pubs.acs.org/crystal Published on Web 04/19/2010 r 2010 American Chemical Society

DOI: 10.1021/cg900110u

2010, Vol. 102064–2067

Facile Synthesis, Shape Evolution, and Photocatalytic Activity of

Truncated Cuprous Oxide Octahedron Microcrystals with Hollows

Hui Yang and Zhi-Hong Liu*

Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry andMaterials 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 thencharacterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). It was observed that the reactiontime 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 spectrophoto-metry, demonstrating that their behavior was influenced by the different morphologies, and the truncated octahedra withhollow microcrystals possessed the highest activity.

Introduction

Cu2O with unique optical and magnetic properties is apromising material with applications in solar energy conver-sion,1 as catalysts for organic reactions,2 electrodes for lithiumion batteries,3 gas sensors,4 biosensors, and magnetic storagedevices,5-7 and as photocatalysts for degradation of organicpollutants and decomposition of water into O2 and H2 undervisible light.8,9 Thus, the controlled synthesis of Cu2O crystalswith uniform morphology becomes an important issue. In thepast decade, synthesis of uniformnano- andmicrocrystalswithwell-controlled sizes and morphologies, such as nanowires,10

nanotubes,11 nanoparticles,12 hollow spheres,7,13 flower-likeshapes,14 nanocubes,15-18 hexapod-shaped Cu2O microcrys-tals,19 octahedral Cu2Onanocages,20-22 andCu2O films,23 hasattracted much attention.

In this work, we report a facile hydrothermal route towardthe synthesis of truncated Cu2O octahedral single microcrys-tals with hollows at low temperature. Moreover, the photo-catalysis activities of the prepared Cu2O microcrystals wereinvestigated by methyl orange photodegradation.

Experimental Section

Synthesis and Characterization of Cu2OMicrocrystals. All of thechemical reagents used were of analytical grade without furtherpurification. Copper chloride (CuCl2 3 2H2O, 1.21 g) and sodiumtartrate [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 trans-ferred to six Teflon-lined stainless steel autoclaves, which were sealedandmaintained at 150 �C for 8, 10, 12, 14, 16, and 18 h. After coolingto room temperature, the products were collected andwashed severaltimes with distilled water and ethanol and then dried at 60 �C for 12 h(the sampleswere 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 ascanning rate of 8�/min with 2θ ranging from 20 to 80�) and scanningelectron microscopy (SEM; Quanta 200, Philips-FEI; prior to SEMimaging, the samples were sputtered with thin layers of gold).

Nitrogen adsorption experiments of as-synthesized samples weremeasured 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 samplesfor the photocatalytic decolorization of methyl orange aqueoussolution was performed at ambient temperature (25 �C). The proce-dure was as follows: 0.01 g of the prepared samples was dispersed into15 mL of methyl orange aqueous solution (300 mg L-1), followed bythe addition of 1 mL of hydrogen peroxide solution (H2O2, 5%). Thesuspensions were magnetically stirred in the dark for over 2 min toensure adsorption equilibrium of methyl orange onto the surface ofCu2O microcrystals, then separated evenly into four parts. An 18 Wdaylight lamp (3 cm above the sample) was used as a light source.Visible light then irradiated the above solutions for 0, 10, 20, and30min, and the corresponding reaction solutionswere filtered and theabsorbance ofmethyl orangeaqueous solutionswas thenmeasuredbya UV-visible spectrophotometer (Lmbda 950).

Results and Discussion

Characterization of As-Prepared Samples. Figure 1 showsthe XRD patterns of the samples prepared by a mild hydro-thermal route at different reaction times. In the 2θ values of29.4, 36.3, 42.3, 61.3, 73.5, and 77.2�, the corresponding maincharacteristic d values of the XRD patterns for the samples ofCu2O were 3.0377, 2.4778, 2.1436, 2.0951, 1.5132, 1.2898, and1.2344 A, respectively, which could be exactly indexed withthose of JCPDS cards (PDF fileNo. 05-0667) and thus showedan absence of other crystalline forms in the prepared samples.

The representative SEM images of the products prepared atdifferent reaction time intervals are shown in Figure 2. Underthe previously mentioned synthetic conditions, micrometerrods (Figure 2a) and hexapod microcrystals (Figure 2b) ofthe Cu2Owere formed after hydrothermal treatment for 8 and10 h, respectively. When the reaction time was prolonged to12 h, the octahedral morphology of the Cu2O appeared, aspresented in Figure 2c. After the reaction time was furtherprolonged to 14 h, the truncated octahedral morphology ofCu2O was formed (Figure 2d). It is worth noting that therealmost exists a hollow on each vertex of this truncatedoctahedron. When the reaction time was further increased to16 h, the truncated octahedron with hollow morphologydisappeared, and different polyhedral morphologies of Cu2O

*To whom correspondence should be addressed. Phone: þ86 29 85307765. Fax: þ86 29 8530 7774. E-mail: liuzh@snnu.edu.cn.

Article Crystal Growth & Design, Vol. 10, No. 5, 2010 2065

appeared (Figure 2e).After the reaction timewas extended to amaximum reaction time of 18 h, the polyhedral morphologiesbegan to be destroyed, and the surface became accidented(Figure 2f).

Growth Mechanism. The formation of Cu2O might beattributed to the reductive action of enol which comes fromthe decomposition of sodium tartrate material under hydro-thermal conditions.

It is believed that the reduction in the surface energy is theprimary driving force for simple particle growth and themorphology evolution. Scheme 1 outlines the growth process

of the truncated octahedral Cu2O.At first, Cu2Omicrometerrods (Figure 2a) were formed through conventional nuclea-tion, growth, and aggregation. Then, the Cu2O hexapodmicrocrystals (Figure 2b) were achieved through self-assem-bly of microrods with the increase of reaction time of 10 h.The slight size difference between these microcrystals mightbe due to a little dissolution during the self-assembly process.After that, the crystals were further grown on the basis ofhexapod microcrystals which could be thought of as theframework of an octahedron, forming the octahedral shapes(Figure 2c). As the reaction proceeded, the saturated solu-tion gradually became low, resulting in the possible dissolu-tion of the sharp vertexes and well-defined edges of theoctahedral crystals, which led to the formation of interestingtruncated octahedra with hollows and ill-defined edges(Figure 2d).Upon continuing the reaction, the crystals beganto grow again along the eliminative vertexes and edges due tothe increase in the saturation of the solution in the abovedissolution process, which made most parts of octahedrachange into polyhedra (Figure 2e). Finally, the polyhedralmorphologies began to be destroyed in varying degrees(Figure 2f) under the actions of autogenous pressure andturbulence.

In addition, the growth mechanism is also discussed withreference to the literature.24-26 According to the literature,24

the transformation of the morphologies among the polyhe-dron, octahedron, and truncated octahedron depend onthe ratio, R, of the growth rate in the Æ100æ to that of theÆ111æ direction. When R=1.73, a perfect octahedron wasformed; when 0.87<R<1.73, the truncated octahedron wasachieved; and when R=0.87, the polyhedron was formed.Usually, the surface energy of the {100} crystal face is higherthan that of the {111} face, which causes the growth ratesalong the Æ100æ direction to be faster than those along theÆ111æ direction.24 Therefore, the octahedron shape of Cu2Owas obtained whenR=1.73 with the increase of reaction timeto 12 h. When the reaction time was further prolonged to14 h, the growth rate along the Æ100æ direction shoulddecrease gradually, as expected, and theR decreased accord-ingly. Thus, when 0.87 < R< 1.73, a truncated octahedronshape of Cu2O was obtained. The growth rate of microcrys-tals along the different positions, including the center and thefour angle directions, which were all vertical to the {100}face, was different in the growth process. Therefore, whilethe growth rate along the center direction was slower thanthat along the four angles, a truncated octahedron with ahollow on each vertex was formed.25 Because differentmicrocrystals have different growth rates and different en-vironments, the growth rates of microcrystals in differentdirections were changed. Thus, polyhedron shapes appearedwhenR=0.87 for some crystals with the increase of reactiontimes to 16 h.

Photocatalytic Activity. Cu2O has been used as a photo-catalyst for the hydrogen production and organic pollutantdegradation under visible light. Previous studies on thephotocatalytic activity of Cu2O were carried out withouttaking account of its morphology.27 To demonstrate thepotential application of the present Cu2O related to themorphology, the photocatalytic activities of the preparedCu2O octahedra and truncated octahedra were investigatedwith reference to the literature.27,28 The degradation ofmethyl orange was selected as the reference, and the chara-cteristic absorption of methyl orange at about 465 nm wasselected for monitoring the adsorption and photocatalytic

Figure 1. XRD patterns of the Cu2O microcrystals prepared by ahydrothermal 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 ahydrothermalmethod at different reaction times: (a) 8, (b) 10, (c) 12,(d) 14, (e) 16, and (f) 18 h.

2066 Crystal Growth & Design, Vol. 10, No. 5, 2010 Yang and Liu

degradation process. Figures S1-S3 (Supporting Infor-mation) show the absorption spectra of an aqueous solutionof methyl orange photodegraded by an analytical gradeCu2O reagent (Figure S1), prepared octahedral microcrys-tals (Figure S2), and truncated octahedral microcrystals(Figure S3) of Cu2O samples. Figure 3 shows the comparisonof photocatalytic activities of different samples. As shown inFigure 3, the photocatalytic activity of each sample wasgradually enhanced with time increasing from 0 to 30 min.The catalytic activity of the perfect octahedral morphologyof the Cu2O sample was much higher than that of theanalytical grade Cu2O reagent, which may be ascribed toits exposed {111} surfaces and relatively small size. Micro-crystals of the truncated octahedra with hollows wereobserved to possess much higher activity than perfectoctahedra. The improvement of the truncated octahedralmicrocrystals in their photocatalytic activitymay be ascribedto their existing hollows in the vertex, which can increasetheir total surface area (the Brunauer-Emmett-Teller(BET) surface areas were determined as 0.0308 m2/g forthe octahedral morphology and 0.1819 m2/g for the trun-cated octahedral morphology). The results demonstratedthat the photocatalytic activities of the microcrystals wererelated to their morphologies.

Conclusions

In summary, Cu2O samples with different morphologies ofmicrometer rods, hexapods, octahedral, and truncated octa-hedral microcrystals have been synthesized by a facile hydro-thermal method, which were obtained via control of thereaction times. The morphologies of the Cu2O samples influ-enced their catalytic activities, and the truncated octahedrawith the hollow microcrystals possessed the highest activity.This investigation has potential as a guide toward the shape-controlled synthesis of Cu2O crystals with their application inthe treatment of organic pollutants.

Acknowledgment. This project is supported by the NaturalScience Foundation of Shaanxi Province of China (No.SJ08B01). We thank Dr. Miriam Gillett-Kunnath (UniversityofNotreDame,USA) for assistance with the English language.

Supporting Information Available: Additional figures. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

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Figure 3. Plots of absorbance (A) versus irradiation time (t) inthe presence of the analytical grade Cu2O reagent (a), Cu2Ooctahedral morphology (b), and Cu2O truncated octahedralmorphology (c).

Scheme 1. Growth Mode of Cu2O Octahedra and Truncated Octahedra

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