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Letter pubs.acs.org/NanoLett
Template-Free Mesoporous Electrochromic Films on Flexible Substrates from Tungsten Oxide Nanorods Sungyeon Heo,† Jongwook Kim,†,§ Gary K. Ong,†,‡ and Delia J. Milliron*,† †
McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, United States
‡
S Supporting Information *
ABSTRACT: Low-temperature processed mesoporous nanocrystal thin films are platforms for fabricating functional composite thin films on flexible substrates. Using a random arrangement of anisotropic nanocrystals can be a facile solution to generate pores without templates. However, the tendency for anisotropic particles to spontaneously assemble into a compact structure must be overcome. Here, we present a method to achieve random networking of nanorods during solution phase deposition by switching their ligand-stabilized colloidal nature into a charge-stabilized nature by a ligand-stripping chemistry. Ligand-stripped tungsten suboxide (WO2.72) nanorods result in uniform mesoporous thin films owing to repulsive electrostatic forces preventing nanorods from densely packing. Porosity and pore size distribution of thin films are controlled by changing the aspect ratio of the nanorods. This template-free mesoporous structure, achieved without annealing, provides a framework for introducing guest components, therefore enabling our fabrication of inorganic nanocomposite electrochromic films on flexible substrates. Following infilling of niobium polyoxometalate clusters into pores and successive chemical condensation, a WOx−NbOx composite film is produced that selectively controls visible and near-infrared light transmittance without any annealing required. The composite shows rapid switching kinetics and can be stably cycled between optical states over 2000 times. This simple strategy of using anisotropic nanocrystals gives insight into mesoporous thin film fabrication with broader applications for flexible devices. KEYWORDS: Tungsten oxide, mesoporous thin films, electrochromic, flexible substrates, ligand-stripping, nanocomposite anocrystalline mesoporous thin films with pore diameters from 2 to 50 nm are widely sought after for sensors,1 catalysts,2 as well as optical3 and electrochemical applications.4 The presence of pores increases the active surface area and simultaneously improves diffusion kinetics, particularly in electrochemical devices.5 In addition, guest components such as dye,6 polymer,7 and inorganic materials8 can be incorporated into a porous structure to create nanocomposite systems with novel properties. For instance, our group demonstrated recently that electrochromic WOx−NbOx nanocomposite films prepared by polymer templating exhibit superior modulation of visible and near-infrared transmittance with fast switching speeds.9 Introducing mesoporosity typically relies on porogens or structure-directing agents comprised of small surfactant molecules or polymers.10−12 Careful tuning of block ratios or molecular weight of block copolymers enables control over porosity, pore dimension, and wall thickness.13−15 Porogens can also be native organic ligands bound to the nanocrystal surface inherently needed to control growth kinetics and confer stable dispersions in the organic solvents. However, for any organic templating agent, thermal removal after deposition on substrates can cause unwanted sintering of nanocrystalline constituents accompanied by irreversible changes in material properties.16 Furthermore, a high temperature template
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removal step is incongruent with a solution-processing manufacturing paradigm that should ideally be scalable via roll-to-roll processes onto low-cost flexible substrates and form factors. In this respect, low-temperature template removal processes have been explored in an effort to lower manufacturing costs, retain nanocrystal properties, and allow future flexible device applications. Most approaches used so far rely on alternative treatment such as UV irradiation,17,18 plasma treatment,19 or dissolution by solvents with cross-linking of nanocrystals using bidendate molecules for structure retention during template removal.20 However, an ideal scenario would be to completely eliminate thermal or chemical processing with its associated potential for disrupting chemical and physical properties. We aimed to develop a template-free process for depositing mesoporous structures from bare nanocrystals. In particular, assemblies of nanorods exhibit unique optical, electrical, and magnetic properties, making them good candidates for photonics and electronics applications.21,22 As such, a lot of research has focused on orientationally ordering nanorods; however, the potential for random packing, namely, Received: June 28, 2017 Revised: July 30, 2017
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Figure 1. Characterization of WO2.72 nanorods with variable aspect ratios. (A,B) STEM images of low (A) and high (B) aspect ratios of nanorods. (C) XRD patterns of low and high aspect ratios of nanorods and reference monoclinic WO2.72 pattern (ICSD code #15254). (D) Absorbance spectra of dispersions of low and high aspect ratio nanorods in tetrachloroethylene.
mesoporous films, but they are also functional. Dynamic change of localized surface plasmon resonance (LSPR) has been proven in indium tin oxide nanocrystal films through electrochemical modulation of carrier concentration.23 Compared to other semiconductor plasmonic nanocrystals such as transparent conducting oxide, which exhibit most of their LSPR absorption in the infrared region from 1400 to 3000 nm wavelengths, tungsten suboxide nanocrystals exhibit electrochemically tunable LSPR at the shorter wavelengths in the nearinfrared region, well overlapped with the solar spectrum.24−28 Moreover, absorbance of both low and high aspect ratios of WO2.72 nanorods spans the solar-relevant region of the nearinfrared owing to contributions from both longitudinal and transverse absorption modes,24,29 making these nanorods desirable for near-infrared absorbing “smart window” materials (Figure 1D). As-synthesized WO2.72 nanorods capped with oleylamine dispersed in toluene were deposited by spin coating to ascertain how nanorods with bound ligands are arranged following solvent evaporation. Dense assemblies with side-by-side packing were observed, similar to previous reports of spincoated ZnO and boehmite nanorods (Figure 2A,B).22,30,31 This arrangement can be explained by the increase in the solution concentration owing to solvent evaporation inducing side-byside packing driven by dominant van der Waals attraction. Both in-plane alignments and tilted domains were observed by crosssection scanning electron microscopy (SEM, Supporting Information Figure S1). These tilted-aligned domains were apparently formed during the deposition process and were not present in the dispersion since small-angle X-ray scattering (SAXS) analysis of a dispersion showed no noticeable peak due to structure factor as was apparent in grazing incidence-SAXS (GISAXS) of nanorod thin films (Supporting Information Figure S2). It is possible that the tilted-aligned domains form as a critical concentration for nucleation of nanorod aggregates is reached before the wet layer has thinned down to the nanorods’ length, then these orientationally ordered aggregates experience centrifugal force that radially tilts them as the film continues to dry.30
disordering of nanorods, has been relatively unexplored despite the interesting opportunities of loose packing. So, we hypothesized that mesopores could be generated by achieving random, open packing in a nanorod assembly. Herein, we demonstrate low-temperature and template-free fabrication of mesoporous thin films of WO2.72 nanorods that form a random, open arrangement due to their chargestabilized nature obtained by ligand-stripping. Key factors enabling fabrication of porous films are the use of anisotropic nanocrystals, specifically nanorods, and the presence of electrostatic repulsion forces after ligand-stripping that prevent close-packing of the nanorods. High porosity up to 58% of the volume of the thin film was achieved by using long nanorods; tuning the nanorods’ aspect ratios enabled control of porosity. This new process enabled low-temperature fabrication of electrochromic nanocomposite WOx−NbOx films on flexible substrates with chemical condensation of polyniobate clusters. These films selectively modulate visible and near-infrared light transmission. This new electrochromic material simultaneously embodies the highest performing material for dual-band (visible and near-infrared) modulation and the most favorable processing conditions that can ultimately enable large scale, low cost manufacturing. We selected tungsten suboxide nanorods as a model system for the following reasons. First, colloidal synthesis can be used to tune aspect ratios by adjusting the concentrations of oleylamine and trimethylamine N-oxide, allowing us to study the effect of aspect ratio on porosity. Low (7; width: 4.3 ± 0.9; length: 30.2 ± 10.1) and high (16; width: 5.3 ± 1.0; length: 84.4 ± 15.1) aspect ratio nanorods were synthesized and analyzed by scanning transmission electron microscope (STEM) analysis (Figure 1A,B). The as-synthesized nanorods are capped with oleylamine ligands, which enables steric stabilization of dispersions in organic solvents such as toluene and hexane. X-ray diffraction (XRD) analysis confirmed both aspect ratios of nanorods were monoclinic WO2.72 phase (Figure 1C). The long axis of the nanorods is along the [010] crystallographic direction, giving rise to a sharp (010) reflection in the XRD patterns. Second, WO2.72 nanorods are not only physical building blocks to demonstrate the fabrication of B
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acidity, to avoid etching the WO2.72 nanorods.35 Fourier transform infrared (FTIR) spectroscopy was used to verify removal of ligands. After treatment with the Meerwein’s salt, the signal of C−H stretching modes around 2900 nm−1 due to oleylamine was largely absent, proving effective removal of ligands (Figure 3A). One interesting feature, also observed in the ligand-stripped PbSe nanocrystals,35 is there is no stretching band of BF4−, expected around 1080 cm−1, nor any apparent carbonyl stretching mode from adsorbed solvent molecules of N,N-dimethylformamide (DMF), expected around 1650 cm−1, both of which are known for coordinating bare nanocrystal surface having positive zeta potential and surface charge following ligand stripping. The absence of these species can be rationalized based on zeta potential measurement of our ligand-stripped WO2.72 nanorods. After stripping ligands, surface zeta potential shifted to a strongly negative value of around −50 mV (Figure 3B). Considering a negatively charged surface, coordination by BF4− is not expected, and DMF coordination would at least be weakened, consistent with the FTIR results. Owing to strong electrostatic repulsions, ligandstripped WO2.72 nanorods were stable in DMF dispersion for more than a month. This negative potential is contrary to our expectations, differing from previous results reporting positively charged ligand-stripped nanocrystals.34 X-ray photoelectron spectroscopy analysis before and after ligand-stripping indicates that the O:W ratio increased after ligand-stripping, so we surmised that the nanocrystal surfaces after stripping were likely hydroxylated (Supporting Information Figure S4). Assuming the isoelectric point of tungsten suboxide nanorods is low (it is around pH 1 for bulk WO3), this negative zeta potential can be rationalized.36,37 In contrast to the case of ligand-capped nanorods, deposition of thin films from dispersions of these ligand-stripped, chargestabilized nanorods resulted in a mesoporous structure associated with the random orientation of the nanorods (Figure 2C,D and Supporting Information Figure S1). The obtained films were macroscopically uniform and optically clear without scattering or haze implying the uniform distribution of nanorods and the subwavelength size of pores. Such an optically clear film is highly desirable for optical coatings and devices. Considering electrochemical applications require good adhesion of the layer, attachment and detachment of scotch tape onto thin films was tested to check mechanical integrity
Figure 2. Top-down SEM images of ligand-capped and ligand-stripped low and high aspect ratio nanorod films. (A,B) Low and high aspect ratios of ligand-capped nanorod films. These films were additionally annealed to remove the organic ligands on the nanocrystal surface. (C,D) Low and high aspect ratios of ligand-stripped nanorod films. Optical microscope images of nanorod films are shown in Figure S3.
Close packed films of ligand-capped nanocrystals are unsuitable for electronic or electrochemical applications since the insulating ligands severely limit electron transport through the film.32 Often, long ligands are exchanged with short ligands following film deposition to improve transport properties.23 Alternatively, ligands can also be replaced or removed in the solution before film processing.33,34 Solution phase ligand stripping offers the additional opportunity to change the nature of the dispersions from sterically stabilized to electrostatic since ligand-stripped nanocrystals have been shown to have substantial zeta potential making them stably dispersible in polar solvents.34 We anticipated stripping the ligands on the WO2.72 nanorods would lead to repulsive internanorod forces that might affect the arrangement of the nanorods following film deposition. While NOBF4 was introduced as a general reagent for removing native ligands from metal oxide nanocrystals,34 the strong Lewis acidity of the nitrosonium cation can lead to etching of certain nanocrystals such as tungsten oxide and ZnO. So, we used Meerwein’s salt (Et3OBF4), which lacks significant
Figure 3. Surface characterization of ligand-capped and ligand-stripped nanorods. (A) FTIR spectra for WO2.72 nanorod films on CaF2 windows before and after ligand-stripping. (B) Zeta-potential of WO2.72 nanorods before and after ligand-stripping. C
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Figure 4. Porosity and pore size distribution of random mesoporous films measured by ellipsometric porosimetry. (A,C) Porosimetry of high (A) and low (C) aspect ratio nanorods deposited as mesoporous films. (B,D) Pore size distribution of high (B) and low (D) aspect ratio nanorod films.
porosity of randomly oriented films prepared from ligandstripped nanorods. In the mesoporous films, high aspect ratio nanorods showed higher porosity (58%) than low aspect ratio nanorod (15%) (Figure 4A,C). The impact of different aspect ratios of nanorods corresponds surprisingly well with experiments using macroscopic scale raw spaghetti in beakers.39 As the length of spaghetti segments increased, packing density decreased; in other words, porosity increased. The simple mathematical random contact equation is consistent with this result, which also agrees with other experimental results on nanorod packing across length scales.40 Hence, we can control the porosity by using different aspect ratios of anisotropic nanocrystals. The porosity obtained with randomly oriented high aspect ratios nanorods is even higher than that of the mesoporous structure that we made previously by the templated approach using isotropic nanocrystals.15 Observed hysteresis of toluene adsorption and desorption closely related to the pore structure is type H2, showing a steep desorption branch, also observed in the porous Vycor glass and is characteristic of irregular pores in which case interconnected networks effects are important.41,42 Although absolute pore radii derived from the data may not be precise due to the necessity of assuming cylindrical pore geometry for fitting the data, given the structural similarity of two samples, we can confidently make a comparison. In this framework, the data shows the pore radius increases together with the porosity when the nanorod aspect ratio is increased (Figure 4B,D). Furthermore, these mesoporous nanorod films can be a platform for designing various nanocomposite films. Here we develop the example of a composite electrochromic thin film. Taking advantage of the low-temperature template-free mesoporous film fabrication process, 300 nm-thick films were deposited onto the tin-doped indium oxide (ITO) coated
(Supporting Information Figure S5). Both films exhibit no apparent delamination of nanocrystals, providing benefits for electrochemical applications of these films without needing an annealing process. Long range random arrangement of ligandstripped nanorods films was also confirmed by GISAXS analysis (Supporting Information Figure S2). Ligand-stripped films did not exhibit a distinct structure factor implying no particular packing or orientation of nanorods, whereas there is a distinct peak in the scattering intensity for films of ligand-capped nanorods, indicating some local arrangement of those nanorods, consistent with SEM. We ascribe the random orientation of the charge-stabilized nanorods and their mesoporous packing to electrostatic repulsions between the strongly negatively charged nanorods after ligand stripping, which prevents close side-by-side packing even though concentration increases during the spin coating process. Considering the randomly packed arrangement of our nanorods in the final state, we surmise the nanorods are ultimately kinetically trapped, that is to say, the nanorods become locked in a jammed state. Slow solvent evaporation by drop casting and spin coating at slow spin speed confirms this hypothesis since more densely packing structures result in those cases (Supporting Information Figure S6). The effects of the nanorod aspect ratio on the film porosity and pore size distribution were examined by ellipsometric porosimetry.38 This method uses the refractive index change of thin films with gradual adsorption and desorption of solvent from the vapor, toluene in our case. For the densely packed structures annealed to remove the porogens, made from ligandcapped nanorods, both low and high aspect ratios gave rise to films with limited porosity of less than 16% and porosity differed little with aspect ratio (Supporting Information Figure S7). By contrast, aspect ratio dramatically influenced the D
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Figure 5. Spectroelectrochemical properties of low-temperature processed composite films. (A) Transmittance spectra and corresponding photographs of a WOx−NbOx composite thin film on an ITO/PET substrate at each potential [4 V (bright), 2.4 V (cool), 1.5 V (dark)] (B) Normalized charging and discharging profiles following potentiostatic steps (2.4 V, 1.5 V, 4 V) [vs Li/Li+ in 1 M Li-TFSI/tetraglyme].
the anisotropic morphology of WO2.72 nanorods. Transforming the initially ligand-capped colloidal dispersion of nanorods to a charge-stabilized dispersion of naked nanorods by ligandstripping allowed depositing nanorods with random orientation thereby creating large volumes of mesopores. This templatefree and low-temperature process enables retention of nanocrystal properties as well as enabling deposition on flexible substrates. Moreover, porosity and pore size can be controlled by using nanorods with different aspect ratios. As an example of a mesostructured composite system made by this method, a flexible WOx−NbOx composite film was fabricated, and its fast dual-band electrochromic switching kinetics were demonstrated. Though nanorods are demonstrated as an example of anisotropic systems, other anisotropic nanocrystals such as nanoplatelets may conceivably be assembled into open structures in a similar way. This methodology not only broadens the existing strategies for forming mesoporous architectures, but also directly enables simple fabrication of highly mesoporous thin films of functional materials for flexible, wearable, and disposable devices.
polyethylene terephtalate (PET) substrates using high aspect ratio WO2.72 nanorods, with a single spin-coating process. Combining this approach with another recently published process to chemically condense niobium polyoxometalate clusters [Nb10O28]6− (Nb10POM) to form electrochromic amorphous NbOx at low temperature,43 we fabricated inorganic electrochromic composite films without any high temperature annealing processes, enabling deposition on low cost flexible substrates, namely PET. Specifically, after deposition of mesoporous films of the WO2.72 nanorods, Nb10POM was infiltrated into the pores in a second spin coating step. Then NbOx was formed by chemical condensation (see details in the Supporting Information). In situ spectroelectrochemical studies of this composite film showed selective modulation in the visible and near-infrared regions as a function of the charging voltage (1.5 V, 2.4 V, 4 V vs Li/Li+) (Figure 5A). At 4 V (“Bright mode”), transmission of the film is high in both visible and near-infrared regions as WOx and NbOx are in their oxidized states. When 2.4 V (“Cool mode”) is applied, transmittance in the near-infrared region is lowered due to charging of WOx, blocking the solar heat gain while largely maintaining the visible transmittance which is useful for daylighting. At 1.5 V (“Dark mode”), NbOx is also intercalated with Li+ so the visible transmittance also decreases. At this voltage, WOx is also further charged so that the film effectively blocks the light and heat. The color indices (CIELab space: L* = 42.6, a* = −1.1, b* = −22.5) in the dark mode indicate a blue-gray color, which is aesthetically desirable. When it is discharged at 4 V, the composite film shows fast discharging kinetics indistinguishable from the rapid discharging speed of the mesoporous WOx without any NbOx (Figure 5B). These rapid kinetics were previously correlated with an organized framework of WOx and the active role of the WOx/ NbOx interfaces, which facilitate Li+ ion extraction, differentiating the properties from those achievable in a simple mixture of WOx and NbOx.9 When simple blending of WO2.72 nanorods with Nb10POM was tried, slow electrochemical switching kinetics and limited optical modulation was exhibited. The composite film also shows excellent cycling stability over 2000 cycles, much more stable than films of the single components, WOx and NbOx, which lose 60% and 30% of their charge capacity, respectively, over the same number of electrochemical cycles (Supporting Information Figure S9). In summary, we exemplified a new methodology to fabricate uniform, optical quality, mesoporous thin films by exploiting
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02730. Experimental and characterization details, XPS, ellipsometric porosimetry, optical microscope, SEM, SAXS, GISAXS, and cyclic voltammetry (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Delia J. Milliron: 0000-0002-8737-451X Present Address §
Laboratoire de Physique de la Matière Condensée, Ecole Polytechnique, CNRS, Université Paris-Saclay, 91128 Palaiseau, France Notes
The authors declare the following competing financial interest(s): D.J.M. has a financial interest in Heliotrope Technologies, a company pursuing commercial development of electrochromic devices. E
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ACKNOWLEDGMENTS The authors would like to acknowledge Dr. Nina Hong from J.A.Woollam for the assistance with measurement and helpful discussion of porosity and pore size distribution data, Dr. Hugo Celio for the analysis of XPS measurements, Dr. Yang Wang for the discussion of the chemical condensation process, and Dr. Evan Runnerstrom for the discussion of the coating process. SAXS and GISAXS data were collected at the Advanced Light Source, Lawrence Berkeley National Laboratory, a user facility supported by the Office of Science, Office of Basic Energy Sciences, of the U.S Department of Energy (DOE) under Contract No. DE-AC02-05CH11231. S.H., J.K., G.K.O., and D.J.M. acknowledge funding from the Welch Foundation (F1848), the National Science Foundation (CHE-1609656), and a U.S. Department of Energy (DOE) ARPA-E grant.
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