Chemical Physics Letters 384 (2004) 283–287

www.elsevier.com/locate/cplett

Highly oxygenated fullerene anions C60O�n formed

by corona discharge ionization in the gas phase

Hideki Tanaka a,*, Kazuo Takeuchi a, Yuichi Negishi b,c, Tatsuya Tsukuda b,c

a RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japanb Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan

c Department of Photoscience, School of Advanced Sciences, The Graduate University for Advanced Studies, Hayama, Kanagawa 240-0193, Japan

Received 3 September 2003; in final form 18 November 2003

Published online: 29 December 2003

Abstract

Oxygenated fullerenes anions were produced by a vaporization source equipped with a corona discharge ionizer in the presence

of a trace amount of oxygen. In situ mass analysis revealed that the species formulated as C60O�n (n6 30) were formed in the source

and that the degree of oxygenation could be altered by the discharge current. Formation of the epoxide structure in the C60O�n was

suggested by XPS measurements for thin films prepared by deposition of the C60O�n beam. The structures and formation processes

of higher analogues (C60)mO�n (m ¼ 2; 3) are briefly discussed.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

Recent advances in fullerene chemistry have explored

possible applications of fullerenes as precursors or

building blocks of novel functional materials [1,2].

Chemical modifications of C60 such as oxygenation and

halogenation have yielded various families of fullerene

derivatives denoted as C60Xm [3–5]. Oxygenated fulle-

renes, C60On, attract much attention as a potential

source for elucidation of the oxidative stability of C60

and applications in areas ranging from photoelectric

devices to biological systems [6–9]. While the reported

synthetic routes of C60On include photooxygenation,

ozonolysis, and epoxidation by peroxybenzoic acids

[10–16], the degree n of the oxygenation has yet been

limited more severely than other addition reactions such

as halogenation. The reasons for this situation are (1)

poor solubility of highly oxygenated fullerenes in con-ventional organic solvents and (2) insurmountable en-

ergy barriers for reactions of fullerenes with oxygen

molecules in the ground state [17,18].

* Corresponding author. Fax: +81-48-462-4702.

E-mail address: htanaka@riken.jp (H. Tanaka).

0009-2614/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2003.11.113

To overcome these difficulties, we have examined the

oxidation reaction of C60 molecules by use of a coronadischarge in the gas phase. The C60 molecules vaporized

into the gas phase were allowed to react with oxygen

species activated by a corona discharge. The degree nwas probed by in situ mass spectrometry in the negative-

ion mode. The chemical identity of the cluster anions

was studied by X-ray photoelectron spectroscopy (XPS)

after deposition onto a substrate. Their structure and

formation process are discussed in this Letter on thebasis of these experimental data, together with the re-

sults of a semi-empirical calculation.

2. Experiment

2.1. Cluster-ion source

The details of the cluster-ion source and the methods

of analysis have been described elsewhere [19–23]. Fig. 1

shows a schematic diagram of the cluster-ion source

equipped with a time-of-flight mass spectrometer. Pow-

der of C60 (purity 99.98%, Matsubo) contained in an

alumina boat was placed in a particle generator made of

stainless steel and vaporized by a furnace heated to 700 K

Fig. 1. Schematic diagram of the experimental setup.

284 H. Tanaka et al. / Chemical Physics Letters 384 (2004) 283–287

in a gas-flowmixture of Ar and O2, whose flow rates were

typically set to 0.99 and 0.01 slm (standard liters per

minute), respectively, by use of digital mass flow con-

trollers (MC-3102E, LINTEC). The vaporized C60 and

the O2 molecules were ionized and/or activated by a

corona discharge ionizer. The corona discharge induced

by applying a negative voltage to an electrode was reg-

ulated by a constant-current power supply (KE2005PN2/100, ME). The pressure monitored by a capacitance

manometer (Baratron Type 622, MKS Instruments) at-

tached to the ionizer was approximately 1.2� 104 Pa at a

total flow rate of 1 slm. The particle generator and

connection tubes were baked before use to minimize

impurities.

2.2. Mass analysis of cluster ions

The cluster ions produced by the corona discharge

ionizer effused through an exit hole 2 mm in diameter

and were admitted to the acceleration region of the mass

spectrometer after passing through two stages of differ-

ential pumping. The cluster anions were extracted per-

pendicularly to the initial-beam axis by applying a

pulsed high voltage ()15 kV, 200 Hz) to a set of elec-trodes. The cluster anions were steered by a set of ion

optics and detected by an inline microchannel plate lo-

cated at the end of the flight path of 1.17 m. Pulsed

signals were counted by a multichannel scaler/averager

(SR430, Stanford Research Systems).

Fig. 2. Negative-ion mass spectrum obtained by a corona discharge

ionizer operated at (a) 100 mA in neat Ar flow. Mass spectra obtained

at (b) 10 mA and (c) 100 mA in Ar flow containing 1% oxygen.

2.3. XPS analysis of clusters collected onto a substrate

XPS measurements were conducted for a thin film of

the clusters prepared by using an ion collector [24] con-

nected downstream from the ion source. The cluster an-

ions produced by the corona discharge ionizer were

electrically collected onto a Si substrate applied with

+200 V. The Si substrate was taken out of the collector

after 1 h of collection and introduced into an X-ray pho-

toelectron spectrometer (ESCALAB250, ThermoVG).

Monochromatic X-ray Al Ka radiation (1486.6 eV)

was used for the measurement. The C 1s peak of pristine

C60 (285 eV) was used as a reference for estimating

the binding energy [25]. Analysis of the spectrum was

carried out by a software package (ECLIPS, ThermoVG).

3. Results and discussion

3.1. Formation of oxygenated fullerene anions C60O�n

Fig. 2a shows a portion of a mass spectrum of

cluster anions produced by the corona discharge ionizer

Fig. 3. C 1s spectra of (a) an oxygenated C60 thin film prepared using

100 mA corona discharge and (b) a pure C60 thin film prepared

without the discharge.

H. Tanaka et al. / Chemical Physics Letters 384 (2004) 283–287 285

at 100 mA in a neat Ar gas flow. The C60 monomer

anion was dominantly observed in this spectrum, while

other ionic species produced by residual oxygen in the

Ar gas were observed as weak peaks. In contrast,

Figs. 2b, c show portions of mass spectra of the clusteranions produced by the ionizer at 10 and 100 mA, re-

spectively, in an Ar flow containing 1% oxygen. While

no C60 monomer anion was practically observed, ex-

tensive production of anions with higher masses was

observed. In both spectra, the observed anions were

composed of mass peaks with an equidistant interval of

16 a.u. This regularity supports that the anions ob-

served dominantly can be assigned to oxygenated ful-lerene anions C60O

�n . We note that interstitial mass

peaks discernible between those of C60O�n (Fig. 2b)

cannot be assigned to species formulated as C60O�n ;

they were possibly originated from impurities such as

N2 and/or H2O molecules present in the connection

tubes. Since the degree of oxygenation n was higher

under higher current conditions where the oxidants

such as O and O3 were expected to be generated moreabundantly [26], we infer that the C60On are formed via

the sequential oxygenation by these reactive species

C60 !O;O3 � � � !O;O3

C60On�1½ �� !O;O3C60On½ �� ð1Þ

The negatively charged oxygenated fullerenes C60O�n

were formed via free-electron capture by C60Om

(06m6 n) and/or the reaction of anionic oxygen species

with C60Om (06m < n) at a certain stage of reaction (1).

3.2. Structures of oxygenated fullerene anions C60O�n

The intensity distributions of Figs. 2b, c appeared to

be uniform without appreciable even–odd alternation or

magic numbers. When the discharge current was in-

creased, the intensity distribution was shifted toward a

higher mass. For example, the distribution recorded at

100 mA, shown in Fig. 2c, started at n � 4, peaked atn � 20, and faded away at n � 30. The absence of an

even–odd alternation in the n-distribution strongly

suggests that the O2 molecules are not physically ad-

sorbed onto the fullerene cage, but the O atoms are

chemically bonded to the cage. It is plausible that the

oxygen atoms are preferentially bonded to the double

bonds between two adjacent hexagonal rings (6/6

bonds) on a C60 molecule, thereby forming epoxidestructures; the existence of these structures is well es-

tablished by extensive experimental and theoretical

studies for neutral C60On molecules [10–16,27]. The

observed maximum number of n (�30) was close to the

number of 6/6 bonds on the C60 molecule. This coinci-

dence also implies the epoxidation of 6/6 bonds on the

C60 molecule.

Fig. 3 shows the C 1s spectra of (a) an oxygenatedC60 thin film prepared using the 100 mA corona dis-

charge and (b) a pure C60 thin film prepared without

the discharge. Upon deposition onto the Si substrate,

the cluster anions C60O�n were neutralized by electron

transfer to the substrate, whose work function (4.9 eV

[28]) is significantly higher than the electron affinity of

the clusters: 2.7 eV for a C60 molecule [17] and 1.6 eV for

a C60O30 estimated by a semi-empirical calculation [29].

While a dominant peak (285 eV) and shake-up satellitepeaks (287–291 eV) due to C60 molecules were observed

in spectrum (b) [30], a new peak was observed at 287 eV

in addition to those associated with the carbons of the

C60 molecules in spectrum (a). The chemical shift of the

additional peak with reference to that for the C60 mol-

ecules (+2 eV) is comparable to that reported for

epoxidized fullerenes [25]. Such a comparison shows

that the deposited oxygenated fullerenes do not formopening structures of the fullerene cages (C@O for-

mation, +4-eV chemical shift), but epoxide structures

(C–O formation, +2-eV chemical shift). Indeed, geom-

etry optimization of C60O30 with semi-empirical PM3

levels yielded a fully epoxidized form of the C60 mole-

cule, as shown in Fig. 4. If we assume that the deposition

of the C60O�n on the substrate does not lead to extensive

rearrangements of the chemical bonding, the presentresults indicate the formation of an epoxide structure for

the gas-phase C60O�n anions.

2100 2400 2700 3000

1400 1700 2000 2300

Inte

nsity

(ar

b. u

nits

)

30 40n = 20

20 30n = 10 40

Mass Number (m/z)

(a)

(b)

Fig. 5. Portions of mass spectra in regions of (a) C120O�n and (b)

C180O�n . Cluster anions were produced under 100 mA discharge in an

Ar flow containing 1% oxygen.

Fig. 4. Energetically optimized geometric structure of fully epoxidized

fullerene C60O30. Open and closed balls represent carbon and oxygen

atoms, respectively.

286 H. Tanaka et al. / Chemical Physics Letters 384 (2004) 283–287

Energetics associated with the formation of C60O�n is

considered here, for which formation of a fully epoxi-

dized fullerene anion is taken as an example,

C60 þ e� þ 30O ! C60O�30 ð2Þ

The heat of reaction of (2) is given by

DH ¼ HfðC60O30Þ � 30HfðOÞ � HfðC60Þ � EaðC60O30Þð3Þ

where Hf and Ea represent the heat of formation and the

electron affinity, respectively. The values of HfðC60Þ,HfðC60O30Þ, and EaðC60O30Þ were calculated to be 35.2,

7.8, and 1.6 eV, respectively, at semi-empirical PM3

levels. The HfðOÞ value has been reported to be 2.6 eV

[28]. By using these values, the heat of reaction DH isestimated to be )107.0 eV. Although reaction (2) over-

simplifies the complex chemical processes in the ionizer,

the overall exothermicity for the C60O�30 formation well

exceeds the energy required to release CO and/or CO2

from the C60O�n [31]. The absence of such fragmentation

channels (Fig. 2) indicates that the excess energy asso-

ciated with the oxidation reaction (1) was effectively

dissipated by inelastic collisions with Ar atoms.

3.3. Polymer anions of oxygenated fullerenes

We found that larger aggregates were formed in the

cluster source in addition to C60O�n . Fig. 5 shows

portions of a mass spectrum of cluster anions in a high-

mass region, which were recorded under the same

conditions as in Fig. 2c. Because the anions observed insuch a mass region were also composed of mass peaks

with an equidistant interval of 16 a.u., the observed

anions can be assigned to C120O�n and C180O

�n . Fig. 5

shows that C120O�25 and C180O

�30 give maximum inten-

sities within the corresponding envelope. The chemical

compositions of (C60)mO�n show that the average

numbers of O atoms per C60 unit decrease with in-

creasing m. This trend suggests that the fullerene cages

of C120O�n and C180O

�n are linked via oxo-bridges; ful-

lerene surfaces that face each other through the oxo-

bridge are not easily oxygenated due to steric hindrance

[31–33]. The oxygenated fullerenes C60On with small n

formed in the early stages of process (1) are allowed to

polymerize with C60 and/or C60On

C60On½ �� !C60On0

C60Onð Þ½ –O– C60On0ð Þ�� !C60On00 � � � ð4Þ

The polymers thus formed are subsequently oxygen-

ated by the oxidants and finally quenched in a manner

similar to that described in Section 3.2. As describedin Section 3.1, the anionic charge was introduced into

the polymer via free electrons and/or anionic oxygen

species at a certain stage of reaction (1) and (4).

4. Summary

Oxygenated fullerene anions C60O�n have been

formed efficiently by the corona discharge ionization of

C60 in the presence of a trace amount of oxygen.

H. Tanaka et al. / Chemical Physics Letters 384 (2004) 283–287 287

Epoxide structure in the C60O�n is suggested by their

uniform intensity distributions and further confirmed by

XPS measurements for the neutralized clusters collected

on the substrate. The degree of epoxidation reported

here is significantly higher than those obtained with wetchemical approaches [10–16]: a fully epoxidized fuller-

ene anion C60O�30 has been detected by mass spectrom-

etry. While it is still unclear whether the ionization of the

clusters is significantly engaged in the formation mech-

anism, the present study demonstrates that gas-phase

reactions involving fullerenes open up a new route for

their chemical modification.

Acknowledgements

We are grateful to Dr. Y. Tajima for helpful discus-

sion and Dr. A. Nakao for XPS measurements. The

present work has been supported in part by a Grant-in-

Aid for Scientific Research (Grant No. 15710098) from

the Ministry of Education, Culture, Sports, Science, and

Technology (MEXT) of Japan and the �Nanotechnology

Support Project� of MEXT, Japan.

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