Robust Sandwich‐Structured Nanofluidic Diodes Modulating Ionic Transport for an Enhanced Electrochromic Performance

Abstract Biomimetic solid‐state nanofluidic diodes have attracted extensive research interest due to the possible applications in various fields, such as biosensing, energy conversion, and nanofluidic circuits. However, contributions of exterior surface to the transmembrane ionic transport are often ignored, which can be a crucial factor for ion rectification behavior. Herein, a rational design of robust sandwich‐structured nanofluidic diode is shown by creating opposite charges on the exterior surfaces of a nanoporous membrane using inorganic oxides with distinct isoelectric points. Potential‐induced changes in ion concentration within the nanopores lead to a current rectification; the results are subsequently supported by a theoretical simulation. Except for providing surface charges, functional inorganic oxides used in this work are complementary electrochromic materials. Hence, the sandwich‐structured nanofluidic diode is further developed into an electrochromic membrane exhibiting a visual color change in response to redox potentials. The results show that the surface‐charge‐governed ionic transport and the nanoporous structure facilitate the migration of Li+ ions, which in turn enhance the electrochromic performance. It is envisioned that this work will create new avenues to design and optimize nanofluidic diodes and electrochromic devices.


The morphology of AAO nanoporous membrane
As shown in Figure S1, the same pore size on the two sides and the well-aligned nanotubular arrays of the cross-section both indicated that the AAO nanoporous membrane used here had cylindrical channels. Furthermore, a high pore density provided a high background current of several microamperes. Figure S1. The top views of two sides (a, b) and the magnified cross-sectional image (c) of the AAO nanoporous membrane.

Ionic conductivity of AAO membrane before and after each layer deposition
From the I-V behaviors ( Figure S2a), we can see that the magnitude of transmembrane ionic current decreases when the AAO membrane is covered by a WO 3 layer because of an enhanced steric hindrance. In this case, the ionic conductance declines from 3.43 to 2.48 μS at -2 V, and from 3.33 to 0.57 μS at +2 V ( Figure S2b). Then, there is a further reduction in the ionic current after the deposition of a NiO layer on the other side of the AAO membrane. The ionic conductance further drops to 1.75 μS and 0.11 μS at -2 V and +2 V, respectively.

Surface morphologies of WO 3 and NiO thin layers
The WO 3 and NiO thin layers on the AAO membrane obtained using magnetron sputtering were composed by nanoparticles as shown in Figure S3. The ions would pass the layers through the tiny gaps between the particles.

WO 3 and NiO distributions in the nanochannels
The element distribution mappings in the cross-sectional region of the AAO nanoporous membrane was measured to understand whether the inner surface of the nanochannels was stained by the metallic oxides. As shown in Figure S4, nickel and tungsten elements substantially exist in the two marginal areas of the membrane, and little could be detected in the nanochannel section. Therefore, it is reasonable to believe that there is little WO 3 and NiO deposited in the nanochannels.
Furthermore, the average pore diameter of original AAO membrane is determined to be 19.2 nm, and that is 18.6 nm after the deposition of WO 3 and NiO layers ( Figure S5). This negligible variation in the pore size further proves that the interior surface of nanochannels is almost not stained by WO 3 or NiO.   Figure S6 showes the measurement setup for investigating the I-V properties of the sandwich-structured nanofluidic diodes. Figure S6. Schematic drawing of the setup for measuring the I-V curves.

I-V behaviors of WO 3 @AAO and NiO@AAO membranes
For comparison, the I-V properties were measured after individually removed the WO 3 layer and NiO layer from the sandwich-structured nanofluidic diodes in 1 mM KCl electrolyte (pH 7.2). As shown in Figure S7, the AAO nanoporous membrane covered with WO 3 layer exhibited a slight ion rectification with a ratio of 3.1. This could be ascribed to the formation of p-n junction ionic channels in heterogeneous WO 3 /AAO nanoporous membrane. [1] However, the ion rectification ratio decreased significantly in an AAO nanoporous membrane covered with NiO layer because the two components both had positive charges with similar density owing to their close pI values.

The influence of steric hindrance and wettability on the asymmetric ionic transport
As shown in Figure S8, the magnitude of the ionic current reduces significantly after the AAO nanoporous membrane is sandwiched by the two WO 3 or NiO layers, indicating an effect of steric hindrance on the ionic transport. The WO 3 and NiO layers demonstrate quite similar steric effect because of their overlapping I-V curves. Note that the AAO membrane exhibits a linear I-V behavior after the coverage of WO 3 or NiO layers on both sides, while an ion rectification behavior appears when it is sandwiched by a WO 3 and a NiO layer. This suggests that the opposite charge on the outer surface of the membrane is a decisive cause of the asymmetric transmembrane ion transmission. Therefore, the steric hindrance of WO 3 and NiO layer could decrease the transmembrane ionic conductance but has little effect on the asymmetric ionic transport. In view of wettability effect, original AAO membrane exhibits hydrophilicity and its surface water contact angle (CA) is determined to be 65° ± 1°. After the deposition of two metallic oxide layers, the CA of NiO-covered side is 51° ± 2° and that of WO 3 side is 66° ± 2° ( Figure S9). There is no significant wettability difference between the two sides of the membrane. Furthermore, the sandwich-structured membrane allow the ionic current to flow preferentially from the WO 3 side, which is in conflict with the effect of wettability on fluids.
Therefore, the wettability has little influence on the transmembrane ionic transport. Figure S9. Surface water contact angles of AAO membrane, NiO and WO 3 -covered side of sandwich-structured membrane.

Theoretical simulation
A theoretical simulation was performed to investigate the ion rectification mechanism of our sandwich-structured nanofluidic diodes. The theoretical model was simplified as a 2D cylindrical nanopore whose inner surface was electrically neutral and the two exterior surfaces carried opposite charges ( Figure S10). The pore diameter and channel length was set as 20 nm and 100 nm.
Where  and  are the electrical potential and the dielectric constant of medium. z i , c i , J i and D i represent the charge number, concentration, particle flux and the diffusion coefficient of species i, respectively. The electrolyte used here was 1 mM KCl aqueous solution. Therefore, the diffusion coefficient of K + and Cl -, D K + and D Cl -, were determined to be 1.96×10 -9 and 2.03×10 -9 m 2 /s, and the dielectric constant  was 80. [2a,3] The boundary conditions for the electrical potential and ion flux are, Where → and denote the unit normal vector and surface charge density. In our model, the charge density on the inner wall of the nanochannel was set to zero, and the surface charge density  1 and  2 were set as -5 × 10 -4 and 5 × 10 -4 C/m 2 , respectively. The bottom edge of the lower reservoir was grounding. The ionic current of species i could be calculated through integrating its particle flux along the cross section of the nanochannel shown in Equation S6, from which the relationship between the ion current and the electrical potential was established.

ds RT
The following I-V behavior obtained through above theoretical calculation shew a good agreement with our experimental demonstrations ( Figure S11). Taking WO 3 as an example in our system, the depositing amount increased with increasing magnetron sputtering time, which directly influenced its coverage on the surface of AAO membrane and its thickness. As shown in Figure S12, the surface coverage increased with the thickness of the WO 3 layer, leading to an enhancement in the rectification ratio ( Figure S13).
With a further increase of the thickness, the formation of large cracks was not was not conducive to the surface-charge dominates ( Figure S12d). Moreover, the compact layer composed of large particles definitely blocked the entrances of ion transport through nanochannels. As a result, there was a decrease in ion rectification ratio and ion current ( Figure S13).

Distributions of T and RH in the period of air-stability study
During the period of air-stability study, the average highest and lowest temperature ranged from 16 to 34 ºC and from 6 to 25 ºC, respectively ( Figure S14a). The relative humidity demonstrated a wide distribution from 3% to 57% shown in Figure S14b.

Optical property of AAO nanoporous membrane
AAO nanoporous membrane had a high transparency with a transmittance of ~70% and an absorbance of ~0.15 within the wavelength ranging from 400 to 850 nm ( Figure S15). Figure S15. The transmittance and absorbance spectra of AAO nanoporous membrane within the wavelength ranging from 400 to 850 nm. Inset: Photograph of AAO nanoporous membrane on a patterned paper.

Cycle ability of the electrochromic performance
The absorbance modulation rate at a wavelength of 750 nm reaches 86.1 % of the initial value after 100 cycles of alternate application of the redox potentials ( Figure S16). The limitation of the cycle stability mainly lies in that it is impossible to extract all ions inserted in the electrochromic layers by a constant oxidation potential.

I-V behaviors of nanofluidic diodes annealed at a low temperature
When reduced the annealing temperature to 300 °C, the sandwich-structured nanofluidic diodes exhibit an ion rectification behavior with a ratio of 8.2 ( Figure S17). The rectification trend is the same as the nanofluidic diodes obtained at 500 °C, but the rectification ratio is lower. The metallic oxide with a specific crystal has a definable isoelectric point that could be used to determine the surface charge polarity. Reducing the annealing temperature decreases the crystallinity of WO 3 and NiO, which in turn decreases the charge density of the two layers.
Therefore, the ion rectification ratio reduces with the reduction of the annealing temperature.