After sputtering of metal atoms onto GO, the GO surface was somewhat reduced due to the high energy of the sputtered metal atoms3. The X-ray photoelectron spectroscopy (XPS) spectrum19 and the cross-sectional field emission scanning electron microscopy (FE-SEM) image of the GO paper without metal deposition are shown in Fig. S1. Although sputtered metal atoms mainly reduced the epoxy groups of the GO paper, the amount of carbonyl groups hardly changed. Cross-sectional SEM images of the GO paper showed that its thickness was about 20 μm and GO sheets were piled up in parallel without cracks. Simultaneously, the GO surface composition changed depending on time and the type of metal used (Figs. 1,2,3,4,5). The remarkable phenomenon observed here is that permeation of metals occurred from M/GO interface into the GO paper bulk at room temperature at a rate depending on humidity.

Figure 1 Time dependences of XPS spectra, surface content of Cu and depth profiles of Cu in the Cu(17 nm)/GO sample. (a) Time dependences of XPS spectra of Cu2p 3/2 at the Cu/GO interface under 30% RH and vacuum. Metallic Cu changed readily to Cu2+ (CuO) under 30% RH but hardly did so under vacuum. (b) Time dependences of XPS spectra of C1s at the Cu/GO interface under 30% RH and vacuum. O in O/(C + O) corresponds to the total amount of O in the oxygenated functions of GO. Weak reduction of GO and formation of Cu(COO) 2 proceed, but oxidation begins after 12 h. (c) Cu content at the Cu/GO interface as a function of time under humid conditions and vacuum. The Cu content drastically decreased under 60% and 90% RH. (d) Depth profiles of the Cu content at the Cu/GO interface at various times under 30% RH and vacuum. Cu has clearly permeated into the GO bulk. Full size image

Figure 2 Cross-sectional SEM images and EPMA mappings of Cu and C. (a) As-deposited sample, sample kept at (b) 30% RH for 1 h, (c) 90% RH for 10 min and (d) 90% RH for 1 h.Scale bars denote the concentrations of Cu and C. Full size image

Figure 3 Surface content and depth profiles of Ag in the Ag(17 nm)/GO sample. (a) Ag content at the Ag/GO interface as a function of time under 30% RH and vacuum. The Ag content markedly decreased with time under 30% RH, but barely decreased under vacuum. (b) Depth profiles of the Ag content at the Ag/GO interface at various times under 30% RH and vacuum. Ag has clearly permeated the GO bulk. Full size image

Figure 4 Time dependences of XPS spectra, surface content of Ni and depth profiles of Ni in the Ni(17 nm)/GO sample. (a) Time dependences of XPS spectra of Ni2p 3/2 at the Ni/GO interface under 30% RH. Ni metal changed to Ni2+ (NiO) under 30% RH immediately after sputtering. (b) Ni content at the Ni/GO interface as a function of time under humid conditions. Ni content drastically decreased with time under 60% and 90% RH, compared with that under 30% RH. (c) Depth profiles of the Ni content at the Ni/GO interface at various times under 30% RH. After 30 min, Ni permeated the GO bulk. Full size image

Figure 5 Surface content and depth profiles of Au and Pt in the Au(17 nm)/GO and Pt(17 nm)/GO samples, respectively. (a) Au and Pt content at the Au/GO and Pt/GO interfaces, respectively, as a function of time under humid conditions and vacuum. The Au content decreased with time under humid conditions, but barely did so under vacuum; in contrast, the Pt content decreased under vacuum but barely did so under 90% RH. (b) Depth profiles of the Au content at the Au/GO interface at various times under 30% RH. Au permeated the GO paper from the surface to around 200 nm into the GO bulk after 7 days. (c) Depth profiles of the Pt content at the Pt/GO interface at various times under 90% RH and vacuum. Pt permeated into the GO bulk after several days under vacuum. Full size image

Figure 1 shows the time dependences of XPS spectra for (a) Cu2p 3/2 and (b) C1s binding energies and (c) the Cu content at the Cu(17 nm)/GO interface under humid conditions and vacuum at 25°C (in Fig. 1 (b), the XPS spectrum of GO before sputtering is also shown). Note that Cu2+ was produced from Cu metal under humid conditions and that the Cu content at the interface decreased from around 30% to 6% at <30% relative humidity (RH) and to 0% at ≥60% RH (after 24 h, Fig. 1 (c)). These results imply that Cu changed to Cu2+ at the Cu/GO interface (or the GO surface) and then immediately moved into the GO paper bulk under humid conditions. The permeation rate was rather high under high humidity (60% and 90% RH), but low under low humidity. Thus, the presence of vapor is important for fast permeation. According to the XPS analysis of Cu2p 3/2 , Cu2+ production was limited under vacuum (Figs. 1 (a) and S2 (a)).

Figure 1 (d) shows depth profiles of the Cu content in the region from the Cu/GO interface to the GO paper bulk under 30% RH and vacuum. In all depth profiles presented here, the GO surface contains a certain amount of Cu because a signal corresponding to Cu is observed at a certain depth even at 0 h. Therefore, the distribution of Cu in depth after a certain time should be evaluated based on comparison with that at 0 h. The decrease of Cu content with time at the surface region (<around 80 nm) suggests fast Cu2+ permeation into the bulk. The permeation rate is estimated to be >7 nm/min under 30% RH (>200 nm/30 min, as shown in Fig. 1 (d)). The low content (<5%) of Cu in the region from the surface to a depth of 200 nm in the present Cu/GO sample after 24 h suggested that all of the coated Cu had permeated deeply into the GO bulk. Furthermore, according to the depth profiles (Fig. 1 (d)) and the decrease in Cu surface content from around 30% to 21.5% under vacuum (Fig. 1 (c)), it is possible that Cu also moves as atoms into the GO bulk, although the permeation rate is rather low. The mechanism of the atom diffusion will be similar to those of Au and Pt as described in the later section.

Figure S3 shows XPS spectra of Cu2p 3/2 and C1s of copper(II) oxalate hemihydrate (Cu(COO) 2 ·0.5H 2 O). According to the C1s spectrum of Cu(COO) 2 , the peak originating in the carboxyl group of Cu(COO) 2 appears near 299.8 eV. Thus, based on Fig. 1 (b) and supplementary Fig. S2 (a) and (b), the formation of Cu(COO) 2 was clear when the peak appeared at 299.8 eV. In particular, under 90% RH, the oxidation of Cu to Cu2+ (i.e., production of Cu(COO) 2 ) was remarkably high, as shown in Fig. S2. Then, the XPS peak intensity decreased with time in the case of 90% RH. These results indicate that Cu2+ in Cu(COO) 2 produced at the surface permeates the GO paper bulk at the same time H+ exchange occurs (mainly at COOH groups), especially under high humidity. The permeation rate visibly decreased with increasing the film thickness of Cu, judging from the comparison of the surface content changes in Fig. 1 (c) (Cu film thickness: 17 nm) and Fig. S2 (c) (Cu film thickness: 400 nm). No decrease in Cu surface content was observed under vacuum for the thick Cu film (400 nm), as seen in Fig. S2 (c). These results do not mean that Cu2+ ion and Cu atom scarcely diffuse for the case of thick films, because the compositions hardly changes for a large amount of Cu of thick film.

Figure 2 shows a mapping of Cu permeating into GO paper, as observed by cross-sectional electron probe microanalysis (EPMA). From cross-sectional EPMA of the as-deposited sample (Fig. 2 (a)), the thickness of deposited Cu on GO paper was estimated to be about 200 nm. Although this is about 10-fold the thickness of the Cu layer on the sample estimated by XPS depth profile analysis, it is in fact plausible in light of the fact that the sputtering time was 10 times longer for that sample. Cross-sectional SEM images and XRD patterns of GO papers revealed closely packed self-assembled GO sheets. These results suggest that the structure of the GO paper was seldom affected by metal permeation. Cu permeation was clearly observed under humid conditions at 25°C (Fig. 2 (b)–(d)). The distribution of Cu extended from the M/GO interface to a depth of about 6 μm under 30% RH for 1 h (Fig. 2 (b)). However, when the Cu-deposited sample was kept under 90% RH, in only 10 min, the Cu permeated the GO paper to a remarkable depth extending to the bulk of the GO paper (Fig. 2 (c)). For the EPMA image of the sample kept under 90% RH for 1 h, the Cu concentration in the GO paper was nearly uniform (Fig. 2 (d)). These EPMA results suggest that Cu permeation into the GO paper took place rather quickly under high humidity.

The oxidation of Cu led to the reduction of GO under humid conditions, as shown in Fig. 1 (b), where after 12 h GO began to oxidize due to the termination of Cu2+ production. Based on the above results, the following reactions at the surface took place under humid conditions if CH was produced. As a matter of course, other reduction reactions with respect to GO proceed simultaneously.

Some of the hydrated Cu2+ are immediately exchanged with H+, mainly at COOH sites, to form Cu(COO) 2 at the surface and then move into the interlayers of the GO paper aided by water molecules and H+ exchange. The permeation model is illustrated in the discussion section.

Figure 3 shows (a) the time dependences of the Ag content at the Ag(17 nm)/GO interface and (b) the depth profiles covering the region from the interface to the bulk. Ag permeation occurred much faster under humid conditions than under vacuum, similar to Cu. Although it was difficult to distinguish between Ag and Ag+ from the binding energy of Ag3d (supplementary Fig. S4), Ag 2 O (that is, Ag+ ion) formation was observed from the XPS spectra of O1s (Fig. S4). In particular, its formation under vacuum was clear because Ag+ in Ag 2 O hardly moved from the GO surface into the bulk. The O atom in Ag 2 O seems to be taken from C−O−C groups because the C−O−C content correspondingly decreased with the formation of Ag 2 O, especially under vacuum (Fig. S4). In any case, the Ag+/Ag redox reaction, similar to reaction (1) together with reaction (2), occurs at the Ag/GO interface. Consequently, Ag changes to Ag+ at the interface and then moves into the bulk through the interlayers of the GO paper with the aid of water molecules. The permeation sites are similar to those of H+, which correspond to bonding sites between C−O−C (and/or OH) groups and water molecules5,20,21. According to cross-sectional EPMA mappings of Ag (Fig. S5), Ag permeated the GO paper at an extremely high rate under 30% RH.

Figure 4 shows XPS spectra of (a) Ni2p 3/2 , (b) the time dependences of Ni content at the Ni/GO interface and (c) depth profiles covering the region from the surface to the bulk. Ni permeation occurred from the Ni/GO interface into the GO bulk, as shown in Fig. 4 (b) and (c), where the permeation rate increased remarkably under 60% and 90% RH. Oxidation of Ni metal to Ni2+ occurred at the surface immediately after sputtering and then Ni metal disappeared with time following a redox reaction similar to those of Cu and Ag. In fact, GO reduction proceeded with time, as shown in supplementary Fig. S6. Figure S7 shows cross-sectional SEM images and EPMA mappings of Ni and C for the Ni-deposited sample. The layer of Ni remained at the interface under 30% RH for 1 h, which suggests that the permeation rate of Ni into the GO paper was low compared with Cu and Ag. In the case of 90% RH, Ni permeated the GO paper to a sufficient depth, similarly to Cu. These EPMA results were in close agreement with the XPS depth profile data.

Figure 5 shows (a) the time dependences of Au and Pt content at the Au/GO and Pt/GO interfaces, respectively and depth profiles for (b) Au and (c) Pt. Au and Pt were always in the form of elemental metals, as clear from supplementary Fig. S8. Au permeation occurred, although the rate was rather low compared to that of the metal ions examined above, as already stated. The permeation rate for Au atoms increased slightly with increasing the humidity. On the other hand, Pt permeation occurred with relative ease under vacuum, but with difficulty under humid conditions. The depth profiles of Au and Pt are different from each other in terms of metal distribution. Au was smoothly distributed in depth, while the Pt distribution reached maximum at around 30 nm after 3 days. Figure S9 shows cross-sectional SEM images and EPMA mappings of Au and C. In the EPMA images of the sample kept for 10 days under 30% RH, slight permeation of Au into the GO paper bulk was observed. However, EPMA could not provide clear evidence of Pt permeation into the GO paper because of the low spatial resolution. According to the XPS depth profile, Au permeated GO from the surface to a depth of around 200 nm, whereas the concentration of Pt in the region from the surface to around 200 nm hardly changed during the experiment. This further confirms that Au and Pt permeate the GO paper bulk as metal atoms at room temperature.