Conductive Graphene–Melamine Sponge Prepared via Microwave

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24776−24783

Conductive Graphene−Melamine Sponge Prepared via Microwave Irradiation Wenlu Liu,†,‡ Haibin Jiang,‡ Yue Ru,‡ Xiaohong Zhang,‡ and Jinliang Qiao*,†,‡ †

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, China

‡

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S Supporting Information *

ABSTRACT: A conductive graphene−melamine sponge (MS) prepared via microwave irradiation is reported in this paper. Graphene oxide supported on the MS was prereduced first at 100 °C and then further reduced in a household microwave oven at over 1000 °C. It was surprising to find that graphene oxide on the MS was reduced perfectly while the three-dimensional structure of the MS was kept well after high-temperature reduction via microwave irradiation. Slight pyrolysis of MS was also found during 5 s microwave irradiation, resulting in nitrogen generation from the pyrolysis of the MS being doped into graphene, which could benefit the electric conductivity of the prepared graphene−MS. The electric conductivity of the prepared graphene−MS is about 0.12−1.0 S/m because of the high reduction degree of graphene oxide and nitrogen doping. On the other hand, different from the pure MS, the newly developed conductive graphene−MS possesses superhydrophobic and superoleophilic properties. Overall, the newly developed conductive graphene−MS contained 94.3 wt % MS and 5.7 wt % N-doped graphene and is a cost-effective material with good elasticity, high conductivity, superhydrophobicity, and superoleophilicity. KEYWORDS: nitrogen-doping, graphene−melamine sponge, microwave reduction, conductivity, superhydrophobic, oil absorption

1. INTRODUCTION Graphene, a two-dimensional (2D) monolayer of carbon atoms packed into a honeycomb lattice, has become one of the most exciting research topics in the last decade.1−4 The typically important properties of graphene are outstanding strength,5 high specific surface areas,6 high thermal conductivity,7 strong chemical durability,8 high electron mobility,9 and intrinsic hydrophobicity.10 Such unique properties qualify graphene as a promising source to fabricate high-performance materials for high-end applications, such as hydrogen storage,11,12 oil cleanup,13 and in high-performance electronics and sensors.14,15 However, the 2D graphene sheets need to assemble into threedimensional (3D) architectures in most of the applications to achieve better performance.16,17 Similar to 3D graphene materials from the 2D graphene sheets, the low-density graphene aerogel with elasticity and high electric conductivity has been studied widely. Cryodesiccation from aqueous graphene oxide (GO) suspensions can convert GO to highperformance graphene aerogels by using appropriate reducing methods.3,18,19 However, these costly processes and complex operations limit the application of graphene aerogels. Cheng et al. reported a 3D flexible and conductive interconnected graphene network grown by chemical vapor deposition.20 However, the demands of long time and strict operating conditions also hindered the large-scale production of such graphene networks. The graphene−polymer sponges as 3D porous graphene-based materials exhibit excellent elasticity, but © 2018 American Chemical Society

the relatively low reduction temperature limits the electric conductivity of the sponges.21−28 It is obvious that it remains a challenge to develop a simple and cost-effective route for the fabrication of 3D graphene foams with elasticity and high electric conductivity. As reported by Voiry et al., GO can be reduced into high-quality graphene by using 1−2 s long microwave pulses.29 However, to the best of our knowledge, microwave irradiation has not been used in the reduction of GO that is supported by the melamine sponge (MS). Herein, we report a new process for the preparation of a 3D graphene sponge by using microwave irradiation (M-GS). The prepared M-GS possesses elasticity, high electric conductivity, and excellent thermal stability. Meanwhile, it also exhibits superhydrophobicity and superoleophilicity.

2. EXPERIMENTAL SECTION 2.1. Materials and Equipment. The aqueous GO suspension was purchased from Nanjing JCNANO Technology Co., Ltd. MSs were supplied by BASF Applied Chemical Co., Ltd. The MS was cut into cubes (2 cm × 2 cm × 2 cm), washed by deionized water by sonicating for 15 min, and then dried in a vacuum oven at 50 °C. The household microwave oven was made by Galanz Microwave Oven and Electric Appliances Manufacturing Co., Ltd. Received: April 15, 2018 Accepted: July 3, 2018 Published: July 3, 2018 24776

DOI: 10.1021/acsami.8b06070 ACS Appl. Mater. Interfaces 2018, 10, 24776−24783

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the M-GS Fabrication Process

Figure 1. (a) XRD patterns of GO and M-GS and (b) Raman spectra of MS, GO, and M-GS.

decreases its microwave absorption capacity.32 As reported by Voiry et al., GO prereduced at 300 °C under argon can be heated up to several thousands of celsius in only few tens of milliseconds by a household microwave oven of 1000 W.29 The heating temperature of GO prereduced at 100 °C with a household microwave oven of 700 W under air should be lower than that of Voiry’s. Furthermore, the experimental results show that the C/ O ratio of our microwaved reduced graphene oxide (rGO) is 8.3, which is slightly larger than (8.1) the C/O ratio of rGO that supported on the MS and thermal annealing at 1000 °C under nitrogen. Therefore, the reduction temperature of the microwaved rGO was deduced to be over 1000 °C. The reduction of GO in the GO−MS foam was confirmed by X-ray powder diffraction (XRD) and Raman spectroscopy measurements. Figure 1a shows the XRD patterns of GO and M-GS. GO exhibits a distinct peak at 2θ = 9.81°, corresponding to the (001) interplanar spacing of 0.73 nm calculated from the 2θ value (d = λ/2 sin θ).33 After reduced by microwave irradiation, the XRD pattern of the M-GS presents a new diffraction peak at 26.30°, which is ascribed to the (002) reflection of graphite domains and corresponds to an interlayer spacing of 0.34 nm.19 The shift of the XRD peaks from 9.8° of GO to 26.30° of M-GS reveals the reduction of GO sheets and a high degree of graphitization of GO.34 This result is consistent with that reported in the literature.35 In the Raman spectra (Figure 1b), two obvious bands at 1346 and 1592 cm−1 appeared in both GO and M-GS. The band at 1346 cm−1 corresponding to the D-band originates from the structure defects. 36 The band at 1592 cm −1 corresponding to the G-band originates from the first-order scattering of the E2g photon of sp2 C atoms.37 The intensity ratio of D to G band for GO is 0.953 while that of M-GS decreases to 0.850, indicating the successful reduction of GO in M-GS. This result is different from most of the published results in the literature that the intensity ratio of D to G band usually increases after GO reduction,3,22,38−40 resulting from the re-establishment of sp2 carbon during the removal of oxygen groups from GO.19 It is clear that microwave irradiation can significantly reduce the defect formation during GO reduction. The instant high temperature caused by microwave irradiation contributes to the reduction of GO, but it could

2.2. Preparation of M-GS. First, the MS was put into the aqueous GO suspension of 1.0 mg/mL and squeezed five times to make sure that the MS was sufficiently wetted with the GO suspension, and then the watery GO−MS foam was heated in a blast oven at 100 °C for 3 h for prereduction, and the coating and drying processes were repeated twice. Then, the prereduced GO−MS foam was placed in a sealed quartz glass box filled with nitrogen and protected by a sealed polypropylene box. Finally, the whole box was heated in a household microwave oven at 700 W for 5 s. 2.3. Characterization. The morphology of M-GS was observed by using a scanning electron microscope (FEI Nova Nano SEM450). The static water contact angle and the oil contact angle were measured with an optical contact angle measuring device (Easy Drop, Germany KRUSS). Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere at a heating rate of 10 °C/min (PerkinElmer). Raman spectroscopy was performed on a LabRAM HR800 Raman microscope using a 532 nm laser beam as the probing light source. The element compositions in the samples were investigated by a X-ray photoelectron spectrometer (ESCALab 250, Thermo Fisher, 2009). The electric conductivity of M-GSs was tested by a Fluck 179C multimeter. X-ray diffraction (XRD) was conducted by using a PANanalytical XRD system with the source wavelength of 1.542 Å at room temperature. The energy-dispersive spectrometry (EDX) measurement was conducted by using EDAX Apollo XT (American EDAX Co., Ltd). Compression test was conducted on INSTRON 3300 at a compressive rate of 10 mm/min.

3. RESULTS AND DISCUSSION Scheme 1 illustrates the fabrication process of the M-GS. First, the MS was fully impregnated with aqueous GO suspension of 1.0 mg/mL to get a MS-supported GO (GO−MS) foam. Then, the GO−MS foam was prereduced in an oven at 100 °C for 3 h. The C/O ratio of GO in GO−MS increased to 4.7 from 3.3 of bare GO. Finally, the prereduced GO−MS foam was treated by microwave irradiation at 700 W under nitrogen atmosphere for 5 s. The prereduction of GO is necessary for microwave heat because low oxygen content can bring high conductivity and provide effective microwave heating.30−32 Large arcs were observed around the prereduced GO−MS foam during microwave irradiation, as expected (Video S1). The microwave absorption of GO strongly depends on its chemical composition and structure, and the increase of oxygen in GO remarkably 24777

DOI: 10.1021/acsami.8b06070 ACS Appl. Mater. Interfaces 2018, 10, 24776−24783

Research Article

ACS Applied Materials & Interfaces

Figure 2. SEM images of (a−c) original MS and (d−f) M-GS.

possibly damage the 3D structure of the MS. Unexpectedly, the 3D structure of the MS was kept well after high-temperature microwave irradiation. The microstructures of MS, GO−MS foam, and M-GS were observed by a scanning electron microscope. The scanning electron microscopy (SEM) images of the MS (Figure 2a−c) show an interconnected and porous 3D network structure with smooth skeletons and through-holes of 30−100 μm. As shown in Figure S1, the GO sheets are attached with the skeletons of MS and the 3D structure of MS is well inherited by the GO−MS foam. After microwave irradiation, the 3D porous structure of M-GS was kept very well and the pore size is constant with MS and GO−MS foam, indicating that the basic structures of MS were not damaged during the microwave irradiation process (Figure 2d). The high-magnification images (Figure 2e,f) further revealed that some of the rGO sheets are larger than the hole of the MS; therefore, only the skeleton surface of the MS can contact with GO and heated to pyrolysis by rGO during a very short time of microwave irradiation. In addition, it can be found from Video S2 that there are only 5 times of arcs appeared during microwave irradiation. The duration of each arc is 50−100 ms;29 thus, the total heating time of the microwave irradiation is 0.25−0.5 s. The intervals between the arcs for cooling were much longer than the heating time, and maximum time of the microwave irradiation involves the annealing procedure. It is also well known that MS is a thermal insulation material with microwave transparency. Therefore, it is reasonable that the 3D structure of the MS can be maintained after ultrafast microwave irradiation. The density of MS and GO−MS is 8.3 and 8.8 mg/cm3, respectively. However, the density of the corresponding M-GS is 5.8 mg/cm3, further implying the pyrolysis of MS as the volume of the sponge remains unchanged. The nitrogen element in the melamine resin is calculated to be about 50.9% according to the molecular formula and the nitrogen atoms could be released as gases N2 and NH3 during the microwave irradiation process.41 Nitrogen doping can dramatically enhance the electrical conductivity of GO sheets42 and the nitrogen generated from the pyrolysis of MS is believed to be doped into graphene during microwave irradiation, which is proved as follows. Therefore, the M-GS we have prepared is a nitrogen-doped (N-doped) 3D graphene− polymer sponge with high-quality rGO, which should have excellent elasticity and good conductivity. The elastic property of the M-GS was investigated by a series of compression tests. As shown in Figure 3a, the M-GS can

Figure 3. (a) Compression−recovery process showing that the M-GS recovers to its original shape after compression by 90%; (b) compressive stress−strain curves of M-GS at 60% strain; and (c) SEM image of M-GS after 100 cycles of the compression−release process.

completely recover to its original shape without mechanical failure after being compressed by 90%. The elasticity of M-GS is further investigated by the cyclic compression test. During the test, the M-GS sample was placed on compression plates without being glued or otherwise attached. Figure 3b displays the cyclic stress−strain curves of M-GS with a maximum strain of 60%. Strikingly, the unloading curve could almost return to the initial point even after 100 compression cycles, indicating the outstanding elasticity of M-GS. Importantly, the 3D interconnected structures are maintained without apparent damage, and the interaction between rGO and the substrate remained intact after 100 cycles of compression test (Figure 3c). The unique structure of M-GS combines the advantages of both polymer and graphene foams, which equips M-GS not only with excellent mechanical properties but also with high electrical conductivity, thus holding a great potential for many applications. MS is generally considered as an insulator, and the electric conductivity of M-GS is 0.122 S/m at its original shape, which is higher than that of the GO−MS foam (0.009 S/ 24778

DOI: 10.1021/acsami.8b06070 ACS Appl. Mater. Interfaces 2018, 10, 24776−24783

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ACS Applied Materials & Interfaces

Figure 4. (a) Photographs illustrating the brightness changes upon compression of M-GS; (b) electrical conductivity of M-GS as the function of compressive strain; and (c) stability of the electric conductivity change of M-GS at different compressive strains.

Figure 5. (a) TG curves of MS, GO−MS foam, and M-GS; (b) XPS wide-scan spectra of GO−MS foam and M-GS; and high-resolution N 1s XPS spectra of (c) GO−MS foam and (d) M-GS.

Figure 4b, the electric conductivity of M-GS reaches 0.97 S/m at the maximum strain of 90%, which is comparable with graphene aerogels.45,46 The gap in the skeletons of M-GS becomes narrow when the sponge is under compression, and more pathways for electron transport are created as there are more chances for graphene sheets to contact each other, resulting in greater electric conductivity.19 The cycling stability was also tested, and

m) and other polymer-based graphene materials reported before.43,44 The change in the conductivity of M-GS when it is compressed was further studied. As illustrated in Figure 4a, a simple circle is formed by a battery, a light-emitting diode lamp, and M-GS. The lamp gradually becomes brighter when M-GS is compressed (Video S3), indicating that the electric conductivity of M-GS gradually increases during compression. As shown in 24779

DOI: 10.1021/acsami.8b06070 ACS Appl. Mater. Interfaces 2018, 10, 24776−24783

Research Article

ACS Applied Materials & Interfaces

Figure 6. (a,b) Water contact angle of original MS and M-GS and (c,d) oil contact angle of M-GS and comparison between water droplet and oil droplet.

curve of the GO−MS foam was similar to that of the original MS, indicating that the simple loading of GO on the MS has no great impact on the thermal stability of the GO−MS foam. The M-GS is different from the MS and GO−MS foam, and its curve exhibits no mass loss at 370−400 °C, suggesting that the portions of the methylene bridges of MS should have been broken down during microwave irradiation. The M-GS maintained thermal stability until the temperature increased to 482 °C, which is much higher than that of the MS and GO−MS foam, indicating the excellent thermal stability of M-GS. The total weight loss of M-GS at 600 °C is slightly lower than that of the original MS, which is attributed to the loading of rGO in MGS. Therefore, the MS content in M-GS could be roughly calculated from the TGA curves as shown in Figure 5a. The results show that 94.3 wt % M-GS is MS and only 5.7 wt % is Ndoped graphene, which is similar to that of the GO−MS foam before high-temperature reduction; therefore, M-GS remains to be an elastomer-like material (Figure 5a). In order to verify the nitrogen doping (N-doping) on graphene during microwave irradiation, the X-ray photoelectron spectroscopy (XPS) was used to analyze the element compositions in GO−MS foam and M-GS. As shown in Figure 5b, the C 1s, N 1s, Na 1s, and O 1s signals can be observed in both GO−MS foam and M-GS in the wide-scan XPS spectra. From a curve deconvolution, the complex N 1s spectra of the GO−MS foam can be well fitted to two peaks with binding energies at 398.3 and 399.4 eV, which are assigned to −NH2 and −CN in MS.22,49 The binding energy of N 1s has undergone significant changes after microwave irradiation. As observed in Figure 5d, the N 1s spectra of M-GS was deconvoluted into four primary peaks of −NH2 (398.3), pyridinic N (398.9), −CN (399.45), and graphitic N (401.1 eV),50 indicating the successful doping of nitrogen atoms into graphene. In addition, the energydispersive spectrometer was also employed to measure the element contents of GO and rGO in M-GS. GO was obtained by freeze-drying the aqueous GO suspension. rGO was separated from M-GS by sonicating M-GS in ethanol for 30 min, and the mixture of separated rGO and MS skeletons was dried at room temperature. Then, the separated rGO was found by SEM for the EDX test. As shown in Figure S4 and Table S2, upon microwave irradiation, the nitrogen content in rGO was 11.78 wt %, whereas the nitrogen content of GO is zero. Meanwhile, the oxygen content decreased from 21.79 wt % in GO to 9.48 wt % in rGO. The results verified that N-doping and the removal of oxygen-containing groups in GO happened simultaneous during microwave irradiation. The newly developed conductive graphene−MS also possesses superhydrophobic and superoleophilic properties; in contrast, original MSs are both hydrophilic and lipophilic. As observed in Figure S5, MS absorbed water immediately after coming in contact with water and sank into the water bottom. In contrast, M-GS floated on water without sinking for over 2 h.

no obvious change occurred under different compressive strains (Figure 4c). The stable elasticity-dependent electrical conductivity makes M-GS a promising pressure-responsive sensor. The relationship between the concentration of aqueous GO (CGO) suspension and the electric conductivity of M-GS was studied. As demonstrated in Figure S2a,b, the loading amount of GO and the electric conductivity of M-GS are both positively correlated with CGO when the treatment time is fixed to 5 s. However, the obtained M-GS becomes inelastic when CGO > 1.0 mg/mL, while electric conductivity dropped sharply if shorter treatment time was used, although elasticity can be maintained. Therefore, the optimum CGO for preparing elastic and conductive M-GS is 1.0 mg/mL. The appropriate microwave treatment time is also investigated. The electric conductivity of M-GS increased along with the treatment time within 5 s. However, as the treatment time continues to increase, the time intervals among the arcs become shorter, leading to continuous high temperature and destruction of the internal 3D structure of the obtained sponge. As shown in Figure S3, fractures appear on the polymer framework and rGO tends to stack together when the treatment time is extended. The possible reason is that the microwave absorption is weak at the first 5 s while increases along with the treatment time, leading to continuous high temperature and decomposition of the polymer framework. As presented in Table S1, the weight of the sponge declined rapidly with the increasing treatment time, which is consistent with our conjecture. Therefore, the optimal concentration for preparing elastic and high-conductivity M-GS should be 1.0 mg/mL, and the treatment time should be 5 s. For comparison, the properties of graphene−MS prepared by conventional thermal annealing at 1000 °C (C-GS-1000) are also investigated. The electric conductivity of C-GS-1000 is 0.09 S/m at the original shape and 0.89 S/m when compressed at a strain of 90%, which are lower than that of M-GS. Moreover, C-GS-1000 cannot rebound when the pressure is removed. The possible reasons are as follows: (1) the electric conductivity of N-doped graphene is higher than that of graphene achieved by thermal annealing at 1000 °C and (2) the long-term heating process of conventional thermal annealing causes massive decomposition of the sponge and less carbon was retained in C-GS-1000 as the weight ratio after thermal annealing is 23.8% when compared with that of microwave irradiation (63.7%). It is fully proved that microwave irradiation is more advantageous than conventional thermal annealing in the preparation of graphene−polymer composites. To confirm that most of the MS could be maintained after the high-temperature treatment of microwave irradiation, TGA was conducted. Figure 5a shows the TGA curves of MS, GO−MS foam, and M-GS in nitrogen atmosphere. The TGA curves of MS have a rapid weight loss in the temperature range of 362− 400 °C, which can be ascribed to the breakdown of methylene bridges.47 Mass losses at higher temperatures are attributed to the thermal decomposition of the triazine ring.48 The TGA 24780

DOI: 10.1021/acsami.8b06070 ACS Appl. Mater. Interfaces 2018, 10, 24776−24783

Research Article

ACS Applied Materials & Interfaces

Figure 7. (a) Absorption capacity of M-GS for various oils and organic solvents and (b) recyclability of M-GS for n-hexane after 10 cycles. The absorption capacity after multiple cycles is normalized by the initial weight of the dried M-GS.

polymer-supported graphene sponge and N-doped graphene− MSs with good elasticity, high electric conductivity, superhydrophobicity, and superoleophilicity . The prepared N-doped graphene−MSs contained 94.3 wt % MS and 5.7 wt % N-doped graphene and also exhibit very high absorption capacities for oils and organic solvents up to 288 times of its own weight, excellent selectivity, and recyclability. Therefore, the newly developed graphene−MS is a promising candidate for the selective removal of organic pollutants from water and many other applications. This new process is also possible to be applied to prepare different types of 3D polymer-supported graphene materials.

Figure 6a,b shows that water droplet was completely adsorbed by the original MS, while remained immobile on the surface of M-GS, and the static water contact angles of original MS and MGS are 0° and 153.8°, respectively. As shown in Figure 6c,d, when a drop of pump oil was deposited on the surface of M-GS, it was absorbed completely by M-GS, and the oil contact angle of M-GS was 0°. For comparison, water droplet attained a nearspherical shape on the surface of M-GS. These results indicate that the M-GS possesses excellent superhydrophobic and superoleophilic surfaces.22 When a piece of M-GS was placed on the surface of pump oil− water mixtures, the pump oil (stained with Sudan red) was completely adsorbed by M-GS (Figure S6a, Video S4). Figure S6b and Video S5 show that M-GS can also effectively absorb high-density organic solvents, such as chloroform (stained with Sudan red). The results indicate that M-GS is a promising adsorbent material for selective removal of oils and organic pollutants with different densities from water. The efficiency of oil absorption can be referred to as weight gain, that is, wt %, defined as the weight of the absorbed substance per unit weight of dry M-GS.13 Several organic liquids were evaluated, including lubricate oil, pump oil, peanut oil, soybean oil, olive oil, chloroform, and n-hexane, which could usually pollute water resources. M-GS showed very high absorption capacity for these organic liquids. It can be seen from Figure 7a that M-GS could absorb these organic liquids from 96 to 288 times of its own weight. This is to say that less than 4.0 kg of M-GS can absorb 1 ton of chloroform, having a great potential in the applications of oil absorption. In fact, MGS shows much higher sorption ability than other reported sorption materials (Table S3).13,22,38,39,51,52 As shown in Figure S7, the oil absorption capacity of the N-doped M-GS was better than that of the N-free graphene−MS, which was prepared using 1.0 mg/mL aqueous GO suspension and ascorbic acid as a reducing agent. It is well-known that N-doping can enhance the interaction between graphene and the molecules of polar solvents;13 therefore, nitrogen doping can promote the oil adsorption capacity of graphene−MS. The recyclability of absorbents was very important for practical application.53,54 Hexane was utilized to explore the recyclability of M-GS. After 10 cycles of adsorption-squeezing tests, no significant fluctuation in the absorption capacity was observed (Figure 7b).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06070.

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SEM images of GO-MS foam and M-GS, effect of concentration of aqueous GO suspension on the loading amount of GO and electric conductivity of M-GS, selected regions of GO and reduced GO, and EDX spectra of GO and reduced GO, picture of MS and M-GS, video snapshots showing the sorption process of pump oil and chloroform, comparison of absorption capacities of N-free graphene−MS and N-doped graphene−MS, weight change of M-GS, elemental analysis of GO, and comparison of absorption capacities of various graphenebased materials (PDF) Large arcs observed around the prereduced GO−MS foam (AVI) Arcs appeared during microwave irradiation (AVI) Lamp brightens when M-GS is compressed (AVI) Adsorption of pump oil by M-GS (AVI) Effective absorption of high-density organic solvents by M-GS (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

4. CONCLUSIONS It has been proved that GO on polymer can be reduced perfectly while the polymer structure can be retained well after short time of microwave irradiation. On the basis of this new finding, a novel process has been developed to successfully prepare

ORCID

Jinliang Qiao: 0000-0002-2608-6223 Notes

The authors declare no competing financial interest. 24781

DOI: 10.1021/acsami.8b06070 ACS Appl. Mater. Interfaces 2018, 10, 24776−24783

Research Article

ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS This work was financially supported by the Ph.D. Programs Foundation of the SINOPEC Beijing Research Institute of Chemical Industry.

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DOI: 10.1021/acsami.8b06070 ACS Appl. Mater. Interfaces 2018, 10, 24776−24783