Sunlight driven photocatalytic reduction of 4-nitrophenol on Pt decorated ZnO-RGO nanoheterostructures

Accepted Manuscript Sunlight driven photocatalytic reduction of 4-nitrophenol on Pt decorated ZnO-RGO nanoheterostructures Suneel Kumar, Vaidehi Pandit, Kaustava Bhattacharyya, Venkata Krishnan PII:

S0254-0584(18)30383-3

DOI:

10.1016/j.matchemphys.2018.04.113

Reference:

MAC 20608

To appear in:

Materials Chemistry and Physics

Received Date: 26 October 2017 Accepted Date: 30 April 2018

Please cite this article as: S. Kumar, V. Pandit, K. Bhattacharyya, V. Krishnan, Sunlight driven photocatalytic reduction of 4-nitrophenol on Pt decorated ZnO-RGO nanoheterostructures, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.04.113. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Sunlight driven Photocatalytic Reduction of 4-Nitrophenol on Pt Decorated ZnO-RGO Nanoheterostructures Suneel Kumar,a# Vaidehi Pandit,a# Kaustava Bhattacharyyab and Venkata Krishnana* a

School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi 175005, Himachal Pradesh, India.

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Email: [email protected]

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Abstract

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Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India.

Low photocatalytic efficiency, rapid charge recombination and limited light absorption in

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wide band gap materials like ZnO severely hinder their practical applications. In order to overcome the above mentioned drawbacks and improve the photocatalytic efficiency, we have developed a series of Pt loaded ZnO-RGO nanoheterostructures. The photocatalytic activity of these nanoheterostructures in different compositions, under natural sunlight irradiation, towards the reduction of 4-nitrophenol has been investigated. The prepared nanoheterostructures showed superior photocatalytic activity and composition having 5

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wt% of RGO and 2 wt% of Pt in ZnO (ZPG5) was found to exhibit the highest activity. Control experiments were performed on ZnO-RGO (ZG) nanocomposites without Pt to determine the role of Pt loading. The rate constant of 4-nitrophenol reduction reaction obtained using

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ZPG5 nanoheterostructure was 0.4203 min-1, which is about 30 folds higher than the rate constant catalyzed using bare ZnO NR (0.0142 min-1) and 6 folds higher than ZG5. Strong

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enhancement in the photocatalytic activity could be evidenced for the Pt loaded ZnO-RGO nanoheterostructures under natural sunlight irradiation, which could be attributed to improved charge separation, enhanced charge transfer across the Schottky barrier between ZnO and Pt and the high ability of RGO to adsorb 4-nitrophenol. The developed nanoheterostructures can be used as highly efficient sunlight activated heterogeneous photocatalysts for several real life applications.

Keywords: Nanoheterostructures; Schottky barrier; photocatalytic activity; 4-nitrophenol

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These authors contributed equally to this work. Page 1 of 27

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1. Introduction In the past few years, there has been increased discharge of phenol and its derivatives, from various industries, such as petrochemical, agrochemical and pharmaceutical, into environment [1, 2]. Most of these phenol derivatives, mainly 4nitophenol (4-NP) have been identified as toxic pollutants and can even be carcinogenic to

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human beings [3, 4]. Thus there is an urgent need to remove such pollutants from environment or to convert them into other useful chemical, by developing some technically viable processes. In this regard, the reduced analogue of 4-NP, i.e., 4-aminophenol (4-AP) has been found to be useful in the synthesis of analgesics and antipyretic drugs [5]. Thus the

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reduction reaction of toxic nitroaromatic compounds, like 4-NP is of great significance. Extensive research has taken place in past to remove various kind of pollutants from

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environment by using semiconductor based nanocomposites as efficient and Green materials [6, 7]. Among various semiconductors, TiO2 and ZnO have been explored in detail for environmental remediation applications [8-12], out of which ZnO is considered as more reliable material for pollutant removal because of its fast charge carrier mobility and prolonged electron life times in comparison to TiO2 [13].

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ZnO being a wide band gap (3.37 eV) semiconductor, can utilize only UV portion of solar spectrum, which constitutes only about 5% [14]. Therefore, it suffers from UV excitation only and low photocatalytic activity due to high recombination rate of photoinduced electron-hole pairs. Recently various successful attempts have been made in

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order to make ZnO visible light active to overcome the above mentioned shortcomings and to utilize the visible portion (43%) of solar spectrum [15-18]. These approaches include

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formation of nanocomposites with other semiconductors, carbon based materials and with noble metals to retard electron-hole recombination. Thus the separated charges (electrons and holes) can move to the reaction sites to activate the surrounding chemical species to promote catalytic activity [19]. Moreover, the low cost, non-toxic nature, photostability and abundance of ZnO makes it a highly desirable material for photocatalytic applications [14]. Carbon based two-dimensional (2D) materials, mainly graphene, have attracted considerable attention in the recent years and have garnered tremendous research interest because of its promising electronic and optical properties [14]. High specific surface area (2630 m2g-1), excellent thermal conductivity (5000 W m-1 K-1) and superior electron mobility of 2D graphene nanosheets makes it an ideal material for various applications, such as Page 2 of 27

ACCEPTED MANUSCRIPT energy generation [20], sensing [21], drug delivery [22] and environmental remediation [14, 23]. Due to high electron mobility (200000 cm2 V-1 s-1), graphene acts as excellent electron shuttling system and serve as electron transporting bridge to reaction sites in nanocomposites [20]. Furthermore, reduced graphene oxide (RGO) serves as excellent support to disperse and stabilize the semiconductor and other metal nanoparticles to form

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semiconductor-RGO binary or semiconductor-RGO-metal ternary heterojunction with welldefined interfaces, which facilitate the transport of charge carriers [24, 25]. The heterojunction formation of ZnO with RGO serve as center to separate photoexcited electron-hole pairs and significantly inhibit their recombination. Such heterojunction of RGO

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with semiconductor materials have attracted considerable attention due to their enhanced photocatalytic performance [14, 26]. In recent works of our group, we demonstrated the

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role of RGO in enhancing the photocatalytic performance of ZnO nanostructures for pollutant removal [14, 23]. However, in our understanding there are still more opportunities to enhance the photoactivity of ZnO-RGO based nanocomposite by incorporation of other component into the above mentioned binary heterojunction.

Noble metals like Au, Ag and Pt are one of potential choices to fabricate

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heterojunctions with enhanced photocatalytic performance for the degradation of nitrocompounds and other pollutants as per previous reports [27-30]. Among noble metals, Pt is one of promising candidate for heterojunction formation with semiconductors because of the enhanced photocatalytic performance of nanaocomposites by its presence [31, 32].

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The introduction of Pt in nanocomposites can enhance the photocatalytic performance (i) by inhibiting electron-hole recombination effectively by acting as a cocatalyst, which traps

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conduction band (CB) electrons of semiconductor, (ii) by the formation of Schottky barrier when Pt is coupled with ZnO, which promotes electron transfer across heterojunction, and (iii) the exposed facets of Pt also provides thermal catalytic sites for adsorbed and intermediate species to promote the catalytic activity [31, 33]. Furthermore, work function of metals is an important parameter, which affects and also determines the electron transfer direction in nanocomposites. Pt has a large work function of 5.93 eV which is greater than that of ZnO work function (5.2-5.3 eV) and hence signify the electron transfer from CB of ZnO to Pt when the metal act as cocatalyst in the nanocomposite [34]. Very recently, Hu et al. [31] have reported the Pt-ZnO nanocomposites with superior photocatalytic degradation of RhB. In addition, ZnO nanorod-Pt nanoheterostructures have Page 3 of 27

ACCEPTED MANUSCRIPT been reported by Wu et al. [35] with enhanced charge transfer across ZnO-Pt heterojunction for the degradation of dyes. Thus it has been well demonstrated that the combination of Pt with semiconductor is an effective strategy to tailor the properties of semiconductor-based nanocomposites by promoting interfacial charge transfer and by retarding the electron-hole recombination to make them more efficient for photocatalytic

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reactions.

Thus combination of Pt to ZnO-RGO to form ternary nanoheterostructure is expected to facilitate charge transfer across the heterojunction and improve the photocatalytic performance significantly. To the best of our knowledge, there has not been any report

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pertaining to ZnO-Pt-RGO ternary nanocomposites for environmental remediation applications. In this work, we have successfully synthesized ZnO nanorods (NR) and coupled

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them with RGO nanosheets and Pt nanoparticles to prepare ZnO-Pt-RGO ternary nanocomposites by using a facile hydrothermal method. The photocatalytic performance of these nanocomposites has been demonstrated for 4-NP reduction under natural sunlight irradiation. A mechanism describing the superior photocatalytic performance of these ternary nanocomposites in comparison to its constituent materials and the influence of Pt in

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enhancing the activity has been described in detail.

2. Experimental section 2.1. Chemicals

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For GO synthesis, graphite powder (crystalline, 300 mesh, 99%) was purchased from Alfa Aesar, India whereas sodium nitrate (NaNO3), sulphuric acid (H2SO4), potassium

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permanganate (KMnO4) and hydrogen peroxide (H2O2) were purchased from Merck, India. While zinc chloride (ZnCl2), sodium hydroxide (NaOH), were also supplied by Merck. Hydrochloric acid (HCl) used in synthesis were purchased from Fischer Scientific. Triton X100, chloroplatinic acid (H2PtCl6), sodium boron hydride (NaBH4) 1-hexanol and hexane by Merck, India. All chemicals were used as received without any further purification. Deionized water (18.2 MΩ-cm) used in synthesis was obtained from double stage water purifier (ELGA PURELAB Option-R7).

2.1.1. Synthesis of graphene oxide

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ACCEPTED MANUSCRIPT Graphene oxide (GO) was synthesized by modified Hummers method [36]. In brief, graphite powder (1 g), sodium nitrate (0.5 g) and conc. H2SO4 (24 mL, 18 M) were mixed in a beaker at 0oC using a magnetic stirrer. This was followed by KMnO4 (3.0 g) addition slowly with vigorous stirring. Then the reaction mixture was kept in an oil bath at 35oC for two hours, with continuous stirring. After this, water (45 mL) was added to terminate the

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reaction, which will increase the temperature up to 90oC. Then the reaction mixture was diluted and treated with H2O2 (4.5 mL, 35%) followed by cooling and washing with HCl and water (1:10). At last, the mixture was sonicated (40 kHz, 500 W) and centrifuged (4500 rpm)

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for 15 min repeatedly to get the GO.

2.1.2. Synthesis of ZnO Nanorods

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ZnO nanorods (NR) were synthesized by following the same hydrothermal procedure as reported in one of our previous works [37]. In brief, 0.2 M solution of ZnCl2 and 0.5 M solution of NaOH were prepared in ethanol. 35 mL of prepared NaOH solution was taken in a beaker and to this solution 5 mL of ZnCl2 was added with vigorous stirring. The resultant suspension was transferred into 50 mL Teflon-lined stainless steel autoclave, sealed tightly

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and maintained at 180oC for 12 h. After the reaction time completion, it was allowed to cool down to room temperature naturally, followed by washing thrice with ethanol and water. The final white product was obtained after drying in an oven at 60oC.

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2.1.3. Synthesis of ZnO-RGO Nanocomposites ZnO-RGO nanocomposites were synthesized by a reported hydrothermal route [38].

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In short, the as prepared ZnO NR (0.2 g) was dispersed in ethanol:water mixture of 1:1 (40 mL) by sonication. This was followed by the addition of GO to the reaction mixture and stirred for 2 h followed by hydrothermal treatment for 12 h at 180˚C. Then, the obtained mixture was washed with ethanol and dried at 60˚C. Grey color powder of ZnO-RGO was obtained as final product. During the hydrothermal treatment, GO was reduced to RGO. By varying the amount of GO added, different compositions were prepared with 1 wt%, 3 wt%, 4 wt%, 5wt%, 6wt% and 10 wt% of GO in ZnO, and these binary nanocomposites were labelled as ZG1, ZG3, ZG4, ZG5, ZG6 and ZG10, respectively.

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ACCEPTED MANUSCRIPT Pt nanoparticles were prepared by following a reported procedure [39]. In short, 10 mL microemulsion were prepared initially as 8% - Triton X-100 (862.5 µL), 0.027% - Water (2.5 µL), 0.027% - 1-hexanol (2.5 µL) and 91.946% - hexane (9195 µL). This was followed by the addition of H2PtCl6 (2.9 µL in 5 mL water) solution drop wise and reducing agent NaBH4 drop wise under vigorous stirring for 20-30 min. The obtained final product was washed

2.1.5. Synthesis of ZnO-Pt-RGO Ternary nanoheterostructures

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with deionized water three times while centrifuging at 9000 rpm for 20 min each time.

First of all, as prepared ZnO-RGO nanocomposites were dissolved in ethylene glycol

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(EG) by ultrasonication. Then it was kept on stirring followed by heating at 78oC. Then H2PtCl6 (27 µL in 1.5 mL of EG) was added and stirred for 2 h at 78oC followed by cooling and

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centrifugation for 30 min at 8000 rpm. The obtained precipitate was washed with deionized water and dried at 70oC overnight. Dark colored ZnO-Pt-RGO nanoheterostructures were obtained. In all ZnO-RGO nanocomposites, 2wt% of Pt was used to form the ternary nanoheterostructures. After the formation of ternary nanoheterostructures, samples having 1 wt%, 3 wt%, 4 wt%, 5wt%, 6wt% and 10 wt% of GO were labelled as ZPG1, ZPG3, ZPG4,

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ZPG5, ZPG6 and ZPG10, respectively.

2.2. Materials characterization

X-ray diffraction (XRD) measurements were performed using the Rigaku Smart Lab 9

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kW rotating anode x-ray diffractometer with Ni-filtered Cu Kα irradiation (λ = 0.1542 nm) at 45 kV and 100 mA in 2θ ranging from 10o - 80o with a scan rate of 2o per minute with

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stepping size of 0.02o. X-ray photoelectron spectroscopic (XPS) measurements were done using the SPECS instrument with a PHOBIOS 100/150 delay line detector (DLD) with 385 W, 13.85 kV and 175.6 nA (sample current). We have used Al Kα (1486.6 eV) dual anode as the source with pass energy of 50 eV. As an internal reference for the absolute binding energy, the C-1s peak (284.5 eV) was used. Initially, the XPS unit was calibrated using Au 4f7/2 line at 83.98 eV from a specimen of Au film and we got a value of 83.977 eV. The data obtained from the instrument was processed using the CASA software. Raman spectroscopic measurements were performed using Horiba LabRAM high resolution UV-VIS-NIR instrument using 633 nm laser. Fourier transform infrared (FTIR) spectra were collected by using Agilent K8002AA Carry 660 instrument. Morphology of the materials was Page 6 of 27

ACCEPTED MANUSCRIPT characterized by using field emission scanning electron microscope (FESEM) FEI Nova Nano SEM-450 and high resolution transmission electron microscope (HRTEM) FEI Tecnai G2 20 Stwin microscope operating at 200 kV. Energy dispersive x-ray spectra (EDAX) were obtained using the same HRTEM instrument. The UV-visible absorption spectra were recorded using Shimadzu UV-2450 spectrophotometer in the wavelength range 200 nm to 800 nm. Optical

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properties were analyzed by UV-vis diffuse reflectance spectroscopy (DRS) using Perkin

polymer was employed as internal reflectance standard.

2.3. Evaluation of photocatalytic activity

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Elmer UV/VIS/NIR Lambda 750 spectrophotometer in which polytetrafloroethylene (PTFE)

The photocatalytic activity of the prepared nanoheterostructures were investigated

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by studying the photocatalytic reduction of 4-NP under natural sunlight irradiation having an intensity of approximately 8.2 X 104 lux, measured by LX-101A digital luxmeter. In a typical experiment, 4-NP (14 mL, 0.1 mM) solution was added to a conical flask along with a freshly prepared aqueous solution of

NaBH4 (1 mL of 0.1M) . After this 1 mL of the

nanoheterostructure solution (2 mg dispersed in 1 mL water) was added to the conical flask.

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The conical flask with all the components was kept on a magnetic stirrer under natural sunlight for 7-10 min and the supernatant was collected at regular intervals of one min in the Eppendorf tubes covered with aluminum foil. The spectra of the supernatant was

to 4-AP.

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recorded using UV-Vis spectrophotometer to monitor the photocatalytic reduction of 4-NP

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3. Results and discussion

3.1. Synthesis and structural studies In this work, ZnO-Pt-RGO ternary nanoheterostructures were prepared by adopting

hydrothermal synthesis method as illustrated in Scheme 1. The amount of RGO were varied as 1 wt%, 3 wt%, 4 wt%, 5wt%, 6wt% and 10 wt% to form different compositions, while the amount of Pt was kept fixed as 2wt% in all nanoheterostructures as optimized amount based on previous report [40].

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Scheme 1. Schematic illustration of the hydrothermal synthesis of ZnO-Pt-RGO (ZPG) ternary nanoheterostructures.

As representative examples, the material characterizations performed on ZG5 and ZPG5 nanoheterostructures are discussed in detail. The crystal phase of the ternary nanoheterostructure and binary nanocomposite along with bare GO and ZnO samples were analyzed by x-ray diffraction (XRD) and data has been presented in Figure 1. Graphene oxide

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(GO) shows characteristic diffraction peak around 2θ = 10o, which could be assigned to (001) reflection plane. The XRD pattern of the ZnO NR shows diffraction peaks at 2θ values of 31.8, 34.4o, 36.3o, 47.5o, 56.6o, 62.9o, 66.4o, 68.0 o, 69.1o and 77.0o which can be indexed to

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(100), (002), (101), (102), (110), (103), (200), (112), (201) and (202) crystal planes, respectively (JCPDS, 89- 0501) [6]. This confirms that the synthesized nanostructure is free

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of impurities as it does not contain any additional peaks other than ZnO peaks. The XRD patterns of ZG5 and ZPG5 nanoheterostructures exhibit the same diffraction peaks as ZnO. There are no characteristic peaks of Pt, which could be attributed to the low concentration of Pt in the samples. It is noteworthy to mention that the intensity of the diffraction peaks decreases in the case of both the nanoheterostructures, which could be ascribed to the small decrease in the crystallinity of material with addition of RGO as already evidenced in literature [14]. Notably, no typical diffraction peaks belonging to RGO are observed in the ZPG5 nanocomposite, which is due to the relatively low content of RGO in nanocomposite as compared to ZnO NR.

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Figure 1. PXRD spectra of GO, ZnO NR, ZG5, and ZPG 5 nanoheterostructures. Raman spectroscopic measurements of GO, ZnO, ZG5 and ZGP5 samples were performed and presented in Figure 2. GO shows two characteristic Raman bands around

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1345 cm-1 and 1592 cm-1 which are assigned as D-band and G-band, respectively. D-band appears due to disordered carbon atoms, while G-band appears because of vibrations of ordered sp2 carbon atoms [23]. Raman spectra of ZnO NR exhibit all the characteristic bands pertaining to its Wurtzite hexagonal crystal structure. These bands correspond to E1 (LO) at

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583 cm-1, E2 at 435 cm-1, A1 (TO) at 382 cm-1 and A1 vibration modes at 331 cm-1. Both the binary and ternary nanoheterostructures (ZG5 and ZPG5) also exhibit Raman bands

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corresponding to ZnO and RGO, which confirms the successful fabrication of the nanoheterostructures. Moreover

small shifting in the D-band and G-band have been

observed in both the nanoheterostructures which signify the electronic interactions and reduction of GO to RGO during the hydrothermal synthesis process [14]. Furthermore, the decrease in ID/IG value which is a measure of defects in carbon materials, (for GO = 1.01, and for ZGP5 nanoheterostructure = 0.98) confirms the removal of oxygen containing functional groups and hence reduction of GO to RGO.

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Figure 2. Raman spectra of (a) ZnO NR, (b) GO, (c) ZG5, and (d) ZPG 5 nanoheterostructures. FTIR studies have been performed in 400-4000 cm-1 range on GO, ZnO NR, ZG5 and ZPG5 samples in order to investigate the presence of various functional groups and data have presented in Figure 3. GO shows the broad peak between 3000–3500 cm-1 in the high

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frequency area together with a sharp peak at 1627 cm−1 corresponding to the stretching and bending vibration of OH groups of water molecules adsorbed on GO surface. The peaks at 1403 cm-1 and 1724 cm-1 can be attributed to O-H deformation vibrations and carbonyl stretching vibrations, respectively. Finally absorption peaks at 1225 cm-1 and 1020 cm-1

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corresponds to epoxy C-O-C bond and C-O stretching vibrations. The presence of these oxygen containing groups are consistent with literature reports and reveals that the

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graphite has been oxidized during synthesis, which is responsible for the strong hydrophilicity of GO [23]. In ZG5 and ZGP5 nanoheterostructures, the intensity of oxygen containing functional groups decreases, which also confirms the reduction of GO to RGO. FTIR spectra of ZnO NP exhibit O-H stretching vibrations in 3000-3500 cm-1 range, while Peaks at 1570 cm-1 and 1021 cm-1 are due to C-O stretching vibrations. In addition to this small peaks at 920 cm-1 and 658 cm-1 can also be evidenced, which correspond to the characteristics peaks of Zn-O stretching vibrations [14].

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Figure 3. FTIR spectra of GO, ZnO NR, ZG5, and ZPG5 nanoheterostructures.

3.2. Morphological and compositional studies

The surface morphology of the prepared samples has been analyzed by scanning

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electron microscopy (SEM) analysis and the obtained results have been presented in Figure 4. GO exhibit 2D sheet like morphology while ZnO clearly illustrate the rod-like morphology with size in range of 100 nm (Figure 4a, b). SEM image of ZG5 nanocomposite can be seen in Figure 4c which shows the presence of ZnO NR over RGO nanosheets to form intimate

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contact at ZnO-RGO heterojunction. In ZPG5 nanoheterostructure also, the ZnO NR are very well dispersed over RGO nanosheets to form ZnO-RGO heterojunction. Due to small size of

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Pt nanoparticles, they cannot be evidenced in SEM micrographs.

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Figure 4. SEM images of (a) GO sheets, (b) ZnO NR, (c) ZG5 and (d) ZPG5 nanoheterostructures.

In order to further investigate the nanostructured morphology of the samples, transmission electron microscopy (TEM) analysis has been performed and the obtained

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images are presented in Figure 5, which shows that the 2D structure of GO sheets with obvious wrinkles still retained after the hydrothermal treatment of GO to RGO during synthesis process. Figure 5b shows the TEM image of Pt nanoparticles which reveals its spherical morphology and size in the range of 2-4 nm. Figure 5c, d & e shows TEM results of

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ZnO NR which presents the rod-like morphology of ZnO, which is in agreement with its SEM results. HRTEM image of ZnO NR (Figure 5d) shows d-spacing equals to 0.28 nm which

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corresponds to (100) diffraction plane of hexagonal ZnO. Figure 5f presents the TEM image of ZG5 nanocomposite which confirms the heterojunction formation between ZnO-RGO in which ZnO NR can be seen well dispersed on the surface of RGO nanosheets. TEM image of ZPG5 nanoheterostructure (Figure 5g) clearly reveal that ZnO NR adhered to RGO nanosheets with Pt-embedded-in-ZnO-RGO nanocomposite. HRTEM image of ZPG5 nanoheterostructure in Figure 5h shows lattice fringes having d-spacing 0.28 nm confirms the presence of hexagonal ZnO, while the d-spacing of 0.22 nm corresponds to characteristic (111) diffraction plane of Pt nanoaprticles dispersed over ZnO NR and RGO nanosheets. The presence of all constituent elements in ZnO (Zn, O), Pt, ZG5 (Zn, O, C) and ZPG5 (Zn, O, Pt, C) have been confirmed by energy dispersive X-ray spectroscopy analysis Page 12 of 27

ACCEPTED MANUSCRIPT (EDAX) and corresponding data has been presented in Figure S1 in supporting information. Furthermore, the EDAX results for ZPG5 nanoheterostructure have been supported by its

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elemental mapping analysis given in Figure S2 (refer supporting information).

Figure 5. TEM images of (a) GO sheets, (b) Pt nanoparticles (c, d & e) ZnO NR, (f) ZG5, (g) ZPG5 and (h) HR image of ZPG5 nanoheterostructure.

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ACCEPTED MANUSCRIPT X-ray photoelectron (XPS) spectroscopic measurements were performed in the region of 0 - 1400 eV with representative samples ZG5 and ZPG5 in order to investigate the chemical composition, electronic state of constituent elements and their interactions between Pt, RGO and ZnO in the nanoheterostructures. The survey scan spectrum of ZG5 and ZPG5 shows the presence of distinct Zn-2p, O-1s, C-1s and Pt-4f peaks in

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nanoheterostructures (Figure 6a). Figure 6b shows the presence of two binding energy peaks at 70.5 eV and 74.0 eV, which corresponds to the Pt-4f7/2 and Pt-4f5/2, respectively. These two binding energy peaks signify the presence of metallic Pt(0) species in ZPG5 nanoheterostructure. The presence of other binding energy peaks after 74.0 eV could be

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attributed to the other states of Pt mainly Pt(II), which could be attributed to incomplete reduction of Pt species during synthesis process. XPS spectrum of ZnO in ZG5

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nanocomposite (Figure 6c) exhibit two distinct binding energy peaks at 1044.5 eV and 1021.4 eV, which could be ascribed to the Zn-2p1/2 and Zn-2p3/2, respectively, confirming the Zn2+ oxidation state in nanocomposite in corroboration with the previous literature [14]. The binding energy at peak at 1034 eV presents the typical satellite peak of ZnO [14]. The O-1s XPS spectrum in Figure 6e exhibit two binding peaks at 532.8 and 529.2 eV,

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which corresponds to the lattice O contribution from both Zn and RGO and nonstoichiometric O peak, respectively. Figure 6d and 6f shows XPS spectra for Zn-2p and O-1s species in ZPG5 nanoheterostructure. The Zn-2p exhibits binding energy peaks at 1023.5 eV and 1046.5 eV, showing a positive shift in the binding energy as compared with the ZG5

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nanocomposite. The higher value of the binding energy could be attributed to electronic interactions between Zn (ZnO) with Pt and RGO. The deconvolution of Zn-2p3/2 peak leads to

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two peaks (Figure 6d insert) at 1023.5 eV and 1019.5 eV, respectively. This in all probabilities indicates that Zn is present in two electronic environments. The peak at 1023.5 eV is representative peak of Zn2+ and signifies its interaction with lattice O atoms from ZnO and RGO in the nanoheterostructures. However, the binding energy peak at 1019.5 eV could signify the Zn2+ interaction with RGO [41]. The O-1s spectrum of ZPG5 shows XPS peaks typically at 529.8 eV and 531.1 eV. The nanoheterostructure therefore shows the presence of two different types of O species. The 531.1 eV peak would be assigned to that of the O from ZnO (lattice) whereas the other at 529.8 eV signify the nonstoichiometric peak could be a result of electronic interaction between ZnO and RGO and Pt to form intimate interface, which promotes charge transfer across heterojunction. Page 14 of 27

ACCEPTED MANUSCRIPT The C-1s (Figure 6g and 6h) high resolution XPS spectrum shows four binding energy peaks at 282.8 eV, 284.4 eV, 285.9 eV and 287.9 eV, which could be attributed to sp2 hybridized C atoms, sp3 and sp2 (C-C, C=C bonds in RGO), C-OH bonds and C=O groups, respectively. Also the low intensity of oxygen containing functional groups peak (mainly 287.9 eV peak) signify reduction of GO to RGO during hydrothermal synthesis. Therefore,

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the XPS studies suggest that the ZnO does have an electronic interaction with the host RGO and do definitely support the formation of the RGO from the GO. The ZnO-Pt-RGO nanoheterojunction could be used as an effective photocatalytic material as the interaction

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between them would lead to alteration in the lifetime of the e--h+ pairs and their stability.

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Figure 6. X-ray photoelectron spectra (XPS) for (a) Survey spectrum (ZG5 and ZPG5), (b) Pt-4f (ZPG5), (c) Zn-2p (ZG5), (d) Zn-2p (ZPG5), (e) O-1s (ZG5), (f) O-1s (ZPG5), (g) C-1s (ZG5) and (h) C-1s (ZPG5), respectively. Page 16 of 27

ACCEPTED MANUSCRIPT 3.3. Optical property studies Diffuse reflectance spectroscopy (DRS) measurements were performed in 200-800 nm range with all samples to determine the band gap of ZnO NR in bare sample as well as in the binary and ternary nanoheterostructures. As it is well known that the composite

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formation with RGO could significantly affects the band gap of semiconductor material by surface interaction between semiconductor and functional groups on the surface of RGO [42]. Furthermore the optical band gap of the material is a very important parameter which determines the light harvesting nature of nanocomposite material. Figure 7 presents the

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DRS spectra of GO, ZnO, ZG5 and ZPG5 nanoheterostructures, it is clear from spectra that ZnO NR shows predominant absorption in UV region as indicated by its absorption edge

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around 385 nm which correspond to the band gap energy of 3.24 eV. There is no fundamental absorption in visible region for ZnO NR. These observation on the absorption edge and band gap of ZnO NR are consistent with literature reports [37]. Unlike ZnO, the GO shows absorption in whole range of 200-800 nm and also improves the light absorption of ZnO NR towards visible region as evidenced from absorption plot wherein one can clearly

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observe the enhanced absorption in the visible region for nanoheterostructures. Furthermore with 5 wt% of RGO in ZG5 nanocomposite, its absorption edge shifts to around 395 nm which corresponds to the band gap value of 3.13 eV as can be seen in KubelkaMunk plots in Figure 7c. This narrowing of band gap can be ascribed to the chemical

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bonding interactions between ZnO and carbon sites in GO as observed in one of our previous report [14]. It is also noteworthy to mention here that with Pt doping in ZnO-RGO

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nanocomposite, the absorption edge shifts to around 400 nm and further narrowing in band gap has been observed for ZPG5 nanoheterostructure (Figure 7d). It has been well reported that unlike Au, and Ag, the hydrogenation catalysts like Pt and Pd don’t exhibit surface Plasmon resonance bands in UV-visible region [43]. This could be attributed to the damping effect of d-d transitions in these metals, which wash out the free electron contribution to the dielectric function [43]. It has also been investigated that loading of these metals cause the localized energy levels below the CB of semiconductor, where the VB electrons can be excited at higher wavelengths [40]. Hence this is responsible for red shift in ZPG5 nanoheterostructure which is evidenced by decreased band gap value (3.02 eV) in Figure 7d. Finally it can be inferred that fabrication of ternary nanoheterostructures comprising of Page 17 of 27

ACCEPTED MANUSCRIPT ZnO-Pt-RGO has resulted in the formation of an efficient heterojunction with improved optical properties with enhanced utilization of visible light along with UV light for

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photocatalytic applications.

Figure 7. (a) UV-vis diffuse reflectance spectra (DRS) of GO, ZnO NR, and ZPG nanocomposite. Plot of transformed Kubelka-Munk function vs the energy of light: (b) ZnO

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NR, (c) ZG5 and (d) ZPG5 nanoheterostructures. UV-visible spectroscopic analysis were performed with GO, ZnO, Pt nanoparticles,

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ZG5 and ZPG5 samples (Figure S3 in supporting infromation) to further confirm their successful preparation. UV-visible absorption spectra of GO exhibited maximum intensity absorption peak at 227 nm which could be attributed to the π-π* transitions of aromatic C-C bonds, and a small shoulder peak at 303 nm corresponds to the n-π* transitions oxygen containing functional groups such as carbonyl (C=O). Bare ZnO NR shows prominent absorption peak at 372 nm which is in agreement with previous reports [37]. UV-visible spectra of Pt nanoparticles exhibit an absorption peak at 272 nm, which is the signature peak of Pt nanoparticles as per literature reports [8]. It is noteworthy to mention here that the

UV-vis

absorption

spectra

of

binary

nanocomposite

(ZG5)

and

ternary

nanoheterostructure (ZPG5) also exhibit the characteristic absorption peak of ZnO (372 nm), Page 18 of 27

ACCEPTED MANUSCRIPT and in addition to this an absorption band in visible region around 500-600 nm can be seen, which may be attributed to RGO and Pt incorporation. This appearance of absorption band in visible region indicates the shift of absorption maximum towards visible region which is significant to utilize both UV and visible region of solar spectrum in photocatalytic reaction. Hence UV-visible spectroscopy results also compliments the DRS Kubelka–Munk plot results

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which shows decrease in band gap of bare ZnO after introduction of RGO and Pt to form binary and ternary nanoheterostructures.

3.4. Photocatalytic reduction of 4-nitrophenol

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The photocatalytic activity of all prepared samples were investigated by studying the reduction of 4-NP in presence of NaBH4 as model reaction using natural sunlight. This

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reaction describes the importance of the prepared nanoheterostructures in the reduction of nitroaromatic compounds. Photocatalytic reduction reaction was investigated by monitoring the concentration change of the characteristic absorbance peak of 4-NP, which is centered at 400 nm. The original UV-vis absorption peak of 4-NP is centered at 317 nm shifted to 400 nm after the addition of freshly prepared NaBH4 (Figure 8a), which indicates the formation

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of p-nitrophenolate ions as per literature reports [44]. Figure 8b presents the UV-vis spectra of 4-NP in the presence of NaBH4 only and reaction was monitored after 2 h. It is very clear from spectra that no reduction of 4-NP takes place with NaBH4 because it cannot reduce the nitrophenolate ions in the absence of catalyst. When ZG5 and ZPG5 nanoheterostructures

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were used as catalyst and the reaction mixture was irradiated under natural sunlight, the absorbance peak at 400 nm starts decreasing in intensity along with the concomitant

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increase in peak at 300 nm within 7 min of irradiation (Figure 8c, d). This absorbance peak at 300 nm corresponds to the 4-aminophenol (AP), reduction product of 4-NP. After 7 min, the peak due to nitroaromatic compounds at 400 nm was no longer observed signifying the successful photocatalytic reduction of 4-NP to 4-AP. The reduction efficiency (%) of 4-NP with respect to time was calculated according to following equation: [14] Reduction efficiency % = 1 −

C X 100 C

Where C0 represents the initial concentration of 4-NP at t=0 and is equivalent to its absorbance, whereas C is the concentration at different irradiation intervals and is also equivalent to the corresponding absorbance. Page 19 of 27

ACCEPTED MANUSCRIPT When the reaction mixture was irradiated under natural sunlight and reaction was monitored regularly at an interval of 1 min, almost 92% photoreduction of 4-NP to 4-AP was achieved with ZPG5 nanoheterostructure (Figure 9a). Figure 9b presents the –ln C/C0 vs time plot, which can be fitted to the pseudo-first order reaction kinetics. The photocatalytic reduction reaction was performed with ZnO, ZPG1, ZPG3, ZPG4, ZG5 and ZPG10

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nanoheterostructures and their obtained rate (%) can be seen as comparative histogram in Figure 9c. In addition to this, the rate constant calculated for all nanoheterostructures for this photoreduction is presented in form of bar graph in Figure 9d. Thus the highest photocatalytic performance was evidenced for ZPG5, which could be attributed to the

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optimized content of RGO in the nanoheterostructure. Samples having more than 5wt% of RGO, shows decreased photocatalytic activity due to shading effect by RGO, which hinder

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the light absorption by nanoheterostructures and hence only fewer number of charge carriers are produced in ZPG6 and ZPG10 catalysts. This directly affects the photocatalytic activity by reducing the charge transfer across ZnO-Pt-RGO heterojunction and moreover excess of RGO content can act as recombination center for photogenrated charge carriers [45]. This explains the reason behind decreased activity of ZPG6 and ZPG10 in comparison to

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ZPG5 nanoheterostructures. The reaction kinetics for this photocatalytic reduction can be fitted into pseudo first order kinetics as mentioned above. The pseudo first order rate constant (k), half life time of reaction (t1/2) and linear regression coefficient (R2) for all the

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catalysts under sunlight irradiation are presented below in Table S1.

It is noteworthy to mention here that rate constant for reaction catalyzed by ZPG5

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nanoheterostructure is 0.4203 min-1, which is about 30 folds higher than rate constant for reaction catalyzed by bare ZnO NR (0.0142 min-1) and 6 folds higher than ZG5. Hence the photocatalytic results clearly indicate the superior activity of the ternary ZPG5 nanoheterostructure in comparison to their binary counterparts and bare samples. These results also signify the active role of noble metal Pt in small optimized amount (2wt %) in ZnO-RGO nanocomposites by forming Schottky barrier with ZnO, which facilitates the charge transfer across ZnO-Pt-RGO heterojunction making the reduction process feasible and quick (within 7 min). Control experiments were also performed under dark conditions with representative samples (ZG5 and ZPG5 nanoheterostructures) and ZnO to check the role Page 20 of 27

ACCEPTED MANUSCRIPT played

by

sunlight

irradiation

in

accelerating

the

catalytic

activity

of

the

nanoheterostructures (Figure 9e, f). It could be evidenced from Figure 9e, f that the binary and ternary nanoheterostructures show very small catalytic activity under dark conditions, the activity is enhanced drastically when the reactions are performed under natural sunlight irradiation. Specifically, the activity of ZPG5 nanoheterostructure is about 20 fold higher

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under sunlight irradiation in comparison to dark conditions. Scheme 2 presents the possible catalytic and photocatalytic reduction of 4-NP to 4-AP with NaBH4, ZG5 and ZPG5

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nanoheterostructures.

Figure 8. Time dependent UV-visible spectra of (a) Pure 4-NP and 4-NP + NaBH4, (b) 4-NP +

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NaBH4 in the absence of catalyst, (c) ZG5 + 4-NP + NaBH4 and (d) ZPG5 + 4-NP + NaBH4 under natural sunlight irradiation.

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Figure 9. (a) Kinetic curves for photocatalytic reduction of 4-NP by bare ZnO and all prepared

ZPG

nanoheterostructures,

including

ZG5

(representative

binary

nanoheterostructure), (b) corresponding –ln (C/C0) vs time curve for photocatalytic reduction of 4-NP, (c) histograms showing comparative photocatalytic reduction efficiency (%) of 4-NP in the presence of varying compositions of the nanoheterostructures, (d) bar graph showing rate constant values for all photocatalysts, (e) kinetic curves for catalytic reduction of 4-NP by control and representative samples (ZG5 and ZPG5) and (f) corresponding –ln (C/C0) vs time curve for catalytic reduction of 4-NP.

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Scheme 2. Schematic representation of possible catalytic and photocatalytic reduction of 4NP in presence of NaBH4, ZG5 and ZPG5 nanoheterostructures.

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It is noteworthy to mention here that suitable CB and VB potentials of ZnO plays crucial role in enhanced photocatalytic activity of nanoheterostructures. The band gap positions can be calculated by applying following equation as reported previously: [37] ECB = X - Ee - 1/2Eg ECB = EVB - Eg

where ECB and EVB are the conduction band and valence band edge potentials, X is the

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electronegativity of the semiconductor (ZnO is 5.79 eV) [46]; Ee is the energy of free electrons on the hydrogen scale (4.5 eV); and Eg is the band gap energy of the semiconductor (ZnO is 3.24 eV)as calculated form DRS data. By applying above mentioned

The

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equation, the ECB and EVB obtained for ZnO were -0.33 eV and 2.91 eV respectively. enhanced

photocatalytic

reduction

in

the

presence

of

ternary

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nanoheterostructure ZnO-Pt-RGO has been presented in Scheme 3 and could be attributed to following reasons:

(1) The large surface area of RGO possess high adsorption ability towards various pollutants including nitroaromatic compounds via π- π stacking interactions which leads to the high concentration of pollutants on catalyst surface. Hence reduction process ultimately enhanced in presence of ZnO-Pt-RGO nanoheterostructures with high adsorption ability for 4-NP. (2) Suitable CB and VB potentials of ZnO, which are -0.33 eV and 2.91 eV respectively, facilitate the electron transfer from photoexcited ZnO to RGO and to Pt nanoparticles at

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ACCEPTED MANUSCRIPT Schottky interface as per their respective work function values. These transferred electrons are involved in the reduction of adsorbed 4-NP molecules on catalyst surface. (3) Moreover, the sacrificial donor NaBH4 used in the reduction reaction fills the photoinduced holes in the VB of ZnO NR and hence contributes to fasten the charge transfer

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process across ZnO-Pt-RGO heterojunction.

Scheme 3. Mechanistic illustration of photoreduction of 4-NP by ZPG nanomposites under

4. Conclusions In

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natural sunlight irradiation.

summary,

we

have

developed

a

series

of

Pt

loaded

ZnO-RGO

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nanoheterostructures having varying RGO content by using a facile hydrothermal synthesis route.

The prepared nanoheterostructures have been characterized thoroughly by

diffraction, microscopic and spectroscopic techniques to determine the formed structure, morphology and optical properties. Among different nanoheterostructures, the one having 5 wt% of RGO showed pronounced photocatalytic activity for reduction of 4-NP to 4-AP, which could be attributed to the enhanced charge transfer by forming the Schottky barrier by Pt loading on ZnO-RGO nanoheterostructures. Furthermore the high pollutant adsorption ability of RGO leads to high concentration of reactant material (4-NP) on the catalyst surface for their faster reduction. A plausible mechanism has been proposed and discussed for this photocatalytic reduction reaction by using these nanoheterostructures. Thus this study Page 24 of 27

ACCEPTED MANUSCRIPT provides an avenue for the preparation of highly efficient, noble metal loaded nanoheterostructures as ideal platform to carry out various heterogeneous photocatalytic reactions.

Acknowledgements

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We are thankful to Advanced Materials Research Centre (AMRC), IIT Mandi for laboratory and the characterization facilities. VK acknowledges the financial support from Department of Science and Technology, India under Young Scientist Scheme (YSS/2014/000456). SK

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acknowledges Research Fellowship from UGC, New Delhi, India.

References

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[1] S. Kumar, A. Kumar, A. Bahuguna, V. Sharma, V. Krishnan, Two-dimensional carbon-based nanocomposites for photocatalytic energy generation and environmental remediation applications, Beilstein Journal of Nanotechnology, 8 (2017) 1571-1600. [2] S. Ahmed, M. Rasul, W.N. Martens, R. Brown, M. Hashib, Heterogeneous photocatalytic degradation of phenols in wastewater: a review on current status and developments, Desalination, 261 (2010) 3-18. [3] S. Saha, A. Pal, S. Kundu, S. Basu, T. Pal, Photochemical green synthesis of calcium-alginatestabilized Ag and Au nanoparticles and their catalytic application to 4-nitrophenol reduction, Langmuir, 26 (2009) 2885-2893. [4] X. Wang, F. Xia, X. Li, X. Xu, H. Wang, N. Yang, J. Gao, Fabrication of Bi-Fe3O4@ RGO hybrids and their catalytic performance for the reduction of 4-nitrophenol, Journal of Nanoparticle Research, 17 (2015) 436. [5] Y.-C. Chang, D.-H. Chen, Catalytic reduction of 4-nitrophenol by magnetically recoverable Au nanocatalyst, Journal of hazardous materials, 165 (2009) 664-669. [6] J. Wang, T. Tsuzuki, B. Tang, X. Hou, L. Sun, X. Wang, Reduced graphene oxide/ZnO composite: reusable adsorbent for pollutant management, ACS applied materials & interfaces, 4 (2012) 30843090. [7] Y. Lin, D. Li, J. Hu, G. Xiao, J. Wang, W. Li, X. Fu, Highly efficient photocatalytic degradation of organic pollutants by PANI-modified TiO2 composite, The Journal of Physical Chemistry C, 116 (2012) 5764-5772. [8] V. Sharma, S. Kumar, V. Krishnan, Homogeneously embedded Pt nanoclusters on amorphous titania matrix as highly efficient visible light active photocatalyst material, Materials Chemistry and Physics, 179 (2016) 129-136. [9] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, TiO 2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants, Carbon, 49 (2011) 2693-2701. [10] V. Sharma, S. Kumar, V. Krishnan, Clustered Au on TiO2 Snowman-Like Nanoassemblies for Photocatalytic Applications, ChemistrySelect, 1 (2016) 2963-2970. [11] X. Liu, L. Zhao, H. Lai, S. Li, Z. Yi, Efficient photocatalytic degradation of 4-nitrophenol over graphene modified TiO2, Journal of Chemical Technology and Biotechnology, (2017). [12] R.G. Nair, S. Mazumdar, B. Modak, R. Bapat, P. Ayyub, K. Bhattacharyya, The role of surface Ovacancies in the photocatalytic oxidation of Methylene Blue by Zn-doped TiO 2: A Mechanistic approach, Journal of Photochemistry and Photobiology A: Chemistry, (2017).

Page 25 of 27

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[13] Y. Li, W. Xie, X. Hu, G. Shen, X. Zhou, Y. Xiang, X. Zhao, P. Fang, Comparison of dye photodegradation and its coupling with light-to-electricity conversion over TiO2 and ZnO, Langmuir, 26 (2009) 591-597. [14] S. Kumar, V. Sharma, K. Bhattacharyya, V. Krishnan, Synergetic effect of MoS 2–RGO doping to enhance the photocatalytic performance of ZnO nanoparticles, New Journal of Chemistry, 40 (2016) 5185--5197. [15] X. Zhang, J. Qin, R. Hao, L. Wang, X. Shen, R. Yu, S. Limpanart, M. Ma, R. Liu, Carbon-doped ZnO nanostructures: facile synthesis and visible light photocatalytic applications, The Journal of Physical Chemistry C, 119 (2015) 20544-20554. [16] J.M. Wu, W.T. Kao, Heterojunction Nanowires of Ag x Zn1–x O–ZnO Photocatalytic and Antibacterial Activities under Visible-Light and Dark Conditions, The Journal of Physical Chemistry C, 119 (2015) 1433-1441. [17] J. Jiang, X. Zhang, P. Sun, L. Zhang, ZnO/BiOI heterostructures: photoinduced charge-transfer property and enhanced visible-light photocatalytic activity, The Journal of Physical Chemistry C, 115 (2011) 20555-20564. [18] K.-J. Kim, P.B. Kreider, C.-H. Chang, C.-M. Park, H.-G. Ahn, Visible-light-sensitive nanoscale Au– ZnO photocatalysts, Journal of nanoparticle research, 15 (2013) 1606. [19] G. Thennarasu, A. Sivasamy, Synthesis and characterization of nanolayered ZnO/ZnCr2O4 metal oxide composites and its photocatalytic activity under visible light irradiation, Journal of Chemical Technology and Biotechnology, 90 (2015) 514-524. [20] S.G. Babu, R. Vinoth, D.P. Kumar, M.V. Shankar, H.-L. Chou, K. Vinodgopal, B. Neppolian, Influence of electron storing, transferring and shuttling assets of reduced graphene oxide at the interfacial copper doped TiO 2 p–n heterojunction for increased hydrogen production, Nanoscale, 7 (2015) 7849-7857. [21] P.A. Pandey, N.R. Wilson, J. Covington, Pd-doped reduced graphene oxide sensing films for H 2 detection, Sensors and Actuators B: Chemical, 183 (2013) 478-487. [22] C.L. Weaver, J.M. LaRosa, X. Luo, X.T. Cui, Electrically controlled drug delivery from graphene oxide nanocomposite films, ACS nano, 8 (2014) 1834-1843. [23] S. Kumar, R. Sharma, V. Sharma, G. Harith, V. Sivakumar, V. Krishnan, Role of RGO support and irradiation source on the photocatalytic activity of CdS–ZnO semiconductor nanostructures, Beilstein Journal of Nanotechnology, 7 (2016) 1684-1697. [24] I.V. Lightcap, T.H. Kosel, P.V. Kamat, Anchoring semiconductor and metal nanoparticles on a two-dimensional catalyst mat. Storing and shuttling electrons with reduced graphene oxide, Nano Letters, 10 (2010) 577-583. [25] S. Kumar, N.L. Reddy, H.S. Kushwaha, A. Kumar, M.V. Shankar, K. Bhattacharyya, A. Halder, V. Krishnan, Efficient Electron Transfer across ZnO-MoS2-RGO Heterojunction for Remarkably Enhanced Sunlight Driven Photocatalytic Hydrogen Evolution, ChemSusChem, 10 (2017) 3588-3603. [26] R. Lv, X. Wang, W. Lv, Y. Xu, Y. Ge, H. He, G. Li, X. Wu, X. Li, Q. Li, Facile synthesis of ZnO nanorods grown on graphene sheets and its enhanced photocatalytic efficiency, Journal of Chemical Technology and Biotechnology, 90 (2015) 550-558. [27] M.M. Mohamed, M.S. Al-Sharif, Visible light assisted reduction of 4-nitrophenol to 4aminophenol on Ag/TiO 2 photocatalysts synthesized by hybrid templates, Applied Catalysis B: Environmental, 142 (2013) 432-441. [28] J. Cai, X. Wu, S. Li, F. Zheng, L. Zhu, Z. Lai, Synergistic effect of double-shelled and sandwiched TiO2@ Au@ C hollow spheres with enhanced visible-light-driven photocatalytic activity, ACS applied materials & interfaces, 7 (2015) 3764-3772. [29] V. Sharma, S. Kumar, V. Krishnan, Shape Selective Au-TiO 2 Nanocomposites for Photocatalytic Applications, Materials Today: Proceedings, 3 (2016) 1939-1948. [30] V. Sharma, G. Harith, S. Kumar, R. Sharma, K.L. Reddy, A. Bahuguna, V. Krishnan, Amorphous titania matrix impregnated with Ag nanoparticles as a highly efficient visible-and sunlight-active photocatalyst material, Materials Technology, (2016) 1-11.

Page 26 of 27

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

[31] X. Hu, Q. Xu, C. Ge, N. Su, J. Zhang, H. Huang, S. Zhu, Y. Xu, J. Cheng, Synthesis and photocatalytic activity of Pt–ZnO hybrid nanocomposite by solution plasma technology, Nanotechnology, 28 (2016) 045604. [32] H. Chen, S. Chen, X. Quan, H. Yu, H. Zhao, Y. Zhang, Fabrication of TiO2− Pt coaxial nanotube array schottky structures for enhanced photocatalytic degradation of phenol in aqueous solution, The Journal of Physical Chemistry C, 112 (2008) 9285-9290. [33] S. Shuang, R. Lv, Z. Xie, Z. Zhang, Surface Plasmon Enhanced Photocatalysis of Au/Pt-decorated TiO2 Nanopillar Arrays, Scientific reports, 6 (2016). [34] N.T. Khoa, S.W. Kim, D.-H. Yoo, S. Cho, E.J. Kim, S.H. Hahn, Fabrication of Au/graphene-wrapped ZnO-nanoparticle-assembled hollow spheres with effective photoinduced charge transfer for photocatalysis, ACS applied materials & interfaces, 7 (2015) 3524-3531. [35] Z. Wu, Y. Xue, H. Wang, Y. Wu, H. Yu, ZnO nanorods/Pt and ZnO nanorods/Ag heteronanostructure arrays with enhanced photocatalytic degradation of dyes, RSC Advances, 4 (2014) 59009-59016. [36] W.S. Hummers Jr, R.E. Offeman, Preparation of graphitic oxide, Journal of the American Chemical Society, 80 (1958) 1339-1339. [37] S. Kumar, V. Sharma, K. Bhattacharyya, V. Krishnan, N-doped ZnO–MoS 2 binary heterojunctions: the dual role of 2D MoS 2 in the enhancement of photostability and photocatalytic activity under visible light irradiation for tetracycline degradation, Materials Chemistry Frontiers, 1 (2017) 1093-1106. [38] K. Huang, Y. Li, S. Lin, C. Liang, H. Wang, C. Ye, Y. Wang, R. Zhang, D. Fan, H. Yang, A facile route to reduced graphene oxide–zinc oxide nanorod composites with enhanced photocatalytic activity, Powder technology, 257 (2014) 113-119. [39] A. Pal, S. Shah, S. Belochapkine, D. Tanner, E. Magner, S. Devi, Room temperature synthesis of platinum nanoparticles in water-in-oil microemulsion, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 337 (2009) 205-207. [40] L.-L. Tan, W.-J. Ong, S.-P. Chai, A.R. Mohamed, Noble metal modified reduced graphene oxide/TiO 2 ternary nanostructures for efficient visible-light-driven photoreduction of carbon dioxide into methane, Applied Catalysis B: Environmental, 166 (2015) 251-259. [41] M.-Q. Yang, Y.-J. Xu, Basic principles for observing the photosensitizer role of graphene in the graphene–semiconductor composite photocatalyst from a case study on graphene–ZnO, The Journal of Physical Chemistry C, 117 (2013) 21724-21734. [42] N. Zhang, Y. Zhang, X. Pan, M.-Q. Yang, Y.-J. Xu, Constructing ternary CdS–graphene–TiO2 hybrids on the flatland of graphene oxide with enhanced visible-light photoactivity for selective transformation, The Journal of Physical Chemistry C, 116 (2012) 18023-18031. [43] A. Wood, M. Giersig, P. Mulvaney, Fermi level equilibration in quantum dot− metal nanojunctions, The Journal of Physical Chemistry B, 105 (2001) 8810-8815. [44] F.-h. Lin, R.-a. Doong, Bifunctional Au− Fe3O4 heterostructures for magne[cally recyclable catalysis of nitrophenol reduction, The Journal of Physical Chemistry C, 115 (2011) 6591-6598. [45] Z. Chen, S. Liu, M.-Q. Yang, Y.-J. Xu, Synthesis of uniform CdS nanospheres/graphene hybrid nanocomposites and their application as visible light photocatalyst for selective reduction of nitro organics in water, ACS applied materials & interfaces, 5 (2013) 4309-4319. [46] Y. Xu, M.A. Schoonen, The absolute energy positions of conduction and valence bands of selected semiconducting minerals, American Mineralogist, 85 (2000) 543-556.

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Highlights * Pt loaded ZnO-RGO nanoheterostructures have been used for photocatalytic reduction of 4-

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nitrophenol * Nanoheterostructure with 5 wt% of RGO and 2wt% of Pt exhibited highest photocatalytic

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activity

* Synergetic effect of fast charge transfer across Schottky barrier and high pollutant adsorption

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ability of RGO has been studied

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* Mechanism for enhanced photocatalytic activity has been proposed