Raman spectroscopy
Occurrence of Gˊ-band
Over the last two decades, the observation of the Raman Gˊ-band has frequently been adopted as evidence for the ordering of sp2-carbon atoms, i.e., the formation of graphitic structures (Jorio et al. 2011). We have previously used the Gˊ-band as an indicator of graphitic-structure formation in Fe-loaded charcoal (Yamagishi et al. 2020, 2022, 2023a, 2023b). The Gˊ-band, which is assigned to the chemical bonds of sp2-carbon as well as D- and G-bands, is usually observed from 2700 to 2650 cm− 1 when the excitation wavelength is 532 nm. The Raman shift and spectral line-shapes of the Gˊ-band are dependent on the ordered structures of sp2-carbon. Thus, the Gˊ-band is a focal point in discussing sp2-carbon structures in charcoal made from wood impregnated with a transition metal in this study. Unless otherwise noted, the Gˊ-band intensity implies the Gˊ-band intensity relative to the G-band intensity observed in the same Raman spectrum.
Figure 1 depicts the typical Raman spectra of the M-850 charcoal samples. Most apparent in these Raman spectra is the occurrence of the positive Gˊ-band for Fe-850, Co-850, and Ni-850 samples. While we previously reported the Raman spectra of many types of Fe-X samples (Yamagishi et al. 2020, 2022, 2023b), the Raman spectra of the Fe-X charcoal obtained in this study were demonstrated as needed. All Raman spectra of Fe-850 and Co-850 samples exhibited a strong Gˊ-band; however, the Gˊ-band intensities of Ni-850 charcoal were weaker than those of Fe-850 and Co-850 charcoal and fluctuated widely with the measurement point, as described later.
As can be seen in Fig. 1, the other five M-850 samples exhibit no Gˊ-band, and their whole spectral line-shapes, including the Raman G- and D- bands, are analogous to that of Non-850 charcoal (Fig. 1a). In addition, no significant differences in Raman spectral line-shapes were observed between Non-X and the five M-X samples when the CT was set at the same temperature, suggesting that the addition of the five kinds of Mn+ has no significant effects on the structures of sp2-carbon when the CT is set at ≤ 850°C. Hence, we focused on the Raman spectral line-shapes of the Gˊ-bands of Fe-X, Co-X, and Ni-X charcoal, especially their intensity and Raman shift, in our discussion of the catalysis of IGM elements.
We previously reported (Yamagishi et al. 2022) that a very weak Gˊ-band was confirmed only in a few Raman spectra at particular sites of Fe-650 charcoal; in contrast, a positive Gˊ-band was observed at almost all the measurement points on Fe-X charcoal made at ≥ 700°C. The wide fluctuation of the Gˊ-band intensity with measurement points was seen in the Raman spectra of Fe-700 and Fe-750 charcoal; however, the Gˊ-band intensity was nearly constant through all the Raman measurements of Fe-X samples made at ≥ 800°C. This was confirmed again by Raman measurements of these Fe-X samples in this study.
Figure 2 shows the Raman spectra of Co-X samples synthesized at the CT from 650 to 800°C. None of the Gˊ-bands were detected in the Raman spectra of Co-650 charcoal. The Gˊ-bands occurred in the spectra of Co-X charcoal synthesized at ≥ 700°C. The Gˊ-band intensity for Co-X (700 ≤ X ≤ 800) charcoal widely fluctuated with the measurement point, and some spectra of Co-700 and Co-750 charcoal exhibited no or a faint Gˊ-band. Although the Gˊ-band was detected as a weak peak at several measurement points on Co-800 charcoal as illustrated in Fig. 2h, most of the Raman spectra of Co-800 charcoal demonstrated a positive Gˊ-band (Figs. 2i and j). All the Raman spectra of Co-850 charcoal possessed a strong Gˊ-band with no significant variation in intensity with measurement points. The fluctuation of the Gˊ-band intensity of Co-700 and Co-750 charcoal seemed to be stronger than that of Fe-700 and Fe-750 charcoal, respectively. Furthermore, the average Gˊ-band intensities of Co-700, Co-750, and Co-800 charcoal are likely to be less than those of Fe-700, Fe-750, and Fe-800 charcoal, respectively.
The Raman spectra of Ni-X samples prepared at the CT from 650 to 850°C are demonstrated in Fig. 3. The Raman spectra of Ni-650 charcoal also exhibited no Gˊ-bands. Unlike Fe-700 and Co-700 charcoal, no Gˊ-bands were observed in the Raman spectra of Ni-700 charcoal. The Gˊ-band was detectable for Ni-X samples made at ≥ 750°C, but no or a faint Gˊ-band occurred at the majority of the measurement points on Ni-750 and Ni-800 samples as shown in Figs. 3c, e, and f. A positive Gˊ-band was seen in almost all the spectra of Ni-850 charcoal, while a few of them were appreciably weak (Fig. 3h), and the significant measurement-point dependence of the Gˊ-band intensity was also observed for Ni-850 charcoal. Additionally, the Gˊ-band intensities of Ni-850 charcoal were clearly weaker than those of Fe-850 and Co-850 charcoal.
The Gˊ-band detection in the Raman spectra of IGM-loaded charcoal suggests that the ordering of sp2-carbon occurs at the lower CT in the order of Ni-X > Co-X > Fe-X. In other words, it is expected that the contribution to the fall of CT at which ordered sp2-carbon structures are formed increases in the order Ni < Co < Fe. Moreover, the rate and uniformity of graphitic-structure formation in charcoal seemed to be in the order of Ni-X < Co-X < Fe-X in the CT range of 750‒850°C.
Based on the SEM images, Suzuki reported that the average carbon-nanoshell diameters were in the order of Fe (55 nm) < Co (70 nm) < Ni (110 nm) for the IGM-loaded charcoal prepared at 900°C (Suzuki K et al. 2017). The SEM observation indicates that the diameters of the metallic IGM particles produced in the IGM-loaded charcoal are also in the same order. Furthermore, the carbonization in the study were essentially adopted the same conditions as those reported by Suzuki, excluding the CT. Thus, the Raman data for the Gˊ-band occurrence and intensity suggests that the formation of ordered structures composed of sp2-carbon is closely correlated with the physicochemical properties of IGM elements, such as M-M and M-C bond energies (Hisama et al. 2018). The generation of IGM carbides in charcoal will be discussed in a later section. Moreover, it can be assumed that the uneven distribution of graphitic structure in charcoal is strongly affected by the size, that is, the mobility, of metallic IGM particles.
Raman shift of Gˊ-band
Several reports have recently asserted that the number of graphene layers is closely related to the Raman parameters of the G´-bands, especially their Raman shifts (Ferrari 2007; Das et al. 2008; Liu et al. 2013; No et al. 2018). It is generally accepted that the G´-band shifts to higher wavenumbers with increasing graphene layers. Thus, we previously discussed graphitic structures formed in Fe-X charcoal in terms of the Raman shift and line-shape of the G´-bands (Yamagishi et al. 2023b). Here, we assess the contribution of metallic IGM and their carbides to the formation of graphitic structures based primarily on the Gˊ-band shifts.
The results of the Gˊ-band positions for Fe-X (650 ≤ X ≤ 850) charcoal are summarized as follows. A faint or very weak G´-band observed for Fe-650 charcoal was positioned at ≤ 2670 cm− 1. Most of the G´-bands of Fe-700 and Fe-750 charcoal were detected in the range 2700–2690 cm− 1; however, a few weak G´-bands were positioned at wavenumbers significantly lower than 2690 cm− 1. The G´-bands of Fe-700 and Fe-750 charcoal seemed to tend to shift to lower wavenumbers with a decrease in their intensity. All the G´-bands of Fe-800 and Fe-850 charcoal occurred from 2700 to 2690 cm− 1. None of the G´-bands showed high asymmetrical line-shapes for all Fe-X samples. The results obtained in this study were consistent with those reported in previous works (Yamagishi et al. 2022, 2023b).
The Raman spectra from 3000 to 2500 cm− 1 of Co-X (700 ≤ X ≤ 850) charcoal are shown in Fig. 4. The spectra shown in Figs. 4a, b, c, d, e, and f are equivalent to the expansion of the spectra shown in Figs. 2c, d, f, g, h, and j, respectively. As mentioned above, the G´-band intensity of Co-700 and Co-750 charcoal widely fluctuated with measurement points. Nevertheless, all the peak-tops of the G´-bands of Co-700 and Co-750 charcoal were observed at ~ 2670 cm− 1 regardless of the relative intensities, unlike Fe-700 and Fe-750 charcoal. The shifts of the G´-bands to higher wavenumbers took place by increasing the CT from 750 (Figs. 4c and d) to 800°C (Figs. 4e and f), and all the peak-top positions of the G´-bands of Co-800 and Co-850 charcoal were located from 2700 to 2690 cm− 1. It should be noted that the G´-bands for Co-800 and Co-850 charcoal were detected at ~ 2690 cm− 1 even though their relative intensities were very weak (Fig. 4e). The results shown in Fig. 4 indicate that Co-X charcoal needs a higher CT to shift the G´-band to higher wavenumbers, as compared with Fe-X charcoal. Moreover, it is expected that the G´-band position of Co-X samples is virtually independent of its intensity for the Raman measurements in this study.
Figure 5 shows the expanded Raman spectra from 3000 to 2500 cm− 1 of Ni-750, Ni-800, and Ni-850 samples. The Raman spectra shown in Figs. 5a, b, c, d, and e correspond to those shown in Figs. 3d, f, g, h, and i, respectively. The G´-band in the Raman spectrum of Ni-750 charcoal (Fig. 5a) occurs at ~ 2655 cm− 1, which are the lowest wavenumbers in this study. The peak-top wavenumbers of the G´-band of Ni-800 charcoal were clearly lower than 2690 cm− 1 and seemed to be almost independent of its intensity. The G´-bands of Ni-850 charcoal were positioned from 2700 to 2690 cm− 1 regardless of their relative intensities. These results reveal that Ni-X charcoal needs a CT higher than 800°C to shift the G´-band to the higher wavenumber region.
In our previous study (Yamagishi et al. 2023b), we predicted that the G´-band occurrence can be attributed to the formation of few- or multi-layer graphene-like structures in Fe-X charcoal and that the G´-band shift to higher wavenumbers is due to the increase in the number of layers of the graphene-like structures. Specifically, we assumed that the G´-bands observed at ~ 2690 and ~ 2660 cm− 1 correspond to the multi- and few-layer graphene-like structures, respectively. If the prediction is acceptable, the results obtained from the Raman measurements of the IGM-loaded charcoal samples can be summarized as follows.
(1) When the CT was ≤ 850°C, the G´-bands were observed in the Raman spectra of IMG-loaded charcoal, Fe-X, Co-X, and Ni-X, among the eight kinds of metal-loaded charcoal. It can be presumed that the contribution of these three metals to the formation of graphitic structures is much greater than that of the other five metals.
(2) The low-wavenumber (LW) G´-bands (≤ 2670 cm− 1) correspond to few-layer graphene-like structures. The structures were generated in Fe-X, Co-X, and Ni-X charcoal in the CT ranges of ≤ 650, 650–700, and 700–750°C, respectively.
(3) The growth from few-layer graphene-like structures to multi-layer graphene-like structures is likely to occur at ~ 700, ~800, and ~ 850°C in Fe-X, Co-X, and Ni-X charcoal, respectively. The high-wavenumber (HW) G´-bands (2700–2690 cm− 1) indicate the formation of multi-layer graphene-like structures.
Powder X-ray diffractometry
In this section, we first explain the powder XRD patterns of the M-X charcoal, excluding those of Fe-X, Co-X, and Ni-X charcoal, that is, the M-X charcoal exhibiting no G´-band at ≤ 850°C. Figure 6 depicts the XRD patterns of M-700 and M-850 (M: Al, Cr, Mn, Cu, Zn) charcoal samples. Since the XRD pattern of Al-700, Cr-700, or Mn-700 was very similar to that of Al-850, Cr-850, or Mn-850, respectively, only the latter is shown in the figure.
XRD peaks assigned to M0, which was reduced from Mn+, are detected only in the XRD patterns of Cu-700 and Cu-850 (Figs. 6d and e). The peak intensities assigned to metallic copper become sharper and higher with increasing CT. This reveals that the reduction from Cu2+ to Cu0 occurs partly in Cu-700 and the growth of metallic copper particles proceeds sufficiently from 700 to 850°C. However, none of the Raman spectra of Cu-850 charcoal exhibited the G´-band, unlike the IGM-850 charcoal samples, indicating that metallic copper has a poorer catalytic ability for the formation of graphitic structures compared with metallic IGM. No signals assignable to the M0 species (metallic M or carbides of M) are observable in the XRD patterns of Al-850, Cr-850, and Mn-850 samples. The reflections attributed to ZnO crystals disappear completely in Zn-850, but no signals assigned to metallic zinc are also detected in the XRD pattern of Zn-850 charcoal. For the four metals other than copper, it was confirmed that no crystallites of the M0 species detectable by XRD are generated, even in the five M-850 samples.
Figure 7 shows the powder XRD patterns of Fe-X charcoal in the 2θ range from 20 to 60 °. The peaks at approximately 26.3 °, which are attributable to the reflection due to the (002) plane of graphite-like structures, are observed for the four Fe-X charcoal samples made at ≥ 700°C. Moreover, the signals corresponding to the chemical species of Fe0 (Fe3C, α-Fe, or γ-Fe) are also detected in all the XRD patterns; however, γ-Fe is not confirmed for Fe-X charcoal made at ≤ 700°C. As reported previously (Yamagishi et al. 2022), the Mӧssbauer absorptions indicated that the chemical species of Fe3+ remained in Fe-X charcoal synthesized even at ≥ 650°C. Although no magnetic hyperfine splitting assigned to Fe2O3 was observed in the Mӧssbauer spectra, we tentatively identified the Fe3+ species as Fe2O3 based on the Mӧssbauer parameters (isomer shift and quadrupole splitting) and presumed that no observation of magnetic hyperfine splitting was caused by the superparamagnetic phenomenon due to the nano-sized Fe2O3 particles (Nasu 2013). Assuming that the Fe2O3 particles in Fe-X samples are too small to be detected by XRD, both the results obtained from the Mӧssbauer and XRD support one another. No peak at ~ 26.3 °, corresponding to graphite-like structures, is observed for Fe-650 as shown in Fig. 7a. Nevertheless, the weak LW G´-bands, which are assigned to few-layer graphene-like structures, were detected for Fe-650 charcoal (Yamagishi et al. 2022). As will be discussed later, this is an important result for graphitic structures generated in the IGM-X charcoal.
The XRD patterns of Co-X (650 ≤ X ≤ 850) samples are illustrated in Fig. 8. While the peak at ~ 44.4 °, corresponding to metallic cobalt, occurs in all the XRD patterns of Co-X charcoal samples, the reflection due to the (002) plane of graphite-like structures is detected only for Co-800 and Co-850 charcoal. The peaks at ~ 44.4 ° observed for Co-650, Co-700, and Co-750 charcoal are not only weak but also spread substantially toward the bottom. The spread or broadening of the peaks may be caused by crystal imperfection and/or insufficient growth of metal particles. Furthermore, the reflection due to Co3C appears as a shoulder at approximately 43.0 ° in the XRD pattern of Co-800 and Co-850 charcoal. While almost all the Raman spectra of Co-700 and Co-750 samples showed the LW G´-band, no peaks assignable to the (002) plane are observed in the XRD patterns of Co-700 and Co-750 samples. In contrast, Co-800 and Co-850 samples exhibit both the Raman HW G´-band and the XRD signal attributed to graphite-like structures. Thus, the HW G´-bands of Co-X charcoal samples correspond to the XRD peak at ~ 26.3 °.
Figure 9 depicts the XRD patterns of Ni-X (650 ≤ X ≤ 850) charcoal. The results shown in the figure reveal that the reduction from Ni2+ to Ni0 also occurs at a CT lower than 650°C. The line-shapes of the XRD peaks assigned to metallic nickel become sharper with increasing CT. Among the five Ni-X charcoal samples, a positive signal due to graphite-like structures is observed in the XRD pattern of Ni-850 alone; however, Ni-800 charcoal exhibits a weak shoulder at ~ 26 °. As noted above, although the G´-bands observed for the other Ni-X samples were the LW G´-bands, Ni-850 charcoal exhibited the HW G´-bands alone. Thus, as with Co-X charcoal, the detection of the HW G´-band is likely to be equivalent to that of graphite-like structures in Ni-X charcoal.
Ni3C can hardly be distinguished from metallic nickel formed into a hexagonal close-packed structure by XRD analysis. Whereas we have not provided strong evidence, shoulder E in Fig. 9e was tentatively assigned to nickel carbide (Ni3C) in this study.
The powder XRD analysis for Fe-X, Co-X, and Ni-X charcoal confirms that the reduction from IGMn+ to IGM0 (IGMn+: Fe3+, Co2+, Ni2+) occurs from a CT ≤ 650°C; however, the XRD peak due to the (002) plane of graphite-like structures is observed for Fe-X, Co-X, and Ni-X charcoal samples synthesized at ≥ 700, ≥ 800, and 850°C, respectively.
Comprehensive explanation for Raman and XRD data of charcoal loaded with iron group metals
In this section, we comprehensively discuss the ordering of sp2-carbon in the JC charcoal formed through IGM catalysis, based on the Raman and XRD data listed in Table 1. The Raman and XRD measurements of the IGM-loaded charcoal, which were synthesized under the carbonization conditions (HR: 10°C/min, HT: 1.0 h, CR: ~50°C/min) used in this study, are summarized as follows:
-
As with the results of Mӧssbauer analysis (Yamagishi et al. 2022), metallic iron and Fe3C were identified in Fe-650 charcoal using XRD. The very weak LW G´-band was detected, but no HW G´-band was observable in Fe-650. In contrast, Fe-800 and Fe-850 charcoal samples exhibited the HW G´-band alone. Both the LW and HW G´-bands were detectable in the Raman spectra of Fe-700 and Fe-750 charcoal.
-
The LW G´-band was detectable in Co-X and Ni-X samples made in the CT range of 700–750°C and 750–800°C, respectively. A positive HW G´-band was observed in almost all Raman spectra of Co-800, Co-850, and Ni-850 samples. Unlike Fe-X charcoal, none of the Co-X and Ni-X charcoal samples exhibited both the LW and HW G´-bands in a sample.
-
The positive XRD peak attributed to graphite-like structures occurred in Fe-X, Co-X, and Ni-X charcoal synthesized at ≥ 700°C, ≥ 800°C, and 850°C, respectively.
-
The XRD patterns revealed that all the metallic IGM were generated in the charcoal samples synthesized below 650°C.
-
The XRD peaks attributed to the metal carbides were detected for Fe-X, Co-X, and Ni-X samples made at ≥ 650°C, ≥ 800°C and 850°C, respectively.
Table 1
Metal-loaded charcoal
|
Fe-loaded charcoal
|
Co-loaded charcoal
|
Ni-loaded charcoal
|
CT (°C)
|
650
|
700
|
750
|
800
|
850
|
650
|
700
|
750
|
800
|
850
|
650
|
700
|
750
|
800
|
850
|
Raman
bands
|
LW Gʹ-band
(< 2675 cm− 1)
|
D*
|
D
|
D
|
N
|
N
|
N
|
D
|
D
|
N
|
N
|
N
|
N
|
D
|
D
|
N
|
HW Gʹ-band
(2700–2690 cm− 1)
|
N
|
D
|
D
|
D
|
D
|
N
|
N
|
N
|
D
|
D
|
N
|
N
|
N
|
N
|
D
|
XRD
reflections
|
(002) plane of GLS
|
N
|
D
|
D
|
D
|
D
|
N
|
N
|
N
|
D
|
D
|
N
|
N
|
N
|
D**
|
D
|
Metallic IGM
|
D**
|
D
|
D
|
D
|
D
|
D
|
D
|
D
|
D
|
D
|
D
|
D
|
D
|
D
|
D
|
IGM carbides
|
D**
|
D
|
D
|
D
|
D
|
N
|
N
|
N
|
D
|
D
|
N
|
N
|
N
|
N
|
D***
|
D: detected, N: not detected, D*: A very weak band was detected at a few points, D**: detected but very weak,
|
D***: A possible shoulder was detected but its assignment is tentative, GLS: graphite-like structures, LW: low wavenumber,
|
HW: high wavenumber, IGM: iron group metals |
When compared to the occurrence of the XRD peak at ~ 26.3 °, it is evident that the HW G´-band is due to graphite-like structures that are equivalent to multi-layer graphene-like structures. Assuming that the LW G´-band corresponds to few-layer graphene-like structures, we discuss these results in terms of sp2-carbon structures in the IGM-loaded charcoal.
No occurrence of the G´-band was confirmed for Fe-600 charcoal that contains no Fe0 species (Yamagishi et al. 2022, 2023a). Hence, the experimental data reveals that the LW G´-band does not occur without the generation of metallic IGM. In contrast, the LW G´-band occurrence seems to be independent of the detection of IGM carbides by XRD. However, the reflections from the IGM carbides should not be found as XRD signals if the carbide layer on the surface of the metallic IGM particles is too thick to be detected by XRD. Thus, the absence of XRD reflections attributed to IGM carbides is not necessarily equivalent to the absence of carbide production.
The HW G´-band occurrence corresponds to the detection of the XRD peak at ~ 26.3 °. Specifically, the HW G´-band provides evidence that few-layer graphene-like structures grow up to multi-layer graphene-like structures. Moreover, it should be noted that the HW G´-band occurrence is virtually equivalent to the detection of graphite-like structures and IGM carbides by XRD
It has been expected for several decades or more that metal carbides are involved in the formation of ordered sp2-carbon via the catalysis of the metal element. However, the role of the carbides remains controversial, and several different mechanisms have been proposed (Zaikovskii et al. 2001; Hernadi et al. 2002; Bokhonov et al. 2002; Takenaka et al. 2004; Louis et al. 2005). Unfortunately, none of these mechanisms are directly applicable to the IGM-loaded charcoal, because their carbon sources are entirely different from woody biomass. Although it is unclear how metallic IGM and IGM carbides are involved in the formation of ordered structures consisting of sp2-carbon, the Raman and XRD data suggest that the formation mechanisms of few-layer and multi-layer graphene-like structures are fundamentally common to the three types of IGM-loaded charcoal. Remarkable differences between Fe-, Co-, and Ni-loaded charcoal can be seen in the CT at which few-layer graphene-like structures (CT1) and multi-layer graphene-like structures (CT2) are generated. Both CT1 and CT2 increase in the order of Fe-X < Co-X < Ni-X. Moreover, it is interesting to note that Fe-X charcoal shows little or no difference between CT1 and CT2; however, marked differences can be seen in Co-X and Ni-X charcoal samples.
While Fe-X charcoal has the advantage of the formation of ordered sp2-carbon structures at a lower CT than Co-X and Ni-X charcoal, the ordered structures formed in Fe-700 and Fe-750 charcoal samples are the mixture of few- and multi-layer graphene-like structures. Co-X and Ni-X charcoal samples need a higher CT to produce ordered structures of sp2-carbon; however, the numbers of layers assigned to graphene-like structures in a Co-X or Ni-X sample are unlikely in a wide range of distribution. Thus, it might be possible to control the number of layers of graphene-like structures in Co- and Ni-X charcoal samples by the setting of carbonization conditions. Additionally, the experimental data in this study suggests that the growth mechanism from turbostratic sp2-carbon structures to graphite-like structures involves two or more steps in IGM-loaded charcoal.
More studies are required to elucidate the chemical behavior of metallic IGM and IGM carbides as catalysts and the formation and growth mechanisms of ordered sp2-carbon structures. Specifically, electron microscopic techniques will be essential for determining graphene-like structures formed in the IGM-loaded charcoal. Moreover, it is necessary for a better understanding of the mechanisms to examine the effects of the heating rate and holding time.