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with Tunable Feature Sizes

Monika M. Biener ,* Jianchao Ye , Theodore F. Baumann , Y. Morris Wang , Swanee J. Shin , Juergen Biener , and Alex V. Hamza

sub-10-nm range and high surface areas, they typically have much higher densities (>100 mg/cm 3 ). [ 11,13,14 ]

Driven by the desire to improve the control over the cel-lular architecture of monolithic ultralow-density bulk mate-rials, and enabled by recent advances in materials engineering, templating has therefore gained increasing interest as an alter-native to the self-assembly strategy. [ 1,4,15 ] Here, the template only provides the blueprint for the three dimensional (3D) architecture but does not have to be a low-density material itself thus relaxing many materials selection constraints. Most importantly, templating opens the door to control the density without affecting the cellular architecture. The challenge, how-ever, is to identify universal porous bulk material templates and coating processes that allow for the fabrication of nm-thick uniform coatings which is a prerequisite for ultralow-density high surface area materials. An exciting new development in this direction is the recent realization of mechanically robust ultralight microlattice bulk materials with uniform, mm-to-cm sized cell architectures by using sacrifi cial templates generated, for example, by a self-propagating photopolymer waveguide technique. [ 4 ] Although engineered microlattice templates pro-vide unprecedented control over cell size and architecture, they are still limited to larger, micrometer-sized features which pre-vents the realization of high volumetric surface areas. This is the domain of nanostructured templates formed by self-organ-ization. Recent interesting developments include the realiza-tion of aerographite from nanostructured ZnO templates [ 1 ] and nanoporous gyroid nickel from block copolymer templates. [ 15 ] Although the aerographite study is limited to a very specifi c materials system and the block copolymer templates offer only a narrow range of accessible length scales, both examples nevertheless clearly demonstrate the power of the templating approach.

Here we report on the development of ultralow-density (>5 mg/cm 3 ) bulk materials with interconnected nanotubular 3D architecture and deterministic, independently control-lable pore size distributions and densities by using an atomic layer deposition (ALD) based sacrifi cial templating approach ( Figure 1 ). Our work was motivated by LLNL’s need to develop brighter, nanosecond-scale X-ray (∼5–10 keV) sources by laser-irradiating ultra-low density (sub-10 mg/cm 3 ) high-Z foam materials. [ 16 ] Deterministic density control is achieved through the atomic-scale thickness control of the ALD process, and cell size control is provided by the sacrifi cial template, nano-porous gold (NPG). Similar to block copolymer templates, NPG is formed by a self-organization process [ 17 ] that yields a narrow unimodal ligament/pore size distribution. In contrast to block copolymer based templates, however, the length scale

Unlocking the full potential of ultralow-density cellular bulk materials requires realization of mechanically robust architec-tures with deterministic control over cell size, density and com-position. A high strength-to-weight ratio makes interconnected tubular networks an attractive design strategy for reducing the superlinear decrease in strength and stiffness of low-density materials with increasing porosity. Here we report the synthesis of ultralow-density, ultrahigh surface area bulk materials with interconnected nanotubular morphology. Deterministic con-trol over density (5 to 400 mg/cm 3 ), pore size (30 nm to 4 µm) and composition is achieved by atomic layer deposition (ALD) templating using nanoporous gold as a tunable template. The materials are thermally stable and, by virtue of their narrow unimodal pore size distributions and thin-walled tubular design, ∼10 times stronger and stiffer than traditional aero-gels of the same density. The three-dimensional nanotubular network architecture opens new opportunities in the fi elds of energy harvesting, catalysis, sensing and fi ltration by enabling mass transport through two independent pore systems sepa-rated by a nanometer-thick 3D membrane.

Monolithic ultralow-density (<10 mg/cm 3 ) porous bulk mate-rials have recently attracted renewed interest [ 1–6 ] due to many promising applications including catalysis, energy storage and conversion, thermal insulation, shock energy absorption and high energy density physics. [ 7,8 ] Because their surface area scales inversely with the architectural feature size, nanostructured materials are the preferred choice whenever an application requires a high surface area. The longest known low-density, high surface area bulk materials are aerogels [ 9 ] whose self-sim-ilar, fractal network structure is formed by crosslinking colloidal nanoparticles. [ 10 ] But despite the progress that has been made since their discovery more than 80 years ago, [ 9 ] control over the aerogel architecture remains to be challenging, especially in the very low density regime <20 mg/cm 3 where with a few exceptions such as silica [ 6 ] and graphene [ 3 ] most aerogel chem-istries fail to produce stable monolithic structures. Other nanostructured porous materials formed by self-assembly techniques include metal organic framework structures [ 11,12 ] and mesoporous materials. [ 13 ] Although these materials can provide well-defi ned, very narrow pore size distributions in the

M. M. Biener, J. Ye, T. F. Baumann, Y. M. Wang, S. J. Shin, J. Biener, A. V. Hamza Nanoscale Synthesis and Characterization Laboratory Lawrence Livermore National Laboratory 7000 East Ave , Livermore , CA 94550 , USA E-mail: biener3@llnl.gov

DOI: 10.1002/adma.201400249

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of NPG can easily be changed by two orders of magnitude, from 30 nm to 4 µm, without changing porosity or ligament connectivity using a simple annealing process. [ 18 ] NPG has fi rst been used as template by Ding et al., who coated 100-nm-thick free-standing NPG membranes with platinum by placing them at the interface between a metal ion solution and vapor of a reducing agent. [ 19 ] Unfortunately, this diffusion limited technique is diffi cult to extend towards bulk materials. Atomic layer deposition, on the other hand, is ideally suited to deposit uniform and conformal thin fi lms with atomic scale thickness control on ultra-high aspect ratio materials. [ 20–22 ] Recently, we have shown that the ALD approach can successfully be applied to several hundred micrometer-thick NPG samples to improve the mechanical and catalytic properties of the material. [ 23 ] Here, we demonstrate the synthesis of ultralow-density Al 2 O 3 and TiO 2 bulk materials with interconnected nanotubular mor-phology and independent deterministic control over a very wide range of density and feature size by removing the Au template through a standard gold etch process [ 24 ] followed by supercrit-ical drying. The resulting materials are transparent, very uni-form, and exhibit excellent mechanical properties due to their uniformity and thin-walled tubular morphology that resembles nature’s design of bamboo. The concept was demonstrated for Al 2 O 3 and TiO 2 ALD coatings that provide high thermal sta-bility [ 25 ] and photocatalytic functionality [ 26 ] , but can easily be extended to other ALD processes.

Figure 2 shows representative transmission electron micro scopy (TEM) and optical images from a 200-µm-thick nanotubular TiO 2 disk with 50 nm pores and a density of ∼150 mg/cm 3 that was obtained by coating a non-annealed NPG disk (Figure 2 A) with ∼2 nm TiO 2 (30 ALD cycles) and removing the Au core through a wet etch process. The sample is crack-free, transparent and, despite of its low density, mechani-cally robust. Shrinkage was not observed. The interconnected

nanotubular morphology of the material is confi rmed by SEM (Supporting Figure S1) and TEM images shown in Figure 2 B and Figure 2 C. Higher magnifi cation TEM images (Figure 2 C) reveal that the wall thickness of the nanotubular structure is very uniform. The measured wall thickness of ∼2 nm is consistent with the expected ALD growth rate (0.07 nm/cycle) and den-sity measurements. The gravimetric surface area of the mate-rial was determined from the measured density of nanotubular monoliths and the Brunauer–Emmett–Teller (BET) surface area of NPG templates. For example, a nanotubular foam derived from a 2.5 nm thick Al 2 O 3 coating (128 mg/cm 3 , ∼58 nm pores) has a specifi c surface area of ∼330 m 2 /g (considering both inner and outer surface of the tubular structure) whereas a 0.5 nm Al 2 O 3 coating on the same template results in a surface area of ∼1355 m 2 /g (∼30 mg/cm 3 ). Direct BET measurements on nanotubular samples were performed to confi rm the validity of this approach (details can be found in the supporting infor-mation). Furthermore, the experimentally determined values are in excellent agreement with surface area values calculated from the density of the coating material (Al 2 O 3 : 2.8 g/cm 3 , TiO 2 : 3.65 g/cm 3 ) [ 27,28 ] and the measured coating thickness.

The samples show excellent thermal stability. Nanotubular TiO 2 foams do neither change their nanoscale morphology nor their macroscopic dimensions up to 600 °C, where the pore walls become porous due to crystallization of the initially amorphous TiO 2 coating (not shown). This is consistent with thermogravimetric analysis (TGA) data that show less than 5% weight loss in the temperature interval 25–900 °C (Supporting Figure S2). The corresponding nanotubular Al 2 O 3 foams are stable up to 800 °C. More details on the thermal stability of the nanotubular materials will be reported in a separate study.

Rutherford backscatter spectrometry (Supporting Figure S3) and the measured weight loss during etching demonstrate that the Au template is completely removed by the etch process.

Adv. Mater. 2014, 26, 4808–4813

Figure 1. Processing and tunability of low-density nanotubular bulk materials. (A) NPG is used as a sacrifi cial template. Conformal and continuous thin fi lms are deposited using ALD. Finally, the template is removed by a wet etch process, and the resulting nanotubular material is supercritically dried. (B) The volumetric surface area and density can be independently and deterministically controlled by tuning the ligament size of the NPG template and the thickness of the ALD coating.

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Although we successfully removed the NPG template from samples with up to 10 nm thick ALD fi lms, we generally observed that thicker coatings (>2 nm) slow down the etch rate, especially for samples with smaller pores. The observation that the Au core can be effi ciently removed through thin conformal and apparently continuous metal oxide ALD fi lms may be related to the fact that the ALD fi lms studied in this work, were deposited at low temperatures and thus cannot be expected to be fully dense which provides the opportunity to use them as 3D membranes.

Deterministic control over feature size and density is dem-onstrated in Figure 3 . The ligament size of the template, and with that the inner diameter of the hollow nanotubes in the fi nal material, can be easily adjusted from ∼50 nm to ∼3 µm by annealing in the temperature range of 100–900 °C (Figure 3 C). The annealing induced coarsening of the charac-teristic sponge-like morphology of NPG is accompanied by a color change, for example, from dark brown for the samples with ∼50 nm pores to light gold for samples with ∼500 nm pores (Figure 3 A). The corresponding ligament/pore size dis-tributions are shown in Supporting Figure S4. Figure 3 B shows optical images of nanotubular Al 2 O 3 samples with pore sizes ranging from ∼50 to ∼500 nm that were obtained by coating the NPG templates shown in Figure 3 A with 10 Al 2 O 3 ALD cycles. All samples were crack-free, and visual inspection reveals that their transparency decreases with increasing pore size, probably refl ecting an increase in scattering as the feature

size increases (Figure 3 B). The gravimetric surface area of NPG, and thus the density of the nanotubular foam is inverse propor-tional to the ligament diameter. For example, annealing at 300 °C increases the ligament diameter from ∼50 to ∼150 nm (Supporting Figure S4), and thus decreases the gravi-metric surface area of NPG and the density of nanotubular Al 2 O 3 (10 cycles) from ∼3.6 m 2 /g to ∼1.2 m 2 /g and ∼120 to ∼40 mg/cm 3 , respectively (Figure 3 D). Alternatively, the density can be adjusted by the number of ALD cycles (Figure 3 E). We successfully tested this approach in the range from 2 to 40 cycles. One ALD cycle did not yield a stable monolithic sample, and for very thick coatings etching becomes increasingly dif-fi cult. The lowest density sample that stayed monolithic through the whole process was a nanotubular TiO 2 foam with a density of only 5 mg/cm 3 , which is well below the den-sity that can currently be realized with sol-gel chemistry (> 30 mg/cm 3 ).

The interconnected hollow tube archi-tecture provides the material with excellent mechanical properties as demonstrated by the nanoindentation test results that are summarized in Figure 4 . Representative load-displacement curves with single or mul-tiple loading-holding-unloading sequences can be found in the supporting online mate-rial (Supporting Figure S5). As expected, both

Young’s Modulus E and contact pressure (Meyer hardness) H decrease with decreasing density, but for any given density, the values for E and H are higher (sometimes even by more than one order of magnitude) than the previously published values for alumina, [ 29 ] silica, [ 6 ] and CNT [ 30 ] aerogels. Only engineered microlattices [ 4 ] show a comparable stiffness vs . density correla-tion, but their specifi c surface area is at least one order of mag-nitude lower due to their much larger feature size.

The stiffer response of nanotubular foams becomes even more pronounced by plotting the relative Young’s modulus E / E s vs . relative density ρ/ρ s where s denotes the property of full density Al 2 O 3 (TiO 2 ): E s = 150 (160) GPa, [ 28,31 ] H s = 8 (7.6) GPa, [ 28,31 ] and ρ s = 2.8 (3.65) g/cm 3 . [ 27,28 ] The corresponding E s / H s / ρ s values used for normalization of the SiO 2 , Al 2 O 3 and CNT aerogel as well as of the microlattice data are sum-marized in Tabel S1 of the supporting information. The data reveal a quadratic scaling relationship between Young’s mod-ulus and density, E ∼ ρ 2 , which is consistent with a cell wall bending dominated deformation behavior similar to that typi-cally observed for open-cellular materials [ 8 ] and engineered mic-rolattices. [ 4 ] By contrast, a steeper scaling behavior, E ∼ ρ 3–3.7 , has been reported for low density silica, [ 6,32,33 ] alumina, [ 29 ] and carbon nanotube (CNT) [ 30 ] aerogels. An exception to this gen-eral behavior are ultra-low density SiO 2 aerogels prepared by a two-step sol-gel chemistry approach. [ 6 ] The elastic modulus of these aerogels also exhibits a power law scaling behavior with an exponent of ∼2 which has been attributed to a superior ligament

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Figure 2. Synthesis and morphology of nanotubular TiO 2 . (A) SEM image of the sacrifi cial NPG template. (B) Low and (C) high magnifi cation TEM images collected from nanotubular TiO 2 obtained by coating the NPG template (as prepared, 50 nm feature size) with 30 cycles of TiO 2 ALD and removing the Au core by wet etching. The high magnifi cation TEM image (C) reveals a wall thickness of ∼2 nm. (D) Transparent nanotubular TiO 2 disk (5 mm diameter and 200-µm-thick) with 50 nm pores and a density of ∼150 mg/cm 3 .

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connectivity. [ 6 ] A lower scaling exponent is critical for the sta-bility at ultra-low densities as it slows down the deterioration of the mechanical properties with decreasing density compared to aerogels with their steeper scaling behavior. The relative contact hardness H/H s shows a similar scaling behavior with a scaling coeffi cient of ∼2.3.

The superior mechanical properties of the nanotubular materials tested here seem to be a consequence of both shape and connectivity of the individual ligaments. For example, Kucheyev et al. [ 29 ] have shown that the mechanical properties of alumina aerogels with a nanoleafl et morphology are dra-matically improved over those with a traditional string-of-pearl morphology. The nanotubular morphology makes the material even stiffer as shown by Rayneau-Kirkhope et al. [ 34 ] who dem-onstrated that a hollow tubular strut morphology can signifi -cantly improve the load bearing capability of cellular materials. While the shape of the ligaments can explain differences in the mechanical properties for a given density, the connectivity of the ligament seems to be responsible for differences in the scaling behavior which determines the mechanical stability at ultralow densities. For example, Woigner et al. [ 33 ] proposed that the scaling behavior depends on the connectivity of the building block. A unique feature of the templated nanotubular material is that their ligament connectivity does not change with density which is solely determined by wall thickness and feature size. This is contrasted, for example, by CNT aerogels whose aspect ratio (length to diameter) increases with decreasing density which seems to be responsible for the steeper E ∼ ρ 3 scaling behavior limiting their E and H at ultra-low densities.

The approach described here can easily be expanded to other ALD processes that are capable of uniformly coating ultrahigh aspect ratio templates such as NPG. Promising ALD processes include, for example, tungsten (WF 6 /B 2 H 6 ) [ 21 ] and ZnO (die-thyl-zinc/water) [ 22 ] deposition. If required, precisely dimen-sioned foam parts can be obtained by using an appropriately shaped template, thus avoiding the diffi culties that arise from machining ultralow-density foams. Additional coating of the free-standing nanotubular material can be used to further tune the pore size of the material with several Angstrom resolu-tion (given by the growth rate of the ALD process) which will be important for the development of tunable membranes for fi ltration. Finally, the unique morphology of our nano tubular material with two independent and interwoven pore systems separated by a nm-thick 3D membrane opens the door to the development of new 3D membrane concepts that mimic nature’s complex three-dimensional membrane structures that provide organs such as the lung and kidneys with their func-tionality. First proof-of-principle experiments are currently per-formed and have already shown promising results.

Experimental Section Disk (diameter ∼5 mm, thickness 200–300 µm) and cylinder (diameter and height ∼2 mm) shaped NPG templates with a porosity of ∼70% were prepared by selective dissolution (dealloying) of Ag 0.7 Au 0.3 alloy samples in concentrated nitric acid (48 h, ∼65 wt% HNO 3 ) as previously described. [ 35 ] The as-prepared material has a narrow pore size distribution with an average ligament diameter of ∼58 nm and a specifi c surface area of ∼3.6 m 2 /g. As described previously and as

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Figure 3. Control over feature size and density. Optical images from (A) 200-µm-thick NPG disks with different ligament diameters ranging from 50 nm (no heat) to 500 nm (annealing at 500 °C) and (B) the corresponding nanotubular Al 2 O 3 samples obtained by coating the template with 10 Al 2 O 3 ALD cycles. (C) The characteristic length scale of NPG templates can be adjusted from ∼50 nm to ∼3 µm by annealing in the temperature range of 100–900 °C. Deterministic control over density by adjusting (D) the coating thickness through the number of (here: Al 2 O 3 ) ALD cycles and (E) the ligament diameter of the NPG template by annealing (keeping the ALD coating thickness constant). The error bars refl ect the standard deviation from the average ligament diameter obtained by geometrical evaluation.

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shown in Figure 3 c and Supporting Figure S4, the feature size of NPG can be increased by two orders of magnitude using a simple annealing procedure [ 23 ] (the average ligament diameter for each sample was determined from ligament diameter distributions that were obtained by geometrical evaluation).

The NPG templates were coated with nanometer-thick Al 2 O 3 or TiO 2 ALD fi lms as described previously. [ 23 ] In short, we used the well-established trimethyl-aluminum (AlMe 3 /H 2 O) [ 20 ] and titanium tetrachloride (TiCl 4 /H 2 O) [ 36 ] ALD processes in a warm wall reactor (wall and stage temperature of 125 °C for Al 2 O 3 and 110 °C for TiO 2 ). Long exposure and purge times (300 s each) were used to ensure uniform coatings throughout the porous material. The resulting growth rate for Al 2 O 3 and TiO 2 fi lms is ∼0.25 nm and ∼0.07 nm per cycle, respectively.

To remove the NPG templates, the ALD coated samples were immersed in 2 mL of KI/I 2 solution (0.5 KI: 1 I 2 : 8.5 H 2 O in weight). [ 24 ] After sitting for 5 days, the residual transparent foams were washed by DI water and acetone. The wet foams were then dried in a Polaron supercritical point drier. The acetone in the foams was exchanged by liquid CO 2 for 24 hours, after which the temperature of the vessel was ramped up to 45 °C with a pressure maintained at ∼1500 psi. The vessel was then slowly depressurized to atmosphere. Additional details regarding sample characterization and nanomechanical testing can be found in the online supporting information.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements Work at LLNL was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52–07NA27344. Project 13-LWD-031 was funded by the LDRD Program at LLNL. We gratefully acknowledge Prof. Andrea Hodge, University of Southern California, who provided polished samples for mechanical testing. Additional information regarding the synthesis and characterization of the materials can be found in the Supplementary Materials.

Received: January 16, 2014 Revised: May 12, 2014

Published online: May 30, 2014

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