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DOI: 10.1016/j.jmrt.2018.11.008
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Synthesis of g-C3N4/N-doped CeO2 composite for photocatalytic degradation of an herbicide
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Mohan Kumar Kesarlaa, Manuel Octavio Fuentez-Torresb, Manuel Antonio Alcudia-Ramosb, Filiberto Ortiz-Chic, Claudia Guadalupe Espinosa-Gonzálezc, Miguel Alemand, Jose Gilberto Torres-Torresb, Srinivas Godavarthic,
Corresponding author
a Department of Chemistry, Madanapalle Institute of Technology & Science, Post Box No: 14, Kadiri Road, Angallu (V), Madanapalle, 517325 Chittoor District, Andhra Pradesh, India
b Universidad Juárez Autónoma de Tabasco, Centro de Investigación de Ciencia y Tecnología Aplicada de Tabasco (CICTAT), C.P. 86690 Cunduacán, Tabasco, Mexico
c Cátedras Conacyt – Universidad Juárez Autónoma de Tabasco, Centro de Investigación de Ciencia y Tecnología Aplicada de Tabasco (CICTAT), C.P. 86690 Cunduacán, Tabasco, Mexico
d Centro de Nanociencias y Micro y Nanotecnologías, Instituto Politécnico Nacional, Mexico City, Mexico
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Received 20 July 2018, Accepted 22 November 2018
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Table 1. Surface area, pore volume and pore sizes of various catalysts.
Abstract

In photocatalysis, surface engineered CeO2 could be vital due to oxygen vacancies arise from multiple valency, i.e. Ce3+ and Ce4+. This study reports photocatalytic properties of g-C3N4/CeO2 composite synthesized by a facile method in the presence of l-arginine. Physicochemical properties of g-C3N4/CeO2 material were analyzed through various characterization techniques such as XRD, UV–Vis, physisorption, etc., and correlated with its photocatalytic activity. Observed bandgap of the synthesized composite material was in the visible region, around 2.8eV which is less than that of typical ceria, but higher than bandgap of exfoliated g-C3N4. On the further side, N doping into CeO2 was confirmed through XPS analysis. It is estimated that synthesis method aided for the N doping, which further played key role in lowering the bandgap of g-C3N4/CeO2 composite. Finally, Photocatalytic activity of g-C3N4/CeO2 composite was analyzed through degradation of an herbicide i.e. diuron, and the study revealed the good performance of the catalyst.

Keywords:
CeO2
Graphitic carbon nitride
Photocatalysis
N doping
Herbicide
Full Text
1Introduction

Worldwide increasing trends of energy consumption and removal of toxic chemicals from air, soil and water, are some of the major concerns for scientific community, and one common solution is to develop efficient photocatalytic materials [1]. Till date several materials, such as TiO2[2,3], MoS2[4], ZnO [5], BiO [6,7] etc., have been studied for this purpose. Among them photocatalytic studies on g-C3N4 have been increased due to its eco friendliness, low cost and extraordinary properties, such as, its band gap lies in visible region and it is the stable allotrope of carbon nitride due to the existence of strong covalent bond between carbon and nitrogen [8]. Moreover, it possesses high surface area due to 2D stacking of π conjugated planes analogous to graphite. The main disadvantage of g-C3N4 is high rate of recombination of photo induced electron-hole pairs [9–11], and several modification endeavors have been reported such as doping nonmetal into g-C3N4[12] creating heterojunction with other semiconductor materials, etc. Among various possibilities, photocatalysis by forming heterojunction between two semiconductors, known as z-scheme photocatalysis [13–15], have been attracted interest as they will allow to utilize large portion of the solar spectrum and reduce the recombination rate by driving the reduction and oxidation reactions at different sites of the catalyst.

On the other hand, cerium is more abundant among the rare earth elements, and its oxide form, i.e. ceria, became most important material in wide variety of applications such as drug delivery [16], fuel cell [17], and one of the most widely used co-catalysts, due to its properties. Co-existence of Ce3+ and Ce4+ states makes ceria very attractive, and changing their ratio, thereby oxygen vacancies, will provide the opportunity to tune surface properties for its use in various applications especially in catalysis. In addition to the size of the particles, the ratio of ceria oxide states depends on the synthesis method and conditions [18,19], and therefore final g-C3N4/CeO2 composite properties specific to synthesis method or process. Till date, few studies have been reported on g-C3N4/CeO2 composite developed through different methods, such as, mixing-calcination route [20,21], co-pyrolysis [22], ultrasonic deposition [23], template-assisted synthesis [24], hydrothermal synthesis [25,26] towards photocatalytic applications such as degradation of dyes, CO2 reduction, etc. However, to our knowledge, studies on effect of doping and/or degradation of pesticide/herbicides are limited for g-C3N4 based materials. In this study, we developed ceria nanoparticles on exfoliated g-C3N4 nanosheets in a facile in-situ synthesis process without using any complex equipment and toxic raw materials. For the first time, this paper reports the nitrogen doping into ceria in g-C3N4/CeO2 composite. Also, an herbicide i.e. diuron, was employed as model contaminant for testing photocatalytic properties of the synthesized hetero junction material and further correlated with its physicochemical properties.

2Materials and methods2.1Synthesis of g-C3N4

Synthesis of bulk g-C3N4 using melamine (Sigma–Aldrich, 99% AR) as precursor involves a process of polycondensation with simultaneous elimination of ammonia [27] In typical synthesis process, 10g of melamine was taken into an alumina crucible with lid, then placed in a muffle furnace and heated at 550°C for 2h with a ramp of 9°C. Then the obtained product was made into fine powder using a mortar.

2.2Exfoliation of bulk g-C3N4 via thermal oxidation

Thermal oxidation method was employed to exfoliate bulk g-C3N4 to develop nanosheets [28] and thereby increasing surface area. 1g of bulk g-C3N4 was placed in open ceramic container and was heated at 550°C for 3h with ramp 9°C/min. After this process, huge decrease in weight and the density of material was observed which confirm the exfoliation.

2.3Synthesis of g-C3N4-Ceria composite

100mg of thermal exfoliated g-C3N4 was dispersed in 50mL of ethanol by ultrasonication, and then 100mg of l-arginine was added which turns pH of the reaction mixture to 8.5. Then 200mg of CeCl3.7H2O was introduced into it and stirred on magnetic stirrer for 30min. Then the reaction mixture was placed in a hot air oven overnight at 70°C. Then the reaction mixture was centrifuged and washed thrice with ethanol and dried in oven for 3h at 90°C.

2.4Characterization

Powder X-ray diffraction (XRD) patterns for the samples were collected using Bruker D8 advance eco diffractometer. Chemical states of the sample were identified using X-ray photoelectron spectrum (XPS), which was recorded using K-Alpha spectrometer from Thermo Scientific. Transmission Electron Microscopic (HR-TEM) images were taken with JEOL JEM 2100. The nitrogen physisorption experiments were carried out in a Micromeritics TriStar II, at −196.2°C. The samples were degassed at 300°C for 2h with nitrogen gas. The surface area was calculated using the standard BET method. Pore size distributions were obtained from the BJH method using the adsorption of the isotherms. UV–Vis adsorption spectra were recorded in a Varian CARY 300, at room temperature with a range of 200–800nm. Scanning electron microscopic images were recorded using JEOL JSM-IT 500 instrument.

2.5Photocatalytic activity tests

The photocatalytic activity of g-C3N4, exfoliated g-C3N4 and the g-C3N4/CeO2 composite were identified via degradation of diuron using benchtop xenon lamp unit “suntest CPS+” solar simulator equipped with air-cooled 1500W xenon lamp. In a typical experiment, 30mg of photocatalyst was dispersed in 200mL of 25ppm diuron solution and kept under dark for 60min with stirring to establish adsorption–desorption equilibrium. During the irradiation, 3mL solution was collected using a syringe with a filter for every 20min and analyzed using UV-Vis spectrophotometer. The rate of degradation of diuron was calculated by C/C0 where C was the concentration of the sample collected after specified time interval and C0 was that of the diuron after dark adsorption–desorption equilibrium. The cyclic runs for g-C3N4/CeO2 composite were carried out by collecting the catalyzer through centrifugation, dried and then used in the next cycle. Superoxide radicals (·O2), active holes (h+) and hydroxyl radicals (·OH) are the major reactive oxidative species in photodegradation experiments. The role of these attacking species were determined through in-situ trapping experiments using p-benzoquinone, ammonium oxalate and isopropanol as scavengers for ·O2, h+ and ·OH radicals, respectively [29].

3Results and discussion

Through this synthesis process, we successfully synthesized CeO2 nanoparticle from CeCl3 with the help of l-arginine and they are anchored in-situ on to the nanosheets of g-C3N4. The following are the purpose of using arginine in the reaction (i) it modifies the exfoliated g-C3N4 sheets surface and (ii) It helps in formation of ceria nanoparticles and (iii) excellent coordination between ceria nanoparticles and the guanidine group of arginine protects the ceria from agglomeration and hence making them to distribute uniformly on the exfoliated g-C3N4 sheets. Similar observation was made by Gadhari et al. in the synthesis of copper-cobalt ferrite on graphene oxide [30].

3.1XRD analysis

The crystalline phase of synthesized materials was analyzed using XRD technique and presented in Fig. 1. Characteristic peaks for g-C3N4 around 13° and 27.8° correspond to (100) and (002) crystalline planes, respectively, are observed. According to JCPDS 87-1526, peaks at 13.0 and 27.8° indicate the formation of hexagonal phase of g-C3N4. The later peak shifted to slightly higher diffracted angle, i.e. 28.2°, for exfoliated g-C3N4 and thus indicating successful exfoliation [31]. The peaks in X-ray diffraction pattern of g-C3N4/CeO2 composite synthesized using l-arginine developed the broader peaks at 47.3°, 56.0° and 76.67° correspond to (220), (311) and (331) of CeO-np respectively, in accordance to JCPDS. No. 00-034-0394 and indicates the cubic structure of ceria. Diffraction peak around 28.5° corresponds to Ceria (111) plane is not apparent, as it is overlapped by higher intensity peak of carbon nitride (002) plane. Similar overlapping of peaks is also observed elsewhere [25]. From above discussion, the formation of g-C3N4/CeO2 composite is established.

Fig. 1.
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XRD patterns of the synthesized materials.

3.2UV visible

Understanding optical properties, absorption ability and bandgap, is important as they will affect the photocatalytic performance of the catalyst. Fig. 2(a) shows UV-Visible absorption spectra of the synthesized materials measured at room temperature. From Fig. 2(a) following points are observed: (1) all samples showed absorption response in the visible region of the spectrum; (2) bulk g-C3N4 absorption edge is at around 500nm; (3) all other samples showed a red shift in their absorption edge w.r.t bulk g-C3N4 in the following order; bulk g-C3N4<the composite<exfoliated g-C3N4. Following the equation (αhv)=A(Eg)n/2, where α, h, ν, Eg, A and n are absorption coefficient, Planck constant, light frequency, band gap energy, a constant and a variable dependence on the type of bandgap, respectively, band gap values were determined through Tauc plot by extrapolating the curve to zero of the x-axis from the graph of (αhv)n/2 vs , as shown in Fig. 2(b). The calculated band gap values are 2.71, 2.86 and 2.8eV for bulk g-C3N4, exfoliated g-C3N4 and the composite samples, respectively and it is well known that increase in the bandgap values for exfoliated g-C3N4 is due to quantum confinement effect. However, composite showed blue shift in the band gap compared to that of the exfoliated and typical ceria [32]. The lower bandgap of the composite is due to Nitrogen doping into ceria, as it leads to bandgap narrowing by forming intermediate energy levels by mixing of 2p orbital of N with 2p orbital of O. [33,34]. It is also established that the presence of Ce3+ and oxygen defects affects the band gap value of ceria [35–37].

Fig. 2.
(0.24MB).

(a) UV–vis absorption spectra of the synthesized material and (b) (αhν)n/2 vs of bulk g-C3N4, exfoliated g-C3N4 and CeO2/g-C3N4 composite.

3.3BET analysis

To understand the specific surface area and pore size distribution of the samples, Brunauer–Emmett–Teller (BET) technique was employed. Fig. 3(a) shows the N2 adsorption-desorption isotherms of g-C3N4, exfoliated g-C3N4 and the composite. All samples showed type IV isotherms with H3 hysterisis loops indicate mesoporous structure of synthesized materials. Fig. 3(b) shows Barrett–Joyner–Halenda (BJH) curves, which are obtained from the desorption branch of the BET isotherm. Variation in surface area between the samples listed in Table 1, along with average pore volume and average pore size. From the table, the surface area of exfoliated g-C3N4 was higher by one order of magnitude to that of bulk g-C3N4. On the other hand, surface area of the composite is in between those of exfoliated g-C3N4 and bulk g-C3N4. Hence surface area of the samples showed the following trend g-C3N4<composite<Exfoliated g-C3N4 and is in agreement with the previous reports [26]. The decrease in surface area of the heterojunction material is because of the probable change in the dimensions due to occupation of pores by the ceria nanoparticles.

Fig. 3.
(0.24MB).

(a) N2 adsorption-desorption isotherms of g-C3N4, exfoliated g-C3N4 and CeO2/g-C3N4 composite and (b) Barrett–Joyner–Halenda (BJH) curves.

Table 1.

Surface area, pore volume and pore sizes of various catalysts.

Catalyst  Surface area (m2/g)  Pore volume (cm3/g)  Pore size (Å) 
g-C3N4  18.19  0.08  156.05 
Exfoliated g-C3N4  198.16  0.68  149.70 
Ceria-exfoliated g-C3N4  51.42  0.24  255.28 
3.4XPS analysis

Surface chemistry of the composite was analyzed by employing XPS technique. XPS survey scan is shown in Fig. S1 of the supplementary material and demonstrates that composite mainly consists Ce, O, N and C elements with no other impurity peaks. High-resolution spectrum of C 1s, N 1s, O 1s and Ce 3d peaks presented in Fig. 4(a)–(d), respectively. Core level spectrum of C 1s peak can be fitted with 3 peaks with peak positions around 293.6, 287.9 and 284.4eV which can be ascribed to two-coordinated C atoms (C2c), three-coordinated C atoms (C3C) and carbon contamination, respectively as shown in Fig. 4(a). Deconvoluted N 1s spectrum is shown in the Fig. 4 (b) with five peaks occurring at 404.2, 400.9, 399.8, 398.8 and 397.1eV which are corresponding to N–N bonds, C–N–H bonds, three coordinated (N3c) nitrogen, two coordinated (N2c) nitrogen in the sp2-hybridized nitrogen and Ce–N bond [38], respectively. With the above information, it is concluded that sp2-bonded graphitic carbon nitride is presented in the composite. Ammonia [39] from C3N4 during the preparation of the composite or nitrogen functionalization from arginine, could be reason behind the formation of Ce–N bond by incorporation of nitrogen into ceria and thereby resulting the lower band gap material as observed in the UV–Vis spectra analysis. Peak fitting for O 1s is shown in Fig. 4(c) and peaks around 533, 531.7, 530.7 and 529.0eV are assigned to surface –OH group, oxygen vacancies, O2 in Ce3+ and O2 in Ce4+, respectively.

Fig. 4.
(0.59MB).

High resolution deconvoluted XPS spectra of CeO2/g-C3N4 composite (a) C1s, (b) N1s, (c) O1s and (d) Ce 3d.

Depending on the reaction process during the synthesis, but in general, presence of Ce3+ ions and oxygen vacancies reduce the co-ordination number to seven from eight in case of Ce4+ based fluorite structure. The co-existence of Ce3+ and Ce4+ can be analyzed by deconvoluting the high-resolution Ce 3d spectra as shown in Fig. 4(d). Following the convention established by Burroughs et al. [40], 3d3/2 spin-orbit states labeled as μ and 3d5/2 states as ν. Full spectrum can be deconvoluted into eight peaks with peak centered around 882.3,885.0, 888.2, 898.1, 900.6, 902.4, 907.0 and 916.4eV assigned to bands ν, ν’, ν”, ν”’, μ°, μ, μ”and μ”’ respectively. Among them, ν, ν”, ν”’, μ, μ”and μ”’ represent Ce4+ states and remaining i.e. ν’ and ν”’, represent Ce3+ states. Presence of Ce3+ states is also evidenced by the presence of oxygen vacancies peak in O 1s spectrum. Hence, it can be concluded that surface of ceria is not completely oxidized and results a possible modification of electronic structure of ceria.

3.5Microscopic analysis

From HRTEM images as shown in Fig. 5(a–c), it is clear that the formation of ceria nanoparticles on the exfoliate g-C3N4 nanosheets. The particle size of ceria was about 2–3nm and is uniform throughout the exfoliated g-C3N4 (Fig. 5c). Also, formation of ceria was confirmed using elemental mapping in SEM-EDX. Fig. 5(d) shows the exfoliated g-C3N4/CeO2 composite and Fig. 5(e) and (f) shows the presence of cerium and oxygen, respectively.

Fig. 5.
(0.93MB).

(a)–(c) HR-TEM images of CeO2/g-C3N4 composite at different magnifications with scale bars 100nm, 50nm and 20nm, respectively (d) SEM image of CeO2/g-C3N4 composite and (e) and (f) elemental mapping of Ce and O, respectively.

3.6Photocatalytic activity

The photocatalytic activity tests were carried by identifying diuron characteristic absorbance peak intensity at 250nm. It is noteworthy to mention that there is diuron adsorption onto the tested photocatalysts after adsorption-desorption equilibrium. Degradation of diuron with time using the studied photocatalysts is shown in Fig. 6(a). Photolysis of diuron also carried under same conditions, as can be seen from Fig. 6 (a) almost no change in diuron concentration is observed after two hours of light irradiation. However, in the presence of photocatalyst, there is clear reduction in diuron concentration with time and concentration reduced by around 15, 34 and 46% for bulk g-C3N4, exfoliated g-C3N4 and the g-C3N4/CeO2 composite, respectively. It was previously established by other authors that bulk g-C3N4 photocatalytic activity is lower compared to that of exfoliated g-C3N4[41] due to high recombination rate of electron hole pair and lower surface area of bulk g-C3N4. Following Langmuir–Hinshelwood pseudo first order kinetic equation: ln(C0/C)=kt where k is rate constant (h−1), first order rate constants determined, and are 0.20048 and 0.28308/h for exfoliated g-C3N4 and the composite, respectively. During the first 20min of light irradiation, both exfoliated and the composite showed almost same concentration change. However, with progress in the time, the diuron concentration reduction slowed when compared to that of the composite. It is clear from UV–Vis analysis that in-situ preparation promoted nitrogen doping into ceria and thereby reduction of band gap of the resulting composite material. This relatively low band gap material helped in fast transfer of the electron from valence band to conduction band and relatively higher photocatalytic activity. Hence, diuron photocatalytic degradation efficiency is higher for the composite than those of bulk and exfoliated g-C3N4.

Fig. 6.
(0.35MB).

(a) Degradation of diuron with bulk, exfoliated and CeO2/g-C3N4 composite. (b) Reusability of CeO2/g-C3N4 composite sample. (c) Photocatalytic activity of CeO2/g-C3N4 composite in the presence of scavengers.

Catalyst stability or reusability of g-C3N4/CeO2 composite was tested through four consecutive cycles as shown in Fig. 6(b). From the graph, the tested photocatalyst showed consistent performance over all cycles in degradation of diuron through photocatalysis. It is therefore g-C3N4/CeO2 composite is stable photocatalyst and will be good candidate for environmental protection through photocatalysis. On the other hand, the main radical responsible was tested through scavengers experiments as shown in Fig. 6(c). It is clear from the figure that diuron degradation is almost suppressed when employed p-benzoquinone as scavenger, but, in the presence of ammonium oxalate or isopropanol, degradation continues with slightly lower rate compared to that of scavenger. This indicates that •O2 are dominating active species in oxidation of diuron using g-C3N4/CeO2 photocatalyst.

4Conclusions

In summary, nitrogen doping into ceria was achieved while synthesizing CeO2/g-C3N4 composite material with no additional process steps. Morphological studies revealed excellent distribution of ceria nanoparticle with size around 3nm on g-C3N4 nanosheets. Doping of Nitrogen into CeO2 resulted in lowering the bandgap of CeO2/g-C3N4. Photocatalytic properties through degradation of diuron showed improved performance of the composite when compared to that of bulk and exfoliated g-C3N4. This higher performance can be ascribed to improved separation efficiency of photo-induced electron-hole pair by forming heterojunction between ceria and g-C3N4.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We greatly acknowledge Dr. Lorenzo Martinez Gomez and Mr. Jose Juan Ramos Hernandez of ICF, UNAM for providing diffraction and elemental analysis. Dr. K. Mohan Kumar greatly acknowledges the financial assistance from the UGC project No. MRP-6396/16 (SERO/UGC).

Appendix A
Supplementary data

The following are the supplementary data to this article:

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