Tuning HOMO and LUMO of three region (UV, Vis and IR) photoluminescent nitrogen doped graphene quantum dots for photodegradation of methylene blue
Introduction
Carbon based nanomaterials have found great attention due to their low toxicity, high biocompatibility, physical and chemical stability, easy preparation and low cost [[1], [2], [3], [4]]. Among these nanomaterials, graphene quantum dots (GQDs), one of the zero-dimensional carbon families are attracting more attention due to their application in various fields such as biosensing, cellular imaging, drug delivery, solar cells, water splitting and wastewater treatment [[5], [6], [7], [8], [9]]. Because of their weak absorption spectrum in the visible region of light, GQDs are not a good choice for photocatalytic degradation [[10], [11], [12], [13]]. To effectively tune the absorption region of GQDs, a promising method is to doping GQDs with heteroatoms such as fluorine, nitrogen and sulfur [[14], [15], [16], [17], [18], [19], [20], [21]].
Doped GQDs have been considerably fabricated for sensing and catalytic application and selection of doping elements depends on the structure and size of heteroatoms [[22], [23], [24]]. Nowadays NGQDs are prepared by both top-down and bottom-up methods, however top-down methods always suffer from some disadvantages, such as need to expensive equipment, low quantum yield and difficult condition to control the size of dots. Bottom-up methods lead to controllable condition over morphology and the size distribution [[25], [26], [27], [28]]. Among bottom-up methods such as solvothermal [29,30], hydrothermal [[31], [32], [33]], pyrolysis [[34], [35], [36], [37]] and electrochemical synthesis [[38], [39], [40]]. hydrothermal is a very simple, cheap and exact method that we report to synthesis highly N doped luminescent GQDs. Nitrogen elements can substitute with carbons in the middle or on the edges of structure as graphitic-N, pyrrolic-N and pyridinic-N and modify the optical properties of GQDs by changing the band gap edges or creating new levels of energy between them [41,42]. This process helps to optimize the place of luminescence peak and the amount of emitted photons for specific applications. Various researchers have investigated and synthesized NGQDs for photodegradation applications, whereas there are very rare studies on the role of LUMO and HOMO energy levels positions on the photodegradation.
Herein we have used highly amount nitrogen doping to extend the absorption spectrum on the whole of light region (UV, Vis and IR) for photocatalytic applications. Also we calculated the density of states of a simple structure of NGQD to find the effect of N concentration on the band gap and absorption spectrum. To explain the reaction for photodegradation of methylene blue, lowest unoccupied molecular orbital (LUMO) and highest occupied molecular-orbital (HOMO) states of the NGQDs were determined. Explaining the mechanism of photodegradation using NGQDs under illumination could be informative to promote their utilization in solar application.
Section snippets
Synthesis of NGQDs
For hydrothermal synthesis of NGQDs, 0.30 g citric acid and 0.35 g urea (Sigma-Aldrich) were stirred in 25 cc deionized water for 2 min in room temperature. The solution was putted in 40 cc autoclave and was heated in 160 °C for 4 h. After synthesis process the colorless solution was changed to orange color. Schematic illustration for formation of NGQDs is shown in Fig.1.
Characterization
Energy-dispersive X-ray (EDX) analysis was performed by a MIRA3 TESCAN instrument. Transmission electron microscopy (TEM) was
Computational method
All calculations were performed with the plane-wave density functional theory by the Quantum ESPRESSO package [43]. In self consistent functional (SCF) calculation, we have used 5*5*1 k-point grids and the generalized gradient approximation (GGA) developed by Perdew, Burke, and Ernzerhof (PBE) [44], used for the exchange-correlation energy functional. For electron wave function, a plane-wave basis set with 40 eV cut-off energy has been used. The unit cells are separated from them replica by 20
Morphology and chemical composition of NGQDs
Morphology and chemical composition of NGQDs was characterized by TEM images, EDX, Raman and XRD analysis. TEM image of the prepared sample and its histogram is shown in Fig. 2-a. Homogeneous and well dispersed distribution of particles is completely obvious in the TEM image, also the histogram shows the particle size is in the range of 4−7 nm and the average size of particles is about 5 nm.
The elemental composition of prepared NGQDs was analyzed by EDX as is shown in Fig. 2-b. The amount of
Conclusion
Low cost, gram scale and highly N doped graphene quantum dots (NGQDs) with a broad absorption spectrum was obtained using hydrothermal method by citric acid and urea as the precursors. Cyclic voltammetry measurements were utilized to directly determine the LUMO and HOMO energy levels. The broadband emission of the synthesized N-GQDs could be used in a wide range of applications like photocatalysts, solar cells and light emitting devices. For instance the photocatalytic performance of NGQDs was
CRediT authorship contribution statement
Mohamad Taghi Dejpasand: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Esmaiel Saievar-Iranizad: Supervision. Amir Bayat: Formal analysis, Methodology, Writing - review & editing. Aref Montaghemi: Formal analysis. Saeed Rahemi Ardekani: Formal analysis.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank Research Council of the Tarbiat Modares University for financial supports.
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