J. Phys. Chem. 1995, 99, 8244-8248
8244
Photoelectrochemical Decomposition of Surfactants on a TiOflCO Particulate Film Electrode Assembly Hisao Hidaka,*9?Yuichi Asai,? Jincai Zhao; Kayo Nohara,? Ezio Pelizzetti,s and Nick Serponeg Department of Chemistry, Meisei University, Hino, Tokyo 191, Japan, Dipartimento di Chimica Analitica, Universita di Torino, 10125 Torino, Italy, and Laboratory of Pure & Applied Studies in Catalysis, Environment & Materials, Department of Chemistry, Concordia University, Montreal, Canada Received: September 1, 1994; In Final Form: December 29, 1994@
The photooxidative degradation of an anionic surfactant, sodium dodecylbenzene sulfonate (DBS), has been examined on a Ti02 thin film immobilized onto a transparent semiconducting oxide (TCO support) electrode assembly. Some of the characteristics of this assembly have been explored. Changes in the rates of decomposition as a function of applied bias and as a function of the nature of the supporting electrolyte were surveyed. The efficiency of the photodegradation was governed by the electrode potential.
Introduction Promotion of charge separation following irradiation and minimization of electrodhole recombination can occur in a semiconductor film electrode in which Ti02 particulates are fixed on a transparent conductive oxide (TCO) glass plate.',2 This type of electrode is analogous to a heterogeneous suspension system which acts as a "short-circuited microphotocell". Active radical species formed by electrons and holes decompose organic pollutants concurrently with photocurrent generation. Photodegradation mediated by Ti02 particulates has been shown to provide a technological method for the purification and treatment of water and air pollutant^.^ To date, Ti02 aqueous suspensions have been mostly employed." One disadvantage of such suspensions in photodegradations is the need to ultimately remove Ti02 particles by filtration and/or centrifugation, in contrast to the TiO2-fixed electrode assembly examined here (and by others',5) which avoids these potentially costly operations. A technology based on a TiO2-immobilized system presents certain advantages in practical utilizations. In this paper, we examine the photodegradation of the anionic surfactant sodium dodecylbenzene sulfonate (DBS) on a TiO2immobilized thin film (onto a TCO) electrode assembly. The changes in the rates of oxidation as a function' of applied bias, as a function of the nature of the supporting electrolyte, and relative to photoelectrochemical properties of the Ti02 particulate thin film electrode are examined.
Experimental Section Ti02 powders (Degussa P-25, anatase) dispersed in aqueous solution were loaded onto a 20 x 50 mm2 TCO glass plate (Asahi Glass Co. Ltd., transparent conductive oxide coated glass plate of fluorine-doped SnO2) and subsequently dried in air. The plate was sintered for 2 h at 300 O C in a furnace to prepare a particulate film electrode; the electron microscopic appearance of the electrode surface is shown in photos 1 and 2 in Figure 1; The TCO coating was 2.1 p m thick, whereas the thickness of the Ti02 thin film was 4.2 pm. The quantity of Ti02 on the area of 20 x 30 mm2 was about 5 mg. The DBS solution (0.1 mM, 50 mL) was contained in a
* Address all correspondence to this author. Meisei University. Universita di Torino. Concordia University. @Abstractpublished in Advance ACS Abstracts, April 15, 1995. +
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Pyrex glass photoreactor. The anode electrode was a Ti021 TCO plate, and the counter electrode (the cathode) was a Pt plate (20 x 20 mm2); the reference electrode was a AgIAgC1 electrode connected to the assembly via a salt bridge. Applied voltages were from a dc potentiostat; the UV illumination was provided by a Toshiba mercury lamp (1 > 250 nm). The potentials at the Ti02 electrode were measured with an electrometer. The temporal changes in the concentrations of the photodegraded DBS surfactant were monitored by high performance liquid chromatography (HPLC) using UV detection. Figure 2A,B illustrates the photoelectrolysis setups for potentiometric and photocurrent measurements, respectively. The latter measurements were carried out using a potential step method with a Nikko-keisoku NPGFZ-2501A potentiolgalvanostat. The temporal disappearance of DBS was also undertaken electrolytically using a pair of Pt electrodes (as anode and cathode).
Results and Discussion ElectrochemicalSetup in the Photooxidations. The photooxidation of the DBS surfactant was carried out on a transparent conductive oxide (TCO) glass electrode loaded with a Ti02 membrane using an electrochemical setup. This setup has the advantage over the dispersion systems as the photooxidations can be performed on a continuous basis without having to remove the photocatalyst's particulates. In all cases, as will be made evident below, the photocatalyzed oxidation of DBS on the Ti02 electrode followed apparent firstorder kinetics at different applied voltages. Figure 3 shows the dependence of the rate constant for the oxidative decomposition of the surfactant on the applied voltage under dark and illuminated conditions. When the Ti02 electrode was employed as the anode, the degradation rate increased slightly with increasing applied voltage initially, and subsequently it increased linearly with further increases in applied voltages. We presume that adsorption of the DBS anion is enhanced and/or the generation and separation of electrodhole pairs are accelerated under the higher voltage conditions. By contrast, the rate of photooxidation of the surfactant decreases with applied voltages when the Ti02 electrode was used as the cathode. Moreover, under otherwise identical conditions, DBS was scarcely decomposed on the TCO electrode without the Ti02 coating. It is also evident that under dark conditions the degradation of DBS was much slower. 0 1995 American Chemical Society
Photoelectrochemical Decomposition of Surfactants
J. Phys. Chem., Vol. 99, No. 20, 1995 8245
Figure 1. Scanning electron micrographs of the TiOz/SnO;?membrane: (photo 1, left) TiO*/SnOz surface (after calcination); (photo 2, right) TiOz/SnOz cross section.
Electrochemical Characteristics of the TCO/Ti02 Assembly. Electrode potentials versus applied voltage plots for the photodegradation of a DBS solution (0.1 mM) in a TiO2/ TCO electrode system and in the presence of carbonate, sulfate, and chloride electrolytes are shown in Figure 4. The potential (vs a Ag/AgCl reference electrode) at the Ti02/TCO electrode in the presence of an electrolyte increased anodically (to more positive values) with increasing applied voltage until about 2-3 V. Additional applied bias led to gradual attainment of a maximal (equilibrium) potential: approximately 1.8 V in NaCl (0.1 M), about 2.0 V in Na2S04 (0.1 M), and ca. 1.0 V in Na2C03 (0.1 M). In an electrolyte-free system, the potential remains negative at -0.3 V regardless of applied voltage. It is interesting to note that the equilibrium potential for the DBS (0.1 mM)/Na2S04 (0.1 M) or NaCl (0.1 M) system coincides approximately with the standard redox potential (EO) of the anion in aqueous media; E" = +2.01 V (vs NHE) for the S2Og2-/ S042- couple and Eo = +1.36 V (vs NHE) for C12/Cl- couple (note that the Ag/AgCl electrode is at +0.22 V vs the NHE). For the DBS (0.1 mM)/Na2C03 (0.1 M) system, hydrolysis of the carbonate anion renders the solution alkaline, pH = ca. 11.5, and influences the electrode potential; note that the redox potential for the C03-/C032- couple is approximately +1.5 V. In an electrolyte-free diluted DBS (0.1 mM) solution, the electrode potential should correspond to the redox potential of H20 in the reaction (eq l),
0,
+ 4H+ + 4e- - H 2 0
(1)
estimated at the actual pH (ca. +OS5 V, NHE). The measured potential is cathodic (ca. -0.3 V vs Ag/AgCl) as the electrons of the valence band are shifted to the conduction band by the characteristic photoactivation of the semiconductor electrode. Hole annihilation at the valence band occurs as a result of the oxidation of DBS, which is likely mediated by the 'OH radicals h+vB 'OH process). (OH-
+
-
The photodegradation of a DBS solution at constant applied voltage (5 V) is illustrated in Figure 5. Addition of supporting electrolytes such as Na2S04 or NaCl (0.1 M) enhances somewhat the photodegradation of the surfactant in comparison with that carried out in an electrolytefree system. We have reported extensively in earlier reports4v6 that the active oxidizing species in photooxidations is the 'OH radical. Since the C032- ion is a well-known 'OH scavenger: the photooxidation of DBS is therefore expected and is observed to be much slower in carbonate media. The Ti02 electrode potential in the presence of electrolytes stays anodic. As a result, the redox activity of photogenerated electrons and holes should be accelerated. Consequently, the active oxygen species (e.g., 'OH, HOi, and/or 02'- radicals) that likely oxidize DBS are probably generated in greater quantity. Insofar as the photodegradation of DBS in the presence of Na2C03 is concerned, the increase in [OH-] from the hydrolysis of the carbonate ion increases the solution pH and consequently renders the Ti02 particulate surface negatively charged (the isoelectric point of Ti02 lies in pH less than 6). Under these conditions, adsorption of negatively charged DBS molecules onto the photocatalyst surface is minimized, if not suppressed, and photodegradations should be retarded even further relative to other electrolytes (cf. Figure 5). Figure 6 depicts the photodegradationof DBS by electrolysis (Pt/Pt electrodes) and by a wet photoelectrochemical cell (PEC). In electrolysis, DBS is not decomposed in the absence of supporting electrolyte, whereas in the presence of Na2S04 some decomposition is observable. This is caused by electrophoretic diffusion of DBS to the electrode surface in the presence of electrolyte. Since the solution resistance (liquid junction potential) decreases, the electrode potential is easily polarized. The potential in the anode becomes anodic compared to the potential in the electrolyte-free solution. Utilization of the photoelectrochemical cell assembly leads to a more rapid decomposition of DBS (cf. Figure 6). In this case, the
Hidaka et al.
8246 J. Phys. Chem., Vol. 99, No. 20, 1995
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for a supporting electrolyte. recombination of photogenerated electronhole pairs is diminished, and more of the active oxidizing radical species are thereby formed. Photocurrent in the T i O n C O Electrode System. The photocurrent developed as a function of electrode potential from -0.5 V to f0.5 V in a Ti02/TCO electrode system is shown in Figure 7A under a variety of conditions. This electrode assembly showed the same photocatalytic behavior as a Ti02 electrode consisting of either a single crystal or a polycrystal. In the dark and under aerated conditions, no photocurrent was generated at positive electrode potentials; at negative potentials, a cathodic photocurrent was generated (Figure 7A). By contrast, under illumination and in the presence of 0 2 (or air), an anionic photocurrent was evident even at negative electrode potentials. Figure 7B illustrates the analogous generation of dark currents for the TCO support both under dark/air and light/air conditions. Note the greater photocurrent generated when the light active Ti02 is implicated.
We now discuss the current/potential data of Figure 7A,B in light of recent work by Kamat and co-workers' and by Hodes et a1.8 It is worth pointing out that the potential distribution in nanostructured materials is currently unknown and is presently the subject of debate. It was not the focus of this paper to be drawn into this debate. The anodic photocurrent ( f 6 0 PA) under aerated conditions was greater than that obtained ( f 3 0 PA) in oxygenated solutions; the zero current potential was -0.28 to -0.32 V (vs Ag/AgCl) in air and -0.15 to -0.20 V in an 0 2 atmosphere. The photocurrent in the anodic direction is likely caused by hole oxidation of H20. Under aerated conditions, the electrons photoactivated from the valence band to the conduction band are consumed by reduction with 0 2 molecules. To the extent that these photogenerated electrons are not transferred to the TCO electrode, the measured photocurrent through the outer circuit decreases; oxygen acts as an electron scavenger. The cathodic photocurrent generated via reduction of 0 2 is more
Photoelectrochemical Decomposition of Surfactants
J. Phys. Chem., Vol. 99, No. 20, 1995 8247
0.100)
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Figure 5. Temporal changes in the concentration of DBS (0.1 mM) in a photoelectrochemical TiOflCO cell assembly in various electrolyte (0.1 M) systems at a constant bias of 5 V. S.E.stands for a supporting electrolyte.
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Irradiation time (min) Figure 6. Temporal changes in the photodegradation of DBS, where COis 0.1 mM DBS, in a photoelectrochemical TiOzRCO cell assembly or in an electrolytic cell assembly using a Pt/Pt electrode at a constant bias of 5 V. (TiOZRCO system is under no bias of potential.)
easily obtained at the lower potentials than that under airsaturated conditions (Figure 7A). At the more positive potentials, the photocurrent increased anodically. This fact implies that the formation of such active oxygen species as 0 2 ’ - and/ or HO2’ is inversely proportional to the photocurrent. Thus, insofar as the photocurrent varied with potential, the photooxidative decomposition of DBS also varied directly with electrode potential. The dark current generated by the TCO electrode was about 0.1 PA; under illumination the current was -3 PA. Hence, the TCO electrode is also photoactivated. The band gap energy (3.8 eV) of TCO, which consists of fluorine-doped Sn02, corresponds to an onset of absorption at about 326 nm, a wavelength well within the range of the radiation field delivered by the Hg lamp (’250 nm) employed. Photodecomposition of DBS under a Constant Applied Bias. The above mentioned results show that the photodegradation of DBS depended upon an applied bias. The rates of this photooxidation (DBS concentration, 0.1 mh4) in a 0.1 M NazS04 aqueous electrolyte solution at various values of
1
Current &A) Figure 7. (A) Photocurrent and current generation as a function of potentials in a T i 0 2 / T C 0 electrode system under illuminated or dark conditions, respectively, in air or oxygen-saturated media. (B) Dark current and photocurrent in a TCO electrode system in the presence of an electrolyte: 0.1 M Na2S04.
constant applied bias from -0.5 V to + O S V are depicted in Figure 8 A,B. Photodegradation occurs via pseudo-first-order kinetics (see for example Figure 6). The maximal rate (determined from HPLC experiments having a W detector), k = ca. 2.3 x lo-, min-I, for the photodegradation of DBS is exhibited at an applied potential of +0.3 V. At this applied bias, the photocurrent reached a maximal value immediately after photoreaction and gradually decreased to a constant value (plateau at ca. 50 PA) at 2 h of irradiation time (cf. Figure 8B). We attribute the maximal photocurrent to the oxidation of formic acid, which was the principal intermediate product before C02 e v o l ~ t i o n .Electrons ~ that appear in the conduction band of Ti02 and lead to increased photocurrent originate from reactions 2 and 3:
+ h+ - HCOO’ + Hf HCOO’ + TiO, - CO, + H+ + TiO, (e-& HCOOH
(2)
(3)
The photocurrent flows through the external circuit. The “current doubling” phenomenon reported by othersi0evidently also takes place in the photooxidation process of the DBS surfactant.
8248 J. Phys. Chem., Vol. 99, No. 20, 1995
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by trapped photoholes generated at the TiOz/solution interfaces that we identify with active oxygen species (e.g., *OHradicals). The efficiency of the photodegradation is governed by the electrode potential that can be altered by the addition of supporting electrolytes to the medium. The electrode potential influences the formation of radical species on the small Ti02 particulates, while the applied voltage affects the diffusion transport of a degradable substrate to the electrode surface. The photocurrent was accessed using a wet photoelectrochemical cell during the decomposition of various organic pollutants. Since the photocurrent decreased with increasing the photodegradation rate, the electrons are consumed (scavenged by 0 2 ) with eventual formation of active oxygen species ('OH, HOi, andlor 0 2 . 3 . Considerations of optimal conditions are important for photocurrent generation and for improvement of the rate of degradations.
Acknowledgment. Support of our work in Tokyo is sponsored by a Grant-in-Aid for Scientific Research from the Ministry of Education (No. 06640757) and the Cosmetology Research Foundation. The work in Torino is partially financed by the Regione Piemonte and the CNR (Roma) and that in Montreal by the Natural Sciences and Engineering Research Council of Canada.
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