4538
J. Phys. Chem. 1996, 100, 4538-4545
Surface Modification of Small Particle TiO2 Colloids with Cysteine for Enhanced Photochemical Reduction: An EPR Study† Tijana Rajh, Agnes E. Ostafin, Olga I. Micic,‡ David M. Tiede, and Marion C. Thurnauer* Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: July 18, 1995; In Final Form: October 29, 1995X
Surface complexation of colloidal titanium dioxide nanoparticles (40-60 Å) with cysteine was investigated by electron paramagnetic resonance (EPR) and infrared (diffuse reflectance infrared Fourier transformDRIFT) spectroscopies. Cysteine was found to bind strongly to the TiO2 surface, resulting in formation of new trapping sites where photogenerated electrons and holes are localized. Illumination of cysteine-modified TiO2 at 77 K resulted in formation of cysteine radicals with the unpaired electron localized on the carboxyl group. Upon warming to 150 K, these radicals are transformed into sulfur-centered radicals as observed by EPR spectroscopy. We have demonstrated the existence of two surface Ti(III) centers on cysteine-modified TiO2 particles having different extents of tetragonal distortion of the octahedral crystal field. Upon addition of lead ions, a new complex of cysteine that bridges surface titanium atoms and lead ions was detected by IR spectroscopy. Illumination of lead/cysteine-modified TiO2 did not result in the formation of sulfur-centered radicals. Instead, a symmetrical, lattice defect type EPR signal for trapped holes was observed. Addition of methanol to this system resulted in the formation of a ‚CH2OH radical at 8.2 K. After the temperature was raised to 120 K, doubling of the signal associated with electrons trapped at the particle surface (Ti(III)surf) was observed. On further increase of the temperature to 200 K, the EPR signal for trapped electrons disappeared due to the reduction of Pb2+ ions, and metallic lead precipitated at room temperature. Conversion of photogenerated holes in the presence of methanol into trapped electrons can lead to the doubled quantum efficiency of metallic lead precipitation.
Introduction Semiconductor photocatalysis has proven to be a promising technology for use in the cleanup of water contaminated with hazardous industrial byproducts.1-6 Titanium dioxide, in particular, could be the catalyst of choice for a large variety of applications because it is cheap and nontoxic and has redox properties which are favorable both for oxidation of many organic pollutants and for reduction of a number of metal ions in aqueous solution. Electron paramagnetic resonance (EPR) spectroscopy has been used recently to identify the nature of radical species formed at the surface of aqueous suspensions of TiO2 particles and to study the specific mechanisms of the redox reactions occurring at the TiO2/solvent interface.7-10 Excitation of TiO2 with light energy greater than its band gap (3.2 eV) generates electron-hole (e-/h+) pairs that can be exploited in various processes at the particle interface. hν
TiO2 98 (e-cb + h+vb)TiO2
(1)
Photogenerated carriers migrate to the particle surface and participate in reduction and oxidation processes at the surface. Because the lifetime of charged pairs is very short, only very fast reactions with adsorbed species lead to efficient charge separation. Higher reaction efficiencies can be obtained when TiO2 is prepared as nanometer-sized particles in colloidal suspensions. As the particle size is reduced, a larger percentage of the particle is comprised of surface molecules which act as surface trapping † Portions of this work are published in the Proceedings of the Fifth International Symposium on Chemical Oxidation: Technology for the Nineties, Nashville, Tennessee, February 15-17, 1995. ‡ Present address: National Renewable Energy Institute, Golden, CO 80401. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4538$12.00/0
sites for photogenerated charges. It was suggested that interfacial electron transfer in TiO2 colloids occurs via surface Ti(IV) atoms which are coordinated with solvent molecules.11 Meanwhile, the hole transfer occurs via surface oxygen atoms covalently linked to the surface titanium. The energy level of these surface traps is generally located in the mid-gap region; therefore, the redox potentials of photogenerated charges are reduced relative to the potentials of conduction band electrons and valence band holes. Thus, modification of the particle surface by adsorption of one or more monolayers of adsorbate might block the surface states and significantly change the chemistry occurring at the TiO2/solvent interface. In addition, the particle surface can be engineered in a way that increases the separation distance of the trapped charges. Thus, both the redox and kinetic properties of the particulate semiconductor can be enhanced. A possible strategy for blocking the surface states of TiO2 and changing its chemical affinity is to derivatize the surface with molecules having multiple functional groups. Here we report that modification of the surface of small particle TiO2 with the amino acid cysteine (HOOCCH(NH2)CH2SH) results in enhanced reduction properties of photogenerated electrons. The interfacial charge transfer processes that occur in these surface-modified TiO2 colloids were investigated by EPR spectroscopy. Also, the mechanism of the current doubling process in the presence of methanol was examined. The stabilization of charge separation resulting in an increased separation distance between trapped electrons and holes was monitored by EPR spectroscopy from 4.2 K to room temperature. Experimental Section All the chemicals were reagent grade and used without further purification (Aldrich or Baker). Triply distilled water was used. © 1996 American Chemical Society
Surface Modification of TiO2 Colloids The pH was adjusted to pH 4 with NaOH or HCl. Oxygen was removed by out-gassing with argon or nitrogen. Colloidal TiO2 was prepared by dropwise addition of titanium(IV) chloride to cooled water. Temperature and rate of component mixing of reactants were controlled by an apparatus developed for automatic colloid preparation. A multiport adjustable-temperature liquid nitrogen cooled gas flow system was used to control both the temperature of TiCl4 prior to its addition to water and the temperature of the reaction vessel. A peristaltic pump with variable size outlet ports was used to control the drop size as well as the drop rate of TiCl4. Following TiCl4 hydrolysis, the solution was dialyzed against distilled water at 4 °C. The concentration of TiO2 (0.1-0.6 M) was determined from the concentration of the peroxide complex obtained after dissolving the colloid in concentrated H2SO4.12 Apparatus. EPR: Samples were excited at 77 K by a Questek 2400 excimer laser (308 nm, 150 mJ/pulse; 3000 pulses). After laser irradiation, the samples were transferred to a variable-temperature Dewar mounted in the EPR spectrometer (Varian E-9). Samples were checked for background EPR signals before irradiation. The g-factors were calibrated by comparison to a Mn2+ standard in SrO matrix (g ) 2.0012 ( 0.0002).13 Lead ions were added as Pb(CH3COO)2. FTIR: Measurements were performed on a Nicolet 510 Fourier transform infrared spectrometer equipped with a Spectra-Tech Inc. diffuse reflectance accessory. The resolution was 4 cm-1. All samples were 8 wt % of sample in KBr matrix. Typically 100 scans were performed for each spectrum. All results are presented as normalized Kubelka-Munk plots. Lead ions were added as Pb(NO3)2. Atomic absorption: The concentration of unreacted lead ions in the solution was measured with a Buck Scientific 200A atomic absorption spectrometer. The colloids were filtered through Amicon-made Diaflo ultrafiltration membrane filters YM10. Cyclic voltammetry: The redox potential of a lead-cysteine complex was obtained by cyclic voltammetry where glassy carbon was used as the working electrode, SCE was used as the reference electrode, 10-2 M KCl was used as the supporting electrolyte, and Ar was bubbled through the solutions. The potentials were scanned from 0 to -1.4 V vs SCE and back. UV-vis absorption spectra were recorded on a Shimadzu MPS-2000 instrument. A 50 W xenon lamp (Orion Corporation) was employed for steady state illumination. The pulse radiolysis system has previously been described.14 For steady state γ-radiolysis a 60Co source was used at a dose rate of 9.7 krad min-1.
Results and Discussion Characterization of Unmodified and Modified TiO2 Catalysts. Electron-hole pairs generated during the illumination of TiO2 particles (eq 1) can be exploited for various redox processes. Conduction band electrons that have a potential of -0.3 V (vs NHE) at pH 315 are trapped on the particle surface in 30 ps, and valence band holes with a potential of +2.9 V at pH 3 are trapped in the 100 ns range.16 Because the energy levels of the surface traps lie within the band gap, the actual reduction and oxidation properties of photogenerated charges are reduced with respect to free charge carriers and are dependent on the redox potential of surface trapping sites. EPR investigations of surface trapping sites in colloidal TiO2 suggest that electrons are trapped at metal centers as Ti(III),7 while the holes are trapped on the surface OH groups as a (TiO2)nTiIVO‚
J. Phys. Chem., Vol. 100, No. 11, 1996 4539
Figure 1. EPR (X-band) spectra of degassed aqueous TiO2 colloids (0.2 M) irradiated with 308 nm excimer laser at 77 K: (a) pH 3.5, recorded at 8 K; (b) in the presence of 0.1 M S-methylcysteine, pH 3.5, recorded at 8 K; (c) in the presence of cysteine-3,3-d2; (d) in the presence of 0.1 M cysteine, pH 3.5, recorded at 8 K.
radical intermediate (Scheme 1a)8-10
e- + (TiO2)nTiIVOH f (TiO2)nTiIIIOH, h+ + (TiO2)nTiIVOH f (TiO2)nTiIVO‚ + H+
(2)
where (TiO2)n represents bulk material. The photoinduced EPR signal (Figure 1a) consists of the oxygen radical signal having an anisotropic g-tensor with gz ) 2.007, gy ) 2.014, and gx ) 2.0249 and electrons trapped as Ti(III) centers with g ) 1.987 for the bulk lattice and g ) 1.926 for the surface Ti(III).7 We have found that in the presence of the monodentate ligand cysteine the characteristic oxygen radical signal in the EPR spectrum obtained after illumination of TiO2 is not observed; instead we observed an EPR spectrum mainly composed of g-factors of 2.004 and 2.022 (Figure 1d). The appearance of the new signal in the presence of cysteine shows that the surface environment has changed after addition of ligands and that the hole trapping sites are changed by cysteine modification of the TiO2 surface. These results indicate that cysteine is strongly bound to the colloid surface and passivates the surface states which act as hole traps. Surface Structure of Cysteine-Modified TiO2. Results obtained by infrared spectroscopy of a dried colloid sample suggest that at pH 4 the carboxyl group of cysteine is involved in the binding of cysteine to the TiO2 surface (Figure 2). This is suggested by the change in the vibrational modes of the carboxyl group in cysteine-modified TiO2 (middle curve) compared to pure cysteine (bottom curve). In contrast, the stretching vibration of the SH group (∼2550 cm-1) was not affected by adsorption. The infrared spectrum of cysteine (zwitterion form) has an antisymmetric stretching vibration at 1610 cm-1 for the carboxyl group.17,18 The energy of this
4540 J. Phys. Chem., Vol. 100, No. 11, 1996
Rajh et al.
SCHEME 1: Schematic Presentation of Structure and Electron Transfer Reactions in (a) TiO2, (b) TiO2/Cysteine, and (c) TiO2/Cysteine/Pb2+ in the Presence of Methanol
a The first column describes the structure, while the second and third columns describe electron transfer reactions at 8 and 200 K as detected by EPR spectroscopy following 308 nm irradiation at 77 K.
1610 cm-1 band decreases to approximately half of its intensity in free cysteine. We have observed similar changes following adsorption onto TiO2 colloids of related polydentate ligands that contain carboxylic acid groups.20 This is consistent with the previous observations of binding via carboxylic acid groups to the TiO2 surface in acetate, oxalate, and phthalate solutions.21 From the infrared spectrum we propose the following three equilibrium structures for cysteine bound to TiO2: O C
Ti –O
–O
NH2 CH
CH2
SH
NH2 CH
C
Ti
CH2
SH
O –O
NH2 C
Ti
CH
CH2
SH
–O
Figure 2. Cysteine adsorption on 50 Å TiO2 colloid. Infrared spectra of dried samples of (bottom) 0.1 M cysteine at pH 4; (middle) 0.1 M cysteine-modified 0.3 M TiO2 colloid at pH 4, dried and thoroughly washed with CH3OH; (top) 0.1 M cysteine-modified 0.3 M TiO2 in the presence of 0.1 M Pb(NO3)2 at pH 4, dried and thoroughly washed with CH3OH.
vibration changes after adsorption onto the TiO2 colloid, and new bands at 1520 and 1380 cm-1 (shoulder) appear. These are characteristic of antisymmetric and symmetric vibrations of carboxylate salts (-COOM).18,19 Also after adsorption the
The different C-O bond orders for these structures contribute to the two modes of vibration of adsorbed cysteine. The same features of the IR spectrum were previously found for metal xanthates where metal ions were coordinated with two sulfur atoms from the -CS2 group.22 On the basis of these results obtained with IR spectroscopy, we propose a structure for the cysteine-modified small particle TiO2 colloids which is shown in Scheme 1b. Hole Trapping Sites. We have found by IR spectroscopy that oxygens from the carboxyl group of cysteine are covalently
Surface Modification of TiO2 Colloids
J. Phys. Chem., Vol. 100, No. 11, 1996 4541
linked to the TiO2 lattice. Thus, it might be expected that the carboxyl group can act as the initial trapping site for photogenerated holes, followed by transfer of the holes within the cysteine molecule. We have observed this hole transfer following illumination at low temperatures (77 K). The first EPR spectrum we observe for trapped holes is composed of g-factors 2.004 and 2.002 and the broad shoulder at g ∼ 2.035 (Figure 1d). The same signal, with g-factors 2.004 and 2.022, was observed when the TiO2 surface was modified with S-methylcysteine (Figure 1b) or with deuterium-substituted cysteine (cysteine-3,3-d2, Figure 1c). Thus, we attribute the observed spectrum composed of g ) 2.004 and 2.022 signals to the carboxyl radical of cysteine:
h+ + (TiO2)nTiIVO2CCH(NH2)CH2SH f (TiO2)nTi+(IV)‚‚‚O˙ OCCH(NH2)CH2SH (3) The Q-band spectrum of cysteine-modified TiO2 is composed of the same g-factors, showing that the splitting is not due to the hyperfine splitting but to the g-tensor anisotropy, suggesting the carboxyl radical as the primary center for hole trapping. Also, the literature data for the g-tensor of the carboxyl radical23 support this interpretation. The broad shoulder at lower field range in cysteine-modified TiO2 (Figure 1c) was affected by deuteration of the methylene group adjacent to the -SH group. These results suggest that this shoulder may come from the contribution of the small amount of holes trapped at the R-carbon bound to the amino group. The trapping at these sites may come from the small amount of cysteine bound to TiO2 through the mercapto group. However, due to the small intensity of the signal, the structure could not be resolved. When the temperature is increased to 150 K, further hole transfer occurs. We find that the characteristic EPR signal for holes trapped on the carboxyl group (g ) 2.022 and 2.004) disappears, and a new spectrum composed of g-factors of 2.052 and 2.021 appears (Figure 3a). We attribute this spectrum to a sulfur-centered radical,24 indicating that at 150 K the holes are transferred within the bound cysteine molecule to form a radical frequently detected in the radiolysis or photolysis of organosulfur compounds and characteristic of sulfur-centered radicals containing two sulfur atoms (RS‚-S(H)R-)25,26
Figure 3. EPR (X-band) spectra of degassed aqeuous solutions of (a) TiO2 colloids (0.2 M) irradiated with 308 nm excimer laser at 77 K in the presence of 0.1 M cysteine, pH 3.5 (- - -), recorded at 4.2 K; (s) temperature raised to 150 K and recorded at 4.2 K and spectrum intensity increased by a factor of 2 in order to clearly show features of the S-centered radical; (b) the spectrum obtained following γ-radiolysis of N2O saturated solution of 0.01 M cysteine at 77 K (0.263 Mrad) recorded at 8 K; (c) the spectrum obtained after illumination at TiO2 colloids (0.2 M) modified with 0.1 M S-methylcysteine, pH 3.5 (- - -) recorded at 8K; (s) recorded at 150 K and spectrum intensity increased by factor of 2 in order to clearly show features of the carboxyl radical.
scavengers such as N2O. Hydroxyl radicals react with cysteine via a hydrogen abstraction reaction forming sulfur-centered radicals that associate with unreacted cysteine molecules to form RS‚-S(H)R species:26
OH‚ + -O2CCH(NH2)CH2SH +
h+ + (TiO2)nTiIVO2CCH(NH2)CH2SH +
-
O2CCH(NH2)CH2SH f -O2CCH(NH2)CH2S‚-
-
O2CCH(NH2)CH2SH f (TiO2)nTiIVO2CCH(NH2)CH2S‚-S(H)CH2CH(NH2)CO2- + H+ (4)
No EPR signal under the same conditions for the TiO2 colloid in the absence of cysteine at 150 K was observed.9 The spectrum for the sulfur-centered radical was also obtained after γ-radiolysis of an N2O-saturated 0.01 M cysteine at 77 K (Figure 3b). During γ-radiolysis at 77 K only OH‚ and H‚ radicals are formed in ice:27
H2O Df H2O+ + e-
(5)
H2O+ + e- f H2O* f H‚ + OH‚
(6)
H2O+ + H2O f OH‚ + H3O+
(7)
H3O+ + e- f H‚ + H2O
(8)
Trapped electrons are formed in negligible yield28 and can be converted into OH‚ radicals in the reaction with electron
S(H)CH2CH(NH2)CO2- + H2O (9) H‚ radicals recombine very fast, so that the signal for the trapped H atom is observed only at temperatures well below 77 K.27 The redox potental for the oxidation of cysteine to form a sulfur radical intermediate requires the potential of the photogenerated hole to be more positive than +0.92 V vs NHE.29 After the formation of sulfur-centered radicals in the case of photolysis of cysteine-modified TiO2, the cysteine radicals most probably undergo dimerization with free cysteine molecules present in the solution to form RSSR•- radicals (E°(RSSR•-/ 2RS-) ) +0.65 V vs NHE)29 at temperatures higher then 150 K. Thus, illumination of a cysteine-modified TiO2 colloid results in stabilization of charge separation where holes are involved in the oxidation of two cysteine molecules into cystine, and electrons are accumulated on the TiO2 particles. When the sulfur atom was blocked with a methyl group (Smethylcysteine), the formation of the sulfur-centered radical was not observed following either illumination of surface-modified TiO2 colloids (Figure 3c) or γ-radiolysis experiments. Instead,
4542 J. Phys. Chem., Vol. 100, No. 11, 1996 SCHEME 2: Coordination of Surface Ti Atoms in Small Particle TiO2; Thick Dashed Lines Denote Bonding to TiO2 Lattice
illumination of S-methylcysteine resulted in the formation of the carboxyl radical at all investigated temperatures. At 150 K (Figure 3c) the signal associated with the carboxyl radical did not change (g ) 2.004 and 2.022). Thus, the system did not undergo further stabilization of charge separation with increased distance between charges (Figure 3c). The origin of the signal at g ) 1.997 is not yet understood, although the analysis of the area under the curves suggests that it should be associated with trapped electrons. Electron Trapping Sites. The presence of cysteine also affects the electron trapping sites. While the narrow EPR signal at g ) 1.988 due to the electron trapping associated with the Ti(III)latt centers in the particle interior7 is not affected with the addition of cysteine, surface-trapped electrons become trapped on two distinct surface trapping sites (gA ) 1.958 and gB ) 1.934) after surface modification. The signal from untrapped, conduction band electrons is too broad to be observed. The two asymmetric EPR signals are obtained for trapped electrons in the presence of cysteine and have been reported previously for TiO2 colloids in the presence of methanol.10 In the latter case it was shown that these two signals belong to two distinct species which decay with different kinetics at different temperatures. We interpret these results in terms of cystal field theory. It is generally assumed that Ti ions are found in a tetragonally distorted octahedral crystal field.30,31 The g-factor of the EPR signal will depend on the extent of tetragonal distortion. In the small particle TiO2, Ti(III) centers are coordinated either (i) with lattice oxygen atoms only, with little tetragonal distortion (interstitial Ti centers), or (ii) with bound OH groups at the surface and oxygen atoms from the lattice, thus having a higher extent of tetragonal distortion (surface Ti centers). (See Scheme 2, structure I for definitions of these sites.) The g-factor in the transition metal ion complexes is dependent on the following parameters
g0 ) 1/3(g⊥ + g|) ) 2.0023 -
4λ 8λ 3(δ + λ/2) 3∆ 5λ2 (10) 3(δ + λ/2)2
where δ and ∆ are the separations of orbital energies due to the tetragonal and octahedral components, respectively, and λ is a spin-orbit coupling constant. While the g-factor of the interstitial Ti(III) centers is not affected when the TiO2 surface is modified with cysteine, the g-factor for surface Ti(III) centers changes. This shift is probably the result of the change in the axial crystal field that becomes stronger than in the OH-coordinated structure, and the covalent character of the bond is enhanced. Because of this covalent character of coordination bonding of the carboxyl group, the g-factor of the EPR signal increases due to the
Rajh et al. reduced spin-orbit coupling constant λ (eq 10).31 We propose two surface structure that contribute to the surface Ti(III) signal after surface modification. In the case where two surface bonds of Ti are bound to oxygen from a cysteine molecule (Scheme 2, structure II), the crystal field does not become significantly distorted and the small shift (∆g ) 0.010) from the g-factor of hydroxyl group coordinated Ti(III) centers is observed. When all four surface groups are replaced with cysteine (Scheme 2, structure III), the crystal field becomes more tetragonally distorted due to the larger extent of covalent bonding. Thus, the shift of 0.034 is observed, and the value of the g-factor becomes close to the value reported for Ti(III) ions chelated with oxygen atoms from carboxyl groups (g ≈ 1.956).32 Therefore, we associate the two signals for surface Ti(III) centers to the presence of two distinct chelated Ti(III) species which exist at the colloid surface. We attribute one to the case of one adsorbed cysteine molecule per Ti atom (Scheme 2, structure II) having the EPR signal at g ) 1.934 and the second to two adsorbed cysteine molecules per Ti atom with the EPR signal at g ) 1.958 (structure III).
Photodeposition of Lead Surface Structure. Upon addition of lead ions into solutions with surface-modified TiO2 colloids, a new complex of cysteine with the heavy metal was observed with IR spectroscopy (Figure 2). Lead ions bind with cysteine in a strong 1:1 complex (Pb2+/ cysteine) in which cysteine acts as a tridentate ligand.33 However, the IR spectrum of dried cysteine-modified TiO2 colloids containing lead ions suggests that lead is chelated with the carboxyl and mercapto groups of cysteine, so that in this TiO2/cysteine/Pb2+ system cysteine acts as a bidentate ligand. The IR spectrum of the TiO2/cysteine/Pb2+ system is composed of bands characteristic for the symmetrical stretching vibration of the linear O-C-O vibrations34 at 2400 and 1405 cm-1 (Figure 2, top). The stretching vibration of SH disappears. These results suggest that cysteine chelates lead ions into a sixmembered ring (see Scheme 1c), as both the carboxyl and mercapto vibrations are affected. The symmetrical vibration of the chelate has a high degree of coupling with the symmetrical stretching of the carboxyl group which in this way becomes enhanced compared to the asymmetrical stretching. Thus, disappearance of the asymmetrical stretching modes (15001650 cm-1) indicates formation of a symmetrically resonant group where the double-bond character is lost and the carboxyl group is resonating between surface Ti and Pb atoms. The carboxyl group bridges Ti and Pb ions (Ti-O-C-O-Pb), and in this way Pb becomes linked to the particle surface and becomes a continuation of the particle lattice. Hole Trapping Sites. Illumination of the TiO2/cysteine/Pb2+ system resulted in the formation of a radical having a relatively intense EPR signal with g ) 2.004 (Figure 4), possibly due to hole trapping on excess cysteine present in the solution and/or axially symmetrical lattice defects associated with trapped holes at the sites with tetragonal symmetry (O•-latt, gx ) gy).9,35 This symmetry is required because the g-tensor component of 2.007 for surface asymmetrical oxygen on TiO2 colloid is not observed. It should be noted that illumination of this system did not result in the formation of a sulfur-centered radical at 150 K most likely because the lone pair electrons of sulfur that participate in oxidation are bound to the lead ion. In the absence of excess cysteine, the holes were trapped on the counter ion of the lead salt. In the case of acetate ions, the EPR signal of methyl radical (four lines, splitting constant aCH3
Surface Modification of TiO2 Colloids
J. Phys. Chem., Vol. 100, No. 11, 1996 4543 chelation with two cysteine molecules (Scheme 2) disappeared, while the signal for surface-trapped electrons at the sites attributed to chelating with one cysteine molecule increased four times relative to the trapped hole signal.
Ti(III)surf III f Ti(III)surf II
(13)
These results indicate that the transition from surface state III to surface state II is thermodynamically favorable, indicating that the redox potential of the trapped site coordinated with one carboxyl group is less negative than the redox potential of the site coordinated with two carboxyl groups simultaneously (Scheme 3). Upon an increase of the temperature to room temperature for several minutes, all photogenerated electrons are scavenged by metal ions O
Pb
2(TiIIIlattO2)nTiIVOCCH(NH2)CH2S O
Figure 4. EPR (X-band) spectra of degassed aqueous TiO2 colloids (0.2 M) in the presence of cysteine (0.1 M) and Pb2+ ions (5 × 10-2 M) irradiated with 308 nm excimer laser at 77 K recorded at different temperatures.
SCHEME 3: Energy Level Diagram of the Observed Electron Trapping Sites
) 23 G) is observed as a result of acetate oxidation by the photogenerated holes.36 Similar effects on illumination of a TiO2 suspension in the presence of acetate ions were previously observed.8 Electron Trapping Sites. The EPR signal for trapped electrons did not change at 8 K after the addition of lead ions, and the electrons were trapped on lattice and/or surface Ti atoms according to the equation: O ecb–
+
O
Pb
(TiO2)nTiIVOCCH(NH2)CH2S
Pb
(TiIIIlattO2)nTiIVOCCH(NH2)CH2S O or
Pb
(TiO2)nTiIIIsurfOCCH(NH2)CH2S
(11)
At 120 K, however, the relative intensity of the signal for lattice type Ti(III) trapped electrons decreased, and surface-trapped Ti(III) electrons with g ) 1.958 (associated with structure III) and g ) 1.934 (associated with structure II) increased.
Ti(III)latt f Ti(III)surf
(12)
We can conclude from this that the electron transfer from the interior of the particle to surface-trapped states occurs first (Scheme 3). When the temperature was raised to 200 K, the signal for the surface-trapped electrons at the sites attributed to
Pb
2(TiO2)nTiIIIsurfOCCH(NH2)CH2S 2(TiO2)nTiIVO2CCH(NH2)CH2SH + Pb° (14)
and precipitation of metallic lead observed. Formation of metallic lead was identified by a Pb-containing dark brownish-gray precipitate formed after steady state illumination of the TiO2/cysteine/Pb2+ system. Because Pb°, PbO2, and PbS all form dark precipitates, we have analyzed the precipitate to distinguish between these possible products. The precipitate can be oxidized by hydrogen peroxide (E°(H2O2/ OH-) ) 0.88 V) with the formation of colorless Pb2+. Addition of sodium borohydride (E°(NaBH4/NaBH3, H+) ) -0.827 V) did not affect the precipitate, indicating that the precipitate cannot be reduced any further. It should be noted that PbO2 could not be reduced directly to metallic Pb because addition of sodium borohydride to the solution of Pb2+ ions in the presence of TiO2 does not produce metallic lead. The possibility of forming PbS is ruled out by the following experiments. Formation of PbS requires generation of HS- from cysteine. Illumination of cysteine-modified TiO2 did not lead to the reduction of cysteine which would result in HS- formation but led to the accumulation of trapped electrons having a broad optical absorption band at λmax ≈ 700-800 nm. Current Doubling Mechanism. We have found that the yield of trapped electrons is enhanced after the addition of methanol (Figure 5), which is an effective hole scavenger (E°(CH3OH/‚CH2OH) ) +1.2 V).37 It has been shown previously that electrochemical oxidation of methanol results in the formation of the electron-donating species ‚CH2OH(E°(‚CH2OH/CH2O) ) -0.95 V).38 The net effect is that from one photon two electrons are formed, and that phenomenon is known in electrochemistry as a current-doubling effect.39 The observation by EPR of current doubling from irradiation of colloidal TiO2 systems containing methanol was reported previously.10 In the presence of methanol, the EPR signal of the lead/cysteinemodified TiO2 colloid at 8.2 K (Figure 5a) is composed of a partially obscured set of triplet lines with separation of ∼18 G and a set of doublet lines with 130 G separation (arising from the methanol radical (‚CH2O(H)) and formyl radical (‚CHO), respectively).10,40 The signals associated with trapped electrons are those due to Ti(III) in the bulk lattice (g ) 1.988) and of Ti(III) at the surface with g ) 1.958 and 1.934. Thus, immediately after charge separation the holes are transferred to
4544 J. Phys. Chem., Vol. 100, No. 11, 1996
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Figure 6. Dependence of the Fermi level of TiO2 particles on the number of injected electrons from the monovalent cation radical of the methyl viologen zwitterion. Insert: Oscilloscope traces obtained (a) in 0.08 M TiO2 and (b) in 0.07 M TiO2 colloids modified with 2-mercaptopropionate ions.
Figure 5. EPR (X-band) spectra of degassed aqueous TiO2 colloids (0.2 M) in the presence of cysteine (0.1 M), Pb(CH3COO)2 (5 × 10-2 M), and methanol (1 M), irradiated with 308 nm excimer laser at 77 K recorded at different temperatures shown in the figure.
adsorbed methanol which is oxidized to the methanol radical where
h+ + (TiO2)nTiIVO‚‚‚HOCH3 f (TiO2)nTiIVO‚‚‚HOCH2‚ + H+ (15) methanol forms a hydrogen bond with surface oxygens. The most probable site for hydrogen bond formation is at the oxygen that bridges two surface Ti(IV) atoms and has a free electron pair, since the majority of surface OH groups are replaced with cysteine molecules41 (see Scheme 1c). As photogenerated holes are transferred to methanol (E°(CH3OH/‚CH2OH) ) +1.2 V) rather than to cysteine molecules (E°(RSSR•-/2RS-) ) +0.65 V), cysteine acts as a bridging agent from the colloid particle to lead ions. After complexation with lead ions, cysteine loses its oxidizing abilities and becomes an electron-accepting species. In the absence of Pb2+, illumination of cysteine-modified TiO2 in the presence of methanol resulted in oxidation of cysteine with carboxyl radical formation (eq 3), and the EPR signal of methanol radical was not observed. The large negative potential of the methanol radical induces electron injection into colloidal TiO2 at 120 K (Scheme 3) with formation of surface-trapped electrons and formaldehyde:
(TiO2)nTiIVO‚‚‚HOCH2‚ f (TiO2)nTiIIIO + OCH2 + H+ (16) Consequently, the yield of electrons is doubled. This is demonstrated in Figure 5b where the signal intensity for Ti(III)surf at g ) 1.958 approximately doubles compared to the signal in Figure 5a. This spectrum disappears at 200 K due to the reduction of Pb2+. It should be noted that direct reduction of Pb2+ ions was not observed in homogeneous solutions due to the negative potential of one-electron reduction of lead ions.38 Effects of Cysteine Modification on TiO2 Photoreduction Chemistry. The reduction of Pb2+ ions in TiO2 aqueous suspensions was not observed previously.42,43 Thus, we have considered four possible ways that cysteine can modify the properties of TiO2 colloids allowing the reduction of Pb2+ ions: (1) electron accumulation, (2) strong adsorption of lead ions, (3) modification of lead ion reduction potentials after
complexation, and (4) the modification of the redox properties of TiO2 itself. We have found that illumination of cysteine-modified TiO2 leads to the accumulation of trapped electrons, as the holes are effectively scavenged by cysteine molecules which are probably oxidized to cystine. In this way accumulated electrons can be used for subsequent simultaneous injection of multiple charges, and the reduction of lead does not proceed via a one-electron reduction process (E°(Pb2+/Pb+) ) -1.0 V vs NHE), but a twoelectron reduction of lead ions with much lower reduction potential becomes possible. We have also found that there is stronger adsorption of Pb2+ ions on cysteine-modified particles as compared to neat TiO2 colloids (three times) so that the efficiency of electron transfer from the Ti(III) centers to cysteine-bridged lead ions is significantly enhanced. From cyclic voltammograms we have found that the redox potential for the two-electron transfer process in the Pb/cysteine complex becomes more negative (E°(Pbcys+/Pb°cys-) ) -0.252 V vs NHE compared to E°(Pb2+/Pb°) ) -0.126). The flat band potential of TiO2 (Vfb) at pH 3 is -0.240 V vs NHE15 and more positive than the potential for precipitation of metallic lead from the Pb2+/cysteine complex. This result indicates that the position of the TiO2 Fermi level also changed with surface modification. Thus, we have examined the change of the position of the Fermi level in modified and unmodified TiO2 colloids with a pulse radiolysis technique44 and found that the position of the Fermi level in modified colloids is shifted by at least negative 0.10 V. In this experiment the monovalent zwitter radical ion of methyl viologen ZW- (E°(ZW°/ZW-) ) -0.443 V vs NHE) is formed during the radiolysis process. This radical ion injects the charge into unmodified TiO2 colloids but cannot inject a charge into the surface-modified TiO2 colloids (Figure 6). Thus, the position of the Fermi level must be more negative than the redox potential of the solution of the highest concentration of ZW- which was investigated (Vfb
