PAPER www.rsc.org/pps | Photochemical & Photobiological Sciences
Photoelectrocatalytic treatment of pentachlorophenol in aqueous solutionusing a rutile nanotube-like TiO2/Ti electrode
Shaogui Yang,*a Xie Quan,b Xinyong Lib and Cheng Suna
Received 20th March 2006, Accepted 9th June 2006First published as an Advance Article on the web 26th June 2006DOI: 10.1039/b604077b
Taking pentachlorophenol (PCP) as a reference, we investigated the photoelectrocatalytic degradationof organic pollutants using a rutile nanotube-like TiO2/Ti film electrode. The nanotube-like TiO2
electrode was prepared by first oxidizing the surface of a titanium sheet to form rutile TiO2 and thentreating it to form the tubular structure in NaOH aqueous solution. The occurrence of PCPdegradation was indicated by the decrease in pH, concentration of PCP and TOC, and by the formationof chloride ions. The photoelectrochemical (PEC) efficiency of the nanotube-like TiO2/Ti electrode hasbeen determined in terms of degradation of PCP and the incident photo-to-current conversionefficiency (IPCE). The experimental results showed that PCP could be degraded more efficiently by aphotoelectrocatalytic process than by a photocatalytic or electrochemical oxidation alone. A significantphotoelectrochemical synergetic effect was observed. The kinetic constant of PEC degradation of PCPusing a nanotube-like TiO2 electrode was over 60% higher than that using a TiO2 film electrode. It isnoted that chloride ion and hydrogen ion concentration increased with irradiation time in the PECdegradation of PCP; PCP was gradually mineralized and the complete minimization of PCP took moretime than its degradation.
1. Introduction
The efficiency of photocatalytic degradation of organic pollutantsin solution using commonly available TiO2 photocatalysts hasbeen improved by electrochemical means.1–4 This technique whichis usually called photoelectrocatalytic (PEC) degradation tech-nology, combines UV light irradiation with the application of acontrolled anodic potential through a supported catalyst, resultingin a decrease in the recombination rate of photogenerated electronsand holes. PEC degradation of organic pollutants has becomea growing area of environmental research.1–6 PEC oxidationtechnology has proven to be more efficient than photocatalytic(PC) oxidation technology because the high degree of chargerecombination could be successfully solved by using an anodicbias.2–5 However, the development of a practical water treatmentsystem related to the photoelectrocatalytic oxidation has not yetbeen successfully achieved due to the low photocatalytic efficiencyand stability of the photoanode (such as the TiO2 film electrode).
In recent years, due to their high surface area, TiO2-basednanotubes began to attract wide attention because of theirpotentials in many areas such as highly efficient photocatalysis7
and photovoltaic cells.8,9 During the past several years, variousmethods have been developed for the synthesis of one-dimensionaltitania (nanotube, and nanowire) that have included sol–gelsynthesis,10–12 freeze–drying,13 electrodeposition,14–15 sonochemicaldeposition,16 anodic oxidation17–19 and chemical treatments of finetitania particles.20–23 Moreover, thin films and coatings of oriented
aState Key Laboratory of Pollution Control and Resource Reuse, School ofthe Environment, Nanjing University, Nanjing, 210093, P.R. China. E-mail:[email protected]; Tel: +86-025-83593239bSchool of Environmental and Biological Science and Technology, DalianUniversity of Technology, Dalian, 116024, P.R. China
nanostructures are often more desirable for applications involvingcatalysis, filtration, sensing, photovoltaic cells, and environmentalapplications because of their high active surface area.
It is known that the stability of the TiO2 film coated electrodeis very important in environmental applications, when a biaspotential, especially a high bias potential, is applied to theelectrode. The adhesion of the rutile TiO2 onto the conductingsubstrate may be better than that of the anatase phase, because theanatase phase is prepared usually at temperature below 500 ◦C24,25
while the rutile phase is done at higher temperatures (>600 ◦C).The adhesion of the rutile form of nanotube-like TiO2 ontothe Ti substrate may be better than that of the anatase form.From a practical point of view, the adhesion of the nanotube-likeTiO2 is significant to endure bias potential, especially high biaspotential, in electrochemically assisted photocatalytic reactions.The stability and photocatalytic activity of the nanotube-likeelectrode prepared by anodic oxidation on a titanium plate werediscussed in our previous work.18 Therefore, a rutile nanotube-likeelectrode ought to have not only high photocatalytic activity butalso good stability. However, only a few reports concerning therutile form of nanotube-like TiO2 electrodes have been found. Aphotoelectrochemical reaction by applying a bias potential on therutile form of a nanotube-like TiO2 electrode is interesting.
Pentachlorophenol, PCP, has been extensively used for woodpreservation and as a biocide. This compound is carcinogenic andtoxic to plants, animals, and human even at low concentrations.26
PCP is also known as an environmental precursor for the for-mation of polychlorinated dibenzo-p-dioxins (PCDDs) and poly-chlorinated dibenzofurans (PCDFs), which are more harmful.27
Previous investigators have examined the feasibility of oxidiz-ing PCP in an aqueous solution using different semiconductorpowders as photocatalysts.28 They reported that PCP could be
decomposed by a suspension photocatalyst in the presence ofoxygen. The oxidation of PCP using an anatase form of a TiO2 filmelectrode by applying a low bias potential was reported. Howevera photocatalytic reaction of PCP oxidation using the rutile formof a nanotube-like TiO2 electrode by applying a high electric fieldhas not yet been reported.
By combining these two interesting concepts, we firstly reporthere the preparation of a rutile nanotube-like titania electrode bytreating the rutile titania film with a hydrothermal method. Thephotoelectrocatalytic degradation of PCP in a Na2SO4 aqueoussolution using the rutile nanotube-like TiO2 electrode was alsoinvestigated.
2. Experimental
2.1 Materials
The reagents pentachlorophenol (99% purity), sodium hydroxide,sodium sulfate, hydrofluoric acid (38–40%) nitric acid, hydrochlo-ric acid and methanol were purchased from the ShenYang Chem-ical Company. All compounds were of analytical grade exceptfor hydrofluoric acid and methanol and used as received withoutfurther purification. Methanol was of HPLC grade. Concentratedstock solutions of PCP were made by dissolving the PCP powderin 0.05 mol L−1 NaOH solution. Subsequent concentrations forexperiments were obtained by diluting the stock solution withdeionized water.
2.2.1 Preparation of the TiO2 film electrode. Two titaniumplates (40 mm × 60 mm) were employed as substrate for thepreparation of the Ti/TiO2 working electrode with only oneface exposed to solution. They were mechanically polished withdifferent abrasive papers, rinsed in ultrasonic bath containing colddistilled water, then chemically etched by immersion in a HF :HNO3 : H2O (1 : 4 : 5 in volume) mixed solution,29 and finallyrinsed with acetone and deionized water. They were then dried inair at room temperature. Pretreatment by this procedure ensuresa good adhesion of the deposit to the substrate. It is interestingto note that when exposed to air, the treated titanium sample israpidly covered with metal oxides composed not only of TiO2 butalso of lower (conducting) titanium oxides. These were oxidizedin a furnace under an atmosphere of air as following: increasingthe temperature from room temperature to 700 ◦C at a rate of5 ◦C min−1, holding at this temperature for 1 h, and then coolingto room temperature to obtain two TiO2/Ti electrodes (labelledas T1 and T2, respectively).
2.2.2 Preparation of the nanotube-like TiO2 electrode. Fur-ther treatment of the TiO2/Ti film electrode (T2) formed bythermal oxidation with a NaOH (10 M) aqueous solution in aTeflon vessel at 150 ◦C was carried out for 20 h. After beingwashed with a 0.1 mol L−1 HCl aqueous solution and deionizedwater, the pH value was about 7.0. T2 was dried in a furnace at100 ◦C so forming a rutile nanotube-like TiO2 electrode (labelledas T2nanotubes).
The anatase TiO2 film electrodes was prepared by the sol–gelmethod described previously in detail.30
2.3 Characterization of the TiO2/Ti electrodes
The morphology and composition of the nanotube-like TiO2
electrode were characterized using scanning electron microscopy(SEM; JSM-5600LV) with an accelerating voltage of 20 kV. Thecrystallinity of the nanotube-like TiO2 electrode was determinedby X-ray diffraction (XRD) using a diffractometer with Cu Karadiation (Model, Shimadzu LabX XRD-6000). The accelerat-ing voltage and the applied current were 40 kV and 30 mA,respectively.30
2.4 Photoelectrocatalytic experiments
Photoelectrocatalytic measurements of PCP degradation wereperformed in a single photoelectrochemical compartment usinga three-compartment electrochemical cell, which was describedin detail previously.18 The experiment was performed under thefollowing conditions: under UV irradiation (a 300 W high-pressuremercury lamp from the factory of Beijing Light with a maximumwavelength of 365 nm, Io = 0.6 mW cm−2), vigorous stirring, 0.6 V(vs. SCE) of electric bias, 0.1 mol L−1 sodium sulfate as supportingelectrolyte. The reactor was open in air without airflow. The initialconcentration of PCP was 20 mg L−1 and the pH of the solutionswas 5.0 in all cases.
2.5 Analytical methods
Samples (1 mL) were withdrawn from the reaction vessel atspecified time intervals, filtered with 0.45 lm Millipore filter toremove any particles that might clog the column. The filtratewas then analyzed as required. The determination of PCP residuewas performed by HPLC (PU-1580, UV-1575, Jasco Corporation,Japan) with a Kromasil ODS (5 lm, 4.6 mm × 250 mm) reversephase column. The mobile phase was 1.0 mL min−1 of methanoland water (v : v = 4 : 1) and determined at wavelength k = 220 nmquantitatively.
Total organic carbon analyzer (TOC-VCPH, Shimadzu, Japan)was employed for mineralization degree analysis of the PCPsolutions. Prior to injection into the TOC analyzer, 10–15 mL werecollected from the aqueous solutions and filtered with a 0.45 lmMillipore filter to remove any particles. Chloride ion concentrationwas determined by titration method, in which 0.01 mol L−1 AgNO3
was used as measuring reagent and potassium dichromate was theindicator.18
The intensity of UV irradiation was measured with a UV-irradiance meter (UV-A, Instruments of Beijing Normal Univer-sity in China). The pH of the solutions was measured with a BasicpH meter (PB-20, Beijing Sartorius Co., Ltd).30
3. Results and discussions
3.1 Characterization of the nanotube-like TiO2/Ti electrode
T1 and T2nanotubes are characterized by X-ray diffraction as shownin Fig. 1. From the figure, there is no obvious difference betweenthe TiO2 film (T1) and nanotube-like TiO2 (T2nanotubes) electrodes.It is evident from Fig. 1 that the peaks at 2h = 27.6 in the spectrum
Fig. 1 XRD pattern of the TiO2 film electrode obtained by thermaloxidation at 700 ◦C and of the nanotube-like TiO2 electrode formed byhydrothermal method.
of TiO2 are easily identified as the crystal of the rutile phase. Thisagrees with the direct thermal oxidation of titanium from 500 to800 ◦C.31 Leng et al.32 have obtained the same result for TiO2
formed thermally in air or oxygen atmosphere. Furthermore, theaverage crystalline size calculated using the diffraction peaks fromthe Scherrer’s formula is about 40 nm.
SEM images of the TiO2 film electrode (T1) and nanotube-like TiO2 electrode (T2nanotubes) are presented in Fig. 2 and Fig. 3,respectively. Fig. 2 indicates that the T1 sample consists of granularcrystals. As clearly seen in Fig. 3, T2nanotubes was remarkablydifferent from T1, that is, the granular particles that were presentin T1 were no longer observed in T2nanotubes. T2nanotubes consists ofnumerous fiber-like nanotubes grew from micro-size TiO2 particleson the titania electrode. The energy dispersive X-ray spectra (notshown here) indicate that the nanotubes in T2nanotubes are composedof Ti and O, ignoring some undetectable light elements andbackground signals, and the ratio of Ti to O is 1 : 2.
Fig. 2 SEM image of the TiO2 film obtained by the thermal oxidation at700 ◦C.
Fig. 3 The SEM image of the nanotube-like TiO2 electrode formed after20 h of reaction at 150 ◦C in NaOH solution at (A) low magnification(×10000) and (B) high magnification (×50000).
3.2 Photoelectrochemical properties of the photoanodes
3.2.1 Photocurrent generation from the photoanodes. TiO2
films are photoactive as they undergo charge separation whensubjected to the following band gap excitation (eqn (1))
TiO2 + hm → TiO2 (eCB + hVB) (1)
Where eCB and hVB represent the conduction band electrons andvalence band holes. The photogenerated electrons can be readilycollected at a conducting surface in a photoelectrochemical cell.The photoelectrochemical behavior of the film electrode reflectsdirectly the competition between hole-mediated and electron-consumed reactions.33 Therefore, photocurrent is an importantparameter in photoelectrocatalysis.
In this study, the photocurrent measurements for TiO2 film andnanotube-like TiO2 electrodes were performed in the presenceof 20 mg L−1 PCP, 0.1 M Na2SO4 solution, generally speakingsodium sulfate does not produce a background photocurrent as
high as sodium chloride.34 The obtained current–voltage plotis shown in Fig. 4. In the dark, the anodic current was low.Upon illumination of the film and nanotube TiO2 electrodes,a significant increase in the anodic current was observed dueto the photoexcitation of electrons to the conduction band. Ascan be seen from Fig. 4, photocurrents of film and nanotubeTiO2 electrodes both increased greatly with the increase of theapplied anodic potential. Their photocurrents are both alwayshigher than the dark current, this being clear evidence ofthe synergic effect between photochemical and electrochemicalprocesses.
Fig. 4 Effect of applying bias potential on both the photocurrent densityand IPCE. Working electrode: rutile TiO2 film or nanotube-like TiO2
electrode; counter electrode: Pt electrode; potential vs. SCE; electrolyte:0.1 M Na2SO4, pH: 4.9; light power: 300 W.
3.2.2 IPCE. The photoelectrochemical property of a photo-catalytic electrode is generally determined by the incident photo-to-current conversion efficiency (IPCE)35–36 which is defined asthe number of electrons generated by light in the external circuitdivided by the number of incident photons and given by thefollowing formula:
IPCE = Ihc/kPe = 1240I
kP(2)
where I is the photocurrent density (lA cm−2), k is the excitationwavelength (nm), P is the incident light intensity (W m−2) and thevalue of hc/e is a constant (1240). The photocurrent spectra andIPCE values of the film and nanotube TiO2 electrodes are shownrespectively in Fig. 4. This indicates that the photoelectrochemicalproperty of the nanotube-like TiO2 electrode is obviously higherthan that of the TiO2 film electrode. It may be ascribed to the highersurface area and the more powerful hole of the nanotube-like TiO2
electrode than that of the TiO2 film electrode.
3.3 Comparison of photoelectrocatalytic degradation of PCP
The primary role of an electrocatalyst for photooxidation pro-cesses is to provide very low valence band edge energy, EVB, and tomake the surface electrons–holes to act as powerful oxidizing sitesfor generating radical oxidants in reactions with the medium.37
The direct oxidation of PCP molecules by high energy holes willoccur as well, though the low concentration of PCP may diminishthe contribution of this reaction pathway. Thus, the main goalsfor preparation of titania nanotubes is obtaining high energy
holes, maximizing their surface concentration and enhancing thephotocatalytic activity.
The photoelectrocatalytic degradation experiments of PCP werecarried out using the rutile nanotube-like TiO2 electrode (T2),and the rutile titania nanofilm electrode (T1) formed by thermaloxidation. The bias potential applied on the TiO2 film andnanotube-like TiO2 electrodes was 0.6 V (vs. SCE). The resultis presented in Fig. 5. It can be easily seen from Fig. 5 thatthe nanotube-like TiO2 and the film electrodes exhibit differentphotoelectrocatalytic activity, the phenomenon accords with ourprevious study.18 The degradation rate of PCP on the nanotube-like TiO2 electrode is faster than that on the TiO2 film electrode.In 3 h, 48% of PCP was mineralized using the nanotube-like TiO2
electrode, while about 40% of PCP was mineralized using the TiO2
film electrode. The kinetics constants of PCP photoelectrocatalysisusing the nanotube-like TiO2 electrode and the TiO2 film electrodewere 0.0251 min−1 and 0.0154 min−1, respectively. These resultsindicated that the kinetic constant of PCP oxidation on thenanotube-like TiO2 electrode was over 60% higher than that onthe TiO2 film electrode. It showed that the photoelectrocatalyticactivity of the nanotube-like TiO2 electrode was higher than thatof the TiO2 film electrode. This may be attributed to the differenceof specific surface area between the nanotubes and the film.Nanotube-like TiO2 has larger specific surface area than TiO2
film and could adsorb more organic matter for degradation. Inaddition, Fig. 5 shows that the complete minimization of PCPtook more time than its degradation.
Fig. 5 The variation of the relative concentrations of PCP and TOC byphotoelectrocatalytic technology with TiO2 film or nanotube-like TiO2
3.4 PCP, pH, TOC and chloride ion concentration of PCPsolutions
In the photoelectrocatalytic degradation of PCP in aqueoussolutions, the attack of •OH radical to PCP may replace chlorineatoms, producing more hydroxyl functional groups that canbe oxidized to aldehyde and/or further oxidation to open upthe aromatic ring. The opening up of the aromatic ring mayproduce organic dicarboxylic acids, such as oxalic acid, beforecomplete mineralization. It can be inferred that the pH willdecrease during the course of the reaction and chloride ions will
evolve. It is apparent that before complete mineralization occurs,some intermediates may be produced. Therefore, in order to gain abetter comprehension of the process for the photoelectrocatalyticdegradation of PCP, we monitored the change in concentration ofPCP, TOC, pH and chloride ions in solution with the increase ofthe irradiation time.
3.4.1 PCP. The decay of PCP concentration as a functionof reaction time is depicted in Fig. 6. Experiments of PCPdegradation were performed in four different processes, (a)electrochemical process (EC) alone, (b) direct photolysis (DP),(c) photocatalysis (PC) and (d) photoelectrocatalysis (PEC).Experiments involving monitoring of the pH, TOC and chlorideion concentration under the same conditions were also conductedin each of the four processes. As shown in the figure, PCPwas not degraded in the electrochemical process in the wholeperiod investigated. Direct photolysis as control experiment can beconsidered in the PEC experiment and the degradation efficiencyof PCP reached 57%. The photocatalysis had an effect on thedegradation of PCP and the degradation efficiency of PCP was82%. When both electrochemical and photocatalytic processes aresimultaneously applied, 100% of PCP removal was obtained. Thisvalue is 18% higher than the summation of the electrochemicaland photocatalytic process acting alone. Such a synergetic effectbetween photo- and electro-degradation has been recognized byYang et al.29 and Vinodgopal et al.1 at low external potential biasusing conducting glass electrode or titanium foil with TiO2 film.
Fig. 6 Removal efficiency of PCP in EC, DP, PC and PEC processes; pH:4.9, supporting electrolyte: 0.1M NaSO4, continuously stirring, no airflow,light intensity: 0.6 mW cm−2. The rutile nanotube-like electrode was usedas working electrode.
3.4.2 pH and TOC. The change of pH and TOC during thephotoelectrocatalytic degradation of PCP in aqueous solution aredisplayed in Fig. 7 and 8. The two figures show that during theelectrochemical experiment, pH and TOC of the solution remainedunchanged during 6 h observation, but decreased as the irradiationtime increased under the direct photolysis, photocatalysis andphotoelectrocatalysis experiments. The decrease of pH and TOCin the PEC experiment was the fastest, that in the PC is mediumand that in the DP is the slowest. Apparently photolysis may causea dechlorination process, which produce proton and chloride ion.Moreover, under the photocatalysis the dechlorination was muchenhanced followed by the breaking down of the aromatic moiety.
Fig. 7 The change of pH value in the PEC, PC, DP and EC processesof 20 mg L−1 pentachlorophenol degradation occurred in a solution ofpH 4.9, supporting electrolyte: 0.1 M NaSO4, continuously stirring, noairflow, light intensity: 0.6 mW cm−2. The rutile nanotube-like electrodewas used as working electrode.
Fig. 8 The change in TOC value in the PEC, PC, DP and EC processesof 20 mg L−1 pentachlorophenol degradation occurred in a solution ofpH 4.9, supporting electrolyte: 0.1 M NaSO4, continuously stirring, noairflow, light intensity: 0.6 mW cm−2. The rutile nanotube-like electrodewas used as working electrode.
3.4.3 Chloride ions. The confirmation of chloride evolutionis illustrated in Fig. 9. During long irradiation periods (6 hirradiation), under solely UV irradiation, dechlorination of nearlytwo atomic chlorine was observed, most probably due to directphotolysis. While in the presence of TiO2 nanotube-array film andUV light, the dechlorination of PCP molecules was apparentlyfaster than direct photolysis. It seems that the photocatalyticdegradation lead to the dechlorination of nearly four atomic chlo-ride after 8 h irradiation. In the photoelectrocatalysis experiment,the dechlorination of PCP molecules was the fastest among EP,DP, PC and PEC processes. It seems that the photoelectrocatalyticdegradation lead to complete dechlorination after 6 h irradiation.
It should be noted that the anatase form of TiO2 is consideredto be more efficient than the rutile form in the photocatalyticdegradation of organic compounds.38 In our present work weannealed the titanium sheet at high temperature (700 ◦C) in orderto obtain a stable TiO2 film and then the TiO2 film was treated toform the tubular structure in NaOH aqueous solution. The results
Fig. 9 Chloride ion evolution in the PEC, PC, DP and EC processesof 20 mg L−1 pentachlorophenol degradation occurred in a solution ofpH 4.9, supporting electrolyte: 0.1 M NaSO4, continuously stirring, noairflow, light intensity: 0.6 mW cm−2. The rutile nanotube-like electrodewas used as working electrode.
shown in Fig. 6, 7, 8 and 9 indicated that rutile nanotube-like TiO2
has also good capability in the photoelectrocatalytic degradationof PCP.
3.5 Comparison with an anatase TiO2 film electrode and stabilityof rutile nanotube-like electrode
In order to test the practical advantage of the rutile nanotube-likeelectrode, experiments on comparison with an anatase titania filmelectrode and the stability of the rutile nanotube-like electrodewere carried out in the same conditions. The results are presentedin Fig. 10. It can be clearly seen from the figure that the rutilenanotube-like electrode displayed higher photoelectrocatalyticactivity compared to the anatase film electrode. In 3 h, 45% of PCPwas mineralized by the rutile nanotube-like electrode, while about42% of PCP was mineralized by the anatase TiO2 film electrode.
Fig. 10 The change of PCP and TOC in the PEC degradation of 20 mg L−1
The experiments on the reproducibility (stability) of the rutileTiO2 nanotube-like electrode were performed using the same TiO2
nanotube-like electrode to degrade PCP ten times. After each use,the TiO2 nanotube-like electrode was cleaned with ultrasonic.On the basis of PCP degradation percent, we assume that the
reproducibility of the TiO2 nanotube-like electrode is only in therange 3–5% in the same condition.18
4. Conclusions
A rutile nanotube-like TiO2/Ti electrode was obtained by a hy-drothermal method. The photocurrent and IPCE of the nanotube-like electrode were obviously higher than those of the rutileTiO2/Ti film electrode. The kinetic constant of PEC degradationof PCP using the nanotube-like TiO2 electrode was over 60%higher than that using the TiO2 film electrode. Degradation ofPCP was faster by photoelectrocatalysis than photocatalysis aloneor electrochemical catalysis alone. The removal efficiency of theformer process was higher than the sum of those by individualphotocatalysis and electrochemical catalysis. This result indicatesthat there is a synergetic effect on the degradation of PCP inaqueous solution when irradiation and applied bias potential aresupplied simultaneously to the nanotube-like TiO2/Ti electrode.The application of a bias potential promotes the photoactivityof the rutile nanotube-like TiO2/Ti electrode, and thus enhancesthe photocurrent. In a photoelectrocatalytic process, PCP canbe degraded completely and mineralized for photoelectrocatalyticprocess in a desirable time.
Acknowledgements
This work was supported jointly by Postdoctoral Nature ScienceFoundation of P.R. China (No.: 20050380238) and the Post-doctoral Nature Science Foundation of Jiangsu province, China(0501010B).
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