Nanocomposites of Ag3PO4 and Phosphorus-Doped Graphitic

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Nanocomposites of Ag3PO4 and PhosphorousDoped Graphitic Carbon Nitride for Ketamine Removal Changsheng Guo, Miao Chen, Linlin Wu, Yingying Pei, Chunhua Hu, Yuan Zhang, and Jian Xu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00295 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Nanocomposites of Ag3PO4 and Phosphorous-Doped Graphitic Carbon Nitride for

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Ketamine Removal

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Changsheng Guo, †, ※ Miao Chen, †, ‡, ※ Linlin Wu, † Yingying Pei, † Chunhua Hu, ‡ Yuan

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Zhang, † Jian Xu †, *

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Research Academy of Environmental Sciences, Beijing, 100012, China

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8

Education, School of Resources Environmental and Chemical Engineering, Nanchang

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University, Nanchang, 330031, China

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese

Key Laboratory of Poyang Lake Environment and Resource Utilization, Ministry of

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ABSTRACT

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As one of the most abused illicit drugs, ketamine (KET) has been widely detected in

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different water environment around the globe, which necessitates the development of

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effective approaches for KET removal from water. In the present study, several novel

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Ag3PO4/P-g-C3N4 heterojunction composites were successfully constructed using in-situ

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growth method, and the samples were characterized by a serious of instruments. The

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synthesized samples were deployed for KET degradation. Results showed that

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Ag3PO4/P-g-C3N4 (1:1) exhibited the most excellent photocatalytic degradation

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performance on KET with the pseudo-first-order rate constant of 0.0326 min-1 at neutral

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pH value, which was 3- and 6-fold faster than Ag3PO4 and P-g-C3N4, respectively. The

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elevated photocatalytic performance of Ag3PO4/P-g-C3N4 was attributed to the

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synergistic effects of high charge separation capacity and the Z-scheme heterojunction

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structure. Low concentrations of dissolved organic matter, nitrate or bicarbonate

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accelerated the KET degradation by Ag3PO4/P-g-C3N4, but high levels of these

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constitutes would inhibit the KET degradation. The scavenging experiments revealed that

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photogenerated superoxide radicals and holes were the main reactive species in the KET

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removal. Twelve degradation intermediates of KET over Ag3PO4/P-g-C3N4 were

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identified and the possible degradation pathway was proposed. Demethylation,

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dehydrogenation, hydroxylation deamination, ring open and Na-modification were the

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major pathways for KET degradation. The Ag3PO4/P-g-C3N4 also exhibited relatively 2

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good photocatalytic performance on KET degradation in surface water and secondary

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effluent.

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KEYWORDS: Ag3PO4/P-g-C3N4; ketamine; degradation mechanism; intermediate;

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pathway

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1. INTRODUCTION

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Abuse of illicit drugs and their incomplete elimination in the sewage treatment

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plants (STPs) have led to their frequent detection in different types of aquatic

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environments.

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“club drug” worldwide for the purpose of entertainment, in addition, KET is also

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prescribed as anesthetic drugs used in humans and animals, and as antidepressant to

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relieve the symptom of depression. 4, 5 It was frequently detected in effluents and aquatic

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environments such as surface waters,

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groundwater.

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was up to 341 ng·L-1 and 206 ng·L-1 respectively in Taiwan.

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KET concentration ranged from 1.5 ~ 16.3 ng·L-1 within seasonal variations. 10 The KET

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concentration in STPs influents in England was up to 447.3 ng·L-1.

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conventional water treatment processes including biodegradation, hydrolysis, photolysis

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and sorption could not effectively eliminate KET from water,

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techniques to degrade KET is urgent and essential.

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1-3

7, 8

As a typical illicit drug, ketamine (KET) has been widely used as the

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hospital wastewaters

1

and even drinking and

For instance, the concentration of KET in rivers and hospital effluents 6, 9

In Beijing urban rivers

1, 6, 11, 12

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Because the

developing new

Advanced oxidation processes (AOPs) are commonly employed to degrade illicit 3

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drugs in water.

As a promising AOP technique, photocatalytic oxidation has been

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extensively used to eliminate persistent pollutants in water.

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(g-C3N4) is a catalytic material that was able to remove tetracycline,

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bisphenol A, 21 and other organic pollutants. 22-24 However, the degradation efficiency of

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organic contaminants over g-C3N4 was limited by the property of its low sunlight

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utilization capability and high recombination rate of photogenerated holes (h+) and

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electrons (e-). 25, 26 Phosphorus-doped g-C3N4 (P-g-C3N4) could broaden the visible light

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absorption region with the band energy of 2.55 eV. 27 In addition, to make full use of the

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whole solar spectrum and elevate the charge transfer efficiency, heterojunctions are

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generally prepared to modulate the light absorption property.

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electron mediator a heterojunction could separate the photoinduced holes and electrons

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into two photo-systems, which could help isolate the reduction and oxidation reaction

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sites and enhance the photocatalytic performance.

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catalyst, which has drawn much concern because it has high oxidative capability and O2

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evolution under solar light illumination.

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was not stable in solutions, which was photochemically decomposed or self-corroded

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under conditions without sacrificial chemicals. 33, 36 Combination of Ag3PO4 and g-C3N4

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therefore would be an option to overcome the above shortcomings.

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hybrid composite could enhance the stability of the composite in the degradation of

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methylene blue. 38 Our previous studies also proved that heterojunctions may prevent the

33-35

30-32

16-18

Graphitic carbon nitride

28, 29

19

phenols,

20

For instance, with an

Ag3PO4 is a visible-light driven

However, in practical applications Ag3PO4

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Ag3PO4@g-C3N4

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light corrosion and show superior photocatalytic activities because of their effective

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charge separation capacity and enhanced specific surface areas.

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knowledge, the heterojunction composite of Ag3PO4/P-g-C3N4 has not been synthesized

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previously.

39, 40

To the best of our

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In this work, several Ag3PO4/P-g-C3N4 hybrid materials with different mass ratios

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were prepared via thermal polymerization coupled with in situ precipitation method. The

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optimal composite was applied to eliminate KET, and the impact of parameters including

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pH, bicarbonate (HCO3-), nitrate (NO3-) and dissolved organic matter (DOM) in the

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degradation process was investigated. The reaction mechanism, intermediates and

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possible pathways of KET during the photocatalytic degradation were proposed as well.

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As far as we know, it is the first time that the hybrid composite of Ag3PO4/P-g-C3N4 was

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deployed to eliminate illicit drugs under visible light irradiation.

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2. MATERIALS AND METHODS

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2.1 Chemicals and reagents

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Melamine, silver nitrate (AgNO3), urea, dibasic sodium phosphate (Na2HPO4),

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ammonium monohydric phosphate ((NH4)2HPO4), sodium hydroxide (NaOH), sodium

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bicarbonate

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ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), 1,4-benzoquinone (BQ),

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5,5-diemthyl-1-pyrroline N-oxide (DMPO) and sodium azide (NaN3) were of analytical

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grade and obtained from Sinopharm Chemical Reagent (Shanghai, China). Humic acid

(NaHCO3),

hydrochloric

acid

(HCl),

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ethanol,

isopropanol

(IPA),

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(HA), nitroblue tetrazolium (NBT) and terephthalic acid (TA) were of analytical grade

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and obtained from Sigma-Aldrich (St. Louis, MO, USA). HPLC grade reagents

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(methanol, acetonitrile and formic acid) were obtained from Fisher (Poole, UK).

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Ketamine was purchased from Cerilliant Corporation (Round Rock, TX, USA). A

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Milli-Q system (Millipore, MA, USA) was used to produce Milli-Q water. The reagents

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were used directly without further purification.

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2.2 Preparation of Ag3PO4/P-g-C3N4

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The method to synthesize P-g-C3N4 was similar to the previous publication.

41

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Ag3PO4/P-g-C3N4 composites were prepared by a facile in situ coprecipitation approach

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at ambient temperature. In a typical process, 50 mL Milli-Q water and 50 mL ethanol

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were mixed well in a beaker, then a certain quality of P-g-C3N4 were added, and the

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suspension was sonicated for half an hour. Different amounts of AgNO3 were dissolved

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in above suspension and magnetically stirred for 30 min in the dark. Then, 50 mL

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Na2HPO4 solution at different concentrations was dropwise added in the mixture which

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was vigorously stirred. With magnetically stirring for one more hour, the precipitates in

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the solution were collected by centrifugation, rinsed by Milli-Q water and ethanol

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respectively for 3 times, and dried under vacuum at 60oC for 24 h. The collected yellow

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powder was Ag3PO4/P-g-C3N4 hybrid composite. The materials were denoted as A/CN

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(X), where X (1:10, 1:5, 1:2, 1:1, 2:1, 5:1, 10:1) represented the mass ratios of Ag3PO4 to

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P-g-C3N4 in the Ag3PO4/P-g-C3N4 composites. Ag3PO4 catalyst was also synthesized 6

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according to the above steps without adding P-g-C3N4. The preparation of

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Ag3PO4/P-g-C3N4 composite was shown in Scheme 1. The dosage of reagents used was

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shown in Supporting Information Table S1.

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Scheme 1. Illustration of the preparation of Ag3PO4/P-g-C3N4 composite. 2.3 Characterization

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An X-ray diffractometry (XRD, Rigaku D/Max-2500) using a radiation of Cu Kα

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(λ= 0.15406 nm) was used to determine the samples’ crystality. The morphology and the

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particle size of the samples was analyzed by a transmission electron microscopy (TEM,

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JEM-100CXII), high resolution transmission electron microscopy (HRTEM, JEM-2100F)

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and scanning electron microscopy (SEM, Hitachi, s-4800) coupled with the

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energy-dispersive X-ray spectroscopy (EDX, Oxford Aztec X-MaxN 80). The Fourier

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transform infrared (FT-IR) spectra was recorded with a spectrometer (Nicolet

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5SX-FTIR). UV-visible diffuse reflectance spectra (UV-vis DRS) were analyzed by a

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UV-vis spectrophotometer (Hitachi, U-3010) with BaSO4 as the reference. The chemical 7

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states and surface compositions of photocatalysts were identified by an X-ray

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photoelectron spectroscopy (XPS, PHI Quantera SXM). The N2 adsorption-desorption

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isotherms were obtained by an automatic analyzer (BET, Tristar Ⅱ 3020M).

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Photoluminescence (PL) spectrum was investigated by a fluorescence spectrophotometer

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(Hitachi, F-4500) with the excitation wavelength at 360 nm. The photocurrent tests were

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conducted by the electrochemical workstation (Chenhua, CHI 660E, China) with a 300 W

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Xenon lamp (Institute of Electric Light Source, Beijing).

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2.4 Photocatalytic degradation experiments

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The photocatalytic degradation was conducted in an XPA-7 photochemical reactor

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(Xujiang Machinery Factory, Nanjing, China) at room temperature. In a typical

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degradation procedure, 0.05 g photocatalyst and 50 mL KET aqueous solution were

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added in a quartz tube under the visible-light irradiation by an 800 W Xenon lamp

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(Institute of Electric Light Source, Beijing) with a 420 nm cut-off filter. Before

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irradiation, the suspension in the quartz tube was stirred magnetically in darkness for half

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an hour to reach the adsorption/desorption equilibrium between catalyst and KET. An

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aliquot of 0.5 mL reaction solution was withdrawn at specific time intervals, filtered

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through a 0.22 μm membrane filter (JinTeng, Tianjin) and ready for analysis. The control

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experiments were conducted without photocatalysts. Detailed information on

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instrumental analysis was provided in Supporting Information.

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2.5 Analysis of hydroxyl radical (·OH) and superoxide radical (·O2-) 8

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TA and NBT were used as probe molecules to measure the generation of ·OH

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and ·O2-over Ag3PO4/P-g-C3N4 composite in aqueous solution, respectively. ·OH could

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react with TA to produce 2-hydroxyterephthalic acid, which was a highly fluorescent

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product. 42 The fluorescence intensity of 2-hydroxyterephthalic acid is proportional to the

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number of ·OH generated in the system. 43 NBT had absorption peak at the wavelength of

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259 nm. ·O2- could react with NBT to generate insoluble purple formazan, which couldn’t

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show the absorption peak at 259 nm.

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photocatalytic experiment, except that KET solutions were substituted with 50 mL 5×10-4

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M terephthalic acid solution (which was dissolved in 2×10-3 M NaOH solution to

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guarantee its solubility)

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withdrawn samples at given time were detected by fluorescence spectrophotometer

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excited at 315 nm and UV-vis spectrophotometer (Shimadzu, UV1800, Japan),

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respectively. Electron spin resonance (ESR) technique was conducted to further verify

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the presence of ·OH and ·O2-, with detailed information provided in Text S3.

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3. RESULTS AND DISCUSSION

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3.1 Characterization of the samples

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3.1.1 XRD analysis

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44

The experimental procedures were similar to the

and 50 mL 1.25×10-6 M NBT solution,

44

respectively. The

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Figure 1a illustrates the XRD spectra of the P-g-C3N4, Ag3PO4 and

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Ag3PO4/P-g-C3N4 photocatalysts. For P-g-C3N4, the diffraction peaks at 13.1o and 27.5o

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were corresponded to the (100) and (002) diffraction planes of g-C3N4, respectively 9

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(JCPDS87-1526).

For Ag3PO4, the crystal structure was consistent with the

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body-centered cubic phase (JCPDS06-0505).

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showed characteristic peaks in accordance with g-C3N4 and Ag3PO4 peaks. The

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characteristic peaks at 21.1o, 29.9 o, 33.3 o, 36.8o, 42.7o, 48.0o, 52.9o, 55.2o, 57.3o, 61.8o,

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66.1o, 70.1o, 72.1o and 73.9o were indexed to the (110), (200), (210), (211), (220), (310),

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(222), (320), (321), (400), (330), (420), (421) and (332) diffraction planes, respectively.

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38

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increasing amount of Ag3PO4 particles. The absence of diffraction peaks of P-g-C3N4 or

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Ag3PO4 of the synthesized materials suggested the low percentage of P-g-C3N4 or

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Ag3PO4 in the Ag3PO4/P-g-C3N4 nanocomposites.

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The Ag3PO4/P-g-C3N4 photocatalyst

The P-g-C3N4 peak intensities decreased while the Ag3PO4 peak increased with the

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Figure 1. XRD patterns (a), FT-IR spectra (b) and UV-vis DRS spectra (c) of the

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as-prepared samples; Plots of (ahv)1/2 versus hv for the band gap energy of samples (d).

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3.1.2 SEM and EDX analysis

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SEM images of P-g-C3N4, Ag3PO4, and Ag3PO4/P-g-C3N4 composites are presented

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in Figure S1. P-g-C3N4 exhibited the structure of porous mesoporous, and Ag3PO4

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materials were orbicular with the diameter size of 100 ~ 200 nm. As shown in Figure

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S1b-e, Ag3PO4 particles also exhibited the spherical and smooth morphology, suggesting

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its successful dispersion on P-g-C3N4 materials surface by ion exchange. The elemental

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mapping scanning of A/CN (1:1) with different EDX elemental distribution maps were 11

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shown in Figure S2. Ag and P elements were dispersed on the g-C3N4 surface, in

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addition, EDX elemental maps illustrated the different elements distribution in the

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composite of Ag3PO4/P-g-C3N4. As shown in Figure S2b-e, the elements of C, O, P, N,

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and Ag were uniformly dispersed on the surface of the obtained photocatalysts, implying

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that P-g-C3N4 and Ag3PO4 were tightly combined.

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3.1.3 TEM analysis

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Figure 2. TEM images of the synthesized composites. (a) A/CN (1:5); (b) A/CN (1:2); (c)

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A/CN (1:1); (d) A/CN (2:1); (e) A/CN (10:1) and (f) Ag3PO4.

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TEM images are shown in Figure 2. The pure Ag3PO4 showed an irregular

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spherical structure with a diameter about 50 ~ 200 nm (Figure 2f). For all 12

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Ag3PO4/P-g-C3N4 composite, P-g-C3N4 (color in dark grey) exhibited a thin and lamellar

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structure without steadfast outline. Ag3PO4 particles (color in black) with a similar size

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were uniformly deposited on the P-g-C3N4 surface. The results indicated that the

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P-g-C3N4 catalysts could be regarded as the supporting structure to bound Ag3PO4

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particles in the Ag3PO4/P-g-C3N4 composites. Ag nanoparticles with diameter of 2 ~ 10

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nm could be observed on the Ag3PO4/P-g-C3N4 surface, which could serve as the center

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to capture e- from conductive band (CB) of Ag3PO4 and h+ from valence band (VB) of

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P-g-C3N4. HRTEM image of A/CN (1:1) was shown in Figure S3, which verified the

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presence of metallic Ag and formation of hybrid heterojunction on Ag3PO4/P-g-C3N4

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composite. Ag3PO4 and P-g-C3N4 had large direct-contact areas, and the intimate contact

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between them resulted in the formation of heterojunction structure. This structure could

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promote the stability of the composites and favor the charge transfer in the heterojunction

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system, which could elevate the photocatalytic performance of Ag3PO4/P-g-C3N4 by

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facilitating the separation efficiency of photo-induced e--h+ pairs.

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metallic Ag in Ag3PO4/P-g-C3N4 composite could be further proved by XPS results.

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3.1.4 FT-IR analysis

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The presence of

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FT-IR spectra of synthesized photocatalysts are shown in Figure 1b. The peak at 559

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cm-1 in the Ag3PO4 spectrum was attributed to the O=P-O stretching vibration, and the

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1005 cm-1 peak was corresponding to the P-O-P bending vibration in PO43-. 48 Except for

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Ag3PO4, the peaks of other samples at 1633 cm-1 ~ 1223 cm-1 were ascribed to the typical 13

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stretching vibration of C=N and C-N, and the absorption peak at 817 cm-1 corresponded

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to the triazine units of g-C3N4. 50 For the composites containing P-g-C3N4, the wide peaks

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ranging from 3500 ~ 3000 cm-1 could be assigned to the stretching vibrations of NH2 or

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NH groups.

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peaks at 817 cm-1, 1406 cm-1 and 1649 cm-1 decreaed with the increasing contents of

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Ag3PO4 in a series of Ag3PO4/P-g-C3N4 composites. All characteristic peaks of g-C3N4

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and Ag3PO4 were found in the A/CN (2:1, 5:1, 10:1) composites, indicating the

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successful synthesis of Ag3PO4/P-g-C3N4 composite by photo-deposition and ion

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exchange measures.

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3.1.5 UV-vis DRS analysis

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The intensity of the peaks at 559 cm-1 and 1005 cm-1 increased while the

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UV-Vis DRS spectra illustrating the optical properties of the samples are presented

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in Figure 1c. The absorption edge of P-g-C3N4, Ag3PO4 and A/CN (1:1) were

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approximately 481 nm, 550 nm and 561 nm, respectively, indicating their excellent light

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absorption capacity. The absorption intensity of Ag3PO4/P-g-C3N4 composites increased

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with the increasing Ag3PO4 ratio, which could enhance the composites photocatalytic

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activity under visible light irradiation. In comparison to P-g-C3N4, the visible light

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absorption region had been observably enhanced by the interaction between Ag3PO4 and

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P-g-C3N4 in the composites, indicating Ag3PO4/P-g-C3N4 hybrids were responsive to

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visible light with high photocatalytic activities.

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The band gap energy of the photocatalysts is evaluated with the formula (1): 51 14

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αhv = A(hv-Eg)n/2

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(1)

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where α is the absorption coefficient, h is Planck constant, v is the light frequency, A is a

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constant and Eg is band gap energy. n is decided by the optical transition types of

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semiconductors. n=1 and n=4 are expression of the direct and indirect transition

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semiconductor, respectively. According to the plots of (αhv)1/2 versus hv in Figure 1d, the

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band gap of Ag3PO4, P-g-C3N4 and A/CN (1:1) were calculated to be 2.14 eV, 2.49 eV

243

and 2.16 eV, respectively. The doping of phosphorus on the g-C3N4 caused the different

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band gap between the prepared P-g-C3N4 (2.49 eV) and typical g-C3N4 (2.70 eV). 52

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3.1.6 XPS analysis

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Figure 3. XPS survey spectra of P-g-C3N4 and Ag3PO4/P-g-C3N4 (a), high resolution C 1s

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spectrum (b), N1s spectrum (c), O 1s spectrum (d), Ag 3d spectrum (e) and P 2p

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spectrum (f) of Ag3PO4/P-g-C3N4 composite. 16

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The XPS result was shown in Figure 3. The elements of C, O, P, N, and Ag were

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observed in the XPS survey spectrum of the A/CN (1:1) composite (Figure 3a). The high

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resolution spectra of C 1s, N 1s, O 1s, Ag 3d and P 2p have been calibrated by the

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standard carbon peak (284.8 eV). As illustrated in Figure 3b, the C 1s peak locating at

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284.8 eV could be assigned to the C-C bonds or C-N bonds of the graphitic carbon in

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P-g-C3N4.

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present in the sample. 54 The N 1s spectrum (Figure 3c) can be deconvoluted to 3 peaks at

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398.5, 399.6 and 400.9 eV. The first peak could be attributed to the tertiary nitrogen

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groups (N-(C)3), 55 the second one could be attributed to the aromatic N atoms bonded to

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the carbon C=N-C, 56 and the third one could be ascribed to amino group (N-H). 57 Figure

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3d showed that the O 1s spectrum was divided into 2 peaks at 530.5 eV and 532.4 eV,

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which were assigned to the oxygen in crystal lattice and oxygen absorbed in the

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composite, respectively.

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high-resolution Ag 3d spectrum (Figure 3e) were assigned to the Ag 3d5/2 and Ag 3d3/2

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orbitals, respectively. 59 The peak at 367.8 eV could be further divided into 367.8 eV and

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368.5 eV peaks, meanwhile, the 373.8 eV peak could be further deconvoluted into 373.8

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eV and 374.8 eV peaks, respectively. The peaks at 367.8 eV and 373.8 eV could be

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ascribed to the Ag+ in the Ag3PO4, 60 and peaks of 368.5 eV and 374.8 eV were assigned

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to metallic Ag,

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that the metallic Ag existed in the Ag3PO4/P-g-C3N4, which was formed during its

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The peak at 288.0 eV was attached to the sp2-hybridizied carbon (N-C=N)

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The two strong peaks at 367.8 eV and 373.8 eV by the

which was coincidence with previous studies.

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62, 63

The result proved

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synthesis process. The banding energy centered at 133.0 eV (Figure 3f) could be

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attributed to the typical P-N coordination or the PO43- in the Ag3PO4/P-g-C3N4 composite.

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3.1.7 N2 adsorption-desorption analysis

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The specific surface area and corresponding pore size distribution curves of

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P-g-C3N4 and A/CN (1:1) catalyst are shown in Figure S4. The synthesized samples can

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be recognized as mesoporous materials due to its type IV isotherms. The

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Brunauer-Emmett-Teller (BET) surface area of P-g-C3N4, A/CN (1:1), and Ag3PO4 were

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31.23, 22.53 and 0.1404 m2·g-1, respectively. The specific surface area of Ag3PO4 was

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smaller than that of P-g-C3N4, and the combination of Ag3PO4 and P-g-C3N4 led to the

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decrease of specific surface area of A/CN (1:1) composite. The larger specific surface

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area could result in the enhanced photocatalytic activity because more active sites could

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be provided.

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3.1.8 PL and photocurrent test

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PL emission spectra were deployed to evaluate the separation rate of photoinduced

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charge carriers in the synthesized materials and the results were presented in Figure S5a.

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All samples except A/CN (1:1) showed the emission peaks at around 469 nm, indicating

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that the combination of Ag3PO4 with P-g-C3N4 could improve the separation rate of the

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e--h+ pairs. As shown in Figure S5b, the composite of A/CN (1:1) possess a much higher

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photocurrent density and better stability than that of Ag3PO4 and P-g-C3N4, indicating the

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higher separation rate of photoinduced carriers. The enhanced photocurrent density can 18

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be ascribed to heterojunction structure and Ag nanoparticles formed on the A/CN (1:1)

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composite, which strengthened its photocatalytic activity remarkably.

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photocurrent tests manifested the high separation rate of e--h+ pairs, contributing to the

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highest photocatalytic degradation efficiency of ketamine by A/CN (1:1) composite.

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3.2 Photocatalytic degradation of KET

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3.2.1 Degradation of KET over different synthesized samples

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The PL and

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Figure 4. Degradation curves (a) and kinetic curves (b) of KET over different synthesized

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samples under visible light illumination.

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The photolytic degradation of KET was negligible without the catalyst (Figure 4a).

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With the catalysts, the loss of KET in the first 30 min in the dark was less than 10%,

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indicated that the adsorption of KET by synthesized samples can be ignored. The KET

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photocatalytic degradation over different samples fitted well with the pseudo-first-order

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kinetic equation:

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-ln(Ct/C0) = kt

(2)

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where k is the reaction rate constant, t is the reaction time, C0 is the initial concentration

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and Ct is the concentration at time t. The pseudo-first-order curves of KET degradation

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are illustrated in Figure 4b. The different mass ratios between P-g-C3N4 and Ag3PO4 in

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Ag3PO4/P-g-C3N4 composites could remarkably impact the degradation rate of KET.

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P-g-C3N4 showed the lowest photocatalytic performance on KET with a rate constant of

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0.0053 min-1, and Ag3PO4 had a rate constant of 0.0116 min-1. With the mass content of

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Ag3PO4 increasing, the photocatalytic performance of Ag3PO4/P-g-C3N4 increased,

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however, further increasing Ag3PO4 content (A/CN (2:1), A/CN (5:1) and A/CN (10:1))

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reduced the degradation rate. The A/CN (1:1) composite exhibited the most excellent

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performance on KET degradation, with a removal efficiency of 99.95% after 90 min

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irradiation, while the removal efficiencies of KET over P-g-C3N4 and Ag3PO4 under the

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same condition were 41.64% and 64.50%, respectively. The degradation rate constant of

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KET over A/CN (1:1) composite was 0.0326 min-1, 6.16- and 2.82-fold faster than

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P-g-C3N4 and Ag3PO4, respectively. According to TEM result, when optimal amount of

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Ag3PO4 was successfully combined with P-g-C3N4, heterojunction structure could be

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formed, which could enhance the separation rate of photo-carriers and promote the

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photocatalytic performance of Ag3PO4/P-g-C3N4 composites.

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Ag3PO4 dispersed on the P-g-C3N4 surface could result in a lower interfacial charge

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transfer between them, which brings a relative low separation efficiency of photo-carriers

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on the composites and leads to a lower degradation rate of KET. 20

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However, excessive

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In the following

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experiment the A/CN (1:1) composite was used for KET degradation.

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Figure 5. The photocatalytic degradation of KET by Ag3PO4/P-g-C3N4 composite at

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different pH values (a); and at different concentration of HCO3- (b); DOM (c) and NO3-

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(d).

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3.2.2 Effect of pH

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The influence of solution pH on the KET photocatalytic degradation was shown in

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Figure 5a. KET degradation rate constants at pH 3, 5, 7, 9 and 11 were 0.0016 min-1,

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0.0267 min-1, 0.0326 min-1, 0.0299 min-1, and 0.0159 min-1, respectively (Figure S6a).

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Best removal efficiency of KET (88.89%) was achieved over Ag3PO4/P-g-C3N4 21

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composite at pH 7 after 60 min irradiation. Under strongly acidic (pH=3) or alkaline

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(pH=11) conditions, the KET degradation was significantly suppressed due to the

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inhibited formation of reactive species (RS).

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were more hydroxyl ions that could form ·OH, which could accelerate the degradation

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efficiency compared with under acidic condition.

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condition, the ·OH would be eliminated, resulting in the low elimination efficiency. 68

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3.2.3 Role of bicarbonate (HCO3-)

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It has been reported that at pH 9 there

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However, under strong alkaline

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Figure 5b shows the KET degradation in the presence of HCO3- in the

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Ag3PO4/P-g-C3N4 system. The degradation of KET was enhanced with low concentration

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of HCO3- (