Gas sensing of graphene and graphene oxide nanoplatelets to ClO2 and its decomposed species

Superlattices and Microstructures 135 (2019) 106248

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Gas sensing of graphene and graphene oxide nanoplatelets to ClO2 and its decomposed species Yingang Gui a, *, Zeshen Hao a, Xin Li b, Chao Tang a, Lingna Xu a a b

College of Engineering and Technology, Southwest University, Chongqing, 400715, China State Grid Hunan Maintenance Company, Changsha, Hunan, 410004, China

A R T I C L E I N F O

A B S T R A C T

Keywords: ClO2 and its decomposition products Graphene Graphene oxide Surface adsorption DFT

Graphene is a type of two-dimensional carbon nanomaterial with a large specific surface area and a stable lattice structure. It has good electrochemical properties and widely used for gas detection based on its strong adsorption capacity. ClO2 is a strong oxidant which has been extensively applied to disinfection and chemical industry. When ClO2 leaks into the air, it can easily decompose into HClO, Cl2 and HCl under heat, collision and strong light. In this paper, graphene is selected as a novel gas sensor for ClO2 leakage detection. Based on density functional theory (DFT), adsorption systems of ClO2, HClO, Cl2, and HCl on graphene and graphene oxide were built. The adsorption energy, charge transfer, DOS, PDOS, and energy band of these adsorption systems were discussed. The results show that graphene and graphene oxide have better adsorption capacity to ClO2 comparing with its decomposition products, which reflects in the obvious increase of conductivity and decrease of band gap during the adsorption process. Therefore, graphene and graphene oxide can be used to detect the leakage of ClO2 efficiently. And it can eliminate the interferences from its decomposition products.

1. Introduction ClO2 is widely used in chemical productions, disinfection of water treatment, and sterilization for buildings due to its strong oxidizability [1–3]. The ClO2 molecular structure looks like a broken line with an angle slightly less than 120� [4]. Due to its strong oxidizability, it can react strongly with many chemicals, and even cause explosions [5,6]. It is stable under low temperature and dark environments. It is quite sensitive to light, heat, and collision, which can easily make it decompose to HClO, Cl2 and O2 when exposes to air. In addition, HClO is extremely unstable, and easily decomposes to HCl, a kind of highly volatile gas [7,8]. Moreover, due to the operational errors and aged equipment, it might lead to the cracks of pressure vessels, the fractures of valves and the breakage of gas pipelines will cause the leakage of ClO2. Therefore, it is of great significance to find a safe and effective method to perceive the leakage of ClO2 by detecting the concentration of ClO2 in the air, and eliminate the interferences from its decomposition products after the leakage of ClO2 during its preparation, storage and application. At present, the methods for ClO2 detection include iodometry, spectrophotometry, current titration, fluorescence spectropho­ tometry, and chromatography [9–13]. Among these methods, Spectrophotometry is one of the most adopted methods using in ClO2 detection. The principle of this method is collecting ClO2 in the air and detect it with reagents. The drawback of this method is dis­ commodious, and it is easy to be affected by its decomposition products and limited by the detection range, which makes it difficult to * Corresponding author. E-mail address: [email protected] (Y. Gui). https://doi.org/10.1016/j.spmi.2019.106248 Received 7 June 2019; Received in revised form 19 August 2019; Accepted 5 September 2019 Available online 6 September 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.

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Cl O C H (a) Intrinsic graphene 118.4

102.5 1.288

1.518

1.698

0.985 (c) HCl

(b) ClO2

(d) HClO

2.020 (e) Cl2

Fig. 1. Molecular mode ls of graphene and target gas.

2.500e-1 3.228

3.296

3.097

3.038

1.250e-1 0 1.250e-1 2.500e-1

(b) HCl

(a) ClO2

(c) HClO

(d) Cl2

Fig. 2. ClO2, HCl, HClO, Cl2 adsorption system model and electron density difference of these systems.

realize the real-time monitoring of ClO2. Therefore, it is urgent to find an effective way to detect ClO2 concentration in the air rapidly, accurately, and conveniently. Since Andre Geim and Konstantin Novoselov successfully prepared two-dimensional graphene materials in 2004 by mechanical exfoliation of highly oriented pyrolytic graphite [14]. Graphene has been put into commercial production due to its ripe technology. Moreover, on account of its excellent mechanical, electrical, light, and thermal conductivity properties, it has been widely used in various fields including gas detection [15]. Therefore, using graphene based gas sensor shows a good research prospect in leakage detection of ClO2. In this study, the adsorption principle of graphene based gas sensor was discussed in-depth. The gas sensing mechanism of graphene and graphene oxide to ClO2 and its decomposition products (HClO, Cl2, and HCl) was carried out, to develop a graphene based gas sensor which can be applied in leakage detection of ClO2 efficiently. 2. Calculation methods and modeling parameters All calculations were performed on the Dmol3 module of Materials Studio based on density functional theory (DFT) [16–19]. In order to ensure the accuracy and reliability of the calculation results. The generalized gradient approximation (GGA) was selected as the processing method, combining with the Perdew-Burke-Ernzerhof (PBE) functional [20–23]. For accurately evaluate the weak interaction like van der Waals force, Grimme of DFT-D was proposed to correct the total energy of the system [24,25]. For convergence accuracy, the energy, max force and max displacement were set to 1 � 10 5 Ha, 2 � 10 3Ha/Å, 5 � 10 3Å. SCF tolerance was set to 1 � 10 6Ha [26,27], and DIIS size was set to 6 [28,29]. The k-point set was set to 4 � 4 � 1 by using Monkhorst-Pack method [30]. The Basis set was set to Double numerical plus polarization (DNP) [31]. In addition, DFT Semi-core Pseudopots (DSPP) was chosen to describe the core electrons [32]. A 4 � 4 � 1 graphene supercell was built as shown in Fig. 1(a). To avoid the interaction between adjacent layers of supercell, the thickness of the vacuum layer was set to 15 Å. Molecular models of ClO2, HClO, Cl2, and O2 are optimized showed in Fig. 1(b)–(e). Where green, red, white, and gray spheres represent chlorine, oxygen, hydrogen, and carbon atoms, respectively. The unit of bond length is Å. The adsorption parameters of graphene to gas molecules, including adsorption energy (Ead), charge transfer (Q) and adsorption distance (D) were calculated. The adsorption energy (Ead) was calculated by following formula [33–35]: Ead ¼ Emolecule=graphene

Egraphene

Emolecule

2

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Table 1 Adsorption parameters of gas molecules on intrinsic graphene. Gas molecule

Ead (eV)

ClO2 HCl HClO Cl2

0.350 0.270 0.338 0.662

Q

G

F

graphene/HCl

Q

(c)

8 6 4 2 0 -2 -4 -6 -8 -10 -12

Z

G

G

3.228 3.296 3.097 3.038

Band gap is 0.891 eV

graphene/ClO2

0.891 eV

Q

F

G

(b)

Band gap is 1.954 eV

Energy/eV

1.986 eV

Z

(a)

D (Å)

0.077 0.009 0.006 0.028

graphene/HClO

1.954 eV

G

Q

F

(d)

Z

8 6 4 2 0 -2 -4 -6 -8 -10 -12

G

Z

G Band gap is 1.314 eV

Energy/eV

F

G

Band gap is 1.986 eV

Energy/eV

8 6 4 2 0 -2 -4 -6 -8 -10 -12

intrinsic graphene

2.002 eV

8 6 4 2 0 -2 -4 -6 -8 -10 -12

Energy/eV

Band gap is 2.002 eV

Energy/eV

8 6 4 2 0 -2 -4 -6 -8 -10 -12

Q (e)

1.314 eV

G

F

graphene/Cl2

Q

(e)

Z

G

Fig. 3. Energy band diagram of different adsorption systems.

where Emolecule/graphene is the total energy of the system after gas adsorbed on graphene surface. Egraphene and Emolecule are the energy of the optimized graphene, and isolated gas molecule, respectively. In addition, gas molecules with strong adsorption capacity are screened out. The changes of adsorption parameters, density of states (DOS), and partial density of states (PDOS) during the adsorption process were further studied. 3. Results and discussion 3.1. Adsorption of ClO2, HCl, Cl2, and HClO on intrinsic graphene surface Various adsorption systems of ClO2, HCl, Cl2, and HClO on intrinsic graphene surface were built. Fig. 2 and Table 1 show the most stable structures and corresponding parameter. As shown in Table 1, the adsorption energy is in following order: Cl2 > ClO2 > HClO > HCl. Combined with charge transfer, it is found that ClO2 has better sensing properties. As shown in Fig. 2, we can intuitively analyze the charge transfer by calculating the electron density difference, where the red region and the blue region represent the increase and decrease of charge, respectively. The equipotential line of electron density difference reflects the probability of charge transfer. There is no covalent bond forming during the adsorption process, signifying that the adsorption of the target gas on the intrinsic graphene surface is physisorption. As shown in Fig. 3, the band gap from Fig. 3(a)–(e) are 2.002, 0.891, 1.986, 1.954 and 1.314 eV, respectively. The intrinsic gra­ phene has a huge band gap of 2.002 eV, which explains the semiconductor properties of intrinsic graphene. Moreover, when ClO2 or Cl2 molecules adsorb on the surface of graphene, the band gaps obviously reduce for the adsorption systems, reflecting that it is easier for electrons transfer from the top of valence band to the bottom of conduction band, also signifying the improvement of conductivity for these adsorption systems. In addition, when HCl or HClO molecules adsorb on graphene, the band gap for adsorption system slightly falls to 1.986 and 1.954 eV, respectively. By analyzing the energy band of different systems, it can be found that the intrinsic graphene has better selectivity for ClO2 molecules. As ClO2 molecules have the best adsorption performance, different adsorption structures were built as the adsorption systems labeled as F1, F2, F3 and F4 in Fig. 4 and Table 2. 3

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3.228

3.445

3.228

3.242

3.070

3.043

2.963

2.500e-1 1.250e-1 0 1.250e-1 2.500e-1

(d) F4

(c) F3

(b) F2

(a) F1

Fig. 4. Electron density difference of four ClO2 adsorption structures. Table 2 Adsorption parameters of ClO2 under different adsorption systems. Adsorption system F1 F2 F3 F4

Ead (eV) 0.446 0.349 0.204 0.286

Q (e) 0.131 0.077 0.181 0.204

DC-Cl (Å)

D1C-O (Å)

D2C-O (Å)

3.445 3.228 – –

3.228 – 2.963 3.043

3.242 – – 3.070

Annotation: It is specified that the oxygen atom closer to the surface of the graphene is O1, and the farther is O2.

It is found that there is still obvious difference between four ClO2 adsorption structures by analyzing the adsorption parameters. Thus the stability of the adsorption systems are also different. Take F2 and F3 systems for example, the amount of charge transfer of F2 is 0.077 e, and the amount of charge transfer of F3 is 0.181 e. It can be found that the amount of charge transfer between the C atom and O atom is significantly more than that between C atom and Cl atom. Moreover, the adsorption capacity is in following order: F1>F2>F4>F3. The comprehensive comparison of these four adsorption structures shows that charge transfer can be obviously observed for different adsorption systems. Therefore, it can be judged that the intrinsic graphene shows great adsorption property to ClO2 gas molecules, which makes it a potential gas sensing materials for ClO2 detection. The DOS and PDOS were calculated to understand the electronic properties, and analyze the atomic orbital interactions for different ClO2 adsorption systems as shown in Fig. 5. When ClO2 molecules adsorb on the surface of intrinsic graphene, the total DOS has significantly increase at the Fermi level, and in the range of 0 to 1.5 eV, 2 to 4 eV, and 6 to 9 eV, which explains the improvement of conductivity. In addition, the DOS of F1–F4 systems moves right, which is beneficial for the charge transfer. According to the PDOS of several major atomic orbitals in F1–F4 systems, it can be found that there is a hybridization between O-2p, Cl-3p, and C2p orbitals in the range from 2 to 4 eV, and near 7.5 eV, that is the reason why the total DOS increases. Hence, ClO2 exhibits good adsorption over the intrinsic graphene. Comparing the DOS and PDOS of these four adsorption structures, the overlapping area of F3 and F4 is slightly less than that of F1 and F2. Combined with the adsorption parameters in Table 2. It can be judged that the adsorption performance of F1 and F2 is better than that of F3 and F4. Basing on the great adsorption capacity, obvious change of DOS and PDOS for different adsorption sites, graphene can be employed as gas sensors to detect ClO2. 3.2. Structure and adsorption for ClO2, HCl, Cl2, and HClO over graphene oxide The adsorption properties of ClO2 and its decomposition products over graphene oxide (GO) were also studied to further improve the gas sensing performance of gas sensor in practical application. GO is an important derivative of graphene obtained by chemical oxidation and stripping [36]. A large number of epoxy groups, hydroxyl groups, and other oxygen-containing functional groups distribute on its surface. And it has some outstanding properties that intrinsic graphene does not possess [37,38]. To study its adsorption properties to the gases, different GO structures containing epoxy group, and hydroxyl group were built labeled as GO-OH and GO-O as shown in Fig. 6, where the red, white and gray spheres represent oxygen, hydrogen and carbon atoms, respectively. Moreover, obvious deformation occurs on the carbon atom layer due to the strong chemical bonds. The adsorption parameters of different adsorption systems were shown in Table 3 and Fig. 7. After gas molecules adsorption, it can be obviously found that the adsorption energy changes slightly for different adsorption systems. The weak adsorption energy explains that target gas molecules adsorption is physisorption, and the process is reversible. Under external excitation, gas molecule will desorb from GO surface. Besides, comparing the charge transfer of adsorptions over intrinsic graphene and two kinds of GO, the adsorption capacity for gas molecules over GO fragment is stronger than that over intrinsic graphene, and the charge transfer amount of ClO2 is more than HCl, HClO and Cl2. The hydroxyl group of GO plays an important role in improving the adsorption properties. It can be judged that the adsorption capacity of GO to ClO2, HClO and Cl2 significantly enhances, and the sensing properties is in order of ClO2>Cl2>HClO > HCl. By analyzing the band gap, it is found that the band gap reduces obviously after ClO2 molecule adsorption. Also the hydroxyl group takes an intensely effect to the change of band gap, and it is beneficial for the improvement of electrical conductivity. However, for HCl, HClO and Cl2, the band gap changes slightly. The final conductivity is in following ordered as: ClO2 adsorption system > Cl2 adsorption system > HClO adsorption system > HCl adsorption system. 4

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2.5

Graphene/ClO2 Graphene

2.0

12

PDOS/eV

DOS/eV

16

C-2p Cl-3p Cl-3d O-2s O-2p

8 4

1.5 1.0 0.5

0 -12

-9

-6

-3

0

3

0.0

6

Energy/eV

-9

-6

2.5

Graphene/ClO2 Graphene

PDOS/eV

DOS/eV

12 8 4

1.5 1.0 0.5

-9

-6

-3

0

3

0.0

6

-9

-6

Energy/eV (c) DOS of F2 system

20

PDOS/eV

DOS/eV

3

6

C-2p Cl-3p Cl-3d O-2s O-2p

2.0

12 8 4

1.5 1.0 0.5

-9

-6

-3

0

3

0.0

6

-9

-6

Energy/eV

-3

0

3

6

Energy/eV (f) PDOS of F3 system

(e) DOS of F3 system

20

2.5

Graphene/ClO2 Graphene

16

C-2p Cl-3p Cl-3d O-2s O-2p

PDOS/eV

2.0

12

DOS/eV

-3 0 Energy/eV (d) PDOS of F2 system

2.5

Graphene/ClO2 Graphene

16

8 4 0 -12

6

C-2p Cl-3p Cl-3d O-2s O-2p

2.0

16

0 -12

3

(b) PDOS of F1 system

(a) DOS of F1 system

20

0 -12

-3 0 Energy/eV

1.5 1.0 0.5

-9

-6

-3 0 Energy/eV

3

0.0

6

(g) DOS of F4 system

-9

-6

-3 0 Energy/eV (h) PDOS of F4 system

3

6

Fig. 5. DOS and PDOS of ClO2 adsorbed intrinsic graphene in F1–F4 systems, the dashed lines represent the Fermi level.

5

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1.493

1.462

(a) Hydroxyl group on the surface (GO-OH)

(b) Epoxy group on the surface (GO-O)

Fig. 6. The top view and side view of the optimized GO fragment models. Table 3 Adsorption parameters of target gas adsorption on GO fragments. Gas molecule

Functional group type

ClO2

-OH -O-OH -O-OH -O-OH -O-

HCl HClO Cl2

-0.7

-0.25

0.247 0.117 0.004 0.001 0.023 0.026 0.051 0.030

Eg(eV)

1.803 3.041 3.072 3.128 2.766 2.663 2.742 2.727

0.128 0.929 0.767 1.637 0.746 1.613 0.728 1.485

2.5

intrinsic graphene GO-OH GO-O

-0.20

D (Å)

intrinsic graphene GO-OH GO-O

2.0

-0.5 -0.4 -0.3 -0.2

-0.15

Band gap/eV

Charge transfer/e

Adsorption energy/eV

Q (e)

0.556 0.406 0.245 0.218 0.315 0.313 0.633 0.607

intrinsic graphene GO-OH GO-O

-0.6

Ead (eV)

-0.10 -0.05 0.00

ClO2

HCl

(a)

HClO

Cl2

1.0

0.5

-0.1 0.0

1.5

ClO2

HCl

(b)

HClO

Cl2

0.0

ClO2

HCl

(c)

HClO

Cl2

Fig. 7. Comparison of adsorption energy (a), charge transfer (b) and band gap (c) for different adsorption systems.

Considering that ClO2 molecule has obvious charge transfer over GO surface containing hydroxyl group and epoxy group, ClO2 adsorption system models and electron density difference of these systems are obtained as shown in Fig. 8. It can be found that the hydroxyl group can still bind to the carbon atom layer of graphene stably, when ClO2 adsorbs over GO fragment containing hydroxyl group. The adsorption distance is 1.803 Å, and the charge transfer between the ClO2 molecule and the hydroxyl group is obvious, indicating that there is a strong adsorption. There is also a small amount of charge transfer between the ClO2 molecule and the surface of the carbon atomic layer, while the amount of charge transfer is significantly less than that between the ClO2 molecule and the hydroxyl group. Comparing the ClO2 adsorption systems, it can be found that the adsorptions on GO-O and the intrinsic graphene are weaker than that on GO-OH. Therefore, the hydroxyl group is like a bridge which can increase the mobility of the charge and improve the conductivity of adsorption system. In order to compare the adsorption property of the GO surface before and after gas adsorption, the band structure, DOS, and PDOS 6

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3.041 1.803

2.500e-1 1.250e-1 0 1.250e-1 2.500e-1

(b) ClO2/GO-O

(a) ClO2/GO-OH

Fig. 8. ClO2 adsorption system models and electron density difference of these systems.

are calculated as shown in Figs. 9 and 10, respectively. The band gap of GO surface containing hydroxyl group is 0.789 eV. After adsorbing ClO2 molecules, the band gap obviously falls to 0.128 eV, which explains that the electrical conductivity of this adsorption system improves. On the other hand, the band gap of GO surface containing epoxy group is 1.641 eV. The adsorption of ClO2 brings the band gap decrease to 0.929 eV. Similarly, it is of benefit to improving the electrical conductivity of adsorption system. By analyzing band structure of systems before and after gas adsorption, it can be found that GO has a good adsorption capacity for ClO2 molecules. Meanwhile, the changes of the band gap can lead to the change of resistance of gas sensor. The DOS and PDOS of ClO2 molecule adsorption over GO surface containing hydroxyl group and epoxy group are shown in Fig. 10. When ClO2 molecule adsorb on the surface of GO-OH, the DOS increases significantly at the Fermi level. Meanwhile, the band gap of ClO2 adsorption system is 0.128 eV, which reflects the improvement of electrical conductivity. In addition, hybridization peaks occur near 3 eV from O-2p orbit, O-2p of hydroxyl group, Cl-3d orbit, and 6 eV by Cl-3p orbit, O-2p orbit, C-2p orbit, O-2p of hydroxyl group orbit, respectively. For ClO2 adsorption on GO-O as shown in Fig. 10(c)–(d), almost no change occurs at 0 eV, but there has a slightly change below and above Fermi level, indicating that the ClO2 molecule obviously contributes to improve the conductivity, and the PDOS from 1.5 eV to 9 eV in this case is also a good illustration of this phenomenon. Obvious orbit hybridizations occur near 3 and 6 eV caused by C-2p, O-2p of epoxy group, Cl-3p, Cl-3d, and O-2p orbits. After ClO2 adsorption, the band gap falls to 0.929 eV. Comparing the parameters of these two adsorption systems synthetically, both of epoxy group and hydroxyl group conduce to Band gap is 0.789 eV

G

F

Q (a)

Z

G

G

F

GO-O

Q (c)

8 6 4 2 0 -2 -4 -6 -8 -10 -12

Z

G

GO-OH/ClO2

0.128 eV

F

G

Q (b)

G

Z

G

GO-O/ClO2

0.929 eV

G

F

Q (d)

Fig. 9. Band structures of systems before and after gas adsorption. 7

Z

Band gap is 0.929 eV

Energy/eV

1.641 eV

Band gap is 0.128 eV

Energy/eV

8 6 4 2 0 -2 -4 -6 -8 -10 -12

Band gap is 1.641 eV

Energy/eV

8 6 4 2 0 -2 -4 -6 -8 -10 -12

GO-OH

0.789 eV

Energy/eV

8 6 4 2 0 -2 -4 -6 -8 -10 -12

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3.0

ClO2/GO-OH GO-OH

20

2.5

PDOS/eV

DOS/eV

16 12 8

2.0 1.5 1.0

4 0 -12

C-2p O-2p(GO-OH) Cl-3p Cl-3d O-2p(ClO2)

0.5 -9

-6

-3 0 Energy/eV

3

0.0

6

-9

(a) DOS of ClO2/GO-OH system

2.0 1.5

8

1.0

4

0.5

-6

-3 0 3 Energy/eV (c) DOS of ClO2/GO-O system

6

C-2p O-2p(GO-O) Cl-3p Cl-3d O-2p(ClO2)

2.5

PDOS/eV

DOS/eV

12

-9

3

3.0

16

0 -12

-3 0 Energy/eV

(b) PDOS of ClO2/GO-OH system

ClO2/GO-O GO-O

20

-6

0.0

6

-9

-6

-3 0 3 Energy/eV (d) PDOS of ClO2/GO-O system

6

Fig. 10. DOS and PDOS of ClO2 adsorption over GO surface, the dashed lines represent the Fermi level.

increasing the conductivity of adsorption system. Compared with the PDOS of ClO2 adsorption over intrinsic graphene, the adsorption parameters of gas molecules on GO is better. Moreover, both of intrinsic graphene and GO show better adsorption to ClO2 than its decomposition products: HCl, HClO and Cl2. It demonstrates that GO is feasible material for ClO2 leakage detection. 4. Conclusion In this work, intrinsic graphene and GO were chosen to detect ClO2 and its decomposed products (HCl, HClO, Cl2). Based on density functional theory (DFT), adsorption systems were constructed with Materials Studio to calculate the adsorption parameters. Band structure and DOS were discussed to synthetically analyze the interactions between gas molecules and graphene based materials. A new gas sensor applied to the leakage detection of ClO2 safely and efficiently has been proposed. Specific gas sensing mechanism and conclusion can be obtained: 1. Intrinsic graphene and GO exhibit good sensitivity to ClO2, but shows weak sensitivity to its decomposed products. The calculation results show that the sensing properties for these gas molecules is in the following order: ClO2 > Cl2 >HClO > HCl. Moreover, GO based gas sensing material is better than intrinsic graphene by comparing the adsorption parameters, which mainly reflects in the increase of charge transfer amount and the decrease of band gap. 2. The adsorption is an endothermic process, indicating it occurs spontaneously. The ClO2 adsorption system has more charge transfer and also has an obvious decrease of the band gap. Electron transfer of ClO2 decomposed products is less than that of ClO2. While Cl2, HClO and HCl adsorption only lead to a slight decrease of the band gap. It explains that graphene based gas sensing material has better sensitivity to ClO2 molecule comparing to its decomposed products.

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3. All of the gas molecules adsorption causes an increase of conductivity of the whole system. Combining with the analysis of DOS and band structure, the conductivity is ordered as following: ClO2 > Cl2 > HClO > HCl. And the ClO2 molecule makes more contri­ butions to the increase of conductivity. The calculation results show that GO and intrinsic graphene based sensors provide a convenient method to detect the leakage of ClO2 efficiently, sensitively and accurately. Also it will avoid the interferences from its decomposed products when ClO2 leaks into the air. Acknowledgements This work is supported by supported by the National Natural Science Foundation of China (Grant No. 51907165), the Chongqing Research Program of Basic Research and Frontier Technology (Grant No. cstc2018jcyjAX0068), and the National Key R&D Program of China (Grant No. 2017YFB0902700, 2017YBF0902702). References [1] E.M. Aieta, J.D. Berg, A review of chlorine dioxide in drinking water treatment, J. AWWA (Am. 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