CuCu2OTiO2 Nanojunction Systems with an Unusual ElectronHole ...

FULL PAPER DOI: 10.1002/asia.201300019

Cu Cu2O TiO2 Nanojunction Systems with an Unusual Electron–Hole Transportation Pathway and Enhanced Photocatalytic Properties Jun Xing,[a] Zu Peng Chen,[a] Fang Yuan Xiao,[a] Xue Yan Ma,[a] Ci Zhang Wen,[a] Zhen Li,[b] and Hua Gui Yang*[a, c] Abstract: Multicomponent Cu Cu2O TiO2 nanojunction systems were successfully synthesized by a mild chemical process, and their structure and composition were thoroughly analyzed by X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy. The asprepared Cu Cu2O TiO2 (3 and 9 h) nanojunctions demonstrated higher photocatalytic activities under UV/Vis light irradiation in the process of the

degradation of organic compounds than those of the Cu Cu2O, Cu TiO2, and Cu2O TiO2 starting materials. Moreover, time-resolved photoluminescence spectra demonstrated that the quenching times of electrons and holes in Cu Cu2O TiO2 (3 h) is higher than that of Cu Cu2O TiO2 (9 h); this leads Keywords: copper · heterogeneous catalysis · photochemistry · semiconductors · titanium

Introduction

is the most typical heterostructure characterized by a staggered energy-level alignment that can lead to the spatial separation of e–h on different sides of the heterojunction.[9] However, one clear drawback of this mechanism is that the redox potential of transferred electrons/holes is greatly reduced by the release of a portion of the potential energy. By contrast, direct Z-scheme-based heterostructures simultaneously maintained charge separation and stronger oxidative holes and reductive electrons, which were isolated on different semiconductors by directly quenching the weaker oxidative holes and reductive electrons in the solid heterostructure interfaces.[10, 11] These heterostructures would have more advantageous photocatalytic properties than those of type-II heterostructures. However, it is still a challenge to construct direct Z-scheme-based heterostructures with suitable semiconductors to realize desirable vectorial electron transfer and essential interface properties. To date, various TiO2 Cu-based nanojunctions have been designed and studied because of their superior photocatalytic performance, including Cu particles loaded with TiO2,[12, 13] Cu2O TiO2 heterojunction thin-film cathodes,[14] Cu- or Cu2O-doped TiO2,[15, 16] and TiO2 Cu2O heterojunctions.[17] However, these heterojunctions lack effective structure control, and multicomponent Cu Cu2O TiO2 heterojunctions that can promote electron transfer and e–h separation have not been reported. Herein, we describe the successful synthesis of Cu Cu2O TiO2 core–shell heterojunctions by a mild chemical process. The prolonged lifetime of photogenerated carriers and enhanced photocatalytic activities of these nanojunctions are also demonstrated. Based on experimental results, it can be confirmed that the existence of

A photocatalytic reaction is a process in which photogenerated electrons and holes (e–h) migrate to the surface of photocatalysts and cause redox reactions. It takes place on semiconductor materials and has various applications in photocatalytic water splitting, CO2 reduction, pollutant decomposition, sensors, and so forth.[1–7] During the photocatalytic reaction, recombination of photogenerated e–h is a crucial factor for low photocatalytic efficiency. Compared with single-phase photocatalysts with inherent electronic structures and low separation efficiencies of photoinduced charge carriers, hybrid or integrated multi-semiconductor systems possess significant advantages in promoting the separation of e–h pairs and keeping reduction and oxidation reactions at two different reaction sites.[8] The type-II heterojunction [a] J. Xing, Z. P. Chen, F. Y. Xiao, X. Y. Ma, C. Z. Wen, Prof. H. G. Yang Key Laboratory for Ultrafine Materials of Ministry of Education School of Materials Science and Engineering East China University of Science and Technology 130 Meilong Road, Shanghai 200237 (P.R. China) Fax: (+ 86) 21-64252127 E-mail: [email protected] [b] Dr. Z. Li ARC Centre of Excellence for Functional Nanomaterials Australian Institute for Bioengineering and Nanotechnology The University of Queensland Brisbane, QLD 4072 (Australia) [c] Prof. H. G. Yang Centre for Clean Environment and Energy, Gold Coast Campus Griffith University, Gold Coast QLD 4222 (Australia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201300019.

Chem. Asian J. 2013, 8, 1265 – 1270

to a better photocatalytic performance of Cu Cu2O TiO2 (3 h). The improvement in photodegradation activity and electron–hole separation of Cu Cu2O TiO2 (3 h) can be ascribed to the rational coupling of components and dimensional control. Meanwhile, an unusual electron–hole transmission pathway for photocatalytic reactions over Cu Cu2O TiO2 nanojunctions was also identified.

1265

2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org

Hua Gui Yang et al.

a Cu metal core can change the photoinduced e–h transportation pathway between Cu2O and TiO2 in the Cu Cu2O TiO2 system, which can behave as a direct Z-scheme photocatalyst. This is quite different from type-II heterojunctions, which are frequently applied to explain the e–h transmission pathway of TiO2 Cu2O-based composite systems.[17]

Results and Discussion The formation process for Cu Cu2O TiO2 nanojunctions is illustrated in Scheme 1. First, pure copper dendrites were prepared by a hydrothemal method, according to a previous

Figure 1. FESEM (a), TEM (b and c), and corresponding high-resolution (HR) TEM (d and e) images of Cu Cu2O TiO2. Scale bars in d) and e) are 5 nm. To obtain Cu Cu2O TiO2 nanojunctions, as-prepared Cu Cu2O was treated by a LPD method for 3 h.

Scheme 1. Formation process of the Cu Cu2O TiO2 nanojunctions.

shell possess thicknesses of about 60 and 30 nm, respectively. X-ray photoelectron spectroscopy (XPS) data (Figure S2 in the Supporting Information) further confirm the successful synthesis of Cu Cu2O TiO2 nanojunctions with a tiny amount of Cu2 + (Figure S3 in the Supporting Information).[18, 19, 21–23] Previously, Yang et al. reported that the diameter and length of the TiO2 nanorods in the TiO2/ H2Ti5O11·H2O nanocomposites increased with increasing LPD treatment time.[20] For comparison, we obtained Cu Cu2O TiO2 with a thicker TiO2 layer by using a similar strategy. After LPD treatment for 9 hours, the surface of the starting material Cu Cu2O is fully wrapped in spinous TiO2 with a diameter of about 25 nm, as shown in Figures S5 and S6 in the Supporting Information. UV/Vis absorption spectra of pure anatase TiO2 and Cu Cu2O TiO2 (3 h) are shown in Figure 2 a. Cu Cu2O TiO2 (3 h) exhibits a broad absorption from 200 to 800 nm. A previous report indicated that the UV/Vis absorption spectrum of Cu2O was characterized by a broad absorption peak from 400 to 600 nm centered at 500 nm.[24] Qian et al. synthesized copper nanowires with a diameter of about 85 nm, which exhibited surface plasmon resonance (SPR) absorptions in the visible-light region.[18] The exact position of the absorption peak (570–650 nm) also depends on its size and shape.[18, 25] Here, the UV/Vis adsorption spectrum of the microsized Cu Cu2O TiO2 (3 h) hierarchical structure also shows analogous SPR adsorptions. Thus, the absorption of both ultraviolet and visible light by Cu Cu2O TiO2 (3 h) should be attributed to synergetic effects from outer-shell TiO2, innershell Cu2O, and the Cu metal core. Time-resolved PL spectra were used to study the influence of shell thickness on the recombination dynamics of e– h pairs in Cu Cu2O TiO2 nanojunctions; the emission decay kinetics are shown in Figure 2 b. It is clear that the radiative recombination dynamics for the photogenerated e–h pairs are longer in Cu Cu2O TiO2 (3 h) than those in Cu Cu2O TiO2 (9 h). This observation indicates that the thin TiO2 layer leads to a prolonged lifetime of e–h pairs, which can be attributed to improved spatial e–h separation.

report.[18] It was demonstrated that the surface of copper could be slowly oxidized to Cu2O in an ambient atmosphere until further exposure of copper was prevented by the oxide layer.[18, 19] Inspired by this, in the second step, Cu Cu2O was obtained by exposing freshly synthesized copper in air for more than two weeks. A thin Cu2O layer was demonstrated by XRD, as shown in Figure S1 in the Supporting Information. The three peaks at 43.3, 50.4, and 74.18 can be indexed as face-centered cubic (fcc) copper (Joint Committee on Powder Diffraction Standards (JCPDS) no. 04-0836), and the peak at 36.48 can be appropriately ascribed to the (111) diffraction of cuprite Cu2O (JCPDS no. 05-0667). Finally, anatase TiO2 was uniformly crystallized on the surface of Cu Cu2O by a liquid-phase deposition (LPD) method to form multicomponent Cu Cu2O TiO2 nanojunctions (the amount of TiO2 is beyond the XRD detection limit). Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of the as-obtained Cu Cu2O TiO2 nanojunctions are shown in Figure 1. The FESEM image in Figure 1 a shows that the nanojunctions display a three-dimensionally dendritic structure assembled from a lone trunk and multiple branches with lengths of about 1 mm. Spinous TiO2 particles uniformly cover the surface of the material (Figure 1 b and c and Figure S2 in the Supporting Information). HRTEM images of the spinous area and the adjacent area are given in Figure 1 d and e. Two sets of fringes with a distance of 0.30 nm and an angle of 1208 match well with {110} planes of cubic fcc Cu2O. A lattice fringe of 0.35 nm is consistent with {101} planes of anatase TiO2. Yang and Zeng developed a mild LPD method to fabricate various TiO2-based nanocomposites. TiO2 in the composites was highly crystalline.[20] Here, the high crystallinity of TiO2 in Cu Cu2O TiO2 nanojunctions might be attributed to this unique LPD. All of the above results indicate that Cu Cu2O TiO2 nanojunctions were successfully synthesized with well-defined multiple core–shell structures. The outer TiO2 shell and inner Cu2O

Chem. Asian J. 2013, 8, 1265 – 1270

1266


www.chemasianj.org

Hua Gui Yang et al.

same nanojunctions to release detectable holes for COH formation. To further identify the outstanding e–h separation capabilities of Cu Cu2O TiO2, photocatalytic decomposition of an aqueous solution of methyl orange (MO) with prepared samples and reference samples, including Cu Cu2O, Cu TiO2 and Cu2O TiO2, were carried out under UV/Vis irradiation. XRD patterns and SEM images of reference sample Cu2O are shown in Figure S7 in the Supporting Information. Under UV/Vis irradiation, almost no change in the concentration of MO was observed in the absence of any photocatalyst. Figure 3 a shows the time dependence of the concentration of an aqueous solution of MO in the presence of photocatalysts under UV/Vis irradiation. After 80 minutes of irradiation, the Figure 2. a) UV/Vis absorption spectra of anatase TiO2, which was obtained by keeping an 8 mm aqueous soluphotocatalytic decomposition of tion of TiF4 at 70 8C for 9 h in an oil bath (Figure S4 in the Supporting Information), and Cu Cu2O TiO2 MO by Cu Cu2O TiO2 nanonanojunctions (3 h). b) Time-resolved photoluminescence (PL) spectra of Cu Cu2O TiO2 nanojunctions LPD junction systems was nearly treatment times of 3 and 9 h. Inset: spectra from 26 to 42 ns. c) Fluorescence spectra of the UV/Vis-irradiated Cu Cu2O TiO2 (3 h) in 3 mm terephthalic acid at different irradiation times. d) Time dependence of the fluo90 %; however, only a small rescence intensity at 426 nm determined by photocatalytic reaction of Cu Cu2O TiO2 nanojunctions and startamount of MO could be deing material Cu Cu2O under different conditions, including irradiation with light at lex 420 nm (& and ~) composed by Cu Cu2O, Cu ! * and UV/Vis light (3, , and ). TiO2, and Cu2O TiO2 under the same conditions. The most efficient degradation of MO by the Cu Cu2O TiO2 nanoActive hydroxyl radicals (COH) obtained upon irradiation are considered as the most important reactive oxidative spejunction suggests excellent photoinduced e–h separation and cies (ROS) in photocatalytic reactions. Terephthalic acid can strong oxidative holes for such a nanojunction. It is clear react with COH in basic solution to generate 2-hydroxyterthat Cu Cu2O TiO2 (3 h) exhibits better photocatalytic acephthalic acid (TAOH), which emits a unique fluorescence tivity than Cu Cu2O TiO2 (9 h) for the first 30 minutes; this signal with a peak at around 426 nm.[26, 27] Herein, terephis the same order as that determined by ROS measurements. As demonstrated by time-resolved PL spectroscopy, the e–h thalic app:ds:acid was used as a fluorescence probe to study quenching time was significantly delayed in Cu Cu2O TiO2 the photodegradation effects of Cu Cu2O TiO2 samples. As shown in Figure 2 c, the relationship between fluorescence (3 h). This result is much better than that of Cu Cu2O TiO2 intensity and irradiation time is linear, which demonstrates (9 h) with a thicker TiO2 layer, which can prolong the e–h the stability of Cu Cu2O TiO2 (3 h). The formation of COH transmission pathway, and thus, hinder their separation. The stability of the highly efficient Cu Cu2O TiO2 nanojunction on Cu Cu2O and Cu Cu2O TiO2 nanojunctions excited by UV/Vis or visible light is illustrated in Figure 2 d. For UV/ photocatalyst was further evaluated by recycling experiVis irradiation, both Cu Cu2O TiO2 (3 h) and Cu Cu2O ments for the degradation of MO.[28, 29] Figure 3 b shows that TiO2 (9 h) present better photodegradation capabilities than as-prepared Cu Cu2O TiO2 retains a similar degradation that of the binary starting material Cu Cu2O; Cu Cu2O rate after five cycles, suggesting that the Cu Cu2O TiO2 nanojunctions exhibit high photocatalytic activity and stabilTiO2 (3 h) shows the best photocatalytic performance. For visible-light (l 420 nm) irradiation, Cu Cu2O TiO2 (3 h) ity during the degradation of MO, although slightly more Cu2 + is present during the photocatalytic reaction, according and Cu Cu2O TiO2 (9 h) show almost invariable fluorescence intensity with time. These results indicate that Cu to XPS spectra (Figure S3 in the Supporting Information). Cu2O TiO2 nanojunctions can only generate COH under Based on the above observations and an energy-band diagram of the multicomponent nanojunctions, the reaction UV-light irradiation, whereas visible light cannot excite the

Chem. Asian J. 2013, 8, 1265 – 1270

1267


www.chemasianj.org

Hua Gui Yang et al.

Figure 4. Proposed reaction pathway for photoexcited e–h pairs in Cu Cu2O TiO2 nanojunctions. The band positions for Cu2O and TiO2 were obtained from the literature.[30–32] NHE = normal hydrogen electrode.

/Fe2 + , in most cases or recombine directly in elaborately designed composites, and hydrogen-generation electrons and oxygen-generation holes are separated spatially.[2, 10, 11] The transmission pathway we propose herein is more feasible because p-type semiconductor Cu2O forms a contact with Cu metal to form an Ohm junction, which facilitates electron transmission from Cu2O to Cu.[33] Holes in TiO2 are more easily trapped by OH under alkaline conditions,[26] and holes in p-type Cu2O and electrons in n-type TiO2 recombine instead of remaining to charge the components.[34–36] It is known that combinations of Cu2O and TiO2 can form type-II heterojunctions, in which electrons transfer from Cu2O to TiO2, whereas holes transfer in the opposite direction to achieve e–h separation.[17, 32, 37] The transmission pathway proposed herein is quite different from previous reports. We provide further evidence for our proposed electron transmission from XPS data for Ti 2p and a terephthalic acid photodegradation test, which was carried out under vacuum and ambient conditions. The lack of a Ti3 + peak in the Ti 2p spectrum (Figure S3 in the Supporting Information) indicates that the chance of electron transmission from the CB of Cu2O to the CB of TiO2 is not likely for the generation of Ti3 + by trapping electrons on the Ti4 + site.[32] In addition, it has been reported that oxygen molecules are strong electron scavengers, which can combine with electrons from the CB and thus prolong the lifetime of holes in the VB to enhance photocatalytic efficiency.[38, 39] We confirmed the role of O2 in Cu Cu2O TiO2 (3 h) and P25 TiO2 by comparison of a terephthalic app:ds:acid photodegradation test performed in vacuum and under ambient conditions, as shown in Figure 5. Compared with the results obtained under vacuum, Cu Cu2O TiO2 (3 h) showed reduced photocatalytic activity, whereas P25 TiO2 showed enhanced photocatalytic activity under ambient conditions. These results indicate that electrons make less contribution to the terephthalic acid photodegradation test in Cu Cu2O TiO2 (3 h; Z-scheme heterostructure) than that in P25 TiO2 (typeII heterostructure composed of anatase and rutile phases).

scheme in Figure 4 is proposed. For UV/Vis irradiation, electrons and holes are photogenerated on both Cu2O and TiO2. Because the Fermi energy level of Cu is lower than that of p-type Cu2O, electrons on the conduction band (CB) of Cu2O migrate to Cu until the two Fermi levels are aligned. Holes in the valance band (VB) of Cu2O recombine with electrons in the CB of TiO2. More holes in the VB of TiO2 are separated from electrons and engage in COH formation. For irradiation with light of lex 420 nm, only Cu2O is photoexcited. Electrons in the CB of Cu2O are trapped by highly conductive Cu, while migration of holes from the VB of Cu2O to VB of TiO2 is difficult, and thus, the small amount of COH generated can barely be detected. This transmission pathway is analogous to that of the Z-scheme system, in which holes in a hydrogen-evolution photocatalyst and electrons in an oxygen-evolution photocatalyst recombine with a redox mediator, such as IO3 /I and Fe3 +

Figure 5. Fluorescence spectra of UV/Vis-irradiated Cu Cu2O TiO2 (3 h) and Degussa P25 TiO2 in 3 mm terephthalic acid. The ROS tests were performed under vacuum or in air.

Figure 3. a) Time profiles of the photocatalytic degradation of MO for different photocatalysts under UV/Vis irradiation and b) photodegradation of a recycled sample of Cu Cu2O TiO2 (3 h).

Chem. Asian J. 2013, 8, 1265 – 1270

1268


Hua Gui Yang et al.

www.chemasianj.org

for recording the fluorescence spectra was l = 320 nm. The light source employed in photoreactions was a 300 W Xe lamp (CEL-HXUV300) with a wavelength range of 200–2500 nm. For visible-light irradiation, a l = 420 nm UV cutoff was used to eliminate lex 420 nm irradiation.

We assume that the reduction in the photocatalytic activity of Cu Cu2O TiO2 (3 h) under oxygen-rich conditions can be ascribed to perturbation of the Z-scheme transmission pathway by O2, which acts as an electron scavenger. For comparison, under vacuum conditions the solid Cu Cu2O TiO2 Z scheme is favored for a lack of O2 electron scavengers, and further enhances the transmission efficiency of the Cu Cu2O TiO2 Z scheme.

The photodegradation of an aqueous solution of MO was carried out under ambient temperatures. Typically, photocatalysts powders (50 mg) were dispersed in MO solution (50 mL, 10 mg L 1) under stirring. The suspension was stirred magnetically for 1 h in the dark to reach adsorption equilibrium. The concentration of MO was determined by using a UV/Vis spectrophotometer. After irradiation every 20 min, the reaction solution was sampled to measure the change in concentration of MO. Characterization of the Materials

Conclusion

Crystallographic information for the Cu Cu2O and Cu Cu2O TiO2 nanojunctions was obtained by XRD (Bruker D8 Advanced Diffractometer, CuKa radiation, 40 kV). The morphologies and structures of the samples were characterized by HRTEM (JEOL JEM-2100F) and FESEM (Hitachi S4800); surface binding elements were analyzed by XPS (Kratos Axis Ultra DLD). All binding energies were referenced to the C1s peak (284.8 eV) arising from surface hydrocarbons (or possible adventitious hydrocarbon). Prior to peak deconvolution, X-ray satellites and inelastic background (Shirley-type) signals were subtracted from all spectra. Fresh samples were redispersed in ethanol and dropped onto a carbon-coated copper grid with irregular holes for TEM analysis. The UV/Vis diffuse reflectance spectra were obtained by using a UV/Vis spectrophotometer (Carry-500).

We have prepared multicomponent Cu Cu2O TiO2 nanojunctions by a facile method. The Cu Cu2O TiO2 nanojunctions have photogenerated carriers with prolonged lifetimes and exhibit high photocatalytic activities and stabilities. Rational coupling of components and dimensional control enable improved e–h separation and photo-oxidation activity of the photocatalysts. Furthermore, we propose that the e–h transmission pathway of a Cu Cu2O TiO2 nanojunction behaves as a Z-scheme photocatalyst instead of as a type-II heterojunction based on theoretical and experimental analyses.

Time-resolved fluorescence spectroscopy was performed at room temperature with a Horiba Jobin Yvon FluoroLog-3 fluorescence spectrophotometer. The instrument works on the principle of the time-correlated single-photon-counting (TCSPC) technique. The powder samples of the Cu Cu2O TiO2 nanojunctions were excited by a l = 340 nm light-emitting diode (LED), and the fluorescence emission decay spectra were monitored at l = 350 nm by TCSPC. Acquisition of the emission decay spectrum was not stopped until the counting number of the fluorescence signal reached 50 007. The thickness of the quartz sample holder was 1 cm.

Experimental Section Synthesis of Nanojunctions The synthesis of Cu Cu2O TiO2 nanojunctions was carried out by using a mild chemical process. 1) Copper dendrites were fabricated by a modified hydrothermal method.[18] In a typical experiment, distilled water (27.6 g), NaOH (9 g), glycerol (18 mL), disodium hydrogen phosphate (1.947 g), copper sulfate pentahydrate (0.5673 g), and sodium dodecyl benzene sulfonate (0.055 g) were introduced into a 50 mL Teflon-lined autoclave under vigorously stirring. Then the autoclave was maintained at 120 8C for 20 h. After cooling to room temperature, the red fluffy powder was harvested by centrifugation and washed with ethanol and distilled water several times. 2) The as-prepared copper product was dried in vacuum and stored in air. After storage for more than two weeks, the surface of the copper was oxidized to cuprous oxide. 3) Finally, Cu Cu2O TiO2 nanojunctions were obtained by a LPD method. An aqueous solution of TiF4 was prepared by a method reported previously.[23] Briefly, the red powder (0.02 g) obtained from step 2) was added to an 8 mm aqueous solution of TiF4 (40 mL) and maintained at 55 8C for 3 or 9 h to obtain Cu Cu2O TiO2 nanojunctions. The product in this step was separated by gravitational settling, washed with distilled water until the supernatant became clear, dried in vacuum, and then preserved in a dryer containing silica-gel desiccant. The reference sample Cu TiO2 (3 h) was synthesized according to the method described above without preserving the as-prepared Cu for more than two weeks.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (20973059, 91022023, 21076076, 21203061), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Shanghai Municipal Natural Science Foundation (12ZR1407500), the Major Basic Research Programme of Science and Technology Commission of Shanghai Municipality (10JC1403200), and the Australian Research Councils Future Fellowships (no. FT120100913).

[1] X. Hu, G. Li, J. C. Yu, Langmuir 2010, 26, 3031 – 3039. [2] J. Xing, W. Q. Fang, H. J. Zhao, H. G. Yang, Chem. Asian J. 2012, 7, 642 – 657. [3] A. Kudo, Y. Miseki, Chem. Soc. Rev. 2009, 38, 253 – 278. [4] J. Yu, W. Wang, B. Cheng, Chem. Asian J. 2010, 5, 2499 – 2506. [5] X. Zou, X. Li, Q. Zhao, S. Liu, J. Colloid Surf. Sci. 2012, 383, 13 – 18. [6] G. Dai, J. Yu, G. Liu, J. Phys. Chem. C 2011, 115, 7339 – 7346. [7] L. Jiang, G. Zhou, J. Mi, Z. Wu, Catal. Commun. 2012, 24, 48 – 51. [8] X. Chen, S. Shen, L. Guo, S. S. Mao, Chem. Rev. 2010, 110, 6503 – 6570. [9] S. S. Lo, T. Mirkovic, C. H. Chuang, C. Burda, G. D. Scholes, Adv. Mater. 2011, 23, 180 – 197. [10] H. Tada, T. Mitsui, T. Kiyonaga, T. Akita, K. Tanaka, Nat. Mater. 2006, 5, 782 – 786. [11] X. Wang, G. Liu, Z. G. Chen, F. Li, L. Wang, G. Q. Lu, H.-M. Cheng, Chem. Commun. 2009, 3452 – 3454.

Cu2O TiO2 was also prepared by a two-step method. Firstly, Cu2O was synthesized followed previous report.[40] Then, Cu2O TiO2 nanojunctions were obtained by the LPD method mentioned above. Photoreactivity Measurements Various photocatalysts, including the Cu Cu2O starting material and multicomponent Cu Cu2O TiO2 nanojunctions (10 mg), were dispersed in 40 mL of an aqueous solution containing 0.01 m NaOH and 3.0 mm terephthalic acid. Before exposure to light irradiation, the solution was stirred in the dark for 30 min and then the photocatalysts were separated from solution by centrifugation. The clear supernatant was used for fluorescence spectroscopy measurements. During the photoreactions, no oxygen was bubbled into the suspension. The excitation light employed

Chem. Asian J. 2013, 8, 1265 – 1270

1269


www.chemasianj.org

[12] Y. H. Xu, D. H. Liang, M. L. Liu, D. Z. Liu, Mater. Res. Bull. 2008, 43, 3474 – 3482. [13] K. Y. Song, Y. T. Kwon, G. J. Choi, W. I. Lee, Bull. Korean Chem. Soc. 1999, 20, 957 – 960. [14] W. Siripala, A. Ivanovskaya, T. F. Jaramillo, S.-H. Baeck, E. W. McFarland, Sol. Energy Mater. Sol. Cells 2003, 77, 229 – 237. [15] S. Chen, S. Zhang, W. Liu, W. Zhao, J. Nanosci. Nanotechnol. 2009, 9, 4397 – 4403. [16] K. Lalitha, G. Sadanandam, V. D. Kumari, M. Subrahmanyam, B. Sreedhar, N. Y. Hebalkar, J. Phys. Chem. C 2010, 114, 22181 – 22189. [17] Y. Hou, X. Li, X. Zou, X. Quan, G. Chen, Environ. Sci. Technol. 2009, 43, 858 – 863. [18] Z. Liu, Y. Yang, J. Liang, Z. Hu, S. Li, S. Peng, Y. Qian, J. Phys. Chem. B 2003, 107, 12658 – 12661. [19] L.-I. Hung, C.-K. Tsung, W. Huang, P. Yang, Adv. Mater. 2010, 22, 1910 – 1914. [20] H. G. Yang, H. C. Zeng, J. Am. Chem. Soc. 2005, 127, 270 – 278. [21] J. Fan, Y. Dai, Y. Li, N. Zheng, J. Guo, X. Yan, G. D. Stucky, J. Am. Chem. Soc. 2009, 131, 15568 – 15569. [22] Z. Ai, L. Zhang, S. Lee, W. Ho, J. Phys. Chem. C 2009, 113, 20896 – 20902. [23] H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng, G. Q. Lu, Nature 2008, 453, 638 – 641. [24] Y. Qu, X. Li, G. Chen, H. Zhang, Y. Chen, Mater. Lett. 2008, 62, 886 – 888. [25] D. B. Pedersen, S. Wang, S. H. Liang, J. Phys. Chem. C 2008, 112, 8819 – 8826. [26] T. Hirakawa, Y. Nosaka, Langmuir 2002, 18, 3247 – 3254.

Chem. Asian J. 2013, 8, 1265 – 1270

Hua Gui Yang et al.

[27] H. G. Yang, G. Liu, S. Z. Qiao, C. H. Sun, Y. G. Jin, S. C. Smith, J. Zou, H. M. Cheng, G. Q. Lu, J. Am. Chem. Soc. 2009, 131, 4078 – 4083. [28] W. Wang, B. Cheng, J. Yu, G. Liu, W. Fan, Chem. Asian J. 2012, 7, 1902 – 1908. [29] G. Dai, J. Yu, G. Liu, J. Phys. Chem. C 2012, 116, 15519 – 15524. [30] Y. Bessekhouad, D. Robert, J.-V. Weber, Catal. Today 2005, 101, 315 – 321. [31] Y. Xu, M. A. A. Schoonen, Am. Mineral. 2000, 85, 543 – 556. [32] A. Paracchino, V. Laporte, K. Sivula, M. Grtzel, E. Thimsen, Nat. Mater. 2011, 10, 456 – 461. [33] J. A. Assimos, D. Trivich, J. Appl. Phys. 1973, 44, 1687 – 1693. [34] N. Serpone, E. Borgarello, M. Grtzel, J. Chem. Soc. Chem. Commun. 1984, 342 – 344. [35] N. Serpone, P. Maruthamuthu, P. Pichat, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol. A 1995, 85, 247 – 255. [36] A. J. Nozik, R. Memming, J. Phys. Chem. 1996, 100, 13061 – 13078. [37] J. Zhang, H. Zhu, S. Zheng, F. Pan, T. Wang, ACS Appl. Mater. Interfaces 2009, 1, 2111 – 2114. [38] N. Wu, J. Wang, D. N. Tafen, H. Wang, J. G. Zheng, J. P. Lewis, X. Liu, S. S. Leonard, A. Manivannan, J. Am. Chem. Soc. 2010, 132, 6679 – 6685. [39] J. Tang, J. R. Durrant, D. R. Klug, J. Am. Chem. Soc. 2008, 130, 13885 – 13891. [40] S. Hacialioglu, F. Meng, S. Jin, Chem. Commun. 2012, 48, 1174 – 1176. Received: January 5, 2013 Published online: March 12, 2013

1270
