Elsevier

Catalysis Today

Volume 284, 15 April 2017, Pages 77-83
Catalysis Today

Polystyrene CuO/Cu2O uniform films inducing MB-degradation under sunlight

Catalysis Today Special Issue “Materials for photocatalytic degradation of contaminants of environmental concern” dedicated to Prof. András Dombi
https://doi.org/10.1016/j.cattod.2016.10.018Get rights and content

Highlights

  • Cu/CuOx films have been sputtered on polystyrene (PS) Petri-dishes.

  • Cu/CuOx-PS showed fast MB discoloration/degradation under visible light.

  • A mechanism of MB-degradation is suggested.

  • Cu/CuOx-PS surface has been characterized by up-to-date techniques.

Abstract

This study reports on a Cu-sputtered film on polystyrene (PS) leading to the discoloration/degradation of methylene blue (MB) under low intensity solar simulated irradiation. Direct current magnetron sputtering (DCMS) was used to graft uniform, adhesive Cu/Cu oxides on the polystyrene substrate. The kinetics of Cu-PS mediated MB-discoloration adding H2O2 was observed to take place within 90–120 min. The surface potential and pH variation was followed on the Cu-PS surface during MB-discoloration. Insight is provided for the observed changes relating them to the dye discoloration mechanism. The concentration, mean- free path and lifetime of the oxidative radical leading to MB-degradation were estimated. The Cu/Cu-oxides on the PS were characterized by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) evidence for redox catalysis involving Cu(I)/Cu(II)-species was detected during MB-discoloration. Also by XPS the surface atomic percentage concentration was determined for the topmost Cu-PS layers. The Cu-PS coatings were also investigated for their optical and crystallographic properties.

Introduction

Photocatalytic degradation of pollutants is a promising approach to face the increasing environmental contamination. Under light, photoactive titanium dioxide (TiO2) has been shown to degrade organic compounds/dyes due to its photocatalytic properties [1], [2]. However, because of its wide band gap (3.2 eV) TiO2 can absorb only UV light (λ< 387 nm), about 4–5% of the whole solar spectrum. Sensitization of the TiO2 by doping with Cu/CuO has been shown to be attractive approach to extend the TiO2 response into the visible region [3], [4]. This approach is used in this study to improve the TiO2 reaction kinetics under the visible light irradiation during discoloration of the probe dye methylene blue (MB).

The use of photocatalysts in the form of colloids, powders and flakes is costly since it has to be separated after the cleaning process. Therefore, TiO2 deposited on various substrates like glass and thin polymer films have been investigated for over three decades mainly being activated by UV-light to avoid the time, cost and materials involved in the separation step. TiO2 colloids have been deposited on supports by calcination on heat resistant substrate supports. On low heat resistant surfaces/polymers, the support will no withstand precluding TiO2 annealing to attain good adhesion. Colloids annealed at lower temperatures do not withstand friction, are easily wiped out, and are not reproducible or mechanically resistant [5]. To avoid the problems inherent to colloidal particle deposition on low thermal resistant substrates, this study focuses on the sputtering of Cu/Cu2O/CuO films on PS. The interest of using polystyrene (PS) resides in the fact that it is a low cost, inert, no-toxic widely available thermoplastic polymer.

During the last few years there has been a growing interest in the preparation of PS films as a substrate for TiO2 nanoparticles in the area of photocatalytic degradation of organic compounds. The interest to degrade the MB dye resides in the fact that MB- is widely used in printing/dyeing industries inducing eye irritation, vomiting and cyanosis [6]. Yang et al., [7] reported TiO2 anchored on PS degrading MB under UV-light within 3–4 h. TiO2 Degussa P25 PS was dip-coated on PS by Singh et al., [8] and led to partial MB-degradation within a few hours.

To extend the TiO2 absorption into the visible range, several laboratories have recently reported TiO2-F and TiO2-N doped PS films prepared by colloidal methods degrading organic compounds under sunlight light within a few hours J. K. Zhou et al., report degradation of MB [9], K. M. Parida et al., report degradation of methyl-orange (MO) and MB, an anionic and a cationic dye [10]. V. Viano et al., report the degradation of MB [11], L. Huang et al., [12] reports MO degradation and Z. Zheng et al., [13] et al., have reported Cu(I)O and Cu(II)O active in the photocatalytic degradation of pollutants/dyes under visible light up to 680 nm.

The fact that CuO and Cu2O absorb light in the range of the solar radiation with a band-gap < 2 eV and is a low cost oxide has led to the its use in environmental processes [14], [15], [16]. Recently, ppb amounts of Cu were sputtered on TiO2 or TiO2-ZrO2 giving rise to materials with improved self-cleaning applications by way of the Cu intra-gap states in the TiO2 matrix [16], [17]. The nature of these intra-gap states in TiO2 is far from been understood at the present time and more work is needed in this direction [18].

The novelty presented in this work is the grafting of Cu/CuO uniform, adhesive and stable thin films on polystyrene (PS) leading to a MB-degradation in acceptable times under low intensity sunlight. The times of MB-degradation found in the present study were shorter than the times reported until now irradiated by UV-light. This implies a reduction in the use of electrical energy. Optimization of the sputtered film thickness to attain the most suitable MB-degradation times was carried out during the course of this study. The characterization of the Cu/CuO PS film properties is also reported in the present work.

Section snippets

Materials, irradiation procedures, Cu-sputtering and Cu- thickness calibration, DRS of Cu-films and determination of MB-discoloration

The MB and H2O2 were Merck reagents (Buchs, Switzerland) and used as received. NaN3 Fluka was used as singlet oxygen scavenger (1O2). 1,4-benzoquinone (BQ) and methanol were used respectively as O2•− radical and HO.-scavengers. Ethylenediamine tetra-acetic acid (EDTA-2Na) was used as CuOxvb hole scavenger. To complex the Cu-species leached out from the sputtered Cu/CuOx-film, batho-cuproin disulfonate (4 μM) and sodium ascorbate (10 μM) were added to the MB-solution before contacting the

Effect of the Cu-film thickness and effect of added H2O2 on the MB-discoloration kinetics and photocatalyst recycling

Fig. 1b indicates for Cu-sputtered at 300 mA a thickness of 100 nm after 100s. Therefore, 1 nm was sputtered within 1s, this is equivalent to about 5 Cu-atomic-layers each one of 0.2 nm. Taking a 0.3 nm lattice distance between the Cu-atoms, about 1015 Cu-atoms can be estimated per cm2. The Cu-deposition proceeds with 5 × 1015 atoms/cm2xs. Fig. 1b shows in the thickness calibration a ±10% experimental error. Fig. 1c shows the discoloration kinetics of MB under low intensity solar light on Cu-PS. It is

Conclusions

This study presents the preparation of an innovative uniform, stable and adhesive sputtered Cu-PS photocatalyst at low temperature by DC-magnetron sputtering. This is the first report on uniform coatings of Cu/CuO on PS catalysts leading to the degradation of a model dye. The MB-discoloration kinetics in the presence of H2O2 was completed within 90–120 min. The degradation of MB occurs within a time about 3 times shorter than its mineralization, suggesting the formation of long-lived

Acknowledgments

We thank the EPFL Institute of Chemistry and Chemical Engineering (EPFL-SB −ISIC Switzerland for the financial support of this study. The authors wish to express their recognition to Prof. András Dombi, a key figure in the topic of photocatalytic materials for the degradation of contaminants of environmental concern.

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    The authors have contributed equally to this study.

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