn/Cu ratio values, which are 15.57/1.00, 32.4/1.00, and 47/1 for the Zn15Cu, Zn30Cu, and Zn45Cu samples, respectively The morphologies of some ZnxCu materials are presented by SEM and TEM images in Figs. 4 The ZnO and system of CuO–ZnO are uniformly spherical The SEM images on the larger scale (μm) show that the ZnO particles are aggregated but the aggregation of CuO–ZnO did not occur All the materials with different Zn/Cu ratios were in the size of 27 ± nm The optical nature of synthesized materials was analyzed through the UV-Vis diffused reflectance spectra technique, corresponding to the results in Fig. 5 The optical spectrum of pure ZnO exhibits with strong absorption spectra in range of 200–400 nm, and the sharp absorption edge around 400 nm The characteristic edge of ZnO was observable in the ZnxCu material, and the band-gap energy (Eg) of ZnO and the ZnxCu (x = 45, 30, 20, 15, 10) system was calculated from the UV-visible absorption spectra of ZnO by a Tauc plot (Senasu et al 2020; Souza et al 2017) (Fig. 6b) The Eg values decreased with increasing Cu content and found to be 3.07, 3.05, 2.99, 2.92, 2.84, and 2.62 eV (Zn15Cu) Element OK Cu K Zn K Totals (Zn45Cu) Element OK Cu K Zn K Totals Weight (%) 17.32 4.92 77.76 100.00 Atomic (%) 46.08 3.29 50.62 Weight (%) 17.08 1.67 81.25 100.00 Atomic (%) 45.69 1.13 53.19 Fig. 3 EDX spectra of different Z nxCu materials 13 Environmental Science and Pollution Research Fig. 4 A. SEM images of a Zn15Cu, b Zn30Cu, and c Zn45Cu; B TEM images of a ZnO and b Z n45Cu Fig. 5 UV-Visible DRS spectra of pure and Cu doped ZnO nanoparticles 1.0 0.8 ZnO Zn45Cu (a) (b) Zn10Cu Zn30Cu Zn20Cu 0.8 Zn15Cu 0.6 Zn10Cu Zn20Cu Abs Abs (a.u) 0.6 Zn15Cu Zn30Cu 0.4 Zn45Cu 0.4 ZnO 0.2 0.2 300 13 400 500 Wavelength (nm) 600 2.6 2.7 2.8 2.9 3.0 Eg (eV) 3.1 3.2 3.3 Environmental Science and Pollution Research 1.4 1.2 Abs 1.0 0.8 100 (a) Initial RhB 20 ppm Ads 60 in the dark = Lighting Lighting 30 Lighting 60 Lighting 90 Lighting 120 Lighting 180 (b) Zn10Cu Zn15Cu 80 Photocatalytic efficiency (%) 1.6 0.6 0.4 Zn20Cu Zn45Cu 60 40 20 0.2 0.0 460 480 500 520 540 560 580 600 20 40 60 Wavelength (nm) 80 100 120 140 160 180 Time (min) Fig. 6 a Absorption spectra of RhB as a function of irradiation time after the photocatalytic degradation using 0.1 g/L Zn45Cu exposed to the sun light b Photocatalic efficiency decompose 20 ppm RhB under sunlight by 0.1 g/L synthetic materials coupled CuO/ZnO nanocomposite shifted the band gap energy into the visible light region Photocatalytic activity To apply the ZnO/CuO materials as photocatalysts in natural environment, we investigated their photocatalytic activities for the RhB degradation in a solution with a pH of approximately under solar light The catalyst dosage of 0.1 g/L was fixed to remove 20 ppm RhB from aqueous solution The results are shown in Fig. 6 Figure 6 a shows the UV-Vis spectra of the RhB aqueous solute taken out at different reaction times during the photodecomposition process using the Zn45Cu material As can be seen, the maximum Abs of 20 ppm RhB solution at 554 nm before and after presence of Zn45Cu placed in the dark only slightly decreased (abs from 1.50 down 1.44, corresponding to 4%) When irradiation time increased, the 100 100 80 80 60 40 (a): 0.1 g/L Zn45Cu 10 ppm RhB 20 ppm RhB 30 ppm RhB 50 ppm RhB 20 0 100 200 300 400 Time (min) 500 600 700 Photocatalytic efficiency (%) Photocatalytic efficiency (%) Furthermore, all ZnO/CuO materials have a high value of Abs in the Vis region compared to pristine ZnO: the abs for ZnO is 0.2 while abs for ZnxCu samples (x = 45, 30, 20, 15, 10) are about 0.35 to 0.7; the abs for ZnxCu samples in the region below 370 nm is also sharply reduced compared to pristine ZnO This may be related to their morphologies, particle size, and surface nanostructures, improving the crystallinity and reducing the defects (Ungula et al 2017); another reason may be due to the strong sp-d exchange interaction between the band electrons of ZnO and the localized electrons of C u2+ ions substituting for the Z n2+ ions (Ramya et al 2018) or the substitution of Cu ions in the ZnO lattice (Kama rulzaman et al., 2016) or a separating phase between ZnO and CuO These results confirm the formation of CuOloaded ZnO hierarchical structures Also, the formation of (b) 20 ppm RhB 0.05 g/L Zn45Cu 0.1 g/L Zn45Cu 0.2 g/L Zn45Cu 0.5 g/L Zn45Cu 60 40 20 0 50 100 150 Time (min) Fig. 7 Photodegradation efficiency of RhB under sunlight at different initial concentrations of RhB and Zn45Cu 13 Environmental Science and Pollution Research maximum absorbance decreased gradually After 240 upon sunlight irradiation, RhB degradation reached to 82% Figure 6b indicates that Cu content was in the sample, and the light absorbance in the visible region of ZnO/CuO systems increases while the photocatalytic efficiency of RhB degradation by the material did not increase accordingly The ZnO/CuO system with ratio of Zn/Cu = 45 shows the highest photocatalytic efficiency compared to other Zn/ Cu ratio samples, followed by Z n10Cu; the efficiencies of Zn15Cu, Zn20Cu and Zn30Cu were insignificant The influence of doped Cu content to the photocatalytic performance of ZnO/CuO materials did not follow a rule in the previously published papers (Acedo-Mendoza et al 2020; Harish et al 2017; Kumari et al 2020) Maybe, in the region of Zn/Cu atom ratio = 10÷20, the excess amount of Cu cannot be incorporated in the ZnO host lattice sites, CuO was segregated from the ZnO crystal lattice leading to the new phase, and the photocatalytic behavior in the visible light of ZnO/ CuO system is mainly due to CuO activity, so the higher the Cu content, the higher the photocatalytic efficiency is However, in the region of Zn/Cu atom ratio = 20÷45, that is, the lower the Cu content, the Cu ion readily penetrates the ZnO crystal lattice during phase information, causing some structural deviations, and enhance the RhB decomposition photocatalytic efficiency of the ZnO/CuO system; the photocatalytic behavior in the visible light of CuO/ZnO system may be due to the combined action between CuO and ZnO, or the interaction between ZnO and CuO Therefore, the optimum content of Cu in Zn45Cu is the important factor to affect the photocatalytic activity of the coupled ZnO/CuO photocatalyst The Zn45Cu sample will be used in the next studies Photodegradation efficiency of RhB under sunlight at different 1.8 0.05 g/L; y = - 0.05704 + 0.00214x; R2 = 0.95307 0.1 g/L; y = - 0.16141 + 0.00486x; R2 = 0.97397 0.2 g/L; y = - 0.41004 + 0.00815x; R2 = 0.97895 0.5 g/L; y = - 0.02257 + 0.00604x; R2 = 0.98899 1.6 1.4 (b) RhB 10 ppm; R2 = 0.96722 y = - 0.40518 + 0.01127x; RhB 20 ppm; R2 = 0.97397 y = - 0.16141 + 0.00486x; RhB 30 ppm; R2 = 0.96091 y = - 0.44227 + 0.00632x; RhB 50 ppm; R2 = 0.99423 y = - 0.1032 + 0.00138x; 2.0 (a) 1.2 1.5 1.0 Log(Co/C) Log (Co/C) initial concentrations of the Z n45Cu catalyst and RhB is shown in Fig. 7 Figure 7 shows that the RhB degradation efficiency using 0.1 g/L Zn45Cu under the solar light decreased significantly when increasing initial RhB dye concentrations from 10 to 50 mg/L The RhB degradation efficiencies after 180 with 10, 20, 30, and 50 ppm decreased to about 98, 82, 73, and 21% respectively Furthermore, RhB degradation efficiencies gradually increased after 180 with increasing the catalyst from 0.05 to 0.5 g/L It implies that the reaction rate depended on both the initial concentrations of Zn45Cu and RhB To understand this dependency, we used the pseudo-first-order reaction to describe the ln(C/Co) against the time (Fig. 8) As can be seen in Fig. 8, all correlation coefficient (R2) values were higher than 0.9, demonstrating that the RhB degradation behavior using Z n45Cu catalyst was in accordance with the pseudo-first-order kinetic The stability of the Zn45Cu nanoparticles was evaluated by catalytic degradation of RhB recycles The material was recovered and reused three cycles After photocatalytic experiments, the catalyst was taken out from the reaction vessel by centrifugation, rinsed with ethanol and deionized water, before drying in the oven at 80°C for 12h The RhB degradation efficiencies after the regenerations are shown in Fig. 9 As can be seen, the RhB degradation efficiencies at all times decreased insignificantly It means that the photocatalytic activity of Zn45Cu nanoparticles is relatively stable It should be noted that RhB removal in the presence of 0.1 g/L synthesized materials for 60 without the light was only 4% (lighting in Fig. 6a), suggesting the negligible RhB adsorption on Zn45Cu nanoparticles Similar experiments were also carried out with all Z nnCu materials We found that RhB concentrations were only reduced below 6% 0.8 0.6 0.4 1.0 0.5 0.2 0.0 0.0 50 100 150 Time (min) 200 250 100 200 300 400 500 600 700 Time (min) Fig. 8 Kinetic study of RhB photodegradation process with Zn45Cu catalyst under the sunlight a 0.1 g/L Zn45Cu and different initial RhB concentrations b 20 ppm RhB and differrent Zn45Cu concentrations 13 Environmental Science and Pollution Research Photocatalytic effficiency (%) 100 (a) Run (1) Run (2) Run (3) 80 60 40 20 20 40 60 80 100 120 140 160 180 Time (min) Fig. 9 The reusability of Z n45Cu for RhB degradation after 60 without the light It again implies that all the synthesized ZnnCu material has very low RhB adsorption effectiveness To confirm the effect of adsorption process, we evaluate the changes in surface functional group by FTIR spectra (Fig. 10) and surface charge change by zeta potential of the material before and after RhB degradation Figure 10 shows that FTIR spectra of Zn45Cu present a peak around 450 cm−1 and 750 cm−1, which are generally assigned to the stretching vibration of Zn–O and Cu–O bonds (Andrade et al 2017); the broad peak at about 3500 cm−1 was assigned for the –OH group In addition, two small peaks at around 2900 c m−1 and some peaks at around 1000 cm−1 show the C–H bonds (Kadam et al 2018) and the peaks at 1400 c m−1 and 1500 c m−1 correspond to the banding of C–H bonds (Manohar et al 2020) All these bands were presented on the FTIR spectra of Z n45Cu after RhB adsorption and degradation process, and the intensity of peaks slightly decreased compared with the FTIR spectra of RhB Furthermore, the FTIR spectra of Z n45Cu after interaction with RhB only appeared a new peak at 1638 cm−1 It can be seen that the 1649 cm−1 peak of RhB shifted to shorter wavenumber, while other characteristic peaks of RhB were not observed The results of FTIR spectra indicate that the RhB adsorption on the Zn45Cu surface was insignificant The results of zeta potential of Zn45Cu sample at pH and were found to be + 14.6 and + 16 mV, respectively Nevertheless, after interaction process with RhB at neutral media, the zeta potential of Z n45Cu sample was found to be + 19 mV Since RhB is a cation dye, if the RhB adsorption occurred on the surface of material, the zeta potential would increase significantly However, in our case, the zeta potential of Z n45Cu sample before and after degradation changed slightly It implies that the adsorption of RhB onto the surface of material was negligible In other words, the RhB removal from aqueous solution was mainly by photocatalytic mechanism The photocatalytic reaction mainly occurred due to the presence of the active species of electrons ( e−), holes ( h+), superoxide radical anions ( O·− ), hydroxiperoxyl radical ) · ( HO2 , and hydroxyl radicals (·OH) (Kumaresan et al 2020) Among them, hydroxyl radicals (·OH) are most active (Anitha and Muthukumaran 2020; Lavín et al 2019) To study the photodegradation mechanism, AO, AgNO3, and BuOH were conducted during photoreaction respectively The result is shown in Fig. 11 The presence of AO and Ag+ in the reaction system (decomposition of 10 ppm RhB solution by 0.1 g/L Z n45Cu under the sunlight at room temperature) at the first 90 leads to significantly increased RhB decomposition compared to the reaction system without them, and slightly increased from 100 to 180 (compared to the reaction system without them) These results revealed that the loss 100 Zn45Cu+RhB Photocatalytic efficiency (%) Transmition (%) Zn45Cu RhB 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) 80 (b) 60 Zn45Cu 0.1 g/L RhB 10 ppm Zn45Cu 0.1 g/L RhB 10 ppm + AgNO3 Zn45Cu 0.1 g/L RhB 10 ppm + AO Zn45Cu 0.1 g/L RhB 10 ppm + BuOH 40 20 20 40 60 80 100 120 140 160 180 Time (min) Fig. 10 FTIR spectra of RhB and Zn45Cu sample before and after adsorption Fig. 11 Effect of different scavenger on photodegradation process 13 Environmental Science and Pollution Research of electron or hole or the present of Ag could accelerate photodegradation process We all know that Ag absorbs light at about 420 nm, and Ag has been also adsorpted on the surface of material (ZnO–CuO); therefore, it accelerates photodegradation process Only the presence of n-butanol in the reaction system immediately significantly reduced the decomposition efficiency of RhB at all times; after 180 min, the efficiency was only about 35% We suggest that hydroxyl radicals are mainly activated species involved for RhB photocatalytic activity of ZnO/CuO system (Zn45Cu) This result was also confirmed by the influence of the reaction medium on the photocatalytic efficiency The photocatalytic degradation of 20 ppm RhB by 0.1 g/L Zn10Cu was carried out at pH 3, 7, and 10 (Fig. 12) The 20 ppm RhB removal efficiency in different media is very obvious, at 120 is 30%, 53%, and 92% respectively for pH 3, 7, and 10 At pH 10, the effect was almost maximum after only 90 According to Anitha et al and Lavín et al (Anitha and Muthukumaran 2020; Lavín et al 2019), the photocatalysis process takes place according to the following reactions: 2O2 + 2e− → O2 ⋅− (2) 2H+ + 2O2 ⋅− → 2HO⋅ (3) 2HO⋅ → O2 + H2 O2 (4) H2 O2 + 2e− → OH⋅ + OH− (5) 1∕2 O2 + H2 O + 2e → OH⋅ + OH− (6) h+ + H2 O → H+ + OH⋅ (7) OH⋅ + RhB → degradation products (8) 100 Photocatalytic efficiency (%) Zn10Cu 0.1 g/L, RhB 20 ppm pH pH pH 10 80 60 40 20 0 20 40 60 80 100 120 140 Time (min) Fig. 12 Effect of pH on photocatalytic degradation of RhB 13 160 180 In the acidic environment, it is favorable for the reactions from (2) to (6) and (8) and in the alkaline medium, it is favorable for the Reactions (7) and (8) And RhB could exist between two forms in acidic and alkaline media as shown in the previously published paper (Birtalan et al 2011) At low pH, RhB exists in cationic form, the material surface is also positively charged, and the electrostatic repulsion makes it difficult for them to come close for a reaction occur And the presence of A g+ (e− scavenger) did not reduce photocatalytic efficiency; it means that Reactions (2), (5), and (6) which occur insignificantly lead to (8) reaction which is weak In alkaline medium, RhB exists in neutral form, and the electrostatic repulsion makes it easier for them to transfer to the material surface for reaction to occur; additionally, the RhB molecular structure has bond angles below 90°, which are unstable and easy to decompose So, the reaction in alkaline medium occurs more easily The band gap energies (Eg) of ZnO and CuO are reported to be about 3.23 and 1.4 eV, respectively, whereas the electron affinity (χ) is 4.35 and 4.07 eV, respectively (Harish et al 2017) During sunlight irradiation, electrons in the valence band (e−VB) of CuO and ZnO were excited (e* VB), and jump into the conduction band (CB), leaving holes in the VB of CuO However, this energy is not enough for e*VB of ZnO to pass the Eg = 3.23 eV to jump into the CB to generate electrons and holes at the CB and the VB, but e * VB of CuO can induce it As the above discussion, when the Cu content in the ZnO/CuO system is very low, the Cu atom can penetrate into the ZnO crystal structure, causing structural deviation, giving up the VB overlap (VBO) between ZnO and CuO rather than bandwidth changes (Liu et al 2008) Thus, it leads to increase in Eg of the ZnO/CuO system falling below 1.4 eV Then, initial e * VB of ZnO migrated on the VBO, and easily moved to the CBO of the system The electrons and holes were generated in both the VB and CB of CuO and ZnO At the same time, the overlap makes the holes and electron migrate from CuO to ZnO and vice versa that increases photocatalytic capacity of Zn45Cu These e* VB react with dissolved oxygen molecules, and form super oxide radical anion ( O2·−), which further indirectly turn into highly reactive hydroxide radicals (OH·) Moreover, the holes in the valence band of CuO which can react with OH− ion form highly reactive hydroxyl radicals Hydroxide radicals react strongly with oxidants, and generate either photogenerated electrons or holes which finally oxidize the RhB molecules, or hydroxide radicals oxidize directly with RhB ZnO/CuO system may be a favorable p–n junction which helps the separation of generated electron-hole pairs under visible light irradiation (Harish et al 2017) Based on the above detailed discussion, we can suggest the mechanism of the photocatalytic process of RhB by Zn45Cu 450°C under the sunlight follows the reactions: Environmental Science and Pollution Research CuO∕ZnO + hv → CuO∕ZnO (e∗ (VB)) ↔ CuO∕ZnO((e∗ (VBO) ) (9) ( ∗ (( + ) ) − CuO∕ZnO (e (VBO)) → CuO∕ZnO h (VBO + e (CBO) (10) ) ( ) ) (( CuO∕ZnO h+ (VBO) + e− ( CBO) ↔ CuO∕ZnO h+ (VB) + CuO∕ZnO(e− (CB ) (11) e− + O2 → O2 ⋅− (inactive) (2) h+ + H2 O → H+ + OH⋅ (7) OH⋅ + RhB → degradation products (8) Conclusions We have investigated the hybrid photocatalytic ZnO/CuO nanomaterials for RhB degradation The materials based on were successfully fabricated by sol-gel method and characterized by XRD, EDX, SEM, TEM, UV-Vis-DRS, FTIR, and zeta potential Cu was doped into ZnO in both ways: Cu replaced Zn site in wurtzite structure of ZnO to form the Zn1−xCuxO structure and build up CuO lattice for forming the Z n1−xCuxO/CuO nanocomposites as the Cu content increases; the ZnO/CuO nanomaterials were in sphere shape, the average size is about 30 nm, and the bandgap energy decreased with the increase in Cu content The Zn45Cu was the best photocatalyst for the RhB degradation under the solar light; the RhB degradation efficiencies gradually increased with increasing the catalyst dose and decreased significantly when increasing initial RhB dye; the photocatalytic activity of Zn45Cu nanoparticles is relatively stable 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