Carbon 183 (2021) 612e619

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Research Article

Tracking the light-driven layer stacking of graphene oxide
Masaki Hada a, b, *, 1, Satoshi Ohmura c, 1, Yuki Yamamoto a, 1, Yoshiya Kishibe a,
Wataru Yajima a, Ryo Shikata a, Tomohiro Iguchi d, Keishi Akada a, Shoji Yoshida a,
Jun-ichi Fujita a, Shin-ya Koshihara e, Yuta Nishina d, f, **aFaculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, 305-8573, Japan
Tsukuba Research Center for Energy Materials Science (TREMS), University of Tsukuba, Tsukuba, 305-8573, Japanc
Faculty of Engineering, Hiroshima Institute of Technology, Hiroshima, 731-5193, Japand
Graduate School of Natural Science and Technology, Okayama University, Okayama, 700-8530, Japane
School of Science, Tokyo Institute of Technology, Tokyo, 152-8551, Japanf
Research Core for Interdisciplinary Sciences, Okayama University, Okayama, 700-8530, Japanba r t i c l e i n f o

a b s t r a c t

Article history:
Received 6 April 2021
Received in revised form
16 July 2021
Accepted 20 July 2021
Available online 22 July 2021

Layer stacking of two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, is critical for controlling their physical and transport properties. By exploiting a speciﬁc stacking
order, the electronic band structures of such 2D materials can be tuned, from which unusual and exotic
properties may emerge. Graphene oxide (GO) undergoes layer stacking along with its photo- andthermal-induced reduction process; however, the underlying mechanism and dynamics during its layer
stacking have not been revealed. In this study, we demonstrate time-resolved electron diffraction formonitoring the structural dynamics during the layer stacking of GO induced by ultraviolet photoexcitation. The experimental results accompanied by the density functional theory calculations reveal thatAB stacking of graphitic domains of GO layers coincides within ~40 ps with photoinduced removal of theepoxy-oxygen from the basal plane of GO via the strong interactions between the GO layers.
© 2021 Elsevier Ltd. All rights reserved.

Keywords:
Time-resolved electron diffraction
Ultrafast dynamics
Graphene oxidep-stacking
Density functional calculations

1. Introduction
As represented by the transition from graphene to graphite,layer stacking or the addition of an individual graphene layercompletely modiﬁes the material's electronic band structure [1e4].
Monolayer graphene has an electronic band structure with gapless
Dirac corns [5,6], and stacking of graphene layers results in semiconductor and semimetal phases [7,8]. Recently, stacking graphenebilayers at a certain twist angle has been found to induce superconductor phase due to the interlayer interaction derived from thelattice mismatch between the layers [9e11]. Stacked layer graphene can be applied for ﬁeld-effect transistors [12], lithium
diffusion ﬁlms in batteries [13], ultrahard carbon ﬁlms [14,15], andcatalytic ﬁelds [16]. Thus, layer stacking of graphene or other twodimensional (2D) materials has attracted great fundamental andtechnological interest. Bilayer or few-layer graphene is fabricatedmainly by exfoliation of graphite [17,18] or by chemical vapor
deposition [19]. Reduction of graphene oxide (GO) is an alternativetechnique to fabricate the stacked graphene [20e22]. However,there are several challenges in the fabrication of the stacked-layer
graphene from GO. The mechanism underlying layer stacking of
GO and even its structural dynamics remain to be fully elucidated.
In this study, we demonstrate ultrafast time-resolved electron
diffraction for capturing the onset of GO layer stacking induced byultraviolet (UV) photoexcitation.
GO is a mass-producible derivative of graphene with several
different oxygen functional groups on its basal plane and edge
[23e27], and has been used for various applications, such astransparent conductive ﬁlms [28], ﬁbers [29], aerogels [30], membranes [31], energy storage devices [32], sensing devices [33],luminescent materials [34], fuel cells [35], biomaterials [36], liquid
crystals [37], and polymer composites [38]. Reduction of GO is* Corresponding author. Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, 305-8573, Japan.
** Corresponding author. Research Core for Interdisciplinary Sciences, Okayama University, Okayama, 700-8530, Japan.
E-mail addresses: hada.masaki.fm@u.tsukuba.ac.jp (M. Hada), nisina-y@cc.okayama-u.ac.jp (Y. Nishina).
1
These authors equally contributed.
https://doi.org/10.1016/j.carbon.2021.07.058
0008-6223/© 2021 Elsevier Ltd. All rights reserved.

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Carbon 183 (2021) 612e619

realized by removal of the oxygen functional groups [39]. Our
previous study combined ultrafast time-resolved electron diffraction, ultrafast time-resolved mid-infrared vibrational spectroscopy,
and time-dependent density functional theory (DFT) calculations toshow that photoexcitation drives selective epoxy-oxygen removalfrom the basal plane of GO [40]. The selective removal of epoxyoxygen is caused by the anti-bonding nature of epoxy-oxygen on
the basal plane of GO in the photoexcited state. A previous report
showed UV photoexcitation-induced structural dynamics on asingle GO layer by analyzing the position shifts of the electron
diffraction peaks from the f1010g and f1120g planes of GO. Herein,
ultrafast time-resolved electron diffraction is intrinsically sensitive
to the changes in the periodicity of molecules or molecular domains[41]; therefore, the structural dynamics of GO layer stacking canalso be observed from the intensity changes of electron diffraction
peaks. Interlayer interaction is quite weak in GO; therefore, GO
layers are stacked independently from each other [42,43]. In
contrast, reduced GO tends to undergo layer stacking with strong
interlayer interactions [44]. Thus, in the present study, we focus on
the intensity changes of these electron diffraction peaks to understand the structural dynamics of GO layer stacking (layer-to-layer
dynamics) induced by UV photoexcitation.

Fig. 1. Scanning electron micrograph of GO ﬂakes.

pseudo-wavefunctions and pseudo charge density were 30 and 250
Ry, respectively. For valence electrons, we included 2s and 2p statesof C and O. The energy functional was minimized using an iterative
scheme based on the preconditioned conjugate-gradient method[51,52]. Total energy was converged to within a tolerance of

2. Materials and methods
2.1. Preparation of GO ﬁlms
GO was prepared using a modiﬁed Hummers’ method [25,45].
The details of the synthesis of GO are given in Ref. [40]. We prepared two GO ﬁlms, one for ultrafast time-resolved electron
diffraction and the other for scanning electron microscopy (SEM).
For ultrafast time-resolved electron diffraction, a 1 wt-% GOdispersion (H2O/Solmix ¼ 1:1) was spin-coated at a rotation speedof 2000 rpm onto an ultrathin (30-nm thickness) silicon nitride
membrane. For SEM, a 0.2 wt-% GO dispersion (diluted dispersion)
was spin-coated at a rotation speed of 2000 rpm onto a thermal
SiO2 substrate. In some very thin portions of the inhomogeneous
GO ﬁlm prepared from the diluted dispersion, monolayer andbilayer GO ﬂakes were able to be observed. Fig. 1 shows the scanning electron micrograph (S-4800, Hitachi High-Tech Corp.) of avery thin portion of the GO ﬁlm. The sizes of GO ﬂakes are in therange of a few to tens of microns. The GO ﬂakes of various sizes arestacked in layers.
2.2. Ultrafast time-resolved electron diffraction
Details of the ultrafast time-resolved electron diffraction setup
are given elsewhere [46,47], and the experimental details areprovided in a previous study [40]. The ultrafast time-resolved
electron diffraction apparatus produced ultrashort (~1 ps) electron pulses. A 75-keV electrostatic ﬁeld accelerated the employedelectron beam. The repetition rate of electron pulses was 1 kHz. The
GO ﬁlm (~70 nm, approximately 60 GO layers) on the ultrathin
silicon nitride substrate was excited by UV pulses (a wavelength of
266 nm) synchronized with the electron pulses.
2.3. Numerical details
We obtained the electronic states using the projectoraugmented-wave (PAW) method [48,49] within the frozen coreapproximation in the framework of density functional theory (DFT).
The generalized gradient approximation formulated by Perdew,
Burke, and Ernzerhof (GGA-PBE) was used for the exchangecorrelation potential [50]. The plane-wave cutoff energies for the

Fig. 2. (a) Electron diffraction pattern from GO. (b) The inset shows the geometric
illustration of the f1010g and f1120g planes of graphene derived from unoxidized
domains of GO. (c) The radial average of the electron diffraction patterns after UVphotoexcitation at an incident ﬂuence of 0e4 mJ/cm2. The horizontal arrows indicate
the peak shifts, and the vertical arrow indicates the intensity change. (A colour versionof this ﬁgure can be viewed online.)613

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Carbon 183 (2021) 612e619

Fig. 3. (a, b) Schematic illustrations of graphene models stacked in AB and AA orders, respectively (side views and top views). (c) Calculated electron diffraction patterns fromrandomly ordered, AA stacked, and AB stacked graphene models. (A colour version of this ﬁgure can be viewed online.)

5.0  108 Ry/electron. The G point was used for Brillouin zone
sampling. We applied van der Waals corrections to the GGA-PBE
calculations by the DFT-D2 approach of Grimme [53], which is anempirical correction to standard DFT that takes into accountdispersive interactions based on damped, atomic pairwise potentials. This approach has recently been applied to carbon systems,
including graphite and graphene bilayers [54]. The simulation was
performed using the highly parallelized plane-wave DFT program,for which implementation details can be found in Refs. [[52,55e57]]. To investigate electronic charges on each atom, weused the population analysis [58,59] by expanding the electronic
wave functions in the atomic-orbital basis sets (Note S1).

lattice periodicity. The CeC bonds of graphene correspond to conjugated double bonds. Once epoxy-oxygen is present on the basal
plane of graphene, the two bonds of the conjugated double bonds
become single bonds. The conjugated double bond is shorter than asingle bond. The average length of CeC bonds decreases upon theremoval of epoxy-oxygen. The average length of the CeC bonds of
GO was 1.431 Å and shortened to 1.417 Å by epoxy-oxygen removalwith UV photoexcitation at an incident ﬂuence of 4 mJ/cm2. In ourprevious report, time-dependent density functional theory calculations combined with time-resolved electron diffraction, timeresolved infrared vibrational spectroscopy, and X-ray photoelectron spectroscopy showed that the oxygen atoms of the epoxyfunctional group (6e7%) with anti-bonding nature in the photoexcited state were removed from the basal plane of the GO layer[40]. Thus, the previous study effectively described the photoinduced structural dynamics within a single GO layer; however, it did
not provide insights into the photoinduced layer-to-layer dynamics
of GO. Spectroscopy is useful for understanding the reduction
mechanism and structural dynamics within a single GO layer;
however, it is difﬁcult to elucidate the interlayer dynamics of GOwith spectroscopic measurements. For example, Raman spectroscopy before and after UV photoexcitation (Figure S1) does not show
any information about the changes in the interlayer structure of GO
induced by UV photoexcitation.
Fig. 2c also shows the intensity changes of the electron diffrac-

3. Results and discussion
Fig. 2a shows the typical electron diffraction pattern from a GO
ﬁlm. The diffraction pattern shows the rings from the f1010g and f
1120g planes of the GO framework. The geometries of the f1010g
and f1120g planes of the graphene sheet are presented in Fig. 2b.
As shown in the radial average of the electron diffraction pattern(Fig. 2c) after UV photoexcitation with 266-nm wavelength pulse,
the Q-values of the f1010g and f1120g planes of GO were shifted
higher, which reﬂects the lattice shrinkage of GO, i.e., the shrinkageof the average length of CeC bonds. The Q-value represents thereciprocal value of the lattice periodicity (Q ¼ 1= d), where d is thetion peaks from the f1010g and f1120g planes of GO. The intensity614

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Carbon 183 (2021) 612e619

constant after photoexcitation. More precisely, the intensity ratio(If1010g =If1120g ) of the f1010g and f1120g diffraction rings is ~2
before photoexcitation, and it decreases to less than two after
photoexcitation above the incident ﬂuence of 2e3 mJ/cm2.
The experimentally obtained intensity changes of the diffraction
peaks from the f1010g and f1120g planes of GO can be estimatedby the kinematic theory of diffraction based on graphene models.
Fig. 3a and b shows the AB stacking and AA stacking of graphene
layers, respectively. Randomly ordered graphene models are shownin the supporting information in more detail (Figure S2). The
calculated electron diffraction patterns from randomly ordered, AB
stacked, and AA stacked graphene layers are presented in Fig. 3c.
The electron diffraction patterns are calculated based on kinematictheory [60]. The proﬁles of the calculated electron diffraction patterns from the randomly ordered graphene layers and the AA
stacked graphene layers are identical, which can be clearly understood by viewing the AA stacked graphene layer from above(Fig. 3b). As shown in Fig. 3c, the intensity of the diffraction peakfrom the f1010g plane of AB stacked graphene layers is four times
lower than those of AA stacked or randomly ordered graphene
layers. On the other hand, the diffraction peaks from the f1120g
plane of randomly ordered, AB stacked, and AA stacked graphene
layers share the same intensity. The ratio of the f1010g and f1120g
diffraction intensities (If1010g =If1120g ) of randomly ordered graphene or AA stacked graphene is 2. The value for AB stacked graphene is 0.5. Thus, GO is randomly ordered on the substrate forelectron diffraction before photoexcitation. The experimentallyobtained decrease in intensity ratio by UV photoexcitation (Fig. 2c)would suggest a change in the stacking order of GO from random
order to AB stacked order.
Noting the structural dynamics of the stacking order of GO upon
the UV photoexcitation can lead to further understanding of its
mechanism. We discuss the structural dynamics of GO from thechanges in the electron diffraction patterns following UV photoexcitation at an incident ﬂuence of 2 mJ/cm2. The changes in the
average CeC bond length of GO can be calculated from the shifts ofthe diffraction peaks from the f1010g and f1120g planes of GO.
Fig. 4a shows the temporal evolution of the average CeC bondlength of GO. The time constant of the change in the average CeC
bond length of GO is 40 ± 8 ps, as shown in a previous report.
This lattice shrinkage is caused by removal of epoxy-oxygen fromthe basal plane of GO. Fig. 4b andc shows the temporal evolutions of
the normalized intensity changes of the f1010g and f1120g
diffraction rings of GO, respectively. The electron diffraction intensity from the f1010g plane decreases by approximately 3 % by
UV photoexcitation with a time constant of 35 ± 10 ps (Fig. 4b). The
time constant of the intensity decrease corresponds well to that of
the change in the average CeC bond length of GO. In contrast, theelectron diffraction intensity from the f1120g plane does notchange upon UV photoexcitation (Fig. 4c). This tendency is also
explained by the calculated electron diffraction patterns shown in
Fig. 3c, where the randomly ordered graphene is transformed into
locally AB stacked graphene by UV photoexcitation. The correspondence of the time constant of the shrinkage of the average CeC
bond length induced by the removal of epoxy-functional groups
and that of change in layer order from randomly ordered GO to AB
stacked GO suggests that the epoxy-oxygen removal and change inlayer stacking have a strong correlation. The correlation can bequantitively analyzed by comparing the ratio of the removed
epoxy-oxygen and the degree of the AB stacking of the GO layers.
According to a previous study, the average length of the CeC bonds
shortens by 0.014 Å (from 1.431 Å to 1.417 Å) when 6e7% of epoxyoxygen is removed from the basal plane of the GO layer [40]. Fig. 4aFig. 4. (a) Temporal evolution of the change in the average CeC bond length of GO.
Temporal evolution of the electron diffraction intensity from the f1010g plane (b) and
f1120g plane (c). Red solid lines indicate the exponential ﬁtting curves. The red dashedlines facilitate visual perception of the horizontal. (A colour version of this ﬁgure canbe viewed online.)of the diffraction peak from the f1010g plane decreases by UVphotoexcitation; in contrast, that from the f1120g plane remains615

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Carbon 183 (2021) 612e619

Fig. 5. Model bilayer graphene structures (top and side views) with epoxy-functional groups at CeO distances of 1.5 Å (a), 2.6 Å (b), and 3.1 Å (c), and without epoxy-functionalgroups (d). The right panels show the HOMOs and LUMOs of the graphene models. (A colour version of this ﬁgure can be viewed online.)

occupied molecular orbitals (HOMOs) and lowest unoccupied
molecular orbitals (LUMOs) of the corresponding bilayer GO
models are shown in the right panels. The HOMOs and LUMOs ofthe bilayer GO models are shown in more detail in Figure S4. Eachstructure of the bilayer GO model features an epoxy-functionalgroup on its upper layer. The average distances between the twonearest carbon atoms and the oxygen atom (CeO distances) are 1.5,
2.6, and 3.1 Å. As shown in Fig. 5a and b, the wave function is
attracted by the epoxy-functional group when the epoxyfunctional group is close to the graphene layer. The electronprobability density is the squared amplitude of the wave function.
The wave function of the bilayer GO model with a CeO distance of
3.1 Å (Fig. 5c) is similar to that without an epoxy-functional group(Fig. 5d), which indicates that the interaction between two graphene layers of the GO model is also similar to that of the graphenebilayer. In these conﬁgurations, the wave functions in the HOMOe2of two graphene layers of the GO model (Fig. 5c) is close to eachother. Thus, the result from the DFT calculation reﬂects that theremoval of the epoxy-functional group from the basal plane of GO
strongly inﬂuences the stacking of GO.
We performed additional quantitative analyses of the DFT calculations. The differential electronic charge between the upper andlower graphene layers in Fig. 6a is deﬁned to be Qdiff by thefollowing equation:shows that the average length of the CeC bonds shrinks by 0.0021 Åwith photoexcitation by a single UV pulse at an incident ﬂuence of
2 mJ/cm2, which suggests that ~1 % of epoxy-oxygen moves awayfrom the basal plane of the GO layer with a single UV pulse. On theother hand, as shown in Fig. 3c, the electron diffraction intensityfrom the f1010g plane would decrease by 75 % when the randomly
ordered GO layers have fully transformed into AB stacked GO layers.
The experimental result that the electron diffraction intensity fromthe f1010g plane decreases by approximately 3 % upon UV
photoexcitation suggests that ~4 % of randomly ordered GO layers
partially change into AB stacked GO layers upon irradiation by asingle UV pulse. Thus, the adjacent 4 % carbon atoms with epoxyoxygen locally form AB stacking layers upon ~1 % epoxy-oxygen
removal from the basal plane of GO.
To understand the causality between epoxy-oxygen removal
and layer stacking, we performed DFT calculations. Fig. 5aed shows
the model bilayer graphene structures with and without the epoxyfunctional group, i.e., bilayer GO models. Each layer consists of an8  8 graphene supercell (128 carbon atoms) in the periodic
boundary condition of the hexagonal net (Figure S3). In z-direction,
the periodic boundary condition is employed with a cell of adimension of 25 Å, which is large enough to avoid the interaction
between periodic images of layers. In the initial structure optimization, iterations were repeated until the atomic forces reachedvalues lower than 0.02 eV/Å. To generate the layer without oxygen,structural optimization was performed locally around carbon
atoms used to bond to an oxygen atom starting from the sameatomic conﬁguration of GO. The distances between layers with andwithout epoxy-oxygen are 3.5 and 3.4 Å, respectively. The highest

Qdiff ¼

X

Q ðcarbonÞlower XQ ðcarbonÞupper ;(1)

where Q ðcarbonÞlower and Q ðcarbonÞupper are the electronic chargesof the carbon atoms in the lower graphene and in the upper616

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Carbon 183 (2021) 612e619

Fig. 6. (a) The differential electronic charge between the upper and lower graphene layers, Qdiff , as a function of CeO distance. (b) Potential energy curves of the bilayer GO models
with CeO distances of 1.5 and 2.6 Å and without epoxy-oxygen. The origin of the axis representing energy is taken to be the energy at the AA stacking. (c) The direction of the upper
layer sliding. The horizontal axis of (b) is the position of the upper layer sliding in the ½1010 direction (the blue arrow). (d) Enlarged view of the red dashed square shown in (b). (Acolour version of this ﬁgure can be viewed online.)

the hydroxyl-functional groups and trilayer GO model, are shownin Figures S6 and S7; all of them also suggest the enhancement of
AB stacking after the removal of epoxy-oxygen.

graphene, respectively. The positive value of Qdiff indicates that theelectronic charge is localized at the oxygen site. Fig. 6a shows Qdiff
as a function of a CeO distance. As shown in the ﬁgure, Qdiff has apositive value (0.4e0.5 elementary charges) with the CeO distanceof ~1.5 Å, and it decreases as the CeO distance increases. At CeO
distance over ~3.1 Å, Qdiff is approximately 0.1 elementarycharges, which is close to the conﬁguration of bilayer graphene. The
potential energy curves of the bilayer GO models with CeO distances of 1.5 and 2.6 Å and without an epoxy-functional group areshown in Fig. 6b. As shown in Fig. 6c, the upper layer slide in the

4. Conclusion
In summary, the ultrafast structural dynamics of the stacking of
GO induced by UV photoexcitation were investigated using a
combination of ultrafast time-resolved electron diffraction and DFT
calculations. Randomly ordered GO layers become stacked in AB
order simultaneously with removal of the epoxy-functional groupon the basal plane of GO. The wave function of the bilayer GO model
approaches that without an epoxy-functional group withincreasing CeO distance. The stacking of GO layers was caused by
the strong interaction between layers. The elucidated mechanismof GO layer stacking may allow tuning of the desired stacking orderof graphene or functionalized graphene layers and thus realize
control of their physical and transport properties, expanding theapplication scope of 2D materials [61].

½1010 direction, where we deﬁne the distance of 0 Å as the AA
stacked bilayer GO. The distances equal to 0 Å and 4.27 Å correspond to AA stacked bilayer GO (black dashed lines), and the distances equal to 1.42 Å and 2.85 Å correspond to AB stacked bilayer
GO (blue dashed lines). The potential energy curve of the model
without the epoxy-functional group has minima at the position of
the AB stacked bilayer. As shown in the enlarged view (Fig. 6d), thepotential energy curve of the bilayer GO model with CeO distancesof 1.5 Å is relatively ﬂat without potential minima around the position of AB-stacking; in contrast, that of the bilayer GO model with
CeO distances of 2.6 Å exhibits potential minima at the position of
the AB stacked bilayer GO. This calculation result suggests thatelongation of the distance between the epoxy-functional group andthe basal plane of GO creates the potential minimum at the positionof the AB stacked bilayer GO and enhances the interaction between
the GO layers. The relationship of potential energy to CeO distancein AB stacking and AA stacking also supports the positedenhancement of AB stacking by epoxy-oxygen removal (Figure S5).
The other possible conﬁgurations, such as bilayer GO model with

CRediT authorship contribution statement
Masaki Hada: Conceptualization, Methodology, Formal analysis,
Investigation, Writing e original draft. Satoshi Ohmura: Methodology, Software, Formal analysis, Investigation, Writing e originaldraft. Yuki Yamamoto: Formal analysis, Investigation, Writing eoriginal draft. Yoshiya Kishibe: Investigation. Wataru Yajima:
Investigation. Ryo Shikata: Investigation. Tomohiro Iguchi:
Investigation. Keishi Akada: Investigation. Shoji Yoshida:617

M. Hada, S. Ohmura, Y. Yamamoto et al.

Carbon 183 (2021) 612e619

Investigation. Jun-ichi Fujita: Data curation, Investigation. Shin-ya
Koshihara: Data curation, Writing e review & editing. Yuta Nishina: Conceptualization, Investigation, Resources, Writing e review& editing.

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Declaration of competing interest
The authors declare that they have no known competing
ﬁnancial interests or personal relationships that could haveappeared to inﬂuence the work reported in this paper.
Acknowledgments
This work was ﬁnancially supported by Kakenhi Grants-in-Aid(Nos. JP18H05208, JP19H00847, JP20H01832, and JP20H04657)
and JST CREST (No. JPMJCR18R3). This work was also supported by
the Leading Initiative for Excellent Young Researchers of the Ministry of Education, Culture, Sports, Science and Technology (MEXT),
Japan. Computations were carried out using the computer facilities
at the Research Institute for Information Technology, Kyushu University and at the Supercomputer Center, Institute for Solid State
Physics, University of Tokyo.
Appendix A. Supplementary data
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.carbon.2021.07.058.
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