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Fabrication of CdSe Nanoparticles Sensitized Long TiO2 Nanotube Arrays for Photocatalytic Degradation of Anthracene-9-carbonxylic Acid under Green Monochromatic Light Lixia Yang,†,‡,| Shenglian Luo,*,†,‡,| Ronghua Liu,§,| Qingyun Cai,§ Yan Xiao,†,‡,§ Shaohuan Liu,§ Fang Su,§ and Lingfei Wen§ College of EnVironmental Science and Engineering, Key Laboratory of EnVironmental Biology and Pollution Control, Ministry of Education, and State Key Laboroator of Chemo/Biosensing and Chemometrics, Hunan UniVersity, Changsha 410082, People’s Republic of China, and School of EnVironment and Chemical Engineering, Nanchang Hangkong UniVersity, Nanchang 330063, People’s Republic of China ReceiVed: August 30, 2009; ReVised Manuscript ReceiVed: January 30, 2010
CdSe nanoparticles with well dispersion were decorated on inner and outer surfaces of 4 µm long TiO2 nanotubes through a simple direct current electrotechnique, resulting in a composite functional material with a perfect construction. The applied depositing voltage plays a determinative role during the CdSe nanoparticles formation process, getting through the breakdown potential of TiO2 and providing intense active sites for the CdSe crystal growth on the TiO2 nanotubes. The CdSe/TiO2 composite nanotube arrays exhibit high absorption in the visible light region due to the narrow band gap of CdSe, and depict sensitive photoelectrochemical response under visible light illumination. Photocatalytic degradation of anthracene-9-carbonxylic acid (ACA), one of the derivants of persistent organic pollutants (POPs), was successfully achieved on CdSe/TiO2 nanotubes when exposed to the 550 nm green monochromatic light. 1. Introduction Recently, particular interest has been given to the selforganized titania nanotube (NT) arrays fabricated by anodization because of the high orientation, uniformly stable structure, large internal surface area, and excellent electron percolation pathways for vectorial charge transfer between interfaces.1-3 Compared with TiO2 powders, TiO2 NT semiconductors have shown unique chemical and physical properties because the NTs have much more free space in their interior as well as outer space that can be filled with active materials such as chemical compounds and noble metals,4-7 giving them a fundamental advantage over powders. The anodized titania NTs are amorphous, in an externally applied electric field, polarization of titania results in electron hopping between neighboring chains as described by Ti3+-O-Ti4+ f Ti4+-O-Ti3+, leading to an enhanced conductivity of the amorphous titania.8 Consequently, electrodepositing synthesis can be an efficient way employed to modify the titania NT arrays with active materials with well distribution. Based on the principle of sensitizing large band gap with smaller band gap semiconductors to extend the photoresponse of TiO2 into the visible, CdSe, one of the most important II-VI group semiconductors, was modified in the TiO2 NTs. CdSe quantum dots with particle diameters of 2.3-3.7 nm were assembled on TiO2 NTs assisted by a bifunctional linker molecule through the soaking method.9,10 However, we think the well dispersion of CdSe is difficult to achieve in long nanochannels by this method due to the high mass-transfer * To whom correspondence should be addressed. † College of Environmental Science and Engineering, Hunan University. ‡ Key Laboratory of Environmental Biology and Pollution Control, Ministry of Education, Hunan University. | School of Environment and Chemical Engineering, Nanchang Hangkong University. § State Key Laboroator of Chemo/Biosensing and Chemometrics, Hunan University.
resistance from the capillaries across section effect. Furthermore, “mulberry-like” CdSe nanoclusters with sizes from 50 to 100 nm anchored on the short TiO2 NTs were fabricated by the photoassisted electrodeposition method.11 As is known to us, the 2.5 µm long TiO2 NTs exhibit much higher photocatalytic activities than do the 400 nm ones.12 Herein, the long TiO2 NTs up to 4 µm were employed as photocatalysts which were sensitized with CdSe nanoparticles fabricated by the simple electrodeposition method. The applied external electric field can effectively accelerate the transfer of the electrolytes in the long TiO2 nanochannels, facilitating the electrodeposition of active metals. In this study, CdSe nanoparticles with diameters ranging from 5 to 50 nm were electrodeposited on the inner and outer surfaces of the long TiO2 NTs. By virtue of the high surface area, open pore structure with enhanced adsorption capacity in visible light region, anthracene-9-carbonxylic acid, a derivant of POPs, was degraded on CdSe/TiO2 NTs under green light (550 nm) illumination. Moreover, the titania NTs were grown from a Ti sheet, the integrality makes the practical application of the CdSe/TiO2 NT arrays simpler and more cost-effective compared with TiO2 powders, avoiding the filtration step after photoreaction or the immobilizing process required for photocatalyst particles. 2. Experimental Section Preparation of TiO2 NT Arrays. Titanium foils (250 µm thick, 99.8%; Sigma Aldrich) were ultrasonically cleaned in acetone and ethanol prior to anodization. These foils were anodized at 30 V constant voltage for 2 h in a two-electrode electrochemical cell in an ethylene glycol (EG) solution containing 0.3 wt % NH4F and 1% volume of H2O, resulting in the formation of long TiO2 NTs. The short TiO2 NT arrays for the control experiments were anodized at 15 V for 2 h in an inorganic electrolyte containing 0.5 M NaHSO4 and 0.1 M NaF. In all cases, a platinum foil was used as the counter electrode.
10.1021/jp910489h 2010 American Chemical Society Published on Web 03/01/2010
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Figure 1. Schematic illustration of photocatalytic degradation of anthracene-9-carbonxylic acid in the presence of CdSe/TiO2 nanotube arrays. The red arrows indicate the flow direction of the anthracene9-carbonxylic acid-contaminated water.
Yang et al. The distance between the two electrodes was maintained at 2 cm. All experiments were conducted at about 25 °C. Electrodeposition of CdSe Nanoparticles. A typical threeelectrode electrochemical cell was equipped with a Pt wire as a counter electrode, saturated calomel electrode (SCE) as a reference, and the TiO2 NTs (on Ti foil) as the working electrode. The electrolyte solution contained CdCl2, SeO2, and Na2SO4 (supporting electrolyte), and the pH value of the solution was adjusted by NaOH or H2SO4. CdSe was deposited at -0.7 V, -6 V vs SCE at room temperature. The particle sizes and loading contents of CdSe were controlled by tuning the depositing duration. After the deposition, the CdSe modified TiO2 NTs were washed several times with distilled water, and then heated in a nitrogen atmosphere at 450 °C for 5 h for photoelectrochemical measurements. Characterization. TiO2 NT arrays with CdSe nanoparticles were investigated by use of a field emission scanning electron microscope (FESEM, Hitachi, model S-4800). Energy dispersive X-ray (EDX) spectrometers fitted to electron microscopes were
Figure 2. (a) SEM images of the top view of the CdSe modified short TiO2 nanotubes, (b) lateral view showing that the nanotubes in panel a are 380 nm long. (c-f) Top view of the CdSe modified long TiO2 nanotubes; (d) lateral view showing the nanotubes are 4 µm long; (e and f) lateral view of high resolution showing the CdSe distribution in the long TiO2 nanotubes.
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Figure 3. UV-vis diffuse-reflectance spectra of unmodified and 25 nm diameter CdSe modified TiO2 nanotubes with (a) 380 nm and (b) 4 µm in length. Photocurrent response (c) and I-V characteristics (d) of 25 nm diameter CdSe modified 4 µm TiO2 NTs and 380 nm TiO2 NTs (excitation g420 nm, 100 mW/cm2, electrolyte 0.1 M Na2S solution, scan rate 1 mV/s).
TABLE 1: Bath Composition and Experimental Parameters for the Deposition of CdSe with Variable Sizes specimen in Figure 4
electrodeposition potential/V
a, 5 nm b, 15 nm c, 25 nm d, 50 nm
-6 -6 -0.7 -0.7
composition of plating soln CdCl2 CdCl2 CdCl2 CdCl2
(0.001 mol/L), SeO2 (0.1 mmol/L) (0.001 mol/L), SeO2 (0.1 mmol/L) (0.1 mol/L), SeO2 (0.2 mmol/L), Na2SO4 (0.1 mol/L) (0.1 mol/L), SeO2 (0.2 mmol/L), Na2SO4 (0.1 mol/L)
used for elemental analysis. The crystal structure of the TiO2 sample was characterized with use of an X-ray diffractometer (XRD, M21X, MAC Science Ltd., Japan) with Cu Ka radiation (λ ) 1.54178 Å). Optical and Electrochemical Measurements. Absorption spectra were recorded with a UV-vis spectrophotometer (Vary 300, USA) equipped with an integrating sphere with a radius of 150 mm. Photoelectrochemical studies were carried out in a three-electrode configuration with CdSe/TiO2 NTs (on Ti foils) as the working electrodes, Pt foils as a counter electrode, and SCE as a reference with Na2S or KOH solution as the electrolytes. The illuminated currents were recorded by an electrochemical working station (CH Instruments, model CHI 660B). Samples were tested under visible light and monochromatic lights of 420, 450, 550, and 600 nm from a 500 W xenon arc lamp (CHF-XQ-500W, Beijing Changtuo Co. Ltd.) by applying UV-cut filter and monochromatic light filters. The excitation intensity can be controlled by tuning the applied current of the Xe lamp and determined by a NOVA Oriel 70260 with a thermodetector. Photocatalytic degradation of ACA was performed under illumination of a monochromatic light of 550 nm. An ACA solution (15 mL) with an initial concentration of 10-5 mol/L was circulated through a quartz tube exposed to the light in which the CdSe/TiO2 NT arrays of a geometrical size of 4 × 1 cm2 were placed. The configuration scheme is depicted in Figure 1. The CdSe/TiO2 nanotube arrays/Ti sheet
pH
deposition duration
4.5 4.5 2.7 2.7
80 s 200 s 2h 4h
can be conveniently settled and replaced during the photocatalysis due to its integrity. The photocatalysis process was monitored by an in suit model, using the UV-vis spectrophotometer. Every experiment was repeated 3 times to ensure the reliability. 3. Results and Discussion 3.1. Effect of TiO2 Nanotubes Length on the Photocatalytic Activities of CdSe/TiO2 NTs. Figure 2 shows FESEM images of CdSe/TiO2 samples, exhibiting the topology of the CdSe nanoparticles modified short/long TiO2 NTs. As is shown in Figure 2a, about 25 nm diameter CdSe nanopaticles electrodeposited in the solution containing CdCl2 (0.1 mol/ L), SeO2 (0.2 mmol/L), and Na2SO4 (0.1 mol/L) at -0.7 V for 0.5 h were well distributed on the top surface of the TiO2 NTs prepared in NaHSO4-NaF electrolyte. The lateral view in Figure 2b depicts that the TiO2 NTs are 380 nm in length and the CdSe nanoparticles were deposited on the inner spaces of the NTs. SEM images in Figures 2c-f depict the CdSe modified 4 µm TiO2 NTs prepared with the EG solution, on which CdSe nanoparticles were deposited at -0.7 V for 2 h. Figure 2c shows the top morphology of the CdSe/ TiO2 NT arrays, depicting that the nanoparticles have size dispersions mainly in the range of about 25-30 nm. Panels d and e of Figure 2 depict the lateral view of the composite NTs with increasing magnification times. In Figure 2d, CdSe
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Figure 4. SEM images of CdSe/TiO2 NTs with variable CdSe particle sizes: (a) less than 5 nm, (b) about 15 nm, (c) 25 nm, and (d) 50 nm. The corresponding parameters of the electrodeposition are listed in Table 1.
nanoparticles (the light dots) distributed all over the TiO2 NTs. In Figure 2e, under illumination of the electron beam of the FESEM, the TiO2 NT wall thickness is so thin that the tubes are essentially transparent, showing that CdSe nanoparticles were deposited on the tube inner walls. Furthermore, some CdSe nanoparticles are attached on the external walls of the TiO2 NTs. In Figure 2f, the lateral view of the part close to the bottom of some broken NTs depicts that the CdSe nanopartilces can be deposited on the deeply inner surface of the NTs. Compared with the results reported in the literature,9-11 only our study gives the clear characterization of the detailed morphology of the CdSe nanoparticles decorated long TiO2 nanotube arrays, confirming that the active materials can be filled in the long nanochannels with uniform distribution. The corresponding EDS and XRD spectra of the CdSe/TiO2 NTs are provided in Figure S1 in the Supporting Information. Panels a and b of Figure 3 show the UV-vis diffusereflectance spectra of the TiO2 NTs and CdSe/TiO2 NT samples. Interestingly, as curve 2 in Figure 3a shows, the diffusereflectance spectrum depicts that the short unmodified TiO2 NTs can absorb the visible light. However, no absorption in the visible range (see curve 2 in Figure 3b) is created on 4 µm long TiO2 NTs anodized in EG solution. Only after CdSe modification can the CdSe/4 µm TiO2 NTs absorb the visible light, which can be understood as a consequence of the large number of CdSe nanoparticles on TiO2 NTs. Specifically, the ordered and porous TiO2 support has interior surfaces on which CdSe nanoparticles can be deposited, resulting in an enhancing absorption capacity in the visible light region while collecting and transmitting electrons through the TiO2 NT network.
Our investigation results revealed only the TiO2 NTs prepared in inorganic electrolytes (pH
