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Photocatalytic degradation of methylene blue in water solution by multilayer TiO2 coating on HDPE
Applied Surface Science 258 (2011) 1738–1743
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photocatalytic degradation of methylene blue in water solution by multilayer TiO2 coating on HDPE Jussi Kasanen, Janne Salstela, Mika Suvanto, Tuula T. Pakkanen ∗ Department of Chemistry, University of Eastern Finland, P.O. Box 111, FIN-80101 Joensuu, Finland
a r t i c l e
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Article history: Received 21 April 2011 Received in revised form 8 September 2011 Accepted 9 October 2011 Available online 14 October 2011 Keywords: Coatings Methylene blue Photocatalysis Polymers Surface morphology TiO2
a b s t r a c t A multilayer photocatalytic TiO2 coating on a high-density polyethylene (HDPE) disk was found to degrade aqueous methylene blue in a batch reactor study. The TiO2 coating was fabricated by a low-temperature method using polyurethane resin (PU) as a barrier layer for HDPE and as a binding agent for two TiO2 layers. Adequate adhesion between the HDPE substrate and PU barrier in aqueous environment was ensured with an oxygen plasma treatment. The photocatalytic effect of immersed TiO2 coating on the degradation of methylene blue in aqueous solution was monitored by UV–vis spectrometry as a function of UV-illumination time. Samples were allowed to adsorb methylene blue in the dark for 1 h before the UV-degradation experiments were started. The percentages of methylene blue degraded during 6 h UV illumination ( = 365 nm) varied from 80% to 92%. The degradation followed pseudo-first order reaction kinetics, and the observed rate constants (kobs ) were between 0.27 and 0.43 h−1 . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide is a widely used multipurpose photocatalyst because of its stability, chemical resistance, favorable redox potential, high photocatalytic activity, and relatively low cost [1]. Unlike zinc oxide [2] or cadmium sulfide [3] it is not prone to photocorrosion and it is suitable for use in aqueous environment [4]. TiO2 can be used as a photocatalyst in water purification to degrade organic pollutants [5,6]. Photomineralization of organic dyes to H2 O and CO2 is a much researched area and of particular interest for the textile industry, which produces liquid dye waste in large quantities [4,7]. Azo dyes continue to be heavily used, and disposing of them is difficult. TiO2 could well be suitable for the disposal of dyes because the reaction conditions in liquid environment can be adjusted so as to increase photocatalytic efficiency. Among other relevant factors are pH, temperature, concentrations of the dye and oxygen, and the intensity of UV light [7,8]. Nanosized TiO2 in powder form has distinct advantages over TiO2 microparticles so far as photocatalytic activity is concerned, the most important one being the large specific surface area [9]. One practical limitation on the use of nanosized TiO2 for photodegradation of wastewater is that it is hard to separate from solution, and catalyst loss may be large, with accompanying increase in costs [10,11]. Catalyst loss can be reduced substantially by immobilizing
titanium dioxide on a substrate such as ceramic material, glass, polymer, or steel [12]. Without immobilization of the TiO2 , adhesion and scratch resistance are likely to be poor because of the weak forces between particles and substrate [13]. TiO2 in powder form can be fixed to polymer substrate in various mechanical ways. Fixing into polyethylene can be done by ironing [14], hot pressing [15], or heating of PE/TiO2 film [16]. Another way to fix TiO2 mechanically, besides heat, is to apply a liquid adhesive. Two adhesives already tested are silicone elastomer and polyurethane lacquer [17,18]. The target of the present work was to study the functioning and effectiveness of a self-cleaning TiO2 -based multilayer photocatalytic coating on HDPE in water purification. In our coating, polyurethane resin was used to fix TiO2 particles on PU-protected HDPE substrate, and methylene blue was employed as organic model substance in photocatalytic degradation studies. The degradation of methylene blue was monitored by UV–vis spectrometry, and reaction rates were calculated from the measured absorbances. Also, the effect of the amount of polyurethane binding agent in the TiO2 coating on photocatalytic degradation of methylene blue was examined. Finally, we tested the influence of pH adjustment on the degradation rate. 2. Experimental 2.1. Preparation of multilayer photocatalytic TiO2 coating
∗ Corresponding author. Tel.: +358 13 2513340; fax: +358 13 2513390. E-mail address: tuula.pakkanen@uef.fi (T.T. Pakkanen). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.10.028
The preparation process and structure of the TiO2 -based multilayer photocatalytic coating on HDPE are described in detail
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elsewhere [18]. The TiO2 coating consists of a polyurethane (PU) barrier layer and diluted PU binder for fixing the TiO2 particles (Degussa P25 from Degussa AG). Two PU binder dilutions (1:12 and 1:8 mass ratios; PU dispersion/ion-exchanged water) were used. For reference purposes, some samples were prepared without PU binder. Water is a challenging medium for the coating because it can penetrate between the hydrophobic HDPE substrate and hydrophilic protective PU layer. If initial adhesion is low the whole photocatalytic coating may disengage from the substrate [18]. Adequate adhesion between HDPE and PU was ensured with a 30 s oxygen plasma treatment of the substrate at 200 W power. 2.2. Scanning electron microscopy (SEM) imaging of surfaces Surface morphology of sample disks was studied with a Hitachi S4800 field emission scanning electron microscope (Toronto, Ontario, Canada). The samples were attached to the sample holder with copper adhesive tape and coated with gold (8 nm). An accelerating voltage of 5 kV was applied, and the general working distance was 8.3 mm. 2.3. Water contact angle measurements Water contact angles of TiO2 -containing sample surfaces were measured with and without PU binder to evaluate wettability. Contact angles were also recorded after 6 h UV illumination to check for possible superhydrophilicity. Static contact angles of four water droplets were individually measured with a Cam 200 contact angle meter with automatic liquid dispenser (KSV Instruments Ltd., Helsinki, Finland). The measurement process is described in detail elsewhere [18].
Fig. 1. (a) Experimental setup for degradation of methylene blue indicator solution and (b) composition of sample disk in (a).
2.4. Photocatalytic studies with methylene blue A reactor made of stainless steel was used in UV experiments. The reactor consisted of a lid with quartz window and a base part with liquid container. The diameter of the reactor was 25.5 mm and that of the sample disk 24.0 mm, and the effective surface area of the disk was 4.5 cm2 . The experimental setup is presented in Fig. 1. A TiO2 /HDPE sample disk was placed in the bottom part of the reactor on a sample holder (O-ring) made of stainless steel (thickness 0.7 mm). Aqueous solution of methylene blue (C = 1.0 × 10−5 M, Reag. Ph Eur, Merck KGaA, Darmstadt, Germany), mixed with a magnetic stirrer, was used for degradation studies. A high intensity UV lamp (UVP Black Ray B100AP, Upland, CA) was stabilized 10 min before the UV-illumination experiments. All experiments were performed at normal air pressure. The reactor was kept open to the air through four tubes sealed with septa and opened up with 21-gauge injection needles to prevent evaporation of the methylene blue solution. Before the photocatalytic studies, the sample disks were immersed in methylene blue solution for 1 h in a dark place to allow them to adsorb the dye. The initial absorbance of the dye solution after the adsorption period was recorded with a UV–vis spectrometer (Perkin-Elmer Lambda 900 UV/vis/NIR, Waltham, MA) at wavelength 664 nm. The immersed sample disks were then irradiated for 6 h in steps of 1 h, and absorbances of the methylene blue solutions were measured after each step. For absorbance measurements, a 2.5 ml sample of the methylene blue solution was removed to a 10 mm wide quartz glass cuvette. After the measurement the solution was returned to the reactor. The reported absorbances are averages for two parallel photocatalytic reactions carried out with identical TiO2 sample disks. The effect of pH on the photocatalytic degradation was investigated by altering the pH value by ∼2 units up (pH 7.64) and down
(pH 3.46) from the initial value (pH 5.56) of the aqueous methylene blue solution. The pH values were adjusted by addition of hydrochloric acid (0.1 and 0.01 M) or sodium hydroxide (0.1 M). The pH values were measured with an Orion pH meter model 420A (Orion Pacific Pty Ltd, Sydney, Australia), and absorbances of the pH-adjusted solutions were measured in the manner described above. 2.5. Reflectance measurements of methylene blue residues Total reflectance measurements were used to evaluate the adsorption of methylene blue on the TiO2 /HDPE disks. Measurements were made with a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer (Waltham, MA) equipped with a 150 mm Spectralon® integrating sphere (PELA-1000, manufactured by Labsphere, North Sutton, NH). The measurement range was 600–700 nm (maximum absorbance of methylene blue 664 nm), and Labsphere certified reflectance standards were used. 3. Results and discussion 3.1. Surface morphology of TiO2 /PU coating The TiO2 particles used in the coating of HDPE were fixed with PU binder. The PU dispersion was diluted with water, and two dilutions (PU/H2 O mass ratios of 1:12 and 1:8) were applied on TiO2 surfaces. Fig. 2 shows the influence of PU binding on the surface morphology of TiO2 particle layer. According to the SEM image (Fig. 2(a)) the TiO2 surface consists mainly of large agglomerates. After spreading and curing of the PU binder (Fig. 2(b)) the amount of TiO2 available for photocatalysis is still adequate, although some of it is embedded under the PU
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Fig. 2. SEM images of TiO2 -containing surfaces on protective PU-coated HDPE: (a) without PU binder and (b) with PU binder dilution 1:8 after curing. Magnification in images is 7k.
binder (seen as a darker tone). With use of high speed spin coating, the binder can be forced between the TiO2 agglomerates and only a very small volume (6.6 l/cm2 ) is needed for good fixing. 3.2. Water contact angles on TiO2 -containing surfaces A large contact area between the photocatalyst surface (TiO2 ) and water is desired for self-cleaning purposes. Measurement of water contact angles is an accurate method for determining the interaction between a liquid and a solid. The hydrophilicity of our photocatalytic TiO2 coatings was determined by measuring apparent water contact angles on TiO2 surfaces. When two TiO2 layers are fixed onto protective PU attached to HDPE substrate the contact angle stabilizes at 29◦ indicating that wettability is good but superhydrophilicity is not reached. After TiO2 is fixed with PU binder (dilutions 1:12 and 1:8) the contact angle of the sample surface increases significantly to ∼70◦ , showing that the PU binder rejects water to some extent. However, after 6 h UV illumination of the sample containing TiO2 in aqueous methylene blue, the situation changes dramatically and the surface is nearly superhydrophilic. After 6 h illumination the contact angles are 4◦ for dilution 1:12 and 11◦ for dilution 1:8. The wettability of both surfaces is very good during the illumination, which suggests that TiO2 is available for adsorption and photocatalysis and PU binder does not substantially inhibit or disturb the degradation of methylene blue. 3.3. UV-induced TiO2 photocatalytic degradation of methylene blue To measure the photocatalytic activity of our TiO2 coating on HDPE substrate disks in the degradation of methylene blue, we
immersed the sample disks in aqueous methylene blue solution and exposed them to UV-illumination for 6 h. Absorbances of the solutions were measured with a UV–vis spectrometer at wavelength 664 nm. The measured absorbances and relative amounts of methylene blue in solutions (absorbance relative to absorbance at 0 h illumination) are reported in Table 1. Absorbance measurements after 1 h adsorption of the dye solution (Table 1, time = 0 h) showed that the polyurethane resin used as barrier and for fixing of TiO2 particles adsorbs methylene blue. The absorbance of the methylene blue solution dropped from 0.8384 to 0.5189 when the HDPE/protective PU sample was immersed in the methylene blue solution for 1 h in dark (Table 1). When two TiO2 layers were added onto the protective PU the adsorption of the dye decreased slightly. The HDPE substrate had only a minor effect on the absorbance of the methylene blue solution. The adsorption of cationic dyes onto polyurethane is facilitated by a polar interaction between the dye and polyurethane [19]. The results after 6 h UV illumination (Table 1) show that the drop in relative amount of methylene blue is faster and much larger for samples containing TiO2 (without PU binder; 85%, with PU binder 1:12; 85%, with PU binder 1:8; 92%) than for samples without TiO2 (with only HDPE; 4%, with HDPE/protective PU; 47%), indicating photocatalytic degradation of methylene blue. In the absence of TiO2 , decrease in the absorbance of methylene blue solution is mostly due to adsorption of the dye on PU. Fig. 3(a) shows that methylene blue on the HDPE/protective PU sample was not degraded during UV illumination but remained adsorbed on the PU layer. However, the sample with two layers of TiO2 on HDPE/protective PU (Fig. 3(b) and Table 1) was almost completely clean after 6 h UV illumination. The traces of methylene blue at the edges of the disk are due to the uneven distribution of
Fig. 3. Photographs of HDPE substrates coated with protective PU after immersion in aqueous methylene blue solution and 6 h UV illumination. Sample (a) does not contain TiO2 and sample (b) contains two layers of TiO2 .
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Table 1 Absorbances of aqueous methylene blue solutions and relative amounts of methylene blue measured by UV–vis spectrometry after 0–6 h UV illumination. UV illumination time (h)
0a 1 2 3 4 5 6 a b
HDPE
HDPE + protective PU
HDPE + protective PU + TiO2
HDPE + protective PU + TiO2 + PU binder 1:12
HDPE + protective PU + TiO2 + PU binder 1:8
Absorbance
Methylene blue (%)b
Absorbance
Methylene blue (%)b
Absorbance
Methylene blue (%)b
Absorbance
Methylene blue (%)b
Absorbance
Methylene blue (%)b
0.7233 0.7233 0.7231 0.7069 0.7030 0.6960 0.6949
100 100 100 97.7 97.2 96.2 96.1
0.5189 0.4355 0.3741 0.3334 0.3106 0.2927 0.2751
100 83.9 72.1 64.3 59.9 56.4 53.0
0.5970 0.4350 0.3103 0.2299 0.1703 0.1201 0.0879
100 72.9 52.0 38.5 28.5 20.1 14.7
0.6407 0.5153 0.3768 0.2610 0.1848 0.1289 0.0941
100 80.4 58.8 40.7 28.8 20.1 14.7
0.5733 0.4264 0.2857 0.1777 0.1156 0.0696 0.0477
100 74.4 49.8 31.0 20.2 12.1 8.3
Samples were kept immersed in aqueous methylene blue solution in a dark room for 1 h before measurement. Absorbance of pure methylene blue solution was 0.8384. Absorbance relative to absorbance at 0 h illumination (%).
TiO2 during spin coating, as verified by reflectance measurements after UV illumination. Reflectances of sample disks were measured in wavelength range 600–700 nm with an integrating sphere to evaluate the adsorption of methylene blue on samples (Fig. 4). Fig. 4 shows that the reflectances of the HDPE substrate and HDPE/protective PU are about the same (∼80%), but a significant difference emerges after the samples have been immersed in methylene blue solution and UV-illuminated. Pure HDPE adsorbs some of the dye, and reflectance is decreased by about 2% indicating adsorption rather than degradation (Table 1). However when protective PU is applied to HDPE, the reflectance of the sample after immersion in methylene blue drops drastically to 19% at wavelength range of 645–655 nm. According to Table 1, the TiO2 sample with PU binder dilution 1:8 is photocatalytically more effective than a sample with dilution 1:12. Reflectance measurements (Fig. 4) show that, after UV illumination, slightly more methylene blue is adsorbed on the sample with dilution 1:8. 3.4. Effect of pH on TiO2 photocatalytic degradation of methylene blue To test whether pH has an effect on the photocatalytic efficiency of TiO2 , we adjusted the pH of the aqueous methylene blue solution (1 × 10−5 M) ∼2 units up and down (7.64 and 3.46) from the original value (5.56). The experiments were done with TiO2 samples in the absence of PU binder to exclude the effect of the binder. Absorbances of samples were recorded before UV illumination and at 1 h intervals for 6 h (Table 2).
Fig. 4. Reflectance of samples measured with an integrating sphere. Immersed means that the sample has been immersed in aqueous methylene blue and UVilluminated for 6 h.
Table 2 shows a significant effect of pH on the adsorption of methylene blue on the sample disks. The initial absorbances, after 1 h adsorption, show adsorption of the dye to be clearly favored in slightly alkaline environment (pH 7.64) as compared with acidic environment (pH 3.46) or the original solution (pH 5.56) without pH adjustment. However, the relative decomposition rate of methylene blue was best without any pH adjustment. As can be seen in Fig. 5, the reflectance value is highest for the sample at pH 5.56 and lowest for the sample in alkaline solution. The difference in reflectances at pH 5.56 and pH 7.64 is 25%. TiO2 degrades methylene blue at pH 7.64, but the process is much slower because deactivation of the catalyst occurs in alkaline environment [15]. The adsorption of cationic methylene blue is favored in alkaline solution because when pH is higher than the point of zero charge (Pzc) of the TiO2 , the TiO2 surface becomes negatively charged (Pzc for TiO2 -P25 ≈ 6.8) [20]. Possibly increased adsorption of methylene blue on the TiO2 surface, combined with adsorption onto the protective PU, leads to excessive overall adsorption and deactivation of the catalyst. According to the results in Table 2, at pH 7.64 most of the methylene blue was adsorbed on the photocatalytic coating during 1 h adsorption (the absorbance was only 0.3596). Evidently excessive adsorption of the dye blocks the active surface sites of TiO2 for oxygen and water, and no active oxygen species that could degrade methylene blue are formed. 3.5. Kinetics of methylene blue degradation We determined the reaction kinetics for the degradation of aqueous methylene blue solution (C = 1 × 10−5 M) using the UV–vis absorbance data. The results in Table 1 indicate that the
Fig. 5. Reflectance of samples used in the pH study, measured with an integrating sphere. Immersed indicates that the sample has been immersed in aqueous methylene blue for 1 h and UV-illuminated for 6 h.
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Table 2 Absorbances of aqueous methylene blue solutions at different pH values and amount of methylene blue after UV illumination, measured as UV–vis absorbance. UV-illumination time (h)
0a 1 2 3 4 5 6 a
HDPE + protective PU + TiO2 (pH 3.46)
HDPE + protective PU + TiO2 (pH 5.56)
HDPE + protective PU + TiO2 (pH 7.64)
Absorbance
Methylene blue (%)
Absorbance
Methylene blue (%)
Absorbance
Methylene blue (%)
0.7628 0.6301 0.4745 0.3719 0.2842 0.2168 0.1552
100 82.6 62.2 48.8 37.3 28.4 20.3
0.5970 0.4350 0.3103 0.2299 0.1703 0.1201 0.0879
100 72.9 52.0 38.5 28.5 20.1 14.7
0.3597 0.3119 0.2263 0.1844 0.1351 0.0852 0.0679
100 86.7 62.9 51.3 37.5 23.7 18.9
Samples were kept in a dark room, immersed in aqueous methylene blue solution, for 1 h before measurement.
Table 3 Rate constants and R-squared values for photocatalytic degradation of methylene blue at different sample compositions.
kobs (h−1 ) R2
HDPE + protective PU + TiO2 + PU binder 1:12
HDPE + protective PU + TiO2 + PU binder 1:8
HDPE + protective PU + TiO2 (pH 3.46)
HDPE + protective PU + TiO2 (pH 5.56)
HDPE + protective PU + TiO2 (pH 7.64)
0.33 1.00
0.43 1.00
0.27 1.00
0.32 1.00
0.29 0.98
Fig. 6. (a) Relative amounts of methylene blue and (b) natural logarithms of absorbance of methylene blue plotted as a function of UV-illumination time.
concentration of methylene blue does not decrease linearly as a function of UV illumination time. The kinetics of photocatalytic degradation of methylene blue on TiO2 dispersion under UV irradiation has often been modeled with a simple Langmuir-Hinshelwood equation (Eq. (1)) [15]: r=−
d[MB] k K[MB] =− 1 + K[MB] dt
(1) 4. Conclusions
where r is the rate of disappearance of the reagent, [MB] is the reagent concentration, k is the rate constant, and K is the observed equilibrium constant. The Langmuir-Hinshelwood equation can be simplified to a pseudo-first order expression (Eq. (2)) if the concentration of reagent is very low (in this study [MB] = 1 × 10−5 M). r=−
d[MB] = kobs [MB] dt
(2)
Integration of Eq. (2) leads to Eq. (3):
ln
[MB] 0
[MB]
= kobs t
binder dilution 1:8 (kobs = 0.43 h−1 vs. 0.32 h−1 ), because PU binder assists the adsorption of methylene blue to the nearby TiO2 particles. Both increase and decrease of pH from 5.56 had a negative effect on the photocatalysis, and the best results were obtained with PU binder dilution 1:8 at pH 5.56.
(3)
When we plot ln([MB]0 /[MB]) vs. t we get a straight line with slope k (Fig. 6). The values in Fig. 6 are taken from Table 1. Fig. 6 shows that the pseudo-first order assumption describes the experimental data well. Table 3 presents the rate constants and R-squared values, including values for samples at different pH. As compared with samples without binding agent there is a significant improvement in the degradation rate in the presence of PU
We have successfully applied our multilayer photocatalytic TiO2 coating for the degradation of methylene blue in aqueous solution. The waterborne heat-cured polyurethane coating on HDPE substrate protects the substrate from photocatalytic degradation. Oxygen plasma treatment of the substrate ensures adequate adhesion between substrate and protective layer so that the coating does not peel off when samples are immersed in methylene blue solution. Fixing of TiO2 on the protective PU with PU binder ensures attachment of TiO2 particles during adsorption and photocatalysis steps. With the best TiO2 samples, which contained PU binder at dilution 1:8, the degradation of the dye was over 90%; with the more diluted binder (1:12) it was 85%. Reflectance measurements of sample disks after the UV illumination showed some residual adsorption of methylene blue in every sample indicating that degradation of the dye was not complete. Adjustment of pH from the original value slowed the degradation: methylene blue was adsorbed on the protective polyurethane, especially in slightly
J. Kasanen et al. / Applied Surface Science 258 (2011) 1738–1743
alkaline solution (pH 7.64), causing intense color and a major decrease in reflectance of the sample. Best decomposition results were obtained at pH 5.56, where degradation of the dye followed pseudo-first order reaction kinetics with a rate constant of 0.43 h−1 . References [1] A. Fujishima, X. Zhang, D.A. Tryk, Heterogeneous photocatalysis: from water photolysis to applications in environmental cleanup, Int. J. Hydrogen Energy 32 (2007) 2664–2672. [2] Y. Li, W. Xie, X. Hu, G. Shen, X. Zhou, Y. Xiang, X. Zhao, P. Fang, Comparison of dye photodegradation and its coupling with light-to-electricity conversion over TiO2 and ZnO, Langmuir 26 (2009) 591–597. [3] A.P. Davis, C.P. Huang, The photocatalytic oxidation of sulfur-containing organic compounds using cadmium sulfide and the effect on CdS photocorrosion, Water Res. 25 (1991) 1273–1278. [4] K. Rajeshwar, M.E. Osugi, W. Chanmanee, C.R. Chenthamarakshan, M.V.B. Zanoni, P. Kajitvichyanukul, R. Krishnan-Ayer, Heterogeneous photocatalytic treatment of organic dyes in air and aqueous media, J. Photochem. Photobiol. C-Photochem. Rev. 9 (2008) 171–192. [5] J.A. Byrne, B.R. Eggins, N.M.D. Brown, B. McKinney, M. Rouse, Immobilisation of TiO2 powder for the treatment of polluted water, Appl. Catal. B-Environ. 17 (1998) 25–36. [6] U.I. Gaya, A.H. Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems, J. Photochem. Photobiol. C-Photochem. Rev. 9 (2008) 1–12. [7] M. Stylidi, D.I. Kondarides, X.E. Verykios, Pathways of solar light-induced photocatalytic degradation of azo dyes in aqueous TiO2 suspensions, Appl. Catal. B-Environ. 40 (2004) 271–286. [8] I.K. Konstantinou, T.A. Albanis, TiO2 -assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B-Environ. 49 (2004) 1–14. [9] N.S. Allen, M. Edge, J. Verran, J. Stratton, J. Maltby, C. Bygott, Photocatalytic titania based surfaces: environmental benefits, Polym. Degrad. Stab. 93 (2008) 1632–1646.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photocatalytic degradation of methylene blue in water solution by multilayer TiO2 coating on HDPE Jussi Kasanen, Janne Salstela, Mika Suvanto, Tuula T. Pakkanen ∗ Department of Chemistry, University of Eastern Finland, P.O. Box 111, FIN-80101 Joensuu, Finland
a r t i c l e
i n f o
Article history: Received 21 April 2011 Received in revised form 8 September 2011 Accepted 9 October 2011 Available online 14 October 2011 Keywords: Coatings Methylene blue Photocatalysis Polymers Surface morphology TiO2
a b s t r a c t A multilayer photocatalytic TiO2 coating on a high-density polyethylene (HDPE) disk was found to degrade aqueous methylene blue in a batch reactor study. The TiO2 coating was fabricated by a low-temperature method using polyurethane resin (PU) as a barrier layer for HDPE and as a binding agent for two TiO2 layers. Adequate adhesion between the HDPE substrate and PU barrier in aqueous environment was ensured with an oxygen plasma treatment. The photocatalytic effect of immersed TiO2 coating on the degradation of methylene blue in aqueous solution was monitored by UV–vis spectrometry as a function of UV-illumination time. Samples were allowed to adsorb methylene blue in the dark for 1 h before the UV-degradation experiments were started. The percentages of methylene blue degraded during 6 h UV illumination ( = 365 nm) varied from 80% to 92%. The degradation followed pseudo-first order reaction kinetics, and the observed rate constants (kobs ) were between 0.27 and 0.43 h−1 . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide is a widely used multipurpose photocatalyst because of its stability, chemical resistance, favorable redox potential, high photocatalytic activity, and relatively low cost [1]. Unlike zinc oxide [2] or cadmium sulfide [3] it is not prone to photocorrosion and it is suitable for use in aqueous environment [4]. TiO2 can be used as a photocatalyst in water purification to degrade organic pollutants [5,6]. Photomineralization of organic dyes to H2 O and CO2 is a much researched area and of particular interest for the textile industry, which produces liquid dye waste in large quantities [4,7]. Azo dyes continue to be heavily used, and disposing of them is difficult. TiO2 could well be suitable for the disposal of dyes because the reaction conditions in liquid environment can be adjusted so as to increase photocatalytic efficiency. Among other relevant factors are pH, temperature, concentrations of the dye and oxygen, and the intensity of UV light [7,8]. Nanosized TiO2 in powder form has distinct advantages over TiO2 microparticles so far as photocatalytic activity is concerned, the most important one being the large specific surface area [9]. One practical limitation on the use of nanosized TiO2 for photodegradation of wastewater is that it is hard to separate from solution, and catalyst loss may be large, with accompanying increase in costs [10,11]. Catalyst loss can be reduced substantially by immobilizing
titanium dioxide on a substrate such as ceramic material, glass, polymer, or steel [12]. Without immobilization of the TiO2 , adhesion and scratch resistance are likely to be poor because of the weak forces between particles and substrate [13]. TiO2 in powder form can be fixed to polymer substrate in various mechanical ways. Fixing into polyethylene can be done by ironing [14], hot pressing [15], or heating of PE/TiO2 film [16]. Another way to fix TiO2 mechanically, besides heat, is to apply a liquid adhesive. Two adhesives already tested are silicone elastomer and polyurethane lacquer [17,18]. The target of the present work was to study the functioning and effectiveness of a self-cleaning TiO2 -based multilayer photocatalytic coating on HDPE in water purification. In our coating, polyurethane resin was used to fix TiO2 particles on PU-protected HDPE substrate, and methylene blue was employed as organic model substance in photocatalytic degradation studies. The degradation of methylene blue was monitored by UV–vis spectrometry, and reaction rates were calculated from the measured absorbances. Also, the effect of the amount of polyurethane binding agent in the TiO2 coating on photocatalytic degradation of methylene blue was examined. Finally, we tested the influence of pH adjustment on the degradation rate. 2. Experimental 2.1. Preparation of multilayer photocatalytic TiO2 coating
∗ Corresponding author. Tel.: +358 13 2513340; fax: +358 13 2513390. E-mail address: tuula.pakkanen@uef.fi (T.T. Pakkanen). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.10.028
The preparation process and structure of the TiO2 -based multilayer photocatalytic coating on HDPE are described in detail
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elsewhere [18]. The TiO2 coating consists of a polyurethane (PU) barrier layer and diluted PU binder for fixing the TiO2 particles (Degussa P25 from Degussa AG). Two PU binder dilutions (1:12 and 1:8 mass ratios; PU dispersion/ion-exchanged water) were used. For reference purposes, some samples were prepared without PU binder. Water is a challenging medium for the coating because it can penetrate between the hydrophobic HDPE substrate and hydrophilic protective PU layer. If initial adhesion is low the whole photocatalytic coating may disengage from the substrate [18]. Adequate adhesion between HDPE and PU was ensured with a 30 s oxygen plasma treatment of the substrate at 200 W power. 2.2. Scanning electron microscopy (SEM) imaging of surfaces Surface morphology of sample disks was studied with a Hitachi S4800 field emission scanning electron microscope (Toronto, Ontario, Canada). The samples were attached to the sample holder with copper adhesive tape and coated with gold (8 nm). An accelerating voltage of 5 kV was applied, and the general working distance was 8.3 mm. 2.3. Water contact angle measurements Water contact angles of TiO2 -containing sample surfaces were measured with and without PU binder to evaluate wettability. Contact angles were also recorded after 6 h UV illumination to check for possible superhydrophilicity. Static contact angles of four water droplets were individually measured with a Cam 200 contact angle meter with automatic liquid dispenser (KSV Instruments Ltd., Helsinki, Finland). The measurement process is described in detail elsewhere [18].
Fig. 1. (a) Experimental setup for degradation of methylene blue indicator solution and (b) composition of sample disk in (a).
2.4. Photocatalytic studies with methylene blue A reactor made of stainless steel was used in UV experiments. The reactor consisted of a lid with quartz window and a base part with liquid container. The diameter of the reactor was 25.5 mm and that of the sample disk 24.0 mm, and the effective surface area of the disk was 4.5 cm2 . The experimental setup is presented in Fig. 1. A TiO2 /HDPE sample disk was placed in the bottom part of the reactor on a sample holder (O-ring) made of stainless steel (thickness 0.7 mm). Aqueous solution of methylene blue (C = 1.0 × 10−5 M, Reag. Ph Eur, Merck KGaA, Darmstadt, Germany), mixed with a magnetic stirrer, was used for degradation studies. A high intensity UV lamp (UVP Black Ray B100AP, Upland, CA) was stabilized 10 min before the UV-illumination experiments. All experiments were performed at normal air pressure. The reactor was kept open to the air through four tubes sealed with septa and opened up with 21-gauge injection needles to prevent evaporation of the methylene blue solution. Before the photocatalytic studies, the sample disks were immersed in methylene blue solution for 1 h in a dark place to allow them to adsorb the dye. The initial absorbance of the dye solution after the adsorption period was recorded with a UV–vis spectrometer (Perkin-Elmer Lambda 900 UV/vis/NIR, Waltham, MA) at wavelength 664 nm. The immersed sample disks were then irradiated for 6 h in steps of 1 h, and absorbances of the methylene blue solutions were measured after each step. For absorbance measurements, a 2.5 ml sample of the methylene blue solution was removed to a 10 mm wide quartz glass cuvette. After the measurement the solution was returned to the reactor. The reported absorbances are averages for two parallel photocatalytic reactions carried out with identical TiO2 sample disks. The effect of pH on the photocatalytic degradation was investigated by altering the pH value by ∼2 units up (pH 7.64) and down
(pH 3.46) from the initial value (pH 5.56) of the aqueous methylene blue solution. The pH values were adjusted by addition of hydrochloric acid (0.1 and 0.01 M) or sodium hydroxide (0.1 M). The pH values were measured with an Orion pH meter model 420A (Orion Pacific Pty Ltd, Sydney, Australia), and absorbances of the pH-adjusted solutions were measured in the manner described above. 2.5. Reflectance measurements of methylene blue residues Total reflectance measurements were used to evaluate the adsorption of methylene blue on the TiO2 /HDPE disks. Measurements were made with a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer (Waltham, MA) equipped with a 150 mm Spectralon® integrating sphere (PELA-1000, manufactured by Labsphere, North Sutton, NH). The measurement range was 600–700 nm (maximum absorbance of methylene blue 664 nm), and Labsphere certified reflectance standards were used. 3. Results and discussion 3.1. Surface morphology of TiO2 /PU coating The TiO2 particles used in the coating of HDPE were fixed with PU binder. The PU dispersion was diluted with water, and two dilutions (PU/H2 O mass ratios of 1:12 and 1:8) were applied on TiO2 surfaces. Fig. 2 shows the influence of PU binding on the surface morphology of TiO2 particle layer. According to the SEM image (Fig. 2(a)) the TiO2 surface consists mainly of large agglomerates. After spreading and curing of the PU binder (Fig. 2(b)) the amount of TiO2 available for photocatalysis is still adequate, although some of it is embedded under the PU
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Fig. 2. SEM images of TiO2 -containing surfaces on protective PU-coated HDPE: (a) without PU binder and (b) with PU binder dilution 1:8 after curing. Magnification in images is 7k.
binder (seen as a darker tone). With use of high speed spin coating, the binder can be forced between the TiO2 agglomerates and only a very small volume (6.6 l/cm2 ) is needed for good fixing. 3.2. Water contact angles on TiO2 -containing surfaces A large contact area between the photocatalyst surface (TiO2 ) and water is desired for self-cleaning purposes. Measurement of water contact angles is an accurate method for determining the interaction between a liquid and a solid. The hydrophilicity of our photocatalytic TiO2 coatings was determined by measuring apparent water contact angles on TiO2 surfaces. When two TiO2 layers are fixed onto protective PU attached to HDPE substrate the contact angle stabilizes at 29◦ indicating that wettability is good but superhydrophilicity is not reached. After TiO2 is fixed with PU binder (dilutions 1:12 and 1:8) the contact angle of the sample surface increases significantly to ∼70◦ , showing that the PU binder rejects water to some extent. However, after 6 h UV illumination of the sample containing TiO2 in aqueous methylene blue, the situation changes dramatically and the surface is nearly superhydrophilic. After 6 h illumination the contact angles are 4◦ for dilution 1:12 and 11◦ for dilution 1:8. The wettability of both surfaces is very good during the illumination, which suggests that TiO2 is available for adsorption and photocatalysis and PU binder does not substantially inhibit or disturb the degradation of methylene blue. 3.3. UV-induced TiO2 photocatalytic degradation of methylene blue To measure the photocatalytic activity of our TiO2 coating on HDPE substrate disks in the degradation of methylene blue, we
immersed the sample disks in aqueous methylene blue solution and exposed them to UV-illumination for 6 h. Absorbances of the solutions were measured with a UV–vis spectrometer at wavelength 664 nm. The measured absorbances and relative amounts of methylene blue in solutions (absorbance relative to absorbance at 0 h illumination) are reported in Table 1. Absorbance measurements after 1 h adsorption of the dye solution (Table 1, time = 0 h) showed that the polyurethane resin used as barrier and for fixing of TiO2 particles adsorbs methylene blue. The absorbance of the methylene blue solution dropped from 0.8384 to 0.5189 when the HDPE/protective PU sample was immersed in the methylene blue solution for 1 h in dark (Table 1). When two TiO2 layers were added onto the protective PU the adsorption of the dye decreased slightly. The HDPE substrate had only a minor effect on the absorbance of the methylene blue solution. The adsorption of cationic dyes onto polyurethane is facilitated by a polar interaction between the dye and polyurethane [19]. The results after 6 h UV illumination (Table 1) show that the drop in relative amount of methylene blue is faster and much larger for samples containing TiO2 (without PU binder; 85%, with PU binder 1:12; 85%, with PU binder 1:8; 92%) than for samples without TiO2 (with only HDPE; 4%, with HDPE/protective PU; 47%), indicating photocatalytic degradation of methylene blue. In the absence of TiO2 , decrease in the absorbance of methylene blue solution is mostly due to adsorption of the dye on PU. Fig. 3(a) shows that methylene blue on the HDPE/protective PU sample was not degraded during UV illumination but remained adsorbed on the PU layer. However, the sample with two layers of TiO2 on HDPE/protective PU (Fig. 3(b) and Table 1) was almost completely clean after 6 h UV illumination. The traces of methylene blue at the edges of the disk are due to the uneven distribution of
Fig. 3. Photographs of HDPE substrates coated with protective PU after immersion in aqueous methylene blue solution and 6 h UV illumination. Sample (a) does not contain TiO2 and sample (b) contains two layers of TiO2 .
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Table 1 Absorbances of aqueous methylene blue solutions and relative amounts of methylene blue measured by UV–vis spectrometry after 0–6 h UV illumination. UV illumination time (h)
0a 1 2 3 4 5 6 a b
HDPE
HDPE + protective PU
HDPE + protective PU + TiO2
HDPE + protective PU + TiO2 + PU binder 1:12
HDPE + protective PU + TiO2 + PU binder 1:8
Absorbance
Methylene blue (%)b
Absorbance
Methylene blue (%)b
Absorbance
Methylene blue (%)b
Absorbance
Methylene blue (%)b
Absorbance
Methylene blue (%)b
0.7233 0.7233 0.7231 0.7069 0.7030 0.6960 0.6949
100 100 100 97.7 97.2 96.2 96.1
0.5189 0.4355 0.3741 0.3334 0.3106 0.2927 0.2751
100 83.9 72.1 64.3 59.9 56.4 53.0
0.5970 0.4350 0.3103 0.2299 0.1703 0.1201 0.0879
100 72.9 52.0 38.5 28.5 20.1 14.7
0.6407 0.5153 0.3768 0.2610 0.1848 0.1289 0.0941
100 80.4 58.8 40.7 28.8 20.1 14.7
0.5733 0.4264 0.2857 0.1777 0.1156 0.0696 0.0477
100 74.4 49.8 31.0 20.2 12.1 8.3
Samples were kept immersed in aqueous methylene blue solution in a dark room for 1 h before measurement. Absorbance of pure methylene blue solution was 0.8384. Absorbance relative to absorbance at 0 h illumination (%).
TiO2 during spin coating, as verified by reflectance measurements after UV illumination. Reflectances of sample disks were measured in wavelength range 600–700 nm with an integrating sphere to evaluate the adsorption of methylene blue on samples (Fig. 4). Fig. 4 shows that the reflectances of the HDPE substrate and HDPE/protective PU are about the same (∼80%), but a significant difference emerges after the samples have been immersed in methylene blue solution and UV-illuminated. Pure HDPE adsorbs some of the dye, and reflectance is decreased by about 2% indicating adsorption rather than degradation (Table 1). However when protective PU is applied to HDPE, the reflectance of the sample after immersion in methylene blue drops drastically to 19% at wavelength range of 645–655 nm. According to Table 1, the TiO2 sample with PU binder dilution 1:8 is photocatalytically more effective than a sample with dilution 1:12. Reflectance measurements (Fig. 4) show that, after UV illumination, slightly more methylene blue is adsorbed on the sample with dilution 1:8. 3.4. Effect of pH on TiO2 photocatalytic degradation of methylene blue To test whether pH has an effect on the photocatalytic efficiency of TiO2 , we adjusted the pH of the aqueous methylene blue solution (1 × 10−5 M) ∼2 units up and down (7.64 and 3.46) from the original value (5.56). The experiments were done with TiO2 samples in the absence of PU binder to exclude the effect of the binder. Absorbances of samples were recorded before UV illumination and at 1 h intervals for 6 h (Table 2).
Fig. 4. Reflectance of samples measured with an integrating sphere. Immersed means that the sample has been immersed in aqueous methylene blue and UVilluminated for 6 h.
Table 2 shows a significant effect of pH on the adsorption of methylene blue on the sample disks. The initial absorbances, after 1 h adsorption, show adsorption of the dye to be clearly favored in slightly alkaline environment (pH 7.64) as compared with acidic environment (pH 3.46) or the original solution (pH 5.56) without pH adjustment. However, the relative decomposition rate of methylene blue was best without any pH adjustment. As can be seen in Fig. 5, the reflectance value is highest for the sample at pH 5.56 and lowest for the sample in alkaline solution. The difference in reflectances at pH 5.56 and pH 7.64 is 25%. TiO2 degrades methylene blue at pH 7.64, but the process is much slower because deactivation of the catalyst occurs in alkaline environment [15]. The adsorption of cationic methylene blue is favored in alkaline solution because when pH is higher than the point of zero charge (Pzc) of the TiO2 , the TiO2 surface becomes negatively charged (Pzc for TiO2 -P25 ≈ 6.8) [20]. Possibly increased adsorption of methylene blue on the TiO2 surface, combined with adsorption onto the protective PU, leads to excessive overall adsorption and deactivation of the catalyst. According to the results in Table 2, at pH 7.64 most of the methylene blue was adsorbed on the photocatalytic coating during 1 h adsorption (the absorbance was only 0.3596). Evidently excessive adsorption of the dye blocks the active surface sites of TiO2 for oxygen and water, and no active oxygen species that could degrade methylene blue are formed. 3.5. Kinetics of methylene blue degradation We determined the reaction kinetics for the degradation of aqueous methylene blue solution (C = 1 × 10−5 M) using the UV–vis absorbance data. The results in Table 1 indicate that the
Fig. 5. Reflectance of samples used in the pH study, measured with an integrating sphere. Immersed indicates that the sample has been immersed in aqueous methylene blue for 1 h and UV-illuminated for 6 h.
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Table 2 Absorbances of aqueous methylene blue solutions at different pH values and amount of methylene blue after UV illumination, measured as UV–vis absorbance. UV-illumination time (h)
0a 1 2 3 4 5 6 a
HDPE + protective PU + TiO2 (pH 3.46)
HDPE + protective PU + TiO2 (pH 5.56)
HDPE + protective PU + TiO2 (pH 7.64)
Absorbance
Methylene blue (%)
Absorbance
Methylene blue (%)
Absorbance
Methylene blue (%)
0.7628 0.6301 0.4745 0.3719 0.2842 0.2168 0.1552
100 82.6 62.2 48.8 37.3 28.4 20.3
0.5970 0.4350 0.3103 0.2299 0.1703 0.1201 0.0879
100 72.9 52.0 38.5 28.5 20.1 14.7
0.3597 0.3119 0.2263 0.1844 0.1351 0.0852 0.0679
100 86.7 62.9 51.3 37.5 23.7 18.9
Samples were kept in a dark room, immersed in aqueous methylene blue solution, for 1 h before measurement.
Table 3 Rate constants and R-squared values for photocatalytic degradation of methylene blue at different sample compositions.
kobs (h−1 ) R2
HDPE + protective PU + TiO2 + PU binder 1:12
HDPE + protective PU + TiO2 + PU binder 1:8
HDPE + protective PU + TiO2 (pH 3.46)
HDPE + protective PU + TiO2 (pH 5.56)
HDPE + protective PU + TiO2 (pH 7.64)
0.33 1.00
0.43 1.00
0.27 1.00
0.32 1.00
0.29 0.98
Fig. 6. (a) Relative amounts of methylene blue and (b) natural logarithms of absorbance of methylene blue plotted as a function of UV-illumination time.
concentration of methylene blue does not decrease linearly as a function of UV illumination time. The kinetics of photocatalytic degradation of methylene blue on TiO2 dispersion under UV irradiation has often been modeled with a simple Langmuir-Hinshelwood equation (Eq. (1)) [15]: r=−
d[MB] k K[MB] =− 1 + K[MB] dt
(1) 4. Conclusions
where r is the rate of disappearance of the reagent, [MB] is the reagent concentration, k is the rate constant, and K is the observed equilibrium constant. The Langmuir-Hinshelwood equation can be simplified to a pseudo-first order expression (Eq. (2)) if the concentration of reagent is very low (in this study [MB] = 1 × 10−5 M). r=−
d[MB] = kobs [MB] dt
(2)
Integration of Eq. (2) leads to Eq. (3):
ln
[MB] 0
[MB]
= kobs t
binder dilution 1:8 (kobs = 0.43 h−1 vs. 0.32 h−1 ), because PU binder assists the adsorption of methylene blue to the nearby TiO2 particles. Both increase and decrease of pH from 5.56 had a negative effect on the photocatalysis, and the best results were obtained with PU binder dilution 1:8 at pH 5.56.
(3)
When we plot ln([MB]0 /[MB]) vs. t we get a straight line with slope k (Fig. 6). The values in Fig. 6 are taken from Table 1. Fig. 6 shows that the pseudo-first order assumption describes the experimental data well. Table 3 presents the rate constants and R-squared values, including values for samples at different pH. As compared with samples without binding agent there is a significant improvement in the degradation rate in the presence of PU
We have successfully applied our multilayer photocatalytic TiO2 coating for the degradation of methylene blue in aqueous solution. The waterborne heat-cured polyurethane coating on HDPE substrate protects the substrate from photocatalytic degradation. Oxygen plasma treatment of the substrate ensures adequate adhesion between substrate and protective layer so that the coating does not peel off when samples are immersed in methylene blue solution. Fixing of TiO2 on the protective PU with PU binder ensures attachment of TiO2 particles during adsorption and photocatalysis steps. With the best TiO2 samples, which contained PU binder at dilution 1:8, the degradation of the dye was over 90%; with the more diluted binder (1:12) it was 85%. Reflectance measurements of sample disks after the UV illumination showed some residual adsorption of methylene blue in every sample indicating that degradation of the dye was not complete. Adjustment of pH from the original value slowed the degradation: methylene blue was adsorbed on the protective polyurethane, especially in slightly
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alkaline solution (pH 7.64), causing intense color and a major decrease in reflectance of the sample. Best decomposition results were obtained at pH 5.56, where degradation of the dye followed pseudo-first order reaction kinetics with a rate constant of 0.43 h−1 . References [1] A. Fujishima, X. Zhang, D.A. Tryk, Heterogeneous photocatalysis: from water photolysis to applications in environmental cleanup, Int. J. Hydrogen Energy 32 (2007) 2664–2672. [2] Y. Li, W. Xie, X. Hu, G. Shen, X. Zhou, Y. Xiang, X. Zhao, P. Fang, Comparison of dye photodegradation and its coupling with light-to-electricity conversion over TiO2 and ZnO, Langmuir 26 (2009) 591–597. [3] A.P. Davis, C.P. Huang, The photocatalytic oxidation of sulfur-containing organic compounds using cadmium sulfide and the effect on CdS photocorrosion, Water Res. 25 (1991) 1273–1278. [4] K. Rajeshwar, M.E. Osugi, W. Chanmanee, C.R. Chenthamarakshan, M.V.B. Zanoni, P. Kajitvichyanukul, R. Krishnan-Ayer, Heterogeneous photocatalytic treatment of organic dyes in air and aqueous media, J. Photochem. Photobiol. C-Photochem. Rev. 9 (2008) 171–192. [5] J.A. Byrne, B.R. Eggins, N.M.D. Brown, B. McKinney, M. Rouse, Immobilisation of TiO2 powder for the treatment of polluted water, Appl. Catal. B-Environ. 17 (1998) 25–36. [6] U.I. Gaya, A.H. Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems, J. Photochem. Photobiol. C-Photochem. Rev. 9 (2008) 1–12. [7] M. Stylidi, D.I. Kondarides, X.E. Verykios, Pathways of solar light-induced photocatalytic degradation of azo dyes in aqueous TiO2 suspensions, Appl. Catal. B-Environ. 40 (2004) 271–286. [8] I.K. Konstantinou, T.A. Albanis, TiO2 -assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B-Environ. 49 (2004) 1–14. [9] N.S. Allen, M. Edge, J. Verran, J. Stratton, J. Maltby, C. Bygott, Photocatalytic titania based surfaces: environmental benefits, Polym. Degrad. Stab. 93 (2008) 1632–1646.
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