Synthesis of macroporous ZnO-graphene hybrid monoliths with potential for functional electrodes

Accepted Manuscript Synthesis of macroporous ZnO-graphene hybrid monoliths with potential for functional electrodes

L. Monica Veca, Florin Nastase, Cristina Banciu, Marian Popescu, Cosmin Romanitan, Marius Lungulescu, Radu Popa PII: DOI: Reference:

S0925-9635(18)30151-1 doi:10.1016/j.diamond.2018.05.010 DIAMAT 7114

To appear in:

Diamond & Related Materials

Received date: Revised date: Accepted date:

9 March 2018 23 April 2018 13 May 2018

Please cite this article as: L. Monica Veca, Florin Nastase, Cristina Banciu, Marian Popescu, Cosmin Romanitan, Marius Lungulescu, Radu Popa , Synthesis of macroporous ZnO-graphene hybrid monoliths with potential for functional electrodes. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Diamat(2017), doi:10.1016/j.diamond.2018.05.010

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ACCEPTED MANUSCRIPT Synthesis of macroporous ZnO-graphene hybrid monoliths with potential for functional electrodes L. Monica Veca *1, Florin Nastase 2, Cristina Banciu3, Marian Popescu 1, Cosmin Romanitan 1,4, Marius Lungulescu3, Radu Popa 1

Centre of Nanotechnologies, National Institute for Research and Development in Microtechnologies,

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2

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IMT-Bucharest, 126 A Erou Iancu Nicolae, 077190, Bucharest, Romania

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Research Centre for Nanotechnologies and Carbon-based Nanomaterials, National Institute for

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Research and Development in Microtechnologies, IMT-Bucharest, 126 A Erou Iancu Nicolae, 077190, Bucharest, Romania* 3

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National Institute for Research and Development in Electrical Engineering ICPE-CA, 313 Splaiul

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Unirii, 030138, Bucharest, Romania 4

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Faculty of Physics, University of Bucharest, 405 Atomistilor, 077125, Magurele, Romania

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*Corresponding author: E-mail: [email protected] ; Tel: +40-21-269.07.70 Fax: +40-21-269.07.72

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ACCEPTED MANUSCRIPT ABSTRACT The favorable optoelectronic and catalytic properties of zinc oxide, combined with the large specific surface area of graphene molded in 3D network, are able to offer a promising pathway towards the development of hybrid structures with improved electrical, optical, and catalytic properties, that are well suited as active elements for a wide range of applications,

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especially those related to energy storage and conversion. Using a template-assisted method we

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demonstrate the synthesis of 3D, porous network monoliths of hybrid ZnO-graphene thin-films in

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various controlled configurations. The thin-film composites are realized by thermal CVD growth of few-layers graphene on macroporous nickel foam, followed by an ALD coating process of the

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ZnO film. Notably, the graphene shell fully preserved its network structure after performing

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complete chemical etching of the nickel scaffold. The challenge of ZnO layer deposition directly on the graphene surface was solved by hydrophobicity adjustment using UV-ozone treatment

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prior to the atomic layer deposition process. Structural and morphological characterization

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revealed the formation of hexagonal crystalline zinc oxide films, conformally deposited on the few-layer graphene surface, while the electrical conduction analysis indicates excellent

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conductivity and interlayer electrical contact for these hybrids. Our results demonstrate that this

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controlled fabrication process is able to provide highly functional materials with great potential

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for various electrode applications.

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ACCEPTED MANUSCRIPT 1. INTRODUCTION For efficient detection, as well as for energy storage and conversion, electrode materials represent a prevalent bottleneck that attracts extended design and synthesis research. Zinc oxide is one of the most versatile transition metal oxides triggering great interest for a wide-range of

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applications. Its non-toxicity, biocompatibility, chemical stability, high isoelectric point and electrochemical activity, advocated its potential for applications as biosensors electrode [1-3].

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The wide band gap (3.37 eV), high carrier mobility (200-300 cm2V-1s-1) [4], and high theoretical

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specific capacity (987 mAhg-1) [5] also stimulated its extensive use as electrode material in solar cells, Li-ion batteries and supercapacitors. Despite its impressive properties, ZnO suffers from

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both large volume expansion (228%) and poor electronic conductivity, causing detachment of

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the active layer from the current collector, or performance limiting charge recombination effects, drawbacks that are reflected in reduced cycling stability and low retention rates in rechargeable

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batteries, as well as in modest energy conversion efficiencies. One way to circumvent capacity

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degradation caused by functional metal oxide materials swelling is to downscale their grain size to nanometers [6] and assemble them on porous materials. Moreover, increasing the specific

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surface area of the functional materials confers enhanced properties for various applications

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based on increasing the level of sensitizer adsorption, the lithium storage capacity, chargetransfer reactions efficiency, or surface electrochemical activity [7]. Combining ZnO with conducting materials such as carbon nanostructures (e.g., carbon nanotubes [8], graphene [9], graphene oxide [10]) has shown not only to improve the electrical conductivity of the final hybrid but also to release the strain energy [11,12]. However, the performances of these hybrids with 3D-aerogel architecture can be further improved by addressing their mechanical stability, electrical conductivity and effective surface area. For example, the electrical conductivity of 3

ACCEPTED MANUSCRIPT graphene oxide and of its derivatives is rather low for practical applications, suggesting the need for efficient methods to reconstruct the carbon sp2 network, while their effective surface area can be further improved by developing methods to avoid restacking and aggregate formation [13]. As these ZnO-C hybrids already offer significant technological impact in a broad field of

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applications, spanning from Li-ion batteries [14,15,16], to supercapacitors [17,18,19], to dye-

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sensitized solar cells [20,21], developing new synthesis methods for robust and conductive structures of ZnO-graphene hybrids with a large surface area has become one of the main

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research focus in the field of electrode materials.

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In this work, a macroporous ZnO-graphene hybrid monolith was developed using atomic layer deposition (ALD) technique to deposit conformal zinc oxide films directly on a 3D

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graphene network. The 3D graphene layer was obtained by thermal chemical vapor deposition

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(CVD) on standard commercial nickel foam [22,23], which has a double role: as the catalyst in the well-established carbon segregation-precipitation process involved in graphene growth, and

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as the 3D scaffold. Such graphene networks, synthesized using the template-assisted method,

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were already proven to reach high electrical conductivity and a larger specific surface area (up to 850 m2g-1 [22]) than those obtained by chemical methods (usually 100-700 m2g-1 [13,24]). When

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completely etching the nickel scaffold, the graphene shell firmly preserved the 3D geometry of the original Ni network; although no poly(methyl methacrylate) (PMMA) support was employed for the graphitized structure during this chemical etching process. The ZnO film was deposited using ALD technique that in contrast to classical wet or dry methods (hydrothermal synthesis [25,26], chemical deposition [27] or magnetron RF-sputtering [28,29]) has the capability to produce conformal films with well controlled thickness and good adhesion on substrates with 4

ACCEPTED MANUSCRIPT complex configuration. A uniform ZnO coating would secure the enhancement of energy and power density of the ZnO-graphene electrode. The ALD also holds the advantage of fine control of the active material composition (mass ratio of 3D-graphene to active material) which is an important parameter in supercapacitors, biosensors and Li-ion batteries. Despite its low

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deposition rate, ALD is an efficient process as it enables the coating of multiple wafers in one

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run [30,31]. The direct growth of the active metal oxide on graphene surface poses a

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technological challenge, due to graphene hydrophobicity. Among the various approaches used to modify the graphene surface, namely oxygen plasma, ozone treatment, UV-irradiation or surface

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chemical modification [32,33,34,35], we have selected the UV-ozone treatment. This is an

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effective method to introduce the oxygen moieties needed for the efficient ZnO deposition while creating low defect density and preserving the superior transport properties of as-grown CVD

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graphene. Thus, we introduce herein an effective process to prepare porous graphene-ZnO

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functional hybrids of very good electrical conductivity, features providing a great potential for various electrode applications, with the added flexibility of systematic adjustment in size and

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thickness of the active oxide films, depending upon the final properties targeted in the specific

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application.

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2. Experimental section

2.1 Materials and methods The 3D graphene layer was synthesized from methane and hydrogen gas mixture, on open-cell nickel foam catalyst (Gelon LIB, China), using the atmospheric pressure thermal chemical vapor deposition technique in a horizontal furnace (EasyTube 2000, FirstNano, CVD Equipment Corporation, USA). The high purity process gases employed for the graphene 5

ACCEPTED MANUSCRIPT growth, methane (>99.9995 %), hydrogen (> 99.995 %), and argon (> 99.999 %), were purchased from SIAD Romania and used as received. The specified characteristics of the Ni foam are: porosity of 110 ppi (pores-per-inch), thickness of 2.5 mm and surface density of at least 250 g/m2. Prior to ALD deposition, the as-grown 3D-graphene (as well as the 3D-graphene

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after Ni scaffold etching), underwent a surface modification by exposure to UV-ozone using an

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UV-Ozone cleaner (Novascan Technologies, USA). Conformal deposition of the zinc oxide layer

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was carried out by employing an atomic layer deposition equipment (OpAL, Oxford Instruments, UK) using diethylzinc (DEZ 99.9998% - Zn Puratrem, USA) as precursor, ultra-pure water as

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oxidizing agent, and ultra-high purity nitrogen (99.9999 vol.%) as carrier and purge gas. For the

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graphene and ZnO depositions, the structural features (crystalline structure, crystallite size and lattice constants) were investigated using a 9kW rotating anode X-ray diffraction system

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(SmartLab, Rigaku, Japan) that employs Cu Kα1 radiation (=1.54056 Å). The morphology and

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growth uniformity were ascertained on a high-resolution field-emission scanning electron

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microscope (FE-SEM) (Nova NanoSEM 630, FEI Company, USA) operated at 15 kV acceleration voltage. Raman spectra were recorded at room temperature on a high-resolution

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micro-Raman spectrometer (LabRAM HR 800, Horiba, Japan) using Ar ion laser excitation

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wavelength (ex= 514nm) whose power was set to less than 1 mW in order to minimize local heating. Fourier transformed infrared (FTIR) spectra were recorded in transmission mode on FTIR spectrometer (Jasco FTIR-4200, USA) in the range of 400 - 4000 cm-1, at resolution of 4 cm-1 and 50 scans for each spectrum, using the KBr pellet method. Electrical measurements for assessing the lateral (in-plane) effective conductivity of the graphene foam monoliths or of a silica deposited ZnO coating were performed based on the van der Pauw 4-probe technique at room temperature on square shaped samples, using a semiconductor characterization system 6

ACCEPTED MANUSCRIPT (4200-SCS/C, Keithley Instruments, USA) coupled to a shielded wafer probing station (Easyprobe EP6, Suss MicroTec, Germany). Electrical contacts on the surface of the selfstanding graphene sample or of the graphene-ZnO hybrid were made by using very small dotlike spots of conductive silver paste placed on the corners, subsequently cured at 150 C. Source

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currents employed were 5 mA for foam monoliths and 1 mA for the thin ZnO film deposited on

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the SiO2/Si substrate. All the direct and reciprocal van der Pauw configurations gave less than

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5% deviations to their average.

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2.2 Synthesis of 3D graphene monoliths

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The thermal CVD growth of the 3D graphene layer [36] was performed on 5 cm x 5 cm nickel foam substrates, ultrasonically cleaned in acetone and isopropyl alcohol before being loaded into

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the CVD furnace preheated at 1000°C. After 15 minutes of annealing under argon and hydrogen

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atmosphere - needed for removing any nickel oxide that might have formed on the foam surface the growth process was initiated by adding the methane gas. The growth process was carried out

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for 60 minutes in a mixture of 200 sccm CH4, 325 sccm H2 and 1000 sccm Ar. The holder

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containing the samples was retracted from the heated furnace area into the heat protected sealed chamber under Ar and H2 atmosphere, where they rapidly cool down to room temperature. The

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self-sustained 3D graphene specimens were obtained by etching out the nickel scaffold in an aqueous solution of HNO3 (3.9 M) at 85°C until the specimen was no longer presenting paramagnetic properties, in our case around 11 hours. The as-grown graphene on nickel foam (i.e., before removing the metallic support) will be further denoted as GF-Ni, while the selfsustained 3D graphene will be denoted as GF.

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ACCEPTED MANUSCRIPT 2.3 Synthesis of ZnO–graphene hybrid films The ALD processes were performed on disk-shaped samples (11 mm diameter) cut out from the initial GF-Ni specimens. To enhance the wettability and surface free energy, the samples (GF-Ni and GF) were initially exposed to UV ozone at 60°C for 15 minutes. The pre-treated samples

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were immediately transferred into the ALD reactor, preheated to 200°C. After evacuating the

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ALD reaction chamber to 6 x 10-3 Torr, vapors of both precursors (DEZ and H2O) were purged alternately into the reaction chamber for either 200 or 1000 consecutive DEZ/H2O cycles. During

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the entire process, the pulsing time was kept constant at 50 ms for DEZ and 20 ms for H2O. The purge time was set at 2000 ms for DEZ and 5000 ms for H2O. The resulted specimens - onward

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denoted as ZnO[200]/GF or ZnO[200]/GF-Ni and ZnO[1000]/GF-Ni, according to the number of ALD

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cycles - were further analytically characterized for process confirmation and optimization.

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3. RESULTS AND DISCUSSIONS

Porous and robust graphene layer was grown on nickel foam by thermal CVD, with the

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nickel foam serving as both the growing catalyst in a segregation-precipitation process, and the

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3D template. The SEM examination (Fig. 1b) reveals the formation of a continuous graphene network with specific wrinkles and folds, which replicates the structure of the Ni foam. Further structural characterizations indicate the formation of single, double and few-layer graphene with insignificant development of defects. In order to get a rough statistical picture of the film quality after various processing steps, we performed micro-Raman measurements in ~10 random surface points (Table 1). As depicted in Fig. 1a-bottom, Raman spectroscopy of the as-grown graphene 8

ACCEPTED MANUSCRIPT (GF-Ni) shows the typical G and 2D peaks, as well as the low-amplitude peaks corresponding to the so-called D+D" and 2D' bands [37]. Table 1. Statistical averaged data extracted from randomly collected micro Raman spectra after

FWHM [cm-1]

Intensity ratio

2D

layers

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55

5

0.042

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59

6

0.35

0.052

17

66

8

0.37

0.012

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4

0.40



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52

10

0.45

0.036

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Sample set

2D

D

I2D/IG

ID/IG

0.014

0.43

1586

2720 1355

0.38

ZnO[200]/GF-Ni

1582

2717 1353

ZnO[1000]/GF-Ni

1586

2721 1361

1584

as grown

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GF-Ni

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(after Ni etching)

ZnO[200]/GF

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2721

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self-sustained

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after UV ozone

GF

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2717 1363

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GF-Ni, 1584

G

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G

No. of

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Peak position

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each processing step.

1582

2716 1353

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ACCEPTED MANUSCRIPT The sharp G peak, corresponding to the E2g symmetry one-phonon mode of the sp2 carbon atoms at Γ, was centered at a statistical average of 1584 cm -1, its position varying between 1582 cm-1 and 1585 cm-1. The 2D band, showing a more complex shape generated by a second-order two-phonon scattering, was centered in average at 2717 cm-1, within the 2712 cm-1

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and 2720 cm-1 interval. The D band, that is a defect-activated band corresponding to the K-point

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appeared very rarely and of very low-amplitude in our samples.

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phonons of A1g symmetry (breathing mode of the benzene rings), was centered at 1363 cm-1 and

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(a)

Fig. 1 Comparison of structural and surface morphology characterizations for the 3D graphene: (a) Typical Raman spectra of: (bottom) as grown 3D graphene on nickel (GF-Ni); (middle) UV 10

ACCEPTED MANUSCRIPT ozone treated 3D graphene on nickel; (top) self-sustained 3D-graphene (GF); (b) corresponding SEM images for (bottom) GF-Ni and (top) GF.

The data presented in Table 1 indicate that the as-grown graphene (GF-Ni) layer presents

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the characteristics of a high-quality sp2 material, devoid of defects such as grain discontinuities

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or other structural disorder. Factors supporting this conclusion are the small ID/IG intensity ratio,

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low FWHM of the G peak (close to 20 cm-1), as well as its low separation to the 1580 cm-1 ideal value [38]. Interestingly, the results also indicate that these quality characteristics persisted well

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after the various processing and deposition steps performed on the as-grown graphene. Even

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though these processes give rise to statistically increasing D peaks - indicating the emergence of some degree of structural defects, especially after the UV ozone treatment, overall their intensity

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remains under 5% from that of the G peaks. This physicochemical stability is a very important

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feature for any type of electrode application.

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According to Ferrari’s studies [39], the shape and amplitude of the 2D peak provide reliable information on the number of layers in few-layer graphene, as well as on its stacking

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type. Hence, we used automated Levenberg-Marquardt least-squares fitting [40] of two or four

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Lorentzians to our 2D peaks to estimate the number of layers in each sampling point measured. To enhance the estimation granularity for the number of layers, we first classified the 2D peaks, based on their shapes and relative amplitude, in the 1, 2, 5, 10, >10-layers category, as previously described [39] and then used a parametric regression (not reported here) based on the averaged position of the two central Lorentzian components (2D1A, 2D2A), finally obtaining a good logarithmic fit. As shown in the last column of Table 1, the statistical average of the number of graphene layers estimated in this way shows slight increases from the as-grown graphene to the 11

ACCEPTED MANUSCRIPT post-treatment films. We could explain this effect by the possible changes in the Raman response caused by increased film folding. A significant difference was observed in the case of GF specimens, with an estimated doubling of the number of layers to the initial GF-Ni state. This could be statistically affected by the contributions from locally touching walls of the GF samples

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after the removal of the metallic scaffold. We also note that, based on the decomposable shapes

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of their Raman 2D peaks, our graphene films seem to present predominant and stable AB

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(Bernal) stacking [39]. The fact that our 60 minutes CVD growth leads to a few-layer graphene film makes the structure well suited for electrode applications where the Ni support needs to be

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etched away, as the developed mechanical robustness allows the graphene network to keep its

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shape and properties very well.

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As already mentioned, some as-grown graphene samples were also used to obtain graphene-ZnO hybrids by completely removing the Ni catalyst and thereby obtaining a 3D self-

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sustained composite. This structure has the advantage to result in light foam-shaped functional

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electrodes; however, it requires the additional step of removing the Ni scaffold to get selfsustained graphene while keeping the quality of the sp2 structure and the original porous network

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configuration, as well as minimizing additional contamination. To this aim, we employed a nitric

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acid etching procedure (see Experimental Section). As illustrated in Fig. 1b-top, our 60 minutes CVD-grown few-layer graphene film was able to maintain its foam structure very well, without the need of using any polymer support in the etching process. Avoiding the provisional PMMA reinforcement of the GF-Ni film during acid treatment significantly promotes the chemical purity of the final free-standing graphene foam. Typical Raman spectrum for the resulted GF film (Fig. 1a, top), as well as the extracted Raman data statistics (GF data in Table 1), indicate a very good quality, defect-free, few-layer graphene with no additional doping. We also performed FTIR 12

ACCEPTED MANUSCRIPT analysis to verify the chemical purity of the graphene shell, and only the functional groups characteristic to asymmetric and symmetric C-H stretching bands (2920 and 2849 cm-1) and the C=C vibration of the benzene ring (~ 1540 cm-1) were detected (Fig. S1). While the absence of defects and dangling bonds on the graphene basal plane is highly

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desirable for efficient electric transport, it also renders the difficulty of obtaining a uniform

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deposition of metal oxide significantly higher [33]. Effective ZnO deposition on the thermal

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CVD graphene film requires thus additional physicochemical pre-treatment, able to enhance the nucleation. As noticed in the Raman spectra, both the as-grown (GF-Ni) and the self-sustained

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(GF) 3D graphene networks are low in defect density which makes the surface of graphene inert

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to zinc nucleation thus requiring effective means for the development of reactive species on the sp2 carbon surface that are capable to react with the organometallic precursor. To create these

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nucleation sites, we applied a UV-ozone treatment before the ALD process. This enriches the GF

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and GF-Ni surfaces with reactive oxygen groups, thereby favoring the linkage of the zinc component to the graphene surface. The UV- ozone parameters were optimized for minimal

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damage to the graphene layer while still ensuring sufficient density of the oxygen groups on the

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surface to enhance ZnO deposition effectiveness. We reached this balance for UV-ozone exposure for 15 minutes at 60°C. It appears that this dry-oxidation process induced only a minor

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increase of the defect density. Indeed, the typical Raman spectrum for the oxidized GF-Ni samples (Fig. 1a, middle), as well as the statistics of the defect-related parameters (ID/IG ratio, position and FWHM of the G peak, see Table 1) confirm that the carbon layer keeps a very good crystalline graphene quality after the UV-ozone step. We also detected no visible development of the D' shoulder of the G peak (usually, at ~1620 cm-1), that originates from disorder induced

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ACCEPTED MANUSCRIPT intravalley resonance Raman scattering [41,42]. Thus, we conclude that the UV ozone treatment was soft enough for structural preservation. As noticed from Table 1 data, the UV ozone process causes several slight changes in the Raman response, all indicating concurrently a soft process that leads to the formation of covalent

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bonds between graphene and the oxygen atoms and the associated p-doping effect of

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oxygenation observed in several experiments [43,44,45]: increase of the ID/IG ratio (0.014 to

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0.042), decrease of I2D/IG (0.43 to 0.38), blue-shifts of the G band (1584 cm-1 to 1586 cm-1) and of the D band (2717 cm-1 to 2720 cm-1). slight blue shifts for the 2D and G bands, from 2717 cmand 1584 cm-1, to 2720 cm-1 and 1586 cm-1, respectively, as well as a slight decrease of the

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1

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I2D/IG ratio. According to Dong et al. [43], these trends corroborate towards a possible conclusion that the UV-ozone treatment causes a moderate p-doping of the material.

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The oxidized GF and GF-Ni samples were subjected to 200, or 1000 ALD cycles of

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alternate H2O/DEZ exposure. For further reference, these samples will be denoted as: ZnO[200]/GF(-Ni) and ZnO[1000]/GF-Ni, respectively. As subsequent Raman and FTIR

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spectroscopy has confirmed, the hydrophilic moieties introduced by the ozone treatment were

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low in density, however they were able to serve as active nucleation sites in the reaction with ALD precursors for effective ZnO growth. As observed in the FTIR spectrum (Fig. S1) typical

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functional groups introduced by the UV ozone treatment correspond to the asymmetric stretch of C-O-C (~1230 cm-1), C-O (~1084 cm-1), C=O (~1725 cm-1), – OH stretch (~3400 cm-1), indicating the creation of epoxy, alkoxy, carbonyl, carboxylic, and hydroxyl groups on the surface of graphene. Subsequent concurrent characterizations validated ZnO film coating of the graphene network. It seems that as low as 200 ALD cycles were sufficient for uniform coverage in case of the free-standing (GF) samples, while for the Ni-supported (GF-Ni) samples a longer 14

ACCEPTED MANUSCRIPT cycling is needed. ALD deposition on GF-Ni and GF without the UV ozone pre-treatment results in almost no material coverage except for the edges or several defects introduced on the graphene basal plane during the process of metallic scaffold etching. The grazing incidence X-ray diffraction (XRD) pattern recorded on the ZnO[200]/GF

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specimens (Fig. 2a) reveals the coexistence of polycrystalline zinc oxide with the underlying

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few-layers graphene film. The broad and equally intense reflections at 31.48° for the (100) plane, 34.41° for (002), 36.20° for (101), as well as the smaller 47.41° for (102) and 56.59° for (110)

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reflections, established the presence of ZnO with hexagonal Wurtzite structure and no preferential growth orientation. Using the Scherrer’s approximation for the FWHM of the ZnO

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(101) reflection peak, we obtain a 7 nm estimate for the crystallite size of the deposited zinc

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oxide layer after 200 ALD cycles. Regarding the carbon-related reflections, we notice a predominant peak at 2=26.48°, indicating an interlayer d002 spacing of 3.36 Å, as well as

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smaller contributions from (100), (101) and (004) planes. In the nitric acid etching process, the

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GF specimens developed into graphite nitrate intercalation compounds [46,47] of high-stage, due to the low concentration of the intercalate solution (i.e., HNO3). Hence, for our self-sustained

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samples, where only the most dominant (00n+1) intercalate reflection peak is clearly detectable

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at 2=25.31° (Fig. 4, *-marked peak), we can estimate a graphite nitrate intercalation stage between 5 and 6, corresponding to HNO3 intercalant gallery heights between 7.64 Å and 7.80 Å, respectively [47].

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(0 0 0 n+1) graphite nitrate

(a)

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Fig. 2 (a) XRD pattern and (b) Raman spectrum of ZnO[200]/GF. The inset shows the zoom-in

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Raman spectrum in the range of 300-1300 cm-1 to emphasize the vibration bands of ZnO film.

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The Raman spectroscopy analysis (Fig. 2b) confirms the same hybrid composition. As

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shown in the inset of Fig. 2b the ZnO vibration bands are detected at 438.5 cm-1 for the E2high acoustic mode due to oxygen sublattice, and at 574.7 cm-1 and ~1140-1151 cm-1 for the A1(LO)

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optical mode, and its 2A1(LO) overtone, respectively [48]. As expected from the estimated small

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size (~7 nm) of ZnO crystallites, the E2 phonon frequency is red-shifted from its 475 cm−1 position detected in case of bulk ZnO particles [49]. Additionally, as summarized in Table 1 for

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both GF-Ni and GF specimen types, after zinc oxide deposition there are concurrent red-shifts for both the G and 2D bands, which is likely an indication of the ZnO-C bond formation in the final hybrids [11]. Consistent with the Raman spectroscopy, the FTIR spectra presented in Figure S1, exhibit a shift of the C=O and C-O stretching bands concurrently with the band at ~ 1460 cm1

typically observed in the carboxylates groups complexed with Zn [50], as well as an increase in

intensity of the bands corresponding to the deformation peaks of carboxylic (-OH) and –OH 16

ACCEPTED MANUSCRIPT groups at 1403 cm-1 and 3400 cm-1, respectively [51]. Simultaneously, it can be observed the appearance of Zn-O stretching band at about 458 cm-1, suggesting the formation of ZnO structures and of the ZnO-GF band at 647 cm-1, which could be a direct indication of the ZnO-C bond formation with ZnO anchored on the GF surface [52].

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A SEM comparison of the surface morphologies for the self-standing graphene and for the Ni-supported graphene samples, after 200 ALD coating cycles reveals differences with

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respect to both coverage uniformity, and ZnO grain size. As shown in Fig. 3, ZnO[200]/GF

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samples typically present highly consistent coverage with larger (~30 nm average diameter) grains, while ZnO[200]/GF-Ni has a decreased and less uniform coverage with smaller grains (~10

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nm average diameter) anchored on the graphene surface. The physicochemical origin of this

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different behavior needs further investigations. In the absence of the UV-ozone treatment the zinc oxide deposition is almost absent on the surface of GF-Ni except for the materials

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singularities such as crystallite edges or wrinkles while the density of the ZnO on the GF is lower

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in comparison with those pre-treated in UV-ozone (Fig. S2).

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Fig. 3 SEM images of the ZnO-graphene hybrid film surfaces, after 200 DEZ-H2O ALD cycles: (a) ZnO[200]/GF and (b) ZnO[200]/GF-Ni.

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Hence, it results that for the applications where the Ni scaffold is functionally essential,

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the complete ZnO coverage needs a larger number of ALD DEZ/H2O cycles. Fig. 4 presents the

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SEM analysis of Ni-supported graphene specimens after increasing the DEZ/H2O ALD cycling to 1000 cycles. We observe now a complete ZnO film coverage of the graphene underlayer. In

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this case, the film thickness was estimated from SEM micrographs to be 200 nm. At this

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thickness, the surface morphology of the ZnO already presents coalesced grains of platelets

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appearance crystallites, occasionally generating flower-like nanostructures.

Fig. 4 SEM images at various levels of details for the ZnO[1000]/GF-Ni hybrids film surfaces. XRD and Raman characterizations of these higher mass depositions are presented in Fig. 5. The XRD pattern (Fig. 5a) indicates the superposition of the three materials, confirming the 18

ACCEPTED MANUSCRIPT Bragg peaks for the ZnO hexagonal Wurtzite structure - without angular position shifts when compared with the database zincite card number 900-418 - and random growth orientation of the

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ZnO crystallites. Using again Scherrer's approximation for the (101) peak, the mean crystallite

size of the ZnO was estimated to be around 25 nm. The graphene film has a predominant (002)

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orientation with a calculated d002 interlayer distance of 0.34 nm. In accordance with the XRD

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data, the room temperature Raman spectrum presented in Fig. 5b contains clearly evidenced modes at 442.5 cm-1 (E2high), 575.8 cm-1 A1(LO) and a small 2A1(LO) overtone at ~1250cm-1,

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specific to the ZnO Wurtzite hexagonal phase.

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Fig.5(a) XRD pattern and (b) Raman spectra of 3D graphene on Ni foam after 1000 ALD cycles (ZnO[1000]/GF-Ni)

The main electrical properties of the graphene-based composites were evaluated using the four-point van der Pauw method, coupled with foam conductivity models. Electrochemical measurements were beyond the scope of this analysis. We measured the lateral conductivity of the free-standing foam samples (supported on a SiO2/Si substrate for more stable electrical 19

ACCEPTED MANUSCRIPT contacting) both before (GF), and after ZnO coating (ZnO[200]/GF). The effective conductivities of these networks, analyzed at 5 mA source current, were found to be ~30.8 S/cm for GF samples, while for ZnO[200]/GF samples higher conductivities, of ~35.4 S/cm, were obtained. These different but close values primarily confirm the good adhesion, and attest a good electrical

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contact, between the two layers. If we consider the very large porosity of the structure we

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deduce that these levels of effective conductivity are high. Indeed, as a term of comparison, for

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our ~98% porous foams, the bare Ni foam samples measured 8.26  102 S/cm, which is more

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than 2 orders of magnitude lower than for bulk Ni (1.43  105 S/cm). The high conductivities obtained for the foam structures reflect the very good continuity and uniformity of the low-defect

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graphene shell, confirming also in this way the SEM and Raman measurements that are possible only at the sample accessible surfaces. We assessed also the conductivity of the ZnO deposited

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material, by growing ZnO films on SiO2/Si substrate using the same ALD process parameters

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and number of cycles (~40 nm ZnO layer for 200 DEZ/H2O cycles), resulting in 0.12 S/cm.

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Despite of this low value, the addition of the ZnO coating causes an effective conductivity increase for the graphene network - from ~30.8 S/cm to ~35.4 S/cm – most

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probably due to the large total surface area of the foam walls, that leads to a slight decrease of

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the foam porosity corresponding to the additional filling of the pores with the ZnO material. Various conductivity calculation models are available for open-cell foams, that relate the effectively measured value to the bulk material conductivity [53]. Finding that the Liu approximate models for [54,55] with two model parameters - of the form eff = bulk  (1-s)t, where  is the porosity - both gave very close approximations when tested against the bare Ni foam data (98% porosity), we assumed that they fit well our foam morphology. Next, using these 20

ACCEPTED MANUSCRIPT models and the 30.8 S/cm value measured for the graphene network we could calculate the “bulk” lateral conductivity of the graphene shell, that resulted to be ~ 4.7  103 S/cm which is almost comparable to that of the HOPG. Through the same translation models [54,55], using this bulk value and the 35.4 S/cm conductivity of the ZnO[200]/GF network leads to a porosity

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reduced to 97.7%; this means that the filling degree due to the ZnO coating is at least this shift to

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the initial estimate of 98%. The small porosity decrease is, in fact, the price coming with the

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advantage that the functional ZnO layer does not affect the effective conduction of the

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composite.

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4. CONCLUSIONS

In summary, this study demonstrates that macroporous monoliths of ZnO-graphene hybrids can

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be easily developed by controlled deposition of a crystalline zinc oxide film on 3D graphene

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network. Structural characterization revealed the formation of defect-free few-layer graphene which maintains its integrity even after different processing steps, such as etching the nickel

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scaffold and dry-oxidation employed to introduce the nucleation sites needed for efficient ZnO

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deposition. The conformal and continuous coating of the zinc oxide layer on the surface of the few-layer graphene film was achieved using ALD process. If in the case of GF, 200 ALD cycles

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are enough to fully cover the graphene surface a larger number of cycles are necessary for GF-Ni specimens. The morphology and crystallite size of the ZnO film seems to depend on the number of ALD-cycles employed, whereas the ZnO crystallites have no preferential growth orientation. With increasing the number of ALD cycles the morphology of the ZnO changes from nanoparticles to coalesced grains of platelets like crystallites with few flower-like nanostructures. Electrical characterizations confirmed the low defect degree of the graphene network shell by 21

ACCEPTED MANUSCRIPT revealing high conductivity values for the self-supported graphene and, importantly, showed that the ZnO layer does not affect the effective conduction of the composite. The proposed strategy provides a method for the fabrication of both ZnO-GF and ZnO-GF-Ni electrodes which can be easily incorporated in various applications. Their enhanced specific surface area offers the

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benefit of obtaining large electrode/electrolyte contact areas for charge-transfer reactions; of

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holding promise for energy conversion and storage applications.

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shortening the Li-ion diffusion lengths, as well as of increasing the active material loading,

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ACKNOWLEDGMENTS: The authors wish to thank Dr. Florin Comanescu for acquiring and preprocessing the micro-Raman raw data. This work was supported by a grant of the Romanian

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National Authority for Scientific Research and Innovation, CNCS – UEFISCDI, project number

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- PN-III-P2-2.1-PED-2016-1159 (129PED/2017).

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

Synthesis of few layers graphene on macroporous nickel foam using CVD method.

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Graphene shell fully preserved its structure after chemical etching of the nickel.

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Graphene hydrophobicity adjustment using UV-ozone treatment prior to ALD coating.

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Uniform and crystalline ZnO film was deposited on the graphene surface using ALD.

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Excellent electrical conductivity and interlayer electrical contacts of the hybrids.

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