J. Chem. SOC., Faraday Trans. I, 1984,80, 1631-1643 Photoreduction of Methyl Orange Sensitized by Colloidal Titanium Dioxide BY GRAHAM T. BROWN AND JAMFS R. DARWENT* Department of Chemistry, Birkbeck College, University of London, Malet Street, London WClE 7HX Received 17th October, 1983 Colloidal TiO, can sensitize the photoreduction of Methyl Orange to a hydrazine derivative. In steady-state experiments the maximum rate occurs at pH 4.7 in the absence of 0,, but shifts to pH 3.0 when 0, is present. Similar behaviour is observed with Methyl Red despite the difference in pK, values for the two dyes. This suggests that the maximum rate results from changes in the surface composition of TiO,. Flash-photolysis experiments show that electron transfer from TiO, to Methyl Orange depends on the concentration of protonated dye (DH) and the potential of an electron in the conduction band (eCB) of TiO,, so the rate of electron transfer = 8.3 x lo7 [DH][e,,]/ [H+]0.38 (mol dm1-3)-0.62 s-l.Electron transfer to 0, was also measured indirectly in this work and showed a similar logarithmic dependence on pH, rate = 5.5 x 103[0,][ecB]/ [H+]o.42 (mol dm-3)-0.58 s-l. These results can be described by a Taffel relationship when the overvoltage is controlled by the pH of the solution. Charge-transfer reactions at a junction between a semiconductor and an electrolyte may provide a route for the conversion of solar energy.l? Recent research has shown that dispersions of oxide semiconductor particles can photodissociate water into H, and 023-5, and liquid-junction photovoltaic cells could prove to be an attractive alternative to conventional solar cells.6*7 These reactions also play a central role in photography and the photo-oxidation of pigments.In the last two years a number of research groups have studied the electron-transfer reactions in colloids of Ti0,,8-10 CdS2* and CdSe.ls The colloids are transparent and stable for a period of weeks and so the reactions can be monitored directly by conventional photochemical techniques such as flash p h o t o l y ~ i s ~ - ~ ~ ~ l8 and resonance Raman spectroscopy.16~ l 7 Aromatic azo compounds are one of the most important classes of commercial dyes. In general, these dyes have very short excited-state lifetimes because of rapid trans-cis photoisomerization.19 They are stable to visible and near-u.v.light2O? 21 but can be photoreduced in the presence of sensitizers such as chlorophyll22+ 23 or mandelic and provide a useful probe for photoredox reactions. We have carried out a detailed study of the reduction of Methyl Orange photo- sensitized by colloidal TiO, using time-resolved and steady-state techniques. The results provide an insight into the photoreduction of aromatic azo compounds and the effect of pH on interfacial electron-transfer reactions from TiO,. Methyl Orange was chosen since its chemical structure is simpler than that of most commercial azo dyes. As a result the products were more easily analysed. Similar results were found with Direct Red 81 and we expect our observations t o apply to many commercial azo dyes.16311632 PHOTOREDUCTION OF METHYL ORANGE EXPERIMENTAL Steady-state irradiations were carried out using an Applied Photophysics clinical irradiator employing a 900 W xenon lamp and monochromator. Microsecond flash-photolysis experiments were performed with an Applied Photophysics K200 system using copper sulphate filter solutions to remove excitation wavelengths below 290 nm. Visible absorption spectra were recorded with a Perkin-Elmer 554 and 402 spectrophotometer for 1 and 10 cm quartz cells, respectively. Samples for anaerobic experiments were outgassed by repeated freeze-thaw cycles to at least lo-, Torr. This treatment did not affect the absorption spectra of colloidal TiO,. Unless otherwise stated all of the solutions contained 5 x lop3 mol dm-3 acetate or 5 x mol dm-3 borate as pH buffers and 0.1 % polyvinyl alcohol (PVA) as a support for the TiO,. The extinction coefficients taken for protonated and unprotonated Methyl Orange were 4.57 x lo4 and 2.68 x lo4 dm3 mol-l cm-l, re~pectively.~~ PVA (molecular weight 72000), Methyl Orange, Methyl Red, TiC1, (98.5%) and all other chemicals were supplied by B.D. H. (A.R. grade). Water was double distilled. The TiO, sol was made by the method described by Henglein.12 Laser light scattering showed that the average diameter of the particles was 40 nm. When these colloids were stabilized with PVA (0.1 % ) they were transparent and stable for months. RESULTS AND DISCUSSION STEADY-STATE IRRADIATION When an anaerobic solution of Methyl Orange and TiO, (1 0-4 mol dm-3, pH 9.25) is illuminated with ultra-band-gap radiation (1 = 310 nm), the visible absorption band of Methyl Orange disappears and a new peak grows in at 247 nm (see fig.1). No bleaching is observed in the absence of TiO, and visible radiation (A > 400 nm) does not result in the disappearance of dye, which indicates that only light absorbed by TiO, will lead to destruction of Methyl Orange. Indeed, fig. 2 shows that the initial rate of dye loss is directly proportional to the number of photons absorbed by TiO, (5 x 10-5-5 x mol dm-3). On increasing the concentration of TiO, above 5 x mol dmP3 the rate levels off, since most of the incident light is absorbed in a smaller volume close to the window of the absorption cell. Methyl Orange is photoreduced rapidly in this region until the rate is controlled by diffusion of dye from the bulk of the solution.This does not apply to low concentrations of TiO,, where light is absorbed more evenly throughout the 1 cm cell. The reaction obeyed first-order kinetics with respect to Methyl Orange. The spectra in fig. 1 suggest that Methyl Orange (I) is photoreduced to a hydrazine derivative (11): The product (11) absorption maximum at 247 nm was also observed when Methyl Orange was chemically reduced (Zn/AcOH) and disappeared over a period of hours when air was admitted to the solution. From fig. 1 the extinction coefficient of (11) at 247 nm is 2.1 x lo4 dm3 mo1-l cm-l, which is comparable to the extinction coefficient of 1,2-diphenylhydrazine in ethanol ( E , ~ ~ = 2 x lo4 dm3 mo1-1 cm-1).26 Below pH 5 , loss of Methyl Orange was accompanied by spectral changes consistent with the production of protonated hydrazine.After irradiation, addition of base showed that the same product was formed over the pH range 1.5-9.5. Gratzel et al. have shown that the potential of an electron in the conduction bandG. T. BROWN AND J. R. DARWENT 1633 t \ 300 500 h/nm Fig. 1. Photosensitized reduction of Methyl Orange. (TiO,, loF4 mol dmp3; PVA, 0.1 %; borate buffer, 5 x lop3 mol dm-3; R = 310 nm). 0 1 .o d I v) 3 0.75 2 2 \ az" 0.5 0.25 0 [Ti02]/10-3 mol dm-' I5 0.1 0.2 0.3 0.40.5 1.02.0 I I I 1 1 1 I I Fig. 2. Rate of photosensitized reduction of Methyl Orange against TiO, concentration and fraction of light absorbed by TiO, (unless otherwise stated, conditions as in fig.1).1634 PHOTOREDUCTION OF METHYL ORANGE I 1 I 0 2 6 10 PH Fig. 3. (a) Initial rate of Methyl Orange (a) and Methyl Red (0) reduction against pH in anaerobic conditions [TiO,, 5 x mol dm-3; acetate buffer (pH < 6), 5 x mol drnv3; otherwise conditions as in fig. I]. (b) Initial rate of Methyl Orange reduction against pH in aerobic conditions. of TiO, particles shifts linearly with pH to more cathodic 27 Similar effects are well known for oxide electrodes28 and If this was the controlling factor in the reduction of Methyl Orange, the rate could be expected to increase steadily with pH. Fig. 3(a) shows the rate of reduction does increase up to pH 4 but then passes through a maximum at pH 4.6. It would be tempting to explain this on the basis of the equilibrium: + R-NzN-R’ R--N=N-R’ + H+ (2) H pK, = 3.5 (Methyl Orange)3o pK, = 4.8 (Methyl Red).30 The maximum rate, however, occurs at the same pH for Methyl Orange and Methyl Red, despite their significantly different pK, values (the pK, of Methyl Orange was not appreciably affected by colloidal TiO,).Also, varying the concentration of acetate buffer from 0 to mol dm-3 had no effect on the rate of reduction. This suggests that the maximum rate does not result from the pK, of Methyl Orange or catalysis by acetate ions. It may, however, reflect changes in the surface composition of theG . T. BROWN AND J. R. DARWENT 1635 m 0.4 0.3 2 - 0.2 K 3 '0 00 g 0.1 - 0 n A 0 - 1 I 1 I Y 0' 2 4 6 8 10 , t - A s PH Fig. 4. Yield of Methyl Orange reduced per photoflash against pH for aerobic (0) and anaerobic (0) conditions [Methyl Orange, 5 x lop6 mol dm-3; otherwise conditions as fig.3 (a)]. TiO, particles. Schindler et ~ 1 . 3 ~ studied the protonation of TiO, and found two equilibria : --TIOH: -TIOH + H+, pK, = 4.95 (3) -TIOH -TiO- + H+, pK, = 7.8. (4) The steady-state reduction of Methyl Orange will depend on the rate of reaction of photogenerated electrons and holes in TiO,. The fate of these reactants will be influenced by the surface of the particles. When equilibria (3) and (4) are compared with the experimental results, it appears that the optimum surface for reduction of Methyl Orange, in this system, contains both -TiOH and --TIOH; groups. When oxygen is present the maximum rate of reduction occurs at the pK, of Methyl Orange [fig.3 (b)] and the rate drops rapidly to zero as the pH is increased. This results from competition between oxygen and Methyl Orange for reducing species. The following flash-photolysis results show that only protonated Methyl Orange can compete with oxygen for interfacial electron transfer from TiO, and that the rate constant for electron transfer to protonated dye increases with pH. DYNAMICS OF INTERFACIAL ELECTRON TRANSFER TO METHYL ORANGE ANAEROBIC CONDITIONS When samples of Methyl Orange (< 6 x lop6 mol dm-3) and colloidal TiO, (5 x lop4 mol dmp3) were studied using microsecond flash photolysis, loss of dye was followed at the maxima of the visible absorption bands (510 and 470 nm for the protonated and unprotonated species). In anaerobic conditions, the amount of dye reduced per flash was independent of the dye concentration (above 10-smol dm-3) and showed a small decrease on going from pH 1.5 to 8.5 (fig.4). Since the steady-state results show that a hydrazine was the main product at all pH values, fig. 4 indicates that the yield of reducing species, i.e. electrons trapped in the con- duction band of TiO, after the photoflash, is essentially independent of pH. In contrast the dynamics of dye reduction show a much more complex pH dependence, as illustrated in fig. 5 and 6.1636 PHOTOREDUCTION OF METHYL ORANGE Fig. 5. (a) Decay and (b) recovery of Methyl Orange after photoflash (pH 1.45; A = 510 nm). (c) Slow decay of Methyl Orange after photoflash (pH 8.6; A = 470 nm). At pH 1.5 the initial rapid decay of the dye [fig.5 (a)] was followed by a slow partial recovery (ca. 30%) [fig. 5(b)]. Below pH 6 the initial decay obeyed pseudo-first-order kinetics with respect to the Methyl Orange concentration. Fig. 6(a) shows that the observed first-order rate constants (kobs) are proportional to the dye concentration, so that the reaction is overall second order with: rate = kobs[eCB] = k,[Methyl Orange][e,,] (0 where [e,,] represents the concentration of electrons trapped in the conduction bandG. T. BROWN AND J. R. DARWENT 80 6 0 - I v) I 0 - - E “E 4 0 - -a A? \ n 1637 - - 200 - I m . D & 30 I 0 1 2 3 L [Methyl Orange] /pmol dm-’ - I m . III 0 -r“ 200 100 0 i 1 I I I 1 2 3 G [Methyl Orange] /pmol dm-’ Fig. 6. (a) kobs for Methyl Orange reduction against dye concentration under anaerobic conditions [PH 2.2 (O), 3.2 (0) and 5.75 (a)]. (b) kobs under aerobic solutions [PH 2.3 (O), 3.2 (0) and 4.2 (a)].2ot 0 2 4 6 PH Fig. 7. Second-order rate constants for electron transfer from TiO, to Methyl Orange against pH (k, calculated for total Methyl Orange concentration).1638 PHOTOREDUCTION OF METHYL ORANGE of TiO, after the photoflash and kD is the rate constant for electron transfer to Methyl Orange. The effect of pH on k , is shown in fig. 7, where the rate rises to a maximum at pH 3.0 and then decreases with increasing pH. EFFECT OF OXYGEN When samples were equilibrated with air the rate of electron transfer from TiO, to Methyl Orange appeared faster but plots of kobs against dye concentration had the same slope as found for anaerobic solutions and the intercept when no dye is present was no longer zero [see fig.6(b)]. When the samples were purged with oxygen there was a five-fold increase in the intercept whereas the slope of the graph remained the same. Purging the solution will increase the oxygen concentration by a factor of five so this suggests that the intercept is due to electron transfer from TiO, to oxygen. In aerobic conditions, eCB can react with 0, and Methyl Orange, which are both in large excess compared with [e,,], so that removal of eCB is controlled by two parallel first-order reactions : -d[ecB]/dt = (k,[0,] + k,[Methyl Orange])[e,,] = ko bs Ee CBI (ii) where k, is the rate of electron transfer to oxygen. For reduction of Methyl Orange : -d[Methyl Orangelldl = k,[Methyl Orange][e,,] = kD[Methy1 Orange][ecB], exp (- kobs t ) (iii) and hence [Methyl Orange] - [Methyl Orange], = ([Methyl Orange], - [Methyl Orange],) exp (-kobs t ) (iv) and the amount of dye reduced per flash is controlled by the relative magnitude of k,[0,] and k,[Methyl Orange] : kD[Methyl Orange][e,,], k,[0,] + k,[Methyl Orange] (v) [Methyl Orange], - [Methyl Orange], = where the subscripts 0 and a refer to the concentrations immediately after the photoflash (0) and after complete removal of eCB (a).The observed rate constant (kobs), which was calculated from the changes in Methyl Orange concentration, thus results from electron transfer to 0, and Methyl Orange [eqn (ii)] and by extrapolating the data to zero dye concentration it is possible to determine k,.Oxygen also has a marked effect on the amount of dye reduced per flash, as would be expected from eqn (v). This is shown in fig. 4. In anaerobic conditions the yield is effectively constant but with aerobic solutions it falls to zero at pH 5 , since k,[0,] is then greater than kD[Methyl Orange]. The flash-photolysis results are consistent with the following scheme, where TiO, particles are considered as short-circuited photochemical electrodes and reduction of Methyl Orange occurs via electron transfer from TiO, to protonated dye molecules (DH): TiO, + hv + TiO,(h++cB) (light absorption) ( 5 ) TiO,(h+-ecB) + TiO, (charge recombination) (6)G . T. BROWN AND J. R. DARWENT 1639 h+ + PVA -+ H+ +oxidized PVA (hole removal) (7) eCB + 0, -+ 0;- (oxygen reduction) (8) eCB + DH -+ DH ' - (electron transfer to dye) (9) DH'- + eCB + H+ -+ DH; (hydrazine formation) (10) 2DH .- -+ DH; + D- (disproportionation) (1 1) where D- represents ( ~ ~ , ) , ~ ~ , ~ , ~ = ~ ~ , ~ , ~ ~ ~ .Reactions (5)-(7) will occur during the 10 ,us photoflash. (The lifetime of a photogenerated ecB-h+ pair is thought to be of the order of nanoseconds in moderately doped Ti0,.27) Oxidation of PVA and water will trap electrons in the conduction band of TiO,. In the absence of oxygen these reducing species have a lifetime of several hourss and are able to reduce Methyl Orange in a dark reaction. The sharp decrease in k , above the pK, of Methyl Orange (fig. 7) suggests that the rate-determining step is electron transfer to the protonated dye in this pH range.When oxygen is present electrons may be transferred to produce Og-, which is thought to be formed as a chemisorbed species on the surface of Ti0,.32 Below pH 5 the semi-reduced dye will be protonated, since its pK, should be similar to that reported for semi-reduced azobenzene (pK, = 7.1).33 This radical can accept a second electron from TiO, [reaction (lo)] or undergo disproportionation [reaction (1 l)], which will regenerate some of the dye and account for the partial recovery shown in fig. 5 (b). The recovery obeys a mixture of first- and second-order kinetics from which 64 k,, z 2 x lo8 dm3 mol-l s-l assuming that the absorption of DH'- is negligible compared with that of DH at 510 nm. The second-order rate constants (k,) in fig.7 result from electron transfer to the protonated dye but k , refers to the total dye concentration. To correct for this (vii) where k , is the rate constant for electron transfer from TiO, to DH and K, is the equilibrium constant for reaction (2). Fig. 8 shows a plot of log k , against pH, which shows that log k, varies linearly with pH: k, = 8.3 x 107/[H+]0.38 (mol dm-3)-0.62 s-1. Similarly log k,, the rate constant for reduction of oxygen, also increases linearly with pH as shown in fig. 8: k, = 5.5 x 103/[H+]0.43 (mol dm-3)-0.57 s-l. This may reflect changes in the conduction band potential of TiO, as previously observed for reduction of methyl viologen.lo The conduction band potential of TiO, (ECB) is known to shift cathodically with ECB = EEB-0.059 pH (viii) where EEB is the value of ECB at pH 0.Since the standard potential of the couples DH/DH'- and O,/O;- will be independent of pH, the rate of interfacial electron transfer from TiO, to DH and 0, should increase with pH. Such behaviour has been demonstrated by Gratzel et al. for the photoreduction of methyl viologen by colloidal PH:lo7 27-291640 PHOTOREDUCTION OF METHYL ORANGE 8 .O 6.0 m A? M - 5.0 4 .O 1 .o 2 .o 3.0 Fig. 8. Plot of log k, (e) and log k, (0) against pH, where k, and k, are second-order rate constants for electron transfer to 0, and DH at a given pH. PH TiO,. They found that the rate constant varied with the overvoltage ( V ) , and hence pH, in accord with the Taffel equation:l07 27 log (k,,/k",) = - ( 1 - a) nFV/2.303 RT = - (1 - a) nF(E, - Ec,)/2.303 RT hence log (ket/kzt) = - n( 1 - a)(E& - E, -0.059 pH)/0.059 = n( 1 -a) pH - n( 1 - a)(E& - E0)/o.059 (4 where n is the number of electrons in the transfer step, F is the Faraday constant, kEt is the rate constant for electron transfer at the standard potential for the redox couple, Eo is the standard redox potential for DH/DH'- and O,/O;l- and a is the transfer coefficient.In the present case there is reasonable agreement, over the limited pH range available, between eqn (ix) and fig. 8. From these a = 0.62 for DH and 0.57 for O,, which suggests that a symmetrical transition state is involved in both reactions. HIGH pH Above pH 7 the kinetics of anaerobic dye reduction show a distinctiy different behaviour, which was illustrated by fig.5 (c). The reaction occurs over several secondsG. T. BROWN AND J. R. DARWENT 1641 2 .o 1.5 " I wl I - - 0 E E a 1.0 2 *O m e =I 0.5 0 1 2 3 4 5 [Methyl Orange] 11O-j mol dm-3 Fig. 9. Plot of kobs (second order) against dye concentration at pH 8.6 [conditions as in fig. 3 (41- and can be described equally badly by second-order kinetics or a mixture of two first-order reactions. At high pH cis-trans isomerization is also observed, but the photostationary state is re-established within milliseconds of the photoflash so that it does not interfere with the kinetics of dye reduction. Cis-trans isomerization was not observed below pH 7 since the reaction is acid ~atalysed~~ and at low pH it occurs too rapidly to be detected by microsecond flash photolysis. At pH 8.6 the disappearance of dye can be fitted to second-order kinetics and the observed second-order rate constants again increase linearly with dye concentration (fig.9), showing that at high pH the rate can be formulated as: rate = k,,,[Methyl Orange] [eCBl2 ( 4 and the reaction is overall third order. At pH 8.6 interfacial electron transfer will involve unprotonated dye (D-) and the unprotonated semi-reduced dye (D2. -), which both have more negative reduction potentials than DH or DH-. After the photoflash the following reactions could result in the observed kinetic behaviour : D- + eCB D2 * - (12) D2'-+eCB+2H+ + DH;. (13) A disproportionation reaction [similar to reaction (1 l)] would lead to overall second- order kinetics and does not seem to be involved at high pH, since the reaction is third 54 FAR 11642 PHOTOREDUCTION OF METHYL ORANGE order overall.This may reflect the low concentrations of D2'- and the high degree of charge repulsion between the two anions. If reaction (1 3) is slow and rate determining, then fast reduction of D- and reoxidation of D2'- could establish the equilibrium reaction (1 2), in which case rate = K,,k,,[Methyl Orange][e,,12 (xii) kobs = K12k13 = 4.8 x lo6 rnol-, dm6 s-l (xiii) at pH 8.6, where K,, is the equilibrium constant for reaction (12) and k,, is the rate constant for electron transfer from TiO, to the unprotonated dye radical D2' -. CONCLUSIONS Colloidal TiO, can photosensitize the reduction of aromatic azo compounds such as Methyl Orange and Methyl Red. For Methyl Orange and Methyl Red steady-state reduction occurs with a maximum rate at pH 4.7 in the absence of oxygen. This may reflect surface protonation of TiO,.In aerobic solutions the maximum rate shifts to lower pH, since 0, is reduced more rapidly than the unprotonated dye. Flash photolysis shows that the rate of electron transfer from TiO, to Methyl Orange depends on the concentration of protonated dye and the potential of an electron in the conduction band of TiO,. Consequently the rate passes through a maximum at pH 3.0 (fig. 7). This is lower than the pH at which the maximum rate is observed in the steady-state experiments. The origin of this difference is not clear; however, the rate of steady-state dye reduction will also depend on the rate at which the hole (h+) reactions occur, i.e.light absorption, charge recombination and oxidation of PVA and water. These reactions are not reflected in the flash-photolysis measurements, which were made > 10 ps after the photoflash. The rate constant for electron transfer from TiO, to oxygen was measured indirectly in these experiments and also showed a logarithmic dependence on pH, which can be attributed to changes in the potential of the TiO, conduction band. 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