Ternary systems based on ZnO/CeO2/Cu2O for the degradation of phenol and carbamazepine

https://doi.org/10.1016/j.jallcom.2020.158167Get rights and content

Highlights

  • Precipitation and hydrothermal synthesis of the mixed systems ZnO/CeO2/Cu2O.

  • Structural and spectroscopic characterization has been performed.

  • Electron Paramagnetic Resonance and UV vis DR demonstrated the presence of residual Cu2+ ions well dispersed on the surface.

  • Phenol and carbamazepine were rapidly completed abated by the material prepared via hydrothermal synthesis.

Abstract

In this paper we prepared via different synthetic processes, two ternary systems based on ZnO, CeO2 and Cu2O for the abatement of organic pollutants. The system ZnO/CeO2 was already known to be efficient in the degradation of emergent contaminants, the addition of cuprous oxides allows also to enhance reductive properties to the material thanks to its specific potential. The mixed oxides were characterized via power X Ray Diffraction, UV visible Diffuse Reflectance and Electron Paramagnetic Resonance. The materials obtained through hydrothermal synthesis shown better performances in the abatement of phenol and carbamazepine.

Introduction

The search for new materials that work as photocatalysts is constantly evolving. In recent years, much attention has been paid to various methods for modifying the classic first generation semiconductors such as TiO2. Among the different types of material engineering, the most used are the intrinsic and extrinsic doping, the formation of solid solutions and the creation of heterojunctions at the interface between two materials that have no tendency to form solid solutions. The development of semiconductor-semiconductor composite heterojunctions for efficient photocatalytic pollutant degradation is of great interest. ZnO is one of the most abundant, cheap and non-toxic semiconductor, already employed for degradation of different pollutants [1], [2], [3].

The photocatalytic degradation process depends on various factors: the light-absorption ability of the semiconductor, which in turn depends on its surface area and morphology, optical band gap and in particular the charge separation capability [4]. The photocatalytic performances of ZnO can be improved either through doping with metals or nonmetals or via the preparation of ZnO heterostructures with other semiconductor elements [5]. Composite nanostructures of transition metal oxides are of great interest because of their tunable optical as well as catalytic properties. Recently, our and other [6], [7], [8] research groups started to investigate the role of cerium as dopant of oxides. In our approach, cerium ions are not soluble in the ZnO matrix. This composite system has shown an amazing, unexpected activity in the mineralization of acesulfame K, a so-called emerging pollutant, performed under visible light [9]. This result led us to an in-depth study of this new material. We observed that the CeO2/ZnO system is biphasic and the role of the solid−solid interface plays a crucial role in the photo-activity of the material.

Coupled semiconductors formed by ZnO and other metal oxides or sulfides (e.g., CuO, Cu2O, CdO, CuS, CdS, and ZnS) have been employed as photocatalysts, enhancing the charge carrier separation ability (electrons and holes) between adjacent semiconductors [10]. In particular, cupric and cuprous oxides (CuO and Cu2O) are nontoxic p-type metal oxide semiconductor having an optical band gap in the range from 1.2 to 2.2 eV [11]. Moreover, Cu2O is considered very interesting thanks to its relative band position with respect to the normal hydrogen electrode (NHE) (- 1.2 eV for the conduction band and +1.3 eV for the valence band). It has been demonstrated that it instigates reduction and oxidation of different pollutants via the formation of hydroxyl and superoxide radicals (with formation energies of approximately +1.23 and - 0.28 eV, respectively), which are the main reactive species in photocatalytic reactions [12]. Accordingly, addition of Cu2O to ZnO in a ZnO–Cu2O–CuO nanocomposite configuration could result in photocatalytic activity superior to that of the intrinsic ZnO semiconductor system. As it will be shown below, it is indeed the role of these interfaces (or heterojunctions) that allows a better understanding of this novel promising photocatalytic system. In the present work we prepared nanocomposite systems coupling CeO2/ZnO materials with Cu2O particle with two different synthetic methods. The preparation process influences the surface area and the crystallinity of the photocatalysts affecting also their photocatalytic properties. We tested these materials in the degradation of a classic probe molecule like phenol and of an emergent pollutant such as carbamazepine.

Section snippets

Experimental section

All reactants employed were purchased by Sigma-Aldrich and used without any further purification treatment. Distilled water was used in the synthesis procedures. Synthesis procedure will be described for each system in the following corresponding section.

The investigated samples were mainly prepared via hydrothermal and precipitation synthetic routes, described hereafter.

Structural and optical characterization

The ZnO nanoparticles synthetized via precipitation and hydrothermal methods were analysed from a structural point of view by means of X-ray powder diffraction. The structural analysis of these materials has been already well described elsewhere [2], [15], the diffrattograms have been reported in the Supporting Information section (see Fig. S1), showing the typical pattern of ZnO wurtzitic hexagonal phase (00–036–1451 ICDD pattern) [16].

The optical properties of the synthetized samples were

Conclusions

The formation of heterojunction between ZnO and CeO2 has been observed experimentally in the past, the addition of Cu2O nanoparticles on the surface of the mixed systems leads to the formation of a very active material in the abatement of phenol and carbamazepine molecules. Residual traces of Copper (II) not reduced have been evidenced by UV Vis analysis and EPR measurements. These species seem to be well dispersed over the surface of the semiconductors. The role of these species in the photo

CRediT authorship contribution statement

Erik Cerrato: Conceptualization, Data curation, investigation. Elettra Rebolini: Validation, data curation. Debora Fabbri: Supervision. Paola Calza: Resources, Supervision. Maria Cristina Paganini: Conceptualization, Supervision Writing - original draft, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

Financial support from the Italian MIUR through the PRIN Project 20179337R7, MULTI-e “Multielectron transfer for the conversion of small molecules: an enabling technology for the chemical use of renewable energy” and the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No 765860 (AQUAlity) is gratefully acknowledge-edged.

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