金属氧化物(Fe2O3,CuO,NiO)改性对TiO2纳米管阵列光电催化活性的增强效应

丛燕青 李 哲 王 齐 张 轶 徐 谦 伏芳霞

(浙江工商大学环境科学与工程学院,杭州310012)

金属氧化物(Fe2O3,CuO,NiO)改性对TiO2纳米管阵列光电催化活性的增强效应

丛燕青*李 哲 王 齐 张 轶 徐 谦 伏芳霞

(浙江工商大学环境科学与工程学院,杭州310012)

采用阳极氧化法和阴极电沉积法制备了Fe2O3,CuO和NiO纳米粒子改性的高度有序的TiO2纳米管(TiO2-NT)阵列.运用场发射扫描电子显微镜(FE-SEM),透射电子显微镜(TEM),X射线衍射(XRD)和紫外-可见漫反射光谱等手段对Fe2O3/TiO2-NT、CuO/TiO2-NT和NiO/TiO2-NT复合电极进行表征.以苯酚为模拟污染物,考察复合电极的光电性能.结果表明,金属氧化物(Fe2O3,CuO,NiO)纳米粒子成功沉积在TiO2-NTs的管口、内壁和管底.金属氧化物改性复合电极的光电催化活性比未改性的TiO2-NTs提高了2倍以上.Fe2O3/TiO2-NTs在可见光区显示出最高的吸收强度.以Fe2O3/TiO2-NTs为阳极处理苯酚废水,光照120 min后苯酚去除率达到96%,而未改性的TiO2-NTs的苯酚去除率只有41%.此外,Fe2O3/TiO2-NTs在生成低毒中间产物方面表现出良好的性能.较高的复合电极光电催化活性主要是由于TiO2纳米管和过渡金属氧化物纳米粒子间构筑的高界面面积异质纳米结构,有效地促进了电子转移,抑制了光生电子-空穴对的复合.

TiO2纳米管;Fe2O3;CuO;NiO;光电催化;可见光

1 Introduction

Abatement of environmental pollutants by green technologies is significantly attractive research topic.It is particularly interest in the treatment processes using the solar energy since sunlight is a renewable natural energy.Photoelectrochemical (PEC)process is recognized to be one of the most promising ways to clean our environment.1Semiconductor electrodes employed in PEC process can be excited by solar light to generate the electron-hole pairs to remove the pollutants.Assisted electrochemical process promotes the separation of electron-hole pairs and further improves the efficiency of pollutants degradation.An efficient photocatalyst should maximize the utilization of solar energy and minimize the recombination of photoexcited electron-hole pairs.Therefore,the properties of semiconductor materials are crucial for achieving high efficiency in PEC process.2

Various semiconductors have been extensively investigated since Fujishima and Honda3firstly suggested the water splitting with TiO2under UV illumination in 1972.TiO2is one of the most studied semiconductors because of its high photocatalytic activity,chemical stability,low cost,and nontoxicity.4-7However,the widespread usage of TiO2is limited by its large band gap energy(3.0-3.2 eV),which can only utilize the ultraviolet region of the solar spectrum.To enhance the photocatalytic activity of TiO2under visible light,considerable efforts have been attempted to improve the absorption in the visible spectrum,including dye sensitization,8-10anion or cation doping,11-13noble metal deposition,14-16and incorporation with transition metal oxides.17,18Another main drawback of TiO2is the high recombination rate of photo-generated holes and electrons.19Faster recombination largely decreases the quantum efficiency of PEC processes.Therefore,it is essential to suppress the recombination of electron-hole pairs.Among various strategies aimed at improving the absorption in the visible region and separating the electron-hole pairs,the incorporation of transition metal oxides with TiO2has been approved to be an effective method.20-23Zhang and co-workers23have synthesized TiO2/ Cu2O composite film and obtained high degradation efficiency of methylene blue.Although some studies have reported the incorporation of transition metal oxides with TiO2,there was little information about the molecular-scale architecture control and systematical study on various metal oxides.

In this work,highly ordered vertically oriented TiO2nanotube(TiO2-NT)arrays were fabricated by electrochemical anodization of Ti foil.The self-organized oriented NT structure could provide large surface areas and facilitate vectorial charge transfer from the solution to the substrate,which were expected to accelerate the separation of electron-hole pairs and harvest sunlight more efficiently.Simple transition metal oxides (Fe2O3,CuO,NiO)nanoparticles were uniformly incorporated with TiO2-NTs by a novel electrochemical deposition method. The photocatalytic activity of the NT electrodes under visible light irradiation could be enhanced by modifying the surface structure and composition with the special metal oxides.Field emission scanning electron microscopy(FE-SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),and UV-visible diffuse reflectance spectroscopy were used to characterize the structure and optical properties of composite electrodes.The PEC activities of composite electrodes were evaluated by phenol removal.

2 Experimental

2.1 Preparation of TiO2-NT electrodes modified by metal oxides

TiO2-NT electrodes were prepared by the electrochemical anodization method on a Ti foil(0.25 mm thick,99.7%purity). Prior to anodization,the Ti foil was polished with sandpaper, and then ultrasonically cleaned with acetone,ethanol,and distilled water.Anodization was performed in a two-electrode system with the pretreated Ti foil as the working electrode and Pt sheet as the counter electrode under constant voltage at room temperature.The anodizing voltage varied from 0 to 20 V with an increasing certain rate and was kept at 20 V for 120 min. The electrolyte was a mixed solution of 0.5%(w)NaF and 0.5 mol·L-1Na2SO4.All reagents were analytical grade.After anodic oxidation,the samples were rinsed with deionized water and dried in air.The as-formed TiO2-NTs were annealed in a muffle furnace with 2°C·min-1heating rate and kept at 773 K for 2 h to convert the amorphous phase to the crystalline one.

Fe2O3nanoparticles were deposited into the crystallized TiO2-NTs using an electrodeposition method.A two-electrode system was used with TiO2-NTs as the cathode and a Pt sheet as the anode.First,the TiO2-NT electrodes were soaked in a 0.1 mol·L-1Fe(NO3)3aqueous solution for 10 min,always subjected to ultrasound sonication before soaking.Then the TiO2-NT electrodes were transferred into a new medium that only contained an inert supporting electrolyte(0.1 mol·L-1Na2SO4).The potentiostatic DC electrodeposition was carried out at a constant voltage of 8 V for 20 min and the temperature of the electrolyte was maintained at 85°C.After the electrodeposition in this medium,Fe nanoparticles were deposited into the interior tubes of TiO2-NT electrodes(denoted as Fe/ TiO2-NTs).About 1.0%(w)deposition amount of Fe in the NTs was obtained after several repetitions.Then the Fe/ TiO2-NT electrodes were connected as the anode and the Pt sheet as the cathode.The material was electrochemically oxi-dized in 1 mol·L-1KOH aqueous solution at a voltage of 8 V for 2 min at room temperature.After this electrochemical oxidization,Fe/TiO2-NTs were converted into the corresponding oxides Fe2O3/TiO2-NTs.The resulting Fe2O3/TiO2-NT samples were rinsed with distilled water and dried at a low temperature.

The preparation processes of CuO/TiO2-NTs and NiO/TiO2-NTs were the same as that of Fe2O3/TiO2-NTs,except that the deposition solution was 0.1 mol·L-1Cu(NO3)2and 0.1 mol·L-1Ni(NO3)2aqueous solutions,respectively.

2.2 Characterization

The morphologies and the cross-section views of TiO2-NT electrodes modified by metal oxides were characterized using a field emission scanning electron microscope(FE-SEM;Hitachi S-4700 II)and a transmission electron microscope(TEM; Philips-FEI Tecnai G2 F30 S-Twin),equipped with energy-dispersive X-ray spectroscopy(EDX;EDAX Analyzer DPP-II). The crystal properties of the prepared samples were determined from X-ray diffraction(XRD)using a diffractometer with Cu Kαradiation(Netherlands PNAlytical XʹPert PRO). The accelerating voltage and applied current were 40 kV and 40 mA,respectively.Light absorption properties were measured using UV-Vis diffuse reflectance spectra(Shimadzu, UV-3150)with a wavelength range of 220-600 nm.Electrochemical impedance spectroscopy(EIS)was performed using a CHI 660D instrument(Chenhua,Shanghai)in a three-electrode system,with a saturated Ag/AgCl electrode and a Pt sheet as reference and counter electrodes,respectively.

2.3 Photoelectrochemical activity test

The PEC activity of the composite electrodes was evaluated using phenol as a model pollutant.All the experiments were carried out in a two-electrode glass cell(100 mL)with constant magnetic stirring,using 0.2 mol·L-1Na2SO4as the electrolyte.The initial concentration of the phenol aqueous solution was 10 mg·L-1.The composite electrode was used as anode and Cu sheet was cathode.Applied voltage was provided by the DC Constant Current Power(WYL603 type,Hangzhou Yuhang Siling Electronic Equipment Co.,Ltd.).The anode was irradiated using a tungsten-halogen lamp(500 W),which generates a continuous light distribution across the visible spectrum and relatively weak emission in the ultraviolet portion of the spectrum.A UV cut-off filter(λ＞420 nm)was used for visible light irradiation.The light intensity on the photoanode was～80 mW·cm-2.All experiments were carried out under ambient conditions.The determination of phenol and its degradation intermediates were carried out using high performance liquid chromatography(HPLC,Agilent 1200)by comparing the retention time of the standard compounds.The separation was performed using a Diamonsil C18 reversed phase column(150 mm×4.6 mm×5 mm)at the flow rate of 1.0 mL·min-1and the column temperature of 25°C.The eluent consisted of methanol/purified water(30:70(volume ratio)).The analyses were performed with a UV detector at a wavelength of 254 nm.

3 Results and discussion

3.1 Characterization of photocatalysts

Fig.1 shows the SEM morphologies of the as-synthesized TiO2-NTs,Fe2O3/TiO2-NTs,CuO/TiO2-NTs,and NiO/TiO2-NTs. The hollow TiO2-NTs are almost uniform and have a highly ordered tubularstructure.The averageinnerdiameterof TiO2-NTs is～80 nm,and their average outer diameter is～110 nm(Fig.1(a)).Fe2O3/TiO2-NTs,CuO/TiO2-NTs,and NiO/TiO2-NTs have the similar tubular structure.The corresponding metal oxide nanoparticles were distinctly deposited on TiO2tubular substrates.The surface of TiO2-NT substrates was not blocked by nanoparticles.To identify the distribution of nanoparticles in TiO2-NTs,the cross-section views of the composite electrodes were analyzed by TEM images.According to Fig.2,the length of TiO2-NTs is around 1.2 μm.Fe2O3nanoparticles are deposited on the mouth,the tube wall,and the base of TiO2-NTs.The average diameter of Fe2O3nanoparticles is about 35 nm.Note that the deposition process has not destroyed the structure of the ordered TiO2-NT arrays,and Fe2O3nanoparticles can be fabricated into the bottom of TiO2-NTs. The EDX spectrum in Fig.2 confirms the existence of Fe,Ti, and O,whereas the Cu signal originates from the Cu substrate used in the imaging process.The measured atomic ratio of Fe/ Ti was 1.21%.The existence of NiO in NiO/TiO2-NTs was also confirmed(figure not shown).The measured atomic ratio of Ni/ Ti was 1.35%.The atomic ratio of Cu/Ti could not be determined due to the interference of Cu substrate used in EDX measurement.Fig.3 shows the XRD patterns of different composite electrodes annealed at 773 K.TiO2is converted from amorphous state to anatase state with a fine preferential growth of the self-organized highly oriented TiO2-NT arrays in the(101)direction.The peak of the(101)crystal(2θ=26.2°) can be seen from all the patterns.The characteristic peaks cor-responding to CuO,NiO,and Fe2O3are also identified in Fig.3. It indicates that TiO2-NTs,Fe2O3/TiO2-NTs,CuO/TiO2-NTs, and NiO/TiO2-NTs have been successfully synthesized.

Fig.1 SEM images of TiO2-NTs and the metal oxide modified TiO2-NTs(a)TiO2-NTs,(b)CuO/TiO2-NTs,(c)NiO/TiO2-NTs,(d)Fe2O3/TiO2-NTs

Fig.2 TEM images with different magnifications(a,b)and energy-dispersive X-ray(EDX)spectroscopy(c)of Fe2O3/TiO2-NT electrode

Fig.3 XRD patterns of(a)TiO2-NTs,(b)NiO/TiO2-NTs, (c)Fe2O3/TiO2-NTs,and(d)CuO/TiO2-NTs

Fig.4 UV-Vis diffuse reflectance spectra of the composite TiO2-NT electrodes modified by different metal oxides

UV-Vis diffuse reflectance spectra of the composite TiO2-NT electrodes modified by different metal oxides are shown in Fig.4.In the wavelength range from 220 to 325 nm,Fe2O3/ TiO2-NTs,CuO/TiO2-NTs,and NiO/TiO2-NTs have lower absorbance intensity than the unmodified TiO2-NTs.When the wavelength is longer than 325 nm,however,the absorbance intensities of Fe2O3/TiO2-NTs,CuO/TiO2-NTs,and NiO/TiO2-NTs are higher than that of TiO2-NTs.Especially for Fe2O3/ TiO2-NTs,its absorbance intensity is significantly higher than other electrodes in the visible light region.Considering the large proportion(47%)of visible light in solar spectrum,the composite TiO2-NT electrodes modified by Fe2O3,CuO,and NiO are superior to the unmodified TiO2-NTs.

3.2 Photoelectrocatalytic degradation of phenol

To investigate the PEC activity of composite electrodes,phenol degradation experiments were carried out using the composite electrodes as the photoanodes.Fig.5 shows the comparison of phenol removal efficiency under irradiation.The removal rates of phenol using the three composite electrodes have been significantly improved relative to the unmodified TiO2-NT electrodes.After 120 min treatment,phenol removal efficiencies of Fe2O3/TiO2-NT,NiO/TiO2-NT,and CuO/TiO2-NT anodes were 96%,93%,and 90%,respectively,while it was only 41%for the unmodified TiO2-NT anode.The PEC activity of the composite NT electrodes was over twice that of the unmodified TiO2-NT electrode.The improved PEC performance was apparently attributed to the modification of metal oxides.

3.3 Comparison of different processes

Fig.5 Comparison of phenol removal efficiency under irradiation using different electrodes

The electrocatalytic,photocatalytic,and photoelectrocatalytic processes were performed to investigate the role of different processes on phenol degradation using Fe2O3/TiO2-NTs as the anode since it has the best PEC activity.In electrocatalytic process,applied potential was performed and the experiments were carried out in the dark.In photocatalytic process,light irradiation was performed at open circuit(without applied potential).In photoelectrocatalytic process,applied potential and light irradiation were simultaneously used.All other operating conditions were the same.Fig.6 shows the comparison of different processes on phenol degradation.After 120 min treatment,phenol removal efficiency could reach 96%in the photoelectrocatalytic process,while it was only 15%for photocatalytic process and 4%for electrocatalytic process.It was evident that the photoelectrocatalytic process has synergetic effects in enhancing the removal efficiency of phenol in comparison with the individual photocatalytic or electrocatalytic process.

Comparison of Fe2O3/TiO2-NTs and TiO2-NTs on phenol degradation under UV-Vis light and visible light irradiation is shown in Fig.7.Phenol removal efficiency of Fe2O3/TiO2-NTs is ca 2.3 times higher under UV-visible light irradiation and 8 times higher under visible light irradiation than that of TiO2-NTs.It is obvious that the modification of Fe2O3on TiO2-NTs significantly improves the PEC activity under visible light irradiation.

Fig.6 Comparison of different processes on phenol degradation using Fe2O3/TiO2-NTs as the anode

Fig.7 Comparison of Fe2O3/TiO2-NTs and TiO2-NTs on phenol degradation under UV-Vis light and visible light irradiation

3.4 Determination of phenol degradation intermediates

Fig.8 shows the HPLC chromatograms of phenol degradation at different treatment time.The main intermediates were identified to be benzoquinone,hydroquinone,and maleic acid by comparing the retention time of the standard compounds (Table 1).It can be seen that phenol was continuously degraded with time.Benzoquinone is an important intermediate of phenol degradation.Fig.9 shows that the yields of benzoquinone under irradiation using TiO2-NT,Fe2O3/TiO2-NT,CuO/ TiO2-NT,and NiO/TiO2-NT electrodes.Benzoquinone yields on the composite NT anodes first increased and then decreased with time.This was quite beneficial to the detoxification of wastewater because benzoquinone was regarded as one of the most toxic intermediates.24Benzoquinone yields of Fe2O3/ TiO2-NT,NiO/TiO2-NT,CuO/TiO2-NT,and TiO2-NT anodes were 1%,4%,7%,and 9%at 120 min,respectively.Fe2O3/ TiO2-NTs showed good performance to generate the low toxic intermediates.A possible reason was that some benzoquinone intermediate was simultaneously degraded when phenol was converted to benzoquinone.Fe2O3/TiO2-NTs had the highest PEC activity among these composite electrodes and could generate the most oxidizing reagents to degrade the pollutants(see Fig.5).Therefore,sufficient oxidants could further degrade the benzoquinone to achieve the lower yields of benzoquinone.

Fig.8 HPLC chromatograms of phenol degradation at different treatment time under UV-Vis light irradiation

Table 1 HPLC retention time of phenol and its intermediates

Fig.9 Yields of benzoquinone under irradiation using different electrodes

3.5 EIS analysis

TiO2-NT electrodes modified by simple transition metal oxides were analyzed by electrochemical impedance spectroscopy(EIS).Experiments were carried out in 0.1 mol·L-1NaOH solution under dark condition.Fig.10 shows the Nynquist plots of TiO2-NT electrodes and TiO2-NTs modified by various metal oxides.For each electrode,only one arc could be observed in the complex plane,which was related to the porous nature of the electrodes.The radius of the arc reflects the charge transfer resistance at the surface of the electrode.25It is obvious that the arc radius on TiO2-NTs modified by various metal oxides is smaller than that on unmodified TiO2-NTs.This indicates that the modification of transition metal oxides improves the interfacial charge transfer of TiO2-NTs.

3.6 Possible mechanism

Fig.10 EIS Nynquist plots of TiO2-NT electrode and TiO2-NT electrodes modified by various metal oxides under dark condition

Fig.11 Current density-potential curves of different electrodes under chopped visible light irradiation(a)TiO2-NTs,(b)Fe2O3/TiO2-NTs,(c)CuO/TiO2-NTs,(d)NiO/TiO2-NTs

Current density-potential curves of various electrodes were tested in 0.1 mol·L-1NaOH solution using a three-electrode system(Fig.11).The photocurrent density of various electrodes increased as the applied voltage rose.Composite electrodes showed a better photoresponse under visible light irradiation than TiO2-NT electrode.The photocurrent of Fe2O3/TiO2-NTs (or NiO/TiO2-NTs)was ca 3 times higher than that of TiO2-NTs at 0.4 V(vs Ag/AgCl).CuO/TiO2-NTs had a lower photocurrent relative to Fe2O3/TiO2-NTs(or NiO/TiO2-NTs),but its photocurrent was still over 2 times higher than that of TiO2-NTs. The recombination peaks of photogenerated electron-hole pairs were found at lower applied voltage,however,they could decrease as the applied voltage bias increased.In addition,Fe2O3/ TiO2-NT and CuO/TiO2-NT electrodes showed higher dark current density than TiO2-NTs and NiO/TiO2-NTs,which indicated that Fe2O3and CuO nanoparticles had higher conductivity and reduced the charge transfer resistance of the Fe2O3/TiO2-NT and CuO/TiO2-NT electrodes.These results were consistent with EIS analysis in Fig.10.We tentatively put forward that Fe2O3and CuO nanoparticles may perform as the channel for electron migration and improve the separation of photogenerated electron-hole pairs since they have higher conductivity than TiO2.

The differences in PEC activity among these composite electrodes were probably associated with their band structure and surface chemical nature.Table 2 lists the valence band(VB) and conduction band(CB)positions of TiO2,Fe2O3,NiO,and CuO.26Fe2O3and CuO have the low band gap,which would favor the absorption of the solar energy in visible light region. Since the CB position of Fe2O3(or CuO)is more positive than that of TiO2,some photogenerated electrons in Fe2O3/TiO2-NTs (or CuO/TiO2-NTs)may transfer from TiO2to Fe2O3(or CuO), leaving more holes to carry out the oxidation reaction.However,it might also serve as a recombination center since the photogenerated holes may move from TiO2to Fe2O3(or CuO).Therefore,the deposition amount of Fe2O3(or CuO)should be appropriately controlled.A schematic diagram of the photogenerated charge separation and electron transport on Fe2O3/ TiO2-NT or CuO/TiO2-NT electrode is shown in Fig.12(a).The low band gap of Fe2O3(or CuO)played an important role in the enhanced PEC activity of Fe2O3/TiO2-NTs(or CuO/ TiO2-NTs).NiO/TiO2-NTs also showed a higher PEC activity compared to the TiO2-NTs,but the band gap of NiO was larger than that of TiO2.Therefore,the possible mechanism of enhanced PEC performance on NiO/TiO2-NTs was different from Fe2O3/TiO2-NTs(or CuO/TiO2-NTs).Since NiO is a p-type semiconductor and TiO2is an n-type semiconductor,a number of p-n junctions would be formed when NiO was deposited on TiO2.As shown in Fig.12(b),the inner electric field was formed at the equilibrium,which made p-type NiO region had the negative charge and n-type TiO2region had the positive charge.Thus,the photogenerated holes moved to the negative field,while the electrons transferred to the positive field under the inner electric field.As a result,electron-hole pairs were effectively separated and the PEC activity of NiO/TiO2-NTs was significantly improved compared with the unmodified TiO2-NTs.

Table 2 Valence band(VB)and conduction band(CB)positions of TiO2,Fe2O3,NiO,and CuO

Fig.12 Schematic diagrams of pollutants degradation on MO/TiO2-NT electrodes under irradiation(a)possible pathway of the photogenerated charge separation and electron transport on Fe2O3/TiO2-NT or CuO/TiO2-NT electrode; (b)p-n junction formation model on NiO/TiO2-NT electrode

When the phenol molecules were adsorbed on the surface of electrodes,some photogenerated holes directly reacted with phenol molecules to produce phenol+•radicals.Further,the reactive phenol+•radicals were transformed into degradation intermediates.And other holes reacted with H2O to produce hydroxyl radicals(HO·),which further oxidized organic compounds into H2O and CO2.18The possible reactions on the heteronanostructures comprised of TiO2-NTs and transition metal oxide(MO)nanoparticles could be expressed as follows:

Note that Fig.12 shows just a tentative mechanism and the differences in PEC activity among various composite electrodes have not been completely clarified.The PEC activity is also sensitive to the interface,which is more complicated.The crystal-face exposed to the electrolyte at the interface may cause a different structure of the electric double layer.Thus this study illustrates the enhanced PEC activity of TiO2-NTs modified by simple transition metal oxides,and more work will be done to further clarify the mechanism.

4 Conclusions

The present study has demonstrated that TiO2-NTs,Fe2O3/ TiO2-NTs,CuO/TiO2-NTs,and NiO/TiO2-NTs could be successfully synthesized by a simple electrochemical anodization and electrodeposition method.The obtained TiO2and composite NT electrodes had a uniform and highly oriented tubular structure.The characteristic peaks corresponding to Fe2O3,CuO, and NiO were identified by XRD and the main phase of TiO2-NTs was anatase.Nanostructured composite electrodes showed a PEC activity more than 2 times higher than the pure TiO2-NTs.After 120 min treatment,phenol removal efficiency using Fe2O3/TiO2-NT,NiO/TiO2-NT,and CuO/TiO2-NT electrodes could reach 96%,93%,and 90%,respectively,while it was only 41%for the unmodified TiO2-NT anode.Moreover, Fe2O3/TiO2-NTs showed good performance to generate the low toxic intermediates.The low band gap of Fe2O3(or CuO) played an important role in the enhanced PEC activity of Fe2O3/ TiO2-NTs(or CuO/TiO2-NTs).The enhanced performance of NiO/TiO2-NTs was attributed to the formation of p-n junctions. The results indicate that TiO2-NTs modified by simple transition metal oxides(Fe2O3,CuO,NiO)are promising candidates for environmental applications.

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Enhanced Photoeletrocatalytic Activity of TiO2Nanotube Arrays Modified with Simple Transition Metal Oxides(Fe2O3,CuO,NiO)

CONG Yan-Qing*LI Zhe WANG Qi ZHANG Yi XU Qian FU Fang-Xia
(College of Environmental Science and Engineering,Zhejiang Gongshang University,Hangzhou 310012,P.R.China)

Composite electrodes consisting of highly ordered,vertically oriented TiO2nanotube(TiO2-NT) arrays modified with Fe2O3,CuO,and NiO nanoparticles were successfully fabricated by a simple electrochemical anodization and electrodeposition method.Field emission scanning electron microscopy (FE-SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),and UV-Vis diffuse reflectance spectroscopy were used to characterize the structure and optical properties of the resulting Fe2O3/TiO2-NT,CuO/TiO2-NT,and NiO/TiO2-NT composite electrodes.The photoelectrochemical(PEC) activities of the composite electrodes were evaluated using phenol as a model pollutant.Results indicated that transition metal oxide nanoparticles were deposited on the mouth,tube wall,and base of the TiO2-NTs. The PEC activity of the composite electrodes was over twice that of an unmodified TiO2-NT electrode.The Fe2O3/TiO2-NT electrode showed the highest absorption intensity in the visible light region.After treatment for 120 min,the phenol removal efficiency using the Fe2O3/TiO2-NT anode could reach 96%,while it was only 41%for the unmodified TiO2-NT anode.Moreover,the Fe2O3/TiO2-NT electrode tended to generate intermediates of low toxicity.The higher PEC activity of the composite electrodes was attributed to the presence of hetero-nanostructures with high interfacial area comprised of TiO2-NTs and transition metal oxide nanoparticles,which efficiently facilitated electron transfer and inhibited the recombination of photogenerated electron-hole pairs.

TiO2nanotube;Fe2O3;CuO;NiO;Photoelectrocatalysis;Visible light

10.3866/PKU.WHXB201203221

∗Corresponding author.Email:yqcong@yahoo.cn;Tel:+86-571-88071024-7018.

The project was supported by the National Natural Science Foundation of China(20976162,21103149,20906079),Natural Science Foundation of Zhejiang Province,China(R5100266),and Significant Science and Technology Project of Zhejiang Province,China(2010C13001).

国家自然科学基金(20976162,21103149,20906079),浙江省自然科学基金(R5100266)及浙江省科技厅重大专项(2010C13001)资助项目

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