Journal of Colloid and Interface Science 421 (2014) 165–169
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
Nanospace-enhanced photoreduction for the synthesis of copper(I) oxide nanoparticles under visible-light irradiation Takahiro Ohkubo a,⇑, Mitsuhiro Ushio a, Koki Urita b, Isamu Moriguchi b, Bashir Ahmmad c, Atsushi Itadani a,d, Yasushige Kuroda a a
Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan Division of Chemistry and Material Science, Graduate School of Engineering, Nagasaki University, 1-14 Bunkyomachi, Nagasaki 852-8521, Japan Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan d Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshidaushinomiyacho, Sakyou-ku, Kyoto 606-8501, Japan b c
a r t i c l e
i n f o
Article history: Received 30 September 2013 Accepted 25 January 2014 Available online 4 February 2014 Keywords: Single-wall carbon nanotube Cuprous oxide (Cu2O) Nanoparticle Photoreduction Adsorption XAFS
a b s t r a c t Nanoparticles of copper(I) oxide (cuprous oxide; Cu2O) were able to be synthesized from nano-restricted copper acetate (Cu(OAc)2) in micropores of single-wall carbon nanotubes (SWNTs) by visible-light photoreduction. The specific structure of confined Cu(OAc)2 in the micropore is indispensable for the reduction process to Cu2O by the irradiation, because, in general, aqueous solution of Cu(OAc)2 can be reduced under UV-light irradiated conditions. The present results strongly suggest that the micropore of SWNTs whose pore width is in the micropore-size range can play as nanoreactor space for the synthesis of Cu2O through the nano-restricted precursor whose reactivity is different from that in the bulk phase. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Metal and semiconductor nanomaterials have been central research subjects in nanoscience and nanotechnology because they are promising candidates to unlock the frontier of catalysts, synthesis, and sustainable chemistry. Especially, nanomaterials such as nanoparticles and nanoporous solids of semiconductor oxides have been widely studied to develop noble materials for photochemical applications. Titanium dioxide (TiO2), for example, has been widely used as a photocatalyst that can decompose water molecules into oxygen and hydrogen under UV-light irradiation [1]. Recent progress enables us to synthesize TiO2 having special forms such as tubular structure [2–4]. Cuprous oxide (Cu2O) has also gathered much attention because it is a semiconductor oxide whose band gap (direct forbidden band gap) is about 2.17 eV [5] and, therefore, Cu2O can be applied to potential materials for catalysts and solar cells. The number of reports related to the synthesis and characterization of noble Cu2O materials is apparent evidence to represent the importance of Cu2O in application fields [6–16]. Copper acetate (Cu(OAc)2) is generally a suitable precursor for the production of Cu2O because Cu(OAc)2 can be easily exited by ⇑ Corresponding author. Fax: +81 86 251 7843. E-mail address: [email protected] (T. Ohkubo). http://dx.doi.org/10.1016/j.jcis.2014.01.035 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
UV-light irradiation to reduce Cu2+ into Cu+, or Cu0 in some cases, by a ligand-to-metal charge-transfer (LMCT) transition process [10]. Confined molecules and ions restricted in nanospace whose pore size is comparable to the adsorbed species generally tend to form characteristic structures that depend on both pore size and geometry. The characteristic structure leads to the physically or chemically unique properties of the adsorbed species. For instance, Holt et al. reported the fast transport property of air and water through less than 2 nm micropore of carbon nanotubes [17]. The property of the restricted compounds must be results of strong confinement in the nanospace and thus-fabricated distortion of adsorbed species. Actually, recent papers have demonstrated the quasi-compression effect in nanospaces, especially in the micropore [18–20]. Therefore, micropores originally have the faculty to produce high-pressure molecular or ionic states even if both an adsorbent and an adsorbate are in the atmospheric condition. We already reported structural anomalies of hydration structure around metal ions such as rubidium [21,22], zinc [23,24], and cobalt [25] ions, as well as coordination structure around a Cu ion of Cu(OAc)2 [26] confined in the carbon micropores. Here, Cu(OAc)2 is a simple coordination compound which can form a dinuclear complex with acetate bridging [27]. Our previous study on Cu(OAc)2 restricted in the micropore of activated carbons (ACs)
T. Ohkubo et al. / Journal of Colloid and Interface Science 421 (2014) 165–169
strongly indicates that the structure of the dinuclear complex is quite different from that in the bulk. Therefore, the reactivity of nano-restricted Cu(OAc)2 should be different from that in the bulk phase because of the remarkably distorted states in micropores. Accordingly, we describe the new synthetic route to Cu2O from Cu(OAc)2 by using single-wall carbon nanotube (SWNT) as a nanoreactor under the appropriate light-irradiated condition without any additional reductants. 2. Experimental section In the present study, we used SWNTs synthesized by the HiPco method (purchased from Unidym Inc.; diameter of a tube = 1.1 ± 0.2 nm) and studied with two kinds of SWNTs; as-synthesized SWNT and oxidized one in an air-flow condition (100 cm3 min 1) at 623 K followed by washing with hydrochloric acid (12 mol dm 3) and distilled water. In the article, we denote such an oxidized SWNT as ox-SWNT. Also, we used copper acetate (Cu(OAc)2) monohydrate (99.9%; Wako Ltd.) as a precursor, which was not purified anymore. Each nanotube sample (30 mg) was stirred in the aqueous solution of Cu(OAc)2 (5 cm3, 0.5 mol dm 3) over 24 h to impregnate the electrolyte into the micropore. In the case of the synthesis of Cu2O, water vapor was adsorbed at a relative pressure greater than 0.9 after the deposition of Cu(OAc)2 followed by washing, drying in the desiccator, and pre-evacuation at 423 K in a quartz reactor cell (denoted by SWNT-Cu or ox-SWNT-Cu, respectively). Here, we describe the samples in evacuated and saturated water-vapor conditions as x-evac and x-H2O (x = SWNT-Cu or ox-SWNT-Cu), respectively. Light irradiation was performed by a 300 W xenon light source (MAX-303; Asahi Spectra). Each sample was shielded in a quartz reactor cell with saturated water vapor. The light beam was passed through both a mirror module which limits the irradiation wavelength between 385 and 740 nm and an appropriate shortpass filter. The power of irradiated light through each shortpass filter was measured using a power meter (Photo-Radiometer Model HD 2302.0 coupled with the probe LP471RAD) which can detect the power between 400 and 1050 nm. Powder X-ray diffraction (XRD) profiles of SWNTCu and ox-SWNT-Cu samples were collected by using the diffractometer (MiniFlexII; Rigaku Ltd.) with monochromatic Cu Ka radiation (k = 0.154184 nm) under 30 kV and 15 mA as well as synchrotron-irradiated X-ray at SPring-8 (BL02B2) to investigate the adsorbed states of the precursor and the resultant under in situ condition. Here, the wavelength of the monochromatic Xray at SPring-8 was estimated by the analysis of standard profiles from CeO2 crystal. Also, scattering parameter (s = 4psinh/k) was used to show our XRD results to avoid confusion when we compare the results obtained by using different wavelengths. Here, h is a half of scattering angle (2h). A conventional high-resolution transmission electron microscope (TEM; JEM-2010, JEOL Ltd.) equipped with a Gatan detector (ORIUS SC1000) was used at an accelerating voltage of 120 kV. All samples for TEM measurements were dispersed in ethanol, and then fixed on a copper grid coated with holey carbon. X-ray absorption fine structure (XAFS) measurements were performed on the Cu K-absorption edge (8980 eV) using hand-made XAFS cell with windows of Kapton film at the National Laboratory for High Energy Accelerator Research Organization (KEK). We used IFEFFIT code [28] with FEFF6 procedure [29] for the analysis of XAFS spectra including both extended XAFS (EXAFS) and X-ray absorption near edge structure (XANES) spectra. 3. Results and discussion At first, the structure of Cu(OAc)2-dispersed samples was examined with a synchrotron-irradiated X-ray source. Fig. 1 shows
(d) Intensity / arb. unit
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(c)
(b)
(a) 10
20
s/
30
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40
50
nm)
Fig. 1. In situ powder XRD profiles of (a) SWNT-Cu-evac, (b) SWNT-Cu-H2O, (c) oxSWNT-Cu-evac, and (d) ox-SWNT-Cu-H2O at room temperature.
in situ powder XRD profiles of x-evac and x-H2O (x = SWNT-Cu or ox-SWNT-Cu) at room temperature. No peak assigned to any crystals such as Cu(OAc)2 itself was observed in the evacuated state, indicating a highly dispersed state of Cu(OAc)2 or any other Cu compounds on the surface of SWNT and ox-SWNT. On the other hand, diffraction patterns assigned to Cu2O appeared after saturated water-vapor adsorption on both SWNT-Cu and ox-SWNTCu. The diffraction peaks at s = 25.5, 29,4, 41.6, and 48.8 nm 1 were able to be assigned to the reflections of (1 1 1), (2 0 0), (2 2 0), and (3 1 1) for Cu2O, respectively (JCPDS No. 00-005-0667). Such a reaction did not proceed when we used copper nitrate as a precursor as shown in Supplementary material (Fig. S1). Therefore, a carboxylic ligand is indispensable for the reduction process into Cu2O on the surface of SWNT and ox-SWNT. Fig. 1 also suggests the dependence of adsorbent species; highly crystallized Cu2O can be synthesized on the surface of ox-SWNT. An oxidation of SWNT can generally make open-ended tubular structure and more functional groups. Here, it must be noted that we could successfully synthesize Cu2O with another kind of open-ended SWNT whose pore diameter was 1.4 nm having less amount of surface functional groups as shown in Supplementary material (Table S1 and Fig. S2). Hence, Cu2O cannot be synthesized from an ion-exchanged Cu species on the surface of SWNT but from physically adsorbed Cu(OAc)2. A lot of methods to synthesize Cu2O with reducible chemicals have been studied. On the other hand, Long et al. reported the photochemical synthesis of Cu2O particles from Cu(OAc)2 solved in polar solvents such as water and alcohols under UV-light irradiation [10]. Also, Nishida et al. recently proposed a method to synthesize core-shell type Cu nanoparticles surrounded by Cu2O crystals by a photoreduction process on multi-wall carbon nanotube (MWNT) under the UV-light irradiation [30]. Hence, it is remarkably needed to study a light-irradiation effect on SWNT-Cu-H2O and ox-SWNTCu-H2O to elucidate the reduction process to Cu2O. Fig. 2 shows powder XRD profiles of ox-SWNT-Cu samples as a function of wavelength of irradiated light to ox-SWNT-Cu-H2O for 1 h. Here, the ratio of the power irradiated through each shortpass filter shown in the figure was (b):(c):(d) = 0.75:0.71:1. No peak assigned
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(e) |FT|
(220)
Intensity / arb.unit
(200)
(f)
(d)
(d) (c)
(c)
(b)
(b)
(a) (a) 10
20
30
40
50
s / nm-1 (Cu Kα; λ =0.154 nm) Fig. 2. Powder XRD profiles of ox-SWNT-Cu treated with saturated water vapor under various light-irradiation conditions: (a) in dark, (b) with 385–425 nm, (c) with 385–460 nm, and (d) with 460–740 nm. Indexes are assigned to Cu2O crystal. All measurements were carried out in the atmospheric condition.
to Cu2O could be observed for ox-SWNT-Cu which was adsorbed by water molecules in dark condition, indicating a necessary of light irradiation to obtain Cu2O from Cu(OAc)2-dispersed SWNT. Also, Fig. 2 obviously indicates that reduction process into Cu2O can be enhanced when we treat an ox-SWNT-Cu-H2O sample with visible light whose wavelength is longer than 460 nm. Of course, the light used for the sample of Fig. 2(d) was 25–30% stronger than that used for any other samples, however, the crystallinity of Cu2O synthesized was obviously higher when the wavelength of irradiated light was longer than 460 nm. Such an excitation of Cu(OAc)2 by visible-light irradiation must be a nano-confinement effect of SWNT because, as already shown by Long et al., aqueous Cu(OAc)2 can be exited with UV-light irradiation. Actually, the local structure around a Cu atom of nano-confined Cu(OAc)2 was different from that in bulk phase, as shown in Fig. 3. Here, the samples of oxSWNT-Cu-H2O in dark condition and visible-light irradiated one are denoted as ox-SWNT-Cu-H2O(dark) and ox-SWNT-Cu-H2O(vis.), respectively. The peak assigned to Cu–Cu structure of a Cu(OAc)2 molecule in the bulk phase was observed between 0.19 and 0.26 nm (without phase-shift correction) in Fig. 3(d) [26]. On the other hand, the interatomic structure between Cu atoms of a Cu(OAc)2 molecule of both ox-SWNT-Cu-evac and ox-SWNT-CuH2O(dark) were observed around 0.28 nm as weak peaks. The result indicates that the interatomic distances between Cu atoms of a confined Cu(OAc)2 were elongated and similar to that of Cu2O crystal. The elongation must be caused by a distortion of Cu(OAc)2 confined in the micropore because such an elongation around a Cu atom of ox-SWNT-Cu(dark) was similarly observed in the case of Cu(OAc)2 confined in the micropores of activated carbons as shown in Supplemental material (Fig. S3) [26]. Meanwhile, any crystal-like products could not be obtained when we used MWNT whose inner diameter is about 6 nm as shown in Supplementary material (Fig. S4). Therefore, these results strongly suggest that Cu2O crystals can be synthesized from Cu(OAc)2 where
0
0.1
0.2
0.3
0.4
r / nm Fig. 3. Radial structure functions around a Cu atom of (a) ox-SWNT-Cu-evac, (b) oxSWNT-Cu-H2O(dark), (c) ox-SWNT-Cu-H2O(vis.), (d) Cu(OAc)2, (e) aqueous solution of Cu(OAc)2, and (f) Cu2O.
the specific structure of Cu(OAc)2 formed in the micropore of SWNT can be excited under visible-light irradiation. Fig. 4 shows XANES spectrum and a differential curve of the spectrum of ox-SWNT-Cu-H2O(vis.). Here, the in-situ spectra of ox-SWNT-Cu-evac, ox-SWNT-H2O(dark) and, in addition, other reference samples were also shown for comparison. The band assigned to 1s ? 4p transition of Cu+ species was clearly observed for ox-SWNT-Cu-H2O(vis.), [26,31] although the XANES spectra of both ox-SWNT-Cu-evac and ox-SWNT-Cu-H2O(dark) is similar to that of aqueous solution of Cu(OAc)2. In addition, differential curves of the XANES spectra demonstrate that there was negligibly weak band assigned to 1s ? 4p transition of Cu2+ on ox-SWNT-CuH2O(vis.), indicating a lot of copper species confined in the micropore of ox-SWNT could be reduced to Cu2O by the visible-light irradiation. A necessity of adsorbed water molecules for the reduction process is supported by the dependence of adsorbed amount of water to the crystalline size of Cu2O as shown in Supplementary materials (Figs. S5 and S6). The materials also support that an oxygen atom from the adsorbed water into the micropore must be the source of oxygen of Cu2O because water molecules cannot adsorb in carbon mesopore or macropore but only into micropore in the rage of P/P0 = 0.1–0.3 where the crystal structure of Cu2O can be clearly observed [32]. The role of water in the reaction with aqueous Cu(OAc)2 under UV-light irradiation had been minutely studied by Long et al., indicating the source of oxygen of synthesized Cu2O as water [10]. Thus, in our reaction system, physically adsorbed water molecules in the micropore are indispensable for the production of Cu2O under visible-light irradiation. Our present results evidence the production of Cu2O crystals on SWNT or ox-SWNT through a reaction between Cu(OAc)2 and water in the micropore under visible-light irradiation. However, structural information such as crystal shapes and sizes of Cu2O synthesized is not clear although the crystalline type and valence states were clearly shown by XRD and XANES spectra, respectively. Fig. 5 shows a TEM image of ox-SWNT-Cu-H2O(vis.), indicating the production of nanoparticles whose average particle diameter is
T. Ohkubo et al. / Journal of Colloid and Interface Science 421 (2014) 165–169
Normalized absorbance / arb. unit
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reactivity of the precursor must be different from that of the bulk crystal. Such a distorted precursor can be reduced by visible-light irradiation to form a core of Cu2O followed by the growth of nanoparticles of Cu2O outside the micropore.
(b) 4. Conclusions
(a)
8980
9000
9020
E / eV
Our present study clearly demonstrates the low-energy and specific reaction route to Cu2O from a specific precursor that is restricted or encapsulated inside the micropore of SWNT whose pore width is about 1 nm, although less evidence to support the reaction mechanism of Cu2O from nano-restricted Cu(OAc)2 was elucidated. Such a distorted precursor can be photoreduced by visible-light irradiation even though Cu(OAc)2 in bulk phase can be reduced under UV-light irradiation. We can reveal the possibility of SWNTs to be applied as a nanoreactor for the synthesis of inorganic materials such as Cu2O.
Differential curve
Acknowledgments
0
8980
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9020
E / eV Fig. 4. Cu K-edge XANES spectra (upper) and its differential curves (bottom): oxSWNT-Cu-evac (dotted line), ox-SWNT-Cu-H2O(dark) (red line), ox-SWNT-CuH2O(vis.) (blue line). XANES spectra of (a) aqueous solution of Cu(OAc)2, (b) Cu2O, and (c) CuO are also shown for comparison. Lines of (1 and 2) indicate the band positions assigned to 1s ? 4p transitions of Cu+ and Cu2+, respectively.
This work was partially supported by Grant-in-Aid for Scientific Research (A) (No. 21245006) from Japan Society for the Promotion of Science (JSPS), Inamori Foundation, The Yakumo Foundation for Environmental Science, Tokyo Ohka Foundation for the Promotion of Science and Technology, and Mukai Science and Technology Foundation. Also, this work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal Nos. 2010G148 and 2012G023) and the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2011B1885 and 2012A1266). In addition, we wish to express our appreciation to Dr. S. Kim and Dr. N. Tsuji (JASRI) for their assistance on the XRD experiments at SPring-8 and to Professor Y. Takaguchi (Okayama Univ.) for his support to measure the power of visible-light source. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.01.035. References
Fig. 5. TEM image of ox-SWNT-Cu-H2O(vis.).
about 10 nm. Therefore, Cu2O nanoparticles synthesized are obviously larger than that of the inner diameter of SWNT. As mentioned above, our present results suggest the nano-restricted Cu(OAc)2 in the micropore of SWNTs is indispensable for the synthesis of Cu2O by the photoreduction process with visible-light irradiation. Therefore, Cu2O nanoparticles must be formed from the nano-restricted Cu(OAc)2 whose structure is different from that of Cu(OAc)2 as shown in Fig. 3, although the clear mechanism to produce Cu2O nanoparticles whose diameters are larger than the pore size of SWNT has not been elucidated. Since the nano-restricted precursor might be distorted because of the pore geometry of SWNT, which can be expected by our previous report [26],
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
Nanospace-enhanced photoreduction for the synthesis of copper(I) oxide nanoparticles under visible-light irradiation Takahiro Ohkubo a,⇑, Mitsuhiro Ushio a, Koki Urita b, Isamu Moriguchi b, Bashir Ahmmad c, Atsushi Itadani a,d, Yasushige Kuroda a a
Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan Division of Chemistry and Material Science, Graduate School of Engineering, Nagasaki University, 1-14 Bunkyomachi, Nagasaki 852-8521, Japan Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan d Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshidaushinomiyacho, Sakyou-ku, Kyoto 606-8501, Japan b c
a r t i c l e
i n f o
Article history: Received 30 September 2013 Accepted 25 January 2014 Available online 4 February 2014 Keywords: Single-wall carbon nanotube Cuprous oxide (Cu2O) Nanoparticle Photoreduction Adsorption XAFS
a b s t r a c t Nanoparticles of copper(I) oxide (cuprous oxide; Cu2O) were able to be synthesized from nano-restricted copper acetate (Cu(OAc)2) in micropores of single-wall carbon nanotubes (SWNTs) by visible-light photoreduction. The specific structure of confined Cu(OAc)2 in the micropore is indispensable for the reduction process to Cu2O by the irradiation, because, in general, aqueous solution of Cu(OAc)2 can be reduced under UV-light irradiated conditions. The present results strongly suggest that the micropore of SWNTs whose pore width is in the micropore-size range can play as nanoreactor space for the synthesis of Cu2O through the nano-restricted precursor whose reactivity is different from that in the bulk phase. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Metal and semiconductor nanomaterials have been central research subjects in nanoscience and nanotechnology because they are promising candidates to unlock the frontier of catalysts, synthesis, and sustainable chemistry. Especially, nanomaterials such as nanoparticles and nanoporous solids of semiconductor oxides have been widely studied to develop noble materials for photochemical applications. Titanium dioxide (TiO2), for example, has been widely used as a photocatalyst that can decompose water molecules into oxygen and hydrogen under UV-light irradiation [1]. Recent progress enables us to synthesize TiO2 having special forms such as tubular structure [2–4]. Cuprous oxide (Cu2O) has also gathered much attention because it is a semiconductor oxide whose band gap (direct forbidden band gap) is about 2.17 eV [5] and, therefore, Cu2O can be applied to potential materials for catalysts and solar cells. The number of reports related to the synthesis and characterization of noble Cu2O materials is apparent evidence to represent the importance of Cu2O in application fields [6–16]. Copper acetate (Cu(OAc)2) is generally a suitable precursor for the production of Cu2O because Cu(OAc)2 can be easily exited by ⇑ Corresponding author. Fax: +81 86 251 7843. E-mail address: [email protected] (T. Ohkubo). http://dx.doi.org/10.1016/j.jcis.2014.01.035 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
UV-light irradiation to reduce Cu2+ into Cu+, or Cu0 in some cases, by a ligand-to-metal charge-transfer (LMCT) transition process [10]. Confined molecules and ions restricted in nanospace whose pore size is comparable to the adsorbed species generally tend to form characteristic structures that depend on both pore size and geometry. The characteristic structure leads to the physically or chemically unique properties of the adsorbed species. For instance, Holt et al. reported the fast transport property of air and water through less than 2 nm micropore of carbon nanotubes [17]. The property of the restricted compounds must be results of strong confinement in the nanospace and thus-fabricated distortion of adsorbed species. Actually, recent papers have demonstrated the quasi-compression effect in nanospaces, especially in the micropore [18–20]. Therefore, micropores originally have the faculty to produce high-pressure molecular or ionic states even if both an adsorbent and an adsorbate are in the atmospheric condition. We already reported structural anomalies of hydration structure around metal ions such as rubidium [21,22], zinc [23,24], and cobalt [25] ions, as well as coordination structure around a Cu ion of Cu(OAc)2 [26] confined in the carbon micropores. Here, Cu(OAc)2 is a simple coordination compound which can form a dinuclear complex with acetate bridging [27]. Our previous study on Cu(OAc)2 restricted in the micropore of activated carbons (ACs)
T. Ohkubo et al. / Journal of Colloid and Interface Science 421 (2014) 165–169
strongly indicates that the structure of the dinuclear complex is quite different from that in the bulk. Therefore, the reactivity of nano-restricted Cu(OAc)2 should be different from that in the bulk phase because of the remarkably distorted states in micropores. Accordingly, we describe the new synthetic route to Cu2O from Cu(OAc)2 by using single-wall carbon nanotube (SWNT) as a nanoreactor under the appropriate light-irradiated condition without any additional reductants. 2. Experimental section In the present study, we used SWNTs synthesized by the HiPco method (purchased from Unidym Inc.; diameter of a tube = 1.1 ± 0.2 nm) and studied with two kinds of SWNTs; as-synthesized SWNT and oxidized one in an air-flow condition (100 cm3 min 1) at 623 K followed by washing with hydrochloric acid (12 mol dm 3) and distilled water. In the article, we denote such an oxidized SWNT as ox-SWNT. Also, we used copper acetate (Cu(OAc)2) monohydrate (99.9%; Wako Ltd.) as a precursor, which was not purified anymore. Each nanotube sample (30 mg) was stirred in the aqueous solution of Cu(OAc)2 (5 cm3, 0.5 mol dm 3) over 24 h to impregnate the electrolyte into the micropore. In the case of the synthesis of Cu2O, water vapor was adsorbed at a relative pressure greater than 0.9 after the deposition of Cu(OAc)2 followed by washing, drying in the desiccator, and pre-evacuation at 423 K in a quartz reactor cell (denoted by SWNT-Cu or ox-SWNT-Cu, respectively). Here, we describe the samples in evacuated and saturated water-vapor conditions as x-evac and x-H2O (x = SWNT-Cu or ox-SWNT-Cu), respectively. Light irradiation was performed by a 300 W xenon light source (MAX-303; Asahi Spectra). Each sample was shielded in a quartz reactor cell with saturated water vapor. The light beam was passed through both a mirror module which limits the irradiation wavelength between 385 and 740 nm and an appropriate shortpass filter. The power of irradiated light through each shortpass filter was measured using a power meter (Photo-Radiometer Model HD 2302.0 coupled with the probe LP471RAD) which can detect the power between 400 and 1050 nm. Powder X-ray diffraction (XRD) profiles of SWNTCu and ox-SWNT-Cu samples were collected by using the diffractometer (MiniFlexII; Rigaku Ltd.) with monochromatic Cu Ka radiation (k = 0.154184 nm) under 30 kV and 15 mA as well as synchrotron-irradiated X-ray at SPring-8 (BL02B2) to investigate the adsorbed states of the precursor and the resultant under in situ condition. Here, the wavelength of the monochromatic Xray at SPring-8 was estimated by the analysis of standard profiles from CeO2 crystal. Also, scattering parameter (s = 4psinh/k) was used to show our XRD results to avoid confusion when we compare the results obtained by using different wavelengths. Here, h is a half of scattering angle (2h). A conventional high-resolution transmission electron microscope (TEM; JEM-2010, JEOL Ltd.) equipped with a Gatan detector (ORIUS SC1000) was used at an accelerating voltage of 120 kV. All samples for TEM measurements were dispersed in ethanol, and then fixed on a copper grid coated with holey carbon. X-ray absorption fine structure (XAFS) measurements were performed on the Cu K-absorption edge (8980 eV) using hand-made XAFS cell with windows of Kapton film at the National Laboratory for High Energy Accelerator Research Organization (KEK). We used IFEFFIT code [28] with FEFF6 procedure [29] for the analysis of XAFS spectra including both extended XAFS (EXAFS) and X-ray absorption near edge structure (XANES) spectra. 3. Results and discussion At first, the structure of Cu(OAc)2-dispersed samples was examined with a synchrotron-irradiated X-ray source. Fig. 1 shows
(d) Intensity / arb. unit
166
(c)
(b)
(a) 10
20
s/
30
nm-1 (λ =0.100
40
50
nm)
Fig. 1. In situ powder XRD profiles of (a) SWNT-Cu-evac, (b) SWNT-Cu-H2O, (c) oxSWNT-Cu-evac, and (d) ox-SWNT-Cu-H2O at room temperature.
in situ powder XRD profiles of x-evac and x-H2O (x = SWNT-Cu or ox-SWNT-Cu) at room temperature. No peak assigned to any crystals such as Cu(OAc)2 itself was observed in the evacuated state, indicating a highly dispersed state of Cu(OAc)2 or any other Cu compounds on the surface of SWNT and ox-SWNT. On the other hand, diffraction patterns assigned to Cu2O appeared after saturated water-vapor adsorption on both SWNT-Cu and ox-SWNTCu. The diffraction peaks at s = 25.5, 29,4, 41.6, and 48.8 nm 1 were able to be assigned to the reflections of (1 1 1), (2 0 0), (2 2 0), and (3 1 1) for Cu2O, respectively (JCPDS No. 00-005-0667). Such a reaction did not proceed when we used copper nitrate as a precursor as shown in Supplementary material (Fig. S1). Therefore, a carboxylic ligand is indispensable for the reduction process into Cu2O on the surface of SWNT and ox-SWNT. Fig. 1 also suggests the dependence of adsorbent species; highly crystallized Cu2O can be synthesized on the surface of ox-SWNT. An oxidation of SWNT can generally make open-ended tubular structure and more functional groups. Here, it must be noted that we could successfully synthesize Cu2O with another kind of open-ended SWNT whose pore diameter was 1.4 nm having less amount of surface functional groups as shown in Supplementary material (Table S1 and Fig. S2). Hence, Cu2O cannot be synthesized from an ion-exchanged Cu species on the surface of SWNT but from physically adsorbed Cu(OAc)2. A lot of methods to synthesize Cu2O with reducible chemicals have been studied. On the other hand, Long et al. reported the photochemical synthesis of Cu2O particles from Cu(OAc)2 solved in polar solvents such as water and alcohols under UV-light irradiation [10]. Also, Nishida et al. recently proposed a method to synthesize core-shell type Cu nanoparticles surrounded by Cu2O crystals by a photoreduction process on multi-wall carbon nanotube (MWNT) under the UV-light irradiation [30]. Hence, it is remarkably needed to study a light-irradiation effect on SWNT-Cu-H2O and ox-SWNTCu-H2O to elucidate the reduction process to Cu2O. Fig. 2 shows powder XRD profiles of ox-SWNT-Cu samples as a function of wavelength of irradiated light to ox-SWNT-Cu-H2O for 1 h. Here, the ratio of the power irradiated through each shortpass filter shown in the figure was (b):(c):(d) = 0.75:0.71:1. No peak assigned
167
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(e) |FT|
(220)
Intensity / arb.unit
(200)
(f)
(d)
(d) (c)
(c)
(b)
(b)
(a) (a) 10
20
30
40
50
s / nm-1 (Cu Kα; λ =0.154 nm) Fig. 2. Powder XRD profiles of ox-SWNT-Cu treated with saturated water vapor under various light-irradiation conditions: (a) in dark, (b) with 385–425 nm, (c) with 385–460 nm, and (d) with 460–740 nm. Indexes are assigned to Cu2O crystal. All measurements were carried out in the atmospheric condition.
to Cu2O could be observed for ox-SWNT-Cu which was adsorbed by water molecules in dark condition, indicating a necessary of light irradiation to obtain Cu2O from Cu(OAc)2-dispersed SWNT. Also, Fig. 2 obviously indicates that reduction process into Cu2O can be enhanced when we treat an ox-SWNT-Cu-H2O sample with visible light whose wavelength is longer than 460 nm. Of course, the light used for the sample of Fig. 2(d) was 25–30% stronger than that used for any other samples, however, the crystallinity of Cu2O synthesized was obviously higher when the wavelength of irradiated light was longer than 460 nm. Such an excitation of Cu(OAc)2 by visible-light irradiation must be a nano-confinement effect of SWNT because, as already shown by Long et al., aqueous Cu(OAc)2 can be exited with UV-light irradiation. Actually, the local structure around a Cu atom of nano-confined Cu(OAc)2 was different from that in bulk phase, as shown in Fig. 3. Here, the samples of oxSWNT-Cu-H2O in dark condition and visible-light irradiated one are denoted as ox-SWNT-Cu-H2O(dark) and ox-SWNT-Cu-H2O(vis.), respectively. The peak assigned to Cu–Cu structure of a Cu(OAc)2 molecule in the bulk phase was observed between 0.19 and 0.26 nm (without phase-shift correction) in Fig. 3(d) [26]. On the other hand, the interatomic structure between Cu atoms of a Cu(OAc)2 molecule of both ox-SWNT-Cu-evac and ox-SWNT-CuH2O(dark) were observed around 0.28 nm as weak peaks. The result indicates that the interatomic distances between Cu atoms of a confined Cu(OAc)2 were elongated and similar to that of Cu2O crystal. The elongation must be caused by a distortion of Cu(OAc)2 confined in the micropore because such an elongation around a Cu atom of ox-SWNT-Cu(dark) was similarly observed in the case of Cu(OAc)2 confined in the micropores of activated carbons as shown in Supplemental material (Fig. S3) [26]. Meanwhile, any crystal-like products could not be obtained when we used MWNT whose inner diameter is about 6 nm as shown in Supplementary material (Fig. S4). Therefore, these results strongly suggest that Cu2O crystals can be synthesized from Cu(OAc)2 where
0
0.1
0.2
0.3
0.4
r / nm Fig. 3. Radial structure functions around a Cu atom of (a) ox-SWNT-Cu-evac, (b) oxSWNT-Cu-H2O(dark), (c) ox-SWNT-Cu-H2O(vis.), (d) Cu(OAc)2, (e) aqueous solution of Cu(OAc)2, and (f) Cu2O.
the specific structure of Cu(OAc)2 formed in the micropore of SWNT can be excited under visible-light irradiation. Fig. 4 shows XANES spectrum and a differential curve of the spectrum of ox-SWNT-Cu-H2O(vis.). Here, the in-situ spectra of ox-SWNT-Cu-evac, ox-SWNT-H2O(dark) and, in addition, other reference samples were also shown for comparison. The band assigned to 1s ? 4p transition of Cu+ species was clearly observed for ox-SWNT-Cu-H2O(vis.), [26,31] although the XANES spectra of both ox-SWNT-Cu-evac and ox-SWNT-Cu-H2O(dark) is similar to that of aqueous solution of Cu(OAc)2. In addition, differential curves of the XANES spectra demonstrate that there was negligibly weak band assigned to 1s ? 4p transition of Cu2+ on ox-SWNT-CuH2O(vis.), indicating a lot of copper species confined in the micropore of ox-SWNT could be reduced to Cu2O by the visible-light irradiation. A necessity of adsorbed water molecules for the reduction process is supported by the dependence of adsorbed amount of water to the crystalline size of Cu2O as shown in Supplementary materials (Figs. S5 and S6). The materials also support that an oxygen atom from the adsorbed water into the micropore must be the source of oxygen of Cu2O because water molecules cannot adsorb in carbon mesopore or macropore but only into micropore in the rage of P/P0 = 0.1–0.3 where the crystal structure of Cu2O can be clearly observed [32]. The role of water in the reaction with aqueous Cu(OAc)2 under UV-light irradiation had been minutely studied by Long et al., indicating the source of oxygen of synthesized Cu2O as water [10]. Thus, in our reaction system, physically adsorbed water molecules in the micropore are indispensable for the production of Cu2O under visible-light irradiation. Our present results evidence the production of Cu2O crystals on SWNT or ox-SWNT through a reaction between Cu(OAc)2 and water in the micropore under visible-light irradiation. However, structural information such as crystal shapes and sizes of Cu2O synthesized is not clear although the crystalline type and valence states were clearly shown by XRD and XANES spectra, respectively. Fig. 5 shows a TEM image of ox-SWNT-Cu-H2O(vis.), indicating the production of nanoparticles whose average particle diameter is
T. Ohkubo et al. / Journal of Colloid and Interface Science 421 (2014) 165–169
Normalized absorbance / arb. unit
168
(2) (1)
(c)
reactivity of the precursor must be different from that of the bulk crystal. Such a distorted precursor can be reduced by visible-light irradiation to form a core of Cu2O followed by the growth of nanoparticles of Cu2O outside the micropore.
(b) 4. Conclusions
(a)
8980
9000
9020
E / eV
Our present study clearly demonstrates the low-energy and specific reaction route to Cu2O from a specific precursor that is restricted or encapsulated inside the micropore of SWNT whose pore width is about 1 nm, although less evidence to support the reaction mechanism of Cu2O from nano-restricted Cu(OAc)2 was elucidated. Such a distorted precursor can be photoreduced by visible-light irradiation even though Cu(OAc)2 in bulk phase can be reduced under UV-light irradiation. We can reveal the possibility of SWNTs to be applied as a nanoreactor for the synthesis of inorganic materials such as Cu2O.
Differential curve
Acknowledgments
0
8980
9000
9020
E / eV Fig. 4. Cu K-edge XANES spectra (upper) and its differential curves (bottom): oxSWNT-Cu-evac (dotted line), ox-SWNT-Cu-H2O(dark) (red line), ox-SWNT-CuH2O(vis.) (blue line). XANES spectra of (a) aqueous solution of Cu(OAc)2, (b) Cu2O, and (c) CuO are also shown for comparison. Lines of (1 and 2) indicate the band positions assigned to 1s ? 4p transitions of Cu+ and Cu2+, respectively.
This work was partially supported by Grant-in-Aid for Scientific Research (A) (No. 21245006) from Japan Society for the Promotion of Science (JSPS), Inamori Foundation, The Yakumo Foundation for Environmental Science, Tokyo Ohka Foundation for the Promotion of Science and Technology, and Mukai Science and Technology Foundation. Also, this work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal Nos. 2010G148 and 2012G023) and the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2011B1885 and 2012A1266). In addition, we wish to express our appreciation to Dr. S. Kim and Dr. N. Tsuji (JASRI) for their assistance on the XRD experiments at SPring-8 and to Professor Y. Takaguchi (Okayama Univ.) for his support to measure the power of visible-light source. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.01.035. References
Fig. 5. TEM image of ox-SWNT-Cu-H2O(vis.).
about 10 nm. Therefore, Cu2O nanoparticles synthesized are obviously larger than that of the inner diameter of SWNT. As mentioned above, our present results suggest the nano-restricted Cu(OAc)2 in the micropore of SWNTs is indispensable for the synthesis of Cu2O by the photoreduction process with visible-light irradiation. Therefore, Cu2O nanoparticles must be formed from the nano-restricted Cu(OAc)2 whose structure is different from that of Cu(OAc)2 as shown in Fig. 3, although the clear mechanism to produce Cu2O nanoparticles whose diameters are larger than the pore size of SWNT has not been elucidated. Since the nano-restricted precursor might be distorted because of the pore geometry of SWNT, which can be expected by our previous report [26],
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