Mechanistic insights into light-driven graphene-induced peroxide decomposition: Radical generation and disproportionation

Article abstract of DOI:10.1039/c6cc02618d

Interaction between adsorbed t-butyl peroxybenzoate and photoexcited graphene rendered trapped phenyl and t-butoxy radicals. Post-irradiation thermal desorption showed benzene, t-butanol, and isobutylene oxide as the end products. The required hydrogen at

Full text of DOI:10.1039/c6cc02618d

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Mechanistic insight into light-driven graphene-induced peroxide
decomposition: radical generation and disproportionation
Received 00th January 20xx,
Accepted 00th January 20xx
Ya-Lan Chu, Yen-An Chen, Wei-Chin Li, Jean-Ho Chu, Chun-Hu Chen* and Chao-Ming Chiang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Interaction between adsorbed t-butyl peroxybenzoate and
Here t-butyl peroxybenzoate (TBPB), a composite of DBP
photoexcited graphene rendered trapped phenyl and t-butoxy and DTBP with the -O-O- moiety flanked unsymmetrically by a
radicals. Post-irradiation thermal desorption showed benzene, t- benzoyl group (PhCO-) and a t-butyl group ((CH3)3C-), was
butanol, and isobutylene oxide as the end products. The required chosen deliberately as the radical precursor because homolytic
hydrogen atoms were obtained via the radical disproportionation. O-O bond scission would first produce two oxygen-centered
Graphene enabled radical species to be captured and their on- radicals; however, the benzoyloxy fragment (PhCOO
·
) may lose
CO2 to give the phenyl radical while the t-butoxy unit
((CH3)3CO ) can convert to the methyl radical by releasing 1
surface chemistry to be revealed.
·
equivalent of acetone.9 It is curious to know whether
phenylation or methylation of graphene can be steered by
adopting such a hybrid. Metal-supported single layer graphene
(SLG) was selected over exfoliated graphene flakes considering
its suitability for the surface infrared characterization. Because
SLG exhibited higher reactivity than its double- and multi-layer
counterparts,8,9 it should be more prone to functionalization.
SLG was prepared by chemical vapour deposition (CVD) in
which hydrogen-pretreated Cu foils were exposed to a mixture
of regulated hydrogen and methane flows in a quartz tube
There has been a lot of graphene hype ever since the discovery
of this two-dimensional wonder material, stimulating immense
interests across multiple disciplines.1,2 Chemists are not
immune to this fascination.3 Chemical functionalization is a
remedy to the zero bandgap and solubility problems of
graphene.4-7 However, the chemical inertness, due to its fully
conjugated sp2 carbon network and lack of curvature, presents
a formidable challenge to modify graphene with covalently
bonded organic groups. Free radical addition approach is one
of the solutions to address this issue. Brus and co-workers
pioneered the photochemical treatment using dibenzoyl
peroxide (DBP) as phenyl radical precursor.8 A lower electrical
conductivity as compared to the starting graphene was
obtained, indicating the attachment of sp3 moieties onto the
basal plane of graphene, but direct evidence for the phenyl
radical remained elusive. Photoinduced methylation of
graphene was achieved by Liu et al. using supported graphene
films immersed into a di-t-butyl peroxide (DTBP) solution and
irradiated by a mercury lamp.9,10 Direct photolysis was
suggested as the source of methyl radicals but with no
mechanistic details. Because all these feats were accomplished
by wet-chemical routes, radical detection might be difficult at
the graphene-liquid interface. Equivalent investigation under
ultrahigh vacuum (UHV) conditions, using the in situ tools
available within the realm of surface science, would
circumvent the obstacle and unveil the various light triggered
radical-radical and/or radical-graphene interactions.
o
furnace at 1000 C (the details are described in the Electronic
Supplementary Information (ESI†)). The single-layer and
defect-free nature was confirmed by confocal Raman
spectroscopy (Fig. S1, ESI†). The copper-supported graphene
sample (SLG/Cu) was then transferred into an UHV apparatus
capable of temperature programmed desorption (TPD) and
reflection-absorption infrared spectroscopy (RAIRS)
measurements (see the experimental details in ESI†). Multiple-
ion TPD, a technique for monitoring the interaction between
adsorbate and substrate, exhibits in Fig. 1(a) that after dosing
SLG/Cu with 0.1 L (a unit of exposure, 1 L = 10-6 Torr sec) TBPB
at 200 K desorption of intact peroxide molecules led to two
separate features, characterized by a set of concurrent m/z
194 (parent ion), 105 (PhCO+, the base peak for the TBPB11), 77
+
+
(Ph+), 59 ((CH3)2COH+), 51 (C4H3 ), 43 ((CH3)CO+), 41 (C3H5 ) and
31 (CH3O+) signals. Sequential dosing experiments from 0.02 to
2.0 L (see Fig. 1(b)) revealed that the 277 K high-temperature
state which we assign to desorption from defects along
domain boundaries of the CVD grown graphene12 (confirmed
by the small Raman D band at 1340 cm-1) was populated and
saturated first. As the exposure is elevated to 1 L twin peaks
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1210
(a)
0.1 L TBPB/SLG/Cu @200 K
223 K
277 K
924
2x10-3
(e) t-butanol
m/z 105
43
(d) 300 K anneal
(c) TBPB + 365 nm
60 min
59
41
710
31
77
C=O
1765
51
C-O
1054
194
1267
O-O
833
300
200
(b)
400
500
1023
1453
(b) TBPB
238 K
231 K
1363
1251
2980
(a) DTBP
1197
877
1479
1038
224 K
Exposure 2.0 L
x 0.5
3200
3000
2800
2000
1800
1600
1400
1200
1000
800
277 K
1.0 L
Wavenumber (cm-1)
0.5 L
0.1 L
Fig. 2 RAIR spectra of (a) DTBP dosed on SLG/Cu at 120 K, (b) 0.1 L
TBPB dosed on SLG/Cu at 200 K, (c) sample (b) after 60 minutes UV
irradiation, (d) sample (c) following vacuum annealing to 300 K, and (e)
t-butanol dosed on SLG/Cu at 120 K. The drawings serve to assist in
vibration mode assignments; no molecular-level structural details are
implied.
0.02 L
150
(c)
200
250
300
0.1 L TBPB/SLG/Cu @200 K
photo-depletion
365 nm irradiation @120 K
0 min
15
30
45
60
located 45o from the incident light to avoid shadowing the
surface). However, photodissociation of TBPB can be
300
400
500
200
Temperature (K)
Fig. 1 (a) Multiplex TPD spectra measured after exposing 0.1 L TBPB to confirmed by comparing the RAIR spectra measured prior and
SLG/Cu at 200 K. (b) Trend of m/z 105 (the most abundant fragment subsequent to the UV exposure. Following the 0.1 L TBPB
for TBPB) ion signals as a function of TBPB exposure. The 0.1 L adsorption at 200 K, key vibration modes are noted at 1765
exposure would correspond to a submonolayer coverage. (c) Trend of cm-1 (typical C=O stretch for the ester carbonyl),13 833 cm-1 (O-
m/z 105 signals as a function of UV irradiation time.
O single-bond stretch for peroxide),14 710 cm-1 (ring C-H out-
of-plane bend) in Fig. 2(b). According to the surface dipole
can be observed at 224 K and 231 K and are attributed to selection rule, the domination of the out-of-plane mode
desorption from the monolayer and thick films on the suggests that at such a low coverage the phenyl ring of TBPB
domaininterior of graphene, respectively. The fact that the lies flat on the surface, attributable to the
π-π stacking
monolayer desorption temperature is lower than the between phenyl and graphene. The strong carbonyl signal
multilayer indicates that the adsorbate-surface interactions indicates that the C=O bond is perpendicular to the surface
are slightly weaker than the adsorbate-adsorbate interactions and extends into vacuum. The assignments for the rest of the
(TBPB is a low vapour pressure liquid, 0.2 torr at 75oC). With IR bands are facilitated by first recognizing the similarities
increasing exposure these two low-temperature peaks merge between adsorbed TBPB and DTBP whose IR bands are more
into one and continue to grow. RAIR spectra of adsorbed TBPB well-defined15 and featured by 2980 (CH3 stretch), 1479 (CH3
without light irradiation were taken at various annealing asymmetric deformation), 1363 (CH3 symmetric deformation),
temperatures (Fig. S2, ESI†). As illustrated in Fig. S2 (b) and (c), 1251 (skeletal C-C stretch), 1197 ((CH3)3C-O stretch), 1038 (CH3
heating above the molecular desorption temperature rock), and 877 (O-O stretch) cm-1 absorptions illustrated in Fig.
rendered the spectra to become featureless. The lack of 2(a). By eliminating the peaks belonging to the t-butoxy,
spectral changes associated with molecular transformation vibrations associated with the benzoyloxy group, including
suggests that TBPB molecules simply adsorb and desorb 1453 (ring stretch), 1267 (ring C-H in-plane bend), 1054 (ester
reversibly. We were unable to thermally activate them on C-O stretch), and 1023 cm-1 (ring C-H in-plane bend) become
graphene.
evident in Fig. 2(b). As shown in Fig. 2(c), signature bands
Intriguingly, following the adsorption of TBPB at 200 K corresponding to the peroxy (833 cm-1) and the ester (1765
peroxide species were found to be depleted from the and 1054 cm-1) functionalities clearly diminish or vanish in the
graphene by UV irradiation at 120 K using 365 nm high-power RAIR spectrum taken after 60 minutes UV irradiation,
LEDs (the details are described in ESI†), as evidenced by a suggesting the rupture of the weak O-O bond instantaneously
series of post-irradiation temperature programmed desorption followed by Ph-C bond scission (decarboxylation) within TBPB
(PITPD) traces displayed in Fig. 1(c) where the diminishing molecules.16,17 The remaining modes, such as CH3 stretch, CH3
trend of m/z 105 intensities with prolonged UV exposure time deformations, (CH3)3C-O stretch, and ring C-H bends, might
is highlighted. Two obvious channels can account for the loss represent the photogenerated t-butoxy and phenyl radical
of TBPB: photodesorption and photodissociation. An attempt species (PhCOO-OC(CH3)3
→
→ (CH3)3CO･ + Ph-COO･; Ph-COO･
to detect the desorbing species in the presence of light was
Ph･ + CO2) co-trapped on the copper-supported graphene
proven unsuccessful because the sensitivity of the quadrupole at 120 K. Control experiments were performed by replacing
mass spectrometer (QMS) was attenuated while the sample SLG/Cu with a bare Cu(111) as the substrate. Completely
was in the irradiation position (QMS head was retracted and different chemistry was found. It turns out that TBPB was
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223 K
223 K
subjected to thermolysis even at 200 K and the O-O bond was
disrupted, resulting in IR spectra featured by a strong
absorption band at 1410 cm-1 which is assigned to the
symmetric carboxylate O=C-O stretching mode for
chemisorbed benzoate species in an upright conformation (Fig.
S3, ESI†).18,19 Conversion to the benzoate-Cu was fully
accomplished at 250 K and the species can persist up to 500 K.
In sharp contrast, the IR features faded out entirely (see Fig.
2(d)) through 300 K vacuum annealing on SLG/Cu; therefore
covalently bonded species were excluded. Namely, neither
phenylation nor methylation was achieved. In the successful
wet-chemical functionalization,8-10 active species were
generated under ambient conditions, whereas radicals in our
experiments were created at the cryogenic temperature under
which the radical reactivity would be attenuated and incapable
of challenging graphene. In fact, clean graphene surface could
always be reproduced by annealing the sample to 700 K under
vacuum. The integrity of the graphene layer was not affected
by repeated dose and thermal desorption cycles. Closer
inspection of Fig. 2(c) also finds new features emerging at 924
and 1210 cm-1 which match remarkably well with the IR
frequencies for adsorbed t-butanol (see Fig. 2(e)).
Inspired by this observation, extensive mass search was
conducted in the PITPD experiments to elucidate the fate of
radicals produced photochemically on graphene. First, the
absence of methane and acetone suggests that the t-butoxy
radical did not proceed to form the methyl radical. By checking
the differences in the TPD results with and without UV
irradiation (see middle and top panels in Fig. 3), benzene,
instead of benzoic acid (PhCOOH), was identified (see Fig. 3(a)).
Thus the decarboxylation step is further consolidated. In
(a)
(b)
223 K
(c)
TBPB
277 K
277 K
277 K
153 K
175 K
160 K
5000
3800
175 K
2000
TBPB + 365 nm
11000
m/z 51
m/z 31
3800
m/z 77
m/z 41
m/z 43
m/z 43
m/z 59
7000
m/z 78
m/z 72
50000
standards
70000
11000
150 200 250 300
150 200 250 300
Temperature (K)
150 200 250 300
Fig. 3 (a) Diagnostic multiple-ion TPD spectra for identifying benzene at
m/z 51, 77, and 78 (parent ion) measured after 0.1 L TBPB adsorption
on SLG/Cu at 200 K (top), after 0.1 L TBPB adsorption on SLG/Cu at 200
K, followed by UV irradiation at 120 K for 60 min (middle), and after
exposing pure benzene directly to SLG/Cu (bottom), (b) The same type
of experiments except for identifying t-butanol at m/z 31, 43, and 59
(base peak), and (c) The same type of experiments for identifying
isobutylene oxide at m/z 41, 43, and 72 (parent ion).
tail C-O linkage to yield a cyclic ether (isobutylene oxide), or it
undergoes further rearrangement to afford a 2-butanone
(methyl ethyl ketone) isomer. This rationale gains strong
support from the desorbing species at 153 K (see middle panel
in Fig. 3(c)) that was otherwise absent in the dark (top panel)
following TBPB adsorption and UV irradiation. The m/z signals
+
at this temperature decrease in the order of 41 (C3H5 ) > 43
+
(C3H7 ) > 72 (C4H8O+, parent ion), and the trend is mirrored
fairly well with the relative intensity pattern obtained from
pure isobutylene oxide (Fig. 3(c) bottom panel) rather than 2-
butanone whose most abundant fragment ion would be m/z
43 (data not shown). The termination at the cyclic ether is
novel but not unprecedented. The same species was once
spectroscopically identified for the thermal decomposition of
DTBP in pellets of KBr.21 Noteworthy is the product, t-butanol,
contributes to the additional m/z 41 and 43 fragments shown
at 175 K. In fact, it is logical to expect t-butoxy radicals to
engage in self-disproportionation, thereby creating t-butanol
and isobutylene oxide as well. One of the driving forces for
radical disproportionation reactions is provided by the high
exothermicity resulting from the newly formed C(sp2)-H, O-H,
and C-O bonds. Although the paths available to a pair of
radicals certainly include recombination (coupling), additional
mass fragments, pertinent to ethane (CH3-CH3), biphenyl (Ph-
Ph), DTBP ((CH3)3CO-OC(CH3)3), benzoyl peroxide (PhCOO-
OOCPh), toluene (Ph-CH3), and t-butoxy benzene ((CH3)3CO-Ph)
were surveyed but not found in PITPD measurements. We
interpret the preference for disproportionation over
recombination rests on the large number of transferable
hydrogens to donate on the t-butoxy radicals and the entropy
effect. On the other hand, the more electronegative oxygen
facilitates alkoxy radical itself as an acceptor to abstract
hydrogen, so the dual role of t-butoxy radical can account for
the diversity of the final products in this case.
addition, t-butanol ((CH3)3CO･ + H･ → (CH3)3COH) was also
recognized in Fig. 3(b). Benzene and t-butanol can both be
ratified by the same relative abundance of the major mass
fragments in the TPD profiles taken after exposing vapours of
the pure compounds to SLG/Cu (see bottom panels in Fig. 3(a)
and (b)). The agreement in desorption maxima together with
their low-temperature (160 K for benzene and 175 K for t-
butanol) implies that the photochemical generation and
termination of radicals are facile and require little activation
energies. The last step, desorption of the end-products is
presumably rate-limiting. Because the initial adsorption of the
reactant was carried out at 200 K (higher than the product
desorption temperatures),
a consequence due to the
extraneous impurities admitted into the UHV chamber should
be ruled out.
To explain the origin of the hydrogen atoms, radical
disproportionation20 reactions must be invoked under solvent-
free conditions. In typical radical disproportionation one
radical plays the role of an acceptor while the other acts as a
hydrogen donor. Each t-butoxy radical has nine methyl
hydrogens to donate; therefore, cross disproportionation of a
･
OC(CH3)3/･Ph pair should give rise to two stable products: A
new C(sp2)-H bond is constructed to make benzene at the
expense of breaking a methyl C-H bond. In the meantime, the
transient ･OC(CH3)2CH2･ species may readily build a head-to-
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Graphene played a crucial part in the discovery. TBPB does different types of interactions of the guest radicals can be
not absorb the 365 nm photons (see Fig. S4 in ESI† for the UV established. Disproportionation serves admirably to explain
spectrum) but graphene does.22 As Brus and coworkers have the origin of the hydrogens for the end-products, and the
suggested,8 photoexcited graphene (GR*) can serve as a donor-acceptor roles in the abstraction mechanism are well
photosensitizer responsible for transferring one electron into resolved. These findings demonstrate graphene as a unique
the LUMO of TBPB. In concomitance with the relocation of mechanistic tool to probe free radical reaction pathways that
another electron from the TBPB HOMO to graphene, such a are not observable under ambient conditions. In light of this
two-way electron-exchange mechanism results in a net energy prototypical study, further experiments on unstable species
transfer (ET) process and formation of ground-state graphene such as nitrene and carbene are being undertaken to explore
(GR) and excited-state TBPB (TBPB*) that cannot be reached the generalization of the concepts. Given the photosensitizer
by direct photon excitation. TBPB* is expected to decompose role of graphene, it is also desirable to use graphene-based
via homolytic cleavage of O-O bond to produce (CH3)3CO･and semiconductor composites as a platform for photocatalysis.
Ph-COO . The π-π interactions, indicated by IR, brought TBPB
･
We thank the financial support from the Ministry of
and graphene in close proximity and the intimate interfacial Science and Technology of ROC under Contract No. MOST 103-
contact facilitated the ET pathway. Graphene also behaved like 2113-M-110-005 and 104-2113-M-110-007.
a solid matrix exploited in the matrix isolation technique which
enables reactive chemical species to be isolated and studied
Notes and references
spectroscopically. The chemical inertness of the matrix
material prevents loss of reactive species. Here graphene was
proven to be unreactive towards phenyl and t-butoxy radicals
generated from adsorbed TBPB by UV exposure. These
photogenerated radicals were intercepted at the cryogenic
temperature on graphene, then became mobile and started to
explore many configurations until finding the right partners
(another radical to be exact) in two dimensions (2D) during the
annealing stage of matrix warm-up.
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In conclusion, our study represents the first example of
using UV-LED as the photon source to initiate decomposition
of an unsymmetrical peroxide composed of t-butoxy and
benzoyloxy groups, bound to the metal-supported graphene
under UHV conditions. Direct surface IR detection infers that
both O-O and Ph-C bonds were cleaved upon irradiation,
suggesting that t-butoxy and phenyl radicals were in situ
generated and trapped on graphene. Desorption of t-butanol
and benzene observed in PITPD resonates with the forming of
the proposed radicals. Our proposed mechanism is depicted in
Fig. 4. Although no evidence supports covalent interactions
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Fig. 4 Proposed reaction mechanism.
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