CdS QD heterostructure for photodegradation and disinfection of pollutants in waste water

Journal Pre-proof Effectiveness of reactive oxygen species generated from rGO/CdS QD heterostructure for photodegradation and disinfection of pollutants in waste water Gul Afreen, Mohd Shoeb, Sreedevi Upadhyayula PII:

S0928-4931(19)33099-1

DOI:

https://doi.org/10.1016/j.msec.2019.110372

Reference:

MSC 110372

To appear in:

Materials Science & Engineering C

Received Date: 22 August 2019 Revised Date:

3 October 2019

Accepted Date: 27 October 2019

Please cite this article as: G. Afreen, M. Shoeb, S. Upadhyayula, Effectiveness of reactive oxygen species generated from rGO/CdS QD heterostructure for photodegradation and disinfection of pollutants in waste water, Materials Science & Engineering C (2019), doi: https://doi.org/10.1016/ j.msec.2019.110372. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Protein leakage

Membrane permeability

ee-

ee-

.

CdS QD

ROS

DNA damage

Bacteria Cell e e

RB39

MB

Diffusion

Interruption of electron transport

RhB

MO

Surface bound particle

1

Effectiveness of reactive oxygen species generated from rGO/CdS QD

2

heterostructure for photodegradation and disinfection of pollutants in

3

waste water

4

Gul Afreen1, Mohd Shoeb2, Sreedevi Upadhyayula1,*

5

1

6

India

7

2

8

Aligarh 202002, India

9

*Corresponding author. Tel. No: +9111-26591083, E-mail id: [email protected]

Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110016,

Department of Applied Chemistry, Z. H. College of Engg. & Tech., Aligarh Muslim University,

10

Declarations of interest: None

11

Keywords: Heterostructure; Reactive oxygen species; Photodegradation; Disinfection;

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Organic pollutants; Bacteria.

13

ABSTRACT

14

The present study investigates the role of reactive oxygen species (ROS) generated on surface

15

of nanophotocatalyst in wastewater treatment discharged from exponentially growing

16

industries. A facile synthetic route is presented to produce reduced graphene oxide/CdS

17

quantum dot (rGO/CdS QD) heterostructure by monowave-assisted solvothermal method

18

where room temperature ionic liquid 1-ethyl-3-methylimidazolium thiocyanate serves as a

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“green” precursor. The prepared photocatalyst was tested for: (1) photodegradation

20

performance against various cationic dyes, anionic dyes, and antibiotics as model organic

21

water pollutants; and (2) disinfection performance against gram-positive S. aureus and gram-

22

negative E. coli bacterial strains as pathogenic water pollutants. The negative surface charge

23

of rGO/CdS QD precisely attracted the cationic dye molecules to its surface and degraded the

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dyes at a higher rate. Moreover, excellent antibacterial activity of rGO/CdS QD were

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observed against S. aureus and E. coli with a minimum inhibitory concentration of 16 µg ml–1

26

and 32 µg ml–1, respectively. A plausible mechanism of the photocatalytic activity suggested

27

that ROS with strong oxidizing ability reacts with the organic pollutants to mineralize them

28

into CO2, H2O or some other small molecules, and reacts with pathogens to damage the

29

macromolecules like proteins, lipids, DNA, etc in the bacterial cells. Among all the surface

30

generated ROS, hydroxyl radicals was found to be the main contributor in the

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photodegradation and disinfection mechanism.

1

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1. Introduction

33

Increased industrial mutability has benefited humankind with various utilities in every

34

field.[1] However, parallelly it has generated colossal chemical and biological waste which

35

has polluted the environment especially natural water bodies, thereby limiting the access to

36

fresh drinking water and sanitation.[1,2] A tremendous increase of organic contaminants like

37

dyes and drugs and biological contaminants like noxious bacteria has polluted water-table

38

that affects life forms including human beings. Wastewater discharged from textile, leather,

39

and foodstuffs industries contain toxic and carcinogenic dyes and are one of the major

40

sources of water pollution. These organic dye compounds in the effluents reduce the

41

transmission of sunlight affecting photosynthesis adversely, and harming the aquatic

42

ecological community.[3] On the other hand, antibiotics, commonly used as therapeutic

43

drugs, are one of the largest groups of pharmaceutical compounds.[4] They are discharged

44

into the water bodies from pharmaceutical industries, excessive farming, and human and

45

animal excreta.[5] Consequently, humans, aquatic organisms, and flora become exposed to

46

them, and they pose a threat to the ecosystem and human health because of the ecotoxicity

47

and rapid development of antibiotic resistant genes. Tetracycline (TC) and Paracetamol (PC)

48

are two broad-spectrum antibiotics widely used worldwide to treat several microbial

49

infections both in humans and animals.[4,6] They are poorly metabolized, stable, and their

50

removal by conventional water treatment method is ineffective. Moreover, the waterborne

51

microbial infections or cross-infections caused by the consumption of fecally contaminated or

52

untreated surface water which contains pathogens such as bacteria, fungi, viruses, etc. pose

53

serious impact on human health and safety.[7] As a consequence, there is an urgent need for

54

the development of a sustainable process for the removal of water contaminants and hence,

55

for water reuse.

56

Photocatalysis, a “green” approach, has recently emerged as a promising solution to

57

such water pollution problems due to inexhaustible solar energy source, mild reaction

58

conditions, non-generation of secondary pollutants, and high efficiency.[8] The inorganic

59

semiconductors such as ZnO and TiO2 are activated by light to generate reactive species (h+,

60

e–, ·OH, ·O2–, H2O2) for degradation of organic compounds into non-toxic products and

61

pathogen inactivation.[9,10] However, these photocatalytic materials are active only in the

62

UV range due to large band gap (~3.2 eV for TiO2, ~3.37 eV for ZnO). Further, their short

63

term photostability is due to the high recombination rates of photoinduced electron−hole pair

64

resulting in low photo-quantum efficiency and poor photoactivity which discourages

65

industrial application.[3,11] Progresses have been made in the development of heterostructure 2

66

materials like Bi2S3@Ag3PO4/Ti, AgBr@MoS2 nanosheets, dual metal-organic framework,

67

Ag2S@WS2, Phosphorous-based Ti, Polydopamine/Ag3PO4/GO for photodisinfection

68

applications in the near infrared region.[12–18] Recently, CdS nanoparticles, categorized as

69

binary II-VI chalcogenide nanomaterials, have gained attention as photocatalysts due to

70

narrow band gap (~2.42 eV) in the visible light range and their more negative conduction

71

band potential as compared to TiO2.[19] However, the photocatalytic efficiency is still

72

limited by the high recombination rate of electron–hole pairs, aggregation to form clusters,

73

and photocorrosion. The activity and stability of CdS nanoparticles can be improved by

74

dispersing them on reduced graphene oxide (rGO). Depending on the extent of oxidation,

75

rGO has a redox potential in the range of -0.11 to -0.30 V below CdS conduction band of -

76

0.52 V which facilitates the thermodynamically controlled electron transfer from CdS to

77

rGO.[20] This factor along with the high electrical conductivity of rGO can reduce

78

electron−hole recombination rates by quick electron transfer from CdS. In addition,

79

combining CdS with rGO prevents agglomeration of CdS and thus improves photostability.

80

Earlier researchers have reported the typical synthesis of CdS/rGO nanocomposites in the

81

organic phase using expensive precursors at high temperature and/or pressure conditions.

82

Moreover, strong reductants like hydrazine hydrate, sodium borohydride, or other reductants

83

were used to convert solid graphene oxides into graphene sheets which may cause extra

84

damage.[21–24] In this work, monowave-assisted synthesis of heterostructure of CdS

85

quantum dots (QD) with reduced graphene oxide (rGO), denoted as rGO/CdS QD, was

86

performed by using ionic liquids (IL) as green alternative precursor to more typically used

87

hazardous organic solvents. Room temperature ILs have unique properties like negligible

88

vapour pressure, good ionic conductivity, wide liquid temperature range, high thermal

89

stability, better dissolving ability, and good monowave absorbing capability. Herein, 1-ethyl-

90

3-methylimidazolium thiocyanate ([EMIM].SCN) ionic liquid was used as sulfur precursor

91

and as stabilizing agent.

92

Chemical structure of the organic pollutants directly affects the photodegradation

93

performance of catalysts. In this work, two cationic dyes (methylene blue (MB) and

94

Rhodamine B (RhB)), two anionic dyes (Reactive black 39 (RB39) and Methyl orange

95

(MO)), and two antibiotic drugs (Tetracycline (TC) and Paracetamol (PC)) have been

96

selected as model organic pollutants (Fig. 1) to evaluate and compare the photodegradation

97

efficiency of the as-prepared rGO/CdS QD heterostructure with CdS QDs. Moreover, Gram

98

negative bacteria, Escherichia coli, and Gram positive bacteria, Staphylococcus aureus, have

99

been selected as pathogenic bacterial pollutants due to their abundance in wastewater and 3

100

high resistance to conventional antimicrobial agents.[25–28] To the best of our knowledge,

101

few studies have been reported on photocatalysis of dye mixtures using rGO/CdS

102

heterostructures, whereas, none of the studies have reported the effect of this photocatalyst on

103

the degradation of antibiotic waste and disinfection of pathogenic bacteria. Compared with

104

pristine CdS QD, the rGO/CdS QD heterostructure with enhanced visible-light absorption

105

remarkably improves the reactive oxygen species (ROS) generation of CdS QD for the

106

degradation of pollutants. The predominant ROS were analysed by radical trapping and

107

fluorescence experiments. The experimental results were further utilized to propose the

108

complete photocatalytic degradation and disinfection mechanism.

109 110

Fig 1. Chemical structures of model organic pollutants used in this study.

111

2. Experimental section

112

2.1 Chemicals

113

Graphite flakes, conc. H2SO4, conc. H3PO4, HCl (30%), KMnO4, H2O2 (30%),

114

Cd(NO3)2·9H2O, KSCN, ethanol, all the dyes and antibiotics were of analytical grade. All

115

materials were used without further purification.

116

2.2 Preparation of the rGO/CdS QD heterostructure

117

2.2.1

118

Graphene oxide (GO) was prepared from the oxidation of graphite flakes by following the

119

modified Hummer’s method.[29] Herein, a mixture of concentrated H2SO4 and H3PO4 in the

Preparation of Graphene oxide (GO)

4

120

ratio of 9:1 was added to graphite flakes (1 wt equiv) and then KMnO4 (6 wt equiv) was

121

slowly added, producing a slight isotherm to 35−40 °C. The reaction mixture was stirred at

122

50 °C for 12 h followed by cooling to room temperature. The mixture was then placed on an

123

ice bath and 30% H2O2 was poured slowly into it. The resulting mixture was sifted through

124

B.S.S. standard molecular sieves (300 µm), followed by filtration through polyester fibre,

125

centrifugation at 4000 rpm for 4 h, and successive washing with water, 30% HCl, and

126

ethanol. The material obtained was coagulated with ether and the suspension was filtered.

127

The solid obtained was dried overnight under vacuum at room temperature.

128

2.2.2

129

Cd(NO3)2·9H2O was coprecipitated with KSCN in ethanol, filtered, and dried to form

130

cadmium thiocyanate (Cd(SCN)2). For a typical synthesis of rGO/CdS QD heterostructure,

131

1:1 molar ratio of Cd(SCN)2 and [EMIM]·SCN was dissolved in ethylene glycol.

132

Simultaneously, the as-prepared GO was dispersed in ethylene glycol by ultrasonication for

133

10 min. These two solutions were mixed together and continuously stirred at room

134

temperature for 15 min. The mixture was then transferred to a 30 ml Anton-Paar monowave

135

300 reactor. The reactor was heated to 200 °C and the reaction occured under monowave

136

irradiation in solvothermal conditions for 30 min and then cooled down to room temperature

137

naturally. The product was collected after filtration, washing, and vacuum-drying at 70 °C for

138

12 h. CdS QD was prepared separately by following the same protocol, only without the

139

addition of GO.

140

2.3 Characterization

141

The structural properties and crystallite size of the as-prepared CdS QD and rGO/CdS QD

142

were analyzed by the X-ray diffractometer instrument (Rigaku MiniFlex™ II benchtop XRD)

143

using Cu Kα radiation (1.5429 Å) in the 2θ range of 5−80°. The crystallite size (d) of the

144

photocatalysts was calculated using the Debye-Scherrer formula as shown in equation (1):

Preparation of rGO/CdS QD

146

kλ (1) β cosθ where, k= 0.9 is the shape factor, λ is the X-ray wavelength of Cu Kα radiation (1.5429 Å), θ

147

is the Bragg diffraction angle, and β is the full width at half maximum height (FWHM) of the

148

(110) diffraction peak. Textural properties of the photcatalysts were analyzed by

149

physisorption of N2 at –196 °C in a Micromeritics Accelerated Surface Area and Porosity

150

analyser to determine the BET surface area and pore-size distribution. The optical properties

151

of the prepared photocatalysts were determined by measuring the absorbance (A) using

145

d=

5

152

double beam UV–Vis spectrophotometer (Perkin Elmer Lambda-35) in the wavelength range

153

of 200–800 nm. The electronic band gap energy (Eg) was calculated by using Tauc

154

relationship. The Fourier Transform Infrared spectra (FTIR) were recorded in the range of

155

500–4000 cm-1 in a Thermo Scientific Nicolet 6700 FTIR Spectrometer. The samples were

156

first mixed with spectroscopic grade KBr in the ratio of 1:100 and 13 mm diameter discs

157

were made for recording the spectra. The Raman spectra were recorded with Renishaw Micro

158

Raman Spectrometer using argon ion laser operating at 514 nm, step of scanning of 2 cm−1,

159

and integration time of 0.5 s. Morphology of the photocatalysts was visualized using

160

Scanning Electron Microscope (SEM, JEOL JSM-6510LV) at an accelerating voltage of

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20 kV and Transmission Electron Microscopy (TEM, JEOL JEM-1400) with an accelerating

162

voltage of ∼150 kV. The elemental analysis was performed using Energy Dispersive X-ray

163

(EDX, Oxford Instruments INCA x-sight spectrometer).

164

2.4 Assessment of photocatalytic degradation performance

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The photodegradation performance of rGO/CdS QD was measured against MB, RhB, RB39,

166

and MO dyes as well as TC and PC antibiotics as model organic pollutants under visible light

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irradiation. In the photocatalytic degradation experiments, 30 µg ml−1 of rGO/CdS QD was

168

mixed in 100 ml dye and antibiotic solutions (25 mg l−1 stock solution) each under stirring in

169

the dark for 1 h to ensure the establishment of an adsorption/desorption equilibrium before

170

irradiation. The photocatalytic activities were carried out in a tube-shaped glass reactor

171

equipped with a magnetic stirrer and the entire arrangement was placed inside an opaque case

172

to prevent the exposure to external light. Photo-irradiation was performed using a 300 W

173

xenon lamp equipped with cutoff filters to provide visible light (λ > 420 nm). At fixed time

174

intervals (upto 60 min for dyes and 120 min for antibiotics), 5 ml aliquots were withdrawn

175

and the catalysts were removed by centrifugation. The filtrate was analyzed at given

176

irradiation time by recording variations in the maximum absorption band using a UV–vis

177

spectrophotometer (Perkin Elmer Lambda-35) in 200–800 nm wavelength range. The

178

pollutant concentrations were calibrated using Beer–Lambert law at λmax values of 664.5, 550,

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596, 463, 250, and 357 nm for MB, RhB, RB39, MO, PC, and TC, respectively. The

180

photodegradation efficiency was calculated using the following equation (2):

181

Photodegradation % =

∗ 100

(2)

182

where, Co is the initial concentration of dye or antibiotic and C is the absorbance after

183

different time intervals. Trapping experiments were performed to find out the ROS produced

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during photodegradation through dissolving various scavengers such as disodium 6

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ethylenediaminetetraacetate dehydrate (EDTA-2Na, used to capture h+), benzoquinone (BQ,

186

used to capture O2˙−), and isopropyl alcohol (IPA, used to capture ˙OH). Hydroxyl radicals

187

were further estimated by fluorescence technique using terephthalic acid (TA) as a probe. In

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this process, the photocatalyst was dispersed in a solution of 5×10-4 M TA and 2×10-3 M

189

dilute aqueous NaOH. The resulting suspension was exposed to visible light irradiation and

190

maximum fluorescence emission intensity with an excitation wavelength of 315 nm were

191

recorded at regular intervals. The intensity of the fluorescence signal of 2-

192

hydroxyterephthalic acid (TAOH) at 425 nm was the basis of hydroxyl radical analysis.

193

2.5 Assessment of bacterial disinfection performance

194

2.5.1. Growth condition

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Clinical isolates of S. aureus (+) and E. coli (–) were obtained from Department of

196

Microbiology, JN Medical College, A.M.U., Aligarh. Stock cultures were maintained on

197

Luria-Bertani (LB) agar at 4 °C. The primary cultures of S. aureus and E. coli were prepared

198

from the stock into the LB broth and incubated at room temperature for 48 h (stationary

199

phase). The primary cultures were re-inoculated in fresh LB broth for ~12 h (mid-log phase,

200

106 cfu ml-1). The antimicrobial activity were performed on the cultures of mid-log phase.

201

2.5.2. Antibacterial test

202

The disinfection performance of rGO/CdS QD were evaluated qualitatively by the disc

203

diffusion assay. The mid-log phase cultures of S. aureus and E. coli were centrifuged at 4000

204

rpm for 5 min at 4 °C. The pellets were washed with 1× phosphate buffer saline (PBS) and

205

resuspended in 500 µl normal saline solution (NSS). Approximately 106 cfu ml-1 of the

206

suspended cells were spreaded uniformly on LB agar plates. Various concentrations of CdS

207

QD and rGO/CdS QD (25, 50, 75, 100 µg ml–1) were loaded onto the pre-sterilized filter

208

paper discs distributed evenly on the seeded agar plates. The petriplates were incubated at 37

209

°C for 16 h and the diameters of zone of inhibition were recorded.

210

The growth curves of S. aureus and E. coli after rGO/CdS QD exposure were plotted

211

in terms of change in optical density (O.D) as a function of time. NSS suspended cells of S.

212

aureus and E. coli (200 µl) were inoculated in 50 ml of fresh LB broths and were

213

supplemented with 25, 50, 75, and 100 µg ml–1 of rGO/CdS QD. The prepared mixture were

214

incubated at 37 °C on a rotary shaker at 200 rpm. Time dependent growth was monitored

215

after every 4 h for 24 h by measuring the O.D at A600nm using a UV-Vis spectrophotometer.

216

The experiments were performed in triplicate and the results were shown as mean ± standard

217

deviation. 7

218

2.5.3. MIC determination

219

The minimum inhibitory concentration (MIC) is the lowest dose of an antimicrobial agent

220

that inhibits the growth of microorganism after a specified interval of time. The MIC values

221

of CdS QD and rGO/CdS QD were calculated by 96-well plate microdilution method against

222

S. aureus and E. coli according to the previously developed protocols.[30]

223

2.5.4. Morphology investigation by SEM analysis

224

The morphology of the S. aureus and E. coli post-treatment with CdS QD and rGO/CdS QD

225

was visualized by culturing the bacterial cells in LB culture media containing 10 µg ml–1 of

226

disinfectants followed by centrifugation at 6000 rpm for 8 min. The bacterial cells were

227

collected, washed twice with PBS, and resuspended in double-distilled water. The suspension

228

was filtered and was fixed in a glutaraldehyde solution (2.5% glutaraldehyde in 0.2 M sodium

229

cacodylate/HCl buffer, pH 7.5) at 4 °C for 2 h. The filters were then rinsed with sodium

230

cacodylate/HCl buffer and postfixed with fresh 1% osmium oxide solution for 1 h. The

231

specimen were rinsed repeatedly with double-distilled water followed by successive

232

dehydration with ethanol solutions of 50% for 30 min, 75%, 85%, and 95% and 100% each

233

for 10 min. Finally, the specimen was critical point dried to remove ethanol and was

234

examined under a SEM by mounting on an aluminium stub and coating with gold sputter.

235

2.5.5. Determination of intracellular ROS generation

236

The intracellular ROS was measured using 2,7dichlorofluorescin diacetate (DCF-DA) which

237

passively enters the cell and reacts with ROS to form highly fluorescent dichlorofluorescein

238

(DCF). S. aureus cells grown in LB media were incubated with DCF-DA (30 µg/mL) on a

239

shaker at 37 °C for 30 min. The bacterial cells loaded with DCF-DA were then treated with

240

different concentration of rGO/CdS QD for 30 min and centrifuged to pellet the cells. The

241

cell pellet was suspended in PBS to measure the fluorescence at an excitation wavelength of

242

~485 nm using a Shimadzu RF 540 spectrofluorometer. The role of ROS in antibacterial

243

activity were further validated by measuring the growth of S. aureus in terms of OD600 nm

244

values as a function of time in the presence or absence of 5mM histidine which acts as a ROS

245

scavenger supplemented with different concentration of rGO/CdS QD.

246

3. Results and discussion

247

3.1 Characterization of rGO/CdS QD

248

The XRD patterns of CdS QD and rGO/CdS QD in the 2θ range of 5-80° is shown in Fig.

249

2(a). It can be seen that the pure CdS QD and rGO/CdS QD exhibited similar XRD pattern. 8

250

The diffraction peaks at 25.7°, 43.0°, and 51.1° corresponded to (111), (220), and (311)

251

crystal planes of face-centered cubic (fcc) CdS (JCPDS card No. 75-0581), respectively. The

252

broadened peaks signified the relatively smaller crystallite size of the CdS QD which can be

253

ascribed to slow release of S2– ions from [EMIM]·SCN complex.[24] Typical diffraction

254

peak of the rGO species at 26° (002) was convincingly shielded by the characteristic peak of

255

CdS at 25.7° due to their relatively low diffraction intensity in the heterostructure. Further,

256

the crystallinity of rGO/CdS QD was observed to be similar to that of pure CdS QD. This is

257

attributed to the suitable platform offered by the rGO for the nucleation of CdS QD during

258

the monowave irradiation process, such that the crystallinity of the CdS in the heterostructure

259

was sustained.[31] The average crystallite sizes of pure CdS QD and CdS QD in rGO/CdS

260

QD calculated by the Debye-Scherrer formula at (110) diffraction peak were 8 nm and 5 nm,

261

respectively.

262

The BET surface area and pore size distribution of rGO/CdS QD heterostructure

263

determined by nitrogen adsorption-desorption measurements is plotted in Fig. 2(b).

264

According to BDDT (Brunauer-Deming-Deming-Teller) classification, the nitrogen sorption

265

isotherm of rGO/CdS QD belonged to Type-IV that signifies the presence of mesopores (2–

266

50 nm). The isotherm exhibited H3 shape hysteresis loops characteristic of the mesoporous

267

structures. The pore size distribution (Fig. 2(b) inset) covered a broad range attributed to the

268

existence of mesopores. The BET surface area for CdS QD and rGO/CdS QD were calculated

269

to be 5.5 and 20.1 m2 g-1, respectively. The BET surface area obviously increased on adding

270

rGO, which is advantageous for better adsorption and thus enhancing of photocatalytic

271

efficiency.

272

Fig. 2(c) shows the optical properties of the photocatalysts measured by using UV–

273

Vis spectrophotometer. The photocatalytic activity is strongly governed by the absorption

274

range of light. Macroscopically, introduction of graphene changed the color of CdS QD from

275

lemon yellow to olive green in the heterostructure, indicating the enhancement of visible light

276

absorption as is also evidenced from the increased absorbance of rGO/CdS QD in the visible-

277

light region (λ ≥ 420 nm) in the UV-Vis spectra. CdS QD showed its characteristic absorption

278

peak at ~500 nm. An additional peak ~262 nm in the heterostructure is attributed to the π-

279

plasmon excitation of graphene sheet.[22]

280 281 282

The electronic band gap energy (Eg) of the CdS QD and rGO/CdS QD was calculated by using Tauc relationship as given in equation (3): αhν = C hν − E

(3)

9

283

where, α is the absorption coefficient (α = 2.303A/t, here A is the absorbance and t is the

284

thickness of the cuvette), C is a constant, h is the Planck’s constant, and ν is the photon

285

frequency. The value of n (= 1/2, 3/2, 2 or 3) depends on the nature of the electronic

286

transition responsible for absorption where n = 1/2 is for direct band gap semiconductor. An

287

extrapolation of the linear region of Tauc function versus photon energy (hν) plot gives the

288

value of the electronic band gap energy (Eg) as shown in Fig. 2(d). The energy band gap

289

values as calculated from the Tauc relationship were 2.73 eV for CdS QD and 2.50 eV for

290

rGO/CdS QD. The narrowing of CdS QD band gap is attributed to the absorption by rGO as

291

well as the chemical bonding or charge transfer interaction between CdS and rGO matrix.

292

FTIR spectra were plotted over the range of 500–4000 cm−1 in Fig. 2(e) to evaluate

293

the degree of reduction of GO sheets. The spectrum of GO exhibited the characterisitic GO

294

bands at 714 cm-1 (C–O–C stretching vibrations), 1047 cm-1 (C–O stretching vibrations of

295

epoxy groups), 1224 cm−1 (C–O stretching vibrations of phenolic group), 1452 cm−1 (O–H

296

bending vibrations of carboxylic group), 1734 cm−1 (C=O stretching vibrations of COOH

297

group), and 3430 cm-1 (O–H stretching vibrations).[19,32] The peak at 1610 cm-1 was related

298

to H–O–H bending vibration of adsorbed water molecules or to the stretching frequencies of

299

unoxidized C=C bond. The intensity of the bands which are linked to the oxygen-containing

300

functional groups were reduced in the FTIR spectrum of rGO/CdS QD heterostructure as

301

compared to GO, thereby confirming the reduction of GO. Moreover, the peak position of the

302

skeletal vibration of the graphene sheets (C=C stretching) shifted from 1610 cm-1 to 1570

303

cm−1 in rGO/CdS QD. The large peak at 3244 cm-1 corresponded to hydroxyl species from

304

water intercalated between the dried sheets. The peaks due to the skeletal vibration of Cd−S

305

bond at ~595 cm−1 was observed in the spectrum of rGO/CdS QD.

306

The GO reduction in rGO/CdS QD was further validated by the Raman spectra as

307

shown in Fig. 2(f). Pure CdS QD showed two peaks at ~280 cm−1 and ~580 cm−1

308

corresponding to longitudinal optical photon mode (1-LO) and overtone (2-LO) of CdS QD,

309

respectively.[31] These two peaks were also observed in the Raman spectrum of rGO/CdS

310

QD but were slightly shifted to lower frequency due to the smaller size effect of CdS QD

311

when assembled on rGO sheets. The Raman spectrum of GO exhibited two prominent bands

312

at 1348 cm-1 (D-band) and at 1580 cm-1 (G-band) as shown in Fig. S1 of Supplementary

313

Information. The D-band or defect induced band refers to a symmetry forbidden band

314

occurring due to the longitudinal plane vibration or k-point phonon of A1g symmetry.

315

Whereas, the G-band or graphitic band refers to the long wavelength longitudinal phonon

316

mode of graphene (E2g phonon) occurring due to the sp2 carbon network of the graphene 10

317

plane.[33] The rGO/CdS QD also showed the D band and G band, both red shifted which

318

indicates the softening of the phonons due to electron enrichment in the graphene network.

319

Moreover, the D/G relative intensity increased from 0.87 in GO to 0.97 in rGO/CdS QD

320

which indicates the higher number of smaller sp2 hybridized domains and hence, the

321

reduction of GO.[21,22]

Intensity (a.u.)

3

(311)

(c)

rGO/CdS QD

CdS QD rGO/CdS QD

200

250 160

200

120

Absorbance (a.u.)

Adsorbed Volume (cm /g-STP)

(220)

300

3

(b)

rGO/CdS QD CdS QD

Pore Volume ( cm /g)

(111)

(a)

80 40

150

0 0

10

20

30 40 50 60 Pore Diameter (nm)

70

80

100 50

60

70

0 0.0

80

(e)

0.4 0.6 Relative Presure (P/Pο)

0.8

1.0

2

2.0

2.5

3.0 3.5 hυ (eV)

4.0

4.5

5.0

4000

300

400

500 600 Wavelength (nm)

(f)

GO rGO/CdS QD

Transmittance (a.u.)

rGO/CdS QD CdS QD

0.2

3500

3000

2500

2000

1500

1000

500

700

800

CdS QD rGO/CdS QD

G band

40 50 2 theta (deg)

D band

30

(α hυ ) (a.u.)

(d)

20

Raman Intensity (a.u.)

10

300

600

900

1200

1500

1800

322 323

Fig. 2. (a) XRD patterns of CdS QD and rGO/CdS QD; (b) N2 adsorption–desorption isotherm for

324

rGO/CdS QD. Inset: pore size distribution; (c) UV-Vis spectra of CdS QD and rGO/CdS QD; (d)

325

Energy band gap of CdS QD and rGO/CdS QD calculated by Tauc relationship; (d) FTIR spectra of

326

GO and rGO/CdS QD; and (f) Raman spectra of CdS QD and rGO/CdS QD.

-1

Wavenumber (cm )

-1

Wavenumber (cm )

327

SEM images were captured to visualize the morphology of rGO/CdS QD and to

328

analyze the effect of rGO on the microscopic structure of CdS QD as shown in Fig. 3(a).

329

When GO was added during the monowave synthesis process, the CdS nanospheres were

330

tightly and uniformly spreaded over the graphene sheets, implying strong interaction between

331

CdS and rGO. These results indicate that rGO nanosheets highly influence the as-prepared

332

CdS QD by hindering their aggregation. The microscopic structure information was further

333

obtained from TEM images as shown in Fig. 3(b,c). The CdS QD were present in the form of

334

spherical particles as shown in Fig. 3(b). In Fig. 3(c), the rGO sheets were all curled and

335

wrinkled and were covered with spherical and lesser agglomerated CdS QDs.[34] The larger

336

specific surface area in presence of rGO was beneficial in promoting its adsorption capability

337

and interaction with pollutants.[35] The elemental composition of C, Cd, S and Pt (due to the 11

338

use of coater) was revealed by EDX spectra in Fig. S2 of Supplementary Information, and the

339

molar ratio of Cd:S was ~1:1.

340 341

Fig. 3. (a) SEM image of rGO/CdS QD; (b,c) TEM images of CdS QD (b) and rGO/CdS QD (c).

342

3.2 Photocatalytic degradation performance of rGO/CdS QD

343

Rapid industrialization, population increase, and depleting water-tables due to drought are

344

some of the current global issues which expanded the interest for cleaner production. Though

345

waste water recycling offers potential solution, yet the lethal organic components like dyes,

346

antibiotics, etc are very stubborn to be removed and thus cause ecological perils. The

347

photodegradation experiments were initiated once the adsorption equilibrium was obtained by

348

stirring the solution for 60 min in dark.

349

3.2.1

350

The photodegradation rates of different cationic (MB, RhB) and anionic (RB39, MO) dyes on

351

pure CdS QD and rGO/CdS QD heterostructure under visible light irradiation at room

352

temperature were compared and is shown in Fig. 4. It can be noted that pure CdS QD was an

353

active photocatalyst under visible light due to its narrow band gap energy. However, a

354

dramatic improvement of photocatalytic activity was observed for rGO/CdS QD under visible

355

light irradiation due to the formation of trapping sites and lower recombination rate that

356

increases the interfacial charge transfer. The photocatalytic systems were stirred for 60 min in

357

dark prior to irradiation. The dye concentration obtained after attaining equilibrium was taken

358

as the initial concentration as shown in Fig. 4(a,b). It can be reasonably concluded that the

359

adsorption of dye on catalyst surface had negligible contribution to dye concentration

360

variation. Su et al. reported that the dyes could absorb visible light and become excited. The

361

self-sensitization can cause degradation of excited state when the energy of their conduction

362

band matches with the photocatalysts.[36] To demonstrate the degradation of dyes by

363

photocatalysis rather than photosensitization, the blank runs was performed where dyes were

364

illuminated with visible light in the absence of photocatalyst (Fig. S3 of Supplementary

Photodegradation of dyes

12

365

Information). No apparent change in the dye concentration was observed which indicates that

366

dye was degraded only by photocatalysis process and not by photosensitization. The

367

photodegradation efficiency of CdS QD and rGO/CdS QD against the dyes is plotted in Fig.

368

4(c). Specifically, rGO/CdS QD have exhibited higher photodegradation than CdS QD within

369

60 min. The dyes decomposed in the vicinity of rGO/CdS QD and CdS QD followed the

370

order: MB (97.1%, 75%) > RhB (90.7%, 72%) > RB39 (81%, 65%) > MO (67%, 48%),

371

respectively.

372 373 374

The photodegradation mechanism can be explained by Langmuir-Hinshelwood pseudo-first-order kinetic model as given in equation (4): −ln

= kt

(4)

375

Fig. 4(d,e) shows the rate constants (k) of different dyes for pure CdS and rGO/CdS

376

QD heterostructure. For both the photocatalysts, the rate constants followed the order: MB >

377

RhB> RB39 > MO. Clearly, rGO/CdS QD had the rate constants 2.58 (MB), 1.86 (RhB),

378

1.60 (RB39), and 1.72 (MO) times higher than that of pure CdS QD. Compared to anionic

379

dyes, the cationic dyes showed higher rate constants, and hence higher photodegradation. The

380

lower rate of photodegradation for anionic dyes (RB39 and MO) is explained by the

381

electrostatic repulsion between negatively charged groups in dyes and the negatively charged

382

backbone of CdS based photocatalyst.

383 384

Fig. 4. (a,b) Variation of normalized C/C0 concentration of dye with irradiation time over (a)

385

rGO/CdS QD and (b) CdS QD; (c) Photodegradation efficiency rGO/CdS QD and CdS QD against

386

dyes; (d,e) Pseudo first order kinetics and rate constants of dye degradation over (d) CdS QD and (e)

387

rGO/CdS QD.

13

388

3.2.2

Photodegradation of antibiotics

389

In the past few years, the disposal of pharmaceutical waste materials has turned out to be

390

leading concern worldwide. Therefore, it is essential to eliminate pharmaceutical waste from

391

the municipal and industrial effluents preceding its discharge. The photocatalytic activity was

392

evaluated using TC and PC antibiotics as model organic pollutants following the same

393

procedure as in the case of dyes. As shown in Fig. 5, pure CdS QD displayed poor

394

photoactivity as compared to rGO/CdS QD heterostructure. Blank run was performed on PC

395

by illuminating it in visible light radiation without adding photocatalyst (Fig 5(a,b)).

396

Negligible change in PC concentration indicates that antibiotic was degraded only by

397

photocatalytic process. The photodegradation percent of TC and PC for CdS QD and

398

rGO/CdS QD is plotted in Fig. 5(c). Pure CdS QD degraded only 65% of TC and 72% of PC

399

after 120 min. Whereas, rGO/CdS QD exhibited superior photocatalytic performance by

400

degrading 84% TC and 90% PC. The better performance of rGO/CdS QD is attributed to the

401

heterostructure formation and effective photoinduced electron–hole pair separation. In case of

402

both the photocatalysts, the efficiency for PC degradation was more than TC degradation. For

403

better insights into the photocatalytic process, pseudo-first order kinetic modelling was

404

performed with the experimental results and the fitted model as per equation (3) is shown in

405

Fig. 5(d,e). The rate constant (k) values for TC and PC degradation were calculated to be

406

0.00875 and 0.01061 min-1 with CdS QD whereas 0.01603 and 0.01956 with rGO/CdS QD,

407

respectively. It is obvious that rate constant of rGO/CdS QD was 1.83-fold higher for TC and

408

1.84-fold higher for PC than that of pure CdS QD. Thus, addition of rGO accelerated the

409

photocatalytic degradation rate.

14

410 411

Fig. 5. (a,b) Variation of normalized C/C0 concentration of antibiotic with irradiation time over (a)

412

CdS QD and (b) rGO/CdS QD (b); (c) Photodegradation efficiency of rGO/CdS QD and CdS QD

413

against antibiotics; (d,e) Pseudo first order kinetics and rate constant of antibiotic degradation over

414

(d) CdS QD and (e) rGO/CdS QD.

415

Overall, the rate constants followed the order: MB > RhB > RB39 > PC > MO > TC with

416

high values of R2, as listed in Table 1. The results of the present work is compared with some

417

of the recent reported literatures as listed in Table 2.

418

Table 1. Rate constants and R2 values of the pollutants photodegradation. Pollutants

CdS QD -1

rGO/CdS QD 2

-1

2

MB

k (min ) 0.0231

R 0.99

k (min ) 0.0595

R 0.99

RhB

0.0212

0.99

0.0394

0.99

RB39

0.0175

0.98

0.0281

0.99

PC

0.0106

0.99

0.0196

0.98

MO

0.0109

0.98

0.0188

0.99

TC

0.0088

0.99

0.0160

0.98

419 420

Table 2. Assessment of the present work compared with the previously reported work. Photocatalyst

Light source

Pollutant

rGO/CdS QD

Visible

MB RhB RB39

Reaction time (min) 60 60 60

Degradation (%) 97 91 81

Reference Present work

15

MO TC PC

60 120 120

67 84 90

BP-RP

Visible

MB

360

91

[37]

Ag-BPNS-GO

Solar light

MB

180

94

[38]

FeCd (2%):ZnO

Visible

MB RhB

140

82 76

[39]

Cobalt ferrite/polyaniline

Visible

MO

540

80

[40]

AgI/BiVO4

Visible

TC

120

90

[41]

rGO/ZnTe

Solar light

TC

45

65

[42]

CdS/TiO2

Visible

TC

480

87

[43]

TiO2/KAl(SO4)2

Visible

PC

540

95

[6]

421

3.2.3

Reusability of rGO/CdS QD

422

Catalyst reusability is an important aspect of heterogeneous catalysis. The rGO/CdS QD

423

heterostructure was easily separated from the clean water after MB degradation through

424

centrifugation and filtration. Fig. 6(a) shows the photoactivity of the recycled rGO/CdS QD

425

for four consecutive MB dye photodegradation recycling experiments of 60 min each. After

426

four cycles, only a decrease of ~1% in photodegradation efficiency of rGO/CdS QD was

427

observed, confirming the high recyclability and photocatalytic response of rGO/CdS QD

428

heterostructure.

429

3.2.4

430

The photodegradation performance of rGO/CdS QD is governed by ROS generated via

431

visible light irradiation. To determine the essential reactive species involved in the

432

photodegradation, a series of trapping experiments were performed using different

433

scavengers like disodium ethylenediaminetetraacetate (EDTA-2Na), benzoquinone (BQ), and

434

isopropyl alcohol (IPA) to trap h+, O2˙− and ˙OH formed during the photo-oxidation process.

435

The percent photodegradation of MB dye with/without different scavengers is compared in

436

Fig. 6(b) after irradiating the aqueous solution containing MB and rGO/CdS QD for 60 min

437

under analogous conditions. The scavenger-free reaction system was found to have maximum

438

decomposition (97.1%) of MB. On the other hand, addition of IPA resulted in the least MB

439

degradation (20%) due to quenching of ˙OH formed in the reaction. This may be attributed to

440

the hydroxyl radical species actively involved in the photodegradation mechanism.

441

Photodegradation efficiency was also decreased in the presence of EDTA-2Na (72%) since it

Evaluation of ROS through trapping and fluorescence studies

16

442

traps h+ during the reaction. However, photodegradation efficiency was least decreased

443

(86%) with BQ that traps photogenerated O2˙− and hence, signifies that although all the

444

reactive species have some contributions toward the photodegradation, hydroxyl radicals are

445

the important ROS during the photodegradation reactions.

446

To further validate the ROS generated during the photodegradation process,

447

fluorescence experiment was performed using terephthalic acid (TA) as a probe, which reacts

448

with ˙OH radicals to generate a fluorescence active species, 2-hydroxyterephthalic acid

449

(TAOH). Under visible light illumination, the fluorescence intensity of TAOH is proportional

450

to the amount of ˙OH generated. Fig 6(c) shows the maximum fluorescence intensity at 425

451

nm of the rGO/CdS QD measured at 10 min intervals when excited at 315 nm. The gradual

452

increase in fluorescence intensity with increasing irradiation time indicates that ˙OH radicals

453

were generated during the photoinduced catalysis, which degrades the pollutant molecules.

454 455

Fig. 6. (a) Four-cycle photodegradation efficiency test of rGO/CdS QD against MB dye; (b) Percent

456

photodegradation of MB by rGO/CdS QD in the presence of scavengers; (c) Fluorescence intensity

457

changes of rGO/CdS QD with time in a basic solution of TA under visible light irradiation.

458

3.3 Photocatalytic disinfection performance of rGO/CdS QD

459

The extensive release and misuse of antibiotics in past decades created the multiple drug

460

resistance in pathogenic bacteria. E. coli and S. aureus are associated with infections in

461

various parts of the body.[44] Compared with conventional methods like antibiotic and

462

chlorine disinfection, photocatalytic disinfection based on the ROS is a cleaner alternative

463

due to negligible formation of potentially harmful disinfection byproducts (DBPs), effective

464

over a wide spectrum of pathogens, and utilize solar energy for the disinfection process.

465

3.3.1

466

The antibacterial activity of rGO/CdS QD against S. aureus and E. coli was compared

467

qualitatively with CdS QD by disk diffusion assay. Fig. 7(a) shows the effect of different

468

concentration of CdS QD and rGO/CdS QD (25, 50, 75, 100 µg ml–1) on the zone of

469

inhibitions in case of both S. aureus and E. coli. The diameter of inhibition zone increased

Antibacterial activity of rGO/CdS QD

17

470

with the increase in photocatalysts concentration. Hence, both CdS QD and rGO/CdS QD

471

exhibited a concentration-dependent bactericidal activity. As compared to CdS QD, the

472

rGO/CdS QD exhibited higher inhibition zone and hence, higher disinfection efficiency. The

473

diameters of inhibition zone of S. aureus were slightly larger than diameters of inhibition

474

zone of E. coli under the same conditions (Fig. S4 of Supplementary Information), which

475

indicates that gram negative E. coli is harder to inactivate due to the presence of cell

476

membrane. The minimum inhibitory concentration (MIC) values of CdS QD and rGO/CdS

477

QD were found to be 64 and 16 µg ml–1 in case of S. aureus and 128 and 32 µg ml–1 in case

478

of E. coli, respectively.

479

The disinfection performance of rGO/CdS QD were further investigated

480

quantitatively by measuring the growth curves of S. aureus and E. coli treated with different

481

concentrations of rGO/CdS QD (25, 50, 75, 100 µg ml–1) as shown in Fig. 7(b,c). The results

482

were found to be consistent with the qualitative analysis data. The effective concentration of

483

the rGO/CdS QD was found to be 25 µg ml–1. On the other hand, the bacterial growth were

484

hardly increased within 8 h when 50 µg ml–1 of rGO/CdS QD was used. The growth of S.

485

aureus and E. coli was inhibited more severely with the increase of rGO/CdS QD

486

concentration. A noticeable difference in the growth rate between the two bacterial strains

487

was observed after a lag phase of 4 h. In case of S. aureus, the exponential phase was delayed

488

by 12 h and 16 h at rGO/CdS QD concentrations of 75 and 100 µg ml–1, respectively,

489

whereas, it was 8 h and 12 h in case of E. coli under the same conditions. These results

490

demonstrate that the rGO/CdS QD exhibited strong antibacterial activity. Control -1 25 µg ml -1 50 µg ml -1 75 µg mLl -1 100 µg ml

(a)

25

1.6

15 10 5

1.2

0.8

E. coli + CdS QD

50 75 Concentration (µg ml-1) S. aureus + CdS QD

E. coli + rGO/CdS QD

100 S. aureus + rGO/CdS QD

1.2

0.8

0.4

0.4

0 25

Control -1 25 µg ml -1 50 µg ml -1 75 µg ml -1 100 µg ml

1.6

O.D at 600 nm

20

O.D at 600 nm

Zone of inhibition diameter (mm)

2.0

2.0

30

(b)

0.0

0

4

8

12 Time (h)

16

20

24

(c)

0.0

0

4

8

12 Time (h)

16

20

24

491 492

Fig. 7. (a) Zone of inhibition as a function of various concentration of CdS QD and rGO/CdS QD

493

against S. aureus and E. coli; (b,c) Growth curves of (b) S. aureus and (c) E. coli, treated with

494

different concentrations of rGO/CdS QD.

495

3.3.2

Morphology change of bacteria after exposure to CdS QD and rGO/CdS QD

496

SEM images in Fig. 8 are shown to compare the morphology changes of S. aureus and E. coli

497

cells prior to and after treating with CdS QD and with rGO/CdS QD. The untreated S. aureus 18

498

were round in shape with intact surfaces, while the untreated E. coli were rod-like with intact

499

surfaces (Fig. 8(a,d)).[19,45] After exposure of S. aureus to CdS QD and rGO/CdS QD for

500

24 h, a significant portion of the bacterial cells were decomposed and their morphology

501

changed significantly from round aggregates to tiny particles. The cell membranes were

502

permeabilized and structural disruption had occurred. The bacterium membrane was

503

collapsed and the cytoplasm was leaked out (Fig. 8(b,c)). When E. coli cells were exposed to

504

CdS QD and rGO/CdS QD for 24 h, the bacterial cells were transformed from rod shape to

505

globular shape and the leaked cytoplasm through damaged outer and inner membrane were

506

visible (Fig. 8(e,f)). As compared to CdS QD, the SEM images of rGO/CdS QD showed

507

higher morphological changes of bacterial cells which eventually led to growth inhibition and

508

cell death.

509 510

Fig. 8. SEM images of (a-c) S. aureus and (d-f) E. coli. (a,d) Control, (b,e) treated with CdS QD, and

511

(d,f) treated with rGO/CdS QD.

512

3.3.3

Measurement of intracellular ROS generation

513

ROS-mediated oxidative stress is a major inducer of cytotoxicity and apoptotic death

514

of bacterial cells. Intracellular ROS generation mediated by rGO/CdS QD in the S. aureus

515

cells was measured using DCF-DA dye. The dye becomes oxidized and releases intense

516

green fluorescence in the presence of ROS.[46] Fig. 9(a) shows the fluorescence intensity as

517

a function of rGO/CdS concentration. It is observed from the graph that the amount of ROS

518

increased with the increase in rGO/CdS QD concentration. The role of ROS (˙OH, O2˙−, h+) 19

519

in antibacterial activity of rGO/CdS QD was further examined by pretreating the bacterial

520

cells with 5mM of histidine, a well-known scavenger of ROS. Fig 9(b) compares the ROS

521

quenching effect of histidine on the growth curve of S. aureus cells with the untreated

522

control. It is clear from the graph that histidine significantly reduced the killing effect of

523

rGO/CdS QD in a dose-dependent manner and hence, confirms that ROS plays a critical role

524

in photocatalytic disinfection process. 2.0

100

(b) 1.6

80

O.D at 600 nm

Flouroscence Intensity (%)

(a)

60

40

1.2

0.8

0.4

Control -1 25 µg ml -1 50 µg ml -1 75 µg ml

20 0.0

0

25 75 50 -1 rGO/CdS QD concentration (µg ml )

0

5

10

15

20

25

525 526

Fig. 9. (a) Intracellular ROS generation determined by DCF-DA fluorescence, (b) The effect of

527

histidine (5 mM) on growth curve of S. aureus in presence of rGO/CdS QD.

Time (h)

528

3.4 Plausible mechanism

529

3.4.1. Plausible mechanism of photodegradation by rGO/CdS QD

530

Photocatalysis of this type of heterostructure depends on the charge separation extent when

531

irradiated with light. The results obtained from the trapping experiments revealed that both

532

radicals and holes are active oxidative species in the photocatalytic reaction system. On this

533

basis, a schematic illustration of the photocatalytic degradation mechanism followed by

534

rGO/CdS QD heterostructure is shown in Fig. 10. The degradation mechanism mainly

535

follows three steps: 1) absorption of the incident photons of sufficient energy on

536

photocatalyst’s surface, 2) the charge carrier generation, separation, and recombination, and

537

3) occurrence of photochemical reaction in proximity of the surface. In this work, the higher

538

photodegradation efficiency of rGO/CdS QD is attributed to increased absorption of photons

539

owing to higher surface area and reduced e−−h+ recombination rate due to high carrier

540

mobility of rGO and thereby better redox reaction on surface. When illuminated under visible

541

light radiation, the photon energy excites the electron from the valence band of the CdS QD

542

to the conduction band and e−−h+ pairs are generated by charge separation. High carrier

543

mobility of graphene makes rGO behaves as electron acceptor and transporter to efficiently 20

544

increase the charge separation and hinder the recombination of photoinduced e−−h+ pairs. The

545

photogenerated electrons reacts with surface adsorbed oxygen or dissolved oxygen to form

546

superoxide anion radical (O2˙−). These O2˙− will degrade the pollutant and also through

547

successive reactions will create hydroxyl radicals (˙OH). Simultaneously, the holes react with

548

adsorbed water molecules or OH– to produce hydroxyl radicals (˙OH). Nonetheless, holes in

549

the valence band oxidize pollutants by capturing electrons from other pollutant molecules.

550

Finally, these ROS with strong oxidizing ability reacts with the organic pollutants to

551

completely mineralize them into CO2, H2O or some other small molecules. Our results depict

552

that the ˙OH and h+ species have the major contribution toward the degradation of the organic

553

pollutants in an aqueous medium. The chain of reaction is as follows:

554

CdS + hν → CdS % + h&

555

CdS e

+ rGO → CdS + rGO e

556

rGO e

+ O) → O)∙ + rGO

(R3)

557

rGO e

+ O)∙ + H & → HO)

(R4)

558

HO) + H & → H) O)

(R5)

559

H) O) + e → ˙OH + OH

(R6)

560

H) O + CdS h& → ˙OH + H &

(R7)

561

(R1) (trapped)

(R2)

Organic pollutants + O)∙ + ˙OH + h& → CO) + H) O + other degradation products

562

(R8)

563

3.4.2. Plausible mechanism of disinfection by rGO/CdS QD

564

The antibacterial activity in photocatalytic water disinfection generally ascribed to the ROS

565

generation and subsequent damage to macromolecules like proteins, lipids, DNA, etc in the

566

bacterial cells. Destruction of bacterial cells in the presence of rGO/CdS QD can occur by

567

light induced oxidative stress mechanism. The plausible disinfection mechanism of rGO/CdS

568

QD is shown schematically in Fig. 10. When S. aureus and E. coli were incubated with

569

rGO/CdS QD, the rGO sheet revolves around the phospholipid layer of the cell membrane.

570

This results in a partial loss of cell membrane integrity leading to the permeabilization of the

571

cell membrane. A stream of rGO/CdS QD enters the cell and ROS is generated in the system.

572

This oxidative stress accelerated by rGO/CdS QD creates a disturbance in the intracellular

573

antioxidant defence process and results in the oxidation of lipids, nucleic acids, and proteins.

574

This leads to the complete destruction of cell membrane and inhibition of cellular growth. As

575

observed from the antibacterial activity data, S. aureus was more susceptible to destruction

576

since it is a gram positive bacteria which contains a peptidoglycan layer (20-80 nm) without 21

577

outer membrane. On the other hand, E.coli which is a gram negative bacteria with a thin

578

peptidoglycan layer (7-8 nm) but with an outer membrane was found to be less susceptible to

579

destruction. Therefore, the outer membrane might be preventing the penetration of rGO/CdS

580

QD into the bacterial cell. The chain of reaction for degradation of bacterial cells is as

581

follows:

582 583

rGO/CdS QD + O∙) + ˙OH + cell membrane + water → Membrane disruption → Exposed cellular components → Bacterial inactivation

(R9)

584 585

Fig 18. Schematic illustration of the plausible mechanism of photodegradation and disinfection by

586

rGO/CdS QD heterostructure

587

4. Conclusion

588

Photocatalytic performance of rGO/CdS QD heterostructure under visible light irradiation

589

against various dyes, antibiotics, and pathogenic bacteria was investigated and compared with

590

pure CdS QD. The rGO/CdS QD was synthesized via a novel monowave-assisted method

591

using [EMIM].SCN ionic liquid as a green precursor. The as-prepared photocatalyst was

592

characterized to confirm the reduction of GO and the dispersion of CdS QD on rGO sheets.

593

The photodegradation efficiency of the photocatalysts were evaluated against cationic dyes

594

(MB, RhB), anionic dyes (RB39, MO), and antibiotics (TC, PC) as model organic pollutants.

595

The photocatalytic performance of heterostructure was found to be better than pure CdS QD,

596

which is attributed to the suppression of e−−h+ recombination leading to higher ROS 22

597

generation in the presence of rGO. The photoactivity trend obtained from experiments was

598

modeled using pseudo-first order kinetics and the rate constants followed the order: MB >

599

RhB > RB39 > PC > MO > TC, which was inline with the percent photodegradation.

600

Trapping experiments performed using EDTA-2Na, BQ, and IPA as scavengers as well as

601

fluorescence studies using TA confirmed that ˙OH was the active ROS in the

602

photodegradation. Further, the bacterial disinfection performance of rGO/CdS QD (25, 50,

603

75, 100 µg ml–1) was evaluated against S. aureus and E. coli through disc diffusion assay and

604

growth kinetics assay. The heterostructure owned excellent concentration dependent

605

antibacterial properties as compared to CdS QD. The S. aureus cells were more susceptible to

606

inactivation as compared to E. coli cells due to the absence of cell membrane. Bacterial cell

607

death resulted from the disruption of cell membrane, cytoplasm leakage, and damage to

608

macromolecules as evidenced from SEM images. The plausible mechanism of ROS-mediated

609

photodegradation of organic pollutants and disinfection of bacterial pathogens in the presence

610

of rGO/CdS QD was discussed. Hence, these findings may present new possibilities for the

611

application of single eco-friendly photocatalyst for simultaneous degradation of different

612

categories of water pollutants for water reuse.

613

Conflicts of interest

614

The authors declare no competing financial interest.

615

Acknowledgements

616

Gul Afreen and Mohd Shoeb acknowledge UGC−MANF for fellowship.

617

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28

Highlights: •

Green synthesis of rGO/CdS QD heterostructure by using ionic liquid as precursor

•

Photodegradation of dyes & antibiotics and disinfection of pathogenic bacteria

•

Photocatalytic activity of rGO/CdS QD is mediated by Reactive Oxygen Species (ROS)

•

Pseudo-first order rate constant follow the order: MB > RhB > RB39 > PC > MO > TC

•

S. aureus is more susceptible to disinfection by rGO/CdS QD than E.coli