Photocatalytic study of two-dimensional ZnO nanopellets in the decomposition of methylene blue

Abstract

We report several significant photodecomposition rates of methylene blue (MB) obtained before and after the refluxing process of own-synthesized two-dimensional (2D) zinc oxide (ZnO) nanopellets. Each photodecomposition rate of MB was found highly dependent on the weight of photocatalyst. The existing photodecomposition rate has been successfully improved to a factor of 22.0 times through refluxing process in excessive pyridine where the surface capping ligand (oleic acid) is removed from the 2D ZnO nanopellets. On the other hand, the refluxed photocatalyst (0.04 g) in this study was found to exhibit excellent recyclability up to three cycles. Furthermore, X-ray powder diffraction spectrums for the refluxed photocatatalyst, respectively, before and after three cycles of photocatalytic reactions, has generated the same patterns showing that the photocatalyst is stable and feasible to be used as an efficient photocatalyst material. Hence, these 2D ZnO nanopellets would provide a new alternative route as a highly efficient photocatalyst for wastewater treatment.

Introduction

Recently, quality drinking water has become a major concern worldwide due to the ever-increasing population and decreasing energy resources. Due to this reason, the efficient treatment of wastewaters have become immediate importance among science communities around the globe as there is a growing need to come out with the state-of-the-art technologies that are capable to solve the problems. Ideally, an effective wastewater treatment is to mineralize completely all the toxic contaminants in wastewater without leaving any hazardous residues. In addition, the wastewater treatment process should be cost-effective and feasible for large-scale applications.

Unfortunately, by the time being, only a limited number of employed treatment technologies such as activated carbon and biological methods, which somehow fulfilled the requirements mentioned above but there are still feeble aspects to be considered. Thus, it is necessary to find an alternative solution, which may offer a prominent route in industrial wastewater treatment process. Recently, nanotechnology has been widely adopted as an efficient way in dealing with clean water issues. In conjunction with the applications of nanomaterials in wastewater treatment, the utilization of semiconductor nanomaterials as an effective photocatalyst in wastewater treatment had been drawn much attention among scientific communities.

Recent advances in colloidal synthesis enable the precise design of high performance photocatalyst in terms of activity, selectivity and resistance to deactivation. Various advanced-synthetic techniques have been pursued in designing congruent photocatalyst materials for the decomposition of organic dyes [1], [2], [3]. The understanding of the properties, which may affect catalytic performance, is of great importance. However, not all the semiconductor materials are suitable to be used as efficient photocatalysts due to their stabilities against oxidation. For example, the well-known CdSe semiconductor nanocrystals have been widely used in bio-labeling [4], [5], [6] as well as optoelectronic devices [7], [8]. Nonetheless, the application of CdSe nanocrystals as photocatalyst is strictly hampered due to its stability against photo-oxidation. Hereby, its application as photocatalyst materials has remained a query since the formation of amorphous oxide layer was observed under ambient condition [9]. Thus, the idea of oxide-based semiconductors to be used as photocatalysts have been introduced thereafter to disclose the ideal photocatalyst characteristics like good photo- and thermal-stability, high selectivity, and excellent recyclability.

In accordance to the oxide-based semiconductor materials, there have been reports about the novelty in utilizing TiO₂ as a photocatalyst material. Utilization of this material for the decomposition of organic compounds has generated many promising results [10], [11], [12]. For last two decades, the demands of titanium worldwide have been raised due to various sophisticated mechanical application in aerospace, automobile fabrication, marine industry and novel biomaterials [13]. Due to the demanding importance of titanium in the industries mentioned above, the commercialization value for TiO₂ have steadily increased this few years and exceeding the value of ZnO [14]. Hence, alternative potential resources for titanium, which render the comparable efficiency, have to be considered. Among the oxide-based semiconductors that hold the promise as a potential photocatalyst, ZnO has reflected comparable efficiency to that of TiO₂. Analogous to TiO₂, ZnO could serve as an ideal photocatalyst candidate that is cost-effective and environment-friendly. Other than that, the comparable intrinsic bandgap energy of ZnO (3.37 eV) [15] to that of TiO₂ (3.2 eV) [16] also makes it (ZnO) a suitable candidate in absorbing UV-light as an excitation source. Recently, various reports have shown that ZnO are good photocatalyst materials. For instances, Sakthivel et al. found that ZnO nanoparticles exhibit higher rate than that of TiO₂ nanoparticles. They attributed this to the ability of ZnO in absorbing large fraction of the solar spectrum and more light quanta than that of TiO₂ [17]. Furthermore, Sun et al. have shown that photocatalytic rate and TOC removal efficiency of ZnO can be improved through modification of its morphology into dumbbell-shape [18].

For few decades, ZnO has been reported as a unique material that exhibits semiconducting, piezoelectric, and pyroelectric multiple properties [19]. Due to its unique intrinsic properties, which are relatively anisotropic, ZnO has been synthesized in various shapes and dimensions either it is in 1D, 2D or 3D. Additionally, many reports regarding the application of ZnO nanostructures especially in optoelectronic device have been logged [19], [20], [21], [22], [23], [24]. However, most of these ZnO nanostructures are 1D ZnO. As a result, the intensive efforts put in the studies of 1D nanostructures have neglected the development of 2D ZnO nanostructures. Until now, there are limited reports on the application of 2D ZnO nanostructures especially as photocatalyst materials.

Hereby, for the first time we use own-synthesized 2D ZnO nanopellets in this study for the photodecomposition of MB. The as-synthesized 2D ZnO nanopellets are prepared via low-cost process using environment-friendly zinc oleate compound through simple one-pot pyrolysis reaction [1]. These nanopellets exhibit low photocatalyst rate due to the presence of oleic acid on its surface. However, we have successfully improved its (2D ZnO nanopellets with the weight of 0.04 g) photocatalytic rates to a factor of 22.0 times through refluxing process in excessive pyridine. There is no large variation in terms of both size and shape distributions after reflux. Hence, this treatment process may open a new route in the removal of capping ligand from nanomaterials surface and preserve its originality especially for nanomaterials prepared via organometallic approach.

Besides that, there are several other beneficial aspects of using 2D ZnO nanopellets as photocatalyst found in this study like the consistent efficiency, stability, ease of material synthesis, and advanced nanostructure of ZnO. Thus, in contrast to the study carried out by Formo et al. [25], the as-synthesized 2D ZnO nanopellets are more stable and exhibiting excellent recyclability up to three cycles, and no reactivation process is required to recover the loss of photocatalytic efficiency which is caused by poisoning effects. Moreover, the approaches used in photocatalytic reaction of ZnO, which are fixed-bed [26] and free-flow suspension, would as well influence the efficiency of photocatalysts. The free-flow suspension approach adopted in this study would provide more surface area of 2D ZnO nanopellets during photocatalytic process than that of fixed-bed and this would drastically contribute to higher photocatalytic rate. Furthermore, not only the solution synthesis of 2D ZnO nanopellets is producing higher yield if compared to that of gas phase synthesis, it is also a more economical route in industrial-scale wastewater treatment [27], [28].