Synthesis of graphene–ZnO nanocomposites by a one-step electrochemical deposition for efficient photocatalytic degradation of organic pollutant

Journal Pre-proof Synthesis of graphene–ZnO nanocomposites by a one-step electrochemical deposition for efficient photocatalytic degradation of organic pollutant A. Henni, N. Harfouche, A. Karar, D. Zerrouki, F.X. Perrin, F. Rosei PII:

S1293-2558(19)30469-8

DOI:

https://doi.org/10.1016/j.solidstatesciences.2019.106039

Reference:

SSSCIE 106039

To appear in:

Solid State Sciences

Received Date: 19 April 2019 Revised Date:

8 July 2019

Accepted Date: 10 October 2019

Please cite this article as: A. Henni, N. Harfouche, A. Karar, D. Zerrouki, F.X. Perrin, F. Rosei, Synthesis of graphene–ZnO nanocomposites by a one-step electrochemical deposition for efficient photocatalytic degradation of organic pollutant, Solid State Sciences (2019), doi: https://doi.org/10.1016/ j.solidstatesciences.2019.106039. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Masson SAS.

Synthesis of graphene–ZnO nanocomposites by a one-step electrochemical deposition for efficient photocatalytic degradation of organic pollutant A. Henni 1a*, N. Harfouche 2a*, A. Karar1, D. Zerrouki 1, F.X. Perrin 2, F. Rosei 3 1

Lab. Dynamic Interactions and Reactivity of Systems, Kasdi Merbah University, Ouargla

30000, Algeria. 2

Laboratoire Matériaux Polymères-Interfaces-Environnement Marin, Université du Sud

Toulon, Var, BP 132, 83957 La Garde Cedex, France. 3

Institut National de la Recherche Scientifique, Centre Énergie, Matériaux et

Télécommunications, Varennes, Québec J3X 1S2, Canada. * Corresponding author. E-mail address: [email protected], [email protected] Tel.: +213 661 405 407 (Abdellah Henni) E-mail address: [email protected] (Nesrine Harfouche) a

These authors contributed equally to this work

Abstract In this study, the ZnO/graphene composite was synthesized by a one-step electrochemical

deposition

(ECD)

approach

using

cyclic

voltamperometry

and

chronoamperometry. Scanning electron microscopy and X-ray diffraction observations revealed that ZnO nanorods of wurtzite structure and rGO nanosheets were successfully associated in the composite film. Integration of 3D graphene in ZnO nanorods leads to a high performance in photocatalytic degradation of methylene blue. The results indicate that the ZnO/graphene achieves a higher degradation efficiency under light irradiation than that of the pure ZnO nanorods. The composite prepared using cyclic voltamperometry exhibits an

1

exceptionally higher photocatalytic activity than that prepared with chronoamperometry method. Thus, this report highlighted the multi-faceted role of graphene in the enhancement of ZnO photo-conversion efficiency. Keywords: electrodeposition; reduced graphene oxide; ZnO; material composite; photocurrent; photocatalytic activity.

Introduction Amongst the inorganic semiconductors, n-type zinc oxide (ZnO) has been widely studied for potential application in various technologies due to its large binding energy and wide band gap [1]. In addition, ZnO is an abundant, low cost, and eco-friendly compound [2,3] that can be used to grow transparent coatings for photovoltaic cells [4] and light-emitting devices [5,6]. Zinc oxide thin films can be easily synthesized using a variety of techniques like electrodeposition [7,8], sol–gel [9] and hydrothermal synthesis [10]. In particular, electrodeposition is a well-known process that could be applied in the growth of metal oxide films from aqueous or nonaqueous solutions [11,12]. Numerous attempts have been reported to improve the pristine properties of oxide such as heterostructuring, alloying and doping [13–20]. Metal or non-metal doping can boost the properties of semiconductors like electrical, optical, and catalytic [21–25]. The synthesis of semiconductors with narrower band gap is another effective approach, but reusability of the composites is questionable due to the no stability of the components of semiconductors [26]. Generally, and in most research on ZnO and rGO nanocomposites are focused on the dispersion of graphene in the matrix of metal oxides. An efficient method consists in hybridizing ZnO with graphene nanosheets for high performance devices [27]. A one-step synthesis of graphene-ZnO nanocomposites by a hydrothermal process was developed. Graphene can act as a fast electron transfer channel within a composite material, thanks to its 2

π-π interactions, thus reducing the recombination of the photo-generated electron-hole pairs and leading to improved photoconversion efficiency [28,29]. Due to its decorating oxygen functional groups which confer versatility and hydrophilicity, GO presents an attractive precursor material to anchor oxide particles during the synthesis process [30]. The reduced GO form is usually achieved by a chemical or electrochemical reduction [31,32]. The distribution of graphene in these oxides is random, which is unfavorable for certain applications. In this case, it is necessary to use the laminate structure of nanocomposite [33]. Dusza et al. demonstrated that graphene transfer on ZnO surface can be successfully applied in fabrication of ultra-thin and large area photodetector structures using spin-coating technique [34]. Baitimirova et al. the effect of the number of alternating graphene sublayers and the thickness of ZnO interlayers on the structure and the optical properties of the fabricated nanolaminates [35]. In this work, we report the improvement in properties of ZnO by rGO. ZnO/rGO composite thin film fabrication is carried out using electrodeposition process with two methods on ITO substrates for photocurrent generation and photocatalysis applications. This is important because few studies have been devoted to ZnO with H2O2 as a precursor, and no study has elaborated the ZnO/rGO composite using H2O2 as a precursor.

Experimental Chemical synthesis of rGO

Graphene oxide (GO) was prepared by the improved Hummer’s method as in our previous studies [36]. In a typical procedure for the reduction of GO to chemically reduced graphene oxide (rGO), 100 mg of GO was put in a 250 ml round-bottom flask with 100 ml of water, yielding an inhomogeneous brown-yellow dispersion. This dispersion was sonicated using an ultrasonic homogenizer until it becomes with no visible particulate matter. Hydrazine 3

hydrate (1 ml) was then added and the solution heated for 24h at 100 °C under a water-cooled condenser over which a progressive precipitation of black solid (GO reduced) is observed. The obtained product was isolated by filtration, and washed extensively with water and methanol. After drying, the solid product obtained is the rGO (Fig. S1).

Electrochemical synthesis of ZnO and ZnO/rGO All electrochemical experiments were carried out using an Autolab PGSTAT204 Galvanostat–Potentiostat. A three-electrode assembly was used for all electrochemical measurements, ITO as the working electrode (WE), a Pt as the counter electrode (CE) and a saturated calomel electrode (SCE) as the reference electrode. Films are electrodeposited onto ITO coated glass substrates (25 Ω/sq). The substrates are ultrasonically cleaned in acetone, in ethanol for 10 min, and finally in distilled water. ZnO and ZnO/rGO coatings are electrodeposited on ITO substrate using two methods: (i) potentiodynamic method between -1.2 to 1.0 V vs. SCE and (ii) potentiostatic method at -1.0 V vs. SCE for 40 min. ZnO and ZnO/rGO films are electrodeposited from the baths which containing 0,6 mg/mL rGO, 5 × 10-3 mol/L ZnCl2, 5 × 10-3 mol/L H2O2 and 0.1 mol/L KCl (pH is between 5.5 and 6). The bath temperature is controlled at 65°C with a water circulating system. The electrodeposition is carried out without stirred knowing that rGO particles were kept in suspension. After electrodeposition, the ZnO/rGO coated electrode is washed with distilled water. Characterization The morphology of the ZnO and ZnO/rGO thin films was investigated by scanning electron microscopy (SEM) (Supra 40 VP Colonne GEMINI Zeiss SEM equipped with an energy dispersive (X-ray detector Oxford X-max- 20). SEM images were collected at an accelerating voltage of 5 kV. Absorbance spectra were recorded using a Shimadzu UV-1800 spectrophotometer. The films thicknesses and Roughness were measured using a

4

Profilometer. The crystal structure analyses are performed by X-ray diffraction (XRD) using a Siemens D5000 X-ray diffractometer equipped with a Cu Kα source (λ = 1.5406 Å). Photoelectrochemical Measurements The photocurrent measurements were recorded in a range of potentials between 0.2 and 1.0 V by illuminating the the front side of samples using a UV illuminator using 25 s on/off cycles. Photocatalytic activities The Photocatalytic activity were measured using methylene blue (MB) aqueous solution as an organic pollutant. The same sized ZnO and ZnO/rGO electrodes were dipped in a MB solution. Before illumination, the solution with electrodes were placed in the dark for 20 min to reach the adsorption/desorption equilibrium. After, it was subjected to irradiation by Xe lamp (350 W).

Results and discussion 3.1 Electrochemical deposition The ZnO electrodeposition mechanism is based the generated OH- by hydrogen peroxide reduction at the ITO interface. The reaction can be expressed as follows: Zn2++ 2OH- → Zn(OH) 2 → ZnO + H2O

(1)

During the formation process of ZnO nanostructured thin film, rGO can diffuse into the ZnO simultaneously. For this, we proceed by introducing the amounts of rGO nanoparticles. The good suspension of rGO in electrolyte permits to ensure the insertion of rGO nanoparticles and obtain homogeneous thin films. Fig.1 corresponds to the chronoamperogram of ZnO and ZnO/rGO nanocomposites recorded for 40 min, to favour film growth. In addition, the applied potential at -1.0 V is a suitable

5

value to avoid the reduction of Zn2+ and the formation of metallic zinc. The curves confirm that ZnO are good electrical conductors. Whatever the absence or presence of rGO during electrodeposition, a similar appearance is obtained: in the first time the increasing current density is explained by nucleation, followed by a plateau of the current density corresponding to the growth of nuclei. As clearly observed, the cathodic current increased with the presence of the rGO content, indicating an increased growth rate. The current density collected at the electrode is kept at about -2.5 and -4.7 mA.cm-2 after 1200 s of growth for ZnO (Fig. 1a) and ZnO/rGO (Fig. 1b), respectively. The curves confirm that rGO improves the electrocatalytic properties of ZnO. The cyclic voltamperograms (CV) relative to ZnO films deposition from a simple ZnCl2 and from a mixed ZnCl2 + rGO bath rGO are shown in Fig. 1. The cyclic voltamperograms in the range from -0.8 to -1.2 V present an increase of the current density. This can be by changing the surface by depositing a layer of ZnO. In the opposite direction, the curve shows no oxidation process (no anodic current) during ZnO deposition, which explains the good layer stability achieved. Upon addition of rGO, an increased current was observed due to synchronous electroreduction of both rGO and H2O2 at the working electrode. The appearance of the cathodic peak at more negative values can be attributed to the incorporation and diffusion of rGO. Fig. 1. Chronoamperometry (CA) curve related to (a) ZnO and (b) ZnO/rGO formation under imposed potential -1.0 V vs. SCE. Cyclic voltammetry (CV) scans obtained during the formation of (c) ZnO and (d) ZnO/rGO on ITO. 3.2 Structural studies To further demonstrate the successful synthesis of the ZnO/rGO composite, we used various characterization techniques. The XRD patterns of the composite obtained after 40 min of electrodeposition is displayed in Fig. 2. Several well-defined diffraction peaks were

6

observed and indexed to the (100), (101), (002), (102) and (110) planes of the wurtzite ZnO (JCPDS no. 36-1451), respectively. The additional peaks correspond to the reflections of the ITO substrate (JCPDS no. 41-1445). No preferential orientation was observed since peak intensities in our diffractgrams are close. With the presence of rGO, the intensity of the (101), (002) and (100) planes of the ZnO structure decreased considerably. A broad diffraction peak was observed at about 2θ =25° which is the characteristic peak of graphite. This indicates the presence of some stacked layer structure of graphene sheets in the electrodeposited ZnO-rGO nanocomposite. The diffraction peak of graphene is relatively less intense in the film electrodeposited by cyclic voltammetry and this probably due to more exfoliated graphene sheets. In addition, the diffraction peaks of the ZnO/rGO are shifted to lower degree compared with that of ZnO only, this indicates that the presence of the carbonaceous material has changed the lattice constant of the ZnO hexagonal structure. Fig. 2. XRD pattern of electrodeposited (a,b) ZnO and (c,d) ZnO/rGO on ITO prepared following the two techniques: (a,c) cyclic voltamperometry and (b,d) chronoamperometry. 3.3 Morphological studies A layered, wrinkle-like structure is visible in SEM images of rGO (Fig. S3). In addition, agglomerated sheets forming a disordered solid are observed. Thus we note the absence of charge on the SEM images, this indicates that the synthesized graphene sheets are electrically conductive. Fig. 3 shows the SEM images of ZnO and ZnO/rGO grown on ITO substrates. ZnO obtained without rGO forms aligned hexagonal nanorods (NRs) with the c-axis preferred orientation perpendicular to the substrate with a little more random orientation for films grown by CV. ZnO rods prepared by CV show a lower density and larger size compared to those prepared

7

by CA (11×108 NRs/cm2 and ca. 250 nm diameter for CV with respect to 22×108 NRs/cm2 and ca. 200 nm diameter for CA). Table 1 displays the thickness and Roughness of the obtained films. Fig. 3. SEM images of (a,b) ZnO and (c,d) ZnO/rGO hybrid on ITO prepared following the two techniques: (a,c) cyclic voltamperometry and (b,d) chronoamperometry. (e) EDX spectrum of ZnO/rGO nanocomposite. The introduction of rGO sheets during the electrodeposition process induce a modification of the morphological characteristics of NRs (Fig. 3c,d), this explain the effect of rGO on the formation of nucleation sites, the direction and the rate of growth [37]. Compared to the perfect vertically aligned growth of ZnO NRs observed in the absence of rGO especially using CA method, the shape of ZnO structures formed in the presence of rGO appear poorly defined. It can be seen that single ZnO nanorods with random orientation and flower-like assembly of ZnO nanorods were grown on the ITO substrate in the presence of rGO. Figure 3c shows that ZnO particles are covered by thin veil-like graphene sheets. The EDX spectrum of the ZnO/rGO film shows the presence of zinc, carbon and oxygen, providing further evidence for the presence of graphene nanosheets in the film (Fig. 3e). Table 1. Displays effect of the rGO on thickness and Roughness. Sample

Method

Thickness

Roughness Ra

(nm)

(µ µm)

ZnO

CA

200

0.0189

ZnO

CV

220

0.0128

ZnO/rGO

CA

180

0.0093

ZnO/rGO

CV

350

0.0388

CA: Chronoamperometry CV: Cyclic voltammetry 8

3.3 Photoelectrochemical Performance Generally, the performance of photoelectrodes is evaluated according to that ability to adsorb light because it has a direct influence on photogenerated holes and electrons. The photocurrent-time response of ZnO and ZnO/rGO under intermittent illumination was studied in 0.1 M Na2SO4 at increasing values of applied potentials. The variation of the photocurrent generated versus the applied potential is shown in Fig. 4. For this we used a range of potentials located at depletion zone (cf. Fig. S5). We notice that in dark conditions, the current density is close to zero, thereby describing the fundamental state thermodynamic of the electrode. Under lighting, the curve presents a significant increase of current with a rectangular response. Fast and uniform photocurrent responses were observed for each switch-on and switch-off event in all electrodes. The transients of the photocurrent generated by all the electrodes have almost the same shape. Thus, we observe that these photocurrents have positive values which is a property of an n-type was observed. This could be explained by the presence of charge carriers moving from the VB to the CB under UV illumination (photon excitation). The photocurrent of the ZnO electrode at 1.0 V merely reaches values of 20 µA/cm2, which could be attributed to the recombination of electron-hole pairs in the electrode and the slow transmission of charge. It is observed that the photocurrent intensities of ZnO/rGO electrodes have been clearly improve compared with pure ZnO electrodes. After modification with rGO, the ZnO/rGO photoelectrode exhibited higher photocurrent (more than 60 µA/cm2 at 1.0 V), reaching values almost three times higher than that of ZnO. The rGO with superior conductivity here act as electron collector and transporter thus the carriers' lifetime is lengthened [38]. According Abdul Hamid et al. the rGO may have contributed to the band gap broadening for the rGO/ZnO sample by increasing the carrier density in the valence band 9

[39]. On the other hand, Luo et al. have demonstrated the absorption edge of the ZnO/rGOZnO composites remains the same as the ZnO, indicating that the band gap has not been changed. However, the absorption intensity increases for ZnO/rGO ; it can be ascribed to the increase in surface electric charge of ZnO in the composite [40]. On the other hand, the photocurrent generated with cyclic voltamperometry method is greater than that obtained with chronoamperometry. Three types of interfacial configurations (ZnC, OC, HC) of both OG and ZG can be built respectively to represent the contact between ZnO and graphene [41]. Fig. 4. Photocurrent (PC) characteristic curves of ZnO and r-GO prepared with two different techniques (CA and CV). Photocatalytic activities Photocatalysis is an environmentally friendly approach based on semiconductor materials. The photocatalytic activities of the electrodes were evaluated based on the decrease in the colour intensity of MB using a Xe-illuminator as light source. The solution of MB was stirred in the dark for one hour to achieve adsorption–desorption equilibrium between the electrode and MB molecules before irradiation with light. At various irradiation time intervals, 10 mL suspensions were collected. The concentration of remnant MB was then monitored using a UV-vis spectrometer. To evaluate the stability of the elaborated electrodes cycling experiments were conducted.

Fig. 5. Time-dependent absorption spectra of the MB solution which is about MB degradation under stimulated light with ZnO/rGO nanocomposite as photocatalyst using (a) CA method and (b) CV method. (c) Photo-degradation rate (%) of MB stimulated at different irradiation time.

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To demonstrate the photocatalytic activity induced the ZnO and rGO, the photocatalytic activities and photo-degradation rate of MB was compared in the presence of our electrodes, as shown in Fig. 5. There was a little degradation without the presence of electrodes in MB solution under Xe irradiation. But in the presence of electrodes, the absorption peaks at 663 nm decreased rapidly under light irradiation with the extension of exposure time. The degradation ratio is defined as (1-C/C0)×100%, in which C0 is the initial concentration after the equilibrium adsorption and C is the reaction concentration of MB. Using the CA method, the photo-degradation rate is around 70% after 3h, while it is around 97% in the case of ZnO/rGO electrode obtained with the CV method (Fig. 5c). This indicates that the presence of rGO in the ZnO structure has enhanced photoactivity, this is related to the synergistic effect between rGO and ZnO [42]. Improved electrode properties can be attributed to the good electroactive property of ZnO and the high electrical conductivity of synthesized rGO [43]. Therefore, direct electron transfer from the VB of ZnO to rGO is thermodynamically favourable (Fig. 6), and much more feasible than to the CB of ZnO. This electron transfer is responsible for improving photocatalytic performance under light. Thus, for large wavelength values, the rGO shows a good extinction capacity of the emission, which indicates that the presence of rGO with ZnO can better capture the photo-induced electron and improve the separation of charge by a process of charge transfer [44,45]. Also, what it contributes to the degradation of MB is the fast transfer of the charges generated on the surface. After the separation of holes and electrons, there are serval reactions involved for the formation of hydroxyl radicals (OH•), it is well known that these hydroxyl radicals are they are very reactive oxidative species (oxidants) which react with the dye rapidly and degrading them to H2O and CO2 [46]. The good separation of the photo-generated electrons and the holes at the interface rGO and ZnO is responsible to improve of the photocatalytic activity of our electrodes [47]. The stability of the as-prepared photocatalyst was investigated by repeating

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the degradation of MB for three cycles. As illustrated in Fig. 7S, the removal efficiency of MB was still 97% after two cycles and around 95% after three cycles.

Fig. 6. Schematic illustration of the photocatalytic degradation mechanism of the ZnO/rGO composite.

Conclusion

We reported a facile and effective method for the synthesis of ZnO/rGO hybrid thin films. The electrochemical properties and structural analysis of ZnO/rGO electrode have been studied comprehensively. In comparison with the ZnO electrode, the of ZnO/rGO nanocomposite electrodes demonstrated an improved photoelectrochemical performance. The results obtained indicate that the ZnO / rGO nanocomposites have a much better photocatalytic activity compared to the annostructured ZnO, thanks to the good absorption of light between the 3D rGO and the ZnO and to the superior separation efficiency of the electron-hole pairs to improve the photo-degradation efficiency. A better MB degradation efficiency is obtained with CV method. The ZnO/rGO nanocomposite is a promising system for the degradation of organic compounds. The coupling of ZnO with the rGO gave ideal results for the degradation of the methylene blue, where the total elimination of the dye is obtained after 180 min.

Conflicts of interest There are no conflicts to declare. Reference

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19

a)

b)

-1

-2

-2

-3

-3

i (mA/cm2 )

i (mA/cm2 )

-1

-4 -5

-4 -5

-6

-6

-7

-7 0

10

20

30

0

40

10

20

30

40

t (min)

t (min) 0,1

c)

0,5

d)

0,0

2

i (mA/cm )

i (mA/cm2)

0,0

-0,1 -0,2 -0,3

-0,5 -1,0

-0,4

-1,5

-0,5

-2,0

-0,6

-2,5

-1,5

-1,0

-0,5

0,0

E (V vs. SCE)

0,5

1,0

-1,5

-1,0

-0,5

0,0

0,5

E (V vs. SCE)

1,0

20

Intensity (a.u)

30

*

40

2 (°) 50

ITO

ZnO (110)

ZnO (102) ITO

ITO ZnO (100) ZnO (002) ZnO (101)

RGO (002)

* *

60

(d)

(c)

(b)

(a)

70

(b)

(a)

1 μm

1 μm

(c)

(d)

1 μm

(e)

1 μm

Photocurrent (μA)

a) (a) ZnO (CV) (b) ZnO (CA) (c) rGO/ZnO (CV) (d) rGO/ZnO (CA)

60

(c) (d)

30

(b) (a)

0.4 V

0.2 V

0

0.6 V

170

90

0.8 V

250

1.0 V

330

410

t (s)

Photocurrent (μA)

b) r r

0.2

0.4

0.6

E (V)

0.8

1.0

Absorbance

(a) 0 min (b) 60 min (c) 90 min (d) 120 min (e) 150 min (f) 180 min

1.5 1.0

(a)

ZnO/rGO (CA)

2.0

(b)

Absorbance

a)

2.0

(c) (d) (e) (f)

0.5

1.5 1.0

b) (a) 0 min (b) 60 min (c) 90 min (d) 120 min (e) 150 min (f) 180 min

(a)

ZnO/rGO (CV)

(b)

(c) (d)

0.5 (e) (f)

0.0

0.0 550

600

700

650

120

Degradation rate (%)

100

ZnO/rGO (CV)

80

ZnO/rGO (CA)

60

ZnO

40 Blank

20 0 0

40

80

120

Time (h)

600

650

(nm)

(nm)

c)

550

160

200

700

O•-2

O2 N Me

+

N

e-

S

N -

Me

Cl

Me

e-

Me

rGO

e- e- e- e- e- e- e-

BC Eg

Photocatalytic Degradation

BV h+ h+ h+ h+ h+ h+ h+

H2O, CO2

OH• H2O

ZnO

rGO

-

Simple and effective method for the synthesis of ZnO/rGO hybrid thin films.

-

ZnO / rGO nanocomposites have a much better photocatalytic activity compared to the annostructured ZnO

-

A better MB degradation efficiency is obtained with cyclic voltammetry method