Enhanced photocatalytic performance of TiO2 nanotube based heterojunction photocatalyst via the coupling of graphene and FTO

Accepted Manuscript Title: Enhanced photocatalytic performance of TiO2 nanotube based heterojunction photocatalyst via the coupling of graphene and FTO Author: Xiaoyou Niu Jianyuan Yu Likun Wang Chen Fu Jixia Wang Li Wang Hongli Zhao Jingkai Yang PII: DOI: Reference:

S0169-4332(17)30913-3 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.220 APSUSC 35596

To appear in:

APSUSC

Received date: Revised date: Accepted date:

26-1-2017 16-3-2017 24-3-2017

Please cite this article as: X. Niu, J. Yu, L. Wang, C. Fu, J. Wang, L. Wang, H. Zhao, J. Yang, Enhanced photocatalytic performance of TiO2 nanotube based heterojunction photocatalyst via the coupling of graphene and FTO, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.03.220 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.

Enhanced Photocatalytic Performance of TiO2 Nanotube Based Heterojunction Photocatalyst via the Coupling of Graphene and FTO

Wang a, Hongli Zhao a,b,*, Jingkai Yang d,* a

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Xiaoyou Niu a, Jianyuan Yu a,c, Likun Wang a, Chen Fu a, Jixia Wang a, Li

cr

College of Materials Science and Engineering, Yanshan University, Qinhuangdao, 066004, China

State Key Laboratory of Metastable Materials Science and Technology,

us

b

c

an

Qinhuangdao, 066004, China

Department of environmental and chemical engineering, Tangshan

d

M

University, Tangshan, 063000, China

National Defense Science and Technology, Yanshan University,

Ac ce p

Abstract

te

d

Qinhuangdao, 066004, China

The TiO2 Nanotube (TONT) based heterojunction photocatalyst was developed via the coupling of reduced graphene oxide (rGO) and SnO2: F film (FTO). Based on the characterization of Raman analysis, XRD, SEM, TEM, XPS and ESR, the crystal phase, morphology, heterojunction interfacial interaction and the photoinduced electron chemical environment of the samples are studied. In the photodegradation of methylene blue (MB) solution under UV irradiation, the rGO-TONT/FTO heterojunction photocatalyst exhibits the improved photocatalytic reaction rate, 3 times greater than that of pure TONT. The enhanced photocatalytic mechanism 1  

Page 1 of 30

was discussed by PL. The effectively separate charge in heterojunction structure of rGO-TONT/FTO is responsible for the enhanced photocatalytic activity. Wherein, the abundant oxygen vacancies at TiO2 surface and the

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chemically bonded interface in rGO-TONT heterojunction also contributes

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to the interfacial electron transfer. Besides, the introduction of rGO

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enhanced its optical absorption capacity.

Keywords: TiO2 Nanotube; Graphene; FTO; Heterojunction; Charge

an

separation; Photocatalytic Performance.

M

Corresponding author at: State Key Laboratory of Metastable Materials

te

China

d

Science and Technology, Yanshan University, Qinhuangdao, 066004,

Ac ce p

E-mail address: [email protected] (H. L. Zhao), [email protected] (J. K. Yang)

1. Introduction

TiO2-based artificial photosynthetic systems, toward efficient utilization

of solar energy in numerous fields such as photon devices, dye-sensitized solar cell, optical decomposition of water, especially in photocatalysis, have witnessed a persistent upsurge interest in recent years [1,2]. Although TiO2 has been the most promising candidate for photocatalyst, its own defects like incapacity of absorbing visible light and photoinduced 2  

Page 2 of 30

electron-hole pairs are easy to recombination, greatly limits the photocatalytic quantum efficiency [3,4]. Based on this, various types of materials

have

been

ingeniously

combined

to

realize

artificial

ip t

high-efficiency photocatalyst. Among numerous strategies, the construction

cr

of semiconductor heterojunction, for instance, TiO2-CdS, TiO2-SnO2, TiO2-graphene, TiO2-CNT [5-8], has been drawn wide attention on its

us

facile charge transportation at the interface and inhibit charge

an

recombination.

In particular, Graphene-TiO2 heterojunction photocatalyst has exhibited

M

enhanced photocatalytic activity because of the unique properties of graphene [5,9,10], including a huge theoretical specific surface area,

d

high charge mobility, and low Fermi level. The huge surface area is

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beneficial for the photocatalytic process, however, it is difficult for TiO2

Ac ce p

nanoparticles to disperse on graphene surface due to the agglomeration of nanoparticles. It is noteworthy that the hollow structure of TONT has an enormous surface area and the disordered arrangement on graphene surface will reduce the occurrence of agglomeration. Furthermore, a sufficient and intimate interface contact could be formed, in which the photoinduced electrons could shuttle through the heterojunction interface, accept and store in graphene and give rise to the inhibition of charge recombination [9,11]. Additionally, with a huge 2-dimensional open surface area, graphene can be easily accessed by π-π stacking between reactants and aromatic 3  

Page 3 of 30

regions of graphene, and can significantly improve the photocatalytic activity [1]. Though a certain extent upgrade of photocatalytic efficiency has been

ip t

made, the nature of the powder sample and cyclic usage restriction were

cr

seriously impeding their further applications and increasing the using cost [12,13]. If graphene-TONT composites were loaded onto a substrate, it

us

would expect to solve the drawbacks of cyclic utilization by the film

an

catalyst. Additionally, it is known that three-phase semiconductor system with different band gap can significantly promote the separation of the

M

photoinduced electron-hole [12,14]. In our continuing efforts to raise the photocatalytic performance, we developed rGO-TONT/FTO heterojunction

d

photocatalyst where FTO acted as substrate. The valence band (VB) and

te

conduction band (CB) of FTO are more negative than that of TiO2, which

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facilitates the photoinduced electrons injecting from the CB of TiO2 to the CB of FTO while trapping photoinduced holes in TiO2, and thus promotes the interfacial charge transfer and separation that similar to graphene [15]. Here, GO (the intermediate product of graphene prepared by chemical

exfoliation method) is used as source material for graphene. The oxygen functional groups on the GO surface give rise to the available structure-directing role of GO in an aqueous medium which can be beneficial for the construction of graphene-semiconductor composites [11,16]. Furthermore, the oxygen functional groups could be eliminated via 4  

Page 4 of 30

a hydrothermal process. By using anatase TiO2 particles and GO as precursors, the reduction of GO and TONT can be achieved simultaneously in a hydrothermal process. Subsequently, the powder dispersion liquid of

ip t

rGO-TONT composites was deposited onto FTO glass. It is worth

performance

was

successfully

tuned

through

cr

mentioning that our research work provides a case where photocatalytic a

multicomponent

us

combination strategy to overcome the drawbacks of single component

an

photocatalyst. Importantly, rGO-TONT/FTO with the coupling of graphene and FTO exhibits a particularly improved photocatalyst activity, and the

M

film photocatalyst greatly extends its application range and reduces the

2. Experimental

d

using cost.

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2.1. Photocatalysts preparation

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In a typical synthesis of the rGO-TONT composites, 0.1g GO (99%

purity) and 1g TiO2 (99% anatase phase) were placed into 55ml deionized water with ultrasonic 2 h to achieve homogeneous dispersions. Subsequently, 22g NaOH was added to the mixing dispersions, then poured into a 100 ml Teflon-sealed autoclave with heated at 120°C for 24 h. The black colored precipitates were washed by 0.1M HCl, then stirred for 10 h. After that, the precipitates was washed again by deionized water, and dried at 60°C, finally calcined at 400°C for 1 h in nitrogen atmosphere. For comparison, rGO and pure TONT were obtained under the same 5  

Page 5 of 30

hydrothermal process but without the addition of TiO2 and GO, respectively. The rGO-TONT powder was dispersed into a 1% polyvinyl alcohol

ip t

solution. The suspension liquid was sprayed on the FTO glass (with square

cr

resistance of 10.5 Ω, and the thickness of FTO is about 600 nm) by air compressor, and maintained the substrate temperature at 200°C to get

us

rGO-TONT/FTO. Contrastive samples of TONT/FTO, rGO-TONT/Glass,

an

and TONT/Glass (the glass act as substrate without photocatalytic activity) were obtained under the same spraying process. The spray layer of

M

rGO-TONT and TONT were maintained at the same thickness of 3 µm. 2.2. Photocatalysts characterization

d

X-ray diffraction (XRD) measurement was conducted on a Rigaku

te

D/max-2500HB/PC with monochromatic Cu Kα radiation (λ=1.5418 Å).

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Morphology characterization and element distribution were performed on a Hitachi S4800 scanning electron microscopy (SEM) and equipped with Energy-dispersive X-Ray Spectroscopy (EDX). Transmission electron microscope (TEM) images were obtained by JEM-2010 with an accelerating voltage of 200 kV. Raman spectra were collected by invia-Reflex micro-Raman spectroscopy system with a 532 nm excitation wavelength at room temperature. The absorbance value of MB solution was recorded by UV1900 Double-beam Spectrophotometer.  The total organic carbon (TOC) content was measured using a Shimadzu TOC-L. The UV-vis 6  

Page 6 of 30

diffuse reflectance spectroscopy of powder samples were measured using a Shimadzu U3900H spectrophotometer and BaSO4 used as a reference. X-ray photoelectron spectroscopy (XPS) analysis was performed on a

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ThermoFisher K-Alpha, which consists of monochromatic Al Ka as the

cr

X-ray source. Photoluminescence spectrum (PL) spectra were investigated

on a fluoromax-4 spectrophotometer at excitation wavelength of 310 nm. 

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Electron spin resonance (ESR) measurement was carried out at 173K with

an

a JEOL JES-FA200 spectrometer under center field 336.00mT, microwave frequency of 9.87 GHz, modulation frequency of 9203 kHz and power of

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0.998 mW. 

2.3. Photocatalytic activity measurements

d

In a typical photodegradation process of MB solution, 40 ml MB

te

solution (10 mg/L) was placed into a 50 ml quartz beaker, and the sample

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(2 cm×2 cm) was placed at the bottom of the beaker. The sample was irradiated by the Hg lamp with radiation wavelength of 365 nm and kept the irradiation distance of 10 cm. The photocatalytic degradation reaction started from the irradiation of Hg lamp and the whole process was carried out in a closed box to insulate all other lights. In the course of irradiation, the solution was taken out at definite interval time of 20 min to measure the absorbance by UV-Vis spectrophotometry.  After a complete process, the sample was washed with water and reused for the next cycle to test the photocatalytic stability. 7  

Page 7 of 30

3. Results and discussion 3.1. Morphology and Structural characterization of catalysts The forming process of rGO-TONT/FTO heterojunction photocatalyst

ip t

is illustrated in Fig.1. The rGO-TONT composites were synthesized by a

cr

hydrothermal process, and then deposited it onto FTO to obtain rGO-TONT/FTO. The morphology of rGO-TONT was investigated by

us

typical SEM and TEM images (Fig.2). As shown in Fig.2a, the rGO surface

an

is covered with the densely distributed TONT. Additionally, the rGO sheet in composite becomes incomplete as compared to GO in Fig.2b. It can be

M

seen clearly that TONT with hollow structure is displayed in TEM images. The outer diameter of pure TONT is consistent with 6-7 nm, and the length

d

is 200-400 nm (Fig.2c). However, the outer diameter of TONT in

te

composites (Fig.2d) is 8-9 nm which is larger than that of pure TONT. This

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is because the residual oxygen functional groups on rGO surface neutralize the dangling bond on the titanate sheets edge, resulting in the decrease of the dynamics requiring for the scrolling process of precursor titanate sheets [17,18]. As a result, the scrolling of titanate sheets is not tight enough. As seen in Fig.2d, the surface of rGO is occupied by the randomly distributed TONT, which leads to the formation of an intimate interfacial contact, a factor significant for the electrons transfer [7]. Fig.3 shows the SEM image of rGO-TONT/FTO. It can be seen that the rGO layer and TONT have been integrated by a close contact. The two-dimensional planar structure of rGO 8  

Page 8 of 30

as well as the giant π-π conjugation system between aromatic molecules and rGO aromatic domains facilitating the great adsorption of aromatic dye on the surface thus leads a higher photocatalytic efficiency [9]. According

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to the inset EDX spectrum, the rGO-TONT/FTO shows peaks

cr

corresponding to Ti, O and C elements, meanwhile, a small amount of Sn is

also observed which originates from FTO. The C content in different areas

us

is in general uniformity with the atomic content of 9.03%, 9.47% and

an

8.41%, indicating the well-distributed of rGO in rGO-TONT composites. The structural changes of GO during the hydrothermal process were

M

characterized by Raman spectra (Fig.4). The D/G intensity ratio (ID/IG) is a measure of the defect concentration in GO or graphene [19,20]. The ID/IG

d

value of rGO-TONT (0.90) and rGO (0.94) is higher than 0.79 of GO,

te

indicating more defects introduced in the course of hydrothermal process.

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Besides, the increase of ID/IG value is a clear indication for decreased average size of sp2 domains, suggesting the reduction of GO upon the hydrothermal process. Furthermore, a red shift of the G band is observed in rGO, indicative of the tensile stress produced in rGO [20]. Whereas no shift appears in rGO-TONT, possibly associated with relaxation of stress by the interaction between TONT and rGO sheets. Accordingly, we suspect that the chemically bonded between rGO and TONT has been formed. The interlayer distance of GO and rGO are calculated using Bragg's Law based on the position of (002) diffraction peak (Fig.5). The interlayer 9  

Page 9 of 30

distance of GO calculated from the (002) peak is 0.79 nm (2θ= 11.2º). However, this decrease to 0.38 nm in rGO according to the wide (002) peak (2θ= 23.2º), indicating the eliminated of oxygen functional groups from

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GO interlayer. Therefore, the decrease of interlayer distance as well as the

cr

increase of ID/IG value in Raman analysis strongly confirms the reduction

of GO to graphene. The diffraction peaks of TONT and rGO-TONT are

us

corresponding with the anatase phase (JCPDS File, No. 21-1272), while the

an

peaks of TONT become weaker and broader as compared to rGO-TONT. The calculated crystal size of TONT is 9.8 nm, smaller than that of

M

rGO-TONT (10.3 nm). This is because the two-dimensional planar structure and surface defects of rGO providing more nucleation sites and

d

much more energy for grain growth, and thus promote TiO2 crystallization.

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No typical diffraction peak assigned to rGO is observed, probably because

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of the low content of rGO in rGO-TONT. The XRD patterns of rGO-TONT/FTO show the presence of anatase TiO2 and SnO2 (JCPDS File, No. 46-1088), which suggests the co-existence of TiO2 and SnO2 in rGO-TONT/FTO heterojunction photocatalyst. 3.2. Photocatalytic performance The photocatalytic performance of rGO-TONT/FTO, rGO-TONT/Glass, TONT/FTO,

and

TONT/Glass

have

been

examined

by

the

photodegradation of MB solution under the UV-light irradiation. The relative concentration of MB was measured during the photodegradation at 10  

Page 10 of 30

a given time interval of 20 min. As seen in Fig.6.a, rGO-TONT/FTO exhibits the best photocatalytic performance toward photodegradation of MB. Under the irradiation time of 100 min, rGO-TONT/FTO shows the

ip t

highest degradation rate of 91.3%, while the degradation rate of

cr

rGO-TONT/Glass, TONT/FTO and TONT/Glass are 70.7%, 67.5%, and 57.8%, respectively. The corresponding photocatalytic degradation kinetics

us

of MB solution is assumed to be calculated by the Pseudo-first-order

an

reaction ln (C/C0) = −kt, where C0 is the original concentration (mg/L) of MB solution, C is the concentration (mg/L) of MB solution at a given

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photocatalytic reaction time t (min), and k is the apparent rate constant (min−1). The photocatalytic reaction rate constant of rGO-TONT/FTO is

d

0.0244 min−1, nearly 3 times greater than 0.0086 min−1 of TONT/Glass

te

(Fig.6.b). Nevertheless, rGO-TONT/Glass and TONT/FTO exhibit the

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photocatalytic reaction rate constant of 0.0123 min−1 and 0.0112 min−1, respectively, about 1.4 times greater than that of TONT/Glass. It strongly suggests that the enhanced photocatalytic activity of rGO-TONT/FTO should be ascribed to the coupling effect of rGO and FTO. To further verify the photocatalytic performance of the as-prepared

samples, the TOC content during the photocatalytic degradation of MB was determined (Fig.6.c). After 100 min of photocatalytic degradation reaction, the TOC removal percentage for rGO-TONT/FTO was 85.2%, higher than that of rGO-TONT/Glass, TONT/FTO, and TONT/Glass (68.5%, 64.5%, 11  

Page 11 of 30

50.2%, respectively). The TOC removal rate was also calculated by Pseudo-first-order reaction ln (C/C0) = −kt. The calculated results show that the removal rate constant for rGO-TONT/FTO, rGO-TONT/Glass,

ip t

TONT/FTO, and TONT/Glass are 0.0191 min−1, 0.0116 min−1, 0.0104

cr

min−1 and 0.00697 min−1, respectively. The improvement of TOC removal rate for rGO-TONT/FTO is consistent with the MB photodegradation rate,

us

which confirmed the improvement of photocatalytic performance.

an

It should be noted that the samples with the addition of rGO all showed an increased photocatalytic performance compared to TONT/Glass.

M

Therefore, the UV-vis diffuse reflectance spectra were measured over the TONT and rGO-TONT powder to study the optical absorption properties.

d

As seen in Fig.6.d, the introduction of rGO results in the increased light

te

absorption intensity in both the visible light and UV light regions compared

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to TONT. Thus, for this photocatalytic reaction, the increased light absorption intensity should be a reason for the increased photocatalytic performance over the samples of rGO-TONT/FTO and rGO-TONT/Glass. In addition, a  slight red shift in the absorption edge of rGO-TONT is also observed, indicated that the introduction of rGO induces the extended photoresponding range. The principle of enhanced photocatalytic activity in rGO-TONT/FTO heterogeneous photocatalyst was elaborated in Fig.7. Under band gap excitation, an electron is excited from the VB of TiO2 into the CB, leaving 12  

Page 12 of 30

a hole in the VB. The electron-hole migrate to catalyst surface is mainly responsible for the catalytic reaction. When rGO-TONT heterojunction is formed, the space-charge region at the interfaces creates a built-in electrical

ip t

potential which drives the photoinduced electrons shuttling the interface

cr

into rGO layer via a percolation mechanism and leaves unbalanced holes in TiO2 [21]. The photoinduced holes can be used to oxidize H2O or OH- to

us

produce highly reactive hydroxyl radicals. With the high charge mobility

an

and large capacity of electronic-storage, rGO can serve as the playground of photoinduced electrons. The photoinduced electrons thereby scavenged

M

by dissolved oxygen to generate hydroxyl radicals. It is noteworthy that the migration of electrons between rGO and TONT leading a negative shift of

d

the Fermi level and a narrowing of the TiO2 band gap, which facilitates the

te

charge separation and thus improves the photocatalytic activity [1,21]. On

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the other side, another heterojunction interface is formed by the coupling of TONT with FTO. The CB and VB position of FTO is more negative than those of TiO2, in terms of the energetics, photoinduced electrons will inject into the CB of FTO from the CB of TiO2, while trapping photoinduced holes in the VB of TiO2 until the Fermi level equilibrium [22]. With this band offset of FTO, photoinduced electron-hole can be efficiently separated, which is beneficial for an improved photocatalytic performance. In principle, the chemical interaction in heterojunction interface of rGO and TONT is perceived to be essential for the effective electrons transfer 13  

Page 13 of 30

during the photocatalytic process. Therefore, XPS was conducted on rGO-TONT and pure TONT to investigate the interaction between TONT and rGO sheets. In the C1s spectra, four fitted peaks are centered at 283.0

ip t

eV, 284.9 eV, 286.3 eV, and 288.6 eV as displayed in Fig.8a. The main peak

cr

at 284.9 eV is ascribed to sp2 carbon species, while the weak peaks at 286.3 eV and 288.6 eV are attributed to the oxygen bound species C-O and C=O

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root in residual oxygen functional groups on rGO surface, respectively

an

[23,24]. Notably, the weak peak at 283.0 eV is corresponding with O-Ti-C bond that originated from carbon atom substituting for the lattice oxygen in

M

TiO2 [25,26], which provides a clear evidence for the formation of chemical bonds in rGO-TONT heterojunction. The chemical interaction in

d

rGO-TONT heterojunction interface gives a promising way to sufficiently

te

utilizing the excellent electrical properties of rGO and promoting the

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transfer of photoinduced electrons. The binding energies of Ti2p3/2 and Ti2p1/2 (Fig.8c) were located at

458.7 eV and 464.4 eV, which is typical TiO2 [23,27]. The deconvoluted XPS spectra of O1s in TONT (Fig.8b) and rGO-TONT (Fig.8d) shows the main peak at 529.9 eV, which is ascribed to Ti-O in TiO2. In addition, the minor peak at 531.6 eV is assigned to hydroxyl group adsorbed on TiO2 surface [28-30]. It is reported that the presence of hydroxyl group is beneficial for trapping photoinduced holes to participate in the photocatalytic reaction. 14  

Page 14 of 30

Fig.9 shows Raman spectra of the rGO-TONT/FTO and TONT/FTO for comparison. The spectra exhibits similar active modes of TiO2, corresponding to 151 cm −1 or 154 cm−1 (Eg), 198 cm −1 (Eg), 397 cm −1 (B1g),

ip t

516 cm −1 (A1g) and 639 cm−1 (Eg) in bulk anatase [31,32]. However, the

cr

modes of 1355 cm −1 and 1595 cm −1 in rGO-TONT/FTO are assigned to the typical D band and G band of graphene, respectively, which manifests the

us

existence of graphene in the sample that was not detected in XRD patterns.

an

No modes belong to SnO2 due to the Raman detection depth limited. The peak corresponding to Eg mode for rGO-TONT/FTO (154 cm−1) presents a

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faint blue-shift compared to TONT/FTO (151 cm−1) as shown in the Fig.9 inset. It is known that such a shift in nanomaterials is often associated with

d

phonon confinement effects [33], while the calculated crystallite sizes of

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TONT and rGO-TONT are 9.8 nm, 10.3 nm, respectively, a gap not large

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enough to produce a Raman shift. The Eg mode is relevant to the planar O-O interactions, and is susceptible to the oxygen deficiency rather than the Ti-O stretching vibration [34]. Therefore, the blue-shift is attributed to the oxygen vacancies existing in TiO2, similar to those observed in mesoporous CNT/TiO2 hybrids and the oxidation-induced spectral changes in nanophase TiO2 [35,36]. It is noteworthy that, the oxygen vacancies on TiO2 surface often serve as charge carrier traps, which can effectively weaken photoinduced charge recombination [37]. The presence of oxygen vacancies was further confirmed via ESR 15  

Page 15 of 30

spectra (Fig.10) where signals due to the  single electron trapped in oxygen vacancy with a g-value of 2.005 [38,39]. The signals of g = 1.936 to 1.989 can be assigned to Ti3+ ions, formed by photoinduced electron

ip t

captured by Ti4+ in the anatase lattice [40,41]. Accordingly, the high

cr

intensity of ESR resonance that originates from trapped electron in oxygen

vacancies and Ti4+ indicates the efficient electron transfer. The ESR

us

resonance at g=1.896 (Fig.10.a, b) assigned to the typical single ionised

an

oxygen vacancy in SnO2, and the strong ESR signal may partly originated from the surface superoxide centers Sn4+–O–2 (g =2.039, 2.005, 1.989)

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[42,43], which was overlap with the resonance signal of TiO2. Nevertheless, the rGO-TONT/FTO shows the highest ESR resonance intensity, indicating

d

the superior photoinduced charge separation properties as compared to

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rGO-TONT/Glass, TONT/FTO and TONT/Glass.  Then the lifetime of the

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photoinduced charge in rGO-TONT/FTO could be lengthened, with consequent the improvement of the photocatalytic activity. Herein, the surface states assisted reaction of rGO-TONT/FTO is

depicted in Fig.11. Since abundant oxygen vacancies are localized at TiO2 surface, photoinduced electrons are easily trapped in (process 1). The built-in electrical potential in the heterojunction interfaces will guide the photoinduced electrons-holes flowing into opposite directions, therefore, the excited electrons in the CB of TiO2 can migrate to graphene and FTO simultaneously until the Fermi level equilibrium (process 2). Moreover, the 16  

Page 16 of 30

electrons trapped in oxygen vacancies can also migrate to graphene and FTO due to the built-in electrical potential produced in the heterojunction structure (process 3) [37,44]. Therefore, the existence of oxygen vacancies

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on the TiO2 surface is of great beneficial for the suppressed of charge

cr

recombination.

Photoluminescence signal is originated from the recombination

and separation process of

an

information of the capture, transfer

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progress of photoinduced electron-hole pairs, hence is good at giving the

electrons-holes [45,46]. To further confirm that the enhanced photocatalytic

M

activity is attributed to the effective separation of electron-hole pairs, PL spectrum (Fig.12) measurements were carried out on samples of

d

rGO-TONT/FTO, rGO-TONT/Glass, TONT/FTO and TONT/Glass. The

te

samples show multiple PL peaks ranging from 369 nm to 560 nm. The

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luminescence peaks at 369 nm, 395 nm, 411 nm are ascribed to band edge luminescence originated from the excitons self-trapped on [TiO6] octahedra as reported in literature [47,48]. However, the visible emission peaks at 450 nm, 465 m, 480 nm, 490 nm, 558 nm are attributed to the luminescence associated with oxygen-related defects (eg. oxygen vacancies) localized in TiO2 surface [47,49], in agreement with the results of Raman spectra (Fig.9). The sample of TONT/Glass shows the highest luminescence intensity, which means the highest recombination rate of electron-hole pairs. Since the PL quenching is the result of photoinduced electrons transfer in 17  

Page 17 of 30

heterojunction interface, the effective separation of photoinduced electrons-holes can be manifested by the decay of PL peak. Moreover, rGO-TONT/FTO shows the most significantly decay of PL intensity,

ip t

indicating the transfer of photoinduced electrons from the CB of TiO2 to the

cr

empty electronic states of rGO and FTO. This result in the nonradiative

decay of the TiO2 excited state that is consistent with the previously reports

us

in graphene-TiO2 and TiO2-carbon nanofiber arrays for promoted charge

an

separation [16,47]. The decay trend of PL intensity of rGO-TONT/FTO, rGO-TONT/Glass, and TONT/FTO, TONT/Glass is in line with the

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photocatalytic degradation rate of MB solution, which is in the order of rGO-TONT/FTO > rGO-TONT/Glass > TONT/FTO > TONT/Glass. This

d

clearly suggests that effective charge separation in rGO-TONT/FTO

Additionally,

the

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activity.

te

heterojunction should be responsible for the improvement of photocatalytic improved

photocatalytic

activity

of

rGO-TONT/FTO heterojunction photocatalyst should also be a result of the presence of surface oxygen vacancies, large specific surface area of TONT and the chemically bonded heterogeneous structure in rGO-TONT. In order to study the long-term stability of rGO-TONT/FTO

heterojunction photocatalyst, the cycle stability test for phtocatalytic activity was carried out. As shown in Fig.13, there is no significant decrease in photodegradation rate after 5 cycles, indicating the good photocatalytic

stability

of

the

rGO-TONT/FTO

heterojunction

18  

Page 18 of 30

photocatalyst. 4. Conclusions The

chemically

bonded

rGO-TONT

heterojunction

has

been

ip t

successfully synthesized via a hydrothermal process. Benefited from the

cr

chemical interaction in heterojunction interface of rGO-TONT, the distinctive electron acceptor role of rGO is available. Furthermore, the

us

rGO-TONT nanocomposites has been deposited onto FTO to gain

The

built-in

electrical

potential

an

rGO-TONT/FTO, and their photocatalytic performance was investigated. in

heterojunction

structure

of

M

rGO-TONT/FTO would drive the photoinduced electrons to travel in different directions. Moreover, abundant oxygen vacancies at the TiO2

d

surface also contribute to the separation of charge. As a result, the

te

as-synthesized rGO-TONT/FTO heterojunction photocatalyst exhibits 3

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times greater of photocatalytic reaction rate over TONT/Glass upon the photodegradation of MB under UV irradiation. The higher photocatalytic activity in rGO-TONT/FTO heterojunction photocatalyst should be attributed to the coupling effect of additional heterojunctions, in which rGO and FTO act as electron reservoir to accept and store the photoinduced electrons from TiO2, and thus contribute to the spatial charge separation, as well as the increased optical absorption capacity due to the addition of rGO. In addition, rGO-TONT/FTO heterojunction photocatalyst shows a prominent photocatalytic efficiency and photocatalytic stability, and 19  

Page 19 of 30

expects to seek a promising application in the field of control environmental pollution. Acknowledgments

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This work was financially supported by the National Key Research and

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Development Program of China (NO. 2016YFB0303902), the National Natural Science Foundation of China (NO. 51602278) and the Natural

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Science Foundation of Hebei Province (NO. E2016203149).

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Figure captions Fig.1. Schematic flowchart of the preparation of rGO-TONT/FTO. The rGO-TONT composites were synthesized by hydrothermal treatment, and

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then deposited it onto FTO to obtain rGO-TONT/FTO.

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Fig.2. SEM images of (a) rGO-TONT, (b) GO. TEM images of (c) TONT, (d) rGO-TONT.

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Fig.3. SEM images of rGO-TONT/FTO. Inset EDX spectrum showing the

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element distribution at three regions in the SEM image of rGO-TONT/FTO.

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Fig.4. Raman spectra of (a) GO, (b) rGO and (c) rGO-TONT produced under same hydrothermal conditions without calcined.

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Fig.5. XRD patterns of (a) GO, (b) rGO produced under hydrothermal

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treatment at 120°C for 24h, (c) TONT and (d) rGO-TONT produced under

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the same hydrothermal conditions, besides, calcined at 400°C for 1h, (e) rGO-TONT/FTO fabricated by deposited rGO-TONT composites onto FTO glass.

Fig.6. (a) Photodegradation of MB solution under the UV light irradiation (main wavelength at 365nm) over samples of rGO-TONT/FTO, rGO-TONT/Glass, TONT/FTO and TONT/Glass at same depositing thickness of 3µm; (b) Apparent rate constants for the photocatalytic reaction over the samples; (c) The change of TOC content in MB solution under the same photocatalytic conditions; (d) UV-vis diffuse reflectance 29  

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spectra of the TONT and rGO-TONT powder. Fig.7. Schematic diagram of the photocatalysis principle and photoinduced electron-hole separation in rGO-TONT/FTO, I, II and III represent TiO2,

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FTO and rGO, respectively.

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Fig.8. XPS spectra of (a) C1s in rGO-TONT, (b) O1s in TONT, (C) Ti2p in TONT and rGO-TONT, (d) O1s in rGO-TONT.

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Fig.9. Raman spectra of (a) TONT/FTO, (b) rGO-TONT/FTO. The inset

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displays the Raman blue-shift of the Eg modes in rGO-TONT/FTO. Fig.10. ESR spectra of (a) rGO-TONT/FTO, (b) TONT/FTO, (C)

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rGO-TONT/Glass and (d) TONT/Glass.

Fig.11. Schematic illustration of surface state assisted reaction of

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rGO-TONT/FTO. (1) photoinduced electrons are trapped in oxygen

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vacancy; (2) photoinduced electrons are transferred to graphene and FTO;

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(3) the photoinduced electrons trapped in oxygen vacancy are transferred to graphene and FTO.

Fig.12. Photoluminescence spectra of (a) rGO-TONT/FTO, (b) rGO-TONT/Glass, (c) TONT/FTO, (d) TONT/Glass. Fig.13. The recyclability tests of rGO-TONT/FTO in the photocatalytic degradation of MB.  

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