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Nano/Subnanometer Pd Nanoparticles on Oxide Supports Synthesized by AB-type and Low-Temperature ABC-type Atomic Layer Deposition: Growth and Morphology† Junling Lu‡ and Peter C. Stair*,‡,§ ‡ Department of Chemistry and Center for Catalysis and Surface Science, Northwestern University, Evanston, Illinois 60208, and §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439
Received April 7, 2010. Revised Manuscript Received May 24, 2010 The synthesis of uniformly dispersed nano/subnanometer Pd nanoparticles on oxide supports with atomic layer deposition (ALD) has been studied in terms of growth and morphology. In situ quartz crystal microbalance (QCM) measurements showed that AB-type Pd ALD grew more favorably on TiO2 than on Al2O3 at 200 °C by the sequential exposure of Pd(II) hexafluoroacetylacetonate (Pd(hfac)2) and formalin. The growth rate of AB-type Pd ALD decreased on the Al2O3 surface at a lower deposition temperature, and there was negligible growth at 110 °C. However, a new ABC-type Pd ALD, which we developed recently, operates at significantly lower temperature by growing both protected Pd nanoparticles and the support simultaneously. Additionally, these two types of Pd ALD demonstrated very different growth behaviors. Scanning transmission electron microscopy (STEM) studies showed that the size of the Pd nanoparticles could be well controlled by varying AB-type Pd ALD cycles at 200 °C, and low-temperature ABC-type Pd ALD provides a novel way to synthesize highly uniform, ultrafine, supported Pd nanoparticles directly on highsurface-area supports, regardless of loading. Both types of Pd ALD indicate that ALD is a promising technique for synthesizing advanced catalysts with precise control.
Introduction Nanometer-sized metal particles (1-10 nm), frequently named metal nanoparticles, supported on an inert support are objects of great interest in heterogeneous catalysis because of their unique chemical and physical properties, which may be distinct from those of both bulk-phase and isolated atoms.1-5 A variety of metal-nanoparticle synthesis methods have been extensively explored, including coprecipitation, impregnation, and chemical vapor deposition, wherein the control of metal particle size is obtained by adjusting experimental conditions such as the reaction temperature, reactant concentrations, reaction medium, stirring speed, protective ligands, and applied potential depending on the synthesis method.6-10 However, fine control of size and the size distribution remains an elusive goal for synthetic chemists in this field. Atomic layer deposition (ALD), also known as atomic layer epitaxy (ALE), was originally developed for the production of thin film materials in the late 1970s by Suntola et al.11,12 It relies on self-limiting, sequential binary reactions between gaseous † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. E-mail: pstair@ northwestern.edu.
(1) Henry, C. R. Surf. Sci. Rep. 1998, 31, 235. (2) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (3) Freund, H. J.; Baumer, M.; Kuhlenbeck, H. Adv. Catal. 2000, 45, 333. (4) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (5) Somorjai, G. A.; Park, J. Y. Top. Catal. 2008, 49, 126. (6) Schwarz, J. A.; Contescu, C.; Contescu, A. Chem. Rev. 1995, 95, 477. (7) Augustine, R. L. Heterogeneous Catalysis for the Synthetic Chemist; Marcel Dekker: New York, 1996. (8) Toebes, M. L.; van Dillen, J. A.; de Jong, Y. P. J. Mol. Catal. A: Chem. 2001, 173, 75. (9) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116(), 7401. (10) Niesz, K.; Grass, M.; Somorjai, G. A. Nano Lett. 2005, 5, 2238. (11) Suntola, T.; Antson, J. U.S. Patent 4,058,430, 1977. (12) Suntola, T.; Hyvarinen, J. Annu. Rev. Mater. Sci. 1985, 15, 177.
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precursor molecules and a substrate in a layer-by-layer fashion.12-14 Uncontrolled deposition through the condensation and decomposition of reactants is prevented by the choice of deposition temperature. Consequently, ALD could allow one to control the film thickness, structure, composition and conformal coatings precisely on an atomic level. These impressive features have expanded its application in the synthesis of both metal and metal oxide catalytic materials. In particular, the initial stage of ALD growth has been a primary focus in forming well-dispersed nanoparticles. Smeds et al. obtained a higher Ni dispersion on an Al2O3 support using ALD synthesis than with a commercial Ni catalyst.15 Backman et al. showed that the cobalt dispersion in ALD-prepared Co/SiO2 catalysts was also high.16 Very recently, Christensen et al. found that the size of Pt nanoparticles on SrTiO3 nanocubes could be controlled precisely by adjusting the number of ALD growth cycles using (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3) and dry air at 300 °C.17 Using the same reagents at 320 °C, King et al. also successfully deposited Pt nanoparticles with a particle size of ∼2 nm on the inner surface of a carbon aerogel.18 Herrera et al.19 reported the synthesis of highly dispersed titanium, molybdenum, and tungsten oxides on mesoporous silica and vanadium oxide on titanium oxide (13) Leskela, M.; Kemell, M.; Kukli, K.; Pore, V.; Santala, E.; Ritala, M.; Lu, J. Mater. Sci. Eng., C 2007, 27, 1504. (14) George, S. M. Chem. Rev. 2010, 110, 21. (15) Smeds, S.; Salmi, T.; Lindfors, L. P.; Krause, O. Appl. Catal., A 1996, 144, 177. (16) Backman, L. B.; Rautiainen, A.; Lindblad, M.; Jylha, O.; Krause, A. O. I. Appl. Catal. A 2001, 208, 223. (17) Christensen, S. T.; Elam, J. W.; Rabuffetti, F. A.; Ma, Q.; Weigand, S. J.; Lee, B.; Seifert, S.; Stair, P. C.; Poeppelmeier, K. R.; Hersam, M. C.; Bedzyk, M. J. Small 2009, 5, 750. (18) King, J. S.; Wittstock, A.; Biener, J.; Kucheyev, S. O.; Wang, Y. M.; Baumann, T. F.; Giri, S. K.; Hamza, A. V.; Baeumer, M.; Bent, S. F. Nano Lett. 2008, 8, 2405. (19) Herrera, J. E.; Kwak, J. H.; Hu, J. Z.; Wang, Y.; Peden, C. H. F. Top. Catal. 2006, 39, 245.
Published on Web 06/15/2010
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precovered mesoporous silica using a liquid-phase ALD method. These highly dispersed oxide catalysts exhibited superior catalytic performance relative to those prepared using conventional incipient wetness impregnation. Pd(II) hexafluoroacetylacetonate (Pd(hfac)2) has been shown to be a good candidate for Pd ALD because of its high vapor pressure compared to that of other β-diketonate derivatives of Pd.20 Senkevich et al. achieved linear growth at 80 °C on Ir metal surfaces, where atomic hydrogen, formed from molecular hydrogen disscociation on the Ir surface, was used as a reducing reagent.21 To increase the substrate generality, Ten Eyck et al. used a remote hydrogen plasma for Pd ALD growth on Ir, W, and Si surfaces.22 However, the lack of suitable reducing reagents has inhibited the application of thermal Pd ALD on different surfaces, especially on high-surface-area oxides. Recently, Elam et al. solved this problem with formalin as the reducing reagent.23 They obtained a stable Pd ALD growth rate of 0.2 A˚/cycle following a nucleation period of slow growth on an Al2O3 support at 200 °C. Hao and Elam et al. also found that these Pd/Al2O3 ALD catalysts showed high activity and hydrogen selectivity in methanol decomposition at relatively low temperature.24 The successful growth of Pd on an Al2O3 support implied that ALD could be a new way to synthesize Pd catalysts on oxide supports. However, the morphology of Pd nanoparticles formed by Pd ALD has not been well characterized, especially in the initial stage of growth. Furthermore, the deposition temperature and growth behavior on different high-surface-area oxide supports has not been explored. In a recent communication, we briefly described a new ALD method—low-temperature ABC-type ALD—for the synthesis of highly uniform ultrafine Pd nanoparticles.25 Here we compare AB-type and ABC-type Pd ALD in terms of growth and particle morphology. AB-type Pd ALD demonstrates the ability to control Pd particle size by the number of ALD cycles, and lowtemperature ABC-type Pd ALD has the capability to alter the density of Pd nanoparticles with a constant ultrafine particle size.
Experimental Section Pd ALD. Pd ALD was performed in a viscous flow reactor system with a continuous flow of ultra-high-purity nitrogen (99.999%) inert gas at a mass flow rate of 360 sccm.25,26 The reaction chamber pressure was about 1 to 2 Torr during the Pd ALD process. The Pd(hfac)2 precursor (Sigma-Aldrich, >97%) contained in a stainless steel bubbler was heated to 60 °C to produce a practical vapor pressure. Nitrogen with a flow rate of 90 sccm passed through the bubbler and carried the Pd(hfac)2 precursor to the reaction chamber. The formalin (Sigma-Aldrich) reducing reagent is a solution of 37% formaldehyde in water with 10-15% methanol as a stabilizer. The inlet lines were heated to at least 100 °C to prevent condensation. For the traditional AB-type ALD, the first precursor exposure time, the first purge time, the second precursor exposure time, and the second purge time are expressed as t1-t2-t3-t4, where all units here are given in seconds (s). In the same way, the timing for ABC-type ALD is t1-t2-t3-t4-t5-t6. In some cases, several BC (20) Igumenov, I. K.; Belosludov, V. R.; Stabnikov, P. A. J. Phys. IV 1999, 9, 15. (21) Senkevich, J. J.; Tang, F.; Rogers, D.; Drotar, J. T.; Jezewski, C.; Lanford, W. A.; Wang, G. C.; Lu, T. M. Chem. Vap. Deposition 2003, 9, 258. (22) Ten Eyck, G. A.; Senkevich, J. J.; Tang, F.; Liu, D. L.; Pimanpang, S.; Karaback, T.; Wang, G. C.; Lu, T. M.; Jezewski, C.; Lanford, W. A. Chem. Vap. Deposition 2005, 11, 60. (23) Elam, J. W.; Zinovev, A.; Han, C. Y.; Wang, H. H.; Welp, U.; Hryn, J. N.; Pellin, M. J. Thin Solid Films 2006, 515, 1664. (24) Feng, H.; Elam, J. W.; Libera, J. A.; Setthapun, W.; Stair, P. C. Chem. Mater. 2010, 22, 3133–. (25) Lu, J. L.; Stair., P. C. Angew. Chem., Int. Ed. 2010, 49, 2547. (26) Elam, J. W.; Groner, M. D.; George, S. M. Rev. Sci. Instrum. 2002, 73, 2981.
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Figure 1. In situ QCM measurements of mass gain in Pd ALD by sequential exposures of Pd(hfac)2 and formalin: (a) on Al2O3 at different temperatures and (b) on Al2O3 and TiO2 at 200 °C. cycles were used for each A cycle. For example, t1-t2-5*(t3-t4t5-t6) indicates that five BC cycles were employed after each A cycle. The number of Pd ALD cycles indicates the number of Pd(hfac)2 exposures regardless of the number of BC exposures. AB-type Pd ALD was performed by exposing Pd(hfac)2 and formalin (5-10-5-10) at the selected temperature. Lowtemperature ABC-type Pd ALD was performed by exposing either Pd(hfac)2 þ TMA þ H2O (5-10-2-10-5-10) at 80 °C or Pd(hfac)2 þ TTIP þ H2O (5-10-2-10-5-10) at 110 °C, where TMA is trimethylaluminum (Sigma-Aldrich, 97%) and TTIP is titanium tetraisopropoxide (Sigma-Aldrich, 97%). The TTIP reservoir was heated to 100 °C to achieve a sufficient vapor pressure and a reasonable dosing time.27 For some samples, ABC-type Pd ALD of Pd(hfac)2 þ 5*(TTIP þ H2O) (5-105*(2-10-5-10)) was performed to deposit additional TiO2 after each Pd deposition. In Situ QCM Measurements. A quartz crystal microbalance (QCM) in the reaction chamber was used to monitor the ALD growth. A commercial QCM housing was modified to allow a nitrogen purge over the back side of the quartz crystal, which prevents the deposition of reactants on the both sides of the quartz crystal surfaces.26 To monitor the Pd growth on Al2O3 and TiO2 supports with the in situ QCM, a 1- to 10-nm-thick layer of Al2O3 or TiO2 was first deposited on the QCM at the same temperature as used for Pd ALD. Here, the Al2O3 layer was grown by sequential exposures of TMA and Millipore water (2-5-2-5) for about 100 cycles; TiO2 was grown by sequential exposure of TTIP and Millipore water (4-5-4-5) for about 100 cycles.23,25 Subsequently, either AB-type or ABC-type Pd ALD was performed at the selected temperature. (27) Lu, J. L.; Kosuda, K. M.; Van Duyne, R. P.; Stair, P. C. J. Phys. Chem. C 2009, 113, 12412.
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Figure 2. Details of in situ QCM measurements of ABC-type Pd ALD: (a) mass gain in one Pd ALD cycle of Pd(hfac)2 þ TMA þ H2O at 80 °C; (b) ΔmPd compound and ΔmAl2O3 vary with sequential Pd ALD cycles; (c) mass gain in one Pd ALD cycle of Pd(hfac)2 þ TTIP þ H2O at 110 °C; (d) ΔmPd compound and ΔmTiO2 vary with sequential Pd ALD cycles; (e) mass gain in one Pd ALD cycle of Pd(hfac)2 þ 5*(TTIP þ H2O) at 110 °C; and (f) ΔmPd compound and ΔmTiO2 vary with sequential Pd ALD cycles.
Pd ALD on High-Surface-Area Supports. A spherical silica gel (Silicycle S10040M, surface area ∼100 m2/g) was used as an initial substrate. The silica gel (200 to 300 mg) was uniformly dispersed in a fixed-bed powder holder that was loaded into the ALD reactor. Prior to Pd ALD, the silica gel was coated with 10 cycles of Al2O3 or TiO2 by alternatively exposing TMA and H2O (50-150-200-200) at 177 °C or TTIP and H2O (60-240-120240) at 150 °C.27 A MgO (Sigma-Aldrich, 98%) support was also applied for growing Pd nanoparticles with AB-type Pd at 200 °C. The oxide supports were pretreated with ozone for 10 min at the reaction temperature before performing Pd ALD. For AB-type Pd ALD, the timing sequences were 300, 300, 300, and 300 s for Pd(hfac)2 þ formalin at 200 °C. For ABC-type Pd ALD, the timing sequences were 300, 300, 50, 150, 300, and 300 s for Pd(hfac)2 þ 16488 DOI: 10.1021/la101378s
TMA þ H2O at 80 °C and 300, 300, 60, 240, 300, and 300 s for Pd(hfac)2 þ TTIP þ H2O at 110 °C. Characterization. About 50 mg of Pd ALD samples were dissolved in a mixture of 1 mL of HNO3, 3 mL of HCl, and 1 mL of HF for 48 h.28 Thereafter, the loading of Pd was determined by inductively coupled plasma atomic emission spectroscopy (Varian VISTA ICP-AES, IMSERC at Northwestern University). Scanning transmission electron microscopy (STEM) measurements were performed on a JEOL JEM-2100F fast transmission electron microscopy system (EPIC at Northwestern University) operated at 200 kV. The size distribution of Pd nanoparticles was (28) Castano, P.; Pawelec, B.; Fierro, J. L. G.; Arandes, J. M.; Bilbao, J. Fuel 2007, 86, 2262.
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Figure 3. (a) Mass gain in one ALD cycle for both Pd(hfac)2 þ TMA þ H2O and TMA þ H2O at 80 °C. (b) The ratio ΔmAl2O3/Δm1
varies with sequential ALD cycles for both Pd(hfac)2 þ TMA þ H2O and TMA þ H2O at 80 °C. (c) Mass gain in one ALD cycle for both Pd(hfac)2 þ TTIP þ H2O and TTIP þ H2O at 110 °C. (d) The ratio ΔmTiO2/Δm1 varies with sequential ALD cycles for both Pd(hfac)2 þ TTIP þ H2O and TTIP þ H2O at 110 °C. Table 1. Comparison of Mass Gain in ABC-type Pd ALD with AB-type Al2O3 and TiO2 ALDa
ALD
Pd compound (ng/cm2)b
new support (ng/cm2)b
total mass gain (ng/cm2)c
Pd(hfac)2 þ TMA þ H2O 10.2 25.1 35.3 35.5 TMA þ H2O 2.2 6.7 9.9 Pd(hfac)2 þ TTIP þ H2O 8.9 TTIP þ H20 a The later two cases were examined by exposing TMA þ H2O (2-105-10) at 80 °C and TTIP þ H2O (2-10-5-10) at 110 °C, respectively. b Mass gain per ABC cycle. c Mass gain per ABC or AB cycle.
obtained by measuring the particle sizes of more than 200 Pd nanoparticles from several different STEM images.
Results and Discussion AB-type Pd ALD. Figure 1a illustrates the gain in sample mass during AB-type Pd ALD on Al2O3 in the temperature range of 110 to 200 °C. Negligible Pd ALD growth was observed on Al2O3 at 110 °C even after 100 Pd ALD cycles with sequential exposures of Pd(hfac)2 and formalin (5-10-5-10). With increasing reaction temperature, the Pd ALD growth on Al2O3 accelerated along with a significantly shorter initial induction period. The slow Pd growth at lower temperature and the induction period is due to Al(hfac)* surface species, formed from Pd(hfac)2 dissociation, which poison the surface and inhibit Langmuir 2010, 26(21), 16486–16495
Pd growth.29 Figure 1b compares Pd ALD on TiO2 and Al2O3 at 200 °C with identical doses and purge times. In the first several cycles, the growth rate is clearly higher on TiO2 than on Al2O3 at 200 °C as seen in the inset of Figure 1b. The reason is either reduced poisoning of the TiO2 surface or a higher reactivity of Pd(hfac)2 on the TiO2 surface or a combination of these two. The growth rate of Pd on TiO2 was 26.03 ng/cm2 per cycle (0.22 A˚ per cycle) in the stable region. This is higher than the growth rate on Al2O3 (18.41 ng/cm2 per cycle or 0.15 A˚ per cycle). The difference is most likely caused by increased surface roughness because the growth for both cases is essentially on a Pd layer after such a large number of Pd ALD cycles. Here we have to mention that the Pd growth rate obtained on Al2O3 at 200 °C was smaller than that reported in the literature (0.21 A˚ per cycle).23 Regardless of the temperature or substrate, a significant mass gain (ca. 55 to 80 ng/cm2) was observed as a result of the first Pd(hfac)2 exposure (as seen in the insets of Figure 1) whereas subsequent growth was inhibited by deposited surface species. Because the Pd nanoparticles occupy only a small fraction of the surface, it is apparent that the presence of a fresh support surface plays a key role in the significant mass gain after the first Pd(hfac)2 exposure. ABC-type Pd ALD. The observation of significant mass gain after the first Pd(hfac)2 exposure, even at low temperature, suggested that the deposition of additional fresh support on the surface would provide new nucleation sites for Pd deposition (29) Goldstein, D. N.; George, S. M. Appl. Phys. Lett. 2009, 95, 143106.
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Figure 4. STEM images of as-prepared xPd/Al2O3-200 °C samples: (a) 1Pd/Al2O3-200 °C, (b) 4Pd/Al2O3-200 °C, (c) 10Pd/Al2O3-200 °C, and (d) 25Pd/Al2O3-200 °C.
Figure 5. Increase in Pd particle size with the number of AB-type Pd ALD cycles at 200 °C.
during the second Pd(hfac)2 exposure. By sequentially repeating the exposures of Pd(hfac)2 and creating a new support, it seemed possible to deposit Pd continuously even at low temperature.25 We call this new method low-temperature ABC-type Pd ALD. Obviously, this new process requires that the precursors for growing the new support do not react with the Pd(hfac)2 precursor in order to keep Pd accessible after multiple deposition cycles. In other words, by skipping the formalin reduction step the hfac ligands retained on Pd could provide a protecting layer to prevent the support from capping the nanoparticles. 16490 DOI: 10.1021/la101378s
In a recent communication, we reported that there is essentially no reaction between Pd(hfac)2 and TMA at 80 °C or TTIP at 110 °C. The growth of ABC-type Pd ALD on both Al2O3 and TiO2, Pd(hfac)2 þ TMA þ H2O, Pd(hfac)2 þ TTIP þ H2O, and Pd(hfac)2 þ 5*(TTIP þ H2O), respectively, is essentially linear, which is very different from that of AB-type Pd ALD by sequential exposure to Pd(hfac)2 and formalin at 200 °C (Figure 1). Additionally, the growth rates of Pd(hfac)2 þ TMA þ H2O and Pd(hfac)2 þ 5*(TTIP þ H2O) were both remarkably higher than for the Pd(hfac)2 þ TTIP þ H2O sequence. The growth behavior in ABC-type Pd ALD has been investigated in more detail. ΔmPd compound (metal Pd and the retained hfac ligands), ΔmAl2O3, and ΔmTiO2 were defined as the mass gain after Pd(hfac)2, TMA þ H2O, and TTIP þ H2O (or 5*(TTIP þ H2O)) exposure in each Pd ALD cycle, respectively (Figure 2a,c,e). Figure 2b,d,f illustrates ΔmPd compound, ΔmAl2O3, and ΔmTiO2 resulting from each cycle in the sequence. As shown in Figure 2b, the Pd compound mass gain, ΔmPd compound decreased from ca. 51.5 ng/cm2 (or 9.9 10-11 mol/cm2, simply based on Pd(hfac)2) in the first Pd ALD cycle to a constant value of ca. 10.2 ng/cm2 (or 2.0 10-11 mol/cm2) per cycle after about three Pd ALD cycles. The corresponding mass gain due to the new Al2O3 support, ΔmAl2O3 exhibited a similar trend, decreasing from ca. 33.0 ng/cm2 (or 3.2 10-10 mol/cm2) in the first Pd ALD cycle to a constant value of ca. 25.1 ng/cm2 (or 2.5 10-10 mol/cm2) per cycle after about three Pd ALD cycles. The mole equivalents of the new Al2O3 support were almost 10 times larger than for the Pd compound, which indicates that Pd(hfac)2 reacted with only a small fraction of the surface hydroxyl groups that are reactive Langmuir 2010, 26(21), 16486–16495
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Figure 6. STEM images of Pd ALD samples synthesized by four cycles of AB-type Pd ALD on different oxide supports at 200 °C: (a) 10-cycle Al2O3-coated silica gel, (b) 10-cycle TiO2-coated silica gel, (c) MgO, and (d) silica gel. Table 2. Mass Gain of Pd and Al2O3 on a 100 mg 10-Cycle Al2O3-Coated Silica Gel Support Synthesized by Different Cycles of AB-type and ABC-type Pd ALDs AB type
ABC type
Pd ALD cycles
Pd mass gain (mg)
Pd mass gain (mg)
Al2O3 mass gain (mg)
1 4 6 10 12 15
0.3 1.2 1.3 2.3 2.7 3.6
0.4 2.3
19.3 27.8
3.6 4.4 5.6
32.8 35.0 37.4
towards TMA. The significant decrease in mass gain per cycle from both the Pd compound and the new support during the first three cycles can be attributed to a decrease in the available number of certain types of active hydroxyl groups for Pd(hfac)2 and in the support surface area. Pd(hfac)2 þ TTIP þ H2O showed a similar behavior, as shown in Figure 2d, but with a much lower mass gain in the stable region. ΔmPd compound and ΔmTiO2 were only 2.2 and 6.7 ng/cm2 (4.2 10-12 and 8.4 10-11 mol/cm2) per cycle in the stable region, respectively. Evidently, the formation of the new Al2O3 support was much more facile than the formation of the new TiO2 support. This result indicates that the growth rate of protected Pd nanoparticles is proportional to the new support surface area created after each B þ C exposure. This conclusion is further supported by an examination of Pd(hfac)2 þ 5*(TTIP þ H2O). Mass gains of ca. 16.8 ng/cm2 (3.2 10-11 mol/ cm2) and ca. 21.0 ng/cm2 (2.6 10-10 mol/cm2) were obtained after each Pd(hfac)2 exposure and five consecutive cycles of TTIPþ H2O, respectively, as shown in Figure 2f. Langmuir 2010, 26(21), 16486–16495
Figure 7. Descriptive scheme for AB-type Pd ALD: (a) an initial support with nucleation sites; (b) A, Pd(hfac)2 precursor is introduced onto the surface and a portion of the hfac ligands poison the surface (red curve); (c) B, formalin is introduced onto the surface to remove the hfac ligands on the Pd nanoparticles and create hydrogen adatoms on Pd nanoparticles as new nucleation sites; and (d) the size of the Pd nanoparticles gradually increases after multiple AB cycles at 200 °C.
Goldstein et al.29 reported that TMA can undergo ligand change reactions with Al(hfac)* surface species: 3AlðhfacÞ þ AlðCH3 Þ3 f AlðhfacÞ3 þ 3AlðCH3 Þ DOI: 10.1021/la101378s
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Figure 8. STEM images of as-prepared xPd/Al2O3-80 °C samples: a 1Pd/Al2O3-80 °C sample at (a) low and (b) high magnification; a
4Pd/Al2O3-80 °C sample at (c) low and (d) high magnification; and a 15Pd/Al2O3-80 °C sample at (e) low and (f) high magnification. The inset shows the Pd particle size distribution on the 15Pd/Al2O3-80 °C sample.
From eq 1, a significant mass loss would be expected if this were the dominant surface reaction during TMA exposure because the hfac ligand (207 amu) is significantly heavier than the CH3 ligand (15 amu). Figure 3a defines the mass gain after each TMA exposure, Δm1, and after each H2O exposure, ΔmAl2O3, in the ABC-type Pd ALD and AB-type ALD. The ratio ΔmAl2O3/Δm1 is diagnostic of the number of ligands lost on adsorption of the metal-containing precursor and hence of the ALD growth 16492 DOI: 10.1021/la101378s
mechanism. As shown in Figure 3b, the ratio of ΔmAl2O3/Δm1 was the same for TMA þ H2O in ABC- and AB-type ALDs. Therefore, the dominant reaction was TMA reacting with the remaining hydroxyl groups after Pd(hfac)2 exposure, but the reaction between Al(hfac)* surface species and TMA made only a minor contribution. Because one hfac ligand is replaced by one hydroxyl after a TMA þ H2O (B þ C) exposure, based on eq 1, neither -O-Al(OH)2* nor (-O-)2Al(OH)* species are added at Langmuir 2010, 26(21), 16486–16495
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Figure 9. STEM images and size distribution histogram of Pd nanoparticles on an as-prepared 4Pd/TiO2-110 °C sample.
this location. Meanwhile, these species are added at other locations on the surface. This process introduces surface roughness and an increase in oxide surface area that compensates for the loss of oxide surface area caused by Pd deposition. This process helps to explain the nearly constant ΔmPd compound and ΔmAl2O3 mass gain in the stable region of ABC-type Pd ALD on Al2O3 (Figure 2b). Pd(hfac)2 þ 5*(TTIP þ H2O) showed a negative mass gain after the first TTIP þ H2O exposure in each Pd ALD cycle (Figure 2e and the Supporting Information in ref 25), which indicates that TTIP can also undergo ligand-exchange reactions with Ti(hfac)* surface species. 4TiðhfacÞ þ TiðOC3 H7 Þ4 f TiðhfacÞ4 þ 4TiðOC3 H7 Þ
ð2Þ
As seen in Figure 3d, the ratio ΔmTiO2/Δm1 was the same for ABC- and AB-type ALD in TTIP þ H2O sequences. As with ALD sequence Pd(hfac)2 þTMA þ H2O, the dominant reaction was TTIP with surface hydroxyl groups rather than hfac species in the ALD sequence of Pd(hfac)2 þ TTIP þ H2O. However, the ligand-exchange reaction appeared to make a significant contribution during the first TTIP þ H2O exposure in the Pd(hfac)2 þ 5*(TTIP þ H2O) ALD sequence. The number of mole equivalents corresponding to ΔmPd compound was larger by an order of magnitude when five cycles of TTIP þ H2O were used instead of one, and this number is close to the mole equivalents deposited in one TTIP þ H2O exposure. Table 1 provides a comparison of the mass gains in ABC-type Pd ALD and AB-type Al2O3 and TiO2 ALD in the stable region. For the conventional AB-type Al2O3 ALD, the mass gain was ca. 35.5 ng/cm2 per cycle. In ABC-type Pd ALD, the new Al2O3 support mass gain was only 25.5 ng/cm2 per cycle. The lower Al2O3 support mass gain for ABC-type ALD is explained by Pd surface species occupation, which is formed by Pd(hfac)2 exposure before introducing TMA to the surface and a minor negative contribution due to the mass decrease resulting from TMA ligand-exchange reactions with Al(hfac)* surface species. Pd(hfac)2 þ TTIP þ H2O showed similar behavior in that the mass gain of the new TiO2 support formed in each ABC cycle (ca. 6.7 ng/cm2 per cycle) was smaller than that formed during conventional AB-type TiO2 ALD (8.9 ng/cm2 per cycle). In other words, the mass gain of the new support is smaller in ABC-type Pd ALD, consistent with the picture in which the protected Pd nanoparticles were not covered up during the ALD process. Morphology of Pd Nanoparticles from AB-type Pd ALD. To investigate the relationship between Pd particle size and the number of Pd ALD cycles, individual samples were prepared with 1, 4, 10, and 25 cycles of AB-type Pd ALD at 200 °C on spherical silica gel initially coated with 10 cycles of Al2O3 (xPd/Al2O3Langmuir 2010, 26(21), 16486–16495
Figure 10. Descriptive scheme for ABC-type Pd ALD: (a) an initial support with nucleation sites; (b) A, Pd(hfac)2 precursor is introduced onto the surface to form Pd nanoparticles protected by hfac ligands (red curves), with a fraction of the surface poisoned by hfac ligands; (c) the first reagents B and C are sequentially introduced to form a new support surface with new nucleation sites and to activate the poisoned surface by removing the hfac ligands; (d) a certain density of hfac-ligand-protected Pd nanoparticles and a new support are formed on the initial support surface after multiple ABC cycles.
200 °C). As shown in Figure 4, Pd nanoparticles were uniformly dispersed on the Al2O3 support with a reasonably narrow size distribution. As seen in Figure 5, the Pd particle size increased almost linearly from 1.1 ( 0.5 to 2.9 ( 0.9 nm. (The error bars are the standard deviations calculated from STEM measurements of more than 200 Pd nanoparticles on each sample.) The increase in Pd particle size is a consequence of the reaction between the Pd-H* surface species formed in the previous ALD cycle and the Pd(hfac)2 precursor introduced in the following exposures during AB-type Pd ALD.23 According to ICP-AES measurements, the Pd mass gain was obtained from 0.3 to 3.6 mg between 1-cycle and 15-cycle samples on the 100 mg 10-cycle Al2O3-coated silica gel (Table 2). AB-type Pd ALD on different oxide supports was also briefly investigated. The particle size and density of Pd nanoparticles grown on TiO2 and MgO were very similar to those observed on Al2O3 (Figure 6a-c). However, the AB-type Pd ALD growth was negligible on SiO2 after four cycles on the basis of the detection limit of the STEM system that we used (Figure 6d). This indicates DOI: 10.1021/la101378s
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Figure 11. STEM images of activated xPd/Al2O3-80 °C samples: an activated 1Pd/Al2O3-80 °C sample at (a) low and (b) high magnification and an activated 15Pd/Al2O3-80 °C sample at (c) low and (d) high magnification.
that the hydroxyl groups on SiO2 are less reactive than those on Al2O3, TiO2, and MgO. Figure 7 presents a schematic model of AB-type Pd ALD. The initial support surface with a number of nucleation sites for Pd(hfac)2 is shown in Figure 7a. When the initial support surface is exposed to Pd(hfac)2, Pd(hfac)* species diffuse, aggregate, and anchor on the surface and part of the hfac ligands poison the surface and inhibit Pd growth (Figure 7b). Formalin is then introduced onto the surface to remove the hfac ligands on the Pd nanoparticles, with Pd-H* surface species, Hhfac, and CO as the products of the reaction. Here, formalin can hardly remove the hfac ligands bound to the support at 200 °C. The Pd-H* surface species are the active sites for the next Pd(hfac)2 reaction (Figure 7c).23 With an increasing number of deposition cycles, the size of the Pd nanoparticles is gradually increased (Figure7d). Morphology of Pd Nanoparticles from ABC-type Pd ALD. On the as-prepared samples, STEM images at low and high magnification show that the protected Pd nanoparticles were uniformly dispersed on the Al2O3 support after various numbers of ALD cycles (xPd/Al2O3-80 °C, Figure 8). The Pd particle size was unchanged for 1, 4, and 15Pd/Al2O3-80 °C samples, but the density of the Pd nanoparticles increased. The constancy of the Pd particle size can be understood in terms of particle capping by hfac ligands, which prevent the addition of Pd atoms to the particles during subsequent deposition cycles; the increase in the Pd nanoparticle density is caused by the creation of new nucleation sites during the BC portion of the ABC-type Pd ALD. To the best of our knowledge, the formation of such a high density of uniform ultrafine Pd nanoparticles directly on a high-surface-area Al2O3 support is unprecedented. As seen in Table 2, the Pd and Al2O3 16494 DOI: 10.1021/la101378s
mass gain varied from 0.4 to 5.6 mg and from 19.3 to 37.4 mg for the 1 to 15-cycle samples on the 100 mg 10-cycle Al2O3-coated silica gel, respectively. Compared with the AB-type Pd ALD materials, the ABC-type Pd samples gave a higher Pd mass gain for the same number of cycles. Figure 9 shows the STEM images and the histogram of an asprepared sample of four ABC-type Pd ALD cycles on TiO2 at 110 °C (4Pd/TiO2-110 °C). Again, ultrafine Pd nanoparticles with a uniform size of ca. 1 nm were dispersed on the high-surface-area TiO2 surface. The Pd particle size distribution of these samples was significantly narrower than for the materials prepared using the traditional AB-type ALD at 200 °C (Figures 4 and 6). The narrow size distribution is due to both the lower reaction temperature, which decreases adsorbate diffusion rates, and the protective ligands that prevent further particle growth. Figure 10 presents a descriptive scheme of this new method. Pd(hfac)2 is first introduced onto the surface to grow hfac-ligandprotected Pd nanoparticles (Figure 10b), and a minor fraction of the surface is poisoned by the hfac ligands. A new support is created on the exposed oxide by reaction with the Bþ C precursors; meanwhile, the hfac-poisoned surface is activated by B exposure via ligand-exchange reactions, where a surface roughness is introduced that increases the surface area (Figure 10c). Repeated Pd(hfac)2 exposure, poisoned surface activation, and new support creation produce a controllable density of hfacligand-protected Pd nanoparticles and new oxide support on the initial support (Figure 10d). Ultimately, the protective hfac ligands can be removed to activate the Pd nanoparticles. One convenient method is to expose the ALD sample to formalin at 200 °C in the ALD chamber after Langmuir 2010, 26(21), 16486–16495
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deposition, which is essentially the same process as the B step in AB-type Pd ALD. As seen in Figure 11, the Pd particle size did not change after exposure to formalin at 200 °C for 1 h. Compared with the 1Pd/Al2O3-200 °C sample synthesized at 200 °C (Figure 4a), no sintering that would lead to a broader Pd particle size distribution was observed. The absence of sintering is most likely a consequence of the surrounding oxide support formed during the BC portion of the ABC cycles.
Conclusions Conventional AB-type and novel low-temperature ABC-type Pd ALD growths on oxide supports have been investigated in terms of particle growth and morphology. Our results show that AB-type Pd ALD grew more favorably on TiO2 than on Al2O3 at 200 °C using Pd(hfac)2 and formalin as reagents. Moreover, the growth rate of AB-type Pd ALD decreased on the Al2O3 surface with a lower deposition temperature until the rate was negligible at 110 °C. By growing both protected Pd nanoparticles and the support sequentially, ABC-type Pd ALD can achieve growth at significantly lower temperature. More interestingly, the surface poisoned by hfac ligands was activated after B exposure, which
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introduces roughness to increase the surface area and explains the constant growth in the stable region. STEM studies show that the Pd particle size can be tuned from 1.1 ( 0.5 to 2.9 ( 0.9 nm by increasing the number of cycles from 1 to 25 using AB-type Pd ALD on Al2O3 at 200 °C. AB-type Pd ALD was negligible on SiO2 at 200 °C, indicating that the hydroxyl groups on SiO2 are less reactive than on Al2O3, MgO, and TiO2. Low-temperature ABC-type Pd ALD achieves control of the density of uniform ultrafine Pd nanoparticles and produces an extremely narrow size distribution. Our results demonstrate that ALD is a promising technique for synthesizing next-generation catalysts with precise control in a direct deposition process. Acknowledgment. This work was financially supported by Dow Chemical Company under the Dow methane challenge project. The ALD system construction was funded by DOE (DEFG02-03ER15457), AFOSR (MURI F49620-02-1-0381 and DURIP FA-9550-07-1-0526), and DTRA JSTO FA9550-06-10558. We thank Jeffrey W. Elam, Jeffrey T. Miller, Neng Guo, and Kathryn M. Kosuda for technical assistance and David D. Graf and Lin Luo for constructive discussions.
DOI: 10.1021/la101378s
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