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Enhancement of Photoexcited Charge Transfer by {001} Facet-Dominating TiO2 Nanoparticles Masato M. Maitani,* Keita Tanaka, Dai Mochizuki, and Yuji Wada* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan
bS Supporting Information ABSTRACT: The recent discovery of a synthetic method for chemically reactive {001} facetdominating TiO2 nanoparticles using hydrofluoric acid provided a new aspect of interfacial chemistry in photo catalysts, batteries, and photoelectrochemical cells using TiO2. We reveal the effects of the reactive {001} facet on the photoexcited charge transfer from organic fluorophores, 9-substituted anthracene derivatives (AnX; X = H and COOH) and tetracene, to TiO2 nanoparticles by differing the fraction of {001} facet. The kinetic analysis of the fluorescence quenching by TiO2 nanoparticles based on SternVolmer relation is employed to estimate the quenching rate constant as a function of the fraction of {001} facet. The results imply a significant enhancement of the photoexcited charge transfer from fluorophores to TiO2 nanoparticles by the reactive {001} facet with a factor of more than 10 in the quenching rate constant at maximum. SECTION: Surfaces, Interfaces, Catalysis
N
anosized metal oxide semiconductors have been actively studied for the past few decades, especially for their potential applications for photocatalytic reactions, photobreaching of toxic compounds, artificial photosynthesis, and photovoltaics.15 Among all metal oxide semiconductors, TiO2 is one of the most important materials because of the abundance of titanium as a natural resource, corrosion resistivity, transparency in the visible region, and nontoxicity. The kinetically favorable anatase crystalline phase is the most widely studied due to its ease of preparation, stability of the crystal up to ∼600 °C, and advantages for catalytic and electronic properties. The anatase crystal of TiO2 has a few fundamental low-index facet systems, such as {101}, {001}, {100}, {110}, and {103}.2 Typical anatase crystal of TiO2, including naturally grown crystals, consists of mostly the {101} facet due to the much greater stability of this surface than other surfaces due to its lower surface energy. Therefore the characteristics of TiO2 nanoparticles, in most cases, have been supposed to be attributed to the {101} facet, regradless of whether it is mentioned in the literature, since it had been extremely difficult to selectively synthesize other unstable surfaces, such as the (001) surface.2,6 Recently, however, Lu et al. proposed a novel synthesis to selectively increase the fraction of {001} facet and thus introduced a new area of study, the chemically reactive (001) surface, into the widely studied TiO2 nanoparticles.7 The proposed novel process is composed of the addition of a relatively large amount of hydrofluoric acid (HF) for hydrothermal growth of TiO2. Although a few groups also reported the follow-up work of {001} facet-dominating TiO2 preparation, most groups used the same procedure using HF.811 The role of HF in TiO2 synthesis was also suggested to be that the fluorine-termination of TiO2 surface r 2011 American Chemical Society
stabilizes the (001) surface more than the other facets and results in a larger fraction of intrinsically unstable (001) surface; the result of the first-principle calculations also supported this explanation.7 The typical fraction of {001} facet in reports varies from 40 to 80%, usually determined by the characteristic bipyramidal structure of particles revealing the trapezoidal (101) and rectangular (001) surfaces observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM).712 Since the surface energy of the {001} facet is higher than that of the {101} facet, recent reports indicated a few characteristic features of the {001} facet, a site-selective reaction, enhancement of reactivity, adsorption of molecules, and water dissociation with {001} facetdominating TiO2 nanoparticles1315 for application to photocatalytic reactions10,11 and photovoltaics.12,16 Additionally, a few theoretical investigations based on first-principle calculations also reported the characteristic reactivity of the {001} facet.17,18 Nevertheless, detailed analysis of the effect of surfaces in different facet systems on photoexcited charge transfer has not been experimentally studied, although a few reports have provided significant differences in electron transfer on different facet systems, such as {101} and {001},13,15 as the fundamental process dominating the chemical reactions and photoelectronic and photocatalytic processes. Herein we study the photoexcited charge transfer from fluorophores, 9-substituted anthracene derivatives (AnX; X = H and COOH) and tetracene, to TiO2 nanoparticles by analyses of fluorescence quenching as a function of the fraction of {001} facet in order to elucidate the effect of Received: August 25, 2011 Accepted: October 4, 2011 Published: October 04, 2011 2655
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Table 1. Synthesis Condition and Geometrical Factors of TiO TiO2a
69%(001)
56%(001)
47%(001)
30%(001)
HF concentration/wt%
43
27
12
0
fraction of {001} facetb/%
69
56
47
30
diameter (a-axis)b/nm
41.6 ( 8.8
27.7 ( 7.1
22.9 ( 4.2
12.0 ( 2.8
diameter (c-axis)b/nm
7.7 ( 2.5
8.2 ( 1.9
9.2 ( 2.7
8.1 ( 1.1
89
97
99
150
80
98
103
162
8.3
4.1
3.1
1
estimated surface areab/m2 3 g1 BET surface areac/m2 3 g1 Stotal/Stotal (30%(001))
Sample names are assigned based on the fraction of {001} facets determined by geometrical analysis in the SI, section S1-6. b Values are estimated from observed particle size by SEM and TEM as described in the SI, section S1-6. c BET surface area was determined by nitrogen adsorptiondesorption isotherms as described in the SI, section S1-3. a
Figure 1. SEM images of TiO2 nanoparticles with different fractions of {001} facet. Insets indicate the particle size distributions determined by measured values in SEM and TEM images (SI, section S1-2).
reactive {001} facet on the photoexcited charge transfer process as one of the fundamental processes on the TiO2 surface. The SternVolmer plot of the fluorescence quenching process reveals 10-fold enhancement in the quenching rate constant with the TiO2 nanoparticles with the greatest {001} facet fraction as compared to those with the least. We controlled the fraction of {001} facets by using the reported hydrothermal growth method with varying the concentration of HF solution (Table 1). All of synthesized TiO2 revealed a complete anatase phase determined by X-ray diffraction (Supporting Information (SI), section S1-1). The morphology of synthesized TiO2 nanoparticles was observed by a field emission scanning electron microscope (FE-SEM, S-5500, Hitachi, Japan) (Figure 1), and a transmission electron microscope (TEM, JEM-2010F, JEOL, Japan) (SI, section S1-2). Insets reveal the distribution of particle size in the a- and c-axes measured with ∼300 particles in SEM and TEM images. On the basis of an assumption of the anatase crystalline geometry with the reported ideal interplane angle between (101) and (001) to be 68.3°,2,3,611 the fractions of {001} facet of synthesized TiO2 nanoparticles were determined to be 69, 56, 47, and 30% (Table 1). As observed in
Figure 2. (a) Fluorescence spectra of AnH quenched by TiO2 nanoparticles in acetonitrile suspension. (b) SternVolmer plots of AnX (X = H and COOH) and tetracene (Tetra) quenched by TiO2 with {001} facet fractions of 69%(001) (filled plots) and 30%(001) (open plots). Inset indicates the same SternVolmer plots in a small range of TiO2 number density, ∼0.2 1017 particles/L. The number density of TiO2 in the colloidal suspension system in the x-axis is calculated from the weight concentration of the added colloidal suspension by applying the single particle weight of each TiO2 system based on the geometry analysis displayed above in order to use this value for the concentration of TiO2.
previous reports,8 the area fraction of {001} facet became larger by increasing the concentration of added HF solution in the hydrothermal growth process. The narrow distribution of particle size within ∼10% allows us to treat these particles as a monodispersive system; this is also supported by the fact that the surface area estimated from the averaged particle sizes revealed good correlation (within 10% error) with the BrunauerEmmett Teller (BET) surface area measured by nitrogen physisorption (Table 1 and SI, section S1-5). These results justify the fact 2656
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that prepared TiO2 nanoparticles have enough uniformity to be applied for following analyses with the averaged size representing particle geometry. The fluorescence spectra of 9-substituted anthracene derivatives (AnX (X = H and COOH)) and tetracene in the presence and absence of TiO2 colloids were utilized to examine the photoexited charge transfer process from the fluorophores to TiO2 nanoparticles in acetonitrile, as it is well studied for its luminescence characteristics and its quenching processes.1924 As well studied previously, all synthesized TiO2 particles revealed fluorescence quenching with all fluorophores (Figure 2a). Figure 2b shows SternVolmer plots of the fluorescence intensity (at λ = 401 nm (AnH), 465 nm (AnCOOH), and 475 nm (tetracene)) as a function of the number density of nanoparticles in the suspension, [TiO2], with {001} facet fractions of 69% and 30%. The plot clearly reveals that the more {001} facet-dominating TiO2 (69%(001)) shows much higher quenching capability than that with less {001} facet fraction (30%(001)). Furthermore, AnCOOH revealed the highest quenching slope as a function of the number density of TiO2 nanoparticles. The fluorescence process is expressed as the excitation of fluorophores, F, (eq 1) and following radiation (eq 2) and nonradiation (eq 3) processes. Additionally, the photoexcited charge transfer process appears in the presence of a quencher, Q, (eq 4). F þ hυ f F ð1Þ F f F þ hυ0
ð2Þ
F f F
ð3Þ
F þ Q f F þ þ Q •
ð4Þ
To provide a more precise comparison of the quenching rate constant, we consider the chemisorption and collision frequency between the fluorophores and TiO2 by taking the geometrical factors into account. In our case, the chemisorption occurs with AnCOOH as the fluorophores.2126 Therefore, the fluorescence would arise from both of the fluorophores chemisorbed on the TiO2 surface and dissolved in the solvent of the colloidal suspension. The fluorescence intensity of chemisorbed AnCOOH was confirmed to be negligible in a control experiment. Therefore the quenching rate constant, kqd, of fluorophores by TiO2 in the suspension system is derived from the SternVolmer relation with the fluorescence intensity ratio between that in pure solution and that in TiO2 colloidal suspension, I0/I, the concentration of fluorophores in pure solution, C0, that adsorbed on TiO2 after TiO2 addition, Csurf, the number density of TiO2, [TiO2], and time constant of fluorophores in pure solution, τ0 (eq 5; derived in SI, section S2); the term (C0 Csurf) is the decreased concentration of fluorophores dissolved in the solvent of the suspension after TiO2 addition, since the total amount of fluorophore is preserved. kqd ¼
ðI0 =IÞ fC0 =ðC0 Csurf Þg τ0 ½TiO2 3 fC0 =ðC0 Csurf Þg
ð5Þ
Here, we consider the geometrical factors of our system, the size effect of TiO2 nanoparticles on the fluorescence quenching reactions. The rate constant is proportional to the collision frequency, which is composed of the averaged relative speed of
Figure 3. (a) Compensated fluorescence quenching rate constants, kqdCom, derived from SternVolmer plots, which appears to be independent of the concentration of TiO2. (b) Compensated fluorescence quenching rate constants normalized at the value of 30%(001) as a function of the {001} facet fraction of TiO2 nanoparticles.
the reactants and the collision cross section (SI, eq S2-9). The relative speed of reactants, fluorophores and TiO2 would qualitatively decrease with the increase of the diameter of TiO2 nanoparticles as according to the EinsteinStokes equation (SI, eq S2-10), although the equation can be applied to spherical particles. By contrast, we observed the opposite effect: more {001} facet-dominating nanoparticles with larger diameter revealed higher quenching capability. This indicates that higher quenching capability with more {001} facet-dominating TiO2 cannot be attributed to the difference in the diffusion constants of TiO2 nanoparticles. To consider the collision cross section of fluorophores and TiO2, we used the ratio of surface area, Stotal/ Stotal (30%(001)), as the compensation factor of the quenching rate constant (SI, Table S1-1); the cross section is proportional to the surface area of a single TiO2 particle (eq 6; derived in the SI, section S2). kqdCom ¼ kqd =ðStotal =Stotal ð30%ð001Þ ÞÞ
ð6Þ
The compensated rate constants, kqdCom, and those normalized at the value of 30%(001) are plotted as a function of the TiO2 concentration in the experimental conditions (Figure3a) and the area fractions of the {001} facet (Figure 3b), respectively. The analysis treated here is justified by the results that the compensated rate constants in Figure 3a reveal constant values in the whole range of TiO2 concentration in our experiments. The significant increase of quenching rate constant is observed with a higher fraction of {001} facet by a maximum factor of ∼15 (69%(001) vs 30%(001)). This results can be attributed to the nature of the reactive {001} facet of anatase TiO2 contributing to the effective photoexcited charge transfer process. AnCOOH 2657
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The Journal of Physical Chemistry Letters revealed a higher quenching rate constant than AnH probably due to the interaction between COOH and the TiO 2 surface. 2326 In summary, the influence of a novel reactive {001} facet on the photoexcited charge transfer process from fluorophores to TiO2 nanoparticles was studied by fluorescence quenching in the TiO2 suspension system and analyzed based on the Stern Volmer relation. A significant enhancement of the photoexcited charge transfer process was observed with more {001} facetdominating TiO2 nanoparticles attributed to the highly reactive {001} facet. These results could acsribe the reported increases of efficiency in photovoltaics12,16 and photocatalysys10,11 using {001} facet-dominating TiO2 to a more effective photoexcited charge transfer process on (001) surface. A direct analysis of the charge-transfer process from photosensitizer molecules to {001}-facet TiO2 in a photovoltaic system is now being carried out for future work.
’ EXPERIMENTAL SECTION Hydrothermal growth procedures similar to those in previous reports were applied to synthesize the anatase TiO2 nanoparticles.7,8 Typically, tetrabutoxytitanium(IV) (Ti(OBu)4, 25 mL) and aqueous hydrofluoric acid solution (HF, 3 mL) were mixed in a dried Teflon-lined autoclave (100 mL capacity) under ambient conditions, and then the autoclave was tightened and kept at 180 °C for 24 h. The concentration of the HF solution was varied from 43 to 0 wt % to control the fraction of exposed {001} facet (Table 1). The white powdery products were washed with ethanol and distilled water several times by sonication followed by decantation. The defluorination was carried out by washing samples with aqueous NaOH solution (0.1M) and then rinsing with distilled water until pH 7.0 was achieved in order to remove fluorinated titanium products on the TiO2 surface (SI, section S1-7).7,8,12 After all cleaning procedures, the samples were dried at 300 °C for 0.5 h in atmosphere.27 The fluorescence spectra of 9-substituted anthracene derivatives (AnX (X = H and COOH)) and tetracene in the presence and absence of TiO2 colloids were measured to study the luminescence quenching processes. A TiO2 stock suspension (∼4 g/L) in acetonitrile/2-propanol (25:1) was prepared by sonication for more than 30 min revealing monodispersive colloidal suspension without secondary aggregations. (S14) The fluorescence spectra of fluorophore solutions in acetonitrile were collected in Ar-purged 1 cm-quartz-cell under photoexcitation (λ = 355 nm (AnH), 390 nm (AnCOOH), and 413 nm (Tetracene), F7000, Hitachi, Japan). A specific amount of the TiO2 stock suspension was successively added dropwise in the fluorophore solution, and then the fluorescence spectra were measured at room temperature after waiting for 15 min with Ar bubbling to reach equilibrium. Since there was no significant change in trend of results observed with the excitation of AnCOOH at a wavelength of 355 nm and 390 nm, we consider that the TiO2 band excitation in the UV region for AnH is still negligible for the results revealed here. ’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed characterization and analyses with controlled experiments included. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (M.M.M.); yuji-w@apc. titech.ac.jp (Y.W.). Phone/fax: +81-5734-2879.
’ ACKNOWLEDGMENT The authors thank Dr. Eichi Suzuki, Dr. Takako Kanai, and Dr. Tetsuji Yano at Tokyo Tech for material characterizations. ’ REFERENCES (1) Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988. (2) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53–229. (3) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515–582. (4) O’Regan, B.; Gr€atzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 335, 737–740. (5) Hagfeldt, A.; Gr€atzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 1995, 95, 49–68. (6) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B 2001, 63, 155409. (7) Yang, H. G.; Sun, H. S.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638–641. (8) Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. Synthesis of Titania Nanosheets with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties. J. Am. Chem. Soc. 2009, 131, 3152–3153. (9) Sun, C. H.; Yang, X. H.; Chen, J. S.; Li, Z.; Lou, X. W.; Li, C.; Smith, S. C.; Lu, G. Q.; Yang, H. G. Higher Charge/Discharge Rates of Lithium-Ions across Engineered TiO2 Surfaces Leads to Enhanced Battery Performance. Chem. Commun. 2010, 46, 6129–6131. (10) Liu, M.; Piao, L.; Ju, S.; Yan, Z.; He, T.; Zhou, C.; Wang, W. Anatase TiO2 Single Crystals with Exposed {001} and {110} Facets: Facile Synthesis and Enhanced Photocatalysis. Chem. Commun. 2010, 46, 1664–1666. (11) Liu, S.; Yu, J; Jaroniec, M. Tunable Photocatalytic Selectivity of Hollow TiO2 Microspheres Composed of Anatase Polyhedra with Exposed {001} Facets. J. Am. Chem. Soc. 2010, 132, 11914–11916. (12) Wang, X.; Liu, G.; Wang, L.; Pan, J.; Lu, G. Q.; Chen, H. M. TiO2 Films with Oriented Anatase {001} Facets and Their Photoelectrochemical Behavior as CdS Nanoparticle Sensitized Photoanodes. J. Mater. Chem. 2011, 21, 869–873. (13) Fang, W. Q.; Gong, X.-Q.; Yang, H. G. On the Unusual Properties of Anatase TiO2 Exposed by Highly Reactive Facets. J. Phys. Chem. Lett. 2011, 2, 725–734. (14) Johansson, E. M. J.; Plogmaker, S.; Walle, L. E.; Sch€ olin, R.; Borg, A; Sandell, A; Rensmo, H. Comparing Surface Binding of the Maleic Anhydride Anchor Group on Single Crystalline Anatase TiO2 (101), (100), and (001) Surfaces. J. Phys. Chem. C 2010, 114, 15015–15020. (15) Tachikawa, T.; Nan Wang, N.; Yamashita, S.; Cui, S.-C.; Majima, T. Design of a Highly Sensitive Fluorescent Probe for Interfacial Electron Transfer on a TiO2 Surface. Angew. Chem., Int. Ed. 2010, 49, 8593–8597. (16) Yu, J.; Fan, J.; Lv, K. Anatase TiO2 Nanosheets with Exposed (001) Facets: Improved Photoelectric Conversion Efficiency in DyeSensitized Solar Cells. Nanoscale 2010, 2, 2144–2149. (17) Vittadini, A.; Casarin, M.; Selloni, A. Chemistry of and on TiO2Anatase Surfaces by DFT Calculations: A Partial Review. Theor. Chem. Acc. 2007, 117, 663–671. (18) C-akır, D.; G€ulseren, O.; Mete, E.; Ellialtıoglu, S-. Dye Adsorbates BrPDI, BrGly, and BrAsp on Anatase TiO2(001) for DyeSensitized Solar Cell Applications. Phys. Rev. B 2009, 80, 035431. 2658
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(19) Hirayama, S. Effect of Substituent on the Behavior of the Excited Singlet and Triplet States in Carbonyl Derivatives of Anthracene of the Type 9-X•CO•A. J. Chem. Soc., Faraday Trans. I 1982, 78, 2411–2421. (20) Masnovi, J. M.; Seddon, E. D.; Kochi, J. K. Electron Transfer from Anthracenes. Comparison of Photoionization, Charge-Transfer Excitation and Electrochemical Oxidation. Can. J. Chem. 1984, 62, 2552–2559. (21) Kamat, P. V.; Ford, W. E. Photochemistry on Surfaces: Triplet Triplet Energy Transfer on Colloidal TiO2 Particles. Chem. Phys. Lett. 1987, 135, 421–426. (22) Kamat, P. V. Photoelectrochemistry in Particulate Systems. 9. Photosensitized Reduction in a Colloidal TiO2 System Using Anthracene9-Carboxylic Acid as the Sensitizer. J. Phys. Chem. 1989, 93, 859–864. (23) Kamat, P. V.; Fox, M. A. Photosensitization of TiO2 Colloids by Erythrosin B in Acetonitrile. Chem. Phys. Lett. 1983, 102, 379–384. (24) Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. Photoelectrochemistry in Particulate Systems. 4. Photosensitization of a TiO2 Semiconductor with a Chlorophyll Analogue. J. Phys. Chem. 1986, 90, 1389–1394. (25) Kalyansundaram, K.; Vlachopoulos, N.; Krishnan, V.; Monnier, A.; Gr€atzel, M. Sensitization of Titanium Dioxide in the Visible Light Region Using Zinc Porphyrins. J. Phys. Chem. 1987, 91, 2342–2347. (26) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, P.; Liska, N.; Vlachopoulos, N.; Gr€atzel, M. Conversion of Light to Electricity by cis-X2Bis(2,20 -bipyridyl-4,40 -dicarboxylate)ruthenium(II) Charge-Transfer Sensitizers (X = C1, Br, I, CN, and SCN) on Nanocrystalline TiO2 Electrodes. J. Am. Chem. Soc. 1993, 115, 6382–6390. (27) A nonannealed sample also showed the same trend in rate constant, although the absolute values were shifted before and after annealing, probably due to water or surface contamination on TiO2.
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