Design and synthesis of polyvinyl alcohol derived conjugated polyene modified TiO2 for visible-light degradation of chloramphenicol

Journal Pre-proof Design and synthesis of polyvinyl alcohol derived conjugated polyene modified TiO2 for visible-light degradation of chloramphenicol Olayinka S. Awofiranye, Sekomeng J. Modise, Eliazer B. Naidoo

PII:

S2213-3437(20)30103-2

DOI:

https://doi.org/10.1016/j.jece.2020.103755

Reference:

JECE 103755

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

4 November 2019

Revised Date:

2 February 2020

Accepted Date:

5 February 2020

Please cite this article as: Awofiranye OS, Modise SJ, Naidoo EB, Design and synthesis of polyvinyl alcohol derived conjugated polyene modified TiO2 for visible-light degradation of chloramphenicol, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103755

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Design and synthesis of polyvinyl alcohol derived conjugated polyene modified TiO2 for visible-light degradation of chloramphenicol Olayinka S. Awofiranye, Sekomeng J. Modise*, Eliazer B. Naidoo. Chemistry Department, Faculty of Applied and Computer Sciences, Vaal University of Technology.

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Vanderbijlpark Campus, Andries Potgieter, Boulevard, 1900 Vanderbijlpark, South Africa.

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*Corresponding author: Sekomeng J.Modise; e-mail: [email protected]

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Graphic abstract

Highlight    

Modification of TiO2 with conjugated polyene enhanced its optical property. Incorporation of conjugated polyene reduced the electron recombination rate of TiO2 CPE-TiO2 presents a lower band-gap, and better visible light absorption compares with pure TiO2. CPE-TiO2 exhibits a better photocatalytic activity for the degradation of chloramphenicol.

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Abstract Visible-light active conjugated polyene modified TiO2 (CPE-TiO2) nanocomposite was prepared by a facile homogenized sol-gel procedure. After synthesis, the materials were

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appropriately characterized and used in visible light photodegradation of chloramphenicol (CAP)

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using LED-light as a source of irradiation. The results indicated that the UV-vis DRS spectra of the CPE-TiO2 with a lower band-gap showed a better absorption ability in the wavelength range

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of 400-800 nm. This result was further confirmed by photoluminescence analysis, which indicated a less recombination rate of electron/hole pairs. Notably, CPE-TiO2 nanocomposite exhibited

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higher photocatalytic properties as compared to pure TiO2 under visible light. For the

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photocatalytic degradation of CAP using CPE-TiO2, the results showed a significant improvement in degradation efficiency (80.47%) as compared to pure TiO2 (36.12%) at the optimum pH of 7

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and most of this antibiotic was degraded within 210 min of visible light irradiation. Co-existence of multiple bonds in poly-conjugated carbon chain with a reduced band gap in CPE-TiO2 composite was able to enhance charge separation and migration as well as improve photocatalytic efficiency.

words:

Conjugated-polyene,

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Key

Visible-light,

Chloramphenicol

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Titanium-dioxide,

Photodegradation,

1.

Introduction Titanium dioxide (TiO2) is a suitable and highly efficient catalyst and extensively studied

in heterogeneous photocatalysis under UV irradiation [1,2,3]. The disadvantage of UV light is the expense and high energy demand for large scale applications [4]. Among other semiconductor photocatalysts, TiO2 is preferred due to its strong photocatalytic ability, high photo-stability, and

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photocatalytic relevance of TiO2 has some limitations [10,11,12].

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cost-effectiveness [5,6,7,8], as well as its stability in an aqueous medium [9]. However, the

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The problem of a relatively wide band-gap (3.2 eV) of TiO2 results in its inefficiency under visible light which accounts for about 46% of solar radiation that reaches the earth surfaces [13].

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As reported, TiO2 exhibits low absorption of about 7% of visible light, which reduces its application as an efficient photocatalyst in wastewater treatment under solar irradiation [14].

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Consequently, there are several efforts in the area of TiO2 modifications to reduce its band-gap,

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prevent the recombination of electron-hole pairs, and enhance its visible light activity [15,16]. Many studies on TiO2 modifications focus on different techniques such as doping with metals, non-metals, and functionalizing with other molecules [17,18,19]. Doping with transition metals usually generate a discrete energy level, which often results in low mobility of electrons

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and holes in the dopant energy levels and thus limits the activity enhancement of the catalyst [20]. On the other hand, the effective activity of non-metal doped TiO2 mainly depends on some other factors such as dopant concentration, dopant energy level, dopant distribution and the oxygen vacancy on the lattice surface [21]. The absolute dependence on these factors tends to limit the efficiency of TiO2 doped with non-metal elements.

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Lately, functionalizing TiO2 with other materials, particularly organic substances have significantly improved its photocatalytic efficiency under visible light [22]. The organic sensitizer application can lengthen the spectra response of TiO2 to the visible-light region [22]. Consequently, the application of dye molecules as organic sensitizers has significantly improved photocatalytic efficiency of TiO2 in the visible region of the light spectrum [23]. However, most

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dyes are found to be highly hazardous in the environment [15]. Very recently, conducting polymers, mostly conjugated polymers (CP), have attracted the

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keen interest of scientific researchers due to their electric and optical activities, chemical stabilities,

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and high absorption coefficient within the solar spectrum [22]. Conducting polymers blended well with a range of wide band-gap inorganic semiconductors (such as TiO2), and they are reported to

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possess rapid charge separation capacity [24]. The effective photocatalytic performance of CP depends on rapid generation and transfer of photoelectrons to the conduction band of the metal

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oxide [25]. The combination of a p-type conducting polymer with an n-type semiconductor helps to overcome the problem of inadequate response to visible light, and high rate of electron-hole

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recombination [25]. CPs have an extended p-conjugated electron system, which possesses high stability and relatively high carrier mobility [26]. It was suggested that such polymer could serve as an active electron donor and stable photosensitizer, and the integration of ᴨ-conjugation with

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semiconductor could improve its photocatalytic activity [24]. Such catalyst could be effective in the photo degradation of antibiotics. Antibiotics are toxic and non-biodegradable, even at low concentration due to their ability

to damage genomes and consequently causing genetic mutation [27,28]. Chloramphenicol (CAP) an antibiotic drug has been reportedly detected at a significant concentration level of 28 ngL-1 in urban water supplies [26,29]. Removal of CAP from wastewater through treatment plants is 4

inefficient [29]. Moreover, the antibacterial property of CAP limits its removal through biological treatment processes [26]. However, photocatalytic degradation of CAP presents a good option considering the broad range applicability of this technique. Related research [30,31,32,33,34,35], has shown that the photocatalytic degradation efficiency and mineralization of CAP under visible light needs further improvement. On the other hand, the application of light-emitting diode (LED) sources in photo-reactors has increasingly attracted keen attention because they generate minimal

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heat and exhibit good linearity of emitted light intensity [35,36]. Consequently, they are used for

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a wide range of applications, including the photocatalytic degradation of environmental pollutants

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[35,37].

On the other hand, conjugated polyenes (CPEs) are poly-unsaturated organic compounds

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that contain alternating double and single bonds. The conjugal interaction of the double bonds in conjugated polymers results in some unique optical light absorptive properties that are useful as

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visible light active photocatalyst [38]. Furthermore, hybrid structures of conjugated polymers with metal oxide semiconductors have been studied for visible-light active photocatalysis [39]. To the

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best of our knowledge, the potential of conjugated polyene obtained from acid dehydration of polyvinyl alcohol has not been explored in enhancing the photocatalytic efficiency of TiO2. In this study, we synthesized and explored the functionalized modification of TiO2 with a

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conjugated polyene (CPE) for enhanced optical properties and photocatalytic activity under LEDvisible light source. The CPE was generated by acid dehydration of PVA and was used in the synthesis of CPE-TiO2 composite. The effects of CPE addition to TiO2 precursor solution on functionality, morphology, crystal structure, and optical properties of the synthesized materials were determined through appropriate characterization. Finally, the photocatalytic activity of the

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catalysts was carefully investigated under LED irradiation by the degradation-monitoring of CAP in an aqueous solution. 2. Materials and methods 2.1 Materials All chemicals used in the study are of analytical grade, purchased from Sigma Aldrich

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chemical company South Africa and applied without further purifications. These include

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chloramphenicol (>99% purity), 99% hydrolyzed polyvinyl chloride (PVA), 98% Methanol (CH3OH), Titanium (IV) Isopropoxide (97%), and hydrochloric acid (32%). All reagents were

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prepared in solutions using Milli-Q water from a Millipore Milli-Q Ultrapure Gradient A10

2.2.

Experimental procedures

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2.2.1. Synthesis of CPE from PVA

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purification system.

A modified method reported by Tretinnikov and Sushko [31] was adopted. A 0.6% (g/v)

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solution of PVA was prepared by dissolving exactly 0.6 g of PVA in 100 mL of Ultrapure water. To this 0.6% g/v aqueous solution of PVA, a 2% solution of hydrochloric acid was added in the amount needed to obtain PVA (polymer unit) to HCl ratio of 50:1 (2 mol% HCl). The solution was

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heated on a magnetic hot plate at 110oC for 45 min for thermal dehydration to obtain linear conjugated polyene (CPE) molecule. 2.2.2. Homogenized sol-gel synthesis of pure TiO2 Titanium nanoparticles were synthesized using a facile homogenized sol-gel method. Titanium-isopropoxide was used as a precursor organic molecule for the synthesis of TiO2 nanoparticles. This precursor was mixed with methanol in the volume ratio 3:2 in a 250 mL beaker. 6

Exactly 10 mL of ultrapure water was introduced dropwise into the solution while heating at 70oC, under homogenizer with stirrer working at 224 xg for 20 min. The resulting solution was allowed to seed for 30 min before taken for centrifugation at 448 xg for 10 min. The wet particles were then removed and dried in an oven at 100oC for 120 min and calcined at 350oC for 120 min. 2.2.3. Synthesis of CPE-TiO2 composite photocatalyst

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The homogenized sol-gel synthesis procedure used for the preparation of pure TiO2 was

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repeated for CPE-TiO2. But, 10 mL of the dehydrated solution of PVA derived conjugated polyene (CPE) was added dropwise during the sol-gel synthesis instead of pure distilled water to form the

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CPE-TiO2 composite nanoparticles. After drying in the oven at 100oC for 120 min, the sample was

2.2.4. Characterization of the materials

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this time calcined at 250oC for 120 min to avoid the breakdown of the polymer molecules.

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The crystalline nature of the as-prepared samples of TiO2 nanoparticles and CPE-TiO2 nanocomposites were obtained, using X-ray diffraction (XRD). The XRD patterns were recorded

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in the 2θ range of 5-90o with a scan rate of 0.02o s-1 using a Bruker-D8-AXS diffractometer system equipped with a Cu Ka radiation (λ = 0.15406 A) (Bruker Co., Berlin, Germany). Fourier transform infrared (FT-IR) spectra were recorded using an FTIR analyzer (Perkin-Elmer, Spectrum 400),

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where KBr served as the reference sample. The morphologies of the obtained products were inspected by a JEM-2100 Plus transmission electron microscopy (TEM) operating at 10 kV. The optical absorption properties of the samples were assessed by an ultraviolet and visible (UV-Vis DRS) spectrophotometer (Lambda 17, Perkin-Elmer) and UV-Vis spectrophotometer (UV- 1700, SHIMADU). The photoluminescence (PL) spectra of photocatalysts were recorded using a

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Fluorescence Spectrophotometer (FP-6500, Tokyo Japan) equipped with a Xenon lamp using an excitation wavelength of 240 nm with an emission wavelength range from 220 and to 800 nm.

2.2.5. Photocatalytic degradation of CAP The photocatalytic oxidation of 25 mgL−1 CAP was carried out under visible light

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irradiation set up, equipped with 72 W LED lamp (rated voltage was DC 12 V, λ >400 nm) with a lampshade, beaker reaction vessel and magnetic stirrer. Batch experiments were carried out using

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400 mL glass beaker at room temperature (25 ± 1°C). Prior to each test, the lamp was turned on

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and warmed up to for 10 min to establish a constant light output. Batch tests were performed as follows: 15 mg of (the catalysts) was added to 200 mL solution of chloramphenicol (CAP) at the

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concentration level of 25 mgL-1. Prior to irradiation the mixture was magnetically stirred for 40 min in the dark to allow for adsorption equilibrium of CAP on the catalyst surfaces. The solution

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was subsequently exposed to visible light under magnetic stirring at 50.4 xg, which marks the starting point of the photodegradation test. An air diffuser pump was connected to the reactor to

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uniformly disperse air into the solution mixture, with a low rate of 0.003 m3min-1. At regular intervals, 3 mL samples were withdrawn, filtered using an Acrodisc Premium 25 mm Syringe (APS) filter with GxF/0.45 µm GHP Membrane, to separate the photocatalyst suspensions. The

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filtrate was then taken for further analysis. The concentration of CAP in the filtrate was determined by measuring the absorption intensity at its predetermined maximum absorbance wavelength of 275 nm using a UV-Vis spectrophotometer (UV- 1700, SHIMADU) with a 1 cm path length spectrometric quartz cell. The corresponding concentration of the measured absorbance was determined from the calibration curve of 0.99945 (R2 value). The concentration of total organic carbons (TOC) in the solution was measured using 8

a TOC analyzer (Shimadzu TOC-VCSH). The degradation efficiency of CAP contained in the aqueous solution was determined according to equation 1:

Degradation Efficiency (%) = C0 - Ct / Ct Χ 100

(1)

Where C0 was the initial concentration of chloramphenicol before the degradation experiment, and

Results and discussion

3.1

Thermal dehydration of PVA to produce CPE

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3.

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Ct was the concentration of chloramphenicol at certain degradation time t (min) during irradiation.

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The very first stage of thermal degradation of polyvinyl alcohol (PVA) involves the cleavage of the hydroxyl group and hydrogen atoms from the polymer in an elimination kind of

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reaction. This elimination process results in the formation of polyene and water [40]. Fig. 1(a) presents the UV-vis absorption spectra of PVA and CPE over a visible range of

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900 nm to 350 nm. It was observed that PVA has no absorption peaks in the visible region while the prepared CPE showed intense peaks, particularly at 393, 410, and 490 nm. These visible absorption peaks indicate the formation of a new chromophore (C=C) which is expected in the polyene molecule. Linear polyene with more than or equals to eight number of conjugated double

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bonds (C=C ≥ 8) is characterized by intense electronic absorption bands (EAB) that lie in the visible region of light spectrum [40]. The FTIR spectra of PVA and CPE, as shown in Fig. (1b) also presents different % transmittance at varying wavelengths for the materials. The CPE spectral clearly showed a sharp peak between 1600 cm-1 and 1640 cm-1 for C=C stretching vibration, which

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is indicative of polyene group presence [41]. Strong broad peaks can be observed at 3293 cm-1 on both PVA and CPE, attributed to O-H stretching. Indicating the presence of hydroxyl group.

PVA CPE

a

Absorbance

8

6

4

2

393 410

b

100

490

80

1685 1629

60

40

3293 20

0 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

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Wavelength (nm)

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0 350 400 450 500 550 600 650 700 750 800 850 900

PVA CPE

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360

Transmitance (%)

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Fig 1: (a) The UV-vis electronic absorption and (b) FTIR spectrum of conjugated polyene (CPE) and

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polyvinyl alcohol (PVA)

The linear polyene formed from thermal dehydration of PVA provides electrical and

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optical ability which are useful to serve as an electron carrier for TiO2, thereby enhancing photodegradation as illustrated later. The proposed reaction scheme for acid dehydration of PVA

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is as shown in Scheme 1.

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OH

OH

+ nH2O m

n

m-n

PVA

PVA/Polyene unit OH

+ mH2O n

m Conjugated polyene

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m-n

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PVA/Polyene unit

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Scheme 1: Proposed reaction mechanism for the formation of conjugated polyene from polyvinyl alcohol.

3.2 Surface functionality and morphology of the materials

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The FT-IR spectra of TiO2 and CPE-TiO2 nanocomposite in the range of wave number

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500-3900 cm-1 is presented in Fig. 2. For TiO2 spectra, the appearance of a strong absorption band with no shoulder peaks at around 460-900 cm-1, is attributed to stretching vibration of T-O and TO-T and confirms the presence of titanium oxide [42,43]. A distinguished bonding peak located

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around 1620 cm-1-1640 cm-1 attributed to conjugated C=C was observed in the CPE spectrum [40]. A similar peak was observed in CPE-TiO2, which is not obviously present in the TiO2 spectrum. The presence of conjugated C=C is significant in the photocatalytic process [44], by reducing the

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electron-hole recombination rate, justified by the reduced intensity of photoluminescence spectrum at a higher wavelength for CPE-TiO2 as compared to that of TiO2 as shown in Fig. 8.

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80

60

1630

40

20

3305 523

CPE-TiO2

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Transmitance (%)

100

TiO2

3500

3000

2500

2000

1500 -1

1000

500

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Wavelenght (cm )

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CPE 0 4000

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Fig. 2: The FTIR spectra of prepared conjugated polyene (CPE), pure TiO2 and CPE-TiO2 materials

The morphology and the composition of the materials were examined using transmission

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electron microscopy (TEM) with EDS (Fig. 3). The TEM micrograph shows that the particles are mostly circular in shape with few other irregularly shaped particles. Pure TiO2 indicates a more

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separated circularly shaped nanoparticles, while for the CPE-TiO2 nanocomposite, a few agglomerated particles were observed. This slight clustering may be due to the presence of an organic polymer in the composite material as indicated by the presence of carbon atoms in the EDS spectrum (Fig. 3d). The materials were mostly composed of Ti and O for pure TiO2 (Fig. 3b),

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Ti, O and C for the composite (Fig. 3d), an indicative of the purity of the sample as shown by TEM-EDS spectral (Fig. 3b and 3d). The trace amount of Cu atom observed in the spectrum is because of the copper grid of the TEM which holds the thin layer of the sample for the passage of electrons.

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Fig. 3: TEM micrograph of prepared (a) pure TiO2 (c) CPE-TiO2 and EDS spectra of (b) pure TiO2 (d)

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CPE-TiO2

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3.3 Thermogravimetric analysis of the materials

Thermogravimetric analysis was carried out to determine the weight loss for both the pure

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TiO2 and CPE-TiO2 synthesized materials and to compare their percentage loss in weight at varying heating temperatures. The gravimetric analysis was also necessary to evaluate the stability temperature for the prepared material and to determine the calcination temperatures, particularly for CPE-TiO2. The TGA and DTA curves obtained for both materials are given in Fig.4a and 4b,

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respectively. Four distinct temperature regions are identified for CPE-TiO2 while three regions were for TiO2. The first region between 45oC and 85oC characterized by low weight loss (between 1-5% for both materials) can be attributed to the loss of some physically adsorbed volatile impurities on the surface of the materials. In the temperature range of 110oC to 230oC, we have the second weight loss due to the liberation of chemically bonded water molecules as often reported in the literature [45]. This very low (<1%) loss in weight is accompanied by a clearly 13

defined endothermic peak at about 110oC on the DTA spectral. After this, at the range of 280oC to 350oC, the combustion of hydrocarbon polymer takes place, leading to a weight loss of approximately 25% for CPE-TiO2 and about just 2% for pure TiO2. This implies that, at a temperature above 280oC, CPE-TiO2 becomes unstable. The broad exothermic peak observed from 450oC to 620oC can be attributed to the combustion of remnant carbon atoms on the surface of

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CPE-TiO2 while such was not observed on the DTA of pure TiO2. Finally, above 620oC the TGA

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for both catalysts remains unchanged. indicating a constant weight.

102

TiO2

96 94 92 90 88 86

CPE-TiO2 600

-0.05

TiO2

CPE-TiO2

-0.10

800

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400

Temperature (oC)

0.00

b

-0.15

84 200

0.05

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Weight (%)

98

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a

Derivative weight (%/min)

100

1000

200

400

600

800

Temperature (oC)

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Fig 4: (a) The thermogravimetric analysis (TGA) and (b) the derivative thermal analysis (DTA) of pure TiO2 and CPE-TiO2 materials

3.4 Crystalline structure and particle size determination

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The XRD patterns diffractogram of TiO2 and CPE-TiO2 samples are as shown in Fig. 5.

The sharp diffraction lines appearance can be related to the crystalline anatase phase of TiO 2, according to the JCPDS database 00-021-1272 [46]. The crystalline sizes of the materials were calculated using Debye-Scherrer's equation, from the [101] diffraction peak, as shown in equation (2):

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𝐷=

𝐾𝜆

(2)

𝛽𝐶𝑜𝑠𝜃

Where D is the crystalline size, λ represent the Cuka 1 radiation wavelength (λ) = 1.54060Å, K is the crystalline shape factor with an approximate value (K) = 0.9, 𝛽 is equal to the full width of the peak at an intensity equal to half of the maximum peak (FWHM), measured in radians and 2𝜃 is the diffracted angle at the maximum peak intensity. Figure 5 compares the X-ray diffraction

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patterns for both the prepared pure TiO2 and CPE-TiO2. It indicates that there are several

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crystalline peaks particularly for pure TiO2 at 2𝜃 values of 25o (101), 38o (004) and 48o (200). Since these peaks correspond to the anatase phase [47], it then implies that only anatase existed in

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the synthesized materials. It was noticed that the composite material (CPE-TiO2) shows less

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crystalline peaks; however, this does not affect the anatase phase of the material as shown by the peaks represented at various 2𝜃. The crystallite size was calculated and was found to be in the

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range 3-3.5 nm for TiO2 and 7-7.6 nm for CPE-TiO2 materials. Gaussian fit particle size distribution from TEM micrographs image (Fig. 6) was used to estimate the average particle size

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and size count distribution. The average or mean particle size was measured to be 4.1 nm with a standard deviation (W/2) of 0.43, giving an average particle size of 4.1.0 ± 0.43 nm for TiO2 and 5.8 ± 0.25 nm for CPE-TiO2. The band-gap values and estimated crystallite particle size are

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presented on Table 1.

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Table 1: The estimated crystallite sizes and band-gap of the synthesized pure and modified TiO2 materials, from XRD plots.

Materials

FWHM

2theta

Theta

Theta

𝛽

Crystallite

(𝛽)

(2𝜃)

(𝜃)

(𝜃) in radians

(in radians)

size (D)

Band gap (eV)

(nm) 10.7509

25.2166

12.753

0.22258

0.1876

TiO2

2.2757

25.50599

12.753

0.22258

0.3972

7.51

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CPE-TiO2

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3.41

1000

(101)

CPE-TiO2 TiO2

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600

(035) (200) (010)

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Intensity (a.u.)

800

400

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200

(004) (010)

(006)

0

0

20

40

60

80

100

2 Theta (degree)

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Fig. 5: X-ray diffraction (XRD) spectra of the synthesized pure TiO2 and CPE-TiO2 materials

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2.4 3.18

35 30

a

30

Ave. Size = 4.1 nm

Ave. Size = 5.8 nm

b 25

Frequency

Friquency

25 20 15

20

15

10

10

5

5 0

0 0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16

Diameter (nm)

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Diameter (nm)

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Fig. 6: Gaussian fit particle size distribution plot of synthesized (a) TiO2 and (b) CPE-TiO2, from

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TEM micrograph.

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3.5 Optical and electrochemical analysis of the materials

The optical property of semiconductors is an important characteristic for the evaluation of the light absorption capacity of photocatalysts [48]. Figure 7a shows the UV-vis diffuse reflectance

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spectra (UV-vis DRS) of TiO2 and CPE-TiO2. It is well known that pure TiO2 lacks effective visible light (>400 nm) absorption. However, the composite presents a significant increase in light absorption within a wide range of wavelengths (400 nm to 700 nm) at the visible region. This

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increase in absorption indicates that CPE-TiO2 has a better UV-vis light absorption capacity as compared to that of TiO2, which also implies that at visible light region CPE-TiO2 composite possesses a better photon absorption. It has been reported that coupling TiO2 with a conjugated molecule will result in redshift (decrease in band-gap) in absorption value due to the formation of new energy state level [49]. In this case, the coupling of CPE with TiO2 also showed a decrease in band gap value of TiO2 by 0.78eV (Fig.7b), which confirms that the band gap of TiO2 is lowered 17

by CPE. This agrees with enhanced photocatalytic activity in the photocatalytic degradation of CAP. Since photodegradation efficiency of a semiconductor largely depends on good light absorption and rapid transfer of charge carriers to the surface for redox reaction [3,50] it implies that CPE-TiO2 has a better visible light absorption capacity and more rapid electron transfer. This fact is also well supported by a reduced electron-hole recombination rate as presented in

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photoluminescence spectra measured (Fig. 8). The application of Photoluminescence (PL) is a good and effective method to study the

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migration, separation, and recombination activities of photogenerated electron-hole pairs in

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semiconductor photocatalysts [51,52]. The emission signal in PL is established to have originated from the recombination of the induced charge carrier. Hence, the charge generation, separation,

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and recombination properties of TiO2 and CPE-TiO2 composite were explored by PL measurement. Figure 8 shows the PL spectra of TiO2 and CPE-TiO2 with two emission peaks of

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both centered at shorter wavelength 420 nm and longer wavelength around 560 nm measured at room temperature. The bands originated from the recombination transition of free electrons and

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holes. However, the emission maximum at about 560 nm is relatively stronger than other emission peaks with CPE-TiO2 composite presenting a lower emission peak. From the literature, the lower photoluminescence signal intensity shows a more efficient charge separation in the photocatalyst

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[50,53]. For the CPE-TiO2 composite, the PL emission intensity at longer wavelength decreases, indicating that the incorporation of CPE favors the separation of photogenerated charge carrier, therefore, preventing the recombination of electron-hole pairs. This increase in the electronic transfer can be attributed to pie-conjugated unit provided by CPE [54]. Consequently, enhanced photocatalytic performance of CPE-TiO2 as observed by higher (80.47%) as compared to pure TiO2 (36.12%) CAP photodegradation efficiency under visible light (Fig.11). 18

The electrochemical impedance spectroscopy (EIS) (Nyquist plots) was as well carried out to further provide insight, regarding the available distinctive electronic properties of the CPE-TiO2 as compared to pure TiO2. Figure 9 presents the Nyquist plots for the transport process of photogenerated electron/hole pairs. The comparative sizes of the arc radius corresponded to charge transfer resistance. A much smaller arc was observed for CPE-TiO2 as compared to that of pure TiO2. This indicates a less impendence or resistance to the flow and transfer of generated electrons

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in CPE-TiO2 during photoexcitation process [55]. This smaller observed arc further confirms the

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fast charge transfer and more effective charge separation in CPE-TiO2 composite compared to that

3.0

a

30

1.5

1.0

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0.5

0.0

400

500

b

re (h)2(eVcm-1)2

CPE-TiO2 TiO2

2.0

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Absorbance

2.5

-p

which occurs in pure TiO2 [35,56].

600

700

20

CPE-TiO2 TiO2

10

0

800

1.5

Wavelength/nm

2.0

2.4

2.5

3.0

3.18

Energy (eV)

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Fig.7: (a) UV-vis DRS spectrum and (b) Tauc plot of pure TiO2 and CPE-TiO2 materials

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3.5

Intensity (a.u.)

1000

CPE-TiO2 TiO2

560 nm 100

520 nm

10

400

500

600

700

-p

Wavelenght (nm)

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420 nm

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Fig.8: The photoluminescence spectrum of the synthesized pure TiO2 and CPE-TiO2 materials

Fig 9: Electrochemical impedance spectroscopy (EIS) Nyquist plots of the synthesized pure TiO2 and CPE-TiO2 (the insert is the zoom in form).

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3.6 Photocatalytic activity test The evaluation of photocatalytic performance was carried out after the successful synthesis and characterization. The materials were tested for the photocatalytic degradation of CAP as a representative antibiotic pollutant under visible light irradiations. The same experiment was performed under identical conditions using both TiO2 and CPE-TiO2 as catalysts, for comparison

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purpose. Figure 10a presents the photocatalytic activities of pure TiO2 and CPE-TiO2 composite as well as photolysis in the degradation of CAP using LED as a source of visible radiation. A

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noticeable, though very low, removal (7.78%) of CAP was observed after 210 min during

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photolysis. This slight decrease in concentration after irradiation without the catalyst may be due to the presence of a light-absorbing chromophore (C=C), which is present in the structure of CAP.

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However, a much dramatic concentration decrease was observed in the presence of the catalysts. Between the two photocatalysts, CPE-TiO2 had a higher percentage removal (80.47%) as

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compared to that of pure TiO2 (36.12%) (Fig 10b). This is due to the relatively lower bang-gap (2.4eV), causing a very high activation of the catalyst by the visible light source (LED). The

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photocatalytic performance of CPE-TiO2 was enhanced by the modification with CPE that resulted in its band-gap reduction (Fig. 6b).

The effect of pH on the photodegradation of CAP by the materials was investigated,

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considering pH 4, 7, 8, and 10 for this study. Figure11 presents the results of CAP degradation at varying pH values for 210 min irradiation time. It was observed that pH of the solution has a significant effect on the removal of CAP from solution. At lower pH 4 and 7, the removal percentage was maintained at 80.39% and 80.47% respectively. It drops down to 74.81% at higher pH of 8 and to 74.29% at a pH of 10. A previous study showed that the degradation rate of CAP is highly dependent on solution pH and that the degradation rate constants in acidic medium are 21

higher than that in alkaline medium [29]. The electrostatic interactions between the charged TiO2 particles and pollutant is also a strong factor that affects the influence of pH on the degradation. TiO2 is positively charged below 6.5 (point of zero charges for TiO2), and negatively charged above pH 6.5 [57]. Table 2 presents the percentage (%) degradation of the 25 mgL-1 aqueous solution of CAP after 210 min of degradation at pH of 7.

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The total organic carbons (TOCs) is a common indicator for good assessment of mineralization of organic pollutants. Figure 12 shows the relative percentage of TOC removal by

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CPE-TiO2 and pure TiO2 during degradation. It was observed that the TOC removal percentage

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was not high enough, particularly for pure TiO2. This may be due to some produced organic intermediates that are difficult to mineralize since the degradation of CAP is accompanied by the

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production of some aromatic organic intermediates [26]. This generated intermediate takes longer time, possibly more than the set irradiation time, to degrade. Figure 13 presents a schematic

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diagram, showing the mechanism of photocatalytic degradation of chloramphenicol using CPETiO2 nanoparticles. The incorporation of CPE into TiO2 structure could enhance visible light

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absorption and increase the electron production for photo-oxidation process. The reusability test for the composite material catalyst (CPE-TiO2) was carried out. After the initial 210 min of irradiation in the CAP solution, the CPE-TiO2 composites were washed with ultrapure

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water to remove any adsorbed CAP. The thorough washing of the photocatalysts permits for more adaptability in their reuse without cross-contamination. The composite was then filled with fresh CAP solution.

As illustrated in Fig. 14, each attempt at photocatalytic degradation only resulted in a slightly lower percentage removal of the organic pollutants. However, after three attempts the CPE-TiO2

22

composites performed more poorly than it was in the original state. A similar result was reported after four attempts with cellulose-polymer modified TiO2 nanomaterial in the degradation of methylene blue [58]. The first degradation run for CAP gave 80.42% removal, and that of the third times gave 79.39%, indicating that CPE-TiO2 composite had good stability and could be reused for at least three times.

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The result obtained from the present study indicates that the material performance is comparable

90 1.0 0.9

80

a

70

C/Co

0.7 0.6 0.5

0.3

CPE-TiO2 TiO2

0.2 0.1

Photolysis

0.0 -40 -20

0

20

40

60

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0.4

60 50

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36.12%

40 30 20

7.78% 10 0

80 100 120 140 160 180 200

Time (min)

80.47%

b

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% Degradation

0.8

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with similar studies from the literature (Table 2).

Photolysis

TiO2

CPE-TiO2

Catalyst

Fig. 10: Visible light photodegradation of CAP; (a) degradation rate with respect to time and (b) Relative

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% degradations with no catalyst, pure TiO2 and CPE-TiO2

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90 CPE-TiO2 TiO2

80

% Degradation

70 60 50 40 30

10 0 pH 4

pH 7

pH 8

pH 10

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pH

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20

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Fig.11: Effect of pH on the photodegradation of CAP using pure TiO2 and CPE-TiO2 as catalysts under

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visible light

90

70 60

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TOC Removal (%)

80

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100

69.45%

50 40

24.98%

30 20 10

0

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TiO2

CPE-TiO2

Catalyst

Fig. 12: Total organic carbon (TOC) removal percentage for CAP photocatalytic degradation after 210 min irradiation time

24

Sensitizer (CP)E)

eS

H2O + O2 + e-

e-

2 OH + O-

(1) OH

CB

e-

CPE

e-

OH

hv

(2)

So

OH

O N O

e-

NH O Cl

e-

TiO2

e-

OH

OH

H2O + h+ VB

OH + H+ +

NH4 , HCl, CO2, H2O

h+

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h+

Cl

Chloramphenicol

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h+

1st run 80.47%

0.8

4th run 68.72%

0.6

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C/C0

3rd run 79.39%

2nd run 79.66%

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1.0

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Fig. 13: The mechanism of photocatalytic degradation of chloramphenicol using CPE-TiO2 nanoparticles

0.4

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0.2

0

100

200

300

400

500

600

700

800

900

Time (min)

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Fig. 14: Recycled photocatalytic degradation of chloramphenicol in aqueous solution using CPE-TiO2 nanocomposite.

25

Table 2: Performance comparison of CPE-TiO2 with some similar literature reports.

Light source % removal after irradiation Not stated Ampicillin Visible -light 100 after 180 min Hydrothermal Tetracycline Visible83 after 120 Light min Not stated Amitriptyline UV 45.4 after 60 min Sol-gel Chloramphen Visible80.47 after icol Light 210 min

Reference

[59] [60] [61] Present study

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ChitosanTiO2/ Ag2O Carbonaceous doped TiO2 PolyanilineTiO2 CPE-TiO2

contaminant

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Photocatalyst Synthetic method

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4. Conclusion

Polyvinyl alcohol derived polyene, polyene/TiO2, and pure TiO2 anatase were successfully

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synthesized and the materials were appropriately characterized. The TEM-EDS results show a well

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interactive polymer/TiO2 nanocomposite material with mostly circularly shaped particles of an average size of 5.8 nm. The result obtained from the calculated crystal particle size from the XRD crystalogram gave 3.41 nm and 7.51 nm crystallite sizes for both pure TiO2 and CPE-TiO2,

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respectively. Also observed was the improvement in the visible light absorption capacity of the polyene sensitized TiO2 over pure TiO2 with an excellent enhancement of electron-hole separation as indicated from UV-vis DRS and photoluminescence results. A significant reduction in band-

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gap (TiO2 = 3.18 eV) was observed with CPE-TiO2 (2.4 eV) as estimated from Tauc plot, which also implies a better light absorption capacity at the visible region of the light spectrum. The CPETiO2 nanocomposite demonstrated 80.47% removal of CAP at the optimum pH of 7 against 36.12% for pure TiO2 as photocatalysts. Credit Author Statement

26

S.J Modise conceived the presented idea. O.S Awofiranye developed the theory, design and carried out the experimental work. S.J Modise and E.B. Naidoo verified the analytical methods, supervised and encouraged O.S Awofiranye to investigate the proposed theory in this work. All authors discussed the results and contributed to the final preparation of the manuscript.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgment

The authors are grateful for the support of the Research Directorate, Vaal University of

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Conflict of interest

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Technology, South Africa.

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The authors hereby declare no conflict of interest.

27

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