Photoregenerable, Bifunctional Granules of Carbon-Doped g-C3N4 as

Research Article pubs.acs.org/journal/ascecg

Photoregenerable, 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 G. Warrier,† and U.S. Hareesh*,†,‡ †

Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram 695019, India ‡ Academy of Scientific and Innovative Research (AcSIR), New Delhi 110025, India § R&D Center, Noritake Co. Limited, Miyoshi-cho, Miyoshi, Aichi 470-0293, Japan ∥ Nanochemistry Research Institute, Department of Chemistry, Curtin University, GPO Box UI987, Perth WA6845, Australia S Supporting Information *

ABSTRACT: Environmental remediation employing semiconducting materials offer a greener solution for pollution control. Herein, we report the development of high surface area porous architecture of C3N4 nanosheets by a simple aqueous spray drying process. g-C3N4 nanosheets obtained by the thermal decomposition of urea-thiourea mixture are spray granulated to microspheres using 2 wt% poly vinyl alcohol (PVA) as binder. The post granulation thermal oxidation treatment resulted in in situ doping of carbon leading to improved photophysical properties compared to pristine g-C3N4. The C3N4 granules with surface area values of 150 m2/g rendered repetitive adsorption of tetracycline antibiotic (∼75% in 60 min) and the extended absorption in the visible region facilitated complete photocatalytic degradation upon sunlight irradiation (>95% in 90 min). The delocalized π bonds generated after carbon doping and the macro-meso porous architecture created by the granulation process aided high adsorption capacity (70 mg/g). The photoregenerable, bifunctional materials herein obtained can thus be employed for the adsorption and subsequent degradation of harmful organic pollutants without any secondary remediation processes. KEYWORDS: Graphitic carbon nitride (g-C3N4), Spray granulation, Carbon doping, Adsorptive photocatalyst, Photoregenerable, Tetracycline

1. INTRODUCTION Utilization of photocatalytic semiconductors for the degradation of organic pollutants and harmful microbes has been amply demonstrated through a variety of systems like TiO2, ZnO, ZnS, etc.1−4 The widespread use of pharmaceutical compounds like antibiotics has, in recent times, posed a major threat to the environment due to issues associated with its degradation and safe disposal. Consequently, we are faced with the emergence of newer hazards, like antibiotic resistant bacteria, that call for immediate remedial measures.5,6 The greener approach for the alleviation of such issues is the use of sunlight active photocatalysis with appreciable efficiencies and recyclability.7 A recent entrant to the category of semiconductor photocatalysts is graphitic carbon nitride (g-C3N4) with a band gap value of 2.7 eV.8,9 This organic semiconductor characterized by exceptional chemical and thermal stability found wide applications in water splitting, organic pollutant degradation, organic catalysis and as electrocatalysts under visible light.10−13 However, the poor surface area, the faster recombination of © 2016 American Chemical Society

excitons, and the low visible light absorption render g-C3N4 less effective.14 Multiple strategies are adopted to overcome these short comings. Consequently approaches like doping, thermal etching, heterostructure formation, and liquid phase exfoliation are employed to improve the catalytic efficiencies.11,15−21 Among these approaches, self-doping in C3N4 alters electronic properties favorably for increased absorption over an extended wavelength regime.22−25 The thermal oxidation etching process induces exfoliation of the stacked C3N4 layers resulting in significantly improved surface area values.16 Conventional adsorption of pollutants is currently practiced through materials like activated charcoal, silica gel, fly ash, MOF, etc by virtue of their high surface area and adsorption efficiency. However, these adsorbents necessitate stringent conditions for desorption making the regeneration process Received: October 3, 2016 Revised: November 28, 2016 Published: December 20, 2016 1610

DOI: 10.1021/acssuschemeng.6b02383 ACS Sustainable Chem. Eng. 2017, 5, 1610−1618

Research Article

ACS Sustainable Chemistry & Engineering environmentally unfriendly and expensive.26 It is therefore desirable to develop materials that combine adsorption with photocatalytic activity resulting in the formation of bifunctional adsorptive photocatalysts. This approach is particularly advantageous as it does not require secondary remediation measures for the removal of the pollutants. The adsorbed pollutant is photocatalytically degraded under sunlight irradiation offering fast regeneration of the adsorptive photocatalyst and an environmentally benign solution for the complete removal of the harmful contaminants. Use of free-flowing microspheres is advantageous in packed bed systems and is thus favored for industrial applications.27 One of the techniques employed for the fabrication of microspheres is the silica template approach that warrants post synthetic etching processes using fluorinated solvents that are time-consuming and environmentally hazardous.28−31 The alternate approach of solvothermal synthesis utilizes organic solvents that pose issues related to its safe disposal.32,33 Aqueous spray drying is an environmentally benign solution amenable for industrial scale powder granulation.27,34 Spray drying consists of atomizing a solution into liquid drops in a hot air flow to get dry solid particles after water evaporation. In this work, spray drying was employed for the first time to develop adsorptive photocatalysts based on granulated g-C3N4 with high surface area. This template free spray drying process followed by thermal oxidation in air atmosphere enabled the formation of microspheres having wider pore size distribution. As an added benefit, the incomplete burn out of the PVA binder induced self-doping of carbon in C3N4 sheets leading to improved light absorption and reduced exciton recombinations. The adsorption and subsequent photocatalytic activity of the as prepared sample was evaluated by tetracycline (TC) degradation under sunlight irradiation. The TC adsorption on spray dried C3N4 has been further analyzed by using the different kinetics and equilibrium adsorption isotherm models. TC molecules adsorbed in the granules were successfully degraded by sunlight induced photocatalysis and the recyclability studies showed no significant loss of activity. The study thus provides processing pathways for the realization of photocatalysts that are bifunctional and photoregenerable.

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

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

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distribution of granule sizes, from 5 to 20 μm is a characteristic feature of the spray drying process. The morphological features of the granules are however retained after the calcination and thermal oxidation etching processes (Figure 2c,d). Figure S2 illustrates the SEM images of CSDC at different magnifications and Figure S3a and b shows the TEM image and EDX analysis of sheet like bulk C3N4 (BC). The TEM images of the spray dried granules (Figure S4) revealed the nature of porosity. The high magnification image of the granule presented in Figure S4 confirmed the porous nature of the granules. The granules appeared as an assembly of C3N4 sheets with slits and pores in between them. The template free method of granulation from aqueous slurries used in our studies is an environmentally benign process devoid of any chemical etching commonly employed for template based spheriodisation process. Optical Properties. UV−visible Absorption Analysis. The electronic band structures of the samples were analyzed by diffused reflectance spectra (Figure 3a). The BC showed shoulder around 440 nm. The absorption spectra of CSDC granules indicated a red shift extending up to 800 nm with increased absorption intensity. This red shift and increased intensity was attributed to the in situ doping of carbon in the spray dried granules, which was also clear from the color

RESULTS AND DISCUSSION Crystal Structure. XRD pattern of spray dried C3N4 (CSDC) showed consistent peaks with that of the bulk C3N4 (BC) indicating that the C3N4 phase has been retained after spray drying and thermal oxidation (Figure 1). The main peak at 27.48° corresponded to the (002) plane and signified the interplanar stacking of aromatic units.35 The low angle reflection peak (100) at about 13.1° was attributed to the crystal plane of tri-s-triazine units.16 Compared with bulk C3N4, the peak at 27.48° was shifted to 27.73°, indicating a decreased gallery distance between the layers of C3N4 sheets. The IR spectra of the CSDC granules and BC are presented in Figure S1 which showed identical peaks as that of bulk C3N4 reported earlier.36 Morphological Features. The morphology of spray dried samples was analyzed by SEM imaging. Figure 2 depicting the SEM images of spray dried granules indicated a gradation in granule sizes up to 20 μm. The magnified image of a typical granule suggested that the C3N4 sheets are spheriodised using the PVA binder (Figure 2a,b). C3N4 sheets dispersed in the aqueous slurry containing very low amounts of PVA binder yielded near spherical granules on atomization. The larger 1611


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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.

changing from pale yellow to brown. The bandgap value was estimated from Tauc equation1.37 (αh ϑ)1/ n = A(h ϑ − Eg )

(1)

Where α, hν, Eg, and A were absorption coefficient, the photon energy, bandgap and a constant, respectively. In the equation n, is a number indicating the nature of transition in the material with values of 1/2 and 2 for direct and indirect transitions, respectively. C3N4 is an indirect bandgap material and the procedure to measure bandgap is given in the Supporting Information. Figure 3b indicated a reduction of bandgap from 2.80 eV for BC to 2.54 eV for the CSDC granules. The reduction in bandgap facilitated by carbon doping induced extension of absorption to longer wavelength region as reported earlier.22,24 The band edge potentials of both spray dried C3N4 and bulk C3N4 was estimated using the eqs 2 and 3.38 E VB = χ − E e + 0.5Eg

(2)

ECB = E VB − Eg

(3)

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

by glucose incorporation and by pretreatment with absolute ethanol.22,24 The large delocalized π bonds originating from carbon doping induces reduced exciton recombinations.41 Chemical Composition Analysis. The C/N ratios of pure C3N4 and spray dried samples were done by elemental analysis and the value of 0.627 calculated for the spray dried sample was slightly higher than that of pure C3N4 (0.564) as has been observed previously.22 The increased C/N ratio in CSDC sample indicated the inclusion of residual carbon arising out of the incomplete burn out of PVA during the heat treatment at 550 °C. This in situ doping of carbon was an outcome of the nitrogen rich environment created during the thermal oxidation etching of C3N4 in the temperature range of 400−550 °C. The self-doping of carbon was further confirmed through XPS analysis. XPS analysis of bulk (BC) and spray dried C3N4 (CSDC) is presented in Figure 5. XPS survey spectrum of both samples revealed that the elemental composition is carbon and nitrogen (Figure S5). Both exhibited similar C 1s and N 1s spectra without any significant peak shift indicating similar chemical states of C 1s and N 1s in both the samples. The deconvoluted C 1s spectra showed four peaks at 283.7, 284.4 286.4 and 288.2 eV which corresponds to graphitic CC or the cyano-group, adventitious carbon, C-NH2 species and N−C−N coordination in the graphitic carbon nitride respectively (Figure 5a and c).42 The N 1 peak (Figure 5b) of bulk C3N4 is deconvoluted into two peaks at 397.7 and 399.1 eV due to sp2 N in the triazine rings and bridging nitrogen atoms, respectively.43 The intensity

where EVB, ECB, and χ represented respectively the valence band edge potential, conduction band edge potential, and the electronegativity of the semiconductor in Mulliken’s scale (which is the geometric mean of constituent atoms). Ee and Eg provided the energy of free electrons on the hydrogen scale (4.5 eV vs NHE) and the band gap of semiconducting photocatalyst, respectively. The χ value for the C3N4 is 4.64.38,39 The valence band (VB) edge potentials are 1.54 and 1.41 eV and conduction band (CB) edge potentials are −1.26 and −1.13 eV for bulk and spray dried granules, respectively. Both the CB and VB edge potentials shifted significantly after carbon doping improving the absorption in the visible region. Photoluminescence Emission. The mass normalized PL emission spectra presented in Figure 4 compare the separation efficiency of photogenerated electron−hole pairs in C3N4 sheets and granules. The broad emission peak at 460 nm is a characteristic of C3N4 due to band−band emission ascribed to the n−π* electronic transitions involving lone pairs of nitrogen atom in C3N4.15 The significant reduction in peak intensity observed for spray dried C3N4 granules revealed lowered exciton recombinations as reported in many other systems.40 This is ascribed to carbon doping arising out of the residual carbon from the incomplete binder (PVA) burn out process. This has been earlier observed in carbon doped C3N4 obtained 1612


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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).

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

π bonds is also reported to favor increased adsorption of organic moieties due to strong π−π interactions.24,41 Granulation Process. The synthesis procedure involving urea and thiourea as a precursor mix produced C3N4 sheets in good yield and enhanced surface area compared to single precursors. Nevertheless, the obtained particles are still aggregated due to which a mechanical exfoliation process like ball milling was essential to arrive at aqueous slurry of good dispersibilty. Atomization of the slurry with PVA as binder yielded aggregation and spheriodisation of C3N4 sheets resulting in micron sized granules. Subsequent calcination of the spray dried granules in air at 550 °C produced porous granules due to thermal oxidation etching as reported before.

of C 1s peak at 283.7 eV is increased while the N 1s peak at 397.7 eV is decreased in the CSDC sample (Figure 5c and d). This is presumed to be due to the deficiency of N in CSDC prompted by the thermal oxidation etching process and subsequent in situ doping of carbon from the residue of PVA burn out. The C/N ratio estimated from the XPS analysis indicated an increase from 0.86 for bulk C3N4 to 1.08 for spray dried calcined granules. This further substantiated the postulation of in situ carbon doping in spray dried samples. It is already reported that the replacement of bridging nitrogen by carbon due to doping, could create large delocalized π bonds that enhances the electrical conductivity and impedes the electron hole recombinations. The presence of large delocalized 1613


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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.

The formation steps are shown in Figure 6. After the calcination process the color of the material is changed from yellow to brown indicating the in situ doping of carbon, as confirmed from the optical as well as chemical composition analysis. Surface Area and Pore Characteristics. The spherical arrangement of sheets during the atomization process lead to micron sized granules and the subsequent post granulation thermal oxidation etching process yielded high surface area granules with increased porosity. The bulk C3N4 obtained from the urea-thiourea mixture yielded surface area values of 67 m2/ g. On spray granulation and thermal oxidation, the surface area values increased many fold to 151 m2/g. The adsorption isotherms provided in Figure 7a indicated type II b adsorption behavior, characteristic of adsorption on surface and in cavities formed by the agglomeration of sheet like structures. The pore size distribution by BJH method (Figure 7b) provided pore sizes in the range of 2−150 nm. It is likely that the agglomerates initially consisted of meso and macropore architecture. The spray dried granules had a wider pore distribution and their total pore volume and surface area (0.46 cc/g and 151 m2/g) were significantly better than the bulk sample (0.33 cc/g and 67 m2/g). The textural properties of the bulk and spray dried C3N4 is summarized in Table S1. The presence of large number of surface active sites is presumed to facilitate the adsorption and transfer of pollutant molecules through the interconnected porous network structure favoring superior photocatalytic activity. Bifunctional Application: Adsorption and Photocatalytic Degradation of Tetracycline (TC) Antibiotic. To demonstrate the bifunctional nature of granules, adsorption and photocatalytic experiments were carried out under dark and sunlight irradiation respectively using tetracycline (TC) as a model pollutant. Figure 8 presents the change in concentration of TC with time for the CSDC granules in comparison with BC. The TC does not undergo self-photolysis as its concentration remains unchanged with time. The spray dried C3N4 granules showed higher percentage adsorption (79.4%) compared to bulk C3N4 (8%). Adsorption of TC is monitored for 180 min and it is observed that adsorption equilibrium is reached within 60 min (Figure S6).The porous morphology and enhanced surface area of the granules induced high rates of adsorption and the adsorbed TC was degraded to greater than 90% within 30−90 min of sunlight exposure. The spray dried granules containing the in situ doped carbon created

Figure 8. Adsorption with photocatalytic degradation of 100 μm TC using spray dried C3N4 granules (CSDC) and bulk C3N4 (BC).

large number of delocalized π bonds which effectively contributed to the enhanced adsorption and photocatalytic degradation, whereas bulk g- C3N4 degraded 54% only. The extended visible light activity due to small amount of doped carbon and enhanced surface area ensured high photocatalytic activity in spray dried C3N4 granules. As the spray dried granules indicated high adsorption, the samples were further analyzed for its adsorption features. Adsorption Studies. Adsorption Capacity. The adsorption capacity qe (mg g−1) of the sample was estimated by evaluating the samples in various concentrations of TC for a fixed period of time (60 min). Figure 9 illustrates the variation in qe as a function of different TC concentrations. As the TC concentration increases, the qe also increases, reaching a maximum value at 100 μM [qm (mg g−1)]. Further increase in the concentration of TC reduces the qe due to adsorption equilibrium. The spray dried C3N4 granules (CSDC) showed the highest adsorption capacity of 70 mg g−1 and was significantly higher than that for the bulk C3N4 (BC) (5 mg g−1). The carbon doping in spray dried granules induced π−π interactions with the aromatic moieties of tetracycline leading to the enhanced adsorption of the latter.23,41 Through DFT calculations, it is established that the carbon doping in C3N4 leads to substitution of bridging N atoms with C atoms. This induces formation of large delocalized π bonds among the substituted carbons and the hexatomic rings.24 The aromatic π system in tetracycline molecule effectively interact with the delocalized π system of the carbon doped C3N4. This strong 1614


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Table 1. Summary of the Parameters Related to Freundlich Isotherm Freundlich isotherm

r2 1/n Kf

0.99928 0.51702 0.710959

⎛1⎞ t 1 = ⎜⎜ ⎟⎟ × t + qt q K × qe2 ⎝ e⎠ 2

π−π interaction leads to enhanced tetracycline adsorption on the surface of carbon doped spray dried C3N4 granules. Moreover the macro-meso porous architectures realized by the granulation process and the consequent increase in the surface area favored improved adsorption properties compared to pristine C3N4. Adsorption Isotherm. The rate of adsorption followed pseudo-second order kinetics and the isotherm fitted well with the Freundlich isotherm model. This is attributed to the presence of larger pore volume in the spray dried granules which can host larger sized antibiotic molecules in their pore channels. The adsorption data of 100 μM TC was evaluated using the isotherm models of Langmuir, Freundlich, and Dubinin−Kaganer−Radushkevich (DKR). The TC adsorption on spray dried g-C3N4 predominantly followed the Freundlich isotherm model where the linear form is represented as44 ⎛1⎞ ⎜ ⎟ln C + ln K f e ⎝n⎠

100 μM

isotherm

surface heterogeneity, a value closer to zero implies more heterogeneity. The rate at which the TC is adsorbed from an aqueous solution of the spray dried granules is analyzed using different kinetic models. The adsorption data of 100 μM TC fits the pseudo-second order kinetics which can be represented as44

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

ln qe =

concentration

−1

(5)

−1

where K2 (g mg min ) is pseudo-second order rate constants and qt is the amount of TC adsorbed on the surface per unit mass of adsorbent (mg g−1) at the contact time of t. Figure 11 shows typical pseudo-second order kinetics plot for

(4)

−1

where Ce (mg L ) is the equilibrium TC concentration, Kf (mg1−1/n g−1 L1/n) is the Freundlich constant related to the Gibb’s free energy of adsorption, and n (g L−1) is another Freundlich constant related to the adsorption intensity. Figure 10 shows the Freundlich isotherm plot for the adsorption of TC on the CSDC granules. Table 1 summarizes the parameters of Freundlich isotherm. The Langmuir and DKR isotherm eqs (Figures S8 and S9) and plots of TC adsorption are presented in Figure S7. The slope is a measure of adsorption intensity or

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

the adsorption of TC on the surface of the composite prepared at 100 μM. Table 2 summarizes the details of pseudo-second order kinetics. The regression correlation coefficient ⟨R2⟩ value is close to 1 and the estimated qe value using this model is comparable with the experimentally determined value of qe. Thus, it verifies that the adsorption of TC on spray dried gC3N4 (CSDC) follows pseudo-second order kinetics. In short, the adsorption studies indicated that the CSDC (0.5 g L−) possessed higher adsorption capacity (qm) of 70 mg g−1 for 100 μM TC solution compared to a value of 5 mg g−1 for bulk C3N4 (BC) sheets. The higher adsorption capacity observed for Table 2. Summary of the Parameters Related to PseudoSecond Order Kinetics

Figure 10. Freundlich adsorption isotherm of 100 μm TC using spray dried C3N4 (CSDC) granules. 1615

concentration

100 μM

kinetics

pseudo-second order

r2 K2 qe (calculated) qe (experimental)

0.97616 0.002 75 70 DOI: 10.1021/acssuschemeng.6b02383 ACS Sustainable Chem. Eng. 2017, 5, 1610−1618

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ACS Sustainable Chemistry & Engineering the spray dried sample could be due to the larger surface area and pore volume of the spray dried sample compared to the bulk sample. Moreover, as is clear from Figure 7b, the spray dried sample contained significant amount of larger pores compared to the bulk sample. This adsorption rate followed the pseudo-second order kinetics and fitted well with the Freundlich isotherm model. Reactive Oxygen Species Scavenging Experiments. Different reactive oxygen scavengers (ROS) were employed to detect the reactive species. 10 mM of isopropyl alcohol (IPA) (the quencher of hydroxyl radical), 6 mM AgNO3 (the quencher of electron), 6 mM benzoquinone (BQ) (the quencher of superoxide anion radical), 10 mM triethanolamine (TEA) (the quencher of holes) were respectively added in the photocatalytic reaction mixture and irradiated under sunlight.45 Figure 12 compares the formation of active species, through

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

standard redox potential of OH*/H2O (+2.68 eV) and OH*/ OH− (1.99 eV). Thus, the photogenerated holes present in the valence band cannot react with H2O or OH− to generate active oxidative species OH*. Hence holes (h+) in the valence band of spray dried C3N4, degrades the TC molecules directly.41,46 The degree of mineralization of TC degradation process is monitored by conducting chemical oxygen demand (COD) experiment (Figure S8). The COD value of 100 μm TC solution is 202 mg/L where as after adsorption and photocatalysis the values are 28 and 60 mg/L respectively. This increase in value after adsorption is attributed to the formation of intermediate carbonaceous species during photocatalysis. The efficiency of the developed granules for repeated adsorption and photocatalysis are evaluated through cyclic studies. Figure 14 presents the data on adsorption and

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

trapping experiments, during the photocatalytic reaction. It is found that photocatalytic degradation of TC was not affected by the addition of isopropyl alcohol indicating that the mechanism is not dominated by hydroxyl radicals. On the other hand, a significant reduction in activity was observed by the addition of AgNO3 confirming the mechanistic role of electrons in the photodegradation process. The degradation, however, was drastically quenched by the addition of triethanolamine and benzoquinone indicating that the dominant reactive species controlling the photocatalytic degradation of tetracycline are holes and super oxide anions radicals, respectively. Therefore, it can be concluded that the species affecting the degradation is in the order of holes > superoxide anions > electrons. Based on the ROS experiments, UV−vis absorption, and PL emission analysis a possible mechanism for the photocatalytic degradation of the TC molecule is illustrated in Figure 13. From the band edge potential calculations, it is noted that both the CB and VB edge potentials altered significantly after carbon doping leading to the improved visible light absorption ability. Also the high surface area enabled more surface active sites for the adsorption of TC molecules on spray dried granules and exhibited more photocatalytic degradation. Under visible light irradiation C3N4, generates excitons. The reduction potential of O2/O2− is −0.33 eV and hence the photogenerated electrons could easily react with O2 and reduces it into superoxide radical (O2−) anions that degrade the TC molecules. On the other hand, EVB value of g-C3N4, is +1.41 eV and is lower than the

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

photocatalytic degradation of tetracycline under sunlight irradiation after five cycles of repetition. There is hardly any loss of activity in the adsorption and photocatalytic performance of the granules after five cycles. The XRD and FTIR patterns of samples recorded after the cyclic studies, presented in Figures S9 and S10, indicated no changes in phase and functional groups, respectively. The prepared material is thus photoregenerable exhibiting good stability even multiple 4 cycles. 1616


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five year plan project on “IntelCoat” (CSC0114). Author S.P. thanks CSIR for the research fellowship.

The extended π−π interactions resulting from in situ carbon doping combined with high surface area and favorable pore characteristics lead to high amount of TC adsorption by the spray dried granules. The adsorbed as well as the TC retained in solution was subsequently degraded under sunlight irradiation. The improved visible light absorption, reduced exciton recombinations, thorough carbon doping, and enhanced surface area by virtue of the porous architecture aided better photocatalytic activity. Thus, the template free, aqueous spray drying process leading to the formation of bifunctional and photoregenerable C3N4 granules offers an environmentally benign solution for the emerging water pollutants without any secondary treatment.

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CONCLUSIONS Bulk C3N4 sheets derived from the pyrolysis of urea-thiourea mixture was converted to high surface area granules employing a template free spray drying process followed by thermal oxidation at 500 °C. The spray granulation and oxidation process introduced carbon doping in C3N4 resulting in reduced band gap and extended absorption in the visible light region. The C3N4 granules thus formed are demonstrated to perform the dual functions of both adsorption and photocatalysis for the efficient degradation of tetracycline. Recyclability studies indicated no loss of activity even after five cycles. The template free spray drying approach offers a greener pathway for the development of photoregenerable, bifunctional photocatalysts for the effective degradation of organic contaminants in the environment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02383. Detailed experimental procedures for the synthesis of spray dried C3N4 granules, tetracycline adsorption measurements, tetracycline adsorption with photocatalytic degradation, regeneration, characterization techniques employed; detailed procedure for band gap measurement, FTIR, XPS survey spectrum, SEM, TEM ,and elemental analysis of bulk and spray dried C3N4; textural property table, adsorption of tetracycline in the dark, Langmuir and DKR isotherm plots, COD measurements, XRD and IR of spray dried C3N4 granules before and after photocatalysis (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]; Tel: 04712535504. ORCID

U.S. Hareesh: 0000-0001-6455-8220 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Mr. Chandran and Mr. Kiran Mohan are thankfully acknowledged for the SEM and HRTEM micrographs, respectively. Ms. Athira A. S. is kindly acknowledged for the COD measurements. The authors are grateful to Council of Scientific and Industrial Research (CSIR, Government of India) for the 12th 1617


Research Article

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Photoregenerable, Bifunctional Granules of Carbon-Doped g-C3N4 as

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