polyacrylic acid composite for efficient photodegradation of chlortetracycline

Journal Pre-proof Performance optimization of CdS precipitated graphene oxide/polyacrylic acid composite for efficient photodegradation of chlortetracycline Wenjia Kong (Data curation) (Writing - original draft), Yue Gao (Formal analysis), Qinyan Yue (Supervision) (Funding acquisition), Qian Li (Conceptualization) (Funding acquisition), Baoyu Gao (Project administration) (Funding acquisition), Yan Kong (Data curation), Xindong Wang (Writing - review and editing), Ping Zhang (Software), Yu Wang (Resources)

PII:

S0304-3894(19)31734-0

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121780

Reference:

HAZMAT 121780

To appear in:

Journal of Hazardous Materials

Received Date:

11 October 2019

Revised Date:

22 November 2019

Accepted Date:

27 November 2019

Please cite this article as: Kong W, Gao Y, Yue Q, Li Q, Gao B, Kong Y, Wang X, Zhang P, Wang Y, Performance optimization of CdS precipitated graphene oxide/polyacrylic acid composite for efficient photodegradation of chlortetracycline, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121780

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Performance

optimization

of

CdS

precipitated

graphene

oxide/polyacrylic acid composite for efficient photodegradation of chlortetracycline

Wenjia Kong 1, Yue Gao 2,*, Qinyan Yue 1,**, Qian Li 1, Baoyu Gao 1, Yan Kong 1,

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Xindong Wang 1, Ping Zhang 3, Yu Wang 4

Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of

Environmental Science and Engineering, Shandong University, Qingdao 266237, China 2

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

Environment, Tsinghua University, Beijing 100084, China 3

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Shandong Urban Construction Vocational College, Jinan 250103, China

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Shandong Construction Project Environmental Assessment Service Center, Jinan 250012, China

* Corresponding author: E-mail address: [email protected]

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Graphical Abstract

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 Corresponding author: E-mail address: [email protected]

Highlights 

GO/PAA-CdS was prepared by facile polymerization-precipitation method.



GO/PAA-CdS showed excellent photocatalytic activity to CTC under visible 1

light. 

The polymer network favored the size control and distribution of CdS nanoparticles.



DFT calculation helped to predict reactive sites and degradation pathways of CTC.

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Abstract Here a CdS embedded poly acrylic acid (PAA)/graphene oxide (GO) polymeric composite was prepared for the efficient degradation of chlortetracycline (CTC) driven

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by visible light irradiation. The structure-activity relationship of GO/PAA-CdS was

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confirmed through the photocatalytic evaluation of a series of samples prepared by varying GO concentration, molar ratio of Cd:S and the amount of crosslinking agent.

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Through the composition, morphology, photoelectrochemical characterizations and degradation kinetic studies, it could be confirmed that the enhanced photocatalytic

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activity is attributed to the controlled growth of CdS nanoparticles by polymer net structure and effective electron transfer along GO nanosheets. The photodegradation of

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CTC was confirmed to be mainly governed by ·O2- and ·OH radicals generated from

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GO/PAA-CdS. The degradation intermediates of CTC were confirmed by LC-MS, and possible degradation pathways were proposed based on the prediction of radical attacking sites according to Fukui function values obtained through Density Functional Theory (DFT). Moreover, it was found that the catalytic activity of the photocatalyst was maintained after several cycles confirming the enhanced anti-photocorrosion of

2

GO/PAA-CdS. This research provided an efficient approach by a novel photocatalyst for the removal of CTC from wastewater.

Key words: CdS nanocomposites, photocatalytic, chlortetracycline, visible light

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Introduction As one of the most frequently used pharmaceutics in our daily life,

tetracyclines have been applied in human and veterinary medicines [1].

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Chlortetracycline (CTC) is the first discovered tetracycline possessing a four-

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ring system with multiple substituent groups on the molecule skeleton [2]. As a broad-spectrum antibiotic, CTC has been extensively used in therapeutics and

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growth promotion for livestock industry [3]. The frequently detection of CTC in environment is a strong evidence of incomplete absorption or excessive

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application in practice [4]. The exposure of CTC to microorganism has

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aggravated the antibiotic resistance [5]. It has been reported that CTC could be discharged into surface water, then cultivated in sediments and soils causing

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adverse effects on ecosystem and human health [6]. Thus, it’s crucial to eliminate CTC from aqueous environment to avoid further hazards. Researchers have done various attempts to eliminate CTC from water bodies,

such

as

adsorption

[7],

membrane

filtration

[8],

biodegradation

by

microorganisms [9, 10] and so on. However, these technologies suffered from 3

secondary pollution, low-efficient, high demands of chemical reagents and high cost [11, 12]. Recently, the Advanced oxidation processes (AOPs) including Fenton, persulfate oxidant and photocatalysis have shown significant promotion in the degradation and removal of antibiotics [13]. It has been reported that trace CTC in aqueous environment undergoes photochemical decay due to its significant sensitivity to light irradiation [14]. Thus, the photocatalysis process

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has been considered as one of the most desirable methods to treat CTC containing wastewater due to its recyclability, high efficiency and cost effectiveness. To

better utilize the sustainable solar energy in practical application, it’s important

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to develop visible light induced photocatalysts. CdS semiconductor has attracted

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much attention because of its high absorption coefficient in visible light range [15, 16]. However, the severe photo-corrosion of pristine CdS hindered its

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application in practice. Two main attempts have been applied to reduce the

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photo-corrosion and enhance the photocatalytic activity. One is binding multi components into one composite to form heterojunctions or facilitate the electron

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transfer for better performance, such as the addition of semiconductors [17-22], metals [23, 24], carbon dots [25], graphene oxide (GO)/ reduced graphene oxide

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(rGO) [26, 27]. Among these, GO or rGO with unique 2D sheet structure and oxygen containing groups has been proved to effectively prevent the photocorrosion of pristine CdS nanoparticles and performs great potential in modification [28]. Jiang synthesized CdS/rGO through hydrothermal treatment and the products appeared to efficiently reduce the electron-hole recombination 4

during the photodegradation of phenol [29]. Another approach to modify the photocatalyst has arouse via blending with polymeric composites. The 3D network and abundant functional groups of it could provide nucleation sites for the nanoparticles growth and contribute to the even dispersion of size-controlled nanoparticles to avoid agglomerates [30]. Midya et.al. reported the preparation of ZnO/CdS embedded crosslinked chitosan via microwave irradiation with

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enhanced photocatalytic and antibacterial performances [31]. An et.al. employed polyacrylamide (PAM) microgel to immobilize CdS nanoparticles and the

electrostatic interactions between CdS and PAM reduced the photo-corrosion

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compared with pure CdS [32]. Polyacrylic acid (PAA) as a widely used monomer

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in polymerization could graft with other components through bonding interaction to form a hybrid 3D network with excellent swelling and adsorption

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performances [33]. The polymerization between GO and PAA could be realized

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as both components embrace numerous oxygen containing groups acting as graft sites. Inspired by the above approaches, it’s feasible to build up a growth matrix

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for CdS nanoparticles consisting of GO and PAA, which could theoretically facilitate the transfer of photogenerated electrons and simultaneously contribute

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to the immobilization and dispersion of CdS nanoparticles. As reported previously, the photocatalytic performance was closely related to the synergistic effect between semiconductor and growth matrix [34]. Thus, it is critical to confirm the structure-activity relationship for the optimum photocatalytic

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performance of GO/PAA-CdS, and to figure out whether the anti-corrosion under visible light and stability of the photocatalyst were promoted. In this research, the photodegradation of CTC by GO/PAA-CdS photocatalyst was first proposed. The concentration of GO, molar ratio of Cd:S and the amount of crosslinking agent were optimized. The degradation kinetics of CTC by a series of samples were investigated combined with the characterization results to confirm the

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role of the main component during the photocatalytic process. The photodegradation

mechanism of GO/PAA-CdS was first discussed and the degradation pathway of CTC was proposed according to the prediction of radical attack sites on CTC via computation

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calculations based on Density Functional Theory (DFT).

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2 Materials and methods 2.1 Chemical reagents

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The chemical reagents applied in this research have been described in Text S1.

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2.2 Preparation of GO/PAA-CdS

The preparation of GO/PAA-CdS could be divided into two successive processes

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including the solution polymerization of GO/PAA hydrogel and the in-situ growth of CdS nanoparticles (as shown in Fig. 1). According to the preliminary study [35], the

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highest photocatalytic efficiency was obtained at weight ratio of GO:AA=0.005:1 which was kept constant during the preparation process. Briefly, 10 mL of 2.0 mg/mL GO suspension was added into the solution polymerization system followed by the addition of initiators, neutralized AA and crosslinkers. The system was then continuously stirred at 50 oC for another 4 h. The resultant GO/PAA hydrogel was 6

freeze-dried and ground to powder for the next step. In a typical precipitation process, 0.30 g of the above GO/PAA powder was placed in 100 mL of 0.0025 mol/L Cd(NO3)2 solution until the adsorption equilibrium and then washed with distilled water for 3 times. Next, the 100 mL of 0.005 mol/L Na2S solution was added dropwise under constant temperature shaking at 50 oC for 4 h. The resultant products were washed with distilled water, freeze-dried and ground for further use.

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To investigate the effects of preparing conditions on the photocatalytic

performance, the amount of crosslinker N,N'-methylene-diacrylamide (MBA,

represented by the weight ratio of MBA to AA, mMBA:mAA), the initial molar

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ratio of Cd:S (nCd:nS) and the concentration of GO suspension (cGO) were varied

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to synthesize a series of samples for the optimization of preparing conditions. Three other control samples, GO/PAA, PAA-CdS, and pristine CdS were

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synthesized as concluded in Text S2 to evaluate the contribution of certain

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compounds in the photocatalytic performance. All the as-prepared samples were

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labelled according to the preparing variables as listed in Table S1.

Fig. 1 The synthetic route of GO/PAA-CdS.

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2.3 Solid phase characterization The Fourier-transform infrared (FTIR) spectra was employed to characterize the chemical components of GO/PAA-CdS samples on a ThermoFisher-Nicolet 6700 spectrometer with Attenuated Total Reflectance (ATR) accessory. The morphology with the qualitative and quantitative information of the samples were measured through scanning electron microscopy-energy dispersive X-ray

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spectroscopy (SEM-EDX, Hitachi SU8010) and high-resolution transmission electron microscope (HRTEM, Titan G2-60-300). The Inductive Coupled

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Plasma-Atomic Emission Spectrometry (ICP-AES, Agilent 5110) was used to

measure the elemental composition of the samples in solid state. The crystallized

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structure of the sample was recorded on an X-ray diffractometer (XRD, Bruker

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D8 Advance) with Cu Kα radiation. The valent state of GO/PAA-CdS samples were analyzed by a Raman microprobe (Horiba Jobin Yvon, LabRAM HR800)

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and (Valence band) X-ray photoelectron spectroscopy ((VB-)XPS, Thermo ESCALAB 250XI). The zeta potential of GO/PAA-CdS samples were

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determined by a Zetasizer (Nano ZS, Malvern). The optical property of the catalysts and the kinetic degradation of CTC was analyzed by UV-vis diffuse

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reflectance spectroscopy (UV-vis DRS, UV 2450 Shimadzu) in the range of 300800 nm and Photoluminescence spectroscopy (PL, FLS 980 Edinburgh) with the excitation wavelength of 430 nm. The radicals were detected with electron spin resonance (ESR, JES FA200) with dimethyl pyridine N-oxide (DMPO) acting as the trapping agents for ·OH and ·O2- radicals. The photoelectrochemical analysis 8

were conducted by an electrochemical workstation (CHI 760E) and the detailed methods were concluded in Text S3. 2.4 Photocatalytic degradation of CTC A typical photodegradation experiment of CTC was conducted as follows: 0.03 g of GO/PAA-CdS particles were added into 200 mL of 30 mg/L CTC

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solution and magnetically stirred in darkness for 1.5 h to reach adsorption equilibrium (pH=6). Next, the reactor was irradiated with visible light (λ>460

nm) via a xenon lamp (CEL-HXF300, 300W) under stirring with a water-cooling

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jacket (solution temperature maintained at 25±1 oC). The distance from light source to the solution surface was set at 8 cm with an approximate irradiation

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intensity of 1150 mW/cm2. At certain time interval, 2 mL of solution was taken

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out and filtered through 0.22 μm millipore film for determination of the residual CTC. The control group was conducted in darkness throughout the process with

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other experimental conditions set the same as above mentioned. The removal efficiency was calculated by Eq. (1) and the photodegradation process was fitted

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with Langmuir-Hinshelwood equation [36] (Eq. (2)).

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Efficiency (%)=(1-C/C0)×100% ln(C0/C)= -kt

(1) (2)

where C0 and C (mg/L) represent the concentration at initial time and instant time t (min) of CTC, respectively; k (min-1) represents the rate constant of photodegradation. 9

The effect of solution pH was investigated by varying the pH of CTC solution with 0.1 mol/L of HCl and NaOH solution. The recycling test of the CTC photodegradation over the catalyst was carried out as follows: after the first run of adsorption-photocatalytic reaction, the catalysts were filtered, washed with distilled water and dried. The stability of the catalyst was evaluated by characterization of used catalysts and the measurement of Cd (II) in the reaction

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supernatant liquid as a result of photo-corrosion by atomic absorption spectrometer (AAS, TAS-990).

ROS (Reactive oxygen species) quenching experiment was conducted with

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1,4-p-benzoquinone, sodium oxalate and t-butanol as the quencher of superoxide

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radicals (·O2-), hydroxyl (·OH) radicals and photo-generated holes (h+),

2.5 Analytical methods

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respectively.

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The concentration of CTC was measured by High Performance Liquid Chromatography (HPLC, Waters 2489) at 274 nm with mobile phase (volume

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ration: acetonitrile/methanol/0.01M oxalic acid=22:11:67). The degradation

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products were analyzed by Liquid Chromatography-Mass Spectrometry (LCMS) system (Thermo, LCQ FLEET). The mineralization rate was confirmed through measurement of the Total Organic Carbon (TOC) by TOC analyser (Shimadzu, VCPH) All the experiments were run in triplicate. The error bars of graphs represented one standard deviation (SD). The theoretical calculation based on DFT was carried out by Gaussian 09 and Multiwfn [37]. The values of 10

Fukui function (f0) representing the preference of radical attack was used to evaluate the reactive sites on CTC molecules and the detailed illustrations could be found in Text S4. 3 Results and discussions 3.1 Compositional, structural and morphology of GO/PAA-CdS catalysts As the synthesis of GO/PAA-CdS catalysts consists of two successive steps,

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the combination among the main compounds should be investigated to better

illustrate the synthetic process. The FTIR spectra of GO/PAA and GO/PAA-CdS

are displayed in Fig. S1. It’s obvious that the spectra of these two different

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samples share several characteristic peaks related to the functional groups on

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PAA polymer chains and GO sheets. The wide and intensive peaks in the range of 3250-3450 cm-1 are ascribed to the -OH groups on the GO/PAA matrix and

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the absorbed water molecules. The peaks at 2947 and 2850 cm-1 are attributed to

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the asymmetrical and symmetrical vibrations of -CH2 on the side chains of PAA, respectively. The symmetric vibration of carboxyl groups on PAA polymer

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chains and GO sheets defections contributes to the peaks at 1548 and 1449 cm-1 [38]. In addition, the peaks at 1647 and 1400 cm-1 are related to the skeletal

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vibration of graphitic structure and C-OH stretching vibration on GO sheets, respectively. The above results reveal the polymerization between GO sheets and PAA polymer chains. After the in-situ precipitation of CdS, a new peak at 1118 cm-1 corresponding to the Cd-S vibration has been observed, indicating the successful precipitation of CdS on GO/PAA. Besides, the position and relative 11

intensity of each peak shows negligible difference after the precipitation process, which suggests that the introduction of CdS did not change the basic structure of GO/PAA polymer and the CdS nanoparticles are well dispersed in the polymeric matrix. To further verify the bonding configuration between CdS nanoparticles and GO/PAA as well as the chemical compositions, XPS measurement of GO/PAA

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and GO/PAA-CdS were performed as shown in Fig. S2 and Fig. 2. For the accurate analysis, all the peaks were calibrated by C1s peak at 284.8 eV. In the

low-resolution spectra of these two samples, the peaks of C1s and O1s are

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observed as main species while the peaks of Cd 3d and S 2p only appear in the

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spectrum of GO/PAA-CdS. The formation of CdS can also be confirmed in Fig. 2(C, c) where the Cd 3d peaks (411.08 and 404.34 eV) and S 2p (161.74 and

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160.50 eV) peaks could be clearly identified. In Fig. 2A, the O1s peaks at 530.40,

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531.34 and 532.32 eV are corresponded to the C-O-C, C-O and C=O bonds in GO/PAA-CdS composites, respectively. It’s obvious that the above three peaks

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related to the oxygen functional groups moved to higher binding energy located at 530.98 eV (C-O-C), 532.04 eV (C-O) and 535.70 eV (C=O) in Fig. 2a [39]. It

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can be calculated that the peak location differences between the corresponding peaks before and after the introduction of CdS were all higher than 0.5 eV, the minimum value for the confirmation of chemical bonding between the reactants [40]. As for the spectrum of C 1s in Fig. 2B, four distinct peaks appeared after peak fitting. The peaks at 287.65, 285.20 eV, 284.61 and 284.18 eV are related 12

to C=O, C-O, C=C (sp3 hybridized) and C-C (sp2 hybridized) originates from the π-π bonding along the GO sheets and polymer skeleton, respectively. Similarly, for GO/PAA-CdS, the peak area ratios slightly changed and the peaks of C=O and C-O moved to 284.19 and 284.63 eV with an increase amount of 0.58 and 0.59 eV, respectively. However, for the peaks of C=C and C-C, no obvious change was observed in the peak shift. The above results indicate that the oxygen

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precipitation of CdS through chemical bonding.

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functional groups (C=O and C-O in COOH) are the main reactive sites for the

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Fig. 2 The high-resolution XPS spectra of GO/PAA (a, b) and GO/PAA-CdS (A, B, C, D) samples.

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The catalysts with different amounts of GO were analyzed by Raman spectra to evaluate the introduction of GO sheets. As can be seen in Fig. 3A, the peaks representing CdS (600 cm-1) and the D (1355 cm-1) or G band (1603 cm-1) of GO sheets were detected in the catalysts with significant intensity, indicating the successful combination between GO/PAA matrix and CdS. With the increasing concentration of GO, the peak intensity ratio of ID/IG gradually increased from 13

0.98 to 1.03, resulting in a more disordered graphite structure owing to the grafted among the oxygen containing groups on the GO sheets and AA chains [41]. It can also be noted that no obvious change was observed in the characteristic peak of CdS, suggesting that the introduction of GO would do little to the structure of CdS. The XRD spectra of GO/PAA and GO/PAA-CdS with different initial nCd:nS are

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shown in Fig. 3B. For the spectrum of GO/PAA, there’s no obvious diffraction peak of GO which may be due to the even distribution of GO nanosheets in the PAA polymeric

matrix. For the spectra of GO/PAA-CdS samples, the optimal crystal structure of CdS

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was obtained at nCd:nS=1:2 with intensive peaks at 26.54o (111), 44.00o (220), 52.20o

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(311), 71.0o (331) and 80.89o (422) corresponding to the specific peaks for cubic CdS particles as compared to the standard database (PDF#75-0581) [42]. The elements

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chemical state and composition on the catalysts varied the nCd:nS in the final product

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which would further affect the CdS crystals and thereby the weakness of peak intensity was observed. As nCd:nS increased to 1:1, the Cd2+ first adsorbed on the polymer matrix

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couldn’t be fully precipitated by S2-, resulting in the reducing amount of CdS nanoparticles on the whole polymer matrix which further negatively affected the

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catalyst crystal structure. On the contrary, as nCd:nS decreased to 1:4, there may be excessive S2- binding on the surface of CdS particles contributing to the generation of unbonded S2- and thus the degree of crystallinity decreased.

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Fig. 3 The Raman spectra of GO/PAA-CdS with different GO concentration (A); the XRD spectra of GO/PAA and GO/PAA-CdS with different molar ratio of nCd:nS (B).

The SEM images of GO/PAA-CdS(1:2) are shown in Fig.4(A, B). The catalyst

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possesses obvious network structure with CdS nanoparticles uniformly dispersed on the rough surface. The polymer network provides excellent matrix for the CdS

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nanoparticles growth and could limit the size of the particles to a relatively

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homogeneous spherical shape. As observed from SEM-EDX mapping results in Fig. 4(E, F), the catalyst mainly consists of Cd, S, C, O and Na. The C, O and Na

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atoms originates from the GO/PAA polymer matrix which is covered by Cd and S atoms with a rough atom ratio of 1:1, indicating that there exists little Cd 2+ or

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S2- on its surface. Besides, according to the ICP-AES results (Table S2), the

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atomic ratio of Cd:S was also approximately 1:1 in the whole solid sample, indicating the even distribution of CdS on the catalyst surface and inside the network. The initial nCd:nS shows significant influence on the photocatalytic activity of the catalysts as a result of different loading amount of Cd and S on the polymeric matrix surface which would be further discussed in the following photocatalytic kinetics studies. 15

In the HRTEM images (Fig. 4 (C, D)), CdS nanoparticles represented by black dot with a diameter of 2-10 nm are evenly distributed in the polymer matrix. The clear and continuous stripes detected in the black dots giving the information of lattice spacing of 0.336 and 0.288 nm for (111) and (200) planes of CdS nanoparticles are highlighted, respectively, indicating the crystallized structure

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of the cubic CdS which is in consistent with the XRD results.

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Fig. 4 The SEM images (A, B), HRTEM images (C, D), SEM-EDX mapping (E, F) of GO/PAA-CdS(1:2). 3.2 Optical absorption and photoelectrochemical properties of GO/PAA-CdS The UV-vis DRS of pristine PAA, GO, CdS, and catalysts with different GO concentrations and molar ratios of nCd:nS are shown in Fig. S3 and Fig. 5A. As observed from the spectra, there is no obvious absorption edge for non-

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semiconductor PAA and GO in the visible range and the absorption of light is mainly contributed by the sample color while darker samples could absorb light

from broader range. In Fig. 5A, the CdS and GO/PAA-CdS samples exhibits

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obvious light absorption in visible region. Pristine CdS nanoparticles possesses the strongest light absorption in the whole region, followed by the catalysts with

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different decrease in absorption intensity. The reduction in light absorption is due

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to the combination of CdS with GO/PAA matrix which results in a smaller amount of CdS in a certain amount of catalysts and the coverage of polymer

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network may hinder the light incidence in its solid phase to a slight extent. For the catalysts with various initial molar ratios of Cd:S, the ones with n Cd:nS=1:2

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shows the best light absorption property which is quite close to that of pristine

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CdS particles, especially in the range of 300-500 nm. For the catalysts with different amounts of GO, there remains a positive relationship between the visible light absorption property and GO amount. The results indicates that the proper value of nCd:nS and larger amount of GO would be favorable for the light absorption of GO/PAA-CdS catalysts. In addition, the adsorption edge of the prepared catalysts shows slight red shift compared with pristine CdS 17

nanoparticles which could be further confirmed by the Kubelka-Munk method and the obtained Tauc plots with band energies are shown in the inset of Fig. 5A. The estimated band gap energy (Eg) of the catalysts (2.32-2.38 eV) are lower than that of pristine CdS particles (2.4 eV). The narrower band gap would be beneficial for the photoexcitation to generate more electron-hole pairs by visible light irradiation, thus enhancing the photocatalyzed chemical redox reactions

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[43].

The PL spectra could be applied to analyze the charge separation property in

the GO/PAA-CdS catalysts and the results are shown in Fig. 5B. The PL spectra

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of different samples possesses similar trends with two main characteristic peaks:

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the peak located at 474 nm originates from the band gap excitation, while the peak at 534 nm is corresponded to the electron-hole pairs on the surface and

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excessive Cd2+ (or S2-) in the catalyst composites [44]. According to the spectra,

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the pristine CdS shows highest peak intensity compared with the GO/PAA-CdS, suggesting the high recombination rate of electron-hole pairs which may lead to

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poor photocatalytic activity. The catalyst with more addition of GO exhibits lower peak intensity at 474 nm due to the effective electron transfer along GO

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sheets which could significantly slow down the recombination of electron-hole pairs [32]. The catalyst with nCd:nS=1:2 performed the lowest recombination rate of photogenerated electrons-hole pairs. The catalysts prepared under the initial nCd:nS=1:1 and 1:4 resulted in excessive of Cd 2+ or S2- leading to a higher

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recombination rate, as they can act as the quenchers for the electrons and holes, respectively, resulting in the shortage of active species lifetime. The photoelectrochemical measurements including the photocurrent responses and electrochemical impedance spectra (EIS) of the samples were also carried out to evaluate the photogenerated electrons separation and transfer efficiency. As shown in Fig. 5C, the photocurrent responses of all the tested samples are

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obvious, however, the intensity and stability of photocurrent response of pure

CdS are poorer than the corresponding properties of the other GO/PAA-CdS

composites. And the 2GO/PAA-CdS shows the most significant response

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towards the visible light with excellent stability. In Fig. 5D, the arc radius of the

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GO/PAA-CdS samples are much smaller than that of pure CdS, confirming a lower electrochemical impedance for the transfer of photogenerated electrons.

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Similarly, the 2GO/PAA-CdS with the smallest arc radius possesses the best

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charge separation. These results of photoelectrochemical tests were in consistent with the optical absorption properties, indicating that the moderate GO

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concentration and nCd:nS could elevate the photo-response capability and favor

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the electron-hole separation.

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Fig. 5 The UV-vis spectra with the corresponding Tauc plots (A), PL spectra (B), transient photocurrent response (C) and electrochemical impedance spectra (EIS) (D) of pure CdS and a series of GO/PAA-CdS samples.

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3.3 Photocatalytic performance of GO/PAA-CdS

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3.3.1 The effect of nCd:nS on the photocatalytic activity of GO/PAA-CdS As shown in Fig. S4, the pristine GO aerogel and GO-CdS showed comparable

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adsorption capacity towards CTC due to the π-π interaction between GO graphitic plain and the CTC tetracene structure. The PAA and PAA-CdS performed better in adsorption process because of numerous functional groups on PAA network acting as binding sites, and the composites of GO/PAA and GO/PAA-CdS performed best as they combined the π-π interaction and active 20

binding sites together which greatly enhanced the adsorption performance. For the CdS contained samples, the poor adsorption could further negatively influence the photodegradation process. Specially, the GO-CdS performed worse than pristine CdS in the photodegradation process because the severe stacking of GO nanosheet greatly hindered its function as electron carrier and affected the light absorption of CdS.

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The CdS nanoparticles are the critical components in the GO/PAA-CdS catalysts where they could be photoexcited to generate electrons and holes for the trigger of photodegradation of CTC. As shown in Fig. 6A, without the

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addition of CdS or as nCd:nS varies, all the tested samples could maintain similar

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adsorption capacity. The results indicate that the adsorption of CTC is mainly contributed by PAA/GO polymeric matrix. Nearly no removal efficiency was

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observed in the sole CTC system or with the addition of PAA/GO, revealing the

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difficulty of CTC self-degradation in a short time and the fundamental role of photocatalyst in the photodegradation process. With the varying n Cd:nS, the

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adsorption capacity of CTC under darkness hardly changed while significant variance was measured in photocatalytic performance. The maximum removal

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efficiency and rate constant k value are obtained at initial nCd:nS=1:2, while both values obviously decrease as the molar ratio shifts to a higher or lower level. The final molar ratio of Cd:S on the catalysts is largely dependent on the initial nCd:nS during the precipitation process. The catalysts were tested by SEM-EDX and ICP-AES. The SEM-EDX results of GO/PAA-CdS(1:1) and GO/PAA21

CdS(1:4) are shown in Fig. S5 and the ICP-AES results are listed in Table S2. The atomic ratio of Cd:S in the overall photocatalyst obtained from ICP-AES was close to the data obtained from SEM-EDS results, indicating the even distribution of CdS nanoparticles in the composites (including the catalyst surface and pores inside the network structure). And the atomic ratio did little to the polymer network while affected the form of CdS nanoparticles as shown in

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Fig. S6. Combined with the ICP-AES and SEM-EDX results, for GO/PAACdS(1:1) catalyst, there remains a large amount of unprecipitated Cd 2+ which results in the less portion of active CdS nanoparticles and excessive

-p

photogenerated electron quenching sites, thereby hinders the light adsorption and

re

redox reaction, leading to poor photocatalytic activity [45]. The results are in consistent with the XRD studies where the initial molar ratio 1:2 shows the

lP

optimum crystal structure. Besides, the Cd2+ ions absorbed on the surface are

na

unstable and easily to be removed from the catalyst under light irradiation, causing harmful Cd2+ leaking [46]. For GO/PAA-CdS(1:4), the excessive S2- ions

ur

were observed which may be absorbed on the surface of CdS and further shortening the life time of photogenerated hole. Thus, the appropriate nCd:nS

Jo

should be optimized at 1:2 with proper Cd:S atom ratio in the resultant products. 3.3.2 The effect of GO concentration on the photocatalytic activity of GO/PAACdS It could be observed in Fig. 6B that the adsorption capacity of the catalysts improves slightly with the increasing amount of GO. The photodegradation 22

process under visible light is promoted with the increasing adsorption capacity because better attachment of target pollutants on the photocatalysts is in favour of the efficient transfer of photogenerated radicals from catalyst surface to the adsorbed CTC molecules. The prepared catalysts GO/PAA-CdS and PAA-CdS possesses higher adsorption capacities and removal efficiency, compared with those of pristine

ro of

CdS particles. The catalyst with support of polymer (PAA or GO/PAA) shows

significant enhancement in the adsorption process due to the abundant functional groups on the polymer structure. The CTC could be adsorbed on the polymer

-p

network through the electrostatic interaction between -COOH and -NH(CH3) or

re

the hydrogen bonding among -OH groups, thus significantly enhancing the adsorption process under darkness. In addition, as the amount of GO increases,

lP

the removal ratio is dramatically levelled up from 38.0% to 86.0% with an

na

increasing k value from 0.0029 to 0.0152 (as shown in the inset column), respectively, revealing that there remains a positive relationship between the

ur

applying amount of GO and photodegradation performance of the catalysts. This phenomenon is highly ascribed to the efficient separation of photogenerated

Jo

electron/hole pairs as illustrated in the PL spectra [47]. The results indicate that the support of GO/PAA could improve the photocatalytic activity compared with that of pristine CdS by the significant increasing adsorption capacity mainly due to PAA component and the introduction of GO plays a vital role in the photocatalytic performance of the prepared catalysts. 23

3.3.3 The effect of MBA amounts on the photocatalytic activity of GO/PAA-CdS Inspired by our previous studies in which the polymer was applied as efficient adsorbents for metal ions removal [33], the amount of crosslinkers in synthesis process showed significant effect on the adsorption property by changing the network structure. In this research, the precipitation of CdS nanoparticles included the adsorption process of Cd2+ which may be influenced by the MBA

ro of

amounts and show potential effect on the photocatalytic activity of the catalyst.

According to the photodegradation kinetic results in Fig. 6C, the adsorption

capacity of GO/PAA-CdS decreases with the increasing amount of MBA from

-p

0.2 wt% to 0.6 wt%. The looser structure allows excessive amounts of CTC to

re

enter the network structure and combines with the adsorption sites on the polymer surface, resulting in an enhancing adsorption capacity [48]. But the

lP

photodegradation of CTC under visible light irradiation reaches maximum both

na

in the total removal efficiency and rate constant k value for the catalyst with 0.4 wt% MBA. To better understand this, the catalysts were analyzed by SEM to

ur

observe the difference in network structure and the distribution of CdS nanoparticles as shown in Fig. S6. Compared with the images of catalyst with

Jo

0.4 wt% MBA in Fig. 4, the network structure was hard to find at 0.6 wt% MBA because larger amount of crosslinkers would result in a tighter structure which showed nearly flat surface. The CdS particles tended to aggregate because of the limited adsorption sites on the plain surface, and finally large clusters were obtained. However, the polymer matrix would become much looser with larger 24

pores as the amount of MBA decreased to 0.2 wt%. The CdS nanoparticles are unevenly distributed on the surface at this applying amount of MBA. The steric hindrance among the functional groups which is beneficial for the control of nanoparticles size during in-situ growth weakens, resulting in ununiform CdS clusters on some specific sites. Thus, the growth of CdS nanoparticles would be greatly influenced by the polymeric network structure and further produces an

ro of

obvious effect on the photocatalytic activity.

According to the above results, the optimized preparing conditions are set as follows: cGO=2 g/L, n(Cd):n(S)=1:2 and m(MBA):m(AA)=0.4 wt%. The best

-p

photocatalytic activity could be ascribed to the enhancing electron-hole

re

separation by GO sheets and effective visible light absorption of well dispersed CdS nanoparticles. Besides, the comparison of photodegradation efficiency with

lP

those of other reported photocatalysts listed in Table S3 indicates the excellent

na

performance of the as-prepared photocatalysts. 3.3.4 The effects of solution pH on the photodegradation of CTC

ur

In order to investigate the influence of solution pH on the removal efficiency of CTC, the adsorption-photodegradation kinetic studies in a series of solutions

Jo

with pH ranging from 2 to 10 were studied (Fig. 6D). The optimum adsorptionphotodegradation of CTC was obtained at around pH=6, while too acid or alkaline solution would result in poor performance both in adsorption and photodegradation processes.

25

As adsorption process accounts for maximum 15% removal ratio in the initial photodegradation process leading to obvious influence on the successive photodegradation of CTC. According to the literatures, the CTC molecules are amphoteric compounds contains three functional sites with pK a of 3.3, 7.44 and 9.27 [49]. Thus, the dominated species distribution of CTC in the whole pH range were calculated and the results are shown in Fig. S7. The main species of CTC

ro of

in aqueous solution appear as CTCH3+, CTCH2±, CTCH3- and CTCH32- with the increasing solution pH values. Similarly, the protonation or deprotonation of the

functional groups on GO/PAA-CdS catalysts happens during the varying of pH

-p

values which has been confirmed by the Zeta potential measurement as shown in

re

Fig. S8. The surface of GO/PAA-CdS is negatively charged almost in the whole pH range apart from pH=2 in which slightly positive charge was observed. The

lP

absolute value of the zeta potential decreases as the pH value rises from 3 to 7

na

and then maintains almost steady which is in consistent with the previous researches [50, 51]. The average Zeta potential of the catalyst was calculated to

ur

be -11.97 mV. The lower pH value in acid solution could enhance the electrostatic interaction between CTC(+) and GO/PAA-CdS catalyst because

Jo

more -COOH groups on GO/PAA groups tend to be deprotonated to generate COO- to combine with CTC(+), however, the excessive amounts of H + in acid solution would obviously destroy the crosslinked sites causing collapse of the polymer network structure, resulting in negative effects of adsorption capacity. Besides, according to the XRD spectra of GO/PAA-CdS after photodegradation 26

in acid solution (Fig. S12B), the diffraction peaks corresponded to CdS nanoparticles remained well which indicated that the crystal structure of CdS was maintained. As the pH increases above 8 the deprotonation of catalysts reaches the maximum and the CTC appears to be negatively charged, thus repulsion force between these two matters largely increases causing significant hindering in the adsorption process and further affects the photodegradation process. Thus, the

ro of

maximum removal efficiency is obtained at around pH=6 as a result of the

synergistic functions of catalyst surface charge, CTC charge and network

Jo

ur

na

lP

re

-p

structure.

Fig. 6 The photodegradation of CTC by the as-prepared samples under different preparing conditions (A, B, C), and as a function of solution pH (D). 3.3.5 Photocatalytic mechanism insights 27

To explore the photocatalytic mechanism and the function of GO in the photocatalytic performance of the prepared catalysts, a comparative quenching experiment between GO/PAA-CdS and PAA-CdS was carried out. As shown in Fig. 7(A, B), the photodegradation process is impeded with the excessive addition of different quenchers, confirming the existence of ·O2-, ·OH and h+ during the photodegradation process catalysed by both GO/PAA-CdS and PAA-

ro of

CdS. For PAA-CdS, the reduction in photodegradation efficiency was observed,

and the maximum difference was obtained after the addition of sodium oxalate for the quenching of photogenerated holes, indicating that the holes are the main

-p

ROSs in the current reaction system, followed by ·O2- and ·OH. This could be

re

attributed to the recombination of electron-hole pairs on the catalyst surface and there are less free electron or hole for the generation of active ·O2- and ·OH [31].

lP

However, after the introduction of GO nanosheets, the maximum difference can

na

be detected when ·OH radicals were quenched, suggesting the dominate role of it. In addition, the amount of ·O2- also increases while that of the photogenerated

ur

holes sharply decreases. The above results reveal that the addition of GO changed the radical distribution in the reaction system. It should be attributed to the

Jo

efficient transfer of the photogenerated electrons from CdS nanoparticles to GO nanosheets via the contact interface due to its excellent electronic conductivity. Thus, the photogenerated electrons and holes could contribute more to the generation of ·O2- and ·OH, respectively, and then significantly improve the degradation of CTC. The separation of photogenerated electron-hole pairs are 28

promoted, effectively inhibiting the recombination between them, thus contributing to the enhanced photocatalytic activity of GO/PAA-CdS. To further confirm the existence of the radicals mentioned above under visible light excitation, the ESR spectra were employed. Dimethyl pyridine N-oxide (DMPO) was chosen to act as spin-trapping agent to capture the radicals separately. The resonance intensities of DMPO-·O2- and DMPO-·OH were

ro of

measured. As shown in Fig. 7(C, D), no signal for ·OH or ·O 2- are detected in

darkness, while strong ESR signals of them can be observed after being irradiated

for 5 min. The results indicate that the photo-generated electron-hole pairs could

-p

deduce the production of ·O2- and ·OH radicals for the degradation of CTC

re

molecules. In addition, the ·OH radicals may contribute more to the photodegradation process as concluded from the quenching experiments and

Jo

ur

na

lP

ESR spectra.

29

ro of -p

re

Fig. 7 The radical quenching experiments for PAA-CdS (A) and GO/PAA-CdS (B);

lP

the ESR spectra of ·OH (C) and ·O2- (D). To find out the photocatalytic mechanism of GO/PAA-CdS, it’s fundamental

na

to know the band structure of the photocatalyst, especially the potential of conduction band (ECB) and valence band (EVB). We took 2GO/PAA-CdS as the

ur

tested sample for the determination of valence band XPS (VB-XPS) and Mott-

Jo

Schottky (M-S) analysis. According to Fig. S9A, the VB edge level for GO/PAACdS is determined to be 1.86 eV referred to the distance from VB edge level to the Fermi level. And the M-S plot shows the flat band potential (E fb) of 0.35 V vs Ag/AgCl (0.55 V vs NHE) obtained from the intercept on x axis of the linear region (Fig. S9B). As illustrated in previous literatures, the E fb is equal to the difference between Fermi level (EF) and water-reduction potential (E(H+/H2) = 30

0.40 V vs NHE) [52]. Thus, based on the Eg value from the Tauc plot (Fig. 5A), the EF, ECB and EVB values were calculated to be 0.15 V, -0.35 V and 2.01 V, respectively. As the conduction band minimum is more negative than the reduction potential of oxygen E(O2/·O2-)=-0.046 eV and the valence band maximum is more positive than E(·OH/OH-)=1.99 eV[53]. Thus, it’s obvious that ·O2- and ·OH radicals could be generated by GO/PAA-CdS under visible

ro of

light. And the generated ·O2- could also be transferred to ·OH under desired

conditions as reported previously. In accordance with the above results, the

approximate band structure and generation pathway of radicals for the prepared

ℎ𝑣

catalyst (ℎ+ + 𝑒 − )

𝑂2 + 𝑒 − →· 𝑂2 −

lP

ℎ+ + 𝑂𝐻 − →· 𝑂𝐻

(3)

re

catalyst →

-p

product was presented in Fig. 8 and the following equations:

(4) (5) (6)

· 𝑂𝑂𝐻 + 𝐻 + + 2𝑒 − →· 𝑂𝐻 + 𝑂𝐻 −

(7)

na

· 𝑂2 − + 𝐻 + →· 𝑂𝑂𝐻

ℎ𝑣

CTC*

ur

CTC →

(8) (9)

CTC* +· OH → CO2 + 𝐻2 𝑂

(10)

Jo

CTC* +· O2 − → CO2 + 𝐻2 𝑂

31

ro of

-p

Fig. 8 The photodegradation mechanism of CTC catalyzed by GO/PAA-CdS with band structure of the photocatalyst.

re

3.4 Identification of photodegradation intermediates and possible pathways According to Fig. S10, the characteristic peaks of CTC in the kinetic UV-vis spectra

lP

weakened as the photodegradation proceeded and the TOC results indicated that almost

na

56% of CTC was mineralized in 2.5 h. The photodegradation intermediates of CTC in the reaction system after 30 min and 90 min light irradiation were identified through

ur

LC-MS spectra as shown in Fig. S11. A highly intensive peak at m/z=479 is corresponding to the original CTC molecules at time 0. As the photodegradation

Jo

continued, the peak intensity of CTC decreased until nearly unable to be identified.

32

ro of -p re

Fig. 9 The optimized geometry (a) and isosurface (b) of f0 for CTC molecule.

lP

In order to better deduce the possible photodegradation pathway of CTC by GO/PAA-CdS under visible light irradiation, the predication of active sites

na

vulnerable for radical attack on CTC was conducted. The condensed Fukui

ur

function representing the possibility of radical attack (f0) of the atoms on CTC molecule were obtained based on Hirshfeld charge distribution via computation

Jo

calculation [54, 55]. The geometry optimization and corresponding calculations were based on Density Functional Theory (DFT) at B3LYP/6-31G* level. The iso-surface of f0 is shown in Fig. 9B, providing a visualized result of Fukui function after the optimization-calculation process. It should be noted that the more intense the electron density difference is, the more possible the atoms are 33

for the attack of radicals. The values of condensed Fukui function in Table S3 could provide a quantitative version that a higher f0 value always leads to a higher possibility. It’s obvious that the top two f0 values go to Cl25 (f0=0.07459) and O24 (f0=0.06821) atoms on the list, suggesting the photodegradation process of CTC via radials attack would more likely be initiated at these two sites. Combined with the intermediates detected from LC-MS, the degradation

ro of

products have been arranged in different degradation pathways predicting major changes after radical attack. Thus, the following pathways are proposed.

As shown in Fig. 10, the dechlorination reaction occurred at Cl25 due to the

-p

attack of radicals resulting in the formation of intermediate with m/z=445 and

re

then substituted by -OH to generate 461a which is a common degradation pathway referring to the previous literatures [56]. When the radicals attacked

lP

O24, the H was abstracted from the phenolic hydroxyl generating a tentatively

na

identified product m/z=478 [57]. Then it underwent the hydroxylation reaction on C3 and C2 leading to the formation of m/z=495. Next, the bond between C7

ur

and C10 atoms was broken as consequence of intramolecular cyclization along with the ketonization on the adjacent hydroxyls by ·O2- and ·OH, producing the

Jo

product m/z=473 [58]. Meanwhile, the intermediate m/z=495 could also be converted to m/z=475, 467 and 461c through the ketonization-dehydroxylation, demethylation and dechlorination process, respectively [59]. The third pathway was proposed to be initiated at the rest active sites on CTC, especially the oxygen atoms in hydroxyl (or carbonyl) groups and the C atoms attached or adjacent to 34

them which agrees well with the iso-surface of f0. The intermediates with m/z>479 indicated net mass gain in transformation products were mainly due to the oxidation via radical attack. The multiple hydroxylation on C4 and C7 followed by the dechlorination and ketonization could result in the formation of m/z=497, 515, 481 and 451 intermediates during the photodegradation process. In addition, according to the previous studies, the loss of H2O at C12 with high

ro of

f0 on the CTC skeleton may happen, generating the m/z=461b and then being

degraded to m/z=433 intermediates with the loss of two methylene groups [4]. Subsequently, the tentative intermediates generated through the four main

-p

pathways promoted above went through the ring cleavage, dehydroxylation,

re

dichlorination, demethylation, and deamination processes. The smaller molecules (m/z=353, 319, 261, 231, and 165) with tricyclic or bicyclic aromatic

lP

structure were identified which have been reported previously [60]. Finally, the

Jo

ur

water.

na

above compounds were mineralized into inorganic salts, carbon dioxide and

35

ro of -p re lP na ur Jo Fig. 10 The possible degradation pathways of CTC by GO/PAA-CdS.

36

Fig. 11 The repeated photodegradation plots of CTC by GO/PAA-CdS ; the

ro of

photodegradation rate constant k with the corresponding removal efficiency. 3.5 Stability of the photocatalyst

The reusability and stability of the catalyst GO/PAA-CdS were explored. As

-p

is shown in Fig. 11A, the degradation kinetics follows the same trend in

re

successive ten photocatalytic cycles. The obtained k value and removal efficiency for each run are presented in Fig. 11B. The results indicate that there remains

lP

slight decrease on the removal rate, however, the total removal efficiency could

na

still reach almost 80%, suggesting the good reusability of the catalyst. The slight decrease may be due to the occupation of active sites by the photodegradation

ur

products. The amounts of Cd containing in the reaction solution after the photodegradation process by GO/PAA-CdS and pure CdS are shown in Fig.

Jo

S12(A). It’s obvious that negligible amount of Cd could be detected in the reaction system catalysed by GO/PAA-CdS and nearly no increasing amount is observed during the cycled experiments. However, the pure CdS particles without the coverage of GO/PAA polymeric matrix exhibits higher leakage of Cd as compared with that of GO/PAA-CdS and shows an increasing trend during 37

the cycled experiments. The XRD spectra of GO/PAA-CdS (Fig. S12B) before and after the photodegradation process were recorded and no obvious difference could be observed in the peak position or peak intensity, indicating nearly no change in the catalysts. Besides, the UV-vis spectra comparison in Fig. S12C showed little decrease in light absorption which may be due to the attached degradation intermediates and the obtained Eg value was nearly the same. And

ro of

in the kinetic UV-vis spectra of CTC solution catalyzed by GO/PAA-CdS (Fig.

S10), there’s nearly no newly formed characteristic peak for the hydrated Cd 2+ which also proved that little CdS was photo-corroded. Thus, we could draw a

-p

conclusion from the above results that the GO/PAA-CdS catalysts possess

re

excellent cycling stability which contributes to its application in the practical conditions.

lP

Conclusions

na

The GO/PAA-CdS photocatalyst was prepared via a facile successive polymerization and in-situ precipitation methods and showing high efficiency in the photodegradation

ur

of CTC in wastewater. The photocatalytic performance of GO/PAA-CdS shows a positive relationship with GO concentration for the enhanced transfer of

Jo

photogenerated electrons. The structure-activity relationship was confirmed through the optimization of preparing conditions (cGO=2.0 g/L, nCd:nS=1:2, mMBA:mAA=0.4 wt%), and the photocatalytic performance could be significantly improved due to the enhancing anti-photocorrosion, uniform distribution and fine crystal structure of CdS nanoparticles. The solution pH shows great effects on the photodegradation process of 38

CTC ascribed to the varying forms of CTC molecules and the charge distribution of catalyst, and the maximum photodegradation rate constant is obtained at pH=6. The photodegradation mechanism can be confirmed to be hydroxyl radicals dominated. The active sites preferable for the radical attack are proved to be the oxygen containing groups and the C atoms attached to them with higher condensed Fukui function values obtained through DFT calculations. The possible photodegradation pathway of CTC

ro of

mainly includes the hydroxylation, dehydrogenation, dichlorination, and elimination of

water. The as-prepared photocatalyst also exhibits excellent recyclability and photostability during the cycled experiments. This study provides a promising approach for

re

-p

efficient removal of organic contaminants from wastewater.

Author Contribution Statement

Jo

ur

na

lP

Wenjia Kong: Data curation, Writing- Original draft preparation; Yue Gao: Formal analysis; Qinyan Yue: Supervision, Funding acquisition; Qian Li: Conceptualization, Funding acquisition; Baoyu Gao: Project administration, Funding acquisition; Yan Kong: Data curation; Xindong Wang: Writing- Reviewing and Editing; Ping Zhang: Software; Yu Wang: Resources.

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

Acknowledgements

39

The authors are grateful for financial support from Taishan Scholar Foundation of Shandong Province (No.ts201511003), National Natural Science Foundation of China (51978384 and 21677088), Young Scholars Program of Shandong University (2015WLJH34), and Major Technological Innovation Engineering Project of Shandong

Jo

ur

na

lP

re

-p

ro of

Province (No. 2018CXGC1010).

40

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