Solid State Communications 142 (2007) 99–103www.elsevier.com/locate/ssc

Electronic behaviours of calcined materials from a(S-nickel-S-phenylene-O)-strontium-(O-phenylene-S-selenium-S)

hybrid copolymer

T. Furukawaa, H. Matsuib, H. Hasegawab, S. Karuppuchamyc,∗, M. Yoshiharab,c

a Department of Chemistry, Kawaijuku Educational Institution, 3-13-31, Toyosaki, Kita-ku, Osaka, 531-0072, Japanb Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashiosaka, Osaka, 577-8502, Japan

c Molecular Engineering Institute, Kinki University, 11-6, Kayanomori, Iizuka, Fukuoka, 820-8555, Japan

Received 23 October 2006; accepted 15 January 2007 by E.V. SampathkumaranAvailable online 21 January 2007

Abstract

The calcination of a (S-nickel-S-phenylene-O)-strontium-(O-phenylene-S-selenium-S) hybrid copolymer under an argon atmosphere at400–600 ◦C was performed. 600 ◦C-calcined material was found to be composed of nano-sized NiS, SrS and Se particles in the matrix ofthe carbon clusters. ESR spectral examinations of the calcined material suggest the possibility of an electron transport in the process of carbonclusters → NiS → SrS → Se with a photoresponsive oxidation–reduction function.c© 2007 Elsevier Ltd. All rights reserved.

PACS: 71.20.Rv; 71.20.Nr; 71.24.+q; 72.80.Tm

Keywords: A. Nanostructures; A. Polymers; A. Semiconductors; D. Electronic transport

1. Introduction

Photosynthesis function is simplified as a PSII-PSI two-step electron excitation, and the construction of such multi-step electron excitation is considered to be important fordeveloping a new photo-science. Semiconductors such as TiO2and modified metal oxides [1–6] have been expected to exhibitsuch an excitation and shown to decompose water to H2and O2 under photo-irradiation, however, their quantum yieldswere low and limited wavelengths below ca. 500 nm wereused. The construction of stable charge separation withoutthe recombination of holes and excited electrons under wholevisible-light irradiation is considered to be important forachieving an effective oxidation–reduction function. We haverecently reported the syntheses of metal-organic moietieshybrid copolymers [7–10], in which an electron transfer fromthe organic moieties to the metal atoms took place. We haveassumed that the calcination of such hybrid copolymers undera reducing atmosphere provides new types of nano-sized

∗ Corresponding author. Tel.: +81 948 22 7201x613; fax: +81 948 22 7336.E-mail address: chamy@me-henkel.fuk.kindai.ac.jp (S. Karuppuchamy).

0038-1098/$ - see front matter c© 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2007.01.011

inorganic semiconductors/carbon clusters composite materials,which affect the feature of electron moving and the lightabsorption ability. Recently, we have shown the formation ofsuch composite materials by calcining several metal-organicmoieties hybrid copolymers [11–17]. Especially, Pt-loadedCeO2/carbon clusters/Ho2O3 composite material was found todecompose water to H2 and O2 with the H2/O2 ratio of 2 undervisible light irradiation [17].

In this work, we describe the compositions and electronicproperties of calcined materials from (S-nickel-S-phenylene-O)-strontium-(O-phenylene-S-selenium-S) hybrid copolymer I(Scheme 1). The calcination of I is expected to provide acomposite material composed of carbon clusters and Ni, Sr,and/or Se derived-inorganic compounds. And, when pluralsemiconductors are involved in the material, a multi-stepelectron transfer is expected to take place.

2. Experimental

2.1. Reagents

Commercially available nickel bromide, selenium bromide,strontium isopropoxide, 1,4-benzenedithiol, 4-mercaptophenol,

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Table 1ICP and elemental analyses of copolymer I and calcined materials Ic’s

Material % (Found) Molar ratioNi Se Sr C H S Ni:Se:Sr

I 4.92 2.66 15.89 31.69 3.32 8.68 2.5:1:5.4

Ic-400 5.91 4.04 15.21 25.90 1.56 10.99 2.0:1:3.4Ic-500 6.82 4.80 17.21 22.76 1.28 14.51 1.9:1:3.1Ic-600 6.40 4.66 16.11 24.08 1.02 12.81 1.8:1:3.1

Scheme 1. Synthesis of copolymer I.

1,4-hydroquinone, anhydrous ethanol, magnesium oxide,1,1-diphenyl-2-picrylhydrazyl (DPPH), triethylamine, and 1,4-benzoquinone were used.

2.2. Syntheses of copolymers

A solution of nickel bromide (131 mg, 0.6 mmol) inanhydrous ethanol (80 mL) was added into a solution of4-mercaptophenol (151 mg, 1.2 mmol) and magnesium oxide(15 g) in a cylindrical filter in anhydrous ethanol (20 mL), andthe mixture was refluxed for 6 h to obtain a precursor solutionA. Similar treatment of selenium bromide (120 mg, 0.3 mmol)with 4-mercaptophenol (151 mg, 1.2 mmol) in the presence ofmagnesium oxide (15 g) in anhydrous ethanol (70 mL) gaveprecursor B solution. A solution of strontium isopropoxide(247 mg, 1.2 mmol) in anhydrous ethanol (50 mL) was addedinto the mixture of A and B, and the resulting mixture wasrefluxed for 6 h. The obtained precipitates were washed withanhydrous ethanol by using a Soxhlet extractor and dried atroom temperature under vacuum to obtain copolymer I.

A solution of nickel bromide (131 mg, 0.6 mmol) inanhydrous ethanol (80 ml) was added into a solution of1,4-benzenedithiol (151 mg, 1.2 mmol) and magnesium oxide(15 g) in a cylindrical filter in anhydrous ethanol (20 ml), andthe mixture was refluxed for 6 h. Precipitates were washed withanhydrous ethanol using a Soxhlet extractor and dried at roomtemperature under vacuum to obtain copolymer II. Similartreatment of selenium bromide (123 mg, 0.6 mmol) with 1,4-benzenedithiol (151 mg, 1.2 mmol) gave copolymer III.

A solution of strontium isopropoxide (247 mg, 1.2 mmol)and 1,4-benzenedithiol (151 mg, 1.2 mmol) in anhydrousethanol (50 ml) was refluxed for 6 h to obtain copolymer IV.

2.3. Calcination of copolymers

0.5 g of copolymers in a porcelain crucible was heated witha heating rate of 5 ◦C/min under an argon atmosphere for 1 husing a Denken KDF 75 furnace.

2.4. Water decomposition

A mixture of calcined material and degassed water in asealed 1 mL-glass tube was irradiated with a halogen lampat room temperature. Evolved gases were analyzed using gaschromatography.

2.5. Apparatus

TG-DTA analysis was performed using a Rigaku TG-DTA-MS8010 unit. ICP analyses were performed by inductivelycoupled plasma atomic emission spectroscopy (ICP-AES)using Shimazu ICP-7500. Elemental analyses were performedfor C and H using Yanaco MT-6 corder and for S usingYanaco YS-10. X-ray diffraction (XRD) spectra were measuredusing Rigaku Mini Flex. Trasmission electron microscopy(TEM) observations were carried out using Jeol TEM-3010microscope. Electron spin resonance (ESR) spectra weretaken using Jeol JES-TE 200 spectrometer. Visible-light wasgenerated using Hoya-Schott Megalight 100 halogen lamp. H2,O2, and CO2 analyses were performed using Shimadzu GC-8Agas chromatography.

3. Results and discussion

In order to find a procedure for synthesizing copoly-mer I, the reactivities of starting metal compounds with1,4-hydroquinone and 1,4-benzenedithiol were examined. Bothnickel bromide and selenium bromide was shown to reactwith 1,4-benzenedithiol to obtain the corresponding copoly-mers but did not react with 1,4-hydroquinone. Meanwhile,strontium isopropoxide reacted with both 1,4-benzenedithioland 1,4-hydroquinone to give the corresponding copolymers.Thus, copolymer I was synthesized by reacting strontium iso-propoxide with the mixture of nickel precursor A and seleniumprecursor B (Scheme 1). Elemental and ICP analyses showedthat the Ni:Se:Sr molar ratio in copolymer I was 2.5:1:5.4 (Ta-ble 1).

The TG-DTA measurement of copolymer I with a heatingrate of 5 ◦C/min under a nitrogen atmosphere (Fig. 1) showedan endothermic reaction below 200 ◦C due to the eliminationof volatile components, an exothermic reaction at 300–550 ◦Cdue to carbonization, and a weight decrease above 550 ◦C maybe due to the growth of carbon clusters and metal compounds.Copolymer I was thus calcined with a heating rate of 5 ◦C/minunder an argon atmosphere for 1 h at 400 ◦C, 500 ◦C, and600 ◦C, respectively, to obtain black-coloured materials Ic-400,Ic-500, and Ic-600, respectively. Results of ICP and elemental

T. Furukawa et al. / Solid State Communications 142 (2007) 99–103 101

Fig. 1. TG-DTA analyses of copolymer I.

Fig. 2. X-ray diffraction of copolymer I and calcined materials Ic’s.

Fig. 3. TEM images of copolymer I and calcined materials Ic’s.

analyses of the materials were summarized in Table 1. The riseof calcination temperature decreases the contents of hydrogen,suggesting that the carbonization of copolymer I has beenproceeded. The Ni:Se:Sr molar ratios in the calcined materialswere found to be 1.8–2.0:1:3.1–3.4, indicating that each metalcomponent was involved in the calcined materials.

The XRD spectra measurements (Fig. 2) showed that Ic-600had peaks at 2θ = 34.8◦, 45.6◦, and 52.6◦ due to NiS and at2θ = 29.6◦ and 42.4◦ due to SrS. Meanwhile, no peak dueto selenium component was detected, suggesting that XRD-undetectable selenium component may be involved. In order toexamine selenium component, the XPS spectra of Ic-600 wasmeasured to show a peak at 54.2 eV due to a 3d of Se metal.TEM images of the calcined materials (Fig. 3) showed that thematerials had ultrafine particles with the diameters of a few nmin the matrix of carbon clusters. These findings indicated thatthe calcined materials had nano-sized NiS, SrS, and Se particlesin the matrix of the carbon clusters.

In order to examine the electronic properties of the calcinedmaterials, the ESR spectra were obtained (Fig. 4). Eithercopolymer I or the calcined materials showed a signal at337 mT (g = 2.003) that is considered to be due toa free electron formed on the phenylene group for thecopolymer and on the carbon clusters for the calcined materialsthrough an electron transfer between the inorganic compoundsand the carbon clusters [11–15]. The radical spin quantities

Table 2Radical spin quantities (rsq) of copolymer I and calcined materials Ic’s

Materials rsq (spins/mg)

I 3.13 × 1018

Ic-400 1.24 × 1020

Ic-500 3.10 × 1020

Ic-600 5.55 × 1020

Fig. 4. ESR spectra of copolymer I and calcined materials Ic’s.

Fig. 5. ESR spectra of calcined material (Ic-600) under visible light irradiation.

Fig. 6. ESR spectra of calcined material (Ic-600) in the presence of an oxidant(1,4-benzoquinone) or a reductant (triethylamine).

(rsq) determined by a double integrating calculation of thedifferential absorption line with the use of DPPH were shownin Table 2. Ic-600 had the highest rsq value, which wasabout 180 times that of copolymer I, indicating that thecalcination treatment enhanced the degree of charge separation.The ESR signal intensity of Ic-600 was found to decrease inresponse to visible-light irradiation, but to recover in responseto eliminating the light (Fig. 5), indicating that Ic-600 hada photo-response characteristics. Fig. 6 showed that the peakintensity of Ic-600 was increased with the addition of areductant (triethylamine) and decreased with the addition of anoxidant (1,4-benzoquinone), indicating that the radical speciesis cationic.

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Scheme 2. Synthesis of copolymers II, III and IV.

Fig. 7. ESR spectra of calcined material (IIc-600) in the presence of an oxidant(1,4-benzoquinone) or a reductant (triethylamine).

Table 3Variation of radical spin quantities (rsq) of calcined materials IIc-600, IIIc-600and IVc-600

Materials rsq (×1019 spins/g)IIc-600 IIIc-600 IVc-600

Oxidant (1,4-benzoquinone) 2.33 × 1019 0.69 × 1019 1.46 × 1019

Reductant (triethylamine) 0.94 × 1019 0.67 × 1019 0.54 × 1019

In order to investigate further the electron transfer pro-cess in Ic-600, nickel-S-phenylene copolymer II, selenium-S-phenylene copolymer III, and strontium-S-phenylene copoly-mer IV were synthesized (Scheme 2), and the copolymers werecalcined under an argon atmosphere at 600 ◦C for 1 h to ob-tain black-coloured materials IIc-600, IIIc-600, and IVc-600,respectively. XRD and TEM measurements showed the pres-ence of nano-sized NiS particles (2θ = 34.8◦, 45.6◦, and52.6◦) for IIc-600 and SrS particles (2θ = 29.6◦ and 42.4◦)for IVc-600, while no XRD peak was detected for IV-600. ESRspectral examinations of calcined materials IIc-600, IIIc-600,and IVc-600 were performed. The signal intensity of IIc-600(NiS system) was found to increase with the addition of the ox-idant and decreased with the addition of the reductant (Fig. 7and Table 3), indicating that an electron transfer from the car-bon clusters to the NiS particles took place to form a cationradical on the carbon clusters. In the case of IVc-600 (SrS sys-tem), the addition of the oxidant increased the peak intensity,but a slight decrease of the intensity was detected with the ad-dition of the reductant (Fig. 8 and Table 3), indicating that anelectron transfer from the carbon clusters to the SrS particlestook place to form a cation radical on the carbon clusters. Itis noted that the signal intensity of IIIc-600 (Se system) didnot vary with the addition of the oxidant and the reductant(Fig. 9 and Table 3), probably due to Se metal without a band.

Fig. 8. ESR spectra of calcined material (IVc-600) in the presence of anoxidant (1,4-benzoquinone) or a reductant (triethylamine).

Fig. 9. ESR spectra of calcined material (IIIc-600) in the presence of anoxidant (1,4-benzoquinone) or a reductant (triethylamine).

Scheme 3. Plausible electron transfer process.

From these results, we wish to propose an electron transfer pro-cess of Ic-600 as follows. The band gaps of NiS and SrS are1.2 eV and 2.4 eV, respectively. Therefore, a conceivable elec-tron transfer process in Ic-600 is a multi-step electron transferof carbon clusters → NiS → SrS → Se (Scheme 3), to giverise to a photoresponsive oxidation–reduction function with anoxidation site at the carbon clusters and a reduction site at Semetal.

In order to examine the oxidation–reduction ability ofIc-600, a preliminary water-decomposition experiment wascarried out by irradiating a stirred mixture of water (0.3 mL)and Ic-600 (50 mg) with a 100 W halogen lamp at roomtemperature for 72 h to obtain H2 of 0.603 µmol/g, however,no O2 evolution was observed. H2 evolution suggested that areduction energy sufficient to decompose water at the reductionsite, i.e. the metallic Se, was obtained and the oxidation ofH2O at the oxidation site, i.e. the carbon clusters, certainlytook place. A possible assumption for the non-generation of O2is that an activated oxygen species generated at the oxidationsite were consumed possibly by the reaction with the carbon

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clusters. The formation of CO2 was observed by the gaschromatography analysis of a gas after irradiating a mixture ofwater and Ic-600 with a halogen lamp, suggesting that activatedoxygen species at the oxygen site may react with the carbonclusters to yield CO2.

4. Conclusions

We have been successful in achieving multi-stage electrontransfer with H2 evolution for the present system, but noO2 evolution was observed. Here, if the surface of thecarbon clusters is covered with other metal oxides such asTiO2, RuO2, etc, a stable and effective oxidation–reductionfunction could be achieved. We believe that such electrontransfer could be achieved by the combinations of carbonclusters and various photosensitive semiconductors. We alsobelieve that our findings will provide a valuable source formany useful materials, including optical, magnetic, electronicdevices, artificial photosynthesis catalysts and so on.

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