- Email: [email protected]
Enhanced photocatalytic activity of porous cuprous oxide dodecahedron nanocrystals synthesized by solvothermal method
Materials Letters 159 (2015) 172–176
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced photocatalytic activity of porous cuprous oxide dodecahedron nanocrystals synthesized by solvothermal method Lufeng Yang a,b, Deqing Chu a,b,n, Limin Wang b,c,nn, Huilou Sun a, Ge Ge a a
College of Environment and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin 300387, PR China c College of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 February 2015 Received in revised form 18 May 2015 Accepted 28 June 2015 Available online 30 June 2015
The porous cuprous oxide (Cu2O) dodecahedron nanocrystals have been successfully prepared via a solvothermal reduction process at a moderate temperature. The average dimensions of these Cu2O dodecahedron nanostructures were about 750 nm in diameter. The crystal growth mechanism of the novel architectures has been discussed. And then, the as-prepared Cu2O samples were used as photocatalysts in the degradation of methylene blue (MB). Due to the porous nature of the products, the photocatalytic performance has been significantly improved. We believe that the current work will offer some ideas for further fabricating porous nanostructures and exploring their applications. & 2015 Elsevier B.V. All rights reserved.
Keywords: Cuprous oxide Porous materials Semiconductors Crystal growth Photocatalytic property
1. Introduction
2. Experimental
As an important p-type semiconductor, Cu2O has a direct small band gap of 2.2 eV [1] and is considered to be a promising material in solar cells conversion [2–4] and photocatalysis areas [5]. Up to now, Cu2O has been prepared by several different methods, such as electrodeposition [6], thermal relaxation [7], sonochemical methods [8], vacuum evaporation [9], and the liquid-phase reduction [10]. Meanwhile, various morphologies of Cu2O, such as nanowires [11], octahedral [12], hollow spheres [13], nanocubes [14], and nanorods [15] have been prepared. However, it remains a great challenge to develop simple and feasible approaches for the shape-controlled synthesis of well-defined Cu2O architectures. Here we report a new method of the synthesis and characterization of porous Cu2O dodecahedron crystals. The synthesis was performed using a solvothermal reduction process at a moderate temperature. The adsorption ability and photocatalytic activity of the samples were evaluated by the photocatalytic degradation of MB aqueous solution under UV/visible light illumination.
A typical synthesis procedure of the porous Cu2O dodecahedron nanostructures was as follows: 2 mmol (0.3993 g) of copper acetate (Cu(Ac)2·H2O) was dispersed in 25 mL of N,N-dimethylformamide (DMF) under vigorous stirring at room temperature, followed by addition of 1 g of polyvinyl pyrrolidone (PVP). After being stirred vigorously for 10 min, the mixed solution was put into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 170 °C for 8 h and then cooled to room temperature naturally. The resulting products were centrifuged, washed several times with distilled water and anhydrous ethanol, and dried at 80 °C in a vacuum for 6 h. The photocatalytic experiments were carried out by adding 100 mg of Cu2O samples into 100 mL of MB aqueous solution with the concentration of 10 mg L 1. The suspension was stirred for 30 min in the dark to obtain adsorption equilibrium. Then, the suspension was irradiated with a 365 nm UV-light. As a comparison, the visible-driven photocatalytic experiments were also carried out. The concentration of MB was detected by a UV–vis spectrophotometer.
n Corresponding author at: College of Environment and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China. Fax: þ 86 22 83955762. nn Corresponding author at: State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin 300387, PR China. E-mail addresses: [email protected] (D. Chu), [email protected] (L. Wang).
http://dx.doi.org/10.1016/j.matlet.2015.06.098 0167-577X/& 2015 Elsevier B.V. All rights reserved.
3. Results and discussion Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provided insight into the morphology of the
L. Yang et al. / Materials Letters 159 (2015) 172–176
173
Fig. 1. (a) SEM images of the porous Cu2O dodecahedron nanocrystals (the inset shows the close observation of the product); (b) TEM image of the Cu2O nanostructure; (c) a representative XRD pattern recorded for Cu2O dodecahedron architectures; and (d) N2 adsorption–desorption isotherm and BJH pore size distribution plots (inset) of the porous Cu2O dodecahedron nanostructures.
Cu2O crystals. A panoramic morphology of the product is displayed in Fig. 1(a), indicating the high yield and uniformity. A magnified SEM image showing the close observation of the nanostructures is given in the illustration in Fig. 1(a). It reveals that the detailed morphology of Cu2O product is well-defined porous dodecahedron with diameters of 750 nm and the surface of Cu2O nanostructures are made of numerous nanoparticals. As shown by the arrows in Fig. 1(b), the surface of the Cu2O nanocrystal was covered with lots of pores. The crystal phase of Cu2O is characterized by XRD, and the data are shown in Fig. 1(c). The characteristic peaks of the cubic phase of Cu2O crystals (JCPDS 05-0667) were observed. No peaks of impurity are detected in the XRD pattern, indicating the formation of pure Cu2O under these experimental conditions. To further confirm the surface architectures of the as-prepared Cu2O samples, N2 adsorption and desorption measurements were performed. As shown in Fig. 1(d), the isotherm of the Cu2O samples exhibit a hysteresis loop at p/p0 range of 0.8–1.0. This phenomenon clearly indicates that the Cu2O samples exhibit a large textural porosity [12]. The pore size distribution of the Cu2O shows that a narrow peak appeared in the pore size region from 2.1 to 200.8 nm. The material had an average pore size of 20.6 nm. The Brunauer–Emmett–Teller (BET) surface area of the Cu2O nanocrystals was calculated to be 15.4 m2 g 1, which is much higher than that of the Cu2O intermidates (5.3, 7.9, 8.6, and 10.3 m2 g 1 after solvothermal treatment for 2, 4, 6, and 10 h, respectively). Fig. 2 illustrates the morphology evolution of Cu2O at different stages. Initially, Cu2 þ is reduced by DMF to generate Cu2O particles, and then Cu2O nanospheres (Fig. 2(a) and (b)) are grown by self-assembly of small primary nanoparticles through an orientedattachment mechanism after solvothermal treatment for 2 h. In
this stage, the morphology evolution of Cu2O nanostructures had not been completed, meanwhile some Cu2O crystallites grew along their specific crystallographic planes and eventually evolved into the original polyhedral nanostructures (as shown in the black dotted bordered rectangles of Fig. 2(a)). Seed particles continuously adsorb onto these nanospheres to allow further growth through a ripening process. Then the intermediate structures develop into structurally well-defined dodecahedron nanostructures (Fig. 2(c) and (d)) via a surface reconstruction process in 4 h. The growth control of Cu2O crystals by adsorption of PVP has been extensively studied [16]. The PVP molecules should be adsorbed preferentially on the {111}, which helps to form unique structure of Cu2O. After conducting the reaction for 6 h, Cu2O dodecahedron with rough surface were constructed (Fig. 2(e)). A close-up view of Cu2O nanostructure in Fig. 2(f) demonstrates that the surfaces of Cu2O crystals present many tiny holes. Further prolonging the solvothermal time to 8 h, we got perfect porous Cu2O dodecahedron nanostructures (Fig. 1(a)). It is well known that the structures of products prepared by solution reactions depend on the rate of nucleation and growth of the reaction products. The results show that the rate of growth of Cu2O is faster than that of its nucleation. Due to the preferential growth of Cu2O in a certain direction, the porous dodecahedron-like products were obtained. Further observation (inset in Fig. 1(a) and (b)) revealed that a large amount of irregular pores of tens of nanometers was randomly distributed in the nanocrystals, as a result of the dissolution-recrystallization process. Surprisingly, with the extension of the reaction time to 10 h, lots of broken dodecahedron-like Cu2O particles can be discerned from Fig. 2(g) and (h), which is the result of increasing rate of the dissolution of Cu2O nanocrystals, probably. To evaluate the photocatalytic activity of the products, we
174
L. Yang et al. / Materials Letters 159 (2015) 172–176
Fig. 2. SEM images of the Cu2O samples obtained after solvothermal treatment for different time: (a) t¼ 2 h; (c) t¼ 4 h; (e) t¼ 6 h; and (g) t ¼10 h ((b), (d), (f), and (h) are magnified images of (a), (c), (e), and (g), respectively).
measured the optical property changes of MB aqueous solution in the presence of Cu2O dodecahedron or other Cu2O intermediates under irradiation of UV/visible light for a given time, respectively. The time-dependent absorption spectra of MB solution containing Cu2O catalyst during the irradiation are illustrated in Fig. 3(a and
c). It can be seen that the maximum absorbance at 665 nm decreases rapidly with irradiation time as shown in Fig. 3(a and c). After it had been irradiated under UV light for 90 min, the degradation rate of MB is 98.3%, indicating that most of the amount of MB was degraded (Fig. 3(a)). However, the degradation rate of
L. Yang et al. / Materials Letters 159 (2015) 172–176
175
Fig. 3. (a and c) Absorbance spectra change of the MB in aqueous solution (10 mg L 1, 100 mL) in the presence of Cu2O dodecahedron nanocrystals under UV/visible light irradiation; (b and d) Photodegradation plots of MB under UV/visible light irradiation for different time in the presence/absence of catalysts (solvothermal treatment for 2, 4, 6, 8 and 10 h, respectively, and commercial Cu2O as control sample); and (e) UV–vis absorbance spectra of the Cu2O samples prepared for different solvothermal time (2, 4, 6, 8 and 10 h, respectively).
MB is only 90.7%, after it had been irradiated under visible-light for the same time (Fig. 3(c)). It reveals that UV irradiation will enhance photodegradation of MB. Possibly because the energy of UV light is stronger than visible-light, which could improve the photocatalytic efficiency. The result also indicates that Cu2O shows great potential as a visible-light-driven photocatalyst, otherwise, pure TiO2 catalyst take effect only under UV light. By monitoring the MB absorption peak at 665 nm, plots of the degradation ratio versus reaction time were obtained for different Cu2O samples (obtained after solvothermal treatment for 2, 4, 6, 8, and 10 h, respectively, and commercial Cu2O as control sample) under identical conditions (Fig. 3(b and d)). It clearly shows that the Cu2O
nanocrystals and other Cu2O products are effective photocatalysts for the direct degradation of MB than commercial Cu2O sample. Due to its vastly surface area, the catalytic properties of the porous Cu2O dodecahedron nanostructures are better than those of other Cu2O intermediates. The morphology of the Cu2O sample obtained after solvothermal treatment for 10 h is irregular. In addition, its BET surface area is smaller than that of the porous Cu2O dodecahedron nanostructure (solvothermal treatment for 8 h). This leads to its low utilization rate of UV/visible light, which explains the reason for its lower photocatalytic activity. The UV–vis absorption spectra (Fig. 3(e)) suggested that the intensity of sample I was much stronger. The order of UV–vis
176
L. Yang et al. / Materials Letters 159 (2015) 172–176
absorption intensities was as follows: samples I 4II4 III4IV 4V. The photocatalytic activities of Cu2O samples obeyed the same order. It is suggested that the photocatalytic activities of the Cu2O samples had positive correlation with the UV–vis absorption intensity. Among the five samples, sample I had the highest UV–vis absorption intensity, so the UV/visible light utilization efficiency of sample I was highest. It could explain the high photocatalytic activity of sample I. In conclusion, apart from the total surface area (the main factor), UV–vis absorption intensity and utilization efficiency of UV/visible light also contributed to the photocatalytic activities.
4. Conclusions In summary, a moderate chemical solution method was demonstrated for the synthesis of the porous Cu2O dodecahedron nanostructures. The simple synthesis approach casts new light on the controllable fabrication of Cu2O porous architectures. Furthermore, the photocatalysis measurements show that the porous Cu2O dodecahedron nanocrystals exhibit outstanding photocatalytic properties for the degradation of MB under UV/visiblelight mainly due to the unique porous nanostructures.
References [1] M. Shen, T. Yokouchi, S. Koyama, T. Goto, Dynamics associated with Bose– Einstein statistics of orthoexcitons generated by resonant excitations in cuprous oxide, Phys. Rev. B 56 (1997) 13066. [2] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, G.K. Paul, T. Sakurai, Thin film deposition of Cu2O and application for solar cells, Sol. Energy 80 (2006) 715–722.
[3] T. Mahalingam, J.S.P. Chitra, J.P. Chu, H. Moon, H.J. Kwon, Y.D. Kim, Photoelectrochemical solar cell studies on electroplated cuprous oxide thin films, J. Mater. Sci.: Mater. Electron. 17 (2006) 519–523. [4] D. Tryk, A. Fujishima, K. Honda, Recent topics in photoelectrochemistry: achievements and future prospects, Electrochim. Acta 45 (2000) 2363–2376. [5] Y.-G. Zhang, L.-L. Ma, J.-L. Li, Y. Yu, In situ Fenton reagent generated from TiO2/Cu2O composite film: a new way to utilize TiO2 under visible light irradiation, Environm. Sci. Technol. 41 (2007) 6264–6269. [6] L. Huang, H. Wang, Z. Wang, A. Mitra, D. Zhao, Y. Yan, Cuprite nanowires by electrodeposition from lyotropic reverse hexagonal liquid crystalline phase, Chem. Mater. 14 (2002) 876–880. [7] S. Deki, K. Akamatsu, T. Yano, M. Mizuhata, A. Kajinami, Preparation and characterization of copper (I) oxide nanoparticles dispersed in a polymer matrix, J. Mater. Chem. 8 (1998) 1865–1868. [8] R.V. Kumar, Y. Mastai, Y. Diamant, A. Gedanken, Sonochemical synthesis of amorphous Cu and nanocrystalline Cu2O embedded in a polyaniline matrix, J. Mater. Chem. 11 (2001) 1209–1213. [9] H. Yanagimoto, K. Akamatsu, K. Gotoh, S. Deki, Synthesis and characterization of Cu2O nanoparticles dispersed in NH2-terminated poly(ethylene oxide), J. Mater. Chem. 11 (2001) 2387–2389. [10] S. Ram, C. Mitra, Formation of stable Cu2O nanocrystals in a new orthorhombic crystal structure, Mater. Sci. Eng. A 304 (2001) 805–809. [11] M. Hafez, F. Al-Marzouki, W.E. Mahmoud, Single crystalline quasi aligned one dimensional P-type Cu2O nanowire for improving Schottky barrier characteristics, Mater. Lett. 65 (2011) 1868–1870. [12] X. Zhang, G. Wang, H. Wu, D. Zhang, X. Zhang, P. Li, et al., Synthesis and photocatalytic characterization of porous cuprous oxide octahedra, Mater. Lett. 62 (2008) 4363–4365. [13] H. Zhang, Q. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, One-pot synthesis and hierarchical assembly of hollow Cu2O microspheres with nanocrystals-composed porous multishell and their gas-sensing properties, Adv. Funct. Mater. 17 (2007) 2766–2771. [14] L. Gou, C.J. Murphy, Solution-phase synthesis of Cu2O nanocubes, Nano Lett. 3 (2003) 231–234. [15] Z. Cheng, J. Xu, H. Zhong, X. Chu, J. Song, Repeatable synthesis of Cu2O nanorods by a simple and novel reduction route, Mater. Lett. 65 (2011) 1871–1874. [16] d-F. Zhang, H. Zhang, L. Guo, K. Zheng, X.-D. Han, Z. Zhang, Delicate control of crystallographic facet-oriented Cu2O nanocrystals and the correlated adsorption ability, J. Mater. Chem. 19 (2009) 5220.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced photocatalytic activity of porous cuprous oxide dodecahedron nanocrystals synthesized by solvothermal method Lufeng Yang a,b, Deqing Chu a,b,n, Limin Wang b,c,nn, Huilou Sun a, Ge Ge a a
College of Environment and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin 300387, PR China c College of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 February 2015 Received in revised form 18 May 2015 Accepted 28 June 2015 Available online 30 June 2015
The porous cuprous oxide (Cu2O) dodecahedron nanocrystals have been successfully prepared via a solvothermal reduction process at a moderate temperature. The average dimensions of these Cu2O dodecahedron nanostructures were about 750 nm in diameter. The crystal growth mechanism of the novel architectures has been discussed. And then, the as-prepared Cu2O samples were used as photocatalysts in the degradation of methylene blue (MB). Due to the porous nature of the products, the photocatalytic performance has been significantly improved. We believe that the current work will offer some ideas for further fabricating porous nanostructures and exploring their applications. & 2015 Elsevier B.V. All rights reserved.
Keywords: Cuprous oxide Porous materials Semiconductors Crystal growth Photocatalytic property
1. Introduction
2. Experimental
As an important p-type semiconductor, Cu2O has a direct small band gap of 2.2 eV [1] and is considered to be a promising material in solar cells conversion [2–4] and photocatalysis areas [5]. Up to now, Cu2O has been prepared by several different methods, such as electrodeposition [6], thermal relaxation [7], sonochemical methods [8], vacuum evaporation [9], and the liquid-phase reduction [10]. Meanwhile, various morphologies of Cu2O, such as nanowires [11], octahedral [12], hollow spheres [13], nanocubes [14], and nanorods [15] have been prepared. However, it remains a great challenge to develop simple and feasible approaches for the shape-controlled synthesis of well-defined Cu2O architectures. Here we report a new method of the synthesis and characterization of porous Cu2O dodecahedron crystals. The synthesis was performed using a solvothermal reduction process at a moderate temperature. The adsorption ability and photocatalytic activity of the samples were evaluated by the photocatalytic degradation of MB aqueous solution under UV/visible light illumination.
A typical synthesis procedure of the porous Cu2O dodecahedron nanostructures was as follows: 2 mmol (0.3993 g) of copper acetate (Cu(Ac)2·H2O) was dispersed in 25 mL of N,N-dimethylformamide (DMF) under vigorous stirring at room temperature, followed by addition of 1 g of polyvinyl pyrrolidone (PVP). After being stirred vigorously for 10 min, the mixed solution was put into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 170 °C for 8 h and then cooled to room temperature naturally. The resulting products were centrifuged, washed several times with distilled water and anhydrous ethanol, and dried at 80 °C in a vacuum for 6 h. The photocatalytic experiments were carried out by adding 100 mg of Cu2O samples into 100 mL of MB aqueous solution with the concentration of 10 mg L 1. The suspension was stirred for 30 min in the dark to obtain adsorption equilibrium. Then, the suspension was irradiated with a 365 nm UV-light. As a comparison, the visible-driven photocatalytic experiments were also carried out. The concentration of MB was detected by a UV–vis spectrophotometer.
n Corresponding author at: College of Environment and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China. Fax: þ 86 22 83955762. nn Corresponding author at: State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin 300387, PR China. E-mail addresses: [email protected] (D. Chu), [email protected] (L. Wang).
http://dx.doi.org/10.1016/j.matlet.2015.06.098 0167-577X/& 2015 Elsevier B.V. All rights reserved.
3. Results and discussion Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provided insight into the morphology of the
L. Yang et al. / Materials Letters 159 (2015) 172–176
173
Fig. 1. (a) SEM images of the porous Cu2O dodecahedron nanocrystals (the inset shows the close observation of the product); (b) TEM image of the Cu2O nanostructure; (c) a representative XRD pattern recorded for Cu2O dodecahedron architectures; and (d) N2 adsorption–desorption isotherm and BJH pore size distribution plots (inset) of the porous Cu2O dodecahedron nanostructures.
Cu2O crystals. A panoramic morphology of the product is displayed in Fig. 1(a), indicating the high yield and uniformity. A magnified SEM image showing the close observation of the nanostructures is given in the illustration in Fig. 1(a). It reveals that the detailed morphology of Cu2O product is well-defined porous dodecahedron with diameters of 750 nm and the surface of Cu2O nanostructures are made of numerous nanoparticals. As shown by the arrows in Fig. 1(b), the surface of the Cu2O nanocrystal was covered with lots of pores. The crystal phase of Cu2O is characterized by XRD, and the data are shown in Fig. 1(c). The characteristic peaks of the cubic phase of Cu2O crystals (JCPDS 05-0667) were observed. No peaks of impurity are detected in the XRD pattern, indicating the formation of pure Cu2O under these experimental conditions. To further confirm the surface architectures of the as-prepared Cu2O samples, N2 adsorption and desorption measurements were performed. As shown in Fig. 1(d), the isotherm of the Cu2O samples exhibit a hysteresis loop at p/p0 range of 0.8–1.0. This phenomenon clearly indicates that the Cu2O samples exhibit a large textural porosity [12]. The pore size distribution of the Cu2O shows that a narrow peak appeared in the pore size region from 2.1 to 200.8 nm. The material had an average pore size of 20.6 nm. The Brunauer–Emmett–Teller (BET) surface area of the Cu2O nanocrystals was calculated to be 15.4 m2 g 1, which is much higher than that of the Cu2O intermidates (5.3, 7.9, 8.6, and 10.3 m2 g 1 after solvothermal treatment for 2, 4, 6, and 10 h, respectively). Fig. 2 illustrates the morphology evolution of Cu2O at different stages. Initially, Cu2 þ is reduced by DMF to generate Cu2O particles, and then Cu2O nanospheres (Fig. 2(a) and (b)) are grown by self-assembly of small primary nanoparticles through an orientedattachment mechanism after solvothermal treatment for 2 h. In
this stage, the morphology evolution of Cu2O nanostructures had not been completed, meanwhile some Cu2O crystallites grew along their specific crystallographic planes and eventually evolved into the original polyhedral nanostructures (as shown in the black dotted bordered rectangles of Fig. 2(a)). Seed particles continuously adsorb onto these nanospheres to allow further growth through a ripening process. Then the intermediate structures develop into structurally well-defined dodecahedron nanostructures (Fig. 2(c) and (d)) via a surface reconstruction process in 4 h. The growth control of Cu2O crystals by adsorption of PVP has been extensively studied [16]. The PVP molecules should be adsorbed preferentially on the {111}, which helps to form unique structure of Cu2O. After conducting the reaction for 6 h, Cu2O dodecahedron with rough surface were constructed (Fig. 2(e)). A close-up view of Cu2O nanostructure in Fig. 2(f) demonstrates that the surfaces of Cu2O crystals present many tiny holes. Further prolonging the solvothermal time to 8 h, we got perfect porous Cu2O dodecahedron nanostructures (Fig. 1(a)). It is well known that the structures of products prepared by solution reactions depend on the rate of nucleation and growth of the reaction products. The results show that the rate of growth of Cu2O is faster than that of its nucleation. Due to the preferential growth of Cu2O in a certain direction, the porous dodecahedron-like products were obtained. Further observation (inset in Fig. 1(a) and (b)) revealed that a large amount of irregular pores of tens of nanometers was randomly distributed in the nanocrystals, as a result of the dissolution-recrystallization process. Surprisingly, with the extension of the reaction time to 10 h, lots of broken dodecahedron-like Cu2O particles can be discerned from Fig. 2(g) and (h), which is the result of increasing rate of the dissolution of Cu2O nanocrystals, probably. To evaluate the photocatalytic activity of the products, we
174
L. Yang et al. / Materials Letters 159 (2015) 172–176
Fig. 2. SEM images of the Cu2O samples obtained after solvothermal treatment for different time: (a) t¼ 2 h; (c) t¼ 4 h; (e) t¼ 6 h; and (g) t ¼10 h ((b), (d), (f), and (h) are magnified images of (a), (c), (e), and (g), respectively).
measured the optical property changes of MB aqueous solution in the presence of Cu2O dodecahedron or other Cu2O intermediates under irradiation of UV/visible light for a given time, respectively. The time-dependent absorption spectra of MB solution containing Cu2O catalyst during the irradiation are illustrated in Fig. 3(a and
c). It can be seen that the maximum absorbance at 665 nm decreases rapidly with irradiation time as shown in Fig. 3(a and c). After it had been irradiated under UV light for 90 min, the degradation rate of MB is 98.3%, indicating that most of the amount of MB was degraded (Fig. 3(a)). However, the degradation rate of
L. Yang et al. / Materials Letters 159 (2015) 172–176
175
Fig. 3. (a and c) Absorbance spectra change of the MB in aqueous solution (10 mg L 1, 100 mL) in the presence of Cu2O dodecahedron nanocrystals under UV/visible light irradiation; (b and d) Photodegradation plots of MB under UV/visible light irradiation for different time in the presence/absence of catalysts (solvothermal treatment for 2, 4, 6, 8 and 10 h, respectively, and commercial Cu2O as control sample); and (e) UV–vis absorbance spectra of the Cu2O samples prepared for different solvothermal time (2, 4, 6, 8 and 10 h, respectively).
MB is only 90.7%, after it had been irradiated under visible-light for the same time (Fig. 3(c)). It reveals that UV irradiation will enhance photodegradation of MB. Possibly because the energy of UV light is stronger than visible-light, which could improve the photocatalytic efficiency. The result also indicates that Cu2O shows great potential as a visible-light-driven photocatalyst, otherwise, pure TiO2 catalyst take effect only under UV light. By monitoring the MB absorption peak at 665 nm, plots of the degradation ratio versus reaction time were obtained for different Cu2O samples (obtained after solvothermal treatment for 2, 4, 6, 8, and 10 h, respectively, and commercial Cu2O as control sample) under identical conditions (Fig. 3(b and d)). It clearly shows that the Cu2O
nanocrystals and other Cu2O products are effective photocatalysts for the direct degradation of MB than commercial Cu2O sample. Due to its vastly surface area, the catalytic properties of the porous Cu2O dodecahedron nanostructures are better than those of other Cu2O intermediates. The morphology of the Cu2O sample obtained after solvothermal treatment for 10 h is irregular. In addition, its BET surface area is smaller than that of the porous Cu2O dodecahedron nanostructure (solvothermal treatment for 8 h). This leads to its low utilization rate of UV/visible light, which explains the reason for its lower photocatalytic activity. The UV–vis absorption spectra (Fig. 3(e)) suggested that the intensity of sample I was much stronger. The order of UV–vis
176
L. Yang et al. / Materials Letters 159 (2015) 172–176
absorption intensities was as follows: samples I 4II4 III4IV 4V. The photocatalytic activities of Cu2O samples obeyed the same order. It is suggested that the photocatalytic activities of the Cu2O samples had positive correlation with the UV–vis absorption intensity. Among the five samples, sample I had the highest UV–vis absorption intensity, so the UV/visible light utilization efficiency of sample I was highest. It could explain the high photocatalytic activity of sample I. In conclusion, apart from the total surface area (the main factor), UV–vis absorption intensity and utilization efficiency of UV/visible light also contributed to the photocatalytic activities.
4. Conclusions In summary, a moderate chemical solution method was demonstrated for the synthesis of the porous Cu2O dodecahedron nanostructures. The simple synthesis approach casts new light on the controllable fabrication of Cu2O porous architectures. Furthermore, the photocatalysis measurements show that the porous Cu2O dodecahedron nanocrystals exhibit outstanding photocatalytic properties for the degradation of MB under UV/visiblelight mainly due to the unique porous nanostructures.
References [1] M. Shen, T. Yokouchi, S. Koyama, T. Goto, Dynamics associated with Bose– Einstein statistics of orthoexcitons generated by resonant excitations in cuprous oxide, Phys. Rev. B 56 (1997) 13066. [2] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, G.K. Paul, T. Sakurai, Thin film deposition of Cu2O and application for solar cells, Sol. Energy 80 (2006) 715–722.
[3] T. Mahalingam, J.S.P. Chitra, J.P. Chu, H. Moon, H.J. Kwon, Y.D. Kim, Photoelectrochemical solar cell studies on electroplated cuprous oxide thin films, J. Mater. Sci.: Mater. Electron. 17 (2006) 519–523. [4] D. Tryk, A. Fujishima, K. Honda, Recent topics in photoelectrochemistry: achievements and future prospects, Electrochim. Acta 45 (2000) 2363–2376. [5] Y.-G. Zhang, L.-L. Ma, J.-L. Li, Y. Yu, In situ Fenton reagent generated from TiO2/Cu2O composite film: a new way to utilize TiO2 under visible light irradiation, Environm. Sci. Technol. 41 (2007) 6264–6269. [6] L. Huang, H. Wang, Z. Wang, A. Mitra, D. Zhao, Y. Yan, Cuprite nanowires by electrodeposition from lyotropic reverse hexagonal liquid crystalline phase, Chem. Mater. 14 (2002) 876–880. [7] S. Deki, K. Akamatsu, T. Yano, M. Mizuhata, A. Kajinami, Preparation and characterization of copper (I) oxide nanoparticles dispersed in a polymer matrix, J. Mater. Chem. 8 (1998) 1865–1868. [8] R.V. Kumar, Y. Mastai, Y. Diamant, A. Gedanken, Sonochemical synthesis of amorphous Cu and nanocrystalline Cu2O embedded in a polyaniline matrix, J. Mater. Chem. 11 (2001) 1209–1213. [9] H. Yanagimoto, K. Akamatsu, K. Gotoh, S. Deki, Synthesis and characterization of Cu2O nanoparticles dispersed in NH2-terminated poly(ethylene oxide), J. Mater. Chem. 11 (2001) 2387–2389. [10] S. Ram, C. Mitra, Formation of stable Cu2O nanocrystals in a new orthorhombic crystal structure, Mater. Sci. Eng. A 304 (2001) 805–809. [11] M. Hafez, F. Al-Marzouki, W.E. Mahmoud, Single crystalline quasi aligned one dimensional P-type Cu2O nanowire for improving Schottky barrier characteristics, Mater. Lett. 65 (2011) 1868–1870. [12] X. Zhang, G. Wang, H. Wu, D. Zhang, X. Zhang, P. Li, et al., Synthesis and photocatalytic characterization of porous cuprous oxide octahedra, Mater. Lett. 62 (2008) 4363–4365. [13] H. Zhang, Q. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, One-pot synthesis and hierarchical assembly of hollow Cu2O microspheres with nanocrystals-composed porous multishell and their gas-sensing properties, Adv. Funct. Mater. 17 (2007) 2766–2771. [14] L. Gou, C.J. Murphy, Solution-phase synthesis of Cu2O nanocubes, Nano Lett. 3 (2003) 231–234. [15] Z. Cheng, J. Xu, H. Zhong, X. Chu, J. Song, Repeatable synthesis of Cu2O nanorods by a simple and novel reduction route, Mater. Lett. 65 (2011) 1871–1874. [16] d-F. Zhang, H. Zhang, L. Guo, K. Zheng, X.-D. Han, Z. Zhang, Delicate control of crystallographic facet-oriented Cu2O nanocrystals and the correlated adsorption ability, J. Mater. Chem. 19 (2009) 5220.