Electrochemically assisted deposition of sol–gel films on graphene nanosheets

Journal Pre-proofs Electrochemically assisted deposition of sol–gel films on graphene nanosheets Lu Fang, Qing-Qing He, Ming-Jie Zhou, Ji-Peng Zhao, Ji-Ming Hu PII: DOI: Reference:

S1388-2481(19)30272-3 https://doi.org/10.1016/j.elecom.2019.106609 ELECOM 106609

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Electrochemistry Communications

Received Date: Revised Date: Accepted Date:

11 August 2019 10 November 2019 11 November 2019

Please cite this article as: L. Fang, Q-Q. He, M-J. Zhou, J-P. Zhao, J-M. Hu, Electrochemically assisted deposition of sol–gel films on graphene nanosheets, Electrochemistry Communications (2019), doi: https://doi.org/10.1016/ j.elecom.2019.106609

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Electrochemically assisted deposition of sol–gel films on graphene nanosheets Lu Fang, Qing-Qing He, Ming-Jie Zhou, Ji-Peng Zhao and Ji-Ming Hu* Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China

*Corresponding author. E-mail: [email protected]. Fax: +86-57187951895.

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ABSTRACT: Due to physical collisions and electron transfer between the working electrode and conductive nano-materials, electrochemical reactions can occur on the surface of both. Here, we explore the use of an electrochemically assisted sol–gel deposition technique (EAT) to modify the surface of nanomaterials. Taking graphene as an example, the fabricated freestanding graphene–silica nanosheet composite exhibits a sandwich structure, with mesoporous silica films covering both sides of the graphene. The thickness of the nanocomposites can be tuned (up to ~50 nm) by controlling the potential and duration of the deposition, and the concentration of the sol–gel precursor. The approach ensures, uniquely, that the silica films are securely attached to the graphene substrate. This indirect deposition technique could also be used for the surface modification of other nanomaterials.

KEYWORDS: Electrodeposition; collision; silica; graphene; sol–gel

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1. Introduction Graphene (Gr) is one of the most well-studied nanomaterials of the 21st century. It is an intriguing material with many particular characteristics and has been widely used in the areas of sensors, electronics, field effect transistors, clean energy devices, and so on [1,2]. The functionalization and surface modification of sheets of Gr (or its oxide, GO) are of crucial importance for a range of applications. Such modifications enable graphene to be processed by solvent-assisted techniques and prevent the agglomeration of single-layer graphene [3,4]. Much work has been done on the surface functionalization of these materials with inorganic oxides [5,6], organic molecules [7,8], metal nanoparticles [9], and others, by both covalent [10,11] and non-covalent modification techniques [12,13]. As graphene has a smooth surface and hardly any functional groups, it is difficult to attach inorganic oxides to this material by either a chemical or a physical route [14]. For this reason GO, which has hydroxyl, epoxide, diol, ketone, and carboxyl functional groups, is more frequently used as the substrate for further modification [15]. Oxides can be attached to the GO surface either with the aid of organosilane pretreatments or directly by a hydrothermal method at high temperatures [16-17]. However, these methods are time consuming and require the use of chemicals. Moreover, the resulting oxide layer has limited thickness because of the lack of a continuous driving force for growth of the oxide layer. In addition, the oxide layer has poor adhesion to the GO substrate. The electrochemically assisted sol–gel technique is commonly used to modify bulk conductive materials with silane or oxide films [18-20]. It utilizes the OH- (or H+) produced by the electrode reaction to catalyze the condensation polymerization of silane into a film. However, it is very hard to process free-standing nanoparticles, like graphene nanosheets, by electrodeposition because the electrons cannot pass easily and stably between the power supply and the graphene

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nanoparticles. Some previous works have reported that graphene nanosheets can be first attached to a conductive substrate by vapor deposition, spin coating, electrodeposition, etc., and the graphene nanoparticle-covered substrate then serves as a working electrode for the further electrodeposition of sol–gel films [21-23]. However this method is complicated and can only be conducted on a graphene-attached electrode. To date, free-standing graphene nanosheets with deposited oxide layers have not yet been prepared by electrodeposition. Here, we explore a new electrochemically assisted sol–gel approach to prepare graphene– silica free-standing nanosheets. The resulting nanosheet composite has a sandwich structure and silica films with a thickness of tens of nanometers cover both sides of the graphene sheet. In this method, graphene nanosheets disperse in the sol without any further processing. The deposition of the silica film occurs due to collisions between graphene nanosheets and the electrode. In these circumstances, there is electron transport from the inert platinum electrode to the graphene surface, and the cathode charge transfer process results in localized alkalization in the electrolyte near the surface of graphene, inducing the local in situ deposition of silica sol–gel film. A platinum foil was selected as the working electrode because it is inert and prevents graphene– silica nanocomposites from sticking to the electrode. A schematic illustration of the electroassisted preparation of silica (eSiO2) films on free-standing graphene nanosheets is depicted in Fig. 1. The graphene–silica nanoparticles prepared by this method have many excellent properties, including a high specific surface area, tunable thickness and good adhesion between the silica film and the graphene substrate. The latter characteristic (good adhesion) is particularly attractive, and can be attributed to use of the electro-assisted technique. 2. Materials and methods

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2.1 Materials High quality graphene and graphene oxide were purchased from XFNANO (Nanjing XFNANO Technology Co., Ltd., China). Hexadecyl trimethyl ammonium bromide (CTAB, 99%), tetramethoxysilane (TMOS, 99%) and tetraethyl orthosilicate (TEOS, 99%) were obtained from Aladdin. 2.2 Electrosynthesis of Gr@eSiO2 nanocomposites The electrodeposition process was carried out in a three-electrode system using a CHI 630D potentiostat (Chenhua Co., Ltd., China). The electrolyte was a sol solution containing an aqueous solution of 50 mL 0.2 M sodium nitrate (NaNO3), 50 mL ethanol, and 5.22 g CTAB with pH ca. 4.0. Graphene (10 mg, ca. 0.1 mg/mL) was ultrasonicated in the above solution for at least 3 hours to produce a stable dispersion, then 5–10 mL TMOS was added and the solution sonicated for an additional several minutes. Electrodeposition was carried out by chronoamperometry under stirring. The working electrode was a platinum (Pt) foil electrode (40 cm2). The reference electrode and counter electrode were an Ag/AgCl electrode (with saturated potassium chloride) and a platinum foil electrode, respectively. Potentials in the range -700 to -1100 mV (vs. Ag/AgCl) were applied to the working electrode while the electrodeposition time was varied between 10 and 40 min. Thereafter, the electrolyte was centrifuged, followed by repeated washing with ethanol. Template extraction was conducted in ethanol/HCl mixture (24 mL HCl (37%)/500 mL ethanol) for 6 hours in a water bath at 60 oC under stirring. After centrifuging and repeated washing, the resulting graphene–silica composite (designated Gr@eSiO2) was dried at 80 oC. As a control experiment, graphene was dispersed in the sol solution for 20 min without applying any potential. Pure silica nanoparticles were also prepared by electrodeposition. The process was the same as for Gr@eSiO2, with an applied potential of -900 mV for 20 minutes,

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except that no graphene was added to the sol. The resulting nanoparticles are referred to as “eSiO2”. 2.3 Characterization The Gr@eSiO2 composite nanoparticles were characterized using Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, Thermo Scientific, USA), Raman spectroscopy (Lab.HR800, Jobin Yvon, France), scanning electron microscopy (SEM, SU 8010, Hitachi, Japan), transmission electron microscopy (TEM, HT-7700, Hitachi, Japan), high-resolution transmission electron microscopy (HRTEM, JEM 2100F, JEOL, Japan), atomic force microscopy (AFM, Multimode 8, Bruker, Germany), and thermogravimetric analysis (TGA, STA409PC, METZSCH, Germany). The nitrogen adsorption–desorption test was also performed (ASAP 2020, Micromeritics, USA). The cross-section of the Gr@eSiO2 nanosheets was observed by scanning transmission electron microscopy (STEM, JEM 2100F, JEOL, Japan). 3. Results and Discussion Figure 2a shows the FTIR spectra of Gr (1), eSiO2 (2) and Gr@eSiO2 (3) nanoparticles. Gr hardly has any infrared absorption because of the presence of symmetrical C–C bonding and shared π bonding [24, 25]. Gr@eSiO2 and eSiO2 have similar peaks, i.e. the Si–O–Si bending vibration at 467 cm-1, the Si–OH stretching at 798 cm-1 and the Si–O–Si stretching at 1087 cm-1, indicating the successful generation of SiO2. The Raman spectrum of graphene has three characteristic peaks: the G peak at 1580 cm-1, Gʹ peak at 2700 cm-1 and D peak at 1350 cm-1. The G peak is caused by the in-plane optical vibration and the Gʹ peak corresponds to second-order zone boundary phonons [26,27]. The D peak, caused by first-order zone boundary phonons, is present only in defective graphene. Figure 2b shows that the original graphene hardly has any defects as it has a small D peak. After deposition of silica, the Raman spectrum does not change

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much as silica has weak Raman signals with strong polarity. The intensity of the D peak (ID) and the value of ID/IG (the intensity ratio of D and G peaks), a measure of the amount of graphene defects, are both little changed after deposition of the silica film (ID/IG: 0.055 to 0.077), indicating that deposition of silica does not affect the intrinsic structure of the graphene substrate. The SEM images show that the silica particles are deposited uniformly on the graphene nanosheets and eventually produce a very rough structure (Figs. 3c and 3d), in contrast with the smooth surface morphology of the bare graphene nanosheets (Figs. 3a and 3b). The thickness of the Gr@eSiO2 composite nanosheets prepared at -900 mV for 20 min is ca. 35 nm, and the silica film consists of silica nanospheres with a diameter of ~10 nm. There is no aggregation of graphene nanosheets after electrodeposition of the silica layer. After removing the silica layer by immersion in NaOH solution for 30 min, the surface smoothness can be recovered and the nanosheets hardly ever aggregate (Fig. 3e). The electrodeposition process provides strong adhesion between the silica deposit and the graphene sheet, such that the deposit still sticks firmly to the graphene surface after ultrasonication for 2 hours (Fig. 3f). Unattached silica nanoparticles are not observed by SEM. Although we are still unclear why there is such good adhesion between graphene and the silica films, use of the electrodeposition technique must be a key reason. Usually it is very difficult to prepare a sol–gel film thicker than 1 m without it cracking, powdering and peeling off when using dip-coating, spray-coating or spin-coating techniques [28]. However, much thicker (up to 10 m [29]), more porous and rougher sol–gel films which do not powder or peel off can be prepared using the electro-assisted technique because the OH − ions produced in solution near the electrode surface when applying negative potentials will promote the sol–gel process, and also because of the separation of the gelation process from solvent evaporation during electrodeposition [20].

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The hydrothermal method is one of the most common ways of modifying graphene-like nanoparticles, and is often used to fabricate oxide-covered GO nanosheets [16,17]. For the purpose of comparison, silica-modified GO (GO@hSiO2) was prepared by a hydrothermal method (Supplementary data, Fig. S1a). A larger amount of silica nanoparticles was found in unattached form. This may be related to the alkaline nature of the whole solution during the synthesis; under these conditions the bulk sol solution is unstable, facilitating the sol polycondensation reaction and the precipitation of particles into the solution rather than their deposition on the GO nanosheets. The more localized alkalization in electrodeposition ensures the stability of the bulk solution, restricting the production of unattached silica nanoparticles. Although the existence of functional groups on the GO surface ensures a certain chemical interaction effect, resulting in the deposition of silica films of nearly identical thickness (~30 nm) as Gr@eSiO2, their adhesion is very poor as the films detach after sonication (Fig. S1b). It is not surprising that a silica film cannot be deposited on the graphene substrate by the hydrothermal method (Fig. S2), because of the lack of active functional groups on the graphene surface. The TEM and HRTEM images (Figs. 4c, 4d and 4f) show that the Gr@eSiO2 has a disordered, polyporous structure. The element mapping of Gr@eSiO2 under HRTEM reveals the existence of both graphene and SiO2 components. STEM directly confirms the formation of a “sandwich structure” (Fig. S3). In contrast with the ordered porous structure of the silica film on GO@hSiO2 (see Fig. S1g), the disordered structure of the silica film in Gr@eSiO2 nanocomposites can be attributed to the absence of functional groups on the graphene surface, so the CTAB template fails to absorb perpendicularly on the surface. The mesoporous structure of Gr@eSiO2 is further confirmed by nitrogen adsorption/desorption measurements (Figs. 4g and 4h). Ordinary graphene has a type III adsorption/desorption isotherm. The adsorption and

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desorption branches do not coincide because the graphene spontaneously produces C some wrinkles and agglomerates to form pore structures. Analysis of pore size distribution (inset of Fig. 4g) shows an average pore width of 4.60 nm. The adsorption/desorption isotherm of Gr@eSiO2 exhibits a type IV N2 adsorption branch with a well-defined type H4 hysteresis loop O between the adsorption and desorption branches (ca. 0.4–1.0 P/P0) due to the narrow slit-shaped pores. The adsorption data also shows a high BET surface area of 362.9 m² g-1, far bigger than that of the original graphene nanoparticles (40 m² g-1). The specific surface area is lower than that of conventional mesoporous silica due to the pores having a larger size, resulting from the faster condensation process promoted by electrochemical propulsion. The XRD pattern further confirms the disordered structure of Gr@eSiO2 (Fig. S4). Thus, with properties including high porosity, a negligible amount of un-attached nanocomposite and strong adhesion, Gr@eSiO2 is a surprising material with potential applications in many fields. This new electrochemically assisted deposition sol–gel method may be extended to produce other graphene-based materials and even to modify other substrates with a wide range of films and particles. The negative shift of deposition potential promotes the generation of OH-, and thereby increases the local pH, accelerating the condensation process of the solution (Fig. S5). It is not surprising that the thickness of the film increases with increasing negative potential. Figure 5a shows a plot of the relation between the thickness of Gr@eSiO2 and the applied potential. The data were obtained by SEM measurements of an average of 40 individual images. Some representative SEM images are displayed in the Supplementary data (Fig. S6) and Fig. 5e. The plot shows a continual increase in the thickness, varying from ca. 26 nm to 48 nm, on tuning the potential from -700 to -1100 mV (vs. Ag/AgCl). This result highlights a major advantage of electrodeposition, which allows controllable deposition of silica onto the surface of graphene.

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The thickness was also controlled by varying the time of deposition and the concentration of TMOS in the solution. Fig. 5b shows an increase in the thickness varying from ca. 27.76 nm to 43.96 nm by tuning the time from 10 to 40 minutes under -900 mV, but the growth rate of the oxide layer decreases with deposition time. Figure 5c shows that the thickness increases from ca. 29.55 nm to 48.60 nm by tuning the volume of TMOS in the solution from 4.5 to 10 mL. Selective SEM images are also displayed in Fig. 5e and Figs. S7–8 (Supplementary data). Thermogravimetric analysis (TGA) gives consistent results, i.e. that the silica component in Gr@eSiO2 composites increases with negative shifts in the electrodeposition potential, increases in the deposition time and increases in the content of TMOS in the solution (Fig. 5d). TGA measurements clearly show that it is very difficult to deposit silica on graphene under open circuit potential, which is consistent with the SEM observation, further indicating that the electro-assisted technique plays an important role in synthesizing Gr@eSiO2 nanosheets. In addition, the presence of NaNO3 and template CTAB in the sol appears to promote the electrodeposition of silica on graphene (Fig. S9). Here, NaNO3 acts not only as a supporting electrolyte but also as the main source of OH- when applying a negative potential (via the reduction reaction: NO3- + H2O + 2e-  NO2- + 2OH-). The catalyzing effect of CTAB on sol– gel film growth during electrodeposition has already been described in the literature [30]. The current measured after applying -0.9 V for 20 min (Fig. S9d) clearly shows the positive role of these two components in hydroxyl ion generation. We are still not exactly clear about the mechanism of silica electrodeposition on graphene nanosheets. One can consider that the generation of a silica film is a result of direct cathodic reactions taking place at the graphene surface when conductively colliding with the Pt electrode. This “direct electrodeposition” mechanism explains the reduced growth rate of silica film with

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increasing deposition time (see Fig. 5b), since the conductive contact between the graphene nanosheets and the Pt electrode would be reduced along with the deposition of insulating silica film on the graphene surface. The direct electrodeposition mechanism also explains the good adherence of sol–gel film on the graphene surface, as this is commonly observed in electrodeposited silica film on large conductive substrates. Alternatively, one could also assume that the silica film is simply deposited on the graphene surface when the graphene nanosheets enter the vicinity of the Pt electrode surface, where the solution is alkalinized, and polycondensation reactions occur when negative potentials are applied to the Pt electrode. According to this “indirect electrodeposition” mechanism, the graphene nanosheets are physically embedded with silica nanoparticles, which become detached from the platinum surface during the electrodeposition. It would be expected that the sol–gel film formed via the “indirect electrodeposition” mechanism would be less firmly attached to the substrate, as is the case in the hydrothermal method. The “direct electrodeposition” mechanism is more likely to happen in the initial stage while the “indirect electrodeposition” mechanism may operate later. The successful collision of Gr nanosheets with conductive substrates has already been investigated and demonstrated by Harim Kwon et al., via the use of the ultra-microelectrode technique [31]. This supports the possibility of direct electrodeposition on the Gr substrate. Further investigations will be carried out in the future.

4. Conclusions Electrodeposition is a powerful approach for fabricating well-attached mesoporous silica sol–gel films on graphene nanosheets. The thickness of graphene–silica nanocomposites could reach ~50 nm, and could be tuned by controlling the deposition potential, deposition time and the

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concentration of sol–gel precursor. The electro-assisted mechanism ensures good adhesion between the sol–gel films and the graphene substrate. It is expected that this electro-assisted technique could be extended to other types of conductive nano-sized substrates for controllable construction of sol–gel films.

Acknowledgments This research was supported by Natural Science Foundation of Zhejiang Province (no. LZ17E010001), Natural Science Foundation of China (no. 51671174) and National Key R&D Program of China (2017YFB1200800). Appendix A. Supplementary data Supplementary data to this article can be found online at ….

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Figure captions

Fig. 1. A schematic illustration of the electrochemically assisted deposition of mesoporous silica films on graphene nanosheets. Fig. 2. (a) IR spectra and (b) Raman spectra of Gr (1), eSiO2 (2) and Gr@eSiO2 (3). Fig. 3. SEM images of (a–b) Gr and (c–d) Gr@eSiO2 nanosheets at different magnifications; (e) Gr@eSiO2 immersed in NaOH solution for 30 minutes (inset is partial enlargement); (f) Gr@eSiO2 after ultrasonic oscillation for 2 h. Fig. 4. TEM images of (a, b) Gr and (c, d) Gr@eSiO2 at different magnifications. (e, f) HRTEM images and elemental mapping of Gr@eSiO2. Nitrogen adsorption/desorption isotherm of (g) Gr and (h) Gr@eSiO2 (inset is pore size attribution). Fig. 5. The thickness of Gr@eSiO2 nanosheets obtained at different (a) deposition potentials, (b) deposition times and (c) TMOS concentrations. (d) TGA curves and (e) selected SEM images of Gr@eSiO2 nanosheets prepared under different conditions. The deposition potential, time and the volume of TMOS in the sol is also labelled.

Highlights

·Silica films are electrodeposited on free-standing graphene nanosheets. ·The thickness of the silica film can be tuned by controlling the electrodeposition processes. ·The adhesion between the graphene nanosheets and silica film is very strong.

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G

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Fig. 5.

Declaration of interests

☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Author Contribution Statement

Lu Fang: Investigation, Data curation, Writing- Original draft preparation. Qing-Qing He: Writing- Reviewing and Editing. Ming-Jie Zhou: Data curation. Ji-Peng Zhao: Methodology, Investigation. Ji-Ming Hu: Conceptualization, Funding acquisition, Writing- Reviewing and Editing.

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