Interplay between ferromagnetism and photocatalytic activity generated by Fe3+ ions in iron doped ZnO nanoparticles grown on MWCNTs

https://doi.org/10.1016/j.physe.2020.114581Get rights and content

Highlights

  • Fe doped ZnO Nps grouped into quasi-spherical structures were grown in situ on MWCNT.

  • XPS measurement reveals that Fe2+ and inner/surface Fe3+ ions coexist in samples.

  • Inner Fe3+ is responsible for ferromagnetism and photogenerated e-h pairs separation.

  • Surface Fe3+ influence the photodegradation efficiency by O2 generation.

  • The best photocatalytic performance was obtained for MWCNT-ZnO:Fe5% sample (97%).

Abstract

In order to limit the photogenerated charges recombination and to ensure an efficient photocatalytic degradation of RhB pollutant, a MWCNT-ZnO:Fe composite material was designed. Fe doped ZnO Nps grouped into quasi-spherical structures of about 100–250 nm were grown in situ on MWCNTs using sol-gel method. The XPS and UPS analysis allowed the identification of both Fe2+ and Fe3+ ionic species at substitutional and surface positions. The substitutional Fe3+ will generate an inverse Burstein-Moss effect and, as a result, the Fermi level is will be pushed down towards the valence band by 0.7, 0.45 and 0.29 eV for 2, 3 and 5% Fe doping respectively. The Fe doped samples show ferromagnetic behavior as determined from both ESR and magnetization measurements. The ferromagnetic order, the inverse Burstein-Moss effect, and the hole capture into the MWCNT valence band favors the charge separation of photogenerated e-h pairs and the reactive oxygen species (ROS) generation in ZnO. The proposed energy bands setup allowed to elucidate both ferromagnetic order formation and the photodegradation mechanism. Thus, an Fe doping degree of 5% ensured, besides the highest magnetization, a 97% photodegradation efficiency and a pollutant mineralization degree of 92%.

Introduction

Nowadays, progress regarding the degradation of water discharged organic pollutants, coming mainly from the textile and food industries, is one of the research topics that has attracted an increasing interest. Over the years, several methods have been tested for the dyes removal such as: adsorption [1], ion exchange [2], membrane technology [3], coagulation and sedimentation [4]. Among them, photocatalysis represents a promising method which is able not only to degrade a wide range of organic pollutants but also achieve a complete mineralization of them [5]. Generally, the photocatalysis is associated with a complex of phenomena which occurs under light irradiation: formation of the electron-hole pairs, their migration to the photocatalyst surface, their interaction with the water molecules and the generation of reactive oxygen species (ROS). The latter process is directly involved in the degradation of organic compounds and, under certain conditions, leads to almost complete mineralization of them. For these reasons, the ROS are considered key factors in breaking and complete degradation of the pollutant molecules.

Many attempts have been made to develop nanomaterials with increased photocatalytic efficiency. One of the proposed strategies is based on the combination between carbon nanostructures and semiconductor nanoparticles, thus resulting in the formation of nanocomposites with new and unique properties [6]. MWCNTs are a one-dimensional class of nanomaterials endowed with a high mobility of charge carriers, superior mechanical properties and having a high surface area [7]. The mechanism by which MWCNTs contribute to the increase of photocatalytic activity is not yet fully elucidated. There are studies showing that MWCNTs act as acceptors of photogenerated electrons in semiconductor nanoparticles or as sensitizers, pumping electrons into the nanoparticles conduction band or they act as escape paths for photogenerated holes [8]. All these processes are strongly dependent on the energy band sat-up at the interfaces inside the nanocomposite. In this way it inhibits the charge recombination process [8]. Furthermore, due to their high surface area, MWCNTs are a good support for in situ growing of semiconductor nanoparticles thus assuring a uniform dispersion and increasing the contact between photocatalyst and the water containing the pollutant molecule [9,10].

A further improvement of the photocatalytic performance of these nanocomposites materials could be done by controlling the semiconductor properties such as: morphology, particle size, defect concentration, and doping [[11], [12], [13]]. Among various oxide semiconductor materials, zinc oxide is a very attractive one since it exhibits a high photocatalytic activity due to its specific longer lifetime of excitons, absorption of a wide UV spectrum fraction and high mineralization degree [[14], [15], [16]]. Previous studies have shown that by doping it with transitional metals such as Mn [17], Cu [18], Co [19], Ni [20], the photocatalytic activity of ZnO has been improved through the band gap narrowing and the generation of charge traps thus delaying of the recombination process [21]. Another strategy to improve the ZnO photocatalytic activity is by coupling with other semiconductors which assure an efficient separation of photoexcited electron-hole pairs at the interface and extend the adsorption range of ZnO [[22], [23], [24]].

Several studies reported the enhancement of photocatalytic activity by anchoring ZnO nanoparticles on the carbon nanotubes surface [7,25]. Only few reports on the optical, electronic and photocatalytic properties of carbon nanotubes decorated with doped ZnO are found [26,27] but the photocatalytic mechanism have not been investigated in detail.

The purpose of this research is to develop a nanocomposite material based on MWCNTs and Fe doped ZnO nanoparticles with high photocatalytic activity by assuring an efficient electron-hole separation and to evidence the key role played by the quantity, distribution and valence state of dopant ions on the photocatalytic mechanism and ferromagnetic order. Thus, a series of Fe-doped ZnO nanoparticles with different iron concentration were grown in-situ onto the MWCNTs surface by using a simple one step method. The influence of Fe doping in both Fe2+ and Fe3+ ionization states on the structural, optical, magnetic and photocatalytic properties of prepared composite materials was studied. The photocatalytic mechanism and the ferromagnetic behavior will be discussed by analyzing the alignment of energy bands and by coupling of ultra violet photoelectron spectroscopy (UPS), XPS, optical spectroscopies, electron spin resonance (ESR), magnetometry, and ROS generation results.

Section snippets

Materials

The following materials and reagents were used for the preparation of MWCNTs decorated with ZnO:Fe: Carbon nanotubes, multiwalled DxL 110 × 170 nm x 5–9 μm (Aldrich), sulfuric acid- H2SO4 97% (Merck), nitric acid-HNO3 65% (Alpha Aesar), zinc acetate - Zn(CH3COO)2 × 2H2O (Alpha Aesar), iron nitrate-Fe(NO3)3 × 9H20) diethylene glycol (DEG, C4H10O3 (Merck), absolute ethanol C2H5OH–EtOH (Merck) and deionized water. All chemicals are analytical grade without further purification and were used as

Results and discussions

The phase structure and crystallinity were analyzed by XRD measurements. The diffraction patterns of all samples are presented in Fig. 1. The peaks positioned at 2θ = 26.3, 42.5, 54.1° can be assigned to the (002), (101), (004) planes of the graphitic structure of MWCNTs (JCPDS No: 00-058-1638). Additional peaks overlap to the MWCNTs XRD patterns and were associated with the ZnO hexagonal wurtzite structure (JCPDS No:01-081-9217).

These peaks are well-defined and intense indicating the good

Conclusions

In summary, Fe-doped ZnO nanoparticles have been grown in-situ on the MWCNTs surface by sol-gel process for photocatalytic applications under UV light irradiation. By XRD measurements was evidenced that Fe ions with both 2+ and 3+ oxidation states can enter in the ZnO lattice, the preponderance of a certain valence state being influenced by the doping degree. The presence in ZnO lattice of both surface uncoupled Fe3+ ions and of exchange correlated Fe2+ ions was highlighted also by ESR

Author statement

Adriana Popa: Conceptualization, Investigation, Writing – original draft, Writing – review & editing, Ovidiu Pana: Conceptualization, Writing – original draft, Writing -Review & Editing, Maria Stefan: Investigation, Validation, Dana Toloman: Conceptualization, Investigation, Manuela Stan: Investigation, Cristian Leostean: Investigation Ramona Crina Suciu: Investigation, Grigore Vlad: Investigation, Sorin Ulinici: Investigation, Gabriela Baisan: Investigation, Sergiu Macavei: Investigation,

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.

Acknowledgements

The authors would like to express appreciation for the financial support of Ministry of Education and Research, Operational Program Competitiveness, POC Project 18/01.09.16, SMIS Code 105533 and for infrastructure access provided by the Romanian Ministry of Research and Innovation through the Project no. 32PFE/19.10.2018.

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