Green Synthesis and Characterization of a ZnO-ZrO₂ Heterojunction for Environmental and Biological Applications

Abstract

The zinc oxide-zirconium dioxide (ZnO-ZrO₂) heterojunction was prepared by a green method using rubber leaves as reducing and capping agents. Various physicochemical techniques were used to study the chemical composition and the structural and optical properties of the synthesized nanocomposite. The nature of the heterojunction was confirmed through X-ray diffraction and the average sizes of ZnO and ZrO₂ crystallites were found to be 70 and 24 nm, respectively. The photocatalytic potential of the ZnO-ZrO₂ heterojunction was examined against rhodamine 6G (Rh-6G), and 97.30 percent of the dye was degraded due to the synergistic effect of the light and the catalyst. The commercial ZnO nanopowder was used as a reference catalyst and 86.32 percent degradation was noted under the same reaction conditions. The in vitro antioxidant activity was also performed to scavenge the 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) free radicals, where the activity of the ZnO-ZrO₂ heterojunction was found to be higher than the ascorbic acid.

1. Introduction

The generation of free radical species is a serious health issue as it causes neurodegenerative disorders, heart diseases, and cancer that can be mild to severe. Free radicals are related to the production of various chemicals, where they eject electrons from other species due to their highly unstable nature, resulting in the denaturation of the target species [1]. In the human body, these reactive molecules are produced continuously and have the ability to damage cellular components such as DNA, lipids, proteins, and other short-lived molecules. The highly reactive free radicals stabilize themselves by accepting hydrogen or electrons, forming diamagnetic, stable molecules [2]. Thus, it is very essential to develop materials that can stabilize the free radicals and prevent the denaturation of cellular components [3]. The removal of organic dyes from water is an important preventative measure for the protection of the environment [4]. Many of the organic dyes come from the food, paper, plastic, photographic, printing, cosmetic, dying, pharmaceutical, leather, and textile industries [4,5,6,7,8]. Textile dyes affect the photosynthetic activity of water bodies due to their complex aromatic structures, which make them very stable and difficult to treat [9]. These dyes contaminate the water, causing damage to the aquatic organisms as well as human life because of carcinogenic, allergic, and mutagenic effects [10]. Among various dyes, rhodamine 6G is mainly used in textile industries as a coloring agent. It has a reddish-purple color, is water-soluble, and is used in fluorescence microscopy and the detection of antigens in many samples. Rhodamine 6G dye is used to detect antigens in water samples and has a wide range of applications in both the textile industry and biochemistry laboratories. Rhodamine 6G is also used in biochemistry for fluorescence spectroscopy and fluorescence microscopy. Rhodamine dyes are neurotoxic and highly carcinogenic, causing serious problems and tissue-borne sarcomas. Drinking water contaminated with rhodamine dye damages the respiratory system and causes irritation to the eyes and skin [11,12].

Wastewater is treated through different techniques such as photochemical, chemical, adsorption, biodegradation, and many other methods [13]. However, the most efficient method for environmental remediation and energy production is photocatalysis. Composite photocatalysts are being prepared to overcome the disadvantages of photocatalysts with a single phase. Many of the composite catalysts are prepared with materials that exhibit improved photocatalytic performance, such as semiconductor nanoparticles with metal nanoparticles, nano-crystal composites with conjugated polymers, and so on [14,15]. ZnO nanoparticles are generally safe to use, inexpensive, and exhibit antibacterial properties; they are also utilized in drug delivery, cancer therapy, in the energy sector, and in biological fields [16]. They play significant roles as gas sensors, catalysts, super-conducting materials, and ceramic resistors [17]. The ZnO nanoparticles possess electrical as well as optical properties with a band gap (3.37 eV) that marks them as UV-absorbers. Due to their low fabrication temperature and high electron mobility, they are used for the fabrication of the electron transport layer to prepare solar cells [18,19]. ZrO₂ nanoparticles are also an efficient and stable photocatalyst belonging to the n-type semiconductor group. They show oxygen vacancies that create many holes on their surface for improved oxidation with a band gap of 5 eV. ZrO₂ nanoparticles exhibit photochemical stability, mechanical strength, and optical properties. They have wide applications in electronic devices, for fuel synthesis as solid-state electrolytes, and act as a degradation photocatalyst in textile industries [20]. Different methods have been used to prepare ZnO-ZrO₂ nanocomposites, including the wet chemical method, green method, evaporative synthesis, green process, and sol-gel method [21,22,23,24,25,26]. Besides these synthetic routes, some limited data have been reported on the plant-mediated synthesis of binary nanocomposites using different metal oxides. The green process is an eco-friendly, cost-effective, and simpler method used for the synthesis of different kinds of nanomaterials [27]. The phytochemicals possess hydroxyl or thiol groups that act as reducing agents, and the hydrogen ions are replaced with a metal cation [1,3]. Though ZnO-ZrO₂ has been synthesized via chemical methods, to our knowledge, no literature regarding the plant-aided synthesis of ZnO-ZrO₂ was found. Moreover, the multifunctional nature of ZnO-ZrO₂ has not been tested, as we have examined the photocatalytic activity against rhodamine 6G and the antioxidant activity against ABTS.

Here, we are reporting the green synthesis of a ZnO-ZrO₂ heterojunction utilizing the extract of natural rubber leaves and characterizing the heterojunction by FTIR, DRS, EDX, SEM, and XRD techniques. Rh-6G was degraded in its aqueous solution under sunlight through the synthesized nanocomposite. Mathematical equations were used to examine the percentage and rate constant for degradation. The antioxidant activity was carried out to stabilize the ABTS free radicals.

2. Materials and Methods

2.1. Materials

Highly pure chemicals, including zinc chloride dehydrates (ZnCl₂·2H₂O), zirconium chloride tetra-hydrates (ZrCl₂·4H₂O), ethanol (C₂H₅OH), rhodamine 6G (C₂₈H₃₁N₂O₃Cl), potassium persulphate (K₂S₂O₈), zinc oxide nanopowder (ZnO), and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (C₁₈H₁₈O₄N₆S₄), were purchased from Merck and utilized with further purification. Deionized water was used to prepare all of the solutions used for this work.

2.2. Extract Preparation

The rubber (Ficus elastic) leaves were collected from Chehlla campus, University of Azad Jammu and Kashmir, Muzaffarabad, and were washed twice with ordinary water, followed by the deionized water. Next, 500 mL of boiled deionized water was transferred into an airtight jar containing 20 mg of the washed rubber leaves and soaked for 24 h. The crude extract was then filtered, followed by centrifugation at 4000 rpm for 15 min. The clear upper portion was stored at 4 °C for further use.

2.3. Synthesis of the ZnO-ZrO₂ Heterojunction

The appropriate amount ZnCl₂·2H₂O was dissolved in 500 mL to prepare a 5 mM solution, and 80 mL of this solution was mixed with 20 mL of the prepared extract. The mixture was heated (50 °C) and stirred (250 rpm) until the formation of a gel was observed, and was then marked as gel-a. Similarly, a 5 mM solution of ZrCl₂·4H₂O was also prepared in 500 mL, and 80 mL of this solution was mixed with 20 mL of the prepared extract with constant heating (50 °C) and stirring (250 rpm) until the formation of a gel was observed, which was marked as gel-b. For the synthesis of the ZnO-ZrO₂ heterojunction, gel-a and gel-b were mixed together with vigorous stirring for 4 h at 80 °C, and the reaction mixture was then aged for 24 h. The well-settled product was then washed thrice with deionized water and calcined at 450 °C for 3 h in an oxygen atmosphere in a muffle furnace (Model: 1700C Bench-Top; Made by MTI corporation, Richmond, CA, USA), followed by its storage in a polyethylene bottle.

2.4. Instrumentation

XRD, SEM, EDX, DRS, and FTIR spectroscopy were used to study the physicochemical properties such as crystal structure, surface morphology, and crystal size percent composition of the ZnO-ZrO₂ heterojunction. The crystallite size was calculated using the Debye-Scherrer equation, and the crystalline nature was examined using a Philips X’Pert XRD model. A SEM model, the JEOL JSM-5600LV, was used to investigate the microstructure and surface topology. To confirm the elemental composition, an energy-dispersive X-ray (EDX model JEOL JSM-5600LV; made in Tokyo, Japan) examination was performed in conjunction with a scanning electron microscope (SEM). An FTIR model, Nicolet 560 operating in the 400–4000 cm⁻¹ range, was used to investigate the surface functional moieties. A DRS spectrum in the 200–1000 nm range was used to measure the transmittance edge, and a Tauc’s plot was used to estimate the band gap energy.

2.5. Photocatalytic Assay

The photocatalytic degradation of rhodamine 6G was surveyed in the presence of the ZnO-ZrO₂ heterojunction under simulated solar light irradiation. The photochemical reaction was performed in a glass reactor containing 20 mg of the catalyst and 50 mL of the dye solution and was stirred in the dark for 30 min. The reaction was then illuminated with simulated solar light, and the UV-Visible analysis was carried out after a specific interval. Equations (1) and (2) were applied to the data to determine the percentage degradation and degradation rate constant. Afterward, the stability of the ZnO-ZrO₂ heterojunction was checked by introducing fresh rhodamine 6G solution into the reactor, and the process was repeated five times to observe the photocatalytic efficiency during the five-fold reutilization. Commercial ZnO nanopowder (99.5%) was used as a reference catalyst, and the degradation of rhodamine 6G was carried out under the same reaction condition.

$$\% \mathrm{Degradation}=\frac{Co-Ct}{Co}\times 100$$

(1)

$$ln\left(\frac{C}{C_o}\right)=-kt$$

(2)

2.6. Antioxidant Assay

For the antioxidant assay, 5, 25, 50, 100, 200, or 400 μg of the ZnO-ZrO₂ heterojunction was ultrasonically dispersed in 1 mL of deionized water at room temperature to prepare stock suspensions. The prepared 5 mM potassium persulphate and 14 mM ABTS solutions were mixed at a 1:1 (v/v) ratio and placed in the dark for 16 h. The generation of ABTS free radicals was analyzed through UV-Visible analysis at 734 nm. Next, 0.2 mL of each stock suspension was mixed with 0.15 mL of the ABTS^•+ solution and, after 30 min, each sample was subjected to UV-Visible analysis at 734 nm. Equation (3) was utilized to calculate the percent free radicals scavenging activity. The antioxidant activity of the ZnO-ZrO₂ heterojunction was compared with ascorbic acid.

$$\%\mathrm{RSA}=\left[\frac{{\mathrm{A}}_{\mathrm{o}}+{\mathrm{A}}_{\mathrm{i}}}{{\mathrm{A}}_{\mathrm{o}}}\right]\times 100$$

(3)

3. Results

3.1. Surface Area Analysis

The N₂ adsorption-desorption process was followed for the measurement of the surface area of the ZnO-ZrO₂ heterojunction using the BET method; the isotherm obtained is shown as the inset in Figure 1. This type of isotherm is obtained for macroporous/nanoporous materials and displays unlimited adsorption at high relative pressure. The BET method was applied to measure the surface area, which was found to be 153.85 m²/g. The H4-type hysteresis loop started at 0.43 and ended at 0.84 p/p°, with the most pronounced adsorption occurring at 0.61 p/p°. The pore size distribution plot for the ZnO-ZrO₂ heterojunction is given in Figure 1. The BJH equation was applied to the N₂ adsorption-desorption data to determine the pore size (18.04 nm) and pore volume (0.201 cc/g).

3.2. XRD Analysis

The XRD analysis of ZnO-ZrO₂ heterojunction was carried out in the range of 20° to 70°, where separate sets of peaks were observed for ZnO and ZrO₂. The diffraction bands appeared at 2 theta angle, with corresponding hkl values of 31.88 (100), 34.49 (002), 36.33 (101), 47.69 (102), 56.71 (110), 62.99 (103), and 68.10 (112) that are consistent with bands listed in the JCPDS card 01-075-0576. All of these peaks confirmed the synthesis of the hexagonal-shaped ZnO crystallites, where the length of a and b is 0.32427 nm, and coordinate c is equal to 0.51948 nm. The average crystallite size of the ZnO crystallites was determined through the Debye-Scherer equation to be 69.87 nm with a lattice strain of 0.157 percent. The synthesis of tetragonal-shaped ZrO₂ crystallites with a space group of P42/nmc and space number of 137 was confirmed the presence of Bragg’s reflections in the diffractogram (Figure 2) at 2-theta angles 30.43, 35.47, 50,32, and 60.49, corresponding to hkl planes (101), (110), (112), and (211). The length of the a and b coordinates is 0.35916 nm, and coordinate c is 0.51790 nm long, while all three angles are equal to 90°. The average crystallite size calculated for ZrO₂ crystals is 24.29 nm, with a 0.563 percent lattice strain. The small crystallite size of ZrO₂ crystallites as compared to ZnO crystallites might be due to its broad diffraction bands, which have higher FWHM values. The sharp and narrow Bragg’s reflections suggest the highly crystalline nature of ZnO crystallites compared to that of ZrO₂. Moreover, the noisy diffraction spectrum may be due to the presence of some amorphous content in the sample.

3.3. EDX Analysis

The elemental composition and purity of the synthesized ZrO₂-ZnO NC were investigated through EDX, as shown in Figure 3. The sharp peaks at 1, 8.6, and 9.5 keV were attributed to Zn, whereas the peaks at 2, 2.5, 15.75, and 17.6 keV are due to the presence of Zr in the sample. The presence of a peak at 0.5 keV for O suggests the formation of a bimetallic oxide. The other, less intense peaks at 0.2 and 0.3 keV are due to Cl and C, which were the result of the use of chloride salt in the original preparations of the Zn and Zr solutions and the carbon tape used during EDX analysis. The weight percentage of Zn, Zr, O, and Cl are 42.7, 34.1, 22.6, and 0.4%, respectively, whereas the determined atomic percentage of Zn, Zr, O, and Cl are 17.28, 13.80, 9.15, and 0.17%, respectively.

3.4. SEM Analysis

The structural morphology of the ZnO-ZrO₂ heterojunction was examined through SEM and the low (a) and high (b) magnification micrographs are shown in Figure 4. The unevenly distributed particles formed a complex network, where some regions are highly agglomerated, leading to the formation of flake-like structures, while in other regions it looks like a highly porous material. Upon close observation, the highly magnified SEM micrograph (Figure 4b) reveals that the particles’ morphology are completely different as they exhibit different shapes. Some of the particles are tightly packed with each other and their boundaries have been demolished, leading to the formation of large particles. Other individual particles are also seen in the micrographs that exhibit well-defined boundaries, and most of these particles are present on the surface of the larger particles. The micrographs also show that cavities are formed due to the irregular arrangement of the particles. The particles’ size was estimated from the high-resolution SEM image using ImageJ software, and ranged from 44 to 96 nm, with an average size of 68 nm.

3.5. DRS Analysis

Investigating the band gap energy of a material reveals the minimum energy required for the excitation of an electron from a valance band to a conduction band. The most important method to find band gap energy is DRS [28]. The light absorption phenomena were studied using the DRS technique, and the spectrum shown in Figure 5 reveals that the maximum absorption occurred in the UV region, and the absorption decreased with increasing wavelength. The DRS spectrum exhibits two absorption bands for electronic transition at 284.54 nm and 389.06 nm for ZrO₂ and ZnO, respectively. Similar absorption bands were also reported in the literature at a slightly lower wavelength of 370 nm due to the transition of electrons of ZnO, whereas other sharp absorption bands at 208 nm and at 228 nm were for ZrO₂ [23,24]. The band gap energy was determined via a Tauc’s plot for ZnO and ZrO₂, and was found to be 3.29 eV and 3.81 eV for ZrO₂ in the ZnO-ZrO₂ heterojunction shown in inset in Figure 5.

3.6. FTIR Analysis

The chemical composition of the synthesized ZnO-ZrO₂ heterojunction was carried out through FTIR analysis and the spectrum obtained is shown in Figure 6. The wide, intense band centered at 3345 cm⁻¹ is ascribed to the vibration of a hydroxyl moiety [24]. The two short bands at 2923.11 cm⁻¹ and 2847.88 cm⁻¹, as well as the bands at 1636.89 cm⁻¹ and 1541.11 cm⁻¹, are attributed to the bending vibration of water molecules [29]. The carbon-carbon stretching of organic compounds is assigned to the band at 1466.82 cm⁻¹, whereas the bending vibrations of M-O-H bonds found on the surface groups are responsible for the band at around 1316.36 cm⁻¹ [24]. The two peaks at 1160.19 cm⁻¹ and 1057.97 cm⁻¹ are due to the vibration of oxygen-metal-oxygen and metal-oxygen-metal [21,25]. The broad band centered at 494.33 cm⁻¹ is due to the metal-oxygen (Zn-O/Zr-O) stretching in the lattice structure [25,30].

3.7. Photocatalytic Activity

The photocatalytic degradation of rhodamine 6G was demonstrated using the ZnO-ZrO₂ heterojunction in the presence of stimulated solar light. Before the exposure of simulated solar light, the reaction mixture was stirred in the dark for 30 min, and the changes in absorbance maxima via a double beam spectrophotometer were observed. Equations (1) and (2) were applied to the experimental data to calculate the percent degradation and degradation rate constant; the results are depicted in Figure 7b,c, respectively. The self-photolysis experiment was performed under the simulated solar light in the absence of a catalyst, where a slight decrease was seen in the absorbance maxima at 526 nm, and 3.07 percent degradation of rhodamine 6G was observed in 55 min (Figure 7a). Similarly, the catalytic efficiency of the ZnO-ZrO₂ heterojunction was examined in the absence of a light source, and a decrease was found in the absorbance maxima, as shown in Figure 7a. This decrease can be attributed to the adsorption of dye molecules on the surface catalyst. A total of 27.79 percent decrease was observed in the concentration of rhodamine 6G with a transfer rate of 0.00662/min. The photocatalytic efficacy of the ZnO-ZrO₂ heterojunction observed during the degradation of rhodamine 6G under irradiation with simulated solar light is shown in Figure 7a. The decrease in the absorbance maxima was attributed to the degradation of rhodamine 6G under the influence of both the catalyst and the light source. The slow degradation was seen up to 15 min, after which the process was sped up with the extension of light irradiation time. The data show that 97.30 percent of rhodamine 6G was degraded in 55 min, with a degradation rate of 0.0615/min. The photocatalytic activity of the synthesized ZnO-ZrO₂ heterojunction was compared with commercially available ZnO nanopowder under the same reaction conditions. The commercial ZnO nanopowder, with a particle size of 100 nm exhibiting 25 m²/g surface area, was used as a reference catalyst; it was found that 86.32 percent of rhodamine 6G was degraded in 55 min with a degradation rate of 0.04096/min. The photodegradation efficacy of the ZnO nanopowder was found to be less than that of the ZnO-ZrO₂ heterojunction.

When the reaction mixture was lit up with simulated solar light, the outermost electrons of both ZnO and ZrO₂ were excited into their respective conduction bands and a positive hole was generated, as shown in Figure 7d. The electrons from the conduction band of zirconium dioxide migrated into the conduction band of zinc oxide as the energy levels of zinc oxide fit well into the band gap of zirconium dioxide. On the other hand, the holes transferred from the valence band of zinc oxide to the valence band of zirconium dioxide, and thus the electron-hole pair recombination could be decreased in the zinc oxide-zirconium dioxide heterojunction. The e⁻ and H⁺ react with oxygen and water to form (OH^•) radicals, which can easily oxidize the rhodamine 6G as they are very reactive, producing water and carbon dioxide [31].

3.8. Antioxidant Activity

The in vitro antioxidant activity was conducted against ABTS free radicals using the ZnO-ZrO₂ heterojunction as an antioxidant, with the results obtained depicted in

Table 1. Antioxidant potential of the ZnO-ZrO₂ heterojunction and ascorbic acid against ABTS free radicals at different concentrations.

Samples	Concentrations (μg mL−1)	%RSA	IC50 Value (μg mL−1)	Variance (S2)	Standard Deviation (S)	Correlation b/w the Dose and Obtained Result
ZnO-ZrO2 heterojunction	5	18.08	149.20	3.13	1.93	0.084
	25	25.87
	50	37.52
	100	50.63
	200	68.03
	400	81.92
Ascorbic Acid	5	14.1	171.04	3.72	1.77	0.077
	25	22.15
	50	34.13
	100	47.83
	200	64.98
	400	78.39

. The dose-dependent free radical scavenging activity of the ZnO-ZrO₂ heterojunction increased with the increasing concentration of the antioxidant. This increase in activity with increasing concentration might be due to the larger number of antioxidants present in the solution, which can counter a larger number of ABTS free radicals. The data were also expressed in terms of half-maximal inhibitory concentration (IC₅₀) values, which has an inverse relation with the potency of an antioxidant [32]. The lower IC₅₀ values of the ZnO-ZrO₂ heterojunction confirm its high potency against ABTS free radicals as compared to the ascorbic acid used as a standard antioxidant.

4. Conclusions

A simple and eco-friendly method was used for the synthesis of a multifunctional ZnO-ZrO₂ heterojunction. The current study proves that the plant-aided process is a very fast and economical technique used for the fabrication of nanomaterials, and a suitable alternative to the toxic chemical and expensive physical methods. The nature of the ZnO-ZrO₂ heterojunction was confirmed via XRD analysis, where the separate sets of diffraction bands for ZnO and ZrO₂ were observed in the XRD pattern. The hexagonal-shaped ZnO and tetragonal-shaped ZrO₂ have different crystallite sizes. The physicochemical studies revealed the well-crystalline nature with heterogamous surface morphology of the ZnO-ZrO₂ heterojunction. The optical investigation disclosed that the maximum light absorption occurred in the UV region, and the nature of the heterojunction is more effective in the sufficient separation of excited elections and generated holes. The preliminary experiment confirmed that the higher photocatalytic efficacy is due to the synergistic effect of light and the ZnO-ZrO₂ catalyst. The synthesized ZnO-ZrO₂ acts as a very efficient antioxidant agent against ABTS free radicals, and it is assumed that it will also show high antioxidant activity against other free radicals.