Intercalated Species in Multilayer Graphene Oxide: Insights Gained

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Intercalated Species in Multilayer Graphene Oxide: Insights Gained From In-Situ FTIR Spectroscopy With Probe Molecule Delivery Richard Bertram Church, Kaiwen Hu, Giuliana Magnacca, and Marta Cerruti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05953 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016

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The Journal of Physical Chemistry

Intercalated Species in Multilayer Graphene Oxide: Insights Gained from in-situ FTIR Spectroscopy with Probe Molecule Delivery Richard Church,1 Kaiwen Hu,1 Giuliana Magnacca,2 Marta Cerruti*,1 1

Department of Mining and Materials Engineering, McGill University, Montreal, Quebec, Canada H3A 0E8

2

Department of Chemistry and NIS Interdepartmental Center, University of Turin, Turin, Italy 10125

ABSTRACT: In this work we identify the molecular properties responsible for intercalation in multilayer (ML-) graphene oxide (GO) through in-situ monitoring of probe molecule delivery via Fourier Transform Infrared Spectroscopy (FTIR), focusing on the probe molecule’s ability to displace the naturally intercalated species in ML-GO. By monitoring changes occurring in the FTIR spectra in real time, we show that small molecules with the ability to form hydrogen bonds are strongly retained in ML-GO. These molecules can easily replace the intercalated CO2, while molecules incapable of hydrogen bonding are unable to do so, even if their size should allow them to intercalate between the ML-GO layers. These results provide direct evidence for the importance of hydrogen bonding, molecule size and functional group orientation on the intercalation process, and introduce in-situ FTIR spectroscopy with probe molecules as a valuable technique to characterize ML-GO.

INTRODUCTION Graphite oxide can be produced by oxidizing graphite through various chemical treatments including but not limited to, the Staudenmaier, Brodie, Hofmann, Hummers’ and Tour methods.1-2 Through exfoliation techniques, graphite oxide can be separated into individual sheets, creating graphene oxide (GO).1 The chemical structure of the GO is complex, containing epoxide, hydroxyl, carbonyl, carboxyl and ether functional groups.1 The concentration and dispersion of these groups is dependent upon a variety of factors, primarily the graphite starting material and the oxidation route, and as a result GO is a non-stoichiometric material.2-4 The thermal1, 3, 5-6 or electrochemical reduction7-8 of multilayer (ML-) GO films allows for the bulk production of high quality reduced graphene oxide (rGO). Certain intercalated species, namely H2O and CO2, occur naturally in ML-GO and understanding intercalation mechanisms in ML-GO is crucial.3 For example, differences in the intercalation properties are responsible for the selective permeability of MLGO membranes developed as molecular sieves.9-13 Also, the reduction pathways of ML-GO and the quality of the final rGO product are dependent upon the intercalated species present in the film.1, 3, 5 In-situ Fourier Transform Infrared Spectroscopy (FTIR) has been used to track chemical transformations during GO reduction. Early insitu FTIR experiments and molecular dynamics simulations showed that CO2 is the dominant gas produced when GO films intercalated with H2O are annealed at low temperatures; this CO2 remains intercalated between MLGO layers.1,3, 14 In this work, we have studied the interaction between probe molecules in the gas phase and ML-

GO films using in-situ FTIR spectroscopy, focusing on the ability of these molecules to displace the intercalated H2O molecules at room temperature (RT) and CO2 produced during annealing. We showed that small molecules able to form hydrogen bonds, such as water and alcohols, can replace the intercalated CO2, while molecules incapable of hydrogen bonding, including the smallest molecules tested, cannot do so. Size and hydroxyl group orientation determine the alcohol ability to displace CO2. These findings prove the crucial role of hydrogen bonding in controlling intercalation between ML-GO layers.

EXPERIMENTAL METHODS For in-situ FTIR measurements ML-GO samples were prepared using concentrated GO aqueous suspensions (6.2 mg mL-1, Graphene Supermarket prepared using Hummers’ method) by a drop casting method.1, 3 Elemental characterization of the GO suspension can be found in Table S1. The suspensions were cast onto a plastic Petri dish and allowed to dry under ambient conditions. Self-supporting films, ranging in mass from 1 to 4 mg, were removed using a razor and placed into a gold envelope, which contained a hole to allow IR radiation to pass through (Figure S1). The envelope was then inserted into a homemade quartz cell. The homemade quartz cell consisted of two components: a KBr window for IR measurements and a quartz sleeve for thermal treatments (Figure S2). The cell was connected to a vacuum line for probe gas deliveries and FTIR measurements (Figure S3). All probe gas deliveries and heat treatments were performed in-situ under vacuum (residual pressure 10-3 mbar). Once the probe species interactions and FTIR

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Additional ML-GO samples were prepared using ultra high concentration GO aqueous suspensions (6.2 mg mL-1, Graphene Supermarket) by a drop casting method. The suspensions were cast onto polyester films and allowed to dry under ambient conditions. Self-supporting films with a mass of ~1 mg were removed using a razor for thermal treatment, Raman spectroscopy and x-ray diffraction (XRD) analysis. The annealed ML-GO was wrapped in copper foil and treated in a vacuum tube furnace at 125oC for ~17 minutes. Both the ML-GO and annealed ML-GO films were placed on scotch tape. The XRD measurements were performed using a Bruker Discover D8 XRD system and a Vantec 2D detector. The spectra were recorded from 3o to 103o (2θ) using a Cu Kα radiation source. The Raman measurements for ML-GO and annealed ML-GO were collected using a Bruker Senterra confocal Raman microscope with a 633 nm laser. The integration time was set at 30 s and 36 1-µm resolution points were collected for each sample.

RESULTS AND DISCUSSION Figure 1A shows the IR spectra of untreated ML-GO films before and after probe deliveries at beam temperature (BT). The spectra of ML-GO samples prior to any probe molecule delivery (i and ii) are in good agreement with literature spectra for ML-GO.1, 3, 5 When the cell is outgassed to 10-3 mbar the peaks corresponding to the O—H stretching (νO—H) and bending (δH2O) of physisorbed H2O molecules decrease in intensity.15 These peaks increase in intensity after contact with H2O vapor (iii), and decrease after outgassing (iv), showing H2O adsorption/desorption on ML-GO is repeatable. After contact with D2O vapor (v) bands appear in the spectrum, corresponding to the O—D stretching (νO—D) and bending (δD2O) modes of D2O.15 This is contrasted by a decrease in the νO—H signal of H2O, while there is no accompanying decrease for the δH2O signal, possibly due to the overlap with the signal for the skeletal vibrations (C=C) of the graphitic planes in GO.1, 5 The intensity of the νO—D and δD2O peaks decreases upon outgassing (vi). Figure 1B shows 13CO2 and CO deliveries to untreated ML-GO films in the 2000-2500 cm-1 spectral region (full spectra in Figure S6). All the spectra contain a peak located at ~2337 cm-1, corresponding to the asymmetric νC=O stretching mode (symmetry species Σu+) of intercalated CO2. This

single peak is indicative of intercalated or physisorbed species and indicates that the CO2 molecules are linearly coordinated to the surface sites on the ML-GO samples.1, 3, 16 In fact, gaseous CO2 would be represented by a double peak corresponding to the complex roto-vibrational signal of the molecules.1, 16 After exposure to 13CO2 (i) all 13CO2 molecules remain in the gaseous phase, as indicated by the 13CO2 roto-vibrational signal. Upon outgassing, this peak disappears, showing that 13CO2 is completely removed from the cell (ii). A similar result is obtained for CO: the double peak corresponding to the rotovibrational signal of gaseous CO (iii) vanishes upon outgassing (iv).

A)

νO—H

δH2O i.

Absorbance (a.u.)

measurements were completed residual probe molecules were outgassed from the cell utilizing the vacuum pumps. Information for all of the probe species utilized in this work can be found in Table S2. All IR measurements were recorded using a Bruker Vector 22 IR spectrometer equipped with a deuterated-triglycine sulfate (DTGS) detector at beam temperature (BT). BT is the temperature of the sample in the IR beam and is estimated to be some 20 to 30oC higher than room temperature. The IR spectrometer resolution was set at 4 cm-1, the scan time at 128 seconds and the scanner velocity at 10.0 kHz. Heat treatments were performed in a homemade tube furnace for 5 minutes at 125oC under outgassing (residual pressure 10-3 mbar) (Figures S4 and S5).

ii. 0.5 a.u.

iii. iv.

νO—D

δD2O

4000

2800

1600

Wavenumber (cm-1)

B)

2500

v. vi.

400

Gaseous 13CO 2

Intercalated CO2

Absorbance (a.u.)

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i.

ii.

Gaseous CO

iii. 0.1 a.u. iv.

2400

2300

2200

2100

2000

Wavenumber (cm-1) Figure 1: IR spectra of ML-GO with probe molecule delivery prior to thermal treatment. A) (i) Ambient conditions, (ii) -3 Vacuum (10 mbar), (iii) H2O contact (1 min) and (iv) Out13 gas; (v) D2O contact (1 min) and (vi) Outgas. B) (i) CO2 (30 min) contact and (ii) Outgas; (iii) CO contact (10 min) and (iv) Outgas.

Probe molecule delivery experiments were performed upon treating ML-GO samples under outgassing conditions at 125oC for 5 minutes. The spectrum of ML-GO following thermal treatment (Figure 2A, i) agrees with literature results, showing a single peak at ~2337 cm-1 corresponding to the Σu+ mode of intercalated CO2.3 This peak vanishes 1 minute after the delivery of H2O vapor (ii). This disappearance is accompanied by an increase in the intensity of the νO—H and δH2O signals of H2O. Following outgassing of the H2O molecules (iii), the peak corresponding to the νO—H mode of H2O has a higher intensity


then immediately following thermal treatment (indicated by the arrow, iii). D2O vapor admission to thermally treated ML-GO yielded similar results (Figure S7). We obtained similar results upon admission of alcohol vapors (Figure 2B): in the presence of methanol (MeOH) vapor for 5 minutes, the peaks related to νC—H and νC—O appear,17 while the signal of intercalated CO2 disappears (ii). Upon outgassing, the νC—H and νC—O signals decrease in intensity but do not disappear (iii). A similar result is obtained upon delivery of ethanol (EtOH) vapor (iv-v). This demonstration of alcohol intercalation is in good agreement with previous works for alcohol intercalation into ML-GO. Two monolayers of MeOH and EtOH have been independently intercalated into Hummers’ ML-GO without any phase transformation.18 Furthermore, Huang et al. has demonstrated that EtOH intercalation is only minimally affected by annealing ML-GO at temperatures up to 180oC.12

4000

Intercalated CO2

δH2O

ii.

A)

4000 iii.

Intercalated CO2 i.

2800

1600

Wavenumber (cm-1)

400

Intercalated CO2 νC—H

i. νC—O ii.

νC—H

νC—O

νC—H

ii.

iii. iv.

2800

1600

Wavenumber (cm-1)

B)

νO—H

B)

4000

i.

νO—H

PrOH remain upon outgassing (iv). In contrast with these results, the intensity of the Σu+ signal of intercalated CO2 in the annealed film does not significantly change even after 30 (ii) or 60 minutes (iii) of contact with i-PrOH, and the νC—H and νC—O peaks related to i-PrOH are completely removed upon outgassing (iv). Minor differences are noticeable in both the high and low frequency regions of the post vacuum treatment spectra (i) in Figure 3A and Figure 3B. This can likely be attributed to differences in the reduction chemistry of the two samples. The reduction chemistry is strongly affected by the functional groups and defects present in the original samples, along with the intercalated water content,1, 3, 14 all of which likely differed between the two samples.

Absorbance (a.u.)

Absorbance (a.u.)

A)

Absorbance (a.u.)

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Absorbance (a.u.)

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400

Intercalated CO2 i.

νC—O

νC—H

ii.

iii.

iv.

iii. νC—O iv.

4000

2800

1600

Wavenumber (cm-1)

400

v.

2800

1600

Wavenumber (cm-1)

400

Figure 2: IR spectra of GO with water and alcohol delivery -3 following thermal treatment. A) (i) Vacuum (10 mbar), (ii) -3 H2O contact (1 min) and (iii) Outgas. B) (i) Vacuum (10 mbar), (ii) MeOH contact (5 min) and (iii) Outgas; (iv) EtOH contact (5 min) and (v) Outgas.

Larger alcohols, 1-propanol (n-PrOH, Figure 3A) and 2propanol (i-PrOH, Figure 3B), behave differently. The Σu+ signal of intercalated CO2 decreases in intensity after being subjected to n-PrOH for 5 minutes (ii), and new peaks appear corresponding to νC—H and νC—O of n-PrOH.19 Only after 30 minutes of contact does the Σu+ signal of intercalated CO2 disappear (iii). The νC—H and νC—O peaks of n-

Figure 3: IR spectra of GO following thermal treatment with -3 propanol isomers delivery. A) (i) Vacuum (10 mbar), (ii) nPrOH contact (5 min), (iii) n-PrOH contact (30 min) and (iv) -3 Outgas. B) (i) Vacuum (10 mbar), (ii) i-PrOH contact (30 min), (iii) i-PrOH contact (60 min) and (iv) Outgas.

In Figure 4 we show the 2000-2500 cm-1 spectral regions after delivering small molecules that do not contain hydroxyl groups (complete spectra in Figure S8). The results are identical to those shown in Figure 1B: only the rotovibrational signals of gaseous 13CO2 (i) and CO (iii) appear, and these peaks are removed upon outgassing (ii and iv) and the peak relative to the Σu+ mode of the intercalated CO2 is unchanged. We obtained similar results with the smallest probe gas tested, He. While no IR signal is expected in relation to the presence of He gas ad-


The Journal of Physical Chemistry sorbed on the sample, the intercalated CO2 peak remains unchanged upon He delivery (v and vi).

Gaseous 13CO 2

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intercalated CO2

i. 0.2 a.u. ii.

Gaseous CO

iii. iv. v. vi.

2500

2400

2300

2200

2100

2000

Wavenumber (cm-1) Figure 4: IR spectra of GO following thermal treatment with 13 probe molecule delivery. (i) CO2 contact (30 min) and (ii) outgas; (iii) CO contact (10 min) and (iv) Outgas; (v) He contact (10 min) and (vi) Outgas.

We first provide insights into the intercalated and physisorbed species in ML-GO at RT (Figure 1). Both H2O and small amounts of CO2 are strongly intercalated, and resist extensive outgassing. Although intercalated CO2 is known to form extensively upon annealing,1, 3 the presence of CO2 before any thermal treatment was not pointed out; it is likely due to the natural degradation of metastable GO under ambient conditions.20 While D2O vapor can displace intercalated H2O molecules, 13CO2 and CO cannot. D2O vapor intercalation occurs quicker (~1 minute) than in the liquid phase (~24 h), due to the faster diffusion of single molecules in the gas phase than large clusters in the liquid phase.21 The signal relative to intercalated CO2 strongly increases upon heating the ML-GO films (Figure 2). While the formation of intercalated CO2 in ML-GO during heat treatment was already shown under air, argon and static vacuum,1, 3, 5 the persistence of this signal under outgassing shows the strength of the interaction between such CO2 and ML-GO. In addition to the newly formed CO2, some of the H2O intercalated at RT remains after treatment, as evidenced by the strong νO—H signal present in all spectra (i) for Figure 2 and Figure 3. We confirmed this by annealing a D2O intercalated ML-GO sample (Figure S7). These observations are further supported by the Raman spectra presented in Figure S9, which show the Gband at ~1595 cm-1 and the D-band at ~1330 cm-1. The bands correspond to the vibrations in the sp2 lattice and the presence of defects in the sp2 lattice, respectively.7 The thermal reduction of ML-GO is typically accompanied by the loss of functional groups and intercalated water, as well as the migration of functional groups to form segregated sp2 patches.1, 14, 22 Due to the similar D/G ratios for the untreated and treated ML-GO in Figure S9 it can be concluded that minimal functional group loss has occurred and that the mean lateral size of the sp2 carbon

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domains have remained unchanged by the vacuum treatment. The intercalated CO2 formed upon heat treatment can be quickly displaced by H2O or D2O admission. Our next experiments (Figures 2-4) sought to identify the mechanisms that allow H2O to displace the strongly intercalated CO2. The interlayer distance of our ML-GO samples after annealing is 0.74 nm (Figure S10). H2O, with a critical diameter of 0.28 nm (Table S2), easily penetrates between the GO sheets, and displaces CO2 in less than one minute.23 Larger alcohols such as MeOH, EtOH and n-PrOH can displace CO2 but take longer times (~5-30 min).24-25 This is in line with previous works on ML-GO molecular sieves, where a diameter of ~0.45 nm represented a cut-off between small species with high permeation rates through ML-GO membranes and larger organic solutes, which require more time for permeation.9, 11 Still, size is not the only factor that governs a molecule’s ability to intercalate. While i-PrOH is only marginally larger than n-PrOH as a result of the altered hydroxyl group placement (Table S2), and both molecules theoretically fit between ML-GO sheets, only n-PrOH can displace the CO2. Furthermore, both n-PrOH and i-PrOH have similar hydrogen bonding energies in the liquid phase, 15.04 kJ mol-1 and 14.82 kJ mol-1 respectively; the marginally lower value found for i-PrOH is attributed to steric hindrance from the altered hydroxyl group position.26-27 This small difference indicates that the inability of i-PrOH to intercalate is a result of a difference in structure, and not in hydrogen bond strength. However, even smaller probe molecules such as 13CO2, CO and He cannot displace the intercalated CO2.23 So, which molecular mechanism allowed H2O and the alcohol species to displace the intercalated CO2? A candidate is hydrogen bond formation through the hydroxyl groups on these probe species; indeed, it has long been known that polar species can intercalate between the GO sheets.28 Following thermal reduction at 125oC there are still oxygen-containing groups between the GO sheets. These groups can act as hydrogen bond acceptors for these hydroxyl groups. Both H2O and all the alcohols tested should be able to intercalate and displace CO2. We propose that i-PrOH is unable to intercalate as a result of the altered hydroxyl group placement, which makes it harder for the i-PrOH molecules to hydrogen bond with the interlayer oxygen-containing species, diffuse between the layers, and displace intercalated CO2. This work also may be useful to identify the difficulties found by researchers trying to apply experimental techniques such as N2 adsorption at 77 K to characterize MLGO.29 We have shown that small inert gases such as He are incapable of displacing the intercalated H2O or CO2 in ML-GO. Similarly, N2 molecules are likely unable to intercalate, and therefore cannot probe between the GO sheets. This might explain the discrepancy between the experimentally determined BET and the theoretically calculated surface areas of ML-GO. Further work may be carried out to investigate this hypothesis.29


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CONCLUSIONS

ACKNOWLEDGMENT

Here we have identified the importance of hydrogen bonding, molecule size and functional group orientation, on intercalation into ML-GO. This is a direct proof showing that molecular size is not the only limiting factor for intercalation into ML-GO, a concept previously introduced in describing the use of ML-GO as a molecular sieve.9 The hydrogen bonding mechanism allowed for H2O, D2O, MeOH, EtOH and n-PrOH to easily intercalate into the ML-GO membranes. The intercalation process for the above molecules proved to be capable of displacing the naturally intercalated CO2 produced when the ML-GO was subjected to thermal treatment. Previous studies have shown the importance of hydration on CO2 permeability for GO membranes, with CO2 permeability increasing with a higher degree of hydration.30-31 Our insitu FTIR measurements have supported this observation by showing that preferential intercalation occurs for molecules with the highest hydrogen bonding capabilities. The work also demonstrated the effects of molecule size and functional group orientation on the intercalation process, as intercalation time increased with increasing molecule size, while the bulkier i-PrOH was incapable of intercalation on a similar timeframe.

This work was supported by the Canada Research Chair Program, the Natural Sciences and Engineering Research Council of Canada, the McGill Engineering Doctoral Award, the Center for Self-Assembled Chemical Structures, the Fonds de Recherche du Québec – Nature et Technologies, the European FP7 PHOTOMAT (proposal n0. 318899) and the H2020 Mat4Treat (MSC Actions RISE, grant agreement no. 645551) projects. Additionally, Giuliana Magnacca is grateful to the University of Turin and Compagnia di San Paolo for funding Project Torino_call2014_L2_126 through “Bando per il finaziamento di progetti di ricerca di Ateneo – anno 2014” (Project acronym: Microbusters).

In-situ FTIR with probe gas delivery is a viable candidate to characterize the surface and interlayer properties of ML-GO, as it directly shows the interaction between MLGO and probe species. Using this technique, we can expand our knowledge of intercalation mechanisms by testing species with differing functional groups or complex molecular structure. Future works will also include quantitative measurements to determine the amount of probe species intercalated or lost in the ML-GO samples during vacuum treatment and probe molecule delivery, which can be accomplished, for example, using microcalorimetry of adsorption32-33 or flow microbalance34.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The supporting information for this work contains elemental analysis for the GO, images of the experimental setup, including the homemade quartz cell and vacuum line, additional in-situ FTIR spectra, Raman spectra, XRD spectra and structural information about the probe species in this work. (PDF).

AUTHOR INFORMATION Corresponding Author

*Email: [email protected] *Telephone: 1 (514) 398-5496 Notes The authors declare no competing financial interests.

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GO after 125oC treatment

After probe molecule delivery

4000

Intercalated CO2

2800

1600

Wavenumber (cm-1)

Gas delivery

400

Absorbance (a.u.)

CO2 Displacement

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


No CO2 Displacement

4000

Water

2-Propanol

Helium

2800

1600

Wavenumber (cm-1)

400

TOC Graphic

ACS Paragon Plus Environment

7

Intercalated Species in Multilayer Graphene Oxide: Insights Gained

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