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Morphological Characterization of ALD and Doping Effects on Mesoporous SnO Aerogels by XPS and Quantitative SEM Image Analysis 2

Juan-Pablo Correa-Baena, Kateryna Artyushkova, Carlo Santoro, Plamen Atanassov, and Alexander G. Agrios ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00019 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 29, 2016

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ACS Applied Materials & Interfaces

Morphological Characterization of ALD and Doping Effects on Mesoporous SnO2 Aerogels by XPS and Quantitative SEM Image Analysis Juan Pablo Correa Baena,†,1,2 Kateryna Artyushkova,3 Carlo Santoro,3 Plamen Atanassov,3 and Alexander G. Agrios*,1,2 1

Civil and Environmental Engineering, 2Center for Clean Energy Engineering, University of

Connecticut, Storrs, Connecticut 06269 and 3Department of Chemical & Biological Engineering, Center for Micro-Engineered Materials (CMEM), University of New Mexico, Albuquerque, NM 87131, USA KEYWORDS: Aerogels, tin oxide, porous electrodes, image processing, surface characterization.

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ABSTRACT Atomic layer deposition (ALD) is unsurpassed in its ability to create thin conformal coatings over very rough and/or porous materials. Yet while the coating thickness on flat surfaces can be measured by ellipsometry, characterization of these coatings on rough surfaces is difficult. Here, two techniques demonstrated to provide such characterization of ALD-coated TiO2 over mesoporous SnO2 aerogel films on glass substrates, and insights are gained as to the ALD process. First, X-ray photoelectron spectroscopy (XPS) is used to determine the coating thickness over the aerogel, and the results (0.04 nm/cycle) agree well with ellipsometry on flat surfaces up to a coating thickness limit of about 6 nm. Second, quantitative analysis of SEM images of the aerogel crosssection is used to determine porosity and roughness, from which coating thickness can be inferred. The analysis reveals increasing porosity from the aerogel/air interface to the aerogel/substrate, indicating a thicker ALD coating near the air side, which is consistent with tortuous diffusion through the pores limiting access of ALD precursors to deeper parts of the film. SEM-derived porosity is generally useful in a thin film since bulk methods like nitrogen physisorption or mercury porosimetry are impractical for use with thin-film samples. Therefore, in this study SEM was also used to quantitatively characterize the morphologogical changes in SnO2 aerogel thin films due to doping with Sb. This study can be used as a methodology to understand morphological changes in different types of porous and/or rough materials.


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Introduction Atomic layer deposition (ALD) is a versatile technique for making thin films by exposing a substrate to alternating pulses of two different gases in an otherwise inert atmosphere.1 Metal oxide films can be made using metal-salt or organometal precursors and water as the two reacting gases. Self-limiting chemistry results in a single atomic layer with each deposition cycle. The film thickness can therefore be precisely controlled by choosing the number of deposition cycles. Crucially, since it is based on gas diffusion, the technique is relatively indifferent to surface morphology. Therefore, ALD has an unparalleled capability to create homogeneous, conformal, dense coatings over very rough materials (such as nanorod films),2 and even throughout the pores of porous materials (such as aerogels).3-5 Aerogels are 3D structures composed of interconnected nanoparticles, exhibiting low density, and high porosity and surface area.6 They can be made from a variety of materials. Metal oxide aerogels are typically made by facile sol-gel processes and dried supercritically.7-10 The epoxy-assisted solgel method has been widely used and proved to be a reliable and fast process that can be used to produce aerogels of various metal oxides using inexpensive precursor salts.9-12 The properties of aerogels are beneficial for different applications including photocatalysis,13-14 solar cells4-5, 15-16 and water splitting devices.17 The open pore structure of aerogels allows for fast diffusion of liquid components, such as electrolytes, and increased interfacial contact between liquids and particles due to the inherent high surface area. Typically, bulk measurement techniques, especially nitrogen adsorption/desorption, are used to study the porosity of aerogels. Pore volume and size in the range of 1-5 cm3g-1 and 1-500 nm, respectively, are common in these materials.17-19


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We recently reported Sb-doped SnO2 aerogels, both in bulk form20 and as thin films,21 and applied ALD coatings of TiO2 to the latter for application as photoanodes in nanostructured solar cells.21 The use of ALD with such porous materials introduces new challenges of characterization. The thickness of ALD layers on very flat surfaces (e.g., a Si wafer) can be measured optically by ellipsometry, but this cannot be used on rough surfaces. It is not known how the thickness of an ALD coating varies through the depth of a porous film. At the same time, standard characterizations of porous materials, like the aforementioned nitrogen sorption with BET and BJH analyses, is practical only for bulk samples. To obtain sufficient sample from thin films requires depositing large areas of them, and scraping them off into a container, which obviously can alter the pore structure. Atomic force microscopy (AFM) can be used to determine the porosity of thin samples, but the wide range of heights in a very porous material is challenging to image by AFM and often requires long acquisition times. In addition, fragile samples like aerogels can be damaged by the tip. Here, we report on the use of two non-contact analyses for the characteriztion of thin-film aerogel porosity and ALD coatings over aerogels. First, X-ray photoelectron spectroscopy (XPS) was used to determine the thickness of an ALD coating over an aerogel film, based on intensity ratios of signals from atoms in the coating and the aerogel. Second, quantitative analysis of SEM images of the cross-section of ALD-coated aerogels was used to determine a porosity profile. This method was previously shown to give results that match porosity values from AFM analysis.22 Based on the porosity profile, we identify a variation of ALD coating thickness through the depth of the aerogel film. Finally, we used porosities extracted from SEM images to characterize morphological changes in an SnO2 aerogel (without ALD coatings) induced by doping with Sb. Our work demonstrates how the relatively facile technique of SEM microscopy can be used for


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quantitative structural characterization of thin, porous materials that are difficult to characterize by other methods. Experimental Materials Preparation The sol-gel and supercritical drying procedures for bulk20 and thin film aerogels21 are explained in detail in our previous publications. Samples were sintered at 450˚C for 30 min after heating at a rate of 3 ˚C/min. SEM Microscopy and Analysis SEM (FEI Quanta FEG250 SEM in high vacuum mode) at 50 K magnification was used to analyze the morphological features of the films. Aerogels on FTO glass electrodes were used for cross-sectional SEM by snapping the glass through the area of interest (i.e. the aerogel film). Five different SEM images were acquired from five different areas on the samples to provide statistical representation of morphology. In order to ensure that the intensity variation is solely caused by the sample morphology and not by the different instrumental conditions, all the images were recorded at exactly the same experimental settings (i.e., voltage, magnification, brightness and contrast, current and gain). Digital image processing was done using an in-house developed GUI23 in Matlab using the Image Processing Toolbox. The porosity and roughness of the sample were obtained from SEM images as shown previously.24 The typical surface roughness can be separated in: i) high-frequency component of the images that represent fine irregularities called roughness and ii) low-frequency component of the images that represent wider spaced irregularities called waviness. It is crucial to examine separately waviness


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and roughness for analysis of morphology at different scales. High pass filter was used to subtract the low-frequency components, and low-pass filter was used to subtract the high-frequency components. Through filtering, we have separated images into their high-frequency and lowfrequency components, representing 63 - 256 nm and 700-1600 nm scales of roughness and porosity, respectively.25 Roughness and skewness parameters calculated from these component images describe morphological properties at two different scales that may be related to different properties of aerogels. Roughness parameter represents vertical heterogeneities while skewness is directly related to porosity. We will refer to the high-frequency parameters as micro-roughness and micro-skewness and low-frequency parameters as meso-roughness and meso-skewness. XPS XPS measurements were performed with a Kratos Axis Ultra DLD X-ray photoelectron spectrometer using a monochromatic Al Kα source operating at 225W. Survey and high resolution C1s, O1s, Ti 2p and Sn 3d were acquired at 80 and 20 eV pass energy, respectively. Charge compensation was accomplished using low energy electrons. Standard operating conditions for good charge compensation were –3.1 V bias voltage, -1.0 V filament voltage and a filament current of 2.1 A. The data were acquired from 3 different areas per sample. The thicknesses of TiO2 on top of aerogel substrates were calculated using the substrate/overlayer model. 26 The area under the Ti 2p peak was used as a signal from overlayer while the area under the Sn 3d peak was used to represent a substrate. Nitrogen Sorption A Micromeritics ASAP 2020 accelerated surface area and porosimetry analyzer was used to perform automated nitrogen physisorption measurements on bulk aerogel samples. Specific


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surface area and pore volume were calculated by the Micrometrics software based on the standard Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models, respectively. Results and Discussion Characterization of TiO2 Overlayers XPS was used to characterize the thickness of the TiO2 overlayers on the aerogel materials. Estimating the thickness of an ALD overlayer is typically done by ellipsometry on a silicon wafer.21 This process can be inaccurate as the bonding of the precursors to the host material (e.g. Si/SiO, FTO, etc) can vary from substrate to substrate.27 Ellipsometry for samples at 150 and 300 cycles of TiO2 yielded a rate of approximately 0.04 nm/cycle for both depositions. By using XPS overlayer model, we can elucidate thickness of the TiO2 overlayer on top on underlying substrate (Figure 1). The XPS derived thickness matched well the ellipsometry data as seen in Figure 1a, except 300 cycles that showed a lower TiO2 thickness than that observed with ellipsometry. Figure 1b and c shows high resolution Sn 3d and Ti 2p spectra for 0,50, 150 and 300 cycles. For 0 cycles no signal of Ti is detected in the Ti 2p spectrum, as expected. As the number of cycles increases, the signal from Sn decreases while the signal from Ti increases reaching a maximum at 150 cycles. No signal of Sn is observed at 300 cycles due to the measurement’s limitation by XPS which is between 5-10 nm. Errors in the absolute thickness are expected for overlayers on top of SnO2 thicker than the XPS sampling depths of Sn. Below this limit, however, deriving thin film thickness from XPS measurements is useful for such nanometer-thin overlayers and we show that they are in good agreement with the ellipsometry-derived TiO2 layer thickness.


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Figure 1. TiO2 thickness as obtained from ellipsometry and XPS (a) and the high resolution Sn 3d (b) and Ti 2p (c) spectra for subset 0, 50, 150 and 300 cycles.

XPS provides the integral thickness information while morphological information is overlooked. To add more insight into properties of the overlayers grown on rough substrates such as aerogels, the microscopic analysis was performed. TiO2 overlayers (by ALD) are often grown on mesoporous scaffolds of different kinds to provide a core-shell architecture useful for certain device configurations. The high aspect ratio of these porous materials, however, makes it difficult to coat such layers evenly through several microns thick film. This is due to the tortuous path that precursors have to go through to reach the substrate.

To study the coverage of TiO2 on aerogel films we used roughness and porosity analysis of SEM images as presented in Figure 2. First, we analyzed statistical parameter from top-view images of the films to understand how morphological properties change when TiO2 is added by ALD. As evident from the SEM images (Figure 2a, right), the morphology of the films is considerably


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affected by the TiO2 overlayers on the SnO2 nanoparticle array. The micro-roughness (63-256 nm) is the most affected by the TiO2 coating, decreasing dramatically over the first 50 cycles (or a few nm overlayer) (Figure 2a). This is expected as the micropores of the aerogel are being filled up by the overlayer, making the surface smoother. A less pronounced effect is seen in the meso range (0.7 – 2 µm), where further smoothing of the surface occurs. Porosity, on the other hand, does not change significantly with the overlayer thickness in either the micro or meso scale. TiO2 deposited by ALD should coat the interior of pores and reduce porosity, but the change in porosity from a few nm coating on pores tens of nm thick may be beyond the resolution of this measurement. The SEM analysis was also performed on the cross-section of an aerogel film coated with 150 cycles of TiO2 by ALD (Figure 2 b, right). Visually, it is difficult to observe any trend from bottom to top, but quantitative image analysis reveals a clear trend in roughness. Figure 2b, left, plots porosity and roughness at the intermediate scale of 174-700 nm, showing a strong decreasing trend in roughness from top to bottom, while the porosity has no significant trend. These results are in good agreement with changes in morphology with the number of ALD cycles in Figure 2a. The bottom part of the film has the higher roughness (similar to that of uncoated aerogels, Figure 2a) and, therefore, low or no coverage. Presumably, precursors have trouble reaching the bottom part of the film during the ALD process due to diffusion limitations. From the middle through the top, the film shows a decrease in roughness, suggesting these areas have more TiO2 coverage as they follow the same trend as that in Figure 2a. We can deduce that diffusion of precursors inside an intricate 3D network, like an aerogel, results in a gradient in deposited layer thickness, increasing from the substrate toward the surface directly exposed to the reactant gases. Some reports have demonstrated such limitations for 1D high aspect ratio structures,28 however, to our best knowledge, we report the first quantitative analysis probing the coating of 3D nano architectures.


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This facile technique is very useful for micro and meso-structured configurations where ALD is performed.

Figure 2. Statistical parameters calculated from SEM images as a function of TiO2 ALD cycles. Roughness and porosity calculated from (a) top-view SEM images of films without and with TiO2 overlayers and (b) a cross-sectional analysis of a film coated with 150 cycles. Morphological Properties In our previous works,20-21 we thoroughly studied the role of Sb doping in the optoelectronic and photovoltaic properties of these aerogels and found that aerogels of Sb-doped SnO2 can be cast as


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thin films and used electrodes in DSSCs. The morphological properties of bulk and film aerogels are further investigated herein. The SEM images were acquired from a flat part at the top of the cylindrical monolith and the top of the film adhered to an FTO glass slide after calcination at 450˚C. Figure 3 shows SEM images at different magnifications for aerogels of doped and undoped SnO2 for films (Figure 3, top row) and bulk aerogels (Figure 3, bottom row). As visible in the images, the morphology of the films is drastically different from that of bulk materials. Undoped SnO2 aerogels (Figure 3a, e, c and g) did not appear significantly different from aerogels with 20% Sb doping (Figure 3b, f, d and h). Aerogel films appear to have macropores over 500 nm in diameter (Figure 3e and f) whereas these are not present in the bulk analogs (Figure 3g and h).

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Figure 3. SEM images of films (a, b, e, f) and bulk (c, d, g, h) for undoped (a, e, c, g) and 15% Sb doped (b, f, d, h) SnO2 aerogels at different magnifications. To fully understand and quantify the effect of the dopant on the overall morphology of the aerogel materials, we calculated roughness Ra and skewness Rsk parameters from SEM images, as described in our previous work.29 Figure 4 summarizes the results for both of these parameters for


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bulk and film aerogels. Meso-skewness of bulk aerogels decreases slightly with the increase in antimony doping content (Figure 4a, black line). The declining trend is in good agreement with nitrogen physisorption-derived Barrett-Joyner-Halenda (BJH) pore volume of bulk aerogels (Figure 4a, red line), which also shows a decreasing trend with an increase of Sb concentration. This validates the use of SEM image-derived mesoporosity as a measure of morphological properties of materials for which N2 physisorption is not possible (e.g. mesoporous thin films). The calculated mesoscale (700-1600 nm) Rsk of films exhibits slightly lower values but the same trend as for the bulk materials. Interestingly, this trend is reversed at the micropore scale (63-256 nm, Figure 4b), i.e., incorporation of Sb results in less mesoporosity but more microporosity. For both films and bulk aerogels, undoped and 5% Sb-doped conditions have similar and comparable microporosity. However, a drastic change in the micropores is seen at Sb doping levels of 15% or higher. Films with 20 and 30% Sb content have double the porosity of the corresponding bulk aerogels. This effect may be related to different shrinkage mechanisms in the bulk and the films which are affected by the addition of the Sb precursors.


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Figure 4. Statistical parameters calculated from SEM images as a function of Sb content in SnO2 aerogels. Low-frequency skewness (a) for bulk and film aerogels compared to nitrogen physisorption-derived Barrett-Joyner-Halenda pore volume for bulk analogs. Roughness (b) of films and bulk aerogels for high frequency. Figure 5a shows the analysis of roughness derived from SEM images for film and bulk aerogels at the micro and meso scales. Roughness at both scales increases with increasing Sb content for both film and bulk aerogels, but it is more pronounced in the film. The roughness of bulk materials was similar in the micro and meso ranges regardless of Sb content in the material. Roughness in films


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differed from that in bulk aerogels. At the mesoscale, the films are similar to the bulk structures in the lower % Sb content (up to 15%), but at 20% Sb the meso-roughness increases by about a third. This is similar to the trend for micro-skewness found in Figure 4b, where at 20% Sb there is a large increase in the microporosity. Films also exhibit an increase in micro-roughness with an increase of Sb content. This increase in the roughness of the bulk and film aerogels may be indicative of increased shrinkage of the structure where nanoparticles agglomerate to form structures with higher roughness on the microscale. The quantification of roughness at different scales can help us understand the difference between the morphology of bulk and film aerogels and can be used as a method to study shrinkage behavior in aerogels. This is crucial for obtaining high surface area thin monolithic films, which are desirable for nano-based device applications. Based on these results, we propose that the difference between the roughness of bulk and film structure, which is particularly large in the microscale, is due to the directionality of the shrinkage during the gelation or calcination steps as depicted in Figure 5b and c. During the gelation process, olation and oxolation occur to form a network of interconnected particles that are further supercritically dried and calcined. It is well known that shrinkage occurs during these steps, but interestingly, our results show that the process is different depending on the casting conditions. For free-standing bulk gels (2 cm high x 1.5 cm in diameter), shrinkage occurs in all directions as there are no physical constraints induced by attachment of the gel to the borosilicate glass vials in which they are processed. However, for thin films grown on FTO glass the adhesion force is sufficient to obstruct lateral shrinkage within the plane of the film, and shrinkage instead occurs in the transverse direction, leading to irregular film thickness and forming deep pores, which is observed as increased roughness at the top of the film (Figure 5b and c).


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Figure 5. (a) Statistical parameters calculated from SEM images as a function of Sb for films and bulk aerogels. Schematic of shrinkage after heat treatment for (b) films and (c) bulk aerogels. Conclusions Nitrogen physisorption, quantitative SEM image analysis, and XPS were used to characterize ALD of TiO2 throughout mesoporous SnO2 aerogels, made as thin films and as bulk monoliths, and the morphological effects of Sb doping of the SnO2. We found good agreement of the SEM-derived porosity with that derived from N2 physisorption. We proposed a model to explain the morphological variations seen in film and bulk aerogels. The SEM technique was also used to understand TiO2 coverage throughout the cross-section in an aerogel film, and it was found that ALD results in more TiO2 deposited at the top of the film (near the film/air interface) than at the bottom (the film/substrate interface). In addition, XPS was used to measure the thickness of TiO2 grown by ALD on SnO2 aerogels and matched well the values estimated from ellipsometry. AUTHOR INFORMATION Corresponding Author A.G.A. [email protected]; Tel: +1 860 486 1350;


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Present Addresses †1Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENT This material is based upon work supported by the National Science Foundation under Grant No. CBET-1332022. J.P.C.B was also supported by NSF Grant No. DGE-0947869. SEM and N2 physisorption were performed at the Center for Clean Energy Engineering at the University of Connecticut. REFERENCES 1.

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Morphological Characterization of ALD and ... - ACS Publications

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