Photocatalytic degradation enhancements of dyes with bi-functionalized zones of modified nanoflower like TiO2 with Pt-C3N4 under sunlight irradiation

Accepted Manuscript Title: Photocatalytic degradation enhancements of dyes with bi-functionalized zones of modified nanoflower like TiO2 with Pt-C3N4 under sunlight irradiation Authors: Behzad Rezaei, Reihaneh Soleimany, Ali A. Ensafi, Neda Irannejad PII: DOI: Reference:

S2213-3437(18)30687-0 https://doi.org/10.1016/j.jece.2018.11.008 JECE 2763

To appear in: Received date: Revised date: Accepted date:

29 July 2018 29 October 2018 4 November 2018

Please cite this article as: Rezaei B, Soleimany R, Ensafi AA, Irannejad N, Photocatalytic degradation enhancements of dyes with bi-functionalized zones of modified nanoflower like TiO2 with Pt-C3N4 under sunlight irradiation, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.11.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Photocatalytic degradation enhancements of dyes with bi-functionalized zones of modified nanoflower like TiO2 with Pt-C3N4 under sunlight

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irradiation

Behzad Rezaei*, Reihaneh Soleimany, Ali A. Ensafi, Neda Irannejad

Department of Chemistry, Isfahan University of Technology, Isfahan 84156–83111, Iran

Author

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*Corresponding

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*Behzad Rezaei. E-mail: [email protected]. Tel: ++983133913268, Fax: ++983133912350.

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Ali A. Ensafi. E-mail: [email protected]

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Reihaneh Soleimany. E-mail: [email protected]

Corresponding author. Tel: ++983133913268, Fax: ++983133912350 E-mail: [email protected]

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Abstract:

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Neda Irannejad. E-mail: [email protected]

In this work, a novel method was proposed to photodegradation of organic dyes as important

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pollutants in wastewaters of textile industries. A one-step hydrothermal method was used to directly synthesis nanoflower like TiO2 on ordinary glass (OG) and modified with Pt-C3N4 nanotubes (NFs TiO2/Pt-C3N4) via electrophoretic deposition (EPD) method. Its photocatalytic activity investigated for degradation of three dyes under sunlight irradiation. The properties of NFs TiO2/Pt-C3N4 NTs were characterized by SEM, FT-IR, EDS, and XRD. Reduction of band

gap energy and improvement of light absorption in the visible region for the modified photocatalyst were evaluated by using Tauc’s plot method. The experimental result showed that the NFs TiO2/Pt-C3N4 NTs achieved degradation efficiency of <97% at 240 min, towards

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methylene blue (MB), methyl violet (MV) and malachite green (MG) which are higher than pure TiO2 (68%, 83%, and 71%, respectively). Also as a new strategy, a part of the NFs TiO2/Pt-C3N4 NTs was used as a photoanode of the dye-sensitized solar cell (DSSC) and the other part, placed at a solution as a degradation zone. So with this bi-functionalized system, the degradation time was reduced from 240 min to 120 min with the same degradation efficiency.

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Keywords: Nanoflowers of TiO2, Pt-C3N4 nanotubes, photodegradation, bi-functionalized

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system, dye-sensitized solar cells

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1. Introduction

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Nowadays, the environmental concerns, particularly wastewater pollutions, are one of the challenging issues for humanity. Among a large number of organic compound pollutants, dyes

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are inseparable material in human life. One of the most significant sources of the organic

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pollutant is finding in the wastewater of the textile industries [1]. During the past decay, many types of research have been done for finding a simple and

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accessible high-performance method to minimize these pollutants. The most important parameters needed to select a suitable catalyst are, fast electron transfer, outside scattering light capability, oxidative power, usable for hazardous organic compounds and availability [2]. To date, a large number of materials like metal oxides (such as TiO2 [3], ZnO [4], WO3 [5], CdS [6], Bi2O3[7] and etc.) are chosen as a photocatalyst. Among them, TiO2 is one of the best

semiconductors due to its nontoxicity, high catalytic activity, oxidative power, chemical inertness, a large range of pH, strong chemical stability, low cost and efficiency for degradation of hazardous organic pollutions [8]. Also, TiO2 as a heterogeneous photocatalysis has attracted

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extensive attention in the field of degradation of chemical pollutants in aqueous phases. However, due to the large band gap of TiO2 (3.2 eV for anatase), its applications are limited to the UV region of the solar spectrum [9]. In order to shift the absorbance region from the UV to the visible spectral range, different modifier and methods were used. Therefore, several types of research have been carried out to modify TiO2 with a transition metal, nonmetal doping, metal

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size of nanostructures and different morphologies [12].

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oxides [10, 11], coupling with other semiconductors and organic dye sensitizing, changing in the

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Graphitic carbon nitride (g-C3N4) as the most attractive semiconductors, is a promising

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visible-light-responsive photocatalyst due to the amazing merit including nontoxicity, easy

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preparation, low cost, high chemical stability, electron-rich properties and moderate band gap

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(approximately 2.7 eV) [13]. Nevertheless, the pure bulk g-C3N4 has several deficiencies, such as low specific surface area, fast recombination of photogenerator of the electron-hole pair and low

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oxidation ability [14]. It was used several methods to avoid the above drawbacks, which doping with alkaline and transition metals such as Na, K, Eu, Pt, Fe, Zr are the best methods that have

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been reported due to the recombination of photogeneration of transition carriers [15]. Therefore,

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the use of this compound as a TiO2 modifier improves its photocatalytic activity in the visible region [16, 17]. One of the other challenging issues in the application of heterogeneous photocatalysis is its refinding for reusing, due to the filtration step for suspended particles of the particulate photocatalyst in the environmental areas. The best way to solve this problem is the deposition of

photocatalyst on a fixed bed. So, it is very important how to create the photocatalyst film at a surface of the suitable substrate. Several techniques use for nanomaterial deposition, such as drop coating [18], spin coating [19], spray coating [20], screen printing [21], doctor Blade [22]

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and electrophoretic deposition [23]. In the way, in order to direct synthesis of TiO2 on OG, hydrothermal synthesis was applied. The most advantages of hydrothermal synthesis are direct preparation in one step and diversity in morphology. This method is environmentally friendly because it occurs at a low temperature in the sealed system, but the reaction time maybe takes a long time. At the present, the electrophoretic deposition (EPD) method is the best way for

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modification of the surface of TiO2. In this case, create a uniform semiconductor is so important.

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EPD method because of taking short time reaction and ability to coating uniform layer has more

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attention than doctor Blade method [24].

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One of the other challenging problems with the uses of photocatalysts is the recombination of

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photogenerated charge carriers. So, the best way to controlling this matter is the use of photo

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electrocatalyst (PEC) system. In 2011, Song Xue et all suggested a suitable PEC system that was bi-functionalized TiO2. For this purpose, a part of photocatalyst film that sensitized by the dye

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called dye sensitized-zone and another part that is put into the pollutant solution called degradation zone [25].

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In this work, nano-flower TiO2 at the surface of OG and Pt-C3N4 NTs were syntheses with one

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step hydrothermal and solvothermal methods, respectively. Then, Pt-C3N4 NTs was coated in different concentrations on it (NFs TiO2/Pt-C3N4 NTs) with EPD method. The photocatalytic activities of NFs TiO2/Pt-C3N4 NTs for degradation of three dyes (MB, MV, and MG) were investigated under sunlight illumination. Also, a bi-functionalized system was used as a new strategy for the degradation of the dyes to increase the time efficiency of the photocatalyst.

2. Experimental 2.1.Chemicals

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In order to prepare the Pt/C3N4 NTs Nanocomposite, Melamine (C3H6N6, Sigma-Aldrich Co.) and Chloroplatinic acid (H2PtCl6.6H2O, Sigma-Aldrich Co.) were used as precursor materials. Titanium (IV)-isopropoxide (TiC12H28O4, 97% Sigma-Aldrich Co.), hydrochloric acid (HCl, 37% Merck) and deionized water were used for the hydrothermal synthesis of nanoflower like TiO2 on OG. For EPD process, ethanol, 2-propanol and Mg(NO3)2 (Sigma-Aldrich Co.) were

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used to form a suitable suspension. Furthermore, to discuss the degradation of dyes, methylene

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blue (MB), methyl violet (MV) and malachite green (MG) (Sigma-Aldrich Co.) were used. For

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discussion about photocatalytic process, Ethylenediaminetetraacetic acid solution (EDTA, 0.5 M

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in H2O), 2-Methyl-2-propanol (tBuOH, 99%), Formic acid sodium salt (HCOONa, 97%) and

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Methanol (CH3OH, 99.8%) were employed from Sigma-Aldrich Co. DSSC was fabricated by

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using cis-Ru(dcbpy)2(NCS)2 (N719), 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI), H2PtCl6.6H2O and Surlyn film. In electrochemical performance sodium sulfate (Na2SO4, 97.0%

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from Sigma-Aldrich Co.) and tetrabutylammonium hexafluorophosphate (NBu4PF6, 98% from

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Sigma-Aldrich Co.) were used.

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2.2.Apparatus

The crystalline structure of the NFs TiO2/Pt-C3N4 NTs films was checked by X-ray

diffraction analysis (XRD) method. XRD was done by a Bruker D8 Advance X-ray diffractometer using Cu-Kα radiation. The surface morphology and structures of films were

characterized by a transmission electron microscope (TEM) by Philips CM30 300kV and field emission scanning electron microscope (FE-SEM) of HITACHI (S-4160). Energy-dispersive spectroscopy (EDS) measurement was investigated by using a Seron AIS 2300 (Korea). Fourier

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transform infrared (FTIR) spectra were obtained by an FTIR JASCO 680-Plus spectrometer in the region of 400–4000 cm-1. The absorption spectra were obtained by using UV-vis spectrophotometry (Jasco V-750). Total organic carbon analyzer (Shimadzu, 5000A) was employed for mineralization degree analysis of dye solutions. Photoelectrochemical properties were characterized in a three-electrode cell with a 0.1 M Na2SO4 electrolyte solution, a saturated

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calomel electrode (SCE), a platinum wire and the photocatalyst film as a reference, counter and

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working electrode, respectively. The electrochemical impedance spectroscopy (EIS) were

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measured by the Eco-Chemie Autolab PGSTAT 302N electrochemical workstation and

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controlled with NOVA software in the frequency range of 0.01 Hz to 100 kHz under 100 mW.cm-2 at open-circuit voltage. In the EPD method to apply the voltage, Isfahan-Tak

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Electronics regulated power supply GW-Instek GFG-2080H was used as a function generator

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(100 mW cm-2).

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that connected to a signal amplifier. All solutions were irradiated by AM 1.5 simulated sunlight

2.3.Directly synthesis of nanoflower-like TiO2@OG

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The hierarchical structure of TiO2@OG is grown by one step hydrothermal processes as

reported by Pengfei Cheng et all [26]. At first, 20 mL of deionized water was mixed with 20 mL of hydrochloric acid and magnetically stirred for 5 min. Then, 0.5 mL titanium isopropoxide was added to the obtained solution and continued to stir for another 15 min. Six pieces of OG substrates with the specific surface area (10×15 mm2 for photocatalytic studies and 10×35 mm2

for the bi-functional system), were put into the bottom of the Teflon-lined stainless steel autoclave. The solution was transferred to the autoclave (60 mL volume) and then heated at 200ºC for 4 h. After cooling at room temperature, they were washed several times with

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deionization water and allowed to dry at 40ºC for 1 h. In the end, the OGs were calcinated at 450ºC for 2 h with a heating rate of 10ºC/min to prepare TiO2@OG.

2.4.Preparation of Pt-C3N4 NTs by solvothermal treatment

Preparation of g-C3N4: according to the previous method [27], pure g-C3N4 was prepared by

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calcination of melamine in the air. So, 5.0 g of melamine powder was put into a 100 mL alumina

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crucible and heated to 250ºC with a heating rate of 5ºC.min-1 and an additional ramping step with

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a heating rate of 10ºC.min-1 to 550ºC, then keeping at this temperature for 2 h. After cooling at

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room temperature, the resulting yellow product is g-C3N4. Pt-C3N4 NTs was prepared by the procedure of Kexin Li et al based on a well-established

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one-step solvothermal method [28]. So, 106 mg of H2PtCl6.6H2O was added into 15 mL of

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ethanol (A). Subsequently, 1.00 g of synthesized powder of g-C3N4, dispersed into 15 mL of

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ethanol (B). Then, solution-(A) was added drop by drop into solution-(B) and stirred for 1 h. After that, this mixture was transferred into an alumina crucible and heated to 150℃ with a

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heating rate of 1ºC.min-1. The final obtained products dried at 60ºC for 24 h and heated at 120ºC

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for 24 h.

2.5. EPD procedure for deposition of Pt-C3N4 NTs on TiO2 According to the previous work [27], different amount of Pt-C3N4 NTs (0.01, 0.02 and 0.03 g)

were dispersed into the 2-propanol: ethanol solution (4:1 ratio) and were sonicated for 30 min.

After that, 20 µL of Mg(NO3)2 0.01 M was added to the resulting mixture and continued stirred for 5 h until prepared stable suspension. In order to create a uniform layer, the pulse-EPD method was used. For this purpose, a prepared photocatalyst film and a stainless-steel plate as an

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anode and cathode, respectively, were put into the above suspension. For deposition, pulsed direct current at 100 Hz with the peak to peak voltage of 6 V was applied in the deposition time of 8 min. In this case, the amount of photocatalyst which deposited on OG is 4.0×10-4 g/mm2. In order to individualize the four kinds of prepared materials, after deposition 0, 0.01, 0.02 and 0.03 g of Pt-C3N4 NTs to NFs TiO2, the photocatalyst films named NFs TiO2/Pt-C3N4 NTs (0, 1, 2

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and 3).

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2.6. Photocatalytic degradation procedure

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The photocatalytic activity of NFs TiO2/Pt-C3N4 NTs (0, 1, 2 and 3) was investigated by decomposition of three types of the dyes under sunlight irradiation (100 mW cm-2). In order to

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decomposition of the dyes, the photocatalyst film was put into the 20 mL aqueous solution of the

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dyes with an initial concentration of 10 mg/L. Before each running, for access

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adsorption/desorption equilibrium, the solution was placed in a dark place and air gas was bubbled (5L/h) into the solution for 30 min. During the degradation tests, the temperature must

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be controlled at room temperature.

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2.7. Fabrication of bi-functionalized photocatalyst film In this case, photocatalyst film that deposited with the specific surface area (10×35 mm2), was

used to fabricate a bi-functionalized film. Since a part of photocatalyst film was sensitized by the dye (dye sensitized-zone) and another part of that was put into the pollutant solution (degradation zone).

In order to prepared dye sensitized-zone, a part of photocatalyst with a specific surface area of 10×15 mm2 was immersed into the dye solution containing 8.0×10-4 mol L-1 of N719 in ethanol for 24 h at a dark place. Then, this part was sandwiched against Pt (I) electrode as a counter

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electrode by using Surlyn film. In the end, according to the design with the DSSC procedure, the standard iodine-based electrolyte was injected into the cell gap with capillary action [27]. (Fig. S1).

The photodegradation of pollutant was carried out in a 50-mL wide-mouth glass that containing 20 mL of MV, MB or MG with an initial concentration of 10 mg/L. Then, the

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degradation zone was put into the solution. Afterward, another Pt (II) electrode (10×35 mm2)

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was placed into the solution with a 5-mm distance from the anode and connected to this electrode

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with copper wire (Scheme 1). The dye-sensitized zone was only exposed to solar radiation.

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Moreover, during the degradation, the air was bubbled by pipe bubbling. As well, the proposed

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mechanism for the destruction of pollutants is clearly shown in scheme 1.

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Scheme 1. (A) The schematic fabrication of bi-functionalized NFs TiO2/Pt-C3N4 NTs (2) system.

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(B) and (C) Working principle of the degradation system. (B) The preparation of bi-

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functionalized NFs TiO2/Pt-C3N4 NTs (2) film with the OG (10 mm×35 mm) coated with the hydrothermal synthesized NFs TiO2 films, an area of NFs TiO2/Pt-C3N4 NTs (2) film (10

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mm×15 mm) sensitized by an N719 dye that used to the dye-sensitized zone. (C) An area of NFs

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TiO2/Pt-C3N4 NTs (2) film (10 mm×15 mm) was put into the dye solution as a degradation zone.

The light collide on the dye-sensitized zone causes excited dye molecules (dye*), then electrons transfer from valence band (VB) of dye* to the conduction band(CB) of the photocatalyst. In the following, the electron moves at photocatalyst network to reach to the

degradation-zone. The existent electron in a high level of the photocatalyst, cause to produce O2˙and then by the creation of OH˙ induce to degradation of dyes. In this regard, by reaction of water at the surface of Pt (II)/FTO electron, that was produced, with the copper wire transfer to

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Pt (I)/FTO. In the following, the iodine/triiodide redox couple (I3-/I-) regenerate the electron according to Scheme 1. All the possible mechanism that carried out in this work [29]: NFs TiO2/Pt-C3N4 NTs/Dye + hν → NFs TiO2/Pt-C3N4 NTs/Dye*

(Eq. 1)

NFs TiO2/Pt-C3N4 NTs/Dye*→ NFs TiO2/Pt-C3N4 NTs/Dye+ + e-CB

(Eq. 2)

1/2 I3- + e-Pt → 3/2 I-

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e-CB diffusion in NFs TiO2/Pt-C3N4 NTs on OG

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NFs TiO2/Pt-C3N4 NTs/Dye+ + 3/2 I- → NFs TiO2/Pt-C3N4 NTs/Dye +1/2 I3-

O2+ e-CB → O2˙-

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O2˙- + H+ → HOO˙

(Eq. 3) (Eq. 4) (Eq. 5) (Eq. 6) (Eq. 7) (Eq. 8)

HOO- + H+→ H2O2

(Eq. 9)

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HOO˙ + e-CB → HOO-

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H2O2 + e-CB → HO- + HO˙

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Pt (II) +H2O → Pt (II) (OH˙) +H+ + ePt (II) (OH˙) → Pt (II)(0) + e- + H+

2.8. The proposed Mechanism for Photodegradation

(Eq. 10) (Eq. 11) (Eq. 12)

From the point of view of photodegradation, the role of the photocatalyst is to initiate oxidation and reduction reaction at the presence of organic pollutants. At first, NFs TiO2/Pt-C3N4 NTs is a photocatalyst with higher degradation efficiency than NFs TiO2 in the vis. light

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irradiation. As shown in Scheme 2, the best-reported reason is matched CB edge potentials and the synergic effect between Pt-C3N4 and TiO2 [30]. As mentioned before, NFs TiO2 cannot be excited by vis. light irradiation, thus the NFs TiO2/Pt-C3N4 NTs can be attributed electron exited from VB to CB of Pt-C3N4, creating holes in the VB of Pt-C3N4 and it can be restored to the ground state by oxidation of each dye. According to the fundamental mechanism of the

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photocatalyst, recombination between hole and electron cause to decrease photodegradation

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efficiency. So, for reach to high efficiency, the couple of electron and hole should be separated

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and quickly transferred across the surface-interface to avoid the recombination [31]. In order to

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improve the photocatalytic performance, form a semiconductor heterojunction by coupling with

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a secondary substance that was shown in Scheme 2.

Scheme 2. The proposed mechanisms for the NFs TiO2/Pt-C3N4 NTs heterojunction

Also, Pt-C3N4 could improve degradation efficiency due to the coupling two semiconductors and also provide a large specific surface area. So, NFs TiO2/Pt-C3N4 NTs represents a good example that demonstrates the enhancement of the photocatalytic properties of this

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heterojunctions.

3. Results and discussion 3.1.Characterization of photocatalyst films

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3.1.1. Morphology and textural property

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The morphology and structure of synthesized TiO2 nanoflowers were observed by FE-SEM.

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As can be seen in Fig. 1(a) and (b), at different magnifications the entire surface of the OG

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substrate was covered uniformly by TiO2 nanoflowers that consist of some nanorods. The

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average length and diameter of each nanorod were revealed 200 nm and 30 nm, respectively.

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Fig. 1. The FE-SEM images of (a),(b) NFs TiO2, (c),(d) g-C3N4 nanosheets, (e),(f) Pt-C3N4 NTs

For preparation TiO2 nanoflowers on the OG substrates, TTIP is hydrolyzed in the presence of

HCl and creates Ti(OH)4. Then, seeds of TiO2 were produced by the condensation process, which ultimately formed denser morphologies in nano dimensions with the high internal specific surface area [32]. The morphology of as-prepared g-C3N4 was shown in Fig. 1(c) and (d) in two

magnifications. It seems that pure g-C3N4 was formed nanosized crystals layers with a smooth surface (graphite-like layered structure). It can be seen in Fig. 1(e) and (f), when g-C3N4 nanosheets were dispersed in ethanol, -NH2 groups changed to –OH. Therefor C3N4 NTs were

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synthesized by the solvothermal method. The average length and diameter of each nanotube is about 500 and 70 nm, respectively, which is confirmed with TEM results (Fig. 2(B)). In this regards, ethanol could successfully reduce Pt4+ to metallic Pt, which follows this reaction [28]. Pt4+ + 2CH3CH2OH → Pt0 + 2CH3CHO + 4H+

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(Eq. 13)

Fig. 2. TEM images of (A) g-C3N4, (B) Pt-C3N4 NTs, (C) EDS of TiO2 on OG and (D) EDS of NFs TiO2/ Pt-C3N4 NTs (2)

The TEM image in Fig. 2(A) also confirmed producing of g-C3N4 nanosheet. In the EDS results, it can be clearly observed Ti and O elements in Fig. 2(C, D) and C and N elements in Fig. 2 (D). Also, because of the low amount of Pt element in NFs TiO2/ Pt-C3N4 NTs (2), cannot

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clearly detect it. It can recognize C and N that is related to g-C3N4 and also C, N and Pt that is indicated by using solvothermal treatment, Pt-C3N4 NTs are successfully synthesized (Fig. S2).

3.1.2. Structural information

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The crystalline structures of g-C3N4 and Pt-C3N4 NTs are investigated using XRD method

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(Fig. 3(A)).

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A peak was shown in the 2θ, 27.3º is related to aromatic systems (111) of the graphitic

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structure of g-C3N4 (Fig. 3 (A). (a)). Presence of Pt in the face-centered cubic has been shown at

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31.4º (111) and 39.6º (110) (Fig. 3 (A). (b)) [27]. In this regard, XRD analysis of NFs TiO2 is

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shown in Fig. 3(A). (c) and based on the diffraction pattern (Fig. (C) (a)), rutile phase of coated NFs TiO2 on OG show peaks in 27.55º, 36.21º and 62.88º that related to (110), (101) and (002)

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plates, respectively. All the XRD peaks accord with Rutile phase (JCPDS No. 87-0710, a = b = 0.458 nm and c = 0.295 nm). Having been dramatically increased for (002) plate,

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notwithstanding other peaks slightly decreased which is because of each nanorod of TiO2 grow in the (001) plate direction that is parallel with substrate surface [33]. The crystallite size of NF

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TiO2 and g-C3N4 was obtained by Debby-Scherrer’s equation (Eq. 14) D = Kλ/ (βcosθ)

(Eq. 14)

where D is the crystal size; K is approximately 1, λ is the wavelength of the X-ray radiation (λ=0.15406 nm), and β is the line width at half-maximum height [34]. The crystallite size found

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41 and 12 nm for NFs TiO2 and g-C3N4, respectively.

Fig. 3. (A) XRD pattern, (B) FT-IR spectra of (a) g-C3N4, (b), Pt-C3N4 NTs, (c) NFs TiO2, (d)

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NFs TiO2/ Pt-C3N4 NTs (2) and (e) NFs TiO2/ Pt-C3N4 NTs (2) after 5 runs, (C) (a) TiO2 Rutile

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Pattern with JCPDS No. 87-0710, (b) Peak list of NFs TiO2 in x’pert

Also, it is clearly obtained that Pt-C3N4 NTs were deposited on NFs TiO2 [35]. Fortunately, there are no significant differences between the XRD spectra of NFs TiO2/ Pt-C3N4 NTs (2) after

5 runs and NFs TiO2/ Pt-C3N4 NTs (2). So, it shows that the crystalline structure of the photocatalyst does not destroy during the photodegradation process. FT-IR spectra in the Fig. 3(B). (a) and (b) showed strong bands series in the 1200-1650 cm-1

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region corresponds to the typical stretching vibration modes of CN heterocycles. Moreover, the peak at 805 cm-1 was assigned to the characteristic breathing mode of the s-triazine units. Also, the broad bands at around 3400-2800 cm-1 centered at 3200 cm-1 are related to the ─NH2 and ═NH stretching vibration modes [36]. In addition, there is no significant difference between two FT-IR spectra. Consequently, the existence of the Pt element does not affect the structure of g-

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C3N4. The broad-band at a center of 655 cm-1, a peak at 1015 cm-1 broad-band at the center of

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3400 cm-1 are attributed to Ti-O-Ti, vibration straight of O-O and physically adsorbed water

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molecule on the surface of the photocatalyst, respectively. Moreover, TiO2 in acidic solution

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caused to create TiO2(OH2+)n that with deposition of Pt-C3N4 on TiO2(OH2+)n, leads to interact

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between H+ and C3N4. So, the peak in 2900 cm-1 can be assigned to the C-H stretching and the

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overtone modes of N-H bend [37]. Also, in the FT-IR spectra of NFs TiO2/ Pt-C3N4 NTs (2) after

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5 runs, indicate that the photocatalyst has been recovered without any damaged.

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3.1.3. Optical and electrochemical properties The band gap and optical properties of NFs TiO2/Pt-C3N4 NTs (0, 1, 2, 3) films were studied

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by UV-vis absorption spectroscopy. Tauc’s plot is one of the best ways for investigating the band gap energy, Eg, in semiconductors by using UV-Vis absorption spectroscopy. The band gap is calculated by this equation: αhν =A(hν-Eg)1/2

(Eq. 15)

where, α, hν, ν and A are absorption coefficient, photon energy, the frequency of vibration and proportionality constant, respectively. By extrapolating the tan (αhν)2 versus hν, band gap energy can be calculated. As previously mentioned, the band gap of Rutile TiO2 is 3 eV which is in the

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UV region of the solar spectrum. But, after modification by the proposed method, the absorbance of photocatalyst films was improved and shifted to the visible region (Fig. 4(B)). As can be seen, the band gap of NFs TiO2/Pt-C3N4 NTs (0, 1, 2, 3) are 2.86, 2.05, 1.95 and 1.65 eV, respectively. In addition, Rutile TiO2 film has two peaks in the UV-Vis region and it means that has two band gaps energy. But according to the Tauc’s plot and other works, just one band gap involve in

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photocatalytic activity [38].

Fig. 4. (A) UV–Vis absorption spectrum, (B) Tauc’s plot, (C) Cyclic Voltammetry curves and (D) EIS plot and equivalent circuit of (a) NFs TiO2, (b) NFs TiO2/Pt-C3N4 NTs (1), (c) NFs TiO2/Pt-C3N4 NTs (3) and (d) NFs TiO2/Pt-C3N4 NTs (2)

Also, in Fig. 4(A) it has been illustrated that with increasing modifier, the absorption was decreased which is due to the probability of charge transfer has been decreased. However, it is

SC RI PT

possible to charge transfer involves in recombination of hole and electron. Because of the obtained results of photodegradation show that NFs TiO2/Pt-C3N4 NTs (2) has more efficiency than others [39].

On the other hand, for evaluation of HOMO (the energy needed to extract electrons from the

U

molecule) and LUMO (the energy needed for electron injection to the molecule) positions of

N

semiconductors such as TiO2, Bredas et all equations were suggested:

A

E (HOMO) = - [4.65 V-Eox(onset)]

M

E (LUMO) = - [4.65 V-Ered(onset)] Eg = LUMO – HOMO

(Eq. 16) (Eq. 17) (Eq. 18)

TE

D

In these equations, the value of 4.65 is constant and named ferrocene value [40]. Based on this suggestion in order to carry out CV method, Ag/AgCl as a reference electrode,

EP

platinum as an auxiliary electrode and a photocatalyst film with a surface area of 10×15 mm2 as a working electrode were used. A solution of 0.1 M of TBAPF6 in acetonitrile anhydrous was

CC

used under a blowing of nitrogen gas at a scan rate of 20 mV s-1. The results in the table. 1 and Fig. 4(C) show that the HOMO and LUMO energy levels which were measured by the CV

A

method and the difference between them were comparable with the Tauc’s plots data.

Table 1. Eg and Rct for NFs TiO2/Pt-C3N4 NTs (0, 1, 2, 3). Eonset OX vs. Ag/AgCl

HOMO level (eV)

Eonset red vs. Ag/AgCl

LUMO level (eV)

Eg from CV (eV)

Optical Eg (eV)

Rct(Ω)

NFs TiO2

1.36

-3.29

-1.51

-6.16

2.87

2.86

3786

NFs TiO2/Pt-C3N4 NTs(1)

0.62

-4.03

-1.43

-6.08

2.05

2.05

2310

NFs TiO2/Pt-C3N4 NTs(2)

0.45

-4.20

-1.20

-5.85

1.65

1.65

1425

NFs TiO2/Pt-C3N4 NTs(3)

0.68

-3.97

-1.26

-5.19

1.94

1.95

1884

SC RI PT

Photocatalyst

In order to investigate the charge transfer and the separation efficiency of photo-generated charge carriers of the different types of photocatalyst films, the electrochemical impedance

U

spectroscopy (EIS) was used. It was carried out in 0.1 M Na2SO4 aqueous solution that

N

illuminated under AM 1.5 simulated sunlight (100 mW cm-2). The Nyquist diagram was

A

investigated to estimate charge transfer resistance (Rct) of NFs TiO2/Pt-C3N4 NTs (0, 1, 2, 3)(Fig.

M

4(D)). In this diagram, each semicircle denotes an Rct and the smallest semicircle radius can represent the best photocatalyst film. It is clearly observed that by modification of TiO2 with Pt-

D

C3N4, Rct is decreased. Also, the best-prepared photocatalyst is NFs TiO2/Pt-C3N4 NTs (2) with

TE

the smallest radius arc in the Nyquist diagram. This result is consistent with the conclusion

CC

EP

obtained from the band gap measurements.

3.2. Photocatalytic activity and bi-functionalized system

A

In the photocatalytic degradation and bi-functionalized studies, the degradation efficiencies

were calculated by this equation: η=

C0 -C C0

(Eq. 19)

where C0 is an initial concentration and C is a concentration of each dye after irradiation time.

SC RI PT U N A M D TE EP

CC

Fig. 5. The photocatalytic degradation of (A) MB, (B) MV, (C) MG at presence (a) bifunctionalized system of NFs TiO2/Pt-C3N4 NTs (2), (b) NFs TiO2/Pt-C3N4 NTs (2), (c) NFs

A

TiO2/Pt-C3N4 NTs (3), (d) NFs TiO2/Pt-C3N4 NTs (1), (e) NFs TiO2, (f) without photocatalyst and (g) without photocatalyst under dark condition. The degradation of pollutants depends on the band gap energy of prepared photocatalyst and also, it should be used the photocatalyst with the lower band gap. The photocatalytic activity of NFs TiO2/Pt-C3N4 NTs (0, 1, 2, 3) was measured by the degradation of MB, MV and MG

solution under sunlight irradiation. After passing 240 min irradiation time, the degradation efficiency of NFs TiO2/Pt-C3N4 NTs (2) for MB, MV, and MG were 96.5%, 97.8%, and 97.2%, respectively (Fig. 5).

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To achieve better degradation during loading time, the bi-functional designed system was used. In this case, the photodegradation of dye is depended on the specific surface area and the amount of adsorbed N719 dye and electron transfer from dye synthesized-zone to degradationzone. As previously mentioned, NFs TiO2/Pt-C3N4 NTs (2) is the best photocatalyst, so it was employed for the bi-functional designed system. The results explored that the degradation

U

efficiency for MB, MV, and MG were 97.2%, 98.8%, and 98% after 120 min irradiation,

N

respectively.

A

Furthermore, it was observed that the degradation of dyes had a negligible self-degradation

M

under sunlight irradiation. The degradation of MB and MG followed photocatalytic degradation

D

pathway as reported by Houas et all [41] and Poulios et all [42], respectively (Fig. S3 and S4).

TE

Moreover, the photocatalytic kinetic of heterogeneous photocatalyst followed the LangmuirHinshelwood model (Eq. 20):

(Eq. 20)

EP

ln(C0/ C) = kt,

CC

where k is the rate constant of photodegradation [43]. To prove the photocatalytic kinetics of the photocatalyst, the dyes concentration variation

A

versus reactions time were followed according to Fig. 6. It was observed that the degradation of dyes is conveyed by a pseudo-first order reaction rate. So, the reaction rate constant (k) of photodegradation and R2 are represented in Table 2.

SC RI PT U N A M D TE

EP

Fig. 6. The kinetic plot of photocatalytic degradation at presence (a) bi-functionalized system of

CC

NFs TiO2/Pt-C3N4 NTs (2), (b) NFs TiO2/Pt-C3N4 NTs (2), (c) NFs TiO2/Pt-C3N4 NTs (3), (d) NFs TiO2/Pt-C3N4 NTs (1), (e) NFs TiO2, (f) without photocatalyst and (g) without photocatalyst

A

under dark condition (A) MB, (B) MV, (C) MG. It can be found that for all types of dye, the degradation rate of NFs TiO2/Pt-C3N4 NTs (2) is

fine. But, bi-functionalized photocatalyst have the best results on the degradation rate. In this case, to compression amount of k values for bi-functionalized NFs TiO2/Pt-C3N4 NTs (2) are

absolutely higher than 0.03 min-1, whereas for NFs TiO2/Pt-C3N4 NTs (2) are lower than 0.02

Table 2. Comparison k value and R2 of photocatalysts film

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min-1.

R2 0.994

MG K(min-1, 10-3) 5.5

R2 0.998

MV K(min-1, 10-3) 5.6

R2 0.926

NFs TiO2/Pt-C3N4 NTs(1)

7.0

0.998

9.4

0.99

9.5

0.989

NFs TiO2/Pt-C3N4 NTs(2)

17.2

0.997

17.7

0.997

19.0

0.977

NFs TiO2/Pt-C3N4 NTs(3) 11.5 Bi-functionalized NFs 29.8 TiO2/Pt-C3N4 NTs (2)

0.997

13.1

0.998

12.8

0.982

0.992

34.2

0.997

38.4

0.985

A

N

U

NFs TiO2

MB K(min-1, 10-3) 4.7

Photocatalyst

M

3.3.The effect of pH and reusability

The pH role of the solution is a significant parameter in the photodegradation process. Fig. 7

D

(A), (C) and (E) demonstrate the degradation efficiency of these dyes at different pH on the

TE

surface of bi-functionalized NFs TiO2/Pt-C3N4 NTs(2) after 100 min. As can be seen, by

EP

increasing the pH of the solution, the degradation efficiency was increased. Also, the optimum amount of pH was 10 for three dyes. By compression of the obtained results, it is clearly

CC

observed that with increasing the pH value, degradation of pollutant was increased. In this regard, the surface charge of MB, MV, and MG are positive. As the pH of the solution increased,

A

the number of negative charge on the surface of photocatalyst increased. So, it leads to better adsorption of the positive charges (MB, MV, and MG) on the surface of photocatalyst with negative charge [17, 44]. The degradation efficiency of these organic pollutants remain stable at the pH= 11 and there is a steady decline in the pH= 12.

SC RI PT U N A M D TE EP

CC

Fig. 7. (A), (C), (E) The pH effect at presence of bi-functionalized NFs TiO2/Pt-C3N4 NTs (2) and (B), (D), (F) reusability of bi-functionalized NFs TiO2/Pt-C3N4 NTs (2) for photodegradation

A

of MB, MV, and MG, respectively.

The efficiency of bi-functionalized NFs TiO2/Pt-C3N4 NTs(2) is excellent after 5 runs (Fig. 7. (B, D, and F)). It is because of that this system has high reusability and stability under sunlight

without any reduces efficiency. As can see this figure, the reusability of the prepared photocatalyst for degradation all of the dyes was not decrease after 5 runs. The most important property of this photocatalyst, which deposited on fixed-bed (OG) is higher reusability than

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particular photocatalyst. For the prepared photocatalyst based on particles which suspended in the dye solution, after each run must be filtered and maybe wasted during the degradation process.

So, by using this designed system, it does not need to filtration step and any

U

photocatalyst does not waste during the photodegradation.

N

3.4. Photocatalytic Property of bi-functionalized NFs TiO2/Pt-C3N4 NTs and the Mineralized

A

Degree

M

Total organic carbon (TOC) analysis was employed to compare the efficiency of photodegradation of these dyes towards mineralization [45]. In this regard, this analysis was

D

carried out for the best efficiency which constantly monitored by UV-Vis spectroscopy. The

TE

experimental results showed that the bi-functionalized NFs TiO2/Pt-C3N4 NTs achieved

EP

degradation efficiency of 97.2%, 98.8% and 98% at 120 min, towards MB, MV, and MG, respectively. However, the TOC analysis indicated only 52%, 57.2% and 55% for each dye,

CC

respectively. So, the less amount of dye molecules were completely produced CO2 and the main of the dyes were break down into small colorless molecules.

A

In order to the illustration of the photocatalytic degradation process of bi-functionalized NFs

TiO2/Pt-C3N4 NTs, the radicals and holes trapping experiments were done. As can be clearly seen in Fig. 8, in comparison with the photodegradation of MG (for an example among the others) that slightly decreased in presence of hole scavenger (EDTA), it is obviously inhibited in

presence of hydroxyl radical scavenger (tBuOH). It can be understood that the hydroxyl radical is the main active oxygen species (AOS) which can degrade the absorbed organic pollutant on

A

N

U

SC RI PT

the surface of NFs TiO2/Pt-C3N4 NTs.

M

Fig. 8. The curves of photogenerated carriers trapping in presence of bi-functionalized NFs

TE

D

TiO2/Pt-C3N4 NTs for photodegradation of MG.

In this regard, HO2˙/O2˙ as the AOS may serve in many reactions. Moreover, in the presence

EP

of 100 mM methanol (Ka= 9.7×108 M-1.s-1) or formate (Ka=3.2×109 M-1.s-1), the majority of OH˙

CC

can be converted to HO2˙/O2-˙. Fig. 8 suggests that hydroxyl radicals play a significant role in bi-

A

functionalized NFs TiO2/Pt-C3N4 NTs system [46].

3.5. Comparison of photocatalytic degradation of pollutants

To confirm the ability of prepared photocatalyst to the degradation of MB, MV and MG its performance compares with the other reported photocatalyst (Table. 4). In some cases with

higher efficiency, for photodegradation of dyes UV light irradiation was used which needed special conditions. As can see in this table, our synthesized photocatalyst have a suitable result

SC RI PT

against the others.

Efficiency % 95>

Rate constant Ref. (×10-3 min-1) 232.6 [47]

MB

UV

120

97

-

[48]

NYT2-Ag

MB

UV

240

100

-

[49]

4

NYT2-Ag

MG

UV

240

100

-

[49]

7

TiO2(PEC)

MB

UV

60

99.9

[50]

8

Au-WO3

MB

Vis

N

-

300

99>

18.0

[51]

9

12C/α-Fe2O3

MB

Solar

30

100

260.8

[52]

10

CQD/ α-Fe2O3

MB

Vis

90

97.3

-

[53]

11

G-TiO2

MB

Vis

110

100

-

[19]

12

C3N4-18/ZnAl-LDH

MB

Vis

240

98

16.8

[54]

13

SnO2 NPs

MV

solar

270

96.2

7.0

[55]

14

SnO2 NPs

MB

solar

240

96

10.7

[55]

15

F-TiO2(B)/SWCNT

MG

Vis

120

91.83

20.9

[56]

16

ITO/ZnO:I/TiO2 Bi-functionalized PtC3N4 NTs (2)/NFs TiO2 Bi-functionalized PtC3N4 NTs (2)/NFs TiO2 Bi-functionalized PtC3N4 NTs (2)/NFs TiO2

RB

Vis

360

97

-

[57]

MB

solar

120

97.4

29.1

This work

MV

solar

120

98.2

37.8

This work

MG

solar

120

98.1

33.9

This work

1

LDH Cu/Al

MV

2

ZnO NPs

3

EP

CC

17

18

A

19

4. Conclusion

Irradiation

A

Dye

M

Photocatalyst

TE

Entry

U

UV

Time (min) 100

D

Table 4. Comparison of photocatalytic degradation of pollutants

In this work, nanoflowers of TiO2 directly synthesized on the ordinary glass by hydrothermal method. The deposition of a uniform and impact layer, as-prepared Pt-C3N4 nanotubes on NFs TiO2, was done by the EPD method. As a conclusion, NFs TiO2/Pt-C3N4 NTs (2) films exhibited

SC RI PT

more advantages than pure NFs TiO2, due to lower Rct and band gap. By this method, the band gap of the photocatalytic film was 1.65 eV. Therefore, photodegradation of pollutant was carried out under sunlight irradiation. Also, the efficiency of degradation by TiO2 is about 60%, whereas for NFs TiO2/Pt-C3N4 NTs (2) is approximately more than 97% for each dye after 4 h irradiation. Nevertheless, when it was used bi-functionalized system, the degradation time of these dyes

U

decreased to 2 h. In this condition, photodegradation improved. It is due to the more charge

N

transfer and separation of e- and h+. This work has a more stability and reusability based on the

M

A

structural, materials and fixed bed.

D

Author Contributions

TE

The paper was written through contributions of all authors. All authors have given approval to

EP

the final version of the paper.

CC

ACKNOWLEDGEMENT

The authors acknowledge, Isfahan University of Technology Research Council (IUT) and

A

Center of Excellency in Sensor and Green Chemistry of IUT for supporting this work.

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