Bandgap engineering in graphitic carbon nitride: Effect of precursors

Journal Pre-proof Bandgap Engineering in Graphitic carbon nitride: Effect of Precursors Veena Ragupathi, Puspamitra Panigrahi, N. Ganapathi Subramaniam

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S0030-4026(19)31499-8

DOI:

https://doi.org/10.1016/j.ijleo.2019.163601

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IJLEO 163601

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Optik

Received Date:

22 July 2019

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11 October 2019

Please cite this article as: Ragupathi V, Panigrahi P, Ganapathi Subramaniam N, Bandgap Engineering in Graphitic carbon nitride: Effect of Precursors, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163601

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Bandgap Engineering in Graphitic carbon nitride: Effect of Precursors

Veena Ragupathi1*, Puspamitra Panigrahi1 and N. Ganapathi Subramaniam2*

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Centre for Clean Energy and Nano Convergence (CENCON), Hindustan Institute of

Technology and Science (Deemed to be University), No. 1 Rajiv Gandhi Salai, Padur-603103,

Quantum Funtional Semiconductor Research Centre (QSRC), Dongguk University, 26

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Phildong3ga, Chung gu, Seoul 100-715, Republic of Korea

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Chennai, India

*

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Corresponding author: [email protected], [email protected]

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Abstract

Bandgap engineering plays a vital role in the optoelectronic application of semiconducting

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material. In this work, Graphitic carbon nitride (g-C3N4) nanoparticles are synthesized by pyrolyzing different precursors and the role of precursor on the reduction bandgap is analyzed. X-ray diffraction pattern confirms the presence of tri-s-triazine units. Fourier transform infrared spectroscopy result reveals the presence of carbon nitride heterocycle. Urea with glycine derived g-C3N4 (UG) shows a layered structure. The absorption band edge of 620 nm and reduction in

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bandgap of 2.35 eV is observed for UG sample. The addition of glycine in urea significantly reduces the optical band gap and thus wider the light absorption range. Photoluminescence measurement reveals the presence of n- π* transition and defect induced luminescence. The quenching of photoluminescence intensity is observed for UG sample and the glycine produces additional NH moieties, which create disorder in the g-C3N4 structure. Reduction in bandgap with lower recombination rate significantly improves light harvesting ability and photocatalytic performance of g-C3N4 material. 1

Keywords: graphitic carbon nitride, optical properties, bandgap, precursors

1. Introduction Graphitic carbon nitride (g-C3N4) is a versatile polymeric, organic semiconducting material and have the band gap of 2.7 eV [1,2]. g-C3N4 material is consists of graphitic, two dimensional (2D), π-conjugated polymeric structures with tri-s-triazine repeating unit and hold weak Vander

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Waal forces between the layers [3,4]. g-C3N4 have promising properties such as excellent optical property, good stability, tunable electronic band structure and photocatalytic and mechanical

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property [3]. In addition to this, it is abundant, low cost and environmental friendly material. Hence, it is widely used in photoelectrochemical water splitting, hydrogen generation, pollutant

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degradation, supercapacitors, solar cell, lithium-ion batteries and chemical sensors etc [5–7]. In 2009, Wang et al. initially reported the g-C3N4 photocatalyst and used for hydrogen

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production from water splitting [8], soon after his pioneering work, many articles have been published in g-C3N4 material [9,10]. However, the growth of g-C3N4 based optoelectronic

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devices are still in the early stage of development. The optoelectronic property of g-C3N4 material primarily depends upon the electronic band structure [11]. In this regard, tuning the bandgap significantly alters the electronic and optical properties of the g-C3N4 material and

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significantly influences the charge transfer and photocatalytic property. [10]. Hence, Researchers have paid more attention to tune the bandgap of g-C3N4 material, which significantly enhances the photocatalytic activity. Generally, doping of metals and nonmetals, fabrication of heterostructures, developing nanostructured morphology and modification of synthesis procedures are employed to alter the band structures of g-C3N4 material [12–14] Bandgap

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engineering in g-C3N4 nanostructure is a fascinating strategy to improve the photocatalytic property through harnessing additional visible light absorption [15,16]. In this regard, bandgap reduction by different precursors is the most effective and simple strategy. Tuning the bandgap effectively increases the light absorption and improves the mobility of charge carriers. In this context, Ong et al., explained the different allotropes of C3N4 and reported the bandgap of 5.49, 4.85, 4.30, 4.13, 2.97, 0.93, 2.88 eV for α, β, cubic, pseudocubic, g-h-triazine, g-triazine, 2

and g-h-heptazine respectively [1]. Among all phases, g-h-heptazine is thermodynamically stable [1,5]. Kang et al. reported the bandgap of 1.9 eV and 2.82 eV for amorphous and crystalline gC3N4 material and suggested higher photocatalytic activity in amorphous g-C3N4 material [17] Dutta et al. reported the bandgap reduction in g-C3N4 material, by coupling organic moieties as a dopant. He performed the density functional theory and observed the bandgap of 2.65 eV for Naphthalene tetracarboxylic dianhydride doped g-C3N4 [18]. Xu et al., synthesized the g-C3N4 using dicyandiamide and melamine precursor and studied the synergic effect of precursor on

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photocatalytic activity [19] In this work, simply varying the precursor strategy was adopted to tune the band structure of graphitic carbon nitride. g-C3N4 nanostructures were synthesized by pyrolyzing different

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precursors. The effect of precursors on the structural and optical properties of the g-C3N4

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material was analyzed.

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2. Experimental

The analytical grade precursors such as thiourea, urea and glycine were used as precursors. For the synthesis of g-C3N4 nanomaterial, 5 g of thiourea, urea and urea with glycine (5%) were put

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into an alumina crucible separately. Then the alumina crucible was heated to 500 ᵒC for 2 h with a heating rate of 3 ᵒC min-1 and then cooled to room temperature. The resultant g-C3N4 obtained

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by thiourea, urea and urea with glycine were denoted as TU, U and UG respectively. 2.1 Characterization

The crystal structure of the synthesized g-C3N4 materials were carried out using Bruker D8 X-ray diffractometer using Cu-Kα radiation (λ = 0.15406 nm). X-ray diffraction (XRD) patterns were

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recorded in the 2θ range of 10-80º with a step size of 0.01º. The functional groups present in the TU, U and UG nanomaterials were analyzed by Bruker Fourier transform infrared spectrometer. Morphology and elemental composition of g-C3N4 material were recorded using CT-200TA, CoXem scanning electron microscopy coupled with Noran 7 (Thermo scientific), Energy dispersive X-ray spectroscopy (EDS). The absorbance measurements were carried out using UV –Visible spectrophotometer (Shimadzu UV 2450) equipped with ISR-240A integrated sphere assembly. Room temperature photoluminescence (PL) measurements were carried out using 3

JASCO FP-8600 UV-VIS-NIR Spectrofluorometer furnished with xenon lamp with an excitation wavelength of 370 nm.

3. Results and Discussion

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3.1 Structural analysis

Figure 1 a) XRD diffraction pattern, b) enlarged profile of dominant (002) peak of g-C3N4

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material synthesized from different precursors

Fig. 1a shows the XRD patterns of g-C3N4 samples derived from different precursors. The XRD result reveals the presence of two diffraction peaks at 13.07° and 27.56°. The small diffraction peak at 13.07° corresponds to the (100) plane of tri-s-triazine units. The intense peak at 27.61°

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attributes to the stacking of the conjugated aromatic ring and assigned as (002) plane [13,20,21]. The highly intense diffraction pattern was observed for thiourea derived g-C3N4 material (TU), which indicate the enhanced crystallinity. Broadened diffraction pattern with lower crystallinity was observed for urea with glycine derived g-C3N4 nanomaterial (UG). The addition of glycine increases the NH2 moieties and disorder in the graphitic structure. TU and U sample shows small impurity peaks at 17.7° and 21.6°, indicating the incomplete condensation of thiourea and urea.

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However, no impurity peak was observed for TG which indicates glycine facilitates the complete reaction. Fig. 1b shows the enlarged version of the XRD pattern and indicates the predominant (002) peak was shifted towards lower angle. Compared to TU and U, the NH2 moiety increases the layer distance and shift the (002) diffraction from 27.61° to 27.31°. The crystallite size of g-C3N4 material derived from different precursors were calculated using Debye Scherer equation and the calculated crystallite sizes of TU, U and UG nanomaterial were 6.43, 5.65 and 4.27 nm

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Figure 2 FTIR spectra of TU, U and UG nanomaterial. Fig.2a shows the FTIR spectra of TU, U and UG nanomaterial and the result reveals similar FTIR pattern for all the samples. Fig.2b shows the FTIR spectra of g-C3N4 material derived from thiourea. The FTIR band at 809.86 cm-1, 1456.0 cm-1 and 1401 cm-1 corresponds to the bending

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and stretching vibration modes of triazine units [22]. The FTIR peak at 1633.38 cm-1 and 1540.8 cm-1 corresponds to the stretching vibration of C=N group. The bands at 1320.37 cm-1 and 1253 cm-1 were assigned to C-N and C-NH-C stretching vibration modes [19,23]. The broad FTIR band at 3167.16 cm-1 was attributed to an amino group. The band at 2343 cm-1 and 3787.84 cm-1 were attributed to C≡N and N-H group respectively. The FTIR band at 728.3 cm-1 was attributed to the breathing mode of heptazine ring and band at 538 cm-1 was assigned to the stretching 5

vibration of C=O [14]. No sulphur related FTIR band was detected, which indicate the complete evaporation of sulphur during calcination. 3.2 Morphological analysis Fig.3 (a-c) depicts the SEM images of g-C3N4 material derived from different precursors. TU and U (Fig. 3 a & b) displays quite similar morphology and shows the highly agglomerated spherical shape nanoparticle. The diameter of the particle was around 20 nm. g-C3N4 material derived from urea shows spherical nanoparticle with glassy morphology. UG material shows

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layered morphology with existence of nanometer size pores (Fig. 3c). The nano-pores might be occurred due to the evaluation of gases. Generally, the layered morphology provides high surface

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area, electron-phonon interaction and improves the charge carrier mobility. Fig. 3d depicts the EDS spectra of TU nanostructures and the all sample shows comparable EDS results and reveals

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the presence of C and N in all the synthesized g-C3N4 material.

Figure 3 (a-c) SEM images of TU, U and UG nanomaterials and d) EDS spectra of TU. 3.3 Optical analysis 6

The absorbance spectra of TU, U and UG samples are plotted in Fig.4. The absorption band edge around 450 nm was observed for g-C3N4 nanostructures derived from thiourea and urea precursors and appeared in pale yellow color. However, the UG sample shows the absorption band edge around 620 nm and appeared in brownish yellow color. The diversity in the absorption edges might be attributed to the different degree of condensation and defects in local structures [24]. From Fig. 4a the absorbance intensity was decreased in the order of TU>U>UG, which might be due to the lower crystalline density and the observed results were well matched with the

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XRD results. The band gap energy (Eg) was determined by the well-known Tauc equation. Fig. 4b shows the

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plot of (ahμ)2 versus hμ, where a is the optical absorption coefficient and hμ is the energy of the incident photon. As shown in the inset Fig. 4b, the calculated Eg values were 2.89, 2.78 and 2.35

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eV for TU, U and UG respectively. The higher bandgap was observed for TU and U sample, which might be caused by quantum confinement effect. UG sample have lower bandgap, which

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might be due to the presence of layered morphology and lower carrier density.

Figure 4 a) Absorbance spectra and b) Tauc Plot of g-C3N4 material derived different precursors.

Photoluminescence (PL) measurement provides information about the charge carrier separation, recombination and trapping of charge carrier. Fig. 5 depicts the Room temperature photoluminescence spectra of g-C3N4 nanostructures derived from different precursors. Fig. 5a 7

shows PL emission spectra of TU and U nanostructures and no difference in PL curves were observed. Yuan et al. reported the bandgap state of g-C3N4 material and composes of an sp3 C–N σ band, sp2 C–N π band and the nitrogen lone pair (LP) [25]. PL emission centered at 438.7 nm was attributed to excitonic recombination and corresponds to n-π* transition [3,26]. The emission peak centered at 566.8 nm might be ascribed to transition

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from nitrogen lone pairs.

Figure 5 a) Room temperature PL spectra of TU, U and b) PL spectra of UG. Fig. 5b displays PL emission spectra of UG nanostructures and exhibits two emission peaks at

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468.78 and 556.56 nm. The small PL peak at 468.78 nm was attributed to direct electron-hole transition from LUMO to HOMO [22]. Compared to TU and U, the emission band was red shifted from 438.7 nm to 468.78 nm and it might be caused by the presence of hybridization of π states in heptazine [27]. Liang et al also reported the red shift in PL intensity [28]. The broadband clearly indicates multiple reflections from the defect centers. It also suggested the

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existence of various different emission centers such as σ–σ*, σ*–LP, π*–LP [25]. The addition of glycine induces defect induced luminescence. Compared to TU and U, quenching of PL intensity was observed for UG and reveals a lower recombination rate of charge carriers. The quenching of recombination of charge carriers plays a significant role in the photocatalytic property of the g-C3N4 material.

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Conclusion In this study, the effect of precursors on the reduction of bandgap was investigated by analyzing the structural and optical properties of the g-C3N4 material. XRD analysis confirms the formation of g-C3N4 material. Compared to TU and U, glycine added sample shows flake like layered morphology with small nanometer size pores. UG shows expanded absorption in the blue-orange region and reduction in the bandgap. Addition of glycine induces defect luminescence in g-C3N4 nanostructures. Wider absorption range and reduction in recombination rate makes UG as a

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promising visible light photocatalyst.

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The authors declare that they have no conflict of interest.

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Conflict of Interest

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Acknowledgment

Authors are thankful to the financial support from the Hindustan Institute of Technology and Science. This work is partially supported by Quantum functional Semiconductor Research

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Figure Captions:

Figure 1 a) XRD diffraction pattern, b) enlarged profile of dominant (002) peak of g-C3N4 material synthesized from different precursors.

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Figure 2 FTIR spectra of TU, U and UG nanomaterials.

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Figure 3 (a-c) SEM images of TU, U and UG nanomaterials and d) EDS spectra of TU.

Figure 4 a) Absorbance spectra and b) Tauc Plot of g-C3N4 material derived different

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Figure 5 a) Room temperature PL spectra of TU, U and b) PL spectra of UG.

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