Needle-like Î±-MnO2

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Visible Light Active Single-Crystal Nanorod/Needle Like #-MnO @RGO Nanocomposites for Efficient Photoreduction of Cr(VI)

Deepak Kumar Padhi, Ayonbala Baral, Kulamani M. Parida, Saroj Kumar Singh, and Malaya Kumar Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10663 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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The Journal of Physical Chemistry

Visible Light Active Single-Crystal Nanorod/Needle Like α-MnO2 @RGO Nanocomposites For Efficient Photoreduction of Cr(VI) Deepak kumar Padhiab, Ayonbala Baralac , Kulamani Paridaad*, S. K. Singhab and M. K. Ghoshac* a

Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and

Industrial Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi-110 001, India. b

Advanced Materials Technology Department, CSIR-Institute of Minerals and Materials

Technology,Bhubaneswar – 751 013, Odisha, India. C

Hydro & Electrometallurgy Department, CSIR-Institute of Minerals and Materials

Technology,Bhubaneswar – 751 013, Odisha, India. d

Centre for Nano Science and Nano Technology SOA University, Bhubaneswar—751 030,

Odisha, India.

*

Corresponding author

E-mail: [email protected] Tel. No. +91-674-2379425 Fax. +91-674-2581637

*

Corresponding author

E-mail: [email protected] Tel. No. +91-674-2379374

ACS Paragon Plus Environment


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Abstract Herein, we report a highly efficient template or surfactant free hydrothermal method to fabricate graphene based single-crystal tetragonal needle and nanorod like α-MnO2 composite, a kind of semiconductor photocatalyst for the reduction of hexavalent chromium [Cr(VI)] under visible irradiation. Hydrothermal reaction time and in-situ introduction of graphene played a important role in tuning the morphology of α-MnO2. The as synthesised RGO/α-MnO2 nanorod composite exhibited outstanding photoreduction ability as compared to RGO/α-MnO2 needle composite. The higher activity of RGO/α-MnO2 nanorod composite was successfully derived from photoluminescence (PL) and photocurrent measurement. The low PL intensity and high photocurrent density of RGO/α-MnO2 nanorod composite concludes that change in aspect ratio as well as presence of RGO favours intimate strong interaction of 2D layer of graphene and 1D α-MnO2 nanorod which facilitates for enhanced photoexcited charge (e-/h+) separation and simultaneously increases its photoreduction ability under visible light irradiation.


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1. Introduction Hexavalent chromium [Cr(VI)] is the most commonly identified heavy metal ions found in wastewater as a result of industrial activities such as metallurgy, electroplating, metal finishing, leather tanning, paint making and dyeing, etc.1-2 Cr(VI) has been listed as one of the 17 chemicals posing the greatest threat to humans by the United States Environmental Protection Agency (US EPA) due to its high toxicity, carcinogenicity and mutagenicity towards.3-4 Moreover, Cr(VI) is non-biodegradable and can easily accumulate in living beings throughout the food chain which causes skin and stomach allergies or ulceration, lung cancer, damage to the liver, kidneys and nerve tissue, and even death in case of humans.5 Hence, industrial effluents or polluted water containing Cr(VI) must be treated before discharge. In this regard, several technologies like ion exchange 6, chemical precipitation7, reverse osmosis 8

, adsorption

9

and photocatalytic reduction10 have been implemented to control of Cr(VI).

Among all, photocatalytic removal of Cr(VI) by semiconductor photocatalysts is considered as one of attractive and gainful method. In the photocatalytic reduction process, Cr(VI) is reduced to Cr(III) which is non-toxic and plays a vital role in the plant and animal metabolism.11-12 Therefore, semiconductor photocatalysts like TiO2, ZnO, SnS2, ZnGeO4, Sn2S3, Bi2WO6 etc. have been reported for the reduction of Cr(VI) to Cr(III).13-18 On the other hand, nanostructured manganese dioxide (MnO2) has been considered as one of the emerging material for Cr(VI) reduction due to its unique optical, electrical, catalytic, magnetic and electrochemical properties

19-20

In this respect, MnO2 has

been extensively used in batteries,21 supercapacitors,22 and as visible light active photocatalysis.23 Owing to its availability, nontoxic nature, cost-effectiveness, large specific surface area,

oxidizing/adsorption abilities and outstanding stability under acidic

conditions,24 MnO2 has been considered as a undeniable photocatalyst for wastewater treatment.25 However, its implementation in the field of photocatalysis is limited due to its



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loss in the form of suspended powder during the photocatalytic reaction and separation process.26 Among reported work on photocatalytic activity of MnO2, Pal et al. fabricated βMnO2 organosol and studied phenol coupling reaction at room temperature.27 Mittal et al. studied photocatalytic degradation of Rose Bengal (RB) dye over MnO2 under visible light irradiation.28 Zhao et al. synthesised thin-layer MnO2 nanosheet-coated Fe3O4 nanocomposite as a magnetically separable photocatalyst towards photo-degradation of methylene blue.26 Boppana et al. reported outstanding photocatalytic water oxidation over K-containing δMnO2 nanosheets under visible light irradiation.29 Above reported works suggest that the charge carrier recombination lower the photocatalytic efficiency of MnO2. To address these above issues, graphene based composite materials with controllable compositional, structural or interfacial features have attracted exponentially increased attention for wide range of photocatalytic application due to the outstanding unique physicochemical properties of graphene.30-31 Moreover, introduction of graphene to semiconductor increases charge carriers separation by sinking the photoexcited electron from the conduction band of the semiconductor and that results in enhancing in photocatalytic efficiency. On the other hand adhesion of inorganic particles on the 2D graphene sheets avoids the aggregation of graphene sheets and simultaneously enhances the stability of composite material. In this regard, a few reports on the photocatalytic removal of Cr(VI) over graphene

based-materials

naoparticle/Graphene,34

like

In2S3

TiO2/RGO,35

nanosheets/graphene,32 ZnO/RGO,36

ZnO/Graphene,33

CdS/RGO,37

Iron

α-Fe2O3/aMEGO,38

GO/Coordination polymer nanobelt(CPNB),39 α-FeOOH nanorod/RGO

40

and Gd(OH)3

nanorod/RGO 41 have already been reported. Out of which the author group have reported αFeOOH nanorod/RGO and Gd(OH)3 nanorod/ RGO composite materials for the photoreduction of Cr(VI).40-41


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Till date, only Yan et al fabricated α-MnO2 nanosheets to NH2-graphene to remove Cr6+ in aqueous solution.24 To the best of our knowledge, no group have reported one pot in-situ fabrication of graphene based single-crystal needle and rod like α-MnO2 composite by facile hydrothermal technique towards the reduction of Cr(VI) under visible light irradiation. In this present work, a facile template free one pot hydrothermal route has been adopted for in-situ fabrication of visible light driven graphene based single-crystal needle and rod like α-MnO2 composite towards enhanced reduction of Cr(VI). The as synthesised α-MnO2 nanorod showed better photocatalytic reduction ability as compared to α-MnO2 needle which was further increased in presence of RGO. Photocurrent and photoluminescence measurements were carried out to confirm the role RGO and enhanced activity of RGO/α-MnO2 nanorod composite towards photo-reduction of Cr(VI) have been discussed in detail. 2. Experimental 2.1 Materials Manganese(II)sulphate monohydrate (MnSO4.H2O), Ammonium peroxy disulphate ((NH4)2S2O8), Sulphuric acid (H2SO4), Hydrochloric acid (HCl), Sodium nitrate (NaNO3) and Sodium hydroxide (NaOH) absolute ethanol, were obtained from SD Fine Chemicals Ltd, India. Natural graphite powder and Potassium permanganate were procured from Sigma Aldrich Chemicals. All the above chemicals and reagents are of analytical grade and were used without further purification. 2.2 Hydrothermal synthesis of needle and rod like α-MnO2 6.70 g of MnSO4.H2O and 13.69 g of (NH4)2S2O8 were dissolved in 50 ml of distilled water under stirring for 30 min at room temperature. The resultant solution was then transferred to a Teflon-line autoclave for hydrothermal treatment at 180 oC for pre-



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determined time periods i.e 6 h and 12 h. The obtained brownish black precipitate at 6 h and 12 h were centrifuged, washed with distilled water followed by absolute ethanol for several times, and then dried in oven at 80 ⁰C overnight. 2.3 Hydrothermal synthesis of RGO/α-MnO2 nanorod and RGO/α-MnO2 needle composite 6.70 g of MnSO4.H2O and 13.69 g of (NH4)2S2O8 were dissolved in 50 ml of distilled water under stirring for 30 min at room temperature. In another pot, 0.05 g of GO (prepared by reported modified Hummers method 42) was sonicated for 1h in 30 ml of ethanol to destroy the restacking of GO layers. Now, the above prepared MnO2 precursor solution and exfoliated graphene oxide solution (EGO) were mixed and stirred for another 30 min at room temperature and the rest of the procedure was kept same as the synthesis of needle and rod like α-MnO2, described earlier and is represented in scheme 1.

Scheme 1. Schematic representation of the synthesis of RGO/α-MnO2 nanorod and RGO/α-MnO2 needle composite.


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2.4 Analytical characterisation Several analytical techniques such as XRD, TEM, XPS, Raman, DRS, PL and photoelectrochemical measurements were done to characterise the as prepared samples. Xray diffraction (XRD) patterns of all the samples were recorded over the range 10

0

< 2θ

Needle-like Î±-MnO2

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