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DOI: 10.1002/adma.200800208

Thermal Oxidation Strategy towards Porous Metal OxideHollow Architectures**

By Jun Liu and Dongfeng Xue*

Hollow and porous structures have been attracting great

attention because of their widespread applications in catalysis,

drug delivery, environmental engineering, and sensor sys-

tems.[1–4] The general approach for the preparation of hollow

structures has involved the use of various removable

or sacrificial templates, including hard ones such as mono-

disperse silica[5] or polymer latex spheres[6] as well as soft ones,

for example, emulsion droplets/micelles.[7,8] Recently, the

Kirkendall effect has been extensively used as an effective

strategy to prepare hollow structures.[9,10] The basis of the

Kirkendall effect is void formation caused by the difference in

diffusion rates between two species. Although much progress

has been made in the synthesis of hollow structures, shells of

the resulting hollow architectures from these approaches are

primarily nonporous.[5–10] As for porous structures, the

available preparation strategies involve the initial formation

of large assemblies of expensive surfactants as templates to

make porous inorganic structures.[11] Here, we report a general

top-down strategy to synthesize uniform porous transition-

metal oxide hollow architectures. The current chemical

strategy is based on the combination of the Kirkendall effect,

volume loss, and gas release. Different from previous work

based on the Kirkendall effect, a reversed reaction route is

designed herein, i.e., a conversion from metal sulfides and

selenides to their corresponding oxides, which makes use of

volume loss and gas release to form the porous shell.

Up to now, two basic approaches (i.e., top-down and

bottom-up techniques) have been proposed to control matter

at the nano/micrometer scale.[12] The bottom-up technique

makes use of self-processes for ordering solid-state architec-

tures at the atomic to mesosopic scale. However, the top-down

approach is similar to the traditional workshop or micro-

fabrication method where tools are used to cut, mill, and shape

[*] Prof. D. Xue, J. LiuState Key Laboratory of Fine ChemicalsDepartment of Materials Science and Chemical EngineeringSchool of Chemical EngineeringDalian University of Technology158 Zhongshan Road, Dalian 116012 (PR China)E-mail: dfxue@chem.dlut.edu.cn

[**] We gratefully acknowledge the financial support of the programfor New Century Excellent Talents in University (Grant No.NCET-05-0278), the National Natural Science Foundation of China(Grant No. 20471012), a Foundation for the Author of NationalExcellent Doctoral Dissertation of PR China (Grant No. 200322),and the Research Fund for the Doctoral Program of HigherEducation (Grant No. 20040141004). Supporting Information isavailable online from Wiley InterScience or from the author.

� 2008 WILEY-VCH Verlag Gmb

materials to the desired shape and scale. As a well-known

transition-metal oxide, copper oxide (CuO) has been exten-

sively studied because of its applications in the field of

lithium-ion batteries,[13] catalysis,[14] and superconductors.[15]

Studies on copper-based and other transition-metal oxide

materials have been widely undertaken in our laboratory.[16]

In this Communication, CuO was used as the example to

demonstrate this general top-down chemical approach. The

obtained hollow architectures with a porous shell might be

more attractive than closed hollow structures in some aspects,

such as for catalysis, because of the dense distribution of pores

in their walls.

Our design strategy for the preparation of porous CuO

hollow architectures is based on a non-equilibrium inter-

diffusion process, and is schematically displayed in Scheme 1.

First, through a solvothermal method, we succeeded in

synthesizing nearly monodisperse copper sulfide (CuS) and

monodisperse cuprous sulfide (Cu2S) with different morphol-

ogies (Scheme 1a). Subsequently, these precursors were

thermally oxidized in air at 700 8C. Simultaneously, core/

shell-structured intermediates formed (Scheme 1b). Since the

diffusion rate of the inner sulfides is larger than that of

atmospheric oxygen during the oxidation reactions, voids are

thus generated, which eventually results in a hollow cavity.

Owing to the volume loss and release of internally born sulfur

dioxide (SO2) in the process of interdiffusion, porous CuO

hollow architectures were finally obtained (Scheme 1c).

X-ray diffraction (XRD) patterns of as-synthesized CuO

products are shown in Figure S1 (see Supporting Information).

All peaks in Figure S1a and S1b match well with monoclinic

CuO (JCPDS No. 05-0661) with cell parameters a¼ 4.684,

b¼ 3.425, c¼ 5.129 A, and b¼ 99.47 8. No other peaks of

impurities were detected, which indicates that these CuS and

Cu2S precursors have been completely converted into CuO.

Energy dispersive X-ray (EDX) analysis was also performed

for these CuO structures, as shown in Figures S2f and S3f (see

Supporting Information). Only copper and oxygen elements

were detected, and their atomic ratio is about 1: 1, which is in

agreement with the stoichiometric ratio of CuO.

The morphology of the as-prepared CuO converted from

CuS is shown in Figure 1c. It can be seen that particles of the

CuO products have a doughnut-shaped architecture, which

preserves their precursor (CuS) morphology (Fig. 1a). The

structural details were revealed in high-magnification scanning

electron microscopy (SEM) images (Figs 1c,d). It can be

observed clearly that these CuO hollow particles have a porous

appearance and are composed of many CuO nanoparticles and

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Figure 1. SEM images of CuS precursor particles and their conversion into CuO products atdifferent reaction stages, summarizing all major morphological changes involved in non-equili-brium interdiffusion, volume loss, and gas release: a) doughnut-shaped CuS precursor, b) formationof a thin layer of intermediates on the surface of the CuS particles by thermal oxidation of CuS in airat 700 8C after 0.5 h reaction, c) formation of porous and hollow CuO doughnut-shaped architectureby complete oxidation of CuS after reaction for 4 h, d) a typical CuO particle showing its porous shell,e) a typical CuO particle showing its hollow interior. The inset of (b) shows a broken core/shellparticle. Scale bars are 500 nm unless otherwise indicated.

Scheme 1. Schematic of the procedure used to fabricate porous CuO hollow architectures: a) uniform doughnut-shaped CuS and spheric Cu2S (notshown) are used to represent the starting precursors, b) formation of CuO at the shell of CuS in the thermal oxidation process, c) continual growth of CuOfrom CuS, which involves the non-equilibrium interdiffusion, volume loss, and release of internally born SO2, and eventual formation of porous CuO hollowarchitectures.

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nanopores (Fig. 1d). The average diameter

and thickness of these doughnut-shaped

particles are about 1.7mm and 600 nm,

respectively. Figure 1e shows a broken

CuO particle, which indicates that these

porous particles are hollow. The low-

magnification SEM image (Fig. S4a, see

Supporting Information) shows that these

CuO particles are uniform in size. This

procedure has also been extended to Cu2S

crystal templates. Using the same strategy,

porous CuO hollow spheres were also

obtained. Figure S4b (see Supporting

Information) is a low-magnification SEM

image, which clearly shows that there exist

a great number of spheric CuO particles

with uniform size (around 1mm). The

magnified SEM images (Fig. 2c,d) show

that the surfaces of these particles are also

composed of many nanoparticles and

nanopores. More careful observation of a

typical porous structure shown in Fig. 2e

indicates that this porous structure is also

hollow. More broken hollow architectures

can be obtained by ultrasonication for

20 min in an ultrasonic water bath (Fig. S5,

see Supporting Information). The current

chemical strategy is versatile and can be

applied to Cu2S with different diameters

(Fig. S8, see Supporting Information).

Since the CuO products were synthesized

at a relatively high temperature (700 8C),

these porous and hollow architectures

have high thermal stability.

In our process (thermal oxidation at

700 8C), the precursor can be converted

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Figure 2. SEM images of Cu2S precursor particles and their conversion into CuO products at different reactionstages, summarizing all major morphological changes involved in non-equilibrium interdiffusion, volume loss,and gas release: a) monodisperse spherical Cu2S precursor, b) formation of a thin layer of intermediates on thesurface of the Cu2S particles by thermal oxidation of Cu2S in air at 700 8C after 0.5 h reaction, c) formation ofporous CuO hollow spheres by complete oxidation of Cu2S after reaction for 4 h, d) a typical CuO particleshowing its porous shell, e) a typical CuO particle showing its hollow interior. The inset of (b) shows a brokencore/shell particle. Scale bars are 500 nm unless otherwise indicated.

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into hollow CuO completely, and the formation of hollow

structures is believed to be the result of non-equilibrium

interdiffusion (the Kirkendall effect), which was introduced

into nanomaterials synthesis and has been well explained by

previous researchers.[9,10,17] At the beginning of the thermal

oxidation of CuS, a thin intermediate shell was formed on the

surface of these particles. This thin layer acts as an interface

and separates the inner CuS from the outside atmospheric

oxygen. However, the interface consists of an intermediate

shell with lots of vacancies. This kind of shell structure allows

the out diffusion of inner sulfides and vacancies. A net

directional flow of precursor/target materials at the template/

reactant interface may lead to the formation of voids in the

products. As evidence, intermediate core/shell structures

have indeed been collected in our experiments in which the

reagents reacted only for a short time (0.5 h) (Fig. 1b, 2b), and a

broken core/shell doughnut-shaped particle and a spherical

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particle are shown in the inset of

Figure 1b and 2b, respectively.

EDX analysis of these core/shell

products shows that they contain

Cu, O, and S (Fig. S2d,3d).

In order to further confirm

the formation mechanism of

these hollow CuO architectures,

we used transmission electron

microscopy (TEM) to investi-

gate the structure of products at

different reaction stages. For a

doughnut-shaped CuS precursor

(Fig. 3a), CuS and O2 in the

atmosphere can react with each

other readily to form CuO in a

high temperature environment.

When the reaction time was

prolonged, the core/shell inter-

mediate product was obtained

(Fig. 3b). Finally, a porous and

hollow CuO structure was col-

lected when the sulfide was

completely converted into oxide

(Fig. 3c). This structural expan-

sion has also been observed by

high-magnification transmission

electron microscopy (HRTEM).

The image of a lattice fringe in

Figure 3d shows an interplanar

distance of d111¼ 0.232 nm in

monoclinic phase CuO. The

selective area electronic diffrac-

tion (SAED) image (Fig. S6b,

see Supporting Information)

shows that the as-prepared

CuO is polycrystalline. TEM

characterization of Cu2S parti-

cles and their conversion into

porous hollow CuO spheres is shown in Fig. S7.

As for the formation of porous structures composed of

nanoparticles and nanopores, it is believed that volume loss

and release of internally born SO2 during the conversion from

CuS and Cu2S into CuO generates some pores in the

architectures. The phase separation for the solid and pores

results in perforated structures. Volume loss has been used

to synthesize porous materials in previous research.[18] As

illustrated in Scheme 1, the newly formed intermediate layer

was very densely coated on the perfect CuS microcrystals,

which can effectively block the release of internally born SO2.

More SO2 was gathered gradually in the interior of the

structure until the pressure was increased to a certain point to

break the structure for the formation of a small pore, and

finally, a porous shell formed. The oxidation rate has an

important effect on the morphology of the final product. When

the oxidation was carried out in a furnace at a previously

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Figure 3. TEM characterization of CuS precursor particles and their conversion into CuO products atdifferent reaction stages. a) TEM image of a doughnut-shaped CuS precursor, b) TEM image of acore/shell intermediate product synthesized by thermal oxidation of CuS in air for 0.5 h, c) TEMimage of a porous hollow CuO doughnut-shaped architecture obtained by thermal oxidation of CuSin air for 4 h, d) HRTEM image of the selected area in image (c). Scale bars are 500 nm unlessotherwise indicated.

Figure 4. UV-Vis absorption spectra of the as-synthesized porousCuO hollow doughnut-shaped architecture (a) and porous CuO hollowspheres (b).

maintained temperature of 700 8C, the non-equilibrium

interdiffusion, volume loss, and gas release accelerated, the

hollow architecture then collapsed, and finally, porous bowl-

shaped CuO products were obtained with doughnut-shaped

CuS as the precursor (Fig. S9, see Supporting Information).

UV-Vis absorption measurements were used to reveal the

optical properties of the as-prepared CuO nano/micocrystal.

UV-Vis spectra of the porous CuO hollow architectures are

presented in Figure 4. Both porous hollow structures exhibit a

broad peak centered at about 250 nm. Based on the absorption

edges (lonset¼ 495 nm, bandgap energy Eg¼ 1241/lonset),[19]

the bandgaps of both the porous hollow doughnut-shaped and

spheric CuO are calculated to be about 2.5 eV, which is much

larger than those of bulk materials (Eg¼ 1.2 eV), which

indicates that the diameter of the tiny subunits (nanoparticles)

of these architectures are in or near the quantum-confined

regime.

During the thermal oxidation process, the size and shape of

the hollow CuO particles are strongly dependent on the CuS

and Cu2S precursors. Therefore, the controlled synthesis of

CuS and Cu2S is important to obtain hollow CuO with

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well-defined shape and size. In our work,

precursors with different morphologies

can be successfully synthesized by a

low temperature solvothermal method.

Figire S10a (see Supporting Information)

shows an XRD pattern for CuS, and all of

the peaks can be readily indexed to the

hexagonal CuS structure (JCPDS No.

06-0464) with cell parameters a¼ 3.792

and c¼ 16.34 A. An XRD pattern of Cu2S

is shown in Figure S10b, which reveals

that pure Cu2S crystals with a cubic

structure (JCPDS No. 84-1770) are pre-

pared. Low-magnification images of CuS

and Cu2S are shown in Figure S11 (see

Supporting Information), which shows

that these precursors are nearly mono-

disperse. The formation process of the

three-dimensional (3D) hierarchical

doughnut-shaped CuS is shown in

Figure S12 (see Supporting Information).

In addition to CuO, some other porous

transition-metal oxide (3D NiO polyhe-

drons and Co3O4 spheres) hollow struc-

tures have been fabricated by thermal

oxidation of the corresponding metal

sulfides (Fig. S13c–f, see Supporting In-

formation), which illustrates the general-

ity of this approach. Furthermore, when

we used metal selenides such as one-

dimensional (1D) copper selenide (CuSe)

microfibers as the precursor, porous

CuO hollow microfibers (Fig. 5a,b) were

obtained, which also confirms the gene-

rality of the current strategy (low-

magnification SEM images of the CuSe precursor and CuO

products are shown in Fig. S13a,b, see Supporting Infor-

mation). It is evident that the current technique for conversion

of metal chalcogenide nanostructure into a metal oxide

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Figure 5. SEM images of porous CuO hollow microfibers obtained by thermal oxidation of CuSe at700 8C for 4 h. The inset of (a) shows the magnified SEM image of a single hollow microfiber, scalebar: 500 nm.

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nanostructure is applicable to structures of different dimen-

sions (1D: CuSe; 3D: Cu2S, CuS, NiS and CoS).

In conclusion, we have demonstrated a thermal oxidation

strategy for the chemical synthesis of porous metal oxide

hollow architectures. High-porosity hollow doughnut-shaped

and spherical CuO structures have been synthesized by

thermal oxidation of the corresponding CuS and Cu2S

precursors, respectively. The combination of non-equilibrium

interdiffusion, volume loss, and gas release has been proposed

to explain the formation of these porous hollow architectures.

The current chemical strategy is quite versatile and can be

extended to other transition-metal sulfides and metal selenides.

A series of porous hollow metal oxides were successfully

obtained by thermal oxidation of 3D NiS polyhedrons and CoS

spheres and 1D CuSe microfibers. Most importantly, it has a

good potential for large-scale fabrication of highly porous

metal oxide materials, which will have promising applications

in catalysis, Li ion batteries, and sensors.

Experimental

CuS and Cu2S: CuS and Cu2S were prepared by a solvothermaltechnique in a Teflon-lined stainless steel autoclave. In a typicalsynthesis of 3D hierarchical doughnut-shaped CuS, 2 mmol ofCu(NO3)2 � 3H2O was dissolved in 25 mL of dimethyl sulfoxide(DMSO) to form a clear solution, and then 0.4 g of poly(vinylpyrrolidone) (PVP, Mw¼ 80 000) and 4 mmol of thiourea were addedto this solution under vigorous stirring. Afterwards, this solution wastransferred into a 30 mL Teflon-lined stainless steel autoclave. Theautoclave was sealed and maintained at 120 8C for 20 h. After thesolution was cooled to room temperature, the obtained black solidproducts were collected by centrifuging the mixture, and were thenwashed with absolute ethanol and deionized water several times anddried at 60 8C for 6 h for further characterization. For monodisperseCu2S sub-microspheres, N,N-dimethylformamide (DMF) was usedinstead of DMSO while keeping other the reaction conditionsunchanged.

Porous CuO Hollow Architectures: CuO was prepared bythermal oxidation of CuS and Cu2S in air under atmospheric pressure.The oxidation was carried out inside a quartz tube inserted in ahorizontal tube furnace at a heating rate of 20 8C min�1, the

www.advmat.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

temperature was then maintained at 700 8Cfor 4 h. After the desired period, the furnacewas gradually cooled to room temperature.

Other Porous Metal Oxide (NiO Polyhed-rons and Co3O4 Spheres) Hollow Architecturesand Porous CuO Hollow Microfibers: Thecorresponding metal sulfides were synthesizedby hydrothermal treatment of 2 mmol of MNO3

(M¼Ni and Co), 4 mmol of KSCN, and 0.2 g ofPVP at 220 8C for 20 h. After that, the productswere thermally oxidized at 700 8C for 4 h.Porous CuO hollow microfibers were obtainedby thermal oxidation of CuSe microfibers,which were synthesized by solvothermal treat-ment of Cu(NO3)2 and Na2SeO3 in formamide.

In general, the thermal oxidation of naturaland precipitated CuS includes four steps: [20]a) formation of lower sulfur content sulfides(Cu1.8S and/or Cu2S) associated with oxidationof the evolved sulfur to SO2, b) oxidation of the

existing sulfides either to oxides (Cu2O and CuO, the latter can beformed also by Cu2O oxidation), or two copper sulfates (Cu2SO4,CuSO4, the latter is formed either by direct oxidation of Cu2O, or bythe reaction between liberated SO2 and Cu2O), c) oxysulfate formation(CuSO4, and CuO �CuSO4) by reaction between CuO, Cu2O, oxygen,and liberated SO2, d) decomposition of oxysulfates with CuOformation.

Characterization: The as-prepared samples were characterizedby XRD on a Rigaku-DMax 2400 diffractometer equipped with agraphite monochromatized Cu Ka radiation flux at a scanning rate of0.02 8S�1 in the 2u range 5 8 to 80 8. SEM images were taken with aJEOL-5600 LV scanning electron microscope, using an acceleratingvoltage of 20 kV. EDX microanalysis of the samples was performedduring SEM measurements. The structures of the as-prepared sampleswere investigated by TEM (Philips, TecnaiG2 20, operated at 200 kV).UV/Vis diffuse reflectance spectra were recorded on UV-vis-NIRspectrophotometer (JASCO, V-550).

Received: January 22, 2008Revised: March 3, 2008

Published online: June 2, 2008

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