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Photo-regenerable, bifunctional granules of carbon doped g-C3N4 as adsorptive photocatalyst for the efficient removal of tetracycline antibiotic Suyana Panneri, Priyanka Ganguly, Midhun Mohan, Balagopal N Nair, Abdul Azeez Peer Mohamed, Krishna Gopa Kumar Warrier, and U. S. Hareesh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02383 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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ACS Sustainable Chemistry & Engineering

Photo-regenerable, bifunctional granules of carbon doped g-C3N4 as adsorptive photocatalyst for the efficient removal of tetracycline antibiotic

1 2 3

Suyana Panneriab, Priyanka Gangulya, Midhun Mohana, Balagopal N. Naircd, Abdul Azeez Peer Mohameda, Krishna G. Warriera and U. S. Hareeshab*

4 5 6 7 8 9 10 11 12

a

Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram - 695019, India. b

Academy of Scientific and Innovative Research (AcSIR), New Delhi, India c R&D Center, Noritake Co. Limited, Aichi 470-0293, Japan d Nanochemistry Research Institute, Department of Chemistry, Curtin University, GPO Box UI987, Perth, WA6845, Australia

13 14 15

Corresponding Author

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E-mail: [email protected], [email protected]

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ABSTRACT

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Environmental remediation employing semiconducting materials offer a greener solution for

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pollution control. Herein, we report the development of high surface area porous architecture of

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C3N4 nanosheets by a simple aqueous spray drying process. g-C3N4 nanosheets obtained by the

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thermal decomposition of urea-thiourea mixture are spray granulated to microspheres using 2

33

weight % poly vinyl alcohol (PVA) as binder. The post granulation thermal oxidation treatment

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resulted in in-situ doping of carbon leading to improved photophysical properties compared to

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pristine g-C3N4. The C3N4 granules with surface area values of 150 m2/g rendered repetitive

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adsorption of tetracycline antibiotic (~75% in 60 min) and the extended absorption in the visible

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region facilitated complete photocatalytic degradation upon sunlight irradiation (>95% in 90

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min). The delocalized π bonds generated after carbon doping and the macro-meso porous

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architecture created by the granulation process aided high adsorption capacity (70 mg/g). The

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photo-regenerable, bi-functional materials herein obtained can thus be employed for the

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adsorption and subsequent degradation of harmful organic pollutants without any secondary

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remediation processes.

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Key words: graphitic carbon nitride (g-C3N4), spray granulation, carbon doping, adsorptive

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photocatalyst, photo-regenerable, tetracycline

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1. Introduction Utilization of photocatalytic semiconductors for the degradation of organic pollutants and

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harmful microbes has been amply demonstrated through a variety of systems like TiO2,

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ZnO, ZnS etc.1-4 The widespread use of pharmaceutical compounds like antibiotics has,

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in recent times, posed a major threat to the environment due to issues associated with its

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degradation and safe disposal. Consequently, we are faced with the emergence of newer 2 ACS Paragon Plus Environment

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hazards, like antibiotic resistant bacteria, that call for immediate remedial measures.5-6

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The greener approach for the alleviation of such issues is the use of sunlight active

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photocatalysis with appreciable efficiencies and recyclability.7 A recent entrant to the

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category of semiconductor photocatalysts is graphitic carbon nitride (g-C3N4) with a band

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gap value of 2.7 eV.8-9 This organic semiconductor characterized by exceptional chemical

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and thermal stability found wide applications in water splitting, organic pollutant

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degradation, organic catalysis and as electrocatalysts under visible light.10-13 However the

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poor surface area, the faster recombination of excitons and the low visible light

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absorption render g-C3N4 less effective.14 Multiple strategies are adopted to overcome

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these

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heterostructure formation and liquid phase exfoliation are employed to improve the

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catalytic efficiencies.11,

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electronic properties favorably for increased absorption over an extended wavelength

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regime.22-25 The thermal oxidation etching process induces exfoliation of the stacked

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C3N4 layers resulting in significantly improved surface area values.16

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Conventional adsorption of pollutants is currently practiced through materials like

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activated charcoal, silica gel, fly ash, MOF, etc by virtue of their high surface area and

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adsorption efficiency. However, these adsorbents necessitates stringent conditions for

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desorption making the regeneration process environmentally unfriendly and expensive.26

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It is therefore desirable to develop materials that combine adsorption with photocatalytic

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activity resulting in the formation of bifunctional adsorptive photocatalysts. This

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approach is particularly advantageous as it doesn’t require secondary remediation

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measures for the removal of the pollutants. The adsorbed pollutant is phtocatalytcially

short

comings.

Consequently

15-21

approaches

like

doping,

thermal

etching,

Among these approaches, self doping in C3N4 alters

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degraded under sunlight irradiation offering fast regeneration of the adsorptive

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photocatalyst and an environmentally benign solution for the complete removal of the

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harmful contaminants.

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Use of free flowing microspheres is advantageous in packed bed systems and is thus

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favored for industrial applications.27 One of the techniques employed for the fabrication

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of microspheres is the silica template approach that warrants post synthetic etching

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processes using fluorinated solvents that are time consuming and environmentally

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hazardous.28-31 The alternate approach of solvothermal synthesis utilizes organic solvents

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that pose issues related to its safe disposal.32-33 Aqueous spray drying is an

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environmentally benign solution amenable for industrial scale powder granulation.27,

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Spray drying consists of atomizing a solution into liquid drops in a hot air flow to get dry

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solid particles after water evaporation. In this work, spray drying was employed for the

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first time to develop adsorptive photocatalysts based on granulated g-C3N4 with high

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surface area. This template free spray drying process followed by thermal oxidation in air

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atmosphere enabled the formation of microspheres having wider pore size distribution. As

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an added benefit, the incomplete burn out of the PVA binder induced self doping of

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carbon in C3N4 sheets leading to improved light absorption and reduced exciton

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recombinations. The adsorption and subsequent photocatalytic activity of the as prepared

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sample was evaluated by tetracycline (TC) degradation under sunlight irradiation. The TC

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adsorption on of spray dried C3N4 has been further analyzed by using the different

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kinetics and equilibrium adsorption isotherm models. TC molecules adsorbed in the

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granules were successfully degraded by sunlight induced photocatalysis and the

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recyclability studies showed no significant loss of activity. The study thus provides

34


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processing pathways for the realization of photocatalysts that are bifunctional and photo-

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regenerable.

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Results and Discussion

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Crystal Structure

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XRD pattern of spray dried C3N4 (CSDC) showed consistent peaks with that of the bulk C3N4

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(BC) indicating that the C3N4 phase has been retained after spray drying and thermal oxidation

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(Figure 1). The main peak at 27.48° corresponded to the (002) plane and signified the

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interplanar stacking of aromatic units.35 The low angle reflection peak (100) at about 13.1° was

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attributed to the crystal plane of tri-s-triazine units.16 Compared with bulk C3N4, the peak at

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27.48° was shifted to 27.73°, indicating a decreased gallery distance between the layers of C3N4

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sheets. The IR spectra of the CSDC granules and BC are presented in Figure S1 which showed

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identical peaks as that of bulk C3N4 reported earlier.36

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110 111 112 113

Figure 1. XRD pattern of bulk C3N4 (BC) and spray dried C3N4 (CSDC) granules (inset shows the shift in main intensity peak).

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Morphological Features 5 ACS Paragon Plus Environment


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The morphology of spray dried samples was analyzed by SEM imaging. Figure 2 depicting the

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SEM images of spray dried granules indicated a gradation in granule sizes up to 20 microns. The

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magnified image of a typical granule suggested that the C3N4 sheets are spheriodised using the

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PVA binder (Figure 2 a and b). C3N4 sheets dispersed in the aqueous slurry containing very low

119

amounts of PVA binder yielded near spherical granules on atomization. The larger distribution of

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granule sizes, from 5-20 micron is a characteristic feature of the spray drying process. The

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morphological features of the granules are however retained after the calcination and thermal

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oxidation etching processes (Figure 2 c and d). Figure S2 illustrates the SEM images of CSDC

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at different magnifications and Figure S3a and b shows the TEM image and EDX analysis of

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sheet like bulk C3N4 (BC). The TEM images of the spray dried granules (Figure S4) revealed the

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nature of porosity. The high magnification image of the granule presented in Figure S4

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confirmed the porous nature of the granules. The granules appeared as an assembly of C3N4

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sheets with slits and pores in between them. The template free method of granulation from

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aqueous slurries used in our studies is an environmentally benign process devoid of any chemical

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etching commonly employed for template based spheriodisation process.

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Figure 2. SEM images of spray dried C3N4 granules (a) and (b) before calcination, (c) and (d)

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after calcination.

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Optical Properties

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UV-Visible absorption analysis

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The electronic band structures of the samples were analyzed by diffused reflectance spectra

136

(Figure 3a). The BC showed shoulder around 440 nm. The absorption spectra of CSDC granules

137

indicated a red shift extending up to 800 nm with increased absorption intensity. This red shift

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and increased intensity was attributed to the in situ doping carbon present in the spray dried

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granules, which was also clear from the colour changing from pale yellow to brown. The

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bandgap value was estimated from Tauc equation1.37

141

ℎ/ = ℎ − )

142

Where α, hν, Eg and A were absorption coefficient, the photon energy, direct bandgap and a

143

constant, respectively. In the equation n, is a number indicating the nature of transition in the

144

material with values of ½ and 2 for direct and indirect transitions respectively. C3N4 is an

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indirect bandgap material and the procedure to measure bandgap is given in the supporting

146

information. Figure 3b indicated a reduction of bandgap from 2.80 eV for BC to 2.54 eV for the

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CSDC granules. The reduction in bandgap facilitated by carbon doping induced extension of

148

absorption to longer wavelength region as reported earlier.22, 24

Equation (1)

149



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Figure 3. (a) UV-Visible absorption spectra and (b) band gap estimation of bulk C3N4 (BC) and

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spray dried C3N4 (CSDC) granules.

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The band edge potentials of both spray dried C3N4 and bulk C3N4 was estimated using the

154

equation 2 and 3.38

155

= χ − 0.5

Equation (2)

156

= −

Equation (3)

157

where EVB, ECB and χ represented respectively the valence band edge potential, conduction band

158

edge potential, and the electronegativity of the semiconductor in Mulliken’s scale (which is the

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geometric mean of constituent atoms). Ee and Eg provided the energy of free electrons on the

160

hydrogen scale (4.5 eV vs. NHE) and the band gap of semiconducting photocatalyst respectively.

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The χvalue for the C3N4 is 4.64.38-39 The valence band (VB) edge potentials are 1.54 and 1.41 eV

162

and conduction band (CB) edge potentials are -1.26 and -1.13 eV for bulk and spray dried

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granules respectively. Both the CB and VB edge potentials shifted significantly after carbon

164

doping improving the absorption in the visible region.


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Photoluminescence emission

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The mass normalized PL emission spectra presented in Figure 4 compare the separation

167

efficiency of photogenerated electron-hole pairs in C3N4 sheets and granules. The broad emission

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peak at 460 nm is a characteristic of C3N4 due to band-band emission ascribed to the n-π*

169

electronic transitions involving lone pairs of nitrogen atom in C3N4.15 The significant reduction

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in peak intensity observed for spray dried C3N4 granules revealed lowered exciton

171

recombinations as reported in many other systems.40 This is ascribed to carbon doping arising

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out of the residual carbon from the incomplete binder (PVA) burn out process. This has been

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earlier observed in carbon doped C3N4 obtained by glucose incorporation and by pre-treatment

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with absolute ethanol.22, 24 The large delocalized π bonds originating from carbon doping induces

175

reduced exciton recombinations.41

176 177

Figure 4. PL emission spectra of bulk and spray dried C3N4 at an excitation wavelength of 360

178

nm.

179

Chemical Composition Analysis

180

The C/N ratios of pure C3N4 and spray dried samples were done by elemental analysis and the

181

value of 0.627 calculated for the spray dried sample was slightly higher than that of pure C3N4



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(0.564) as has been observed previously.22 The increased C/N ratio in CSDC granules sample

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indicated the inclusion of residual carbon arising out of the incomplete burn out of PVA during

184

the heat treatment at 550 °C. This in situ doping of carbon was an outcome of the nitrogen rich

185

environment created during the thermal oxidation etching of C3N4 in the temperature range of

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400-550 0C. The self doping of carbon was further confirmed through XPS analysis.

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XPS analysis of bulk (BC) and spray dried C3N4 (CSDC) is presented in Figure 5. XPS

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survey spectrum of both samples revealed that the elemental composition is carbon and

189

nitrogen (Figure S5). Both exhibited similar C 1s and N 1s spectra without any

190

significant peak shift indicating similar chemical states of C 1s and N 1s in both the

191

samples. The deconvoluted C 1s spectra showed four peaks at 283.7, 284.4 286.4 and

192

288.2 eV which corresponds to graphitic C=C or the cyano-group, adventitious carbon,

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C-NH2 species and N-C-N coordination in the graphitic carbon nitride respectively

194

(Figure 5 a and c).42 The N 1s peak (Figure 5 b) of bulk C3N4 is deconvoluted into two

195

peaks at 397.7 eV and 399.1 eV due to sp2 N in the triazine rings and bridging nitrogen

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atoms respectively.43 The intensity of C 1s peak at 283.7 eV is increased while the N 1s

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peak at 397.7 eV is decreased in the CSDC sample (Figure 5 c and d). This is presumed

198

to be due to the deficiency of N in CSDC prompted by the thermal oxidation etching

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process and subsequent in situ doping of carbon from the residue of PVA burn out. The

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C/N ratio estimated from the XPS analysis indicated an increase from 0.86 for bulk C3N4

201

to 1.08 for spray dried calcined granules. This further substantiated the postulation of in

202

situ carbon doping in spray dried samples. It is already reported that the replacement of

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bridging nitrogen by carbon due to doping, could create large delocalized π bonds that

204

enhances the electrical conductivity and impedes the electron hole recombinations. The


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presence of large delocalized π bonds is also reported to favor increased adsorption of

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organic moieties due to strong π−π interactions.24, 41

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208 209

Figure 5. High resolution of XPS patterns (a) C 1s and (b) N 1s of bulk C3N4 (BC) and (c) C 1s

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and (d) N 1s of carbon doped spray dried C3N4 granules (CSDC).

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Granulation process

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The synthesis procedure involving urea and thiourea as a precursor mix produced C3N4 sheets in

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good yield and enhanced surface area compared to single precursors. Nevertheless, the obtained

214

particles are still aggregated due to which a mechanical exfoliation process like ball milling was

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essential to arrive at aqueous slurry of good dispersibilty. Atomization of the slurry with PVA as

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binder yielded aggregation and spheriodisation of C3N4 sheets resulting in micron sized granules.

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Subsequent calcination of the spray dried granules in air at 550 °C produced porous granules due

218

to thermal oxidation etching as reported before. The formation steps are shown in Figure 6.

219

After the calcination process the colour of the material is changed from yellow to brown



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indicating the in-situ doping of carbon, as confirmed from the optical as well as chemical

221

composition analysis.

222 223 224

Figure 6. Schematic representation of the formation of spray dried porous C3N4 granules with carbon doping (CSDC).

225

Surface area and pore Characteristics

226

The spherical arrangement of sheets during the atomization process lead to micron sized granules

227

and the subsequent post granulation thermal oxidation etching process yielded high surface area

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granules with increased porosity. The bulk C3N4 obtained from the urea-thiourea mixture yielded

229

surface area values of 67 m2/g. On spray granulation and thermal oxidation, the surface area

230

values increased many fold to 151 m2/g. The adsorption isotherms provided in Figure 7 a

231

indicated type II b adsorption behavior, characteristic of adsorption on surface and in cavities

232

formed by the agglomeration of sheet like structures. The pore size distribution by BJH method

233

(Figure 7 b) provided pore sizes in the range of 2-150 nm. It is likely that the agglomerates

234

initially consisted of meso and macropore architecture. The spray dried granules had a wider

235

pore distribution and their total pore volume and surface area (0.46 cc/g and 151 m2/g) were

236

significantly better than the bulk sample (0.33 cc/g and 67 m2/g). The textural properties of the 12 ACS Paragon Plus Environment

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bulk and spray dried C3N4 is summarized in Table S1. The presence of large number of surface

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active sites is presumed to facilitate the adsorption and transfer of pollutant molecules through

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the interconnected porous network structure favoring superior photocatalytic activity.

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241 242

Figure 7. (a) Nitrogen adsorption-desorption isotherms and (b) BJH pore size distribution

243

curves of bulk C3N4 (BC) and spray dried C3N4 (CSDC) granules.

244 245

Bifunctional application: Adsorption and Photocatalytic degradation of tetracycline (TC)

246

antibiotic

247

To demonstrate the bi-functional nature of granules, adsorption and photocatalytic experiments

248

were carried out under dark and sunlight irradiation respectively using tetracycline (TC) as a

249

model pollutant. Figure 8 presents the change in concentration of TC with time for the CSDC

250

granules in comparison with BC. The TC does not undergo self photolysis as its concentration

251

remains unchanged with time. The spray dried C3N4 granules showed higher percentage

252

adsorption (79.4%) compared to bulk C3N4 (8%). Adsorption of TC is monitored for 180 minutes

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and it is observed that adsorption equilibrium is reached within 60 minutes (Figure S6).The 13 ACS Paragon Plus Environment


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porous morphology and enhanced surface area of the granules induced high rates of adsorption

255

and the adsorbed TC was degraded to greater than 90% within 30-90 minutes of sunlight

256

exposure. The spray dried granules containing the in situ doped carbon created large number of

257

delocalized π bonds which effectively contributed to the enhanced adsorption and photocatalytic

258

degradation, whereas bulk g- C3N4 degraded 54 % only. The extended visible light activity due

259

to small amount of doped carbon and enhanced surface area ensured high photocatalytic activity

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in spray dried C3N4 granules. As the spray dried granules indicated high adsorption, the samples

261

were further analyzed for its adsorption features.

262 263

Figure 8. Adsorption cum photocatalytic degradation of 100 µm TC using spray dried C3N4

264

granules (CSDC) and bulk C3N4 (BC).

265

Adsorption studies

266

Adsorption capacity

267

The adsorption capacity qe (mg g-1) of the sample was estimated by evaluating the samples in

268

various concentrations of TC for a fixed period of time (60 min). Figure 9 illustrates the

269

variation in qe as a function of different TC concentrations. As the TC concentration increases,

270

the qe also increases, reaching a maximum value at 100 µM [qm (mg g-1)]. Further increase in the 14 ACS Paragon Plus Environment

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concentration of TC reduces the qe due to adsorption equilibrium. The spray dried C3N4 granules

272

(CSDC) showed the highest adsorption capacity of 70 mg g-1 and was significantly higher than

273

that for the bulk C3N4 (BC) (5 mg g-1). The carbon doping in spray dried granules induced π-π

274

interactions with the aromatic moieties of tetracycline leading to the enhanced adsorption of the

275

latter.23, 41 Through DFT calculations, it is established that the carbon doping in C3N4 leads to

276

substitution of bridging N atoms with C atoms. This induces formation of large delocalized

277

π bonds among the substituted carbons and the hexatomic rings.24 The aromatic π system in

278

tetracycline molecule effectively interact with the delocalized π system of the carbon doped

279

C3N4. This strong π-π interaction leads to enhanced Tetracycline adsorption on the surface of

280

carbon doped spray dried C3N4 granules.

281

realized by the granulation process and the consequent increase in the surface area favoured

282

improved adsorption properties compared to pristine C3N4.

Moreover the macro-meso porous architectures

283 284

Figure 9. Adsorption capacity of different concentrations of TC using spray dried C3N4 (CSDC).

285

Adsorption isotherm

286

The rate of adsorption followed pseudo second order kinetics and the isotherm fitted well with

287

the Freundlich isotherm model. This is attributed to the presence of larger pore volume in the

288

spray dried granules which can host larger sized antibiotic molecules in their pore channels. The



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289

adsorption data of 100 µM TC was evaluated using the isotherm models of Langmuir,

290

Freundlich, and Dubinin–Kaganer–Radushkevich (DKR). The TC adsorption on spray dried g-

291

C3N4 predominantly followed the Freundlich isotherm model where the linear form is

292

represented as44

293

=

294

where Ce (mgL-1) is the equilibrium TC concentration, Kf (mg1-1/n g-1 L1/n) is the Freundlich

295

constant related to the Gibb’s free energy of adsorption and n (g L-1) is another Freundlich

296

constant related to the adsorption intensity. Figure 10 shows the Freundlich isotherm plot for the

297

adsorption of TC on the CSDC granules. Table 1 summarizes the parameters of Freundlich

298

isotherm. The Langmuir and DKR isotherm equations (S8 and S9) and plots of TC adsorption

299

are presented in Figure S7. The slope is a measure of adsorption intensity or surface

300

heterogeneity, a value closer to zero implies more heterogeneity.

!

Equation (4)

301 302

Figure 10. Freundlich adsorption isotherm of 100 µm TC using spray dried C3N4 (CSDC)

303

granules.

304

Table 1. Summary of the parameters related to Freundlich isotherm Concentration

100 µM 16

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305

Isotherm

Freundlich isotherm

306

r2

0.99928

1/n

0.51702

Kf

0.710959

307 308 309 310

The rate at which the TC is adsorbed from an aqueous solution of the spray dried granules is

311

analyzed using different kinetic models. The adsorption data of 100 µM TC fits the pseudo-

312

second order kinetics which can be represented as44

313

"

"

= × "

Equation (5)

$ $ ×

314 315

where K2 (g mg-1 min

-1

316

adsorbed on the surface per unit mass of adsorbent (mg g-1) at the contact time of t. Figure 11

317

shows typical pseudo-second order kinetics plot for the adsorption of TC on the surface of the

318

composite prepared at 100 µM. Table 2 summarizes the details of pseudo-second order kinetics.

319

The regression correlation coefficient value is close to 1 and the estimated qe value using

320

this model is comparable with the experimentally determined value of qe. Thus, it verifies that

321

the adsorption of TC on spray dried g-C3N4 (CSDC) follows pseudo-second order kinetics. In

322

short, the adsorption studies indicated that the CSDC (0.5 gL-) possessed higher adsorption

323

capacity (qm) of 70 mg g-1 for 100 µM TC solution compared to a value of 5 mg g-1 for bulk

324

C3N4 (BC) sheets. The higher adsorption capacity observed for the spray dried sample could be

325

due to the larger surface area and pore volume of the spray dried sample compared to the bulk

326

sample. Moreover, as is clear from Figure 7b, the spray dried sample contained significant

) is pseudo-second order rate constants and qt is the amount of TC



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327

amount of larger pores compared to the bulk sample. This adsorption rate followed the pseudo

328

second order kinetics and fitted well with the Freundlich isotherm model.

329 330

Figure 11. Pseudo second order kinetics of adsorption 100 µm TC using spray dried C3N4

331

(CSDC) granules.

332

Table 2. Summary of the parameters related to pseudo-second order kinetics Concentration

100 µM

Kinetics

Pseudo-second order

r2

0.97616

K2

0.002

qe (calculated)

75

qe (experimental)

70

333 334

Different reactive oxygen scavengers (ROS) were employed to detect the reactive species.

335

10mM of Isopropyl Alcohol (IPA) (the quencher of hydroxyl radical)., 6mM AgNO3 (the

336

quencher of electron), 6mM Benzoquinone (BQ) (the quencher of superoxide anion radical),

337

10mM Triethanolamine (TEA) (the quencher of holes) were respectively added in the

338

photocatalytic reaction mixture and irradiated under sunlight.45 Figure 12 compares the


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339

formation of active species, through trapping experiments, during the photocatalytic reaction. It

340

is found that photocatalytic degradation of TC was not affected by the addition of isopropyl

341

alcohol indicating that the mechanism is not dominated by hydroxyl radicals. On the other hand,

342

a significant reduction in activity was observed by the addition of AgNO3 confirming the

343

mechanistic role of electrons in the photodegradation process. The degradation, however, was

344

drastically quenched by the addition of triethanolamine and benzoquinone indicating that the

345

dominant reactive species controlling the photocatalytic degradation of tetracycline are holes and

346

super oxide anions radicals respectively. Therefore, it can be concluded that the species affecting

347

the degradation is in the order of holes>superoxide anions>electrons.

348 349

Figure 12. Reactive oxygen species trapping of spray dried C3N4 (CSDC) granules using IPA,

350

TEA, BQ, and AgNO3 under sun light irradiation.

351

Based on the ROS experiments, UV-Vis absorption, and PL emission analysis a possible

352

mechanism for the photocatalytic degradation of the TC molecule is illustrated in Figure 13.

353

From the band edge potential calculations, it is noted that both the CB and VB edge potentials

354

altered significantly after carbon doping leading to the improved visible light absorption ability.

355

Also the high surface area enabled more surface active sites for the adsorption of TC molecules 19 ACS Paragon Plus Environment


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356

on spray dried granules and exhibited more photocatalytic degradation. Under visible light

357

irradiation C3N4, generates excitons. The reduction potential of O2/O2- is -0.33 eV and hence the

358

photogenerated electrons could easily react with O2 and reduces it into superoxide radical (O2-)

359

anions that degrade the TC molecules. On the other hand, EVB value of g-C3N4, is +1.41 eV and

360

is lower than the standard redox potential of OH*/H2O (+2.68 eV) and OH*/OH- (1.99 eV). Thus,

361

the photogenerated holes present in the valence band cannot react with H2O or OH- to generate

362

active oxidative species OH*. Hence holes (h+) in the valence band of spray dried C3N4, degrades

363

the TC molecules directly.41,

364

monitored by conducting chemical oxygen demand (COD) experiment (Figure S8). The COD

365

value of 100 µm TC soluton is 202 mg/L where as after adsorption and photocatalysis the values

366

are 28 and 60 mg/L respectively. This increse in value after adsorption is attributed to the

367

formation of intermediate carbonaceous species during photocatalysis.

46

The degree of mineralization of TC degradation preocess is

368 369

Figure 13. Photocatalytic mechanism of tetracycline degradation by spray granulated C3N4

370

(CSDC).


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371

The efficiency of the developed granules for repeated adsorption and photocatalysis are

372

evaluated through cyclic studies. Figure 14 presents the data on adsorption and photocatalytic

373

degradation of tetracycline under sunlight irradiation after five cycles of repetition. There is

374

hardly any loss of activity in the adsorption and photocatalytic performance of the granules after

375

five cycles. The XRD and FTIR patterns of samples recorded after the cyclic studies, presented

376

in Figure S9 and S10 indicated no changes in phase and functional groups respectively. The

377

prepared material is thus photo-regenerable exhibiting good stability even multiple 4 cycles.

378 379

Figure 14. Cyclic tests on the adsorption and photocatalytic degradation of 100 µm TC using

380

spray dried C3N4 (CSDC) granules.

381

The extended π-π interactions resulting from in-situ carbon doping combined with high surface

382

area and favorable pore characteristics lead to high amount of TC adsorption by the spray dried

383

granules. The adsorbed as well as the TC retained in solution was subsequently degraded under

384

sunlight irradiation. The improved visible light absorption, reduced exciton recombinations

385

thorough carbon doping and enhanced surface area by virtue of the porous architecture aided

386

better photocatalytic activity. Thus the template free, aqueous spray drying process leading to

387

the formation of bifunctional and photo-regenerable C3N4 granules offers an environmentally

388

benign solution for the emerging water pollutants without any secondary treatment.



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389

Conclusions

390

Bulk C3N4 sheets derived from the pyrolysis of urea-thiourea mixture was converted to high

391

surface area granules employing a template free spray drying process followed by thermal

392

oxidation at 500 °C. The spray granulation and oxidation process introduced carbon doping in

393

C3N4 resulting in reduced band gap and extended absorption in the visible light region. The C3N4

394

granules thus formed are demonstrated to perform the dual functions of both adsorption and

395

photocatalysis for the efficient degradation of tetracycline. Recyclability studies indicated no loss

396

of activity even after five cycles. The template free spray drying approach offer a greener

397

pathway for the development of photo-regenerable, bifunctional photocatalysts for the effective

398

degradation of organic contaminants in the environment.

399

ASSOCIATED CONTENT

400

Supporting Information: Detailed experimental procedures for the synthesis of spray dried

401

C3N4 granules, tetracycline adsorption measurements, tetracycline adsorption cum photocatalytic

402

degradation, regeneration, characterization techniques employed, Detailed procedure for band

403

gap measurement, FTIR, XPS survey spectrum, SEM, TEM and elemental analysis of bulk and

404

spray dried C3N4, Textural property table, adsorption of tetracycline in the dark, Langmuir and

405

DKR isotherm plots, COD measurements, XRD and IR of spray dried C3N4 granules before and

406

after photocatalysis.

407

Author Information

408

Corresponding Author

409

E-mail: [email protected], [email protected]

410

Tel: 04712535504 (Office), 09446337222 (mobile)

411

Acknowledgements 22 ACS Paragon Plus Environment

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412

Mr. Chandran and Mr. Kiran Mohan are thankfully acknowledged for the SEM and HRTEM

413

micrographs respectively. Ms. Athira A. S. is kindly acknowledged for the COD measurements.

414

The authors are grateful to Council of Scientific and Industrial Research (CSIR, Government of

415

India) for the 12th five year plan project on “IntelCoat” (CSC0114). Author S P thanks CSIR for

416

the research fellowship.

417 418

References

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For Table of Contents only

540 541 542 543 544 545

Photo-regenerable, bifunctional granules of carbon doped g-C3N4 as adsorptive photocatalyst for the efficient removal of tetracycline antibiotic

546 547 548

Suyana Panneri, Priyanka Ganguly, Balagopal N. Nair, Abdul Azeez Peer Mohamed, Krishna G. Warrier and U.S. Hareesh*

549 550 551 552

Synopsis: Porous C3N4 granules as high capacity adsorptive photocatalysts for a greener solution of tetracycline removal

553 554


Page 27 of 41

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TOC 60x48mm (150 x 150 DPI)



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Figure 1. XRD pattern of bulk C3N4 (BC) and spray dried C3N4 (CSDC) granules (inset shows the shift in main intensity peak). 180x156mm (300 x 300 DPI)


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Figure 2. SEM images of spray dried C3N4 granules (a) and (b) before calcination, (c) and (d) after calcination. 42x32mm (300 x 300 DPI)



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Figure 3. (a) UV-Visible absorption spectra and (b) band gap estimation of bulk C3N4 (BC) and spray dried C3N4 (CSDC) granules. 219x186mm (300 x 300 DPI)


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Figure 4. PL emission spectra of bulk and spray dried C3N4 at an excitation wavelength of 360 nm. 36x32mm (300 x 300 DPI)



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Figure 5. High resolution of XPS patterns (a) C 1s and (b) N 1s of bulk C3N4 (BC) and (c) C 1s and (d) N 1s of carbon doped spray dried C3N4 granules (CSDC). 43x37mm (300 x 300 DPI)


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Figure 6. Schematic representation of the formation of spray dried porous C3N4 granules with carbon doping (CSDC). 254x184mm (300 x 300 DPI)



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Figure 7. (a) Nitrogen adsorption-desorption isotherms and (b) BJH pore size distribution curves of bulk C3N4 (BC) and spray dried C3N4 (CSDC) granules. 240x106mm (300 x 300 DPI)


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Figure 8. Adsorption cum photocatalytic degradation of 100 µm TC using spray dried C3N4 granules (CSDC) and bulk C3N4 (BC). 40x32mm (300 x 300 DPI)



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Figure 9. Adsorption capacity of different concentrations of TC using spray dried C3N4 (CSDC). 39x32mm (300 x 300 DPI)


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Figure 10. Freundlich adsorption isotherm of 100 µm TC using spray dried C3N4 (CSDC) granules. 40x32mm (300 x 300 DPI)



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Figure 11. Pseudo second order kinetics of adsorption 100 µm TC using spray dried C3N4 (CSDC) granules. 39x32mm (300 x 300 DPI)


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Figure 12. Reactive oxygen species trapping of spray dried C3N4 (CSDC) granules using IPA, TEA, BQ, and AgNO3 under sun light irradiation. 167x129mm (300 x 300 DPI)



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Figure 13. Photocatalytic mechanism of tetracycline degradation by spray granulated C3N4 (CSDC). 219x123mm (300 x 300 DPI)


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Figure 14. Cyclic tests on the adsorption and photocatalytic degradation of 100 µm TC using spray dried C3N4 (CSDC) granules. 32x32mm (300 x 300 DPI)


Photoregenerable, Bifunctional Granules of ... - ACS Publications

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