GO nanocomposite

Journal Pre-proof Removal of naphthalene from simulated wastewater through adsorption-photodegradation by ZnO/Ag/GO nanocomposite Nthambeleni Mukwevho, Rashi Gusain, Elvis Fosso-Kankeu, Neeraj Kumar, Frans Waanders, Suprakas Sinha Ray

PII:

S1226-086X(19)30508-8

DOI:

https://doi.org/10.1016/j.jiec.2019.09.030

Reference:

JIEC 4789

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

23 May 2019

Revised Date:

12 September 2019

Accepted Date:

13 September 2019

Please cite this article as: Mukwevho N, Gusain R, Fosso-Kankeu E, Kumar N, Waanders F, Ray SS, Removal of naphthalene from simulated wastewater through adsorption-photodegradation by ZnO/Ag/GO nanocomposite, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.09.030

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Removal of naphthalene from simulated wastewater through adsorptionphotodegradation by ZnO/Ag/GO nanocomposite

Nthambeleni Mukwevhoa, Rashi Gusainbc, Elvis Fosso-Kankeua*, Neeraj Kumarb, Frans Waandersa, Suprakas Sinha Raybc

a

Water Pollution Monitoring and Remediation Initiatives Research Group , School of Chemical and Minerals

Engineering, North West University, P. Bag X6001 Potchefstroom 2520, South Africa. b

DST-CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research,

c

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Pretoria 0001, South Africa.

Department of Applied Chemistry, University of Johannesburg, Doornfontein 2028, Johannesburg, South

Africa

Tel: 0027182991659

0000-0002-5293-9920 (Nthambeleni Mukwevho)

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0000-0002-7340-7237 (Rashi Gusain)

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ORCID: 0000-0002-7710-4401 (Elvis Fosso-Kankeu)

0000-0001-5019-6329 (Neeraj Kumar)

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0000-0002-0007-2595 (Suprakas Sinha Ray)

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Graphical Abstract

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* Email: [email protected]; [email protected]; [email protected]

Abstract In this study, a ZnO/Ag/GO nanocomposite was synthesised and used as photocatalyst for effective photodegradation of naphthalene from simulated wastewater under visible light. Chemical and morphological characterisation were successfully done using XRD, PL, UV-vis, FTIR, XPS, FESEM and HRTEM analytical tools. Photocatalytic degradation experiments were first carried out under dark conditions and then under visible-light irradiation. Adsorption study of naphthalene prior to

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photocatalysis using synthesised material was thoroughly done by studying the kinetics and adsorption isotherm models. All as-synthesised materials (ZnO nanoparticles, binary ZnO/Ag, and ternary ZnO/Ag/GO nanocomposites) followed pseudo-second-order kinetics and the Freundlich adsorption isotherm, confirming the adsorption on hetero-structural surface. ZnO/Ag/GO could

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successfully adsorb 80% naphthalene in 20 minutes, with 500 mg.g-1 adsorption capacity. High

adsorption of naphthalene molecules on ZnO/Ag/GO surfaces trigger improved photodegradation

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efficiency upon light irradiation. Incorporation of plasmonic Ag nanoparticles and 2D graphene oxide

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(GO) to ZnO semiconductor improved the photocatalytic degradation efficiency of naphthalene, achieving up to 92% degradation in 50 minutes. The photodegradation of naphthalene follows the Langmuir-Hinshelwood kinetics model and was found acceptable to express the photodegradation

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rate. Furthermore, the ZnO/Ag/GO photocatalyst could easily be recycled and reused for five cycles,

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maintaining up to 85% of its photodegradation efficiency.

Keywords: Naphthalene, adsorption, visible light, photocatalytic degradation, nanohybrid materials

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nanocomposite, kinetics

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) mark an entrance into the environment via incomplete combustion of several organic compounds or oil spills and are also an emerging class of environmental pollutants [1-2]. The high stability of PAHs makes them resistant to biological degradation, thus causing them to stay in the environment for prolonged periods [3]. Many of them

are already listed as priority pollutants in a report distributed by the United States Environmental Protection Agency (US EPA) [4]. Direct/indirect exposure of PAHs to humans and animals could be toxic, oncogenic and mutagenic, with both acute and chronic effects on health [5-6]. One of the regularly found PAHs in nature is naphthalene. Its toxic nature induces respiratory or haematological and ocular health effects [7]. Several technologies have been developed for the remediation of organic contaminants including PAHs from wastewater, such as advanced oxidation processes, photocatalysis, adsorption, volatilisation, biological treatments, and membrane technologies [8-12]. Among all of these, considerable attention has been given to the use of semiconductor-assisted photocatalysis for

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the removal of pollutants from solutions due to the possibility of complete degradation [13-14]. Photocatalytic degradation is one of the most credible techniques to remove pollutants from

wastewater, because of its quick reaction, economic feasibility and environmental friendliness [15-

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16]. For the past two decades, photocatalysts based on TiO2 and its composites have been employed

for organic-compound remediation from wastewater under light illumination [17-18]. Owing to higher

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quantum effectiveness than TiO2, as of late, ZnO has been considered as a potential semiconductor

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photocatalyst for the depollution of different contaminants [19-20]. Although ZnO nanoparticles cannot activate in a visible range due to a large band gap, irradiation of ZnO nanoparticles with UV light leads to the generation of electron-hole charge-pair separation through electron excitation from

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the ground-energy-level valence band (VB) to the high-energy-level conduction band (CB). Furthermore, photocatalytic activities of catalysts depend on the availability of as-formed electron-

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hole pairs and their recombination rate [21]. Photocatalytic performance of nanomaterials can be enhanced by delaying the recombination rate of charged electron-hole pairs. It can possibly be

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achieved by either hybridisation of nanomaterials with noble metals such as Pt, Ag and Au or fabricating composites with various semiconducting photoactive materials [21-23]. Incorporation of noble metals with nanomaterials significantly enhances the visible-light absorption via surface plasmonic effects and generates electron-hole pairs for photocatalysis [24-25]. ZnO/Ag microspheres have already been investigated as photocatalyst under visible light for the photodegradation of methylene blue (MB) dye [26-27]. It was concluded that loading of Ag nanoparticles on ZnO

improved the interfacial charge transfer kinetics and provided high-charged electron-hole-pair recombination time. GO is a 2D nanomaterial with sp2-bonded carbon atoms used for transportation of electrons during photocatalysis due to its superior electrical conductivity, mechanical and catalytic properties [28-29]. Researchers have studied the synergic effects of graphene/GO with semiconductor materials for various photocatalytic applications [30]. It is evident that the nearness of individual Ag nanoparticles and graphene can advance the photocatalytic activity of ZnO nanoparticles. Balamurgan et al.

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deposited a ZnO/Ag/GO nanocomposite film on a glass substrate for the photocatalytic degradation of MB dye [31]. This heterostructure reduces the band gap of ZnO and improves the visible-light

absorption, due to Ag doping which excites more electrons from the low-energy VB to the high-

energy CB and generates resultant holes in the ground-level VB. The GO layer easily trapped the

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excited electrons due to its structure and excellent conductivity which aids to separate the charge pairs. These excited electrons and holes would therefore be available for the effective

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photodegradation of the pollutants. Thus, combining these materials together into a

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heterostructure/composite could result in better performance of semiconductor photocatalysts [32]. ZnO nanocomposite has been used for the remediation of water pollution by other investigators [3334]. Herein, we are reporting the synthesis of a ZnO/Ag/GO nanocomposite as photocatalyst for the

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removal of naphthalene from contaminated wastewater under visible light. The prime objectives of this work were: (i) synthesis of ZnO-based nanomaterials (ZnO, ZnO/Ag and ZnO/Ag/GO) with

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enhanced removal efficiency of naphthalene from solution, (ii) chemical and morphological characterisation of the as-synthesised nanomaterials and (iii) effective adsorption and

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photodegradation of naphthalene using a ZnO/Ag/GO nanocomposite under visible light.

2. Experimental 2.1 Materials

Graphite powder (<20 μm), potassium permanganate (KMnO4, 97%), sulphuric acid (H2SO4, 98%), phosphoric acid (H3PO4, 85%), hydrogen peroxide (H2O2, 30%), hexamethylenetetramine (C6H12N4, 99%), zinc acetate dihydrate (Zn(Ac)2.2H2O, ≥98%), hydrochloric acid (HCl, 37%), silver

nitrate (AgNO3, ≥99.9%), and naphthalene (98%) were procured from Sigma-Aldrich, and also used as received. 2.2 Synthesis of GO and ZnO nanoparticles GO was prepared following solvothermal synthesis with minimal modifications [35]. In a typical exercise, 2 g graphite powder was gently mixed with 50 mL H2SO4 and 5.5 mL H3PO4 in a round-bottom flask using an ice bath to maintain the temperature. Subsequently, 0.038 mol KMnO4 was added to the above reaction mixture and the temperature was raised to 35 °C for 2 hours with

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undisrupted stirring. The temperature of the reaction was increased by adding 100 mL boiling water and maintained at 95 °C for 30 minutes. Finally, the reaction mixture was cooled down to room

temperature and quenched using 325 mL distilled water, following by the addition of 20 mL 37%

H2O2. Synthesised GO was centrifuged and collected in the pellet, then washed several times in 1 M

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fresh HCl solution and distilled water. The collected GO precipitate was dried at 60 °C under reduced

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pressure for 24 hours. To synthesise ZnO nanoparticles, 3.0 g Zn(Ac)2.2H2O was grounded nicely into fine powder using a mortar and pestle for 1 hour and calcined at 350 °C for 4 hours in an alumina

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crucible.

2.3 Synthesis of ZnO/Ag and ZnO/Ag/GO nanocomposite

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To prepare ZnO/Ag/GO, firstly, 1250 mg/L aqueous dispersion of GO was prepared by sonication as above synthesised GO in water. 50 mL GO dispersion (1250 mg/L), 25 mL Zn(Ac)2.2H2O (0.1 M),

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10 mL AgNO3 (0.01 M) and 25 mL C6H12N4 (0.2 M) were added in a round-bottom flask and sonicated for 20 minutes, followed by reflux for 2 hours with continuous stirring. The prepared

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precipitate was collected through centrifugation and washed four times with distilled water to get rid of all impurities and acid content. The resultant product was dried at 60 °C for 24 hours in an air oven. The ZnO/Ag nanocomposite was also synthesised in a similar way without adding any GO dispersion. 2.4 Characterisation An X-ray diffractometer (XRD) was employed to determine the crystallinity and purity of the synthesised compounds. All XRD experiments were run using powdered samples in a Philips

PANalytical X’Pert PRO PW 3040/60, at 40 kV voltage and 1.6 kW energy. Photoluminescence (PL) was carefully recorded on a Horiba Jobin-Yvon NanoLog spectrometer using an Xe lamp generating an excitation wavelength (ʎ) of 325 nm. Fourier transform infrared (FTIR) analyses of the synthesised samples were done on a Perkin Elmer Spectrum 100 FTIR spectrophotometer from 400-4000 cm-1 wavenumber at a 4 cm-1 resolution. All UV-vis absorbance analyses were conducted on a Shimadzu UV-2401PC spectrophotometer for 200-700 nm wavelength. X-ray photoelectron spectroscopy (XPS) measurements using a monochromatic (Al Kα) excitation source were performed on a Kratos Axis Ultra device (Kratos, UK) for the ZnO/Ag/GO nanocomposite. Morphological studies of

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nanomaterials were done using field emission scanning electron microscopy (FESEM) on a Zeiss

Auriga Cobra FIB microscope and high-resolution transmission electron microscopy (HRTEM) on

2.5 Adsorption and photodegradation of naphthalene

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JEOL JEM-2100.

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50 mg.L-1 naphthalene simulated aqueous solution was selected for the evaluation of the photocatalytic potential of the ternary ZnO/Ag/GO nanocomposite. All photocatalysis experiments

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were run under visible-light irradiation. To prepare 50 mg.L-1 naphthalene aqueous solution, 50 mg naphthalene was dissolved into 1000 mL 10% methanol solution (methanol:distilled water = 1:10 V/V) at room temperature under continuous stirring for 30 minutes. Photodegradation of naphthalene

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was achieved by adding a fixed amount (25 mg) of prepared photocatalyst into 100 mL of the 50 mg.L-1 naphthalene solution under continuous stirring. The reaction was first placed in the dark for

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20 minutes to create the adsorption-desorption equilibrium between the photocatalyst and naphthalene molecules, and was then followed by exposure to a 250 W Xe lamp emitting visible light. At regular

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intervals (under dark and visible-light conditions), ~4 mL of a sample aliquot was extracted with the aid of a syringe and filtered using PDVP (0.45 μm) syringe filters to remove all the photocatalyst particles. In order to examine the residual naphthalene concentration in the simulated wastewater, a UV-vis spectrophotometer was used by measuring the absorption peak at 275 nm. The photodegradation efficiency of the photocatalyst was calculated using the following equation: Photodegradation efficiency (%) =

𝐶0 −𝐶𝑡 𝐶0

× 100

(1)

Where C0 and Ct are the initial and residual concentrations, respectively, of naphthalene in solution at time (t). The adsorption capacity (qe) for naphthalene on ZnO/Ag/GO nanocomposite was determined using the following equation: 𝑞𝑒 (mg/g) =

𝐶𝑜 − 𝐶𝑒 ×𝑉 𝑚

(2)

In Equation 2, Co and Ce (mg.L-1) refer to naphthalene concentration at time zero and at equilibrium, respectively, m (g) signifies the mass of nanocomposite and V (L) refers to the volume of the

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naphthalene solution.

3. Results and Discussion 3.1 Structural and morphological characterisation

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3.1.1 XRD analysis

XRD analysis was performed in order to probe the phase, crystallinity and purity of the synthesised

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nanomaterials. Fig. 1 shows the XRD diffractogram of synthesised ZnO nanoparticles, ZnO/Ag and ZnO/Ag/GO nanocomposite. The ZnO diffractogram is in agreement with the standard JCPDS file

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no. 89-1397, and confirmed the hexagonal wurtzite structure of the nanoparticles. The XRD pattern of the ZnO/Ag shows two additional low-intensity peaks, marked with blue asterisks in the graph, along

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with the characteristic peaks of ZnO. These peaks are typical peaks of Ag at 2θ = 38.2° and 44.3°, representing the (111) and (200) planes of cubic Ag as per JCPDS file no. 04-0783. The intensity of

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the diffraction peaks in the ZnO/Ag diffractogram for the (100) and (002) planes are higher than that

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of the ZnO, which might be due to the fact that Ag helps ZnO to grow with a high orientation.

For the ZnO/Ag/GO nanocomposite, the XRD peaks tagged with the “#” symbol are assigned to the hexagonal wurtzite ZnO, while those marked with “*” and “$” could be indexed to Ag and GO, respectively. The introduction of Ag and GO did not disturb the phase and structure of pure hexagonal wurtzite ZnO. The observed diffraction peak of GO is small and can be ascribed to the low diffraction intensity and low GO concentration. The detection of reduced graphene oxide (rGO) in the XRD

pattern is challenging as nanoparticles might get inserted between the adjacent GO layers which disordered the layer spacing [33]. The XRD patterns confirm the successful synthesis of ZnO and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposite. 3.1.2 PL spectra Vacancies or defects and surface/trap states can alter the optical and electrical characteristics of material. The presence of these invisible agents in materials can be determined by PL measurements. Fig. 2a shows the PL emission spectra for the ZnO, ZnO/Ag and ZnO/Ag/GO nanomaterials at room

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temperature. The PL spectrum for pure ZnO nanoparticles exhibits two major emission peaks, one sharp, narrow UV emission peak at 380 nm, with another broad, red emission band at 650 nm [36]. The narrow UV emission peak generally originates because of band gap emission and the broad,

visible emission peak occurs because of various intrinsic defect emissions due to the recombination of

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holes and electrons which are trapped in the ionised oxygen vacancies [37]. PL-spectra intensity

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decreased significantly from the ZnO nanoparticles to ZnO/Ag and further reduced to the ZnO/Ag/GO nanocomposite. Low PL intensity suggests poorer recombination rates of photo-induced charge

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carriers [38-39]. Therefore, the modification of ZnO with Ag and GO decreases the recombination time of photo-induced charged electrons and holes. The slow recombination rate of electron-hole pairs

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is necessary for good photocatalysis.

3.1.3 UV-vis adsorption spectroscopy

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UV-vis adsorption spectroscopy is extensively used to investigate the optical characteristics of nanomaterials. The adsorption spectra of the as-prepared ZnO nanoparticles and binary ZnO/Ag and

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ternary ZnO/Ag/GO nanohybrid materials are shown in Fig. 2b. The adsorption edge values from the spectra were calculated for the ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposite at 370 nm, 380 nm and 398 nm, respectively, according to the method described by Provenzano et al. [40]. The broad shoulder observed in the ZnO/Ag/GO adsorption spectrum could be attributed to the π-π* transition and n-π* transition of GO nanosheets [28]. Band gap (Eg) values for the samples were calculated [16] using Equations 3 and 4:

1/2

(𝛼ℎ𝜐) = 𝐴(ℎ𝜐 − 𝐸𝑔 )

(3)

ℎ𝑐 1240 = λ λ

(4)

𝐸𝑔 =

With Plank’s constant (h), absorption coefficient (α), light frequency (υ), optical energy band gap (c), speed of light (Eg), wavelength (λ) and a constant (A). A Tauc plot (graph between (αhυ)2 vs energy) helped in calculating the band gap energies of the synthesised ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposite by creating a tangent on the graph (Fig. S1). The observed band gap values for the ZnO, ZnO/Ag and ZnO/Ag/GO nanomaterials, with the help of the Tauc plot,

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were found to be 3.4, 3.25, and 2.97 eV, respectively. The band gap value decrease with the

incorporation of Ag and GO into ZnO, from 3.4 to 2.97, implies the appearance of different energy states in nanocomposite. The narrowing of band gap is good for photocatalytic activity as energy

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demand is decreased to excite the electrons from ground-energy-level VB to high-energy-level CB.

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3.1.4 FTIR analysis

Fig. 3 shows the FTIR spectroscopy of the synthesised nanomaterials and helps to investigate the

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functional groups on the nanomaterial surface. The broad peaks appearing at 3392 and 1613 cm-1 for the ZnO nanoparticles are assigned to O-H stretching and bending, respectively from adsorbed water on the ZnO surface. The ZnO/Ag/GO also exhibits a broad band at 3210 cm–1, which corresponds to

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the vibration of O-H groups. The vibrational signatures present in the range of 2900-2800 cm-1 for all nanomaterials are due to aliphatic C-H stretching and indicate the presence of methyl and methylene

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groups. The strong vibrational bands at 1325 cm-1 and ~1612 cm-1 are attributed to the symmetric and asymmetric bands of acetate groups, respectively [41]. These signature bands reveal the presence of

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acetate residues on the ZnO surface. Furthermore, the ZnO/Ag/GO also has few characteristic vibrational peaks of GO at 1385, 1155 and 1026 cm-1, ascribed to O-H bending, epoxy C-O stretching, and alkoxy C-O stretching, respectively [42]. Vibrational signatures in the FTIR graph also confirm the successful construction of the nanomaterials.

3.1.5 XPS analysis Detailed chemical characterisation of ternary ZnO/Ag/GO nanocomposite was investigated through XPS measurements. The survey XPS spectra of the ternary ZnO/Ag/GO nanocomposite is shown in the supplementary information (Fig. S2) and the corresponding high-resolution XPS spectra of each element of the nanocomposite is displayed in Fig. 4. The Zn 2p XPS spectrum (Fig. 4a) displays the spin orbit doublet peaks of Zn 2p3/2 and Zn 2p1/2 at 1021.9 and 1044.9 eV, respectively, and is characteristic of the Zn2+ valence state of ZnO in the ZnO/Ag/GO nanocomposite [43]. The O 1s XPS

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spectrum (Fig. 4b) shows an asymmetric peak, which reveals more than one origin for O 1s. The XPS spectrum for O 1s can be steadily deconvoluted into three main Gaussian peaks, at 530.2, 531.6 and 532.9 eV. The XPS peak at lower binding energy (530.2 eV) is ascribed to the O2- valence state

present in ZnO and GO, while the peak at 531.6 eV is assigned to the oxygen vacancies and defects in

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the ZnO/Ag/GO nanocomposite [44]. These oxygen defects and vacancies can act as electron traps which help to increase the electron-hole charge-pair recombination time and thus enhance the

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photocatalytic characteristics [45]. The O 1s peak at 532.9 eV is credited for the chemisorbed and/or

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loosely bonded oxygen functionalities, e.g. carboxylic and hydroxyl groups on the ZnO/Ag/GO nanocomposite [46-47]. The C 1s spectrum (Fig. 4c) can also be deconvoluted into three peaks, at 284.6, 286.2 and 288.6 eV, which can be assigned, respectively, to C-C/C=C, C-O and COO carbon

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functional groups present on the GO nanosheets [48]. The COO peaks can also be originated from ZnO as the presence of acetate residues onto the surface. The Ag 3d spectrum (Fig. 4d) shows two

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sharp peaks, at 367.5 and 373.3 eV, which are for the Ag 3d5/2 and Ag 3d3/2 orbitals, respectively. The 3d doublet peaks for the Ag XPS spectrum differ with 5.8 eV, which indicates the presence of Ag as

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Ag0 on ZnO/Ag/GO [49]. 3.1.6 FESEM analysis

The structural morphology of ZnO and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposites were explored through FESEM and TEM analyses. Typical FESEM images of the as-prepared ZnO and binary ZnO/Ag and ternary ZnO/Ag/GO nanomaterials are displayed in Fig. 5. The ZnO exhibited mixed morphologies of smaller nanoparticles (40-160 nm) and nanorods with a length of ~5.5 µm and

a width of 100-250 nm (see Fig. 5a, b). Diverse morphologies of ZnO were observed due to the presence of acetate groups which are directing the growth of ZnO crystals [20]. In the ZnO/Ag FESEM image (Fig. 5c), various sizes of ZnO nanoparticles can be seen, with a decoration of tiny Ag nanoparticles. Fig. 5c also reveals that the ZnO particles’ surface is smooth, which implies that no etching of ZnO occurred during preparation. In this case, growth of ZnO and Ag nanoparticles were governed by the presence of various anionic species, such as COO- ions from the zinc precursor, NH4+ and OH– ions from the hexamethylenetetramine [9]. The FESEM image of the ZnO/Ag/GO

distributed onto the surface of the GO nanosheets (Fig. 5d). 3.1.7 EDS analysis

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nanocomposite clearly demonstrates that Ag and ZnO/Ag nanoparticles are homogeneously

The elemental distribution and composition of the ZnO/Ag/GO nanocomposite are illustrated by the

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EDS mapping and spectrum (Fig. 6a, b, respectively). Elements, viz. Zn, O, Ag, and C, were

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uniformly distributed throughout the ZnO/Ag/GO nanocomposite, as shown in Fig. 6b. It was noticed that Ag and ZnO nanoparticles were evenly distributed on C sheets. No extra elements were present in

3.1.8 HRTEM analysis

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the EDS spectrum, confirming that the sample was of high purity.

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For a better insight into the ZnO nanoparticles and the distribution of tiny Ag nanoparticles on ZnO and GO, HRTEM images have been conducted, as shown in Fig. 7. Small-sized ZnO nanoparticles

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were the focus of the TEM images, rather than nanorods. Fig. 7a depicts the non-uniform agglomerated ZnO nanoparticles carrying different shapes, viz. hexagonal and spherical, and having

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sizes less than 100 nm. The TEM image of Ag deposited on the ZnO exhibits the various sizes of the ZnO nanoparticles, with a decoration of tiny Ag nanoparticles, as depicted in Fig. 7b. This observation is consistent with the FESEM results, suggesting the formation of heterogeneous nanocrystals or a nanohybrid of ZnO/Ag. Fig. 7c shows the HRTEM image of the ZnO/Ag, depicting 0.24 nm d-spacing which agrees with the (111) lattice planes of cubic Ag. The average particle size of Ag nanoparticles was noticed to be around 4-30 nm. For the ZnO/Ag/GO, Ag nanoparticles were evenly distributed on flakes/sheets of GO which wrapped around the ZnO/Ag nanoparticles, as seen in

Fig. 7d. It can easily be observed from the HRTEM images that both ZnO/Ag and Ag are dispersed on the GO nanosheets. These heterojunctions in the ZnO/Ag/GO nanohybrid nanocomposite might increase charge separation and thus the photocatalytic efficiency of the photocatalyst. 3.2. Adsorption and photocatalytic degradation of naphthalene 3.2.1 Adsorption of naphthalene To examine the adsorption potential of ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposites as adsorbents for polycyclic aromatic hydrocarbon removal, 0.25 g.L-1

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concentration of adsorbent was added to the 50 mg.L-1 naphthalene aqueous solution. The mixture was continuously stirred for 20 minutes under dark conditions before light irradiation for interaction between adsorbent surfaces and naphthalene molecules. Sample aliquots were extracted at regular

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time intervals and filtered using syringe filters, to check the remaining naphthalene concentration in the water with the help of UV-vis adsorption spectra. Disappearance of the pungent naphthalene

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odour from the solution suggested the removal of naphthalene following adsorption onto the adsorbent surface. The changes in naphthalene concentration in the solution were observed on the

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basis of change in the intensity of the UV-vis absorbance peak at a 275 nm wavelength. The residual naphthalene concentration in the water was calculated using a naphthalene calibration UV-vis

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adsorption curve (Fig. S3). Fig. 8 shows the adsorption efficiency of the ZnO and binary ZnO/Ag and ternary ZnO/Ag/GO in terms of the remaining concentration of naphthalene in the solution with

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respect to contact time between adsorbent and adsorbate. Interestingly, naphthalene adsorption on all the adsorbents was found to be quick, more than 40%

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being adsorbed within 1 minute of exposure of the adsorbent surface to the naphthalene molecules in the simulated contaminated water. The ZnO and ZnO/Ag nanomaterials performed a similar adsorption efficiency and ~65% naphthalene was adsorbed within 5 minutes of the adsorption process and reached the adsorption-desorption equilibrium phase. However, the adsorption of naphthalene molecules reached up to ~80% within 20 minutes following interaction with the surface of the ZnO/Ag/GO nanocomposite material. Continuously increasing adsorption efficiency of ZnO/Ag/GO with time might be as a result of the enhanced active surface areas on the adsorbent surface due to

GO. Also, the GO skeleton in the ZnO/Ag/GO nanocomposite is enriched with π-electrons, which enhances the π-π interactions with the aromatic naphthalene rings and facilitates the adsorption process. 3.2.2 Adsorption kinetics of naphthalene Furthermore, to check the maximum adsorption capacity of ZnO/Ag/GO, adsorption experiments were executed over a span of time using 25 mg of adsorbent for 100 mL naphthalene aqueous solutions containing variable concentrations of naphthalene – 25, 50, 75 and 100 mg.L-1 (Fig. S4).

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This study helped to predict adsorption kinetics and mechanism using ZnO/Ag/GO as adsorbent for naphthalene adsorption following linear equations for pseudo-first-order, pseudo-second-order and intraparticle diffusion models. Kinetics study helps to express the rate-determining step for

ln(𝑄𝑒 − 𝑄𝑡 ) = ln 𝑄𝑒 − 𝑘1 𝑡

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contaminant removal from the solution. Equation 5 depicts the linear pseudo-first-order kinetics: (5)

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Where Qe and Qt are the number of naphthalene molecules (mg) adsorbed per gram of adsorbent at equilibrium (e) and time (t, minutes), respectively, and k1 is the pseudo-first-order rate constant

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(min-1). Qe was calculated using Equation 6:

𝑉 𝑚

(6)

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𝑄𝑒 = (𝐶𝑜 − 𝐶𝑒 ).

Where V is the volume (mL) of wastewater treated using m (weight, mg) of the adsorbent. Similarly,

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Equations 7 and 8 represent the linear pseudo-second-order kinetics and intraparticle diffusion

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models, respectively:

𝑡 1 𝑡 = + 2 𝑄𝑡 𝑄𝑒 𝑘2 . 𝑄𝑒

(7)

𝑄𝑡 = 𝑘𝑖𝑑 𝑡1/2 + 𝐶

(8)

Where k2 and kid are the pseudo-second-order rate constant (g mg−1.min−1) and intraparticle diffusion constant (mg g−1. min−1/2), respectively.

Fig. 9 represents the pseudo-first-order plot [ln(Qe - Qt) vs t], pseudo-second-order plot (t/Qt vs t) and intraparticle diffusion (Qt vs t1/2) plot for naphthalene adsorption on ZnO/Ag/GO. Coefficient of determination (R2) is used as controlling parameter to determine the best-suited kinetic model for the adsorption process. R2 values for the pseudo-first-order kinetic (0.75-0.91) and intraparticle diffusion models (0.75-0.90) were found to be relatively poorer for all naphthalene solutions than for that of the pseudo-second-order kinetic model (~0.99), revealing the best linear fitting for the pseudo-secondorder kinetic model. Low R2 values for the intraparticle diffusion plot indicate that diffusion is not the

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rate-determining step for naphthalene adsorption on ZnO/Ag/GO. Table 1 shows the kinetic parameters for pseudo-second-order kinetics, calculated using the slope and intercept values of the plots. Qe values for all the adsorbents calculated using the pseudo-second-order kinetic model are found to be very close to the experimental values, which again confirm that kinetics

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studies are well in agreement with pseudo-second-order kinetics.

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3.2.3 Adsorption isotherms of naphthalene

Adsorbent-adsorbate interaction and the feasibility of the adsorption process are performed by

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adsorption isotherm plots. For this study, the Langmuir and Freundlich adsorption isotherm models were chosen to study the adsorption of naphthalene on ZnO/Ag/GO. Langmuir adsorption follows the

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homogeneous adsorption by monolayer formation of adsorbate molecules on the adsorbent surface, with uniform energies of adsorption [50]. The Langmuir adsorption isotherm can be expressed using

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mathematical Equation 9, as follows:

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𝑄𝑒 =

𝑄𝑚 . 𝐾𝐿 . 𝐶𝑒 1 + 𝐾𝐿 . 𝐶𝑒

(9)

This equation can further be transformed into linear Equation 10, as: 𝐶𝑒 𝐶𝑒 = 𝑄𝑒 𝑄𝑚

+

1 𝑄𝑚 . 𝐾𝐿

(10)

Where KL (L.mg-1) is the Langmuir constant which explains the energy of adsorption and Qm is the maximum adsorption capacity (mg.g-1) of the adsorbent.

Freundlich adsorption, on the other hand, follows the assumption of multilayer adsorption of adsorbate molecules on the heterogeneous surface of the adsorbent, with different energies [51]. The Freundlich isotherm can be written as mathematical Equation 11, which can be linearized as Equation 12: 1/𝑛

𝑄𝑒 = 𝐾𝑓 . 𝐶𝑒

𝑙𝑛𝑄𝑒 = 𝑙𝑛𝐾𝑓

(11) +

1 𝑙𝑛𝐶𝑒 𝑛

(12)

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Where Kf (mg g-1) is the Freundlich adsorption constant, representing the adsorption capacity, and n is the degree of adsorption, representing adsorption intensity. 1/n values in between (0 < 1/n < 1) show the feasibility of the adsorption process, 1/n = 0 shows the irreversible behaviour and 1/n > 1 depicts the unfeasibility of the adsorption process. The value of 1/n and Kf can easily be calculated (Table 2)

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from the slope and intercept values of the Freundlich adsorption plot ln Qe versus ln Ce (Fig. 10b).

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Fig. 10 (a, b) represents the Langmuir and Freundlich adsorption plots for naphthalene adsorption using ZnO/Ag/GO. A linear adsorption isotherm plot with a high R2 value favours the adsorption

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isotherm model. The R2 value for the Langmuir isotherm plot was found to be 0.67, lower than that of the Freundlich isotherm plot (0.95). Therefore, it signifies that the adsorption process is controlled by

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the Freundlich isotherm model and the adsorbent exhibits heterogeneous surfaces for the adsorption [51–55]. Different morphologies in the ZnO/Ag/GO nanocomposite, i.e. ZnO nanoparticles and GO nanosheets, might provide the heterogeneous surfaces for the multilayer binding of naphthalene via

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adsorption. Table 2 represents the adsorption isotherm parameters for the Langmuir and Freundlich

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adsorption isotherms. Furthermore, the n value (0.21) supports the feasibility of the adsorption process.

Table 3 shows the adsorption capacity of ZnO/Ag/GO nanocomposite and its comparison with already studied adsorbents from previous reports for naphthalene adsorption.

3.2.4 Photocatalytic degradation of naphthalene Furthermore, the photocatalytic impact of ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposites on the photodegradation of naphthalene molecules in simulated contaminated water, was investigated. 100 mL of the 50 mg.L-1 naphthalene aqueous solution, with 25 mg of photocatalyst, was first stirred for 20 minutes under dark conditions to obtain the adsorptiondesorption equilibrium. Thereafter, it was exposed to 250 W UV-visible light for 30 minutes for the photodegradation of naphthalene molecules. Fig. 11a shows the photocatalytic potential of the ZnO

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nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposites for naphthalene degradation. Fig. 11b shows that the absorbance intensity of naphthalene decreased with the course of the photocatalytic reaction using the ZnO/Ag/GO nanocomposite. Fig. 11c depicts that the photocatalytic degradation efficiency of photocatalysts are in the order of

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ZnO < ZnO/Ag < ZnO/Ag/GO. The ZnO exhibits a wide band gap (~3.4 eV) and hence is not able to efficiently adsorb the visible spectrum of the solar light [20]. Ag nanoparticles in ZnO/Ag support the

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adsorption of visible light via surface plasmon resonance (SPR), to generate photo-excitation

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electrons and holes [16, 24]. Ag nanoparticles also support the enhancement in photo-oxidation by entrapping the photo-generated electrons and successfully transferring them to the CB of ZnO, leaving behind the holes, decreasing the recombination period of charge pairs [16, 65]. The

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ZnO/Ag/GO further exhibits an enhanced photocatalytic efficiency for photodegradation of naphthalene molecules in comparison to the ZnO and ZnO/Ag under identical conditions. After

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30 minutes of exposure to the visible light using ZnO as photocatalyst, the remaining concentration of naphthalene in the water was found to be 8.2 mg.L-1. However, using ZnO/Ag/GO, such amount of

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naphthalene molecules remaining in the water was noticed within 10 minutes of exposure to visible light. After 30 minutes’ photocatalytic treatment of naphthalene-contaminated water with a ZnO/Ag/GO catalyst, the remaining concentration of naphthalene in the water was less than 4 mg.L-1. GO nanosheets in ZnO/Ag/GO provide a platform for electrons to transfer to and promote the separation of photo-generated electron-hole charge pairs [66]. GO exhibits superior electrical conductivity and easily captures the electrons to store in the large π-π network of the graphene

nanosheets [67-68]. This increases the recombination period of charge pairs and improves the photocatalytic activity of the photocatalyst. Table 4 shows the photocatalytic efficiency of ZnO/Ag/GO nanocomposite compared to already studied photocatalysts from literature for naphthalene photocatalytic degradation. Among the tabulated photocatalysts, the developed ZnO/Ag/GO photocatalyst has better photocatalytic activity towards naphthalene due the presence of large number of active sites and delayed recombination time for electron-hole charge pairs.

The photo-degradation of naphthalene using ZnO/Ag/GO as photocatalyst was also confirmed using

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FTIR analysis of naphthalene and ZnO/Ag/GO after adsorption and after photocatalysis (Fig. 12). FTIR spectrum of naphthalene exhibits three characteristics peaks at 3045, 1506 and 775 cm-1,

corresponding to the aromatic C-H stretching, C-C stretching and C-H bending, respectively [56, 76].

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FTIR peaks at 3045, 1504 and 775 cm-1 in the ZnO/Ag/GO spectrum after adsorption confirms the

presence of naphthalene molecules on the surface of ZnO/Ag/GO nanocomposite. This reassures the

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adsorption of naphthalene on ZnO/Ag/GO. However, these peaks disappear in the FTIR spectrum of ZnO/Ag/GO after photocatalysis which proves that naphthalene molecules are not present on the

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surface after photocatalysis. Furthermore, UV-vis analysis of naphthalene contaminated solution after photocatalysis clearly confirms the absence of naphthalene molecules. Therefore, the absence of

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naphthalene molecule on the surface of photocatalyst (ZnO/Ag/GO) after photocatalysis evidences the photo-degradation of naphthalene during the course of photocatalytic reaction.

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3.2.5 Kinetics study of photodegradation of naphthalene The photocatalytic degradation rate of naphthalene can be studied using the pseudo-first-order or

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Langmuir-Hinshelwood (LH) kinetics models [51, 77]. LH kinetics is generally used to explain the rate of the heterogeneous catalytic processes. An integrated form of the LH kinetics model can be expressed as Equation 13, as follows: 𝐶𝑜 ln ( ) = 𝑘1 𝑡 𝐶𝑡

(13)

Where k1 = kTK = pseudo-first-order rate constant, while kT is the limiting rate constant of the photocatalytic reaction and K is the adsorption equilibrium constant of substrate onto the photocatalyst. The graph in Fig. 11c shows the plotting of ln (C0/Ct) at the Y-axis against t at the X-axis. The slope value helps to determine the value of k1. The values calculated using the LH kinetics model are tabulated in Table 5; R2 values for all the photocatalysts are near to 1. This shows that the LH kinetics model is in good relation to the photodegradation process and can be employed to study the kinetics.

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Higher reaction rate values (k1) for the ZnO/Ag/GO than for the ZnO and ZnO/Ag further explain the high performance of the catalyst. 50 mg.L-1 naphthalene was photodegraded in 30 minutes using the ZnO/Ag/GO nanocomposite.

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3.2.6 Reusability of photocatalyst

To check the stability and recyclability of the photocatalyst, recycling experiments of naphthalene

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photodegradation using ZnO/Ag/GO were conducted for five cycles. After each cycle, the catalyst was recovered by centrifugation and washed with distilled water several times to study its reusability

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and stability. Fig. 11d shows that from the first to the fifth cycle, there is not much difference in the degradation efficiency of the recycled photocatalyst, as it slightly decreased from 92% to 85%.

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The overall adsorption mechanism (Fig. 13) can be explained as follows: firstly (Step I), adsorption of naphthalene occurred on the surface of the ZnO/Ag/GO nanocomposite mainly via π-π interactions

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due to the aromatic characteristic of both naphthalene and GO; and later (Step II), adsorbed naphthalene photo-catalytically degraded in the presence of visible light. During photocatalysis,

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photo-generated holes and electrons participated in the production of active hydroxyl radicals and superoxide anion radicals [78], respectively, and consequently, these radicals could convert naphthalene into less harmful degraded products.

4. Conclusion In this study, ZnO-based nanocomposites (ZnO/Ag, and ZnO/Ag/GO) were synthesised for comparative photocatalytic study of naphthalene solution. As-prepared materials were characterised

by XRD, PL, UV-vis, FTIR, XPS, SEM and TEM measurements. These nanomaterials were firstly employed for the adsorption and then for photodegradation of naphthalene from water. A 0.25 mg.L-1 dosage of nanomaterial was used in the naphthalene-contaminated water as treatment. Comparison of adsorption and photocatalytic studies clearly reveal that ZnO/Ag/GO nanocomposite is the most active adsorbent and photocatalyst for the adsorption and degradation of naphthalene. Incorporation of GO significantly improves the adsorption potential of nanohybrid materials through π-π interactions. The adsorption process follows the pseudo-second-order kinetics and Freundlich adsorption isotherm. This suggests the presence of heterogeneous surfaces on the nanoadsorbent. High adsorption potential

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of ZnO/Ag/GO, with adsorption capacity 500 mg.g-1, eases the direct interaction of photo-generated electrons with adsorbed naphthalene molecules and hence facilitates quick photodegradation. GO nanosheets also delayed the recombination time of photo-induced electrons and holes suitable for

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photodegradation activities. Photocatalytic degradation of naphthalene in a solution under visible light followed pseudo-second-order kinetics. In this study, a nanocomposite was synthesised with enhanced

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photodegradation activity under visible light, which suggests a sustainable and cost-effective

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Acknowledgements

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treatment system for the remediation of polluted water.

The authors are thankful to the sponsors: the Water Research Commission (WRC, Project 2974) and the North-West University in South Africa. The authors are also grateful to DST/CSIR National

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Centre for Nano-Structured Materials, South Africa for providing facilities for the characterisation of

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materials. All the authors hereby declare that there is no conflict of interest pertaining to this paper.

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71. D. Liu, Z. Wu, F. Tian, B. Ye, Y. Tong, Synthesis of N and La co-doped TiO2/AC

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72. N. Mukwevho, N. Kumar, E. Fosso-Kankeu, F. Waanders, J. Bunt, S.S. Ray, Visible lightexcitable ZnO/2D graphitic-C3N4 heterostructure for the photodegradation of naphthalene,

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Desalin. Water Treat. 163 (2019) 286-296. 73. S. Kohtani, M. Tomohiro, K. Tokumura, R. Nakagaki, Photooxidation reactions of polycyclic aromatic hydrocarbons over pure and Ag-loaded BiVO4 photocatalysts, Appl. Catal., B. 58 (2005) 265–272. 74. S. Xia, L. Zhang, X. Zhou, M. Shao, G. Pan, Z. Ni, Fabrication of highly dispersed Ti/ZnO– Cr2O3 composite as highly efficient photocatalyst for naphthalene degradation, Appl. Catal., B. 176–177 (2015) 266–277.

75. X. Yang, H. Cai, M. Bao, J. Yu, J. Lu, Y. Li, Insight into the highly efficient degradation of PAHs in water over graphene oxide/Ag3PO4 composites under visible light irradiation, Chem. Eng. J. 334 (2018) 355–376. 76. B. P. A. George, N. Kumar, H. Abrahamse, S. S. Ray, Apoptotic efficacy of multifaceted biosynthesized silver nanoparticles on human adenocarcinoma cells, Sci. Rep. 8 (2018) 14368. 77. K. V. Kumar, K. Porkodi, F. Rocha, Langmuir–Hinshelwood kinetics: A theoretical study, Catal. Commun. 9 (2008) 82–84.

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78. E. Fosso-Kankeu, A. K. Mishra, Photocatalytic degradation and adsorption techniques

involving nanomaterials for biotoxins removal from drinking water, in: A. Grumezescu (Ed.), Water Purification. Academic Press, Elsevier. ISBN: 9780128043004,

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https://doi.org/10.1016/B978-0-12-804300-4.00009-5, 2017, pp. 323–354.

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Fig. 1. XRD patterns of the ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposite.

Fig. 2. Optical properties of the ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO

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nanocomposite: (a) PL, (b) UV-vis spectrum.

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Fig. 3. FTIR spectra of the ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposite.

Fig. 4. High-resolution XPS binding energy spectra of ZnO/Ag/GO: (a) Zn 2p, (b) O 1s, (c) C 1s and (d) Ag 3d

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Fig. 5. FESEM images of (a, b) ZnO nanoparticles and (c) binary ZnO/Ag and (d) ternary ZnO/Ag/GO

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nanocomposite.

Fig. 6. EDS (a) mapping and (b) spectrum of the ternary ZnO/Ag/GO nanocomposite

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Fig. 7. HRTEM images of (a) the ZnO nanoparticles and (b, c) binary ZnO/Ag and (d) ternary ZnO/Ag/GO

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nanocomposite.

Fig. 8. Residual naphthalene concentration in solution using 0.25 g.L-1 ZnO, ZnO/Ag and ZnO/Ag/GO as adsorbents for 100 mL, 50 mg.L-1 naphthalene-contaminated aqueous solution, with time

Fig. 9. (a) Pseudo-first-order adsorption kinetics, (b) pseudo-second-order adsorption kinetics and (c) intraparticle diffusion of 25, 50, 75, 100 mg.L-1 naphthalene solution using ZnO/Ag/GO (0.25 mg.mL-1).

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The solid red line in graph b shows the linear fitting.

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Fig. 10. Adsorption (a) Langmuir isotherm and (b) Freundlich isotherm plots for naphthalene adsorption using ZnO/Ag/GO nanocomposite (0.25 mg.mL-1) as adsorbent. The solid red line shows the linear fitting in

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the graphs.

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Fig. 11. (a) Photocatalytic degradation of naphthalene molecules using ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposites as photocatalysts. (b) Decrease in absorbance intensity of naphthalene during photocatalytic degradation using ZnO/Ag/GO. (c) Langmuir-Hinshelwood kinetics

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graph for photocatalytic degradation of naphthalene with ZnO, ZnO/Ag and ZnO/Ag/GO. (d)

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Recyclability experiments for five cycles using ZnO/Ag/GO as photocatalyst.

Fig. 12 FTIR spectra of naphthalene, ZnO/Ag/GO nanocomposite after adsorption and photocatalysis process.

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Fig. 13. Schematic showing the plausible adsorption-assisted mechanism

Table 1: Pseudo-second-order adsorption kinetic parameters for naphthalene solutions of varying concentrations using ZnO/Ag/GO nanocomposite (0.25 g.L-1) as adsorbent Pseudo-Second-Order Kinetic Adsorbents Parameters ZnO ZnO/Ag ZnO/Ag/GO Qe,exp mg.g-1 67.7 67.8 78.6 Qe,calc mg.g-1 68 66.67 79.36 k2, g min−1.L−1 0.043 0.066 0.0437 R2 0.99 0.99 0.99 50 mg/L Qe,exp mg.g-1 130.04 149.25 160 Qe,calc mg.g-1 131.5 152.2 162.6 k2, g min−1.L−1 0.0275 0.01 0.0278 R2 0.99 0.98 0.98 75 mg/L Qe,exp mg.g-1 217 240 244 Qe,calc mg.g-1 217.4 239 241.5 k2, g min−1.L−1 0.0112 0.0099 0.0074 R2 0.99 0.99 0.99 100 mg/L Qe,exp mg.g-1 300 325 340 Qe,calc mg.g-1 299.4 322 335.51 k2, g min−1.L−1 0.0096 0.0093 0.0079 R2 0.99 0.99 0.99 *Qe,exp and Qe,calc are the experimental and calculated amounts of naphthalene molecules adsorbed, respectively.

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Naphthalene Solution 25 mg/L

Table 2: Adsorption isotherm parameters for naphthalene adsorption using 0.25 g.L-1 of ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO nanocomposite materials as adsorbent Adsorbent ZnO

ZnO/Ag

ZnO/Ag/GO

Adsorption isotherm Langmuir R2 Qm, mg.g-1 KL, L.mg-1 R2 Qm, mg.g-1 KL, L.mg-1 R2 Qm, mg.g-1 KL, L.mg-1

0.2 425 1.5 X 10-3 0.83 170 0.037 0.67 500 0.0247

Freundlich R2 1/n Kf, (mg.g−1)(L.mg−1) R2 1/n Kf, (mg.g−1)(L.mg−1) R2 1/n Kf, (mg.g−1)(L.mg−1)

0.895 1.26 4.49 0.99 0.27 6.68 0.95 0.13 8.166

nanocomposite for naphthalene adsorption

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Reference This study 56 57 58 59 60 61 62 63 63 64

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Adsorption capacity, mg.g-1 500 119 24.75 142.68 5.734 63.6 200 46.641 85 300 106.24

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Adsorbent ZnO/Ag/GO Activated carbon dots Starch Modified Feo ZnS-NPs-AC Clay Activated Rice husk Boehmite Nano powders Mesoporous Organosilica (PMO) Activated Carbon (Carbon A) K2CO3 Activated Carbon (Carbon B) Al-MCM-41

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Table 3: Comparison of adsorption capacity of various adsorbents from literature with ZnO/Ag/GO

Table 4: Comparison of adsorption capacity of various photocatalysts from literature with ZnO/Ag/GO nanocomposite for naphthalene photocatalytic degradation Percentage Degradation, %

Photocatalysis reaction time, min

Reference

Fe3O4/H2O2

81.1

480

69

TiO2

66.3

170

70

0.001 La-N-TiO2/AC

93.5

120

71

ZnO/g-C3N4

84.5

240

72

Ag-BiVO4

30

480

73

Ti/ZnO-Cr2O3

90.2

240

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3 wt% GO/Ag3PO4

82.1

7

ZnO/Ag/GO

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Photocatalyst

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Table 5: Values of Langmuir-Hinshelwood kinetics model constants for the ZnO nanoparticles and binary ZnO/Ag and ternary ZnO/Ag/GO

ZnO/Ag

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ZnO/Ag/GO

Parameter R2 K1, m-1 R2 K1, m-1 R2 K1, m-1

Value 0.93 0.014 0.98 0.015 0.94 0.028

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Photocatalyst ZnO

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nanocomposites