Chapter 2: Synthesis of heterocyclic polyimide and usage of the solid source as a graphene synthesis
2.2 Results and Discussions
Polyimide was synthesized by the condensation reaction of its precursor poly(amic acid). To prepare LB films, alkyl amine was reacted to PAA afforded PAA salt that the amphiphilic properties required for the LB process. The synthesis of PAA and its salt has followed the method reported in the literature9, 108-110 and was summarized in Scheme 2.1. Briefly, dianhydride (1.01 mole) and diamine (1.00 mole) was stirred in DMAc solution under nitrogen atmosphere for 23 hrs afforded PAA and followed the addition of hexadecylamine and stirred further 16 hrs to form salts. To examine the imidization reaction of film that cyclization of amic acid to imide, 100 nm thick films of PAAs were spic-coated on SiO2/Si substrates and imidization with heat-treatment at 240 ℃ that was temperature from thermogravimetric analysis (TGA) study and discuss it later. Characterization of the converted film was performed with FT-IR spectroscopy since aromatic PIs become infusible and insoluble.
BTDA-PDA PAA was characterized as a representative example and shown in Figure 2.1(a). The key change of FT-IR was the disappearance of the C-N bond peak observed at 1405 cm-1 and carbonyl C=O peak observed at 1662 and 1722 cm-1 assigned to corresponding carboxylic acid and amide. Then, the appearance of a new peak at 1359 cm-1 assigned to the C-N-C imide bond and carbonyl C=O for imide peak observed at 1720 and 1778 cm-1, respectively.111-112 Moreover, broad O-H peak observed at 2900- 3300 cm-1 disappear after imidization suggested the dehydration occur during imidization. Furthermore, there was no change of C=C bond of aromatic ring observed at 1513 cm-1 before and after imidization.
Scheme 2.1: Synthesis and conversion of PAA (3) to PI (4) and ultimately graphene.
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To investigate the thermal stabilities of the PAAs and PIs, the weight change during the thermal annealing process was monitored by TGA to 1000 ℃ with a heating rate of 100 ℃ min-1 under Ar atmosphere that was a similar condition of the heating process to perform carbonization to graphene.
As shown in Figure 2.1(b), PAAs exhibit weight loss around 200 ℃ that was dehydration of cyclization to imide ring of the amide group. An additional decrease in weight was observed near 600 ℃ and assigned to the release of CO or CO2 gas during carbonization.98 After those weight losses, 50% of the weight was retained even heated up to 1000 ℃. On the contrary, when PMMA was tested to a similar condition as PAAs, significant weight loss was observed around 350 ℃ and further complete decomposition of polymer near at 450 ℃ that corresponding PMMA becomes gaseous precursor as discussed earlier.
Figure 2.1: (a) FT-IR spectra recorded of PAA (red) and PI (black) from BTDA-PDA. (b) TGA graph recorded of various PAAs and PMMA (indicated).
Next, efforts were toward transforming PAAs film to graphene. Using LB techniques, monolayers of PAAs salt, as well as PMMA, were deposited on Cu foils109-110 and those characterizations of surface and thickness were performed with scanning electron microscopy (SEM) and atomic force microscopy (AFM) (Figure H.1); the corresponding surface pressure-area (Π-A) isotherms of LB trough are shown in Figure H.2. The deposited PAA salt films were heated at 240 ℃ for 30 min under Ar gas (100 sccm) afforded imide ring. Resultant films were then heated to 1000 ℃ and keep the temperature for 20 min under H2 gas (100 sccm). After cool to room temperature, the resulting carbonized sheets were transferred onto SiO2/Si substrates as well as TEM grids using common wet transfer techniques,80, 113 and those surface and thickness were then characterized using optical microscopy, SEM, high-resolution TEM (HR-TEM), and AFM that were summarized in Figure 2.2. BTDA-PDA PAA was characterized as a representative film and resultant graphene data was
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labeled as PI-GR. In SEM and AFM images, a relatively smooth surface of the graphene sheet was obtained. Although some wrinkles were observed, they may have originated from the thermal expansion coefficient difference between Cu and graphene.81 The HR-TEM image shown in Figure 2.2(c) along with the fast Fourier transform image (inset) indicated that high-quality graphene was formed. Raman spectra measured of layer characterization of graphene and the sharp peaks of the G and 2D bands with the high 2D/G ratio (2.25) and the lack of a D band suggested resultant graphene was monolayer with a defect-free surface. On the other hand, when monolayer PMMA was used to solid source, resultant graphene showed strong and broad D and G bands with a weak 2D band indicated the incomplete formation of graphene.114-115 Moreover, PMDA based PAA produced a lower quality of graphene than BTDA based PAA from those Raman data Figure 2.2(d).
Figure 2.2: Analysis of PI-GR (BTDA-PDA) transferred on SiO2 (300 nm)/Si substrate. (a) SEM image of PI-GR with inset of photograph of the PI-GR on SiO2 (300 nm)/Si. Scale bars are 4 µm and 0.5 cm, respectively. (b) AFM image of PI-GR with 0 to 10 nm height scale; the scale bar is 4 µm. (c) Atomic resolved HR-TEM image with 5 nm scale bar and the inset image is a Fast Fourier Transform (FFT)
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image. (d) Raman spectra of various PI-GRs obtained from different PIs via their corresponding PAA salts (BTDA-PDA (red), BTDA-ODA (blue), PMDA-PDA (green), and PMDA-ODA (purple)) and PMMA-GR (pink).
To understand the mechanism of PI conversion to graphene, BTDA-PDA PAA solution was directly deposited on SiO2/Si substrates and attempt the heating process. Raman spectra showed that the resultant carbonized film heated at 1000 ℃ formed amorphous carbon. Next, change the heating condition that increases rapidly to 1000 ℃ afforded defective graphene with a large D band in Raman spectra revealed the imidization step that formed heterocyclic aromatic ring was a role in synthesis graphene (Figure C.2). The aforementioned results suggested that the catalytic active Cu surface and the imidization process were essential to achieve high conversions of PI to graphene. Next, efforts were moved to determine the thickness of the polymer film affected the conversion of graphene and various layers of film were deposited on the Cu surface with the LB method. BTDA based PAAs were used to compare with PMMA LB films and the results were summarized in Figure 2.3. Figure 2.3(a) and (b) showed that monolayers and bilayers of PAA resulted in the formation of high-quality graphene whereas the use of three or five layers of PAA as precursors resulted in the formation of defective graphene. Notably, the use of three layers of PAA afforded graphene that was of higher quality than that obtained from five layers of PAA. A difference between using monolayer versus bilayers of PAA as precursors is that the former afforded a surface that was approximately 92% covered with graphene where the latter resulted in essentially complete coverage. On the contrary, the use of five layers of PMMA resulted in higher quality graphene when compared to that obtained from monolayers, bilayers, or trilayers of PMMA (Figure 2.3(c)). We surmise that the quantity of hydrocarbon gas produced upon the decomposition of a monolayer of PMMA was insufficient to afford the formation of high-quality graphene. While the increased amount of gas that may be released from the thicker PMMA films may improve the quality of graphene products, the use of five layers was insufficient (Figure C.3).
Figure 2.3: Raman spectra recorded of 1 (green), 2 (yellow), 3 (orange), and 5 (red) layers of (a) PI- GR (BTDA-PDA), (b) PI-GR (BTDA-ODA), and (c) PMMA-GR.
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As shown in Scheme 2.2, we propose a mechanism that accounts for the conversion of monolayer PAA to PI and finally to high-quality graphene. The as-prepared monolayers of PAA dehydrate upon heating to 240 ℃ and the resulting PIs undergo carbonization in conjunction with atomic rearrangement and coalescence upon further heat treatment.98, 116 It is known that reorientation of aromatic moieties occurs in confined spaces during carbonization even if such species are not precisely aligned.100, 102, 116 Meanwhile, the substrate may assist the conversion of such aromatic intermediates to graphene by decreasing the activation barriers for dehydrogenation and nucleation due to the strong interactions formed between Cu and the aromatic moieties in PI.95-97, 117 As noted in the conclusions derived from the TGA data described above, approximately 50% of the initial quantity of PAA remained after heating the material to 1000 ℃. The residue may contain coalesced aromatic species98, 100 that are different from those found in the amorphous carbon produced on the SiO2/Si substrate. As shown in Figure C.4, an inspection of the temperature-dependent Raman spectra recorded for BTDA-PDA on Cu foils revealed that the size of the graphitic domains (crystallites) increased during the carbonization process, consistent with the mechanism described above. On the other hand, the thicker PAA films (i.e., those comprised of three or five layers) may not have resulted in the formation of high-quality graphene upon annealing because the upper regimes of the carbonized materials were not in contact with the Cu surface and therefore less likely to undergo structural rearrangement.
Scheme 2.2: Proposed mechanism of graphene growth on Cu substrates from various polymer precursors. (a) The change in weight as a function of temperature. Yellow and gray lines represent PAA
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and PMMA, respectively. Formation of graphene from (b) PAA and (c) PMMA according to the corresponding temperature of the graph shown above. Evaporated and re-adsorbed molecules are expressed by color: oxygen (orange), hydrogen (blue), carbon (grey), and nitrogen (green).
Finally, we discuss how the quality of the PI-GR products was dependent on the type of PI precursor employed. From the Raman spectra shown in Figure 2.2(d), we concluded that the PI-GRs grown from the BTDA-based PIs were of higher quality than those prepared from the PMDA-based PIs.
It is known that BTDA-based PIs and PMDA-based PIs have different glass temperatures (Tg 280 ℃ ~ 320 ℃ for the former and 400 ℃ ~ 420 ℃ for the latter) but a similar decomposition temperature (Td
500 ℃). The relatively large temperature range between the Tg and the Td displayed by the BTDA-based PIs may have enabled sufficient rearrangement during heat treatment and resulted in the formation of a well-ordered structure.99 Likewise, the small temperature window between the Tg and the Td exhibited by the PMDA-based PIs was insufficient to enable proper rearrangement. In support of this conclusion, annealing PMDA-based PIs at 450 ℃ improved the quality of graphene that was formed (Figure C.5).