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Polypyrrole-TiO2 composite for removal of 4-chlorophenol and diclofenac
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Polypyrrole-TiO2 composite for removal of 4-chlorophenol and diclofenac ⁎
Siara Silvestria, , Thiago A.L. Burgob,c, Celia Dias-Ferreirad,e,f, João A. Labrinchad, David Maria Tobaldid a
Graduate Program in Environmental Engineering, Federal University of Santa Maria, Santa Maria 97105–900, Brazil Department of Chemistry, Federal University of Santa Maria, Santa Maria 97105-900, Brazil Department of Physics, Federal University of Santa Maria, Santa Maria 97105-900, Brazil d Department of Materials and Ceramic Engineering, Aveiro Institute of Materials, University of Aveiro, Aveiro 3810-193, Portugal e Polytechnic Institute of Coimbra, Research Centre for Natural Resources, Environment and Society (CERNAS), Coimbra 3045-601, Portugal f Universidade Aberta, Lisboa, Portugal b c
A R T I C LE I N FO
A B S T R A C T
Keywords: PPy-TiO2 4-chlorophenol Diclofenac Photocatalysis
In this work, we successfully synthesized TiO2-polypyrrole (PPy) composite material via polymerization method using the sulfuric acid (H2SO4) as an oxidizing agent. Different characterization techniques, including electrostatic force microscopy, confirmed that TiO2 was intimately attached on PPy. The photocatalytic activity of PPyTiO2 composite was studied experimentally for the conversion of diclofenac (DCF)– one of the most frequently detected pharmaceutical compounds in the aquatic environment – and 4-chlorophenol (4-CP) - largely used in the bleaching of cellulose pulp in the paper industry, as herbicide and pesticide - under simulated solar light irradiation at environmental conditions. This is the first time that a composite PPy-TiO2 was used as photocatalyst for the conversion of 4-CP. Results showed that the PPy-TiO2 was able to convert more than 90% of DCF and 40% of 4-CP in just 60 min, showing stability and efficiency over five consecutive cycles of reuse for both pollutants. The produced PPy-TiO2 showed higher photocatalytic efficiency compared to that of TiO2, thus proving itself to be a promising photocatalyst.
1. Introduction 4-chlorophenol (4-CP) is an organochlorine widely used in the bleaching of cellulose pulp in the paper industry, as herbicide and pesticide [1–4]. Its residues can be leached by rainwater to the soil, water sources and groundwater [2,3,5]. Contamination by 4-CP affects fauna, flora and the human health, as it is irritating to the eyes, skin and respiratory system, and can have effects on the central nervous system [6,7]. Diclofenac (DCF - 2-[2-(2,6-dichloroanilino)phenyl]acetic acid) is a pharmaceutical compound extensively used as human and veterinary anti-inflammatory, is resistant and bio accumulative in the environment [8–10] It can be found in wastewater through urinary and animal carcass disposal, spreading along the ecosystem food chain [11,12]. This bioaccumulation may lead to even more danger to aquatic organisms. Its residues are also potential contaminants [13,14] and increase the risk of cardiovascular infarct heart attack or stroke [15]. Studies on hospital effluents always indicate the presence of DCF among the most abundant pharmaceutical compounds [13,16–18]. Advanced oxidation processes (AOPs) are an alternative technology
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to the treatment of several environmental pollutants [19,20]. Photocatalysis is highlighted as an efficient oxidative process for the conversion of pollutants such as dyes, pharmaceuticals, pesticides, herbicides and organic acids [21,22]. The great advantage of AOPs lies in the fact that the contaminant does not simply undergo a phase change, but rather is mineralized through a series of chemical reactions [23–25]. One of the most used photocatalysts is TiO2. TiO2 is a white powder, insoluble and easily dispersed in aqueous medium. The main disadvantage in the use of TiO2 is the great difficulty of removing it from the solution. To solve the problem of removal of the photocatalyst from the reaction medium, the adhesion of TiO2 particles to some substrates has been widely reported. Examples are TiO2 support on SiO2, activated carbon, glass, zeolites, biochar, polymers, etc. [26–35]. Polymer materials in the form of nanocomposites have certain advantages such as high surface area to volume ratio. Conductive polymers are a new group of synthetic polymers that combine the chemical and mechanical properties of the polymers as well as the adsorption and support with the electronic and photocatalytic properties of semiconductor oxides [36]. Nowadays, conducting polymers have several applications in different areas such as coatings for improved corrosion
Corresponding author. E-mail address: [email protected] (S. Silvestri).
https://doi.org/10.1016/j.reactfunctpolym.2019.104401 Received 28 July 2019; Received in revised form 25 October 2019; Accepted 26 October 2019 1381-5148/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Siara Silvestri, et al., Reactive and Functional Polymers, https://doi.org/10.1016/j.reactfunctpolym.2019.104401
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drying.
resistance of steel [37], as dopant to drug molecules [38], hydrogen storage [39], sensors [40–43], biosensors [44], electrochemical batteries [45], super capacitors [46,47], adsorbents [48], and supports [49,50]. They have extended p-conjugation with single- and doublebond alteration along its chain. They behave as a semiconductor material with low charge carrier mobility [51] and their conductivity is increased to reach the metallic range by doping with appropriate dopants [36]. Polypyrrole (PPy)-TiO2 is a composite classified as reticulate doped polymer, leading to specific properties in which a conducting crystalline network of charge transfer is formed building up the continuous conducting network in reticulate doped polymer, resulting in high conductivity [51]. According to Kryszewski [51], this property is not predicted by any theory of conductivity in heterogeneous systems. Simple percolation theory and fractal analysis of such systems cannot be applied because of the different ‘shape’ of the conducting element appearing at different stages of the network formation. In recent years, some studies have been published on the combination of PPy and TiO2. Sun and collaborators [52] prepared PPy-TiO2 composite by reverse microemulsion polymerization and Deng et al. [53] prepared PPy-TiO2 by surface molecular imprinting technique, and both researches used the composites as a photocatalyst for the degradation of methyl orange dye. PPy-TiO2 composites were also synthesized by polymerization of pyrrole with macro/mesoporous TiO2 in presence of dioctyl sulfosuccinate sodium salt with isooctane as solvent, as a photosensitizer and template by Li and collaborators [54]. Sangareswari and co-workers [55] used ammonium persulfate as an oxidant to prepare PPy-TiO2 and evaluated the photocatalytic activity by monitoring the degradation of methylene blue. Gao et al. [50] synthetized PPy/TiO2 using ferric chloride, tetrabutyl titanate, acetic acid, ethanol, followed of calcination at 500 °C. The composite was used to degrade Rhodamine B dye. However, degradation of dyes is not as challenging as the degradation of persistent organic compounds (such as some herbicides and pesticides), and therefore degradation studies using dyes, although useful to some extent, do not provide a realistic assessment of the degradation potential of PPy-TiO2 photocatalysts. In this work, PPy was used as support to TiO2 in order to facilitate the transfer of charges, to prevent electron-hole recombination, and to be easily removed from the solution after the photocatalytic reaction. The PPy-TiO2 was synthetized using only water, pyrrole, TiO2, hydrogen peroxide and sulfuric acid under magnetic stirring. The photocatalytic activity of the produced composite was evaluated by monitoring the degradation of pesticide 4-CP and pharmaceutical compound DCF, being reported for the first time in the literature through the present research. Furthermore, the electronic property of the composite was elucidated by electrostatic force microscopy (EFM).
2.3. Characterization of materials X-ray diffraction (XRD) (Rigaku Miniflex 300 diffractometer) was used to identify the mineralogical phases of the prepared materials. The surface areas and average pore diameters were calculated from N2 adsorption-desorption isotherms (Micromeritics Gemini 2380, US). Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer FT-IR Spectrum spectrometer. The band gap energy of the samples was determined by means of the Tauc method [57,58], using the diffuse reflectance spectra (UV-2600 Plus, Shimadzu). Raman spectra were recorded using a RFS 100/S (Bruker, DE) equipped with a 1064 nm Nd:YAG laser as the excitation source. The scanning electron microscopy (SEM-EDS) (Tescan-VEGA3) was used to observe microstructural features. Atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were performed simultaneously in a Park NX10 (Park Systems, Suwon – Korea) instrument equipped with a SmartScan software version 1.0. RTM 11a. Samples were diluted in water and dropped into square small pieces of fresh cleaved mica and imaged in non-contact mode. The measurements were conducted using a PPP-EFM (Nanosensors, Neuchâtel – Switzerland) Si probe PtIr coated, with a nominal resonance frequency of 75 kHz and 2.8 N m−1 force constant. All measurements were made under ambient conditions at room temperature of 27 ± 5 °C and a relative humidity of 60 ± 10% with a scanning rate of 0.4 Hz with a 2 V bias applied to the sample holder. Images were treated offline using XEI software version 4.3.4 Build 22. RTM 1. Also, XEI software was used to calculate average roughness (Ra) and fractal dimension (D). The photoluminescence measurements were done on a Cary Eclipse fluorescence spectrophotometer (Varian) at room temperature. The fluorescence spectra were recorded on a sample solid state on a with excitation and emissions slits of 5 nm. Sample excitation was performed at 255 nm at room temperature, and the emission was scanned between 300 and 500 nm. 1 H NMR spectra were recorded on a BRUKER AVANCE III HD Spectrometer at the frequency of 600.13 MHz. Data were obtained in 5 mm tubes, temperature of 70 °C, concentration of approximately 0.01 M in dimethylsulfoxide deuterated (DMSO‑d6) as solvent using tetramethylsilane (TMS) as an internal reference. 2.4. Photocatalytic activity In order to evaluate the photocatalytic activity of PPy-TiO2 in the presence of 4-CP and DCF pollutants, 1 g L−1 of photocatalyst (PPyTiO2) was added to 100 mL aqueous solution of pollutant (10 mg L−1), these quantities were previously studied and based on the research [59]. A solar chamber equipped with Xenon lamp (600 W m−2) (Suntest CPS Atlas MTS) was used as reacting system. Samples were kept under constant magnetic stirring, and simulated ambient conditions were maintained in the solar chamber during the photocatalytic experiments – i.e. relative humidity between 40 and 42%, temperature between 22 and 24 °C. Aliquots (3 mL) were collected periodically with a syringe and centrifuged to remove the photocatalyst. The supernatant was analyzed by UV–Vis spectroscopy (T80+ UV/VIS Spectrometer PG instruments) at a wavelength of 226 nm for 4-CP and 274 nm for DCF. The tests were performed in triplicate for each pollutant. To test the reusability of the PPy-TiO2, cyclic experiments of 4-CP and DCF photo conversion were conducted. After each cycle, the photocatalyst was recovered and washed several times with deionized water for the subsequent cycle. In total, 5 cycles were carried out for each pollutant. The radical species generated by PPy-TiO2 were evaluated by the addition of scavengers of •OH, O2-• and h+, such as isopropyl alcohol
2. Experimental 2.1. Materials Pyrrole monomer (98%), TiO2 (P25-Evonik Aeroxide - 76.3 wt% anatase, 10.6 wt% rutile, and 13.0 wt% amorphous phase, with average crystalline size between 15.5 nm and 19.3 nm [56]), DCF and 4-CP were purchased from Sigma-Aldrich. H2SO4 (Synth), H2O2 (50% - 200 vol – Êxodo Científica). Deionized water was used for all the experiments. All these reagents were of analytical grade and used without further purification. 2.2. Preparation of PPy-TiO2 For preparing the PPy-TiO2, 7 mg TiO2 was dispersed into 80 mL of 0.05 M pyrrole aqueous solution and 20 mL H2SO4 0.2 M, and 0.05 mL H2O2. This mixture was magnetically stirred until it became a dark solution. The solution was then filtered and washed with deionized water twice and the precipitate was kept in a desiccator until complete 2
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are generated by oxidation of pyrrole monomer. Secondly by radicalradical coupling and deprotonation between adjacent radical cations, dimers are generated leading to a bipyrrole. By reoxidization and coupling with other radical cations, bipyrroles are connected with each other and the propagation step is repeated consecutively. The nucleophilic attacks of water molecules or impurities in the polymer chain appear stops the chemical polymerization process. The TiO2 does not initially interact with the pyrrole. This fact was proved via NMR analysis, (Supplementary Material S2). The 1H NMR spectrum shows the signal at δ 3.17 being neglected due to solvent water and the δ 2.50 signal of DMSO solvent. The Py and Py-TiO2 spectrum shows a double doublet δ 6.74–6.01 each of which has a proportional area for two hydrogens, suggesting that are attached to pyrrole ring carbons; the signal δ 10.74 is attributed to the hetero nitrogen bonded to hydrogen [39]. The PPy and PPy-TiO2 spectrum shows a triplet from δ 7.12–6.95 are characteristic of hydrogens of the pyrrole ring [40]. The peaks related to the couplings of the hydrogens of the pyrrole ring remain in the same position and with the same intensity when they are in the presence of TiO2. Notably, when the polymer is formed in TiO2 presence the chemical shift difference between the peaks was considerably larger than the splitting due to stereochemical configuration. This is due to the number of intramolecular hydrogen bonds between the neighbouring oxygen groups, referents to TiO2 [60,61].
(IPA), benzoquinone (BQ), and ethylenediamine tetra-acetic acid (EDTA) to the 4-CP solution. An amount of 10−4 mol of each scavenger was added to each 50 mL 4-CP solution (10 mg L−1) with 0.5 g L−1 of PPy-TiO2. All solutions were kept under magnetic stirring inside the solar chamber during irradiation incidence. The aliquots collected were centrifuged and analyzed by UV–Vis spectrometer. 3. Results and discussion 3.1. Preparation of PPy-TiO2 In order to evaluate the influence of each reactant on the synthesis of PPy-TiO2, the reaction followed as described in 2.2 Section above. In Supplementary Material Fig. S1a, the reaction occurred only in the presence of water, pyrrole and TiO2. In Fig. S1b, the reaction b is similar as reaction a, plus addition of hydrogen peroxide. In Fig. S1c, the reaction c occurred in the presence of water, pyrrole, TiO2, plus addition of sulfuric acid. Finally, in the Fig. S1d the reaction d occurred in the presence of water, pyrrole, TiO2, hydrogen peroxide and sulfuric acid. When the coloration of the solutions is observed over time, it is possible to predict that the pyrrole polymerizes in acid medium (Fig. S1c–d) and the polymerization reaction is complete when the dark color is obtained (Fig. S1d) occurs in 4 h in the presence of hydrogen peroxide. Hydrogen peroxide, in the presence of water, pyrrole and TiO2, is not able to polymerize monomer (Fig. S1b), but in acidic medium (Fig. S1d) the reaction is complete. The chemical oxidation polymerization processes with the TiO2 nanoparticles are proposed in Fig. 1. Firstly, radical cations (C4NH5+)
3.2. Characterization of the synthesized photocatalytic PPy-TiO2 Fig. 2a shows the XRD patterns of the PPy, TiO2, and PPy-TiO2. The diffractogram of the PPy does not exhibit any reflection peaks, so it can
Fig. 1. Mechanism of the polymerization of pyrrole to forming polypyrrole in presence of TiO2. 3
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Fig. 2. a) XRD patterns and b) FTIR spectra of PPy, TiO2 and composite PPy-TiO2 samples.
Material S3), the peaks at 393, 511 and 633 cm−1 are attributed to TieO vibration modes of anatase TiO2 and peak at 447 cm−1 is attributed to rutile phase [70,71]. The peaks at 1367 and 1584 cm−1 are assigned to the CeN and C]C backbone stretching and ring stretching of PPy respectively [54,69,72]. The bare TiO2 is characterized by a strong and sharp band at 142 cm−1, which is assigned to the bending type vibration of v6 (Eg) mode [54]. Characteristic peaks of TiO2 can be observed, though when TiO2 interacts with PPy, the characteristic peaks of TiO2 lose intensity and only the peak at 936 cm−1 can be observed in both spectra. Table 1 shown the physical characteristics of precursors and of PPyTiO2. Higher values than PPy for specific surface area, average pore size and pore volume were obtained when TiO2 was added to the polymer. All the samples are mesoporous. Fig. 3 shows the SEM images of polymerized polymer (PPy) and the polymerized polymer with added TiO2 (PPy-TiO2). PPy showed the amorphous surface, lacking of a well-defined shape with layers, typical of PPy, as reported in [50,66,68,73]. When TiO2 was added in PPy, the resulting material seemed somehow more “organized”, showing smaller, more dispersed particles tending to spherical form. The addition of TiO2 on PPy could contribute to the higher surface area of PPy, consistent with the results listed in Table 1. SEM equipped with an energy dispersive spectrometer (EDS) was applied to observe the microstructures of PPy-TiO2 (Fig. 4). As shown in EDS elemental mapping, the main feature elements of C and N of PPy, and O and Ti of photocatalyst oxide were detected in PPy-TiO2
Table 1 Surface area and pore volume analysis of PPy, PPy-TiO2 and TiO2. Sample name
Surface area (m2 g−1)
Pore volume (cm3 g−1)
Pore Size (Å)
PPy TiO2 PPy-TiO2
35.9 52.1 42.9
0.05 0.34 0.10
128 31 99
be identified as an (XRD) amorphous material. The characteristic TiO2 reflections were found in the PPy-TiO2 diffractogram. This shows that the two precursor materials are present in the PPy-TiO2 sample. In Fig. 2b it is possible to observe that the intensity of the characteristic PPy peaks is reduced in the PPy-TiO2, as well as the Ti-O-Ti (490–660 cm−1) vibration peaks compared to bare PPy and TiO2. The interaction amid PPy and TiO2 occurs between Ti and N in the pyrrole during the polymerization process. The spectrum of PPy in Fig. 2b confirmed the formation of PPy. The band at 1565 and a weak band at 1459 cm−1 are assigned to stretching vibration of C]C and CeC in the pyrrole ring [62,63]. The absorption at 1283 cm−1 corresponds to CeH in plane deformation modes [62]. PPy shows characteristic CeN and CeH stretching vibration of pyrrole at 1205 and 1043 cm−1 respectively in the infrared spectrum [64–66]. The band observed at 923 cm−1 and 782 cm−1 may be attributed to the out-of plane ring deformation and to the NeH vibrations in polymer [62,65–67]. These bands are also called as bipolaron bands [68,69]. In the Raman spectra of TiO2 and PPy-TiO2 (Supplementary
Fig. 3. SEM micrographs of a) PPy and b) PPy-TiO2. 4
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Fig. 4. EDS micrographs of PPy-TiO2.
roughness when compared with TiO2 (Ra = 192 nm and 56 nm respectively). Also, large surface electric gradients are observed in the PPy-TiO2 image obtained by EFM (Fig. 5d) so that electric properties measured macroscopically are the result of a large number of contributions from charged domains. Moreover, electric profiles are axially aligned with the surface grooves observed on the PPy-TiO2 topographic image. Finally, the linescans plotted from topography and electric images (Fig. 5e–f) revel and highlight that topography and electric gradients on TiO2 surface are much smoother than on PPy-TiO2 nanocomposite. It is worth mentioning that although TiO2 has a smoother topography, the surface (and the electric patterns) is more complex than that of the nanocomposite, which is evidenced by the fractal
composite. The optical properties of PPy-TiO2, investigated by UV–Vis spectroscopy, are shown in (Supplementary Material S4). The band gap energy of PPy-TiO2 cannot be estimated from the wavelength values. EFM micrographs from TiO2 and PPy-TiO2 composite are presented with standard noncontact images, in Fig. 5. The AFM topography of TiO2 (Fig. 5a) shows quasi-uniform packed particles with some small protrusions and smooth boundaries that is characteristic of TiO2 particles [74–76], while the EFM image (Fig. 5c) shows particle shells more positive than their cores, but not a significant surface electric gradient. On the other hand, the PPy-TiO2 composite presents different features on both topography (Fig. 5b) and electric mappings (Fig. 5d). First, the nanocomposite has a surface with aligned grooves and a higher 5
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Fig. 5. AFM topography (a,b) and the respective EFM (c,d) images for TiO2 and PPy-TiO2 composite. The linescans for topography and EFM are shown in (e) and (f), respectively. Fractal dimension (D) calculated using box-counting method and surface roughness (Ra parameter) are indicated in the images.
however when the TiO2 is added to PPy, the PL response decreased approximately 10 times. When the TiO2 and PPy are in contact it is possible that a transfer of photo-generated electron occurs from the TiO2 to PPy, avoiding the recombination of the photo-generated charge carries in TiO2. Similar findings were observed by Sangareswari [55] and Gao and coworkers [50].
dimensions (D) calculated for all the images. A complex topography and/or surface electric distribution as observed for TiO2 (D = 2.165 for AFM and 2.614 for EFM), must reflect a poorer macroscopic electric performance when compared with PPy-TiO2 composite (D = 2.105 for AFM and 2.308 for EFM), which has much more simple profiles from both topography and electric perspectives. To investigate more the surface between TiO2 and PPy, and the interactions electron/hole the photoluminescence (PL) spectra of samples have been taken (Supplementary Material S5). The TiO2 displayed a broad absorption band at 362 and a broad large absorption band between 377 and 392 nm when excited at 255 nm. The emission band in the present case is observed at 361–393 nm (~3.28–3.16 eV). Furthermore, it was found that the samples exhibit intense UV photoluminescence with emission maxima at 255 nm. These bands may be attributed to the release of trapped luminescent particles and oxygen vacancies [67]. The intensity of PL of TiO2 is around 100 (a.u.),
3.3. Photocatalytic activity The photocatalytic activity of PPy-TiO2 was evaluated monitoring DCF and 4-CP conversion. Fig. 6a show the conversion ratio of DCF with function of irradiation time. The DCF (black line) showed the photolysis, this value showed practically constant along of time, indicating that the DCF is a molecule quite resistant. TiO2 presented a significant conversion of the DCF in 60 min (red line). However, the PPy-TiO2 showed a much higher catalytic activity already in the initial 6
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Fig. 6. Conversion efficiency of (a) DCF and (c) 4-CP in the solar chamber using TiO2 or PPy-TiO2. Reuse of PPy-TiO2 in (b) DCF and (d) 4-CP conversion reactions.
minutes of the reaction (blue line). The nanoparticles have a high surface potential due to their large specific surface area, strong adsorption or chelation for pollutants molecules in waste water [77]. Furthermore, ease of remotion of solution, the composite (PPy-TiO2) may be separated from the waste water out and reused to achieve the purpose of purifying contaminated water, becoming one of the most technologically new wastewater treatment promising. The reuse efficiency of PPy-TiO2 was evaluated by five consecutive cycles, following the same parameters as those used in the photocatalytic activity tests for the DCF (Fig. 6b). The PPy-TiO2 showed to be efficient in DCF conversion for five cycles of 15 min each, with efficiency above 90% at the end of the 5th cycle. During the cycles there was a small reduction in the reaction rate, but this is not a significant reduction in the continuous efficiency of the PPy-TiO2. The photocatalytic efficiency of the PPy-TiO2 prepared in this work was also evaluated in the conversion of 4-CP (Fig. 6c), showing a similar behavior to that of DCF. PPy-TiO2 was more efficient in the conversion of 4-CP than bare TiO2. Reuse tests were also performed on 4-CP conversion (Fig. 6d). The efficiency of conversion of the 4-CP by the PPy-TiO2 after the 5th cycle was 35%, a value very close to the conversion in the first cycle (40%). Thus indicating the stability of the PPy-TiO2, and a minimum increase in the reaction rate over the cycles. An analysis of reaction kinetics shows that the rate of conversion of the pollutants in the presence of PPy-TiO2 is 3 times and 2 times higher than the conversion of DCF and 4-CP, respectively, in relation to the conversion rate only in the presence of TiO2 (Table 2). This fact can be attributed to the transfer of electrons from TiO2 to PPy, avoiding electron-hole recombination [55].
Table 2 Kinetic analysis of pollutants conversion by radiation (photolysis), TiO2 and PPy-TiO2 after 60 min in solar chamber. DCF
4-CP −1
k'app (min Photolysis TiO2 PPy-TiO2
0.001 0.012 0.034
)
R
k'app (min−1)
R2
0.997 0.996 0.999
0.002 0.004 0.008
0.984 0.994 0.999
2
Fig. 7. Effects of different scavengers for the 4-CP photoconversion with PPyTiO2.
7
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July 19.
The conversion reaction of 4-CP is slower than the conversion reaction of DCF (Table 2). Even though the DCF molecule is larger than the 4-CP molecule, it has bonds that can be broken more easily (-C-N-, –C-COOH, and –C-C-) [78] than the bonds directly attached to the benzene ring of 4-CP [2,7,79]. Fig. 7 shows the effect in kinetics reaction with addiction of EDTA, BQ and IPA scavengers for the 4-CP conversion, using the same conditions cited in 2.4 Section. The addition of all the scavengers reduced the reaction kinetics from 0.0164 to 0.0136, 0.0096 and 0.0004 min−1 respectively. This indicates that the PPy-TiO2 is capable of generating the radicals, hole (h+), superoxide (O2-•) and hydroxyl (•OH) responsible for the cleavage of the molecules of the pollutants 4-CP and DCF. When IPA was added, the reaction rate decreases hugely. This means that •OH radicals are the main responsible of 4-CP conversion. FTIR of PPy-TiO2 was realized after the photocatalytic tests reuse with DCF and a similar spectrum to composite before use was obtained, indicating that PPy-TiO2 is stable photochemistriely (Supporting Information S6). An increase in the band intensity relative to the OeH vibration of the adsorbed water was also observed.
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4. Conclusions This work contributing to a better understanding of the contact surface between the semiconductor metal oxide and the conducting polymer and how this interaction favors the photo catalytic degradation process of aqueous pollutants. The physical characteristics of the composite such as specific surface area, average pore size and pore volume are intermediate to the characteristics of the precursors alone. The addition of TiO2 to PPy modifies the morphology of the polymer, making it more organized and spherical. PPy-TiO2 composite showed different characteristics in topographic and electrical mappings, with a surface with aligned grooves and a higher roughness when compared to TiO2. The topography and electric gradients on TiO2 surface are much smoother than on PPy-TiO2 nanocomposite. When TiO2 and PPy are in contact, photo-generated electron transfer from TiO2 to PPy may occur, avoiding recombination of the photo-generated charge carries in TiO2. Thereat, the degradation efficiencies of the composite were evaluated and compared with that of the photocatalyst alone, where the composite showed extremely superior efficiency. Results showed that the PPyTiO2 was able to convert more than 90% of DCF (15 min) and 40% of 4CP (60 min), showing stability and efficiency over five consecutive cycles of reuse for both pollutants. Characterization techniques helped to understand the contribution of the conducting polymer to the photocatalysis mechanism, as well as the degradation of emerging pollutants such as the diclofenac medicine and the 4-chlorophenol pesticide. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of Competing Interest None. Acknowledgements This work was developed within the scope of the project CICECOAveiro Institute of Materials, FCT Ref. UID/CTM/50011/2019, financed by national funds through the FCT/MCTES. S. Silvestri has been thanked by Bolsista CAPES/BRASIL (FAPERGS N° 88887.195036/201800) (Brazilian Federal Agency for Support and Evaluation of Graduate Education) for the financial support. David Maria Tobaldi is grateful to Portuguese national funds (OE), through FCT, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of 8
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Polypyrrole-TiO2 composite for removal of 4-chlorophenol and diclofenac ⁎
Siara Silvestria, , Thiago A.L. Burgob,c, Celia Dias-Ferreirad,e,f, João A. Labrinchad, David Maria Tobaldid a
Graduate Program in Environmental Engineering, Federal University of Santa Maria, Santa Maria 97105–900, Brazil Department of Chemistry, Federal University of Santa Maria, Santa Maria 97105-900, Brazil Department of Physics, Federal University of Santa Maria, Santa Maria 97105-900, Brazil d Department of Materials and Ceramic Engineering, Aveiro Institute of Materials, University of Aveiro, Aveiro 3810-193, Portugal e Polytechnic Institute of Coimbra, Research Centre for Natural Resources, Environment and Society (CERNAS), Coimbra 3045-601, Portugal f Universidade Aberta, Lisboa, Portugal b c
A R T I C LE I N FO
A B S T R A C T
Keywords: PPy-TiO2 4-chlorophenol Diclofenac Photocatalysis
In this work, we successfully synthesized TiO2-polypyrrole (PPy) composite material via polymerization method using the sulfuric acid (H2SO4) as an oxidizing agent. Different characterization techniques, including electrostatic force microscopy, confirmed that TiO2 was intimately attached on PPy. The photocatalytic activity of PPyTiO2 composite was studied experimentally for the conversion of diclofenac (DCF)– one of the most frequently detected pharmaceutical compounds in the aquatic environment – and 4-chlorophenol (4-CP) - largely used in the bleaching of cellulose pulp in the paper industry, as herbicide and pesticide - under simulated solar light irradiation at environmental conditions. This is the first time that a composite PPy-TiO2 was used as photocatalyst for the conversion of 4-CP. Results showed that the PPy-TiO2 was able to convert more than 90% of DCF and 40% of 4-CP in just 60 min, showing stability and efficiency over five consecutive cycles of reuse for both pollutants. The produced PPy-TiO2 showed higher photocatalytic efficiency compared to that of TiO2, thus proving itself to be a promising photocatalyst.
1. Introduction 4-chlorophenol (4-CP) is an organochlorine widely used in the bleaching of cellulose pulp in the paper industry, as herbicide and pesticide [1–4]. Its residues can be leached by rainwater to the soil, water sources and groundwater [2,3,5]. Contamination by 4-CP affects fauna, flora and the human health, as it is irritating to the eyes, skin and respiratory system, and can have effects on the central nervous system [6,7]. Diclofenac (DCF - 2-[2-(2,6-dichloroanilino)phenyl]acetic acid) is a pharmaceutical compound extensively used as human and veterinary anti-inflammatory, is resistant and bio accumulative in the environment [8–10] It can be found in wastewater through urinary and animal carcass disposal, spreading along the ecosystem food chain [11,12]. This bioaccumulation may lead to even more danger to aquatic organisms. Its residues are also potential contaminants [13,14] and increase the risk of cardiovascular infarct heart attack or stroke [15]. Studies on hospital effluents always indicate the presence of DCF among the most abundant pharmaceutical compounds [13,16–18]. Advanced oxidation processes (AOPs) are an alternative technology
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to the treatment of several environmental pollutants [19,20]. Photocatalysis is highlighted as an efficient oxidative process for the conversion of pollutants such as dyes, pharmaceuticals, pesticides, herbicides and organic acids [21,22]. The great advantage of AOPs lies in the fact that the contaminant does not simply undergo a phase change, but rather is mineralized through a series of chemical reactions [23–25]. One of the most used photocatalysts is TiO2. TiO2 is a white powder, insoluble and easily dispersed in aqueous medium. The main disadvantage in the use of TiO2 is the great difficulty of removing it from the solution. To solve the problem of removal of the photocatalyst from the reaction medium, the adhesion of TiO2 particles to some substrates has been widely reported. Examples are TiO2 support on SiO2, activated carbon, glass, zeolites, biochar, polymers, etc. [26–35]. Polymer materials in the form of nanocomposites have certain advantages such as high surface area to volume ratio. Conductive polymers are a new group of synthetic polymers that combine the chemical and mechanical properties of the polymers as well as the adsorption and support with the electronic and photocatalytic properties of semiconductor oxides [36]. Nowadays, conducting polymers have several applications in different areas such as coatings for improved corrosion
Corresponding author. E-mail address: [email protected] (S. Silvestri).
https://doi.org/10.1016/j.reactfunctpolym.2019.104401 Received 28 July 2019; Received in revised form 25 October 2019; Accepted 26 October 2019 1381-5148/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Siara Silvestri, et al., Reactive and Functional Polymers, https://doi.org/10.1016/j.reactfunctpolym.2019.104401
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drying.
resistance of steel [37], as dopant to drug molecules [38], hydrogen storage [39], sensors [40–43], biosensors [44], electrochemical batteries [45], super capacitors [46,47], adsorbents [48], and supports [49,50]. They have extended p-conjugation with single- and doublebond alteration along its chain. They behave as a semiconductor material with low charge carrier mobility [51] and their conductivity is increased to reach the metallic range by doping with appropriate dopants [36]. Polypyrrole (PPy)-TiO2 is a composite classified as reticulate doped polymer, leading to specific properties in which a conducting crystalline network of charge transfer is formed building up the continuous conducting network in reticulate doped polymer, resulting in high conductivity [51]. According to Kryszewski [51], this property is not predicted by any theory of conductivity in heterogeneous systems. Simple percolation theory and fractal analysis of such systems cannot be applied because of the different ‘shape’ of the conducting element appearing at different stages of the network formation. In recent years, some studies have been published on the combination of PPy and TiO2. Sun and collaborators [52] prepared PPy-TiO2 composite by reverse microemulsion polymerization and Deng et al. [53] prepared PPy-TiO2 by surface molecular imprinting technique, and both researches used the composites as a photocatalyst for the degradation of methyl orange dye. PPy-TiO2 composites were also synthesized by polymerization of pyrrole with macro/mesoporous TiO2 in presence of dioctyl sulfosuccinate sodium salt with isooctane as solvent, as a photosensitizer and template by Li and collaborators [54]. Sangareswari and co-workers [55] used ammonium persulfate as an oxidant to prepare PPy-TiO2 and evaluated the photocatalytic activity by monitoring the degradation of methylene blue. Gao et al. [50] synthetized PPy/TiO2 using ferric chloride, tetrabutyl titanate, acetic acid, ethanol, followed of calcination at 500 °C. The composite was used to degrade Rhodamine B dye. However, degradation of dyes is not as challenging as the degradation of persistent organic compounds (such as some herbicides and pesticides), and therefore degradation studies using dyes, although useful to some extent, do not provide a realistic assessment of the degradation potential of PPy-TiO2 photocatalysts. In this work, PPy was used as support to TiO2 in order to facilitate the transfer of charges, to prevent electron-hole recombination, and to be easily removed from the solution after the photocatalytic reaction. The PPy-TiO2 was synthetized using only water, pyrrole, TiO2, hydrogen peroxide and sulfuric acid under magnetic stirring. The photocatalytic activity of the produced composite was evaluated by monitoring the degradation of pesticide 4-CP and pharmaceutical compound DCF, being reported for the first time in the literature through the present research. Furthermore, the electronic property of the composite was elucidated by electrostatic force microscopy (EFM).
2.3. Characterization of materials X-ray diffraction (XRD) (Rigaku Miniflex 300 diffractometer) was used to identify the mineralogical phases of the prepared materials. The surface areas and average pore diameters were calculated from N2 adsorption-desorption isotherms (Micromeritics Gemini 2380, US). Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer FT-IR Spectrum spectrometer. The band gap energy of the samples was determined by means of the Tauc method [57,58], using the diffuse reflectance spectra (UV-2600 Plus, Shimadzu). Raman spectra were recorded using a RFS 100/S (Bruker, DE) equipped with a 1064 nm Nd:YAG laser as the excitation source. The scanning electron microscopy (SEM-EDS) (Tescan-VEGA3) was used to observe microstructural features. Atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were performed simultaneously in a Park NX10 (Park Systems, Suwon – Korea) instrument equipped with a SmartScan software version 1.0. RTM 11a. Samples were diluted in water and dropped into square small pieces of fresh cleaved mica and imaged in non-contact mode. The measurements were conducted using a PPP-EFM (Nanosensors, Neuchâtel – Switzerland) Si probe PtIr coated, with a nominal resonance frequency of 75 kHz and 2.8 N m−1 force constant. All measurements were made under ambient conditions at room temperature of 27 ± 5 °C and a relative humidity of 60 ± 10% with a scanning rate of 0.4 Hz with a 2 V bias applied to the sample holder. Images were treated offline using XEI software version 4.3.4 Build 22. RTM 1. Also, XEI software was used to calculate average roughness (Ra) and fractal dimension (D). The photoluminescence measurements were done on a Cary Eclipse fluorescence spectrophotometer (Varian) at room temperature. The fluorescence spectra were recorded on a sample solid state on a with excitation and emissions slits of 5 nm. Sample excitation was performed at 255 nm at room temperature, and the emission was scanned between 300 and 500 nm. 1 H NMR spectra were recorded on a BRUKER AVANCE III HD Spectrometer at the frequency of 600.13 MHz. Data were obtained in 5 mm tubes, temperature of 70 °C, concentration of approximately 0.01 M in dimethylsulfoxide deuterated (DMSO‑d6) as solvent using tetramethylsilane (TMS) as an internal reference. 2.4. Photocatalytic activity In order to evaluate the photocatalytic activity of PPy-TiO2 in the presence of 4-CP and DCF pollutants, 1 g L−1 of photocatalyst (PPyTiO2) was added to 100 mL aqueous solution of pollutant (10 mg L−1), these quantities were previously studied and based on the research [59]. A solar chamber equipped with Xenon lamp (600 W m−2) (Suntest CPS Atlas MTS) was used as reacting system. Samples were kept under constant magnetic stirring, and simulated ambient conditions were maintained in the solar chamber during the photocatalytic experiments – i.e. relative humidity between 40 and 42%, temperature between 22 and 24 °C. Aliquots (3 mL) were collected periodically with a syringe and centrifuged to remove the photocatalyst. The supernatant was analyzed by UV–Vis spectroscopy (T80+ UV/VIS Spectrometer PG instruments) at a wavelength of 226 nm for 4-CP and 274 nm for DCF. The tests were performed in triplicate for each pollutant. To test the reusability of the PPy-TiO2, cyclic experiments of 4-CP and DCF photo conversion were conducted. After each cycle, the photocatalyst was recovered and washed several times with deionized water for the subsequent cycle. In total, 5 cycles were carried out for each pollutant. The radical species generated by PPy-TiO2 were evaluated by the addition of scavengers of •OH, O2-• and h+, such as isopropyl alcohol
2. Experimental 2.1. Materials Pyrrole monomer (98%), TiO2 (P25-Evonik Aeroxide - 76.3 wt% anatase, 10.6 wt% rutile, and 13.0 wt% amorphous phase, with average crystalline size between 15.5 nm and 19.3 nm [56]), DCF and 4-CP were purchased from Sigma-Aldrich. H2SO4 (Synth), H2O2 (50% - 200 vol – Êxodo Científica). Deionized water was used for all the experiments. All these reagents were of analytical grade and used without further purification. 2.2. Preparation of PPy-TiO2 For preparing the PPy-TiO2, 7 mg TiO2 was dispersed into 80 mL of 0.05 M pyrrole aqueous solution and 20 mL H2SO4 0.2 M, and 0.05 mL H2O2. This mixture was magnetically stirred until it became a dark solution. The solution was then filtered and washed with deionized water twice and the precipitate was kept in a desiccator until complete 2
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are generated by oxidation of pyrrole monomer. Secondly by radicalradical coupling and deprotonation between adjacent radical cations, dimers are generated leading to a bipyrrole. By reoxidization and coupling with other radical cations, bipyrroles are connected with each other and the propagation step is repeated consecutively. The nucleophilic attacks of water molecules or impurities in the polymer chain appear stops the chemical polymerization process. The TiO2 does not initially interact with the pyrrole. This fact was proved via NMR analysis, (Supplementary Material S2). The 1H NMR spectrum shows the signal at δ 3.17 being neglected due to solvent water and the δ 2.50 signal of DMSO solvent. The Py and Py-TiO2 spectrum shows a double doublet δ 6.74–6.01 each of which has a proportional area for two hydrogens, suggesting that are attached to pyrrole ring carbons; the signal δ 10.74 is attributed to the hetero nitrogen bonded to hydrogen [39]. The PPy and PPy-TiO2 spectrum shows a triplet from δ 7.12–6.95 are characteristic of hydrogens of the pyrrole ring [40]. The peaks related to the couplings of the hydrogens of the pyrrole ring remain in the same position and with the same intensity when they are in the presence of TiO2. Notably, when the polymer is formed in TiO2 presence the chemical shift difference between the peaks was considerably larger than the splitting due to stereochemical configuration. This is due to the number of intramolecular hydrogen bonds between the neighbouring oxygen groups, referents to TiO2 [60,61].
(IPA), benzoquinone (BQ), and ethylenediamine tetra-acetic acid (EDTA) to the 4-CP solution. An amount of 10−4 mol of each scavenger was added to each 50 mL 4-CP solution (10 mg L−1) with 0.5 g L−1 of PPy-TiO2. All solutions were kept under magnetic stirring inside the solar chamber during irradiation incidence. The aliquots collected were centrifuged and analyzed by UV–Vis spectrometer. 3. Results and discussion 3.1. Preparation of PPy-TiO2 In order to evaluate the influence of each reactant on the synthesis of PPy-TiO2, the reaction followed as described in 2.2 Section above. In Supplementary Material Fig. S1a, the reaction occurred only in the presence of water, pyrrole and TiO2. In Fig. S1b, the reaction b is similar as reaction a, plus addition of hydrogen peroxide. In Fig. S1c, the reaction c occurred in the presence of water, pyrrole, TiO2, plus addition of sulfuric acid. Finally, in the Fig. S1d the reaction d occurred in the presence of water, pyrrole, TiO2, hydrogen peroxide and sulfuric acid. When the coloration of the solutions is observed over time, it is possible to predict that the pyrrole polymerizes in acid medium (Fig. S1c–d) and the polymerization reaction is complete when the dark color is obtained (Fig. S1d) occurs in 4 h in the presence of hydrogen peroxide. Hydrogen peroxide, in the presence of water, pyrrole and TiO2, is not able to polymerize monomer (Fig. S1b), but in acidic medium (Fig. S1d) the reaction is complete. The chemical oxidation polymerization processes with the TiO2 nanoparticles are proposed in Fig. 1. Firstly, radical cations (C4NH5+)
3.2. Characterization of the synthesized photocatalytic PPy-TiO2 Fig. 2a shows the XRD patterns of the PPy, TiO2, and PPy-TiO2. The diffractogram of the PPy does not exhibit any reflection peaks, so it can
Fig. 1. Mechanism of the polymerization of pyrrole to forming polypyrrole in presence of TiO2. 3
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Fig. 2. a) XRD patterns and b) FTIR spectra of PPy, TiO2 and composite PPy-TiO2 samples.
Material S3), the peaks at 393, 511 and 633 cm−1 are attributed to TieO vibration modes of anatase TiO2 and peak at 447 cm−1 is attributed to rutile phase [70,71]. The peaks at 1367 and 1584 cm−1 are assigned to the CeN and C]C backbone stretching and ring stretching of PPy respectively [54,69,72]. The bare TiO2 is characterized by a strong and sharp band at 142 cm−1, which is assigned to the bending type vibration of v6 (Eg) mode [54]. Characteristic peaks of TiO2 can be observed, though when TiO2 interacts with PPy, the characteristic peaks of TiO2 lose intensity and only the peak at 936 cm−1 can be observed in both spectra. Table 1 shown the physical characteristics of precursors and of PPyTiO2. Higher values than PPy for specific surface area, average pore size and pore volume were obtained when TiO2 was added to the polymer. All the samples are mesoporous. Fig. 3 shows the SEM images of polymerized polymer (PPy) and the polymerized polymer with added TiO2 (PPy-TiO2). PPy showed the amorphous surface, lacking of a well-defined shape with layers, typical of PPy, as reported in [50,66,68,73]. When TiO2 was added in PPy, the resulting material seemed somehow more “organized”, showing smaller, more dispersed particles tending to spherical form. The addition of TiO2 on PPy could contribute to the higher surface area of PPy, consistent with the results listed in Table 1. SEM equipped with an energy dispersive spectrometer (EDS) was applied to observe the microstructures of PPy-TiO2 (Fig. 4). As shown in EDS elemental mapping, the main feature elements of C and N of PPy, and O and Ti of photocatalyst oxide were detected in PPy-TiO2
Table 1 Surface area and pore volume analysis of PPy, PPy-TiO2 and TiO2. Sample name
Surface area (m2 g−1)
Pore volume (cm3 g−1)
Pore Size (Å)
PPy TiO2 PPy-TiO2
35.9 52.1 42.9
0.05 0.34 0.10
128 31 99
be identified as an (XRD) amorphous material. The characteristic TiO2 reflections were found in the PPy-TiO2 diffractogram. This shows that the two precursor materials are present in the PPy-TiO2 sample. In Fig. 2b it is possible to observe that the intensity of the characteristic PPy peaks is reduced in the PPy-TiO2, as well as the Ti-O-Ti (490–660 cm−1) vibration peaks compared to bare PPy and TiO2. The interaction amid PPy and TiO2 occurs between Ti and N in the pyrrole during the polymerization process. The spectrum of PPy in Fig. 2b confirmed the formation of PPy. The band at 1565 and a weak band at 1459 cm−1 are assigned to stretching vibration of C]C and CeC in the pyrrole ring [62,63]. The absorption at 1283 cm−1 corresponds to CeH in plane deformation modes [62]. PPy shows characteristic CeN and CeH stretching vibration of pyrrole at 1205 and 1043 cm−1 respectively in the infrared spectrum [64–66]. The band observed at 923 cm−1 and 782 cm−1 may be attributed to the out-of plane ring deformation and to the NeH vibrations in polymer [62,65–67]. These bands are also called as bipolaron bands [68,69]. In the Raman spectra of TiO2 and PPy-TiO2 (Supplementary
Fig. 3. SEM micrographs of a) PPy and b) PPy-TiO2. 4
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Fig. 4. EDS micrographs of PPy-TiO2.
roughness when compared with TiO2 (Ra = 192 nm and 56 nm respectively). Also, large surface electric gradients are observed in the PPy-TiO2 image obtained by EFM (Fig. 5d) so that electric properties measured macroscopically are the result of a large number of contributions from charged domains. Moreover, electric profiles are axially aligned with the surface grooves observed on the PPy-TiO2 topographic image. Finally, the linescans plotted from topography and electric images (Fig. 5e–f) revel and highlight that topography and electric gradients on TiO2 surface are much smoother than on PPy-TiO2 nanocomposite. It is worth mentioning that although TiO2 has a smoother topography, the surface (and the electric patterns) is more complex than that of the nanocomposite, which is evidenced by the fractal
composite. The optical properties of PPy-TiO2, investigated by UV–Vis spectroscopy, are shown in (Supplementary Material S4). The band gap energy of PPy-TiO2 cannot be estimated from the wavelength values. EFM micrographs from TiO2 and PPy-TiO2 composite are presented with standard noncontact images, in Fig. 5. The AFM topography of TiO2 (Fig. 5a) shows quasi-uniform packed particles with some small protrusions and smooth boundaries that is characteristic of TiO2 particles [74–76], while the EFM image (Fig. 5c) shows particle shells more positive than their cores, but not a significant surface electric gradient. On the other hand, the PPy-TiO2 composite presents different features on both topography (Fig. 5b) and electric mappings (Fig. 5d). First, the nanocomposite has a surface with aligned grooves and a higher 5
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Fig. 5. AFM topography (a,b) and the respective EFM (c,d) images for TiO2 and PPy-TiO2 composite. The linescans for topography and EFM are shown in (e) and (f), respectively. Fractal dimension (D) calculated using box-counting method and surface roughness (Ra parameter) are indicated in the images.
however when the TiO2 is added to PPy, the PL response decreased approximately 10 times. When the TiO2 and PPy are in contact it is possible that a transfer of photo-generated electron occurs from the TiO2 to PPy, avoiding the recombination of the photo-generated charge carries in TiO2. Similar findings were observed by Sangareswari [55] and Gao and coworkers [50].
dimensions (D) calculated for all the images. A complex topography and/or surface electric distribution as observed for TiO2 (D = 2.165 for AFM and 2.614 for EFM), must reflect a poorer macroscopic electric performance when compared with PPy-TiO2 composite (D = 2.105 for AFM and 2.308 for EFM), which has much more simple profiles from both topography and electric perspectives. To investigate more the surface between TiO2 and PPy, and the interactions electron/hole the photoluminescence (PL) spectra of samples have been taken (Supplementary Material S5). The TiO2 displayed a broad absorption band at 362 and a broad large absorption band between 377 and 392 nm when excited at 255 nm. The emission band in the present case is observed at 361–393 nm (~3.28–3.16 eV). Furthermore, it was found that the samples exhibit intense UV photoluminescence with emission maxima at 255 nm. These bands may be attributed to the release of trapped luminescent particles and oxygen vacancies [67]. The intensity of PL of TiO2 is around 100 (a.u.),
3.3. Photocatalytic activity The photocatalytic activity of PPy-TiO2 was evaluated monitoring DCF and 4-CP conversion. Fig. 6a show the conversion ratio of DCF with function of irradiation time. The DCF (black line) showed the photolysis, this value showed practically constant along of time, indicating that the DCF is a molecule quite resistant. TiO2 presented a significant conversion of the DCF in 60 min (red line). However, the PPy-TiO2 showed a much higher catalytic activity already in the initial 6
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Fig. 6. Conversion efficiency of (a) DCF and (c) 4-CP in the solar chamber using TiO2 or PPy-TiO2. Reuse of PPy-TiO2 in (b) DCF and (d) 4-CP conversion reactions.
minutes of the reaction (blue line). The nanoparticles have a high surface potential due to their large specific surface area, strong adsorption or chelation for pollutants molecules in waste water [77]. Furthermore, ease of remotion of solution, the composite (PPy-TiO2) may be separated from the waste water out and reused to achieve the purpose of purifying contaminated water, becoming one of the most technologically new wastewater treatment promising. The reuse efficiency of PPy-TiO2 was evaluated by five consecutive cycles, following the same parameters as those used in the photocatalytic activity tests for the DCF (Fig. 6b). The PPy-TiO2 showed to be efficient in DCF conversion for five cycles of 15 min each, with efficiency above 90% at the end of the 5th cycle. During the cycles there was a small reduction in the reaction rate, but this is not a significant reduction in the continuous efficiency of the PPy-TiO2. The photocatalytic efficiency of the PPy-TiO2 prepared in this work was also evaluated in the conversion of 4-CP (Fig. 6c), showing a similar behavior to that of DCF. PPy-TiO2 was more efficient in the conversion of 4-CP than bare TiO2. Reuse tests were also performed on 4-CP conversion (Fig. 6d). The efficiency of conversion of the 4-CP by the PPy-TiO2 after the 5th cycle was 35%, a value very close to the conversion in the first cycle (40%). Thus indicating the stability of the PPy-TiO2, and a minimum increase in the reaction rate over the cycles. An analysis of reaction kinetics shows that the rate of conversion of the pollutants in the presence of PPy-TiO2 is 3 times and 2 times higher than the conversion of DCF and 4-CP, respectively, in relation to the conversion rate only in the presence of TiO2 (Table 2). This fact can be attributed to the transfer of electrons from TiO2 to PPy, avoiding electron-hole recombination [55].
Table 2 Kinetic analysis of pollutants conversion by radiation (photolysis), TiO2 and PPy-TiO2 after 60 min in solar chamber. DCF
4-CP −1
k'app (min Photolysis TiO2 PPy-TiO2
0.001 0.012 0.034
)
R
k'app (min−1)
R2
0.997 0.996 0.999
0.002 0.004 0.008
0.984 0.994 0.999
2
Fig. 7. Effects of different scavengers for the 4-CP photoconversion with PPyTiO2.
7
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July 19.
The conversion reaction of 4-CP is slower than the conversion reaction of DCF (Table 2). Even though the DCF molecule is larger than the 4-CP molecule, it has bonds that can be broken more easily (-C-N-, –C-COOH, and –C-C-) [78] than the bonds directly attached to the benzene ring of 4-CP [2,7,79]. Fig. 7 shows the effect in kinetics reaction with addiction of EDTA, BQ and IPA scavengers for the 4-CP conversion, using the same conditions cited in 2.4 Section. The addition of all the scavengers reduced the reaction kinetics from 0.0164 to 0.0136, 0.0096 and 0.0004 min−1 respectively. This indicates that the PPy-TiO2 is capable of generating the radicals, hole (h+), superoxide (O2-•) and hydroxyl (•OH) responsible for the cleavage of the molecules of the pollutants 4-CP and DCF. When IPA was added, the reaction rate decreases hugely. This means that •OH radicals are the main responsible of 4-CP conversion. FTIR of PPy-TiO2 was realized after the photocatalytic tests reuse with DCF and a similar spectrum to composite before use was obtained, indicating that PPy-TiO2 is stable photochemistriely (Supporting Information S6). An increase in the band intensity relative to the OeH vibration of the adsorbed water was also observed.
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4. Conclusions This work contributing to a better understanding of the contact surface between the semiconductor metal oxide and the conducting polymer and how this interaction favors the photo catalytic degradation process of aqueous pollutants. The physical characteristics of the composite such as specific surface area, average pore size and pore volume are intermediate to the characteristics of the precursors alone. The addition of TiO2 to PPy modifies the morphology of the polymer, making it more organized and spherical. PPy-TiO2 composite showed different characteristics in topographic and electrical mappings, with a surface with aligned grooves and a higher roughness when compared to TiO2. The topography and electric gradients on TiO2 surface are much smoother than on PPy-TiO2 nanocomposite. When TiO2 and PPy are in contact, photo-generated electron transfer from TiO2 to PPy may occur, avoiding recombination of the photo-generated charge carries in TiO2. Thereat, the degradation efficiencies of the composite were evaluated and compared with that of the photocatalyst alone, where the composite showed extremely superior efficiency. Results showed that the PPyTiO2 was able to convert more than 90% of DCF (15 min) and 40% of 4CP (60 min), showing stability and efficiency over five consecutive cycles of reuse for both pollutants. Characterization techniques helped to understand the contribution of the conducting polymer to the photocatalysis mechanism, as well as the degradation of emerging pollutants such as the diclofenac medicine and the 4-chlorophenol pesticide. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of Competing Interest None. Acknowledgements This work was developed within the scope of the project CICECOAveiro Institute of Materials, FCT Ref. UID/CTM/50011/2019, financed by national funds through the FCT/MCTES. S. Silvestri has been thanked by Bolsista CAPES/BRASIL (FAPERGS N° 88887.195036/201800) (Brazilian Federal Agency for Support and Evaluation of Graduate Education) for the financial support. David Maria Tobaldi is grateful to Portuguese national funds (OE), through FCT, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of 8
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