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1、Electrochemical fabrication of CuAl2O4-Cu hybrid nanorods at room temperatureDing Da-wei1, Cai Wei-min1, 2*, Long Ming-ce1, Wu Ya-hui3 （1. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; 2. State Key Laboratory of Urban Water Resource and Envir
2、onment, Harbin Institute of Technology, Harbin 150090; 3. Department of Environmental Science and Engineering, School of Civil and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China）Abstract: Among various methods used for fabrication of nanoarrays, template synthesis us
3、ing electrodepositon has been proved a low cost and high yield technique for producing large arrays of nanowires or nanorods. Herein, we present an electrochemical fabrication of CuAl2O4-Cu hybrid nanorod arrays in ordered anodic aluminum oxide (AAO) film nanotubes. The nucleation and growth of CuAl
4、2O4 nanocrystallite was favored by a comfortable electro-chemical environment, especially, the applied potential, which was investigated in details. The diameter of the nanorods was in accordance with the size of AAO nanotube (40-50 nm), with lengths of 600 to 700 nm. Photocatalysis experiment of th
5、e as synthesized CuAl2O4-Cu hybrid arrays showed considerable degradation rate of methyl orange under visible-light irradiation, which rendered potential application for the degradation of other recalcitrant organic pollutants in water purification.Keywords: spinel, CuAl2O4-Cu hybrid, template synth
6、esis, photocatalysisIntroductionLow dimensional spinel compounds have drawn considerable attention for their wide variety of technological applications in electronic, catalytic, and magnetic properties (Ammundsen and Paulsen 2001; Luders et al. 2006; Zhang et al. 2005; Jeong et al. 2007; Nasr-Allah
7、2002; Dhak 2006). Aluminum based spinels constitute an important class of its kind. Copper aluminate, CuAl2O4, is well known to be an inverse spinel and possesses interesting electronic and catalytic properties and is found wide application as ceramic pigments, coatings and catalysts (Alejandre et a
8、l. 1999; Shimizu 2000; Kim 2001; Yahiro 2006). By far, a number of approaches have been reported for the synthesis of nanocrystalline CuAl2O4 powders: solid state reaction (L.C.Leu 2007), co-precipitation reaction in solution (Shimizu 2000; Kameswari 1996)，combustion process (Mimani 2001) and sol-ge
9、l technique (Yanyan 2007; Meyer et al. 1999) have been used. Nevertheless, all the efforts to prepare CuAl2O4 spinel mentioned above require high-temperature (T 800) calcination procedures and the product is limited to powers or film (Dhak 2006; Yahiro 2006). A low-cost, easy-prepared new synthesis
10、method is therefore of great attraction considering the morphology control and energy conserving. Previously, we have reported briefly the successful synthesis of nano-scale CuAl2O4 spinels by electrochemical method at room temperature. This nanorod-like CuAl2O4-Cu hybrid was found grow with well co
11、ntrolled morphology in the anodic aluminum oxide nanotube(Ding et al. 2009). The nucleation of crystalline CuAl2O4 was favored by a comfortable electro-chemical environment, of which electropotential was vital for the stable growth of CuAl2O4 crystals within confined dimension. Herein, we discussed
12、in details how potential influence the deposition of spinel CuAl2O4 into the tubular structure. Later, excellent solar spectrum matching properties and photocatalytic activity under visible light irradiation of the modified hybrid structure was presented. The photocatalytic properties of the nanorod
13、s were investigated by using methyl orange as model dye.1 Materials and methods1.1 Preparation of anodic aluminaAluminum foil (99.99%), 0.3-mm thick, is annealed in air at 500 for 4 h in furnace. Al pieces, 3 cm 3 cm in size, were degreased by acetone in ultrasonic for 5 min, rinsed in distilled wat
14、er and electropolished in a mixture of ethanol and perchloric acid until visually mirror-like surface at 20 V. Prior to anodization, the back and edges of electrodes were sealed with wax to produce an electrode with only one face exposed. Anodization was carried out in a 2 M phosphoric acid at 20 fo
15、r 20 min under a voltage of 15 V against a graphite counter electrode. This procedure produces a porous aluminum oxide layer of pore fraction about 40% by volume. The alumina thickness was about 700 nm with pore diameter approximate to 50nm.1.2 Synthesis of CuAl2O4-Cu nanorodsUsing a two-electrode s
16、ystem consisting of a graphite plate as the counter electrode and PAO as the working electrode, electrochemical deposition of CuAl2O4-Cu is performed in an electrolyte of CuSO4 (35 g/L) and MgSO4 (20 g/L) (pH = 3 4) at various potentials, respectively, 11 V, 13 V and 15 V under continuous ac mode fo
17、r 60 seconds, frequency was kept at 200 Hz for all samples. The electrolyte was refreshed after every five samples to compensate Cu2+ depletion and to keep the pH values at 3 4.1.3 Photocatalysis experimentThe CuAl2O4-Cu nanorods filled PAO was firstly immersed in a 0.5 M NaOH at 30 for 3 minutes to
18、 remove the outer alumina nanostructure. The exposed CuAl2O4-Cu nanorods with an active area of 1.2 cm2 were placed in a single quartz photoreactor. 15 mL, 5 mg/L methyl orange (MO) was added and stirred continuously. A UV light filtered 350 W UV-Vis Xenon lamp ( = 0.4 -1 m) with maximal light inten
19、sity of 90 mW/cm2 was used as light source. Concentration of MO after degradation was measured by spectrometer at the absorption peak of MO ( = 465 nm). The degradation rate V was calculated with V = (C0 - Ct) / C0 100%, where C0 and Ct are the absorbance of original solution and solution photodegra
20、ded after t time.2 Results and discussion2.1 Influence of applied potentialFigure 1a shows a scanning electron microscope (SEM) image of the as-electrodeposited PAO under 11 V, 200Hz. As can be seen, the growth of nanorods in alumina pores is quite slow, only a few pores are fully filled. When the a
21、pplied potential raise to 15 V, template pitting was caused and agglomeration of boundless deposited nanoparticles is observed on the surface of PAO template (Fig 1b). When a voltage of 13 V is applied, considerable pore-filling quality is obtained (Fig 1c, d). Since the growth rate of nanorods in p
22、orous alumina varies according to the different resistances of the barrier layer resulted from the previous anodic oxidation of aluminum, over growth of nanorods into nearly micro-size octahedral crystallite on the surface of PAO template is seen (Fig 1c, insert). Figure 1e and f show nanorod bundle
23、s after wet etching of the PAO in 0.5 M NaOH for 5 minute and entire micrograph of individual nanorod after PAO was totally dissolved.Fig.1 a) and b) are the SEM images of electrochemical deposited PAO under 11 V, 200 Hz and 15 V, 200 Hz, respectively. c) - f) are the SEM images of deposited PAO at
24、13 V, 200 Hz, including: c) as deposited PAO and octahedral crystallites at high magnification (insert); d) cross section of as deposited PAO; e) exposed nanorod bundles after 5 minute wet etching of PAO in 0.5 M NaOH, f) scattered individual nanorods released from PAO.Figure 2 shows the X-ray diffr
25、action pattern of the nanorods filled PAO under different electrochemical conditions. For all the samples, diffraction lines due to a spinel phase were observed. The crystal structure of CuAl2O4 is consistent with that of cubic spinel aluminate (JCPDS / ICSD number 061454) Compared with that of samp
26、le a (11 V, 200 Hz), the XRD pattern exhibited much stronger and sharper (311) reflection (2 = 36.82) of the spinel form of CuAl2O4 for the sample b (13 V, 200 Hz). The crystallite size, about 10 nm, was calculated from line broadening of the peak (311) diffraction line according to the Scherrer equ
27、ation, which is in line with a crystallite diameter signified by TEM image. When the applied potential is raised to 15V, a broader and weaker peak of (311) plane of spinel CuAl2O4 is witnessed, which is accompanied by the arisen sharper and stronger peak of copper crystals. This reveals that at a re
28、latively low potential, the nucleation of CuAl2O4 and copper proceed at an even pace; With the increase of potential, the nucleation and growth of CuAl2O4 crystals is dominant in the whole system at first, which is substituted by the preferential growth of copper crystals at higher potential (15 V).
29、 In all, the value of applied potential affect considerably the growth rate of both CuAl2O4 spinel and Cu nanocrystals and benefits the preferential growth of CuAl2O4 nanocrystallite at 13 V in this case.Fig. 2 X-ray diffraction patterns of the composite nanorods filled PAO under different electroch
30、emical condition: a: 11 V, 200 Hz, b: 13 V, 200 Hz, c: 15 V, 200 Hz;The morphology of the CuAl2O4/Cu nanorods was further investigated with transmission electron microscopy (TEM) imaging. Figure 3a shows the full image of CuAl2O4-Cu nanorods connected with a micro-size octahedral CuAl2O4 spinel resu
31、lting from the overgrowth of CuAl2O4 nanocrystallite. The energy-dispersive X-ray spectroscopy (EDXS) analysis of these octahedrons showed that the main elemental components are Cu, Al and O, and the Cu/Al atomic ratio is close to 1:2. Figure 3b is a close look of an individual CuAl2O4 -Cu nanorod o
32、f the sample b (13 V, 200 Hz), which is characterized by the stack of multiple nanoparticles. Figure 3c shows a micrograph of the sample c (15 V, 200 Hz), indicating that due to the weak confinement of nanopores caused by pore-pitting (as show by figure 1b), undirected stack of nanocrystals is more
33、easily occurred. Fig. 3 TEM images of a) a fully grown octahedral CuAl2O4 spinel, b) an individual CuAl2O4/Cu nanorod, c) randomly agglomerated nanoparticles of sample c (15V, 200Hz).It is worthwhile to pay attention to the preferential overgrowth of CuAl2O4 nanocrystals into nearly micro-size octah
34、edron on the apexes of porous aluminum oxide. XPS analysis was performed for the PAO surface, of which the Cu element only exists as Cu2+, whereas no zero-valence copper was detected. From the XRD pattern of sample after wet-etching treatment (Figure 1e), decreased intensity of (311) plane is obviou
35、sly observed comparing with that of as-synthesized (Figure 1c), (see supporting information). The XPS, XRD results together with previous EDXS information provide adequate evidence that the octahedral crystals are CuAl2O4。The dominant growth of CuAl2O4 crystal on the nanostructured pores is probably
36、 a result of the lowered potential necessary for the reduction of Cu2+ on the apexes and enrichment of Al3+ ion released from the dissolution of Al2O3 on the interface.2.2 Mechanism discussionIt should be noted that these CuAl2O4-Cu nanorods were synthesized by electrochemical deposition within the
37、PAO pores in a simple solution of CuSO4 and MgSO4. It is interesting to mention that previous attempt to deposit copper into PAO resulted in single copper nanowires under ac electrodeposition condition (Gerein 2005). The different results can be attributed to the complex nature of alumina barrier la
38、yer and porous structure generated by different anodic oxidation process and to the varied electrodeposition condition applied here (electrolyte, PH, etc). Generally, the anodic and cathodic reactions triggered by bias voltage can be presented as follows. In the cathodic bias, (1) (2) (3)In the anod
39、ic bias, (4) (5) (6) (7)When the cathodic bias is applied, Cu2+ is reduced to Cu+ by gaining one electron (reaction (1) or to Cu by gaining two electrons (reaction (2). In addition, hydrogen ions are reduced to hydrogen gas (reaction (3). In the subsequent anodic reactions, part of the reduced Cu+ a
40、re re-oxidized into Cu2+ (reaction (5), which, together with hydroxyl ions and dissolved Al3+, participate in the formation of copper aluminates by complex coprecipitation of Al3+ and Cu2+ hydroxides and decomposition triggered by exterior potential (simplified by reaction (6) and (7). During the wh
41、ole process, pores are continuously nucleated at the centre by the anodic reaction based on the field-enhanced oxide dissolution mechanism (Parkhutik and Shershulsky 1992; Li et al. 2000; Choi et al. 2005), releasing Al3+ ions (reaction (4) that contribute to the evolution of CuAl2O4 spinel. Unlike
42、other report (Choi 2007), both Cu2+ and Cu+ did not undergo the process of Cu2O and CuO formation in this case, since the formation of CuAl2O4 from solid-state reaction of CuO and Al2O3 only take place at high calcination temperature (G.A.El-Shobaky 2006). More over, no intermediate CuO and Cu2O pha
43、se were detected from the XRD pattern of as synthesized CuAl2O4-Cu nanorods. It must, however, be underlined that the mechanism of CuAl2O4 evolution in the electrochemical environment is under further investigation, given the combination of two complicated chemical and electrochemical processes: cop
44、recipitation of aluminum-Cu () hydroxides at certain pH value and potential induced decomposition.2.3 PhotocatalysisAfter establishing the synthesis and characterization of the new CuAl2O4-Cu nanorods, we further studied their function in visible-light photocatalyis. To this end, methyl orange (MO)
45、is selected as a model dyeing pollutant because it is one of the most important commercial dyes, has a very short excited-state lifetime, and is stable to visible and near UV light (Yu et al. 2007). The degradations of methyl orange under different conditions were presented to evaluate the visible-l
46、ight photocatalysis of as-synthesized CuAl2O4 -Cu nanorods. As show in Figure 4, the methyl orange did not change without photocatalyst under visible illumination (350 W xenon lamp, 400 nm), suggested it was stable under the visible light irradiation. In the system of CuAl2O4 -Cu catalyst, the degra
47、dation rate reached to 80% within 3 hours under the same illumination. This suggested that the degradation of methyl orange was mainly due to the photocatalysis of CuAl2O4 -Cu hybrids.The visible-light photocatalysis of CuAl2O4 -Cu hybrid nanorods can be explained by the low bandgap of CuAl2O4 and m
48、etal enhanced charge separation at the metal/semiconductor interface. As show in figure 4b, the prepared CuAl2O4 -Cu hybrids possess an excellent light-absorption characteristic over the UV-visible region, with an upper limit wavelength around 750 nm. The solar spectrum matching optical properties is significant in the photocatalytic solar energy conversion(Mingce et al. 2008). The apparent bandgap of this hybrid system
