3432
Langmuir 2007, 23, 3432-3439
Photocatalytic Patterning of Monolayers for the Site-Selective Deposition of Quantum Dots onto TiO2 Surfaces Rachel S. Dibbell,† Gregory R. Soja,† Ruth M. Hoth, and David F. Watson* Department of Chemistry, UniVersity at Buffalo, The State UniVersity of New York, Buffalo, New York 14260-3000 ReceiVed October 28, 2006. In Final Form: January 8, 2007 A novel photochemical mechanism is reported for the site-selective deposition of quantum dots onto nanocrystalline TiO2 films. The patterning mechanism involves the combination of surfactant-mediated self-assembly and monolayer photolithography. In the self-assembly process, CdS and CdSe quantum dots were attached to TiO2 surfaces through bifunctional mercaptoalkanoic acid (MAA) linkers. MAAs were adsorbed to the TiO2 surface as the deprotonated carboxylates, primarily through monodentate coordination to Ti4+ sites. CdSe quantum dots were bound to the terminal thiol groups of surface-adsorbed MAAs, with a surface adduct formation constant, Kad, of (2.1 ( 0.7) × 104 M-1. The color and optical density of the quantum dot-functionalized TiO2 films were tunable. Monolayer photopatterning involved the TiO2-catalyzed oxidative degradation of surface-adsorbed mercaptohexadecanoic acid (MHDA). A mechanism is proposed wherein MHDA degradation occurs through both oxidative decarboxylation and the formation of interchain disulfides. These MHDA photodegradation processes regulate the extent to which CdSe quantum dots adsorb onto the TiO2 surface. Illumination through a photomask yielded optically patterned, quantum dot-functionalized TiO2 films that were characterized by scanning electron microscopy and energy-dispersive X-ray analysis.
Introduction The site-selective deposition of metallic and semiconducting nanoparticles onto substrate surfaces represents an important challenge in nanofabrication. Patterned, nanoparticle-functionalized surfaces may have applications in array-based sensors and electronic, photonic, and magnetic devices.1-3 A promising fabrication strategy is to combine top-down patterning with bottom-up self-assembly. In this approach, top-down techniques such as photolithography,4-6 scanning probe lithography,7-10 and microcontact printing,11-13 are used to generate patterned substrates for nanoparticle adsorption. The top-down step imparts long-range order, and the bottom-up step enables precise control over interparticle connectivity. Photolithographic patterning of self-assembled monolayers (SAMs) presents certain advantages over other top-down patterning techniques. First, light is easy to generate and focus, and illumination through a mask enables tunable and parallel †
These authors contributed equally.
(1) Mendes, P. M.; Chen, Y.; Palmer, R. E.; Nikitin, K.; Fitzmaurice, D.; Preece, J. A. J. Phys.: Condens. Matter 2003, 15, S3047-S3063. (2) Mendes, P. M.; Preece, J. A. Curr. Opin. Colloid Interface Sci. 2004, 9, 236-248. (3) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1171-1196. (4) Liu, J.-F.; Zhang, L.-G.; Gu, N.; Ren, J.-Y.; Wu, Y.-P.; Lu, Z.-H.; Mao, P.-S.; Chen, D.-Y. Thin Solid Films 1998, 327-329, 176-179. (5) Sun, S.; Chong, K. S.; Leggett, G. J. Nanotechnology 2005, 16, 17981808. (6) Darling, S. B.; Yufa, N. A.; Cisse, A. L.; Bader, S. D.; Sibener, S. J. AdV. Mater. 2005, 17, 2446-2450. (7) Demers, L. M.; Park, S.-J.; Taton, T. A.; Li, Z.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 3071-3073. (8) Demers, L. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 30693071. (9) Demers, L. M.; Ginger, D. S.; Park, S.-J.; Li, Z.; Chung, S.-W.; Mirkin, C. A. Science 2002, 296, 1836-1838. (10) Chung, S.-W.; Ginger, D. S.; Morales, M. W.; Zhang, Z.; Chandrasekhar, V.; Ratner, M. A.; Mirkin, C. A. Small 2005, 1, 64-69. (11) Yan, L.; Zhao, X.-M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6179-6180. (12) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825-7834. (13) Lee, I.; Zheng, H.; Rubner, M. F.; Hammond, P. T. AdV. Mater. 2002, 14, 572-577.
pattern formation. Second, most photochemical reactions can be carried out under ambient conditions. Finally, the reactivity of SAMs toward nanoparticles can be fine-tuned by varying the terminal functional groups. Monolayer photolithography typically involves the photodegradation of SAMs14-19 or the photochemical modification of functional groups.20,21 The resulting patterned, mixed monolayers22-24 can serve as templates for nanoparticle adsorption.4,5,20,21 Until recently, the diffraction limit prevented nanometer-scale patterning by monolayer photolithography. Leggett and co-workers have addressed this issue by developing scanning near-field photolithography, which has yielded sub50-nm feature sizes in patterned SAMs.25-28 Thus, bottom-up self-assembly onto photolithographically patterned SAMs, herein referred to as “photopatterning and self-assembly”, may represent an attractive alternative to conventional nanofabrication, particularly for nonplanar substrates or for materials that are unstable at high temperatures or under vacuum. We have developed a novel photopatterning and self-assembly technique that enables the fabrication of patterned, nanoparticlefunctionalized TiO2 surfaces. The technique involves the site(14) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342-3343. (15) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626628. (16) Lewis, M.; Tarlov, M.; Carron, K. J. Am. Chem. Soc. 1995, 117, 95749575. (17) Brewer, N. J.; Rawsterne, R. E.; Kothari, S.; Leggett, G. J. J. Am. Chem. Soc. 2001, 123, 4089-4090. (18) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398-1405. (19) Lee, J. P.; Sung, M. M. J. Am. Chem. Soc. 2004, 126, 28-29. (20) Ryan, D.; Nagle, L.; Fitzmaurice, D. Nano Lett. 2004, 4, 573-575. (21) del Campo, A.; Boos, D.; Spiess, H. W.; Jonas, U. Angew. Chem., Int. Ed. 2005, 44, 4707-4712. (22) Tarlov, M. J.; Burgess, J., Donald, R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (23) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024-1032. (24) Chang, W.; Choi, M.; Kim, J.; Cho, S.; Whang, K. Appl. Surf. Sci. 2005, 240, 296-304. (25) Sun, S.; Chong, K. S. L.; Leggett, G. J. J. Am. Chem. Soc. 2002, 124, 2414-2415. (26) Sun, S.; Leggett, G. J. Nano Lett. 2002, 2, 1223-1227. (27) Sun, S.; Leggett, G. J. Nano Lett. 2004, 4, 1381-1384. (28) Sun, S.; Montague, M.; Critchley, K.; Chen, M.-S.; Dressick, W. J.; Evans, S. D.; Leggett, G. J. Nano Lett. 2006, 6, 29-33.
10.1021/la063161a CCC: $37.00 © 2007 American Chemical Society Published on Web 02/17/2007
Photocatalytic Patterning of Monolayers
selective deposition of CdSe quantum dots onto 16-mercaptohexadecanoic acid (MHDA)-functionalized nanocrystalline TiO2 films. Photoexcitation of the TiO2 substrate leads to the catalytic oxidation and degradation of surface-adsorbed MHDA. CdSe quantum dots adhere to the terminal thiol groups of MHDA, which are present only in unilluminated regions of the surface. Illumination through a mask during the MHDA photooxidation step yields optically patterned surfaces. In this article, we describe the self-assembly and photopatterning mechanisms and the fabrication of micropatterned, quantum dot-functionalized TiO2 films. Experimental Section Materials and Instrumentation. Dimethyl cadmium (Me2Cd) and tri-n-octylphosphine (TOP) were obtained from Strem Chemicals. Bis(trimethylsilyl)sulfide [(TMS)2S], tri-n-octylphosphine oxide (TOPO), selenium shot, titanium(IV) tetraisopropoxide, zirconium(IV) tetraisopropoxide, and 3-mercaptopropionic acid (MPA) were obtained from Alfa Aesar. MHDA was obtained from Aldrich. Dithiothreitol (DTT) was obtained from EMD Chemicals. Methanol, tetrahydrofuran (THF), nitric acid, and acetic acid were obtained from a variety of sources. Reagents were used without further purification. FTIR spectra were obtained using a Nicolet Magna-IR 550 spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector with a resolution of 2 cm-1 with Happ-Genzel apodization. UV/visible absorption spectra were obtained using an Agilent 8453 spectrophotometer. SEM images were obtained using a Hitachi S-4000 instrument. TEM images were obtained using a JEOL JEM 2010 instrument. Synthesis of Quantum Dots. CdS and CdSe quantum dots were synthesized by adaptation of the method developed by Murray et al.29,30 TOP solutions of Me2Cd, (TMS)2S, and selenium were prepared in an inert atmosphere glovebox. In a typical CdSe preparation, selenium shot (1.58 g, 20.0 mmol) was dissolved in TOP (50.0 mL) and added to a solution of Me2Cd (2.0 mL, 4.0 g, 28 mmol) in TOP (50.0 mL). TOPO (100 g) was heated to 100 °C under Ar. Rapid addition of the Me2Cd/Se solution to the liquid TOPO yielded a pale-yellow suspension, accompanied by a decrease in temperature to ∼70 °C. The absorption onset of this initial suspension was ∼320 nm. Larger particle sizes were obtained by gradually increasing the reaction temperature. Aliquots of the reaction mixture were removed and quenched with THF. UV/visible absorption spectra were obtained to monitor particle growth. After cooling to below the melting point of TOPO (60 °C), we added sufficient methanol to the reaction mixture to dissolve all solid TOPO while flocculating the TOPO-capped CdSe quantum dots. The flocculate was collected by centrifugation and washed repeatedly in methanol to remove excess TOPO. The quantum dots were then dried in a stream of Ar and suspended in THF. X-ray powder diffraction revealed that the CdSe quantum dots were crystalline with the wurtzite structure.29 The band gap absorption onset of THF suspensions of CdSe quantum dots varied from 360 to 640 nm. From TEM measurements, the largest CdSe particles had diameters of 5.2 ( 0.3 nm. These CdSe samples were used in self-assembly and photopatterning experiments. CdS quantum dots were synthesized by a similar procedure using (TMS)2S as a precursor. In a typical preparation, (TMS)2S (1.25 mL, 1.06 g, 5.94 mmol) was dissolved in TOP (11.0 mL) and added to a solution containing Me2Cd (0.50 mL, 1.0 g, 7.0 mmol) in TOP (12.5 mL). The Me2Cd/(TMS)2S solution was rapidly added to TOPO (25 g) at 100 °C. The rest of the procedure was similar to that for CdSe. X-ray powder diffraction confirmed the wurtzite structure.29 The band gap absorption onset of THF suspensions of CdS quantum dots varied from 410 to 500 nm. From TEM measurements, the (29) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (30) Bowen Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109-4117.
Langmuir, Vol. 23, No. 6, 2007 3433 largest CdS particles had diameters of 3.3 ( 0.2 nm. These CdS samples were used in self-assembly experiments. Fabrication of Nanocrystalline Metal Oxide Films. Nanocrystalline anatase TiO2 films on glass substrates were prepared by the hydrolysis of titanium(IV) tetraisopropoxide following the procedure reported by Heimer et al.31 SEM images revealed that the films were 4 µm thick and consisted of approximately 20-nm-diameter particles. X-ray powder diffraction confirmed the anatase structure. Nanocrystalline ZrO2 films were prepared by a similar method, following the procedure reported by Bonhoˆte et al.32 Self-Assembly. Nanocrystalline metal oxide films were immersed in THF solutions of mercaptoalkanoic acids (MAAs) for at least 3 h. For MHDA, a concentration of 1.73 × 10-3 M yielded maximal surface coverage. The films were rinsed repeatedly in THF upon removal from the MAA solutions. The films were then sonicated briefly in THF to remove any excess MAA. The films were either stored in the dark for future use or immediately derivatized with CdS or CdSe quantum dots. The MAA-derivatized metal oxide films were immersed in THF suspensions of quantum dots for at least 3 h. The films were then rinsed repeatedly with THF and allowed to dry. FTIR Spectroscopy. FTIR spectra were obtained using KBr pellets as the sample matrix. The KBr was heated for 1 h at 100 °C and then kept under vacuum until use. MHDA-derivatized TiO2 films were scraped from the glass substrate using a razor blade. The sample was combined with KBr in a 1:10 ratio, ground, and stored under argon until analysis. The FTIR instrument chamber was purged with dry air for 15 min before sampling. A minimum of 200 scans were collected, with a resolution of 2 cm-1. Photochemical Patterning. All photochemical experiments were performed using a Continuum Powerlite Precision II 8000 Nd:YAG laser system (6-8 ns pulse width, 10 Hz repetition rate). Samples were illuminated at a wavelength of 355 or 532 nm with an intensity of 60 mW/cm2 and a beam diameter of 1.1 cm. MHDA-derivatized TiO2 films on glass slides were immersed in THF solutions within quartz cuvettes such that the TiO2 film faced away from the laser beam. The THF solutions were purged with O2 or Ar. After illumination, the MHDA-derivatized TiO2 films were immersed in CdSe suspensions, as described above. Micropatterned films were prepared using an Edmund Optics 1951 USAF chromium-on-glass resolution target as a photomask. The photomask was placed flush against the cuvette to minimize light diffraction.
Results and Discussion Mercaptohexadecanoic Acid (MHDA) Surface Attachment. FTIR spectra were obtained for TiO2 films that had been immersed for 12-16 h in THF solutions of MHDA (Figure 1). Peak assignments are presented in Table 1. Similar spectra were obtained for TiO2 films that were immersed in THF solutions of mercaptopropionic acid (MPA). The presence of C-H stretching modes, C-O stretching modes, and alkyl wagging and twisting modes in the spectra of MHDA-exposed TiO2 films provides clear evidence for the attachment of MHDA to the TiO2 surface. The spectrum of MHDA-derivatized TiO2 differs from the spectrum of free MHDA in three significant respects: (1) the 1698 cm-1 asymmetric C-O stretching band (νa(CO2)) and the 1430 cm-1 symmetric C-O stretching band (νs(CO2)) of free MHDA33,34 are not present in the spectrum of MHDA-deriva(31) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg. Chem. 1996, 35, 5319-5324. (32) Bonhoˆte, P.; Gogniat, E.; Tingry, S.; Barbe´, C.; Vlachopoulos, N.; Lenzmann, F.; Comte, P.; Gra¨tzel, M. J. Phys. Chem. B 1998, 102, 1498-1507. (33) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (34) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18, 39803992. (35) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334-341. (36) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1999, 15, 2402-2408. (37) Parikh, A. N.; Gillmor, S. D.; Beers, J. D.; Beardmore, K. M.; Cutts, R. W.; Swanson, B. I. J. Phys. Chem. B 1999, 103, 2850-2861.
3434 Langmuir, Vol. 23, No. 6, 2007
Dibbell et al. Table 1. FTIR Frequencies and Assignments for MHDA-Derivatized TiO2 Films
Figure 1. (a) High-frequency and (b) low-frequency windows of the FTIR spectra of MHDA in a KBr pellet (free MHDA) and MHDAderivatized TiO2 films (MHDA-TiO2). The free MHDA baseline absorbance was increased by 0.2 absorbance units in plot a and 0.3 absorbance units in plot b.
tized TiO2; (2) a broad absorption from 1510 to 1655 cm-1, containing a well-resolved band at 1610 cm-1, is present in the spectrum of MHDA-derivatized TiO2 but not in the spectrum of free MHDA; and (3) an intense band at 1403 cm-1 is present in the spectrum of MHDA-derivatized TiO2 and is significantly enhanced relative to the 1410 cm-1 band in the spectrum of free MHDA. The changes in the C-O stretching region of MHDA’s FTIR spectrum indicate that MHDA binds to the TiO2 surface through the carboxylic acid group. The νa(CO2) frequency depends on the carbonyl bond order and is higher for carboxylic acids than carboxylates.33,42,43 Thus, the absence of the 1698 cm-1 νa(CO2) band in the spectrum of MHDA-derivatized TiO2 provides clear evidence for the deprotonation and surface attachment of MHDA (38) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227-250. (39) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; John Wiley and Sons: New York, 1997. (40) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D.; Siperko, L. M. Langmuir 1992, 8, 2707-2714. (41) Tao, Y.-T.; Hietpas, G. D.; Allara, D. L. J. Am. Chem. Soc. 1996, 118, 6724-6735. (42) Meyer, T. J.; Meyer, G. J.; Pfennig, B. W.; Schoonover, J. R.; Timpson, C. J.; Wall, J. F.; Kobusch, C.; Chen, X.; Peek, B. M.; Wall, C. C.; Ou, W.; Erickson, B. W.; Bignozzi, C. Inorg. Chem. 1994, 33, 3952-3964. (43) Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1998, 14, 27442749.
frequency (cm-1)
assignment
ref(s)
2922 2850 1635 1610-1525 1467 1460-1420 1403 1320-1125
CH2 asymmetric stretch CH2 symmetric stretch H-O-H bend (TiO2-adsorbed H2O) CO2- asymmetric stretch CH2 deformation CO2- symmetric stretch (bidentate) CO2- symmetric stretch (monodentate) alkyl wags, twists
33-35 33-35 36 33, 34 33, 34, 37 33, 34, 38, 39 33, 34, 38, 39 37, 40, 41
as the carboxylate. The surface attachment process most likely involves either a dehydration reaction with hydroxyl groups on the TiO2 surface or the protonation of surface oxide sites. We assign the broad absorption from 1510 to 1655 cm-1 as multiple carboxylate νa(CO2) bands that overlap with the bending mode of surface-adsorbed water (δ(H2O)) at 1635 cm-1.36 (The 1635 cm-1 mode was also present in the FTIR spectra of unmodified TiO2 films.) The frequencies of the νa(CO2) band and the symmetric C-O stretching band (νs(CO2)) should depend on the carboxylate coordination mode.38,39 In carboxylato coordination complexes, the νa(CO2) frequency and ∆, the frequency difference between the νa(CO2) and νs(CO2) bands, are greatest for monodentate coordination, intermediate for bidentate bridging coordination, and lowest for bidentate chelation.38,39 The ∆ values for monodentate carboxylato complexes are typically greater than 200 cm-1, whereas those for bidentate bridging and chelating coordination generally range from 50 to 175 cm-1.38 The broad range of unresolved νa(CO2) bands in the FTIR spectrum of MHDA-derivatized TiO2 indicates that MHDA may bind to the TiO2 surface through multiple coordination modes. However, the 1610 cm-1 band is clearly the most intense band within the broad absorption. Because it is the highest frequency band in the νa(CO2) region, we assign the 1610 cm-1 band to the νa(CO2) absorption of mercaptohexadecanoate coordinated to surface Ti4+ sites in the monodentate geometry. The greater intensity of the 1610 cm-1 band, relative to that of the other bands within the broad νa(CO2) absorption, suggests that the primary coordination mode is monodentate. Monodentate coordination should give rise to an increased ∆ value. In free MHDA, the νs(CO2) band lies at 1430 cm-1, with a high-frequency shoulder corresponding to the COH deformation mode (ω(COH)).33,34 The 1410 cm-1 band corresponds to the CH2 scissors deformation mode (ω(CH2)).33,34 In the spectrum of MHDA-derivatized TiO2, the 1430 cm-1 band is absent, but the 1410 cm-1 band is broadened and red-shifted, with a maximum at 1403 cm-1, and has a significantly increased intensity. These spectral changes are consistent with the red shift of the νs(CO2) band from 1430 to 1403 cm-1 upon deprotonation of MHDA and monodentate coordination of the carboxylate to the TiO2 surface. The ∆ value of 207 cm-1 is greater than the value of 191 cm-1 for mercaptohexadecanoate on alumina33 and corresponds closely to various coordination complexes with monodentate carboxylate coordination.38 The unresolved bands from 1420 to 1460 cm-1 may correspond to the νs(CO2) bands for the bidentate bridging or chelating geometry. The presence of several bands within the νs(CO2) region is consistent with the breadth of the νa(CO2) region. In summary, the FTIR spectra provide strong evidence that MHDA is attached to the TiO2 surface in the first step of the self-assembly process. The disappearance of the 1698 cm-1 νa(CO2) band indicates that MHDA coordinates as the carboxylate. The primary coordination geometry is most likely monodentate,
Photocatalytic Patterning of Monolayers
but surface attachment in the bidentate bridging and chelating geometries may also occur. Quantum Dot Adsorption. Mercaptoalkanoic acid (MAA)derivatized TiO2 films that were immersed in CdS or CdSe suspensions for 12-16 h turned yellow to red in color, depending on the quantum dot composition and particle size. UV/visible absorption spectra corresponded to the sum of the TiO2 and quantum dot spectra, clearly indicating that the quantum dots adhered to the MAA-functionalized TiO2 surfaces. Similar adsorption behavior was observed for CdS and CdSe quantum dots and for MHDA- and MPA-derivatized TiO2 films. To simplify this section of the article, CdS and CdSe are collectively referred to as CdE. The color and optical density and therefore the CdE surface coverage were uniform throughout the surface of each TiO2 film. The FTIR spectra of MHDA-derivatized TiO2 films did not change significantly upon exposure to CdE suspensions. The weak S-H stretching band was not present in the spectrum of MHDA-derivatized TiO2 before or after exposure to CdE suspensions. CdE quantum dots did not adhere to underivatized TiO2 films or to TiO2 films that had previously been exposed to solutions of alkanethiols or alkanoic acids (Figure 2). These control experiments unambiguously reveal that bifunctional linker molecules, containing both carboxylic acid and thiol groups, are necessary to attach CdE quantum dots to TiO2 films. Importantly, the physisorption of CdE to underivatized TiO2 films was not observed (Figure 2). FTIR spectra revealed that our as-synthesized CdE quantum dots were coated with the TOPO capping agent, consistent with previous reports.29,30 We speculate that the hydrophobic octyl groups of TOPO prevent the physisorption of CdE quantum dots to the polar TiO2 surface. TOPO is presumably displaced by the thiol group of TiO2-adsorbed MHDA, which should bind more strongly to the CdE surface than the phosphine oxide group of TOPO. The surface coverage of CdE quantum dots was determined from the band gap absorbance. Extinction coefficients were calculated using the Beer-Lambert law, neglecting the contribution of surface-adsorbed TOPO to the mass of CdE samples. The CdE surface coverage increased with MAA and CdE concentrations. At high surface coverages, the absorbance near the CdE band gap absorption maximum was typically 0.4 to 0.6. Equilibrium binding studies were performed to measure the influence of quantum dot concentration on the surface-attachment process. For example, the attachment of CdSe nanoparticles to mercaptopropionic acid (MPA)-derivatized nanocrystalline TiO2 films was accurately modeled by the Langmuir adsorption isotherm,44 yielding a a saturation surface coverage of (4.80 ( 0.04) × 10-7 mol/cm2 and a surface adduct formation constant, Kad, of (2.1 ( 0.7) × 104 M-1 (Figure 3). The measured Kad value is consistent with previously reported values of 103 to 105 M-1 for the adsorption of thiols and thiolates to the surfaces of planar and nanoparticulate CdE semiconductors.45-47 Similar surface coverages were observed for the adsorption of CdS quantum dots to MPA-derivatized TiO2 films. The saturation surface coverages of CdS and CdSe quantum dots on MHDAderivatized TiO2 films were slightly lower than on MPAderivatized TiO2 films. Taken together, the FTIR data, functional group dependence studies, and equilibrium binding data provide strong evidence (44) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361-1402. (45) Thackeray, J. W.; Natan, M. J.; Ng, P.; Wrighton, M. S. J. Am. Chem. Soc. 1986, 108, 3570-3577. (46) Natan, M. J.; Thackeray, J. W.; Wrighton, M. S. J. Phys. Chem. 1986, 90, 4089-4098. (47) Bullen, C.; Mulvaney, P. Langmuir 2006, 22, 3007-3013.
Langmuir, Vol. 23, No. 6, 2007 3435
Figure 2. Absorption spectra of TiO2 films after immersion in MPA (-), propionic acid (‚‚‚), propanethiol (-‚-), or neat THF (--), followed by rinsing and immersion in THF suspensions of (a) CdS or (b) CdSe quantum dots. In plot b, the blank spectrum of each TiO2 film, obtained prior to immersion in solutions and quantum dot suspensions, was subtracted from each spectrum to account for discrepancies in the baseline absorbance.
for the self-assembly mechanism outlined in Scheme 1. The self-assembly mechanism provides a straightforward method for the attachment of CdE quantum dots to TiO2 surfaces. Because CdE synthesis and surface attachment occur in separate steps, it is possible to control the size distribution of surface-adsorbed quantum dots. In addition, the well-characterized surface attachment equilibria enable control over the CdE surface coverage. Therefore, both the color and optical density of CdEfunctionalized TiO2 films are tunable. MAAs have previously been used to link quantum dots to metal oxides in colloidal suspensions and within layered and nanocrystalline films.48-52 However, this article presents the first report of the MAA surface (48) Lawless, D.; Kapoor, S.; Meisel, D. J. Phys. Chem. 1995, 99, 1032910335. (49) Hao, E.; Yang, B.; Zhang, J.; Zhang, X.; Sun, J.; Shen, J. J. Mater. Chem. 1998, 8, 1327-1328. (50) Hirai, T.; Suzuki, K.; Komasawa, I. J. Colloid Interface Sci. 2001, 244, 262-265. (51) Wijayantha, K. G. U.; Peter, L. M.; Otley, L. C. Sol. Energy Mater. 2004, 83, 363-369. (52) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385-2393.
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Dibbell et al.
Figure 4. Dependence of CdSe surface coverage on illumination time (60 mW/cm2) for MHDA-derivatized TiO2 and ZrO2 films: TiO2 at 355 nm (2), TiO2 at 532 nm (9), and ZrO2 at 355 nm (b). Superimposed on the data for TiO2 at 355 nm is a single-exponential fit.
Figure 3. Adsorption of CdSe quantum dots to MPA-functionalized TiO2 films. Dependence of CdSe surface coverage Γ (a) on the concentration of CdSe in suspension and (b) fit to the Langmuir adsorption isotherm. The fit results are superimposed on the Γ vs [CdSe] data in plot a.
attachment mode, the influence of CdE concentration on surface coverage, and the functional group dependence studies that definitively reveal that self-assembly occurs through the bifunctional MAA linker. Photochemically Directed Quantum Dot Adsorption. Illumination of MAA-derivatized TiO2 films with 355-nm light decreased the films’ affinity for both CdSe and CdS quantum dots. The effect was observed for longer-chain MAAs, such as MHDA, and shorter-chain MAAs, such as MPA. The influences of MAA chain length and CdE composition and particle size on the photochemistry are the subjects of ongoing research in our laboratory. In this article, we focus on the photochemically directed surface attachment of CdSe quantum dots to MHDAderivatized TiO2 films. In a typical experiment, MHDAderivatized TiO2 films were illuminated at 355 nm prior to immersion in CdSe suspensions. The CdSe surface coverage, determined from the band gap absorbance, was lower in illuminated regions than in unilluminated regions of the films. CdSe surface attachment was completely “turned off” by 45 min of illumination at 60 mW/cm2. Plots of surface coverage versus illumination time (Figure 4) were accurately fit to a singleexponential decay, suggesting that the photochemical process was first order in photon flux. The decrease of CdSe surface attachment occurred on similar time scales for oxygen-purged and argon-purged samples. Backscattered SEM images confirmed the decreased surface coverage of CdSe quantum dots in illuminated regions of MHDA-
Figure 5. Backscattered SEM image and EDX line scan of the interface between illuminated and unilluminated areas of an MHDAderivatized TiO2 film after reaction with CdSe. The thin gray line shows the intensity of the 3.1 keV Cd peak as a function of position along the thick horizontal white line. The black line equals zero X-ray intensity.
derivatized TiO2 films (Figure 5). The backscattered electron intensity increases with the atomic number of the scattering atom.53 Because the atomic number of cadmium is greater than that of titanium, the regions of the TiO2 surface that are coated with CdSe quantum dots appear brighter than those that are not. Energy-dispersive X-ray analysis (EDX) indicates a sharp decrease in the CdSe surface coverage at the interface between unilluminated and illuminated regions of the film (Figure 5). The decreased CdSe surface coverage is evidenced by the decreased intensity of the gray line, which corresponds to the intensity of the 3.1 keV Cd peak in the EDX spectrum. In control experiments, MHDA-derivatized TiO2 films were illuminated at sub-band-gap energy (532 nm, 60 mW/cm2). No measurable decrease in the CdSe surface coverage was observed within illuminated regions of the films (Figure 4). Similarly, 355-nm illumination of MHDA-derivatized nanocrystalline ZrO2 films did not affect the CdSe surface coverage (Figure 4). These (53) Lloyd, G. E. Mineral. Mag. 1987, 51, 3-19.
Photocatalytic Patterning of Monolayers
Langmuir, Vol. 23, No. 6, 2007 3437 Scheme 1. Self-Assembly Mechanism
Scheme 2. Potentials of the TiO2 Band Edges and an MAA+/MAA Couplea
a
TiO2-catalyzed MAA oxidation is shown.
results imply that the photochemical mechanism involves the band gap excitation of TiO2. The band gap of anatase TiO2 is 3.2 eV,54,55 corresponding to an absorption threshold of 388 nm. The absorption onset of the nanocrystalline anatase TiO2 films used in this study was 385 nm. Therefore, 355-nm light was sufficiently energetic to excite the TiO2 band gap transition, but 532-nm light was not. The band gap of ZrO2 is 5.0 eV,56,57 and the absorption onset of our ZrO2 films was 280-300 nm. Therefore, 355-nm light was not absorbed by the ZrO2 films. We attribute the photochemically induced decrease in CdSe surface coverage to the TiO2-catalyzed oxidative degradation of MHDA. The valence band potential of anatase TiO2 is roughly 1.6 V positive of MAA oxidation potentials.54,55,58 Therefore, the TiO2-catalyzed oxidation of MHDA is thermodynamically favorable (Scheme 2). The photoinduced decrease in CdSe surface coverage was reversible by reacting the illuminated MHDA-derivatized TiO2 films with MHDA or dithiothreitol (DTT). Exhaustively illuminated MHDA-derivatized TiO2 films, which underwent little or no CdSe surface attachment when initially immersed in CdSe suspensions, were subsequently reacted with 10-15 mM THF solutions of MHDA or DTT and then re-immersed in CdSe suspensions. Following the reaction with MHDA or DTT, the CdSe surface coverage increased dramatically. For a typical MHDA-derivatized TiO2 film, the CdSe surface coverage increased from approximately 1/10 to 1/2 the coverage of an unilluminated region within the same film (Table 2). The CdSe surface coverage could be further increased by sequentially reacting the illuminated films with DTT and then MHDA, or MHDA and then DTT. These results suggest that two oxidative degradation mechanisms are active. The first involves the TiO2-catalyzed decarboxylation of MHDA. The oxidative degradation of alkylcarboxylates and alkanoic acids by TiO2 powders and colloids is well-known.59-64 The mechanism typically involves the oxidation (54) Nozik, A. J. Annu. ReV. Phys. Chem. 1978, 29, 189-222. (55) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49-68. (56) Sayama, K.; Arakawa, H. J. Phys. Chem. 1993, 97, 531-533. (57) Sayama, K.; Arakawa, H. J. Photochem. Photobiol., A 1996, 94, 67-76. (58) Esplandiu´, M. J.; Hagenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828838. (59) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 2239-2240. (60) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 5985-5992. (61) Chum, H. L.; Ratcliff, M.; Posey, F. L.; Turner, J. A.; Nozik, A. J. J. Phys. Chem. 1983, 87, 3089-3093.
Table 2. Influence of DTT and MHDA on CdSe Surface Coveragea treatment
Γrel unillum.b
Γrel illum. beforec
Γrel illum. afterd
DTT MHDA
1.0 1.0
0.15 ( 0.18 0.09 ( 0.04
0.5 ( 0.2 0.5 ( 0.1
a MHDA-derivatized TiO2 films were illuminated at 355 nm (60 mW/ cm2, 45 min), immersed in CdSe suspensions, immersed in 10-15 mM THF solutions of DTT or MHDA, and then reimmersed in CdSe suspensions. b Relative surface coverage of CdSe (Γrel) within unilluminated regions of the films. c Γrel in illuminated regions of the films before reaction with DTT or MHDA. d Γrel in illuminated regions of the films after reaction with DTT or MHDA.
of the carboxylate or carboxylic acid to the corresponding carboxylate radical, which decays to an alkyl radical via decarboxylation.59-61,63 The oxidative decarboxylation of MHDA most likely occurs through a similar mechanism in which surfaceadsorbed mercaptohexadecanoate is oxidized by valence band holes, yielding the HS-(CH2)15COO• radical, which subsequently decays by decarboxylation. The decarboxylated MHDA degradation products desorb from the surface, decreasing the thiol surface coverage and causing decreased reactivity toward CdSe quantum dots. This mechanism is consistent with the observed decrease in CdSe surface coverage on 355-nm illuminated TiO2 films (Figure 4) and the reversibility upon reaction with MHDA (Table 2). MHDA presumably coordinates to empty Ti4+ surface sites, which are exposed by the photoinduced decarboxylation of MHDA and the desorption of its degradation products. Importantly, the increased CdSe surface coverage upon reaction with MHDA cannot be attributed to a surface-exchange process in which MHDA displaces carboxylated photodegradation products that otherwise would not desorb from the TiO2 surface. The possibility of surface exchange was investigated by immersing unilluminated MHDA-derivatized TiO2 films in THF solutions of palmitic acid (CH3(CH2)14CO2H) prior to reaction with CdSe quantum dots. The unilluminated films that were exposed to palmitic acid exhibited no change in CdSe surface coverage compared to those that were not. If the palmitic acid had displaced a significant quantity of MHDA from the surface, then a decrease in CdSe surface coverage would have been expected because palmitic acid does not contain a CdSe-binding functional group. The absence of any effect implies that the surface exchange of carboxylic acids is not significant on the time scale of our experiments. Therefore, the reversibility of the photochemistry by reaction with MHDA must be attributed to the dissociation of the MHDA degradation products from the TiO2 surface, which is consistent with the oxidative decarboxylation mechanism. A second possible mechanism for the TiO2-catalyzed degradation of MHDA involves the oxidative cleavage of S-H bonds and the formation of interchain disulfides. Oxidative bond cleavage, leading to dissociation through radical mechanisms, (62) Sakata, T.; Kawai, T.; Hashimoto, K. J. Phys. Chem. 1984, 88, 23442350. (63) Harada, H.; Ueda, T.; Sakata, T. J. Phys. Chem. 1989, 93, 1542-1548. (64) Serpone, N.; Martin, J.; Horikoshi, S.; Hidaka, H. J. Photochem. Photobiol., A 2005, 169, 235-251.
3438 Langmuir, Vol. 23, No. 6, 2007
Dibbell et al.
Figure 6. Backscattered SEM images of micropatterned CdSefunctionalized TiO2 surfaces from illumination through negative and positive masks.
has previously been observed for the degradation of alkanoic acids on TiO2 surfaces.64,65 In our system, one possible termination mechanism involves the formation of interchain disulfides by the reaction of thiolate radicals (S•) on adjacent MHDA molecules. Disulfide formation is supported by the reversibility of the photochemistry upon reaction of illuminated MHDA-derivatized TiO2 films with DTT (Table 2). DTT is known to reduce disulfides to thiols efficiently.66 On the TiO2 surface, the reduction of disulfides to thiols should increase the reactivity toward CdSe quantum dots, consistent with the observed increase in CdSe surface coverage after treatment with DTT. Because reaction with both MHDA and DTT increased the CdSe surface coverage, we conclude that both the oxidative decarboxylation and disulfide formation mechanisms are active in the photochemical process. The existence of two mechanisms is consistent with our observation that the CdSe surface coverage increased further when DTT-treated films were subsequently reacted with MHDA and when MHDA-treated films were subsequently reacted with DTT. We considered whether the direct photooxidation of MHDA thiol groups to sulfonates was responsible for the decreased CdSe surface coverage. Previous studies have shown that alkylsulfonates bind less strongly than thiols to gold surfaces.18,22 Presumably, alkylsulfonates also exhibit decreased reactivity toward CdSe quantum dots. However, three factors suggest that the direct photooxidation of thiols to alkylsulfonates is not the primary mechanism in our system. First, 355-nm illumination of MHDA-derivatized ZrO2 films did not affect the CdSe surface coverage. Changing the substrate from TiO2 to ZrO2 should not impact the efficiency of the direct thiol photooxidation process. Second, the illumination-induced decrease in CdSe surface coverage occurred on similar time scales for MHDA-derivatized TiO2 films immersed in oxygen-purged and argon-purged THF. Therefore, molecular oxygen is not involved in the photochemical mechanism, suggesting that thiols are not oxidized to sulfonates. Finally, more energetic UV light is typically required for the direct photooxidation of alkanethiols to alkylsulfonates.17 The 355-nm light used in our experiments should not have been sufficiently energetic to oxidize MHDA thiols to sulfonates. The TiO2-catalyzed oxidative degradation of MHDA represents a new approach to monolayer photolithography. The mechanism differs from previously reported approaches in two significant respects. First, the TiO2 substrate presents a different surface reactivity than gold, silver, silicon, and silica, which have been used in most previously reported monolayer photopatterning strategies. Second, the TiO2 substrate itself is photochemically active and catalyzes the top-down monolayer photopatterning
step. Using a photocatalytic substrate differs from most monolayer photopatterning strategies, which involve the direct excitation of photochemical reactions within functional groups of the surfactant monolayer. TiO2-catalyzed oxidative degradation reactions occur under longer-wavelength light than, for instance, the direct photooxidation of alkanethiols to alkylsulfonates. Therefore, photopatterning with less energetic UV light is possible. Furthermore, TiO2-catalyzed oxidation may be applicable to a wide range of surfactant species. Sung and co-workers recently reported a related TiO2-catalyzed photopatterning mechanism.19,67 In their approach, the photomask was coated with a patterned TiO2 film that catalyzed the oxidative degradation of alkylsiloxane monolayers on silicon substrates. Our approach differs in that the TiO2 film serves as both the photocatalyst and the substrate for monolayer adsorption. Furthermore, our approach is applicable to the fabrication of high-surface-area, optically dense films. Optical Patterning. Coupling the monolayer photolithography and self-assembly mechanisms enables the site-selective deposition of CdSe quantum dots onto TiO2 surfaces. Quantum dots adhere only to unilluminated regions of the TiO2 surface, in which the MHDA monolayer has not been degraded. In proofof-concept experiments, micropatterned surfaces were prepared by illuminating MHDA-derivatized TiO2 films through photomasks (Figure 6). Lined patterns, consisting of alternating CdSecoated and uncoated regions of the TiO2 surface, diffracted 532nm light. (CdSe quantum dots absorb 532-nm light, but the TiO2 substrate does not; therefore, the lined films diffract 532-nm light.) Measured diffraction patterns (Figure 7) were used to calculate the width of the features within the patterned CdSefunctionalized TiO2 surface and the fidelity of image transfer from the photomask. The smallest feature size had a width of 43 ( 0.1 µm, equal to the width of the photomask spacing. Diffraction of 355-nm light by the photomask prevented the formation of smaller features. In principle, however, the resolution of the photopatterning and self-assembly technique is limited only by the CdSe particle size. Near-field techniques25,27 could be used to produce smaller features. A unique aspect of our photopatterning and self-assembly mechanism is the high surface
(65) Guillard, C. J. Photochem. Photobiol., A 2000, 135, 65-75. (66) Lees, W. J.; Whitesides, G. M. J. Org. Chem. 1993, 58, 642-647.
(67) Lee, J. P.; Kim, H. K.; Park, C. R.; Park, G.; Kwak, H. T.; Koo, S. M.; Sung, M. M. J. Phys. Chem. B 2003, 107, 8997-9002.
Figure 7. Plot of log(I/I0) vs φ for diffraction of 532-nm light by a linearly patterned CdSe-coated MHDA-TiO2 film. (I/I0 is the intensity at angle φ divided by the intensity of nondiffracted light (φ ) 0).) The number above each peak indicates the diffraction order. From the data, the width of the pattern features was calculated to be 43 ( 0.1 µm.
Photocatalytic Patterning of Monolayers
area of the TiO2 substrate, which promotes dense packing of quantum dots and optically dense films. Therefore, the photopatterning and self-assembly process is easily characterized by transmission-based spectroscopic techniques and provides a facile route to preparing optically patterned surfaces. Furthermore, because the photochemical mechanism is reversible by treatment with MHDA and DTT, the formation of ternary and higher-order patterns should be possible.
Conclusions We have reported an efficient self-assembly process for the attachment of CdE quantum dots to nanocrystalline TiO2 films through bifunctional MAA linkers. The MAAs bind to the TiO2 surface as deprotonated carboxylates primarily through monodentate coordination to surface Ti4+ sites. TOPO-coated CdSe quantum dots bind strongly to MPA-derivatized TiO2 films, with a surface adduct formation constant of (2.1 ( 0.7) × 104 M-1. Bifunctional MAA linkers, with both thiol and carboxylic acid functional groups, are required for CdE surface attachment. This self-assembly process provides a straightforward method for the attachment of quantum dots to metal oxide surfaces. The color and optical density of the composite semiconductor films are tunable. TiO2-catalyzed photooxidative degradation reactions were used to prepare micrometer-scale patterns within MHDA monolayers. The oxidation process most likely involves both the decarboxylation of MHDA and the formation of interchain disulfides. The
Langmuir, Vol. 23, No. 6, 2007 3439
TiO2-catalyzed mechanism represents the first example of monolayer photopatterning in which a nanocrystalline substrate itself is photochemically active. The monolayer photopatterning technique was combined with the self-assembly process to yield a novel mechanism for hybrid top-down/bottom-up materials fabrication. The top-down step involves the site-selective degradation of MHDA in illuminated regions of the TiO2 surface. The bottom-up step involves the self-assembly of CdSe quantum dots onto the patterned, MHDA-functionalized surfaces. Because the micropatterned CdSe-functionalized TiO2 films have high optical densities and tunable color, they may have applications in optical sensor arrays or photon-mode information storage devices. More generally, the photopatterning and self-assembly method may provide a solution-phase alternative to conventional nanofabrication techniques, particularly for nonplanar substrates or for materials that are unstable at high temperatures, low pressures, or under deep-UV illumination. Acknowledgment. This work was funded, in part, by the New York State Office of Science, Technology, and Academic Research (NYSTAR). In addition, acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. We thank Peter Bush and Donald MacFarland for their guidance in obtaining SEM images and EDX data. LA063161A
