Pressure Effects with Incorporated Particle Size Dependency in Graphene Oxide Layers through Observing Spin Crossover Temperature

1 Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555 Japan; 179d8024@kumamoto-u.ac.jp (H.K.); 187d9041@kumamoto-u.ac.jp (R.A.); m_nakamura@kumamoto-u.ac.jp (M.N.) 2 Institute of Pulsed Power Science (IPPS), Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan * Correspondence: hayami@kumamoto-u.ac.jp; Tel.: +81-096-342-3469

Recently, interlayers of two-dimensional (2D) materials, such as graphene, boron nitride (BN), and MoS 2 , were found to play an important role for the confinement of molecules and pseudo-pressure effects [9][10][11][12].For instance, pressure corresponding to 1.2 ± 0.3 GPa was observed by trapping pressure-sensitive molecules of triphenyl amine (TPA) and boric acid (BA) into an interlayer of graphene [9].In typical 2D layered materials, the correlations between pressure (P) and the interlayer distance (d) were estimated using the equation of P ≈ E w /d, where E w is the adhesion energy [11,12].As such this is an indication that the pressure effects that occur in the interlayer are significantly affected by the interlayer distance.Thus, 2D materials that possess a tunable interlayer have the possibility of tuning pressure effects, leading to the generation of unique phases and physical properties.
Graphene oxide (GO), an oxidized graphene, is a 2D material that has oxygen functional groups, such as hydroxyl, carboxyl, and epoxy groups [13][14][15][16].These oxygen functional groups on the GO surface were removed by thermal reduction treatment, resulting in reduced graphene oxide (rGO) [17][18][19].Importantly, their interlayer distances decrease from 7.9 Å in GO to 3.4 Å in rGO as a result of the removal of the oxygen functional groups [20][21][22].Therefore, a pseudo pressure effect can be generated via GO/rGO transformation.
In the present study, we aimed to further investigate the pressure effects between the GO/rGO layers.For this purpose, we prepared GO (1)/rGO (2) composites incorporating cylinder shape nanorods (NRs) [Fe(Htrz) 2 (trz)](BF 4 ) with a size of 30 × 200 nm (intermediate size of previously reported particle) as a way to detect the pressure effects (Figure 1).Then, pressure effects in the GO/rGO layers were discussed by monitoring T 1/2 , and a comparison was made with GO/rGO composites incorporating spherical NPs (30 × 30 nm) and bulk particles (100 × 100 nm) respectively.Graphene oxide (GO), an oxidized graphene, is a 2D material that has oxygen functional groups, such as hydroxyl, carboxyl, and epoxy groups [13][14][15][16].These oxygen functional groups on the GO surface were removed by thermal reduction treatment, resulting in reduced graphene oxide (rGO) [17][18][19].Importantly, their interlayer distances decrease from 7.9 Å in GO to 3.4 Å in rGO as a result of the removal of the oxygen functional groups [20][21][22].Therefore, a pseudo pressure effect can be generated via GO/rGO transformation.
In the present study, we aimed to further investigate the pressure effects between the GO/rGO layers.For this purpose, we prepared GO (1)/rGO (2) composites incorporating cylinder shape nanorods (NRs) [Fe(Htrz)2(trz)](BF4) with a size of 30 × 200 nm (intermediate size of previously reported particle) as a way to detect the pressure effects (Figure 1).Then, pressure effects in the GO/rGO layers were discussed by monitoring T1/2, and a comparison was made with GO/rGO composites incorporating spherical NPs (30 × 30 nm) and bulk particles (100 × 100 nm) respectively.

Results and Discussion
The [Fe(Htrz)2(trz)](BF4) NRs were synthesized by the reaction between FeCl2⋅4H2O, NaBF4, and 1-H-1,2,4-triazole, using the ligand-melt method [29].Composite 1 was prepared by mixing GO and [Fe(Htrz)2(trz)](BF4) NRs in a mass ratio of 1:2 in ethanol, which was then filtrated.Composite 2 was obtained by subsequent heating at 473 K for 12 h.The GO/rGO transformation in these composites was confirmed by investigating the current-voltage (I-V) properties.The I-V curve for composite 1 shows mainly an insulator property in accordance with the behavior of GO.The electron conductivity of composite 1 was 7.67 × 10 -11 A, applied at 1 V. On the other hand, composite 2 showed 7.28 × 10 -6 A applied at 1 V, in accordance with the oxygen functional groups being removed to yield rGO.This transformation is also corroborated by the powder X-ray diffraction (PXRD) patterns, as presented in Figure 3.
The scanning electron microscopy (SEM) images of the [Fe(Htrz)2(trz)](BF4) NRs, composite 1, and composite 2 are presented in Figures 2 and S2.The SEM image demonstrated that the size of the NR complex was 29.6 nm in width and 203.4 nm in length.For composites 1 and 2, the NRs incorporated between the GO/rGO layers were observed obviously.Furthermore, the presence of [Fe(Htrz)2(trz)](BF4) was clearly confirmed by the energy dispersive X-ray (EDX) spectroscopy

Results and Discussion
The [Fe(Htrz) 2 (trz)](BF 4 ) NRs were synthesized by the reaction between FeCl 2 •4H 2 O, NaBF 4 , and 1-H-1,2,4-triazole, using the ligand-melt method [29].Composite 1 was prepared by mixing GO and [Fe(Htrz) 2 (trz)](BF 4 ) NRs in a mass ratio of 1:2 in ethanol, which was then filtrated.Composite 2 was obtained by subsequent heating at 473 K for 12 h.The GO/rGO transformation in these composites was confirmed by investigating the current-voltage (I-V) properties.The I-V curve for composite 1 shows mainly an insulator property in accordance with the behavior of GO.The electron conductivity of composite 1 was 7.67 × 10 −11 A, applied at 1 V. On the other hand, composite 2 showed 7.28 × 10 −6 A applied at 1 V, in accordance with the oxygen functional groups being removed to yield rGO.This transformation is also corroborated by the powder X-ray diffraction (PXRD) patterns, as presented in Figure 3.
The scanning electron microscopy (SEM) images of the [Fe(Htrz) 2 (trz)](BF 4 ) NRs, composite 1, and composite 2 are presented in Figure 2 and Figure S2.The SEM image demonstrated that the size of the NR complex was 29.6 nm in width and 203.4 nm in length.For composites 1 and 2, the NRs incorporated between the GO/rGO layers were observed obviously.Furthermore, the presence of [Fe(Htrz) 2 (trz)](BF 4 ) was clearly confirmed by the energy dispersive X-ray (EDX) spectroscopy (Figure 2c,d).The Fourier transform infrared spectra (FT-IR) results also supported the presence of [Fe(Htrz) 2 (trz)](BF 4 ) NRs composited within the GO/rGO interlayers (Figure S3).(Figures 2c and d).The Fourier transform infrared spectra (FT-IR) results also supported the presence of [Fe(Htrz)2(trz)](BF4) NRs composited within the GO/rGO interlayers (Figure S3).As such, the changes of the interlayer distance that is associated with the transformation of GO to rGO were investigated by powder X-ray diffraction (PXRD) measurements (Figure 3).Results shows that pristine GO has a distinct peak at 2θ = 10.15˚, with an interlayer distance of 8.70 Å.As for composite 1, the GO peak was observed at 2θ = 10.17˚ and an interlayer distance of 8.68 Å, where the remaining peaks are ascribed to the presence of [Fe(Htrz)2(trz)](BF4) NRs.In the case of composite 2 (which was treated at 473 K for 12 h), the interlayer distance decreased to 3.5 Å (2θ = 25˚) as a result of the removal of the oxygen functional groups on the GO layers.From these results, it can be anticipated that pressure effects occurred between the interlayers.As such, the changes of the interlayer distance that is associated with the transformation of GO to rGO were investigated by powder X-ray diffraction (PXRD) measurements (Figure 3).Results shows that pristine GO has a distinct peak at 2θ = 10.15 • , with an interlayer distance of 8.70 Å.As for composite 1, the GO peak was observed at 2θ = 10.17 • and an interlayer distance of 8.68 Å, where the remaining peaks are ascribed to the presence of [Fe(Htrz) 2 (trz)](BF 4 ) NRs.In the case of composite 2 (which was treated at 473 K for 12 h), the interlayer distance decreased to 3.5 Å (2θ = 25 • ) as a result of the removal of the oxygen functional groups on the GO layers.From these results, it can be anticipated that pressure effects occurred between the interlayers.In order to investigate the influence of the pressure effects on the SCO behavior, caused by the shorter interlayer distance associated with the structural transformation between GO and rGO, the temperature-dependent magnetic susceptibility for the [Fe(Htrz)2(trz)](BF4) NRs, composite 1, and composite 2 were measured in the temperature range of 300 to 400 K.The magnetic susceptibility for the [Fe(Htrz)2(trz)](BF4) NRs in the form of the χmT vs. T plot can be seen in Figure S4, where χm is the molar magnetic susceptibility and T is the temperature.From these results, [Fe(Htrz)2(trz)](BF4) NRs show SCO behavior at T1/2 = 356 K, with a thermal hysteresis of 29 K.The χgT vs. T plots for composite 1 and composite 2 are shown in Figure 4, where χg is the magnetic susceptibility per gram.
Both composites 1 and 2 exhibited SCO behavior at T1/2 = 357 K and 364 K respectively.The T1/2 value of composite 2 is 7 K higher than that observed in composite 1. Accordingly, these results are in agreement with pressure effects behavior when decreasing the interlayer distance.The pseudo-pressure effects were estimated from the T1/2 value using the Clausius-Clapeyron equation (Equation 1) reported by Colacio and co-workers, where p is hydrostatic pressures [28], as follows: In order to investigate the influence of the pressure effects on the SCO behavior, caused by the shorter interlayer distance associated with the structural transformation between GO and rGO, the temperature-dependent magnetic susceptibility for the [Fe(Htrz) 2 (trz)](BF 4 ) NRs, composite 1, and composite 2 were measured in the temperature range of 300 to 400 K.The magnetic susceptibility for the [Fe(Htrz) 2 (trz)](BF 4 ) NRs in the form of the χ m T vs. T plot can be seen in Figure S4, where χ m is the molar magnetic susceptibility and T is the temperature.From these results, [Fe(Htrz) 2 (trz)](BF 4 ) NRs show SCO behavior at T 1/2 = 356 K, with a thermal hysteresis of 29 K.The χ g T vs. T plots for composite 1 and composite 2 are shown in Figure 4, where χ g is the magnetic susceptibility per gram.Both composites 1 and 2 exhibited SCO behavior at T 1/2 = 357 K and 364 K respectively.The T 1/2 value of composite 2 is 7 K higher than that observed in composite 1. Accordingly, these results are in agreement with pressure effects behavior when decreasing the interlayer distance.In order to investigate the influence of the pressure effects on the SCO behavior, caused by the shorter interlayer distance associated with the structural transformation between GO and rGO, the temperature-dependent magnetic susceptibility for the [Fe(Htrz)2(trz)](BF4) NRs, composite 1, and composite 2 were measured in the temperature range of 300 to 400 K.The magnetic susceptibility for the [Fe(Htrz)2(trz)](BF4) NRs in the form of the χmT vs. T plot can be seen in Figure S4, where χm is the molar magnetic susceptibility and T is the temperature.From these results, [Fe(Htrz)2(trz)](BF4) NRs show SCO behavior at T1/2 = 356 K, with a thermal hysteresis of 29 K.The χgT vs. T plots for composite 1 and composite 2 are shown in Figure 4, where χg is the magnetic susceptibility per gram.
Both composites 1 and 2 exhibited SCO behavior at T1/2 = 357 K and 364 K respectively.The T1/2 value of composite 2 is 7 K higher than that observed in composite 1. Accordingly, these results are in agreement with pressure effects behavior when decreasing the interlayer distance.The pseudo-pressure effects were estimated from the T1/2 value using the Clausius-Clapeyron equation (Equation 1) reported by Colacio and co-workers, where p is hydrostatic pressures [28], as follows: The pseudo-pressure effects were estimated from the T 1/2 value using the Clausius-Clapeyron equation (Equation ( 1)) reported by Colacio and co-workers, where p is hydrostatic pressures [28], as follows: The values of SCO temperature and pseudo-pressure for GO/rGO composite when [Fe(Htrz) 2 (trz)](BF 4 ) of different size and shape are incorporated are summarized in Table 1.As a result of the calculation, the pseudo-pressure originated from the transformation of composite 1 to composite 2 is equal to 24 MPa.We have reported previously that GO/rGO composites incorporating [Fe(Htrz) 2 (trz)](BF 4 ) NPs with a size of 30 nm show an increase of the T 1/2 value from T 1/2 = 351 K in the GO, to T 1/2 = 362 K in rGO due to pressure effects corresponding to 38 MPa [23].The pseudo-pressure effect observed in the composite with NRs (30 × 200 nm) was smaller than that observed in the composite with NPs (30 nm).Considering that no pressure effects were observed for the composite incorporating BPs of 100 nm size, it can be concluded that the accommodated particle size and shape crucially affected the pseudo-pressure effects within the GO/rGO layers.For the case of small particle size, the GO layers stack regularly.GO layers form the ordered stacking structures when incorporating NRs, however, the surface area of the NRs influencing the pressure effects is larger than the NPs with a size 30 nm (Figure 5).It is then proposed that a large surface of NRs leads to small pressure effects.
The values of SCO temperature and pseudo-pressure for GO/rGO composite when [Fe(Htrz)2(trz)](BF4) of different size and shape are incorporated are summarized in Table 1.As a result of the calculation, the pseudo-pressure originated from the transformation of composite 1 to composite 2 is equal to 24 MPa.We have reported previously that GO/rGO composites incorporating [Fe(Htrz)2(trz)](BF4) NPs with a size of 30 nm show an increase of the T1/2 value from T1/2 = 351 K in the GO, to T1/2 = 362 K in rGO due to pressure effects corresponding to 38 MPa [23].The pseudopressure effect observed in the composite with NRs (30 × 200 nm) was smaller than that observed in the composite with NPs (30 nm).Considering that no pressure effects were observed for the composite incorporating BPs of 100 nm size, it can be concluded that the accommodated particle size and shape crucially affected the pseudo-pressure effects within the GO/rGO layers.For the case of small particle size, the GO layers stack regularly.GO layers form the ordered stacking structures when incorporating NRs, however, the surface area of the NRs influencing the pressure effects is larger than the NPs with a size of 30 nm (Figure 5).It is then proposed that a large surface of NRs leads to small pressure effects.

Synthesis
All the materials and reagents were obtained from Wako Pure Chemical Industries (Osaka-shi, Osaka, Japan) and Tokyo Chemical Industry (TCI) Co., Ltd (Chuo-ku, Tokyo, Japan) and used without further purification.The [Fe(Htrz) 2 (trz)](BF 4 ) NRs were prepared according to the previously reported procedure [29].The mixture of FeCl 2 •4H 2 O (200 mg, 1 mmol), NaBF 4 (110 mg, 1 mmol), and 1-H-1,2,4-triazole (5 g, 72.4 mmol) was heated at 150 • C for 5 min.After heating, the resulting melt was cooled to room temperature.The obtained crude product was dispersed in ethanol.The dispersion was centrifuged at 4800 r/min, washed with ethanol, and then collected using a membrane filter (1 µm) so as to give the product as a violet powder.

Graphene Oxide (GO)
The graphene oxide was prepared by Hummer's method with a minor modification [21].The mixture of graphite (2 g), grinded NaNO 3 (2 g), and H 2 SO 4 (92 mL) was stirred for 30 min at 0 • C. Subsequently, KMnO 4 powder (10 g) was added carefully, and the resulting mixture was stirred at 35 • C for 60 min.Then, deionized water (92 mL) was dropped into the mixture slowly, and the mixture was heated at 95 • C for 20 min.Subsequently, deionized water (200 mL) was poured into the reaction mixture.Then, a 30% H 2 O 2 solution (30 mL) was dropped very carefully so as to convert the manganese dioxide and unreacted permanganate into soluble sulfates in an ice bath.The mixture was centrifuged at 3000 r/min to remove the supernatant liquid.The precipitate was washed with a 5% HCl solution three times, and then with distilled water five times.The resulting solid was washed with ionized water three times, and exfoliated by ultrasonication for 2 h.The solution was centrifuged at 8000 r/min for 30 min, then the supernatant dispersion was centrifuged at 15000 r/min for 30 min to give the graphene oxide (GO) dispersion.The mixture of GO in ethanol (30 mg/50 mL) and [Fe(Htrz) 2 (trz)](BF 4 ) NRs in ethanol (60 mg/50 mL) was stirred at 25 • C for 6 h.After stirring, the brown product was centrifuged at 4000 r/min for 30 min, and the crude product was collected using a membrane filter (1 µm), washed with ethanol to give the product.

Figure 5 .
Figure 5. Schematic illustration of the pressure effects with incorporated particle size and shape dependency in the GO/rGO layers.

Figure 5 .
Figure 5. Schematic illustration of the pressure effects with incorporated particle size and shape dependency in the GO/rGO layers.