Electrospinning Directly Synthesized Porous TiO2 Nanofibers

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Electrospinning directly synthesized porous TiO2 nanofibers modified by graphitic carbon nitride sheets for enhanced photocatalytic degradation activity under solar light irradiation Surya Prasad Adhikari, Ganesh Prasad Awasthi, Han Joo Kim, Chan-Hee Park, and Cheol Sang Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01085 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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Figure 1: Schematic illustration for the synthesis of the TiO2/g-C3N4 composite. 48x16mm (600 x 600 DPI)

ACS Paragon Plus Environment

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Figure 2: XRD patterns of (a) g-C3N4, (b) TiO2, (c) TCN-1, (d) TCN-3, (e) TCN-5 and (f) TCN-10. Inset XRD pattern showing the 23-30 degree region. 116x96mm (300 x 300 DPI)


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Figure 3: Raman shift of TiO2 and TCN-5. 99x70mm (300 x 300 DPI)


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Figure 4: FT-IR images of (a) g-C3N4, (b) TiO2, (c) TCN-1, (d) TCN-3, (e) TCN-5 and (f) TCN-10. Inset FTIR spectra showing 2500-4000 cm-1 region. 126x113mm (300 x 300 DPI)


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Figure 5: FE-SEM images of (a) TTIP/PVAC fibers before calcinations, (b) TiO2 fibers after calcination, (c) TTIP/PVAC/g-C3N4 fibers before calcinations, and (d) TCN composite after calcinations. 95x65mm (300 x 300 DPI)


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Figure 6: (a) TEM images of (a) TiO2 NFs, (b) TCN, and (c) HR-TEM images of TCN composite. 118x100mm (300 x 300 DPI)


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Figure 7: (a) XPS survey spectrum and high resolution XPS spectra of (b) C1s, (c) N1s, (d) Ti 2p, (e) O1s and (f) shifting of Ti2p peak in TCN-5 composite. 160x182mm (300 x 300 DPI)


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Figure 8: (a) UV-vis absorption spectra of TiO2, g-C3N4 and TCN-x samples and (b) plots of (αhν)2 vs. photon energy (hν) for the band gap energies of different samples. 53x20mm (300 x 300 DPI)


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Figure 9: (a) Photodegradation of RhB under natural sun light in the corresponding time intervals; (b) Photo images showing the initial 10-ppm RhB solution and the remaining RhB in the solution after being irradiated for 40 min; (c) Photodegradation of RB-5 under natural sunlight in the corresponding time intervals; and (d) Photo images showing the initial 10- ppm RB-5 solution and the remaining RB-5 in the solution after being irradiated for 20 min. 86x53mm (300 x 300 DPI)


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Figure 10: Kinetics of photocatalytic degradation of different catalysts, (a) 10-ppm RhB solution, and (b) 10ppm RB-5 solution. 57x23mm (300 x 300 DPI)


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Figure 11: PL spectra of g-C3N4, TiO2 and TCN-5 111x89mm (300 x 300 DPI)


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Figure 12: Cyclic voltammograms of TiO2, g-C3N4 and TCN-5 102x75mm (300 x 300 DPI)


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Figure 13: Electrochemical impedance spectra of TiO2, g-C3N4 and TCN-5 107x81mm (300 x 300 DPI)


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Figure 14: Schematic diagram of photogenerated charges in the TCN hetero-junction. 120x104mm (300 x 300 DPI)


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Figure 15: Cyclic runs of TCN-5 in the photocatalytic degradation over RhB under natural light irradiation. 74x39mm (300 x 300 DPI)


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Figure S1: FE-SEM images of g-C3N4 before and after sonication (a) and (b), respectively, SEM-EDX image with real atomic compositions (c) and (d), and XPS survey spectrum with constituent elements of g-C3N4. 110x87mm (300 x 300 DPI)


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Figure S2: Porous formation mechanism of TiO2 NFs 44x14mm (300 x 300 DPI)


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Figure S3: FE-SEM images of electrospun PVAc NFs with g-C3N4 sheets in different magnifications. 108x83mm (300 x 300 DPI)


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Figure S4: High resolution XPS spectra of Ti 2p, O1s, C1s, and N1s for the pristine g-C3N4 and TiO2 NFs. 105x79mm (300 x 300 DPI)


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Figure S5: Binding energy shifting comparison of O1s high resolution XPS spectra from pristine TiO2 NFs and TCN-5 composite. 204x299mm (300 x 300 DPI)


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Figure S6: Distribution of pore diameter 103x76mm (300 x 300 DPI)


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Figure S7: Life time comparison of g-C3N4 sheets, TiO2 NFs and TCN-5 from FT-IR spectra before and after 4 cyclic runs 87x55mm (300 x 300 DPI)


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Table S1: BET surface area and Langmuir surface area of different samples 42x12mm (300 x 300 DPI)


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Table S2: Porous characteristics of g-C3N4, TiO2 and TCN-5 samples 76x41mm (300 x 300 DPI)


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Electrospinning directly synthesized porous TiO2 nanofibers modified by graphitic carbon

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nitride sheets for enhanced photocatalytic degradation activity under solar light irradiation

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Surya Prasad Adhikari1, 2, 3, Ganesh Prasad Awasthi1, Han Joo Kim4, Chan Hee Park1, 3*, Cheol Sang

4

Kim1, 3*

5

1

6

Republic of Korea

7

2

Institute of Engineering, Tribhuvan University, Kathmandu, Nepal

8

3

Division of Mechanical Design Engineering, Chonbuk National University, Jeonju 561-756,

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Republic of Korea

Department of Bionanosystem Engineering, Chonbuk National University, Jeonju 561-756,

10

4

11

University, Jeonju 561-756, Republic of Korea

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*Corresponding authors:

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Tel.: +82-63-270-4284; Fax: +82-63-270-2460

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E-mail:[email protected] (C. H. Park),

15

[email protected] (C. S. Kim)

Division of Convergence Technology Engineering, Engineering College, Chonbuk National

16 17 18 19 20 21 22 23 24

1


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Abstract

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We report a direct approach to the fabrication of a composite made of porous TiO2 nanofibers (NFs)

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and graphitic carbon nitride (g-C3N4) sheets, by means of an angled two-nozzle electrospinning

4

combined with calcination process. Different wt % amounts of g-C3N4 particles in a polymer

5

solution from one nozzle, and TiO2 precursors containing the same polymer solution from another

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nozzle, were electrospun and deposited on the collector. Structural characterizations confirm a well-

7

defined morphology of the TiO2/g-C3N4 composite in which the TiO2 NFs are uniformly attached

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on the g-C3N4 sheet. This proper attachment of TiO2 NFs on the g-C3N4 sheets occurred during

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calcination. The prepared composites showed the enhanced photocatalytic activity over the

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photodegradation of rhodamine B and reactive black 5 under natural sunlight. Here, the synergistic

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effect between the g-C3N4 sheets and the TiO2 NFs having anisotropic properties enhanced the

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photogenerated electron-hole pair separation and migration, which was confirmed by the

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measurement of photoluminescence spectra, cyclic voltammograms, and electrochemical

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impedance spectra. The direct synthesis approach that is established here for such kinds of sheet-

15

like structure and porous NFs composites could provide new insights for the design of high-

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performance energy conversion catalysts.

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Keywords: Electrospinning, g-C3N4, Porous TiO2 NFs, Photocatalysis

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2


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Introduction

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The semiconductors photocatalysis for degrading organic pollutants is one of the most potentially

3

important technologies that have been given serious consideration in the past few decades.

4

Therefore, a large number of semiconductor materials like metal oxides and sulfides such as TiO2,

5

ZnO, ZnS, CdS and WO3 were successfully prepared by a variety of methods and many seek to

6

further maximize their efficiency

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most promising and efficient energy conversion catalyst for controlling organic pollutants because

8

of its unique characteristics, including low cost, encouraging optical and electronic properties,

9

nontoxicity, and high chemical and photochemical stability

1, 2, 3, 4, 5

. Among these, TiO2 has been considered as one of the

6, 7, 8

. Moreover, among various TiO2

10

nanostructures, the porous one-dimensional (1D) morphologies of the NFs have drawn a significant

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amount of attention due to their large surface area and distinctively-oriented NFs array, which

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improves the separation efficiency for the charge carrier by reducing the recombination rate. In

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recent years, many researchers have studied the relationship between porous TiO 2 NFs and

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photocatalytic performance, and have reported that TiO2 NFs having porous structures enhanced the

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photocatalytic performance. H. Hou et al. prepared porous TiO2 NFs from electrospinning and

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demonstrated that the (1D) porous fibers exhibit the highest photocatalytic activity compared to

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nanostructures composed of nanowires and naoparticles 9. Similarly, S. K. Choi et al. prepared

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electrospun, mesoporous TiO2 NFs and showed that the enhanced photocatalytic activity was due to

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the effect of porosity and NFs alignment, which could reduce the recombination of electron-hole (e-

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h) pairs through interparticle charge transfer along the NFs framework

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properties, practical application of TiO2 is severely limited because of the narrow range of effective

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wavelengths which belongs to the UV range and also the too-fast recombination of photo-induced

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electron-hole (e-h) pairs. Considering these limitations, a lot of effort has been devoted to

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synthesizing

hetero-structured

composites

of

TiO2

with

3


other

10

. In spite of these

visible-light-responsive

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semiconductors by this means extending the absorption edge of TiO2 to the visible range, and

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reducing the recombination of photoinduced e-h pairs 11, 12, 13, 14.

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Among various visible-light-responsive semiconductors, polymeric g-C3N4 has been utilized as a

4

coupled semiconductor in the most recent times. Because of its narrow band gap and outstanding

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mechanical, thermal, and optical properties, this metal-free photocatalyst is showing massive

6

potential for application to the photocatalytic degradation under visible light. However, in spite of

7

all these properties, the photocatalytic activity of pure g-C3N4 is also limited due to the minimal

8

surface area, low visible-light utilization rate and high recombination rate of photogenerated e-h

9

pairs

15

. Therefore, many attempts were made to improve the photocatalytic properties of g-C3N4,

10

such as the introduction of a mesoporous structure 16, 17, element doping with metals or nonmetals 18,

11

19, 20, 21

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combining π structures of g-C3N4 with highly active semiconductors has attracted much interest.

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Therefore, blending TiO2 with g-C3N4 sheets could be an appropriate technique for reducing the

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recombination of e-h pairs beyond those of TiO2 and g-C3N4. To date, several authors have invoked

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different techniques for synthesizing TiO2/g-C3N4 composites. Recently, Y. Li. et al. reported a

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TiO2/g-C3N4 hybrid from a seed-induced solvothermal method that enabled the fabrication of TiO2

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nanostructures on g-C3N4 surfaces with enhanced photocatalytic activity under visible light

18

et al. prepared a g-C3N4-TiO2 hybrid Z-scheme photocatalyst by a facile calcination route

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space separation efficiency of the photogenerated e-h pairs was enhanced through the Z-scheme

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route. Mostly, g-C3N4-based hybrid photocatalysts, including g-C3N4/TiO2, were prepared

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calcinations of precursor mixtures that yielded composite NPs that are prone to aggregate

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practical applications of g-C3N4 and TiO2 inexorably depend on the morphology of the material.

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Therefore, it is still a big challenge to control the structures of these photocatalysts. Previous

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literature has reported that the sheet-like structures of g-C3N4-based composites exhibit distinctive

, and building hetero-structures

22, 23

. Among them, constructing hetero-structures by

4


24

25

. Yu.

. The

26

. The

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optical and electronics properties compared to bulk g-C3N4 composites. P. Niu et al. revealed that

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the sheet-like structure of g-C3N4 exhibits increased lifetimes of photoexcited charge carriers

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This improved photoexcited charge carrier property was due to the quantum confinement effect.

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Similarly, among various TiO2 nanostructures, the one-dimensional TiO2 NFs or nanorods attracted

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serious interest in view of the dimensional confinement and minimal agglomeration compared to

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nanoparticles (NPs)

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composite made of sheet-like g-C3N4 and porous TiO2 NFs will exhibit encouraging properties,

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tending toward-enhanced the photocatalytic activity. So, we were inspired to develop a simple

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technique for creating such a composite material.

28

27

.

. Therefore, bearing in mind these observations, it is to be expected that a

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To our best knowledge, there has been no report on the fabrication of porous TiO2 NFs grafted onto

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g-C3N4 sheets. Recently, Han C. et al. synthesized a composite of TiO2 NFs and g-C3N4 sheets by

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electrospinning, in which g-C3N4 was embedded inside the NFs

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time, TiO2 porous NFs-having sufficient length were dispersed homogeneously and attached on the

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surfaces of g-C3N4 sheet, with minimal agglomeration of NFs from an angled two-nozzle

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electrospinning process with calcinations. Here, the enormous surface area of g-C3N4 sheets

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provides adequate surfaces for the direct attachment of TiO2 NFs, which make charge separation

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more efficient. Such NFs have a better chance than NPs to be uniformly attached onto g-C3N4

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sheets to form bonding, simply based on geometric considerations. Here also, a series of TiO2/g-

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C3N4 with different g-C3N4 loadings with respect to the polymer weight have been prepared. The

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photodegradation behaviors of rhodamine B (RhB) and reactive black (RB-5) over the prepared

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photocatalyst were measured under sun light. The composite exhibits higher photoactivity than pure

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TiO2 and g-C3N4 under natural sun light irradiation.

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Thus, this proposed method provides a facile and straightforward approach for affixing porous TiO2

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NFs on the surface of g-C3N4 sheets from a simple and low cost electrospinning process. 5


29

. But, in our study, for the first

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Experimental

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Materials

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Commercially available g-C3N4 particles nicanite®, (Carbedon, Finland), acetic acid, titanium

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isopropoxide (TTIP), polyvinyl acetate (PVAc, Mw = 500000) and N,N-Dimethylformamide (DMF)

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were purchased from Sigma Aldrich and used as-received.

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Fabrication of TiO2 NFs-intercalated g-C3N4 sheets

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Here, TiO2/g-C3N4 hybrid photocatalyst was directly prepared by using an angled two-nozzle

8

electrospinning-calcination process. Typically, two different solutions, one containing g-C3N4 and

9

the other containing TiO2 precursors were made from the same polymer solution. First, PVAc

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solution (18 wt %) in DMF was prepared by overnight magnetic stirring at room temperature.

11

Thereafter, g-C3N4 particles (1, 3, 5 and 10 wt % with respect to the weight of the PVAc) were

12

added to the PVAc solution and the mixture was subjected to bath sonication for two hours, to

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disperse the g-C3N4 particles. Similarly, the TiO2 precursor-containing PVAc solution was prepared

14

by mixing 6 g of the PVAc solution and 5 g of clear solution of TTIP (obtained by dropwise

15

addition of acetic acid with continuous stirring). For two-nozzle electrospinning, one syringe

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contained PVAc solution with g-C3N4 particles while the other contained PVAc solution with TiO2

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precursors. The angle between the two nozzle tips was maintained at 80°. A schematic diagram for

18

the preparation of the composite is as shown in Figure 1. Electrospinning was carried out in room

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conditions where the parameters include 18 kV of applied voltage, a tip-to-collector distance of 12

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cm, and solution feed rate of 1 ml/h. The obtained electrospun mats were vacuum dried at 80 °C

21

overnight. Then the vacuum-dried electrospun mats were treated in air at 500 °C for 3h at the

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heating rate of 5 °C/min. At such a high temperature, TTIP [Ti{OCH(CH] molecules could

23

decompose into TiO2, CO2 and H2O. The PVAc component was eliminated in the mean time. Here,

24

CO2 and H2O escaped rapidly, and only TiO2 and g-C3N4 sheets remained. At high temperature, 6


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TiO2 molecules could react and generate anatase/rutile-TiO2. Here, the composite material in which

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TiO2 NFs attached on the surface of the g-C3N4 sheets are denoted as “TCN-x”, where, “TCN”

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refers TiO2/g-C3N4 composite and x refers the wt % of g-C3N4 with respect to the polymer weight,

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namely 1, 3, 5 and 10. Note that the mass ratio of g-C3N4 sheet affects the spinnability of the

5

solution. Above 10 wt % of g-C3N4 sheet, spinnability diminishes and it is difficult to obtain fibers

6

due to the increasing viscosity. Moreover, pristine TiO2 NFs were prepared the same as TiO2/g-

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C3N4 from the solution containing TiO2 precursors, using a single-nozzle electrospinning process

8

with all conditions identical to that of the two-nozzle electrospinning.

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Characterization

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The X-Ray diffraction patterns of the resulting samples were obtained by using a Rigaku X-ray

11

diffractometer (XRD, Rigaku, Japan). Raman shift was investigated by Raman Spectroscopy

12

(Nanofinder® 30, Tokyo Instrument Inc.). Fourier transform infrared (FT-IR) spectra were recorded

13

using an ABB Bomen MB100 Spectrometer (Bomen, Canada). Microscopic morphologies of the

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samples were analyzed by field emission scanning electron microscopy (FE-SEM, Hitachi S-7400,

15

Hitachi, Japan) and transmission electron microscopy (TEM, JEM-2200, JEOL, Japan). The

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specific surface area was determined using multipoint Brunauer, Emmett and Teller (BET) analysis.

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The room-temperature photoluminescence (PL) spectra of the samples were investigating with a

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luminescence spectrometer (LS 55; Perkin-Elmer Inc., USA) with a Xe lamp.

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Electrochemical measurements

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The cyclic voltammograms (CV) were taken in an N2-saturated 0.01M potassium ferrocyanide

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solution prepared with 1M KNO3, with the scanning rate 50 mV/s on a

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Potentiostat/Galvanostat/EIS (WonTech, ZIVE, SPI Korea) that was equipped with three-electrode

23

system. A platinum electrode, 1.6 mm in diameter and coated with the as-prepared catalyst paste

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was used as the working electrode. A standard calomel electrode was used as the reference 7


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electrode and platinum wire was used as the counter electrode. Similarly, electrochemical

2

impedance spectra (EIS) Nyquist plots measurements were taken in the same system over the

3

frequency range of 0.001 to 100 kHz. Before measurement, the working electrode was prepared as

4

follows: 20 mg of the different as-prepared samples were added to 20 µL of a solution of Nafion (5

5

wt %) and propanol. Subsequently, ultrasonication was carried out for 1 h to obtain a homogeneous

6

paste. Then 5 µg of the paste was taken and dripped on the surface of the platinum electrode to

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form a thin catalyst film on the electrode. After drying at 80 °C in an oven for 30 min, the working

8

electrode was used for experiments.

9

Photocatalytic activity test

10

The photocatalytic activities of the prepared composites were evaluated by degrading RhB (10 ppm)

11

and RB-5 (10 ppm) under natural sunlight irradiation. The amount of the as-prepared composite,

12

initial concentration and the volume of the mixed solution were the same in all experiments. During

13

photocatalytic degradation, 20 mg of the as-prepared composite was mixed with 30 ml of the

14

organic dyes solution in a 100-ml beaker. Prior to photocatalytic reaction the mixed solution was

15

stirred in the dark for 30 min to reach desorption-absorption equilibrium. At a given time interval, 1

16

ml of the solution was withdrawn, centrifuged and filtered out, and, absorbance was measured.

17

Then the concentration of the dyes was analyzed using an UV−vis spectrophotometer. Following

18

equation was used to account the degradation rate, Degradation Efficiency % = ( −

C )× CO

%

19

where, Co is the initial concentration and C is the concentration after “t” minutes of irradiation. In

20

the reusability experiment, the recovered photocatalyst was centrifuged, filtered and dried. The

21

dried composite was used for the next run after making up for the lost portion with fresh solution.

22

All of the experiments were conducted in September, 2015 during sunny days in Jeonju, a city of

23

South Korea. 8


Page 33 of 70

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Langmuir

1

Results and discussion

2

Characterization of the TCN samples

3

Figure 2 shows the XRD patterns of the different samples. It is noticeable that the synthesized TiO2

4

NFs and the composites with different wt % ratios of g-C3N4 exhibit similar patterns. The XRD

5

patterns reveal that most of the diffraction peaks of pure TiO2 correspond to the anatase phase. The

6

diffraction peak of pure g-C3N4 appearing at 27.4° matches to the (0 0 2) plane, which corresponds

7

to the characteristic interplanar stacking peak of the aromatic system 30. The peaks of TiO2 at

8

around 25.3°, 37.8°, 38.58°, 37.8°, 47.9°, 53.89°, 55.06°, 62.7°, 68.7°, 70.3° and 75.0° were attributed

9

to the (1 0 1), (0 0 4), (1 1 2), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0) and (2 1 5) lattice

10

planes of anatase TiO2 (JCPDS, 21-1272). The peaks around 27.5°, 41.2° and 56.7° were attributed

11

to the (1 1 0), (1 1 1) and (2 2 0) lattice planes of rutile TiO2 (JCPDS 75-1755) 31. The anatase (1 0

12

1) peak for composites is much broader than that of pristine TiO2 (Figure 2 inset). This broadening

13

of peaks suggests that the lattice structure of TiO2 is distorted due to the intense interaction between

14

TiO2 and g-C3N4. Moreover, the diffraction peak of TiO2 at the higher wt % of g-C3N4 becomes

15

weaker, and also shifted slightly to a smaller value diffraction angle, further showing that the g-

16

C3N4 affects the crystal growth of TiO2. Thus, such broadened, weakened and shifted peaks can be

17

attributed to the intense interaction between TiO2 and g-C3N4 32. It should be noted that no typical

18

peak of g-C3N4 appears in the TiO2/g-C3N4 composite. The reason can be recognized to the low

19

weight loading of g-C3N4.

20

Figure 3 shows the Raman spectra of pure TiO2 and TCN-5 in the range of 0-1200 cm-1. Here,

21

Raman spectra were used to confirm the formation of TiO2 anatase phase and the attachment of g-

22

C3N4 sheet with TiO2 NFs. The pure TiO2 shows peaks at wave numbers of about 142 (Eg), 386

23

(B1g), 518 (A1g +B1g) and 640 (Eg) which can be attributed to the typical anatase phase of TiO2. 9


Langmuir

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Page 34 of 70

1

These observed peaks were assigned to the Ti—O—Ti network structure of TiO2 33. When the NFs

2

were attached on g-C3N4 sheets, there is a considerable increase in the intensity of these four

3

characteristic peaks. The possible reason can be credited to the fact that the porous TiO2 NFs are

4

attached on the surface of g-C3N4 sheets with strong chemical interaction. Moreover, the possible

5

formation of new bonds on the surface of TCN-5 composite was responsible for the enhancement

6

of the intensity at 142, 386, 518, and 640 cm−1. In addition, reducing properties of graphitic carbon

7

in carbon nitride facilitated the formation of a bulk anatase phase after the calcinations of TCN-5,

8

which also supports the enhanced intensity

9

prepared composite shows the slight shifting of Eg towards a higher wave number. This shift is

34, 35, 36

. Compared with pristine anatase TiO2, the as-

37

10

another indication for the formation of new bonds or bond modifications

11

analysis confirms that the TiO2 NFs could in fact be attaching to the surface of g-C3N4 sheets with

12

strong interaction rather than existing purely as a mixture of the two.

13

Regarding the FT-IR spectrum of bare TiO2 shown in Figure 4, the broad-band at around 500 to 800

14

cm-1 is assigned to the Ti—O—Ti stretching vibration mode. Additionally, the peak at 3400 cm-1

15

correspond to the stretching vibration mode of O—H bonds of free water molecules and Ti—OH,

16

and N—H vibrations for NH4+ ions 38. For the spectrum of bare g-C3N4, three characteristic

17

absorption regions were revealed located around 3200 cm-1, 1200-1650 cm-1, and 809 cm-1. Several

18

bands that were found in the 1200—1650 cm-1 corresponds to the typical stretching mode of

19

aromatic CN heterocycles 39. The peaks around 1640, 1462 and 1407 cm-1 is attributable to the

20

stretching vibration modes of heptazine-derived repeating units. In addition, the peaks at 1316 and

21

1240 cm-1 are allocated to the characteristic stretching vibrations of C—N(—C)—C 40. Similarly,

22

the broad adsorption band centered at 3188 cm-1 for g-C3N4 initiates from the stretching vibration

23

of the N—H bond associated with the uncondensed amino group. The strong absorption at 809 cm-1

24

could be attributed to the s-triazine ring system 41. As expected, all of the main characteristic 10


. Thus, the Raman

Page 35 of 70

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Langmuir

1

absorption peaks of TiO2 and g-C3N4 were found to coexist in the TCN-x. Moreover, compared

2

with the peak at 3350 cm-1 for pure TiO2, a slight shift of this peak to the higher wavelength value

3

was observed in TCN-x, suggesting an interfacial interaction between TiO2 and g-C3N4 (Figure 4

4

inset). Such an intense interfacial interaction between the two compounds would enhance the

5

catalytic activity 25.

6

The morphologies of the prepared samples are observed by FE-SEM. The g-C3N4 showed irregular

7

surface morphology with different shapes and sizes of particles. However, its morphology has

8

significantly changed into sheet like structure after sonication. The different morphology and the

9

constituting elements of g-C3N4 are given in Figure S1. It was found that TTIP/PVAc composite

10

electrospun NFs from single nozzle electrospinning process, are uniform with smooth surfaces

11

before calcinations (Figure 5a). It can be observed from the figure that the TiO2 NFs become porous

12

and remained as continuous structures with mostly uniform diameters after calcinations. However,

13

their average diameter was decreased slightly (Figure 5b). The formation mechanism of porous NFs

14

is given in Figure S2. Figure 5c displays the morphology of composite electrospun mat of

15

TTIP/PVAc/g-C3N4 sheet prepared by means of angled two-nozzle electrospinning process. Here,

16

the nanosize particles were electrospun with and attached to PVAc fibers, whereas microsized

17

sheets were electrospun along with a number of PVAc fibers and attached on the composite mat.

18

The image clearly reveals the presence of g-C3N4 sheets in the composite mats. Moreover, the g-

19

C3N4 sheets do not seem to have a significant effect on the smoothness and uniformity of the NFs

20

indicating good performance of the electrospinning conditions. The FE-SEM images of electrospun

21

composite mats given in Figure S3 further clearly confirm the attachment of g-C3N4 sheets and the

22

morphology of the NFs. It is perceptible on the FE-SEM image that the TiO2 NFs are affixed on the

23

g-C3N4 sheet after calcinations in a reasonably unvarying manner (Figure 5d). The attached NFs

24

have a length of several micrometers with porous structure, but the diameter varies around 40 nm to 11


Langmuir

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Page 36 of 70

1

200 nm. In contrast to the NPs, such NFs of significant length promote direct and sufficient contact

2

with g-C3N4 sheet, resulting in less agglomeration. Such less agglomerated and uniformly

3

distributed NFs attached on the g-C3N4 sheets enhanced the surface area and porosity-related

4

characteristics of the composite than pristine TiO2 NFs, hence, further improving the

5

photodegradation efficiency. Moreover, such a very thin sheet, having large aspect ratios, high

6

surface area and a stoichiometric N/C ratio, increase the transport of charges and reduce the

7

recombination probability of photoexcited charge carriers.

8

Corresponding TEM images of TiO2 and TCN composites are displayed in Figure 6. Figure 6b

9

shows homogeneous, long and narrow, porous TiO2 NFs attached to the g-C3N4 sheet with a narrow

10

distribution of diameters ~ 125 nm. Moreover, HR-TEM image of the TCN composite suggests

11

intimate contact of TiO2 NFs and the g-C3N4 sheets. As shown in Figure 6c, a clear three sets of

12

lattice fringes of the interplanar spacing of about 0.48 nm, 0.35 nm and 0.35 nm are observed in the

13

dark area, which are very close to the (0 0 2), (1 0 1) and (1 0 1) planes of TiO2, respectively. The

14

electron beam can pass more easily through the g-C3N4 due to its low atomic weight, so the light

15

area can be determined to be g-C3N4 where it is very difficult to observe the lattice fringe.

16

The XPS survey spectrum and high resolution spectrum of the pristine and composite samples were

17

carried out to further illuminate the surface composition and chemical interaction between the

18

elements. Figure 7a illustrates the XPS survey spectrum of g-C3N4, TiO2 and TCN-5. Obviously,

19

the g-C3N4 contains the photoelectron peaks of C and N elements, and TCN-5 composite contains

20

the photoelectron peaks of C, N, Ti and O elements. However, the pure TiO2 NFs not only contain

21

the photoelectron peaks of Ti and O elements, but also contains the peak of C elements. This carbon

22

peak is ascribed to the residual carbon from the samples and adventitious hydrocarbon from the

23

XPS instrument itself. The deconvoluted peaks of all elements of the composite material are given 12


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Langmuir

1

in the corresponding high-resolution spectrum (Figure 7b-7e). Correspondingly, the high-resolution

2

XPS spectrum of all elements of the pristine TiO2 and g-C3N4 are given in Figure S4. The C 1s peak

3

of composite can be deconvoluted two fitted peaks at 285.65 eV and 287.05 eV (Figure 7b)

4

indicating that carbon posses two diverse chemical states 42. The peak around 285.65 eV is

5

attributed to defects in the g-C3N4 that involves sp2-hybridized carbon atoms. In addition, the other

6

peak located around 287.05 eV is assigned to N—C━N coordination 43, 44. The curves of N 1s

7

region can be divided also into two peaks situated at 397.7eV and 399.8 eV (Figure 7c). The main

8

peak located at 397.7eV belong to sp2 –hybridized pyridinic-like nitrogen(N-sp2C) and the peak

9

located at 399.8 eV corresponds to the tertiary pyrrolic graphitic nitrogen (N—(C)3) 45. The Ti 2p3/2

10

spin-orbital splitting photoelectron of the composite was located at binding energy 459.5 eV in the

11

Ti 2p spectrum. Similarly, peaks at 465.05 eV in the same spectrum correspond to the Ti 2p1/2

12

(Figure 7d) 46. The peak situated at 530.65 eV in the O 1s is ascribable to oxygen anions in the

13

lattice (Ti-O) (Figure 7e) 47. Furthermore, the binding energy values of Ti 2p in the composite are

14

slightly higher than those of pristine TiO2 (Figure 7f). Correspondingly, binding energy value of O

15

1s is also slightly higher in the composite compared to pristine TiO2 (Figure S5). Such shifts for

16

binding energy in the XPS spectra reflect the intense interaction between TiO2 and g-C3N4 48. Hence,

17

the results from XRD, Raman, FT-IR, TEM and XPS analyses strongly verified that the TiO2 NFs

18

are attached on the surface of g-C3N4 sheets forming a hetero-structure rather than a physical

19

mixture.

20

Figure 8a shows the UV-vis absorption spectra of different samples. The absorption intensity of the

21

pure g-C3N4 occurs at wavelengths greater than 450 nm, in good accordance with the band gap of

22

g-C3N4 (~ 2.7eV). However, the absorption wavelength of synthesized TiO2 NFs is less than 400

23

nm, consistent with the intrinsic band gap of anatase TiO2 (~ 3.18 eV) which means that it could

24

only have a response in the UV zone. The absorption edge of TCN-x composites shifts extremely to 13


Langmuir

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Page 38 of 70

1

a longer wavelength in comparison to the pure TiO2 NFs, clearly illuminating that the visible light

2

absorption was enhanced, which was because of the existence of g-C3N4 sheets. All of the TCN-x

3

composites exhibited absorbance in the visible light region, with the absorbance edges between

4

those of pure TiO2 and g-C3N4. It is noteworthy that the unattached TiO2 NFs express absorption in

5

the UV light region, while the TiO2 NFs attached on g-C3N4 sheets demonstrate absorption in the

6

visible light region. The band-gap energy of a semiconductor can be determined by the following

7

equation 49, 50, 51,

�ℎ� 8

1⁄

= � ℎ� − ��

where A is a proportional constant, ℎ� is the photo energy, h is Planck’s constant, � is the

10

frequency of vibration, � is an absorption coefficient and �� is the band gap energy. The value of

11

� depends on the transition in semiconductors. For g-C3N4 and TiO2, value of � is 1/2 for direct transition and 2 for indirect transition. Thus from the Figure 8b, the band gap energies of g-C3N4,

12

TiO2, TCN-1, TCN-3, TCN-5 and TCN-10 are calculated to be 3.35, 3.28, 3.0, 2.94, 2.8 and 2.73

13

eV respectively. The following empirical equations are used to calculate the CB and VB of

14

semiconductors 47,

9

�� = � − � + .5�� 15 16 17 18

where �� is the valence band potential, �

= �� − �� is the conduction band potential, � is the electro

negativity which is calculated from the geometric mean of the constitute elements, � is the

energy of free electrons on the hydrogen scale which is 4.5 eV vs NHE, and �� is the band gap

energy. The � values are 5.81 and 4.64 for TiO2 and g-C3N4, respectively. From the above 14


Page 39 of 70

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Langmuir

2

equations, �� and �

3

Correspondingly, �� and � respectively.

4

Photocatalytic activity

5

On the basis of the above results, photocatalytic activities of the different samples were assessed by

6

decomposing RhB and RB-5 under natural sunlight. Photodegradation was measured at 10 min and

7

5 min intervals for RhB and RB-5 respectively. Then the concentrations of RhB and RB-5 in the

8

solution were plotted as a function of irradiation time. The results of different catalysts are

9

displayed in Figure 9. As displayed in the figure, all TCN-x samples exhibited a better

1

values of g-C3N4 are calculated to be 1.5 and -1.23 eV, respectively. values of TiO2 are calculated to be 2.985 and -0.365 eV,

10

photocatalytic activity than pure TiO2 and g-C3N4 over RhB (Figure 9a) and RB-5 (Figure 9c),

11

suggesting that a synergistic effect exits between g-C3N4 and TiO2. This superior performance of

12

TCN-x over RhB and RB-5 could also be confirmed from the photographs shown in Figure 9b and

13

9d, respectively. In the absence of a photocatalyst, the self-degradation of both RhB and RB-5 is

14

negligible. Moreover, the figures show that the photocatalytic activity is enhanced with the content

15

of g-C3N4 increasing from 1 to 5 wt % of polymer weight. However, further increasing the g-C3N4

16

weight leads to a decrease in the degradation rate under the same photolysis condition. It is

17

noteworthy that the excess amount of carbon nitride in the composite creates an unsuitable ratio

18

between the TiO2 and g-C3N4, which may block the electron transfer from g-C3N4 sheets to TiO2

19

NFs and reduce the photogenerated charge separation. Thus, there is an optimum g-C3N4 content

20

for maximizing the photocatalytic activity. The TCN-5 samples demonstrate the highest

21

photocatalytic activity, which can absorb all most all of RhB and RB-5 after degradation for a short

22

period, 40 min and 20 min, respectively.

15


Langmuir

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Page 40 of 70

1

BET surface area of the samples was determined by a multipoint Brunauer-Emmett-Teller (BET)

2

method using nitrogen adsorption-desorption analysis. Table S1 summarizes the specific surface

3

area of pristine TiO2 and g-C3N4, and TCN-x composites. As shown in Table S1, the BET surface

4

area of TCN-x composites was higher than that of pristine TiO2 and g-C3N4. The reason behind this

5

is that the large surface area of g-C3N4 sheets provides ample space for TiO2 NFs, preventing the

6

agglomerating of NFs and increasing porosity of the composite, effectively increasing the surface

7

area. As can be seen from the table, the BET surface area of the composite gradually increases with

8

increasing amounts of g-C3N4 and showed the highest surface area (95.6313 m2/h) at TCN-5.

9

However, the BET surface area of TCN-10 was smaller than other composites indicating that this

10

may be an unsuitable ratio of TiO2 and g-C3N4. Thus, the higher surface area obtained in TCN-5

11

may be the result of a suitable ratio between TiO2 and g-C3N4. Correspondingly, the porous

12

characteristics, such as porosity, pore volume, pore size distribution and density of g-C3N4, TiO2

13

and TCN-5 samples were analyzed by mercury porosimetry analysis technique (Autopore IV,

14

Micromeritics USA). The different porous characteristics and distribution of pore diameters were

15

given in Table S2 and Figure S6, respectively. As shown in Figure S6, distribution of pore diameter

16

for TCN-5 and TiO2 were almost similar. However, the g-C3N4 showed lower pore size diameter

17

distribution compared to them. Here, the size of the pores means the space between two NFs. As

18

expected, porosity of TCN-5 was slightly higher than TiO2, which is in agreement with the FE-SEM

19

morphology. A possible reason for this is the large surface area of g-C3N4 sheets providing

20

sufficient space for the uniform attachment of TiO2 NFs which increased the porosity of the

21

composite. Hence, the TCN-5 composite, which has higher surface area and porosity, increased the

22

contact chance between pollutants and photocalayst, which is beneficial for improving the

23

photocatalytic degradation activity of the hybrids. However, the surface area and porosity are not

24

the only factors that affect the photoactivity. Other factors such as effective wavelength range and 16


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Langmuir

1

rate of recombination of e-h pairs also affect the photocatalytic activity 52. Hence, the TiO2 NFs

2

which have a higher surface area than g-C3N4, showed lower photocatalytic activity under sunlight

3

irradiation.

4

The Langmuir-Hinshelwood model is mainly used to estimate the kinetics of the photocatalytic

5

degradation

6

and degradation rate (r), which is expressed as 54,

53

. Their model basically narrates the concentration of reactants in water at time t(C)

�=−

�

=

�� + ��

7

where, Kr is the rate constant and Kad is the adsorption equilibrium constant. When the adsorption is

8

relatively weak and the concentration of reactants in a photocatalytic reaction is low, the

9

photocatalytic reaction equation can be simplified to a pseudo-first-order equation with an apparent

10

first-order rate constant (Kapp) as, �� (

� ) = �� = ��

11

where, Co is the initial concentration of the substrate. Figure 10 shows the linear relationship

12

between ln(Co/C) and the irradiation time with Kapp for different catalyst. The Kapp value commonly

13

provides an indication of the activity of the photocatalyst. The Kapp value shows that the

14

photocatalytic activity for all TCN-x is higher than those of the pristine TiO2 and g-C3N4 for both

15

RhB and RB-5, which suggests that there exists a significant synergistic effect between g-C3N4 and

16

TiO2.

17

Figure 11 reveals the PL spectra of different samples in the wavelength range of 300-600 nm. The

18

first peak of TiO2 in the PL spectra around 370 nm is due to the direct recombination between

19

electrons in the conduction band and holes in the valence band 55. Additionally, the PL spectrum

20

peak around 480 nm and 530 nm is attributed to the surface oxygen vacancies and related defects 44.

21

Photo-induced electrons are attached by these oxygen vacancies and defects to form excitons, so 17


Langmuir

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Page 42 of 70

1

that PL signal can occur easily 56. Similarly, pure g-C3N4 exhibits a high intensity emission peak at

2

around 450 nm, which can be ascribed to the band-band PL phenomenon with a light energy that is

3

approximately equal to the band gap energy of g-C3N4 57. This band-band PL signal is a result of the

4

n →π* electronic transition involving lone pairs of nitrogen atoms in g-C3N4. Here, the all PL

5

intensity peaks of TCN-5 are lower in comparison to that of the pristine TiO2 and g-C3N4. Such a

6

lower PL intensity is a general indication of a lower recombination rate of e-h pairs 58.

7

In water-splitting photocatalysis, two processes are involved; oxidation process in which the holes

8

transfer from the photocatalyst to the degradable chemical in solution and a reduction process in

9

which electrons transfer from the photocatalyst to the solution. The better conductivity of g-C3N4

10

offers it as a super charge-carrier transport medium, and its large surface area may also bring about

11

a higher carrier transfer rate between the photocatalyst and the solution/photodegradable chemicals.

12

For better understanding of such a catalytic performance, a potentiostat/galvanostat/EIS analyzer

13

was used to measure the CV to confirm the interfacial charge transfer effect of TiO2, g-C3N4 and

14

TCN-5. Figure 12 reveals CV of different samples, in which clearer reduction and oxidation peaks

15

are observed for TCN-5 than other two pristine samples. As shown in the figure, the oxidation

16

peaks for g-C3N4, TiO2 and TCN-5 are located at 0.45, 0.57and 0.27 V and respectively represent

17

the oxidation of ferrocyanide with the loss of one electron. The peak current of TCN-5 (0.127

18

mA/cm2) is much higher than that of g-C3N4 (0.0505 mA/cm2) and TiO2 (0.039 mA/cm2)-more than

19

2.5 times, signifying a considerably improved rate of electron transfer due to g-C3N4 being a highly

20

conducting substrate. Furthermore, the ratio of the strengths of the oxidation (0.127 mA/cm2, 0.29V)

21

and reduction peaks (0.112 mA/cm2, 0.15V) of TCN-5 is nearly 1, which specifying greatly

22

enhanced reaction reversibility. EIS Nyquist plots were also recorded to further investigate the

23

interfacial charge immigration of the samples (Figure 13). It has been reported in recent research

24

that a smaller arc radius in Nyquist plots are related to a more effective interfacial charge 18


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Langmuir

1

immigration in semiconductor based electrode 59. Arc radius for TCN-5 in the Nyquist plot shown

2

in figure is smaller compared to g-C3N4 and TiO2, demonstrating the reduced interface resistance so

3

that the separation and migration efficiency of e-h pairs will be enhanced. Thus, combine with the

4

PL spectra, CV measurements and EIS Nyquist plots, it is concluded that the g-C3N4 sheet modified

5

by porous TiO2 NFs exhibits the most charge-carrier migration compared to ones.

6

A schematic diagram for the photocatalytic mechanism of TCN hetero-junction was proposed based

7

on the above experimental results and discussion (Figure 14). It was well understood that the

8

enrichment of the photodegradation activities of the composite materials was mainly to be realized

9

via the effective transfer of electron and holes at the interface of the photocatalysts. When TCN-x

10

was exposed to the visible light, g-C3N4 could easily absorb the visible light because of its band gap

11

(2.73 eV), leading to the excitation of electrons and generation of e-h pairs. Given the calculated

12

value above, the CB edge potential of g-C3N4 (-1.23 eV) is more negative than that CB of TiO2

13

(-0.365 eV). Hence the photogenerated electrons transfer easily from the CB of g-C3N4 to the CB of

14

TiO2. It has been reported in various previous research that the difference in the CB edge potential

15

between these two materials were perhaps a more powerful driving force, which promote the

16

transfer of electrons between hetero-junctions and reduce the recombination of charge carriers 58, 60.

17

Since the CB edge potentials of g-C3N4 and TiO2 were more negative than the reduction potential of

18

oxygen E° (O2/.O2̶ ) (-0.046 eV) 61, electrons transferred to the CB of TiO2 NFs,

19

main active groups of superoxide anions radicals (.O2̶) through reacting with dissolved oxygen near

20

the surface of TiO2, which is mainly responsible for the degradation of the pollutants. In addition,

21

since the VB edge potential of TiO2 (2.985 eV) is more positive than the VB of g-C3N4 (1.5 eV),

22

transfer of photogenerated holes occurs from the VB of TiO2 to the VB of g-C3N4 through the close

23

interfacial connection between g-C3N4 and TiO2 61, 62. However, the hole on the VB of g-C3N4 could

24

not generate .OH radicals through reacting with OH− or H2O because of the lower VB level of g19


generating the

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C3N4 compared to the potential of .OH/OH− (1.99 eV) 63. Conversely, the remaining holes on the

2

VB of TiO2 directly oxidize the pollutants and concurrently generate .OH radicals through reacting

3

with H2O because of the higher VB level of TiO2 compared to the potential of .OH/OH−. Since

4

most of the holes are transferred from the VB of TiO2 to the VB of g-C3N4, the number of holes that

5

remain in the VB of TiO2 is considerably reduced. Therefore, in TCN composites, the .O2̶ radicals

6

and electron are the most active species during photocatalysis compared to .OH and holes. However,

7

for pure g-C3N4, most of the photogenerated electrons and holes recombine due to the narrow

8

energy gap (2.73 eV), and only a small fraction participate in the photocatalytic reaction, resulting

9

in lower activity 27. Correspondingly, because of broader band gap (Eg = 3.35 eV), the anatase TiO2

10

could not be exited under 420 nm visible light irradiation, thus showing a weaker photocatalytic

11

degradation activity. In most of the previous studies of TiO2 and g-C3N4 composites, TiO2 NPs were

12

dispersed on the g-C3N4 sheet. Such NPs have fewer chances for uniform distribution over the

13

surface of the g-C3N4 sheet without agglomeration. As mentioned above, NFs have more uniform

14

distribution over the surface of g-C3N4 sheets so more NFs have direct attachment with the g-C3N4

15

sheet. Moreover, such porous structured NFs, having enhanced surface area and porosity, could

16

promote the migration of electrons. Therefore, electron transfer in such a straight path between g-

17

C3N4 sheet and NFs will be much easier in comparison to the zigzag path in agglomerated NPs.

18

The recycling capability of the TCN-5 photocatalyst was investigated, by degrading RhB under

19

visible light, cycled for four times. For this, the recovered photocatalyst was centrifuged, filtered,

20

and dried at 60 °C overnight. This dried composite was used for the next run after making up the

21

lost portion with fresh material. As displayed in Figure 15, the photocatalytic activity of TCN-5 for

22

RhB degradation decreased from 91.26% to 84.56% after the four-run test, which demonstrates that

23

it has satisfactory stability of the photocatalytic activity. Moreover, FT-IR patterns were used to

24

further investigate the lifetime of g-C3N4, TiO2 and TCN-5. For this, the photocatalysts after 4 20


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1

cyclic runs were centrifuged, filtered and dried at 60 °C overnight. Then, the FT-IR spectra of these

2

used materials were taken and compared with fresh one. Figure S7 clearly illustrates that the FT-IR

3

patterns of all samples after four cyclic runs had not changed depreciably compared to the fresh

4

sample. This result hints that the lifetime of these samples are satisfactory for photocatalytic

5

degradation activity.

6

Conclusions

7

In summary, the porous TiO2 NFs became well attached to the surface of the sheet-like structure of

8

g-C3N4 through the use of a simple and facile electrospinning-calcination process. After

9

incorporation of porous TiO2 NFs, the TiO2/g-C3N4 composites exhibited significantly enhanced

10

photocatalytic degradation over RhB and RB-5 than did pristine TiO2 and g-C3N4, under irradiation

11

by natural sunlight. The PL spectra, CV and EIS Nyquist plots measurements suggest that the better

12

conductivity of g-C3N4 provides a super charge-carrier transport medium and a higher carrier

13

transfer rate between the photocatalyst and the solution, by holding down the recombination of e-h

14

pairs in TiO2, leading to the extended lifetime of charge carriers and improved photocatalytic

15

activity.

16

Associated Content

17

Supporting Information

18

FE-SEM images, SEM-EDX image with real atomic compositions and XPS survey spectrum of g-

19

C3N4; porous formation mechanism of TiO2 NFs; FE-SEM images of electrospun PVAc NFs with

20

g-C3N4 sheets in different magnifications; high resolution XPS spectra of Ti 2p, O1s, C1s, and N1s

21

for the pristine g-C3N4 and TiO2 NFs; binding energy shifting comparison of O1s high resolution

22

XPS spectra from pristine TiO2 NFs and TCN-5 composite; BET surface area and Langmuir surface

23

area of different samples; distribution of pore diameter.

24

21


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1

Acknowledgement

2

This paper was supported by the Human Resource Training Program for Regional Innovation and

3

Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-

4

2015H1C1A1035635)

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

22


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Figure 1: Schematic illustration for the synthesis of the TiO2/g-C3N4 composite.

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Figure 2: XRD patterns of (a) g-C3N4, (b) TiO2, (c) TCN-1, (d) TCN-3, (e) TCN-5, and (f) TCN-10.

3

Inset XRD pattern showing the 23-30 degree region.

4 5 6 7 8 9 10 11 12

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Figure 3: Raman shift of TiO2 and TCN-5.

3 4 5 6 7 8 9 10 11 12 13 14

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Figure 4: FT-IR images of (a) g-C3N4, (b) TiO2, (c) TCN-1, (d) TCN-3, (e) TCN-5 and (f) TCN-10.

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Inset FT-IR spectra showing 2500-4000 cm-1 region.

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Figure 5: FE-SEM images of (a) TTIP/PVAC fibers before calcinations, (b) TiO2 fibers after

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calcination, (c) TTIP/PVAC/g-C3N4 fibers before calcinations, and (d) TCN composite after

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calcinations.

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Langmuir

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Figure 6: (a) TEM images of (a) TiO2 NFs, (b) TCN, and (c) HR-TEM images of TCN composite.

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Figure 7: (a) XPS survey spectrum and high resolution XPS spectra of (b) C1s, (c) N1s, (d) Ti 2p,

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(e) O1s and (f) shifting of Ti2p peak in TCN-5 composite.

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Langmuir

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Figure 8: (a) UV-vis absorption spectra of TiO2, g-C3N4 and TCN-x samples and (b) plots of (αhν)2

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vs. photon energy (hν) for the band gap energies of different samples.

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Figure 9: (a) Photodegradation of RhB under natural sun light in the corresponding time intervals;

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(b) Photo images showing the initial 10-ppm RhB solution and the remaining RhB in the solution

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after being irradiated for 40 min; (c) Photodegradation of RB-5 under natural sunlight in the

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corresponding time intervals; and (d) Photo images showing the initial 10- ppm RB-5 solution and

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the remaining RB-5 in the solution after being irradiated for 20 min.

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Langmuir

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Figure 10: Kinetics of photocatalytic degradation of different catalysts, (a) 10-ppm RhB solution,

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and (b) 10-ppm RB-5 solution.

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Figure 11: PL spectra of g-C3N4, TiO2 and TCN-5

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Figure 12: Cyclic voltammograms of TiO2, g-C3N4 and TCN-5

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Figure 13: Electrochemical impedance spectra of TiO2, g-C3N4 and TCN-5

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Figure 14: Schematic diagram of photogenerated charges in the TCN hetero-junction.

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Figure 15: Cyclic runs of TCN-5 in the photocatalytic degradation over RhB under natural light

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irradiation.

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Page 70 of 70

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Electrospinning directly synthesized porous TiO2 nanofibers modified by graphitic carbon

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nitride sheets for enhanced photocatalytic degradation activity under solar light irradiation

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Table of Contents Graphic

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Electrospinning Directly Synthesized Porous TiO2 Nanofibers

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