Article pubs.acs.org/cm
Exploring Pore Formation of Atomic Layer-Deposited Overlayers by in Situ Small- and Wide-Angle X‑ray Scattering Tao Li,† Saurabh Karwal,‡ Bachir Aoun,† Haiyan Zhao,†,§ Yang Ren,† Christian P. Canlas,‡ Jeffrey W. Elam,‡ and Randall E. Winans*,† †
X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
ABSTRACT: In this work, we explore the pore structure of overcoated materials by in situ synchrotron small-angle (SAXS) and wide-angle X-ray scattering (WAXS). Thin films of aluminum oxide (Al2O3) and titanium dioxide (TiO2) with thicknesses of 4.9 and 2.5 nm, respectively, are prepared by atomic layer deposition (ALD) on nonporous nanoparticles. In situ X-ray measurements reveal that porosity is induced in the ALD films by annealing the samples at high temperatures. Moreover, this pore formation can be attributed to densification resulting from an amorphous to crystalline phase transition of the ALD films as confirmed by high-resolution X-ray diffraction and the pair distribution function. Simultaneous SAXS and WAXS results show not only that the porosity is formed by the phase transition but also that the pore size increases with temperature.
■
INTRODUCTION Coking and sintering are two common mechanisms for the undesirable deactivation of supported noble metal nanoparticle (NP) catalysts. However, coking and sintering can be effectively suppressed by “overcoating” the noble metal nanoparticles with metal oxide layers using atomic layer deposition (ALD).1−14 Moreover, the lifetime and selectivity of the overcoated catalyst can be dramatically increased in both gas phase and aqueous phase reactions. In some cases, the ALD overcoating consists of a discontinuous submonolayer and the catalyst is used in the asdeposited state.15,16 In other cases, a much thicker ALD coating is applied and the catalyst is calcined prior to use.1−14 It is believed that nanosized pores formed during this calcination inhibit coking and sintering while maintaining the high catalytic activity.2,12,14 However, the details of this pore formation process are unknown. Understanding the mechanisms of pore formation is desirable because it may provide the necessary insights that can guide the creation of highly controllable ALDovercoated catalysts. Pore formation has not yet been conclusively understood in part because of the limitations of traditional methods such as Brunauer−Emmett−Teller (BET) and Barret−Joyner−Halenda (BJH) for the in situ characterization of porous materials. Therefore, the application of time-resolved techniques capable of characterizing and in situ probing the evolution of pore formation is of great importance. Time-resolved synchrotron Xray techniques such as small-angle (SAXS) and wide-angle Xray scattering (WAXS) are promising candidates for real-time monitoring of pore formation and evolution. Recently, simultaneous SAXS/WAXS has been demonstrated to be a © 2016 American Chemical Society
very powerful technique for studying nanoparticle growth and a variety of nanostructures such as colloid, porous materials, biomolecules, and proteins, ranging in size from 1 to 200 nm.17−21 In this article, we present a model system utilizing nonporous supports to facilitate the investigation of ALD-overcoated materials that lead to catalyst stabilization. Two different metal oxides, Al2O3 and TiO2, are deposited on nonporous spherical alumina nanoparticles (NanoDur alumina) by ALD and then annealed at high temperatures. High-temperature annealing causes the ALD coatings to become more dense because of amorphous to crystalline phase transitions, as confirmed by high-resolution synchrotron X-ray diffraction (XRD) and the pair distribution function (PDF). These changes in turn cause nanopores to form, and the pore size can be measured by the synchrotron SAXS technique. Moreover, the evolution in pore size can be monitored by the in situ SAXS/WAXS technique. These pores are formed as soon as the ALD overlayer material starts to crystallize. In addition, the pore size increases with an increase in the annealing temperature.
■
EXPERIMENTAL SECTION
Material Synthesis. Nonporous spherical alumina nanoparticles with a diameter of ∼50 nm (NanoDur, 99.5%, Alfa Aesar) were pretreated to 1000 °C to ensure that there would be no phase transitions of the support during calcination. Al2O3 and TiO2 ALD was Received: August 3, 2016 Revised: September 15, 2016 Published: September 16, 2016 7082
DOI: 10.1021/acs.chemmater.6b03222 Chem. Mater. 2016, 28, 7082−7087
Article
Chemistry of Materials conducted in a hot-walled, stainless steel viscous flow reactor having an internal diameter of 5 cm.22 Approximately 1.5 g of the NanoDur was placed in a shallow tray covered with a tight-fitting wire cloth lid to contain the powder, while allowing diffusion of ALD precursors.23 Ultra-high-purity (99.999%) nitrogen carrier gas was constantly passed through the reactor at a flow rate of 400 sccm. Al2O3 ALD was performed using alternating exposure of trimethylaluminum (TMA, 95%, Aldrich) and deionized water (H2O, 18 MΩ cm) at 200 °C (Scheme 1). Prior to coating, the
Al2O3 ALD growth per cycle (GPC) is 1.1−1.2 Å/cycle.24 A total of 45 Al2O3 ALD cycles were performed yielding a thickness of ∼4.9 nm. Similarly, TiO2 was deposited using alternating exposures of titanium tetraisopropoxide (TTIP, Aldrich) and H2O at 200 °C with a 90− 120−90−120 timing sequence. TTIP was contained in a stainless steel bubbler heated to 80 °C through which 40 sccm of nitrogen carrier gas was diverted. A total of 85 ALD TiO2 cycles were performed yielding a film thickness of ∼2.5 nm based on the GPC literature value of 0.3 Å/ cycle.25 Calcination. Synthesized materials were calcined at different temperatures of 300, 500, and 800 °C for 3 h in Thermolyne type 48000 furnaces in air. The materials were then allowed to cool to room temperature before being removed from the oven. High-Resolution X-ray Diffraction Spectroscopy (HRXRD). High-resolution synchrotron X-ray diffraction (HRXRD) was conducted at beamline 11-BM with an energy of 30 keV at the Advanced Photon Source (APS) at Argonne National Laboratory. The sample was loaded into a Kapton capillary for the measurements. Pair Distribution Function (PDF). Scattering data for PDF analysis were collected at beamline 11-ID-C at the Advanced Photon Source at Argonne National Laboratory. High-energy X-rays (115 keV) were used combined with a large area detector. The sample was loaded into a Kapton capillary for the ex situ PDF measurements. Small-Angle X-ray Scattering (SAXS)/Wide-Angle X-ray Scattering (WAXS). The SAXS/WAXS experiments were performed at station 12-ID-B with an X-ray energy of 14 keV at the Advanced Photon Source of the Argonne National Laboratory. The twodimensional (2D) images were radially averaged to produce onedimensional plots of scattered intensity I(q) versus q, where q = 4π(sin θ)/λ. Both a Pilatus 2M detector for collecting SAXS images and a Pilatus 300K detector for acquiring the WAXS images were used to collect scattering data for SAXS and WAXS with typical exposure times in the range of 0.1−1.0 s. Samples were pressed into wafers, and
Scheme 1. Representation of ALD-Overcoated Materials before and after Heating
a
After calcination at a high temperature, pores are formed because the overlayer becomes more dense upon crystallization of the initially amorphous layer.
NanoDur was allowed to thermally equilibrate for 10 min and to remove physisorbed moisture and then was cleaned using a 15 min exposure to flowing ozone produced by a commercial ozone generator (Ozone Engineering L11) using a feed of ultra-high-purity oxygen at a flow rate of 400 sccm to produce ∼10% ozone in oxygen. TMA and H2O were alternately injected into flowing nitrogen at 50 sccm. The timing sequences can be expressed as t1−t2−t3−t4, where t1 is the TMA exposure time, t2 is the purge time following TMA exposure, t3 is the H2O exposure time, and t4 is purge time following H2O exposure. The Al2O3 ALD timing sequence was a 50−70−50−70 sequence. The
Figure 1. (a) SAXS data of 45 cycles of Al2O3 overcoating prepared by ALD on NanoDur before and after heating at 300 °C. (b) SAXS data of 45 cycles of Al2O3 overcoating on NanoDur before and after heating at 800 °C. (c) Fitted curve of subtracted data of the Al2O3 overlayer coated on NanoDur calcined at 800 °C. (d) Pore size distribution obtained after fitting the spherical pore model on subtraction result. 7083
DOI: 10.1021/acs.chemmater.6b03222 Chem. Mater. 2016, 28, 7082−7087
Article
Chemistry of Materials the wafers were held inside a Linkam stage (TS1500) for in situ measurements. The data were analyzed and fitted using Irena.26
■
RESULTS AND DISCUSSION In our previous studies, we characterized the overcoated catalyst by SAXS. Typical SAXS curves show the intensity (arbitrary) of SAXS as a function of q (in inverse angstroms). Figure 1a shows the SAXS data of a NanoDur sample coated with 45 cycles of Al2O3 with a layer having a thickness ∼4.9 nm before heating (filled blue triangle) and after being heated at 300 °C (empty red circle). The two curves are identical to each other. In other words, there was no change after the overcoated Al2O3 layer was heated at 300 °C, which indicates there is no pore formation. However, when the sample was calcined to a higher temperature of 800 °C, the SAXS curve (Figure 1b) showed a clear intensity increase in the q range from 0.035 to 0.1 Å−1 compared to that of the unheated sample. The q < 0.035 Å−1 data are not selected because of the larger signal-tonoise ratio. This increase in intensity is possibly due to structural changes in either the NanoDur support or the overcoated Al2O3 layer before and after calcination. In situ SAXS showed that the support (i.e., NanoDur calcined at 1000 °C) is stable and no change in intensity took place during calcination. Thus, the difference in intensity before and after calcination results from the change in the Al2O3 overcoat, indicating pore formation in the overcoat. The SAXS intensity data of the NanoDur-overcoated Al2O3 layer material at an elevated temperature of 800 °C were subtracted from the SAXS intensity data of the starting NanoDur overcoated Al2O3 layer material without heating to produce Figure 1c. A spherical model for the pore size distribution was fit to the subtracted data in the q region of 0.035−0.1 Å−1 using the Modeling II tool in Irena within the IgorPro application.26 From the data fitting (green curve in Figure 1c), the average pore diameter of the overcoated Al2O3 was estimated to be around 10 nm based on volume distribution, as shown in Figure 1d. It should be noted that the pore size varies depending on many factors such as the support, the size and composition of the catalyst, and the thickness of the overlayer. For example, in the case of the SiO2supported Cu catalyst overcoated with 30-layer AlOx, the pore size is around 1 nm from SAXS measurement.12 However, for the 30-cycle TiO2-overcoated Co/TiO2 catalyst, the pore size is around 16 nm.11 Thus, we believe the pore formation and size are controlled by a variety of parameters such as the support, the size and composition of the catalyst, the thickness of the overlayer, the heating rate, and the heating conditions. Our work, applying SAXS to systemically study pore formation under different conditions, is ongoing. The formation of the pores is assumed to be due to the overcoated Al2O3 layer becoming more dense upon changing from an amorphous to a crystallized phase. To investigate this hypothesis, synchrotron high-resolution X-ray diffraction (HRXRD) data were obtained. An initial study was performed on NanoDur overcoat Al2O3 samples before calcination, as shown in Figure 2. The black curve in Figure 2a shows data for the NanoDur support without coating. Upon deposition of the Al2O3 overlayer (red curve in Figure 2b), peaks start to broaden as indicated in the highlighted regions, which may be due to the amorphous nature of the overcoated thin film. After the NanoDur background had been subtracted, the thin Al2O3 overlayer clearly shows two broad peaks, indicating that the thin film is amorphous.
Figure 2. (a) High-resolution XRD data of the NanoDur support and a 45-cycle Al2O3 overcoat on NanoDur before heating. (b) HRXRD data of only the Al2O3 overlayer after subtracting NanoDur from the Al2O3 overcoating on NanoDur without heat.
In addition, the atomic pair distribution function (PDF) was performed to obtain further quantitative structural information at the atomic scale. Recently, PDF analysis has emerged as a powerful and unique technique for investigating crystalline and amorphous materials. Figure 3a is the PDF of NanoDur and NanoDur overcoat Al2O3 samples before calcination. There is a
Figure 3. (a) PDF data of the NanoDur support and a 45-cycle Al2O3 overcoating on NanoDur before heating. (b) Data after NanoDur background subtraction. 7084
DOI: 10.1021/acs.chemmater.6b03222 Chem. Mater. 2016, 28, 7082−7087
Article
Chemistry of Materials
determine the new formed crystalline phase of the Al2O3 overlayer after heating. To further confirm that pore formation is indeed due to densification, another material, TiO2, was coated on the NanoDur support. We deposited 85 cycles of TiO2 with a 2.5 nm film thickness on the NanoDur support. Generally, TiO2 can exist in four distinct phases (amorphous, anatase, rutile, and brutile), unlike Al2O3, which mostly occurs in polycrystalline mixed phases (α, δ, θ, and γ). Before being heated (Figure 5a), the 85-cycle TiO 2 overlayer after subtraction of the NanoDur background (blue curve in Figure 5a) shows a broad peak that can be attributed to the amorphous nature, similar to that of the Al2O3 overcoating shown above in Figure 2. Different from the Al2O3 overcoat layer, the XRD pattern after heating at 500 °C clearly shows seven sharp diffraction peaks, indicating crystallization (red curve in Figure 5b). After indexing, the new peaks are the (101), (004), (200), (105), (211), (204), and (116) planes of anatase TiO2. As the calcined temperature increases to 800 °C, the peaks become much sharper, indicating growth of the TiO2 nanocrystals. Interestingly, the 2.5 nm thick overcoated TiO2 does not change to the rutile phase, while in bulk, the transformation of anatase to rutile occurs at approximately 500−600 °C.13,27 Because the pore size and pore formation mechanism can be obtained from scattering and diffraction measurements, respectively, the whole process of pore opening during heating can be monitored using in situ SAXS/WAXS. Figure 6a shows a contour plot of the in situ SAXS/WAXS data of the NanoDur TiO2 overlayer sample during the whole calcination process with the temperature increasing from 30 to 1000 °C at a heating rate of 20 °C/min. q ranges up to 2.0 Å−1. The first clear indication of a phase change occurred during the initial heating up to around 500 °C, as denoted by the arrow in Figure 6a. The new peak occurred at the q position of 1.83 Å−1 that is related to the (101) peak of the anatase phase of TiO2. To improve our understanding of the growth of anatase TiO2, the (101) peak area over a q range from 1.75 to 1.96 A−1 has been integrated and is plotted in Figure 6b. Below 470 °C, the values of the integrated area remain the same within the noise level. Above 470 °C, the values become larger because the (101) peak becomes sharper, indicating the ALD overlayer is more crystallized. The q values remain almost the same during the calcination, indicating there is no lattice expansion or shrinkage.17,28 The scattering intensity remains the same below around 500 °C, indicating there is no pore formation. Above that
greater difference at short range (
