# Temperature dependent intercalation of molten 1-hexadecanol into Brodie graphite oxide

Andreas Nordenström, Artem Iakunkov, Nicolas Boulanger, Gui Li, Christoph Hennig, Igor Baburin, Mads Jørgensen, Innokenty Kantor, Alexander V. Talyzin

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Andreas Nordenström, Artem Iakunkov, Nicolas Boulanger, Gui Li, Christoph Hennig, et al. Temperature dependent intercalation of molten 1-hexadecanol into Brodie graphite oxide. Carbon, 2023, 203, pp.770–784. 10.1016/j.carbon.2022.12.030. hal-05094701

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# Temperature dependent intercalation of molten 1-hexadecanol into Brodie graphite oxide

Andreas Nordenström<sup>a, 1</sup>, Artem Iakunkov<sup>a, 1</sup>, Nicolas Boulanger<sup>a</sup>, Gui Li<sup>a</sup>, Christoph Hennig<sup>b, c</sup>,
Igor Baburin<sup>d</sup>, Mads Jørgensen<sup>e, f</sup>, Innokenty Kantor<sup>f, g</sup>, Alexander V. Talyzin<sup>a, *</sup>

## ARTICLE INFO

### Keywords:

- Graphene oxide
- Graphite oxide
- Intercalation
- Swelling
- Phase transition

## ABSTRACT

Intercalation of very long molecules into the structure of multi-layered graphene oxide (GO) was studied using example of 1-hexadecanol (C16), an alcohol molecule with 16 carbon atoms. Brodie graphite oxide (BGO) immersed in excess of liquid C16 just above the melting point shows expansion of c-unit cell parameter from ~6 Å to ~48.76 Å forming a structure with two densely packed layers of C16 molecules in a perpendicular orientation relative to the GO planes (α-phase). Heating of the BGO-C16 α-phase in excess of C16 melt results in reversible phase transition into β-phase at 336–342K. The β-phase shows much smaller unit cell parameter of 29.83 Å (363K). Analysis of data obtained using vacuum-driven evaporation of C16 from the β-phase provides evidence for structure of β-phase consisting of five layers of C16 molecules in parallel to GO plane orientation. Therefore, the transition from α-to β-phase corresponds to change in orientation C16 molecules from perpendicular to parallel relative to GO planes and decrease in the amount of intercalated solvent. Cooling of the β-phase in absence of C16 melt is found to result in the formation of γ-phase with inter-layer distance of ~26.5 Å corresponding to one layer of C16 molecules intercalated perpendicularly relative to the GO planes. Structures with one and two layers of C16 molecules parallel to GO planes were identified in samples with rather small initial loading of C16. Surprisingly rich variety of structures revealed in the BGO-C16 system provides opportunities to create materials with precisely controlled GO inter-layer distance.

## 1. Introduction

Graphite oxide can be defined as a hydrophilic layered material prepared by oxidation of graphite and showing swelling in polar solvents. The interest in GO is often related to the possibility to disperse it on single sheets thus producing graphene oxide [1,2]. The dispersions can be drop-casted [3], spin-coated [4] or vacuum filtrated to prepare GO thin films [5], papers [6] or membranes [3,7–10].

Graphite oxides synthesized by different methods show strong variation in many properties [11], e.g. thermal exfoliation [12,13], swelling [7,14–16], mechanical strength of individual flakes [17], sorption of polar solvents [18] and sorption capacity towards radionuclides [19].

The Brodie method involves oxidation of graphite by sodium chlorate and fuming nitric acid [20]. However, most of modern studies have been done using materials produced by Hummers-Offeman method (most commonly named as Hummers' method) which involves oxidation of graphite by KMnO₄ in H₂SO₄ and NaNO₃ [21], notably with many modifications reported over past decade [22–26]. Oxidation of graphite results in adding of several types of functional groups on the basal planes, most importantly hydroxyls and epoxy, while carboxyls and carbonyls are the main functionality on the edges of flakes. Non stoichiometric composition of GO in combination with complete disorder in positions of oxygen groups is general for all kinds of graphite/graphene oxides [2,27–31]. As a result of oxidation, the inter-layer distance of GO

<footnote><sup>a</sup> Department of Physics, Umeå University, Umeå, SE-90187, Sweden</footnote>

<footnote><sup>b</sup> Institute of Resource Ecology, Helmholtz Zentrum Dresden Rossendorf, Bautzner Landstrasse 400, 01328, Dresden, Germany</footnote>

<footnote><sup>c</sup> The Rossendorf Beamline, European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38043, Grenoble, France</footnote>

<footnote><sup>d</sup> Theoretische Chemie, Technische Universität Dresden, 01062, Dresden, Germany</footnote>

<footnote><sup>e</sup> MAX IV Laboratory, Lund Universiy, Fotongatan 2, 225 94, Lund, Sweden</footnote>

<footnote><sup>f</sup> Department of Chemistry and iNANO, Aarhus University, Langelandsgade 140, DK-8000, Aarhus C, Denmark</footnote>

<footnote><sup>g</sup> Department of Physics, The Technical University of Denmark, Fysikvej, DK-2880, Kgs. Lyngby, Denmark</footnote>

<footnote>* Corresponding author.
E-mail address: alexandr.talyzin@umu.se (A.V. Talyzin).</footnote>

<footnote n="1">¹ Contributed equally.</footnote>
increases to ~6-8Å [1,32] compared to ~3.3 Å in precursor graphite [33].

Swelling is one of the most remarkable properties of GO [34]. It is well known that GO swells in polar solvents with expansion of inter-layer distance from 6-8Å up to 40-50 Å depending on solvent [35–37]. Remarkably, swelling in many solvents is very different for GO produced by Brodie method (BGO) and Hummers’ method (HGO). For example, BGO is intercalated by one layer of methanol, ethanol and propanol at ambient conditions (in excess of solvent) while increase of pressure or decrease of temperature leads to reversible phase transition into two-layered solvate phase [4, 16, 18, 38–40]. Similar phase transitions were also found for BGO in many other polar solvents [15] while HGO exhibits much stronger expansion of lattice and gradual changes of inter-layer distance upon pressure and temperature variations due to effects of interstratification and interstratification [38,41]. Most likely the difference is related to more defect state of HGO and more inhomogeneous distribution of oxygen-containing functional groups over the graphene oxide surface [14]. Therefore, BGO is more important as a model system for understanding GO swelling.

Expansion of GO lattice by swelling defines permeation properties of graphene oxide membranes and provides the possibility to make porous pillared materials [42–44]. Pillared GO materials with high surface area could be useful for many possible applications, e.g. gas storage [43], materials for supercapacitors [23] and membranes [45]. The main route to prepare pillared porous GO materials includes first expanding pristine GO lattice by swelling in proper solvent followed by insertion of “pilling” molecules in dissolved state and attaching them to the graphene oxide planes [43]. It is obvious that expansion of GO structure by swelling is the limiting factor on the size of inserted pillaring molecules. Therefore, understanding and control of GO swelling are of major interest for preparation of pillared structures.

Swelling in 1-alcohols provides one of the most convenient route to control inter-layer distance of GO. Alkyl alcohols are inexpensive, nontoxic and easily available in a large set of homologous series of molecules. Already in 1960-s it was reported that insertion of progressively longer 1-alcohols into GO structure results in increase of inter-layer distance up to ~50 Å for 1-alcohol with 18 carbon atoms in the chain [36,46]. However, these studies were limited only to X-ray diffraction method and almost completely forgotten for over 50 years. In modern times, most of the studies of GO swelling were performed only for methanol, ethanol and propanol [16,38,40,47,48]. Recently we reported also detailed studies of GO swelling in alcohols with chain length of up 9 carbon atoms (1-nonanol) [49,50]. Swelling by ~2.5 Å was reported at ambient temperature for smallest alcohols (methanol, ethanol, propanol) [37], while for butanol and pentanol the structure expands significantly stronger (7-9Å). The expansion is larger for alcohols with longer carbon chain [36]. D. MacEwan and J.Cano Ruiz [36] suggested that alcohol molecules with less than four carbon atoms are intercalated with parallel to graphene oxide sheets orientation. The orientation of inserted molecules was suggested to change from “lying down” to “standing up” for alcohols with higher number of carbon atoms. They also suggested that temperature dependent change of GO-alcohol structure can be related to change from parallel to stand up orientation of intercalated alcohol molecules [36]. However, our earlier studies of BGO in methanol and ethanol provided evidence for layered nature of alcohol intercalation. The phase transition between 1-layer and 2-layer solvate structure was found in BGO immersed in methanol, ethanol and propanol upon cooling or upon increase of pressure [16,40,47]. The layered intercalation of alcohols was confirmed also for all alcohols up to carbon number 9 (1-nonanol) [49,50]. For example, ambient solvate of BGO in liquid octanol includes four layers of solvent molecules, which can be removed one by one using vacuum annealing [49,50].

The data reported in the old papers from 1960-s are very scarce providing only the values of inter-layer distance for very long alcohols with carbon number up to 18 and not showing recorded XRD patterns [36,46]. The study by D. MacEwan and J.Cano Ruiz [36] reported the

values of inter-layer distance recorded for a full set of alcohols (1–18 carbon atoms) at ambient temperature and for some alcohols the value recorded at 70 °C. The values of inter-layer distance reported for 70 °C in Ref. [36] for alcohols with number of carbon atoms 12–15 are 11–13 Å smaller than inter-layer distance recorded at ambient temperature, thus indicating possible phase transformation. Interestingly, the change in inter-layer distance is significantly larger than thickness of one alcohol layer thus indicating rather different phase transition compared to transitions observed in BGO intercalated with small alcohols. The temperature dependent structural change in BGO solvates with long alcohols was confirmed also in the PhD thesis by R.Kruger from 1980 [51]. However, the data from this thesis (written in German) were never published in peer-reviewed journals except for some numbers recently cited in our review paper [34]. Moreover, the BGO-alcohol solvates in the R.Kruger’s study were prepared by using ethanol solution of long alcohols, thus forming a triple phase system which further complicates the analysis [51].

In this study we performed a complex study of BGO swelling in 1-hexadecanol (named in following as C16 according to the number of carbon atoms), as a function of temperature using X-ray diffraction, TGA and DSC. The swelling of BGO in excess of molten C16 results in the formation of a structure with two layers of vertically standing C16 molecules and inter-layer distance of ~49 Å at temperature just above the melting point of C16. This phase is also stable at ambient temperature thus providing an example of solid GO solvate with some of the largest ever-reported inter-layer distance.

Phase transition with strong decrease of the inter-layer distance occurs in BGO-C16 system upon heating at around 336–342K. It is demonstrated that the phase transition corresponds to incongruent melting of ambient temperature phase with partial release of C16 from the structure and simultaneous change in orientation of remained intercalated C16 molecules from nearly perpendicular to GO to parallel layered structure. The transition is reversible and C16 released from ambient temperature phase above the point of transition is intercalated back into BGO structure upon cooling. Several other phases with smaller inter-layer distance were observed in BGO-C16 system if some alcohol was removed at high temperature by vacuum heating and in experiments with heating/cooling of BGO mixed with progressively smaller amount of C16. The structural changes observed in BGO-C16 are likely to be common for BGO intercalation with other long chain alcohols. Therefore, BGO-C16 can be considered as a model system for studies GO swelling in 1-alcohols with melting point above ambient temperature.

## 2. Material and methods

### 2.1. Synthesis of BGO

Graphite (10.25 g) purchased from “Alfa Aesar”, (natural, ~325 mesh) was mixed in three neck flask with 85.44 g grinded sodium chlorate (NaClO₃ Sigma Aldrich, <99%). 60 mL of fuming nitric acid (HNO₃ Sigma Aldrich, >90%) was slowly added through dropping funnel under intense mechanical mixing. Three neck flask was connected to water trap in order to collect gases evolving in the process of reaction. After adding the fuming nitric acid over a period of about 2 h, mixture of graphite and sodium chlorite became viscous and 10 min later turned into a homogeneous paste. At that moment, magnetic stirring was used instead of mechanical and the mixture was left overnight at 400 rpm stirring in room temperature. Note that leaving the mixture without stirring for prolonged time might result in explosion. After 20 h of stirring, additional 30 ml nitric acid was added into reaction volume to liquefy mixture. After that, temperature was increased to 60 °C for 6 h. Reaction products were washed by 1 L 10% hydrochloric acid, 2 L of water and freeze-dried. The yield of reaction was 14.5 g of brown colored powder.
## 2.2. Characterization

The BGO batch used in this study was characterized after preparation using XRD, XPS, FTIR, TGA/DSC (Figs. S1–S4 in SI file). Detailed analysis of characterization data for BGO prepared using identical method, differences compared to HGO and swelling of BGO in variety of polar solvents were also provided in our earlier publications [11,14,34,49,52, 53]. The batch studied here showed C/O = 2.65 and negligible component due to C=O in C1s XPS spectra indicating rather low number of hole defects typical for HGO [19]. The averaged inter-layer distance provided by XRD is given by d(001) = 6.0–6.2 Å. Note that it depends on humidity at ambient air conditions. 1-Hexadecanol (99% purity) was purchased from Sigma Aldrich.

XPS spectra were recorded with a Kratos Axis Ultra electron spectrometer equipped with a delay line detector. A monochromatic Al Kα source operated at 150 W, a hybrid lens system with a magnetic lens, providing an analysis area of 0.3× 0.7 mm, and a charge neutralizer were used for the measurements. The binding energy scale was adjusted with respect to the C1s line of aliphatic carbon, set at 285.0 eV. All spectra were processed with the Kratos software. TGA was done using a Mettler Toledo TGA/DSC1 STARe System. Experiments were performed from room temperature up to 700 °C at a heating rate of 3 K/min under nitrogen or air flow (40 mL min⁻¹).

DSC scans with samples of BGO-C16 were recorded using STARe system DSC 3 by Mettler Toledo under flow of nitrogen, typically with 2 K/min. The powder samples were studied using aluminum DSC capsules.

## 2.3. Temperature dependent XRD experiments

Part of the experiments with heating and cooling was performed at the Rossendorf Beamline (BM20) [54], ESRF, using a Cryostream 800 System (Oxford Cryosystems) and a radiation wavelength of λ = 0.7381 Å (16.798 keV). Experimental data were collected in transmission geometry using a DECTRIS Pilatus3 X 2 M Si area detector and extracted with the Bubble software [55]. The detector geometry parameters were calibrated with PyFAI using the NIST standard LaB₆. The intensity of X-ray beam was adjusted using filters to avoid beam-induced damage.

Another part of data was recorded at the DanMAX beamline at the MAX IV Laboratory. The temperature was controlled using a Cryostream 800+ (Oxford Cryosystems). The diffraction data were collected up to a maximum 2θ of 28° (Qmax ~3.75 Å⁻¹) using a wavelength of 0.8266 Å (15 keV) on a DECTRIS Pilatus3 X 2 M CdTe area detector (DECTRIS). The wavelength and geometry of the instrument were refined using a LaB₆ standard (NIST SRM660c) using the program PyFAI [56]. The raw area detector data were azimuthally integrated to intensity vs. 2θ using the MatFRAIA algorithm [57]. The beam was attenuated to avoid radiation induced damage to the samples.

Glass capillaries with diameters of 0.7–1 mm and transmission geometry were used in both sets of XRD experiments at ESRF and at MAX IV. Typically, the powder samples of BGO and C16 were ground together in different proportions, loaded into glass capillaries which were sealed to prevent solvent evaporation at higher temperatures. The mixture was then heated above the melting point of C16 using a controlled temperature ramp with simultaneous recording of XRD images (typically 1s per diffractogram). Some 10s exposures were also recorded at selected temperature points.

Some of the XRD experiments were performed using an in house Panalytical X'pert X-ray diffractometer with Cu Kα radiation. Cu Kαaverage (λ = 1.5418 Å) was used for calculation of d-spacing of the low angle reflections. A TTK600 stage by Anton Paar was used in Panalytical diffractometer for heating and cooling of BGO-C16 samples in reflection geometry. The same cell was also used for vacuum treatment of the BGO-C16 samples at temperatures up to 383K. The typical recording time was 10–60 min per pattern depending on the selection of the angular range.

## 3. Results

### 3.1. Phase transition in BGO immersed in excess of molten C16

Bulk 1-Hexadecanol (C16), C₁₆H₃₃OH, is solid at ambient temperature and melts at 322K. Therefore, two methods were tested to intercalate C16 into BGO structure. The first method is to expose BGO powder to molten C16 at elevated temperatures providing strong excess of C16 in order to achieve a saturated sorption/intercalation. The second method is to expose BGO powder to ethanol solution of C16. Both methods were earlier tested successfully for BGO intercalation with sugar alcohols, which are also solids at ambient conditions [52]. However, in situ XRD experiments with BGO powder exposed to saturated ethanol solution of C16 showed only reversible swelling in ethanol without any indications of C16 intercalation. Pristine BGO was recovered after evaporation of ethanol solution of C16.

In contrast, intercalation of BGO was found to be rapid and complete within minutes of exposure to C16 melt as illustrated by in situ XRD experiments presented below.

The sample of BGO and C16 mixed in proportion 1:7 (g/g) was loaded into glass capillary and heated using 0.5 K/min rate up to 363K with XRD patterns recorded every 1 s (Fig. 1).

As expected, the XRD pattern recorded from initial powder mixture shows reflections from BGO and many reflections from crystalline C16. The XRD pattern of pure BGO was also recorded separately (see Fig. S15). The XRD pattern of BGO showed only four reflections: strong (001), rather weak (002), (010) and (110). The angle range shown in Fig. 1 shows only one reflection from BGO, d(001) = 6.06 Å, in the XRD pattern of initial mixture recorded at ambient temperature.

Heating the mixture above the melting point of C16 results in swelling of BGO (Fig. 1). The pattern recorded at 330K (Fig. 2) shows no reflections from BGO and pure C16. Instead, sharp reflections from a new phase emerge. The low temperature BGO-C16 phase (in following named as α-phase) shows very strong and sharp (001) reflection, set of (00ℓ) peaks up to ℓ = 12 and reflections from in plane BGO lattice on the same positions as in precursor BGO. Using these reflections, unit cell parameters of α-phase were calculated as a = 2.485 Å and c = 48.761 Å

<figure><img src="image_4.png" /><figcaption>Fig. 1. Low angle part of XRD patterns recorded from BGO: C16 powder mixed in 1:7 proportion (by weight) upon heating with 0.5 K/min ramp up to 352K. Inset shows temperature dependence of d(001) of BGO phases formed by swelling in excess of molten C16. The sharp transition from low temperature α-phase into high temperature β-phase occurs at 338K. Selected XRD patterns included in this figure are shown also in Fig. 2. (λ = 0.8266 Å).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** XRD diffraction patterns recorded from initial mixture of BGO and C16 powder (1:7 g/g) at 298K, low temperature α-phase of BGO intercalated by C16 at 330K, high temperature β-phase at 363K and back to α-phase after cooling to 298K. (λ = 0.8266 Å). Peaks from pure solid C16 are marked by stars (*). (A colour version of this figure can be viewed online.)</figcaption></figure>

(330K) (Fig. 2). Therefore, the lattice of BGO expands by ~43 Å as a result of swelling in molten C16.

This value corresponds almost exactly to the length of two C16 molecules (22 Å + 22 Å = 44 Å). This observation provides a hint to possible arrangement of intercalated C16 in two layers of molecules in a “stand up” orientation nearly perpendicular to GO planes. Note that pure 1-hexadecanol forms a monoclinic structure with space group A 2/ a with Z = 8 and unit cell parameters a = 8.980 Å, b = 4.939 Å, c = 87.91 Å, β = 122.63° at ambient temperature. The c-unit cell parameter in this structure corresponds nearly exactly to the length of four C16 molecules arranged in close packed planes [58].

Only one reflection in the XRD pattern of BGO-C16 recorded at 330K cannot be indexed using the expanded GO structure. This additional reflection has d = 4.204 Å and should be assigned to partial ordering of C16 molecules intercalated between GO layers (Fig. 2). Note that this relatively broad peak was interpreted in earlier studies as an evidence of “solid” state of C16 intercalated in BGO structure [36]. The C₁₆H₃₃OH, is an almost rod-like molecule with a width of ~4 Å. Therefore, the reflection with d = 4.204 Å is likely to originate from the lattice formed inside of GO inter-layer space by some closely packed C16 molecules. Clearly, the structure of BGO encapsulated C16 is strongly disordered as evidenced by absence of other XRD reflections. However, reasonably good qualitative agreement is found when experimental XRD pattern is compared to theoretical pattern simulated using the model shown in Fig. 3. The model was constructed using general considerations and using the structure of pure 1-hexadecanol [58]. The theoretical XRD pattern is dominated by strong reflections from (00 *ℓ*) set as in experimental data. The experimental reflection observed at ~4.2 Å corresponds to overlapped (120) and (200) reflections in the theoretical model. All other reflections are significantly weaker even in the idealized ordered theoretical model. Even lower intensity of these reflections could be expected for strongly disordered true structure of GO.

Heating the BGO-C16 sample results in a sharp transition at 338K (Fig. 1). The peaks from the low temperature α-phase are replaced by a new set of (00*ℓ*) peaks with the c-unit cell parameter decreased by 18.2 Å. The pattern shown as an example of high temperature phase in Fig. 2 shows (001), (002) and (003) reflections which correspond to unit cell parameter of c = 29.831 Å (363K). This phase will be named in following as β-phase. The reflections from (00*ℓ*) set of β-phase also show

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Experimental (328K) and theoretical XRD pattern of the low temperature BGO structure (α-phase) intercalated with C16 molecules showing qualitative agreement. (λ = 0.738 Å) Inset shows idealized theoretical model with two layers of C16 molecules in “stand up” (relative to GO planes) orientation. (A colour version of this figure can be viewed online.)</figcaption></figure>

significant broadening compared to α-phase. The FWHM of (001) reflection of α-phase is 0.064° just below the phase transition showing increase to 0.081° in the β-phase. The absence of the reflection with d = 4.204 Å and higher broadening of (00*ℓ*) reflections indicates stronger disorder in the structure of β-phase of BGO-C16 (Fig. 2).

The phase transition between α- and β-phases of BGO-C16 is reversible, the low temperature α-phase is recovered after cooling (Fig. 2).

XRD data do not provide unambiguous structural information sufficient to identify structure of α- and β-phases due to the absence of specific reflections related to lattice of C16. The data only demonstrate strong change in the inter-layer distance (~18 Å), which is somewhat smaller compared to the length of vertically standing C16 molecule (22 Å) and indicate stronger disorder of C16 molecules in the β-phase. However, the same inter-layer distance can hypothetically be provided by intercalation of densely packed molecules or by fewer molecules in “pillared” structure. Moreover, the same inter-layer distance in the expanded GO structure can be modeled using vertically standing intercalated molecules or by model with several layers of C16 parallel to GO layers. Therefore, some additional experiments were performed in order to obtain information about the orientation and amount of intercalated C16.

The reversible α-to β-phase transition was confirmed using DSC method (Fig. 4). In good agreement with literature data [59,60], the reference DSC scan recorded from pure C16 revealed an endothermal peak from melting (0.25 kJ/g, 59.64 kJ/mol) with onset temperature 321 K and two exothermal peaks during the cooling: first peak explained by solidification of melt and second peak due to polymorphic phase transition in solid C16.

The DSC scan recorded from pre-melted mixture of BGO and C16 powders (1:5 g/g proportion) reveals new features, which correspond to reversible α-to β-phase transition. The enthalpy of transition from α- and β-phase of BGO-C16 according to the DSC is 7.47 kJ/mol BGO (0.42 ± 0.04 kJ/g BGO) with onset temperature of 340 K. Considering total mass of material (C16 + BGO) the enthalpy of transition is 99.9 kJ/g. The temperature of transition between α- and β-phase of BGO-C16 is in good agreement with XRD data. The transition is reversible according to DSC scan, which showed 3 exothermal features during the cooling. First peak
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** DSC scans recorded from: a) pure C16 and b) sample of BGO:C16 in proportion 1:5 by weight. Onset temperatures and enthalpies are shown for main features. The sample was first heated above melting point of 1-hexadecanol in order to form low temperature phase of BGO intercalated by C16. On the next step the mixture was cooled back to ambient temperature and after that subjected to 2 heating-cooling cycles between ambient temperature and 363K. The first of these two cycles is shown in the figure. The second cycle showed the same features at the same temperatures and very similar enthalpy values. Complete time line of the experiment is shown in SI file. (A colour version of this figure can be viewed online.)</figcaption></figure>

is found at 336 K (transition from β-phase to α-phase of BGO-C16) and two other peaks correspond to solidification of C16 melt and poly-morphic transition (as in the DSC scan of pure C16). The same features were recorded also during next heating-cooling cycle (full scan is shown in Fig. S5).

The composition of α-phase of BGO-C16 at the temperature point of melting can be easily determined by comparing melting enthalpy of pure C16 and C16 with known amount of added BGO. The C16 absorbed by BGO does not contribute to the enthalpy of C16 melting. The same method was used to determine composition of BGO solvates with small alcohols in our earlier publications [16,50]. In this particular experiment (Fig. 4) the melting enthalpy had decreased to 23.44 kJ/mol indicating that the amount of “free” (not intercalated) solvent which undergoes melting decreased by factor of 2.54 due to sorption by BGO.

This corresponds to sorption of 3.15 g/g or 0.23 mol/mol (C16/BGO). Molar mass 18 was used for BGO according to composition CO<sub>0.38</sub> (corresponding to C/O = 2.65 found by XPS).

The DSC and XRD data shown above do not provide direct information about the composition of β-phase. Therefore, two alternatives could be suggested:

- Transition occurs solely due to a change in orientation of intercalated C16 molecules, e.g. from stand up orientation perpendicular to GO planes to orientation parallel GO planes. This type of transition was suggested in early studies of GO intercalation with alcohols [36].

- The change in inter-layer distance revealed by XRD corresponds to change in composition. Part of C16 intercalated into BGO at lower temperature escapes from the structure above the point of transition into β-phase resulting in smaller inter-layer distance. This scenario was demonstrated earlier for BGO immersed in shorter alcohols (number of carbon atoms C = 1–9) [16,18,49,50].

The second scenario suggests that some C16 is released from the solid BGO-C16 and returned into the liquid melt above the point of transition. The data presented below support this scenario. As discussed in our earlier studies, this type of transition can be considered as incongruent melting of the α-phase [18].

It is interesting to compare the enthalpy of α-to β-phase transition with enthalpy of pure C16 melting. The enthalpy of transition (0.42 kJ/g BGO) corresponds to 0.13 kJ/g C16, considering only the amount of absorbed C16 into BGO/C16 structure. This 0.13 kJ/g value corresponds to 0.11 mol/mol of C16/BGO. That is just a little less than half of the C16 inserted into BGO in α-phase (0.23 mol/mol C16/BGO). Of course, the structural state of C16 intercalated into BGO structure can be rather different compared to bulk C16 which also affects the enthalpy of transition. However, as a rough estimation the DSC data suggest that less than half of C16 present in α-phase melts above the point of transition into the β-phase and escapes from the structure.

More precise and direct evaluation of β-phase composition appeared to be not straightforward. The isopiestic method used in our earlier studies for BGO intercalated with small alcohols is impossible to apply to the BGO-C16 system. The method suggests to expose BGO powder to vapors of solvent at certain temperature and to evaluate sorption from vapor by weight change. C16 has a very small vapor pressure at the temperature interval of BGO stability (up to ~120 °C) making the sorption too slow for measurements. Therefore, some less direct methods had to be used.

## 3.2. Phase transformations in samples of BGO-C16 pre-mixed with amount of C16 smaller compared to composition of α-phase

### 3.2.1. DSC scans and structural analysis of samples subjected to heating/cooling cycles

A simple set of DSC experiments with progressively smaller amount of C16 added to BGO was planned initially as a method to determine approximate composition of β-phase. A sample with initial BGO:C16 ratio exactly corresponding to composition of β-phase was expected to show no anomaly due to β to α-phase transition in the cooling part of DSC scan. The transition requires the presence of molten C16 in amount sufficient to convert β-phase into α-phase. Therefore, the decrease in amount of loaded C16 (relative to composition of α-phase) was expected to result in progressively smaller transition DSC peak until it completely disappears indicating that the loaded amount of BGO and C16 corresponds to exact composition of β-phase. As discussed below, this set of experiments did not provide exact information about composition of β-phase. However, this set of experiments revealed existence of new anomaly assigned to phase transition from β-phase to the new ambient temperature phase (γ-phase).

A set of powder samples deficient in amount of C16 relative to composition of α-phase (3.15 g/g C16/BGO) was prepared using
proportions 1:2, 1:1.37, 1:1, 1:0.7, 1:0.6, 1:0.5 g/g BGO:C16. The samples were tested by DSC using the same procedure as in experiments shown in Fig. 4. All samples were first subjected to C16 melting in the DSC cell, cooled back to ambient temperature and then subjected to two heating-cooling scans between 283K and 383K. The DSC scans recorded in first and second cycles were nearly identical in all experiments confirming reversible changes. Fig. 5 shows scans recorded in the second heating-cooling cycle.

Since the amount of C16 in these samples is smaller compared to

saturated composition of α-phase, all solvent is absorbed by BGO above the melting point of C16 leaving some BGO in a state with not saturated sorption. The absence of liquid C16 in the second cooling–heating cycle is reliably detected in DSC experiments (no anomalies due to melting-freezing) (Fig. 5).

The DSC scan recorded from the sample with initial loading 1:2 g/g showed peaks due to transition from α-phase to β-phase during at 339 K and reverse transition on cooling part (336 K). As expected, these peaks are smaller (compared to 1:5 composition in Fig. 4) due to decreased

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** DSC scans recorded from BGO and C16 pre-mixed in different proportions, melted together in DSC cell and recorded with heating/cooling rate 2 K/min under nitrogen. The temperatures correspond to onset value and enthalpy values (kJ/g) are related to the weight of loaded BGO. (A colour version of this figure can be viewed online.)</figcaption></figure>
amount of C16, insufficient to convert all BGO into α-phase. Surprisingly, the DSC scan for 1:2 composition (Fig. 5a) showed a peak at 328 K upon heating and asymmetric peak at 326 K upon cooling, indicating the presence of an initially unexpected phase transformation (marked by question sign in Fig. 5a). These new anomalies were found also in the DSC scans recorded using samples with smaller C16 loading (Fig. 5b–f).

Decrease in amount of C16 added to BGO results in gradual decrease of peak at ~339 K (corresponding to α-to β-phase transition) until it completely disappears in 1:0.6 and 1:0.5 samples (Fig. 5). Using known enthalpy of transition from α-to β-phase measured using 1:5 sample (0.42 kJ/g BGO, 7.47 kJ/mol) it is possible to estimate amount of α-phase formed in other samples. For example, DSC scan recorded using 1:2 sample showed enthalpy of transition 2.75 kJ/mol, which corresponds to 37% of pure α-phase. Using the same calculation the amount of α-phase was 16%, 7% and 2% for samples with initial ratios of 1:1.37, 1:1 and 1:0.7.

The anomaly at ~328K (heating) is at maximum for BGO:C16 composition of 1:1.37 g/g (0.10 kJ/g BGO or 1.72 kJ/mol) and decreases for smaller loading of C16. This peak is not present in DSC scans recorded for the sample with excess of C16 (Fig. 4). Therefore, the trend shown in Fig. 5 suggests that the peak at 328K (heating) corresponds to some structure change related to the β-phase of BGO:C16.

The DSC data shown in Fig. 5 are interpreted here as evidence for the existence of additional phase (named as γ-phase in following). According to DSC data shown in Fig. 5, reversible phase transition between γ-phase and β-phase occurs at ~328K (heating) and is observed in all C16-deficient samples. Pure γ-phase can be obtained when β-phase is cooled down in absence of “free” molten C16. Note that α- and β-phases are stable in equilibrium with molten alcohol. The γ-phase appears only in C16 deficient samples when molten C16 is not present.

The structure of all samples shown in Fig. 5 was verified using XRD patterns recorded after the DSC scans (Fig. S6 in SI file) at ambient temperature. The XRD data revealed that samples with composition 1:2 and 1:1.37 showed weaker reflections from α-phase (inter-layer distance of ~49 Å) and reflections corresponding to (00/') set with c = ~27 Å. That is about 5 Å smaller than c-unit cell parameter of β-phase. Therefore, we assign these reflections to γ-phase revealed in DSC scans. Samples with initial loading 1:1 and smaller showed only traces of α-phase and strong peaks which correspond to set of (00/') with unit cell c = 26.7 Å (γ-phase).

XRD patterns recorded from all C16-deficient (compared to the composition of α-phase) samples showed also a broad peak around 4.14 Å (not found in β-phase) and (001) reflection of pure BGO, especially strong in samples with smaller C16 content. The presence of not intercalated BGO complicates exact evaluation of β-phase composition, which was initial motivation for the whole set of DSC scans with decreased C16-loading. Obviously, C16 melt appeared to be not sufficiently fluid (unlike e.g. methanol) leaving some BGO material not intercalated. The peak due to transition from α-to β-phase and back is rather small in 1:0.7 sample and disappears in DSC scans for compositions 1:0.6 and lower. However, when BGO is clearly present in these samples, the true composition of β-phase must include amount of C16 higher than that provided by loading ratio.

### 3.2.2. Structure of C16-deficient samples studied in situ at key temperature points

The DSC experiments revealed the presence of an additional phase, the γ-phase, obtained by cooling of the β-phase to ambient temperature in absence of C16 melt. However, the XRD data recorded only at ambient temperature after DSC scans do not provide information about the structure of BGO:C16 phases formed at high temperature. Therefore, another set of experiments was performed with samples of BGO pre-mixed with progressively smaller amount of C16. XRD patterns of these samples were recorded in situ at key temperature points. The powder mixture (BGO + C16) was first heated above the melting point of C16 (328 K) to induce intercalation of BGO. The first XRD pattern was

recorded after cooling of the pre-melted samples at 295 K, then XRD patterns were recorded in the temperature region of the α-phase+β-phase (333 K), the β-phase (363 K) and after cooling back to ambient temperature (295 K) (Figs. 6 and 7). Rapid heating/cooling was used between these temperature points.

XRD patterns recorded in the experiment with an initial loading of BGO:C16 of 1:1 are shown in Fig. 6. The melting of C16 resulted in the formation of only α-phase even when amount of melt is not sufficient to saturate all the BGO. The XRD pattern shown in Fig. 6a shows mixture of the α-phase with d(001) = 48.42 Å and BGO. The pattern recorded at 333K shows already a presence of second phase (β-phase) with d(001) = 28.95 Å (Fig. 6b). Note that 333 K is above the temperature of transition related to the γ-phase detected by DSC (~328K, Fig. 5). Heating to 363K results in transition of all α-phase into β-phase with d(001) = 29.84 Å (Fig. 6c). Cooling this sample back to ambient temperature results in a partial reverse transition. The XRD pattern in Fig. 6d shows presence of two phases: α-phase with d(001) = 47.75 Å and equally strong set of reflections which correspond to inter-layer distance of 26.7 Å. The latter corresponds to γ-phase.

Similar results were also obtained with samples pre-mixed with larger amount of C16 (1:2 sample, Fig. S8) except for a change in the relative intensity of the observed phases. Samples with smaller amount of C16 (1:0.66 (Fig. S9), 1:0.5 (Fig. S10), 1:0.35 samples (Fig. S11) also showed only reflections from BGO and the α-phase after pre-melting, coexistence of α- and γ-phases at 333K, pure β-phase at 363K and a mixture of α- and γ-phases after cooling back to ambient temperature. Lowering the amount of loaded C16 also results in a progressively larger part of sample remaining in not intercalated state (pristine BGO) even above the melting point of the alcohol. When the amount of C16 is decreased to proportion 1:0.2 g/g only a smaller part of the BGO is converted into intercalated structures as evidenced from XRD patterns showing rather strong peaks from pure BGO (Fig. 7). For this sample, the co-existence of two phases (α and γ) was found already at the first stage of experiment, after melting C16 and cooling back to ambient temperature.

The XRD pattern in Fig. 7a shows two (00/') sets, one from the α-phase and a second from the γ-phase. The pattern was recorded at ambient temperature after rapid heating to 333 K followed by rapid cooling. Note that in this case the γ-phase was obtained directly by intercalation at 333 K, without reaching the temperature of the β-phase transition. The pattern recorded in situ at 333 K (Fig. 7b) shows the dominating set of peaks from β-phase. As expected, the peaks from the α-phase are not apparent in the pattern recorded at 363 K (Fig. 7c) and dominated by the (001) reflection from pure BGO and reflections from β-phase. It also shows minor reflections which are likely to originate from a structure with two layers (2 L-phase) of C16 molecules in parallel to GO orientation, d(001) = 14.0 Å. The last XRD pattern recorded after cooling back at 295 K (Fig. 7d) shows only the set of (00/') reflections from the γ-phase and the (001) reflection of pristine BGO. No α-phase is found in this sample due to rather low C16 loading. The existence of the 2 L phase is even more obvious in the samples pre-mixed with smaller amount of C16 (1:0.05 g/g BGO/C16) (Fig. S12).

The data collected in the set of experiments with decreasing amount of C16 do not allow to determine exactly composition of β-phase due to inhomogeneous intercalation of BGO in the C16-deficient samples. The inhomogeneous intercalation is most likely related to the relatively high viscosity of the C16 melt. This would also explain why the α-phase of BGO:C16 (with composition 1:3.15 g/g) is found after melting even for the sample with much smaller C16 loading (1:0.05 g/g) (Fig. S12). Reasonably “pure” γ-phase (pure from α-phase but in a mixture with non-intercalated BGO) was observed in this set of experiments only when the β-phase cooled back to ambient temperature in absence of “free” C16 melt. However, once again, the presence of non-intercalated BGO in all samples from this set does not allow exact evaluation of the γ-phase composition.
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Low angle part of the XRD patterns recorded from samples with a 1:1 g/g ratio of BGO:C16 and annealed for 30 min at 328 K (slightly above melting point of C16): a) recorded after cooling to 295 K b) at 333 K c) at 363 K and d) at 295 K after cooling. The d-spacings shown in black are from the α-phase, in red from the β-phase, in blue-from the γ-phase. (λ = 0.738 Å).</figcaption></figure>

### 3.2.3. *In situ* heating/cooling XRD experiment with BGO-C16 sample deficient in amount of C16 compared to composition of α-phase

The structural changes in the BGO-C16 system with initial loading of C16 not sufficient to convert all material into the α-phase were studied additionally *in situ* at elevated temperatures using synchrotron radiation XRD.

The sample with an initial loading of BGO:C16 1:3 g/g (slightly deficient compared to the composition of α-phase) was heated to ~352K and cooled back with XRD patterns recorded every 1s (Figs. 8 and 9, S13-15). Heating this sample slightly above the melting point of C16 resulted in an incomplete conversion of BGO into the α-phase. It is evidenced by the presence of a weak (001) reflection of BGO together with a set of peaks corresponding to α-phase (Fig. 8, Fig. S15). Further heating resulted in appearance of a second minor phase appearing at ~336K. New (001) peak with d(001) = 28.9 Å at 336K shifted gradually to 31.5 Å at 341K (Fig. 8). Around 345K the remaining α-phase is rapidly and completely transformed into the β-phase.

As expected, the heating-cooling run showed incomplete reversibility of the α-to β-transition (Fig. 8 a,b). The data are interpreted as following. The smaller amount of C16 loaded into the initial mixture was not sufficient to convert all the BGO into the α-phase but sufficient for saturation of all BGO in the β-phase. The C16 escaping from BGO interlayers above the temperature point of transition is immediately absorbed by pure BGO present in the sample with some solvent remaining in liquid phase. As a result, only peaks from the β-phase are observed in XRD pattern recorded from this sample above 343K and no peaks from pure BGO (Fig. 8a). Since the part of liquid C16 was removed from melt

by sorption, a completely reversible transition from β-to α-phase during cooling is not possible. The XRD patterns recorded during the full heating-cooling cycle are represented in Fig. 9b showing positions and intensity of all major reflections as a function of temperature.

The cooling part of the cycle shows a sharp transition point at 338K which corresponds to sorption of all the molten C16 and transition from the β-phase into the α-phase. However, part of the β-phase is preserved below 338K due to the deficiency of C16. The XRD reflections from the β-phase are shifted to lower d-spacing upon further cooling in the temperature region 338K–325K (Fig. 9a) with the c-unit cell parameter decreasing from a maximal value of c = 31.205 Å (β-phase) down to a value of c = 26.59 Å at ambient temperature (γ-phase).

Fig. 9 clearly shows the existence of a transitional two-phase regions with a gradual change of the d(001) in the γ-phase both during heating and cooling. The α-phase is dominating in the two-phase regions. The interlayer distance is gradually changing upon cooling from 338 K to 325 K. Note that the temperatures 336 K and 322 K were found in DSC scans for the β-to α-phase and β-to γ-phase transitions, respectively (Fig. 5). In both cases, the transition corresponds to a change from a parallel orientation of C16 molecules with respect to the GO planes in the β-phase to a perpendicular orientation in the α- and γ-phases. The transformation from β-to γ-phase is completed at ~325 K and further cooling to room temperature do not result in any further changes (Fig. 9b).
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Low angle part of XRD patterns recorded from samples with a 1:0.2 g/g ratio of BGO:C16 and annealed for 30 min at 328K (slightly above the melting point of C16): a) recorded after cooling at 295 K, b) at 333 K, c) at 363 K, and d) at 295 K after cooling. The d-spacings shown in black are from α-phase, in red the β-phase, and in blue the γ-phase. Brown and green indexes correspond to the suggested 2-layered and 1-layered structures, respectively (see text). The (001) reflection from non-intercalated BGO is present in all patterns. (λ = 0.738 Å).</figcaption></figure>

### 3.3. Vacuum driven evaporation of C16 as evidence of layered structure of β-phase

The composition and structure of the β- and γ-phases could not be reliably determined using the data described above which motivated us to perform an additional experiment using vacuum annealing of BGO-C16. Experiments with gradual removal of solvent by vacuum treatment allow to provide information about parallel orientation of solvent layers intercalating BGO structure if the layers can be evaporated one by one forming distinct phases detectable by XRD.

Similar experiments helped to establish layered structure of BGO intercalated by 1-octanol in our earlier study. Evaporation of 1-octanol revealed set of phases corresponding to removal of octanol layer by layer starting from 4-layered BGO solvate phase [49]. These “kinetic” phases corresponding to 3-layered, 2-layered and 1-layered intercalation of 1-octanol were later confirmed by TGA method [50].

The ideal experiment would include the removal of C16 from the BGO-C16 structure both at lower temperature for the α-phase and at higher temperature for the β-phase. However, C16 melt has significantly smaller vapor pressure than liquid octanol. Our attempt to study thermal decomposition of BGO-C16 material at 363K under nitrogen was not successful due to the extremely slow rate of C16 evaporation. The rate of C16 evaporation under a flow of nitrogen was also too slow for detecting stepwise weight changes using TGA. However, vacuum annealing at 343K and 363K resulted in a sufficiently rapid loss of C16 intercalated into BGO structure within the temperature interval of GO stability.

The sample was prepared using 1:3.5 g/g BGO/C16 ratio and

converted into pure α-phase by heating at 333K. The α-phase sample was then heated to 363K transforming it into β-phase with slight excess of C16 melt and subjected to vacuum treatment for different periods of time while recording in situ XRD patterns (Fig. 10).

Vacuum annealing at 363 K results first in evaporation of all “free” molten C16 followed by gradual removal of solvent from the β-structure. The structural changes in the β-phase were detected by XRD already after 15 min of vacuum annealing at 363 K. The pattern recorded after 1000 min of annealing showed only three reflections: the first corresponds to pure BGO with d(001) = 5.84 Å and two reflections which correspond to the (001) and the (002) from a structure with a c-unit cell parameter of 9.81 Å. This value of inter-layer distance is in agreement with BGO structure intercalated with one layer of C16 molecules parallel to GO planes. The patterns recorded after 45 min, 95 min, and 320 min of vacuum annealing showed also reflections interpreted as d(001) = 14.20 Å and d(002) = 7.21 Å, from BGO intercalated with two layers of C16 molecules parallel to GO. The samples annealed for 15 min and 45 min showed reflections from one more additional phase with d(001) = 19.5 Å which are interpreted as a 3-layer solvate. The patterns recorded after 45 min of vacuum annealing also showed a set of reflections corresponding to d(001) = 23.10 Å (Fig. 10), interpreted as 4-layer solvate. Note that in this experiment we used a standard in-house diffractometer with set up not allowing to record very low angles.

The whole set of data presented in Fig. 10 can be summarized as following. The initial β-phase has a c-unit cell parameter of 30.20 Å at 363K. This phase is not stable under vacuum conditions due to evaporation of C16 molecules. Prolonged vacuum treatment results in almost
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** a) Low angle part of XRD patterns recorded from BGO: C16 powders mixed in 1:3 proportion by weight at ambient temperature (302K), upon rapid heating above the melting point of C16 (328 K) followed by a controlled 0.5 K/min heating ramp up to 352 K. Only about 50% of recorded patterns are shown for clarity. The red oval emphasizes the region which indicates the appearance of the new reflection originating from γ-phase. b) XRD patterns recorded upon cooling from 353 K to 322 K. The partial transition from the pure β-phase into a two phase mixture of the α- and β-phases occurs at 339 K. (λ = 0.738 Å).</figcaption></figure>

complete evaporation of C16 molecules resulting in the formation of almost pure BGO. However, a set of intermediate phases is observed in process of gradual C16 removal showing reflections with c-unit cell parameters 23.7 Å, 19.2 Å, 14.6 Å and 9.8 Å. Assuming the thickness of one C16 molecule layer to be about 4.5-5Å this set of c-parameters could be interpreted as corresponding roughly to 4-layer, 3-layer, 2-layer and 1-layer structures with solvent layers oriented parallel to the GO planes. Only the set of peaks with d(001) = 23.7 Å (instead of ~25 Å) is slightly off the trend expected for the layered model. Note that none of these intermediate phases was observed in the pure state. Similar set of phases

was observed also in a separate experiment with vacuum annealing performed at lower temperature of 343K (Fig. S16).

Summarizing this part, we suggest that the structure of β-phase includes five layers of C16 molecules in a parallel to GO orientation. The layers can be removed one by one using vacuum driven evaporation of C16 from the BGO inter-layers. Therefore, the transition from the α-phase to the β-phase of BGO-C16 is likely to correspond not only to partial removal of intercalated molecules (incongruent melting) but also a change in orientation of C16 molecules from two perpendicular orientation layers in α-phase (Fig. 3) to highly disordered layers parallel to GO in β-phase.

The remaining question is the change in structure and composition of the γ-phase obtained after cooling β-phase back to ambient temperature. According to the DSC data the cooling of the β-phase in the absence of excess molten C16 results in a sharp transition and according to XRD data in a gradual decrease of c-unit cell parameter from ~30 Å to ~26-27 Å. As noted above, the distance between the GO layers in the γ-phase is in very good agreement with one layer of C16 molecules oriented normal to the GO planes. Ideally, experiment with vacuum annealing of the γ-phase could possibly help to rule out an alternative geometry of C16 intercalation with a layered structure of the alcohols. Unfortunately, vacuum-driven evaporation of C16 is too slow at ambient temperature and cannot be used as a method to distinguish between either a structure model with C16 molecules arranged perpendicular or parallel with the GO planes.

To get further confirmation of this structural model we performed an additional set of experiments. The vacuum annealing was performed at 363K, as in experiment shown in Fig. 10 for different periods of time. The difference here is that XRD was recorded after cooling back to ambient temperature. According to the data shown in Fig. 10 vacuum annealing results in formation of several phases corresponding to 5 L (β-phase), 4 L, 3 L, 2 L and 1 L structures at 363K. However, XRD in this experiment was recorded after cooling back to ambient temperature. The C16 intercalated into the BGO structure could be expected either to remain in parallel orientation or change orientation from parallel to vertical relative to the GO sheets after the cooling. If the amount of C16 added to the BGO corresponds exactly to the composition of β-phase, the transition into pure γ-phase should be observed and composition of β-phase thus established approximately.

Experiments shown in Fig. 11 were performed with bulk powder samples. The change of weight due to vacuum annealing was recorded to monitor evaporation of C16.

The bulk sample of BGO:C16 was mixed with slight excess of solvent (1:3.5 g/g BGO/C16) compared to the ideal composition of α-phase found by DSC and pure α-phase was prepared by heating at temperature slightly above melting point of C16. This powder sample of α-phase was split on 7 parts (about 50 mg each) and annealed in oven at 363K under vacuum conditions. The samples were taken out of furnace after heating for different time periods. The weight of samples was measured before and after vacuum annealing providing information about change of BGO:C16 ratio.

Assuming that all weight loss occurs due to evaporation of C16, the ratio has changed from 1:3.5 in the initial sample to 1:3.3, 1:2.05, 1:2.02, 1:1.61, 1:0.88, 1:0.61 and 1:0.36 (g/g) for samples annealed for of 40 min, 90 min, 150 min, 220 min, 300 min, 420 min and 660 min, respectively. According to the *in situ* experiment performed at 363K, vacuum annealing of the β-phase results in the formation of intermediate layered phases due to a gradual removal of C16 from the BGO interlayers (Fig. 10). According to the data shown in Fig. 11 not all of these phases are preserved after cooling back to room temperature.

The sample vacuum annealed for 40 min showed only peaks of α-phase after cooling back to ambient temperature, as expected from the composition 1:3.3 g/g (not shown in Fig. 11). The sample annealed for 150 min showed new set of peaks assigned to the γ-phase. The mixture of α- and γ-phases was observed in the sample annealed for 220 min. Finally, almost pure γ-phase with c-unit cell parameter of 27.08 Å was
<figure><img src="image_13.png" /><figcaption>Fig. 9. a) Temperature dependence of the d(001) recorded upon cooling BGO-C16 in the temperature interval of phase transitions. Arrows indicate the temperature points corresponding to β-to α-phase transition (338K) and β to γ-phase transition (325K). b) Waterfall plot showing evolution of d-spacing for main XRD reflections of BGO-C16 in a complete heating-cooling cycle. (λ = 0.738 Å).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 10. XRD patterns recorded in situ during vacuum annealing from the sample of BGO:C16 with 1:3.5 g/g initial loading and vacuum annealed at 363K. Reference XRD pattern recorded at ambient temperature (α-phase) is shown as reference. The sample was first heated to 363K to convert the α-phase into the β-phase and then the vacuum annealing applied for different periods of time 15–1000 min (λ = 1.5406 Å). (A colour version of this figure can be viewed online.)</figcaption></figure>

found in the sample annealed for 300 min with final composition 1:0.88 g/g BGO:C16. Note that the XRD pattern of the γ-phase includes reflection with d = 4.16 Å (Fig. 11) which is absent in β-phase (Fig. 2. This reflection serves as an indicator of a partial ordering of C16 molecules in the α- and γ-phases.

The amount of C16 intercalated in γ-phase is about 4 time lower (by weight) compared to the composition of the α-phase while the c-unit cell parameter is only about ~1.7 times smaller. Therefore, the structure of the γ-phase is likely to be composed of “diluted” layer with some empty space between vertically standing molecules as opposed by close-packed C16 molecules in the α-phase. Assuming that transition from the β-phase into the γ-phase occurs only due to change in orientation of C16 molecules, the 1:0.88 g/g BGO/C16 is our best estimate for compositions of both γ- and β-phases.

The γ-phase was not found in the samples subjected to vacuum annealing longer than 300 min. Longer vacuum annealing (420 min and 660 min) provided (after cooling) samples with rather similar XRD

<figure><img src="image_15.png" /><figcaption>Fig. 11. XRD pattern of the BGO:C16 sample mixed in 1:3.5 wt proportion heated to 363K and cooled back to ambient temperature (pure α-phase). Indices corresponding to reflections from the α-phase are shown in black. The samples were vacuum annealed for different periods of time at 363K and XRD patterns were recorded after cooling at ambient temperature. The weight loss during vacuum annealing was measured and the new BGO:C16 proportion was calculated (shown in the figure for each sample). (λ = 1.5406 Å).</figcaption></figure>

patterns which were interpreted as a mixture of two phases: one with c-unit cell parameter of 14.2 Å and second with 9.76 Å. These values are in agreement with the structures of BGO intercalated with two layers (2 L) and one layer (1 L) of C16 molecules in parallel with the GO planes orientation. In contrast to the data recorded from the vacuum annealed samples at 363K, no intermediate phases with c-unit cell parameter corresponding to 3-layer or 4-layer intercalation were observed in the XRD patterns recorded at ambient temperature (Fig. 11). Considering the relatively small change in composition of the samples obtained after annealing for 300 min (1:0.88 g/g) and 420 min (1:0.61 g/g) it is unlikely that 3-layered and 4-layered phases actually exist at ambient temperature. As expected, none of the annealed samples showed presence of β-phase after cooling back to ambient temperature.

The enthalpy of transition between γ- and β-phases was evaluated using DSC scan (Fig. 12a) performed using sample of γ-phases produced by vacuum driven evaporation of C16 melt from α-phase of BGO-C16. The time required to produce reasonably pure γ-phase sample was determined using experiments shown in Fig. 11. Using the weight measurements before and after annealing the overall composition of
<figure><img src="image_16.png" /><figcaption>Fig. 12. a) DSC scan performed with the purpose to determine enthalpy of γ-to β-phase transition. Almost pure sample of γ-phase was used in this experiment as evidenced by XRD shown in the panel b. b) XRD pattern of sample studied by DSC showing strong reflections from γ-phase, rather weak reflections from α-phase and no reflections from BGO. (A colour version of this figure can be viewed online.)</figcaption></figure>

studied sample was 1:1 g/g BGO/C16. The XRD recorded from this sample confirmed that it consists of nearly pure γ-phase (Fig. 12b). The DSC scan reveals major peak related to reversible γ-to β-phase transition (0.12 kJ/g BGO) and minor peak from α-to β-phase transition (0.02 kJ/g). Using earlier determined enthalpy of α-to β-phase transition (0.42 ± 0.04 kJ/g BGO, Fig. 4) the overall amount of α-phase impurity in the sample can be estimated to be as small as 4.8%. Therefore, the enthalpy of γ-to β-phase transition corrected by excluding contribution from α-phase is 0.13 ± 0.01 kJ/g BGO.

Notably the enthalpy of γ-to β-phase transition is about 3-fold smaller compared to the enthalpy of α-to β-phase transition. That is in good agreement with the difference in compositions of α- and γ-phases (1:3.15 g/g BGO/C16 and 1:0.9 g/g BGO/C16 respectively) which is also about 3-fold.

Summarizing all the data, two BGO-C16 phases were found at ambient conditions: the α-phase with a unit cell c parameter of about 49 Å and the γ-phase with unit cell c axis of ~28 Å. Considering that the inter-layer distance of BGO is 6 Å, we suggest that the structure of the α-phase corresponds to two close packed layers of C16 molecules in orientation perpendicular relative to GO planes. The γ-phase is composed by one layer of C16 molecules also in a perpendicular orientation. Intermediate phases produced by vacuum annealing appear at ambient temperature only when most of C16 removed and correspond to 2-layer or 1-layer intercalation with the C16 molecules arranged parallel to the GO planes.

Analysis of all data presented above suggests that the DSC peak (Fig. 5) observed in the C16-deficient samples at ~328K (heating) corresponds to change in orientation of C16 molecules from perpendicular to the GO planes to layered parallel to GO planes. The reversible transition between γ- and β-phases occurs only in absence of “free” C16 melt.

## 4. Discussion

The experiments presented above revealed several BGO structures intercalated by C16. These results are summarized in the “reaction map” showing all observed structures and the corresponding BGO:C16 compositions (Fig. 13).

Only two phases shown in Fig. 13 should be considered as thermodynamically stable in the BGO-C16 system: α-phase and the β-phase. The α-phase is formed when BGO is exposed to molten C16 below ~340K in an amount sufficient to provide saturated sorption/intercalation. The composition of this phase was determined exactly using DSC as 1:3.15 g/g (BGO/C16). The value of the inter-layer distance (~49 Å), composition and structural modeling allows to suggest the structure of the

α-phase containing two close packed layers of C16 molecules with approximately perpendicular orientation relative to the GO planes (Fig. 3). Heating the α-phase above the point of phase transition (~340K) results in the formation of β-phase with smaller c-unit cell parameter (~30-31 Å) and a significantly smaller amount of intercalated C16. The composition of this phase was determined approximately in a set of experiments with heating/cooling of BGO:C16 mixed in different proportions and vacuum annealing experiments. This phase was observed in reasonably pure state for composition of 1: 0.88 (BGO:C16, g/g) which is possibly still slightly underestimating amount of intercalated C16.

The β-phase remains stable in a C16 melt at least up to ~420K. At higher temperatures slow thermal deoxygenation of GO is known to occur. Significant and rapid degradation of the BGO-C16 structure was found at temperatures above ~480K (Fig. S17b). The transition between α- and β-phases is reversible if the molten C16 released from the α-phase above the temperature point of transition remains in the system and can be re-absorbed by BGO upon cooling. We suggest that the structure of the β-phase is strongly disordered with five C16 layers parallel to GO planes and average thickness of each “layer” about 4.5-5Å (Fig. 13).

As mentioned above, only α- and β-phases of BGO-C16 can be considered as thermodynamically stable in an equilibrium with C16 melt. When the melt is removed (e.g. by evaporation under vacuum conditions), cooling of the β-phase results in transition (detected by DSC and XRD) into the γ-phase. Our results suggest that this transition occurs without change of composition and is related to a change in orientation of C16 molecules from layers parallel to GO planes to approximately perpendicular orientation providing inter-layer distance of ~26.5-27.5 Å (γ-phase). The structure of the γ-phase is likely to correspond to one layer of C16 molecules oriented approximately perpendicular to the GO layers. Note that the length of C16 molecules (~22 Å) is in almost perfect agreement with the distance between GO layers in the γ-phase (~21 Å). The structure of γ-phase is likely to be “diluted” (not close packed) since the C16 content in the one (vertical)-layer structure (1:0.88 g/g) is about four times smaller compared to the amount of C16 in the two (vertical) layers of α-phase (1:3.18). Therefore, one vertical layer of C16 molecules in the α-phase corresponds to composition 1:1.6 g/g BGO/C16, that is double value compared to C16 content in the γ-phase.

The evidence of the layered structure of the β-phase is provided by vacuum annealing experiments. Layer by layer evaporation of C16 molecules revealed “kinetic” phases, which correspond to 4-layer, 3-layer, 2-layer and 1-layer structures. The absence of any specific XRD reflections suggests completely disordered orientation of the C16
<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Temperature-composition “reaction map” schematically representing “saturated” structures formed when BGO is immersed in liquid C16 (α- and β-phases) and structures obtained in experiments with different C16 loading or by vacuum driven evaporation of C16 from the β-phase. (A colour version of this figure can be viewed online.)</figcaption></figure>

molecules intercalated into BGO in the β-phase. The absence of XRD reflections corresponding to the structure of the intercalated C16 has motivated the name “liquid” to describe the β-phase in early studies [36].

This model with random in-plane orientation of C16 molecules in the β-phase is in agreement with the complete disorder of β-phase indicated by XRD data. The ordering of C16 molecules in ambient temperature phases is indicated by appearance of an additional broad XRD reflections with d-spacing of about 4.2 Å which corresponds roughly to the diameter of alcohol molecule.

It is also interesting to compare the β-phase of BGO-C16 with

structure of BGO intercalated by 1-octanol, chain alcohol with 8 carbon atoms (C8). Octanol is twice shorter molecule compared to hexadecanol, it is liquid at ambient temperature and has higher vapor pressure thus allowing to use vapor sorption method to find saturated composition of BGO-octanol solvate structure [49]. Intercalation of octanol into BGO was found in earlier studies to result in formation of 4-layer structure at ambient temperature and at lower temperature in 5-layer structure with octanol layers parallel to GO planes. The thickness and density of parallel layers formed by 1-hexadecanol and 1-octanol molecules should be rather similar. The c-unit cell parameter (~28 Å) and composition (1.1 g/g) of 5-layer BGO-C8 structure is in remarkable agreement with
parameters of 5-layer β-phase of BGO-C16 (c-unit cell ~30 Å and ~0.9 g/g). The thickness of octanol layer intercalated into BGO structure was determined using phase transition between 4-layer and 5-layer structures providing value of 4.5 Å [49], also in good agreement with data obtained for layer thickness in BGO intercalated by C16 in the β-phase.

It can be concluded that the high temperature phases of BGO-C16 and BGO-octanol are similar, both exhibiting completely disordered intercalation of the alcohol molecules with solvent molecules forming layers parallel to GO planes. However, the low temperature phases are different. Cooling in the BGO-octanol system results in a transition corresponding to insertion of additional alcohol layer parallel to GO planes (increase of the c-unit cell parameter by 4.5 Å). The transition in BGO-C16 system provides increase in inter-layer distance by much larger value of ~18 Å suggesting not only insertion of additional alcohol molecules but also change in orientation of already intercalated C16 from parallel to GO planes to approximately perpendicular.

Further studies are required to establish the full trend for BGO intercalation with progressively longer alcohols and the exact length of 1-alcohols molecules required for change in the orientation of C16 molecules from parallel (number of carbon atoms C = 1–9 in earlier studies) to vertical (C = 16 in this study) relative to GO plane.

We note that our study provides the first fundamental insight into the GO-C16 system and is not focused on any possible applications of these materials. However, one can note that BGO intercalated by long alcohols demonstrate remarkably high expansion of inter-layer distance achieving ~49 Å for 1-hexadecanol. That is one of the largest values reported in literature for expanded GO structures. Therefore, BGO-C16 can be considered as an attractive precursor for preparation of porous pillared structures. The main applications of porous pillared structures could be gas storage, sorption of pollutants or preparation of electrodes for energy storage devices (e.g. for supercapacitors). However, GO is not electrically conductive and some reduction is required to convert BGO-C16 into rGO-C16. It is possible that mild thermal reduction or chemical reductions could possibly lead to the formation of porous rGO structures pillared by long alcohol molecules thus suitable for applications in e.g. supercapacitors.

## 5. Conclusions

In conclusion, the first detailed temperature dependent study of graphite oxide intercalation by long chain normal alcohol with 16 carbon atoms (1-hexadecanol, C16) revealed several phase transitions. Brodie graphite oxide (BGO) immersed in excess of liquid C16 just above the melting point is found to form a structure with two layers of hexadecanol molecules in orientated perpendicular relative to graphene oxide and with an inter-layer distance of ~49 Å (α-phase). That is one of the largest ever observed inter-layer distances reported for intercalated or pillar GO structures. The α-phase composition was determined using DSC as 1:3.15 g/g BGO/C16. Incongruent melting of the α-phase at 336–342K results in formation of the β-phase with much smaller c-unit cell parameter of 29.83 Å (363K) and significantly smaller C16 content determined approximately as 1:0.9 g/g BGO/C16. Experiments with vacuum evaporation of C16 from the structure of β-phase suggest a layered structure with C16 molecules parallel to the graphene oxide planes. Cooling the solid β-phase in absence of C16 melt (required for the reverse transition into α-phase) results in the formation of the γ-phase with an inter-layer distance of ~26-27 Å suggestively composed by diluted single layer of C16 molecules in a perpendicular orientation relative to the GO planes. The reversible transition between the β- and γ-phases observed by DSC and XRD method is due to a change in orientation of the C16 molecules from stand-up to parallel relative to graphene oxide planes. Vacuum driven evaporation of C16 from the BGO-C16 structure also demonstrated the possibility to prepare structures with inter-layer distance corresponding to two-layer and one-layer intercalation of C16 parallel to GO planes.

## CRediT authorship contribution statement

**Andreas Nordenström:** Investigation, Formal analysis, Data curation, Visualization, Writing. **Artem Iakunkov:** Investigation, Formal analysis, Data curation, Visualization. **Nicolas Boulanger:** Investigation. **Gui Li:** Investigation. **Christoph Hennig:** Methodology, Visualization, Data curation. **Igor Baburin:** Investigation, Formal analysis. **Mads Jørgensen:** Methodology, Visualization, Data curation. **Innkenty Kantor:** Methodology, Visualization, Data curation, All co-authors: Writing - Review & Editing. **Alexander V. Talyzin:** Conceptualization, Methodology, Investigation, Data curation, Validation, Formal analysis, Writing – original draft, Supervision, Funding acquisition.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

A. T. acknowledges funding from the European Union's Horizon 2020 research and the innovation program under grant agreement no. 881603. A. T. also acknowledges support from Energimyndigheten (no. 50620-1). A. T. also acknowledge the Vibrational Spectroscopy Platform of Umeå University and A. Shchukarev for support with the XPS characterization.

We acknowledge MAX IV Laboratory for time on Beamline DanMAX under Proposal 20210963. Research conducted at MAX IV is supported by the Swedish Research council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. DanMAX is funded by the NUFI grant no. 4059-00009B.

We acknowledge ESRF for time at BM20 (ROBL) beamline and technical support of experiments.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2022.12.030.

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