Electrochemical Aspects of Photocatalysis: Application to Detoxification and Disinfection Scenarios K. Rajeshwar Department of Chemistry and Biochemistry, Box 19065, The University of Texas at Arlington, Arlington, TX76019-0065 J. G. lbanez
Depto. de lngenieria y Ciencias Quimicas, Universidad Iberoamericana, Prol. Reforma 880,01210 Mexico, D. F., Mexico Federal legislation aimed a t pollution control as well a s public awareness of environmental damage have grown dramatically in the past decade. Chemists and chemical engineers can obviously play a key role i n the refinement and development of existing and new technologies for combating pollution in air and water. I n this regard, the synergism between chemistry and the environment has been highlighted in a recent issue of this Journal (I). The present article focuses on the use of irradiated semiconductor particles for treating pollutants (both organic and inorganic) and microorganisms. Arecent article i n this Journal describes a simple experiment that demonstrates the effkacv of the use of Ti02 for the ~hotooxidationof an 2,. Re\wws on sernicondurtor phutocstalysis i nI are also avadshle 3 4 1 .Conc(.pts n!htvd to thc chemistry of semiconductors, the electronic properties of solids, and photoelectrochemistry were also discussed recently in this Journal ( 7 , s ) . In the present article we highlight the useful role that electrochemical concepts can play i n a fundamental understanding of the behavior of semiconductor particles in detoxification and disinfection. We thus show that a unified treatment can be developed that
. .
hiphl~ghr.. r h v rhtrnw.d rule of electrons 311d holes nr the phorocnrnlgir p;firt~rlt.rncd~urninterface pro\&; .t rutlmal explortat~onfor rhr obsrrvcd influence of rxper~mentrl\iirti~ble. rcl;(tcd 10, for exnrnple photon flux. phut~c.ir.ilysri ~ r l i ( ~modificnlion, .e and nrnhirnt nrmosphme
emphasizes the symmetry inherent in the conjugate chemical reactions undergone by the electron-hole pairs at the semiconductor partieleimedium interface The Model System Figure 1 describes the photodriven events a t a single semiconductor particle. Absorption by the particle of photons with energy (hv) greater than the bandgap causes photoexcitation of electron-hole pairs (7). A fraction of these (i.e., those carriers that do not recombine: e + ht + heat or light) arrive a t the surface where they undergo reduction and oxidation reactions. Thus, with reference to Figure 1 and from a n environmental perspective, 0x1 could be a heavy metal ion (e.g., pb2+), and Redz could be either an organic substrate molecule (e.g., chlorohydrocarbon) or a microorganism (e.g., Escherichia coli). With a metal substrate, we obtain reduction ofthe metal ion (to a lower oxidation state) and oftenits immobilization onto the semiconductor surface. This is environmentally significant; the toxicity of many metals vary with their ox;Thus, Rajeshwar has recently shown (10) dation state (9). that toxic Cr(V1) can be converted to the environmentally more benign form CI.(III) using a UV-irradiated semiconductor. An adjustment of process pH also affords immobilization of this species a s the insoluble Cr(OH13 on the semiconductor particle surface. 1044
Journal of Chemical Education
Red
0x1
Red,
.
0x2
Figure 1. A schematic of photodriven events at a TiO, particle in contact with a medium containing an oxidant, Ox, (e.g.,0,) and a reductant Red, (e.g.,an organic substrate). What happens to the holes arriving a t the surface? They can either oxidize H z 0 (with the intermediate generation of OH radicals) or directly photodecompose a suitable substrate, whether i t is a n organic molecule or a microorganism. The molecular details of the hole reaction pathway are obscure a t present. Generally, the OH radicals have been frequently implicated in the photodecomposition of organics a t oxide semiconductor surfaces (3-6) and in the bactericidal activity of these materials (11). We choose TiOz a s a model photocatalyst system for the following discussion. This choice is motivated by the following factors. TiO, is plentiful, inexpensive,and environmentally friendly. The majority of the research thus far has been carried out with TiOz (2-6). N o other photocatalyst material has matched (at least thus far) the attributes of TiO2in terms of corrosion stability and photocatalysis efi&iency. TiOz is a top:50 chemical in the U.S. (12) and is widely used in the paint and pigment industry. The anatase modification has generally outperformed the rutile polymorph a s a photocatalyst. The optical bandgaps of these two modifications are slightly different: 3.23 eV (384 nm) for anatase and 3.02 eV (411 nm) for mtile. Photodriven Events at the Ti02 Particle-Water Interface Revisited Consider the chemical processes involving e- and ht in more detail. The simple recombination of the e--ht pairs often involves a mediating surface state (located i n the
bandgap of the semiconductor) and results in no net chemical change in the system. An alternative scenario involving the initial oxidation of water provides a more interesting starting point for discussion. Thus, the photogenerated h' can oxidize H z 0 as a n initial step.
Now Oz functions a s a n electron acceptor to chemically reverse this process. Figure 2. Short-circuiting of an irradiated TiO, microelectrode particle by the OdOH- redox couple. If the carrier fluxes due to the reactions i n eqs 1 and 2 ;ire exactly h;il:inccd, ihe holes and electrons have been cfIi.ctivels rcco~nb~ned. This can he demonstrated bv simple addition of eqs 1 and 2. Again there is no net chemisiry, and Oz has functioned a s a chemical surface state for mediating t h e e--hi recombination process. This is illustrated in Figure 2. More complicated chemical scenarios can be envisioned that involve the surface hydroxyl group a t the TiOz surface a s the carrier recombination mediators, but the conclusion remains the same. The foregoing discussion underlines the fact that net chemistrv can onlv occur a t the TiOa.surface if either reactlon is intercepted at an intrnncdiate srage. Alternatively, d~ffwrnt rhr e and h' at the - surface must react w ~ t h redox couples i n the contacting medium as illustrated in Figure 1. In pollutant destruction, either eq 1 or eq 2 constitutes one-half of the conjugate reaction pair; the other half of the pair or partner comprises the pollutant molecule, ion, or microorganism. To illustrate, eqs 3 and 2' form a conjugate pair i n the photocatalytic degradation of a n organic substrate. Tio, 4 0 H + 4hi -+ 40Ht hv
40H +organic substrate + products 0, + 2H,O
+ 4e- --t 4 0 K
organic substrate + 0, + 2H20 +products
V vs. SHE
(3a)
(3b) (2')
(4)
A second example involves the photoreduction of a toxic metal ion, (e.g., pbzt) a t the TiOz surface. In this instance, eqs 1and 5 form a conjugate pair. Tio, 2H,O + 4h' 7 0, + 4Hf
ISthe Ti02 System Photocatalytic or Photosynthetic? Let us now consider the energetics of eqs 4 and 6 as written. Equation 4 i s photocatalytic because i t involves a negative Gibbs free energy change (AGO < 0).Thus, the reaction is driven in the spontaneous direction by the light, and the radiant energy simply serves to overcome the activation barrier for the process. Equation 6 involves a positive AG" of +48 kJ and is photosynthetic. Here the light drives the reaction in the thermodynamically up-hill direction. Indeed, most reactions of this type are photosynthetic except when the metal ions are reduced a t very positive potentials, that is, when they have standard reduction po-
Figure 3. A potential (or energy) ladder (see ref 13) consisting of selected redox couples of environmental significance. ApH 7 electrolyte is assumed in all the cases except for the OJH,O couple where a pH 6 solution is also shown. tentials that are more positive relative to the OzHzO redox couple. Representative examples of both types of reactions can be culled from the standard potential data contained in Figure 3. Only mercury deposition will be photocatalytic a t Ti02 under these conditions; silver deposition will be photocatalytic for a solution slightly acidic of the neutral point. In each case, the spontaneous direction of electron flow between the conjugate redox couples is in the positive potential direction (down the energy hill). Arecent article in this Journal (13)may be consulted for a review of concepts related to the thermodynamics of electrochemical reactions. Photosynthetic Systems
An important ramification of the preceding discussion is that the back reaction is important only for photosynthetic systems. For example, consider the deposition of copper a t Volume 72 Number 11 November 1995
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Current A u .-
3 0
(1)
hd
H,O Figure 4. The short-circuiting of a copper-modified TO2 particle under band gap irradiation. The 0, that is evolved via the hole reaction is reconverted back to H 2 0 at the Cu site that mediates the electron conjugate process. Ti02 (see Fig. 3). The hack reaction between Cu and Oz (which has a negative AGO) serves to chemically reverse the forward reaction (eq 1). In other words, we have a situation akin to that in Figure 2 except that the C U ~ ' ' ~redox couple has acted a s a mediator or electron relay (see Fig. 4); the Cu2+ ions have short-circuited the TiOz microelectrode. We will discuss experimental examples of this effect later along with other subtleties. In many instances, however, the thermodynamically downhill (AGO < 0)back reaction is kinetically slow. Also the conjugate reaction partner plays a pivotal role in dictating whether a given reaction is photocatalytic or photosynthetic. For example, the reaction 2cuZt + 2H20 + ZCu + O2+ 4Ht
Figure 5. A Wagner diagram (see ref 15) of the conjugate processes at an irradiated TiO, particle. The hole process is shown as curve 1; two different cases are shown for the conjugate electron process involving one with sluggish (curve 2) and facile (curve 3) kinetics. The (dashed)tie lines correspond to particle potentials ( A and 6~at which the anodic and cathodic branches are balanced.
(7)
is photosynthetic (AGO = +I64 kJ). On the other hand, addition of a hole scavenger, such as acetic acid, to the medium causes the resultant reaction,
to be photocatalytic (AGO = -136 kJ1mol) (14).
Current h6
(2)
Electrochemical Aspects of Photocatalysis Rate Limitations Exercised on Detoxification or Disinfection by the Conjugate Reaction Partner
A useful model framework for discnssine.. coniueate . .. reactions at a p h o t o c ~ t a l y ~ partirle ~c c m hc himowed from the c o r n ~ s i mfield (15. 16 The u n d d v i n g ~ d e ais that, on a a t steadystate, the net rates of "microelectrode" the oxidation and the reduction components must be the same; the anodic and the cathodic current branches must have the same mamitude (17-20). This is illustrated in ~~'1'102, F ~ g u r c5 s and 6. ~01.a"n-type semiconductorsu~~h the anod~cc u r n m hranch reflects the flux uf the mmorlty carriers: holes. The cathodic component originates i n the flow of majority camers: electrons. The potential scale in Figures 5 and 6 refers to the location of the so-called Fermi level in the electrode or approximately the potential of the conduction band edge i n the semiconductor hulk, relative to a n external reference, such a s SCE or NHE. 1046
Journal of Chemical Education
w
' Voltage
(+)
Figure 6. Wagner diagrams for a targeted photoreduction at TiO, when the cathodic process (a)is not mass-transport-limitedand (b) is mass-transport-limited.The medium is assumed to be agitated when mass-fransport-limited.Curves 1 and 2 correspond to a slow and fast anodic process for the same photon-flux incident on the Ti02 particle sudace. The tie lines have the same significance as in Figure 5.
Current
Current
t .Y
I
;I I ;I I
-
h6
(14)
h6
(13)
h6
(11)
Voltage
(+)
I
Current
eflen on a pnoloanod~cprocess a1 T!Oi In Ine Flgure 8 Photon-fl~x presence of an LnJsLa y lac1 e conpgate rcacr on A I near oepenaence of reacl on ye o on gnt nlens~~y s preo cled n lh s case Jn ode s an0 70 Olner nolafon as n FtgLre 7 for rnal in F g ~ r e 7a (ppm levels) a s i s usually the case, then the cathodic branch will be mass-transport limited and will exhibit a plateau (curve 3, Fig. 6b). In this case, the available hole flux may be more than sufficient to balance the maximum cathodic current that can be sustained a t the particle surface. Economically speaking, the cases illustrated in Figure 6a thus may be more appropriate to a metal recovery or "photoelectrowinning" scheme wherein higher levels (millimolar) of metal ions exist in the waste stream. Silver recovery from a photoprocessing operation is such a case. Figure 7. Influence of photon flux on the rate of a photoanodic process at TiO, in the presence of a conjugate (electron-driven)process with (a)no mass-transoort limitation and fbl finitemass-transoort limitatio". i h e light intensity increases from i, io 4 in both cases. The tie lines have the same significance as in Figure 5. Photoanodic Detoxification Consider Fieure 5 first. Here the desired reaction is the photoanodic &toxificarion of a pollutant rsce eq 41. Two sccn;moi arc shown in Fiaure 5 whereln this oxidation reaction (curve 1) is coupled with a sluggish (curve 2) and facile (curve 3) cathodic conjugate reaction. For a sluggish reaction, the particle a t steady state attains a mixed-potential represented by the point A. For a facile reaction, this potential shifts in the desirable direction of higher (i.e., more positive) reaction overpotential (point B). So the rate of the conjugate reaction partner (eq 2 in the preceding example) must be optimized to afford faster photodecomoosition of the oreanic oollutant. ~ & u r 6a e contains symmktrica~considerations for a photoreduction-target system (curve 3). I n this case, a faster conjugate photooxidation reaction (curve 2 instead of curve 1)leads to a shift of the reaction overootential in the desired negative direction. Thus, if the'removal of a toxic metal ion is the goal (see eq 6) a n efficient hole scavenger can be added to accelerate the clean-up process. However, if the toxic metal ion is present only in low concentration
-
Photon-Flux Effects What happens when the photon flux is increased? Recall that the photon flux directly affects only the minority carrier current branch (21, 22). In the test system (n-TiOz) under ronsidrration, it is the anodic current component that will be alkcted. Aeain two scenarios call hc c~l\.iiion(d depending on whether the concentration of the oxidant 0x1 (see Fig. 1)is high (Fig. 7a) or low (Fig. 7b). In both the cases, the photopotential of the TiOz particles shifts in the negative direction with increasing light intensity. Interestingly enough, a linear dependence of the photocatalysis rate on the photon flux is not expected in either case. Such a trend necessitates a n exceptionally facile cathodic reaction as illustrated in Figure 8. This is unlikely because most cathodic processes of relevance to environmental remediation (e.g., Oz reduction, metal ion reduction) are electrochemically irreversible (i.e., kinetically slow). As a corollary, a sublinear dependence of the ohotocatalvsis rate on the ohoton flux translates to a coneomitant decrease in the quantum yield with increasing photon flux because those e--ht pairs not consumed by the conjugate reaction pair will simply recombine. The scenario illustrated in Figure 8 is most likely for photocatalytic reactions with large negative AGO. (This possibility was pointed out by the reviewer. Volume 72 Number 11 November 1995
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along with the conjugate hole reaction partner.
-
40H-+ 4ht -+ 40H anodic OH+ holescavenger products
(3a) (3b)
Acloser examination of this scheme shows that the reactions of eqs 2' and 3a together form a "short-circuiting" loop. The net result is a reduction in the metal deposition efficiency. Analogous considerations apply by symmetry to a targeted photoanodic detoxification process. Comparisons with ExperimentalData
Addition of cuZt ions was found to result in a n increased yield of oxidation products from toluene a t UV-irradiated TiOz powder (23). Here, cn2+ ions may function a s facile electron acceptors (relative to 0 2 ) thus increasing the rate of the cathodic current branch. The consequence is a concomitant increase i n the magnitude of the (photoanodic) conjugate reaction (see Fig. 5). f k t h e r interesting exam lc of a metal Ion rtX:ct on pho)'.I- r;dox couplt! (2.1 . At high tocatalysis concerns tht: 'F; I c e . . 50 mM . concentrntions of Fe '.a drcreasc in the ohotocatalysis yield was noted for a variety of hydrocardons. I n this instance, the ~ e redox ~ couple " ~functions ~ in a similar (deleterious) role as Oz in Figure 2; i t short-circuits the TiOz microelectrode.
,-~-~. ~
A similar trend was reported for cuZ+ions i n a study on the photocatalytic decomposition of phenol a t TiOz (25). Both these systems represent examples of instances wherein the photogenerated e--ht pairs react with the same redox couple leading to no net chemical change in the system (see Fig. 2). Catalytic modification (i.e., metallization) of the semiconductor surface can result in a n enhancement of the photocatalytic yield via concomitant increases in the rate of the conjugate cathodic reaction (Oz reduction). This effect (which is predicted by Fig. 5) has been discussed by other authors for the photooxidation of organic substrates a t platinum-modified (17) and palladium-modified (26) TiOz. Examples of photon-flux effects on photocatalytic yields appear to be rather limited. However, the majority of the studies to our knowledge do indeed report a sublinear dependence-more specifically, a square-root dependence. Thus, this trend can be applied, for example, to the photooxidation of phenol a t TiOz (251, the photoreduction of Cr(V1) a t TiOz (lo),the bactericidal action of TiOz on E. coli (111, a n d t h e photodecomposition of t a r t r a t e a t a-Fez03 (27).
Time (min) F g ~ r 9c nf1,ence of O2 on the ell c ency of Ti02 (a)to redxe Cr(VI) 1 19) or Ib) lo n E coh (20)Tne bacler c.oa acl vlly of T 0, s expresseo as tne percenlage s-rvwal of co ony-forming Jnds (CFU's!
Competitive Reaction Pathways
Competitive reactions can also be treated within the framework of the preceding discussion related to Figures 5 and 6. For example, the photocatalytic deposition of a metal in the presence of Oz a t TiOz may additionally involve the following competitive pathway. cathodic 1048
40K O2+ ZHzO + 4e- ---+ Mh+4e-+M
Journal of Chemical Education
~~~~
(9)
The Effect of Or: An Interesting Symmety
We finally present two examples for the contrasting influence of Oz on photoreduction of Cr(VI) (Fig. 9a) and the rate of killing of E. coli (Fig. 9b) a t TiOz. These data are taken from recent research in the laboratory of one of the present authors (19,201. In the former case (Fig. gal, Oz i s deleterious because i t competes with Cr(VI) for the photoeenerated electrons. On the other hand. the ~hotoanodic thole, r t ~ x t i o nraIe is enhancrd in Figure 9b bv improving the kinetics of the rathr~dic rcaction coniucarr a s in the model case in Figure 5. This is done by inc>e&ing the concentration of the electron acceptor (Oz in this case) in the
electrolyte. Interestingly enough, too high a concentration would have perhaps reversed this trend (by short-circuiting this cell), although this limit was fortuitously not obtained in Figure 9b due to the finite solubility of Oz i n water. Concluding Remarks and Perspectives This article has hopefully highlighted the fact that electrochemical concepts are very useful for analyzing the behavior of irradiated semiconductor particles. Although these particles have been shown to be effective in the treatment of pollutants, much of the data thus far have been gathered under controlled laboratory conditions, and on rather pristine samples. However, many practical application scenarios appear to hold much promise. For example, a n intriguing approach is to coat TiOz on the interior walls and floors of buildings (e.g., hospitals) for the inactivation of odor-causing bacteria and for other disinfection purposes (28). Such real-world systems ought to provide an acid test for the effectiveness of models such a s those discussed in this paper. Additionally and from a pedagogical perspective, laboratory experiments built around these systems would have the virtue of demonstrating t h a t chemistry-and specifically electrochemistry and materials chemistry-can indeed play a useful role in providing a cleaner and healthier environment. Acknowledgment The authors thank Sanjay Basak and Mai Zhou for assistance with the graphics. We also thank the reviewer for
several pertinent suggestions for revising an initial version of this manuscript. Literature Cited I . J . Chrm Educ. 1993. 70151. 2. Nogueira, R. F. P; Jardim. W. F. J Chpm Educ. 1993. 70. 861. 3. Ollis. D.F; Pelissetti. E.: Serpone, N.Enuiron. Sci. Xchnoi. 1991.25. 1523. 4. Kormann. C.; Bahnemann, D. W : Hoffman". M. R. Environ. Sci. l k h n o l . 1991.25,
.....
LO&
5. Fox. M.A.Chemtech. 1992.680. . press. 6. Rajeahwar, K. J . Appl. E i e t r ~ h e m in 7. Kel1er.S. W.: Mallouk. T E. J Chem. Mxc. 1993. 70,855. 8. Ibaner, J. G.: Snlorzl, 0.:Gomer-d'!-Camp. E. J Cham. Edlre. 1991, 68, 873, and referenecr therein. : Florence. 1.M. Chrm Brrr 19R199.iAup~sfi.791. 9. Morrison,G. M. P.;Baile& G E 10. Lin,W-Y.;Wei. C.; Rajrrha,sr. K. J. Elsrtrwhvin. Suc. 1993. 140, 24-7. 11. Wei, C.; Lin. W-Y.; Zainal. 3.; Urdliams, N. E.:3hu. K.: Kruzic. A. P: Sn,ith. B I. Rajeshwar,K.Eni,imn Sct. %iiehiioi. 1994.28.934, . 199S.,April 121. 11. 1 2 Ckem o n d E n ~News 13. Runo. J. R.: Peters, 1). G. J. Chcm. Edur. 1993. 70. 708. 14. Reiche. H.; Dunk?. W. W.: Bard. A. J. ,I. Phy.. Chem. 1979.83.2248. 15. Wagner C.:Traud, W. 2.Elekfrnchsm. 193844,391. 16. Steigerwald. R. P Cormaim 1968.24. 1. 17. KTaeutler B.:Bsrd, A. J. J A m Ciirm. Soc 1978, 100,5985. 18. Bard. A. J. Seimce 1980.207. 139. 19. Milier. D. S.: Bard. A. J.; McLendon, G.; F e w s o n , J. J . Am. Chem Sor. 1981, 103. 5336. 20. Bard. A. J. J. Phva. Ckem 1982.86. 172.
24. ~ujihira.satoh. oh.^: o s a , chrm. ~ L p r r . 1981. 1053. 25. Okamoio. K;Yamamoto.Y.:Tanaka, H.;Tanaka, M.; ltaya,A.Bull. Chem. Soe. Jpn. 1985.58.2015. 26. Wang, C-M.;Helicr.A.; Gerischer. H. J A m Chem Soc 1992,114,5230, 27. h i a n d . J. K:Bard, A. J. J Ph,ys. Chem. 1987.91.5076. 28. Hashimoto, K.: Fujiahima, A. Paper No. 928. 183rd Meeting of the Electrochemical Society. Hawaii. May. 1993.
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