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TiO2 core-shell nanocomposites
Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 151–155
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Solution synthesis of Cu2 O/TiO2 core-shell nanocomposites Xiaodan Su a,b , Jingzhe Zhao a,∗ , Yunling Li a,b , Yanchao Zhu b , Xiaokun Ma b , Fang Sun b , Zichen Wang b,∗∗ a b
College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130023, PR China
a r t i c l e
i n f o
Article history: Received 5 January 2009 Received in revised form 11 August 2009 Accepted 12 August 2009 Available online 19 August 2009 Keywords: Cu2 O TiO2 Nanocomposites Core-shell structure Photo-electrical properties Hydrolyzation
a b s t r a c t We report here a simple approach to the synthesis of Cu2 O/TiO2 core-shell nanocomposites with uniform octahedral structure in solution phase. First, fresh synthesized Cu2 O octahedra were used as precursor, and butyl titanate (Ti(OBu)4 ) diluted by ethanol was preliminary hydrolyzed by the water adsorbed on the surface of Cu2 O, so a very thin TiO2 condensed on the Cu2 O surfaces. Then, when a mixture of water and ethanol was dropped into the reaction system, the Ti(OBu)4 would further hydrolyze and condense around the Cu2 O to form TiO2 , so octahedral Cu2 O/TiO2 core-shell composites with uniform and compact TiO2 shells were obtained. This method is suitable for the formation of uniform integrated TiO2 shells and their thickness can be controlled by adjusting the ratio of water/ethanol (W/E). According to the surface photovoltage spectroscopy of the Cu2 O/TiO2 composites, we think the material would have a potential application in photocatalysis and photoelectric transition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, with the development of material science, the synthesis of functional core-shell composites have became an attractive topic [1]. Combination of two or more materials will provide the formed composites with several outstanding physical and chemical performances, especially for oxides and semiconductors [2,3]. Cuprous oxide (Cu2 O) as a p-type semiconductor has been widely used in hydrogen production, superconductors, solar cells, and photocatalysis [4–6]. Now, Cu2 O with different kinds of structure and morphologies has already been obtained [7,8]. But the narrow energy band gap of Cu2 O (2.0 eV) limits its performance on photocatalytic activity. In order to solve this problem, so much work has been done on combining Cu2 O with other conductive materials [9]. As an n-type semiconductor with a wide energy band gap, titanium dioxide (TiO2 ) is frequently studied in optic and electric fields, especially as an excellent photocatalytic material applied in photodegradation and solar energy conversion [10,11]. Many core-shell materials composed of TiO2 and other metal or metal oxides have
∗ Corresponding author. Tel.: +86 731 8809278; fax: +86 731 8809278. ∗∗ Corresponding author. Tel.: +86 431 85155358; fax: +86 431 85155358. E-mail addresses: [email protected], zhao [email protected] (J. Zhao), [email protected] (Z. Wang). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.08.011
been synthesized for their better application performance [12–15]. However, the wide energy band gap of TiO2 (3.2 eV) limits its absorption in the high-energy portion (UV) of the solar spectrum [16]. One of the most promising ideas to extend the light absorbing property of TiO2 and to enhance its photocatalytic efficiency is to couple TiO2 with narrow band gap semiconductors. So, combined Cu2 O with TiO2 has attracted people’s attention. Yu and co-workers developed a new way to utilize TiO2 under visible light irradiation on the basis of TiO2 /Cu2 O composite film, in which the TiO2 /Cu2 O composite was prepared by a simple electrochemical method [17]. Yasomanee and Bandara reported the preparation of Cu2 O/TiO2 thin film on ITO conducting glass. Upon UV–vis irradiation on the films, the energy can be stored, which could lead to the generation of H2 from H2 O in the dark [18]. A Cu2 O/TiO2 heterojunction thin film cathode has also been synthesized by McFarland and co-workers for receiving high efficiency on photoelectrolysis [19]. Until now, sol–gel method, chemical vapor decomposition method, hydrothermal technique and reversed micelle method have been used to produce TiO2 nanostructures [20–23]. In our work, we report a simple solution method to synthesize Cu2 O/TiO2 octahedral core-shell composites, which is composed of two hydrolyzing processes: the primary hydrolyzing by surface absorbed water of Cu2 O and the followed main hydrolyzing by water/ethanol solution. The method is simple to perform, and can make the TiO2 shell uniform and compact.
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2. Experimental 2.1. Synthesis In a typical synthesis, first, 40 mL of NaOH (1 M) solution was added into the mixture of CuSO4 solution (0.25 M, 80 mL) and sodium oleate solution (0.005 M, 40 mL) in a water bath at 80 ◦ C, and then 20 mL of 2.5% hydrazine hydrate was quickly poured into the mixture under stirring. Cu2 O precipitates were obtained after 3 h reaction, then, separated by centrifugation, washed with distilled water and ethanol in sequence. Second, the wet Cu2 O was dispersed into 50 mL of ethanol by ultrasonic treatment, and 20 mL of 0.1 M Ti(OBu)4 ethanol solution was dropped into the above mixture under stirring. 2 h later, water/ethanol (W/E) solution in certain ratios was dropped into the mixture, and the reaction was going on for another 2 h. The resultant composites were centrifuged, rinsed with absolute ethanol for several times, and subsequently dried at room temperature. In further preparation, the resultant Cu2 O/TiO2 composites (0.01 mol) were dipped in H2 SO4 solution (0.02 M 100 mL) for 2 h at room temperature, and the deposits of Cu/SiO2 nanocomposites were filtrated, and then dried in a vacuum system.
2.2. Characterization The shape, size, and core-shell structure of the Cu2 O/TiO2 composites were characterized using high resolution transmission electron microscopy (HRTEM JEOL JEM-3010). The crystallographic structure and part information on chemical composition of the composites were identified by Powder X-ray diffraction (XRD) using Shimadzu model XRD-6000 with Cu K␣ radiation. Surface composition analysis was further carried out on an X-ray photoelectron spectroscope (XPS, ESCALAB Mark II). Detailed composition characterization was carried out with energy-dispersive X-ray (EDX) analysis (equipped with the HRTEM). The photovoltaic character of the nanocomposites was detected by surface photovoltage spectroscopy (SPS). In brief, the lock-in-based SPV measurement system is constituted of a source of monochromatic light, a lock-in amplifier (SR830, Standford Research System) with a light chopper (SR540 Standford Research System), and a computer. A 500 W xenon lamp and a monochromator provide monochromatic light (Zolix Instruments Co., Ltd.). Transmission electron microscopy (JEOL-1230) was also used here to characterize the morphology of Cu/TiO2 composites.
Fig. 1. (a and c) are the TEM and HRTEM images of the Cu2 O/TiO2 sample (W/E = 2/18 mL); (b) is the TEM image of pre-synthesized Cu2 O; (d) is the SEAD image of the Cu2 O/TiO2 sample.
XRD measurements were also carried out to determine the composition and phases of the Cu2 O/TiO2 composites. Fig. 2 is the XRD pattern of the above mentioned Cu2 O/TiO2 sample with low W/E ratio. All the sharper diffraction peaks at 29.6◦ , 36.4◦ , 42.3◦ and 61.4◦ well correspond to the reflections of cubic symmetry Cu2 O (1 1 0), (1 1 1), (2 0 0) and (2 2 0), respectively (JCPDS, No.78-2076). The absence of TiO2 in the XRD pattern of the composite as well as SEAD detection may be due to its amorphous structure and low content. In order to learn clearly the composition and element status of the composite surface, X-ray photoelectron spectroscopy (XPS) analysis was further performed on the Cu2 O/TiO2 sample synthesized in low W/E ratio of 2/18 mL, the results are shown in Fig. 3. The binding energies at 458.3 and 463.8 eV corresponding to Ti 2p3/2 and Ti 2p1/2 (Fig. 3a), confirm the presence of TiO2 at the composite surface. [24] XPS also detected Cu 2p3/2 signal at 934.8 eV and Cu 2p1/2 signal at 954.5 eV with satellite peaks, as given in Fig. 3b.
3. Results and discussion For obtaining better uniform core-shell structure, a simple twostep hydrolyzation procedure was used here to generate Cu2 O/TiO2 octahedral core-shell composites. Fig. 1a is the TEM image of the Cu2 O/TiO2 sample synthesized in low W/E ratio of 2/18 mL, from which we can see that the composites have an octahedral structure with a size of 100–250 nm and light shells condensed on the cores of Cu2 O. This octahedral structure of the composites is due to the morphology of core Cu2 O octahedra, which can be seen in the TEM image of the pre-synthesized Cu2 O precursor as shown in Fig. 1b. Fig. 1c is the HRTEM image taken from one edge part of the Cu2 O/TiO2 particle. It can be seen that on the surface of dark octahedral Cu2 O there is a light shell with the thickness of 6–10 nm. The light shell is proposed to be amorphous TiO2 . Fig. 1d gives the selected area electron diffraction (SAED) image of this Cu2 O/TiO2 sample, in which the inerratic dot-matrix can be seen clearly, and it corresponds to the pattern of Cu2 O. This indicates that the octahedral Cu2 O core is single crystal.
Fig. 2. XRD pattern of the Cu2 O/TiO2 composites synthesized by two-step hydrolyzation (W/E = 2/18 mL).
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Fig. 3. XPS spectra of Ti 2p (a), Cu 2p (b) and O 1s (c) of the Cu2 O/TiO2 composite (W/E = 2/18 mL).
This means that the copper species in composite surface are mainly dispersed as Cu2+ , for XPS analysis is a surface sensitive technique. The Cu(II) 2p binding energies of the composite sample are slightly higher than the reported data of pure CuO [24,25]. According to several previous studies, the higher binding energy was possibly caused by the interaction between support and active component, in our case it should be attributed to an atomic distribution of Ti element to copper oxide, thus Cu2+ at the surface of the composite sample may be in the form of Cu–O–Ti [26]. Some CuO crystallites have also been detected to locate at the surface of the Cu2 O core in the HRTEM analysis. No CuO peaks detected in XRD analysis should be attributed to less amount of CuO in the composite sample. O1s binding energies in Fig. 3c reveal larger amount of OH− (531.9 eV) existing at the composite surface except lattice oxygen (530.0 eV). [24] The abundant OH− would benefit further growth of TiO2 at the composite surface. We think the primary hydrolyzing process (hydrolyzing based on adsorbed water) is important in our reaction system. Fresh synthesized Cu2 O without drying has much adsorbed water at its surface, so Ti(OBu)4 can hydrolyze by the adsorbed water and locate as a thin flat of compact TiO2 shell on the surface of Cu2 O octahedra. The structure of the Cu2 O/TiO2 composite obtained in the first hydrolyzation is shown in Fig. 4a, there is a thin light shell on the surface of the Cu2 O octahedron with the thickness of just a few nanometers, which is supposed to be TiO2 shell as revealed from the EDX pattern of Fig. 4b. We think, it is this thin flat of TiO2 that can make Ti(OBu)4 condense around octahedra easily during the followed main hydrolyzing step, which is favorable for the formation of uniform and compact TiO2 shell. So, Cu2 O/TiO2 nanocomposites with uniform and compact TiO2 shell can be obtained. The TiO2 shell’s thickness of the Cu2 O/TiO2 samples prepared with our strategy can be controlled through adjusting the ratios of water/ethanol added in the main hydrolyzing step. It is found that the thickness of TiO2 shell increased with the increasing of W/E ratio. Fig. 5 is the TEM and HRTEM images of the Cu2 O/TiO2 sample synthesized with a W/E ratio of 10/10 mL. As shown in Fig. 5a, all the Cu2 O octahedra were coated with uniform and compact TiO2 shell in the thickness of about 20 nm. And from the TEM image of one Cu2 O/TiO2 composite particle in Fig. 5b, the integral and uniform TiO2 shell can be seen more clearly. Fig. 5c is the HRTEM image of the edge of one particle, from which we can see that the thick TiO2 shell of this Cu2 O/TiO2 sample is amorphous in character, which is same as that synthesized at low water/ethanol ratio solution (W/E = 2/18 mL). It is deduced that the increasing of W/E
Fig. 4. TEM (a) and EDX (b) images of the Cu2 O/TiO2 sample synthesized by only primary hydrolyzing step (hydrolyzed with the surface adsorbed water of Cu2 O cores).
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Fig. 7. TEM image of the Cu/TiO2 sample.
Fig. 5. (HR)TEM and EDX images of the Cu2 O/TiO2 samples (W/E = 10/10 mL). (a) is the TEM image of the sample, (b) is the TEM image of one particle, (c) is the HRTEM image of the edge of one particle and (d) is the EDX pattern of this sample.
ratio would lead to fast hydrolyzation of Ti(OBu)4 , so more TiO2 turned out in a short time, and these fresh TiO2 easily formed in amorphous rather than crystal structure. Fig. 5d is the EDX pattern of the sample, Ti element can be seen clearly in the result. As Cu2 O and TiO2 are two typical p-type and n-type semiconductors, respectively, much attention has been focused on their optics,
electronics and photoelectric performance [27]. In this work, Cu2 O was coated with TiO2 , and it is hoped that this Cu2 O/TiO2 core-shell composite would show better chemical and physical properties. So we characterized the Cu2 O/TiO2 composites by surface photovoltage spectroscopy (SPS). At the same time, the surface photovoltage (SPV) of precursor Cu2 O and pure TiO2 was also detected for comparison. Fig. 6 is the SPS of Cu2 O, TiO2 and Cu2 O/TiO2 composites (W/E = 10/10 mL). From these responses we can see that the Cu2 O has a wide SPV response band from 300–600 nm with a main peak at about 400 nm. The peak of TiO2 is about 330 nm and its SPV response wavelength reaches to 450 nm. Two obvious SPV response bands were observed in the SPS of the prepared Cu2 O/TiO2 composites, one is at about 350 nm, and the other is in the range of 500–600 nm. We think the formation of the main peak of 350 nm was resulted from the composite of Cu2 O and TiO2 , and the appearance of the peak at long wave band was due to the Cu2 O mainly. These results indicate that the Cu2 O/TiO2 composites have a wider response wavelength compared with simplex TiO2 , and it extended the response of TiO2 from ultraviolet to almost visible wave band. So, this Cu2 O/TiO2 composite shows a potential application in photocatalysis and photoelectric transition.
Fig. 6. Surface photovoltage spectrum of the precursor Cu2 O, pure TiO2 and the obtained Cu2 O/TiO2 composites.
X. Su et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 349 (2009) 151–155
Furthermore, disproportionation also has been used to treat the Cu2 O/TiO2 composites (W/E = 10/10 mL) by using a certain concentration of H2 SO4 solution. As Cu2 O can disproportionate to Cu(0) and Cu(II) with help of acid (Cu2 O + 2H+ = Cu + Cu2+ + H2 O), in the present study, we chose dilute H2 SO4 (0.02 M) as etching agent to deal with Cu2 O/TiO2 core-shell powders for obtaining Cu/TiO2 nanocomposites upon Kirkendall effect [28,29]. Herein, H2 SO4 and Cu2 O can react with each other and form a diffusion pair, and the coupled reaction/diffusion at the solid/solution interface could lead to the quick formation of Cu nanoparticles within the TiO2 shell, resulting in the formation of Cu/TiO2 core-shell nanostructure. Fig. 7 is the TEM image of the resulting Cu/TiO2 sample. From the image we can see that the light shells of TiO2 were kept integrity after the reaction, and the solid Cu2 O core has already been eroded, leaving one or more moveable Cu cores. These formed Cu/TiO2 moveable core-shell nanocomposites may have a good use in catalysis applications [30]. 4. Conclusion In summary, a simple solution method has been given by us to synthesize Cu2 O/TiO2 core-shell nanocomposites with uniform octahedral structure. This approach can make the TiO2 layer uniformly coated on each Cu2 O octahedron. Moreover, by using simple disproportionation method, we also have obtained Cu/TiO2 moveable core-shell nanocomposites. The Cu2 O/TiO2 and Cu/TiO2 composites would have potential applications in the field of optics, electronics, optoelectronics and catalysis. Acknowledgements This work was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS, SEM), the Natural Science Foundation of Jilin Province for Excellent Young Scholars (Grant No. 20040117). We also acknowledge the financial support from Scientific Research Foundation of Hunan University. References [1] Y. Lei, W.K. Chim, Highly ordered arrays of metal/semiconductor core-shell nanoparticles with tunable nanostructures and photoluminescence, J. Am. Chem. Soc. 127 (2005) 1487–1492. [2] M.H. Liao, C.H. Hsu, D.H. Chen, Preparation and properties of amorphous titaniacoated zinc oxide nanoparticles, J. Solid State Chem. 179 (2006) 2020–2026. [3] X.G. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, Epitaxial growth of highly luminescent cdse/cds core/shell nanocrystals with photostability and electronic accessibility, J. Am. Chem. Soc. 119 (1997) 7019–7029. [4] R. Liu, E.A. Kulp, F.E. Oba, W. Bohannan, F. Ernst, J.A. Switzer, Epitaxial electrodeposition of high-aspect-ratio Cu2 O(110) nanostructures on InP(111), Chem. Mater. 17 (2005) 725–729. [5] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Taracon, Nano-sized transitionmetaloxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496–499. [6] S. Guo, Y. Fang, S. Dong, E. Wang, Templateless, surfactantless, electrochemical route to a cuprous oxide microcrystal: from octahedra to monodisperse colloid spheres, Inorg. Chem. 46 (2007) 9537–9539.
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Solution synthesis of Cu2 O/TiO2 core-shell nanocomposites Xiaodan Su a,b , Jingzhe Zhao a,∗ , Yunling Li a,b , Yanchao Zhu b , Xiaokun Ma b , Fang Sun b , Zichen Wang b,∗∗ a b
College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130023, PR China
a r t i c l e
i n f o
Article history: Received 5 January 2009 Received in revised form 11 August 2009 Accepted 12 August 2009 Available online 19 August 2009 Keywords: Cu2 O TiO2 Nanocomposites Core-shell structure Photo-electrical properties Hydrolyzation
a b s t r a c t We report here a simple approach to the synthesis of Cu2 O/TiO2 core-shell nanocomposites with uniform octahedral structure in solution phase. First, fresh synthesized Cu2 O octahedra were used as precursor, and butyl titanate (Ti(OBu)4 ) diluted by ethanol was preliminary hydrolyzed by the water adsorbed on the surface of Cu2 O, so a very thin TiO2 condensed on the Cu2 O surfaces. Then, when a mixture of water and ethanol was dropped into the reaction system, the Ti(OBu)4 would further hydrolyze and condense around the Cu2 O to form TiO2 , so octahedral Cu2 O/TiO2 core-shell composites with uniform and compact TiO2 shells were obtained. This method is suitable for the formation of uniform integrated TiO2 shells and their thickness can be controlled by adjusting the ratio of water/ethanol (W/E). According to the surface photovoltage spectroscopy of the Cu2 O/TiO2 composites, we think the material would have a potential application in photocatalysis and photoelectric transition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, with the development of material science, the synthesis of functional core-shell composites have became an attractive topic [1]. Combination of two or more materials will provide the formed composites with several outstanding physical and chemical performances, especially for oxides and semiconductors [2,3]. Cuprous oxide (Cu2 O) as a p-type semiconductor has been widely used in hydrogen production, superconductors, solar cells, and photocatalysis [4–6]. Now, Cu2 O with different kinds of structure and morphologies has already been obtained [7,8]. But the narrow energy band gap of Cu2 O (2.0 eV) limits its performance on photocatalytic activity. In order to solve this problem, so much work has been done on combining Cu2 O with other conductive materials [9]. As an n-type semiconductor with a wide energy band gap, titanium dioxide (TiO2 ) is frequently studied in optic and electric fields, especially as an excellent photocatalytic material applied in photodegradation and solar energy conversion [10,11]. Many core-shell materials composed of TiO2 and other metal or metal oxides have
∗ Corresponding author. Tel.: +86 731 8809278; fax: +86 731 8809278. ∗∗ Corresponding author. Tel.: +86 431 85155358; fax: +86 431 85155358. E-mail addresses: [email protected], zhao [email protected] (J. Zhao), [email protected] (Z. Wang). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.08.011
been synthesized for their better application performance [12–15]. However, the wide energy band gap of TiO2 (3.2 eV) limits its absorption in the high-energy portion (UV) of the solar spectrum [16]. One of the most promising ideas to extend the light absorbing property of TiO2 and to enhance its photocatalytic efficiency is to couple TiO2 with narrow band gap semiconductors. So, combined Cu2 O with TiO2 has attracted people’s attention. Yu and co-workers developed a new way to utilize TiO2 under visible light irradiation on the basis of TiO2 /Cu2 O composite film, in which the TiO2 /Cu2 O composite was prepared by a simple electrochemical method [17]. Yasomanee and Bandara reported the preparation of Cu2 O/TiO2 thin film on ITO conducting glass. Upon UV–vis irradiation on the films, the energy can be stored, which could lead to the generation of H2 from H2 O in the dark [18]. A Cu2 O/TiO2 heterojunction thin film cathode has also been synthesized by McFarland and co-workers for receiving high efficiency on photoelectrolysis [19]. Until now, sol–gel method, chemical vapor decomposition method, hydrothermal technique and reversed micelle method have been used to produce TiO2 nanostructures [20–23]. In our work, we report a simple solution method to synthesize Cu2 O/TiO2 octahedral core-shell composites, which is composed of two hydrolyzing processes: the primary hydrolyzing by surface absorbed water of Cu2 O and the followed main hydrolyzing by water/ethanol solution. The method is simple to perform, and can make the TiO2 shell uniform and compact.
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2. Experimental 2.1. Synthesis In a typical synthesis, first, 40 mL of NaOH (1 M) solution was added into the mixture of CuSO4 solution (0.25 M, 80 mL) and sodium oleate solution (0.005 M, 40 mL) in a water bath at 80 ◦ C, and then 20 mL of 2.5% hydrazine hydrate was quickly poured into the mixture under stirring. Cu2 O precipitates were obtained after 3 h reaction, then, separated by centrifugation, washed with distilled water and ethanol in sequence. Second, the wet Cu2 O was dispersed into 50 mL of ethanol by ultrasonic treatment, and 20 mL of 0.1 M Ti(OBu)4 ethanol solution was dropped into the above mixture under stirring. 2 h later, water/ethanol (W/E) solution in certain ratios was dropped into the mixture, and the reaction was going on for another 2 h. The resultant composites were centrifuged, rinsed with absolute ethanol for several times, and subsequently dried at room temperature. In further preparation, the resultant Cu2 O/TiO2 composites (0.01 mol) were dipped in H2 SO4 solution (0.02 M 100 mL) for 2 h at room temperature, and the deposits of Cu/SiO2 nanocomposites were filtrated, and then dried in a vacuum system.
2.2. Characterization The shape, size, and core-shell structure of the Cu2 O/TiO2 composites were characterized using high resolution transmission electron microscopy (HRTEM JEOL JEM-3010). The crystallographic structure and part information on chemical composition of the composites were identified by Powder X-ray diffraction (XRD) using Shimadzu model XRD-6000 with Cu K␣ radiation. Surface composition analysis was further carried out on an X-ray photoelectron spectroscope (XPS, ESCALAB Mark II). Detailed composition characterization was carried out with energy-dispersive X-ray (EDX) analysis (equipped with the HRTEM). The photovoltaic character of the nanocomposites was detected by surface photovoltage spectroscopy (SPS). In brief, the lock-in-based SPV measurement system is constituted of a source of monochromatic light, a lock-in amplifier (SR830, Standford Research System) with a light chopper (SR540 Standford Research System), and a computer. A 500 W xenon lamp and a monochromator provide monochromatic light (Zolix Instruments Co., Ltd.). Transmission electron microscopy (JEOL-1230) was also used here to characterize the morphology of Cu/TiO2 composites.
Fig. 1. (a and c) are the TEM and HRTEM images of the Cu2 O/TiO2 sample (W/E = 2/18 mL); (b) is the TEM image of pre-synthesized Cu2 O; (d) is the SEAD image of the Cu2 O/TiO2 sample.
XRD measurements were also carried out to determine the composition and phases of the Cu2 O/TiO2 composites. Fig. 2 is the XRD pattern of the above mentioned Cu2 O/TiO2 sample with low W/E ratio. All the sharper diffraction peaks at 29.6◦ , 36.4◦ , 42.3◦ and 61.4◦ well correspond to the reflections of cubic symmetry Cu2 O (1 1 0), (1 1 1), (2 0 0) and (2 2 0), respectively (JCPDS, No.78-2076). The absence of TiO2 in the XRD pattern of the composite as well as SEAD detection may be due to its amorphous structure and low content. In order to learn clearly the composition and element status of the composite surface, X-ray photoelectron spectroscopy (XPS) analysis was further performed on the Cu2 O/TiO2 sample synthesized in low W/E ratio of 2/18 mL, the results are shown in Fig. 3. The binding energies at 458.3 and 463.8 eV corresponding to Ti 2p3/2 and Ti 2p1/2 (Fig. 3a), confirm the presence of TiO2 at the composite surface. [24] XPS also detected Cu 2p3/2 signal at 934.8 eV and Cu 2p1/2 signal at 954.5 eV with satellite peaks, as given in Fig. 3b.
3. Results and discussion For obtaining better uniform core-shell structure, a simple twostep hydrolyzation procedure was used here to generate Cu2 O/TiO2 octahedral core-shell composites. Fig. 1a is the TEM image of the Cu2 O/TiO2 sample synthesized in low W/E ratio of 2/18 mL, from which we can see that the composites have an octahedral structure with a size of 100–250 nm and light shells condensed on the cores of Cu2 O. This octahedral structure of the composites is due to the morphology of core Cu2 O octahedra, which can be seen in the TEM image of the pre-synthesized Cu2 O precursor as shown in Fig. 1b. Fig. 1c is the HRTEM image taken from one edge part of the Cu2 O/TiO2 particle. It can be seen that on the surface of dark octahedral Cu2 O there is a light shell with the thickness of 6–10 nm. The light shell is proposed to be amorphous TiO2 . Fig. 1d gives the selected area electron diffraction (SAED) image of this Cu2 O/TiO2 sample, in which the inerratic dot-matrix can be seen clearly, and it corresponds to the pattern of Cu2 O. This indicates that the octahedral Cu2 O core is single crystal.
Fig. 2. XRD pattern of the Cu2 O/TiO2 composites synthesized by two-step hydrolyzation (W/E = 2/18 mL).
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Fig. 3. XPS spectra of Ti 2p (a), Cu 2p (b) and O 1s (c) of the Cu2 O/TiO2 composite (W/E = 2/18 mL).
This means that the copper species in composite surface are mainly dispersed as Cu2+ , for XPS analysis is a surface sensitive technique. The Cu(II) 2p binding energies of the composite sample are slightly higher than the reported data of pure CuO [24,25]. According to several previous studies, the higher binding energy was possibly caused by the interaction between support and active component, in our case it should be attributed to an atomic distribution of Ti element to copper oxide, thus Cu2+ at the surface of the composite sample may be in the form of Cu–O–Ti [26]. Some CuO crystallites have also been detected to locate at the surface of the Cu2 O core in the HRTEM analysis. No CuO peaks detected in XRD analysis should be attributed to less amount of CuO in the composite sample. O1s binding energies in Fig. 3c reveal larger amount of OH− (531.9 eV) existing at the composite surface except lattice oxygen (530.0 eV). [24] The abundant OH− would benefit further growth of TiO2 at the composite surface. We think the primary hydrolyzing process (hydrolyzing based on adsorbed water) is important in our reaction system. Fresh synthesized Cu2 O without drying has much adsorbed water at its surface, so Ti(OBu)4 can hydrolyze by the adsorbed water and locate as a thin flat of compact TiO2 shell on the surface of Cu2 O octahedra. The structure of the Cu2 O/TiO2 composite obtained in the first hydrolyzation is shown in Fig. 4a, there is a thin light shell on the surface of the Cu2 O octahedron with the thickness of just a few nanometers, which is supposed to be TiO2 shell as revealed from the EDX pattern of Fig. 4b. We think, it is this thin flat of TiO2 that can make Ti(OBu)4 condense around octahedra easily during the followed main hydrolyzing step, which is favorable for the formation of uniform and compact TiO2 shell. So, Cu2 O/TiO2 nanocomposites with uniform and compact TiO2 shell can be obtained. The TiO2 shell’s thickness of the Cu2 O/TiO2 samples prepared with our strategy can be controlled through adjusting the ratios of water/ethanol added in the main hydrolyzing step. It is found that the thickness of TiO2 shell increased with the increasing of W/E ratio. Fig. 5 is the TEM and HRTEM images of the Cu2 O/TiO2 sample synthesized with a W/E ratio of 10/10 mL. As shown in Fig. 5a, all the Cu2 O octahedra were coated with uniform and compact TiO2 shell in the thickness of about 20 nm. And from the TEM image of one Cu2 O/TiO2 composite particle in Fig. 5b, the integral and uniform TiO2 shell can be seen more clearly. Fig. 5c is the HRTEM image of the edge of one particle, from which we can see that the thick TiO2 shell of this Cu2 O/TiO2 sample is amorphous in character, which is same as that synthesized at low water/ethanol ratio solution (W/E = 2/18 mL). It is deduced that the increasing of W/E
Fig. 4. TEM (a) and EDX (b) images of the Cu2 O/TiO2 sample synthesized by only primary hydrolyzing step (hydrolyzed with the surface adsorbed water of Cu2 O cores).
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Fig. 7. TEM image of the Cu/TiO2 sample.
Fig. 5. (HR)TEM and EDX images of the Cu2 O/TiO2 samples (W/E = 10/10 mL). (a) is the TEM image of the sample, (b) is the TEM image of one particle, (c) is the HRTEM image of the edge of one particle and (d) is the EDX pattern of this sample.
ratio would lead to fast hydrolyzation of Ti(OBu)4 , so more TiO2 turned out in a short time, and these fresh TiO2 easily formed in amorphous rather than crystal structure. Fig. 5d is the EDX pattern of the sample, Ti element can be seen clearly in the result. As Cu2 O and TiO2 are two typical p-type and n-type semiconductors, respectively, much attention has been focused on their optics,
electronics and photoelectric performance [27]. In this work, Cu2 O was coated with TiO2 , and it is hoped that this Cu2 O/TiO2 core-shell composite would show better chemical and physical properties. So we characterized the Cu2 O/TiO2 composites by surface photovoltage spectroscopy (SPS). At the same time, the surface photovoltage (SPV) of precursor Cu2 O and pure TiO2 was also detected for comparison. Fig. 6 is the SPS of Cu2 O, TiO2 and Cu2 O/TiO2 composites (W/E = 10/10 mL). From these responses we can see that the Cu2 O has a wide SPV response band from 300–600 nm with a main peak at about 400 nm. The peak of TiO2 is about 330 nm and its SPV response wavelength reaches to 450 nm. Two obvious SPV response bands were observed in the SPS of the prepared Cu2 O/TiO2 composites, one is at about 350 nm, and the other is in the range of 500–600 nm. We think the formation of the main peak of 350 nm was resulted from the composite of Cu2 O and TiO2 , and the appearance of the peak at long wave band was due to the Cu2 O mainly. These results indicate that the Cu2 O/TiO2 composites have a wider response wavelength compared with simplex TiO2 , and it extended the response of TiO2 from ultraviolet to almost visible wave band. So, this Cu2 O/TiO2 composite shows a potential application in photocatalysis and photoelectric transition.
Fig. 6. Surface photovoltage spectrum of the precursor Cu2 O, pure TiO2 and the obtained Cu2 O/TiO2 composites.
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Furthermore, disproportionation also has been used to treat the Cu2 O/TiO2 composites (W/E = 10/10 mL) by using a certain concentration of H2 SO4 solution. As Cu2 O can disproportionate to Cu(0) and Cu(II) with help of acid (Cu2 O + 2H+ = Cu + Cu2+ + H2 O), in the present study, we chose dilute H2 SO4 (0.02 M) as etching agent to deal with Cu2 O/TiO2 core-shell powders for obtaining Cu/TiO2 nanocomposites upon Kirkendall effect [28,29]. Herein, H2 SO4 and Cu2 O can react with each other and form a diffusion pair, and the coupled reaction/diffusion at the solid/solution interface could lead to the quick formation of Cu nanoparticles within the TiO2 shell, resulting in the formation of Cu/TiO2 core-shell nanostructure. Fig. 7 is the TEM image of the resulting Cu/TiO2 sample. From the image we can see that the light shells of TiO2 were kept integrity after the reaction, and the solid Cu2 O core has already been eroded, leaving one or more moveable Cu cores. These formed Cu/TiO2 moveable core-shell nanocomposites may have a good use in catalysis applications [30]. 4. Conclusion In summary, a simple solution method has been given by us to synthesize Cu2 O/TiO2 core-shell nanocomposites with uniform octahedral structure. This approach can make the TiO2 layer uniformly coated on each Cu2 O octahedron. Moreover, by using simple disproportionation method, we also have obtained Cu/TiO2 moveable core-shell nanocomposites. The Cu2 O/TiO2 and Cu/TiO2 composites would have potential applications in the field of optics, electronics, optoelectronics and catalysis. Acknowledgements This work was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS, SEM), the Natural Science Foundation of Jilin Province for Excellent Young Scholars (Grant No. 20040117). We also acknowledge the financial support from Scientific Research Foundation of Hunan University. References [1] Y. Lei, W.K. Chim, Highly ordered arrays of metal/semiconductor core-shell nanoparticles with tunable nanostructures and photoluminescence, J. Am. Chem. Soc. 127 (2005) 1487–1492. [2] M.H. Liao, C.H. Hsu, D.H. Chen, Preparation and properties of amorphous titaniacoated zinc oxide nanoparticles, J. Solid State Chem. 179 (2006) 2020–2026. [3] X.G. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, Epitaxial growth of highly luminescent cdse/cds core/shell nanocrystals with photostability and electronic accessibility, J. Am. Chem. Soc. 119 (1997) 7019–7029. [4] R. Liu, E.A. Kulp, F.E. Oba, W. Bohannan, F. Ernst, J.A. Switzer, Epitaxial electrodeposition of high-aspect-ratio Cu2 O(110) nanostructures on InP(111), Chem. Mater. 17 (2005) 725–729. [5] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Taracon, Nano-sized transitionmetaloxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496–499. [6] S. Guo, Y. Fang, S. Dong, E. Wang, Templateless, surfactantless, electrochemical route to a cuprous oxide microcrystal: from octahedra to monodisperse colloid spheres, Inorg. Chem. 46 (2007) 9537–9539.
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