Huiqi Hou

Jianfu Zhao

Ling Chen

Tian Li

Yanling Qiu

Zhiliang Zhu

Comptes Rendus Chimie

Available online at www.sciencedirect.comC. R. Chimie 11 (2008) 120e124http://france.elsevier.com/direct/CRAS2C/Full paper / MémoireTransmission flash photolysis of titanium dioxidephotooxidation of pyrogallolYanling Qiu a,*, Jianfu Zhao b, Huiqi Hou c, Tian Li b, Ling Chen b, Zhiliang Zhu aa Key Laboratory of Yangtze River Water Environment of the Ministry of Education, College of Environmental Science and Engineering,Tongji University, 1239 Siping Road, Shanghai 200092, Chinab State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University,1239 Siping Road, Shanghai 200092, Chinac Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, ChinaReceived 15 December 2006; accepted after revision 13 August 2007Available online 24 October 2007AbstractLaser-flash photolysis was performed to explore the primary micromechanism of pyrogallol’s oxidation by TiO2 photocatalysis.Time-resolved transient transmission measurements indicated that the initial photocatalytic reaction mechanism for pyrogallol isthe generation of photoexcited electronseholes, with the holes, instead of OH radicals, oxidizing pyrogallol into pyrogallolcations. To cite this article: Y. Qiu et al., C. R. Chimie 11 (2008). 2007 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.Keywords: Flash photolysis; Titanium dioxide; Pyrogallol; Transient absorption1. IntroductionThe basic mechanism of TiO2 photodegradation oforganic compounds is widely accepted as photo excita-tion generating electronseholes transferring to the TiO2surface where an oxidation and reduction reactions oc-cur. It is well known that OH radicals, which havea very high oxidation reduction potential (ORP) andoxidation reactivity, are generated as a product ofsuch photoexcited reactions. Research has concentratedon the oxidation reactions between the OH radicals andvariable substrates [1e5], but little attention has been* Corresponding author.E-mail address: ylqiu@mail.tongji.edu.cn (Y. Qiu).1631-0748/$ - see front matter  2007 Académie des sciences. Publisheddoi:10.1016/j.crci.2007.08.003given to the origin of the OH radicals or the transientspecies associated with the oxidation.Flash photolysis is one of the most powerful meansto study transient absorption. It may provide informa-tion on the intermediate product of the primary reactionthat would only exist for a very short time. This infor-mation is important in determining the photoinduced re-action mechanism [6e10].Pyrogallol, also called pyrogallic acid or 1,2,3-trihydroxybenzene, is a white crystalline powder andpowerful reducing agent belonging to the phenol fam-ily. It is used as a photographic film developer, in thedyeing of hair and suturing materials, and for oxygenabsorption in gas analysis. It also has antiseptic proper-ties. Pyrogallol is not likely to be used as a modern hairdye due to the suspected toxicity of the compound.by Elsevier Masson SAS. All rights reserved.Fig. 1. Absorbance spectra of TiO2 suspension.121Y. Qiu et al. / C. R. Chimie 11 (2008) 120e124In this study, transmission laser flash photolysis isemployed to study the primary TiO2 photooxidationof pyrogallol. The initial oxidation dynamic of pyro-gallol photooxidation was determined from thetransient absorption spectra and the spectral changecaused by pyrogallol concentration and dissolvedoxygen.2. Experimental2.1. Instrumentation and materialsTime-resolved transient transmission measurementswere performed using the third harmonic (355 nm,5 ns full width at half-maximum) from a Q-switchedNd:YAG laser (Quanta-Ray, GCR-150). The transientsignals were recorded with LKS 60 laser photolysisequipment. Steady-state UVevis absorption wasmeasured with a UVevis spectrophotometer (Shimadzu,UV-1700). TiO2 powder suspension was sonicated in anultrasonic cleanser (Branson, 3510).TiO2 powder (P25, Degussa), pyrogallol(C6H3(OH)3, HPLC grade) and high-purity nitrogenwere used during the experiments.2.2. Sample preparationTiO2 powder was suspended in 500 mL deionizedwater by sonicating for 10 min. The pH value of thissuspension was 5.5. Through steady-state absorptionspectra scanning, a TiO2 concentration with an idealextinction value at 355 nm was selected as the subse-quent experimental TiO2 concentration.Concentrated pyrogallol solution was added to theTiO2 suspension and stirred to form a uniform mixture.The desired volume of the mixture and concentrationsof pyrogallol and TiO2 were realized by adding suitablequantities of deionized water. The mixture was pouredinto a quartz cuvet, which was transparent on all sides,and the cuvet was placed inside the flash photolysisapparatus to produce a transmission system for the sub-sequent experiment. All procedures for the samplepreparation were performed with shielding from UVlight.3. Results and discussion3.1. Steady-state absorption spectraFig. 1 shows the UVevis scanning spectra of 10, 20and 25 mg/L TiO2 suspension. It was found that the ab-sorbance over wavelengths of 200e600 nm had a verygood linear correlation with the TiO2 concentration.For example, at a wavelength of 355 nm, the linear re-gression equation for the absorbance and concentrationof TiO2 suspension was: A¼ 0.0327Cþ 0.0376(R2¼ 0.9981). The TiO2 concentration of 20 mg/Lwas selected as the subsequent experimental TiO2 con-centration as it had the most ideal extinction value at355 nm, namely the absorbance equaled 0.680, whichwas neither very high nor very low. After the suspensionwas left for 1 h, the absorption spectra of TiO2 suspen-sion was measured again and showed scarcely any dif-ference compared to the original spectra, whichindicated that the TiO2 suspension was homogeneousand stable.Different amounts of pyrogallol concentrated solu-tion were added to 20 mg/LTiO2 suspension to give dif-ferent concentrations of pyrogallol and TiO2 mixedsolutions. An absorption spectra measurement (Fig. 2)showed no influence on the absorbance at 355 nm com-pared to that of pure TiO2 suspension. That is, pyrogal-lol had no absorbance at 355 nm and it had no directphotolysis at this wavelength. This guaranteed that thefunction of TiO2 would be recognized if there wasany photolysis of pyrogallol during the followingexperiment.3.2. Electronehole generation and recombinationFig. 3 depicts the transient absorption spectra ob-served for the 50 mg/L pyrogallolþ 20 mg/L TiO2mixture after 80 ns, 240 ns and 15 ms of laser excita-tion. The spectra recorded immediately after laserexcitation (Dt¼ 80 ns) shows peak absorptions200 300 400 500 6000.00.51.01.52.02.53.03.54.04.5Absorbance/a.u.Wavelength/nm20 mg/L TiO220 mg/L TiO2 + 100 mg/L pyrogallol20 mg/L TiO2 + 200 mg/L pyrogallolFig. 2. Absorbance spectra of TiO2þ pyrogallol mixture comparedto pure TiO2 suspension.122 Y. Qiu et al. / C. R. Chimie 11 (2008) 120e124around 325 nm and 600 nm. Both the electron and thehole absorb at l¼ 600 nm [11]. Electrons and holesthen recombined rapidly and the peak absorbancedropped to zero after 15 ms of excitation. The peakabsorption at 325 nm may indicate some new tran-sient species formed, which could exist for a relativelylonger time than the electrons and holes. Todetermine the characteristic of this transient species,310 nm was selected as the subsequent detectingwavelength because the absorbance at 310 nm ismore stable than that at 325 nm in the spectral rangeof the same peak.Fig. 3. Time-resolved absorption spectra recorded after 355-nm laserpulse excitation of a 50 mg/L pyrogallolþ 20 mg/L TiO2 mixedsolution.3.3. Transient absorption with and without pyrogallolThe transient absorption of the 50 mg/L pyrogal-lolþ 20 mg/L TiO2 mixture and 20 mg/L TiO2 pure so-lution after excitation detected at 310 nm is presented inFig. 4. The recombination reaction of holes and elec-trons finished within 800 ns if no pyrogallol was addedto the TiO2 suspension. However, when pyrogallol wasadded, the transient absorbance did not drop to zero af-ter hole and electron recombination finished, but a lowobvious absorbance lasted for a relatively long period oftime. This must be attributable to some newly generatedtransient species with an absorption at 310 nm.From the time-resolved absorption curves in Fig. 4, itis also found that the recombination of holes and elec-trons finished in about 800 ns for pure TiO2. Some liter-ature has reported that the average lifetime of holes andelectrons is only a few tens of picoseconds [12]. Thisdoes not conflict with our observation because the re-combination competes with the trapping of free chargesby coordination defects at the surface and by lattice de-fects in the nanoparticle bulk. After photoexcitation,some of the electronehole pairs recombine rapidlyand some migrate to the TiO2 surface. Among the car-riers on the surface of TiO2, 45% of the carriers descendinto surface traps and react with substances adsorbed onthe surface slowly thereafter (after about 200 ns), and10% of the carriers enter deep inside the TiO2 nanopar-ticles and do not react at all [11,13,14]. The variation oftransient absorption observed in this experiment, fromits peak to zero, was due to the complicated generationand recombination of electrons and holes.Fig. 4. Transient absorption spectra obtained at 310 nm after flashphotolysis, in 50 mg/L pyrogallolþ 20 mg/L TiO2 mixed solutionand 20 mg/L TiO2 pure solution.Fig. 5. Transient absorption spectra obtained at 310 nm after flashphotolysis, in different concentrations of pyrogallolþ 20 mg/LTiO2 mixed solution.123Y. Qiu et al. / C. R. Chimie 11 (2008) 120e1243.4. Transient absorption for different concentrationsof pyrogallolFig. 5 shows the transient absorption results at310 nm obtained as a function of pyrogallol concentra-tion in the TiO2 solution. The absorbance signal in-creased with increasing pyrogallol concentration. Thisis due to more transient species being generated whenthe amount of pyrogallol absorbed on the surface ofTiO2 was higher. There are two possibilities for the tran-sient species. One is pyrogallol cations oxidized byholes and the other is the OH adducts formed fromOH radicals.3.5. Transient species determinationIn order to determine the transient species presentafter laser photolysis of pyrogallolþ TiO2 solution,the influence of dissolved oxygen was studied. Amixture of 50 mg/L pyrogallolþ 20 mg/L TiO2 ina quartz cuvet was purged with high-purity nitrogenfor 20 min to drive away dissolved oxygen. A flashphotolysis experiment was conducted immediatelyafter deoxygenation. The transient absorption spectraof both deoxy and air-equilibrated samples were sim-ilar, which suggested that the presence of oxygen hadlittle effect on the transient absorption. If the transientspecies was an OH adduct, it should be easilyoxidized by oxygen and the absorption signal of theoxygen-containing sample should attenuate obviouslycompared to the N2-saturated sample. This lack ofsensitivity to oxygen suggests that the transient spe-cies was not any OH adduct, but pyrogallol cationsthat could not be oxidized by oxygen.3.6. Primary mechanism of the photooxidation ofpyrogallol by titanium dioxideBased on the previous results, it is deduced that theinitial TiO2 photocatalytic reaction mechanism for py-rogallol is the generation of photoexcited electronseholes followed by the holes, instead of OH radicals,oxidizing pyrogallol into pyrogallol cations. The reac-tions are described asTiO2 þ hn / hþ þ e ð1Þhþ þ e / heat ð2Þhþ þ pyrogallol / pyrogallolþ ð3Þ4. ConclusionsTiO2 photocatalytic oxidation reactions of pyrogal-lol were observed by time-resolved transmission flashphotolysis. Compared with pure TiO2 suspension, tran-sient absorption spectra of the pyrogallol and TiO2 mix-ture clearly indicated that some transient speciesbesides electronseholes were generated after laser-flashexcitation. The higher the concentration of pyrogallol inthe mixture, the stronger the transient absorption signaldetected. The transient species was confirmed bya deoxygenation experiment to be pyrogallol cationsoxidized by photo-generated holes, instead of pyrogal-lol OH adducts oxidized by OH radicals. Based onthe above results, the primary oxidation mechanism ofpyrogallol by TiO2 was deduced as the generation ofelectronseholes immediately after photo excitation,followed by the holes transferring to the surface of theTiO2 and oxidizing pyrogallol into its cations.AcknowledgementsWe thank Dr. Bin Ouyang and Wenbo Dong for theirgenerous help with the data analysis and valuable dis-cussions. This work was supported by the NationalHigh-Tech R & D Program of China (2002AA601250).References[1] C.S. Turchi, D.F. Ollis, J. Catal. 122 (1990) 178.[2] E.M. Ceresa, L. Burlamacchi, M. Visca, J. Mater. Sci. 18 (1983)289.[3] Y. Nosaka, Y. Yamashita, H. Fukuyama, J. Phys. Chem. B 101(1997) 5822.[4] Y. Mao, C. Schoneich, K.D. Asumus, J. Phys. Chem. 95 (1991)10080.[5] T.M. El-Morsi, W.R. Budakowski, A.S. Abd-El-Aziz, et al.,Environ. Sci. Technol. 34 (2000) 1018.124 Y. Qiu et al. / C. R. Chimie 11 (2008) 120e124[6] D. Bahnemann, A. Henglein, J. Lilie, et al., J. Phys. Chem. 88(1984) 709.[7] G.T. Brown, J.R. Darwent, P.D.I. Fletcher, J. Am. Chem. Soc.107 (1985) 6446.[8] M. Graetzel, A. Frank, J. Phys. Chem. 86 (1982) 2964.[9] T. Tachikawa, S. Tojo, M. Fujitsuka, et al., Tetrahedron Lett. 45(2004) 3753.[10] P.F. Schwarz, N.J. Turro, S.H. Bossmann, et al., J. Phys. Chem.B 101 (1997) 7127.[11] I.A. Shkrob, M.C. Sauer, J. Phys. Chem. B 108 (2004) 12497.[12] D.P. Colombo, R.M. Bowman, J. Phys. Chem. 100 (1996) 18445.[13] T. Tachikawa, S. Tojo, M. Fujitsuka, et al., J. Phys. Chem. B108 (2004) 5859.[14] S. Goldstein, G. Cazpski, J. Rabani, J. Phys. Chem. 98 (1994) 6586.

Transmission flash photolysis of titanium dioxide photooxidation of pyrogallol

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Transmission flash photolysis of titanium dioxide photooxidation of pyrogallol

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