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Nanocrystals with Different Morphologies and Photocatalytic Activities
J. Mater. Sci. Technol., 2011, 27(4), 289-295.
Microwave Synthesis of Cuprous Oxide Micro-/Nanocrystals with Different Morphologies and Photocatalytic Activities Qingwei Zhu1) , Yihe Zhang1,3)† , Jiajun Wang2) , Fengshan Zhou1) and Paul K. Chu3) 1) State Key Laboratory of Geological Processes & Mineral Resources, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China 2) Department of Powder Metallurgy and Special Materials, General Research Institute of Non-Ferrous Metal, Beijing 100088, China 3) Department of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China [Manuscript received September 21, 2010, in revised form December 11, 2010]
Cuprous oxide micro-/nanocrystals were synthesized by using a simple liquid phase reduction process under microwave irradiation. Copper sulfate was used as the starting materials and macromolecule surfactants served as the templates. The morphologies phase and optical properties of them are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible diffuse reflection absorptive spectra (UV-vis/DRS), respectively. The crystals had four different shapes, namely spheres, strips, octahedrons, and dandelions. The photocatalytic behavior of the cuprous oxide particles were investigated by monitoring the degradation of rhodamine B. In spite of the different morphologies, all of the cuprous oxide micro-/nanocrystals exhibited photocatalytic activities under visible light irradiation in the following order: dandelions, strips, spheres, and octahedral crystals. The photocatalytic degradation rates of rhodamine B are 56.37%, 55.68%, 51.83% and 46.16%, respectively. The morphology affects significantly the photocatalytic performance.
KEY WORDS: Cuprous oxide; Synthesis; Morphology; Photocatalytsis
1. Introduction As a transition metal oxide, cuprous oxide has been studied extensively for its distinctive properties. It is a p-type semiconductor with a direct band gap of ca. 2.2 eV, which can be activated by irradiation of visible light. Hence, it has been used as a visible-light driven photocatalyst to break down organic contamination in water[1–5] as well as solar cells[6–10] . Moreover, cuprous oxide can be used in smoke suppression, thermal degradation of poly-vinyl-chloride [11] , and dissociation of nitrous oxide [12] due to its low toxicity and good environmental acceptability. Cuprous oxide crystals with different morphologies possess different physical and chemical properties. Much effort has been made to synthesize cuprous oxide crystals with different shapes such as octahedron, whisker, cactus, box, cube, sphere, wire, tube, rod, bi-pyramid, star, † Corresponding author. Prof., Ph.D.; Tel./Fax: +86 10 82323433; E-mail address: [email protected] (Y.H. Zhang).
and flower[13–22] . The morphology and size often affect significantly the physical and chemical property of crystal. Therefore, there have been some studies on the photocatalytic activities of the crystals with different shapes and/or sizes[17, 23–27] . In this work, a simple method is designed to synthesize four types of cuprous oxide micro/nanocrystals using copper sulfate as the starting materials, glucose and hydrate hydrazine as the reducing agents, and macromolecule surfactants as the templates. The phase, morphologies and optical properties of four types of crystals were characterized by XRD, SEM, and UV-vis/DRS. Their photocatalytic activities are determined by monitoring the photocatalytic degradation of rhodamine B. 2. Experimental 2.1 Synthesis of samples All the reagents except copper sulphate were
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Table 1 Reaction materials and heating time in the synthesis of the samples and the specific surface areas (SBET ) of samples No. 1 2 3 4
CuSO4 CuSO4 CuSO4 CuSO4
NH3 H2 O NaOH NaOH NaOH
Reactants glucose PVP N2 H4 P(VP-NVP-St) N2 H4 P(NVP-St) N2 H4 P(VP-NVP)
analytical grade and used without further purification. Copper sulfate was purified via recrystallization and water used in these experiments was deionized. The macromolecule surfactants such as poly-vinyl-pyridine(PVP), copolymer of vinyl pyridine, vinyl pyrrolidone and styrene{P(VP-NVP-St)}, copolymer of vinyl pyrrolidone and styrene {P(NVPSt)}, and copolymer of vinyl pyridine and vinyl pyrrolidone{P(VP-NVP)} were synthesized in the laboratory using radical polymerization with ammonium peroxydisulfate as initiator and their average molecular weight are 7,000–14,000. To synthesize the cuprous oxide microcrystals with spherical structure, 50.0 mL of 0.5 mol/L copper sulfate solutions and 1.0 mL of 5% (M/V) PVP were mixed in a beaker at room temperature. Under constant magnetic stirring, 15.0 mL of 5.0 mol/L ammonia solution was added to the above solution quickly. Then, 20.0 mL of 2.5 mol/L glucose was added and red. The beaker was heated in a microwave oven and the solution boiled constantly for 30 min at 600 W. Finally, the precipitates were filtrated and washed with deionized water and absolute ethanol and dried in a vacuum dryer at 80o C for 2 h. To synthesize the cuprous oxide crystals with the other three types of shapes, 50.0 mL of 0.5 mol/L copper sulfate solutions and 1.0 mL of 5% (M/V) macromolecule surfactants were mixed in a beaker at room temperature. For dandelion-shaped crystals, 50 mL of 1 mol/L tartaric acid was also added. Under constant magnetic stirring, after adding 50.00 mL of 1.0 mol/L sodium hydroxide quickly, 5.0 mL of 1.0 mol/L hydrazine hydrate was added to the above solution. Then the beaker was moved to a microwave oven and heated at 600 W. Followed by boiling constantly for 5 min, the mixtures were separated by filtrating and the residues were washed with deionized water and absolute ethanol and dried in a vacuum dryer at 80◦ C for 2 h. Different crystal shapes were obtained by changing the reactants and heating time in the synthesis process as illustrated in Table 1 in detail. 2.2 Characterizations of samples The crystal phase of the synthesized particles was identified by X-ray diffraction (XRD, Rigaku D/Max2000) using CuKα radiation (2 kV rotating anode, λ=0.154056 nm). The samples were scanned from 10◦ to 110◦ at a scanning rate of 0.06◦ /s. The microstruc-
– – – tartaric acid
Time 30 min 5 min 5 min 5 min
Morphology sphere strip octahedron dandelion
SBET /(m2 /g) 29.70 51.70 41.94 54.06
tures and morphologies of the particles were determined by scanning electron microscopy (SEM, Hitachi S-4300). The samples were coated with gold to improve the conductivity during SEM observation. The specific surface area of the particles was determined by an automated surface area and pore size analyzer (Autosorb-1, Quantachrome, USA) using the nitrogen adsorption. The ultraviolet-visible diffuse reflection absorptive spectra (UV-vis/DRS) were acquired on a UV-visible spectrophotometer (Hitachi-UV3010) equipped with an integration sphere at room temperature. 2.3 Photocatalytic degradation of rhodamine B To assess the photocatalytic activity of various cuprous oxide micro-/nanocrystals, the photocatalytic reaction was conducted in a 250 mL glass beaker in a reactor equipped with a magnetic force stirrer and circular cooling copper tube. A 500 W tungsten halogen lamp without filter chip was used as the visible light source. 0.5 g of cuprous oxide powders were dispersed in 200 mL of 5 mg/L rhodamine B aqueous solution. The suspension was sonicated for 1 h in darkness to disperse the powders well in the solution before irradiation with visible light. At regular intervals of 1 h, 25 mL of the suspension was sampled and separated by centrifugation at 12,000 r/min for 10 min. The concentration of remaining rhodamine B was determined by measuring their spectroscopy in the wavelength of 250–700 nm using a UV-vis spectrophotometer (Hitachi UV-3010). 3. Results and Discussion 3.1 SEM images of the samples Figure 1 illustrates the SEM images revealing four kinds of crystal morphologies. And the specific surface areas of various cuprous oxide crystals are shown in Table 1. The specific surface areas of the as synthesized cuprous oxide crystals with sphere, strip octahedron and dandelion shape are 29.70, 51.70, 41.94 and 54.06 m2 /g, respectively. In Fig. 1(a), the crystals are spherical and the size is about 2∼12 μm. The spherical crystals are synthesized in an ammonia solution with PVP as the template and glucose as the reducing agent. Figure 1(b) discloses strip-shaped crystals with a thickness of about 20 nm and width of 300∼400 nm. The strip-shaped crystals are obtained from a sodium hydroxide solution with P(VP-NVP-St) as the tem-
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291
Fig. 1 SEM images of cuprous oxide with different morphologies: (a) sphere, (b) strip, (c) octahedron, (d) dandelion
plate and hydrazine hydrate as the reductant. The octahedral cuprous oxide particles shown in Fig. 1(c) are obtained using P(NVP-St) as the template whereas the dandelion-shaped cuprous oxide crystals in Fig. 1(d) are synthesized using P(VP-NVP) as the template. The SEM results suggest that it is possible to control the morphology of the cuprous oxide crystals by changing reaction conditions, among which the self-assembly-induced template is significantly important. In the aqueous solution, as a result of their amphiphilic nature, surfactant molecules acting as templates often spontaneously self-assemble into microscopic structures shaped spheres, cylinders, vesicles, and membranes depending on outside factors[28] . During the process of synthesis of materials with nanometer or submicrometer scale, the particles growing on the self-assembly-induced template are often affected by such factors as liquid interfacial tension, capillary force, and different hydrophilic or hydrophobic forces. And selective adsorption of ions or molecules onto different surfaces also affects the particle shape. Any of these factors can become the key controlling parameter for the resulting morphologies[29] . In this study, the formation of the spherical crystals may be resulted from the effect of PVP template. Because ammonia solution exists, cupric tetraammino ions would generate before they were reduced. Cupric tetraammino ions combined with PVP to selfassemble into spherical microscopic structure in the aqueous solution due to interfacial tension and hydrophilic and hydrophobic forces. During the process of reducing, crystallographic planes of cuprous oxide
microcrystals were affected by PVP in the interface to form spherical structure. For strip-shaped crystals, it is possible that the longitudinal growth of cuprous oxide nanocrystals was postponed because of self-assembly membrane spatial structure of P(VPNVP-St) in the aqueous solution. The formation of Cu2 O octahedra can be explained as the corporate effect coming from the growth units of anion coordinative polyhedra theoretical model and the polymer selective adsorption. According to the growth units of anionic coordinative polyhedra theoretical model, there are two anion coordinative polyhedra in Cu2 O. These two coordinative polyhedra connect each other and constitute the different growth unit with the different dimension and the dimensions of the growth units can vary with the change of the growth condition[13] . In our experiments, when P(NVP-St) was introduced, due to the selective interaction between P(NVP-St) and various crystallographic planes of cuprous oxide, the growth along the [111] direction could be greatly postponed but the growth along the (100) direction could be enhanced. So the (111) faces preferentially appear and the octahedra are obtained. For dandelion-shaped structure, we suggest that it is resulted from self-assembly of nanosized cuprous oxide crystallites with the assistance of P(VP-NVP) and complexing agent tartaric acid. In aqueous solution, copper ion can combine tartaric acid to form coordination complexes with polyhydroxy compounds. When copper ions were reduced to cuprous oxide crystallites by hydrazine hydrate, the growth of various faces of these crystallites was controlled in the interface with P(VP-NVP) to produce variform tiny polyhedral par-
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Fig. 2 XRD patterns of (a) spherical, (b) strip-shaped, (c) octahedral and (d) dandelion-shaped cuprous oxide crystals
ticles. And these variform tiny polyhedral particles agglomerated into dandelion-shaped structure due to their high surface free energy. 3.2 XRD patterns of the samples Figure 2 depicts the XRD patterns of the products which possess a cubic structure which can be indexed according to the JCPDS card No. 05-0667. Five characteristic peaks are observed at 2θ values of 29.6◦ , 36.5◦ , 42.4◦ , 61.5◦ , 73.7◦ , and 77.6◦ corresponding to the crystal planes of (110), (111), (200), (220), (311) and (222) of crystalline cuprous oxide, respectively. The other characteristic peaks caused by impurities of copper oxide, such as 35.5◦ , 38.7◦ , 48.7◦ , are found from the strip-shaped cuprous oxide crystals (Fig. 2(b)). According to JCPDS card No.45-0937, they correspond to the planes of (002), (111) and (¯202) of crystalline copper oxide, respectively. Because of the large specific surface area, a small quantity of cuprous oxide particles on the surface is oxidized to copper oxide during synthesis process. In curves (a), (c), and (d) in Fig. 2, no noise peaks can be observed and it can be inferred that the synthesized spherical, dandelion-shaped, and octahedral cuprous oxide crystals have high purity. These three kinds of crystals were not oxidized during synthesis process, which is possibly due to the protection of surfactants coating in the surface of the particles, while the surfactants with membrane structure coating in the surface of strip-shaped cuprous oxide nanocrystals were more likely to be partially broken. 3.3 Optical properties of the samples The optical response of the samples was characterized by using UV–vis diffuse reflectance spectrometry (UV-vis/DRS). The UV-vis/DRS of the cuprous oxide samples are shown in Fig. 3(a). All of the samples show a clear absorption edge at 650 nm. The maxi-
Fig. 3 (a) UV-vis/DRS of the as prepared cuprous oxide crystals; (b) plot of (αhν)2 vs hν of various cuprous oxide micro-nanocrystals
mum absorbance values of the spherical, strip-shaped, octahedral, and dandelion-shaped cuprous oxide microcrystals are at 410 nm, 427 nm, 453 nm, and 453 nm, respectively and in the visible range. The cuprous oxide crystals with different morphologies exhibit different sensitivities to visible lights and the dandelionshaped cuprous oxide crystal has the strongest absorbency and the octahedral one has the weakest one. It is possibly because dandelion-shaped cuprous oxide crystals have a porous structure and active crystallite facets and thus light reflection is reduced. In comparison, the inactive facets of the octahedral cuprous oxide crystals reflect most of the incoming irradiation and the absorbency is subsequently the smallest. The strip-shaped cuprous oxide crystals also show higher absorbance than the spherical and octahedral ones because the particle is tiny and has larger specific surface. Our results also suggest that optical absorption is often affected by the morphology of the crystals[30] . Since cuprous oxide is a P-type direct gap semiconductor[31] , the relationship between absorption coefficient (α) and photon energy (hν) can be expressed as[32] : 1
αhν = B(hν − Eg ) 2
(1)
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Fig. 4 Absorbance of remaining rhodamine B degraded at different time in the wavelength of 553 nm. No light irradiation in the first hour
In this equation, B is a constant which does not depend on photo energy and Eg is the band gaps energy. The band gap can be estimated by the intercepts of the tangents to the (αhν)2 vs photon energy (hν) plots. Plots of (αhν)2 vs hν for cuprous oxide samples with different morphologies are shown in Fig. 3(b). The direct band gaps of spherical, strip-shaped, octahedral and dandelion-shaped cuprous oxide micro/nanocrystals are estimated to be 2.10, 2.06, 2.15 and 2.04 eV, respectively. It thus can be inferred that cuprous oxide can be activated by visible light. The red shift of the direct band gaps displays the effect of the morphologies of crystals. The microcrystals with different morphologies have different dominant active facets and will response different excitation energy and consequently have different direct band gaps. 3.4 Photocatalytic activities of the samples The photocatalytic activities of the cuprous oxide crystals with different morphologies are evaluated by monitoring the decomposition of rhodamine B in an aqueous solution under visible light irradiation. Direct decomposition of rhodamine B without cuprous oxide cannot almost be detected under visible light irradiation in our control experiment. The decreasing absorbance of remaining rhodamine B degraded directly is depicted in Fig. 4 which exhibits that the absorbance does not almost change after being illuminated for 4 h. From Fig. 4, rhodamine B is degraded to different degrees through visible-light driven photocatalysis by the various cuprous oxide crystals. On the other hand, the absorbance of rhodamine B decreases slightly without visible light irradiation due to the small absorption of rhodamine B by the cuprous oxide crystals. As rhodamine B has a strongest absorbance in 553 nm, it is rational to obtain the degradation rate by the following equation.
293
Fig. 5 Photocatalytic degradation rate of rhodamine B by various cuprous oxide crystals
Di =
A0 − Ai × 100% A0
(2)
where Di (%) is the degradation rate when irradiation time is i hours, A0 the initial absorbance and Ai is the absorbance when irradiation time is i hours in the wavelength of 553 nm. The degradation rates of rhodamine B by different cuprous oxide crystals when irradiation time is 4 h was described in Fig. 5. It reveals that under visible light irradiation, 56.37%, 55.68%, 51.83% and 46.16% of rhodamine B (monitored at 553nm) can be decomposed by dandelion-shaped, strip-shaped, spherical and octahedral cuprous oxide crystals, respectively. It can be inferred that all the cuprous oxide crystals synthesized have some photocatalytic activities which are much higher than those reported in the previous literature[33,34] and the shape factor seems to be of overriding importance to photocatalytic activities. The different photocatalytic activities of the cuprous oxide crystals can be explained by reaction activities. According to the literature reported by Xu et al.[35] and Zhang et al.[36] , the (111) facets are more active than (100) facets due to the dangling bonds of (111) surfaces, while no dangling bands but saturated chemical bonds exist in the (100) facets. So the cuprous oxide crystals with dominant (111) facets have a higher adsorption and photocatalytic activity than those with dominant (100) surfaces and (100) surfaces. Due to their dominant (110) facet, the octahedral cuprous oxide crystals show the lowest photocatalytic activities among these four kinds of crystals though they have a larger specific surface area (41.94 m2 /g) comparing to the spherical Cu2 O crystals (29.70 m2 /g). The dandelion-shaped Cu2 O crystals are composed of lots of tiny polyhedral crystals with dominant (111) surfaces by aggregation and have a largest specific surface area (54.06 m2 /g), so they exhibit the best activities in the photocatalytic degradation of rhodamine B. For strip-shaped and spherical Cu2 O crystals, the dominant facets also are of
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the (111) type, but they display somewhat lower photocatalytic activities than dandelion-shaped ones because of their lesser specific surface (51.70 and 29.70 m2 /g, respectively). Also, the stability of surface of crystal is an important factor of photocatalytic activity. The strip-shaped Cu2 O crystal was oxidized slightly on the surface resulting in its relatively lower catalytic activity. On the other hand, the shape of the crystals affects the absorbencies and direct band gaps and then impacts on the photocatalytic performance due to the different activity of the dominant facet. The specific surface dominated by the structural and crystal size is an important factor affecting the reaction activity. High absorbency in the visible region results in the visible-light induced photocatalytic activity[37] . In general, the higher the absorbency of the photocatalyst, the higher the utilization of light as well as electron-hole pairs generated by excited electrons. The potential of the photocatalytic redox activity is improved and the catalytic activity is increased. The dandelion-shaped cuprous oxide crystals with the active dominant (111) facets have a much stronger absorbency and smaller direct band gap (∼2.04 eV), so they exhibit much higher activity in the photocatalytic degradation of rhodamine B. The decreasing of the absorbencies and direct band gaps of strip-shaped, spherical and octahedral Cu2 O crystals in order result in the diminution of their photocatalytic activities accordingly. As a result, the different morphologies of cuprous oxide crystals lead to the different specific surface areas, direct band gaps and UV-vis absorbencies which cause the different photocatalytic activities. 4. Conclusion Spherical, strip-shaped, octahedral, and dandelion-shaped cuprous oxide micro-/nanocrystals are synthesized via simple reduction of copper sulfate using different reducing agents and macromolecule surfactants under microwave irradiation. The macromolecule surfactants acting as the templates play an important role in controlling the morphologies of the resulting cuprous oxide crystals. Different reducing agents and alkaline solutions also influence the morphology of the cuprous oxide crystals. It is possible to control the morphology of the cuprous oxide crystals by changing the reducing agents, templates, and alkaline solution. Investigation of the photocatalytic activity shows that dandelion-shaped cuprous oxide crystals have the highest activity and can degrade 56.37% of rhodamine B, the octahedral cuprous oxide crystals exhibit the lowest activity and can only degrade 46.16% of rhodamine B, and the strip-shaped and spherical cuprous oxide crystals can decompose 55.68% and 51.83% of rhodamine B, respectively. Different morphology results in different photocatalytic activities of cuprous oxide crystals under visible light
illumination due to different active dominant facets, direct band gaps, specific surface areas and absorbencies. It is suggested to improve the photocatalytic activities by tuning the morphologies of cuprous oxide crystals. Acknowledgements This study was supported by the Open Foundation of National Laboratory of Mineral Materials of China University of Geosciences (Grant No. 08A006), the Key Project of Chinese Ministry of Education (No. 107023), Special Fund of Co-construction of Beijing Education Committee, and City University of Hong Kong Strategic Research Grant (SRG) No. 7008009. REFERENCES [1 ] H.M. Yang, J. Ouyang, A.D. Tang, Y. Xiao, X.W. Li, X.D. Dong and Y.M. Yu: Mater. Res. Bull., 2006, 41, 1310. [2 ] C.C. Hu, J.N. Nian and H. Teng: Sol. Energy Mater. Sol. Cells, 2008, 92, 1071. [3 ] J.N. Nian, C.C. Hu, and H. Teng: Int. J. Hydrogen Energ., 2008, 33, 2897. [4 ] S. Kakuta and T. Abe: Solid State Sci., 2009, 11, 1465. [5 ] C.H. Han, Z.Y. Li and J.Y. Shen: J. Hazard. Mater., 2009, 168, 215. [6 ] Y. Hames and S.E. San: Sol. Energy, 2004, 77, 291. [7 ] K. Han and M. Tao: Sol. Energy Mater. Sol. Cells, 2009, 93, 153. [8 ] V. Georgieva and M. Ristov: Sol. Energy Mater. Sol. Cells, 2002, 73, 67. [9 ] S.S. Jeong, A. Mittiga, E. Salza, A. Masci and S. Passerini: Electrochim. Acta, 2008, 53, 2226. [10] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, G.K. Paul and T. Sakurai: Sol. Energy, 2006, 80, 715. [11] B. Li: Polym. Degrad. Stabil., 2001, 74, 195. [12] B.Z. Sun, W.K. Chen, X. Wang and C.H. Lu: Appl. Surf. Sci., 2007, 253, 7501. [13] X.J. Zhang, G.F. Wang, A.X. Gu, H.Q. Wu and B. Fang: Solid State Commun., 2008, 148, 525. [14] Y. Yu, F.P. Du, J. C. Yu, Y.Y. Zhuang and P.K. Wong: J. Solid State Chem., 2004, 177, 4640. [15] Y.L. Qu, X.Y. Li, G.H. Chen, H.J. Zhang and Y.Y. Chen: Mater. Lett., 2008, 62, 886. [16] Y.S. Luo, B.H. Yu, Y. Tu, Y. Liang, Y.G. Zhang, J.P. Liu and Z.J. Jia: Mater. Res. Bull., 2008, 43, 2166. [17] L. Huang, F. Peng, H. Yu and H.J. Wang: Solid State Sci., 2009, 11, 129. [18] X.Y. Liu, R.Z. Hu, S.L. Xiong, Y.K. Liu, L.L. Chai, K.Y. Bao and Y.T. Qian: Mater. Chem. Phys., 2009, 114, 213. [19] H.S. Shin, J.Y. Song and J. Yu: Mater. Lett., 2009, 63, 397. [20] M.H. Cao, C.W. Hu, Y.H. Wang, Y.H. Guo, C.X. Guo and E.B. Wang: Chem. Commun., 2003, (15), 1884. [21] G. Jimenez-Cadena, E. Comini, M. Ferroni and G. Sberveglieri: Mater. Lett., 2010, 64, 469. [22] Z.H. Liang and Y.J. Zhu: Mater. Lett., 2005, 59, 2423. [23] X.J. Zhang, G.F. Wang, H.B. Wu, D.Zhang, X.Q. Zhang, P. Li and H.Q. Wu: Mater. Lett., 2008, 62, 4363.
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Microwave Synthesis of Cuprous Oxide Micro-/Nanocrystals with Different Morphologies and Photocatalytic Activities Qingwei Zhu1) , Yihe Zhang1,3)† , Jiajun Wang2) , Fengshan Zhou1) and Paul K. Chu3) 1) State Key Laboratory of Geological Processes & Mineral Resources, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China 2) Department of Powder Metallurgy and Special Materials, General Research Institute of Non-Ferrous Metal, Beijing 100088, China 3) Department of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China [Manuscript received September 21, 2010, in revised form December 11, 2010]
Cuprous oxide micro-/nanocrystals were synthesized by using a simple liquid phase reduction process under microwave irradiation. Copper sulfate was used as the starting materials and macromolecule surfactants served as the templates. The morphologies phase and optical properties of them are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible diffuse reflection absorptive spectra (UV-vis/DRS), respectively. The crystals had four different shapes, namely spheres, strips, octahedrons, and dandelions. The photocatalytic behavior of the cuprous oxide particles were investigated by monitoring the degradation of rhodamine B. In spite of the different morphologies, all of the cuprous oxide micro-/nanocrystals exhibited photocatalytic activities under visible light irradiation in the following order: dandelions, strips, spheres, and octahedral crystals. The photocatalytic degradation rates of rhodamine B are 56.37%, 55.68%, 51.83% and 46.16%, respectively. The morphology affects significantly the photocatalytic performance.
KEY WORDS: Cuprous oxide; Synthesis; Morphology; Photocatalytsis
1. Introduction As a transition metal oxide, cuprous oxide has been studied extensively for its distinctive properties. It is a p-type semiconductor with a direct band gap of ca. 2.2 eV, which can be activated by irradiation of visible light. Hence, it has been used as a visible-light driven photocatalyst to break down organic contamination in water[1–5] as well as solar cells[6–10] . Moreover, cuprous oxide can be used in smoke suppression, thermal degradation of poly-vinyl-chloride [11] , and dissociation of nitrous oxide [12] due to its low toxicity and good environmental acceptability. Cuprous oxide crystals with different morphologies possess different physical and chemical properties. Much effort has been made to synthesize cuprous oxide crystals with different shapes such as octahedron, whisker, cactus, box, cube, sphere, wire, tube, rod, bi-pyramid, star, † Corresponding author. Prof., Ph.D.; Tel./Fax: +86 10 82323433; E-mail address: [email protected] (Y.H. Zhang).
and flower[13–22] . The morphology and size often affect significantly the physical and chemical property of crystal. Therefore, there have been some studies on the photocatalytic activities of the crystals with different shapes and/or sizes[17, 23–27] . In this work, a simple method is designed to synthesize four types of cuprous oxide micro/nanocrystals using copper sulfate as the starting materials, glucose and hydrate hydrazine as the reducing agents, and macromolecule surfactants as the templates. The phase, morphologies and optical properties of four types of crystals were characterized by XRD, SEM, and UV-vis/DRS. Their photocatalytic activities are determined by monitoring the photocatalytic degradation of rhodamine B. 2. Experimental 2.1 Synthesis of samples All the reagents except copper sulphate were
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290
Table 1 Reaction materials and heating time in the synthesis of the samples and the specific surface areas (SBET ) of samples No. 1 2 3 4
CuSO4 CuSO4 CuSO4 CuSO4
NH3 H2 O NaOH NaOH NaOH
Reactants glucose PVP N2 H4 P(VP-NVP-St) N2 H4 P(NVP-St) N2 H4 P(VP-NVP)
analytical grade and used without further purification. Copper sulfate was purified via recrystallization and water used in these experiments was deionized. The macromolecule surfactants such as poly-vinyl-pyridine(PVP), copolymer of vinyl pyridine, vinyl pyrrolidone and styrene{P(VP-NVP-St)}, copolymer of vinyl pyrrolidone and styrene {P(NVPSt)}, and copolymer of vinyl pyridine and vinyl pyrrolidone{P(VP-NVP)} were synthesized in the laboratory using radical polymerization with ammonium peroxydisulfate as initiator and their average molecular weight are 7,000–14,000. To synthesize the cuprous oxide microcrystals with spherical structure, 50.0 mL of 0.5 mol/L copper sulfate solutions and 1.0 mL of 5% (M/V) PVP were mixed in a beaker at room temperature. Under constant magnetic stirring, 15.0 mL of 5.0 mol/L ammonia solution was added to the above solution quickly. Then, 20.0 mL of 2.5 mol/L glucose was added and red. The beaker was heated in a microwave oven and the solution boiled constantly for 30 min at 600 W. Finally, the precipitates were filtrated and washed with deionized water and absolute ethanol and dried in a vacuum dryer at 80o C for 2 h. To synthesize the cuprous oxide crystals with the other three types of shapes, 50.0 mL of 0.5 mol/L copper sulfate solutions and 1.0 mL of 5% (M/V) macromolecule surfactants were mixed in a beaker at room temperature. For dandelion-shaped crystals, 50 mL of 1 mol/L tartaric acid was also added. Under constant magnetic stirring, after adding 50.00 mL of 1.0 mol/L sodium hydroxide quickly, 5.0 mL of 1.0 mol/L hydrazine hydrate was added to the above solution. Then the beaker was moved to a microwave oven and heated at 600 W. Followed by boiling constantly for 5 min, the mixtures were separated by filtrating and the residues were washed with deionized water and absolute ethanol and dried in a vacuum dryer at 80◦ C for 2 h. Different crystal shapes were obtained by changing the reactants and heating time in the synthesis process as illustrated in Table 1 in detail. 2.2 Characterizations of samples The crystal phase of the synthesized particles was identified by X-ray diffraction (XRD, Rigaku D/Max2000) using CuKα radiation (2 kV rotating anode, λ=0.154056 nm). The samples were scanned from 10◦ to 110◦ at a scanning rate of 0.06◦ /s. The microstruc-
– – – tartaric acid
Time 30 min 5 min 5 min 5 min
Morphology sphere strip octahedron dandelion
SBET /(m2 /g) 29.70 51.70 41.94 54.06
tures and morphologies of the particles were determined by scanning electron microscopy (SEM, Hitachi S-4300). The samples were coated with gold to improve the conductivity during SEM observation. The specific surface area of the particles was determined by an automated surface area and pore size analyzer (Autosorb-1, Quantachrome, USA) using the nitrogen adsorption. The ultraviolet-visible diffuse reflection absorptive spectra (UV-vis/DRS) were acquired on a UV-visible spectrophotometer (Hitachi-UV3010) equipped with an integration sphere at room temperature. 2.3 Photocatalytic degradation of rhodamine B To assess the photocatalytic activity of various cuprous oxide micro-/nanocrystals, the photocatalytic reaction was conducted in a 250 mL glass beaker in a reactor equipped with a magnetic force stirrer and circular cooling copper tube. A 500 W tungsten halogen lamp without filter chip was used as the visible light source. 0.5 g of cuprous oxide powders were dispersed in 200 mL of 5 mg/L rhodamine B aqueous solution. The suspension was sonicated for 1 h in darkness to disperse the powders well in the solution before irradiation with visible light. At regular intervals of 1 h, 25 mL of the suspension was sampled and separated by centrifugation at 12,000 r/min for 10 min. The concentration of remaining rhodamine B was determined by measuring their spectroscopy in the wavelength of 250–700 nm using a UV-vis spectrophotometer (Hitachi UV-3010). 3. Results and Discussion 3.1 SEM images of the samples Figure 1 illustrates the SEM images revealing four kinds of crystal morphologies. And the specific surface areas of various cuprous oxide crystals are shown in Table 1. The specific surface areas of the as synthesized cuprous oxide crystals with sphere, strip octahedron and dandelion shape are 29.70, 51.70, 41.94 and 54.06 m2 /g, respectively. In Fig. 1(a), the crystals are spherical and the size is about 2∼12 μm. The spherical crystals are synthesized in an ammonia solution with PVP as the template and glucose as the reducing agent. Figure 1(b) discloses strip-shaped crystals with a thickness of about 20 nm and width of 300∼400 nm. The strip-shaped crystals are obtained from a sodium hydroxide solution with P(VP-NVP-St) as the tem-
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291
Fig. 1 SEM images of cuprous oxide with different morphologies: (a) sphere, (b) strip, (c) octahedron, (d) dandelion
plate and hydrazine hydrate as the reductant. The octahedral cuprous oxide particles shown in Fig. 1(c) are obtained using P(NVP-St) as the template whereas the dandelion-shaped cuprous oxide crystals in Fig. 1(d) are synthesized using P(VP-NVP) as the template. The SEM results suggest that it is possible to control the morphology of the cuprous oxide crystals by changing reaction conditions, among which the self-assembly-induced template is significantly important. In the aqueous solution, as a result of their amphiphilic nature, surfactant molecules acting as templates often spontaneously self-assemble into microscopic structures shaped spheres, cylinders, vesicles, and membranes depending on outside factors[28] . During the process of synthesis of materials with nanometer or submicrometer scale, the particles growing on the self-assembly-induced template are often affected by such factors as liquid interfacial tension, capillary force, and different hydrophilic or hydrophobic forces. And selective adsorption of ions or molecules onto different surfaces also affects the particle shape. Any of these factors can become the key controlling parameter for the resulting morphologies[29] . In this study, the formation of the spherical crystals may be resulted from the effect of PVP template. Because ammonia solution exists, cupric tetraammino ions would generate before they were reduced. Cupric tetraammino ions combined with PVP to selfassemble into spherical microscopic structure in the aqueous solution due to interfacial tension and hydrophilic and hydrophobic forces. During the process of reducing, crystallographic planes of cuprous oxide
microcrystals were affected by PVP in the interface to form spherical structure. For strip-shaped crystals, it is possible that the longitudinal growth of cuprous oxide nanocrystals was postponed because of self-assembly membrane spatial structure of P(VPNVP-St) in the aqueous solution. The formation of Cu2 O octahedra can be explained as the corporate effect coming from the growth units of anion coordinative polyhedra theoretical model and the polymer selective adsorption. According to the growth units of anionic coordinative polyhedra theoretical model, there are two anion coordinative polyhedra in Cu2 O. These two coordinative polyhedra connect each other and constitute the different growth unit with the different dimension and the dimensions of the growth units can vary with the change of the growth condition[13] . In our experiments, when P(NVP-St) was introduced, due to the selective interaction between P(NVP-St) and various crystallographic planes of cuprous oxide, the growth along the [111] direction could be greatly postponed but the growth along the (100) direction could be enhanced. So the (111) faces preferentially appear and the octahedra are obtained. For dandelion-shaped structure, we suggest that it is resulted from self-assembly of nanosized cuprous oxide crystallites with the assistance of P(VP-NVP) and complexing agent tartaric acid. In aqueous solution, copper ion can combine tartaric acid to form coordination complexes with polyhydroxy compounds. When copper ions were reduced to cuprous oxide crystallites by hydrazine hydrate, the growth of various faces of these crystallites was controlled in the interface with P(VP-NVP) to produce variform tiny polyhedral par-
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Fig. 2 XRD patterns of (a) spherical, (b) strip-shaped, (c) octahedral and (d) dandelion-shaped cuprous oxide crystals
ticles. And these variform tiny polyhedral particles agglomerated into dandelion-shaped structure due to their high surface free energy. 3.2 XRD patterns of the samples Figure 2 depicts the XRD patterns of the products which possess a cubic structure which can be indexed according to the JCPDS card No. 05-0667. Five characteristic peaks are observed at 2θ values of 29.6◦ , 36.5◦ , 42.4◦ , 61.5◦ , 73.7◦ , and 77.6◦ corresponding to the crystal planes of (110), (111), (200), (220), (311) and (222) of crystalline cuprous oxide, respectively. The other characteristic peaks caused by impurities of copper oxide, such as 35.5◦ , 38.7◦ , 48.7◦ , are found from the strip-shaped cuprous oxide crystals (Fig. 2(b)). According to JCPDS card No.45-0937, they correspond to the planes of (002), (111) and (¯202) of crystalline copper oxide, respectively. Because of the large specific surface area, a small quantity of cuprous oxide particles on the surface is oxidized to copper oxide during synthesis process. In curves (a), (c), and (d) in Fig. 2, no noise peaks can be observed and it can be inferred that the synthesized spherical, dandelion-shaped, and octahedral cuprous oxide crystals have high purity. These three kinds of crystals were not oxidized during synthesis process, which is possibly due to the protection of surfactants coating in the surface of the particles, while the surfactants with membrane structure coating in the surface of strip-shaped cuprous oxide nanocrystals were more likely to be partially broken. 3.3 Optical properties of the samples The optical response of the samples was characterized by using UV–vis diffuse reflectance spectrometry (UV-vis/DRS). The UV-vis/DRS of the cuprous oxide samples are shown in Fig. 3(a). All of the samples show a clear absorption edge at 650 nm. The maxi-
Fig. 3 (a) UV-vis/DRS of the as prepared cuprous oxide crystals; (b) plot of (αhν)2 vs hν of various cuprous oxide micro-nanocrystals
mum absorbance values of the spherical, strip-shaped, octahedral, and dandelion-shaped cuprous oxide microcrystals are at 410 nm, 427 nm, 453 nm, and 453 nm, respectively and in the visible range. The cuprous oxide crystals with different morphologies exhibit different sensitivities to visible lights and the dandelionshaped cuprous oxide crystal has the strongest absorbency and the octahedral one has the weakest one. It is possibly because dandelion-shaped cuprous oxide crystals have a porous structure and active crystallite facets and thus light reflection is reduced. In comparison, the inactive facets of the octahedral cuprous oxide crystals reflect most of the incoming irradiation and the absorbency is subsequently the smallest. The strip-shaped cuprous oxide crystals also show higher absorbance than the spherical and octahedral ones because the particle is tiny and has larger specific surface. Our results also suggest that optical absorption is often affected by the morphology of the crystals[30] . Since cuprous oxide is a P-type direct gap semiconductor[31] , the relationship between absorption coefficient (α) and photon energy (hν) can be expressed as[32] : 1
αhν = B(hν − Eg ) 2
(1)
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Fig. 4 Absorbance of remaining rhodamine B degraded at different time in the wavelength of 553 nm. No light irradiation in the first hour
In this equation, B is a constant which does not depend on photo energy and Eg is the band gaps energy. The band gap can be estimated by the intercepts of the tangents to the (αhν)2 vs photon energy (hν) plots. Plots of (αhν)2 vs hν for cuprous oxide samples with different morphologies are shown in Fig. 3(b). The direct band gaps of spherical, strip-shaped, octahedral and dandelion-shaped cuprous oxide micro/nanocrystals are estimated to be 2.10, 2.06, 2.15 and 2.04 eV, respectively. It thus can be inferred that cuprous oxide can be activated by visible light. The red shift of the direct band gaps displays the effect of the morphologies of crystals. The microcrystals with different morphologies have different dominant active facets and will response different excitation energy and consequently have different direct band gaps. 3.4 Photocatalytic activities of the samples The photocatalytic activities of the cuprous oxide crystals with different morphologies are evaluated by monitoring the decomposition of rhodamine B in an aqueous solution under visible light irradiation. Direct decomposition of rhodamine B without cuprous oxide cannot almost be detected under visible light irradiation in our control experiment. The decreasing absorbance of remaining rhodamine B degraded directly is depicted in Fig. 4 which exhibits that the absorbance does not almost change after being illuminated for 4 h. From Fig. 4, rhodamine B is degraded to different degrees through visible-light driven photocatalysis by the various cuprous oxide crystals. On the other hand, the absorbance of rhodamine B decreases slightly without visible light irradiation due to the small absorption of rhodamine B by the cuprous oxide crystals. As rhodamine B has a strongest absorbance in 553 nm, it is rational to obtain the degradation rate by the following equation.
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Fig. 5 Photocatalytic degradation rate of rhodamine B by various cuprous oxide crystals
Di =
A0 − Ai × 100% A0
(2)
where Di (%) is the degradation rate when irradiation time is i hours, A0 the initial absorbance and Ai is the absorbance when irradiation time is i hours in the wavelength of 553 nm. The degradation rates of rhodamine B by different cuprous oxide crystals when irradiation time is 4 h was described in Fig. 5. It reveals that under visible light irradiation, 56.37%, 55.68%, 51.83% and 46.16% of rhodamine B (monitored at 553nm) can be decomposed by dandelion-shaped, strip-shaped, spherical and octahedral cuprous oxide crystals, respectively. It can be inferred that all the cuprous oxide crystals synthesized have some photocatalytic activities which are much higher than those reported in the previous literature[33,34] and the shape factor seems to be of overriding importance to photocatalytic activities. The different photocatalytic activities of the cuprous oxide crystals can be explained by reaction activities. According to the literature reported by Xu et al.[35] and Zhang et al.[36] , the (111) facets are more active than (100) facets due to the dangling bonds of (111) surfaces, while no dangling bands but saturated chemical bonds exist in the (100) facets. So the cuprous oxide crystals with dominant (111) facets have a higher adsorption and photocatalytic activity than those with dominant (100) surfaces and (100) surfaces. Due to their dominant (110) facet, the octahedral cuprous oxide crystals show the lowest photocatalytic activities among these four kinds of crystals though they have a larger specific surface area (41.94 m2 /g) comparing to the spherical Cu2 O crystals (29.70 m2 /g). The dandelion-shaped Cu2 O crystals are composed of lots of tiny polyhedral crystals with dominant (111) surfaces by aggregation and have a largest specific surface area (54.06 m2 /g), so they exhibit the best activities in the photocatalytic degradation of rhodamine B. For strip-shaped and spherical Cu2 O crystals, the dominant facets also are of
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the (111) type, but they display somewhat lower photocatalytic activities than dandelion-shaped ones because of their lesser specific surface (51.70 and 29.70 m2 /g, respectively). Also, the stability of surface of crystal is an important factor of photocatalytic activity. The strip-shaped Cu2 O crystal was oxidized slightly on the surface resulting in its relatively lower catalytic activity. On the other hand, the shape of the crystals affects the absorbencies and direct band gaps and then impacts on the photocatalytic performance due to the different activity of the dominant facet. The specific surface dominated by the structural and crystal size is an important factor affecting the reaction activity. High absorbency in the visible region results in the visible-light induced photocatalytic activity[37] . In general, the higher the absorbency of the photocatalyst, the higher the utilization of light as well as electron-hole pairs generated by excited electrons. The potential of the photocatalytic redox activity is improved and the catalytic activity is increased. The dandelion-shaped cuprous oxide crystals with the active dominant (111) facets have a much stronger absorbency and smaller direct band gap (∼2.04 eV), so they exhibit much higher activity in the photocatalytic degradation of rhodamine B. The decreasing of the absorbencies and direct band gaps of strip-shaped, spherical and octahedral Cu2 O crystals in order result in the diminution of their photocatalytic activities accordingly. As a result, the different morphologies of cuprous oxide crystals lead to the different specific surface areas, direct band gaps and UV-vis absorbencies which cause the different photocatalytic activities. 4. Conclusion Spherical, strip-shaped, octahedral, and dandelion-shaped cuprous oxide micro-/nanocrystals are synthesized via simple reduction of copper sulfate using different reducing agents and macromolecule surfactants under microwave irradiation. The macromolecule surfactants acting as the templates play an important role in controlling the morphologies of the resulting cuprous oxide crystals. Different reducing agents and alkaline solutions also influence the morphology of the cuprous oxide crystals. It is possible to control the morphology of the cuprous oxide crystals by changing the reducing agents, templates, and alkaline solution. Investigation of the photocatalytic activity shows that dandelion-shaped cuprous oxide crystals have the highest activity and can degrade 56.37% of rhodamine B, the octahedral cuprous oxide crystals exhibit the lowest activity and can only degrade 46.16% of rhodamine B, and the strip-shaped and spherical cuprous oxide crystals can decompose 55.68% and 51.83% of rhodamine B, respectively. Different morphology results in different photocatalytic activities of cuprous oxide crystals under visible light
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