Etching Route to ZnSe Nanotube Arrays and Their

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Replacement/Etching Route to ZnSe Nanotube Arrays and Their Enhanced Photocatalytic Activities Lingling Chen, Weixin Zhang,* Cheng Feng, Zeheng Yang, and Yumei Yang School of Chemical Engineering, Hefei University of Technology and Anhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei, Anhui 230009, China S Supporting Information *

ABSTRACT: Well-aligned ZnSe nanotube arrays with diameters of 300−400 nm and wall thicknesses of 60−70 nm have been controllably prepared based on a replacement/etching method, with ZnO nanorod arrays on zinc substrate as sacrificial templates. An obvious difference of the solubility product (Ksp) between the ZnSe wall and ZnO core materials is crucial for the direct replacement of one type of anions by the other. Ammonia as the chemical etching agent is also important for dissolving ZnO nanorod core. The photocatalytic activities of the as-prepared ZnSe nanotube arrays have been studied for the degradation of methyl orange aqueous solution and compared with those of ZnO nanorod arrays and the intermediates including ZnO/ZnSe core/sheath nanorod arrays and partially dissolved ZnO core/ZnSe sheath nanorod arrays, respectively. The results indicate that ZnSe nanotube arrays exhibit superior photocatalytic performance to the other three nanostructured arrays, which can be mainly attributed to their full hollow interior nanotubes providing more accessibility to the dye molecules and more reactive adsorption/ desorption sites for photocatalytic reactions. Furthermore, the ZnSe nanotube arrays have successfully overcome the shortcomings related to photocatalyst recovery and stability, which the powder-form photocatalysts usually face. ZnSe nanotube arrays as photocatalysts are expected to be promising in sewage water treatment.

1. INTRODUCTION Over the past decade, nanotubes constructed of noncarbonbased inorganic semiconductor nanostructures popularly known as inorganic nanotubes have become a symbol of the fast and new developing research area of nanotechnology.1 The existence of tubular forms of matter with nanoscale diameters has opened up an exciting field of research in physics and chemistry.2 Tenne et al.3 synthesized a variety of concentric polyhedral and cylindrical WS2 inorganic nanotubes (ranging in size from 10 to 100 nm), which were then followed by intense experimental and theoretical research on hollow cylindrical structures that led to the development of numerous inorganic nanotubes with diverse properties.4−9 The physical properties of these nanotubes strongly depend on their characteristics such as chirality and radius. Their potential applications range from highly porous catalytic and ultralight anticorrosive materials to electron field emitters and nontoxic strengthening fibers. As one of the important Zn-based II−VI semiconductors, zinc selenide (ZnSe) has been considered to be a prospective material for light-emitting devices, solar cells, sensors, and optical materials due to its wide direct band gap (2.67 eV) and large exciton binding energy (21 meV).10−13 ZnSe nanostructures with various morphologies,14−19 such as nanocrystals, nanorods, nanowires, nanobelts, nanotubes, and nanospirals, have been fabricated by a variety of methods. Previous studies were mainly focused on the optical, photoconducting, and other physical properties of ZnSe nanostructures. Recently, photocatalytic activities in ZnSe nanostructures have received much attention.20−25 Qian et al.20 reported that the photocatalytic activity of wurtzite ZnSe ultrathin nanobelts in the photodegradation of fuchsin acid is higher than that of TiO2 © 2012 American Chemical Society

nanoparticles. The ZnSe nanobelts were synthesized in a mixture of distilled water and ethanolamine at 220 °C for 24 h by a solvothermal route. Yang et al.21 synthesized ZnSe spherical flowerlike nanoarchitectures with diameters of 3.5− 4.5 μm and ZnSe microspheres with diameters of about 3 μm via a solvothermal reaction and subsequent annealing in Ar. Their photocatalytic activities in the degradation of methyl orange indicate that the photocatalytic ability of the ZnSe spherical flowerlike nanoarchitectures is stronger than that of the ZnSe microspheres. In our previous work, we synthesized well-aligned arrays of single-walled Cu7S4 nanotubes and double-walled Cu7S4 and Cu2−xSe nanotube arrays by using Cu(OH)2 nanorod arrays as sacrificial templates.26−29 Herein, well-aligned arrays of ZnSe nanotube arrays have been successfully prepared based on a replacement/etching method, using ZnO nanorod arrays on Zn substrate as sacrificial templates. Well-aligned ZnO/ZnSe core/ sheath nanorod arrays and partially dissolved ZnO core/ZnSe sheath nanorod arrays as the intermediates have also been prepared. Their photocatalytic activities have been investigated in the degradation of methyl orange solution, and the ZnSe nanotube arrays have exhibited photocatalytic performance superior to that of the other three nanostructured arrays. Compared with the commonly used powder-form photocatalysts, the ZnSe nanotube array catalyst has the advantages of easy recovery and stable structure. Further experiments show Received: Revised: Accepted: Published: 4208

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Scheme 1. Schematic Illustration for the Formation of ZnSe Nanotube Arrays

2.4. Characterization of the Samples. X-ray powder diffraction (XRD) patterns were recorded on a Japan Rigaku D/max-rB X-ray diffractometer with Cu Kα radiation (λ = 0.154 178 nm), operated at 40 kV and 80 mA. Field-emission scanning electron microscopy (FESEM) measurements were carried out on a field-emission microscope (JEOL JSM-6700F) operated at an acceleration voltage of 5 kV. Transmission electron microscopy (TEM) images were taken with a Hitachi H-800 transmission electron microscope, operated at an acceleration voltage of 200 kV. The variation of the methyl orange concentration was evaluated by the photoluminescence intensity, which was determined by a Hitachi F-4500 fluorescence spectrophotometer. The photoluminescence (PL) measurements of the samples which grew on the Zn foil (10 × 10 × 0.25 mm3) were performed on a Hitachi F-4500 FL spectrophotometer at room temperature by putting the sample on the Zn foil into a colorimetric cuvette.

that this novel photocatalyst can be recycled many times without obvious loss of catalytic activity.

2. EXPERIMENTAL SECTION 2.1. Synthesis of ZnO Nanorod Arrays. The synthesis of ZnO nanorod array precursors has been described in our previous report.30 A typical procedure to synthesize the ZnO nanorod array precursors was performed as follows. The reagents include a piece of cleaned Zn foil (10 × 10 × 0.25 mm3) and a mixed solution of 30 mL of diluted ammonia (12 wt %) and 0.4 mL of NaOH solution (10 M). A hydrothermal reaction was conducted in a Teflon-lined stainless steel autoclave (capacity, 50 mL) at 60 °C for 7 days. When the reactions finished, the Zn foil was taken out, washed with distilled water and ethanol several times, respectively, and then dried in air. 2.2. Synthesis of ZnSe Nanotube Array on Zinc Substrate. A typical procedure was performed as follows. The Se2− source solution was first prepared by mixing a certain amount of Se powder and NaBH4 in NaOH solution (0.005 M), with reference to the reported literature.31 The ZnO nanorod array on zinc substrate was put into the Se2− source solution and hydrothermally treated at 170 °C for 2 h. The products were washed with distilled water and absolute ethanol, respectively, and then dried at 50 °C. The obtained ZnO/ZnSe core/sheath nanorod array was subsequently immersed in a diluted ammonia solution (6 wt %) for 4 and 10 h to remove the inner ZnO core partly and completely, to produce partially dissolved ZnO core/ZnSe sheath nanorods and the final ZnSe nanotube array, respectively. 2.3. Photocatalytic Experiments. The photocatalytic activities of ZnSe nanotube arrays were evaluated by determining the photodegradation of methyl orange in an aqueous solution at room temperature. A Xe lamp (150 W) of a Hitachi F-4500 fluorescence spectrophotometer was used as an ultraviolet (UV) light source (λex = 400 nm). The ZnSe nanotube arrays on zinc substrate were placed in the bottom of a cuvette containing 1 mL of methyl orange solution (5 × 10−5 M). Prior to irradiation, the ZnSe nanotube arrays were held in the dye solution in the dark for a period to establish an adsorption/desorption equilibrium between them. After irradiation for the desired time, the ZnSe nanotube arrays on zinc substrate were removed, and the in situ photoluminescence (PL) spectra of methyl orange aqueous solution were recorded by using the same fluorescence spectrophotometer. For comparison of their photocatalytic activities, the intermediates including ZnO/ZnSe core/sheath nanorod arrays and partially dissolved ZnO core/ZnSe sheath nanorod arrays as the photocatalysts have also been respectively investigated.

3. RESULTS AND DISCUSSION The Ksp value of ZnSe (3.6 × 10−26) is much smaller than that of ZnO (6.8 × 10−17). This implies that ZnO nanorods can act as both precursors to synthesize more stable chalcogenides and sacrificed templates to obtain structures with hollow interiors. Our strategy for the synthesis of the ordered array of ZnSe nanotubes is illustrated in Scheme 1. The first step includes the synthesis of the ZnO nanorod array grown on a zinc substrate (step I). When the as-prepared ZnO nanorod array is immersed in Se2− solution, Se2− will replace O2− on the surface of each ZnO nanorod to produce ZnO/ZnSe core/sheath nanostructures (step II). The unreacted ZnO core can be controlled to be dissolved partially in diluted ammonia solution (step III). An interlayer would exist between the inner ZnO core and the outer ZnSe sheath. At last, after the inner remnant ZnO core is dissolved completely in the ammonia solution, ZnSe nanotube arrays will be obtained (step IV). 3.1. Structures and Morphologies of Nanostructured Arrays. The as-prepared products have been characterized by X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). Figure 1 shows the XRD patterns of the as-prepared nanostructured arrays on zinc substrate. Figure 1a shows the XRD pattern of the ZnO nanorod arrays (step I) which were synthesized according to our previous report.30 All other diffraction peaks can be indexed to hexagonal ZnO (JCPDS 36-1451), except for the peaks marked with an asterisk which come from the zinc substrate. The XRD pattern of sample b, which was prepared by immersing the as-prepared ZnO nanorod arrays in Se2− solution (step II) in Figure 1b, can be indexed to a mixture of cubic ZnSe (JCPDS 37-1463) and 4209

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Followed by dissolution of the ZnO core in the ammonia solution for a shorter time (4 h), ZnO core/ZnSe sheath nanorod arrays with partially open tips (sample c) are obtained as shown in Figure 2c. The sheath thickness is about 60−70 nm. When the ZnO/ZnSe core/sheath nanorod arrays are immersed in the ammonia solution for a longer time (10 h), Figure 2d shows that the inner ZnO core has been removed completely and ZnSe nanotube arrays (sample d) can be obtained, which have diameters of 300−400 nm and wall thicknesses of about 60−70 nm similar to those of ZnO/ZnSe core/sheath nanorod arrays. Figure S1 in the Supporting Information shows when the open tips occur on the ZnO core/ ZnSe sheath nanorod arrays. Figure S1a,b in the Supporting Information shows that the nanorod array sample dissolved in the diluted ammonia solution for 2 h has close tips. When the time for the etching is prolonged to 8 h, the tips of the nanorods become open gradually, as shown in Figure S1c,d in the Supporting Information. When the etching time is further prolonged to 10 h, almost all the nanorods in the array have open tips. Therefore, enough etching with diluted ammonia solution is the main reason that causes the open tips of the asprepared ZnSe nanotube arrays. Figure 3 presents the TEM images and the corresponding selected area electron diffraction (SAED) patterns of the four

Figure 1. XRD patterns corresponding to samples a, b, c, and d, respectively: (a) sample a (step I), (b) sample b (step II), (c) sample c (step III), and (d) sample d (step IV).

hexagonal ZnO. The XRD pattern of sample c, which was prepared by dissolving sample b in diluted ammonia solution for 4 h (step III), in Figure 1c also show the coexistence of cubic ZnSe (JCPDS 37-1463) and hexagonal ZnO, but the intensities of ZnO diffraction peaks decrease greatly. However, when sample b is dissolved in the diluted ammonia solution for 10 h (step IV), the XRD pattern in Figure 1d indicates that the diffraction peaks can only be indexed to cubic ZnSe (JCPDS 37-1463) with lattice constant a = 5.668 Å, indicating that the ZnO core has been completely dissolved. Figure 2 shows the morphologies of the as-prepared samples corresponding to the above four steps. Figure 2a presents the

Figure 3. TEM images and the corresponding selected area electron diffraction (SAED) patterns corresponding to samples a, b, c, and d, respectively: (a) ZnO nanorod arrays, (b) ZnO/ZnSe core/sheath nanorod arrays, (c) partially dissolved ZnO core/ZnSe sheath nanorod arrays, and (d) ZnSe nanotube arrays.

samples, respectively. The TEM image of the ZnO nanorod prepared on the zinc substrate is shown in Figure 3a, revealing that the nanorods have smooth surfaces with a diameter of about 200 nm. The inserted SAED pattern indicates that the hexagonal ZnO nanorod is a single crystal, which agrees well with the interplanar spacings of the (001) and (100) planes. When the ZnO nanorod precursor reacts with Se2− solution hydrothermally at 170 °C for 2 h, a layer of ZnSe is produced on the surface of the nanorod (Figure 3b). Across the rod, the intensity profile shows a clear variation, and the edge surfaces show lighter contrast, suggesting that ZnSe is covered on the surface of the ZnO nanorod to form a continuous coating layer. The diameter of the ZnO/ZnSe core/sheath nanorod is about 200−300 nm. From the inserted SAED pattern, some rings can be seen besides the hexagonal ZnO crystalline pattern. After the ZnO/ZnSe core/sheath nanorod array is immersed in the diluted ammonia solution for a short time to dissolve the ZnO core partially, an interlayer between the outer ZnSe sheath and the inner ZnO core is formed (Figure 3c). The thickness of the outer ZnSe sheath is about 60−70 nm, which is consistent with its FESEM image. Also, the inset SAED pattern shows some diffused rings present besides hexagonal ZnO crystal spots.

Figure 2. FESEM images corresponding to samples a, b, c, and d, respectively: (a) ZnO nanorod arrays, (b) ZnO/ZnSe core/sheath nanorod arrays, (c) partially dissolved ZnO core/ZnSe sheath nanorod arrays, and (d) ZnSe nanotube arrays.

FESEM images of ordered ZnO nanorod arrays growing almost perpendicular to the zinc substrate (sample a). The ZnO nanorods have smooth surfaces with diameters of 200−300 nm on average and lengths of about several micrometers. After ZnO nanorod arrays react with aqueous Se2− solution hydrothermally at 170 °C for 2 h, ZnO/ZnSe core/sheath nanorod arrays (sample b) are obtained. Figure 2b shows that they have diameters of 300−400 nm and lengths of about 3 μm, which keep the similar morphology of ZnO nanorod templates. 4210

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is generally used for the photocatalytic degradation process if the initial concentration of the pollutant is low.32

Finally, when the inner ZnO core is removed completely, the ZnSe nanotube is obtained, as shown in Figure 3d. The wall thickness of the ZnSe nanotube is about 60−70 nm with a diameter of 200−300 nm. The SAED pattern displays three concentric diffraction rings. These rings are assigned to the (111), (220), and (311) planes of cubic ZnSe, respectively, indicating that the wall of the nanotube is composed of tiny ZnSe nanocrystallites. 3.2. Photocatalytic Activity of the Nanostructured Arrays. Figure 4 displays the time profiles of photodegradation

ln(C0/C) = kt

(1)

where C0 and C are the concentrations of dye in solution at times corresponding to 0 and t, respectively, and k is the pseudo-first-order rate constant. Figure 5 shows the photo-

Figure 5. Photocatalytic degradation kinetics of methyl orange solution in the presence of different catalysts: (a) ZnO nanorod arrays, (b) ZnO/ZnSe core/sheath nanorod arrays, (c) partially dissolved ZnO core/ZnSe sheath nanorod arrays, and (d) ZnSe nanotube arrays.

Figure 4. Time profiles of photocatalytic degradation of the methyl orange solution in the presence of different catalysts and under exposure to UV light, respectively: (a) ZnO nanorod arrays, (b) ZnO/ ZnSe core/sheath nanorod arrays, (c) partially dissolved ZnO core/ ZnSe sheath nanorod arrays, and (d) ZnSe nanotube arrays.

catalytic degradation kinetics of methyl orange in solution on the basis of the data plotted in Figure 4. The rate constants obtained from the regression lines in Figure 5 are listed in Table 1. The k values for the four different catalysts under

of the methyl orange solution (5 × 10−5 M) with the four different nanostructured arrays as photocatalysts respectively and under exposure to UV light. The degradation ratio C/C0 is reduced from the PL spectra of methyl orange solution with different photocatalysts at different irradiation times (Figure S2 in the Supporting Information), where C is the concentration of methyl orange solution at the irradiation time t and C0 is the initial concentration. It can be seen that the degradation ratios of the methyl orange solutions with the four different photocatalysts for 7 h of UV light irradiation are 33.4% (Figure 4a), 41.3% (Figure 4b), 52.5% (Figure 4c), and 76.5% (Figure 4d), respectively. Obviously, the ZnSe nanotube array sample exhibits photocatalytic activity superior to that of the other three samples. To further verify the photocatalysis effect of the ZnSe nanotube arrays, two comparison experiments are conducted. One is that the methyl orange solution is under exposure to UV light for 5 h, without the presence of any catalysts, and it can be seen that its degradation is negligible (Figure S3 in the Supporting Information). Another is that the degradation of the methyl orange solution is tested with the ZnSe nanotube array as catalyst but in a dark environment, without exposure to UV light, and the degradation ratio at 5 h is only about 10.2% (Figure S4 in the Supporting Information), which may result from the adsorption of the dye on ZnSe nanotube arrays. From the comparisons, it can be concluded that the significant decrease of the concentration of methyl orange solution in the presence of ZnSe nanotube array catalyst is mainly ascribed to its photocatalytic effect. For a detailed analysis of the photocatalytic degradation kinetics of the methyl orange solution in our experiments, we apply the pseudo-first-order model as expressed by eq 1, which

Table 1. Relationship between the Photocatalytic Activities of the Four Samples and Their Compositions and Morphologies

exposure to a Xe lamp for 7 h are 0.066 h−1 (Figure 5a), 0.093 h−1 (Figure 5b), 0.140 h−1 (Figure 5c), and 0.254 h−1 (Figure 5d), respectively. As can be seen, a rather good correlation to the pseudo-first-order reaction kinetics (R2 > 0.988) is obtained. It shows more clearly that the ZnSe nanotube array sample has the highest methyl orange degradation rate. Usually, powder-form photocatalysts have drawbacks related to catalyst recovery when used in liquid−solid heterogeneous catalysis processes. On the other hand, when the catalyst particles are in nanoscale, they often face the problem of serious agglomeration due to high surface energy. These issues give rise to difficulties in the catalyst recycle, therefore, hindering its 4211

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applications. However, the as-prepared ZnSe nanotube array film as a kind of photocatalyst has overcome these shortcomings successfully. First, the nanotube arrays on the Zn substrate rule out the possible particle agglomeration effectively and can take full advantage of the nanosized effect. Second, as a film on substrate, it can be easily separated with the solutions. Third, it can be recycled just by simple treatments, such as washing with deionized water and absolute ethanol, respectively, and then being dried at 50 °C. Recycle testing shows that the degradation ratio of the methyl orange solution changes from 76.5% of the first run to 73.5% of the seventh run, as shown in Figure 6, indicating that the ZnSe nanotube array Figure 7. Time profiles of photocatalytic degradation of different initial concentrations of methyl orange solutions in the presence of ZnSe nanotube arrays under exposure to UV light: (a) 1 × 10−5, (b) 5 × 10−5, and (c) 10 × 10−5 M.

shortage of photons to activate ZnSe inhibits the degradation of methyl orange at a higher initial concentration.33,34 Generally, photocatalytic activities of semiconductors are subjected to the following two factors: (1) light-irradiationinduced formation of conduction band electrons and valence band holes; (2) recombination of electron−hole pairs. In order to clarify the effects of different nanostructured arrays on the two processes in photocatalysis, diffuse reflectance UV−visible spectra and photoluminescence spectra were investigated to compare their light absorption and recombination of electron− hole pairs. 1. Light Absorption. Figure 8 shows the diffuse reflectance UV−visible spectra of the as-prepared samples. Under

Figure 6. Cycling runs in the photocatalytic degradation of methyl orange solution in the presence of ZnSe nanotube arrays and under UV light irradiation.

catalyst exhibits no significant loss of activity for the photodegradation of methyl orange solution after seven recycles and has good stability during the photocatalytic oxidation of the pollutant molecules. From Table 1, it can be seen that the photocatalytic activity of the four samples varies with their composition and structure, and ZnSe nanotube arrays are superior to the other three samples. The significant difference results first from replacing ZnO with ZnSe and second from the hollow structure of ZnSe nanotubes with open tips. It can be considered that the nanotube arrays (sample d) with full hollow interiors have more reactive adsorption/desorption sites for photocatalytic reactions than the nanorod arrays (samples a and b) or the partially dissolved ZnO core/ZnSe sheath nanorod arrays (sample c). Furthermore, the nanotube arrays with open tips can provide more accessibility to the dye molecules than the other three samples. Moreover, we have studied the effect of solution thickness on light absorption with the ZnSe nanotube arrays as the catalyst. Figure 7 shows that the degradation ratios of methyl orange for 7 h of UV light irradiation are 94.7, 76.5, and 55.2% when the initial concentrations of methyl orange solutions are 1 × 10−5 M (Figure 7a), 5 × 10−5 M (Figure 7b), and 10 × 10−5 M (Figure 7c), respectively. Obviously, the degradation ratio of methyl orange decreases with the increase of the initial solution concentration when the ZnSe nanotube arrays are used as the photocatalysts. Lower degradation ratio of methyl orange at higher initial concentration may be attributed to the fact that an increased amount of methyl orange can occupy more ZnSe active sites, which subsequently suppress generation of the oxidants.33 Furthermore, the methyl orange solution with a higher concentration may absorb more photons, consequently decreasing available protons to activate ZnSe. Thus the

Figure 8. Diffuse reflectance UV−visible spectra of different catalysts, respectively: (a) ZnO nanorod arrays, (b) ZnO/ZnSe core/sheath nanorod arrays, (c) partially dissolved ZnO core/ZnSe sheath nanorod arrays, and (d) ZnSe nanotube arrays.

ultraviolet irradiation, electrons in the valence band of the semiconductor could absorb the photon energy and jump to the conduction band, leaving holes in the valence band. Light absorption is mainly determined by the band structure and the dipole matrix elements. As shown in Figure 8a,d, ZnSe nanotube arrays exhibit more absorption than ZnO nanorod arrays in the UV region, indicating that ZnSe nanotube arrays could generate more photogenerated charges than ZnO nanorod arrays, facilitating the photodegradation of the methyl orange dye.35 Compared with ZnO nanorod arrays, both of the ZnSe/ZnO heterophotocatalysts (ZnO/ZnSe core/sheath nanorod arrays and partially dissolved ZnO core/ZnSe sheath 4212

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excellent catalytic activity, easy catalyst recovery, and good reuse stability, which are expected to be promising in sewage water treatment.

nanorod arrays) also exhibit more absorptions (Figure 8b,c). Obviously, ZnSe leads improved absorptions to the ultraviolet. Partially dissolved ZnO core/ZnSe sheath nanorod arrays (Figure 8c) show more absorptions than ZnO/ZnSe core/ sheath nanorod arrays (Figure 8b), which may be caused by the higher specific surface area of partially dissolved ZnO core/ ZnSe sheath nanorod arrays with more hollow spaces. 2. Recombination of Electron−Hole Pairs. The photocatalysis efficiency is determined by the competition between the charge separation process and recombination process, both in the bulk and on the surface of the photocatalyst. PL spectra can be used to evaluate the charge recombination rate as reported in the literature,36 because PL spectra directly arise from the radiative recombination processes of the electrons and the holes between two different energy states. The higher the PL emission intensity, the higher the recombination rate of the photogenerated electrons and holes and the lower the photocatalytic activity. Figure 9 shows the photoluminescence

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing FESEM images corresponding to samples dissolved in diluted ammonia solution; room-temperature PL spectra of methyl orange solution under exposure to UV light in the presence and absence of different catalysts and in the presence of ZnSe nanotube arrays. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-551-2901450. Fax: +86-551-2901450. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (NSFC Grants 20871038, 20976033, and 21176054), the Fundamental Research Funds for the Central Universities (2010HGZY0012), and the Education Department of Anhui Provincial Government (TD200702).

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REFERENCES

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Figure 9. PL spectra of different catalysts: (a) ZnO nanorod arrays, (b) ZnO/ZnSe core/sheath nanorod arrays, (c) partially dissolved ZnO core/ZnSe sheath nanorod arrays, and (d) ZnSe nanotube arrays.

spectra of the as-prepared samples. It can be seen that the ZnSe nanotube arrays have a lower recombination rate than the other three samples, which is consistent with its higher photocatalysis efficiency.

4. CONCLUSIONS In summary, controlled synthesis of well-aligned arrays of ZnSe nanotubes has been demonstrated by using self-prepared ZnO nanorod arrays as sacrificial templates. The key step of the process involves formation of the ZnSe wall and the dissolution of the ZnO core by chemical means. A great difference in the solubility product (Ksp) is a linchpin for the direct exchange between the two kinds of anions involved. The photocatalytic activities of the as-prepared ZnSe nanotube arrays have been studied for the degradation of methyl orange aqueous solution and compared with those of ZnO nanorod arrays and the intermediates including ZnO/ZnSe core/sheath nanorod arrays and partially dissolved ZnO core/ZnSe sheath nanorod arrays, respectively. Significantly, the results demonstrate that ZnSe nanotube arrays exhibit photocatalytic performance superior to that of the other three nanostructured arrays, which mainly results from their full hollow interior nanotubes providing more accessibility to the dye molecules and more reactive adsorption/desorption sites for photocatalytic reactions. As photocatalysts the ZnSe nanotube arrays have the virtues of 4213

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dx.doi.org/10.1021/ie202044v | Ind. Eng. Chem. Res. 2012, 51, 4208−4214