Enhanced Photocatalytic Removal of Tetrabromobisphenol A by

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Enhanced Photocatalytic Removal of Tetrabromobisphenol A by Magnetic CoO@graphene Nanocomposites under Visible-light Irradiation Yulin Tang, Linfan Dong, Shun Mao, Hongbo Gu, Tyler Malkoske, and Bingdi Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00379 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Applied Energy Materials

1

Enhanced

Photocatalytic

2

Tetrabromobisphenol

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CoO@graphene Nanocomposites under Visible-light

4

Irradiation

5

Yulin Tanga,b, Linfan Donga,b, Shun Maoa,b, Hongbo Guc, Tyler Malkoskea,b, Bingdi Chend,*

6

a. State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental

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Science & Engineering, Tongji University, Shanghai, 200092, P.R. China.

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b. Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, P.R.

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China.

A

Removal by

of

Magnetic

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c. Shanghai Key Lab of Chemical Assessment and Sustainability, Department of Chemistry,

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Tongji University, Shanghai, 200092, P.R. China.

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d. Shanghai East Hospital, The Institute for Biomedical Engineering & Nano Science, Tongji

13

University School of Medicine, Shanghai, 200443, P.R. China.

14

KEYWORDS

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CoO@graphene nanocomposites, photocatalyst, visible-light irradiation, photocatalysis,

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tetrabromobisphenol A

ACS Paragon Plus Environment

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT A modified, facile, and ultrasonic-assisted approach was developed to synthesize

2

CoO@graphene nanocomposites. The chemical, electronic and structural characteristics of CoO

3

nanoparticles grown on graphene nanosheets were investigated by Scanning transmission X-ray

4

microscopy through spatially resolved X-ray absorption near edge structure spectroscopy and

5

other techniques. Enhanced octahedral Co2+ (Oh) structures of CoO nanoparticles were

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successfully loaded on the surface of graphene nanosheets and the photocurrent of

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CoO@graphene nanocomposites was 10.6 times higher than that of CoO nanoparticles. The

8

removal efficiency of recyclable magnetic CoO@graphene nanocomposites reached up to 73.4%

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for tetrabromobisphenol A (TBBPA) degradation under visible-light irradiation. Free radical

10

trapping experiments revealed that •OH radicals were the photogenerated radical species driving

11

the high catalytic activity. Moreover, beta scission and debromination are suggested as two

12

possible pathways of TBBPA degradation under visible light. CoO@graphene nanocomposites

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maintained high photocatalytic activity after reuse over four cycles, which suggests that the

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synthesized materials have a promising application for TBBPA removal from wastewater.

15

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1. Introduction

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Semiconductor metal oxide materials have been intensively studied during the past several

18

decades in energy and environmental applications such as photoelectrodes, water splitting and

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photocatalysis.1-4 Among all the materials, TiO2, ZnO, and Cu2O have been sufficiently studied

20

and developed as photocatalysts. However, these materials are effective only under UV

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irradiation and show poor performance under solar and visible light owing to their wider band

22

gap (3.0-3.2 eV),5-7 thereby restricting their practical application and the efficient utilization of


2

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solar energy.8 Co-based oxides are one of the most widely used photocatalysts to utilize sunlight

2

as an energy source due to their excellent visible-light response, high photocatalytic efficiency,

3

chemical stability, superior magnetic and photocatalytic properties.9

4

Among Co-based oxide photocatalysts, cobalt monoxide (CoO) is regarded as an important

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p-type semiconductor oxide with unique electronic configuration and magnetic properties and

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the band gap of 2.2-2.8 eV. Though CoO shows relatively high photocatalytic efficiency, the

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poor dispersion, easy recombination of photo-generated electrons and holes as well as the small

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surface area still hinder its photocatalytic performance. In order to conquer these issues, different

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strategies have been investigated, such as controlling morphology and doping functionalized

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carbon materials.10-12 Graphene, a two-dimensional carbon structure matrix, has been regarded as

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a desirable support for catalysts because of its excellent chemical and thermal stabilities, large

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specific surface area, and excellent electrical conductivity to enhance electron-hole separation

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and charge transport.13, 14 Recently, CoO@graphene hybrid materials have attracted scientific

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interest because of their electrochemical properties, such as a large reversible capacity and large

15

quantity of accessible active sites, which make them suitable for lithium-ion batteries.15-17

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Meanwhile, coupled CoOx graphene hybrid material has been developed as a novel oxygen

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evolution/oxygen

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CoO@graphene nanocomposites have been shown to have very good lithium battery

19

performance.15-20 Since the CoO@graphene nanocomposites have excellent electrochemical and

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redox performances, it’s supposed to have the good photocatalytic performance. However, the

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photocatalytic performance of CoO@graphene hybrid materials is not well reported. Moreover,

22

the photocatalytic mechanisms and activities of these materials have not been fully understood.

reduction

bi-functional

electrocatalyst.

In

our

previous

studies,


3


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Tetrabromobisphenol A (TBBPA), one of the highest production volume brominated ﬂame

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retardants (BFRs) in the world, has attracted more attention from researchers due to the

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increasing occurrence of TBBPA in the environment and its toxicity and endocrine disrupting

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activity.21, 22 Recently, the photodegradation of TBBPA was performed with UV irradiation,

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UV/photocatalyst degradation, solar light/phtocatalysts, and other related reactions.22-24

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However, effective wavelength range for accomplishing this issue is mainly in the UV region.

7

Photocatalysts that can respond in the visible region (~45%) have attracted much attention,

8

because UV light (~5%) is only a small portion of the sunlight spectrum.25 To date, a

9

visible-light photocatalytic TBBPA removal process with the CoO@graphene nanocomposites

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has not been investigation.

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In this study, the facile synthesized CoO@graphene nanocomposites were characterized by

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numerous techniques including transmission electron microscopy (TEM), X-ray diffraction

13

(XRD), Raman spectrum, UV-vis spectroscopy, photocurrent and electrochemical impedance

14

spectroscopy and photoluminescence technique. X-ray absorption near edge structure (XANES)

15

spectroscopy and Scanning transmission X-ray microscopy (STXM), 26 are powerful techniques

16

to characterize the chemical information at spatial scale which influence on their catalytic

17

activity of materials. They have also been used to investigate the electronic and structural nature

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of CoO nanoparticles adhered to graphene nanosheets. In addition, photocurrent and

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Electrochemical impedance spectroscopy (EIS) measurements demonstrate CoO@graphene

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nanocomposites have better visible-light responses and smaller charge-transfer resistance than

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CoO nanoparticles. Total organic carbon (TOC) and Ultra-performance Liquid Chromatography

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Coupled with Q-TOF Mass Spectrometry (UPLC/Q-TOF-MS) analysis are carried out to

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evaluate the mineralization efficiency and possible degradation pathways of TBBPA. Finally, the


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recovery experiment of CoO@graphene nanocomposites by an external magnet, and the cyclic

2

test for the degradation ability were also conducted to evaluate the stability of the materials. This

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study provides a facile way for preparing CoO@graphene nanocomposites, which has potential

4

application in the elimination of TBBPA from aqueous solution.

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2. Experimental section

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2.1 Materials and reagents

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Co4(CO)12 was purchased from Alfa Aesar China (Tianjin) Co., Ltd., and graphene (XF-nano),

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TBBPA (gradient grade, >97%) and ammonium oxalate (AO) were obtained from

9

Sigma-Aldrich Company (MO, USA). Methanol (HPLC gradient grade, ≥99.9%) and

10

p-Benzoquinone (BQ) were purchased from Adamas Reagent Co., Ltd (Shanghai, China).

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Cobaltous oxide (CoO) was obtained from Meryer Co., Ltd (Shanghai, China). All the reagents

12

and chemicals were used as received, without further purification. Ultrapure water was prepared

13

with a Milli-Q integral 15 system (Millipore, MA, USA).

14

2.2 Synthesis of the photocatalysts

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The synthesis of CoO@graphene with different amounts of graphene is similar to our previous

16

report of CdS@graphene nanohybrids.27 In our previous study, the influence of Co4(CO)12

17

concentrations on the CoO@graphene composite structures and electrochemical performance has

18

been investigated, which concluded that the CoO and graphene mass ratio of 1:1 is optimal.19 In

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a typical procedure, 40 mg of graphene nanosheets were dispersed in 100 mL of hexane by

20

sonication and 78 mg of Co4(CO)12 was dissolved into 80 mL of hexane by magnetically stirring

21

at 300 rpm at room temperature. Then the two suspensions were mixed and sonicated (240W) for

22

1 hour. The resulting products were centrifuged, washed with ethanol to remove any remaining

23

ions on the materials, then dried at 60 °C in air to evaporate the ethanol and complete the


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production of ultrasonic-synthesized CoO@graphene (U-CoO@graphene) material. Finally, the

2

materials were thermally treated at 550 °C for 2 hours in a tube furnace under nitrogen

3

atmosphere at a heating rate of 3 °C/min. After the reaction, CoO@graphene nanocomposites

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were cooled and collected. CoO@graphene(2.0) and CoO@graphene(0.5) were obtained in the

5

same procedure with double and half amounts of graphene in typical procedure, respectively.

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2.3 Characterization

7

The morphology and structure of the samples were obtained by a transmission electron

8

microscopy (TEM, JEM-200CX). The high-resolution transmission electron microscopy

9

(HRTEM) images and selected area electron diffraction (SAED) images of samples were

10

characterized on a high-resolution transmission electron microscopic instrument (HRTEM,

11

JEOLJEM-2100, Japan). X-ray diffraction (XRD) was conducted on a Bruker D8 X-ray

12

diffractometer (Germany) with Cu Kα radiation. The accelerating voltage and the applied current

13

were 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) data was recorded on a Perkin

14

Elmer PHI 5000C ESCA system with Mg Kα excitation line (hm=1253.6 eV).

15

Thermogravimetric analysis (TGA) was conducted using a TA SDT-Q600 thermogravimetric

16

analysis instrument in air at a heating rate of 10 °C/min. UV-vis diffuse reflection spectroscopy

17

(DRS) measurement was performed using a UV spectrophotometer (Shimadzu UV-2550). The

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Raman spectra were obtained using a Raman confocal microscope (Invia, Renishaw) with 514

19

nm laser excitation. The magnetic properties were tested on a magnetic property measurement

20

system (Lakeshore 735 VSM Controller 7300 Series Magnetometer). The surface zeta potentials

21

were determined by using a DLS Particle Size analyzer (Zetasizer Nano-ZS, Malvern, U.K.). The

22

specific surface area and pore size distribution of the photocatalysts were performed on ASAP

23

2010 micropore physisorption analyzer (Micromeritics, USA). The concentration of leaching


6

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Co2+ after the reaction were analyzed by an inductively coupled plasma-mass spectrometer

2

(ICP-MS) (PlasmaQuad 3, Thermo Fisher Inc.).

3

Scanning transmission X-ray microscopy investigations were carried out at the BL08U1A

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beamline with a spatial resolution of 30 nm. This third-generation synchrotron facility was

5

operated using a 3.5 GeV in the Shanghai Synchrotron Radiation Facility (SSRF) and according

6

to detailed principles and design of STXM described elsewhere.28 These image sequences were

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also employed to extract X-ray near-edge structures (XANES). XANES spectra were extracted

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from groups of pixels within the image regions of interest using the IDL package aXis2000.29

9

Cobalt distribution maps were obtained through analyzing a series of stack-scanned images at

10

energies around the relevant absorption edges and the stack images were aligned via a spatial

11

cross correlation analysis.

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2.4 Photocatalytic activity

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The photocatalytic activities of CoO@graphene nanocomposites were evaluated by the

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photodegradation of TBBPA in aqueous solution under visible-light irradiation. The

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photocatalytic activity experiments were conducted in a XPA-7 photochemical reaction chamber

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(Xujiang Electromechanical Plant, Nanjing, China) equipped with 150 mL quartz tubes. The

17

distance between quartz tubes and the lamp center was 5.5 cm. A 350 W xenon lamp equipped

18

with a 420 nm cutoff filter was used as the visible light source. The intensity of the lamp was

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13.2 MW/cm2, measured by a radiometer (CEL-NP2000, Beijing Aulight Co. China).

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In a typical experiment, 1.5 mg samples of the catalyst were first added to 150 mL of TBBPA

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solution (4.0 mg/L) by sonication for 1 min, and pH adjusted to 8.0 ± 0.1 with 0.01 mM HCl or

22

NaOH. Prior to irradiation, the mixture was magnetically stirred in the dark for 0.5 h to achieve

23

adsorption equilibrium between the TBBPA and CoO@graphene as Figure S1. Then the lamp


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was turned on and the photodegradation was initiated. 1.5 mL of the suspension was sampled at

2

predetermined time intervals and immediately centrifuged to remove the photocatalyst. The

3

concentration of TBBPA at interval times was analyzed by HPLC (Waters, ACQUITY UPLC

4

H-Class, USA). The degradation is expressed as C/C0, where C0 is the initial TBBPA

5

concentration and C is the residual concentration for each irradiation time interval.

6

Reactive oxygen species generated during photodegradation were evaluated firstly by

7

radical-trapping experiments, in which a certain amount of specific scavengers were added to

8

reaction solutions. Subsequently, the generated •OH radicals were detected by terephthalic acid

9

photoluminescence probing technique (TA-PL) with an Edinburgh FL/FS900 spectrophotometer.

10

In the TA-PL experiment, 5×10-4 M TA solutions were set in 2×10-3 M NaOH to replace the

11

TBBPA solutions used in the photodegradation experiments. An excitation wavelength of 370

12

nm was used.

13

The photocurrent and electrochemical impedance spectroscopy (EIS) was obtained on a

14

standard three-electrode system with an electrochemical analyzer (CHI660B, Shanghai Chenhua

15

Instrument Corp., Shanghai, China). Aqueous 0.1 M Na2SO4 was used as an electrolyte solution

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with a saturated calomel electrode (SCE) as the reference electrode and platinum foil electrode as

17

the counter electrode. The working electrode was the film of prepared CoO@graphene

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nanocomposites coated on a fluorine-doped tin oxide (FTO) conductive glass with an active area

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of ca. 0.5 cm2. The visible-light irradiation source was a 350 W xenon lamp equipped with a 420

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nm cutoff filter. The photocurrent was obtained at 0.0 V vs SCE in 0.1 M Na2SO4 aqueous

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solution. In addition, the EIS measurement was recorded at AC voltage amplitude of 0.5 V and a

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frequency range of 1 MHz to 5 mHz at 0.5 V.30

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2.5 TBBPA and TOC analysis


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Detailed detection conditions of TBBPA by HPLC can be found in our previous study.31 TBBPA

2

degradation

3

Ultra-performance Liquid Chromatography coupled with Q-TOF Mass Spectrometry

4

(UPLC/Q-TOF-MS) in electrospray negative ion mode. A Waters BEH C18 column (2.1 mm φ

5

100 mm, 1.7µm) was used in UPLC separation. A mixture of water and acetonitrile was used as

6

mobile phase, with gradient elution at a flow rate of 0.25 mL/min.

intermediates

from

the

photocatalytic

reactions

were

identified

by

7

Total organic carbon (TOC) analysis was performed using a shimadzu TOC-L analyzer to

8

assess the mineralization capacity of the photocatalyst. All the samples were filtered by 0.22 µm

9

PTFE membranes to remove CoO@graphene nanocomposites before analysis. The degree of

10

debromination of TBBPA was determined by measuring the Br- concentration using an ICS-5000

11

ion chromatographic system equipped with conductivity detectors and AS14 anion columns.

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3. Results and discussion

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3.1 Characterization of CoO@graphene nanocomposites

Figure 1. TEM images of CoO@graphene (a, b) and high-resolution TEM image of CoO@graphene (c, d), and inset in (d) is the electronic diffraction pattern corresponding to the CoO adhered to graphene.


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Figure 2. XPS full spectrum of CoO@graphene (a), XPS spectra of C 2s (b), O 1s (c) and Co 2p (d) of CoO@graphene, XRD patterns of CoO, graphene, CoO@graphene (e), TGA analysis of CoO@graphene (f). 1

Figure 1 presents the TEM and HRTEM microstructures of CoO@graphene nanocomposites.

2

Highly loaded CoO nanoparticles, the average size of which was about 12-16 nm, were

3

homogeneously anchored on the surface of two-dimensional graphene sheets. While the TEM

4

images of CoO@graphene, CoO@graphene(0.5), and CoO@graphene(2.0) (Figure S2) indicate

5

the amounts of graphene influence the dispersion and uniform combination between CoO and

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graphene sheets. SAED images (inset in Figure 1d) further confirmed the structure of

7

CoO@graphene nanocompostites. As can be seen, three diffraction rings, which correspond to

8

(111), (200) and (220) planes, demonstrate the face-centered cubic structure of CoO

9

nanoparticles.32 The HRTEM image of a single CoO nanoparticle shows an obvious crystal

10

lattice corresponding to (220) plane with an interval of 0.213 nm. In addition, Figure 1c displays

11

the combination of graphene nanosheets and CoO nanoparticles in CoO@graphene

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nanocomposites, which may possess a synergistic effect in photocatalytic degradation.


10

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The chemical compositions of CoO@graphene nanocomposites were illustrated by XPS

2

analysis. The full XPS spectrum in Figure 2a reveals the existence of C, O and Co elements in

3

the nanocomposites, elucidating the well synthesized CoO@graphene. The strong C 1s peak at

4

284.4 eV in Figure 2b can be ascribed to the graphitic carbon in graphene, while the peaks at

5

286.2 and 287.8 eV are related to the hydrocarbons adsorbed and their oxidative forms. A

6

prominent peak at 533.8 eV and a shoulder peak at 531.2 eV in the O 1s XPS spectra in Figure

7

2c can be attributed to the lattice oxygen and surface oxygen in CoO. It reveals that there was

8

few additional residual oxygen-containing groups on [email protected] Additionally, the peak

9

corresponding to oxygen species in Co3O4 at 530 eV was not detected, suggesting that the

10

nanoparticles attached to the graphene are CoO. The spin-orbital splitting energy between the

11

peaks of Co 2p3/2 (781.6 eV) and Co 2p1/2 (781.6 eV) in Figure 2d is approximately 15.8 eV,

12

which is regarded as the characteristic of CoO phase. At the same time, two satellite peaks are

13

considered as Co2+ shake-up peaks of CoO. 33, 34

14

XRD was also employed to investigate the structural difference between CoO@graphene

15

nanocomposites and CoO. Figure 2e shows that the relating diffraction peaks of CoO@graphene

16

nanocomposites at 2θ values of 36.5°, 42.1°, 61.3°, 73.6° and 77.5° correspond well with (111),

17

(200), (220), (311) and (222) crystallographic phases of cubic CoO (JCPDS No. 43-1004), which

18

is consistent with the SAED image of CoO@graphene nanocomposites. The absence of peaks

19

due impurities and consistence of graphene and CoO implied well-synthesized CoO@graphene

20

nanocomposites. The average crystallite size can be approximately calculated by Scherrer’s

21

equation.35 Taking the peaks at 42.1° of CoO@graphene nanocomposites as examples, the

22

average crystallite size is calculated to be about 11.7 nm, which is consistent with the results

23

observed in TEM. The Raman spectra of pure CoO and CoO@graphene nanocomposites were


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also investigated in Figure S3. A broad peak at 1352 cm-1 (marked as D) and a relatively sharp

2

peak at 1585 cm-1 (marked as G), two main characteristic peaks of graphene, are clearly detected

3

in CoO@graphene nanocomposites, indicating that the thermal treatment under nitrogen had

4

little influence on the structure of graphene. The D peak can be attributed to the in-plane

5

vibrations of sp2 hybridized carbon atoms with dangling bonds of disordered graphite, while G

6

peak usually results from the vibrations of all sp2 disordered carbon atoms in the graphene

7

layer.36 In addition, the peaks of CoO@graphene nanocomposites at 515, 465 and 670 cm-1, can

8

be ascribed to F2g, Eg and A1g modes of CoO.37 This result is consistent with the XRD findings

9

indicating CoO nanoparticles have better crystallization and structure.

10

A typical TGA analysis was tested under nitrogen atmosphere to quantify the contents of CoO

11

and graphene in hybrid CoO@graphene nanocomposites as shown in Figure 2f. The results show

12

that the weight loss is only 1.3% before 180 °C, resulting from the loss of moisture.

13

Comparatively, the major weight loss was observed from 230 °C to 420 °C with the appearance

14

of maximum degradation rate at 364.15 °C, mainly owing to the fast oxidation of graphene to

15

CO2 with increasing temperature. The weight of CoO@graphene nanocomposites remained

16

unchanged at 50.9% from 680 °C to 800 °C, indicating good thermal stability of CoO.

17

The nitrogen adsorption-desorption isotherms of CoO@graphene nanocomposites (Figure S4)

18

can be classified as type III of isotherms with IUPAC-type H3 hysteresis loops in the p/p0 range

19

of 0.50-1.0, indicating the existence of narrow necks in the materials.38 The pore size distribution

20

curves of CoO@graphene nanocomposites were calculated by the BJH method, shown in the

21

insert of Figure S4. It exhibits a pore distribution peaks at about 3 nm with pore volume of about

22

1.19 cm3/g, indicating CoO@graphene has a typical mesoporous structure. The specific surface

23

area of CoO@graphene, calculated according to the BET method, is 290.78 m2/g. The relative


12

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1

high specific surface area can provide more photocatalytic sites, while the mesoporous structure

2

effectively reduces the phtotocatalyst agglomeration and facilitate the contact between

3

contamination and oxidative radicals produced during photocatalysis. Therefore the

4

photocatalytic performance improved. A previous study has already confirmed that the electron

5

transition process of Co (II) compound from 2p orbital to 3d orbital would finally result in two

6

main peaks in the L-edge spectrum, with the lower energy peak (marked as L3 edge) and the

7

higher energy peak (marked as L2 edge).39 Herein, CoO@graphene nanocomposites were further

8

investigated by spatially resolved XANES spectroscopy.

Figure 3. XANES spectra of U-CoO@graphene and CoO@graphene (a), and STXM chemical imaging of CoO@graphene (b-d). 9

As can be seen in Figure 3a, CoO@graphene nanocomposites contain an L3 edge and an L2 edge,

10

and the Co element existed in the Co(II) value, which is similar to the pure CoO spectrum of

11

XANES found in the literature.39 The increasing ratio also indicates the enhanced octahedral

12

Co2+ (Oh) structure in CoO nanoparticles adhered to graphene after thermal treatment, which

13

reflects the change of electronic structure and crystallinity.40, 41 A selected sample region of the

14

CoO@graphene nanocomposites in Figure. 3b-d is characterized by STXM microscope at the Co

15

L-edge. According to the XANES spectrum shown in Figure 3a, 776.2 eV is selected as the main

16

absorption edge energy, and 770 eV is the energy before the absorption edge. In Figure 3(b,c),

17

the black-and-white images are scanning stack images of Co elements on the L-edge, of which

18

(b) are images before and (c) images on the absorption edge. The colored images (d), derived


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Page 14 of 31

1

from images before and on the absorption edge, represent the density of the Co element. Figure

2

3d illustrates the heterogeneous distribution of Co elements on graphene. This phenomenon is

3

consistent with the findings in the TEM and XRD results, and provides further verification of the

4

superiority of CoO@graphene nanocomposites.

Figure 4. UV-Vis diffuse reflection spectra of CoO@graphene (a), the adsorption and degradation efficiency of TBBPA in an aqueous solution (Initial TBBPA= 4mg/L, pH=8.0, photocatalysts dosage= 10mg/L) (b). 5

As a photocatalyst, it is meaningful to investigate the band gap of CoO@graphene

6

nanocomposites.42 As shown in Figure 4a, CoO@graphene nanocomposites show small diffuse

7

reflectance rate in the visible-light region, which implies good visible-light responses.43 The

8

band gap can be obtained by the modified Kubelka-Munk function, as shown in the insert of

9

Figure 4a.43 Through drawing a tangent line on the abrupt drop of the curve, the band gap energy

10

of CoO@graphene nanocomposites is estimated to be 2.60 eV. At the same time, the low band

11

gap energy of CoO@graphene nanocomposites would facilitate the production of more

12

electron-hole pairs and enhance visible-light absorption. Photogenerated electrons could be

13

transported by high conductivity graphene nanosheets thereby improving the photocatalytic

14

efficiency.

15

3.2 Photocatalytic activity of samples


14

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Figure 4b shows the visible-light driven photocatalytic activities of CoO@graphene,

2

CoO@graphene(0.5) and CoO@graphene(2.0) for TBBPA. Single CoO and graphene has little

3

photocatalytic degradation but adsorption of TBBPA. In the dark reaction period, the removal

4

efficiencies of TBBPA by CoO@graphene, CoO@graphene(0.5) and CoO@graphene(2.0) were

5

17.2 %, 15.5 % and 18.8 %, respectively. They were attributed to the adsorption capacity of

6

these nanomaterials. Among the different photocatalysts, it can be found that the higher amounts

7

of graphene in CoO@graphene leads to stronger adsorption behavior. After 120 min visible-light

8

irradiation, 60.6%, 73.4%, 68.7% and 63.1% of TBBPA were degraded by U-CoO@graphene,

9

CoO@graphene,

CoO@graphene(0.5)

and

CoO@graphene(2.0).

The

increased

size,

10

crystallization degree, distribution density of CoO adhered on CoO@graphene after thermal

11

treatment may accounts for its improved photocatalytic performance, compared with

12

U-CoO@graphene (Figure S5-6).44 The amounts of graphene in photocatalysts also influence the

13

removal of TBBPA.45,

14

CoO@graphene showed best photocatalytic capacity. CoO cannot be sufficiently distributed on

15

graphene due to the low amounts of graphene.47

46

Among different amounts of graphene in hybrid photocatalysts,

While too much graphene wrapped the CoO

Figure 5. Effect of pH on photocatalytic degradation (Initial TBBPA= 4 mg/L, photocatalysts dosage= 10 mg/L) by CoO@graphene nanocomposites (a), TOC removal by CoO@graphene nanocomposites under different pH (b).


15


Page 16 of 31

1

and influence the visible-light irradiation on CoO (Figure S2).48 Therefore CoO@graphene was

2

the best choice under the current experimental conditions. Photodegradation by CoO@graphene

3

nanocomposites was described by pseudo-second order kinetics (Figure S7). The rate constant k

4

for CoO@graphene nanocomposites was calculated as 0.0053 g/mg/min/s.

5

pH of the solution is considered as the most predominant parameter influencing photocatalytic

6

degradation of TBBPA. Figure 5 displays the influence of initial pH (4.0, 8.0, and 12.0) of the

7

aqueous solution on TBBPA degradation and mineralization. As shown in Figure 5a,

8

photocatalytic degradation efficiency obviously decreases with the increase of pH. The

9

isoelectric zeta point is around 3.6, and zeta potentials are continuously decreased when solution

10

pH is increased (Figure S8). TBBPA has carboxyl and piperazinyl groups as its two

11

proton-binding sites (Figure S9), which may be molecular or anionic forms depending on

12

solution pH.49 With the increase of solution pH, TBBPA molecules decreased and were

13

gradually displaced by anionic forms resulting in weaker π-π interaction. In addition, negatively

14

charged CoO@graphene nanocomposites were expected to repel anionic forms of TBBPA at

15

high pH. The weakened π-π interaction and enhanced electrostatic repulsion finally reduce the

16

chances of TBBPA contacting with reactive oxygen species (hydroxyl radical, superoxide anion

17

radical and holes) produced on/near the photocatalyst. Therefore, a decrease in photocatalytic

18

efficiency of TBBPA with increasing pH was observed. However, when pH increased from 4.0

19

to 12.0 during the reaction time of 120 min, the TOC removal has negligible change, showing

20

that only 18.2% of TBBPA can be mineralized by CoO@graphene nanocomposites during

21

visible-light exposure.

22

3.3.Reactive oxygen species identification


16

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

To investigate the main active oxidant in the photodegradation process of TBBPA by

2

CoO@graphene nanocomposites, radical-trapping experiments were conducted by adding three

3

different radical scavengers, benzoquinone (BQ, a quencher of •O2-), isopropanol (IPA, a

4

quencher of •OH), and (NH4)2C2O4 (AO, a quencher of H+) to the reaction system.47, 50 The

5

control experiment was performed under the same conditions without any radical scavengers.

6

Compared to the addition of BQ and AO, the degradation of TBBPA is obviously inhibited by

7

IPA as shown in Figure 6a. Therefore, the above results indicate that generated •OH radicals play

8

a more important role during photocatalytic degradation of TBBPA. To further investigate the

9

formation of •OH radicals, terephthalic acid photoluminescence (PL) probing technique was

10

used.51

11

photoluminescence intensity peak at 425 nm after 120 min reaction under visible light, which

12

indicates the existence of •OH radicals. Pure CoO does not show an obvious peak at 425 nm due

13

to its low photocatalytic activity for TBBPA.

From

Figure

6b,

CoO@graphene

nanocomposites

present

a

characteristic

Figure 6. Radical-trapping experiments (Initial TBBPA= 4mg/L, CoO@graphene dosage= 10 mg/L, pH =8.0) (a), PL spectra of CoO, CoO@graphene (b). 14 15

3.4.Photocatalytic mechanism discussion

16

To investigate the enhanced photocatalytic activity of CoO@graphene compared to CoO and

17

photocatalytic mechanism, the Mott-Schottky measurement was performed. The negative slope


17


Page 18 of 31

1

in Figure S10 indicates CoO is p-type. The Mott-Schottky plots gives the flat-band potential of

2

CoO@graphene and CoO at around 2.69 V and 3.70 V, respectively, with the flat-band potential

3

of CoO@graphene less than that of CoO by about 1 V. The estimated carrier density of

4

CoO@graphene and CoO are 2.55×1019 and 1.63×1019 cm−3, respectively (the calculation

5

process is stated in the Supporting Information). The CoO@graphene has about 1.6 times higher

6

carrier density than that of CoO. The increased carrier density of CoO@graphene can enhanced

7

the band bending, thus shifting to the Fermi level, so that the charge separation would be

8

facilitated.52, 53

9

The improved photocatalytic activity can be ascribed to the synergistic effect of the

10

combination of CoO and graphene and therefore enhanced charge separation in

11

[email protected] Firstly, electron (e-)-hole (h+) pairs separated on CoO under visible-light

12

irradiation (reaction (1)). Light harvesting was improved by graphene due to its large surface

13

area, thereby large number of photo-induced electrons from the valence band to the conduction

14

band increased.55 Subsequently, photo-induced electrons can be transferred quickly and easily to

15

the graphene (reaction (2)), where electrons and dissolved oxygen form the superoxide anion

16

radical (reaction (3)).56 Simultaneously, holes in the valence band of CoO react with water

17

molecules or hydroxide anion to form hydroxyl radicals (reaction (4-5)).57, 58 Eventually, the

18

above active species (holes, superoxide anion radical and hydroxyl radical) react with TBBPA

19

molecules absorbed on the surface of CoO@graphene and further oxidize TBBPA into smaller

20

intermediates (reaction (6)). In addition, graphene can effectively facilitate the efficiency of

21

photodegradation because of its large adsorption ability for TBBPA, increasing the exposure of

22

TBBPA to oxidative radicals. Possible mechanism for the photocatalytic enhancement is

23

proposed as follows and is shown in Figure 7.


18

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

CoO+hυ→CoO(h++e-)

(1)

2

CoO(e-)+graphene→CoO+graphene(e-)

(2)

3

Graphene(e-)+O2→•O2-+graphene

(3)

4

CoO(h+)+H2O→•OH

(4)

5

CoO(h+)+OH-→•OH

(5)

6

CoO(e-)+•OH/•O2-/h++TBBPA→intermediates

(6)

Figure 7. Proposed schematic mechanism for visible light photocatalytic degradation of TBBPA by CoO@graphene. 7

EIS measurements were performed to understand electrical conductivity and synergistic effect

8

between CoO and graphene. As shown in Figure 8a, CoO@graphene has far smaller

9

electron-transfer resistance those of graphene and pure CoO, because of the smaller radius of the

10

semicircular Nyquist plots.59 The remarkable decrease in charge transfer resistance of

11

CoO@graphene can be attributed to the incorporated graphene of excellent conductivity. The

12

smaller charge transfer resistance plays an important role in enhancing photocatalytic ability

13

owing to the easier transition of photo-generated electrons from the conduction band of adhered

14

CoO to graphene. This facilitates the spatial separation of the photo-generated electrons and

15

holes and prevents direct recombination. 30


19


Page 20 of 31

Figure 8. The electrochemical impedance spectra (EIS) of CoO, graphene and CoO@graphene (a), and photocurrent transient responses of CoO, CoO@graphene (b). 1

To investigate the synergistic effect between CoO and graphene sheets, photocurrent

2

measurements were conducted under visible light and darkness via several on-off cycles (Figure

3

8b). With the separation and transportation of photo-induced electrons to the working electrodes,

4

the transient photocurrent therefore formed. The photocurrent of the CoO@graphene (0.233 µA)

5

was much larger than that of the CoO (0.022 µA), demonstrating the synergistic effect has

6

obvious improvement in the separation efficiency of photoinduced electrons and holes of

7

CoO@graphene.

8

3.5.Products identification and possible degradation pathways

9

The major intermediates in the visible-light driven the degradation of TBBPA catalyzed by

10

CoO@graphene nanocomposites were analyzed by UPLC/Q-TOF-MS. Eleven intermediates,

11

assigned as products 1-11, including their molecular weight, structures and retention time are

12

summarized in detail in Table S2. The mass spectra of products 1-11 are shown in Figure

13

S6-S18.

14

2,6-dibromo-4-isopropylphenol

15

2,6-dibromo-4-(prop-1-en-2-yl)phenol

16

2-(3,5-dibromo-4-hydroxyphenyl)-2-hydroxyacetic acid (4), according to the intermediates

Among

all

the

products, (1),

products

1-4

are

tentatively

identified

as

2,6-dibromo-4(2-hydroxypropan-2-yl)phenol

(2),

(3),

and


20

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

found in previous reports on the degradation of TBBPA.60-62 Products 6-8, debrominated

2

intermediates, are identified as tribromobisphenol A (TriBBPA), dibromobisphenol A (DiBBPA,

3

two isomers), and monobromobisphenol A (MonoBBPA), by comparing the mass spectrums

4

with identified products reported in published literatures.63, 64 Products 9 and 11(two isomers

5

labeled as 11a and 11b), which have been observed in earlier studies,65 are identified as

6

3-bromo-5-(2-(3,5-dibromo-4-hydroxyphenyl)progan-2-yl)benzene-1,2-diol

7

5-(2-(3,5-dibromo-4-hydroxyphenyl)propan-2-yl)benzene-1,2,3-triol

8

5,5’-(propane-2,2-diyl)bis(3-bromobenzene-1,2-diol) (11b). In addition, products 5 and 10 (two

9

isomers labeled as 10a and 10b) are detected for the first time in the photodegradation of Product

11

4-((2-(3,5-dibromophenyl)propan-2-yl)oxy)benzoic acid. Product 10 is tentatively identified as

12

4-(2-(3,5-dibromo-4-hydroxyphenyl)propan-2-yl)benzene-1,2-diol

13

3-bromo-5-(2-(3-bromo-4-hydroxyphenyl)propan-2-yl)benzene-1,2-diol (10b). Based on the

14

above discussion on the intermediates identified by UPLC / Q-TOF-MS methods and previous

15

studies,60, 61, 66 possible pathways for photocatalytic degradation of TBBPA by CoO@graphene

16

nanocomposites are proposed. As shown in Figure 9, the proposed reaction mechanisms for the

17

degradation of TBBPA include two different pathways (labeled as routine I and routine II),

18

which correspond to different •OH radical attack sites on TBBPA molecules. For routine I, •OH

19

radical attack the C-C bond, resulting in the cleavage between the isopropyl group and one of the

20

benzene rings to generate product R1 and 2,6-dibromo-4-isopropylphenol (1). Product R1 may be

21

further oxidized into CO2 and H2O by •OH radicals due to its relatively small molecular weight.

22

Product

23

2,6-dibromo-4-(prop-1-en-2-yl)phenol

can

be

transferred

into

indentified

or

TBBPA.

5

tentatively

(11a)

10

1

is

(9),

(10a)

2,6-dibromo-4(2-hydroxypropan-2-yl)phenol (3),

as

or

(2), and


21


Page 22 of 31

1

2-(3,5-dibromo-4-hydroxyphenyl)-2-hydroxyacetic acid (4) by •OH substitution, elimination and

2

the combination of substitution and oxidation, respectively. In addition, product 1 containing an

3

active benzyl carbon-hydrogen bond is more easily attacked by •OH radical. Therefore product 1

4

may be oxidized into product R2, which subsequently reacted with excessive product 1 through

5

substitution and debromination to yield product 5.

6

The formation of Br- during TBBPA degradation is justified as the TBBPA lost one or two Br-

7

atoms due to attack by •OH in the medium.61, 67 For routine II, C-Br bonds of TBBPA were

8

attacked by •OH and sequential debromination of TBBPA occurred to form Tri-BBPA (6),

9

Di-BBPA (7) and Mono-BBPA (8). Further •OH substitution occurred resulting in the generation

10

of

3-bromo-5-(2-(3,5-dibromo-4-hydroxyphenyl)progan-2-yl)benzene-1,2-diol

11

4-(2-(3,5-dibromo-4-hydroxyphenyl)propan-2-yl)benzene-1,2-diol

12

3-bromo-5-(2-(3-bromo-4-hydroxyphenyl)propan-2-yl)benzene-1,2-diol

13

5-(2-(3,5-dibromo-4-hydroxyphenyl)propan-2-yl)benzene-1,2,3-triol

14

5,5’-(propane-2,2-diyl)bis(3-bromobenzene-1,2-diol) (11b). Bromide ion is also detected in the

15

current study at considerably lower concentrations. This is rationalized as the debromination rate

16

of TBBPA is about 15.3% (Figure S11). Overall, CoO@graphene nanocomposites can be

17

activated by visible light to generate reactive oxygen species for the degradation of TBBPA.

(10a) (10b) (11a)

(9), or and or


22

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Proposed possible pathways of photocatalytic degradation of TBBPA under visible light irradiation in the presence of CoO@graphene. 1

3.6. Reusability of CoO@graphene nanocomposites

2

The magnetization curves of the CoO@graphene nanocomposites at room temperature are shown

3

in Figure 10a. They show that CoO@graphene nanocomposites have good magnetic properties

4

with saturation magnetization (Ms) of 12.9 emu/g. The room temperature coercive field is 0.0258

5

T and the permanence magnetization is 1.962 emu/g, indicating the presence of ferromagnetic

6

portion. The increase of uncompensated moments at the disordered particle surface, which is

7

resulted from the reduced coordination of the surface spins, is considered as the reason for

8

ferromagnetic portion.68 The relatively large saturation magnetization value of CoO@graphene

9

nanocomposite is sufficient for its immediate recycling from solutions using an external


23


Page 24 of 31

1

permanent magnet, as showed in the insert of Figure 10a. This magnetic behavior plays an

2

important role in separation and reuse of the magnetic CoO@graphene nanocomposite for the

3

purification of wastewater.

Figure 10. Magnetization curves of CoO@graphene (a), and four cycles photodegradation of TBBPA over CoO@graphene (b). 4

Reusability is usually considered as an important factor in the application of catalysts in terms

5

of economy. After separating from solution using a magnet, recycled CoO@graphene

6

nanocomposites was examined in terms of the photocatalytic degradation towards fresh TBBPA.

7

Figure 10b displays the repeat photocatalytic performance under visible- light irradiation for four

8

cycles. Though the photoactivity of CoO@graphene nanocomposites is decreased around 8.6%

9

after four cycles, it still retains relatively good photocatalytic activity. XPS spectra of used

10

CoO@graphene also indicate the good stability of it. Moreover, the concentration of the leaching

11

Co2+ after the reaction is low (Figure S12). Therefore CoO@graphene nanocomposites recycled

12

using magnets can effectively reduce the cost of regeneration and may have practical use in

13

wastewater purification.

14

4. Conclusions

15

Well-dispersed CoO@graphene nanocomposites were synthesized by a modified, facile, and

16

ultrasonic-assisted method. The samples were characterized by various spectroscopic and

17

analytical techniques. TEM, STXM and XANES indicated the higher density of Co elements and


24

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

enhanced octahedral Co2+ (Oh) structure in CoO nanoparticles adhered to graphene. XRD and

2

Raman analysis confirmed the increased crystallinity of CoO nanoparticles on the surface of

3

graphene nanosheets. The UV-vis DRS spectral analysis illustrated the narrower band gap.

4

Photocurrent and EIS measurements displayed the charge transfer resistance of CoO@graphene

5

nanocomposites resulting in its superior visible-light responses. The synergistic effect of CoO

6

and graphene is evident in the photocatalytic performance of the catalysts and CoO@graphene

7

nanocomposites show excellent photocatalytic degradation for TBBPA. The enhanced

8

photocatalytic activity was mainly attributed to the intense light absorption from increased

9

crystallization, well-dispersed CoO and narrow band gap energy.

10

ASSOCIATED CONTENT

11

Supporting Information

12

The Supporting Information is available free of charge on the ACS Publications website at DOI:

13

XXXXXXXXXXX.

14

TEM, Roman, Nitrogen adsorption-desorption isotherm, XRD, Degradation kinetics, Zeta

15

potential,Species distribution of TBBPA, Mott-Schottky plots, XPS spectra after reaction, ICP

16

and other reaction intermediates.

17

AUTHOR INFORMATION

18

Corresponding Author

19

Tel.: +86 21 6598 8029; Fax: +86 21 6598 3706.

20

*E-Mail: [email protected].

21

Funding Sources


25


Page 26 of 31

1

Natural Science Foundation of China (21776224) and National Water Pollution Control and

2

Treatment Key Technologies RD Program (2015ZX07406-001).

3

ACKNOWLEDGMENT

4

We thanks L. Zhang and X. Zhen for their supporting during runs at the Shanghai Synchrotron

5

Radiation Facility (SSRF). This work was supported by Natural Science Foundation of China

6

(21776224) and National Water Pollution Control and Treatment Key Technologies RD Program

7

(2015ZX07406-001).

8

REFERENCES

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

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(11)

(12)

(13) (14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

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