technologies Article
Preparation of TiO2-Poly(3-Chloro-2-Hydroxypropyl Methacrylate) Nanocomposite for Selective Adsorption and Degradation of Dyes M. Shamim Hossan and Bungo Ochiai * Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan;
[email protected] * Correspondence:
[email protected]; Tel.: +81-238-26-3092 Received: 19 July 2018; Accepted: 28 September 2018; Published: 2 October 2018
Abstract: We report a new nanocomposite TiO2 -poly(3-chloro-2-hydroxypropyl methacrylate) (TiO2 -PCHPMA) for selective adsorption/degradation of cationic dyes and degradation of anionic dyes. TiO2 -PCHPMA was prepared by free radical polymerization of CHPMA in the presence of TiO2 modified with 3-(trimethoxysilyl)propyl methacrylate. TiO2 -PCHPMA adsorbed cationic methylene blue (MB), but did not adsorb anionic methyl orange (MO) in their aqueous solutions. The adsorption efficiency for MB reached 99% within 5 min at 28 ◦ C, and adsorbed MB could be recycled in 96% efficiency. The adsorption accelerated degradation of MB under UV irradiation. The degradation of anionic MO proceeded completely with TiO2 -PCHPMA under UV irradiation, and the efficiency was not affected by the PCHPMA layer. TiO2 -PCHPMA is potentially applicable as a material capable of selective removal and recovery of cationic dyes, and degradation of other dyes from industrial effluents. Keywords: dye; selectivity; degradation; adsorption; water treatment
1. Introduction Dyes are extensively used in various industrial processes, and their annual market is more than 7 × 105 t/year [1]. More than 10% of the amounts of dyes are discharged into the environment during industrial process [2]. Specifically, synthetic dyes composed of structures with stable aromatic backbones are not biodegradable, and their polluting nature is of considerable environmental concern [3]. Materials for degradation and removal of such dyes are in high demand. TiO2 has been widely used as an efficient photocatalyst for degradation of various pollutants due to its high activity associated with the low cost, low toxicity, and high photo- and chemical stability [4–53]. The structure of TiO2 can be tailored by surface modification or doping for enhancement of the performance of TiO2 on photocatalytic activity, availability of pollutant, processability etc. [49,50]. For example, TiO2 has been hybridized or modified with metals [5–7], organic substances including polymers [23–48], and carbon materials [8–10] including carbon nanotubes [11–13], activated carbon [14–16], graphene [17–19], and graphene oxide [20–22]. Such modifications sometimes deteriorate the photocatalytic performance due to prevention of charge transfer. Accordingly, appropriate designs are necessary to develop TiO2 materials with additional properties. Selectivity is a major drawback of TiO2, but is very important in targeting pollutants in the treatment of waste water, which typically contains high concentration of non-hazardous substances. For selective photocatalytic degradation, the surface structure is the important factor in controlling the selectivity of adsorption. Ionic surfaces on TiO2 enable selective degradation of cationic and anionic
Technologies 2018, 6, 92; doi:10.3390/technologies6040092
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pollutants by interactions with organic pollutants with opposite charges [42,43,50–52]. For example, TiO2 with positive and negative surface charges fabricated using Na2 CO3 and NH4 OH is highly active for selective degradation of cationic methylene blue (MB) and anionic methyl orange (MO), respectively, from their aqueous mixture [52]. Molecular imprinting was also employed for selective degradation. TiO2 coated with thin layers of polymers imprinting 2-nitrophenol and 4-nitrophenol was prepared by polymerization of o-phenylenediamine in the presence of TiO2 nanoparticles and the target molecules. The resulting imprinted TiO2 composites photodegraded the targets predominantly over other coexisting pollutants [44]. Control of polarity was also used for selective adsorption [45]. Hydrophobic TiO2 can be prepared by modifying the surface of TiO2 nanoparticles (P25) with long alkyl chains such as n-octyl groups (MS-P25oct). The hydrophobic character of MS-P25oct enabled adsorption of hydrophobic substances, which led to selective degradation of 4-nonylphenol (NLP). Contrary to this TiO2 , a core-shell nanocomposite consisting of P25 and mesoporous silica (CMS-P25) with a more hydrophilic surface was developed for selective degradation of NLP and phenol (PL) from an aqueous mixture of NLP, PL, and nonane, while the mechanism of the selection is unclear [46]. Such selectivity depends on the characters of the functional groups and the spaces to capture target pollutants, and the modification to improve affinity basically assisting selective degradation. Challenges for such modification involve higher selectivity, higher stability, and higher capacity. In this study, we designed TiO2 -poly (3-chloro-2-hydroxypropyl methacrylate) (TiO2 -PCHPMA) as a photocatalyst for recycling and degradation of dyes with selectivity depending on the ionic character of the dyes. PCHPMA has been employed as a scaffold of modification with various functionalities for selective interactions in aqueous media by the electrostatic interaction of the chloroalkyl moieties and the moderate hydrophilicity of the hydroxy groups [54]. In the course of our study, TiO2 -PCHPMA was synthesized and examined for removal of organophosphorus pesticides for comparison with TiO2 modified with polystyrene [55]. However, this TiO2 -PCHPMA showed weaker interaction with the examined pesticides than the polystyrene analogue. The pesticides consist of hydrophobic and non-ionic structures, and the similarity with polystyrene resulted in the higher removal efficiency of TiO2 modified with polystyrene. The utility of PCHPMA depends on the chloroalkyl moieties for selective interactions, namely the electrostatic interactions based on the Lewis-basic chloride group. We targeted electrostatic selection of ionic dyes for separation and degradation. The adsorption and degradation experiments of dyes using TiO2 -PCHPMA were conducted, and revealed that TiO2 -PCHPMA is an efficient material for adsorption of MB and photocatalytic degradation of MO. As a result of selective adsorption, MB could be either recycled or degraded. This clear difference originated from the electrostatic adsorption of MB on the layer of PCHPMA and the moderate hydrophilicity of PCHPMA allowing diffusion of MO around the TiO2 core. 2. E.xperimental Section 2.1. Materials Tetra-isopropyl orthotitanate (TIPOT), 3-chloro-2-hydroxypropyl methacrylate (CHPMA), and 3-(trimethoxysilyl)propyl methacrylate (MPS) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Dibenzoyl peroxide (BPO), methylene blue (MB), methanol (MeOH), ethanol (EtOH), 1,4-dioxane, acetic acid, and aqueous ammonia solution (28 wt%) were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Methyl orange (MO) was purchased from Wako Pure Chemical Industry, Ltd. (Tokyo, Japan). Water was purified with MINIPURI TW-3000RU (Nomura Micro Science Co., Ltd., Kanagawa, Japan). All materials used were analytical grade reagents, and used without further purification.
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2.2. Measurements and Instrument Scanning electron microscopy (SEM) measurements were conducted on a Hitachi SU8000 microscope (Hitachi, Ltd., Tokyo, Japan). Energy-dispersive X-ray (EDX) analysis was carried out on a JEOL JSM-6510A scanning electron microscope (JEOL Ltd., Tokyo, Japan) equipped with a JEOL JED 2300 EDX spectrometer. The samples for EDX were molded as disks and placed on a carbon tape. Transmission electron microscopy (TEM) measurements were carried out on a JEOL TEM-2100F field emission electron microscope. Hydrodynamic size and zeta potential were measured by dynamic light scattering (DLS) analysis conducted on a Malvern Zetasizer Nano ZS instrument (Malvern Instruments, Ltd., Malvern, UK). Fourier transform infrared (FTIR) spectra were measured on a JASCO FT/IR-460 spectrometer (JASCO Co., Tokyo, Japan). Thermogravimetric analysis (TGA) was carried out on a Seiko (Tokyo, Japan) TG/DTA 6200 (EXSTER6000) instrument at a heating rate of 10 ◦ C/min under N2 atmosphere. A UV visible light (input current 1.35 A, 121 W, frequency 50 Hz) model HLR100T-2 (SEN lights corp., Osaka, Japan) was utilized as the illumination light source. Optical absorbance was measured on a HACH DR 5000 UV visible spectrometer (HACH Com., Loveland, Colorado, USA). Centrifugation was carried out on a Kubota KN-70 centrifugation machine (Kubota Corp., Osaka, Japan). Cyclic voltammetry (CV) measurements were carried out on an ECstat-301 potentio/galvnostat analyzer (EC Frontier, Kyoto, Japan). The working electrodes were prepared by drying a Pt electrode dipped into 0.1 g/L THF solutions of the samples. Ag and Pt were employed as the reference and counter electrodes, respectively. Supporting electrolyte was 0.1 M NaCl solution. 2.3. Preparation of TiO2 Nanoparticles from TIPOT TIPOT (7.68 g, 2.70 × 10−2 mol) was dispersed in 20 mL of EtOH with magnetic stirring for 20 min. Then, a mixture of acetic acid (1.2 mL) and deionized water (250 mL) was added to the dispersion under vigorous magnetic stirring, and the reaction was continued for 20 h at 60 ◦ C. After the reaction, the product was collected by centrifugation and washed with EtOH. This centrifugation–dispersion washing process was repeated 5 times. TiO2 nanoparticles were obtained by drying at 60 ◦ C for 24 h under vacuum (2.12 g, 2.65 × 10−2 mol, 98.2% yield). 2.4. Preparation of TiO2 -MPS TiO2 (2.02 g) was dispersed in EtOH (50 mL) by ultrasonication for 30 min. Then, deionized water (1.25 g), NH3 solution (0.62 g), and MPS (1.11 g, 2.32 × 10−4 mol) were added to the dispersion. The mixture was magnetically stirred for 24 h at 60 ◦ C under a nitrogen atmosphere. The product was collected after 5 times of centrifugation–dispersion with EtOH. TiO2 -MPS was obtained by drying at 60 ◦ C for 24 h under vacuum (1.82 g, 58.1% yield). 2.5. Preparation of TiO2 -PCHPMA TiO2 -MPS (1.53 g) was dispersed in 1,4-dioxane (20 mL) in a round-bottomed flask immersed in a thermostat oil bath maintained at 70◦ C. CHPMA (3.10 g, 3.47 × 10−4 mol) and BPO (15 mg, 1.2 × 10−6 mol) were added to the flask. The free radical polymerization was conducted at 70 ◦ C for 24 h under a nitrogen atmosphere. The product was washed 5 times with centrifugation–dispersion in EtOH. TiO2 -PCHPMA was obtained by drying at 60 ◦ C for 24 h under vacuum as a yellowish solid (1.31 g, 28.3% yield). 2.6. Adsorption Experiment The solutions of MB and MO (10 mg/L) were prepared by dissolving in purified water. Experiments on the adsorption of these dyes were carried out using 8 mg of adsorbents TiO2 -PCHPMA or PCHPMA for 10 mL of the MB and MO solutions at 28 ◦ C in an AS ONE I-cover incubator installed with a magnetic stirrer. The mixture was centrifuged at 5000 rpm for 15 min after a desired time interval. The dyes adsorbed were eluted with MeOH 3 times by the centrifugation–dispersion process.
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The elutant was concentrated by evaporating MeOH with a BÜCHI R-205 Rotavapor (Nihon Büchi Co., Tokyo, Japan), and the elutant with adjusted volume was subjected to UV-visible measurement. The final concentrations of MB and MO in the supernatants after the adsorption were also measured by UV-visible measurements. The concentrations were calculated based on the calibration curves of Technologies 4 of 16 MB and MO. 2018, 6, x The adsorption ratio (rad in %) and the amount of adsorbed dyes (qt in mg/g) on TiO2 -PCHPMA adsorption were also measured by UV-visible measurements. The concentrations were calculated in a certain time interval were calculated from the following Equations (1) and (2), respectively; based on the calibration curves of MB and MO. The adsorption ratio ( in %) and the amount c0 − coft adsorbed dyes ( in mg/g) on TiO2-PCHPMA = the × 100Equations 1 and 2, respectively; in a certain time interval were calculatedrad from following
c0
− = c0 − ct× 100
qt =
m
×v
(1)
(1) (2)
− where c0 (mg/L) is the initial concentration; ct (mg/L) is the residual concentration of cationic dyes at = × (2) time t in liquid phase after adsorption; v (L) is the volume of solution of dyes; and m (g) is the weight of the adsorbent. where (mg/L) is the initial concentration; (mg/L) is the residual concentration of cationic dyes Adsorption experiments under varied pH(L) conditions were carried out by adjusting(g) the pH with at time in liquid phase after adsorption; is the volume of solution of dyes; and is the 0.1 Mweight HCl and 0.1adsorbent. M NaOH aqueous solutions. of the Adsorption experiments under varied pH conditions were carried out by adjusting the pH with
2.7. Degradation Experiment 0.1 M HCl and 0.1 M NaOH aqueous solutions.
The aqueous solutions of MO and MB (10 mg/L) were mixed with TiO2 or TiO2 -PCHPMA (0.40 mg/mL) in Pyrex glass beakers with magnetic stirring for 20 min in dark conditions. 2 or aqueous solutions of MO and MB were mixed with TiO2-PCHPMA (0.40 Then, theThe suspensions were UV-irradiated for(10 themg/L) desired time. After theTiO UV irradiation, the suspension in Pyrex glass beakers withrpm. magnetic for 20 was min in dark conditions. the was was mg/mL) centrifuged for 15 min at 5000 The stirring supernatant decanted, and theThen, volume suspensions were UV-irradiated for the desired time. After the UV irradiation, the suspension was adjusted with water for UV-visible measurements. The precipitate was washed 3 times with the centrifuged for 15 min at 5000 rpm. The supernatant was decanted, and the volume was adjusted centrifugation–dispersion process with hot water (45 ◦ C) for MO, and MeOH for MB to elute the dyes. with water for UV-visible measurements. The precipitate was washed 3 times with the The elutants were concentrated with a BÜCHI R-205 Rotavapor, and the elutants with adjusted volume centrifugation–dispersion process with hot water (45 °C) for MO, and MeOH for MB to elute the dyes. wereThe subjected UV-visible measurement. elutantstowere concentrated with a BÜCHI R-205 Rotavapor, and the elutants with adjusted The degradation ratio of MO and MB in the presence of TiO2 -PCHPMA in a certain time (rdeg in volume were subjected to UV-visible measurement. %) was calculated from the following Equation The degradation ratio of MO and MB in the(3); presence of TiO2-PCHPMA in a certain 2.7. Degradation Experiment
time (
in %) was calculated from the following Equation 3;
rdeg =
=
a0 − a t − × 100 a0 × 100
(3)
(3)
where a0 is theisabsorbance of of initial and atisisthe theabsorbance absorbance of dye inliquid the liquid where the absorbance initialdye dyeconcentration concentration and of dye in the phase after degradation at time t. phase after degradation at time t.
Scheme for preparation of TiOof methacrylate)methacrylate) (TiO22-poly(3-chloro-2-hydroxypropyl FigureFigure 1. 1.Scheme forthe the preparation TiO2 -poly(3-chloro-2-hydroxypropyl PCHPMA). (TiO2 -PCHPMA).
3. Results and Discussion 3.1. Characterization TiO2-PCHPMA was prepared by sequential reactions of condensation of TIPOT, coating with MPS, and radical polymerization of CHPMA in the presence of TiO2-MPS (Figure 1). The morphology of TiO2, TiO2-MPS, and TiO2-PCHPMA was observed by SEM (Figure 2a–c). The particles were agglomerated and non-uniform in shape. The ranges of the sizes of the primary particles of TiO2,
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3. Results and Discussion 3.1. Characterization TiO2-PCHPMA was prepared by sequential reactions of condensation of TIPOT, coating with MPS, and radical polymerization of CHPMA in the presence of TiO2-MPS (Figure 1). The morphology of TiO2, TiO2-MPS, and TiO2-PCHPMA was observed by SEM (Figure 2a–c). The particles were agglomerated and non-uniform in shape. The ranges of the sizes of the primary particles of TiO2, TiO2-MPS, and TiO2-PCHPMA were approximately 15–45 nm, 20–60 nm, and 22–90 nm, respectively. The gradual increase in size is attributed to the formation of the MPS and PCHPMA layers on the TiO2 surface. The morphology was also changed after the surface modifications of TiO2 with MPS and PCHPMA. Technologies 2018, 6, x 5 of 16 The particles of TiO 2 were simply aggregated and showed their bare surface. By contrast, TiO2 -MPS TiO2-MPS, andfused, TiO2-PCHPMA were approximately 15–45 nm, 20–60 nm, respectively. and TiO2-PCHPMA were and the interfaces unclear duenm, toand the22–90 organic components serving as The gradual increase in size is attributed to the formation of the MPS and PCHPMA layers on the glue. This tendency was more significant in the image of TiO -PCHPMA having a higher content of 2 TiO2 surface. The morphology was also changed after the surface modifications of TiO2 with MPS and PCHPMA. The particles of of TiO simply aggregated and showed their bare surface. Byobserved by TEM organic component. The morphology TiO2 2were , TiO -MPS, and TiO -PCHPMA was also 2 2 contrast, TiO2-MPS and TiO2-PCHPMA were fused, and the interfaces unclear due to the organic (Figure 2d–f). The TEM image of TiO -PCHPMA shows the size of the primary particles in the range of components serving as glue.2This tendency was more significant in the image of TiO2-PCHPMA a higher content of organic The SEM morphology of TiO TiO2- grains with sizes 2, TiO 2-MPS, and 1–8 nm, which ishaving significantly smaller thancomponent. that in the image. This image shows PCHPMA was also observed by TEM (Figure 2d–f). The TEM image of TiO2-PCHPMA shows the identical to those size observed in the SEM image consisting of agglomerated primary particles. This difference of the primary particles in the range of 1–8 nm, which is significantly smaller than that in the image. This image shows with sizes to those observed in the SEM image of TiO2 -MPS and indicates that theSEM grains were grown bygrains the fusion ofidentical nanoparticles. The TEM images consisting of agglomerated primary particles. This difference indicates that the grains were grown TiO2-PCHPMA also show grains consisting of aggregated nanoparticles, while the grains are not fused. by the fusion of nanoparticles. The TEM images of TiO2-MPS and TiO2-PCHPMA also show grains The organic layers probably reduced the interparticle interactions, and dispersibility. consisting of aggregated nanoparticles, while the grains are not fused. The improved organic layers the probably reduced the interparticle interactions, and improved the dispersibility.
Figure 2. Scanning electron microscopy (SEM) images of (a) TiO2 , (b) TiO2 -MPS, and (c) TiO2 -PCHPMA (scale bar = 500 nm); and transmission electron microscopy (TEM) images of (d) TiO2 , (e) TiO2 -MPS, and (f) TiO2 -PCHPMA (scale bar = 50 nm).
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Figure 2. Scanning electron microscopy (SEM) images of (a) TiO2, (b) TiO2-MPS, and (c) TiO2PCHPMA (scale Technologies 2018, 6, 92 bar = 500 nm); and transmission electron microscopy (TEM) images of (d) TiO2, (e)6 of 15 TiO2-MPS, and (f) TiO2-PCHPMA (scale bar = 50 nm).
The formation of TiO22-PCHPMA -PCHPMA was was confirmed confirmed by by EDX, FTIR, and TGA analyses. Figure Figure 3a indicates TiO22, TiO22-MPS, and TiO22-PCHPMA indicates the EDX spectra of TiO -PCHPMA for for evaluation evaluation of the elemental compositions. The The EDX EDX spectrum spectrum of of TiO TiO22 exhibited exhibited major major peaks peaks originating originating from from Ti Ti and and O. A minor peak of C is attributable to the organic residue of TIPOT TIPOT and the the carbon carbon tape tape substrate. substrate. The EDX spectrum spectrum of TiO22-MPS -MPS shows shows aa new new peak of Si, indicating the successful modification of TiO22 with MPS. The EDX spectrum of TiO22-PCHPMA shows another new peak of Cl. The The relative relative intensities of the peaks of C and O compared TiO2,2, supporting compared to the peak of Ti are larger than those of TiO supporting the the increase increase of the organic component originating from the PCHPMA layer. layer. 2-PCHPMA The structure waswas evaluated by FTIR spectroscopy (Figure(Figure 3b). Broad structure ofofTiO TiO evaluated by FTIR spectroscopy 3b). 2 -PCHPMA absorption peaks originating from thefrom Ti–O the bond were observed 620around cm−1 in 620 the FTIR Broad absorption peaks originating Ti–O bond were around observed cm−1spectra in the of unmodified TiO and spectrum[56]. of theThe precursor TiOof 2, TiO2-MPS, 2-MPS FTIR spectra of unmodified TiO TiO22-PCHPMA -MPS, and[56]. TiO2The -PCHPMA spectrum the 2 , TiO −1, showed characteristic peaks originating from MPSoriginating at 2986, 1717, 1192,1717, and 1680, 1042 cm precursor TiO2 -MPS showed characteristic peaks from1680, MPS1461, at 2986, 1461, −1 , assigned assigned thecm C–H, C=O, andto C=C stretching, C–HC=C bending, C–O–C and Ti–O–SiC–O–C stretching 1192, andto 1042 the C–H, C=O, and stretching, C–H bending, and vibrations, respectively [47,57–58]. The FTIR spectrum of FTIR TiO2-PCHPMA a weak absorption Ti–O–Si stretching vibrations, respectively [47,57,58]. The spectrum ofexhibits TiO2 -PCHPMA exhibits a −1 assignable peak 750 peak cm−1 around assignable C–Cl bondstoas observed in the of the PCHPMA. The weak around absorption 750to cmthe the C–Cl bonds as spectrum observed in spectrum of − 1 −1 absorption peaks at 1740 and 1196atcm assignable theassignable characteristic stretching vibration of the PCHPMA. The absorption peaks 1740are and 1196 cm toare to the characteristic stretching C=O and of C–O in the group inester the PCHPMA structure. Asstructure. a result ofAsthe grafting of vibration thebonds C=O and C–Oester bonds in the group in the PCHPMA a result of the PCHPMA, the FTIR spectrum TiO2-PCHPMA shows an absorption of the C–Hofstretching vibration grafting of PCHPMA, the FTIRof spectrum of TiO2 -PCHPMA shows an absorption the C–H stretching −1 with −1 −1 of at 2987 cmat−1 2987 withcm a higher relative intensity the absorption peaks at 1170 and 620 cm the vibration a higher relativeand intensity and the absorption peaks at 1170 and 620 cmTi– O–Si Ti–Oand bonds with weaker than the spectrum of TiO2-MPS. of theand Ti–O–Si Ti–O bonds withrelative weakerintensities relative intensities than the spectrum of TiO2 -MPS.
Figure 3. (b)(b) Fourier transform infrared (FTIR) spectra, and Figure 3. (a) (a) Energy-dispersive Energy-dispersiveX-ray X-ray(EDX) (EDX)spectra, spectra, Fourier transform infrared (FTIR) spectra, ◦ C/min, N ) of TiO , TiO -MPS, and TiO -PCHPMA. (c) thermogravimetric analysis (TGA) curves (10 2 2 2, TiO2-MPS, 2 and TiO2and (c) thermogravimetric analysis (TGA) curves (10 °C/min, N22) of TiO PCHPMA.
The compositions of the organic structures were estimated by TGA (Figure 3c). The TGA curveThe of compositions TiO2 -PCHPMA exhibited weight loss as observed for various TiOTGA 2 -polymer of the organicthree-stage structures were estimated by TGA (Figure 3c). The curve ◦ composites [11,20,48,57,58]. The first approximately 10% weight loss below 200 C originates from the of TiO2-PCHPMA exhibited three-stage weight loss as observed for various TiO2-polymer composites evaporation of physiosorbed water and alcohols on the nanocomposite surface. The second weight loss [11,20,48,57,58]. The first approximately 10% weight loss below 200 °C originates from the ◦ C correlated mainly with the degradation of the organic took place ofof approximately at 200–450 evaporation physiosorbed10% water and alcohols on the nanocomposite surface. The second weight ◦ C is attributed to the loss of water components [59–61]. The third weight loss occurring above 450mainly loss took place of approximately 10% at 200–450 °C correlated with the degradation of the produced by the condensation ofthird neighboring terminal hydroxyl initial 2 [62,63]. organic components [59–61]. The weight loss occurring abovegroups 450 °Cof is TiO attributed to The the loss of weight losses of the modified TiO are slower than that of bare TiO probably due to the modification 2 2 water produced by the condensation of neighboring terminal hydroxyl groups of TiO2 [62,63]. The of the organic moieties being more hydrophobic than TiO2 , but the weight loss became faster after 300 ◦ C due to the degradation of the introduced organic moieties. The TGA profile of TiO2 -PCHPMA
initial weight losses of the modified TiO2 are slower than that of bare TiO2 probably due to the modification of the organic moieties being more hydrophobic than TiO2, but the weight loss became faster after 300 °C due to the degradation of the introduced organic moieties. The TGA profile of TiO2PCHPMA2018, shows Technologies 6, 92 a more significant effect of the organic component on the slower weight loss at 7 ofthe 15 earlier stage and the faster weight loss at the later stage. The final weight loss of TiO2-PCHPMA reached 36% and the 10% difference from that of TiO2 is comparable to the weight fraction of the shows more significant effect of the organic component on the slower weight loss at the earlier stage organica component. and the faster at FTIR, the later stage. final weight loss ofthe TiO reached 36% and 2The SEM,weight TEM, loss EDX, and TGAThe analyses confirmed successful fabrication of TiO 2 -PCHPMA the 10% difference from that of TiO2 is comparable to the weight fraction of the organic component. PCHPMA. The SEM, TEM, EDX, FTIR, and TGA analyses confirmed the successful fabrication of TiO -PCHPMA. 3.2.2Adsorption of Dyes The ability of TiO2-PCHPMA for adsorption and degradation of dyes was evaluated using 3.2. Adsorption of Dyes cationic MB and anionic MO. First, adsorption experiments were conducted for MB and MO at 28 C, Theinitial abilitypH of values TiO2 -PCHPMA degradation of dyes wasadsorbed evaluated and the of 5.5 and for 6.5,adsorption respectivelyand (Figure 4). MB was rapidly on using TiO2cationic MB and anionic MO. First, adsorption experiments were conducted for MB and MO at2 PCHPMA, while MO was negligibly adsorbed. This tendency is identical to those of both bare TiO 28 C, and the initial pH values of 5.5 and 6.5, respectively (Figure 4). MB was rapidly adsorbed [42] and PCHPMA. PCHPMA adsorbs MB (3 mg/g-PCHPMA) more efficiently than MO (0.1 12) efficiencies was lower owing to the interactionefficiency between bases MB. basic Additionally, the adsorption probably owing to the electrostatic interaction between bases and MB. Additionally, the adsorption of anionic MO onto TiO2 -PCHPMA were constantly low above pH 4, indicating the selectivity of the 2-PCHPMA were constantly low above pH 4, indicating the efficiencies The of anionic MO efficiency onto TiOat absorption. adsorption pH 3 was higher, probably because MO having a pKa value of selectivity of the under absorption. The adsorption efficiency at pH 3 was higher, probably MO 3.5 is protonated this acidic condition. The decrease at pH 2 is attributable to the because competition having a pK a value of 3.5 is protonated under this acidic condition. The decrease at pH 2 is attributable with other protonated species in the same manner with MB. to theThe competition other protonated species in the same manner with MB. adsorptionwith ability of TiO 2 -PCHPMA was compared with those of other composites of TiO2 in The adsorption of TiO 2-PCHPMA was compared with those of other composites of TiO2 literatures reporting ability adsorption and degradation of MB and MO (Table 2). The adsorption ability of in literatures reporting adsorption and degradation of MBThe andnegligible MO (Table 2). The adsorption ability TiO2 -PCHPMA toward MB is higher than reported ones. adsorption of MO indicates of TiO 2-PCHPMA toward MB is higher than reported ones. The negligible adsorption of MO indicates the high selectivity of TiO2 -PCHPMA. the high selectivity of TiO2-PCHPMA.
Figure 5. Effect of pH on the adsorption of methylene blue (MB) and methyl orange (MO) by Figure 5. Effect of pH on the adsorption of methylene blue (MB) and methyl orange (MO) by TiO2TiO2 -PCHPMA at 28 ◦ C (conditions: dyes = 10 mg/L, TiO2 -PCHPMA = 0.8 mg/mL, 20 min). PCHPMA at 28 °C (conditions: dyes = 10 mg/L, TiO2-PCHPMA = 0.8 mg/mL, 20 min). Table 2. Adsorption/degradation efficiencies of methylene blue (MB) and methyl orange (MO) by Table 2. Adsorption/degradation efficiencies of methylene blue (MB) and methyl orange (MO) by TiO hybrid composites. 2 based TiO2 based hybrid composites.
Catalysts
Dye
Catalysts
Dye
TiO2 /poly[acryla mide-co-(acrylic acid)]
MB
TiO2 treated with Na2 CO3
MO
Adsorption
Degradation
Adsorption Conditions: concentration of dye Efficiency Conditions: (mg/L), composite concentration(%)of (g/L) and adsorption time (min)
Conditions: Efficiency UV irradiation (%) Conditions: time (min)
dye (mg/L), composite (g/L)87and 5, 0.2, and 15 adsorption time (min) 23 10, 1, and 30
Degradation Effici UV ency irradiation91 (%) 40 time (min) 120
> 99
References
Effici ency (%)
Refer ences [40]
[52]
TiO2/poly[acryla TiO2 treated with MB MB 10, 1, and 5, 30 0.2, and 15 96 mide-co-(acrylic 87 60 40 > 99 91 [40] NH4 OH acid)] MB 10, 0.8, and 20 99 180 94 TiO -PCHPMA This work TiO22 treated with MO 10, 0.8, and 20 1< 180 >99 MO 10, 1, and 30 23 120 > 99 Na2CO3 [52] 2 TiO treated with MB 96 3.4. Desorption of MB 10, 1, and 30 60 > 99 NH4OH MB adsorbed on TiO2 -PCHPMA was washed out with MeOH with 3 times of the MB 10, 0.8, and 20 99 180 94 This TiO2-PCHPMA centrifugation–dispersion process (Figure 6). MB was quantitively adsorbed on TiO2 -PCHPMA work MO 10, 0.8, and 20 1< 180 >99 as confirmed by the absence of the absorption of MB in the supernatant. Adsorbed MB was recovered by washing in a desorption efficiency of 96%, indicating that MB was degraded negligibly during the 3.4. Desorption of MB adsorption–desorption process. The slight loss of MB probably originated from the trace degradation, MB adsorbed on TiO2-PCHPMA was washed out with MeOH 3 times of the loss of very tiny nanoparticles adsorbing MB not precipitated by thewith centrifugation, andcentrifugation– remaining MB dispersion process (Figure 6). MB was quantitively adsorbed on TiO -PCHPMA as confirmed by the 2 on TiO2 -PCHPMA by the strong adsorption. absence of the absorption of MB in the supernatant. Adsorbed MB was recovered by washing in a
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desorption desorptionefficiency efficiencyofof96%, 96%,indicating indicatingthat thatMB MBwas wasdegraded degradednegligibly negligiblyduring duringthe theadsorption– adsorption– desorption process. The slight loss of MB probably originated from the trace degradation, desorption process. The slight loss of MB probably originated from the trace degradation,loss lossofofvery very tiny tinynanoparticles nanoparticlesadsorbing adsorbingMB MBnot notprecipitated precipitatedby bythe thecentrifugation, centrifugation,and andremaining remainingMB MBon onTiO TiO2-2PCHPMA by Technologies 2018, 6,the 92strong 9 of 15 PCHPMA bythe strongadsorption. adsorption.
UV visible absorption spectra aqueous solution supernatant after adsorption Figure Figure 6.6.6. UV visible absorption spectra ofofinitial aqueous solution of of MB, supernatant after adsorption UV visible absorption spectra ofinitial initial aqueous solution ofMB, MB, supernatant after adsorptionofof Figure 2-PCHPMA, 2-PCHPMA MB on TiO and the elutant washed out from the TiO adsorbing the MB (concentrations: of MB on TiO -PCHPMA, and the elutant washed out from the TiO -PCHPMA adsorbing the MB 2 2 adsorbing the MB (concentrations: MB on TiO2-PCHPMA, and the elutant washed out from the TiO2-PCHPMA 2-PCHPMA TiO = 1.5 mg/mL and MB = 10 mg/L, adsorption for 20 min at 28 °C, desorption by washing and MB =for 10 20 mg/L, adsorption for 20 min 28 ◦ C, with 2 -PCHPMA TiO2(concentrations: -PCHPMA = 1.5 TiO mg/mL and MB== 1.5 10 mg/mL mg/L, adsorption min at 28 °C, desorption byatwashing with MeOH). desorption by washing with MeOH). MeOH).
3.5. Degradation of Dyes 3.5. 3.5.Degradation DegradationofofDyes Dyes The photodegradation of MO was was conducted under under UV irradiation irradiation in the presence presence (Figure 7) 7) The Thephotodegradation photodegradationofofMO MO wasconducted conducted underUV UV irradiationininthe the presence(Figure (Figure 7) and absence (Figure 8a) of TiO -PCHPMA. The intensity of the optical absorption of MO in the 22-PCHPMA. The intensity of the optical absorption of MO in the and andabsence absence(Figure (Figure8a) 8a)ofofTiO TiO 2-PCHPMA. The intensity of the optical absorption of MO in the solution containing TiO -PCHPMA decreased decreased with time, time, and the absorption became unobservable 22-PCHPMA solution containing TiO solution containing TiO2-PCHPMA decreasedwith with time,and andthe theabsorption absorptionbecame becameunobservable unobservable after 180 min. MO was not adsorbed on TiO -PCHPMA as previously mentioned, and result 22-PCHPMA as previously mentioned, and this after 180 mins. MO was not adsorbed on TiO after 180 mins. MO was not adsorbed on TiO2-PCHPMA as previously mentioned, andthis thisresult result clearly indicates that that MO was was completely degraded degraded in 180 mins. min. By contrast, MO was not degraded clearly clearlyindicates indicates thatMO MO wascompletely completely degradedinin180 180 mins.By Bycontrast, contrast,MO MOwas wasnot notdegraded degraded under UV irradiation in the absence of TiO2 -PCHPMA, indicating that the degradation was completely under indicating that the degradation was 2-PCHPMA, indicating that the degradation was under UV UV irradiation irradiation inin the the absence absence of of TiO TiO2-PCHPMA, mediated by TiO -PCHPMA. The degradation experiment of MO was also conducted inconducted the presence 2 completely mediated by TiO 2-PCHPMA. The degradation experiment of MO was also completely mediated by TiO2-PCHPMA. The degradation experiment of MO was also conductedinin of synthesizedofTiO irradiationUV for comparison.for The degradationThe efficiency for MO by TiO2 2 under UV the TiO thepresence presence ofsynthesized synthesized TiO2 2under under UVirradiation irradiation forcomparison. comparison. Thedegradation degradationefficiency efficiency was 60% at 1 h, while that by TiO -PCHPMA was 65%. The identical efficiencies indicate that the 2 for was 65%. The identical efficiencies forMO MOby byTiO TiO2 2was was60% 60%atat11h,h,while whilethat thatby byTiO TiO2-PCHPMA 2-PCHPMA was 65%. The identical efficiencies PCHPMA chains in TiO -PCHPMA did not affect the diffusion and the degradation of MO due to 2 chains in TiO2-PCHPMA did not affect the diffusion and the degradation indicate indicatethat thatthe thePCHPMA PCHPMA chains in TiO2-PCHPMA did not affect the diffusion and the degradation negligible interaction. interaction. ofofMO MOdue duetotonegligible negligible interaction.
Figure 7.7.UV-visible UV-visible absorption spectra of aqueous solution of MO during degradation in the absorption spectra ofofaqueous solution ofofMO during degradation ininthe Figure7. UV-visible absorption spectra aqueous solution MO during degradation thepresence presence presence of TiO -PCHPMA under UV light irradiation with inset photo images of the ofofTiO under UV light irradiation with inset photo images of the initial solution of 2-PCHPMA 2 TiO2-PCHPMA under UV light irradiation with inset photo images of the initial solution initial ofMO MO solution of MO and after the supernatant after degradation of MO (concentrations: = 10 mg/L, 2MO and the supernatant degradation of MO (concentrations: MO = 10 mg/L, TiO -PCHPMA and the supernatant after degradation of MO (concentrations: MO = 10 mg/L, TiO2-PCHPMA==0.4 0.4 TiO = 0.4 mg/mL, 31 ◦ C). mg/mL, 31 2 -PCHPMA mg/mL, 31°C). °C).
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Figure 8. UV-visible absorption spectra of aqueous solution of (a) MO and (b) MB under UV light Figure 8. UV-visible absorption spectra of aqueous solution of (a) MO and (b) MB under UV light Figure 8. UV-visible absorption spectra of aqueous solution of (a) MO and (b) MB under UV light 2-PCHPMA for the control experiment (concentrations: MO and MB irradiation in the absence of TiO irradiation in the absence of TiO2 -PCHPMA for the control experiment (concentrations: MO and irradiation in the absence of TiO2-PCHPMA for the control experiment (concentrations: MO and MB = 10 mg/L, 31 °C). MB = 10 mg/L, 31 ◦ C). = 10 mg/L, 31 °C).
degradation experiment forwas MBalso wascarried also carried outpresence in the presence of TiO2-PCHPMA TheThe degradation experiment for MB out in the of TiO2 -PCHPMA under 2-PCHPMA The degradation experiment for As MBa was also carried out in the presence of TiOat under UV irradiation (Figure 9a). result, MB was degraded in 94% efficiency 3 h, and the UV irradiation (Figure 9a). As a result, MB was degraded in 94% efficiency at 3 h, and the efficiency under UV irradiation (Figure 9a). As a result, MB of was degraded in 94% efficiency at 39b). h, and the efficiency was higher than that in the presence bare TiO (80% efficiency) (Figure However, 2 was higher than that in the presence of bare TiO2 (80% efficiency) (Figure 9b). However, the residual efficiency was higher than that in the presence of bare TiO 2 (80% efficiency) (Figure 9b). However, theafter residual solid washing was slightly colored blue, presumably MB was solid washing wasafter slightly colored blue, presumably indicating that MB wasindicating covalentlythat anchored thecovalently residual solid aftertowashing was slightly colored blue, presumably indicating that MB was anchored the chloroalkyl groups under UV-irradiation, and the degradation efficiency to the chloroalkyl groups under UV-irradiation, and the degradation efficiency involved slight loss of covalently anchored toof the chloroalkyl groups under UV-irradiation, and of the degradation efficiency 2-PCHPMA toward MB slight loss MB by covalent linkage. The degradation ability TiO MBinvolved by covalent linkage. The degradation ability of TiO 2 -PCHPMA toward MB higher than that of 2-PCHPMA toward MB involved slight loss of MB by covalent linkage. The degradation ability of TiO than that of TiO2 is probably due to the adsorption of cationic MB assisting the approach of TiOhigher 2 is probably due to the adsorption of cationic MB assisting the approach of MB towards the surface higher than thatthe of surface TiO2 is of probably toAcceleration the adsorption of cationic MB assisting the approach of MB towards the TiO2due core. of degradation by adsorption also reported of the TiO2 core. Acceleration of degradation by adsorption was also reported for TiOwas 2 modified by MBfor towards the surface the TiO2 core. Acceleration of degradation by adsorption wasand also reported 4OH [52]. TiO2 modified byof polyacrylamide [34], polyacid)] [acrylamide-co-(acrylic acid)]The [40], polyacrylamide [34], poly [acrylamide-co-(acrylic [40], and NH4 OH [52]. opticalNH intensity for The TiO2optical modified by polyacrylamide [34], poly [acrylamide-co-(acrylic acid)] [40], and NH4OH [52]. intensity of absorption of MB was not changed in the absence of TiO -PCHPMA 2 of absorption of MB was not changed in the absence of TiO2 -PCHPMA under UV irradiation under in The optical intensity of absorption of MB was not changed in the absence of TiO2-PCHPMA irradiation in the manner MO (Figure thatofthe degradation of MBunder was the UV same manner as MOsame (Figure 8b), as indicating that8b), theindicating degradation MB was also mediated byalso UV irradiation the2-PCHPMA. same manner as MO (Figure 8b), indicating that the degradation of MB was also byinTiO TiOmediated 2 -PCHPMA. mediated by TiO2-PCHPMA.
Figure 9. UV visible absorption spectra of initial aqueous solution of MB, supernatants afterafter Figure 9. UV visible absorption spectra of initial aqueous solution of MB, supernatants degradation of MB by (a) TiO -PCHPMA and (b) TiO , and elutants washed out from TiO -PCHPMA Figure 9. UV visible absorption spectra of initial aqueous solution of MB, supernatants after 2 2-PCHPMA degradation of MB by (a) 2TiO2-PCHPMA and (b) 2TiO2, and elutants washed out from TiO and TiO after the degradation experiment (concentrations: TiO -PCHPMA and TiO = 0.6 mg/mL 2 2 2-PCHPMA 2-PCHPMA degradation of MBthe bydegradation (a) TiO2-PCHPMA and (b) TiO2, and elutants washed out from 2 after 2 = 0.6 and TiO experiment (concentrations: TiO and TiO2TiO mg/mL and and MB 10 mg/L, degradation for 3 h, desorption by washing with MeOH). 2 10 2 2 andMB TiO== after the degradation experiment (concentrations: TiO -PCHPMA and TiO = 0.6 mg/mL and mg/L, degradation for 3 h, desorption by washing with MeOH). MB = 10 mg/L, degradation for 3 h, desorption by washing with MeOH).
3.6. Mechanism of Photocatalysis 3.6. Mechanism of Photocatalysis 3.6. Mechanism of Photocatalysis The photocatalytic activity of a semiconducting photocatalyst highly depends on the surface The photocatalytic activitystructure. of a semiconducting highly oncoupling the surface modification and the band energy For example, photocatalyst modifications of TiO2 depends with silane The photocatalytic activity of a structure. semiconducting photocatalyst highlyofdepends on the surface 2 with silane coupling modification and the band energy For example, modifications TiO reagents including MPS decrease the surface modification the photocatalytic activity probably by modificationincluding and the band energy structure. Formodification example, modifications of TiO2 activity with silane couplingthe decrease surface thethe photocatalytic probably the reagents blocking of active MPS catalytic sites the [64,65]. Figure 10a shows UV-vis absorption spectrumby of reagents including MPS decrease the surface modification the photocatalytic activity probably by the blockingThe of maximum active catalytic sites [64,65]. Figure 10a wavelength shows the UV-vis absorption PCHPMA. absorption peak and the onset appeared at 218 andspectrum 244 nm, of blocking of active catalytic sites [64,65].peak Figure 10a onset showswavelength the UV-visappeared absorption218 spectrum of PCHPMA. The maximum absorption and and respectively. These wavelengths are shorter than thethe wavelengths of the UV emissionatlines of the244 Hgnm, PCHPMA. The maximum absorption peak and the onset wavelength appeared at 218 and 244 nm, respectively. These are shorter the wavelengths of the UVtoemission of the Hg lamp (253 and 365 nm).wavelengths This result indicates thatthan PCHPMA does not contribute the lightlines absorption. respectively. areresult shorterindicates than the that wavelengths of the UVnot emission lines of lamp (253These and wavelengths 365 nm). This PCHPMA does contribute to the theHg light lamp (253 and 365 nm). This result indicates that PCHPMA does not contribute to the light
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The band gap2018, calculated from the onset wavelength is 5.1 eV. Figure 10b shows the CV of Technologies 2018, 11curves 16 Technologies 6,6,xx 11 ofof 16 TiO2 -PCHPMA, TiO2 , and PCHPMA in 0.1 M NaCl aq. A peak reduction of TiO2 was observed absorption. Therespect bandgap gap calculatedfrom from theonset onset wavelength 5.1eV. eV.Figure Figure 10bshows shows the CV The band the wavelength isis5.1 10b the CV fromabsorption. –0.38 V with to calculated Ag/AgCl, agreeing well with reported values [8,66,67]. The reduction curves of of TiO TiO2-PCHPMA, TiO2,2, and and PCHPMA PCHPMA in in 0.1 0.1 M M NaCl NaCl aq. aq. AA peak peak reduction reduction of of TiO TiO22 was was 2-PCHPMA, TiO curves potential of TiO2 -PCHPMA was observed from −0.28 V with respect to Ag/AgCl, while no peak was observedfrom from–0.38 –0.38VVwith withrespect respectto toAg/AgCl, Ag/AgCl,agreeing agreeingwell well with withreported reportedvalues values[8,66,67]. [8,66,67].The The observed observed in the voltammogram of PCHPMA. The small positive shift (+0.1 V) suggests the slightly 2-PCHPMAwas reductionpotential potentialof ofTiO TiO2-PCHPMA wasobserved observedfrom from−0.28 −0.28VVwith withrespect respectto toAg/AgCl, Ag/AgCl,while whileno no reduction higher andwas lowest unoccupied molecular orbital (LUMO)The level of positive TiO due to the change 2 -PCHPMA peak was observed inthe thevoltammogram voltammogram ofPCHPMA. PCHPMA. The small positive shift(+0.1 (+0.1 V)suggests suggests the of peak observed in of small shift V) the the chemical structure of the unoccupied surface of TiO effect can be ignored considering the lower 2 , but the 2-PCHPMA slightlyhigher higher andlowest lowest unoccupied molecular orbital (LUMO) level ofTiO TiOon dueto to the 2-PCHPMA slightly and molecular orbital (LUMO) level of due the catalytic abilities reported TiO2 modified with coupling reagents. Accordingly, the primary change ofthe theof chemical structure ofthe thesurface surface ofsilane TiO2,2,but butthe theeffect effect canbe beignored ignored onconsidering considering change of chemical structure of of TiO can on thelower lowercatalytic catalytic abilities ofreported reported TiO modified withsilane silanecoupling coupling reagents. Accordingly, the photocatalytic reaction was not enhanced by the modification. The enhanced degradation occurred 22modified the abilities of TiO with reagents. Accordingly, the primary photocatalytic reaction wasPCHPMA notenhanced enhanced byaround themodification. modification. The enhanced degradation primary photocatalytic was not by the The by the rapid adsorption of reaction MB on the layer the surface ofenhanced TiO2 in adegradation similar manner occurred bythe the rapid adsorptionof ofMB MB onpoly[acrylamide-co-(acrylic thePCHPMA PCHPMAlayer layeraround aroundthe theacid)] surface ofTiO TiO inNH similar 22in occurred by rapid adsorption on the surface of aasimilar to TiO by polyacrylamide [34], [40], and 2 modified 4 OH [52]. 4OH manner to TiO modified by polyacrylamide [34], poly[acrylamide-co-(acrylic acid)] [40], and NH 22modified 4OH manner to TiO by polyacrylamide [34], poly[acrylamide-co-(acrylic acid)] [40], and NH A plausible mechanism for the degradation of dyes in the presence of TiO2 -PCHPMA under UV [52].AAplausible plausiblemechanism mechanismfor forthe thedegradation degradationof ofdyes dyesin inthe thepresence presenceof ofTiO TiO2-PCHPMA underUV UV 2-PCHPMAunder [52]. irradiation is displayed in Figure 11. The PCHPMA chain grafted on TiO2 attracts cationic MB to allow irradiation isis displayed displayed in in Figure Figure 11. 11. The The PCHPMA PCHPMA chain chain grafted grafted on on TiO TiO22 attracts attracts cationic cationic MB MB to to irradiation the rapid approach of MB over the surface of TiO2 . Adsorbed MB is smoothly degraded by oxidation allowthe the rapid rapid approach approachof of MB MB over over the the surface surface of of TiO TiO2.2. Adsorbed Adsorbed MB MB isis smoothly smoothly degraded degraded by by allow species such asspecies O2 . Bysuch contrast, anionic MOanionic does not interact with the PCHPMA chain, and approaches oxidation species such asOO2.2.By Bycontrast, contrast, anionic MO doesnot not interact withthe thePCHPMA PCHPMA chain, and oxidation as MO does interact with chain, and the surface of TiO by diffusion. adiffusion. result, the of MO proceeds identically to TiO2 and 2 surface approaches the surface ofTiO TiO22As bydiffusion. Asadegradation aresult, result,the thedegradation degradation ofMO MOproceeds proceedsidentically identically approaches the of by As of TiO2 -PCHPMA. toTiO TiO22and andTiO TiO2-PCHPMA. 2-PCHPMA. to
Figure 10.UV-vis (a)UV-vis UV-vis absorption spectrumof ofPCHPMA PCHPMAin in MeOH and (b)(b) cyclic voltammetry (CV)(CV) Figure 10. (a) absorption spectrum inMeOH MeOH and cyclic voltammetry Figure 10. (a) absorption spectrum of PCHPMA and (b) cyclic voltammetry (CV) curves of2TiO TiO TiO and PCHPMA deposited on Pt working electrodes (electrolyte, 0.1 M 2-PCHPMA, 2,and curves of TiO , PCHPMA deposited on Pt working electrodes (electrolyte, 0.1 M 2-PCHPMA, 2 curves of TiO -PCHPMA, TiO , and PCHPMA deposited on Pt working electrodes (electrolyte, 0.1 M 2 −1−1 − 1 NaCl aq.: counter electrode, Pt wire: reference electrode, Ag wire: sweep rate = 100 mVs ). NaCl aq.: counter electrode, Pt wire: reference electrode, Ag wire: sweep rate = 100 mVs ). NaCl aq.: counter electrode, Pt wire: reference electrode, Ag wire: sweep rate = 100 mVs ).
Figure 11. Plausible mechanism for the degradation of dyes inthe the presence of TiO under Figure 11.Plausible Plausible mechanism for the degradationof ofdyes dyesin in the presence ofTiO TiO under 2-PCHPMA 2 -PCHPMA Figure 11. mechanism for the degradation presence of under 2-PCHPMA UV irradiation. UV irradiation. UV irradiation.
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4. Conclusions We developed TiO2 -PCHPMA for selective separation and degradation of dyes. The structure of TiO2 -PCHPMA was confirmed by SEM, TEM, EDX, FTIR, and TGA. The grafted chains of PCHPMA enabled the selective adsorption of cationic MB under a wide pH range from 4 to 12 due to the alkyl chloride group electrostatically interacting with MB, while PCHPMA had no interaction with anionic MO. Adsorbed MB could be either recovered quantitatively by simple washing with methanol or degraded under UV irradiation. The ability of selective adsorption and degradation of TiO2 -PCHPMA is beneficial for its potential application to water treatment not only by degradation of pollutants but also by collection for reuse. We are now investigating the effect of TiO2 -PCHPMA on various other dyes. Author Contributions: Conceptualization, M.S.H. and B.O.; Methodology, M.S.H. and B.O.; Validation, M.S.H. and B.O.; Formal Analysis, M.S.H.; Investigation, M.S.H.; Resources, M.S.H.; Data Curation, M.S.H.; Writing-Original Draft Preparation, M.S.H.; Writing-Review & Editing, M.S.H. and B.O.; Visualization, M.S.H. and B.O.; Supervision, B.O.; Project Administration, B.O.; Funding Acquisition, B.O. Funding: This research received no external funding. Acknowledgments: M.S.H. thanks Ministry of Education, Culture, Sports, Science, and Technology, Japan, for providing a Japanese government (MEXT) scholarship. Conflicts of Interest: The authors declare no conflict of interest.
References 1.
2. 3. 4. 5. 6.
7. 8. 9. 10.
11. 12. 13. 14.
Robinson, T.; McMulan, G.; Marchant, R.; Nigam, P. Remediation of dyes in textile effluents: A critical review on current treatment technologies with a proposed alternative. Bioresour. Technol. 2001, 77, 247–255. [CrossRef] Pearce, C.I.; Lioyd, J.R.; Guitrie, J.T. The removal of color from textile wastewater using whole bacterial cells: A review. Dyes Pigm. 2003, 58, 179–196. [CrossRef] Karan, C.K.; Bhattacharjee, M. Self-healing and moldable metallogels as the recyclable materials for selective dye adsorption and separation. ACS Appl. Mater. Interfaces 2016, 8, 5526–5535. [CrossRef] [PubMed] Nakata, K.; Fujishima, A. TiO2 photocatalysis: Design and applications. J. Photochem. Photobiol. C: Photochem. Rev. 2012, 13, 169–189. [CrossRef] Li, H.; Bian, Z.; Zhu, J.; Huo, Y.; Li, H.; Lu, Y. Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity. J. Am. Chem. Soc. 2007, 129, 4538–4539. [CrossRef] [PubMed] Zhao, D.; Chen, C.; Wang, Y.; Ma, W.; Zhao, J.; Rajh, T.; Zang, L. Enhanced photocatalytic degradation of dye pollutants under visible irradiation on Al(III)-modified TiO2 : Structure, interaction, and interfacial electron transfer. Environ. Sci. Technol. 2008, 42, 308–314. [CrossRef] [PubMed] Naya, S.I.; Nikawa, T.; Kimura, K.; Tada, H. Rapid and complete removal of nonylphenol by gold nanoparticle/rutile titanium(IV) oxide plasmon photocatalyst. ACS Catal. 2013, 3, 903–907. [CrossRef] Sakthivel, S.; Kisch, H. Daylight photocatalysis by carbon-modified titanium dioxide. Angew. Chem. Int. Ed. 2003, 42, 4908–4911. [CrossRef] [PubMed] Ren, W.; Ai, Z.; Jia, F.; Zhang, L.; Fan, X.; Zou, Z. Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO2 . Appl. Catal. B 2007, 69, 138–144. [CrossRef] Dong, F.; Guo, S.; Wang, H.; Li, X.; Wu, Z. Enhancement of the visible light photocatalytic activity of c-doped TiO2 nanomaterials prepared by a green synthetic approach. J. Phys. Chem. C 2011, 115, 13285–13292. [CrossRef] Wang, W.; Serp, P.; Kalck, P.; Faria, J.L. Photocatalytic degradation of phenol on MWNT and titania composite catalysts prepared by a modified sol–gel method. Appl. Catal. B 2005, 56, 305–312. [CrossRef] Woan, B.K.; Pyrgiotakis, G.; Sigmund, W. Photocatalytic carbon-nanotube–TiO2 composites. Adv. Mater. 2009, 21, 2233–2239. [CrossRef] Vijayan, B.K.; Dimitrijevic, N.M.; Finkelstein-Shapiro, D.; Wu, J.; Gray, K.A. Coupling titania nanotubes and carbon nanotubes to create photocatalytic nanocomposites. ACS Catal. 2012, 2, 223–229. [CrossRef] Tryba, B.; Morawski, A.W.; Inagaki, M. Application of TiO2 -mounted activated carbon to the removal of phenol from water. Appl. Catal. B 2003, 41, 427–433. [CrossRef]
Technologies 2018, 6, 92
15. 16. 17.
18.
19. 20.
21.
22. 23. 24. 25. 26. 27. 28. 29. 30.
31.
32. 33. 34. 35. 36. 37.
13 of 15
Asiltürk, M.; S¸ ener, S. TiO2 -activated carbon photocatalysts: Preparation, characterization and photocatalytic activities. Chem. Eng. J. 2012, 180, 354–363. [CrossRef] Alam, M.G.; Tawfik, A.; Ookawara, S. Enhancement of photocatalytic activity of TiO2 by immobilization on activated carbon for degradation of pharmaceuticals. J. Environ. Chem. Eng. 2016, 4, 1929–1937. [CrossRef] Zhang, Y.; Tang, Z.R.; Fu, X.; Xu, Y.J. TiO2 -graphene nanocomposites for gas-phase photocatalytic degradation of volatile aromatic pollutant: Is TiO2 -graphene truly different from other TiO2 -carbon composite materials? ACS Nano 2010, 4, 7303–7314. [CrossRef] [PubMed] Liang, Y.T.; Vijayan, B.K.; Gray, K.A.; Hersam, M.C. Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production. Nano Lett. 2011, 11, 2865–2870. [CrossRef] [PubMed] Zhao, D.; Sheng, G.; Chen, C.; Wang, X. Enhanced photocatalytic degradation of methylene blue under visible irradiation on graphene@TiO2 dyade structure. Appl. Catal. B 2012, 111–112, 302–308. [CrossRef] Liu, B.J.; Bai, H.; Wang, Y.; Liu, Z.; Zhang, X.; Sun, D.D. Self-assembling TiO2 nanorods on large graphene oxide sheets at a two-phase interface and their anti-recombination in photocatalytic applications. Adv. Funct. Mater. 2010, 20, 4175–4181. [CrossRef] Sang, Y.; Zhao, Z.; Tian, J.; Hao, P.; Jiang, H.; Liu, H.; Claverie, J.P. Enhanced photocatalytic property of reduced graphene oxide/TiO2 nanobelt surface heterostructures constructed by an in situ photochemical reduction method. Small 2014, 10, 3775–3782. [CrossRef] [PubMed] Li, L.; Yu, L.; Lin, Z.; Yang, G. Reduced TiO2 graphene oxide heterostructure as broad spectrum-driven efficient water-splitting photocatalysts. ACS Appl. Mater. Interfaces 2016, 8, 8536–8545. [CrossRef] [PubMed] Zhu, Y.; Dan, Y. Photocatalytic activity of poly(3-hexylthiophene)/titanium dioxide composites for degrading methyl orange. Sol Energy Mater Sol Cells 2010, 94, 1658–1664. [CrossRef] Zhang, H.; Zong, R.; Zhao, J.; Zhu, Y. Dramatic visible photocatalytic degradation performances due to synergetic effect of TiO2 with PANI. Environ. Sci. Technol. 2008, 42, 3803–3807. [CrossRef] [PubMed] Min, S.X.; Wang, F.; Feng, L.; Tong, Y.C.; Yang, Z.R. Synthesis and photocatalytic activity of TiO2 /conjugated polymer complex nanoparticles. Chin. Chem. Lett. 2008, 19, 742–746. [CrossRef] Dimitrijevic, N.M.; Tepavcevic, S.; Liu, Y.; Rajh, T.; Silver, S.C.; Tiede, D.M. Nanostructured TiO2 /polypyrrole for visible light photocatalysis. J. Phys. Chem. C 2013, 117, 15540–15544. [CrossRef] Gao, F.; Hou, X.; Wang, A.; Chu, G.; Wu, W.; Chen, J.; Zou, H. Preparation of polypyrrole/TiO2 nanocomposites with enhanced photocatalytic performance. Particuology 2016, 26, 73–78. [CrossRef] Liao, G.; Chen, S.; Quan, X.; Chen, H.; Zhang, Y. Photonic crystal coupled TiO2 /polymer hybrid for efficient photocatalysis under visible light irradiation. Environ. Sci. Technol. 2010, 44, 3481–3485. [CrossRef] [PubMed] Luo, Q.; Li, X.; Li, X.; Wang, D.; An, J.; Li, X. Visible light photocatalytic activity of TiO2 nanoparticles modified by pre-oxidized polyacrylonitrile. Catal. Commun. 2012, 26, 239–243. [CrossRef] Wang, D.; Zhang, J.; Luo, Q.; Li, X.; Duan, Y.; An, J. Characterization and photocatalytic activity of poly(3-hexylthiophene)-modified TiO2 for degradation of methyl orange under visible light. J. Hazard. Mater. 2009, 169, 546–550. [CrossRef] [PubMed] Mahanta, D.; Manna, U.; Madras, G.; Patil, S. Multilayer self-assembly of TiO2 nanoparticles and polyaniline-grafted-chitosan copolymer (CPANI) for photocatalysis. ACS Appl. Mater. Inter. 2011, 3, 84–92. [CrossRef] [PubMed] Li, X.; Wang, D.; Cheng, G.; Luo, Q.; An, J.; Wang, Y. Preparation of polyaniline-modified TiO2 nanoparticles and their photocatalytic activity under visible light illumination. Appl. Catal. B 2008, 81, 267–273. [CrossRef] Wang, D.; Wang, Y.; Li, X.; Luo, Q.; An, J.; Yue, J. Sunlight photocatalytic activity of polypyrrole–TiO2 nanocomposites prepared by ‘in situ’ method. Catal. Commun. 2008, 9, 1162–1166. [CrossRef] Tang, Q.; Lin, J.; Wu, Z.; Wu, J.; Huang, M.; Yang, Y. Preparation and photocatalytic degradability of TiO2 /polyacrylamide composite. Eur. Polym. J. 2007, 43, 2214–2220. [CrossRef] Yang, C.; Zhang, M.; Dong, W.; Cui, G.; Ren, Z.; Wang, W. Highly efficient photocatalytic degradation of methylene blue by PoPD/TiO2 nanocomposite. PLoS ONE 2017, 12, e0174104. [CrossRef] [PubMed] Wang, F.; Min, S.; Han, Y.; Feng, L. Visible-light-induced photocatalytic degradation of methylene blue with polyaniline-sensitized TiO2 composite photocatalysts. Supperlatt. Microstruct. 2010, 48, 170–180. [CrossRef] Xu, S.; Gu, L.; Wu, K.; Yang, H.; Song, Y.; Jiang, L.; Dan, Y. The influence of the oxidation degree of poly(3-hexylthiophene) on the photocatalytic activity of poly(3-hexylthiophene)/TiO2 composites. Sol. Energy Mater. Sol. Cells. 2012, 96, 286–291. [CrossRef]
Technologies 2018, 6, 92
38.
39.
40.
41.
42.
43. 44.
45.
46. 47. 48.
49.
50. 51. 52. 53. 54.
55.
56. 57.
14 of 15
Luo, Q.; Wang, X.; Wang, D.; An, J.; Li, X.; Yin, R. TiO2 /cyclized polyacrylonitrile hybridized nanocomposite: An efficient visible-light photocatalyst prepared by a facile “in situ” approach. Mater. Sci. Eng. B 2015, 199, 96–104. [CrossRef] Muthirulan, P.; Nirmala, C.K.; Sundaram, M.M. Facile synthesis of novel hierarchical TiO2 @Poly(o-phenylenediamine) core–shell structures with enhanced photocatalytic performance under solar light. J. Environ. Chem. Eng. 2013, 1, 620–627. [CrossRef] Kangwansupamonkon, W.; Jitbunpot, W.; Kiatkamjornwong, S. Photocatalytic efficiency of TiO2 /poly[acrylamide-co-(acrylic acid)] composite for textile dye degradation. Polym. Degrad. Stabil. 2010, 95, 1894–1902. [CrossRef] Fukahori, S.; Ichiura, H.; Kitaoka, T.; Tanaka, H. Photocatalytic decomposition of bisphenol A in water using composite TiO2 -zeolite sheets prepared by a papermaking technique. Environ. Sci. Technol. 2003, 37, 1048–1051. [CrossRef] [PubMed] Liu, B.; Wen, L.; Nakata, K.; Zhao, X.; Liu, S.; Ochiai, T.; Murakami, T.; Fujishima, A. Polymeric adsorption of methylene blue in TiO2 colloids-highly sensitive thermochromism and selective photocatalysis. Chem. Eur. J. 2012, 18, 12705–12711. [CrossRef] [PubMed] Zang, G.; Choi, W.; Kim, S.H.; Hong, S.B. Selective photocatacatalytic degradation of aquatic pollutants by titania encapsulated into FAU-type zeolites. J. Hazard. Mater. 2011, 188, 198–205. [CrossRef] [PubMed] Shen, X.; Zhu, L.; Liu, G.; Yu, H.; Tang, H. Enhanced photocatalytic degradation and selective removal of nitrophenol by using surface molecular imprinted titania. Environ. Sci. Technol. 2008, 42, 1687–1692. [CrossRef] [PubMed] Inumaru, K.; Murashima, M.; Kasahara, T.; Yamanaka, S. Enhanced photocatalytic decomposition of 4-nonylphenol by surface-organografted TiO2 : a combination of molecular selective adsorption and photocatalysis. Appl. Catal. B 2004, 52, 275–280. [CrossRef] Nakamura, K.J.; Ide, Y.; Ogawa, M. Molecular recognitive photocatalytic decomposition on mesoporous silica coated TiO2 particle. Mater. Lett. 2011, 65, 24–26. [CrossRef] Bao, L.; Meng, M.; Sun, K.; Li, W.; Zhao, D.; Li, H.; He, M. Selective adsorption and degradation of rhodamine B with modified titanium dioxide photocatalyst. J. Appl. Polym. Sci. 2014, 40890, 1–12. [CrossRef] Choi, H.; Kim, Y.J.; Varma, R.S.; Dionysiou, D.D. Thermally stable nanocrystalline TiO2 photocatalysts synthesized via sol-gel methods modified with ionic liquid and surfactant molecules. Chem. Mater. 2006, 18, 5377–5384. [CrossRef] Kumar, S.G.; Devi, L.G. Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics. J. Phys. Chem. A 2011, 115, 13211–13241. [CrossRef] [PubMed] Lazar, M.A.; Walid, A.; Daoud, W.A. Achieving selectivity in TiO2 -based photocatalysis. RSC Adv. 2013, 3, 4130–4140. [CrossRef] Robert, D.; Piscopo, A.; Weber, J.V. First approach of the selective treatment of water by heterogeneous photocatalysis. Environ. Chem. Lett. 2004, 2, 5–8. [CrossRef] Lazar, M.A.; Daoud, W.A. Selective adsorption and photocatalysis of low-temperature base-modified anatase nanocrystals. RSC Adv. 2012, 2, 447–452. [CrossRef] Genevrier, A.C.; Boissiere, C.; Nicole, L.; Grosso, D. Distance dependence of the photocatalytic efficiency of TiO2 revealed by in situ ellipsometry. J. Am. Chem. Soc. 2012, 134, 10761–10764. [CrossRef] [PubMed] Gokaltun, A.; Çelebi, B.; Tuncel, A. Octadecylamine-attached poly(3-chloro-2-hydroxypropyl methacrylate-co-ethylene dimethacrylate) microspheres as a new stationary phase for microbore reversed phase chromatography. Anal. Meth. 2014, 6, 5712–5719. [CrossRef] Molina, E.A.; Néstor, R.V.; Rodríguez, D.M.; Figueiras, C.C. Synthesis and characterization of TiO2 modified with polystyrene and poly(3-chloro-2hydroxypropyl methacrylate) as adsorbents for the solid phase extraction of organophosphorous pesticides. J. Chem. 2016, 2016, 1–11. [CrossRef] Cozzoli, P.D.; Kornowski, A.; Weller, H. Low-temperature synthesis of soluble and processable organic-capped anatase TiO2 nanorods. J. Am. Chem. Soc. 2003, 125, 14539–14548. [CrossRef] [PubMed] Wang, C.; Mao, H.; Wang, C.; Fu, S. Dispersibility and hydrophobicity analysis of titanium dioxide nanoparticles grafted with silane coupling agent. J. Ind. Eng. Chem. Res. 2011, 50, 11930–11934. [CrossRef]
Technologies 2018, 6, 92
58.
59.
60.
61.
62.
63. 64. 65.
66. 67.
15 of 15
Yuvaraj, H.; Kim, W.S.; Kim, J.T.; Kang, I.P.; Gal, Y.S.; Kim, S.W.; Lim, K.T. Synthesis of poly(methyl methacrylate) encapsulated TiO2 nanocomposite particles in supercritical CO2 . Mol. Cryst. Liq. Cryst. 2009, 514, 355–365. [CrossRef] Wang, J.; Sun, W.; Zhang, Z.; Jiang, Z.; Wang, X.; Xu, R.; Li, R.; Zhang, X. Preparation of Fe-doped mixed crystal TiO2 catalyst and investigation of its sonocatalytic activity during degradation of azo fuchsine under ultrasonic irradiation. J. Colloid Interface Sci. 2008, 320, 202–209. [CrossRef] [PubMed] Sun, X.; Liu, H.; Dong, J.; Wei, J.; Zhang, Y. Preparation and Characterization of Ce/N-Codoped TiO2 Particles for Production of H2 by Photocatalytic Splitting Water Under Visible Light. Catal. Lett. 2010, 135, 219–225. [CrossRef] Ba-abbad, M.M.; Kadhum, A.A.H.; Mohamad, A.B.; Takriff, M.S.; Sopian, K. Synthesis and catalytic activity of TiO2 nanoparticles for photochemical oxidation of concentrated chlorophenols under direct solar radiation. Int. J. Electrochem. Sci. 2012, 7, 4871–4888. Wang, Y.Q.; Chen, S.G.; Tang, X.H.; Palchik, O.; Zaban, A.; Koltypin, Y.; Gedanken, A. Mesoporous titanium dioxide: sonochemical synthesis and application in dye-sensitized solar cells. J. Mater. Chem. 2001, 11, 521–526. [CrossRef] Yu, J.C.; Zhang, L.; Yu, J. Direct sonochemical preparation and characterization of highly active mesoporous TiO2 with a bicrystalline framework. Chem. Mater. 2002, 14, 4647–4653. [CrossRef] Pazokifard, S.; Mirabedini, S.M.; Esfandeh, M.; Mohseni, M.; Ranjbar, Z. Silane grafting of TiO2 nanoparticles: dispersibility and photoactivity in aqueous solutions. Surf. Interface Anal. 2012, 44, 41–47. [CrossRef] Siddiquey, I.A.; Ukaji, E.; Furusawa, T.; Sato, M.; Suzuki, N. The effects of organic surface treatment by methacryloxypropyltrimethoxysilane on the photostability of TiO2 . Mater. Chem. Phys. 2007, 105, 162–168. [CrossRef] Pelouchova, H.; Janda, P.; Weber, J.; Kavan, L. Charge transfer reductive doping of single crystal TiO2 anatase. J. Electro. Chem. 2004, 566, 73–83. [CrossRef] Renault, C.; Nicole, L.; Sanchez, C.; Costentin, C.; Balland, V.; Limoges, B. Unraveling the charge transfer/electron transport in mesoporous semiconductive TiO2 films by voltabsorptometry. Phys. Chem. Chem. Phys. 2015, 17, 10592–10607. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
