Optical graphene quantum dots gas sensors: Theoretical study

Accepted Manuscript Optical Graphene Quantum Dots Gas Sensors: Theoretical Study

D. Raeyani, S. Shojaei, S. Ahmadi-Kandjani PII:

S0749-6036(17)32838-0

DOI:

10.1016/j.spmi.2017.12.050

Reference:

YSPMI 5447

To appear in:

Superlattices and Microstructures

Received Date:

30 November 2017

Accepted Date:

26 December 2017

Please cite this article as: D. Raeyani, S. Shojaei, S. Ahmadi-Kandjani, Optical Graphene Quantum Dots Gas Sensors: Theoretical Study, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi. 2017.12.050

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Optical Graphene Quantum Dots Gas Sensors: Theoretical Study

D. Raeyani, S. Shojaei,*, S. Ahmadi-Kandjani Photonics Department, Research Institute for Applied Physics & Astronomy (RIAPA), University of Tabriz, 51655-163,Tabriz, Iran

Abstract: In this work, we theoretically studied the changes of graphene quantum dots (GQD) absorption spectra under the influence of different gases to indicate optical gas sensing features of GQDs. The adsorption of gas molecules such as CO2, N2 and Ar on GQDs have been theoretically investigated through time-dependent density functional theory (TDDFT) calculations. Our study revealed that UV-Vis absorption spectrum of GQDs in the presence of CO2 undergoes considerable changes than that of N2 and Ar. The shift of maximum absorption wavelength for adsorption of CO2, N2 and Ar in same distance from GQD in addition to density of state (DOS) and orbital analyses have been obtained. To verify our theoretical results, comparison with experimental study has been done and good agreement has been observed. Comparing with electrical property of GQD, optical properties showed an efficient tool to be implemented in gas adsorption and paves the way towards GQD optical gas sensors. Keywords: Time Dependent Density Functional Theory, Graphene Quantum Dot, Optical absorption, Optical Gas Sensing.

*Corresponding authors: E. mail: [email protected], [email protected] (S. Shojaei).

Tel: +98 4133392995, Fax: +98 4133347050

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1-Introduction: In recent years, global warming as a result of greenhouse gases effect is being concerned. Carbon dioxide is one of the greenhouse gases that derive from different ways, including: combustion of coal and organic materials, liquid fermentation, respiration processes of living organisms and volcanic outgassing. Also detecting CO2 is necessary in measuring the amount of air pollution, fire detection, checking metabolism of human body, production of carbonated drinks and etc. Moreover, since the industrial revolution till now a significant amount of CO2dissolved from atmosphere in the ocean, causing ocean acidification and endangering life of underwater organisms. According to these points, it is necessary to detect the exact amount of CO2 gas. Graphene Quantum dots (GQDs) are one of the Nano sized materials, which contain very small pieces of Graphene (a single atomic layer of graphite)[1].They are known as an attractive material for optoelectronic devices. High optical absorption and tunable band gap are the main reasons for this attention[2–6]. Also they show an important sensory property [7–10] because of their large surface to volume ratio[11] and they are used in both electrical and optical gas sensing devices[8,12]. Up to now, GQDs have been fabricated in different ways which is fall into two methods: topdown [13–18] and bottom-up[19]. We have synthesized them using hydrothermal method as a kind of top- down approach and studied experimentally the optical gas sensing property in our previous work [20]. Optical gas sensing has a lot of great advantages compared with other methods, including following features as omitting electrical signals, the risk of acting as an ignition source disappears in gas leakage locations and the electromagnetic noise doesn’t have any effect on this type of sensors. In addition, it is possible to cover a wide area with only one optical sensor using optical fibers.[21,22] There are also many theoretical studies about GQDs [23,24] and their optical[25– 30]and electrical properties [31–34]. However to the best of our knowledge, there is no systematic research about theoretical study of GQD’s optical gas sensing

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property. Herein, we study the CO2 gas detection of GQDs using optical absorption spectra by theoretical time-dependent density functional theory (TDDFT) method. To compare different gas interactions, we also study the effect of two other gases, N2 and Ar. All calculations were based on B3LYP level which is presented as one of the most accurate functionals in computational chemistry[35–37]. Also we report frontier molecular orbital (MO) analysis and density of state (DOS) spectrum of GQDs and considered the effect of gas adsorption on these studies. In addition to verify our calculations, we studied effect of CO2 gas on UV-Vis absorption spectra of GQDs, experimentally. 2-Computational Methods: The GQDs are modeled by 24 carbon atoms forming7 benzene rings in a circular shape, that is the most stable geometry[29]. To saturate the dangling bonds at the edge of GQD, two different models are used: using Hydrogen atoms at the edge sides and second one OH functional groups. These two structures are shown in Fig 1. CO2, N2 and Ar are the gas molecules which are used in this study. After optimizing the gas molecules and GQD structures, gas molecules were placed in different distances (6, 4, 3, 2.5, 2.25, 2, 1.75, 1.5, 1.25, 1, 0.75, 0.5 and 0.25 Å) from GQDs and UVVis optical absorption was calculated for each distance. According to previous works[38–41], physisorption and chemisorption of gas molecules mostly happened in this range of distance. The distance between gas molecule and GQD’s surface is influenced by gas molecule kinetic energy and the bonding type. According to kinetic energy gas molecule may form a chemical or physical bond or none of them. From point of view, beside physisorption and chemisorption distances, the gas molecules may take unstable distances from surface of GQD for a few periods of time. Chosen range of distances above, we should find the possible distance of physisorption and chemisorption for different gases using MO analysis also this method considered most of possible conditions for a better simulating. All calculations including optimization and UV-Vis absorption were carried out using the Gaussian 09 program package [42], with TDDFT [43,44] at the level of B3LYP[45]. From previous research, absorption and fluorescence wavelengths for GQDs obtained by B3LYP hybrid functional are satisfactorily close to the experimental results [27]. Also 6-311 G basis set are used for all calculations. The frontier MO and UV-Vis absorption calculations are prepared with GaussView software [46] and DOS, HOMO, LUMO and band gap are obtained by Gauss Sum

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software[47]. Pressure and temperature are kept constant and we used only the Gaussian package presuppositions. 3- Experimental Methods In order to verify our findings in experiment, we musured absorption spectrum of GQD in the presence of gases. Here, we briefly introduce our experimental method. To prepare GQDs, a mixture of graphite and organic salt reacts in a vacuumed autoclave vessel at 250 °C for 24 hour, leading to formation of Graphite intercalation compounds (GICs). Then GICs are solvated and broken into GQDs in a water solution. Finally, we used dialysis tubing to remove remaining salt. For optical gas sensing measurements, GQDs deposited on SiO2 substrates and placed in a chamber with quartz windows which was located in an optical setup.

4-Results and Discussion 4-1 Geometry Optimizations The optimized structures for GQDs are presented in Fig. 1. Bond length between carbon atoms in Fig. 1a denoted by C1-C2, C1-C3 and C1-C4 obtained as 1.423,1.443 and 1.421Å, respectively, also bond angles between carbon atoms donated by C2C1-C3, C2-C1-C4 and C4-C1-C3 obtained as120.1, 119.7 and 120.0˚, respectively. Bond lengths and bond angles between carbon atoms for other GQD, which presented in Fig. 1 b, denoted by C1-C2, C1-C3 , C1-C4 and C2-C1-C3, C2-C1-C4 , C4C1-C3are calculated to be 1.422, 1.444, 1.422 Å and 120.0, 119.9, 120.0˚, respectively. These results are in good agreement with previous works[48,49]. The bond length in the edge side of GQD between Hydrogen and Carbon atom in Fig. 1 b was predicted as1.082 Å. For other structure which contains OH functional group, C-O and O-H bond lengths fall in the range between 1.377 to 1.440 Å and 0.975 to 0.982 Å, respectively. Also C-O-H bond angle was calculated as between 108.4 to 111.0˚. CO2 and N2 gas molecules can be placed on GQDs in both parallel and vertical modes. Based on previous studies [38], parallel mode is the most stable state for gas adsorption. Hence, we examined only the parallel mode for our structures (Fig. 2). These optimized structures including GQDs and gases were calculated at the B3LYP level using 6-311G basis set.

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Fig.1.The molecular structure of (a) OH-substituted GQD (b) H-substituted GQD.

Fig. 2.The molecular structure of (a) Ar (b) N2 (c) CO2 adsorbed on GQD.

4-2 UV-Vis absorption In this subsection, we should calculate the optical absorption of isolated GQD and GQD in the presence of gas molecule above GQD at different distances, using TDDFT /B3LYP level combined with 6-311G basis set. The absorption spectra of GQDs are reported in Fig. 3 which shows spectra for both designed structure of GQDs with different edge atoms, including H-substituted and OH-substituted. Maximum absorption wavelengths were calculated to be 301 and 351nm for Hsubstituted and OH-substituted GQD, respectively. Since the thickness of pellet and concentration of sample have not been considered in our calculations, Fig. 3 displays the absorption spectra using changes of epsilon in Y-axis. According to LamberBeer law reads: A= ε b c (1)

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Where A is Absorbance, b is the thickness of pellet and c is the concentration of sample, the spectrum with epsilon in Y-axis has the same relative absorbance as an absorption spectrum would have[50]. The spectra of GQD and their maximum absorption wavelengths are similar and in the range of previous reports [29,30]. Also this wavelength associated with π-π* transition of C-C bonds in GQDs [27,51].Fig. 4 also presents the absorption spectra of GQD based on our experiments and theoretical studies. It shows that maximum absorption wavelength has a difference about 40nm between calculated and experimental results and for the other structure of GQD with OH atoms on edge sides, it is about 90nm.This shift is between 50 to100 nm in other works [29]. To explain the difference between maximum absorption wavelength of experimental and theoretical results, it is worth to note that increasing the 2pz atomic orbitals, energy levels of pi MOs will increase. Therefore energy gap for π-π* transition becomes narrower, and the wavelength of absorbed light becomes longer accordingly[52]. In the case of synthesized GQDs, since the most of 2pz atomic orbitals are saturated with functional groups, energy gap for ππ* transition increases and wavelength of light absorbed becomes shorter than theoretical calculations which contain completely free 2pz atomic orbitals in designed structure. The wavelength (λ max) and oscillator strengths (f) for the absorption maximum of isolated GQD and GQD with CO2 , N2 and Ar gas molecules are reported in table 1. It is clear that by adsorption of CO2 gas molecule on GQDs, the absorption wavelength changes to higher wavelengths. The maximum shift of wavelength is about 3293.0 nm and it happened at distance of 1.25 Å between GQD and CO2 gas molecule. At the same distance, this shift for adsorption of N2and Ar gases compared with CO2 are lower and they became 625.4 and 446.2 nm, respectively. Furthormore, from the time resolved muserment reported in Fig. 7, it can be seen that compared with the case of N2 gas, CO2 induce a considerable variation of absorbance. All time resolve tests performed at 250nm using pure concentration of gases. Argon molecules are nonreactive and regularly used as a recovery gas in sensing experiments. Nitrogen same as Argon is known as inert gas; therefore they don’t show much impact on GQDs absorption wavelength. Our results display a significant change in absorption spectra of GQDs during CO2 adsorption. This observation indicates potential of GQDs as an optical gas sensor for CO2 gas. Fig. 5 shows the absorption spectra of GQD for different distances of CO2 gas adsorption. The difference between convolution curve of all spectra and isolated GQD’s absorption spectra in this Fig shows the change of absorption spectra in

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agreement of our experimental studies. In experimental spectra, the intensity of optical absorption shows a greater increase in higher wavelengths by gas adsorption (Fig. 6). The same will happen for convolution curve of GQD+CO2spectra compared with optical absorption of isolated GQD, in theoretical result. Gas response of GQD in absorption wavelength of 468 nm calculated to be %129 and it increases for higher wavelengths. The gas response in previous works[38]using electrical properties of GQD was calculated about %15.5 (Table. 2).This suggests that electrical method doesn’t show a significant sensing capability for GQD so there are many studies, trying to improve GQDs electrical sensing property by doping or using other methods[40,53].

Fig. 3.Calculated optical absorption spectra for two types of GQDs.

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Fig. 4. UV-Vis absorption spectra of GQD obtained from both experimental and theoretical investigations.

System Isolated GQD

GQD+ CO2

GQD+ N2

GQD+ Ar

Distance (Å)

λ max (nm)

f

-

351.4

0.537

4 3 2.5 2.25 2 1.75 1.5 1.25 1 4 3 2.5 2.25 2 1.75 1.5 1.25 1 4 3

448.7 447.8 448.5 458.0 458.9 467.9 1111.1 3293.0 1132.2 351.9 361.1 370.8 380.3 400.4 475.1 795.1 625.4 736.8 351.5 351.7

0.027 0.023 0.023 0.019 0.024 0.074 0.003 0.001 0.008 0.406 0.180 0.235 0.243 0.316 0.147 0.017 0.059 0.055 0.536 0.532

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2.5 2.25 2 1.75 1.5 1.25 1

356.6 353.9 356.9 366.8 419.5 446.2 725.0

0.506 0.480 0.371 0.175 0.034 0.007 0.0004

Table 1.The maximum absorption wavelengths (λ max) and oscillator strengths (f) for different distances of gas adsorption on GQD.

Fig. 5.Calculated UV-Vis absorption spectra of GQD for different distances of CO2gas. (Distances are in angstrom)

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Fig. 6.UV-Vis absorption spectra of GQD when exposed to air and CO2 gas.

Fig. 7.Time-resolved tests for GQD sample performed at 250 nm after exposure to multiple air-gas (Pure CO2 and N2) cycles.

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Theoretical response

Our work 𝐼𝐺𝑄𝐷 + 𝐶𝑂 ‒ 𝐼𝐺𝑄𝐷 2

𝐼𝐺𝑄𝐷

Previous study[38] 𝐸𝐺𝑄𝐷 + 𝐶𝑂 - 𝐸𝐺𝑄𝐷 ≅%129

2

𝐸𝐺𝑄𝐷

≅%15.5

Table 2.Theoretical gas sensing response for our work and previous study.(Where E is absolute energy[38] and I is optical absorption intensity)

4-3 Orbital Analysis Frontier molecular orbitals of GQD for both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shown in Fig. 8. It can be seen that HOMO of GQD is localized on C-C bonds and collection of Pz orbitals for each carbon atom has overlapped with its adjacent carbon atom, while for oxygen atoms at edge side of GQD HOMO is shown as node. Overlap on C-C bonds is also seen in LUMO of GQDs, but in the opposite site of it. After placing CO2 gas molecule on GQD in distance of 4 to 3 Å, no change is observed in HOMO and LUMO of GQDs but from 2.5 to 1.25 Å, the LUMO shifts towards CO2 molecule and from 1.25 to 1, both HOMO and LUMO shift towards the CO2 gas molecule (Fig. 9). In terms of quantum mechanics, the interaction of frontier molecular orbitals shows the interaction between two reactants [54]. So we find that only in distance of 2.5 Å to the bottom, bonding formation occurs. From 1.25 Å to the bottom the interaction becomes stronger and probably in this range, bonding type is chemical.

Fig. 8.The molecular orbitals of (a) HOMO and (b) LUMO for GQD.

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Distance HOMO (Å)

LUMO

3

2.5

1.25

Fig. 9. The molecular orbitals of HOMO and LUMO for different distance of CO2 gas from GQD.

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4-4 DOS Study For further investigations, density of state (DOS) of GQD with and without gas adsorption were also calculated using the same method (TDDFT/B3LYP/6-311G). In DOS of isolated GQD, the band gap energy (Eg) is 3.276 eV and HOMO and LUMO peaks are in -5.019 and -1.743eV respectively (table 2), which are similar to previous studies [40,49,53]. Also by adding CO2, in distance of 6 to 2 Å there is no significant change in DOS of GQD but in distance of 1.25 Å band gap and HOMO and LUMO peaks changed to 0.935, -4.571 and -3.636 eV respectively (table 2). This study carried out for two other gases and the results are demonstrated in table 2. For a better comparison of different gas impact on DOS of GQD, their DOS spectra are shown in Fig.12. The distance between GQD and gas molecule for all DOS spectra in Fig 12 is about 1 Å. The figure displays that most changes are respectively seen in CO2, N2 and Ar gas adsorptions.

Table 3.Calculated Band Gap, HOMO peak and LUMO peak energies of CO2, N2 and Ar adsorbed on GQD for different adsorption distances. CO2 Distance Band (Å) Gap Isolated GQD 4

N2 HOMO LUMO Peak Peak

Ar

Band Gap

HOMO LUMO Peak Peak

Band Gap

HOMO LUMO Peak Peak

3.276

-5.019

-1.743

3.265

-5.033

-1.768

3.277

-5.029

-1.752

3.276

-5.019

-1.743

3

3.265

-5.038

-1.776

3.268

-5.037

-1.769

3.275

-5.021

-1.746

2.5

3.246

-5.056

-1.810

3.229

5.050

-1.821

3.277

-5.029

-1.752

2.25

3.211

-5.068

-1.857

3.177

-5.053

-1.876

3.283

-5.038

-1.755

2

3.048

-5.071

-2.023

3.050

-5.020

-1.980

3.300

-5.051

-1.751

1.75

2.557

-5.045

-2.488

2.642

-4.887

-2.245

3.341

-5.069

-1.728

1.5

1.652

-4.927

-3.275

1.357

-4.216

-2.859

3.419

-5.084

-1.665

1.25

0.935

-4.571

-3.636

2.162

-4.554

-2.392

3.362

-4.992

-1.630

1

1.438

-4.354

-2.916

2.140

-4.574

-2.434

2.491

-4.473

-1.982

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Fig. 12.The electronic density of states for GQD, CO2+GQD, N2+GQD and Ar+GQD in adsorption distance of 1 Å. 5-Conclusions We have performed TDDFT calculations on optical property of GQD to investigate the adsorption of gas molecules including CO2, N2 and Ar. The optical absorption spectra of GQD which calculated using B3LYP/6-311G is in good agreement with previous theoretical predictions and our experimental results. All DOS, experimental time resolved tests and optical absorption spectra show significant change for CO2 during gas adsorption of GQDs, against other gases. Also we found that compared with electrical method, optical property of GQD can be used as an efficient sensing method. Both theoretical and experimental results show that GQDs have a good

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ability to detect CO2 gas using optical absorption property. CO2 gas compared with N2 and Ar gases have a greater impact on UV-Vis Absorption spectra of GQD. It is indicated that GQDs could be used as an optical gas sensor for CO2 gas.

Acknowledgement

We are grateful to Professor Wojtek Wlodarski from School of Electrical and Computer Engineering and School of Applied Sciences, RMIT University( Australia) for her helpful comments and discussions.

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►Optical absorption of graphene quantum dot has been calculated ► the effect of three types of gases on optical absorption has been studied in details ► good agreement with experiments has been obtained.