Full length articleNanofiber immobilized CeO2/dendrimer nanoparticles: An efficient photocatalyst in the visible and the UV
Graphical abstract
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.
References (87)
- et al.
Degradation of phenol by nanomaterial TiO2 in wastewater
Chem. Eng. J.
(2006) - et al.
Efficient degradation of Azorubin S colourant in the commercial jam-jelly food samples using TiO2-CoFe2O4 nanocomposites in visible light
Mater. Res. Bull.
(2017) - et al.
Decontamination and disinfection of water by solar photocatalysis: recent overview and trends
Catal. Today
(2009) - et al.
Formic acid enhanced effective degradation of methyl orange dye in aqueous solutions under UV-Vis irradiation
Water Res.
(2016) Photocatalytic purification and remediation of contaminated air and water
C.R. Acad. Sci. Ser. IIc: Chim.
(2000)- et al.
A review on the visible light active titanium dioxide photocatalysts for environmental applications
Appl. Catal. B Environ.
(2012) - et al.
TiO2 photocatalysis: design and applications
J. Photochem. Photobiol. C
(2012) - et al.
Recent developments of zinc oxide based photocatalyst in water treatment technology: a review
Water Res.
(2016) - et al.
An overview on limitations of TiO2-based particles for photocatalytic degradation of organic pollutants and corresponding countermeasures
Water Res.
(2015) - et al.
Solution-processed CeO2/TiO2 nanocomposite as potent visible light photocatalyst for the degradation of bromophenol dye
Chem. Eng. J.
(2014)