One-step synthesis of hexylresorcinol calix[4]arene-capped ZnO–Ag nanocomposites for enhanced degradation of organic pollutants

Abstract

In this study, hexylresorcinol calix[4]arene (HRCA) is introduced into the reaction system, and HRCA-capped ZnO–Ag nanocomposites are prepared via a simple one-step reflux method. HRCA is used not only as a reducing agent for deoxidizing Ag⁺ to Ag, but also as a protectant for wrapping around the microstructure of the formed ZnO–Ag. The prepared samples are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and water contact angle. The Ag nanoparticles and ZnO adhere to each other and HRCA molecules encapsulate on the surface of ZnO–Ag nanocomposites. HRCA-capped ZnO–Ag nanocomposites with different Ag contents are investigated for use in photodegradation of organic pollutants (rhodamine B (RhB) and levofloxacin hydrochloride). The sample with 10.20 mol% Ag, denoted as ZA3, exhibits the highest catalytic activity for photodegradation of RhB and levofloxacin hydrochloride. Moreover, ZA3 exhibits high stability during photodegradation of organic pollutants even after multiple reuses. The possible photocatalytic mechanism is discussed. We believe that O₂⁻ and h⁺ are the chief active species responsible for the photocatalytic activity of HRCA-capped ZnO–Ag nanocomposite system. HRCA-capped ZnO–Ag nanocomposite is expected to be an effective photocatalyst with potential application to sewage treatment under sunlight.

Introduction

Water pollution is one of the most serious problems related to environmental protection, and industrial wastewater is particularly harmful. Therefore, it has become necessary to develop effective and efficient purification measures for pollutants removal from wastewater. Several techniques for water pollution control have been developed, such as biodegradation, adsorption, and photocatalysis degradation (photodegradation) [1]. In particular, photodegradation is a promising technique for sewage disposal that avoids generation of secondary toxic materials [2], [3], [4].

Nanocomposites of metal oxide semiconductors (such as ZnO or TiO₂) with different morphologies and sizes have been evaluated for use as photocatalysts, owing to their unique properties and potential application to environmental pollutant elimination [2], [3]. Among the various metal oxide semiconductors, ZnO has been used extensively as a photocatalyst for pollutants removal due to its excellent chemical stability. Further, it is characterized by a large surface area, low cost, and high electron mobility [2], [4], [5], [6]. However, the rapid recombination of electron–hole pairs limits the practical application of ZnO as a photocatalyst. A highly effective strategy for enhancing ZnO photocatalytic activity is to combine it with metallic nanocrystals, such as those of Ag [7], [8], [9], Au [10], or Pt [11], [12]. Ag is a noble metal with high conductivity and low cost and is abundantly available. Thus, ZnO–Ag nanocomposites with various morphologies have been synthesized, including pompons [13], nanorods [14], and microspheres [15], which offer superior photocatalytic performance.

In addition, various organic compounds have been used as protectants or shape-directing agents in ZnO synthesis, such as ascorbate ions [16], trisodium citrate [17], cetyl trimethylammonium bromide (CTAB) [13], and oleic acid [18]. Hexylresorcinol calix[4]arene (HRCA) is another compound with potential application in ZnO synthesis. HRCA is composed of four resorcinols linked by methylene; therefore, the π electrons of the resorcinols have a larger conjugate range, and the absorption wavelength of HRCA moves toward the long wavelength direction.

In this study, HRCA and AgNO₃ are introduced to the ZnO nanocomposite reaction system to study their effect on the resultant photocatalysts. The photocatalytic activity of the prepared photocatalysts is studied. HRCA molecules cap ZnO surface via the negative oxygen ions [19], and the final color of the prepared ZnO is pink. At this point, the effect of HRCA molecules adsorbed on the ZnO surface is similar to that of a photosensitizer in dye-sensitized solar cells [20], [21], which is expected to utilize visible solar light. HRCA also acts as a reducing agent for reduction of Ag⁺ to Ag during synthesis. To the best of our knowledge, this is the first report on application of resorcinol calix[4]arene to ZnO synthesis. The synthesized HRCA-capped ZnO–Ag nanocomposites exhibit high photocatalytic activity during degradation of organic pollutants (rhodamine B (RhB) and levofloxacin hydrochloride) under simulated sunlight.