Journal of Colloid and Interface Science 308 (2007) 460–465www.elsevier.com/locate/jcis

3-D ordered macroporous cuprous oxide: Fabrication, optical, andphotoelectrochemical properties

Xun Li, Feifei Tao, Yuan Jiang, Zheng Xu ∗

State Key Laboratory of Coordination Chemistry and Laboratory of Solid State Microstructure, School of Chemistry and Chemical Engineering,Nanjing University, Jiangsu 210093, People’s Republic of China

Received 24 October 2006; accepted 14 December 2006

Available online 30 January 2007

Abstract

Cuprous oxide 3-D ordered macroporous material was constructed by electrochemical deposition using a polystyrene colloidal crystal astemplate. The highly ordered macroporous structure with a hexagonal array can be extended over hundreds of square micrometers. The photonicstop bands of both the PS colloidal crystal and Cu2O 3DOM were found. Due to the highly ordered porous structure, the optical absorption andthe charge carrier transportation are better in Cu2O 3DOM than in bulk Cu2O, which makes the reduction of oxygen faster on Cu2O 3DOM thanon bulk Cu2O under visible light illumination. The higher photocurrent efficiency under visible light illumination makes the 3DOM Cu2O moresuitable for solar applications.© 2006 Elsevier Inc. All rights reserved.

Keywords: Cuprous oxide; Macroporous; Photonic stop band; Photoelectrochemistry; Visible light; Reduction of oxygen

1. Introduction

Three-dimensional ordered macroporous (3DOM) materi-als (especially 3DOM semiconductor materials) with uniformpore diameters more than 50 nm, bicontinuous networks, andlarge accessible surfaces have a wide range of applicationsin photonic bandgap crystals [1,2], solar cells [3,4], and gassensors [5,6]. To date, many 3DOM semiconductor materialshave been synthesized, such as Si [1], Ge [7–9], GeO2 [7],TiO2 [10–12], CdS [13–15], CdSe [13,15–17], CdTe [15,17],ZnO [18,19], ZnSe [15], PbSe [15], GaAs [15], SnO2 [5,6],and SnS2 [2]. Most of them use colloidal crystal as a tem-plate [20]. The critical step of this approach is filling up withsemiconductor materials, because incomplete fill always resultsin the disruption of original ordered arrays after the templateis removed. Nowadays, many fill-up techniques, such as sol–gel chemistry [5–7,10–12], chemical vapor deposition (CVD)[1,2,8], nanocrystal deposition [16,17], and electrodeposition[9,13,15,19], have been developed. Of these, the CVD route al-

* Corresponding author. Fax: +86 25 83314502.E-mail address: zhengxu@netra.nju.edu.cn (Z. Xu).

0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.12.044

ways deposits material on the top few layers of the colloidaltemplate. For nanocrystal deposition, it is difficult to fully fillthe interstitial space with semiconductor nanocrystals and sub-sequent sintering may destroy the macroporous array. The sol–gel chemical approach can deeply fill the space within a col-loidal crystal, but after the original colloidal template and smallmolecular by-products are removed by pyrolysis, the replicasare easy to crack due to shrinkage. Comparably, electrodeposi-tion is an effective approach to fabricating 3DOM semiconduc-tor materials, by which the morphology and thickness of themacroporous film can be easily controlled via simply adjust-ing the electrochemical parameters such as current, potential,or deposition time. Most important, it is a bottom-up deposi-tion technique and allows a high filling ratio of materials in theinterstice of the template because the macroporous film growsupwards from the conductive substrate.

Cu2O has been used as a biocide since the early 19th cen-tury and continues to be one of the most important componentsof modern antifouling and algicide products [21–23]. It is ur-gently necessary to enhance its antifouling and algicide effectvia nanotechnology. Perhaps it is a good method to embed an-tifouling and algicide drugs into Cu2O macroporous materials.Both the cooperative antifouling effect and slowly releasing the

X. Li et al. / Journal of Colloid and Interface Science 308 (2007) 460–465 461

drugs will remarkably enhance their antifouling effect. Also,Cu2O is a p-type semiconductor and has a direct band gap ofabout 2.17 eV [24], which makes it very suitable for solar en-ergy applications, such as overall water splitting [25], solar cell[26], electrochemical photovoltaic cell [27], electrode materialfor lithium ion batteries [28], and photocatalyst for degradationof organic pollutants under visible light [29]. So the photo-electrochemical properties of Cu2O in aqueous solution are aninteresting subject. Kelly’s group reported the current–potentialcharacteristics for a 0.5-µm-thick film of Cu2O under chopped350 nm illumination from an Ar ion laser and found that oxygenis reduced to hydrogen peroxide through a multistep reaction onthe photocathode of Cu2O. They predicted that Cu2O could beused as a p-type photoelectrode in an electrochemical photo-voltaic cell [27]. It is more valuable for practical applicationif the illumination light source can be extended into the visi-ble range. Based on a band gap of about 2.17 eV, Cu2O will beexpected to have excellent photoelectrochemical properties un-der visible light. Several works on bulk Cu2O photocathodesunder white light illumination have appeared [30–32]. How-ever, to date, the fabrication and photoelectrochemical behaviorof Cu2O 3DOM photocathode under visible light illuminationhave not yet been reported.

In this paper, we report that, using hexagonal close-packedcolloidal arrays of polystyrene (PS) assembled on an ITO sub-strate as a template, a 3DOM Cu2O superstructure was success-fully constructed by electrochemical deposition. The photonicstop band of Cu2O 3DOM was observed, which was red-shiftedto ∼908 nm in comparison with PS colloidal crystal templatedue to the higher infraction index of Cu2O; enhanced photocur-rent efficiency of Cu2O 3DOM was also found. A study for theantifouling effect is under way and will be published elsewhere.

2. Experimental

2.1. Materials

Styrene (C.P.), potassium persulfate (A.R.), ethanol (A.R.),acetone (A.R.), cupric sulfate (A.R.), ammonia (25%, A.R.),sodium sulfate (A.R.), lactic acid (A.R.), sodium hydroxide(A.R.), tetrahydrofuran (THF) (A.R.), and glass slides coatedwith ITO (indium tin oxide) (Rs = 30 �) were all commer-cially available products. Styrene was washed three times with10 wt% NaOH solution, followed by another three times withdeionized water, before use. ITO/glass was washed with ace-tone, ethanol, and deionized water, under sonication before use.Other materials were used without further purification.

2.2. Synthesis of polystyrene (PS) microspheres

PS microspheres with a diameter of 350 ± 17 nm weresynthesized using the surfactant-free emulsion polymerizationmethod [33]. In brief, an aqueous emulsion composed of 60wt% ethanol, 34.9 wt% deionized water, 5 wt% styrene, and0.1 wt% potassium persulfate was polymerized for 24 h at 70 ◦Cunder nitrogen. After completion of the polymerization, the la-

tex particles were washed with deionized water several timesby centrifugation and filtration.

2.3. Fabrication of PS colloidal crystal template

The PS colloids were coated on an ITO/glass substrate bymeans of vertical deposition (VD) [34]. The substrate (2.5 ×1.0 cm) was placed in a vial containing 0.8 wt% PS colloidalsuspension with a tilt angle of 10◦ [35]. The vial was thenheated in an incubator at 65 ◦C [36] until the solvent was com-pletely evaporated.

2.4. Fabrication of Cu2O 3-D ordered macroporous (3DOM)materials

The Cu2O 3-D ordered macroporous materials were con-structed by electrochemical reduction of copper(II) lactate in al-kaline solution [37]. The electrolyte solution consisted of 0.4 MCuSO4 and 3 M lactic acid and the pH of the solution wasadjusted to 12 by 10 wt% NaOH solution. Cu2O was grownpotentiostatically in a three-electrode system controlled by anEG&G PAR 273A potentiostat. ITO-substrates-covered PS col-loidal crystal was used as the working electrode with a copperwire counter electrode and a saturated calomel reference elec-trode (SCE). Potentiostatic deposition was performed at −0.5 Vvs SCE for 15 min and the deposition temperature was kept con-stant at 45 ◦C. The Cu2O 3-D ordered macroporous materialswere obtained after removing PS in THF solution. For com-parison, the bulk Cu2O was electrodeposited on the bare ITOsubstrates with the same area and the same electrodepositiontime by the same methods.

2.5. Characterization

The morphologies of the PS microspheres, PS colloidal crys-tals, and Cu2O 3DOM materials were imaged with a JEOLJEM-5610LV scanning electron microscope (SEM) operatingat 15 kV. The powder XRD analysis was performed using a Shi-madzu XRD-6000 X-ray diffractometer with graphite mono-chromatized CuKα radiation (λ = 0.15406 nm). The near-infrared transmission spectra of PS colloidal crystals and Cu2O3DOM were recorded by a Shimadzu UV-3100 UV/vis/near-infrared spectrophotometer. The UV/vis absorption spectra ofthe Cu2O were recorded by a Perkin–Elmer Lambda 35 UV/visspectrometer.

2.6. Electrochemical characterization

The photoelectrochemical measurements were performed ina three-electrode system [27], using an EG&G PAR 273A po-tentiostat, a platinum counter electrode, and a saturated calomelreference electrode (SCE). Both Cu2O 3DOM materials andbulk Cu2O on ITO with an area of 1 × 1 cm were used as work-ing electrodes. The electrolyte containing 0.5 M Na2SO4 wasbubbled with oxygen for 5 min before measurement. A 441.6-nm He/Cd laser (KIMMON IK5452R-E) was used as a lightsource with an output power of 89.7 mW.

462 X. Li et al. / Journal of Colloid and Interface Science 308 (2007) 460–465

Fig. 1. (A) SEM image of PS microspheres; (B) a cross-section view and (C) a top view of the PS colloidal crystal template arrays on an ITO substrate; (D) opticaltransmission spectrum curves of the PS colloidal crystal.

3. Results and discussion

3.1. PS colloidal crystal array

Fig. 1A shows a typical SEM image of the PS microspheressynthesized by the surfactant-free emulsion polymerization.The average particle size is around 340 nm. With the verticaldeposition method under proper conditions, ordered PS col-loidal crystals can be formed on the ITO substrate. Fig. 1Bshows that the crystal is hexagonal close-packed with (111)crystalline plane parallel to the substrate. The cross section ofthe sample reveals the 3D ordered arrays of the colloid par-ticles. Fig. 1C shows a top view of the PS colloidal crystalin the range of hundreds of square micrometers. The arrange-ment of the PS spheres shows a typical hexagonal close-packed(hcp) structure with very few defects. These colloidal crys-tals made from PS spheres, were bright-colored because ofoptical diffraction on regular multilayers. The optical transmis-sion spectrum of the sample showed the existence of a pro-nounced photonic stop band centering at 785 nm due to theBragg reflection on the (111) plane (Fig. 1D). The positionof the stop band is size-dependent, followed by the equation[38] λ = 2dhkl(n

2eff − sin2 θ)1/2, where θ is the angle between

the incident light and the normal direction of the (hkl) plane.Because all the measurements in this study were taken alongthe normal direction of the (111) plane (θ = 0◦), the equa-tion above can be simplified to λ = 2d111neff, where neff isthe effective refractive index of the latex/air composite, withn2

eff = n2psf + n2

air(1 − f ); f = 0.74 is the filling factor for a

close packed structure; d111 = (2/3)1/2D is the distance be-

tween crystalline planes (111); D is the PS sphere diameter.Using λ = 785 nm and nps = 1.59, the calculated PS spherediameter is ∼330 nm, which is in agreement with the averagevalue from the SEM image.

3.2. Cu2O 3DOM materials

Fig. 2A shows a typical SEM image of the Cu2O 3DOMobtained from a template composed of 340-nm PS colloidalcrystals. The highly ordered macroporous structure can be ex-tended over hundreds of square micrometers. Cu2O was grownin the space among the highly ordered PS spheres to form theCu2O 3DOM, while Cu2O grew faster at the line defects ofthe PS templates to form the ridge (Fig. 2B). It can be seen thatmost of the pores are highly ordered in a hexagonal array, whichis consistent with the (111) plane of an fcc structure and copiesthe structure of the original latex templates (Fig. 2C). The mag-nification SEM image of the (100) plane of the Cu2O 3DOM isshown in Fig. 2D. There are three small pores in the hexagonalbig pores (Fig. 2C) and four small pores in the square big pores(Fig. 2D), which indicates that the big pores are interconnectedwith small channels. As we known from Fig. 1B, PS spheres inthe array closely contact each other and the electrochemical de-position is unable to reach the places where the PS spheres arein close contact, so small pores remain after PS spheres are re-moved. From Fig. 2C, the wall thickness of ∼40 nm and thecenter-to-center distance of ∼350 nm between the pores can bedetermined.

The photonic stop band of the Cu2O 3DOM centered at908 nm clearly appeared in the optical transmission spectrum

X. Li et al. / Journal of Colloid and Interface Science 308 (2007) 460–465 463

Fig. 2. (A) Typical SEM images of Cu2O 3DOM; (B) the magnification of (A); (C) image of (111) crystalline planes; (D) image of (100) crystalline planes.

of the sample (Fig. 3A). Based on the stop band λ = 908 nmand nCu2O = 2.72 [39], a pore diameter of 341 nm can be ob-tained from the above equation. The calculated pore diameter isin good agreement with the average size of pores from the SEMimage. Fig. 3B is the XRD pattern of the macroporous film,from which four distinct diffraction peaks and a broad amor-phous peak between 15◦ and 38◦ can be observed. The broadpeak belongs to the glass substrate. The four diffraction peakscan be indexed as (110), (111), (200), and (220) planes of fccCu2O, which is in good accordance with PDF No. 78-2076.

3.3. Photoelectrochemical properties of Cu2O 3DOMmaterials

As a p-type semiconductor, Cu2O has been widely studiedbecause it has a potential application in photoelectrochemicalcells. The optical properties of the as-obtained macroporousCu2O and bulk Cu2O have been determined by the UV–vis ab-sorption spectra. Using ITO–glass as a reference, the absorptionspectra of the macroporous Cu2O and bulk Cu2O are shown inFig. 4. It can be seen that the electrodeposited bulk Cu2O hasan obvious absorption edge at ∼590 nm, which is consistentwith the reported band gap energy of 2.1 eV [37]. However, theCu2O 3DOM materials have a broad absorption band from 350to 800 nm and two absorption peaks at ∼580 nm and ∼480 nm.

The thickness of both macroporous Cu2O and bulk Cu2Oused above were measured, based on the SEM images in Fig. 5,and it was found that the bulk Cu2O (∼1.8 µm) deposited on thebare ITO–glass was a little thicker than the macroporous Cu2O(∼1.5 µm) deposited on the PS-coated ITO–glass. As the infil-tration of electrolyte solution is easier on the bare ITO–glass

(A)

(B)

Fig. 3. (A) Near-infrared optical transmission spectrum of Cu2O 3DOM;(B) powder XRD patterns of Cu2O 3DOM.

464 X. Li et al. / Journal of Colloid and Interface Science 308 (2007) 460–465

Fig. 4. UV–vis absorption spectra of macroporous Cu2O and bulk Cu2O.

Fig. 5. SEM images of side-face view: (A) bulk Cu2O electrochemical depo-sited for 10 min; (B) macroporous Cu2O electrochemical deposited for 10 min.

than in the interstices of PS spheres, the growth of the bulkCu2O is a little faster than that of the macroporous Cu2O forthe same electrodeposited time.

Although the Cu2O 3DOM film was thinner than the bulkCu2O, the absorption spectrum of the former was significantlystronger than that of the later, which may be attributed to thehighly ordered porous structure in which the incident light is re-flected and absorbed multipletimes [40]. By the way, the smallpeak at 326 nm is due to switching the light source.

As previous report [27], Cu2O is a good cathodic materialfor photoelectroreduction of oxygen. The enhanced absorptionof Cu2O 3DOM materials in Fig. 4 may improve the photoelec-trochemical properties of Cu2O materials, and it is expectedthat Cu2O 3DOM may be better than bulk Cu2O as a photo-cathodic material. The current–potential characteristics (I–V

(A)

(B)

Fig. 6. Current–potential characteristics for (A) Cu2O 3DOM electrode and(B) bulk Cu2O electrode under chopped 441.6-nm (89.7-mW) illumination in0.5 M Na2SO4.

plot) for both Cu2O 3DOM and bulk Cu2O electrodes weremeasured under chopped illumination at 441.6 nm (Fig. 6). Themaximum photocurrent on bulk Cu2O electrode is 70 µA/cm2,which is comparable to or even better than the results in previ-ous work [30–32]. But more important, the cathodic photocur-rent on Cu2O 3DOM electrode is much larger than that of bulkCu2O electrode under 441.6 nm illumination (Fig. 6). As wecan see from Fig. 4, using 500–650 nm as a light source, theresults should be even better.

This may be attributed to two factors. First, the optical ab-sorption of Cu2O 3DOM materials is stronger than that of bulkCu2O. Second, the recombination possibility of the charge car-riers is less in Cu2O 3DOM than in the bulk Cu2O, because thedistance from the place where the charge carrier generated inCu2O to the interface between Cu2O and electrolyte solutionis quite small, and the efficiency of the charge carrier trans-portation is higher in Cu2O 3DOM than in bulk Cu2O. Strongerabsorption and more efficient charge transportation make ca-thodic photocurrent larger in Cu2O 3DOM than in bulk Cu2Ounder visible light illumination. The sharp peak at every be-ginning of the light being on is relative to the concentration ofoxygen in water. Because the bubbled oxygen was stopped dur-ing 200 s scanning time for current stability, the oxygen in thewater was consumed quickly, and therefore, the photocurrentdecreased immediately. After the light was off for 10 s, oxygenin water was supplied from air, and the O2 reduction current

X. Li et al. / Journal of Colloid and Interface Science 308 (2007) 460–465 465

increased at the beginning of the light being on. Due to the con-sumption rate of O2 being higher than the supplement rate, thecathodic photocurrent decreases in a few seconds; therefore, asharp peak of the photocurrent appears in Fig. 6.

4. Conclusions

In conclusion, both the PS colloidal crystal and Cu2O3DOM have been successfully constructed. A highly orderedarray with hundreds of square micrometers was obtained withfcc structure. The photonic stop band of Cu2O 3DOM wasfound at 908 nm. Furthermore, Cu2O 3DOM is a good can-didate for photocathodic materials. The reduction of oxygen ona photocathode made of Cu2O 3DOM was faster than on bulkCu2O under visible light illumination. The higher photocurrentefficiency under visible light illumination makes the 3DOMCu2O more suitable for solar applications.

Acknowledgments

The authors thank the National Natural Science Founda-tion of China (NNSFC) for financial support under Projects90606005, 20571040, and 20371026. We gratefully acknowl-edge Keyu Wang for scanning electron microscopy analysis andDejun Liu for supplying the He/Cd laser.

References

[1] Y.A. Vlasov, X.Z. Bo, J.C. Sturm, D.J. Norris, Nature 414 (2001) 289.[2] M. Müller, R. Zentel, T. Maka, S.G. Romanov, C.M.S. Torres, Adv. Mater.

12 (2000) 1499.[3] A. Mihi, H. Míguez, J. Phys. Chem. B 109 (2005) 15968.[4] I. Rodriguez, P. Atienzar, F. Ramiro-Manzano, F. Meseguer, A. Corma,

H. Garcia, Photon. Nanostruct. Fundam. Appl. 3 (2005) 148.[5] R.W.J. Scott, S.M. Yang, G. Chabanis, N. Coombs, D.E. Williams, G.A.

Ozin, Adv. Mater. 13 (2001) 1468.[6] M. Acciarri, R. Barberini, C. Canevali, M. Mattoni, C.M. Mari, F. Moraz-

zoni, L. Nodari, S. Polizzi, R. Ruffo, U. Russo, M. Sala, R. Scotti, Chem.Mater. 17 (2005) 6167.

[7] H. Míguez, F. Meseguer, C. López, M. Holgado, G. Andreasen, A. Mifsud,V. Fornés, Langmuir 16 (2000) 4405.

[8] H. Míguez, E. Chomski, F. García-Santamaría, M. Ibisate, S. John,C. López, F. Meseguer, J.P. Mondia, G.A. Ozin, O. Toader, H.M. vanDriel, Adv. Mater. 13 (2001) 1634.

[9] L.K. van Vugt, A.F. van Driel, R.W. Tjerkstra, L. Bechger, W.L. Vos,D. Vanmaekelbergh, J.J. Kelly, Chem. Commun. (2002) 2054.

[10] A. Imhof, D.J. Pine, Nature 389 (1997) 948.[11] B.T. Holland, C.F. Blanford, A. Stein, Science 281 (1998) 538.[12] J.E.G.J. Wijnhoven, W.L. Vos, Science 281 (1998) 802.[13] P.V. Braun, P. Wiltzius, Nature 402 (1999) 603.[14] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 453.[15] Y.C. Lee, T.J. Kuo, C.J. Hsu, Y.W. Su, C.C. Chen, Langmuir 18 (2002)

9942.[16] Y.A. Vlasov, N. Yao, D.J. Norris, Adv. Mater. 11 (1999) 165.[17] L. Yang, Z. Yang, W. Cao, L. Chen, J. Xu, H. Zhang, J. Phys. Chem. B 109

(2005) 11501.[18] H. Yan, C.F. Blanford, B.T. Holland, W.H. Smyrl, A. Stein, Chem. Mater.

12 (2000) 1134.[19] T. Sumida, Y. Wada, T. Kitamura, S. Yanagida, Chem. Lett. (2001) 38.[20] A. Stein, R.C. Schroden, Curr. Opin. Solid State Mater. Sci. 5 (2001) 553.[21] K. Thomas, K. Raymond, J. Chadwick, M. Waldock, Appl. Organomet.

Chem. 13 (1999) 453.[22] K. Schiff, D. Diehl, A. Valkirs, Mar. Pollut. Bull. 48 (2004) 371.[23] D.M. Yebra, S. Kiil, K. Dam-Johansen, C. Weinell, Prog. Org. Coat. 53

(2005) 256.[24] C. Kittel, in: Introduction to Solid State Physics, Wiley, New York, 1986.[25] M. Hara, T. Kondo, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka, J.N.

Kondo, K. Domen, Chem. Commun. (1998) 357.[26] L. Papadimitriou, N.A. Economou, D. Trivich, Solar Cells 3 (1981) 73.[27] P.E. de Jongh, D. Vanmaekelbergh, J.J. Kelly, J. Electrochem. Soc. 147

(2000) 486.[28] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407

(2000) 496.[29] J. Ramírez-Ortiz, T. Ogura, J. Medina-Valtierra, S.E. Acosta-Ortiz,

P. Bosch, J.A. de los Reyes, V.H. Lara, Appl. Surf. Sci. 174 (2001) 177.[30] K. Nakaoka, J. Ueyama, K. Ogura, J. Electrochem. Soc. 151 (2004) C661.[31] K.E.R. Brown, K.-S. Choi, Chem. Commun. (2006) 3311.[32] R.P. Wijesundera, M. Hidaka, K. Koga, M. Sakai, W. Siripala, Thin Solid

Films 500 (2006) 241.[33] A.M. Homola, M. Inoue, A.A. Robertson, J. Appl. Polym. Sci. 19 (1975)

3077.[34] P. Jiang, J.F. Bertone, K.S. Hwang, V.L. Colvin, Chem. Mater. 11 (1999)

2132.[35] S.H. Im, M.H. Kim, O.O. Park, Chem. Mater. 15 (2003) 1797.[36] M.A. McLachlan, N.P. Johnson, R.M. De La Rue, D.W. McComb,

J. Mater. Chem. 14 (2004) 144.[37] T.D. Golden, M.G. Shumsky, Y. Zhou, R.A. Vanderwerf, R.A. van Leeu-

wen, J.A. Switzer, Chem. Mater. 8 (1996) 2499.[38] R.C. Schroden, M. Al-Daous, C.F. Blanford, A. Stein, Chem. Mater. 14

(2002) 3305.[39] R.R. Reddy, Y.N. Ahammed, K.R. Gopal, D.V. Raghuram, Opt. Mater. 10

(1998) 95.[40] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Chem. Mater. 18 (2006) 867.