Fabrication of ZnO–TiO₂ nanohybrids for rapid sunlight driven photodegradation of textile dyes and antibiotic residue molecules

Highlights

• Synthesis of ZnO–TiO₂ nanohybrids by hydrothermal method.

• ZnO–TiO₂ nanohybrids shows remarkable PL quenching and enhanced photocatalytic properties.

• ZnO–TiO₂ nanohybrids can efficiently decompose antibiotic residue and textile dye molecules.

• Superior photocatalytic efficiency ascribed to higher electron separation due to synergistic effect among ZnO and TiO₂.

Abstract

Semiconductor nanohybrids garner huge interest among environmentalists for the various applications owing to their efficiency, cost-effectiveness and benignity. We report the fabrication of TiO₂ nanoparticles (TNPs) functionalized ZnO nanoflakes (ZNFs) and their outstanding detoxification ability towards the methylene blue (MB), rhodamine 6G (R6G) and Oxytetracycline (OTC) molecules solution in water. TEM studies confirm the formation of 3D architecture of ZnO–TiO₂ nanohybrids while high-resolution transmission electron microscopy assures the uniform functionalization of TNPs over ZNFs. Scanning electron microscopy studies reveal the modulation in the surface morphology with the tenability in volume ratios of Zn and Ti sources. Significant PL quenching in ZnO–TiO₂ nanohybrids spectrum as compared to ZNFs confirms the improvement in the charge separation which is highly favorable for the enhancement in the photodegradation efficiency. The 3D architecture of ZnO–TiO₂ with volume ratio (Zn: Ti: 4:1) showed superior photodegradation activity and decompose OTC, MB and R6G dye solution within 8 min, 6 min and 30 min respectively under natural solar light (~862 W/cm²). Most efficient ZnO–TiO₂ nanohybrid exhibits extremely high rate constant values for MB, R6G and OTC molecules which are 7, 5 and 4.8 times of the rate constant value of pure ZNFs. Extremely superior photodegradation performance of 3D architecture of ZnO–TiO₂ nanohybrid could be ascribed to the charge separation and synergistic effect between the TNPs and ZNFs which is responsible for the high density of electrons in the conduction band of ZnO–TiO₂ nanohybrids. ZnO–TiO₂ nanohybrids reveal the extremely high photodegradation rate for the decomposition of azo dyes and pharmaceutical residue which has not been reported till now.

Introduction

Antibiotic residue and toxic organic chemical molecules from the various chemical, textile and pharmaceutical industries are found to extremely toxic for the aquatic environment ecosystem and human health [1,2]. It has been reported that the usage of several antibiotics all around the world is more than 800 tons in a year [3]. On the other hand, various chemical industries manufactured ~7 × 10⁵ tons of azo dyes in a year [2,4]. The large scale usage of antibiotics by pharmaceutical industries causes several harmful effects on the environment consequently water quality reduced throughout the world [4,5]. Antibiotics and organic dye molecules are highly stable and can sustain in the flowing water due to their complex structures [2,6]. To improve the wastewater quality and sustain the several water resources, the cost-effective, green method should be developed for the complete decomposition of antibiotics residue molecules (oxytetracycline, quinolones and sulfonamides) as well as several organic dye molecules such as crystal violet (CV), methyl orange (MO) and methylene blue (MB). Advanced oxidation process has proven as one of the most effective greenways to decompose various organic molecules with the activation of semiconductor through sunlight illumination [7,8]. Semiconductors based photocatalyst has been proven more advantageous due to their facile preparation methods, cost-effectiveness and nontoxic nature which enhance their production and usage for practical application [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]]. Among different wide bandgap semiconductors, ZnO is one of the well-known photocatalyst material due to its easy availability, high quantum yield, nontoxic and cost-effective nature [[27], [28], [29], [30], [31]]. ZnO is an n-type semiconductor (band gap ~ 3.2 eV) and can be activated with UV light exposure and create electron-hole pair in their particular conduction and valance band. High recombination rate in ZnO lowers the lifetime of the charge carries; consequently, their photocatalytic performances drastically diminished [32]. To minimize the high recombination rate in ZnO, their heterostructures with various other semiconductors are one of the facile cost-effective approaches [33,34]. The creation of semiconductor nanoheterostructures with ZnO effectively controls the recombination rate in ZnO due to synergistic effects which effectively increase the lifetime of charge carriers and reduce the defect states in ZnO as well [35]. It has been reported that reduction in the defect state and increment in the lifetime of photo-induced charge carrier are the two major two key factors which can significantly improve the photodegradation performance of semiconductor-based photocatalyst. Among the large varieties of semiconductors, TiO₂ aroused great interest as a photocatalyst over the last few decades due to its high stability, nontoxicity and phase-dependent optical properties [[36], [37], [38], [39]]. Above mentioned fascinating properties, make TiO₂ nanostructures more promising for a wide range of energy harvesting applications in photovoltaics [39], water splitting [40], antibacterial [41], photocatalysis [42,43], and hydrogen production [44]. Previously, several research groups displayed an improvement in photocatalytic behavior of ZnO by combining them with TiO₂ nanostructures [[45], [46], [47], [48], [49], [50]]. TiO₂ nanostructures attached ZnO effectively minimize the recombination rate in ZnO through synergistic effect and improve the charge separation efficiency.TiO₂ modified ZnO nanostructure offers efficient adsorption in the UV range as compared to pure ZnO [46]. TiO₂ functionalized ZnO nanostructures provides the non-toxicity towards the environment which make it more advantageous over other combinations of semiconductor heterojunctions based photocatalysts [[15], [16], [17]]. Moreover, ZnO–TiO₂ based photocatalysts are found very cost-effective and highly stable which can be more useful for the practical and industrial applications as well. Zha et al. [51] synthesized the hedgehog and fan blade structure of TiO₂/ZnO nanocomposites through the hydrothermal method and employed them for the decomposition of methyl orange dye in UV light exposure. In their photocatalytic studies, 20 mg/L solution of methyl orange dye solution was decomposed in 30 min. In another study, Sun et al. [52] fabricated heterojunction among the TiO₂ nanowire and ZnO nanoparticles and explored their photodegradation ability with the decomposition of methyl orange (MO), MB and RhB dye solutions. In their photodegradation experiment, 10 mg photocatalyst was applied for the degradation of 10 mg/L of MB, MO and RhB solution under Xenon lamp illumination. In the earlier studies, Jo et al. [5] reported the preparation of Co-doped TiO₂-Graphene nanohybrids and used them for the decomposition of OTC and congo red solution under Xe-lamp exposure (300W). In their photodegradation test, 50 mg photocatalyst material were employed to dissociate the 10 mg/L of each molecule (CR and OTC) was degraded in 85 min respectively.

In this work, 1D and 3D architectures of ZnO–TiO₂ nanohybrids were developed via the hydrothermal method. Modulations in the optical, morphological and photocatalytic properties of ZnO–TiO₂ nanohybrids over pure ZNFs have been explored. Improved charge carrier separation has been achieved among the ZnO and TiO₂ nanostructures which largely enhanced the photodegradation efficiency of ZnO–TiO₂ nanohybrids. We have shown the modulations in the density of heterojunctions among ZnO and TiO₂ by tuning the volume ratio of their precursors. Tremendous photocatalytic nature of cost-effective ZnO–TiO₂ nanohybrids has been revealed by the breakdown of MB, R6G and OTC molecule under solar light exposure. Most superior ZnO–TiO₂ nanohybrid sample rapidly decomposes the OTC, MB and R6G molecules in 8 min, 6 min and 30 min respectively.