Chlorinated Graphene via the Photodecomposition of Metal Chlorides

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Chlorinated Graphene via the Photodecomposition of Metal Chlorides Xinyang Ji, Shunyao Wang, Enhao Zhang, Yuanyuan Zhang, Zhuo Ma, and Yunfeng Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b02193 • Publication Date (Web): 17 May 2019 Downloaded from http://pubs.acs.org on May 19, 2019

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ACS Sustainable Chemistry & Engineering

Chlorinated Graphene via the Photodecomposition of Metal Chlorides Xinyang Ji,a Shunyao Wang,d Enhao Zhang,a Yuanyuan Zhang,a Zhuo Ma,c Yunfeng Qiu,a,b* a School

of Chemistry and Chemical Engineering, Harbin Institute of Technology, No.92 West

Dazhi Street, Nan Gang District, Harbin, 150001, People’s Republic of China. b

Key Laboratory of Micro-systems and Micro-structures Manufacturing, Harbin Institute of

Technology, No.2 Yikuang Street, Nan Gang District, Harbin 150080, People’s Republic of China. c

School of Life Science and Technology, Harbin Institute of Technology, No.92 West Dazhi

Street, Nan Gang District, Harbin, 150001, People’s Republic of China. d

The First Affiliated Hospital, Jiamusi University, No.258 Xuefu Street, Jiamusi, 154007,

People’s Republic of China. *Corresponding author’s E-mail: [email protected]

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KEYWORDS. graphene; heteroatom doping; carbonaceous material; self-supported electrode; oxygen evolution reaction.

ABSTRACT. Searching for facile and sustainable chlorination method to modify graphene has drawn tremendous attention due to the modulation of its electronic property and surface chemistry. In contrast to previously developed Cl2 photochlorination method via Cl radicals, present work found that the photodecomposition of metal chlorides under UV light irradiation was able to chlorinate graphene taking into account both appropriate Cl contents and high carrier mobility. Field effect transistor proved that the mobility was around 345.5 cm2 V−1 s−1 when introducing 4.3 at.% Cl dopants. Raman proved that UV irradiation resulted in structural defects to favor the mass transport in electrochemical reactions. Chlorinated verticallyoriented graphene nanosheets on carbon cloth exhibited improved oxygen evolution reaction performance in comparison to pristine graphene. The specific electrochemical active area and charge transfer analysis indicated that Cl-doping was favorable for the exposure of active sites and fast electron transport, and thus driving the electrochemical kinetics in virtue of small Tafel value (59 mV dec-1). The direct growth of graphene nanosheets on carbon cloth endowed the free-standing film robust durability. Present work opens a new avenue for the chlorinated graphene considering all merits in one for electrochemical activities, and will provide promising candidates for application in energy storage and conversion fields.


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Introduction The increasing depletion of fossil fuel and environmental contaminations have required the development of renewable energy1-2. Oxygen evolution reaction (OER) is a basic reaction in water splitting or metal-air battery3-9, which plays an important role in lowering the cell voltage of water splitting and the charging potential of rechargable metal-air battery10-12. However, OER involves the stepwise 4e- transfer process, and such kinetically sluggish process needs highly active electrocatalysts to overcome. Although Ir or Ru based electrocatalysts exhibited promising OER performance, the scarcity and high-cost of noble metal based electrocatalysts are detrimental to the practical application13-15. During the past decades, researchers have endeavored to design and synthesize non-precious(noble) electrocatalysts to replace noble metal16-20. Lots of poinneering work about transition metal oxides, metal oxyhydroxides, metal phosphides, metal dichlchagenides, etc., have been reported to show excellent OER activites21-22. Meanwhile, in parrallel to non-precious metal OER electrocatalysts, metal-free OER electrocatalysts have been also exploited due to its ecofriendly nature for sustainable development of renewable energy1. Among all carbonaceous nanomaterials, graphene has drawn tremendous attention due to its unique two-dimensional atomic crystalline structure, surface chemical properties, facinating optoelectronic preperties, mechanical properties, etc23. So far, heteroatoms, such as B, N, P, S, etc. Elements, can be introduced into graphene skeleton to induce the charge dislocation and modulate spin density, showing intriguing OER property1,

24-26.

Of note,


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various key intermediates are involved in OER process, such as O*, OH*, and OOH*, of which the adsorption energy could be optimized via surface modification, interface engineering, and structural adjustment in graphene27. Theretical studies have predicted that the formation of O* from OH* after releasing a proton and electron, or the formation of OOH* from O*, was regarded as the potential-determinting step. Meanwhile, the transformation from OOH* to O2 was still dominant by the adsorption energy of OOH* and the dissociation barrier of O2 from the surface of graphene. Experimetal results have proven that heteroatoms doping (including B, N, O, P, S, and so on) strategy is able to drive the transformation from O* to OOH* downhill at relatively lower potential, thus greatly leading to the reduction of the overpotential for OER. For instances, N-doped three-dimensional (3D) graphene nanoribbon networks exhited superb OER activity with overpotential of 360 mV at current density of 10 mA cm-2, and Tafel slope of 47 mV dec-128. Taking into account the electron accepting ability of pyridinic N, the adjacent C atom will be positively polarized, resulting in the p-type doping effect for graphene. As a result, the OH* and OOH* intermediates will be favorablely adsorped on the positively charged C atoms (δ+), which are regared as the rate-determing steps for OER in alkaline solutions. Similarly, plasma-etched carbon cloth (P-CC) contains O-doped graphene, where the charge redistribution on carbon atoms occured due to the higher electronegegativity of oxygen in comparison to the surrounding carbon atoms29. P-CC reached the current density of 10 mA cm-2 at 450 mV, much lower than that of CC, consistent with the experimental results of surface-oxidized CNT30. Besides the intrinsic activities dominated by heteroatoms, the defects in graphene after removal of doping atoms also played an important role in the realization of


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trifunctional properties31. In brief, the presence of doped atoms and defects will synergistically induce the charge redistrubution, and thus boosting the OER performance. However, serching for facile method for obtaining both features in graphene is still a great challenge. Graphene can be halogenated via introducing covalent C-F, C-Cl, or C-Br bonds, resulting in the possibility to open the bandgap and modify the surface chemistry. Graphene can be chlorinated under UV irradiation in Cl2 thorough Cl radicals mechanism, showing bandgap of 45 meV32-33. Liu et. al. further proved that p-type doping was caused by the electron transfer from carbon to Cl, and lots of defects can be created after UV irradiation34. It is predicted the p-type doping in chlorinated graphene contained lots of positively charged carbon atoms for the favorable adsorption of OOH*, which is the rate-determing step for OER process28, 35. Meanwhile, the defects will favor the exposure of active sites and faciliate the mass transport31. However, on the basis of previous work, Cl2 photochlorination method always led to very low carrier mobility, namely the resistance increased greatly, which is detrimental to the electron transfer during electrochemical process33-34,

36.

We also found similar results using Cl2

photochlorination method36. Therefore, it is urgent to develop an ecofriendly strategy instead of using the highly toxic Cl2, and yet effective way to simutaneously maintain the intrinsic mobility and defects in graphene. Herein, metal chlorides can assist the chlorination of graphene under UV irradiation. The generated metal oxides can be easily removed by acid washing. The chlorination degree of graphene is not only dependent on different metal chlorides, but also relying on the irradiation


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time. The structural defects were systematically studied by Raman spectra. Field effect transistors of pristine graphene and chlorinated graphene preapred by different metal chlorides were fabricated to reveal the mobility changes. Vertically-oriented graphene were further grown on carbon cloth (denoted as CC@VG), and chlorinated using FeCl3-assisted method. OER for chlorinated CC@VG were studied in detail, and possible mechanism was proposed for the improved OER performance. Experimental Materials Highly oriented pyrolyzed graphite (HOPG) was purchased from Sigma-Aldrich. FeCl3, CuCl2, and NiCl2 were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical grade and used as received without any further purification. Distilled water (18.2 MΩ) is prepared from Millipore water system and used for all experiments. Toray carbon cloth was used for the growth of vertically-oriented graphene according to previous work37. The preparation of Cl-doped graphene Graphene nanosheets were exfoliated from HOPG via scotch tape transfer method. The scotch tape loading with graphene nanosheets was glued on the surface of SiO2(300 nm)/Si substrate at room temperature. After removing the scotch tape, graphene nanosheets were left on the surface of Si substrate. The substrate loading with graphene was annealed to remove all organic residues at 500 oC for 1 h in Ar. 10 μL FeCl3 (0.5 M) solution was spin-coated on a Si


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substrate at 1000 rpm for 60 s, then stored at 60 oC in vacuum for 12 h for complete dryness. The substrate coated with FeCl3 was irradiated in a distance of 10 cm by UV light (500 W Hg lamp) for various time to drive the photodecomposition of FeCl3. Other metal chlorides were deposited on graphene using similar method, and irradiated in the same irradiation set-up. After irradiation experiments, the substrates were immersed in 20 mL H2SO4 (3 M) for 24 h, and thoroughly washed by distilled water to remove the dissolved compounds. The asprepared substrates were stored at 60 oC in vacuum for further measurements. CC@VG films were chlorinated according to the similar procedure. Characterizations Graphene morphologies were characterized on FEI Quanta 200 scanning electronic microscope. Graphene nanosheets and interlayer distance were obtained on field emission TEM (TecnaiG2F30, FEI, US). Metal chlorides and metal oxides were confirmed by X-Ray diffraction (XRD) on DIFFRACTOMETER-6000 with Cu Kα radiation (λ = 0.1542 nm). Defects were studied by Raman spectra on a confocal microscope-based Raman spectrometer (LabRAM XploRA, incident power of 1 mW, pumping wavelength of 532 nm). The thickness of graphene and chlorinated graphene was confirmed by Atomic Force Microscope (AFM, Nanoscope IIIa Vecco). Compositions in graphene and chlorinated graphene were confirmed by X–ray photoelectron spectroscopy (XPS, Thermo Scientific K–Alpha XPS, using Al (Kα) radiation as a probe). Field effect transistors (FET) were fabricated using Cr/Au electrodes, which were built by Coating machine (ZHD-300). FET devices with bottom gated


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configuration were measured on The Lakeshore TTP-4 probe station with Keithley 4200 semiconductor analyzers, and all measurements were performed at room temperature in air. FET measurements FET was bottom gated configuration. Exfoliated graphene from HOPG and corresponding chlorinated graphene were used as channel material. The current and voltage were applied and recorded on Keithley 4200 semiconductor tester. All operations were carried out in air at room temperature without any vacuum or low temperature conditions. After depositing Cr/Au electrodes, the devices were directly measured to avoid the contamination from dust. Mobility was calculated according to the following equation： ∆𝐼𝑑𝑠 𝐿 ∆𝑉𝑔𝑠·𝑊

𝜇 = 𝑉𝑑𝑠·𝐶𝑜𝑥 （1） Where Ids is the current between source and drain electrodes, Vds was the bias between source and drain electrodes, Vgs is the gate voltage between source and gate electrodes, L represents the length of graphene channel (μm), W was the width of graphene channel (μm), Cox was the specific capacitance of dielectric layer SiO2 (F/cm2). Cox is 1.15×10–8 F/cm2 for 300 nm SiO2 dielectric layer. OER measurements Electrochemical measurements were carried out on a CHI760E electrochemical workstation (CH Instruments, Inc., Shanghai). The OER performance was evaluated on a three-electrode


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cell. CC@VG or chlorinated CC@VG as working electrodes were protected by epoxy for accurate determination of the area dipped in solution, Hg/HgO (1.0 M KOH) and graphite rod were used as the reference electrode and counter electrode, respectively. All potentials are converted by the following equation: 𝐸𝑅𝐻𝐸 = 𝐸𝐻𝑔 𝐻𝑔𝑂 +0.927 𝑉 (2), which has been calibrated in Figure S1 using the method in our previous work37. OER was measured in an O2-saturated KOH solution (1.0

M).

Polarization curves were

performed at a scan rate of 5 mV s-1 in the range of 0.2~0.5 V vs. RHE at 20 oC. Tafel slopes were obtained from the LSV curves according to the Tafel equation 𝜂 = 𝑏 𝑙𝑜𝑔 (𝑗 𝑗0) (3), where

b represents the Tafel slope, η corresponds to the overpotential, j and j0 are the current density and the exchange current density, respectively. Overpotential was deduced from equation 𝜂 = 𝐸𝑅𝐻𝐸 ―1.23 𝑉 (4). Electrochemical impedance spectroscopy (EIS) of electrocatalysts was carried out in the range from 0.1 Hz to 106 Hz by using a Solartron 1260A Impedance/Gainphase Analyzer (Ametek, UK). The electrochemical double-layer capacitances (EDLCs, Cdl) was obtained from CV plots, which were recorded under various scan rates from 10 to 150 mV s-1 in a non-faradaic voltage range. Cdl can be derived from the plots of the half values of the difference between cathodic and anodic current densities vs. scan rates. Long-term stability was conducted by continuous CV scan and chloroamperometric scan at current density of 10 mA cm-2 for OER, respectively. Faradaic efficiency measurements


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Rotating ring-disk electrode (RRDE) contains a glassy carbon disk electrode (0.2475 cm2) and a Pt ring electrode (0.1866 cm2). In order to obtain the Faradaic efficiency of CC@VG12h catalyst, the film was cut into small pieces and ground into fine powders. 5 mg powders were dispersed into 1 mL isopropanol/H2O (350 μL/650 μL), and treated by ultrasonication for 1 h. 20 μL ink were drop-casted on disk electrode of RRDE, and dried at 50 oC for 1h. For the detection of H2O2 intermediates, the ring potential was set as 1.5 V vs. RHE in O2-saturated 1 M KOH solution. The disk current was recorded by polarization curve at a scan rate of 5 mV s-1 under 1600 rpm, and the ring current was simultaneously recorded. For the determination of Faradaic efficiency of CC@VG12h catalyst, a continuous OER and ORR were performed on RRDE, of which disk electrode was coated with CC@VG12h. Such experiments were carried out on a RRDE at 1600 rpm with a constant disk current of 300 μA and a ring potential of 0.2 V vs. RHE in N2-saturated 1

M

KOH solution, which is also blanketed by N2 during

measurement. Then the ring current was recorded for 100 s, and corresponding Faradaic efficiency can be calculated from the equation: ε = 𝐼𝑟 𝐼𝑑𝑁 (5), where Ir is the ring current, Id is the disk current, and N is the collection efficiency of RRDE (N is 0.37 in this study.) In order to further confirm the Faradaic efficiency, water-gas displacing method was utilized to record the gas volume to calculate the mole of O2. The theoretical amount of O2 was obtained according to Faraday law. Results and discussion Synthesis and characterizations


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Graphene nanosheets were exfoliated using scotch tape, and transferred on the surface of SiO2(300 nm)/Si substrate in Figure 1a. FeCl3 (0.5 M) was spin-coated on graphene surface at 1000 rpm, and dried at 60 oC in vacuum. The film was irradiated by UV light (500 W Hg lamp) for 12 h to trigger the photodecomposition of FeCl3 and corresponding chlorination of graphene (denoted as CC@VG12h). As seen by AFM image in Figure 1b, the thickness of exfoliated graphene nanosheets was about 0.6 nm. SEM image in Figure 1c showed the film layer after UV irradiation for 12 h. After thorough removal of the top layer, one can see in inset in Figure 1c, the irregular graphene nanosheets were clean. AFM image in Figure 1d showed that the thickness was 1.1 nm, the increased thickness might be due to the coverage of Cl dopants. The presence of Cl was confirmed by XPS of UV irradiated graphene in Figure 1e, and the peaks corresponding to Cl in as-prepared graphene were not observed. Furthermore, fine deconvolution of Cl 2p XPS spectra for chlorinated graphene in Figure 1f showed two subpeaks at binding energy of 200.7 eV and 202.2 eV, which corresponded to covalent C-Cl bonds. In contrast, Cl 2p peaks in as-prepared graphene are absent. We also fitted the C 1s XPS spectra in Figure 1g, and found that C-C bond appeared at binding energy of 284.8 eV for the pristine graphene and Cl-doped graphene. It is also observed that a new peak for chlorinated graphene appeared at 286.6 eV in comparison to pristine graphene, which can be ascribed to C-Cl bond. Basically, the Cl atomic content in graphene was about 4.3 at.% after UV irradiation for 12 h. In brief, photodecomposition of FeCl3 was an effective way for the chlorination of graphene, and one can obtain moderate Cl dopants in graphene with less destruction toward graphene. As seen in the fine XPS spectra, no O-C=O bonds were observed


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in C 1s spectra. In the previously developed Cl2 photochlorination method, Cl radicals generated under UV irradiation were responsible for the covalent modification of Cl on carbon skeleton, and might lead to structural damage due to its highly reactive nature.

Figure 1. UV-triggered photodecomposition of FeCl3-assisted method for chlorination of graphene. (a) Scheme of chlorination process via spin-coating metal chloride solutions, photoirradiation, and acid-washing steps. (b) AFM height image and height graph of exfoliated graphene. (c) SEM image of Fe2O3 oxides layer covered on the surface of graphene after photodecomposition of FeCl3. (d) AFM height image and height graph of chlorinated graphene after removal of Fe2O3 by 3

M

H2SO4. (e) XPS full, (f) Fine Cl 2p and (g) C 1s XPS spectra of

pristine graphene and chlorinated graphene.


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Figure 2. Raman analysis of graphene as a function of irradiation time. (a) Raman spectra of graphene, which was irradiated by UV light for 0, 4, 8, 12, 16 h, respectively. Chlorination experiment was performed in the presence of FeCl3, which was completely removed before Raman measurements. (b) Corresponding ID/IG and I2D/IG ratios vs. irradiation time. Control experiments of graphene after (c) exposure to UV light in the absence of FeCl3, and (d) without UV irradiation in dark for 16 h. Inspired from previous work, chlorination would inevitably result in the presence of structural defects, which can be monitored by Raman. As shown in Figure 2a, Raman spectra for graphene chlorinated at various time from 0 to 16 h were systematically recorded. The ID/IG and I2D/IG ratios vs. irradiation time were plotted in Figure 2b, displaying that ID/IG greatly increased from 0.9 to 3 after 12 h irradiation, while I2D/IG decreased from 2.9 to about 1. Such results are consistent with previous work using Cl2 photochlorination method, showing the


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structural defect due to the introduction of covalent C-Cl bonds. Two control groups are performed to verify the significance of both UV light and FeCl3. The effect of FeCl3 was evaluated in Figure 2c, and it is seen that the Raman spectrum for graphene without coating FeCl3 was almost unchanged in comparison to the fresh graphene after 16 h UV irradiation. Meanwhile, graphene was coated with FeCl3, there are no observable Raman changes after storing the sample in dark for 16 h. Taken together, FeCl3 and UV light are two necessary conditions for the chlorination after enough irradiation time. Are there any other metal chlorides useful for the chlorination of graphene? We next to evaluate two common metal chlorides in Labs, such as NiCl2 and CuCl2. Under similar experimental conditions, CuCl2 solution was coated on the surface of exfoliated graphene and irradiated for various time. Raman spectra in Figure S2a demonstrated that D peak intensity gradually increased as a functional of irradiation time. Similar to FeCl3 case, the ID/IG and I2D/IG ratios vs. irradiation time were plotted in Figure S2b. I2D/IG ratios showed similar change after 16 h irradiation, however the ID/IG increased to around 1, much lower than that of chlorinated graphene by FeCl3. This results might be related to the relatively low Cl dopants in graphene in the presence of CuCl2. The introduction of Cl was further confirmed by a control experiment in Figure S2c, also indicating that the importance of UV irradiation for the photodecomposition of CuCl2. In the case of NiCl2, negative results were obtained in Figure S3. Clearly, D peak was silent for all irradiation time. And the ID/IG and I2D/IG ratios were almost unchanged along with the irradiation time. Therefore, one can see that FeCl3 showed better chlorination ability toward graphene under UV irradiation than the other two metal chlorides, CuCl2 and NiCl2. ACS Paragon Plus Environment

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FET measurements and mobility modulation

Figure 3. Field-effect transistor (FET) measurements. (a) FET device configuration. (b) Optical image of chlorinated graphene covered by Au/Cr electrodes. (c) Transfer and (d) output curves of as-exfoliated pristine graphene. (e) Transfer and (f) output curves of chlorinated graphene for 12 h. Covalent modification of Cl will affect the electronic properties of graphene. FET devices were fabricated to compare the electronic behavior before and after introduction of Cl using present chlorination method. As shown in Figure 3a, a typical bottom-gated FET device contains 300 nm SiO2 as dielectric layer on Si substrate, and source/drain electrodes were Cr/Au (5 nm/45 nm), which were deposited by shadow mask. The exfoliated graphene and chlorinated graphene were used as channel material, as seen in Figure 3b. From transfer curve of graphene in Figure 3c, the mobility for the pristine graphene can be calculated to be 1473.6 cm2 V−1 s−1 using equation (1) at room temperature in air. In sharp contrast, the mobility for


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chlorinated graphene prepared by FeCl3 after 12 h UV irradiation, was found to be 345.5 cm2 V−1 s−1, much lower than that of pristine graphene. The introduction of Cl will compromise the π electron structure and induce local charge distribution, thus causing p-doping effect for graphene32, 34. As seen by these two transfer curves, the Dirac point moved from 20 V to 33 V, showing significant p-type semiconductor behavior after introducing Cl-dopants38. Output curves are illustrated in Figure 3d and 3f, and the linear plots indicated that ohmic contact existed between channel materials and electrodes. On the basis of previous Raman results, it is assumed that FeCl3 could result in heavy Cl-doping, CuCl2 led to medium Cl-doping, whereas NiCl2 was unable to chlorinate graphene under UV irradiation. FET devices were further constructed for graphene treated by CuCl2 and NiCl2 in Figure S4, respectively. The mobility for chlorinated graphene by CuCl2 was 473.6 cm2 V−1 s−1, slightly larger than that in the case of FeCl3. However, the mobility of chlorinated graphene by NiCl2 was as high as 1254.9 cm2 V−1 s−1, very close to pristine graphene. It is also found that the Dirac point for graphene treated by NiCl2 was unchanged in comparison to that of pristine. The above results indicated that NiCl2 treatment failed to chlorinate graphene, and thus resulting in almost unchanged electronic properties. Taken together, the mobility of chlorinated graphene by FeCl3 was much higher than that prepared by previous Cl2 photochlorination method36. This feature is appealing for application in boosting the electrochemical reactions. Basically, heteroatom-doping is effective to change the charge density and spin ordering of active sites, thus leading to the optimization of the key intermediates for ideal absorption energy. In addition, the high mobility of electrocatalysts, namely the excellent conductivity, plays


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important role to fasten the electrochemical kinetics. Note that too high Cl-doping content in graphene might significantly lower the intrinsic carrier mobility, thus resulting in lower electrolytic kinetics39.

Figure 4. Characterizations of chlorinated CC@VG. (a) Low-magnification SEM image of CC@VG12h. (b and c) High-magnification SEM image of CC@VG12h as indicated by square in (a). (d) Low-resolution TEM image of VG12h nanosheets, which was taken off from the CC by ultrasonication. Inset is the high-resolution TEM image. (e) XRD patterns of CC@VG (black curve), CC@VG coated with FeCl3 (red curve), CC@VG covered with Fe2O3 film (blue curve, film was irradiated for 12 h), respectively. OER measurements


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In order to realize the application in electrolysis, we further fabricated chlorinated vertically-oriented graphene on carbon cloth (CC@VG). VG was successfully synthesized by PECVD method in large scale on commercial carbon cloth40-41. Using optimal chlorination conditions, FeCl3 UV photodecomposition method was selected to chlorinate VG due to the simultaneous realization of high mobility and Cl contents. As seen in Figure 4a, SEM image displayed that chlorinated VG nanosheets were evenly distributed on the surface of CC. Highmagnification SEM images in Figure 4b and 4c demonstrated the graphene nanosheets were closely interconnected. Low-resolution TEM image in Figure 4d indicated the sheet-like nanostructure, and inset is the high-resolution TEM image, showing that interlayer distance of 0.34 nm for carbon layer. The curved carbon nanosheet are not well ordered, making distortion of hexagonal carbon unit, thus causing local polarization of carbon. Raman spectra in Figure S5 also indicated the defective structure in chlorinated VG by the increased ID/IG. The distorted carbon layer and structural defects have been proven to be favorable for boosting the electrochemical performance. As is known, metal chlorides might decompose under UV irradiation to form metal oxides42. XRD patterns in Figure 4e solidly confirmed the gradually disappearance of the characteristic peaks for FeCl3 (4 h and 8 h shown in Figure S6), while the newly generated peaks after 12 h can be assigned to Fe2O3 (PDF#89-0599).


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Figure 5. OER measurements of CC@VGT. (a) Polarization curves in O2-saturated 1 M KOH. (b) Corresponding Tafel plots. (c) Overpotential at 10 mA cm-2 and 50 mA cm-2 vs. chlorination time, as well as Tafel slopes vs. chlorination time. (d) Δj vs. scan rates from 10 to 150 mV s-1. (e) Nyquist plots of CC@VGT. (f) Corresponding Rct vs. chlorination time and Cdl vs. chlorination time. Taken together, chlorinated VG contains a certain amount of Cl dopants, distorted carbon layers, structural defects, and direct growth manner on CC. Those factors will endow chlorinated CC@VG some advantages, including exposed active sites, charge dislocation, fast electron/ion transport path, and robust mechanical nanostructure, which are highly demanded for electrochemical reactions43-45. We next to evaluate the OER activities for chlorinated


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graphene. As shown in Figure 5a, polarization curves are measured as a function of UV irradiation time, which are iR-corrected (95%). And corresponding Tafel slopes were obtained from the plots of E vs. Log[j (mA cm-2)] in Figure 5b. The overpotential at 10 and 50 mA cm-2, as well as Tafel slopes were summarized in Figure 5c, respectively. CC@VG8h, CC@VG12h, and CC@VG16h showed similar overpotential at 10 mA cm-2, which are 402, 399, and 405 mV, respectively, lower than that of pristine CC@VG (469 mV). Interestingly, CC@VG12h showed the smallest overpotential of 438 mV at current density of 50 mA cm-2 among all chlorinated samples. Meanwhile, the Tafel slope for CC@VG12h was as low as 59 mV dec-1, comparable or even lower than some published carbonaceous OER electrocatalysts in Table S146-49. The CV curves in Figure S7 for CC@VGT are recorded in non-faradaic range to calculate the electric double layer capacitance (Cdl). From the Δj vs. scan rates plots in Figure 5d, Cdl can be calculated from the slopes. Basically, the electrochemical active surface area (ECSA) is proportional to Cdl according to equation 𝐸𝐶𝑆𝐴 = 𝐶𝑑𝑙 𝐶𝑠 (6). The specific capacitance value of

Cs is in the range of 0.01-0.06 mF cm-2 for a smooth surface electrode, and can be regarded as a constant value in present case50. Meanwhile, the Nyquist plots for all electrocatalysts were measured in Figure 5e. The Cdl and charge transfer resistance (Rct) are summarized in Figure 5f. It is seen that the Cdl values gradually increased from 0.63 mF cm-2 for CC@VG0h to 1.87 mF cm-2 for CC@VG12h, then slightly decreased to 1.76 mF cm-2 for CC@VG16h. The largest Cdl value indicated that CC@VG12h possessed high ECSA, namely more exposed active sites for OER. In addition, Rct for CC@VG12h was 32 Ω, lower than chlorinated CC@VG at other


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irradiation time. Small Rct for CC@VG12h is favorable for fast electron transfer during electrochemical kinetics for OER. In order to further prove the significance of Cl dopants for the OER performance, the polarization curves of CC, pristine CC@VG, annealed CC@VG12h, and commercial Ir/C were carried out in Figure 6a. CC was almost inert to the OER. After growth of VG, the overpotential was 466 mV to reach the current density of 10 mA cm-2. Cl dopants will induce the charge dislocation and spin ordering on graphene surface, thus active sites will be created for the adsorption of the key intermediates, such as OOH*, OH*, O2*. F, Cl, or Br modified graphene was unstable above 500 oC, which could be completely eliminated. CC@VG12h was annealed at 700 oC to remove Cl elements, and showing overpotential of 461mV at current density of 10 mA cm-2, and 492 mV at current density of 50 mA cm-2. However, the current density was only 11.8 mA cm-2 at 492 mV for CC@VG. This results indicated two things, the first one is that the presence of Cl played an important role to boost the OER performance in CC@VG12h. The second one is that there might exist polarized sites at defects in annealed CC@VG12h, and those defective structure might contain active sites for the adsorption of the key oxygencontaining intermediates. The overpotential of commercial Ir/C was 359 to reach current density of 10 mA cm-2, better than that of CC@VG12h (399 mV). However, Ir/C needed overpotential larger than 500 mV to reach the current density of 50 mA cm-2, much higher than that of CC@VG12h (438 at 50 mA cm-2). Tafel slopes in Figure 6b indicated the similar trend for control groups. The Tafel slope of annealed CC@VG was 106 mV dec-1, lower than


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that of pristine CC@VG (127 mV dec-1), but larger than that of CC@VG12h (59 mV dec-1). The Tafel slope for Ir/C was 95 mV dec-1, much larger than that of CC@VG12h. Most importantly, CC@VG12h possessed satisfied long-term stability after 1000th continuous CV scans in Figure 6c, the overpotential only increased 7 mV. Meanwhile, chronoamperometric curves in Figure 6d indicated that the current density retention was 93.1%, higher than that of commercial Ir/C (81.0%). Basically, powdered Ir/C was deposited on the surface of current collector in the presence of Nafion, which will somehow block the active sites. Another side-effect is that Ir nanoparticles will be aggregated and peeled off from the substrate into the solutions to lose the activity. Taken together, UV irradiation for CC@VG was useful to introduce covalent C-Cl bonds in graphene, meanwhile, structural defects might be created. These two aspects are of great importance for OER activity. As confirmed in Figure 6e, the CC@VG12h showed the largest Cdl of 1.87 mF cm-2 in comparison to the control groups. After removal of Cl dopants, the Cdl decreased to 0.61 mF cm-2, but still larger than that of pristine CC@VG (0.50 mF cm-2), indicating the existence of defects in annealed sample. Nyquist plots in Figure 6f displayed that Rct for annealed CC@VG12h was 46 Ω, larger than that of CC@VG12h (32 Ω), but smaller than that of pristine CC@VG (51 Ω). Of note, the presence of Cl was of profound importance to improve the ECSA and charge transfer rate, thus finally boosted the OER performance.


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Figure 6. OER measurements for control groups. (a) Polarization curves of CC, CC@VG, CC@VG12h, annealed CC@VG12h, and Ir/C, respectively. (b) Corresponding Tafel slopes. (c) Polarization curves of CC@VG12h at the first and 1000th cycles, respectively. (d) Chloroamperometric curves of CC@VG12h and Ir/C, respectively. (e) Δj vs. scan rates from 10 to 150 mV s-1. (f) Corresponding Nyquist plots. To further understand the OER process catalyzed by CC@VG12h, the by-product of peroxide intermediate has been analyzed by RRDE in Figure 7a51-53. When the ring potential was set as 1.5 V, it is seen that the ring current was below 9 μA, much lower than that of disk current, displaying the negligible H2O2 formation. It is also observed the gradual decrease of ring


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current when increasing potential toward vast O2 generation range, indicating that the formation of hydrogen peroxide intermediates can be neglected at high potential. The fast increase of current after 1.6 V can be ascribed to the formation of O2 via anodic four-electron transfer reaction of 4𝑂𝐻 ― →𝑂2 +2𝐻2𝑂 + 4𝑒 ― . The Faradaic efficiency can be obtained by recording the ring current when set the ring potential at 0.2 V vs. RHE with disk current of 300 μA. Basically, the O2 generated on disk electrode via continuous OER process will diffuse across the ring electrode, where ORR will effectively occur due to the superior activity of Pt at 0.2 V vs. RHE. As seen in Figure 7b, the ring current is zero when fixing the disk current at 0 μA, whereas the ring current was as high as 106 μA when the disk current was fixed at 300 μA. According to equation (5), the Faradaic efficiency was about 95.6%, indicating that the anodic oxidation current was ascribed to the OER process. Meanwhile, the Faradaic efficiency was also evaluated by water-gas displacing method in Figure S8, showing almost 98.7% Faradaic efficiency, consistent with the RRDE results.

Figure 7. Faradaic efficiency measurements. (a) Disk and ring current of CC@VG12h catalyst on a RRDE at 1600 rpm. The ring potential was set as 1.5 V in O2-saturated 1.0 M KOH solution.


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(b) Ring current of CC@VG12h catalyst on a RRDE at 1600 rpm with a ring potential of 0.2 V in N2-saturated 1.0 M KOH solution, which is also blanketed by N2 during measurement. The disk current was maintained at 300 μA. Mechanism analysis Of note, heteroatoms-doping method for graphene has been proven as an effective method to obtain promising electrochemical performance. Very recently, AgCl has been used to chlorinate graphene under UV irradiation for local chlorination42. In present chlorinated CC@VG, one can see that the UV irradiation is able to drive the decomposition of metal chlorides. There is also another possibility that Cl radicals might be existed during the irradiation by high energy UV light. Meanwhile, despite water cooling system is used during the irradiation process, the surface temperature of substrate might be also elevated to accelerate the decomposition of metal chlorides into metal oxides. Of note, ozone will be generated during UV irradiation, which will further assist the transformation of metal chloride into metal oxides. It is also found that FeCl3, CuCl2, and NiCl2 showed different chlorination ability toward graphene. To our knowledge, ferric chloride is one of the most acidic salts in aqueous solution, which is mainly caused by the strong hydrolysis of Fe3+ according to the following equation: Fe𝐶𝑙3 +3𝐻2𝑂 ⇌𝐹𝑒(𝑂𝐻)3 +3𝐻𝐶𝑙 (7)54-55. Alternatively, hydrated FeCl3 will decompose due to the presence of thermal effect under UV irradiation as mentioned above. As a result, HCl is generated on the basis of the following two equations: Fe𝐶𝑙3 ∙ 6𝐻2𝑂

∆


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∆

𝐹𝑒(𝑂𝐻)𝐶𝑙2 +3𝐻𝐶𝑙 + 5𝐻2𝑂 (8) and Fe𝐶𝑙3 +3𝐻2𝑂 𝐹𝑒(𝑂𝐻)3 +3𝐻𝐶𝑙 (9). Subsequently, the ∆

ferric hydroxides will decompose via the following equation: 2𝐹𝑒(𝑂𝐻)3 𝐹𝑒2𝑂3 +3𝐻2𝑂 (10). Inspired from previous work, HCl could serve as Cl-doping source for graphene56-58. Specifically, chlorination of graphene by HCl will proceed smoothly in the presence of high energy of UV light irradiation for enough time. In contrast, hydrated NiCl2 or CuCl2 possesses worse HCl generation performance due to its relatively weaker hydrolysis ability in comparison to hydrated FeCl3. According to above mentioned characterizations, the introduction of Cl using our method showed much higher mobility in comparison to previously developed Cl2 photochlorination method. For instance, the mobility of chlorinated graphene prepared by Liu et. al was only 1 cm2 V−1 s−1. As is known, about 8 at.% Cl atoms are covalently bonded to carbon, leading to the severe destruction of conjugated carbon skeleton32. As a result, the mobility will be decreased in Cl2 photochlorination systems. In present chlorinated graphene, the mobility is finely preserved by introducing relatively lower Cl contents. For electrochemical application, the presence of proper amount of Cl elements and high conductivity are both important for the creation of active sites and charge transfer rate. Fortunately, our method could balance such competition. Chlorinated graphene prepared in the presence of FeCl3 not only contains 4.3 at.% Cl contents, but also exhibited high mobility of 345.54 cm2 V−1 s−1. The presence of Cl will induce the localized charge distribution and change the spin density in graphene, favoring the adsorption of the key intermediates including OOH*, OH*, O2* on the active sites. Theoretical studies have predicted that the rate


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determining step is related to the transformation from O* to OOH*. Importantly, the presence of Cl also facilitate the interaction of H-carrying key intermediates, such as OH* and OOH*, on the surface of chlorinated graphene via the H-Cl hydrogen bonding effect59-60. Meanwhile, the VG nanosheets were directly grown on the surface of CC, endowing the free-standing film robust mechanics. We further measured the TEM images after long-term OER stability test in Figure 8a, it is shown that the irregular nanosheets with interlayer distance of 0.34 nm were well maintained. Meanwhile, XPS full spectra in Figure 8b indicated that the presence of Cl after long-term OER test.

Figure 8. Post-characterizations of CC@VG12h. (a) Low-resolution TEM image of CC@VG12h after long-term OER test. Inset is the interlayer distance of stacked graphene nanosheets. (b) XPS full spectra of CC@VG, the as-prepared and after OER test CC@VG12h. Conclusions In summary, graphene can be chlorinated via the photodecomposition of metal chlorides under UV light irradiation. In sharp contrast to previous Cl2 photochlorination method,


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present work used widely available metal chlorides to assist the chlorination instead of using highly toxic Cl2 gas. The most important feature for present chlorination method is that one can simultaneously obtain appropriate amounts of Cl content and high carrier mobility of graphene. As a result, the presence of covalent C-Cl bonds induced charge dislocation and spin density change, thus facilitated the adsorption of key OER intermediates. In virtue of the ClH hydrogen bonds, H-carrying OER intermediates including OH* and OOH* will be effectively anchored on chlorinated graphene. In addition, the structural defects resulted in large electrochemically active surface area in chlorinated graphene, which is confirmed by Cdl, and in turn leading to the high exposure of active sites and fast mass transport rate for OER process. Overall, present work developed a mild and efficient method for the chlorination of graphene taking into account both the introduction of Cl content and the high carrier mobility. Of note, other metal bromides and metal iodides might be useful for preparing Br or I doped carbonaceous nanomaterials, and providing novel candidates for optoelectronic device, nanocatalysis, and electrolysis fields. ASSOCIATED CONTENT Supporting Information. Raman analysis of graphene prepared by the assistance of CuCl2 and NiCl2; Field-effect transistor (FET) measurements for control groups; Raman Spectra of CC@VG irradiated by UV light for 0 and 12 h in the presence of FeCl3; XRD patterns of UV irradiated CC@VG for 4 and


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8 h, respectively; CV curves of all electrocatalysts in non-faradaic potential range; Comparison table with previous electrocatalysts; water-gas displacing method for O2 evolution. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Qiu Y.). Phone: +86(0)451 86403583, Fax: +0451 86403583. ORCID Yunfeng Qiu: 0000-0002-0163-4908 Author Contributions Y. Qiu conceived the experiments. Y. Ji, S. Wang, and E. Zhang designed and conducted the preparation, characterizations, FET and electrochemical experiments. Y. Zhang synthesized vertically-oriented graphene on carbon cloth. Y. Qiu and Z. Ma wrote the manuscript. All the authors reviewed and approved the manuscript. Notes The authors declare no competing financial interest ACKNOWLEDGMENT We gratefully acknowledge the financial support of from Beijing National Laboratory for Molecular Sciences (BNLMS) (No. BNLMS20160119), Key Laboratory of Micro-systems and


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Table of content The photodecomposition of metal chlorides is a facile and ecofriendly strategy to chlorinate graphene which exhibits promising OER activity.


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Graphical Abstract 84x36mm (300 x 300 DPI)



Figure 1 165x119mm (300 x 300 DPI)


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Figure 2 165x125mm (300 x 300 DPI)



Figure 3 164x87mm (300 x 300 DPI)


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Figure 4 165x154mm (300 x 300 DPI)



Figure 5 165x169mm (300 x 300 DPI)


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Figure 6 165x189mm (300 x 300 DPI)



Figure 7 165x66mm (300 x 300 DPI)


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Figure 8 165x89mm (300 x 300 DPI)


Chlorinated Graphene via the Photodecomposition of Metal Chlorides

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