Environ. Sci. Techno/. 1995,29,841-845
Enhanced Rates of Photocatalytic Degradation of an Azo Dye Using SnOfiiO2 Coupled Semiconductor Thin Films
SCHEME 1
Diagram Illustrating the Principle of Charge Separation in a SnOfliOz Coupled Semiconductor System
K. VINODGOPAL* Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408
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PRASHANT V. KAMAT* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
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Introduction The photocatalytic properties of semiconductor particles have been investigated extensively in slurries and on immobilized fims (1-3). Charge recombination between the photogenerated electrons and holes is often a major limiting factor as it impedes charge transfer at the semiconductor-electrolyte interface. By applying an anodic bias to an n-type particulate film semiconductor electrode such as TiOzon an optically transparent electrode (indium tin oxide-coated conducting glass plate), it is possible to drive away the photogenerated electrons from the semiconductor particle, thereby achieving more efficient charge separation in these immobilized semiconductor nanocrystalline films (4-6). The anodic biasing consequently avoids the need for any oxygen as an electron scavenger. Within such a context, recent attention has been focused on thin nanocrystalline SnOz semiconductor particulate films prepared from colloidal suspensions (7, 8 ) . These thin semiconductor films possess a highly porous structure and also possess interesting electrochemical and photoelectrochemical properties. In the past, semiconductors such as these have not been considered as suitable photocatalysts because of their inability to reduce oxygen. One interesting way to overcome such limitations and achieve efficient charge separation would be to couple the nanocrystalline SnOzcolloids withTiO2. While their spectral responses are quite similar as both are large bandgap bandgap energy, for SnOz = 3.8 eV semiconductors (Eg, while Eg for Ti02 = 3.2 eV), the conduction band (CB) of SnOz (EcBfor Sn02= 0 Vvs NHE at pH 7) is lower than that of the TiOz (ECB = -0.5 Vvs NHE at pH 7) so that the former can act as a sink for the photogenerated electrons. Since the holes move in the opposite direction from the electrons, photogenerated holes can be trapped within the TiOz particle thereby making charge separation more efficient. The principle of charge separation in such a coupled semiconductor system is shown in Scheme 1. Although the advantages of using such coupled systems have been demonstrated earlier in flash photolysis studies (9,101,very limited effort has been made to employ this concept in photocatalysis ( 11 ) . If better oxidative efficiencies can be achieved by application of an anodic bias to semiconductor electrodes * Address correspondence to either author.
0013-936X/95/0929-0841$09.00/0
@ 1995 American Chemical Society
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OTE or by coupling semiconductorsystems such as TiOzparticles with SnOz,it is reasonable to expect an improvement in photocatalytic degradation rates by combining these two techniques, i.e., by applying a bias potential to coupled semiconductor films immobilized on optically transparent electrodes. We present here for the first time the results from electrochemically assisted photocatalytic experimentsusing coupled TiOz/SnOz semiconductor thin films in the degradation of textile dye effluent. We show that by using such a system the oxidative efficiency of photocatalytic semiconductor systems in degrading acommercial azo dye such as Acid Orange 7 (A07) can be improved. Azo dyes
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Acid Orange 7 are ubiquitous commercial chemicals that present unique environmental problems (12-15). The largest discharge of these dyes into the environment occurs via the dye effluents from textile mills and presents an acute problem for municipal waste treatment facilities especially in the southeastern part of the United States (16).Quite apart from the aesthetic desirability of colored streams resulting from dye waste, the azo dyes in particular can undergo natural anaerobic degradation to potentially carcinogenic amines (15). The results presented here on the decolorization of the azo dye A 0 7 represents a major step forward in the development of new advanced oxidation processes for the treatment of such industrial waste.
Experimental Section Materials. Optically transparent electrodes (OTE) (5 cm x 0.9 cm) were cut from an indium tin oxide-coated glass
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Wavelength (nm) FIGURE 1. Absorption spectrum of a 42 ppm (0.2 mM) Acid Orange 7 azo dye solution recorded at different time intervals following electrochemically assisted photocatalysis at an OTU(Sn02 Ti02) electrode (0.36 mg/cm2 of SnOz and 0.18 mg/cd of TiOz). The OTV(Sn02 TiOz) electrode was maintained at an anodic bias of 0.83 V vs SCE, and the solution was continuously bubbled with a slow stream of nitrogen. The absorption spectra were recorded at time intervals of (a) 0, (b) 10, (c) 30. and (d) 60 min.
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plate (1.3 mm thick, 20 Q2) obtained from Donelley Corporation, Holland, MI. SnOncolloidalsuspension (18%) was obtained from Alfa Chemicals and used without further purification. The particle diameter of these colloids is in the range of 30-50 A. TiO? powder (product name P25, particle size 30 nm, surface area 50 m2/g)was a gift sample from Degussa Corporation. Acid Orange 7 also known as Orange I1 was obtained from fddrich and purified by recrystallization and column chromatography. Preparation of Particulate Film Electrodes. An aliquot of 0.1 mL of the concentrated Sn02suspension was diluted with 9.7 mL of water and 0.2 mL of NH40H. Addition of NH40H was essential for controlling the stability of the colloids and improving the adsorption properties of the film. Depending on the final concentration desired, more or less of the concentrated SnOzsolution was added. The semiconductor mixtures were obtained by adding appropriate amounts of the Ti02 to the diluted Sn02solutions described above and sonicating the mixture for approximately 10min. Depending on the particular electrode desired, 200 ,uL of the semiconductor suspension was applied to half the area (-2 cm2) of the OTE plate and was dried in air on a warm plate. The semiconductor-coated glass plates were then annealed in the oven at 673 K for 1 h. These electrodes with immobilized semiconductor film were used as the working electrode in the electrochemical cell and will be referred to henceforth as OTE/semiconductor. Adsorption studies carried out with cationic dyes such as thionine or cresyl violet indicate the coupled fdms to be highly porous, similar to those of SnOz and Ti02films (17).
Photocatalysis Experiments. All the electrochemical and photoelectrochemicalmeasurements were carried out in a standard two-compartment cell in which working and 842
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counter electrodes were separated by a fine glass frit. The electrode assembly consisted of an OTE/semiconductor working electrode, a platinum wire gauze counter electrode, and a saturated calomel reference electrode (SCE) with a solution of the A07 dye as the electrolyte. The initial concentration of the dye was in most cases -1.2 x M (42 ppm). Nitrogen or oxygen atmospheres were maintained by gentle bubbling of the individual gases through the dye solution in the working electrode compartment. A Princeton Applied Research (PAR) Model 173 potentiostat and Model 175 universal programmer were used in all the electrochemical measurements. All experiments were done at room temperature (-296 K). A collimated light beam from a 250-W xenon lamp filtered through a CuS04 solution (cutoffwavelength ‘325 nm) was used for excitation of the electrode in the front face (i.e., the side with the semiconductor film). Absorption spectra were recorded with a Perkin-Elmer 3840 diode array spectrophotometer.
Results and Discussion The absorption spectra of an aqueous solution of Acid Orange 7 recorded following the excitation of an OTE/ (SnOz + TiOd are shown in Figure 1. To facilitate efficient charge separation in the semiconductor film, an anodic bias of 0.83 V vs SCE was applied, which is less than the oxidation potentialofA07 (1.OVmeasuredat aglassycarbonelectrode vs SCE). Thus, the choice of 0.83 V as an anodic bias potential in these electrochemically assisted photocatalytic experiments ensures that no direct oxidation of A 0 7 can occur during these experiments. The solution was continuously bubbled with a stream of nitrogen, indicating that electrochemically assisted photocatalysis (ECAP) can be carried out under anaerobic conditions. The absorption
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FIGURE 2. Dependence of Acid Orange 7 degradation rate on the nature of semiconductor film: (a) OTU(Sn02 TiOz), (b) OTUSn02, and (c) OTE/TiOz maintained at a bias potential of 0.83 V vs SCE. Curve d shows the photocatalytic degradation of OTE/TiOz at no applied bias potential but with oxygen bubbling through the solution. Curve e is the dark control for the degradation of A07 using the coupled film electrodes at f0.83 V in nitrogen-saturated solutions. The electrode for the coupled film in curves a and e had the same composition as in Figure 1. The other electrodes used to obtain curves b-d had the same weight of semiconductor, Le., 0.54 mg/cm2. In all of these experiments, nitrogen was bubbled through the dye solution (pH -6, unbuffered) except curve d.
peaks corresponding to the orange dye disappear completely following photolysis, indicating degradation of the dye. Even more surprising is the rate at which degradation is achieved. A 42 ppm aqueous solution of the dye is 95% degraded following 30 min of irradiation of the semiconductor electrode, giving a pseudo-first-order rate constant (k)of 7.3 x min-'. For comparison, an electrochemically assisted photocatalysis experiment was also carried out with A07 dye solution using both the OTE/SnOZand OTE/TiOZ separately under identical conditions (Le., at an anodic bias of $0.8 V vs SCE and with nitrogen gas bubbling). The total mass of the semiconductor on the electrode was the same in all three cases (0.18 mg/cm2). While the observed pseudo-first-order degradation rate of the Sn0210TE systemwas identicalwith the Ti02 electrodes (k = 9 x min-l), both were almost an order of magnitude slower as compared to the OTE/ (SnOz+ TiOd system. A comparison of the relative degradation rates under three different conditions is shown in Figure 2. This is a very good indication that the coupled semiconductor film made from Sn02 and Ti02 is a much more efficient photocatalytic system for degrading these dyes. We also studied the degradation of A07 using a TiOz film coated on the OTE but with no applied potential and oxygen gas bubbling through the solution. Such a setup is similar to the conditions employed in a typical photocatalysis experiment in which a photocatalyst is immobilized on a glass or fiber surface. The degradation rate of A07 was rather slow with a pseudo-first-order rate constant of 4.6 x low3min-' (curve d in Figure 2). On the other hand, no degradation was observed with the OTE/ SnOz systemwith oxygen bubbling and no applied potential, consistent with the inability of SnOz to reduce oxygen. Suitable control experiments were also carried out to confirm the observation that the enhancement in the
degradation rate is truly a photocatalytic effect arising from the mixed semiconductor electrode. At a bias potential of +0.83V, with no illumination, no degradation was observed in either nitrogen- or oxygen-saturated solutions. This is a good indication that at a bias potential of 0.83 V direct oxidationof the dye does not occur. Since the dark controls for both NZ-and 02-saturatedsolution give similar results, only the degradation rate from the former is shown (curve e in Figure 2). As an illustration of the effectiveness of ECAP in general, we have also measured the degradation rates of A07 using the coupled semiconductor film electrodes in nitrogensaturated solutions as a function of the applied anodic bias voltage (Figure 3). When no external potential is applied, very little degradation is observed. At an anodic bias of $0.43 V, the dye decays with a pseudo-first-order rate constant of 1.6 x min-'. When the potential is increased to $0.83 V, the rate of degradation is significantly faster. The rapid disappearance of the 480-nm absorption band in Figure 1 suggests that the chromophore responsible for the characteristic color of the azo dye is breaking down. Earlier studies on similar dyes by our group and others have shown that the dye is cleaved at the azo link, leading to a sulfonated phenyldiazene and naphthaquinone (1820). It is expected then that these intermediates can also undergo hydroxyl radical-induced oxidative degradation. Evidence suggesting that such a degradation is occurring in our electrochemically assisted photocatalytic experiments is seen in the absorption spectra (Figure 1) where all the absorption bands of the orange dye, including those between 200 and 300 nm, are significantly affected and reduced. The speed with which decolorization is achieved (95% in ~ 3 min) 0 compares very favorably with other reported results of hydroxyl radical-mediated degradation of azo dyes (18). VOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Weight of semiconductor (mg/cm2) FIGURE 4. Comparison of photocatalytic degradation rates of A07 with SnOz. TiOl. and Sn0dTi02 particulate films coated on OTE electrodes. In each set of experiment. the total weight of the semiconductor catalyst was kept constant at the indicated value. The bias potential was 0.83 V VI SCE. and the electrolyte was 42 ppm A07 in water at unbuffered pH -6.
We havealsoattempted todetermine theoptimumratio of the two semiconductors necessary to obtain fastest degradation of A07. The plot of the electrochemically assisted photocatalytic degradation rates of A07 as a functionofthe total massofthesemiconductorwhen both TiOz and SnOz are present in equal amounts is shown as a bar diagram in Figure 4. For comparison, the rates obtained when either Sn02orTiOz is used separately (but atthesamemass) isalsoincludedinthebardiagram.Higher loadings of TiOzdoes seem to slow down the degradation process primarily due to the increased opacity of the resulting films. The behaviorofthedecayratesasafunctionofincreasing amounts of one semiconductor in the mixture relative to theother was also investigated. Figcre 5 shows degradation rates for the coupled system in which the mass of TiO' is kept constant at 0.18 mglcm'. but the amount of SnOz is increased progressively from 0 to 0.36 mglcm'. At vely low relative amounts of SnOz. the rates of photocatalytic degradation are similar to that of the individual semiconductor systems. For example, when the amount of TiOz is 844 m ENVIRONMENTAL SCIENCE &TECHNOLOGY i VOL. 29.
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FIGUAEJ. Dependence ofA07 degradationonthe enemally applied anodic bias. The OTE/(Sn02+ TiOJ electrode was maintained at (a) no applied potential. (blO.43 V. and IC) 0.83 V vs SCE during the photolysis. The solution in the working electrode compartment was continuously bubbled with a slow stream of nitrogen.
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due to any increased resistivity of the more heavily loaded electrodes or due to greater electron-hole recombination since a comparison of Figures 5 and 6 reveals that, at a given total mass of semiconductor, a decrease in degradation rates is only observed when the amount of Sn02 is lower thanTiOz. In both Figures 5 and 6, the rates of decay obtained when either SnOz and Ti02 alone are used have been plotted to facilitate comparison with the mixed systems. The lower rates seen in these latter two cases for the same total mass of the semiconductor used in the mixtures justify our conclusion that the accelerated degradation observed with the coupled systems is not a mass effect. Under Wexcitation, charge separation occurs in closely contacted pairs of SnOz and Ti02 particles. Since the electrons and holes quickly move in opposite directions to accumulate on SnOzand Ti02 particles, respectively, their recombination is greatly suppressed. Our preliminae photoelectrochemicalmeasurements carried out with single and coupled semiconductor films show higher photon to photocurrent conversion efficienciesfor SnOn/TiOz coupled films (17). Improved charge separation in coupled colloidal semiconductor systems such as TiOz/CdSand ZnOlCdS have also been demonstrated by laser flash photolysis experiments (8-1 1 ) . The externally applied bias drives away the accumulated electrons via the external circuit, thereby promoting the selective oxidation of the dye on the Ti02 surface. The results presented here show how a simple approach of mixing two semiconductor particulate systems and packing them as an immobilized particulate film on a conducting glass surface leads to the increased rate of photocatalytic degradation. We are currently carrying out HPLC analyses to determine the nature of the intermediates and the degradation products.
Conclusions A very rapid and complete decolorization of the textile azo dye can be achieved using nanocrystalline semiconductor thin films under U V irradiation. By employing a coupled SnOz/TiOzsemiconductor system, it has been possible to enhance the rate of electrochemicallyassisted photocatalytic degradation of the dye in aqueous solution. The improved charge separation as a result of coupling two semiconductor systems with different energy levels and the applied anodic bias is responsible for the enhancement in the rate of photocatalytic degradation.
Acknowledgments We would like to thank Degussa Corporation for supplying us the gift sample of Ti02 powder. K.V. acknowledges the support of Indiana University Northwest through a Summer Faculty Fellowship and a Grant-In-Aid. P.V.K. acknowledges the support of the Office of Basic Energy Sciences of the U.S. Department of Energy. This is ContributionNDRL3750 from the Notre Dame Radiation Laboratory.
literature Cited (1) Bahnemann, D.; Cunningham, J.; Fox, M. A.; Pelizzetti,E.; Pichat, P.; Serpone, N. In Aquatic and Surface Chemistry, 1st ed.; Helz, G. R.,Zepp, R. G., Crosby, D. G., Eds.; Lewis Publishers: Boca Raton, FL, 1994; p 261. (2) Ollis, D.; Pelizzetti, E.; Serpone, N. Environ. Sci. Technol. 1991, 25, 1522. (3) Ollis, D.; El-Akahi, H. Ti02 Photocatalytic Purification and Treatment of Water and Air; Elsevier, New York, 1993. (4) Vinodgopal, K.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1993.. 97.. 9040. (5) Vinodgopal, K.; Stafford, U.; Gray, K.; Kamat, P. V. J. Phys. Chem. 1994, 98, 6797. (6) Kim, D. H.; Anderson, M. A. Environ. Sci. Technol. 1994,28,479. (7) Ford, W.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98, 3822. (8) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1994, 98, 4133. (9) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V.J. Phys. Chem. 1990, 94, 6435. (10) Spanhel, L.; Weller, H.; Henglein, A. J. J. Am. Chem. SOC. 1987, 109, 663. (11) Serpone, N.; Borgarello, E.; Gratzel, M. J. Chem. Soc., Chem. Commun. 1983, 142. (12) Zollinger, H. in Color Chemistry Synthesis, Properties and Applications of Organic Dyes and Pigments; VCH Publishers: New York, 1987. (13) McCann, 7.; Ames, B. N.Proc. Natl. Acad. Sci. USA.1975, 73, 950. (14) Tincher, W. C. Text. Chem. Color. 1989, 21, 33. (15) Boeninger, M. Carcinogenicity and Metabolism of Azo Dyes Especially Those Derived From Benzidine; DHHS (NIOSH) Publication 80-119; U.S. Government Printing Ofice: Washington, DC, July 1990. (16) Vaidya, A. A.; Datye, K. V. Colourage 1982, 14, 3. (17) Vinodgopal, K.; Bedja, I.; Kamat, P. V. Unpublished results. (18) Spadaro, J. T.; Isabelle, L.; Ranganathan, V. Environ. Sci. Technol. 1994,28, 1389. (19) Matsui, M.; Shibata, K.; Takase, Y. Dyes Pigm. 1984, 5, 321. (20) Vinodgopal, K.; Kamat, P. V. Abstracts ofthe First Conference on Advanced Oxidation Technologies, London, Ontario, June 1994.
Received f o r review August 18, 1994. Revised manuscript received December 5, 1994. Accepted December 16, 1994.
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