DOI: 10.1021/cg900110u
Facile Synthesis, Shape Evolution, and Photocatalytic Activity of Truncated Cuprous Oxide Octahedron Microcrystals with Hollows
2010, Vol. 10 2064–2067
Hui Yang and Zhi-Hong Liu* Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, People’s Republic of China Received January 31, 2009; Revised Manuscript Received March 30, 2010
ABSTRACT: Microcrystals of cuprous oxide (Cu2O) samples exhibiting different morphologies such as rods, hexapods, octahedra, and truncated octahedra with hollows were synthesized via a facile hydrothermal method and were then characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). It was observed that the reaction time had a prominent effect on the aforementioned morphologies. The crystal growth processes have been proposed. Furthermore, the photocatalytic activities of the prepared Cu2O microcrystals were investigated by UV-vis spectrophotometry, demonstrating that their behavior was influenced by the different morphologies, and the truncated octahedra with hollow microcrystals possessed the highest activity.
Introduction Cu2O with unique optical and magnetic properties is a promising material with applications in solar energy conversion,1 as catalysts for organic reactions,2 electrodes for lithium ion batteries,3 gas sensors,4 biosensors, and magnetic storage devices,5-7 and as photocatalysts for degradation of organic pollutants and decomposition of water into O2 and H2 under visible light.8,9 Thus, the controlled synthesis of Cu2O crystals with uniform morphology becomes an important issue. In the past decade, synthesis of uniform nano- and microcrystals with well-controlled sizes and morphologies, such as nanowires,10 nanotubes,11 nanoparticles,12 hollow spheres,7,13 flower-like shapes,14 nanocubes,15-18 hexapod-shaped Cu2O microcrystals,19 octahedral Cu2O nanocages,20-22 and Cu2O films,23 has attracted much attention. In this work, we report a facile hydrothermal route toward the synthesis of truncated Cu2O octahedral single microcrystals with hollows at low temperature. Moreover, the photocatalysis activities of the prepared Cu2O microcrystals were investigated by methyl orange photodegradation. Experimental Section Synthesis and Characterization of Cu2O Microcrystals. All of the chemical reagents used were of analytical grade without further purification. Copper chloride (CuCl2 3 2H2O, 1.21 g) and sodium tartrate [Na2(C4H4O6), 2.1 g] were dissolved in 40 mL of H2O, followed by the slow addition of 0.72 g of sodium hydroxide (NaOH). After being stirred, six parts of such mixture were transferred to six Teflon-lined stainless steel autoclaves, which were sealed and maintained at 150 C for 8, 10, 12, 14, 16, and 18 h. After cooling to room temperature, the products were collected and washed several times with distilled water and ethanol and then dried at 60 C for 12 h (the samples were placed onto a surface utensil, then into the drybox). The final products were characterized by X-ray powder diffraction (XRD, recorded on a Rigaku D/MAX-IIIC with Cu target at a scanning rate of 8/min with 2θ ranging from 20 to 80) and scanning electron microscopy (SEM; Quanta 200, Philips-FEI; prior to SEM imaging, the samples were sputtered with thin layers of gold). *To whom correspondence should be addressed. Phone: þ86 29 8530 7765. Fax: þ86 29 8530 7774. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 04/19/2010
Nitrogen adsorption experiments of as-synthesized samples were measured at 77 K on a Micromeritics ASAP 2020 system. Photocatalysis Activities of the Prepared Cu2O Microcrystals. The evaluation of photocatalytic activities of the as-prepared samples for the photocatalytic decolorization of methyl orange aqueous solution was performed at ambient temperature (25 C). The procedure was as follows: 0.01 g of the prepared samples was dispersed into 15 mL of methyl orange aqueous solution (300 mg L-1), followed by the addition of 1 mL of hydrogen peroxide solution (H2O2, 5%). The suspensions were magnetically stirred in the dark for over 2 min to ensure adsorption equilibrium of methyl orange onto the surface of Cu2O microcrystals, then separated evenly into four parts. An 18 W daylight lamp (3 cm above the sample) was used as a light source. Visible light then irradiated the above solutions for 0, 10, 20, and 30 min, and the corresponding reaction solutions were filtered and the absorbance of methyl orange aqueous solutions was then measured by a UV-visible spectrophotometer (Lmbda 950).
Results and Discussion Characterization of As-Prepared Samples. Figure 1 shows the XRD patterns of the samples prepared by a mild hydrothermal route at different reaction times. In the 2θ values of 29.4, 36.3, 42.3, 61.3, 73.5, and 77.2, the corresponding main characteristic d values of the XRD patterns for the samples of Cu2O were 3.0377, 2.4778, 2.1436, 2.0951, 1.5132, 1.2898, and 1.2344 A˚, respectively, which could be exactly indexed with those of JCPDS cards (PDF file No. 05-0667) and thus showed an absence of other crystalline forms in the prepared samples. The representative SEM images of the products prepared at different reaction time intervals are shown in Figure 2. Under the previously mentioned synthetic conditions, micrometer rods (Figure 2a) and hexapod microcrystals (Figure 2b) of the Cu2O were formed after hydrothermal treatment for 8 and 10 h, respectively. When the reaction time was prolonged to 12 h, the octahedral morphology of the Cu2O appeared, as presented in Figure 2c. After the reaction time was further prolonged to 14 h, the truncated octahedral morphology of Cu2O was formed (Figure 2d). It is worth noting that there almost exists a hollow on each vertex of this truncated octahedron. When the reaction time was further increased to 16 h, the truncated octahedron with hollow morphology disappeared, and different polyhedral morphologies of Cu2O r 2010 American Chemical Society
Article
Figure 1. XRD patterns of the Cu2O microcrystals prepared by a hydrothermal approach at different reaction times: (a) 8, (b) 10, (c) 12, (d) 14, (e) 16, and (f) 18 h.
Figure 2. SEM images of the Cu2O microcrystals prepared by a hydrothermal method at different reaction times: (a) 8, (b) 10, (c) 12, (d) 14, (e) 16, and (f) 18 h.
appeared (Figure 2e). After the reaction time was extended to a maximum reaction time of 18 h, the polyhedral morphologies began to be destroyed, and the surface became accidented (Figure 2f). Growth Mechanism. The formation of Cu2O might be attributed to the reductive action of enol which comes from the decomposition of sodium tartrate material under hydrothermal conditions. It is believed that the reduction in the surface energy is the primary driving force for simple particle growth and the morphology evolution. Scheme 1 outlines the growth process
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of the truncated octahedral Cu2O. At first, Cu2O micrometer rods (Figure 2a) were formed through conventional nucleation, growth, and aggregation. Then, the Cu2O hexapod microcrystals (Figure 2b) were achieved through self-assembly of microrods with the increase of reaction time of 10 h. The slight size difference between these microcrystals might be due to a little dissolution during the self-assembly process. After that, the crystals were further grown on the basis of hexapod microcrystals which could be thought of as the framework of an octahedron, forming the octahedral shapes (Figure 2c). As the reaction proceeded, the saturated solution gradually became low, resulting in the possible dissolution of the sharp vertexes and well-defined edges of the octahedral crystals, which led to the formation of interesting truncated octahedra with hollows and ill-defined edges (Figure 2d). Upon continuing the reaction, the crystals began to grow again along the eliminative vertexes and edges due to the increase in the saturation of the solution in the above dissolution process, which made most parts of octahedra change into polyhedra (Figure 2e). Finally, the polyhedral morphologies began to be destroyed in varying degrees (Figure 2f) under the actions of autogenous pressure and turbulence. In addition, the growth mechanism is also discussed with reference to the literature.24-26 According to the literature,24 the transformation of the morphologies among the polyhedron, octahedron, and truncated octahedron depend on the ratio, R, of the growth rate in the Æ100æ to that of the Æ111æ direction. When R = 1.73, a perfect octahedron was formed; when 0.87
