Microwave-synthesized ZrO2/ZnO heterostructures: fast and high charge separation solar catalysts for dyes-waste degradation

In 60 min, microwave-synthesized ZrO2/ZnO heterostructures exhibited high and fast sunlight photodegradation efficiencies for 50 ppm Congo red (CR) and 50 ppm methylene blue (MB) pollutants. ZrO2/ZnO heterostructures were characterized by XRD, SEM, EDX, FTIR, and diffuse reflectance (DR) techniques. The XRD analysis showed that these heterostructures have combined components of tetragonal ZrO2 and hexagonal ZnO phases. The SEM micrographs of all ZrO2/ZnO nanocomposites demonstrate the formation of nanospherical particles (major) and rod-like (minor) structures. The EDX spectra verified the presence of Zr, Zn and O elements with percentage ratios equivalent or close to that used during the experimental preparation. The FT-IR spectra showed the vibrational characteristic absorption modes of ZrO2 and ZnO bonds around 400–600 cm−1. Two band gap energies were estimated corresponding to ZrO2 (5.05–5.16 eV) and ZnO (3.1–3.16 eV) components. Remarkably, in presence of ZrO2/ZnO (30/70 at%) heterostructure, the free solar energy initiated photodegradation efficiencies of 87% and 98% for 50 ppm CR and 50 ppm MB dyes after 60 min, respectively, which indicates the fast and superior photocatalytic activity of microwave-synthesized ZrO2/ZnO heterostructure. As well, this composition reveals good reusability and stability for three photocatalytic cycles. This uppermost photodegradation performance can be assigned to the high separation of charge carriers, especially for the ZrO2/ZnO (30/70 at%) nanocomposite. Graphical abstract Graphical abstract Microwave assisted-hydrothermal ZrO2/ZnO heterostructures. ZrO2/ZnO heterostructure exhibits fast and superior sunlight catalytic activity. Efficiency of 87% and 98% for removal of 50 ppm Congo red and methylene blue. High separation of photo-generated electron hole pairs. Microwave assisted-hydrothermal ZrO2/ZnO heterostructures. ZrO2/ZnO heterostructure exhibits fast and superior sunlight catalytic activity. Efficiency of 87% and 98% for removal of 50 ppm Congo red and methylene blue. High separation of photo-generated electron hole pairs.


Introduction
The elimination of dyes, herbicides, stimulants, pesticides and antibiotics as organic pollutants from the surrounding environment has become a serious world issue in the last years [1][2][3]. Water contaminated with residual dyes from the textile, printing press, paper, cosmetics, leather and other industries is the main source of many environmental problems [4][5][6]. Using solar energy-based semiconductor oxides in photocatalytic degradation of these types of organic pollutants seems to be a promising cost technique [7][8][9]. The light induces heterogeneous catalysis enables the conversion of organic pollutants such as dyes to molecules with higher biodegradability, finally leading to nonhazardous molecules (CO 2 and H 2 O) [10,11]. As a result, the search for advanced solar energy photo-reactive materials is of great importance environmentally and economically [12,13]. In studies relating to the photocatalytic degradation of organic dyes pollutants, zinc oxide (ZnO) nanostructures have been extensively candidates as a photocatalysts, owing to their high activity, long life span, low cost and its environmentally friendly nature [14][15][16]. However, the high recombination rate of the photogenerated charge carriers (electrons and holes pairs) acts as a limiting factor restricting the widespread applications of ZnO in photocatalysis [17,18]. Recently, the ZnO based nanocomposites received a great attention of addressing these obstacles in an operative manner [19,20]. To clarify, both of the metal oxides semiconductors were instantaneously excited and the electrons slip from the conduction band (CB) of one semiconductor to another while the holes moved to the valence band (VB) in the opposite direction [21][22][23]. These reactions have the ability to separate the electron-hole pairs which in turn enhanced the photocatalytic efficiency [21][22][23]. One of the metal oxide semiconductor materials that improve the photocatalytic properties of ZnO nanoparticles is zirconium oxide (ZrO 2 ) [24][25][26]. ZrO 2 belongs to a group of metal oxide semiconductor materials with relatively wide band gap energy over 5 eV [27]. In the past decade, the global research interest in wide band gap semiconductors has been significantly focused on ZrO 2 /ZnO oxides due to their excellent properties as semiconductor materials [24][25][26]. The high electron mobility, high thermal conductivity, wide and direct band gap, good transparency, large exciton binding energy and easiness of synthesizing make ZrO 2 /ZnO structure appropriate for a wide range of applications in optoelectronics, transparent electronics, lasing and sensing [28][29][30]. The replacement of Zn 2+ by Zr 4+ or versa vise can disturbs the charge balance in their lattices and the disturbed charge balance contributes to the adsorption of more hydroxide ions (-OH) on the surface of the particles of the powder, which additionally inhibits the recombination of the electron-hole pairs [21]. Congo red (CR) and methylene blue (MB) dyes have aromatic rings in their structures that are resistant to biodegradation and aerobic degradation. Therefore, it is essential to find operative methods that are useful for environmental protection and human health to remove these dyes from wastewater. In our previous study [21], we found that ZrO 2 /ZnO (50:50) nanocomposite prepared by sol-gel method possesses degradation efficiencies of 99% and 97% for indigo carmine (IC) (5 × 10 −5 M) and MB (2.5 × 10 −5 M) in 300 and 150 min, respectively. In this study, we made use of the major rewards of the microwaveactivation hydrothermal technique to obtain nano-sized ZrO 2 /ZnO heterostructures. These advantages include high purity synthesis circumstances, wide range of heating, perfect control of time, and the accessibility to achieve highly active powders with a narrow particle-size distribution. The effect of variation of Zr/Zn atomic ratio on the physicochemical and photocatalytic properties of the synthesized composites was evaluated. The synthesized ZrO 2 / ZnO heterostructures were used as photocatalysts in the heterogeneous photodegradation of 50 ppm CR and 50 ppm MB dyes. Remarkably, the obtained results revealed that ZrO 2 /ZnO heterostructure with composition of 30/70 at% has the highest ability to degrade the high concentrations of CR (50 ppm) and MB (50 ppm) in a short time of 60 min compared to ZrO 2 /ZnO (50:50%) nanocomposite prepared by sol-gel method (300 and 150 min).
2 Experimental: synthesis, characterization and measurements

Materials and synthesis
Throughout the entire preparation part of this work, double distilled water, as well as the following reagent grade chemicals, were used: zinc acetate dihydrate Zn(CH 3 COO) nanocomposites were attained through the addition of calculated amounts of zirconium salt (dissolved in 10 ml of 4 N HNO 3 ) to 50 ml aqueous solution of Zn(CH 3 COO) 2 ·2H 2 O at the desired atomic percentage (at%) of ZrO 2 = 10, 20, 30, and 40 at%. The mixture was stirred well, after that, 5 ml of ethylene glycol (complexing agent) was added drop by drop to stimulate the gelation process. The pH of the solutions was neutralized to 7 by slow addition of aqueous ammonia solution which speeds up the hydrolysis process and initiates the gelation process. After that, the mixture was poured into the Teflon vessel of the microwave reactor (MW) from Plazmatronika Ltd (Warsaw, Poland). The system runs at (600 W, 2.45 GHz, ERTEC microwave reactor). The duration of the reaction was 20 min, temperature 220°C and power 100%. After the reaction was completed, the reaction vessel was cooled down for 20 min. The obtained powders were sedimented, separated from the solution by filtering and were washed with distilled water and isopropanol.  (100) No observable diffraction peaks related to any secondary phases or any impurities were detected in the patterns. The crystallite sizes (D) of ZrO 2 /ZnO heterostructures were calculated based on Scherrer equation [30]:

Characterization and photocatalytic properties measurements
where λ is the X-ray wavelength, θ is the Bragg diffraction angle, 0.89 is Scherrer's constant and β is the full width at half maximum of the diffraction peaks. The calculated approximate values of the phase ratio, lattice parameters (a, b, c) and unit cell volume (V) of ZrO 2 and ZnO components in ZrO 2 /ZnO heterostructures were determined by leastsquare Rietveld refinement based on FullProf software [31]. For all ZrO 2 /ZnO heterostructures, the profiles of the refinement data showed a well-fitting between the experimental and the calculated data, as represented in Fig. 2. Besides, no indication for any chemical products due to the reaction between ZrO 2 and ZnO were detected and only the patterns confirmed the nanocomposites formation. As represented in Table 1 Table 1, there are some variations in the lattice parameter (a, b, c) and unit cell volume (V) of ZnO and ZrO 2 due to the change in ZrO 2 content in ZrO 2 /ZnO nanocomposites. The average crystallite size of the different ZrO 2 /ZnO heterostructures was calculated to be 45-51 nm, which confirms the formation of small nano-sized composites. Figure 3 illustrates the spacing-filling model and polyhedral crystal structure of hexagonal ZnO and tetragonal ZrO 2 . Figure 4 illustrates the scanning electron micrographs and the corresponding 3D view of ZrO 2 /ZnO heterostructures with ZrO 2 content of 10, 20, 30 and 40 at% prepared by microwave-assisted hydrothermal method. The SEM micrograph of ZrO 2 /ZnO heterostructure with ZrO 2 content of 10 at% shows the presence of two types of particles. The major type of these particles has a spherical shape while the minor type possesses an elongated shape, rod-like structure as shown inset Fig. 4a. When the content of ZrO 2 reached 20 at%, more fine spherical nanoparticles are observed in addition to the rod-like structure. In the case of 30 at% ZrO 2 similar architecture to that of 20 at% ZrO 2 was formed. The micrograph of the ZrO 2 /ZnO heterostructure with ZrO 2 content of 40 at% shows more uniform elongated particles in contact with very fine spherical nanoparticles. Focus shot on gathering of spherical nanoparticles (inset Fig. 4d) illustrates that the particles are homogenous and have a nearly similar size. The SEM micrographs of the synthesized ZrO 2 /ZnO heterostructures clearly show the enhancements in the size and homogeneity for the formed particles with increasing ZrO 2 content. Elemental composition of ZrO 2 /ZnO heterostructures with Zr content from 10 to 40 at% were performed by EDX spectroscopy as shown in Fig. 5. Obviously, the characteristic peaks which corresponding to Zn, Zr and O elements were detected without any sign for the presence of any other impurities elements.

SEM-EDX analysis: morphological and compositions
With increasing the Zr content, the atomic percent (at%) of the Zn, Zr and O elements (inset Fig. 5) show gradual decreases for Zn element with steady increase for Zr

Photocatalytic properties under sunlight
Under free solar energy irradiation, the photodegradation efficiencies of ZrO 2 /ZnO heterostructures with ZrO 2 content of 10, 20, 30 and 40 at% were assessed for decomposition of anionic CR (50 ppm, 100 ml) and cationic MB (50 ppm, 100 ml). Figures 9 and 10 demonstrate the variations of the maximum absorption peak of CR which located at 497 nm and that of MB situated at 668 nm in the presence of ZrO 2 /ZnO (30/70 at%) heterostructure after exposed to sunlight irradiation of 60 min. For ZrO 2 /ZnO (30/70 at%) heterostructure, obvious changes were detected in the maximum absorption peak of the CR (497 nm) and MB (668 nm) dyes after irradiation by sunlight with nearly complete vanishing of these peaks after 60 min. In case of ZrO 2 /ZnO heterostructure with ZrO 2 content of 10, 20 and 40 at% (figures not included here) the decreasing in the maximum absorption peaks of both dyes is still good but less compared to ZrO 2 /ZnO heterostructure of 30 at% ZrO 2 content. Figure 11 illustrates the whole photocatalytic efficiency of ZrO 2 /ZnO heterostructures for CR (50 ppm) and MB (50 ppm) dyes after 60 min of sunlight radiation. For CR, the photodegradation efficiencies were estimated to be 52%, 70%, 87% and 74% for ZrO 2 /ZnO heterostructures with ZrO 2 content of 10, 20, 30 and 40 at %, respectively. In case of MB, total efficiencies of 56%, 75%, 98% and 76% were detected for these catalysts,  possesses more efficient for separation of the photogenerated charge carriers (electron-hole pairs). The obtained results revealed that ZrO 2 /ZnO (30/70 at%) heterostructure synthesized by microwave-assisted hydrothermal method has high activity and fast degradation time (60 min) for organic dyes in comparison with our previous study reporting degradation of MB and IC dyes using sol-gel-assisted ZrO 2 /ZnO (50:50%) nanocomposite in 300 and 150 min, respectively [21]. The mechanism for organic pollutant dyes degradation (CR and MB) in the presence of ZrO 2 /ZnO heterostructure was linked to the excitation of electrons by the solar photon energy (Fig. 12)   . The total steps of the photocatalytic mechanism can be illuminated through the following chemical reaction [33][34][35][36][37][38]: In this work, the obtained results show that the best composition for photocatalysis is ZrO 2 /ZnO heterostructure with content of 30/70 at% and also revealed that the microwave-assisted hydrothermal technique is more effective for the photocatalytic applications of ZrO 2 /ZnO heterostructure compared to sol-gel method used on our previous study on the same nanocomposite [21]. The microwaveassisted hydrothermal technique helps in reducing the photodegradation to 60 min compared to 150 and 300 min with maintaining the same efficiency and also perfect for high concentrations of organic pollutants [21]. The stability and reusability of ZrO 2 /ZnO (30/70 at%) heterostructure (high efficient catalyst) was studied for the decomposition of MB dye at similar reaction circumstances. After the first photocatalytic experiment the catalyst was collected, washed with deionized water and dried at 90°C for 1 h in air atmosphere to be used for the new degradation test. Figure 13 shows the results of MB degradation for three cycles. ZrO 2 /ZnO (30/70 at%) photocatalyst reveals a good photo-stability for MB decomposition with efficiency of 98%, 92% and 85% for the first, second and third test under solar irradiation for 60 min, respectively. The obtained results point out that ZrO 2 /ZnO (30/70 at%) photocatalyst is satisfactorily stable during the photo-decomposition of methylene dye.

Conclusion
ZrO 2 /ZnO heterostructure (30/70 at%) exhibited a superior and fast sunlight photodegradation efficiency for high concentrations of CR (50 ppm) and MB (50 ppm) dyes. The XRD and Rietveld refinement results confirm the existence of two sets of diffraction peaks corresponding to tetragonal ZrO 2 and hexagonal ZnO structures. The morphological study of ZrO 2 /ZnO heterostructures demonstrates the formation of a mixture of nanospherical particles (major) and rod-like (minor) structures in each composite. The EDX analysis shows the occurrence of Zr, Zn and O elements with a percentage ratio equivalent or closer to that used in the experimental preparation. Two band gap energies were estimated in each composite corresponding to ZrO 2 (5.05-5.16 eV) and ZnO (3.1-3.16 eV) components. The solar energy photodegradation efficiency of ZrO 2 /ZnO (30/70 at%) heterostructure for 50 ppm CR and 50 ppm MB were found to be 87% and 98% after 60 min of solar energy irradiation, respectively, confirming the higher photocatalytic activity of ZrO 2 /ZnO heterostructure. ZrO 2 /ZnO (30/70 at%) heterostructure reveals good reusability and stability for three photocatalytic cycles. The significant photodegradation performance can be correlated to the high separation of charge carriers, especially for ZrO 2 /ZnO (30/70 at%) heterostructure.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

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