Accepted Manuscript

Self-assembled nitrogen-doped graphene quantum dots (N-GQDs) over graphene sheets for superb electro-photocatalyticactivity

Rabia Riaz, Mumtaz Ali, Iftikhar Ali Sahito, Alvira Ayoub Arbab,T. Maiyalagan, Aima Sameen Anjum, Min Jae Ko, Sung HoonJeong

PII: S0169-4332(19)30585-9DOI: https://doi.org/10.1016/j.apsusc.2019.02.228Reference: APSUSC 41923

To appear in: Applied Surface Science

Received date: 28 December 2018Revised date: 16 February 2019Accepted date: 26 February 2019

Please cite this article as: R. Riaz, M. Ali, I.A. Sahito, et al., Self-assembled nitrogen-doped graphene quantum dots (N-GQDs) over graphene sheets for superb electro-photocatalytic activity, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.02.228

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Self-Assembled Nitrogen-Doped Graphene Quantum Dots (N-GQDs) over

Graphene Sheets for Superb Electro-Photocatalytic Activity.

Rabia Riaza, Mumtaz Alia, Iftikhar Ali Sahitoc, Alvira Ayoub Arbaba,c, T. Maiyalagan d, Aima

Sameen Anjuma, Min Jae Kob*, Sung Hoon Jeonga*

aDepartment of Organic and Nano Engineering, Hanyang University, Seoul 133-791, Republic of Korea.

bDepartment of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea.

cDepartment of Textile Engineering, Mehran University of Engineering and Technology, Jamshoro, 76062, Pakistan.

dElectrochemical Energy Laboratory, Department of Chemistry, SRM Institute of Science and Technology,

Kattankulathur, 603203, India.

*Corresponding authors: shjeong@hanyang.ac.kr and mjko@hanyang.ac.kr

Abstract

Nitrogen-doped graphene quantum Dots (N-GQDs) are emerging electroactive and visible light

active organic photocatalysts, known for their high stability, catalytic activity and

biocompatibility. The edge surfaces of N-GQDs are highly active, however, when N-GQDs

make the film the edges are not fully exposed for catalysis. To avoid this issue, the N-GQDs

are shaped to branched leaf shape, with an extended network of voids, offering highly active

surfaces (edge) exposed for electrocatalytic and photocatalytic activity. The nitrogen doping

causes a decrease in the bandgap of N-GQDs, thus enabling them to be superb visible light

photocatalyst, for degradation of Methylene blue dye from water. Photoluminescence results

confirmed that by a synergistic combination of the highly conductive substrate; Carbon fabric

coated graphene sheets (CF-rGO) the recombination of photogenerated excitons is significantly

suppressed, hence enabling their efficient utilization for catalysis. Comparatively, uniformly

coated N-GQDs showed 49.3 % lower photocatalytic activity, owing to their hidden active sites.

The degradation was further boosted by 30 % by combining the electrocatalytic activity, i.e.

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electro-photocatalysis of the proposed electrode. The proposed electrode material was analyzed

using TEM, FE-SEM, FTIR, AFM, and WA-XRD, whereas the stability of electrode was

confirmed by TGA, tensile test, bending test, and in harsh chemical environments. The

proposed photo-electrocatalyst electrode is binder-free, stable, flexible and highly conductive,

which makes the electrode quite suitable for flexible catalytic devices like flexible solar cells

and wearable supercapacitors.

Key words; Reduced Graphene Oxide, Nitrogen doped Graphene quantum dots, Carbon fabric,

Flexible, electro-photocatalysis

Introduction

The growing industrial revolution has increased the concentration of organic wastes in water

to a life threatening limit [1]. To solve this problem, different mitigation techniques have been

adopted previously such as adsorption [2], aeration [3], coagulation [4], membrane distillation

[5], and photocatalysis [6]. Amongst all these techniques, currently, photocatalysis is a highly

focused research method to degrade the water contaminants using sunlight as an energy source,

without any secondary by-product pollutants [6]. For the purpose, different metal oxides based

materials are used by various researchers [6], however, because of recent environmental

concerns, metals free and highly active composites of carbon-based green catalysts are highly

demanding [7]. Among carbon-based catalysts, graphene and associated composite structures

are highly focused because of their unique electronic properties, flexibility, catalytic activity,

and intrinsically highly functional surface. Apart from many other advantages, graphene oxide

(GO), the two-dimensional wonder material has a highly functional surface, which gives

chance for the different type of semiconductors can be grown or self-assembled on it [8]. In

such a composite structure, the role of graphene sheets is to provide high mobility to excitons

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and suppressing the recombination [9].

Other than mobility, graphene can also be used as photo-active material if its size is further

reduced to quantum dots [10]. Graphene quantum dots are used in junction with different metal

oxides, to utilize the visible light catalysis in the composites. For the synthesis of graphene

quantum dots (GQDs) sheets are cut using oxidizing agents like nitric acid, sulfuric acid, H2O2

for a different time and temperature [10]–[14]. Adding a nitrogen source during this cutting

process enables the synthesis of doped GQDS with nitrogen, which modifies the properties

critically. Compared to other photocatalytic materials, organic quantum dots are highly

preferable for their eco-friendliness, stability, low-cost, and superb dual electrocatalytic and

photocatalytic activity. Such carbon-based catalysts are replacing the other metals based

quantum dots in the field of bioimaging, solar concentrators, energy down-shift layers in Si-

Solar cells, flexible display and UV-pumped LEDs [15]–[20].

Most of the photocatalytic materials are used in powder form, where the photocatalyst material

has high activity because it is in the mobile phase [6]. However, mobile phase photocatalysis

causes secondary pollution, caused by an incomplete recovery of photocatalyst itself [21]. To

avoid this issue, photocatalysts are coated on different substrates, which provides an additional

benefit of using photocatalyst repeatedly, without any secondary pollution [22], [23]. Usually,

for proper adhesion of photocatalytically active materials, binders are used to improve the

adhesion and avoid peeling off. However, the addition of binders hinders the photocatalytic

performance of active materials, as they are adsorbed on the surface of active materials, making

a hampering layer [24]. However, graphene oxide is an exception to it, as it can be easily

adsorbed on the surface of hydrophilic substrates because of its highly functional surface, and

after reduction, it makes binder-free, flexible and stable coating [25].

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When the coated photocatalytic material absorbs the light energy higher than its bandgap, an

electron and a hole pair is generated. This photogenerated electron and hole pair is highly

reactive and it degrades the organic impurities of water, either by oxidation or reduction [8].

N-GQDs also show similar photodegradation, and in addition to it, high electrical conductivity

of CF-rGO makes it equally suitable for free-standing electrode application, because of

outstanding electrocatalytic activity (ECA) [25]. ECA activity of N-GQDs and rGO is

comparable to that of expensive and rare Platinum, owing to highly defect-rich active surface.

The ECA can be further tuned by different reduction techniques, polymer additives, and novel

architectures. Because of superb ECA of N-GQDs, it is used for several electrochemical cells

e.g. supercapacitors, battery electrodes, counter electrodes of solar cells (DSSCs), fuel cells,

and electrochemical degradation of organic pollutants [25]–[30]. In all these applications,

different conductive substrates like Florine doped tin-oxide glass, metallic substrates are used

to hold the active (N-GQDs and rGO) materials.

On the other hand heavyweight, corrosiveness, non-flexible nature of glass and metallic

substrates are limitations for their use in photocatalytic or electrochemical electrodes in the

future. Therefore, textile substrates are preferable, because of their flexibility, abundance, ease

of availability, and their hydrophilic nature make these materials suitable for the coating of

graphene onto them [28]. Previously, different textile substrates e.g. lyocell, silk, cotton etc.

and their modifications by plasma, bovine serum albumin, etc are reported as free standing and

photocatalytically active free-standing electrode materials [22], [30]. Nearly all textile

materials are inherently insulator unless they are finished with certain electroconductive

material. Hence, the conductivity of graphene coated electrode is comparatively low, i.e. the

lowest possible resistivity is reported in our previous works is 40 ohms/sq [30]. Another

problem associated with insulating fibers coated graphene is that an intense reduction is

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required to achieve a high conductivity, which causes the catalytic sites to be reduced [31].

Most importantly the hydral/thermal degradation of conventional textile materials is a limit to

commercial electro-photocatalytic degradation applications of conventional textile materials-

based electrodes [32]. Therefore, carbon fabric with intrinsically high electrical conductivity

and very high strength can be a good choice for the coating of graphene for flexible applications.

As the edges of N-GQDs are highly enriched with functional groups; which play a critical role

in rendering high catalytic activity. Therefore, here we designed a facile method for assembling

N-GQDs on CF-rGO in a way that maximum edge surfaces could be exposed to catalytic

degradation of organic impurities from water. Additionally, the design of substrate; with rGO

coated on woven carbon fabric also plays a crucial role in providing high surface area by

providing mesoporous and nano-roughness. To the best of our knowledge, this is the first report

on graphene coated carbon fabric with leaflets shaped self-assembled N-GQDs as a textile-

based flexible free-standing electrode. Owing to doped nitrogen, the bandgap/absorbance of N-

GQDs was tuned to visible light (450 nm) and higher electrocatalytically active groups are

generated thus boosting the photocatalytic and electrocatalytic activity significantly. In addition

to light, applying biased voltage on the proposed electrode, electrochemical degradation of the

pollutant further boosts the photo-catalytic degradation i.e. electro-photocatalytic activity

(EPCA). Furthermore, crack and the binder-free flexible electro-photo-catalytically active

electrode is highly stable in water, electrolyte, and high temperatures. Other than water

treatment, the proposed strategy can be implemented to boost the performance of fuel cells,

solar cells, supercapacitors, water splitting, and battery devices.

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Experimental

Materials

Graphite with a particle size less than 100 μm was purchased from Asbury Carbons (USA).

Other chemicals used for the synthesis of rGO and N-GQDs were purchased from Sigma

Aldrich, including concentrated sulfuric acid (99 %), Hydrazine monohydrate, Potassium

permanganate powder (KMnO4), Hydrogen peroxide (H2O2; 35 %), and hydrochloric acid (HCl;

35 %), NH4OH. Dialysis tubing for washing of N-GQDs was purchased from Cellu Sep ®,

USA. The chemicals required for the cyclic voltammetry (CV) and photocatalysis, including

Acetonitrile, Iodine, Lithium chlorate, and Lithium iodide and Methylene blue dye were

purchased from Aldrich Co. Carbon fabric was developed by weaving 10 K carbon tows, with

5 ends/cm and 5 picks/cm. De-ionized water and Nitric acid washed glass-ware was used

throughout the experiment.

Synthesis of Graphene oxide sheets;

The graphene was synthesized by using modified Hummer method as given in our previous

report [25]. Briefly, 5 grams of graphite powder was slowly heated up to 400 degrees, to

degrade all organic impurities. Organic impurities free graphite powder was then stirred in

concentrated sulfuric acid in an ice bath until the chunks of graphite are completely broken. In

the ice-bathed dispersion of graphite, 25 grams of KMnO4 was added very slowly, so that

solution temperature is not raised above 10 °C. Afterward, the solution was set on continuous

stirring for 8 hours at 35 °C, where the color is changed to dark brown. To stop the further

oxidation reaction, the temperature was lowered to less than 10 °C, and 500 ml of de-ionized

water was added to the solution, drop-wise. Here care must be taken to avoid the high increase

of temperature if sudden water is added to the acidic solution. Afterward, 5-8 ml of H2O2 was

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added, and the bright yellow dispersion of graphene oxide sheets was obtained. Afterward, this

solution was washed with 10 percent diluted HCl solution, and de-ionized water to remove any

metal contaminations and acids. The solution was centrifuged at 2000 rpm for 2 hours, to

remove the graphite from the graphene oxide dispersion. For ultra-purification, the graphene

oxide solution was further dialyzed for 1 week with a membrane having a molecular weight

cut off 50 kDa. Upon drying this washed solution, graphene oxide sheets powder was obtained,

which was used for N-GQDs synthesis.

Coating of Graphene oxide on carbon fabric and its reduction

Carbon fabric was heated to 400 °C to remove the polymeric coating of the carbon fibers and

was treated for 20 min under UV-ozone treatment to further clean the impurities. The plasma

treated carbon fabric showed a higher hydrophilic nature, which is easily prone to attract higher

GO loading, due to hydrophilic-hydrophilic interactions. Untreated carbon fabric has week

interaction with GO, so during the reduction process, the rGO coating is peeled off.

Graphene oxide powder was dispersed in water to make a stable dispersion of 1.5 weight

percent. This graphene oxide dispersion was then coated on the carbon fabric by dip and dry

method. Typically, the fabric was kept immersed in the GO dispersion for 3 minutes, so that

the fabric can uptake the GO dispersion, by adsorption. Afterward, the soaked fabric was

completely dried in a 70 °C curing oven, for 30 minutes. Because of the highly functional

surface of graphene oxide, a uniform coating can be achieved, with such a facile method. The

process was repeated for 4 or 5 times and a uniform coating of graphene oxide can be observed

on the fabric. For the reduction of graphene oxide coated fabric, the fabric was treated with

vapors of hydrazine monohydrate, at 90 °C (30 minutes). This strong reduction process then

converts the graphene oxide to reduced graphene, along with inducing high conductivity and

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other inherent electrocatalytic properties of reduced graphene.

Synthesis of N-GQDs

N-GQDs were synthesized by a using well known hydrothermal cutting of as-synthesized

graphene oxide (GO) sheets, in presence of H2O2 and NH4OH [33]. Typically, 200 ml of 0.2 %

(W/W %) of GO solution was prepared in de-ionized water, and after addition of 20 ml of H2O2

and 10 ml of NH4OH, the solution was transferred to the Teflon lined stainless steel

hydrothermal autoclave (250 ml capacity). After treating the solution at 200 °C for four hours,

nano-porous carbon was separated from the graphene quantum dots by filtration. Organic

impurities and unwanted chemical residues were separated by using dialysis and rotary

evaporator, to finally get the N-GQDs powder. For the synthesis of undoped-GQDs, a similar

procedure was followed without the addition of NH4OH.

The powder of N-GQDs was dissolved in the ethanol, acetone, and water (5:1:2) mixture, to

make 3 % solution. Afterward, this solution was drop-casted with a concentration of 0.08 ml

per 2 x 2 cm2 and dried at 70 °C to get the self-assembled layer of N-GQDs. The sample with

an overlayer of N-GQDs was kept at 150 °C for the reduction and making the film of N-GQDs

insoluble in water.

Characterizations

The size and morphology of N-GQDs was analyzed with the transmission electron microscopy

(TEM JEOL JEM-2100F) in transmission mode to analyze the detailed morphology an

d internal structure of N-GOQDs. Field emission electron microscope (FE-SEM; JEOL

JSM 6700 F), with an acceleration voltage of 15 kV was used for analyzing the surface

morphology. Before analysis, the samples were adhered on the FE-SEM stage with conductive

carbon tape and were sputtered with platinum for 30 seconds. Carbon tape and sputtering make

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a proper dissipation of electrons, thereby enabling to obtain a clear image during analysis. The

surface morphology and roughness were further characterized by tapping mode Atomic Force

Microscopy (AFM), XE-70, Park Systems. Also, microscale surface morphology of electrode

at different stages was analyzed by using an optical lens microscope. The crystal lattice of

graphene and crystalline regions of carbon fabric were analyzed using Wide-Angle X-ray

Diffraction (WA-XRD). The WA-XRD analysis was performed with an acceleration voltage of

40 kV, in the 2θ range of 10 to 80 degrees at a scanning speed of 2̊/min. The X-ray source used

for testing was Cu Kα (wavelength= 1.5410 Å), manufactured by Rigaku Denki (Rigaku-

D/MAX-2500). Functional groups were analyzed with Fourier Transform Infra-Red (FTIR)

spectrometer (Manufacturer; Thermo Fisher Scientific Inc., Model; NicoletTM iSTM 10) was

used in the ATR mode. The intensities of elements and their bonding was further analyzed by

X-ray Photo-electron Spectroscopy (XPS), using Multilab ESCA 2000 system VG

(manufacturer; Thermo scientific, USA). To examine the dye concentrtion and optical

properties of N-CQDs at different wavelengths, UV-Visible spectroscopy (Shimadzu Co,

Koyoto Japan) was used. Photo-Luminescence-Excitation (PLE) was further characterized to

study the fluorescence behavior and charge recombination behavior of QDs, using fluorescence

spectrophotometer (SCINCO, South Korea).

The conductivity of the electrode was tested by using a standard four-point probe head system

method, using RM3000 resistivity test unit, manufactured by Jandel Engineering, Switzerland.

The tensile strength was tested on the Universal tensile strength tester, manufactured by Tinius

Olsen Inc, USA (Model; H10K-UTM). For sample preparation of tensile test, BS 5081 standard

testing method was followed. Thermal stability was tested by thermo-gravimetric analysis

(TGA) using Thermo-Gravimetric Analyzer (Q600, TA instruments), in a range of 25-500 °C,

with a ramp of 5 °C/minute. Electrocatalytic activity was tested by using three electrode system,

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on Bio-Logic Co. electrochemical workstation, using iodine electrolyte. The electrolyte is

typically a solution of 0.01 M LiClO4, 1 mM iodine, and 10 mM LiI in acetonitrile. For testing

the photocatalytic activity of the electrode was tested by estimating the extent of dye degraded

by the electrode, under visible light exposure.

Results and Discussion

Morphological analysis

To detailed TEM analysis of N-GQDs shows a random oval and circular morphology, with size

varying between 5 nm to 10 nm, as depicted in Figure 1 (a). The non-uniform shape is attributed

to the spontaneous cutting process of graphene oxide sheets in an oxidative environment, under

high pressure. The high-resolution TEM analysis of N-GQDs (inset of Figure 1 (a)) shows the

amorphous edge, because of highly oxidized edges, enriched with oxygenated and nitrogenated

functional groups. This high functionality was further confirmed with XPS analysis, which

shows two major peaks centered at binding energies of 285 eV and 531.5 eV, corresponding to

C1s and O1s, respectively. However, after coating N-GQDs, an additional peak of N1s appears

at 400 eV, confirming that nitrogen is successfully doped in the GQDs (Figure 1 (b)). High

resolution of N 1s can be resolved in two peaks, with a higher proportion of Pyrrolic nitrogen,

and a minor portion of amino nitrogen, as shown in Figure 1 (c). Presence of nitrogen in such

state can be regarded as attachment of amino groups on the edges of the N-GQDs. Graphitic

doping was not observed in this case, as it is mostly observed in quantum dots synthesized by

a bottom-up approach. Another major change observed as compared to GO is that the

concentration of oxygen peak is significantly high after coating N-GQDs, owing to more edges

66and higher functionality of the N-GQDs edges.

For detailed characterization of functional groups, FTIR analysis was conducted from 1000

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cm-1 - 4000 cm-1, as shown in Figure 1(d). GO coated carbon fabric showed a broad peak

extending from 3000 cm-1 to 3750 cm-1 is because of -OH and -CH stretch. The side stretch of

-CH peaks confirms the presence of hydrogen attached to the carbonyl group, i.e. aldehyde

groups. A strong peak at 1725 cm-1 refers to carbonyl stretch, and very next peak centered at

1648 cm-1 is because of -OH bending. These functional groups enable the formation of a

uniform film of GO, without the need of an additional binder. All these peaks of functional

groups were diminished after the reduction process, however, weak OH stretch and bending

peak and epoxide groups peak were still present after reduction. The reduction process was

designed to assure that functional groups were not removed completely, because these

functional groups keep the surface hydrophilic; which intern facilitates the self-assembly of N-

GQDs [34]. As compared to GO, the concentration of oxygenated groups was drastically high

in N-GQDs coated CF-rGO, which is because of oxidation by H2O2. Other than oxygenated

groups, additional nitrogenated functional groups can also be observed owing to doped nitrogen,

as highlighted in Figure 1 (d).

Wide angle x-ray diffraction (WA-XRD) analysis showed three characteristic peaks for both

CF-rGO and CF-rGO-NGQDs, centered at an angle of 25.5°, 44.1°, and 53.6° as shown in

Figure 1 (e). The diffraction peaks centered at 53.4° correspond to graphitic planes formed in

carbon fiber, and a broad peak at 25.4° and 44.1° corresponds to the (0 0 2) and (1 0 0)

diffraction peak of reduced graphene sheets [24, 25]. From the absence of additional peak of

WA-XRD, it can be confirmed that N-GQDs composed of a single layer of highly

functionalized carbon, with amorphous nature. By coating the N-GQDs there is a minor

increase in the intensity in smaller angle region, and additionally, there is a minor red shift in

the first diffraction peak. This shift is because of the higher functionality of N-GQDs; as the

surface groups cause an increase in the lattice parameter of the GO sheets [14]. From all these

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characterization techniques, it can be proved that single layer N-GQDs, with a highly functional

surface, are successfully synthesized.

Figure 1 (a) TEM analysis of N-GQDs, (b) XPS of GO and N-GQDs, (c) High-resolution

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XPS analysis of N 1s of N-GQDs (d) FTIR and (e) WA-XRD analysis of electrode at

different stages of development.

The coating process of carbon fabric was further analyzed at different stages, where optical

microscopic analysis (x40) confirms the bare carbon fabric (Figure 2 (a)) is uniformly covered

with rGO sheets, as shown in Figure 2(b). This uniform coating of GO is not cracked or peeled

off in reduction process, and a clear microscale roughness can be observed on the CF-rGO,

which is attributed to the woven structure of carbon tows and interspacing between carbon

fibers. The fabric structure plays an important role in proper adhesion, i.e. the GO solution is

infused in a porous fabric structure like a matrix, which is held in fabric structure by physical

interactions, even after reduction. Figure 2 (c) shows the electrode after depositing the N-GQDs

layer, which shows no significant difference at the microscale.

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Figure 2 Optical microscopic (x40) images of (a) carbon fabric, (b) rGO coated carbon

fabric, (c) N-GQDs coated CF-rGO, Field emission scanning electron microscopic

images of (d) bare carbon fabric, (e) rGO coated carbon fabric, (f) High resolution

cross-sectional view of CF-rGO (g) High resolution of rGO sheets.

Field emission scanning electron microscopy (FE-SEM) was used to analyze the surface

morphology of the proposed electrode at different stages of development. Figure 2 (d) shows

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the high-resolution image of bare carbon fibers, with a diameter of 7 µm, and uniform circular

morphology. Individual fiber gets coated with rGO sheets, thus fiber level roughness is retained

in the electrode, as observed in top view of the electrode in Figure 2 (e & f). Similarly, it can

also be observed in the cross-sectional view (Figure 2 (f)) that rGO sheets are covering

individual fiber from all sides, whereas higher resolution inset shows the highly crumpled

structure of rGO attached to a fiber. Top view of graphene coated carbon fabric also shows a

similar crumpled surface (Fig 2(g)) because of rGO sheets. This highly rough surface makes

the high surface area exposed for catalytic activity and degradation of pollutants.

Figure 3 Field emission scanning electron microscopic images of CF-rGO-NGQDs at

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different positions (all with x 500 resolution).

High resolution (x 500) FE-SEM images of N-GQDs coated electrode shows the spontaneous

assembly of N-GQDs into leaflets shape, as shown in Figure 3. A similar nano-leaflets were

observed at different positions of the electrode, confirming the homogeneity of the assembling

technique. These nano-leaflets are formed as a result of the compact aggregation of N-GQDs,

and there are spaces left in between the assembled leaflets. The spaces essentially provide an

interface for the interaction of organic impurities and the N-GQDs. The proposed technique is

very facile, and the underlying mechanism is the phase separation of N-GQDs during the

evaporation of different solvents. As the N-GQDs having different solubility in different

solvents so the aggregation behavior can be precisely controlled to achieve the highly porous

and well-organized structure. It is also interesting to observe that the nano-roughness of rGO

sheets do not affect the self-assembly of N-GQDs.

If the solid leaflet structure is analyzed at further higher resolution (higher than x20,000), then

it can be observed that it is composed of the fine ditch and wall type arrangements (Figure 4

(a,b,c)). The width of each wall varies from 90 nm to 450 nm, and the N-GQDs are stacked in

a way that maximum edge surfaces are exposed for the catalytic activity. In Figure 4(c), the

individual sheet type stacked structure can be observed, as highlighted by arrows. While if the

wall thickness is high, it is fully enriched with the cracks (10 -20 nm width) and the ditches

between the walls are as fine as 200 nm – 350 nm (Figure 4 (d)). Such narrow and compact

arrangements facilitate the maximum accommodation of N-GQDs, without overlap or hiding

the active edges. Also, as compared to the self-assembled structure, the simple coating has a

lesser direct area exposed for catalytic activity. The ion diffusion through plain overlapped

structure of 2D materials is slow, which impedes the electrocatalytic performance of the

electrode. By increasing the concentration of the depositing solution, the drains formed

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between the walls are not well defined as shown in Figure 4 (e). The high concentrations start

filling the gap between the N-GQDs walls, so 0.08 ml per 2 x 2 cm2 was the optimum

concentration for achieving best self-assembled structures.

The surface topology of the rGO coating was further characterized by the tapping mode atomic

force microscope (AFM), as observed in 3D AFM plot of Figure 4 (f). AFM showed the surface

roughness of the N-GQDs coating varies between 160 nm - 250 nm. Out of which 130 nm of

the roughness is because of N-GQDs layer, which was also tested by coating the same

concentration solution on Si-Pt substrate (Figure S1), and 80 nm of roughness is imparted by

crumpled rGO sheets. This N-GQDs nano-roughness observed by AFM results is consistent

with the roughness tested by TEM, thus confirming the high catalytic nano-roughened surface

area of the proposed electrode.

Figure 4 Analysis of N-GQDs layer with (a - d) FE-SEM images of self-assembled N-

GQDs at different resolutions (e) FE-SEM image of high concentration deposited

electrode (f) AFM image of the deposited electrode. 66

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Conductivity and Stability of electrode

Highly conductive and flexible electrodes are greatly focused currently, because of their

emerging applications in wearable electronic devices and wearable energy storage devices.

Here, with an optimized, four dip and dry cycles, the minimum resistance of 2.5 ohm/cm was

achieved, as shown in Figure 1 (a). Bare carbon fabric is also electrically conductive, showing

an electrical resistance of 40 ohms/square. However, the graphene coating further decreases

the resistivity. The decrease in resistance is because of the high conductivity of rGO sheets

coating, which enhanced inter-fiber connections of carbon fabric. Variation of resistivity at

different coating cycles (Figure 5 (a)) shows that there is no significant decrease in resistivity

after four cycles, as all the inter-fiber connections are fully developed. Further higher loading

causes the blockage of micropores, thus decreasing the catalytic interface, hence rGO electrode

with four coating cycles was further subjected for the N-GQDs coating. The conductivity of

electrode plays a major role in electrocatalytic activity and photocatalytic activity by providing

high mobility and superb charge collection. It is also important that the N-GQDs played no

significant role in increasing the conductivity of the electrode.

The stability of the free-standing electrode is equally important as a photocatalytic response,

therefore the stability of the electrode was tested against the different type of stresses and

chemical environments. The stability of against water and electrolyte was tested by immersing

the electrode in electrolyte and water for 1 week, and variation in transmittance (after

immersion) was analyzed, as shown in Figure 5 (b). The transmittance of water remained 100 %,

even after emersion of a long time, thus confirming the stability of the electrode in water.

Similarly, negligible variation in transmittance of the electrolyte solution was observed,

confirming that graphene sheets/N-GQDs will are not peeled off even in iodine-based strong

electrolytes. A minor increase in transmittance of the electrolyte after immersion may be

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because of adsorption of iodine on the surface of N-GQDs. In addition, no physically broken

chunks of the film were observed in immersed water and electrolyte solution, further

confirming the stability of the electrode.

The bending stability was also tested by comparing the variation in electrical resistance of

fabric at different bending positions, as shown in Figure 5 (c). The maximum variation was

recorded less than 0.6 % increase in resistance at the maximum bent position. Similarly, the

stability after the different number of bending cycles was also tested to be negligible. The

flexible nature of graphene can be attributed to the crumbled 2D nano-sheets structure.

Mechanical stability under tensile stresses was analyzed to confirm that the designed process

is not degrading the strength of carbon fiber. Tensile strength response of bare fabric and rGO

coated fabric is compared in Figure 5(d). A minor increase of about 2 % was observed after

coating rGO and N-GQDs; because nano-coating reduces the inter-fiber and inter-yarn slippage.

From this test, it is also confirmed that there is no degradation of carbon fabric during the

reduction and coating process. This stability against strong chemical reducing agents and

temperatures is because of inorganic nature of carbon fiber, whereas most of the other natural

fibers are sensitive to such harsh reduction environments. Additionally, the interface of rGO

sheets with the carbon fiber was also analyzed, by viewing the cross-section after tensile

breakage, using FE-SEM (inset of 5(d)). The cross-sectional view confirms that the rGO sheets

are fully infused in the carbon fabric, and are fully covering the carbon fibers. As a result of

the tensile failure of carbon fiber, the rGO matrix also breaks simultaneously, from the same

point. Such type of breakage behavior reflects that the interface bonding of rGO sheets is

reasonably strong, however owing to its low strength, there is simultaneous breakage of both

fibers and rGO film.

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Similarly, the thermal stability of free-standing carbon and cotton fabric electrode was tested,

to assure its stability at elevated temperatures, as shown in Figure 5 (e). Thermal degradation

behavior of the electrode was tested by the thermogravimetric analyzer, from room temperature

to 500 °C. Bare carbon fabric and rGO-N-GQDs coated carbon fabric both showed no

degradation even at a temperature of 500 °C. On the hand, rGO coated cotton fabric starts to

degrade at 250 °C and was fully degraded at a maximum temperature of 350 °C. Based on

high thermal stability, the proposed electrode can be considered a suitable template for further

secondary growth of other materials, using high-temperature techniques, e.g. chemical vapor

deposition etc.

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Figure 5 (a) Decrease in resistance at the different dip and dry cycles, (b) stability of

electrode in water and electrolyte, (c) bending stability test of electrode, (d) tensile

strength comparison of proposed electrode and bare carbon fabric, (e) TGA analysis of

cotton and carbon fabric-based electrodes.

(a) (b)

(d) (c)

(e)

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Catalytic activity test:

The bandgap of any semiconductor must be tuned as low that it can absorb visible light, to

make it visible light active. It was observed that N-GQDs show a shouldered absorbance peak,

which extends to the visible spectrum, as shown in Figure S2 (a). On the other hand, the

undoped-GQDs have no absorption in the visible region, the only a single weak peak was

observed in the deep UV spectrum, as shown in Figure S2 (b). The nitrogen being electron

donating nature increases the conjugation in N-GQDs, thus a red shift in absorbance is observed

[37]. Analyzing the whole spectrum of absorbance, the absorbance band at high energy around

360 nm corresponds to n-π, or π-π* transitions of electrons. This transition peak exists in both

doped and undoped GQDs, with a minor red or blue shift respectively. However, the other

absorbance peak in the visible region is because of oxygenated and nitrogenated functional

groups, mainly including C=O and C-N-C bonds [38], [39]. These low energy functional

groups generate the new surface states in N-GQDs and cause a significant red shift in the

absorbance, thus enabling the N-GQDs to be visible light active photocatalyst.

To analyze the photocatalytic activity, methylene blue dye was used as a reference containment

to be degraded. The electrode was immersed in the dye solution, and the solution was stirred

under dark, for 45 minutes. This makes the dye to adsorb on the surface of graphene coated

carbon fabric, and the solution after the adoption was considered as a reference. By this way,

the adsorption factor can be omitted from the photocatalytic degradation. After subtraction of

adsorption factor, visible light (400 nm to 900 nm; 12 W, Phillips x 4) was turned on, and the

decrease in concentration of methylene blue was tested after every 15 min, for a total time of

90 min. The percentage decrease in absorption peak of methylene blue dye at 665 nm was

plotted as photocatalytic degradation. Furthermore, during photocatalytic degradation testing,

electrode immersed solution was kept under constant stirring with a magnetic stirrer, to assure

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the degradation of dye is not limited by the diffusion of dye species.

To check the effect of the self-assembly on the photocatalytic degradation, the photocatalytic

degradation of N-GQDs was compared with uniformly coated N-GQDs electrode, developed

with equal dimensions. Initial absorbance or concentration (Co) of dye solution was 2.128,

however, the Self-assembled N-GQDs electrode degraded the dye to absorption of 0.817 in 90

min, as shown in Figure 6 (a). On the other hand, uniformly coated N-GQDs electrode

decreased the absorbance of the same concentration solution to 1.250, in equal time, as shown

in Figure 6 (b). Comparing the photocatalysis of both electrodes in Figure 6(c), it can be

observed that the uniformly coated electrode degraded 41 % of the original concentration,

whereas the self-assembled electrode degraded 61 %. This 49.31 % higher photocatalytic

degradation of our proposed electrode shows that the assembled structure of N-GQDs plays a

significant role in the photocatalytic response. In addition, a test was also performed just in the

absence of photocatalyst, to analyze the dye degradation due to light source, which was

recorded to be negligible, as shown in Figure 6 (c).

The enhanced photocatalytic response of assembled N-GQDs can be attributed to the higher

surface area exposed, for the catalytic activity. Because of the ditches/drains formed between

the walls of N-GQDs in assembled structure, the dye molecules can easily access the N-GQDs.

Substrate geometry also plays a significant role in the catalytic activity, by providing a high

interface. Carbon fibers being the finest of all fibers i.e. the diameter of the individual fiber is

as low as 5 micrometers, which provide the micro roughness. Fabric structure provides the

macro-roughness, and rGO sheets provide the nano-roughened high surface area for

accommodation of maximum amount of N-GQDs. Another factor responsible for the higher

degradation of the proposed textile electrode is the conductivity of carbon fiber, which provides

higher mobility to photogenerated electrons [40]. Higher mobility suppresses the

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recombination of charges, thus boosting the PCA significantly, as confirmed by the

photoluminescence test, explained in Figure S2 (c).

Figure 6 Photocatalytic activity of (a) Uniformly coated N-GQDs on CF-rGO, (b) self-

assembled N-GQDs on CF-rGO, (c) comparison of Assembled and uniformly coated N-

GQDs.

The ECA of the electrode was tested by using cyclic voltammetry (CV), by using iodide-based

electrolyte systems. The negative peak centered at 200 mV was assigned as an oxidation peak,

whereas the positive peak was assigned for the reduction reaction, as respective redox reaction

equations are added in Figure 7. The electrocatalytic activity was tested at a scan rate of 5

mV/sec, ranging from -200 mV to 1000 mV. The Electrocatalytic efficiency of rGO coated

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carbon fabric was compared with platinum coated fluorine-doped tin oxide (FTO) glass, as

shown in Figure 6. Platinum coated FTO glass was selected as a reference because it is most

commonly used electrocatalyst in most of the applications.

Majorly, oxidation and reduction peak height and their voltage difference are taken as a

measure of electrocatalytic activity. The higher height of peaks with a lower peak to peak

difference is referred to be the electrocatalyst with higher efficiency. The height of oxidation

and reduction peaks for proposed electrode was much higher than the platinum coated FTO

glass, i.e. the carbon electrode showed a current density of -4.1 mA/cm2 and 4.92 mA/cm2 for

the oxidation peak and reduction peak, respectively. On the other hand, platinum coated FTO

glass showed lower peak heights, i.e. oxidation and reduction peak heights if -0.38 mA/cm2,

and 0.84 mA/cm2 were observed respectively. Other than lower peak height, peak to peak

difference was also higher in platinum coated FTO glass, thus confirming that ECA

performance of the proposed electrode is higher than the platinum. The ECA of rGO coated N-

GQDs shows a higher Epp value but current density is comparable to Pt, however, after the

uniform coating, there is a significant increase in current density, as shown in Figure S3.

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Figure 7 Comparison of the electrocatalytic activity of rGO coated carbon fabric and

platinum coated FTO

One of the factors in superb ECA is the higher electrical conductivity of CF-rGO electrode,

which makes the faster flow of charges, thus increasing the peak heights. Carbon coated rGO

electrode showed a surface resistance of 2.5 ohm/cm, whereas the resistance of FTO coated

platinum has a resistance of 8 ohms/sq. Here, the abundant active sites induced by N-GQDs

also play a critical role in boosting the ECA by speeding the redox reaction, hence the decrease

in Epp value was observed. Although the highly defect-rich and functional surface of N-GQDs

has abundant functional groups, however in the uniform coating, only the N-GQDs on the outer

most surface can participate, therefore the Epp of uniformly coated N-GQDs is higher than Pt.

However, in the self-assembled N-GQDs structure possesses directly exposed abundant multi-

edge surfaces, which serves as an active site for charge storage reactions. Also, the porous

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assembly of N-GQDs makes easy for the ionic species to diffuse through.

To test the combined electrochemical and photocatalytic degradation of the electrode, an

additional current source was applied during the photocatalysis test. The variable current source

was connected on the edges of rGO coated carbon fabric, while other conditions were kept

similar. While testing electro-photocatalysis, the effect of different voltages was tested and

compared with the photocatalytic activity of the electrode, tested without any voltage (0 Volts).

The photocatalytic degradation of the electrode was marginally accelerated by applying a

biased voltage of 1 volt, i.e. the maximum degradation was 70 % (Figure 8 (a)) and 61 %

(Figure 5 (a)), under 1 volt and 0 volt, respectively. The increase of photocatalytic degradation

(under application of low voltage) was observed to be not as significant as it is commonly

observed in the metals-based substrates.

To further accelerate the photocatalytic degradation, the applied voltage was increased from 1

volt to 5 volts. The electro-photocatalytic degradation by applying 5 volts, is given in Figure 8

(b), where it can be observed that the absorbance of dye solution is decreased from 2.128 to

0.14, under the equal time of treatment. The degradation was observed to be 93 %, i.e. the

degradation was increased by 32 % just by applying the biased voltage. It is also interesting to

note that electro-photocatalytic degradation is highly fast at the start, i.e. 89 % of dye is

degraded in just in the first 60 min. The degradation of dye solution can also be observed after

treatment, as shown in the inset of Figure 8 (b).

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Figure 8 Photocatalytic activity at (a) 1 volt, (b) 5 Volt, (c) comparison of electro-

photocatalytic activity at different volts

Similarly, electro-photocatalytic degradation was also tested at further higher voltages, where

a further increase in degradation was observed. However, owing to the relatively lower

conductivity of the carbon fabric (as compared to metals), higher voltages causes a slight

increase in temperature of the carbon fabric and degradation bath. Thus, at higher voltages, an

additional cooling system is required to maintain the temperature during the electro-

photocatalytic process. Therefore, electro-photocatalysis at a biased voltage of 5 Volts was

considered optimum in this work. The externally applied voltage accelerates the photocatalytic

degradation process by the electrochemical degradation of dye under applied voltage and

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current. Another factor which is important in improving the photocatalytic activity is the

suppression of electron and hole recombination under the external applied electric field.

Photogenerated electrons are attracted to the outer circuit, due to an external voltage applied,

that provides further high mobility of electrons, with suppressed charge recombination. The

same mechanism is explained schematically in Figure 9.

Figure 9 Schematic representation of the electro-photocatalytic activity of the proposed

electrode.

The applied voltage strongly varies the electron recombination process of the active material,

hence varying the degradation rate. The applied voltage depends majorly on the nature of the

active material, and the conductivity of the substrate, target pollutant and degradation

conditions (pH, temperature etc). For instance, the TiO2 co-doped with the Cu and N, coated

on a Ti substrate requires 20 V of biased voltage to proceed with the electro-photocatalytic

reaction [41]. While in the case of fountain and flushing water, RuO2 and TiO2 were used under

the biased voltage of 25-30 volts [42]. For treatment of organic pollutants, the biased voltage

varies between 1.5 V to 3 V, however, in those systems, the substrate used is the stainless steel,

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or other metals foil (Ti commonly) [43]–[56]. Relatively our proposed system requires higher

voltage, which can be attributed to the higher resistivity of carbon fibers, as compared to metal

substrates. Owing to higher resistivity, a marginally higher voltage is required for moving the

electrons to an external circuit. In addition to it, the inorganic, flexible and corrosion resistant

nature of our proposed system makes it superior to previous works. Additionally,

conventionally UV light is used in electro-photocatalysis, due to which the excitons already

have high energy. However, in our proposed strategy, visible light (lower energy), which is

abundant in the solar spectrum is utilized. If the results of electro-photocatalytic activity are

compared with the other recent work performed using metal substrate and visible light, where

78 % of methylene blue is degraded in 80 min by TiO2 and WO2 composites [57]. In other

work, using carbon fiber substrate, the TiO2 degraded the 90 % of dye in 180 minutes, under

UV light [53]. Compared to such metals oxides, UV drove, metallic substrates based electro-

photocatalysts, our proposed strategy provides higher effeciency with higher stability.

Conclusions

By doping nitrogen, the bandgap of N-GQDs was successfully reduced to the visible spectrum,

which was then utilized for the visible light driven photocatalytic degradation of an organic

dye, from the water. N-GQDs were self-assembled in a highly porous plant leaflet shaped

structure, and by this assembled structure possesses highly exposed abundant multi-edge

surfaces, which serves as an active site for both electrocatalytic and photocatalytic reactions.

Woven carbon fabric coated with rGO sheets was used as a substrate for the coating of N-

GQDs, which provides a high surface area and high electron mobility, thus facilitating the

catalytic activity. The self-assembled structure achieved by a facile technique shows a high

electrocatalytic activity, i.e. higher than the Nobel metal Pt. By combining both catalytic

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activities, a fast degradation of organic pollutant (methylene blue) was achieved in Electro-

photocatalytic activity test of the electrode. The self-assembled N-GQDs showed 49.31 %

higher photocatalytic degradation as compared to uniformly coated N-GQDs, thus confirming

the significance of the assembled N-GQDs. This photocatalytic degradation was further

accelerated by 31.81 % by combining electrocatalytic degradation, by testing under an external

voltage of 5 V, while overall 93 % degradation was achieved in 90 minutes. This superb

performance of electrode is because of high electron mobility provided by the conductive

carbon substrate, which reduces the recombination of electron/hole pair, as confirmed by

photoluminescence test. Furthermore, the proposed electrode was tested to be stable in harsh

chemical environments, elevated temperatures (500 °C), tensile stresses (115 MPa) and cyclic

bending. Based on superior EPCA, high stability and conductivity, the proposed electrode can

pave the way for treatment of rising water pollution issues.

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Graphical abstract

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Highlights

A highly flexible and conductive electrode is fabricated using overlayer of Nitrogen

doped Graphene Quantum Dots (N-GQDs) over Carbon Fabric coated with reduced-

Graphene Oxide (CF-rGO).

Visible light active photocatalyst N-GQDs were self assembeled in a highly porous

leaflets structure, so that maximum edge surface could to able to participate in catalytic

reaction.

The electrode has high stability and photoelectrocatalytic activity, for degradation of

organic pollutants.

The proposed electrode is metal free and is stable at high temperature, harsh chemical

environments, and mechanical stresses.

The surface resistance of the all carbon electrode is only 2.5 Ω sq−1.

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