ARTICLE pubs.acs.org/Langmuir
Three-Dimensional Type II ZnO/ZnSe Heterostructures and Their Visible Light Photocatalytic Activities Seungho Cho,†,§ Ji-Wook Jang,†,§ Jungwon Kim,‡ Jae Sung Lee,† Wonyong Choi,‡ and Kun-Hong Lee*,† †
Department of Chemical Engineering and ‡School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk, Korea 790-784
bS Supporting Information ABSTRACT: We report a method for synthesizing three distinct type II 3D ZnO/ZnSe heterostructures through simple solution-based surface modification reactions in which polycrystalline ZnSe nanoparticles formed on the surfaces of singlecrystalline ZnO building blocks of 3D superstructures. The experimental results suggested a possible formation mechanism for these heterostructures. The formation of the ZnO/ZnSe heterostructures was assumed to result from a dissolution recrystallization mechanism. The optical properties of the 3D ZnO/ZnSe heterostructures were probed by UVvis diffuse reflectance spectroscopy. The 3D ZnO/ZnSe heterostructures exhibited absorption in the visible spectral region. The visible photocatalytic activities of 3D ZnO/ZnSe heterostructures were much higher than those of the 3D pure ZnO structures. The activities of the 3D ZnO/ZnSe heterostructures varied according to the structures under visible light. The morphologies and exposed crystal faces of pure ZnO building blocks prior to surface modification had a significant effect on the visible light photocatalytic processes of ZnO/ZnSe heterostructures after surface modification.
1. INTRODUCTION The world’s population is increasing, and this population growth is expected to continue until at least midcentury. Consequently, the demand for energy is also increasing. Fossil fuels are limited in supply and will one day be depleted. Fossil fuel combustion creates carbon dioxide, the number one greenhouse gas that contributes to detrimental changes in the earth’s climate, such as global warming. Thus, researchers living in the current fossil-fuel-based energy system and feeling its limitations have accelerated the development of alternative energy technologies. Solar energy is an inexhaustible resource and is in abundant supply on all continents of the world. Finding a viable way to tap solar energy is urgently needed.1,2 Photocatalytic reactions and materials for solar energy conversion have been studied extensively. Visible light accounts for 4447% of the solar energy spectrum, whereas UV light accounts for 35%. Therefore, efficient use of visible light is an essential prerequisite for the efficient use of solar energy. Various semiconducting materials have been used as photocatalysts that utilize solar energy.2 Among them, metal oxide nanostructures are promising materials because of their high stability and well-established synthesis routes. However, the “Achilles’ heel” of most metal oxides for use in solar materials is that they do not absorb significantly in the visible region of the spectrum because of the deep valence band positions (O 2p orbitals), which hinders their efficient use in solar energy capture because photoexcitation of the semiconductor must proceed by the absorption of light of an energy equal to or exceeding the band gap energy. r 2011 American Chemical Society
Zinc oxide (ZnO, space group = P63mc, a = 0.32495 nm, c = 0.52069 nm), an important and promising metal oxide, is a IIVI semiconductor with a direct band gap of 3.37 eV at room temperature and a large exciton binding energy of approximately 60 meV. ZnO has useful characteristics, such as a large piezoelectric constant and good biocompatibility. ZnO forms a rich variety of nanostructures, ranging from nanorods to nanoflowers.312 In particular, hierarchically self-assembled 3D ZnO superstructures constructed using 1D and 2D ZnO nanoscale building blocks have been extensively investigated13,14 because of their outstanding electronic, optical, and catalytic properties.1517 Three-dimensional ZnO architectures can prevent structural aggregation, which preserves the large catalytically active surface area and the interstitial channels that permit the diffusion of chemicals into the structure, thereby improving the efficiency of surface reactions. ZnO is also a useful material for photoelectrochemical processes and photocatalysis.18 It has an electron mobility (205 300 cm2 V/s) that is higher than that of TiO2 (0.14 cm2 V/s).19 However, ZnO cannot absorb visible light and thus makes use of only 35% of the solar spectrum that reaches the earth because of its wide band gap. Several research approaches have been developed in an effort to overcome this limitation.2 One approach involves introducing anionic nonmetal doping into the ZnO structures. Doping with C, S, or N was postulated to Received: May 10, 2011 Revised: July 5, 2011 Published: July 05, 2011 10243
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Figure 1. (a, b) SEM images of S1 structures synthesized from the reaction of an aqueous solution containing 0.03 M zinc acetate dihydrate and 0.1 M sodium peroxide at room temperature for 5 h. (c) Schematic illustration of a building block of S1. (d, e) SEM images of S2 structures synthesized from the reaction of an aqueous solution containing 0.01 M zinc acetate dihydrate and 0.2 M sodium peroxide at 90 °C for 5 h. (f) Schematic illustration of a building block of S2. (g, h) SEM images of S3 structures synthesized from the reaction of an aqueous solution containing 0.02 M zinc acetate dihydrate and 0.03 M sodium peroxide at 90 °C for 5 h. (i) Schematic illustration of a building block of S3.
reduce the band gap in wide-band-gap metal oxides.20 ZnO doping has been tested with C or N.21,22 The introduction of dopants into the ZnO crystal lattice produced an intermediate energy level in the band gap and reduced the absorption energy.21,23 However, dopant incorporation also increased the number of carrier recombination centers, thereby reducing the photocatalytic activity.24,25 Semiconductor heterojunction formation is another promising approach to simultaneously improving the optical absorption capacity while reducing the propensity for charge recombination.26,27 To utilize visible light, narrow-band-gap semiconductors have been coupled to ZnO nanostructures. ZnO/CdX (X = S, Se, or Te) heterostructures have been described for this purpose.2830 However, many cadmium compounds are carcinogenic, and cadmium metal itself presents a cancer risk. Zinc selenide (ZnSe) is a candidate for producing type II ZnO heterostructures because of its narrower band gap (2.67 eV) than that of ZnO, and its valence and conduction band alignments are staggered relative to those of ZnO. However, investigations of ZnO/ZnSe heterostructures have been sparse.3133 Wang et al. fabricated ZnO/ZnSe core/ shell nanowire arrays by combining chemical vapor deposition and pulsed laser deposition.31 Zhang et al. synthesized ZnO nanorod/ZnSe nanoparticle composites using a hydrothermal method.32 However, these methods required expensive and sophisticated equipment or long reaction times. Recently, our research groups reported a solution method for synthesizing exposed-crystal-face-controlled 3D ZnO superstructures under mild conditions without organic additives.34 These distinct 3D ZnO superstructures were converted to three distinct type II 3D ZnO/ZnSe heterostructures through solution-based surface modifications. The surface modification
process was simple and fast (1 min). The designed type II energy band alignment in ZnO/ZnSe heterostructures was expected to utilize visible light for photocatalysis. We discuss a possible mechanism for the formation of the 3D ZnO/ZnSe heterostructures. The optical properties and visible light photocatalytic activities of three distinct 3D ZnO/ZnSe heterostructures are also examined.
2. EXPERIMENTAL SECTION Synthesis of S1 ZnO Structures. All chemicals used in this study were of analytical grade and were used without further purification. An aqueous solution containing 0.03 M zinc acetate dihydrate (Zn(CH3COO)2 3 2H2O, 99%, Samchun) and 0.1 M sodium peroxide (Na2O2, Samchun) was prepared and maintained at room temperature for 5 h. After the reaction, the solution was filtered through a polycarbonate membrane filter (ISOPORE). The filtered powders were washed several times with deionized (DI) water and dried in an oven at 60 °C for 12 h. Synthesis of S2 ZnO Structures. An aqueous solution containing 0.01 M zinc acetate dihydrate and 0.2 M sodium peroxide was prepared at room temperature. The solution was maintained at 90 °C for 5 h, followed by filtration. The filtered powders were washed several times with DI water and dried in an oven at 60 °C for 12 h. Synthesis of S3 ZnO Structures. An aqueous solution containing 0.02 M zinc acetate dihydrate and 0.03 M sodium peroxide was prepared at room temperature. The solution was maintained at 90 °C for 5 h, followed by filtration. The filtered powders were washed several times with DI water and dried in an oven at 60 °C for 12 h. Synthesis of 3D ZnO/ZnSe Heterostructures (H1, H2, or H3). S1, S2, or S3 (0.1 g) was dispersed in a 50 mL aqueous solution containing 0.004 M sodium selenite (Na2SeO3, 99%, Aldrich) and 1.5 M 10244
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Figure 2. (a, b) SEM images of H1 structures synthesized from the reaction of S1 powders in an aqueous solution containing 0.004 M sodium selenite and 1.5 M hydrazine monohydrate at 90 °C for 5 h. (c) XRD pattern of H1 powders. (d, e) TEM images of an H1 structure. (f) SAED pattern of the building block of the H1 structure. (g) HR-TEM image of the area indicated by the circle in e. (h) TEM image of a part of an H1 structure. (i) Elemental mapping of Zn. (j) Elemental mapping of O. (k) Elemental mapping of Se. hydrazine monohydrate (N2H4 3 H2O, Samchun). The dispersed ZnO solution was loaded into a microwave reactor (Discover SP, CEM Corporation). The solution was maintained at 140 °C for 1 min, cooled to 95 °C as described in Figure S1 of the Supporting Information, and then filtered. The filtered powders were washed several times with DI water and dried in an oven at 60 °C for 12 h. Characterization. The morphology, crystallinity, crystalline nature, chemical composition, and optical properties of the samples were characterized by field-emission scanning electron microscopy (FESEM, JEOL JMS-7401F and Philips Electron Optics B. V. XL30S FEG, operated at 10 keV), X-ray diffraction spectrometry (XRD, Mac Science, M18XHF using Cu KR radiation, λ = 0.15406 nm), high-resolution scanning transmission electron microscopy (HR-STEM, Cs-corrected, JEOL JEM-2200FS with an energy-dispersive X-ray (EDX) spectrometer operated at 200 kV and an electron energy loss spectrometer (EELS)), elemental analysis (Analysennsysteme GmBH, Vario EL II), and UVvis diffuse reflectance spectroscopy (Shimadzu, UV2501PC). BrunauerEmmettTeller (BET) nitrogen adsorptiondesorption was measured using a Micromeritics analyzer (ASAP 2020 V3.01 H analyzer). Photocatalytic Activity Measurements. The photodecomposition of orange-II (4-(2-hydroxy-1-naphthylazo)benzenesulfonic acid, Aldrich) was used to study the photocatalytic properties of the assynthesized superstructures. The 50 mg as-synthesized powders were transferred into 100 mL of a 5.0 105 M orange-II aqueous solution. The photocatalytic reactions in the solution were carried out at room temperature in a closed system using a mercury lamp (1 W cm2, model 66905, Newport Co.) with a UV cutoff filter (λ g 400 nm) placed in an inner irradiation-type 100 mL Pyrex reaction cell. Prior to visible light exposure, the suspension was aged in the dark for 30 min to equilibrate the adsorption and desorption of organic molecules. With stirring, the
suspensions were placed under visible light. The quantity of orange-II in solution was determined by measuring the UVvis diffuse reflectance absorption intensity at 486 nm, the organic’s main absorption peak.
3. RESULTS AND DISCUSSION Recently, we reported a method of synthesizing exposed crystal-face-controlled 3D ZnO superstructures under mild conditions (at room temperature or 90 °C under 1 atm) without organic additives.34 We used three of these distinct 3D ZnO structures as reaction substrates (Figure 1). Figure 1a,b displays SEM images of the first 3D ZnO structures (S1) synthesized from the reaction of 0.03 M zinc acetate dihydrate and 0.1 M sodium peroxide aqueous solution at room temperature over 5 h. Figure 1c shows the schematic model of the building block of S1. The exposed crystal faces of the building blocks were indexed by the crystal orientation analysis reported previously.34 The S1 building block presented dominant {2110} planar exposed surfaces. S1 structures formed from these 2D building blocks. The second 3D ZnO structures (S2), synthesized from the reaction of 0.01 M zinc acetate dihydrate and 0.2 M sodium peroxide aqueous solutions at 90 °C for 5 h, consisted of hexagonal columns, as shown in Figure 1d,e. The axial direction of these hexagonal columns was [0001] (c-axis direction of ZnO crystals, Figure 1f). The third ZnO structures (S3), synthesized from the reaction of 0.02 M zinc acetate dihydrate and 0.03 M sodium peroxide aqueous solutions at 90 °C for 5 h, consisted of 3D architectures with conelike-shaped building blocks (Figure 1g,h). The exposed crystal faces of S3 were mainly composed of {1011}, {1010}, and (0001) planes, and the axial direction of 10245
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Langmuir the cone-shaped building blocks was [0001] (Figure 1i). The three pure 3D ZnO structures with distinct morphologies and exposed crystal faces underwent surface modification reactions in an aqueous Na2SeO3 and hydrazine solution, which led to the formation of 3D ZnO/ZnSe heterostructures for visible light photocatalysts. We used these three 3D ZnO nanostructures (Figure 1) as the reaction substrates in order to investigate the effects of the pristine 3D ZnO structures with distinct exposed crystal faces on the visible light photocatalysis of the heterostructures. Figure 2a shows an SEM image of the structures (H1) produced from the secondary reaction of S1 powders with 0.004 M sodium selenite and 1.5 M hydrazine monohydrate aqueous solution at 140 °C for 1 min. The general shapes of H1 were similar to those of S1. However, closer observation revealed that spherical nanoparticles formed on the surfaces of the 2D plate building blocks (Figure 2b). The XRD pattern (Figure 2c) could be indexed as a hexagonal wurtzite ZnO structure (JCPDS no. 36-1451) with calculated lattice constants of a = 0.3248 nm and c = 0.5211 nm and a cubic zinc blende ZnSe structure (JCPDS no. 37-1463) with a calculated lattice constant of a = 0.5671 nm. These lattice constants were consistent with previously reported data. The broad nature of the ZnSe XRD peaks indicated that the ZnSe nanocrystallites were relatively small. The ZnSe nanocrystallite size was calculated using Scherrer’s equation. The crystallite size was about 6.8 nm. For closer characterization, the 3D structures were investigated by TEM. Figure 2d shows a low-magnification TEM image. As shown in the SEM images, nanoparticles were attached to thin 2D plates (Figure 2d,e). The selected-area electron diffraction (SAED) pattern (Figure 2f) of the building block (a 2D plate covered with spherical nanoparticles) showed a rectangular arrangement of ZnO spots (0001 and 0110 spots), representatively indexed in Figure 2f, and ring-shape patterns corresponding to ZnSe 111, 220, and 331. This was consistent with the XRD analysis results. The regular spots of the ZnO and the ring patterns of ZnSe indicated that the building blocks were composed of single-crystalline ZnO and polycrystalline ZnSe nanostructures, confirmed by HR-TEM. An HR-TEM image (Figure 2g) of the area marked by the circle in Figure 2e revealed a single-crystalline 2D plate and polycrystalline nanoparticles deposited on the 2D plate. The HR-TEM image shows that the plates (building blocks) were single-crystalline with a lattice spacing of 0.26 nm, which corresponded to the distance between the (0002) planes in the hexagonal ZnO crystal lattice. The 0.33 nm lattice spacing corresponded to the distance between the (111) lattice planes of the cubic ZnSe crystal. The interfaces of ZnO/ZnSe heterostructures were clean. The composition of the 3D structures was investigated by EDX and EELS. Figure 2h shows a portion of a building block composed of a 2D plate and attached nanoparticles. The EDX data (Figure S2b in the Supporting Information) showed that the 2D plates were composed of Zn and O, as shown in area I of Figure S2a. C and Cu signals were attributed to the carboncoated copper TEM mesh. The spherical nanoparticle marked in area II of Figure S2a had a relatively high Se signal (Figure S2c). This result and the HR-TEM results showed that the 2D plates were ZnO and the nanoparticles were ZnSe, as confirmed by EELS analysis. The EELS elemental maps demonstrated that Zn (Figure 2i) and O (Figure 2j) were distributed homogeneously within the 2D plate and Zn (Figure 2i) and Se (Figure 2k) were distributed throughout the nanoparticles. Taken together, the SEM, XRD, TEM, EDX, and EELS data support a building block
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Figure 3. (a, b) SEM images of H2 synthesized from the reaction of S2 powders with an aqueous solution containing 0.004 M sodium selenite and 1.5 M hydrazine monohydrate at 90 °C for 5 h. (c) XRD pattern of H2 powders. (d) TEM image of a building block of an H2 structure. (e) HR-TEM image of the area indicated by the circle in d. (f) SAED pattern of the building block of the H2 structure.
structure comprising single-crystalline 2D ZnO building blocks decorated with polycrystalline ZnSe nanoparticles. Figure 3a shows a low-magnification SEM image of H2 structures synthesized from the reaction of S2 powders with 0.004 M sodium selenite and 1.5 M hydrazine monohydrate aqueous solution at 140 °C for 1 min. The 3D shape of S2 was preserved through the reaction. Closer SEM observations revealed that nanoparticles formed on nanorod building blocks of the 3D structures (Figure 3b). The XRD peaks of H2 were indexed as a wurtzite ZnO structure with calculated lattice constants of a = 0.3249 nm and c = 0.5211 nm and a zinc blende ZnSe structure with a calculated lattice constant of a = 0.5669 nm 10246
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Figure 4. (ac) SEM images of H3 synthesized from the reaction of S3 powders with an aqueous solution containing 0.004 M sodium selenite and 1.5 M hydrazine monohydrate at 90 °C for 5 h. (d) XRD pattern of H3 powders.
(Figure 3c). The average ZnSe crystallite size, calculated using Scherrer’s equation, was 6.9 nm, similar to that of the H1 structures. Figure 3d shows a low-magnification TEM image of a building block of H2. Nanoparticles were bound to the 1D core structures. Figure 3e shows an HR-TEM image of the junction area marked by the circle in Figure 3d. The 1D core structure was single-crystalline in nature, with an axis direction of [0001] revealed by the lattice spacing of 0.52 nm corresponding to the interplanar separation of the (0001) planes of a wurtzite ZnO phase. Polycrystalline nanoparticles were attached to the core single-crystalline structures. A lattice spacing of 0.33 nm in a grain of a polycrystalline nanoparticle indicated the distance between (111) planes of a cubic ZnSe phase. SAED analysis (Figure 3f) of the building block generalized these features. As in the case of H1, single-crystalline ZnO spots and polycrystalline ZnSe ring patterns were observed. The qualitative analysis using EDX and EELS (Figure S3 in the Supporting Information) confirmed that polycrystalline ZnSe nanoparticles formed on the surfaces of ZnO building blocks of S2, resulting in 3D ZnO/ ZnSe heterostructures with nanorod building blocks (H2). Figure 4a shows H3 powders prepared from the reaction of S3 powders with 0.004 M sodium selenite and a 1.5 M hydrazine monohydrate aqueous solution at 140 °C for 1 min. As shown in Figure 1g,h, the S3 structures used as reaction substrates had conelike-shaped building blocks. H3 structures also formed 3D
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superstructures with conelike building blocks (Figure 4a). However, as in the case of H1 and H2, nanoparticles were present on the surfaces, as shown in the high-magnification SEM images (Figure 4b,c). The XRD pattern of H3 also showed that the powder contained a wurtzite ZnO crystalline phase and a zinc blende ZnSe crystalline phase similar to those of H1 and H2. The calculated lattice constants of the ZnO crystal were a = 0.3250 nm and c = 0.5212 nm, and the lattice constant of the ZnSe crystal was a = 0.5671 nm. An average ZnSe crystallite size, calculated using Scherrer’s equation, was 6.8 nm. HR-TEM observations and the qualitative analysis (Figure S4 in the Supporting Information) revealed that the solution-exposed surfaces of single-crystalline ZnO building blocks of S3 were converted into polycrystalline ZnSe nanoparticles. Therefore, the characterization results supported the conclusion that three different 3D ZnO/ZnSe superstructures (H1, H2, and H3) were produced from the three different 3D ZnO structures over the course of the surface-modification reaction. Semiconductor nanostructures, such as ZnO, CdSe, CdS, ZnS, and ZnSe, were synthesized using organic materials as the surfactants or capping agents to control the size, morphology, or exposed crystal faces of the semiconductor nanostructures.3537 The organic additives adsorbed to the surfaces of the semiconductor nanostructures after the reaction, or they were incorporated into the crystal structures in some cases.38 Thus, these adsorbed organic additives may decrease the effective surface area of the structures, thereby decreasing the catalytic and sensing efficiencies of the semiconductor nanostructures. Additionally, the organic additives can negatively affect the formation of heterostructures from the two semiconductor nanostructure building blocks. In some reports describing the formation of heterostructures, organic linkages were used. For example, ZnO/CdX (X = Se or Te) heterostructures were prepared by self-assembly directed by organic materials on the surfaces of the ZnO or CdX particles.29,30 The adsorbed organic additives on building blocks and molecular bridge linkages for heterostructure formation is detrimental to electronic and photonic applications because they result in indirect contact between the different phases and increase the barrier for electron transport.33 Elemental analysis revealed that none of the samples, that is, before (S1, S2, and S3) or after (H1, H2, and H3) surface modification, included organic content. This was because organic additives were not used during the synthesis of the 3D ZnO structures and the 3D ZnO/ZnSe heterostructures described here. As shown in the HR-TEM analysis described above, the ZnO/ZnSe heterostructures possessed clean interfaces. Therefore, three distinct 3D ZnO/ZnSe heterostructures were successfully synthesized by this simple method. In this method, the Zn source used to form ZnSe on the ZnO surfaces was the 3D ZnO structures themselves. The formation of ZnO/ZnSe heterostructures was assumed to proceed by one of two mechanisms based on either anion exchange or dissolutionrecrystallization phenomena. The ZnSe that formed on the ZnO was polycrystalline and irregularly shaped, ruling out the anion-exchange mechanism. Structures formed by anion-exchange mechanisms preserve the morphology, crystal symmetry, and orientations of the initial crystals to some degree.39,40 The experimental results support a possible mechanism whereby the 3D ZnO/ZnSe heterostructures formed. Under these alkaline conditions, hydroxyl anions react with the ZnO surfaces (local dissolution of ZnO), resulting in the release of 10247
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Figure 5. UVvis absorption spectra of 3D superstructures.
Zn(OH)42 (reaction 1). Thus, the concentration of Zn(OH)42 near the ZnO surfaces locally increases. ZnOsurface þ H2 O þ 2OH f ZnðOHÞ4 2
ð1Þ
SeO3 2 þ N2 H4 f Se þ N2 þ H2 O þ 2OH
ð2Þ
3Se þ 6OH f 2Se2 þ SeO3 2 þ 3H2 O
ð3Þ
Se2 þ ZnðOHÞ4 2 f ZnSe þ 4OH
ð4Þ
The SeO3 ions in the reaction solution are reduced first by hydrazine to Se atoms or clusters as the reaction temperature increases (reaction 2). Highly reactive Se atoms or clusters are then further reduced or disproportionated in the alkaline solution to generate Se2 (reaction 3).41 Se2 then reacts with Zn(OH)42 to form ZnSe by heterogeneous nucleation and growth on the ZnO surfaces and as-nucleated ZnSe nanocrystallites (reaction 4).42,43 The UVvis absorption spectra of the 3D ZnO nanostructures (S1, S2, and S3) used as reaction substrates and 3D ZnO/ ZnSe heterostructures (H1, H2, and H3) after the surfacemodification reactions are displayed in Figure 5. The spectra revealed significant differences in the optical absorption properties of the 3D ZnO nanostructures and the 3D ZnO/ZnSe heterostructures. The 3D ZnO nanostructures (S1, S2, and S3) showed strong, broad absorptions in the ultraviolet region near the visible region, characteristic of the wide-band-gap ZnO semiconductor materials. However, the 3D ZnO/ZnSe heterostructures (H1, H2, and H3) showed absorptions in the visible region as well as in the ultraviolet region. These visible light absorption bands were attributed to the visible light absorption of ZnSe nanoparticles on the ZnO building block surfaces. In our previous study, the photocatalytic activities of the 3D ZnO superstructures (S1, S2, and S3) varied as a function of their exposed crystal faces under UV light irradiation,34 as shown in Figure S5 of the Supporting Information. As mentioned previously, the use of visible light is indispensible to the efficient utilization of sunlight. As seen in the UVvis absorption spectra (Figure 5), the 3D ZnO/ZnSe heterostructures showed an absorption band over some of the visible spectrum. Thus, photocatalytic activity under visible illumination was expected. The visible light photocatalytic activities of the pure 3D ZnO superstructures (S1, S2, and S3) and 3D ZnO/ZnSe hetero- and superstructures (H1, H2, and H3) were evaluated by measuring the degradation rate of aqueous orange-II solutions in the presence of visible radiation. The concentration of orange-II in
Figure 6. Normalized concentrations (with respect to the optical absorbance at 486 nm) of the 100 mL orange-II solution (initial concentration (Ci) = 5.0 105 M) without catalyst, with 50.0 mg of S1, with 50.0 mg of S2, with 50.0 mg of S3, with 50.0 mg of H1, with 50.0 mg of H2, and with 50.0 mg of H3 as a function of the visible light irradiation time. In each catalyst experiment, the suspension was aged in the dark for 30 min to equilibrate the adsorption and desorption of organic molecules prior to exposure to visible light.
2
Figure 7. Schematic diagram showing the energy band structures of a ZnO/ZnSe heterostructure.
solutions upon exposure to visible light was monitored as a function of time. The initial concentration of orange-II was 5.0 105 M (Ci). Under conditions in which the LambertBeer law is applicable, the concentration c of the absorbing component is proportional to the absorbance A,44 A ¼ εcl
ð5Þ
where l is the length of the light path through the absorbing layer and ε is the molar absorption coefficient. Figure 6 shows the normalized concentration (with respect to the optical absorbance measurements at 486 nm, which are provided in Figure S6 of the Supporting Information) of the 100 mL orange-II solution containing 50.0 mg of catalyst (S1, S2, S3, H1, H2, or H3) under visible light irradiation. In the absence of the catalyst, the orangeII concentration was almost constant during visible light irradiation of the solution, confirming the photostability of orange-II. In each experiment, the suspension was aged in the dark for 30 min to equilibrate the adsorption and desorption of organic molecules prior to exposure to visible light. The visible light photocatalytic activity of the 3D ZnO/ZnSe heterostructures (H1, H2, and H3) was much higher than that of pure 3D ZnO structures 10248
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Langmuir (S1, S2, and S3). The activity of 3D ZnO/ZnSe heterostructures varied according to structure: H1 > H3 > H2. The specific surface areas of the 3D heterostructures were measured using the micromeritics analyzer and the BET equation. The specific areas of H1, H2 and H3 were 24.86, 19.28, and 25.14 m2/g. Thus, we conducted the visible light photocatalytic experiments with different weights of catalysts inversely proportional to the specific surface areas. Figure S7 in the Supporting Information shows the visible light photocatalytic activities of the 3D heterostructures with the same surface areas. The trend was similar to that shown in Figure 6 with 50.0 mg of catalyst: H1 > H3 > H2. Interestingly, this trend was similar to that observed for pure 3D ZnO structures under UV light, as shown in Figure S5 of the Supporting Information: S1 > S3 > S2. The trend implies that the morphologies and exposed crystal faces of the ZnO building blocks prior to surface modification play an important role in the photocatalytic processes of ZnO/ZnSe heterostructures after surface modification. Figure 7 shows the energy band structure of a ZnO/ZnSe heterostructure. The conduction band edge of ZnO is located between the conduction band edge and the valence band edge of ZnSe (type II energy band alignment). In general, semiconductors are photoexcited with light of energy equal to or exceeding their band gap energy. Because ZnSe nanoparticles with a narrower band gap can absorb some regions of the visible spectrum, the 3D ZnO/ZnSe heterostructures (H1, H2, and H3) absorbed visible light, as shown in Figure 5. In addition, the type II energy band alignment of ZnO and ZnSe induced the formation of a diffusion potential. This potential acted as an electromotive force to favor electron injection.45 Conduction band electrons (e) and valence band holes (h+) were generated in ZnSe crystals when the ZnO/ZnSe heterostructures were illuminated with visible light. The electrons could be transferred to the conduction band of ZnO by electron injection, and the holes remained in the ZnSe valence band, which facilitated charge separation of the electronhole pair before recombination. The result is faster charge-carrier transport and a decrease in the probability of charge recombination. Holes in ZnSe could potentially react with water adhering to the surfaces of the ZnSe nanoparticles to form highly reactive hydroxyl radicals (OH 3 ). It is possible that the holes themselves directly oxidized the organic molecules. The electrons in ZnO could potentially react on the ZnO surface with oxygen, which can act as an electron acceptor, to form a superoxide radical anion (O2 3 ). Superoxide radical anions can further form hydroxyl radicals, the powerful oxidation ability of which can degrade organic molecules.46 Therefore, the activities (S1 > S3 > S2) of core ZnO structures, investigated in the previous study,34 may strongly influence the visible light activities of 3D ZnO/ZnSe heterostructures, producing the activity trend H1 > H3 > H2 under visible light.
4. CONCLUSIONS We report a designed method for the synthesis of three distinct type II 3D ZnO/ZnSe heterostructures (H1, H2, and H3) via simple solution-based surface-modification reactions. On the basis of experimental results, a possible formation mechanism for these heterostructures was proposed. The formation of ZnO/ZnSe heterostructures was assumed to result from a dissolutionrecrystallization mechanism. Se2 ions, which formed by the two-step reduction of SeO32, reacted with Zn(OH)42 released from ZnO surfaces to form ZnSe by
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the heterogeneous nucleation and growth on the ZnO building block surfaces. The 3D ZnO/ZnSe heterostructures exhibited absorption bands in the visible spectrum. The visible illumination photocatalytic activities of the 3D pure ZnO structures (S1, S2, and S3) and the 3D ZnO/ZnSe heterostructures (H1, H2, and H3) were examined. The visible light photocatalytic activities of 3D ZnO/ZnSe heterostructures were much higher than those of pure 3D ZnO structures because of the visible light absorption of ZnSe nanoparticles and the enhanced charge-carrier transport of type II heterostructures. The activities of 3D ZnO/ZnSe heterostructures varied according to structure: H1 > H3 > H2 under visible light. This trend was similar to that observed for 3D pure ZnO structures under UV light (S1 > S3 > S2) because the morphologies and exposed crystal faces of pure ZnO building blocks prior to surface modifications play an important role in the visible light photocatalytic processes of ZnO/ZnSe heterostructures after surface modification. We expect that this surface modification method may be exploited for the fabrication of ZnO/ZnSe heterostructure arrays for use in a variety of applications, such as photoelectrochemical water splitting and solar cells.
’ ASSOCIATED CONTENT
bS
Supporting Information. Temperature profile of the ZnO surface-modification reaction. TEM images, corresponding EDX patterns, and EELS mappings of building blocks of H1, H2, and H3. Normalized concentration of the orange-II solution with S1, S2, or S3 powder as a function of the UV irradiation time. Absorption spectra of the orange-II solution photodegradation as a function of visible irradiation time with S1, S2, S3, H1, H2, or H3. Visible light photocatalytic experiments with different weights of catalysts inversely proportional to the specific surface areas. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions §
These authors contributed equally to this work.
’ ACKNOWLEDGMENT This work was supported by grants from the second phase BK21 program of the Ministry of Education of Korea and a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST) (grant no. 2011-0000360). ’ REFERENCES (1) Atwater, H. MRS Bull. 2011, 36, 57. (2) Osterloh, F. E.; Parkinson, B. A. MRS Bull. 2011, 36, 17. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (4) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. (5) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriquez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (6) Wang, Z. L. Condens. Matter 2004, 16, R829. (7) Zeng, H.; Cai, W.; Liu, P.; Xu, X.; Zhou, H.; Klingshirn, C.; Kalt, H. ACS Nano 2008, 2, 1661. 10249
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