Green synthesis, characterisation, and performance evaluation of ZnO/TiO₂ nanocomposite for cationic and anionic dye removal from aqueous solutions under solar irradiation

The study investigates the photocatalytic activity of green synthesised ZnO/TiO₂ nanocomposites using hibiscus leaf extract. The synthesised nanocomposite was used to degrade cationic dye and anionic dye in an aqueous solution by solar radiation. The effect of different ZnO molar ratios on the characterisation and photocatalytic behaviour of the nanocomposite was studied. In addition, factors affecting the degradation of dyes, including different concentrations of catalyst and pH of the solution were investigated. Reusability study of the catalyst was also carried out to investigate the stability of the catalyst.

Hibiscus leaves were collected from the National Institute of Technology, Karnataka campus. The chemicals used in the study were titanium tetraisopropoxide (TTIP) (99%), ethanol (96%), zinc nitrate (ZnNO₃) (99%), TiO₂ (99%), MB powder (85%), and MO powder (85%). The pH of the aqueous solutions was adjusted using solutions of 2 N NaOH and 1 N H₂SO₄. Double distilled water was used for the preparation of leaf extract and all other solutions. All the chemicals used were of analytical grade and were not further purified.

A sufficient quantity of hibiscus leaves was gathered from the NITK campus. The leaves underwent multiple rinses with distilled water to eliminate any dirt or dust particles and were subsequently air dried. 50 g of the cleaned leaves were added to 500 mL of double distilled water and boiled at 100 °C until the colour of the solution changed to yellow. The prepared extract was then allowed to cool. Once cooled, the prepared extract was filtered using Whatman filter paper grade no. 1, pore size of 11 μm.

A solution of 7.55 mL TTIP and 92.45 mL of ethanol was poured into a glass beaker. To this solution, 30 mL of distilled water and 15 mL of hibiscus leaf extract were added dropwise with continuous mixing at room temperature for 4 h. The mixture obtained after stirring was washed several times with distilled water and filtered using vacuum filtration. The yellow colour thick paste obtained was kept overnight in air-heated oven at 110 °C. The solid TiO₂ was then calcined at 570 °C for 3 h in a muffle furnace.

The composites in the present study were synthesised using a previously established method (Abd El-Kader et al. 2021) with modifications. 30 mL of the previously prepared hibiscus leaves extract was taken in a glass beaker and heated at 70 °C. 11 g of ZnNO₃was added step-by-step to the solution with continuous stirring for 90 min until a deep yellow colour paste was formed. The paste was then collected in a crucible and burned for 2 h at 450 °C in a muffle furnace. A light-yellow powder obtained after burning was ground using a mortar and stored in an air-tight container.

About 18.1 mL of hibiscus leaf extract was heated at 700 °C. To this solution, 1.8 g of both previously prepared TiO₂ and ZnO nanoparticles were added together with continuous mixing until a greyish-white paste was obtained. The paste was collected in a crucible and heated at 700 °C for 3 h in a muffle furnace. The obtained ZnO/TiO₂ nanocomposite of the ratio 1:1 was ground using a mortar and stored in an air-tight container for characterisation and further experiments. Similarly, ZnO/TiO₂ nanocomposite of different ratios (1:2 and 2:1) was prepared by varying the weight concentrations of ZnO and TiO₂ (Abd El-Kader et al. 2021). In the following sections ZnO/TiO₂ of ratio 1:1, 1:2, and 2:1 is abbreviated as Z1T, ZT2, and Z2T, respectively, and bare TiO₂ as T.

Fourier Transform Infrared Spectroscopy in the range 400–4,000 cm⁻¹ (FTIR, Spectrum 2 FTIR Spectrometer, PerkinElmer, Singapore) was carried out to investigate molecular vibrations and functional groups present in the catalyst. The crystal structure and phase of the catalyst were identified with the help of X-ray Diffraction (XRD, Empyrean 3^rd Gen, Malvern PANalytical, Netherlands) using Cu-Kα radiation (Shimadzu, XD-3 H) in the diffraction angle (2θ) range of 20°–80° with 10°/min scanning speed. Energy Dispersive X-ray Spectrometer (EDS; Octane Elite, EDAX AMETEK) in conjunction with FESEM was used to determine elemental composition. The surface morphology and shape of the catalyst were determined using High-Resolution Field Emission Scanning Electron Microscopy (HR-FESEM, GEMINI 300, Carl Zeiss, Germany). The absorbance and bandgap energy of the catalyst were determined using UV–Vis Diffuse Reflectance Spectroscopy (UV–Vis DRS, Lambda 950, Perkin Elmer, Singapore).

To avoid the excessive use of catalysts it is advisable to find the optimum dosage of catalyst for the effective degradation of dyes. To study the effect of catalyst dosing a dye concentration of 8 mg/L and a working volume of 100 mL was taken for both MB and MO. The concentration of the catalyst i.e., ZnO/TiO₂ of weight ratio 1:2 (ZT2) was varied from 60 to 200 mg/L for MB and from 100 to 1,300 mg/L for MO with a time of contact of 150 and 240 min for MB and MO, respectively. Tables 2 and 3 show the kinetic data of photocatalytic degradation of MB and MO for varying catalyst dosage. In Figure 9(a) it can be seen that with the increase in catalyst dosage, the photodegradation rate increased up to a loading of 100 mg/L for MB. MB showed maximum dye degradation within 130 min of contact time with a catalyst dosage of 100 mg/L and reached a stabilised state. When the catalyst dosage for MB was further increased beyond 100 mg/L, it led to the reduction of the photodegradation constant (k) from 0.02379 to 0.02236 min⁻¹. When the catalyst dosage was increased from 100 to 1,000 mg/L the degradation efficiency of MO increased from 32.13 to 89.39%. For MO a maximum removal of 89.39% occurred at 1,000 mg/L with a contact time of 240 minutes. The removal efficiency was reduced to 87.5% when the catalyst loading was further increased to 1,300 mg/L. As the catalyst dosage was increased the degradation rate increased due to an increase in active sites on the surface of the catalyst resulting in greater electron/hole pair for degradation. Above the optimum dosage, the degradation of dye remained almost the same. And when catalyst loading was further increased it led to the reduction in dye degradation. This may be due to the hindrance and blocking of light penetration due to the excessive amount of catalyst particles. Also, particle aggregation is significant at high dosages of catalyst which may lead to the reduction in active sites on the catalyst surface (Habib et al. 2013). Hence the optimum dosage of catalyst for MB and MO was taken as 100 and 1,000 mg/L, respectively. MO being an azo dye is more stable and difficult to degrade because of the presence of the azo group, hence MO requires more catalyst dosage and longer contact time than MB for efficient degradation (Waghchaure et al. 2022). There was not any discernible decrease in MO or MB concentration during dark adsorption, this may be due to the limited adsorption and activation property of the catalyst in the absence of solar radiation. The same results were reported in the study conducted by (Rashid Al-Mamun et al. 2022). It is to be noted that no significant dye decolourisation of MO occurred without a catalyst under solar irradiation indicating that the dye was resistant to self-photolysis in aqueous solution under solar light whereas photolysis removed 40% of MB dye in 150 min without the catalyst.

The zero-point charge pH_zpc of TiO₂ and ZnO are about 6.8 and 9, respectively (Mostafa et al. 2022). At a pH lower or higher than the pH_zpc, ZnO/TiO₂ composite is positively or negatively charged, respectively (Habib et al. 2013). At acidic pH (pH 2 or pH 6) the composite will be positively charged and the negatively charged MO anions will get adsorbed onto the surface of the catalysts enhancing the degradation of the dye. In alkaline medium ZnO/TiO₂ composite becomes negatively charged and the adsorption of MO anion on the catalyst surface decreases due to Coulomb repulsion between them leading to a reduced degradation rate. In addition, in acidic medium, the photogenerated hole reacts with water to produce hydroxyl radicals as given in Equation (6), which has a strong oxidation capacity to oxidise organic pollutants (Equation (7)) (Ge et al. 2009).

MB is a cationic dye, and it does not dissociate into ions in the same way as a salt would. Instead, it exists as a positively charged molecule. The electrostatic interaction between negatively charged composite and positively charged MB results in greater degradation in alkaline medium (Abdellah et al. 2018). Also, at higher pH hydroxyl radicals will be formed through the reaction of OH- with photogenerated holes.

In this study, ZnO/TiO₂ composite was synthesised from hibiscus leaf extract by using the green synthesis method. The composites were prepared for different weight ratios of ZnO (ZnO:TiO₂ − 1:2, 1:1, and 2:1). Among the three catalysts prepared, ZT2 with a weight ratio of 1:2 showed the maximum efficiency. After 150 min of contact time, under solar radiation, 96.13% of MB was degraded. The degradation rate constant of ZT2 was 1.9 times the degradation rate constant of green synthesised TiO₂ suggesting that the photocatalytic activity of ZnO/TiO₂ composite is better than pure TiO₂ which can be attributed to the formation of heterojunction which effectively separates the photogenerated charge carriers, reducing their recombination. The degradation efficiency of Z2T was less compared to TiO_2, as a higher dose of ZnO to TiO₂ decreases the active sites of TiO₂ due to saturation and overlapping, decreasing the degradation rate. The XRD and FTIR characterisation confirmed the formation of ZnO/TiO₂ composite. XRD analysis showed the well-crystallised anatase TiO₂ and wurtzite ZnO. The presence of Zn–O–Ti bond at the broad band 1,800–2,300 cm⁻¹ centred at 2,028 cm⁻¹ in FTIR analysis indicates that ZnO has been embedded into the surface of TiO₂. FESEM revealed the flaky hexagonal structure of ZnO and reflected the morphology of TiO₂ as a spherical structure. FESEM images of the composite revealed the distribution of ZnO over TiO₂. EDS analysis confirmed the presence of Ti, Zn and O elements in the composite. According to the UV–Vis study, the bandgap of ZT2 and TiO₂ were 2.98 and 3.14 eV, respectively, indicating the formation of heterojunction caused a red shift in the absorption spectrum making the composite more active in the visible region. The photocatalytic activity ZT2 catalyst was evaluated by degrading cationic MB and anionic MO. In addition, factors affecting the degradation of dyes including the catalyst dosage and initial pH were investigated. The optimum catalyst dosage for a dye concentration of 8 mg/L was found to be 100 and 1,000 mg/L for MB and MO, respectively. MB showed maximum degradation at alkaline pH whereas MO showed maximum degradation at acidic pH. Furthermore, the synthesised catalyst ZT2 showed good recyclability. Therefore, ZnO/TiO₂ shows great potential for pollution abatement under solar light.

The authors thank the Central Research Facility, National Institute of Technology Karnataka, Surathkal, for the facilities provided for material characterisation. K.A. acknowledges Ministry of Education, India for the PG fellowship.

No funding was received for conducting this study.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.