Electronic properties and molecular electrostatic potential mapping of edge functionalized GQDs with ZnO, CuO, and TiO2

Graphene quantum dots (GQDs) are a cutting-edge material that has excellent electrical, thermal, and mechanical characteristics, as well as minimal toxicity and powerful photoluminescence. These unique properties give GQDs the ability to be used in different electrical, optical, and optoelectronic applications. GQDs with hexagonal and triangular cuts with armchair and zigzag ends (ATRI, AHEX, ZTRI, and ZHEX) were decorated with different metal oxides such as ZnO, CuO, and TiO2. Metal oxides (MOs) were employed to tune the band gap and electrical properties of GQDs for usage in certain applications. The influence of functionalization on the GQDs electronic properties was examined utilizing HOMO–LUMO orbital distribution and molecular electrostatic potential (MESP) mapping calculations. The model structures were calculated using density functional theory (DFT: B3LYP/ LanL2DZ). The band gap energies of AHEX C42, ZHEX C54, ATRI C60, and ZTRI C46 were found to be 3.508, 2.886, 3.177, and 0.305 eV, respectively. The findings show that addition of MOs increases the total dipole moment (TDM) while decreasing the band gap energy ∆E. The most effective metal MO on GQDs band gap and electronic properties was TiO2, which enhanced the band gap energy ∆E for AHEX C42-TiO2, ZHEX C54-TiO2, ATRI C60-TiO2, and ZTRI C46-TiO2 to 0.391, 0.530, 0.287, and 0.250 eV, respectively. Accordingly, GQDs seem to be excellent for certain applications. Accordingly, GQDs functionalized with ZnO could be used for sensors, due to their increased responsiveness and energy gap variation while GQDs functionalized with TiO2 is excellent to be applied as optoelectronic materials.


Introduction
Carbon nanomaterials (CNMs) such as carbon nanotubes, fullerenes, nanoporous carbon, carbon nanofibers, graphene (G), and G derivatives have recently attracted a lot of attention.CNMs have great focus due to their significant properties, such as extraordinary surface properties, including large surface area, porosity, high chemical stability, low density, easy modification, and controlled structural behavior (Gusain et al. 2020).Accordingly, CNMs are employed in a variety of applications, including nanoscale electronic components (Ajayan and Tour 2007), energy storage (Sun et al. 2017), batteries (Wu et al. 2019), and conversion systems (Raman et al. 2013).CNMs can also help with environmental applications, including sensing (Li et al. 2019a) and pollution removal (Jun et al. 2018).Furthermore, CNMs have important biological applications such as drug delivery (Wong et al. 2013), electrochemical biosensors (Kour et al. 2020), and disease monitoring, treatment, and control (Mitragotri et al. 2015).
The most significant CNMs are G-based nanomaterials, which are applied in different fields of application such as super capacitors (Yang et al. 2019), antimicrobial agents (Anand et al. 2019), and environmental remediation applications (Wang et al. 2019a).GQDs are one of the most significant recent G-based materials, known as zero-dimensional (0D) nanomaterials, with unique electronic, fluorescent, photoluminescent, optical, and biological properties.These properties have sparked increased interest in GQDs to be used in a variety of applications, including sensing and bio-imaging, solar cell applications, and several other applications (Li et al. 2019b).Because GQDs have various constraints that limit their applicability, adjusting the characteristics of GQDs is critical if the material is to be applied in a variety of applications (Chung et al. 2021).GQDs may be functionalized to change their optical, chemical, and electrical characteristics, allowing them to be employed in a variety of applications (Ghaffarkhah et al. 2022).
GQDs with functionalization, substitution, and/or doping have lately become a significant stage in developing improved materials with the qualities necessary to fulfil application requirements (Nande et al. 2023).Edge-functionalized GQDs also have significantly improved power conversion efficiency and have recently been used in solar cell applications (Tian et al. 2018;Mahalingam et al. 2021).Also, the sensing properties of GQDs with hydroxyl groups OH on the edges have been improved for gas sensing applications (Sharma and Jha 2019).Furthermore, substituting nitrogen (N) into GQDs is utilized to modify both their chemical compositions and their different crystalline forms.N-doped GQDs are particularly well suited for optical, sensing, energy, and biological applications (Arunragsa et al. 2020).The charge transport layer for a high-rate and long-lasting Li 4 Ti 5 O 12 -based Li-ion battery is N-functionalized GQDs.N-functionalized GQDs serve as a stable charge transport and protection layer for Li-ion batteries, resulting in high transport performance and a long life (Gu et al. 2020).Furthermore, GQD functionalization using MOs has received a lot of interest for modifying the physical, chemical, thermal, and stability characteristics (Khan et al. 2019).
Accordingly, GQDs functionalization with NiO microspheres recorded great improvements to be applied for lithium storage applications (Yin et al. 2018).Additionally, aminofunctionalized GQDs had a strong positive response to Fe 3+ detection (Wang et al. 2019b).Chitosan/carboxyl-functionalized GQDs thin films have improved optical and surface plasmon resonance sensing characteristics and could detect Hg 2+ (Ramdzan et al. 2019).
Theoretical molecular modeling is a powerful tool for studying the physical, chemical, and biological properties of chemical compounds, particularly nanomaterials (Bayoumy et al. 2020).Molecular modeling has different levels of theories utilized for studying chemical compounds and reactions' properties (Budyka 2019;Abdelsalam et al. 2019).One of the most accurate theoretical approaches is DFT, which studies the properties of nanomaterials and provides indications close to what an experiment does (Hegazy et al. 2022;Omar et al. 2022).DFT calculations are commonly used in investigating the properties of CNMs and G-based nanomaterials, and their decorations (Ezzat et al. 2019a(Ezzat et al. , 2018)).Moreover, molecular structure could be studied and provided with significant information about compounds and their interactions by calculating infrared (IR) spectrum theoretically, which confirms the expressed-value IR result (El-Mansy et al. 2021;Refaat et al. 2019).
Because of the importance of G and GQDs, and their functionalized derivatives, the current study was conducted to investigate the electronic properties of different GQDs cuts functionalized with various MOs, including ZnO, CuO, and TiO 2 .DFT simulations were performed to study the change in electronic properties and charge distribution as a result of functionalization.Electronic properties were studied in terms of TDM, band gap energy (ΔE), HOMO-LUMO orbital distribution, and MESP mapping parameters.Moreover, the theoretical IR spectrum was computed and compared to the experimentally obtained ATR-FTIR spectrum of the prepared GQDs.

Calculation details
Following DFT: B3LYP/ LanL2DZ (Becke 1993;Lee et al. 1988;Vosko et al. 1980) calculations, the four different forms of GQDs were interacted with metal oxides such as ZnO, CuO, and TiO 2 .Models were processed with the GAUSSIAN09 (Frisch et al. 2010) package at the Molecular Modeling and Spectroscopy Laboratory, National Research Centre, Egypt.TDM, ΔE, HOMO-LUMO orbital distribution, and MESP mapping were conducted on the studied model structures to investigate the electronic properties of the chosen GQDs and how they interacted with ZnO, CuO, and TiO 2 .

Building model molecule
According to achive the porpouse of these work, functionalization of GQDs were considered with kind of MOs for tuning electronic properties and band gap.Functionalization of GQDs with MOs make it able to be used in specific applications (Guo et al. 2021).GQDs is a zero-band gap material with edge effect which has the ability to be enhanced with variety of organic and inorganic materials according to the target application (Kadian et al. 2021).According to Boltzmann statistics, the total quantity and type of electronic charge carriers in a MO are a consequence of the band gap energy.Electronic conduction is classified as n-hopping or p-hopping depending on whether the primary charge carriers are electrons or holes (Mather and Martinez-Arias 2007).
Accordingly, ZnO and TiO 2 (as n-type)with a direct wide bandgap and a relatively large exciton binding energy at room temperature which is attractive for sensing and optoelectronic applications (Schmidt-Mende and MacManus-Driscoll 2007; Sadaf et al. 2011) were chosen beside CuO with narrow band gap, variety of chemo-physical properties, low cost, and high surface area to volume ratio (Sansonov 1982) a good 1097 Page 4 of 20 candidate for sensing and uniqe optical and electrical properties (Filipic and Cvelbar 2012).Accordingly, the interaction mechanizm were supposed to be GQDs with hexagonal and triangular edges were decorated with ZnO, CuO, and TiO 2 .The symbols ATRI and AHEX correspond to triangular and hexagonal GQDs with armchair endings, respectively.In addition, symbols ZTRI and ZHEX stand for triangular and hexagonal GQDs with zigzag ends, respectively.The impact of MOs decoration on the electronic properties of GQDs was investigated in terms of TDM, HOMO-LUMO orbital distribution, and MESP mapping calculations.The four forms of GQDs, ATRI, AHEX, ZTRI, and ZHEX, were enhanced by connecting the MOs under investigation to one edge of each of the different forms.Figures 1a, 2a, 3a, and 4a demonstrate the model structures of the four forms of GQDs: ATRI, AHEX, ZTRI, and ZHEX, which contain 42, 54, 60 and 46 carbon atoms.All forms were used that the edge morphology have different response with functionalization beside the effect of edge cutting morphology on tuning the electrical properties and band gap (Li et al. 2011;Abdelsalam et al. 2018).
Each of those forms are decorated with the different MOs including ZnO, CuO, and TiO 2 .The interaction of ZnO, CuO and TiO 2 with GQDs occurs once from the metal atom (as in Fig. 1b) and once from the oxygen atom (as in Fig. 1c) (Ezzat et al.

TDM and HOMO-LUMO orbital distribution
TDM and HOMO -LUMO band gap energy (∆ E) are the most important indicators of electrical characteristics as well as responsiveness and probably of the investigated interactions (Ezzat et al. 2018;Badry et al. 2018;Ibrahim 2009).Moreover, it is commonly known that the indicator of enhancement of chemical systems correlates with greater TDM and lower calculated (∆E) (Ibrahim and Mahmoud 2009;Politzer et al. 1985).Accordingly, the simulated model structures were optimized to calculate the TDM, HOMO-LUMO orbital distribution and bandgap energy.
The HOMO-LUMO orbital distribution of AHEX C42 GQD and its interaction with ZnO, CuO, and TiO 2 are shown in Fig. 5.The AHEX C42 HOMO-LUMO orbital dispersion as shown in Fig. 5a is distributed uniformly over the surface and edges.The HOMO-LUMO orbitals were rearranged after metal oxides were attached to the AHEX C42 GQD.As in Fig. 5 b and f the HOMO orbitals were switched with LUMO orbitals in the same position for AHEX C42 GQD interacted with ZnO and TiO 2 while in case of OZn (Fig. 5c) the orbitals were rearranged around the carbon ring arms.In the case of AHEX C42 GQD with CuO (Fig. 5d), the HOMO-LUMO dispersion is concentrated in the center of the surface flower like, while for AHEX C42 GQD with OCu (Fig. 5e) having the same features with the HOMO orbitals were switched with LUMO orbitals in the same position.As listed in Table 1, TDM of AHEX C42 increased from 0.000 Debye to 8.771, 6.045, 7.281, 4.387 and 2.881 for AHEX C42 and its interaction with ZnO, OZn, CuO, OCu and OTiO, respectively.At the same time, ∆E decreased from 3.508 eV to 1.859, 0.777, 2.231, 1.466 and 0.391 eV for AHEX C42 and its interaction with ZnO, OZn, CuO, OCu and OTiO, respectively.Accordingly, the most enhanced structure of these studied models was found to be AHEX C42-OTiO and AHEX C42-OZn which have the smallest band gap value and the most probable and stable interaction.Accordingly, all interacted MOs enhanced the electrical properties of AHEX C42 GQD and enhanced the sensitivity and reactivity of it.
In addition, as illustrated in Fig. 6a, the HOMO-LUMO orbital dispersion for GQD ZHEX C54 is spread throughout the surface and edges.The GQD ZHEX C54 interacted with ZnO, OZn and TiO 2 (Fig. 6b, c, and f), the HOMO-LUMO orbital shape were changed and move slightly to dispersion to be redistributed in the edges with some orbitals on the surface.For GQD ZHEX C54 interaction with CuO and OCu (Fig. 6d and e), the HOMO-LUMO orbital was localized on the edges and disappeared from the center which means that the electrons were localized on the edges.TDM of ZHEX C54 improved from 0.000 Debye to 9. 455, 4.635, 7.718, 3.595, and 5.505 when it interacted with ZnO, OZn, CuO, OCu, and OTiO, as shown in Table 2. Simultaneously, ∆E dropped from 2.886 eV to 1.589, 0.632, 1.833, 1.171, and 0.530 eV, respectively.As a result, ZHEX C54-OZn and ZHEX C54-OTiO were the most upgraded structure.Also, for this form of GQD all the MOs enhanced the electrical characteristics and band gap.
The HOMO-LUMO orbital distribution for GQD ATRI C60 is illustrated in Fig. 7a.The HOMO-LUMO orbital for GQD ATRI C60 is distributed throughout the surface and edges.The interaction of ATRI C60 with ZnO makes slight ineffective change as shown in Fig. 7b.In case of OZn and TiO 2 (Fig. 7c and f) caused the HOMO-LUMO orbital to be reallocated in the surface's center and switched around the carbon ring edge.While because of the interaction of ATRI C60 with CuO (Fig. 7d), the HOMO-LUMO orbital was concentrated on the central sector, which appears as a rectangular shape.When ZHEX C54 interacted with ZnO, OZn, CuO, OCu, and OTiO, its TDM increased from 0.000 Debye to 9. 214, 5.710, 7.507, 4.183, and 4.472, as shown in Table 3.At the same time, ∆E decreased from 3.177 eV to 1.634, 0.764, 2.014, 1.306, and 0.287 eV, respectively.Consequently, the ZHEX C54-OZn and ZHEX C54-OTiO were the most improved structure.
Finally, Fig. 8 illustrates the HOMO-LUMO orbital distribution for GQD ZTRI C46 and GQD ZTRI C46 interacted with metal oxides.From the Fig. 8a, the GQD ZTRI C46 HOMO-LUMO orbital is spread across the edge of the structure.As ZTRI C46 interacts with ZnO (through Zn atom or O atom) and TiO 2 (Fig. 8 b, c and e), the HOMO-LUMO orbital was reallocated at the edge surrounding the MO.In the case of ZTRI C46-CuO and ZTRI C46-OCu (Fig. 8 d and e), the HOMO-LUMO orbital was spread on all surfaces.As shown in Table 4, when ZTRI C46 interacted with ZnO, OZn, CuO, OCu, and OTiO, its TDM increased from 2.211 Debye to 9.246, 5.713, 7.727, 3.688, and 9.044 Debye, respectively.∆E of ZTRI C46 decreased from 0.305 eV to 0.291, 0.260, and 0.250 eV for the interaction with ZnO, OZn, and OTiO, while it is increased to 1.190, and 0.659 eV for the interaction with CuO and OCu respectively.From these results,

Molecular electrostatic potential (MESP)
MESP is useful because it can relate the impacts of the total charge distribution to electronegativity, dipole moment, partial charge, and the chemical reactivity sides of the molecule.MESP investigates the reactivity of the components of intermolecular interaction to characterize  the reactivity and stability of chemical structures.As a result, the electrostatic potential of the molecule is a suitable reference for determining the reactivity of the molecules to positively or negatively charged substances (Lu and Chen 2020).On the molecule's surface, different MESP values are shown in different colours in the following order: red > orange > yellow > green > blue.Highest potential is commonly found in red regions, whereas the most unattractive potential is found in blue.The colour difference indicated as red on the MESP surface is corresponding to a negative area, the blue colour refers to a positive region, and the green colour represents zero electrostatic potential.As shown in Figs. 9 and 10 a, the MESP map were perpendicular to the GQD HEX sheets before being decorated with MOs, but after being decorated with MOs, the color map spread in plane with the GQD sheet shown  in Figs. 9 and 10 b-f.The map was spread in a horizontal plane with yellow colour in the case of GQD TRI sheets, as shown in Figs.11a and 12a.In addition, in the case of ZnO and TiO 2 , red colour increased around the MO, but in the case of CuO, the red colour increased over the whole GQD sheet, as shown in Figs.11 and 12 d and e.According to MESP calculations, GQD interaction with MO activated its surface.And the most active MO was CuO, followed  by TiO 2 .This indicates that MO activates the GQD surface, and enhances its electronic properties and detection sensitivity, with GQD ZTRI C46 being the most enhanced GQDs form.As a result, GQDs decorated with CuO may be utilized as a sensing material, whereas GQDs decorated with TiO 2 could be used in electrical devices and/or as optoelectronic material.

IR results of GQDs theoretical and experimental
DFT:B3LYP at the LanL2DZ basis set was used to determine the IR frequencies of GQDs (AHEX C42, ZHEX C54, ATRI C60, and ZTRI C46).The theoretical IR spectrum of GQDs of various forms was computed and compared to the experimentally obtained ATR-FTIR spectrum of the produced GQDs. Figure 13 demonstrates the computed IR spectra of GQDs of various forms, as well as the ATR-FTIR absorbance spectrum of manufactured GQDs (14).Table 5 compare the theoretical IR frequencies of GQDs with experimental data, including the various forms of AHEX C42, ZHEX C54, ATRI C60, and ZTRI C46.After correction using the basis set scale factor, the theoretical IR information was provided.Table 5 demonstrates the fingerprint of the molecular structure of GQDs.The main characteristic bands representing GQDs, which are attributed to the bands observed around 1636 cm −1 can be attributed to C-C stretching vibrations (Dager et al. 2019).In addition, the band at 1189 cm −1 can be assigned to in-plane C-H bending (Tang et al. 2017).Therefore, the presence of C-C in-plane symmetric stretching can be assigned to the band at 1049 cm −1 .Finally, the bands around ∼884 and ∼579 cm −1 represent C-H out-of-plane bending (Danial et al. 2022).The characteristic bands of the GQDs theoretical IR spectra of several kinds of GQDs are almost the same as in the experimental spectrum (Fig. 13 and 14).

Raman results of GQDs
Figure 15 demonstrates the Raman spectra of GQDs.The main characteristic peaks that represent the pure GQD were observed (Ahirwar et al. 2017) at 1358 and 1605 cm -1 .The two observed peaks represent the D and G bands of Raman shifts, and the normal

Conclusions
DFT molecular modeling approach was used to study the electronic properties for GQDs and GQDs attached with MOs.The calculations have been carried out for hexagonal and triangular GQDs with armchair and zigzag terminations at B3LYP/LAN-L2DZ basis set.The attachment of MOs significantly increased the TDM with very high values observed in GQDs attached with ZnO.The increase in the TDM reflects the fact that the given structures are highly reactive with their surrounding media.∆E was calculated for GQDs and GQDs attached with MOs.The band gap energy of AHEX C42, ZHEX C54, ATRI C60 and ZTRI C46 was found to be 3.508, 2.886, 3.177 The most effective metal oxide on GQDs was TiO 2 that enhanced ∆E for AHEX C42-TiO 2 , ZHEX C54-TiO 2 , ATRI C60-TiO 2 and ZTRI C46-TiO 2 to 0.391, 0.530, 0.287 and 0.250 eV, respectively.MESP was calculated for the studied structures and it was found that MOs activate the GQD surface and enhance its electronic properties and detection sensitivity.GQD ZTRI C46 was found to be the most enhanced GQDs form.As a result, GQDs decorated with CuO may be utilized as a sensing material, where as GQDs decorated with TiO 2 could be used in electrical devices and/or as optoelectronic material.DFT:B3LYP at the LANL2DZ basis set was also used to determine the IR frequencies of GQDs (AHEX C42, ZHEX C54, ATRI C60, and ZTRI C46).The theoretically calculated IR spectra of GQDs of various forms were compared to the experimental

Table 5
The computed IR (Scaled) frequencies of GQDs such as AHEX C42, ZHEX C54, ATRI C60, and ZTRI C46 using the DFT:B3LYP/ LanL2DZ model were compared to the experimental resultsThe bold is defined as the assignment of the FITR bands