Green exfoliation of graphitic carbon nitride towards decolourization of Congo-Red under solar irradiation

Journal Pre-proof Green Exfoliation of Graphitic Carbon Nitride towards decolourization of Congo-Red under solar irradiation Sambhu Prasad Pattnaik, Arjun Behera, Rashmi Acharya, Kulamani Parida

PII:

S2213-3437(19)30579-2

DOI:

https://doi.org/10.1016/j.jece.2019.103456

Reference:

JECE 103456

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

28 May 2019

Revised Date:

19 September 2019

Accepted Date:

5 October 2019

Please cite this article as: Prasad Pattnaik S, Behera A, Acharya R, Parida K, Green Exfoliation of Graphitic Carbon Nitride towards decolourization of Congo-Red under solar irradiation, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103456

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Green Exfoliation of Graphitic Carbon Nitride towards decolourization of Congo-Red under solar irradiation

Sambhu Prasad Pattnaik*a, Arjun Behera a, Rashmi Acharya a and Kulamani Parida* a

aCentre

for Nano Science and Nano Technology, Institute of Technical Education and

Research, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar-751030,

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India. *Corresponding Author. FAX: +91 6 74 2581637; Tel: +91 674 2379425

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Email address: [email protected], [email protected]

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Graphical abstract

Reaction mechanism pathway for de-colorization of Congo-Red under solar irradiation.

Highlights (for review) 

The g-C3N4 nano particles as visible light driven photocatalyst in bulk were synthesized by urea pyrolysis.

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The bulk g-C3N4was exfoliated using the aqueous bi-thermal method.

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The mechanism of aqueous bi-thermal method for exfoliation was discussed in detail.

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The multi stage procedure yielded good quality exfoliated g-C3N4nano sheets.

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It was observed that 1 g/ L exfoliated g-C3N4 could decolorize up to 95.1% of a 100

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ppm aqueousCongo Red solution under solar irradiation for 60 min.

Reactive radical species (ROS) like super oxide (・O2.-) and hydroxyl (・OH) were

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responsible towards decolourization.

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ABSTRACT

Exfoliated g-C3N4 nanoparticles were synthesized by the aqueous bi-thermal methods and

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were evalute during different analytical tools and techniques like XRD, PL, FTIR, UV-VisDRS, FESEM, and HRTEM etc. The exfoliated g-C3N4 obtained using this green exfoliation

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technique provided material with improved band gap. The results of UV-VDRS spectrophotometer together with Mott-Schottky graphs of exfoliated g-C3N4 nanoparticles

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corroborated this observation. The sought after characteristics like effective separation and very low rate of recombination of photo-induced electron and holes and enhanced band-gap

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of exfoliated g-C3N4 are expected to increase the photocatalytic performance of semiconductor. The photo catalytic ability of the exfoliated g-C3N4were evaluated from solar decolourization studies of 100 ppm aqueous solution of Congo-Red (CR).The decolourization up to 95.1% could be achieved with 1 h exposure to sun light at catalyst concentration of 1 g/ L. The experiments included scavenger tests for active species identification and prediction

of CR decolourization mechanism. This project demonstrates a low cost, green method for exfoliation of g-C3N4 scalable further. Keywords: Bi-thermal, Green Synthesis, g-C3N4, exfoliation, 1. Introduction The nano structured polymeric graphitic carbon nitride (g-C3N4) belongs to the new class of next-generation visible light-induced semiconductor photo catalyst material[1-3] that finds application in wide ranging fields like photo-electrochemical research[4-6], clean fuel

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production i.e. hydrogen production from water reduction reaction [7], temperature

sensing[8], chemical sensing [9], bio- sensing [10-11], bio-imaging[9-11], white-lightemitting diodes[12], devices and energy-related applications (batteries[13], super

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capacitors[14] etc.

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Worldwide research focus on photocatalysis utilizing graphitic carbon nitride (g-C3N4) got a tremendous boost with the inspiring work by Wang et al. in 2009[1]. Since then nano

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structured polymeric g-C3N4 has become possibly the most studied among metal free semiconductor photo catalysts as it holds immense potential for diverse range of applications

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stated above apart from one of the most pressing requirement of mankind at present i.e. aqueous pollutant degradation [ 3,15-20 ]. As a photocatalystg-C3N4 have multiple

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advantages when compared to other photo catalytic materials. It is nontoxic (environment friendly, therefore makes handling easier), stable from thermal and chemical s tress in an

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aqueous suspension phase during photo catalytic reactions, hard and recyclable material to work with. The g-C3N4 is also best owed with superior properties like wear resistance, water resistivity, low density and biocompatibility. As a photocatalyst g-C3N4 utilizes abundantly available photons of visible spectrum of solar light towards photocatalytic activity because of its moderate band gap (~2.7 eV). Moreover it has appropriate electronic band structure and last but not the least its ease of cost effective synthesis that adds uniqueness to its character

[21]. Bulk g-C3N4 is mostly synthesized by a facile method from easily available, low cost earth-abundant organic precursors containing nitrogen, such as melamine, urea, dicyanamidethiourea, cyanamide, guanidine hydrochloride etc. [22] the precursors are subjected to pyrolytic polycondensation with de-ammoniation process to obtain bulk g-C3N4 which is intrinsically a layered compound structurally analogous to graphite. The bulk gC3N4 comprises of stacked 2D structure of tri-s-triazine units coupled with planar amino clusters [23] and the individual 2D layers are bound to adjacent layers by Van der Waal

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forces. The tri-s-triazine ring structure comprising of p-conjugated graphitic 2D layers made by sp2 hybridization of C (carbon) and N (nitrogen) atoms.[ 24] imparts g-C3N4 a high

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thermal endurance (up to 600 0C in air) and chemical resistance (in both acidic and basic media) [25]. The nitrogen atoms contains lone pair of electrons which provide the tri-s-

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triazine planner entities with superior electronic structure [25-26]. The n-type semi-conductor

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g- C3N4 has a tuneable band gap that offers an adjustable conduit to achieve easily manageable lowest unoccupied molecular orbital (LUMO) as well as highest occupied molecular orbital (HOMO) that radically influences the photoelectronic performance of bulk

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g-C3N4 [27]. Additionally, it makes g-C3N4 modification process easier primarily by construction of heterojunctions with other semiconductors or elemental doping, thereby

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diversifying its application range [28]. It may be noted that the pristine bulk g- C3N4

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synthesized by pyrolytic poly-condensation process inherently lowers the specific surface area of g-C3N4 causing compromised photo catalytic performance mainly due to lowering of specific surface area and therefore lesser availability of active sites for photo catalytic reaction, inadequate use of visible radiation and fast re-combination of photo-induced holeelectron pairs.[ 29] These shortcomings of bulk g- C3N4 are necessarily to be limited to the extent possible before subjecting it to real world applications. In this contest exfoliating bulk

g- C3N4 is very viable amongst strategies to take care of aforesaid demerits of bulk C3N4. To enhance the photocatalytic performance, the bulk g-C3N4, may be exfoliated under certain conditions. It may be reiterated that the bulk g- C3N4 comprises of layers of polymeric tri-striazine structures bound to adjacent layers because of van der Walls forces. Hence it is perfectly possible to obtain a few layer thick sheets, while we aim to get a solo layer of gC3N4 upon the breakdown of these weak forces. The exfoliated g-C3N4 nano sheets obtained from bulk have more specific surface area,

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active sites, and charge transfer conductance. These prerequisites vastly increase the photocatalytic activity of g-C3N4 substantially. Therefore exfoliation is desirable to realise

full potential of exfoliated g-C3N4 before it is put to real use. No wonder, the exfoliation of g-

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C3N4 to obtain two-dimensional (2D) nanosheets is one of the hot research topic that attracts global researcher’s interest. There exists a large numbers of articles and patents related to

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exfoliation of g-C3N4 of which we cite a few. [30-35]. Various methods with varying

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difficulty levels and engaging different solvents, chemicals, acids etc. are in use to exfoliate bulk g-C3N4. Very few recent publications [36-37] describe use of only water as exfoliant. In this regard our facile method of exfoliation of bulk g-C3N4 [38] is green enough to use only

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water under a bi-thermal condition (patent pending) and delivers a few 2D layers of g-C3N4 nano sheets. The method is cost effective and amenable to large scale production of

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exfoliated g-C3N4 in line with norms set by pollution regulators. The bulk exfoliated g-C3N4

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samples synthesized by the aqueous bi-thermal method were tested to find out phase purity, chemical composition and morphology. The UV–Vis DRS and PL experiments were under taken to determine their range of light absorption and separation of photo-generated charge carriers respectively. The FTIR spectra of bulk and exfoliated g-C3N4 were compared out to determine if there is any change in the molecular structure of g-C3N4 due to exfoliation procedure. Mott–Schottky analysis from electrochemical measurements was performed to

compare the CB positions of bulk and exfoliated g-C3N4in electrochemical potential scale. The band gap of exfoliated g-C3N4 sample was studied to predict its performance as a photocatalyst. The photocatalytic activity was explored under solar irradiation by the breakdown of Congo red, a well-known contaminant in effluent water of textile industries and the mechanism of photo decolorization postulated.

2. Experimental

Analytical grade Reagents were used during the work. 2.2 Synthesizing the g-C3N4 nanoparticles

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2.1 Chemicals

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Urea (about 8 g) was taken in a closed crucible made of silica and kept in a muffle furnace.

The furnace was programed to a ramp rate 5 0C per minute and heated the crucible at 550 0C

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for 2 h. The calcined material was cooled to room temperature and ground carefully to get

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powdered form of bulk g-C3N4. The exfoliation of bulk g-C3N4 was carried out by aqueous bi-thermal method discussed earlier [38]. One round-bottom flask of capacity 250 ml was taken. 1.0 g of bulk g-C3N4 in powder form was added to the flask along with a magnetic

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stirring bar and 100 ml water. A reflux condenser was fitted to the flask. The flask was heated by an electric heater with provision for magnetic stirring. The heating with stirring continued

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to boil the water to provide the reflux that continued for about 8 h. The flask was cooled to

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ambient temperature, the magnet removed and the g-C3N4 dispersed in water taken to a separate container. It was then subject to deep freezing for about 8 h to form ice. Later the frozen ice with g-C3N4 powder was taken out. The frozen ice was permitted to melt. The gC3N4 dispersed in water was refluxed in the round-bottom flask. The back to back boiling and freezing may be treated as one bi-thermal cycle in exfoliation protocol. The g-C3N4 along

with water was subjected to 10 such bi-thermal cycles and exfoliated g-C3N4 was separated

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by filtration, dried at 70 0C for 10 h, ground to powder and labelled.

2.3 Characterization

The characterization the g-C3N4 samples before green exfoliation and after were undertaken

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using various analytical methods for chemical, physical and photoelectrochemical properties. PXRD images of different g-C3N4 samples was taken on Rigaku Miniflex XRD instrument

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(power setting30 kV and 15 mA) 2θ ranging from 5 to 70 0 with scan rate of 2 0 per min and using λ=1.54 Å of Cu Kα radiation. Using a JEOL TEM-2010 instrument with electron

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acceleration voltage set at 200 kV, HRTEM images of g-C3N4 nanoparticles were acquired. A JASCO make, model V-750 UV-Visible spectrophotometer was used to record the UV-

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Visible absorption spectra of samples. The FTIR and PL spectral measurements were carried out with JASCO make FTIR 4600 spectrophotometer and spectrofluorometer model FP 8300

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(excitation selected at 320 nm) respectively. X-ray photoelectron spectroscopy (XPS)

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measurements were performed on a Thermo Fischer Scientific ESCALAB Xi+ spectrometer equipped with a mono-chromatized aluminum source (Al Kα radiation hν = 1486.7 eV). The specific surface area and pore size distribution of the g-C3N4 samples were evaluated using Quantachrome’s quadrasorb SI instrument. 2.4 Photo-electrochemical measurements

The electrochemical workstation (IVIUMn STAT) configured with three electrodes under dark and illuminated conditions were used to determine the photo-electrochemical properties. Therefore-mentioned electrolytic cell consists of one g-C3N4 film photo-anode, reference electrode Ag/AgCl and a counter electrode Pt. The electrodes were immersed in 0.1 M Na2SO4 as electrolyte (pH maintained at 6.8). The under-mentioned procedure was followed for preparation of photo-anodes. Repeated cleaning of the FTO electrodes were carried out first with acetone, then ethanol and finally distilled water using an ultrasonicator. 20 mg each

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respectively of iodine and g-C3N4 powder were uniformly dispersed in acetone40 ml and sonicated for about 15 minutes. The g-C3N4 films on FTO were coated by method of

electrophoresis. The active surface area of electrode maintained at 1.0 cm2. The FTOs so coated

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over were dried at temperature 70°C (24 h duration) in a hot air oven to obtain photo-anodes. The photo-anodes were exposed to light from one Xenon lamp (300 W) having a UV light cut-

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of filter. The Mott–Schottky plots were logged with frequency of 500 Hz,

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2.5 Photocatalytic Activity

Photocatalytic properties of g-C3N4 samples were tested by photo de-colourisation of CR in aqueous solution under solar radiation. Using magnetically stirred 100 ml stoppered conical

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flasks tests were conducted for 1h duration under solar irradiation with g-C3N4 concentration at 1.0 gL-1 and 20 mL of CR solution whose concentration maintained at 100.0 mg L-1. The g-

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C3N4suspension was kept under darkness under stirring to attain sorption-desorption

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equilibrium for 60 mins before exposure to sun light. At the end of each experiment, the said suspension is taken for centrifugation for 8 minutes at a speed of 9,000 rpm. The concentration of CR in supernatant solution was measured at 497 nm using a JASCO V-750 UV-Visible spectrophotometer. The tests were conducted in triplicate during the month of May (hot summer days) from 11.00 to 12.00 hours. The average reading of luminosity was

found to be 102,000 lux during the tests. As per equation 1, the percentage of de-colorization (DE) was calculated. % DE = 100 x (C0- C)/C0………………….. (1) Where, C0 and C represent the concentration of the CR solution in starting and final condition. Active species causing decolourization during photocatalytic decolourization process was examined by conducting tests under controlled conditions using different scavengers like

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Isopropyl alcohol, Para-benzoquinone, Dimethyl Sulphoxide and EDTA-2Na for effectively trapping hydroxyl radicals, super oxide radicals, holes and electrons in that order.

Exfoliated g-C3N4 photocatalyst was subjected to repeated usability studies following the

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protocol as under. The photocatalyst was subject to separation by centrifugation at 8000 rpm for 10 m after completion at the end of de-colorization test, washed three times with double

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distilled water followed by washing in ethanol three times and then dried in a hot air oven for

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5 h at 70 oC. The retrieved sample was used again in subsequent test and the protocol followed again under similar conditions for a third time to test for the deterioration of photocatalyst, if any and its reusability.

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3. Results and discussions

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3.1 Crystal and Morphological structure of g-C3N4 The PXRD pattern of g-C3N4 samples may be seen in Figure 1. Both bulk and exfoliated the

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g-C3N4 samples exhibit similar diffraction patterns. These exhibited patterns match with the hexagonal phase of g-C3N4 (JCPDS Card No. 87-1526). No impurity phase found. The two peaks at around 13° and 27° identify the g-C3N4 samples. The minor characteristic peak near 13° signifies (100) facet. Its respective d-spacing found to be about 0.69 nm resulting from in-plane structural stacking motif, which is comparable to the size of in-planar tri-s-triazine unit [39]. It is also observed that peak reflection at 12.8◦ is slightly depressed in case of

exfoliated samples, caused by decreasing planar size of layers because of exfoliation of bulk g-C3N4.The strongest characteristic peak of the g-C3N4 at about 27° represents (002) facet that suggests in-planar length of about 0.323 nm. It’s typically observed in graphite due to interaction of stacked layer of conjugated aromatic systems [40, 41]. The intensity of this peak is reduced in the case of exfoliated samples compared to that of bulk due to reduction of size of stacked g-C3N4 layers. The d-spacing for major and minor peaks did not change with exfoliation. However the peak position [02 ϑ] of minor peak changed marginally from 12.86

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to 12.76 and major peak from 27.6 to 27.7 after exfoliation.Using Scherer’s formula [42], the crystallite sizes for g-C3N4 bulk and exfoliated respectively were calculated to be 16 nm and 33 nm respectively. After exfoliation, the crystallite size of g-C3N4 increased to nearly double

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that of bulk.

Figure 1.PXRD patterns of g-C3N4samples. The representative HRTEM micrographs of bulk and exfoliated g-C3N4 are displayed as per Figure2. From HRTEM images it is observed that bulk and exfoliated g-C3N4 appears to consist of the agglomerated assembly of irregular stacked layers. The calculated d-value from

samples of exfoliated g-C3N4 was noted as 0.326 nm. This value is in agreement with d-value calculated from PXRD measurement for (002) plane for exfoliated sample of g-C3N4. It may be inferred from HRTEM images that g-C3N4 both bulk and exfoliated samples showed randomly packed granular construction with ample porosity. In g-C3N4 sample (exfoliated), the layers look arranged in a haphazardly packed array giving way to plenty of oversized voids leading to irregular and thin lamellar structures porous in nature. This spongy

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framework structure aids to the photo-catalytic capability of exfoliated g-C3N4.

Figure 2. (a, b) HRTEM images showing synthesized exfoliated g-CN samples, (c) SAED image of (002) plane of g-CN (exfoliated) matches with results obtained from PXRD measurements and (d) elemental mapping of g-CN (exfoliated).

3.3 Formation mechanism The exfoliation of g-C3N4 was carried out by aqueous bithermal method (patent pending) and described as under. In case of bulk g-C3N4 the layers of tri-s-triazine structure are stacked one

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above another like a pack of cards. Each layer is bound to adjacent layer by van der Waals forces. When van der Waals forces are made to diminish by aqueous bi- thermal method, the bulk structure falls apart leading to separation of individual layers and thus g-C3N4 exfoliated.

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The group of forces collectively known as the vander Walls forces have their genesis in weak electro static interactions like, dipole-induced dipole , dipole-dipole and dipole-induced

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dipole instantaneous (London Dispersion).These electrostatic force of attraction are governed

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| F | = ke| q1q2 | / r2……….. (2)

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by Coulomb's Law of the Electrostatic as follows:

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Where r =separation distance, ke = Coulomb's constant. The Coulomb's constant is related to

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dielectric constant εm of a medium by the expression

ke= 1 / (4╥ εm)……….. (3)

After substituting for ke in equation 1, it becomes

|F| = (|q1q2|) / 4π r2εm……….. (4)

When the two terms in the denominator of equation (3) are considered, then the inter layer forces of attraction between two adjacent layers of bulk g-C3N4 can be made to diminish if it is kept in water medium (dielectric constant, εm= 80 of water being on the relatively higher side) and reduce further if intercalated with water molecule, thereby increasing the r i.e. expanding the distance between the adjacent layers. As there are hydrogen atoms dangling at the edges of polymeric chain in the C3N4 layers which can easily be attached to water

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molecules through hydrogen bonding making g- C3N4 hydrophilic.In actual practice when bulk g- C3N4 is soaked in water, the water molecules seeps in to the tri-s-triazine layers

through broken surface of layers and / or inside of adjacent layers through capillary action

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and swelling of bulk g- C3N4 takes place. The high dielectric constant of water and its

intercalation improves loosening of tri-s-triazine layers. The heat provided through refluxing

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stage accelerates loosening of layers. The loose tri-s-triazine layers are separated further into

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nano sheets once the intercalated water expands during freezing and leading to exfoliation of bulk g-C3N4.

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A multi cycle protocol is more effective towards bulk g-C3N4 exfoliation.

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Scheme 1: Formation protocol of Exfoliated g-CN.

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3.4 Chemical composition of g-C3N4 samples

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Both g-C3N4 samples bulk and exfoliated displayed similar peak features in their FTIR spectra indicating identical functional groups, structure and composition (Fig.3). A number of

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strong bands in the 1100–1700 cm−1 section are seen in the spectrum relevant to characteristic stretching mode of conjugated Carbon Nitrogen rings [43]. Additionally, the absorption band

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at 820 cm−1 attributed to breathing of triazine units is observed implying that both bulk and exfoliated g-C3N4 nano sheets have the same structures chemically. In case of both bulk and

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exfoliated g-C3N4 , wide absorption band at 3000–3500 cm-1 was allocated to stretching vibration modes of N–H and O–H bonds. It may be inferred from the FTIR spectra that the

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structural arrangement and stoichiometric composition of bulk g-C3N4 did not change after exfoliated to g-C3N4 nanosheets.

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Figure 3.FTIR spectra of g-CN samples

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The compositions and the chemical states of as-prepared bulk and exfoliated g-C3N4 samples were further established by carrying out X-ray photoelectron spectroscopy

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(XPS) measurements, as shown in Figure 4 and 5. As expected, the survey spectra of g-

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C3N4 samples exhibited two major peaks corresponding to elements carbon (C) and nitrogen (N) (Figure 4). The high-resolution C1s and N1s spectra of both the bulk and exfoliated g-C3N4 were further deconvoluted into three Gaussian–Lorenzian peaks, in

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that order (Figure 5 a and b).

Figure 4: Survey spectra of exfoliated g-C3N4 . From the Figure 5 a and b, it was found that there was only a small binding energy shift of C 1s and N 1s, suggesting that the chemical states of both carbon and nitrogen in the exfoliated g-C3N4 were almost same as those of the bulk g- C3N4. In Figure 5 a, the C2 peak located at ~287.8 eV was identified as sp2-bonded carbon (N-C-N) of the g-C3N4 inside the aromatic ring and the main contributor to their C1s spectra. In comparison, the lowest energy contribution from C3 peak located at ~284.6 eV was ascribed to

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graphitic C=C or / and the cyano- group related to it and seen in case of both bulk and exfoliated g-C3N4. The size of the C3 peak was smaller in case of the exfoliated g-C3N4

sample. The C1 peak at ~289 eV is assigned to sp2 C atoms in the aromatic ring attached

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to the NH2 group. The size of C1 peak also got diminished in case of exfoliated samples.

Figure 5 a and b: deconvoluted peaks for C1s and N1s respectively. Moreover, the high resolution N 1s XPS spectra of the bulk and exfoliated g-C3N4 Figure 5 b shows that N 1s had been deconvoluted into three distinct peaks at ~398.2 eV (N1), ~399.1 eV (N2) and ~400.2 eV (N3) eV, indicating three different types of N

bonding in the g-C3N4. The prominent peak N1 could be ascribed to sp2 hybridized nitrogen (C-N=C) and the peak with medium intensity N2 is assigned to bridging N atoms in N-(C)3 or N bonded with H atoms. The least prominent peak N3 can be assigned to tertiary nitrogen linked to terminal amino functions (C–N–H). The XPS studies showed that the chemical states of in the exfoliated g-C3N4 were the same as in the bulk g-C3N4.The results obtained for the C1s and N1s spectra are consistent with those reported [44-47] for graphitic carbon nitride powders.

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Nitrogen Adsorption-Desorption Study: To characterize the specific surface areas and porosity of both bulk and exfoliated g-C3N4 samples, the nitrogen adsorption–

desorption isotherms and corresponding pore size distribution (PSD) were measured, as

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depicted in Fig. 6. It may be understood from Fig. 6 (a) that both bulk and exfoliated gC3N4 samples have type IV (BDDT classification) shape of isotherms with an H3-type

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hysteresis loop in the IUPAC classification, thereby indicating the aggregation of plate-

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like particles. This result agrees with the sheet-like morphology of exfoliated g-C3N4 (Figs. 2) from HRTEM micrograph. In the case of bulk g-C3N4, less N2 adsorbed volumes is seen, indicating structure with lower porosity. The exfoliated g-C3N4 samples

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could adsorb a much higher amount of N2 gas than the bulk one does, indicating the formation of enlarged 2D sheets. The formation of sheet like structure results in

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improvement of surface area from ~37 m2/g for the bulk g-C3N4 sample to

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~ 64 m2/g for the exfoliated g-C3N4 sample (Table 1). It is possible to have surface area of bulk g-C3N4 influenced by the thermal oxidation [49] during synthesis. As shown in Fig. 5b, the corresponding PSD curves further confirm the introduction of more porosity in the exfoliated g-C3N4 nanosheets. The specific surface area, pore volume and peak pore size of bulk and exfoliated g-C3N4, samples are presented in Table 1. Compared to bulk g-C3N4, the exfoliated g-C3N4 retains larger surface area and pore

volume. This behaviour is expected as the surface area and pore volume increases with

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exfoliation.

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Fig. 6. N2 adsorption–desorption isotherms (a) and the corresponding pore-size

Sample name

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Bulk g-C3N4

peak pore size

(cm3/g)

(nm)

37.3

0.20

24.9

63.6

0.42

28.7

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Exfoliated

Total pore volume

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(m2/g)

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distribution curves (b) for bulk and exfoliated g-C3N4.

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g-C3N4

Table 1: Surface area and pore dimensions of bulk and exfoliated g-C3N4 Samples.

The enlarged specific surface area and pore volume would be beneficial for mass transfer and provide more possible redox reaction sites. Therefore the exfoliated g-C3N4, obtained from aqueous bi-thermal protocol with increased porous surface areas may

effectively promote the kinetics of the photocatalytic reaction in comparison to its bulk counterpart. 3.5. Band gap potential of g-C3N4 samples. The band gap of g-C3N4 samples bulk and exfoliated were determined from their photochemical characteristics by using UV–Visible DR spectra and Mott-Schottky (M.S.) plots. Both g-C3N4 samples exhibited light absorption. After exfoliation into 2D nanosheets, the exfoliated g-C3N4absorption edge revealed a perceptible blue shift possibly due to

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quantum confinement effect leading to ultra-thinning of 2D nanosheets. This fact also points that the light-absorbing ability of exfoliated g-C3N4 slightly reduces, which is indicated by

colour of bulk g-C3N4 changing from pale yellow to off white on exfoliation. It may be noted

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that g-C3N4 is a photo-catalyst with direct band gap [52]. The band gap energy values for g-

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C3N4bulk and exfoliated were deduced from their UV-Visible absorption spectra respectively. This under-mentioned equation governs absorption of light adjacent to

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semiconductor band gap,

αhϑ = C (hϑ -Eg)n …………………………………….(5)

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whereν, h, α, Eg, C and n denotes frequency of light, Planck’s constant, absorption coefficient, , band gap energy and constants respectively. The equation 5 is used to find the

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direct band gap energy of g-C3N4 by taking n =2 and plotting (αhν)2 values against light

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energy (eV scale) .Further extrapolation of the linear component of (αhν)2 to intersect the Xaxis when α, the co-efficient of absorption takes 0 value as shown in figures 7 (b) and 7 (c). The band gap energy for bulk g-C3N4 was estimated between 2.75eV and 3.0 eV for the exfoliated g-C3N4. It was apparent from the band gap value measurement that the g-C3N4 that absorbs visible light and the band gap value improved with exfoliation. Hence the photocatalytic capability of exfoliated of g-C3N4 is expected to be more than that of bulk.

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Figure.7 (a) UV-Visible Diffusion reflectance spectra of bulk and exfoliated g-C3N4 (b,c)

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Direct bandgap estimation for bulk and exfoliated g-C3N4 in that order from the (αhν)2 vs Eg plots.

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The M-S plots of g-C3N4 samples (Fig.8) were investigated at 500 Hz frequency to

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understand conductor type and band alignment. The negative slope of the linear part of plots suggests n-type features in g-C3N4 samples. Utilizing the MS equation, the flat band potential (fbp) of bulk and exfoliated g-C3N4 samples were assessed. From X-axis intercept of line segment of M-S plots, the fbp of g-C3N4 samples was calculated to -1.65 and -1.78 eV against Ag/AgCl electrode for g-C3N4 bulk and exfoliated respectively. The g-C3N4 being a n-type semiconductor, its flat band potential may be considered to almost same as its CB

potential. When the potential measurements are aligned with N.H.E scale they became -1.05 and -1.18 eV for g-C3N4 bulk and exfoliated in that order. Using the band gap values of gC3N4 samples i.e. 2.75 and 3.0 eV obtained from the UV-DRS measurement (figure 7) the Valence band (VB) potentials of bulk and exfoliated g-C3N4 were estimated to be +1.7 and

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+1.8eV in that order.

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Figure. 8: The M.S. plot of g-C3N4 measured using frequency of 500 Hz.

As expected, the negative gradient of linear segment of plots established that synthesized g-

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C3N4 photo-electrodes were indeed n-type semiconductors. The structural disparity of the gC3N4 samples were exhibited in minimum edge of CB and uppermost brink of VB energies.

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The individual electronic band structures of bulk and exfoliated g-C3N4 samples decided their corresponding band gap. The CB edge of exfoliated g-C3N4 i.e. a few atom layer thick tri-striazine 2D nano sheet, moved towards a more negative side and the VB edge relocated to a more positive position due to quantum confinement effect (QCE)[49] as depicted in the scheme 2. Typically, when the CB value becomes more negative, it is responsible for creation of more reductive photo-generated electrons and relatively lower charge carrier

recombination caused due to improved efficacy for charge transport, [50]. Additionally, the improved mobility of photo-induced holes and increased photo-oxidation ability could also be described by enhanced value of VB [50]. It may be concluded that the g-C3N4 sample

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exfoliated is supposed to display superior photocatalytic activities than g-C3N4bulk.

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Scheme 2. Diagram of bandgap structure of bulk and exfoliated g-C3N4.

3.6 Charge transfer and recombination in g-C3N4 samples

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The photoluminescence spectra of bulk g-C3N4 and its exfoliated part at excitation wavelength 320 nm at room temperature are shown in Figure.9. The PL spectra displays near similar band-to-band fluorescence emission that is characteristic for both samples and gave insight to the electron relocation and recombination phenomena. The emission peaks appearing at different wavelengths for a semiconductor photocatalyst were attributed to holeelectron recombination. The recombination probability of photo-induced electron and hole

pairs in bulk g-C3N4 is observed to be more than that of exfoliated g-C3N4. Hence the exfoliated g-C3N4sample is supposed to show improved photo-catalytic activity than bulk g-

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C3N4 due to its lower re-combination rate of photo-generated charge carriers.

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Figure 9 : The PL (Photo luminescence) spectra g-C3N4 samples.

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The effective separation of charges at semiconductor catalyst interface and its opposition to charge transfer of synthesized g-C3N4 were also evaluated from electrochemical impedance

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spectra (EIS) as represented in Nyquist plots. The Nyquist semicircle’s diameter may be thought of as function of resistance at the touch point between working electrode and

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electrolyte solution. Consequently, the size of semicircle reveals photocatalytic reaction rate taking place on the surface of semiconductor catalyst. The smaller radius of semicircle points to faster transfer of photo induced charges and therefore effective charge separation making improvement to photoactivity [51]. It is obvious from the figure 10 that displays the Nyquist plot of g-C3N4 samples that a semicircle with smaller diameter is acquired for exfoliated gC3N4 revealing its better conductance to transfer of charges and hence enhanced charge

separation. The greater separation of charges is expected to show better photo-catalytic

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performance of exfoliated g-C3N4 vis a vis its bulk variant.

Figure 10 : The Nyquist plots of g-C3N4 samples

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The g-C3N4 samples were exposed to light and photon generated current measured with a

voltage sweep rate = 20 mVs-1, using a solution of 0.1 M Na2SO4 at pH 6.8. The electrolyte

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pH remained unchanged after completion of test indicating low corrosion potential of g-C3N4

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samples and hence may be put to practical use. Figure 11 denotes the LSV graphs of g-C3N4 samples under light exposure. The anodic current density improved from nearly zero mA/cm2

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at a potential of 1.0 V to about 0.23 mA/cm2 and 0.44 mA/cm2 for g-C3N4 bulk and exfoliated respectively by increasing the potential applied to 1.5 V.Increase in the photocurrent density

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indicated improvement in the amount of photo-generated electrons for the semiconductor photo-catalyst and the slope for the plot of exfoliated g-C3N4 turned out to be more than that

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of bulk g-C3N4 indicating a large amount of photocurrent production due to greater concentration of photo-electrons. This suggests increased charge separation for exfoliated gC3N4 resulting in availability of greater number of electrons towards photocatalytic activities. The LSV test results, EIS measurements and PL behaviour are found to be in agreement expecting exfoliated g-C3N4 to exhibit better photocatalytic ability.

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4. Photocatalytic Activities.

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Figure 11 : The LSV graphs of g-C3N4 samples under light exposure.

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Fig.12. (a) The change in absorbance spectra of solar irradiated CR solution in presence of g-

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C3N4 as photocatalyst. (b) The concentration change of CR solution with respect to time. (c) Reaction kinetics of CR de-colourization (d) Effect of exfoliated g-C3N4 on photode-

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colorization of CR noted in recycle tests. The photocatalytic activities of g-C3N4 were tested against photo-decolourization of 100

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mg L-1 of CR solution exposed to solar light. Initially the CR solution was exposed to solar radiation for 1 hour minus the g-C3N4 powders and found to be less than 1% of CR decolourized.

To check the viability of CR adsorption on the active sites of the photocatalyst, the CR solution was stirred in absence of light for 60 minutes with 1g L-1 of photocatalyst. Decolourisation of nearly 3% of CR suggested that loss of colour intensity due to adsorption

of CR onto g-C3N4 photocatalyst could be neglected. In any case, the CR solution was stirred in absence of light for 1 h for achieving adsorption-desorption equilibrium and then exposed to solar light. The aliquots of the irradiated CR solution were sampled, centrifuged and analysed at different time intervals and photocatalytic activity of g-C3N4 towards decolourisation of CR solution due to exposure of solar light were quantified .Fig.12 (c) shows rate of decolourisation of CR solution by g-C3N4 photocatalysts. As it may be seen from table 2, maximum 95.1% of 100 mgL-1 CR solution could be

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decolourised by 1.0 gL-1 of exfoliated g-C3N4 against 81.8% in case of bulk g-C3N4 when irradiated with solar light for 1hour. Obviously the exfoliatedg-C3N4 turned out to be more effective in decolourisation tests. The Fig. 12a depicts UV–Vis absorbance spectra of CR

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concentration versus time, solar irradiated with g-C3N4 samples. The rate constants of CR

decolourisation were found to be 2.0 X 10-2 min-1 and 4.0 X 10-2 min-1, in that order for bulk

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and exfoliated g-C3N4 samples. The CR decolourisation by g-C3N4 samples conformed to

R2

kobs (min-1)

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Catalyst

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pseudo-first-order reaction.

t1/2 (min)

% of decolourization

0.98

2.0 X 10-2

34.65

81.8

Exfoliated g-

0.93

4.0 X 10-2

17.33

95.1

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Bulk g-C3N4

C3N4

Table 2. Parameters of photo de-colorization kinetic studies of CR under solar irradiation.

It’s noticed from the graph that absorption spectra reduces for every time interval of 15 minutes and was found lowest at 60 minutes. Decline in absorption intensity reveals decline in concentration of CR solution along with time which results from the good photocatalytic activity of exfoliated g-C3N4. It might be attributed to its porous nanosheet like morphology as seen from PXRD study and TEM results. Also, lowest PL intensity of sample along with LSV and EIS studies suggested decline in charge carrier recombination and quicker transport for photocatalytic reactions. All of these enhanced the photocatalytic activity of exfoliated g-

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C3N4 resulting in higher percentage of CR decolourization. The exfoliated g-C3N4 appears to have a higher decolourization efficiency under solar irradiation than other photocatalysts reported for similar photocatalytic degradation of CR, as presented in Table 3. [Pollutant]

source

mg L-1

BiFe2O3 NP

Solar

10

CS/n-CdS

Xe 300 W

20

P25 TiO2

W 300W

40

Fe/CMS-1

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Co3O4/TiO2/GO

Time

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gm L-1

min

%

Refs

Removal

60

77

51

1.5

180

86

53

1.0

210

~90

54

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1.0

UV 15 W

100

1.0

180

100

55

UV

50

0.33

120

100

56

Xe 300 W

10

0.25

90

91

57

100

1.0

60

95

This work

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CuO/NiO

[Catalyst]

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Light

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Photocatalyst

Exfoliated g-C3N4 Solar

Table 3 Comparative study for the photodegradation of CR by different nanomaterials

The photostability of the exfoliated g-C3N4 under exposure to visible-light was tested in recycling tests after 1hour of photocatalytic reaction in CR solution as per protocol mentioned earlier. The decline in CR concentration during every cycle is shown in Figure 9. It was noted after 3 cycle runs of photo-decolourization of CR, the photocatalytic ability of exfoliated g-C3N4 stayed same suggesting photo stability of exfoliated g-C3N4 under experimental conditions. This fact is corroborated by the XRD, XPS and FTIR measurement of exfoliated g-C3N4 samples before and after recycle tests which did not show any alteration

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in the structure and composition of material as depicted in the spectra (figure 13) after tests

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implying the photo-stability of exfoliated g-C3N4.

Figure 13. (a) The XRD spectra, (b) FTIR spectra, (c) The XPS N1s spectra of exfoliated g-C3N4 samples before and (d) that of after catalyst recycle tests towards decolourization of CR. 4.2 Mechanism of CR decolourization:

When solar radiation strikes at dispersed gC3N4 in CR solution the photo-generated holes and electrons could either merge or get transported to the interface, where it is possible to combine with adsorbed hydroxyls and molecular oxygen to produce reactive oxygen species (ROS) like H2O2, ·O2−and·OH radicals. The generated ROS in turn undertake photocatalytic reactions toward decolourisation of CR solution [38]. The reactive species participating in the photocatalytic decolourisation process of CR could be recognised by using various scavenging agents like IPA, DMSO, EDTA and p-BQ and their specific contribution mapped

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with respect to the process in general. The effect of reactive species on percentage of

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photocatalytic decolourisation of CR solution is depicted in figure 14(a).

Figure 14. (a) The bar graph portraying the effects of active species in decolourisation of CR (b) absorbance spectra of NBT before and after the test with g-C3N4. (c) The PL spectra of

alkalined5 × 10−5 M 2-hydroxyterephthalic acid solution with g-C3N4 identifying the generation of OH• radicals.

It may be deduced from the Figure. 14 (a) that superoxide radical (O2.-) is mainly responsible towards the CR decolourization process. The other responsible species which contributes to overall CR decolourizations reactions is OH• radical. The CB and VB position of exfoliated g-C3N4 were calculated to be −1.18 and +1.8 eV Vs. NHE in that order, from Mott–Schotky

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plots. The position of conduction band in exfoliated g-C3N4 is comparatively more negative than E0 of •O2-(−0.046 eV vs NHE). Hence, electrons in the CB of exfoliated g-C3N4 are

capable of producing •O2− when reacting with oxygen in dissolved form [58]. The formation

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of •O2− radical was independently confirmed when 5x10-5 M nitro blue tetrazolium (NBT) was used as a sensing probe and its UV-visible spectra taken by means of a uv-visible

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spectrophotometer. 0.01 g of exfoliated g-C3N4 was dispersed in 10 mL of prepared NBT

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solution and irradiated by Sun light for 1 hour. Then the catalyst was separated by centrifugation and uv-visible spectra obtained of NBT solution. From the uv-visible spectra acquired after exposure to solar light, the concentration of NBT turned out to have decreased

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suggesting the generation of •O2-during the light exposure in presence of photo catalyst. Figure 14 (c) portray the generation of •O2-radical under solar irradiation with exfoliated g-

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C3N4. This observation proves the generation of •O2-that causes effective decolouration of

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CR. It is of interest to note that the holes in the VB position (+1.8 eV) of exfoliated g-C3N4 are incapable to generate •OH directly as it lies above E0 of •OH/OH− = +1.99 eV vs NHE. Therefore the •OH radicals are ultimately formed from decomposition of H2O2.The OH• radical formation during the CR decolorization process could be directly confirmed by using terephthalic acid (TA) as a detecting probe. The TA grabs OH• radical to generate the 2-

hydroxyterephthalic acid and fluorescence intensity thereof could be related to the OH• radicals concentration. ・OH + TA → TAOH ……………………………… (6) The fluorescence intensity was calculated in an alkaline 5 × 10−5 M soln. of terephthalic acid in presence of exfoliated g-C3N4 (1 gm L−1) after exposure to 60 min of solar light. The fluorescence spectra measured after excitation at 315 nm confirmed the formation of ・OH radicals.

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The following equations represent the overall photo-decolourisation process,

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E-GCN + h →E-GCN (e-) + E-GCN (h+) ……………………… (7) E-GCN (e-) + O2 → •O2- ………………………………………… (8)

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E-GCN (e-) + •O2- +2H+ → H2O2 ………………….…………… (9)

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E-GCN (e-) + H2O2 → •OH + OH- ………………….….……… (10) CR + •OH → de-colorization …………………………..……… (11)

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CR + •O2- → de-colorization …………………………………… (12)

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Or net reaction may be rewritten as

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CR + •OH + •O2-→ de-colorization ……………………….…… (13)

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Scheme 3. Pictorial representation of proposed reaction mechanism of decolourization of

5. Conclusion

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Congo red due to exfoliated g-C3N4 under solar irradiation

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The bulk graphitic carbon nitride was prepared by urea pyrolysis route. The green exfoliation technique i.e. an aqueous bi-thermal method has been successfully applied for exfoliation of

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bulk g-C3N4. The g-C3N4 samples, bulk and exfoliated were suitably characterized. The formation of exfoliated g-C3N4 was deduced from XRD patterns. The band edge potentials were recalculated in NHE scale and plotted. It is obvious from the plots that green exfoliation has led to the blue shift of band gap possibly due to quantum confinement effect of few-layer thick of exfoliated g-C3N4 nanosheets. The CB position has become more negative and VB position shifted to more positive side of NHE scale after exfoliation implying possible photo-

generation of more reductive electrons and more oxidative holes leading to improvement in visible radiation photocatalytic behaviour of exfoliated g-C3N4 prepared via the aqueous bithermal exfoliation process. The photocatalytic performance of exfoliated g-C3N4 was evaluated during decolourization of CR solution under solar light exposure and possible mechanism for the same was proposed. Acknowledgment: SPP acknowledges Siksha ‘O’ Anusandhan deemed to be University, Bhubaneswar for

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providing XRD characterisation of samples and support towards filing the patent of aqueous

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bi-thermal method of exfoliation.

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Prime Novelty Statement:

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The 2D g-C3N4nano sheets obtained by the aqueous bi-thermal exfoliation method (Mechanism of exfoliation discussed).The decolorization of Congo red dye solution was

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studied under solar light using the g-C3N4nano sheets. A mechanism of dye decolorization

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was proposed based on the results of scavenger tests for reactive oxygen species.

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