Elsevier

Applied Surface Science

Volume 479, 15 June 2019, Pages 608-618
Applied Surface Science

Full length article
Nanofiber immobilized CeO2/dendrimer nanoparticles: An efficient photocatalyst in the visible and the UV

https://doi.org/10.1016/j.apsusc.2019.02.119Get rights and content

Highlights

  • CeO2 nanoparticles show photocatalytic activity in the UV and at visible light.

  • Dendrimer coating of CeO2 moves the band gap towards the visible.

  • Immobilization enhances the photocatalytic activity and allows simple recycling.

  • Hydroxyl radical intermediates are detected by fluorescence spectroscopy.

  • Photosensitization is not involved.

Abstract

The contribution of a nanofiber based support and dendrimer coating on the visible light photocatalytic activity of CeO2 (ceria) nanoparticles towards phenol and azorubine dye was investigated. CeO2 nanoparticles were immobilized on electrospun nanofiber mats obtained from pullulan/poly(vinyl alcohol)/poly(acrylic acid) (Pul/PVA/PAA) blends and the CeO2 surface was modified with generation 3.0 PAMAM dendrimer. The catalytic activity was studied between pH 4 and 11 and it was compared with free CeO2- and free TiO2-nanoparticles for reference. The supported catalyst was photocatalytically active towards both phenol and azorubine and immobilization increased its reactivity by a factor of three. The mat type nature of the immobilized catalyst was also beneficial for simple recycling. XPS spectra revealed the presence of a Ce3+ state and the formation of hydroxyl radicals was shown using fluorescence spectroscopy.

Introduction

Water purification is an important global issue. Industrial wastewaters contain a variety of organic pollutants which are stable, toxic, and resistant to biodegradation. Phenolic compounds and dyes are two representatives of such pollutants [1]. Azorubine is a typical azo dye containing β-naphthylamine in the chemical structure [2]. The carcinogenic character of arenes requires the development of economically feasible elimination processes of these compounds from wastewaters, such as degradation [3].

The photocatalytic degradation process is an eco-friendly, economically viable, and effective method for the degradation of organic pollutants in wastewaters [[4], [5], [6]]. This method is based on sufficient illumination with light using a photocatalyst – typically a semiconductor metal oxide – and oxygen as oxidizing agent [7]. The most popular semiconductors in water treatment are TiO2 and ZnO due to their promising photocatalytic activity and nontoxicity [[8], [9], [10], [11]]. Their main drawback is a wide band gap which moves the photocatalytic activity towards the UV spectral range (~5% of the solar spectrum). Research is therefore focusing on alternative metal oxide semiconductors such as cerium oxide [[12], [13], [14], [15], [16], [17], [18]]. Due to its band gap of Eg = 2.8–3.1 eV, ceria (CeO2) is not only a photocatalyst in the UV range but it has also the potential to show visible light photocatalytic activity [[19], [20], [21], [22], [23], [24]].

Pushing photocatalytic activity towards the visible light range is an active area of research [25]. A promising surface treatment [26,27] for metal oxide nanoparticles is the introduction of amino groups [28]. Poly(amido-amine) (PAMAM) is a dendrimer with ethylene diamine core and different applications such as drug delivery [29], cancer treatment [30], and sensors [31,32]. Recently, decorating TiO2 with dendritic polymers was investigated and a shift in photocatalytic activity towards visible light was shown [[33], [34], [35]].

The second challenge in photocatalytic degradation is the removal of the catalyst from the reaction mixture for recovery and reuse. Recovery of photocatalysts was achieved by immobilizing them on various supports such as activated carbon and brick grain particles [36], glass fibers [37], silica [38], and different polymer matrixes [[39], [40], [41], [42]]. Polymer nanofibers have interesting properties as catalyst support such as a large specific surface area and a high porosity to facilitate mass transport [[43], [44], [45], [46], [47], [48], [49]]. Choosing an environmentally friendly supporting polymer can prevent follow-up problems related to polymer accumulation in the environment. Pullulan is a biodegradable and edible polysaccharide based on starch which is suited for the syntheses of nanofiber-based catalyst supports. Upon thermal crosslinking, pullulan becomes water insoluble [45,46].

Here, we report the photocatalytic activity of a nanocomposite based on cerium oxide nanoparticles decorated with PAMAM dendrimer which were successfully immobilized on a mat of electrospun pullulan/poly (vinyl alcohol)/poly(acrylic acid) (Pul/PVA/PAA) nanofibers. The efficiency of the catalyst was investigated both upon UV and visible light illumination and it was compared with the activity of TiO2 nanoparticles as a well-known photocatalyst in the UV [50]. Controlling activity parameters were identified and a possible mechanism for the photocatalytic degradation of both phenol and azorubine is presented.

Section snippets

Materials

Cerium (IV) oxide (ceria, CeO2-NP) (nanopowder, <25 nm), (3-glycidyloxypropyl) trimethoxysilane (>98%) (GPTMS), PAMAM dendrimer with ethylene diamine core, generation 3.0 solution (20 wt% in methanol), PVA (Mw = 89,000–98,000 Da, DH = 99%), PAA sodium salt (Mw = 5100), Titanium(IV) oxide P25, sodium hydroxide, hydrochloride acid (ACS reagent 37%), potassium persulfate (K2S2O8), ammonium oxalate (AO), 4-Hydroxy-1-naphthalenesulfonic acid sodium salt (HNSA), phenol, terephthalic acid, and sodium

Synthesis and characterization of DCN

The synthesis of the DCN nanocomposite was straight forward as illustrated in Scheme 1. Stirring CN-2 in ethanol introduced hydroxyl groups at the CeO2–NPs surface, which allowed coupling with GPTMS in the following step. GPTMS was used as a reactive silane coupling agent for its capacity as a chemical linker between inorganic (cerium oxide) and organic (dendrimer) materials [[52], [53], [54], [55]]. Such treated nanocomposite had epoxy terminal groups which were available to covalently attach

Conclusions

We have demonstrated the photocatalytic activity of a series of nanofiber immobilized CeO2-NPs both under UV and visible light illumination. The highest photocatalytic activity was observed for nanofibers coated with a monolayer of CeO2-NPs and activity was lost with a higher degree of coating. Surface treating the nanofiber immobilized CeO2-NPs with dendrimer enhanced the photocatalytic efficiency considerably and shifted the band gap towards visible light illumination. The new nanocomposite

Acknowledgements

We thank Rahel Bollinger and Michael Edelmann for their support with the TOC measurements. We thank Roman Kontic and Jan Inauen for performing the XPS and XRD measurements.

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