12512
J. Phys. Chem. B 2004, 108, 12512-12517
Efficient, Rapid Photooxidation of Chemisorbed Polyhydroxyl Alcohols and Carbohydrates by TiO2 Nanoparticles in an Aqueous Solution Ilya A. Shkrob,* Myran C. Sauer, Jr., and David Gosztola Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: May 25, 2004; In Final Form: June 14, 2004
Time-resolved transient absorption spectroscopy has been used to study electron dynamics in aqueous anatase nanoparticles (pH ) 4, 4.6 nm diameter) in the presence of hole scavengers: chemisorbed polyols and carbohydrates. These polyhydroxy compounds are rapidly oxidized by the holes on the nanoparticles; 5060% of these holes are scavenged within the duration of 3.3 ns fwhm, 355 nm excitation laser pulse. The scavenging efficiency rapidly increases with the number of anchoring hydroxyl groups and varies considerably as a function of the carbohydrate structure. A specific binding site for the polyols and carbohydrates is suggested that involves an octahedral Ti atom chelated by the -CH2(OH)-CH2(OH)- ligand. This mode of binding accounts for the depletion of undercoordinated Ti atoms observed in the XANES spectra of coated nanoparticles. We suggest that these binding sites trap a substantial fraction of holes before the latter descend to surface traps and/or recombine with free electrons. The resulting oxygen hole center rapidly loses a C-H proton to the environment, yielding a metastable, titanium-bound, ketyl radical.
1. Introduction Nanoparticles and nanocrystalline films of anatase (TiO2) find numerous applications in photovoltaics and photocatalysis (e.g., refs 1 and 2). Photoexcitation of these nanoparticles yields electron-hole pairs whose recombination competes with trapping of the charges by defects.3-7 In this study, we report rapid, efficient scavenging of the holes by polyhydroxyl alcohols and sugars which are chemisorbed at the aqueous TiO2 nanoparticle surface. This study continues the themes addressed in the previous work from our laboratory. 8 It has been recognized that photoexcitation of TiO2 nanoparticles yields at least two kinds of surface-trapped holes.9-12 According to EPR spectroscopy,9,10 both of the hole centers are O 2p radicals, such as TiIV-O• (ref 9) and adsorbed hydroxyl radicals, HO•ads.10 According to the product analyses of Ishibashi et al.12a and spin-trapping experiments of Nosaka et al.,12b the quantum yield for generation of hydroxyl radicals is small (ca 0.7% of that for the iodide-oxidizing hole); however, this species is very important for photocatalysis by TiO2 nanoparticles in aqueous solutions, as it is more reactive toward hydrogen donors, such as monohydroxyl alcohols, than other hole centers. Gao et al.11 demonstrated that only HO•ads oxidizes methanol and other monohydroxyl alcohols in aqueous TiO2 solutions, whereas the predominant hole center is unreactive toward this substrate and decays via recombination with the electron.8,13 Importantly, this conclusion pertains to aqueous solutions of the nanoparticles only. Time-resolved IR studies of Yamakata et al.14,15 showed that at the rutile surface exposed to methanol and 2-propanol vapor, the holes are scavenged in less than 50 ns, which was the time resolution of their setup. Using time-resolved near-IR spectroscopy, Yoshihara et al.16 observed this rapid hole scavenging (in less than 10 ns) for nanocrystalline TiO2 films in contact with liquid methanol. Warman et al.17 observed this scavenging for suspensions of TiO2 particles in alcohol-alkane * Corresponding author. Tel 630-252-9516, FAX 630-252-4993, e-mail:
[email protected].
mixtures, by means of time-resolved microwave conductivity. Apparently, only in aqueous solutions do these monohydroxyl alcohols scavenge the holes inefficiently. By contrast, polyhydroxylated compounds, such as poly(vinyl alcohol) (e.g., refs 18 and 19) and glycerol,8 are very efficient hole scavengers for TiO2 nanoparticles, even in an aqueous solution. In the present work, photooxidation of polyols and sugars by aqueous TiO2 nanoparticles is studied in a systematic fashion. 2. Experimental Section The details of the laser spectroscopy setup are given elsewhere.8,20 Transient absorbance was observed following 355 nm laser photoexcitation of 2.4 × 10-4 M aqueous solution of 46 ( 5 Å diameter anatase nanoparticles (1400 units of TiO2 per particle) at pH ) 4. This solution was prepared as described in refs 2 and 21 and then aged for a year. The particle size and crystallinity were characterized by transmission electron microscopy prior to use. Polyhydroxyl alcohols and sugars of the highest available purity were obtained from Aldrich and used as received. The 355 nm, 3.3 ns fwhm, laser pulse was derived from the third harmonic of a Nd:YAG laser (Quantel Brilliant). The analyzing light from a pulsed Xe arc lamp crossed at 45° with the 355 nm laser beam inside a 1.35 mm optical path flow cell with suprasil windows. The typical fluence of 355 nm photons at the sample was 0.05 J/cm2, and the optical density (OD) of the photolysate at 355 nm was 0.8. The transmitted light was analyzed using either a grating monochromator or a set of 10 nm fwhm interference filters. A fast Si photodiode (350 MHz) was used to detect the transient absorbance (∆ODλ) at wavelength λ < 1.1 µm; for detection at longer wavelengths, fast InGaAs and Ge photodiodes were used. The photodiode signal was amplified and sampled using a 400 MHz digital signal analyzer. The overall response time of the detection system was ca. 3 ns. The measurement of the quantum yield of the electron was carried out as described in ref 20.
10.1021/jp0477351 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/24/2004
Photooxidation of Chemisorbed Polyols and Carbohydrates
J. Phys. Chem. B, Vol. 108, No. 33, 2004 12513
To prevent the buildup of permanent electron absorbance during the laser photolysis of the polyol solutions, the sample was saturated with oxygen and flowed at 1.5 cm3/min during the photolysis (the repetition rate of the laser was 1-2 Hz). Control experiments using N2-saturated solutions (with no polyols) showed that oxygen does not react with the electron on the microsecond scale,8 i.e., its only function is to oxidize the electrons between the consecutive laser shots. 3. Results 3.1. Transient Absorption Spectroscopy and Scavenging Dynamics. Typical transient kinetics at 900 nm in photoexcited aqueous TiO2 nanoparticles are shown in Figures 1a and 1c (trace (i)). Very similar decay kinetics (save for the normalization factor) were observed at other wavelengths of the analyzing light in the near-IR (0.85-1.35 µm). 8 Only the electron absorbs at these long wavelengths.3,8,18,19,22-25 Assuming the molar absorptivity of these electrons to be 700 ( 50 M-1 cm-1 at 800 nm (which is within 10% of the 900 nm absorptivity),8,16,23,24 ca. 1-1.2 electron-hole pairs per nanoparticle were present in the reaction mixture at the end of a 0.04 J/cm2 (10-7 Einstein/cm2) excitation pulse (t ) 0+). The quantum yield for the electron, as determined from the prompt absorption signal at t ) 0+, was ca. 0.2, i.e., recombination within the excitation pulse was relatively inefficient (near-unity quantum yield is commonly assumed for the initial charge separation).5,13 The observed quantum yield can be compared with the limiting quantum yields for I2- formation in aqueous TiO2 solutions containing I- (an efficient hole scavenger) obtained by other workers: 0.16,25 0.2,26 0.42,19 and 0.9.13 The latter estimate of 0.9 was obtained at very low fluence of the excitation photons (1000) 5 electron-hole pairs were generated per nanoparticle. At such high generation rates, the recombination of trapped electrons and holes occurs on the picosecond time scale, and only a small fraction of the initially generated pairs survives into the nanosecond regime. Even though this recombination was slowed due to the low charge density, >80% of the electrons decayed in less than 100 ns, by intraparticle recombination.8 A small fraction of these electrons decays on a slower time scale, over many decades in time (to at least 1 ms), exhibiting dispersive power law kinetics (see the double logarithmic plot in Figure 1c).8 This recombination becomes faster on the nanosecond time scale (Figures 1a and 1c). Between 5 and 300 ns, this kinetic trace can be approximated by a sum of two exponentials with time constants of 6.5 and 73 ns (these time constants vary slightly with laser fluence). Since the duration of the 355 nm excitation pulse (3.3 ns fwhm) is shorter than either one of these time constants, relatively few electrons decay within the duration of the excitation pulse, i.e., the latter integrates over the entire population of these electrons. As there is little decrease in ∆OD900 per decade of time once the power law regime sets in, the ratio of the t ) 300-350 ns absorbance and the prompt absorbance at t ) 0+ is considered as a measure of the fraction of the electrons that “escape” the recombination with the hole on the sub-microsecond time scale. Qualitatively, the spectral and kinetic changes observed upon the addition of polyols (C2-C6) and carbohydrates to the
Figure 1. Decay kinetics of electron absorbance at 900 nm, following short-pulse 355 nm laser excitation of aqueous anatase nanoparticles. Traces (i) in Figures 1a and 1c are from the solutions without the hole scavenger. The initial increase in the optical density follows the time profile of the photoexcitation laser pulse. The straight line in Figure 1c (where the kinetics is given on a double logarithmic scale) corresponds to the power law decay. In the presence of a hole scavenger (D-arabitol), the decay kinetics flatten out and the electron absorbance at 300-400 ns increases 5-fold, whereas the prompt absorbance increases by 5% only. The molar concentration c of the scavenger was (from bottom to the top, as indicated by an arrow): 0, 0.02, 0.041, 0.084, 0.26, 0.92, and 1.8 mol/dm3. In Figure 1b, the traces δ∆OD900(t) ) ∆OD900(t) - ∆OD900(t ) 350 ns) normalized at t ) 0+ for the same series are plotted on the same graph (different kinetics are shown with different colors). It is seen that all of these normalized kinetics are the same within the experimental error.
nanoparticle solution do not depend on the hole scavenger structure and closely resemble those observed by Shkrob and Sauer8 for glycerol (C3). We refer the reader to that study for more detail, as it provides justification for the kinetic analyses described below. At λ ) 900 nm, the experimental decay kinetics ∆ODλc(t) of the transient absorbance in the presence of the hole scavenger obeys an empirical relation (t > 0+ )
∆OD900c(t) ≈ f(c)∆ODλc)0(t ) 0+) + [1 - f(c)] ∆ODλc)0(t) (1) where c is the molar concentration of the hole scavenger,
f(c) ≈ f∞ Khc/(1 + Khc)
(2)
is the fraction of promptly scavenged holes, and Kh is the scavenging efficiency (Table 1). The same behavior was observed for all wavelengths λ between 850 and 1350 nm, i.e., across the entire spectral region where only electrons absorb in aqueous TiO2 nanoparticles.8 The time evolution of the IR absorbance from electrons at rutile surfaces exposed to alcohol vapor follows the same phenomenology as that given by eq 1.14,15
12514 J. Phys. Chem. B, Vol. 108, No. 33, 2004 TABLE 1: Hole Scavenging Efficiency Kh for Polyhydroxyl Alcohols and Sugars (Eq 2) in Aqueous Solution of TiO2 Nanoparticles at 25 °C polyola linear chain polyols 1,3-propandiol ethylene glycol (C2) glycerol (C3) meso-erythritol (C4) D-arabitol (C5) D-mannitol (C6) sugarsa inositol R-D-glucose D-fructose D-galactose D-glucal D-galactal R-D-lactose
Kh, M-1 0.33 ( 0.03 0.44 ( 0.04 2.3 ( 0.03 5.85 ( 0.85 25 ( 3.6 18.5 ( 2.5 9.64 ( 0.7 10.1 ( 0.35 33 ( 12 9.8 ( 3 1.5 ( 0.3 3.7 ( 0.9 27.1 ( 9.3
a See Scheme 1 in the Supporting Information for the chemical formulas of these sugars.
The empirical formula (eq 1) posits that λ > 800 nm kinetics observed in the presence of a polyol is given by a weighted sum of (i) a flat kinetics (∆ODλ(t) ) const) and (ii) the ∆ODλc)0 kinetics obtained for c ) 0 (i.e., without the hole scavenger; trace (i) in Figure 1a), with the weighting coefficients chosen so that the prompt absorbance ∆OD900c(t ) 0+) does not change ( 0+ obey eq 1 and no growth component due to the delayed conversion of the holes to electrons is apparent, electron transfer from the polyol-derived radical into the nanoparticle on the submicrosecond time scale can also be ruled out. 3.2. Scavenging Efficiency. In Figures 2a and 2b, the ratio ∆OD900c(t ) 315-370 ns)/∆OD900c(t ) 0+) (which approximates the fraction f(c) in eq 2) is plotted vs the molar concentration c of the hole scavenger. All these dependencies can be described by eq 2, and the scavenging constants Kh thus obtained are given in Table 1. Whereas all polyols and carbohydrates increased the 900 nm absorbance for t > 100 ns, addition of up to 50 vol. % of monohydroxyl alcohols (methanol, ethanol, 1-propanol, and 2-propanol) had no effect on the kinetics (see also refs 8 and 13). With the exception of D-arabitol (C5), for linear polyols the constant Kh is proportional to (n - 1)2, where n is the carbon
Shkrob et al. number, i.e., the scavenging efficiency rapidly increases with the number of hydroxyl groups per molecule. Having two hydroxyl groups in the 1,2-position (as in 1,2-ethanediol vs 1,3propanediol) increases Kh. Closing a polyol into a ring considerably decreases Kh (compare D-mannitol and inositol). This decrease can be accounted for by postulating that, in sugars, neighboring hydroxyl groups oriented to the same side of the ring result in higher scavenging efficiency than the same groups oriented to the opposing sides. The example of D-glucal and D-galactal (Table 1) suggests that the difference in the scavenging efficiency for these two orientations is ca. 2.5 times. As more neighboring hydroxyl groups are added, this difference becomes smaller (compare D-glucose and D-galactose, Table 1). The highest constants Kh were found for a disaccharide, D-lactose, and a furanose, D-fructose (Table 1). 3.3. XANES Spectroscopy of Surface Modified Nanoparticles. Given that hole scavenging is very rapid and structurespecific, it is likely that the reaction involves a scavenger molecule that is chemisorbed at the particle surface in a particular way. Recent XANES studies show that for small aqueous TiO2 nanoparticles, surface titanium atoms change the coordination from octahedral to square pyramidal.2,27,28 These undercoordinated sites exhibit enhanced reactivity toward bidentate ligands, such as catechol, that bind to these sites and change the coordination of Ti atoms back to octahedral.2 Since the active site of these ligands, -CH(OH)dCH(OH)-, closely resembles that of the polyols, -CH2(OH)-CH2(OH)-, an experiment was conducted to discern whether the polyols and carbohydrates also bind to undercoordinated Ti atoms at the nanoparticle surface. Titanium K-edge XANES spectra were obtained using the APS facility at Argonne. The details of the experiment and the analysis are given in refs 2 and 27. The typical Ti K-edge XANES spectra are shown in Figure 3. To obtain these spectra, polylols were added in a concentration sufficient to obtain the total coverage of Ti atoms at the particle surface, and the solutions were dried. The XANES signal from undercoordinated Ti sites is seen in Figure 3 as a sharp feature centered at 4.97 keV. 2,27 This feature is present in the TiO2 nanoparticles but absent in crystalline anatase. The polyol- and carbohydratecoated nanoparticles exhibit a 4.97 keV signal that is intermediate to these two extremes, suggesting that undercoordinated Ti atoms are partially converted to octahedral Ti atoms by the adsorbate. The efficiency of this “repair” decreases with the number of hydroxyl groups in the molecule. At full surface coverage, ethylene glycol (C2) and glycerol (C3) repair 85% of the undercoordinated surface sites, erythritol (C4) and fructose (C6) repair 54% of these sites, and D-arabitol (C5), D-glucose (C6) and D-glucal (C6) repair 43% of these sites. This observation suggests that extra hydroxyl groups may not assist in the “repair” of undercoordinated Ti atoms as efficiently as they do in smaller polyols (we stress that this observation refers to dried samples; the efficiency of binding in the aqueous solutions was not addressed in this XANES study). Still, our data indicate that, once at the surface of the nanoparticle, all polyols efficiently bind to undercoordinated Ti atoms in such a way that this atom becomes octahedrally coordinated. 4. Discussion Given the scarcity of structural data on the interaction between polyols and the surface of TiO2 nanoparticles in aqueous solution, we can only speculate on the mechanism for hole scavenging by these polyhydroxy compounds. As stated in section 3.1 and ref 8, no evidence for the occurrence of rapid
Photooxidation of Chemisorbed Polyols and Carbohydrates
Figure 2. The ratio ∆OD900(t ) 315-370 ns)/∆OD900(t ) 0+) plotted vs molar concentration c of the hole scavenger for (a) polyols and (b) carbohydrates (the same conditions as in Figure 1). The solid lines drawn through the symbols are the least-squares fits obtained using eq 2. The scavenging constants Kh obtained from these plots are given in Table 1. The polyols were (i) ethylene glycol, C2, (ii) glycerol, C3, (iii) meso-erythritol, C4, (iv) D-arabitol, C5, (v) D-mannitol, C6, and (vi) 1,3-propanediol. The carbohydrates were (i) inositol, (ii) R-Dglucose, (iii) D-galactose, (iv) fructose, (v) D-glucal, (vi) D-galactal, and (vii) R-D-lactose.
Figure 3. Titanium K-edge XANES spectra of surface modified 46 Å diameter TiO2 nanoparticles (dried aqueous solutions). The feature near 4.97 keV is from undercoordinated Ti atoms in the chemically active surface sites. Reference traces (i) and (x) are from uncoated, dried TiO2 nanoparticles and macroscopic anatase crystals, respectively. The surface of the aqueous nanoparticles was modified by addition of (ii) ethylene glycol, (iii) glycerol, (iv) meso-erythritol, (v) D-arabitol, (vi) D-glucal, (vii) R-D-glucose, (viii) fructose and (ix) R-D-lactose. Addition of these polyhydroxylated compounds results in the reduced 4.97 keV peak from undercoordinated Ti atoms at the nanoparticle surface due to the “repair” of the site by binding of the -CH2(OH)CH2(OH)- ligand to these atoms (a possible structure for the resulting complex is shown in Figure 4).
two-electron oxidation in polyol solutions of TiO2 nanoparticles was found in our experiments. Once the hole is scavenged by a polyol and the resulting hole center deprotonates, the product
J. Phys. Chem. B, Vol. 108, No. 33, 2004 12515 of this reaction (the ketyl radical)9 neither injects an electron back to the nanoparticle nor recombines with an electron present on the same nanoparticle (on the time scale of our experiment). The electron injection into the TiO2 nanoparticles from the ketyl radicals is known to be slow in the water bulk (e.g., for hydroxymethyl and propan-2-olyl, bimolecular rate constants, in terms of nanoparticle concentration, are 2 × 105 and 3 × 106 M-1 s-1, respectively).29 The electron injection that involves the ketyl radicals adsorbed at the TiO2 surface might be considerably faster. Still, the existing experimental data suggest that this reaction is slow even at the surface. Using time-resolved IR spectroscopy, Yamakata et al.15 observed the formation kinetics for the CdO stretch absorbance from acetone (which is formed via electron injection from propan-2-olyl) in the laser excitation of the rutile surface exposed to 2-propanol vapor. Whereas the change in the electron decay kinetics (due to hole scavenging) was instantaneous (
