Facile Method for Fabricating Boron-Doped TiO2 Nanotube Array with Enhanced Photoelectrocatalytic
Facile Method for Fabricating Boron-Doped TiO 2Nanotube Array with Enhanced Photoelectrocatalytic Properties
Jingyuan Li,Na Lu,Xie Quan,*Shuo Chen,and Huimin Zhao
Key Laboratory of Industrial Ecology and En V ironmental Engineering (Ministry of Education),School of En V ironmental and Biological Science and Technology,Dalian Uni V ersity of Technology,Dalian 116024,People’s Republic of China
Highly ordered boron-doped TiO 2nanotube arrays were fabricated via a facile electrodeposition method.X-ray photoelectron spectroscopy (XPS)analysis revealed incorporated B atoms in the lattice of a TiO 2nanotube array.The X-ray diffraction (XRD)spectrum indicated improved crystallinity of boron-doped TiO 2nanotube arrays,relative to undoped TiO 2nanotube arrays.A shift of the absorption edge toward the visible region and a new absorption shoulder (380–510nm)of boron-doped TiO 2nanotube arrays were observed via diffuse re?ectance spectroscopy (DRS).In photoelectrochemical measurements,under either ultraviolet (UV)or visible-light irradiation,the photocurrent conversion ef?ciency was enhanced because of boron doping.The photoelectrocatalysis of phenol under simulated solar irradiation was performed using boron-doped or undoped TiO 2nanotube arrays,and the kinetic constant of a boron-doped TiO 2nanotube array photoelectrode was increased by ca.28%,compared to that of an undoped TiO 2nanotube array photoelectrode.
1.Introduction
TiO 2electrodes have been extensively investigated for their superior semiconducting material properties,1,2and among the TiO 2photoelectrodes with various morphologies and architec-tures,the anodic TiO 2nanotube array exhibits more promising photochemical and photocatalytic properties,because of its nanotube-array architecture,which enhances the electron per-colation pathway for vectorial charge transfer,promotes ion diffusion in the semiconductor/electrolyte interface,2–4and restrains photogenerated electron–hole pairs from recombina-tion.5Recently,TiO 2nanotube arrays have been investigated in photoelectrochemical and photoelectrocatalytic regions,such as solar cells,4,6photoelectrocatalytic water splitting,7and the degradation of organic pollutants.8We found that the mineral-ization ef?ciency of pentachlorophenol,using TiO 2nanotube array photoelectrodes,was much higher than that of TiO 2nanoparticle ?lm photoelectrodes under ultraviolet (UV)light,and the photoelectrocatalytic activity was greater than the photocatalytic activity,because of charge separation that was caused by an external bias supply.8However,the utilization ef?ciency of solar light for TiO 2nanotube arrays is still poor,because TiO 2nanotube arrays with a wide band gap are photochemically excited only by UV light.Several studies have shown that doping TiO 2with nonmetals,such as nitrogen,9,10carbon,11,12or boron,13would increase its visible-light photo-response.In our previous work,boron-doped TiO 2nanotube arrays were fabricated for the ?rst time via a vapor method,and it was found that boron doping in TiO 2nanotube arrays contributed to the increased visible photoresponse and the enhanced photoelectrocatalytic activity.14However,the vapor method for boron doping should involve a gaseous boric source,which is not as safe and convenient as using a nitrogen source.Ion implantation is another main strategy for doping nonmetals into TiO 2nanotube arrays,whereas this process may cause morphology disintegration and amorphization,which requires secondary treatment to compensate.10Therefore,the doping methods still need innovation and development.
Electrodeposition is a common technique that can be used to achieve a uniform deposition of elements on a conducting substrate.However,to our knowledge,until now,it has not been used as a doping method to incorporate impurity atoms in the lattice of a nanostructured ?lm.In consideration of the controllability of the electrodeposition technique and the unique nanostructure of the TiO 2nanotube array,which allows the electrolyte to permeate throughout the internal and external nanotube layer,7we extrapolate that boron-doped TiO 2nanotube arrays can be achieved using the electrodeposition technique.
In this paper,we have presented a facile electrodeposition method for doping TiO 2nanotube arrays with elemental boron,and we have investigated the effects of boron doping on the properties of the TiO 2nanotube array,including crystal structure,optical properties,and photoelectrocatalytic activity.2.Experimental Section
2.1.Fabrication of Boron-Doped TiO 2Nanotube Arrays.Titanium sheets (0.5mm thick,25mm width ×40mm length,99%purity)were sonicated in acetone for 3min and in deionized (DI)water for 20min,and then they were chemically etched in a mixture of acids (HF:HNO 3:H 2O )1:4:5,by volume)for 30s,followed by rinsing with DI water and,?nally,drying in a nitrogen stream for 30s at 25°C.The electrolyte was 1M (NH 4)2SO 4+0.5wt%NH 4F,15and then the pH value was adjusted to 5.0using 0.3M H 2SO 4,to prevent the anodized titanium sheet from forming a precipitate that is produced during the anodization process.The anodization was conducted with increasing potential (from 0V to 20V,then holding at 20V for 2h),using a direct current (dc)power (Beijing Dahua,PRC)in a two-electrode electrochemical setup that consisted of the pretreated titanium sheet anode and a platinum sheet cathode.After anodization,the sample was immediately rinsed with DI water.
A potentiostat (model DJS-292)and a three-electrode cell with an anodized titanium sheet as the working electrode,a saturated calomel electrode (SCE)as the reference electrode,and a platinum sheet as the counter electrode were used in the electrodeposition process.The ef?cient volume of the electro-
*To whom correspondence should be addressed.Tel.:+86-411-84706140.Fax:+86-411-84706263.E-mail:quanxie@https://www.sodocs.net/doc/4044161.html,.
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380410.1021/ie0712028CCC:$40.75 2008American Chemical Society
Published on Web 04/24/2008
chemical cell is 80mL.The electrodeposition processes were performed using current densities of 10μA/cm 2for 27min in the 0.1M H 3BO 3electrolyte,and an anodized titanium sheet that was not subjected to electrodeposition was prepared for comparison.Afterward,the resulting samples were rinsed with DI water,dried in air,and then annealed at 500°C for 2h with heating and cooling rate of 2°C/min.
2.2.Characterization.The morphology of the boron-doped TiO 2nanotube array was characterized by environmental scanning electron microscopy (ESEM)(model Quanta 200FEG).The crystal structures of the boron-doped and undoped TiO 2nanotube arrays were investigated using X-ray diffracto-metry (XRD,Shimadzu,model LabX XRD-6000)equipment with Cu K R radiation.The chemical composition of the boron-doped and undoped TiO 2nanotube array was examined using X-ray photoelectron spectroscopy (XPS,model Escalab 250)equipment with Al K R monochromatic radiation (1486.6eV).The optical absorption properties of the two samples were investigated using diffuse re?ectance spectroscopy (DRS),using an ultraviolet -visible light (UV–vis)spectrophotometer (Jasco,model UV-550).
2.3.Photocurrent Measurements.A schematic diagram of the experimental setup for the photocurrent measurements is shown in Figure 1.The photocurrent measurements,using 1M KOH as the electrolyte,were conducted in a standard three-electrode cell with boron-doped or undoped TiO 2nanotube arrays as the photoanode,a platinum sheet as the cathode,and SCE as the reference electrode,which were connected to a CHI electrochemical analyzer (CH Instruments,model 650B,Shang-hai Chenhua).A 350-W high-pressure mercury lamp (Beijing Huiyixin),which provided an illumination intensity of 0.75mW/cm 2was used as the UV light source;a high-pressure xenon short arc lamp (CHF-XM35-500W,Beijing Changtuo)with an illumination intensity of 80mW/cm 2provided full-spectrum illumination;a ?lter was added to this high-pressure xenon short arc lamp to allow visible light (λ>400nm)to pass through with an illumination intensity of 37mW/cm 2.Radiometers were used to measure the intensity of the incident UV light (model UV-A,Photoelectric Instrument Factory,Beijing Normal Uni-versity)and the visible light (model FZ-A,Photoelectric Instrument Factory,Beijing Normal University).
2.4.Direct Photolysis and Photoelectrocatalysis.The ex-perimental setup for the photoelectrocatalysis (PEC)of phenol was the same as that described for photocurrent measurements,as shown in Figure 1.A constant potential of 0.4V vs SCE was applied using the CHI electrochemical analyzer during the PEC processes.The high-pressure xenon short arc lamp served as the light source,with an illumination intensity of 80mW/cm 2at the position of the photoanode.Eighty milliliters of an aqueous solution with an initial phenol concentration of 20mg/L,with 0.01M Na 2SO 4as the supporting electrolyte,was added
into a quartz cell (50mm length ×40mm width ×60mm height).All the experiments were conducted with stirring at room temperature.Direct photolysis (DP)of the phenol,without any photocatalyst,was conducted under the same conditions as those for PEC.The concentration of phenol was determined using a high-performance liquid chromatography (HPLC)system (Separations Module 2695,Waters)that was equipped with a photodiode array detector (Module 2996,Waters).The mobile phase consisted of methanol and water (in a 2:1ratio),and the detection wavelength was 280nm.3.Results and Discussion
As shown in Figure 2,the nanotubes of boron-doped TiO 2nanotube array are open at the top,with an average diameter of ca.100nm,which is the same as that for an undoped anodized TiO 2nanotube array,14,15and there is no distinct deposit on the ?lm surface.The cross-sectional image of the boron-doped sample (Figure 2)provide direct evidence that the self-organized and ordered nanotube arrays are vertical to the substrate,with a nanotube layer thickness of ca.1.0μm.
Figure 3shows the B 1s XPS spectrum of (a)the boron-doped TiO 2nanotube array and (b)the undoped TiO 2nanotube array.Figure 3a shows that a B 1s peak at ca.191.4eV is observed for the boron-doped sample,whereas the XPS spectra contain no distinct peak at the same binding energy for
the
Figure 1.Schematic diagram of photoelectrochemical reactor.Legend:1,light source;2,quartz cell;3,anode;4,cathode;5,reference electrode;6,stirring device;7,stirring bar;8,CHI electrochemical analyzer;and 9,
computer.
Figure 2.ESEM top view image and cross-section (inset picture)of a boron-doped TiO 2nanotube
array.
Figure 3.B 1s XPS spectra of (a)a boron-doped TiO 2nanotube array and (b)an undoped TiO 2nanotube array.
Ind.Eng.Chem.Res.,Vol.47,No.11,20083805
undoped sample in Figure 3b.According to the standard B 1s binding energies in TiB 2(187.5eV,Ti -B)and B 2O 3(193.0eV,B -O),16the B 1s binding energy of 191.4eV might be assigned to the mix state Ti -O -B of TiB 2and B 2O 3,which is consistent with the published value of the B 1s signal for boron-doped TiO 2in XPS analysis.13,14,16Ti 3+cations should be produced by the reduction of Ti 4+cations,because of charge compensation.17However,there is no discernible difference in the Ti 2p peaks between boron-doped and undoped samples (?gure not shown).This phenomenon might be attributed to the subtle amount of Ti 3+cations that are produced by boron incorporation,relative to the large amount of Ti 4+cations.Accordingly,the results of XPS analysis indicate that the B atoms can be incorporated into an ordered TiO 2nanotube array.However,the XPS measure sensitivity for boron is poor,and the amount of boron dopant is low,relative to TiO 2;therefore,the B 1s peak is weak and,to some extent,can be in?uenced by instrument noise.
The XRD patterns of boron-doped and undoped TiO 2nanotube arrays are compared in Figure 4.The crystal structures of these two samples both correspond predominantly to anatase-phase TiO 2at 2θ)25.3°after annealing at 500°C,which implies that the boron-doped and undoped samples mainly consist of anatase-phase TiO 2.The (101)anatase peak intensity of the boron-doped sample is larger than that of undoped sample,which demonstrates that the crystallinity of TiO 2could be improved by boron doping.Correspondingly,the grain bound-aries and amorphous regions that can serve as charge-carrier recombination centers are reduced.7
Figure 5shows the optical absorption coef?cient (R )of boron-doped and undoped TiO 2nanotube arrays,as a function of the wavelength (λ).The increased optical absorption coef?cient in the main absorption region at 270–320nm was observed for the boron-doped sample.Furthermore,a red shift of optical absorption edge and a new optical absorption shoulder at 380–510nm are obtained for the boron-doped sample.
In terms of the reasons for the red shift of element-doped TiO 2,two types of debatable theories exist:one is that “bandgap narrowing”leads to a red shift by mixing the impurity state with valence-band states,such as a mixture of N 2p or C 2p with O 2p states;9,12the other one is that the “introduction of donor or acceptor levels”in the band gap results in a visible response.18–21Zhao et al.13reported that the B 2p states mixing with the O 2p states in boron-doped TiO 2lead to the bandgap narrowing.However,in this paper,the absorption shoulder in the visible-light region with a steady absorption coef?cient from 380-420nm cannot be interpreted appropriately using this “bandgap narrowing”mechanism,because a narrowed interband
transition should exhibit an abrupt decrease in the absorption coef?cient in the absorption-edge region,rather than such an absorption shoulder.Therefore,the red shift and the new absorption shoulder in the visible range are likely due to the excitation of electrons from the impurity energy levels,located above the valence-band edge (O 2p states),provided by the substituted B atoms,to the conduction band edge (Ti 3d states).It is,to some extent,consistent with the cases of nitrogen-and carbon-doped TiO 2.18–21
It is well-known that the interband electron transition is accompanied by relaxation and recombination in TiO 2semi-conducting material,and consequently,only partial photons absorbed by TiO 2can contribute to the generation of a photocurrent.Therefore,it is necessary to take into account the photocurrent–voltage characteristics for further investigation of the photoelectrochemical properties of boron-doped TiO 2nano-tube arrays.The net photocurrent density was calculated by subtracting the dark current density from the measured photo-current density,and the dark currents were negligible in all cases.As shown in Figure 6a,the saturated photocurrent density of a boron-doped TiO 2nanotube array photoelectrode generated under UV light at 1.0V vs SCE is ca.20%higher than that of an undoped TiO 2nanotube array photoelectrode,which is consistent with that of the boron-doped sample fabricated via the vapor method in our previous work.14The superior photo-current conversion ef?ciency may be contributed to improved optical absorption capability and increased crystallinity caused by boron doping.Similarly,there is a noticeable increase in photocurrent density under visible light for the boron-doped TiO 2nanotube array photoelectrode,compared to the undoped TiO 2nanotube array photoelectrode,as shown in Figure 6b.The photocurrent density of the boron-doped sample at 1.0V vs SCE is ca.18%higher than that of the undoped sample.The absorption shoulder in the visible region and the red shift of the absorption edge may have important roles in the enhanced visible-light photocurrent conversion ef?ciency of the boron-doped sample.However,the photocurrent density of the boron-doped sample in visible light is still far from that in UV light,despite the presence of a new absorption shoulder.It may result from harvest losses of charge carriers that are due to high recombination in these impurity energy levels under visible-light excitation.18,19
According to the superior photocurrent conversion ef?ciency of the boron-doped TiO 2nanotube array photoelectrode under UV and visible illumination,it is reasonable that the photocur-rent density of the boron-doped sample is greater than that of the undoped sample under full spectrum illumination (see
Figure
Figure 4.XRD spectra of boron-doped and undoped TiO 2nanotube arrays after annealing at 500°
C.
Figure 5.Difuse re?ectance spectroscopy (DRS)spectra of (;)boron-doped TiO 2nanotube arrays and (---)undoped TiO 2nanotube arrays.
3806Ind.Eng.Chem.Res.,Vol.47,No.11,2008
6c).The photoconversion ef?ciencies of light energy to chemical energy without applied bias were calculated according to related expressions in refs 3and 18,and values of ca.0.4%and 0.6%are obtained for the undoped and boron-doped TiO 2nanotube array photoelectrodes,respectively.
To investigate the applied potential of boron-doped TiO 2nanotube arrays for use as photocatalysts,DP and PEC tests were performed.As seen from Figure 7,the resulting concentration -reaction time curves show noticeable differences in the removal ef?ciencies among these tests.Clearly,35%of the phenol was removed via DP degradation after 2h.In contrast,via PEC degradation for 2h,56%and 66%of the phenol was removed with undoped and boron-doped samples,respectively.
In terms of kinetics study,the DP and PEC degradations of phenol under these experimental conditions follow a pseudo-?rst-order kinetics equation,in accord with previous studies that DP or PEC degradations of organic contaminants of low initial concentration ?t the pseudo-?rst-order kinetics equation.22,23The kinetic constants (k )are listed in Table 1.The kinetic constants of the DP and PEC degradations were increased from 0.252h -1to 0.431h -1,which indicated that the overall degradation rate was signi?cantly increased by the addition of the TiO 2
nanotube array as a photocatalyst.Furthermore,the kinetic constant of the boron-doped sample is increased by ca.28%,as compared to that of the undoped sample,which indicates that enhanced photoelectrocatalytic activity was obtained by boron doping via this electrodeposition method.The reasons may be as follows:boron-doped TiO 2nanotube array photo-electrodes of superior photoelectrochemical properties are able to afford more photogenerated electrons and holes to decompose phenol;on the other hand,the surface defects that are induced by the incorporated B atoms are able to serve as catalytic centers,which could enhance the photoelectrocatalytic activity of TiO 2.244.Conclusions
Boron-doped TiO 2nanotube arrays were obtained via an electrodeposition method with a current density of 10μA/cm 2for https://www.sodocs.net/doc/4044161.html,pared to undoped TiO 2nanotube array photoelectrodes,the photocurrent conversion ef?ciencies of the boron-doped sample were enhanced under ultraviolet (UV)light,visible light,and a full solar spectrum,respectively.The tests of photoelectrocatalysis (PEC)degradation of phenol under full spectrum illumination demonstrated a higher catalytic activity for the PEC process than that of the direct photolysis (DP)process,and boron doping could further accelerate the PEC reaction rate.The effects of boron doping on ordered TiO 2nanotube array photoelectrodes may result from improved crystallinity and impurity energy levels that are located in the TiO 2band gap.More work is still needed to further understand the mechanism of this doping method,and to develop this electrodeposition method for doping with other elements,or with multiple elements,to increase the utilization ef?ciency of solar light for TiO 2nanotube arrays.Acknowledgment
This work was supported jointly by the National Science Foundation of Distinguished Young Scholars of China (under Project No.20525723)and National Nature Science Foundation China (under Project No.20407005).Literature Cited
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Recei V ed for re V iew September05,2007
Re V ised manuscript recei V ed February26,2008
Accepted March01,2008
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