Photocatalytic Performance of Cuxo/Tio2 Deposited by Hipims on Polyester Under Visible Light Leds: Oxidants, Ions Effect, and Reactive Oxygen Species Investigation

materials

Article Photocatalytic Performance of CuxO/TiO2 Deposited by HiPIMS on Polyester under Visible Light LEDs: Oxidants, Ions Effect, and Reactive Oxygen Species Investigation

Hichem Zeghioud 1 , Aymen Amine Assadi 2,*, Nabila Khellaf 3, Hayet Djelal 4, Abdeltif Amrane 2 and Sami Rtimi 5,* 1 Department of Process Engineering, Faculty of Engineering, Badji Mokhtar University, P.O. Box 12, 23000 Annaba, Algeria; [email protected] 2 Ecole Nationale Supérieure de Chimie de Rennes, Université de Rennes 1, CNRS, UMR 6226, Allée de Beaulieu, CS 50837, 35708 Rennes CEDEX 7, France; [email protected] 3 Laboratory of Organic Synthesis-Modeling and Optimization of Chemical Processes, Badji Mokhtar University, P.O. Box 12, 23000 Annaba, Algeria; [email protected] 4 Ecole des Métiers de l’Environnement, Campus de Ker Lann, 35170 Bruz, France; [email protected] 5 Ecole Polytechnique Fédérale de Lausanne, EPFL-STI-LTP, Station 12, CH-1015 Lausanne, Switzerland * Correspondence: [email protected] (A.A.A.); sami.rtimi@epﬂ.ch (S.R.)

Received: 29 December 2018; Accepted: 22 January 2019; Published: 29 January 2019

Abstract: In the present study, we propose a new photocatalytic interface prepared by high-power impulse magnetron sputtering (HiPIMS), and investigated for the degradation of Reactive Green 12 (RG12) as target contaminant under visible light light-emitting diodes (LEDs) illumination. The CuxO/TiO2 nanoparticulate photocatalyst was sequentially sputtered on polyester (PES). The photocatalyst formulation was optimized by investigating the effect of different parameters such as the sputtering time of CuxO, the applied current, and the deposition mode (direct current magnetron sputtering, DCMS or HiPIMS). The results showed that the fastest RG12 degradation was obtained on CuxO/TiO2 sample prepared at 40 A in HiPIMS mode. The better discoloration efficiency of 53.4% within 360 min was found in 4 mg/L of RG12 initial concentration and 0.05% Cuwt/PESwt as determined by X-ray fluorescence. All the prepared samples contained a TiO2 under-layer with 0.02% Tiwt/PESwt. By transmission electron microscopy (TEM), both layers were seen uniformly distributed on the PES fibers. The effect of the surface area to volume (dye volume) ratio (SA/V) on the photocatalytic efficiency was also investigated for the discoloration of 4 mg/L RG12. The effect of the presence of different chemicals (scavengers, oxidant or mineral pollution or salts) in the photocatalytic medium was studied. The optimization of the amount of added hydrogen peroxide (H2O2) and potassium persulfate (K2S2O8) was also investigated in detail. Both, H2O2 and K2S2O8 drastically affected the discoloration efficiency up to 7 and 6 times in reaction rate constants, respectively. Nevertheless, the presence of Cu (metallic nanoparticles) and NaCl salt inhibited the reaction rate of RG12 discoloration by about 4 and 2 times, respectively. Moreover, the systematic study of reactive oxygen species’ (ROS) contribution was also explored with the help of iso-propanol, methanol, and potassium dichromate as •OH radicals, holes (h+), and superoxide ion-scavengers, respectively. − • Scavenging results showed that O2 played a primary role in RG12 removal; however, OH radicals’ + and photo-generated holes’ (h ) contributions were minimal. The CuxO/TiO2 photocatalyst was found to have a good reusability and stability up to 21 cycles. Ions’ release was quantified by means of inductively coupled plasma mass spectrometry (ICP-MS) showing low Cu-ions’ release.

Keywords: photocatalysis; reactive green 12; CuxO/TiO2; polyester; HiPIMS; visible light LEDs

Materials 2019, 12, 412; doi:10.3390/ma12030412 www.mdpi.com/journal/materials Materials 2019, 12, 412 2 of 16

1. Introduction The textile industry is one of the largest consumers of water on our planet, and the second most polluting industry after the oil and gas industry [1,2]. Textile effluent containing synthetic dyes have toxic and carcinogenic compounds, which are stable and non-biodegradable due to the high molecular weight, the presence of azo bonds and amide groups in the molecular structure. Many of the chemical and physical treatment processes have shown insufficient results toward the degradation of theses pollutants. Recently, heterogeneous photocatalysis is one of the advanced oxidation processes that increasingly interests researchers. Photocatalysis was seen to transform/mineralize these synthetic dyes to lesser harmful by-products before their discharge into the environment. In the last decade, numerous works have been mostly focused on the designing and the development of new photocatalytic materials [3–8]. In this direction, different methods and preparations were investigated [3]. Sol–gel [4], hydrothermal [5], combined sol–gel/hydrothermal [6], chemical vapor deposition (CVD) [7], liquid phase deposition (LPD) methods [8] among many others were reported in the literature. In the aim of understanding the relationship between physico-chemical proprieties of photocatalyst and photocatalytic performances, various strategies have been adopted. Many studies investigated the effect of (1) semiconductor type such as TiO2, ZnO, and SnO2 [9,10]; (2) the number (mono-, bi-, or tri-doping) and the ratio of doping agents (N, P, Fe, Ag, etc.) [11–14]; (3) the support type such as polyester, cellulose, polypropylene, or polystyrene [15–17]; and (4) the substrate functional groups before the catalyst deposition [18,19]. It had been widely reported that band gap energy, surface area, particles size, and chemical stability were the most important parameters controlling the reactivity of a photocatalytic material [20–22]. TiO2 was reported to be the most suitable photocatalyst because of its high stability, low toxicity, low cost, chemical inertness, wide band gaps, and resistance to photo-corrosion [23–25]. Conventional direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) have been used during the last decade to prepare photoactive thin films [26,27]. These films were reported to exhibit redox catalysis leading to bacterial inactivation [19,24,27], organic dyes degradation [25], antifungal [28], corrosion resistance [29], and self-cleaning [30]. Sequentially sputtered TiO2/Cu showed bacterial inactivation in the minute range [31,32]. Co-sputtered TiO2/CuxO using HiPIMS was reported to lead to compact photoactive films showing reduced ions’ release [31]. The main goals of the present study are (i) to explore the HiPIMS deposition of CuxO on TiO2 under-layer; (ii) to optimize the deposition parameters leading to stable thin film materials showing fast degradation of a toxic textile dye (Reactive Green 12) as target hazardous compound; (iii) to use the visible light light-emitting diodes (LEDs) system as efficient and economic light source; and (iv) to test the effect of the presence of some oxidant, mineral pollutant, or salts on the performance of the photoactive material. This latter goal is a step further to mimic real textile industry wastewater effluents normally presenting salts and oxidative agents. The photo-generation and the contribution of reactive oxygen species (ROS) are investigated in detail.

2. Experimental

2.1. Materials Hydrogen peroxide (35 wt %, Merck KGaA, Darmstadt, Germany), potassium dichromate (>99 wt %, Carlo Erba Reagents S.A.S., Chaussée du Vexin, France), 2-propanol (>99.5%, Merck KGaA, Darmstadt, Germany), potassium peroxydisulfate (≥90 wt %, Merck KGaA, Darmstadt, Germany), chloroform (>99.97%, Acros-Organics, Thermo Fisher Scientific, Geel, Belgium), methanol (99.99%, Thermo Fisher Scientific, Geel, Belgium), copper (98.5 wt %, Merck KGaA, Darmstadt, Germany), and sodium chloride (99.5 wt %, Acros-Organics, Thermo Fisher Scientific, Geel, Belgium) were used. Reactive Green 12 dye (>99%, MW = 1837 g mol−1) procured from the textile manufacture of Constantine (Algeria) was used as received; the aqueous solutions were prepared using MilliQ water with a resistance of 15.0 MΩ·cm. The chemical structure and properties of Reactive Green 12 (RG12) dye were recently reported [18]. Materials 2019, 12, 412 3 of 16

Materials 2019, 12, x FOR PEER REVIEW 3 of 15 2.2. Catalyst Preparation with a resistance of 15.0 MΩ.cm. The chemical structure and properties of Reactive Green 12 (RG12) The CuxO/TiO2 coatings were sputtered using reactive HiPIMS process. Ti and Cu targets were dye were recently reported [18]. purchased from Lesker (Kurt J. Lesker Company Ltd., East Sussex, UK) (99.99% pure). The sputtering −5 chamber2.2. Catalyst was operated Preparation at a high vacuum with a residual pressure less than 4 × 10 Pa. This chamber was equipped with two confocal targets 5 cm in diameter. A HiP3 5KW Solvix generator (Advanced The CuxO/TiO2 coatings were sputtered using reactive HiPIMS process. Ti and Cu targets were Energy, Fort Collins, CO, USA) was used for the HiPIMS deposition and was operated at an average purchased from Lesker (Kurt J. Lesker Company Ltd., East Sussex, UK) (99.99% pure). The sputtering power of 100 W (5 W·cm−2) with a pulse-width of 50 µs and a frequency of 1000 Hz. A TiO under-layer chamber was operated at a high vacuum with a residual pressure less than 4 × 10−5 Pa. This 2chamber was sputteredwas equipped (for with 1 min) two before confocal the targets deposition 5 cm in of diameter. CuxO to A reduce HiP3 5KW the voids Solvix of generator polyester (Advanced and to permit a highEnergy, dispersion Fort Collins, of Cu speciesCO, USA) on was the used surface for [the33 ].Hi ThePIMS target-to-substrate deposition and was distance operated was at an ﬁxed average at 10 cm in orderpower to obtainof 100 W homogeneous (5 W·cm−2) with and a pulse-width adhesive Cu ofxO 50 ﬁlms. µs and The a frequency sample holder of 1000 was Hz. rotatingA TiO2 under- at a speed of 18layer rpm. was sputtered (for 1 min) before the deposition of CuxO to reduce the voids of polyester and to Tablepermit1 ashows high dispersion a summary of Cu about species the on preparedthe surface catalysts [33]. The target-to-substrate used in this study. distance Different was fixed current intensitiesat 10 cm were in order applied to obtain to the homogeneous Cu-sputtering and target—(1)adhesive Cux HiPIMSO films. The mode sample (20 A,holder 40 Awas and rotating 80 A) and (2) directat a speed current of 18 mode rpm. (direct current magnetron sputtering, DCMS) at 300 mA. The Cu-deposition Table 1 shows a summary about the prepared catalysts used in this study. Different current times were 5, 10, 20, and 100 s. The obtained photocatalytic thin layers were used in the degradation of intensities were applied to the Cu-sputtering target—(1) HiPIMS mode (20 A, 40 A and 80 A) and (2) 4 mg/Ldirect of RG12current solution, mode (direct as shown current in magnetron Figure1. sputtering, DCMS) at 300 mA. The Cu-deposition times were 5, 10, 20, and 100 s. The obtained photocatalytic thin layers were used in the degradation Table 1. Summary of the prepared catalysts used in this study. of 4 mg/L of RG12 solution, as shown in Figure 1. Sputtering Time of Copper Catalyst Sputtering Mode Sputtering Power Table 1. Summary of the preparedUpper catalysts Layer used (s) in* this study. Catalyst Sputtering Mode Sputtering Time of Copper Upper Layer (s) * Sputtering Power CuxO/TiO2 DCMS/DCMS 5, 10, 20, and 100 0.5 A/0.3 A CuCuxO/TiOO/TiO2 2 DCMS/DCMSHiPIMS/DCMS 5, 10, 5, 20, 10, and 20, 100 and 100 0.5 20 A/0.3 AA CuCuxO/TiOO/TiO2 2 HiPIMS/DCMSHiPIMS/DCMS 5, 10, 5, 20, 10, and 20, 100 and 100 2040 A/0.3 A/0.3 A A CuCuxO/TiOO/TiO2 2 HiPIMS/DCMSHiPIMS/DCMS 5, 10, 5, 20, 10, and 20, 100 and 100 4080 A/0.3 A/0.3 A A CuxO/TiO2 HiPIMS/DCMS 5, 10, 20, and 100 80 A/0.3 A * The TiO2 under-layer was sputtered for 1 min. * The TiO2 under-layer was sputtered for 1 min.

FigureFigure 1. Photocatalytic 1. Photocatalytic apparatus apparatus forfor Reactive Green Green 12 12 (RG12) (RG12) degradation. degradation.

2.3. Characterization2.3. Characterization of Materials of Materials TheThe UV–Vis UV–Vis spectra spectra of of all all samplessamples werewere recorded using using a aVarian Varian Cary Cary®50 ®UV–Vis50 UV–Vis spectrophotometerspectrophotometer (Agilent, (Agilent, Les Les Ulis, Ulis, France), France), wherewhere the the spectra spectra were were obtained obtained at the at thewavelength wavelength range of 200–800 nm (λmax = 615 nm). range of 200–800 nm (λmax = 615 nm). TransmissionTransmission electron electron microscopy microscopy (TEM) (TEM) of of the th sputterede sputtered polyester polyester (PES)(PES) fabrics was was carried carried out out using a FEI Tecnai Osiris (Thermo Fisher Scientific, Hillsboro, OR, USA) operated at 200 kV. The using a FEI Tecnai Osiris (Thermo Fisher Scientiﬁc, Hillsboro, OR, USA) operated at 200 kV. The spot spot size was set to 5 µm, dwell time 50 µs, and the real time of 600 s. The samples were embedded size was set to 5 µm, dwell time 50 µs, and the real time of 600 s. The samples were embedded in in epoxy resin (Embed 812) and cross-sectioned with an ultra-microtome (Ultracut E) up to thin layers epoxy resin (Embed 812) and cross-sectioned with an ultra-microtome (Ultracut E) up to thin layers of 80–100 nm thick. These thin layers were then placed on TEM holey carbon grid in order to image the samples. Materials 2019, 12, 412 4 of 16

X-rayMaterials diffraction 2019, 12, x FOR (XRD) PEER REVIEW INEL EQUINOX instrument (INEL, Stratham, NH, USA), power4 of 15 3.5 kW and coupled with a CPS120detector (INEL, Stratham, NH, USA) was used to register peaks from 2 of 80–100 nm thick. These thin layers were then placed on TEM holey carbon grid in order to image θ to 120the .samples. X-ray diffraction (XRD) INEL EQUINOX instrument (INEL, Stratham, NH, USA), power 3.5 kW 2.4. Photocatalytic Experiments and coupled with a CPS120detector (INEL, Stratham, NH, USA) was used to register peaks from 2 to The120 θ photocatalytic. activity of the synthesized photocatalyst was evaluated by following the degradation of 15 mL of the RG12 dye solution in petri dish as a reactor under magnetic stirring 2.4. Photocatalytic Experiments (see Figure1). The reaction system included an ultraviolet light-emitting diodes (LEDs) system (visible lightThe photocatalytic LEDs, Innolux, activity Santa of Clara,the synthesized CA, USA) photocatalyst as an irradiation was evaluated source (byλ =following 420 nm) the with a measureddegradation intensity of 15 of mL 1 of mW/cm the RG122. dye The solution initial RG12in petri solution dish as a wasreactor stirred under in magnetic the dark stirring for 30 (see min to reachFigure adsorption–desorption 1). The reaction system equilibrium included an before ultravio thelet LEDs light-emitting irradiation diodes was (LEDs) ON. The system concentration (visible of light LEDs, Innolux, Santa Clara, CA, USA) as an irradiation source (λ = 420 nm) with a measured dye solution samples (V = 3 mL) was analyzed using a UV–Vis spectrophotometer (Agilent, Les Ulis, intensity of 1 mW/cm2. The initial RG12 solution was stirred in the dark for 30 min to reach France) at regular time intervals. The protective grid is made of stainless steel and is used to protect adsorption–desorption equilibrium before the LEDs irradiation was ON. The concentration of dye the catalystsolution from samples the possible(V = 3 mL) mechanical was analyzed damage using caused a UV–Vis by spectrophotometer the stirrer. (Agilent, Les Ulis, TheFrance) RG12 at regular discoloration time intervals. efficiency The (%)protective of the materialgrid is made was of evaluated stainless steel by the and following is used to Equationprotect (1) the catalyst from the possible mechanical damage caused by the stirrer. A − A The RG12 discoloration efficiencyn( %(%)) =of (the0 materialt ) × was100 evaluated by the following Equation (1) (1) A0 where A and At are the initial intensity of absorbanceAA0  t peak at λ (λ = 615 nm) and the intensity 0 n(%) ( ) x100 max max (1) of absorbance peak at time t in UV–Vis spectra of RG12,A0 respectively. The coated fabrics were also tested for stability by testing their recycling performance. where A0 and At are the initial intensity of absorbance peak at λmax (λmax = 615 nm) and the intensity Theof ions absorbance released peak from at time the t in fabrics UV–Vis were spectra quantified of RG12, respectively. using inductively coupled plasma mass spectrometryThe coated (ICP-MS) fabrics using were Finnigan also tested™, for Element2 stability high-performanceby testing their recycling high-resolution performance. ICPMS The ions model (Zug,released Switzerland). from the The fabrics ICP-MS were resolutionquantified using was of inductively 1.2 × 105 coupledcps/ppb plasma with amass detection spectrometry limit of (ICP- 0.2 ng/L. CleanMS) Teflon using bottles Finnigan wereTM used, Element2 to prepare high-performance the calibration standardshigh-resolution through ICPMS successive model dilutions(Zug, of 0.1 gSwitzerland). L−1 ICPMS stockThe ICP-MS solutions resolution (TechLab, was Metz,of 1.2 × France). 105 cps/ppb The with washing a detection solution limit from of 0.2 the ng/L. samples Clean were Teflon bottles were used to prepare the calibration standards through successive dilutions of 0.1 g L−1 digested with nitric acid 69% (1:1 HNO3 + H2O) to remove the organics and to guarantee that there ICPMS stock solutions (TechLab, Metz, France). The washing solution from the samples were were no remaining ions adhered to the reactor wall. digested with nitric acid 69% (1:1 HNO3 + H2O) to remove the organics and to guarantee that there 3. Resultswere no and remaining Discussion ions adhered to the reactor wall. 3. Results and Discussion 3.1. Effect of the Photocatalyst Preparation Parameters on the RG12 Discoloration and the Microstructure 3.1. Effect of the Photocatalyst Preparation Parameters on the RG12 Discoloration and the Microstructure The photocatalytic degradation of RG12 under the LEDs is shown in Figure2. We noted that The photocatalytic degradation of RG12 under the LEDs is shown in Figure 2. We noted that whenwhen applying applying a current a current intensity intensity of of 40 40 AA toto thethe target, the the resulting resulting thin thin film film showed showed the fastest the fastest RG12RG12 degradation degradation followed followed by by HiPIMS HiPIMS atat 2020 A, DCMS (300 (300 mA), mA), and and HiPIMS HiPIMS at 80 at A 80 with A with 100 s 100as s as depositdeposit time. time.

30 4 ppm_20A_05s 40 4 ppm_20A_10s 4 ppm_40A_05s 4 ppm_40A_10s 4 ppm_80A_05s 4 ppm_80A_10s 25 4 ppm_DC_05s 35 4 ppm_DC_10s

30 20 25

15 20

15 10 10 5 Decolorization efficiency (%) efficiency Decolorization Decolorization efficiency (%) efficiency Decolorization 5

0 0

0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 Time (min) Time (min) (a) (b)

Figure 2. Cont. Materials 2019, 12, x FOR PEER REVIEW 5 of 15 Materials 2019, 12, 412 5 of 16

Materials35 2019, 12, x FOR PEER REVIEW 60 5 of 15 4 ppm_20A_20s 4 ppm_20A_100s 4 ppm_40A_20s 55 4 ppm_40A_100s 30 35 4 ppm_80A_20s 60 4 ppm_80A_100s 4 ppm_DC_20s 4 ppm_20A_20s 50 4 ppm_20A_100s4 ppm_DC_100s 4 ppm_40A_20s 55 4 ppm_40A_100s 45 25 30 4 ppm_80A_20s 4 ppm_80A_100s 4 ppm_DC_20s 50 4 ppm_DC_100s 40 45 25 35 20 40 20 3530 15 3025 15 2520 10 2015 10 5 1510 Decolorization efficiency (%) efficiency Decolorization 5 (%) efficiency Decolorization 105 Decolorization efficiency (%) efficiency Decolorization Decolorization efficiency (%) efficiency Decolorization 0 50 0 0 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 Time (min) Time (min) Time (min) Time (min) (c)( c) (d)( d)

FigureFigure 2. Photocatalytic 2. Photocatalytic degradation degradation of of RG12RG12 withwith diffedifferentrent current current intensity intensity used used in photocatalystin photocatalyst Figure 2. Photocatalytic degradation of RG12 with different current intensity used in photocatalyst preparationpreparation (initial (initial pollutant pollutant concentration concentration 44 mg/L) onon (a(a) )samples samples sputtered sputtered fo rfo 5 rs 5on s polyesteron polyester preparation (initial pollutant concentration 4 mg/L) on (a) samples sputtered for 5 s on polyester (PES); (PES);(PES); (b) (samplesb) samples sputtered sputtered for for 10 10 s; s; ( c(c) )samples samples sputtered for for 20 20 s; s; and and (d ()d samples) samples sputtered sputtered for 100 for 100 (bs.) sampless. sputtered for 10 s; (c) samples sputtered for 20 s; and (d) samples sputtered for 100 s. From another hand, at this value of current intensity (40 A), increasing the sputtering time led FromFrom another another hand, hand, at at this this value value of of currentcurrent intensitynsity (40 (40 A), A), increasing increasing the the sputtering sputtering time time led led to an increase in the photocatalytic discoloration efﬁciency of RG12, where 27.3%, 33.1%, 37.6%, to anto increase an increase in thein the photocatalytic photocatalytic discoloration discoloration efficiency of of RG12, RG12, where where 27.3%, 27.3%, 33.1%, 33.1%, 37.6%, 37.6%, and and and 53.4% dye elimination after 360 min under irradiation was observed with 05, 10, 20, and 100 s 53.4%53.4% dye dye elimination elimination after after 360 360 min min underunder irradiationation waswas observed observed with with 05, 05, 10, 10,20, 20,and and 100 100s s depositiondeposition time, time, respectively, respectively, as as shown shown below below inin Figure 22.. The The TiO TiO2 under-layer2 under-layer did didnot show not show any any deposition time, respectively, as shown below in Figure 2. The TiO2 under-layer did not show any photocatalytic activity (3–6% RG12 removal). This is in accordance with previous results found for photocatalyticphotocatalytic activityactivity (3–6%(3–6% RG12RG12 removal).removal). This is in accordance with with previous previous results results found found for for methylenemethylene blue blue [25 [25].]. This This can can bebe attributedattributed to to the the low low amount amount of TiO of2 TiO(and active(and activesites) available sites) available for methylene blue [25]. This can be attributed to the low amount of TiO2 (and2 active sites) available for for RG12.RG12. RG12. This can be attributed to the optimal mass-to-volume ratio of the HiPIMS deposited film at 40 This can be attributed to the optimal mass-to-volume ratio of the HiPIMS deposited ﬁlm at A.This It hascan beenbe attributed reported tothat the sm optimalall-sized mass-to-volume nanoparticles induce ratio favorableof the HiPIMS photocatalytic deposited bacterial film at 40 40 A. It has been reported that small-sized nanoparticles induce favorable photocatalytic bacterial A. Itinactivation has been kineticsreported due that to smthe all-sizedlarge surface nanoparticles area per unit induce mass favorable [24]. The distributionphotocatalytic of CubacterialxO inactivationinactivationnanoparticles kineticskinetics on the duedue TiO toto2 under-layer thethe largelarge surfaceonsurface the polyester areaarea perper was unitunit found massmass to be [[24].24 uniform]. The The anddistribution distribution did not induce of of Cu Cu xxOO nanoparticlesnanoparticlescracks on the onon substrate. thethe TiOTiO22 under-layerFigureunder-layer 3 shows on the the TEM polyester imaging was of the found sputtered to to be be layersuniform uniform (HiPIMS and and did at did 40 not notA) induceon induce crackscrackspolyester on on thethe substrate.(PES).substrate. The FigureTiO Figure2 under 3 showsshows was the thesputtered TEM TEM imaging imagingto reduce of of thethe the porositysputtered sputtered of layersthe layers PES (HiPIMS leading (HiPIMS at to 40 atthe A) 40 on A) continuous distribution of the CuxO upper layer [31,33]. onpolyester polyester (PES). (PES). The The TiO TiO2 under2 under was was sputtered sputtered to toreduce reduce the the porosity porosity of of the the PES PES leading leading to to the the continuouscontinuous distribution distribution ofof thethe CuCuxxOO upper layer [31,33]. [31,33].

Figure 3. TEM imaging of CuxO/TiO2 deposited by high-power impulse magnetron sputtering (HiPIMS) at 40 A.

FigureFigure 3. TEM3. TEM imaging imaging of of Cu CuxO/TiOxO/TiO2 depositeddeposited by by high-power high-power im impulsepulse magnetron magnetron sputtering sputtering (HiPIMS) at 40 A. (HiPIMS) at 40 A.

The charge transfer between the TiO2 and the CuxO at the surface and the organic pollutant depends on the diffusion length of the photo-generated charges at the interface of the ﬁlm under light

Materials 2019, 12, x FOR PEER REVIEW 6 of 15

MaterialsThe2019 charge, 12, 412 transfer between the TiO2 and the CuxO at the surface and the organic pollutant6 of 16 depends on the diffusion length of the photo-generated charges at the interface of the film under light irradiation. The charge transfer/diffusion is also a function of the TiO2 and of the Cu-particles’ size irradiation. The charge transfer/diffusion is also a function of the TiO2 and of the Cu-particles’ size and shape as previously reported [34,35]. and shape as previously reported [34,35]. X-ray diffraction of the sputtered CuxO/TiO2 on PES (sputtered for 100 s at 40 A) showed the X-ray diffraction of the sputtered CuxO/TiO2 on PES (sputtered for 100 s at 40 A) showed the absence of sharp peaks that can be attributed to the CuxO clusters. This reflected the low crystallinity absence of sharp peaks that can be attributed to the CuxO clusters. This reﬂected the low crystallinity of of the deposited Cu clusters. The absence of clear crystal phase could be attributed to the short the deposited Cu clusters. The absence of clear crystal phase could be attributed to the short sputtering sputtering time leading to the formation of very small Cu nanoparticles/clusters. The low Cu and Ti time leading to the formation of very small Cu nanoparticles/clusters. The low Cu and Ti loadings led loadings led to ~80 nm thin film on the porous PES. The diffractogram showed a high noise-to-signal to ~80 nm thin ﬁlm on the porous PES. The diffractogram showed a high noise-to-signal ratio due to ratio due to the low amount of Cu species on the PES. the low amount of Cu species on the PES.

3.2. Effect of key Parameters: Pollutant Conc Concentrationentration and Surface-Area to Volume Ratio Figure 4 4shows shows thethe photocatalyticphotocatalytic RG12RG12 discolorationdiscoloration whenwhen usingusing differentdifferent initialinitial dyedye concentrations. It It is is readily readily seen seen from from Figure Figure 44 thatthat thethe discolorationdiscoloration isis fasterfaster forfor lowerlower initialinitial concentrations, withwith optimaloptimal kineticskinetics forfor 44 mg/L.mg/L. This This can can be be explained explained by the unavailability of active site for the adsorption of all dye molecules and thei theirr degradation at high initial concentrations [25]. [25]. Furthermore, the increase in dye concentration led to an increase in medium opacity and then less permeability to the applied light, which reduced thethe photo-generated reactive species important for the photocatalytic process [25,36]. It has been recently reported that porous Cu2O nanostructures the photocatalytic process [25,36]. It has been recently reported that porous Cu2O nanostructures were alsowere able also to able adsorb to adsorb ions such ions assuch radioactive as radioactive iodine iodine issued issued from nuclearfrom nuclear ﬁssion fission [37]. [37].

60 4 ppm_40A_100s 55 10 ppm_40A_100s 16 ppm_40A_100s 50 45 40 35 30 25 20 15

Decolorization efficiency (%) 10 5 0 0 30 60 90 120 150 180 210 240 270 300 330 360 Time (min) Figure 4. Photocatalytic degradationdegradation ofofRG12 RG12 with with different different initial initial concentrations concentrations (current (current intensity, intensity, 40 A40 andA and sputtering sputtering time, time, 100 100 s). s).

In order toto investigateinvestigate the effect ofof thethe photocatalystphotocatalyst surface (in other words,words, the available catalytic active active sites), sites), the the effect effect of ofincreasing increasing surf surfaceace area area to volume to volume (dye (dyevolume) volume) ratio (SA/V) ratio (SA/V) on the onphotocatalytic the photocatalytic efficiency efficiency was stud wasied studiedusing higher using ratios higher for ratios the discoloration for the discoloration of 4 mg/L ofRG12. 4 mg/L We 2 RG12.noted that We notedthe used that photocatalytic the used photocatalytic surface in the surface previous in the experiment previous experimentwas fixed to was 16 cm fixed2, which to 16 gave cm , −1 whicha surface gave area a surfaceto volume area ratio to volume (SA/V) ratioof 1.06 (SA/V) cm−1. The of 1.06 results cm are. The shown results in Figure are shown 5. It was in Figure readily5. Itseen was that readily increasing seen that SA/V increasing increased SA/V the discoloration increased the discolorationefficiency. This efficiency. can be a Thisttributed can be to attributedthe increase to thein the increase number in of the active number sites of available active sites for available RG12 molecules for RG12 and molecules intermediate and intermediate by-products’ by-products’ adsorption adsorptionand degradation. and degradation. A ratio of 1.06 A ratio showed of 1.06 53.4% showed of discoloration 53.4% of discoloration efficiency; efficiency; however, however, a ratio of a ratio2.66 −1 ofcm 2.66−1 led cm to 90.1%led tounder 90.1% visible under light visible LEDs light irradiation LEDs irradiation for 360 min. for Similar 360 min. results Similar were resultsreported were by reportedHuang et byal. Huangfor the methyl et al. for orange the methyl discoloration orange on discoloration Pt-modified on TiO Pt-modified2 supported TiO on 2naturalsupported zeolites on natural[38]. zeolites [38].

MaterialsMaterials2019 2019, ,12 12,, 412 x FOR PEER REVIEW 77 of of 16 15 Materials 2019, 12, x FOR PEER REVIEW 7 of 15

100 100 4 ppm_40A_100s (SA/V=1.06) 4 ppm_40A_100s (SA/V=1.06) 90 4 ppm_40A_100s (SA/V=2.1) 90 4 4ppm_40A_100s ppm_40A_100s (SA/V=2.1) (SA/V=2.65) 4 ppm_40A_100s (SA/V=2.65) 80 80 70 70 60 60 50 50 40 40 30 30

Decolorization efficiency (%) efficiency Decolorization 20 Decolorization efficiency (%) efficiency Decolorization 20 10 10 0 0 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 Time (min) Time (min) Figure 5. Photocatalytic degradation of RG12 with different catalyst surface area to volume ratio Figure 5.5. PhotocatalyticPhotocatalytic degradation degradation of of RG12 RG12 with with different differe catalystnt catalyst surface surface area toarea volume to volume ratio (initial ratio (initial concentrations, 4 mg/L; current intensity, 40 A; and sputtering time, 100s). (initialconcentrations, concentrations, 4 mg/L; 4 currentmg/L; current intensity, inte 40nsity, A; and 40 A; sputtering and sputtering time, 100 time, s). 100s).

3.3.3.3. ImplicationImplication ofof RadicalsRadicals andand ROS ROS Species Species within within RG12 RG12 Discoloration Discoloration 3.3. Implication of Radicals and ROS Species within RG12 Discoloration • ⦁ •2–⦁– ++ TheThe contributioncontribution ofof individualindividual reactivereactive oxidizing oxidizing species species ( ⦁(OH,OH, OO2 ⦁ ,, andand hh )) inin thethe RG12RG12 The contribution of individual reactive oxidizing species ( OH, O2 –, and h+) in the RG12 degradationdegradation waswas studiedstudied usingusing selectiveselective scavengersscavengers (MeOH(MeOH and and iso-propanol iso-propanol for for• OH,⦁OH, potassiumpotassium degradation was studied using selective scavengers (MeOH and iso-propanol for ⦁OH, potassium •−2⦁− 3 dichromatedichromate for for O 2O⦁ ,, andand CHCl CHCl3 and and MeOH MeOH for for electronic electronic holes). holes). The The choice choice of selective of selective scavenger scavenger was dichromate for O2 −, and CHCl3 and MeOH for electronic holes). The choice of selective scavenger basedwas based on the on second-order the second-order kinetic ratekinetic constant rate ofcons thetant reaction of the between reaction the between ROS and the each ROS scavenger and each as was based on the second-order kinetic rate constant of the reaction between the ROS and each shownscavenger in Supplementary as shown in Supplementary Materials Table Materials S1. Table S1. scavenger as shown in Supplementary Materials Table S1. FigureFigure6 a6a shows shows that that the the presence presence of of MeOH MeOH (holes (holes scavenger) scavenger) in in reaction reaction medium medium enhanced enhanced the the Figure 6a shows that the presence of MeOH (holes scavenger) in reaction medium enhanced the discolorationdiscoloration efﬁciencyefficiency ofof thethe reactivereactive dye.dye. Furthermore,Furthermore, increasingincreasing thethe MeOHMeOH amountamount ledled toto thethe discoloration efficiency of the reactive dye. Furthermore, increasing the MeOH amount led to the accelerationacceleration ofof the the photocatalytic photocatalytic RG12 RG12 abatement abatement until until an an optimum optimum value value of of alcohol alcohol of of 50 50 mmol/L mmol/L acceleration of the photocatalytic RG12 abatement until an optimum value of alcohol of 50 mmol/L leadingleading to to 74.3% 74.3% of of discoloration discoloration efﬁciency efficiency after after 360 360 min min of of irradiation. irradiation. leading to 74.3% of discoloration efficiency after 360 min of irradiation.

80 65 No addition 8075 No addition 65 No 25,0 addition mmol/l_MeOH 60 No 4,5 addition mmol/l_ IsoPro 75 60 70 25,0 50,0 mmol/l_MeOH mmol/l_MeOH 4,5 11,3 mmol/l_ mmol/l_ IsoPro IsoPro 70 55 65 50,0 83,0 mmol/l_MeOH mmol/l_MeOH 55 11,3 43,5 mmol/l_ mmol/l_ IsoPro IsoPro 65 83,0 mmol/l_MeOH 50 43,5 mmol/l_ IsoPro 60 50 6055 45 55 45 50 40 50 40 45 35 4540 35 40 30 35 30 3530 25 25 3025 20 25 20 20 15 2015 15 15 10 Decolorization efficiency (%) efficiency Decolorization Decolorization efficiency (%) efficiency Decolorization 10 10

Decolorization efficiency (%) efficiency Decolorization 5 Decolorization efficiency (%) efficiency Decolorization 10 5 5 50 0 0 0 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150Time 180 210(min) 240 270 300 330 360 0 30 60 90 120 150Time 180 210 (min) 240 270 300 330 360 Time (min) Time (min) (a) (b) (a) (b) Figure 6. Cont.

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70 60 No addition No addition 65 2,0 mmol/l_CHCl3 55 0,35 mmol/l_K2Cr2O7 0,70 mmol/l_K2Cr2O7 60 4,5 mmol/l_CHCl3 50 1,10 mmol/l_K2Cr2O7 55 45 50 40 45 35 40 35 30 30 25 25 20 20 15 15

Decolorization efficiency (%) efficiency Decolorization 10 Decolorization efficiency10 (%) 5 5 0 0 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 Time (min) Time (min) (c) (d)

FigureFigure 6. 6. PhotocatalyticPhotocatalytic degradation degradation of ofRG12 RG12 in inthe the presence presence of ( ofa) (methanol;a) methanol; (b) (iso-propanol;b) iso-propanol; (c) chloroform;(c) chloroform; and and (d) (potassiumd) potassium dichromate. dichromate. (Initial (Initial RG12 RG12 conc concentrationentration = 4 = mg/L; 4 mg/L; sample, sample, HiPIMS HiPIMS at 40at 40A; A;and and sputtering sputtering time, time, 100 100 s.) s.)

OnOn the the other other hand, hand, as as shown shown in in Figure Figure 66b,b, thethe presencepresence ofof iso-propanoliso-propanol inin thethe reactionreaction mediummedium slightlyslightly enhanced enhanced the the photocatalytic photocatalytic RG12 RG12 discol discoloration,oration, where where 58.8% 58.8% of of RG12 RG12 was was destroyed with with 43.543.5 mmol/L mmol/L of of alcohol alcohol after after 360 360 min min compared compared to to 53.4% 53.4% without without scavenger scavenger addition. addition. FigureFigure 66a,ba,b showsshows thatthat thethe presencepresence ofof alcoholalcohol (MeOH(MeOH oror iso-propanol)iso-propanol) slightlyslightly enhancedenhanced thethe photocatalytic reactionreaction rate, rate, which which can can be explainedbe explaine byd the by inhibitorythe inhibitory effect playedeffect played by hydroxyl by hydroxyl radicals radicalsin the degradation in the degradation mechanism mechanism of RG12. of SimilarRG12. Similar results results were found were found by Rong by andRong Sun and in Sun their in worktheir workreported reported on the on photocatalytic the photocatalytic degradation degradation of triallyl of triallyl isocyanurate isocyanurate (TAIC) [(TAIC)39]. The [39]. additional The additional effect in effectthe inhibition in the inhibition of discoloration of discoloration efficiency efficiency in the presence in the presence of MeOH of MeOH compared compared to iso-propanol to iso-propanol can be canattributed be attributed to the holes’to the effectholes’ because effect because of the high of th reactivitye high reactivity of iso-propanol of iso-propanol with the with hydroxyl the hydroxyl radicals • ⦁ 9 −91 −−11 −1 • ⦁ 88 −1−1−1 −1 radicals(kiso-prop, (kOHiso-prop,= 1.9OH ×= 1.910 ×M 10 MS S)[40) [40]] compared compared to theto the MeOH MeOH (kMeOH, (kMeOH,OHOH= =9.7 9.7 × 1010 MM S S) [41].)[41 ]. ItIt isis worthworth to to mention mention at thisat this level level that that Di Valentin Di Valentin and Fittipaldi and Fittipaldi [42] studied [42] thestudied photo-generated the photo- generatedhole scavenging hole scavenging using different using organic different adsorbates organic adsorbates on TiO2. They on TiO established2. They established a scale of scavenging a scale of scavengingpower showing power glycerol showing > tert-butanolglycerol > tert-butanol > iso-propanol > iso-propanol > methanol > >methanol formic acid. > formic This suggestedacid. This suggestedthat methanol that couldmethanol exhibit could electronic-holes exhibit electronic-h scavenging,oles scavenging, but negligible but compared negligible to compared the •OH-radical to the ⦁scavenging.OH-radical Furthermore,scavenging. Furthermore, Shen and Henderson Shen and [43 ]Henderson reported on [43] the molecularreported on and the dissociative molecular forms and dissociativeof methanol forms on TiO of2 surfacemethanol for on holes TiO scavenging.2 surface for They holes showed scavenging. that methoxy They showed (dissociative that methoxy form of (dissociativemethanol) effectively form of methanol) scavenged theeffectively photo-generated scavenged holes, the photo-generated and not methanol holes, itself. and This not did methanol not allow itself.an unequivocal This did not separation allow an between unequivocal•OH-radical separation and between h+ scavenging ⦁OH-radical when usingand h methanol.+ scavenging when usingFigure methanol.6c shows that the presence of 4.5 mmol/L of chloroform slightly improved the discoloration efficiencyFigure of 6c RG12 shows by 11%that afterthe 360presence min underof 4.5 light. mmol/L This of enhancement chloroform canslightly be attributed improved to the discolorationholes-scavenging efficiency characters of RG12 of Clby ions 11% generated after 360 mi fromn under the chloroform light. This enhancement decomposition/dissolution. can be attributed •− to theOxygen holes-scavenging superoxide anion characters (O2 ) is oneof of theCl veryions important generated radicals from studied the becausechloroform of its decomposition/dissolution.crucial role in the photocatalytic process. The contribution of this photo-generated radical was studied 2⦁– by addingOxygen potassium superoxide dichromate anion (O (K2)Cr is2 Oone7) to of the the reaction very important medium. Theradicals results studied of adding because are shown of its crucialin Figure role6d. in The the presence photocatalytic of potassium process. dichromate The contribution decreased theof reactionthis photo-generated rate of RG12 discoloration, radical was studiedwhere a by reduction adding potassium of 32.7% in dichromate discoloration (K2Cr efficiency2O7) to the was reaction recorded medium. in the The presence results of of 1.1 adding mmol/L are shownK2Cr2O in7. Figure An inversely 6d. The proportional presence of relationship potassium dichromate between the decreased added quantity the reaction of K2Cr rate2O 7ofand RG12 the discoloration,discoloration efficiencywhere a reduction of RG12 was of 32.7% also noticed. in discoloration This can beefficiency explained was by recorded the positive in the role presence played byof •− 2 2 7 2 2 7 1.1O2 mmol/Lin the K eliminationCr O . An ofinversely RG12 textile proportional dye. relationship between the added quantity of K Cr O and the discoloration efficiency of RG12 was also noticed. This can be explained by the positive role ⦁ played by O2 – in the elimination of RG12 textile dye.

3.4. Effect of the Addition of Oxidizing Agents: Cases of H2O2 and K2S2O8

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Materials 2019, 12, x FOR PEER REVIEW 9 of 15 3.4. Effect of the Addition of Oxidizing Agents: Cases of H O and K S O In order to enhance the photocatalytic reaction, several2 2 intensification2 2 8 ways can be tested. The effectIn of orderthe adding to enhance oxidants the such photocatalytic as hydrogen reaction, peroxide several (H2O intensification2) and potassium ways peroxydisulfate can be tested. (KThe2S2 effectO8) in of the the reaction adding oxidantsmedium such(mimicking as hydrogen real textile peroxide industry (H2O2 )effluents) and potassium on the peroxydisulfate photocatalytic reaction(K2S2O8 )kinetics in the reactionwas investigated. medium (mimickingFigure 7a shows real textilethat adding industry hydrogen effluents) peroxide on the (H photocatalytic2O2) strongly improvedreaction kinetics the reaction was investigated. rate, where 47.4% Figure and7a shows 100% thatdiscoloration adding hydrogen were, respectively, peroxide (H achieved2O2) strongly after 30improved and 300 themin reaction in the presence rate, where of 4.6 47.4% mmol/L and 100%H2O2. discolorationHowever, only were, 48% respectively, of discoloration achieved efficiency after was30 and noticed 300 min without in the any presence addition of 4.6after mmol/L 300 min H unde2O2.r However, light. The onlystrong 48% enhancement of discoloration caused efficiency by the ⦁ presencewas noticed of H without2O2 can anybe explained addition by after (i) 300the minparticipation under light. of H The2O2 strongin the generation enhancement of causedOH from by one the • handpresence [44] ofand H2 (ii)O2 thecan reduction be explained of the by (i)recombination the participation rate ofas Hshown2O2 in below the generation in Equations of OH (2)–(4) from from one anotherhand [44 hand] and [45,46]. (ii) the It reduction is worth ofto themention recombination at this level rate that as shownthe ⦁OH below is 1.5 intimes Equations more oxidant (2)–(4) fromthan • Hanother2O2 and hand 1.4 times [45,46 more]. It is than worth ozone to mention [47,48]. at this level that the OH is 1.5 times more oxidant than

H2O2 and 1.4 times more than ozone [47,48]. eHOHOcb 22 2 (2) − • e cb + H2O2 → 2HO (2)    OHOOHOHO222 2 (3) •− • − O2 + H2O2 → OH + OH + O2 (3) hv H O 2(• OH) (4) hv + H2O222 → 2( OH) (4)

110 No addition 100 No addition 100 0,4 mmol/l_H2O2 1,1 mmol/l_K2S2O8 1,2 mmol/l_H2O2 90 2,2 mmol/l_K2S2O8 90 2,4 mmol/l_H2O2 3,1 mmol/l_K2S2O8 4,6 mmol/l_H2O2 80 80 70 70 60 60 50 50 40 40 30 30

20 20 Decolorization efficiency (%) efficiency Decolorization Decolorization efficiency (%) efficiency Decolorization 10 10

0 0 0 30 60 90 120 150 180 210 240 270 300 330 360 0 30 60 90 120 150 180 210 240 270 300 330 360 Time (min) Time (min) (a) (b)

FigureFigure 7. 7. PhotocatalyticPhotocatalytic degradation degradation of of RG12 RG12 in inthe the presence presence of of(a)( hydrogena) hydrogen peroxide peroxide (H2 (HO22) Oand2) and(b) potassium(b) potassium peroxydisulfate peroxydisulfate (K2S (K2O28).S2 (InitialO8). (Initial RG12 RG12concentration concentration = 4 mg/L; = 4 mg/L; sample, sample, HiPIMS HiPIMS at 40 A; at and40 A; sputtering and sputtering time, 100 time, s.) 100 s.)

TheThe effect effect of of persulfate persulfate ions ions on on the the photocatal photocatalyticytic degradation degradation of of RG12 RG12 was was investigated investigated by by varyingvarying its its concentration from from 1.1 to 3.1 mmol/L. Figure Figure 77bb shows that thethe presencepresence ofof potassiumpotassium peroxydisulfateperoxydisulfate (K (K22SS22OO8)8 )drastically drastically increased increased the the reaction reaction rate rate of of RG12 RG12 discoloration, discoloration, where where 67.5% 67.5% 2− andand a full discoloration (100%) werewere achievedachieved withinwithin 210210 minmin withwith 1.11.1 and and 3.1 3.1 mmol/L mmol/L SS2O2O882−, , − respectively.respectively. This This can can be due to thethe electron-scavengingelectron-scavenging propertiesproperties ofof SS22OO88− [49],[49], which which led led to to the the •− ◦ 2− formationformation of of sulfate radicalradical anionsanions (SO(SO44–)) with with a a high high oxidizing power ((EE° == 2.62.6 eV)eV) byby S 2SO2O882− reductionreduction according according to to the the Equations Equations (5) and (6) [50]. [50].

2− 22− •− 2− S2OSO828+ e ecbcb→ SO SO4 4+  SO 44 (5)(5)

•− − + SO → SO 2 2 + h  (6) SO4 44 SO4 hvbvb (6) •− The effect of SO ⦁ in the enhancement of RG12 discoloration can be rationalized in (i) the The effect of SO4 – in the enhancement of RG12 discoloration can be rationalized in (i) the preventionprevention of of electron/hole recombination recombination rate rate resu resultedlted from from the the interaction interaction with with conduction conduction band band electronselectrons leaving leaving behind behind positives positives holes holes [51]; [51]; (ii) (ii) the the abstraction abstraction of of a a hydrog hydrogenen atom atom from from saturated saturated carbon [44]; (iii) the generation of hydroxyl radicals by interaction with H2O molecules according to

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Equationcarbon [44 (7)]; (iii)[52]; the and generation (iv) the possible of hydroxyl reaction radicals of sulfate by interaction radical anions with H 2andO molecules the dye molecules according by to directEquation atta (7)ck [53]. [52]; and (iv) the possible reaction of sulfate radical anions and the dye molecules by direct attack [53]. 2    SO•−42 H O SO2 4− • OH H + (7) SO4 + H2O → SO4 + OH + H (7) At this stage,stage, itit isis worthworth to to mention mention that that copper copper oxide oxide can can be be easily easily “sulfated”. “sulfated”. This This may may affect affect the thecatalytic catalytic performance performance of the of catalyst.the catalyst.

3.5. Effect of Different Water Matrices on RG12 Degradation In order to approach a real case of textile effluent, efﬂuent, the effect of mineral ions’ and salts’ presence on the photocatalytic reaction rate was investigated by by testing testing both both Cu Cu and and NaCl NaCl as as inorganic inorganic ions. ions. Cu and NaCl were tested separately, and then combinedcombined together. It is readily seen from Figure 88 that the presencepresence ofof CuCu hashas aa negative negative effect effect on on the the discoloration discoloration efﬁciency efficiency which which can can be be explained explained as asfollows: follows: (i) (i) the the possible possible complexation complexation of Cuof Cu and an organicd organic species species or some or some intermediate intermediate by-products by-products [54]; (ii)[54]; according (ii) according to the to work the ofwork Kumawar of Kumawar et al. [55 et], al. there [55], is anthere optimum is an op concentrationtimum concentration when adsorbed when adsorbedmetal ions metal change ions the change behavior the from behavior enhancer from to enhancer inhibitor becauseto inhibitor of changing because surfaceof changing charge surface of the chargephotocatalyst of the photocatalyst or the target pollutant.or the target Similar pollutant. results Similar on Cu-inhibitory results on Cu-inhibitory effect was foundeffect was by Tercero found byEspinoza Tercero et Espinoza al. [56]; and et (iii)al. [56]; the presence and (iii) ofthe transition presence metal of transition ions can altermetal the ions electron-transfer can alter the electron- pathway, transferdecrease pathway, the reduction decrease of oxygen, the reduction and even of suppressoxygen, and the degradationeven suppress [57 the]. degradation [57].

100 No addition NaCl 90 Cu NaCl+Cu 80

20 Decolorization efficiency (%) 10

0 0 30 60 90 120 150 180 210 240 270 300 330 360 Time (min) Figure 8.8.Photocatalytic Photocatalytic degradation degradation ofRG12 of RG12 in the presencein the presence NaCl and NaCl Cu. (Initial and RG12Cu. (Initial concentration RG12 concentration= 4 mg/L; sample, = 4 mg/L; HiPIMS sample, at 40 HiPIMS A; and sputtering at 40 A; and time, sputtering 100 s). time, 100 s).

− The addition of NaCl as chloride ions sourcesource (Cl(Cl–)) to to the the dye dye solution solution in in our our experiment experiment led to a reduction of discoloration efﬁciencyefficiency byby aboutabout 20%20% afterafter 360360 minmin underunder LEDsLEDs lightlight (420(420 nm).nm). The effect of anions in the photocatalytic medi mediumum has been reported to have an inhibitory characters due to (1) the competitive effect between the anions and the target pollutant toward the active sites available on the surface of the photocatalyst [[58];58]; (2) the anion radicals’ scavenging • properties suchsuch asas holesholes and and hydroxyl hydroxyl radicals, radicals, which which led led to to the the formation formation of ionicof ionic radicals radicals such such as Cl as, • •− • + ⦁ ⦁ ⦁ ⦁ ClNO, 3NO, and3 , and HCO HCO3 3less− less reactive reactive than thanOH OH [20 ];[20]; and and (3) the(3) hthe-scavenging h+-scavenging proprieties proprieties of chloride of chloride ions ionsaccording according to Equation to Equation (8) [59 (8)]. [59]. − + •− + Cl + h vb → Cl + H (8)   Cl hvb Cl H (8) The simultaneous addition of Cu and NaCl (both at the same time) led to a drastic enhancement in theThe discoloration simultaneous rate addition of RG12, of where Cu and 98% NaCl of RG12(both at discoloration the same time) was led observed to a drastic within enhancement 360 min of inillumination, the discoloration as shown rate in of Figure RG12,8 .where 98% of RG12 discoloration was observed within 360 min of illumination, as shown in Figure 8. RG12 removal can be influenced by the multitude of species present in the real wastewater effluent. Mathematical simulation of the degradation of RG12 was seen to fit a second-order model as described by the equation (see supplementary Table S1):

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RG12 removal can be influenced by the multitude of species present in the real wastewater effluent. Mathematical simulation of the degradation of RG12 was seen to fit a second-order model as Materials 2019, 12, x FOR PEER REVIEW 11 of 15 described by the equation (see supplementary Table S1):

d[RG12[RG12]] = −=−k[RG12[RG12][AS][]AS] dt wherewhere [AS][AS] isis thethe steady-statesteady-state concentrationconcentration ofof activesactives speciesspecies (radical(radical hydroxyl,hydroxyl, sulfatesulfate ion radical,radical, reactivereactive oxygen anion anion superoxide, superoxide, etc.), etc.), [RG12] [RG12] is the is theconcentration concentration of RG12 of RG12 in water, in water,k is the ksecond-is the second-orderorder rate constant rate constant and t isand the treactionis the reaction time. time. TakingTaking intointo account account the the instantaneous instantaneous concentrations concentrations of of the the photo-generated photo-generated ROS, ROS, the the kinetics kinetics of theof the degradation degradation of of RG12 RG12 in in water water can can be be described described according according to to the the pseudo-ﬁrst-order pseudo-first-order equationequation asas givengiven below:below: − kapp.t . [RG12[RG12](t) =][RG12= [RG12]]0 expexp −1 wherewhere kkappapp, isis thethe pseudo-ﬁrst-orderpseudo-first-order apparentapparent raterate constantconstant (min(min−1)) and and it was obtainedobtained byby linearlinear regressionregression ofof ln(ln(CCtt/C/Coo) vs. time t. FigureFigure9 9 summarizes summarizes the the reaction reaction rate rate constants constantsk kappapp of RG12RG12 degradationdegradation withwith andand withoutwithout thethe additionaddition ofof variousvarious chemicalschemicals (scavengers,(scavengers, oxidants,oxidants, oror inorganicinorganic pollutants).pollutants). TheThe constantsconstants variedvaried fromfrom veryvery lowlow valuesvalues ofof 0.000510.00051 minmin−11 inin the the presence presence of of Cu Cu ions ions to to very very high high values values (0.0143 (0.0143 min min−1−) in1) inthe the presence presence of ofH2O H22.O In2. the In presence the presence of H of2O2 H, 2thereO2, therewas an was enhancement an enhancement of 7 times of 7 timesof the of RG12 the RG12discoloration discoloration rate followed rate followed by a 6 bytimes’ a 6 times’enhancement enhancement with K2 withS2O8. K It2 Sis2 Oreadily8. It is seen readily that seen the constant that the constantrate of reaction rate ofreaction in the presence in the presence of methanol of methanol (0.004 (0.004min−1) min is −higher1) is higher than with than withthe addition the addition of iso- of iso-propanolpropanol (0.002 (0.002 min min−1). −1).

0,014

0,012 0,01432 0,01136 0,01309 0,010 )

1 0,008 -

0,006 k (min 0,00444 0,004 0,00308 0,00244 0,00243 0,002 0,00104 5,34E-4 5,14E-4 0,000 n io O7 Cu Cu anol OH 2 + p e NaCl ddit M Cr CHCl3 H2O2 S2O8 Cl a 2 K2 K Na No Iso-Pro FigureFigure 9.9. Reaction rate constant of RG12RG12 photocatalyticphotocatalytic degradation in thethe presencepresence ofof differentdifferent chemicalchemical products.products.

InIn thethe aimaim toto evaluateevaluate thethe stabilitystability andand thethe reusabilityreusability ofof thethe synthesizedsynthesized materialmaterial supportedsupported onon PESPES usedused in this study, successive photocatalytic photocatalytic cycles cycles were were applied applied for for the the degradation degradation of ofRG12 RG12 (4 (4mg/L mg/L in each in each experiment) experiment) with with the the same same catalyst. catalyst. As Asit can it can be seen be seen in Figure in Figure 10, the10, thephotocatalyst photocatalyst has hasa good a good stability stability under under LEDs LEDs illumination illumination approved approved by by the the slight slight decrease decrease in in discoloration discoloration efﬁciencyefficiency (about(about 7%)7%) afterafter 2121 runs.runs.

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55 1st Run 50 21 th Run

10 Decolorization efficiency (%) efficiency Decolorization 5

0 60 20 80 40 00 60 00 60 20 80 40 00 60 1 1 2 3 3 72 72 73 73 74 75 75 Time (min) FigureFigure 10.10. RecyclingRecycling runsruns ofof thethe catalystcatalyst forfor thethe photocatalyticphotocatalytic degradationdegradation ofof RG12RG12 upup toto 2121 runsruns (initial(initial concentrations:concentrations: 4 4 mg/L; mg/L; catalyst catalyst dose, dose, 1 sheet 1 sheet;; current current intensity, intensity, 40 40A; and A; and sputtering sputtering time, time, 100 100s). s). Table2 shows the released Cu and Ti ions from the fabrics after 1, 5, 10, and 21 washing cycles. Table 2 shows the released Cu and Ti ions from the fabrics after 1, 5, 10, and 21 washing cycles. The results presented in Table2 are cumulative quantification of the released ions during all the The results presented in Table 2 are cumulative quantification of the released ions during all the cycles. It is readily seen from Table2 that the Ti ions are found at very low quantities during the first cycles. It is readily seen from Table 2 that the Ti ions are found at very low quantities during the first 5 cycles and then reduced drastically to almost zero ppb at the end of the recycling experiment. Table2 5 cycles and then reduced drastically to almost zero ppb at the end of the recycling experiment. Table also shows that the Cu ions’ release slows down at the end of the recycling (after the 10th cycle) to 2 also shows that the Cu ions’ release slows down at the end of the recycling (after the 10th cycle) to stabilize at about 2–3 ppb/cycle. These results were slightly similar to previous reports using HiPIMS stabilize at about 2–3 ppb/cycle. These results were slightly similar to previous reports using HiPIMS deposition of CuxO or TiO2/Cu deposited on polyester for photocatalytic applications [15,31,60]. deposition of CuxO or TiO2/Cu deposited on polyester for photocatalytic applications [31,15,60]. The The cumulative ions’ release is still far below the threshold fixed by regulatory bodies for Cu and/or cumulative ions’ release is still far below the threshold fixed by regulatory bodies for Cu and/or Ti Ti ions. ions. Table 2. Inductively coupled plasma mass spectrometry (ICP-MS) quantification of ions’ release during theTable recycling 2. Inductively experiment. coupled plasma mass spectrometry (ICP-MS) quantification of ions’ release during the recycling experiment. Cu Ions Ti Ions Cu Ions Ti Ions Cycle 1 17 6 CycleCycle 5 1 29 17 6 9 CycleCycle 10 5 32 29 9 10 CycleCycle 21 10 34 32 10 10 Cycle 21 34 10 4. Conclusions 4. Conclusions CuxO thin films deposited using HiPIMS on polyester under different sputtering energies were successfullyCuxO thin synthesized. films deposited The sputteringusing HiPIMS mode on and polyes theter applied under currentdifferent intensity sputtering were energies optimized. were Thesuccessfully photocatalytic synthesized. performance The sputtering of the photocatalyst mode and the was applied evaluated current for the intensity degradation were optimized. of RG12 under The visiblephotocatalytic light LEDs performance irradiation of with the photocatalyst different initial was dye evaluated concentrations for the anddegradation surface areaof RG12 to volume under ratios.visible Thelight effect LEDs of irradiation different concentrations with different of initia variousl dye chemicals concentrations (scavengers, and surface oxidants, area or to inorganic volume ions)ratios. on The the effect photocatalytic of different process concentrations was also of studied various mimicking chemicals a(scavengers, real textile oxidants, effluent containing or inorganic a largeions) varietyon the ofphotocatalytic compounds. Theprocess monitoring was also of studied ROS showed mimicking that the a superoxidereal textile ionseffluent played containing the main a rolelarge in variety RG12 degradationof compounds. contrary The mo tonitoring hydroxyl of radicalsROS showed and the that photo-generated the superoxide ions holes. played The presencethe main ofrole H 2inO 2RG12or K degradation2S2O8 presented contrary a high to hydroxyl oxidizing ra power,dicals enhancingand the photo-generated the photocatalytic holes. activity The presence of the sputteredof H2O2 or catalyst. K2S2O8 Thepresented presence a high of Cu oxidizing as mineral power, pollution enhancing and NaCl the asphotocatalytic salt decreased activity the reaction of the rate,sputtered but the catalyst. addition The of presence both of themof Cu (Cuas mineral and NaCl pollution salts) improvedand NaCl theas salt removal decreased efficiency the reaction of the studiedrate, but pollutant. the addition The of recycling both of ofthem the catalyst(Cu and showed NaCl salts) a high improved stability ofthe the removal catalyst efficiency up to 21 RG12of the discolorationstudied pollutant. cycles. The ICP-MS recycling showed of the stable catalyst ions’ showed release a after high the stability 5th cycle of the for catalyst both ions. up This to 21 allows RG12 potentialdiscoloration industrial cycles. applications ICP-MS showed of the stable reported ions’ HiPIMS release coatings after the in 5th the cycle future. for both ions. This allows potential industrial applications of the reported HiPIMS coatings in the future.

Materials 2019, 12, 412 13 of 16

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/12/3/412/s1, Table S1: Second order rate constants of different scavengers with radical species. Author Contributions: Conceptualization: S.R and A.A.A.; Methodology: A.A.A., A.A. and S.R.; Software and validation: H.Z., N.K. and H.D.; Investigation and Experimental: S.R., H.Z. and A.A.A.; Resources: S.R., A.A. and A.A.A.; Writing—original draft preparation: H.Z. and A.A.A.; Writing—review and editing A.A. and S.R.; Supervision: A.A., A.A.A., H.D. and N.K. Acknowledgments: S.R. acknowledges the support of EPFL-IPhys (especially, Prof. László Forró) and M. Bensimon. Authors would like to thank ENSCR support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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